## 4 Conclusions  
The influence of the output voltage, discharge frequency, and air flow rate on the composition of air plasma and NO\({}_{\text{x}}\) concentrations produced by gliding arc discharge was determined. Emission spectroscopy investigations demonstrated that the excited nitrogen molecules (N\({}_{2}\)*) were the dominant species in the GAD air plasma. The air plasma consistedof a significant amount of N2+ and NO species. Meanwhile, the intensities of the emission lines attributed to the nitrogen ion (N+) or atomic oxygen (O) were relatively low.  
The increase in the frequency from 230 kHz to 270 kHz at constant voltage and air flow rate enhanced the intensities of the emissions peaks of N2* by -7.7%. Meanwhile, the intensities of the N2+, O, and NO\({}_{\gamma}\) species were reduced by 10%, 6%, and 10%, respectively, when the frequency was increased. As a result, the vibrational temperature varied in the range of 5930-6405 K. The produced NO\({}_{\rm x}\) concentration was reduced from 1380 ppm to 1195 ppm, and the selectivity of NO was increased from 80.8% to 82.8%, with the increase in the frequency from 230 to 270 kHz, respectively. The increase in the air flow rate from 6.7 L/min to 15.6 L/min enhanced the intensities of the emissions lines of the N2*, N2+, N+, O, and NO\({}_{\gamma}\) species in air plasma. The NO and NO\({}_{2}\) gas concentrations were reduced by -24% and -53% when the flow rate was enhanced from 6.7 to 15.6 L/min, respectively. Therefore, the selectivity of the NO was increased from 78.4% to 83.1%. The enhancement of the voltage increased the intensities of the emission lines attributed to the N2+, O, and NO\({}_{\gamma}\) species in air plasma by 1.95, 3.58, and 2.16 times, respectively. The highest intensity of the emission lines related to N2* particles was at 130 V, and the further enhancement in the voltage reduced the intensity of the N2* emission lines. This is direct evidence that the ionization degree of air gas and the dissociation of the nitrogen and oxygen molecules were enhanced. The vibrational temperature of N2* increased by \(\sim\)13% with the increase in voltage when the direct air flow rate was 11.2 L/min. The increase in the voltage drastically enhanced the formation of the NO and NO\({}_{2}\) in the GAD plasma. It can be stated that the concentrations of the produced NO\({}_{\rm x}\) are significantly more sensitive to the used voltage than the air flow rate. The increase in the voltage allowed us to enhance the concentrations of the NO and NO\({}_{2}\) up to \(\sim\)4 times and 14 times, respectively. Meanwhile, the selectivity of NO decreased from 91.5-90.8% to 76-74.5%, with increasing voltage for various air flows. These investigations demonstrated that the highest production of the NO\({}_{\rm x}\) is obtained at the highest output voltage and the lowest air flow rates. Meanwhile, the determined vibrational temperatures only indicate the temperature at a certain measured location, and the obtained \(T_{vib}\) values cannot be considered maximum temperatures for discharge formation parameters.  
Conceptualization, L.M.; methodology, R.U., M.A. and L.M.; software, R.U. and L.M.; validation, R.U., M.A. and L.M.; formal analysis, R.U., M.A. and L.M.; investigation, R.U., M.A. and L.M.; resources, R.U., M.A. and L.M.; data curation, R.U., M.A. and L.M.; writing--original draft preparation, L.M.; writing--review and editing, L.M.; visualization, R.U. and M.A.; supervision, L.M. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Research Council of Lithuania under Grant P-MIP-22-257.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
The authors would like to express their gratitude to [PERSON] for the measurements of the NO\({}_{\rm x}\) concentrations.  
The authors declare no conflicts of interest.