Multi-Phase Processes in the Atmospheric Sulfur Cycle

Sulfur enters the atmosphere from natural as well as anthropogenic sources (Table 1). An overview of the atmospheric sulfur cycle is given in Figure 1. Natural sources generally emit reduced sulfur compounds, for example H2S (hydrogen sulfide), CS2 (carbon disulfide), OCS (carbonyl sulfide) and CH3SCH3 (dimethyl sulfide, DMS). H2S, a product of sulfate reduction by bacteria in anaerobic environments such as swamps and marshes, has a average tropospheric volume mixing ratio of 20–50 pptv (Delmas et al., 1980; Herrmann and Jaeschke, 1984). DMS is also a volatile product from biological activity, predominantly of marine phytoplankton. The mixing ratio of DMS is of order 500–150 pptv (Maroulis and Bandy, 1977; Andreae and Raemdonck,1983). CS2 is probably largely of anthropogenic origin (Turco et al., 1979), although comparatively little is known about this gas. The CS2 mixing ratio in background air is approximately 150–40 pptv (Maroulis and Bandy, 1980; Jones et al., 1983). The “background” troposphere, the main focus of this paper, is defined as the part of the troposphere that is not directly affected by anthropogenic emissions of trace compounds, a predominantly marine environment.