AVS 49th International Symposium
    Surface Science Tuesday Sessions
       Session SS2-TuA

Paper SS2-TuA3
XPS Study on Surface Segregation of Bromide in Bromide-doped NaCl Crystals by Water Vapor Exposure

Tuesday, November 5, 2002, 2:40 pm, Room C-110

Session: Atmospheric Surface Chemistry
Presenter: K. Inazu, University of California, Irvine
Authors: K. Inazu, University of California, Irvine
J.T. Newberg, University of California, Irvine
J.C. Hemminger, University of California, Irvine
Correspondent: Click to Email
Drastic decrease of trophospheric ozone in the Arctic at polar spring has been observed accompanying an increase of particulate bromide.@footnote 1@ The chain reactions involving sea salt bromide were proposed to be responsible for the ozone depletion.@footnote 2@ While current model calculations for the tropospheric ozone depletion have applied the available abundance of bromide for the atmospheric reactions estimated from the bromide concentration in bulk sea water, the availability of bromide in sea salt particles must be reevaluated if the selective surface enrichment of bromide takes place on sea salt particles. In order to verify the feasibility of the surface bromide enrichment in sea salt particles, water-vapor exposure experiments were conducted for bromide-doped NaCl (100) single crystals with different bromide concentration using X-ray photoelectron spectroscopy and scanning electron microscopy. Segregation of bromide to the surface of the NaCl crystals uniformly doped with bromide at Br/Cl ratio below the level in sea water was observed even when the crystals were exposed to water vapor at lower pressure than the deliquescence points of NaCl and NaBr. The initial segregation rate of bromide was dependent on water vapor pressure and steeply increased when the employed water vapor pressure approached the deliquescence point of NaBr. At the initial stage of the segregation, segregated NaBr crystallites were highly dispersed on the crystal with ca. 0.2 µm on a side. As segregation proceeded by further water vapor exposure, the NaBr crystallites fused each other to achieve three-dimensional growth similarly to the case of water-induced reorganization of the ultra thin nitrate film formed by the reaction of NaCl single crystal with nitric acid.@footnote 3@ @FootnoteText@ @footnote 1@L. A. Barrie et al., Nature, 334, 138(1988).@footnote 2@K. W. Oum et al., Geophys. Res. Lett., 25, 3923-3926(1998). @footnote 3@J. C. Hemminger, Int. Rev. Phys. Chem., 18, 387(1999).