An Integrated Assessment 

2005 

National Acid 

Precipitation 

Assessment 

Program Report 

to Congress: 
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Members of Congress: 

I am pleased to transmit the National Acid Precipitation Assessment Program Report to 

Congress:An Integrated Assessment.This report presents the latest scientific information and 

analysis concerning the costs, benefits, and environmental effectiveness of the Acid Rain 

Program.This Program was mandated by Title IV of the 1990 Clean Air Act Amendments to 

reduce sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions from electric generating 

sources.The SO2 program includes the use of an innovative emissions cap and trade program. 

Title IX of the 1990 Clean Air Act Amendments requires the National Acid Precipitation 

Assessment Program (NAPAP) to report to Congress the costs, benefits, and effectiveness of 

the Acid Rain Program and characterize what deposition reductions would be necessary to 

prevent adverse ecological effects in acid sensitive ecosystems.This report fulfills the requirements of Title IX. 

NAPAP coordinates federal acid rain research and monitoring under the auspices of the 

National Science and Technology Council (NSTC) Committee on Environment and Natural 

Resources (CENR).The NAPAP member agencies are the U.S. Environmental Protection Agency, 

the U.S. Department of Energy, the U.S. Department of Interior/U.S. Geological Survey, the 

U.S. Department of Interior/National Park Service, the National Aeronautics and Space 

Administration, and the National Oceanic and Atmospheric Administration.This assessment 

involved many individuals from across the federal agencies representing numerous scientific 

disciplines. NAPAP sought review from several academic perspectives as well, and the results 

reported here were subjected to extensive peer review. 

The NAPAP Report analyzes the results of the Acid Rain Program through 2002.The report 

uses quantitative and qualitative indicators to assess the effectiveness of the cap and trade 

approach to reduce emissions, improve air quality and reduce acid deposition while minimizing compliance costs. It closely tracks emission reductions since implementation of Title IV in 

1995 and includes analysis of emission reductions, compliance, the operation of the sulfur 

dioxide (SO2) allowance market and program costs.The report also identifies emerging areas 

of acid rain research, long-term environmental monitoring and assessment and urges continued support for integrated assessments. 

The report also contains the status and trends of emission reductions in SO2 and NOx and 

associated environmental impacts.The report concludes that Title IV has been successful in 

improving air quality, visibility and achieving broad-scale reductions in sulfate deposition. 

During Phase I of the program, SO2 emissions from power plants were 25–40% below 
allowable levels, resulting in larger human health and environmental benefits during those 

years. Such emissions will continue to decline to approximately 9.86 million tons in 2010. 

While there have not been similar broad-scale reductions in nitrogen deposition, the report 

indicates that nitrogen deposition has declined in some areas with resulting benefits. Overall 

a 40% reduction in NOx emissions from power plants, compared with 1990 levels, is expected 

by 2010 through Title IV and the NOx SIP Call. 

As emissions of SO2 approach the cap level established in Title IV and compliance occurs 

under Title IV and the NOx SIP Call, air quality is expected to show continued improvement 

and there will be some additional reduction in deposition. Some lakes and streams, particularly in the Northeast, are beginning to show signs of recovery from acidification.The report, 

however, also indicates that emission reductions beyond Title IV alone are not sufficient to 

allow sensitive forests and aquatic systems to recover. In this regard, the report analyzed 

three scenarios that included reductions beyond those directly required by Title IV.While 

the scenarios were able to include emission reductions from the NOx SIP Call and certain 

state programs, they did not, however, include other existing Clean Air Act programs, including mobile source reductions. 

Since the analysis for this report was concluded, EPA finalized several rules that will result in 

further substantial reductions of SO2 and NOx, including the Clean Air Interstate Rule, the 

Non-Road Diesel Rule and the Best Available Retrofit Technology guidance for regional haze. 

States will also reduce SO2 and NOx as necessary to attain or maintain the new fine particulate matter and ozone National Ambient Air Quality Standards (NAAQS).As part of ongoing 

work, NAPAP intends to further analyze the improvements in air quality and the resulting 

benefits for air quality and the environment. 

John H. Marburger, III 

Director 

Acknowledgements 

The NAPAP Office of the Director would like to thank all those who wrote, edited, and reviewed this NAPAP Report 

to Congress. Special thanks are extended to the extramural peer reviewers, who, in addition to providing a technical 

review, were asked to consider the relationship of this report to public policy. Their comments and suggestions make 

this a significantly better report. 

Peer Reviewers 

Dallas Burtraw, Resources for the Future 

Thomas Butler, Cornell University 

Kenneth Colburn, Northeast States for Coordinated Air Use Management (NESCAUM) 

Charles Driscoll, Syracuse University 

David Grigal, University of Minnesota 

Primary Authors and Contributors 

INTRODUCTION AND EXECUTIVE SUMMARY 

Tamara Saltman, U.S. Environmental Protection Agency 

CHAPTER 1: PROGRAM ELEMENTS AND IMPLEMENTATION 

U.S. Environmental Protection Agency/Clean Air Markets Division 

CHAPTER 2: RESULTS OF THE ACID RAIN PROGRAM: STATUS AND TRENDS OF EMISSIONS AND ENVIRONMENTAL IMPACTS 

(1990–2002) 

