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 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 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 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 r(eff), and geometric standard deviation a) 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, 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 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.