prepared by Fateh Ali Bajwa

1. Environmental Chemistry
23.1
CHEMISTRY OF TROPOSPHERE AND STRATOSPHERE

2. Introduction to environmental chemistry;
 Environmental chemistry deals with the chemicals and other pollutants in the
environment. In this we study the sources, reactions, transportation of the
chemicals and other toxic substances especially created by human activity in the
environment and their adverse effects on human beings. This branch of chemistry
is interrelated with all other branches of science, i.e. biology, physics, medicine,
agriculture, public health and sanitary engineering, etc.

3. 23.1.1 Describe the various chemical reactions occurring in the
atmosphere(w.r.t formation of acid rain, ozone, ammonium
nitrates and sulphates and carbon dioxide);
 Formation of acid rain: Acid rain which now-a-days is termed as acid
deposition, was first discovered by Angus Smith in Great Britain in the mid
seventeenth century but this phenomenon gained importance as a serious
environmental problem in 1950’s. Initially it was referred to the precipitation which
was more acidic than natural rain. Due to the presence of CO2 in the atmosphere
the natural rain itself forms carbonic acid:
CO2(g) + H2O(aq) H2CO3(aq)
The pH of unpolluted rain water should be 5.6. The rain water has pH less than 5 is
considered truly acidic.

4. In the atmosphere SOz(oxides of sulphur) and Nox(oxides of nitrogen) are
transformed by reactions with oxygen and water into H2SO4 and HNO3, respectively.
These acids get mixed with rain. The acid deposition includes both wet(rain, snow,
fog) and dry acidic deposition.
Sulphur dioxide(SO2) dissolves in water to form sulphurous acid (H2SO3).
Sulphur dioxide + water sulphurous acid
SO2(g) + H2O(aq) H3SO3(aq)
In the presence of oxygen in the air, this acid is slowly oxidized to sulphuric acid
(H2SO4).i.e.
SO2 + ½ O2 + H2O (hydrocarbon,smoke,metal oxides) H2SO4

5. In the presence of oxygen and water, nitrogen dioxide is converted to nitric acid.
4NO2(aq) + 2H2O(aq) + O2(g) 4HNO3
Acid rain has a pH value of 4 or less. In some countries due to release of HCl by
volcanic eruption there is temporary acid rain.

6.  Formation Of Ozone: Ozone is a chemical with the symbol O3. This means
one molecule of ozone is made of 3 oxygen atoms. Ozone is also called trioxygen.
Ozone is formed from dioxygen by the action of ultraviolet light and
also atmospheric electrical discharges. It is present in low concentrations
throughout the Earth's atmosphere. In total, ozone makes up only 0.6 ppm of the
atmosphere.
Ozone is important to planet Earth. There is a portion of the stratosphere with a
high concentration of ozone, called the ozone layer. The ozone layer filters out
damaging ultraviolet radiation from the Sun, like a kind of sun screen. Without this
ozone layer things would not have been able to live on the surface of our planet.
.

7. However, ozone is toxic to animals and plants above concentrations of about 0.1
ppm. In humans, it can cause nasal and throat irritation, and nausea. Extended
exposure can cause lung oedema. 0.100 ppm is the maximum allowable limit for
industrial, public, or occupied spaces in England, Japan, France,
the Netherlands and Germany.
As described before, ozone is produced by the photochemical reaction between
oxygen molecules and oxygen atoms in the atmosphere, therefore:
O(g) + O2(g) O3(g)
Since 1976, there has been an alarming decrease in the amount of ozone in the
stratosphere over the south pole and recently in the north pole as well.

8. What is causing the depletion of ozone in the stratosphere?
Scientists have discovered that the depletion of the ozone layer is caused by the
chlorofluorocarbons. Chlorofluorocarbons, commonly called CFCs, are compounds
containing the elements carbon, fluorine and chlorine. CFCs were widely used as
propellants for aerosols and as coolants in refrigerators and air conditioners. They were
also used in the manufacture of packing foam. Chlorofluorocarbons (CFCs) play an
effective role in removing O3 in the stratosphere due to following reactions.
a) CFCl3 CFCl2 + Cl. b)Cl. + O3 ClO. + O2
c) ClO. + O Cl. + O2

9. Formation of Ammonium sulphates and nitrates: a) Formation of ammonium
sulphate: Soluble atmospheric particulates give rise to reduced visibility and thermal
radiation while airborne and may give rise to material damage and reduced soil
fertility upon deposition through rain water wash out. As ammonium sulphate has
been shown to account for 60 % or more of soluble atmospheric particulates in a
number of places in the UK, the mechanism of its formation is of fundamental
importance for both determining its source and also to suggest possible schemes for
its control. The emission of sulphur dioxide from the combustion of most fossil fuels
has led to many studies of possible routes for its oxidation to sulphate in the
presence of ammonia or an ammonium ion.

