Key Concepts

After completing this chapter, you will be able to:

1. Describe the most important chemical ingredients of precipitation and explain which ones may cause acidity to develop.
2. Outline the spatial patterns of acidic precipitation in North America, and identify factors influencing this distribution.
3. Explain the difference between the wet and dry deposition of acidifying substances and how their rates vary.
4. Describe how water chemistry is affected as precipitation interacts with vegetation and soil and explain the implications for surface waters.
5. Explain the importance of reducing emissions of sulfur and nitrogen gases to mitigating the acidification of surface waters.

Introduction

Acidification is a process that is characterized by increasing concentrations of hydrogen ions (H⁺) in soil or water. It can cause metals and their compounds to ionize, producing ions (such as Al³⁺) in concentrations high enough to be toxic to plants, animals, and microorganisms. Consequently, increasing acidification is usually interpreted as a degradation of environmental quality. Acidification is caused by many influences, both natural and anthropogenic, but the most widespread problems are associated with a phenomenon commonly referred to as acid rain.

Acid rain has been an important problem in parts of North America since at least the 1950s, but it did not become a high-profile issue until the early 1970s. This rather sudden attention resulted from the discovery that acid rain was a widespread problem in Western Europe, and the realization that the same conditions likely occurred in North America. This awareness stimulated research in the U.S. and Canada, which demonstrated that acid rain was causing a widespread acidification of lakes and streams, and possibly of soil. The acidification of aquatic ecosystems was resulting in important ecological damage, including the loss of many fish populations. Buildings and other materials were also being damaged because acidity erodes metals, paint, and some kinds of quarried stone.

Strictly speaking, the term “acid rain” refers only to acidic rainfall, which along with snowfall accounts for wet deposition. However, acidifying chemicals are also deposited from the atmosphere when it is not raining or snowing, through the dry deposition of certain gases and particulates. A suitable phrase to define this complex of processes is “the deposition of acidifying substances from the atmosphere”, or more simply, acidifying deposition. In this chapter we examine natural and anthropogenic causes of the acidification of ecosystems. We focus on the chemical qualities of acidic precipitation and dry deposition, their effects on terrestrial and aquatic ecosystems, and how acidification can be avoided or mitigated.

Chemistry Of Precipitation

Scientists have adopted a functional definition of acidic precipitation as having a pH less than 5.65. This was chosen as the cut-off because at pH 5.65, an aqueous solution of carbonic acid (H₂CO₃) is in equilibrium with atmospheric CO₂, as follows: CO₂ + H₂O ⇌ H₂CO₃ ⇌ H+ + HCO₃^– ⇌ 2H+ + CO₃^2-

This definition assumes that “non-acidic” precipitation is essentially distilled water, in which the acidity is determined only by the atmospheric concentration of CO₂ and the amount of carbonic acid that subsequently develops. This is why the threshold below which precipitation is deemed “acidic” is set at the slightly acidic pH of 5.65, rather than at the strict zero-acidity pH of 7.0 (see In Detail 20.1). The U.S. Environmental Protection Agency (EPA) describes acid rain has usually having a pH between 4.2 and 4.4 (EPA 2020).

It is, however, too simplistic to consider atmospheric moisture as consisting merely of distilled water in a pH equilibrium with gaseous CO₂. Additional chemicals are also present in trace concentrations in precipitation. For example, on windy days, dust containing calcium and magnesium is blown into the atmosphere, and precipitation containing these elements may develop a pH higher than 5.65. This is especially true of agricultural and prairie landscapes, where the ground surface is often bare of plant cover and soil particles may be easily eroded into the atmosphere. In some other regions, a relatively high concentration of naturally occurring sulfate in the atmosphere may result in precipitation having a pH less than 5.65.

The most abundant cations (positively charged ions) in precipitation are hydrogen ion (H⁺), ammonium (NH₄⁺), calcium (Ca²⁺), magnesium (Mg²⁺), and sodium (Na⁺). The most abundant anions (negatively charged ions) are sulfate (SO₄^2-), chloride (Cl^–), and nitrate (NO₃^–). Other ions are also present, but only in trace amounts that have little influence on the pH (see In Detail 23.2).

One of the longest-running records of precipitation chemistry is from a research site at Hubbard Brook, New Hampshire, in a region exposed to intense acidifying deposition. During 1967-1971, when acid rain was relatively severe, the average pH of precipitation at Hubbard Brook was 4.1. This level of acidity then relaxed somewhat to pH 4.9 in 1991-1995 because of decreased industrial emissions, particularly of the acid-forming gas SO₂, and then even more so in 2009-2013 because of further decreases in SO₂ emissions (Table 20.1; see In Detail 20.2 for an explanation of equivalents). Sulfate and nitrate are the most important anions in precipitation, and from 1967-1971 they accounted for 88% of the anion equivalents. During 2009-2013 these two still contributed 87% of the anion equivalents, although their total amounts were considerably smaller. These data suggest that most of the acidity in the precipitation occurs as dilute solutions of sulfuric and nitric acids. The precipitation events at Hubbard Brook that are most acidic are associated with storms that have passed over the large metropolitan regions of Boston, New York, and New Jersey. These areas have enormous emissions of SO₂ and NO_x, which are the precursor gases of much of the SO₄^2– and NO₃^– in acidic precipitation.

