Mechanistic Insights into N<sub>2</sub>O<sub>5</sub>-Halide Ions Chemistry at the Air–Water Interface
Bo Tang, Qi Bai, Ye‐Guang Fang, Joseph S. Francisco, Chongqin Zhu, Wei‐Hai Fang
Abstract
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.