Role of water in alkali halide heterogeneous chemistry relevant to the atmosphere: A surface science study

Water is a ubiquitous atmospheric constituent. The interaction of water in its various forms (vapor, liquid, ice) with other atmospheric constituents has a significant impact on the chemistry of the atmosphere. Another class of compounds that are of considerable importance in atmospheric chemistry are alkali halide salts such as sea salt particles. Heterogeneous reactions of alkali halides with gas phase pollutants are believed to be an important source of halogens in the troposphere. There is an increasing amount of evidence that the presence of water plays an important role in the heterogeneous chemistry of alkali halide particles. It is the goal of this dissertation to contribute to the understanding of the interaction of water with alkali halide surfaces and its atmospheric implications.; Surface processes are of fundamental importance in heterogeneous atmospheric chemistry, but they are often difficult to study because of their inherent complexity. As this dissertation shows, the use of modern surface science techniques offer valuable insights into these complex processes and as such offer complementary alternatives to the traditional atmospheric chemistry experiments. The surface science techniques used in this dissertation are X-ray photoelectron spectroscopy (XPS), scanning polarization force microscopy (SPFM) and scanning electron microscopy (SEM).; Presented here are the results of the XPS and SEM studies undertaken to determine the nature and content of surface adsorbed water on NaCl as a function of surface defects. The details of HNO₃ uptake on NaCl and the effect of surface adsorbed water on this uptake are also discussed. Our results show that the amount of “strongly adsorbed water” (SAW) on the surface of NaCl particles depends on the particle size and hence, on the concentration of surface defects. Unlike the (100) single crystal the more defective surfaces show dissociative water uptake at room temperature upon exposure to water vapor well below the deliquescence point of NaCl. The binding energy and temperature dependence characteristics of the SAW related oxygen signal on the NaCl particles are consistent with OH− like species.; We also show that the dissociative adsorption of HNO₃(g) on NaCl to form NaNO₃(s) and HCl(g) follows single-site Langmuir adsorption behavior. The dissociative adsorption accompanied by water-induced recrystallization of the NaNO₃ product on the NaCl surface gives rise to the experimentally observed HNO₃ pressure dependence of the reactive sticking coefficient for reaction under steady-state reaction conditions.; Surface segregation of bromine in bromide doped NaCl single crystals has been studied using XPS, SEM and SPFM. Our results show, for the first time, that substantial segregation of Br− to the surface of NaCl samples with low level Br− dopant concentrations occurs under conditions of water vapor exposure. The segregation phenomenon is likely to play an important role in atmospheric phenomena involving surface reactions of sea salt particles and aerosols.