The effect of humidity on the discharge mode transition of air discharge plasma

Cold atmospheric plasma in air commonly operates in the O3 mode and NOx mode, which easily interconvert through a transition mode, depending on discharge conditions. Given that the humidity varies considerably in different weather, it is important to elucidate the effect of humidity on the discharge mode transition, but few studies have been reported thus far. In this study, air plasmas were generated by a surface dielectric barrier discharge with different discharge powers of 6, 9, and 12 W, and the relative humidity of air was controlled at 1.5% (dry air), 40%, or 80% for a comparative study. It was found that an increase in humidity suppressed the production of O3 but promoted that of NO2 when the discharge power was 6 W, whereas it promoted the production of O3 but suppressed that of NO2 when the discharge power was 12 W. This implies that air humidity could have a bidirectional effect on the discharge mode transition, which was validated by experiments with a moderate power of 9 W. In that case, the discharge in dry air maintained the transition mode at a quasi-stable state, but it transited either into the NOx mode when the humidity was 40% or into the O3 mode when the humidity was 80%. A competition between reaction pathways dominated by N2(ν) or water-originated compounds may be the cause, and our findings indicate that the effect of humidity should be taken seriously in the research and development of air discharge plasmas.