Ozone exists in two layers of the atmosphere - the troposphere and stratosphere. Stratospheric ozone protects life on Earth by absorbing harmful UV radiation from the sun. Chlorofluorocarbons and other ozone depleting substances released chemicals that destroyed stratospheric ozone, causing thinning of the ozone layer. Thinning of the ozone layer allows more UV radiation to reach the Earth's surface, increasing health risks like skin cancer as well as risks to plants, animals, and ecosystems. International agreements like the Montreal Protocol aimed to phase out ozone depleting substances and allow recovery of the ozone layer.

1. 1
ATMOSPHERIC OZONE AND ITS
DEPLETION
by
Prof. A. Balasubramanian
Centre for Advanced Studies in Earth Science
University of Mysore, India

2. 2
Ozone is a form of oxygen:
Ozone (O3) is formed by the combination of
three oxygen atoms. Normal oxygen which we
breathe is colourless and odourless.
Ozone is much less common than normal
oxygen. Out of 10 million air molecules, about
2 million are normal oxygen, but only 3 are
ozone.

3. 3
An unstable gas with a strong and irritating odor
(which explains its name), ozone is corrosive, a
strong oxidant and very toxic.
For all of these reasons it absolutely unsuitable
to sustain life. Ozone is generally produced by
generating high-power electrical discharges in
air or in oxygen.
Naturally found in the upper layers of the
atmosphere.

4. 4
In Liquid phase Ozone is fairly unstable in a
watery solution; its half-life in water is about 20
minutes.
In air, ozone has a half-life of 12 hours, which
makes the stability of ozone in air superior.
Liquid ozone has a deep blue, almost black,
colour, and is opaque in layers exceeding 2 mm.
in thickness.

5. 5
Discovery :
It was first discovered in the 1830s by the
German scientist Christian Schönbein.
He identified a new compound in laboratory
experiments using oxygen, and named the
molecule “ozein,” meaning “to smell” in Greek.
In 1881, John Hartley experimented with ozone
and found that it was strongly absorbing the
ultraviolet light.

6. 6
The oxygen we breathe is in the form of oxygen
molecules (O2) - two atoms of oxygen bound
together.
Physical Properties of Ozone:
Ozone absorbs radiation strongly in the
ultraviolet region of the atmospheric spectrum
between 220-290 nm.

7. 7
This protects the Earth and its inhabitants from
the harmful ultraviolet radiation of the Sun.
Without this protective layer, more ultraviolet
radiation would reach the surface of the Earth
and cause damage to plant, animal and human
life.
Molecular weight : 47.998 g/mol. Melting
point : -193 °C.
Liquid density (1.013 bar at boiling point) :
1349.08 kg/m3
.

8. 8

9. 9
Gas density (1.013 bar and 0 °C (32 °F)) : 2.154
kg/m3
. Specific gravity : 1.612.
Pure ozone is a blue gas, with a strong irritating
smell.
When inhaled, it causes headache and nausea.
In smaller proportions it smells pleasant.
It is about 1.5 times heavier than air and has a
vapor density of 24, corresponding to the
formula O3.

10. 10
It is more soluble than oxygen in water, about
49% by volume at 0°C.
It gets liquefied to a deep blue colour liquid,
when cooled in liquid air.
It boils at 161.2 K and solidifies to violet-black
crystals, which melt at 80.6 K.
It dissolves readily in turpentine oil and acetic
acid.
The Chemical Properties of Ozone are very
unique. Let us learn some of them, here.

11. 11
Decomposition:
Ozone is an unstable compound.
Pure ozone decomposes explosively, while
ozonised oxygen decomposes slowly at room
temperature.
The decomposition is accelerated by the
presence of manganese dioxide, platinum black
and copper oxide etc.

12. 12
Bleaching agent :
Due to the oxidizing action of ozone, it acts as a
mild bleaching agent as well as a sterilizing
agent. It acts as a bleaching agent for vegetable
coloring matter.
Oxidizing property:
Ozone acts as a powerful oxidizing agent due to
the reaction, .

