## 1 Introduction  
In an interaction between non-thermal atmospheric pressure plasma (NTAPP) and water, a process appears, such as plasma-activated water (PAW), that provides wide applications [1, 2]. PAW, as an adaptable medium, is being paid much attention to promising applications in biomedical [3], food, and agriculture [4]. Following the generation of enormous chemically active species in the gas phase by NTAPP in the interaction with water, PAW initiates chemical reaction processes inside the water leading huge amount of chemically active species. Since PAW contains mainly reactive oxygen and nitrogen species inside the water, it can be considered an alternative method for disinfection [5]. NTAPPs have been emerging with different configurations in and contact with water [6] for treatment. In the case of discharge formation inside the water, the conductivity of water strongly affects the discharge conditions. The discharge volume and energy efficiency significantly change with water conductivity [7] that causes difficulties to ignite discharge as well as discharge instabilities inside the water. To overcome this problem, the discharge is generated in a gas phase over a water surface [8]. In this way, dielectric barrier discharge (DBD) is developed as an NTAPP over the water surface for PAW to control and stabilize the discharge. NTAPPs are sources of producing chemically reactive species that act as oxidizing agents. The reactive species are applied for PAW due to the advanced oxidation process [9]. Hydrogen peroxide, as a long-lived species, along with the other produced species inside the PAW, contributes as an indirect plasma treatment for biological and medical applications [10]; thus, it is necessary to characterize the active species generated inside the PAW. Hydrogen peroxide, as one of the most stable species generated inside the PAW, plays a significant role in the antimicrobial properties of PAW [11,12]. Hydrogen peroxide is also able to improve the taste of water and unpleasant smell [13]. There are enormous articles in which the pH and oxidation-reduction potential (ORP) levels were investigated as well as the physiochemistry of the treated/activated water and applications. The findings of these investigations have been performed with different plasma sources and experimental conditions. It is known that the plasma source with various electrode configurations [14], voltage waveform [15], frequency [16] affect discharge conditions and produced active species. Thus, it is pretty difficult or impossible to compare the results. For instance, argon plasma using a high-voltage needle with positive and negative pulsed discharges was considered on produced active species in the gas phase and hydrogen peroxide concentration in the liquid [17]. [PERSON] et al. [18] utilized a needle electrode configuration, but the effect of oxygen, nitrogen, and air plasmas was considered on nitrite and nitrate concentrations with AC discharge. [PERSON] et al. [19] reported a volume DBD reactor with sinusoidal and nanosecond discharge in the ambient air. Argon/oxygen plasmas were reported using a plasma jet [4] and microjet [20]. [PERSON] et al. [1] classified plasma sources with various gas species conducted for PAW. In this work, we are motivated to investigate the effect of the gas species on the pH and chemical strength (ORP) levels of the water as well as the concentration of nitrite, nitrate, and hydrogen peroxide inside the treated/activated water with four typical gas species of air, nitrogen, oxygen, and argon under the same experimental conditions. This study aimed to characterize generated active species in the gas phase and inside the water as well as activated water properties by changing the gas feeding. In this study, the NTAPPs were generated using DBD over the water surface, where primary active species are generated by using different gas feedings as mentioned above. To have a uniform and broad plasma treatment, a two-parallel plate configuration was employed.