Air pollution has occurred naturally since the formation of the Earth’s atmosphere; however, the industrial era has resulted in human activities greatly contributing to global atmospheric pollution. One of the more highly publicized and controversial aspects of atmospheric pollution is that of acidic deposition. Acidic materials can be transported long distances, some as much as hundreds of kilometers. Acidic deposition can impact buildings, sculptures, and monuments that are constructed using weatherable materials like limestone, marble, bronze, and galvanized steel. While acid soil conditions are known to influence the growth of plants, agricultural impacts related to acidic deposition are of less concern due to the buffering capacity of these types of ecosystems. When acidic substances are deposited in natural ecosystems, a number of adverse environmental effects are believed to occur, including damage to vegetation, particularly forests, and changes in soil and surface water chemistry.
Air pollution has occurred naturally since the formation of the Earth’s atmosphere; however, the industrial era has resulted in human activities greatly contributing to global atmospheric pollution.[1,2] One of the more highly publicized and controversial aspects of atmospheric pollution is that of acidic deposition. Acidic deposition includes rainfall, acidic fogs, mists, snowmelt, gases, and dry particulate matter. The primary origin of acidic deposition is the emission of sulfur dioxide (SO2) and nitrogen oxides (NO x ) from fossil fuel combustion; electric power generating plants contribute approximately two-thirds of the SO2 emissions and one-third of the NO x emissions.
Acidic materials can be transported long distances, some as much as hundreds of kilometers. For example, 30%–40% of the S deposition in the northeastern U.S. originates in industrial midwestern U.S. states. After years of debate, U.S. and Canada have agreed to develop strategies that reduce acidic compounds originating from their countries.[5,6] In Europe, the small size of many countries means that emissions in one industrialized area can readily affect forests, lakes, and cities in another country. For example, approximately 17% of the acidic deposition falling on Norway originated in Britain and 20% in Sweden came from eastern Europe.
The U.S. EPA National Acid Precipitation Assessment Program (NAPAP) conducted intensive research during the 1980s and 1990s that resulted in the “Acidic Deposition: State of the Science and Technology” that was mandated by the Acid Precipitation Act of 1980. NAPAP Reports to Congress have been developed in accordance with the 1990 amendment to the 1970 Clean Air Act and present the expected benefits of the Acid Deposition Control Program,[6,7] http://www.nnic.noaa.gov/CENR/NAPAP/. Mandates include an annual 10 million ton or approximately 40% reduction in point-source SO2 emissions below 1980 levels, with national emissions limit caps of 8.95 million tons from electric utility and 5.6 million tons from point-source industrial emissions. A reduction in NO x of about 2 million tons from 1980 levels has also been set as a goal; however, while NO x has been on the decline since 1980, projections estimate a rise in NO x emissions after the year 2000. In 1980, the U.S. levels of SO2 and NO x emissions were 25.7 and 23.0 million tons, respectively.
Acidic deposition can impact buildings, sculptures, and monuments that are constructed using weatherable materials like limestone, marble, bronze, and galvanized steel,[7,8] http://www.nnic.noaa.gov/CENR/NAPAP/. While acid soil conditions are known to influence the growth of plants, agricultural impacts related to acidic deposition are of less concern due to the buffering capacity of these types of ecosystems.[2,5] When acidic substances are deposited in natural ecosystems, a number of adverse environmental effects are believed to occur, including damage to vegetation, particularly forests, and changes in soil and surface water chemistry.[9,10]
Typical sources of acidic deposition include coal- and oil-burning electric power plants, automobiles, and large industrial operations (e.g., smelters). Once S and N gases enter the earth’s atmosphere they react very rapidly with moisture in the air to form sulfuric (H2SO4) and nitric (HNO3) acids.[2,3] The pH of natural rainfall in equilibrium with atmospheric CO2 is about 5.6; however, the pH of rainfall is less than 4.5 in many industrialized areas. The nature of acidic deposition is controlled largely by the geographic distribution of the sources of SO2 and NO x (Fig. 1). In the midwestern and northeastern U.S., H2SO4 is the main source of acidity in precipitation because of the coal-burning electric utilities. In the western U.S., HNO3 is of more concern because utilities and industry burn coal with low S contents and populated areas are high sources of NO x .
