Progress Report on the EPA Acid Rain Program

Agencywww.epa.gov/acidrain

(J)

2 Printed on paper that contains at least 30 percent postconsumer fibe

1EPA
1 Introduction

2 The Acid Rain Program 

5 What Has the Program Achieved so Far?

11 Acid Deposition and the Environment 

20 Summary of Accomplishments and Next Steps

Table of Contents
The Acid Rain Program: A success in reducing emissions and costs. How is the environment responding?

I

n 1990, Congress established the Acid Rain Program under the Clean Air Act. The principal goal of the program is to achieve reductions

of 10 million tons of sulfur dioxide (SO2) and 2 million tons of nitrogen oxides (NOx), the primary components of acid rain. These pollutants, in their various forms, lead to the acidification of lakes and streams rendering some of them incapable of supporting aquatic life.

In addition, they impair visibility in our national parks, create respiratory problems in people, weaken forests, and degrade monuments

and buildings.

These environmental and public health problems caused by acid rain have affected us for several decades. We have, however, started on

the path to recovery. Through efforts made by the United States Environmental Protection Agency’s (EPA’s) Acid Rain Program, emissions

of SO2 and NOx are declining. The centerpiece of the Acid Rain Program is a creative, market-based approach for bringing about significant, cost-effective reductions in SO2. As a result of the program and its innovative approaches, we will enjoy multiple environmental and

health benefits in a cost-effective manner.

Introduction
The Acid Rain Program

T

o address the problem of acid rain—more accurately known as

acid deposition—Congress established the National Acid

Precipitation Assessment Program (NAPAP) in 1980 to study the

causes and impacts of acid deposition. This research revealed acid

deposition’s broad environmental and health effects and also documented that the pollution causing acid deposition can travel hundreds

of miles, crossing state and national boundaries. In addition, these

studies identified electric power generation as responsible for twothirds of SO2 emissions and one-third of NOx emissions.

With NAPAP’s scientific underpinning, Congress created the Acid Rain

Program under Title IV (Acid Deposition Control) of the 1990 Clean Air

Act Amendments. The long-range transport of acid deposition and the

amount of emissions linked to electric power generation led Congress

to require significant reductions of SO2 and NOx emissions from electric utilities. By 2010, utilities would need to lower their emissions by

8.5 million tons compared to their 1980 levels. In addition, they would

need to reduce their NOx emissions by 2 million tons each year compared to levels before the Clean Air Act Amendments. 
Marketing Emission Reductions: 

The Cap and Trade Program

For SO2, the Acid Rain Program places a mandatory ceiling, or cap, on

emissions nationwide from electric utilities, and allocates emissions

to these pollution sources in the form of allowances. An allowance is

an authorization to emit 1 ton of SO2. At the end of the year, sources

must hold an allowance for each ton of SO2 they emitted. Extra

allowances may be banked (or carried over) for future use or sold to

other companies. This system of emissions trading, known as “cap

and trade,” is innovative in its use of the market to achieve greater

environmental results for a given cost than are possible through traditional approaches. Traditional approaches might include requirements

to install specific pollution control equipment. In addition to providing

an economic incentive to reduce SO2, this system allows utilities the

flexibility to decide how they will achieve the necessary emission

reductions. For example, they could install pollution control equipment

such as “scrubbers,” switch to less polluting fuel, conserve energy,

rely more on renewable energy, trade SO2 allowances, or use any combination of these options. Furthermore, utilities can change their

approach as new opportunities arise, without needing government

approval. This freedom allows them to determine for themselves the

most cost-effective timing and method of emission reductions.

In return, sources must continuously measure and report their emissions, providing emissions information unprecedented in its accuracy and

completeness. Detailed measurement ensures that emission goals are

attained and that sources are in compliance. Since the start of the program in 1995, no company in the trading program has failed to comply.

NOx is treated differently. Rather than using a cap and trade program,

the Acid Rain Program reduces NOx emissions by designating an emission rate for each source. The program gives utilities the opportunity

to find cost efficiencies while ensuring that overall emission reductions are achieved by allowing emission rates to be averaged across a

utility’s boilers. The primary difference between the NOx and SO2

What Is Acid Rain?

Acid rain, more accurately known as acid deposition, begins

with the burning of fossil fuels, such as coal, gas, or oil, for

energy. The resulting air pollution contains SO2 and NOx.

These gases react in the atmosphere with water, oxygen, and

oxidants to form various acidic compounds, most often

tracked as sulfate, nitrate, and hydrogen ions. Often carried by

winds for hundreds of miles, these compounds may be

deposited on earth as dry deposition, a process where acidic

particles or gases settle on or are absorbed by plants, land,

water, or building materials. The acidic compounds may also

be deposited through rain, snow, and cloud water, which is

known as wet deposition. In other words, there are many pathways by which acid “rain” reaches the earth. 

