A global model of the coupled sulfur/oxidant chemistry in the troposphere: The sulfur cycle

Abstract

A sulfur cycle chemistry scheme with dimethyl sulfide (DMS), SO2, sulfate, H2S, and methanesulfonic acid (MSA) is included in the OsloCTM2 model, and concentrations of sulfur are calculated interactively with the oxidant chemistry. This allows more consistent estimates of aqueous phase oxidation of SO2 to sulfate by O3, H2O2, and HO2NO2. The year 1996 is chosen as the standard, and a model run with 1996 meteorology and emissions is compared with 1996 observations. The results agree well with observations overall, although the model tends to overestimate SO2 and underestimate sulfate in Northern Hemisphere winter owing to an oxidation limitation. A global budget for 1996 quantifying the various processes is investigated. Our model results give a global lifetime (global burden) of 1 day (0.25 Tg(S)) and 3.8 days (0.5 3 Tg(S)) for SO2 and sulfate. Differences between the Southern Hemisphere, characterized by natural emissions and by loss of SO2 by O3 and H2O2 oxidation, and the Northern Hemisphere, characterized by anthropogenic emissions and by large loss by dry deposition, are revealed. Significant changes in sulfur emissions have occurred over the last decades with decrease in the Unites States and Europe and increase in Southeast Asia. U.S., European, and Chinese SO2 emissions have changed by −17.6%, −47.5%, and +93%, respectively. To study the impact of emission changes on the atmospheric composition, we have calculated distributions using the Global Emissions Inventory Activity (GEIA) 1985 inventory. The changes in sulfur emissions have significant changes on the sulfur concentrations and also some effect upon the oxidants. Increased emissions of NOx and hydrocarbons in China enhance O3, but increased sulfur inhibit the increase. The SO2 oxidation by OH, which can lead to formation of new sulfate particles, is given special attention. The model run using GEIA 1985 anthropogenic emission inventory is compared with other model studies.