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ATMOSPHERIC OZONE AND ITS

DEPLETION

by

Prof. A. Balasubramanian

Centre for Advanced Studies in Earth Science

University of Mysore, India

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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.

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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.

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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.

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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.

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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.

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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 .

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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.

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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.

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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.

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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, .

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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).

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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;

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.