13. 13
The nascent oxygen formed due to its
decomposition is responsible for the oxidation
of a number of substances.
Reaction with mercury:
When ozone is passed through mercury, it loses
its meniscus and sticks to the glass due to the
formation of mercurous oxide.
This is called tailing of mercury.

14. 14
Uses of ozone:
Ozone is used for air purification at the crowded
places like cinema halls and tunnel railways.
Due to its strong oxidizing power it also
destroys the foul smell in slaughter houses. In
sterilizing drinking water by oxidizing all germs
and bacteria.
For preservation of meat in cold storages.

15. 15
For bleaching delicate fabrics such as silk,
ivory, oils, starch and wax. It helps to locate a
double bond in any unsaturated organic
compound by ozonolysis.
Pharmaceuticals:
Ozone is used in chemical synthesis and for
treatment of wastewater.

16. 16
Food and beverage:
Ozone's very strong oxidation properties are
sufficient to kill micro-organisms on food
stuffs. Further, ozone short shelf life prevents
products from any residual contamination.
These properties have been successfully used in
fish farming water treatment, greenhouse
nutritive solution recycling and sanitation of
food products.

17. 17
Pulp and paper:
Environment-friendly paper pulp bleaching.
Ozone is produced from oxygen at the point of
use for stability reasons.
Ozone is used under variable concentrations for
pulp bleaching (ECF or TCF Pulps), to reduce
the residual fluorescence coming from the
optical whiteners of the waste papers and for
treatment of specific effluents.

18. 18
Environmental control:
Ozone decreases 'hard' COD (Chemical Oxygen
Demand)
Occurrence and Distribution :
Ozone also occurs in very small amounts in the
lowest few kilometres of the atmosphere, a
region known as the troposphere.

19. 19
It is produced at ground level through a reaction
between sunlight and volatile organic
compounds (VOCs) and nitrogen oxides (NOx),
some of which are produced by human
activities such as driving cars.
Ground-level ozone is a component of urban
smog and can be harmful to human health.

20. 20
While ozone can be found through the entire
atmosphere, the greatest concentration occurs at
altitudes between 19 and 30 km above the
Earth's surface.
This band of ozone-rich air is known as the
"ozone layer".
Most ozone is produced naturally in the upper
atmosphere or stratosphere.
Ozone concentrations are highest between 19
and 23 km.

21. 21
Most of the ozone in the stratosphere is formed
over the equator where the level of sunshine
striking the Earth is greatest. It is transported by
winds towards higher latitudes.
The Two Ozone Layers :
The term “ozone layer” generally refers to a
relatively high concentration of ozone in the
stratosphere, a layer of very dry air around 15 to
35 kilometers (9 to 22 miles) above the Earth’s
surface.

22. 22
However, about 10 percent of the total ozone is
found in the troposphere, the lowest portion of
the atmosphere.
Tropospheric Ozone :
he ozone between the surface and the
tropopause forms only a fraction of the ozone
over most locations.

23. 23
Nevertheless, it absorbs solar UV more
efficiently than an equal amount of stratospheric
ozone.
This is because scattering caused by dust and
aerosols increases the distance that rays of
sunlight travel on their way to the surface.
In spite of this benefit, tropospheric ozone is
often referred to as “bad” ozone because of its
adverse effects in high concentrations.

24. 24
If the same ozone were somehow to drift into
the stratosphere, it would be called “good”
ozone.
Stratospheric Ozone:
Most references to the ozone layer mean the
ozone found in the stratosphere.

25. 25
There it forms a vaporous shield that protects
life on Earth from the lethal effects of the sun’s
UV radiation.
Hence, it has been called the Earth’s sunscreen.
Normal ozone concentration is about 300 to 350
D.U.
Stratospheric ozone depletion is so severe that
levels fall below 200 Dobson Units (D.U.), the
traditional measure of stratospheric ozone.

26. 26
Measuring Stratospheric Ozone :
Ozone in the stratosphere can be measured
directly using instruments on aircraft, rockets,
and–especially–balloons.
Many of the same kinds of sensing systems
used for measuring ozone at the surface have
been modified for these roles.

