Acid Rain And Deposition


1 INTRODUCTION Early Concern

The first mention of acid rain in print was by Robert Boyle in which he referred to "nitrous or salino-sulphureous spirits" in air in his 1692 book A General History of the Air. The Scottish chemist Robert Angus Smith began to study acid rain in Manchester, England, in 1852 and extended the work in England, Scotland, and Germany for 20 years. His 1872 book, Air and Rain: The Beginnings of a Chemical Climatology, pointed out the link between sulfur pollution and "acid rain." He warned that acid rain was damaging plants and materials downwind of industrial regions, but his warning went largely unheeded.

While some research was conducted on acid deposition in the ensuing years, it was not until the 1950s and 1960s that E. Gorham, conducting research in England and Canada, built the major foundations for our present understanding of the causes of acid precipitation and its impact on aquatic ecosystems. However, it took the work of a Swedish scientist, S. Oden, in the 1960s, to arouse the scientific community and general public to engage in the debate about acid deposition. One newspaper account described his ideas about an insidious "chemical war" among the nations of Europe. Thus, by the 1970s, it was finally realized that Eastern Europe, Germany, Scandinavia, Canada, and the United States were experiencing widespread damage to forests and lakes as well as damage to stone and metal buildings and other structures from acid rain. In Germany, the term Waldsterben (forest death) was coined. Forests in parts of the Czech Republic, Slovakia, and Russia were practically devastated due to acid rain and heavy-metal ion deposition from uncontrolled industrial and power plant emissions. China and India are also experiencing significant effects of acid

Handbook of Weather, Climate, and Water: Atmospheric Chemistry, Hydrology, and Societal Impacts, Edited by Thomas D. Potter and Bradley R. Colman. ISBN 0-471-21489-2 © 2003 John Wiley & Sons, Inc.

rain, with the Taj Mahal losing much of its stonework surface material to acid deposition.

Acid Rain Chemistry

Acid rain is actually precipitation of various ions, both anions and cations, through precipitation, such as rain, snow, fog, as well as dry particles or aerosols. A typical ion balance is

[H+] + [Na+] + [Na4+] + 2[Ca2+]

= 2[S042-] + 2[S032 ] + [NO J] + [CP] + [OH"] + [HC03"] + 2[C032-] (1)

The primary naturally occurring trace gas that affects the pH of precipitation is carbon dioxide (C02), which forms carboxylic acid in water. The aqueous reactions of carbon dioxide are as follows:

C02 gas + H20 H2C03

H2C03 HC03 +H+

Since pKa of (4) is as high as 10.3, reaction (2) has the greatest influence on the acidity of natural atmospheric systems. For a partial pressure of C02 of 350 ppmv, Henry's law constant (KH) is as follows:

Kh = [H2C03]/[C02 gas] = 3.97 x 10~2 mol/L atm and the equilibrium constant (K3) of reaction (3) is given by:

K3 = [H+][HC03"]/[H2C03] = 4.5 x 10"7 mol/L

By combining and rearranging these two expressions, one arrives at the following equation:

[HC03-] = ([C02 gas] x K„ x AT3)/[H+]

If the concentration of bicarbonate in water is equal to the hydrogen ion concentration, then by substitution, one arrives at the following:

([C02 gas] x Kh x k3)/[h4 = 5.97 x 10"12 mol2/L~2


The bracketed quantities denote molar concentrations, with the cations on the left, the anions on the right.

Note that Henry's law can be expressed in terms of a pseudo-Henry's law constant to account for the increased uptake of gas in the liquid due to reactions in the liquid. For example,

However, 5.6 is not necessarily the natural pH of rain since other naturally occurring species also play a role. Nitrogen oxides are formed naturally during lightning discharges, and sulfur species are released into the atmosphere over the oceans from biological activity as dimethyl sulfide (DMS). Hydrocarbon acids such as carboxylic acids, HCOO„ and methylcarboxylic acids, CH3COO„ also contribute to the acidity, especially in remote, forested regions. On the other hand, base cations from soil dust, such as Ca, Mg, K, and P, etc., are alkaline and increase the pH. Thus, the natural acidity of precipitation can vary considerably depending on the upwind sources and, as will be discussed, meteorological conditions.