10. Aerosol formation by the photo-oxidation of sulphur dioxide in air has been the
subject of a considerable number of studies. The results have suggested that sulphur
trioxide is formed as an intermediary and then rapidly hydrated to molecular H2SO4.
SO2 SO3 ( Photo-oxidation in air)
SO3 + H2O H2SO4
The presence of ammonia in the atmosphere then leads to the formation of
ammonium sulphate.
It has been reported that the rate of photo-oxidation of sulphur dioxide is
increased by other atmospheric trace gases such as ammonia, olefins and

11. nitrogen oxides. However, in the absence of ultra-violet radiation other schemes have
to be found for sulphate formation.
However, in the presence of ammonia the formation of sulphuric acid is catalyzed
as follows:
NH3 + H2O NH4OH NH4
+ + OH- (Partial)
This reaction catalyzes the aqueous phase oxidation as follows:
2SO2 + O2 2SO3
SO3 + H2O H2SO4 2H+ + SO4
2- (Complete)

12. The respective ions thus produced combine together to give ammonium sulphate
while hydrogen and hydroxide ions neutralize themselves forming water. Thus we can
summarize the formation of ammonium sulphate in the environment as follows:
NH3 + SO2 NH3 . SO2 ( In the presence of water)
NH3.SO4 + H2O (NH4)2SO4
NH3.SO4 is an intermediate which immediately converts into ammonium sulphate.
SOURCE: http://www.spq.pt/magazines/RPQ/286/article/780/pdf

13. b) Formation of ammonium nitrate: Ammonium Nitrate forms in the atmosphere by
the reaction of ammonia and nitrogen oxides. Both sources can be generated
naturally, for example biological decay producing ammonia and lightening producing
nitrogen oxides, but human activities are more common sources.
Common human sources are ammonia from agricultural spraying and nitrogen oxides
from the combustion of fossil fuels.
Ammonia ions are more commonly associated with sulfate ions in most places, but
for example in California sulfates are relatively lower and nitrates more common.
Ammonium Nitrate may be removed from the atmosphere from solution in rain or by
dust deposition. Ammonium Nitrate may aggregate on salt particles.

14.  Formation Of Carbon Dioxide: Carbon dioxide in Earth's atmosphere is a trace
gas, currently (early 2015) having an average concentration of 400 parts per
million by volume (or 591 parts per million by mass). Atmospheric concentrations
of carbon dioxide fluctuate slightly with the change of the seasons. Concentrations
fall during the Northern Hemisphere spring and summer as plants consume the
gas, and rise during the northern autumn and winter as plants go dormant or die
and decay. Concentrations also vary on a regional basis, most strongly near the
ground with much smaller variations aloft. In urban areas concentrations are
generally higher and indoors they can reach 10 times background levels.

15. Combustion of fossil fuels and deforestation have caused the atmospheric
concentration of carbon dioxide to increase by about 43% since the beginning of
the age of industrialization. Most carbon dioxide from human activities is released
from burning coal and other fossil fuels. Other human activities, including
deforestation, biomass burning, and cement production also produce carbon dioxide.
Volcanoes emit between 0.2 and 0.3 billion tons of carbon dioxide per year,
compared to about 29 billion tons of carbon dioxide per year emitted by human
activities. Up to 40% of the gas emitted by some volcanoes during sub-aerial
eruptions is carbon dioxide. Generally combustion yields carbon dioxide and water
like in the following reaction:
CH4 + 2O2 CO2 + H2O

16. 23.1.2 Discuss the release of oxides of C,S,N and VOCs
which are associated with combustion of hydrocarbon
based fuel:
 Oxides of Carbon( CO & CO2): Carbon Monoxide is a colorless, odorless and
highly toxic gas. It is three times lighter than air. It is soluble in water.
Fuel burning in various types of transportation i.e., motor vehicles, railways and
aircraft is the major source(75%) of carbon monoxide in the atmosphere. Other
sources of carbon monoxide emission are forest fires, combustion of fossil fuel and
agricultural products. Carbon monoxide is also emitted from industries in which any
type of fuel is burnt in air. These industries include iron and steel, petroleum, cement,
brick kilns, paper and pulp, etc. Incomplete combustion and dissociation of CO2 at
high temperature also produces CO.
Carbon monoxide is highly poisonous gas and causes suffocation if inhaled. It
binds blood haemoglobin more strongly than oxygen thus excluding oxygen

