15.5.3 caused by photo chemical oxidants. Let us discuss

15.5.3             Smog

      Smog is a mixture of
smoke and fog which forms droplets that remain suspended in the air. It is common air
pollutant present in many cities.There is two types of
smog. One is Classical smog  caused by coal smoke and fog
while the other photo chemical smog caused by photo chemical oxidants. Let us discuss in detail

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London Smog or Classical smog

Classical smog
was first observed in London in December 1952and hence it is also known as
London smog. It consists of coal smoke and fog. It occurs in cool humid climate.  This atmospheric
smog found in many large cities. The chemical composition is the mixture of SO2,
SO3 and humidity. It generally occurs in the morning and becomes
worse when the sun rises. This is mainly due to the induced oxidation of SO2
to SO3, which reacts with humidity yielding sulphuric acid aerosol. Chemically it is reducing smog because of high
concentration of SO2.


Smog is primarily responsible
for acid rain.

 Smog results in poor visibility and it affects
air and road transport.

 It also
causes bronchial irritation.

(ii)Los Angel Smog or Photo Chemical Smog

Chemical smog was first observed in Los Angeles in 1950. It occurs in warm, dry
and sunny climate.This type of smog is formed by the
combination of smoke, dust and fog with air pollutants like oxides of nitrogen
and hydrocarbons in the presence
of sunlight. It forms when the sun shines and become worse in the afternoon. Chemically it is oxidizing in nature because of high concentration of
oxidising agents NO2 and O3

important reaction occurring in the  presence of sunlight is described below.

(i) N2  and O2 react to form nitric
oxide  which is emitted into air with
exhaust gases.

+ O2(g)  ……(1)     

(ii) Nitric oxide  is oxidised 
to form nitrogen di oxide

2NO(g) + O2(g) ¾¾® 2NO2(g)   ….… (2)

Nitrogen di oxide formed absorbs energy from sunlight and undergoes photochemical     

       decomposition to give nitric oxide and
very reactive free oxygen atom

NO2(g)     NO(g) + (O)…..      (3)

(iv) Reactive oxygen combines
with O2 to form ozone.

                                               O2(g)   + (O)   ¾¾® O3(g)……  .           

 The ozone formed (from equation 4) reacts
rapidly with  nitric oxide  ( formed in equation 1) to regenerate nitrogen
di oxide  (equation 2)

(v) NO
and O3 are strong oxidising agent and can react with  unburnt hydrocarbons in polluted air to form
oxidised hydrocarbons like formaldehyde, acrolein PAN  etc.

+ O3 , O2 , O, NO2, NO ¾¾® Oxidised hydrocarbons

                                                                                      (PAN, Formaldehyde Acrolein)      

     Effects  of photo chemical smog


            The three main components of photo chemical
smog are nitrogen oxide, ozone and oxidised hydro carbon like  formaldehyde(HCHO), Acrolein (CH2=CH-CHO),peroxy
acetyl nitrate, PAN




 i) Photo chemical
smog causes irritation to eyes, skin and lungs, increase in chances of asthma.


Rubber materials has strong affinity for ozone and  is cracked and aged by smog.


PAN is toxic to plants, attacks younger leaves and cause bronzing and
glazing  of their surfaces


of Photo chemical smog

(i) The formation of photochemical smog can be suppressed by preventing
the release of nitrogen oxides and hydrocarbons in the atmosphere from the motor
vehicles by using catalytic convertors
in engines.

 (ii) It is also suppressed by spraying
compounds like free radical traps in the atmosphere. They generate free radicals
which readily combine with free radicals pre cursors of photo chemical smog.

(iii)Plantation of
certain trees like Pinus, Pyrus, Querus Vitus and juniparus can metabolise
nitrogen oxide.