In addition, pollutants such as the nitrogen and sulfur oxides also contribute to acidity and are the focus of most of the concern regarding acid deposition. However, ammonia, often associated with agricultural operations, is alkaline. This chapter will examine the sources, the chemical transformations involved in the production of acid deposition, transport, deposition amounts and trends, and the effects on soils, plants, animals, and materials.

2 SOURCES Natural

To put pollution contributions into perspective, it is worthwhile first to understand the role that naturally occurring materials play in acid deposition. Natural sources of sulfur account for 25 to 30% of the total, unless there are large volcanic eruptions, such as El Chichon in 1982 or Mount Pinatubo in 1991. Mount Pinatubo was estimated to emit 9Tg of S into the stratosphere (total sources are 94 to 123TgS/yr), where the e_1 residence time for sulfuric acid aerosols is approximately one year.

Kco* = [C02 x H20 + HC03 C032-]/PCOj

Oxides of nitrogen (NOt = NO + N02) are also produced naturally. As discussed in Chapter 4, natural sources such as soil emissions, lightning, stratospheric-tropo-spheric exchange, and a portion of biomass burning account for approximately one third of total NOv.

Hydrocarbons are also involved in acid deposition. Carboxylic acids, HCOO„ and methylcarboxylic acids, CH3COO„ are important hydrocarbon acids derived from direct terrestrial emissions as well as oxidation of emissions by marine or terrestrial biota.

Base cations are generally derived from soils through lofting of aeolian dust by wind. Deserts, such as the Sahara Desert in Africa and the Gobi Desert in China, generate large dust clouds each year that are transported thousands of kilometers. The dust from the Sahara Desert often reaches both North and South America and may provide significant base cations for vegetation in the rain forests. The dust from the Gobi Desert is often seen over Japan and the Korean Peninsula. The base cation deposition in Europe was studied for 1989. Using a 10 x 20 km2 grid, maps were produced showing that base cations neutralize S042- +N03~ by much more in southern regions than in northern regions. South of 45° to 50°N, more than 50% was neutralized, with more than 75% in some locations; in Norway and Sweden, the amount neutralized generally fell to less than 10%. The variations can be explained in terms of the amounts of acid ions and base cations in the air. Soil-derived dust in the United States used to provide the base cations to help neutralize the effects of sulfur and nitrogen. However, the amount of base cations in precipitation has been declining in the past 2 to 3 decades in the United States probably because of changes in farming and construction practices that leave fewer disturbed regions from which wind can raise dust.


Anthropogenic sources of sulfur accounted for 77% of global sulfur emissions in 1980. Combustion of fossil fuel for electric power production is responsible for most of the anthropogenic contributions to acid deposition, accounting for 67% of the anthropogenic S02 emissions in the United States in 1996. Industrial fuel combustion accounts for 17% of the U.S. S02 emissions, with various other sources accounting for the rest. Fuel used for transportation generates 7% of the S02 emissions, which has been linked to regional haze patterns in such places as the Los Angeles Basin and portions of the eastern United States.

NO is a by-product of combustion of all hydrocarbon fuels, both fossil fuel and fresh biomass, due to the high temperatures involved. In the United States in 1996, 30% of the NO emissions came from on-road vehicles, 28% from electric utilities, 19% from nonroad engines and vehicles, 13% from industrial fuel combustion, and 10% from other sources. On a global basis, anthropogenic NO emissions are highest where industry, fossil fuel power plants, and surface transportation are most densely sited, i.e., the northern midlatitudes.

Anthropogenic ammonia emissions are associated with fertilizers and livestock feedlots. Organic acids also contribute to the anthropogenic burden of acid deposi tion. The major organic acids found in the gas phase are formic acid (HCOOH) and acetic acid (CH3COOH), with other organic acids found in minor amounts. Sources include automobile exhaust, biomass burning, and some food processing plants.

