Heterogeneous Reactions of N 2 O 5 with Nitrate- and Chloride-Containing Solutions: Isotopic Evidence for the Nitration of N 2 O 5
Year: 2025
Type: article
Abstract: High Resolution Image Download MS PowerPoint Slide 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.
Language: English
Subfield: Atmospheric Science
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: hybrid
OpenAlex ID: W4416092983