Method for generating an atmospheric plasma jet which comprises: - flowing a process gas that advances in a flow direction (202, 402, 502) through a tubular duct (201, 401, 501) made of dielectric material with an inlet section and an outlet section (207, 410) at atmospheric pressure; - positioning a first pair of coaxial electrodes (203-204, 307-308, 404-405, 503-504) in contact with the external surface of said tubular duct (201, 401, 501); said first pair of electrodes (203-204, 307-308, 404-405, 503-504) being connected to a high-frequency generator (208, 301); - said high-frequency generator (208, 301) generating a filamentary plasma within said tubular duct (201, 401, 501); said method being characterized in that it further comprises: - positioning a second pair of coaxial electrodes (205-206, 309-310, 406-407, 505-506) in contact with the external surface of said tubular duct (201, 401, 501); said first pair of electrodes (203-204, 307-308, 404-405, 503-504) being placed in position upstream of said second pair of electrodes (205-206, 309-310, 406-407, 505-506) in relation to the flow direction of said process gas in said tubular duct (202, 402, 502); said second pair of electrodes (205-206, 309-310, 406-407, 505-506) being connected to a Radio-Frequency generator (209, 303); - said filamentary plasma extending at least to said second pair of electrodes (205-206, 309-310, 406-407, 505-506); - said Radio-Frequency generator (209, 303) generating a second RF plasma; - flowing out said RF plasma and said filamentary plasma to outside the tubular duct (201, 401, 501) through said outlet section (207, 410), such plasmas at the outlet comprising at least one neutral gas at the outlet having temperature not higher than about 100°C. - flowing a process gas that advances in a flow direction (202, 402, 502) through a tubular duct (201, 401, 501) made of dielectric material with an inlet section and an outlet section (207, 410) at atmospheric pressure; - positioning a first pair of coaxial electrodes (203-204, 307-308, 404-405, 503-504) in contact with the external surface of said tubular duct (201, 401, 501); said first pair of electrodes (203-204, 307-308, 404-405, 503-504) being connected to a high-frequency generator (208, 301); - said high-frequency generator (208, 301) generating a filamentary plasma within said tubular duct (201, 401, 501); - positioning a second pair of coaxial electrodes (205-206, 309-310, 406-407, 505-506) in contact with the external surface of said tubular duct (201, 401, 501); said first pair of electrodes (203-204, 307-308, 404-405, 503-504) being placed in position upstream of said second pair of electrodes (205-206, 309-310, 406-407, 505-506) in relation to the flow direction of said process gas in said tubular duct (202, 402, 502); said second pair of electrodes (205-206, 309-310, 406-407, 505-506) being connected to a Radio-Frequency generator (209, 303); - said filamentary plasma extending at least to said second pair of electrodes (205-206, 309-310, 406-407, 505-506); - said Radio-Frequency generator (209, 303) generating a second RF plasma; - flowing out said RF plasma and said filamentary plasma to outside the tubular duct (201, 401, 501) through said outlet section (207, 410), such plasmas at the outlet comprising at least one neutral gas at the outlet having temperature not higher than about 100°C.