The majority of ozone in the earth's atmosphere is found in the stratosphere(15-50km above the
earth's surface) This high level ozone plays a crucial role, protecting animals and plants from the
suns harmful ultra violet rays, and stabilising the earth's climate. This ozone layer is incredibly
unstable, since it is constantly being formed and broken down through interactions with UV radiation.
But, as it is so reactive the ozone layer can easily be broken down by pollutant gases rising from
the earth. CFC's and nitrous oxide are thought to be particularly responsible for the breakdown of
the ozone layer. Since the hole in the ozone layer has developed cases of skin cancer, particularly
have grown.
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The primary role of the ozone layer is to prevent the sun's harmful ultra violet rays from
reaching life on earth. This UV radiation is of a suitable energy to damage living cells, causing
sunburn which can lead to skin cancer. The areas of the planet with thinner ozone coverage have
much higher incidences of skin cancer. This is particularly true around the equator. It is thought
that a reduction of 1% of the ozone in the stratosphere could lead to:
- An increase in skin cancers in animals and humans
- A suppression of the human immune systems
- Some inhibition to plant life, and an increased susceptibility to pests
- Reduction in growth of phytoplankton, endangering the food chain
- A decrease in aquatic lifeforms
Ozone absorbs UV radiation in its formation and breakdown:
O2absorbs UV light to form 2 oxygen radicals:
O2 + hv O + O (1)
The oxygen radicals can then react in one of three ways:
O+O2 O3 (2)
O+O O2 (3)
O+O3 2O2 (4)
Reaction 2 is of course the reaction in which ozone is produced. This ozone can absorb ultra-violet
radiation of wavelength 10.1-14.0 x 1014Hz and undergo photodissociation:
O3 + hv O2 + O (5)
It is this reaction which is the most vital in shielding the earth from the sun's UV radiation.
Ozone in the stratosphere also plays a role in absorbing some of the infra-red radiation which
travels from the earth into space, keeping the earth warm. In this role it is partly responsible
for global warming. The primary culprits for this, however, are the pollutant gases from human
activities which seem to exaggerate the effect, making the earth too warm.
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In the early 1980's scientists who had been studying the atmosphere above the Antarctic realised
that ozone was disappearing from the area every spring. Data showed that the total amount of ozone
in the atmosphere declined by 2% between 1978 and 1985. This may not seem much, but ozone is a
trace element in the stratosphere maintaining a delicate balance. There are only a few parts of
ozone per billion.
Why were ozone levels decreasing? The answer came when Chlorofluorocarbons, were found in the
stratosphere. These chemicals had originally been developed because they were so stable. They were
used as refrigerants in air conditioners and fridges, Blowing agents in expanded
plastics(e.g. polystyrene), Aerosol Propellants, and Cleaning Solventsto dissolve grease,
particularly in dry cleaning. The stability of these gases mean that they can exist in the
atmosphere for as long as 75 years. During this time they can be carried by winds into the
stratosphere. It is here where they breakdown, due to the sun's high energy radiation, into hydrogen
fluoride and chlorine. It is the chlorine and the fluorine which are responsible for the destruction
of the ozone layer. Nitrogen oxides are also thought to be partly responsible for the decay of the
ozone layer, but they can also rebuild it as shown by the equations in Low Level Ozone.
Here chlorine will be used as the example:
When CFC's are destroyed they form Cl radicals. These chlorine radicals react with ozone:
Cl+O3 O2+ClO (6)
The ClO formed is another reactive free radical, which can react with oxygen atoms.
ClO+O Cl+O2 (7)O+O2 O3 (2)
So, reaction 4 and 6 are competing with each other to remove oxygen from the stratosphere. As the
concentration of Cl radicals is much lower than that of O atoms, this does not seem important.
However the rate of reaction of ozone with Cl is over 1500 times greater than that of the one with
O atoms. Another important consideration is that the Cl atoms are regenerated in reaction 7, so are
a catalyst which can go on to break down more and more ozone.
The hole in the ozone layer is particularly bad over Antarctica, although most of the CFC's are
released in the Northern Hemisphere. This is because winds blow the CFC's to the poles. Below is a
graph of the October levels of ozone above the Antarctic between 1980 and 1991:
The reduction over time is clear. The pictures corresponding to the graph are below. They show
how the ozone hole has grown over the past years. It is thought to continue growing until about 20
years after the complete phasing out of CFC's.
After much research by scientists all over the world governments decided that something had to
be done to stop the ozone depletion of the ozone layer. In 1987 24 countries signed a treaty aiming
to save the ozone layer. This Montreal Protocol called for a reduction in the production of ozone
depleting substances, aiming to reduce CFC's by 50% by the year 2000. Since then alternatives have
been found to fulfil the roles of CFC's and the amounts being released to the atmosphere have
dramatically reduced. However the ozone hole is still a problem needing to be carefully monitored.
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