COMPOSITION AND FREEZING OF AQUEOUS H2SO4 HNO3 SOLUTIONS UNDER POLAR STRATOSPHERIC CONDITIONS/

The results of laboratory investigations of the freezing behavior of a queous acid solutions indicate that in the stratosphere H2SO4/H2O aero sol droplets would not freeze at temperatures above the ice frost poin t in the absence of HNO3; however, in the presence of typical levels o f HNO3 liquid sulfuric acid aerosols take up significant amounts of HN O3 and H2O vapors and freeze much more readily. This is a consequence of the very rapid change in composition of the liquid droplets as the temperature drops to within two to three degrees of the equilibrium te mperature at which HNO3 and H2O vapors would co-condense to form a liq uid solution. In the high latitude stratosphere this HNO3/H2O 'dew poi nt' is typically around 192-194 K at 100 mbar.