Ozone Layer
Depletion
© 2014 - Brilliant Classes by Piyush
Prepared by
Piyush & Ilaxi
Following topics are covered in a given
module
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What is Ozone?
What is Ozone Layer?
Causes of Ozone layer depletion
Ozone depletion substances
Role of CFCs ,HCFCs, Chlorine atoms and other
pollutants
Ozone hole
Mechanism of hole formation
Effects of Ozone depletion
As a human being what can we do to protect Ozone
layer?
© 2014 - Brilliant Classes by Piyush
What is ozone?
• Ozone is a gas made up of molecule that
are formed by three oxygen atoms.
• Its molecule formula is O3.
• Ozone is formed when the sunlight hits
oxygen molecules (O2) and breaks them up
into individual atoms.
• These individual atoms then join up with
O2 molecules and make O3 or ozone.
© 2014 - Brilliant Classes by Piyush
What is the ozone layer?
• The ozone layer refers to the ozone in the
stratosphere; where more than 90% of the
earth's ozone resides.
• The ozone layer absorbs much of the sun’s
UV light. So, the ozone layer helps to
protect us.
• The ozone layer, situated in the
stratosphere about 15 to 30 km above the
earth's surface.
© 2014 - Brilliant Classes by Piyush
© 2014 - Brilliant Classes by Piyush
Causes of Ozone Depletion
• Depletion of the ozone layer is due to the emission of a
set of chemicals that cause ozone to convert into
oxygen.
• Ozone blocks ultraviolet radiation’
• Ozone-Depleting Substances are:
• CFCs,
• HCFCs,
• halons,
• methyl bromide,
• carbon tetrachloride
• methyl chloroform.
© 2014 - Brilliant Classes by Piyush
Depletion of Ozone Layer
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• Most of the depletion of the ozone layer
has been attributed to pollutants containing
chloride (chlorofluorocarbons or CFCs).
CFCs were used in refrigeration and air
conditioning systems and as propellants in
spray cans.
• These chemicals serve as a catalyst in a
chemical reaction that converts ozone to
oxygen.
The presence of ice crystals
accelerates the process.
• CFCs are not consumed in the reaction but
remain in the stratosphere to continue the
destruction of the ozone.
© 2014 - Brilliant Classes by Piyush
© 2014 - Brilliant Classes by Piyush
• HCFCs are CFCs that contain hydrogen. This
makes them more reactive to the OH radical,
decreasing their tropospheric lifetime.
• “Hard” CFCs are unreactive to OH and other
reactive radicals in the troposphere.
• They are also pretty insoluble in water.
• That means their tropospheric lifetimes are
easily long enough that the majority of
tropospheric CFCs pass through the
tropopause into the stratosphere.
© 2014 - Brilliant Classes by Piyush
• Many CFCs absorb in the “UV window”
(centered at 205 nm) between strong O2
and O3 absorption. That means most can
photo dissociate in the bottom half of the
stratosphere.
• Photodissociation
releases
chlorine
atoms:
• For example: CFCl3 + light  CFCl2 + Cl (l
< 225 nm)
© 2014 - Brilliant Classes by Piyush
• Chlorine atoms deplete odd oxygen (Ox) largely by
the following cycle
• Cl + O3  ClO + O2
• ClO + O  Cl + O2
• Chlorine undergoes a series of reactions to form a
variety of compounds.
• Some of these are active in depleting ozone: Cl,
ClO
• Some of these do not directly deplete ozone;
these are chlorine reservoirs: HCl, ClONO2, HOCl
© 2014 - Brilliant Classes by Piyush
• The most important (long-lived) stratospheric
chlorine reservoir is HCl .
• The reservoirs can become activated by
various processes such as Photodissociation or
reaction with OH.
• Loss of stratospheric chlorine occurs when
they cross-back into the troposphere and are
removed from the atmosphere.
• Most common route: HCl crosses back,
dissolves in water, and is washed out.
© 2014 - Brilliant Classes by Piyush
Ozone Hole
• The ozone hole is defined as the area having less
than 220 Dobson units (DU) of ozone in the
overhead column (i.e., between the ground and
space).
• The “ozone hole” is a sudden, marked depletion
of ozone – a loss of 50% or more of total column
ozone – in the lower stratosphere of the
Antarctic in the weeks after the Spring sunrise.
• In 1985 the area of the hole was 10 million sq.
km (and growing yearly).
© 2014 - Brilliant Classes by Piyush
Hole Formation Based on Two different
mechanisms:
• Meteorological mechanism
• Movement of air from one place to another in the
upper stratosphere.
• Cold temperature in the upper atmosphere causes
nitric acid to freeze into crystals forming wispy
pink clouds.
• Forms a vortex of tightly twisted winds thus
forming a hole in the upper atmosphere.
© 2014 - Brilliant Classes by Piyush
Chemical Mechanism
• Different chemicals are responsible for the
destruction of the ozone layer.
• chlorofluorocarbons (CFC’s) is man-made,
non-toxic and inert in the troposphere.
• In the stratosphere they are photolysed,
releasing reactive chlorine atoms that
catalytically destroy ozone.
• A combination of low temperatures and
elevated chlorine and bromine concentrations
are responsible for the destruction of ozone in
the upper stratosphere thus forming a “hole”.
© 2014 - Brilliant Classes by Piyush
Effects of Ozone depletion
• Skin
Cancer
(melanoma
and
nonmelanoma)
• Premature aging of the skin and other
skin problems
• cataracts and other eye damage
• Immune system suppression
© 2014 - Brilliant Classes by Piyush
What can we do?
• If you have a refrigerator made before 1995 it
probably uses refrigerant made from CFCs.
Get a new refrigerator and be sure to have the
old one disposed of properly.
• Old air conditioners, made before 1994,
typically used a CFC called Freon.
• Even newer air conditioners use chemicals
called HCFCs. Even though HCFCs are better
for the ozone, they still contributes to the
depletion, so try and use as little air
conditioning as possible.
© 2014 - Brilliant Classes by Piyush
• A lot of foam products contain CFCs. Try and
use different packing materials such as
crumpled old newspapers.
• Protect yourself against sunburn. Minimize
sun exposure during midday hours (10 am to 4
pm). Wear sunglasses, a hat with a wide brim,
and protective clothing with a tight weave.
• Use a broad spectrum sunscreen with a sun
protection factor (SPF) of at least 15 and 30 is
better.
© 2014 - Brilliant Classes by Piyush
© 2014 - Brilliant Classes by Piyush