THE OZONE `LAYER'

This band of concentrated ozone is found in the stratosphere, an upper layer of the atmosphere, 10 to 50 kilometres above the earth's surface. Ozone shields life on earth from the sun's UV radiation. Plants and animals living at different altitudes are adapted to different amounts and wavelengths of UV radiation. Some scientists believe that the evolution of life on earth has been made possible by the existence of the protective ozone shield.

OZONE DEPLETION

In 1982 a team from the British Antarctic Survey working in Antarctica found that much of the ozone overhead had vanished. Suspecting that their well-worn instruments might be faulty, the team returned, with new equipment, in 1983 and 1984 - to find even less ozone. The report of their findings caused a sensation, as ozone depletion of this scale was totally unforseen. In reaction to the British Antarctic Survey results, NASA (National Aeronautics and Space Administration) scientists re-examined data from their weather satellite, Nimbus 7. They found that the satellite had indeed registered the same massive depletions, but the computers had been programmed to reject such findings as impossible!

It is now known that a depletion, often termed a hole, of the ozone layer over Antarctica occurs each spring, and lasts for approximately six weeks. It is as wide as the United States, and up to 50% depletion can occur.

The ozone layer was first discovered by Professor Gordon Dobson of Oxford University in 1957. Today ozone concentration is measured in Dobson Units.

CAUSES OF OZONE DEPLETION

Ozone is destroyed mainly by chlorine-based compounds, chlorofluorocarbons (CFCs), and chlorinated solvents such as carbon tetrachloride, as well as the fire-extinguishing chemicals, halons. Chlorine molecules are highly reactive and readily break apart the three-atom ozone into the two-atom form of oxygen. It is estimated that one chlorine atom can destroy 100 000 ozone molecules. Halons release bromine atoms which are even more potent ozone destroyers.

CFCs
When they were discovered in 1928, CFCs appeared to be perfect industrial chemicals: odourless, non-toxic, non-flammable and chemically inert. This last property is particularly useful as it means that CFCs do not react chemically with other materials, such as the working parts of refrigerators, the contents of aerosols or plastics. However, when CFCs are released into the atmosphere, they slowly drift up to the ozone rich stratosphere where they break down and release atoms of chlorine. Each chlorine atom may react repeatedly to destroy many ozone molecules. Unfortunately CFCs are very stable, i.e. they do not easily break down or change form. Thus they can remain in the atmosphere for more than 100 years.

USES OF CFCs

* Aerosols: CFCs used to be common aerosol propellants. Today, however, all domestic aerosols made in South Africa are free of CFCs. Certain medical aerosols, such as asthma pumps, contain CFCs as a safe alternative has not yet been found. Fewer than 4% of aerosols made in South Africa contain CFCs.

* Refrigeration and air-conditioning: Refrigerators, freezers and air-conditioners rely on CFCs as refrigerants. A refrigerant is a gas which, when compressed, becomes liquid. During compression heat is taken up. Once the pressure is released, the liquid changes to a gas and heat is released. During this process the fridge is cooled.

* Foam: CFCs are used to create millions of air pockets in plastic, thus forming foamed plastics such as polystyrene and polyurethane foams. These provide insulation, cushioning and packaging.

CONSEQUENCES OF OZONE DEPLETION

Ozone depletion will result in increased UV radiation, specifically the biologically damaging UV-B, reaching the earth. Consequences of this include:

* A higher incidence of cataracts and skin cancer. A 1% rise in UV radiation is estimated to produce a 2 - 3% increase in the frequency of skin cancer.

* Suppression of the immune system of both humans and animals which could result in an increase in the spread of infectious diseases.

* Inhibition of photosynthesis, and thus plant growth, would reduce world food production.

* UV radiation penetrates water by up to 20 metres in clear conditions. Phytoplankton and zooplankton, vulnerable to UV radiation, form the base of the marine food web. Increased UV radiation could damage the marine ecosystem. In addition, phytoplankton play an important role in the uptake of carbon dioxide, and reducing this would contribute to the greenhouse effect (see Enviro Facts "Global Warming").

ACTION ON OZONE

Evidence of ozone depletion motivated growing international efforts to phase out the use of ozone-depleting chemicals, resulting in the Montreal Protocol. This international agreement calls on signatory countries to phase-out CFC use by 1996. Originally formulated in Montreal in 1987, the agreement was amended in 1990 and 1992. South Africa signed the Protocol in 1990 and the amended version in 1992. South African industry has reacted to the Montreal Protocol as follows.

Aerosol industry: By the end of 1992, all CFCs had been phased out of use, other than those used for medical purposes. Alternatives to CFCs are a butane-propane mix, or dimethylether.

Foam industry: Expect to have phased out CFC use by 1996. Alternatives include carbon dioxide, hydrocarbons, hydrofluorocarbons, partially-halogenated chlorofluorocarbons and water vapour.

Refrigeration and air-conditioning industry: Faces the greatest difficulty in phasing out CFCs by 1996. CFC refrigerants recovered from old equipment can be reclaimed at AECI Midlands factory, a practice that should reduce production requirements.

Fire fighting industry: Halons are used in many fire-fighting systems. By the end of 1993, all signatories to the Montreal Protocol will have stopped manufacturing halons, and new installations will not be permitted to use halons.

WHAT YOU CAN DO

* The pipes of refrigerators and air-conditioners contain CFCs. Avoid leakages from these appliances by following these tips:

- Don't scrape the ice from inside freezers using knives or other hard objects which may puncture the inside-casing.

- If your refrigerator, freezer or air-conditioner (especially in your car) needs a repair, go to a reputable dealer and ensure that the service mechanic collects the gas so that it does not escape into the atmosphere. Also ensure that a test for leaks is carried out before re-gassing.

- When purchasing new air-conditioners or refrigerators, ensure that they do not use CFCs.

* Do not buy halon based fire extinguishers (eg. total flood systems used to protect computer equipment).

DID YOU KNOW

* Most ozone depleting substances have a lifetime of about 100 years, so we can expect ozone depletion to continue well into the next century.
Ozone depletion will reach its maximum in the next 10 years, and recovery to pre-1960 levels will occur between 2050 and 2066.

* Assessment of the ozone problem is complicated by particulate air pollution (e.g. particles of dust or ash) which has a light scattering effect and reduces incoming UV radiation; and increased ground level ozone, which like stratospheric ozone, decreases UV radiation.

* Since 1978 ozone levels at Durban have dropped on average by 0,2% each year.

FURTHER READING

THE GAIA ATLAS OF PLANET MANAGEMENT. N. Myers (ed). Pan Books, London, 1985.

ENVIRONMENTAL MANAGEMENT IN SOUTH AFRICA. R. Fuggle and M. Rabie. Juta, Cape Town, 1992.

All books available from Russel Friedman Books, PO Box 73, Halfway House 1685. Tel. 011-7022300/1.

USEFUL CONTACTS

Wildlife Society of Southern Africa. PO Box 44344, Linden, 2104. Tel. 011-486 3294/5 or 0938.

Aerosol Manufacturers Association. PO Box 483, Olifantsfontein, 1665. Tel. 011 - 884 6319.

AECI. PO Box 1122, Johannesburg, 2000. Tel. 011 - 223 9113.