Origin of hydroxyl radicals in a weakly ionized plasma-facing liquid

Abstract

The hydroxyl radical (center dot OH) is omnipresent and indispensable in environments from Earth's atmosphere to biological system. Plasma-facing liquids (PFLs, e.g., raindrops in the vicinity of lightning and laboratory-scale plasma applications) have attracted attention because they facilitate storage of reactive oxygen and nitrogen species (RONS), including center dot OH, produced by plasma. As the hydroxyl radical is considered as one of the most powerful oxidants and the most important intermediate of chemical processes in PFLs, it has become increasingly significant; however, some of center dot OH generation mechanisms, particularly photo-induced productions, remain ambiguous. In this contribution, we show that N(III) [i.e., nitrite anion (NO2-) and nitrous acid (HONO)] and hydrogen peroxide (H2O2) have not been significantly considered as sources of center dot OH in PFLs. The contribution of N(III) and H2O2 photolysis to % OH production is estimated by numerical and experimental approaches. The results of the model experiments show that the concentration of center dot OH generated by UV photolysis of N(III) due to plasma emission is approximately 20% of the total concentration in a PFL. Thus, the unique feature of a plasma jet results in self-enhancing and self-focusing center dot OH production in PFLs. Furthermore, we demonstrate that additional light exposure is the effective way to enhance center dot OH production in a PFL.