## 2 Experimental Set-Up and Procedures  
Figure 1a shows a systematic diagram of a non-thermal APPJ using Ar and O\({}_{2}\)/Ar admixture discharges. The ceramic tube is the main part in the APPJ, with 2 mm and 1.5 mm for the outer and inner diameters, respectively. The APPJ dimensions and powering characteristics are given in Table 1. The APPJ consists of three electrodes: two isolated hollow cylindrical copper electrodes around the ceramic tube, with 20 mm separation from each other, one electrode powered by an AC high-voltage power source with a voltage ranging from 2.5 to 25 kV and a variable frequency of up to 60 kHz, and another electrode grounded at 40 mm away from the nozzle and 20 mm from the high voltage electrode. Moreover, the aluminum inner capillary with a 1.5 mm diameter represents the third electrode of the discharge, with the whole ceramic tube length ended with the nozzle and described in Figure 1a with the dotted line inside the tube, also described briefly in our previous work [18]. Gas flows into the ceramic tube from two gas inlets, one for argon gas and the other for oxygen to admix inside the aluminum capillary tube, which is maintained in a floating potential. The flow rates were measured using a volumetric flow meter connected to a needle valve to control the flow rates. The APPJ system creates discharges using: (i) flow rates ranging from 0.2 to 4 slm for Ar discharges (dry argon); (ii) O\({}_{2}\)/Ar admixture discharges (wet argon) with different O\({}_{2}\) ratio admixtures, as shown in Table 2.  
Figure 1: Cont. The thermal, electrical, spectroscopical, and photographical characteristics of plasmas generated by the APPJ were measured: (i) measurements used a digital Canon camera (Ota City, Tokyo, Japan) with an exposure time of 40 ms for fast-imaging the emerging jet afterglow discharge in a darkened laboratory. The length and width of the jet measurements can be accurately measured relative to the ground electrode as a reference (the ground electrode with length and width 10 and 3 mm, respectively), as shown in Figure 1b. (ii) Jet dimensions can be calculated from APPJ images according to the dimensions of the ground electrode, as a calibration scale, using Microsoft Paint, as shown in Figure 1c. The calibrations and measurements are discussed briefly in our previous work, ref. [18]. (iii) An avaspec-2048 spectrometer (Louisville, CO, USA) with a charge coupled device (CCD) \begin{table}
\begin{tabular}{c c c} \hline \hline  & Ceramic & Material \\ \cline{2-3}  & Cylindrical & Shape \\ \cline{2-3} Tube dimensions & 70 mm & Length \\ \cline{2-3}  & 2 mm & Outer diameter \\ \cline{2-3}  & 1.5 mm & Inner diameter (Nozzle) \\ \cline{2-3}  & AC high voltage & Power source \\ \cline{2-3} Powering characteristics & 2.5 to 25 kV & Applied voltage \\ \cline{2-3}  & 60 kHz & Frequency \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Tube dimensions and powering characteristics. Figure 1: (a) Schematic diagram of APPJ experimental set-up; (b) Typical example of a ground electrode; (c) Plasma jet width and length as calibration scale. \begin{table}
\begin{tabular}{c c} \hline \hline \multicolumn{1}{c}{**O\({}_{2}\) Flow Rate**} & **Equivalent O\({}_{2}\) Ratio** \\ \hline
2.5 mslm & 0.25\% \\
5 mslm & 0.50\% \\
10 mslm & 1\% \\
15 mslm & 1.5\% \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Equivalent O\({}_{2}\) flow rate ratio. detector was used to measure the axial distribution emission spectra of the APPJ, from the nozzle to far away. The spectral range of the spectrometer was 200 to 1100 nm with a spectral resolution of 1.4 nm. A Fiber Optics Cable (FOC) was connected to the spectrometer and collected emitted light from the APPJ via a lens at the end of the FOC. (iv) A Fluoroptic thermometer (Model No.604, Luxtron Corporation, Santa Clara, CA, USA) was used to measure the average plasma temperature for Ar and O\({}_{2}\)/Ar discharges.