Increasing acidity in natural water and soil has become a global concern. Acidification and climate change are interrelated as the sources responsible for acidification and greenhouse gases are the same.

Unpolluted rain water normally is always slightly acidic, because CO₂ in atmosphere dissolves in it forming carbonic acid (H₂CO₃).The pH of unpolluted rain water is about 5.5-5.7 but due to presence of SO₂ and NO₂ gases as pollutants in the atmosphere, the pH of rain water is further lowered, often as low as 2.4 and this type of precipitation is generally referred to as acid rain.

Acid rain means the presence of excessive acids in rain water. Acid rain is a blend of H₂SO₄, HNO₃, and HCl. 

The contribution of various gases is as follows:

H₂SO₄ = 60-70%

HNO₃ = 30-40%

HCl = 10%

These oxides travel long distances in the atmosphere and undergo several physical and chemical transformations to produce more hazardous substances.

Every source of energy that we use be it coal, wood or petroleum products has sulphur and nitrogen. These elements when burnt in atmospheric oxygen are converted into their respective oxides SO₂ and NO_x which are highly soluble in water.

In the atmosphere the following reactions take place

In case of sulphur

S + O₂ → SO₂

2SO₂ + O₂ → 2SO₃

SO₃ in humid atmosphere forms droplets of H₂SO_4.

In case of nitrogen following reactions take place

NO + O → NO₂ + O₂

NO₂ + O₃→ NO₃ + O₂

NO₂ + NO₃ → N₂O₅

N₂O₅ in humid conditions form nitric acid

N₂O₅ + H₂O → 2HNO₃

EFFECTS OF ACID RAIN

·          Acid rain causes extensive damage to buildings and structural materials such as marble, limestone and slate. Taj Mahal has suffered lot due to SO₂ and H₂SO₄ acid fumes or other pollutants from Mathura Refinery.

·          Acid rain contaminates potable ground water with toxic compounds present in it. These toxic compounds enter into human body and effect human respiratory, nervous and digestive system.

·          It produces smog which is a safety hazard.

·          It increases the acidity of lakes and rivers which kills fish, algae and the aquatic system gets collapsed.