## 3 Results and Discussion
### Optical Emission Spectra of APPJ  
Optical emission spectroscopy (OES) of the APPJ was performed at the core of the jet's vertical axis using fiber optics with the following parameters: (i) the intensity of the emission spectra (IES) of the emitted jet versus the wavelength; (ii) the peak-to-peak voltage was 11.2 kV, with a 25 kHz frequency; (iii) the wavelength ranged from 250 to 850 nm; (iv) detection was carried out at two different axial locations--the nozzle and 10 mm apart from the nozzle; (v) the investigated flow rates for dry argon discharges were 1.8 and 3.2 slm and for wet argon discharges were 1 slm of argon with different ratios of oxygen admixture: 0.25% and 1.5%. Figure 11 shows the OES of dry argon discharge with an applied flow rate of 1.8 slm taken from the two axial locations: nozzle and 10 mm apart from the nozzle. The dry argon APPJ emission spectrum at the nozzle is presented in Figure 11a. A high emission intensity with a strong line is found for the hydroxide band (OH) at 309.6 nm ((A\({}^{2}\)\(\times\)-X\({}^{2}\)[]) transition), for argon lines as follows: ArI: (696.7, 727.3, 738.46, 751.47, 763.76, 772.53, 795.53, 801.47, 811.53, 826.45, and 842.46 nm) (3\(\diameter 39^{5}\)(2P\({}^{5}\)\({}^{3}\)/2)4\(\diameter\) transition), and for the nitrogen band (N\({}_{2}\)): (337.26, 357.77, 380.36 nm) ((C\({}^{3}\)I\({}_{\rm u}\)-B\({}^{3}\)I\({}_{\rm g}\)) transition). Figure 11b shows the emission spectrum detected at 10 mm from the jet nozzle for dry argon, a moderate emission intensity lower than that of the nozzle measurements. The OH band appears lower with 80% than IES at the nozzle, and the nitrogen band (N\({}_{2}\)) with higher IES than at the nozzle by 90%, and with lines, as follows: {337.26, 357.77,  
Figure 10: Oxygen ratio (with 1 slm of argon flow rate) vs. temperature of the emerging jet for different axial distributions. 380.36, 405.22, and 433.62 nm) ((C\({}^{3}\)I\({}_{\rm u}\)-B\({}^{3}\)I\({}_{\rm g}\)) transition); besides argon lines appeared at the jet nozzle. The intensity of APPJ emission spectra decreased when the argon flow rate increased to 3.2 slm for all spectra measured at the two different axial locations ( nozzle and 10 mm), as shown in Figure 12a,b. A reduction in the APPJ emission spectra was measured when the dry argon flow rate increased to more than 2.4 slm for all spectra measured due to a transition from the laminar flow mode to the turbulent flow mode. Figure 11: Optical emission spectra of APPJ using argon discharge at an applied flow rate of 1.8 slm for (**a**) nozzle and (**b**) 10 mm. Figure 13a displays the optical emission spectra of wet argon discharges with ratios of 0.25% of oxygen admixture with 1 slm of argon at the two different axial locations (nozzle and 10 mm). A high IES with a strong OH band at 309.6 nm was measured at the nozzle location; a low IES for weak N\({}_{2}\) bands was measured: {337.26, 357.77, 380.36, 380.36, 405.22, 415.85, 420.19, 427.3, and 433.62 nm} ((C\({}^{3}\)I\({}_{\rm u}\)-B\({}^{3}\)T\({}_{\rm g}\)) transition); Ar lines appeared as presented in Figure 11a for the case of a dry argon discharge; and oxygen (O) radical lines were observed: {777.84 and 843.8 nm} (3s\({}^{2}\)3p\({}^{5}\)(2P\({}^{5}\)\({}^{3}\)/2)4s transition). For a wettability with 0.25% of oxygen and at 10 mm from the nozzle, as shown in Figure 13b, the same lines and bands appeared in the emission spectra recorded at the nozzle, but IES decreased by 60% for the OH band and increased by 80% for the N\({}_{2}\) bands: {337.26, 357.77, 380.36, 380.36, 405.22, and 433.62 nm} ((C\({}^{3}\)I\({}_{\rm u}\)-B\({}^{3}\)T\({}_{\rm  
763.76, 795.53, 801.47, 811.53, and 826.45 nm) (3s\({}^{2}\)3p\({}^{5}\)(2P\({}^{\circ}\)\({}_{3/2}\))4\(p\) transition), with a 27% lower emission spectrum intensity; oxygen lines at 777.84 and 843.8 nm (3s\({}^{2}\)3p\({}^{5}\)(2P\({}^{\circ}\)\({}_{3/2}\))4\(p\) transition) were recorded.