@article{niyogi_chang_saxena_holt_alapaty_booker_chen_davis_holben_matsui_et al._2004, title={Direct observations of the effects of aerosol loading on net ecosystem CO2 exchanges over different landscapes}, volume={31}, ISSN={["0094-8276"]}, DOI={10.1029/2004gl020915}, abstractNote={We present the first direct, multisite observations in support of the hypothesis that atmospheric aerosols affect the regional terrestrial carbon cycle. The daytime growing season (summer) CO2 flux observations from six sites (forest, grasslands, and croplands) with collocated aerosol and surface radiation measurements were analyzed for high and low diffuse radiation; effect of cloud cover; and effect of high and low aerosol optical depths (AOD). Results indicate that, aerosols exert a significant impact on net CO2 exchange, and their effect may be even more significant than that due to clouds. The response appears to be a general feature irrespective of the landscape and photosynthetic pathway. The CO2 sink increased with aerosol loading for forest and crop lands, and decreased for grassland. The cause for the difference in response between vegetation types is hypothesized to be canopy architecture.}, number={20}, journal={GEOPHYSICAL RESEARCH LETTERS}, author={Niyogi, D and Chang, HI and Saxena, VK and Holt, T and Alapaty, K and Booker, F and Chen, F and Davis, KJ and Holben, B and Matsui, T and et al.}, year={2004}, month={Oct} }
 @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{wright_yu_kasibhatla_mcgraw_schwartz_saxena_yue_2002, title={Retrieval of aerosol properties from moments of the particle size distribution for kernels involving the step function: cloud droplet activation}, volume={33}, ISSN={["0021-8502"]}, DOI={10.1016/S0021-8502(01)00172-0}, abstractNote={Aerosol properties such as the number of particles that activate to form cloud drops and the mass contained within specified size ranges (as in the PM 2.5 and PM 10 regulatory standards) require integration over only part of the full size range of the particle distribution function (PDF) and may be formally expressed as integrals over kernels involving the Heaviside step function. Determination of these properties requires essentially that the size spectrum be partitioned into two (or more) portions, and poses a special challenge for aerosol modeling with the method of moments. To assess the ability of moment-based methods to treat kernels involving step functions, several algorithms for the estimation of aerosol properties associated with cloud activation have been evaluated. For 240 measured continental distributions employed here as test cases, the full size spectrum of the PDF was partitioned into three distinct portions based upon characteristic critical radii for activation in cumulus and stratiform clouds, and mass- and number-concentration metrics were evaluated for each portion. The first six radial moments yielded results accurate to within about 10% or better, on average, and the numbers of particles activated as cloud drops and the aerosol mass taken into cloud water were estimated to an accuracy of 5% or better. Of the moment-based approaches evaluated, the multiple isomomental distribution aerosol surrogate (MIDAS) (Wright, J. Aerosol Sci. 31 (2000) 1) technique performed best. Accurate results were also obtained with the randomized minimization search technique (RMST) (Yue et al., Geophys. Res. Lett. 24 (1997) 651; Heintzenberg et al., Appl. Opt. 20 (1981) 1308).}, number={2}, journal={JOURNAL OF AEROSOL SCIENCE}, author={Wright, DL and Yu, SC and Kasibhatla, PS and McGraw, R and Schwartz, SE and Saxena, VK and Yue, GK}, year={2002}, month={Feb}, pages={319–337} }
 @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{yu_saxena_zhao_2001, title={A comparison of signals of regional aerosol-induced forcing in eastern China and the southeastern United States}, volume={28}, ISSN={["0094-8276"]}, DOI={10.1029/2000GL011834}, abstractNote={This paper compares the temperature change patterns and the signals of regional aerosol‐induced forcing in eastern China and the southeastern US during the latter half of the twentieth century. Both regions show decreasing trends in the mean maximum temperature over recent decades and the cooling effect of Pinatubo volcanic aerosols. In contrast to the southeastern US, we found a slight overall warming trend in eastern China. Our analysis suggests that in addition to greenhouse warming effect, observed high concentrations of absorbing aerosols over eastern China during winter and spring might be one of the major reasons for the observed warming trend.}, number={4}, journal={GEOPHYSICAL RESEARCH LETTERS}, author={Yu, SC and Saxena, VK and Zhao, ZC}, year={2001}, month={Feb}, pages={713–716} }
