Heterogeneous studies of atmospheric pollutants, nitric acid, nitrogen pentoxide, and hydroxide radicals, with sodium chloride, sodium nitrate, and synthetic sea salt using multiple analytical methods

Heterogeneous reactions involving sea salt aerosols and oxides of nitrogen are a potential source of photolyzable halogen compounds in the marine boundary layer. Specifically, the reactions of HNO₃ and N₂O ₅ with NaCl, the major component in sea salt aerosols, results in production of gas phase HCl and ClNO₂ leaving NaNO₃ remaining within on particles. Experiments were conducted to elucidate a number of issues: (1) the reaction probability of each of the gases with NaCl; (2) the phase of NaNO₃ aerosols and mixed aerosols of NaNO₃ and NaCl under low relative humidity conditions; and (3) the extent to which the presence of nitrate affects the reactivity of chloride ions toward OH radicals. A number of analytical techniques were utilized including a Knudsen cell coupled to a quadrupole mass spectrometer, scanning electron microscopy with energy dispersive analysis of X-rays, and long path Fourier transform infrared spectroscopy.; Knudsen cell studies were conducted using less than one layer of salt particles and a new model was applied to account for the true available salt surface area. Reaction probabilities were determined to be (1.0 ± 0.8) × 10−3 and (2.9 ± 1.7) × 10 −3 for HNO₃ and N₂O₅, respectively. Additionally, new models are proposed for each reaction involving water adsorbed on the salt surface. The data suggest that the reaction of HNO₃ with effluoresced sea salt particles is less important than previously thought relative to reactions with N₂O₅ and ClONO₂, which generate chlorine atom precursors, leaving nitrate remaining on the particles.; Hydration and dehydration studies of NaNO₃ particles indicate that they exist as unusual metastable, amorphous solids at low relative humidities, and undergo continuous hygroscopic growth with increasing relative humidity. Mixed aerosol particles of NaCl and NaNO₃ are shown to be comprised of a core of NaCl surrounded by a mixed layer of nitrate and chloride. Additionally, the presence of nitrate in these particles is found to actually enhance the depletion of chloride upon reaction with OH radicals, compared to reaction with NaCl particles alone.