Mechanistic Insights into NO-Halide Ions Chemistry at the Air-Water Interface.

The activation of halogens (X = Cl, Br, I) by NO is linked to NO sources, ozone concentrations, NO reactivity, and the chemistry of halide-containing aerosol particles. However, a detailed chemical mechanism is still lacking. Herein, we explored the chemistry of the NO···X systems at the air-water interface. Two different reaction pathways were identified for the reaction of NO with X at the air-water interface: the formation of XNO or XONO, along with NO. In the case of the Cl system, the ClNO 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 X from XNO 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 ClNO is stable at the air-water interface. Contrary to the widely held belief regarding X producing from the reaction of XNO 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 Br and I, respectively. Finally, Br and I decompose to form Br and I These findings have significant implications for experimental interpretation and offer new insights into halogen cycling in the atmosphere.