An atomizing method comprises providing an atomizer including a mist generator and a plasma producer. The plasma producer includes a tube of dielectric material, an external electrode on an outside surface of the tube of dielectric material, an internal electrode on an inside surface of the tube of dielectric material, and a high-frequency power supply for applying a high-frequency voltage between the external electrode and the internal electrode. Mist from the mist generator is plasma-activated while passing through the tube of dielectric material and emitted from an end of the tube of dielectric material. The external electrode and the internal electrode are located such that location of the external electrode and location of the internal electrode does not overlap or merely partially overlap in the longitudinal direction of the tube of dielectric material. Peroxynitrous acid is continuously generated by generating hydrogen peroxide and nitrous acid using plasma-activated water mist.