Acid
rain is the common name for acidic deposits that fall to Earth from the
atmosphere. The term was coined in 1872 by the Scottish chemist Robert
Angus Smith (1817–1884) to describe the acidic precipitation in
Manchester, England. In the twenty-first century scientists study both
wet and dry acidic deposits. Even though there are natural sources of
acid in the atmosphere, acid rain is primarily caused by emissions of
sulfur dioxide (SO2) and nitrous oxide (N2 O) from electric utilities
burning fossil fuels, especially coal. These chemicals are converted to
sulfuric acid and nitric acid in the atmosphere and can be carried by
the winds for many miles from where the original emissions took place.
(See Figure 5.1.) Other chemicals contributing to acid rain include
volatile organic compounds (VOCs). These are carbon-containing chemicals
that easily become vapors or gases. VOC sources include paint thinners,
degreasers, and other solvents and burning fuels such as coal, natural
gas, gasoline, and wood.
Wet deposition occurs when the
acid falls in rain, snow, or ice. Dry deposition is caused by tiny
particles (or particulates) in combustion emissions. They may stay dry
as they fall or pollute cloud water and precipitation. Moist deposition
occurs when the acid is trapped in cloud or fog droplets. This is most
common at high altitudes and in coastal areas. Whatever its form, acid
rain can create dangerously high levels of acidic impurities in water,
soil, and plants.
Measuring Acid Rain
The acidity
of any solution is measured on a potential hydrogen (pH) scale numbered
from zero to fourteen, with a pH value of seven considered neutral.
(See Figure 5.2.) Values higher than seven are considered more alkaline
or basic (the pH of baking soda is eight); values lower than seven are
considered acidic (the pH of lemon juice is two). The pH scale is a
logarithmic measure. This means that every pH change of one is a tenfold
change in acid content. Therefore, a decrease from pH seven to pH six
is a tenfold increase in acidity; a drop from pH seven to pH five is a
one hundredfold increase in acidity; and a drop from pH seven to pH
four is a one thousandfold increase.
Pure,
distilled water has a neutral pH of seven. Normal rainfall has a pH
value of about 5.6. It is slightly acidic because it accumulates
naturally occurring sulfur oxides (SOx) and nitrogen oxides (NOx) as it
passes through the atmosphere. Acid rain has a pH of less than 5.6.
Figure
5.3 shows the average rainfall pH measured during 2005 at various
locations around the country by the National Atmospheric Deposition
Program (NADP), a cooperative project between many state and federal
government agencies and private entities. Rainfall was most acidic in
the mid-Atlantic region and upper Southeast, particularly Ohio,
Pennsylvania, West Virginia, Maryland, Delaware, Virginia, eastern
Tennessee, and Kentucky. The areas with the lowest rainfall pH contain
some of the country's most sensitive natural resources, such as the
Appalachian Mountains, the Adirondack Mountains, Chesapeake Bay, and
Great Smoky Mountains National Park. Overall, precipitation is much more
acidic in the eastern United States than in the western United States
because of a variety of natural and anthropogenic (human-caused) factors
that are discussed below.
SOURCES OF SULFATE AND NITRATE IN THE ATMOSPHERE
Natural Sources
Natural
sources of sulfate in the atmosphere include ocean spray, volcanic
emissions, and readily oxidized hydrogen sulfide, which is released from
the decomposition of organic matter found in the Earth. Natural sources
of nitrogen or nitrates include NOx produced by micro-organisms in
soils, by lightning during thunderstorms,
FIGURE 5.1
and
by forest fires. Scientists generally speculate that one-third of the
sulfur and nitrogen emissions in the United States comes from these
natural sources. (This is a rough estimate as there is no way to measure
natural emissions as opposed to those that are manmade.)
Sources Caused by Human Activity
According
to the U.S. Environmental Protection Agency (EPA), in "What Is Acid
Rain?" (June 8, 2007, http://www.epa.gov/acidrain/what/index.html), the
primary anthropogenic contributors to acid rain are SO2 and NOx,
resulting from the burning of fossil fuels, such as coal, oil, and
natural gas.
