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

Sulfur chemistry has been incorporated in the National Center for Atmospheric Research Community Climate Model in an internally consistent manner with other parameterizations in the model. The model predicts mixing ratios of dimethylsulfide (DMS), SO₂, SO₄^2-, H₂O₂. Processes that control the mixing ratio of these species include the emissions of DMS and SO₂, transport of each species, gas- and aqueous-phase chemistry, wet deposition, and dry deposition of species. Modeled concentrations agree quite well with observations for DMS and H₂O₂, fairly well for SO₂, and not as well for SO₄^2-. The modeled SO₄^2- tends to underestimate observed SO₄^2- at the surface and overestimate observations in the upper troposphere. The SO₂ and SO₄^2- 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 ∼50-60% of the sulfate burden in the troposphere was derived from cloud chemistry. Because cloud chemistry is an important source of sulfate in the troposphere, the importance of H₂O₂ concentrations and pH values was investigated. When prescribing H₂O₂ concentrations to clear-sky values instead of predicting H₂O₂, the global-averaged, annual-averaged in-cloud production of sulfate increased. Setting the pH of the drops to 4.5 also increased the in-cloud production of sulfate. In both sensitivity simulations, the 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.