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Ozone layer

Ozone is a chemical that is related to oxygen. Oxygen is usually found as a stable particle, called a molecule, that is made up of two atoms. However when another oxygen atom is added the oxygen molecule becomes ozone. This happens when oxygen is exposed to ultra-violet radiation. The oxygen molecule completely changes, instead of being essential for breathing life, it takes on quite different properties.

There is worldwide concern about the decrease in ozone. You may have heard a lot of talk about "the ozone layer". This is not a layer like a thick blanket, but is an area in the outer atmosphere where you can expect to find rather more ozone than elsewhere. These ozone molecules can be found anywhere, but they are more concentrated about twenty kilometres above the earth, in an area called the stratosphere. Here ultra-violet radiation from the sun reaches the outer atmosphere and hits oxygen molecules combining them to form ozone. Even so, the concentration of ozone is only about one molecule in every 100,000.

The ozone layer, or ozonoshere, is this area where you would expect to find this higher concentration of ozone. If all the ozone was brought to earth and spread evenly it would lie no thicker than a sunday newspaper.

Ozone screens out most (90%) of the harmful ultraviolet rays from the sun. The ultra-violet radiation that penetrates through the atmosphere has a number of effects both on plants and people. UV radiation causes sun tanning, and eye irritation, but also skin cancer. UNEP figures indicate that a 1% decrease in world ozone would increase skin cancers worldwide by 6%.

The ozone layer is threatened by a group of chemicals, originally thought to be "miracle" chemicals, called Chloroflourocarbons. They are used in all sorts of places, in refrigerators, as solvents, to propel materials from aerosol cans, and to create foams for creams. They easily evaporate and get up into the stratosphere. There they attack the ozone breaking it back down to oxygen.

People were alerted to what was happening in the mid 1970s. UNEP launched a project in 1975 to identify hazards threatening the ozone layer. In 1985 the ozone layer over the antarctic region was found to be particularly thin. This thin layer became known as "the hole"in the ozone layer.

The 'hole'in the ozone layer comes and goes each year. Each year, the ozone layer is studied to see if the "hole" is bigger or smaller. Tha annual spring thinning of the ozone layer over Antacrtica was as serious in 2003 as in the record year of 2000. It also lasted longer than ever, according to the World Meteorological Organisation. The thinnest part of the ozone layer was the largest on record. It seems the "small hole" of 2002 was a one-off, and that it may be 2010 before there is evidence of healing and over 50 years for full recovery.

In 1987 in Montreal a Protocol was adopted and signed by 62 countries and the European Community. The agreement pledged to reduce the use of CFCs to half and to freeze the use of halons.

There are a number of connections between the ozone layer and the greenhouse effect.

  • CFCs are greenhouse gases as well as ozone depleters.

  • Depletion of the ozone layer allows more radiation on earth to hit the earth increasing the input of heating

  • Some CFC substitutes are greenhouse gases themselves eg hydrocarbons

  • CFCs will contribute to both global warming and the ozone depletion long after they have been banned.


Chloroflurocarbons (CFCs) are synthetic chemicals that contribute to the depletion of the ozone layer found in the stratosphere. Ozone is made up of three oxygen atoms and CFCs contain chlorine atoms, chlorine chemistry is very complex. A chlorine atom can destroy ozone catalytically.

After being released CFCs remain in the troposphere, the layer of atmosphere above the earth where our weather is generated, they eventually rise into the upper stratosphere, above the ozone concentration. Once up there ultraviolet radiation from the sun breaks the CFC molecules apart so freeing the chlorine atoms, these attack ozone. A chlorine atom first steals one of the oxygen atoms from ozone so breaking the molecule apart to form chlorine monoxide and a stable oxygen molecule. These atoms and molecules are not still but are constantly jostling around in the stratosphere. When a chlorine monoxide molecule bumps into another oxygen atom, the two oxygen atoms combine easily, freeing the chlorine atom to continue destroying another ozone molecule.

Some chemical processes interfere with this catalytic cycle. Nitrogen dioxide (NO2) can bind to chlorine monoxide and form a chlorine reservoir - whilst chlorine is bound in this way it cannot react with ozone.

The unique climatic conditions at the south pole minimize such interference, allowing the catalytic conversion of ozone destruction to continue without interference so causing the ozone hole.

Reduction in the use of CFC's will not reduce the effect that chlorine atoms are now having on the ozone layer. In order not to make the situation worse further emissions should be halted if the ozone layer is to stand any chance of recovery.



