Ozone depletion, gradual thinning of Earth's ozone layer in the upper atmosphere caused by the release of chemical compounds containing gaseous chlorine or bromine from industry and other human activities. The thinning is most pronounced in the polar regions, especially over Antarctica.

1. By- HIMANSHU JAIN
JUNIOR
1JUNIOR

2.  Ozone (O3) is a highly-reactive form of oxygen.
 Unlike oxygen (O2), ozone has a strong scent and is blue in color
 Ozone exists within both the tropospheric and stratospheric
zones of the Earth’s atmosphere
 In the troposphere, ground level ozone is a major air pollutant
and primary constituent of photochemical smog
 In the stratosphere, the ozone layer is an essential protector
of life on earth as it absorbs harmful UV radiation before it
reaches the earth.
2JUNIOR

3. Ozone is naturally formed in the stratosphere when solar UV
radiation dissociates O2 into two free oxygen atoms. One of the
liberated oxygen atoms then combines with O2 to form O3 a
process that may be summarized as;
O + O2 + M O3 + M
where M is any atom or molecule (e.g. O2, N2) capable of
absorbing excess energy liberated in the exothermic chemical
reaction.
3JUNIOR

4. Ozone itself is then naturally broken down by ultraviolet
radiation or in collisions i.e.,
O3 + hν O2 + O
O + O3 O2 + O2
but there is always a net balance between the rate of formation
and destruction producing a slight excess of ozone formation,
and hence allowing the formation of the thin ozone layer in the
Earth’s stratosphere.
The most widely used CFCs have been CFC-11 (CFCl3) and CFC-12
(CF2Cl2). These chemicals were commonly used as propellants in
spray cans, shaving foams, hair spray, deodorants, paints,
insecticides, etc.
4JUNIOR

5. Originally chosen by industry due to their stability they are
highly unreactive and have a very long residence time – about 50
to 100 years in the lower atmosphere. However, the lower
atmosphere is very fluid and allows abundant mixingCFCs
eventually move upward and enter the stratosphere. In the
stratosphere, they are broken down by UV radiation releasing
chlorine, which destroys ozone in a fast and efficient chemical
reaction.
CFCl3 + UV light CFCl2 + Cl
5JUNIOR

6. Chlorofluorocarbons are created
and used in refrigerators and
air conditioners. These
chlorofluorocarbons are not
harmful to humans and have
been a benefit to us. Once
released into the atmosphere,
chlorofluorocarbons are
bombarded and destroyed by
ultraviolet rays. In the process
chlorine is released to destroy
the ozone molecules
6JUNIOR

7. Molecular oxygen is broken down in the stratosphere by solar
radiation to yield atomic oxygen, which then combines with
molecular oxygen to produce ozone. The ozone is then
destroyed by chlorine atoms.
7JUNIOR

8. 8JUNIOR

9. Ozone depletion causes increases in UV rays effects on
aquatic ecosystems by:
1. Decreasing the abundance of phytoplankton – affects the
food stock for fishes and the absorption of CO2
2. Decreasing the diversity of aquatic organisms – reduces
food stock and also destroys several fish and amphibians.
9JUNIOR

10.  Skin cancer
 Premature aging (photo aging) of the skin (different from
normal chronological aging)
 Cataracts and eye disorders (corneal sunburn and blindness)
 Immune system damage
10JUNIOR

11.  Damage to plant cell DNA molecules - makes plants more
susceptible to pathogens and pests
 Reductions in photosynthetic capacity in the plant - results in
slower growth and smaller leaves
 Causes mutations in mammalian cells and destroys membranes
11JUNIOR

12.  The most dramatic evidence for the effect of CFCs on the
Earth’s ozone shield has been observed in the spring over the
Antarctica with smaller depletions over the Arctic in the late
winter and early spring. This is due to the special climate in
these regions in the winter.
 During the Antarctic winter air temperatures above the
continent fall below –80 °C forming a large weather system
called ‘vortex’ which isolates the Antarctic continent from the
rest of the world and form a giant containment vessel for
pollutants.
 These conditions produce very low stratospheric temperatures
below –80 °C, which in turn lead to the formation of high
altitude clouds commonly known as polar stratospheric clouds
(PSCs).
12JUNIOR

13.  The polar stratospheric clouds are formed due to condensation of
nitric acid (HNO3), sulphuric acid (H2SO4), and water (H2O) in the
stratospheric region.
 When freezing, the hydrates of HNO3 become stable and are
commonly known as nitric acid dihydrate (NAD=HNO3.2H2O) and
nitric acid trihydrate (NAT=HNO3.3H2O). The particles that
contain appreciable HNO3 are termed as Type I PSCs.
 Alternatively, the liquid aerosol can freeze to form sulphuric acid
tetrahydrate (SAT) or other sulphate hydrates especially in the
absence of HNO3, these are known as Type II PSCs.
 These clouds are responsible for chemical changes and promote
rapid ozone loss during the winter cycle in September and October
each year resulting in the so-called ‘ozone hole’.
13JUNIOR

