30 photochemical smog
TRANSCRIPT
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What do you
think why such a
difference is
there?
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L-30 UNIT-VI
Photochemical smog
Air Pollution and Control(Elective - I)
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Unit- VIL-29 Chemistry of air pollution, Chain
reactions of hydrocarbons, nitrogen
oxide, Sulphuric oxides and
intermediates, photochemical smog
formationL-30 Air pollution indices -aerosols,
fog, smog index.
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Smog
The word smog comes from two words:
smoke and fog. Characterize visible combination of smoke and
fog
Occurs in highly motorised areas and whereinversion conditions prevail in the
atmosphere.
e.g. Los Angeles
It is because of action of sunlight on H-C
(Hydrocarbons) and Nitrogen Oxides (NOx)
emitted from the automobiles
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Smog FormationSun
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Composition of smog Nitrogen oxides
Ozone
Aldehydes
Unreacted carbon
Particular matter Peroxyaceyl nitrate (PAN)
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Chemical compounds involved
Molecule Name
and Formula
Two-Dimensional
Representation
Three-
Dimensional
Representation
Nitric Oxide, NO
Nitrogen Dioxide,
NO2
Ozone, O3
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Acetaldehyde,
C2H4O (an example of
an Aldehydes)
Peroxyacetyl Nitrate,
C2H3O5N
1,3-Butadiene,
C4H6 (an example of a
volatile organic
hydrocarbon)
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smog:smog: variation in day
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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 nitrogendioxide, increasing its concentration.
As the sunlight becomes more intense later in
the day, nitrogen dioxide is broken down andits by-products form increasing
concentrations of ozone.
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At the same time, some of the nitrogen
dioxide can react with the volatile organiccompounds to produce toxic chemicals such
as PAN.
As the sun goes down, the production ofozone is halted. The ozone that remains in
the atmosphere is then consumed by several
different reactions.
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Colour 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
appearance.
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Photochemical smog / LA smogLos Angeles
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Photochemical smog / LA smog
LA
Santiago
Las Vegas
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Reaction with NO2 UV light energy is absorbed by NO It moves into high energy state and becomes
unstable.
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(X).
If nitric oxide is present it reacts with ozone to form
NO2 and O2
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Chemical reactions :-
NO2 + hv NO* NO + O
O + O2
O3
O3 + NO NO2 +O2
NO2 behaves like catalyst
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Aldehydes
Peroxyacyl radical
Acetyl radical
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Acetaldehyde
Methyl
Formyl radical
Acetone
Methyl
Acetyl
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Factors affecting photochemical
reactions Light intensity
Hydrocarbon reactivity.
Ratio of H-C to nitric oxide
Presence of light absorbers
Meteorological variables
Height and intensity of atmospheric
inversion
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Effects on human health
Headaches
Eyes, nose and throat irritations Impaired lung function
Coughing and wheezing
smog: effectssmog: effects
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PANs cause eyes to water and respiratory
problems, also catalyses formation ofsulphuric acid,
ozone in the troposphere is: an eye irritant and also irritates respiratory system
causes deterioration of rubber as it breaks C=C-and bleaches dyes
affects leaves and therefore growth/damages crops
reduced visibility secondary pollutants are toxic to plants
corrosion of building materials by acids
and ozone.
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Objective Questions1. _________________ is main driving factor
for photochemical smog formation.2. NO2 acts as ________________.
3. Increase in ozone concentration during day
time indicates _____________________
formation.
4. Cracking of rubber takes place because of______.
5. Photochemical smog occurs in highly
area.
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Theory QuestionsQ1. Write detailed short note on Photo-chemistry of
air pollution
OR
Explain Photochemical reactions in detail.
Q2. Explain effects of photochemical smog.
Vid Cli 1
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Video Clip-1
Vid Cli 2
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Video Clip-2