 @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{yu_zender_saxena_2001, title={Direct radiative forcing and atmospheric absorption by boundary layer aerosols in the southeastern US: model estimates on the basis of new observations}, volume={35}, ISSN={["1352-2310"]}, DOI={10.1016/S1352-2310(01)00187-X}, abstractNote={In an effort to reduce uncertainties in the quantification of aerosol direct radiative forcing (ADRF) in the southeastern United States (US), a field column experiment was conducted to measure aerosol radiative properties and effects at Mt. Mitchell, North Carolina, and at an adjacent valley site. The experimental period was from June 1995 to mid-December 1995. The aerosol optical properties (single scattering albedo and asymmetry factor) needed to compute ADRF were obtained on the basis of a procedure involving a Mie code and a radiative transfer code in conjunction with the retrieved aerosol size distribution, aerosol optical depth, and diffuse-to-direct solar irradiance ratio. The regional values of ADRF at the surface and top of atmosphere (TOA), and atmospheric aerosol absorption are derived using the obtained aerosol optical properties as inputs to the column radiation model (CRM) of the community climate model (CCM3). The cloud-free instantaneous TOA ADRFs for highly polluted (HP), marine (M) and continental (C) air masses range from 20.3 to −24.8, 1.3 to −10.4, and 1.9 to −13.4 W m−2, respectively. The mean cloud-free 24-h ADRFs at the TOA (at the surface) for HP, M, and C air masses are estimated to be −8±4 (−33±16), −7±4 (−13±8), and −0.14±0.05 (−8±3) W m−2, respectively. On the assumption that the fractional coverage of clouds is 0.61, the annual mean ADRFs at the TOA and the surface are −2±1, and −7±2 W m−2, respectively. This also implies that aerosols currently heat the atmosphere over the southeastern US by 5±3 W m−2 on annual timescales due to the aerosol absorption in the troposphere.}, number={23}, journal={ATMOSPHERIC ENVIRONMENT}, author={Yu, SC and Zender, CS and Saxena, VK}, year={2001}, month={Aug}, pages={3967–3977} }
 @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} }
 @article{anderson_brogniez_cazier_saxena_lenoble_mccormick_2000, title={Characterization of aerosols from simulated SAGE III measurements applying two retrieval techniques}, volume={105}, ISSN={["2169-897X"]}, DOI={10.1029/1999JD901120}, abstractNote={We investigated the retrieval of aerosol properties and the extinction due to aerosols at the ozone and water vapor channels from simulated measurements at variations of the planned Stratospheric Aerosol and Gas Experiment (SAGE) III aerosol channels. The aerosol quantities surface area, volume, and effective radius are retrieved through the application of two distinct algorithms in the form of the randomized‐minimization‐search technique (RMST) and the constrained linear inversion (CLI) method. These aerosol quantities are important as inputs in climate, photochemical, and radiative forcing models and are useful in comparing diverse measurements. Ten analytical size distributions fitted to aerosol populations measured in situ are used with a Mie scattering code in conjunction with a Monte Carlo technique to simulate SAGE III measurements. These models consist of variations of prevolcanic and postvolcanic size distributions that exhibit various spectral shapes. Neither the complex components nor the uncertainties of the refractive indices are considered. We developed an objective scheme to estimate the systematic, random, and total uncertainties of each retrieved quantity that considers the contribution of the particles that lie outside the retrieved size range. Results, based on the 10 selected aerosol models, indicate that in the seven‐eight SAGE III channel retrievals, both algorithms obtain estimated total errors in the range 8–50% for the surface area with an average total error (R*) of ∼25%; for the volume the range is 5–25% with an R* of ∼12%, and for the effective radius, the range is 6–36% with an R* of 20% though both inversion techniques are applied in different size ranges. The inversion of the six longest channels to study aerosol properties in both the lower stratosphere and the upper troposphere leads to RMST R* values of ∼32, ∼15, and ∼20% and CLI R* values of ∼48, ∼22, and ∼31% for the surface area, volume, and