Reaction of Chlorine Nitrate with Hydrogen Chloride and Water at Antarctic Stratospheric Temperatures

Abstract

Laboratory studies of heterogeneous reactions important for ozone depletion over Antarctica are reported. The reaction of chlorine nitrate (ClONO 2 ) with H 2 O and hydrogen chloride (HCl) on surfaces that simulate polar stratospheric clouds [ice and nitric acid (HNO 3 )—ice and sulfuric acid] are studied at temperatures relevant to the Antarctic stratosphere. The reaction of ClONO 2 on ice and certain mixtures of HNO 3 and ice proceeded readily. The sticking coefficient of ClONO 2 on ice of 0.009 ± 0.002 was observed. A reaction produced gas-phase hypochlorous acid (HOCl) and condensed-phase HNO 3 ; HOCl underwent a secondary reaction on ice producing dichlorine monoxide (Cl 2 O). In addition to the reaction with H 2 O, ClONO 2 reacted with HCl on ice to form gas-phase chlorine (Cl 2 ) and condensed-phase HNO 3. Essentially all of the HCl in the bulk of the ice can react with ClONO 2 on the ice surface. The gaseous products of the above reactions, HOCl, Cl 2 O, and Cl 2 , could readily photolyze in the Antarctic spring to produce active chlorine for ozone depletion. Furthermore, the formation of condensed-phase HNO 3 could serve as a sink for odd nitrogen species that would otherwise scavenge the active chlorine.