AVS 46th International Symposium
    Surface Science Division Monday Sessions
       Session SS3-MoM

Paper SS3-MoM7
The Effect of Water on the Nitric Acid Pressure Dependence of the Reaction Between Gas Phase Nitric Acid and NaCl Surfaces.@footnote 1@

Monday, October 25, 1999, 10:20 am, Room 612

Session: Water-Surface Interactions
Presenter: J.C. Hemminger, University of California, Irvine
Authors: J.C. Hemminger, University of California, Irvine
S. Ghosal, University of California, Irvine
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The reactions of oxides of nitrogen with sea salt particles, that liberate halogenated compounds into the gas phase, may play an important role in the chemistry of the marine troposphere. Recent laboratory measurements have shown that the reactive sticking coefficient of HNO@sub 3@ on NaCl particles exhibits a significant dependence on the HNO@sub 3@ pressure, when the measurements are carried out under conditions of steady state reaction. This pressure dependence has been rationalized in terms of a two-site Langmuir model for dissociative adsorption of the HNO@sub 3@. However, our XPS experiments show that the dissociative adsorption of HNO@sub 3@(g) on NaCl to form NaNO@sub 3@(s) and HCl(g) follows single-site Langmuir adsorption behavior. We also used XPS to show that the amount of "strongly adsorbed water" on the surfaces of NaCl particles strongly depends on the particle size. Particles of 1-10 micron diameter show large quantities of adsorbed water that remain on the sample up to temperatures of 200 degrees C. Particles in the size range of 500 micron diameter have less, but still easily measurable, amounts of strongly adsorbed water. We present a model for the HNO@sub 3@ pressure dependence of the reactive sticking coefficient of HNO@sub 3@ on NaCl under steady state reaction conditions. The origin of the pressure dependence in the model is the competition between site blocking on the surface by the build-up of the NaNO@sub 3@ reaction product and the water induced 3-d recrystallization of the NaNO@sub 3@ that frees up reactive sites for further reaction. @FootnoteText@ @footnote 1@ This work was supported by the National Science Foundation Grant ATM-9707285.