Gaseous

Pollutants in 

Atmosphere

Particulate

Pollutants in 

Atmosphere

Pollutants in 

Cloud Water

and 

Precipitation

SOURCES

Dry Deposition

Wet

Deposition

Dry Deposition

VOC

Natural RECEPTORS

Anthropogenic

NOx

SO2

VOC

SO2

NOx

NOx

A combination of natural and manmade activities result in the deposition of acidic compounds.

Figure 1. Origins of Acid Rain
programs is that the SO2 cap and trade approach ensures total

power plant emissions never exceed the mandated ceiling.

The program’s ultimate objective is to protect the environment and

improve human health by reducing both SO2 and NOx emissions.

EPA believes these reductions will benefit the nation by: 

• Restoring acidified lakes and streams so they can once again

support native aquatic life.

• Protecting air quality and public health.

• Improving visibility, especially at scenic vistas in national parks.

• Reducing the damage to forests along the mountain ranges of the

East Coast.

• Protecting our historic buildings and monuments from degradation. 

Predicted costs for reducing acid rain continue to

decline...

In 1990, EPA estimated that complying with the program’s SO2 emissions reduction goals would cost utilities approximately $4.6 billion

per year by 2010 (the date when the annual 10 million ton emissions reduction is expected to be reached). In 1994, the cost was

reevaluated by the General Accounting Office and the estimate was

lowered to about $2 billion by 2010. Based on recent compliance

cost information, a 1998 Resources for the Future report estimated

costs of SO2 emissions reductions to be less than $1 billion by

2010. These independent studies show that real-life experiences

with the program reveal greater cost savings than initially expected.

1989* 1990 1994 1998

7.4

4.6

1.9 0.87

Cost in Billions ($1990)

Edison Electric Institute

Key

EPA

General Accounting Office

Resources for the Future

Year

*Estimate assumes no trading program and the associated cost savings.

0

3

6

9

12

15

Cost estimates of implementing the Acid Rain Program have declined.

Figure 2. Estimated Cost of the Acid Rain Program at Full 

Implementation (2010)
What Has the Program Achieved so Far?

Emission Trends

The first step in protecting the environment and human health

from acid rain is reducing emissions. The Acid Rain Program

is proving to be extremely effective in this regard. In 1995,

the first year of compliance under the program, S02 emissions

dropped dramatically, by 3 million tons, reducing total S02 emitted

nationally as shown in Figure 3. Over the first 4 years of the program, emissions from the largest, highest emitting utility units were

about 5 million tons below their 1980 levels, as shown in Figure 4.

These deep cuts in emission levels include reductions of about 30

percent below the allowable emission levels made in advance of

future limits. These early reductions mean environmental benefits

can begin sooner. Figure 5 presents another encouraging sign: the

largest emission reductions are occurring in the heaviest emitting

areas, particularly the Midwest.

While overall N0x emissions have remained constant since the 1980s

(see Figure 6), the average emission rate of utilities participating in

Phase I of the N0x program has decreased by 42 percent since 1996.
This reduction translates to a 35 percent drop in actual NOx emissions

from electric utilities currently affected by the Acid Rain Program.

Starting in 2000, additional utility units will be affected with more

stringent SO2 emissions limitations nationwide. NOx rates (i.e., the

amount of NOx emitted per unit of coal used) will be reduced to

between 0.40 and 0.86 lbs/mmBtu, depending on the boiler type.

Air Quality

Emission reductions from the Acid Rain Program are contributing to

improved air quality. Data collected for the past 10 years, shown in

Figure 7, indicate that ambient SO2 concentrations are declining, as

are ambient SO4, or sulfate, concentrations. Sulfates are compounds formed from SO2 emissions and are transported long 

Comb-Util Comb-Industry Metals On-Road Comb-Other Non-Road All Other

Year

Emissions (million tons)

0

5

10

15

20

25

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997

National emissions of SO2 have been decreasing over the past decade, punctuated by a

significant decrease attributable to the start of the Acid Rain Program in 1995. Utilities

account for about 67 percent of the total estimated SO2 emissions. Other contributors

include industrial sources and metallurgical processes as well as cars, trucks, and construction equipment.

Source: EPA. 1998. National Air Quality and Emission Trends Report.

0

2

4

6

8

10

12

1980 1985 1990 1995 1996 1997

SO Emissions (million tons) 2

9.4 9.3 8.7

4.5 4.8 4.8

1998

4.7

Year

Emission reductions have been substantial since the program began in 1995.

Ultimately, the program will result in a 10 million ton reduction in SO2 from 1980

levels in 2010.

Source: EPA, Acid Rain Program.

Comb-Util Comb-Ind Non-Road Comb-Other All Other

Year

On-Road

Emissions (million short tons)

0

5

10

15

20

25

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997

National NOx emissions have not declined. They have remained at around 23 million

tons per year since the 1980s. Electric utilities generate about 30 percent of the total

estimated NOx emissions, while motor vehicles and other industrial sources also contribute significantly. Although NOx emission rates have declined and cleaner technologies are being used, total NOx emissions have not decreased significantly because

electricity generation and vehicle use—and, therefore, fuel combustion—have increased.

