@article{tulve_geller_hagerthey_julius_lavoie_mazur_paul_frey_2024, title={Challenges and opportunities for research supporting cumulative impact assessments at the United States environmental protection agency's office of research and development}, volume={30}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85182004820&partnerID=MN8TOARS}, DOI={10.1016/j.lana.2023.100666}, journal={Lancet Regional Health - Americas}, author={Tulve, N.S. and Geller, A.M. and Hagerthey, S. and Julius, S.H. and Lavoie, E.T. and Mazur, S.L. and Paul, S.J. and Frey, H.C.}, year={2024} }
 @article{hu_frey_boroujeni_2023, title={Contribution of Cold Starts to Real-World Trip Emissions for Light-Duty Gasoline Vehicles}, volume={14}, ISSN={["2073-4433"]}, url={https://doi.org/10.3390/atmos14010035}, DOI={10.3390/atmos14010035}, abstractNote={For catalytic converter-equipped light-duty gasoline vehicles (LDGV), the hot-stabilized tailpipe emissions for pollutants such as carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) are well controlled. However, there are few reported real-world measurements of cold starts. Idling cold start and hot-stabilized trip exhaust emissions were measured for 37 LDGVs using a portable emissions measurement system (PEMS). Five vehicles were also measured for transient driving cold starts. On average, it took approximately 400, 150, 330, and 120 s to accumulate 90 percent of the idle cold start increments for fuel use, CO, HC, and NOx, respectively. Driving cold start increments were substantially higher than idling cold start increments, whereas cold start duration was typically shorter. For example, driving cold start contributed approximately 64%, 68%, 58%, and 4.5% of the trip total CO, HC, NOx, and carbon dioxide (CO2), respectively. This study is unique in quantifying the cold start contribution on a trip basis with real-world data. Although the cold start increment is sensitive to driving compared to idling, in either case, cold starts contribute substantially to total exhaust mass emissions. Furthermore, driver decisions regarding driving versus idle can substantially affect the contribution of cold starts, especially for CO and NOx.}, number={1}, journal={ATMOSPHERE}, author={Hu, Jiangchuan and Frey, H. Christopher and Boroujeni, Behdad Yazdani}, year={2023}, month={Jan} }
 @article{rastogi_frey_wei_2023, title={Identifying emissions hotspots and strategies to reduce real-world fuel use and emissions for passenger rail: A spatially resolved approach}, volume={896}, ISSN={["1879-1026"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85164224998&partnerID=MN8TOARS}, DOI={10.1016/j.scitotenv.2023.165110}, abstractNote={The objectives of this work are to model spatially resolved passenger locomotive fuel use and emission rates, locate emissions hotspots, and identify strategies to reduce trip train fuel use and emissions. Train fuel use and emission rates, speed, acceleration, track grade, and track curvature were quantified based on over-the-rail measurements, using portable emission measurement systems, for diesel and biodiesel passenger rail service on the Amtrak-operated Piedmont route. Measurements included 66 one-way trips and 12 combinations of locomotives, consists, and fuels. A locomotive power demand (LPD) based emissions model was developed based on the physics of resistive forces opposing train motion, taking into account factors such as speed, acceleration, track grade, and curvature. The model was applied to locate spatially-resolved locomotive emissions hotspots on a passenger rail route, and also identify train speed trajectories with low trip fuel use and emissions. Results show that acceleration, grade, and drag are the major resistive forces affecting LPD. Hotspot track segments have 3 to 10 times higher emission rates than non-hotspot segments. Real-world trajectories are identified that reduce trip fuel use and emissions by 13 % to 49 % compared to the average. Strategies for reducing trip fuel use and emissions include dispatching energy-efficient and low-emitting locomotives, using a 20 % blend of biodiesel, and operating on low-LPD trajectories. Implementing these strategies will not only decrease trip fuel use and emissions but reduce the number and intensity of hotspots and, thus, lowering the potential for exposure to train-generated pollution near railroad tracks. This work provides insights on reducing railroad energy use and emissions, which would lead to a more sustainable and environmental-friendly rail transportation system.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Rastogi, Nikhil and Frey, H. Christopher and Wei, Tongchuan}, year={2023}, month={Oct} }
 @article{tulve_guiseppi-elie_geller_ward-caviness_paul_lavoie_rivers_frey_2023, title={Redefining exposure science to advance research supporting cumulative impacts, environmental justice, and decision-making}, volume={33}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85176085804&partnerID=MN8TOARS}, DOI={10.1038/s41370-023-00610-5}, abstractNote={Exposure science is often described as characterizing and predicting the intersection of chemical, biological, and physical agents with receptors, including individuals, geographically defined groups, and communities, in both space and time [1].However, to address today's complex scientific challenges, it is imperative that the working definition of exposure science be holistic, consistent with the concept of the exposome [2], and inclusive of the importance of non-chemical stressors (Fig. 1).Thus, the definition should characterize and predict the intersection of agents, including chemical and non-chemical stressors, with receptors in both space and time.By expanding the definition of exposure science to include non-chemical stressors, the scientific community stresses the importance of understanding how all exposures relate to health, well-being, and quality of life outcomes.Extreme weather events resulting from climate change are increasingly common and impact all communities by threatening the essential ingredients for good health, well-being, and quality of life, such as clean air, safe drinking water, and clean land.Climate change exacerbates the inequitable distribution of chemical and other non-chemical stressors that result in disproportionate exposures and greater vulnerability to already overburdened communities.Stressors include not only chemical contaminants in air, water, and land (e.g., particulates in air, perfluorinated compounds in water, metals in soil), but also nonchemical stressors (e.g., climatic stressors, relationships and social connectivity, access to resources such as health care and food stores, public infrastructure, job stability) associated with the built, natural, and social environments.Overburdened communities often contain historically marginalized groups and include "fenceline communities" that are adjacent to facilities that discharge chemicals into the environment [3][4][5][6][7][8][9].Reports published by the U.S. EPA [3], World Health Organization [10], and American Lung Association [11], amongst others, describe efforts to identify and characterize cumulative impacts that affect overburdened communities.Cumulative impacts describes the concept that health, well-being, and quality of life outcomes are dependent on a wide variety of factors that can occur concurrently and persistently in a person's life.Without}, number={6}, journal={Journal of Exposure Science and Environmental Epidemiology}, author={Tulve, N.S. and Guiseppi-Elie, A. and Geller, A.M. and Ward-Caviness, C.K. and Paul, S.J. and Lavoie, E.T. and Rivers, L. and Frey, H.C.}, year={2023}, pages={843–845} }
 @article{ahn_aredah_rakha_wei_frey_2023, title={Simple Diesel Train Fuel Consumption Model for Real-Time Train Applications}, volume={16}, ISSN={["1996-1073"]}, url={https://doi.org/10.3390/en16083555}, DOI={10.3390/en16083555}, abstractNote={This paper introduces a simple diesel train energy consumption model that calculates the instantaneous energy consumption using vehicle operational input variables, including the instantaneous speed, acceleration, and roadway grade, which can be easily obtained from global positioning system (GPS) loggers. The model was tested against real-world data and produced an error of −1.33% for all data and errors ranging from −12.4% to +8.0% for energy consumption of four train datasets amounting to a total of 5854 km trips. The study also validated the proposed model with separate data that were collected between Valencia and Cuenca, Spain, which had a total length of 198 km and found that the model was accurate, yielding a relative error of −1.55% for the total energy consumption. These results show that the proposed model can be used by train operators, transportation planners, policy makers, and environmental engineers to evaluate the energy consumption effects of train operational projects and train simulation within intermodal transportation planning tools.}, number={8}, journal={ENERGIES}, author={Ahn, Kyoungho and Aredah, Ahmed and Rakha, Hesham A. and Wei, Tongchuan and Frey, H. Christopher}, year={2023}, month={Apr} }
 @article{yuan_frey_wei_2022, title={Fuel use and emission rates reduction potential for light-duty gasoline vehicle eco-driving}, volume={109}, ISSN={["1879-2340"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85134667996&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2022.103394}, abstractNote={Eco-driving offers potential to reduce fuel use and emission rates for light-duty gasoline vehicles (LDGVs). The objective is to quantify real-world route-level and segment-level fuel use and emission rates reduction potential for LDGV eco-driving. Three million seconds of real-world speed trajectory data were analyzed based on predominantly naturalistic driving of 160 drivers on eight mesoscale routes. The routes were further divided into 199 segments. A Vehicle Specific Power modal model was used to estimate trajectory-average fuel use and emission rates of CO2, CO, hydrocarbons, NOx, and particulate matter and to identify eco-driving trajectories. For route-level eco-driving, fuel use and emission rates reduction potential ranges from 6% to 40%, compared to average fuel use and emission rates estimated based on all trajectories. Eco-driving focused on fuel savings typically reduced air emissions and vice versa. Route-level eco-driving typically but not always concurrently reduces segment-level fuel use and emission rates. These co-benefits and tradeoffs can be used to guide LDGV eco-driving decisions.}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Yuan, Weichang and Frey, Christopher and Wei, Tongchuan}, year={2022}, month={Aug} }
 @article{wei_frey_2022, title={Intermodal comparison of tailpipe emission rates between transit buses and private vehicles for on-road passenger transport}, volume={281}, ISSN={["1873-2844"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85129721755&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2022.119141}, abstractNote={Modal shift from private vehicles (PVs) to transit buses has the potential to reduce energy use and emissions from on-road passenger transport. Comparisons between these modes may be sensitive to key factors, such as vehicle size, fuel and powertrains, passenger load, and travel routes. The objectives are to evaluate the sensitivity of emission rates to route alignment, and compare emission rates between PVs and buses accounting for variability in key factors. Real-world bus speed trajectories were measured on actual bus routes for four origin-destination pairs (ODPs). To evaluate the sensitivity of emission rates to route alignment, hypothetical alternative bus routes were posited based on shortest distance, shortest travel time, or observed PV routes for each ODP. Trajectories and emission rates for PVs were quantified based on prior measurements of two routes per ODP using portable emission measurement systems. Trip-based tailpipe CO2, CO, total hydrocarbons (THC), NOx, and particulate matter (PM) emission rates were estimated for each ODP for gasoline and gasoline-hybrid PVs based on a Vehicle Specific Power modal model and for compressed natural gas, diesel, and diesel-hybrid buses based on the Transit Bus Emissions Model. Break-even passenger load (BEPL) was quantified to assess the minimum bus passenger load needed to achieve lower per passenger-trip emissions compared to PVs. Bus emission rates per bus-trip on actual bus routes are generally higher than those on hypothetical routes. As a bounding analysis, compared to single-occupancy PVs, fully occupied buses are estimated to have 82%–94% lower CO2, 99% lower to 308% higher CO, 99% lower to 145% higher THC, 67% lower to 62% higher NOx, and 94%–99% lower PM emission rates per passenger-trip depending on vehicle size, fuel and powertrain, passenger load, and route. BEPL varies depending on vehicle size, fuel and powertrain, route, and pollutant. The relative importance of key factors affecting intermodal comparisons differs by pollutants. The intermodal comparison is also affected by interactions among key factors, such as passenger load and route alignment, which reinforces the need for joint consideration of key factors.}, journal={ATMOSPHERIC ENVIRONMENT}, author={Wei, Tongchuan and Frey, H. Christopher}, year={2022}, month={Jul} }
 @article{breen_xu_frey_breen_isakov_2022, title={Microenvironment Tracker (MicroTrac) model to estimate time-location of individuals for air pollution exposure assessments: model evaluation using smartphone data}, volume={12}, ISSN={["1559-064X"]}, DOI={10.1038/s41370-022-00514-w}, abstractNote={{"Label"=>"BACKGROUND"} A critical aspect of air pollution exposure assessments is determining the time spent in various microenvironments (ME), which can have substantially different pollutant concentrations. We previously developed and evaluated a ME classification model, called Microenvironment Tracker (MicroTrac), to estimate time of day and duration spent in eight MEs (indoors and outdoors at home, work, school; inside vehicles; other locations) based on input data from global positioning system (GPS) loggers. {"Label"=>"OBJECTIVE"} In this study, we extended MicroTrac and evaluated the ability of using geolocation data from smartphones to determine the time spent in the MEs. {"Label"=>"METHOD"} We performed a panel study, and the MicroTrac estimates based on data from smartphones and GPS loggers were compared to 37 days of diary data across five participants. {"Label"=>"RESULTS"} The MEs were correctly classified for 98.1% and 98.3% of the time spent by the participants using smartphones and GPS loggers, respectively. {"Label"=>"SIGNIFICANCE"} Our study demonstrates the extended capability of using ubiquitous smartphone data with MicroTrac to help reduce time-location uncertainty in air pollution exposure models for epidemiologic and exposure field studies.}, journal={JOURNAL OF EXPOSURE SCIENCE AND ENVIRONMENTAL EPIDEMIOLOGY}, author={Breen, Michael S. S. and Xu, Yadong and Frey, H. Christopher and Breen, Miyuki and Isakov, Vlad}, year={2022}, month={Dec} }
 @inproceedings{eluri_frey_2021, title={An illustrative case study of the contribution of commuting to PM<sub>2.5</sub>exposures in Hong Kong}, volume={2021-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85120584014&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Eluri, S. and Frey, H.C.}, year={2021} }
 @article{rastogi_frey_2021, title={Characterizing Fuel Use and Emission Hotspots for a Diesel-Operated Passenger Rail Service}, volume={55}, ISSN={["1520-5851"]}, url={https://doi.org/10.1021/acs.est.1c00273}, DOI={10.1021/acs.est.1c00273}, abstractNote={Spatially varying diesel locomotive fuel use and emission rates (FUERs) are needed to accurately quantify local emission hotspots and their health impacts. However, existing locomotive FUER data are typically not spatially resolved or representative of real-world locomotive operation. Therefore, existing data are of limited use in quantifying the spatial variability in real-world FUERs. The objectives of this work are to quantify spatial variability in locomotive FUERs and identify factors differentiating hotspots from non-hotspots. FUERs were measured based on real-world measurements conducted for the Piedmont passenger rail service using a portable emission measurement system. FUERs were quantified based on 0.25 mile track segments on the Piedmont route. Hotspots were defined as segments in the top quintile of segment-average FUERs. On average, hotspots contributed 40-50% to trip fuel use and emissions. Hotspots were typically associated with low-to-medium speed, and high acceleration and grade. In contrast, non-hotspots were associated with high speed, and low acceleration and grade. Hotspots were typically located near populated areas and, thus, may exacerbate air pollutant exposure. The method demonstrated here can be applied to other passenger train services to assess key trends in hotspot locations and factors that explain the occurrence of hotspots.}, number={15}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, publisher={American Chemical Society (ACS)}, author={Rastogi, Nikhil and Frey, H. Christopher}, year={2021}, month={Aug}, pages={10633–10644} }
 @article{hossain_frey_louie_lau_2021, title={Combined effects of increased O-3 and reduced NO2 concentrations on short-term air pollution health risks in Hong Kong}, volume={270}, ISSN={["1873-6424"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85098531159&partnerID=MN8TOARS}, DOI={10.1016/j.envpol.2020.116280}, abstractNote={The reduction of NOx emissions in a VOC-limited region can lead to an increase of the local O3 concentration. An evaluation of the net health effects of such pollutant changes is therefore important to ascertain whether the emission control measures effectively improve the overall protection of public health. In this study, we use a short-term health risk (added health risk or AR) model developed for the multi-pollutant air quality health index (AQHI) in Hong Kong to examine the overall health impacts of these pollutant changes. We first investigate AR changes associated with NO2 and O3 changes, followed by those associated with changes in all four AQHI pollutants (NO2, O3, SO2, and particulate matter (PM)). Our results show that for the combined health effects of NO2 and O3 changes, there is a significant reduction in AR in urban areas with dense traffic, but no statistically significant changes in other less urbanized areas. The increase in estimated AR for higher O3 concentrations is offset by a decrease in the estimated AR for lower NO2 concentrations. In areas with dense traffic, the reduction in AR as a result of decreased NO2 is substantially larger than the increase in AR associated with increased O3. When additionally accounting for the change in ambient SO2 and PM, we found a statistically significant reduction in total AR everywhere in Hong Kong. Our results show that the emission control measures resulting in NO2, SO2, and PM reductions over the past decade have effectively reduced the AR over Hong Kong, even though these control measures may have partially contributed to an increase in O3 concentrations. Hence, efforts to reduce NOx, SO2, and PM should be continued.}, journal={ENVIRONMENTAL POLLUTION}, author={Hossain, Md Shakhaoat and Frey, H. Christopher and Louie, Peter K. K. and Lau, Alexis K. H.}, year={2021}, month={Feb} }
 @article{che_li_frey_tang_sun_wei_hossain_hohenberger_leung_lau_2021, title={Factors affecting variability in gaseous and particle microenvironmental air pollutant concentrations in Hong Kong primary and secondary schools}, volume={31}, ISSN={["1600-0668"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85090243272&partnerID=MN8TOARS}, DOI={10.1111/ina.12725}, abstractNote={School-age children are particularly susceptible to exposure to air pollutants. To quantify factors affecting children's exposure at school, indoor and outdoor microenvironmental air pollutant concentrations were measured at 32 selected primary and secondary schools in Hong Kong. Real-time PM10 , PM2.5 , NO2 and O3 concentrations were measured in 76 classrooms and 23 non-classrooms. Potential explanatory factors related to building characteristics, ventilation practice, and occupant activities were measured or recorded. Their relationship with indoor measured concentrations were examined using mixed linear regression models. Ten factors were significantly associated with indoor microenvironmental concentrations, together accounting for 74%, 61%, 46% and 38% of variations observed for PM2.5 , PM10 , O3 and NO2 microenvironmental concentrations respectively. Outdoor concentration is the single largest predictor for indoor concentrations. Infiltrated outdoor air pollution contributes to 90%, 70%, 75% and 50% of PM2.5 , PM10 , O3 and NO2 microenvironmental concentrations, respectively, in classrooms during school hours. Interventions to reduce indoor microenvironmental concentrations can be prioritized in reducing ambient air pollution and infiltration of outdoor pollution. Infiltration factors derived from linear regression models provide useful information on outdoor infiltration and help address the gap in generalizable parameter values that can be used to predict school microenvironmental concentrations.}, number={1}, journal={INDOOR AIR}, author={Che, Wenwei and Li, Alison T. Y. and Frey, Henry Christopher and Tang, Kimberly Tasha Jiayi and Sun, Li and Wei, Peng and Hossain, Md Shakhaoat and Hohenberger, Tilman Leo and Leung, King Wai and Lau, Alexis K. H.}, year={2021}, month={Jan}, pages={170–187} }
 @article{hossain_che_frey_lau_2021, title={Factors affecting variability in infiltration of ambient particle and gaseous pollutants into home at urban environment}, volume={206}, ISSN={["1873-684X"]}, url={https://doi.org/10.1016/j.buildenv.2021.108351}, DOI={10.1016/j.buildenv.2021.108351}, abstractNote={The majority of urban residents live in places with air quality exceeding World Health Organization guidelines. To quantify infiltration of outdoor pollution, and key factors affecting it, common outdoor air pollutants (PM2.5, NO2, and O3) were measured at 49 homes in Hong Kong. Infiltration factors (Finf) were derived based on linear regression of simultaneous indoor and outdoor measurements for each pollutant at each home. Finf estimated based on data for home occupancy, during which people were actually exposed, differed by up to 22%, 73% and 63% for PM2.5, NO2 and O3, respectively, from estimates based on whole monitoring data. This indicates the importance of separating occupancy time to quantify Finf for exposure estimation. The inter-home variability in occupancy Finf ranged from 0.11 to 1.00 (mean: 0.75) for PM2.5, 0.14 to 1.00 (mean: 0.53) for NO2, and 0.05 to 0.95 (mean: 0.47) for O3. Ventilation practices (e.g., window opening duration and air-conditioning on/off) explained 48%, 20%, and 10% of the inter-home variations in PM2.5, NO2 and O3 Finf, respectively. Use of air purifiers explained an additional 8%–9% of variations for PM2.5 and NO2. Thus, there is potential to reduce outdoor infiltration by modifying occupant behaviours. Compared to PM2.5 (R2 = 0.63), the developed models explained less variability in Finf for NO2 (R2 = 0.40) and O3 (R2 = 0.10). These two gases are chemically reactive. Further investigation, supported by additional measurements of related chemical species, is needed to improve understanding of infiltration process of reactive gases such as NO2 and O3.}, journal={BUILDING AND ENVIRONMENT}, publisher={Elsevier BV}, author={Hossain, Md Shakhaoat and Che, Wenwei and Frey, H. Christopher and Lau, Alexis K. H.}, year={2021}, month={Dec} }
 @article{yuan_frey_2021, title={Multi-scale evaluation of diesel commuter rail fuel use, emissions, and eco-driving}, volume={99}, ISSN={["1879-2340"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85112349776&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2021.102995}, abstractNote={Diesel commuter rail emissions affect populations near rail corridors. An approach is demonstrated to quantify mesoscale and microscale diesel commuter rail fuel use and emission rates (FUERs) of CO2, CO, NOx, particulate matter, and total hydrocarbons based on two U.S. systems. A speed trajectory simulator, an energy model, and an emissions model were calibrated, evaluated, and applied. FUERs and potential reductions from eco-driving were quantified based on simulated trajectories. Hotspots were defined as sections with ≥ 90th percentile of section-average FUERs by species. A few key variables explain 74–80% of variability in mesoscopic and microscopic FUERs. On average, FUERs are 7–8 times greater for hotspots than non-hotspots. Eco-driving was estimated to reduce segment-average FUERs by 3–33% and eliminate 2–11% hotspots. However, mesoscale oriented eco-driving can, at some locations, increase microscopic FUERs. The approach is adaptable to other diesel commuter rail systems.}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Yuan, Weichang and Frey, H. Christopher}, year={2021}, month={Oct} }
 @article{sandhu_frey_bartelt-hunt_jones_2021, title={Real-world activity, fuel use, and emissions of heavy-duty compressed natural gas refuse trucks}, volume={761}, ISSN={["1879-1026"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85096371806&partnerID=MN8TOARS}, DOI={10.1016/j.scitotenv.2020.143323}, abstractNote={Over 50% of new refuse truck sales have been compressed natural gas (CNG). Compared to diesel, CNG is less expensive on diesel gallon equivalent (dge) basis. This study quantifies the real-world fuel use and tailpipe exhaust emissions from three front- and three side-loader refuse trucks, each with a spark ignition CNG engine, three-way catalyst, and similar gross weight. Measurements were made at 1 Hz using a portable emissions measurement system (PEMS). Inter-cycle and inter-vehicle variability is quantified. Effect of vehicle weight was analyzed and comparisons were made with MOVES predicted cycle average emission rates. In total, about 220,000 s of data covering 490 miles of operation were recorded. The average fuel economy was 1.9 miles per dge. On average the trucks spent 53% of time in idle, which includes trash collection activity. The average speeds were 10 mph and 5 mph, for front- and side-loader trucks, respectively. Overall, compared to side-loader trucks, front-loader trucks had 55% better fuel economy and 60% lower emission rates. Compared to diesel trucks, CNG truck cycle average NOx and PM emission rates, at 1.2 g/mile and 0.006 g/mile respectively, were substantially lower while CO and HC rates, at 29 g/mile and 6 g/mile respectively, were considerably higher. Fuel use and CO2 emissions rates increased by 10% due to increase in truck weight during trash collection, while CO emissions rates increased by up to 30%. Compared to measured values, MOVES estimated cycle average fuel use and CO2 emissions were 25% lower, CO emissions are 70% lower, and NOx emissions were 200% higher. Results from this study can be used to improve solid waste life cycle and tailpipe emission factor models and, when combined with previous studies on diesel refuse trucks, evaluate the effect on fuel use and emissions from adoption of CNG refuse trucks.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, author={Sandhu, Gurdas S. and Frey, H. Christopher and Bartelt-Hunt, Shannon and Jones, Elizabeth}, year={2021}, month={Mar} }
 @article{wei_frey_2021, title={Sensitivity of light duty vehicle tailpipe emission rates from simplified portable emission measurement systems to variation in engine volumetric efficiency}, volume={6}, ISSN={["2162-2906"]}, url={https://doi.org/10.1080/10962247.2021.1923586}, DOI={10.1080/10962247.2021.1923586}, abstractNote={ABSTRACT Light-duty gasoline vehicle (LDGV) tailpipe emission rates can be quantified based on pollutant concentrations measured using portable emission measurement systems (PEMS). Emission rates depend on exhaust flow. For simplified and micro-PEMS, exhaust flow is inferred from engine mass air flow (MAF) and air-to-fuel ratio. For many LDGVs, MAF is broadcast via the on-board diagnostic (OBD) interface. For some vehicles, only indirect indicators of MAF are broadcast. In such cases, MAF can be estimated using the speed-density method (SDM). The SDM requires an estimate of the engine volumetric efficiency (VE), which is the ratio of actual to theoretical MAF. VE is affected by intra-vehicle variability in the engine load and inter-vehicle variability in engine characteristics (e.g., the type of valvetrain). The suitability of SDM-based estimates of MAF in conjunction with simplified and micro-PEMS has not been adequately evaluated. Therefore, the objectives are to: (1) quantify VE accounting for intra- and inter-vehicle variability; and (2) evaluate the accuracy of SDM-based vehicle emission rate estimation approaches. Seventy-seven naturally-aspirated LDGVs were measured using PEMS. For each vehicle, VE was estimated using three approaches: (1) constant VE calibrated to actual fuel use; (2) average estimates of VE for Vehicle Specific Power modes imputed from OBD data; and (3) modeled VE using multilinear regression (MLR). The MLR models were developed based on engine load and engine characteristics. The best model was selected based on various statistical diagnostics. When engines were under load, variability in VE was most sensitive to variations in engine load. During idling, VE differed between engines depending on engine characteristics. The constant and modeled VE estimation approaches enable the accurate estimation of microscale and mesoscale emission rates, with errors typically within ±10% compared to values imputed from OBD data. Thus, accurate emission rates can be obtained from simplified and micro-PEMS. Implications: Simplified and micro portable emission measurement systems (PEMS) enable widespread measurement of vehicle exhaust emission. As a cost saving measure, they estimate exhaust flow indirectly rather than via measurement, typically based on engine mass air flow (MAF). For some vehicles, MAF is not reported by the on-board diagnostic (OBD) system but can be inferred from other reported variables and volumetric efficiency (VE). However, VE is typically proprietary. Methods demonstrated here for estimating VE enable accurate quantification of emission rates, thereby enabling use of these PEMS for policy-relevant applications such as technology assessments, trends analysis, and emissions inventories.}, number={9}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, publisher={Informa UK Limited}, author={Wei, Tongchuan and Frey, H. Christopher}, year={2021}, month={Jun} }
 @inproceedings{wei_frey_yuan_2020, title={Application of transit bus emissions model to emission estimation: Case study for a wolfline transit bus line at North Carolina State University}, volume={2020-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85104853939&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Wei, T. and Frey, H.C. and Yuan, W.}, year={2020} }
 @inproceedings{frey_2020, title={Effect of changes since 2017 on the quality, credibility, and integrity of scientific review of national ambient air quality standards}, volume={2020-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85104833715&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C.}, year={2020} }
 @article{wei_frey_2020, title={Evaluation of the Precision and Accuracy of Cycle-Average Light Duty Gasoline Vehicles Tailpipe Emission Rates Predicted by Modal Models}, volume={2674}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85094930128&partnerID=MN8TOARS}, DOI={10.1177/0361198120924006}, abstractNote={ A vehicle specific power (VSP) modal model and the MOtor Vehicle Emission Simulator (MOVES) Operating Mode (OpMode) model have been used to evaluate and quantify the fuel use and emission rates (FUERs) for on-road vehicles. These models bin second-by-second FUERs based on factors such as VSP, speed, and others. The validity of binning approaches depends on their precision and accuracy in predicting variability in cycle-average emission rates (CAERs). The objective is to quantify the precision and accuracy of the two modeling methods. Since 2008, North Carolina State University has used portable emission measurement systems to measure tailpipe emission rates for 214 light duty gasoline vehicles on 1,677 driving cycles, including 839 outbound cycles and 838 inbound cycles on the same routes. These vehicles represent a wide range of characteristics and emission standards. For each vehicle, the models were calibrated based on outbound cycles and were validated based on inbound cycles. The goodness-of-fit of the calibrated models was assessed using linear least squares regression without intercept between model-predicted versus empirical CAERs for individual vehicles. Based on model calibration and validation, the coefficients of determination ( R2) typically range from 0.60 to 0.97 depending on the vehicle group and pollutant, indicating moderate to high precision, with precision typically higher for higher-emitting vehicle groups. The slopes of parity plots for each vehicle group and all vehicles typically range from 0.90 to 1.10, indicating good accuracy. The two modeling approaches are similar to each other at the microscopic and macroscopic levels. }, number={7}, journal={TRANSPORTATION RESEARCH RECORD}, author={Wei, Tongchuan and Frey, H. Christopher}, year={2020}, month={Jul}, pages={566–584} }
 @article{wei_frey_2020, title={Factors affecting variability in fossil-fueled transit bus emission rates}, volume={233}, ISSN={["1873-2844"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85084988997&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2020.117613}, abstractNote={Globally, there are over 10 million transit buses. Exhaust emissions from transit buses include carbon dioxide (CO2), carbon monoxide (CO), total hydrocarbons (THC), nitrogen oxides (NOx), and particulate matter (PM). Key factors affecting bus emission rates have been evaluated separately or in limited combinations in prior studies, including bus size, fuel and powertrain, passenger load, driving cycle, and model year. However, bus emission rates are jointly affected by all of these factors. To systematically evaluate these factors, a transit bus emissions model (TBEM) was developed. TBEM is calibrated based on generic compressed natural gas (CNG) and diesel bus types represented in the U.S. Environmental Protection Agency MOtor Vehicle Emission Simulator and empirical cycle average emission rates from the Integrated Bus Information System. The importance of the factors varies depending on the pollutant. For emission rates per vehicle-kilometer, model year is an important factor for NOx and PM, fuel and powertrain is an important factor for CO and THC, and driving cycle and bus size are important factors for CO2. For emission rates per passenger-kilometer, passenger load is generally an important factor for each pollutant. For a given fuel and powertrain and pollutant, smaller buses have lower emission rates per vehicle-kilometer than larger buses. However, a full large bus has lower emission rates per passenger-kilometer than a full small bus. There are tradeoffs among bus types regarding emission rates, especially for THC and PM. The comparison of bus emission rates is dependent on interactions between these key factors. For example, the effect of bus size and passenger load on emission rates is larger for lower speed driving cycles. For 2010 and newer model year buses and for moderate to high speed driving cycles, diesel buses have the lowest NOx emission rates whereas for low speed cycles, CNG buses have the lowest NOx emission rates. However, for 2007 to 2009 model year buses, CNG buses have the lowest NOx emission rates regardless of driving cycle. The study will be useful in helping transit planners and policy makers to develop strategies to reduce transit bus fleet emissions and in providing accurate emission factors for use in bus life cycle inventories and emission inventories.}, journal={ATMOSPHERIC ENVIRONMENT}, author={Wei, Tongchuan and Frey, H. Christopher}, year={2020}, month={Jul} }
 @article{khan_frey_rastogi_wei_2020, title={Geospatial Variation of Real-World Tailpipe Emission Rates for Light-Duty Gasoline Vehicles}, volume={54}, url={https://doi.org/10.1021/acs.est.0c00489}, DOI={10.1021/acs.est.0c00489}, abstractNote={Spatial variability in real-world on-road tailpipe light duty gasoline vehicle nitrogen oxides, hydrocarbon, carbon monoxide, and carbon dioxide emission rates, the locations of emissions hotspots, and factors that explain spatial variability are quantified. A sample of 205 vehicles were measured on four pre-defined round trip study routes using Portable Emission Measurement Systems. The trips on each route were divided into segments, averaging ¼ mile in length. Segment-average emission rates were estimated based on measured 1 Hz emission rates. Emission hotspots are defined as segments with ≥ 90th percentile of segment-average emission rates. The hotspots have average emission rates 2 to 4 times greater, depending on the pollutant, than other segments. Hotspots are of heterogeneous characteristics including road attributes and vehicle activity metrics. For example, some hotspots were on arterial roads with an upstream signalized intersection and positive road grade, whereas some hotspots were on interstates with positive grade. Vehicle activity metrics, including average vehicle specific power and relative positive acceleration, help identify the hotspots. To reliably identify a fleet-average hotspot, data are needed for at least 36 to 130 vehicles, depending on the pollutant.}, number={14}, journal={Environmental Science & Technology}, publisher={American Chemical Society (ACS)}, author={Khan, Tanzila and Frey, H. Christopher and Rastogi, Nikhil and Wei, Tongchuan}, year={2020}, month={Jul}, pages={8968–8979} }
 @inproceedings{rastogi_frey_2020, title={Locomotive fuel use and emission rates: Effects of transients and train consists}, volume={2020-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85104863944&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Rastogi, N. and Frey, H.C.}, year={2020} }
 @article{che_frey_fung_ning_qu_lo_chen_wong_wong_lee_et al._2020, title={PRAISE -HK: A personalized real-time air quality informatics system for citizen participation in exposure and health risk management}, volume={54}, ISSN={["2210-6715"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85078175966&partnerID=MN8TOARS}, DOI={10.1016/j.scs.2019.101986}, abstractNote={Exposure to air pollutants causes a range of adverse health effects. These harmful effects occur whenever and wherever people come into direct contact with air pollution. Therefore, individual actions that reduce the frequency, duration, and severity of personal contact with air pollution can reduce health risks. We developed a system that empowers the public with personalized information on air quality and exposure health risk. This system, the Personalised Real-Time Air Quality Informatics System for Exposure – Hong Kong (PRAISE-HK, http://praise.ust.hk/), is embodied in an interactive mobile application. PRAISE-HK is based on real-time data on emissions, high resolution urban morphology, meteorology, physical and chemical processes affecting pollutant transport and transformations, extensive measurements of air pollution concentrations in typical locations such as homes, schools, offices, and transportation, and big data integration of sensor monitoring to accurately estimate current and short-term forecasted street-level air quality. The street-level air quality simulation has been validated against reference monitoring data. Ongoing and planned future enhancements to PRAISE-HK include prediction of personal exposure and health response. PRAISE-HK is an example of the use of collective intelligence in a smart city to engage citizens in learning about and managing their own exposure to air pollution.}, journal={SUSTAINABLE CITIES AND SOCIETY}, author={Che, Wenwe and Frey, H. Christopher and Fung, Jimmy C. H. and Ning, Zhi and Qu, Huamin and Lo, Hong Kam and Chen, Lei and Wong, Tze-Wai and Wong, Michelle K. M. and Lee, Ophelia C. W. and et al.}, year={2020}, month={Mar} }
