Proposed mechanism for the catalyzed oxidation of CO by O3: Are NO and NO2 catalysts or intermediates?
My take on this problem is as follows. Suppose a flask contains an equimolar mixture of $\ce{CO}$ and $\ce{O_3}$. The net reaction between the two gases will be 

$$\ce{CO (g) + O3 (g) -&gt; CO2 (g) +  O2 (g)} \tag{1}$$

But it needs a catalyst. If $\ce{NO_2}$ gas is introduced, then the reaction proceeds via the two reactions given in the OP's question:

$$\ce{NO_2 (g) + CO (g) -&gt; CO2 (g) +  NO (g)} \tag{2}$$
and $$\ce{NO (g) + O3 (g) -&gt; NO2 (g) +  O2 (g)} \tag{3}$$

So the nitrogen oxides cycle around. On the other hand, if $\ce{NO}$ gas is introduced, then the reaction proceeds via 'flipped' equations:

$$\ce{NO (g) + O3 (g) -&gt; NO2 (g) +  O2 (g)} \tag{4}$$
and $$\ce{NO_2 (g) + CO (g) -&gt; CO2 (g) +  NO (g)} \tag{5}$$

and again the nitrogen oxides cycle around. So it does not really matter whether $\ce{NO_2}$ gas or $\ce{NO}$ gas is introduced first: the two nitrogen oxides are the overall catalytst and either could be considered to be an intermediate. Basically, this is a distinction without a difference.