The formation of nitric acid and sulphuric acid as secondary pollutants in the atmosphere results in a kind of pollution known as acid rain. It includes all types of rain and snow whose excessive acidity causes environmental problems like destruction of vegetation and marine life and the corrosion and etching of buildings that are exposed to the atmosphere. Chemically speaking, all rain is acidic with or without air pollution. This is due to the natural presence of carbon dioxide in the atmosphere which dissolves in rain drops or rain water (even moisture present in the atmosphere does the same function) to form carbonic acid.
CO2 + H2O → H2CO3
Due to the above reaction, carbon dioxide can dissolve in water until the solution becomes saturated. This results in the rainwater attaining an acidic pH of 5.6. Due to this, the purest form of rain (in the absence of any of the air pollutants) reaches the earth as an acidic solution of pH 5.6. In view of the above, the acid rain is described as rain whose pH is lower than 5.6.
Caused of Acid Rain:
Due to the presence of oxides to nitrogen in the atmosphere During thunderstorm and lightning; atmospheric nitrogen and oxygen combine to form nitric oxide, which gets converted to nitrogen dioxide which reacts with water to form nitric acid and is washed down as acid rain.
Due to the presence of sulphur dioxide in the atmosphere Sulphur dioxide is released into the atmosphere from a number of sources. In the atmosphere, sulphur dioxide reacts with atmospheric oxygen to form sulphur trioxide by photolytic and catalytic oxidation processes. The formed sulphuric trioxide reacts with water forming sulphuric acid which comes down on earth as acid rain.
The presence of hydrogen sulphide and chlorine in the atmosphere also contributes to the formation of acid rain. The acid rain is not confined to the region where the harmful pollutants (like oxides of nitrogen and sulphur) are emitted. Since winds carry atmospheric contaminants across national borders, the acid rain may fall hundreds of miles away from the pollution sources. Acid rain, in fact, has become an international issue. It is the responsibility of all countries to make sure that the pollutants are not discharged into the atmosphere; these should be checked at the initial source only.
Harmful Effects of Acid Rain:
Acid rain causes extensive damage to the environment.
Vegetation: The acid rain makes the soil acidic. This adversely affects the plants and animals. The acid rain contains H–, and NO42 ions which when added to soil leaches the nutrients from the soil. The acid rain also damages the leaves of plants and trees. This is responsible for retarding forests and other vegetation.
Fertility of Soil: The activity of symbiotic nitrogen fixing bacteria present in the nodules of leguminous family is inhibited. This is responsible for destroying or reducing the fertility of the soil.
Aquatic Life: Acid rain renders the river or even ocean waters acidic, thereby, adversely affecting marine animals. Changes in pH of fresh water affect the .reproduction and survival of many species of fish.
Building and Monuments: Acid rain causes extensive damage to buildings and monuments made from marble, limestone, slate and mortar. Limestone which is very common building material, is attacked readily. CaCO3 + H2SO4 →CaSO4 + H2O + CO2 Most of the monuments are made from marble which is mostly CaCO3 and so are attacked by acid rain. The damage caused to the buildings and monuments by acid rain is irreversible. A most glaring example is slow degradation of Taj Mahal in Agra (UP, India). The pollutants in and near Agra arise from iron foundries, rubber manufacture, brick kilns, and oil refinery at Mathura.
Ecological Balance: Acid rain is responsible for wiping out many bacterial and blue green algae thereby disrupting the whole ecological balance.
Human Health: Acid rain has been found to be very dangerous to human health. Acidic conditions can play havoc within human nervous system, respiratory system and digestive system by making the person an easy prey to various neurological diseases. It is of utmost importance to control the acid rain. Short-term control of acid deposition (soil) can be achieved by using lime.