A side effect of air pollution is acid rain, which is now of common occurrence. You might have seen, that some industries build very tall smoke stacks to protect the air quality in their immediate vicinity. But this allows wind currents to carry the fine particles that have absorbed sulphur dioxide and nitrogen oxides to far off places. Sulphur dioxide and nitrogen oxides react with water in the atmosphere to give rise to sulphuric acid and nitric acids. These acids, formed in the air, come down to the eanh along with rain. The pH of acid rain depends on the concentration of sulphur dioxide and nitrogen dioxide in the atmosphere. The pH of acid rain varies from 5.6 to as low as 3.0, which makes it almost as acidic as vinegar. The pH of natural rainfall is also slightly acidic and is 5.6. This is because rain water reacts with carbon dioxide in air and forms weak carbonic acid.
Now let us see the effects of acid rain on the ecosystem. It causes direct damage to the leaves of p!ants. Forests in many pans of the industrialized world are drying because of acid rain. It also causes the leaching of nutrients out of the soil, some of which are very essential for plant growth, e.g., calcium and magnesium. These nutrients are carried out of soil into streams and ponds. Besides this, it causes a lot of other effects, as indicated in Fig. Acid rain also affects lakes. Their waters become devoid of life, except for some algae and fungi that grow at the bottom. Acidity leads to increase in dissolved metals, particularly aluminium. This element affects the gills of the fish. and they die of suffocation. Birds living near lakes that have high aluminium content, are poisoned by aluminium because they feed on the aquatic insects. Acid rain also corrodes materials such as marble stonework and metals.
Then, how could one minimise the occurrence and the effects of acid rains? Most of the sulphur dioxide produced, that leads to acid rain, is due to the burning of sulphur-containing coal. Coal, therefore, could be washed prior to burning. Low sulphur coal could be substituted for high sulphur coal. Devices such as scrubbers could be installed in tall chimneys of furnaces to prevent sulphur dioxide from entering the air. A new method of burning coal in which a mixture of coal and limestone are used, is now under development.
This technique could reduce the emission of sulphur and nitrogen oxides. Experiments have been done to restore the lakes, streams and soils that have been affected by acid rain. Lime is added to lakes and in fields where soil becomes too acidic. But it helps only for a short time and does not completely solve the problem. However, limestone cannot be spread in the forests. So. don't you think. the most effective solution is to reduce the emission of sulphur and nitrogen oxides from all devices that produce them?