Emissions of SO2 and NO x increased in the 20th century due to the accelerated industrialization in developed countries and antiquated processing practices in some undeveloped countries. However, there is some uncertainty as to the actual means by which acidic deposition affects our environment,[11,12] http://nadp.sws.uiuc.edu/isopleths/maps1999/. Chemical and biological evidence, however, indicates that atmospheric deposition of H2SO4 caused some New England lakes to decrease in alkalinity.[13,14] Many scientists are reluctant to over-generalize cause and effect relationships in an extremely complex environmental problem. Although, the National Acid Deposition Assessment Program has concluded there were definite consequences due to acidic deposition that warrant remediation[6,7] http://www.nnic.noaa.gov/CENR/NAPAP/. Since 1995, when the 1990 Clean Air Act Amendment’s Title IV reduction in acidic deposition was implemented, SO2 and NO x emissions have, respectively, decreased and remained constant during the late 1990s.
Both H2SO4 and HNO3 are important components of acidic deposition, with volatile organic compounds and inorganic carbon also components of acidic deposition-related emissions. Pure water has a pH of 7.0, natural rainfall about 5.6, and severely acidic deposition less than 4.0. Uncontaminated rainwater should be pH 5.6 due to CO2 chemistry and the formation of carbonic acid. The pH of most soils ranges from 3.0 to 8.0. When acids areadded to soils or waters, the decrease in pH that occurs depends greatly on the system’s buffering capacity, the ability of a system to maintain its present pH by neutralizing added acidity. Clays, organic matter, oxides of Al and Fe, and Ca and Mg carbonates (limestones) are the components responsible for pH buffering in most soils. Acidic deposition, therefore, will have a greater impact on sandy, low organic matter soils than those higher in clay, organic matter, and carbonates. In fresh waters, theprimary buffering mechanism is the reaction of dissolved bicarbonate ions with H+ according to the following equation:
Few direct human health problems have been attributed to acidic deposition. Long-term exposure to acidic deposition precursor pollutants such as ozone (O3) and NO x , which are respiratory irritants, can cause pulmonary edema.[5,6] Sulfur dioxide (SO2) is also a known respiratory irritant, but is generally absorbed high in the respiratory tract.
Indirect human health effects due to acidic deposition are more important. Concerns center around contaminated drinking water supplies and consumption of fish that contain potential toxic metal levels. With increasing acidity (e.g., lower pH levels), metals such as mercury, aluminum, cadmium, lead, zinc, and copper become more bioavailable. The greatest human health impact is due to the consumption of fish that bioaccumulate mercury; freshwater pike and trout have been shown to contain the highest average concentrations of mercury.[5,15] Therefore, the most susceptible individuals are those who live in an industrial area, have respiratory problems, drink water from a cistern, and consume a significant amount of freshwater fish.
A long-term urban concern is the possible impact of acidic deposition on surface-derived drinking water. Many municipalities make extensive use of lead and copper piping, which raises the question concerning human health effects related to the slow dissolution of some metals (lead, copper, zinc) from older plumbing materials when exposed to more acidic waters. Although metal toxicities due to acidic deposition impacts on drinking waters are rare, reductions in S and N fine particles expected by 2010 based on Clean Air Act Amendments will result in annual public health benefits valued at $50 billion with reduced mortality, hospital admissions and emergency room visits.
Different types of materials and cultural resources can be impacted by air pollutants. Although the actual corrosion rates for most metals have decreased since the 1930s, data from three U.S. sites indicate that acidic deposition may account for 31%–78% of the dissolution of galvanized steeland copper,[7,8] http://www.nnic.noaa.gov/CENR/NAPAP/. In urban or industrial settings, increases in atmospheric acidity can dissolve carbonates (e.g., limestone, marble) in buildings and other structures. Deterioration of stone products by acidic deposition is caused by: 1) erosion and dissolution of materials and surface details; 2) alterations (blackening of stone surfaces); and 3) spalling (cracking and spalling of stone surfaces due to accumulations of alternation crusts. Painted surfaces can be discolored or etched, and there may also be degradation of organic binders in paints.
