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The reaction of gaseous N2O5 with sea salt and its components is a potential source of halogen atoms in the marine boundary layer. There are two possible reaction paths when water is present on the salt surface. Reaction with the chloride ion forms nitryl chloride (ClNO2), a photolyzable compound: N2O5+NaCl→ClNO2+NaNO3, while hydrolysis of N2O5 generates HNO3 that can react further with NaCl to form gaseous HCl: N2O5+H2O (on NaCl)→2 HNO3, HNO3+NaCl→HCl+NaNO3. We report here Knudsen cell studies at 23°C of the reaction of N2O5 with NaCl, using less than one layer of salt particles. A model, which takes into account the effective salt surface area exposed to the gas, was applied, allowing for the determination of uptake coefficients without introducing uncertainties associated with diffusion into multiple layers of salt particles. The net uptake coefficient for the sum of both channels for the N2O5 reaction was measured to be γN2O5=(2.9±1.7)×10−3, where the error cited is the 2σ statistical error. The cumulative error is estimated to be better than a factor of three. Both ClNO2 and HCl were observed as gaseous products from the N2O5-salt reaction and the branching ratio for ClNO2 was 0.73±0.28 (2σ). A limited number of experiments were carried out for the reaction with synthetic sea salt, resulting in an uptake coefficient of about an order of magnitude larger than for NaCl, and a ClNO2 yield of 100%. We propose a mechanism for this reaction in which surface-adsorbed water plays a key role in the competition between hydrolysis of N2O5 to generate HNO3 and the reaction with NaCl to generate ClNO2. Reaction with NaCl is shown to be a potentially important source of ClNO2, and thus, of highly reactive chlorine atoms in urban marine regions at dawn. Application of our model to previous data from this laboratory for the reaction of chlorine nitrate (ClONO2) with fractional layers of NaCl gives a corrected uptake coefficient of γClONO2=(2.4±1.2)×10−2
(2σ), which suggests that the ClONO2–NaCl reaction may contribute significantly to the observed concentrations of Cl2 in the marine boundary layer.