TERM PAPER
PHYSICS PHY102
TOPIC: Ozone Layer Depletion
Date of Allotment:01/09/2010
Date of Submission:24/11/2010
Submitted to: Submitted by:
Mr. Karanveer Dhingra Mr. Nand Kishore Singh
Deptt. of Physics Roll No:
Regd. No:
Section:
ACKNOWLEDGEMENT
I NAND KISHORE SINGH, student of B.Tech-M.Tech (CSE)
(Section) express my deep gratitude to Mr. Karanveer Dhingra. I am
thankful to him for support that led me to the completion of this term
paper.
I am very thankful to my parents who encouraged me and
provided me all the necessary resources that had made possible for me
to be able to accomplish this work.
I also thank all my friends who assisted me in completing this work.
CONTENTS
1.Introduction to Ozone Depletion
2.Ozone Cycle Overview
3.Observations on Ozone Layer Depletion
4.The Ozone Hole and Its Causes
5.Consequences of Ozone layer Depletion
6.The Ozone hole
7.Ozone depletion and global warming
8.Misconceptions about ozone depletion
Introduction the Ozone Layer was implemented in 1987.
This legally binding international treaty
Ozone is both beneficial and harmful to us. called for participating developed nations to
Near the ground, ozone forming as a result reduce the use of CFCs and other ozone
of chemical reactions involving traffic depleting substances. In 1990 and again in
pollution and sunlight may cause a number 1992, subsequent Amendments to the
of respiratory problems, particularly for Protocol brought forward the phase out date
young children. However, high up in the for CFCs for developed countries to 1995.
atmosphere in a region known as the
stratosphere, ozone filters out incoming Protecting the ozone layer is essential.
radiation from the Sun in the cell-damaging Ultraviolet radiation from the Sun can cause
ultraviolet (UV) part of the spectrum. a variety of health problems in humans,
Without this ozone layer, life on the earth including skin cancers, eye cataracts and a
would not have evolved in the way it has. reduction in the body’s immunity to disease.
Furthermore, ultraviolet radiation can be
Concentrations of ozone in the stratosphere damaging to microscopic file in the surface
fluctuate naturally in response to variations oceans which forms the basis of the world’s
in weather conditions and amounts of energy marine food chain, certain varieties of crops
being released from the Sun, and to major including rice and soya, and polymers used
volcanic eruptions. Nevertheless, during in paints and clothing. A loss of ozone in the
the 1970s it was realised that man-made stratosphere may even affect the global
emissions of CFCs and other chemicals used climate.
in refrigeration, aerosols and cleansing
agents may cause a significant destruction of International agreements and other
ozone in the stratosphere, thereby letting legislation have gone a long way to
through more of the harmful ultraviolet- safeguarding this life supporting shield.
radiation. Then in 1985 evidence of a large Nevertheless, for there to be real and long-
“ozone hole” was discovered above the lasting success, everyone must become part
continent of Antarctica during the of the solution. Individual efforts taken
springtime. This has reappeared annually, together can be powerful forces for
generally growing larger and deeper each environmental change. There are a number
year. More recently, fears have emerged of things that we, as individuals, can do to
about significant ozone depletion over the both protect the ozone layer. These include
Arctic, closer to the more populous of the proper disposal of old refrigerators, the use
Northern Hemisphere. of halon-free fire extinguishers and the
recycling of foam and other non-disposable
In response to this and additional fears about packaging. Finally, we should all be aware
more widespread global ozone depletion, the that while emissions of ozone depleters are
Montreal Protocol on substances that deplete now being controlled, the ozone layer is not
likely to fully repair itself for several between photochemical production and
decades. Consequently, we should take recombination.
precautions when exposing ourselves to the
Sun. Ozone can be destroyed by a number of free
radical catalysts, the most important of
Ozone Cycle Overview which are the hydroxyl radical (OH), the
nitric oxide radical (NO), atomic chlorine
Three forms (or allotropes) of oxygen are (Cl) and bromine (Br). All of these have
involved in the ozone-oxygen cycle: oxygen both natural and man-made sources; at the
atoms (O or atomic oxygen), oxygen gas (O2 present time, most of the OH and NO in the
or diatomic oxygen), and ozone gas (O3 or stratosphere is of natural origin, but human
triatomic oxygen). Ozone is formed in the activity has dramatically increased the level
stratosphere when oxygen molecules photo of chlorine and bromine. These elements are
dissociate after absorbing an ultraviolet found in certain stable organic compounds,
photon whose wavelength is shorter than especially CFCs, which may find their way
240 nm. This produces two oxygen atoms. to the stratosphere without being destroyed
The atomic oxygen then combines with O 2 in the troposphere due their low reactivity.
