One possible mechanism would be the collision of two N0 molecules to form the unstable but perfectly legitimate N202 molecule, followed either by its decomposition to N0 again or its collision with 02 to form N02:

This mechanism is shown in fig 5 opposite. If we assume that the buildup and breakdown of N202 are very fast reactions in comparison with the collision of N202 and 02, and that the first two reactions are in equilibrium, then we can describe the association and dissociation reactions involving constants k1 and k2 in terms of an equilibrium constant:
  k1[NO]2 = k2[N2O2]

The slower collision of N202 with 02 is called the rate-determining step, since the overall reaction rate depends on how fast the slowest step occurs. If the reaction takes place by collision of N202 and 02, then the rate of production of N02 will be proportional to the concentrations of these two molecules: