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

Abstract

Abstract. 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. Bromide may be supplied to the snow surface by upward migration from sea ice, by frost flowers being wind-blown to the snow surface, or by wind-transported aerosol generated by sea spray. We investigate here the relative importance of these processes by analyzing ions in snow 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 winter snow, 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 diffusion 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 comes from sea spray rather than frost flowers. Estimations based on our results suggest that the marine snowpack contains about 10 times more Na + than the frost flowers, so that both the marine snowpack and frost flowers need to be considered as sea salt sources. Our data suggest that ozone depletion chemistry can significantly enhance the Br − content of 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.

High summertime aerosol organic functional group concentrations from marine and seabird sources at Ross Island, Antarctica, during AWARE

“…This sodium depletion is the result of Na 2 SO 4 precipitating out from sea ice brine before frost flowers wick up the remaining salt solution. Blowing snow could also contribute to submicron particles (Domine et al, 2004), but this source has not been associated with a substantial sodium deficiency in submicron particle composition (Gordon and Taylor, 2009).…”

Organics in environmental ices: sources, chemistry, and impacts

“…Impurities in snow can be molecules or ions adsorbed to or dissolved in crystals, aerosol particles which originated as a condensation nucleus (Parungo and Pueschel, 1973), gases and particles scavenged during precipitation (Lei and Wania, 2004), or species deposited onto snow on the ground (Domine et al, 2004). The amount and location of chemical species incorporated into growing ice crystals depends on the snow formation mechanism, i.e., whether the crystal forms by freezing supercooled water or by water vapor condensation.…”

Seasonal evolution of snow permeability under equi-temperature and temperature-gradient conditions

“…Circulation can be induced over flat snow surfaces by processes such as turbulence (Sokratov and Sato, 2000;Clifton et al, 2008) and over rough surfaces by wind pumping (Colbeck, 1989). One noteworthy consequence of these processes is the deposition of atmospheric particles such as sulfate and sea salt to snow, affecting the chemical composition of snow (Cunningham and Waddington, 1993;Domine et al, 2004;Harder et al, 1996). Examples of the importance of understanding these chemical changes include the interpretation of ice core analyses in terms of past atmospheric composition (Legrand and Mayewski, 1997) and the quantification of the sources of seasalt-derived bromine that destroys tropospheric ozone in polar regions (Simpson et al, 2005;Bottenheim et al, 2009;Spicer et al, 2002).…”