Trends and sources of perchlorate in Arctic snow.

Samples from the Devon Island ice cap (Nunavut, Canada) were used to calculate the annual input of atmospheric formed perchlorate. Depth samples collected in the spring of 2006 were dated between 1996 and 2005. An optimized ion chromatography tandem mass spectrometry (IC-MS/MS) method with direct injection allowed detection of perchlorate in all analyzed samples. Concentrations varied between 1 and 18 ng L(-1), showed seasonality, and were correlated with the total ozone levels from the area. A significant correlation was observed between chloride and perchlorate only for data sets corresponding to peak perchlorate concentrations. Data available suggests that perchlorate from the Arctic snow was formed in the atmosphere following two different mechanisms. Stratospheric chlorine radicals reacted with ozone year around, producing concentrations of perchlorate correlated with the total ozone level. The second pathway was specific to the summer months, when the amounts of perchlorate were correlated with the chloride concentrations, suggesting a possible tropospheric formation. Analysis of a deep ice core sample confirmed that perchlorate was present in precipitation at similar concentration more than 2000 years ago. Perchlorate ion represents a sink for the stratospheric chlorine, being removed via precipitation. The estimated amount of perchlorate that reached the Arctic in 2005 was 41-86 t.