## 3 Results and Discussion
### The Effects of Pulse Rising and Falling Time on Reactive Species  
Figures 13 and 14 show the effects of pulse rising and falling time on the emission intensity of SPS of N\({}_{2}\) and FNS of N\({}_{2}\)\({}^{+}\) under air and nitrogen. The rising and falling time of the voltage pulse are both set synchronously from 50 ns to 250 ns. At the top of the reactor in Figure 13, the emission intensity of FNS of N\({}_{2}\)\({}^{+}\) decreases when the rising and falling time of the voltage pulse increase from 50 ns to 150 ns. The reason for this is that the shorter pulse has a higher electron average energy [37], which is beneficial to generate N\({}_{2}\)\({}^{+}\) (B\({}^{2}\Sigma_{\mathrm{u}}\)\({}^{+}\)). Thus, the emission intensity of FNS of N\({}_{2}\)\({}^{+}\) is stronger when the pulse rising and falling time is shorter. In addition, the emission intensity of FNS of N\({}_{2}\)\({}^{+}\) increases when increasing the pulse rising and falling time from 150 ns to 250 ns in Figure 13b. This means that more discharge channels appear when increasing the rising and falling time of the voltage pulse from 150 ns to 250 ns. In Figure 14a, the emission intensity of SPS of N\({}_{2}\) at the top of the reactor decreases when the rising and falling time of the voltage pulse increase from 50 ns to 150 ns, and then it increases with increasing the rising and falling time of the voltage pulse further, which shows the same trend with the curve regarding the changes of the emission intensity of FNS of N\({}_{2}\)\({}^{+}\) at the top in Figure 13b. In addition, the emission intensity of SPS of N\({}_{2}\) at the bottom of the reactor increases first by increasing the rising and falling time of the voltage pulse from 50 ns to 100 ns, and then it decreases with the rising and falling time of the voltage pulse increase from 100 ns, which has the same trend of the curve regarding the changes of the discharge currents in air shown in Figure 6. These two phenomena mean that the generation of N\({}_{2}\) (C\({}^{3}\Pi_{\mathrm{u}}\)) relates to the changes of the discharge channel and electron  
Figure 12: Time evolution of emission intensity of SPS of N\({}_{2}\) under (**a**) air during the rising time of the voltage, (**b**) air during the falling time of the voltage pulse, (**c**) pure nitrogen during the rising time of the voltage pulse, and (**d**) pure nitrogen during the falling time of the voltage pulse with different pulse durations. density, which can be reflected by the discharge current and emission spectrum of the FNS of N\({}_{2}\)\({}^{+}\), respectively. In Figure 14b, the emission intensity of N\({}_{2}\) (C\({}^{3}\)I\({}_{\rm{u}}\rightarrow\) B\({}^{3}\)I\({}_{\rm{g}}\)) decreases with the increase of the pulse rising and falling time, and this is due to the decrease of the electron average energy with increasing the rising and falling time of the voltage pulse [37]. Figure 15 shows the effects of the rising and falling time of the voltage pulse on the time evolution of the emission intensity of the SPS of the nitrogen molecule during one single voltage pulse. Firstly, the intensities decrease with increasing the pulse rising and falling time, which is consistent with the results in Figure 14. Secondly, the moment the peak of the emission intensity of the SPS of N\({}_{2}\) appears is delayed with increasing the rising  
Figure 14: Effects of pulse rising and falling time on emission intensity of SPS of N\({}_{2}\) collected from the top and bottom of the reactor in (**a**) air and (**b**) nitrogen. Figure 13: Effects of pulse rising and falling time on the emission intensity of FNS of N\({}_{2}\)\({}^{+}\) collected from the top and bottom of the reactor in (**a**) air and (**b**) pure nitrogen. and falling time of the voltage pulse, and this is due to the delay of the FP of the discharge current shown in Figure 4. However, the emission intensity of the SPS of N\({}_{2}\) during the pulse falling time is too weak to be observed.