Earth's sunscreen – the ozone layer

Key text

A natural balance keeps us well supplied with ozone

Up in the stratosphere, small amounts of ozone are constantly being made by the action of sunlight on oxygen. At the same time, ozone is being broken down by natural processes. The total amount of ozone usually stays constant because its formation and destruction occur at about the same rate.

Human activity has recently changed that natural balance. Certain manufactured substances (such as chlorofluorocarbons and hydrochlorofluorocarbons) can destroy stratospheric ozone much faster than it is formed.

Ozone is a natural sunblock

Go outside on a fine day and feel the sun warm your face. What happens when a cloud passes over? You’ll notice that the cloud takes away some of the heat and light coming from the sun. In much the same way that a cloud blocks the heat on a hot day, the ozone layer in the stratosphere blocks out the sun’s deadly ultraviolet rays. It acts as our planet’s natural sunblock.

The sun doesn’t just produce heat and light. It throws out all sorts of other types of electromagnetic radiation, including ultraviolet radiation (Box 1: Meet the ultraviolet family). Because ultraviolet radiation can damage DNA it is potentially harmful to most living things, including plants (Box 2: Can plants get sunburn?).

Unfortunately our bodies can't detect ultraviolet radiation directly. We can be unaware of the harm it is doing until it is too late – for example, at the end of a day in the sun without adequate protection.

When there is less ozone in the stratosphere, more ultraviolet radiation hits us

Even a 1 per cent reduction in the amount of ozone in the upper atmosphere causes a measurable increase in the ultraviolet radiation that reaches the Earth's surface. If there was no ozone at all, the amount of ultraviolet radiation reaching us would be catastrophically high. All living things would suffer radiation burns, unless they were underground, in protective suits, or in the sea,

So what exactly is ozone?

Ozone is a form of oxygen. Each ozone molecule is made of three oxygen atoms, so its chemical formula is O₃. But unlike oxygen, ozone is a poisonous gas, and an increase in its concentration at ground level is not something that we want. But in the stratosphere, where ozone exists naturally, it blocks out the sun's ultraviolet rays and is a life-saver.

Ozone-depleting substances usually contain chlorine or bromine

The synthetic chemicals called chlorofluorocarbons (CFCs) are now well-known as environmental ‘baddies’, even though they are useful and completely non-toxic substances. They get their bad name because they are ozone-eaters (properly called ozone-depleting substances). CFCs are not the only ozone-depleting substances, but they are the most abundant. Some ozone-depleting substances are naturally occurring compounds.

Ozone-depleting substances are long-lived because it takes them several years to drift up into the stratosphere. When they arrive, they are broken apart by exposure to ultraviolet radiation and that releases the chlorine atoms. These are the real ozone-killers. The chlorine atoms react with ozone, to form oxygen and chlorine monoxide.

Ozone loss occurs mainly at the poles

The ozone-destroying reactions take place most rapidly only under certain conditions in the stratosphere. These conditions – extreme cold, darkness and isolation, followed by exposure to light – occur over the polar regions after the long polar winter has finished and the first spring sun appears.

Antarctica is the worst affected area, probably because the air above it is most isolated from the rest of the atmosphere (Box 3: How ozone is lost). Scientists often refer to the part of the atmosphere where ozone is most depleted as the ‘ozone hole’, but it is not really a hole – just a vast region of the upper atmosphere where there is less ozone than elsewhere.

Ozone-poor air can spread out from the polar regions and move above other areas. In addition, direct ozone loss elsewhere is slowly increasing, although it is not occurring at the same rate as over the poles.

Scientists around the world regularly monitor ozone-depleting substances and the amount of ozone in the stratosphere. In Australia, the Australian Bureau of Meteorology and the CSIRO Division of Atmospheric Research jointly manage the Cape Grim Baseline Air Pollution Station. The Cape Grim station is located in remote north-western Tasmania, in the path of strong westerly winds that carry air thousands of kilometres across the Southern Ocean. Air at Cape Grim is regularly sampled in order to monitor atmospheric composition. (Another reason to monitor ozone-depleting substances is because most are also powerful greenhouse gases.)