17. from normal respiration. The CO poisoning can be reversed by giving high pressure
oxygen. Exposure to high concentration of CO results in headache, fatigue,
unconsciousness and eventually death( if such exposure is sustained for longer
period).
Carbon dioxide is a very unique gas, it cannot be termed as toxic but higher
amounts leads to global warming and without it life on earth would have been
impossible as it serves to absorb the infrared radiations emitted by Earth thus
warming it and keeping the Earth’s temperature to a constant. However in high
concentration it super warms the Earth and resulting in global warming.
Carbon Dioxide is released both by natural and artificial or anthropogenic

18. means but the amount of CO2 released due to human activities is far more than that
obtained from the natural sources which is causing an imbalance in CO2 levels in the
atmosphere and thus causing serious implications.
Carbon dioxide is added to the atmosphere by human activities. When
hydrocarbon fuels (i.e. wood, coal, natural gas, gasoline, and oil) are burned, carbon
dioxide is released. During combustion or burning, carbon from fossil fuels combine
with oxygen in the air to form carbon dioxide and water vapor.
These natural hydrocarbon fuels come from once-living organisms and are made
from carbon and hydrogen, which release carbon dioxide and water when they burn.

19. Not only does the burning of forests release carbon dioxide, but deforestation can
also affects the level of carbon dioxide. Trees reduce the amount of carbon dioxide
from the atmosphere during the process of photosynthesis, so fewer trees means
more carbon dioxide left in the atmosphere.

20.  Oxides of Sulphur (SOz): Air is polluted with SO2 due to combustion of coal (
containing 1-9%S), crude oil and other fossil fuel in power plants and petroleum
industry, etc.
a) S + O2 SO2 b) 2SO2 + O2 3SO3
These gases (SO2 & SO3) because of their pungent odor are very irritant and
suffocating. Through various reactions in the atmosphere they form sulphate aerosols.
These aerosols cause severe respiratory troubles particularly among older people.
Sulphur dioxide is the major source of acid deposition in the atmosphere.

21.  Oxides of Nitrogen (Nox): Oxides of nitrogen are produced by combustion of all
fossil fuels including coal- and gas-fired power stations and motor vehicles.
Whereas fuel itself can produce some nitrogen (for example, oil and coal contain
around 0.5 – 1.5% of nitrogen, and natural gas contains less than that , most of
nitrogen oxides' production comes from the reaction of atmospheric nitrogen and
oxygen within the combustion chamber.
The two main nitrogen oxides are nitric oxide (NO), or nitrogen monoxide, and
nitrogen dioxide (NO2) the sum of which is equal to NOx. Nitric oxide (NO) is a
colorless gas. Nitrogen dioxide (NO2) is a gas of reddish-brown color with a distinct
sharp, biting odor.

22. Combustion of fuels always produces both NO2 and NO. But almost 90% of the
NOX combustion product is in the form of NO which is then oxidized to nitrogen
dioxide (NO2) in the air.
Therefore, only a small percentage of NO2 found in the atmosphere is directly
emitted there in this form. The rest has been formed as a result of chemical reactions
in the atmosphere itself.
Road transport (motor vehicles) is by far the largest contributor of nitrogen
emissions. For example, based on 1998 UK figures, road transport contributed nearly
half of all nitrogen emissions, followed by contributions from electric power
generating plants which only contributed around 20% of total nitrogen emissions.

23.  Volatile Organic Compounds (VOCs): Volatile organic compounds (VOCs) are
defined as organic compounds which easily evaporate and enter the atmosphere.
VOCs may include a wide range of organic air pollutants, from pure hydrocarbons
to partially oxidized hydrocarbons to organic compounds containing chlorine, sulfur,
or nitrogen.
Historically, the definition of VOCs did not include methane compounds (non-
methane VOCs: NMVOCs) since the atmospheric concentration of methane was
considered to be a stable natural background. But it was ultimately recognized that
methane is also an anthropogenic air pollutant that comes from intensive animal and
rice production.