 @article{yuan_frey_2020, title={Potential for metro rail energy savings and emissions reduction via eco-driving}, volume={268}, ISSN={["1872-9118"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85083432807&partnerID=MN8TOARS}, DOI={10.1016/j.apenergy.2020.114944}, abstractNote={Metro rail energy efficiency needs to be improved to compensate for growing capacity demand. Eco-driving aims to reduce energy consumption without affecting safety and passenger comfort. Estimates of energy savings from train eco-driving are typically based on theoretical speed trajectory optimization models. However, achievable energy savings from eco-driving should be assessed based on realistic trajectories. A Markov chain speed trajectory simulator calibrated to measured trajectories was used to simulate realistic inter-run variability in 1 Hz trajectories. The simulator was calibrated and applied to the Washington Metropolitan Area Transit Authority Metrorail system. Estimated energy consumption for each trajectory includes auxiliary loads and tractive effort to overcome resistive forces. Inter-run variability in estimated energy consumption implies opportunities for energy savings via eco-driving. Energy savings was quantified by comparing the lowest and average segment energy consumption. A segment is the one-way rail track between adjacent stations of each line. Simulated trajectories are similar to measured trajectories based on mean absolute error and coefficient of determination (R2) for the same operation mode sequence. Based on 100 simulations per segment, energy savings ranging from 5% to 50% among segments and from 14% to 18% at the system level can be achieved without modifying travel time. Energy savings lead to reduced electricity consumption and, therefore, reduced power generation emissions. The method demonstrated here to quantify opportunities for metro train energy conservation and emissions mitigation is broadly applicable to electric metro and commuter trains and rail segments. Implications for energy-efficient passenger rail planning and operation are discussed.}, journal={APPLIED ENERGY}, author={Yuan, Weichang and Frey, H. Christopher}, year={2020}, month={Jun} }
 @inproceedings{yuan_frey_2020, title={Potential for subway energy savings and emissions reductions based on inter-run variability in speed trajectories: A case study of Baltimore subway}, volume={2020-June}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85104829385&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Yuan, W. and Frey, H.C.}, year={2020} }
 @article{frey_gadre_singh_kumar_2020, title={Quantification of Sources of Variability of Air Pollutant Exposure Concentrations Among Selected Transportation Microenvironments}, volume={2674}, ISSN={["2169-4052"]}, DOI={10.1177/0361198120929336}, abstractNote={ The National Research Council has identified the lack of sufficient microenvironmental air pollution exposure data as a significant barrier to quantification of human exposure to air pollution. Transportation microenvironments, including pedestrian, transit bus, car, and bicycle, can be associated with higher exposure concentrations than many other microenvironments. Data are lacking that provide a systematic basis for comparing exposure concentrations in these transportation modes that account for key sources of variability, such as time of day, season, and types of location along a route such as bus stops and intersections. The objectives of this work are: to quantify and compare particulate matter (PM2.5), CO, and O3 exposure concentrations in selected active and passive transportation microenvironments; and to quantify the effect of season, time of day, and location with respect to variability in transportation mode exposure concentrations. Measurements were made with an instrumented backpack and were repeated for multiple days in each season to account for the effect of inter-run variability. Results include mean trends, spatial variability, and contribution to variance. Pedestrian and cycle mode exposure concentrations were approximately similar to each other and were substantially higher than for bus and car cabins for both PM2.5 and O3. Based on over 30 days of field measurements conducted over three seasons and for two times of day on weekdays, transportation mode and season were the largest contributors to variability in exposure for PM2.5 and O3, whereas location type alone and in combination with transport mode helped explain variability in CO exposures. }, number={9}, journal={TRANSPORTATION RESEARCH RECORD}, author={Frey, H. Christopher and Gadre, Disha and Singh, Sanjam and Kumar, Prashant}, year={2020}, month={Sep}, pages={395–411} }
 @article{frey_adams_adgate_allen_balmes_boyle_chow_dockery_felton_gordon_et al._2020, title={The need for a tighter particulate-matter air-quality standard}, volume={383}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85089417407&partnerID=MN8TOARS}, DOI={10.1056/NEJMsb2011009}, number={7}, journal={New England Journal of Medicine}, author={Frey, H.C. and Adams, P.J. and Adgate, J.L. and Allen, G.A. and Balmes, J. and Boyle, K. and Chow, J.C. and Dockery, D.W. and Felton, H.D. and Gordon, T. and et al.}, year={2020}, pages={680–683} }
 @article{frey_zheng_hu_2020, title={Variability in Measured Real-World Operational Energy Use and Emission Rates of a Plug-In Hybrid Electric Vehicle}, volume={13}, url={https://doi.org/10.3390/en13051140}, DOI={10.3390/en13051140}, abstractNote={Compared to comparably sized conventional light duty gasoline vehicles (CLDGVs), plug-in hybrid electric vehicles (PHEVs) may offer benefits of improved energy economy, reduced emissions, and the flexibility to use electricity as an energy source. PHEVs operate in either charge depleting (CD) or charge sustaining (CS) mode; the engine has the ability to turn on and off; and the engine can have multiple cold starts. A method is demonstrated for quantifying the real-world activity, energy use, and emissions of PHEVs, taking into account these operational characteristics and differences in electricity generation resource mix. A 2013 Toyota Prius plug-in was measured using a portable emission measurement system. Vehicle specific power (VSP) based modal average energy use and emission rates are inferred to assess trends in energy use and emissions with respect to engine load and for comparisons of engine on versus engine off, and cold start versus hot stabilized running. The results show that, compared to CLDGVs, the PHEV operating in CD mode has improved energy efficiency and lower CO2, CO, HC, NOx, and PM2.5 emission rates for a wide range of power generation fuel mixes. However, PHEV energy use and emission rates are highly variable, with periods of relatively high on-road emission rates related to cold starts.}, number={5}, journal={Energies}, author={Frey, H. Christopher and Zheng, Xiaohui and Hu, Jiangchuan}, year={2020}, month={Mar} }
 @article{bunds_casper_frey_barrett_2019, title={Air Pollution at College Football Games: Developing a Methodology for Measuring Air Pollutant Exposure in a Sport Event Microenvironment}, volume={23}, ISSN={1525-9951}, url={http://dx.doi.org/10.3727/152599518x15403853721484}, DOI={10.3727/152599518X15403853721484}, abstractNote={International organizations such as the United Nations have begun to recognize the connection between sport events and air pollution. However, to date, there has been a dearth of research attempting to measure air pollution at sporting events. To address the lack of research on air
 pollution at sporting events, in this article we developed a methodology for measuring the air pollution at a college football game utilizing both stationary and mobile monitoring systems. Stationary monitors allowed for the creation of a microenvironment wherein we could examine the overall
 particulate matter exposure within the stadium and tailgate lot areas. Mobile monitors allowed for the examination of specific causes of spikes in air pollution. We found grills, generators, and cars caused pollutant spikes, which were, at times, over 20 times worse than the recognized levels
 for moderate air quality. Additionally, while the overall pollution in the microenvironment was elevated, pollutant levels during postgame (i. e., where there is a rush of individuals leaving at the same time) were consistently and considerably higher. Implications for practice and future
 research include event policy considerations and the application of the methodology across alternative sporting venues/contexts.}, number={3}, journal={Event Management}, publisher={Cognizant, LLC}, author={Bunds, Kyle S. and Casper, Jonathan M. and Frey, H. Christopher and Barrett, Martin}, year={2019}, month={May}, pages={399–412} }
 @article{yuan_frey_wei_rastogi_vandergriend_miller_mattison_2019, title={Comparison of real-world vehicle fuel use and tailpipe emissions for gasoline-ethanol fuel blends}, volume={249}, ISSN={["1873-7153"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063525708&partnerID=MN8TOARS}, DOI={10.1016/j.fuel.2019.03.115}, abstractNote={Differences in fuel use and emission rates of carbon dioxide (CO2), carbon monoxide (CO), hydrocarbons (HC), nitrogen oxide (NOx), and particulate matter (PM) were quantified for three gasoline-ethanol blends and neat gasoline measured for one flexible-fuel vehicle (FFV) and four non-FFVs using a portable emission measurement system (PEMS). The purpose was to determine if non-FFVs can adapt to a mid-level blend and to compare the fuel use and emission rates among the fuels. Each vehicle was measured on neat gasoline (E0), 10% ethanol by volume (E10) “regular” (E10R) and “premium” (E10P), and 27% ethanol by volume (E27). Four real-world cycles were repeated for each vehicle with each fuel. Second-by-second fuel use and emission rates were binned into Vehicle Specific Power (VSP) modes. The modes were weighted according to real-world standard driving cycles. All vehicles, including the non-FFVs, were able to adapt to E27. Octane-induced efficiency gain was observed for higher octane fuels (E10P and E27) versus lower octane fuels (E0 and E10R). E27 tends to lower PM emission rates compared to E10R and E10P and CO emission rates compared to the other three fuels. HC emission rates for E27 were comparable to those of E10R and E10P. No significant difference was found in NOx emission rates for E27 versus the other fuels. Intervehicle variability in fuel use and emission rates was observed. Lessons learned regarding study design, vehicle selection, and sample size, and their implications are discussed.}, journal={FUEL}, publisher={Elsevier BV}, author={Yuan, Weichang and Frey, H. Christopher and Wei, Tongchuan and Rastogi, Nikhil and VanderGriend, Steven and Miller, David and Mattison, Lawrence}, year={2019}, month={Aug}, pages={352–364} }
 @article{khan_frey_2019, title={Effect of Air-Conditioning on Light Duty Gasoline Vehicles Fuel Economy}, volume={2673}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063930471&partnerID=MN8TOARS}, DOI={10.1177/0361198119838507}, abstractNote={ With more stringent U.S. fuel economy (FE) standards, the effect of auxiliary devices such as air-conditioning (AC) have received increased attention. AC is the largest auxiliary engine load for light duty gasoline vehicles (LDGVs). However, there are few data regarding the effect of AC operation on FE for LDGVs based on real-world measurements, especially for recent model year vehicles. The Motor Vehicle Emission Simulator (MOVES) is a regulatory model for estimating on-road vehicle energy-use and emissions. MOVES adjusts vehicle energy-use rates for AC effects. However, MOVES-predicted FE with AC has not been evaluated based on empirical measurements. The research objectives are to quantify the LDGVs FE penalty from AC and assess the accuracy of MOVES2014a-predicted FE with AC. The AC effect on real-world fleet-average FE was quantified based on 78 AC-off vehicles versus 55 AC-on vehicles, measured with onboard instruments on defined study routes. MOVES2014a-based FE penalty from AC was evaluated based on real-world estimates and chassis dynamometer-based FE test results used for FE ratings. The real-world FE penalty ranges between 1.3% and 7.5% among a wide range of driving cycles. Fuel consumption at idle is 13% higher with AC on. MOVES underestimates the real-world FE with AC by 6%, on average. MOVES overestimates the AC effect on cycle-average FE ranging between 13.5% and 18.5% for real-world and MOVES default cycles, and between 11.1% and 14.5% for standard cycles. }, number={5}, journal={TRANSPORTATION RESEARCH RECORD}, author={Khan, Tanzila and Frey, H. Christopher}, year={2019}, month={May}, pages={131–141} }
 @article{che_frey_li_lao_lau_2019, title={Indoor Exposure to Ambient Particles and Its Estimation Using Fixed Site Monitors}, volume={53}, ISSN={["1520-5851"]}, url={https://doi.org/10.1021/acs.est.8b04474}, DOI={10.1021/acs.est.8b04474}, abstractNote={Ambient PM2.5 concentrations measured at fixed site monitors (FSM) are often biased with respect to exposure concentrations because of spatial variability and infiltration. Based on comparison of ambient concentrations from 14 FSMs and of exposure concentrations measured indoors and outdoors at two schools in Hong Kong for winter and summer seasons, the magnitude and sources of exposure error based on using FSMs as a surrogate for exposure are quantified. An approach for bias correcting surrogate exposure estimates from FSMs is demonstrated. The approach is based on a proximity factor (PF) that accounts for differences in spatial locations, proximity to emissions and deviation from dominant wind direction, and an infiltration factor (IF) that varies by season. The combination of the PF and IF reduce bias in mean school exposure estimates from ±90% to ±20%. Bias in exposure estimates from using FSMs as surrogates tend to be smaller for which the exposure site and FSM are aligned with wind direction, have similar sampling height, and are in close proximity. The methodology demonstrated to assess concordance between FSMs and exposure measurement sites can be applied more broadly to help reduce exposure error, which may help to interpret seasonal variations in health estimates.}, number={2}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, publisher={American Chemical Society (ACS)}, author={Che, Wenwei and Frey, H. Christopher and Li, Zhiyuan and Lao, Xiangqian and Lau, Alexis K. H.}, year={2019}, month={Jan}, pages={808–819} }
 @article{yuan_frey_rastogi_2019, title={Quantification of Energy Saving Potential for A Passenger Train Based on Inter-Run Variability in Speed Trajectories}, volume={2673}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85063956284&partnerID=MN8TOARS}, DOI={10.1177/0361198119838516}, abstractNote={ Passenger train energy consumption is dependent on speed trajectories. The variability of passenger train energy consumption owing to the variability in speed trajectories can help identify ways to reduce train energy use via improved operations. Empirical fuel use data from a portable measurement emission measurement system (PEMS) and empirical speed trajectories measured using a global positioning system (GPS) receiver were used to verify and quantify real-world energy consumption variability and the variability in empirical speed trajectories, respectively. To identify potential realistic speed trajectories that can lead to energy saving (i.e., eco-driving), a Markov chain based speed trajectory simulator was used to simulate inter-run variability in speed trajectories. An energy index model (EIM) was used to compare energy consumption among different speed trajectories. The results show inter-run variability in fuel use associated with inter-run variability in the empirical speed trajectories. There is also inter-segment variability in fuel use related to the segment length and grade. The Markov chain based speed trajectory simulator can simulate realistic inter-run variability in speed trajectories based on calibration using empirical speed trajectories. The number of empirical speed trajectories used for simulator calibration affects the range of simulated inter-run variability. The EIM provides an accurate estimation of the empirical fuel use. Eco-driving, such as reducing the peak speed, can reduce energy consumption without compromising travel time. The methodology shown in this study is not system-specific and can be applied to other passenger train systems. }, number={5}, journal={TRANSPORTATION RESEARCH RECORD}, author={Yuan, Weichang and Frey, H. Christopher and Rastogi, Nikhil}, year={2019}, month={May}, pages={153–165} }
 @article{kumar_patton_durant_frey_2018, title={A review of factors impacting exposure to PM2.5, ultrafine particles and black carbon in Asian transport microenvironments}, volume={187}, ISSN={1352-2310}, url={http://dx.doi.org/10.1016/J.ATMOSENV.2018.05.046}, DOI={10.1016/J.ATMOSENV.2018.05.046}, abstractNote={The World Health Organization estimates 3.7 million deaths in 2012 in low- and middle-income Asian countries due to outdoor air pollution. However, these estimates do not account for the higher exposures of specific particulate matter (PM) components – including fine particles (PM2.5), ultrafine particles (UFP) and black carbon (BC) – typical of transport microenvironments (TMEs). With the rapidly growing number of on-road vehicles in Asia, human exposure to PM is an increasing concern. The aim of this review article is to comprehensively assess studies of PM2.5, UFP, and BC in Asian TMEs in order to better understand the extent of exposure, the underlying factors leading to exposure, and how Asian exposures compare to those found in Europe and the United States of America (USA). The health impacts of exposure to PM2.5, UFP, and BC are described and the key factors that influence personal exposure in TMEs (i.e., walk, cycle, car, and bus) are identified. Instrumentation and measurement methods, exposure modeling techniques, and regulation are reviewed for PM2.5, UFP, and BC. Relatively few studies have been carried out in urban Asian TMEs where PM2.5, UFP, and BC had generally higher concentrations compared to Europe and USA. Based on available data, PM2.5 concentrations while walking were 1.6 and 1.2 times higher in Asian cities (average 42 μg m−3) compared to cities in Europe (26 μg m−3) and the USA (35 μg m−3), respectively. Likewise, average PM2.5 concentrations in car (74 μg m−3) and bus (76 μg m−3) modes in Asian cities were approximately two to three times higher than in Europe and American cities. UFP exposures in Asian cities were twice as high for pedestrians and up to ∼9-times as high in cars than in cities in Europe or the USA. Asian pedestrians were exposed to ∼7-times higher BC concentrations compared with pedestrians in the USA. Stochastic population-based models have yet to be applied widely in Asia but can be used to quantify inter-individual and inter-regional variability in exposures and to assess the contribution of TMEs to total exposures for multiple pollutants. The review also highlights specific gaps in the Asian TME data set that need to be filled since UFP and BC studies were rare as were studies of pedestrian and cyclist exposure.}, journal={Atmospheric Environment}, publisher={Elsevier BV}, author={Kumar, Prashant and Patton, Allison P. and Durant, John L. and Frey, H. Christopher}, year={2018}, month={Aug}, pages={301–316} }
 @article{khan_frey_2018, title={Comparison of real-world and certification emission rates for light duty gasoline vehicles}, volume={622}, ISSN={["1879-1026"]}, url={https://doi.org/10.1016/j.scitotenv.2017.10.286}, DOI={10.1016/j.scitotenv.2017.10.286}, abstractNote={U.S. light duty vehicles are subject to the U.S. Environmental Protection Agency (EPA) emission standards. Emission compliance is determined by certification testing of selected emissions from representative vehicles on standard driving cycles using chassis dynamometers. Test results are also used in many emission inventories. The dynamometer based emission rates are adjusted to provide the certification levels (CL), which must be lower than the standards for compliance. Although standard driving cycles are based on specific observations of real-world driving, they are not necessarily real-world representative. A systematic comparison of the real-world emission rates of U.S. light duty gasoline vehicles (LDGVs) versus CL, and emission standards has not been previously reported. The purpose of this work is to compare regulatory limits (both CLs and emission standards) and the real-world emissions of LDGVs. The sensitivity of the comparisons to cold start emission was assessed. Portable Emission Measurement Systems (PEMS) were used to measure hot stabilized exhaust emissions of 122 LDGVs on a specified 110 mile test route. Cold start emissions were measured with PEMS for a selected vehicle sample of 32 vehicles. Emissions were measured for carbon dioxide (CO2), carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx). For each vehicle, a Vehicle Specific Power (VSP) modal emission rate model was developed. The VSP modal rates were weighted by the standard driving cycles and real-world driving cycles to estimate the respective cycle average emission rates (CAERs). Measured vehicles were matched with certification test vehicles for comparison. For systematic trends in comparison, vehicles were classified into four groups based on the Tier 1 and Tier 2 emission regulation, and the vehicle type such as passenger car and passenger truck. Depending on the cycle-pollutant and the vehicle groups, hot stabilized CAERs are on average either statistically significantly higher than or significantly not different from the CLs, with the exception of CO on the US06 cycle, for which real-world rates are lower than CLs. Compared to the emission standards, hot stabilized CAERs are on average significantly lower. However, comparisons of CAERs and standards are sensitive to cold start emissions. For some combinations of pollutants and vehicle groups, cold start inclusive CAERs are higher than the corresponding CLs and as high as the standards. The CLs, which are based on standard driving cycles, tend to underestimate real-world emission rates. Therefore, emission inventory estimates using certification test results are potentially underestimated.}, journal={SCIENCE OF THE TOTAL ENVIRONMENT}, publisher={Elsevier BV}, author={Khan, Tanzila and Frey, H. Christopher}, year={2018}, month={May}, pages={790–800} }
 @article{kumar_rivas_singh_ganesh_ananya_frey_2018, title={Dynamics of coarse and fine particle exposure in transport microenvironments}, volume={1}, ISSN={["2397-3722"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85048165474&partnerID=MN8TOARS}, DOI={10.1038/s41612-018-0023-y}, abstractNote={Abstract}, number={1}, journal={NPJ CLIMATE AND ATMOSPHERIC SCIENCE}, author={Kumar, Prashant and Rivas, Ioar and Singh, Anant Pratap and Ganesh, Vikas Julius and Ananya, Monirupa and Frey, H. Christopher}, year={2018} }
 @article{tanvir_frey_rouphail_2018, title={Effect of Light Duty Vehicle Performance on a Driving Style Metric}, volume={2672}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85060916631&partnerID=MN8TOARS}, DOI={10.1177/0361198118796070}, abstractNote={ Eco-driving involves alterations to driving style to improve energy efficiency. The observed driving style reflects the combined effects of roadway, traffic, driver, and vehicle performance. Although the effect of roadway and traffic characteristics can be inferred from microscale driving activity data, the effect of vehicle performance on driving style is not properly understood. This paper addresses two questions: (1) how different is an individual driver’s driving style when operating vehicles with differences in performance?; and (2) how dissimilar are the driving styles of different drivers when operating vehicles that have similar performance? To answer these questions, we have gathered microscale vehicle activity measurements from 17 controlled real-world driving schedules and two years of naturalistic driving data from five drivers. We also developed a metric for driving style termed “envelope deviation,” which is a distribution of gaps between microscale activity (1 Hz) and fleet average envelope. We found that there is significant inter-driver heterogeneity in driving styles when controlling for vehicle performance. However, no significant inter-vehicle heterogeneity was present in driving styles while controlling for the driver. Findings from this study imply that the choice of vehicle does not significantly alter the natural driving style of a driver. }, number={25}, journal={TRANSPORTATION RESEARCH RECORD}, author={Tanvir, Shams and Frey, H. Christopher and Rouphail, Nagui M.}, year={2018}, month={Dec}, pages={67–78} }
 @article{li_che_frey_lau_2018, title={Factors affecting variability in PM 2.5 exposure concentrations in a metro system}, volume={160}, ISSN={0013-9351}, url={http://dx.doi.org/10.1016/J.ENVRES.2017.09.006}, DOI={10.1016/J.ENVRES.2017.09.006}, abstractNote={The objectives of this study were to: (1) evaluate PM2.5 inflow to metro train cabins when doors open at stations; (2) assess the spatial and temporal variability in PM2.5 exposure concentration; and (3) quantify the relationship between in-cabin concentration versus outdoor and non-ambient PM2.5. We measured in-cabin PM2.5 concentrations using portable monitors at the door-side and center of a train cabin simultaneously on a Hong Kong metro line. In addition, platform and in-cabin pollutant concentrations near a train door were simultaneously measured. Short-term spikes in PM2.5 concentrations typically occur near train doors when doors open, related to inflow of ambient air aboveground and tunnel air underground. In-cabin PM2.5 exposure concentrations are typically lower away from the doors when the doors open. PM2.5 concentrations inside train cabins and on station platform operating above-ground are more influenced, compared to underground, by outdoor PM2.5. Moreover, non-ambient sources contribute approximately 50% of train in-cabin and station platform PM2.5 concentrations during underground operation. The results help more accurately quantify commuting PM2.5 exposure on a metro system, and can be used to improve population-based exposure simulation models.}, journal={Environmental Research}, publisher={Elsevier BV}, author={Li, Zhiyuan and Che, Wenwei and Frey, H. Christopher and Lau, Alexis K.H.}, year={2018}, month={Jan}, pages={20–26} }
 @article{delavarrafiee_frey_2018, title={Real-world fuel use and gaseous emission rates for flex fuel vehicles operated on E85 versus gasoline}, volume={68}, ISSN={["2162-2906"]}, url={https://doi.org/10.1080/10962247.2017.1405097}, DOI={10.1080/10962247.2017.1405097}, abstractNote={ABSTRACT Flex fuel vehicles (FFVs) typically operate on gasoline or E85, an 85%/15% volume blend of ethanol and gasoline. Differences in FFV fuel use and tailpipe emission rates are quantified for E85 versus gasoline based on real-world measurements of five FFVs with a portable emissions measurement system (PEMS), supplemented chassis dynamometer data, and estimates from the Motor Vehicle Emission Simulator (MOVES) model. Because of inter-vehicle variability, an individual FFV may have higher nitrogen oxide (NOx) or carbon monoxide (CO) emission rates on E85 versus gasoline, even though average rates are lower. Based on PEMS data, the comparison of tailpipe emission rates for E85 versus gasoline is sensitive to vehicle-specific power (VSP). For example, although CO emission rates are lower for all VSP modes, they are proportionally lowest at higher VSP. Driving cycles with high power demand are more advantageous with respect to CO emissions, but less advantageous for NOx. Chassis dynamometer data are available for 121 FFVs at 50,000 useful life miles. Based on the dynamometer data, the average difference in tailpipe emissions for E85 versus gasoline is −23% for NOx, −30% for CO, and no significant difference for hydrocarbons (HC). To account for both the fuel cycle and tailpipe emissions from the vehicle, a life cycle inventory was conducted. Although tailpipe NOx emissions are lower for E85 versus gasoline for FFVs and thus benefit areas where the vehicles operate, the life cycle NOx emissions are higher because the NOx emissions generated during fuel production are higher. The fuel production emissions take place typically in rural areas. Although there are not significant differences in the total HC emissions, there are differences in HC speciation. The net effect of lower tailpipe NOx emissions and differences in HC speciation on ozone formation should be further evaluated. Implications: Reported comparisons of flex fuel vehicle (FFV) tailpipe emission rates for E85 versus gasoline have been inconsistent. To date, this is the most comprehensive evaluation of available and new data. The large range of inter-vehicle variability illustrates why prior studies based on small sample sizes led to apparently contradictory findings. E85 leads to significant reductions in tailpipe nitrogen oxide (NOx) and carbon monoxide (CO) emission rates compared with gasoline, indicating a potential benefit for ozone air quality management in NOx-limited areas. The comparison of FFV tailpipe emissions between E85 and gasoline is sensitive to power demand and driving cycles.}, number={3}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, publisher={Informa UK Limited}, author={Delavarrafiee, Maryam and Frey, H. Christopher}, year={2018}, pages={235–254} }
 @article{altshuler_ayala_collet_chow_frey_shaikh_stevenson_walsh_watson_2018, title={Trends in on-road transportation, energy, and emissions}, volume={68}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85054087647&partnerID=MN8TOARS}, DOI={10.1080/10962247.2018.1512734}, abstractNote={Air Quality Consultant, Danville, CA, USA; Air Pollution Control Officer and Executive Director, Sacramento Metropolitan Air Quality Management District, Sacramento, CA, USA; Executive Engineer, Toyota Motor North America, Inc., Ann Arbor, MI, USA; Desert Research Institute, Reno, NV, USA; State Key Laboratory of Loess and Quaternary Geology (SKLLQG), Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, People’s Republic of China; Glenn E. Futrell Distinguished University Professor of Environmental Engineering, Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Raleigh, NC, USA; Director of Science, Health Effects Institute, Boston, MA, USA; Meteorology and Measurements Division, Bay Area Air Quality Management District, San Francisco, CA, USA; Consultant on Clean Transportation, Arlington, VA, USA}, number={10}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Altshuler, Samuel L. and Ayala, Alberto and Collet, Susan and Chow, Judith C. and Frey, H. Christopher and Shaikh, Rashid and Stevenson, Eric D. and Walsh, Michael P. and Watson, John G.}, year={2018}, pages={1015–1024} }
 @misc{frey_2018, title={Trends in onroad transportation energy and emissions}, volume={68}, ISSN={["2162-2906"]}, url={https://doi.org/10.1080/10962247.2018.1454357}, DOI={10.1080/10962247.2018.1454357}, abstractNote={ABSTRACT Globally, 1.3 billion on-road vehicles consume 79 quadrillion BTU of energy, mostly gasoline and diesel fuels, emit 5.7 gigatonnes of CO2, and emit other pollutants to which approximately 200,000 annual premature deaths are attributed. Improved vehicle energy efficiency and emission controls have helped offset growth in vehicle activity. New technologies are diffusing into the vehicle fleet in response to fuel efficiency and emission standards. Empirical assessment of vehicle emissions is challenging because of myriad fuels and technologies, intervehicle variability, multiple emission processes, variability in operating conditions, and varying capabilities of measurement methods. Fuel economy and emissions regulations have been effective in reducing total emissions of key pollutants. Real-world fuel use and emissions are consistent with official values in the United States but not in Europe or countries that adopt European standards. Portable emission measurements systems, which uncovered a recent emissions cheating scandal, have a key role in regulatory programs to ensure conformity between “real driving emissions” and emission standards. The global vehicle fleet will experience tremendous growth, especially in Asia. Although existing data and modeling tools are useful, they are often based on convenience samples, small sample sizes, large variability, and unquantified uncertainty. Vehicles emit precursors to several important secondary pollutants, including ozone and secondary organic aerosols, which requires a multipollutant emissions and air quality management strategy. Gasoline and diesel are likely to persist as key energy sources to mid-century. Adoption of electric vehicles is not a panacea with regard to greenhouse gas emissions unless coupled with policies to change the power generation mix. Depending on how they are actually implemented and used, autonomous vehicles could lead to very large reductions or increases in energy consumption. Numerous other trends are addressed with regard to technology, emissions controls, vehicle operations, emission measurements, impacts on exposure, and impacts on public health. Implications: Without specific policies to the contrary, fossil fuels are likely to continue to be the major source of on-road vehicle energy consumption. Fuel economy and emission standards are generally effective in achieving reductions per unit of vehicle activity. However, the number of vehicles and miles traveled will increase. Total energy use and emissions depend on factors such as fuels, technologies, land use, demographics, economics, road design, vehicle operation, societal values, and others that affect demand for transportation, mode choice, energy use, and emissions. Thus, there are many opportunities to influence future trends in vehicle energy use and emissions.}, number={6}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, publisher={Informa UK Limited}, author={Frey, H. Christopher}, year={2018}, pages={514–563} }
 @article{li_che_frey_lau_lin_2017, title={Characterization of PM2.5 exposure concentration in transport microenvironments using portable monitors}, volume={228}, ISSN={["1873-6424"]}, url={https://doi.org/10.1016/j.envpol.2017.05.039}, DOI={10.1016/j.envpol.2017.05.039}, abstractNote={Recently, portable monitors have been increasingly used to quantify air pollutant concentrations at high spatiotemporal resolution. A sampling campaign was conducted to measure the fine particulate matter (PM2.5) and carbon monoxide (CO) exposure concentrations in transport microenvironments (TMEs) in Hong Kong in January and June 2015 using TSI DustTrak and Q-Trak portable monitors. The objectives were to: (1) calibrate DustTrak and Q-Trak; (2) evaluate variability between seasons and microenvironments; (3) estimate indoor/outdoor relationships; and (4) determine minimum sample size. Calibration equations, obtained through side-by-side measurement against stationary reference methods in winter and summer, were applied to correct the measured PM2.5 data set. In general, PM2.5 concentrations in all TMEs were significantly higher in winter than in summer. The mean PM2.5 concentration in winter was lower for underground sections of the Mass Transit Railway (MTR) metro system (31 μg/m3) than for other TMEs, whereas in summer TMEs had mean PM2.5 concentrations in the range of 10–15 μg/m3, with above-ground MTR train as an exception, at 23 μg/m3. PM2.5 concentrations measured in TMEs were strongly correlated with nearby air quality monitoring stations (AQMSs) measurements in winter, but in summer there was little correlation. The minimum sample size estimates varied more among TMEs in summer versus winter because of the differences in PM2.5 concentration distributions related to changes in ambient PM2.5 concentrations and ventilation practices. This study provides a feasible protocol on the calibration and application of portable monitors in TME air quality measurement and develops a method for estimating minimum sample size.}, journal={ENVIRONMENTAL POLLUTION}, publisher={Elsevier BV}, author={Li, Zhiyuan and Che, Wenwei and Frey, H. Christopher and Lau, Alexis K. H. and Lin, Changqing}, year={2017}, month={Sep}, pages={433–442} }
 @inproceedings{wei_khan_frey_2017, title={Development of simplified models of CNG, diesel, and hybrid transit bus energy use}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85039153359&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Wei, T. and Khan, T. and Frey, H.C.}, year={2017} }
 @inproceedings{frey_2017, title={Identifying and transcending ideological barriers in the climate change public policy debate}, volume={2017-October}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85052332630&partnerID=MN8TOARS}, booktitle={Air and Waste Management Association, A and WMA - Finding Common Ground on Climate Change Mitigation and Adaptation}, author={Frey, H.C.}, year={2017}, pages={8–17} }
 @inproceedings{khan_frey_2017, title={Key factors affecting inter-county variability in moves county-level energy inventory}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85039169682&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Khan, T. and Frey, H.C.}, year={2017} }
 @inproceedings{rastogi_frey_2017, title={Procedure for evaluation of retrofitted scr-based emission control system for a passenger railroad locomotive}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85039160003&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Rastogi, N. and Frey, H.C.}, year={2017} }
 @inproceedings{singh_frey_2017, title={Procedure for measuring and comparing fuel use and gaseous emissions for gas-direct injection versus port fuel injection vehicles}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85039147517&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Singh, S. and Frey, H.C.}, year={2017} }
 @inproceedings{yuan_frey_sun_2017, title={Quantification of transit train activity data for energy consumption estimation}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85039168169&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Yuan, W. and Frey, H.C. and Sun, Y.}, year={2017} }
 @article{frey_delavarrafiee_singh_2017, title={Real-World Freeway and Ramp Activity and Emissions for Light-Duty Gasoline Vehicles}, volume={2627}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85033794699&partnerID=MN8TOARS}, DOI={10.3141/2627-03}, abstractNote={ There are few data on differences in real-world emissions by in-use vehicles when they operate on freeway ramps compared with operations on the freeway itself. The objective of this paper is to quantify the variability in link-based emissions rates for on-ramps and off-ramps in comparison to rates on freeways. Real-world measurements were made with the use of a portable emissions measurement system (PEMS) for selected vehicles, ramps, and freeway segments. The methodology included development of a study design for field data collection of vehicle activity and emissions, execution of the study design, quality assurance of the raw data, and analysis of the quality-assured data. Four light-duty gasoline vehicles were driven on two routes, each composed of on-ramp, freeway, and off-ramp links. Data were collected for morning peak, evening peak, and off-peak time periods. A PEMS test was used to measure exhaust emissions of oxides of nitrogen (NOx), hydrocarbon (HC), and carbon monoxide (CO). The emissions rates for on-ramps were shown to be substantially higher than rates on freeways for NOx, HC, and CO. Some of this variability in emissions rates can be explained by link average vehicle specific power, which can vary by time of day and from one location to another. The variability in emissions rates by route and time of day indicates that there can be complex interactions between traffic flow, road geometry, and emissions rates. Recommendations are offered for additional study and regarding how these results can be used by researchers and practitioners. }, number={2627}, journal={TRANSPORTATION RESEARCH RECORD}, author={Frey, H. Christopher and Delavarrafiee, Maryam and Singh, Sanjam}, year={2017}, pages={17–25} }
