## 3 Results and Discussion
### The Influence of Solution Concentration on NOx Removal Efficiency  
As can be seen from Table 1, the concentration of KCL solution affects its conductivity significantly. Therefore, when KCL solution is used as the ground electrode, the solution concentration is one of the important factors affecting the NOx removal effect. The variation of energy density with input voltage at different solution concentrations is shown in Figure 6. When the solution concentration is 0.001 mol/L, the sustaining voltage at the start of discharge is about 60 V, which is 10 V higher than the sustaining voltage at the start of discharge corresponding to other solution concentrations. In addition, the energy density under each input voltage is the largest at such concentration, indicating that the discharge in the DBD reactor requires greater energy consumption. As the concentration of the solution increases, the energy density corresponding to the same voltage becomes smaller. Interestingly, when the solution concentration is 0.1 mol/L and 1.0 mol/L, respectively, the profiles of the energy density versus input voltage basically coincide. That is, after the conductivity reaches a certain level, it has little influence on the discharge effect of the DBD reactor.  
The influence of solution concentration on NO and NOx removal efficiency is displayed in Figure 7. The liquid temperature is 20 \({}^{\circ}\)C and the power discharge frequency is 8 kHz. It can be seen from Figure 7a that under the same energy density, increasing the solution concentration can improve NO removal efficiency, and this effect is particularly significant when the solution concentration is lower than 0.1 mol/L. Specifically, when the KCL solution with a concentration of 0.001 mol/L is used as the ground electrode, the energy density at the initial discharge reaches 2.2 kJ/L, and the maximum NO removal efficiency is only 58% at the energy density of 4.8 kJ/L. The NO removal effect is enhanced remarkably as the solution concentration increases to 0.01 mol/L and a NO removal efficiency of 100% is achieved at the energy density of 4.2 kJ/L. However, the profiles of the NO removal efficiency versus energy density at the solution concentration of 0.1 mol/L and 1.0 mol/L are basically the same, and both the NO removal efficiency reach 100% at the energy density of 3.0 kJ/L. Considering the relationship between the concentration and conductivity of the KCL solution, the results demonstrate that the influence of solution conductivity on NO removal effect becomes negligible as it reaches 10 ms/cm or higher. The variation of NOx removal efficiency with solution concentration is similar to that of NO, as shown in Figure 7b. At the solution concentration of 0.001 mol/L, the maximum  
Figure 6: The variation of energy density with input voltage at different solution concentrations.  
NOx removal efficiency is 28.75% as the energy density increases to 4.88 kJ/L. Half of the energy density is required for the same NOx removal efficiency when a KCL solution with a concentration of 0.01 mol/L is used. Furthermore, under the same energy density, the NOx removal efficiency is significantly improved with the increase of solution concentration. When the solution concentration is 1.0 mol/L and the energy density is 3.15 kJ/L, the NOx removal efficiency reaches up to 85.24%.