Trapping intermediates of the NO2 hydrolysis reaction on ice
Maurais J.,1* Robidas, R.,2 Ayotte, P.2* presenter
1josee.maurais@usherbrooke.ca, Université de Sherbrooke, Canada
2
Université de Sherbrooke, Canada
The hydrolysis of NO2 at the air/ice interface plays an important role in regulating the oxidative
capacity of the atmosphere 1. Indeed, the reaction yields nitric acid and nitrous acid, compounds
that acidify precipitations, but also create photochemical sources of NOx and OH radicals. Inthe gas phase, NO2 is at equilibrium with its dimers, N2O4, which can exist in the form of
constitutional isomers and stereoisomers, but whose abundance and reactivity towardshydrolysis continue to be debated 2. Computational chemistry results reported in the literaturestate that the isomer trans-ONONO2 is a key intermediate species in the hydrolysis reaction
mechanism, but experimental evidence to confirm this hypothesis is still lacking. These
uncertainties regarding the reaction mechanism hamper heterogeneous NO2 hydrolysis to beaccurately accounted for in atmospheric chemistry models 3.
Using Electric Field Standing Waves (EFSW) to enhance the sensitivity and selectivity ofReflection-Absorption IR Spectroscopy (RAIRS) 4 , along with molecular beam deposition
techniques, it is shown that the relative abundance of the nitrogen oxides in the gas phase canbe controlled experimentally, enabling the control over the distribution of cis-ONONO2 andtrans-ONONO2 metastable isomers of N2O4 in NO2 vapors 5. The study of the reactivity of eachspecies towards heterogeneous hydrolysis on ice can thus be done independently allowing
invaluable molecular-level insight into the reaction mechanism using vibrational spectroscopy.
A detailed account of the molecular-level interactions of nitrogen oxides with water molecules
was elaborated using computational chemistry investigations of mixed (NO2)n•(H2O)m clusters(n = 1,2; m = 0-8), allowing the experimental identification of the reaction intermediates at theice surface. In addition to understanding how the structural changes and shifts in the vibrationalfrequencies inform and reflect on the relative reactivity of the different reaction intermediates,
the molecular modeling results obtained indicate that the isomers trans-ONONO2 and cisONONO2 tend to self-ionize towards ON+ONO2-, a trend that is confirmed experimentally.
These insights in the molecular-level understanding of the hydrolysis reaction mechanism will
hopefully improve its inclusion thereby improving future atmospheric chemistry models.

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Maurais, J.; Wespiser, C.; Kang, H.; Ayotte, P. J. Phys. Chem. A 2022, 126, 2353-2360.
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