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 NOx 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 I2. These findings have significant implications for experimental interpretation and offer new insights into halogen cycling in the atmosphere.