A three-dimensional global model investigation of seasonal variations in the atmospheric burden of anthropogenic sulfate aerosols

Abstract

A global three-dimensional chemical transport model is used to investigate seasonal variations of anthropogenic sulfur in the troposphere. Particular emphasis is placed on detailed comparisons of the modeled surface sulfur dioxide (SQ2) and sulfate (SO4) concentrations and sulfate wet deposition fluxes with measurements from the Eulerian Model Evaluation 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. Initial comparisons of model results with measurements reveal a systematic tendency of the model to overestimate SO2 concentrations and underestimate SO4 concentrations while producing a reasonable fit to measured wet deposition fluxes. Through a series of sensitivity tests we find that the addition of a nonphotochemical pathway for converting SO2 to SO4 in the boundary layer with a pseudo first-order rate of constant of 1-2 x 10-6S-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 heterogeneous reactions between SO2 and atmospheric aerosols that typically are not included in models of the atmospheric sulfur cycle. Despite the vastly improved simulation of surface SO2 and SO4 when this hypothetical heterogeneous oxidation pathway is included, the model is unable to simultaneously simulate the large seasonal cycle in surface SO4 observed over eastern North America and the almost total absence of a seasonal cycle in surface SO4 over Europe. The seasonal cycles in model-predicted column SO4 burdens are similar, but not identical, to those for surface SO4 because of regional differences in transport, free tropospheric oxidation, and in-cloud removal. We find that the summer-to-winter ratio in column SO4 is larger over eastern North America than it is over Europe; however, both are larger than that for eastern Asia, where wintertime column SO4 is predicted to exceed summertime column SO4.