Table 20.1. Chemistry of Precipitation at Hubbard Brook. The data represent the average concentration (in microequivalents per litre, or µeq/L) of various ions in precipitation during three 5-year periods. The small difference between the sums of cation and anion equivalents is due to analytical inaccuracies, which are inevitable in even the best chemical data. Source: Data from Buso et al. (2003).

Spatial Patterns

Acidic precipitation has been a widespread phenomenon in the eastern U.S. (Figures 20.2 and 20.3), eastern Canada, Europe, eastern Asia, and elsewhere. In eastern North America prior to the mid-1950s, precipitation with pH below 4.6 affected only relatively local areas, mostly in southern Ontario, New York, Pennsylvania, and New England. Since then, this area has expanded considerably. At present, most of southeastern Canada and the eastern United States experience acidic precipitation, although pH levels in rainwater have risen in recent years (Figure 23.3). One of the most important aspects of acidic precipitation is the vast size of the areas it affects.

Precipitation chemistry varies between regions (Figures 20.2 and 20.3). The variation reflects the patterns of emission of SO₂ and NO_x, the degree of oxidation of those gases to SO₄^2- and NO₃^–, the prevailing direction travelled by polluted air masses, and the amount of acid-neutralizing dust in the atmosphere. Atmospheric dust is particularly important where vegetation cover is sparse, such as in agricultural regions where tiny soil particles are easily eroded into the atmosphere by strong winds blowing over bare fields. Unpaved roads are also an important source of atmospheric dust.

Figure 20.2. Hydrogen Ion Concentrations as pH of Precipitation for 2000. Depicted is the distribution in rainfall pH in the United States for the year 2000. Source: Source: National Atmospheric Deposition Program/National Trends Network.

Figure 20.3. Hydrogen Ion Concentrations as pH of Precipitation for 2019. Depicted is the distribution in rainfall pH in the United States for the year 2019.  Source: Source: National Atmospheric Deposition Program/National Trends Network.

Rapid changes in precipitation chemistry may occur at the border between a forested landscape and areas dominated by prairie or agricultural land. A study in southern Ontario examined precipitation at eight places in a forested area with thin soil and Precambrian bedrock, and at three sites just south of the Shield in agricultural terrain with calcium-rich soil (Dillon et al., 1977). The average pH of precipitation among the Shield sites was 4.1-4.2, while at sites to the south it was 4.8-5.8. Precipitation was less acidic in the agricultural area because of the local neutralizing influence of dust blown up from fields and roads.

Another important characteristic of acidic precipitation is that, unlike SO₂ and metal particulates, its intensity does not increase closer to large point sources of emissions, such as a coal-fired power plant or a smelter.

Fog moisture may also be quite acidic in eastern North America and elsewhere. Fogwater collected at high elevations and at coastal locations is commonly more acidic than pH 4.0, and it can be as acidic as pH 2.5-3.0. At forested sites where fog is a common occurrence, large amounts of acidity and other chemicals are filtered out of the atmosphere by trees. This occurs because tiny suspended water droplets coalesce on the surface of foliage and bark as fog passes through a forest canopy, a process that removes much of the fogwater from the atmosphere. This phenomenon is illustrated in Table 20.2 for a conifer forest on a mountain that often experiences foggy conditions. The total input of atmospheric moisture to that forest was 264 cm/year, with rain and snow accounting for 68% and fogwater 32% (fog was present 40% of the time). However, the concentrations of many chemicals are much higher in fogwater than in precipitation – because the suspended water droplets are tiny, the dissolved chemicals are less “diluted” by their aqueous matrix. As a result, the rates of deposition of dissolved substances to the forest can be higher than from precipitation. At this particular location, fogwater accounted for 62% of the H⁺ deposition and 81% of the inputs of SO₄^2- and NO^3-.

Table 20.2. Chemistry and Deposition of Fogwater. The deposition of water and ions was studied in a conifer forest at a high-elevation site on Mount Moosilauke, New Hampshire. Fogwater deposition occurs when suspended water droplets are “filtered” from the atmosphere by trees. The concentrations are average values, in microequivalents per liter. Precipitation refers to rain plus snow, while percent fogwater refers to the deposition that was due to that source. Source: Modified from Lovett et al. (1982).

Transboundary Air Pollution

Acidifying substances and their gaseous precursors are often transported over long distances in the atmosphere, far from their sources of emission. The acidifying chemicals do not respect political boundaries, so emissions occurring in one country can degrade ecosystems and valuable resources in other countries. This transboundary context has helped to focus the attention of governments on the problem of acidifying deposition from the atmosphere.

