## 3 Results and Discussions
The intensity of the atomic oxygen peak (at 777.5 nm) slightly increased from 3825 counts to 4070 counts with the decrease in the frequency from 270 kHz to 230 kHz. The intensity of the emission line of NO\({}_{\gamma}\) (peak at 236 nm) species was slightly enhanced from 14,540 counts to 16,200 counts with the reduction in the frequency. It was observed that the intensity of NO\({}_{\gamma}\) was increased by ~11% with the decrease in the frequency from 270 to 230 kHz. The emission line intensity  
Figure 3: Emission spectra of the air plasma when the discharge frequency was 230 kHz and 270 kHz, voltage was 130 V, tangential flow rate was 9.44 L/min, and direct air flow rate was 13.4 L/min. Figure 2: Emission spectra of the air plasma when the discharge frequency was 270 kHz measured with an AOS4 spectrometer. of the N\({}_{2}\)+ particle was enhanced from 9270 counts to 10,300 counts with the reduction in the frequency (Figure 4b). The nitrogen molecule is broken into nitrogen atoms by an electron impact-induced dissociation reaction. Nitrogen atoms created in the plasma further react with oxygen atoms, and during this recombination reaction, the NO particles are formed [24]. The observed trends in the emission lines of O, NO\({}_{\gamma}\), N\({}_{2}\)+, and N\({}_{2}\)* species suggest that the ionization degree of the air plasma slightly increased as the frequencies decreased. The emission spectra of the air plasma measured at a 130 V output voltage, at a 270 kHz frequency, and various direct air flow rates are presented in Figure 5. The results indicate that the main peaks are attributed to the O, NO\({}_{\gamma}\), N\({}^{+}\), N\({}_{2}\)+, and N\({}_{2}\)* particles (Figure 5a,b) with the low-intensity peaks of the oxygen ion and molecular oxygen ion. Increasing the air flow rate from 6.7 L/min to 13.4 L/min resulted in higher intensities of the air plasma emission lines. The influence of the air flow rate on the intensities of the main nitrogen and oxygen species is given in Figure 6. Analysis of the most intense emission lines showed that as the gas flow rate increased from 6.7 L/min to 15.6 L/min, the densities of the particles in the  
Figure 4: Variation in intensities of the N\({}_{2}\)* (**a**) and NO\({}_{\gamma}\), O, and N\({}_{2}\)+ (**b**) species on the discharge frequency at 130 V voltage, when the air flow rates were 9.44 L/min (tangential) and 13.4 L/min (direct). Figure 5: Emission spectra of the air plasma at different direct air flow rates when the discharge frequency was 270 kHz and output voltage was 130 V.  
plasma increased (Figure 6). The intensities of the excited nitrogen molecules obtained at 316 nm and 337 nm wavelengths increased from 27,200 counts to 55,350 counts and from 41,900 to 77,000 counts when the air flow rate was enhanced from 6.7 L/min to 15.6 L/min, respectively (Figure 6a). The intensity of the molecular nitrogen ion peak observed at 391 nm was increased from 8070 counts to 11,000 counts. Meanwhile, the emission line intensities of the excited oxygen were enhanced more than twice from 2480 to 5160 counts with an increase in the air flow rate from 6.7 L/min to 15.6 L/min, respectively (Figure 6b). The intensity of the highest NO\({}_{\gamma}\) peak (236 nm) was enhanced from \(\sim\)10,400 counts to 17,650 counts (up by 60%) with the increase in the direct air flow rate (Figure 6b). The obtained results demonstrate that the intensities of the O, NO\({}_{\gamma}\), and N\({}_{2}\)+ species were higher at higher voltages and air flow rates (Figure 7). The intensities of the N\({}_{2}\)* peaks were enhanced with the increase in the air flow rate from 11.2 L/min to 13.4 L/min. The N\({}_{2}\)* peak intensity obtained at 337 nm increased from \(\sim\)12% up to \(\sim\)26%, with an increase in the voltage from 90 V to 250 V and the air flow rate.