# The Role of HNO2 in the Generation of Plasma-Activated Water by Air Transient Spark Discharge
###### Abstract  
Transient spark (TS), a DC-driven self-pulsing discharge generating a highly reactive atmospheric pressure air plasma, was employed as a rich source of NOx. In dry air, TS generates high concentrations of NO and NO2, increasing approximately linearly with increasing input energy density (\(E_{d}\)), reaching 1200 and 180 ppm of NO and NO2, at \(E_{d}\) = 400 J/L, respectively. In humid air, the concentration of NO2 decreased down to 120 ppm in favor of HNO2 that reached approximately 100 ppm at \(E_{d}\) = 400 J/L. The advantage of TS is its capability of simultaneous generation of the plasma and the formation of microfloplets by the electrospray (ES) of water directly inside the discharge zone. The TS discharge can thus efficiently generate plasma-activated water (PAW) with high concentration of H2O2-(aq), NO2-(aq) and NO3-(aq), because water microfloplets significantly increase the plasma-liquid interaction interface. This enables a fast transfer of species such as NO, NO2, HNO2 from the gas into water. In this study, we compare TS with water ES in a one stage system and TS operated in dry or humid air followed by water ES in a two-stage system, and show that gaseous HNO2 rather than NO or NO2, plays a major role in the formation of NO2-(aq) in PAW that reached the concentration up to 2.7 mM. promotion, pest control) [12; 13; 14; 15; 16]. With PAW, the biocidal effects of plasma can be indirectly applied in cases where a direct plasma treatment is not possible. The composition of PAW and thus also its potential application varies with the used plasma source and working gas [15; 17]. The liquid phase RONS in PAW include long-lived species: ozone (O\({}_{3(aq)}\)), hydrogen peroxide (H\({}_{2}\)O\({}_{2(aq)}\)), nitrites (NO\({}_{2}^{-}\)(aq)), or nitrates (NO\({}_{3}^{-}\)(aq)), besides short-lived species that are challenging for diagnostics: hydroxyl OH, peroxyl HO\({}_{2}\) radicals, atomic O, N and H, singlet molecular O\({}_{2}\) (\({}^{1}\Delta\)), superoxide anion O\({}_{2}^{-}\) and other ions. Transport of reactive species from gas to water is the primary source of the liquid RONS, but the composition of liquid RONS does not perfectly mirror the concentrations of gaseous RONS. First, the concentration of liquid phase RONS depends on the solubility of their gas phase counterparts. Second, the composition of PAW is influenced by a rich set of the subsequent chemical reactions in the liquid phase [18; 19]. The highest achievable (saturated) concentration of species in liquid phase \(c_{i}^{satur}\) is determined by [PERSON]'s law:  
\[c_{i}^{satur}=k_{H}^{i}p_{i} \tag{1}\]  
Here \(p_{i}\) is partial pressure and \(k_{H}^{i}\) is the [PERSON]'s law coefficient of i-species. For instance, H\({}_{2}\)O\({}_{2}\) (with \(k_{H}\) = 108,000 mol.kg\({}^{-1}\) atm\({}^{-1}\) at 298.15 K [20]) is much more soluble than many other gas phase RONS, having \(k_{H}\) by seven orders of magnitude higher than O\({}_{3}\) and NO\({}_{2}\), and by eight orders of magnitude higher than NO. The high solubility of H\({}_{2}\)O\({}_{2}\) leads to the depletion of its gas phase concentration near the liquid interface [21; 22]. The highest achievable concentration of H\({}_{2}\)O\({}_{2(aq)}\) in the liquid phase is thus limited by the number of available H\({}_{2}\)O\({}_{2}\) molecules in the gas phase. On the other hand, for weakly soluble molecules, such as O\({}_{3}\), the assumption of the constant concentration and partial pressure in the gas phase is valid. Here, the saturated concentration in the liquid phase could be achieved without a noticeable decrease of O\({}_{3}\) concentration in the gas phase. In plasma-liquid interactions, the situation is much more complicated, because solubility determined by Henry's law coefficient is valid for systems in thermodynamic equilibrium. This is certainly not the case of NTP interacting with water.