What do you
think why such a
Air Pollution and Control
(Elective - I)
L-29 Chemistry of air pollution, Chain
reactions of hydrocarbons, nitrogen
oxide, Sulphuric oxides and
intermediates, photochemical smog
L-30 Air pollution indices -aerosols,
fog, smog index.
The word ‘smog’ comes from two words:
smoke and fog.
Characterize visible combination of smoke and
Occurs in highly motorised areas and where
inversion conditions prevail in the
e.g. – Los Angeles
It is because of action of sunlight on H-C
(Hydrocarbons) and Nitrogen Oxides (NOx)
emitted from the automobiles
Early morning traffic increases the emissions
of both nitrogen oxides and VOCs as people
drive to work.
Later in the morning, traffic dies down and
the nitrogen oxides and volatile organic
compounds begin to be react forming nitrogen
dioxide, increasing its concentration.
As the sunlight becomes more intense later in
the day, nitrogen dioxide is broken down and
its by-products form increasing
concentrations of ozone.
At the same time, some of the nitrogen
dioxide can react with the volatile organic
compounds to produce toxic chemicals such
As the sun goes down, the production of
ozone is halted. The ozone that remains in
the atmosphere is then consumed by several
Nitrogen dioxide is responsible for the brownish
colour of the haze.
The particulates formed and the brown colour of
nitrogen dioxide give the air a dirty, ‘smoggy’
Photochemical smog / LA smog
Reaction with NO2
UV light energy is absorbed by NO
It moves into high energy state and becomes
Then decomposes in to NO and O (nascent oxygen)
Nascent oxygen quickly reacts with atmospheric
oxygen to form ozone.
This reaction takes place in presence of third body
If nitric oxide is present it reacts with ozone to form
NO2 and O2
Chemical reactions :NO2 + hv
O + O2
O3 + NO
NO2 behaves like catalyst
NO + O
Factors affecting photochemical
Ratio of H-C to nitric oxide
Presence of light absorbers
Height and intensity of atmospheric
Effects on human health
Eyes, nose and throat irritations
Impaired lung function
Coughing and wheezing
PAN’s cause eyes to water and respiratory
problems, also catalyses formation of
ozone in the troposphere is:
an eye irritant and also irritates respiratory system
causes deterioration of rubber as it breaks –C=Cand bleaches dyes
affects leaves and therefore growth/damages crops
secondary pollutants are toxic to plants
corrosion of building materials by acids
_________________ is main driving factor
for photochemical smog formation.
NO2 acts as ________________.
Increase in ozone concentration during day
time indicates _____________________
Cracking of rubber takes place because of
Photochemical smog occurs in highly
Q1. Write detailed short note on ‘Photo-chemistry of
Explain ‘Photochemical reactions’ in detail.
Q2. Explain effects of photochemical smog.