Abstract
We present data collected from a number of snow-covered environments including two polar locations (Summit, Greenland and the South Pole) and two mid-latitude regions (a remote site in northern Michigan, and Niwot Ridge, Colorado). At each site, concentrations of CH 3 I and C 2 H 5 I were enhanced within the interstitial air near the snow surface, compared to levels in boundary layer air. Fluxes of CH 3 Br from surface snow to the atmosphere were observed at each site except Niwot Ridge, where CH 3 Br appeared to have a sink. The mid-latitude sites showed significant emissions of CH 3 Cl , mostly originating at the ground surface and traveling up through the snow, while at the polar locations CH 3 Cl emissions from firn air were relatively small. In general, methyl halide mixing ratios in firn air were significantly greater at Summit than at the South Pole, with Summit showing a strong diurnal cycle in the production of alkyl halides that was well correlated with actinic radiation and firn temperature. We suggest that the most likely route to alkyl halide formation is through an acid catalyzed nucleophilic substitution of an alcohol type function by a halide, both of which should be preferentially segregated to the quasi-liquid layer at the surface of the snow grains. A series of experiments using a snow-filled quartz chamber irradiated by natural sunlight allowed estimation of emission trends that were hard to measure in the natural snowpack. These static chamber experiments confirmed significant production of the primary alkyl halides, following the order CH 3 Cl > CH 3 Br > C 2 H 5 Cl > CH 3 I > C 2 H 5 Br > C 2 H 5 I > 1 - C 3 H 7 Br > 1 - C 3 H 7 I . Our observations at all four locations, including polar and mid-latitude sites, imply that alkyl halide production may be associated with all surface snows.
Highlights
Post-depositional processes within surface snows have become an increasingly important topic overA.L
Firn concentrations of CH3Cl, CH3Br, and C2H5Br peaked at solar noon, while the C2H5I peak corresponded to the 6 pm profile, and CH3I shows similar firn concentrations at both noon and 6 pm
We presented near-surface snow alkyl halide observations from four remote sites
Summary
Post-depositional processes within surface snows have become an increasingly important topic overA.L. Results, which showed photochemically driven production of HCHO from organic chromophores in the snowpack interstitial air, suggested that net flux to the BL would both increase the oxidative capacity of the BL and increase ozone depletion in the Arctic by inducing Br2 emissions from the snow (Sumner and Shepson, 1999). Evidence of these processes occurring in natural snowpacks was subsequently collected at Alert, Canada in 2000, where Br2 was shown to be twice as concentrated in firn pore air just below the snow surface compared to overlying air (Foster et al, 2001). Most research dealing with halogens in arctic regions or areas impacted by snowfall have focused on bromine oxidation of ozone
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