Ask The Scientist
What is ozone, and why is it
Ozone is simply a molecule
consisting of 3 oxygen atoms, which reacts strongly with other molecules. Ozone
is created in the stratosphere when high energy uv radiation causes on O2
molecule to split. The free oxygen atoms collide and react with other O2
molecules to form O3.
Production is highest where the
solar uv is the greatest eg near the tropics, but once created, the ozone is
then circulated towards the poles by the atmosphere. The amount of ozone in the
stratosphere can vary with location, season and even day to day climatic
The process of ozone creation is
what makes the O3 in the atmosphere very effective at shielding the
Earth from harmful uv radiation, which can cause many biological problems, such
as skin cancer. However, due to its high reactivity, the uv found in the
tropospher at ground level can aslo be dangerous as a toxic pollutant which is
harmful to plants and lung tissue, and is a major cause of smog.
When was the ozone hole
The discovery of the annual depletion of ozone above the Antarctic was first
announced in a paper by Joe Farman, Brian Gardiner and Jonathan Shanklin which
appeared in Nature in May 1985. Later, NASA scientists re-analyzed their
satellite data and found that the whole of the Antarctic was affected.
Why does the ozone hole form
over Antarctica ?
The answer is essentially
'because of the weather in the ozone layer'. In order for rapid ozone
destruction to happen, clouds (known as PSCs, Stratospheric Clouds Mother of
Pearl or Nacreous Clouds) have to form in the ozone layer. In these clouds
surface chemistry takes place. This converts chlorine or bromine (from CFCs and
other ozone depleting chemicals) into an active form, so that when there is
sunlight, ozone is rapidly destroyed. Without the clouds, there is little or no
ozone destruction. Only during the Antarctic winter does the atmosphere get cold
enough for these clouds to form widely through the centre of the ozone layer.
Elsewhere the atmosphere is just too warm and no clouds form. The northern and
southern hemispheres have different 'weather' in the ozone layer, and the net
result is that the temperature of the Arctic ozone layer during winter is
normally some ten degrees warmer than that of the Antarctic. This means that
such clouds are rare, but sometimes the 'weather' is colder than normal and they
do form. Under these circumstances significant ozone depletion can take place
over the Arctic, but it is usually for a much shorter period of time and covers
a smaller area than in the Antarctic.
How long has the Antarctic
ozone layer been studied?
Ozone was first measured from British Antarctic stations during the
International Geophysical Year of 1957/58. It was originally studied because of
its influence on the temperature structure of the atmosphere, and also as a
tracer for the circulation of stratospheric air. In the 1970s, ozone became the
focus of attention as a possible indicator of long-term changes in the
atmosphere. Scientists realised that ozone might be affected by the increasing
concentration in the atmosphere of man-made gases such as nitric oxide and CFCs.
In 1995, Paul Crutzen, Mario Molina and Sherwood Rowland received the Nobel
prize for this pioneering work.
Does the ozone hole affect the
rest of the world?
At the moment, catastrophic ozone depletion is only seen in the Antarctic during
the spring, but surrounding areas experience lowered ozone levels as the ozone
hole decays at the end of the spring. As the ozone hole rotates, it may extend
over populated areas for a short while when it is very elongated. For example it
covered the tip of South America and the Falkland Islands for over a week in
October 1994 (Fig. 12).
Limited ozone depletion can occur
above the Arctic, but at present it is confined to parts of the region and only
lasts for a few days at a time. If CFC releases had continued at the high rates
of the mid 1980s, a continental sized ozone hole might have appeared over the
Arctic. Elsewhere in the northern hemisphere, stratospheric ozone amounts over
temperate latitudes have fallen by 5 to 10% during the winter.
How does the ozone hole damage
All living cells, whether microbes, plants or animals, contain a complex
molecule called DNA which carries the genetic code. This is the set of
instructions which describes the structure and biochemistry of an organism.
Unfortunately, DNA readily absorbs high-energy UV-B radiation and becomes
damaged so that the instructions cannot be read properly. If the amount of UV-B
entering the cell increases (as during the ozone hole), the risk of damage also
increases and may result in malfunction or death of the organism. Some Antarctic
organisms such as algae, lichens and mosses also contain a pigment called
chlorophyll. This absorbs visible light as the energy source of photosynthesis
for making organic compounds. Chlorophyll also absorbs UV-B light so that the
system becomes bleached and non-functional. Even enzymes and other proteins are
damaged by this high-energy radiation. Living organisms therefore have to
protect themselves from UV-B. Humans can cover their skin with artificial
sunscreens, but natural protection systems have also evolved. Many microbes,
plants and other animals synthesize protective pigments. Our skin cells
synthesize brown melanin to protect against sunburn (which is caused by UV-B
radiation), and so do Antarctic lichens on rocks near the edge of the polar
ice-cap. A variety of suncreen pigments are produced by Antarctic organisms on
land, in freshwater and in the sea. That is why exposed, snow-free rocks are
often covered with bright orange and yellow lichens. Some lichens and microbes
even live inside translucent rocks to shelter from high radiation levels and
Does the Greenhouse effect
cause the ozone hole?
