## 3 Results and Discussions  
The emission spectra of air plasma produced at 90 V and measured with an acousto-optic emission spectrometer are given in Figure 2. The emission lines observed at 315.8 nm, 337 nm, 353 nm, 357.6 nm, and 380.3 nm are related to the (0,1), (0,0), (2,1), (1,0), and (2,0) vibrational modes of excited nitrogen molecules, respectively [5, 12, 21, 25]. The emission lines observed at 391.1 nm and 419 nm are related to the first negative transition of the molecular nitrogen ion (N\({}_{2}\)\({}^{+}\)) (Figure 2). It was indicated that the emission lines in the interval of 280-450 nm are usually ascribed to the second positive system of N\({}_{2}\) (C\({}^{3}\Pi_{\text{u}}\)\(\rightarrow\)B\({}^{3}\Pi_{\text{g}}\)) and N\({}_{2}\)\({}^{+}\)'s first negative system (B\({}^{2}\Sigma^{+}_{\text{u}}\)\(\rightarrow\) X\({}^{2}\Sigma^{+}_{\text{g}}\)). The formation of N\({}_{2}\)\({}^{+}\) is also due to the direct ionization reaction produced through collision with high-energy electrons. Those peaks are the main emission lines observed in the air, nitrogen, or air-nitrogen plasmas [5, 10, 21, 24, 27]. The lower intensity peaks observed at the 777.5 nm and 844.7 nm wavelengths are attributed to the atomic oxygen (O) with transition bands of 3p\({}^{5}\)P \(\rightarrow\) 3s\({}^{5}\)S and 3p\({}^{3}\)P \(\rightarrow\) 3s\({}^{3}\)S, respectively [3, 29, 30, 31]. The peak at 306.8 nm is associated with the OH radical. The existence of OH radicals in the emission spectra of the air plasma is explained by the presence of the low amount of water vapor in the ambient air used [29]. The low-intensity peaks at 399.1 nm and 434.1 nm are attributed to the nitrogen ion [23, 32]. The optical emission spectra of an air plasma created by gliding arc discharge at 130 V, with a direct air flow rate of 13.4 L/min, and different frequencies are given in Figure 3. The emission spectra of air plasma produced at various discharge frequencies demonstrated that the excited nitrogen molecules are the dominant particles in plasma. The formation of the excited states of NO\({}_{\text{Y}}\) particles was confirmed by the presence of peaks at \(\sim\)236 nm, 246 nm, 258 nm, and 283 nm (Figure 3a). The formation of an NO \(\upbeta\)-system (B\({}^{2}\Pi-X^{2}\Pi\)) and NO \(\upgamma\)-system (A\({}^{2}\Sigma^{+}-X^{2}\Pi\)) were obtained in nitrogen, air, or nitrogen-oxygen plasma at a range of 200-300 nm [3, 23, 24, 29, 33]. The atomic oxygen peaks were obtained at 777.5 nm and 844.7 nm (Figure 3b). The low-intensity lines at \(\sim\)221 nm, \(\sim\)399 nm, and \(\sim\)434 nm are attributed to the atomic nitrogen ions in air plasma [5]. The emission spectra of the air plasma demonstrated low-intensity lines of oxygen ions (464 nm) and molecular oxygen ions (636 nm) [5, 23, 33]. The information about the particles presented in the plasma and variation in the densities of the produced species could be obtained from the emission spectra. To determine the variation in the main species obtained in the air plasma on the discharge frequencies, several emission lines were chosen. The dependence of the emission line intensities of NO\({}_{\text{Y}}\) (236 nm), N\({}_{2}\)\({}^{*}\) (316 nm and 337 nm), N\({}_{2}\)\({}^{+}\) (391 nm),and O (777.5 nm) on the discharge frequency are given in Figure 4a,b. It was obtained that the intensities of the atomic oxygen peak at ~777 nm was several times lower compared to the intensities of N\({}_{2}\)\({}^{+}\) or N\({}^{+}\) particles (Figures 3 and 4b). It was obtained that the intensity of N\({}_{2}\)\({}^{+}\) particles was enhanced by ~11% with the decrease in the frequency from 270 to 230 kHz. Meanwhile, a slight reduction in the emission line intensities assigned to the excited N\({}_{2}\) particles was observed at lower frequencies. The intensity of the N\({}_{2}\)\({}^{\star}\) peak was reduced from 62,600 counts to 58,080 counts (by ~7%) with the decrease in the discharge frequency (Figure 4a).