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.
The papers in this section provide a big picture look at the problem of acid deposition—primarily as acid rain—with a particular focus on North America and Europe. Many of them articulate how the effects of acid rain link across air, land, and water systems, as well as across the scientific, political, and economic aspects of the problem. Gorham 1955 provides one of the first accounts of the chemical composition of rainwater, with attention to the natural and anthropogenic drivers that affect it. This type of approach was critical to early investigators interested in understanding cause and effect. Likens, et al. 1972 and Likens and Bormann 1974 are classic early overviews of the acid rain problem by some of the most well-known investigators to pursue this topic. Likens and Bormann 1974 focuses in part on the argument for federal action to regulate fossil fuel emissions and pursue alternative energy sources. Subsequent works that are accessible to those with a baseline understanding of ecosystem science include Driscoll, et al. 2001, which synthesizes the body of research on acid deposition in the northeastern United States, and Likens, et al. 2002, which details close to forty years of research done on S cycling at Hubbard Brook Experimental Forest. Important works describing the problem beyond North America include Irwin, et al. 2002, which reports a trend analysis of atmospheric data from a period of rapid change in acid deposition across Europe, and Dianwu, et al. 1988, which describes patterns of acid deposition and acidification of receiving ecosystems in some of the most impacted areas of China. Finally, Vitousek, et al. 1997 is a well-known paper documenting changes to the global N cycle, including those due to our reliance on fossil fuels.
Dianwu, Z., X. Jiling, X. Yu, and W. H. Chan. 1988. Acid rain in southwestern China. Atmospheric Environment 22.2: 349–358.
Details the problem of acid rain and acidification in the most impacted area of China. The focus is on the variability in precipitation and aerosol chemistry measurements from across the country.
Driscoll, C. T., G. B. Lawrence, A. J. Bulger, et al. 2001. Acidic deposition in the Northeastern United States: Sources and inputs, ecosystem effects, and management strategies. BioScience 51.3: 180–198.
A comprehensive look at the response of northeastern US ecosystems to regulation of air pollution, including recommendations for future regulation to recover damaged regions.
Gorham, E. 1955. On the acidity and salinity of rain. Geochimica et Cosmochimica Acta 7.5–6: 231–239.
A classic work that was among the first to explore the chemistry of rainwater, including natural and anthropogenic factors affecting its composition and the concentrations of key constituents.
Irwin, J. G., G. Campbell, and K. Vincent. 2002. Trends in sulphate and nitrate wet deposition over the United Kingdom: 1986–1999. Atmospheric Environment 36:2867–2879.
Presents a trend analysis, including some daily records, demonstrating the decline in acid deposition over Europe during the late 20th century. Overall, declines in sulfate deposition have been greater (by approximately one-third) than nitrate deposition.
Likens, G. E., and F. H. Bormann. 1974. Acid rain: A serious regional environmental problem. Science 184.4142: 1176–1179.
One of the first high-profile published papers describing the severity of the acid rain problem in the northeastern United States by two of the most well-known researchers on the topic, Gene Likens and Herbert Bormann. Not only does the article describe the science behind acid rain but it also calls for alternative energy sources and air quality regulation.
Likens, G. E., F. H. Bormann, and N. M. Johnson. 1972. Acid rain. Environment 14.2: 33–40.
A classic, early overview of the acid rain problem accessible to a general audience.
Likens, G. E., C. T. Driscoll, D. C. Buso, et al. 2002. The biogeochemistry of sulfur at Hubbard Brook. Biogeochemistry 60:235–316.
Presents a synthesis of thirty-four years of study on S cycling at the Hubbard Brook Experimental Forest. This paper demonstrates the decline in atmospheric S deposition and subsequent changes in sulfate losses in surface waters at one of the most studied sites in the United States.
Vitousek, P. M., J. D. Aber, R. W. Howarth, et al. 1997. Human alteration of the global nitrogen cycle: Sources and consequences. Ecological Applications 7.3: 737–750.
A classic work describing how humans have fundamentally changed the cycling of N. This paper includes the impacts of fossil fuel emissions, among many other anthropogenc drivers of change.
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