The Greenhouse Effect (producing global warming) and ozone depletion are two
separate problems, however there are links between them. Warming at the earth's
surface is caused by certain gases in the atmosphere which can trap energy from
the sun. An increase in the amount of these gases produces an increase in the
surface temperature. The largest increase is in carbon dioxide from burning
coal, oil, gas and forests, but other gases such as methane (from cattle and
rice fields) play a part. A link with ozone depletion is that CFCs are gases
which also contribute to greenhouse warming.
A further link is that although
the Greenhouse Effect warms the surface, it allows the higher atmosphere, where
ozone is present, to cool. This means that more stratospheric clouds may form
and so make the ozone hole worse.
Even if the problem of ozone
depletion is solved, global warming will still remain. It will cause a rise in
sea-level and change the regions where crops can be grown. The issue will be
harder to tackle than ozone depletion, but is one which concerns everyone on our
What is the Montreal Protocol?
The Montreal Protocol is an international agreement which was drawn up in
September 1987. It originally aimed to half the use of CFCs by 1999. However,
reviews of the protocol held in 1990 in London and 1992 in Copenhagen imposed
more stringent controls, so that all production of CFCs, CCl4 and halons should
cease by the year 2000. Many countries have even agreed to stop using CFCs
before this deadline. Production of other ozone depleting gases is to stop in
the early years of the 21st century. Unfortunately the ozone hole will not
immediately disappear as CFCs are such stable gases that they will remain in the
atmosphere for decades after release.
Independent reviews by panels of
scientist (eg the UK Stratospheric Ozone Review Group reports) present
conclusive evidence that CFCs are still increasing in the atmosphere and that
chlorine from them is also increasing and is responsible for ozone depletion.
Thanks to the provisions of the Montreal Protocol and its subsequent amendments
the level of ozone depleting gases in the atmosphere will start dropping by the
end of the 1990s.
Where were CFCs used?
CFCs were used in a wide variety of products. Thanks to public pressure the use
of CFCs by the aerosol industry declined rapidly. The other major uses were
"foam blowing" for upholstery padding, freezer linings, fast-food
cartons, cavity-wall insulation, and as the fluid in refrigeration and
air-conditioning systems. CFCs were also used as solvents in industrial and
electronic cleaning processes. Halons (bromo-fluoro-carbons) were extensively
used in fire extinguishing systems. CFCs were introduced because they were
generally odourless, non-toxic, stable, non-flammable and compressible
substances. It was their high stability which allowed them to get into the
stratosphere where they were broken down to release active chlorine. The two
simplest CFCs are CFC11 and 12 which have the chemical formulae CFCl3 and
Is the ozone hole recovering ?
Some reports in the
media suggest that the ozone layer over Antarctica is now recovering. This
message is a little confused. Recent measurements at surface monitoring stations
show that the loading of ozone destroying chemicals at the surface has been
dropping since about 1994 and is now about 6% down on that peak. The
stratosphere lags behind the surface by several years and the loading of ozone
depleting chemicals in the ozone layer is at or near the peak. Satellite
measurements show that the rate of decline in ozone amount in the upper
stratosphere is slowing, however the total ozone amount is still declining. The
small size of the 2002 ozone hole was nothing to do with any reduction in ozone
depleting chemicals and it will be a decade or more before we can unambiguously
say that the ozone hole is recovering. This assumes that the decline in ozone
depleting chemicals continues and that there are no other perturbations to the
ozone layer, such as might be caused by a massive volcanic eruption or Tunguska
like event. It will be the middle of this century or beyond before the ozone
hole ceases to appear over Antarctica. What we saw in 2002 is just one extreme
in the natural range of variation in the polar stratosphere and is the
equivalent of an extreme in 'stratospheric weather'. By contrast the 'weather'
in 2003 moved to the opposite extreme and we saw one of the largest ozone holes
How can we mend the ozone
The only way to med the ozone hole is to stop releasing CFCs and other ozone
depleting gases into the atmosphere. The restrictions of the Montreal Protocol
and its extensions are helping to do this.