to create O3. Ozone molecules absorb UV Once in the stratosphere, the Cl and Br
light between 310 and 200 nm, following atoms are liberated from the parent
which ozone splits into a molecule of O2 and compounds by the action of UV light, e.g.
an oxygen atom. The oxygen atom then (‘h’ is Planck’s constant, ‘ν’ is frequency of
joins up with an oxygen molecule to the electromagnetic radiation)
regenerate ozone. This is a continuing
process which terminates when an oxygen CFCl3 + hν → CFCl2 + Cl
atom recombines with an ozone molecule to
make two O2 molecules: The Cl and Br atoms can then destroy
ozone molecules through a variety of
O + O3 → 2 O2 catalytic cycles. In the simplest example
of such a cycle, a chlorine atom reacts
with an ozone molecule, taking an oxygen
atom with it and leaving a normal oxygen
molecule. The chlorine monoxide (ClO)
can react with a second molecule of ozone
(O3) to yield another chlorine atom and
two molecules of oxygen. The chemical
shorthand for these gas-phase reactions is
:
Cl + O3 → ClO + O2
The overall amount of ozone in the ClO + O3 → Cl + 2O2
stratosphere is determined by a balance
The overall effect is a decrease in the above a point on the Earth’s surface,
amount of ozone. More complicated which is normally expressed in Dobson
mechanisms have been discovered that units, abbreviated as ‘DU”. Marked
led to ozone destruction in the lower decreases in column ozone in the
stratosphere as well. Antarctic spring and early summer
compared to the early 1970s and before
A single chlorine atom would keep on
have been observed using instruments
destroying ozone for up to two years
such as the Total Ozone Mapping
were it not for reactions that remove
Spectrometer (TOMS).
them from this cycle by forming reservoir
species such as hydrogen chloride (HCl)
and chlorine nitrate (ClONO2). On a per
atom basis, bromine is even more
efficient than chlorine at destroying
ozone, but there is much less bromine in
the atmosphere at present. As a result
both chlorine and bromine contribute
significantly to the overall ozone
depletion. In the Earth’s stratosphere,
fluorine atoms react rapidly with water
and methane to form strongly bound HF,
while organic molecules which contain Reactions that take place on polar
iodine react so rapidly in the lower stratospheric clouds (PSCs) play an
atmosphere that they do not reach the important role in enhancing ozone
stratosphere in significant quantities. depletion. PSCs form more readily in the
Furthermore, a single chlorine atom is extreme cold of Antarctic stratosphere.
able to react with 100,000 ozone This is why ozone holes first formed, and
molecules. This fact plus the amount are deeper, over Antarctica.
chlorine released into the atmosphere by
In middle latitudes it is preferable to
CFCs yearly demonstrates how dangerous
speak of ozone depletion rather than
CFCs are to the environment.
holes. Declines are about 3% below pre-
Observations on Ozone Layer Depletion: 1980 values for 35-60˚N and about 35-
60˚S. In the tropics, there are no
The most pronounced decrease in ozone significant trends.
has been in the lower stratosphere.
However, the ozone hole is most usually Ozone depletion also explains much of the
measured not in terms of ozone observed reduction in stratospheric and
concentrations at these levels (which are upper tropospheric temperatures. The
typically of a few parts per million) but by source of warmth of the stratosphere is
reduction in the total column ozone, the absorption of UV radiation by ozone,
hence reduced ozone leads to cooling. form cloud particles. There are three
Some stratospheric cooling is also types of PSC clouds, nitric acid trihydrate
predicted from increases in greenhouse clouds, slowly cooling water-ice clouds,
gases such as CO2; however the ozone- and rapid cooling water ice clouds, that
induced cooling appears to be dominant. provide surfaces for chemical reactions
that lead to ozone destruction.
The Ozone Hole and Its Causes:
The photochemical processes involved
The Antarctic ozone hole is an area of the
are complex but well understood. The key
Antarctic stratosphere in which the
observation is that, ordinarily, most of the
recent ozone levels have dropped to as
chlorine in the stratosphere resides in
low as 33% of their pre-1975 values. The
stable reservoir compounds, primarily
ozone hole occurs during the Antarctic
HCl and ClONO2. During the Antarctic
spring from September to early
winter and spring, however, reactions on
December, as strong westerly winds
the surface of the polar stratospheric
starts to circulate around the continent
cloud particles convert these reservoir
and create an atmospheric container.
compounds into reactive free radicals (Cl
Within this polar vortex, over 50% of the
and ClO). The clouds can also remove NO 2
lower stratospheric ozone is destroyed
from the atmosphere by converting it to
during the Antarctic spring.
nitric acid, which prevents the newly
As explained above, the primary cause of formed ClO from being converted back
ozone depletion is the presence of into ClONO2.
