A global three-dimensional model of tropospheric sulfate

Abstract

A three-dimensional model is used to simulate the global tropospheric distributions of dimethylsulfide (DMS), SO2, SO42−, and methanesulfonic acid (MSA). The model uses meteorological input from a general circulation model (GCM) developed at the Goddard Institute of Space Studies (GISS) with 4° × 5° horizontal resolution, nine layers in the vertical, and a time resolution of 4 hours. Model results are compared with observations from surface sites, ships, and aircraft. The model reproduces generally to within 30% the observed SO2 and SO42− concentrations over the United States and Europe; these concentrations are highly sensitive to the supply of H2O2 as an in-cloud SO2 oxidant. Sulfate concentrations and wet deposition fluxes observed at remote marine sites can be accounted for using a global DMS source of 22 Tg S yr−1 in the model. However, this source overestimates DMS air concentrations by a factor of 2 unless we assume the presence of another DMS oxidant besides OH and NO3. Inclusion of another DMS oxidant in our model also improves the simulation of the MSA to SO42− concentration ratio in marine air. Simulated SO42− concentrations in the northern hemispheric free troposphere are much lower than in previous global models and are more consistent with the few observations available. The difference reflects in part our accounting of efficient scavenging of SO2 and SO42− in wet convective updrafts. Global mean tropospheric lifetimes computed in our model are 1.0 days for DMS, 1.2 days for SO2, 3.9 days for SO42−, and 6.2 days for MSA. Fossil fuel combustion and industrial activities represent 68% of global non-sea-salt sulfur emissions. About 50% of SO2 globally is converted to SO42− aerosol (principally by in-cloud oxidation) while the remainder is removed by deposition (30% by dry, 20% by wet). In-cloud oxidation of SO2 represents 85% of the global SO42− source.