Acid Rain: Causes Effects and Solutions

acid rain
Acid Rain: An invisible threat to life and property

-Dr. Arvind Singh 

Generally, the pH of rain water is acidic (pH 5.6) due to the presence of a weak acid known as carbonic acid which is formed from the reaction of atmospheric carbon dioxide with water. Therefore, the precipitation in which pH below 5.6 constitutes the acid rain or the contamination of rain water with strong acids (sulphuric acid and nitric acid) is known as acid rain. The term acid rain was coined by Robert Angus Smith in 1872 and it is an example of secondary pollution.
The corrosive nature of acid rain causes many forms of environmental damage, acid pollutants also occur as dry particles and gases, which when washed from the ground by rain add to acids in rain, form on even more corrosive solution. This is called acid deposition.

Causes of acid rain:

Air pollution is the chief cause of acid rain. The gaseous pollutants like sulfur dioxide (SO2) and nitrogen oxides (NO & NO2) produced by combustion of fossil fuels (coal, oil, and natural gas) from smelters, power plants, automobile exhausts, forest fires, domestic fires, etc. are the causative agents of acid rain. These oxides as primary pollutants are swept up into the atmosphere and can travel to long distances. In atmosphere sulfur dioxide is oxidized to sulphuric acid (H2SO4) while nitrogen oxides are oxidized to nitric acid (HNO3). Sulfur dioxide present in the air gets oxidized to sulfur trioxide by the oxygen of the air. The sulfur trioxide combines with the water vapors present in the air to form sulphuric acid. Nitrogen oxides present in the air combines with oxygen and water vapors to form nitric acid. Therefore, acid rain is a cocktail of sulphuric acid and nitric acid, and the ratio of the two may vary depending on the relative quantities of sulfur dioxide and nitrogen oxides emitted in the atmosphere. On average 60-70% of the acidity is due to sulphuric acid while 30-40% is due to nitric acid. When rainfall occurs these acids fall as secondary pollutants in the form of rain, snow or fog on the ground.

Acid rains are drifted by prevailing winds to elsewhere where precipitation takes place. Thus the oxides produced in one place can cause an influence at another place by turning into acids. Canada and Sweden are the two such victims. Canada gets acid rains from petrochemical units in North America. Heavy winds pick up acid rain from the industrial units in Britain and France to Sweden. Equally grim are the acid rains in Norway, Denmark and Germany. It is said that 90% of the acid rain of Norway and 75% of Sweden are due to drifted sulfur dioxide and nitrogen oxides. Thus acid rain is emerging as a major political issue in the world.

Consequences of acid rain:

Damage from acid rain is widespread in North America, Europe, Japan, China, and South-east Asia. In the United States of America, coal-burning power plants contribute to about 70% of sulfur dioxide. In Canada, oil refining metal smelting and other industrial activities account for 61% of the sulfur dioxide pollution. Motor vehicle exhaust fumes are the main source of nitrogen oxides to the atmosphere.

Acid rain leaches metals like aluminum, mercury, calcium, zinc, and lead from the soils and rocks, and drains them into the rivers, lakes, and ponds thus polluting water and poisoning the aquatic flora and fauna. Fishes die due to the accumulation of aluminum in their gills.

Acid rain affects the trees by creating holes in the waxy coating (cuticle) of leaves causing brown dead spots that affect the plant's photosynthesis. Such trees are also prone to pest infestation, diseases, cold and drought. In West Germany, approximately 8% of the forests died and nearly 20 million acres of forests are critically affected by acid rain. Forests in Switzerland, Netherlands, and Czechoslovakia have also been damaged by acid rains. Thus, acid rain affects the productivity of forests. It also affects the productivity of grassland and crop plants.

Acid rain causes the acidification of lakes and rivers thus lowering their productivity. More than 10,000 lakes in Sweden have become acidified. Thousands of lakes in the United States of America, Canada, Norway have become unproductive due to acidity. The population of fishes has decreased to a serious extent. The fishless areas are now fish graveyards. Acid rain also causes soil acidity thus lowering the productivity of the soil.

Acid rain also has far-reaching impacts on wildlife. When one species is affected then the whole food chain is disrupted and eventually the entire ecosystem is disturbed. Although surface water polluted by acid rain does not directly harm people the toxic substances leached from soil can pollute the water supply. Fishes caught in these waters may be harmful for human consumption. Acid along with other chemicals in the air produced urban smog, which causes respiratory problems. Acid rain causes skin injury in human beings.

Acid rain damages not only forests, lakes and rivers but also causes extreme damage to buildings and sculptural materials of marble, limestone, slate, etc. These materials react with acid rain and get corroded or damaged because calcium carbonate reacts with sulphuric acid. In Greece and Italy, several invaluable statues have been deteriorated. The historical monument Taj Mahal in India has been affected by acid rain (due to the emission of gaseous pollutants from the oil refinery at Mathura). British Parliament building also suffered damage due to acid rain. Many building materials including steel, paint, plastics, cement are also affected by acid rain.

What is ‘Acid Shock’?

‘Acid Shock’ is a major problem created by acid rain in cold areas when the acidic ice melts, and the pH value becomes suddenly less. Due to this acid shock, a situation arises which causes the death of large numbers of aquatic lives at a time. ‘Acid Shock’ also badly affects human health.

Acid rain in India:

Being a developing nation the level of industrialization in India is not very high hence till date acid rain is not a big problem. However, industrial areas in which the pH value of rain water below or close the critical value have been recorded in Delhi, Nagpur, Pune, Bombay, and Kolkata. This is due to sulfur dioxide from coal-based power plants and petroleum refinery. According to a study made by Bhabha Atomic Research Centre Air Monitoring Section; the average pH value of rain water at Kolkata is 5.80, Hyderabad 5.73, Chennai 5.85, Delhi, 6.21 and Mumbai 4.80. The situation may deteriorate in the future due to the increased installation of thermal power plants by National Thermal Power Corporation, and the consequent increase in coal consumption. There is an urgent need for proper regular monitoring to provide timely warnings about the acidification of our environment.

Control of acid rain:

The best way to stop the formation of acid rain is to reduce the emission of sulfur dioxide and nitrogen oxides into the atmosphere. Therefore, less energy needs to be used from fossil fuels in industries, power plants, and vehicles. Coal should be desulphurized before being used as fossil fuel. Anti-pollution devices like scrubbers need to be used in the smoke-stacks of factories to prevent the release of sulfur dioxide and nitrogen oxides in the atmosphere.

Frequent use of catalytic converters in automobiles can also be helpful in checking the emissions of sulfur dioxide and nitrogen oxides in the atmosphere. In catalytic converters, the gases are passed over metal coating beads that transform harmful chemicals into less harmful ones. The catalytic converters are generally used in cars to minimize the effects of exhaust fumes on the atmosphere. By a process, liming powdered limestone can be added to neutralize the acidity of the soil.


It can be concluded that acid rain is an invisible menace causing acidification of soil, rivers, lakes, and ponds. It damages the vegetation and also corrodes the historical monuments, buildings, and statues. Therefore, it is inevitable to minimize and check the emissions of the oxides of sulphur and nitrogen in the atmosphere to avoid the escalating danger of acid rain. 
Dr. Arvind Singh is M. Sc. and Ph. D. in Botany with an area of specialization in Ecology. He is a dedicated Researcher having more than four dozen published Research Papers in the Journals of National and International repute. His main area of Research is Restoration of Mined Lands. However, he has also conducted Research on the Vascular Flora of Banaras Hindu University-Main Campus, India.

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