Spatial distribution of biogenic sulphur compounds (MSA, nssSO42-) in the Northern Victoria Land - Dome C - Wilkes Land area (East Antarctica)

During the 1992-2002 Antarctic expeditions, in the framework of the International Trans-Antarctic Expedition (ITASE) project, about 600 sites were sampled (superficial snow, snow pits and firn cores) along traverses in the northern Victoria Land-Dome C-Wilkes Land region. The sites were characterized by different geographical (distance from the sea, altitude) and climatological (annual mean accumulation rate, temperature) conditions and were affected by air masses from different marine sectors (Ross Sea, Pacific Ocean). Mean anion and cation contents were calculated at each site, in order to evaluate the spatial distribution of chemical impurities in snow. Here we discuss the distribution of non-sea-salt sulphate (nssSO4 2-) and of methanesulphonic acid (MSA) mainly originating from atmospheric oxidation of biogenic dimethyl sulphide; these compounds play a key role in climate control processes by acting as cloud condensation nuclei. The spatial distribution of nssSO4 2- and MSA is discussed as a function of distance from the sea, altitude and accumulation rate. Depositional fluxes of nssSO4 2- and MSA decrease as a function of distance from the sea, with a higher gradient in the first 200 km step. There is an analogous trend with the site altitude, and the first 1600 m step is relevant in determining the nssSO4 2- and MSA content in snow. The nssSO4 2-/MSA ratio depends on the distance from the sea and the biogenic source strength. At coastal sites, where biogenic inputs are dominant, this ratio is ∼2. As biogenic input decreases (low MSA content) inland, the ratio increases, indicating the presence of alternative sources of nssSO4 2- (crustal, volcanic background) or advection of low-latitude air masses. By plotting total flux as a function of accumulation rate, dry depositional contributions were evaluated for nssSO4 2- and MSA in the Ross Sea and Pacific Ocean sectors. Non-sea-salt sulphate wet deposition prevails at sites where the accumulation rate (expressed as water equivalent) is higher than 70 kg m-2a-1 (Ross Sea sector) or 370 kg m-2a-1 (Pacific Ocean sector). MSA threshold values in these sectors are respectively 90 and 220 kg m-2a-1.