Elucidating the mechanism of reactive uptake of N$_2$O$_5$ in aqueous aerosol

TL;DR

This study uses advanced molecular simulations to reveal that N$_2$O$_5$ hydrolysis in aqueous aerosols occurs much faster than previously thought, challenging traditional models and suggesting an interfacial reactivity mechanism.

Contribution

The paper introduces machine learning-based reactive potentials and importance sampling simulations to elucidate the rapid hydrolysis mechanism of N$_2$O$_5$ in aerosols, proposing a new interfacial reactivity model.

Findings

Hydrolysis rate of N$_2$O$_5$ is 4.1 ns$^{-1}$, much faster than assumed.

Hydrolysis involves coordinated fluctuations of charge separation and solvation.

Interfacial reactivity model explains experimental observations.

Abstract

Nearly one third of all nitrogen oxides are removed from the atmosphere through the reactive uptake of N$_2$O$_5$ into aqueous aerosol. The primary step in reactive uptake is the rapid hydrolysis of N$_2$O$_5$, yet despite significant study, the mechanism and rate of this process are unknown. Here we use machine learning-based reactive many body potentials and methods of importance sampling molecular dynamics simulations to study the solvation and subsequent hydrolysis of N$_2$O$_5$. We find that hydrolysis to nitric acid proceeds through the coordinated fluctuation of intramolecular charge separation and solvation, and its characteristic rate is 4.1 ns$^{-1}$, orders of magnitude faster than traditionally assumed. This large rate calls into question standard models of reactive uptake that envision local equilibration between the gas and the bulk solution. We propose an alternative model based on interfacial reactivity that can explain existing experimental observations and is corroborated by explicit simulations.