Attribution of modeled atmospheric sulfate and SO 2 in the Northern Hemisphere for June – July 1997

Anthropogenic sulfate aerosol is a major contributor to shortwave radiative forcing of climate change by direct light scattering and by perturbing cloud properties and to local concentrations of atmospheric particulate matter. Here we analyze results from previously published calculations with an Eulerian transport model for atmospheric sulfur species in the Northern Hemisphere in June–July, 1997 to quantify the absolute and relative contributions of specific source regions (North America, Europe, and Asia) and SO 2-to-sulfate conversion mechanisms (gas-phase, aqueous-phase and primary sulfate) to sulfate and SO 2 column burdens as a function of location and time. Although material emitted within a given region dominates the sulfate and SO 2 column burden in that region, examination of time series at specific locations shows that material imported from outside can make a substantial and occasionally dominant contribution. Frequently the major fraction of these exoge-nous contributions to the sulfate column burden was present aloft, thus minimally impacting air quality at the surface, but contributing substantially to the burden and, by implication , to radiative forcing and diminution of surface irradi-ance. Although the dominant sulfate formation pathway in the domain as a whole is aqueous-phase reaction in clouds (62%), in regions with minimum opportunity for aqueous-phase reaction gas-phase oxidation is dominant, albeit with considerable temporal variability depending on meteorological conditions. These calculations highlight the importance of transoceanic transport of sulfate, especially at the western margins of continents under the influence of predominantly westerly transport winds.