Ice-catalyzed ionization of hydrochloric acid

Ionization of hydrochloric acid (HCl) on stratospheric ice particles is bel ieved to be a key step in the depletion of stratospheric ozone. Ab initio c alculations based on a model HCl-water cluster show that HCl ionization on ice surfaces is a barrierless process. Since this mechanism is rapid and pr oduces chloride anions that are exposed to ambient stratospheric chlorine r eservoir molecules, it is likely to be important for stratospheric chemistr y. It complements a previously suggested mechanism where HCl forms part of the ice lattice before ionizing. The mechanism proposed here is also consis tent with experimentally observed ionization of HCl on ice at low temperatu res and under vacuum, where the HCl is not expected to be encapsulated in t he ice lattice.