We examine the balance between processes that contribute to the global and regional distributions of sulfate aerosol in the Earth's atmosphere using a set of simulations from the NCAR CCM3. The analysis suggests that the seasonal cycle of SO2 and sulfate are controlled by a complex interplay between transport, chemistry and deposition processes. The seasonal cycle of these species are not strongly controlled by temporal variations in emissions, but by seasonal variations in volume of air processed by clouds, mass of liquid water serving as a site for aqueous chemistry, amount of oxidant available for the conversion from SO2 to sulfate, vertical transport processes, and deposition. A tagging of the sulfate by emission region (Europe, N. America, Asia, and rest of world [ROW]), chemical pathway (gaseous vs. in-cloud), and type of emissions (anthropogenic vs. biogenic) is used to differentiate the balance of processes controlling the production and loading from this material. Significant differences exist in the destiny of SO2 molecules emitted from the several regions. An SO2 molecule emitted from the ROW source region has a much greater potential to form sulfate than one emitted from, for example, Europe. A greater fraction of the SO2 molecules that originate from ROW is oxidized compared with other areas, and once formed, the sulfate has a longer residence time (that is, it is not readily scavenged). The yield of sulfate from ROW sources of SO2 is a factor of four higher than that of Europe. A substantially higher fraction of the SO2 emitted over Europe is oxidized to sulfate through the ozone pathway compared to other regions. The analysis suggests that there are significant differences in the vertical distribution, and horizontal extent, of the propagation of sulfate emitted from the several source regions. Sulfate from Asian source regions reaches the farthest from its point of origin, and makes a significant contribution to burdens in both hemispheres, primarily from plumes reaching out in the upper troposphere. Sulfate from other source regions tends to remain trapped in its hemisphere of origin.
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