Photochemical smog is initiated by the photochemical dissociation of $\mathrm{N}{\mathrm{O}}_2$ and the resulting secondary reactions involving unsaturated hydrocarbons, other organic compounds and free radicals, leading to the formation of organic peroxides and ozone.



Hydrocarbons $+{\mathrm{O}}_3, {\mathrm{O}}_2, \mathrm{O},{\mathrm{NO}}_2, \mathrm{NO}$; peroxides, peroxyacetyl nitrate, formaldehyde, ozone aldehyde, acrolein, etc. oxidised hydrocarbons and ozone in the presence of humidity cause photochemical smog, which dissipates at night.