## 3 Results and Discussion
### The Influence of Solution Temperature on NOx Removal Efficiency  
Gas temperature is one of the critical factors affecting the reaction rate of each chemical reaction in the DBD reactor. During the experiment, the temperature of the ground electrode directly determines the temperature of the reaction gas, which in turn affects the NOx removal effect. In order to clarify the influence of gas temperature on NOx removal efficiency, the NOx removal experiments at different solution temperatures are conducted in this part. The results are exhibited in Figure 8 and the data in the left and right columns are obtained at the KCL concentrations of 0.001 mol/L and 0.1 mol/L, respectively. Figure 8a,b depict the variation of the energy density with input voltage at solution temperatures of 30\({}^{\circ}\), 40\({}^{\circ}\), 50\({}^{\circ}\), and 60\({}^{\circ}\). When the solution concentration is 0.001 mol/L, the energy density of the DBD reactor decreases with the increase of solution temperature. For example, the energy density is 4.5 kJ/L at the input voltage of 110 V when the solution temperature is 30\({}^{\circ}\), and a 20% drop is obtained as the temperature rises to 60\({}^{\circ}\). Therefore, increasing gas temperature can improve the discharge effect of the reactor under such concentration. On the contrary, the difference in energy density between different temperatures is very small when the solution concentration is set to 0.1 mol/L (Figure 8b). This shows that after the solution conductivity of the ground electrode reaches a certain value, the gas temperature has little effect on the energy consumption per unit volume of the DBD plasma reactor. The influence of gas temperature on NO removal efficiency at the solution concentrations of 0.001 mol/L and 0.1 mol/L are shown in Figure 8c,d. Obviously, when the concentration of the ground electrode is 0.001 mol/L, the increase in solution temperature can effectively promote the removal of NO, and a higher energy density is required to achieve the same NO removal efficiency at low temperatures. As the solution concentration increases to 0.1 mol/L, the curves in Figure 8d are basically the same under different temperatures, indicating that the influence of gas temperature on NO removal effect becomes negligible. In particular, a 100% NO removal efficiency is achieved when the energy density reaches 2.7 kJ/L or higher regardless of the gas temperature. Figure 7: The influence of solution concentration of the ground electrode on NO/NOx removal efficiency at different SEDs: (**a**) NO removal efficiency; (**b**) NOx removal efficiency. A similar variation trend is observed as to the influence of gas temperature on NOx removal efficiency. It can be seen from Figure 8e that the increase in temperature significantly improves the removal of NOx when the solution concentration is 0.001 mol/L. In fact, corresponding to the solution temperature of 30\({}^{\circ}\), 40\({}^{\circ}\), 50\({}^{\circ}\), and 60\({}^{\circ}\), the conductivity of the KCL solution is 0.250 ms/cm, 0.253 ms/cm, 0.256 ms/cm, and 0.260 ms/cm, respectively, and the absolute change in conductivity is too small to cause such a large increase in NOx removal efficiency. The main reason for this phenomenon is that there is only a layer of 1 mm thick glass between the reaction gas and the ground electrode, and the NOx removal reaction rate is elevated as the solution temperature increases. The change of NOx removal efficiency with solution temperature at the solution concentration of 0.1 mol/L is shown in Figure 8. The influence of gas temperature and KCL solution concentration on NOx removal efficiency is 0.001 mol/L. The SED changes with input voltage at 0.001 mol/L; (**b**) The SED changes with input voltage at 0.1 mol/L; (**c**) NO removal efficiency at 0.001 mol/L; (**d**) NO removal efficiency at 0.1 mol/L; (**e**) NOx removal efficiency at 0.001 mol/L, and (**f**) NOx removal efficiency at 0.1 mol/L.