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Consequently, acid rain is a pollution problem that does not respect state or national boundaries because the atmospheric pollutants often undergo longrange transport. For example, most acid rain that falls in Norway, Sweden, and the Netherlands originates as sulfur and nitrogen oxides emitted in other countries in Europe. Indeed, the modern recognition of acid rain as a problem stems from observations made in Sweden in the 1950s and 1960s, which were due to emissions from outside its borders. China now has serious acid rain problems due to its high emissions of S02. Acidification is more serious in southern and southwestern than in northern China, where airborne alkaline dust originating in deserts neutralizes the acid. Some of the acid rain that originates m iZ'h.iTici is carried by the wind to Japan and, on occasion, all the way to North America. As the late economist-philosopher John Kenneth Galbraith noted, "Acid rain falls on the just and the unjust and also equally on the rich and poor."

The Ecological Effects of Acid Rain and of Photochemical Smog

Acid rain has a variety of ecologically damaging consequences, and the presence of acid particles in air may also have direct effects on human health. However, the effects of acid rain on soil vary dramatically from region to region. In this section, we investigate the chemical processes underlying the ecological effects of acid rain.

Nitric oxide is not especially soluble in water, and the acid (sulfurous) that sulfur dioxide produces upon dissolving in water is a weak one. Consequently, the primary pollutants NO and S02 themselves do not make rainwater particularly acidic. However, some of the mass of these primary pollutants is converted over a period of hours or days into the secondary pollutants sulfuric acid and nitric acid, both of which are very soluble in water and are strong acids. Indeed, virtually all the acidity in acid rain is due to the presence of these two acids. In eastern North America, sulfuric acid greatly predominates because some electrical power is generated from power plants that use high-sulfur coal. In western North America, nitric acid attributable to vehicle emissions is predominant, since the coal mined and burned there is low in sulfur.

Figure 4-2 shows a contour map of the average pH of precipitation in different regions of the world. The lowest pH ever recorded, 2.4, occurred for a rainfall in April 1974 in Scotland. Indeed, central-west Europe, including the United Kingdom, has a serious acid rain problem, as can be seen from the pH = 4.0 and 4.5 contours surrounding the area in Figure 4-2. In North America, the greatest acidity occurs in the eastern United States and in southern Ontario, since both regions lie in the path of air originally polluted by emissions from power plants in the Ohio Valley. On the other hand, much of the acidity that falls in upper New York State stems from emissions in southern Ontario.

In addition to the acids delivered to ground level during precipitation, a comparable amount is deposited on the Earth's surface by means of dry deposition, the process by which nonaqueous chemicals are deposited onto solid and liquid surfaces at ground level when air containing them passes over the

Mapa Del Mundo
FIGURE 4-2 Global pattern of acidity of precipitation, [Source: Redrawn from J. H, Seinfeld and S, N. Randis, Atmospheric Chemistry and Physics (Chichester: |ohn Wiley, 1998).]

surfaces. Much of the original S02 gas is not oxidized in the air but rather is removed by dry deposition from air before reaction can occur: Oxidation and conversion to sulfuric acid occurs after deposition. Wet deposition processes encompass the transfer of pollutants to the Earth's surface by rain, snow, or fog—i.e., by aqueous solutions.

Neutralization of Acid Rain by Soil

The extent to which acid precipitation affects biological life in a given area depends strongly on the composition of the soil and bedrock in that area. If the bedrock is limestone or chalk, the acid can be efficiently neutralized ("buffered"), since these rocks are composed of calcium carbonate, CaC03, which acts as a base and reacts with acid, producing bicarbonate ion, Hcor , as an intermediate:

CaC03(s) + H+(aq)->Caz+(aq) + HC03"(aq)

HC03~(aq) + H+(aq)->H2C03(aq)->C02(g) + H20(aq)

The reactions here proceed almost to completion due to the excess of H+ that is present. Thus the rock dissolves, producing carbon dioxide and calcium ion to replace the hydrogen ion. These same reactions are responsible for the deterioration of limestone and marble statues; fine detail, such as ears, noses, and other facial features, are gradually lost as a result of reaction with acid and with sulfur dioxide itself. Also, neutralization by calcium carbonate and similar compounds that are commonly present as suspended particles in atmospheric dust is the mechanism by which carbonic acid in normal rainfall and acid rain over some areas has a pH greater than expected.