Source: EPA. 1998. National Air Quality and Emission Trends Report.

Figure 3. Trend in Sulfur Oxide Emissions for Six Principal 

Source Categories, 1987 to 1997

Figure 4. SO2 Emissions From 263 Large Utility Units

Figure 6. Trend in Nitrogen Oxide Emissions for Five 

Principal Source Categories, 1987 to 1997
2,500,000

Emissions Scale

(In Tons)

2,000,000

1,500,000

1,000,000

500,000

0

1980 1990 1996 1997

1997 1996 1990 1980

20

15

10

5

0

Annual SO2 Emissions

(Millions Tons)

Key

Emission reductions are occurring where they are needed— in some of the highest emitting areas of the country. For example, electric

utilities affected by the Acid Rain Program in 1997 in both Ohio and Indiana reduced SO2 emissions by about 44 percent and 50 percent, respectively, from 1990 levels. This decrease is important environmentally, but it also supports an economic premise of the Acid

Rain Program’s market-based approach: the highest emitting plants have more incentive to make substantial emission reductions

because they can achieve these reductions at a lower cost per ton. Experience is validating this expectation, and concerns that the

biggest emitters of SO2 would simply buy allowances and continue to emit at their historical levels have proved unwarranted thus far. 

Source: EPA, Acid Rain Program.

Figure 5. 1980 to 1997 SO2 Emissions From Utilities
distances. These fine particles aggravate respiratory health, can

lead to premature mortality, and degrade visibility, resulting in a hazy

view of the horizon. 

Downward trends in SO2 emissions lead to decreases in airborne sulfate particles and improved air quality in areas distant from the original source of air pollution. The relationship between regional SO2

emissions and sulfate concentrations in air quality in the Northeast

is illustrated in Figure 8. This graph compares long-term trends

(1978 through 1996) in annual mean aerosol sulfate concentrations

at two rural locations in New York State (Mayville and Whiteface

Mountain) with upwind SO2 emissions for the Midwest (Minnesota,

Wisconsin, Illinois, Michigan, Indiana, Ohio, West Virginia, Kentucky,

and western Pennsylvania). The underlying trend in sulfate concentrations parallels SO2 emissions, with both trends decreasing. Sulfates

declined sharply in 1995, corresponding to a 36 percent reduction in

regional SO2 emissions. Air quality improved by approximately 30 percent for Mayville, New York, and an impressive 47 percent for the

more distant Whiteface Mountain, New York, location.

Midwest Emissions

1

3

5

5

7

9

11

Mayville

Whiteface Mt.

1978 1981 1984 1987 19931990 1996

SO ( g/m ) μ 4 3 SO (10 metric tons) 2 8

Alaska is in EPA Region 10; Hawaii, EPA Region 9; and Puerto Rico, EPA Region 2. Concentrations are PPM.

Percentage is the percent drop in SO2 concentration in that Region. Trend line shows changes in SO2 concentration

from 1988 on the left side to 1997 on the right side. Trends are decreasing in all Regions, with most prominent

trends occurring in the Northeast and Mid-Atlantic states.

Source: EPA

Declining emissions in the Midwest correlate to improved air quality in

Mayville and Whiteface Mountain, New York. Ambient sulfates dropped

significantly in 1995, the first year of Title IV implementation.

Source: Husain, L., Dutkiewicz, V.A., and Dass, M. 1998. Evidence for Decrease in

Atmospheric Sulfur Burden in the Eastern United States Caused by Reduction in SO2

Emissions. Geophysical Research Letters Vol. 25 No. 7.

Figure 7. Regional SO2 Trends 1988 to 1997 Figure 8. Relationship Between Emissions and 

Air Quality
Lower Emissions Mean Less Acid Deposition

Declining emissions from fossil fuel combustion will bring about

decreases in concentration of acidic compounds and total acid deposition. Measured as sulfate, nitrate, and hydrogen ions, acid deposition

is tracked by scientists and EPA with the use of data from several

monitoring networks. 

Field data collected by the National Atmospheric Deposition

Program/National Trends Network (NADP/NTN), a network of more than

200 monitoring stations, show that sulfate levels in precipitation have

declined sharply since the Acid Rain Program began requiring emission

reductions in 1995. As shown in Figure 9, sulfate deposition levels have

fallen by about 25 percent in the Northeast and Mid-Atlantic regions,

where ecosystems have proven more sensitive to acidic deposition.

Figure 10 shows that no dramatic regional changes in nitrate concentration levels have been recorded due to steady NOx emissions levels.

The Clean Air Status and Trends Network (CASTNet) measures dry deposition1 of sulfur and nitrogen, among other pollutants, at approximately

70 sites. CASTNet data show that dry deposition sulfate concentration

levels also have declined by up to 30 percent in the Northeast and the

Mid-Atlantic (see Figure 11). As anticipated, no statistically significant

regional trends have been measured for nitrate concentration levels.

