Acid rain

It was over 150 years ago when an Englishman, Robert Smith, first used the term “acid rain”. In 1858 he wrote: “It has often been observed that the stones and bricks of buildings, especially under projecting parts, crumble more readily in large towns, where much coal is burnt, than elsewhere… I was led to attribute this effect to the slow but constant action of acid rain.” But it was not until the late 1960s that people in different parts of Europe noticed that life was disappearing from lakes and that trees in forests were dying. There were already rain gauges monitoring chemicals in rain when, in 1968, Professor Svante Oden, a Swedish scientist, showed that pollution from industrial areas of Europe could travel over hundreds of kilometers. The link had been made between pollution from industry and the dying fish and trees.

The true effects of acid rain have really become noticeable in only the last 20 years. At first the suggestion that acid rain could be poisoning lakes in Sweden was met with disbelief. But acid rain is a creeping pollution. Its effects have been building up over many decades.

What causes acid rain?

Tall chimneys carry away the sulphur dioxide and other pollutant gases. They are not allowed to pollute the air breathed by the factory workforce or the local residents.

Unfortunately, the acid gases do not disappear. In the air, sulphur dioxide meets water vapour, and reacts to form sulphurous acid. This is slowly oxidized by oxygen in the air to sulphuric acid. When it rains or snows, down comes the sulphuric acid as acid rain or acid snow. Oxides of nitrogen are converted into nitric acid.

Rain is naturally weakly acidic. Carbon dioxide from the air dissolves in it to form the weak acid, carbonic acid. However, sulphuric acid is a thousand times stronger than carbonic acid.

SUMMARY NOTE

Sulphur dioxide and oxides of nitrogen are present in the exhaust gases of coal- and oil-fired power stations. In the atmosphere, these gases react with water vapour to form acid rain.

What harm does acid rain do? The main damage so far has been to lakes and fish, to trees and to building materials such as stone, concrete, metal and cement.

Fish cannot survive in acidic lake water. Thousands of lakes in Canada, Norway and Sweden are now empty of fish. These countries produce very little of the acid rain and snow that falls on them. The pollution is carried to Scandinavia by the prevailing winds from the UK and from Germany, France and other highly industrialized European countries. Canada receives much of its pollution from the USA.

Some countries suffer more than others from attack by acid rain. If the soil contains basic substances, these will partially neutralize the rain as it trickles through the soil. Cold countries like Sweden receive acid snow for many months. During the spring thaw, a huge volume of acidic water is released into the lakes. The soil has no chance to neutralize it. The acidic lake water destroys water plants. As a result, light can penetrate further through the water. Acid lakes look beautifully clear and blue.

The effect of acid rain on trees is considerable. It dissolves minerals from the soil, removing the ions of calcium, potassium, aluminium and lead. Some of these ions are nutrients, which should be absorbed by the roots of the trees. Acid rain deprives the trees of these nutrients.

Building materials suffer from attack by acid rain. Concrete and stone react with acids, forming calcium carbonate. Metals are attacked by acids. Iron railings, steel window-frames, motor vehicles and bridges are all corroded faster when rain water is acidic.

SUMMARY NOTE

Acid rain makes lakes too acidic for fish to live in. It removes minerals from the soil. Trees are deprived of essential minerals and die. Acid rain attacks construction materials such as limestone, cement, concrete and metals.