Robert Cook, U.S. Department of Energy Oak Ridge National Laboratory 

Chris Eagar, U.S. Department of Agriculture Forest Service 

Mark Fenn, U.S. Department of Agriculture Forest Service 

Richard Haeuber, U.S. Environmental Protection Agency 

Tom Huntington, U.S. Department of Interior/Geological Survey 

Sandy McLaughlin, U.S. Department of Energy Oak Ridge National Laboratory 

Peter Murdoch, U.S. Department of Interior/U.S. Geological Survey 

Tamara Saltman, U.S. Environmental Protection Agency 

David Schmeltz, U.S. Environmental Protection Agency 

Mary Striegel, U.S. Department of Interior/National Park Service 

Carl Trettin, U.S. Department of Agriculture Forest Service 
CHAPTER 3: ASSESSING ACID DEPOSITION: ADVANCES IN THE STATE OF THE SCIENCE 

Bryan Bloomer, U.S. Environmental Protection Agency


Robert Cook, U.S. Department of Energy Oak Ridge National Laboratory


Chris Eagar, U.S. Department of Agriculture Forest Service


Mark Fenn, U.S. Department of Agriculture Forest Service


Richard Haeuber, U.S. Environmental Protection Agency


Tom Huntington, U.S. Department of Interior/Geological Survey


Sandy McLaughlin, U.S. Department of Energy Oak Ridge National Laboratory


Mary Ross, U.S. Environmental Protection Agency


David Schmeltz, U.S. Environmental Protection Agency


Mary Striegel, U.S. Department of Interior/National Park Service


Carl Trettin, U.S. Department of Agriculture Forest Service


CHAPTER 4: EXPECTATIONS FOR FULL IMPLEMENTATION OF TITLE IV IN 2010 

Bryan Bloomer, U.S. Environmental Protection Agency 

B.J. Cosby, University of Virginia


Tamara Saltman, U.S. Environmental Protection Agency


Ann Watkins, U.S. Environmental Protection Agency


CHAPTER 5: BEYOND TITLE IV: ECOLOGICAL IMPACTS OF FURTHER EMISSION REDUCTIONS 

B.J. Cosby, University of Virginia


Richard Haeuber, U.S. Environmental Protection Agency


Tamara Saltman, U.S. Environmental Protection Agency


About the National Acid Precipitation Assessment Program 

The National Acid Precipitation Assessment and to periodically assess the costs, benefits, and 

Program (NAPAP) is a cooperative federal effectiveness of Title IV. The NAPAP member agencies are 

program first authorized in 1980 to coordinate the U.S. Environmental Protection Agency, the U.S. 

acid rain research and report the findings to Department of Energy, the U.S. Department of Agriculture, 

Congress. The research, monitoring, and the U.S. Department of Interior, the National Aeronautics 

assessment efforts by NAPAP and others in and Space Administration, and the National Oceanic and 

the 1980s culminated in Title IV of the 1990 Atmospheric Administration. This report is the third 

Clean Air Act Amendments, also known as published by NAPAP since 1990 assessing Title IV. 

the Acid Deposition Control Program. In a 

bold new approach to environmental protec- In 1997 NAPAP began to operate under the auspices of 

the Committee on Environment and Natural Resources tion, Title IV includes a market-based 

program that provides economic incentives (CENR) of the National Science and Technology Council. 

for controlling emissions of sulfur dioxide NAPAP’s goal continues to be providing credible technical 

from electricity generating facilities. Title IX findings on acid deposition and its effects to inform the 

public decision-making process. To ensure that this goal is of the CAAA reauthorized NAPAP to 

conduct acid rain research and monitoring	 met, NAPAP coordinates its activities through the Air Quality 

Research Subcommittee of the CENR. 
Table of Contents


Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1


Long-Term Environmental Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3


Key Components of a Model Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3


Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4


NAPAP Mission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4


Structure of the Report . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5


Chapter 1. Program Elements and Implementation 

Emission Reductions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6


Program Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6


SO2 Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6


NOx Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7


Title IV Affected Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8


Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8


SO2 Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8


NOx Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9


SO2 Allowance Market . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10


Program Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12


Phase I Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13


Phase II Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13


Administrative Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14


Program Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15


Emissions Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15


Air Quality, Deposition, and Ecological Monitoring . . . . . . . . . . . . . . . . . . . . . . .15


Chapter 2. Results of the Acid Rain Program: Status and Trends of 

Emissions and Environmental Impacts (1990–2002) 

Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17


SO2 Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17


NOx Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17


Geographic Distribution of Emissions from Power Generation . . . . . . . . . . . . . . . .20


Geographic Trends in SO2 Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20


Geographic Trends in NOx Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22


Air Quality, Deposition, and Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23


Air Quality: Sulfur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26


Air Quality: Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27


Acid Deposition: Sulfur in Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28


Acid Deposition: Nitrogen in Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30


Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

Ecological Recovery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36


Freshwater Acid-Sensitive Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36


Forest Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41


Coastal Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43


Materials and Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43


Chapter 3. Assessing Acid Deposition: Advances in the 

State of the Science 

Air Quality, Deposition, and Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44


Aquatic and Forest Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46


Freshwater Acid-Sensitive Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46


Forest Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48


Coastal Ecosystems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49


Materials and Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49


Human Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .51


Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .52


Atmospheric Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55


Nonlinear Responses to Decreases in SO2 and NOx Emissions . . . . . . . . . . . .53


Water/Watershed Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .53


Health Effects Modeling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56


Chapter 4. Expectations for Full Implementation of 


Title IV in 2010


Emissions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58


Acid Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58


Acidification of Freshwater Lakes and Streams . . . . . . . . . . . . . . . . . . . . . . . . . . .61


Human Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63


Other Benefits of Title IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64


Chapter 5. Beyond Title IV: Ecological Impacts 

of Further Emission Reductions 

Analysis of the Environmental Impact of Further Air Emission Reductions . . . . . . .67


