A polar cirrus case study is discussed with the help of a one-dimensional model with explicit aerosol and ice microphysics. It is demonstrated that continuous cooling of air in regions with small amounts of ice and slow ice deposition rates of water vapor drives 5 significant in-cloud supersaturations over ice, with potentially important consequences for heterogeneous halogen activation. Radiatively important cloud properties such as ice crystal size distributions are investigated, showing the presence of high number concentrations of small crystals in the cloud top region at the tropopause, broad but highly variable size spectra in the cloud interior, and mostly large crystals at the cloud 10 base. It is found that long-lived and vertically extended Arctic cirrostratus are highly efficient at dehydrating the upper troposphere. Estimating nitric acid uptake in cirrus clouds with an unprecedented treatment of diffusional burial in growing ice crystals suggests that such clouds could also denitrify upper tropospheric air masses efficiently, but a closer comparison to observations is needed to draw a definite conclusion on this 15 point. It is also shown that low temperatures, high relative humidities, and the absence of ice above but close to the cloud top region cause efficient uptake of nitric acid in background aerosol particles.