In Western Europe, for example, Scandinavians justifiably argued that most of the acidifying deposition that has affected extensive regions of their landscape has resulted from emissions of SO₂ and NO_x in Germany and England. This international European context was the first well-demonstrated case of so-called LRTAP, an acronym for the long-range transport of atmospheric pollutants.

Similar transboundary circumstances occur elsewhere. In eastern North America, there are large populations and industrial centers in the northeastern United States. Emissions of SO₂ and NO_x from those areas often waft into eastern Canada, worsening damage caused there by local emissions. Canada also exports some of its emissions to the United States, although Canadian emissions account for much less of the total deposition of sulfur and nitrogen compounds in the eastern U.S.

Dry Deposition

Dry deposition occurs during intervals between precipitation events, and it includes the following:

• The direct uptake of gaseous SO₂ and NO_x by vegetation, soil, and water
• Gravitational settling of larger particles
• The filtering of suspended particulates by vegetation

Dry deposition occurs to all kinds of habitat, but forest is particularly effective at absorbing gases and particles from the atmosphere. This is because trees have such a large and complex surface area of foliage and bark, which greatly enhances the rate of dry deposition.

Dry deposition can result in large inputs of substances from the atmosphere, including some that generate acidity when they are chemically transformed within the ecosystem. For instance, atmospheric SO₂ readily dissolves into the surface water of lakes and streams. This gas is also freely absorbed by plants – it enters foliage through tiny abundant pores on the surface known as stomata, and then dissolves into the moist film of water that covers the surface of cells in a sub-stomatal cavity. In this sense, the SO₂ behaves just like CO₂, a vital nutrient, which is also absorbed by plants in this manner. The absorbed SO₂ becomes oxidized to the anion sulfite (SO₃^2-), which is rapidly oxidized to sulfate (SO₄^2-). Because the sulfate is balanced electrochemically mostly by hydrogen ions, acidity is generated by the transformation of SO₂ gas into the SO₄^2- ion (see Figure 20.4).

Figure 20.4. Acidification Caused by Transformations of Sulfur and Nitrogen. This diagram describes the acidifying effects of important transformations of nitrogen and sulfur compounds in soil or water. Source: Modified from Reuss and Johnson (1985).

NO_x gas may be similarly dry-deposited and then oxidized to nitrate (NO₃^–), which also generates an equivalent amount of H⁺. The gas ammonia (NH₃) and the cation ammonium (NH₄⁺) can also be dry-deposited to soil or water, where they can be oxidized by bacteria to nitrate plus equivalent quantities of H⁺.

The rates of dry deposition of sulfur and nitrogen compounds are greatest when there are high concentrations of gaseous NO_x and SO₂ in the atmosphere. Such conditions typically occur in urban areas and close to large industrial sources of emissions. In those places, dry deposition accounts for much larger inputs of acidifying substances than does wet deposition. In more remote, less-contaminated environments, far from sources of emission, inputs with precipitation are typically larger than dry deposition.

Soil Acidity

Soil acidity is an important factor that affects the growth of plants. Soil acidification is a natural process that has been demonstrated by studies of succession in ecosystems.

One well-known study was done at Glacier Bay in Alaska. The melting of a glacier in a long fiord is exposing a mineral substrate of till that has a pH of about 8.0 and contains up to 10% carbonate minerals of calcium and magnesium (Crocker and Major, 1955). Once exposed, this material becomes modified by colonizing plants and climatic factors. Rainfall is especially important, because much of it percolates through the soil and leaches dissolved chemicals to beyond the rooting depth of the plants. These influences result in an increased acidity of the soil, which reaches about pH 4.8 after 70 years of succession, by which time a conifer forest is established. The acidification is accompanied by large and progressive declines in the amounts of Ca, Mg, and carbonates in the soil during the succession.

In part, the acidification is caused by the uptake of the nutrients Ca, Mg, and K by trees and other plants, a process that is accompanied by the excretion of H+ and a decrease in the buffering capacity of the soil (this is related to the ability of the soil to resist further acidification). The leaching of calcium and other cations out of the soil by rainwater also contributes to acidification.

Various chemical changes occur as rainwater percolates through the soil and interacts with minerals, organic matter, microbes, and roots:

• Roots and microorganisms selectively absorb, release, and transform chemicals
• Ions are exchanged at the surfaces of clay particles, minerals, and organic matter
• Insoluble minerals are made soluble by so-called weathering processes, including reactions with acids
• Secondary minerals are formed, such as certain clays and insoluble precipitates of iron and aluminum oxides

These reactions cause important changes to occur in the soil, such as acidification, the leaching of calcium and magnesium, and the solubilization of metals, particularly toxic ions of aluminum (such as Al³⁺). All of these processes occur naturally wherever the input of water from precipitation exceeds the amount returned to the atmosphere by evapotranspiration, so there is a surplus to percolate downward through the soil. These reactions are also influenced by the kind of vegetation growing on the site. For instance, pines, spruces, and oaks tend to cause soil to acidify. In addition, the deposition of acidifying substances from the atmosphere can potentially increase the rates of some of these processes in soil, and thereby increase the leaching of toxic Al³⁺ and H⁺ into streams and lakes.

