Abstract. This study examines how aerosols measured from the ground and space over the US Southeast change temporally over a regional scale during the past decade. PM 2.5 (particulate matter with aerodynamic diameter > 2.5 micrometers) data consist of two datasets that represent the measurements that are used for regulatory purposes by the US EPA (Environmental Protection Agency) and continuous measurements used for quickly disseminating air quality information. AOD (aerosol optical depth) data come from three NASA sensors: the MODIS sensors onboard Terra and Aqua satellites and the MISR sensor onboard the Terra satellite. We analyze all available data over the state of Georgia from 2000-2009 of both types of aerosol data. The analysis reveals that during the summer the large metropolitan area of Atlanta has average PM 2.5 concentrations that are 50 % more than the remainder of the state. Strong seasonality is detected in both the AOD and PM 2.5 datasets, as evidenced by a threefold increase of AOD from mean winter values to mean summer values, and the increase in PM 2.5 concentrations is almost twofold over the same period. Additionally, there is agreement between MODIS and MISR onboard the Terra satellite during the spring and summer, having correlation coefficients of 0.64 and 0.71, respectively. Monthly anomalies were used to determine the presence of a trend in all considered aerosol datasets. We found negative linear trends for both the monthly AOD anomalies from MODIS onboard Terra and the PM 2.5 datasets, which are statistically significant. Decreasing trends were also found for MISR onboard Terra and MODIS onboard Aqua, but those trends were not statistically significant. The observed decrease in AOD and PM 2.5 concentrations may be indicative of the brightening over the study region during the past decade.

Aerosols comprise a critical portion of the Earth's climate due to their radiative properties. More emphasis is now being placed upon understanding radiative effects of aerosols on a regional scale. The primary goal of this research is to estimate the aerosol direct radiative effect (DRE) and examine its dynamical nature in the Southeastern U.S. based on satellite data obtained from the moderate-resolution imaging spectroradiometer (MODIS) and multi-angle imaging spectroradiometer (MISR) instruments onboard the Terra satellite from 2000 to 2011. This 12-year analysis utilizes satellite measurements of aerosol optical depth (AOD), surface albedo, cloud fraction, and single-scattering albedo over the Southeastern U.S. as inputs to a first-order approximation of regional top of the atmosphere DRE. Results indicate that AOD is the primary driver of DRE estimates, with surface albedo and singlescattering albedo having some appreciable effects as well. During the cooler months, the minima (less negative) of DRE vary between -6 and -3 W/m 2 , and during the warmer months, there is more variation with DRE maxima varying between -24 and -12.6 W/m 2 for MODIS and -22.5 and -11 W/m 2 for MISR. Yet if we take an average of the monthly DRE over time (12 years), we estimate ΔF = -7.57 W/m 2 for MODIS and ΔF = -5.72 W/m 2 for MISR. Regional assessments of the DRE show that background levels of DRE are similar to the 12-year average of satellite-based DRE, with urbanized areas having increased levels of DRE compared to background conditions. Over the study period, DRE has a positive trend (becoming less negative), which implies that the region could lose this protective top of the atmosphere cooling with the advancement of climate change impacting the biogenic emissions of aerosols.KEY WORDS: aerosols, aerosol direct radiative effect, MODIS, MISR, climate CITATION: Alston and Sokolik. A First-order Assessment of direct Aerosol radiative effect in the Southeastern U.S. Using over a decade long Multisatellite data record.

Aerosols and their radiative properties play an integral part in understanding Earth’s climate. It is becoming increasingly common to examine aerosol’s radiative impacts on a regional scale. The primary goal of this research is to explore the impacts of regional aerosol’s forcing at the surface and top-of-atmosphere (TOA) in the south-eastern U.S. by using a 1-D radiative transfer model. By using test cases that are representative of conditions common to this region, an estimate of aerosol forcing can be compared to other results. Speciation data and aerosol layer analysis provide the basis for the modeling. Results indicate that the region experiences TOA cooling year-round, where the winter has TOA forcings between −2.8 and −5 W/m2, and the summer has forcings between −5 and −15 W/m2 for typical atmospheric conditions. Surface level forcing efficiencies are greater than those estimated for the TOA for all cases considered i.e., urban and non-urban background conditions. One potential implication of this research is that regional aerosol mixtures have effects that are not well captured in global climate model estimates, which has implications for a warming climate where all radiative inputs are not well characterized, thus increasing the ambiguity in determining regional climate impacts.