Photochemical smog is a type of air pollution formed when sunlight interacts with vehicle emissions and other pollutants in the atmosphere. It was first observed in Los Angeles in the 1940s and results from complex chemical reactions involving hydrocarbons, nitrogen oxides, and sunlight that produce secondary pollutants like ozone, aldehydes, and ketones. Photochemical smog can cause serious health effects like lung and heart problems as well as damage plants and materials. It is controlled by reducing vehicle and industrial emissions through catalytic converters, using alternative fuels, and promoting practices that lessen air pollution.

1. DONE BY: MAHASWARI JOGIA
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2. Photochemical Smog is an air pollution, formed when photons of sunlight
hit molecules of different kinds of pollutants in the atmosphere.
It was first observed in Los Angeles, U.S.A in the mid 1940’s and since then
it is detected in many metropolitan cities in the world.
Conditions for formation of photochemical smog are air stagnation,
abundant sunlight, high concentration of hydro carbons and nitrogen oxides.
In India cities such as Mumbai and Delhi are ideal candidates for the
formation of photochemical smog.
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3. Smog arises from photochemical reactions in the lower atmosphere by
the interaction of hydro carbon and nitrogen oxide released by exhausts of
automobiles and some stationery sources.
This interaction results in a series of complex reactions producing
secondary pollutants such as ozone, aldehyde, ketones, peroxyacyl nitrates.
Basically this mechanisms are complex and not fully understood.
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4. FORMATION OF PHOTOCHEMICAL SMOG
When fossil fuels are burnt, a variety of pollutants are emitted into the earth’s
troposphere. Two of the pollutants that are emitted are hydrocarbons (unburnt
fuels) and nitric oxide (NO). When these pollutants build up more than
sufficient, a chain reaction occurs from the interaction with sunlight in which
NO is converted into nitrogen dioxide (NO2). This NO2 in turn absorbs energy
from sunlight and breaks up into nitric oxide and free oxygen atom
NO2(g) + hv NO(g) + O(g) (i)
Oxygen atoms are very reactive and combine with the O2 in air to produce
ozone.
O(g) + O2 (g) O3 (g) (ii)
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5. The ozone formed in the above reaction (ii) reacts rapidly with the NO(g)
formed in the reaction (i) to regenerate NO2. NO2 is a brown gas and at
sufficiently high levels can contribute to haze.
NO (g) + O3 (g) NO2 (g) + O2 (g) (iii)
Ozone is a toxic gas and both NO2 and O3 are strong oxidizing agents and can
react with the unburnt hydrocarbons in the polluted air to produce chemicals
such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN).
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7. SOURCES FOR PHOTOCHEMICAL SMOG
vehicles emitting smoke
industrial emitting smoke
volatile organic compound released from paints and adhesives
power plants
unburnt hydro carbons
trees which emit isoprene and terpene
PAN are secondary pollutants formed from peroxyacid radicals and
NO2 .
CH3CHO + OH• CH3C•O + H2O
CH3C•O + O2 CH3C(O)OO • (acetylperoxy)
CH3C(O)OO• + •NO2 CH3C(O)OONO2 (PAN)
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8. FIGURE: SOURCES OF PHOTO CHEMICAL SMOG
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9. EFFECTS OF PHOTOCHEMICAL SMOG
 Photochemical smog causes serious health problems. Both ozone and
PAN act as powerful eye irritants. Ozone and nitric oxide irritate the nose
and throat and their high concentration causes headache, chest pain,
dryness of the throat, cough and difficulty in breathing.
 Photochemical smog leads to cracking of rubber and extensive damage
to plant life. It also causes corrosion of metals, stones, building materials,
rubber and painted surfaces.
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10.  It can cause anything from minor pain to deadly diseases such as lung
cancer ,wheezing . Smog slowly ruins people's lungs to an extent as great as
that of cigarettes.
It causes plant disease smog-injury cells on the upper part of the leaf
collapses, the leaf looks water soaked.
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HOW CAN PHOTOCHEMICAL SMOG BE
CONTROLLED?
The main method of lowering the levels of nitrogen oxides is by a process
called ‘catalytic reduction’, which is used in industry and in motor vehicles. A
catalytic converter fitted to a car’s exhaust system will convert much of the
nitric oxide from the engine exhaust gases to nitrogen and oxygen.
Certain plants such as ;pinus, pyrus, can metabolize NO and therefore their
plantation could help control the formation of photochemical smog.
Reduction of VOCs from motor vehicles. These include the use of liquefied
petroleum gas or compressed natural gas rather than petrol, and
implementing various engine and emission controls now being developed by
manufacturers.

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If renovating or building, use energy-efficient designs and materials.
Make it your goal to purchase 'green power'—power generated from
clean, renewable energy sources.
Keep your motor vehicle regularly serviced and the tyres inflated to the
manufacturer's specifications. This will ensure the car is running efficiently
and not emitting excessive pollutants.
Instead of using a car, try riding a bike or walking, and use buses, trams or
trains whenever you can.

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REFERENCES:
1. ENVIRONMENT POLLUTION AND CONTROL ENGINEERING BY
C.S. RAO
2. NCERT CHEMISTRY BOOK UNIT 14 “ENVIRONMENTAL
CHEMISTRY”
3. EPA 90/04 ‘Photochemical Smog’ MARCH 2004 PUBLICATION.
4. POWER POINT PRESENTATION BY By,K.Suganthi Prasana ON
‘PHOTOCHEMICAL SMOG’

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THANK YOU!