## 2 NTAPP Sources and Plasma Chemistry  
### Non-Thermal Plasma Sources  
Non-thermal plasmas have been achieved under normal atmospheric pressure by avoiding gas heating with the help of dielectric barrier discharge (DBD) [54; 55]. DBD has been performed with the help of a strong electric field [56; 57; 58]. The source structure for a plasma discharge system consists of two electrodes, where either or both are concealed by a dielectric layer. Figure 2 depicts various DBD plasma sources. DBD has an electron density in the range of 10\({}^{8}\)-10\({}^{15}\) cm\({}^{-3}\), an electron temperature in the range of 1-10 eV, a gas temperature of approximately room temperature, and a plasma dissipation energy of approximately 1-10 W [29]. When the breakdown voltage is achieved, luminous DBD is produced at atmospheric pressure when neutral gas is ionized after the collision of electrons with the working gas [29]. A distance between electrodes, flow rates, and nature of working gas (mainly that of noble gases such as He or Ar) determines the magnitude of applied voltage. NTAAP can be supplied by direct, pulsed direct, or high frequency alternating currents (radiofrequency and microwave). Interaction between the NTAAP and air can be minimized by using purging gas or shielding. The main feature of DBD is the existence of dielectric material along with the electrode. Gas is ionized above the dielectric surface in accordance with the electric field. The working frequency range of DBD is generally 50-500 kHz, and the voltage is approximately 1-30 kV [57; 58]. DBD plasma or plasma jets are generally selected for water purification [47], bio-sterilization [48], decontamination [48], and biological applications. Figure 2a-c provide schematics of surface DBD, specifically, a floating electrode DBD plasma source, facing electrode DBD plasma source, and surface DBD plasma source, respectively. It consists of two parallel plates in a planar or facing pattern. The electrode separation in a DBD is made to be in the range of micrometers to a few centimeters. Plasma is formed in a non-uniform electric field. In DBD, the electrode gap can be changed according to the application requirements. DBD plasma is applicable for large area surface treatment. Figure 2d-f illustrate the use of a volumetric luminous plasma jet formed by a dielectric tube and one or two electrodes. The plasma is produced inside the dielectric tube and then appears in the ambient air along with the gas flow. In all types of sources,  
Figure 2: Schematic diagrams of DBD plasma sources: (**a**) floating electrode DBD plasma source, (**b**) facing electrode DBD plasma source, (**c**) surface DBD plasma source, (**d**) floating electrode plasma jet, (**e**) direct plasma jet, and (**f**) indirect plasma jet treating a sample. plasma is produced within the high voltage and ground electrodes, which are separated by a dielectric tube. Plasma jets are improved designs of DBD plasma sources whose plasma plumes elongate to the order of a few centimeters. The plasma plume ejected from a discharge tube into an open space looks like a continuous luminous plume to human eyes; however, it propagates discontinuously as a plasma bullet [59]. Reactive species, electrons, and ions generated by the plasma discharge are delivered to the target via the dielectric tube and ambient air. The differences between plasma jets mainly originate from variations of the electrode, electrode material, dielectric material, working gas, frequency, applied voltage, and electrode separation. In this regard, [PERSON] et al. [60] and [PERSON] et al. [61] described the control of RONS production in liquid by NTAPP with different shielding gases. [PERSON] et al. [62; 63] discussed the variation of RONS with the gap distance, electrode separation, and plasma propagation length. [PERSON] et al. [64] and [PERSON] et al. [31] studied the effects of working gas on the concentration of RONS. Similarly, [PERSON] et al. [65] and [PERSON] et al.