During the 1970s and ’80s the phenomenon called acid rain was one of the most well-known environmental problems in Europe and North America, appearing frequently in news features and mentioned, on occasion, in situation comedies of the day. Since that time, the visibility of acid rain in the media has been supplanted by stories about climate change, global warming, biodiversity issues, and other environmental concerns. Acid rain still occurs, but its impact on Europe and North America is far less than it was in the 1970s and ’80s, because of strong air pollution regulations in those regions.
The term acid rain is a popular expression for the more formal and scientific term acid deposition. Acid deposition includes more than just acid precipitation in the form of falling rain. Acid deposition can occur as snow, sleet, hail, and fog, too, and it also includes “dry deposition” made up of acidic particles and gases, which can affect landscapes during rain-free periods. Precipitation of each of these forms is considered “acidic” if it has a pH of about 5.2 or below. (Water with a pH of 7 is neutral; however, rainwater and surface water lean slightly acidic.) The excess acids often come from human activities—particularly from the burning of fossil fuels (coal, oil, natural gas) and the smelting of metal ores—and also from volcanic activity. Sulfur dioxide (SO2) and nitrogen oxides (NOx; the combination of NO and NO2) are the chemical compounds that are most responsible, produced in the U.S. mostly through the burning of coal by electrical utilities.
Where industrial emissions are large and emission controls too weak to reduce SO2 and NOx emissions, acid deposition will fall hours or days later, far downwind of power plants and other emission sources. In these areas, the pH of precipitation can average between 4.0 and 4.5 annually, and the pH of individual rainstorms can sometimes drop below 3.0. In addition, cloud water and fog in polluted areas may be many times more acidic than rain falling over the same region.
If the precipitation falls in acid-sensitive areas—that is, areas without acid-neutralizing chemicals such as limestone, which acts as a buffer to acidic conditions (as long as the supply of limestone in the environment lasts)—the pH of the water and the soil decreases, bringing heightened risk to many forms of life. Acid deposition can reduce the pH of surface waters and lower biodiversity by contributing to a decline in the health of fishes and invertebrates. It weakens trees and increases their susceptibility to damage from other stressors, such as drought, extreme cold, and pests. Acid rain can also leach important plant nutrients, such as calcium and magnesium, from the soil and release aluminum, bound to soil particles and rock, in its toxic dissolved form. Acid rain that falls in urban areas contributes to the corrosion of surfaces exposed to air pollution and is responsible for the deterioration of limestone and marble buildings and monuments.
The acid rain problem in Europe and North America has largely abated because of stronger SO2 and NOx emission controls, such as the U.S. Clean Air Act of 1970, the Canada–United States Air Quality Agreement in 1991, and similar measures in Europe. In the United States the first phase of emission reductions took effect in 1995, and subsequent reductions followed. Such effective emission controls have not spread to all countries, however. As developing countries such as India and China have industrialized, their emissions of SO2 and NOx have increased. This same pattern can be observed in some of the quickly growing urban areas in Latin America and Africa, which has resulted in more acidic rain and other acid deposition falling both within and downwind of those regions.