WET DEPOSITION OF NON-SEA-SALT SULFATE 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 i s not attributable to pollutants emitted within Europe less than four days previously. These are both natural and anthropogenic sources of t his sulphur. An important natural source, especially during the summer , is dimethylsulphide (DMS) produced by marine phytoplankton. To ident ify the contribution of marine biogenic sulphur we have measured stabl e sulphur isotope ratios in precipitation We show that biogenic sulphu r is significant in summer but contributes little in winter and that a round 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 precip itation sulphate accounted for around 30% of background sulphate in Ju ly but was negligible in December Investigation of five day back-traje ctories for the period indicated little opportunity for re-circulation of European emissions, suggesting that other (non-DMS) natural source s and non-European anthropogenic emissions were responsible for most o f the background sulphur.