27. 27
Several kinds of optical instruments have been
developed for measuring ozone from the
surface, including the Dobson
spectrophotometer and various instruments that
use filters or diffraction gratings to measure
narrow bands of ultraviolet.

28. 28
Ozone Layer Depletion :
In the 1970s, scientists discovered that
chlorofluorocarbons (CFCs) could destroy
ozone in the stratosphere.
Compounds that contain chlorine and bromine
molecules, such as methyl chloroform, halons,
and chlorofluorocarbons (CFCs), are stable and
have atmospheric lifetimes long enough to be
transported by winds into the stratosphere.

29. 29
When these ozone-depleting substances (ODS)
break down in the atmosphere, they release
chlorine or bromine, which attack ozone.
Each chlorine or bromine atom reacts with
ozone, repeatedly combining with and breaking
apart as many as 100,000 ozone molecules
during its stratospheric life.

30. 30
CFCs, which have a long history of use as
refrigerants, solvents, foam-blowing agents and
in other applications, have been almost
completely phased out worldwide.
In addition, restrictions are now in place to
phase out hydrochlorofluorocarbons (HCFCs),
compounds used as substitutes for the more
damaging CFCs.

31. 31
A combination of low temperatures, elevated
chlorine, and bromine concentrations in the
upper stratosphere are responsible for the
destruction of ozone.
CFC's account for almost 80% of the total
depletion of ozone. Other ozone-depleting
substances (ODS), include
hydrochlorofluorocarbons (HCFCs), and
volatile organic compounds (VOCs).

32. 32
These are often found in vehicle emissions, by
products of industrial processes, refrigerants,
and aerosols.
As ozone depletes in the stratosphere, it forms a
'hole' in the layer.
This hole enables harmful ultraviolet rays to
enter the Earth's atmosphere.

33. 33
EFFECT OF OZONE LAYER DEPLETION
A. Effects on Human and Animal Health -
profound impact on human health with potential
risks of eye diseases, skin cancer and infectious
diseases.
Skin cancer:
Exposure to ultraviolet rays poses an increased
risk of developing several types of skin cancers,
including malignant melanoma, and basal and
squamous cell carcinoma.

34. 34
Eye damage:
Direct exposure to UV radiations can result in
photokeratitis (snow blindness), and cataracts.
Immune system damage:
Effects of UV rays include impairment of the
immune system. Increased exposure to UV rays
weakens the response of the immune system.

35. 35
Accelerated aging of skin:
Constant exposure to UV radiation can cause
photo allergy, which results in the outbreak of
rashes in fair-skinned people.
Other effects:
Ozone chemicals can cause difficulty in
breathing, chest pain, throat irritation, and
hamper lung functioning. UV radiation is
known to damage the cornea and lens of the
eye.

36. 36
Chronic exposure to UV-B could lead to
cataract of the cortical and posterior subcapsular
forms. UV-B radiation can adversely affect the
immune system causing a number of infectious
diseases.
In light skinned human populations, it is likely
to develop nonmelanoma skin cancer (NMSC).
Sunburn /Sun-Damaged Skin /Snow Blindness
/ Skin Cancer /Immune System Deficiencies.

37. 37
Decreases immunity-
Some species have become more vulnerable to
diseases and death . Retinal damage and
blindness in some species.
Effects on Amphibians:
Ozone depletion is listed as one of the causes
for the declining numbers of amphibian species.
Ozone depletion affects many species at every
stage of their life cycle.

38. 38
Some of the effects are :
a) Hampers growth and development in larvae.
b) Changes the behavior and habits, Causes
deformities in some species.
Effects on Marine Ecosystems:
Plankton (phytoplankton and bacterioplankton)
are threatened by increased UV radiation.

39. 39
Ultraviolet rays can influence the survival rates
of these microscopic organisms, by affecting
their orientation and mobility. This eventually
disturbs and affects the entire ecosystem.
B. Effects on Terrestrial Plants: Impact on
Plants:
In some species of plants, UV radiation can
alter the time of flowering, as well as the
number of flowers produced by a plant.

