Photochemical smog

• Motor vehicle emissions contain NOx and unburnt hydrocarbons (CxHy). They are converted
to ozone, aldehydes, peroxyacetyl nitrate (PAN), peroxy benzoyl nitrate (PBN), etc. in the
presence of sunlight and temperatures above 15 0C.

• This is known as photochemical smog as those chemicals are formed in the presence of
sunlight
• Smog is a yellowish haze which reduces visibility and causes eye irritation.
• The word smog is used to describe the combination of smoke and fog.
• The starting reaction of photochemical smog is dissociation of NO2 to NO and ‘O’.
• The steps in the formation of a photochemical smog are given below.
(i) NO2 absorbs sunlight and undergoes photolysis.

$N{ O }_{ 2 }\longrightarrow NO+O$ .

(ii) The resulting atomic oxygen combines with ${ O }_{ 2 }$ molecules
a) to form ozone.

$O+{ O }_{ 2 }+M\longrightarrow { O }_{ 3 }+M$

(M is known as third body which absorbs excess energy. M can be an airborne particle
or a gas.)
b) to form OH radicals.

$O+{ H }_{ 2 }O\longrightarrow 2OH$

(iii) The resulting HO
•
converts other airborne chemicals into radicals and they start a set of
reactions to produce aldehydes, PAN, PBN, etc.

Effects of photochemical smog are given below.
• Effects on human health and comfort : Photochemical smog affects the respiratory system.
It causes coughing, wheezing, etc.
• Damage to materials : Ozone causes rubber to deteriorate through fission of the double
bond and also reduces the quality of fabrics and bleaches dyes.
• Effects on the atmosphere : Aerosol particles scatter light and reduce the visibility.
• Toxicity to plants : Most of the photochemical smog products are toxic to plants. Plant
growth is inhibited by the prodcts from photochemical smog. This
can effect the food production.