Effect of black carbon on the optical properties and climate forcing of sulfate aerosols

We study the optical properties of anthropogenic sulfate aerosols containing black carbon using a recently developed exact solution of the scattering problem for a spherical particle (sulfate aerosol) containing an eccentrically located spherical inclusion (black carbon). We present the expression for the change of planetary albedo due to addition of an absorbing, but optically thin aerosol layer and estimate the effect of the black carbon within the sulfate aerosol layer on the aerosol direct radiative forcing. The black carbon within the sulfate aerosol reduces the expected sulfate direct cooling effect by about 0.034 W/m 2 for each 1% of the black carbon to sulfate mass mixing ratio. Thus the presence of black carbon within sulfate in the background aerosol does not significantly change the previous estimates of the global aerosol direct cooling effect. However, in regions where the black carbon in sulfate concentrations are of the order of 5% or more, the local and regional effects are significant. Briegleb, 1993) suggest that the effect of anthropogenic sulfates and aerosols produced by tropical biomass burning may be sufficient to counteract any possible warming trend due to an increase of atmospheric greenhouse gases generated since the beginning of the industrial revolution. The direct cooling effect of aerosols will be reduced by the presence of black carbon. Black carbon (soot) within aerosol particles increases the absorption of solar radiation and de- creases the reflection (albedo). In this paper we (1) calculate the optical properties of sul- fate aerosol containing black carbon using a recently devel- oped exact solution of the scattering problem by a spherical particle (sulfate aerosol) containing an eccentrically located spherical inclusion (black carbon), (2) present the expression for the change of planetary albedo due to the addition of an absorbing aerosol layer, and (3) estimate the effect of the black carbon within a sulfate aerosol layer on the aerosol direct radiative forcing.