In many parts of the United States, the rains of summer and snows of winter bring not just hope and vitality but a subtle threat to the environment. The rain and snow must fall through an atmosphere polluted by sulfur oxides and nitrogen oxides, reacting to produce acid precipitation—sulfuric acid (H2SO4) and nitric acid (HNO3). Acid rain can have many harmful effects, including the acidification of lakes—with significant declines in fish population—and damage to vegetation and forests, building corrosion, and possibly damage to human health. Although emission levels of SO2 have decreased significantly since 1980, they are still precarious.
The acidity of any solution is measured on a scale known as the pH scale, which is a measure of hydrogen ion concentration. Hydrogen ions have a positive electrical charge (they have lost their electron). A solution with an equal number of positive and negative electrical charges is neutral and has a pH of 7. Each decrease in pH by 1 unit represents a factor of 10 increase in the solution’s acidity. A solution with a pH of 6 has ten times as many hydrogen ions as one with pH = 7. Figure 8.14 shows the pH of some common substances. Unpolluted rain is slightly acidic (pH = 5.6) because of its interaction with atmospheric carbon dioxide to produce carbonic acid (H2CO3).
Acid rain has affected the eastern United States and Canada and northern and eastern Europe (see Fig. 8.15) as well as China and southeast Asia. Particularly strong evidence of increased acidification in the United States has been observed in lakes in New York’s Adirondack Park in the second half of the twentieth century. More than 200 high mountain lakes there have lost their native fish populations. Rainwater with pH values between 4.0 and 4.5 (10 to 40 times more acidic than pure rain) were common
A 71 7 7 7 1 7 7 7n 7 Th 7 7
1 2 3 4 5 6 7 8 9 10 11 12 13
Figure 8.14 Acidity is expressed by pH, which is a logarithmic number. A 1-unit change in pH represents a change in acidity by a factor of 10. A solution with a pH of 5 is 100 times more acidic than pure water, which is neutral at a pH of 7.
in the northeastern and southeastern states, especially in the 1950s to 1980s, and values between 3.0 and 4.0 were detected in some individual storms. Annual average pH levels less than 4.0 have been observed in some northern European countries. Figure 8.16(a) and (b) show the decrease in the annual average pH levels for precipitation in the eastern United States from 1955 to 1973. However, as controls were put on sulfate emissions, higher pH levels returned in this area. Figure 8.16(c) and (d) show this dramatic improvement for wet sulfate precipitation in the eastern United States.
Figure 8.17 (page 255) shows the change in the distribution of pH for Adirondack Park lakes between the 1930s and 1970s. (This figure correlates with the acidification changes shown in Figure 18.16(a) and (b).) The fish population is severely impacted if the pH drops below 5.0. (Not all fish, shellfish, or the insects they eat can tolerate the same acid concentrations.) The lakes most susceptible to acidification effects are those that are shallow and surrounded by hard, insoluble bedrock with thin surface soils and that have low buffering capacity to neutralize the acids. Metals, such as aluminum, in surrounding soil can be leached into the lake, which can contribute to problems in the fish’s gills. The effects of acidification are blunted if the surrounding soil is alkaline (usually rich in limestone) or if the water contains basic ions derived from the weathering of rock.
Acid rain also affects crops by suppressing the bacterial decomposition of organic matter and by leaching (removing) nutrients such as calcium and magnesium from soil. In more serious cases, acid rain causes lesions on leaves, which reduces the area for photosynthesis and stunts plant growth. Since 1980, there has been a clear loss in the vitality
Figure 8.17 Change in pH and fish population for 200 Adirondack lakes (in New York) above 600 meters altitude between the 1930s and 1970s. (Source: U. S. EPA)
of U. S. and European forests, but the role of acid rain in this situation is not clear. Forest damage in Europe has been especially severe; in 1995, about 20% of Europe’s trees were moderately or severely defoliated. It has been suggested that the acid precipitation adds stress to trees and weakens them to attack by disease or insects.