Most ozone-depleting substances are banned or strictly controlled

Many substances other than chlorofluorocarbons are also ozone-depleting. Examples are carbon tetrachloride (used in dry cleaning), and methyl bromide (used as an insecticide for soil fumigation). An Australian scientist (Jonathan Banks) has been internationally recognised for his work in finding a replacement for methyl bromide (Box 4: Australia finds a replacement for methyl bromide).

CFCs, previously used as refrigerants, foam-blowing agents and propellants in spray cans, are now banned in Australia (and many other countries). Their temporary replacements, the hydrochlorofluorocarbons, are still slightly ozone-depleting, though not to the same extent. HCFCs are also being phased out.

An international agreement called the Montreal Protocol limits the production and use of ozone-depleting substances. A slowing down in the rate of ozone loss has been measured, and the concentration of CFCs in the atmosphere is levelling off. But because of a long lag time, ozone depletion will get worse at least until the year 2000 and the ozone hole will continue for some decades after that. If all countries keep to the targets set by the international community in the amendments to the Montreal Protocol, the ozone in the stratosphere should eventually recover.

Box 1. Meet the ultraviolet family

Ultraviolet is normally used to describe radiation with wavelengths between about 100 and 400 nanometres. (A nanometre is one-millionth of a millimetre.) As with all electromagnetic radiation, the shorter the wavelength the greater the energy carried.

The ultraviolet family can be divided into three parts:

UV-A (315-400 nanometres) has the longest wavelengths of the family and is the least damaging. However, it does cause sunburn and has been implicated in causing sun-induced premature ageing of skin and some cancers. This is the form of ultraviolet produced in most solariums.

UV-B (280-315 nanometres) can cause skin cancer and eye damage. It also causes sunburn. Radiation with a wavelength close to 280 nanometres is strongly absorbed by proteins, altering and often damaging their function. In this way, UV-B can reduce the immune response and it also interferes with photosynthesis in some crop plants. A very small amount of exposure to UV-B is necessary to produce vitamin D in human skin.

UV-C (100-280 nanometres) is the most dangerous member of the family. Wavelengths around 260 nanometres are absorbed by DNA and so nearly all life forms are irreparably damaged by this radiation.

The good news is that the stratospheric ozone layer absorbs all UV-C, the most deadly form, and even a thinned ozone layer is unlikely to let much through.

The intact ozone layer does, however, let through some UV-A, especially when the sun is high in the sky, and a very small amount of UV-B. The proportion of both of these reaching ground level will increase with ozone loss.

Many species have some natural protection against UV-A. For example, we can produce melanin, a dark pigment, in the outer layer of our skin. However, pale-skinned people can't produce enough melanin to protect against the amount of ultraviolet radiation that occurs across all of Australia for most of the year (even on cloudy days). Even dark-skinned people, who naturally have high melanin concentrations in their skin, can suffer sunburn after long periods of exposure.

Box 2. Can plants get sunburn?

Tests have shown that plants vary in their sensitivity to UV-B. Most species tested so far have been crops. In experiments subjecting rice plants to a 33 per cent increase in UV, the plants were visibly damaged and the yield of rice-grain fell by 20 per cent. A 33 per cent increase in UV-B at mid-latitudes is not considered likely to occur. However, a 20 per cent increase is a possibility.

Some rice varieties are more resistant to UV than others. As might be expected, these have been grown in high-altitude areas for generations – where the thinner atmosphere ensures naturally higher UV levels. It is quite possible that non-domesticated, wild relatives of food crops may contain valuable genes coding for UV resistance or for repair mechanisms for UV-induced damage. Also, many plants can produce UV-absorbing compounds; and increased exposure to UV stimulates greater production, up to a point. Just as different races of humans can produce different quantities of protective melanin, so plants differ.

