Formation of HCl in the Antarctic atmosphere

A comprehensive glaciochemical study has been conducted at several Antarctic locations on the Antarctic plateau (South Pole Station, Dome C) and in more coastal regions (a few stations of Terre Adelie, East Antarctica). The objective was to investigate the sulfur, nitrogen, and halogen atmospheric cycles in very remote areas. In this paper the spatio-temporal variations of the Cl/Na ratio are reported for several hundred samples collected in snow pits or from firn and ice cores using contamination-free techniques. The Cl/Na weight ratio in snow is generally very close to that of bulk seawater (1.8) near the coast and begins to increase at the edge of the Antarctic plateau. In central areas, both relatively high and very low values are observed (excess chloride or excess sodium with respect to the 1.8 reference value), depending on the time period. Determination of all major ions (not fully reported in the present work) has provided an in-depth understanding of the chemical composition of Antarctic precipitation, explaining excess chloride and excess sodium by the presence in snow of HCl or Na2SO4, respectively, formed by the reaction of excess sulfate (biogenic H2SO4) with sea-salt particles in the aerosol phase. This reaction results in the release of gaseous HCl into the atmosphere. Short-term (seasonal) or long-term (climatic) variations observed in the sequences analyzed suggest that this reaction occurs more completely when weather conditions are calm and marine aerosol is aged. In central areas this alteration of marine aerosol can lead to excess chloride of up to 50% of total chloride deposition, and Na2SO4 can be equivalent to the sulfuric acid deposition. These results demonstrate the importance of the interaction between sulfur and chlorine cycles in the Antarctic atmosphere.