@article{petters_saxena_slusser_wenny_madronich_2003, title={Aerosol single scattering albedo retrieved from measurements of surface UV irradiance and a radiative transfer model}, volume={108}, number={D9}, journal={Journal of Geophysical Research. Atmospheres}, author={Petters, J. L. and Saxena, V. K. and Slusser, J. R. and Wenny, B. N. and Madronich, S.}, year={2003}, pages={4288–1} }
 @article{barnard_saxena_wenny_deluisi_2003, title={Daily surface UV exposure and its relationship to surface pollutant measurements}, volume={53}, ISSN={["1047-3289"]}, DOI={10.1080/10473289.2003.10466134}, abstractNote={Abstract For the past 30 years, the stratospheric ozone layer has decreased in the Northern Hemisphere. The main effect of this ozone decrease was an expected increase in the UV radiation at the Earth’s surface, but there has been no clear evidence of an increasing urban trend in surface UV. This study shows that specific air pollutants can reduce the increased surface levels of UV radiation and offers an explanation for why the expected surface UV increases have not been observed, especially in urban regions. A U.S. Environmental Protection Agency (EPA) UV monitoring site at the University of California at Riverside combined with air pollution data from a site operated by the California Air Resources Board in Rubidoux, CA, provided the basis of this study. The 1997 South Coast Ozone Study (SCOS-97) provided three key ingredients: black carbon, PM10 concentrations, and collocated radiometric measurements. The Total Ozone Mapping Spectrometer (TOMS) satellite data were used to provide the stratospheric ozone levels that were included in the statistical model. All of these input parameters would be used to test this study’s hypothesis: the expected increase of surface UV radiation, caused by decreases in stratospheric ozone, can be masked by increases in anthropogenic emissions. The values for the pollutants were 7:00 a.m.-5:00 p.m. averages of the instrument’s values taken during summer 1997. A statistical linear regression model was employed using the stratospheric ozone, black carbon, PM10 , and surface ozone concentrations, and the sin (Θ) and cos (Θ). The angle Θ is defined by Θ = 2π (Julian date/365). This model obtained a coefficient of determination of 0.94 with an uncertainty level (p value) of less than 0.3% for all of the variables in the model except ground-level ozone. The final model, regressed against a data set from a remote, western North Carolina site, resulted in a coefficient of determination of 0.92. The model shows that black carbon can reduce the Diffey-weighted UV levels that reach the surface by as much as 35%, depending on the season.}, number={2}, journal={JOURNAL OF THE AIR & WASTE MANAGEMENT ASSOCIATION}, author={Barnard, WF and Saxena, VK and Wenny, BN and DeLuisi, JJ}, year={2003}, month={Feb}, pages={237–245} }
 @article{menon_saxena_durkee_wenny_nielsen_2002, title={Role of sulfate aerosols in modifying the cloud albedo: a closure experiment}, volume={61}, ISSN={["1873-2895"]}, DOI={10.1016/S0169-8095(01)00140-5}, abstractNote={At a remote mountain-top location in the southeastern US, measurements were made to estimate the contribution of anthropogenic aerosols to the cloud albedo. The influence of the long-range transport of anthropogenic emissions on cloud microphysical and optical properties at the mountain top site was investigated. The sources of the cloud forming air masses were determined from back-trajectory analysis. Cloud water sulfate content was used as a surrogate for anthropogenic pollution. The effects of particulate sulfate on cloud condensation nuclei (CCN) concentration, cloud droplet number concentration (N), cloud droplet effective radii (Reff) and cloud albedo were analyzed. A non-linear relationship between CCN and sulfate mass was obtained with a lowered sensitivity of CCN at high values of sulfate. Differences in N and sulfate from polluted to less polluted type air masses were much larger than that in Reff. This could be due to the variability in cloud liquid water content (LWC) as Reff is more related to LWC and cloud thickness than is N. The variability in cloud liquid water path (LWP) results in the optical depth being more sensitive to changes in Reff than to N for differences in cloud pollutant content. As part of a “closure experiment”, the cloud albedo calculated from in situ measurements for a 3-year period (1993–1995) compared well with that inferred from the Advanced Very High Resolution Radiometer (AVHRR) data. This accomplishes the objective of our closure experiment and proves that albedo of non-precipitating, thin, isolated clouds can be resolved against the dark forested background by AVHRR. The cloud reflectivity inferred from satellite measurements and that calculated from in situ observations were found to vary with the cloud water sulfate and N. Non-linear increases in satellite inferred cloud albedo with LWP suggest the importance of determining the contribution of cloud dynamic feedbacks on the indirect effect.}, number={3}, journal={ATMOSPHERIC RESEARCH}, author={Menon, S and Saxena, VK and Durkee, P and Wenny, BN and Nielsen, K}, year={2002}, month={Mar}, pages={169–187} }
