Molecular admixtures and impurities in atmospheric pressure plasma jets

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A more complete understanding of reactive chemistry generated by atmospheric pressure plasma jets (APPJs) is critical to many emerging medical, agricultural, and water treatment applications. Adding molecular gases to the noble working gas which flows through the jet is a common method to tailor the resulting production of reactive oxygen and nitrogen species (RONS). In this paper, results are discussed from a computational investigation of the consequences of H2O and O2 admixtures on the reactive chemistry of He APPJs flowing into humid air. This investigation, performed with a 2-dimensional plasma hydrodynamics model, addresses the RONS that are initially produced and the evolution of that chemistry on longer time scales. Without an admixture, the impurities in 99.999% pure helium are a major source of RONS. The addition of H2O decreases the production of reactive nitrogen species (RNS) and increases the production of reactive oxygen species (ROS). The addition of O2 significantly decreases the production of RNS, as well as hydrogen-containing ROS, but increases the production of ROS without hydrogen. This selectivity comes from the lower ionization energy of O2 compared to N2 and H2O, which then allows for charge exchange reactions. These charge exchange reactions change the RONS which are produced in the afterglow by dissociative recombination. The consequences of impurities were also examined. Humid air impurities as low as 10 ppm in the helium can account for 79%-98% of the production of most RONS in the absence of an intentional admixture. The degree to which the impurities affect the RONS production depends on the electrode configuration and can be reduced by molecular admixtures.