HCl vapour pressures and reaction probabilities for ClONO 2 + HCl on liquid H 2 SO 4 –HNO 3 –HCl–H 2 O solutions

Henry's Law solubility constants for HCl have been measured for liquid H2SO4–HNO3–HCl–H2O solutions; the results are in good agreement with predictions from published semiempirical models. The ClONO2+ HCl reaction on the surfaces of such solutions with compositions simulating those of stratospheric aerosols has been investigated; as the composition changes following the temperature drop characteristic of the high-latitude stratosphere the reaction probability γ increases rapidly. Furthermore, the γ values remain essentially unchanged when HNO3 uptake is neglected; the controlling factor appears to be the solubility of HCl. These results corroborate our earlier suggestion that supercooled liquid sulfate aerosols promote chlorine activation at low temperatures as efficiently as solid polar stratospheric cloud particles.