Photochemical production of reactive light NMHCs, alkyl halides, and alkyl nitrates associated with surface snow at Summit, Greenland


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.