## 3 Results and Discussion
### Plasma-Treatment Stabilization
The treated liquid was sampled at various times after discharge initiation, and samples were examined using DUV absorption spectroscopy to observe the effects of treatment duration on absorption in the DUV wavelength region that is related to RONS concentration. The measured absorption spectra and absorption peak intensity at 200 nm are presented in Figure 4.  
The absorption of the treated liquid increased with an increase of plasma-treatment duration, indicating the increase of RONS concentrations. However, after 4 min, the spectrum shape did not change (Figure 4a), and the absorption spectrum remained at the same level of intensity (Figure 4b), indicating the same level of RONS concentrations in the treated liquids. Figure 4: (**a**) DUV absorption spectra and (**b**) intensity of absorption peak at 200 nm of flowing DI water treated with AAGD, sampled at 1, 3, 4, 5, and 10 m after treatment initiation. Figure 3: Fitting and deconvolution of deep ultraviolet (DUV) absorption spectrum of DI water treated with ambient-air glow discharge (AAGD). To avoid absorption-spectrum saturation, PAW samples were diluted by 10-fold, and minor variations in the mean intensity and standard deviation of absorption spectra of PAW after 4 min originate from dilution inaccuracy caused by the used pipette (PIPETMAN Classic P1000, Gilson, Middletown, WI, USA) and human error. For fast measurement, 2.7 mL DI water was prepared in a DUV absorption spectroscopy measurement cell, and 0.3 mL of treated water was suspended to the cell immediately after treatment, resulting in 3 mL of the 10-fold-diluted PAW required for measurements. Pipette accuracy was \(\pm\) 8 \(\mu\)L, resulting in about 2.5% possible error in suspending 0.3 mL of the liquid, which is represented in Figure 4b. Considering error bars reported in the Figure 4b, it could be concluded that absorbance was stable after 4 min of irradiation within experiment uncertainty. The stabilization of DUV absorption spectra after 4 min indicates that RONS concentrations reached the saturation level, leading to liquid-conductivity and plasma-discharge-parameter stabilization. Therefore, it was necessary to wait for more than 4 min after the initiation of discharge to achieve reproducible RONS concentration. Absolute values of RONS concentrations in the flowing liquid after 4 min of treatment measured by high-performance liquid chromatography and ion chromatography were 282 \(\mu\)M for H\({}_{2}\)O\({}_{2}\), 4.4 \(\mu\)M for NO\({}_{2}\)\({}^{-}\), and 520 \(\mu\)M for NO\({}_{3}\)\({}^{-}\) (same as reported elsewhere for identical conditions) [19]. Obtained concentrations of H\({}_{2}\)O\({}_{2}\) and NO\({}_{3}\)\({}^{-}\) were at least 1 order of magnitude higher than those reported for conventional He or Ar plasma jets or DBD systems (which are typically in order of \(\mu\)M, considering the volume of produced liquid and duration of the treatment) [32; 33; 34; 35]. However, NO\({}_{2}\)\({}^{-}\) concentration produced by AAGD was negligible compared to that of H\({}_{2}\)O\({}_{2}\) and NO\({}_{3}\)\({}^{-}\), which correlates well with the deconvolution of DUV absorption spectra in Figure 3. Interestingly, the ratio of RONS concentrations in the present work for AAGD was similar to that observed for spark discharge (mostly H\({}_{2}\)O\({}_{2}\) and NO\({}_{3}\)\({}^{-}\)) and the opposite for glow discharge reported elsewhere (mostly NO\({}_{2}\)\({}^{-}\)), which could be related to differences in discharge conditions [57]. Obtained concentration of NO\({}_{2}\)\({}^{-}\) was lower compared to conventional systems employing Ar or He, which could be a limitation for the use of AAGD in certain applications requiring large NO\({}_{2}\)\({}^{-}\) concentrations.