Photochemical smog is produced by action of sunlight on hydrocarbons and nitrogen oxides. NO is converted to NO2 by the effect of sunlight. This NO2 further absorbs sunlight and disintegrates into NO and O2. Thus, a chain reaction begins.
NO(g)+

1

2

O2(g)

hv

→

NO2(g)
NO2

hv

→

NO(g)+O(g)
These oxygen atoms react with O2 to form ozone as :
O2(g)+O(g) ⇌ O3(g)
O3(g)+NO(g) → NO2(g)+O2(g)
These two gases formed, react with hydrocarbons to produce other chemicals.
3CH4+2O3 →

3CH2=O

formaldehyde

+3H2O
Other chemicals produced are :

CH2=CH−CH=O

Acrolein

CH3− C || O −O−O−NO2

Peroxyacetyl‌nitrate(PAN)