## 3 Results and Discussion
### Spectroscopic Characteristics and Gas Temperature Measurement of the Air Plasma  
As a powerful tool, OES is widely used to determine plasma properties, especially for studying excited and active species and excitation, vibrational, and rotational temperatures. In this study, OES was used to validate the uniformity of plasma generation in each channel of a ten-layer DBD reactor at a targeted high flow rate of 1000 L/min, and to confirm the optimum operating flow rate. The optical fiber was placed in the vicinity of each layer directly facing the plasma zone at the measurement points shown in Figure 3a, where it could detect the strongest emission intensity using careful and subtle adjustment. Figure 7a presents the optical emission spectrum in the range of 200-900 nm for the air discharge in the fifth flow channel of the DBD reactor, where the applied voltage and gas flow rate were fixed at 19 kV and 1000 L/min, respectively. It can be seen that the typical peak lines of the air plasma which mainly result from the excited N\({}_{2}\) (i.e., N\({}_{2}\) second positive band: N\({}_{2}\) (C\({}^{3}\Pi_{u}-B^{3}\Pi_{g}\)), N\({}_{2}\)\({}^{+}\) first negative band: N\({}_{2}\)\({}^{+}\) (B\({}^{2}\Sigma_{u}-X^{2}\Sigma_{g}^{+}\)), and N\({}_{2}\) first positive band: N\({}_{2}\) (B\({}^{3}\Pi_{g}-A^{3}\Pi_{u}\))) were observed, as the main constituents of air are N\({}_{2}\) (78.08%) and O\({}_{2}\) (20.95%). Moreover, atomic nitrogen (N) at 747.55 nm and 868.55 nm were also determined. In contrast, only NO (A\({}^{2}\Sigma-X^{2}\Pi\)) was detected in the O\({}_{2}\)-related lines, which may be due to the extremely strong emission intensity of the excited nitrogen species. These peak lines were identified by comparing them with references [51; 52; 53; 54; 55]. Moreover, the optical emission spectrum of each channel during the plasma discharge at 1000 L/min exhibited an extremely similar emission intensity under the same ignition conditions, with a standard deviation of the emission intensities in the 10 layers of 646,  
The gas temperature of the air plasma was evaluated with respect to the gas flow rate. The fiber-optic temperature probe was placed at the fifth layer of the ten-layer DBD reactor, and the gas temperatures of the plasma-off were recorded when the temperature reached a plateau (approximately 30 min after gas injection), while those of the plasma-on were recorded 10 min after plasma ignition. Figure 7d shows the gas temperature of the air plasma generated in the ten-layer DBD reactor as a function of the gas flow rate, where the applied voltage was fixed at 22 kV, and the initial gas temperature before plasma-on (i.e., gas temperature with plasma-off) varied from 35.5 \({}^{\circ}\)C at 1000 L/min to 40.43 \({}^{\circ}\)C at 50 L/min. The increase in the gas temperature after plasma-on ranged from a maximum of 11.42 \({}^{\circ}\)C at 50 L/min to a minimum of 5.23 \({}^{\circ}\)C at 680 L/min. As can be observed, all the gas temperatures of the plasma generated in the ten-layer DBD reactor, irrespective of  
Figure 7: (**a**) Optical emission spectrum of the air plasma discharge at the fifth layer and (**b**) emission intensities at the 337.93 nm line in each flow channel of the ten-layer DBD reactor, ignited at 19 kV and 1000 L/min; (**c**) emission intensities at the 337.93 nm line of the air plasma discharge in the fifth channel at different gas flow rates; (**d**) gas temperature of the air plasma as a function of gas flow rate at 22 kV. the change in the gas flow rates, were maintained at a low temperature (42.41-51.85 \({}^{\circ}\)C), corroborating the generation of LTP. Moreover, the plasma gas temperature significantly dropped from 51.85 \({}^{\circ}\)C at 50 L/min to 44.12 \({}^{\circ}\)C at 520 L/min, and then gradually declined to 42.41 \({}^{\circ}\)C at 1000 L/min. This phenomenon can be attributed to the increased heating effect at lower flow rates, which is caused by the heated gate valve due to the collision of high-flow-rate wind induced on the primary side.