The emitted S02, NO, and NH3 are transformed to aerosols and components of precipitation through both gas-phase and aqueous-phase chemical reactions. Sulfur dioxide is transformed in the gas phase primarily by:

Ozone can also lead to the oxidization of S02. Such reactions would be especially important at night when OH radical concentrations are very small due to the absence of solar radiation. One way this can happen is for ozone to react with an alkene, such as ethene or propene by adding to the carbon double bond, creating a primary ozonide. Since ozonides are not stable, this can rapidly split into what is called a Criegee intermediate, named after the German chemist who proposed the mechanism. A Criegee intermediate can react with S02 in a series of steps that also result in the oxidation of S02, which can also be oxidized directly by ozone, but the reaction rate is slow. Note that the rate of oxidation of S02 has a seasonal cycle in middle latitudes, being as much as an order of magnitude lower in winter than in summer.

In the aqueous phase, other reactions can occur. For example:

These reactions establish equilibria of the various sulfur species, with mole fractions dependent on the pH of the solution and both Henry's law constant [for (14)] and equilibrium constants [for (15) and (16)]. Dissolved S02 (13) is favored at pH below 2, the bisulfite ion (15) for 2 < pH < 7, and the sulfite ion (16) for pH > 7.

Aqueous-phase reactions with H202 in cloud, fog, and raindrops are considered to be the dominant mechanisms for the oxidation of S02 to H2S04. Thus, H202 could be rate limiting. Field and modeling studies show that to explain the seasonal concentrations of H202 (higher in summer than in winter) the initial rate of aqueous phase H202 photoformation has to be linearly dependent on solar actinic flux, i.e., radiation that induces photochemical reactions. Organic chromophores are suggested to be responsible for the H202 photoformation. One implication of this study is that the seasonal variability in the nonlinearity between S02 emissions and regional sulfate deposition may be largely explained. Other peroxides can also oxidize S02, but exist in lower concentrations than does H202.


S02 + H20 -» S02 x H2O S02 xH20-> HSO3-+H+ HS03~ -» S032- +H+

Other reactions leading to the oxidation of S(IV) include ozone, 02 catalyzed by transition metal ions such as Fe3+ and Mn2+, and carbonaceous particles. While the reactions with H202 are generally most important (2 to 20% per hour, independent of pH), the others are much weaker in general and have very strong pH dependences. Above a pH of 5, the reaction with ozone is comparable to that for H202, with the other reactions somewhat weaker.

Note that other sulfur species, such as hydrogen sulfide (H2S) and carbonyl sulfide (OCS), emitted by biological sources, can also be oxidized, as well as dimethyl sulfide (DMS), emitted from marine sources. While OH is the primary source of DMS oxidation, N03 also reacts rapidly with DMS, and halogens, such as bromine, chlorine, and iodine are also potential reactants with DMS in the marine boundary layer.

Nitric oxide (NO) is rapidly oxidized to N02, especially by reacting with ozone: NO + 03 N02 + 02 (17)

From there, it is transformed to nitric acid by interaction with the hydroxyl radical:

This reaction is about 10 times more rapid than that of (13).

Nitric acid can also be formed by the reaction with various organics, such as the alkanes and aldehydes. In this case, hydrogen is abstracted from the organic molecule. This reaction may account for 15% of the nitric acid formation, occurring primarily at night.

Both sulfate and nitrate aerosols are very hygroscopic and increase in diameter rapidly with increases in relative humidity above 50% to 70%. In the absence of cloud formation, they form the bulk of regional aerosols downwind of heavily industrialized/urbanized regions, such as the eastern United States. As acid haze becomes thicker and stays near the surface, it can become acid fog, such as has been observed in California and in eastern U.S. mountains. An aerosol/fog cycle can be set up in which aerosol particles grow by water condensation on existing nuclei, dilute and dissolve in fog droplets, where they undergo chemical conversions. The process can go the other way as solute concentrations increase due to evaporation of the water, leading back to aerosols. Thus, as the temperature cycles during the day, the fog-aerosol-fog cycle can be made.

An excellent overview of the chemistry of acid precipitation can be found in Finlayson-Pitts and Pitts (2000).


In addition to source regions and transformation mechanisms and rates, winds and other meteorological conditions also play important roles in determining where acid precipitation will occur. The pollution plumes will be transported at the rate of the


prevailing winds. The source gases will be transformed at various rates depending on such factors as amount of solar radiation, concentrations of OH and water vapor, temperature, and the extent of clouds.