Factors Affecting Soil Acidity

Soil acidity is influenced by numerous chemical transformations and ion exchanges. Some are carried out by organisms, while others are non-biological reactions. The following are the most important factors affecting soil acidity.

• Carbonic Acid. In many terrestrial ecosystems, such as grassland and forest, the surface litter and upper soil are rich in organic matter and roots. Decomposition and respiration result in high concentrations of CO₂ (often exceeding 1%) in the atmosphere within the soil. The high CO₂ concentrations result in carbonic acid (H₂CO₃) forming in the soil water, which contributes to its acidification. This effect is strongest in soil with a pH greater than about 6.0, and it is unimportant in acidic soil with pH less than about 5.5.
• The Nitrogen Cycle. Soil acidity can also be affected by microbial transformations of nitrogen compounds and by their uptake and release by plants (Figure 20.3). Ammonium (NH₄⁺) and nitrate (NO₃^–) are especially important because plants must take up one or both of these essential nutrients, the choice depending largely on soil acidity. In soil with pH less than about 5.5, almost all inorganic nitrogen occurs as NH₄⁺. The NH₄⁺ may originate from the ammonification of organic nitrogen to form ammonia (NH₃), a process carried out by many species of microorganisms. The ammonia absorbs one H⁺ to form ammonium. If the NH₄+ ion is absorbed by a plant root, one H⁺ is excreted into the soil to maintain electrochemical neutrality, so there is no net change in acidity. However, if NH₄⁺ is added directly to soil (such as by atmospheric deposition or by the application of a fertilizer), then plant uptake of NH₄⁺, accompanied by the release of H⁺, has an acidifying effect. In soils with pH greater than 5.5, most of the inorganic nitrogen occurs as NO₃^–, which is produced by the oxidation of NH₄⁺ through the process of nitrification. Nitrification is carried out by the bacteria Nitrosomonas and Nitrobacter, which are intolerant of acidity. The oxidation of NH₄⁺ to NO₃^– generates two H⁺ (Figure 20.3). If the NH₄⁺ originated from ammonification of organic nitrogen (which consumes one H⁺ for each NH₄⁺ produced), the net effect is the release of one H⁺ for each NO₃^– produced from organic nitrogen. However, if the NO₃^–is then absorbed by a root, one OH^– is excreted to the soil to maintain electrochemical neutrality, which is equivalent to the consumption of one H⁺. In that case, the net effect on soil acidity is zero. It is well known to farmers and agronomists that the addition of ammonium to soil can have a severely acidifying influence. This happens because the NH₄+ becomes nitrified into NO₃-, which generates large amounts of acidity. There are two main types of ammonium inputs: the treatment of agricultural fields with fertilizer containing inorganic nitrogen (such as urea or ammonium nitrate), and the deposition of NH₃ gas and NH₄⁺ from the atmosphere.
• The Sulfur Cycle. Much of the sulfur in soil occurs in organically bound forms. Microbial processes can transform this organic sulfur into more highly oxidized compounds, including sulfides and elemental sulfur, but if oxygen is abundant, these become further oxidized to sulfate. Overall, the oxidation of organic sulfur to SO₄^2- releases one equivalent of H⁺ per equivalent of SO₄^2- produced (this is the same as two H⁺ per SO₄^2- SO₄^2- is absorbed by a root, an equivalent quantity of OH^– is excreted to conserve electrochemical neutrality, so there is no net effect on acidity. However, if atmospheric deposition causes a direct input of SO₄^2- to the soil, followed by uptake by plant roots, the net effect is a reduction of acidity. In addition, if the soil is deficient in oxygen (anaerobic), as commonly occurs in wet sites, then the SO₄^2- can be transformed by microbes into a sulfide compound, which results in the consumption of an equivalent amount of H⁺ and a reduction in acidity.

Reactions associated with the sulfur cycle usually have a smaller effect on soil acidity than those involving the nitrogen cycle. In certain situations, however, the sulfur cycle is dominant. For instance, when a wetland is drained, its previously anaerobic sediment becomes aerobic. This allows bacteria to oxidize reduced sulfide compounds into sulfate. Some drained wetlands develop an extremely acidic condition known as acid sulfate soil. For 10 or more years after drainage occurs, usually to develop agricultural land, the pH can be lower than 3.0. This severely impairs crop growth, although the acidity can be neutralized by adding calcium carbonate (lime) to the soil.

Sometimes, sulfide minerals such as pyrite (iron sulfides) become exposed to atmospheric oxygen. This allows specialized Thiobacillus bacteria to oxidize the sulfides, a process that produces sulfate and oxidized iron ions, according to the following reaction: 4 FeS₂ + 15 O₂ + 14 H₂O → 4 Fe(OH)₃ + 16 H⁺ + 8 SO₄^2-
.

