How Acid Rain is formed

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How Acid Rain is formed

Rainfall with a pH* level below 5.6 has been termed as Acid Rain and Robert Angus Smith, a British chemist, was the first to note this in 1852. Scientists have learned that Acid Rain has been causing great damage to our abiotic and biotic environment over the years.

*[pH scale has a range of 0 to 14 and anything below 7 is acidic whereas anything above 7 is alkaline.]

Sulphur- di-oxide (SO2) and oxides of Nitrogen (NOx) are the principal contributors to the Acid Rain.

Fossil fuels containing Sulphur when burnt produce Sulphur- di-oxide which is a colorless gas with pungent smell. This gas is also produced during the processing of crude oil and production of steel. Sea spray, plankton, decomposed vegetation and volcanoes are regular sources of Sulphur. Nitrogen oxide is a compound of Nitrogen with any amount of oxygen atoms. Nitrogen monoxide and Nitrogen dioxide, that is, the oxides of Nitrogen, are processed naturally from volcanoes, forest fires and bacterial action in soil.* Transportation and industrial combustion also produce these gases. These gases, after several chemical reactions, are changed into acids and sometimes contribute to the Acid Rain.

*[10% of all Sulphur-di-oxide is produced from sea spray, plankton, wasted vegetation and volcanoes, 69.4%
from industrial combustion and 3.7% from transportation.

Lightening, volcanoes, forest fire and action in soil contribute 5% of Nitrogen oxides, transportation 43% and industrial combustions 32%.

These are from fifteen years old accounts. China and India enhanced the rate of industrialization more or less from that time. The current statistics will be formidable undoubtedly.]

Nitrogen monoxide and Nitrogen-di-oxide are all oxides of Nitrogen. These gases are by-products of firing processes of extreme high temperatures (automobiles, utility plants), and in chemical industries (fertilizer production).
Natural processes such as bacterial action in soil, forest fires, volcanic action, and lightning make up five percent of Nitrogen oxide emission. Transportation makes up 43 percent, and 32 percent belongs to industrial combustion.

[“Acid Rain.” The New World Book Encyclopedia. 1993.]

Again these gases are changed into acids mainly in two phases: (a)
Gas phase (b) Aqueous phase.


Gas phase oxidation of sulphur-di-oxide may be presented in three different reactions.

by ultraviolet light. The reaction is slow and the produce is insignificant due to absence of any catalyst. This is known as photo-oxidation.

(b) Sulphuric acid is formed by the oxidation of Ozone.

(c) Sulphur-di-oxide is oxidized first to form a sulphite ion:

SO2 (g) + o2 (g) SO32-

The sulphite ions coming in contact of the moisture in the atmosphere form Sulphuric acid:

SO32- +H2O (l) H2SO4 (aq)

This is a rapid reaction and this is very common.

Aqueous phase oxidation of Sulphur-di-oxide:

[S(IV)][SO2(aq)] + [HSO32]+[SO32]

Dissociation two parts:

SO2 (aq) H++HSO3-

HSO3- (aq)H+ + SO32-

Iron and manganese may be the catalysts. A part of the Sulphur-di-oxide is changed to Sulphuric acid and this is observed mainly in clouds and actually in the highly polluted air and in presence of ammonia and ozone which contribute as catalysts.

Nitrogen oxides

Gas phase reactions of Nitrogen oxide

NO2 (g)+H2O (l)HNO3 (aq)+HNO2(aq) [gas phase]

Nitrogen oxides are oxidized in the highly polluted clouds and Nitric acid is formed where iron, hydrogen peroxide and manganese act as catalysts.

Nitrogen oxides react with water and form Nitric acid in huge quantity because of automobile combustions.

Aqueous phase reactions:

2NO2(g)+H2O(l) 2H++NO3- +NO2-

NO (g)+NO2(g)+H2O (l) 2H+ +2NO2-

3NO2(g)+H2O(l) 2H+ +2NO3-+2NO(g)

These reactions yield limited results and reactions are fine if iron or manganese acts as catalyst.

Acid Rain has been counted as a great threat to the habitation of the living beings on the earth:

Soil and Plants

Minerals and other beneficial materials of the soil which the plants do take as nutrients are getting dissolved and carried away by the Acid Rain. Leaves lose the waxy protective coat and plants are exposed to diseases. The plants stand hapless as the diseased leaves fail to trap the photon particles of the sun and fail in producing food.

Aquatic animals

Lakes and other water bodies, the traditional hubs of the aquatic animals are polluted with the acid water. The fish find it hard to take in oxygen and other nutrients and sometimes mucus is formed in their gills and restrict their oxygen-inhaling capacity. The Acid Rain acts against the reproduction of the animals of the water bodies. Fertilizers of the field washed away by the cursed rainfall are causing great harm to the aquatic vegetation.

Human beings

Encroachment of acid in the human body has become a great danger. A great damage is done

to the drinking water and to the food as the crops and protein food from the small animals have become doubtful. Diseases affecting nerve, heart, lungs, eyes, nose and even brain are on the rise. Even chances of death are not to be ignored.

Buildings and Monuments

Acid rain has been working against the magnificent buildings or the heritage buildings of mankind.

Sandstone, marble, limestone, ceramics and paints are getting dissolved and eroded and thus a great number of lasting beauty which stands as the pride of man may gradually be lost.

The industrially developed Europe, the entire Europe, England and France and Holland and Germany, from the west to the east and from the north to the south, have experiences of Acid Rains and of environmental pollution. Again, the countries on the other side of the Atlantic that is, America and Canada have observed the wounds in the forests and lakes. China and India have sped up the process of industrialization and it has been marked that none have time or means to consider the unavoidable disaster that has been appearing or will sure to appear in the form of Acid Rain.

The only silver lining in our time rests in the fact that awareness of the environmental pollution caused by the Acid Rain has been seriously identified.


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