Abstract
As sources of reactive halogens, snowpacks in sea ice regions control the oxidative capacity of the Arctic atmosphere. However, measurements of snowpack halide concentrations remain sparse, particularly in the high Arctic, limiting our understanding of and ability to parameterize snowpack participation in tropospheric halogen chemistry. To address this gap, we measured concentrations of chloride, bromide, and sodium in snow samples collected during polar spring above remote multi-year sea ice (MYI) and first-year sea ice (FYI) north of Greenland and Alaska, as well as in the central Arctic, and compared these measurements to a larger dataset collected in the Alaskan coastal Arctic by Krnavek et al. (2012). Regardless of sea ice region, these surface snow samples generally featured lower salinities, compared to coastal snow. Surface snow in FYI regions was typically enriched in bromide and chloride compared to seawater, indicating snowpack deposition of bromine and chlorine-containing trace gases and an ability of the snowpack to participate further in bromine and chlorine activation processes. In contrast, surface snow in MYI regions was more often depleted in bromide, indicating it served as a source of bromine-containing trace gases to the atmosphere prior to sampling. Measurements at various snow depths indicate that the deposition of sea salt aerosols and halogen-containing trace gases to the snowpack surface played a larger role in determining surface snow halide concentrations compared to upward brine migration from sea ice. Calculated enrichment factors for bromide and chloride, relative to sodium, in the MYI snow samples suggests that MYI regions, in addition to FYI regions, have the potential to play an active role in Arctic boundary layer bromine and chlorine chemistry. The ability of MYI regions to participate in springtime atmospheric halogen chemistry should be considered in regional modeling of halogen activation and interpretation of satellite-based tropospheric bromine monoxide column measurements.
Highlights
Snowpack-driven halogen chemistry has a profound impact on atmospheric composition in the polar regions (Simpson et al, 2007b; Abbatt et al, 2012)
We find surface snow from both multi-year sea ice (MYI) regions and first-year sea ice (FYI) regions is generally enriched in chloride compared to seawater
Snow in FYI regions is most often enriched in bromide, while snow in MYI regions shows both depletion and enrichment of bromide
Summary
Snowpack-driven halogen chemistry has a profound impact on atmospheric composition in the polar regions (Simpson et al, 2007b; Abbatt et al, 2012). Due to the coupling between the atmosphere and the cryosphere (Domine and Shepson 2002), these surface changes are expected to impact the production of molecular halogens from the snowpack. The snowpack is a source of Cl2 and BrCl (Custard et al, 2017), leading to increased chlorine levels in the lower troposphere (Liao et al, 2014; Custard et al, 2016) This atmospheric chlorine chemistry increases the oxidation of hydrocarbons (Jobson et al, 1994), reducing the lifetime of the greenhouse gas methane (Platt et al, 2004). Given the influence of the snowpack on the chemical composition of the overlying atmosphere (Domine and Shepson, 2002), understanding the environmental factors that control snowpack chemical composition, in remote sea ice regions, is crucial
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