This phenomenon, known as acid-mine drainage (or as acid-rock drainage), causes severe acidification of soil and surface waters. It can cause a pH less than 2.0 to develop, with high concentrations of sulfate and toxic ions of aluminum and iron. Acid-mine drainage is an important problem where coal and metal mining have exposed mineral sulfides to the atmosphere (see In Detail 23.3).

• Uptake of Basic Cations by Plants. Terrestrial plants obtain many of their nutrients by absorbing ions from the soil in which they are growing. (A few nutrients, however, are absorbed mainly from the atmosphere, particularly CO₂) Calcium, magnesium, and potassium are important nutrients that are absorbed from soil as cations (Ca²⁺, Mg²⁺, and K⁺), whose absorption is offset by a release of H⁺. In natural ecosystems, the absorbed Ca²⁺, Mg²⁺, and K⁺ are eventually returned to the soil with plant litter, so there is no long-term effect on soil acidity. However, if biomass is removed from the site, as occurs in agriculture and forestry, these cations are removed, resulting in acidification of the soil.
• Leaching of Ions. In most soils, the anions nitrate and chloride readily leach downward into the groundwater. They do this because they are highly soluble in water and are only weakly retained at anion-exchange sites on organic matter and clays. The leached anions may eventually reach surface waters such as streams and lakes. This is also true of sulfate, especially in relatively young soils of glaciated regions (older soils of more southern regions often have a greater capacity to retain sulfate). In areas with large inputs of acidifying substances, the amounts of NO₃^– and SO₄^2- in soil may be high enough to result in substantial rates of leaching. As these anions leach from the soil, they are accompanied by cations such as Ca²⁺, Mg²⁺, H⁺, and Al³⁺, which results in acidification, nutrient loss, and toxicity (associated with the Al³⁺) in terrestrial and aquatic ecosystems. For instance, one monitored watershed, Hubbard Brook in New Hampshire, was found to have lost 50% of its soil calcium between 1963 and 1992 (Likens et al., 1998).

Atmospheric Deposition and Soil

The potential effects of atmospheric deposition on soil acidity have been studied in experiments in which simulated rainwater solutions were added to soil contained in plastic cylinders, known as lysimeters. These experiments have shown that extremely acidic solutions can cause these changes in soil chemistry:

• An increase in acidity
• Increased leaching of calcium, magnesium, and potassium, resulting in their depletion and greater vulnerability of the soil to acidification
• Increased solubilization of toxic metal ions, especially of aluminum, but also iron, manganese, and others
• An overwhelming of the ability of soil to absorb sulfate, after which this ion leaches freely, at a rate similar to its input (because SO₄^2- is an anion, its leaching is accompanied by base cations and toxic Al³⁺ and H⁺, which may contribute to the acidification and toxicity of surface waters.)

One experiment involved the treatment of a sandy soil from jack pine stands with simulated rainwater, adjusted with sulfuric acid to a pH of 5.7, 4.0, 3.0, or 2.0 (Table 20.3). Even treatment with the extremely acidic pH 2.0 had little effect on soil acidity, and the percolating solutions had a pH higher than 6.5 in all treatments. However, the leaching of Ca, Mg, total bases (Ca + Mg + K + Na), and sulfate were much higher in the pH 2.0 treatment. Overall, this experiment found that the soil was quite resistant to effects of acid loading. Eventually, however, the resistance could be overcome by treatment with highly acidic solutions, and perhaps by long-term exposure to more moderate acidities. Keep in mind that experiments such as these are short-term investigations, whereas soil acidification in nature is a slow, long-term process.

Table 20.3. Experimental Leaching of Soil by Acidic Solutions. The data are concentrations of chemicals in water that drained from lysimeters containing sandy soil collected from two jack pine (Pinus banksiana) stands in northern Ontario. The experiment ran for three years, with simulated rainfall of various pH levels added at 100 cm/year as weekly 1-2 hour events. The data are averages of three replicates. Source: Modified from Morrison (1983).

Researchers monitoring soil chemistry at particular places in the field can determine whether acidification has occurred, although such studies do not necessarily identify the causes of the change. For instance, the conversion of agricultural land into conifer forest usually results in acidification of the soil. In southern Ontario, the afforestation of abandoned farmland with pine or spruce caused the soil to acidify from pH 5.7 to pH 4.7 after 46 years of forest development (Brand et al., 1986). It is less understood whether already-forested sites will become more acidic because of atmospheric inputs of acidifying substances. A study in southern Ontario re-sampled forest soils after a 16-year interval, in a region where the average pH of precipitation is about 4.1, but no further soil acidification was observed (Linzon and Temple, 1980).

Overall, studies conducted in the United States, Canada and Europe, have come to ambiguous conclusions about the effects of atmospheric deposition on soil acidification. Except for cases where the atmosphere is severely polluted by SO₂, such as near a metal smelter, there is no convincing evidence that atmospheric deposition has acidified soil on a wide scale. It appears that soil acidification is a potential long-term risk associated with this type of pollution.

