## 3 Results
### Optimized Condition of Ozone Generation  
Figure 4 shows the time-dependent concentration of ozone and the outlet gas temperature with respect to the applied flow rate. Ozone was generated from a compact air-cooled coaxial DBD according to the applied flow rate. The saturated concentrations of ozone were 235.1, 474.1, 500.5, 469.2, and 460.3 ppm at applied flow rates of 10, 15, 20, 25, and 30 L min\({}^{-1}\), respectively, and the power consumption was 60 W. Figure 4b shows the outlet gas temperature according to the applied flow rates. Twenty minutes after the discharge of plasma, the gas temperatures were 125.4, 107.5, 98.4, 86.3, and 79.9 \({}^{\circ}\)C at the applied flow rates of 10, 15, 20, 25, and 30 L min\({}^{-1}\), respectively, and the consumption power was 60 W. The ozone concentration was the highest at a flow rate of 20 L min\({}^{-1}\). The gas velocity was low, and the gas temperature increased rapidly at a flow rate of 10 L min\({}^{-1}\). As shown in Figure 4b, when the flow rate of the discharge gas was low, the gas temperature increased owing to the Joule heating effect of the electrodes. Ozone decomposes rapidly at high  
Figure 3: The optical emission spectrum of the compact air-cooled coaxial DBD in the range of 200–900 nm.  
temperatures. The half-life of ozone at various temperatures of \(-50\), \(-35\), \(-25\), \(20\), \(120\), and \(250\)\({}^{\circ}\)C was 3 months, 18 days, 8 days, 3 days, 1.5 h, and 1.5 s, respectively. As shown in reaction (3), ozone will always be formed when there is a process of oxygen dissociation. At high temperatures, when the concentration of atomic oxygen is high, the equilibrium of reaction (4) moves to the left as the ozone dissociation proceeds and the ozone concentration is low. However, the equilibrium shifts to the right at low temperatures [25]. In this system, the gas temperature at higher flow rates is lower because of the self-cooling effect. However, at high discharge gas flow rates, the concentration of ozone decreases because of the short residence time of the discharge gas in the electrode. In general, the key parameters for stable plasma generation and high ozone production include the applied frequency and voltage, kinds of gases, electrode configuration, gas flow rate, applied power, electrode cooling, etc. Also, long-term operation using only ambient air is very important for our application. So, we designed the coaxial DBD plasma with an air-cooled internal electrode for stable and long-term operation. According to our results, eventually, the compact air-cooled coaxial DBD was optimized to generate ozone at an applied power of 60 W and a flow rate of 20 L min\({}^{-1}\) through repeated experiments. It was confirmed that a decrease in the gas temperature and residence time of the discharge gas in the electrode are important for high-concentration ozone production. According to the results, the developed compact air-cooled coaxial DBD was optimized to generate ozone at an applied power of 60 W and a flow rate of 20 L min\({}^{-1}\). Consequently, the nonthermal plasma processes of the compact air-cooled coaxial DBD generated high concentrations of ozone at low temperatures.  
Figure 4: Time-dependent (**a**) concentration of ozone according to the applied flow rates measured by an ozone monitor and (**b**) outlet gas temperature measured by a thermometer. We carried out each measurement three times per given condition.