Tropospheric sulfur simulation and sulfate direct radiative forcing in theGoddard Institute for Space Studies general circulation model

Global simulations of tropospheric sulfur are performed in the Goddard Inst itute for Space Studies (GISS) general circulation model (GCM) and used to calculate anthropogenic sulfate direct radiative forcing, Prognostic specie s are in cloud oxidant H2O2, dimethylsulfide (DMS), methanesulfonic acid (M SA), SO2 and sulfate, Compared with most previous models (except others wit h prognostic H2O2), this model has relatively high anthropogenic SO2 and su lfate burden, We show that this is due partly to the depletion of the progn ostic H2O2 and that moist convection delivers significant levels of SO2 to the free troposphere in polluted regions. Model agreement with surface obse rvations is not remarkably different from previous studies. Following some previous studies, we propose that an additional in-cloud or heterogeneous o xidant is Likely to improve the simulation near the surface. Our DMS source is lower than sources in previous studies, and sulfur values in remote reg ions are generally lower than those observed. Because of the high flux of S O2 to the free troposphere and the relatively low natural source, our model indicates a larger global anthropogenic contribution to the sulfate burden (77%) than was estimated by previous global models. Additional high-altitu de observations of the sulfur species are needed for model validation and r esolution of this issue. Direct radiative forcing calculations give an annu al average anthropogenic sulfate forcing of -0.67 W/m(2) We compare the rad iative forcings due to online (hourly varying) versus offline (monthly aver age) sulfate and find Little difference on a global average, but we do find differences as great as 10% in some regions, Thus, for example, over some polluted continental regions the forcing due to offline sulfate exceeds tha t of online sulfate, while over some oceanic regions the online sulfate for cing is larger, We show that these patterns are probably related to the cor relation between clouds and sulfate, with positive correlations occuring ov er some polluted continental regions and negative correlations over high-la titude oceanic regions.