 @article{wenny_saxena_frederick_2001, title={Aerosol optical depth measurements and their impact on surface levels of ultraviolet-B radiation}, volume={106}, ISSN={["2169-8996"]}, DOI={10.1029/2001JD900185}, abstractNote={Surface measurements of total and diffuse UV irradiance at the seven narrowband wavelength channels of the ultraviolet multifilter rotating shadow‐band radiometer (UVMFR) were used to determine total column ozone and aerosol optical depth for two 6‐month periods in 1997 and 1999 at a site in the Blue Ridge Mountains of North Carolina. The retrieved column ozone displayed a seasonal dependence and consistent agreement with the Total Ozone Mapping Spectrometer (TOMS). The mean ratio of retrieved ozone to TOMS ozone was 0.98 with standard deviations of 0.02 and 0.01 for 1997 and 1999, respectively. Aerosol optical depth at 317, 325, 332, and 368 nm was derived for a 6‐month period of 1999. The seasonal trend exhibited is influenced by the persistent summertime haze that occurs in the region. The retrieved aerosol optical depths are used as input in a radiative transfer model to investigate the effect of their realistic values on the calculation of the UV index (UVI) forecasted by the National Weather Service. The percentage change in calculated surface erythemally weighted UV (versus calculations using the standard UVI aerosol inputs) ranges from a 4% increase to a nearly 50% decrease, dependent upon the aerosol optical depth and amount of absorption by aerosols. Based on our measurements, it was found that during the summertime the UV index can deviate by up to −5 index units from the forecast using the standard aerosol inputs.}, number={D15}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Wenny, BN and Saxena, VK and Frederick, JE}, year={2001}, month={Aug}, pages={17311–17319} }
 @article{im_saxena_wenny_2001, title={An assessment of hygroscopic growth factors for aerosols in the surface boundary layer for computing direct radiative forcing}, volume={106}, ISSN={["2169-8996"]}, DOI={10.1029/2000JD000152}, abstractNote={Aerosol optical properties in the southeastern United States were measured at two research sites in close horizontal proximity but at different altitudes at Black Mountain (35.66 °N, 82.38 °W, 951 m msl) and Mount Gibbes (35.78 °N, 82.29 °W, 2006 m msl) to estimate the direct radiative forcing in the lowest 1 km layer of the troposphere during the summer of 1998. Measurements of light scattering and light absorption at ambient relative humidity (RH) are categorized by air mass type (polluted continental, marine with some continental influence, continental) according to 48‐hour back‐trajectory analysis. At a wavelength of 530 nm the average total scattering coefficient (σsp) measured at the valley site was 1.46×10−4 m−1 for polluted continental air masses, 7.25×10−5 m−1 for marine air masses, and 8.36×10−5 m−1 for continental air masses. The ratio of σsp at the mountain site to σsp at the valley site was 0.64, 0.58, and 0.45 for polluted continental, marine, and continental air masses, respectively. The hygroscopic growth factor (σsp(RH = 80%)/σsp(RH = 30%)) was calculated to be almost a constant value of 1.60±0.01 for polluted continental, marine, and continental air masses. As the RH increased from 30% to 80%, the backscatter fraction decreased by 23%. On the basis of these measurements, direct radiative climate forcing (ΔFR) by aerosols in the lowest 1 km layer of the troposphere was estimated. The patterns of ΔFR for various values of RH were similar for the three air masses, but the magnitudes of ΔFR(RH) were larger for polluted continental air masses than for marine and continental air masses by a factor of about 2 due to higher sulfate concentration in polluted continental air masses. The average value of ΔFR(RH = 80%)/ΔFR(RH = 30%) was calculated to be almost a constant value of 1.45±0.01 for all three types of air masses. This implies little dependence of the forcing ratio on the air mass type. The averaged ΔFR for all the observed ambient RHs, in the lowest 1 km layer during the 3‐month summer period, was −2.95 W m−2 (the negative forcing of −3.24 W m−2 by aerosol scattering plus the positive forcing of +0.30 W m−2 by aerosol absorption) for polluted continental air masses, −1.43 W m−2 (−1.55 plus +0.12) for marine air masses, and −1.50 W m−2 (−1.63 plus +0.14) for continental air masses. The ΔFR for polluted continental air masses was approximately twice that of marine and continental air masses. These forcing estimates are calculated from continuous in situ measurements of scattering and absorption by aerosols without assumptions for Mie calculations and global mean column burden of sulfates and black carbon (in g m−2) used in most of the model computations.}, number={D17}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Im, JS and Saxena, VK and Wenny, BN}, year={2001}, month={Sep}, pages={20213–20224} }
