# Influence of Oxygen Impurity on Nitrogen Atmospheric-Pressure Plasma Jet
## 1 Introduction  
Atmospheric pressure plasma jets (APPIs) are inexpensive to produce because they do not require vacuum equipment [1; 2; 3]. Therefore, they are considered promising for plasma treatment applications in material surface treatment [4; 5; 6; 7] and biomedicine [8; 9; 10; 11]. The DC pulse nitrogen APPJ with a quartz tube discussed in this paper was proved to improve the power conversion efficiency of Perovskite solar cells better and within a shorter treatment time compared with DBD plasma and UV ozone treatment [6]. However, the amount of excited species on the treated surface cannot easily be evaluated through experiments because of the limitations of measuring techniques.  
Studies have simulated [12; 13] APPJ propagation in a transient state and used models to calculate the electronic and magnetic properties and chemical reactions. As a result, streamer and electric field propagation characteristics have been obtained. To improve mass transfer in the fluid flow downstream, combinations of laminar [14], large-eddy simulation (LES) [15], and k-\(\varepsilon\) and k-\(\omega\) turbulent models [16; 17] in commercial computational fluid dynamics (CFD) software or user-defined code [18; 19; 20] have been used.  
Some studies have experimentally investigated the impact of adding oxygen impurity to the inlet gas used for the APPJ. For example, [PERSON] and [PERSON] [21] added oxygen (0.25%, 0.50%, 1%, and 1.5%) to an argon (Ar) APPJ and found that the oxygen content reduced the plasma temperature as well as the length and intensity of the emission spectra at the nozzle outlet and in the downstream, while oxygen-related species (e.g., O, OH, and NO) increased. Similarly, [PERSON] and [PERSON] [22] mixed 0.1-1.2% O\({}_{2}\) in nitrogen (N\({}_{2}\)) microwave-induced plasma (MIP) and found that increasing oxygen content in the inlet mixture reduced the temperature, electric density, and N\({}_{2}\), and N\({}_{2}\) ion species and increasedthe NO and OH species. [PERSON] et al. (2013) utilized ambient air, nitrogen, a hydrogen-nitrogen mixture, and hydrogen as plasma inlet gas, and this research emphasized that the thermal effect of different inlet gas would affect the selective etching performance.  
[PERSON] et al. (2014) performed He dielectric barrier discharge (DBD) simulations and found that as the oxygen content in the inlet mixture was increased from 7.5% to 15%, and O atoms increased. [PERSON] (2015) performed He DBD simulations and found that as the oxygen content in the inlet mixture was increased from 0.003% to 0.1%, the electronic density of the plasma and the nitrogen second positive system (SPS) excitation decreased. Other researchers studied humid air as an inlet gas impurity (2016) and found that it increased OH in the downstream (2017; 2018; 2019; 2020).  
In this study, we simulated a nitrogen DC pulse APPJ with a quartz tube design (2016; 2016) by using an axisymmetric model that takes into account the fluid dynamics, heat transfer, mass diffusion, and species transfer equations. We used the steady-state assumption because we focused on the treated surface downstream. We added different amounts of oxygen impurities to the nitrogen gas source and investigated the heat and species in the downstream above the substrate.