## 3 Results and Discussions
The NO gas concentration increased from 480 ppm to 1785 ppm with the increase in applied voltage, respectively (Figure 11a). The highest NO\({}_{\mathrm{2}}\) gas values were obtained at the lowest air flow rate. The NO\({}_{\mathrm{2}}\) concentration was increased from 49 ppm to 595 ppm with the increase in voltage from 90 V to 250 V, respectively (Figure 11b). The increase in the flow rate up to 13.4 L/min reduced the NO\({}_{\mathrm{2}}\) concentration from 10 to 20% depending on the used voltage. The lowest NO\({}_{\mathrm{2}}\) concentrations were obtained at the 15.6 L/min flow rate and ranged from 36 ppm to 520 ppm with the increase in the voltage. The selectivity of the NO was reduced from 91.5 to 76.0% with the increase in the voltage when the air flow was 15.4 L/min. Meanwhile, the selectivity of NO decreased from 90.8% to 74.9% when the flow rate was reduced to 11.2 L/min. The obtained results indicate that the selectivity of NO was only slightly reduced with the increase in the flow rate in the measured voltage ranges. The selectivity of NO was reduced in the range of 0.7-2.4%, depending on the used voltage, when the air flow rate was changed. Meanwhile, the selectivity of the NO\({}_{\mathrm{2}}\) gas was significantly enhanced up to 2.82 times with the increase in the output voltage from 90 V to 250 V depending on the used air flow rates. Higher energy densities are obtained at the lower air flow rates as the same amount of energy is applied to a smaller volume of air gas per unit of time. Thus, a higher energy density will result in greater dissociation of nitrogen and oxygen molecules to atomic nitrogen and oxygen that can further form NO and NO\({}_{\mathrm{2}}\). Additionally, it should be noted that the formation of NO is increased at higher temperatures through the following chemical reactions [5; 9; 16; 29]:  
Figure 3.7  
Figure 3.8 Variation in the NO (**a**) and NO\({}_{\mathrm{2}}\) (**b**) gas concentrations produced in gliding arc discharges on a direct air flow rate at 270 kHz. The NO created in the plasma could be partly lost due to a chemical reaction with oxygen outside the plasma volume through the following reaction:  
Figure 3.9  
This reaction will dominate if the concentration of the oxygen molecules is relatively high in plasma. The lifetime of the gliding arc is longer at lower air flow rates. Thus, the residence time of air in the arc discharge zone is also extended, which will lead to a more efficient conversion of NO to NO\({}_{2}\) gas in the plasma. Meanwhile, the lifetime of the arc is reduced with the increase in the air flow rate to 15.6 L/min. The shorter residence time reduces the extent of NO oxidation to NO\({}_{2}\), leading to lower concentrations of NO\({}_{2}\) compared to NO gas. It was observed that the increase in the air gas temperature from 220 \({}^{\circ}\)C to 420 \({}^{\circ}\)C resulted in an increase in the NO/NO\({}_{2}\) ratio from 1.5 to 4.4, respectively [9]. Such an increase in the NO/NO\({}_{2}\) ratio was explained by the following dominant chemical reaction:  
Figure 3:0  
It was observed that when oxygen and nitrogen molecules collide with a high-energy third particle (electron, molecule, or atom), the NO particles are formed [5]. Our results indicate that the increase in the air flow rate from 6.7 to 15.6 L/min at the output voltage of 130 V increased the NO/NO\({}_{2}\) ratio from 3.63 to 4.92, respectively. Thus, the selectivity of the NO was increased. It should be noted that the increase in the flow rate reduces the ionization degree of plasma, as higher densities of the N\({}_{2}\) and O\({}_{2}\) are obtained in the air plasma. However, the dissociation energy of the oxygen molecule is only 5.15 eV, while for the nitrogen molecule, 9.79 eV of energy is required. Additionally, the ionization energy of the O\({}_{2}\) and N\({}_{2}\) is 13.6 eV and 15.58 eV, respectively.