Mechanistic Insights into N 2 O 5 -Halide Ions Chemistry at the Air-Water Interface.

The activation of halogens (X = Cl, Br, I) by N 2 O 5 is linked to NO x sources, ozone concentrations, NO 3 reactivity, and the chemistry of halide-containing aerosol particles. However, a detailed chemical mechanism is still lacking. Herein, we explored the chemistry of the N 2 O 5 ···X - systems at the air-water interface. Two different reaction pathways were identified for the reaction of N 2 O 5 with X - at the air-water interface: the formation of XNO 2 or XONO, along with NO 3 - . In the case of the Cl - system, the ClNO 2 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 2 from XNO 2 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 2 is stable at the air-water interface. Contrary to the widely held belief regarding X 2 producing from the reaction of XNO 2 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 3 - and I 3 - , respectively. Finally, Br 3 - and I 3 - decompose to form Br 2 and I 2. These findings have significant implications for experimental interpretation and offer new insights into halogen cycling in the atmosphere.