The main species and chemical processes in low‐temperature atmospheric‐pressure He + O2 plasmas are identified using a comprehensive global model. The simulation results highlight the significance of Penning processes at low oxygen concentration, and the increasingly important role of electron attachment as the oxygen concentration increases. With increasing the oxygen concentration, the electron energy dissipation shifts from elastic collisions with He to dissociative excitation and attachment of O2 molecules, and large ions (${\rm O}_{3}^{ + } $, ${\rm O}_{4}^{ + } $) become the dominant charged species. Generation and loss of ROS (O, O(1D), O(1S), O2(a1Δg), O2(b1∑ g+), O3) relevant for biomedical applications are discussed.

Normal saline is a common biological solution which provides much better living environment for Staphylococcus aureus than deionized water, but the plasma‐activated normal saline is found to have a stronger bactericidal effect than the plasma‐activated deionized water. A model is developed for the explanation, from which various kinds of reactive chlorine/oxy‐chlorine species (RCS), such as HClO, are found to be generated in the plasma‐activated normal saline. The production pathways of RCS are elucidated, in which O3 plays as an important intermediate species. Compared to the plasma‐activated deionized water, the concentrations of reactive oxygen/nitrogen species are lower, but the bactericidal effect is higher, implying that the RCS play a crucial role for the sterilization.

Cold atmospheric-pressure He + N 2 and He + O 2 plasmas are chosen as the representatives for electropositive and electronegative plasmas, of which the discharge characteristics are studied and then compared to each other by fluid models. As the increase of the impurity (N 2 or O 2) fraction from 0 to 10%, for He + N 2 plasmas the electron density and ion density increase, the spatiotemporal distributions of electron density, ion density, electron temperature and electron generation rate change a little. On contrast, for He + O 2 plasmas the electron density decreases, the ion density first increases and then decreases, the electron temperature increases in the bulk region, but decreases in the sheath region, and the plasmas transform from g mode to α mode as the significant change of electron generation rate distributions. Larger electric field is needed in the bulk region to sustain the electronegative plasma, so the electrical characteristics of He + O 2 plasmas transform form capacitive to resistive with increasing O 2 fraction. Meanwhile, the ion-coupling power increases dramatically, which can be estimated by a formula based on the electronegativity. A new criterion for determining the sheath boundary, |∇E| = 5 kV/cm 2 , is put forward, which is found suitable for both the electropositive and electronegative plasmas.