In this paper the electron density and temperature of atmospheric helium and argon plasmas operated under similar experimental conditions are compared. The conditions are chosen such that both plasmas are ionizing. It is found that a helium plasma has a higher electron temperature and a lower electron density than an equi-operational argon plasma, i.e. an argon plasma that is operated at the same external conditions. This is mainly caused by the higher excitation potential of the first excited state and the lower mass of helium, respectively. Due to these differences in electron density and temperature the densities of the helium ground state and of the excited states are much larger than their corresponding Saha equilibrium values for a wide range of conditions. The consequence of this is that the spectroscopic methods, which are used to determine the electron density and temperature, have a very limited validity region in the case of helium. For argon the deviations are much smaller so that these methods can often be applied safely.

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Molecular rare gas ions (MRIs) may offer a fast volume recombination channel in rare gas plasmas via dissociative recombination (DR). In this paper, the concentration of MRI is calculated by balancing the most important processes for formation and destruction of these ions. It is shown that the effective loss frequency of charged particles in the plasma due to DR of MRI is not determined by the formation of these ions, but by the DR process itself. Also the consequences for the particle and energy balances of atmospheric 'size-stabilized' helium and argon plasmas are discussed, as well as the influence on the lower boundary for partial local Saha equilibrium.