is present at different altitudes of atmosphere. Layer of ozone is present in
upper region of stratosphere, known as ozone layer. Approximately 97% of all
the ozone in the atmosphere(10 km
to 50 km) is present in the stratosphere The highest
concentration of ozone is between the altitudes of 12 km and 35 km in the
stratosphere. This zone (12 km to 35 km) of atmosphere is called ozonosphere or
the stratosphere ozone layer and is considered as a protective shield on the
earth’s surface. This layer shields the earth from the harmful ultraviolet radiation
of the sun. Depletion of the ozone layer is considered as a threat to all forms
15.6.1 Depletion of Ozone Layer (Ozone hole)
When UV light from the sun
strikes the earth’s atmosphere, it causes photolysis of oxygen molecules into
reactive oxygen atom.
O2 + h ¾¾® 2(O) ———- (1)
This reaction is followed by
another in which a reactive oxygen atom recombines with an oxygen molecule
O2 + (O) ¾¾® O3 ———- (2)
The ozone formed in reaction
distributes itself in the stratosphere forming a layer. Stratosphere ozone then
absorbs harmful ultraviolet radiation.
O3 + h ¾¾® O2 + (O) ———- (3)
ozone layer filters the incoming UV radiations and provides natural protection.
The reaction (3) destroys ozone but it also produces reactive oxygen (Reaction-2)
producing more ozone to compensate the loss. Thus in the stratosphere, ozone is
continuously created and destroyed by the sun’s radiation. This results in
equilibrium concentration of ozone. The equilibrium is disturbed when reactive
chlorine atoms, released from photolysis of CFC’s, enter into the atmosphere. Fig. 15.3 Ozone hole
atoms create an imbalance by destroying ozone molecules. Generally substance
that cause depletion of ozone or make it thinner are called Ozone Depletion Substances abbreviated
as ODS. The loss of ozone molecules in the upper atmosphere is termed as depletion of stratospheric ozone. When
this happens, the ozone layers capacity to filter out harmful U-V rays from the
sun decreases. Nitric oxide and Chloro Fluoro carbon are found
to be most responsible for depletion of ozone layer.
i) Oxides of Nitrogen:
Nitrogen oxides introduced directly
into the stratosphere by the supersonic jet aircraft engines in the form of
exhaust gases. These oxides also released by combustion of fossil fuels and
nitrogen fertilizers. Inert nitrous oxide in the stratosphere is photo
chemically converted into more reactive nitric oxide. Oxides of nitrogen catalyse the decomposition of ozone and
are themselves regenerated. Ozone gets depleted as shown below
The net reaction
Thus ozone decomposition rate increase in
stratosphere in the presence of nitrogen oxides. Reaction of NOx with ozone
causes 40% depletion.
ii) Chloro Fluoro Carbons (CFC) Freons
The chloro fluoro
derivatives of methane and ethane are referred by trade name Freons
Fluoro Carbon compounds are stable, non-toxic, noncorrosive and non-inflammable,
easily liquefiable gases and are used in refrigerators, air- conditioners and
in the production of plastic foams. CFC’s are the exhaust of supersonic air
craft’s and jumbo jets flying in the upper atmosphere. They slowly pass from
troposphere to stratosphere. They stay for very longer period of 50 – 100
years. In the presence of ultraviolet radiation from sun, CFC’s break up into
chlorine free radical
chlorine free radicals formed react with stratospheric ozone to form chlorine
monoxide radical and oxygen molecule.
Reaction of chlorine monoxide
radical with atomic oxygen produces more chlorine free radical.
Due to this
continuous attack of Cl? thinning of ozone layer takes place which lead to
formation of ozone hole. It is
estimated that for every reactive chlorine atom generated in the
stratosphere1,00,000 molecules of ozone are depleted.
Now a days air-conditioning and refrigeration industries use hydrochloro
and hydrofluoro carbons (HFC) as short term ozone friendly substitutes for
15.6.2 The Ozone hole over Antarctica
Depletion of ozone layer take place
in all parts of the stratosphere but ozone hole mainly observed in the
stratosphere over Antartica. In most
parts of stratosphere reacts with nitrogen
dioxide and Cl. reacts with
hydrocarbons as shown below
This prevents the reaction of active
chlorine radical with ozone and the stops the chain reaction.
The Ozone hole in Antartica
In Antartica, the climatic conditions are
quite different. In summer season NO2 and CH4 react with
chlorine monoxide radical and chlorine radical respectively and prevents ozone
depletion. But in winter season special types of clouds called Polar
stratospheric clouds (PSC) are formed over antartica. PSC’s composed of either
nitric acid trihydrate at about 196K (Type I) or ice formed at 188K (TypeII).
These clouds hydrolysis chlorine nitrate to hypochlorus acid and also hydrogen
chloride to chlorine molecule as shown in following reaction
spring season (September and October) sun shines over antartica and the sun’s
warmth break up the clouds .HOCl and Cl2 undergo photolysis to form
reactive chlorine radical. The chlorine free radicals formed initiate the chain
reaction of ozone depletion
Due to polar stratospheric clouds, polar vortex
surrounds Antarctica. This rigid and cuts off Antarctica from ozone air of non-polar
regions. Hence ozone hole remains unfilled. After the spring the intensity of
sunlight increases and polar vortex breakdown. The ozone rich air from
surroundings rushes up to fill the ozone hole.
Impact of Ozone Depletion
The formation and
destruction of ozone is a regular natural process, which never disturbs the
equilibrium level of ozone in the stratosphere. Any change in the equilibrium
level of the ozone in the atmosphere will adversely affect life in the
biosphere in the following ways.
Depletion of ozone layer will
allow more UV rays to reach the earth surface. Consequently, the temperature of
the earth’s surface will increase, which would cause melting of continental
glaciers and ice sheets.
It increases evaporation of surface water and decrease the
moisture content of the soil.
Depletion of ozone layer would
cause skin cancer and also decreases the immunity level in human beings.
The heavily fertilised crops would be more
adversely affected due to their exposure to UV radiations and hence plants are
damaged. Plants give poor yield.
UV radiation affects the
growth of phytoplankton, as a result ocean food chain is disturbed and even damages the fish productivity.