Scenarios Analyzed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67


Changes in Acid Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68


Changes in Sulfur Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68


Changes in Nitrogen Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70


Water Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70


Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .73


References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75

List of Figures and Tables for NAPAP Report


Figure 1. Sensitive ecological receptors in the United States 

Figure 2. Title IV affected sources—2002 

Figure 3. Percent of SO2 emission reductions by compliance option for 

Acid Rain Program sources—2001 

Figure 4. SO2 allocations for electric generating units, 1995–2002 

Figure 5. Nominal SO2 allowance prices, 1994–2002 

Figure 6. SO2 allowances transferred, 1994–2002 

Figure 7: Estimated costs of implementation of Title IV SO2 emission reductions in 2010, 1989–1998 

Figure 8. SO2 emissions from all affected sources (electric generating units), 1980–2002 

Figure 9. SO2 emissions from all sources, 1980–2001 

Figure 10. NOx emissions from all affected sources (electric generating units), 1990–2002 

Figure 11. Energy use and NOx emissions by state, 1990–2002 

Figure 12. NOx emissions from all sources, 1990–2001 

Figure 13. SO2 emission trends from power generation facilities by state, 1990–2002 

Figure 14. NOx emission trends from power generation facilities by state, 1990–2002 

Figure 15. Location of NADP/NTN, CASTNET, and IMPROVE monitoring sites and pollutants measured 

by major networks 

Figure 16. Comparison of annual ambient sulfur dioxide (SO2) concentrations in the rural 

Eastern United States from CASTNET monitoring data, 1989–1991 vs. 2000–2002 

Figure 17. Comparison of annual ambient sulfate (SO4

2-) concentrations in the rural Eastern United States 

Figure 18. Comparison of annual ambient nitric acid (HNO3) concentrations in the rural 

Eastern United States 

Figure 19. Comparison of annual ambient nitrate (NO3 -) concentrations in rural Eastern United States 

Figure 20. Comparison of annual ambient ammonium (NH4

+) concentrations in rural Eastern United States 

Figure 21. Comparison of annual wet sulfate (SO4

2-) deposition in the United States 

Figure 22. Comparison of annual total (wet plus dry) sulfur deposition in the United States 

Figure 23. Comparison of annual wet nitrate (NO3 -) deposition in the United States 

Figure 24. Comparison of annual concentrations of nitrate (NO3 -) in wet deposition in the United States 

Figure 25. Comparison of annual total (wet plus dry) nitrogen deposition in the United States 

Figure 26. Trends in visibility in Class 1 areas, 1992–2001 

Figure 27. Relationship between acid neutralizing capacity (ANC) and the number of species present in 

aquatic freshwater ecosystems in Shenandoah National Park 

Figure 28. Map of the Temporally Integrated Monitoring of Ecosystems (TIME) monitoring network 
Figure 29. Trend in acidification of eastern lakes and streams, 1990–2000 

Figure 30. Projected change in wet sulfate deposition in 2010 with Title IV compared to conditions in 

1989–1991 and 2000–2002 

Figure 31. Projected change in inorganic wet nitrogen deposition in 2010 with Title IV compared to 

conditions in 1989–1991 and 2000–2002 

Figure 32. Projected change in the number of chronically acidic lakes and streams in 2030 

with and without Title IV 

Figure 33. Stages of recovery for acidic lakes and streams 

Figure 34. Projected percent change in annual sulfur deposition from base case in 2020 with additional 

SO2 and NOx emission reductions 

Figure 35. Projected percent change in annual nitrogen deposition from base case in 2020 with additional 

SO2 and NOx emission reductions 

Figure 36. Change in the number of chronically acidic lakes and streams in 2030 with additional SO2 and 

NOx emission reductions 

TABLES 

Table 1. Nitrogen contribution to estuaries in the United States from atmospheric deposition 

(direct and indirect) 
Executive Summary


Acid deposition, more commonly known as acid rain, 

occurs when emissions of sulfur dioxide (SO2) and nitrogen 

oxides (NOx) react in the atmosphere (with water, oxygen, 

and oxidants) to form various acidic compounds. These 

acidic compounds then fall to earth in either a wet form 

(rain, snow, and fog) or a dry form (gases, aerosols, and 

particles). Prevailing winds transport the acidic compounds 

hundreds of miles, often across state and national borders. 

At certain levels the acidic compounds, including small particles such as sulfates and nitrates can cause many negative 

human health and environmental effects. While ecosystems 

are subject to many stresses, including land-use changes, 

climate change, and variations in hydrologic and meteorologic cycles, the scientific literature has clearly demonstrated 

that these pollutants can: 

• Degrade air quality, 

• Impair visibility, 

• Damage public health, 

• Acidify lakes and streams, 

• Harm sensitive forests, 

• Harm sensitive coastal ecosystems, and 

• Accelerate the decay of building materials, paints, and 

cultural artifacts such as buildings, statues, and 

sculptures. 

Title IV was passed by Congress as part of the 1990 Clean 

Air Act Amendments to reduce emissions of SO2 and NOx 

from fossil fuel-burning power plants in order to protect 

ecosystems suffering damage from acid deposition and to 

improve air quality. At the same time, the National Acid 

Precipitation Assessment Program (NAPAP) was asked to 

periodically assess and report to Congress on the 

implementation of the Acid Rain Program, recent scientific 

knowledge surrounding acid deposition and its effects, and 

the reduction in acid deposition necessary to prevent 

adverse ecological effects. This NAPAP Report focuses primarily on emission reductions from power plants, both in 

terms of assessing past reductions under the Acid Rain 

Program and in projecting the ecological effects of additional reductions of SO2 and NOx. 