 @inproceedings{rastogi_frey_2016, title={Assessing the impact of real-world driving cycles, fuel economy and tailpipe exhaust emissions on eco-rating of passenger cars}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85014833745&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Rastogi, N. and Frey, H.C.}, year={2016}, pages={2758–2783} }
 @article{hu_frey_washburn_2016, title={Comparison of Vehicle-Specific Fuel Use and Emissions Models Based on Externally and Internally Observable Activity Data}, volume={2570}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85014459431&partnerID=MN8TOARS}, DOI={10.3141/2570-04}, abstractNote={ On-road vehicles consume a substantial amount of fuel and emit air pollutants, including carbon monoxide, hydrocarbon, and nitrogen oxides. Increasing concern about air quality has meant increasing interest in accurate estimates of microscale vehicle energy use and emissions (EU&E). Vehicle EU&E rates are related to vehicle-specific power (VSP). For a given vehicle, VSP is a function of vehicle speed, acceleration, and grade, each of which can be categorized as an externally observable variable (EOV). However, internally observable variables (IOVs), such as manifold absolute pressure (MAP) and engine revolutions per minute (RPM), are more predictive of EU&E. Field measurements were conducted for 10 on-road vehicles using a portable emissions measurement system, an on-board diagnostic scan tool, and GPS receivers to measure exhaust concentrations and engine activity and to estimate road grade. VSP and the product of MAP and RPM ( PM×R) are useful as indicators of engine load. IOV-based models were developed to predict EU&E rates. The IOV-based models typically performed better than EOV-based models. The models demonstrated could be integrated into next-generation traffic simulation programs, as well as into vehicle engine control units, to provide feedback on real-time emissions. }, number={2570}, journal={TRANSPORTATION RESEARCH RECORD}, author={Hu, Jiangchuan and Frey, H. Christopher and Washburn, Scott S.}, year={2016}, pages={30–38} }
 @article{frey_2016, title={Dose-Response Models are Conditional on Determination of Causality}, volume={36}, ISSN={["1539-6924"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84992199746&partnerID=MN8TOARS}, DOI={10.1111/risa.12672}, abstractNote={Causality is a key underlying issue in this special collection of articles regarding uncertainties in health risk estimates for “criteria” air pollutants in the United States and estimates of human health benefits of the National Ambient Air Quality Standards (NAAQS). Specifically, does exposure to a particular air pollutant at a particular concentration cause an adverse effect in the exposed population? However, before addressing this question, some context is required. It is not possible to discuss the risk assessment process related to the NAAQS without consideration of the legal and policy context of these standards. Therefore, some of this context is provided here. There is also a critical distinction between how information is used to inform the process of revising or setting a standard versus how it is used in benefit-cost assessment. The statutory mandate for the NAAQS is from the Clean Air Act (CAA). The CAA requires the Administrator of the U.S. Environmental Protection Agency to list those air pollutants that “cause or contribute to air pollution which may reasonably be anticipated to endanger public health or welfare.” These pollutants are referred to as “criteria pollutants.” Here, we will focus on NAAQS aimed at protecting public health, which are referred to as “primary” standards. The legislative and historical backgrounds for NAAQS are described in the U.S. Environmental Protection Agency’s (EPA’s) policy assessment for each of the pollutants regulated under the NAAQS. For example, see the ozone policy assessment for more details.(1) Section 109(b)(1) of the CAA directs that a primary standard allows “an adequate margin of safety” and be “requisite to protect the public health.” This language is vague but has been interpreted as the re-}, number={9}, journal={RISK ANALYSIS}, author={Frey, H. Christopher}, year={2016}, month={Sep}, pages={1751–1754} }
 @article{graver_frey_hu_2016, title={Effect of Biodiesel Fuels on Real-World Emissions of Passenger Locomotives}, volume={50}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84994320268&partnerID=MN8TOARS}, DOI={10.1021/acs.est.6b03567}, abstractNote={Few data are available regarding the effect of biodiesel on exhaust emission rates of two-stroke engines used in many passenger locomotives. Using a portable emissions measurement system (PEMS), duty cycle average nitrogen oxides (NOx), hydrocarbons (HC), carbon monoxide (CO), particulate matter (PM), and carbon dioxide (CO2) emission rates were measured for three locomotives operating on ultra-low sulfur diesel (ULSD) and soy-based B10, B20, and B40 biodiesel blends. Measurements were conducted in the rail yard (RY) and over-the-rail (OTR) during passenger service. Compared to ULSD, B20 biodiesel had statistically significant average emission rate reductions in the RY of 58% for CO, 45% for PM, and 6% CO2 and OTR of 59% for HC, 50% for CO, 26% for PM, and 5% for CO2. The average differences in NOx emission rates for both the RY and OTR, and HC in the RY, were not statistically significant. The OTR findings typically agreed qualitatively with the RY findings; however, OTR provides a better basis for estimating the real-world impact of fuel switching. The results indicate substantial potential to reduce in-use locomotive emissions for existing older locomotives, with the exception of NOx.}, number={21}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Graver, Brandon M. and Frey, H. Christopher and Hu, Jiangchuan}, year={2016}, month={Nov}, pages={12030–12039} }
 @article{khan_frey_2016, title={Evaluation of Light-Duty Gasoline Vehicle Rated Fuel Economy Based on In-Use Measurements}, volume={2570}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84976319253&partnerID=MN8TOARS}, DOI={10.3141/2570-03}, abstractNote={ A study demonstrated a systematic method to assess the degree of concordance between estimated real-world and the U.S. Environmental Protection Agency (EPA) rated fuel economy for light-duty gasoline vehicles based on real-world emissions measurements. Cold engine start effects are accounted for in the real-world fuel economy and in comparison with EPA ratings. Portable emissions measurement systems were used to measure emissions of carbon dioxide, carbon monoxide, hydrocarbons, and nitrogen oxides of 122 vehicles on a specified test route consisting of various roads. Fuel economy of measured vehicles was estimated considering two cases: EPA standard driving cycles and real-world driving cycles. Average cold start emissions measurements from 32 vehicles were used in sensitivity analysis of fuel economy with respect to cold start. Real-world fuel economy estimates and comparison with rated fuel economy are not highly sensitive to cold start effects. On average, both the city and highway ratings are conservative compared with real-world fuel economy. However, there are proportions of vehicles that have lower real-world fuel economy than rated. The method demonstrated should be applied to additional vehicles and real-world driving cycles. Insights from these comparisons could motivate development of a more accurate rating scheme and identification of advice to consumers to improve their fuel economy. }, number={2570}, journal={TRANSPORTATION RESEARCH RECORD}, publisher={SAGE Publications}, author={Khan, Tanzila and Frey, H. Christopher}, year={2016}, pages={21–29} }
 @inproceedings{singh_frey_2016, title={Gasoline vehicle fuel efficiency and emissions for gas-direct injection versus port-fuel injection}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85014864810&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Singh, S. and Frey, H.C.}, year={2016}, pages={2823–2844} }
 @inproceedings{khan_frey_2016, title={Geospatial variation of real-world emissions from a passenger car}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85014820615&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Khan, T. and Frey, H.C.}, year={2016}, pages={2799–2822} }
 @article{graver_frey_2016, title={Highway Vehicle Emissions Avoided by Diesel Passenger Rail Service Based on Real-World Data}, volume={2}, url={https://doi.org/10.1007/s40864-016-0044-y}, DOI={10.1007/s40864-016-0044-y}, abstractNote={Avoided emissions attributable to the reduction in personal automobile trips for passenger rail riders are quantified based on real-world measurements. The North Carolina Department of Transportation (NCDOT) sponsors the Piedmont passenger rail service between Raleigh and Charlotte, NC. Per passenger-kilometer locomotive emissions were quantified based on portable emissions measurement system measured exhaust concentrations and duty cycles, or the fraction of trip time spent in each throttle notch setting of the prime mover engine, from 68 one-way trips of six Tier 0+ and Tier 1+ locomotives, and actual ridership data. Motor Vehicle Emissions Simulator (MOVES) software was used to estimate light-duty gasoline vehicle (LDGV) emission factors. Moving a passenger from an LDGV to a Piedmont train would lead to a net reduction in carbon dioxide (CO2) and carbon monoxide (CO) emissions by 44–94 %, respectively, between Raleigh and Charlotte, based on the assumption that the driver is the only LDGV passenger. However, locomotive nitrogen oxides (NO x ), hydrocarbons (HC), and particulate matter (PM) emission factors were 4–11 times higher than for the LDGV, respectively. Delays for either the train or highway vehicles did not substantially alter the key findings. If a Tier 4 locomotive was used, NO x , PM, and HC emission rates would be 90–99 % lower than current NCDOT locomotives. The use of real-world data representative of actual train operations provides an accurate basis for comparing rail and personal vehicle energy use and emissions and for identifying key factors affecting variability in the comparison.}, number={3-4}, journal={Urban Rail Transit}, publisher={Springer Science and Business Media LLC}, author={Graver, Brandon M. and Frey, H. Christopher}, year={2016}, month={Dec}, pages={153–171} }
 @inproceedings{delavarrafiee_frey_2016, title={Procedure for real-world measurement and data analysis of a flex fuel vehicle operated on E85}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85014816584&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Delavarrafiee, M. and Frey, H.C.}, year={2016}, pages={2689–2714} }
 @article{sandhu_frey_bartelt-hunt_jones_2016, title={Real-world activity, fuel use, and emissions of diesel side-loader refuse trucks}, volume={129}, ISSN={["1873-2844"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84957899410&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2016.01.014}, abstractNote={Diesel refuse trucks have the worst fuel economy of onroad highway vehicles. The real-world effectiveness of recently introduced emission controls during low speed and low engine load driving has not been verified for these vehicles. A portable emission measurement system (PEMS) was used to measure rates of fuel use and emissions on six side-loader refuse trucks. The objectives were to: (1) characterize activity, fuel use, and emissions; (2) evaluate variability between cycles and trucks; and (3) compare results with the MOVES emission factor model. Quality assured data cover 210,000 s and 550 miles of operation during which the trucks collected 4200 cans and 50 tons of waste material. The average fuel economy was 2.6 mpg. Trash collection contributed 70%–80% of total fuel use and emissions. The daily activity Operating Mode (OpMode) distribution and cycle average fuel use and emissions is different from previously used cycles such as Central Business District (CBD), New York Garbage Truck (NYGT), and William H. Martin (WHM). NOx emission rates for trucks with selective catalytic reduction were over 90% lower than those for trucks without. Similarly, trucks with diesel particulate filters had over 90% lower particulate matter (PM) emissions than trucks without. Compared to unloaded trucks, loaded truck averaged 18% lower fuel economy while NOx and PM emissions were higher by 65% and 16%, respectively. MOVES predicted values are highly correlated to empirical data; however, MOVES estimates are 37% lower for NOx and 300% higher for PM emission rates. The data presented here can be used to develop more representative cycles and improve emission factors for side-loader refuse trucks, which in turn can improve the accuracy of refuse truck emission inventories.}, journal={ATMOSPHERIC ENVIRONMENT}, author={Sandhu, Gurdas S. and Frey, H. Christopher and Bartelt-Hunt, Shannon and Jones, Elizabeth}, year={2016}, month={Mar}, pages={98–104} }
 @article{che_frey_lau_2016, title={Sequential Measurement of Intermodal Variability in Public Transportation PM2.5 and CO Exposure Concentrations}, volume={50}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84983049668&partnerID=MN8TOARS}, DOI={10.1021/acs.est.6b01594}, abstractNote={A sequential measurement method is demonstrated for quantifying the variability in exposure concentration during public transportation. This method was applied in Hong Kong by measuring PM2.5 and CO concentrations along a route connecting 13 transportation-related microenvironments within 3-4 h. The study design takes into account ventilation, proximity to local sources, area-wide air quality, and meteorological conditions. Portable instruments were compacted into a backpack to facilitate measurement under crowded transportation conditions and to quantify personal exposure by sampling at nose level. The route included stops next to three roadside monitors to enable comparison of fixed site and exposure concentrations. PM2.5 exposure concentrations were correlated with the roadside monitors, despite differences in averaging time, detection method, and sampling location. Although highly correlated in temporal trend, PM2.5 concentrations varied significantly among microenvironments, with mean concentration ratios versus roadside monitor ranging from 0.5 for MTR train to 1.3 for bus terminal. Measured inter-run variability provides insight regarding the sample size needed to discriminate between microenvironments with increased statistical significance. The study results illustrate the utility of sequential measurement of microenvironments and policy-relevant insights for exposure mitigation and management.}, number={16}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Che, W. W. and Frey, H. Christopher and Lau, Alexis K. H.}, year={2016}, month={Aug}, pages={8760–8769} }
 @article{frey_2015, title={A scientific perspective on the challenges and opportunities for: Multipollutant approaches to air quality management}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84920474504&partnerID=MN8TOARS}, number={January}, journal={EM: Air and Waste Management Association's Magazine for Environmental Managers}, author={Frey, H.C.}, year={2015}, pages={24–29} }
 @article{frey_2015, title={Clean Air Scientific Advisory Committee advice on the ozone standards}, volume={65}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84946154078&partnerID=MN8TOARS}, number={May}, journal={EM: Air and Waste Management Association's Magazine for Environmental Managers}, author={Frey, H.C.}, year={2015}, pages={9–10} }
 @article{graver_frey_2015, title={Comparison of Over-the-Rail and Rail Yard Measurements of Diesel Locomotives}, volume={49}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84946601235&partnerID=MN8TOARS}, DOI={10.1021/acs.est.5b02497}, abstractNote={Locomotive prime mover engine emission rates are typically measured at steady-state for discrete throttle notches using an engine dynamometer weighted by a standard duty cycle. However, this method may not represent real-world locomotive emissions. A method for in-use measurement of passenger locomotives, using a portable emissions measurement system (PEMS), was developed to estimate duty cycle average emission rates. We conducted 48 measurements of one-way trips between Raleigh and Charlotte, NC, on 7 locomotives and 18 sets of measurements in the rail yard (RY). Real-world duty cycles differed from those used for regulatory analyses, leading to statistically significant lower cycle average NOx and HC emission rates. Compared to RY measurements, notch average NOx emission rates measured over-the-rail (OTR) at the highest two notch settings were, on average, 19% lower for four locomotives. At the highest notch, OTR CO2 emission rates were, on average, 12% lower than RY rates for five locomotives. For a more accurate representation of real-world emission rates, OTR measurements are preferred. However, using steady-state notch average RY emission rates and standard duty cycles may be tolerable for some applications. OTR versus RY cycle average emission rates typically differed by less than 10%.}, number={21}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Graver, Brandon M. and Frey, H. Christopher}, year={2015}, month={Nov}, pages={13031–13039} }
 @article{che_frey_lau_2015, title={Comparison of Sources of Variability in School Age Children Exposure to Ambient PM2.5}, volume={49}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84964267743&partnerID=MN8TOARS}, DOI={10.1021/es506275c}, abstractNote={School age children are particularly susceptible to exposure to ambient fine particle (PM2.5). To provide insight into factors affecting variability in ambient PM2.5 exposure, distributions of daily PM2.5 exposures for school age children are estimated for four seasons in three climatic zones of the United States using a stochastic microenvironmental exposure model, based on ambient concentration, air exchange rate, penetration factor, deposition rate, census data, meteorological data, and time pattern data. Estimated daily individual exposure varies largely among seasons, regions, and individuals. The mean ratio of ambient exposure to ambient concentration (Ea/Ca) ranges from 0.46 to 0.61 among selected regions and seasons, resulting from differences in air exchange rate. The individual Ea/Ca varies by a factor of 2 to 3 over a 95% frequency range among simulated children, resulting from variability in children's time patterns. These patterns are similar among age groups, but vary with the day of the week and outdoor temperature. Variability in exposure is larger between individuals than between groups. The high end ratio of the Ea/Ca, at the 95th percentile of inter-individual variability, is 30% to 50% higher than the mean Ea/Ca ratio. Results can be used to intepret and adjust exposure errors in epidemiology and to assist in development of exposure mitigation strategies.}, number={3}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Che, W. W. and Frey, H. Christopher and Lau, Alexis K. H.}, year={2015}, month={Feb}, pages={1511–1520} }
 @inproceedings{sandhu_frey_bartelt-hunt_jones_2015, title={Comparison of real-world and MOVES estimated emissions for heavy-duty diesel refuse trucks}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84983775253&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Sandhu, G.S. and Frey, H.C. and Bartelt-Hunt, S. and Jones, E.}, year={2015}, pages={1702–1711} }
 @article{sandhu_frey_bartelt-hunt_jones_2015, title={In-use activity, fuel use, and emissions of heavy-duty diesel roll-off refuse trucks}, volume={65}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84942103066&partnerID=MN8TOARS}, DOI={10.1080/10962247.2014.990587}, abstractNote={The objectives of this study were to quantify real-world activity, fuel use, and emissions for heavy duty diesel roll-off refuse trucks; evaluate the contribution of duty cycles and emissions controls to variability in cycle average fuel use and emission rates; quantify the effect of vehicle weight on fuel use and emission rates; and compare empirical cycle average emission rates with the U.S. Environmental Protection Agency’s MOVES emission factor model predictions. Measurements were made at 1 Hz on six trucks of model years 2005 to 2012, using onboard systems. The trucks traveled 870 miles, had an average speed of 16 mph, and collected 165 tons of trash. The average fuel economy was 4.4 mpg, which is approximately twice previously reported values for residential trash collection trucks. On average, 50% of time is spent idling and about 58% of emissions occur in urban areas. Newer trucks with selective catalytic reduction and diesel particulate filter had NOx and PM cycle average emission rates that were 80% lower and 95% lower, respectively, compared to older trucks without. On average, the combined can and trash weight was about 55% of chassis weight. The marginal effect of vehicle weight on fuel use and emissions is highest at low loads and decreases as load increases. Among 36 cycle average rates (6 trucks × 6 cycles), MOVES-predicted values and estimates based on real-world data have similar relative trends. MOVES-predicted CO2 emissions are similar to those of the real world, while NOx and PM emissions are, on average, 43% lower and 300% higher, respectively. The real-world data presented here can be used to estimate benefits of replacing old trucks with new trucks. Further, the data can be used to improve emission inventories and model predictions. Implications: In-use measurements of the real-world activity, fuel use, and emissions of heavy-duty diesel roll-off refuse trucks can be used to improve the accuracy of predictive models, such as MOVES, and emissions inventories. Further, the activity data from this study can be used to generate more representative duty cycles for more accurate chassis dynamometer testing. Comparisons of old and new model year diesel trucks are useful in analyzing the effect of fleet turnover. The analysis of effect of haul weight on fuel use can be used by fleet managers to optimize operations to reduce fuel cost.}, number={3}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Sandhu, Gurdas S. and Frey, H. Christopher and Bartelt-Hunt, Shannon and Jones, Elizabeth}, year={2015}, month={Mar}, pages={306–323} }
 @article{zhou_tanvir_lei_taylor_liu_rouphail_frey_2015, title={Integrating a simplified emission estimation model and mesoscopic dynamic traffic simulator to efficiently evaluate emission impacts of traffic management strategies}, volume={37}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84941658996&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2015.04.013}, abstractNote={This paper presents a computationally efficient and theoretically rigorous dynamic traffic assignment (DTA) model and its solution algorithm for a number of emerging emissions and fuel consumption related applications that require both effective microscopic and macroscopic traffic stream representations. The proposed model embeds a consistent cross-resolution traffic state representation based on Newell’s simplified kinematic wave and linear car following models. Tightly coupled with a computationally efficient emission estimation package MOVES Lite, a mesoscopic simulation-based dynamic network loading framework DTALite is adapted to evaluate traffic dynamics and vehicle emission/fuel consumption impact of different traffic management strategies.}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Zhou, Xuesong and Tanvir, Shams and Lei, Hao and Taylor, Jeffrey and Liu, Bin and Rouphail, Nagui M. and Frey, H. Christopher}, year={2015}, month={Jun}, pages={123–136} }
 @article{liu_frey_2015, title={Measurement and Evaluation of Real-World Speed and Acceleration Activity Envelopes for Light-Duty Vehicles}, volume={2503}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-85137320390&partnerID=MN8TOARS}, DOI={10.3141/2503-14}, abstractNote={ Accurate estimation of vehicle activity is critically important for the accurate estimation of emissions. To provide a benchmark for estimation of vehicle speed trajectories such as those from traffic simulation models, this paper demonstrates a method for quantifying light-duty vehicle activity envelopes based on real-world activity data for 100 light-duty vehicles, including conventional passenger cars, passenger trucks, and hybrid electric vehicles. The vehicle activity envelope was quanti-fied in the 95% frequency range of acceleration for each of 15 speed bins with intervals of 5 mph and a speed bin for greater than 75 mph. Potential factors affecting the activity envelope were evaluated; these factors included vehicle type, transmission type, road grade, engine displacement, engine horsepower, curb weight, and ratio of horsepower to curb weight. The activity envelope was wider for speeds ranging from 5 to 20 mph and narrowed as speed increased. The latter was consistent with a constraint on maximum achievable engine power demand. The envelope was weakly sensitive to factors such as type of vehicle, type of transmission, road grade, and engine horsepower. The effect of road grade on cycle average emissions rates was evaluated for selected real-word cycles. The measured activity envelope was compared with those of dynamometer driving cycles, such as the federal test procedure, highway fuel economy test, SC03, and US06 cycles. The effect of intervehicle variability on the activity envelope was minor; this factor implied that the envelope could be quantified based on a smaller vehicle sample than used for this study. }, number={2503}, journal={TRANSPORTATION RESEARCH RECORD}, author={Liu, Bin and Frey, H. Christopher}, year={2015}, pages={128–136} }
 @article{salamati_rouphail_frey_liu_schroeder_2015, title={Simplified method for comparing emissions in roundabouts and at signalized intersections}, volume={2517}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84975815213&partnerID=MN8TOARS}, DOI={10.3141/2517-06}, abstractNote={ An empirically based macroscopic method is presented. It estimates and compares the pollutant emissions generated from signalized intersections and roundabouts. This method was built on a large sample size of real-world second-by-second vehicle trajectories, traffic volumes, and other traffic characteristics collected at signalized intersections and roundabouts in six U.S. states. The basis for predicting and estimating pollutant emissions was the concept of vehicle-specific power. The method enables inclusion of emissions standards and vehicle classes, such as Tier 1 (T1) and Tier 2 (T2) passenger cars (PCs) and passenger trucks (PTs). More than 1,980 vehicle trajectories were analyzed. Traffic variables including intersection capacity, demand-to-capacity ratio ( d/ c), cycle length, green-to-cycle length ratio, signal progression (i.e., arrival type), and number of lanes were included in the model for analysis and comparison between signals and roundabouts. Application of the method to a case study showed that on average under low d/ c (<0.7), roundabouts generated lower emission rates than signalized intersections. As demand approached capacity under high traffic volumes, signalized intersections with favorable progression (i.e., most demand arrived during green phase) generally produced lower emission rates than roundabouts. Signalized intersections with poor progression (i.e., most demand arrived during red phase) generated more emissions than roundabouts. Results also showed that during oversaturation periods (when d/ c > 1), the amount of produced emissions increased steadily in roundabouts but increased a large amount at signals. }, number={2517}, journal={Transportation Research Record}, author={Salamati, K. and Rouphail, N.M. and Frey, H.C. and Liu, B. and Schroeder, B.J.}, year={2015}, pages={48–60} }
 @article{liu_frey_2015, title={Variability in Light-Duty Gasoline Vehicle Emission Factors from Trip-Based Real-World Measurements}, volume={49}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84945292175&partnerID=MN8TOARS}, DOI={10.1021/acs.est.5b00553}, abstractNote={Using data obtained with portable emissions measurements systems (PEMS) on multiple routes for 100 gasoline vehicles, including passenger cars (PCs), passenger trucks (PTs), and hybrid electric vehicles (HEVs), variability in tailpipe emission rates was evaluated. Tier 2 emission standards are shown to be effective in lowering NOx, CO, and HC emission rates. Although PTs are larger, heavier vehicles that consume more fuel and produce more CO2 emissions, they do not necessarily produce more emissions of regulated pollutants compared to PCs. HEVs have very low emission rates compared to tier 2 vehicles under real-world driving. Emission factors vary with cycle average speed and road type, reflecting the combined impact of traffic control and traffic congestion. Compared to the slowest average speed and most congested cycles, optimal emission rates could be 50% lower for CO2, as much as 70% lower for NOx, 40% lower for CO, and 50% lower for HC. There is very high correlation among vehicles when comparing driving cycles. This has implications for how many cycles are needed to conduct comparisons between vehicles, such as when comparing fuels or technologies. Concordance between empirical and predicted emission rates using the U.S. Environmental Protection Agency's MOVES model was also assessed.}, number={20}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Liu, Bin and Frey, H. Christopher}, year={2015}, month={Oct}, pages={12525–12534} }
 @article{anya_rouphail_frey_schroeder_2014, title={Application of AIMSUN microsimulation model to estimate emissions on signalized arterial corridors}, volume={2428}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907481905&partnerID=MN8TOARS}, DOI={10.3141/2428-09}, abstractNote={ Tailpipe emissions from on-road vehicles have negative impacts on the air quality of a region and are influenced by several factors including driver behavior, road characteristics, and traffic conditions. Typically vehicles on arterial roads frequently stop and then accelerate, a behavior that produces more tailpipe emissions than does maintaining a steady speed. Microscopic simulation models generate synthetic vehicle activity that can be used to estimate emissions on a facility. For emissions to be estimated accurately from an arterial facility, the simulated activity must closely represent the activity observed in the field. This study investigated whether a selected set of simulation parameters that used speed trajectories observed in the field at high temporal resolution could improve the emissions estimated from a simulated signalized arterial corridor. The simulated trajectories and those observed in the field were characterized by vehicle-specific power (VSP). Emissions were estimated with VSP modal emission rates and the time spent by vehicles in each VSP mode. The emissions were compared for routes along an urban arterial facility and for shorter segments within routes. At both spatial scales, the emissions from parameter-adjusted simulation vehicles that had the same average number of stops and travel time distribution as vehicles observed in the field were closer to real-world emissions than were the simulation vehicles under default model parameters. Adjusting the parameters also allowed the model to capture emissions hot spots along the routes more accurately. Currently, microscopic simulation models are calibrated with traffic performance measures that are improving. The findings from this paper present the potential for using these improving emissions estimates from simulated vehicle activity for the additional calibration of microsimulation models. }, number={2428}, journal={Transportation Research Record}, author={Anya, A. and Rouphail, N. and Frey, H. and Schroeder, B.}, year={2014}, pages={75–86} }
 @article{che_frey_lau_2014, title={Assessment of the Effect of Population and Diary Sampling Methods on Estimation of School-Age Children Exposure to Fine Particles}, volume={34}, ISSN={["1539-6924"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84921063491&partnerID=MN8TOARS}, DOI={10.1111/risa.12238}, abstractNote={Population and diary sampling methods are employed in exposure models to sample simulated individuals and their daily activity on each simulation day. Different sampling methods may lead to variations in estimated human exposure. In this study, two population sampling methods (stratified‐random and random‐random) and three diary sampling methods (random resampling, diversity and autocorrelation, and Markov‐chain cluster [MCC]) are evaluated. Their impacts on estimated children's exposure to ambient fine particulate matter (PM2.5) are quantified via case studies for children in Wake County, NC for July 2002. The estimated mean daily average exposure is 12.9 μg/m3 for simulated children using the stratified population sampling method, and 12.2 μg/m3 using the random sampling method. These minor differences are caused by the random sampling among ages within census tracts. Among the three diary sampling methods, there are differences in the estimated number of individuals with multiple days of exposures exceeding a benchmark of concern of 25 μg/m3 due to differences in how multiday longitudinal diaries are estimated. The MCC method is relatively more conservative. In case studies evaluated here, the MCC method led to 10% higher estimation of the number of individuals with repeated exposures exceeding the benchmark. The comparisons help to identify and contrast the capabilities of each method and to offer insight regarding implications of method choice. Exposure simulation results are robust to the two population sampling methods evaluated, and are sensitive to the choice of method for simulating longitudinal diaries, particularly when analyzing results for specific microenvironments or for exposures exceeding a benchmark of concern.}, number={12}, journal={RISK ANALYSIS}, author={Che, W. W. and Frey, H. Christopher and Lau, Alexis K. H.}, year={2014}, month={Dec}, pages={2066–2079} }
 @article{jiao_frey_2014, title={Comparison of Fine Particulate Matter and Carbon Monoxide Exposure Concentrations for Selected Transportation Modes}, ISSN={["2169-4052"]}, DOI={10.3141/2428-07}, abstractNote={ Daily commutes may contribute disproportionately to overall daily exposure to urban air pollutants such as particulate matter less than 2.5 &Mu;m (PM2.5) and carbon monoxide (CO). PM2.5 and CO concentrations were measured and compared across pedestrian, bus, and car modes during lunchtime and the afternoon peak hour within a 3–week time period on preselected round-trip routes. Variability in the concentration ratios of PM2.5 and CO for the selected transportation modes was quantified, and factors affecting variability in concentrations were identified. Exposure concentrations of transportation modes were sensitive to mode and were affected by factors such as vehicle ventilation and proximity to on-road emission sources. In general, pedestrian and bus modes had higher PM2.5 concentrations than did the car mode. However, the car mode had the highest average CO concentrations among the selected modes. Near-road pedestrian PM2.5 concentrations generally covaried with fixed site monitor (FSM) measurements, but there was little correlation between pedestrian CO concentrations and FSM data. Field studies such as this one are needed to develop input data for simulation models of population-based exposure to predict more accurately exposure concentrations for transportation modes. }, number={2428}, journal={TRANSPORTATION RESEARCH RECORD}, author={Jiao, Wan and Frey, H. Christopher}, year={2014}, pages={54–62} }
 @inproceedings{graver_hu_frey_2014, title={Comparison of real-world emission rates of a locomotive engine operating on diesel and biodiesel}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84939813222&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Graver, B.M. and Hu, J. and Frey, H.C.}, year={2014}, pages={991–999} }
 @inproceedings{che_frey_lau_fung_2014, title={Geographic and seasonal variations in air exchange rate and their impacts on the estimation of children's exposure to ambient PM<sub>2.5</sub>}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84924657052&partnerID=MN8TOARS}, booktitle={Indoor Air 2014 - 13th International Conference on Indoor Air Quality and Climate}, author={Che, W. and Frey, H.C. and Lau, A.K.H. and Fung, J.C.H.}, year={2014}, pages={836–839} }
 @article{sandhu_frey_bartelt-hunt_jones_2014, title={In-use measurement of the activity, fuel use, and emissions of front-loader refuse trucks}, volume={92}, ISSN={["1873-2844"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84901001305&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2014.04.036}, abstractNote={Field measurements were made for six front-loader refuse trucks for over 560 miles (901 km) and 47 h of operation using a portable emissions measurement system, electronic control unit data logger, and global positioning system receivers. Daily activity, fuel use rates, and emission rates are quantified in terms of operating mode bins defined by the U.S. Environmental Protection Agency for the MOVES emission factor model. On average, 44 (±4) percent of time was spent at idle, 5 (±1) percent braking or decelerating, 11 (±2) percent coasting, 23 (±3) percent cruising or accelerating at low speed (up to 25 mph, 40.2 kmph), 10 (±2) percent cruising or accelerating at moderate speed (25–50 mph, 40.2 to 80.4 kmph), and 7 (±3) percent cruising or accelerating at high speed (50 mph, 80.4 kmph or higher). Fuel use and emission rates varied among operating modes by factors of 6–24. The estimated daily activity cycle average fuel economy ranges from 2.3 to 3.2 mpg (0.98–1.4 kmpl). The PM emission rates for trucks with diesel particulate filters are 98 percent lower compared to those without. Variation in truck weight lead to differences in average fuel use and emission rates of 20 percent or less, except for hydrocarbons. The variation in the empirically-based daily activity cycle average rates were highly correlated with MOVES estimates, except for hydrocarbons. The data collected here are useful for quantifying daily activity specific to front-loaders, and for developing fuel use and emission estimates and models for this type of vehicle.}, journal={ATMOSPHERIC ENVIRONMENT}, author={Sandhu, Gurdas S. and Frey, H. Christopher and Bartelt-Hunt, Shannon and Jones, Elizabeth}, year={2014}, month={Aug}, pages={557–565} }
 @article{hu_frey_sandhu_graver_bishop_schuchmann_ray_2014, title={Method for Modeling Driving Cycles, Fuel Use, and Emissions for Over Snow Vehicles}, volume={48}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84904409293&partnerID=MN8TOARS}, DOI={10.1021/es501164j}, abstractNote={As input to a winter use plan, activity, fuel use, and tailpipe exhaust emissions of over snow vehicles (OSV), including five snow coaches and one snowmobile, were measured on a designated route in Yellowstone National Park (YNP). Engine load was quantified in terms of vehicle specific power (VSP), which is a function of speed, acceleration, and road grade. Compared to highway vehicles, VSP for OSVs is more sensitive to rolling resistance and less sensitive to aerodynamic drag. Fuel use rates increased linearly (R2>0.96) with VSP. For gasoline-fueled OSVs, fuel-based emission rates of carbon monoxide (CO) and nitrogen oxides (NOx) typically increased with increasing fuel use rate, with some cases of very high CO emissions. For the diesel OSVs, which had selective catalytic reduction and diesel particulate filters, fuel-based NOx and particulate matter (PM) emission rates were not sensitive to fuel flow rate, and the emission controls were effective. Inter vehicle variability in cycle average fuel use and emissions rates for CO and NOx was substantial. However, there was relatively little inter-cycle variation in cycle average fuel use and emission rates when comparing driving cycles. Recommendations are made regarding how real-world OSV activity, fuel use, and emissions data can be improved.}, number={14}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Hu, Jiangchuan and Frey, H. Christopher and Sandhu, Gurdas S. and Graver, Brandon M. and Bishop, Gary A. and Schuchmann, Brent G. and Ray, John D.}, year={2014}, month={Jul}, pages={8258–8265} }
