Halogen-Bond-Driven IONO2 Reactivity at the Air–Water Interface: A Source of Nighttime Interhalogens

Abstract Iodine-containing interhalogens (ICl/IBr) govern halogen cycling, strongly modulating atmospheric oxidizing capacity and driving tropospheric ozone depletion in the marine boundary layer. However, well-known hypoiodous acid (HOI)-driven mechanisms fail to account for enhancements in nighttime interhalogen production observed in semipolluted coastal air, with the signals correlating positively with I2, NOx and aerosol surface area. This discrepancy points to an unidentified dark source, and iodine nitrate (IONO2) formed nocturnally from reactions of iodine species with nitrate radicals acts as a critical yet overlooked candidate. Here, we investigate two competing heterogeneous reaction pathways of IONO2 at the air–water interface of sea spray aerosols (SSA) using Born–Oppenheimer molecular dynamics combined with electronic-structure analyses. The interfacial hydrolysis of IONO2 is found to be hindered by a discernible free-energy barrier and is thermodynamically endergonic. By contrast, direct reactions with Cl– and Br– are barrierless, strongly exergonic, and proceed via a halogen-bond-mediated SN2 mechanism with strict stereoelectronic control for backside-attack geometry. By redefining IONO2 as a reactive interfacial precursor of interhalogens rather than merely a nighttime reservoir, our findings provide a molecular-level explanation for the anomalous nighttime halogen activation observed in polluted coastal environments and highlight the pivotal role of halogen bonding in heterogeneous atmospheric chemistry.