Air pollution causes acid rains. Power plants and other industries generate airborne acids and pollutants, which combine with the rainfall. If the pH value of rain or snow or fog is less than 5.6, it is termed acid rain. Acid rain causes severe damage to the environment and depletes the soil of its nutrients. This affects the growth of trees and renders lakes and rivers less habitable for fish and other animals. The Eastern parts of North America, European region, Japan, China and Southeast Asia are most affected, on account of acid rain (Hart).
Despite these negative characteristics, acid pollution has shielded the earth. It has reflected a significant portion of the sunlight, back into the space. Consequently, the process of global warming has slowed down. Many scientists have declared that acid pollution had prevented the early onset of global warming in the Twentieth century (Hart).
Industrialization and urbanization generate the highest rate of emission of the oxides of sulfur and nitrogen into the atmosphere.
These areas receive frequent acid rains. The acidity of rainfall is directly related to the levels of emissions of pollutants by industries. Therefore, acidic precipitation is caused by the emission of industrial gases and pollutants into the atmosphere. The chief causes of acid rain are sulfur dioxide and nitrogen oxide, which generate sulfuric and nitric acids, due to chemical reactions in the atmosphere (Acid rain ).
The burning of fossil emits sulfur dioxide, and this is almost 85% in Europe. Power generating plants and chemical plants produce sulfur dioxide gas.
Nitrogen dioxide and Nitrogen oxide are chiefly emitted by thermal power stations that burn fossil fuels. Fossil fuels, like coal, natural gas, and oil produce greenhouse gases, like sulfur dioxide and nitrogen oxide on combustion. These oxides combine with water to form sulfuric acid and nitric acid (Acid rain ).
The acid pollutants reach the higher altitudes of the atmosphere and finally return to the earth as rainfall, snow or fog. Atmospheric processes regulate the transport of acidic substances and their deposit. The chemical and atmospheric processes are interdependent and complex. The physical processes of atmosphere include the transportation of pollutants by the winds and the formation of clouds. Rainfall and deposit of pollutants are controlled by these physical processes, whereas the chemical reactions regulate the formation of different compounds and their deposit (Acid rain ).
Acid rains initiate chemical reactions on coming into contact with other objects. They are corrosive in nature and produce an immediate chemical action with other substances. The number of free hydrogen atoms that result from dissolving a substance in water, determines its acidity. The acidity of a substance is measured by the pH scale, which comprises of units from 0 to 14. Substances whose pH value is less than 7 are acidic. On the other hand, alkalis accept hydrogen atoms from acids (Acids in the Environment. The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide). The action of alkalis is opposite to that of the acids. Alkalis or bases have pH values greater than 7. A pH value of 7 indicates a neutral substance that is neither acidic nor alkaline. Pure water is assigned a pH value of 7 (Park)
Acid rains leach the soil of nutrients that are essential for the growth of plants. Toxic substances such as aluminum and mercury are dissolved by acid rain, from the soil of some areas. These dissolved toxins contaminate water sources; and are absorbed by plants, when they take up water. The deposit of acidic substances depends on the nature of the soil. Alkaline soils neutralize acid deposits; whereas thin mountain soils, which are formed by granite or gneiss, temporarily store acid deposits, for a short period (Acids in the Environment. The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide).
Acid rain washes away the nutrients from the soil, and this has an adverse effect on the growth of plants and trees. Acid rain bleaches the wax coating of leaves resulting in the development of dead spots in trees. If the number of such spots on a tree increases, then the tree would be unable to perform photosynthesis (Acids in the Environment. The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide).
Trees become vulnerable to infections through leaves with dead spots. In this manner, acid rain weakens the trees and exposes them to threats like insect infestations, drought and cold. Acid rains mainly affect forests than farm crops. This is because the soil in farms is capable of neutralizing and absorbing acidic substances in large quantities. However, farm crops in mountain soils depict greater vulnerability to acid rains. The thin soils in those regions cannot neutralize or absorb acidic substances (Acids in the Environment. The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide).
Finally, the acid rain reaches streams, lakes and marshes. In winter, the waters are covered with snow; which absorbs toxic substances. In spring, the snow melts and makes the water acidic (Acids in the Environment. The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide).
In general, water has a pH value between 6 and 8. However, in some lakes of the northeastern United States and southeastern Canada, the pH value of water was found to be less than 5. Scientists have determined that acid rain had causes these waters to become acidic. Aquatic life does not survive in such acidic water. Several of the lakes and ponds in the Adirondack Mountain region of New York State are acidic. This acidity has rendered them bereft of aquatic life (Acids in the Environment. The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide).
Streams in the middle Appalachian Mountain range are found to be acidic. Moreover, the major rivers of Norway have become acidic, due to acid rains. As a result, the salmon fish population in these rivers has reduced to a considerable extent (Acids in the Environment. The Hutchinson Unabridged Encyclopedia with Atlas and Weather guide).
Land animals that feed on aquatic life would be affected adversely. Furthermore, several research studies have shown that the snail population, in the water that has been contaminated by acid rain was on the decline. In addition, songbirds in the Netherlands feed on snails; and a decrease in the population of snails, would gravely affect these birds, as they would be unable to procure snails to feed themselves. Moreover, the egg shells of songbirds could be weakened, because these birds receive calcium from snail shells. Acid rains damage structures in cities and many historical monuments like the Parthenon in Athens and the Taj Mahal in India have been badly affected by acid pollution (Hart).
It has been observed that freshwater clams and mayfly are on the verge of extinction. Moreover, frogs feed on mayflies and their extinction could adversely affect the population of frogs. If the pH level of water approaches 5, fish cannot hatch their eggs. If the pH value of water reaches 4.5, then it becomes sterile water, and is no longer capable of supporting any form of life (Hart).
Several countries have made endeavors to mitigate the problem of acid pollution; and to control the emission of sulfuric dioxide, which is the primary cause of acid rains and acid pollution. For instance, in 1985, the Thirty Percent Club was formed by 19 countries. The aim of this club was to reduce emissions by 30% by the end of 1993. The Member States of the European Union have also consented to reduce the emissions of SO2 by 20% (Acid rain. The Crystal Reference Encyclopedia).
Wildlife and plants are affected to a significant extent by acid rain. Animals that feed on plants would be deprived of food and the ecological balance would be endangered. Some aquatic species could cease to exist due to acidity. In urban areas, acid rains damage buildings and statues. Urban smog forms due to the combination of acids and other chemical pollutants. People in cities suffer from this pollution, which causes respiratory problems and accelerates their death.