SEASONAL-VARIATIONS OF ATMOSPHERIC SULFUR-DIOXIDE AND DIMETHYLSULFIDE CONCENTRATIONS AT AMSTERDAM ISLAND IN THE SOUTHERN INDIAN-OCEAN

Daily measurements of atmospheric sulfur dioxide (SO2) concentrations were performed from March 1989 to January 1991 at Amsterdam Island (37-degrees 50'S-77-degrees 30'E), a remote site located in the southern Indian Ocean. Long-range transport of continental air masses was studied using Radon (Rn-222) as continental tracer. Average monthly SO2 concentrations range from less than 0.2 to 3.9 nmol m-3 (annual average = 0.7 nmol m-3) and present a seasonal cycle with a minimum in winter and a maximum in summer, similar to that described for atmospheric DMS concentrations measured during the same period. Clear diel correlation between atmospheric DMS and SO2 concentrations is also observed during summer. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces the seasonal variations in the measured atmospheric SO2 concentrations within +/- 30%. Comparing between computed and measured SO2 concentrations allowed us to estimate a yield of SO2 from DMS oxidation of about 70%.