Upper Atmosphere
Basics
Unit 2
Ozone and the formation of the ozone hole
Ozone is one of the most interesting trace gases in our atmosphere. In the
stratosphere it protects us from harmful ultra-violet radiation from the Sun. In
the troposphere, the layer of the atmosphere closest to Earth, high
concentrations of ozone are, however, a problem. Ozone is harmful to humans, it
irritates our throats and lungs and makes it difficult to breathe.
One of the most important scientific findings at the end of the last century was
the discovery of the ozone hole over Antarctica. In this unit we will look at how
this ozone hole formed and what measures have been taken so far to close it
(and we will try and do this without using too much chemistry!).
The discovery of the hole
Since the 1970's, measurements of stratospheric ozone have been made in
Antarctica. These measurements show that the ozone concentration has fallen
over time. There are many stories surrounding the discovery of the ozone hole.
The first measurements of really low ozone levels
were made over Antarctica in 1985. The levels
were so low that the scientists who made them
thought they weren't true, and that their
instruments were faulty. It wasn't until later,
when new instruments were used, that these low
values were found to be true.
At the same time, ozone levels were being made
from space aboard a satellite using an instrument
called TOMS (Total Ozone Mapping
Spectrometer). This instrument also didn't pick
up the low ozone values because values
recorded below an certain value were assumed to
be errors. It was only later, when the raw data
was reprocessed, that the results confirmed what
nobody wanted to believe.
On the trail of the missing ozone ...
© US Environmental Protection
Agency
Very intensive research then began and former
warnings about the potential harmful impact of
chlorofluorocarbons (CFC's) on ozone levels were
rediscovered. As a result, CFC's were banned as
part of the Montreal Protocol. Discovery of the
ozone hole showed that we humans are capable
of altering climate globally. It also proved that
rapid world-wide action can be taken to slow
down climate change.
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Part 1: Ozone Formation
Formation of ozone
If it wasn't for stratospheric ozone, life wouldn't be possible on Earth.
Ozone prevents harmful ultra-violet radiation from the Sun (light with
wavelengths less than 320 nm) reaching the ground. If allowed to reach
Earth, this radiation would severly damage the cells that plants and
animals are made up of. Ozone was first formed in the Earth's
atmosphere after the release of oxygen, between 2000 and 600 million
years before the first humans appeared.
Ozone formation and
destruction
1. Ozone formation - ultra-violet radiation from the
sun (shown in yellow) splits oxygen molecules into
oxygen atoms which then react with more O2 to form
ozone. Another molecule (M) is needed to absorb
some of the huge amount of energy involved in the
reaction.
Two forms of oxygen are found in
the stratosphere. Molecular
oxygen (O2), which is made up of
two atoms of oxygen (O), and
ozone (O3) which, as you can see
from its chemical formula, is
made up of three oxygen atoms.
Ozone is formed when intensive
ultra-violet radiation from the Sun
breaks down O2 into two oxygen
atoms. These highly reactive
oxygen atoms can then react with
more O2 to form O3.
In a similar way, ozone is
destroyed by solar
radiation. Ultraviolet radiation
hits ozone and breaks it back
down into molecular oxygen (O2)
and atomic oxygen (O). The
oxygen atom O then reacts with
another ozone molecule to form
two oxygen molecules.
2. Destruction of ozone by solar radiation.
Formation in the tropics, accumulation in polar regions
Since solar radiation is strongest over the tropical regions, most of the ozone is
formed in there. The Sun doesn't just drive this tropical ozone formation but
also allows tropospheric air to rise to higher altitudes here.
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Even though ozone is produced in
the tropics, concentrations
aren't particularly
high since intensive solar
radiation means that ozone loss in
this area is also high. Ozone is
transported from the tropics
towards the poles where it
accumulates. Ozone can
accumulate in these
regions as solar radiation isn't
strong enough here to cause
much ozone destruction. Unless
disturbed by the formation of the
Antarctic ozone hole in spring,
ozone levels are generally highest
in the cold subpolar regions.
Concentrations are slightly lower
at the poles themselves,
particularly in winter, as no
additional ozone can be
formed here because there isn't
any sunlight!
3. This scheme shows ozone transport. The
simulation below shows how measured ozone
concentrations vary between the poles and the equator
(low values = blue, high values = red). It also
shows air containing low ozone levels rising at the
Equator to high latitudes. Data from GOME (DLR, IUP
Bremen).