Sulfate can be transported up to 1100 km in normal downwind directions and up to 400 km in the normal upwind directions {i.e., during the reduced opportunities for transport in that direction), while nitrogen oxides are transported as nitrates as far as 200 to 800 km. It is found that turnover times for anthropogenic sulfate are 4.7± 1.1 days in the eastern United States.

The transport of ammonia and ammonium depends on the emissions of S02 and NOv along the trajectory of the air mass containing them. The transport distance for ammonia and ammonium in northern Europe depends on the amount of S02 and NOr present. When they are present, transport is reduced significantly because ammonium aerosols are formed rapidly. NHr is most likely to be deposited in the country of origin in Europe, given the sizes of the countries, while for SOv only 25 to 30% would be deposited and for NO,, only 10%.

Figure I Annual pH of rain for the United States in 1990. The black lines indicate contours of equal pH. See ftp site for color image.


Acid deposition occurs in two primary forms—wet and dry. Wet deposition comprises rain, snow, and fog. Dry deposition involves turbulent transport of aerosol and gases to the surface layer. The relative amounts of wet and dry deposition depend on a number of factors, such as the amount of precipitation, whether the

Figure I Annual pH of rain for the United States in 1990. The black lines indicate contours of equal pH. See ftp site for color image.

elevation is above the cloud line, how far the site is from the primary sources of the acid ions, etc. At U.S. Environmental Protection Agency (EPA) National Dry Deposition Network stations in the eastern United States in 1991, dry sulfate deposition accounted for approximately 10 to 60% (mean approximately 40%) of total sulfate deposition, with wet deposition accounting for the rest. For nitrates, the dry deposition fraction varied from 20 to 65% (mean approximately 45%). Due to the seasonal cycle in the rate of oxidation of S02, deposition rates for S02 tend to be higher than for S04 in winter, with the reverse occurring in summer.

The acidity of deposition depends on the difference between anions and cations in the precipitate. Thus, the nitrate and sulfate ions reduce the pH, while ammonium and soil-derived dust increase the pH. Figure 1 shows a map of the pH of rain for the United States in 1990, indicating that the pH is lowest just southeast of the Great Lakes, a consequence of the high amount of fossil fuel combustion in and to the west of the region.

6 MEASUREMENT Instruments

Various instruments are used in the study of acid deposition. Since emission rates are generally estimated based on factors associated with fuel consumption, not many measurements are made at the source regions. Standard meteorological instruments and networks are used for the meteorological data input. The collectors generally use polypropylene funnels and bottles. The bottles may be refrigerated to 4°C to reduce evaporation and/or heated to melt snow. When wet and dry deposition collectors are used together, a lid is placed over the dry deposition bucket at the onset of precipitation, then back over the wet deposition bucket at the end of precipitation. However, it should be noted that measurement of dry deposition is notoriously difficult, and that buckets do not adequately represent the manner in which the local surfaces collect dry deposition. The three conceptual ways in which dry deposition is measured are: (1) direct collection on surrogate or natural surfaces, (2) flux measurements by eddy correlation or profile techniques, and (3) indirect estimation using atmospheric concentration monitoring and estimated deposition velocities. Which approach is used varies depending on the funds available and the accuracy to which the information is desired.

Once the samples are collected, they are taken to a laboratory for analysis. The analytical methods used by the National Acid Deposition Program/National Trends Network (NADP/NTN) in the United States are likely typical of such programs. A glass electrode is used to measure pH; conductivity is measured using a platinum electrode; chloride, nitrate, orthophosphate and sulfate are measured with ion chro-matrography with a detection limit of 0.03 mg/L for all but orthophosphate, which is measured with a detection limit of 0.02 mg/L; ammonium is measured using automated phenate colorimetry with a detection limit of 0.02 mg/L; calcium, magnesium, potassium, and sodium are measured with flame atomic absorption spectro-


photometer with a detection limit of 0.003 except for calcium, for which the detection limit is 0.09 mg/L. Sodium and/or magnesium can be used to estimate the fraction of material derived from sea salt. This is useful in determining how to apportion the sulfate values between terrestrial and oceanic sources.

The Acid Precipitation in Ontario Study (APIOS) deposition monitoring program has similar instrumentation with slightly different detection limits. The NADP/NTN detection limits were improved by instrument changes in 1985, while the APIOS instrumentation was established in 1980 and not updated as of 1990.

