Acid Rain
Acid rain is the collective name given to any rain containing acidic impurities. These impurities are generally the product of industrial pollution.
Normal, unpolluted rain has a pH of 5.6 due to the presence of dissolved carbon dioxide. Acid rain is regarded as rain with a pH more acidic than this value (a pH value less than 5.6). The greater acidity of acid rain is due to different oxides of nitrogen and sulfur dissolved in the rain. These oxides are generally referred to as NOx and SOx to indicate their variable composition. These pollutants are released into the atmosphere by the combustion of fossil fuels such as coal, oil, and gas.
Acid rain has been implicated in a number of environmental problems. In the 1960s and 1970s, fish stocks were depleted in a large number of Scandinavian lakes and this occurred at exactly the same time as an increase in the acidity of the lake water. With lakes and other bodies of fresh water the acid pH itself is not the killer, but the problem is that the increased acidity allows toxic elements, such as aluminum, to exist in greater concentration in solution. It is the increased level of these poisons that causes the damage to wildlife.
A more obvious problem, particularly in the United States, is the damage caused to trees and forests. Where trees are exposed more directly to rain, such as at the edge of forests or on the sides of hills, there is an increased level of damage evident. This environmental damage takes the form of tree die-back where the outer branches are progressively denuded of their leaves. The damage caused by acid rain also can be seen by studying lichens. Lichens are intolerant to the presence of sulfur dioxide in acid rain and different species of lichen have different threshold levels before death occurs. This means it is possible to quantify the approximate level of pollution by studying which species of lichen are present and which have disappeared from an area.
Acid rain does not just affect plants and animals. It is also strong enough to damage stone. Many old buildings and statues show pitting caused by the acidic rain which dissolves the stone.
Many of the developed countries of the world have entered into agreements to reduce the levels of sulfur and nitrogen oxides released into the atmosphere. The measures taken will eventually reduce the amount of acid rain and the damage caused will consequently be reduced. One action that has been taken is the cleaning of gases released from factory and power station chimneys. The simplest technique for cleaning these emissions is known as scrubbing and consists of firing a fine spray of water through the waste gases. This dissolves the sulfur and nitrogen oxides before they reach the atmosphere. The acidic water produced is collected and treated before release from the factory.
Acid rain is a problem associated with industrialization and the burning of fossil fuels. Countries without high levels of industrialization generally do not show evidence of acid rain damage, unless they are neighboring an industrial country. The gases responsible for acid rain are blown around in the atmosphere. As a result, the damage is not always recorded near to the site of production. To assess where the acid rain and pollution originate factors such as the direction of the prevailing wind must be taken into account.
Acid rain is a problem that was first recognized in the mid-twentieth century. It was discovered by checking records of plants and looking at levels of damage to buildings that this form of pollution had existed since the Industrial Revolution. By the end of the twentieth century, a number of international protocols were successfully reducing the levels of pollutants in the atmosphere.
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