24. Automobiles are the major source of VOCs. In addition to this, petroleum, coal,
wood, incinerators, refuse burning and solvent evaporator also contribute towards the
emission of hydrocarbons into the atmosphere.
Forests are the primary natural sources of VOC emissions. And tropical forests are
estimated to produce about half of all global natural non-methane VOC emissions.
Plants synthesize many organic molecules and release some VOCs (including a
range of terpenes) into the atmosphere. In total, around 1000 different compounds
(with some of which themselves being families with thousands of their own members)
are known to be emitted by natural sources.

25. 23.1.3 Outline problems associated with the release of
pollutants (e.g. acid rain and hazardous inorganic and organic
compounds like PeroxyAcetyl Nitrate;
The toxic air pollutants of greatest concern are those that cause serious health
problems or affect many people. Health problems can include cancer, respiratory
irritation, nervous system problems, and birth defects.
Some health problems occur very soon after a person inhales a toxic air pollutant.
These immediate effects may be minor, such as watery eyes. Or they may be serious,
such as life-threatening lung damage.
Other health problems may not appear until many months or years after a
person's first exposure to the toxic air pollutant. Cancer is one example of a delayed
health problem.

26.  Acid Rain: Problems associated with acid rain are as follows;
i) Acid rain on soil and rocks leaches heavy metals (Al, Hg, Pb, Cr, etc.) with it and
discharges these metals into rivers and lakes. This water is used by human beings for
drinking purpose. These metals accumulate in human body to a toxic level. On the
other hand, aquatic life present in lakes also suffers because of high concentration of
these metals. Especially high concentration of aluminium ions clogs the fish gills. It
causes suffocation and ultimately death of fish.
ii) Acid rain attacks the calcium carbonate present in the marble and limestone of
buildings and monuments. Thus these buildings are getting dull and eroded day by
day.

27. iii) Acid rain increases the acidity of the soil. Many crops and plants cannot grow
properly in such soil. It also increases the toxic metals in the soil that poisons the
vegetation. Even old trees are being affected due to acidity of soil.. Their growth is
retarded. They get dry and die.
iv) Acid rain directly damages the leaves of trees and plants, thus limiting their
growth. Depending upon the severity of the damage, plants growth can be
hampered. Plants capability to resist cold or diseases reduces ultimately death of
plant occurs.

28.  Oxides of Carbon (CO & CO2): As described before, CO2 is not a toxic gas
however is essential for photosynthesis in plants and maintaining Earth’s
temperature within a bearable range but if it exceeds the normal limit it causes
global warming which at some stage might be fatal.
On the other hand, CO is a highly toxic gas and exposures to carbon monoxide
may lead to: i)Toxicity of the central nervous system and heart. ii) Severe effects on
the baby of a pregnant woman. iii) Headaches and dizziness. iv) Problems with
getting oxygen supplied to some body parts which may be life-threatening.

29.  Oxides of Sulphur (SOz): Sulphur trioxide, SO3, is formed by the oxidation of sulphur
dioxide, SO2, and rapidly converts into sulphuric acid, H2SO4, by reacting with water.
The sulphuric acid formed along with nitric acid cause acid rain.
However, SO2 remains in the atmosphere in relatively larger amounts and it can cause
various problems, like; Sulphur dioxide can harm crops and trees, textiles, building
materials, animals, and people either as a result of exposure to long-term low
concentrations or short-term high concentrations. It turns leaves yellow and decreases the
growth rate of crops. Sulphur dioxide corrodes metal, and causes building materials and
textiles to deteriorate and weaken. Sulphur dioxide irritates the throat and lungs and, if
there are fine dust particles in the air, can damage a person's respiratory system. Sulphur
oxides combine with other substances in the air to produce a haze that reduces visibility.

30.  Oxides of Nitrogen (Nox): NOx causes a wide variety of health and
environmental impacts because of various compounds and derivatives in the
family of nitrogen oxides, including nitrogen dioxide, nitric acid, nitrous oxide,
nitrates, and nitric oxide.
i) Ground-level Ozone (Smog) - is formed when NOx and volatile
organic compounds (VOCs) react in the presence of heat and sunlight. Children,
people with lung diseases such as asthma, and people who work or exercise outside,
are susceptible to adverse effects such as damage to lung tissue and reduction in
lung function. Ozone can be transported by wind currents, and can cause health
impacts far from original sources. Other impacts from ozone include damaged
vegetation and reduced crop yields.

