Characteristics of a direct current （DC） discharge in atmospheric pressure helium are numerically investigated based on a one-dimensional fluid model. The results indicate that the discharge does not reach its steady state till it takes a period of time. Moreover, the required time increases and the current density of the steady state decreases with increasing the gap width. Through analyzing the spatial distributions of the electron density, the ion density and the electric field at different discharge moments, it is found that the DC discharge starts with a Townsend regime, then transits to a glow regime. In addition, the discharge operates in a normal glow mode or an abnormal glow one under different parameters, such as the gap width, the ballast resistors, and the secondary electron emission coefficients, judged by its voltage-current characteristics.
A zero-dimensional model is used to study the processes of physical and chemical reactions in atmospheric plasma with different ionization degrees near the ground （0 km）. The temporal evolutions of CO, C02 and other main reactants （namely OH and O2）, which affect the conversion of CO and C02, are obtained for afterglow plasma with different initial values. The results show that the consumption rate of CO is largest when the initiM electron number density neo=1012 cm-3, i.e. the ionization degree is 0.000004%. The number density of CO2 is relatively small when neo=1016 cm-3, i.e. the ionization degree is 0.04%, whereas they are very close under the condition of other ionization degrees. Considering the total number densities of CO and C02 and the consumption rate of CO comprehensively, the best condition is neo=1013 cm-3, i.e. the ionization degree is 0.00004% for reducing the densities of CO and CO2 in the atmospheric plasma. The temporal evolutions of N＋, Ar＋, CO＋ and CO＋ are also shown, and the influences on the temporal evolutions of CO and C02 are analyzed with increasing ionization degree.
