Ozone Depletion, Causes and Effects
- Natural gas
- An allotrope of oxygen consisting of three atoms of oxygen
- Chemical symbol – 03.
- It is found in two different layers of the atmosphere.
Why ozone is ‘bad in troposphere’ and ‘good in stratosphere’?
Ozone in the troposphere – because it dirties the air and helps to form smog, which is
not good to breathe.
Ozone in the stratosphere – because it protects life on Earth by absorbing some of the
sun’s harmful Ultra Violet (UV) rays.
Ozone protects oxygen at lower altitudes from being broken up by the action of
ultraviolet light and also keeps most of the ultraviolet radiation from reaching the
Reduces the risks of mutation and harm to plant and animal life.
How UV rays are harmful for us?
They cause direct damage to the genetic material or DNA of animal and plant cells.
Exposure of mammals to UV light act on the immune system, thereby making the body
more susceptible to diseases.
Too much UV rays can cause skin cancer and will also harm all plants and animals.
Significant decrease in the concentration of ozone in a particular region of the atmosphere, is
called as ‘ozone depletion’.
Change in equilibrium
Equilibrium between the formation and destruction of ozone, has been upset by influx
of several substances into the atmosphere which react with ozone and destroy it.
Rate of destruction of ozone > Rate of formation of ozone
Best example of ozone depletion – ozone hole in the, atmosphere over the Antarctic
which has only about 50 percent of the ozone that originally occurred there.
Made of – chlorine, fluorine and carbon.
Use – used as refrigerants, propellants in aerosol sprays, foaming agents in plastic
manufacturing, fire extinguishing agents, solvents for cleaning electronic and metallic
components, for freezing foods etc.
Why they are used – because of their properties like non-corrosiveness, noninflammability, low toxicity and chemical stability, etc.
Lifetime or residence time – 40-150 years
Removal – cannot be eliminated by usual scavenging processes like photodissociation,
rain-out and oxidation.
Escape of CFCs – CFCs enter into the atmosphere by gradual evaporation from their
source. E.g. CFCs can escape into the atmosphere from a discarded refrigerator. Because
CFCs are thermally stable they can survive in the troposphere. But in the stratosphere,
they are exposed to UV radiation.
Chemical reaction – UV rays breakup CFCs and frees the chlorine atoms which react
with ozone molecule and form chlorine monoxide (ClO). ClO further reacts with oxygen
atom resulting in free chlorine atom which again react with ozone and divides them and
cycle goes on.
A single chlorine atom destroys thousands of ozone molecules before encountering
reactive nitrogen or hydrogen compound that eventually return chlorine to its
CFC substitutes’ characteristic’s – they must be safe, low cost, increased energy
efficiency, effective refrigerants with low ozone layer depletion potential and low GWP.
- CFC-12 (R-12) – widely used refrigerant. HFC 134a (R-134a) is the most promising
alternative (R-143a) and (R-152a) can also be used.
Sources – explosions of thermonuclear weapons, industrial emissions and agricultural
Escape of N2O – Nitrous oxide (N20) is released from solid through denitrification of
nitrates under anaerobic conditions gradually reach the middle of the stratosphere,
where it is photolytically destroyed to yield nitric oxide which in turn destroys ozone.
Bromine containing compounds called halons and HBFCs, i.e. hydrobromo fluorocarbons
[both used in fire extinguishers and methyl bromide (a widely used pesticide)].
100x of Chlorine i.e. Each bromine atom destroys 100x of more ozone molecules than
what a chlorine atom does.
Sulphuric acid particles
They free chlorine from molecular reservoirs, and convert reactive nitrogen into inert
forms thus preventing the formation of chlorine reservoirs.
Monitoring the ozone layer
Some organizations monitor the atmosphere and form a network of information
- World Meteorological Organization (WMO)
- World Weather Watch (WWW)
- Integrated Global Ocean Services Systems (IGOSS)
- Global Climate Observing System (GCOS)
Q.) Why has an “ozone hole” appeared over Antarctica when ozone-depleting substances are present throughout the stratosphere?
Because atmospheric and chemical conditions unique to this region increase the
effectiveness of ozone destruction by reactive halogen gases.
Formation of the Antarctic ozone hole requires temperatures low enough to form polar
stratospheric clouds (PSCs), isolation from air in other stratospheric regions, and
Role of polar stratospheric clouds in ozone depletion
Correlation between the cycle of ozone depletion and the presence of polar
stratospheric clouds (PSCs) i.e. the ice particles of the cloud provided substrates for
chemical reactions which freed chlorine from its reservoirs. Usually the reaction
between HCL and ClON02 is very slow, but this reaction occurs at a faster rate in the
presence of a suitable substrate which is provided by the stratospheric clouds at the
It will result in formation of molecular chlorine and nitric acid.
Molecular chlorine → broken down to atomic chlorine and ozone depletion continue
PSCs activate chlorine + absorb reactive nitrogen.
Arctic Ozone Depletion
In 1996 it was the greatest depletion of ozone ever seen, in the northern hemisphere.
It had been caused because of cooling of the upper atmosphere in the northern
Other reasons – increasing cold temperature in the arctic stratosphere which encourages
the formulation of PSCs.
The formation of ozone hole in the Antarctic region has been a cause of concern. What could be the reason for the formation of this hole?
(a) Presence of prominent tropospheric turbulence; and inflow of chlorofluorocarbons .
(b) Presence of prominent polar front and stratospheric clouds and inflow of
(c) Absence of polar front and stratospheric clouds: and inflow of methane and
(d) Increased temperature at polar region due to global warming.
Measurement of ozone
Dobson spectrophotometer and the filter ozonometer called M83, and total ozone
mapping spectrometer (TOMS) in the Nimbus-7 satellite – tools to measure ozone.
Effect of ozone depletion
Skin Cancer – exposure to UV rays from sun can lead to increased risk for developing of
several types of skin cancers. Malignant melanoma, basal and squamous cell carcinoma
are the most common cancers caused by exposure to UV rays.
Eye Damage – UV rays are harmful for our eyes too. Direct exposure to UV rays can lead
to Cataract problems, and also Photokeratitis or snow blindness.
Damage to Immune system – our immune system is also highly vulnerable to UV rays.
Increased exposure to UV rays can lead to weakening of the response of immune system
and even impairment of the immune system in extreme cases.
Aging of skin – exposure to UV rays can lead to acceleration of the aging process of your
skin. This will result in you looking older than what you actually are. It can also lead to
photo allergy that result in outbreak of rashes in fair skinned people
In humans, exposure to UV rays can also lead to difficulty in breathing, chest pain, and
throat irritation and can even lead to hampering of lung function.
UV rays affect other life forms too. It adversely affects the different species of
amphibians and is one of the prime reasons for the declining numbers of the amphibian
species. It affects them in every stage of their life cycle – from hampering the growth
and development in the larvae stage, deformities and decreases immunities in some
species and to even retinal damage and blindness in some species.
UV rays also have adverse effect on the marine ecosystem. It adversely affects the
planktons which plays a vital role in the food chain and oceanic carbon cycle. Affecting
phytoplankton will in turn affect the whole ocean ecosystem.
UV rays will also affect the plants. UV radiations can alter the time of flowering in some
plant species. It can also directly affect the plant growth by altering the physiological
and developmental processes of the plants.