A study of the aerosol radiative properties needed to compute direct aerosol forcing in the southeastern United States

To assess the direct radiative forcing due to aerosols in southeastern Unit ed States where a mild cooling is under way, an accurate set of data descri bing the aerosol radiative properties are needed. We report here aerosol op tical depth (AOD) and diffuse-to-direct solar irradiance ratio (DDR) at thr ee operational wavelengths (415, 500, 673 nm) determined by using Multifilt er Rotating Shadowband Radiometers (MFRSR) at two sites (a mountain top sit e: Mount Gibbes, 35,78 degreesN, 82.29 degreesW, 2006 m mean sea level (msl ); a valley site: Black Mountain, 35,66 degreesN, 82.38 degreesW, 951 m msl ), which are separated horizontally by 10 km and vertically by 1 km. The ch aracteristics AOD and DDR were determined from the field measurements obtai ned during 1995, It was found that the representative total AOD values at 5 00 nm at the valley site for highly polluted (HP), marine (M) and continent al (C) air masses were 0.68 +/- 0.33, 0.29 +/- 0.19, and 0.10 +/- 0,04, res pectively. 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 t hat the major portion of the atmospheric aerosol was located in the lowest 1 km surface boundary layer (SBL). There was a significant linear correlati on between the DDR and the total AOD at both sites. A simple, fast, and ope rative search-graph method was used to retrieve the columnar size distribut ion (number concentration N, effective radius r(eff), and geometric standar d deviation a) from the optical depth observations at the three operational wavelengths. The ground albedo, single-scattering albedo, and imaginary pa rt of the refractive index are calculated using a mathematically unique pro cedure involving a Mie code and a radiative transfer code in conjunction wi th the retrieved aerosol size distribution, AOD, and DDR, It was found that N, r(eff) and sigma (g) were in the range of 1.9 x 10 to 1.7 x 10(4) cm(-3 ), 0.09-0.68 mum, and 1.12-2.70, respectively. The asymmetry factor and sin gle-scattering albedo were in the ranges of 0.63-0.75 and 0.74-0.99 respect ively. The ground albedo over the forested terrain and the imaginary part o f refractive index were found to be in the range of 0.08-0.29 and 0,005-0.0 51, respectively.