Terrestrial Vegetation

Numerous studies have demonstrated that plants may be injured by treatment with simulated “acid rain.” In almost all of the studies, however, the pH that caused acute injuries was more acidic than is normally found in ambient precipitation.

For example, experiments in Norway exposed young conifer stands to simulated acid rain for three years (Tveite, 1980). The control treatment was pH 5.6-6.1, while the acidified treatments used pH 4.0 or 3.0. On average, control saplings of lodgepole pine (Pinus contorta) grew 15–20% less than plants receiving the acidic treatments. Growth of Scotch pine (P. sylvestris) and birch (Betula pendula) was also stimulated by the acidic treatments, while spruce (Picea abies) was unaffected. However, the moss-dominated ground vegetation was severely damaged by the most acidic treatment (pH 3.0).

Laboratory experiments are also useful for determining the effects of rainwater pH on plants, because the environmental conditions can be well controlled. In general, such experiments do not find reductions of growth until the pH becomes more acidic than about 3.0 (for comparison, the average acidity of precipitation is about pH 4.0 in regions where acid rain is considered a severe problem). Moreover, the productivity of some tolerant species may be stimulated by rainwater even more acidic than pH 3.0. For example, seedlings of white pine (Pinus strobus) grew more quickly when exposed to acidic mist ranging from pH 2.3 to 4.0 than at pH 5.6 (Wood and Bormann, 1976). In another experiment, seedlings of 11 tree species were treated with solutions of various pHs, but acute injury to foliage was caused only after a week of treatment at pH 2.6, which is an unnaturally acidic exposure (Percy, 1986).

In general, it appears that trees and other vascular plants have little risk of suffering acute injury from exposure to ambient acid rain. However, stresses associated with acidic precipitation could possibly decrease plant growth, even in the absence of acute injuries. These “hidden injuries” (see Chapter 20) might be caused by a subtle disruption of plant metabolism, or indirectly by changes in soil chemistry. Because acidic precipitation affects extensive regions, even a small decrease in plant productivity could have important economic and ecological consequences.

Hidden injuries, if they do occur, are most relevant to forest and other kinds of natural vegetation. Agricultural land becomes acidified mostly through management practices, such as cropping and the use of nitrogen fertilizer. Moreover, agricultural soil is routinely treated with liming agents to reduce its acidity.

Clearly, the effects of acidifying depositions on soil and vegetation are somewhat ambiguous and variable by species (Figure 20.5). However, as the following sections will show, the effects on vulnerable freshwater ecosystems can be severe.

Figure 20.5. Red Spruce. During the 1990s, acid rain was found to have been a causal factor in the decline of red spruce trees in the northeastern United States. From the 1950s to the 1990s red spruce trees in the U.S. suffered massive growth reduction, freezing injury, and death believed to be caused by acid rain and air pollution (DeHayes et al. 1999). The photo on the left of red spruce trees was taken in Monongahela National Forest in West Virginia. The image on the right depicts the mechanisms through which acid rain is thought to damage some trees. Sources: “Red Spruce” by cm195902 is licensed under CC BY 2.0, and Hubbard Brook Ecosystem Study.

Reducing Emissions

Ultimately, the extensive damage caused by acidifying deposition from the atmosphere can only be resolved by reducing the emissions of acid-forming gases. Although this fact is intuitively clear, the issue of emissions reduction remains controversial for the following reasons:

• Scientists do not know exactly how much the emissions of SO₂ and/or NO_x must be reduced to prevent damage by acidifying deposition.
• Various emission-reduction strategies are possible, and they vary in their economic consequences. Would it be more effective to target large point-sources of emissions, such as power plants and smelters, while paying less attention to smaller but numerous sources, such as automobiles and oil-burning home furnaces? Or should both large and small sources be aggressively curtailed?
• To be effective, emissions reductions must be coordinated among neighboring countries. For example, what would happen if the government of one country (perhaps with large emissions of SO₂ and NO_x) does not regard acidifying deposition to be a high-priority problem, but neighboring countries do?

Not surprisingly, industries and regions that are responsible for large emissions of acid-forming gases have tended to resist the imposition of substantial legislated reductions of their releases. In general, they argue that the scientific justification for the reductions is not yet convincing, while the costs of controls are known to be large and potentially disruptive to the economy.

In addition, how low should the rates of atmospheric deposition of sulfur and nitrogen compounds be, in order to avoid further acidification of sensitive surface waters or to allow their recovery? The critical rates of deposition of acidifying compounds are influenced, in part, by the vulnerability of the receiving ecosystems – areas with shallow, nutrient-poor soil can sustain much lower inputs of acidifying substances than areas rich in calcium.

Although there are many uncertainties about the specific causes and magnitude of the damage caused by the atmospheric deposition of acidifying substances in the natural and built environment, it is obvious that what goes up (emissions of acid-precursor gases) must eventually come down (as acidifying deposition) (Image 20.4). This common sense idea is supported by a great deal of scientific evidence. This knowledge, combined with public awareness and concern about acidification in many countries, has spurred politicians to begin to take effective action. This is resulting in reduced emissions of SO₂ and NO_x, particularly in relatively wealthy countries in North America and Western Europe.