The EPA notes in "Clearinghouse for
Inventories and Emissions Factors"
(http://www.epa.gov/ttn/chief/trends/trends06/nationaltier1upto2006basedon2002finalv2.1.xls)
that approximately 70% of SO2 emissions produced in 2006 were because
of fuel combustion by fossil-fueled electric utilities. Fuel combustion
at industrial facilities contributed another 13%. Lesser sources
included transportation vehicles and industrial processes. Highway
vehicles were the primary source of NOx emissions, accounting for 36% of
the total in 2006. Off-highway vehicles (such as bulldozers)
contributed 22%. Fuel combustion in power plants was another major
source,
FIGURE 5.2
accounting for 20% of the total. Lesser sources included industrial processes and waste disposal and recycling
NATURAL FACTORS THAT AFFECT ACID RAIN DEPOSITION
Major
natural factors contributing to the impact of acid rain on an area
include air movement, climate, and topography and geology. Transport
systems—primarily the movement of air—distribute acid emissions in
definite patterns around the planet. The movement of air masses
transports emitted pollutants many miles, during which the pollutants
are transformed into sulfuric and nitric acid by mixing with clouds of
water vapor.
FIGURE 5.3
In drier
climates, such as those of the western United States, windblown alkaline
dust moves more freely through the air and tends to neutralize
atmospheric acidity. The effects of acid rain can be greatly reduced by
the presence of basic (also called alkali) substances. Sodium,
potassium, and calcium are examples of basic chemicals. When a basic and
an acid chemical come into contact, they react chemically and
neutralize each other. By contrast, in more humid climates where there
is less dust, such as along the eastern seaboard, precipitation is more
acidic.
Areas most sensitive to acid rain contain hard,
crystalline bedrock and thin surface soils. When no alkaline-buffering
particles are in the soil, runoff from rainfall directly affects surface
waters, such as mountain streams. In contrast, a thick soil covering or
soil with a high buffering capacity, such as flat land, neutralizes
acid rain better. Lakes tend to be most susceptible to acid rain because
of low alkaline content in lake beds. A lake's depth, its watershed
(the area draining into the lake), and the amount of time the water has
been in the lake are also factors.
EFFECTS OF ACID RAIN ON THE ENVIRONMENT
In
nature the combination of rain and oxides is part of a natural balance
that nourishes plants and aquatic life. However, when the balance is
upset by acid rain, the results to the environment can be harmful and
destructive. (See Table 5.1.)
Aquatic Systems
Even
though pH levels vary considerably from one body of water to another, a
typical pH range for the lakes and rivers in the United States is six
to eight. Low pH levels kill fish, their eggs, and fish food organisms.
The
TABLE 5.1
Effects of acid rain on human health and selected ecosystems and anticipated recovery benefits
Human health and ecosystem
Effects
Recovery benefits
SOURCE:
"Appendix I. Effect of Acid Rain on Human Health and Selected
Ecosystems and Anticipated Recovery Benefits," in Acid Rain: Emissions
Trends and Effects in the Eastern United States, U.S. General Accounting
Office, March 2000, http://www.gao.gov/archive/2000/rc00047.pdf
(accessed July 27, 2007)
Human health
In
the atmosphere, sulfur dioxide and nitrogen oxides become sulfate and
nitrate aerosols, which increase morbidity and mortality from lung
disorders, such as asthma and bronchitis, and impacts to the
cardiovascular system.
Decrease emergency room visits, hospital admissions, and deaths.
Surface waters
Acidic
surface waters decrease the survivability of animal life in lakes and
streams and in the more severe instances eliminate some or all types of
fish and other organisms.
Reduce the acidic levels of surface waters and restore animal life to the more severely damaged lakes and streams.
Forests
Acid
deposition contributes to forest degradation by impairing trees' growth
and increasing their susceptibility to winter injury, insect
infestation, and drought. It also causes leaching and depletion of
natural nutrients in forest soil.
Reduce
stress on trees, thereby reducing the effects of winter injury, insect
infestation, and drought, and reduce the leaching of soil nutrients,
thereby improving overall forest health.
Materials
Acid
deposition contributes to the corrosion and deterioration of buildings,
cultural objects, and cars, which decreases their value and increases
costs of correcting and repairing damage.
Reduce the damage to buildings, cultural objects, and cars, and reduce the costs of correcting and repairing future
damage.
Visibility
In
the atmosphere, sulfur dioxide and nitrogen oxides form sulfate and
nitrate particles, which impair visibility and affect the enjoyment of
national parks and other scenic views.
Extend
the distance and increase the clarity at which scenery can be viewed,
thus reducing limited and hazy scenes and increasing the enjoyment of
national parks and other vistas.
degree of damage depends on
several factors, one of which is the buffering capacity of the watershed
soil—the higher the alkalinity, the more slowly the lakes and streams
acidify. The exposure of fish to acidified freshwater lakes and streams
has been intensely studied since the 1970s. Scientists distinguish
between sudden shocks and chronic (long-term) exposure to low pH levels.