 @article{im_saxena_wenny_2001, title={Temporal trends of black carbon concentrations and regional climate forcing in the southeastern United States}, volume={35}, ISSN={["1352-2310"]}, DOI={10.1016/S1352-2310(00)00520-3}, abstractNote={The effect of black carbon (BC) on climate forcing is potentially important, but its estimates have large uncertainties due to a lack of sufficient observational data. The BC mass concentration in the southeastern US was measured at a regionally representative site, Mount Gibbes (35.78°N, 82.29°W, 2006 m MSL). The air mass origin was determined using 48-h back trajectories obtained from the hybrid single-particle Lagrangian integrated trajectory model. The highest average concentration is seen in polluted continental air masses and the lowest in marine air masses. During the winter, the overall average BC value was 74.1 ng m−3, whereas the overall summer mean BC value is higher by a factor of 3. The main reason for the seasonal difference may be enhanced thermal convection during summer, which increases transport of air pollutants from the planetary boundary layer of the surrounding urban area to this rural site. In the spring of 1998, abnormally high BC concentrations from the continental sector were measured. These concentrations were originating from a biomass burning plume in Mexico. This was confirmed by the observations of the Earth probe total ozone mapping spectrometer. The BC average concentrations of air masses transported from the polluted continental sector during summer are low on Sunday to Tuesday with a minimum value of 256 ng m−3 occurring on Monday, and high on Wednesday to Friday with a maximum value of 379 ng m−3 occurring on Friday. The net aerosol radiative forcing (scattering effects plus absorption effects) per unit vertical depth at 2006 m MSL is calculated to be −1.38×10−3 W m−3 for the southeastern US. The magnitude of direct radiative forcing by aerosol scattering is reduced by 15±7% due to the BC absorption.}, number={19}, journal={ATMOSPHERIC ENVIRONMENT}, author={Im, JS and Saxena, VK and Wenny, BN}, year={2001}, month={Jul}, pages={3293–3302} }
 @article{yu_saxena_wenny_deluisi_yue_petropavlovskikh_2000, title={A study of the aerosol radiative properties needed to compute direct aerosol forcing in the southeastern United States}, volume={105}, ISSN={["2169-897X"]}, DOI={10.1029/2000JD900346}, abstractNote={To assess the direct radiative forcing due to aerosols in southeastern United States where a mild cooling is under way, an accurate set of data describing the aerosol radiative properties are needed. We report here aerosol optical depth (AOD) and diffuse to‐direct solar irradiance ratio (DDR) at three operational wavelengths (415, 500, 673 nm) determined by using Multifilter Rotating Shadowband Radiometers (MFRSR) at two sites (a mountain top site: Mount Gibbes, 35.78°N, 82.29°W, 2006 m mean sea level (msl); a valley site: Black Mountain, 35.66°N, 82.38°W, 951 m msl), which are separated horizontally by 10 km and vertically by 1 km. The characteristics AOD and DDR were determined from the field measurements obtained during 1995. It was found that the representative total AOD values at 500 nm at the valley site for highly polluted (HP), marine (M) and continental (C) air masses were 0.68±0.33, 0.29±0.19, and 0.10±0.04, respectively. The fact that the ratio of the mean 1 km layer optical depth to total mean optical depth at 500 nm from the valley site was 71% indicates that the major portion of the atmospheric aerosol was located in the lowest 1 km surface boundary layer (SBL). There was a significant linear correlation between the DDR and the total AOD at both sites. A simple, fast, and operative search‐graph method was used to retrieve the columnar size distribution (number concentration N effective radius reff, and geometric standard deviation σg) from the optical depth observations at the three operational wavelengths. The ground albedo, single‐scattering albedo, and imaginary part of the refractive index are calculated using a mathematically unique procedure involving a Mie code and a radiative transfer code in conjunction with the retrieved aerosol size distribution, AOD, and DDR. It was found that N, reff, and σg were in the range of 1.9×10 to 1.7×104 cm−3, 0.09–0.68 μm, and 1.12–2.70, respectively. The asymmetry factor and single‐scattering albedo were in the ranges of 0.63–0.75 and 0.74–0.99 respectively. The ground albedo over the forested terrain and the imaginary part of refractive index were found to be in the range of 0.08–0.29 and 0.005–0.051, respectively.}, number={D20}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Yu, SC and Saxena, VK and Wenny, BN and DeLuisi, JJ and Yue, GK and Petropavlovskikh, IV}, year={2000}, month={Oct}, pages={24739–24749} }