Diurnal and seasonal variation of atmospheric dimethylsulfoxide at Amsterdam Island in the southern Indian Ocean

Atmospheric dimethylsulfoxide (DMSO), an important dimethylsulfide (DMS) ox idation by-product, has been monitored on a daily basis at Amsterdam Island in the southern Indian Ocean during three periods (January-September 1997, January-June 1998, and January-October, 1999). In addition, during an inte nsive experiment carried out in January 1998, DMSO diurnal variation has be en studied in conjunction with parallel measurements of atmospheric DMS. Mo nthly average atmospheric DMSO mixing ratios range from 0.3 to 5.8 parts pe r trillion by volume and present a well-distinguished seasonal cycle with a minimum in winter and a maximum in summer similar to that observed for atm ospheric DMS simultaneously measured. DMSO presents also a well-distinguish ed diurnal variation with maximum values around 0900 and minimum during nig ht. A photochemical box model using measured atmospheric DMS concentrations as input data reproduces quite well the diurnal variation of DMSO when ass uming a 50% yield from the DMS+OH addition channel [Hynes and Wine, 1996]. However, in absolute terms the simulated DMSO concentrations were about a f actor of 2 higher than those measured. Photochemically driven heterogeneous losses of DMSO on aerosol and/or clouds could account for this difference.