What do you think why such a difference is there?

L-30 UNIT-VI Photochemical smog

Air Pollution and Control

(Elective - I)

Unit- VI

L-29 Chemistry of air pollution, Chain reactions of hydrocarbons, nitrogen oxide, Sulphuric oxides and intermediates, photochemical smog formation

L-30 Air pollution indices -aerosols, fog, smog index.

Smog

The word ‘smog’ comes from two words: smoke and fog.

Characterize visible combination of smoke and fog

Occurs in highly motorised areas and where inversion 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

smogsmog

primary

pollutants

volatile hydrocarbons -VOC

nitrogen oxides

secondary pollutants

ketones,

aldehydes,

PANs,

peroxides,

ozone,

Smog FormationSun

Composition of smog

Nitrogen oxides

Ozone

Aldehydes

Unreacted carbon

Particular matter

Peroxyaceyl nitrate (PAN)

Chemical compounds involved

Molecule Name

and Formula

Two-Dimensional

Representation

Three-

Dimensional

Representation

Nitric Oxide, NO

Nitrogen Dioxide,

NO2

Ozone, O3

Acetaldehyde,

C2H4O (an example of

an Aldehydes)

Peroxyacetyl Nitrate,

C2H3O5N

1,3-Butadiene,

C4H6 (an example of a

volatile organic

hydrocarbon)

smog:smog: variation in day

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 PAN.

As the sun goes down, the production of ozone is halted. The ozone that remains in the atmosphere is then consumed by several different reactions.

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.

18

Photochemical smog / LA smogLos Angeles

Photochemical smog / LA smog

LA

Santiago

Las Vegas

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

Chemical reactions :-

NO2 + hv NO* NO + O

O + O2 O3

O3 + NO NO2 +O2

NO2 behaves like catalyst

Aldehydes

Peroxyacyl radical

Acetyl radical

Acetaldehyde

Methyl

Formyl radical

Acetone

Methyl

Acetyl

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

Effects on human health

Headaches

Eyes, nose and throat irritations

Impaired lung function

Coughing and wheezing

smog: effectssmog: effects

PAN’s cause eyes to water and respiratory problems, also catalyses formation of sulphuric 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.

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.

Theory QuestionsQ1. Write detailed short note on ‘Photo-chemistry of

air pollution’

OR

Explain ‘Photochemical reactions’ in detail.

Q2. Explain effects of photochemical smog.

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