Spatially sequenced humidification decouples ionization and ozone chemistry in a floating-electrode plasma jet

Abstract

Adding H2O to atmospheric-pressure plasma jets can suppress ozone (O3) but also attenuate the discharge that generates reactive species. We examine whether downstream H2O injection can separate these effects in a floating-electrode tube atmospheric-pressure plasma jet. Transient fluid simulations, optical diagnostics, gas- and liquid-phase measurements, and GL261 cell assays show that inlet H2O increases channel impedance and weakens floating-electrode charging and ionization-wave propagation through electron-energy loss, attachment, and excited-He quenching. By contrast, the same nominal H2O load injected downstream preserves the upstream conductive channel and second ionization wave while redirecting effluent chemistry. Relative to Dry-He, downstream injection reduces gas-phase O3 by more than 90%, retains approximately 93% of the He I emission and 90% of the liquid-phase DCFH response, and produces the highest mean intracellular DCFH signal. Separating energy deposition from H2O-mediated chemistry thus enables low-O3 operation without global discharge attenuation.