## 2 NTAPP Sources and Plasma Chemistry
### Reactive Species Formed in NTAPP Discharge  
As mentioned above, NTAPP generates various reactive oxygen species (ROS), such as OH radicals, O2\({}^{-}\), H2O, O, O3, 1O2, and reactive N species (RNS), including NO, NO2, NO2, N2O5, and atomic N [42]. Some of them are short-lived, such as OH radicals, O, NO, N, and 1O2[33]. The RONS are formed due to the energy associated with the collisions between accelerating electron and neutrals and are referred as primary reactive species. Electrons (e\({}^{-}\)), ionized neutrals and gas (M\({}^{+}\)), excited neutrals and gas (M\({}^{+}\)), N, O, atomic H, NO, and O2*- are formed in the gas phase immediately after the collision [34; 65]. These species are called primary reactive species [67], and their intensities within the plasma region are very high. Primary reactive species have very short lifetimes; for example, the lifetimes of OH radicals, NO, and O2*- are -2.7 us -1.2 us, 1.4 us, and -1.3 us, respectively [68]. Some of these reactive species immediately undergo radiative decay, and others are combined with other reactive species, neutrals, and water molecules. The primary reactive species are transformed into other reactive species such as H2O2, NO2, NO3, and O3[30] in the ambient environment to form secondary reactive species [67]. The RONS produced in the gas phase reach the liquid phase (or another target) and are dissolved, forming tertiary reactive species [67]. O3, H2O2, NO3\({}^{-}\), and NO2\({}^{-}\) are long-lived reactive species because they last for a few milliseconds to several days [69]. H2O2, NO2, and NO3 are soluble in water, and NO2 and NO3 are immediately changed into NO2\({}^{-}\) and NO3\({}^{-}\), respectively [34; 70]. When these reactive species dissolve in liquid, the pH of the target liquid decreases by up to 2 depending upon the plasma source, working gas, power source, treatment time, and sample volume [70]. However, the chemistry of ROS and RNS formation in a target differs depending on whether the target is dry or aqueous [70]. Figure 3 provides a schematic diagram of RONS formation within the discharge region, gas region, plasma/target interface, and inside the target. As mentioned above, for purification, bio-sterilization, and decontamination of water, OH radicals, H2O2, and O3 play vital roles. These reactive species are formed in the plasma and plasma-liquid interface through numerous possible pathways. The main reaction pathways are explained in this section. When plasma is in contact with water, OH radicals are produced via chemical and physical processes. They can be in either aqueous or gaseous form. OH radicals can be formed by the dissociation of water via electron collisions, UV photolysis, and collisions with metastable or excited particles. The pathways of OH radicals consumption are even more complicated in discharges of high energy density due to the complexity of water dissociation and fast aqueous reactions [68] via Equations (1)-(6):  
\[\mathrm{e}^{-}+\mathrm{H}_{2}\mathrm{O}^{+}\rightarrow\mathrm{H}+\mathrm{OH} \left(1\;\mathrm{eV}\right) \tag{1}\]  
\[\mathrm{e}^{-}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{H}+\mathrm{OH}^{+}+ \mathrm{e}^{-}\left(\mathrm{direct\ dissociation,\,4.4\;eV}\right) \tag{2}\]In plasma-liquid interface, the penning ionization is also possible, as in Equations (3) and (4) [33]:  
\[\mathrm{e}+\mathrm{O}_{2}\rightarrow\mathrm{O}(^{1}\mathrm{D})+\mathrm{O}(^{3} \mathrm{P}) \tag{3}\]  
\[\mathrm{O}(^{1}\mathrm{D})+\mathrm{H}_{2}\mathrm{O}\to 2\mathrm{OH}\ (3.1\ \mathrm{eV}) \tag{4}\]  
Form UV photolysis of water, it can be achieved from Equation (5) and (6) [71]:  
\[\mathrm{UV}+\mathrm{H}_{2}\mathrm{O}\rightarrow\mathrm{H}_{2}\mathrm{O} \tag{5}\]  
\[\mathrm{UV}+\mathrm{H}_{2}\mathrm{O}^{\ast}\rightarrow\mathrm{OH}+\mathrm{H} \tag{6}\]  
\(\mathrm{H}_{2}\mathrm{O}_{2}\) is an important biologically reactive molecule involved in bacterium inactivation and cell oxidation stress [72]. Chemically, \(\mathrm{H}_{2}\mathrm{O}_{2}\) is a ROS with 1.8 V oxidation potential.