WHAT ARE ACIDS?

There are many different kinds of acids. The proteins in our food, and in our bodies, are made up of amino acids. Motor cars start because of the sulphuric acid in their batteries. Swimming pools need hydrochloric acid, commonly known as `pool acid'.

Some acids are weak, e.g acetic acid (vinegar) and lemon juice. They are not harmful and are used in preparing our food. Othershowever, such as sulphuric acid (battery acid) are strong and can burn holes in our clothes.

WHAT SHOULD RAIN BE LIKE?

The only place on earth where pure water is found is in a laboratory. Rain water always contains small amounts of impurities. These impurities come from dust particles or are absorbed from the gases in the air.

If pure water is exposed to the air it absorbs carbon dioxide to form carbonic acid and becomes slightly acidic, dropping from pH 7 i.e. neutral, to pH 5,6. Even in remote, unpopulated areas rain can reach a pH of 4,5. However, a pH of less than 4,5 in rain is almost certainly caused by pollution.

HOW ACID RAIN IS FORMED

Acid rain is caused by the release of the gases SO2 (sulphur dioxide) and NOX (nitrous oxides). The main sources of SO2 in South Africa are coal-fired power stations and metal working industries. The main sources of NOX emissions are vehicles and fuel combustion.

Sulphur dioxide reacts with water vapour and sunlight to form sulphuric acid. Likewise NOX form nitric acid in the air. These reactions takes hours, or even days, during which polluted air may move hundreds of kilometres. Thus acid rain can fall far from the source of pollution.

When mist or fog droplets condense they will remove pollutants from the air and can become more strongly acid than acid rain. Even snow can be acid. Gases and particles, not dissolved in water, with a low pH can also be deposited directly onto soil, grass and leaves. It is possible that even more acidity is deposited in this way than by rain! Not much is known about this process, and it is particularly difficult to study.

EFFECTS OF ACID RAIN

* Acid rain can increase the acidity of lakes, dams and streams and cause the death of aquatic life.

* Acid rain can increase the acidity of soil, water and shallow groundwater.

* Acid rain has been linked with the death of trees in Europe and North America. In spite of a great deal of research, no one yet knows exactly how acid rain harms forests. Most of the forests of Europe consist of huge areas of one tree species. This encourages the spread of plant pests and diseases. It seems likely that acid rain weakens the trees, perhaps helped by other pollutants such as ozone, and then leaves the trees open to attack by disease. Acid rain also disrupts the availability of soil nutrients. The final death of a tree may result from a combination of stresses such as heat, cold, drought, nutrient disruption and disease. It seems that the slow-growing, longer lived forests of the North may be more susceptible than the faster growing, shorter lived forests of South Africa.

* Acid rain erodes buildings and monuments. Acid particles in the air are suspected of contributing to respiratory problems in people.

ACID RAIN IN SOUTH AFRICA

The coal burning power stations and large metal working industries of South Africa are concentrated in the Eastern Transvaal Highveld, the industrial heart of the country. Electricity is distributed nationally from here and many of our exports are produced in this region. This area produces most of the country's pollution: In 1987 it was responsible for 1,84 million tonnes of sulphuric acid and 0,84 million tonnes of nitric acid. Not surprisingly acid rain occurs in this region and can average pH 4,2 and sometimes fall as low as pH 3,7. Whether this is harming our crops or forests is not known. There are some signs of forest damage, but other causes are also possible. Research into this issue is being carried out in South Africa by CSIR, Eskom and other bodies.

SOLVING THE PROBLEM

No simple overnight solution is possible. We need to use energy more efficiently at home, in our vehicles and in industry. We will have to think hard about alternative energy sources such as nuclear power and hydro-electric power. It is possible to remove acidic emissions from coal-burning, but this is very expensive. It may be possible to breed crops and trees that resist pollution, but this would only be a partial solution. In practice we will probably need a combination of all these ideas and innovations.

ANALYSING RAIN

* Collect some rain in very clean plastic buckets which have been rinsed with distilled water. Put out the buckets just before it rains.

* Place the buckets at least 2 m above a lawn to reduce dust contamination. Soil dust will usually tend to be alkaline and neutralize acid.

* The pH of the rain can be measured with a carefully calibrated pH meter. Indicators such as litmus paper or a swimming pool testing kit can be used, but they will be less accurate than a pH meter.

TOPICS FOR DEBATE

* Plants need sulphur and nitrates as nutrients. Should we treat acid rain as free fertilizer?

* Would it be better for South Africa to provide electricity for all its people, rather than spend money controlling the power station emissions that cause acid rain?

FURTHER READING

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

USEFUL CONTACTS

Earthlife Africa. PO Box 11383, Johannesburg, 2000. Branches nation-wide.

Eskom: Communications Department. PO Box 1091, Johannesburg, 2000. Tel. 011-8002499

CSIR. P.O. Box 395, Pretoria, 0001. Ematek Tel. 012 841 4111, Forestek Tel. 012-841 2674.

ACID RAIN TESTING KIT "Acid Drops". Enables children to test rain to see how acid it is. Available from Roopers Ltd, PO Box 82, Tunbridge Wells, Kent, TN3 8BZ, England.