<5 8 11 14 17 20 23 26 29 32 >35

Figure 9. Wet Sulfate Deposition in Acid Rain Reduced (kg/ha) Figure 10. Nitrate Concentration in 

Acid Rain Unchanged (μeq/L)

1

Actual measurement of dry deposition (acid deposition in the form of particles or gases) is very complex. CASTNet measures concentration levels and meteorology and then estimates what is deposited on the surface.

-6.0 -4.5 -3.0 -1.5 0 1.5 3.0 4.5 6.0

These maps represent snapshots of wet sulfate deposition over time. As illustrated in the 1995 to 1997 map, following the 1995

implementation of the Acid Rain Program, total sulfate deposition fell in a dramatic and unprecedented reduction of up to 25 percent over a large area of the Eastern United States. The greatest reductions occur in the Northeastern United States, where many

of the most acid sensitive ecosystems are located. (Units are in kilograms per hectare).

Source: Lynch, J.A., Bowersox, V.C. and Grimm, J.W., 1999. Changes in Sulfate Deposition in Eastern U.S.A. Following Enactment of Title IV of the

Clean Air Act Amendments of 1990. Submitted to Atmospheric Environment. In Press.

1989 to 1991 1995 to 1997

<5 8 11 14 17 20 23 26 29 32 >35

As compared to the 1983 to 1994 trend, in both 1995 and 1996,

nitrate concentrations have been estimated to be about the same as

levels expected. White crosses indicate locations of NADP/NTN sites

used in this analysis. (Units are in microequivalents per liter (μeq/L).)

Source: Lynch, J.A., Bowersox, V.C. and Grim, J.W. Acid Rain Reduced in

Eastern U.S.A. Submitted to Environmental Science & Technology. In Review. 
Source: Holland, D.M., Principe, P., and Sickles, J.E, II. 1999. Trends in Atmospheric Sulfur and Nitrogen Species in the Eastern United States. Atmospheric Environment, Vol. 33, 37-49.

Laurel Hill

Penn State

Connecticut Hill

Kane Forest

M.K. Goddard

Washington Crossing

Arendtsville

Beltsville

Cedar Creek

Parsons

Alhambra

Ann Arbor

Bondville

Perkinstown

Salamonie Lake

Unionville

Wellston

Deer Creek

Lykens

Oxford

Vincennes

Coffeeville

Edgar Evins

Georgia State

Pisgah

Prince Edward

Sand Mountain

Shenandoah

Speedwell

Virginia Tech

Ashland

Caddo Valley

Coweeta

Sumatra

10% 0% -10% -20% -30% -40% -50% -60%

Allegheny Front

Allegheny Plateau

Mid-Appalachians

Midwest

Ohio River Valley

South

Unique Sites

10% 0% -10% -20% -30% -40% -50% -60%

Percent Increases Percent Reductions

SO4

2- 

Key

(Sulfate)

Figure 11. Widespread Reductions in Sulfate Concentrations at CASTNet Sites Between 1989 and 1995 (μg/m3)
Acid Deposition and the Environment

The ultimate consequence of acid deposition is its impact on

the environment and public health. In both wet and dry forms,

acid rain has multiple effects, sometimes through mechanisms that scientists are still trying to understand. In general, the

chemical composition of rainwater varies from place to place, often

due to air pollution and other chemicals found in a particular area.

Figure 12 presents the pH, a measure of acidity, of precipitation

measured by the NADP network in 1997. 

The following sections discuss areas that will benefit from the Acid

Rain Program’s emission reductions: surface water, visibility, forests,

human health, and materials and structures.

Surface Water: Lakes and Streams

In a national surface water survey conducted under NAPAP in the

mid-1980s, acid rain was the dominant cause of acidification in 75

percent of the acidic lakes and about 50 percent of the acidic
streams (NAPAP, 1991). Acid deposition can deplete the ability of

lakes and streams to sustain some fish and other aquatic species’

capability to grow, reproduce, or survive. Although the diversity and

abundance of fish species present in a body of water is related to

many factors, in some cases, the types of species able to survive

can be directly correlated to surface water acidity. Figure 13 shows

critical pH levels below which various species of fish cannot survive.

The loss of fish occurs primarily in surface waters resting atop shallow

soils that are not able to buffer, or counteract, acidity, most commonly

in the Northeast and Mid-Atlantic regions (see Figure 14). Furthermore,

as acidity increases, aluminum is leached from the soil causing 

concentrations in lakes and streams to increase.

Aluminum, particularly inorganic aluminum, is

highly toxic to aquatic life. The Adirondack

Mountains in New York and the Mid-Appalachian

highlands contain many of the U.S. waters most

sensitive to acidification. Other sensitive areas

include Florida, the upper Midwest, and the highelevation West. In eastern Canada, where the

soil is similar to that found in the Adirondacks,

the Canadian government estimates that about

14,000 lakes are acidic.

