## 3 Results and Discussion
### Spectral Diagnosis  
To better understand the output of the NOx removal experiment, spectral diagnosis is performed at the solution concentration of 1.0 mol/L. As shown in Figure 9a, when pure nitrogen is introduced into the DBD reactor at the power supply frequency of 8 kHz and the input voltage of 150 V, the spectral line intensity of the second positive band of N\({}_{2}\)(C\({}^{3}\Pi_{u}\)\(\rightarrow\)\({}^{3}\Pi_{g}\)) molecule is very obvious. Significant peaks are observed at the wavelengths of 316 nm, 337 nm, 357 nm, 375 nm, and 406 nm, which indicates that the nitrogen molecules in the discharge area have undergone a transition from N\({}_{2}\)(C\({}^{3}\Pi_{u}\)) state to N\({}_{2}\)(B\({}^{3}\Pi_{g}\)) state. In addition, although 99.99% high-purity nitrogen is used in the experiment, a clear NO-\(\gamma\) band appears in the wavelength range of 200-280 nm. [PERSON] [24] also reported a similar phenomenon, which may be caused by the trace amount of O\({}_{2}\) contained in the sample gas. Since the content of O\({}_{2}\) is very small, the minute quantity of NO is mainly generated through Reactions (4)-(6) [20]. Figure 9b displays the emission lines of the reactor when N\({}_{2}\), NO (500 ppm), and NO\({}_{2}\) (50 ppm) are introduced. Similarly, there is a distinct second positive band of nitrogen molecules in the wavelength range of 300-400 nm. Moreover, a high intensity NO-\(\gamma\) band in the region of 200-280 nm produced by the transition of the excited state NO (A\({}_{2}\)\(\sum^{+}\)) to the ground state is shown, which demonstrates that NO is produced during ionization. When N\({}_{2}\), O\({}_{2}\) (5%), NO (500 ppm), and NO\({}_{2}\) (50 ppm) are filled into the DBD reactor, the emission spectrum of NO is not observed shown as Figure 9c. This is because a large content of oxygen collides with the high-energy electrons to produce O atoms in the reactor. The main reactions that may occur in the DBD reactor are Reactions (8)-(10), i.e., O\({}_{2}\) in the gas source ionizes to form O\({}_{3}\) and reacts with NO to generate NO\({}_{2}\), which reduces NO [25]. Therefore, when the power supply voltage reaches 150 V under the N\({}_{2}\), O\({}_{2}\), NO, and NO\({}_{2}\) system, the NO removal efficiency is 100%. [PERSON] [26] found that when the input power of the plasma reactor is less than 175 W, the ozone in the reactor is mainly consumed by the oxidation of NO to NO\({}_{2}\), and NO decreases sharply as the power increases. When the input power is greater than 175 W, NO is completely oxidized, and NO\({}_{2}\) starts to be slowly oxidized to N\({}_{2}\)O\({}_{5}\), and the NO\({}_{2}\) content decreases. This is consistent with the spectral results of ours. When the input power exceeds a certain value, the main reaction in the reactor is the oxidation of NO\({}_{2}\) by O\({}_{3}\) to N\({}_{2}\)O\({}_{5}\). Figure 9d shows the variation of spectral intensity with peak discharge voltage in the N\({}_{2}\), O\({}_{2}\), NO, and NO\({}_{2}\) system. It can be seen that the spectral intensity is enhanced as the peak discharge voltage increases. The reason for this phenomenon is that when the input voltage increases, both the density of high-energy particles and the energy released by the DBD reactor increase.