Sulfur Chemistry in the NCAR CCM 1 Sulfur Chemistry in the NCAR CCM : Description , Evaluation , Features and Sensitivity to

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, SO 2 , SO 2? 4 , and H 2 O 2 and cloud and rain water pH. Processes that control the mixing ratio of these species include the emissions of DMS and SO 2 , transport of each species (through resolved-scale advection and subgrid-scale convective and diiusive processes), gas and aqueous phase chemistry, and wet deposition and dry deposition of species. Modeled concentrations agree quite well with observations for DMS and H 2 O 2 , and fairly well for SO 2 and SO 2? 4 , although the modeled SO 2? 4 tends to underestimate observed SO 2? 4 in Europe. The SO 2 and SO 2? 4 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 H 2 O 2 concentrations and pH values was investigated. By prescribing H 2 O 2 concentrations to clear sky values instead of predicting H 2 O 2 , 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 SO 2 was available for gas-phase conversion, which contributes more eeciently to the tropospheric sulfate burden than does aqueous-phase conversion. 2 Barth et al.