Photo Chemical smog: Nitrogen monoxide is formed by reaction of N2 and O2 at high temperature in petrol and diesel engines of cars and trucks. NO is oxidized into the air to form NO2 which absorbs energy from sunlight and breaks up into NO and free oxygen atom.
NO2(g) \(\overset{hv}{\longrightarrow}\) NO(g) + O(g)
Oxygen atoms are very reactive and can combine with O2 to form O3
O(g) + O2(g) → O3(g).
Ozone reacts with NO to form NO2 and O2
NO + O3 → NO2 + O2
The brownish haze of photochemical smog is largely due to brown colour of NO2.
Effects: Pungent smelling, smog produced ozone is known to be toxic. It can cause coughing, wheezing, bronchitis and irritation to mucous system.
Control of photochemical smog:
(i) Installation of efficient catalytic converters in the automobiles is one of the ways of reducing smog formation.
(ii) It can be suppressed by certain compounds, which acts as free radical traps. When these compounds are sprayed in the atmosphere they generate free radicals which readily combine with free radical precursors of photochemical smog.