14. In the spring, sunlight penetrates the vortex (during winter it is
nearly always night) and triggers chemistry on the surface of
the ice in the PSCs, breaking down the CFC’s and releasing Cl
which rapidly destroys the ozone and leads to a dramatic decline
in local ozone concentrations (up to 80%) producing what is
known as the ‘ozone hole’.
14JUNIOR

15. In September 1987, 24 nations met to negotiate the final text
and sign the Montreal Protocol on Substances that deplete the
Ozone Layer (ODS).
The agreed Montreal Protocol, which entered into force on
January 1, 1989, limited production of most commonly used
ODSs, i.e. chlorofluorocarbons (CFCs) and halons.
The Protocol required each party's production of
chlorofluorocarbons (CFC-11, 12, 112, 113, 114 and 115) first to
be frozen at 1986 levels and ultimately reduced to 50% of 1986
levels by 1998. Production of halons 1211, 1301 and 2402 were to
be restricted to 1986 levels. The Protocol called for a freeze in
production of halons at 1986 levels beginning in 1992.
15JUNIOR

16. Shortly after the 1987 Protocol was negotiated, new scientific
evidence showed that ozone depletion was occurring at a rate
significantly faster than previously assumed.
Hence, in June 1990, the parties to the Protocol met in London
and agreed to amendments that required more stringent
controls on ODSs included in the original agreement.
The London agreement added further controls on other
important ODSs such as carbon tetrachloride (CTC) and 1,1,1-
trichloroethane (1,1,1-TCE) also known as methyl chloroform
(MC).
16JUNIOR

17. The London Amendment limited production of commonly used
CFCs to 50 percent of 1986 levels by 1995 and 15% by 1997.
Under the amended agreement, CFCs, halons and CTC production
is to be phased out by the year 2000, and methyl chloroform is
to be phased out by 2005.
The 1990 amendment also introduced the concept of transitional
substances, such as the HCFCs. These are envisaged to be
chemical replacement for CFCs and other controlled substances
and have relatively small ozone depletion potential.
A non-binding resolution by the parties calls for a phase out of
HCFCs by the year 2020, if possible, but not later than 2040.
The London Amendment to the Protocol entered into force in
August 1992.
17JUNIOR

18. Scientific data on depletion of the ozone layer presented to the
Parties at their November meeting in Copenhagen revealed that
depletion has been occurring at a rate twice as fast as originally
observed.
For example, at latitudes where 2% depletion had been observed
over the last decade, new evidence showed that actual depletion
is closer to 3 - 5%.
The Copenhagen Amendment calls for an accelerated phase-out
of ODS for the developed countries (CFCs, CTC, and, MC by
1996; HCFCs by 2030).
18JUNIOR

19. Additionally, the Copenhagen Amendment calls for measures
against hydrobromofluorocarbons (HBFCs) and methyl bromide.
The Copenhagen Amendment to the Protocol was adopted in
November 1992 to be effective from January 1, 1994, with
ratification by at least twenty countries. As of May 1994,
twenty-four countries have done so.
The Protocol grants a 10-year grace period on all phase out
dates and interim reduction deadlines for developing countries
whose per capital consumption of Annexure A chemicals is less
than 0.3 kg/year.
19JUNIOR

20. Annexure A chemicals include the five main CFCs: CFC-11, CFC-
12, CFC-113, CFC-114, CFC-115; and halons.
The list of ODS that are regulated by the Montreal Protocol are
thus:
 Chlorofluorocarbons (CFC), CFC-11, 12, 113, 114, 115 as well as
mixtures of these substances
 Halons 1211, 1301 and 2402
 1,1,1-trichloroethane (methyl chloroform)
 Carbon tetrachloride (CTC)
 HCFC, HBFC
 Methyl bromide
20JUNIOR

21.  It is beyond any doubt that the destruction of our ‘ozone UV
shield, if not checked at an early stage, will result in grave
consequences for our living environment.
 The destruction of ozone shield will result in more ultraviolet
radiation reaching the Earth’s surface in the next few decades
causing potential environmental disruption and human health
problems.
 The troubling aspect is that if we stop manufacturing, using and
even emitting all ozone depleting chemicals today, the problem
will not go away immediately. Since what we have already
emitted will be in the atmosphere for another 50 or more years.
 Theoretically, the ‘ozone hole’ should gradually heal as
international regulations cease the flow of ozone destroying
chemical substances in the atmosphere but it is an open question
whether complete repair is possible.
21JUNIOR