Acidification can be chronic or episodic. Lakes

and streams suffering from chronic acidification have a constantly low capacity to buffer

acids over a long period of time. The NAPAP

survey found that more than 500 streams in

the Mid-Atlantic Coastal Plain and more than

1,000 streams in the Mid-Atlantic Highlands

are chronically acidic, primarily due to acidic

deposition. In the New Jersey Pine Barrens

area, more than 90 percent of streams are

acidic, the highest rate in the nation. Many

streams in that area have already experienced

trout losses due to the high level of acidity. Hundreds of lakes in

the Adirondacks have acidity levels unsuitable for the survival of

sensitive fish species [see inset on Adirondacks]. In some sensitive

lakes and streams further south, acidification has severely depleted

even the hardiest species, such as the brook trout [see inset on

streams in the southern Appalachians]. 

Episodic acidification is the rapid increase in surface water acidity,

resulting from large surges of nitrate and/or sulfate, which typically

occur during snowmelt or the heavy rains of early spring. Preventing

these surges in winter and early spring is critical because fish and

other aquatic organisms are in their vulnerable, early life stages.

Estimated pH of precipitation sampled at more than 200 sites nationwide. Most acidic precipitation is found in the

Midwest and Northeast. Normal pH for rainfall is about 5.5.

Source: National Atmospheric Deposition Program/National Trends Network

Lab pH

> 5.3

< 4.3

0 km 500 1,000

Key

Figure 12. 1997 Estimated Acidity (pH) of Precipitation at NADP Sites
Though temporary, episodic acidification can affect aquatic life significantly and has the potential to cause “fish kills.”

Episodic acidification increases the number of lakes and streams

that are susceptible to acid rain. Approximately 70 percent of sensitive lakes in the Adirondacks may be at risk of episodic acidification.

This amount is more than three times the number of chronically

acidic lakes. About 30 percent of sensitive streams in the midAppalachians are likely to become acidic during an episode. This level

is seven times the number of chronically acidic streams in that area.

High-elevation lakes in the Western United States also are at risk.

Acid Neutralizing Capacity

Whether surface waters can resist acidification depends on the ability

of the water and watershed soil to neutralize the acid deposition it

receives. The best measure of this ability is acid neutralizing capacity

(ANC), which is determined by the amount of dissolved compounds

that will counteract acidity. Surface water with an ANC of 200 micro

equivalents per liter is normal. ANC less than 50 micro equivalents

Adirondacks: Slow to Recover

In spite of declining emissions and even declining surface water sulfate

concentrations in the Adirondacks, lakes in this region are not showing

any measurable increase in acid neutralizing capacity. Why have other

Northeastern lakes, such as those in New England, begun to recover, but

not Adirondack lakes? 

Recovery of Adirondack surface waters may be affected by several factors:

relatively constant deposition and presence of nitrate in surface waters,

loss of soil’s ability to neutralize excess acidity (“base cation depletion”),

naturally occurring acid sources, underestimated dry deposition, and

lengthy lag time between deposition reduction and ecosystem recovery. 

Recent research and modeling efforts suggest that although conditions

would likely have been substantially worse without the Acid Rain

Program, full recovery of the Adirondack ecosystems may require reductions in sulfur and nitrogen beyond those currently expected. Watershed

dynamics are proving to be complex, highlighting the need for continued

environmental monitoring in order to fully evaluate the effects of controlling atmospheric sulfur and nitrogen deposition. 

7.0 6.5 6.0 5.5 5.0 4.5 Acidic

Brook Trout

Lake Trout

Smallmouth Bass

Rainbow Trout

Common Shiner

Yellow Perch

PH is a measure of acidity. The lower the pH, the more acidic the water. Fish species have different

abilities to withstand excess acidity.

Solid symbols for each type of organism are placed in favorable pH ranges; empty symbols are

placed in less favorable ranges. No symbol is placed in pH ranges that generally do not support

populations of a particular type of organism.

Source: National Acid Precipitation Assessment Program. 1991. 1990 Integrated Assessment Report.

NAPAP Office of Director, Washington, DC.

Figure 13. Critical pH for Selected Fish in Lakes and Streams
per liter is considered highly sensitive to

acidification. An ANC of zero or less is acidic.

Surface water pH is another direct measure

of the balance of acid and base ions in a

lake or stream. However, pH does not indicate whether the cause of the acidity is acid

deposition or other sources, like organic

soils or drainage from mining activities.

ANC depends largely on the surrounding

watershed’s physical characteristics such as

geology, soils, and size. Waters that are

acidic tend to be located in small watersheds that have few alkaline minerals and

shallow soils. Conversely, watersheds that

contain alkaline minerals, such as limestone, tend to have waters with a high ANC. 