effective radius, respectively. In the seven wavelength retrievals, both algorithms retrieved the extinction coefficients at the unused channel to within their measurement uncertainties except at the 0.385 and 1.550 μm channels located at the tail ends of the SAGE III aerosol extinction spectrum. The calculated extinction due to aerosols at the water vapor channel at 0.940 μm and the ozone channel at 0.600 μm produced R* values of <10 and <15% for both techniques. We have shown that the application of either technique, when properly tailored to the SAGE III system, not only can obtain useful aerosol information in most cases but also can estimate reasonably the extinction due to aerosols at other wavelengths within the SAGE III wavelength range.}, number={D2}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Anderson, J and Brogniez, C and Cazier, L and Saxena, VK and Lenoble, J and McCormick, MP}, year={2000}, month={Jan}, pages={2013–2027} }
 @article{menon_saxena_logie_2000, title={Chemical heterogeneity across cloud droplet size spectra in continental and marine air masses}, volume={39}, ISSN={["0894-8763"]}, DOI={10.1175/1520-0450(2000)039<0887:CHACDS>2.0.CO;2}, abstractNote={Abstract Variations in the chemical composition of cloud droplets of different sizes are predicted in models. Measurements made in natural clouds to verify this prediction are extremely limited, however. During the spring of 1995 and the summers of 1995 and 1996, a size-fractionating version of the California Institute of Technology active-strand cloud water collector was operated on a mountaintop platform in Mount Mitchell State Park, North Carolina (35°44′05"N, 82°17′15"W), to examine differences in drop chemistry between large and small cloud droplets. The size-fractionated measurements also were compared with the chemical composition collected from a passive string–type collector that collects bulk samples. Back-trajectory analysis was used to categorize the source of cloud-forming air masses that arrived at the site as polluted continental, continental, and marine. The differences in cloud drop acidity and chemical constituents were investigated for these different air masses. On average, smaller dro...}, number={6}, journal={JOURNAL OF APPLIED METEOROLOGY}, author={Menon, S and Saxena, VK and Logie, BD}, year={2000}, month={Jun}, pages={887–903} }
 @article{saxena_menon_1999, title={Sulfate-induced cooling in the southeastern US: An observational assessment}, volume={26}, ISSN={["1944-8007"]}, DOI={10.1029/1999GL900555}, abstractNote={The effect of sulfates on cloud microphysical and optical properties have been studied using data from a rural mountaintop location in the Mt. Mitchell State Park, North Carolina. Although the amended Clean Air Act has limited the sulfur emissions beginning 1990, we found the sulfate concentrations greater during 1993–97 compared to that during 1986–89. Cloud albedo inferred from satellite data and calculated from surface observations does not indicate a monotonic increase with increasing sulfates. The direct and indirect (cloud‐mediated) sulfate forcing are assessed to be 4.8 and −4 W m−2. These values exceed current model predictions as evaluated by other investigators. Surface temperature records of the region during 1949–94 indicate a cooling trend tacitly supporting our assessment.}, number={16}, journal={GEOPHYSICAL RESEARCH LETTERS}, author={Saxena, VK and Menon, S}, year={1999}, month={Aug}, pages={2489–2492} }
 @article{wenny_schafer_deluisi_saxena_barnard_petropavlovskikh_vergamini_1998, title={A study of regional aerosol radiative properties and effects on ultraviolet-B radiation}, volume={103}, ISSN={["2169-897X"]}, DOI={10.1029/98JD01481}, abstractNote={A field experiment was conducted in western North Carolina to investigate the relationship between aerosol optical properties and atmospheric transmission. Two research measurement sites in close horizontal proximity but at different altitudes were established to measure the transmission of UV radiation through a slab of atmosphere. An identical set of radiation sensing instruments, including a broadband UV‐B radiometer, a direct Sun pyrheliometer, a shadowband radiometer, and a spectral photometer, was placed at both sites, a mountaintop site (Mount Gibbes 35.78°N, 82.29°W, 2004 m elevation) and a valley site (Black Mountain, North Carolina 35.66°N, 82.38°N, 951 m elevation). Aerosol size distribution sampling equipment was located at the valley site. Broadband solar pseudo‐optical depth and aerosol optical depths at 