| Summary: | 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 NO<sub>2</sub>, increasing approximately linearly with increasing input energy density (<i>E<sub>d</sub></i>), reaching 1200 and 180 ppm of NO and NO<sub>2</sub>, at <i>E<sub>d</sub></i> = 400 J/L, respectively. In humid air, the concentration of NO<sub>2</sub> decreased down to 120 ppm in favor of HNO<sub>2</sub> that reached approximately 100 ppm at <i>E<sub>d</sub></i> = 400 J/L. The advantage of TS is its capability of simultaneous generation of the plasma and the formation of microdroplets 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 H<sub>2</sub>O<sub>2</sub><sup>−</sup><sub>(aq)</sub>, NO<sub>2</sub><sup>−</sup><sub>(aq)</sub> and NO<sub>3</sub><sup>−</sup><sub>(aq)</sub>, because water microdroplets significantly increase the plasma-liquid interaction interface. This enables a fast transfer of species such as NO, NO<sub>2</sub>, HNO<sub>2</sub> 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 HNO<sub>2,</sub> rather than NO or NO<sub>2</sub>, plays a major role in the formation of NO<sub>2</sub><sup>−</sup><sub>(aq)</sub> in PAW that reached the concentration up to 2.7 mM.
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