 @article{boroujeni_frey_2014, title={Road grade quantification based on global positioning system data obtained from real-world vehicle fuel use and emissions measurements}, volume={85}, ISSN={["1873-2844"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84891950017&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2013.12.025}, abstractNote={Real-world vehicle fuel use and emission rates depend on engine load, which is quantified in terms of Vehicle Specific Power (VSP). VSP depends on vehicle speed, acceleration, and road grade. There is not a standard method for measuring road grade from a moving vehicle. A method for quantifying grade is evaluated based on statistical analysis of multiple runs using low cost consumer grade Global Positioning System (GPS) receivers with in-built Barometric Altimeter (GPS/BA). The average grade precision is ±0.71, ±0.46, and ±0.31 percentage points, for sample sizes of 9, 18, and 36 GPS/BA runs, respectively, among 2213 individual 0.08 km road segments. In addition, 4 sets of repeated measurements were performed on the same routes using a high cost, high accuracy Differential GPS (DGPS). Both sets of GPS-based grade estimates compared well with those derived from LIght Detection And Ranging (LIDAR) data. GPS/BA and DGPS grade estimates were similar, except for high magnitude grades of 8–10 percent for which DGPS estimates are more accurate. DGPS is more sensitive to loss of signal; thus, a hybrid approach for substituting GPS/BA data for missing DGPS data at specific locations along a route is demonstrated. The local and overall effects of road grade on fuel use and emission rates are investigated for an example light duty gasoline vehicle.}, journal={ATMOSPHERIC ENVIRONMENT}, author={Boroujeni, Behdad Yazdani and Frey, H. Christopher}, year={2014}, month={Mar}, pages={179–186} }
 @article{mannshardt_sucic_jiao_dominici_frey_reich_fuentes_2013, title={Comparing exposure metrics for the effects of fine particulate matter on emergency hospital admissions}, volume={23}, ISSN={["1559-064X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84886725530&partnerID=MN8TOARS}, DOI={10.1038/jes.2013.39}, abstractNote={A crucial step in an epidemiological study of the effects of air pollution is to accurately quantify exposure of the population. In this paper, we investigate the sensitivity of the health effects estimates associated with short-term exposure to fine particulate matter with respect to three potential metrics for daily exposure: ambient monitor data, estimated values from a deterministic atmospheric chemistry model, and stochastic daily average human exposure simulation output. Each of these metrics has strengths and weaknesses when estimating the association between daily changes in ambient exposure to fine particulate matter and daily emergency hospital admissions. Monitor data is readily available, but is incomplete over space and time. The atmospheric chemistry model output is spatially and temporally complete but may be less accurate than monitor data. The stochastic human exposure estimates account for human activity patterns and variability in pollutant concentration across microenvironments, but requires extensive input information and computation time. To compare these metrics, we consider a case study of the association between fine particulate matter and emergency hospital admissions for respiratory cases for the Medicare population across three counties in New York. Of particular interest is to quantify the impact and/or benefit to using the stochastic human exposure output to measure ambient exposure to fine particulate matter. Results indicate that the stochastic human exposure simulation output indicates approximately the same increase in the relative risk associated with emergency admissions as using a chemistry model or monitoring data as exposure metrics. However, the stochastic human exposure simulation output and the atmospheric chemistry model both bring additional information, which helps to reduce the uncertainly in our estimated risk.}, number={6}, journal={JOURNAL OF EXPOSURE SCIENCE AND ENVIRONMENTAL EPIDEMIOLOGY}, author={Mannshardt, Elizabeth and Sucic, Katarina and Jiao, Wan and Dominici, Francesca and Frey, H. Christopher and Reich, Brian and Fuentes, Montserrat}, year={2013}, pages={627–636} }
 @article{graver_frey_2013, title={Comparison of Locomotive Emissions Measured During Dynamometer Versus Rail Yard Engine Load Tests}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887758744&partnerID=MN8TOARS}, DOI={10.3141/2341-03}, abstractNote={ For environmental emissions certification, locomotive prime mover engines undergo rigorous measurements with the use of federal reference methods (FRMs). However, there are nonregulatory reasons for engine emissions measurement, such as to compare relative differences in fuel use and emissions as a result of engine rebuild or between engines in a fleet. Portable emissions measurement systems (PEMSs) are widely used for highway vehicle and nonroad construction equipment but have had limited locomotive applications. The objectives are to (a) demonstrate an approach for quantifying locomotive emission rates with a PEMS during dynamometer and rail yard engine load tests, (b) compare the emissions measured in dynamometer versus rail yard load tests, and (c) assess the relative change in fuel use and emissions from engine rebuild. Measurements were conducted on 3,000-hp prime movers, including an EMD 16-645 for a GP40 and two EMD 12-710s for F59PHs. Fuel use and PEMS-based emission rates for nitric oxide, carbon monoxide, hydrocarbons, and particulate matter were compared between dynamometer and rail yard load tests and with data from previous literature. Fuel use and oxides of nitrogen (NOx) emission rates after engine rebuild were lower for the GP40 prime mover, and the fuel use and NOx emission rates for the F59PH rebuilt engines were lower than those of the rebuilt GP40 engine. PEMS is not a substitute for locomotive FRMs if compliance certification is needed but provides useful data for comparative assessment. }, number={2341}, journal={TRANSPORTATION RESEARCH RECORD}, author={Graver, Brandon M. and Frey, H. Christopher}, year={2013}, pages={23–33} }
 @article{sandhu_frey_2013, title={Effects of errors on vehicle emission rates from portable emissions measurement systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84883041079&partnerID=MN8TOARS}, DOI={10.3141/2340-02}, abstractNote={ Portable emissions measurement systems (PEMS) are useful for quantification of real-world vehicle activity, energy use, and emissions. However, there is no standard methodology for processing PEMS data; this can lead to errors in reported results. PEMS typically include tailpipe exhaust gas and particle analyzers, Global Positioning System (GPS) receivers, engine sensors, and electronic control unit (ECU) data loggers. The sensitivity of estimated emission rates to random errors in measurements is quantified. Methods are evaluated for identification and correction of improper synchronization of PEMS, ECU, and GPS data streams and for road grade estimation. Estimated fuel use and emission rates for light- and heavy-duty vehicles are sensitive to errors in intake manifold absolute pressure and engine revolutions per minute values and in indicators of air-to-fuel ratio including carbon dioxide and oxygen concentrations. Synchronization can be aided by maximizing the Pearson correlation coefficient between two indicator variables and confirming the result by matching concurrent increases in indicator variables. The effect of improper synchronization on estimated modal emission rates is quantified. Modal average emission rates based on vehicle-specific power (VSP) are more sensitive to improperly synchronized engine versus GPS data. Improperly synchronized data streams result in decreased variability between the lowest and highest modal average emission rates. Estimation of road grade from a linear least squares slope of elevation over a specified distance is demonstrated. VSP-based modal fuel use and pollutant emission rates are less sensitive to differences in road grade than to errors in synchronization. }, number={2340}, journal={Transportation Research Record}, author={Sandhu, G. and Frey, H.}, year={2013}, pages={10–19} }
 @article{salamati_coelho_fernandes_rouphail_frey_bandeira_2013, title={Emissions estimation at multilane roundabouts}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84897133284&partnerID=MN8TOARS}, DOI={10.3141/2389-02}, abstractNote={ This paper reports on research that explored how multilane roundabouts located on urban corridors have affected traffic performance and pollutant emissions generated from vehicles. The research also compared the emissions of vehicles moving through the roundabouts as they used either the left or right entry lanes. The methodology can be generalized to measure the emissions of any multilane roundabout. The paper identified a representative speed profile for each speed trajectory type, no stop, one stop, and multiple stops, from field data collected at four multilane roundabouts in Aveiro, Portugal. The vehicle-specific power emissions methodology was employed to estimate the second-by-second emissions generated from a vehicle during different acceleration–deceleration cycles. Congestion-specific vehicle speed profiles for two-lane roundabout approaches were used to develop regression models to predict the percentage of vehicles that would experience different speed trajectory types in the roundabout. The analysis tested hypotheses about how differences in the following characteristics have affected the amount of emissions generated from vehicles in each lane: (a) the speed profiles in each lane (left versus right), (b) the conflicting flows for the left and right lanes, (c) the lane flow, and (d) the overall congestion levels. Under low congestion levels, vehicles in the right lane emitted more pollutant because they had on average higher speed and sharper acceleration and deceleration rates. For high congestion levels, given equal flow rates for the left and right lanes, vehicles in the left lane produced more emissions because vehicles in the left lane experienced longer stop-and-go cycles and had different speed profiles than did vehicles in the right lane. }, number={2389}, journal={Transportation Research Record}, author={Salamati, K. and Coelho, M. and Fernandes, P. and Rouphail, N. and Frey, H. and Bandeira, J.}, year={2013}, pages={12–21} }
 @article{salamati_coelho_fernandes_rouphail_frey_bandeira_2013, title={Emissions estimation at multilane roundabouts effects of movement and approach lane}, number={2389}, journal={Transportation Research Record}, author={Salamati, K. and Coelho, M. C. and Fernandes, P. J. and Rouphail, N. M. and Frey, H. C. and Bandeira, J.}, year={2013}, pages={12–21} }
 @article{abolhasani_frey_2013, title={Engine and Duty Cycle Variability in Diesel Construction Equipment Emissions}, volume={139}, ISSN={["1943-7870"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84873375511&partnerID=MN8TOARS}, DOI={10.1061/(asce)ee.1943-7870.0000548}, abstractNote={AbstractThis paper explores methods for analyzing onboard mass emissions data and developing modal models on the basis of case study examples for nine selected nonroad construction vehicles. Data for these vehicles were obtained from the U.S. Environmental Protection Agency (EPA). Several modeling methods were explored, including stratification of the data into operating modes and supplementing the modal models with ordinary least-squares regression and multiple least-squares regression. The modal approach offers the advantages as conceptually the simplest, reducing the influence of autocorrelation in the model and providing substantial explanatory power. The normalized relationship between predicted mode-specific average emissions and exhaust flow is stable, similar, and consistent for all vehicles. For a given engine, the average emission rate can vary by more than a factor of two when comparing highest to lowest rates among different duty cycles. Some engines are common to different types of equipment,...}, number={2}, journal={JOURNAL OF ENVIRONMENTAL ENGINEERING}, author={Abolhasani, Saeed and Frey, H. Christopher}, year={2013}, month={Feb}, pages={261–268} }
 @article{jiao_frey_2013, title={Method for Measuring the Ratio of In-Vehicle to Near-Vehicle Exposure Concentrations of Airborne Fine Particles}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84887641213&partnerID=MN8TOARS}, DOI={10.3141/2341-04}, abstractNote={ Human exposure to fine particulate matter of less than 2.5 microns in aerodynamic diameter is causally linked to cardiovascular and pulmonary diseases. In-vehicle exposure may account for 10% to 20% of daily average exposure. However, exposure models are typically based on areawide air quality data that poorly predict in-vehicle concentration. A practical method is demonstrated for conducting field measurements to quantify the ratio of in-vehicle to outside vehicle concentration (I/O) for a wide range of conditions that affect intravehicle variability in exposure concentration. A field data collection study design is developed on the basis of sources of intravehicle variability in I/O that include ventilation air source, window status, fan setting, air-conditioning (AC) use, vehicle speed, road type, travel direction, and time of day. Three replicates of measurements were made for 16 combinations of these factors on 110 mi of roads comprising eight one-way routes between typical commuter origin–destination pairs. Two portable particle monitors recorded in-vehicle and near-vehicle ambient concentrations on 1-min averages for four particle size ranges. The comparability of the monitors was quantified. Near-vehicle concentrations varied with road type, time of day, and traffic conditions. The I/O ratio was approximately independent of near-vehicle concentration and varied with window status, source of ventilation air (fresh or recirculated), and for cases with recirculation and closed windows, fan setting, and AC use. The study design can be extended to additional vehicles to account for potential sources of inter-vehicle variability. Data collected here can be used to improve exposure simulation models. }, number={2341}, journal={TRANSPORTATION RESEARCH RECORD}, author={Jiao, Wan and Frey, H. Christopher}, year={2013}, pages={34–42} }
 @article{boroujeni_frey_sandhu_2013, title={Road Grade Measurement Using In-Vehicle, Stand-Alone GPS with Barometric Altimeter}, volume={139}, ISSN={["0733-947X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84881240089&partnerID=MN8TOARS}, DOI={10.1061/(asce)te.1943-5436.0000545}, abstractNote={Real-world vehicle fuel use and emission rates are sensitive to road grade. There is a need for a practical method for measuring road grade in combination with on-board measurement of vehicle activity, energy use, and emissions using portable emissions measurement systems (PEMS). This paper focuses on quantification of the accuracy and precision of a low-cost method using a stand-alone global positioning system (GPS) receiver with an in-built barometric altimeter. Approximately 100 one-way runs were made on each of several study routes. The sensitivity of average grade estimates to the averaging distance over which grade is estimated is quantified. The repeatability of vehicle location and distance traveled is quantified. The run-to-run variability and confidence intervals for average estimates of grade are quantified. The accuracy of the grade estimates is evaluated in comparison to LIDAR-based estimates. The low-cost method is shown to be accurate, but imprecision in the measurements leads to a need for typically at least 10 or more repeated runs, depending on the desired precision of the average estimate of grade.}, number={6}, journal={JOURNAL OF TRANSPORTATION ENGINEERING-ASCE}, author={Boroujeni, Behdad Yazdani and Frey, H. Christopher and Sandhu, Gurdas Singh}, year={2013}, month={Jun}, pages={605–611} }
 @article{jiao_frey_cao_2012, title={Assessment of Inter-Individual, Geographic, and Seasonal Variability in Estimated Human Exposure to Fine Particles}, volume={46}, ISSN={["0013-936X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869388772&partnerID=MN8TOARS}, DOI={10.1021/es302803g}, abstractNote={Health effects associated with ambient fine particle (PM(2.5)) exposure are typically estimated based on concentration-response (C-R) functions using area-wide concentration as an exposure surrogate. Persons 65 and older are particularly susceptible to adverse effects from PM(2.5) exposure. Using a stochastic microenvironmental simulation model, distributions of daily PM(2.5) exposures were estimated based on ambient concentration, air exchange rate, penetration factor, deposition rate, indoor emission sources, census data, and activity diary data, and compared for selected regions and seasons. Even though the selected subpopulation spends an average of over 20 h per day indoors, the ratio of daily average estimated exposure to ambient concentration (E(a)/C) is approximately 0.5. The daily average E(a)/C ratio varies by a factor of 4-5 over a 95% frequency range among individuals, primarily from variability in air exchange rates. The mean E(a)/C varies by 6-36% among selected NC, TX, and NYC domains, and 15-34% among four seasons, as a result of regional differences in housing stock and seasonal differences in air exchange rates. Variability in E(a)/C is a key factor that may help explain heterogeneity in C-R functions across cities and seasons. Priorities for improving exposure estimates are discussed.}, number={22}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Jiao, Wan and Frey, H. Christopher and Cao, Ye}, year={2012}, month={Nov}, pages={12519–12526} }
 @article{lewis_rasdorf_frey_leming_2012, title={Effects of Engine Idling on National Ambient Air Quality Standards Criteria Pollutant Emissions from Nonroad Diesel Construction Equipment}, ISSN={["0361-1981"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84867783739&partnerID=MN8TOARS}, DOI={10.3141/2270-09}, abstractNote={ It is difficult to assess the effects of engine idling on air pollutant emissions from highway construction equipment because of a lack of combined activity and emissions data. A methodology is presented for quantifying the impact of idling on National Ambient Air Quality Standards criteria pollutant emissions, including nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), and particulate matter (PM). The methodology and results are based on field data collected from 35 items of nonroad diesel construction equipment. Engine idle time was quantified in terms of equipment operational efficiency (η), which was defined as the ratio of nonidle time to total equipment use time (nonidle time plus idle time). With η and the ratio of idle to nonidle emission rates (re) for each pollutant, the percentage increase in the total quantities of each pollutant emitted (NE) was calculated for each item of equipment for the observed values of η and re. Results showed that as η increased (or idle time decreased), NE decreased. Mathematical models with η as a predictor variable were developed to estimate values of NE for each pollutant. Regression equations in the form of y = ax3 + bx2 + cx + d were developed for NOx and HC, with R2 values of 97% and 87%, respectively. Regression equations in the form of y = mx + b were developed for CO and PM, with R2 values of .50 and .65, respectively. Recommendations include using the models as performance measurement tools to assess idle restriction regulations and operational strategies of highway construction equipment. }, number={2270}, journal={TRANSPORTATION RESEARCH RECORD}, author={Lewis, Phil and Rasdorf, William and Frey, H. Christopher and Leming, Michael}, year={2012}, pages={67–75} }
 @inproceedings{sandhu_frey_2012, title={Energy use and emissions from diesel and biodiesel blends for earthmoving equipment}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84907338054&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Sandhu, G.S. and Frey, C.H.}, year={2012}, pages={609–616} }
 @inproceedings{graver_christopher frey_2012, title={Estimation of light-duty gasoline vehicle emissions avoided by passenger rail service in North Carolina}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84876198041&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Graver, B.M. and Christopher Frey, H.}, year={2012}, pages={1001–1007} }
 @article{rasdorf_lewis_marshall_arocho_frey_2012, title={Evaluation of On-Site Fuel Use and Emissions over the Duration of a Commercial Building Project}, volume={18}, ISSN={["1943-555X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861877818&partnerID=MN8TOARS}, DOI={10.1061/(asce)is.1943-555x.0000071}, abstractNote={AbstractCommercial construction operations are heavy users of nonroad equipment and diesel engines. These engines are large contributors of carbon dioxide (CO2) and other engine-based pollutants and consume large quantities of diesel fuel. This paper presents a case study to identify construction activities that cause the most emissions and fuel use on a construction project and to determine when and where these occur. The paper links emissions and fuel use of construction activities with a project schedule to establish a temporal relationship. RSMeans was used to obtain equipment productivity. Equipment emission rates and fuel use rates were obtained from the EPA’s NONROAD Model. A sensitivity analysis was conducted to investigate the effect of changes in scheduling, equipment size, and EPA engine tier on emissions and fuel use. Site construction activities proved to be the heaviest polluters and caused a significant spike in emissions early in the project. Further study is recommended to validate the tr...}, number={2}, journal={JOURNAL OF INFRASTRUCTURE SYSTEMS}, author={Rasdorf, William and Lewis, Phil and Marshall, Steven Kelly and Arocho, Ingrid and Frey, H. Christopher}, year={2012}, month={Jun}, pages={119–129} }
 @article{lee_frey_2012, title={Evaluation of representativeness of site-specific fuel-based vehicle emission factors for route average emissions}, volume={46}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84862562432&partnerID=MN8TOARS}, DOI={10.1021/es204451z}, abstractNote={An approach to evaluate the representativeness of site-specific fuel-based vehicle emission factors, such as would be obtained using Remote Sensing Devices (RSDs) is demonstrated based on real-world data for 23 selected light duty gasoline vehicles. Real time vehicle route-average emissions rates were measured using a Portable Emissions Measurement System (PEMS) for a variety of road types and traffic characteristics. Several hypothetical remote sensing sites were selected to estimate site-specific fuel-based emission factors. The average fuel-based emission factors increased with vehicle specific power (VSP) and varied by a factor of 3 and 4 for NO(x) and CO, respectively. The route average emission factors varied by approximately 20% for either NO(x) or CO. The site-specific emission factors varied among specific sites by 20 and 30% for NO(x) and CO, respectively. Fuel-based HC emission rates had little variability with engine load, among routes, or between sites. Arbitrarily chosen sites can lead to potential biases for CO and NO(x) if measured emission factors are used for route average rates and, therefore, for area-wide inventories. However, site-specific emission factors have the potential to be representative of area-wide emission rates if the distribution of positive VSP at the site is similar to that of routes or area-wide cycles of interest.}, number={12}, journal={Environmental Science and Technology}, author={Lee, T. and Frey, H.C.}, year={2012}, pages={6867–6873} }
 @article{marshall_rasdorf_lewis_frey_2012, title={Methodology for Estimating Emissions Inventories for Commercial Building Projects}, volume={18}, ISSN={1076-0431 1943-5568}, url={http://dx.doi.org/10.1061/(ASCE)AE.1943-5568.0000073}, DOI={10.1061/(ASCE)AE.1943-5568.0000073}, abstractNote={AbstractConstruction of commercial buildings has an environmental impact because of emissions from nonroad equipment. Construction produces 7% of the U.S. greenhouse gas emissions, of which 76% originates from engines. However, there has not been an established methodology for estimating construction activity emissions at the project level for buildings and other constructed facilities. Therefore, determining where and when construction produces emissions is important, necessitating that emissions inventories (i.e., databases of project emissions by type, cost, etc.) be developed to quantify these emissions. The pollutants addressed in this paper include carbon dioxide (CO2), nitrogen oxides (NOx), total hydrocarbons (THC), carbon monoxide (CO), particulate matter (PM), and sulfur dioxide (SO2). This paper presents a methodology that provides a direct link between building construction activities and emissions. The use of this methodology is demonstrated for a commercial building case study, thereby provi...}, number={3}, journal={Journal of Architectural Engineering}, publisher={American Society of Civil Engineers (ASCE)}, author={Marshall, Steven Kelly and Rasdorf, William and Lewis, Phil and Frey, H. Christopher}, year={2012}, month={Sep}, pages={251–260} }
 @article{cao_frey_2012, title={Modeling of Human Exposure to In-Vehicle PM2.5 from Environmental Tobacco Smoke}, volume={18}, ISSN={["1549-7860"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84861953590&partnerID=MN8TOARS}, DOI={10.1080/10807039.2012.672894}, abstractNote={ABSTRACT Environmental tobacco smoke (ETS) is estimated to be a significant contributor to in-vehicle human exposure to fine particulate matter of 2.5 μm or smaller (PM2.5). A critical assessment was conducted of a mass balance model for estimating PM2.5 concentration with smoking in a motor vehicle. Recommendations for the range of inputs to the mass-balance model are given based on literature review. Sensitivity analysis was used to determine which inputs should be prioritized for data collection. Air exchange rate (ACH) and the deposition rate have wider relative ranges of variation than other inputs, representing inter-individual variability in operations, and inter-vehicle variability in performance, respectively. Cigarette smoking and emission rates and vehicle interior volume are also key inputs. The in-vehicle ETS mass balance model was incorporated into the Stochastic Human Exposure and Dose Simulation for Particulate Matter (SHEDS-PM) model to quantify the potential magnitude and variability of in-vehicle exposures to ETS. The in-vehicle exposure also takes into account near-road incremental PM2.5 concentration from on-road emissions. Results of probabilistic study indicate that ETS is a key contributor to the in-vehicle average and high-end exposure. Factors that mitigate in-vehicle ambient PM2.5 exposure lead to higher in-vehicle ETS exposure, and vice versa.}, number={3}, journal={HUMAN AND ECOLOGICAL RISK ASSESSMENT}, author={Cao, Ye and Frey, H. Christopher}, year={2012}, pages={608–626} }
 @article{frey_choi_kim_2012, title={Portable Emission Measurement System for Emissions of Passenger Rail Locomotives}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869852000&partnerID=MN8TOARS}, DOI={10.3141/2289-08}, abstractNote={ The purpose of this study was to demonstrate a method for measuring passenger railroad locomotive emissions with the use of a portable emission measurement system (PEMS) based on rail yard load tests of three locomotives, including one GP40 and two F59PHIs. These locomotives have mechanically governed diesel prime mover engines (PMEs) with an approximately 3,000-hp output. Each locomotive has a head end power (HEP) engine that produces approximately 600 hp for generating electricity used in the passenger cars. The engine measurements were based on ultralow sulfur diesel fuel. Each engine was instrumented to measure manifold absolute pressure, engine revolutions per minute, intake air temperature, and exhaust concentrations of selected gases and particles. These data were used to quantify exhaust and fuel flow. The exhaust concentrations of nitric oxide, carbon monoxide (CO), carbon dioxide, hydrocarbons, and particulate matter were measured. The PMEs are operated at each of many throttle notch settings. For the HEP engines, three electrical loads were applied on the basis of power usage for one, two, and four passenger cars, respectively. More than 97% of the raw data survived a multistep quality assurance process. The data obtained from the PEMS for the main engines were found to be comparable on a fuel basis to data reported by others, particularly for oxides of nitrogen and CO. The key results from this work are the establishment of a simplified methodology for future tests and the development of baseline data. }, number={2289}, journal={TRANSPORTATION RESEARCH RECORD}, author={Frey, H. Christopher and Choi, Hyung-Wook and Kim, Kangwook}, year={2012}, pages={56–63} }
 @article{sandhu_frey_2012, title={Real-world measurement and evaluation of duty cycles, fuels, and emission control technologies of heavy-duty trucks}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84867838839&partnerID=MN8TOARS}, DOI={10.3141/2270-21}, abstractNote={ The purpose of this study is to assess the robustness of relative comparisons of emission rates between fuels and technologies to differences in real-world duty cycles on the basis of in-use measurements of five heavy-duty diesel vehicles (HDDVs). Prior comparisons of biodiesel and ultra-low-sulfur diesel emissions, recent changes in emission standards applicable to HDDVs, and typical emission control technologies used in these vehicles are reviewed. The study methodology includes field measurements with a portable emission measurement system and related instruments and sensors for five HDDVs operated in normal service by professional drivers on multiple round-trip routes within North Carolina. Duty cycles and emission rates are quantified on the basis of the manifold absolute pressure, which is an indicator of engine load. Variability in engine load for each observed round trip is quantified on the basis of the cumulative distribution function of normalized manifold absolute pressure. The effect of variability in duty cycles on fuel-based emission rates for nitrogen oxides, carbon monoxide, hydrocarbons, and particulate matter is evaluated. Comparisons are made for emissions of three trucks operated on B20 biodiesel and ultra-low-sulfur diesel. Furthermore, comparisons of five trucks with model years ranging from 1999 to 2010 illustrate the impact of different emission standards and emission control technologies on real-world emission rates. A key finding is that relative comparisons pertaining to fuels and technologies are robust to variability in observed duty cycles. }, number={2270}, journal={Transportation Research Record}, author={Sandhu, G. and Frey, H.}, year={2012}, pages={180–187} }
 @inbook{frey_zhu_2012, title={Techno-economic analysis of combined cycle systems}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84904033299&partnerID=MN8TOARS}, DOI={10.1016/B978-0-85709-013-3.50011-0}, booktitle={Combined Cycle Systems for Near-Zero Emission Power Generation}, author={Frey, H.C. and Zhu, Y.}, year={2012}, pages={306–328} }
 @article{chang_fuentes_frey_2012, title={Time series analysis of personal exposure to ambient air pollution and mortality using an exposure simulator}, volume={22}, ISSN={["1559-064X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84865498111&partnerID=MN8TOARS}, DOI={10.1038/jes.2012.53}, abstractNote={This paper describes a modeling framework for estimating the acute effects of personal exposure to ambient air pollution in a time series design. First, a spatial hierarchical model is used to relate Census tract-level daily ambient concentrations and simulated exposures for a subset of the study period. The complete exposure time series is then imputed for risk estimation. Modeling exposure via a statistical model reduces the computational burden associated with simulating personal exposures considerably. This allows us to consider personal exposures at a finer spatial resolution to improve exposure assessment and for a longer study period. The proposed approach is applied to an analysis of fine particulate matter of <2.5 μm in aerodynamic diameter (PM(2.5)) and daily mortality in the New York City metropolitan area during the period 2001-2005. Personal PM(2.5) exposures were simulated from the Stochastic Human Exposure and Dose Simulation. Accounting for exposure uncertainty, the authors estimated a 2.32% (95% posterior interval: 0.68, 3.94) increase in mortality per a 10 μg/m(3) increase in personal exposure to PM(2.5) from outdoor sources on the previous day. The corresponding estimates per a 10 μg/m(3) increase in PM(2.5) ambient concentration was 1.13% (95% confidence interval: 0.27, 2.00). The risks of mortality associated with PM(2.5) were also higher during the summer months.}, number={5}, journal={JOURNAL OF EXPOSURE SCIENCE AND ENVIRONMENTAL EPIDEMIOLOGY}, author={Chang, Howard H. and Fuentes, Montserrat and Frey, H. Christopher}, year={2012}, pages={483–488} }
 @article{lewis_leming_frey_rasdorf_2011, title={Assessing Effects of Operational Efficiency on Pollutant Emissions of Nonroad Diesel Construction Equipment}, ISSN={["0361-1981"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-83655197006&partnerID=MN8TOARS}, DOI={10.3141/2233-02}, abstractNote={ Operational efficiency is a simple way of addressing nonproductive time of diesel-powered construction equipment that performs a construction activity and is defined as the ratio of nonidle activity time to total activity time including idle time. The effects of diesel equipment operational efficiency on total emissions for construction activities have not been fully established; thus a fundamental understanding of operational efficiency is needed to evaluate the effects on total pollutant emissions of diesel-powered construction equipment. This paper presents a methodology to assess the effects of equipment operational efficiency on the total pollutant emissions of construction equipment performing construction activities. This methodology estimates the percentage of additional pollutants emitted due to a reduced operational efficiency, based on the ratio of idle to nonidle emissions. Results for a case study of six bulldozers indicate that pollutant emissions, in addition to the minimum amount required to complete a specific quantity of work, increase as operational efficiency decreases. This conclusion allows the savings resulting from increased efficiency to be determined. Furthermore, additional pollutant emissions for a specified operational efficiency increase as the ratio of idle to nonidle emissions increases. The ratio of idle to nonidle emissions varies according to pollutant and is approximately 1:2 for hydrocarbons and carbon monoxide, 1:5 for nitrogen oxides, and 1:7 for particulate matter. }, number={2233}, journal={TRANSPORTATION RESEARCH RECORD}, author={Lewis, Phil and Leming, Michael and Frey, H. Christopher and Rasdorf, William}, year={2011}, pages={11–18} }
 @article{cao_frey_2011, title={Assessment of Interindividual and Geographic Variability in Human Exposure to Fine Particulate Matter in Environmental Tobacco Smoke}, volume={31}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79953034889&partnerID=MN8TOARS}, DOI={10.1111/j.1539-6924.2010.01523.x}, abstractNote={Environmental tobacco smoke (ETS) is a major contributor to indoor human exposures to fine particulate matter of 2.5 μm or smaller (PM2.5). The Stochastic Human Exposure and Dose Simulation for Particulate Matter (SHEDS‐PM) Model developed by the U.S. Environmental Protection Agency estimates distributions of outdoor and indoor PM2.5 exposure for a specified population based on ambient concentrations and indoor emissions sources. A critical assessment was conducted of the methodology and data used in SHEDS‐PM for estimation of indoor exposure to ETS. For the residential microenvironment, SHEDS uses a mass‐balance approach, which is comparable to best practices. The default inputs in SHEDS‐PM were reviewed and more recent and extensive data sources were identified. Sensitivity analysis was used to determine which inputs should be prioritized for updating. Data regarding the proportion of smokers and “other smokers” and cigarette emission rate were found to be important. SHEDS‐PM does not currently account for in‐vehicle ETS exposure; however, in‐vehicle ETS‐related PM2.5 levels can exceed those in residential microenvironments by a factor of 10 or more. Therefore, a mass‐balance‐based methodology for estimating in‐vehicle ETS PM2.5 concentration is evaluated. Recommendations are made regarding updating of input data and algorithms related to ETS exposure in the SHEDS‐PM model. Interindividual variability for ETS exposure was quantified. Geographic variability in ETS exposure was quantified based on the varying prevalence of smokers in five selected locations in the United States.}, number={4}, journal={Risk Analysis}, author={Cao, Y. and Frey, H.C.}, year={2011}, pages={578–591} }
 @article{liu_frey_2011, title={Comparison of driving schedule project level MOVES emission factors to empirical data}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84859175012&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Liu, B. and Frey, H.C.}, year={2011}, pages={2445–2454} }
 @article{sun_frey_2011, title={Comparison of real-world activity, fuel use, and emissions for selected light duty gasoline vehicles based on driving cycles}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84859094415&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Sun, Y. and Frey, H.C.}, year={2011}, pages={600–607} }
 @article{zhai_frey_rouphail_2011, title={Development of a modal emissions model for a hybrid electric vehicle}, volume={16}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79957940661&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2011.05.001}, abstractNote={With the growing market share of hybrid electric vehicles (HEVs), new methods are needed for estimating their actual energy use and emissions in order to support emission inventories. This research quantifies criteria associated with startup and shutdown of the internal combustion engine (ICE) for a 2001 Toyota Prius HEV and develops a modal tailpipe emissions model under hot stabilized conditions. The engine is found to be “off” below thresholds of engine power demand that are speed and acceleration dependent. Vehicle specific power (VSP) is used as the basis for modeling emissions. The predicted cycle emissions of CO, NOx and HC for individual cycles are subject to large relative errors, but the overall emission predictions for the average of multiple cycles have relative errors within 10% for each selected pollutant. The ICE engine status identification method is recommended for application to model HEV emissions under actual driving conditions.}, number={6}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Zhai, Haibo and Frey, H. Christopher and Rouphail, Nagui M.}, year={2011}, month={Aug}, pages={444–450} }
 @article{graver_frey_2011, title={Fuel use and emission rates for F59PH locomotive engines tested on an engine dynamometer}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84859054969&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Graver, B.M. and Frey, H.C.}, year={2011}, pages={614–619} }
