## 3 Results and Discussions
In reducing the air flow, the arc can propagate a longer distance, and higher temperatures are obtained in the more distant interelectrode zone, i.e., below the measurement point. This would explain a higher intensity of the emission lines with the increase in the air flow rate. The increase in vibrational temperatures is related to the enhanced collision frequency of electrons. The energy transferred from the electrons to the oxygen or nitrogen molecules is higher as the collision frequency is increased. [PERSON] et al. [32] indicated that the highest vibrational and rotational temperatures in the nitrogen plasma are obtained with the elongation of the arc downstream in the axial direction. The produced concentrations of the NO and NO\({}_{2}\) gasses were measured at various discharge parameters and using various air flow rates. The variation in the NO and NO\({}_{2}\) concentrations on the discharge frequency is presented in Figure 3.4 The results indicate that the NO concentration was reduced from 1115 ppm to 990 ppm (at 130 V) with the increase in the frequency from 230 kHz to 270 kHz, respectively (Figure 9a). The NO\({}_{2}\) gas concentration decreased from 265 ppm to 205 ppm with the increase in frequency when the voltage of 130 V was used (Figure 9b). The increase in the output voltage led to the enhancement of NO and NO\({}_{2}\) gas concentrations (Figure 9). The NO\({}_{2}\) and NO concentrations were increased up to 2.1 times and 1.55 times, with the increase in voltage from 130 V to 210 V, respectively. The selectivity of the NO was enhanced from 80.8 to 82.8% with the increase in frequency. The influence of the direct air flow rate on the produced NO and NO\({}_{2}\) concentrations is given in Figure 3.5 It was obtained that the increase in the air flow rate from 6.7 L/min to 15.6 L/min reduced the NO gas concentration from 1180 ppm to 900 ppm. The concentration of the produced NO\({}_{2}\) gas was reduced from 325 ppm to 184 ppm, with the increase in the air flow rate. The selectivity of NO was increased from 78.4% to 83.1% with the increase in air flow rate. The reduction in the NO and NO\({}_{2}\) concentrations is due to the lower ionization degree of the air plasma at the constant output voltage. As the flow rate is increased, the amount of the produced excited and ionized nitrogen and oxygen molecules or atoms in the total plasma volume is reduced. It was obtained that the NO\({}_{\kappa}\) concentration varied from 0.45 to 1.5 vol.%, depending on the GAD parameters. However, it is important to highlight that the air flow rate was from 1 L/min to 5 L/min [35] [PERSON]. [PERSON] et al. [16] observed the reduction in the NO and NO\({}_{2}\) concentrations by 46% and 67%, respectively, when the flow rate was enhanced from 2 to 4 L/min. The total NO\({}_{\kappa}\) concentration varied  
Figure 3.6 Variation in the NO (**a**) and NO\({}_{2}\) (**b**) gas concentrations on the discharge frequency at various output voltages, when the direct air flow rate was 13.4 L/min. from 385 ppm to 2880 ppm depending on the used power and flow rate. The increase in the air flow rate enhanced the air gas velocity, and the treatment duration of air was reduced in the plasma. Therefore, the total NO\({}_{\mathrm{x}}\) concentration was reduced from 1505 ppm to 1090 ppm (Figure 10). The dependence of the NO and NO\({}_{\mathrm{2}}\) gas concentrations on the discharge output voltages at various direct air flow rates was measured (Figure 11). It was demonstrated that the NO and NO\({}_{\mathrm{2}}\) concentrations were enhanced with the increase in the voltage and decrease in the air flow rate. The lowest NO values were at 15.6 L/min air flow rates and varied from 390 ppm to 1655 ppm with the increase in voltage from 90 V to 250 V, respectively (Figure 11a). The NO gas concentration was enhanced from 5% to 12% when the flow rate of 13.4 L/min was used. The highest NO concentrations were obtained at an 11.2 L/min flow rate.