Thankfully, U. S. annual SO2 emissions have decreased by 40% since the 1980s, and acid rain levels have dropped 65%. The EPA estimate of acid rain deposited in the Adiron – dacks shows a drop from 415,000 tons in 1990 to 220,000 tons in 2005. Figure 8.16(c) and (d) show this dramatic improvement for wet sulfate precipitation. Lakes in the Adirondacks that are being monitored have showed a significant decrease in sulfate concentrations since 1990, coinciding with decreases in atmospheric sulfur concentrations. The same is true for nitrates. With a reduction in lake acidity, some of these lakes are expected to gradually recover their fish populations.
The explanation of the formation of (sulfuric) acid rain, as given by the equations 2SO2 + O2 ^ 2SO3 and SO3 + H2O ^ H2SO4, is too simple and probably is only part of the process. The term “acid rain” is also a misnomer because acid is deposited on the earth by other means as well. There are also other forms of wet deposition, such as acid snow and acid fog. In dry deposition, acid-forming substances in dry form (such as sulfates) fall to earth by gravity and are transformed to acids on the earth by precipitation. Wet and dry forms of acid deposition seem to be of about equal importance; collectively, they will continue to be referred to as acid rain throughout this book.
Many atmospheric processes and factors influence the transformation of SO2 and NO^ into acid rain. Such processes can take a long time. It might take four days before the pollutants either fall to earth as dry deposition or react with moisture in the air to
form acids. Consequently, acid rain can be transported many, many miles from the source to the deposition point, crossing state and national boundaries. Canada claims that SO2 emissions are causing serious deterioration of its eastern lakes and forests.
Taller smokestacks built in recent years have put pollutants higher into the atmosphere, allowing them to remain aloft longer. The prevailing westerly winds place a large burden on eastern Canada and the northeastern United States from industries in the Midwest. Some possible solutions to the acid rain problem involve burning lower sulfur fuel or the renovation of older plants with flue-gas desulfurization units or fluidized beds for cleaner combustion of coal. These methods are discussed in following sections. Providing teeth and incentives to these approaches are the Clean Air Act laws of 1970 and 1990 (administered by the EPA) that require reductions in SO2 and NO^ emissions. (See Focus On 8.3.)
Indoor Air Pollution
All the attention so far in this chapter, and in the actions of the public and the government until the past few years, has been in the area of outdoor air pollution. However, studies in the early 1980s found that indoor air pollutant concentrations can be many times greater than outside levels, even exceeding EPA standards. Because 80% of our time is spent indoors, some environmental scientists feel that breathing indoor air may cause or aggravate half of all illnesses in the United States and contribute to thousands of deaths a year.
The home or office is a place where oil, gas, kerosene, wood, and tobacco are burned and where furniture and building supplies emit volatile chemicals (see Table 8.4). More than 200 different chemicals have been detected in indoor air. A common example is
Table 8.4 INDOOR AIR POLLUTANTS
Source: Newsweek, January 7, 1985, p. 58.
formaldehyde, which is found in plywood, particle board, and foam insulation. Formaldehyde can cause irritation to the eyes, skin, and respiratory system, and it has driven many people out of a home or building. Radon gas is another important pollutant, accounting for more than 50% of our radiation dose and thousands of cases of lung cancer per year (see Chapter 15). The link between indoor pollutants and disease is difficult to prove because of the low concentrations, much like low levels of radiation discussed in Chapter 15. However, headaches, nausea, rashes, eye and skin irritation, and abdominal and chest pains might be symptomatic of a “sick building” syndrome.
An even more shocking air-pollution statistic is that almost 2 million people per year (mostly women and children in the developing world) die annually from air pollution associated with indoor cooking stoves. About one-third of the world’s population, 2 billion people, rely on indoor fires and inefficient cookstoves to prepare their daily meals. Wood, charcoal, crop waste, and dung are some of the fuels. The science behind more efficient cookstoves is discussed in Chapter 17.
One of the reasons for high indoor air-pollution levels in newer homes is increased “tightening” of the home (weather stripping, caulking, vapor barriers) to reduce infiltration and reduce home heating and cooling costs. (See Chapter 5.) Air exchange rates have decreased by a factor of 2 to 4 in most energy-efficient houses. Increased ventilation (through air to air-heat exchanges) can often reduce pollutant concentrations to safe levels.