The origin of sea salt in snow on Arctic sea ice and in coastal regions

F. Domine (1) , R. Sparapani (2) , A. Ianniello (2) , H. J. Beine (2)

1 LGGE - Laboratoire de glaciologie et géophysique de l'environnement
2 C.N.R.

Snow, through its trace constituents, can have a major impact on lower tropospheric chemistry, as evidenced by ozone depletion events (ODEs) in oceanic polar areas. These ODEs are caused by the chemistry of bromine compounds, that originate from sea salt bromide. According to current ideas, bromide may be supplied to the snow surface either by upward migration from sea ice or by frost flowers being wind-blown to the snow surface. We investigate here the relative importance of both these processes by analyzing mineral ions in snow samples collected near Alert and Ny-Ålesund (Canadian and European high Arctic) in winter and spring. Vertical ionic profiles in the snowpack on sea ice are measured to test upward migration of sea salt ions and to seek evidence for ion fractionation processes. Time series of the ionic composition of surface snow layers are investigated to quantify wind-transported ions. Upward migration of unfractionated sea salt, to heights of at least 17 cm, was observed in snow sampled in winter, at temperatures near -30°C, leading to Cl^- concentration of several hundred µM. Upward migration thus has the potential to supply ions to surface snow layers. Time series show that wind can deposit aerosols to the top few cm of the snow, leading also to Cl^- concentrations of several hundred µM, so that both migration from sea ice and wind transport can significantly contribute ions to snow. At Ny-Ålesund, sea salt transported by wind was unfractionated, implying that it does not come from frost flowers. In the Arctic, frost flowers thus do not appear necessary to lead to large sea salt concentrations in surface snow, and to supply the bromide needed for ODEs. The data obtained also indicate that ODEs lead to significant deposition of Br^- to snow. We speculate that this can also take place in coastal regions and contribute to propagate ODEs inland. Finally, we stress the need to measure snow physical parameters such as permeability and specific surface area, to understand quantitatively changes in snow chemistry.

Domaines: Océan, Atmosphère

F. Domine, R. Sparapani, A. Ianniello, H. J. Beine. The origin of sea salt in snow on Arctic sea ice and in coastal regions. Atmospheric Chemistry and Physics Discussions, 2004, 4 (4), pp.4737-4776. ⟨hal-00327906⟩