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 HNO3 content 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.