## 3 Results and Discussion  
### Effect of Electrode Length on NO Oxidation  
Figure 6 shows the effect of electrode length on NO oxidation under different SED. The experimental results show that the 100- and 150-mm electrodes have a higher oxidation degree of NO\({}_{\chi}\) than 50-mm electrode. The energy consumption is lower when the electrode length is 100 mm. Electrode length determines the length of discharge interval, which mainly affects the residence time of gas in the discharge interval of DBD reactor [18; 21]. Plasma chemical reaction is mainly divided into two stages. In the primary stage, the electrons are accelerated to form high-energy electrons by electric field in the discharge range, and the high-energy electrons collide with gas molecules leading to the ionization, excitation and dissociation of neutral molecules. In the secondary stage, free radicals, excited-state molecules and ions interact to cause radical-radical, radical-neutrals and ion-ion reactions in the downstream of the discharge range [22]. Therefore, the free radicals, excited molecules and atoms generated in the discharge range have a significant impact on pollutant removal. Under oxygen-enriched condition, the concentration of oxidative radicals in the discharge range of DBD reactor gradually increased and reached equilibrium within several discharge cycles, and then it was basically stable with time [23]. It can be seen from the experimental results in Figure 6 that, when the electrode length is 50 mm, the maximum oxidation degree of NO\({}_{\chi}\) is lower than that of electrode lengths 100 and 150 mm due to the short residence time of gas in DBD reactor, and the oxidation radicals do not reach the equilibrium concentration. When the electrode length is in the range of 100-150 mm, the concentration of oxidation free radicals reaches equilibrium and does not change with the increase of discharge time. Increasing the electrode length does not lead to higher NO\({}_{\chi}\) oxidation degree, but higher energy consumption. Figure 6: Effect of electrode length on oxidation degree of NO\({}_{\chi}\)  
It is worth noticing that the oxidation degree of NO\({}_{\text{X}}\) rises first and then shows a decline trend with the increase of SED under oxygen-enriched condition, which is different from the NO\({}_{\text{X}}\) removal efficiency behavior under the condition of non-oxygen [19]. To analyze the mechanism, we measured the concentration changes of NO and NO\({}_{2}\) in the process of NO oxidation in DBD reactor under oxygen-enriched condition. Figure 7 shows that the concentration of NO decreases first and then increases, while the concentration of NO\({}_{2}\) increases first and then decreases with the increase of SED. The main plasma chemical reactions involved in oxygen enriched conditions are listed in Table 1.  
The discharge voltage increases with the increase of SED, as shown in Figure 8a. As a result, the reduced electric field strength E/N (ratio of the electric field strength to the gas particle number density) is increased (as shown in Figure 8b), which makes the mean electron energy increase [16]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reactions** & **NO.