Description: Snow in the environment acts as a host to rich chemistry and provides a matrix for physical exchange of contaminants within the ecosystem. The goal of this review is to summarise the current state of knowledge of physical processes and chemical reactivity in surface snow with relevance to polar regions. It focuses on a description of impurities in distinct compartments present in surface snow, such as snow crystals, grain boundaries, crystal surfaces, and liquid parts. It emphasises the microscopic description of the ice surface and its link with the environment. Distinct differences between the disordered air–ice interface, often termed quasi-liquid layer, and a liquid phase are highlighted. The reactivity in these different compartments of surface snow is discussed using many experimental studies, simulations, and selected snow models from the molecular to the macro-scale. Although new experimental techniques have extended our knowledge of the surface properties of ice and their impact on some single reactions and processes, others occurring on, at or within snow grains remain unquantified. The presence of liquid or liquid-like compartments either due to the formation of brine or disorder at surfaces of snow crystals below the freezing point may strongly modify reaction rates. Therefore, future experiments should include a detailed characterisation of the surface properties of the ice matrices. A further point that remains largely unresolved is the distribution of impurities between the different domains of the condensed phase inside the snowpack, i.e. in the bulk solid, in liquid at the surface or trapped in confined pockets within or between grains, or at the surface. While surface-sensitive laboratory techniques may in the future help to resolve this point for equilibrium conditions, additional uncertainty for the environmental snowpack may be caused by the highly dynamic nature of the snowpack due to the fast metamorphism occurring under certain environmental conditions. Due to these gaps in knowledge the first snow chemistry models have attempted to reproduce certain processes like the long-term incorporation of volatile compounds in snow and firn or the release of reactive species from the snowpack. Although so far none of the models offers a coupled approach of physical and chemical processes or a detailed representation of the different compartments, they have successfully been used to reproduce some field experiments. A fully coupled snow chemistry and physics model remains to be developed.

Description: The acidity of sea ice and sea ice aerosols plays a key role in the reactivity of the cryosphere, leading to or mediating processes such as the oxidation of bromine, which results in tropospheric ozone depletion events. We performed laboratory experiments to assess the acidity and subsequently used an environmental scanning electron microscope to observe the particles that emanate from the sublimating sea ices. For the acidity assessment, we propose a spectrophotometric method based on sulfonephthalein indicators employed in the frozen state. The diffuse reflectance UV-Vis approach thus allows estimating the local acidity at the level of molecular interactions and at environmentally relevant temperatures. Our results show a strong freezing-induced acidity increase in sea water, especially as regards solutions of low salinity. Importantly, the microscopic observation of sea ice sublimation revealed a major dependence of the temperature and concentration on the emanating aerosol-sized salt particles. In this context, the sublimation temperature of the ice is a dominant physical factor to determine the size of the residua: Below −20 °C, micron-sized pieces of salt emerge, whereas above this temperature large chunks of salt are detected. Concentration also plays a role in particle size distribution: Micron-sized particles are observed exclusively at salinities below 3.5 psu, while below 0.085 psu particles with a median smaller than 6 μm arise from sea ices at any subzero temperature.Generally, we relate our findings to the production of the polar atmospheric sea salt aerosols and acid-catalyzed reactivity reactions (e.g., photochemical bromine recycling).