The role of the ocean in the global atmospheric sulfur cycle

Abstract

The background atmospheric SO2 concentration above the sea surface has been established at 0.1 × 10−6g/m3 (35 pptv). Several hypotheses to explain the origin of SO2 above the ocean, far from continental sources, have been discussed. We have shown that one of the mechanisms of SO2 formation is the biogenic production of reduced organic sulfur compounds in the ocean. Dimethyl sulfide (DMS) has been observed in oceanic areas, with concentrations ranging from 11 to 1150 × 10−9g l−1 in surface waters and from 0.2 to 400 × 10−9g m−3 (0.07 to 144 pptv) in marine atmosphere. This compound is emitted into the atmosphere and oxidized there by photochemically induced processes. That leads partly to SO2 formation, and subsequently, to sulfate. This hypothesis is corroborated by a correlation between atmospheric SO2 concentrations and the relation between atmospheric SO2 concentrations and biological activity. The flux of sulfur by this process (biological activity → organic sulfides → SO2 → SO2=) is estimated to range between 27 and 72×106 tons of sulfur per year. This biogenic production is comparable to atmospheric sulfur production by sea spray, which is about 44×106 T S/yr. Therefore, the oceanic production by these two processes could be estimated as being from 71 to 116×106 T S/yr.