A 3-DIMENSIONAL GLOBAL-MODEL INVESTIGATION OF SEASONAL-VARIATIONS IN THE ATMOSPHERIC BURDEN OF ANTHROPOGENIC SULFATE AEROSOLS

A global three-dimensional chemical transport model is used to investi gate seasonal variations of anthropogenic sulfur in the troposphere. P articular emphasis is placed on detailed comparisons of the modeled su rface sulfur dioxide (SO2) and sulfate (SO4) concentrations and sulfat e wet deposition fluxes with measurements from the Eulerian Model Eval uation Field Study (EMEFS) and Cooperative Program for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe (EMEP) field programs in North America and Europe, respectively. Initi al comparisons of model results with measurements reveal a systematic tendency of the model to overestimate SO2 concentrations and underesti mate SO4 concentrations while producing a reasonable fit to measured w et deposition fluxes. Through a series of sensitivity tests we find th at the addition of a nonphotochemical pathway for converting SO2 to SO 4 in the boundary layer with a pseudo first-order rate of constant of 1-2 x 10(-6) s(-1) provides the most reasonable method of bringing the model results into better agreement with the EMEFS and EMEP data sets . We propose that this additional pathway may be related to heterogene ous reactions between SO2 and atmospheric aerosols that typically are not included in models of the atmospheric sulfur cycle. Despite the va stly improved simulation of surface SO2 and SO4 when this hypothetical heterogeneous oxidation pathway is included, the model is unable to s imultaneously simulate the large seasonal cycle in surface SO4 observe d over eastern North America and the almost total absence of a seasona l cycle in surface SO4 over Europe. The seasonal cycles in model-predi cted column SO4 burdens are similar, but not identical, to those for s urface SO4 because of regional differences in transport, free troposph eric oxidation, and in-cloud removal. We find that the summer-to-winte r ratio in column SO4 is larger over eastern North America than it is over Europe; however, both are larger than that for eastern Asia, wher e wintertime column SO4 is predicted to exceed summertime column SO4.