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 imp ortance to the ozone balance and oxidation of organics in marine regio ns. Sea salt particles, containing NaCl as the main constituent, are b elieved to be the major source of reactive tropospheric chlorine. To d evelop a fundamental understanding of the processes involved, we have carried out studies which utilize X-ray photoelectron spectroscopy (XP S) to follow the surface composition of NaCl single crystals as a func tion 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 thic kness on the order of 1-2 monolayers. Subsequent exposure of the nitra te layer to water at various pressures, well below the deliquescence p oints of NaCl and NaNO3, induced surface ionic mobility in a quasi-liq uid layer. Phase separation occurred, with microcrystallites of NaNO3 recrystallizing three-dimensionally on the substrate surface. This exp osed fresh chlorine from the bulk NaCl, making it available for furthe r reaction. The large deficits of Cl- found in many sea salt particles can be explained by this cycling effect. Roughening of the NaCl surfa ce from nitric acid corrosion generated surface defects which enhanced water adsorption. Some active sites induced H2O dissociation and gene ration of surface OH- species, which was detected by XPS. Experiments on the more defective NaCl (111) surface confirmed the role of surface defects.