## 2 Materials and Methods  
A gliding arc discharge plasma setup was constructed at the Lithuanian Energy Institute, and a detailed schematic view of the GAD plasma setup can be found in ref. [23]. The GAD plasma system consisted of a power supply (AC power generator and high-voltage transformer) and gas flow control and supply system. A G2000 high-voltage/medium frequency generator (Redline Technologies Elektronik GmbH, Baesweiler, Germany) was employed to create the gliding arc discharge. Two \"knife-like\" shape electrodes were used. Each electrode was 52 mm long and 25 mm wide (at the top) with a gap of 1 mm at the top of the positive and negative electrodes. The AC generator output voltage varied from 90 V up to 250 V, the frequency was ranked from 230 kHz up to 270 kHz, and the signal duty cycle was kept at 0.35 during each discharge. It should be noted that the turn ratio of the used transformer was 1:33.  
The gas supply system consisted of an air compressor, mass flow controllers, plastic pipes, metal tubes, and steel connections. Compressed air was used to form plasma. Two FLD-type flowmeters were used to supply the air. Air was injected into the plasma reactor in two ways: from the top of the electrodes (directly) and tangentially [23] (Figure 1). Such an introduction of air flows allowed for the creation of more stable and uniform temperature plasma within the reactor volume. During all experiments, the tangentially injected air flow rate was kept at a constant value of 9.44 L/min. When the air was supplied from the top of the electrodes, the air flow rate was changed from 6.7 L/min to 15.6 L/min. The GAD plasma was generated at atmospheric pressure at room temperature.  
The air plasma emission spectra were investigated with two spectrometers: a flame emission spectrometer, Flame UV-VIS (Ocean Insight, Orlando, FL, USA), and an acousto-optic emission spectrometer, IFU AOS4 (IfU Diagnostic Systems GmbH, Lichtenau, Ger  
Figure 1: Schematic of experimental setup used for the creation of gliding arc discharge and the investigation of plasma.  
many). The integration time was set to 0.375 s, with 10 scans accumulated per measurement, and each spectrum was recorded three times over a wavelength range of 200-1000 nm, when spectra were captured using the flame emission spectrometer. The spectra were recorded in the range of 250-800 nm (with a spectral resolution of 0.05 nm at 250 nm and 0.5 nm at 800 nm), and each spectrum was measured 3 times when the IFU AOS4 equipment was used. An optical fiber was placed between the electrodes at a 10 cm distance and at 35 mm from the top of the electrodes [23] (Figure 1). The IFU AOS4 spectrometer was used to determine the intensity of emission lines more precisely. The intensities of all emission lines presented in this manuscript were determined from the spectra obtained with a flame UV-VIS spectrometer. An integration time for a lower resolution spectrometer was selected after comparing air spectra to emission spectra recorded with a higher-resolution spectrometer, which allowed us to measure the air spectrum with the main emission lines.  
The measurements of the concentrations of the NO and NO\({}_{2}\) gasses produced using air gas with the gliding arc discharge plasma reactor were determined using an SWG 300-1 (MRU instruments Inc., Neckarsulm, Germany) gas analyzer. The measurements were carried out 3 times for each experimental point, and the average values were calculated.