31. ii) Acid Rain - NOx and sulfur dioxide react with other substances in the air to form
acids, which fall to earth as rain, fog, snow or dry particles. Some may be carried by
wind for hundreds of miles. Acid rain damages; causes deterioration of cars, buildings
and historical monuments; and causes lakes and streams to become acidic and
unsuitable for many fish.
iii) Particles - NOx reacts with ammonia, moisture, and other compounds to form nitric
acid and related particles. Human health concerns include effects on breathing and
the respiratory system, damage to lung tissue, and premature death. Small particles
penetrate deeply into sensitive parts of the lungs and can cause or worsen respiratory
disease such as emphysema and bronchitis, and aggravate existing heart disease.

32. iv) Water Quality Deterioration - Increased nitrogen loading in water bodies,
particularly coastal estuaries, upsets the chemical balance of nutrients used by aquatic
plants and animals. Additional nitrogen accelerates “eutrophication,” which leads to
oxygen depletion and reduces fish and shellfish populations.
v) Global Warming - One member of the NOx, nitrous oxide, is a greenhouse gas. It
accumulates in the atmosphere with other greenhouse gasses causing a gradual rise
in the earth's temperature. This will lead to increased risks to human health, a rise in
the sea level, and other adverse changes to plant and animal habitat.
vi) Toxic Chemicals - In the air, NOx reacts readily with common organic chemicals
and even ozone, to form a wide variety of toxic products, some of

33. which may cause biological mutations. Examples of these chemicals include the
nitrate radical, nitroarenes, and nitrosamines.
v) Visibility Impairment - Nitrate particles and nitrogen dioxide can block the
transmission of light, reducing visibility in urban areas and on a regional scale in our
national parks.

34.  Volatile Organic Compounds ( VOCs): VOCs may produce the following effects
:i)Some aromatic compounds such as benzene, toluene and xylene are potential
carcinogens and may cause leukemia, ii) Contribute to sick building
syndrome indoors and iii) As facilitators in ozone formation, VOCs may indirectly
contribute to respiratory problems and other ozone-related problems, etc.
Short-Term (Acute) to high levels of VOCs: Eye, nose and throat irritation,
headaches, nausea / vomiting, dizziness and worsening of asthma symptoms
Long-Term (Chronic) to high levels of VOCs causes increased risk of: Cancer, liver
damage, kidney damage and Central Nervous System damage, etc.

35.  Peroxyacetyl Nitrate (PAN): PAN is one of a class of common air pollutants
formed by the action of sunlight on volatile organic compounds and nitrogen
oxides. The acute toxicity of PAN is less than that of ozone, similar to NO2 and
higher than SO2. Following acute exposure, severe lung lesions and, at the higher
levels, damage to the epithelium of upper parts of the respiratory tract were found
in animals. It seems that concentrations of 1.19-1.49 mg/m3 lie not far from the
threshold required for pulmonary function effects in sensitive individuals.
However, these PAN concentrations are well above the maximum ambient
concentrations usually experienced within the USA and Canada (0.003-0.078
mg/m3). It appears unlikely that present ambient PAN concentrations would affect
pulmonary functions responses to ambient ozone.

36. In human, the lowest level causing eye irritations was 0.64 mg/m3 for 2 h.
Concentrations of 0.99 and 4.95 mg/m3 were identified as no-observed-effect level
(NOEL) and no-observed-adverse-effect level (NOAEL) for pathological and
histological changes in the respiratory system (nasal passages). PAN is a weak point
mutagen or clastogen. The data are not sufficient to evaluate its carcinogenicity.

37. 23.1.4 Describe causes and effects of
Urban Smog;
 What is Smog? Urban smog, commonly known as smog is basically derived
from the merging of two words; smoke and fog. Smog is also used to
describe the type of fog which has smoke or soot in it. Smog is a
yellowish or blackish fog formed mainly by a mixture of pollutants in the
atmosphere which consists of fine particles and ground level ozone.
Smog which occurs mainly because of air pollution, can also be defined
as a mixture of various gases with dust and water vapor. Smog also refers
to hazy air that makes breathing difficult.

