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, NOx, 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.