For atmospheric pressure plasma jet, which is expected to have applications in medicine and other fields, atomic and molecular processes have been shown to determine the electron temperature and density using optical emission spectroscopy. Blue light, which was not observed in previous studies, was detected in the plasma plume. This phenomenon will be called “blue afterglow.” The continuum spectrum of atmospheric pressure plasma was previously thought to be be due to electron-neutral atom bremsstrahlung. However, the blue afterglow was considered to be origineated from excited states or ions of molecular nitrogen generated by Penning effect. It can be concluded that the blue afterglow is a plasma with a lower electron temperature than the helium plasma.