NATURAL SULFUR SPECIES FROM THE NORTH-ATLANTIC AND THEIR CONTRIBUTION TO THE UNITED-KINGDOM SULFUR BUDGET

Measurements of dimethyl sulphide (DMS) and its oxidation products, methane sulphonic acid (MSA), sulphate, dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO2), and sulphur dioxide (SO2) in the atmosphere have been made during all seasons of 1988/1990 at coastal sites in northwest Scotland and Eire. The annual average air concentration of DMS was estimated from results obtained on 92 days in 1989/1990 to be 58 ng (S) m-3 and that of MSA to be 6 ng (S) m-3. Backward air mass trajectories were used for each sampling period to explain the observed concentrations in terms of the origins of the sampled air. Seasonal and diurnal cycles were observed for DMS with maximum concentrations > 300 ng (S) m-3 occurring at night and during the spring in air of oceanic origin. Maximum MSA concentrations (up to 120 ng (S) m-3) were observed during the spring. Using a steady state approximation, the sea-to-air flux of DMS from the Atlantic north of 45-degrees was estimated to be 0.93-mu-mol m-2 d-1 or 96 Gg (S) yr-1. Three methods were used to correct the observed air concentrations of the various sulphur species for anthropogenic emissions. The resultant flux of biogenic sulphur into the United Kingdom from the North Atlantic, net of any anthropogenic sulphur, was estimated to be 30 - 96 Gg (S) yr-1, or 2 - 5 % of the total anthropogenic emission rate of sulphur from the U.K. A simple 10-step model was used to describe the equilibria existing between the various sulphur species and CO2 in cloud water at cloudwater concentrations of 0.1, 0.5, and 2.5 g m-3 and the resultantacidity calculated. Values of pH as low as 3.4 were predicted, with MSA having a negligible effect and sulphate dominating. Biogenic sulphur species from the North Atlantic may therefore make a small but quantifiable contribution to the atmospheric sulphur budget of the U.K. and may play an important role in determining rainwater acidity in the west of the country.