# Influence of Oxygen Impurity on Nitrogen Atmospheric-Pressure Plasma Jet
## 3 Results and Discussion
### Species Distribution
#### 3.2.1 Distribution of Major Molecules (N\({}_{2}\), O\({}_{2}\))  
Figure 6 shows the nitrogen and oxygen distribution on the surface at different radii with the given inlet gas impurities. Below the exit (r = 0 mm), the nitrogen concentration was around 85 mole/m\({}^{3}\), and the oxygen concentrations with inlet gas conditions of \(10^{-4}\) O\({}_{2}\), \(10^{-3}\) O\({}_{2}\), and \(10^{-2}\) O\({}_{2}\) were 0.013 mole/m\({}^{3}\), 0.085 mole/m\({}^{3}\), and 0.76 mole/m\({}^{3}\), respectively. At a distance from the center, the oxygen concentration increased because of the air intruding from the bottom of the quartz tube; at the same time, the nitrogen concentration decreased because nitrogen was oxidized to nitrogen oxide (NO\({}_{x}\)).  
Figure 5: Temperature profile above and on the surface under the quartz tube region, in which the substrate is treated by the plasma.  
2.2 N-series Species Distribution (N\({}_{2}\)(A), N\({}_{2}\)(B), N\({}_{2}\)(C), N\({}_{2}\)(a), N)  
The distribution of N-series excited state molecules on the surface is shown in Figure 7. The excited molecules created from the nozzle were clearly dissipated along the downstream. As shown in Figure 7a-e, the concentrations of N\({}_{2}\)(A), N\({}_{2}\)(B), N\({}_{2}\)(C), N\({}_{2}\)(a), and N were \(10^{-7}\), \(10^{-12}\), \(10^{-14}\), \(10^{-12}\), and \(10^{-4}\) mole/m\({}^{3}\) below the exit, and they decreased with increasing distance from the center.  
Further, the concentrations of N\({}_{2}\)(A), N\({}_{2}\)(B), N\({}_{2}\)(C), and N\({}_{2}\)(a) decreased with increasing oxygen at the inlet, possibly owing to the lower N\({}_{2}\) concentration from the plasma source. By contrast, the concentration of N was only slightly higher for the case of \(10^{-2}\) O\({}_{2}\).  
2.3 Distribution of O-Series and NO\({}_{\rm x}\) Components (O, O\({}_{3}\), N\({}_{2}\)O, NO, NO\({}_{2}\), and NO\({}_{3}\))  
Figure 8 shows the concentration of the O-series and NO\({}_{\rm x}\) components. The inlet gas impurities significantly influence these components. Because oxygen is one of the major reactants with these components, an increase in the amount of oxygen mixed with the inlet gas results in the production of a higher concentration of O-series and NO\({}_{\rm x}\) from the nozzle.  
Figure 6: Species distribution of (**a**) N\({}_{2}\) and (**b**) O\({}_{2}\) on the surface.  
Atomic O is a highly active chemical, and it rapidly reaches a stable concentration at atmospheric pressure in combination with intruding surrounding air, as illustrated in Figure 8a. N-series excited components and N\({}_{2}\) react with oxygen from the gap under the quartz tube in the downstream and produce O\({}_{3}\), NO, NO\({}_{2}\), and NO\({}_{3}\). Therefore, O\({}_{3}\) and NO\({}_{3}\) increased by almost ten times below the exit, as shown in Figure 8b,f, respectively, and the same could be seen for NO and NO\({}_{2}\) with impurities of 10\({}^{-4}\) O\({}_{2}\) and 10\({}^{-3}\) O\({}_{2}\), as shown in Figure 8d,e, respectively. An impurity of 10\({}^{-2}\) O\({}_{2}\) seemed to cause saturation for NO and NO\({}_{2}\) such that the concentration decreased along the radius. By contrast, N\({}_{2}\)O was relatively stable and exhibited little variation when mixed with downstream air, as shown in Figure 8c.  
A jump was seen at approximately 12.77 mm in almost all concentration distribution results in Figures 6-8. This position could be considered a demarcation between the plasma jet and the incorporation of air, as also seen in Figure 3.