Photochemical smog consists of excessive amount of X, in addition...

Chemical pollutants in photochemical smog are nitrogen oxides (NO and $N{O}_{2}$ ), volatile organic compounds, ozone $\left({O}_{3}\right)$ , peroxyacetyl nitrates.

In the presence of sunlight of the following reactions take place

$N{O}_{2}\overset{hv}{→}NO + O$

$O + {O}_{2} \to {O}_{3}$

${O}_{3} + NO \to N{O}_{2} + {O}_{2}$

Hence, it consists of excessive amount of ozone melecules.