## 4 Conclusions  
A gliding arc discharge plasma set-up was constructed, and the electrical characteristics of the air plasma discharge were investigated. It was revealed that the voltage and current waveforms of the discharge have a serrate-like shape, and that the increase in the output voltage has only a slight influence on the discharge voltage. Meanwhile, the highest arc current values near the discharge breakdown point were decreased from \(\sim\)1.54 A to \(\sim\)0.96 A with the increase in the voltage from 90 V to 250 V. It was observed that the increase in the voltage from 50 to 250 V increased the length of arc and enhanced the duration of the arc discharge from 1.71 ms up to 5.21 ms, respectively. The emission spectroscopy studies of the GAD air plasma indicated high-intensity N2 lines, a moderate intensity of the N2\({}^{+}\) and NO lines, and only a low intensity of the N+ and O lines. The existence of only some traces of the emission lines of oxygen ions (O\({}^{+}\) and O\({}_{2}\)\({}^{+}\)) was confirmed. The increase in the voltage enhanced the ionization degree of the plasma and increased the densities of the N2\({}^{+}\), N+, O, and NO\({}_{\rm\gamma}\) species in the air plasma. The intensity of the emission lines of the O, NO\({}_{\rm\gamma}\), and N2\({}^{+}\) species was increased \(\sim\)17, \(\sim\)15, and \(\sim\)8 times with the increase in V\({}_{\rm G}\) from 50 to 250 V. Meanwhile, the intensity of the emission lines attributed to excited N2 particles was enhanced only \(\sim\)3.5 times when this voltage was increased from 50 V to 170 V. A further increase in voltage resulted in a 10% reduction in the intensity of the N2 emission lines.  
The results indicated that the increase in the voltage lead to the production of a higher concentration of nitrite and nitrate ions and hydrogen peroxide in the BG-11 medium. The concentration of the hydrogen peroxide in the BG-11 medium was enhanced from 5.9 to 17.4 mM, the pH value was reduced from 7.37 to 6.99, and the conductivity of the medium was enhanced from 2.40 mS/cm to 3.34 mS/cm, when the voltage was increased from 90 V to 250 C, respectively. It should be noted that the conductivity of the medium was increased to almost double after the air plasma treatment. The performed studies indicated that by controlling the output voltage, it is possible to regulate the composition of the air plasma and produce a higher amount of NO species, which are important for the application of GAD air plasma in the treatment of biological objects and for the creation of plasma-activated liquid or water.  
Conceptualization, K.J. and L.M.; methodology, K.J., Z.K., R.U., M.A., S.K., A.S. ([PERSON]), A.S. ([PERSON]), V.S. and L.M.; software, K.J., Z.K., A.S. ([PERSON]) and L.M.; validation, K.J., Z.K., R.U., M.A., S.K., A.S. ([PERSON]), A.S. ([PERSON]), V.S. and L.M.; formal analysis, K.J., Z.K., R.U., M.A., S.K., A.S. ([PERSON]), A.S. ([PERSON]), V.S. and L.M.; investigation, K.J., Z.K., R.U., M.A., S.K., A.S. ([PERSON]), A.S. ([PERSON]), V.S. and L.M.; resources, R.U., M.A. and K.J.; data curation, K.J., Z.K., R.U., A.S. (Antanas Striks), A.S. (Artnas Strike) and L.M.; writing--original draft preparation, K.J., Z.K., A.S. (Antanas Striks) and L.M.; writing--review and editing, K.J., V.S., A.S. (Artnas Stirke) and L.M.; visualization, R.U., Z.K., K.J. and L.M.; supervision, K.J. and 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 are contained within the article.  
The authors declare no conflicts of interest.