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

## TL;DR

This study investigates how N2O5 reacts with nitrate and chloride in solution, revealing a new mechanism involving nitration and chlorination.

## Contribution

The paper introduces an SN2 mechanism for N2O5 reactions, supported by isotopic evidence and modeling.

## Key findings

- N2O5 reacts with NO3– before chlorination, producing Cl14NO2 and Cl15NO2.
- The rate constant ratio kCl–/kNO3– is between 2 and 6 across different NO3– concentrations.
- An SN2 mechanism better explains the observed isotope and uptake data than previous SN1 models.

## Abstract

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 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 SN1 NO2
+ mechanism. We propose an alternate SN2 mechanism
involving N2O5 activation and deactivation that
fits both isotope and uptake data.