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 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 similar to 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.