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-SO42−), 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-SO42− in rainwater were 3.19, 3.04 and 4.57 μ eq l−1 at Amsterdam, Crozet, and Kerguelen, respectively while the VWM of MS− were 0.24, 0.15 and 0.30 μ 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-SO42− 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-SO42− 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-SO42− concentrations at Amsterdam Island, indicating that the biogeochemical sulfur cycle at this area is mainly influenced by biogenic emissions.