COHERENCE BETWEEN SEASONAL CYCLES OF DIMETHYL SULFIDE, METHANESULFONATE AND SULFATE IN MARINE AIR

THE effect of cloud condensation nuclei (CCN) in controlling cloud albedo and hence global climate has prompted questions about the factors that control CCN populations. Charlson et al. 1 suggested that marine phytoplankton might control CCN populations. The effect would be strongest over the oceans, where low stratiform clouds are common, and CCN are composed mostly of ammonium sulphate believed to be produced by atmospheric oxidation of gaseous dimethyl sulphide (DMS) emitted by phytoplankton, Here we present twenty months of data from a clean marine site at 40-degrees-S, which confirm the connection between atmospheric DMS and aerosol sulphur species that is central to the hypothesis of Charlson et al. 1. The relationships between DMS, aerosol sulphate and CCN are nonlinear, implying that at this site there would be significant nonlinearities in the climate feedback mechanism. In addition, our data on atmospheric sulphur species show a strong seasonal cycle, indicating that it should be possible to look for large, natural, seasonal variations in cloud albedo as a rigorous test of the Charlson et al. hypothesis.