Nicola J. Blake, Aaron L. Swanson, Donald R. Blake, and F. Sherwood Rowland
Department of Chemistry, University of California at Irvine, Irvine, CA 92697-2025
Jack E. Dibb
Climate Change Research Center, Institute for the Study of Earth, Oceans and Space, University of New Hampshire, Durham, NH 03824
Abstract. Investigations of polar snow during the past
decade indicate that the composition of surface and near-surface snow changes
significantly within days to months after deposition. Recently, several
groups have shown that photochemical processes occurring within the firn
(unconsolidated snow) alter the pore air composition on a temporal scale
of minutes to hours. For example, photochemical formation of nitrogen
oxide (NOx) species in firn pore air has been observed both at Summit,
Greenland, Alert, Canada, South Pole, and at Neumayer Station, Antarctica,
possibly as a result of nitrate ion photolysis. Our summertime 1999
measurements at Summit, Greenland revealed mixing ratios of alkenes, halocarbons,
and alkyl nitrates that are typically a factor of 2-10 higher within the
snow-pack (firn) pore air than in the atmosphere 1 m above the snow.
Concentration profiles of to a firn depth of 2 m show that maximum values
of these trace gases occur between the surface and 60 cm depth. Peak
ethene (C2H4) levels were as high as 200 pptv. Diurnal variations
are consistent with a near-surface photo-induced production mechanism.
Active photochemistry in the firn may play an important role in the oxidative
capacity of the atmospheric boundary layer over snow-covered surfaces,
and may influence post-depositional chemistry, which in turn could affect
certain aspects of ice core record interpretation.