# The Role of HNO2 in the Generation of Plasma-Activated Water by Air Transient Spark Discharge
## 3 Results and Discussion
### Solvation of Gaseous RONS to ES Microdroplets  
The solubility of gases depends on their Henry's law coefficient. NO, NO\({}_{2}\) and HNO\({}_{2}\) generated by TS differ significantly from the solubility point of view. The solubility of NO is the lowest (with Henry's law coefficient \(k_{H}\) = 0.0018 mol kg\({}^{-1}\) atm\({}^{-1}\) at 298.15 K [44]). It is easy to show that in an experiment with gas flow 1.1 L/min and water flow rate 100 uL/min, even if we achieve a saturated NO solution, the decrease of the gas phase concentration of NO is not detectable as it is much smaller than the accuracy of our analytic techniques. Thus, the decrease of NO concentration generated by TS in dry air with ES compared to the results without ES (Figure 6), cannot be explained by the solvation of NO into water. We assume that there are other processes responsible for this NO decrease. We must consider instabilities of TS caused by the water flow supplied for the ES, as shown in Figure 5b with long time scale voltage waveforms. Taking into account the appearance of short periods without spark pulses and energy consumed on evaporation of the sprayed microdroplets, we assume that water sprayed into the discharge has a cooling effect. This could explain a lower efficiency of TS to generate NO at higher input energy densities (Figure 6). The solubility of NO\({}_{2}\) is slightly higher than the solubility of NO, almost by about one order of magnitude. However, the decrease of NO\({}_{2}\) in the experiment with ES compared to the dry air (Figure 7) also cannot be dominantly due to the NO\({}_{2}\) solvation to liquid water. Moreover, there was even less NO\({}_{2}\) in the experiment in humid air than in dry air or air with ES. We thus suppose that the observed changes of NO\({}_{2}\) concentration are rather related to the shift in NO\({}_{2}\)/HNO\({}_{2}\) formation chemistry due to the presence of H\({}_{2}\)O (reactions (11)-(14)). The solubility of HNO\({}_{2}\) is much higher (by three orders of magnitude) than the solubility of NO\({}_{2}\). Thus, it is possible that the measured gaseous HNO\({}_{2}\) concentration is influenced by the solvation effect. However, it is questionable since the sum of NO\({}_{2}\) and HNO\({}_{2}\) (H\({}_{y}\)NO\({}_{2}\)) is the same in all experiments (Figure 10). It is therefore possible that the decrease of HNO\({}_{2}\) in the experiment in dry air with ES compared to the experiment in humid air is caused by the change in NO\({}_{2}\)/HNO\({}_{2}\) chemistry due to differences in the humidity. In the single reactor experiments in 1 SS, when comparing the results with and without ES, it is impossible to separate changes of NO\({}_{2}\) and HNO\({}_{2}\) concentrations due to solvation to water, changes in chemical pathways of their formation and decay, cooling effect of water, or discharge instabilities. In order to assess the role of individual RONS in PAW generation, additional experiments were performed in 2 SS with two reactors. The synthetic air (dry or humidified) was treated by TS in the first reactor. This treated air containing HyNOx was lead as the inlet gas into the second reactor with ES but without TS. We measured changes of the gas composition and aqueous NO\({}_{2}\)\({}^{-}\)(aq) and H\({}_{2}\)O\({}_{2}\)(aq) in the accumulated water (see Section 3.3). Figure 11 shows the comparison of NO concentration generated by TS in dry and humid air in 2 SS with and without ES treatment in the second reactor (water flow rate 500 uL/min). These results confirm a low solubility of NO, because its concentration in gas is within the experimental uncertainty not influenced by ES, both for dry and humid air. The solubility of NO\({}_{2}\) is almost by one order of magnitude higher than the solubility of NO. There is probably some decrease of NO\({}_{2}\) due to post-discharge ES treatment, especially at higher energy densities, i.e., when the NO\({}_{2}\) concentration is higher (Figure 12).