415 nm, 500 nm, and 673 nm were measured for the lowest 1‐km layer of the troposphere. The measurements exhibited variations based on an air mass source region as determined by back trajectory analysis. Broadband UV‐B transmission through the layer also displayed variations relating to air mass source region. Spectral UV transmission revealed a dependence upon wavelength, with decreased transmission in the UV‐B region (300–320 nm) versus UV‐A region (320–363.5 nm). UV‐B transmission was found to be negatively correlated with aerosol optical depth. Empirical relations were developed to allow prediction of solar noon UV‐B transmission if aerosol optical depth at two visible wavelengths (415 and 500 nm) is known. A new method was developed for determining aerosol optical properties from the radiation and aerosol size distribution measurements. The aerosol albedo of single scatter was found to range from 0.75 to 0.93 and the asymmetry factor ranged from 0.63 to 0.76 at 312 nm, which is close to the peak response of human skin to UV radiation.}, number={D14}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Wenny, BN and Schafer, JS and DeLuisi, JJ and Saxena, VK and Barnard, WF and Petropavlovskikh, IV and Vergamini, AJ}, year={1998}, month={Jul}, pages={17083–17097} }
 @article{erlick_frederick_saxena_wenny_1998, title={Atmospheric transmission in the ultraviolet and visible: Aerosols in cloudy atmospheres}, volume={103}, ISSN={["2169-8996"]}, DOI={10.1029/1998JD200053}, abstractNote={This study considers the effects of aerosols in cloudy atmospheres on the wavelength dependence of atmospheric transmission in the ultraviolet and visible parts of the solar spectrum. Normalizing the transmission to that of a cloud‐ and aerosol‐free atmosphere, we examine the competing influences of clouds and aerosols on the shape of the transmission function when clouds and aerosols are mixed; while pure water clouds cause the normalized transmission to decrease with wavelength from around 320 nm through the visible, aerosol particles cause the normalized transmission to increase with wavelength from 320 nm through the visible. The results show that when clouds are superimposed on an aerosol profile with the cloud drops and aerosol particles externally mixed, the shape of the normalized transmission spectrum is dominated by the effect of the cloud drops, unless the optical depth of the aerosols begins to approach the optical depth of the cloud. This is the case for an optically thin stratus cloud and an urban aerosol profile. When cloud drops and aerosol particles are internally mixed through coagulation, the shape of the normalized transmission spectrum is again dominated by the effect of the cloud drops, unless there is an unrealistically high volume fraction of strongly absorbing aerosols embedded in the droplets. While measurements of the mass fraction of absorbing aerosols such as soot in cloud and rainwater range from 3.0×10−9 to 6.9×10−6, a soot volume fraction of 1×10−4 is necessary to cause the normalized transmission to increase with wavelength from 320 nm through the visible. The model results are also shown to be consistent with Brewer spectrophotometer irradiance measurements under cloudy and hazy conditions.}, number={D24}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Erlick, C and Frederick, JE and Saxena, VK and Wenny, BN}, year={1998}, month={Dec}, pages={31541–31555} }
 @article{bahrmann_saxena_1998, title={Influence of air mass history on black carbon concentrations and regional climate forcing in southeastern United States}, volume={103}, ISSN={["2169-897X"]}, DOI={10.1029/98JD02475}, abstractNote={Atmospheric black carbon (BC) mass concentrations in the southeastern United States have been measured at a regionally representative site near Mount Mitchell, North Carolina (35°44′05″N, 82°17′15″W, 2038 m elevation), the highest peak in the eastern United States, during a 9 month period from June to October 1996 and March to June 1997. BC concentrations are measured by an aethalometer, which operates by measuring the attenuation of light through a sample. All measured BC concentrations are reported in terms of air mass histories determined from back trajectory analysis using the Hybrid Single‐Particle Lagrangian Integrated Trajectory (HY‐SPLIT) model. Air masses influencing the site have been classified as polluted, marine, and continental according to SOx and NOx. emission inventories. The average BC mass concentrations for each sector are 216.6±47.8 ng m−3 for polluted air masses, 65.6±23.5 ng m−3 for marine air masses, and 169.9±50.6 ng m−3 for continental air masses. A positive relationship between cloud condensation nuclei and BC concentrations suggest at times the BC measured at the site may be internally mixed. The average BC concentration found in cloud water is 74.2 μg kg−1. Derived BC to sulfate mass ratios ranged from 0.01 to 0.06.}, number={D18}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Bahrmann, CP and Saxena, VK}, year={1998}, month={Sep}, pages={23153–23161} }
