Mechanism of Discharge Mode Transition in Air Plasma: the Inducing Effect of Humidity on Ozone to Nitrogen Oxide Mode by Kinetics Modeling

The products of air plasma have high application value in fields such as nitrogen-fixing energy conversion, biomedicine and environmental protection, which correspond to the ozone (O₃) and nitrogen oxides (NO_x) mode respectively, and different discharge paraments have different impacts on these two modes. However, the air inevitably contains moisture and the water content varies in different regions and climates, the humidity has the significant effect on the transition between the O₃ and NO_x mode, but there are fewer studies on the intrinsic mechanism of transition. Therefore, the transition process from the O₃ to NO_x mode of air plasma and intrinsic mechanism were revealed by calculating the evolutionary trend and mechanism of key species through the global model in this study. The results show that the air humidity changes the evolution trend of key short-live species, particularly increasing the number density of hydroxyl (OH) and hydroperoxyl (HO₂). Additionally, which can also affect the evolution trend of long-lived characteristic species, particularly reducing the number density of O₃ and increasing the number density of NO₂, thus promoting the transition from O₃ to NO_x mode. Moreover, the focus of OH and HO₂ in promoting the transition process are different, the OH and HO₂ are more inclined to directly and indirectly suppress the O₃ mode, and indirectly and directly enhance the NO_x mode respectively, meanwhile also accompanied by the generation of new HO₂ and OH that maintains the sustainability of the chemical reaction chain. The results of this study contribute to understand the intrinsic mechanism of humidity promoting transition from O₃ to NO_x mode and provide theoretical references for the application in practical environment of air plasma.