CLIMATE FORCING BY CARBONACEOUS AND SULFATE AEROSOLS

An atmospheric general circulation model is coupled to an atmospheric chemistry model to calculate the radiative forcing by anthropogenic su lfate and carbonaceous aerosols. The latter aerosols result from bioma ss burning as well as fossil fuel burning. The black carbon associated with carbonaceous aerosols is absorbant and can decrease the amount o f reflected radiation at the top-of-the-atmosphere. In contrast, sulfa te aerosols are reflectant and the amount of reflected radiation depen ds nonlinearly on the relative humidity. We examine the importance of treating the range of optical properties associated with sulfate aeros ol at high relative humidities and find that the direct forcing by ant hropogenic sulfate aerosols can decrease from -0.81 W m(-2) to -0.55 W m(-2) if grid box average relative humidity is not allowed to increas e above 90%. The climate forcing associated with fossil fuel emissions of carbonaceous aerosols is calculated to range from + 0.16 to + 0.20 W m(-2), depending on how much organic carbon is associated with the black carbon from fossil fuel burning. The direct forcing of carbonace ous aerosols associated with biomass burning is calculated to range fr om -0.23 to -0.16 W m(-2). The pattern of forcing by carbonaceous aero sols depends on both the surface albedo and the presence of clouds. Mu ltiple scattering associated with clouds and high surface albedos can change the forcing from negative to positive.