Acidification, the process of making acid, is not just caused by deposits of acidic rain but also by chemicals in snow and fog and by gases and particulates when precipitation is not occurring. The major human-made causes of acid deposition are (1) emissions of sulfur dioxide from power plants that burn coal and oil and (2) emissions of nitrogen oxides from automobiles. These emissions are transformed into sulfuric acid and nitric acid in the atmosphere, where they accumulate in cloud droplets and fall to Earth in rain and snow, wet deposition. Other sources of acid deposition are gases like sulfur dioxide and nitrogen oxides, as well as very small particulates. These gases and particulates are usually deposited when it is not raining or snowing which is called dry deposition.
While large areas of Europe and North America are exposed to acidifying depositions, only certain types of ecosystems are affected by these depositions. The most vulnerable ecosystems usually have a thin cover of soil, containing little calcium and sitting upon solid rock made up of hard minerals such as granite or quartz. Many freshwater lakes, streams, and rivers have become acidic, resulting in the decline or local destruction of some plant and animal populations. It is not yet certain that land-based ecosystems have been affected by acidic deposition. After acid rain was discovered in Europe, scientists began measuring the acidity of rain in North America. Initially, they found that the problem was concentrated in the northeastern states of New York and Pennsylvania because the type of coal burned there was more sulfur containing.
Acid rain is measured through pH tests that determine the concentration of hydrogen ions in a liter of fluid. The pH scale is used to measure acidity or alkalinity. It runs from 0 to 14. Water has a neutral pH of 7. The greater the concentration of hydrogen ions and the lower the pH number, the more acidic a substance is and the lower the concentration of hydrogen ions and the higher the pH number, the more alkaline or basic a substance is. So a pH greater than 7 indicates an alkaline substance while a pH less than 7 indicates an acidic substance. It is important to note that a change of only one unit in pH equals a tenfold change in the concentration of hydrogen ions. For example, a solution of pH 3 is 10 times more acidic than a solution of pH 4. Normal rain and snow measure about pH 5.60. In environmental science, the definition of acid precipitation refers to a pH less than 5.65.
Measured values of acid rain vary according to geographical area. When pH levels are drastically upset in soil and water, entire lakes and forests are endangered. Evergreen trees in high elevations are especially vulnerable. Although the acid rain itself does not kill the trees, it makes them more susceptible to disease. Also, high acid levels in soil cause leaching of other valuable minerals such as calcium, magnesium, and potassium. Small marine organisms cannot survive in acidic lakes and rivers, and their depletion affects the larger fish who usually feed on them, and, ultimately, the entire marine-life food chain. Snow from acid rain is also damaging; snowmelt has been known to cause massive, instant death for many kinds of fish. Some lakes in Scandinavia and New York's Adirondack Mountains are completely devoid of fish life. Acid rain also eats away at buildings and metal structures. From the Acropolis in Greece to Renaissance buildings in Italy, ancient structures are showing signs of corrosion from acid rain. In some industrialized parts of Poland, trains cannot exceed 40 miles (65 kilometers) per hour because the iron railway tracks have been weakened from acidic air pollution. Usually, waters affected by acid rain are treated by adding limestone or lime, an alkaline substance (base) that reduces acidity. Fishery biologists especially are interested in liming acidic lakes to make them more habitable for sport fish. In some parts of Scandinavia, for instance, liming is used extensively to make the biological damage of acidification less severe.
While neutralizing ecosystems that have become acidic, treats the symptoms but not the sources of acidification. Although exact sources of acid rain are difficult to pinpoint and the actual amount of damage caused by acid deposition is uncertain, it is agreed that acid rain levels need to be reduced. Scientific evidence supports the notion that what goes up must come down, and because of public awareness and concerns about acid rain in many countries, politicians have begun to act decisively in controlling or eliminating human causes of such pollution. Emissions of sulfur dioxide and nitrogen oxides are being reduced, especially in Western Europe and North America. For example, in 1992 the governments of the United States and Canada signed an air-quality agreement aimed at reducing acidifying depositions in both countries. While countries in Western Europe and North American have actively carried out actions to reduce emissions of gases leading to acid deposition for a number of years, countries in other parts of the world have only recently addressed the issue. In Eastern Europe, Russia, China, India, Southeast Asia, Mexico, and various developing nations, acid rain and other pollution problems are finally gaining notice. For example, in 1999, scientists identified a haze of air pollution that hovers over the Indian Ocean near Asia during the winter. The 3.8 million-square-mile haze (about the size of the combined area of all fifty American states) is made up of small by-products from the burning of fossil fuels. Such a cloud has the potential to cool Earth, harming both marine and terrestrial life.