In the sea, phytoplankton are also at risk. (Phytoplankton are important because they remove carbon dioxide from the air. They are also at the base of many marine food chains.)

Further reading

• Ecos, No. 68, 1991, pages 28-30
  Plants in the sun (by Roger Beckmann)

• The effects of ozone depletion (Environmental Protection Agency, USA)

Box 3. How ozone is lost

The final link in the destruction of ozone requires light. During the winters in the polar regions, the sun never rises. Polar stratospheric clouds form in the cold conditions, and chemical reactions that are necessary for ozone destruction occur. But the final link in the chain does not start until the sun returns in spring. Then, the ozone is destroyed rapidly. Fortunately, the polar stratospheric clouds also disappear as the stratospheric temperature warms up. Net ozone loss then ceases, and the layer will gradually be replenished, but not quite to its former level. Then, the following spring, further depletion will occur.

Arctic ozone loss does occur, and is worsening, but it is not as severe as that measured over Antarctica. The reason is that strong winds develop around Antarctica in the winter, isolating the atmosphere there from the rest of the world. As a result of being shut off from any warmer air from elsewhere, temperatures in the stratosphere fall so low that the formation of polar stratospheric clouds is greater than over the Arctic. (And there is no ‘fresh’ air coming in to dilute the build-up of reactants in the chain of destruction.) In September, ozone loss occurs at its greatest rate over Antarctica as the sun rises. In November, warm air from the rest of the world starts to break through and ozone-poor air moves away thus reducing the average concentration of ozone in the southern hemisphere.

Recently, scientists have become concerned about ozone loss over other regions of the world. Although these losses are nowhere near as great at that recorded over Antarctica, they are worrying because of their possible effects on humans and our crop plants. Over North America, for example, ozone levels fell by about 0.5 per cent per year from 1978 to 1990. The details of the mechanisms causing these mid-latitude losses remain obscure. Polar stratospheric clouds don't seem to be involved. The only region where stratospheric ozone has not diminished is the tropics – and that’s just as well because levels of ultraviolet radiation are at their highest there.

Over Australia, the average amount of stratospheric ozone has also declined, with the losses being greatest in the more southern latitudes. As a result, ultraviolet radiation at ground level has increased. Hobart, for example, has recorded increases of UV-B of about 4 to 6 per cent since 1980.

Box 4. Australia finds a replacement for methyl bromide

Unlike chlorofluorocarbons (CFCs), methyl bromide gets into the atmosphere naturally – particularly in volcanic eruptions – but human activity is probably responsible for releasing much more. Molecule for molecule, it is actually a more efficient ozone-killer than the CFCs, but it breaks down faster than they do.

By 2001, the countries of the developed world are planning to cut their emissions of methyl bromide by a quarter and to reduce emissions to zero by 2025.

Dr Jonathan Banks, a researcher in CSIRO’s Stored Grain Research Laboratory at the Division of Entomology, has been at the forefront of finding ways to avoid using methyl bromide. For example, covering an area of ground with plastic and then waiting for the sun to heat it up is quite an effective way to sterilise soil. If the temperature is carefully monitored, there’s even a bonus: it is possible to kill the harmful bugs without affecting many of the good ones!

Fumigation of produce can also be carried out using other chemicals – for example, carbon dioxide – and a whole suite of different methods can be used to manage pest infestations.

In recognition of his work, Dr Banks received the 1996 Stratospheric Ozone Protection Award from the US Environmental Protection Agency.

Related site

• CSIRO researcher wins ozone award

• Protecting ozone layer earns 'Best of the best' award

Activities

• Bureau of Meteorology (Australia)
  • The ups and downs of ozone – students graph and interpret data relating to ozone measurements.

• Exploratorium (USA)
  • Graphing – provides images showing ozone concentrations (1979-1992). Students generate a data table then draw a graph.
  • Introduction – suggests more advanced graphing exercises.