Sudden,
short-term shifts in pH levels result from snowmelts, which release
acidic materials accumulated during the winter, or sudden rainstorms
that can wash residual acid into streams and lakes. The resulting acid
shock can be devastating to fish and their ecosystems. At pH levels
below 4.9, fish eggs are damaged. At acid levels below 4.5, some species
of fish die. Below pH 3.5, most fish die within hours. (See Table 5.2.)
TABLE 5.2
Generalized short-term effects of acidity on fish
pH range
Effect
SOURCE:
"Generalized Short-Term Effects of Acidity on Fish," in National Water
Quality Inventory: 1998 Report to Congress, U.S. Environmental
Protection Agency, June 2000
6.5–9
No effect
6.0–6.4
Unlikely to be harmful except when carbon dioxide levels are very high (1,000 mg l-1)
5.0–5.9
Not especially harmful except when carbon dioxide levels are high (20 mg I1) or ferric ions are present
4.5–4.9
Harmful to the eggs of salmon and trout species (salmonids) and to adult fish when levels of Ca2, Na+and Cl-are low
4.0–4.4
Harmful to adult fish of many types which have not been progressively acclimated to low pH
3.5–3.9
Lethal to salmonids, although acclimated roach can survive for longer
3.0–3.4
Most fish are killed within hours at these levels
Because
many species of fish hatch in the spring, even mild increases in
acidity can harm or kill the new life. Temporary increases in acidity
also affect insects and other invertebrates, such as snails and
crayfish, on which the fish feed.
Gradual decreases of
pH levels over time affect fish reproduction and spawning. Moderate
levels of acidity in water can confuse a salmon's sense of smell, which
it uses to find the stream from which it came. Atlantic salmon are
unable to find their home streams and rivers because of acid rain. In
addition, excessive acid levels in female fish cause low amounts of
calcium, thereby preventing the production of eggs. Even if eggs are
produced, their development is often abnormal.
Increased
acidity can also cause the release of aluminum and manganese particles
that are stored in a lake or river bottom. High concentrations of these
metals are toxic to fish.
Soil and Vegetation
Acid
rain is believed to harm vegetation by changing soil chemistry. Soils
exposed to acid rain can gradually lose valuable nutrients, such as
calcium, magnesium, and potassium and become too concentrated with
dissolved inorganic aluminum, which is toxic to vegetation. Long-term
changes in soil chemistry may have already affected sensitive soils,
particularly in forests. Forest soils saturated in nitrogen cannot
retain other nutrients required for healthy vegetation. Subsequently,
these nutrients are washed away. Nutrient-poor trees are more vulnerable
to climatic extremes, pest invasion, and the effects of other air
pollutants, such as ozone.
FIGURE 5.4
Some
researchers believe that acid rain disrupts soil regeneration, which is
the recycling of chemical and mineral nutrients through plants and
animals back to the Earth. They also believe acids suppress decay of
organic matter, a natural process needed to enrich the soils. Valuable
nutrients such as calcium and magnesium are normally bound to soil
particles and are, therefore, protected from being rapidly washed into
groundwater. Acid rain, however, may accelerate the process of breaking
these bonds to rob the soil of these nutrients. This, in turn, decreases
plant uptake of vital nutrients. (See Figure 5.4.)
Acid
deposition can cause leafy plants such as lettuce to hold increased
amounts of potentially toxic substances such as the mineral cadmium.
Research also finds a decrease in carbohydrate production in the
photosynthesis process of some plants exposed to acid conditions.
Research is under way to determine whether acid rain could ultimately
lead to a permanent reduction in tree growth, food crop production, and
soil quality. Effects on soils, forests, and crops are difficult to
measure because of the many species of plants and animals, the slow rate
at which ecological changes occur, and the complex interrelationships
between plants and their environment.
trees
trees.
The effect of acid rain on trees is influenced by many factors. Some
trees adapt to environmental stress better than others; the type of
tree, its height, and its leaf structure (deciduous or evergreen)
influence how well it will adapt to acid rain. Scientists believe that
acid rain directly harms trees by leaching calcium from their foliage
and indirectly harms them by lowering their tolerance to other stresses.
According
to the EPA, acid rain has also been implicated in impairing the winter
hardening process of some trees, making them more susceptible to
cold-weather damage. In some trees the roots are prone to damage because
the movement of acidic rain through the soil releases aluminum ions,
which are toxic to plants.