Surface networks

Collection instruments are often set out in networks. The NADP/NTN is an example of such a network. It is part of a cooperative program that includes federal, state, and private research organizations. The objectives of the program are:

1. To measure and characterize the supply of beneficial and injurious chemical substances in atmospheric deposition on a broad regional scale

2. To determine the spatial patterns and temporal trends in the distribution of chemical elements deposited on natural and managed ecosystems

3. To provide information needed to gain a better understanding of the sources, transport, and transformation of materials contributing to or associated with acidic atmospheric deposition in the United States

The NADP/NTN was made operational in July 1978 and continues to the present time. The sites were selected to represent major physiographic, agricultural, aquatic, and forested areas throughout the United States. In general, sites are located in rural areas away from sources that could affect the measurements. The program grew from 22 sites in late 1978 to about 200 sites in 1985, which were still in operation in 1990. The containers are heated to 4°C to melt snow but are not refrigerated. Samples are collected weekly and sent to the Central Analytical Laboratory in Champaign, Illinois.


In the 1980s, a major study, the National Acid Precipitation Assessment Program (NAPAP) was funded by Congress to investigate the situation in the United States. The total cost was $500 million. Areas of investigation included acid deposition and effects on aquatic and terrestrial ecosystems. Both nitrate and sulfate depositions were found to be highest in the northeast United States near the eastern Great Lakes, centered on eastern Michigan, western New York and Pennsylvania, and northern West Virginia, and extending into southern Ontario, albeit with somewhat different geographical distributions. Ammonium deposition peaked in Michigan and southern

Ontario. As a consequence, annual pH of rain is lowest in New York, Pennsylvania, and West Virginia as shown in Figure 1 for 1990.

Similar programs have been carried out in a number of European countries, especially in terms of acid rain effects on forests, with a number of them reported in the Springer Ecological Studies series.


With accelerating economic development in Southeast Asia, anthropogenic NOx emissions are expected to increase dramatically in the near future. It has been estimated that global NOx emissions will increase from an estimated 19Tg N02 in 1990 to 86 Tg N02 in 2020. The largest increases are expected in the power and transport sectors.

Trends of acid deposition should generally follow the regional trends for fossil fuel consumption, with coal and oil providing most of the sulfur, and all components contributing to the nitrogen oxides and organic acids. In the United States, wood was the primary source of fuel until 1880, being used to generate about 3 x 1015 Btu/yr at the peak in 1870. Coal started to be used in increasing amounts around 1850, rising to 15 x 1015 Btu/yr by 1916, staying in the range 10 to 17 x 1015 Btu/yr after that. Oil started to become an important fuel source after 1900, rising to 35 x 1015 Btu/yr by 1977 before leveling off. Natural gas also became important after 1900, rising to 24 x 1015 Btu/yr by 1970 before dropping slightly. Thus, in the United States, acid deposition should have risen steadily from 1900 to at least the 1980s. In the eastern and midwestern United States there has been an estimated 19% decrease in S02 emissions and a 16% decrease in NOv emissions between 1975 and 1987. Since the U.S. Clean Air Act Amendments of 1990 mandated further decreases in sulfur emissions, they have continued to decrease. Between 1989 and 1995, sulfur dioxide decreased 35% and sulfate 26% in rural eastern United States. Nitrogen emissions have not been recognized as being very important until recently for a variety of scientific and political reasons, and it is more difficult to remove NOx than S02 from the flue gases, so the regulations on nitrogen emissions are not as strong as for sulfur. Between 1989 and 1995 nitrogen concentrations in rural eastern United States had fallen only 8%.

Data for historical anthropogenic emissions of S02 are also available for Europe. A gradual increase is seen from 1880 (0.45Tg/yr) to 1940 (1.4Tg/yr), a dip in 1945, then a rapid increase to >36Tg/yr in 1980, followed by a gradual decline thereafter. Ammonia emissions peaked in the mid-1980s.

Continued population growth and development are expected to lead to an increase of 25% in the deposition of nitrogen in the more-developed-country regions by the year 2020. Earth's population is projected to increase from 6 billion in 1999 to 8.5 billion in 2020, and per-capita energy consumption is expected to double compared to 1980. Much of the increase will be felt in Asia. The increases in nitrogen oxides may lead to larger ozone concentrations, thereby increasing the


oxidizing capacity of the atmosphere and its ability to absorb thermal infrared radiation.


