Hemispherical backscattering by biomass burning and sulfate particles derived from sky measurements

Holben et al. [1995] reported measurements of the aerosol spectral optical thickness and derived volume size distribution from measurements of the sky spectral radiance in the aureole (scattering angle less than or equal to 40 degrees). In this paper we use these and other sky data for analysis of the aerosol,hemispheric backscattering ratio beta for two aerosol types: smoke aerosol, prevailing during the biomass burning season in the Amazon and the Cerrado in 1993, and predominately sulfate aerosol, measured during the Sulfate/Smoke Cloud and Radiation-Atlantic field experiment in the eastern United States in 1993. The beta determines the efficiency (per unit of optical thickness) of the aerosol particles to reflect radiation back to space and therefore to generate a negative radiative forcing of climate. The average value of beta, using the Wiscombe and Grams [1976] definition, beta(WG), varied between 0.20 and 0.28 for both aerosol types, as compared to beta(WG) = 0.29 used by Charlson. et al. [1992] and Penner et al. [1992] to calculate radiative forcing by sulfate and smoke aerosol, respectively. The variable beta(WG) is an average value of beta on all the illumination directions. However, high optical thicknesses occur in the Amazon and eastern United States during the period of July to September, when the solar elevation is high. For these months and the latitude range the actual average value of beta is 25% lower than beta(WG) for the same aerosol type. A combination of these two factors results in values of beta, and the corresponding aerosol direct radiative forcing, that are 30-50% lower from these estimates. The climate modeling of Kiehl and Briegleb [1993] uses actual computations of the sulfate aerosol hemispherical backscattering as a function of time and geographic position for a particle radius of 0.2 mu m, which corresponds very closely to the present results. The volume size distribution, used to derive beta, was obtained from sky radiances for scattering angle less than or equal to 40 degrees, assuming spherical homogeneous particles. It can be compared with values of beta derived from the whole sky almucantar radiance (scattering angle less than or equal to 140 degrees) that reflects the; true aerosol scattering phase function. The two values of beta did not differ significantly.