The term acid rain was introduced some decades ago when it was discovered that the rain falling in some parts of the world was more acid than it should be and this was causing damage to forests, soils and water bodies. It is now realised that acid rain is not the only means of transport of the acids to the earth's surface from the atmosphere. Snow and hail are also involved and particulate matter such as dust in the atmosphere can also carry acid compounds. As a result acid deposition is the more correct and preferred term for this phenomenon.
The acidity is caused by emissions of sulphur dioxide and nitrogen oxides into the atmosphere, which react with water and other gases to form HNO3 (nitric acid) and H2SO4 (sulphuric acid), both of which are strong acids. Most of these emissions have been associated with the processes of industrialisation and involve in particular, the burning of fossil fuels by electric generation stations, smelters and a range of other industrial processing. More recently, motor vehicles have also become a significant source of emission of nitrogen and sulphur gases to the atmosphere. Other mechanisms of release of these gases to the atmosphere include volcanic activity, forest fires, lightning, but the increase in the problem in the last two centuries is mainly a result of the burning of fossil fuels. Once in the atmosphere these acids can be transferred across the earth's atmosphere, taken up by rainwater, and eventually reach the earth's surface as precipitation or particulate matter where they can damage vegetation and harm the soil and water environments.
The pH of rain falling to earth should be between 5.5 and 7.0. This is the pH range that results from the normal solution of carbon dioxide (CO2) in the atmosphere in rainwater. It is also the range that most plants and animals can tolerate. Up until the last 200 years it was the range that most living things on earth had been accustomed to for thousands, even millions, of years. The more recent sulphur dioxide and nitrogen oxides emissions to the atmosphere have caused the pH of precipitation falling to the earth's surface to fall in some regions to below 5.0. Countries that have reported major problems associated with acid rain include north-eastern United States and Canada, north-western, central and eastern Europe, India, Japan and China. The areas most affected are those in the lee of major industrial areas but the effects can range over thousands of miles.