It should be noted that power generation 

currently contributes approximately 69% of 

the SO2 emissions and 22% of the NOx emissions nationwide. This contribution is 

decreasing as emissions from power generation continue to decrease, making the other 

sources of these pollutants more prominent. 

Modeling suggests that even if SO2 

emissions from power generation were 

reduced to zero, some lakes and streams 

would remain acidic due to acid deposition. 

However, there are several other regulations 

that reduce emissions of SO2 and NOx from 

these non-power generation sources, such 

as the Tier II mobile source standards, the 

Heavy Duty Diesel standards, and the Clean 

Air Non-Road Diesel Rule, that have also 

been promulgated since 1990. These regulations, primarily designed to bring counties 

into attainment with fine particle and ozone 

air quality standards, also incidentally reduce 

emissions that contribute to acid deposition. 

Implementation of Title IV has successfully 

and substantially reduced emissions of SO2 

and NOx from power generation at a significantly lower cost than expected: 

• In 2002, SO2 emissions were 10.2 million 

tons, 35% lower than 1990 emissions and 

40% lower than 1980 emissions.* 

• In 2002, NOx emissions were 4.5 million 

tons, 33% lower than 1990 emissions. 

In addition, SO2 emissions from all sources 

have decreased by 32% since 1990 and 

emissions of NOx from all sources have 

decreased by 12% since 1990. Power generating sources continue to close in on the 

goal of reducing power plant SO2 emissions 

from 1980 levels by 50% (to 8.95 million 

tons) as required by the 1990 Clean Air Act. 

Power generating sources have also exceed­

* 2002 was the latest year for which emissions data were available when this report was written and reviewed. More recent emissions data are available on 

the EPA Web site at www.epa.gov/airmarkets/emissions. 
ed the goal of a two million ton reduction in 

NOx emissions from projected 2000 levels 

as required by the 1990 Clean Air Act. 

These emission reductions have contributed 

to measurable improvements in air quality, 

reductions in acid deposition, and the beginnings of recovery of acid-sensitive waters in 

some areas: 

• SO2 concentrations in the atmosphere (a 

precursor to fine particles and acid deposition) have decreased since 1990. Average 

annual SO2 concentrations in the 

Northeast in 2000–2002 were 40% lower 

than they were in 1989–1991, concentrations in the mid-Atlantic were 30% lower, 

concentrations in the Southeast were 35% 

lower, and concentrations in the Midwest 

were 45% lower. 

• Sulfate concentrations in the atmosphere (a major component of fine particles, especially in the East) have 

decreased since 1990 as well. Average 

annual sulfate concentrations in the 

Northeast and Midwest in 2000–2002 

were approximately 30% lower than they were in 

1989–1991, and concentrations in the mid-Atlantic and 

Southeast were 25% lower. 

• Wet sulfate deposition, a major component of acid rain, 

has also decreased since 1990. Average annual sulfate 

deposition in the Northeast in 2000–2002 was 40% lower 

than it was in 1989–1991, deposition in the mid-Atlantic 

and Midwest was 35% lower, and deposition in the 

Southeast was 25% lower. 

• Wet nitrate deposition has not decreased regionally from 

historical levels because of the relatively moderate NOx 

reduction from power plants and the continuing large contribution (over 50% of total NOx emissions) from other 

sources of NOx such as vehicles and nonroad vehicles. 

• Although visibility has begun to improve in some parts 

of the U.S., there is still significant impairment of visibility in many national parks and other Class I areas 

throughout the U.S. 

• Acid neutralizing capacity is beginning to rise in some 

surface waters in the Northeast, including lakes in the 

Adirondack Mountains (see graphic below). This is an 

indication that recovery from acidification is occurring in 

those areas. 
• Reductions in fine particles due to reductions in emissions of SO2 and NOx are expected to continue to benefit human health by substantially lowering the incidence 

of respiratory and cardiovascular illnesses. 

Sources in both the more conventional emission rate-based 

NOx program and the cap and trade program for SO2 have 

demonstrated a high level of compliance, and their efforts 

have achieved measurable environmental results. The flexibility for sources to choose how to control their emissions 

inherent in the cap and trade approach for SO2 has been 

successful at reducing compliance costs to a fraction of 

the cost estimated in 1990. It has not resulted in any significant geographic shifts in SO2 emissions. The annual 

cost of the SO2 trading program by 2010 is expected to be 

approximately $1 to $2 billion per year ($2000), far lower 

than the estimated $6 billion annual cost in 1990 ($2000). 

Emissions of SO2 are expected to continue to decline as the 

bank of allowances that was built up in Phase I of the Acid 

Rain Program (as a result of reductions beyond those 

required) is largely depleted. At that point, annual emissions 

will approximate annual allocations of emission allowances. 

Thus, annual emissions will equal 8.95 million tons (the level 

of the cap). Air quality, visibility, and ecosystems are expected to continue to improve as emissions decrease, and deposition is also expected to continue to decrease. A recent 

estimate of the health benefits of Title IV in 2010 indicates 

that the benefits are substantial, including thousands fewer 

premature deaths and hospital admissions and emergency 

room visits. 

