Main species and chemical pathways in cold atmospheric-pressure Ar + H₂O plasmas

Cold atmospheric-pressure plasmas in Ar + H₂O gas mixtures are a promising alternative to He + H₂O plasmas as both can produce reactive oxygen species of relevance for many applications and argon is cheaper than helium. Although He + H₂O plasmas have been the subject of multiple experimental and computational studies, Ar + H₂O plasmas have received less attention. In this work we investigate the composition and chemical pathways in Ar + H₂O plasmas by means of a global model that incorporates 57 species and 1228 chemical reactions. Water vapor concentrations from 1 ppm to saturation (32 000 ppm) are considered in the study and abrupt transitions in power dissipation channels, species densities and chemical pathways are found when the water concentration increases from 100 to 1000 ppm. In this region the plasma transitions from an electropositive discharge in which most power is coupled to electrons into an electronegative one in which most power is coupled to ions. While increasing electronegativity is also observed in He + H₂O plasmas, in Ar + H₂O plasmas the transition is more abrupt because Penning processes do not contribute to gas ionization and the changes in the electron energy distribution function and mean electron energy caused by the increasing water concentration result in electron-neutral excitation and ionization rates changing by many orders of magnitude in a relatively small range of water concentrations. Insights into the main chemical species and pathways governing the production and loss of electrons, O, OH, OH(A) and H₂O₂ are provided as part of the study.