The Vienna Convention for the Protection of the Ozone Layer (1985)

The United Nations Environment Programme (UNEP) addressed the issue of ozone depletion in 1977. Governments committed themselves to protect the ozone layer and to co-operate with each other in scientific research through the Vienna Convention on the Protection of the Ozone Layer in 1985,

The Montreal Protocol on Substances that Deplete the Ozone Layer (1987)

Since then there have been many changes, in particular the Protocol in Montreal in 1987. Ozone Control is often referred to as the "Montreal Protocol". The Protocol aims to reduce and eventually eliminate the emissions of manufactured ozone depleting substances. The Montreal Protocol on Substances that Deplete the Ozone Layer was agreed by Governments in 1987. There have been a number of amendments since.

The London Amendment to the Montreal Protocol (1990)
The Copenhagen Amendment to the Montreal Protocol (1992)
Montreal Amendment to the Montreal Protocol (1997)

The Montreal Amendment entered into force on 1 January 1999. This version of the Montreal Protocol also includes the text of the Amendment adopted by the Parties at the Ninth Conference of the Parties in Montreal 1997.This laid down the following schedule for the phase out of ozone depleting substances

Phase Out Schedule

Control measures that apply to:

  • developed countries are indicated by (c)

  • developing countries are indicated by *

1 January 1994 Halons phased out
1 January 1996 HBFCs phased out
Carbon tetrachloride phased out
All CFCs phased out
Methyl chloroform phased out
1 January 1999 Methyl bromide reduced by 25% from 1991 levels
1 January 1999 * Freeze of Annex A CFCs at 1995-97 average levels
1 January 2001 Methyl bromide reduced by 50% from 1991 levels
1 January 2002 * Freeze of halons at 1995-97 average levels
* Freeze of methyl bromide at 1995-1998 average levels
1 January 2003 Methyl bromide reduced by 70% from 1991 levels
* Annex B CFCs reduced by 20% from 1998-2000 average
* Freeze in methyl chloroform at 1998-2000 average levels
1 January 2004 HCFCs reduced by 35% below base levels
1 January 2005 Methyl bromide phased out
* Annex A CFCs reduced by 50% from 1995-97 average
* Halons reduced by 50% from 1995-97 average levels(7)
* Carbon tetrachloride reduced by 85% from 1998-2000
* Methyl chloroform reduced by 30% from 1998-2000
1 January 2007 * Annex A CFCs reduced by 85% from 1995-97 average
* Annex B CFCs reduced by 85% from 1998-2000 average
1 January 2010 HCFCs reduced by 65%
* CFCs, halons and carbon tetrachloride phased out
* Methyl chloroform reduced by 70% from 1998-2000
1 January 2015 HCFCs reduced by 90%
* Methyl chloroform phased out
* Methyl bromide phased out
1 January 2016 * Freeze of HCFCs at base line figure of year 2015 average
1 January 2020 HCFCs phased out allowing for a service tail of up to 0.5% until 2030 for existing refrigeration and air-conditioning equipment
1 January 2040 * HCFCs phased out

Outstanding environmental issues:

  1. Several gases that are being used as ozone-safe replacements for CFCs - notably hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) - also contribute to global warming.

  2. Global warming may slow the ozone layer's healing process because the warming of the atmosphere near the ground may cause the stratosphere to become even colder.

  3. There is non-compliance with the Montreal Protocol on the part of eight countries. Members of the former Soviet Union have been unable to meet their phase-out schedules due to their recent transition to market economies.

  4. A number of new substances (namely Chlorobromomethane, n-propylbromide and Halon-1202) can be marketed as replacements for stronger ozone-depleting substances controlled under the Protocol even though they themselves have some ozone-depleting potential.

  5. Concentrations of halons have continued to increase due to halons' long atmospheric lifetime and releases from fire extinguishers.


CFC Phaseout Schedules:
Allowed Production and Consumption for Developed Countries (percent of baseline)

1987 1990 1992 1990 1994

Origonal Montreal Protocol London Copenhagen U.S Clean Air Act European Community
1990 100% 100% 85%

1991 100% 100% 80%

1992 100% 80% 75% 50%
1993 80% 80% 25% 25% 15%
1994 80% 50% 25% 25% 0%
1995 80% 50% 0% 0%
1996 80% 15%

1997 80% 15%

1998 80% 15%

1999 50% 0%

2000 50%

From 1 January 2002, discarded fridges need to have the insulation foam removed before the fridge can be recycled or scrapped, to prevent the release of CFCs into the environment.

For information about Fridges and Ozone Depleting Substances Regulation

For latest UN information visit: http://www.unep.org/ozone/

© ep@w Publishing Company Ltd. 2000
2002 Edition