Mechanism of the heterogeneous reaction of hydrogen chloride with chlorine nitrate and hypochlorous acid on water ice

The interactions of HOCl and ClONO2 with pure and HCl-doped water ice have been reinvestigated using infrared spectroscopy in conjunction with static and thermal desorption mass spectrometry in the temperature range 140-180 K to probe the detailed mechanisms of their heterogeneous atmospheric reactions. In agreement with earlier studies, HOCl was found to react with hydroxonium chloride species to directly produce molecular chlorine. This molecular chlorine desorbed from the surface above ca. 155 K either directly or in subsequent thermal desorption experiments, depending upon substrate temperature at the time of reaction. ClONO2 was observed to heterogeneously hydrolyze upon water ice to produce HOCl and hydrated H3O+NO3-. Below ca. 155 K, HOC1 remains adsorbed on the ice surface, while above ca. 155 K, it desorbs directly into the gas phase. These results suggest that a long-lived, adsorbed state of HOC1 is unlikely to play a direct role in heterogeneous chemistry at stratospheric temperatures. The direct reaction of ClONO2 with an ice surface saturated with ionic hydrates of HCl was observed to result in the production of molecular chlorine, which, as for the HOCl reaction, either remained adsorbed or desorbed from the surface depending upon temperature. This provides convincing evidence for the direct heterogeneous reaction of Cl- with ClONO2 under stratospheric conditions. A novel explanation for the enhanced reactivity of ClONO2 toward atmospherically relevant substrates is presented, in which a partially ionized precursor state is formed. Partial ionization of ClONO2 along the Cl-O bond serves to increase the electrophilicity of the chlorine atom, making the site highly prone, to nucleophilic attack by either water or adsorbed chloride ions.