One area in which acid rain
has been linked to direct effects on trees is from moist deposition via
acidic fogs and clouds. The concentrations of acid and SOx in fog
droplets are much greater than in rainfall. In areas of frequent fog,
such as London, significant damage has occurred to trees and other
vegetation because the fog condenses directly on the leaves.
Birds
Increased
freshwater acidity harms some species of migratory birds. Experts
believe the dramatic decline of the North American black duck population
since the 1950s is because of decreased food supplies in acidified
wetlands. Acid rain leaches calcium out of the soil and robs snails of
the calcium they need to form shells. Because titmice and other species
of songbirds get most of their calcium from the shells of snails, the
birds are also perishing. The eggs they lay are defective—thin and
fragile. The chicks either do not hatch or have bone malformations and
die.
In "Adverse Effects of Acid Rain on the
Distribution of the Wood Thrush Hylocichla mustelina in North America"
(Proceedings of the National Academy of Sciences, August 12, 2002),
Ralph S. Hames et al. discuss the results of their large-scale study,
which shows a clear link between acid rain and widespread population
declines in the wood thrush, a type of songbird. Hames and his
colleagues believe that calcium depletion has had a negative impact on
this bird's food source, mainly snails, earthworms, and centipedes. The
bird may also be ingesting high levels of metals that are more likely to
leach out of overly acidic soils. Declining wood thrush populations are
most pronounced in the higher elevations of the Adirondack, Great
Smoky, and Appalachian mountains. Hames and his cohorts warn that acid
rain may also be contributing to population declines in other songbird
species.
Materials
Acid rain can also be harmful
to materials, such as building stones, marble statues, metals, and
paints. Elaine McGee of the U.S. Geological Service reports in Acid Rain
and Our Nation's Capital (1997,
http://pubs.usgs.gov/gip/acidrain/contents.html) that limestone and
marble are particularly vulnerable to acid rain. Historical monuments
and buildings composed of these materials in the eastern United States
have been hit hard by acid rain.
Human Health
Acid
rain has several direct and indirect effects on humans. Particulates
are extremely small pollutant particles that can threaten human health.
Particulates related to acid rain include fine particles of SOx and
nitrates. These particles can travel long distances and, when inhaled,
penetrate deep into the lungs. Acid rain and the pollutants that cause
it can lead to the development of bronchitis and asthma in children.
Acid rain is also believed to be responsible for increasing health risks
for those over the age of sixty-five; those with asthma, chronic
bronchitis, and emphysema; pregnant women; and those with histories of
heart disease.
THE POLITICS OF ACID RAIN
Scientific
research on acid rain was sporadic and largely focused on local
problems until the late 1960s, when Scandinavian scientists began more
systematic studies. Acid precipitation in North America was not
identified until 1972, when scientists found that precipitation was
acidic in eastern North America, especially in northeastern and eastern
Canada. In 1975 the First International Symposium on Acid Precipitation
and the Forest Ecosystem convened in Columbus, Ohio, to define the acid
rain problem. Scientists used the meeting to propose a
precipitation-monitoring network in the United States that would
cooperate with the European and Scandinavian networks and to set up
protocols for collecting and testing precipitation.
In
1977 the Council on Environmental Quality was asked to develop a
national acid rain research program. Several scientists drafted a report
that eventually became the basis for the National Acid Precipitation
Assessment Program (NAPAP). This initiative eventually translated into
legislative action with the Energy Security Act of 1980. Title VII (Acid
Precipitation Act of 1980) of the act produced a formal proposal that
created NAPAP and authorized federally financed support.
The
first international treaty aimed at limiting air pollution was the
United Nations Economic Commission for Europe (UNECE) Convention on
Long-Range Trans-boundary Air Pollution, which went into effect in 1983.
It was ratified by thirty-eight of the fifty-four UNECE members, which
included not only European countries but also Canada and the United
States. The treaty targeted sulfur emissions, requiring that countries
reduce emissions 30% from 1980 levels—the so-called Thirty Percent Club.
The
early acid rain debate centered almost exclusively on the eastern
United States and Canada. The controversy was often defined as a problem
of property rights. The highly valued production of electricity in
coal-fired utilities in the Ohio River Valley caused acid rain to fall
on land in the Northeast and Canada. An important part of the acid rain
controversy in the 1980s was the adversarial relationship between U.S.
and Canadian government officials over emission controls of SO2 and NO2.
More of these pollutants crossed the border into Canada than the
reverse. Canadian officials very quickly came to a consensus over the
need for more stringent controls, whereas this consensus was lacking in
the United States.