 @article{menon_saxena_1998, title={Role of sulfates in regional cloud-climate interactions}, volume={48}, ISSN={["1873-2895"]}, DOI={10.1016/S0169-8095(98)00057-X}, abstractNote={Abstract During the summers of 1993–1996, field experiments were conducted at Mt. Mitchell, NC, in the southeastern US. The effect of anthropogenic pollution on cloud microphysical properties such as liquid water content (ω), cloud droplet number concentration (N), effective radii (Reff), cloud condensation nuclei (CCN) activation spectrum and cloud reflectivity were investigated. Cloud water sulfate concentration was used as a measure of anthropogenic pollution. Back trajectory analysis was used to categorize the cloud forming air masses that arrived at the site. The sulfates and nitrates emission inventories of the U.S. Environmental Protection Agency (EPA) were used to classify air masses as polluted marine (PM), polluted continental (PC) or as highly polluted (HP). Empirical values for the relationships between CCN–N, CCN–sulfate, N–sulfate and Reff–N for different air masses have been obtained. A quantitative nonlinear relationship between CCN and N was obtained. The sublinear relationship between CCN and sulfate mass indicated the susceptibility of the different air masses to the sulfate content. The lesser polluted air masses exhibited greater changes in CCN concentrations for smaller increases in sulfate mass concentrations as compared to the more polluted air masses. The relationship between Reff and N for different air masses, as well as the N–sulfate mass relationship, suggested that the counteracting effect of sulfates on greenhouse warming for the southeastern US would be of magnitude greater than −4.0 W m−2 obtained by modeling studies. The nonlinear relationships between the cloud microphysical/optical properties and the sulfate content of the air mass implies the existence of an optimum level for the sulfate concentration beyond which cloud reflectivity stays unaffected. Analysis of 3 years of observational data established this level at 400–500 μeq l−1 of sulfate in the cloud water.}, journal={ATMOSPHERIC RESEARCH}, author={Menon, S and Saxena, VK}, year={1998}, month={Jun}, pages={299–315} }
 @article{saxena_yu_1998, title={Searching for a regional fingerprint of aerosol radiative forcing in the southeastern US}, volume={25}, ISSN={["0094-8276"]}, DOI={10.1029/98GL02106}, abstractNote={Although aerosols have long been considered to exert a cooling influence on the regional climate due to direct and indirect radiative forcing, persuasive evidence of the response to this forcing has been lacking. Here, we analyze the regional patterns of climate change in the Southeast US during the period 1949–94 to search for a fingerprint of aerosol radiative forcing. The results show that direct and indirect radiative forcing of both natural (such as Pinatubo volcanic aerosols) and anthropogenic aerosols (such as those transported from the polluted regions of US) may be responsible for the regional cooling trend in the Southeast during the past 46 years. Lack of availability of long term measurements precludes a rigorous cause‐and‐effect analysis. Circumstantial evidence presented here amply justifies immediate establishment of a network of measurements of aerosol optical depth and cloud reflectivity in the southeastern US.}, number={15}, journal={GEOPHYSICAL RESEARCH LETTERS}, author={Saxena, VK and Yu, SC}, year={1998}, month={Aug}, pages={2833–2836} }
