Article

Acid Deposition

Eve-Lyn S. Hinckley

in Environmental Science


Published online October 2015 | | DOI: http://dx.doi.org/10.1093/obo/9780199363445.016.0042

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Acid deposition is one of the major environmental problems of the modern world. This broad term refers to a mix of wet and dry material containing high amounts of primarily nitrogen (N) and sulfur (S) oxides (i.e., SO2 and NOx). In combination with water, oxygen, and other chemicals in the atmosphere, these gases form acid rain, fog, and snow. When acidic wet deposition (pH < 5.6) enters terrestrial and aquatic ecosystems, the dissociation of acid releases hydrogen ions into soils and surface waters, lowering pH and causing a cascade of ecosystem effects. These include accelerating the weathering and release of base cations from soils, increasing the acidity of lakes and streams, mobilizing aluminum and mercury, which are toxic to fish and other wildlife, and weakening forests. The concentration of acids in the precipitation, the chemistry of the receiving soils and surface waters, and the types of wildlife present determine the degree to which these effects are observed. In predominately dry climates, acidic dry deposition lands on surfaces and can cause the same ecological impacts when water combines with dust during large rainstorms, generating acidic runoff. While natural sources of acid deposition occur, such as from volcanic eruptions and decaying vegetation, human activity is largely responsible for changing precipitation chemistry through the burning of fossil fuels. Beginning with the Industrial Revolution of the mid-18th through 19th centuries, the release of fossil fuel emissions via tall smokestacks caused long-range transport of N and S oxides to remote ecosystems. There, they rained down and their impacts have lasted for decades. The most devastating ecological impacts of acid rain have been observed in the forests and surface waters of the northeastern United States, Canada, and Europe. Research beginning in the 1960s at the Hubbard Brook Experimental Forest in New Hampshire (US) was among the first to link the sources of acid rain in the United States—fossil fuel burning power plants in the Mid-west—to the observed consequences for ecosystems hundreds of miles away. Scientific research on this topic is often held up as an environmental success story in North America and Europe, due to air quality regulation at national and international levels that followed the science. In the United States, these efforts began with the amendments to the Clean Air Act of 1970. However, while significant progress was made to reduce fossil fuel emissions, continued reliance on fossil fuels feeds persistent impacts on air quality, downgradient ecosystems, and human health in many regions of the world, including those where success has been declared. For example, in China and India, acid rain and poor air quality continue to be major environmental and human health issues. This article touches on many dimensions of the acid deposition problem—scientific, social, political, and economic—with particular emphasis on the discovery of acid rain and its ecological consequences from the biogeochemical and ecological literature.

Article.  9328 words. 

Subjects: Environment ; Environmental Science, Engineering, and Technology ; Environmental Science

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