BIOGENIC SULFUR EMISSIONS FROM THE GULF OF SAINT-LAWRENCE AND ASSESSMENT OF ITS IMPACT ON THE CANADIAN EAST-COAST

We measured dimethylsulfide (DMS) concentrations over a grid of 79 sta tions covering the central part of the Gulf of Saint Lawrence and duri ng a 24-hour sampling period at a fixed station, in August: 1993. Surf ace water DMS concentrations ranged from <0.9 to 9 nM over the samplin g grid. DMS sea-to-air fluxes ranged from <0.1 to 23 mu mol m(-2) d(-1 ), with a mean of 4.7 mu mol m(-2) d(-1) for August. Atmospheric DMS l evels measured during the cruise ranged from less than 0.5 nmol m(-3) (12 parts per trillion by volume, or pptv) to 25 nmol m(-3) (625 pptv) with a mean value of 11.9 nmol m(-3) (291 pptv). Results from the 24- hour sampling at the fixed station show that ventilation may represent the most important sink for surface DMS when wind speeds exceed appro ximately 8 m s(-1). The relative magnitude of the biogenic and anthrop ogenic S components was estimated by comparing sulfur emissions from t he gulf to the sulfur emissions from surrounding continental regions a nd to a sulfur budget calculated by using a Lagrangian box model. The first approach suggests that DMS emissions from the gulf represented a t least 4% of total (biogenic plus anthropogenic) sulfur emissions fro m the Atlantic Provinces in August 1993. A Lagrangian forward motion t rajectory shows that these biogenic emissions would affect all the Mar itime Provinces and a portion of eastern Quebec. The second approach s uggests that the contribution of the gulf to the total sulfur flux may reach 13% when compared with the anthropogenic inflow of sulfur from the Atlantic Provinces impacting the Maritimes. When the transboundary inflow of sulfur from the United States is also taken into account, t he contribution of the gulf becomes 8%. Thus DMS emissions from coasta l waters may represent a significant source of sulfur to the atmospher e.