## 3 Results and Discussion
### Plasma-Treatment Stabilization  
Reproducibility is one of the most crucial factors for the practical use of treatment with AAGD. A major factor that could have effects on the treatment process in ambient air is humidity, which could vary in a wide range with location and weather-condition changes. Therefore, it is necessary to verify the effect of humidity on experiment reproducibility before analysis of any experiment data. The expected effect of humidity variation is a change in RONS concentrations delivered to the treated liquid (e.g., density change of OH radicals produced in the gas phase from water molecules), which could have a significant effect on the bactericidal activity of treated liquid and plant-growth promotion. The other possible source of problems with reproducibility is discharge initiation and the change of RONS concentrations in the treated flowing liquid during the initial state. Most common long-living RONS produced in liquid exposed to ambient air during plasma irradiation are H\({}_{2}\)O\({}_{2}\), NO\({}_{2}\)\({}^{-}\), NO\({}_{3}\)\({}^{-}\), and dissolved oxygen O\({}_{2\mathrm{aq}}\). All of these species have strong absorption in the deep ultraviolet and absorption spectrum of the liquid at a wavelength range between 190 and 400 nm, and could provide information about the concentrations of each species, as was reported elsewhere [32; 33; 45]. An additional advantage of DUV absorption spectroscopy is the short time required for measurement (below 5 min), and the possibility of sample analysis without special preparation (e.g., filtering in the case of HPLC). DUV absorption spectroscopy allows the sample to be analyzed almost immediately after treatment, avoiding the presence of a large number of chemical reactions in the sample before and during measurements. However, in the case of L-Phe, liquid components could be decomposed by the plasma and result in the production of a large number of new components that could interfere with the RONS absorption spectrum in the deep ultraviolet (DUV) range. To avoid the presence of additional absorption peaks in the deep ultraviolet, DI water was treated with ambient-air glow discharge at a controlled humidity to evaluate the effects of humidity and discharge stability on RONS concentrations. An example of the measured spectra of DI water after plasma treatment, fitting the absorption spectrum, and deconvolution of the fitting on RONS absorption peaks is presented in Figure 3.  
RONS concentrations are proportional to absorption-peak intensity in deconvolution according to the Beer-Lambert law (Equation (2)):  
\[\mathrm{Abs}_{x}(\lambda)=\varepsilon(\lambda)\times 1\times c_{x} \tag{2}\]  
where \(Abs_{x}(\lambda)\) is absorbance, \(\varepsilon(\lambda)\) is the molar absorption coefficient, \(l\) is the optical-path length, and \(c_{x}\) is the concentration of the molecule \(X\)[34]. The small error in fitting (within 1.5%) observed in Figure 3 is related to the presence of other species in the treated water that had absorbance in the DUV wavelength region, but were not accounted for in the fitting. Mostly H\({}_{2}\)O\({}_{2}\) and NO\({}_{3}\)\({}^{-}\) absorption peaks were present in deconvolution when the NO\({}_{2}\)\({}^{-}\) peak was negligible. After initiation of the discharge, some instability was observed for several seconds (up to 20 s depending on conductivity) in the case of a 5 mL sample of DI water without flow [19]. However, in the case of flowing liquid, it could take longer to achieve RONS concentration saturation in the liquid that passes through the area of plasma treatment, which means that liquid conductivity increases with an increase of RONS concentration until its saturation point, affecting plasma-discharge parameters. To find the treatment duration required for stable RONS concentration in liquid passing through the plasma-treatment cell, the flowing DI water was treated with AAGD operated at a standard condition.