chlorine containing source gases. In the
The role of sunlight in ozone depletion is
presence of UV light, these gases
the reason why the Antarctic ozone
dissociate, releasing chlorine atoms,
depletion is greatest during spring.
which then go on to catalyze ozone
During winter, even though PSCs are at
destruction. The Cl-catalyzed ozone
their most abundant, there is no light over
depletion can take place in the gas phase,
the pole to drive the chemical reactions.
but it is dramatically enhanced in the
During the spring however the sun comes
presence of polar stratospheric clouds
out, providing energy to drive
(PSCs).
photochemical reactions, and melt the
These polar stratospheric clouds form polar stratospheric clouds, releasing the
during winter, in the extreme cold. Polar trapped compounds. Warming
winters are dark, consisting of 3 months temperatures near the end of spring
without solar radiation (sunlight). The break up the vortex around mid-
lack of sunlight contributes to a decrease December. As warm, ozone rich air flows
in temperature and the polar vortex traps in from lower latitudes, the PSCs are
and chills air. Temperatures hover around destroyed, the ozone depletion process
or below -80˚C. These low temperatures shuts down, and the ozone hole closes.
Most of the ozone that is destroyed is in the globe, the effects could be
the lower stratosphere, in contrast to the substantially more dramatic. As the ozone
much smaller ozone depletion through hole over Antarctica has in some
homogeneous gas phase reactions, which instances grown so large as to reach
occurs primarily in the upper southern parts of Australia, New Zealand,
stratosphere. Chile, Argentina, and South Africa,
environmentalists have been concerned
Consequences of Ozone Layer Depletion:
that the increase in the surface UV could
Since the ozone layer absorbs UVB be significant.
ultraviolet light from the Sun, ozone layer Effects on humans:
depletion is expected to increase surface
UVB levels, which could lead to damage, UVB is generally accepted to be a
including increases in skin cancer. This contributory factor to skin cancer. In
was the reason for the Montreal Protocol. addition, increased surface UV leads to
Although decreases in stratospheric increased tropospheric ozone, which is a
ozone are well-tied to CFCs and there are health risk to humans.
good theoretical reasons to believe that
1.Basal and Squamous Cell Carcinomas:
decreases in ozone will lead to increases
in surface UVB, there is no direct The most common forms of skin cancer in
observational evidence linking ozone humans, basal and squamous cell
depletion to higher incidence of skin carcinomas, have been strongly linked to
cancer in human beings. This is partly UVB exposure. The mechanism by which
because UVA, which has also been UVB induces these cancers is well
implicated in some forms of skin cancer, understood- absorption of UVB radiation
is not absorbed by ozone, and it is nearly causes the pyrimidine bases in the DNA
impossible to control statistics for molecule to form dimmers, resulting in
lifestyle changes in the populance. transcription errors when the DNA
replicates. These cancers are relatively mild
Biological Effects: and rarely fatal, although the treatment of
The main public concern regarding the squamous call carcinoma sometimes
ozone hole has been the effects of requires extensive reconstructive surgery.
By combining epidemiological data with
increased surface UV and microwave
results of animal studies, scientists have
radiation on human health. So far, ozone
estimated that a one percent decrease in
depletion in most locations has been
stratospheric ozone would increase the
typically a few percent and, as noted
incidence of these cancers by 2%.
above, no direct evidence of health
damage is available in most latitudes. Others are Malignant Melanoma, Cortical
Were the high levels of depletion seen in cataracts.
the ozone hole ever to be common across
Increased Tropospheric Ozone- Increased crystals is far different from when it occurs
surface UV leads to increased tropospheric in atmosphere. These conditions have led to
ozone. Ground level ozone is generally ozone hole formation in Antarctica. The
recognized to be a health risk, as ozone is hypothesis was decisively confirmed, first
toxic due to its strong oxidant properties. At by laboratory measurements and
this time, ozone at ground level is produced subsequently by direct measurements, from
mainly by the action of UV radiation on the ground and from high altitude airplanes
combustion gases from vehicle exhausts. of very high concentrations of chlorine
monoxide (ClO) in the Antarctic
Effects on crops: stratosphere.
