Photochemical production of carbonyl sulphide in marine surface waters

Abstract

Carbony1 sulphide (COS) has been proposed as the major source of the stratospheric sulphate layer in times of low volcanic activity¹ because of its long tropospheric residence time², its uniform distribution in both the Northern and Southern Hemispheres³, and the lack of knowledge of any significant sinks except photodissociation in the stratosphere⁴. Known sources of COS include biomass burning⁵, volcanoes^6,7, salt marshes^2,8, fossil fuel combustion⁹, and oxidation of atmospheric CS₂ (refs 10,11). Also, recent measurements^12–14 have shown the surface oceans to be a source of COS. The total flux to the atmosphere from these known sources is substantially greater than the flux to the stratosphere needed to maintain the sulphate layer¹⁰. Thus questions remain about the sources and sinks of COS, and we have now performed a series of experiments in nearshore ocean and estuarine waters to investigate further the oceanic source of this gas. Dissolved COS in near-surface waters was found to be supersaturated with respect to atmospheric COS, and the dissolved concentrations varied diurnally with light intensity. Similar behaviour has been observed by other investigators for carbon monoxide, which is produced photochemically in surface seawater¹⁵. Furthermore, COS production appears to be independent of salinity, photosynthetic activity and microbial activity, and is probably the result of photooxidation of organic matter. COS was produced in the laboratory by UV-irradiation of seawater and solutions of several biochemically relevant organosulphur compounds.