Ozone absorbs ultra-violet radiation
Absorption of the Sun's ultra-violet radiation during ozone formation and
destruction in the stratosphere has three important consequences.
1. less ultra-violet radiation
reaches the lower parts of
the atmosphere and, as a
consequence, the surface
of the Earth is protected
from damaging radiation.
2. since ultra-violet radiation
is important for both ozone
formation and destruction,
the amount of ozone that
can accumulate is limited as ozone formation
increases, ozone
destruction also increases.
3. ultra-violet radiation is
highly energetic. This
energy is transformed to
heat during
reaction leading to a
4. Absorption of ultra-violet radiation by ozone and
warming of the
other compounds in the stratosphere.
stratosphere. This is the
Adapted from Chemie Didaktik Uni Duisburg.
reason why the
temperature trend in the
stratosphere is opposite to
that seen in the
troposphere.
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Thickness of the ozone layer
The term "ozone layer" is often misunderstood. Its not really a single layer in
the atmosphere where ozone is concentrated. Rather it means that a higher
fraction of ozone molecules are found in the stratosphere (at altitudes between
18 and 40 km) compared to levels the troposphere below or the mesosphere
above. In reality, only about 10 out of every million molecules in the atmosphere
are actually ozone but 90% of all the ozone present in the atmosphere is found in
the stratosphere.
Dobson units (DU)
You will often see ozone levels reported in Dobson units (DU). 300 DU is a
typical value. But what does this mean? If we assume that all the ozone
molecules in the atmosphere were concentrated in a small layer at the ground
(rather than being spread over the whole stratosphere and troposphere) then
then thickness of this layer would be about 3 mm. Since 1 DU is equivalent to a
layer of pure ozone molecules 0.01 mm thick, a 3 mm layer of ozone is
equivalent to a value of 300 DU.
5. Visualisation of a Dobson Units
source: Univ. of Cambridge Ozone Hole Tour.
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Part 2: Ozone hole and CFC´s
Chlorofluorocarbons (CFC's) and the ozone hole
The story of the ozone hole is a good example of how an apparently
harmless class of chemical compounds can become a real danger to life
on Earth. It also shows how governments, industry and society can
work together to identify an environmental problem and solve it.
The ozone hole has taught us that human actions can change the natural
state of our climate in unexpected ways. It has also shown us that, if the
world community works together in a targetted way, global
environmental problems can be effectively dealt with. CFC's are just one
class of chemical substance that depletes the ozone layer, but they are
the most important one.
Properties of CFCs and their use
Chlorofluorocarbons are simple organic
compounds where all of the hydrogen
atoms have been replaced with the
halogens chlorine and fluorine (e.g.
CFC-11 CFCl3 and CFC-12 CF2Cl2).
Commercially they are well known
as FREON's. CFC's were used, for
example, as refrigerants, as solvents in
the electronic industry, as aerosol
propellants, in fire extinguishers, as
dry cleaning solvents, as degreasing
agents and in foam packing and
insulation materials.
1. Use of CFC's. Information from the US
Environmental Protection Agency.
CFC's were used extensively because, in the troposphere, they are completely
unreactive and do not affect human health in any way. This gives them extremely
long atmospheric lifetimes and allows then to accumulate in the atmosphere. The
fatal properties of the compounds which humans didn't take into account is that
they are broken down by high energy ultra-violet radiation from the Sun.
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The stratospheric fate of CFC's
2. The development of the ozone hole 2001.
Original animation provided by the NOAA Climate
Monitoring and Diagnostics Laboratory, Boulder,
Colorado.
High energy ultra-violet radiation
from the Sun is absorbed by
stratospheric ozone and, as a
result, doesn't enter the
troposphere. This means that
the ultra-violet light which does
reach the Earth's surface is too
weak to breakdown the CFC's
present there. However, because
CFC's have such a long life time in
the atmosphere, significant
amounts entered the stratosphere
where the ultra-violet radiation
was strong enough to break them
down into very reactive chlorine
and fluorine radical species.
These radicals are capable
of destroying ozone.
This process, however, doesn't necessarily lead to strong ozone depletion
because chlorine radicals (which are mainly responsible for ozone removal) also
undergo other reactions depending on the meteorological conditions. Although
stratospheric ozone is removed at all latitudes by reaction with chlorine and
fluorine radicals, the ozone hole is only formed at the poles, particularly over
Antarctica and only in the spring. Why is this so?