Throughout the 1980s the major
lawsuits involving acid rain all came from eastern states, and the
states that passed their own acid rain legislation were those in the
eastern part of the United States.
Legislative attempts
to restrict emissions of pollutants were often defeated after strong
lobbying by the coal industry and utility companies. These industries
advocated further research for pollution-control technology rather than
placing restrictions on utility company emissions.
The NAPAP Controversy
In
1980 Congress established NAPAP to study the causes and effects of acid
deposition and recommend policy approaches for controlling acid rain
effects. About two thousand scientists worked on this unique
inter-agency program, which ultimately cost more than $500 million. Even
though its first report was due in 1985, the program was plagued by
problems that resulted in numerous delays. In 1985 the first executive
director, Christopher Bernabo, resigned and was replaced by Lawrence
Kulp. In 1987 the study group released to Congress Interim Assessment:
The Causes and Effects of Acidic Deposition, a massive four-volume
preliminary report that caused a storm of controversy. The report
contained detailed scientific information in its technical chapters
about acid rain. The executive summary, written by Kulp, was released to
the public and widely criticized for mis-representing the scientific
findings of the report and downplaying the negative effects of acid
rain. Philip Shabecoff notes in "Acid Rain Report Unleashes a Torrent of
Controversy" (New York Times, March 20, 1990) that critics claimed Kulp
had slanted the summary to match the political agenda of the
administration of President Ronald Reagan (1911–2004), which advocated
minimum regulation of business and industry.
Some of the scientific findings in the 1987 report included:
Acid rain had adversely affected aquatic life in about 10% of eastern lakes and streams.
Acid rain had contributed to the decline of red spruce at high elevations by reducing this species' cold tolerance.
Acid rain had contributed to erosion and corrosion of buildings and materials.
Acid rain and related pollutants had reduced visibility throughout the Northeast and in parts of the West.
The
report concluded, however, that the incidence of serious acidification
was more limited than originally feared. At that time the Adirondacks
area of New York was the only region showing widespread, significant
damage from acid. Furthermore, results indicated that
electricity-generating power plants were responsible for two-thirds of
SO2 emissions and one-third of NOx emissions.
Controversy
over Kulp's role led to him being replaced by James Mulhoney. The new
director ordered reassessments and revisions of the interim report. This
was completed in 1991. However, by that time President George H. W.
Bush (1924–) was in power, and he had made acid rain legislation a
component of his election campaign. As a result, political forces,
rather than NAPAP, largely drove the nation's emerging policy toward
acid rain.
THE ACID RAIN PROGRAM—CLEAN AIR ACT AMENDMENTS, TITLE IV
Congress
created the Acid Rain Program under Title IV (Acid Deposition Control)
of the 1990 Clean Air Act Amendments. The goal of the program is to
reduce annual emissions of SO2 and NOx from electric power plants
nationwide. The program set a permanent cap on the total amount of SO2
that could be emitted by these power plants. According to the EPA, in
Acid Rain Program: 2005 Progress Report (October 2006,
http://www.epa.gov/airmarkets/progress/docs/2005report.pdf), this cap
was set at 8.9 million tons (approximately half the number of tons of
SO2 emitted by these plants during 1980). The program also established
NOx emissions limitations for certain coal-fired electric utility
plants. The objective of these limitations was to achieve and maintain a
two-million-ton reduction in NOx emission levels by 2000 compared with
the emissions that would have occurred in 2000 if the limitations had
not been implemented.
In the 1999 Compliance Report:
Acid Rain Program (July 2000,
http://www.epa.gov/airmarkets/progress/docs/1999compreport.pdf), the EPA
indicates that the reduction was implemented in two phases. Phase I
began in 1995 and covered 263 units at 110 utility plants in 21 states
with the highest levels of emissions. Most of these units were at
coal-burning plants located in eastern and midwestern states. They were
mandated to reduce their annual SO2 emissions by 3.5 million tons. An
additional 182 units joined Phase I voluntarily, bringing the total of
Phase I units to 445.
Phase II began in 2000. It
tightened annual emission limits on the Phase I group and set new limits
for more than two thousand cleaner and smaller units in all forty-eight
contiguous states and the District of Columbia.
A New Flexibility in Meeting Regulations
Traditionally,
environmental regulation has been achieved by the "command and control"
approach, in which the regulator specifies how to reduce pollution, by
what amount, and what technology to use. Title IV, however, gave
utilities flexibility in choosing how to achieve these reductions. For
example, utilities could reduce emissions by switching to low-sulfur
coal, installing pollution-control devices called scrubbers, or shutting
down plants.