Deposition reductions from the Acid Rain

Program are expected to benefit surface

water chemistry. Recently, many acidic surface waters have shown declines in sulfate

concentrations consistent with the program’s

emissions reductions. 

Percent ANC < 0 eq/L

< 1%

1%-5%

5%-10%

10%-20%

>20%

West Upper Midwest

Adirondacks

New

England

Mid-Atlantic

Highlands Mid-Atlantic

Coastal Plain

Florida

Southeastern 

Highlands

μ

Source: NAPAP. 1991. 1990 Integrated Assessment Report.

Fresh Water Fish and Rising

Acidification

Compared to the Adirondacks, streams

in Shenandoah National Park are in the

early stages of acidification. Recent

declines in fish population and species

diversity indicate, however, that episodic

acidification is taking its toll. In a

University of Virginia study on trout

reproduction in the Southern

Appalachian Mountains, researchers

found nearly 100 percent death in the

trout eggs and newly hatched fish after

a severely acidic rainfall and steep

increase in stream water acidity. This

sharp acidic surge, due to acidic rainfall,

altered stream chemistry, resulting in

conditions fatal to fish at young and vulnerable stages. [Trout Unlimited, 1998.]

Figure 14. Percentage of Acidic Surface Waters in Surveyed Regions

Key
Recovery in New

England

The Acid Rain Program has led

to significant emissions cuts

from Northeastern and

Midwestern utilities. These

reductions can be expected to

spur the recovery of lakes and

streams in the Northeast, particularly in New England. Some

acidified ecosystems are already

showing signs of improvement

as emissions and acid deposition decline. A recent study

examining acidified lakes in the

Northeast found that lakes atop

thin gravel soils (and thus sensitive to acid rain) in New England

have shown statistical decreases

in surface water sulfate concentrations and concurrent increases in ANC. These trends (as

shown in Figure 15) are significant because ANC indicates an

ecosystem’s long-term ability to

resist acidification.

The study of New England lakes

used surface water data from

EPA’s Environmental Monitoring

and Assessment Program’s LongTerm Monitoring Project, which

provides estimates of the recovery of lakes and streams

exposed to large-scale emissions

reductions. Ecosystem recovery

speed depends heavily on at

least three factors: 1) emissions

and acid deposition reduction

rates; 2) ANC; and 3) time delay

for ecosystem response. 

82 83 84 85 86 87 88 89 90 91 92 93 94 95

120

110

100

90

80

70

60

50

15

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0

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ANC Sulfate

Bourn Pond, Vermont

Surface-water chemistry data indicated that acidic conditions are

improving in this New England lake. Acidic compounds are

decreasing while acid neutralizing capacity is increasing.

Source: Stoddard, Driscoll, Kahl, and Kellogg. 1998. Can site-specific trends

be extrapolated to a region? An acidification example for the Northeast,

Ecological Applications 8 (2) 1998.

Figure 15. Improvements in Surface Water Acidity 

For Bourn Pond, Vermont 
Visibility

The pollutants associated with

acid deposition also reduce

visibility. Visibility impairment

occurs when particles and

gases in the atmosphere,

including sulfates and

nitrates, scatter and absorb

light. Visibility tends to vary by

season and geography

because it also is affected by

the angle of sunlight and

humidity. High relative humidity heightens pollution’s effect

on visibility because particles,

such as sulfates, accumulate

water and grow to sizes at

which they scatter more light,

creating haze. 

Sulfate particles from SO2

emissions account for more

than 50 percent of the impaired visibility in the Eastern United

States, particularly in combination with high summertime humidity.

In the West, nitrogen and carbon also impair visibility, and sulfur has

been implicated as a major cause of visibility impairment in many of

the Colorado River Plateau national parks, including the Grand

Canyon, Canyonlands, and Bryce Canyon.

The Interagency Monitoring of Protected Visual Environments

(IMPROVE) network monitors visibility primarily in the nation’s national parks. Reductions in particulate sulfate, usually correlated to visibility improvements, have been measured at 13 eastern IMPROVE

sites (see Figure 17). It is too soon to tell how much of these

improvements can be attributed to the Acid Rain Program.

Chesapeake Bay

Nitrogen plays a significant role in both short- and long-term

acidification of surface waters. In addition, new research reveals

that atmospheric nitrogen deposition may have significant

adverse impacts on estuaries (where the ocean meets fresh

water), other coastal waters, and large river basins. Nitrogen is a

key cause of the eutrophication (oxygen depletion) of water bodies, which occurs when excess nutrients enter an estuary and

cause excess algae growth. This additional growth limits the

amount of light and oxygen available to other organisms in the

estuary, making it difficult to survive. Further losses in available

oxygen occur once the algae begins to die off. 

Most of the excess nutrients entering the

Chesapeake Bay and its

tributaries come from drainage waters,

such as pollutant discharge and run-off

from farms. Still, scientists estimate

that up to 25 percent of the nitrogen

in the Chesapeake Bay comes from

atmospheric deposition. Long-range

transport of atmospheric nitrogen deposition

is also an important factor. The Acid Rain

Program will help reduce these airborne

Watershed sources that contribute to eutrophication.

