Estimation of seasonal dimethyl sulphide fluxes over the North Atlantic Ocean and their contribution to European pollution levels

A monthly resolved emission budget which aims to reproduce the observed seasonal variations of dimethyl sulphide (DMS) production over the oceans is derived for the North Atlantic Ocean. The monthly cycle of DMS concentrations in seawater and their distribution over the North Atlantic Ocean was determined from a collation of available field measurements. DMS fluxes are calculated on the basis of Liss and Merlivat's (1986) formulation of sea-to-atmosphere gas exchange, using six-hourly meteorological data from the European Center for Medium-Range Forecasts for the year 1988. This allows to show the influence of meteorological conditions on the DMS fluxes to the atmosphere. Results show averaged fluxes over the North Atlantic Ocean in the range 39–390 μg S m−2 d−1, with a monthly summer maximum between 67 and 1260 μg S m−2 d−1, about 20 times larger than the winter minimum. The contribution of natural oceanic sulphur to the acidic deposition levels over Europe is evaluated with the help of a three-dimensional Eulerian dispersion model and is compared to the separate contributions of European and North American anthropogenic sources. The relative contribution of biogenic sulphur to total sulphur deposition over the west coast of Europe does not exceed 2% on a yearly basis although it can be close to 10% during the summer months. Considering the uncertainty ranges associated both to the estimation of DMS fluxes and to the subsequent dispersion in the atmosphere, the actual contribution of North Atlantic DMS to European pollution levels can vary by a factor of 3.