Despite anticipated environmental improvements, research 

over the last few years indicates that recovery from the 

effects of acidification is not possible for many areas that 

continue to be exposed to acid deposition. Many published 

articles, as well as the modeling presented in this report, 

show that the SO2 and NOx emission reductions achieved 

under Title IV from power plants are now recognized as insufficient to achieve recovery or to prevent further acidification 

in some regions. Additional SO2 and NOx emission reductions 

from power plants and other source sectors are needed to 

improve air quality, reduce deposition, and further reduce the 

number of acidic lakes and streams in many regions of the 

U.S. Some of these additional emission reductions may be 

achieved through implementation of existing or future regulations to address transport of ozone and fine particles and 

mercury deposition, including the NOx SIP call in the Eastern 

U.S., tier II and diesel rules affecting mobile sources, state 

implementation plans to achieve the ozone and fine particle 

National Ambient Air Quality Standards, and the recent rules 

to reduce interstate transport of fine particles and ozone and 

mercury from power plants. 

Long-Term Environmental 

Monitoring 

Emissions, air quality, deposition, and ecological monitoring are critical components of 

Title IV. These monitoring efforts allow 

researchers and policymakers to assess the 

effectiveness of the Title IV program. 

Emissions monitoring is conducted by 

affected sources; the other types of monitoring are conducted by a wide variety of 

federal and state agencies, universities, and 

other organizations. The agencies of the 

National Acid Precipitation Assessment 

Program (NAPAP) continue to have a strong 

commitment to the research and monitoring 

that makes assessments like this NAPAP 

Report to Congress possible. 

Key Components of a 

Model Program 

Title IV serves as a model structure of an air 

pollution control program because of its significant and measurable progress towards 

meeting its emission reduction goals and the 

low costs associated with its implementation. 

The program’s guiding principles of the cap 

on emissions, complete accountability, and 

simplicity are largely responsible for its 

effectiveness. The cap is the core prerequisite of an effective and efficient trading program that guarantees the environmental 

goals are achieved and sustained. 

Accountability, including reliable emissions, 

air quality, atmospheric deposition, and ecological monitoring and regular assessment of 

progress towards the goals, promotes both 

public confidence in the environmental 

integrity of the program and business confidence in the financial integrity of the 

allowance market. The simple, clear, 

straightforward nature of Title IV has resulted in high levels of compliance, low administrative costs, and timely achievement of its 

environmental goals. 
Introduction 

NAPAP Mission 

The National Acid Precipitation 

Assessment Program (NAPAP) is 

comprised of the U.S. Environmental 

Protection Agency, the U.S. Department of 

Energy, the U.S. Department of 

Interior/U.S. Geological Survey, the U.S. 

Department of Interior/National Park 

Service, and the National Oceanic and 

Atmospheric Administration. Under Title IX 

of the 1990 Clean Air Act Amendments, 

Congress reauthorized NAPAP to continue 

coordinating acid rain research and monitoring, as it had done during the previous 

decade, and to provide Congress with 

periodic reports. In particular, Congress 

asked NAPAP to assess all available data 

and information to answer two questions: 

• What are the costs, benefits, and 

effectiveness of Title IV? This question addresses the costs and economic impacts of 

complying with the Acid Rain Program as 

well as benefit analyses associated with 

the various human health and welfare 

effects, including reduced visibility, 

damages to materials and cultural 

resources, and effects on ecosystems. 

• What reductions in deposition rates are 

needed to prevent adverse ecological 

effects? This complex question addresses 

ecological systems and the deposition levels 

at which they experience harmful effects. 

The results of the assessment of the effects 

of Title IV and of the relationship between 

acid deposition rates and ecological effects 

were to be reported to Congress quadrennially, beginning with the 1996 report to 

Congress. The objective of this Report is to 

address the two main questions posed by 

Congress and fully communicate the results 

of the assessment to decision-makers. Given 

the primary audience, most of this report is 

not written as a technical document, 

although information supporting the conclusions is provided 

along with references. 

Impacts of SO2 and NOx Emissions 

Title IV of the 1990 Clean Air Act Amendments (CAAA) 

requires significant reductions of sulfur dioxide (SO2) and 

nitrogen oxide (NOx) emissions. These emissions contribute 

to acid deposition, ultimately leading to a wide range of 

environmental impacts, as well as the formation of fine particles and gases that can harm health and impair visibility. 

Some places and people are more susceptible or sensitive to 

the impacts of SO2 and NOx emissions than others. Some 

people (especially children, the elderly, and those with existing respiratory or cardiovascular conditions) are more likely 

to be harmed by fine particles formed from SO2 and NOx 

emissions and ozone formed from NOx emissions. 

Areas where acid deposition damages ecosystems or where 

gases and particles impair visibility are called “sensitive ecological receptors.” Sensitive ecological receptors include 

lakes and streams throughout the Appalachian Mountains; 

forests in the Appalachian Mountains, the Colorado Front 

Range, and western coastal mountain ranges; and many 

east and gulf coast estuaries and coastal waters (see 

Figure 1). Many national parks and wilderness areas, including Shenandoah National Park, Great Smoky Mountains 

National Park, Acadia National Park, and Grand Canyon 

National Park have impaired visibility due in part to 

emissions of SO2 and NOx from power generation sources. 

Adirondack Park has a large number of lakes that have been 

damaged by acidic deposition. 

Many of these health and environmental impacts can occur 

hundreds of miles from where the pollutants are emitted. 

