# 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  
We measured NO2(aq) and H2O2(aq) in plasma-activated water collected during 1 SS experiments with TS combined with ES, as well as in water collected during 2 SS experiments, with TS and ES separated into two reactors. As for control, we also performed experiments in 1 SS with ES, but without TS. The used gas was either synthetic air (dry or humidified), or NO and NO2 from calibration pressure tanks, mixed with N2 or synthetic air, respectively. In the control experiments with ES, the applied voltage (6 kV) was not sufficient for TS generation, but it was above the onset voltage of the corona discharge. Due to this, there was probably some formation of reactive species inside the ES reactor, and we detected a low concentration of H2O2(aq), -20 mM. This weak corona discharge in air, however, produced no detectable amount of nitrogen oxides and no NO2(aq) was detected in water. Relatively small amount of NO2(aq) (-80 mM, Figure 13, red point) was detected in control experiment in 1 SS with ES and without TS, using gas from the pressure tank containing -1400 ppm NO in N2. This proves a low solubility of NO. The formation of NO2(aq) from the dissolved NO(aq) proceeds via the following reaction:  
\[\text{NO}_{\text{(aq)}}+\text{NO}_{\text{2(aq)}}+\text{H}_{\text{2}}\text{O} \to 2\text{NO}_{\text{2}}^{-}{}_{\text{(aq)}}+2\text{H}^{+} \tag{15}\]  
Thus, the dissolved NO2(aq) is also needed to produce NO2(aq). In the gas phase only around 25 ppm NO2 was detected and this limited the NO2(aq) production. We can conclude that without NO2, even very high concentration of NO has almost a negligible effect on the aqueous RONS formation. On the other hand, the solvation of NO2 from the gas phase into water can lead to NO2(aq) formation even without a presence of NO(aq), via the following reaction:  
\[\text{NO}_{\text{2(aq)}}+\text{NO}_{\text{2(aq)}}+\text{H}_{\text{2}}\text{O} \to\text{NO}_{\text{2}}^{-}{}_{\text{(aq)}}+\text{NO}_{\text{3}}^{-}{}_{ \text{(aq)}}+2\text{H}^{+} \tag{16}\]  
Figure 13: Concentration of NO2(aq) in collected water as function of gas phase NO2 concentration, ES with Qw = 500 uL/min, different gas mixtures, and combined TS with ES (Qw = 100 uL/min). One NO\({}_{2}\)\({}^{-}\)(aq) molecule is produced from two dissolved NO\({}_{2}\)\({}^{-}\)(aq) molecules. If we assume that NO\({}_{2}\)\({}^{-}\)(aq) is proportional to the gas phase NO\({}_{2}\) concentration, Reaction (16) could explain why the measured concentration of NO\({}_{2}\)\({}^{-}\)(aq) in water can be well fitted as a quadratic function of the gas phase NO\({}_{2}\) concentration (Figure 13, black points), when NO\({}_{2}\) from the pressure tank mixed with dry synthetic air was used. In the dry synthetic air treated by TS in the first reactor and ES in the second reactor (2 SS), the measured concentration of NO\({}_{2}\)\({}^{-}\)(aq) in water was higher, above the fitting curve valid for NO\({}_{2}\) from the pressure tank (Figure 13, green points). We assume that it can be explained by the presence of both NO and NO\({}_{2}\) in the sprayed gas, i.e., both NO and NO\({}_{2}\) can also contribute to the NO\({}_{2}\)\({}^{-}\)(aq) formation via Reactions (15) and (16). If NO\({}_{2}\) in the gas phase plays a dominant role on NO\({}_{2}\)\({}^{-}\)(aq) formation in water, one could expect less NO\({}_{2}\)\({}^{-}\)(aq) in the water collected during 2 SS experiments in humid synthetic air, where the gas phase NO\({}_{2}\) concentration produced by TS in the first reactor was lower compared to that generated by TS in the dry air. On the contrary, significantly higher NO\({}_{2}\)\({}^{-}\)(aq) concentration was observed (Figure 13, blue points). Here, the TS discharge in the first reactor generated not only NO and NO\({}_{2}\), but also directly by gaseous HNO\({}_{2}\), which can then form NO\({}_{2}\)\({}^{-}\)(aq) directly by its rapid solvation into water microdroplets generated by ES in the second reactor.