 @article{cao_frey_2011, title={Geographic differences in inter-individual variability of human exposure to fine particulate matter}, volume={45}, ISSN={["1352-2310"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80052319017&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2011.07.034}, abstractNote={Human exposure to fine particulate matter (PM(2.5)) is associated with short and long term adverse health effects. The amount of ambient PM(2.5) that infiltrates indoor locations such as residences depends on air exchange rate (ACH), penetration factor, and deposition rate. ACH varies by climate zone and thus by geographic location. Geographic variability in the ratio of exposure to ambient concentration is estimated based on comparison of three modeling domains in different climate zones: (1) New York City; (2) Harris County in Texas, and (3) a six-county domain along the I-40 corridor in North Carolina. Inter-individual variability in exposure to PM(2.5) was estimated using the Stochastic Human Exposure and Dose Simulation for Particulate Matter (SHEDS-PM) model. ACH is distinguishably the most sensitive input for both ambient and nonambient exposure to PM(2.5). High ACH leads to high ambient exposure indoors but lower non-ambient exposure, and vice versa. For summer, the average ratio of exposure to ambient concentration varies by 13 percent among the selected domains, mainly because of differences in housing stock, climate zone, and seasonal ACH. High daily average exposures for some individuals are mainly caused by non-ambient exposure to smoking or cooking. The implications of these results for interpretation of epidemiological studies are discussed.}, number={32}, journal={ATMOSPHERIC ENVIRONMENT}, author={Cao, Ye and Frey, H. Christopher}, year={2011}, month={Oct}, pages={5684–5691} }
 @article{graver_frey_choi_2011, title={In-Use Measurement of Activity, Energy Use, and Emissions of a Plug-in Hybrid Electric Vehicle}, volume={45}, ISSN={["0013-936X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80054685657&partnerID=MN8TOARS}, DOI={10.1021/es201165d}, abstractNote={Plug-in hybrid electric vehicles (PHEVs) could reduce transportation air emissions and energy use. However, a method is needed for estimating on-road emissions of PHEVs. To develop a framework for quantifying microscale energy use and emissions (EU&E), measurements were conducted on a Toyota Prius retrofitted with a plug-in battery system on eight routes. Measurements were made using the following: (1) a data logger for the hybrid control system; (2) a portable emissions measurement system; and (3) a global positioning system with barometric altimeter. Trends in EU&E are estimated based on vehicle specific power. Energy economy is quantified based on gasoline consumed by the engine and grid energy consumed by the plug-in battery. Emissions from electricity consumption are estimated based on the power generation mix. Fuel use is approximately 30% lower during plug-in battery use. Grid emissions were higher for CO₂, NO(x), SO₂, and PM compared to tailpipe emissions but lower for CO and hydrocarbons. EU&E depends on engine and plug-in battery operation. The use of two energy sources must be addressed in characterizing fuel economy; overall energy economy is 11% lower if including grid energy use than accounting only for fuel consumption.}, number={20}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Graver, Brandon M. and Frey, H. Christopher and Choi, Hyung-Wook}, year={2011}, month={Oct}, pages={9044–9051} }
 @article{frey_sandhu_sun_lee_swidan_liu_babaee_2011, title={Incorporating vehicle portable emissions measurement systems into the classroom}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84859028524&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Sandhu, G.S. and Sun, Y. and Lee, T. and Swidan, H. and Liu, B. and Babaee, S.}, year={2011}, pages={620–637} }
 @article{graver_frey_2011, title={Modeling of hybrid and plug-in hybrid electric vehicle energy use and emissions based on in-use measurement}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84859038443&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Graver, B.M. and Frey, H.C.}, year={2011}, pages={608–613} }
 @article{liu_frey_2011, title={Modeling of in-vehicle human exposure to ambient fine particulate matter}, volume={45}, ISSN={["1873-2844"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79960586024&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2011.04.019}, abstractNote={A method for estimating in-vehicle PM2.5 exposure as part of a scenario-based population simulation model is developed and assessed. In existing models, such as the Stochastic Exposure and Dose Simulation model for Particulate Matter (SHEDS-PM), in-vehicle exposure is estimated using linear regression based on area-wide ambient PM2.5 concentration. An alternative modeling approach is explored based on estimation of near-road PM2.5 concentration and an in-vehicle mass balance model. Near-road PM2.5 concentration is estimated using a dispersion model and fixed site monitor (FSMs) data. In-vehicle concentration is estimated based on air exchange rate and filter efficiency. In-vehicle concentration varies with road type, traffic flow, windspeed, stability class, and ventilation. Average in-vehicle exposure is estimated to contribute 10–20 percent of average daily exposure. The contribution of in-vehicle exposure to total daily exposure can be higher for some individuals. Recommendations are made for updating exposure models and implementation of the alternative approach.}, number={27}, journal={ATMOSPHERIC ENVIRONMENT}, author={Liu, Xiaozhen and Frey, H. Christopher}, year={2011}, month={Sep}, pages={4745–4752} }
 @inproceedings{lee_frey_2011, title={Variability in gram per gallon vehicle emission factors with engine load and among driving cycles}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84859179711&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Lee, T. and Frey, H.C.}, year={2011}, pages={2335–2341} }
 @article{frey_choi_graver_sandhu_2010, title={Comparative assessment of passenger railroad locomotives using portable emission measurement systems}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79956260289&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Choi, H.-W. and Graver, B.M. and Sandhu, G.S.}, year={2010}, pages={5249–5255} }
 @article{frey_rasdorf_lewis_2010, title={Comprehensive Field Study of Fuel Use and Emissions of Nonroad Diesel Construction Equipment}, ISSN={["0361-1981"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78651305557&partnerID=MN8TOARS}, DOI={10.3141/2158-09}, abstractNote={ Limited field data are available for analyses of fuel use and emissions of nonroad diesel construction equipment. This paper summarizes the results of field research that used a portable emissions monitoring system to collect fuel use and emissions data from eight backhoes, six bulldozers, three excavators, four generators, six motor graders, three off-road trucks, one skid-steer loader, three track loaders, and five wheel loaders while they performed various duty cycles. These tests produced approximately 119 h of field data for petroleum diesel and approximately 48 h for B20 biodiesel. Engine attribute data including horsepower, displacement, model year, engine tier, and engine load were collected to determine these factors’ influence on fuel use rates and emission rates of nitrogen oxides, hydrocarbons, carbon monoxide, carbon dioxide, and opacity. Mass per time fuel use rates were developed for each item of equipment, as were mass per time and mass per fuel used emission rates for each pollutant. For petroleum diesel, fuel use and emission rates of each pollutant were found to increase with engine displacement, horsepower, and load and to decrease with model year and engine tier. The results were qualitatively similar for B20 biodiesel. Fuel-based emission rates were found to have less variability and less sensitivity to engine size and load than time-based emission rates. Where possible, development of emission inventories based on fuel consumed, rather than time of activity, is preferred. }, number={2158}, journal={TRANSPORTATION RESEARCH RECORD}, author={Frey, H. Christopher and Rasdorf, William and Lewis, Phil}, year={2010}, pages={69–76} }
 @article{choi_frey_2010, title={Estimating Diesel Vehicle Emission Factors at Constant and High Speeds for Short Road Segments}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78651279535&partnerID=MN8TOARS}, DOI={10.3141/2158-03}, abstractNote={ Vehicle emission estimates with high spatial and temporal resolution are needed to estimate near-roadway air quality and human exposure to emissions. The U.S. Environmental Protection Agency's (EPA) MOBILE6 emission factor model is based on test cycles with transient speeds and durations on the order of 10 min. MOBILE6 does not report emission factors for average speeds higher than 65 mph. However, for near-roadway studies, emission factors are needed for short highway segments that represent only a few seconds of vehicle travel time at approximately constant speed and speeds greater than 65 mph. Constant-speed and high-speed correction factors for nitrogen oxides, hydrocarbons, carbon dioxide (CO2), and carbon monoxide were developed on the basis of 59,286 s of EPA dynamometer emissions data for diesel vehicles from which speed–acceleration modal models were derived. The constant speed correction factor is the ratio of the emission factor at constant speed to the emissions factor during transient cycles with an equivalent average speed. For example, the constant speed correction factor for CO2 increases from 0.71 to 1 as speed increases from 31 to 78 mph. High-speed correction factors are based on the ratio of emission factors at a constant speed greater than 65 mph to emission factors at a constant speed of 65 mph. For example, at 80 mph, the high-speed correction factor is approximately 1.5 for CO2. Sensitivity analysis of emission estimates to these correction factors was conducted for speeds of 40 to 80 mph. }, number={2158}, journal={TRANSPORTATION RESEARCH RECORD}, author={Choi, Hyung-Wook and Frey, H. Christopher}, year={2010}, pages={19–27} }
 @article{liu_frey_cao_2010, title={Estimating In-Vehicle Concentration of and Exposure to Fine Particulate Matter Near-Roadway Ambient Air Quality and Variability in Vehicle Operation}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78651334013&partnerID=MN8TOARS}, DOI={10.3141/2158-13}, abstractNote={ Fine particulate matter (PM2.5) exposure is associated with short-term and chronic respiratory effects. It is necessary to investigate human exposure to PM2.5 to support the assessment of the association between exposure and adverse health effects. The methodology used in the current version of Stochastic Exposure and Dose Simulation Model for Particulate Matter for in-vehicle PM2.5 concentration is reviewed. An alternative approach for estimating in-vehicle PM2.5 concentration, based on the use of a dispersion model to estimate near-road PM2.5 concentration and a mass balance model to estimate in-vehicle concentration, has been specified and applied. The objectives of this paper are to (a) demonstrate the application of the alternative approach to various scenarios, (b) analyze the sensitivity of the modeling results to the key inputs, and (c) evaluate this approach. Typical inputs for the alternative approach are reviewed. Sensitivities of these inputs to the modeling results and estimated human exposure have been analyzed. The in-vehicle PM2.5 concentration was estimated to vary by more than a factor of two, depending on factors such as the cabin air exchange rate and filter efficiency of the ventilation system. Recommendations are made for field data that would be useful to better characterize variability in factors that affect in-vehicle PM2.5 concentration. }, number={2158}, journal={TRANSPORTATION RESEARCH RECORD}, author={Liu, Xiaozhen and Frey, H. Christopher and Cao, Ye}, year={2010}, pages={105–112} }
 @inproceedings{rasdorf_lewis_frey_2010, title={Estimating productivity-based fuel eifficiency and emissions factors of diesel construction equipment performing earthmoving activities}, ISBN={9781897911358}, booktitle={Proceedings of the 6th International Conference on Innovation in Architecture, Engineering, and Construction}, publisher={University Park, Pa. : Department of Architectural Engineering, Pennsylvania State University}, author={Rasdorf, W. and Lewis, P. and Frey, H. C.}, editor={Chimay J. Anumba, N. M. Bouchlaghem and John Messner and Parfitt, M.KevinEditors}, year={2010} }
 @article{rasdorf_frey_lewis_kim_pang_abolhassani_2010, title={Field Procedures for Real-World Measurements of Emissions from Diesel Construction Vehicles}, volume={16}, ISSN={["1943-555X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78649383748&partnerID=MN8TOARS}, DOI={10.1061/(asce)is.1943-555x.0000027}, abstractNote={Construction vehicles are a source of nonroad mobile air pollutant emissions. Emissions from construction vehicles are typically quantified based on steady-state modal engine dynamometer tests using uninstalled stationary engines. However, these tests do not represent real-world activity. Therefore, there is a need to quantify energy use and air pollutant emissions from construction vehicles based on in-use measurement methods. The purpose of this paper is to outline standard procedures for field data collection for construction vehicles. This methodology is based on second-by-second measurement of in-use activity and air pollutant emissions using a portable emissions measurement system. The field data collection methodology includes the development of a study design, installation and use of instrumentation, and field measurements. After the field data collection, the raw data undergo a quality assurance procedure to check for and correct the synchronization between the engine data and emission data. This...}, number={3}, journal={JOURNAL OF INFRASTRUCTURE SYSTEMS}, author={Rasdorf, William and Frey, Christopher and Lewis, Phil and Kim, Kangwook and Pang, Shih-Hao and Abolhassani, Saeed}, year={2010}, month={Sep}, pages={216–225} }
 @inbook{zhu_frey_2010, title={Integrated gasification combined cycle (IGCC) power plant design and technology}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84882848834&partnerID=MN8TOARS}, DOI={10.1533/9781845699468.1.54}, abstractNote={: 
The main process areas of integrated gasification combined cycle (IGCC) plants without and with carbon dioxide (CO2) capture are described. Key factors in IGCC plant design are described for major process areas, including gasification, water–gas shift, gas turbine, CO2 capture, and other emissions control technologies. The advantages and limitations of coal IGCC plants are discussed. The main development trends of IGCC technologies are reviewed and summarized.}, booktitle={Advanced Power Plant Materials, Design and Technology}, author={Zhu, Y. and Frey, H.C.}, year={2010}, pages={54–88} }
 @inproceedings{frey_choi_graver_sandhu_2010, title={Measurement and comparison of locomotive engine emissions}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79956274785&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Choi, H.-W. and Graver, B.M. and Sandhu, G.S.}, year={2010}, pages={5272–5280} }
 @article{choi_frey_2010, title={Method for in-use measurement and evaluation of the activity, fuel use, electricity use, and emissions of a plug-in hybrid diesel-electric school bus}, volume={44}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77951823059&partnerID=MN8TOARS}, DOI={10.1021/es903330k}, abstractNote={The purpose of this study is to demonstrate a methodology for characterizing at high resolution the energy use and emissions of a plug-in parallel-hybrid diesel-electric school bus (PHSB) to support assessments of sensitivity to driving cycles and comparisons to a conventional diesel school bus (CDSB). Data were collected using onboard instruments for a first-of-a-kind prototype PHSB and a CDSB of the same chassis and engine, operated on actual school bus routes. The engine load was estimated on the basis of vehicle specific power (VSP) and an empirically derived relationship between VSP and engine manifold absolute pressure (MAP). VSP depends on speed, acceleration, and road grade. For the PHSB, the observed electrical discharge or recharge to the traction motor battery was characterized on the basis of VSP. The energy use and emission rates of the PHSB from tailpipe and electricity use were estimated for five real-world driving cycles and compared to the engine fuel use and emissions of the CDSB. The PHSB had the greatest advantage on arterial routes and less advantage on highway or local routes. The coupled VSP-MAP modeling approach enables assessment of a wide variety of driving conditions and comparisons of vehicles with different propulsion technologies.}, number={9}, journal={Environmental Science and Technology}, author={Choi, H.-W. and Frey, H. Christopher}, year={2010}, pages={3601–3607} }
 @article{graver_frey_2010, title={Quantifying the microscale activity, energy use, and emissions of a plug-in hybrid electric vehicle based on in-use data}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-79956211810&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Graver, B.M. and Frey, H.C.}, year={2010}, pages={23–45} }
 @article{frey_zhang_rouphail_2010, title={Vehicle-Specific Emissions Modeling Based upon on-Road Measurements}, volume={44}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77951826102&partnerID=MN8TOARS}, DOI={10.1021/es902835h}, abstractNote={Vehicle-specific microscale fuel use and emissions rate models are developed based upon real-world hot-stabilized tailpipe measurements made using a portable emissions measurement system. Consecutive averaging periods of one to three multiples of the response time are used to compare two semiempirical physically based modeling schemes. One scheme is based on internally observable variables (IOVs), such as engine speed and manifold absolute pressure, while the other is based on externally observable variables (EOVs), such as speed, acceleration, and road grade. For NO, HC, and CO emission rates, the average R(2) ranged from 0.41 to 0.66 for the former and from 0.17 to 0.30 for the latter. The EOV models have R(2) for CO(2) of 0.43 to 0.79 versus 0.99 for the IOV models. The models are sensitive to episodic events in driving cycles such as high acceleration. Intervehicle and fleet average modeling approaches are compared; the former account for microscale variations that might be useful for some types of assessments. EOV-based models have practical value for traffic management or simulation applications since IOVs usually are not available or not used for emission estimation.}, number={9}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Frey, H. Christopher and Zhang, Kaishan and Rouphail, Nagui M.}, year={2010}, month={May}, pages={3594–3600} }
 @article{frey_hubbell_2009, title={A risk-based assessment and management framework for multipollutant air quality}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77952278636&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Hubbell, B.}, year={2009}, pages={1068–1080} }
 @article{oezkaynak_frey_burke_pinder_2009, title={Analysis of coupled model uncertainties in source-to-dose modeling of human exposures to ambient air pollution: A PM2.5 case study}, volume={43}, ISSN={["1352-2310"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-59649124689&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2008.12.008}, abstractNote={Quantitative assessment of human exposures and health effects due to air pollution involve detailed characterization of impacts of air quality on exposure and dose. A key challenge is to integrate these three components on a consistent spatial and temporal basis taking into account linkages and feedbacks. The current state-of-practice for such assessments is to exercise emission, meteorology, air quality, exposure, and dose models separately, and to link them together by using the output of one model as input to the subsequent downstream model. Quantification of variability and uncertainty has been an important topic in the exposure assessment community for a number of years. Variability refers to differences in the value of a quantity (e.g., exposure) over time, space, or among individuals. Uncertainty refers to lack of knowledge regarding the true value of a quantity. An emerging challenge is how to quantify variability and uncertainty in integrated assessments over the source-to-dose continuum by considering contributions from individual as well as linked components. For a case study of fine particulate matter (PM2.5) in North Carolina during July 2002, we characterize variability and uncertainty associated with each of the individual concentration, exposure and dose models that are linked, and use a conceptual framework to quantify and evaluate the implications of coupled model uncertainties. We find that the resulting overall uncertainties due to combined effects of both variability and uncertainty are smaller (usually by a factor of 3–4) than the crudely multiplied model-specific overall uncertainty ratios. Future research will need to examine the impact of potential dependencies among the model components by conducting a truly coupled modeling analysis.}, number={9}, journal={ATMOSPHERIC ENVIRONMENT}, author={Oezkaynak, Haluk and Frey, H. Christopher and Burke, Janet and Pinder, Robert W.}, year={2009}, month={Mar}, pages={1641–1649} }
 @article{coelho_frey_rouphail_zhai_pelkmans_2009, title={Assessing methods for comparing emissions from gasoline and diesel light-duty vehicles based on microscale measurements}, volume={14}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-58849133274&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2008.11.005}, abstractNote={This paper assess whether a real-world second-by-second methodology that integrates vehicle activity and emissions rates for light-duty gasoline vehicles can be extended to diesel vehicles. Secondly it compares fuel use and emission rates between gasoline and diesel light-duty vehicles. To evaluate the methodology, real-world field data from two light-duty diesel vehicles are used. Vehicle specific power, a function of vehicle speed, acceleration, and road grade, is evaluated with respect to ability to explain variation in emissions rates. Vehicle specific power has been used previously to define activity-based modes and to quantify variation in fuel use and emission rates of gasoline vehicles taking into account idle, acceleration, cruise, and deceleration. The fuel use and emission rates for light-duty diesel vehicles can also be explained using vehicle specific power -based modes. Thus, the methodology enables direct comparisons for different vehicle fuels and technologies. Furthermore, the method can be used to estimate average fuel use and emission rates for a wide variety of driving cycles.}, number={2}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Coelho, Margarida C. and Frey, H. Christopher and Rouphail, Nagui M. and Zhai, Haibo and Pelkmans, Luc}, year={2009}, month={Mar}, pages={91–99} }
 @article{zhai_frey_rouphail_goncalves_farias_2009, title={Comparison of Flexible Fuel Vehicle and Life-Cycle Fuel Consumption and Emissions of Selected Pollutants and Greenhouse Gases for Ethanol 85 Versus Gasoline}, volume={59}, ISSN={["1096-2247"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70350516518&partnerID=MN8TOARS}, DOI={10.3155/1047-3289.59.8.912}, abstractNote={Abstract The objective of this research is to evaluate differences in fuel consumption and tailpipe emissions of flexible fuel vehicles (FFVs) operated on ethanol 85 (E85) versus gasoline. Theoretical ratios of fuel consumption and carbon dioxide (CO2) emissions for both fuels are estimated based on the same amount of energy released. Second-by-second fuel consumption and emissions from one FFV Ford Focus fueled with E85 and gasoline were measured under real-world traffic conditions in Lisbon, Portugal, using a portable emissions measurement system (PEMS). Cycle average dynamometer fuel consumption and emission test results for FFVs are available from the U.S. Department of Energy, and emissions certification test results for ethanol-fueled vehicles are available from the U.S. Environmental Protection Agency. On the basis of the PEMS data, vehicle-specific power (VSP)-based modal average fuel and emission rates for both fuels are estimated. For E85 versus gasoline, empirical ratios of fuel consumption and CO2 emissions agree within a margin of error to the theoretical expectations. Carbon monoxide (CO) emissions were found to be typically lower. From the PEMS data, nitric oxide (NO) emissions associated with some higher VSP modes are higher for E85. From the dynamometer and certification data, average hydrocarbon (HC) and nitrogen oxides (NOx) emission differences vary depending on the vehicle. The differences of average E85 versus gasoline emission rates for all vehicle models are −22% for CO, 12% for HC, and −8% for NOx emissions, which imply that replacing gasoline with E85 reduces CO emissions, may moderately decrease NOx tailpipe emissions, and may increase HC tailpipe emissions. On a fuel life cycle basis for corn-based ethanol versus gasoline, CO emissions are estimated to decrease by 18%. Life-cycle total and fossil CO2 emissions are estimated to decrease by 25 and 50%, respectively; however, life-cycle HC and NOx emissions are estimated to increase by 18 and 82%, respectively.}, number={8}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Zhai, Haibo and Frey, H. Christopher and Rouphail, Nagui M. and Goncalves, Goncalo A. and Farias, Tiago L.}, year={2009}, month={Aug}, pages={912–924} }
 @article{lewis_frey_rasdorf_2009, title={Development and Use of Emissions Inventories for Construction Vehicles}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-76149135725&partnerID=MN8TOARS}, DOI={10.3141/2123-06}, abstractNote={ Real-world data are insufficient to estimate actual emissions from construction vehicles and to develop effective decisions aimed at reducing emissions. A methodology is developed here for inventorying construction fleet emissions on the basis of representative real-world measurements of construction vehicles by means of a portable emissions measurement system (PEMS). The PEMS enables measurements of actual duty cycles and their corresponding fuel use and emissions. The methodology is demonstrated via application to a fleet of publicly owned construction vehicles used primarily for highway maintenance. Selected backhoes, front-end loaders, and motor graders, representing various model years and engine emissions standards, were measured with PEMS during use. Tests were performed for B20 biodiesel and petroleum diesel (PD) fuels. Emission factors from the PEMS data, combined with owner records of annual fuel consumption for over 1,000 vehicles, were used to estimate annual inventories of tailpipe emissions of nitrogen oxides, particulate matter, hydrocarbons, and carbon monoxide. The emissions inventory was stratified by pollutant, vehicle type, fuel type, and engine tier and was used to assess fleet management strategies aimed at reducing emissions. Case study results illustrate that total fleet emissions would be reduced by 3% to 24% when B20 fuel was being used exclusively instead of PD exclusively, 11% to 50% when all Tier 0 and Tier 1 engines were replaced with Tier 2 engines, and 31% to 72% when B20 fuel was used exclusively in the highest engine tier available. Recommendations are made about development and practical applications of emissions inventories for construction fleet management. }, number={2123}, journal={TRANSPORTATION RESEARCH RECORD}, author={Lewis, Phil and Frey, H. Christopher and Rasdorf, William}, year={2009}, pages={46–53} }
 @article{frey_kuo_villa_2009, title={Effects of Idle Reduction Technologies on Real World Fuel Use and Exhaust Emissions of Idling Long-Haul Trucks}, volume={43}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-69549124649&partnerID=MN8TOARS}, DOI={10.1021/es900186e}, abstractNote={Idling long-haul freight tucks may consume nearly one billion gallons of diesel fuel per year in the U.S. There is a need for real-world data by which to quantify avoided fuel use and emissions attributable to idle reduction techniques of auxiliary power units (APUs) and shore-power (SP). Field data were obtained from 20 APU-equipped and SP-compatible trucks observed during 2.8 million miles of travel in 42 states. Base engine fuel use and emission rates varied depending on ambient temperature. APU and SP energy use and emission rates varied depending on electrical load. APUs reduced idling fuel use and CO2 emissions for single and team drivers by 22 and 5% annually, respectively. SP offers greater reductions in energy use of 48% for single drivers, as well as in emissions, except for SO2. APUs were cost-effective for single drivers with a large number of APU usage hours per year, but not for team drivers or for single drivers with low APU utilization rates. The findings support more accurate assessments of avoided fuel use and emissions, and recommendations to encourage greater APU utilization by single drivers and to further develop infrastructure for SP.}, number={17}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Frey, H. Christopher and Kuo, Po-Yao and Villa, Charles}, year={2009}, month={Sep}, pages={6875–6881} }
 @inproceedings{graver_frey_2009, title={Estimation of air carrier emissions at Raleigh-Durham international airport}, volume={4}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77952383333&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Graver, B.M. and Frey, H.C.}, year={2009}, pages={2502–2516} }
 @article{frey_choi_pritchard_lawrence_2009, title={In-use measurement of the activity, energy use, and emissions of a plug-in hybrid electric vehicle}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77952244297&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Choi, H.-W. and Pritchard, E. and Lawrence, J.}, year={2009}, pages={1127–1144} }
 @article{frey_kim_2009, title={In-use measurement of the activity, fuel use, and emissions of eight cement mixer trucks operated on each of petroleum diesel and soy-based B20 biodiesel}, volume={14}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-71249088383&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2009.08.004}, abstractNote={In-use micro-scale fuel use and emission rates were measured for eight cement mixer trucks using a portable emission measurement system. Each vehicle was tested on petroleum diesel and B20 biodiesel. Average fuel use and emission rates increase monotonically versus engine manifold absolute pressure. A typical duty cycle includes loading at a cement plant, transit while loaded from the cement plant to work site, creeping in a queue of vehicles at the worksite, unloading, and transit without load from the site to the plant. For B20 versus petroleum diesel, there is no significant change in the rate of fuel use, CO2 emissions, and NO emissions, and significant decreases in emissions for CO, hydrocarbons, and particulate matter. For loaded versus unloaded onroad travel, fuel use and CO2 emissions rates are approximately 60% higher and the rates for other pollutants are approximately 30–50% higher. A substantial portion of cycle emissions occurred at the work site. Inter-vehicle and intra-cycle variability are also quantified using the micro-scale methodology.}, number={8}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Frey, H. Christopher and Kim, Kangwook}, year={2009}, month={Dec}, pages={585–592} }
 @article{pang_frey_rasdorf_2009, title={Life Cycle Inventory Energy Consumption and Emissions for Biodiesel versus Petroleum Diesel Fueled Construction Vehicles}, volume={43}, ISSN={0013-936X 1520-5851}, url={http://dx.doi.org/10.1021/es802916u}, DOI={10.1021/es802916u}, abstractNote={Substitution of soy-based biodiesel fuels for petroleum diesel will alter life cycle emissions for construction vehicles. A life cycle inventory was used to estimate fuel cycle energy consumption and emissions of selected pollutants and greenhouse gases. Real-world measurements using a portable emission measurement system (PEMS) were made forfive backhoes, four front-end loaders, and six motor graders on both fuels from which fuel consumption and tailpipe emission factors of CO, HC, NO(x), and PM were estimated. Life cycle fossil energy reductions are estimated it 9% for B20 and 42% for B100 versus petroleum diesel based on the current national energy mix. Fuel cycle emissions will contribute a larger share of total life cycle emissions as new engines enter the in-use fleet. The average differences in life cycle emissions for B20 versus diesel are: 3.5% higher for NO(x); 11.8% lower for PM, 1.6% higher for HC, and 4.1% lower for CO. Local urban tailpipe emissions are estimated to be 24% lower for HC, 20% lower for CO, 17% lower for PM, and 0.9% lower for NO(x). Thus, there are environmental trade-offs such as for rural vs urban areas. The key sources of uncertainty in the B20 LCI are vehicle emission factors.}, number={16}, journal={Environmental Science & Technology}, publisher={American Chemical Society (ACS)}, author={Pang, Shih-Hao and Frey, H. Christopher and Rasdorf, William J.}, year={2009}, month={Aug}, pages={6398–6405} }
 @article{pang_frey_rasdorf_2009, title={Life cycle inventory energy consumption and emissions for biodiesel versus petroleum diesel fueled construction vehicles}, volume={43}, DOI={10.1021/os802916u}, number={16}, journal={Environmental Science & Technology}, author={Pang, S. H. and Frey, H. Christopher and Rasdorf, W. J.}, year={2009}, pages={6398–6405} }
 @article{choi_frey_2009, title={Light duty gasoline vehicle emission factors at high transient and constant speeds for short road segments}, volume={14}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-71249122701&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2009.09.001}, abstractNote={Vehicle emissions estimates are needed at high spatial and temporal resolution to estimate near-roadway air quality and human exposures. The MOBILE6 emission factor model is based on transient test cycles of less than 65 mph. Correction factors for high speed and constant speed are developed based on vehicle-specific power-based modal models for light duty gasoline vehicles, using data from portable emission measurement systems. At 80 mph versus 65 mph, the estimated average emission rates are greater by 30%, 20%, 80%, and 10% for NOx, HC, CO, and CO2. The ratio of constant to average of transient speed emission rates range from 0.49 to 0.94 for NOx at speeds of 20 mph and 80 mph. The high speed and constant speed correction factors are applied to estimate vehicle emissions for a freeway segment that includes vehicle cruising speeds between 65 and 80 mph. The potential error for not accounting for constant speed operation on a short segment of highway could be 49% at moderate speed and 24% at high speed.}, number={8}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Choi, Hyung-Wook and Frey, H. Christopher}, year={2009}, month={Dec}, pages={610–614} }
 @article{frey_choi_kim_2009, title={Measurement of the energy use and emissions of passenger rail locomotives using a portable emission measurement system}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77952268050&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Choi, H.-W. and Kim, K.}, year={2009}, pages={1145–1157} }
 @article{liu_frey_cao_deshpande_2009, title={Modeling of in-vehicle PM2.5 exposure using the stochastic human exposure and dose simulation model}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77952286694&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Liu, X. and Frey, H.C. and Cao, Y. and Deshpande, B.}, year={2009}, pages={1087–1100} }
 @misc{bogen_cullen_frey_price_2009, title={Probabilistic Exposure Analysis for Chemical Risk Characterization}, volume={109}, ISSN={["1096-0929"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-65549124897&partnerID=MN8TOARS}, DOI={10.1093/toxsci/kfp036}, abstractNote={This paper summarizes the state of the science of probabilistic exposure assessment (PEA) as applied to chemical risk characterization. Current probabilistic risk analysis methods applied to PEA are reviewed. PEA within the context of risk-based decision making is discussed, including probabilistic treatment of related uncertainty, interindividual heterogeneity, and other sources of variability. Key examples of recent experience gained in assessing human exposures to chemicals in the environment, and other applications to chemical risk characterization and assessment, are presented. It is concluded that, although improvements continue to be made, existing methods suffice for effective application of PEA to support quantitative analyses of the risk of chemically induced toxicity that play an increasing role in key decision-making objectives involving health protection, triage, civil justice, and criminal justice. Different types of information required to apply PEA to these different decision contexts are identified, and specific PEA methods are highlighted that are best suited to exposure assessment in these separate contexts.}, number={1}, journal={TOXICOLOGICAL SCIENCES}, author={Bogen, Kenneth T. and Cullen, Alison C. and Frey, H. Christopher and Price, Paul S.}, year={2009}, month={May}, pages={4–17} }
 @article{frey_kuo_2009, title={Real-World Energy Use and Emission Rates for Idling Long-Haul Trucks and Selected Idle Reduction Technologies}, volume={59}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-68649109612&partnerID=MN8TOARS}, DOI={10.3155/1047-3289.59.7.857}, abstractNote={Abstract Long-haul freight trucks typically idle for 2000 or more hours per year, motivating interest in reducing idle fuel use and emissions using auxiliary power units (APUs) and shore-power (SP). Fuel-use rates are estimated based on electronic control unit (ECU) data for truck engines and measurements for APU engines. Engine emission factors were measured using a portable emission measurement system. Indirect emissions from SP were based on average utility grid emission factors. Base engine fuel use and APU and SP electrical load were analyzed for 20 trucks monitored for more than 1 yr during 2.76 million mi of activity within 42 U.S. states. The average base engine fuel use varied from 0.46 to 0.65 gal/hr. The average APU fuel use varied from 0.24 to 0.41 gal/hr. Fuel-use rates are typically lowest in mild weather, highest in hot or cold weather, and depend on engine speed (revolutions per minute [RPM]). Compared with the base engine, APU fuel use and emissions of carbon dioxide (CO2) and sulfur dioxide (SO2) are lower by 36–47%. Oxides of nitrogen (NOx) emissions are lower by 80–90%. Reductions in particulate matter (PM), carbon monoxide (CO), and hydrocarbon emissions vary from approximately 10 to over 50%. SP leads to more substantial reductions, except for SO2. The actual achievable reductions will be lower because only a fraction of base engine usage will be replaced by APUs, SP, or both. Recommendations are made for reducing base engine fuel use and emissions, accounting for variability in fuel use and emissions reductions, and further work to quantify real-world avoided fuel use and emissions.}, number={7}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Frey, H. Christopher and Kuo, Po-Yao}, year={2009}, month={Jul}, pages={857–864} }