This long-range transport makes it critical to reduce 

emissions that contribute to acid deposition and other environmental impacts, even those that occur far from sensitive 

waters, forests or population centers. The purpose of Title 

IV is to reduce the ecological impacts of acid deposition by 

reducing emissions of SO2 and NOx from power generation 

sources. These emission reductions have and will continue 

to benefit the nation by: 

• Improving air quality and protecting public health 

• Restoring acidified lakes and streams so they can once 

again support fish and other aquatic life 
• Improving visibility, especially at scenic vistas in national 

parks 

• Reducing the damage to sensitive forests, such as those 

along the Appalachian Mountains and in the Colorado 

Front Range 

• Reducing the damage to nitrogen-sensitive coastal 

waters along the east and gulf coasts 

• Protecting our historic buildings and monuments from 

degradation 

Structure of the Report 

This report is directed to Congress but provides valuable economic and scientific information to all public officials who are 

responsible for determining or evaluating air quality policy. Its 

goal is to present highly technical information pertinent to 

current public policy issues in a format that can be understood 

by the nonscientific reader. Where more scientific or economic 

detail is desired, references are noted in the text with superscripts and provided at the end of the report. 

The status of implementation of Title IV by EPA and the affected utilities at the end of 2002 is presented in the first 

chapter. This is followed by an analysis of the observed 

changes, both past and present, in emissions 

of acid rain precursors, air pollutant concentrations, deposition of acidic species, and 

various ecological endpoints, particularly 

surface water quality, in the second chapter. 

The third chapter covers advances in the 

state of the science regarding atmospheric 

deposition monitoring, the impacts of acid 

deposition on aquatic and terrestrial 

ecosystems, visibility, materials and cultural 

resources, and human health. The fourth chapter presents estimates of the changes in air 

quality, deposition, freshwater acidification, 

and the human health benefits of full implementation of Title IV in 2010. The fifth chapter addresses the question posed by Congress 

in the 1990 CAAA regarding the ecological 

impacts of further emission reductions and 

the uncertainties in the estimated 

relationships between emission reductions 

and the resulting ecological effects. 
Chapter 1: Program Elements 

and Implementation 

Title IV has been successfully implemented according to the rules and guidance established by 

the Acid Rain Program. Emissions have 

decreased significantly since 1990, and source 

compliance has been extremely high since the 

program began. The market-based system for 

reducing SO2 emissions is functioning well; 

there is a significant amount of market activity 

and SO2 allowance prices, which have varied 

from $65–$210 per ton, are significantly lower 

than had been predicted in 1990. 

Emission Reductions 

Title IV has successfully reduced emissions 

of SO2 and NOx from power generation 

sources. In 2002, there were approximately 

3,200 electric power units subject to the 

SO2 provisions of Title IV.* By 2002, these 

sources had reduced their combined SO2 

emissions by approximately 40% from 1980 

levels and 35% from 1990 levels. Although 

not all sources in the SO2 trading program 

are also covered by the NOx program, the 

sources in the SO2 program reduced their 

NOx emissions by 33%. The approximately 

1,000 sources affected by Title IV NOx 

requirements reduced their NOx emissions 

by approximately 27% from 1990 levels in 

2002. The emission reductions achieved under Title IV are discussed in more detail in 

Chapter 2. 

Program Design 

SO2 PROGRAM 

The SO2 component of Title IV represents a dramatic departure from traditional command and control regulatory 

approaches that establish source-specific emission 

limitations. Instead, the program uses an overall emissions 

cap for SO2 that ensures emission reductions are achieved 

and maintained as well as a trading system that facilitates 

lowest-cost emission reductions. The program features tradable SO2 emission allowances, where one allowance is a limited authorization to emit one ton of SO2. A fixed number of 

allowances are issued by the government, and they may be 

bought, sold, or banked for future use by utilities, brokers, or 

anyone else interested in holding them. Existing units are allocated allowances for each year; new units do not receive 

allowances and instead must buy them. Recent research on 

the dynamics of the SO2 allowance market indicates that this 

is not a substantial barrier to entry; i.e., new sources have 

had no trouble acquiring the allowances they need to compete 

effectively in the market.1 At the end of the year, all affected 

sources are obliged to surrender to the EPA the number of allowances that correspond to their annual SO2 emissions (one 

allowance for each ton of SO2). 

Title IV requires a two-phased tightening of allowances (or 

SO2 emission allocations) to fossil fuel-fired power plants in 

order to reach the permanent cap on the number of 

allowances at 8.95 million tons annually. Once the bank of 

unused allowances is depleted, the cap on allocations limits 

emissions to the level of the cap (8.95 million tons). This 

permanent cap is approximately half the amount of SO2 

emitted by these power plants in 1980. Phase I of the SO2 

program (1995–1999) affected 263 of the larger (>100 

megawatt (MW)), higher emitting units which are located 

primarily in the Eastern United States. Phase I SO2 

allowance allocations were distributed to each source based 

on the following formula: the product of an emission rate of 

2.5 lb SO2/million British thermal units (mm BTU) of heat 

input and its average heat input for 1985–1987. Some 

Phase II sources chose to “opt-in” to Phase I and comply 

early, bringing the total number of units in Phase I to over 

400.** Phase II began in 2000 and extends to all existing 

* These units, as well as an additional 273 units which were retired or not yet operating, are listed in Appendix A of the annual Acid Rain Program 

Progress Report available at www.epa.gov/airmarkets/cmprpt. 

**The number of sources in Phase 1 varied slightly by year due to changes in the number of substitution, compensating, and opt-in units in any given 

year. These units, which were not identified in the 1990 Clean Air Act as Phase I sources, entered the SO2 or NOx programs early in order to 

reduce costs or take advantage of the SO2 allowance market. 
electric utility units serving generators larger than 25 MW 

and all new fossil fuel-fired generation units throughout the 

country. This has brought the total number of units covered 

by the SO2 program to approximately 3,200 sources (see 

Figure 2). In Phase II, all Phase I and Phase II SO2 affected 

sources are allocated allowances equivalent to an amount 

no greater than the product of 1.2 lb SO2/mm BTU and 

their average heat input for 1985–1987. 

