Acid deposition is any type of precipitation - rain, snow, sleet, hail, or fog - that has a lower pH (and is therefore more acidic) than normal. However, the term almost always used for all of these is acid rain. This higher acidity can cause problems in ecosystems and the environment, and remains one of the major environmental concerns from fuel use despite the immense progress made to address it since the 1970s.
Acid rain is produced when water in the air combines with nitrogen oxides and sulfur dioxide, two types of pollutants, and then falls down the surface of the Earth. These pollutants may also collect on the Earth's surface and the rain may combine with it upon arrival to the Earth, so the term "acid deposition" is often preferred over acid rain.
Normal rain has a pH of around 5.6 so it is slightly acidic, and this is due to the combination of carbon dioxide with the water in the air to form carbonic acid. This acidity is naturally dealt with by the Earth, as alkaline minerals such as calcium, magnesium and potassium act to offset the acidity. The carbonic acid is in fact important for the Earth, as it weathers bedrock and releases bicarbonates into the soil and water, which can further neutralize stronger acidic inputs. Acid rain, however, has a pH of around 4.2. This means that acid rain is around 25 times more acidic than normal rain due to the logarithmic nature of the pH scale. This higher acidity can be harmful to the environment as will be explored below.
Acid rain has many damaging effects on vegetation, lakes, fish, buildings and other structures. It can also cause respiratory diseases in humans, especially those that have bad health. When lakes and other bodies of water become too acidic, typically less than a pH of 6.0, plants and aquatic life begin to suffer. Fish reproduction ability falters, with death or deformity being widespread among younger fish. Amphibians and invertebrates suffer similarly, which means that acidic lakes have very little to no animal life.
The most notable impacts have been measured in northeastern sections of North America and Western Europe, with the sources being traced primarily to coal-fired power plants which emit large amounts of NOx and SO2. The acid rain levels can be seen in Figure 3 in the United States, and due to a combination of wind flows from the US and the production of pollutants within Canada, Eastern Canada also suffers largely from acid rain. Various regulations and restrictions of pollutants have been implemented since acid rain became a concern, and these power plants must use air pollution control devices to meet them.
After acid rain falls to the Earth, the effects depend on the type of rock and soil it encounters. Limestone is able to neutralize the acid content of the rain, while bedrocks consisting of granite, quartz, and gneiss are not able to. The northeastern regions of Figure 2 are made up largely of bedrocks of this type, therefore they are very vulnerable to acid rain. Lakes at higher elevations in these regions tend to be more acidic, and are considered "dead" if they are not able to support life.
The build up of acid snow on mountains is also a negative consequence, especially since a large amount of the acidic content may be released all at once during the spring in mountain runoff. This chemical shock is devastating to plant life downstream. Many high mountain lakes have been known to lose their fish populations due to increased acidity.
Following harsh realizations of the effects of acid rain, many limits to pollution were put in place around the world since the 1980s. The emissions of SO2 have decreased by 40% in the United States, with acid rain levels down about 65% in 2005. The same is true for NOx. The acidity of lakes has been found to have decreased in these chemical concentrations, and fish life is expected to gradually recover. This is made possible by the use of various air pollution control devices, such as scrubbers and electrostatic precipitators. Coal power plants and other pollution producing plants require these devices in order to meet restrictions on emissions, which can reduce the emissions of some harmful pollutants by 99%.