In contrast, areas strongly affected by acid rain are those having granite or quartz bedrock, since the soil there has little capacity to neutralize the acid. Figure 4-3 shows areas of North America having low soil alkalinity, that is, low amounts of basic compounds with which acids can react. Large areas susceptible to acidity are the Precambrian Shield regions of Canada and Scandinavia. Acid rain resulting from the massive development of the tar sands to produce synthetic crude oil in northern Alberta, and the S02 and NOx emissions that result, are now affecting areas in Manitoba and northern Saskatchewan that lie upwind from them, since the soils in these two areas have very little neutralizing capacity (Figure 4-3).

Acidity from precipitation leads to the deterioration of soil. When the pH of soil is lowered, plant nutrients such as the cations potassium, calcium, and magnesium are exchanged with Hh~ and thereupon leached from it.

Although sulfur dioxide emission levels fell significantly in recent decades in both Europe and North America, there has not been as large a corresponding change in the pH of the precipitation, especially in northeastern North America. The lack of corresponding reduction in acidity is attributed to a decline over the same period of fly ash emissions from smokestacks and of other solid particles, all of which are alkaline and in the past neutralized a fraction of the sulfur dioxide and sulfuric acid in the same way that calcium carbonate does in soil. Thus the decline in acidity in precipitation in the northeastern United States from 1983 to 1994 amounted to 11%, although the sulfate ion molar concentrations in precipitation fell not by 5.5% (half that of H+), but rather by 15%. The much smaller nitrate levels remained essentially unchanged in this period in this region. The change in sulfate deposition in the northeastern United States and south-central Canada

New Jersey Acid Rain
FIGURE 4-3 Regions of North America with low soil alkalinity for neutralizing acid rain. (Source: D. J. Jacob, Introduction to Atmospheric Chemistry (Princeton, NJ: Princeton University Press, 1999), p. 233.]
Acid Deposition Canadian
FIGURE 4-4 Wet sulfate deposition in eastern North America as a four-year mean (kilograms/ hectare per year). [Source: Canadian National Atmospheric Chemistry Database, Meteorological Service of Canada, Environment Canada.]

from the early 1980s to the late 1990s is shown in Figure 4-4- In Great Britain, rainfall acidity declined by about 40% in the 1986-1997 period due to emission controls there.

Because of acid rainwater falling and draining into them, tens of thousands of lakes in the Shield regions of both Canada and Sweden have become strongly acidified, as have lesser numbers in the United States, Great Britain, and Finland. Lakes in Ontario are particularly hard hit, since they lie directly in the path of polluted air and since the soil there contains little limestone. In a few cases, attempts have been made to neutralize the acidity by adding limestone or calcium hydroxide, Ca(OH)2, to the lakes; however, this process must be repeated every few years to sustain an acceptable pH. Adding phosphate ion to lakes can also control acidity, since it stimulates plant growth during the natural denitrification process by which nitrate ion, N03~, is converted to reduced nitrogen with the consumption of large quantities of hydrogen ions, as shown in the reduction half-reaction

2 NOb~ + 12 H+ + 10 e~->N2 + 6 H20

In recent years, a new source of sulfuric acid in lakes has appeared—the oxidation of sulfur in shallow wetlands dried up by global warming and thereby exposed to the air.

As Problem 4-1 shows, the oxidation of ammonium ion, N*H4-, to nitrate ion produces hydrogen ions. Indeed, the large emissions of ammonia into the air from manure in areas of livestock and poultry farming result in the atmospheric deposition of ammonium ion, which then is oxidized by soil microbes. The resulting H~ contributes to the acidification of soil.


Deduce the balanced redox half-reaction of conversion of ammonium ion, NH4+, to nitrate ion, N03~, and thereby show that H" is also produced in this process.

In Australia, soil acidity has a completely different origin. Acidification is associated there with the removal of nitrate ion by the harvesting of plant and animal crops and by soil leaching. Presumably the loss of nitrate prevents its natural buffering of acidity by the reaction shown above. As in Canadian lakes, the effects of the acidification have been partially reversed in Australia by the addition of lime to the soil.

