The Hydrolysis of Chlorine Nitrate on Ice Is Autocatalytic

A key chemical reaction in stratospheric ozone depletion is the reaction of chlorine nitrate (ClONO2) with water. This reaction is known to be catalyzed in the presence of ice particles found in polar stratospheric clouds (PSCs). However, the mechanism and time scale of the reaction, and the role of the ice surface, are not well understood. This surface second harmonic generation study shows that the submonolayer ClONO2 hydrolysis on basal ice (Ih) surfaces at 185 K occurs autocatalytically, with the product molecule HOCl acting as the autocatalyst. The other product, HNO3, acts to delay the reaction. The hydrolysis reaction is surprisingly slow, with induction times that range from 100 to 1000 s, depending upon how much reactant is initially present. It is proposed that the ice surface serves as a reservoir, enabling the reaction to be acid catalyzed.