This study assesses a potential use of the recently developed nonthermal atmospheric pressure plasma technology in hydroponic plant cultivation. Two types of plasma‐activated liquid solutions were compared, namely, l‐Phenylalanine (l‐Phe), which had a benzene ring structure, and l‐Alanine (l‐Ala), which did not have a benzene ring structure. Antimicrobial property and plant growth enhancement were simultaneously obtained when plasma‐activated l‐Phe was used. We found that the benzene ring structure in solution contributed to antimicrobial property. Further, colony forming unit assay indicated that 99% of Escherichia coli (E. coli) were eliminated after 24‐h incubation and radish sprout growth increased by about 40% after 2 days of cultivation.

In this work, we developed a portable device with low production and operation costs for generating an ambient air low-current arc (AALCA) that is transferred to the surface of a treated liquid. It was possible to generate a stable discharge, irrespective of the conductivity of the treated liquid, as a sequence of corona, repeating spark, and low-current arc discharges. The estimated concentration of reactive oxygen and nitrogen species (RONS) in plasma-treated water (PTW) produced using AALCA treatment was two orders of magnitude higher than that of PTW produced using conventional He nonequilibrium atmospheric pressure plasma jets or dielectric barrier discharges. The strong bactericidal effect of the treatment using AALCA and the water treated using AALCA was confirmed by survival tests of Escherichia coli. Further, the possibility of treating a continuous flow of liquid using AALCA was demonstrated.

We treat deionized water using an atmospheric‐pressure radical source to investigate the role of neutral radicals in the bactericidal efficacy of radical‐activated water (RAW). The bactericidal efficacy of RAW is investigated using a colony‐forming unit assay with Escherichia coli. Both the bactericidal efficacy and reactive oxygen and nitrogen species (RONS) concentration in stored RAW are investigated over a period of a month. E. coli was completely sterilized even in RAW stored for 7 days, indicating the strong bactericidal effect. However, deterioration of the bactericidal efficacy of RAW stored for 14 days is confirmed, which does not have a clear correlation with the time evolution of the RONS concentration. This result becomes a strong evidence that RAW has long‐term bactericidal efficacy.

We report a very low energy cost (EC) for nitric monoxide (NO) radical formation found in atmospheric spark discharge. For this purpose non-equilibrium air discharges working in spark and glow regimes were studied showing the optimum performance in the vicinity of the spark-to-glow transition. The minimum EC for NO production of about 80 eV/molecule (calculated based on the total direct current power applied to the discharge generator) is achieved in the spark regime before the transition, whereas the maximum NO yield (of about 10 4 part per million) corresponds to the glow regime. Based on the estimated power absorbed in plasma the EC below 4 eV/molecule is achievable in the spark regime, which is close to the Zeldovich reaction enthalpy of NO formation (about 3 eV/molecule). The result implies that the energetic efficiency of a single spark likely exceeds that of the modern Haber-Bosch cycle (with EC of about 5 eV/molecule in the case of ammonia, according to Patil et al (2015 Catal. Today 256 49). The found low EC is associated with the discharge non-equilibrium caused by short spark duration.

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