What is Acid Rain?

Acid rain is defined as the rainfall with pH (a unit of measuring acidity or basicity) values of 5.6 or less due to the chemical reaction of sulfur dioxide and nitrogen oxides with water vapor in the atmosphere. The term “acid rain” was first coined by a British chemist, Robert Angus Smith in 1872 after observing the impact of acid rain on buildings with heavy industrial activities. Acid rain can either occur in the form of wet deposition (as rainfall) or dry deposition (dry particles of sulfuric and nitric acid).

Acid rain is mainly formed when sulfur dioxide and nitrogen oxides combine with moisture in the atmosphere to form acids that are then deposited on land surface either through rain, snow or dew. The process occurs mainly in those areas that have large presence of fossil-fuel burning industries and automobile density. Sulfur dioxide is mainly produced from coal- burning industries, smelters, petroleum refineries and thermal power plants while nitrogen oxides are mainly produced from motor vehicles and automobiles that run on petrol and diesel. In India, cases of Acid rain have been reported from Agra, Mumbai and Delhi.