Affected sources must demonstrate compliance with the 

SO2 provisions of Title IV at the end of each year. Sources 

are granted a 60-day grace period during which additional 

SO2 allowances may be purchased, if necessary, to cover 

each unit’s emissions for the year. At the end of the grace 

period (the Allowance Transfer Deadline), the allowances a 

unit holds in its Allowance Tracking System (ATS) account 

must equal or exceed the unit’s annual SO2 emissions for 

the previous year. Title IV requires affected sources to monitor emissions continuously and to report their emissions 

quarterly. Failure to surrender sufficient allowances results 

in two significant automatic penalties: a fine and a reduction 

in the number of allowances allocated in the following year. 

Any remaining SO2 allowances not needed for compliance 

may be sold or banked for future use. Sources may use their 

banked allowances as needed to comply with the program in 

future years until the bank is depleted. 

NOX PROGRAM 

There are no tradable emission allowances in 

the NOx reduction portion of the Acid Rain 

Program. Instead, sources control how much 

NOx is emitted from coal-fired boilers based 

on the use of cost-effective control technologies for each unit of fuel consumed (lb 

NOx/mm BTU). There were also two phases 

to the NOx control program: Phase I began 

in 1996 (delayed one year because of litigation) and Phase II in 2000. During Phase I, 

which applied to specific coal-fired boilers 

statutorily affected by Phase I SO2 requirements, the NOx emissions rate was set at 

0.50 lbs/mm BTU for dry bottom wall-fired 

units and 0.45 lbs/mm BTU for tangentiallyfired units. Beginning in 2000, Phase II 

plants were required to meet emissions rates 

between 0.40 lbs/mm BTU and 0.86 lbs/mm 

BTU depending on the type of boiler. 

Although the NOx program does not include 

emission trading, sources are provided a degree of flexibility through emission averaging 

provisions, whereby a company can meet its 

emission limitations by averaging the 
emission rates of two or more boilers. This 

allows sources to reduce their emissions at 

lower cost by allowing them to over-control 

at units where it is technically easier to control emissions. In addition, specific Phase II 

units elected to comply with Phase I limits 

beginning in 1997. These “early election 

units” are not subject to the more stringent 

Phase II limits until 2008. In 2002 there were 

over 1,000 units required to meet a NOx 

emission limit under Title IV provisions. 

Since NOx emission reductions are achieved 

by limiting the rate of NOx output per unit of 

heat input and not through an emission cap, 

emissions could increase through a combination of increased utilization of existing units 

and/or construction of new units. 

Sources affected by the NOx portion of Title 

IV must also demonstrate that they have complied with the NOx provisions at the end of the 

year. Sources demonstrate compliance with 

the NOx program by achieving an annual emission rate at or below mandated levels as outlined in their EPA-approved compliance plans. 

Title IV Affected Sources 

Generally, boilers or combustion turbines that 

burn fossil fuel, serve generators with nameplate capacity greater than 25 MW, and produce electricity for sale are subject to the 

Title IV SO2 requirements. There are several 

types of units, however, that meet these criteria that are not affected by the program: 

• Simple combustion turbines, regardless 

of their nameplate capacity, if they began 

to produce electricity for sale before 

November 15, 1990. 

• Cogeneration units whose annual electrical sales remain below the threshold 

established by regulation (one-third of operating capacity or 219,000 MW per hour, 

whichever is greater). 

• Specific qualifying facilities and 

independent power producers that are 

contractually bound to sell electricity at a 

price that was established before 

November 15, 1990. 

Despite these exceptions, almost all non-cogeneration units 

built in the future that serve generators with total 

nameplate capacity greater than 25 MW and produce electricity for sale will be affected units that must participate in 

the Acid Rain Program. Of the units subject to the SO2 portion of Title IV, some are also covered by the Title IV NOx 

Program requirements. All units where coal accounted for 

more than 50% of its heat input for at least one year during 

the 1990 through 1995 time period and that are configured 

for a specific type of boiler (cell burner, cyclone, dry bottom 

wall-fired, tangentially-fired, vertically-fired, or wet bottom) 

are affected by the Title IV NOx Program. For more details 

on Acid Rain Program applicability criteria, see 40 CFR 

72.6, the Acid Rain Program applicability regulations established under Title IV. 

Compliance 

SO2 PROGRAM 

The Title IV SO2 program has seen a near-perfect 

compliance record since the program took effect in 1995. 

During the compliance process, the number of allowances 

surrendered at an individual unit must be equal to the number of tons emitted at the unit.* During Phase I 

(1995–1999) compliance was 100%; in each year since the 

beginning of Phase II a small number of units (1–6, depending on the year) were short allowances to cover their emissions. For example, in 2002, due to an administrative error 

on the part of the company, a single unit was short a total 

of 33 allowances to cover its emissions for the 2002 

compliance year. Thirty-three year 2003 allowances were 

taken from this unit as an “offset.” In addition to the offset, 

the operator of this unit was assessed an automatic monetary penalty totaling over $90,000 ($2002) as prescribed in 

the Clean Air Act. 

