Abstract. 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 NO x 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.

[1] An off-line three-dimensional tropospheric chemical transport model, parallelTropospheric Off-Line Model of Chemistry and Transport (p-TOMCAT), has been extended by incorporating a detailed bromine chemistry scheme that contains gas-phase reactions and heterogeneous reactions on both cloud particles and background aerosols. Bromine emission from bromocarbon photo-oxidation and from sea-salt bromine depletion and bromine removal through dry and wet deposition are included. Using this model, tropospheric bromine chemistry and ozone budgets are studied. The zonal mean of the inorganic gas-phase bromine compounds (Br x ) is calculated to be high (4-8 pptv) in the lower troposphere of the midlatitudes to high latitudes in each hemisphere, with decreasing trends with altitude (down to $2-3 pptv in the upper troposphere). The lowest Br x (<2 pptv) is over low latitudes, corresponding to small sea-salt Br emission and a high rate of precipitation scavenging. A mean lifetime of $5 days is obtained for the tropospheric Br x . Sea-salt emission plays the dominant role in total Br x in the lower troposphere while organic Br-containing compounds are important in upper layers. High daytime BrO mixing ratios (>1 pptv) are found over the high-latitude ocean surface, corresponding to high tropospheric column BrO values of up to 1.6 Â 10 13 molecules/cm 2 in the monthly mean. The addition of bromine chemistry to the model leads to a reduction in tropospheric ozone amounts by 4-6% in the Northern Hemisphere and up to $30% in the Southern Hemisphere high latitudes. The net ozone loss depends not only on total Br x , but also on solar irradiance, especially at high latitudes. The hydrolysis reaction of bromine nitrate, which occurs on cloud and aerosol surfaces (BrONO 2 + H 2 O aq ! HOBr + HNO 3 ), has a significant influence on ozone chemistry through its effect on NO x as well as on reactive BrO and Br.

[1] A global three-dimensional chemical transport model has been used to simulate atmospheric bromoform using a variety of prescribed surface emission scenarios and a simple atmospheric chemistry scheme. Model simulations indicate that global emissions of bromoform calculated previously using top-down methods are too low, and emissions are likely to be significantly larger than suggested in the World Meteorological Organization's reports on the Scientific Assessment of Ozone Depletion of 1998 and 2002. Our simulations suggest that global emissions of bromoform are in the range of 400-600 GgCHBr 3 /yr and that a large proportion of emissions are situated in tropical regions. Both these factors are likely to have an important influence on estimates of the quantity of bromine transported from the surface to the lower stratosphere by short-lived bromocarbon species and its subsequent impact on ozone in this region. Further simulations including methyl bromide, bromoform and the other major short-lived bromocarbons emitted from the ocean (CH 2 Br 2 , CH 2 BrCl, CHBr 2 Cl, CHBrCl 2 ) provide estimates on the amount of reactive bromine in the troposphere and lower stratosphere derived from these compounds.