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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.

Industrial acid rain is a substantial problem in China, Eastern Europe, Russia and areas down-wind from them. These areas all burn sulfur-containing coal to generate heat and electricity.

Acid rain is defined as rain with a pH of below 4.0 - 4.5. Normal rain has a pH of about 5.6, which is slightly acidic. This natural acidity is caused by dissolved carbon dioxide dissociating to form weak carbonic acid. 'Acid rain' is caused by sulfur from impurities in fossil fuels and nitrogen from the air combining with oxygen to form sulfur dioxide and nitrogen dioxide. These diffuse into the atmosphere and react with water to form sulfuric and nitric acids which are soluble and fall with the rain. Some hydrochloric acid is also formed.

A possible reaction for the production of acid rain is as follows:

Evidence for an increase in the levels of acid rain comes from analysing layers of glacial ice. These show a sudden decrease 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 sedimentSediment is any particulate matter that can be transported by fluid flow and which eventually is deposited as a layer of solid particles on the bed or bottom of a body of water or other liquid. Sedimentation is the deposition by settling of a suspended ma 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 Scandinavia compared to the oxide levels it gives off.

There is a strong relationship between higher pHs and the lowering of populations of fishAtlantic herring, Clupea harengus one of the most abundant species in the world Photo A fish is a poikilothermic (cold-blooded) water-dwelling vertebrate with gills. There are over 27,000 species of fish, making them the most diverse group of vertebrates. 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 enzymeAn enzyme is a protein, or protein complex, that catalyzes a chemical reaction. Like any catalyst, enzymes work by lowering the activation energy of a reaction, thus allowing the reaction to proceed to its steady state or completion much faster than it ots which enable troutThis article is about fish. The Trout Quintet is a work by Schubert. Rainbow Trout (Oncorhynchus mykiss Trout is the common name given to a number of species of freshwater fishes belonging to the salmon family, Salmonidae''. All fish properly called trout larvaeLarvae are the plural of larva, juvenile form of animals with indirect development. See larva In Roman mythology, the Larvae were the spectres or spirits of the dead; they were the malignant version of the Lares. Some Roman writers describe Lemures as the to escape their eggs. It also mobilises toxic metals such as aluminiumAluminium (or aluminum in North American English) is the chemical element in the periodic table with the symbol Al and atomic number 13. A silvery and ductile member of the poor metal group of elements, aluminium is found primarily as the ore bauxite and in lakes. Aluminium causes some fish to produce an excess of mucusMucus is a slippery secretion of the lining of various membranes in the body ( mucous membranes). Mucus aids in the protection of the lungs by trapping foreign particles that enter the nose during normal breathing. Additionally, it prevents tissues from d around their gillGill In aquatic organisms In aquatic organisms, gills are a respiratory organ for the extraction of oxygen from water and for the excretion of carbon dioxide. Many small aquatic animals absorb oxygen through the surface of their bodies in general, but mors, preventing proper ventilation. Phytoplankton growth is inhibited by high acid levels, and animals which feed on it suffer.

Many lakes are subject to natural acid runoff from acid soils, and this can be triggered by particular rainfall patterns that concentrate the acid. An acid lake with newly-dead fish is not evidence of severe air-pollution.

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 frost, fungus and insects. 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.