Distribution of atmospheric sulphur species over various wetland regions in the southeastern U.S.A.

Atmospheric dimethylsulphide (DMS), sulphur dioxide (SO₂), aerosol non-seasalt sulphate (nss-SO₄ and methanesulphonate (MSA) were measured periodically at Sapelo Island, Georgia, during March-April 1989 and April-May 1990. The spring 1990 measurements also included the sulphur gases hydrogen sulphide (H₂S), carbonyl sulphide (COS) and carbon disulphide (CS₂). In August 1989 single measurements of these compounds were also conducted in various natural environments of southern Louisiana (coastal waters, saltwater marsh, brackish/freshwater marsh, swamp). The median DMS concentration over Sapelo Island was significantly higher in April-May 1990 (92 ppt) than in March-April 1989 (18 ppt) due to enhanced biogenic DMS emission in spring. Atmospheric DMS levels increased sharply (up to 560 ppt) during advection of polluted air from paper mills located on the coastal mainland. Results obtained from measurements in Louisiana suggest that biogenic sulphur fluxes from soils and/or vegetation are significantly enhanced durign rainfall. H₂S concentrations were highly variable at each study site. Corresponding values ranged consistently higher over swamps and marshes (300-820 ppt) compared to tidal flats (3-510 ppt). Generally, DMS was the dominant biogenic sulphur gas emitted from coastal environments while further inland H₂S was the dominant species. Over all environments studied the relative contribution from natural sulphur sources to atmospheric nss-SO₄ levels ranged between approximately 15 and 50% during the individual measurement periods. Major contributions were made from regional anthropogenic sources and, on the southeastern Atlantic coast, from additional long-range transport of air masses from Africa and Europe. The effects of these sources on aerosol and rainwater acidity levels over each region are discussed.