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Acid rain refers to a mixture of deposited material, both wet and dry, coming from the atmosphere containing more than normal amounts of nitric and sulphuric acids. Simply put, it means rain that is acidic in nature due to the presence of certain pollutants in the air due to cars and industrial processes . Acid rain can occur in form of rain, snow, fog or dry material that settle to earth. Acidity is determined on the basis of the pH level of the water droplets. Normal rain water is slightly acidic with a pH range of 5.3-6.0, because carbon dioxide and water present in the air react together to form carbonic acid, which is a weak acid. When the pH level of rain water falls below this range, it becomes acid rain. Acid rain is caused by a chemical reaction that begins when compounds like sulphur dioxide and nitrogen oxides are released into the air. These substances can rise very high into the atmosphere, where they mix and react with water, oxygen, and other chemicals to form more acidic pollutants, known as acid rain. Sulphur dioxide and nitrogen oxides dissolve very easily in water and can be carried very far by the wind. As a result, the two compounds can travel long distances where they become part of the rain, sleet, snow, and fog that we experience.

It occurs due to natural and human activities. Erupting volcanoes contains some chemicals that can cause acid rain. Apart from this, burning of fossil fuels, running of  factories and automobiles due to human activities are few other reasons behind this activity.

Natural sources such as erupting volcanoes, rotting vegetation and sea sprays produce sulfur dioxide and fires, bacterial decomposition and lightening generate nitrogen dioxide. The chemicals released by natural sources gets mixed up with water and oxygen and are disperse over large areas because of wind patterns.

Man-made sources include emission of sulphur dioxide and nitrogen oxides due to combustion of fossil fuels. Roughly two-thirds of all sulphur dioxide and one-fourth of all nitrogen oxides come from generation of electricity through burning of fossil fuels such as coal. These gases react in the atmosphere with water, oxygen, and other chemicals to form various acidic compounds such as sulphuric acid, ammonium nitrate, and nitric acid. The existing winds blow these acidic compounds over large areas across borders and they fall back to the ground in the form of acid rain or other forms of precipitation. Upon reaching the earth, it flows across the surface, absorbs into the soil and enters into lakes and rivers and finally gets mixed up with sea water.

The gases i.e. i.e. sulphur dioxide (SO2) and nitrogen oxides (NOx) are primarily gases occurring from electric power generation by burning coal and responsible for acid rain.

Effects of Acid Rain

Acid rain has significant effects on the world environment and public health.

Effect on Aquatic Environment – Acid rain either falls directly on aquatic bodies or gets run off the forests, roads and fields to flow into streams, rivers and lakes. Over a period of time, acids get accumulated in the water and lower the overall pH of the water body. The aquatic plants and animals need a particular pH level of about 4.8 to survive. If the pH level falls below that the conditions become hostile for the survival of aquatic life.

Effect on Forests – It makes trees vulnerable to disease, extreme weather, and insects by destroying their leaves, damaging the bark and arresting their growth. Forest damage due to acid rain is most evident in Eastern Europe – especially Germany, Poland and Switzerland.

Effect on Soil – As it falls on forest or field soil, it kills useful micro-organisms and leaches nutrients of soil. Many a times, this leads to calcium and other nutrient deficiency, producing infertile soils.

Effect on Architecture and Buildings – Acid rain on buildings, especially those constructed with limestone, react with the minerals and corrode them away. This leaves the building weak and susceptible to decay. Modern buildings, cars, air planes, steel bridges and pipes are all affected by acid rain. Irreplaceable damage can be caused to the old heritage buildings.

Effect on Public Health – When in atmosphere, sulphur dioxide and nitrogen oxide gases and their particulate matter derivatives like sulphates and nitrates, degrades visibility and can cause accidents, leading to injuries and deaths.