Are methyl halides produced on all ice surfaces? Observations from snow-laden field sites

Authors

Aaron L. Swanson, University of California, Irvine
Nicola J. Blake, University of Colorado, BoulderFollow
D R. Blake, University of California, Irvine
F Sherwood Rowland, University of California, Irvine
Jack E. Dibb, University of New HampshireFollow
Barry Lefer, University of Houston - Main
E L. Atlas, University of MiamiFollow

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 CH₃I and C₂H₅I were enhanced within the interstitial air near the snow surface, compared to levels in boundary layer air. Fluxes of CH₃Br from surface snow to the atmosphere were observed at each site except Niwot Ridge, where CH₃Brappeared to have a sink. The mid-latitude sites showed significant emissions of CH₃Cl, mostly originating at the ground surface and traveling up through the snow, while at the polar locations CH₃Cl 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 orderCH₃Cl>CH₃Br>C₂H₅Cl>CH₃I>C₂H₅Br>C₂H₅I>1-C₃H₇Br>1-C₃H₇I. Our observations at all four locations, including polar and mid-latitude sites, imply that alkyl halide production may be associated with all surface snows.

Department

Earth Sciences, Earth Systems Research Center

Publication Date

8-2007

Journal Title

Atmospheric Environment

Publisher

Elsevier

Digital Object Identifier (DOI)

10.1016/j.atmosenv.2006.11.064

Document Type

Article

Recommended Citation

Aaron L. Swanson, Nicola J. Blake, Donald R. Blake, F. Sherwood Rowland, Jack E. Dibb, Barry L. Lefer, Elliot Atlas, Are methyl halides produced on all ice surfaces? Observations from snow-laden field sites, Atmospheric Environment, Volume 41, Issue 24, August 2007, Pages 5162-5177, ISSN 1352-2310, http://dx.doi.org/10.1016/j.atmosenv.2006.11.064.

Rights

Copyright © 2007 Elsevier Ltd. All rights reserved.