The composition of photochemical smog is nitrogen oxides, Volatile organic compounds (VOC), ozone, PAN i.e. peroxyacetyl nitrate.

Now the nitrogen oxide present in the smog undergoes dissociation in the presence of sunlight to NO and nascent oxygen (O) .as shown in the reaction below:

NO₂ (g) → NO (g) + O (g)

The nascent oxygen is very reactive and reacts with the oxygen gas to form Ozone gas along with the inert gas which promotes their reaction. After the reaction inert gas is obtained as it is without reacting with any of the species. As depicted by the reaction below:

O (g) + O_{2 (g)} + M → O₂ (g) + M

Where M = inert gas.