Activity 1. Comparing and contrasting ozone in the stratosphere with lower atmospheric ozone

Teachers notes

Ozone is the same compound in the stratosphere as it is at ground level. However the concentrations of ozone are very different in the two places – in the stratosphere it naturally peaks at about 8 parts per million while at ground level it is measure in parts per hundred million. Ozone in the stratosphere is beneficial – it protects the Earth from ultraviolet radiation. At ground level, ozone is a poisonous gas that can cause inflammation of the respiratory system in higher animals and reduce plant growth.

Activity 2. Correcting false statements

1. Ozone consists of oxygen atoms.
2. Ozone is poisonous.
3. Methane destroys the ozone layer.
4. Chlorofluorocarbons are greenhouse gases.
5. Ozone concentrations are highest in the troposphere.
6. Ultraviolet radiation is highest at the equator.
7. The greatest loss of stratospheric ozone has occurred over the equator.
8. Atmospheric ozone forms in the darkness of the Antarctic winter.

Teachers notes

1. True
2. True
3. False. A true statement would be: Methane does not destroy the ozone layer. It is a greenhouse gas.
4. True
5. False. A true statement would be: Ozone reaches its highest concentration in the stratosphere.
6. True
7. False. A true statement would be: The biggest loss of stratospheric ozone has occured over the poles, especially over the southern pole.
8. False. A true statement would be: Atmospheric ozone forms by the action of ultraviolet radiation on oxygen.

This exercise is valuable because this topic is likely to have been introduced to students via the media and the students may have picked up misconceptions, distortions or incorrect information.

In particular students frequently confuse ozone depletion with the enhanced greenhouse effect. It is important to emphasise that the two effects are completely different.

You could use this activity before beginning the topic to see if students are confused about some aspects.

Activity 3. Properties of chlorofluorocarbons (CFCs)

Teachers notes

CFCs are stable, non-flammable, low in toxicity, inexpensive to produce, and have a convenient boiling point and low critical pressure for liquification. The last two properties mean that CFCs are ideal for the cycle of evaporation and condensation involved in refrigeration and air conditioning.

Their low flammability and toxicity meant that there were few concerns about their use or escape. Their low cost and variety of uses meant that they were produced in large quantities. Because CFCs are stable, they remain indefinitely in the atmosphere and are gradually distributed around the Earth and into the stratosphere.

Activity 4. Analysing the reduction of use of CFCs in Australia

Year	1989	1990	1991	1992	1993	1994	1995
Quantity of CFCs (in tonnes)	14 000	8 000	7 000	6 000	5 300	3 900	2 800

1. Discuss the factors that you think might have been involved in successfully reducing CFC use.

2. Have the changes needed to reduce the CFC use made much difference to your lifestyle?

3. Why are governments more reluctant to legislate to reduce carbon dioxide emissions than they were to reduce CFC use?

Teachers notes

1. Factors involved in the successful reduction of CFC use may include:
   • the ability to substitute other compounds for CFCs (eg, as refrigerants or spray-can propellants);

   • the guidelines of the Montreal Protocol, resulting in legislation in Australia restricting CFC production and use;

   • reviewable targets that are achievable;

   • voluntary action often occurs if industry and the public are informed for a genuine threat to public health and the environment (eg, the high risk of melanoma and the links with ultraviolet radiation);

   • government controls and economic incentives to industry;

   • changes to government purchasing policies;

   • some commercial organisations see advantages in reducing CFC use and publicising their environmentally friendly actions.

2. Most students would probably say that the switch from CFCs had not affected their lifestyles since, in most cases, alternative, ozone-friendlier compounds have been substituted (eg, hydrocarbons in spray-cans).

3. Governments are usually reluctant to make decisions that would have an adverse effect on current lifestyles and standards of living.

