Mechanistic Insights into N₂O₅-Halide Ions Chemistry at the Air-Water Interface

The activation of halogens (X = Cl, Br, I) by N₂O₅ is linked to NO sources, ozone concentrations, NO₃ reactivity, and the chemistry of halide-containing aerosol particles. However, a detailed chemical mechanism is still lacking. Herein, we explored the chemistry of the N₂O₅···X^- systems at the air-water interface. Two different reaction pathways were identified for the reaction of N₂O₅ with X^- at the air-water interface: the formation of XNO₂ or XONO, along with NO₃^-. In the case of the Cl^- system, the ClNO₂ generation pathway is more favorable, while for the Br^- and I^- systems, the formation of BrONO and IONO is barrierless, making them the predominant products. Furthermore, the mechanisms of formation of X₂ from XNO₂ and XONO were also investigated. The high energy barriers of reactions and the high free energies of the products compared to those of the reactants indicate that ClNO₂ is stable at the air-water interface. Contrary to the widely held belief regarding X₂ producing from the reaction of XNO₂ with X^-, our calculations demonstrate that BrONO and IONO initially form stable BrONO···Br^- and IONO···I^- complexes, which then subsequently react with Br^- and I^- to form Br₃^- and I₃^-, respectively. Finally, Br₃^- and I₃^- decompose to form Br₂ and I_2. These findings have significant implications for experimental interpretation and offer new insights into halogen cycling in the atmosphere.