Acid Rain

Acid rain refers to a mixture of wet and dry deposition of chemical compounds from the atmosphere containing higher than normal amounts of nitric acid and sulfuric acid (HNO3 and H2SO4, respectively). Rain with a pH of 5.0 or stronger is considered acidic, which is slightly more acidic than clean or unpolluted rainwater (around 5.2–5.6). Extreme cases have been measured at a pH of between 3.0 and 4.5. Pure, unpolluted water has a pH of around 7.0. The phenomenon of acid rain and deposition has resulted in geographically and regionally pronounced effects, leading in some cases to dead lakes, fish, and trees, and damaged soils, crops, and national monuments in many countries. Nitric acid and sulfuric acid also damage human health. Concern with this problem began in the late 1960s and 1970s and, [Page 6]following substantial research efforts, culminated in government action in Europe, North America, and East Asia in the 1980s and 1990s.

Acid-rain-damaged trees at dusk, petrochemical plant in the distance.

Source: David Woodfall/Getty Images.

The precursors of acid rain formation come from both natural sources, for example, decaying vegetation and volcanoes, and anthropogenic sources, especially emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) from coal and oil combustion. High-sulfur coal-fired power plants and copper smelters are particularly important sources, and in some areas, motor vehicles. Acid rain occurs when these gases react in the atmosphere with rain, snow, or fog and oxygen and other chemicals to form various acidic compounds (acid aerosols). If the weather is dry, the acid chemicals can mix with dust particles or smoke. Either way, the result is a mild mix of HNO3 and H2SO4. When SO2 and NOx are released from fossil-fueled power plants and other sources, prevailing winds can blow the compounds across state and national borders, sometimes over hundreds of miles from the combustion sources. The widespread use of tall chimneys in coal-fired power plants and other industrial facilities in the 1970s and 1980s in North America and Europe led to an increase in this problem.

In the United States, there are two national networks (both supported by the U.S. Environmental Protection Agency) that monitor acid rain and dry-acid deposition: the National Atmospheric Deposition Program and the Clean Air Status and Trends Network. Similar monitoring networks exist in Europe and East Asia. While some areas have naturally acidic lakes and soils, the regional patterns of the problem have been fairly clear for decades. Areas suffering from especially strong levels of acid rain and deposition include the Adirondack and Catskill Mountains (New York), [Page 7]the rest of the northeastern United States, and some mountainous areas of the southeastern United States; southeastern Canada; Poland, Czech Republic, Slovakia, Germany, Greece, Russia, northeastern England, Southern Sweden, Norway, and Finland in Europe; and China, India, Japan, and Malaysia in Asia.

The most serious adverse effects of acid rain are those affecting freshwater aquatic environments (lakes, streams, creeks, marshes, and fish), forests, and human health; serious effects can also occur to buildings, other materials, automotive coatings, and visibility. The lower pH and higher aluminum levels in acid rain can damage a variety of ecosystems. This is especially problematic in watersheds whose soils have a limited ability to neutralize acidic compounds (called “buffering capacity”). At extreme levels, fish will develop tumors, and their eggs will not hatch; mortality rates increase; and biodiversity decreases. In the case of forests, acid rain does not usually kill trees but instead weakens them by damaging their leaves, limits the nutrients available to them, or exposes them to the slow release of toxic substances in soils. Human health effects from acid rain result from the important role of sulfate and nitrate aerosols as fine particles, as well as the role of NOx in tropospheric ozone formation. These pollutants can result in a variety of morbidity and mortality risks associated with lung inflammation, including asthma, chronic bronchitis, emphysema, vascular inflammation, and atherosclerosis. Particulate pollution can also result in lower levels of visibility. Building or monument damage can occur when H2SO4 rain reacts with calcium deposits in the stones to create gypsum, which will eventually flake off.

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