Ozone ( \(\hbox {O}_3\) ) mode and Nitrogen oxides ( \(\hbox {NO}_x\) ) mode are the two types of discharge modes present in air plasma, and different discharge conditions have different effects on the transition of the two modes. Since air often contains moisture and the humidity conditions are different in different regions and climates, the humidity has an important effect on the transition between the \(\hbox {O}_3\) mode and \(\hbox {NO}_x\) mode, but there are few studies on the intrinsic mechanism of the effect of humidity on the transition between the \(\hbox {O}_3\) mode and \(\hbox {NO}_x\) mode of air plasma. Therefore, the intrinsic mechanism of the effect of humidity on the transition between the \(\hbox {O}_3\) mode and \(\hbox {NO}_x\) mode of air plasm was investigated by using the zero-dimensional model of plasma reaction kinetic approach. The process and intrinsic mechanism of the humidity promoting the transition from the \(\hbox {O}_3\) mode to \(\hbox {NO}_x\) mode of air plasma were revealed by analyzing the evolution trends of key short-lived species and long-lived characteristic species, as well as the evolutionary mechanism of these particles. The results of this paper show that the presence of air humidity can change the evolution trend of the number density of short-lived species produced during mode transition process, and in particular will increase the number density of hydroxyl (OH) radical and hydroperoxyl ( \(\hbox {HO}_2\) ) radical. In addition, compared with the dry condition, air humidity also can affect the evolution trend of the number density of long-lived characteristic species generated during the model transition process, and in particular will reduce the number density of \(\hbox {O}_3\) that the dominant product of air plasma \(\hbox {O}_3\) model, and increase the number density of nitrogen dioxide ( \(\hbox {NO}_2\) ) that the dominant product of \(\hbox {NO}_x\) model, thus promoting the transition of air plasma from \(\hbox {O}_3\) mode to \(\hbox {NO}_x\) mode. Moreover, the focus on function of OH radical and \(\hbox {HO}_2\) radical in the process of humidity promoting transition from \(\hbox {O}_3\) mode to the \(\hbox {NO}_x\) mode of air plasma are different. The OH radical and \(\hbox {HO}_2\) radical are more likely to play direct and indirect roles respectively in suppressing \(\hbox {O}_{3}\) -dominant mode, and play indirect and direct roles respectively in promoting the occurrence degree of \(\hbox {NO}_x\) mode, and the above chemical chain of reactions is accompanied by the generation of new \(\hbox {HO}_2\) radical and OH radical, which can maintain the sustainability of the chemical chain of reactions. These results suggest that the humidity of air can influence the chemical reaction chain during the mode transition process and promote the air plasma transition from \(\hbox {O}_3\) mode to \(\hbox {NO}_x\) mode. The results of this paper can help to understand the microscopic intrinsic mechanism of humidity promoting transition of air plasma from \(\hbox {O}_3\) mode to \(\hbox {NO}_x\) mode, and provide theoretical references for the application of air plasma in practical environment.
