Aerosol chemistry and air-snow transfer in coastal Antarctica

Abstract

The aims of this dissertation are to better understand the sources of aerosol particles reaching coastal Antarctica, and the processes that control aerosol deposition to the snow surface and inclusion into the ice. Daily collections of aerosol particles and surface snow samples were made from British Antarctic Survey base, Halley. Aerosol and snow sea salt were found to have maximum concentrations during the austral winter and non sea salt sulphate and methane sulphonic acid concentrations peaked during the summer, confirming previous work by other authors.These species were compared with local meterological events (such as wind speed and direction) to identify a source for particularly high concentration events. Winter sea salt was found to have a local source, consisting probably of concentrated brine pools on surfaces of freshly formed sea ice and needle-like structures, known as frost flowers, which form from the pools. The sea salt component of these high events was also found to be fractionated, with a deficit of sodium sulphate (mirabilite). Methane sulphonic acid and nonsea salt sulphate did not appear to have a local source. Instead, using back trajectories of air mass origins to identify a longer range aerosol source, high concentration events were associated with the air mass having passed over an area of open water several days before reaching Halley. The processes of aerosol deposition to the snow surface were then quantified. Dry, fog and wet deposition, sublimation, wind pumping, blowing and drifting snow were examined experimentally and theoretically. For this coastal Antarctic location, wet deposition was found to be highly dominant (80%). Dry deposition accounted for about 10% and drifting and blowing snow were found to be important in determining whether a snowfall event remained recorded in the accumulated snow record, and ultimately in any ice core. This thesis has suggested that for sea salt, methane sulphonic acid (MSA) and non sea salt sulphate, there may be an alternate way of interpreting concentrations of these species in coastal Antarctic ice cores. Rather than an indication of increased storminess and long range transport, high loadings of sea salt could actually give information on the extent of new, fresh sea ice and could therefore be used to infer the local temperature, sea ice extent and possible wind direction at the time of core formation. Elevated MSA and non sea salt sulphate concentrations in ice cores could also give us information on the extent of open water and not simply an increase in marine biogenic activity and DMS emissions.