Utilities took advantage of their
flexibility under Title IV to choose less costly ways to reduce
emissions—many switching from high- to low-sulfur coal—and as a result,
they have been achieving sizable reductions in their SO2 emissions.
Allowance Trading
Title
IV also allows electric utilities to trade allowances to emit SO2.
Utilities that reduce their emissions below the required levels can sell
their extra allowances to other utilities to help them meet their
requirements.
Title IV allows companies to buy, sell,
trade, and bank pollution rights. Utility units are allocated allowances
based on their historic fuel consumption and a specific emissions rate.
Each allowance permits a unit to emit one ton of SO2 during or after a
specific year. For each ton of SO2 discharged in a given year, one
allowance is retired and can no longer be used. Companies that pollute
less than the set standards will have allowances left over. They can
then sell the difference to companies that pollute more than they are
allowed, bringing them into compliance with overall standards. Companies
that clean up their pollution would recover some of their costs by
selling their pollution rights to other companies.
The
EPA holds an allowance auction each year. The sale offers allowances at a
fixed price. This use of market-based incentives under Title IV is
regarded by many as a major new method for controlling pollution.
From
1995 to 1998 there was considerable buying and selling of allowances
among utilities. Because the utilities that participated in Phase I
reduced their sulfur emissions more than the minimum required, they did
not use as many allowances as they were allocated for the first four
years of the program. Those unused allowances could be used to offset
SO2 emissions in future years. In Acid Rain: Emissions Trends and
Effects in the Eastern United States (March 2000,
http://www.gao.gov/archive/2000/rc00047.pdf), the U.S. General
Accounting Office (now the U.S. Government Accountability Office) notes
that from 1995 to 1998 a total of 30.2 million allowances were allocated
to utilities nationwide; almost 8.7 million,
FIGURE 5.5
or 29%, of the allowances were not used but were carried over (banked) for subsequent years.
Figure
5.5 shows the status of the allowance bank from 1995 through 2005.
Banked allowances increased dramatically in 2000 due to the addition of
the Phase II sources to the Acid Rain Program. Over the next five years
the allowance bank steadily decreased in size. The EPA reports in Acid
Rain Program: 2005 Progress Report that in 2005 a total of 9.5 million
allowances were allocated. Another 6.9 million banked allowances were
carried over from previous years. The EPA expects that the allowance
bank will eventually be depleted as SO2 emissions are further restricted
by the implementation of the Clean Air Interstate Rule.
PERFORMANCE RESULTS OF THE ACID RAIN PROGRAM
There
are three quantitative measures that environmental regulators use to
gauge the performance of the Acid Rain Program: emissions, atmospheric
concentrations, and deposition amounts.
U.S. Progress Report
The following information comes from the EPA's Acid Rain Program: 2005 Progress Report.
sources and emissions
sources
and emissions. The report notes that in 2005 there were 3,456 electric
generating units subject to the SO2 provisions of the Acid Rain Program.
Most emissions were associated with approximately 1,100 coal-fired
units making up the total. In all, program sources emitted 10.2 million
tons of SO2 into the air. (See Figure 5.6.) The EPA expects that the
8.9-million-ton annual cap on emissions will be achieved by 2010. SO2
emissions from sources covered by the program decreased by 41% between
1980 and 2005.
In 2005 the NOx portion of the Acid Rain
Program applied to a subset of the 3,456 units mentioned earlier,
specifically 982 operating coal-fired units generating at least 25
megawatts. Between 1990 and 2005 NOx emissions from power plants subject
to the Acid Rain Program decreased from 5.5 million tons per year to
3.3 million tons per year. (See Figure 5.7.)
According
to the report, in 2000 the program first achieved its goal of reducing
emissions by at least 2 million tons; 8.1 million tons were originally
predicted in 1990 to be emitted in 2000 without the program in place.
The
report indicates that the SO2 and NOx emission reductions were achieved
even though the amount of fuel used to produce electricity in the
United States increased by more than 30% between 1990 and 2005. Coal was
the
FIGURE 5.6
FIGURE 5.7
single-largest fuel source for U.S. electric generating plants in 2005, accounting for 50% of the total.
atmospheric concentrations and deposition amounts
atmospheric
concentrations and deposition amounts. The EPA's Acid Rain Program uses
two complementary monitoring networks to track trends in regional air
quality and acid deposition: the Clean Air Status and Trends Network and
the NADP's National Trends Network. Additional monitoring data are
provided by national, state, and local ambient monitoring systems.
