## 3 Results and Discussions
It was observed that the intensity was enhanced 7.8 times with the increase in the voltage from 50 to 250 V. This result indicates that the ionization degree of the plasma was enhanced. The energy of the electrons was enhanced at higher voltages and the possibility of the ionization of the nitrogen molecules increased as well. The slight reduction of the intensities of the emission lines attributed to the exited N\({}_{2}\) species at higher voltages is also a sign that the quantity of the ionization reactions was increased. The intensity of the emission line at 236 nm, assigned to NO\({}_{\gamma}\), was enhanced from 1300 counts to 18,900 counts with the increase in the voltage from 50 V to 250 V. It was stated that the formation of NO  
Figure 2:7 Emission intensities of various species as a function of the output voltage. species in air plasma is due to a reaction between oxygen and nitrogen molecules and a third body (electrons, atoms, or molecules) with a relatively high amount of energy [6]. Figure 2:8  
The formation of the NO species in plasmas could also be due to the Zeldovich mechanism, as in Equations (9) and (10) [21]:  
Figure 2:9  
Figure 5.0  
It was found that in pure-nitrogen GAD plasma the formation of exited nitrogen molecules has the highest probability of occurring [15]. The emission spectra of the air plasma produced by GAD also demonstrated similar trends (Figures 7 and 8). [PERSON] et al. [41] observed that the formation of various NOx particles very strongly depends on the vibrational excitation of N2 species. It was established that the intensity of the NO, N2+, and N2 emission lines depended on the air flow rate in GAD [35]. It should be noted that the dominant particles in the GAD air plasma were the reactive nitrogen species N2, N2+, N+, and NOx. Meanwhile, the intensities of the emission lines of the atomic oxygen (O) or oxygen ions (O+) in air plasma were quite low, which leads us to the conclusion that the densities of those particles in the plasma were significantly lower compared to the nitrogen species. The existence of the reactive nitrogen species and even a small amount of oxygen reactive species in the emission spectra of air plasma produced by GAD shows that the created plasma set-up is suitable for the treatment of algae or seeds and production of plasma-activated water or liquid. The BG-11 freshwater microalgae growth medium was chosen as a test sample and its physicochemical properties were investigated. The first significant change during the plasma treatment was the increase in the temperature in the medium (Figure 10a), which was recorded when V\({}_{\mathrm{G}}\) exceeded 130 V. Further increases in the output voltage from 170 V to 250 V increased the temperature of the sample to 46 \({}^{\circ}\)C, 49 \({}^{\circ}\)C, and 54 \({}^{\circ}\)C, respectively. The increase in the temperature is due to the higher ionization degree of the plasma induced at higher voltages. The changes recorded after the plasma treatment included changes in pH, which was initially around 7.5. Increasing the voltage from 90 V to 250 V resulted in a decrease in the medium's pH from 7.37 to 6.99 (Figure 10b). As the plasma generator's voltage was increased, the pH value started to decrease, but a significant decrease (~7%) was only observed at V\({}_{\mathrm{G}}\) of 250 V.  
Figure 5.1 The dependence of the medium’s temperature on the used voltage (**a**) and the variation its of pH values (**b**). The effect of increasing the processing voltage was also observed in the medium's conductivity values (Figure 11a). The BG-11 medium, which initially had a conductivity of \(\sim\)1.85 mS/cm, increased its conductivity after the air plasma treatment. The conductivity of the medium was 2.40 mS/cm when the treatment with air plasma was performed using 90 V. Meanwhile, the highest voltage (250 V) resulted in a significant increase in its conductivity, reaching 3.34 mS/cm--an 80% increase compared to the control medium (Figure 11a).