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

[摘要] Snow, through its trace constituents, can have a major impact on lowertropospheric chemistry, as evidenced by ozone depletion events (ODEs) inoceanic polar areas. These ODEs are caused by the chemistry of brominecompounds that originate from sea salt bromide. Bromide may be supplied tothe snow surface by upward migration from sea ice, by frost flowers beingwind-blown to the snow surface, or by wind-transported aerosol generated bysea spray. We investigate here the relative importance of these processes byanalyzing ions in snow near Alert and Ny-Ålesund (Canadian and Europeanhigh Arctic) in winter and spring. Vertical ionic profiles in the snowpackon sea ice are measured to test upward migration of sea salt ions and toseek evidence for ion fractionation processes. Time series of the ioniccomposition of surface snow layers are investigated to quantifywind-transported ions. Upward migration of unfractionated sea salt toheights of at least 17cm was observed in winter snow, leading to Cl^-concentration of several hundred µM. Upward migration thus has thepotential to supply ions to surface snow layers. Time series show that windcan deposit aerosols to the top few cm of the snow, leading also to Cl^-concentrations of several hundred µM, so that both diffusion from seaice and wind transport can significantly contribute ions to snow. AtNy-Ålesund, sea salt transported by wind was unfractionated, implyingthat it comes from sea spray rather than frost flowers. Estimations based onour results suggest that the marine snowpack contains about 10 times moreNa⁺ than the frost flowers, so that both the marine snowpack and frostflowers need to be considered as sea salt sources. Our data suggest thatozone depletion chemistry can significantly enhance the Br^- content ofsnow. We speculate that this can also take place in coastal regions andcontribute to propagate ODEs inland. Finally, we stress the need to measuresnow physical parameters such as permeability and specific surface area tounderstand quantitatively changes in snow chemistry.

[学科分类] 大气科学