Nitrosamines are highly carcinogenic and reactive nitrogen-containing pollutants that are widely detected in atmospheric particulate matter; however, their formation mechanisms remain poorly understood. Herein, we elucidate previously unrecognized yet kinetically viable heterogeneous mechanisms for nitrosamine formation via amine-mediated reactions with dinitrogen tetroxide (N<sub>2</sub>O<sub>4</sub>) at the air–water interface, using Born–Oppenheimer molecular dynamics simulations. The reactions proceed via two distinct pathways: (<em>i</em>) barrierless N-nitrosation of methylamine (MA) or dimethylamine (DMA) by N<sub>2</sub>O<sub>4</sub>, directly yielding nitrosamine cations and nitrate ions (NO<sub>3</sub><sup>−</sup>); and (<em>ii</em>) MA/DMA-mediated hydrolysis of N<sub>2</sub>O<sub>4</sub> produces interfacial HONO rapidly within ~2–16 ps, which can further react with amines to form nitrosamines, albeit with a relatively high reaction barrier of 7.65 kcal mol<sup>−1</sup>. Overall, the amine-mediated interfacial N-nitrosation reactions proceed rapidly and may represent an important source of particulate nitrosamines. Meanwhile, amine-mediated interfacial hydrolysis of N<sub>2</sub>O<sub>4</sub> is a potential source of HONO, proceeding through the combined effects of interfacial water bridging and strong basicity of amines. Our findings reveal a previously overlooked role of heterogeneous interfacial chemistry in elevated particulate nitrosamine formation and coupled HONO production, with important implications for urban reactive nitrogen cycling and its representation in chemical transport models.