# Atmospheric Oxidation of NH3, HNO3 and NH3···HNO3 by OH, NH2, and NO3 Radicals. The Effect of Water Vapor

## TL;DR

This paper studies how ammonia and nitric acid react with radicals in the atmosphere, showing that water vapor significantly affects some of these reactions.

## Contribution

The study reveals the role of proton coupled electron transfer mechanisms and quantifies the impact of water vapor on oxidation reactions involving atmospheric ammonia and nitric acid.

## Key findings

- The reaction of NH3 with OH follows a hydrogen transfer mechanism with a rate constant of 1.24 × 10–13 cm³·molecule⁻¹·s⁻¹.
- Water vapor significantly increases the oxidation rate of ammonia by nitrate radicals by 751%.
- The proton coupled electron transfer mechanism is crucial for most reactions involving ammonia and nitric acid.

## Abstract

Atmospheric ammonia, in both particulate and gaseous
forms, has
major ecological, health, and economic impacts, making it essential
to understand its chemical processes. The reactions of ammonia and
ammonia complexed with nitric acid with hydroxyl radical and the oxidation
of nitric acid by amidogen radical and ammonia by nitrate radical,
both taking into account the effect of water vapor, have been investigated
using quantum mechanical (QCISD and CCSD­(T)) calculations with the
6-311+G­(2df,2p), aug-cc-pVTZ, and aug-cc-pVQZ and extrapolation to
the CBS basis sets. From a mechanistic point of view, the reaction
of NH3 + OH follows a conventional hydrogen transfer mechanism,
but for the rest of reactions considered, the proton coupled electron
transfer mechanism plays a key role. For the reaction of ammonia with
hydroxyl radical we have computed a rate constant of 1.24 × 10–13 cm3·molecule–1·s–1 at 298 K, and the effect of water vapor
is negligible. The calculated rate constant for the HNO3···NH3 + OH reaction is 6.50 × 10–16 cm3·molecule–1·s–1 at 298 K. and our results show that both
HNO3 and NH3 moieties can be oxidized. The effect
of water vapor on the oxidation of nitric acid by an amidogen radical
is significant. We have computed a rate constant of 1.98 × 10–13 cm3·molecule–1·s–1, at 298 K and 100% of RH for the whole
HNO3 + NH2 + H2O reaction, which
is an 11% greater than the calculated value for the naked reaction.
For the oxidation of ammonia by a nitrate radical, the effect of water
vapor is huge. The calculated rate constant at 298 K and 100% of RH
is 16 × 10–15 cm3·molecule–1·s–1 for the whole NH3 + NO3 + H2O reaction, that is, 751% greater
than the value of the naked reaction at 298 K.