As
shown in Figure 2.14 and Figure 2.6 in Chapter 2, atmospheric levels of
SO2 and NO2 averaged nationwide since 1990 have been well below the
national standards for these pollutants.
Table 5.3
shows trends in atmospheric concentrations and deposition for four key
regions in the Acid Rain Program: mid-Atlantic, Midwest, Northeast, and
Southeast. Overall, concentrations of ambient SO2 and wet sulfates
averaged over the period 2003–05 declined in all these regions, compared
with the period 1989–91. The most dramatic differences are evident in
the Northeast, where ambient SO2 concentrations decreased by more than
50%. The results for nitrogen and nitrate compound concentrations are
mixed, with decreases in some areas and increases in others. The same is
true for wet inorganic nitrogen deposition, which decreased in the
mid-Atlantic, Midwest, and Northeast, but increased slightly in the
Southeast.
Canadian Progress Report
In November
2006 Environment Canada released a report on progress made by Canada and
the United States on cross-border air pollution. The study,
Canada–United States Air Quality Agreement: 2006 Progress Report
(http://www.ec.gc.ca/cleanair-airpur/caol/canus/report/2006canus/toc_e.cfm),
is the eighth biennial report related to the 1991 agreement between the
two countries. The report states that Canada has been successful at
reducing SO2 emissions below its national cap. Canada's total SO2
emissions were 2.3 million tonnes (metric tons) in 2004, which is 28%
below the national cap of 3.2 million tonnes. However, Environment
Canada notes that the reductions have not been sufficient to reduce acid
deposition below the levels needed to ensure the recovery of ecosystems
damaged by excess acidity in its eastern provinces.
ARE ECOSYSTEMS RECOVERING?
Monitoring
data clearly indicate decreased emissions and atmospheric
concentrations of SO2 and NOx and some reductions in deposition amounts.
These improvements have not necessarily resulted in recovery of
sensitive aquatic and terrestrial ecosystems. This is due, in part, to
the long recovery times required to reverse damage done by
acidification. The EPA reports that ecosystems
TABLE 5.3
Regional changes in air quality and deposition of sulfur and nitrogen, 1989–91 and 2003–05
Average
Measurement
Unit
Region
1989–1991
2003–2005
Percent change*
SOURCE:
"Table 4. Regional Changes in Air Quality and Deposition of Sulfur and
Nitrogen, 1989–1991 Versus 2003–2005," in Acid Rain Program: 2005
Progress Report, U.S. Environmental Protection Agency, October 2006,
http://www.epa.gov/airmarkets/progress/docs/2005report.pdf (accessed
June 19, 2007)
*Percent change is estimated from raw measurement
data, not rounded; some of the measurement data used to calculate
percentages may be at or below detection limits.
Notes: kg=kilogram. ha=hectare. mg=milligram. L=liter. µg=microgram. m3=cubic meter.
Wet sulfate deposition
kg/ha
Mid-Atlantic
27
20
-24
Wet sulfate concentration
mg/L
Mid-Atlantic
2.4
1.6
-33
Midwest
2.3
1.6
-30
Northeast
1.9
1.1
-40
Southeast
1.3
1.1
-21
Ambient sulfur dioxide concentration
µg/m3
Mid-Atlantic
13
8.4
-34
Midwest
10
5.8
-44
Northeast
6.8
3.1
-54
Southeast
5.2
3.4
-35
Ambient sulfate concentration
µg/m3
Mid-Atlantic
6.4
4.5
-30
Midwest
5.6
3.8
-33
Northeast
3.9
2.5
-36
Southeast
5.4
4.1
-24
Wet inorganic nitrogen deposition
kg/ha
Mid-Atlantic
5.9
5.5
-8
Midwest
6.0
5.5
-8
Northeast
5.3
4.1
-23
Southeast
4.3
4.4
+2
Wet nitrate concentration
mg/L
Mid-Atlantic
1.5
1.0
-29
Midwest
1.4
1.2
-14
Northeast
1.3
0.9
-33
Southeast
0.8
0.7
-9
Ambient nitrate concentration
µg/m3
Mid-Atlantic
0.9
1.0
+5
Midwest
2.1
1.8
-14
Northeast
0.4
0.5
+20
Southeast
0.6
0.7
+17
Total ambient nitrate concentration (nitrate + nitric acid)
µg/m3
Mid-Atlantic
3.5
3.0
-14
Midwest
4.0
3.5
-12
Northeast
2.0
1.7
-13
Southeast
2.2
2.1
-5
harmed
by acid rain deposition can take a long time to fully recover even
after harmful emissions cease. The most chronic aquatic problems can
take years to be resolved. Forest health is even slower to improve
following decreases in emissions, taking decades to recover. Finally,
soil nutrient reserves (such as calcium) can take centuries to
replenish.
