HR: 0800h
AN: A51E-0829    [Abstracts]
TI: Surface Reactions of Dry Alkali Halides Salts With Ozone and the Influence of Water Vapor
AU: * Newberg, J T
EM: jnewberg@uci.edu
AF: Department of Chemistry, 516 Rowland Hall, Irvine, CA 92697
AU: Hemminger, J C
EM: jchemmin@uci.edu
AF: Department of Chemistry, 516 Rowland Hall, Irvine, CA 92697
AB: Heterogeneous reactions of sea-salt aerosol particles and sea ice have been implicated in the chemistry and composition of the marine boundary layer. For example, reactions of important atmospheric oxidants (e.g., hydroxyl radical and ozone) with seasalt halides lead to the formation of reactive halogen radicals that can significantly affect tropospheric ozone concentrations and the deposition of mercury. A number of previous laboratory experiments have investigated the reactivity of ozone with salts by measuring the reactive loss and/or formation of gas-phase species. Results from these studies indicate that reactions at the interface can play an important role in the heterogeneous chemistry of aerosols. However, the efficacy of the surface chemistry of ozone with alkali halide salts has yet to be elucidated with surface spectroscopy studies. Using X-ray photoelectron spectroscopy (XPS), we have investigated changes in the surface composition of various alkali halide salts in our ultra-high vacuum (UHV) instrument as they are exposed to ozone. This was done by monitoring the O(1s), Cl(2p), Br(3d) and I(3d) photoelectron peaks. Peak binding energies (BE) provide species identification and integrated areas provide a quantitative analysis of the surface. Salt samples were either freshly cleaved single crystals or high grade salt crystals pressed into pellets. In order to identify the products formed at the surface of the oxidized salts, reference spectra were obtained from commercially available salts of NaClO$_{x}$ (x = 1 - 4), NaBrO$_{x}$ (x = 3) and KIO$ª_{x}$ (x = 3, 4). The in-vacuo exposure of ozone to dry alkali halides led to an uptake of oxygen on the surface. The exposure of ozone to NaCl and NaBr single crystals leads to a relatively small uptake of oxygen, with O(1s) BE's consistent with the reference spectra of NaClO$_{3}$ and NaBrO$_{3}$, respectively. However, due to the small amount of oxygen uptake, there was no evidence of chloride or bromide oxidation in the Cl(2p) and Br(3d) spectra, respectively. The reaction of ozone with the surface of a KI pellet led to the rapid formation of KIO$_{3}$ on the surface. This was evidenced in both the build up of oxygen on the surface (O(1s) spectra) and the oxidation of iodide to iodate (I(3d$_{5/2}$) spectra). Water vapor exposure to KI below the deliquescence point, both before and after exposure to ozone, changes the surface chemistry. For example, a dry KI surface was exposed to ozone to generate a surface composition containing a mixture of iodide and iodate. Upon exposing this oxidized surface to water vapor below the deliquescence of KI, the iodate disappeared. This water induced chemistry is consistent with the oxidation of iodide by iodate, leading to the liberation of molecular iodine (Dushman reaction). Thus, although KI was exposed water vapor below the deliquescence point, the chemistry at the salt surface appears to be aqueous-like. Unlike the case with ozone reacting with a "dry" KI surface, the availability of surface adsorbed water appears to allow for the evolution of iodine into the gas phase. Similar experiments are being performed on mixed NaCl/NaClO$_{3}$ and NaBr/NaBrO$_{3}$ surfaces. The effects of water vapor exposure to these oxidized salt surfaces will be discussed.
DE: 0300 ATMOSPHERIC COMPOSITION AND STRUCTURE
DE: 0305 Aerosols and particles (0345, 4801)
SC: Atmospheric Sciences [A]
MN: 2004 AGU Fall Meeting