Frozen water is ubiquitous in the global environment: snow at high latitudes or high elevations, solid cloud particles in the upper troposphere or in the stratosphere, sea ice in both polar regions. Although solid water ice crystals dominate the structures of these particles, variable amounts of impurities are incorporated. Due to the specific properties of water, only small fractions of the impurities are included in the ice crystal. Larger fractions are located at the surfaces enhancing the formation of a quasi-liquid surface layer with interesting and specific properties. Due to the large enrichment of the impurities in the surface layer high concentrations of ionic and organic compounds are possible. Under the influence of the solar radiation remarkable photochemical reactions can occur in the surface layer, which are not commonly observed in the atmosphere. These reactions can lead to the formation of highly reactive compounds, which influence the composition of the atmosphere if they are released to the gas phase. A comprehensive understanding of these processes requires information about the nature of the surface layer, about the distribution and reactions of the impurities, and about the exchange of the impurities between the condensed phase and the gas phase. Examples of the formation of reactive compounds and the subsequent release to the atmosphere will be presented including photochemical reactions in surface snow and reactions in so-called frost flowers, which form under certain conditions on thin sea ice.