## 3 Results and Analysis  
### Characteristics of LTP Jet Array  
Figure 3a shows the current and voltage characteristics of the LTP jet array. The peak value of the applied voltage was 9 kV. The current peaks shown in Figure 3a suggest that the discharge mainly occurred at the rising phase of the positive and negative half cycle. The length of the plasma plume of the LTP jet array with the working gas of argon alone was about 2 cm, and it decreased to 1.7 cm and 0.9 cm for the working gas with an O\({}_{2}\) percentage of 0.1% and 1%, respectively. The current peak was about 95 mA for the argon alone case, and it decreased by 50 mA as the O\({}_{2}\) percentage increased from 0 to 1%. The plasma plume of the LTP jet array can be touched by hand directly. The gas temperature of the plasma plume measured by the thermometer was 25 \(\pm\) 2 \({}^{\circ}\)C. The difference in the gas temperature of the LTP jet array between the different O\({}_{2}\) percentages was negligible. Figure 3: (**a**) Current and voltage characteristics of the LPT jet array (with the working gas of argon alone). Because the difference of the CVC between argon, argon + 0.1% O\({}_{2}\), argon + 1% O\({}_{2}\) was negligible, only one group of CVC is plotted. (**b**) The Lissajous figure corresponding to (**a**) was used to calculate the power of the LPT jet array. The power of the LTP jet array was calculated using Equation (1) based on the Lissajous figure shown in Figure 3b [21]:  
\[P=\frac{1}{T}\int_{0}^{T}UIdt=\frac{C_{M}}{T}\int_{0}^{T}U\frac{dU_{M}}{dt}dt=fC_ {M}\int_{0}^{T}UdU_{M}=fC_{M}S \tag{1}\]  
In Equation (1), \(P\), \(U\), \(I\), \(f\), \(C_{M}\), and \(S\) are the power, voltage, current, discharge frequency, capacitor of the LTP jet array, and the area of the closed voltage-current curve in the Lissajous figure, respectively. The capacitance of the plasma jet array measured using the impedance analyzer was 9.756 pF. The power of the argon, argon + 0.1% O\({}_{2}\), argon + 1% O\({}_{2}\) cases was 1.88 W, 1.64 W, and 1.32 W, respectively. The addition of O\({}_{2}\) into the working gas also affected the reactivity of the LTP jet array significantly. The optical emission spectrum (OES) shown in Figure 4 indicated that the OES of atomic oxygen at O (35\({}^{\circ}\)S-3p\({}^{\circ}\)P) 777 nm and O (3p\({}^{\circ}\)4p-3p\({}^{\circ}\)6s) 844 nm increased by about 40% as the O\({}_{2}\) ratio increased from 0 to 0.1% and decreased by 63.3% as the O\({}_{2}\) percentage increased from 0.1% to 1% [22; 23]. Energetic electrons with electron energy >10 eV are needed to generate the excited atomic oxygen corresponding to the OES at 777 nm and 844 nm [22; 24; 25; 26]; therefore, the concentration of atomic oxygen in the ground state was much higher than that in the excited state [27; 28]. For example, the absolutely calibrated TALIF measurement of atomic oxygen in the grounded state in the effluent of the similar argon plasma jet indicated that the atomic oxygen density was in the order of 10\({}^{15}\)/cm\({}^{3}\), and that it decreased with the distance to the nozzle by up to an order of magnitude [29]. The increasing at first and decreasing afterward trend of the OES was also consistent with the evolution of atomic oxygen in the ground state as a function of the O\({}_{2}\) admixture ratio measured by TALIF [10; 30]. More electrons consumed by attachments to oxygen, e + O\({}_{2}\) => O\({}_{2}\)\({}^{-}\), results in the decreasing discharge intensity and decreasing reactivity as the O\({}_{2}\) admixture ratio increased from 0.1% to 1% [31]. Figure 5 shows O\({}_{3}\) and NO density as a function of the O\({}_{2}\) admixture ratio. As the O\({}_{2}\) admixture ratio increased from 0 to 1%, O\({}_{3}\) density increased from 18 ppm to 43 ppm (0.1% O\({}_{2}\)) and 25 ppm (1% O\({}_{2}\)). The peak concentration of NO was 6 ppm when the  
Figure 4: The optical emission spectroscopy of the LTP jet array with the working gas of argon, argon + 0.1% O\({}_{2}\), argon + 1% O\({}_{2}\). \(\text{O}_{2}\) admixture ratio was 0.1%.