How halogen radicals
destroy ozone
Under natural conditions,
ozone levels are relatively
constant since they are both
formed and destroyed by
ultra-violet light. So as ozone
concentrations increase, the
amount of
ozone destroyed also
increases. However, chlorine
radicals (Cl) react with ozone
simply to destroy it. They are
very efficient at removing
ozone because they act as
catalysts. This means that
they are not consumed by the
reaction but are recycled and
can continue to react with
other ozone molecules to
destroy them as well.
3. Chlorine ozone reaction. Dots indicate, that the reaction
partners are radicals.
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The conditions
Decomposition of the CFC's leads to chlorine monoxide radicals (ClO). These
can then react with nitrogen dioxide (NO2) to form chlorine nitrate (ClONO2) or
with nitrogen monoxide (NO) and methane (CH4) to form hydrochloric acid (HCl)
and nitric acid (HNO3). We will not focus on the chemistry here, but it is
important to know that both HCl and ClONO2 do not react with ozone but are
rather stable compounds and remove chlorine from the ozone destruction
mechanism.
It is only under certain meteorological conditions that ozone holes form. It took
over two years of research at the British Research Station at Halley Bay in
Antarctica to finally understand what these conditions are.
1. One factor is the extremely
low temperatures in the
stratosphere. During the night
temperatures can be as low as 80 oC over Antarctica. Under
these conditions, nitric acid and
water form stratospheric ice
clouds. On the surface of the ice,
hydrochloric acid and ClONO2
react with each other to form
nitric acid and molecular chlorine
(Cl2).
2. Molecular chlorine (Cl2) is a
stable
molecule which does not
4. Chemistry on polar stratospheric clouds leads to the
dangerous chlorine radicals Cl (red).
react with ozone. However, it is
easily broken down by ultra-violet
radiation from the Sun to form
two chlorine radicals which can
then attack and destroy ozone.
So high levels of molecular chlorine (Cl2) can be produced in the stratosphere at
the poles during the winter. In the spring, the Sun reappears and levels of solar
ultra-violet radiation increase. This ultra-violet radiation breaks down the Cl2 into
chlorine radicals, these then destroy ozone and an ozone hole forms. As a result,
we see the ozone hole at the same time each year and ozone levels don't recover
until the ice clouds thaw and the chlorine radicals are removed by other reactions.
5. Ozone hole development in Antarctic spring 1998. Data from GOME.
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6. The ozone hole in the polar vortex over
Antarctica in September / October 1996.
Adapted from UKMO data set, published at
Brown University.
3. Certain meteorological conditions are
then required to allow the ozone hole to
develop fully. Formation of the chlorine
monoxide radical (ClO) usually occurs at
high up in the stratosphere, away from
the most of the ozone molecules which
are found at altitudes between 14 and 22
km high. So chlorine radicals shouldn't
really affect most of the ozone in the
stratosphere. This isn't the case,
unfortunately. Special meteorological
conditions around the poles allows a polar
vortex to form and this causes downward
air movement. This transports ClO to
lower altitudes where it plays a part in
ozone destruction.
As you can see, the conditions required to form the ozone hole:
•
•
•
•
cold temperatures during the polar winter
ice cloud formation
special meteorological conditions to form the polar vortex
followed by the polar sun rise in the spring
are so special that it is really unlikely that scientists could ever have predicted the
ozone hole.
The future of the ozone hole
CFCs were banned globally by the
Montreal Protocol on substances that
deplete the Ozone layer-1987 (and
further amendments). As they have
such long atmospheric lifetimes (the
longest lived have lifetimes of the order
of 100 years) it will take around another
50 years until all the CFC's released so
far have been destroyed in the
stratosphere and ozone concentrations
stabilise. Maximum CFC concentrations
in the stratosphere were predicted to
occur in 2000 and the ozone hole has
been rather constant in size over the
past few years. However, there are
exceptions to this trend. In 2002 no
significant ozone hole was seen. The
reason why was simple: it was too warm
and the polar vortex did not form as
usual. Once again, an example that
atmospheric processes sometimes ignore
any prediction! But in 2003 the hole was
back to its previous size, the second
largest ever observed.
7. Developement of the concentrations of the
two most important CFCs (also called FREON
11 and FREON 12). Data from: Walker et al.,
J. Geophys. Res., 105, 14,285-14,296, 2000
[internet plots]. Picture by Gian-Kasper
Plattner (Univ. of Bern, UCLA).
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