The chemical composition of surface snow in the Arctic: Examining marine, terrestrial, and atmospheric influences

We collected surface snow samples during three years (2004, 2005, and 2007) in Arctic Alaska and from the adjoining frozen Arctic Ocean during springtime in environments that span most Arctic terrains including land, thin and thick first-year sea ice, and multi-year sea ice. The snow was analyzed for Br−, Cl−, SO42−, NO3−, NH4+, Na+, K+, Mg2+ and Ca2+. Source fractionation processes and atmospheric influences are important modification mechanisms that influence the ions. We find that Cl−, K+ and Mg2+ are primarily sourced by unfractionated sea salt and only show deviations from sea salt composition at low Na+ concentrations. Bromide and Ca2+ are highly influenced by atmospheric processes that are evident at low sea salt tracer concentrations. Calcium enrichments are due to the addition of non-sea salt (nss) Ca2+ from dust and possibly other sources. Bromide enrichments are due to the addition of nss-Br− and bromide depletions are due to bromine activation to the gas phase. Sulfate is affected by source fractionation at high Na+ concentrations and by atmospheric addition at low Na+ concentrations. Nitrate and NH4+ are not correlated with sea salt and show less concentration variability than sea salt ions. Modifications are related to time-integrated air–snow exchange fluxes for Br−, SO42− and Ca2+ and are in good agreement with observations of related gas-phase and aerosol species. This work reinforces our understanding of air–snow exchanges and their importance for atmospheric chemistry in the Arctic.