Bernhard Ulrich is credited with determining how acid deposition affects soil during the acidification process. His 1966 study set the stage for his later work. His review summarizes the effects of acid deposition on soil cation-anion budgets and lists a number of his key works. As soil acidity increases due to acid deposition (or plant biomass harvesting for that matter), the base cations (e.g., Ca, Mg, K, P) try to neutralize the acidity and are leached from the upper soil horizons in the process. As the process continues, the transition metal and aluminum oxides are dissolved, with these cations becoming more prevalent in the soil solution. Nitric acid is a stronger acid than sulfuric, so it has a greater ability to lower the soil pH. An interesting recent finding is that as the process continues, Al3+ seems to accelerate the base cation leaching process, making Al3+ more readily available. As acid deposition continues over a long period, the acid neutralizing capacity (ANC) or alkalinity decreases.

Additional influences on ANC arise from biogeochemical processes. Trees, for example, enhance the collection of dry deposition as well as remove base cations from the soil. Soil organic matter storage is followed by decay, which releases the trace minerals, nitrogen, and organic acids. In addition, forest defoliation by the gypsy moth has exacerbated the effects of acidic deposition. Changes in stream water composition following severe defoliation of forested mountain watersheds in western Virginia has included increased concentrations of nitrate and acidity, as well as accelerated export of base cations, and pH and ANC reached lower levels than previously observed, especially during storm flow conditions. To date, several years following the defoliation, stream water composition has not returned to pre-defoliation values.

Finally, there are interactions between the various processes. Changing acid-base status changes vegetation amounts and types. Reductions in vegetation cover can lead to reduction in enhanced collection of dry deposition as well as higher surface temperatures, thereby increasing microbial activities.



Paradoxically, one of the first effects of acid deposition on trees and forests is that of stimulating growth, rather than hindering it. Nitrogen in both ammonium (NH4) and nitrate (N03) forms can be utilized by trees in building amino acids required for growth. Thus, nitrogen deposition first has the impact of fertilizing plants. This process eventually ceases in temperate ecosystems when the soil is nitrogen saturated. The impact of nitrogen deposition on carbon uptake by terrestrial ecosystems has been modeled using several different three-dimensional models. Both NOv and NHX deposition were considered. The bulk of the NOv, deposition was found to be in the eastern United States, Europe, and, to a lesser extent, in eastern Asia and Japan. All five models predict that most of the carbon will be sequestered in the forests of eastern United States and Europe. Without N saturation, C sequesterization was found to range from 6 to 13 x 1015 g C/yr, while with N saturation, the range was 5 to 10 x 1015 g C/yr. This implies that N saturation reduces the growth rate of forests, in line with what has been observed in forests in the northeastern United States.

Acid deposition also causes the soil solution pH to be lowered, in part through the increased biomass growth rate, since the tree has to give up hydronium ions in exchange for base cations. It should be noted that the impact of acid deposition on forests is mediated through the soils, with some better able to buffer the acid than others. Calcium carbonate or limestone, for example, has a high buffering capacity, and would take a long time to show serious effects from acid deposition. One response of trees is for the tree roots to try to grow away from the acid soil, which may take the form of growing more in the upper organic layer, rather than in the lower mineral horizons. This makes trees susceptible to other stresses, such as winds and drought. Another effect is that since trees obtain less calcium after long-term acid deposition, the strength of the boles (trunks) and branches is reduced, since plants rely upon calcium for cell wall structure, they are much more susceptible to falling during ice, snow, and wind storms, as was the case in the northeastern United States and southeastern Canada in early 1998.

Starting around the 1970s, researchers in the United States and Europe began to notice that trees were beginning to show evidence of decline for nonhistorical reasons. Acid deposition was identified as a likely suspect in the early 1970s, although the effects of acid deposition had been observed in the sixteenth century in Europe and discussed again in the midnineteenth century.