Image 20.4. Erosion of the Built Environment. The built environment may also be damaged by acidifying deposition from the atmosphere. For example, structures made of limestone, marble, or sandstone become chemically destabilized and eroded by the dry deposition of SO₂ and NO_x and by acidic precipitation. These pollutants are seriously damaging many famous artifacts of cultural heritage, such as this medieval religious sculpture in Germany. Source: “Skulptur aus Sandstein, Dresden” by Slick is licensed under CC0 1.0.

In 1992, the governments of Canada and the U.S. signed a binational treaty aimed at reducing acidifying deposition in both countries. This agreement, known as the Canada–U.S. Air Quality Agreement, calls for large expenditures by industries and governments to substantially reduce the emissions of air pollutants, especially SO₂. These cutbacks are on top of reductions of emissions that both countries had already achieved during the 1980s. In the U.S., emissions of SO₂ have decreased from 14.2 metric tons in 1990 to 1.1 million metric tons in 2017, and emissions of NO_x decreased from 6.1 million metric tons in 1990 to 1.2 million metric tons in 2017 (EPA, 2014). For comparison, the Canadian emissions of SO₂ decreased from 3.4 million metric tons in 1990 to 955 thousand metric tons in 2017, and emissions of NO_x decreased from 2.4 million metric tons in 1995 to 772,474 metric tons in 2017 (EPA, 2018).

A major component of the U.S. initiatives to reduce emissions of SO₂, established under the 1990 Clean Air Act Amendments, is the creation of a marketplace for emissions trading. In essence, any company that is emitting SO₂ at a rate less than it has been permitted by the EPA has a right to sell (or trade) those unused “credits” to another business that is exceeding its emissions target. This is an important initiative because it helps to set a “market value” for emissions of certain pollutants, and also for investments to reduce their release. From the environmental perspective, this marketplace for emissions is a logical instrument because the atmosphere is a common-property resource that is owned and affected by everyone, so any changes in the release of pollutants (whether increases or decreases) have a global effect. In essence, a company whose emissions are smaller than its allowance can realize a profit by selling its credits, while another that has exceeded its target incurs costs. Those costs may be paid either by purchasing emissions credits or by taking action to reduce the emissions, such as installing SO₂-removal technology, switching to a low-sulfur fuel, or in extreme cases, shutting down particularly dirty facilities.

The flexibility associated with these options is considered by many economists and politicians to have been an important benefit of the system of emissions trading. Nevertheless, the establishment of a marketplace that commodifies the release of SO₂ does liberate many companies from the expensive investments that would be required to achieve tangible reductions of their emissions. This fact has engendered controversy, as has the potential establishment of a global marketplace for tradable emissions of greenhouse gases under the Kyoto Protocol.

In any event, are the cuts in emissions large enough to achieve their intended effect of preventing and repairing the acidification of ecosystems? According to a science assessment carried out by Environment Canada, the reductions of SO₂ emissions have resulted in lower rates of acidifying deposition in Canada (based on a comparison of data for 1990–1994 and 1996–2000; Environment Canada, 2005). Nevertheless, an estimated 21-75% of eastern Canada still receives amounts of acidifying deposition that exceed the critical loads (the smaller numbers correspond to a best-case scenario, and the larger to a worst-case one). Environment Canada also suggests that, to protect ecosystems from further damage caused by acidifying deposition, a further 75% reduction of SO₂ emissions will be required by Canada and the U.S. beyond those agreed to under the existing Air Quality Agreement. A similar conclusion was reached by the U.S. Environmental Protection Agency in its own science assessment of the issue (2011).

Furthermore, not much action has been taken to reduce the emissions of NO_x, and this appears to be working against the environmental benefits associated with increased control of SO₂. It is crucial that future regulatory actions include reduced emissions of both SO₂ and NO_x and that acid rain and its environmental damage continues to be monitored.

The 1992 air-pollution treaty between Canada and the U.S. is a helpful accomplishment. Although there have been reductions in the acidity of precipitation and surface waters in some areas, it appears that the reductions of SO₂ emissions are not large enough to fully mitigate many of the damages caused by acidifying deposition. So far, extremely large areas of terrain continue to be affected by acidification caused by atmospheric deposition.

Improving trends in the chemistry of precipitation and streamwater at Hubbard Brook, NH, a monitoring location with outstanding longer-term data of this sort of data, are shown in Figure 20.9. The precipitation data show that the acidity of precipitation is decreasing (the pH is increasing), and that the concentrations of sulfate and nitrate are also decreasing. The reduction of sulfate concentrations is especially large, and likely reflects the fact that regulatory controls have concentrated on SO₂ emissions (the main precursor of sulfate) more so that on NO_x (the precursors of nitrate). The streamwater data also show decreasing acidity and sulfate concentration, as well as a steady decrease in the concentrations of calcium. The latter observation may reflect a progressive loss of calcium from these watersheds, which represents a degradation of the ability of the system to provide acid-neutralizing capability against future inputs of acidifying substances. John Smol (2008) of Queen’s University refers to the phenomenon of progressive calcium losses as a kind of “osteoporosis” that affects and degrades many watersheds whose soil and bedrock are low in calcium.

