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

Photochemical smog is called summer smog because during summer when temperature is warmer and there is more sunlight present and hence Photochemical smog is the dominant type of smog formation. The X molecule is formed by three types of the same atoms. The atom in X has an atomic number of 8 and it belongs to the period 2 and group 16 of the periodic table.

that is photochemical smog consists of excessive amounts of Ozone in addition to aldehydes, ketones, PAN etc.

Photochemical smong also contains ${\mathrm{O}}_3$