It is important to examine the nature of acidity in soil, vegetation, and aquatic environments. Damage from acidification is often not directly due to the presence of excessive H+, but is caused by changes in other elements. Examples include increased solubilization of metal ions such as Al3+ and some trace elements (e.g., Mn2+, Pb2+) that can be toxic to plants and animals, more rapid losses of basic cations (e.g., Ca2+, Mg2+), and the creation of unfavorable soil and aquatic environments for different fauna and flora.
Soil acidification is a natural process that occurs when precipitation exceeds evapotranspiration. “Natural” rainfall is acidic (pH of ∼ 5.6) and continuously adds a weak acid (H2CO3) to soils. This acidification results in a gradual leaching of basic cations (Ca2+ and Mg2+) from the uppermost soil horizons, leaving Al3+ as the dominant cation that can react with water to produce H+. Most of the acidity in soils between pH 4.0 and 7.5 is due to the hydrolysis of Al3+,[17,18] http://www.epa.gov/airmarkets/acidrain/effects/index.html. Other acidifying processes include plant and microbial respiration that produces CO2, mineralization and nitrification of organic N, and the oxidation of FeS2 in soils disturbed by mining or drainage. In extremely acidic soils (pH < 4.0), strong acids such as H2SO4 are a major component.
The degree of accelerated acidification depends both upon the buffering capacity of the soil and the use of the soil. Many of the areas subjected to the greatest amount of acidic deposition are also areas where considerable natural acidification occurs. Forested soils in the northeastern U.S. are developed on highly acidic, sandy parent materials that have undergone tremendous changes in land use in the past 200 years. However, clear-cutting and burning by the first European settlers have been almost completely reversed and many areas are now totally reforested. Soil organic matter that accumulated over time represents a natural source of acidity and buffering. Similarly, greater leaching or depletion of basic cations by plant uptake in increasingly reforested areas balances the significant inputs of these same cations in precipitation.[20,21] Acidic deposition affects forest soils more than agricultural or urban soils because the latter are routinely limed to neutralize acidity. Although it is possible to lime forest soils, which is done frequently in some European countries, the logistics and cost often preclude this except in areas severely impacted by acidic deposition.
Excessively acidic soils are undesirable for several reasons. Direct phytotoxicity from soluble Al3+ or Mn2+ can occur and seriously injure plant roots, reduce plant growth, and increase plant susceptibility to pathogens. The relationship between Al3+ toxicity and soil pH is complicated by the fact that in certain situations organic matter can form complexes with Al3+ that reduce its harmful effects on plants. Acid soils are usually less fertile because of a lack of important basic cations such as K+, Ca2+, and Mg2+. Leguminous plants may fix less N2 under very acidic conditions due to reduced rhizobial activity and greater soil adsorption of Mo by clays and Al and Fe oxides. Mineralization of N, P, and S can also be reduced because of the lower metabolic activity of bacteria. Many plants and microorganisms have adapted to very acidic conditions (e.g., pH < 5.0). Examples include ornamentals such as azaleas and rhododendrons and food crops such as cassava, tea, blueberries, and potatoes.[5,22] In fact, considerable efforts in plant breeding and biotechnology are directed towards developing Al- and Mn-tolerant plants that can survive in highly acidic soils.
Acidic deposition contains N and S that are important plant nutrients. Therefore, foliar applications of acidic deposition at critical growth stages can be beneficial to plant development and reproduction. Generally, controlled experiments require the simulated acid rain to be pH 3.5 or less in order to produce injury to certain plants. The amount of acidity needed to damage some plants is 100 times greater than natural rainfall. Crops that respond negatively in simulated acid rain studies include garden beets, broccoli, carrots, mustard greens, radishes, and pinto beans, with different effects for some cultivars. Positive responses to acid rain have been identified with alfalfa, tomato, green pepper, strawberry, corn, lettuce, and some pasture grass crops.
