## 4 Discussion  
Various types of NTAPP have been tried in skin therapy to utilize active species from atmospheric plasma [35; 36; 37]. The NTAPP used in this experiment was a jet-type with a weak gas flow rate, and it was adjusted until suitable for skin treatment. The discharge conditions were adjusted to ensure continuous and stable RONS generation in the air gas flow [38]. The prominent peaks in the OES in Figure 1c were the second positive system N\({}_{2}\) (C-B) and the first positive system N\({}_{2}\) (B-A) in the near UV region. The atomic oxygen emission line (\(O\big{(}2s^{2}2P^{2}\big{(}4S^{0}\big{)}3s\ \to 2s^{2}2P^{2}\big{(} ^{4}S^{0}\big{)}3p\big{)}\) was measured at 777 nm. The NO-\(\upbeta\) and NO-\(\upgamma\) systems were also weakly detected. This OES shows that nitrogen and oxygen molecules from air are supplied with enough electrons from the plasma to form reactive species [15; 39]. The distance to the sample and flow rate of air gas were adjusted for the adequate heat that the sample receives. Under the conditions of 1.5 lpm and 15 mm used in the skin model, the sample temperature was measured as 37.7 \({}^{\circ}\)C. This is a state below the standard value of 41 \({}^{\circ}\)C (applied part having contact with the patient) of IEC 60601-1 [40]. The leakage current was measured as 0.002 mA. In this condition, plasma can be treated without any electric shocks to the target object and without electroporation [41]. Therefore, the plasma treatment will be painless for the patients. In this condition, the distance from the plasma plume was about 7 mm or more to reach the sample. Therefore, it can be assumed that the reactive species' effect is the main effect in NTAPP skin treatment. Figure 7: Permeation characteristics of reactive species in real skin (**a**) Microscopic measurement result of pig skin (**b**) UV absorption spectra of NTAPP treated water during 10 min of NTAPP treatment and 30 min of post NTAPP treatment according to presence of agarose, agarose + SC layer, and tissue. All measurements were repeated three times. In our experiment, the overall concentration of RONS decreases because the amounts of RONS reaching the sample decreases as the plasma treatment distance increases (Figure 3). For each reactive species, the primary reaction of H\({}_{2}\)O\({}_{2}\) generation is the interaction between OH radical molecules. As the plasma treatment distance increases, the number of electrons colliding with the water surface decreases, resulting in a reduction of the OH radical generation rate. Also, the photolysis effect due to the decline in UV intensity is reduced under the contact distance between the water surface and the plume, and the OH radical concentration decreases. As a result, the concentration of short-lived species decreases as the plasma treatment distance increases are measured only in the interface region and cannot be reached with the bulk liquid [42]. In the case of long-lived active species, they are mainly produced and accumulated at the liquid interface and finally transferred to the liquid bulk region by diffusion [17; 18; 43]. H\({}_{2}\)O\({}_{2}\), NO\({}_{2}\)\({}^{-}\), and NO\({}_{3}\)\({}^{-}\) measured in this experiment, they are formed and destroyed by the reactions summarized in Table 1. H\({}_{2}\)O\({}_{2}\) is affected by Equations (1)-(5), NO\({}_{2}\)\({}^{-}\) is Equations (6)-(9), and NO\({}_{3}\)\({}^{-}\) is Equations (10)-(14). We used an agarose gel as a human tissue to study the barrier effect on RONS from plasma. As shown in Figure 4, the absorbance immediately increased during the plasma treatment time of 10 min in the absence of the agarose film. After the plasma was turned off, RONS was no longer generated, and the absorbance was maintained constant. In the case of plasma exposure through agarose, there is little change in absorbance during the plasma treatment time. After the plasma was turned-off, the absorbance showed a steady increase, and this tendency was maintained for a measurement time of 40 min.