Figure 20.9. Trends in the chemistry of (a) precipitation and (b) stream water chemistry at Hubbard Brook, NH. Source: Data obtained from Gene E. Likens, Hubbard Brook Ecosystem Study, with funding from the National Science Foundation and The A.W. Mellon Foundation.

Some lakes are also benefiting from reduced sulfate loading. A survey of 202 lakes monitored in eastern Canada since the early 1980s found that the acidity of 33% had decreased, while 56% were unchanged, and 11% became more acidic (Environment Canada, 1998. Overall, about 80% of recently sampled lakes in Nova Scotia had a pH <6.0, as did 40% of those in Ontario and 25% in New Brunswick (note that pH <6.0 is considered a critical threshold of tolerance for sensitive fish and other aquatic animals; Environment Canada, 2005). Many of these lakes are naturally acidic because of organic substances leaching from bogs, particularly in Nova Scotia, where brown-waters comprise about 40% of the acidic lakes. Across eastern Canada, however, 0.5-0.6 million lakes are thought to still be vulnerable to acidification under an atmospheric-deposition regime similar to that in recent years.

It must be recognized that pollution control is extremely expensive. For example, it could cost $600 million annually to further reduce the SO₂ emissions by 50% below the current targets for the eastern U.S. and eastern Canada. Because of this cost, policies that favor reduced emissions of SO₂ and NO_x may not be able to survive the frequent challenges mounted by politicians, economists, and business people who do not believe that such actions are necessary.

So far, actions to reduce the emissions of SO₂ and NO_x have been vigorous only in relatively wealthy regions of North America and Western Europe. In less-wealthy countries, the political focus is mostly on industrial and economic growth. However, air pollution and other environmental damages “subsidize” that economic growth and are often paid little heed. As soon as possible, much more political and scientific attention must be devoted to the problems of acidifying deposition and other kinds of pollution in eastern Europe, Russia, China, India, Southeast Asia, Mexico, and other rapidly growing economies. In those countries, emissions of SO₂, NO_x, and other important airborne pollutants are galloping out of control.

Conclusions

Acidification is a natural process that occurs as ecosystems interact with climatic and biological influences, for example in bogs and coniferous forest. Acidification is also caused by anthropogenic influences, particularly emissions of SO₂ and NO_x, which oxidize to form acids while in the atmosphere or after they are dry-deposited to ecosystems. Aquatic and terrestrial ecosystems that are vulnerable to acidification have little acid-neutralizing capacity, largely because of small amounts of calcium and magnesium carbonates in their soil, sediment, or rocks. Acidifying influences also damage the built environment by eroding materials made of limestone, marble, and certain metals such as copper. Although some of the ecological damage caused by acidification to surface waters can be mitigated by liming, this treatment has to be repeated, typically on about a three-year rotation. The best way to avoid the environmental problems associated with acidifying deposition is to reduce the emissions of the key acid-precursor gases (SO₂ and NO_x). To a substantial degree, this is being done in wealthy countries, including the U.S. However, rapidly growing economies, such as India, have not seen reductions in their emissions as they increase their supply of commercial energy by burning sulfurous fossil fuels, particularly coal.

Questions For Review

1. Explain the principle of conservation of electrochemical neutrality? How is it relevant to the chemistry of precipitation and surface waters?
2. What environmental influences cause soil to become acidic? How can this problem be mitigated?
3. Define critical load, and explain factors that influence its value for particular kinds of terrain and surface waters.

Questions For Discussion

1. How do wet and dry depositions of acidifying substances contribute to acidification? Why do their rates and relative importance differ between urban and rural areas?
2. Explain why wealthy countries have been taking action to reduce their emissions of acid-precursor gases, but rapidly growing economies such as India have not. What are likely consequences of these policies?

Exploring Issues

1. There are two broad options for dealing with acidifying deposition: (1) reduced emissions to prevent the problem and (2) liming of water and soil to treat the symptoms. Some people have called emissions reductions the “billion dollar solution,” and liming the “million dollar solution.” This is because of the potentially greater capital costs associated with technologies for reducing SO₂ and NO_x emissions. Which of these options (or both) do you think is most appropriate for dealing with acidification as an environmental problem? Explain your answer.
2. The U.S. and Canada have negotiated a treaty concerning the transboundary movements of air pollutants, with a focus on acidifying deposition from the atmosphere. Major aspects of the treaty are reductions in the emissions of SO₂, and to a lesser degree, NO_x to the atmosphere. You are a scientist working for the EPA and have been given the responsibility of monitoring whether the negotiated emissions reductions have been successful in improving conditions in the state where you live – is acidification becoming less of a problem? How would you design a program of environmental monitoring and research to answer this important question? What ecological and human health questions would you examine?

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