# The Role of HNO2 in the Generation of Plasma-Activated Water by Air Transient Spark Discharge
## 3 Results and Discussion
### Analysis of Plasma-Activated Water
Figure 13 shows an approximately linear dependence of NO\({}_{2}\)\({}^{-}\)(aq) on gaseous NO\({}_{2}\) when applying ES in humid air treated by TS. This most likely indicates a linear dependence of NO\({}_{2}\)\({}^{-}\)(aq) on the gaseous HNO\({}_{2}\) concentration, since HNO\({}_{2}\) concentration is proportional to the NO\({}_{2}\) concentration (Figure 12). If we assume that the amount of NO\({}_{2}\)\({}^{-}\)(aq) in the liquid phase is proportional to the depletion of HNO\({}_{2}\) from the gas phase, we can estimate the expected NO\({}_{2}\)\({}^{-}\)(aq) concentration in the water. The highest HNO\({}_{2}\) decrease observed was approximately 50 \(\pm\) 10 ppm. Taking into account gas and water flow rates, the corresponding estimated NO\({}_{2}\)\({}^{-}\)(aq) concentration in the liquid phase should be 4.4 \(\pm\) 0.9 mM. The measured NO\({}_{2}\)\({}^{-}\)(aq) concentration was somewhat lower: 2.7 \(\pm\) 0.2 mM. This difference could be explained by the instability of NO\({}_{2}\)\({}^{-}\)(aq) at acidic conditions, because the measured pH = 2.5-2.8 in the collected water. The disproportionation reaction:  
\[3\text{NO}_{2}\text{${}^{-}$}\text{(aq)}+3\text{H}\text{${}^{+}$}\to 2\text{ NO}_{\text{(aq)}}+\text{NO}_{3}\text{${}^{-}$}\text{(aq)}+\text{H}_{3}\text{O} \text{${}^{+}$} \tag{17}\]  
leads to exponential-like NO\({}_{2}\)\({}^{-}\) decrease in time. In PAW generated by TS, the observed characteristic decay time was approximately 30-40 min [58]. In our experiments in 2 SS, the collected water sample was mixed with Griess reagent within 5 min after the end of the experiment. During this time, the NO\({}_{2}\)\({}^{-}\) concentration could decrease approximately by about 20% from the concentration reached at the moment when the experiment was stopped. Moreover, conversion of NO\({}_{2}\)\({}^{-}\)(aq) to NO\({}_{3}\)\({}^{-}\)(aq) also occurs during the time of experiment (4 min) inside the vessel where the water accumulates. Taking into account the NO\({}_{2}\)\({}^{-}\)(aq) concentration decrease due to Reaction (17) and the experimental uncertainty, the agreement between the measured NO\({}_{2}\)\({}^{-}\)(aq) concentration and the estimated one from the HNO\({}_{2}\) depletion from the air is satisfactory. In the previous paragraphs we have discussed and compared differences of NO, NO\({}_{2}\) and HNO\({}_{2}\) solubility, and we based our reasoning mainly on differences of Henry's law coefficients of these three species. As mentioned in the Introduction, the Henry's law coefficient is valid for systems in a thermodynamic equilibrium and in plasma-water interactions, the solvation process may be influenced by many other phenomena. Here we must emphasize that in 2 SS, the plasma and water did not interact directly. Direct interaction of plasma with water microdroplets occurred only in 1 SS. However, the concentration of NO\({}_{2}\)\({}^{-}\)(aq) in PAW generated in 1 SS with a direct contact of TS with microdroplets is not higher than NO\({}_{2}\)\({}^{-}\)(aq) concentration achieved in 2 SS experiments in humid air with TS and ES separated into two reactors. It shows the same trend and similar values (Figure 13, cyan vs. blue points). This result justifies the reasons to compare solubility of species with long lifetime based on their Henry's law coefficient. The major difference between PAW generated by TS with simultaneous ES (ISS), and water prepared in 2 SS by spraying humid air treated by TS in the first reactor, is the concentration of produced H\({}_{2}\)O\({}_{\text{2(aq)}}\). In the latter case the H\({}_{2}\)O\({}_{\text{2(aq)}}\) concentration was only around 20 \(\upmu\)M, while the concentration of H\({}_{2}\)O\({}_{\text{2(aq)}}\) in PAW generated by TS with ES was higher, 60-80 \(\upmu\)M. This result can be interpreted so that H\({}_{2}\)O\({}_{\text{2(aq)}}\) is created mainly by solvation of OH radicals with short lifetime and therefore their concentration in the second reactor of 2 SS is negligible.