A description of the global sulfur cycle and its controlling processes in the National Center for Atmospheric Research Community Climate Model, Version 3

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 National Center for Atmospheric Research Commu nity Climate Model, Version 3. The analysis suggests that the seasonal cycl e of SO2 and SO42- are controlled by a complex Db 4 interplay between trans port, chemistry and deposition processes. The seasonal cycle of these speci es is not strongly controlled by temporal variations in emissions but by se asonal variations in volume of air processed by clouds, mass of liquid wate r serving as a site for aqueous chemistry, amount of oxidant available for the conversion from SO2 to SO42-, vertical transport processes, and deposit ion. A tagging of the sulfate by emission region (Europe, North America, As ia, and rest of world [ROW]), chemical pathway (gaseous versus in-cloud), a nd type of emissions (anthropogenic versus biogenic) is used to differentia te the balance of processes controlling the production and loading from thi s material. Significant differences exist in the destiny of SO2 molecules e mitted from the several regions. An SO2, molecule emitted from the ROW sour ce region has a much greater potential to form sulfate than one emitted fro m, for example, Europe. A greater fraction of the SO2 molecules is oxidized that originate from ROW compared with other areas, and once formed, the su lfate has a longer residence time (that is, it is not readily scavenged). T he yield of sulfate from ROW sources of SO2 is a factor of 4 higher than th at of Europe. A substantially higher fraction of the SO2 emitted over Europ e is oxidized to sulfate through the ozone pathway compared to other region s. The analysis suggests that there are significant differences in the vert ical distribution, and horizontal extent, of the propagation of sulfate emi tted from the several source regions. Sulfate from Asian source regions rea ches the farthest from its point of origin and makes a significant contribu tion to burdens in both hemispheres, primarily from plumes reaching out in the upper troposphere. Sulfate from other source regions tends to remain tr apped in their hemisphere of origin.