Agricultural lands are maintained at pH levels that are optimal for crop production. In most cases the ideal pH is around pH 6.0–7.0; however, pH levels of organic soils are usually maintained at closer to pH 5.0. Because agricultural soils are generally well buffered, the amount of acidity derived from atmospheric inputs is not sufficient to significantly alter the overall soil pH. Nitrogen and S soil inputs from acidic deposition are beneficial, and with the reduction in S atmospheric levels mandated by 1990 amendments to the Clean Air Act, the S fertilizer market has grown. The amount of N added to agricultural ecosystems as acidic deposition is rather insignificant in relation to the 100–300 kg N/ha/yr required of most agricultural crops.
Perhaps the most publicized issue related to acidic deposition has been widespread forest decline. For example, in Europe estimates suggest that as much as 35% of all forests have been affected. Similarly, in the U.S. many important forest ranges such as the Adirondacks of New York, the Green Mountains of Vermont, and the Great Smoky Mountains in North Carolina have experienced sustained decreases in tree growth for several decades. Conclusive evidence that forest decline or dieback is caused solely be acidic deposition is lacking and complicated by interactions with other environmental or biotic factors. However, NAPAP research has confirmed that acidic deposition has contributed to a decline inhigh-elevation red spruce in the northeastern U.S. In addition, nitrogen saturation of forest ecosystems from atmospheric N deposition is believed to result in increased plant growth, which in turn increases water and nutrient use followed by deficiencies that can cause chlorosis and premature needle-drop as well as increased leaching of base cations from the soil.
Acidic deposition on leaves may enter directly through plant stomates.[1,22] If the deposition is sufficiently acidic (pH ∼ 3.0), damage can also occur to the waxy cuticle, increasing the potential for direct injury of exposed leaf mesophyll cells. Foliar lesions are one of the most common symptoms. Gaseous compounds such as SO2 and SO3 present in acidic mists or fogs can also enter leaves through the stomates, form H2SO4 upon reaction with H2O in the cytoplasm, and disrupt many metabolic processes. Leaf and needle necrosis occurs when plants are exposed to high levels of SO2 gas, possibly due to collapsed epidermal cells, eroded cuticles, loss of chloroplast integrity and decreased chlorophyll content, loosening of fibers in cell walls and reduced cell membrane integrity, and changes in osmotic potential that cause a decrease in cell turgor.
Root diseases may also increase in excessively acidic soils. In addition to the damages caused by exposure to H2SO4 and HNO3, roots can be directly injured or their growth rates impaired by increased concentrations of soluble Al3+ and Mn2+ in the rhizosphere,[2,25] http://nadp.sws.uiuc.edu. Changes in the amount and composition of these exudates can then alter the activity and population diversity of soil-borne pathogens. The general tendency associated with increased root exudation is an enhancement in microbial populations due to an additional supply of carbon (energy). Chronic acidification can also alter nutrient availability and uptake patterns.[8,22]
Long-term studies in New England suggest acidic deposition has caused significant plant and soil leaching of base cations,[1,21] resulting in decreased growth of red spruce trees in the White Mountains. With reduction in about 80% of the airborne base cations, mainly Ca2+ but also Mg2+, from 1950 levels, researchers suggest forest growth has slowed because soils are not capable of weathering at a rate that can replenish essential nutrients. In Germany, acidic deposition was implicated in the loss of soil Mg2+ as an accompanying cation associated with the downward leaching of SO4 2−, which ultimately resulted in forest decline. Several European countries have used helicopters to fertilize and lime forests.
Ecological damage to aquatic systems has occurred from acidic deposition. As with forests, a number of interrelated factors associated with acidic deposition are responsible for undesirable changes. Acidification of aquatic ecosystems is not new. Studies of lake sediments suggest that increased acidification began in the mid-1800s, although the process has clearly accelerated since the 1940s. Current studies indicate there is significant S mineralization in forest soils impacted by acidic deposition and that the SO4 2− levels in adjacent streams remain high, even though there has been a decrease in the amount of atmospheric-S deposition.
