An isotopic study of atmospheric sulphur at three sites in Wales and at Mace Head, Eire

Sulphur isotope measurements made on aerosol and precipitation samples collected at three sites in Wales and Mace Head, Eire, between March 1993 and September 1994, had highest non‐sea‐salt sulphate δ34S values, up to 11.9‰ for aerosol and 9.7‰ for precipitation, during the summer months. Aerosol methanesulphonate shows a clear summer peak with values up to 3–4 nmol m−3. Assuming samples contained only sulphur from terrestrial/anthropogenic, sea salt, and marine biogenic (from the oxidation of dimethylsulphide) sources, we used the sulphur isotope values to determine the contribution of dimethylsulphide‐derived sulphate to total sulphate. In general the biogenic contribution was small, <10%, although in samples from marine air masses it was as high as 40%. A comparison of aerosol and precipitation δ34S values found no evidence for large isotopic fractionations during the oxidation of sulphur dioxide.