Solubility and equilibrium vapor pressures of HCl dissolved in polar stratospheric cloud materials: Ice and the trihydrate of nitric acid

Measurements of the pressure‐solubility behaviour of HCl in water ice and in the nitric acid trihydrate (NAT) crystal at 200 K are reported. It was found that HCl is about 20 times more soluble in NAT than in ice for stratospheric conditions. A relation between HCl pressure and substrate composition based on the Gibbs‐Duhem equation is developed. This relation, along with other thermodynamic data, can be used to obtain the HCl pressure‐solubility behaviour at different temperatures. Implications of these results for the south polar ozone hole are discussed.