Post-discharge gas composition of a large-gap DBD in humid air by UV–Vis absorption spectroscopy

Large gap dielectric barrier discharges (DBD) provide non-thermal, non-equilibrium plasmas that can generate specific gas chemistry with enhanced bactericidal effects when working in humid air. The present study investigates the post-discharge gas composition of such plasmas operated in humid air using UV–Vis (200–800 nm) absorption spectroscopy. Absorbance spectra have been de-convoluted using direct deconvolution and iterative methods and results are correlated to the DBD electrical parameters. The high-voltage (56 and 70 kV rms) DBD plasma generated at 50 Hz frequency in a closed container over a 20 mm gap in air with relative humidity (RH) of 5–70% has been characterized by I–V and capacitive methods. The post-discharge gas composition at each RH is assessed by UV–Vis absorption spectroscopy for plasma exposure times of 15–120 s. The concentration of ozone and nitrogen oxides (O₃, NO₂, NO₃, N₂O₄) increases with plasma exposure time but a strong decrease in [O₃] levels is obtained with increase in RH. The decrease in [O₃] and an abundance of nitrogen oxides is ascribed to high specific power densities in the closed container and to increasing RH levels. The absorbance residual following deconvolution shows a strong band at 230–270 nm consistent with the presence of pernitric acid (HNO₄) and other HNO_x (x = 1, 3) species. Humid air large gap DBD plasmas in closed containers generate along with O₃, high levels of nitrogen oxides and HNO_x (x = 1, 4) acids leading to increased bactericidal rates. (paper)