Sulfur chelllistry in the National Center for Atlllospheric Research Community Climate Model : Description , evaluation , features , and sensitivity to aqueous chelllistry

Sulfur chemistry lIas been incorporated in the National Center for Atmospheric Research Community Climate Model in an internally consisterlt manner with other parameterizations in the nlodel. The model predicts mixing ratios of dimethylsulfide (DMS), S02, SO~-, H20 2. Processes tllat control the mixing ratio of these species include the emissions of DMS aIld S02, transport of each species, gasand aqueous-phase chemistry, wet deposition, and dry deposition of species. Modeled concentrations agree qllite well with observations for DMS and H20 2, fairly well for S02, and not as well for SO~-. The modeled SO~tends to lln(lerestimate observed SO~at the surface and overestimate observations in tIle upper troposphere. 'TIle S02 and SO~species were tagged according to the chemical production pathway and whether the sulfur was of anthropogenic or biogenic origin. Although aqueous-phase reactions in cloud accounted for 81% of the sulfate production rate, only f'.J50-60% of the sulfate burden in the troposphere was derived from cloud chemistry. Because cloud chenlistry is an important source of sulfate in the troposphere, the importance of H20 2 concentratioIls and pH values was investigated. When prescribing H20 2 concentrations to clear-sky values iIlstead _of predicting 1120 2, the global-averaged, annual-averaged ill-cloud production of sulfate increased. Setting the pH of the drops to 4.5 also increased the in-cloud production of slilfate. In both sensitivity simulations, the increased ill-cloud production of sulfate decreased the burden of sulfate because less S02 was available for gas-phase conversion, wIlich contributes more efficiently to the troposplleric sulfate burden than does aqueous-phase conversion.