38.  How Smog is Formed?
The atmospheric pollutants or gases that form smog are released in the air when
fuels are burnt. When sunlight and its heat react with these gases and fine particles in
the atmosphere, smog is formed. It is purely caused by air pollution. Ground level
ozone and fine particles are released in the air due to complex photochemical
reactions between volatile organic compounds (VOC), sulphur dioxide (SO2) and
nitrogen oxides (NOx). These VOC, SO2 and NOx are called precursors. The main
sources of these precursors are pollutants released directly into the air by gasoline and
diesel-run vehicles, industrial plants and activities, and heating due to human activities.
Smog is often caused by heavy traffic, high temperatures, sunshine and calm
winds. These are few of the factors behind increasing level of air pollution in
atmosphere. During the winter months when the wind speeds are

39. low, it helps the smoke and fog to become stagnate at a place forming smog
and increasing pollution levels near the ground closer to where people are respiring.
It hampers visibility and disturbs the environment. The time that smog takes to form
depends directly on the temperature. Temperature inversions are situations when
warm air does not rise instead stays near the ground. During situations of
temperature inversions if the wind is calm, smog may get trapped and remain over a
place for days.But it is also true that smog is more severe when it occurs farther away
from the sources of release of pollutants. This is because the photo chemical
reactions that cause smog take place in the air when the released pollutants from
heavy traffic drift due to the wind. Smog can thus affect and prove to be dangerous
for suburbs, rural areas as well as urban areas or large cities.

40.  Effects of Smog:
Smog is harmful and it is evident from the components that form it and effects
that can happen from it. It is harmful to humans, animals, plants and the nature as a
whole. Many people deaths were recorded, notably those relating to bronchial
diseases. Heavy smog is responsible for decreasing the UV radiation greatly. Thus
heavy smog results in a low production of the crucial natural element vitamin D
leading to cases of rickets among people.
When a city or town gets covered in smog, the effects are felt immediately. Smog
can be responsible for any ailment from minor pains to deadly pulmonary diseases
such as lung cancer. Smog is well known for causing irritation in the eye. It may also
result in inflammation in the tissues of lungs; giving rise to pain in the chest.

41. Other issues or illnesses such as cold and pneumonia are also related to smog. The
human body faces great difficulty in defending itself against the harmful effects of smog.
Minor exposure to smog can lead to greater threats of asthma attacks; people
suffering from asthma problems must avoid exposure. Smog also causes pre-mature
deaths and affects densely populated areas building it up to dangerous levels. The highly
affected people include old people, kids and those with cardiac and respiratory
complications as they have easy tendency to be at disadvantage of asthma.
The ground level ozone present in the smog also inhibits plant growth and causes
immense damage to crops and forests. Crops, vegetables like soybeans, wheat, tomatoes,
peanuts, and cotton are subject to infection when they are exposed to smog.

42. The smog results in mortifying impacts on environment by killing innumerable animal
species and green life as these take time to adapt to breathing and surviving in such toxic
environments.
Smog is a devastating problem especially due to the fast modernization or
industrialization as the hazardous chemicals involved in smog formation are highly reactive
is spread around in the atmosphere. Smoke and sulphur dioxide pollution in urban areas is
at much lower levels than in the past, as a result of law passed to control emissions and in
favor of cleaner emission technology.
So how should you fight with the forceful impact of smog? It can be reduced by
implementing modifications in your lifestyle, decreasing the consumption of fuels that
are non-renewable and by replacing them with alternate sources of fuel which will reduce
toxic emissions from vehicles.

43. 23.1.5 Describe the roles of CFCs in
destroying ozone in the stratosphere;
 Ozone in the lower atmosphere (Troposphere): About 10% of all ozone (O3) in
the atmosphere is found in the troposphere (up to 16km above the earth's
surface).Ozone in the troposphere has harmful effects on many living things
because it is toxic. In humans, ozone causes eye irritation, compromised lung
functions, aggravation of respiratory conditions like asthma, and increases the
susceptibility to infection. Ozone pollution in the troposphere is often linked
to photochemical smog.
Ozone in the lower atmosphere is formed during electrical discharge from high
voltage appliances as shown in the equations below:
O2(g) → 2O(g)
O2(g) + O(g) → O3(g)

44.  Ozone in the upper atmosphere ( Stratosphere): About 90% of all ozone (O3)
in the atmosphere is found in the stratosphere (16 to 32 km above the earth's
surface).
In the stratosphere ozone acts as the primary UV radiation shield, short
wavelength UV radiation from the Sun (<240nm) provides the energy to split oxygen
molecules into oxygen atoms:
O2(g) → 2O(g) ( UV radiation)
Oxygen atoms then react with oxygen molecules to form ozone:
O(g) + O2(g) → O3(g)

45. Ozone can absorb harmful UV-B and UV-C radiation, preventing it from reaching
the earth's surface:
O3(g) → O2(g) + O(g) ( UV radiation)
The constant formation and destruction of ozone maintains a balance over time.
Human activities, such as the release of chlorofluorocarbons in to the atmosphere,
have disturbed this balance.
 Chlorofluorocarbons (CFCs) and Halons: CFCs and halons belong to
the haloalkanes. Chlorofluorocarbons (CFCs) are compounds containing only
carbon, chlorine and fluorine (no hydrogen).
Halons are compounds containing only carbon, bromine and other halogens
(no hydrogen).

