A three-dimensional study of the tropospheric sulfur cycle

The global tropospheric distributions of seven important sulfur species were simulated with a global three-dimensional chemistry-transport model (IMAGES). Surface emission and deposition velocity maps were established for use as lower boundary conditions in the model. While anthropogenic SO2 emissions are by far the largest sulfur source in the northern midlatitudes, reduced sulfur compounds, notably dimethyl sulfide (DMS) predominate over most remote areas. Simulations were performed for the present-day (similar to 1985) atmosphere. The calculated distributions are compared with available observations. The model results are found to be generally within a factor of (at most) 2-3 of long-term observations. Comparison with campaign measurements is more difficult, mostly due to the strong dependence of sulfur species concentrations on local meteorological conditions. The results, however, indicate the need for future model refinements, especially with respect to biogenic emission estimates and parameterization of cloud processes. A sensitivity study is presented to discuss the uncertainties of the results related to several parameters (the decoupling of wet scavenging and convective transport for soluble species, volcanoes emission and deposition velocities). Results are also discussed in terms of global budgets and related variables and processes. Around 125 Tg S/yr of non-sea-salt (nss) sulfur compounds (DMS, CS2, H2S, COS, and SO2) are injected into the atmosphere. The balance is mainly maintained by nss-sulfates wet and dry deposition, and by SO2 dry deposition (94% of total sulfur deposition). It is found that DMS oxidation represents the main contribution to SO2 chemical production (80% of the chemical sources), and that the major sink of SO2 is provided by in-cloud oxidation (90% of the chemical sinks), under the assumption that all SO2 incorporated into clouds is oxidized. The calculated annual wet deposition of sulfates reaches 3 g S m(-2) yr(-1) over Europe and North America, while it is usually lower than 0.5 g S m(-2) yr(-1) in remote parts of the world. Estimations for the global lifetimes are 0.9 day for DMS, 4 days for CS2, 2.2 days for H2S, 0.6 day for SO2, 0.18 day for DMSO, 6.1 days for MSA, and 4.7 days for nss-sulfates.