HR: 0830h
AN: A11F-0062 [PDF]
TI: Surface Spectroscopy Studies of the Reactive Uptake of Ozone on Alkali Halides
AU: * Newberg, J T
EM: jnewberg@uci.edu
AF: University of California, Irvine
Department of Chemistry and Institute for Surface and
Interface Science, 516 Rowland Hall, Irvine, CA 92697 United States
AU: Hemminger, J C
EM: jchemmin@uci.edu
AF: University of California, Irvine
Department of Chemistry and Institute for Surface and
Interface Science, 516 Rowland Hall, Irvine, CA 92697 United States
AB:
Heterogeneous reactions in the atmosphere have attracted a lot of attention. In particular, reactions involving sea-salt in
the form of aerosol droplets, particles, and/or sea-ice have been implicated to significantly affect the chemistry and
composition of the marine boundary layer. For example, highly reactive chlorine and bromine atoms resulting from the
oxidation of sea-salt halides (Cl$^{-}$ and Br$^{-}$) have been implicated in tropospheric ozone depletion in the arctic and
in lower latitude marine regions, as well as the deposition of mercury. While the heterogeneous processing of sea-salt has
been studied extensively in laboratory, field and model studies, the mechanistic details behind the release of gas-phase
halogens remains unclear and has sparked some interests.
Recently there has been attention focused on the interaction of important atmospheric oxidants (e.g., OH and O$_{3}$) with
halides that reside at the air-particle interface of sea-salt. Such chemical interactions at the surface of particles may
lead to unique chemical transformations that can alter current views of known chemical processing of sea-salt particles.
There are several laboratory investigations which have investigated the surface reactivity of salts by measuring the reactive
loss and/or formation of gas-phase species, indicating that reactions at the interface likely play an important role in
aerosol chemistry. The efficacy of such surface-phase chemistry has yet to be elucidated with surface spectroscopy studies.
X-ray photoelectron spectroscopy (XPS) is a surface spectroscopy technique with submonolayer resolution. Using XPS, we have
investigated changes in the surface chemistry of various alkali halide salts upon exposure to ozone in an ultra-high vacuum
(UHV) instrument. Salt samples were either freshly cleaved single crystals which were prepared from a melt, or purified salt
crystals/powders pressed into pellets. Upon exposure to ozone, oxygen on the salt surfaces was monitored by measuring the
O(1s) photoelectron peak. Initial XPS spectra of salts in the vacuum chamber prior to ozone exposure indicated that the
salts were either devoid of oxygen, or contained a small amount of oxygen from strongly adsorbed water under ultra-high
vacuum.
In all cases, in-vacuo exposure of the salts to ozone lead to an increase in O(1s) photoelectron signal. For NaCl this was
surprising since previous studies have been interpreted to indicate that ozone is essentially unreactive towards solid NaCl.
The initial uptake of oxygen on the surface gave a broad O(1s) peak, indicating that there are several types of oxygen
species on the surface. Reference spectra were obtained for NaClO$_{2}$, NaClO$_{3}$, and NaClO$_{4}$ salts. The O(1s)
binding energies in these reference spectra increased in series with NaClO$_{2}$ $<$ NaClO$_{3}$ $<$ NaClO$_{4}$. Based on
these results, it is suggested that the reaction of ozone with pure NaCl initially leads to the formation of NaClO$_{2}$ and
NaClO$_{3}$, with the formation of NaClO$_{4}$ at higher ozone doses. The oxygen uptake upon ozone exposure to alkali
bromide and iodide salts were also investigated and compared to reference salts. Future work will incorporate the influence
of adsorbed water and subsequent ozone exposures on the surface chemistry of salts.
DE: 0305 Aerosols and particles (0345, 4801)
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting