Sea‐salt aerosol in coastal Antarctic regions

Abstract

Continuous year round records of atmospheric sea‐salt concentrations have been recovered at three coastal Antarctic stations (Halley, Dumont D'Urville, and Neumayer) at temporal resolutions typically between 1 day and 2 weeks. The records were evaluated in terms of their spatial and seasonal variability as well as with respect to changes in the relative ion composition of airborn sea‐salt particles. Annual mean sea‐salt concentrations vary between 1400 ng m−3 at Dumont D'Urville, 850 ng m−3 at Neumayer, and 200 ng m−3 at Halley, respectively. They are thus considerably lower than the mean levels previously observed at the north tip of the Antarctic Peninsula but are, at their lower end, comparable to the level previously reported from Mawson. The representativeness of the atmospheric sea‐salt data appears to be weak due to their high temporal variability, strong impacts of site specific aspects (such as site topography) but also due to the nonuniform sampling techniques applied so far. In accordance with the ice core evidence, the seasonal change in the atmospheric sea‐salt load is found to be clearly out of phase with the seasonal cycle of the open water fraction offshore from the station as (with the exception of Dumont D'Urville) the lowest concentrations are generally observed during the local summer months. Major ion analyses of bulk aerosol and concurrently sampled fresh snow show a strong, systematic depletion of the SO42− to Na+ (Cl−) ratios with respect to bulk sea water, which appeared to be confined to the local winter half year. During that time, sea‐salt SO42− was found to be depleted typically by 60–80% along with a concurrent Na+ deficit, which is in accordance with the precipitation of mirabilite. No significant fractionation of Mg2+, K+, and Ca2+ between seawater and sea‐salt particles is observed. Laboratory experiments failed to simulate the SO42− fractionation in airborne seawater droplets or in the skin of seawater bubbles at low air temperatures. They gave, however, SO42− depletion factors, similar to the field observation in air and snow, in the remaining brine of seawater which was partly frozen below −8°C to an artificial sea ice surface. It is suggested therefore that the mobilization of brine from the sea ice surface constitutes an important sea‐salt source in winter which may dominate the atmospheric sea‐salt load at high latitudes of coastal Antarctica.