## 4 Discussion  
As shown in Figure 2, the reactive species produced by the plasma were affected by varying the gas species and temperature. The OH radicals are produced largely from the nitrogen plasma. In the nitrogen plasma generated in the atmosphere, spectroscopic measurements have indicated the existence of nitrogen atoms [13]. The metastable atomic states of nitrogen \(2^{2}\)D\({}_{5/2}\), argon \(4^{3}\)P\({}_{2}\), and oxygen \(2^{1}\)D\({}_{2}\) have lifetimes of 6.1 \(\times\)\(10^{4}\) s [23], 56 s [24], and 1.0 \(\times\)\(10^{2}\) s [23], respectively. It is thought that metastable nitrogen, which has a longer lifetime than other metastable atoms, remained active and contacted water or water droplets in the air, producing OH radicals at the gas\(-\)liquid interface [25]. In a previous study, we reported that more OH radicals were produced from nitrogen plasmas than from any other gas species [15]. Singlet oxygen was produced more in the higher-temperature carbon dioxide plasma than in the oxygen plasma. This may be because, in the oxygen plasma, the atomic oxygen that produces singlet oxygen reacts with the ambient triplet oxygen, which is abundant, and quickly turns into ozone [26]. In fact, the amount of ozone produced by the oxygen plasma was higher than that produced by the carbon dioxide plasma. In the oxygen and carbon dioxide plasmas, the production of OH radicals, singlet oxygen, and hydrogen peroxide tended to increase as the gas temperature increased. This is similar to the report by [PERSON] et al. on the increase in the production of the same reactive species when the gas temperature of the helium\(-\)oxygen mixture is increased [12]. In other words, the above-mentioned reactive species are considered to increase with increasing temperature in gas species that contain oxygen elements. There was a significant difference in the bactericidal effect of the nitrogen and carbon dioxide plasmas, as shown in Figure 3, depending on the gas species. The nitrogen plasma showed only a weak bactericidal effect of less than 1-log reduction after 60 s of treatment. These results were different from our previous results, which confirmed the high bactericidal effect of nitrogen gas [14]. In the formation of reactive species in the nitrogen plasma, mixing with air is important because the gas does not contain oxygen. Therefore, it is considered that the difference in treatment conditions, such as gas flow rate and treatment distance, had a considerable influence. In contrast, the carbon dioxide plasma at 90 \({}^{\circ}\)  
Figure 4: Correlation between bactericidal effect and reactive species in carbon dioxide plasma. showed a high bactericidal effect of 6-log reduction after 10 s of treatment. In the treatment with the carbon dioxide gas at 90 \({}^{\circ}\)C, the number of surviving bacteria did not decrease by 1-log even after 60 s of treatment. Therefore, these results indicate that the improvement in the bactericidal effect is not due to the gas temperature, but due to the introduction of reactive species generated by the plasma. In the carbon dioxide plasma, the production of reactive species, such as singlet oxygen and OH radicals, changed significantly as the plasma gas temperature increased, suggesting that these reactive species contribute to the bactericidal effect. As shown in Table 2, the bactericidal effect of the carbon dioxide plasma exhibited a strong negative correlation with the ozone and a strong positive correlation with the OH radicals and singlet oxygen. Ozone is also used for disinfection, and [PERSON] et al. reported 90% disinfection of _Escherichia coli_ by 0.94 mg/L (19.6 \(\upmu\)M) of ozone in 0.41 min [27]. In our study, the highest ozone concentration in the carbon dioxide plasma was 2.0 \(\upmu\)M, which is an insufficient concentration for disinfection. If the OH radical is assumed to be the reactive species contributing to its bactericidal effect, the observation is inconsistent with the fact that no bactericidal effect was observed in the nitrogen plasma.