 @article{frey_zhai_rouphail_2009, title={Regional On-Road Vehicle Running Emissions Modeling and Evaluation for Conventional and Alternative Vehicle Technologies}, volume={43}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70350776818&partnerID=MN8TOARS}, DOI={10.1021/es900535s}, abstractNote={This study presents a methodology for estimating high-resolution, regional on-road vehicle emissions and the associated reductions in air pollutant emissions from vehicles that utilize alternative fuels or propulsion technologies. The fuels considered are gasoline, diesel, ethanol, biodiesel, compressed natural gas, hydrogen, and electricity. The technologies considered are internal combustion or compression engines, hybrids, fuel cell, and electric. Road link-based emission models are developed using modal fuel use and emission rates applied to facility- and speed-specific driving cycles. For an urban case study, passenger cars were found to be the largest sources of HC, CO, and CO(2) emissions, whereas trucks contributed the largest share of NO(x) emissions. When alternative fuel and propulsion technologies were introduced in the fleet at a modest market penetration level of 27%, their emission reductions were found to be 3-14%. Emissions for all pollutants generally decreased with an increase in the market share of alternative vehicle technologies. Turnover of the light duty fleet to newer Tier 2 vehicles reduced emissions of HC, CO, and NO(x) substantially. However, modest improvements in fuel economy may be offset by VMT growth and reductions in overall average speed.}, number={21}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Frey, H. Christopher and Zhai, Haibo and Rouphail, Nagui M.}, year={2009}, month={Nov}, pages={8449–8455} }
 @article{lewis_rasdorf_frey_pang_kim_2009, title={Requirements and Incentives for Reducing Construction Vehicle Emissions and Comparison of Nonroad Diesel Engine Emissions Data Sources}, volume={135}, ISSN={["0733-9364"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-65249158552&partnerID=MN8TOARS}, DOI={10.1061/(ASCE)CO.1943-7862.0000008}, abstractNote={Nonroad construction vehicles and equipment powered by diesel engines contribute to mobile source air pollution. The engines of this equipment emit significant amounts of carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter. These pollutants pose serious problems for human health and the environment. Therefore, it is necessary to regulate and control the levels of these pollutants. Furthermore, there are emerging requirements and incentives for “greening” of construction vehicle fleets and operations. Currently, there are two types of standards that regulate air pollution for these types of vehicles: technological standards for engines and quality standards for air. It is also necessary to quantify the levels of emissions that nonroad construction vehicles and equipment produce. Quantification may be based on existing data sources (such as the EPA NONROAD model) or by collecting data directly from the vehicles as they work in the field. The purpose of this paper is to introduce the chall...}, number={5}, journal={JOURNAL OF CONSTRUCTION ENGINEERING AND MANAGEMENT-ASCE}, author={Lewis, Phil and Rasdorf, William and Frey, H. Christopher and Pang, Shih-Hao and Kim, Kangwook}, year={2009}, month={May}, pages={341–351} }
 @article{zhai_frey_rouphail_2008, title={A Vehicle-Specific Power Approach to Speed- and Facility-Specific Emissions Estimates for Diesel Transit Buses}, volume={42}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-55349085068&partnerID=MN8TOARS}, DOI={10.1021/es800208d}, abstractNote={Emissions during a trip often depend on transient vehicle dynamics that influence the instantaneous engine load. Vehicle specific power (VSP) is a proxy variable for engine load that has been shown to be highly correlated with emissions. This study estimates roadway link average emission rates for diesel-fueled transit buses based on link mean speeds, using newly defined VSP modes from data gathered by a portable emissions monitoring system. Speed profiles were categorized by facility type and mean travel speed, and stratified into discrete VSP modes. VSP modal average emission rates and the time spent in the corresponding VSP modes were then used to make aggregate estimates of total and average emission rates for a road link. The average emission rates were sensitive to link mean speed, but not to facility type. A recommendation is made regarding the implementation of link average emission rates in conjunction with transportation models for the purpose of estimating regional emissions for diesel transit buses.}, number={21}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Zhai, Haibo and Frey, H. Christopher and Rouphail, Nagui M.}, year={2008}, month={Nov}, pages={7985–7991} }
 @article{frey_kim_pang_rasdorf_lewis_2008, title={Characterization of Real-World Activity, Fuel Use, and Emissions for Selected Motor Graders Fueled with Petroleum Diesel and B20 Biodiesel}, volume={58}, ISSN={["1047-3289"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-55049085635&partnerID=MN8TOARS}, DOI={10.3155/1047-3289.58.10.1274}, abstractNote={Abstract Motor graders are a common type of nonroad vehicle used in many road construction and maintenance applications. In-use activity, fuel use, and emissions were measured for six selected motor graders using a portable emission measurement system. Each motor grader was tested with petroleum diesel and B20 biodiesel. Duty cycles were quantified in terms of the empirical cumulative distribution function of manifold absolute pressure (MAP), which is an indicator of engine load. The motor graders were operated under normal duty cycles for road maintenance and repair at various locations in Wake and Nash Counties in North Carolina. Approximately 3 hr of quality-assured, second-by-second data were obtained during each test. An empirical modal-based model of vehicle fuel use and emissions was developed, based on stratifying the data with respect to ranges of normalized MAP, to enable comparisons between duty cycles, motor graders, and fuels. Time-based emission factors were found to increase monotonically with MAP. Fuel-based emission factors were mainly sensitive to differences between idle and non-idle engine operation. Cycle average emission factors were estimated for road “resurfacing”, “roading,” and “shouldering” activities. On average, the use of B20 instead of petroleum diesel leads to a negligible decrease of 1.6% in nitric oxide emission rate, and decreases of 19– 22% in emission rates of carbon monoxide, hydrocarbons, and particulate matter. Emission rates decrease significantly when comparing newer engine tier vehicles to older ones. Significant reductions in tailpipe emissions accrue especially from the use of B20 and adoption of newer vehicles.}, number={10}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Frey, H. Christopher and Kim, Kangwook and Pang, Shih-Hao and Rasdorf, William J. and Lewis, Phil}, year={2008}, month={Oct}, pages={1274–1287} }
 @article{özkaynak_frey_hubbell_2008, title={Characterizing variability and uncertainty in exposure assessments improves links to environmental decision-making}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-47749136225&partnerID=MN8TOARS}, number={JULY}, journal={EM: Air and Waste Management Association's Magazine for Environmental Managers}, author={Özkaynak, H. and Frey, H.C. and Hubbell, B.}, year={2008}, pages={18–22} }
 @article{frey_rasdorf_kim_pang_lewis_2008, title={Comparison of Real-World Emissions of B20 Biodiesel Versus Petroleum Diesel for Selected Nonroad Vehicles and Engine Tiers}, ISSN={["0361-1981"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-56749107557&partnerID=MN8TOARS}, DOI={10.3141/2058-05}, abstractNote={ Field data for in-use fuel consumption and emission rates were collected for 15 nonroad vehicles by using a portable emission measurement system. Five backhoes, four front-end loaders, and six motor graders were tested once on petroleum diesel and once on B20 biodiesel. The vehicles represented a variety of engine certification tiers. A methodology was developed for study design, field data collection, data screening and quality assurance, data analysis, and benchmarking of the data. On average, 6.9% of data were lost because of quality issues and more than 3 h of valid data were collected in each test. Time-based emission factors increased monotonically with respect to engine manifold absolute pressure. Fuel-based emission factors were sensitive to differences between operations of engines idling and not idling. Typical duty cycles were quantified in terms of frequency distributions of manifold absolute pressure and used to estimate cycle average emission factors. On average, the use of B20 instead of petroleum diesel led to an insignificant 1.8% decrease in the nitric oxide (NO) emission rate and significant decreases of 18%, 26%, and 25% for opacity, hydrocarbons (HC), and carbon monoxide (CO), respectively. Emission rates decreased significantly in newer, higher-tier vehicles compared with older ones. Fuel use, NO, HC, and CO data were of similar magnitude as independent benchmark data. Specific recommendations were made for future work. }, number={2058}, journal={TRANSPORTATION RESEARCH RECORD}, author={Frey, H. Christopher and Rasdorf, William and Kim, Kangwook and Pang, Ghih-hao and Lewis, Phil}, year={2008}, pages={33–42} }
 @article{zhai_frey_rouphail_gonçalves_farias_2008, title={Comparison of fuel consumption and tailpipe emissions of ethanol 85 and gasoline for flexible fuel vehicles}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449498143&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Zhai, H. and Frey, H.C. and Rouphail, N.M. and Gonçalves, G.A. and Farias, T.L.}, year={2008}, pages={3262–3276} }
 @inproceedings{frey_choi_2008, title={Estimating light duty gasoline vehicle emission factors at high transient and constant speeds for short road segments to support near-roadway air quality studies}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449500370&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Choi, H.-W.}, year={2008}, pages={3137–3148} }
 @article{zhang_frey_2008, title={Evaluation of response time of a portable system for in-use vehicle tailpipe emissions measurement}, volume={42}, ISSN={["0013-936X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-37549066979&partnerID=MN8TOARS}, DOI={10.1021/es062999h}, abstractNote={The objective of this paper is to quantify and evaluate the effects of response time of a portable emission measurement system (PEMS). The PEMS measures tailpipe emissions and vehicle dynamics on a second-by-second basis. Response times of the PEMS for exhaust concentrations were quantified on the basis of fixed periods of measurement of calibration gases for NO, hydrocarbons (HC), CO, and CO2. The time constant was quantified on the basis of the time to reach 63% of the maximum measured value when calibration gas was continuously administered for a period of typically 20 s or more. The time constant was found to be 6 s for NO and 3 s each for CO, HC, and CO2. Measurement errors associated with the response time of the PEMS were quantified. A first-order dynamic discrete model was developed to simulate the instrument measurements. Simulations showed that correction improves the measurement accuracy. Correction with smoothing better improves the measurement accuracy, especially when the noise is relatively large. On a trip level, the average error of the simulated measurements relative to the simulated signal before correction is -4%, which is deemed to be acceptable. For real-world data, smoothing and correction is recommended for major peaks to improve the measurement accuracy.}, number={1}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Zhang, Kaishan and Frey, Christopher}, year={2008}, month={Jan}, pages={221–227} }
 @article{frey_zhang_rouphail_2008, title={Fuel use and emissions comparisons for alternative routes, time of day, road grade, and vehicles based on in-use measurements}, volume={42}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-41649109289&partnerID=MN8TOARS}, DOI={10.1021/es702493v}, abstractNote={The objective here is to quantify the variability in emissions of selected light duty gasoline vehicles by routes, time of day, road grade, and vehicle with a focus on the impact of routes and road grade. Field experiments using a portable emission measurement system were conducted under real-world driving cycles. The study area included two origin/destination pairs, each with three alternative routes. Total emissions varied from trip to trip and from route to route due to variations in average speed and travel time. On an average trip basis, the total NO emissions differed by 24% when comparing alternative routes and by 19% when comparing congested travel time with less congested traffic time. Positive road grades were associated with an approximately 20% increase in localized emissions rates, while negative road grades were associated with a similar relative decrease. The average vehicle-specific power based NO modal emission rates differed by more than 2 orders of magnitude when comparing different vehicles. The results demonstrate that alternative routing can significantly impact trip emissions. Furthermore, road grade should be taken into account for localized emissions estimation. Vehicle-specific models are needed to capture episodic effects of emissions for near-road short-term human exposure assessment.}, number={7}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Frey, H. Christopher and Zhang, Kaishan and Rouphail, Nagui M.}, year={2008}, month={Apr}, pages={2483–2489} }
 @inproceedings{zhai_frey_rouphail_gonçalves_farias_2008, title={Impact of Alternative Vehicle Technologies on Measured Vehicle Emissions}, volume={320}, ISBN={9780784409602}, url={http://dx.doi.org/10.1061/40960(320)38}, DOI={10.1061/40960(320)38}, abstractNote={Emerging vehicle technologies such as hybrids, flex-fuel, electric, and fuel cells are projected to exceed 25% of total light-duty vehicle sales by 2030. Because of their higher energy efficiency, hybrid electric vehicles (HEVs) produce lower emissions of greenhouse gases such as CO 2 than comparable gasoline vehicles. Measurements of selected advanced gasoline and diesel direct injection vehicles and gasoline-electric hybrid vehicles indicate that the HEV had the lowest emissions and highest fuel economy. One study concluded that replacing all conventional vehicles with HEV's could cut total CO and NO x emissions in half. Flex-fuel vehicles that can use either gasoline or ethanol 85 (E85), a blend of 85% ethanol and 15% gasoline by volume have sensors that can analyze the fuel-air mixture and adjust the fuel injection and timing. Since ethanol is an oxygenated fuel, the use of ethanol may reduce emissions of products of incomplete combustion including CO and HC. However, the oxygenate may tend to increase NO x emissions because of lean combustion. The use of blends with a high percentage of ethanol in the mix was found to produce increased emissions of NO x and aldehydes as the ethanol content increased. However, measurements of flex-fuel Chevrolet Lumina vehicles indicated that the use of E85 may decrease vehicle CO, HC and NO x emissions, but increase aldehydes emissions when compared to gasoline vehicles showed that NO x emissions increased with increasing ethanol content for some fuels, but were unaffected by ethanol content for other fuels, depending on fuel volatility. Hochhauser found that use of ethanol fuel may increase vehicle permeation emissions of VOC. Therefore, flex-fuel vehicles do not assure lower emissions for at least some pollutants compared to dedicated gasoline vehicles. The objective of this study is to develop an advanced modeling system to quantify influences of land use and vehicle technologies on on-road vehicle emissions such as CO 2 , CO, and NO x . The main focus here is to demonstrate a methodology for assessing how differences in driving cycles affect link-based average emission rates for selected vehicle technologies. The methodology requires second-by-second data, with a preference for real-world in-use data where possible. The methodology is illustrated here based on data from one flex-fuel vehicle that was measured during actual driving using a portable emission measurement system (PEMS) on both gasoline and E85 and for one HEV that was tested on a chassis dynamometer. Testing for the flex-fueled vehicle was conducted by Instituto Superior Tecnico (IST) in Lisbon, Portugal. The flex-fuel vehicle is a European 2006 flex-fuel Ford Focus wagon with a 1.8 liter engine (92 kW/6000rpm, 165 Nm/4000rpm). By comparison, the U.S. version of the Focus has a 2.0 liter engine (97 kW/6000rpm, 175 Nm/4000rpm). The HEV is a 2001 Toyota Prius with a 1.5 liter gasoline engine.}, booktitle={Transportation Land Use, Planning, and Air Quality}, publisher={American Society of Civil Engineers}, author={Zhai, H. and Frey, H. C. and Rouphail, N. M. and Gonçalves, G. A. and Farias, T. L.}, year={2008}, month={May} }
 @inproceedings{zhai_frey_rouphail_gon?alves_farias_2008, title={Impact of alternative vehicle technologies on measured vehicle emissions}, volume={320}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-57549103716&partnerID=MN8TOARS}, booktitle={Transportation Land Use, Planning, and Air Quality - Proceedings of the 2007 Transportation Land Use, Planning, and Air Quality Conference}, author={Zhai, H. and Frey, H.C. and Rouphail, N.M. and Gon?alves, G.A. and Farias, T.L.}, year={2008}, pages={408–414} }
 @article{frey_rouphail_zhai_2008, title={Link-based emission factors for heavy-duty diesel trucks based on real-world data}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-56749151233&partnerID=MN8TOARS}, DOI={10.3141/2058-04}, abstractNote={ Heavy-duty diesel vehicles contribute a substantial fraction of nitrogen oxides (NOx) and particulate matter (PM) to the on-road vehicle emission inventory. The objectives of this study are to estimate roadway link-based emission rates for heavy-duty trucks for use in emission inventory estimation and to quantify the impact of factors affecting truck emissions. A speed–acceleration modal emission approach is developed from a database gathered via a portable emission measurement system for single rear-axle and tandem-axle dump trucks. Second-by-second real-world truck speed profiles on links are analyzed on the basis of observed patterns of time distributions of speed–acceleration modes. Link-based emission rates are estimated as the product of the fraction of time spent in each mode and the corresponding modal average emission rate. The sensitivity of link-based emission rates to key factors including chassis type, vehicle load, and fuel type is discussed. Single rear-axle trucks have lower emission rates than do tandem-axle trucks for carbon dioxide, PM, nitric oxide (NO), and hydrocarbons (HC) but higher carbon monoxide (CO) emission rates. Loaded trucks have higher fuel use and emissions than unloaded trucks. Replacing diesel fuel with biodiesel fuel for heavy-duty trucks may reduce tailpipe NO exhaust emissions and will reduce emissions of PM, CO, and HC. However, both fuels generate similar CO2 emissions. Benchmark comparisons for link-based emission rates show that NO emission rates increase with mean speed. However, link-based CO and HC emission rates were not as sensitive to speed variation as NO emissions. The link-based emission rate approach is recommended to couple heavy-duty vehicle emission inventory estimation with transportation demand models. }, number={2058}, journal={Transportation Research Record}, author={Frey, H. Christopher and Rouphail, N.M. and Zhai, H.}, year={2008}, pages={23–32} }
 @article{frey_kuo_villa_2008, title={Measurement and modeling of fuel use and exhaust emissions from idling Long-haul freight truck and auxiliary power unit engines}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449497001&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Kuo, P.-Y. and Villa, C.}, year={2008}, pages={3060–3064} }
 @article{frey_kuo_villa_2008, title={Methodology for characterization of long-haul truck idling activity under real-world conditions}, volume={13}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-56349136006&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2008.09.010}, abstractNote={Real-world data are used to quantify the activity of long-haul sleeper cab trucks during extended idling and estimate baseline fuel use and emission rates, and to assess reductions achievable with idle reduction techniques. Twenty new in-service trucks equipped with auxiliary power units and shorepower capability, divided between single drivers and team drivers, were monitored for more than a year. Data were collected from each truck using existing electronic control units, additional installed sensors, and a satellite uplink. For single drivers, there was an average of 2130 h of idling per truck, of which 1520 h occurred for stops of 7 h or more. The auxiliary power units were used 59% of the time for all stops and 68% for stops of 7 h or more. Team drivers averaged only 770 h of idling per truck and 29% overall auxiliary power units usage. There was substantial inter-driver variability, demonstrating the need for adequate sample size to reliably characterize activity patterns.}, number={8}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Frey, H. Christopher and Kuo, Po-Yao and Villa, Charles}, year={2008}, month={Dec}, pages={516–523} }
 @book{frey_rasdorf_kim_pang_lewis_abolhassani_2008, title={Real-world duty cycles and utilization for construction equipment in North Carolina}, number={FHWA/NC 2006-55}, journal={Technical Report- Not held in TRLN member libraries}, institution={Raleigh, N.C.:  North Carolina Department of Transportation}, author={Frey, H. C. and Rasdorf, W. and Kim, K. and Pang, S.-H. and Lewis, P. and Abolhassani, S.}, year={2008} }
 @article{abolhasani_frey_kim_rasdorf_lewis_pang_2008, title={Real-world in-use activity, fuel use, and emissions for nonroad construction vehicles: A case study for excavators}, volume={58}, ISSN={["1047-3289"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-50849117829&partnerID=MN8TOARS}, DOI={10.3155/1047-3289.58.8.1033}, abstractNote={Abstract A study design was developed and demonstrated for deployment of a portable emission measurement system (PEMS) for excavators. Excavators are among the most commonly used vehicles in construction activities. The PEMS measured nitric oxide, carbon monoxide, hydrocarbons, carbon dioxide, and opacity-based particulate matter. Data collection, screening, processing, and analysis protocols were developed to assure data quality and to quantify variability in vehicle fuel consumption and emissions rates. The development of data collection procedures was based on securing the PEMS while avoiding disruption to normal vehicle operations. As a result of quality assurance, approximately 90% of the attempted measurements resulted in valid data. On the basis of field data collected for three excavators, an average of 50% of the total nitric oxide emissions was associated with 29% of the time of operation, during which the average engine speed and manifold absolute pressure were significantly higher than corresponding averages for all data. Mass per time emission rates during non-idle modes (i.e., moving and using bucket) were on average 7 times greater than for the idle mode. Differences in normalized average rates were influenced more by intercycle differences than intervehicle differences. This study demonstrates the importance of accounting for intercycle variability in real-world in-use emissions to develop more accurate emission inventories. The data collection and analysis methodology demonstrated here is recommended for application to more vehicles to better characterize real-world vehicle activity, fuel use, and emissions for nonroad construction equipment.}, number={8}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Abolhasani, Saeed and Frey, H. Christopher and Kim, Kangwook and Rasdorf, William and Lewis, Phil and Pang, Shih-Hao}, year={2008}, month={Aug}, pages={1033–1046} }
 @article{frey_pang_kim_2008, title={Vehicle-specific emissions modeling for non-road construction vehicles based upon real-world measurements}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70449336768&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Pang, S.-H. and Kim, K.}, year={2008}, pages={2743–2749} }
 @article{frey_2008, title={What is uncertainty analysis and how can it be performed?}, volume={180}, DOI={10.1016/j.toxlet.2008.06.016}, journal={Toxicology Letters}, author={Frey, H.Christopher}, year={2008}, pages={3} }
 @article{frey_rouphail_zhai_farias_goncalves_2007, title={Comparing real-world fuel consumption for diesel- and hydrogen-fueled transit buses and implication for emissions}, volume={12}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34247868934&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2007.03.003}, abstractNote={This paper explores the influence of key factors such as speed, acceleration, and road grade on fuel consumption for diesel and hydrogen fuel cell buses under real-world operating conditions. A Vehicle Specific Power-based approach is used for modeling fuel consumption for both types of buses. To evaluate the robustness of the modeling approach, Vehicle Specific Power-based modal average fuel consumption rates are compared for diesel buses in the US and Portugal, and for the Portuguese diesel and hydrogen fuel cell buses that operate on the same route. For diesel buses there is similar intra-vehicle variability in fuel consumption using Vehicle Specific Power modes. For the fuel cell bus, the hydrogen fuel consumption rate was found to be less sensitive to Vehicle Specific Power variations and had smaller variability compared to diesel buses. Relative errors between trip fuel consumption estimates and actual fuel use, based upon predictions for a portion of real-world activity data that were not used to calibrate the models, were generally under 10% for all observations. The Vehicle Specific Power-based modeling approach is recommended for further applications as additional data become available. Emission changes based upon substituting hydrogen versus diesel buses are evaluated.}, number={4}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Frey, H. Christopher and Rouphail, Nagui M. and Zhai, Haibo and Farias, Tiago L. and Goncalves, Goncalo A.}, year={2007}, month={Jun}, pages={281–291} }
 @article{zhai_frey_rouphail_gonçalves_farias_2007, title={Fuel consumption and emissions comparisons between ethanol 85 and gasoline fuels for flexible fuel vehicles}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44649111256&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Zhai, H. and Frey, H.C. and Rouphail, N.M. and Gonçalves, G.A. and Farias, T.L.}, year={2007}, pages={3104–3108} }
 @book{frey_rasdorf_kim_pang_lewis_abolhassani_2007, title={Life cycle inventory and impact analysis framework for nonroad construction vehicles and equipment based upon in-use measurements}, journal={Technical Report- Not held in TRLN member libraries}, institution={Raleigh, N.C.:  National Science Foundation}, author={Frey, H. C. and Rasdorf, W. and Kim, K. and Pang, S.-H. and Lewis, P. and Abolhassani, S.}, year={2007} }
 @inproceedings{frey_rasdorf_pang_kim_abolhasani_lewis_2007, title={Methodology for activity, fuel use, and emissions data collection and analysis for nonroad construction equipment}, booktitle={Proceedings of the Air and Waste Management Association Annual Conference}, publisher={Pittsburgh, PA: AWMA}, author={Frey, H. C. and Rasdorf, W. and Pang, S. H. and Kim, K. and Abolhasani, S. and Lewis, P.}, year={2007} }
 @article{frey_rasdorf_pang_kim_abolhasani_lewis_2007, title={Methodology for activity, fuel use, and emissions data collection and analysis for nonroad construction equipment}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44649103903&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Rasdorf, W. and Pang, S.-H. and Kim, K. and Abolhasani, S. and Lewis, P.}, year={2007}, pages={3124–3128} }
 @inproceedings{frey_rasdorf_pang_kim_lewis_2007, title={Methods for measurement and analysis of in-use emissions of nonroad construction equipment}, booktitle={Proceedings of the EPA Emissions Inventory Conference}, publisher={Raleigh, NC: EPA}, author={Frey, H. C. and Rasdorf, W. and Pang, S. and Kim, K. and Lewis, P.}, year={2007} }
 @article{frey_kuo_2007, title={Potential best practices for reducing greenhouse gas (GHG) emissions in freight transportation}, volume={5}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44649095696&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Kuo, P.-Y.}, year={2007}, pages={3099–3103} }
 @article{vicari_mokhtari_morales_jaykus_frey_slenning_cowen_2007, title={Second-order modeling of variability and uncertainty in microbial hazard characterization}, volume={70}, ISSN={["0362-028X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847215129&partnerID=MN8TOARS}, DOI={10.4315/0362-028X-70.2.363}, abstractNote={This study describes an analytical framework that permits quantitative consideration of variability and uncertainty in microbial hazard characterization. Second-order modeling that used two-dimensional Monte Carlo simulation and stratification into homogeneous population subgroups was applied to integrate uncertainty and variability. Specifically, the bootstrap method was used to simulate sampling error due to the limited sample size in microbial dose-response modeling. A data set from human feeding trials with Campylobacter jejuni was fitted to the log-logistic dose-response model, and results from the analysis of FoodNet surveillance data provided further information on variability and uncertainty in Campylobacter susceptibility due to the effect of age. Results of our analyses indicate that uncertainty associated with dose-response modeling has a dominating influence on the analytical outcome. In contrast, inclusion of the age factor has a limited impact. While the advocacy of more closely modeling variability in hazard characterization is warranted, the characterization of key sources of uncertainties and their consistent propagation throughout a microbial risk assessment actually appear of greater importance.}, number={2}, journal={JOURNAL OF FOOD PROTECTION}, author={Vicari, Andrea S. and Mokhtari, Amirhossein and Morales, Roberta A. and Jaykus, Lee-Ann and Frey, H. Christopher and Slenning, Barrett D. and Cowen, Peter}, year={2007}, month={Feb}, pages={363–372} }
 @article{zhu_frey_2007, title={Simplified performance model of gas turbine combined cycle systems}, volume={133}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34248512929&partnerID=MN8TOARS}, DOI={10.1061/(asce)0733-9402(2007)133:2(82)}, abstractNote={Conventional evaluation approaches for complex technologies, such as gas turbine systems, typically use process simulators for modeling, which are usually complicated and time-consuming. In order to facilitate policy analysis, a simplified desktop model for gas turbine systems based upon the air-standard Brayton cycle is developed in Microsoft EXCEL. The simplified model incorporates key process details and includes a comprehensive cost model. The model is calibrated based on a typical “Frame 7F” heavy duty gas turbine fired with natural gas and syngas. The model generally produces accurate and reasonable estimates for performance and cost comparable to reference data. The model responds appropriately to different syngas compositions, such as based on variation in moisture content and C O2 removal. Changes in syngas composition lead to different syngas heating values and thus affect gas turbine performance and cost. The effects of changes in inputs on key outputs are evaluated. Six key inputs are identifi...}, number={2}, journal={Journal of Energy Engineering}, author={Zhu, Y. and Frey, H. Christopher}, year={2007}, pages={82–90} }
 @inproceedings{frey_zhang_2007, title={Spatial and temporal analysis of real-world empirical fuel use and emissions}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-44649198655&partnerID=MN8TOARS}, booktitle={100th Annual Conference and Exhibition of the Air and Waste Management Association 2007, ACE 2007}, author={Frey, H.C. and Zhang, K.}, year={2007}, pages={2097–2102} }
 @misc{miller_hidy_hales_kolb_werner_haneke_parrish_frey_rojas-bracho_deslauriers_et al._2006, title={Air emission inventories in North America: A critical assessment}, volume={56}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33749135247&partnerID=MN8TOARS}, DOI={10.1080/10473289.2006.10464540}, abstractNote={Abstract Although emission inventories are the foundation of air quality management and have supported substantial improvements in North American air quality, they have a number of shortcomings that can potentially lead to ineffective air quality management strategies. Major reductions in the largest emissions sources have made accurate inventories of previously minor sources much more important to the understanding and improvement of local air quality. Changes in manufacturing processes, industry types, vehicle technologies, and metropolitan infrastructure are occurring at an increasingly rapid pace, emphasizing the importance of inventories that reflect current conditions. New technologies for measuring source emissions and ambient pollutant concentrations, both at the point of emissions and from remote platforms, are providing novel approaches to collecting data for inventory developers. Advances in information technologies are allowing data to be shared more quickly, more easily, and processed and compared in novel ways that can speed the development of emission inventories. Approaches to improving quantitative measures of inventory uncertainty allow air quality management decisions to take into account the uncertainties associated with emissions estimates, providing more accurate projections of how well alternative strategies may work. This paper discusses applications of these technologies and techniques to improve the accuracy, timeliness, and completeness of emission inventories across North America and outlines a series of eight recommendations aimed at inventory developers and air quality management decision-makers to improve emission inventories and enable them to support effective air quality management decisions for the foreseeable future.}, number={8}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Miller, C. Andrew and Hidy, George and Hales, Jeremy and Kolb, Charles E. and Werner, Arthur S. and Haneke, Bernd and Parrish, David and Frey, H. Christopher and Rojas-Bracho, Leonora and Deslauriers, Marc and et al.}, year={2006}, month={Aug}, pages={1115–1129} }
 @article{mokhtari_frey_jaykus_2006, title={Application of classification and regression trees for sensitivity analysis of the Escherichia coli O157 : H7 food safety process risk model}, volume={69}, ISSN={["1944-9097"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33644903201&partnerID=MN8TOARS}, DOI={10.4315/0362-028X-69.3.609}, abstractNote={Microbial food safety process risk models are simplifications of the real world that help risk managers in their efforts to mitigate food safety risks. An important tool in these risk assessment endeavors is sensitivity analysis, a systematic method used to quantify the effect of changes in input variables on model outputs. In this study, a novel sensitivity analysis method called classification and regression trees was applied to safety risk assessment with the use of portions of the Slaughter Module and Preparation Module of the E. coli O157:H7 microbial food safety process risk as an example. Specifically, the classification and regression trees sensitivity analysis method was evaluated on the basis of its ability to address typical characteristics of microbial food safety process risk models such as nonlinearities, interaction, thresholds, and categorical inputs. Moreover, this method was evaluated with respect to identification of high exposure scenarios and corresponding key inputs and critical limits. The results from the classification and regression trees analysis applied to the Slaughter Module confirmed that the process of chilling carcasses is a critical control point. The method identified a cutoff value of a 2.2-log increase in the number of organisms during chilling as a critical value above which high levels of contamination would be expected. When classification and regression trees analysis was applied to the cooking effects part of the Preparation Module, cooking temperature was found to be the most sensitive input, with precooking treatment (i.e., raw product storage conditions) ranked second in importance. This case study demonstrates the capabilities of classification and regression trees analysis as an alternative to other statistically based sensitivity analysis methods, and one that can readily address specific characteristics that are common in microbial food safety process risk models.}, number={3}, journal={JOURNAL OF FOOD PROTECTION}, author={Mokhtari, A and Frey, HC and Jaykus, LA}, year={2006}, month={Mar}, pages={609–618} }
 @inproceedings{zietsman_perkinson_frey_wieters_2006, title={Emissions comparison between dirt roads and paved roads using portable emissions measurement systems}, volume={6}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847791632&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Zietsman, J. and Perkinson, D.G. and Frey, H.C. and Wieters, K.M.}, year={2006}, pages={3421–3439} }
 @article{mokhtari_frey_zheng_2006, title={Evaluation and recommendation of sensitivity analysis methods for application to Stochastic Human Exposure and Dose Simulation models}, volume={16}, ISSN={["1559-064X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33751086727&partnerID=MN8TOARS}, DOI={10.1038/sj.jes.7500472}, abstractNote={Sensitivity analyses of exposure or risk models can help identify the most significant factors to aid in risk management or to prioritize additional research to reduce uncertainty in the estimates. However, sensitivity analysis is challenged by non-linearity, interactions between inputs, and multiple days or time scales. Selected sensitivity analysis methods are evaluated with respect to their applicability to human exposure models with such features using a testbed. The testbed is a simplified version of a US Environmental Protection Agency's Stochastic Human Exposure and Dose Simulation (SHEDS) model. The methods evaluated include the Pearson and Spearman correlation, sample and rank regression, analysis of variance, Fourier amplitude sensitivity test (FAST), and Sobol's method. The first five methods are known as "sampling-based" techniques, wheras the latter two methods are known as "variance-based" techniques. The main objective of the test cases was to identify the main and total contributions of individual inputs to the output variance. Sobol's method and FAST directly quantified these measures of sensitivity. Results show that sensitivity of an input typically changed when evaluated under different time scales (e.g., daily versus monthly). All methods provided similar insights regarding less important inputs; however, Sobol's method and FAST provided more robust insights with respect to sensitivity of important inputs compared to the sampling-based techniques. Thus, the sampling-based methods can be used in a screening step to identify unimportant inputs, followed by application of more computationally intensive refined methods to a smaller set of inputs. The implications of time variation in sensitivity results for risk management are briefly discussed.}, number={6}, journal={JOURNAL OF EXPOSURE SCIENCE AND ENVIRONMENTAL EPIDEMIOLOGY}, author={Mokhtari, Amirhossein and Frey, H. Christopher and Zheng, Junyu}, year={2006}, month={Nov}, pages={491–506} }
 @article{silva_farias_frey_rouphail_2006, title={Evaluation of numerical models for simulation of real-world hot-stabilized fuel consumption and emissions of gasoline light-duty vehicles}, volume={11}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33748436974&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2006.07.004}, abstractNote={The use of numerical models for estimating fuel consumption and emission of HC, CO, NOx and CO2 of gasoline vehicles, under urban driving conditions, is examined. Three models were selected for evaluation: EcoGest, CMEM and ADVISOR. The models were used to simulate a sample of 14 urban trips for two 1999 Ford Taurus vehicles. Trip statistics were monitored on-board of the passenger vehicles across a variety of traffic conditions, using a portable emissions measurement device (PEMS). A key conclusion is that the tested models can be used with relatively high confidence to predict fuel consumption and CO2 emissions. However, results must be viewed with greater caution when it comes to predictions for other pollutants.}, number={5}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Silva, C. M. and Farias, T. L. and Frey, H. Christopher and Rouphail, Nagui M.}, year={2006}, month={Sep}, pages={377–385} }
