Acid Rain:

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?