The most recent comprehensive analysis of
acidified ecosystems was presented by NAPAP in the National Acid
Precipitation Assessment Program Report to Congress: An Integrated
Assessment (2003,
http://www.cleartheair.org/documents/NAPAP_FINAL_print.pdf). The report
presents a literature review summarizing findings from various
government and academic studies. Overall, NAPAP finds that some
ecosystems affected by acid deposition are showing limited signs of
recovery. For example, one study shows that more than 25% of affected
lakes and streams studied in the Adirondacks and northern Appalachians
are no longer acidic. However, little to no improvement has been seen in
examined water bodies in other regions, including New England and
portions of Virginia. The report notes that even though chemical
recovery has begun in some waterways, complete recovery for aquatic life
forms, such as fish, is expected to take "significantly longer."
In
regards to terrestrial ecosystems, NAPAP reports that forests are under
many stresses besides acid rain, such as global warming, land use
changes, and air pollution from urban, agricultural, and industrial
sources. The combined effect of these stressors has greatly limited
forest recovery from acidification. According to NAPAP, "There are as
yet no forests in the U.S. where research indicates recovery from acid
deposition is occurring." However, it is expected that reduced emissions
under the Acid Rain Program will benefit forests in the long term.
The
report acknowledges the future benefits of continued implementation of
the Acid Rain Program, but it concludes that "the emission reductions
achieved by Title IV are not sufficient to allow recovery of
acid-sensitive ecosystems." Recent studies support the idea that
additional emission cuts 40% to 80% beyond those of the existing program
will be needed to protect acid-sensitive ecosystems. NAPAP modeling
indicates that even virtual elimination of SO2 emissions from power
plants will be insufficient to provide this protection. It is believed
that emission reductions from other sources (such as the industrial and
transport sectors) will be necessary.
The Next Step: The Clean Air Interstate Rule
In
2005 the EPA issued the Clean Air Interstate Rule (CAIR; April 5, 2007,
http://www.epa.gov/cair/) to address the transport of air pollutants
across state lines in the eastern United States. CAIR puts permanent
caps on emissions of SO2 and NOx in twenty-eight eastern states and the
District of Columbia. It is expected to reduce SO2 emissions by more
than 70% and reduce NOx emissions by more than 60% compared with 2003
levels. These measures should reduce the formation of acid rain and
other pollutants, such as fine particulate matter and ground-level
ozone.
The CAIR program will use a cap-and-trade system
similar to that used in the SO2 portion of the acid rain program. The
EPA projects that complete implementation of CAIR in 2015 will result in
up to $100 billion in annual health benefits and a substantial
reduction in premature deaths because of air pollution in the eastern
United States. It should also improve visibility in southeastern
national parks that have been plagued by smog in recent years.
PUBLIC OPINION ABOUT ACID RAIN
Every
year the Gallup Organization polls Americans about their attitudes
regarding environmental issues. The most recent poll to assess acid rain
was conducted in March 2007. Participants were asked to express their
level of personal concern about various environmental issues, including
acid rain, water pollution, soil contamination, air pollution, plant and
animal extinctions, loss of tropical rain forests, damage to the ozone
layer, and global warming. The results showed that acid rain ranked last
among these environmental problems.
Analysis of
historical Gallup poll results shows a dramatic decline in concern about
acid rain since the late 1980s. (See Table 5.4.) In 1989 Gallup found
that 41% of respondents felt a great deal of concern about acid rain and
11% felt none at all. By 2007 only 25% of people polled were concerned a
great deal about acid rain and 20% expressed no concern about the acid
rain issue.
TABLE 5.4
Public concern about acid rain, 1989–2007
Great deal
Fair amount
Only a little
Not at all
No opinion
SOURCE:
"I'm going to read you a list of environmental problems. As I read each
one, please tell me if you personally worry about this problem a great
deal, a fair amount, only a little, or not at all. First, how much do
you personally worry about—Acid Rain," in Environment, The Gallup
Organization, 2007, http://www.galluppoll.com/content/?ci=1615&pg=1
(accessed June 19, 2007). Copyright © 2007 by The Gallup Organization.
Reproduced by permission of The Gallup Organization.
