The contribution of in-cloud oxidation of SO2 to wet scavenging of sulfur in convective clouds

The sensitivity of in-cloud oxidation of SO₂ in corrective clouds to a number of chemical and physical parameters is examined. The parameterization of precipitation growth processes is based on the work of Scott (1978) and Hegg (1983). A chemical model predicts gas and aqueous phase distributions of soluble gases and in-cloud uncatalyzed oxidation of SO₂ by O₃ and H₂O₂. Sulfate aerosol and SO₂, CO₂, NH₃, H₂O₂ and O₃ gases and their aqueous phase dissociation products are treated.The results indicate that in-cloud conversion is an important removal mechanism for SO₂ and accounts for a significant fraction of the precipitation sulfate. However, except at low SO₂ concentrations, the precipitation sulfate concentration is insensitive to the initial SO₂ concentration; the sulfate concentration is most sensitive to the initial H₂O₂ and NH₃ concentrations. At low SO₂ concentrations, the precipitation sulfate concentration is determined primarily by the initial sulfate aerosol concentration. The feedback between sulfate production and pH is important in limiting SO₂ oxidation by O₃. If gas phase H₂O₂ of order 1 ppb is the major source of aqueous phase H₂O₂ for S(IV) oxidation, it is likely that the oxidation reaction is oxidant limited. The sulfate concentration is a decreasing function of the precipitation rate. At low rainfall rates (< 1 mm h⁻¹), ice phase growth decreases the sulfate concentration. However, the results are insensitive to an ice phase origin at moderate and high rainfall rates.