46. Chlorofluorocarbons (CFCs) are sold under the trade name of Freons.
CFCs are used as working fluids in refrigerators and air conditioners because they
are gases at room temperature which can be easily liquefied by compression and
because they are stable and non-toxic.
CFCs are used as foaming agents in the production of polystyrene and
polyurethane foam plastics used for insulation and packing materials.
CFCs are used as a propellant in spray cans for paint, insect repellants, deodorants
Halons are used in fire extinguishers because they are dense, non-flammable
liquids.

47.  Chemistry of Ozone depletion by CFCs: CFCs destroy the ozone in the
stratosphere (15 - 20 km above the earth's surface)
(Ozone concentrations are measure in Dobsen units, 1 Dobsen unit represents 1
molecule of O3 for every 1 billion air molecules)
Ozone loss is greatest over Antarctica where the ozone depletion has been
recorded and is commonly referred to as the "ozone hole".
Ozone (O3), an allotrope of oxygen, is poisonous to humans if breathed in, but is
important to life in that it filters out or absorbs short wavelength ultraviolet radiation
(U.V) in the 280 - 320nm range which can cause serious sunburn, skin cancer and eye
disorders.

48. The inertness and lack of water solubility of CFCs mean they are not destroyed nor
are they dissolved in rain water so they stay in the atmosphere for a very long time
and diffuse up to the stratosphere
In the stratosphere, CFCs come into contact with short wavelength ultraviolet
radiation which is able to split off chlorine atoms from the CFC molecules
CCl3F(g) → CCl2F(g) + Cl(g)
These chlorine atoms destroy the ozone layer
Cl(g) + O3(g) → ClO(g) + O2(g)

49. There are significant numbers of oxygen atoms in the stratosphere (since ozone
undergoes a natural photochemical decomposition producing oxygen atoms and
molecules) which leads to the regeneration of chlorine atoms in the stratosphere. So,
1 CFC molecule can destroy many ozone molecules.
ClO(g) + O(g) → O2(g) + Cl(g)

50. 23.1.6 List possible alternatives to the use
of CFCs;
 Substitutes for CFCs: The only long term solution to solve the problem of
depletion of the ozone layer is to phase out the use of CFCs
(Montreal Protocol of 1987 and subsequent modifications)
Some CFCs can be replaced by HCFCs (hydrochlorofluorocarbons), compounds
containing at least 1 H atom.
The C-H bond makes these compounds more reactive in the atmosphere so they are
destroyed more quickly and so are less able to diffuse into the stratosphere.
See the table on the next slide;

51. Name Formula Code Uses
chlorodifluoromethane CHClF2 HCFC-22
air conditioning,
refrigeration,
foams
1-chloro-1,1-difluoroethane CClF2CH3 HCFC-142b Aerosols
1,1-difluoroethane CHF2CH3 HCFC-152a
aerosols,
refrigeration

52. 23.1.7 Explain Greenhouse effect and Global Warming
as resulting in climate change;
The greenhouse effect occurs naturally, providing a habitable climate. Atmospheric
concentrations of some of the gases that produce the greenhouse effect are increasing
due to human activity and most of the world's climate scientists consider that this is a
significant part of the cause of observed climate change.
The oceans are a critical part of the climate system, with vastly greater thermal capacity
than the atmosphere. Most of the net energy increase in the climate system in recent
decades is stored in the oceans. A small, slow increase in their temperature is significant.
Over one-third of human-induced greenhouse gas emissions come from the burning of
fossil fuel to generate electricity. Nuclear power plants do not emit these gases.

53. The "greenhouse effect" is the term used to describe the retention of heat in the
Earth's lower atmosphere (troposphere) due to concentrations of certain trace gases
and water vapor in the atmosphere. These gases are generally known as greenhouse
gases. Concentrations of some of them have increased steadily during the 20th
century and into the 21st, with CO2 rising from under 300 ppm to 400 ppm. A large
part of the increase in all greenhouse gases is attributed to human sources, i.e. it is
anthropogenic, hence the term ‘anthropogenic global warming’ (AGW).
Furthermore, although most sources of anthropogenic emissions can be identified
in particular countries, their effect is in no way confined to those countries – it is
global.

