Mechanistic Studies on the Reaction of HOClO 2 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.