Sulfur chemistry has been incorporated in the NCAR CCM in an internally consistent manner with other parameterizations in the model. The model calculates mixing ratios of DMS, SO2, sulfate, and H2O2 and cloud and rain water pH. Processes that control the mixing ratio of these species include the emissions of DMS and SO2, transport of each species (through resolved-scale advection and subgrid-scale convective and diffusive processes), gas and aqueous phase chemistry, and wet deposition and dry deposition of species. Modeled concentrations agree quite well with observations for DMS and H2O2, and fairly well for SO2 and sulfate, although the modeled sulfate tends to underestimate observed sulfate in Europe. The SO2 and sulfate species were tagged according to the chemical production pathway and whether the sulfur was of anthropogenic or biogenic origin. Although aqueous-phase reaction in cloud accounted for 81% of the sulfate production rate, only about 50-60% of the sulfate burden in the troposphere was derived from cloud chemistry. Anthropogenic sulfate accounts for 74% of the total sulfate burden globally. Because cloud chemistry is an important source of sulfate in the troposphere, the importance of H2O2 concentrations and pH values was investigated. By prescribing H2O2 concentrations to clear sky values instead of predicting H2O2, the global-averaged, annual-averaged in-cloud production of sulfate increased by 16.5%. This increased in-cloud sulfate production decreased the global sulfate burden by 7%. Larger changes were noted in the industrial regions of the northern midlatitudes. Setting the pH of the drops to 4.5, increased the in-cloud production of sulfate by 2.4% (increasing the rate in industrial regions and decreasing the rate in more remote regions of the world). Again the increased in-cloud production rate decreased the global sulfate burden (by 4%). Larger changes were noted regionally, especially in eastern Europe. In both sensitivity simulations, increased in-cloud production of sulfate decreased the burden of sulfate because less SO2 was available for gas-phase conversion, which contributes more efficiently to the tropospheric sulfate burden than does aqueous-phase conversion.
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