Effect of black carbon and sulfate aerosols on the global radiation budget

Global fields of radiative forcing due to natural and anthropogenic sulfate aerosols, black carbon aerosols, and an external mixture of the two have been calculated with a one-dimensional radiative transfer model developed for estimates of the direct radiative forcing by aerosols. Estimates of solar radiative forcing by different aerosols are presented for January and July, based on calculated three-dimensional, global distributions of sulfate and black carbon mass. We show that the radiative forcing by sulfate is negative, as already known, while the forcing due to black carbon aerosols is mainly positive. Considering both black carbon and sulfate together and assuming an external mixture, we calculate a globally averaged radiative forcing of approximately -0.2 W/m2, with a quite nonuniform geographical distribution. The radiative forcing due to aerosols is highly dependent upon the optical properties of the aerosol, while the surface reflectance and the Sun angle influence the direction of the forcing. Our results show that the presence of black carbon, the main absorbing component of anthropogenic aerosol, may reduce the cooling effect of aerosol, thus leading to an increase in the greenhouse warming