HCL dissolved in solid mixtures of nitric acid and ice: Implications for the polar stratosphere

The solubility of HCl in polar stratospheric cloud (PSC) particles plays an important role in the heterogeneous chemistry of the lower polar stratosphere. New laboratory studies are reported showing a strong dependence of the HCl solubility on the HNO 3 content in ice particles. At 200 K and a partial HCl pressure of 10 −6 Torr, the HCl content in NAT is 0.35 mol%, decreasing about a factor of 3 for every 10 fold decrease in the substrate's HNO 3 content. At an HCl pressure of 10 −7 Torr the content is about 40% of that at 10 −6 Torr. HCl dissolved in pure water ice at these partial pressures is less than 0.002 mol%. The surface coverage of HCL on small ice samples was estimated to be about 0.1 monolayer at 10 −6 Torr exposure.