Methane sulfonic acid to non-sea-salt sulfate ratio in coastal Antarctic aerosol and surface snow

Multiple-year time series of the weight ratio R of methane sulfonic acid (MSA) to non-sea-salt sulfate (nss SO4) in aerosols collected at three coastal Antarctic stations Neumayer (70 degrees 39'S, 8 degrees 15'W), Halley (75 degrees 35'S, 26 degrees 19'W), and Dumont D'Urville (66 degrees 40'S, 140 degrees 1'E) are presented here. Discarding the November 1991 to October 1992 time period, during which the Antarctic atmosphere is thought to have been contaminated by volcanic plumes (Pinatubo and Cerro Hudson), the multiple-year data sets indicate mean annual R values of 17.9 +/- 10.7% at Neumayer (1983-1995) and 11.7 +/- 6.7% at Dumont D'Urville (1991-1996). On the basis of a more limited time series (February 1991/February 1992), which partly overlaps the volcanic time period, the Halley data had R values of 25.6 +/- 11.9% from October to April. The R seasonal variations are broadly consistent at the three sites, with maxima in mid to late summer (from December to April) ranging from 18.2 +/- 4.6% at Dumont d'Urville to 29.2 +/- 6.5% at Neumayer and 32.2 +/- 6.9% at Halley. Minima are lower than 10% during winter (from May to September) at Neumayer (8.5 +/- 2.5%) and Dumont d'Urville (6.0 +/- 3.0%). The interpretation of these R values in terms of relative abundance of MSA and nss SO4 produced by the oxidation of dimethylsulfide (DMS) at high southern latitudes is not straighforward. In winter, nss sulfate present at coastal Antarctic sites is likely originating in long-range transported by-products from marine DMS emissions taking place at 50 degrees S and non-DMS sulfate from the continental free troposphere. Midwinter R values show a mean ratio of up to 20% for the pure marine input. From September to November, both MSA and nss sulfate concentrations increase in relation to enhanced marine DMS emissions from 50 degrees to 60 degrees S. R values are only weakly enhanced. In summer, the further increase of MSA and nss sulfate concentrations is related to increased biogenic activity south of 60 degrees S and is characterized by R values in the range of 25 to 35%. The mean ratio for DMS marine emissions south of 60 degrees S is close to 62% at Neumayer and 32 to 38% at Dumont d'Urville. These baseline data obtained at coastal Antarctic sites allow us to investigate the role of various parameters that modulate seasonal variations of R at high southern latitudes including the oxidative capacity of the atmosphere, the temperature of the atmospheric oxidation of DMS, and the aging of marine air masses during their transport from source regions toward coastal Antarctic regions.