Abstract. 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.

We look ahead from the frontiers of research on ice dynamics in its broadest sense; on the structures of ice, the patterns or morphologies it may assume, and the physical and chemical processes in which it is involved. We highlight open questions in the various fields of ice research in nature; ranging from terrestrial and oceanic ice on Earth, to ice in the atmosphere, to ice on other solar system bodies and in interstellar space.

The alkali metal release during pyrolysis of biomass is investigated with a surface ionization method. Wheat straw samples (20 mg) are pyrolyzed in a laboratory unit under N2 atmosphere, and two characteristic temperature intervals for alkali metal emission are identified. A small fraction of the alkali metal content is released in a low-temperature region (180−500 °C) and is attributed to a connection with the decomposition of the organic structure. The two most pronounced emission processes below 500 °C are well described by a first-order rate behavior, and the activation energies are found to be 156 ± 11 and 178 ± 8 kJ/mol. The major part of the alkali metal release takes place in the high-temperature region (>500 °C), and activation energies of alkali metal emission from the ash residues are found in the range 168−238 kJ/mol. A high chlorine content is found to enhance the alkali metal emission from the ash, while the alkali metal release in the low-temperature region cannot be correlated with the chlorine content.

We present the results of coupled quantum mechanics and molecular mechanics (QM/MM) classical molecular dynamics simulations for HCl sticking to the (0001) basal plane of ice Ih. Interatomic forces and energies of hydrogen chloride and up to 24 water molecules in the top ice bilayer were obtained from semiempirical molecular orbital calculations based on the PM3 method. A few PM3 parameters were adjusted so that structural and energetic properties of small neutral and ionic systems match available ab initio and experimental data. This QM region was coupled to the remainder of the ice surface (the MM region), which was treated using the analytic TIP4P force field. The surface temperature was between 0 and 180 K, and the dynamics was followed for 100 ps. On surface impact, HCl binds to a dangling (free) H 2 O oxygen via a ClH-OH 2 hydrogen bond. If the Cl is solvated by one dangling H 2 O hydrogen, HCl adsorbs molecularly. If two dangling hydrogens are available in a surface hexagon, HCl dissociates to a Cl --H 3 O + contact ion pair. The simulations thus predict a mechanism by which HCl can ionize readily on ice surfaces. This mechanism is consistent with a saturation coverage of 0.33 monolayers for ionized HCl on ice surfaces. As a comparison we have also simulated HCl colliding with a cubic (H 2 O) 8 cluster, in which the whole system was treated by the semiempirical method. Hydrogen chloride adsorbs on the cluster and, depending on the temperature, the (H 2 O) 8 cube may open up, thereby initiating HCl ionization. The results are discussed in relation with stratospheric heterogeneous ozone chemistry and available experimental and theoretical results.

The dynamics of HCl collisions with ice surfaces is studied using molecular beam techniques. The experiments are carried out with a water vapor pressure of up to 3 × 10-5 mbar outside the ice surface, which allows experiments to be performed at surface temperatures of 127−180 K. At the higher surface temperatures, the ice has a very dynamic character and constantly undergoes evaporation and condensation. Angular-resolved intensity and time-of-flight distributions are measured with mass spectrometry, and the effects of surface temperature, incident kinetic energy, and HCl surface coverage are investigated. The dominating outcome of the surface interaction is loss of HCl by sticking to the surface with a residence time of more than 1 ms. Small direct scattering and trapping-desorption channels are also observed depending on the conditions. For a pure ice surface the sticking probability is 1.00 ± 0.02 at thermal incident kinetic energies, E, while a small direct scattering channel is observed when E is increased, reaching a probability of 0.015 ± 0.005 at E = 0.53 eV. For HCl-covered ice surfaces at 165 K and with thermal incident energies, the sticking probability is 0.88 ± 0.03 and a trapping-desorption channel (surface residence time less than 30 μs) with a probability of 0.12 ± 0.02 is also observed. A direct scattering channel opens at higher energies, reaching a probability of 0.08 ± 0.02 at E = 0.53 eV. For all surface conditions, the collisions are highly inelastic with large energy loss observed for the directly scattered flux, comparable to the results for the previously studied Ar−ice system.