Static SIMS Studies of Reactions on Mimics of Polar Stratospheric Clouds III: Mechanism of Chlorine Nitrate Decomposition and Reaction

Abstract

Static secondary ion mass spectrometry (SSIMS) was used to probe the interactions of chlorine nitrate (ClONO{sub 2}) or of chlorine nitrate plus HCl with condensed water ice films to probe the molecular interactions that may occur on polar stratospheric clouds. The experiments were carried out in an ultrahigh vacuum chamber on ice films at temperatures between 90 and 170 K and at pressures up to 5 x 10{sup {minus}7} mbar. A reference set of SSIMS spectra obtained at <100 K on gold and on ice were used as the basis set from which to probe molecular transformations and surface reactions. at 90--100 K chlorine nitrate adsorbs on gold or ice as a molecular species. Depth profiling experiments through chlorine nitrate multilayers laid down on an ice film indicated a change in the degree of polarization of the Cl-O bond in the chlorine nitrate molecules as proximity to water molecules increased (i.e., as the chlorine nitrate/ice interface was approached). Upon warming, the chlorine nitrate began to desorb from the ice film at 120 K. Despite the polarizing influence of the water molecules, no hydrolysis products were observed under these conditions. In contrast, the ClONO{sub 2} reacted readily above 120 K whenmore » HCl was present. Formation of nitrate ions, implying the formation of surface-adsorbed nitric acid was indicated by the appearance of an NO{sub 3}{sup {minus}} signal in the negative ion SSIMS spectrum. This was accomplished by a simultaneous increase in the partial pressure of chlorine species in the gas phase. It was thus concluded that chlorine nitrate can react directly in the presence of the strong solvated Cl{sup {minus}} nucleophile to form molecular chlorine (which desorbs) and solvated or condensed nitric acid.« less