Q.Nitrogen dioxide is sometimes described as playing a 'catalytic' role in photochemical smog formation. Using the $\text{NO}_2$-$\text{NO}$-$\text{O}_3$ cycle, explain what this means.

In the $\text{NO}_2$-$\text{NO}$-$\text{O}_3$ cycle, sunlight splits $\text{NO}_2$ into $\text{NO}$ and a free oxygen atom ($\text{NO}_2 \xrightarrow{h\nu} \text{NO} + \text{O}$), and that oxygen atom goes on to form ozone ($\text{O} + \text{O}_2 \rightarrow \text{O}_3$). On its own, this step would use up $\text{NO}_2$ permanently. But the $\text{NO}$ released in that same step is then re-oxidised back to $\text{NO}_2$ by reacting with atmospheric oxygen ($2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$), regenerating the very species that started the cycle. Because $\text{NO}_2$ is consumed in one step and regenerated in another, it is not permanently used up the way a normal reactant would be -- exactly the defining behaviour of a …

Track $\text{NO}_2$ through both steps of the cycle explicitly -- show it being split apart in one step and reformed in th …

Describing $\text{NO}_2$ as simply 'a pollutant' without explaining the regeneration step …