SULFUR ISOTOPE MEASUREMENTS OF SUBMICROMETER SULFATE AEROSOL-PARTICLES OVER THE PACIFIC-OCEAN

The climatic importance of atmospheric submicrometer sulfate aerosol particles necessitates a thorough understanding of the various sources of sulfur to the marine atmosphere [Charlson et al., 1987; Coakley et al., 1987; Charlson et al., 1990]. Over the remote ocean, the primary source of atmospheric non-seasalt (NSS) sulfate is thought to be marine emissions of dimethylsulfide (DMS) [Charlson et al., 1987]. Here we report the results of a study which used stable sulfur isotopes to test this hypothesis. NSS sulfate in submicrometer aerosol particles over remote parts of the South Pacific Ocean had an average delta-S-34 value of +15.6 +/- 3.1 part-per thousand, relative to the Canyon Diablo standard. The sulfur isotopic composition of these particles was 5.5 part-per thousand lighter than sulfate derived directly from seawater, but isotopically heavier than sulfate derived from most continental sources. This observed isotopic fractionation between seawater sulfate and NSS sulfate is consistent with the isotopic fractionation predicted for the transformation of seawater sulfate to atmospheric NSS sulfate via a DMS pathway (-1 to +7 parts per thousand) and hence supports the hypothesis that DMS can be the source of the NSS sulfate.