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

Abstract

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 (RH_ice), 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.