3-DIMENSIONAL MODELING OF THE GLOBAL ATMOSPHERIC SULFUR CYCLE - A 1ST STEP

Atmospheric sulfur is known to be a major source of condensation nuclei (CN). Over continents the major source of sulfur is believed to be SO2 emitted from industrial regions. Over remote marine regions, emission of biogenic dimethylsulfide (DMS) from the ocean surface is thought to be the major source of non-sea-salt sulfate and of CN. Here we apply a global three-dimensional model with a simplified treatment of the atmospheric sulfur cycle to address the question of whether anthropogenic sulfur transported from continents might impact marine CN concentrations over a significant fraction of the world's oceans. Processes treated in the model include transport, convective and eddy mixing, a simplified treatment of the conversion of SO2 and DMS to form the condensate SO42-, removal of SO2 and SO42- by dry deposition and wet deposition of SO42-. Simulations for January, when DMS emissions are largest in the Southern Hemisphere, indicate that natural sulfate aerosol derived from marine emissions of DMS dominates anthropogenic sulfate over the Southern Hemisphere oceans. However, anthropogenic sources dominate CN concentrations over much of the Northern Hemisphere oceans, especially the North Atlantic, as well as continental regions. During July, when DMS emissions are largest in the Northern Hemisphere, natural sulfate concentrations are generally larger than anthropogenic sulfate concentrations in both hemispheres, and concentrations in the Northern Hemisphere are much higher than in the Southern Hemisphere. These results suggest that the influence of industrial emissions of SO2 on marine CN concentrations, and therefore marine cloud albedo, may be restricted to the Northern Hemisphere winter, when solar insolation and oceanic DMS production are lowest. However, these results are from a highly simplified model, and must therefore be regarded as preliminary.