Sulfur dioxide and dimethyl sulfide in marine air at Cape Grim, Tasmania

Results from one long-term study and one summer-time field campaign in which the sulfur dioxide (SO₂) mixing ratio was determined in mid-to-high latitude marine air at Cape Grim, Tasmania, are presented and analysed to provide a perspective on the potential role of SO₂ in the production of aerosol sulphate at this site. Sulfur dioxide at Cape Grim has a distinct seasonal cycle, with a maximum mixing ratio in summer and minimum in winter, consistent with a similar cycle observed previously at Cape Grim for dimethyl sulphide (CH₃SCH₃; DMS) and its other oxidation products. Concentrations of SO₂ in the summer were typically 10 ppt (pmole mole^-1), which is considerably lower than those found at other sites subject to marine air more equatorial in origin. A modelling study suggests that differences between equatorial and mid-to-high latitude marine SO₂ concentrations result from lower levels of the DMS precursor at mid latitudes and a lower rate of SO₂ production at lower temperatures. The analysis also suggests the existence of an additional non-DMS source of boundary layer sulfur, relatively more important in winter when DMS levels are low. Entrainment from the lower free troposphere appears to be one likely source.