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A systematic study of nitrogen luminescence decay time has been performed, exciting nitrogen by Â¿ particles in various oxygen mixtures and air near atmospheric pressure. Results obtained by varying the oxygen ratio from 0 to 20% and total pressure from 200 to 2000 torr are given and explained by kinetic theory of the phenomena. It seems that a resonance collision process is responsible for the changes in the decay times of the second positive nitrogen group. An extrapolation of the measurements gives a decay time of 0.52 ns for air at atmospheric pressure. We proposed to study the luminescence decay time of air excited by the alpha particles of Polonium 210, in a pressure range between 200 and 2000 torr. As is known, nitrogen is mainly responsible for the luminescence of air irrespective of the excitation method; the most intense bands emitted correspond to transitions of the second positive system of the N2 molecule arn the first negative system of the once ionised molecule N2+5. Between 220 and 600 nm, the spectral domain of our experiments, the presence of oxygen is shown by a strong inhibition of the intensity of the nitrogen bands and by the appearance of the auroral line OI at 557.7 nm line, whose intensity is always very small. We therefore compared the luminescence decay time of the air and that of a mixture containing 80% nitrogen and 20% oxygen.