Air-Water Interface Enhance the Formation of N-Centered Radicals From Amine Oxidation

N-centered radicals from amine oxidation are well-established precursors of atmospheric nitrosamines. Their low yield during gaseous oxidation of amines causes significant underestimation of nitrosamine formation, suggesting a potential new pathway. Using Born-Oppenheimer molecular dynamics (BOMD) in combination with metadynamics (MTD) and thermodynamic integration (TI), we investigated the oxidation of methylamine (CH₃NH₂) by hydroxyl radicals (·OH) at the air-water interface. Results indicated that CH₃NH₂ is notably enriched at the interface. The reaction rate constant at the water interface is approximately 2 orders of magnitude greater than that in the gas phase. Moreover, the presence of an interfacial electric field increases the reaction rate. In contrast to gaseous oxidation, which primarily abstracts H atoms from the ─CH₃ group, but at the interface preferentially favor H abstraction from the ─NH₂ group, forming N-centered radicals. This selectivity is primarily attributed to the H-bonding environment around ·OH and, to a lesser extent, the solvation-induced weakening of the N─H bond facilitated by interfacial waters. Consequently, the interface contributes 1.22 times more to carcinogenic nitrosamine formation than gaseous oxidation, increasing to 4.26 times with the presence of an interfacial electric field. These findings reveal a previously overlooked interfacial source that helps explain underestimated atmospheric concentrations of nitrosamines.