Arctic haze, mercury and the chemical composition of snow across northwestern Alaska

We sampled three layers of snow at 16 sites along a 1200 km transect from Nome to Barrow across northwestern Alaska. Samples were analyzed for major element concentrations, specific conductance and pH. Samples from five of the sites were analyzed for trace element concentrations. Pb, Cd, SO42− and non-sea salt SO42− concentrations were significantly higher in layers deposited later in the winter than those deposited earlier. This is consistent with the seasonal increase in atmospheric aerosol loading (arctic haze) that develops as the Arctic polar front expands southward in March and April. Haze contaminant concentrations in the snow pack were as high south of the Brooks Range as they were to the north, suggesting the Brooks Range is not an effective orographic barrier to aerosol transport. Computed yearly non-sea salt SO42− loading rates at the 16 sites ranged from 12 to 281 mg/m2/yr. Elevated concentrations of Hg, Na and Cl were measured near the Arctic Ocean coast but not near the Bering Sea coast. To explain this pattern we suggest that the “effective distance from the coast,” inferred from prevailing wind directions and storm tracks, is critical in governing whether halogen emissions from the ocean are available for photochemical reactions that result in Hg deposition to the snow.