Acid rain is defined as rain with a pH of below 5.6. Normal rain has a pH of about 6, which is slightly acidic. This natural acidity is caused by dissolved carbon dioxide dissociating to form weak carbonic acid. 'Acid rain' is caused by sulphur from impurities in fossil fuels and nitrogen from the air combining with oxygen to form sulphur and nitrogen dioxides. These diffuse in to the atmosphere and react with water to form sulphuric and nitric acids which are soluble and fall with the rain. Some hydrochloric acid is also formed.
The principal natural phenomena that contribute acid-producing gases to the atmosphere are emissions from volcanoes and from biological processes that occur on the land, in wetlands, and in the oceans. The effects of acidic deposits have been detected in glacial ice thousands of years old in remote parts of the globe. Principal human sources are industrial and power-generating plants and transportation vehicles. The gases may be carried hundreds of miles in the atmosphere before they are converted to acids and deposited.
Evidence for an increase in the levels of acid rain comes from analysing layers of glacial ice. These show a sudden increase in pH from the start of the industrial revolution of 6 to 4.5 or 4. Other information has been gathered from studying organisms known as diatoms which inhabit ponds. Over the years these die and are deposited in layers of sediment on the lakes bottom. Diatoms thrive in certain pHs, so the numbers of diatoms found in layers of increasing depth give an indication of the change in pH over the years.
Since the industrial revolution, emissions of sulfur and nitrogen oxides to the atmosphere have increased. Industrial and energy-generating facilities that burn fossil fuels, primarily coal, are the principal sources of increased sulfur oxides. Occasional pH readings of well below 2.4 (the acidity of vinegar) have been reported in industrialized areas. These sources, plus the transportation sector, are the major originators of increased nitrogen oxides.
The problem of acid rain not only has increased with population and industrial growth, it has become more widespread. The use of tall smokestacks to reduce local pollution has contributed to the spread of acid rain by releasing gases into regional atmospheric circulation. Often deposition occurs a considerable distance from its formation, with mountainous regions tending to receive the most (simply because of their higher rainfall). An example of this effect is the frequent high pH of rain which falls in Scandanavia compared to the oxide levels it gives off.
There is a strong relationship between higher pHs and the lowering of populations of fish in lakes. Below 4.5 virtually no fish survive, whereas levels of 6 or higher promote healthy populations. Acid in water inhibits the production of enzymes which enable trout larvae to escape their eggs. It also mobilises toxic metals such as aluminium in lakes. Aluminium causes some fish to produce an excess of mucus around their gills, preventing proper ventilation. Phytoplankton growth is inhibited by high acid levels, and animals which feed on it suffer.
Trees are harmed by acid rain in a variety of ways. The waxy surface of leaves is broken down and nutrients are lost, making trees more susceptible to and frost. Root growth slows and as a result less nutrients are taken up. Toxic ions are mobilised in the soil, and valuable minerals are leached away or (in the case of phosphate) become bound to clay.
The toxic ions released due to acid rain forms the greatest threat to humans. Mobilised copper has been implicated in outbreaks of diarrhoea in young children and it is thought there are links between water supplies contaminated with aluminium and Alzheimer's disease.