Geology, soil properties, and land use are the main determinants of the effect of acidic deposition on aquatic chemistry and biota. Lakes and streams located in areas with calcareous geology resist acidification more than those in granitic and gneiss materials. Soils developed from calcareous parent materials are generally deeper and more buffered than thin, acidic soils common to granitic areas. Land management decisions also affect freshwater acidity. Forested watersheds tend to contribute more acidity than those dominated by meadows, pastures, and agronomic ecosystems.[8,14,20] Trees and other vegetation in forests are known to “scavenge” acidic compounds in fogs, mists, and atmospheric particulates. These acidic compounds are later deposited in forest soils when rainfall leaches forest vegetation surfaces. Rainfall below forest canopies (e.g., throughfall) is usually more acidic than ambient precipitation. Silvicultural operations that disturb soils in forests can increase acidity by stimulating the oxidization of organic N and S, and reduced S compounds such as FeS2.
A number of ecological problems arise when aquatic ecosystems are acidified below pH 5.0, and particularly below pH 4.0. Decreases in biodiversity and primary productivity of phytoplankton, zooplankton, and benthic invertebrates commonly occur.[15,16] Decreased rates of biological decomposition of organic matter have occasionally been reported, which can then lead to a reduced supply of nutrients. Microbial communities may also change, with fungi predominating over bacteria. Proposed mechanisms to explain these ecological changes center around physiological stresses caused by exposure of biota to higher concentrations of Al3+, Mn2+, and H+ and lower amounts of available Ca2+. One specific mechanism suggested involves the disruption of ion uptake and the ability of aquatic plants to regulate Na+, K+, and Ca2+ export and import from cells.
Acidic deposition is associated with declining aquatic vertebrate populations in acidified lakes and, under conditions of extreme acidity, of fish kills. In general, if the water pH remains above 5.0, few problems are observed; from pH 4.0 to 5.0 many fish are affected, and below pH 3.5 few fish can survive. The major cause of fish kill is due to the direct toxic effect of Al3+, which interferes with the role Ca2+ plays in maintaining gill permeability and respiration. Calcium has been shown to mitigate the effects of Al3+, but in many acidic lakes the Ca2+ levels are inadequate to overcome Al3+ toxicity. Low pH values also disrupt the Na+ status of blood plasma in fish. Under very acidic conditions, H+ influx into gill membrane cells both stimulates excessive efflux of Na+ and reduces influx of Na+ into the cells. Excessive loss of Na+ can cause mortality. Other indirect effects include reduced rates of reproduction, high rates of mortality early in life or in reproductive phases of adults, and migration of adults away from acidic areas. Amphibians are affected in much the same manner as fish, although they are somewhat less sensitive to Al3+ toxicity. Birds and small mammals often have lower populations and lower reproductive rates in areas adjacent to acidified aquatic ecosystems. This may be due to a shortage of food due to smaller fish and insect populations or to physiological stresses caused by consuming organisms with high Al3+ concentrations.
Damage caused by acidic deposition will be difficult and extremely expensive to correct, which will depend on our ability to reduce S and N emissions. For example, society may have to burn less fossil fuel, use cleaner energy sources and/or design more efficient “scrubbers” to reduce S and N gas entering our atmosphere. Despite the firm conviction of most nations to reduce acidic deposition, it appears that the staggering costs of such actions will delay implementation of this approach for many years. The 1990 amendments to the Clean Air Act are expected to reduce acid-producing air pollutants from electric power plants. The 1990 amendments established emission allowances based on a utilities’ historical fuel use and SO2 emissions, with each allowance representing 1 ton of SO2 that canbought, sold or banked for future use,[4,6,7] http://www.nnic.noaa.gov/CENR/NAPAP/. Short-term remedial actions for acidic deposition are available and have been successful in some ecosystems. Liming of lakes and some forests (also fertilization with trace elements and Mg2+) has been practiced in European counties for over 50 years.[16,23] Hundreds of Swedish and Norwegian lakes have been successfully limed in the past 25 years. Lakes with short mean residence times for water retention may need annual or biannual liming; others may need to be limed every 5–10 years. Because vegetation in some forested ecosystems has adapted to acidic soils, liming (or over-liming) may result in an unpredictable and undesirable redistribution of plant species.