Comparison of the Transport of Reactive Nitrogen Plasma Species into Water Bulk vs. Aerosolized Microdroplets

Abstract

This work presents the experimental study of the transport of typical air plasma long-lived reactive nitrogen species (RNS: HNO2, NO2, and NO) into deionized water and compares them with the most typical reactive oxygen species (ROS: H2O2 and O3). RONS are generated either by external sources or by a hybrid streamer-transient spark plasma discharge, in contact with bulk water or aerosol of charged electrospray (ES) or non-charged nebulized microdroplets with a large gas/plasma-water interface. It was found that NO’s contribution to NO2¯ ion formation was negligible, NO2 contributed to about 10%, while the dominant contributor to NO2¯ ion formation in water was gaseous HNO2. A higher transport efficiency of O3, and a much higher formation efficiency of NO2¯ from gaseous NO2 or HNO2 than predicted by Henry’s law was observed, compared to the transport efficiency of H2O2 that corresponds to the expected Henry’s law solvation. The improvement of the transport/formation efficiencies by nebulized and ES microdroplets, where the surface area is significantly enhanced compared to the bulk water, is most evident for the solvation enhancement of the weakly soluble O3. NO2¯ ion formation efficiency was strongly improved in ES microdroplets with respect to bulk water and even to nebulized microdroplets, which is likely due to the charge effect that enhanced the formation of aqueous nitrite NO2¯ ions when NO2 or HNO2 are transported into water. Comparisons of the molar amounts of O3, H2O2, and NO2¯ formed in water by hybrid streamer-transient spark plasma discharge with those obtained with single RONS from the external sources enabled us to estimate approximate concentrations of gaseous concentrations of HNO2, NO2, O3, and H2O2. The medium or highly soluble gaseous HNO2 or H2O2, with a low concentration of < 10 ppm are sufficient to induce the measured aqueous NO2¯ or H2O2 amounts in water. This study contributes to a deeper understanding of the transport mechanism of gaseous plasma RONS into water that can optimize the design of plasma–liquid interaction systems to produce efficient and selected aqueous RONS in water.