 @article{mokhtari_frey_2006, title={Evaluation of sampling-based methods for sensitivity analysis: Case study for the E-Coli food safety process risk model}, volume={12}, ISSN={["1080-7039"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33750417713&partnerID=MN8TOARS}, DOI={10.1080/10807030600977251}, abstractNote={ABSTRACT This article evaluates selected sensitivity analysis methods applicable to risk assessment models with two-dimensional probabilistic frameworks, using a microbial food safety process risk model as a test-bed. Six sampling-based sensitivity analysis methods were evaluated including Pearson and Spearman correlation, sample and rank linear regression, and sample and rank stepwise regression. In a two-dimensional risk model, the identification of key controllable inputs that can be priorities for risk management can be confounded by uncertainty. However, despite uncertainty, results show that key inputs can be distinguished from those that are unimportant, and inputs can be grouped into categories of similar levels of importance. All selected methods are capable of identifying unimportant inputs, which is helpful in that efforts to collect data to improve the assessment or to focus risk management strategies can be prioritized elsewhere. Rank-based methods provided more robust insights with respect to the key sources of variability in that they produced narrower ranges of uncertainty for sensitivity results and more clear distinctions when comparing the importance of inputs or groups of inputs. Regression-based methods have advantages over correlation approaches because they can be configured to provide insight regarding interactions and nonlinearities in the model.}, number={6}, journal={HUMAN AND ECOLOGICAL RISK ASSESSMENT}, author={Mokhtari, Amirhossein and Frey, H. Christopher}, year={2006}, month={Dec}, pages={1128–1152} }
 @article{frey_zhu_2006, title={Improved system integration for integrated gasification combined cycle (IGCC) systems}, volume={40}, ISSN={["0013-936X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33644869596&partnerID=MN8TOARS}, DOI={10.1021/es0515598}, abstractNote={Integrated gasification combined cycle (IGCC) systems are a promising technology for power generation. They include an air separation unit (ASU), a gasification system, and a gas turbine combined cycle power block, and feature competitive efficiency and lower emissions compared to conventional power generation technology. IGCC systems are not yet in widespread commercial use and opportunities remain to improve system feasibility via improved process integration. A process simulation model was developed for IGCC systems with alternative types of ASU and gas turbine integration. The model is applied to evaluate integration schemes involving nitrogen injection, air extraction, and combinations of both, as well as different ASU pressure levels. The optimal nitrogen injection only case in combination with an elevated pressure ASU had the highest efficiency and power output and approximately the lowest emissions per unit output of all cases considered, and thus is a recommended design option. The optimal combination of air extraction coupled with nitrogen injection had slightly worse efficiency, power output, and emissions than the optimal nitrogen injection only case. Air extraction alone typically produced lower efficiency, lower power output, and higher emissions than all other cases. The recommended nitrogen injection only case is estimated to provide annualized cost savings compared to a nonintegrated design. Process simulation modeling is shown to be a useful tool for evaluation and screening of technology options.}, number={5}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Frey, HC and Zhu, YH}, year={2006}, month={Mar}, pages={1693–1699} }
 @article{kim_abolhasani_pang_frey_2006, title={Real-world data collection procedure for non-road construction equipment: Problems, solutions, and data quality assurance}, volume={3}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33847776263&partnerID=MN8TOARS}, journal={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Kim, K. and Abolhasani, S. and Pang, S.-H. and Frey, H.C.}, year={2006}, pages={1935–1939} }
 @article{zhang_frey_2006, title={Road grade estimation for on-road vehicle emissions modeling using light detection and ranging data}, volume={56}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33745442029&partnerID=MN8TOARS}, DOI={10.1080/10473289.2006.10464500}, abstractNote={Abstract Vehicle–specific power (VSP) is useful for explaining a substantial portion of variability in real–world vehicle emissions, such as those measured with portable emissions monitoring systems (PEMS). VSP is a function of vehicle speed, acceleration, and road grade. Road grade is shown to significantly affect estimates of both VSP and of real–world emissions via sensitivity analysis and analysis of empirical data. However, road grade is difficult to measure reliably using PEMS. Therefore, alternative methods for estimating road grade were identified and compared. A preferred method for estimating road grade was explored in more detail based on light detection and ranging (LIDAR) data. The method includes buffering LIDAR data onto roadway maps using a geographic information system tool, defining segments of roadway based on criteria pertaining to vertical curvature, quantification of roadway elevations within the buffered segments, and estimation of road grade and banking by fitting a plane to each segment. Factors influencing errors in road grade estimates are discussed. The method was evaluated by application to selected interstate highways and comparison to design drawing data. The development and application of LIDAR–based road grade data are demonstrated via a case study using PEMS data collected in the Research Triangle Park, NC, area. LIDAR data are shown to be reliable and accurate for road grade estimation for vehicle emissions modeling.}, number={6}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Zhang, Kaishan and Frey, H. Christopher}, year={2006}, month={Jun}, pages={777–788} }
 @inproceedings{christopher frey_rouphail_zhai_2006, title={Speed- And facility-specific emission estimates for on-road light-duty vehicles on the basis of real-world speed profiles}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33947608832&partnerID=MN8TOARS}, number={1987}, booktitle={Transportation Research Record}, author={Christopher Frey, H. and Rouphail, N.M. and Zhai, H.}, year={2006}, pages={128–137} }
 @article{zhu_frey_2006, title={Uncertainty analysis of integrated gasification combined cycle systems based on frame 7H versus 7F gas turbines}, volume={56}, ISSN={["1047-3289"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33845944471&partnerID=MN8TOARS}, DOI={10.1080/10473289.2006.10464570}, abstractNote={Abstract Integrated gasification combined cycle (IGCC) technology is a promising alternative for clean generation of power and coproduction of chemicals from coal and other feedstocks. Advanced concepts for IGCC systems that incorporate state-of-the-art gas turbine systems, however, are not commercially demonstrated. Therefore, there is uncertainty regarding the future commercial-scale performance, emissions, and cost of such technologies. The Frame 7F gas turbine represents current state-of-practice, whereas the Frame 7H is the most recently introduced advanced commercial gas turbine. The objective of this study was to evaluate the risks and potential payoffs of IGCC technology based on different gas turbine combined cycle designs. Models of entrained-flow gasifier-based IGCC systems with Frame 7F (IGCC-7F) and 7H gas turbine combined cycles (IGCC-7H) were developed in ASPEN Plus. An uncertainty analysis was conducted. Gasifier carbon conversion and project cost uncertainty are identified as the most important uncertain inputs with respect to system performance and cost. The uncertainties in the difference of the efficiencies and costs for the two systems are characterized. Despite uncertainty, the IGCC-7H system is robustly preferred to the IGCC-7F system. Advances in gas turbine design will improve the performance, emissions, and cost of IGCC systems. The implications of this study for decision-making regarding technology selection, research planning, and plant operation are discussed.}, number={12}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Zhu, Yunhua and Frey, H. Christopher}, year={2006}, month={Dec}, pages={1649–1661} }
 @article{zhao_frey_2006, title={Uncertainty for data with non-detects: Air toxic emissions from combustion}, volume={12}, ISSN={["1080-7039"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33750414559&partnerID=MN8TOARS}, DOI={10.1080/10807030600977178}, abstractNote={ABSTRACT Air toxic emission factor datasets often contain one or more points below a single or multiple detection limits and such datasets are referred to as “censored.” Conventional methods used to deal with censored datasets include removing non-detects, replacing the censored points with zero, half of the detection limit, or the detection limit. However, the estimated means of the censored dataset by conventional methods are usually biased. Maximum likelihood estimation (MLE) and bootstrap simulation have been demonstrated as a statistically robust method to quantify variability and uncertainty of censored datasets and can provide asymptotically unbiased mean estimates. The MLE/bootstrap method is applied to 16 cases of censored air toxic emission factors, including benzene, formaldehyde, benzo(a)pyrene, mercury, arsenic, cadmium, total chromium, chromium VI and lead from coal, fuel oil, and/or wood waste external combustion sources. The proportion of censored values in the emission factor data ranges from 4 to 80%. Key factors that influence the estimated uncertainty in the mean of censored data are sample size and inter-unit variability. The largest range of uncertainty in the mean was obtained for the external coal combustion benzene emission factor, with 95 confidence interval of the mean equal to minus 93 to plus 411%.}, number={6}, journal={HUMAN AND ECOLOGICAL RISK ASSESSMENT}, author={Zhao, Yuchao and Frey, H. Christopher}, year={2006}, month={Dec}, pages={1171–1191} }
 @inproceedings{frey_kim_2005, title={In-use emissions of heavy duty diesel dump trucks on petroleum diesel and B20 biodiesel fuels}, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646469610&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Frey, H.C. and Kim, K.}, year={2005} }
 @article{zheng_frey_2005, title={Quantitative analysis of variability and uncertainty with known measurement error: Methodology and case study}, volume={25}, ISSN={["1539-6924"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-23644460458&partnerID=MN8TOARS}, DOI={10.1111/j.1539-6924.2005.00620.x}, abstractNote={The appearance of measurement error in exposure and risk factor data potentially affects any inferences regarding variability and uncertainty because the distribution representing the observed data set deviates from the distribution that represents an error‐free data set. A methodology for improving the characterization of variability and uncertainty with known measurement errors in data is demonstrated in this article based on an observed data set, known measurement error, and a measurement‐error model. A practical method for constructing an error‐free data set is presented and a numerical method based upon bootstrap pairs, incorporating two‐dimensional Monte Carlo simulation, is introduced to address uncertainty arising from measurement error in selected statistics. When measurement error is a large source of uncertainty, substantial differences between the distribution representing variability of the observed data set and the distribution representing variability of the error‐free data set will occur. Furthermore, the shape and range of the probability bands for uncertainty differ between the observed and error‐free data set. Failure to separately characterize contributions from random sampling error and measurement error will lead to bias in the variability and uncertainty estimates. However, a key finding is that total uncertainty in mean can be properly quantified even if measurement and random sampling errors cannot be separated. An empirical case study is used to illustrate the application of the methodology.}, number={3}, journal={RISK ANALYSIS}, author={Zheng, JY and Frey, HC}, year={2005}, month={Jun}, pages={663–675} }
 @article{mokhtari_frey_2005, title={Recommended practice regarding selection of sensitivity analysis methods applied to microbial food safety process risk models}, volume={11}, ISSN={["1549-7860"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-22244443715&partnerID=MN8TOARS}, DOI={10.1080/10807030590949672}, abstractNote={ABSTRACT A guideline is presented for selection of sensitivity analysis methods applied to microbial food safety process risk (MFSPR) models. The guideline provides useful boundaries and principles for selecting sensitivity analysis methods for MSFPR models. Although the guideline is predicated on a specific branch of risk assessment models related to food-borne diseases, the principles and recommendations provided are typically generally applicable to other types of risk models. Applicable situations include: prioritizing potential critical control points; identifying key sources of variability and uncertainty; and refinement, verification, and validation of a model. Based on the objective of the analysis, characteristics of the model under study, amount of detail expected from sensitivity analysis, and characteristics of the sensitivity analysis method, recommendations for selection of sensitivity analysis methods are provided. A decision framework for method selection is introduced. The decision framework can substantially facilitate the process of selecting a sensitivity analysis method.}, number={3}, journal={HUMAN AND ECOLOGICAL RISK ASSESSMENT}, author={Mokhtari, A and Frey, HC}, year={2005}, month={Jun}, pages={591–605} }
 @inproceedings{zhang_frey_2005, title={Road grade estimation for on-road vehicle emissions modeling using lidar data}, volume={2005}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-33646469896&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Zhang, K. and Frey, C.}, year={2005} }
 @article{mokhtari_frey_2005, title={Sensitivity analysis of a two-dimensional probabilistic risk assessment model using analysis of variance}, volume={25}, ISSN={["1539-6924"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-31944435421&partnerID=MN8TOARS}, DOI={10.1111/j.1539-6924.2005.00679.x}, abstractNote={This article demonstrates application of sensitivity analysis to risk assessment models with two‐dimensional probabilistic frameworks that distinguish between variability and uncertainty. A microbial food safety process risk (MFSPR) model is used as a test bed. The process of identifying key controllable inputs and key sources of uncertainty using sensitivity analysis is challenged by typical characteristics of MFSPR models such as nonlinearity, thresholds, interactions, and categorical inputs. Among many available sensitivity analysis methods, analysis of variance (ANOVA) is evaluated in comparison to commonly used methods based on correlation coefficients. In a two‐dimensional risk model, the identification of key controllable inputs that can be prioritized with respect to risk management is confounded by uncertainty. However, as shown here, ANOVA provided robust insights regarding controllable inputs most likely to lead to effective risk reduction despite uncertainty. ANOVA appropriately selected the top six important inputs, while correlation‐based methods provided misleading insights. Bootstrap simulation is used to quantify uncertainty in ranks of inputs due to sampling error. For the selected sample size, differences in F values of 60% or more were associated with clear differences in rank order between inputs. Sensitivity analysis results identified inputs related to the storage of ground beef servings at home as the most important. Risk management recommendations are suggested in the form of a consumer advisory for better handling and storage practices.}, number={6}, journal={RISK ANALYSIS}, author={Mokhtari, A and Frey, HC}, year={2005}, month={Dec}, pages={1511–1529} }
 @inproceedings{hanna_heinold_paine_frey_baker_karp_feldman_2004, title={A Monte Carlo study of the uncertainties in predictions by ISC3ST and AERMOD of annual average benzene and 1,3-butadiene concentrations around the Houston Ship Channel}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21944432040&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Hanna, S.R. and Heinold, D. and Paine, R. and Frey, H.C. and Baker, D. and Karp, R. and Feldman, H.}, year={2004}, pages={3145–3157} }
 @inproceedings{hanna_vukovich_arunachalam_loughlin_frey_isakov_2004, title={Assessment of uncertainty in benzene concentration estimates in the Houston, TX, area}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21944435254&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Hanna, A.F. and Vukovich, J.M. and Arunachalam, S. and Loughlin, D. and Frey, H.C. and Isakov, V.}, year={2004}, pages={3129–3143} }
 @article{patil_frey_2004, title={Comparison of sensitivity analysis methods based on applications to a food safety risk assessment model}, volume={24}, ISSN={["1539-6924"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-3042765751&partnerID=MN8TOARS}, DOI={10.1111/j.0272-4332.2004.00460.x}, abstractNote={Sensitivity analysis (SA) methods are a valuable tool for identifying critical control points (CCPs), which is one of the important steps in the hazard analysis and CCP approach that is used to ensure safe food. There are many SA methods used across various disciplines. Furthermore, food safety process risk models pose challenges because they often are highly nonlinear, contain thresholds, and have discrete inputs. Therefore, it is useful to compare and evaluate SA methods based upon applications to an example food safety risk model. Ten SA methods were applied to a draft Vibrio parahaemolyticus (Vp) risk assessment model developed by the Food and Drug Administration. The model was modified so that all inputs were independent. Rankings of key inputs from different methods were compared. Inputs such as water temperature, number of oysters per meal, and the distributional assumption for the unrefrigerated time were the most important inputs, whereas time on water, fraction of pathogenic Vp, and the distributional assumption for the weight of oysters were the least important inputs. Most of the methods gave a similar ranking of key inputs even though the methods differed in terms of being graphical, mathematical, or statistical, accounting for individual effects or joint effect of inputs, and being model dependent or model independent. A key recommendation is that methods be further compared by application on different and more complex food safety models. Model independent methods, such as ANOVA, mutual information index, and scatter plots, are expected to be more robust than others evaluated.}, number={3}, journal={RISK ANALYSIS}, author={Patil, SR and Frey, HC}, year={2004}, month={Jun}, pages={573–585} }
 @article{zhao_frey_2004, title={Development of probabilistic emission inventories of air toxics for Jacksonville, Florida}, volume={54}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-8444251311&partnerID=MN8TOARS}, DOI={10.1080/10473289.2004.10471002}, abstractNote={Abstract Probabilistic emission inventories were developed for 1,3-butadiene, mercury (Hg), arsenic (As), benzene, formaldehyde, and lead for Jacksonville, FL. To quantify inter-unit variability in empirical emission factor data, the Maximum Likelihood Estimation (MLE) method or the Method of Matching Moments was used to fit parametric distributions. For data sets that contain nondetected measurements, a method based upon MLE was used for parameter estimation. To quantify the uncertainty in urban air toxic emission factors, parametric bootstrap simulation and empirical bootstrap simulation were applied to uncensored and censored data, respectively. The probabilistic emission inventories were developed based on the product of the uncertainties in the emission factors and in the activity factors. The uncertainties in the urban air toxics emission inventories range from as small as –25 to +30% for Hg to as large as –83 to +243% for As. The key sources of uncertainty in the emission inventory for each toxic are identified based upon sensitivity analysis. Typically, uncertainty in the inventory of a given pollutant can be attributed primarily to a small number of source categories. Priorities for improving the inventories and for refining the probabilistic analysis are discussed.}, number={11}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Zhao, YC and Frey, HC}, year={2004}, month={Nov}, pages={1405–1421} }
 @inproceedings{zhao_frey_2004, title={Development of probabilistic emission inventory of air toxics for Jacksonville, Florida}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21944442584&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Conference and Exhibition, AWMA}, author={Zhao, Y. and Frey, H.C.}, year={2004} }
 @inproceedings{zhao_christopher frey_2004, title={Development of probabilistic emission inventory of air toxics for jacksonville, Florida}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18944389363&partnerID=MN8TOARS}, booktitle={Proceedings of the Air and Waste Management Association's Annual Meeting and Exhibition}, author={Zhao, Y. and Christopher Frey, H.}, year={2004}, pages={3101–3114} }
 @article{chen_frey_2004, title={Optimization under variability and uncertainty: A case study for NOx emissions control for a gasification system}, volume={38}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-10644257613&partnerID=MN8TOARS}, DOI={10.1021/es0351037}, abstractNote={Methods for optimization of process technologies considering the distinction between variability and uncertainty are developed and applied to case studies of NOx control for Integrated Gasification Combined Cycle systems. Existing methods of stochastic optimization (SO) and stochastic programming (SP) are demonstrated. A comparison of SO and SP results provides the value of collecting additional information to reduce uncertainty. For example, an expected annual benefit of 240,000 dollars is estimated if uncertainty can be reduced before a final design is chosen. SO and SP are typically applied to uncertainty. However, when applied to variability, the benefit of dynamic process control is obtained. For example, an annual savings of 1 million dollars could be achieved if the system is adjusted to changes in process conditions. When variability and uncertainty are treated distinctively, a coupled stochastic optimization and programming method and a two-dimensional stochastic programming method are demonstrated via a case study. For the case study, the mean annual benefit of dynamic process control is estimated to be 700,000 dollars, with a 95% confidence range of 500,000 dollars to 940,000 dollars. These methods are expected to be of greatest utility for problems involving a large commitment of resources, for which small differences in designs can produce large cost savings.}, number={24}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Chen, JJ and Frey, HC}, year={2004}, month={Dec}, pages={6741–6747} }
 @article{abdel-aziz_frey_2004, title={Propagation of uncertainty in hourly utility NOx emissions through a photochemical grid air quality model: A case study for the Charlotte, NC, modeling domain}, volume={38}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1842779078&partnerID=MN8TOARS}, DOI={10.1021/es034062b}, abstractNote={One of the major hypothesized sources of uncertainties in air quality model inputs is the emission inventory. A probabilistic hourly NOx emission inventory for 32 units of nine coal-fired power plants in the Charlotte domain for the year 1995 was propagated through the Multiscale Air Quality Simulation Platform (MAQSIP). The inventory was developed using time series techniques. Time series for a 4-d episode were simulated and propagated through the air quality model 50 times in order to represent the ranges of uncertainty in hourly emissions and predicted ozone levels. Intra-unit autocorrelation in emissions and inter-unit dependence were accounted for. The range of uncertainty in predicted ozone was greater when inter-unit dependence was included as compared to when units were treated as statistically independent. Uncertainties in maximum ozone hourly or 8-h concentrations at a specific location could be attributed to a specific power plant based upon regression analysis. Out of 3969 grid cells in the modeling domain, there were 43 and 1654 grid cells with a probability greater than 0.9 of exceeding a 1-h 120 ppb standard and an 8-h 80 ppb standard, respectively. The time series of predicted ozone values had similar autocorrelation as compared to monitored data. The implications of these results for air quality management are addressed.}, number={7}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Abdel-Aziz, A and Frey, HC}, year={2004}, month={Apr}, pages={2153–2160} }
 @article{unal_frey_rouphail_2004, title={Quantification of highway vehicle emissions hot spots based upon on-board measurements}, volume={54}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0942265487&partnerID=MN8TOARS}, DOI={10.1080/10473289.2004.10470888}, abstractNote={The purpose of this study is to demonstrate a methodology for quantification of high emissions hot spots along roadways based upon real-world, on-road vehicle emissions measurements. An emissions hot spot is defined as a fixed location along a corridor in which the peak emissions are statistically significantly greater by more than a factor of 2 than the average emissions for free-flow or near free-flow conditions on the corridor. A portable instrument was used to measure on-road tailpipe emissions of carbon monoxide, nitric oxide, hydrocarbons, and carbon dioxide on a second-by-second basis during actual driving. Measurements were made for seven vehicles deployed on two primary arterial corridors. The ratio of average emissions at hot spots to the average emissions observed during a trip was as high as 25 for carbon monoxide, 5 for nitric oxide, and 3 for hydrocarbons. The relationships between hot spots and explanatory variables were investigated using graphical and statistical methods. Average speed, average acceleration, standard deviation of speed, percent of time spent in cruise mode, minimum speed, maximum acceleration, and maximum power have statistically significant associations with vehicle emissions and influence emissions hot spots. For example, stop-and-go traffic conditions that result in sudden changes in speed, and traffic patterns with high accelerations, are shown to generate hot spots. The implications of this work for future model development and applications to environmental management are discussed.}, number={2}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Unal, A and Frey, HC and Rouphail, NM}, year={2004}, month={Feb}, pages={130–140} }
 @article{frey_zhao_2004, title={Quantification of variability and uncertainty for air toxic emission inventories with censored emission factor data}, volume={38}, ISSN={["1520-5851"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-8544262958&partnerID=MN8TOARS}, DOI={10.1021/es035096m}, abstractNote={Probabilistic emission inventories were developed for urban air toxic emissions of benzene, formaldehyde, chromium, and arsenic for the example of Houston. Variability and uncertainty in emission factors were quantified for 71-97% of total emissions, depending upon the pollutant and data availability. Parametric distributions for interunit variability were fit using maximum likelihood estimation (MLE), and uncertainty in mean emission factors was estimated using parametric bootstrap simulation. For data sets containing one or more nondetected values, empirical bootstrap simulation was used to randomly sample detection limits for nondetected values and observations for sample values, and parametric distributions for variability were fit using MLE estimators for censored data. The goodness-of-fit for censored data was evaluated by comparison of cumulative distributions of bootstrap confidence intervals and empirical data. The emission inventory 95% uncertainty ranges are as small as -25% to +42% for chromium to as large as -75% to +224% for arsenic with correlated surrogates. Uncertainty was dominated by only a few source categories. Recommendations are made for future improvements to the analysis.}, number={22}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Frey, HC and Zhao, YC}, year={2004}, month={Nov}, pages={6094–6100} }
 @article{zhao_frey_2004, title={Quantification of variability and uncertainty for censored data sets and application to air toxic emission factors}, volume={24}, ISSN={["1539-6924"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-4544222855&partnerID=MN8TOARS}, DOI={10.1111/j.0272-4332.2004.00504.x}, abstractNote={Many environmental data sets, such as for air toxic emission factors, contain several values reported only as below detection limit. Such data sets are referred to as “censored.” Typical approaches to dealing with the censored data sets include replacing censored values with arbitrary values of zero, one‐half of the detection limit, or the detection limit. Here, an approach to quantification of the variability and uncertainty of censored data sets is demonstrated. Empirical bootstrap simulation is used to simulate censored bootstrap samples from the original data. Maximum likelihood estimation (MLE) is used to fit parametric probability distributions to each bootstrap sample, thereby specifying alternative estimates of the unknown population distribution of the censored data sets. Sampling distributions for uncertainty in statistics such as the mean, median, and percentile are calculated. The robustness of the method was tested by application to different degrees of censoring, sample sizes, coefficients of variation, and numbers of detection limits. Lognormal, gamma, and Weibull distributions were evaluated. The reliability of using this method to estimate the mean is evaluated by averaging the best estimated means of 20 cases for small sample size of 20. The confidence intervals for distribution percentiles estimated with bootstrap/MLE method compared favorably to results obtained with the nonparametric Kaplan–Meier method. The bootstrap/MLE method is illustrated via an application to an empirical air toxic emission factor data set.}, number={4}, journal={RISK ANALYSIS}, author={Zhao, YC and Frey, HC}, year={2004}, month={Aug}, pages={1019–1034} }
 @article{zheng_frey_2004, title={Quantification of variability and uncertainty using mixture distributions: Evaluation of sample size, mixing weights, and separation between components}, volume={24}, ISSN={["1539-6924"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-3042716474&partnerID=MN8TOARS}, DOI={10.1111/j.0272-4332.2004.00459.x}, abstractNote={Variability is the heterogeneity of values within a population. Uncertainty refers to lack of knowledge regarding the true value of a quantity. Mixture distributions have the potential to improve the goodness of fit to data sets not adequately described by a single parametric distribution. Uncertainty due to random sampling error in statistics of interests can be estimated based upon bootstrap simulation. In order to evaluate the robustness of using mixture distribution as a basis for estimating both variability and uncertainty, 108 synthetic data sets generated from selected population mixture log‐normal distributions were investigated, and properties of variability and uncertainty estimates were evaluated with respect to variation in sample size, mixing weight, and separation between components of mixtures. Furthermore, mixture distributions were compared with single‐component distributions. Findings include: (1) mixing weight influences the stability of variability and uncertainty estimates; (2) bootstrap simulation results tend to be more stable for larger sample sizes; (3) when two components are well separated, the stability of bootstrap simulation is improved; however, a larger degree of uncertainty arises regarding the percentiles coinciding with the separated region; (4) when two components are not well separated, a single distribution may often be a better choice because it has fewer parameters and better numerical stability; and (5) dependencies exist in sampling distributions of parameters of mixtures and are influenced by the amount of separation between the components. An emission factor case study based upon NOx emissions from coal‐fired tangential boilers is used to illustrate the application of the approach.}, number={3}, journal={RISK ANALYSIS}, author={Zheng, JY and Frey, HC}, year={2004}, month={Jun}, pages={553–571} }
 @article{abdel-aziz_frey_2003, title={Development of hourly probabilistic utility NOx emission inventories using time series techniques: Part I - univariate approach}, volume={37}, ISSN={["1873-2844"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0142226858&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2003.09.007}, abstractNote={Abstract Historical data regarding hourly variability in coal-fired power plant unit emissions based upon continuous emission monitoring enables estimation of the likely range of possible values in the near future for purposes of air quality modeling. Analyses were conducted for 32 units for a base case in 1995, an alternative 1998 case, and a 2007 future scenario case. Hourly inter-unit uncertainty was assumed to be independent. Univariate stochastic time series models were employed to quantify hourly uncertainty in capacity and emission factors. Ordinary least-squares regression models were used to quantify hourly uncertainty in heat rate. The models were used to develop an hourly probabilistic emission inventory for a 4-day period. There was significant autocorrelation for time lags 1, 2, 23, and 24 for the capacity and emission factor and a 24 h cyclical pattern for the capacity factor. The uncertainty ranges for hourly emissions were found to vary for different hours of the day, with 95% probability ranges of typically ±20–40% of the mean. For the 1995 case, the 95% confidence interval for the daily inventory was 510–633 t/d, representing approximately ±10% uncertainty with respect to the average value of 576 t/d. Inter-annual changes in the mean and variability were assessed by comparison of 1998 data with 1995 data. The daily inventory for the 2007 scenario had an uncertainty range of ±8% of the average value of 175 t/d. The substantial autocorrelation in capacity and emission factor, and the cyclic effect for capacity factor, indicate the importance of accounting for time series effects in estimation of uncertainty in hourly emissions. Additional work is recommended to account for inter-unit dependence, which is addressed in Part 2.}, number={38}, journal={ATMOSPHERIC ENVIRONMENT}, author={Abdel-Aziz, A and Frey, HC}, year={2003}, month={Dec}, pages={5379–5389} }
 @article{abdel-aziz_frey_2003, title={Development of hourly probabilistic utility NOx emission inventories using time series techniques: Part II - multivariate approach}, volume={37}, ISSN={["1352-2310"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0142226859&partnerID=MN8TOARS}, DOI={10.1016/j.atmosenv.2003.09.008}, abstractNote={Inter-unit dependence in the time series of capacity factors was accounted for in developing time series models for predictions of uncertainty in hourly NOx emissions for base load coal-fired power plants. Analyses were conducted for 32 units from 9 plants for a 1995 base case, and 1998 alternate case, and a future scenario in 2007. Multivariate time series models were employed in the analyses to account for the dependence between emissions from correlated units. The trade-off of using this approach is the complexity involved in the modeling process, including selection of model parameters and computational effort in the simulation process. Sufficient simultaneously recorded data for all correlated units must be available for purposes of model development. The results were compared to those of the inter-unit independent approach employed in a companion paper. Inter-unit correlations for capacity factor were as high as 0.86 and for total emissions were as high as 0.62. The total daily inventory for the 1995 case had a 95% confidence interval of 497–705 t/d which represents an uncertainty range of −15% to +20% of the average value of 587 t/d. The 2007 case had an uncertainty range of −8% to +15%. These uncertainty ranges are wider than the corresponding ranges obtained from the inter-unit independent approach. Simulations from the vector autoregressive time series approach that accounted for inter-unit correlation in capacity factor were more accurate than the inter-unit independent approach when compared to observed data.}, number={38}, journal={ATMOSPHERIC ENVIRONMENT}, author={Abdel-Aziz, A and Frey, HC}, year={2003}, month={Dec}, pages={5391–5401} }
 @inbook{unal_rouphail_frey_2003, title={Effect of arterial signalization and level of service on measured vehicle emissions}, ISBN={0309085756}, number={1842}, booktitle={Energy, air quality, and fuels 2003}, publisher={Washington, DC: Transportation Research Board}, author={Unal, A. and Rouphail, N. M. and Frey, H. C.}, year={2003}, pages={47–56} }
 @article{unal_rouphail_frey_2003, title={Effect of arterial signalization and level of service on measured vehicle emissions}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-1542316140&partnerID=MN8TOARS}, number={1842}, journal={Transportation Research Record}, author={Unal, A. and Rouphail, N.M. and Frey, C.}, year={2003}, pages={47–56} }
 @article{christopher frey_small_2003, title={Integrated environmental assessment, Part I: Estimating emissions}, volume={7}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0038818049&partnerID=MN8TOARS}, DOI={10.1162/108819803766729159}, abstractNote={A key element of industrial ecology involves estimating the environmental impact of products and processes. This assessment generally entails an evaluation of pollutant emissions and sometimes of fate and transport in the environment, exposure of humans or other environmental receptors, and the resulting health, ecological, or economic effects. When all of these steps are effectively executed, the process is often called integrated environmental assessment. The analysis of emissions is especially salient in life-cycle assessment (LCA), where the inventory and impact assessment of emissions from the various stages in the life cycle of a product is a critical component of the overall environmental evaluation. Analysis of emissions plays a similarly central role in substance flow analysis (SFA), another tool widely used in the industrial ecology community. A review of the state of the art of systems modeling for each of the steps in an integrated environmental assessment is provided in a series of columns, beginning with this evaluation of methods for emissions estimation and modeling. Good emissions inventories are necessary as the first step in predicting environmental impact, but they also play an essential role when assessing trends and progress in industrial ecology, evaluating international treaty compliance, and in the enforcement of environmental regulations. This column reviews the principal methods used to estimate emissions and the critical need to recognize and characterize the uncertainty in the resulting estimates and inventories.}, number={1}, journal={Journal of Industrial Ecology}, author={Christopher Frey, H. and Small, M.J.}, year={2003}, pages={9–11} }
