Study of present-day sources and transport processes affecting oxidised sulphur compounds in atmospheric aerosols at Dome C (Antarctica) from year-round sampling campaigns

A year-round study, which was conducted from November 2004 to November 2007, of atmospheric oxidised sulphur compounds (methanesulphonic acid (MSA) and sulphate) was carried out in the east Antarctic Plateau at Dome C (75° 06' S, 123° 20' E, 3220m a.s.l. and 1100km away from the nearest coast). The two sulphur-derived species exhibit a seasonal cycle characterised by maxima in the summer from November to March. Size-segregated sampling performed with Andersen 8-stage impactors revealed that SO ₄ ^2- and MSA have different size distributions in early summer (November) in comparison with mid-late summer (February). In November, the size distribution exhibited two distinct modes, the accumulation (0.4-0.7μm) and the micrometric mode (1.1-2.1μm), which is in contrast to February when only the accumulation mode was observed. The two modes exhibited different speciation; in the finest mode, sulphate and methanesulphonate were present primarily in the acidic form, whereas they were present primarily as sodium or ammonium salts in the micrometric mode. The different size distributions and speciation patterns in the two months are related to different transport pathways from oceanic areas to the central Antarctic Plateau. In the early summer months, air masses came primarily from the Indian Ocean and lingered for a long time over the Antarctic continent. The transport of sulphur compounds is related to sea spray aerosols and the resulting condensation of H ₂SO ₄ and MSA over sea salt particles to form sodium salts. In contrast, a rapid transport of H ₂SO ₄ and MSA formed above the boundary layer over oceanic areas leads to higher concentrations of the acidic species in the fine fraction of aerosols reaching Dome C in February relative to other summer months.