54.  The Greenhouse Effect
The greenhouse effect itself occurs when short-wave solar radiation (which is not
impeded by the greenhouse gases) heats the surface of the Earth, and the energy is
radiated back through the Earth's atmosphere as heat, with a longer wavelength. In
the wavelengths 5-30µm a lot of this thermal radiation is absorbed by water
vapor and carbon dioxide, which in turn radiate it, thus heating the atmosphere and
land and ocean surface. This is natural and what keeps the Earth habitable. Without
the greenhouse effect overnight temperatures would fall and the average surface
temperature would be about minus 18°C, about the same as on the moon, which
lacks the blanket of our atmosphere. We owe the difference of some 33°C
substantially to natural levels of water vapor (60%, or more including clouds) and
carbon dioxide in the Earth’s atmosphere.

55. In respect to enhancing the greenhouse effect, or the likelihood of AGW, the
particular issue is focused in the 8-18µm band where water vapor is a weak absorber
of radiation and where the Earth's thermal radiation is greatest. Increased
concentrations of CO2 and other radiative gases here mean that less heat is lost to
space from the Earth's lower atmosphere, and temperatures at the Earth's surface are
therefore likely to increase. Atmosphere and oceans are the focus of attention.
A number of indicators suggest that atmospheric warming due to increased levels
of greenhouse gases is indeed observable since 1970, despite some masking by
aerosols. Global air temperatures do appear to have risen about 0.6oC over the last
century, though this has been irregular rather than steady, and does not correlate well
with the steady increase in greenhouse gas – notably CO2 – concentrations.

56. The amount, extent and rate of this exceeds natural climate variability, some of the
warmest years on record have been in the last decade. However, the climate is a
complex system and other factors influence global temperatures.
One of these is water vapor, and climate models have assumed that the direct
warming effect of CO2 is amplified by water vapor. However, there is doubt about
whether in practice this occurs to the extent previously thought.
The oceans have also warmed slightly, affecting climate.

57.  Global Warming and Climate Change:
There is clear evidence of changes in the composition of the greenhouse gases in
the lower atmosphere, with CO2 in particular steadily increasing to its present level of
about 400 ppm. In May 2013 the daily mean concentration of carbon dioxide in the
atmosphere of Mauna Loa, Hawaii, the primary global benchmark site, surpassed 400
ppm for the first time since measurements began there in 1958. It has increased by
one-third in the last 200 years, and half of that in the last 30 years. Ice core samples
show that both carbon dioxide and methane levels are higher than at any time in the
past 650,000 years – CO2 there being 170-300 ppm.

58.  Estimates of the individual contribution of particular gases to the greenhouse
effect – their Global Warming Potential (GWP), are broadly agreed (relative to
carbon dioxide = 1). Such estimates depend on the physical behavior of each kind
of molecule and its lifetime in the atmosphere, as well as the gas's concentration.
Both direct and indirect effects due to interaction with other gases and radicals
must be taken into account and some of the latter remain uncertain:
 See table on the following slide;

59. Greenhouse gas
Concentration
change, 1800s -
2010
Anthropogenic
sources
Proportion of total effect
apart from water vapor (approximate)
carbon dioxide 280 - 390 ppm
fossil fuel burning,
deforestation
60%
methane 0.75 - 1.75 ppm
agriculture, fuel
leakage
20%
halocarbons 0 - 0.7 ppb refrigerants 14%
nitrous oxide 275 - 310 ppb
agriculture,
combustion
6%
ozone 15? - 20-30 ppb urban pollution

60. Although water vapor has a major influence on absorbing long-wave thermal radiation,
its GWP is not calculated since its concentration in the atmosphere varies widely and
mainly depends on air temperature. Also its residence time is only about nine days,
compared with years for CO2 and methane. It is classed a positive feedback, not a forcing
agent for the troposphere. In the stratosphere, water vapor from methane oxidation and
possibly from aircraft may be a forcing agent, but the former is included in methane’s
GWP.
The Intergovernmental Panel on Climate Change (IPCC) is a scientific body under the
auspices of the UN, set up in 1988 to review and assess scientific and other information on
human contributions to climate change. It was set up as a partnership between the World
Meteorological Organization (WMO) and the UN Environment Program (UNEP) and 195
countries are members. It does not conduct any research nor does it monitor climate-
related data or parameters. Its remit does not focus on natural causes or trends of climate
change. It is based at the WMO in Geneva.