WATER-INDUCED REORGANIZATION OF ULTRATHIN NITRATE FILMS ON NACL - IMPLICATIONS FOR THE TROPOSPHERIC CHEMISTRY OF SEA-SALT PARTICLES

Reactions of sodium chloride, a major constituent of atmospheric sea s alt particles which have been observed in the marine boundary layer as well as inland, and nitrogen oxides are known to form gas-phase chlor ine species that may ultimately influence tropospheric ozone levels. T herefore, studies were carried out on sodium chloride single crystals which were exposed to nitric acid vapor at similar to 298 K to form su rface bonded nitrate. Upon exposure to water vapor pressures below the bulk dissolution (deliquescence) of sodium chloride and sodium nitrat e, two-dimensional nitrate layers on the surface of sodium chloride cr ystals reorganized to form separate three-dimensional microcrystallite s of sodium nitrate which were observed using transmission electron mi croscopy. X-ray photoelectron spectroscopy results show that the reorg anization of the surface, and the regeneration of a fresh NaCl surface , also takes place under conditions where only 1-2 nitrate monolayers initially existed. These results explain atmospheric observations of h ighly variable chlorine deficits within individual sea salt particles and suggest that a large portion of the particle chloride, not just th e original surface, may become available for reaction.