First direct observation of sea salt aerosol production from blowing snow above sea ice

Two consecutive cruises in the Weddell Sea, Antarctica, in winter 2013 provided the first direct observations of sea salt aerosol (SSA) production from blowing snow above sea ice, thereby validating a model hypothesis to account for winter time SSA maxima in polar regions not explained otherwise. Blowing or drifting snow always lead to increases in SSA during and after storms. Observed aerosol gradients suggest that net production of SSA takes place near the top of the blowing or drifting snow layer. The observed relative increase of SSA concentrations with wind speed suggests that on average the corresponding aerosol mass flux during storms was equal or larger above sea ice than above the open ocean, demonstrating the importance of the blowing snow source for SSA in winter and early spring. For the first time it is shown that snow on sea ice is depleted in sulphate relative to sodium with respect to sea water. Similar depletion observed in the aerosol suggests that most sea salt originated from snow on sea ice and not the open ocean or leads, e.g. on average 93 % during the 8 June and 12 August 2013 period. A mass budget calculation shows that sublimation of snow even with low salinity (< 1 psu) can account for observed increases of atmospheric sea salt from blowing snow. Furthermore, snow on sea ice and blowing snow showed no or small depletion of bromide relative to sodium with respect to sea water, whereas aerosol at 29 m was enriched suggesting that SSA from blowing snow is a source of atmospheric reactive bromine, an important ozone sink, with bromine loss taking place preferentially in the aerosol phase between 2 and 29 m above the sea ice surface. Evaluation of the current model for SSA production from blowing snow showed that the parameterisations used can generally be applied to snow on sea ice. Snow salinity, a sensitive model parameter, depends to a first order on snowpack depth and therefore is higher above first-year than above multi-year sea ice. Shifts in the ratio of FYI and MYI over time are therefore expected to change the seasonal SSA source flux and contribute to the variability of SSA in ice cores, which both represents an opportunity and a challenge for the quantitative interpretation of the sea salt sea ice proxy. It is expected that similar processes take place in the Arctic regions.

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