Abstract The concentration distributions of chemically reactive species in water exposed to low-temperature atmospheric-pressure plasma (APP) have been studied with one-dimensional numerical simulations. Highly reactive species supplied from an APP to the water surface all react in the “reaction boundary layer,” i.e. a thin layer with a thickness of about 100 nm on the solution side of the interface, and are converted to stable species. This study quantitatively shows that, in the case of pure water irradiated by an APP, the simultaneous presence of H₂O₂, NO₂ ⁻, and O₃ in the solution is the only cause to make it an oxidizing medium as they continuously produce ONOOH and HO₃, which then decay to generate OH radicals in the solution bulk. ONOOH and its accompanying HO₂NO₂ have much longer diffusion lengths than HO₃ and therefore their diffusion can also contribute to the oxidizing capability of the solution.