The results of laboratory investigations of the freezing behavior of aqueous acid solutions indicate that in the stratosphere H2SO4/H2O aerosol droplets would not freeze at temperatures above the ice frost point in the absence of HNO3; however, in the presence of typical levels of HNO3 liquid sulfuric acid aerosols take up significant amounts of HNO3 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 temperature at which HNO3 and H2O vapors would co‐condense to form a liquid solution. In the high latitude stratosphere this HNO3/H2O ‘dew point’ is typically around 192–194 K at 100 mbar.

We would like to acknowledge Th. Peter, B.P. Luo, ICS. Carslaw and S.L. Clegg for pointing out the need to add the above eomtraints to equatiom i and 2.