Until recently, acid rain in the United States was considered to be a problem for its northeastern region. Indeed, one of the hardest-hit regions is the Catskill Mountains in New York State, whose surface rocks consist of calcium-poor sandstone and from which most of the nutrients have now been leached. At the Hubbard Brook Experimental Forest in New Hampshire, half the calcium and magnesium in soil was leached by 1996 and, as a result, vegetative growth almost stopped. However, as a consequence of the reduction in S02 emissions, by 2003 over half the lakes in the Adirondack Mountains of New York State showed some significant recovery from acid rain. On average, the ability of lake water to neutralize acids increased by an average of 1.6 micromoles of H+ per liter per year in the 1990s. Unfortunately, full recovery for these lakes to an acid-neutralizing capacity of 50 micromoles per liter is predicted to take another 25-100 years to achieve.

Acid rain now is also a concern in the southeastern United States. Here soils are generally thicker and thus able to neutralize more acid. However, much of that leaching ability now has been exhausted and acid levels in many waterways have increased substantially. It has been discovered that the recovery of such soils, and of those in Germany, is slowed once acid precipitation has declined, because previously stored sulfate ion is then released, causing more cation leaching and penetration of acidity deeper into the ground.

The regulatory scheme used in the United States of requiring reductions in sulfur dioxide emissions in certain geographical regions has been extended by European scientists and regulators into the concept of critical load. This concept recognizes that different levels of risk from acid rain are faced in different regions. Geographic areas that have buffering capacity can withstand a much greater load of acid rain before damage occurs than those without the capacity. Thus, higher sulfur dioxide emissions from a particular region can be allowed if the area in which the resulting sulfuric acid is usually deposited has a high critical load. To determine the critical loads, scientists use computer models that incorporate soil chemistry, rainfall, topography, etc. Use of the concept has had great success in Sweden, for example.

In using critical loads, pollution control becomes effects-based rather than source-based. Although the critical-loads concept has been implemented in regulations in Europe and Canada and is favored by many scientists and politicians in the United States, it has not been implemented there. Although significant progress has been made in reducing emissions of S02, and more reductions are scheduled both for it and for NOx, scientists predict that these efforts will be insufficient to allow a full recovery of lakes and forests in the northeastern United States and south-central Canada.

Acidification reduces the ability of some plants to grow, including those in fresh-water systems. Because of the decrease in this productivity in lakes and streams that feed them, the amount of dissolved organic carbon (DOC) in the surface water has declined. The DOC contains molecules that absorb some of the ultraviolet from sunlight; thus a decline in DOC levels has allowed more penetration of uv light into the lower layers of lakes. In addition, global warming (see Chapters 6 and 7) has resulted in the drying up of some streams that supplied DOC to lakes. Furthermore, stratospheric ozone depletion has also allowed more UV to reach the Earth's surface, including lakes, in the first place. Thus fresh-water lakes have suffered a "triple whammy" from global environmental problems.

Release of Aluminum into Soil and Water Bodies by Acid Rain

Acidified lakes characteristically have elevated concentrations of dissolved aluminum ion, Al3+, and it is now known that many of the biological effects of acid rain are due to increased levels of aluminum ion dissolved in water rather than to the hydrogen ion itself.

Aluminum ions are leached from rocks in contact with acidified water by reaction with the hydrogen ions; under normal, near-neutral pH conditions, the aluminum is immobilized in the rock by their insolubility.

Plots of dissolved aluminum concentration versus water acidity for lakes in the Adirondack Mountains of New York State and for lakes in Sweden are illustrated in Figure 4-5, (The chemistry underlying these processes is further discussed in Chapter 13, as are the reasons why natural waters have pH values of 7 or 8, rather than the 5.3 of rain.)

FIGURE 4-5 Aluminum concentrations versus pH of the water in different fresh-water lakes in (a) the Adirondacks and (b) western Sweden. Notice the logarithmic vertical axis. [Source; M. Havas and ). F. Jaworski, Aluminum in the Canadian Environment (Ottawa; National Research Council of Canada Report 24759, 1986).]

Adirondacks, U.S.A.


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