Activity 5. Graphing and analysing CFC-11 data

Year	1978	1980	1982	1984	1986	1988
Concentration of CFC-11 at Cape Grim, Tasmania (parts per trillion)	135	160	175	190	210	230

1. Graph the data given for CFC-11 concentrations from 1978 to 1988.

2. Assuming the line continues in the same fashion, estimate the concentration of the compound now. Try to find the latest figure and see how it compares with your extrapolated one. (Make sure that you check whether the current figure you look up is for CFC-11 and not all CFCs combined.)

3. Extrapolating backwards, estimate when the concentration of CFC-11 was effectively zero.

4. Write a paragraph explaining why CFCs in the atmosphere concern environmental scientists, even though their concentration is low.

Teachers notes

2. Students will probably estimate the current concentration of CFC-11 at 340 to 360 parts per trillion. In actual fact the concentration has stabilised and the 1996 to 2005 concentrations are closer to 260 parts per trillion.

3. Extrapolating backwards, students should estimate that the concentration of CFC-11 was effectively zero in about 1976. This reflects the explosion of use of CFCs in the 1970s.

4. Students should include in their answers the stable nature of CFCs and their ability to act as catalysts for ozone destruction.

Activity 6. Different aspects of ozone depletion

• how has the Earth's atmosphere changed over the past 4700 million years?
• the work of the World Meteorological Organization;
• the Montreal Protocol;
• how the volume of CFCs that escape from car air conditioners can be reduced;
• how we measure stratospheric ozone.

Activity 7. Can ozone depletion be stopped quickly?

Teachers notes

Ozone is removed from the atmosphere when an oxygen atom (O) and an ozone molecule (O3 ) meet and react. This reaction is slow. Certain compounds such as chlorine (Cl) and chlorine monoxide (ClO) can catalyse the reaction. The chlorine atoms are catalysts, so they are not consumed during the reaction and can go on to catalyse further reactions. So each chlorine atom introduced into the stratosphere can destroy thousands of ozone molecules before it is removed. (Note that the process is even more dramatic for bromine. It has no stable reservoirs so it can destroy millions of molecules of ozone before it is removed. Fortunately bromine is less abundant than chlorine in the stratosphere.)

There is also a time lag before the chlorine molecules reach the stratosphere. It takes about 5 years for chlorine levels at ground levels in the troposphere to drift up to the stratosphere.

Further reading

25 March 2000, pages 24-28
The hole story (by Gabrielle Walker)
Looks at possible connections between the ozone layer and greenhouse warming.

Useful sites

The ozone layer (National Oceanic and Atmospheric Administration, USA)

Written in non-technical language this provides a good introduction to the topic. Ozone formation and destruction is described simply and research into the ozone hole is discussed.
http://www.oar.noaa.gov/climate/t_ozonelayer.html

The ozone layer: life's protective blanket (Australian Antarctic Division)

Looks at why the thinning of the ozone layer occurs over the Antarctic in spring and summer.
http://www-new.aad.gov.au/default.asp?content=dynamic&title=The+ozone+layer&casid=750&docid=578&type=1&children=

Ozone depletion (Atmospheric Research and Information Centre, UK)

Provides a series of fact sheets on ozone depletion, organised under three headings: 'The science of ozone depletion', 'The impacts of ozone depletion' and 'Managing ozone depletion'.
http://www.ace.mmu.ac.uk/Resources/Fact_Sheets/Key_Stage_4/Ozone_Depletion/contents.html

The Nobel Prize in chemistry 1995 (Nobelprize.org, Sweden)

This poster describes the work on ozone chemistry that won a Nobel Prize.
http://nobelprize.org/chemistry/educational/poster/1995/index.html

The changing atmosphere in 2005 (Australian Academy of Science)

The transcript of a lecture by Nobel laureate Sherwood Rowland about chemical reactions in the atmosphere.
http://www.science.org.au/events/lectures-and-speeches/rowland.htm

Australian Government Department of the Environment and Water Resources

• Ozone and synthetic greenhouse gases
  Provides information on ozone depleting substances, synthetic greenhouse gases and Australian regulations to implement the Montreal Protocol.
  http://www.environment.gov.au/atmosphere/ozone/

