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Introduction (continued)

Ozone depletion

Stratospheric ozone loss

The stratosphere contains a low concentration of ozone gas. Even though there is not a great deal of ozone present, the stratosphere contains 90% of the ozone in the atmosphere. (As an indication of the paucity of atmospheric ozone, if all the ozone in the stratosphere were collected and brought to the surface, it would form a layer of pure ozone about 3 mm thick.)

The ozone in the stratosphere, the ozone layer, is essential to life on earth. Despite the low concentrations, the ozone layer protects us from most of the sun's harmful UV radiation.

Ozone is produced when UV radiation strikes oxygen molecules. Once formed, ozone absorbs UV radiation. However, ozone is an unstable molecule, and for every molecule formed in the unpolluted stratosphere, another breaks down. Thus, the ozone layer is the result of natural processes that both produce and destroy the gas, and its concentration depends on the balance between these processes.

Chlorine, bromine and oxides of nitrogen, in their reactive forms, can catalyse the breakdown of ozone in the stratosphere. They are part of the natural ozone destruction cycle. Unfortunately, production and release of chemicals including CFCs and halons have led to an increase in the concentration of ozone-depleting chemicals, resulting in acceleration of the destruction of ozone.

This destruction by human-produced chemicals is particularly efficient over Antarctica in spring (Figure 7), as a result of the presence of ozone-depleting substances in the stratosphere combined with the unique temperature, structure and circulation of the Antarctic stratosphere during the long polar night. Rapid ozone destruction on icy polar stratospheric clouds is initiated by sunlight in spring, giving rise to the Antarctic 'ozone hole'. Stratospheric ozone concentrations over Antarctica in spring drop significantly. The ozone is replenished at the end of spring as sunlight initiates ozone formation and ozone-rich air from the rest of the stratosphere moves over Antarctica.

Ozone decline is not just a polar phenomenon: all regions apart from the tropics have shown a decline in stratospheric ozone over the last two decades, although not as severe as the decline over the poles. As stratospheric ozone has declined, levels of UV radiation at the earth's surface have increased. Australians are now exposed to greater levels of UV radiation.

In humans, exposure to UV radiation can cause sunburn, eye damage, skin cancer and damage to the immune system in susceptible people. Fair-skinned people are most at risk. Many other organisms, including plants, are also at risk from increased UV radiation. Too much UV irradiation reduces plant growth, the sensitivity varying between different species. Although the existence of stratospheric ozone is so important, increases in tropospheric ozone (associated with photochemical smog) in the lower atmosphere are of concern. Tropospheric ozone has a limited atmospheric lifetime and is not transported to the stratosphere to any significant extent.

The Montreal Protocol (http://www.unep.org/ozone/montreal.shtml) and its Amendments and Adjustments regulate the production of CFCs and other ozone-depleting substances. Production of the most damaging ozone-depleting substances was eliminated, except for a few critical uses, by 1996 in developed countries and will be eliminated by 2010 in developing countries.

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