THE ATMOSPHERIC SULFUR CYCLE IN ECHAM-4 AND ITS IMPACT ON THE SHORTWAVE RADIATION

The atmospheric general circulation model ECHAM-4 is coupled to a chem istry model to calculate sulfate mass distribution and the radiative f orcing due to sulfate aerosol particles. The model simulates the main components of the hydrological cycle and, hence, it allows an explicit treatment of cloud transformation processes and precipitation scaveng ing. Two experiments are performed, one with pre-industrial and one wi th present-day sulfur emissions. In the pre-industrial emission scenar io SO2 is oxidized faster to sulfate and the in-cloud oxidation via th e reaction with ozone is more important than in the present-day scenar io. The atmospheric sulfate mass due to anthropogenic emissions is est imated as 0.38 Tg sulfur. The radiative forcing due to anthropogenic s ulfate aerosols is calculated diagnostically. The backscattering of sh ortwave radiation (direct effect) as well as the impact of sulfate aer osols on the cloud albedo (indirect effect) is estimated. The model pr edicts a direct forcing of -0.35 Wm(-2) and an indirect forcing of -0. 76 W m(-2) Over the continents of the Northern Hemisphere the direct f orcing amounts to -0.64 W m(-2). The geographical distribution of the direct and indirect effect is very different. Whereas the direct forci ng is strongest over highly polluted continental regions, the indirect forcing over sea exceeds that over land. It is shown that forcing est imates based on monthly averages rather than on instantaneous sulfate pattern overestimate the indirect effect but have little effect on the direct forcing.