Important atmospheric and environmental processes involve radical/atom reactions at aqueous interfaces. The mechanisms of interfacial radical/atom reactions are, however, not fully understood, due to the lack of experimental techniques for in-situ monitoring of the intermediates of fast interfacial radical reactions. Here, products and intermediates of the prompt reactions of a variety of aqueous species, including atmospherically relevant carboxylic acids and antioxidant of human epithelial lining fluids, initiated by hydroxyl radicals, iodine atoms, and Criegee intermediates at air-aqueous interfaces are detected by a newly-developed mass spectrometry under ambient conditions. Exposure of sub-millimolar reactant aqueous microjets to gas-phase reactants yielded interfacial species, including peroxyl radicals, that are simultaneously identified in situ by mass spectrometry. Interface-specific mechanisms for the heterogeneous reactions are proposed.