Sulfate depletion in snow over sea ice near Syowa Station, Antarctica, in relation to the origin of sulfate depleted sea salt aerosol particles in winter

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Snow samples collected over sea ice near Syowa Station were analyzed for ionic constituents to understand sulfate depletion in sea salt aerosol particles, which have often been observed in winter in the coastal region of Antarctica. Ratios of Cl/Na^+ in snow were higher than that in sea water, whereas SO_4^<2->/Na^+ ratios were lower in snow samples. The degree of SO_4^2 depletion relative to the sea water composition increased with increase in Na^+ concentration. From comparison with a simple chemical mass balance model using the SO_4^<2->/Na^+ ratios in the samples, depletion of SO_4^<2-> in snow is considered to be largely attributed to the fractionation of sulfate by forming Mirabilite (Na_2SO_4・10H_2O) during sea ice growth. The Br/Na^+ ratios of the snow predicted from the fractionation model differ significantly from the observed values : positive deviations were found in dark early July and negative deviations were seen in September. This could have resulted from Br_2 release from the snow after local sunrise. In winter aerosols collected at Syowa Station, depletion of SO_4^<2-> was also found. Based on statistics of meteorological data at Syowa Station, as appropriate conditions of meteorological and salt concentrations in drifting snow particle were fulfilled, the depletion of SO_4^2 in aerosol particles in winter can be explained by sublimation of salted drifting snow particles from the sea ice area.
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