Spatial, Temporal and Interannual Variability of Methanesulfonate and Non-Sea-Salt Sulfate in Rainwater in the Southern Indian Ocean (Amsterdam, Crozet and Kerguelen Islands)

Methanesulfonate (MS-) and non-sea-salt sulfate (nss-SO(2-)4), two of the major oxidation products of atmospheric dimethylsulfide (DMS), have been continuously measured in rainwater at three remote islands in the Southern Indian Ocean: Amsterdam since 1991, Crozet since 1992, and Kerguelen since 1993. The annual volume weighted mean (VWM) concentrations of nss-SO(2-)4 in rainwater were 3.19, 3.04 and 4.57 (mu)eq l^(-1) at msterdam, ACrozet, and Kerguelen, respectively while the VWM of MS- were 0.24, 0.15 and 0.30 (mu)eq l^(-1), respectively. At all three islands, MS- presented a well-distinguished seasonal variation with a maximum during summer whereas the seasonal variation of nss-SO 4 2 was less pronounced, possibly due to the increased anthropogenic influence during the winter period. Furthermore, MS presented significant interannual variations, in particular at Amsterdam and Crozet, which is closely related to the sea-surface temperature (SST) anomalies). Finally, the nss-SO(2-)4 deposition at Crozet Island presented a decreasing interannual trend, reflecting probably reductions in sulfur emissions from Southern Africa. On the contrary no interannual tendency was observed in the nss-SO(2-)4 Aconcentrations at Amsterdam Island, indicating that the biogeochemical sulfur cycle at this area is mainly influenced by biogenic emissions.