NO is mainly converted to NO2 by chemical oxidation in the presence of oxygen. Initial selectivity analysis shows that three electron collision reactions are important for NOX evolution in O2/N2. The rate constants of these reactions decrease with increasing oxygen concentration. This is because oxygen is electronegative and hence reduces the electron concentration. The rate constant of O2 dissociation by electron collision reaction is almost two orders of magnitude higher than that of N2 dissociation. NO formation occurs predominantly through N(2D) + O2 → NO + O. The critical oxygen concentration, defined as the concentration above which the NOX formation rate exceeds the NOX decomposition rate, increases with increasing the initial NO concentration.