Nitric acid partitioning in cirrus clouds: the role of aerosol particles and relative humidity

Owing to the potential of cirrus clouds to vertically redistribute HNO3 in the upper troposphere, the uptake of HNO3 in ice particles is at the focus of recent research. Here, we investigate whether HNO3 residing in freezing aerosol particles could be a relevant source of HNO3 in ice clouds. To this end, model studies on the sensitivity of the uptake of HNO3 in aerosol particles to temperature, relative humidity with respect to ice (RHice), particle composition and amount of available HNO3 were performed. Combining the model results with the history of RHice during cirrus formation and comparison with field measurements indicates that at temperatures <200 K a considerable part of the HNO3 in ice may originate from freezing particles containing HNO3