Heterogeneous Reactions of N₂O₅ with Nitrate- and Chloride-Containing Solutions: Isotopic Evidence for the Nitration...

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, Cl¹⁴NO_2, 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 k_Cl^-/k_NO3^- 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, k_NO3^-/k_H2O, 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 S_N1 NO₂⁺ mechanism. We propose an alternate S_N2 mechanism involving N₂O₅ activation and deactivation that fits both isotope and uptake data.