Airshed

Key

Figure 16. Chesapeake Bay Airshed 

and Watershed

Good visibility day in Great Smoky Mountains.

Visual range is 100 miles.

Bad visibility day at same location. Visual range

is 20 miles.

Source: National Park Service
0%

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40%

E. B. Forsythe Washington, DC Dolly Sods

0%

10%

20%

30%

40%

0%

10%

20%

30%

40%

0%

10%

20%

30%

40% Shenandoah Great Smoky Mts. Mammoth Cave

0%

10%

20%

30%

40%

0%

10%

20%

30%

40%

Upper Buffalo

Sipsey

0%

10%

20%

30%

40%Okefenokee

0%

10%

20%

30%

40%

Chassahowitzka*

0%

10%

20%

30%

40%

Average

Spring

Summer

Fall

Boundary Water Boundary Waters Canoe Ar s Canoe Area

Acadia National Park

Lye Brook

Wilderness Ar Wilderness Area

Upper Buffalo Wilderness Ar Upper Buffalo Wilderness Area

Mammoth Cave National Park

Sipsey Wilderness Ar Sipsey Wilderness Area

Great Smoky Mountains

National Park

Okefenokee National Okefenokee National 

Wildlife Refuge

Chassahowitzka National ahowitzka National

Wildlife Refuge

Edmond B. Forsythe

National Wildlife Refuge

Dolly Sods Wilderness Ar Dolly Sods Wilderness Area

Shenandoah National Park Washington, DC

Improvements in visibility at our national parks correspond to

reductions in the seasonal averaged sulfate concentrations.

Percent reduction in particulate sulfate concentrations is 

calculated by comparing 1995 data to the 1993 to 1994 mean 

of the IMPROVE network sites.

Source: NAPAP. 1998. NAPAP Biennial Report to Congress: An Integrated Assessment.

Winter

Annual

*For winter the ratio is 1995 to 1994

Key

Figure 17. Visibility Improvements in National Parks
Forest Ecosystems

Acid deposition, combined with other pollutant and natural stress

factors, can damage forest ecosystems. Damage could include

increased death and decline of Northeastern red spruce at high elevations and decreased growth of red spruce in the southern

Appalachians. In some cases, acid deposition is implicated in

impairing a tree’s winter hardening process, making it susceptible to

winter injury. In other cases, acid deposition seems to impair tree

health beginning with the roots. As acid rain moves through soils, it

also can strip nutrients from the soil and increase the presence of

aluminum ions, which are toxic to plants.

Long-term changes in the chemistry of some sensitive soils may

have already occurred. In some regions, nitrogen deposition in

forests can lead to nitrogen saturation, which occurs when the forest

soil has taken up as much nitrogen as possible. Saturated, the soil

can no longer retain nutrients and they are leached away (shown in

Figure 18). Nitrogen saturation has been observed in a number of

regions including Northeastern forests, the Colorado Front Range

and mountain ranges near Los Angeles, California. Effects also have

been seen in Canada and Europe. This phenomenon can create

nutrient imbalances in the soils and roots of trees, leaving them

more vulnerable to the effects of air pollutants such as ozone, climatic extremes such as drought and cold weather, or pest invasion.

Figure 19 shows the location of forests sensitive to acid deposition.

The Acid Rain Program can be expected to reduce the stress to forest ecosystem health.

Human Health Effects

In the atmosphere, SO2 and NOx gases are transformed into fine

particles of sulfates and nitrates. These particles—the same that

impair visibility—have serious health effects as well. Particulate matter (PM) is the term used for the mix of particles that, when airborne, may appear as haze, dust, or soot. Within that mix, particles

st n ie tr Nu ke of ta Up t an l P

sli oS

om Fr ents

Loss of Nutri

Increasing Soil Acidity

Increasing Nutrient Flow

Identified as a sensitive ecosystem

and subject to high deposition rates.

Four major forest types assessed by NAPAP.

Source: NAPAP. 1996. NAPAP Biennial Report to Congress: An Integrated Assessment, National

Science and Technology Council Committee on Environment and Natural Resources.

Western Western

Conifer Conifers

Colorado Alpine

Meadows

Eastern Eastern

Hardwoods

High-Elev High-Elevation

Spruce/Fir Spruce/Fir

Southern Pine Southern Pine

and Pine and Pine

Hardwood

Southern California

Urban Forest

As soil and deposition

acidity increases, nutrients, such as base

cations, are leached

from the soil and are

not available for plant

growth.

