# The Role of HNO2 in the Generation of Plasma-Activated Water by Air Transient Spark Discharge
## 3 Results and Discussion
### Generation of RONS in the Gas Phase
Consequently, O\({}_{3}\) also oxidizes NO to NO\({}_{2}\):  
\[\text{NO}+\text{O}_{3}\rightarrow\text{NO}_{2}+\text{O}_{2} \tag{10}\]  
As a result, NO\({}_{2}\) is a product with the second highest measured concentration (Figure 7). In dry air, there are no other significant products of TS. We observed no O\({}_{3}\) at the outlet, probably because Reaction (10) is fast enough and there is a sufficient amount of NO so that O\({}_{3}\) is fully consumed by the oxidation of NO to NO\({}_{2}\) before the gas enters the closest UV absorption cell (the estimated delay between the moment when air is treated by the discharge and when it is analyzed in the short UV cell is 17-20 s). If we assume that Reaction (10) is a dominant source of NO\({}_{2}\), the achieved NO\({}_{2}\) concentration (\(\sim\)100-200 ppm, Figure 7) should be approximately equal to concentration of formed and immediately consumed O\({}_{3}\). To understand O\({}_{3}\) formation without its depletion by NO, we performed experiments in ISS with TS operating in dry O\({}_{2}\). These experiments proved that TS can really generate high concentration of O\({}_{3}\) (Figure 8), when it is not consumed by reaction with NO. Figure 7: NO\({}_{2}\) concentration as function of the TS input energy density, measured by UV-Vis absorption technique, short absorption cell, ISS. We also performed experiments in humid O\({}_{2}\), where the electron impact reaction with H\({}_{2}\)O can result into several radicals (OH, H) or ions (O\({}^{-}\), OH\({}^{-}\), H\({}_{2}\)O\({}^{+}\)) [66,67]. The OH radicals can be also created by collisions of H\({}_{2}\)O with O(\({}^{1}\)D) species. Next, H and OH radicals can decompose O\({}_{3}\) in the reactions:  
\[\text{O}+\text{N}_{2}\rightarrow\text{NO}+\text{N} \tag{4}\]0  
\[\text{O}+\text{N}_{2}\rightarrow\text{NO}+\text{N} \tag{4}\]1  
For this reason, the concentration of O\({}_{3}\) generated in humid O\({}_{2}\) was approximately 15 times lower than in dry O\({}_{2}\). We can also expect lower production of O\({}_{3}\) in the humid air compared to dry air. In turn, lower production of O\({}_{3}\) could explain why the presence of humidity in air decreased the NO\({}_{2}\) concentration (Figure 7). However, the concentration of NO\({}_{2}\) in humid air was not 15 times lower than in dry air. It is probably because NO\({}_{2}\) may be also generated by different reaction channels, for example by the reaction with HO\({}_{2}\) radical. Sufficient amount of HO\({}_{2}\) radicals may be provided by reaction of OH radicals with O\({}_{3}\) (Equation (12)), or with H\({}_{2}\)O\({}_{2}\). The presence of OH radicals influences the reaction mechanism even more significantly by the reaction:  
\[\text{O}+\text{N}_{2}\rightarrow\text{NO}+\text{N} \tag{4}\]2  
leading to the formation of HNO\({}_{2}\). In the humid air treated by TS, the HNO\({}_{2}\) concentration becomes comparable to NO\({}_{2}\) concentration (Figure 9). Some HNO\({}_{2}\) was observed in dry air as well, showing a significant influence of even minor H\({}_{2}\)O impurities (from the carrier gas and/or due to desorption from surfaces) on NO, NO\({}_{2}\) and HNO\({}_{2}\) generation chemistry. \[\text{O}+\text{N}_{2}\rightarrow\text{NO}+\text{N} \tag{4}\]3 O\({}_{3}\) concentration as a function of the TS input energy density, dry O\({}_{2}\) treated by TS in 1 SS without ES, measured by UV-Vis absorption technique, short absorption cell. Overall, the sum of NO\({}_{2}\) and HNO\({}_{2}\) concentrations (H\({}_{y}\)NO\({}_{2}\)) is almost equal in humid and dry air, only below \(E_{d}\)\(\sim\)250 J/L it is slightly lower in humid air compared to dry air. The total amount of H\({}_{y}\)NO\({}_{x}\) (NO + NO\({}_{2}\) + HNO\({}_{2}\)) is within the experimental uncertainty the same in the entire range of \(E_{d}\) (Figure 10). The total amount of H\({}_{y}\)NO\({}_{x}\) is constant in time as well, based on comparison of concentrations obtained in short UV-Vis absorption cell and concentrations measured by FT-IR spectrometry.