Write down the reactions involved during the formation of photochemical smog. 
Or
Discuss the mechanism of photochemical smog formation. 

(i) During the early morning i.e. before the sunrise, the automobile exhaust emits CO, hydrocarbons and oxides of nitrogen. The NO reacts with oxygen to produce NO₂ gas.
(ii) In the presence of sunlight (during sunrise), NO₂ undergoes photolysis (ultraviolet and visible radiation) to form NO and atomic oxygen,

(iii) Some of these oxygen atoms combine with O₂ in the air to produce ozone gas.

(iv) Ozone reacts with NO to form NO₂ and O₂.

The NO again  absorbs ultraviolet radiation and the entire cycle starts again. Reactive oxygen atoms also combine with hydrocarbons coming out of the exhaust of vehicles to form organic free radicals. The formation of organic free radicals results into a number of chain reactions producing many undesirable compounds (organic peroxides, organic hydroperoxides, peroxy acyl nitrates etc.) which constitute photochemical smog.

As this type of smog contains O₃ and NO₂, it is oxidising in character.