 @article{deininger_saxena_1997, title={A validation of back trajectories of air masses by principal component analysis of ion concentrations in cloud water}, volume={31}, ISSN={["1352-2310"]}, DOI={10.1016/1352-2310(96)00152-5}, abstractNote={The chemical characteristics of polluted marine, polluted continental, and highly polluted air masses were studied by applying principal component analysis (PCA) to the cloud water data collected during field studies at a site located in Mt. Mitchell (2038 m msl, 35°44'05″N, 82°17'15″W) State Park, NC. The site intercepts air masses arriving from the East (polluted marine), the West (polluted continental), and the Northwest (highly polluted). The PCA was used to study the relationship between the ionic constituents of the cloud water and the type of air mass in which the cloud formed. By applying PCA to the cloud water chemistry, a set of highly intercorrelated variables (ions) was replaced with a set of uncorrelated principal components. Using PCA, we can identify the most significant acids and salts dissolved in the cloud water. For instance, sulfuric acid was found to be most influential in clouds formed in highly polluted air masses. Sea salt particles were present in the majority of the polluted marine cloud events. Calcium was found to be the most important ion in the polluted continental cloud events. The type of the air mass was determined by 48-h back trajectory analysis using Hybrid Single-Particle Lagrangian Integrated Trajectories (HYSPLIT) model. The results of the HYSPLIT model were confirmed by the PCA of the ionic composition of cloud water collected at the mountain top site.}, number={2}, journal={ATMOSPHERIC ENVIRONMENT}, author={Deininger, CK and Saxena, VK}, year={1997}, month={Jan}, pages={295–300} }
 @article{lin_saxena_1997, title={Changes in stratospheric aerosol parameters over 105-135 degrees E longitude due to eruption of Mount Pinatubo}, volume={28}, ISSN={["0021-8502"]}, DOI={10.1016/S0021-8502(96)00463-6}, abstractNote={The eruption of Mt Pinatubo (15.14°N, 120.35°E) in the Philippines on 15 June 1991 produced the largest volcanic effluents into the stratosphere as observed by satellite measurements. In this paper we demonstrate the application of an inversion technique to satellite observations to infer the stratospheric aerosol size distributions before and after the eruption. The U.S.A. NASA SAGE (Stratospheric Aerosol and Gas Experiment) 11 satellite data were used. As a result, the stratospheric aerosol size distributions were found to be bimodal due to the addition of larger particles from Mt Pinatubo ejection. In addition, aerosol parameters such as extinction coefficient, effective radius, total surface area, and mass loading were used to track the northward and southward dispersion of Pinatubo volcanic plume unmistakably. For example, by 18 July 1991, the Pinatubo plume had reached as high as 22 km in the stratosphere above Taiwan area. The extinction profiles for September and October 1991 cases were enhanced about two orders in magnitude at the altitude of 20–24 km owing to Pinatubo aerosols. Regarding the transport in the southern hemisphere, for five months after eruption, the plume had reached the Antarctic stratosphere, and dispersed vertically as high as 40 km. In middle Antarctic stratosphere, the aerosol extinctions were increased in general by an order of 103-105 due to the intrusion of Pinatubo plume. The second mode at 0.5 μm was found compared with the one at less than 0.1 μm, generally found in background stratosphere.}, number={4}, journal={JOURNAL OF AEROSOL SCIENCE}, author={Lin, NH and Saxena, VK}, year={1997}, month={Jun}, pages={697–712} }
 @article{ulman_saxena_1997, title={Impact of air mass histories on the chemical climate of Mount Mitchell, North Carolina}, volume={102}, ISSN={["2169-897X"]}, DOI={10.1029/97JD01183}, abstractNote={Cloud water acidity and ionic content, as measured at the Mount Mitchell (35°44′05″N, 82°17′15″W, 2038 m msl (highest peak in the eastern United States)) State Park observing site, using a passive cloud water collector, are directly influenced by the trajectories of cloud‐forming air masses which pass over areas of varying levels of pollutant emission. Regions of the United States which are emitters of high‐level pollutants, such as SOx and NOx will thus serve to reduce observed pH levels in cloud water samples and raise the levels of acidifying ions, such as sulfate and nitrate. The 48‐hour backward trajectories for all 39 cloud events during the 1993 field season (May 15, 1993 to October 14, 1993) were computed using the hybrid singleparticle Lagrangian integrated trajectories (HY‐SPLIT) model. Three sectors, identified as the polluted sector, from 290° to 65° azimuth relative to the site, the continental sector, 240° to 290° azimuth, and the marine sector, 65° to 240° azimuth, were used to classify the cloud‐forming air masses. The polluted sector was associated with the lowest overall pH averages, with the marine sector following closely behind. The highest average pH values were received from air masses indicated as having crossed the continental and the marine sectors (in combination), with the largest portions of those air mass trajectories passing through the continental sector (exclusively continental sector air masses were also the most frequent). These observations are in agreement with findings in Colorado where aerosols produced by wind erosion were responsible for decreasing the precipitation acidity.}, number={D21}, journal={JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES}, author={Ulman, JC and Saxena, VK}, year={1997}, month={Nov}, pages={25451–25465} }
