Photochemical smog consists of excessive amount of X, in addition to aldehydes, ketones, peroxy acetyl nitrile (PAN). X is:

Photochemical smog is formed on the chemical reaction of sunlight, nitrogen oxides and volatile organic compounds in the atmosphere.

${\mathrm{N}\mathrm{O}}_2\left(\mathrm{g}\right)\stackrel{\mathrm{h}\mathrm{v}}{\to }\mathrm{N}\mathrm{O}\left(\mathrm{g}\right)+\mathrm{O}\left(\mathrm{g}\right)$

Oxygen atom (nascent) is very reactive and combines with O₂ in air to produce ozone.

$\mathrm{O}\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{O}}_3\left(\mathrm{g}\right)$