Global model of low-frequency-driven cold atmospheric He + air plasmas

He + air mixture is a commonly used background gas for cold atmospheric-pressure plasmas (CAPs) because of its good discharge stability and chemical reactivity. However, the internal physical and chemical characteristics, especially the mechanism of the reactive species in low-frequency (LF)-driven He + air CAPs, are not fully understood. Hence, in this study, a global model incorporating 59 species and 866 volume reactions is developed to investigate the species densities and chemical pathways in 50 kHz-driven He + air CAPs with air contents ranging from 200 to 5000 ppm. In the entire air content range, O₄⁺ is the most abundant cation, and NO₃⁻ is the most abundant anion. HNO₂ is the dominant reactive nitrogen species (RNS), and the dominant reactive oxygen species (ROS) changes from H₂O₂ to O₃ when the air concentration increases above 1000 ppm. Moreover, since He + air CAPs are typically driven by low frequency to radio frequency (RF) power sources, in this work, the species densities in LF (f = 50 kHz)- and RF (f = 13.56 MHz)-driven He + air plasmas are compared.