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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