## 3 Results and Discussions
Thus, the probabilities of the dissociation and ionization of the oxygen or nitrogen molecules will increase with the increase in the discharge power or reduction in the gas flow rate [16; 24]. It was observed that the NO\({}_{2}\) concentration increased from ~1500 ppm to ~7000 ppm with the increase in the GAD power in air plasma [24]. [PERSON] et al. [36] demonstrated that the reduction in the air or O\({}_{2}\) gas flow rates increased the concentrations of NO\({}_{x}\) in GAD plasmas. The highest NO\({}_{x}\) concentration  
Figure 3:1 Dependence of NO (**a**) and NO\({}_{2}\) (**b**) gas concentrations on discharge output voltages. of 1.4 vol.% was obtained when the lowest flow rate of 0.5 L/min was used. The longer interaction time with plasma increases the reactions of nitric oxide with excited oxygen particles and leads to the creation of nitrogen dioxide. The residence time of the air gas in the plasma was enhanced when the air flow rate was reduced and more NO and NO\({}_{2}\) were created in the gliding arc discharge. It was determined that the arc propagation time was reduced from 14.5 ms to 4.2 ms, with the increase in the air flow rate from 0.5 L/min to 1 L/min, respectively. [PERSON] et al. [27] demonstrated that the reduction in the air flow rate from 170 L/min to 40 L/min increased the concentration of NO\({}_{\text{x}}\) from 0.3% to 1.4%, respectively. The NO produced in air plasma quickly reacts with the oxygen to form NO\({}_{2}\), and as a result, the production of NO is reduced at higher discharge power or lower air flow rates [27]. [PERSON] et al. [37] obtained that the increase in the frequency from 7 kHz to 9 kHz enhanced the NO\({}_{\text{x}}\) concentration in nitrogen-oxygen plasma when the total flow rate was 1 L/min. The NO concentration varied from \(\sim\)2000 ppm to \(\sim\)6000 ppm depending on the N\({}_{2}\)/O\({}_{2}\) ratio, used pulse duration, and amplitude. It was stated that the higher frequency generated more energetic electrons in the plasma volume and resulted in more collisions between oxygen and nitrogen molecules. However, the authors observed that by changing the pulse width, the NO\({}_{\text{x}}\) concentration could be higher even at lower frequencies. We also found that the NO\({}_{\text{x}}\) concentration increases with decreasing frequency (Figure 9). Such an effect was related to the higher specific energy input and peculiarities of the arc behavior propagation [37]. The selectivity of NO\({}_{2}\) depends on the O\({}_{2}\) content in the nitrogen-oxygen gas mixture and increases with the increase in the O\({}_{2}\) amount from 20 to 50% [16]. It was demonstrated that the NO radical is considered the most important particle for the creation of NO\({}_{2}\) due to chemical reaction Equation (7) in plasma [24]:  
Figure 3:2  
As the supply voltage is increased, the concentration of NO\({}_{2}\) is enhanced due to the promotion of reaction Equation (7) [24]. This was confirmed by the reduction in the NO/NO\({}_{2}\) ratio with the increase in output voltage or reduction in the air flow rate [35]. The NO/NO\({}_{2}\) ratio at an 11.2 L/min flow rate was reduced from 9.8 to 3.0 with the increase in voltage from 90 V to 250 V, respectively. The obtained results indicate that the densities of the NO species in the plasma were enhanced with the increase in the voltage. However, the more NO produced in the plasma, the higher the amount of NO particles converted into NO\({}_{2}\), which would lead to the NO\({}_{2}\) gas concentration increasing faster than the NO gas concentration. A similar effect would be achieved by reducing the air flow at a constant voltage. The specific energy input is higher at lower air flow rates as more atomic nitrogen and oxygen would form and higher concentrations of NO and NO\({}_{2}\) gas would be produced [35; 36]. [PERSON] et al. [28] observed that the increase in NO\({}_{\text{x}}\) concentrations in the gas phase will result in higher concentrations of nitrites in the liquid phase.