## 1 Introduction  
Skin applications of non-thermal atmospheric pressure plasma (NTAPP) have been studied for the last decades, for both medical or cosmetic purposes. Positive effects of plasma have been reported for blood coagulation [1, 2], wound healing [3, 4], tissue regeneration [5], cancer treatment [6, 7], drug delivery [8, 9] and cosmetic purposes such as acne treatment and anti-aging [10, 11]. These positive effects are attributed to the various reactive oxygen and nitrogen species (RONS) occurring in the plasma. The actions of reactive oxygen species (ROS) and reactive nitrogen species (RNS) are distinguished in biological systems [12]. ROS generally activates redox responses in cells; meanwhile RNS induces nitrosylation or nitrosation, which initiate different cellular signaling pathways that are known to be related to cell differentiation or wound healing processes [7, 13]. The composition of RONS from NTAPP can be tailored by using different feeding gases, which may result in differential biological responses [14, 15]. The gaseous species that can be formed from air include both ROS such as OH radicals, O, H\({}_{2}\)O\({}_{2}\), O\({}_{3}\), H\({}_{2}\)O\({}_{2}\), HO\({}_{2}\), and RNS such as NO, NO\({}_{2}\), HNO\({}_{2}\), HNO\({}_{3}\), and NO\({}_{3}\)[16, 17, 18]. Previous studies show that OH, O, N, and H radicals with a short lifetime show a steadily decreasing trend as the distance from the nozzle increases [16, 17, 18]. On the other hand, the density of long-lived reactive species such as O\({}_{3}\), HNO\({}_{2}\), HNO\({}_{3}\), and H\({}_{2}\)O\({}_{2}\) increases with the distance. When these RONS reach an aqueous interface, the aqueous density of H\({}_{2}\)O\({}_{2}\), HNO\({}_{2}\)/NO\({}_{2}\)\({}^{-}\) and NO\({}_{3}\)\({}^{-}\) increased with time, but the densities of O\({}_{3}\), HO\({}_{2}\)/O\({}_{2}\)\({}^{-}\) and ONOOH/ONOO\({}^{-}\) quickly decreased with depth in silico [18]. These simulations are hard to prove experimentally due to the short lifetimes and short pathlengths of highly reactive RONS in solution. Long-lived species have been measured with colorimetric assays with known indicators [19, 20, 21]. Recently, UV absorption spectroscopy (UVAS) was introduced as a real-time measurement method for plasma-induced RONS [22, 23, 24]. The analysis of absorption spectrum in UV range can distinguish between H\({}_{2}\)O\({}_{2}\), NO\({}_{2}\)\({}^{-}\), NO\({}_{3}\)\({}^{-}\), and O\({}_{2}\) in water. For proper application of NTAPP on the skin, it is essential to understand how deep the generated active species diffuse into the tissue. In tissues, RONS reaching on the surface should penetrate gel-like structures containing cells. Their structural complexity and optical turbidity make it harder to detect RONS inside tissues. As a simplified model, homogeneous gels such as agarose or gelatin have been utilized [22; 25; 26; 27; 28]. Some studies have used mixtures of indicators and gels to visualize the position of each RONS in model tissues [25; 26; 27]. They showed that the diffusion of each RONS increased with time and the depth of diffusion of long-lived species reached about 2\(\sim\)3 mm. Some studies placed gel films on the water to analyze the penetration of RONS through gels [22; 27; 28]. [PERSON] et al. [27] showed that the permeabilities of H\({}_{2}\)O\({}_{2}\), NO\({}_{2}\)\({}^{-}\), NO\({}_{3}\)\({}^{-}\), and O\({}_{3}\) are different through gelatin gels by colorimetric measurements. [PERSON] et al. [22; 28] used the UVAS method and showed that long-lived RONS can penetrate 2\(\sim\)3 mm agarose gel, and there are accumulations of RONS within the agarose. However, considering the complex structure of the skin, a homogeneous gel is not an accurate skin model. [PERSON] et al. [19] used colorimetric assays and showed that the penetration of some kinds of the RONS could be enhanced significantly by tape stripping stratum corneum (SC) from mice skin.