THE GREENHOUSE EFFECT: ACID RAIN AND POLLUTION
The earth's atmosphere is about 900 km thick but living things can exist only up to about 3.2 km above the earthÕs surface. It is in this rather thin layer that most pollutants assemble.
Sources of air pollution:
By far the major source of air pollution comes from the burning of coal and oil in houses, factories, power stations and vehicle engines. When coal and oil are burned, they produce soot and smoke. They also give off a mixture of gases such as sulphur dioxide, carbon monoxide, carbon dioxide, oxides of nitrogen, and various hydrocarbons.
These pollutants do not stay close to the source, but rise into the atmosphere, borne by warm air currents. Some remain airborne for quite a long time and can be carried hundreds of kilometres away from the source. Some of these pollutants therefore enter clouds and fall back to earth as rain (known as wet deposition or acid rain), and some fall back to earth as dry depositions.
Sulphur dioxide is very harmful to plants, as it is taken into their leaves through their stomata. As a result, the leaf cells are destroyed and the leaves are damaged and may even drop off. In addition, when there is a lot of ozone in the atmosphere, some of the sulphur dioxide reacts with other substances in the atmosphere to form sulphuric acid. Other gases, such as nitrogen oxides, can also form acids. Because all this happens at a high level in the atmosphere, the acid is sometimes carried for hundreds of kilometres before it falls to the ground as acid rain or snow. Normal rain is in any case slightly acid, because some carbon dioxide dissolves in it. This forms a very weak acid called carbonic acid. Normal rain has a pH of about 5.7. However, rain containing sulphuric acid may occasionally have a pH as low as 2.4.
Look at a graphic representation of the pollution of the atmosphere and the acid rain, below (click on the thumbnail for larger view):
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