Abstract
The nitrate (NO(3)(-)) ion has been shown to suppress the reactive uptake of dinitrogen pentoxide (N(2)O(5)) to aqueous aerosol, yet a molecular mechanism that explains this effect remains elusive. To explore how N(2)O(5) and NO(3)(-) interact in solution, we use isotopically labeled (15)NO(3)(-) in the presence of Cl(-) to mark (14,14)N(2)O(5) that react with (15)NO(3)(-) by detecting the nitration and chlorination products (14,15)N(2)O(5), (15,15)N(2)O(5), Cl(14)NO(2,) and Cl(15)NO(2). At three Na(15)NO(3) concentrations (0.47, 1.35, and 3.7 M), both Cl(14)NO(2) and Cl(15)NO(2) products are measured as the NaCl concentration increases from 0 to 3.0 M, confirming that N(2)O(5) can reactively exchange with NO(3)(-) prior to chlorination. Using a kinetic multilayer model to compare chlorination and nitration (nitrate exchange) of N(2)O(5), the rate constant ratio k(Cl(-))/k(NO(3)(-)) is determined to be between 2 and 6 for the three (15)NO(3)(-) concentrations. Model results suggest that the initial adsorption of a (14,14)N(2)O(5) molecule produces the majority of Cl(15)NO(2) by exchange with (15)NO(3)(-) before reacting with Cl(-), rather than occurring through multiple absorption and evaporation events involving (14,15)N(2)O(5) or (15,15)N(2)O(5). The ratio of nitrate exchange to hydrolysis, k(NO(3)(-))/k(H(2)O), is inferred to be between 70 and 230, which overestimates nitrate suppression in 6 M NaNO(3) by 10-fold when using previous parametrizations of N(2)O(5) uptake based on an S(N)1 NO(2)(+) mechanism. We propose an alternate S(N)2 mechanism involving N(2)O(5) activation and deactivation that fits both isotope and uptake data.