Wet deposition of non sea-salt sulphate in the United Kingdom: The influence of natural sources

Because of its position to the west of Europe, much of the wet sulphur deposition in the west of the UK is background in the sense that it is not attributable to pollutants emitted within Europe less than four days previously. There are both natural and anthropogenic sources of this sulphur. An important natural source, especially during the summer, is dimethylsulphide (DMS) produced by marine phytoplankton. To identify the contribution of marine biogenic sulphur we have measured stable sulphur isotope ratios in precipitation. We show that biogenic sulphur is significant in summer but contributes little in winter and that around 5–10% of the annual background wet sulphur deposition is due to biogenic sources. During July and December 1993, airflow across the UK was predominantly westerly. The measured biogenic component of precipitation sulphate accounted for around 30 % of background sulphate in July but was negligible in December. Investigation of five day back-trajectories for the period indicated little opportunity for re-circulation of European emissions, suggesting that other (non-DMS) natural sources and non-European anthropogenic emissions were responsible for most of the background sulphur.