# The Role of HNO2 in the Generation of Plasma-Activated Water by Air Transient Spark Discharge
## 4 Conclusions  
Transient spark (TS) discharge in air turns out to be a very efficient source for generation of nitrogen oxides NO and NO\({}_{2}\). More than 1000 ppm of NO was generated with an input energy density of 400 J/L. It was shown that in humid air, the TS discharge also generates a significant amount of HNO\({}_{2}\), providing 10-100 ppm of HNO\({}_{2}\) depending on the input energy density. The air TS discharge in contact with water is very suitable for the generation of plasma-activated water (PAW), because TS can operate in a direct contact with water electrospray (ES) into the discharge zone. The formation of microdroplets with high interface surface area facilitates the transfer of reactive species from plasma into water. HNO\({}_{2}\) was also detected in air treated by TS in combination with water ES in one-stage system (ISS). This indicates an important role of HNO\({}_{2}\) in PAW generation, because HNO\({}_{2}\), although directly produced at lower concentrations, is much more soluble than two other major gas phase species generated by TS, NO and NO\({}_{2}\). To assess the role of HNO\({}_{2}\), measurements were performed in two-stage system (2 SS), where dry or humid air was treated by TS in the first reactor and water ES was applied in the second reactor. The dry air treated by TS in the first reactor contained mainly NO and NO\({}_{2}\) with only traces of HNO\({}_{2}\), while in the humid air, NO\({}_{2}\) decreased and HNO\({}_{2}\) reached almost the same concentration of as that of NO\({}_{2}\). For comparison, we also applied ES on NO and NO\({}_{2}\) from pressure tanks diluted in N\({}_{2}\) or air, respectively. These experiments confirmed that solvation to ES microdroplets caused a significant depletion of HNO\({}_{2}\) from the gas phase, while the depletion of NO was not observed, and the decrease of the NO\({}_{2}\) concentration was questionable. The importance of HNO\({}_{2}\) for PAW generation by TS in air was proved by the measured concentrations of NO\({}_{2}\)\({}^{-}\)(aq) in water. The concentration of NO\({}_{2}\)\({}^{-}\)(aq) was much higher in humid air containing HNO\({}_{2}\) than in dry air with higher concentration of NO\({}_{2}\), but almost no HNO\({}_{2}\) in 2 SS experiments with TS in the first reactor and ES in the second reactor. The NO\({}_{2}\)\({}^{-}\)(aq) concentration in 2 SS experiments in humid air was similar (or even higher) than NO\({}_{2}\)\({}^{-}\)(aq) concentration in PAW generated by a direct contact of the plasma with microdroplets in 1 SS. The advantage of using 2 SS is the stability and repeatability of the process. The TS in humid air without ES microdroplets inside the first reactor is relatively stable and the stable ES of water with higher water flow rate can be also achieved without TS in the second reactor. On the contrary, in 1 SS with a direct contact of the discharge with water, the ES and TS negatively influence each other and both ES and TS are less stable. However, the advantage of using ISS with generation of TS and ES in the same reactor is reaching a higher concentration of H\({}_{2}\)O\({}_{2}\) in the water. The optimization of the HNO\({}_{2}\) generation by TS and of the 2 SS with a shorter distance between TS and ES reactors are planned in future. Further research of undergoing chemistry is also desirable, both experimentally and by chemical kinetic modeling. Comparison of biocidal effects of PAW generated in 1 SS and 2 SS should be also performed to investigate the role of low vs. high H\({}_{2}\)O\({}_{\text{(aq)}}\) concentration in addition to NO\({}_{2}^{-}\) (aq). Nevertheless, this experimental study showed that gaseous HNO\({}_{2}\) produced in plasma discharges operated in humid air is a dominant contributor of nitrites in the plasma-activated water. Conceptualization, M.J., [PERSON] and K.H. ; methodology, M.J., K.H., P.T. and Z.M. ; software, M.J.; validation, M.J. and P.T. ; formal analysis, M.J. and P.T. ; investigation, M.J., M.E.H. and P.T. ; resources, M.J., K.H. and Z.M.