Mechanistic Studies on the Reaction of HOClO₂ with OH Radical on Aqueous Surfaces

Chloric acid (HOClO₂) is a newly detected compound in the atmosphere and is involved in ozone depletion. The reaction of HOClO₂ with OH radical is likely one of its main atmospheric loss processes. Aqueous surfaces have been reported as critical media for reactions of some chlorine acids, such as HOCl/HCl. However, their effect on the reaction of HOClO₂ with OH remains unexplored. Here, we use Born-Oppenheimer molecular dynamics simulations to investigate the reaction of HOClO₂ with OH on an aqueous surface. The interfacial behaviors of the reactant (HOClO₂) and product (ClO₃) involved in this reaction were investigated. Gas phase HOClO₂ can be absorbed on the aqueous surface by forming H-bonds between the acidic H atoms and water. As HOClO₂ penetrates the sublayer, it rapidly dissociates into OClO₂^- and H₃O⁺; however, before this dissociation, a competitive reaction with OH is likely due to the rapid reactivity of the radical. The reaction of HOClO₂ with OH on the aqueous surface follows hydrogen abstraction mechanisms; these include the direct and water monomer/dimer-mediated abstraction pathways. Direct abstraction has no energy barrier and is most kinetically favored, compared to ∼0.5 kcal mol^-1 and ∼3.0 kcal mol^-1 for the water monomer- and dimer-mediated mechanisms, respectively. The resulting ClO₃ can reside on the surface with its Cl atom exposed to the air, increasing the potential for further reactions with other atmospheric species. Overall, the specific reaction of HOClO₂ with OH on the water surface, could lead to new chlorine recycling by converting HOClO₂ into reactive species.