The Interactions of Stratospherically Abundant Compounds with Ultrathin Ice Films

To gain understanding of heterogeneous chemistry that occurs in the stratosphere, ultrahigh vacuum-based analysis techniques have been used to study the interactions of HCl, OClO, and Cl_2 with amorphous and crystalline ice films between 10-100 monolayers thick. The study of ultrathin films allows for the distinction between phenomena occurring on the surface and those occurring within the film bulk. Hydrogen chloride interacts with ice to produce two states, one associated with ionized HCl in the bulk and the second with HCl adsorption on the surface. The bulk state, which is the only one present at low HCl exposures, has been assigned to HCl cdot6H_2O. The adsorbed state has been tentatively assigned to molecularly adsorbed HCl. Adsorption of tert-butyl alcohol on amorphous HCl cdot6H_2O results in tert-butyl chloride formation, indicating that chloride ions in the near surface region can react with adsorbates. It was also determined that the sticking coefficient of HCl on crystalline ice is ~40% of that on amorphous ice. Coadsorption of propene and chlorine on ice results in 1,2-dichloropropane formation. The observation that chlorine adsorbed on ice is reactive suggests that there may be heterogeneous pathways for chlorine consumption in the stratosphere. The interaction of OClO with ice was studied in order to predict whether ice particles perturb OClO photochemistry. The desorption rate parameters and sticking coefficient of OClO on ice were used to estimate the equilibrium OClO coverage on ice at stratospherically relevant temperatures and pressures. The value was estimated to be ~10^{ -4} monolayer. Although this value is too low to lead to a significant amount of ozone depletion via heterogeneous chemistry, the value does match well with the equilibrium coverage measured at stratospherically mimetic conditions. The good agreement between the calculated and measured OClO coverages show that ultrahigh vacuum -based techniques can be used to study atmospheric chemistry. Finally, the photolysis of OClO thin films and OClO adsorbed on ice was investigated.