INVESTIGATION OF THE REACTIVE AND NONREACTIVE PROCESSES INVOLVING CLONO2 AND HCL ON WATER AND NITRIC-ACID DOPED ICE

A flow tube reactor attached to a chemical ionization mass spectrometer was used to investigate the reactions of ClONO2 with H2O and HCl on pure water ice and nitric acid doped ice (referred to as NAT). The uptake coefficients, gamma, for the above reactions were found to be nearly independent of the ice substrate thickness, suggesting that the geometrical surface areas of our ices are close to the true surface areas for such reactions. At low concentrations of HCl, close to those found in the atmosphere, its uptake due to physical adsorption was found to be very rapid, i.e., gamma > 0.3; however it slowed considerably when a monolayer of HCl had been deposited on the surface. The surface of both water and NAT ice are shown to be saturated with HCl after the formation of a monolayer, suggesting that reactions involving HCl are confined to the surface. At high HCl concentrations the surface was altered by the formation of a hydrate or melting. The reaction of ClONO2 with H2O on pure ice was found to generate HOCl, which has an appreciable adsorptivity on the surface. The direct uptake of HOCl on water ice was observed and its adsorptivity was found to decrease at warmer temperatures. A direct reaction between HOCl and HCl to produce Cl2 on both pure ice and a NAT surface was observed. It is proposed that the reaction of ClONO2 with HCl on water ice can proceed through the formation of HOCl and its subsequent reaction with HCl to give Cl2, while the reaction is direct on HNO3-doped ice. The implication of these findings to the surface reactions in the atmosphere are discussed.