Formation of nitric acid/water ice particles in cirrus clouds

Abstract

Nitric acid (HNO₃) in cirrus ice crystals has been measured in the last decade during airborne field campaigns at latitudes 53°S–68°N. The HNO₃content in ice crystals, expressed in terms of HNO₃/H₂O molar ratio, and the fraction of HNO3 in ice derived from those measurements exhibit a clear upward trend with decreasing temperature. The observations are explained by a novel model describing dissolution of HNO₃ in liquid aerosol particles serving as freezing nuclei and subsequent trapping of HNO₃ during ice crystal growth. The efficiency of trapping increases with decreasing temperature. Efficient trapping occurs via diffusional burial of the ambient HNO₃ below about 203 K, because of long residence times of HNO₃ molecules at the ice surface. This opens the possibility for HNO₃-induced modifications of processes affecting ice crystal growth. At warmer temperatures, molecular processes in the ice surface layer cause an increasingly rapid escape of adsorbed HNO₃ into the gas phase and render trapping less efficient despite faster ice growth rates.