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OZONE LAYER

SUBITTED BY:

NAME:YASH MANEK

CLASS:9 E

ROLL NO:36

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The ozone layer is a layer in earths atmosphere which containsrelatively high concentrations of ozone (O3). This layer absorbs 9799%of the sun's high frequency ultraviolet rays which is potentiallydamaging to the life forms on Earth.[1] It is mainly located in the lowerportion of the stratosphere from approximately 20 to 30 kilometers

(12 to 19 mi) above Earth, though the thickness varies seasonallyand geographically.]The ozone layer was discovered in 1913 by theFrench physicists Charles fabry and Henri Buisson Its properties wereexplored in detail by the British meteorologist G. M. B. Dobson, whodeveloped a simple spectrophotometer (the Dobsonmeter) that could beused to measure stratospheric ozone from the ground. Between 1928

and 1958 Dobson established a worldwide network of ozone monitoringstations, which continue to operate to this day. The "Dobson unit", aconvenient measure of the columnar density of ozone overhead, isnamed in his honor.

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Origin of ozone

The photochemical mechanisms that give rise to the ozone layer werediscovered by the British physicist Sidney Chapman in 1930. Ozone inthe Earth's stratosphere is created by ultraviolet light striking oxygenmolecules containing two oxygen atoms (O2), splitting them intoindividual oxygen atoms (atomic oxygen); the atomic oxygen then

combines with unbroken O2 to create ozone, O3. The ozone molecule isalso unstable (although, in the stratosphere, long-lived) and whenultraviolet light hits ozone it splits into a molecule of O2 and an atomof atomic oxygen, a continuing process called the ozone-oxygen cycle, thus creating an ozone layer in the stratosphere, the region fromabout 10 to 50 kilometers (33,000 to 160,000 ft) above Earth's

surface. About 90% of the ozone in our atmosphere is containedin the stratosphere. Ozone concentrations are greatest between about20 and 40 kilometers (12 and 25 mi), where they range from about2 to 8 parts per million. If all of the ozone were compressed to thepressure of the air at sea level, it would be only 3 millimeters thick.

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The ozone layer

Ozone is an unstable gas

It rapidly breaks down

The ozone layer is only a few cm thick If the rate of breakdown is faster than the

rate of formation the ozone layer thins

This could develop into hole

An ozone hole was first observed over theAntarctic in 1985

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The ozone hole

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Ultra Violet light

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UV light and life

UV light is high energy light

When it strikes molecules it can causethem to beak into ions or free radicals

The free radicals in turn damage largemolecules such as proteins and DNA

Damage to DNA causes mutations

It is a mutagen Mutations can lead to cancer

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UV light and life

UV light can have beneficial effects too

It stimulates the transformation dietarysteroids into Vitamin D (calciferol)

UV light is used to sterilise foods andmedical equipment

Some animals can see UV light

Insects use it to guide them to nectarsources in flowers

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UV light & DNA

UV-B has a direct effect on DNA

DNA absorbs UV light of 260nm

The action of UV forms thymine dimers

This can cause a gene mutation when theDNA replicates

UV-B Thymine dimer

ACGCTGGCTTAGT

TGCGACCGAATGA

ACGCTGGCT=TAGT

TGCGACCGAATGADNA

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Specific damage byUV light

Sunburn

Clouding of the cornea and formation ofcataract

Skin cancer (melanoma)

Reduced rates of photosynthesis

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Ozone in the atmosphere

Ozone (O3) forms under the effect of UVlight in the stratosphere

Ozone can also form in the troposphere

Naturally it forms from volcanic activity

It is produced by motor vehicles so it is apollutant

Sunlight acts on NOx to produce O3

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Atmospheric O3 Paris 6eme arr 2006-2007

Data Airparif

0

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

02/07/2006 10/10/2006 18/01/2007 28/04/2007 06/08/2007 14/11/2007 22/02/2008

O3/gm-3

Ozone pollution in the troposphere

Safety

limit

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The cause of the hole Chlorofluorocarbons (CFCs)

The breakdown under the effect of UVlight to release chlorine radicals (Cl)

Especially under cold conditions The chlorine radicals react with O3

converting it to O2 and more radicals ClO

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What are CFCs?

Used as propellants in aerosol spray cans

Used as refrigerants in fridges, freezersand air conditioning units

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What can be done?

Reduce the use of CFCs

They are already banned in aerosols(1987)

BUT they are still used as refrigerants

Recycle fridges and air conditioning plants

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The current situation

The holes developing over the polesuggest that they may be show animprovement

BUT CFC molecules take 30 years to riseup to the stratosphere

The chlorine radicals last a long time

The peak ozone damage was supposed tobe in 2000

Damage could go on another 50 years

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The biggest yet September 2006

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