• Australian halon management strategy
  Describes strategies for management of ozone-depleting substances in 'The Australian Halon Management Strategy (AHMS)' and 'Controlled substances'.
  http://www.environment.gov.au/atmosphere/ozone/publications/halonstrategy.html

Australian Broadcasting Corporation

• Why is the ozone hole in the less polluted southern hemisphere?(Ask an expert, 3 June 2009) Explains why the ozone hole is in the southern hemisphere. http://www.abc.net.au/science/articles/2009/06/03/2588286.htm

• Record ozone loss over Antarctica (News In Science, 3 October 2006)
  Reports that European Space Agency measurements indicate that Antarctica suffered its highest recorded single-year loss in ozone.
  http://www.abc.net.au/science/news/stories/2006/1754508.htm?enviro

• Ozone hole is recovering (News in Science, 31 August 2006)
  Suggests that the ozone layer may be fully recovered by mid-century.
  http://www.abc.net.au/science/news/stories/s1728666.htm

• Antarctic ozone hole may have peaked (News in Science, 19 October 2005)
  Suggests that the hole in the ozone layer has reached a plateau in size.
  http://abc.net.au/science/news/enviro/EnviroRepublish_1485579.htm

Ask the experts (Scientific American)

Answers questions about the stratospheric ozone hole and surface level ozone.
http://www.scientificamerican.com/article.cfm?id=experts-ozone-hole-in-atmosphere-not-on-ground

UV forecast chart (Australian Bureau of Meteorology)

Daily UV forecasts are available here.
http://www.bom.gov.au/weather/national/charts/UV.shtml

The Ozone Hole Tour (Centre for Atmospheric Science, University of Cambridge, UK)

Covers the discovery of the hole, recent ozone loss over Antarctica, the science of the ozone hole and latest research.
http://www.atm.ch.cam.ac.uk/tour/

Science@NASA (USA)

• The incredible shrinking ozone hole
  Reports that the ozone hole was the largest ever in 2000, but closed up earlier than it has in recent years.
  http://science.nasa.gov/headlines/y2000/ast12dec_1.htm?list153136

• Peering into the ozone hole
  Explains why the size of the ozone hole is not simply related to the concentration of CFCs in the stratosphere.
  http://science.nasa.gov/headlines/y2000/ast02oct_1.htm?list41469

The ozone depletion phenomenon (Beyond Discovery, National Academy of Sciences, USA)

A detailed look at the basic research that led to the discovery of stratospheric ozone depletion. (A PDF file of the complete article is available.)
http://www.beyonddiscovery.org/content/view.article.asp?a=73

The science of ozone depletion (Environmental Protection Agency, USA)

This site has many useful annotated links. It is very up-to-date and has an extensive glossary.
http://www.epa.gov/ozone/science/

Ozone depletion FAQ Part I: Introduction to the ozone layer
http://www.faqs.org/faqs/ozone-depletion/intro/

Ozone depletion FAQ Part II: Stratospheric chlorine and bromine
http://www.faqs.org/faqs/ozone-depletion/stratcl/

Ozone depletion FAQ Part III: The Antarctic ozone hole
http://www.faqs.org/faqs/ozone-depletion/antarctic/

Ozone depletion FAQ Part IV: UV radiation and its effects
http://www.faqs.org/faqs/ozone-depletion/uv/

This set of FAQs (frequently asked questions) has been put together by Robert Parson, a chemist at the University of Colorado.

