HOCl/ClONO₂ Catalytic Cycle: Promotion of the Reactive Uptake of N₂O₅ at the Air-Water Interface

Changes in the reactive uptake of dinitrogen pentoxide (N₂O₅) by aerosols play important roles in regulating the levels of O₃, OH, NO_x, and CH₄. However, a quantitative understanding of the uptake mechanism remains incomplete. Herein, we show that a HOCl/ClONO₂ catalytic cycle facilitates the reactive uptake of N₂O₅ at the air-water interface. Our ab initio molecular dynamics (AIMD) simulations reveal that the (N₂O₅)N···O(HOCl) dipole-dipole interaction is the primary interaction between HOCl and N₂O₅ at the air-water interface. This interaction promotes the formation of an N-O bond and leads to the generation of NO₃^-, ClONO₂, and H₃O⁺. Free-energy calculations further reveal that this reaction is both kinetically and thermodynamically more favorable at the air-water interface than in bulk water and has an energy barrier of ∼5.3 kcal/mol. Additionally, the generated ClONO₂ rapidly hydrolyzes at the interface and forms HOCl, NO₃^-, and H₃O⁺. The net reaction is as follows: N₂O₅ + 3H₂O → 2NO₃^- + 2H₃O⁺. This mechanism allows a single HOCl molecule to participate in multiple HOCl/ClONO₂ cycles, thereby enhancing the reactive uptake of N₂O₅ by aerosols. This study provides insights for interpreting experimental results and has broader implications for understanding the chemistry of aerosols and clouds at the air-water interface.