Aerosol iodide accelerates reactive nitrogen cycling in the marine atmosphere.

Reactive nitrogen plays critical roles in atmospheric chemistry, climate, and geochemical cycles, yet its sources in the marine atmosphere, particularly the cause of the puzzling daytime peaks of nitrous acid (HONO), remain unexplained. Here we reveal that iodide enhances HONO production during aqueous nitrate photolysis by over tenfold under typical marine conditions. Laboratory experiments and molecular simulations confirm that HONO formation from nitrate photolysis is a surface-dependent process, and the extreme surface propensity of iodide facilitates nitrate enrichment at interfaces, reducing the solvent cage effect and promoting HONO release. Global model simulations show that this process accelerates atmospheric nitrogen cycling, increasing the levels of nitrogen oxides, hydroxyl radicals, and ozone by over 25%, 30%, and 15%, respectively, and enhancing dimethyl sulfide and methane degradation by over 20% in the marine boundary layers. Our findings highlight the crucial role of iodide in interfacial photochemistry and marine atmospheric nitrogen cycling.