Figure 18. Relationship Between Soil Acidity and Nutrients

Figure 19. Sensitive Forest Ecosystems
are divided by size; coarse PM is less than 10 microns in diameter,

while fine PM is less than 2.5 microns. Although both fine and

coarse particles are of health concern, fine particles are particularly

important because they easily penetrate the deepest portions of the

lungs. Recent studies have found an association between exposure

to fine PM and increased health problems, including premature

death, cardiac and respiratory-related hospital admissions and emergency room visits, aggravated asthma, acute respiratory symptoms

such as aggravated coughing and difficult or painful breathing, new

cases of chronic bronchitis, and work and school absences.

The Acid Rain Program’s reductions of SO2 and NOx, especially from

coal-burning utilities, have brought about significant drops in airborne concentrations of fine PM. The estimated mean value in 1994

dollars of the health benefits associated with decreased sulfate levels and associated fine PM reductions in the Eastern United States

is $10 billion for 1997 and $40 billion per year by 2010.

Materials and Structures

Sulfur dioxide, sulfates, and, to a lesser degree, nitrates, are corrosive to most materials, and thus can severely damage manmade

objects exposed to the atmosphere. NAPAP found evidence that acid

deposition degrades materials beyond natural weathering. The

Eastern United States, with its high concentration of historic buildings and outdoor monuments, also has some of the nation’s highest

levels of acid deposition. 

Materials potentially damaged include monuments and historic buildings; outdoor structures such as bridges and buildings; and automotive paints and finishes. For some materials, such as carbonate,

steel, or nickel, the effects are apparent after about 1 year of exposure; for others, including copper and paints, effects may appear

after about 4 years. Research suggests that materials containing

calcium carbonate, such as limestone and marble, are more damaged by dry acid deposition than wet deposition. Anecdotal evidence

suggests the same is true for car paints. Ultimately, cultural preservation as well as significant monetary benefits from avoided damage

will accompany reductions in SO2 and NOx.
Progress in Solving the Acid Rain Problem...

• Phase I of the Acid Rain Program began achieving significant reductions of SO2 emissions from large electric power generators in

1995. The emissions trading program has proven to be an extremely

cost-effective mechanism, and is facilitating 100 percent compliance

by affected sources and stimulating early emissions reductions.

• Phase II of the Acid Rain Program begins in the year 2000.

Emission reductions will be achieved for a greater number of

smaller plants, and emissions from larger plants will be further

reduced. By 2010, we will attain a S02 emissions cap on electric

utilities of 8.95 million tons (a level approximately one half of the

industrywide emissions in 1980).

• About 350,000 tons of NOx emissions have been eliminated. In

2000, these reductions are expected to exceed 2 million tons

compared to levels without EPA controls. Absent an emissions

cap, however, NOx emissions could be expected to rise in the

future as electrical production increases.

The Environment’s Response to Date...

• In the Northeast and Mid-Atlantic regions of the United States,

where ecosystems are most sensitive to acidic deposition, sulfate

levels in precipitation (wet deposition) have declined by up to 25

percent, mirroring the reductions in SO2

emissions achieved through the implementation of the Acid Rain Program. No distinct

regional trends in wet deposition of nitrate

have been detected. This is consistent with

NOx emission trends, which have remained

fairly constant over the past decade. Trends

in dry deposition of sulfur and nitrogen are

comparable.

• While in most cases it is too early to assess the ecological benefits

of the Acid Rain Program, some surface water chemistry trends in

New England are showing signs of recovery, as indicated by a rise

in acid neutralizing capacity in some lakes.

• Ecosystems most severely impacted by acid deposition, such as

the Adirondacks, have not yet shown signs of recovery. Some scientists believe that further reductions in SO2 and NOx may be

necessary for full recovery of these sensitive ecosystems.

Where Do We Go From Here?

• Efforts are under way to better coordinate research, monitoring,

and assessment activities to further our understanding of the environmental response to the Acid Rain Program and other air quality

control efforts. In addition, these activities will enable better predictions of the environmental benefits from future policies under

consideration such as the implementation of the new ozone and

particulate matter standards.

• Due to the Acid Rain SO2 trading program’s success in cost savings and unprecedented levels of emission reductions, other air

quality efforts include cap and trade programs. For example, EPA

is administering a NOx cap and trade program for large stationary

sources in 12 Northeastern states and has offered to administer

a similar program for 23 states and the District of Columbia. EPA

also is working with these states to reduce NOx during summer

months to reduce regionwide ozone (smog) levels in the Midwest

and Eastern United States. These NOx reductions and expected

reductions from the transportation sector may also provide some

benefits for a number of other nitrogen-related environmental concerns (e.g., acid rain and eutrophication of coastal waters). A

trading program also has been discussed as a possible approach

to facilitate future greenhouse gas emissions reductions.

Summary of Accomplishments and Next Steps
For more information contact...

EPA’s Acid Rain Division:

www.epa.gov/acidrain

National Acid Precipitation Assessment Program:

www.nnic.noaa.gov/CENR/NAPAP/NAPAP_96.htm

National Atmospheric Deposition Program:

nadp.sws.uiuc.edu
U.S. Environmental Protection Agency

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