Depletion of the Ozone Layer

The Earth's atmosphere is divided into several layers.The lowest region, the troposphere, extends from the Earth's surface up to about 10 kilometers (km) in altitude. Virtually all human activities occur in the troposphere. Mt. Everest, the tallest mountain on the planet, is only about 9 km high. The next layer, the stratosphere, continues from 10 km to about 50 km. Most atmospheric ozone is concentrated in a layer in the stratosphere, about 15-30 kilometers above the Earth's surface.

'Normal' oxygen, which we breathe, has two oxygen atoms, double covalently bonded together and is colourless and odourless. It has the formula O₂.

Ozone is a molecule containing three oxygen atoms bonded together and thus has the formula O₃.

• The name ozone is derived from a Greek word meaning "to smell".
• It is blue in colour and has a strong odour (a bit like seaweed - because of that the Victorians thought that there were high ozone levels at the sea-side and it was good for you! In fact that is not the case and it causes breathing difficulties for people when it is found in the lower atmosphere).
• At ground level, ozone is a serious health hazard it is a harmful pollutant that causes damage to lung tissue and plants- it is a major constituent of smog.
• Ozone is much less common than normal oxygen. Out of each 10 million air molecules, about 2 million are normal oxygen, but only 3 are ozone.

Ozone is constantly produced and destroyed in a natural cycle. The overall amount of ozone is essentially stable - there is an equilibrium because ozone production and destruction are balanced keeping the ozone levels stable.

This was the situation until the past several decades when our production of certain chemicals upset that balance and shifted the equilibrium so that the ozone layer that protects the Earth from harmful ultraviolet light became thinner - effectively a 'hole' - over the poles.

The large increases in stratospheric chlorine and bromine, however, have upset the Earth's ozone cycle equilibrium by removing ozone faster than natural ozone creation reactions can work at. Therefore, ozone levels fall.

Since ozone filters out harmful UVB radiation, less ozone means higher UVB levels at the Earth's surface. The more depletion, the larger the increase in incoming UVB.

UVB has been linked to skin cancer, cataracts, damage to materials like plastics, and harm to certain crops and marine organisms. Although some UVB reaches the surface even without ozone depletion, its harmful effects will increase as a result of this problem.

The ozone depletion process begins when CFCs and other ozone-depleting substances (ODS) are emitted into the atmosphere (1). Winds efficiently mix the troposphere and evenly distribute the gases. CFCs are extremely stable, and they do not dissolve in rain. After a period of several years, ODS molecules reach the stratosphere, about 10 kilometers above the Earth's surface (2).

Strong UV light breaks apart the ODS molecule. CFCs, HCFCs, carbon tetrachloride, methyl chloroform, and other gases release chlorine atoms, and halons and methyl bromide release bromine atoms (3). It is these atoms that actually destroy ozone, not the intact ODS molecule. It is estimated that one chlorine atom can destroy over 100,000 ozone molecules before it is finally being removed from the stratosphere (4).

Mouse over the graphic on the right of the screen for an animation of how a CFC breaks down ozone.

The information and graphics on this page were taken from http://www.epa.gov with kind permission - there is more detailed information about the problem of the depletion of the ozone layer on that site

See PPARC Link- it includes an animation of how the hole has grown!