An increase of UV radiation would be Since 1981 the United Nations Environment
expected to affect crops. A number of Programme has sponsored a series of reports
economically important species of plants, on scientific assessment of ozone depletion
such as rice, depend on cyanobacteria based on satellite measurements. The 2007
residing on their roots for the retention of reports showed that the hole in the ozone
nitrogen. Cyanobacteria are sensitive to UV layer was recovering and the smallest it had
light and they would be affected by its been for about a decade. The 2010 reports
increase. found that Over the past decade, global
ozone and ozone in the Arctic and Antarctic
regions is no longer decreasing but is not yet
The Ozone Hole increasing, the ozone layer outside the Polar
region is projected to recover to its pre-1980
The discovery of the Antarctic ozone hole
levels some time before the middle of this
by British Antarctic Survey scientists
century. In contrast the spring time ozone
Farman, Gardiner and Shanklin came as a
hole over the Antarctic is expected to
shock to the scientific community, because
recover much later.
the observed decline in polar ozone was far
larger than anyone had anticipated. Satellite Ozone Depletion and Global Warming:
measurements showing massive depletion of
ozone around the south pole were becoming There are five areas of linkage between
available at the same time. However these ozone depletion and global warming:
were initially rejected as unreasonable by The same CO2 radiative forcing that
data quality control algorithms; the ozone produces global warming is
hole was detected only in satellite data when expected to cool the stratosphere.
the raw data was reprocessed following This cooling in turn, is expected
evidence of ozone depletion in situ produce a relative increase in ozone
observations. depletion in polar area and the
Moreover the polar vortex formed over frequency of ozone holes.
Antarctica is very tight and the reactions Conversely, ozone depletion
which occurs in the surface of the cloud represents a radiative forcing of the
climate system. There are two Ozone depletion chemicals are
opposing effects: Reduced ozone also greenhouse gases. The
causes the stratosphere to absorb increase in concentrations of
less solar radiation, thus cooling the these chemicals have produced
stratosphere while warming the 0.34 +- 0.03 W/m2 of radiative
troposphere, the resulting colder forcing corresponding to about
stratosphere emits less long wave 14% of the total radiative
radiations downward, thus cooling forcing from increases in the
the troposphere. concentrations of well mixed
greenhouse gases.
The long term modeling of the
process, its measurements,
study, design of theories and
testing take decades to
document, gain wide
acceptance, and ultimately
become the dominant paradigm.
Misconception about Ozone
Overall, the cooling dominates. Depletion:
One of the strongest predictions A few of the more common
of the greenhouse effect is that misunderstandings about ozone
the stratosphere will cool. depletion are addressed briefly
Although this cooling has been here:
observed, it is not trivial to
CFCs are too heavy to reach the
separate the effects of changes
stratosphere.
in the concentrations of
greenhouse gases and ozone It is commonly believed that CFC
depletion since both will lead to molecules are heavier than air, so
cooling. However this can be that the CFC molecules cannot
done by numerical stratospheric reach the stratosphere in significant
modeling. Results from the amount. But atmospheric gases are
National Oceanic and not sorted by weight; the forces of
Atmospheric Administration’s wind can fully mix the gases in the
Geophysical Fluid Dynamics atmosphere. Despite the fact that
Laboratory show that above 20 CFCs are heavier than air and with
km (1204 miles), the a long lifetime, they are evenly
greenhouse gases dominate the distributed throughout the
cooling. turbosphere and reach the upper
atmosphere.
Man-made chlorine is insignificant
compared to natural sources.
Another misconception is that “ it is
generally accepted that natural
sources of tropospheric chlorine are
four to five times larger than man-
made one.” While strictly true,
tropospheric chlorine is irrelevant;
it is stratospheric chlorine that
affects ozone depletion. Chlorine
from ocean spray is soluble and
thus is washed by rainfall before it
reaches the stratosphere. CFCs, in
contrast, are insoluble and long- The ozone hole should be above the
lived, allowing them to reach the sources of CFCs:
stratosphere. In the lower
atmosphere there is much more Some people thought that the ozone
chlorine from CFCs and related hole should be above the sources of
haloalkanes than there is in HCl CFCs. However, CFCs are well
from salt spray, and in the mixed in the troposphere and the
stratosphere hoalocarbons are stratosphere. The reason for
dominant. Only CH3Cl which is one occurrence of the ozone hole above
of these halocarbons has a mainly Antarctica is not because there are
natural source, and it is responsible more CFCs concentrated but
for 20% of the chlorine in the because the low temperatures help
stratosphere; the remaining 80% form polar stratospheric clouds. In
comes from man-made sources. fact, there are finding of significant
and localized “ozone holes” above
other parts of the earth.
The “ozone hole” is a hole in the
ozone layer:
There is common misconception
that “ozone hole” is really a hole in
the ozone layer. When the “ozone
hole” occurs, the ozone in the lower
stratospheric is destroyed. The
upper stratosphere is less affected,
so that the amount of ozone over
the continent decreases by 50% or
even more. The ozone hole does
not disappear through the layer, on
the other hand, it is not a uniform
‘thinning’ of the ozone layer. It is a
hole which is a depression, not in
the sense of a hole in the
windshield.
Reference:
www.ace.mmu.ac.uk
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