The heterogeneous reaction of HOCl + HCl → Cl2 + H2O on ice and nitric acid trihydrate: Reaction probabilities and stratospheric implications

Abstract

The heterogeneous reaction HOCl + HCl → Cl2(g) + H2O(s) has been observed to proceed readily on cold water‐ice and nitric acid trihydrate (NAT) surfaces. For reactant partial pressures in the 10−7–10−5 torr range, the reaction probability (γ) on water‐ice is found to be 0.16 ± 0.10 at 202 K, and 0.24 +0.5/−0.15 at 195 K, as measured by the decay of HOCl vapor when the HCl partial pressure is above that of HOCl. On NAT, γ is found to have a strong dependence on the H2O vapor pressure of the NAT crystals: at 202 K, reaction probabilities of 0.17 are measured for H2O vapor pressures approaching those of ice (H2O‐rich NAT), whereas γ drops to 0.002 if the H2O pressure is lower by a factor of five (HNO3‐rich NAT). The yield of Cl2(g) relative to HOCl(g) loss is measured to be 0.87 ± 0.20. These results suggest that the heterogeneous process ClONO2 + HCl → Cl2(g) + HNO3(s), which occurs on polar stratospheric cloud surfaces, may be comprised of two steps: ClONO2 + H2O(s) → HOCl + HNO3(s), followed by HOCl + HCl → Cl2(g) + H2O(s). Also, the HOCl + HCl reaction may be directly of importance in the polar stratosphere providing an additional path for chlorine activation.