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 NO, evolution in O-2/N-2. The rate constants of these reactions decrease with increasing oxygen concentration. This is because oxygen is electronegative and thus reduces electron concentration. The rate constant of 02 dissociation by electron collision reaction is almost two orders of magnitude higher than that of N-2 dissociation. NO formation occurs predominantly through N(D-2) + O-2 -> NO + O. The critical oxygen concentration, defined as the concentration at which the NOx formation rate counterbalances the NO, decomposition rate, increases with increasing initial NO concentration. (c) 2005 American Institute of Chemical Engineers.