Anthropogenic and natural contributions to tropospheric sulfate: A global model analysis

Abstract

A global three-dimensional model is used to examine the export of anthropogenic sulfur from northern midlatitude continents and to assess the relative importance of anthropogenic and natural sources to sulfate levels in different regions of the troposphere. Model results indicate that about 40% of the anthropogenic sulfur emitted in the United States, Europe, or eastern Asia is exported out of the continental boundary layer of these regions; the rest is removed within the regions, primarily by dry deposition of SO2 and wet deposition of SO42−. Export is relatively more efficient in summer than in winter. There is little nonlinearity between the magnitude of sulfur emissions in the northern midlatitude continents and the export of this sulfur to the global atmosphere. Anthropogenic influence on SO42− decreases rapidly with altitude because of efficient scavenging of SO2 and SO42− in deep convective updrafts. Thus it is found that anthropogenic influence accounts on average for less than 20% of SO42− anywhere in the upper troposphere. The main source of SO42− in the tropical upper troposphere in the model is from biogenic dimethylsulfide (DMS) pumped in deep convective events. Volcanic emissions account for 20–40% of SO42− in much of the middle troposphere and for up to 80% over the North Pacific; they also represent a major contributor to SO42− in the upper troposphere at high latitudes. On a global scale, it is estimated that anthropogenic, biogenic, and volcanic emissions account for 70%, 23%, and 7%, respectively, of the global sulfur source, but that they account for 37%, 42%, and 18%, respectively, of the global column of atmospheric SO42−. The disproportionality between source and column contribution reflects the rapid deposition of anthropogenic SO2 and SO42− at low altitudes. It thus appears that the anthropogenic contribution to the SO42− aerosol optical depth is much less than would be expected simply on the basis of emissions.