Snow can cover a significant fraction of the land mass on Earth. The snowpack itself is an active chemical reactor under the influence of trace gas exchange and sunlight. Understanding the partitioning behaviour of trace gases to ice helps assessing the impact of the chemical processes in snow on atmospheric composition, it helps assessing the impact of deposition of trace gases to the local environment, and it helps elucidating the relation between atmospheric composition and the composition of ice core archives used to reconstruct past climates. Snow is a highly dynamic medium that undergoes continuous metamorphosis induced by non-equilibrium processes due to always present and changing temperature gradients. This project is heading towards understanding these non-equilibrium processes in more detail.Nitrous acid (HONO) is an important atmospheric nitrogen oxide species. It is easily photolysed by sunlight and thereby is an important source of OH radicals in the troposphere. Its sources in the troposphere are still not fully established. In the snowpack it can be produced from photolysis of nitrate and through surface reactions of nitrogen dioxide. Its release from the snowpack into the air aloft depends on temperature and snow properties. We have chosen HONO to probe the chemical aspects of non-equilibrium processes due its significance for the oxidant budget in the polar boundary layer.In a collaborative project with the Snow and Avalanche Research Institut (SLF) in Davos, the physical and chemical aspects of metamorphosing snow are being explored since about 3 years. The project combines X-ray micro-tomography of metamorphosing snow samples and chemical uptake experiments under equilibrium and non-equilibrium conditions. We first investigated partitioning of nitrous acid and acetic acid under equilibrium conditions alone and in competition with each other. Acetic acid should help assessing the effects of other acidic species typically present in snow. Using the combination of tomography with chemical experiments, we have observed that the ice surface is smooth up to the scale of tens of micrometers, i.e., it does usually not contain nanostructures. With regard to non-equilibrium conditions, we have performed first successful combined experiments with a novel snow diffusion chamber that allows monitoring diffusion of nitrous acid (labeled with 13N) within a snow sample under the same geometrical and temperature gradient conditions as in a natural snow cover. The last task will be to perform uptake experiments of nitrous acid to ice in a new setup that allows varying ice growth rates over a large range to get more fundamental insights into the dynamics at the air - ice interface.