Comparison of oceanic and continental sources of non-sea-salt sulphate over the Pacific Ocean

Abstract

THE absence of long-term systematic measurements of sulphur species over the oceans has made it difficult to estimate the relative importance of what are believed to be the two major sources of non-sea-salt (n.s.s.) sulphate: reduced sulphur gases produced by biological activity in the oceans and sulphur compounds derived from continental anthropogenic sources. Here we summarize the results of aerosol sulphate measurements made over five- to seven-year periods from 1981 to 1987 at island stations in the Pacific Ocean (Fig. 1). The mean methanesulphonate (MSA)/n.s.s. SO₄²⁻ mass ratio, 0.065, at the relatively pristine sites on Fanning and American Samoa is believed to result solely from the atmospheric oxidation of reduced sulphur gases, primarily dimethylsulphide (DMS), emitted from the ocean. Estimates based on this ratio and mean MSA concentrations indicate that the biogenic source accounts for ∼80% of the annual average n.s.s. SO₄²⁻ over the mid-latitude North Pacific. Thus, if n.s.s. SO₄²⁻ is the primary source of cloud condensation nuclei over the ocean, then the nuclei concentrations over the Pacific appear to be largely controlled by biological sources in the ocean.