 @article{saxena_yu_anderson_1997, title={Impact of stratospheric volcanic aerosols on climate: Evidence for aerosol shortwave and longwave forcing in the southeastern US}, volume={31}, ISSN={["1352-2310"]}, DOI={10.1016/S1352-2310(97)00244-6}, abstractNote={Major volcanic eruptions inject massive amounts of dust and gases into the lower stratosphere and upper troposphere. Stratospheric volcanic aerosols can scatter incoming solar radiation to space, increasing planetary albedo, reducing the total amount of solar energy reaching the troposphere and the earth's surface, and decreasing the daytime maximum temperature (aerosol shortwave forcing). They can also absorb and scatter outgoing terrestrial longwave radiation, increasing the nighttime minimum surface temperature (longwave forcing). However, persuasive evidence of climate response to this forcing has thus far been lacking. Here we examine patterns of annual and seasonal variations in mean maximum and minimum temperature trend during the periods 1992–1994 and 1985–1987 relative to that during the period 1988–1990 at 47 stations in the southeastern U.S. for evidence of such climate responses. The stratospheric volcanic aerosol optical depths over the southeastern U.S. during the period 1985–1994 were inferred from the Stratospheric Aerosol and Gases Experiment (SAGE) 11 satellite extinction measurement. After the long-term trend signals are removed, it is shown that the dominant decreasing trend of mean maximum temperature and the dominant increasing trend of mean minimum temperature over periods 1992–1994 and 1985–1987 relative to that over the period 1988–1990 are consistent with the distribution of stratospheric volcanic aerosols and predictions from aerosol radiative forcing in the southeastern U.S.}, number={24}, journal={ATMOSPHERIC ENVIRONMENT}, author={Saxena, VK and Yu, SC and Anderson, J}, year={1997}, month={Dec}, pages={4211–4221} }
 @article{defelice_saxena_yu_1997, title={On the measurements of cloud condensation nuclei at Palmer Station, Antarctica}, volume={31}, ISSN={["1352-2310"]}, DOI={10.1016/S1352-2310(97)00250-1}, abstractNote={This article presents and discusses the predominant characteristics associated with a first dataset of daily daylight period (i.e. ≃13-15 h long) averaged cloud condensation nuclei spectral measurements at a remote region of the globe, namely Palmer Station, Antartica.}, number={23}, journal={ATMOSPHERIC ENVIRONMENT}, author={Defelice, TP and Saxena, VK and Yu, SC}, year={1997}, month={Dec}, pages={4039–4044} }
 @article{yue_lu_mohnen_wang_saxena_anderson_1997, title={Retrieving aerosol optical properties from moments of the particle size distribution}, volume={24}, ISSN={["0094-8276"]}, DOI={10.1029/97GL00522}, abstractNote={The randomized minimization search technique (RMST) is a powerful tool for retrieving aerosol size distribution from a set of aerosol extinction measurements. This technique is now extended to retrieve aerosol optical properties from a known moment sequence. The new usage of the RMST is demonstrated by using particle size distributions obtained from fits to in situ measurements conducted in the troposphere and stratosphere. Good agreements (with differences less than 5%) between some aerosol optical properties calculated from the moments and those calculated directly from the particle size distributions are obtained. Our results illustrate that the RMST can be applied to parameterize particle optical properties from the lower‐order moments of an aerosol size distribution.}, number={6}, journal={GEOPHYSICAL RESEARCH LETTERS}, author={Yue, GK and Lu, J and Mohnen, VA and Wang, PH and Saxena, VK and Anderson, J}, year={1997}, month={Mar}, pages={651–654} }
 @article{saxena_lin_1990, title={Cloud chemistry measurements and estimates of acidic deposition on an above cloudbase coniferous forest}, volume={24A}, number={2}, journal={Atmospheric Environment}, author={Saxena, V. K. and Lin, N. H.}, year={1990}, pages={329} }