40. 40
Plant growth can be directly affected by UV-B
radiation. Despite mechanisms to reduce or
repair these effects, physiological and
developmental processes of plants are affected.
In forests and grasslands increased UV-B
radiation is likely to result in changes in species
composition (mutation) thus altering the bio-
diversity in different ecosystems.

41. 41
UV-B could also affect the plant community
indirectly resulting in changes in plant form,
secondary metabolism, etc.
C. Effects on Aquatic Ecosystems
While more than 30 percent of the world’s
animal protein for human consumption comes
from the sea alone, it is feared that increased
levels of UV exposure can have adverse
impacts on the productivity of aquatic systems.

42. 42
High levels of exposure in tropics and
subtropics may affect the distribution of
phytoplanktons which form the foundation of
aquatic food webs.
D. Effects on Bio-geo-chemical Cycles
Increased solar UV radiation could affect
terrestrial and aquatic bio-geo-chemical cycles
thus altering both sources and sinks of
greenhouse and important trace gases.

43. 43
They are carbon dioxide (CO2), carbon
monoxide (CO), carbonyl sulphide (COS), etc.
Other effects of increased UV-B radiation
include:
Changes in the production and decomposition
of plant matter;
reduction of primary production changes in the
uptake and release of important atmospheric
gases;

44. 44
reduction of bacterioplankton growth in the
upper ocean;
increased degradation of aquatic dissolved
organic matter (DOM), etc.
E. Effects on Air Quality
Reduction of stratospheric ozone and increased
penetration of UV-B radiation result in higher
photo dissociation rates of key trace gases that
control the chemical reactivity of the
troposphere.

45. 45
F. Effects on Materials - adverse effects on
synthetic polymers, naturally occurring
biopolymers and some other materials of
commercial interest.
UV-B radiation accelerates the photo
degradation rates of these materials thus
limiting their lifetimes.

46. 46
G. Effects on Climate Change
Ozone depletion and climate change are linked
in a number of ways, but ozone depletion is not
a major cause of climate change.
Atmospheric ozone has two effects on the
temperature balance of the Earth.
It absorbs solar ultraviolet radiation, which
heats the stratosphere.

47. 47
H. Effects on Ultraviolet Radiation
The depletion of the ozone layer leads to an
increase in ground-level ultraviolet radiation,
because ozone is an effective absorber of ultra-
violet radiation.
Some of this UV radiation (UV-B) is especially
effective in causing damage to living beings.

48. 48
The largest decreases in ozone during the past
15 years have been observed over Antarctica,
especially during each September and October
when the ozone hole forms.
I. Other Effects:
Ozone present in the lower atmosphere is
regarded as a pollutant and a greenhouse gas,
that can contribute to global warming and
climate change.

49. 49
How is the ozone hole related to global
warming?
Continued global warming will speed up the
process of stratospheric ozone depletion.
The depletion of the ozone increases when the
stratosphere gets colder.
Because global warming traps heat in the
troposphere, a less amount of the heat reaches
the stratosphere, making it colder.

50. 50
The greenhouse gases act as a cover or shield
for the troposphere making it warmer and
keeping the stratosphere cool.
Global warming can make ozone depletion way
worse right when it is supposed to recover in
the next century.

51. 51
International Actions :
The first international action to focus attention
on the dangers of ozone depletion in the
stratosphere and its dangerous consequences in
the long run on life on earth, was focused in
1977 when in a meeting of 32 countries held in
Washington D.C.

52. 52
Montreal Protocol In 1985 the Vienna
Convention established mechanisms for
international co-operation in research into the
ozone layer and the effects of ozone depleting
chemicals (ODCs).
1985 also marked the first discovery of the
Antarctic ozone hole.

53. 53
On the basis of the Vienna Convention, the
Montreal Protocol on Substances that Deplete
the Ozone Layer was negotiated and signed by
24 countries and by the European Economic
Community in September 1987.