Water-Induced Reorganization of Ultrathin Nitrate Films on NaCl:  Implications for the Tropospheric Chemistry of Sea Salt Particles

Abstract

Reactions of sodium chloride, a major constituent of atmospheric sea salt particles which have been observed in the marine boundary layer as well as inland, and nitrogen oxides are known to form gas-phase chlorine species that may ultimately influence tropospheric ozone levels. Therefore, studies were carried out on sodium chloride single crystals which were exposed to nitric acid vapor at ∼298 K to form surface bonded nitrate. Upon exposure to water vapor pressures below the bulk dissolution (deliquescence) of sodium chloride and sodium nitrate, two-dimensional nitrate layers on the surface of sodium chloride crystals reorganized to form separate three-dimensional microcrystallites of sodium nitrate which were observed using transmission electron microscopy. X-ray photoelectron spectroscopy results show that the reorganization 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 highly variable chlorine deficits within individual sea salt particles and suggest that a large portion of the particle chloride, not just the original surface, may become available for reaction.