Heterogeneous Reactions of N ₂ O ₅ with Nitrate- and Chloride-Containing Solutions: Isotopic Evidence for the Nitration of N ₂ O ₅

The nitrate (NO₃⁻) ion has been shown to suppress the reactive uptake of dinitrogen pentoxide (N₂O₅) to aqueous aerosol, yet a molecular mechanism that explains this effect remains elusive. To explore how N₂O₅ and NO₃⁻ interact in solution, we use isotopically labeled ¹⁵NO₃⁻ in the presence of Cl⁻ to mark ¹⁴,¹⁴N₂O₅ that react with ¹⁵NO₃⁻ by detecting the nitration and chlorination products ¹⁴,¹⁵N₂O₅, ¹⁵,¹⁵N₂O₅, Cl¹⁴NO₂, and Cl¹⁵NO₂. At three Na¹⁵NO₃ concentrations (0.47, 1.35, and 3.7 M), both Cl¹⁴NO₂ and Cl¹⁵NO₂ products are measured as the NaCl concentration increases from 0 to 3.0 M, confirming that N₂O₅ can reactively exchange with NO₃⁻ prior to chlorination. Using a kinetic multilayer model to compare chlorination and nitration (nitrate exchange) of N₂O₅, the rate constant ratio kCl⁻/kNO₃⁻ is determined to be between 2 and 6 for the three ¹⁵NO₃⁻ concentrations. Model results suggest that the initial adsorption of a ¹⁴,¹⁴N₂O₅ molecule produces the majority of Cl¹⁵NO₂ by exchange with ¹⁵NO₃⁻ before reacting with Cl⁻, rather than occurring through multiple absorption and evaporation events involving ¹⁴,¹⁵N₂O₅ or ¹⁵,¹⁵N₂O₅. The ratio of nitrate exchange to hydrolysis, kNO₃⁻/kH₂O, is inferred to be between 70 and 230, which overestimates nitrate suppression in 6 M NaNO₃ by 10-fold when using previous parametrizations of N₂O₅ uptake based on an SN1 NO₂⁺ mechanism. We propose an alternate SN2 mechanism involving N₂O₅ activation and deactivation that fits both isotope and uptake data.