X-ray photoelectron spectroscopy studies of the effects of water vapor on ultrathin nitrate layers on NaCl

Understanding the reactive chlorine cycle in the troposphere is of importance to the ozone balance and oxidation of organics in marine regions. Sea salt particles, containing NaCl as the main constituent, are believed to be the major source of reactive tropospheric chlorine. To develop a fundamental understanding of the processes involved, we have carried out studies which utilize X-ray photoelectron spectroscopy (XPS) to follow the surface composition of NaCl single crystals as a function of sequential exposures to gaseous nitric acid and water vapor at room temperature. The uptake of HNO3(g) on NaCl was found to saturate the substrate surface, forming a metastable nitrate layer with a thickness on the order of 1-2 monolayers. Subsequent exposure of the nitrate layer to water at various pressures, well below the deliquescence points of NaCl and NaNO3, induced surface ionic mobility in a quasi-liquid layer. Phase separation occurred, with microcrystallites of NaNO3 recrystallizing three-dimensionally on the substrate surface. This exposed fresh chlorine from the bulk NaCl, making it available for further reaction. The large deficits of Cl- found in many sea salt particles can be explained by this cycling effect. Roughening of the NaCl surface from nitric acid corrosion generated surface defects which enhanced water adsorption. Some active sites induced H2O dissociation and generation of surface OH- species, which was detected by XPS. Experiments on the more defective NaCl (111) surface confirmed the role of surface defects.