Removal of sulphur from the marine boundary layer by ozone oxidation in sea-salt aerosols

THE oxidation of sulphur dioxide to sulphate in the marine boundary layer (MBL) is an important pathway in the global sulphur cycle. Oxidation by ozone in the aqueous phase is an important process in cloud droplets but has not generally been thought to be significant in the clear air of the MBL. Yet the lower part of the MBL contains abundant sea-salt aerosol particles, which are largely water of sufficiently high pH (ref. 2) to support ozone oxidation of SO2 to sulphate. We have argued previously that 5–25% of the total non-sea-salt sulphate (n.s.s. SO^(2−)_4) observed in the MBL may be formed by this mechanism; here we assess its contribution to the cycling of sulphur in (and particularly its removal from) the MBL. We show that, owing to the effects of mass transfer, the n.s.s. SO^(2−)_4 so generated will be predominantly associated with particles of 2–9 μm diameter, and will accordingly dry-deposit at a rapid rate. Because part of the dimethyl sulphide (DMS) emitted by marine organisms is converted to SO_2 in the MBL, this additional removal pathway for sulphur may markedly reduce the proposed feedback between greenhouse warming, oceanic DMS emissions and sulphate haze albedo.