# 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  
The analytical method we used enables the detection of various RONS. In the dry air treated by the TS we detected only three long-lived species: NO, NO\({}_{2}\) and traces of HNO\({}_{2}\). Other possible RONS, such as O\({}_{3}\) or HNO\({}_{3}\) were not detected. NO is a dominant product of the air TS plasma with the highest concentration achieved. The concentration of NO  
Figure 5: Typical long timescale TS voltage waveforms; (**a**) voltage increase regrowth between two voltage drops (gas breakdowns, i.e., TS pulses), showing that voltage is high enough for formation of microdroplets by ES most of the time; (**b**) voltage waveforms showing several tens of breakdowns, comparison of voltage waveforms without (black) and with ES (red; water flow rate \(Q_{w}\) = 300 \(\mu\)L/min) where periods without breakdowns and TS current pulses appear (e.g., time period 6–9 ms). grows almost linearly with increasing input energy density, and the concentration of NO using either dry or humid air are almost identical (Figure 6). Based on preliminary results from chemical kinetic model of TS [62], we assume that NO is generated by the modified Zeldovich mechanism. The original Zeldovich mechanism is initiated by the thermal decomposition of N\({}_{2}\) and O\({}_{2}\) into their atomic states at high temperature (above \(\sim\)1500 K). Despite the fact that TS generates 'cold' non-equilibrium plasma, the temperature during the short spark phase of the TS can be as high as \(\sim\)3000 K [47]. Next, both N and O atomic radicals are able to produce NO:  
\[\text{O}+\text{N}_{2}\rightarrow\text{NO}+\text{N} \tag{4}\]  
\[\text{N}+\text{O}_{2}\rightarrow\text{NO}+\text{O} \tag{5}\]  
The rate coefficient of Reaction (5) with N atoms is four orders of magnitude higher at 3000 K than at room temperature, but the production of N atomic radicals via the thermal decomposition of N\({}_{2}\) is very slow, even at 3000 K (rate coefficient \(\sim 1.5\times 10^{-24}\text{cm}^{3}\,\text{s}^{-1}\)) [63]. As a result, the thermal decomposition of N\({}_{2}\) is a rate-limiting step of the thermal NO formation. In the TS discharge, the thermal mechanism of NO formation is bypassed by alternative reaction pathways for the generation of N and O atomic radicals. The influence of spark on plasma induced chemistry is not based on gas heating only, more important is an achievement of a high degree of ionization, with the electron density \(n_{e}>10^{17}\) cm\({}^{-3}\)[64]. The high degree of ionization results in a high degree of atomization thanks to the dissociative electron-ion recombination reactions:  
\[\text{e}+\text{O}_{2}{}^{+}\rightarrow\text{O}+\text{O}\qquad\text{(O}(^{1} \text{D)}\sim\)40\%,\text{(O}^{1}\text{S)}\sim\)5%) \tag{6}\]  
\[\text{e}+\text{N}_{2}{}^{+}\rightarrow\text{N}+\text{N}\qquad\text{(N}(^{2} \text{D)}\sim\)45\%,\text{N}(^{2}\text{P)}\sim\)5%) \tag{7}\]  
In the next step, the products of reactions (6) and (7) enhance the NO synthesis, especially by N production bypassing the rate limiting step in the Zeldovich mechanism by Reaction (4). Moreover, reactions (4) and (5) can be much faster if one of the reactants is in an excited state [63; 65]. There are many other reactions that can contribute to N or O  
Figure 6: NO concentration as a function of the TS input energy density, measured by UV-Vis absorption technique, short absorption cell, ISS. atoms production. For example, additional O atoms can also be created from O\({}_{2}\) molecules by dissociative electron attachment reaction. A few microseconds after the spark current pulse, air cools down and the N concentration decreases. Consequently, the NO production stops. Remaining O atoms start to generate ozone O\({}_{3}\):  
\[\text{O}+\text{O}_{2}+\text{M}\rightarrow\text{O}_{3}+\text{M} \tag{8}\]  
Alternatively, the atomic oxygen can oxidize NO to form NO\({}_{2}\) in a three-body Reaction (9):  
\[\text{NO}+\text{O}+\text{M}\rightarrow\text{NO}_{2}+\text{M} \tag{9}\]  
However, Reaction (8) producing O\({}_{3}\) is faster.