 @article{frey_li_2003, title={Methods for quantifying variability and uncertainty in AP-42 emission factors: Case studies for natural gas-fueled engines}, volume={53}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0142172026&partnerID=MN8TOARS}, DOI={10.1080/10473289.2003.10466317}, abstractNote={Abstract Quantitative methods for characterizing variability and uncertainty were applied to case studies of oxides of nitrogen and total organic carbon emission factors for lean-burn natural gas-fueled internal combustion engines. Parametric probability distributions were fit to represent inter-engine variability in specific emission factors. Bootstrap simulation was used to quantify uncertainty in the fitted cumulative distribution function and in the mean emission factor. Some methodological challenges were encountered in analyzing the data. For example, in one instance, five data points were available, with each data point representing a different market share. Therefore, an approach was developed in which parametric distributions were fitted to population-weighted data. The uncertainty in mean emission factors ranges from as little as ~±10% to as much as -90 to 21+180%. The wide range of uncertainty in some emission factors emphasizes the importance of recognizing and accounting for uncertainty in emissions estimates. The skewness in some uncertainty estimates illustrates the importance of using numerical simulation approaches that do not impose restrictive symmetry assumptions on the confidence interval for the mean. In this paper, the quantitative method, the analysis results, and key findings are presented.}, number={12}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Frey, HC and Li, S}, year={2003}, month={Dec}, pages={1436–1447} }
 @article{frey_unal_rouphail_colyar_2003, title={On-road measurement of vehicle tailpipe emissions using a portable instrument}, volume={53}, ISSN={["2162-2906"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0042123783&partnerID=MN8TOARS}, DOI={10.1080/10473289.2003.10466245}, abstractNote={Abstract A study design procedure was developed and demonstrated for the deployment of portable onboard tailpipe emissions measurement systems for selected highway vehicles fueled by gasoline and E85 (a blend of 85% ethanol and 15% gasoline). Data collection, screening, processing, and analysis protocols were developed to assure data quality and to provide insights regarding quantification of real-world intravehicle variability in hot-stabilized emissions. Onboard systems provide representative real-world emissions measurements; however, onboard field studies are challenged by the observable but uncontrollable nature of traffic flow and ambient conditions. By characterizing intravehicle variability based on repeated data collection runs with the same driver/vehicle/route combinations, this study establishes the ability to develop stable modal emissions rates for idle, acceleration, cruise, and deceleration even in the face of uncontrollable external factors. For example, a consistent finding is that average emissions during acceleration are typically 5 times greater than during idle for hydrocarbons and carbon dioxide and 10 times greater for nitric oxide and carbon monoxide. A statistical method for comparing on-road emissions of different drivers is presented. Onboard data demonstrate the importance of accounting for the episodic nature of real-world emissions to help develop appropriate traffic and air quality management strategies.}, number={8}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Frey, HC and Unal, A and Rouphail, NM and Colyar, JD}, year={2003}, month={Aug}, pages={992–1002} }
 @article{frey_bammi_2003, title={Probabilistic nonroad mobile source emission factors}, volume={129}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0037331021&partnerID=MN8TOARS}, DOI={10.1061/(asce)0733-9372(2003)129:2(162)}, abstractNote={Quantitative methods for characterizing both variability and uncertainty are applied to case studies of nitrogen oxides (NOx) and total hydrocarbon air pollutant emission factors for construction, ...}, number={2}, journal={Journal of Environmental Engineering}, author={Frey, H. Christopher and Bammi, S.}, year={2003}, pages={162–168} }
 @article{abdel-aziz_frey_2003, title={Quantification of hourly variability in NOx emissions for baseload coal-fired power plants}, volume={53}, ISSN={["1047-3289"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0242322675&partnerID=MN8TOARS}, DOI={10.1080/10473289.2003.10466298}, abstractNote={Abstract The objectives of this paper are to (1) quantify variability in hourly utility oxides of nitrogen (NOx) emission factors, activity factors, and total emissions; (2) investigate the autocorrelation structure and evaluate cyclic effects at short and long scales of the time series of total hourly emissions; (3) compare emissions for the ozone (O3) season versus the entire year to identify seasonal differences, if any; and (4) evaluate interannual variability. Continuous emissions monitoring data were analyzed for 1995 and 1998 for 32 units from nine baseload power plants in the Charlotte, NC, airshed. Unit emissions have a strong 24-hr cycle attributable primarily to the capacity factor. Typical ranges of the coefficient of variation for emissions at a given hour of the day were from 0.2 to 0.45. Little difference was found when comparing weekend emissions with the entire week or when comparing the O3 season with the entire year. There were substantial differences in the mean and standard deviation of emissions when comparing 1995 and 1998 data, indicative of the effect of retrofits of control technology during the intervening time. The wide range of variability and its autocorrelation should be accounted for when developing probabilistic utility emission inventories for analysis of near-term future episodes.}, number={11}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Abdel-Aziz, A and Frey, HC}, year={2003}, month={Nov}, pages={1401–1411} }
 @inproceedings{frey_patil_2002, title={Identification and review of sensitivity analysis methods}, volume={22}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036085488&partnerID=MN8TOARS}, DOI={10.1111/0272-4332.00039}, abstractNote={Identification and qualitative comparison of sensitivity analysis methods that have been used across various disciplines, and that merit consideration for application to food‐safety risk assessment models, are presented in this article. Sensitivity analysis can help in identifying critical control points, prioritizing additional data collection or research, and verifying and validating a model. Ten sensitivity analysis methods, including four mathematical methods, five statistical methods, and one graphical method, are identified. The selected methods are compared on the basis of their applicability to different types of models, computational issues such as initial data requirement and complexity of their application, representation of the sensitivity, and the specific uses of these methods. Applications of these methods are illustrated with examples from various fields. No one method is clearly best for food‐safety risk models. In general, use of two or more methods, preferably with dissimilar theoretical foundations, may be needed to increase confidence in the ranking of key inputs.}, number={3}, booktitle={Risk Analysis}, author={Frey, H. Christopher and Patil, S.R.}, year={2002}, pages={553–578} }
 @article{frey_2002, title={Introduction to special section on sensitivity analysis and summary of NCSU/USDA workshop on sensitivity analysis}, volume={22}, ISSN={["0272-4332"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036085220&partnerID=MN8TOARS}, DOI={10.1111/0272-4332.00037}, abstractNote={This guest editorial is a summary of the NCSU/USDA Workshop on Sensitivity Analysis held June 11–12, 2001 at North Carolina State University and sponsored by the U.S. Department of Agriculture's Office of Risk Assessment and Cost Benefit Analysis. The objective of the workshop was to learn across disciplines in identifying, evaluating, and recommending sensitivity analysis methods and practices for application to food‐safety process risk models. The workshop included presentations regarding the Hazard Assessment and Critical Control Points (HACCP) framework used in food‐safety risk assessment, a survey of sensitivity analysis methods, invited white papers on sensitivity analysis, and invited case studies regarding risk assessment of microbial pathogens in food. Based on the sharing of interdisciplinary information represented by the presentations, the workshop participants, divided into breakout sessions, responded to three trigger questions: What are the key criteria for sensitivity analysis methods applied to food‐safety risk assessment? What sensitivity analysis methods are most promising for application to food safety and risk assessment? and What are the key needs for implementation and demonstration of such methods? The workshop produced agreement regarding key criteria for sensitivity analysis methods and the need to use two or more methods to try to obtain robust insights. Recommendations were made regarding a guideline document to assist practitioners in selecting, applying, interpreting, and reporting the results of sensitivity analysis.}, number={3}, journal={RISK ANALYSIS}, author={Frey, HC}, year={2002}, month={Jun}, pages={539–545} }
 @article{frey_zheng_2002, title={Probabilistic analysis of driving cycle-based highway vehicle emission factors}, volume={36}, ISSN={["0013-936X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036882670&partnerID=MN8TOARS}, DOI={10.1021/es0114308}, abstractNote={A probabilistic methodology for quantifying intervehicle variability and fleet average uncertainty in highway vehicle emission factors is developed. The methodology features the use of empirical distributions of emissions measurement data to characterize variability and the use of bootstrap simulation to characterize uncertainty. For the base emission rate as a function of mileage accumulation under standard conditions, a regression-based approach was employed in which the residual error terms were included in the probabilistic analysis. Probabilistic correction factors for different driving cycles, ambient temperature, and fuel Reid vapor pressure (RVP) were developed without interpolation or extrapolation of available data. The method was demonstrated for tailpipe carbon monoxide, hydrocarbon, and nitrogen oxides emissions for a selected light-duty gasoline vehicle technology. Intervehicle variability in emissions was found to span typically 2 or 3 orders of magnitude. The uncertainty in the fleet average emission factor was as low as +/- 10% for a 95% probability range, in the case of standard conditions, to as much as -90% to +280% when correction factors for alternative driving cycles, temperature, and RVP are applied. The implications of the results for method selection and for decision making are addressed.}, number={23}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Frey, HC and Zheng, JY}, year={2002}, month={Dec}, pages={5184–5191} }
 @article{frey_zheng_2002, title={Quantification of variability and uncertainty in air pollutant emission inventories: Method and case study for utility NOx emissions}, volume={52}, ISSN={["1047-3289"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036715372&partnerID=MN8TOARS}, DOI={10.1080/10473289.2002.10470837}, abstractNote={Abstract The quality of stationary source emission factors is typically described using data quality ratings, which provide no quantification of the precision of the emission factor for an average source, nor of the variability from one source to another within a category. Variability refers to actual differences caused by differences in feedstock composition, design, maintenance, and operation. Uncertainty refers to lack of knowledge regarding the true emissions. A general methodology for the quantification of variability and uncertainty in emission factors, activity factors, and emission inventories (EIs) is described, featuring the use of bootstrap simulation and related techniques. The methodology is demonstrated via a case study for a selected example of NOx emissions from coal-fired power plants. A prototype software tool was developed to implement the methodology. The range of interunit variability in selected activity and emission factors was shown to be as much as a factor of 4, and the range of uncertainty in mean emissions is shown to depend on the interunit variability and sample size. The uncertainty in the total inventory of −16 to +19% was attributed primarily to one technology group, suggesting priorities for collecting data and improving the inventory. The implications for decision-making are discussed.}, number={9}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Frey, HC and Zheng, JY}, year={2002}, month={Sep}, pages={1083–1095} }
 @article{frey_bammi_2002, title={Quantification of variability and uncertainty in lawn and garden equipment NOx and total hydrocarbon emission factors}, volume={52}, ISSN={["1047-3289"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0036228881&partnerID=MN8TOARS}, DOI={10.1080/10473289.2002.10470792}, abstractNote={Abstract Variability refers to real differences in emissions among multiple emission sources at any given time or over time for any individual emission source. Variability in emissions can be attributed to variation in fuel or feedstock composition, ambient temperature, design, maintenance, or operation. Uncertainty refers to lack of knowledge regarding the true value of emissions. Sources of uncertainty include small sample sizes, bias or imprecision in measurements, nonrepresentativeness, or lack of data. Quantitative methods for characterizing both variability and uncertainty are demonstrated and applied to case studies of emission factors for lawn and garden (L&G) equipment engines. Variability was quantified using empirical and parametric distributions. Bootstrap simulation was used to characterize confidence intervals for the fitted distributions. The 95% confidence intervals for the mean grams per brake horsepower/hour (g/hp-hr) emission factors for two-stroke engine total hydrocarbon (THC) and NOx emissions were from -30 to +41% and from -45 to +75%, respectively. The confidence intervals for four-stroke engines were from -33 to +46% for THCs and from -27 to +35% for NOx. These quantitative measures of uncertainty convey information regarding the quality of the emission factors and serve as a basis for calculation of uncertainty in emission inventories (Els).}, number={4}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Frey, HC and Bammi, S}, year={2002}, month={Apr}, pages={435–448} }
 @article{hanna_lu_frey_wheeler_vukovich_arunachalam_fernau_hansen_2001, title={Uncertainties in predicted ozone concentrations due to input uncertainties for the UAM-V photochemical grid model applied to the July 1995 OTAG domain}, volume={35}, ISSN={["1352-2310"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0034739917&partnerID=MN8TOARS}, DOI={10.1016/S1352-2310(00)00367-8}, abstractNote={The photochemical grid model, UAM-V, has been used by regulatory agencies to make decisions concerning emissions controls, based on studies of the July 1995 ozone episode in the eastern US. The current research concerns the effect of the uncertainties in UAM-V input variables (emissions, initial and boundary conditions, meteorological variables, and chemical reactions) on the uncertainties in UAM-V ozone predictions. Uncertainties of 128 input variables have been estimated and most range from about 20% to a factor of two. 100 Monte Carlo runs, each with new resampled values of each of the 128 input variables, have been made for given sets of median emissions assumptions. Emphasis is on the maximum hourly-averaged ozone concentration during the 12–14 July 1995 period. The distribution function of the 100 Monte Carlo predicted domain-wide maximum ozone concentrations is consistently close to log-normal with a 95% uncertainty range extending over plus and minus a factor of about 1.6 from the median. Uncertainties in ozone predictions are found to be most strongly correlated with uncertainties in the NO2 photolysis rate. Also important are wind speed and direction, relative humidity, cloud cover, and biogenic VOC emissions. Differences in median predicted maximum ozone concentrations for three alternate emissions control assumptions were investigated, with the result that (1) the suggested year-2007 emissions changes would likely be effective in reducing concentrations from those for the year-1995 actual emissions, that (2) an additional 50% NOx emissions reductions would likely be effective in further reducing concentrations, and that (3) an additional 50% VOC emission reductions may not be effective in further reducing concentrations.}, number={5}, journal={ATMOSPHERIC ENVIRONMENT}, author={Hanna, SR and Lu, ZG and Frey, HC and Wheeler, N and Vukovich, J and Arunachalam, S and Fernau, M and Hansen, DA}, year={2001}, pages={891–903} }
 @article{frey_burmaster_1999, title={Methods for characterizing variability and uncertainty: Comparison of bootstrap simulation and likelihood-based approaches}, volume={19}, DOI={10.1111/j.1539-6924.1999.tb00393.x}, abstractNote={Variability arises due to differences in the value of a quantity among different members of a population. Uncertainty arises due to lack of knowledge regarding the true value of a quantity for a given member of a population. We describe and evaluate two methods for quantifying both variability and uncertainty. These methods, bootstrapsimulation and a likelihood‐based method, are applied to three datasets. The datasetsinclude a synthetic sample of 19 values from a Lognormal distribution, a sample of nine values obtained from measurements of the PCB concentration in leafy produce, and asample of five values for the partitioning of chromium in the flue gas desulfurization system of coal‐fired power plants. For each of these datasets, we employ the two methods to characterize uncertainty in the arithmetic mean and standard deviation, cumulative distribution functions based upon fitted parametric distributions, the 95th percentile of variability, and the 63rd percentile of uncertainty for the 81st percentile of variability. The latter is intended to show that it is possible to describe anypoint within the uncertain frequency distribution by specifying an uncertainty percentile and a Variability percentile. Using the bootstrap method, we compare results based upon use of the method of matching moments and the method of maximum likelihood for fitting distributions to data. Our results indicate that with only 5‐19 data pointsas in the datasets we have evaluated, there is substantial uncertainty based upon random sampling error. Both the boostrap and likelihood‐based approaches yield comparable uncertainty estimates in most cases.}, number={1}, journal={Risk Analysis}, author={Frey, H. Christopher and Burmaster, D. E.}, year={1999}, pages={109–130} }
 @article{frey_burmaster_1999, title={Methods for characterizing variability and uncertainty: Comparison of bootstrap simulation and likelihood-based approaches}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032940312&partnerID=MN8TOARS}, DOI={10.1023/A:1006962412150}, number={1}, journal={Risk Analysis}, author={Frey, H.C. and Burmaster, D.E.}, year={1999}, pages={109–130} }
 @article{frey_rhodes_1998, title={Characterization and simulation of uncertain frequency distributions: Effects of distribution choice, variability, uncertainty, and parameter dependence}, volume={4}, ISSN={["1080-7039"]}, DOI={10.1080/10807039891284406}, abstractNote={We use bootstrap simulation to characterize uncertainty in parametric distributions, including Normal, Lognormal, Gamma, Weibull, and Beta, commonly used to represent variability in probabilistic assessments. Bootstrap simulation enables one to estimate sampling distributions for sample statistics, such as distribution parameters, even when analytical solutions are not available. Using a two-dimensional framework for both uncertainty and variability, uncertainties in cumulative distribution functions were simulated. The mathematical properties of uncertain frequency distributions were evaluated in a series of case studies during which the parameters of each type of distribution were varied for sample sizes of 5, 10, and 20. For positively skewed distributions such as Lognormal, Weibull, and Gamma, the range of uncertainty is widest at the upper tail of the distribution. For symmetric unbounded distributions, such as Normal, the uncertainties are widest at both tails of the distribution. For bounded distri...}, number={2}, journal={HUMAN AND ECOLOGICAL RISK ASSESSMENT}, author={Frey, HC and Rhodes, DS}, year={1998}, month={Apr}, pages={423–468} }
 @inproceedings{frey_bharvirkar_1998, title={Desktop modeling of the performance, emissions, and cost of gasification systems}, volume={91}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0032274276&partnerID=MN8TOARS}, number={1998 June}, booktitle={Proceedings of the Air & Waste Management Association's Annual Meeting & Exhibition}, publisher={Pittsburgh, PA :; A & WMA}, author={Frey, H.Christopher and Bharvirkar, Ranjit}, year={1998} }
 @inproceedings{frey_1998, title={Methods for quantitative analysis of variability and uncertainty in hazardous air pollutant emissions}, volume={91}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-18344405017&partnerID=MN8TOARS}, number={1998 June}, booktitle={Proceedings of the Air & Waste Management Association's Annual Meeting & Exhibition}, publisher={Pittsburgh, PA :; A & WMA}, author={Frey, H.Christopher}, year={1998} }
 @inbook{frey_1998, title={Quantitative analysis of variability and uncertainty in energy and environmental systems}, booktitle={Uncertainty modeling and analysis in civil engineering}, publisher={Boca Raton: CRC Press}, author={Frey, H. C.}, year={1998}, pages={381–423} }
 @inproceedings{frey_1997, title={Bootstrap methods for quantitative analysis of variability and uncertainty in exposure and risk assessment}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031344095&partnerID=MN8TOARS}, booktitle={Proceedings of the Air & Waste Management Association's Annual Meeting & Exhibition}, author={Frey, H.Christopher}, year={1997} }
 @article{rubin_kalagnanam_frey_berkenpas_1997, title={Integrated environmental control modeling of coal-fired power systems}, volume={47}, ISSN={["1047-3289"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031282206&partnerID=MN8TOARS}, DOI={10.1080/10473289.1997.10464063}, abstractNote={Abstract The capability to estimate the performance and costs of advanced environmental control systems for coal-fired power plants is critical to a variety of planning and analysis requirements faced by utilities, regulators, researchers, and analysts in the public and private sectors. This paper describes a computer model developed for the U.S. Department of Energy (DOE) to provide an up-to-date capability for analyzing a variety of pre-combustion, combustion, and post-combustion options in an integrated framework. A unique feature of the model allows performance and costs of integrated environmental control concepts to be modeled probabilistically as a means of characterizing uncertainties and risks. Examples are presented of model applications, comparing conventional and advanced emission control designs. The magnitude of technological risks associated with advanced technologies now under development are seen to vary markedly across applications. In general, however, integrated environmental control c...}, number={11}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Rubin, ES and Kalagnanam, JR and Frey, HC and Berkenpas, MB}, year={1997}, month={Nov}, pages={1180–1188} }
 @inproceedings{frey_iwanski_1997, title={Methods for characterizing and managing technological risks in advanced power generation systems: Application to gasification repowering in Poland}, number={1997 Oct.}, booktitle={Proceedings of Gasification Technologies Conference, Electric Power Research lnstitute, Palo Alto, CA. October 1997}, author={Frey, H. C. and Iwanski, Z.}, year={1997} }
 @article{agarwal_frey_1997, title={Modeling and evaluation of externally fired combined cycle using ASPEN}, volume={123}, DOI={10.1061/(ASCE)0733-9402(1997)123:3(69)}, abstractNote={A new performance model of the externally fired combined cycle (EFCC) is implemented in ASPEN (Advanced System for Process ENgineering), a chemical process simulator. The model accounts for interactions within and between approximately a dozen major process areas. The new model is applied to case studies to evaluate the sensitivity of process performance to the ranges of uncertainty in key model inputs such as heat losses, carbon conversion, and air leakage rates, and to illustrate how alternative designs, including steam injection and flue gas desulfurization (FGD), can affect plant thermal efficiency and net power output. The model results suggest that careful attention to the design of the FGD system is needed, especially with respect to reheat, and that steam injection may offer cost savings associated with reducing the size of the steam turbine with little or no penalty on plant output or plant efficiency. Uncertainties in the slagging combustor and the ceramic heat exchanger may lead to significant ...}, number={3}, journal={Journal of Energy Engineering}, author={Agarwal, P. and Frey, H. Christopher}, year={1997}, pages={69–87} }
 @article{agrawal_frey_1997, title={Modeling and evaluation of externally fired combined cycle using ASPEN}, volume={123}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-6144248166&partnerID=MN8TOARS}, number={3}, journal={Journal of Energy Engineering}, author={Agrawal, P. and Frey, H.C.}, year={1997}, pages={69–86} }
 @article{diwekar_rubin_frey_1997, title={Optimal design of advanced power systems under uncertainty}, volume={38}, ISSN={["0196-8904"]}, DOI={10.1016/S0196-8904(96)00184-7}, abstractNote={Technical and economic uncertainties are not rigorously treated or characterized in most preliminary cost and performance estimates of advanced power system designs. Nor do current design methods rigorously address the issues of design under uncertainty. However, process costs and other important quality measures, such as controllability, safety, and environmental compliance, largely depend on the process synthesis stage. This conceptual design stage involves identifying the basic flowsheet structures from a typically large number of alternatives. This paper describes recent developments in on-going research to develop and demonstrate advanced computer-based methods for dealing with uncertainties that are critical to the design of advanced coal-based power systems. Results are presented illustrating the use of these new modeling tools for the environmental control design of an advanced energy system based on an integrated gasification combined cycle (IGCC) for electric power generation.}, number={15-17}, journal={ENERGY CONVERSION AND MANAGEMENT}, author={Diwekar, UM and Rubin, ES and Frey, HC}, year={1997}, pages={1725–1735} }
 @article{diwekar_rubin_frey_1997, title={Optimal design of advanced power systems under uncertainty}, volume={38}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031259174&partnerID=MN8TOARS}, number={15-17}, journal={Energy Conversion and Management}, author={Diwekar, U.M. and Rubin, E.S. and Frey, H.C.}, year={1997}, pages={1725–1735} }
 @book{kini_frey_1997, title={Probabilistic evaluation of mobile source air pollution. Volume I, Probabilistic modeling of exhaust emissions from light duty gasoline vehicles}, publisher={Raleigh, NC: North Carolina State University for Center for Transportation and the Environment}, author={Kini, M. D. and Frey, H. C.}, year={1997} }
 @inproceedings{frey_iwanski_1997, title={Probabilistic methodology for risk assessment of new energy technologies and application to gasification repowering for an oil refinery in Poland}, volume={39}, number={16/18}, booktitle={Florence World Energy Research Symposium (1997): FLOWERS ''97: Florence World Energy Symposium}, publisher={Padova, Italy: SGEditorial}, author={Frey, H. C. and Iwanski, Z.}, editor={G. Manfrida and Lior, N.Editors}, year={1997}, pages={267–274} }
 @inproceedings{frey_1997, title={Quantification of uncertainty in remotesensing-based school bus CO and hydrocarbon emission factors}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0031354445&partnerID=MN8TOARS}, booktitle={Proceedings of the Air & Waste Management Association's Annual Meeting & Exhibition}, author={Frey, H.Christopher}, year={1997} }
 @inproceedings{rhodes_frey_1997, title={Quantification of variability and uncertainty in AP-42 emission factors using bootstrap simulation}, number={1997 Oct.}, booktitle={Emission inventory: Planning for the future, proceedings of a specialty conference, October 28-30, 1997, Research Triangle Park, NC}, publisher={Pittsburgh, Pa.: Air & Waste Management Association}, author={Rhodes, D. S. and Frey, H. C.}, year={1997} }
 @inbook{frey_rubin_1997, title={Uncertainty evaluation in capital cost projection}, volume={59}, booktitle={Encyclopedia of chemical processing and design, Vol. 59}, publisher={New York: Marcel Dekker}, author={Frey, H. C. and Rubin, E. S.}, year={1997}, pages={480–494} }
 @inproceedings{frey_1997, title={Variability and uncertainty in highway vehicle emission factors}, number={1997 Oct.}, booktitle={Emission inventory: Planning for the future, proceedings of a specialty conference, October 28-30, 1997, Research Triangle Park, NC}, publisher={Pittsburgh, Pa.: Air & Waste Management Association}, author={Frey, H. C.}, year={1997} }
 @article{frey_rhodes_1996, title={Characterizing, simulating, and analyzing variability and uncertainty: An illustration of methods using an air toxics emissions example}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-21444446390&partnerID=MN8TOARS}, number={4}, journal={Human and Ecological Risk Assessment (HERA)}, author={Frey, H.C. and Rhodes, D.S.}, year={1996}, pages={762–797} }
 @inproceedings{frey_1996, title={Comparison of classroom and video-based instruction: a case study for a graduate-level air pollution control course}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030313917&partnerID=MN8TOARS}, booktitle={Proceedings of the Air & Waste Management Association's Annual Meeting & Exhibition}, author={Frey, H.Christopher}, year={1996} }
 @inproceedings{tran_frey_1996, title={Methods for evaluating the costs of utility NOx control technologies}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030315356&partnerID=MN8TOARS}, booktitle={Proceedings of the Air & Waste Management Association's Annual Meeting & Exhibition}, author={Tran, Loan K. and Frey, H.Christopher}, year={1996} }
 @inproceedings{diwekar_rubin_frey_1996, title={Optimization of environmental control system design for an IGCC power plant}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0029706671&partnerID=MN8TOARS}, booktitle={Proceedings of the American Power Conference}, author={Diwekar, Urmila M. and Rubin, Edward S. and Frey, H.Christopher}, year={1996}, pages={58–63} }
 @inproceedings{frey_agarwal_1996, title={Probabilistic analysis and optimization of new power generation technologies: a case study for the externally-fired combined cycle}, volume={1}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0029707721&partnerID=MN8TOARS}, booktitle={Proceedings of the American Power Conference}, author={Frey, H.Christopher and Agarwal, Pankaj}, year={1996}, pages={52–57} }
 @inproceedings{frey_agarwal_1996, title={Probabilistic modeling and optimization of clean coal technologies: case studies of the externally-fired combined cycle}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0030314477&partnerID=MN8TOARS}, booktitle={Proceedings of the Air & Waste Management Association's Annual Meeting & Exhibition}, author={Frey, H.Christopher and Agarwal, Pankaj}, year={1996} }
 @inproceedings{frey_kini_ranjithan_fu_1996, title={Uncertainty, bias, and variability in emission factors for light duty gasoline vehicles}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-17544390686&partnerID=MN8TOARS}, booktitle={Proceedings of the Air & Waste Management Association's Annual Meeting & Exhibition}, author={Frey, H.Christopher and Kini, Mitesh D. and Ranjithan, S.Ranji and Fu, Sing-Yih}, year={1996} }
 @article{shih_frey_1995, title={Coal blending optimization under uncertainty}, volume={83}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0029328154&partnerID=MN8TOARS}, number={3}, journal={European Journal of Operational Research}, author={Shih, J.-S. and Frey, H.C.}, year={1995}, pages={452–465} }
 @inproceedings{frey_1995, title={Engineering-economic evaluation of SCR NO<sub>x</sub> control systems for coal-fired power plants}, volume={57-2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0029232725&partnerID=MN8TOARS}, booktitle={Proceedings of the American Power Conference}, author={Frey, H.Christopher}, year={1995}, pages={1583–1588} }
 @article{frey_rubin_diwekar_1994, title={Modeling uncertainties in advanced technologies: Application to a coal gasification system with hot-gas cleanup}, volume={19}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0028413599&partnerID=MN8TOARS}, number={4}, journal={Energy}, author={Frey, H.C. and Rubin, E.S. and Diwekar, U.M.}, year={1994}, pages={449–463} }
 @article{frey_rubin_1992, title={Evaluate uncertainties in advanced process technologies}, volume={88}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0026855415&partnerID=MN8TOARS}, number={5}, journal={Chemical Engineering Progress}, author={Frey, H.Christopher and Rubin, Edward S.}, year={1992}, pages={63–70} }
 @article{frey_rubin_1992, title={Evaluation method for advanced acid rain compliance technology}, volume={118}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0026853964&partnerID=MN8TOARS}, number={1}, journal={Journal of Energy Engineering}, author={Frey, H.Christopher and Rubin, Edward S.}, year={1992}, pages={38–55} }
 @article{frey_rubin_1992, title={Evaluation of Advanced Coal Gasification Combined-Cycle Systems under Uncertainty}, volume={31}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0026868636&partnerID=MN8TOARS}, DOI={10.1021/ie00005a010}, abstractNote={Advanced integrated gasification combined cycle (IGCC) systems have not been commercially demonstrated, and uncertainties remain regarding their commercial-scale performance and cost. Therefore, a probabilistic evaluation method has been developed and applied to explicitly consider these uncertainties. The insights afforded by this method are illustrated for an IGCC design featuring a fixed-bed gasifier and a hot gas cleanup system}, number={5}, journal={Industrial and Engineering Chemistry Research}, author={Frey, H.C. and Rubin, E.S.}, year={1992}, pages={1299–1307} }
 @article{diwekar_frey_rubin_1992, title={Synthesizing Optimal Flowsheets: Applications to IGCC System Environmental Control}, volume={31}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0026906174&partnerID=MN8TOARS}, DOI={10.1021/ie00008a014}, abstractNote={In this paper a new process synthesis capability implemented in the public version of the ASPEN chemical process simulator is demonstrated via an illustrative case study of a complex flowsheet. The objective of the case study is to minimize the cost of an advanced integrated gasification combined-cycle (IGCC) plant design featuring hot-gas cleanup, subject to environmental constraints. The problem is formulated as a mixed integer nonlinear programming (MINLP) optimization problem, involving the selection of both an optimal process configuration and optimal design parameters for that configuration. Performance and cost models of the IGCC system developed for the ASPEN simulator, along with the newly developed process synthesis capability, are used. As a first step, alternative in situ and external desulfurization are considered as process alternatives.}, number={8}, journal={Industrial and Engineering Chemistry Research}, author={Diwekar, U.M. and Frey, H.C. and Rubin, E.S.}, year={1992}, pages={1927–1936} }
 @inproceedings{frey_rubin_1991, title={Evaluating advanced technologies for acid rain compliance}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0025843790&partnerID=MN8TOARS}, booktitle={Energy in the 90's}, author={Frey, H.Christopher and Rubin, Edward S.}, year={1991}, pages={172–178} }
 @article{frey_rubin_1991, title={Probabilistic Evaluation of Advanced So<sub>2</sub>/No<sub>X</sub> Control Technology}, volume={41}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0026329024&partnerID=MN8TOARS}, DOI={10.1080/10473289.1991.10466954}, abstractNote={A new probabilistic modeling environment is described which allows the explicit and quantitative representation of the uncertainties inherent in new environmental control processes for SO2 and NOx removal. Stochastic analyses provide additional insights into the uncertainties in process performance and cost not possible with conventional deterministic or sensitivity analysis. Applications of the probabilistic modeling framework are illustrated via an analysis of the performance and cost of the fluidized bed copper oxide process, an advanced technology for the control of SO2 and NOx emissions from coal-fired power plants. An engineering model of a conceptual commercial-scale system provides the basis for the analysis. The model also captures interactions between the power plant, the SO2/NOx removal process, and other components of the emission control system. Results of the analysis address payoffs from process design improvements; the dependence of system cost on process design conditions and the availabi...}, number={12}, journal={Journal of the Air and Waste Management Association}, author={Frey, H.C. and Rubin, E.S.}, year={1991}, pages={1585–1593} }
 @inproceedings{rubin_frey_diwekar_1991, title={Probabilistic modeling of the performance and economics of IGCC systems}, volume={2}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-0026400326&partnerID=MN8TOARS}, booktitle={American Society of Mechanical Engineers, Environmental Control Division Publication, EC}, author={Rubin, E.S. and Frey, H.C. and Diwekar, U.M.}, year={1991}, pages={59–66} }
 @article{rubin_salmento_frey_1988, title={Cost-Effective emission controls for coal-fired power plants}, volume={74}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34248562338&partnerID=MN8TOARS}, DOI={10.1080/00986448808940456}, abstractNote={Integrated environmental control (IEC) concepts involving combined SO2/NOx. removal processes in combination with pre-combustion and combustion control methods offer significant potential for more cost-effective emissions control for coal-fired power plants. IEC system designs involving three advanced flue gas treatment processes in conjunction with conventional and advanced coal cleaning technologies to achieve overall reductions of 90 percent for both SO2 and NOx are compared to a conventional pulverized coal plant design using currently available technology. A new computer modeling framework is used to characterize key uncertainties, and to evaluate overall performance and cost probabilistically using Monte Carlo methods. Results indicate that several IEC systems currently under development offer a good chance of reducing environmental control costs by 20 to 30 percent, although improvements in currently commercial technology are likely to offset some of the gains expected from advanced process designs...}, number={1}, journal={Chemical Engineering Communications}, author={Rubin, E.S. and Salmento, J.S. and Frey, H.C.}, year={1988}, pages={155–167} }