Blowing snow and sea ice surfaces as a source of polar sea salt aerosol (BLOWSEA)

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Small particles (known as aerosol) in the atmosphere play several critical roles. They affect the transmission of sunlight to the underlying surface; they affect the formation of clouds, and they host and enhance important chemical reactions. When they are deposited on ice they leave a record of past conditions that can be accessed by drilling ice cores. The most significant aerosol component over marine areas is sea salt aerosol. Over most of the world's oceans this is created by bubble bursting in sea spray. However there is strong evidence that another source of sea salt aerosol is important in the polar regions, and that this ultimately derives from the surface of sea ice. The existence of this source forms the basis for a proposed method using ice core data for determining changes in sea ice extent over long time periods. Additionally sea salt aerosol, along with salty sea ice surfaces, is the host for the production of halogen compounds which seem to play a key role in the oxidation chemistry of the polar regions. It is therefore important to understand the sources of polar sea salt aerosol and therefore to be able to predict how they may vary with, and feedback to, climate.It was recently proposed that the main source of this polar sea salt aerosol was the sublimation of salty blowing snow. The idea is that snow on sea ice has a significant salinity. When this salty snow is mobilised into blowing snow, sublimation from the (top of) the blowing snow layer will allow the formation of sea salt aerosol above the blowing snow layer, that can remain airborne after the blowing snow has ceased. First calculations suggested that this would provide a strong source of aerosol (greater than that from open ocean processes over an equivalent area). It was proposed that this would have a strong influence on polar halogen chemistry and a noticeable influence on halogens at lower latitudes. However, this was based on estimates of the relevant parameters as there were no data about aerosol production from this source, and almost no data about blowing snow over sea ice in general.Here we propose to take advantage of a very rare opportunity to penetrate the Antarctic sea ice zone during winter, as we have been allocated spaces on an unusual winter cruise into the sea ice zone on the German icebreaker Polarstern. During this cruise, we will be able to confirm that the blowing snow sea ice source exists, and make measurements that will provide a soundly-based parameterisation of the source. This will be done by making measurements of the snow on sea ice, of the blowing snow itself, and of aerosol above the blowing snow, as well as before and after such episodes. Measurements will include salinity, chemistry (looking at the amount of bromine present in each medium), and for blowing snow and aerosol, the amounts and size distributions.By combining our data with meteorological data, and by comparing them to satellite observations that have recently attempted to identify blowing snow episodes, we will be able to make estimates of the spatial and temporal distribution of sea salt aerosol from this source over the entire Antarctic sea ice zone. This will allow us to assess the importance of this source of sea salt (and of halogens) compared to others that have been proposed. We will then use existing models to assess how important such a source is to sea salt deposition in Antarctica, allowing us to determine how sea salt in ice cores is related to sea ice extent. This opens the possibility of turning a qualitative sea ice proxy into a quantitative one. Models will also be used to re-assess the importance of this source for halogen chemistry in the polar regions and globally.In summary this proposal will provide the first targeted measurements of the parameters needed to assess the importance of blowing snow sublimation as a source of sea salt, and to quantify its most relevant impacts.