Chemical pollutants in photochemical smog are nitrogen oxides (NO and $\mathrm{N}{\mathrm{O}}_2$ ), volatile organic compounds, ozone $\left({\mathrm{O}}_3\right)$ , peroxyacetyl nitrates.

In the presence of sunlight of the following reactions take place

$\mathrm{N}{\mathrm{O}}_2\stackrel{\mathrm{h}\mathrm{v}}{\to }\mathrm{N}\mathrm{O}+\mathrm{O}$

$\mathrm{O}+{\mathrm{O}}_2\to {\mathrm{O}}_3$

${\mathrm{O}}_3+\mathrm{N}\mathrm{O}\to \mathrm{N}{\mathrm{O}}_2+{\mathrm{O}}_2$

Hence, it consists of excessive amount of ozone melecules.