Knudsen cell studies of the reactions of N2O5 and ClONO2 with NaCl: development and application of a model for estimating available surface areas and corrected uptake coefficients

Abstract

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 γ_N2O5 = (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 γ_ClONO2 = (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.