Samrat Prithviraj Chauhan Government College
Department Of Chemistry
Photochemical Air Pollution
Submitted by Rohini Narwal
M.Sc. Chemistry Semester 3rd
TABLE OF CONTENTS
Formation of Photochemical smog
Chemistry of Photochemical Smog Formation
Major sources of Photochemical smog
Effect of Photochemical Air Pollution
Control of Photochemical Air Pollution
Characteristics London smog
(i.e. Sulfurous smog )
1. Place of first notice London (1952)
2. Primary pollutants SO₂ + suspended particulates
3. Secondary pollutants H₂SO₄ + sulfonic acid + sulfate
4. Principal sources of primary
Combustion of coal
5. Red-ox behaviour Reducing
6. Relative humidity and
High humidity, cold condition
7. Time of occurrence Winter (specially in the early morning
8. Effect on humans Lung and throat irritation
Los Angeles smog
(i.e. Photochemical smog )
Los Angeles, USA (1944)
Hydrocarbons + NO
Ozone + PAN + peroxy organic
compounds + aldehydes + HNO₃
Combustion of motor vehicle fuels
Low humidity, usually hot and dry
Photochemical smog was first detected through plant
damage in Los Angeles in 1944 and a chemist named Arie
Jan Haagen-Smit identified two major culprits responsible
for photochemical smog are:
1. VOCs (Volatile Organic Compounds)
2. Nitrogen Oxides (NOₓ)
He was the first to recognize that ozone was the primary
source of the haze. Ozone is created when partially
unburned exhaust from automobiles and the
hydrocarbons from oil refineries are hit by sunlight.
Haagen-Smit also demonstrated that the ozone was the
cause of the bleach smell L.A. residents were reporting, as
well as the source of their eye irritation and respiratory
Photochemical smog is a condition that develops when
1. Primary pollutants: Oxides of nitrogen and volatile organic compounds created from fossil
fuel combustion .
2. Secondary pollutants: These primary pollutants interact under the influence of sunlight to
produce a mixture of hundreds of different and hazardous chemical known as secondary
pollutants namely ozone, peroxyacetyl nitrate(PAN).
FORMATION OF PHOTOCHEMICAL SMOG
CHEMISTRY OF PHOTOCHEMICAL SMOG FORMATION
Photochemical smog cycles involving NO,NO₂, and O₃:
1. NO is produced in automobile engine and NO reacts with atmospheric oxygen to produce NO₂.
2. The produced NO₂ experiences photochemical dissociation to produce the reactive species NO
and atomic oxygen(O).
3. The rate of photo dissociation of NO is fairly high and this reaction triggers the production of O₃
in lower atmosphere.
4. Ozone formed can react with NO to regenerate NO₂ which can again participate in photo
N₂ + O₂ 2NO (engine cylinder)
2NO + O₂ 2NO₂
NO₂ + hv (<400nm) NO + O
O₂ + O O₃
O₃ + NO NO₂ + O₂(titration reaction)
Participation of O₃ and NO₂ in the reaction with hydrocarbon:
RCH₂O• + NO₂
HO₂• + RCHO
O₃ and NO₂ produced in the reactions are quite
oxidising and they can oxidise the hydrocarbons
to produce toxic substances like aldehydes,
peroxyacetyl nitrate (PAN) and peroxybenzoyl
(PAN for R=CH₃
PBzN for R=C₆H₅)
Given figure gives the relative concentration profile of NO,NO₂ and O₃ at different
times of a day:
a. NO is a primary pollutant that is produced by automobiles during rush hour.
b. The NO is oxidised to NO₂ which makes the NO₂ levels up.
c. When the sun goes up the NO₂ is converted to O and NO so it goes downs. This is due to
the photochemical reaction.
d. The O is eventually to O₃ (ozone) which goes up.
e. VOCs are primary pollutants such as RH and RCH₃. They are converted to aldehydes
(RCHO) which in turn are converted to PAN. This is why these goes up when the VOC
MAJOR SOURCES OF PHOTOCHEMICAL SMOG
Nitrogen oxides: They are produced
mainly from the combustion of fossil fuels,
particularly in power stations and motor
Volatile Organic Compounds: VOCs are
formed from the incomplete combustion of
fossil fuels, from the evaporation of
solvents and fuels, and from burning plant
matter—such as backyard burning and
Nitrogen Oxides: In nature, bushfires,
lightning and the microbial processes that
occur in soil generate nitrogen oxides.
Volatile Organic Compounds: VOCs
are produced from the evaporation of
naturally-occurring compounds, such as
terpenes, which are the hydrocarbons in
oils that make them burn. Eucalypts have
also been found to release significant
amounts of these compounds. Pictures of bushfires in Australia
EFFECT OF PHOTOCHEMICAL AIR POLLUTION
Ozone attack can lead to pulmonary edema, headache, chest
discomfort, coughing, bronchian constriction, irritation to the
respiratory mucus system etc.
PAN and PBzN are the eye irritants. They are quite strong
oxidising agents and they can oxidise the –SH groups(sulfhydryl
groups) of different proteins, skin cancer.
NOₓ can cause irritation on nose, eye and pulmonary tracts. It can
affect enzymes including catalyse. Antioxidant vitamin E is its
antidote. NOₓ can block the haemoglobin and many other heme
The aldehydic compounds cause eye irritation, dry and sore
throats. They undergo condensation with different biomolecules
Ozone promotes excessive transpiration from the leaves of plants and
consequently, the plants experience dehydration. Ozone can also cause bronzing,
chlorosis and coloured flecks on the leaves. Overall the plants growth is retarded
severely in presence of O₃.
PAN can damage the plants by attacking the younger leaves causing bronzing and
glazing of their surfaces. PAN can probably prevent Hill reaction of photosynthesis.
The –SH groups of plants proteins are also attacked by PAN by its oxidising activity
and acetylating activity.
NOₓ is toxic to plant at higher concentration. It can cause chlorosis and bleaching in
leaves. It can induce leaf-drop.
Ozone can damage several organic materials including textiles, fibres, dyes,
paints, polymers, etc. Ozone attack on natural rubber or synthetic polymers can
reduce their mechanical strength. Ozone can attack the carbon-carbon double
bond and ultimately the carbon-carbon bond experiences the oxidative
cleavage. It causes the cracking of rubber and damages the polymers and
makes rubber brittle.
Carbon-carbon double bond attacked by ozone
CONTROL OF PHOTOCHEMICAL AIR POLLUTION
The most effective way of reducing the amount of secondary pollutants created in
the air is to reduce emissions of both primary pollutants.
Use of catalytic converters in the Automobile Engines: These will remove the
key players NOₓ and unburned hydrocarbons from the exhaust. These will also
remove the toxic substance CO. 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.
Chemical method: Some nontoxic compounds e.g. diethyl hydroxyl amine can be
sprayed to trap the radical precursors of photochemical smog but this method is
yet limited in laboratory scale.
Take care of cars. Getting regular tune-ups, changing oil on schedule, and inflating
tires to the proper level can improve gas mileage and reduce emissions.
Avoid products that release high levels of VOCs. For example, use low VOC paints.
Chemistry of Atmosphere
Book By P.S. Sindhu
Environmental Chemistry with Green Chemistry
Book By Asim K.Das
Colbeck, I & Mackenzie, AR 1994, Air pollution by
photochemical oxidants, Air Quality Monographs Vol 1, Elsevier,
White, V 1998, Air emissions inventory for the Adelaide airshed
1995, Environment Protection Agency, South Australia.