Causes of seasonal and daily variations in aerosol sea-salt concentrations at a coastal Antarctic station
Two years worth of daily aerosol data has been collected from Halley station, Antarctica, between February 1991 and February 1993. The seasonal cycle of sea-salt aerosol was found to peak during the winter months, with an annual mean of 162 ng m-3. Specific site characteristics are used to explain this relatively low value. The winter sea-salt source does not appear to be solely due to the presence of open water. Comparison of individual high salt concentration events in the data, with 3 hourly meteorological records, shows that sea-salt loadings are not linked to high wind speeds, but more moderate ones. The high sea-salt loadings are associated with a change in wind direction that opens up an area of water and then switches to bring sea-salt inland. It is hypothesised that the exposed areas of sea water, which are rapidly frozen in winter creating areas of local, freshly formed ice with a surface covering of concentrated brine, are the source of the winter sea-salt. Fractionation of the sea-salt component in individual high concentration events, is used to reinforce the theory that a surface skim of highly saline brine, on fresh ice, is the winter sea-salt source. The presence of frost flowers is thought to aid incorporation of sea-salt into the atmosphere. Implications for the interpretation of sea-salt data in ice cores are highlighted.