Glossary

DNA (deoxyribonucleic acid). The nucleic acid forming the genetic material of all organisms with the exception of some viruses which have RNA. DNA is present in the nucleus and other organelles such as mitochondria and chloroplasts.

electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Back to Basics: Electromagnetic radiation (Australian Academy of Science) and Electromagnetic Spectrum (NASA's Observatorium, USA).

greenhouse gas. A gas that is transparent to incoming solar radiation and absorbs some of the longer wavelength infrared radiation (heat) that the Earth radiates back. The result is that some of the heat given off by the planet accumulates, making the surface and the lower atmosphere warmer. For more information see The greenhouse effect (CSIRO Atmospheric Research, Australia).

hydrochlorofluorocarbons (HCFCs). Organic compounds like CFCs but with extra hydrogen atoms, and a lower ozone-destroying potential. They have similar properties to CFCs and are being used as temporary substitutes for them.

Montreal Protocol. An intergovernmental document signed by many countries in 1987 (and regularly revised) which established restrictions for the manufacture and use of ozone-depleting substances in an international effort to reduce ozone depletion. The text of the Protocol with the 1990 and 1992 amendments is available.

ozone. Ozone (O₃ ) is a form of oxygen. It is a colourless gas that has a very pungent odour. It exists naturally at low concentrations in the stratosphere where it absorbs ultraviolet radiation. In the troposphere it exists naturally at extremely low concentrations. But these concentrations increase when sunlight acts on various gases, coming mainly from vehicle exhausts, and ozone then becomes a pollutant in the troposphere. Ozone is a highly corrosive gas and is poisonous to most organisms. At concentrations as low as 0.00001 per cent (or 10 parts per hundred million) it can irritate the membranes lining the nose, throat and airways and can trigger or exacerbate asthma attacks.

ozone-depleting substances. Any substance that causes a net loss of ozone in the stratosphere. Such substances must be sufficiently stable to survive the time needed to mix into the stratosphere. Common ozone-depleting substances are the CFCs (there are more than one hundred different types), the HCFCs, carbon tetrachloride and methyl chloroform – all of which contain chlorine; as well as methyl bromide and oxides of nitrogen. Some ozone-depleting substances are naturally occurring, but by the far the greatest ozone-depleting potential comes from compounds synthesised and/or released as a result of human activity.

ozone formation and destruction. Ozone is formed when ultraviolet radiation causes oxygen molecules (O₂ ) in the upper layers of the atmosphere to split apart. If a freed oxygen atom (O) bumps into an oxygen molecule (O₂ ), the three oxygen atoms re-form as ozone (O₃ ).

Ultraviolet radiation can cause ozone to break apart, resulting in an oxygen molecule (O₂ ), and a single oxygen atom that is highly reactive. The oxygen molecule is quickly converted back to ozone. The reactive oxygen atom can play a part in breaking down more ozone molecules if ozone-depleting substances are present.

ozone 'hole'. The ozone 'hole' does not refer to a complete absence of ozone molecules but rather a general decrease in the number of ozone molecules scattered throughout a band of the stratosphere above certain regions of the Earth. The phenomenon is more like a carpet thinning.

polar stratospheric clouds (PSCs). Long faint clouds which form in the stratosphere only when the temperature falls below about -80°C. They are common above the poles in winter. These clouds appear to play a role in the depletion of stratospheric ozone. The ice particles in the cloud provide surfaces on which a reaction takes place to release free chlorine. The chlorine then reacts with ozone to form chlorine monoxide and oxygen.

stratosphere. The layer of atmosphere that lies about 15 to 50 kilometres above the Earth's surface. In the stratosphere, the temperature rises with increasing height, which is the opposite of the situation in the lower atmosphere. Ozone occurs in minute quantities throughout the full depth of the atmosphere, but its concentration peaks within the stratosphere at an altitude of about 35 kilometres. This is referred to as the ozone layer. There is little up-and-down air movement in the stratosphere, so the ozone layer stays in position.

ultraviolet (UV). A form of electromagnetic radiation. UV radiation has shorter wavelengths than visible light and it therefore carries more energy. It is divided into three broad categories: A, B and C. UV-A has the longest wavelength and is the least damaging form, although sufficient exposure will cause sunburn. UV-B damages proteins in unprotected organisms and can cause cancer, while UV-C is extremely dangerous because it can cause mutations in DNA.