The effects of acid precipitation on European forests in the 1980s have been well documented, especially to the Norway spruce [Picea abies (L.) Karst]. A study investigating the spruce decline determined that a long history of acid deposition, mostly sulfate prior to the early part of the century, with nitrate added around 1915, led to the observed effects. The soils were somewhat deficient in calcium and magnesium, and by about 1980, there was a strong nutritional imbalance due to years of ammonium nitrate depositions, nitrate leaching from the soils, and soil acidification. The yellowing of the leaves was attributed to deficiencies in magnesium. While Waldsterben in Europe was less pronounced in the early-to-mid-1990s than in the mid-1980s, probably due to reductions in sulfur emissions, declines in forest health are still quite prevalent, especially in central Europe. Annual forest condition surveys in conjunction with the modeling studies of nitrogen deposition show increased soil acidity in the regions with highest forest decline symptoms. There, the mean plot defoliation was in the range of 20 to 40% in 1997, with evergreens affected more than deciduous trees.


Acid deposition has had an adverse impact on forests in the eastern United States. The decline of the red spruce forests in the northeastern United States has been attributed to acid deposition, as has the decline of red spruce forests in North Carolina. Acid deposition has also adversely affected the sugar maples {Acer saccharum Marsh.) in Pennsylvania and Quebec as well as red oaks {Quercus rubra) and white oaks {Quercus alba) in the eastern United States. Evidence linking acid deposition to U.S. forest condition is found using the U.S. Department of Agriculture Forest Service Forest Inventory and Analysis data in conjunction with acid ion deposition doses using the acid deposition data from NAPAP. Increased mortality rates for white oaks {Quercus alba) in the northeastern United States can be related statistically to increased acid ion doses.

Further evidence for the role of acid deposition affecting oaks is found in oak tree ring studies in North Carolina and Missouri in the United States. The growth spurts in the 1950s for oaks in decline compared with lower growth rates of healthier nearby oaks are consistent with the N fertilization effect; the gradual growth decline subsequently is consistent with impaired tree vitality due to both acid deposition and ozone exposure; and the rapid decline after major droughts in the 1980s is consistent with shallower root depth, leading to greater water stress in drought periods.

Aquatic Ecosystems

Aquatic ecosystems have borne much of the brunt of acid deposition, resulting in significant loss of invertebrate populations and fish production among other things.

There are several processes influencing acid-base chemistry of surface waters. Wet and dry deposition is one. The other important factor is the ANC (alkalinity) of the water body. In turn, the ANC is strongly affected by the soils and bedrock under and near the body. The difference between the sums of base cations and acid anions derived from the soils and bedrock is equal to the ANC. Location of a body of water in a region where the soils and rocks are more likely to contribute base cations than acid anions to the water are less likely to be acidified by acid deposition. The base cations involved at the higher ANC levels are generally, in approximate order of importance, calcium, magnesium, sodium, and potassium. The acid anions are, likewise, carbonate, organic acids, sulfate, chloride, and nitrate. Of course, local conditions affect the amounts and relative orders.

Both aquatic animals and plants are adversely affected by acidification. The processes affected by acidification include change rates and amounts of primary production, nutrient cycling, and decomposition. Aluminum plays an important role in acidified systems since it is detrimental or toxic to both animal and plant life. Normally, aluminum is tightly bound to oxygen or the hydroxyl radical, OH. As the pH is lowered below 6, the concentration of monomelic aluminum rises rapidly. Aluminum in acidified streams has been found to coat the gills of fish, leading to premature mortality.

It has recently been recognized that atmospheric deposition of nitrogen is playing a significant role in the eutrophication in estuaries and coastal waters, such as the Chesapeake Bay in the mid-Atlantic eastern United States. Until a landmark study was published in 1991, it was thought that most of the nitrogen reaching such bodies of water came from agricultural operations. More recent work has determined that approximately 20% of the nitrogen reaching the Chesapeake Bay as wet precipitation is in the form of dissolved organic nitrogen. In addition, a significant fraction comes from ammonium.

Another consequence of lake acidification is increased transparency. Most likely this is due to reduction in dissolved organic carbon or from a change in the chemical nature and light absorption capacity of dissolved organics in the water. This can lead to changes in primary productivity and thermal structure at lower depths. An additional consequence of increased transparency is increased transmission of ultraviolet B (UV-B) (280 to 320 nm) radiation. This leads to reductions in abundances of phytoplankton and zooplankton sensitive to UV-B.

