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It has been shown that sunlit snow and ice plays an important role in processing atmospheric species. Photochemical production of a variety of chemicals has recently been reported to occur in snow/ice and the release of these photochemically generated species may significantly impact the chemistry of the overlying atmosphere. Nitrogen oxide and oxidant precursor fluxes have been measured in a number of snow covered environments, where in some cases the emissions significantly impact the overlying boundary layer. For example, photochemical ozone production (such as that occurring in polluted mid-latitudes) of 3–4 ppbv/day has been observed at South Pole, due to high OH and NO levels present in a relatively shallow boundary layer. Field and laboratory experiments have determined that the origin of the observed NOx flux is the photochemistry of nitrate within the snowpack, however some details of the mechanism have not yet been elucidated. A variety of low molecular weight organic compounds have been shown to be emitted from sunlit snowpacks, the source of which has been proposed to be either direct or indirect photo-oxidation of natural organic materials present in the snow. Although myriad studies have observed active processing of species within irradiated snowpacks, the fundamental chemistry occurring remains poorly understood. Here we consider the nature of snow at a fundamental, physical level; photochemical processes within snow and the caveats needed for comparison to atmospheric photochemistry; our current understanding of nitrogen, oxidant, halogen and organic photochemistry within snow; the current limitations faced by the field and implications for the future.

Concentration measurements of nitric oxide (NO), nitrogen dioxide (NO2), nitrous acid (HONO), nitric acid(HNO3), formaldehyde (HCHO), hydrogen peroxide (H2O2), formic acid (HCOOH) and acetic acid (CH3COOH) wereperformed in air filtered through the pore spaces of the surface snowpack (firn air) at Summit, Greenland, in summer2000. In general, firn air concentrations of NO, NO2, HONO, HCHO, HCOOH, and CH3COOH were enhancedcompared to concentrations in the atmospheric boundary layer above the snow. Only HNO3 and H2O2 normallyexhibited lower concentrations in the firn air. In most cases differences were highest during the day and lowest duringnighttime hours. Shading experiments showed a good agreement with a photochemical NOx source in the surface snow. Patterns of H2O2, CH3COOH, and HNO3 observed within the surface snow-firn air system imply that the number of molecules in the snow greatly exceeded that in the firn air. Deduced partitioning indicates that the largest fractions of the acids were present at the ice grainair interface. In all cases, the number of molecules located at the interface was significantly higher than the amount in the firn air. Therefore, snow surface area and surface coverage are important parameters, which must be considered for the interpretation of firn air concentrations.

Ambient gas-phase and snow-phase measurements of formaldehyde (HCHO) wereconducted at Summit, Greenland, during several summers, in order to understand the roleof air-snow exchange on remote tropospheric HCHO and factors that determine snowpackHCHO. To investigate the impact of the known snowpack emission of HCHO, a gas-phasemodel was developed that includes known chemistry relevant to Summit and that isconstrained by data from the 1999 and 2000 field campaigns. This gas-phase-only modeldoes not account for the high ambient levels of HCHO observed at Summit for severalprevious measurement campaigns, predicting approximately 150 ppt from predominantlyCH4 chemistry, which is ~2550% of the observed concentrations for several years.Simulations were conducted that included a snowpack flux of HCHO based on HCHOflux measurements from 2000 and 1996. Using the fluxes obtained for 2000, thesnowpack does not appear to be a substantial source of gas-phase HCHO in summer. The1996 flux estimates predict much higher HCHO concentrations, but with a strong dielcycle that does not match the observations. Thus, we conclude that, although the flux ofHCHO from the surface likely has a significant impact on atmospheric HCHO above thesnowpack, the timedependent fluxes need to be better understood and quantified. It isalso necessary to identify the HCHO precursors so we can better understand the nature andimportance of snowpack photochemistry.

Nitrogen oxides play a critical role in the chemistry of the atmosphere and indirectly influence global warming through the production of ozone. At Barrow, Alaska, the NOAA long-term surface ozone record indicates an increase of about 2% per year during the summer months. Since NOx (NO+NO2) concentrations above about 30 ppt (parts per trillion) result in net ozone production in the presence of sunlight, we propose that the observed Barrow surface ozone increase is related to anthropogenic nitrogen oxide emissions. A high-sensitivity chemiluminescent instrument for measurements of nitrogen oxides has been built to test this hypothesis. Measurement campaigns have been conducted during summer 1988 and spring 1989, and are continuing during spring and summer 1990. Periods during which the NOy concentrations measured at the GMCC site were unaffected by local (Barrow) emissions were selected from the data record. Observations during these periods suggest that nitrogen oxide concentrations are, at times, very elevated at Barrow and sufficient to account for photochemical 03 production. Based on simultaneous collection of meteorological, sulfur, and NO, data, several sources of nitrogen oxides have been tentatively identified at Barrow., This article is from 'Proceedings of the International Conference on the Role of Polar Regions in Global Change Held in Fairbanks, Alaska on 11-15 June 1990. Volume 2', AD-A253 028, p730-734. See also Volume 1, AD-A253 027.