The excess acidity in rain is a result of air pollutants, mainly sulphur oxides and nitrogen oxide by the transformation of acidic components to acidic particles and vapours. Acid rain thus occurs when these acidic gases react in the Earth’s atmosphere with oxygen, water and other chemicals to form a variety of acidic compounds. Sunlight increases most of these reactions. The primary sources of these pollutants are cars, buses, trucks, trains, and industrial or power plants.
A more precise term for Acid rain is ‘acid deposition,’ which can be classified in two parts: wet and dry deposition.
Wet Acid deposition refers to acidic rain, snowfall and fog. As this acidic rain flows over the ground, it affects a range of animals, plants and living things.
Dry Acid deposition refers to acidic gases and particles. This occurs when the wind blows the acidic particles and gases causes them to stick to cars, trees, homes and buildings.
Acid rain is measured with the help of a scale known as “pH”. As acids release hydrogen ions, the acid content of a solution is based on the concentration of hydrogen ions and is expressed as "pH. When the ‘pH’ of a substance is low, it is more acidic, thus making the ‘pH’ of acidic rain, unusually low. Rain measuring between 0 and 5 on the pH scale is acidic and therefore called "acid rain."
Acid rain affects the whole environment. Polluted rain enters surface waters and permeates into the groundwater making the aluminium in the soil reactive which in turn leaches out the nutrients. This causes some fish to produce extra mucus around their gills, stopping ventilation.
Acidic rain damages trees and plants through the decrease in vital soil nutrients. Biodiversity in areas affected by acid rain is likely to be poor and therefore a great threat to humans in the long run.
Acid rain accelerates weathering and deforestation along with serious acidic deposition in rivers, streams, and other water bodies. It not only disturbs the delicate ecological balance with faster corrosion process, it also causes buildings and monuments of historical value to erode with premature disintegration.
In the U.S., Canada, and Europe, pollutant controls and emissions trading programs have significantly reduced the amount of sulfur dioxide from industrial sources. China, Eastern Europe and Russia are currently the regions most affected by acidification.