The geographic overview of the regional case study areas is instructive. The key factors distinguishing among the regions are geology, soils, climate, hydrology, deposition chemistry, land use, vegetation, and landforms. All play important roles in determining the degree of acidification of aquatic ecosystems. Regions with bedrock highly resistant to chemical weathering are more likely to have low ANC lakes. Calcareous bedrock leads to high ANC waters. However, if the overlying till has different properties, it can counter the influences of the bedrock. In the northeast United States, glaciers brought in till from the calcareous Canadian Shield, leading to high ANC lakes. Among soils, the younger soils, more often found in the northern United States, lead to lower ANC water bodies, while the older soils, more often found in the southeastern United States, lead to higher ANC water bodies due to the accumulated organic matter that can lead to organic acidity.


Acid deposition also affects materials such as rocks and metals used in monuments and building construction through corrosion. Calcareous rock materials such as marble and sandstone are particularly vulnerable since the base cations are leached by the acids just as in soils. Mortar from limestone is also very susceptible to damage, but bricks are largely immune to the effects. Even ancient monuments are affected in a variety of ways including removal of material; development of rusty yellow patinas rich in Fe and Cu; firmly attached black crusts in contact with percolating water, where recrystallized calcite shields amorphous deposits rich in S, Si, Fe, and carbonaceous particles; and black loose deposits of gypsum and fly ash particles. Also, metals that react with hydrogen, nitrate, or sulfate, such as copper and iron, will be slowly eroded. Modern building practices have to consider effects of acid deposition and corrosion in the design phase.


Since acid rain has adverse impacts on animals, plants, and structures, there is concern that levels be reduced from current levels in many places and not increase rapidly in developing regions where fossil fuel combustion is increasing.


After completion of the NAPAP study, but not because of it, the 1990 amendments to the Clean Air Act mandated reductions in sulfur dioxide emissions from power plants in an effort to reduce the impacts of acid deposition on the environment. The key study in this regard was one published in Science showing essentially that what goes up must come down, i.e., that regions within a few hundred miles downwind of S02 (and NOx) emission sources would be impacted by the emissions.

Given the fact that anthropogenic emissions of acid precursors are expected to rise, and that acid deposition has major adverse impacts on both aquatic and land ecosystems, it seems to be worthwhile to set local, national, and international policies that would tend to reduce the projected increases in emissions. The four main routes to cutting pollution emissions are: (1) using low-pollutant fuels, (2) preventing the formation of pollutants such as NO during combustion, (3) screening pollutants from exhaust and flue gases, and (4) energy conservation. Some of these routes would also help reduce the emissions of greenhouse gases. Choosing between these routes or some combination thereof involves consideration of the trade-offs including economic and political issues, e.g., the sources of the various fuels and whether the costs of emissions reductions outweigh the benefits, with the added complication that the groups incurring the costs are not necessarily the ones reaping the benefits.

A variety of policies has been identified that could be adopted to reduce the contribution of transport sector NOx emissions at the local level in the Netherlands. The most important national policies identified relate to vehicles and fuels, pricing policy, public transport policies, and national guidelines for policies on parking and land use, while the most important local policies identified are those for parking, land use, cycling, and restrictions for motorized vehicles.

Regulations that would lead to further reductions in nitric oxide and sulfur emissions were proposed in the United States in late 1999. Oil refiners are being asked to remove 90% of the sulfur from gasoline. The manufacturers of sport utility vehicles (SUVs) and light-duty trucks are being asked to comply with the emission standards for passenger vehicles. Older electric power generating plants, which tried to escape emissions controls under the "grandfather" clause, are being asked to cut their nitrogen emissions. The proposed action affects 392 generating units at both electric generating (EGU) and non-electric-generating (non-EGU) facilities in 12 states. Affected EGUs will be required to reduce NOx emissions to 0.15 lb(mmBtu)-1, while large non-EGUs will be required to reduce NOx emissions by approximately 60% from baseline levels.

If changed regulations are not sufficient, Congress may consider additional legislation to reduce emissions. Of course, there would be a phase-in period, so it might take a decade or two for the changes to have an impact on the environment.


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