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

• Published in: Science (American Association for the Advancement of Science) 1987-11, Vol.238 (4831), p.1258-1260
• Main Authors: Tolbert, Margaret A., Rossi, Michel J., Malhotra, Ripudaman, Golden, David M.
• Format: Article
• Language: English
• Subjects: Antarctica; Atmosphere; Atmospheric chemistry; Atmospheric composition. Chemical and photochemical reactions; Chlorides; Chlorine; Composition; Earth, ocean, space; Environment Pollution; Exact sciences and technology; External geophysics; Gases; Hydrogen; Hydrogen chloride; Ice; Knudsen gauges; Nitrates; Ozone; Ozone depletion; Ozone layer depletion; Physics of the high neutral atmosphere; Properties; Reactivity; Stratosphere; Surface areas; Water pressure
• ISSN: 0036-8075; 1095-9203
• DOI: 10.1126/science.238.4831.1258

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 $\pm $ 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.