Atmospheric sulfur cycle simulated in the global model GOCART: Comparison with field observations and regional budgets

We present a detailed evaluation of the atmospheric sulfur cycle simulated in the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Tr ansport (GOCART) model. The model simulations of SO2, sulfate? dimethylsulf ide (DMS), and methanesulfonic acid (MSA) are compared with observations fr om different regions on various timescales. The model agrees within 30% wit h the regionally averaged sulfate concentrations measured over North Americ a and Europe but overestimates the SO2 concentrations by more than a factor of 2 there. This suggests that either the emission rates are too high, or an additional loss of SO2 which does not lead to a significant sulfate prod uction is needed. The average wintertime sulfate concentrations over Europe in the model are nearly a factor of 2 lower than measured values, a discre pancy which may be attributed largely to the sea-salt sulfate collected in the data. The model reproduces the sulfur distributions observed over the o ceans in both long-term surface measurements and short-term aircraft campai gns. Regional budget analyses show that sulfate production from SO2 oxidati on is 2 to 3 times more efficient and the lifetimes of SO2 and sulfate are nearly a factor of 2 longer over the ocean than over the land. This is due to a larger free tropospheric fraction of SO2 column over the ocean than ov er the land, hence less loss to the surface. The North Atlantic and northwe stern Pacific regions are heavily influenced by anthropogenic activities? w ith more than 60% of the total SO2 originating from anthropogenic sources. The average production efficiency of SO2 from DMS oxidation is estimated at 0.87 to 0.91. in most oceanic regions.