The flexibility provided by Title IV enables the sources subject to the Title IV SO2 program to pursue a variety of compliance options, some of which, such as process innovation, 

might have been discouraged under other types of control 

programs.2 Sources meet their SO2 reduction obligations by 

installing scrubbers, running existing scrubbers more 

efficiently, switching fuels, changing practices or 

procedures to improve energy efficiency, and buying 

allowances. Most sources are complying with the SO2 

requirements in Title IV by blending or switching fuels. In 

2001 this accounted for approximately two-thirds of the 

SO2 emission reductions (see Figure 3). Scrubbers installed 

on 30 Phase I units accounted for almost all of the remaining emission reductions in 2001.1 

* The annual deductions for emissions at each unit after the common stack apportionment was made can be found in Appendix A of the most recent annual 

Acid Rain Program Progress Report available at www.epa.gov/airmarkets/cmprpt. Units sharing a common stack are listed directly under the entry for 

their common stack. 
NOX PROGRAM 

Instead of using allowance trading to facilitate NOx 

emission reductions, Title IV establishes limits on NOx 

emission rates (in lb NOx/mm BTU) for coal-fired electric 

generating units. The various ways a source can meet its 

emission limit are described below. 

• Standard Limitation. A unit with a standard limit simply 

meets the applicable individual NOx limit prescribed for 

its boiler (Group 1: dry bottom wall-fired and tangentiallyfired boilers; and Group 2: cell burner, cyclone, verticallyfired, and wet bottom boilers) under the Title IV NOx 

regulation (40 CFR part 76). 

• Early Election. Under this compliance option, a Phase II 

Group 1 NOx affected unit meets a less stringent Phase I 

NOx limit beginning in 1997, three years before it would 

normally be subject to an Acid Rain NOx limit. In return 

for accepting a NOx limit three years earlier than would 

normally be required, an early election unit does not 

become subject to the more stringent Phase II NOx limit 

until 2008. 

• Emissions Averaging. A company can meet its NOx emissions reduction requirements by choosing to make a 

group of NOx program affected boilers subject to a group 

NOx limit, rather than meeting individual NOx limits for 

each unit. The group limit is established at 

the end of each calendar year, and the 

group rate for the units must be less than 

or equal to the BTU-weighted rate at 

which the units would have been limited 

had each been subject to an individual NOx 

limit. 

• Alternative Emission Limitation (AEL). A 

utility can petition for a less stringent AEL 

if it properly installs and operates the NOx 

emissions reduction technology prescribed 

for that boiler but is unable to meet its 

standard limit. EPA determines whether an 

AEL is warranted based on analyses of 

emissions data and information about the 

NOx control equipment. 
The NOx program has also had a high rate of 

compliance. For example, in 2002, 1,047 

units met their NOx emissions limits through 

compliance with their respective NOx compliance plans. Only one unit failed to meet its 

NOx emissions limit in 2002. That unit had 

excess NOx emissions of 47 tons and was 

assessed a monetary penalty totaling over 

$133,000 ($2002)*. Emissions averaging 

was the most widely chosen compliance option for NOx affected units in 2002; more 

than 50 emission averaging compliance 

plans involving over 600 units were in place 

in 2002. 

SO2 Allowance Market 

The number of allowances (authorizations to 

emit SO2) allocated to each source in any 

given year is determined by the Clean Air 

Act. The presence of the allowance market 

has given some sources the incentive to reduce their SO2 emissions below the level of their allowance 

allocation in order to sell their allowances to other sources 

or bank them for use in future years. Other sources have 

been able to postpone or reduce expenditures for pollution 

control by purchasing allowances from sources that 

controlled beyond their allowance allocation level. The SO2 

allowance market is generally thought of as relatively wellfunctioning due to the “clear, consistent rules that emphasize transparency, fungibility, and market performance 

[that] have been the key factors in creating the investor 

certainty...”3 

Under Title IV, the “bank” or store of unused allowances 

grew throughout Phase I (1995–1999) as sources reduced 

emissions more than required. These “early reductions” reduced the amount of fine particles and acid deposition 

reaching sensitive ecological receptors in the early years of 

the acid rain program, increasing the human health and 

ecological benefits of the program in those years. 

Beginning in 2000, the tighter emission cap in Phase II took 

effect, and sources began to use previously banked 

allowances in addition to allocations from the current year 

in order to comply with Title IV. 

* Detailed annual compliance information by unit can be found in Appendices B1 and B2 of the most recent annual Acid Rain Program Progress Report 

available at www.epa.gov/airmarkets/cmprpt. 
In 2002, a total of 9.5 million allowances were granted to 

sources as prescribed in Title IV. Adding these 2002 

vintage allowances to the unused allowances banked from 

prior years, a total of 18.8 million allowances were 

available for use in 2002. Sources emitted 10.2 million 

tons in 2002, 650,000 tons more than the allowances 

granted in 2002, but far less than the allowable level. 

Figure 4 shows the changing size of the allowance bank 

from 1995 through 2002. 

In a well-functioning market, the price of an allowance 

should equal the expected marginal cost of compliance, the 

cost of reducing the next ton of SO2 emitted from the utility 

sector. Emission reductions continue to cost less than anticipated when Title IV was enacted, and this is reflected in the 

price of an SO2 emission allowance. The cost of an 

allowance was initially estimated at $250 to $400/ton 

during Phase I and at $500 to $1,000 for Phase II.4 As 

shown in Figure 5, actual prices have been significantly lower than that. The market price for allowances has tended to 

fluctuate between $100 and $200 per ton and was as low as 

$65 in 1996.* 

Allowance prices for 2002 continued the downward trend 

that started in the second half of 2001. Prices hovered in 

the $170/ton dollar range early in 2002 and 

decreased slightly during the summer 

months, ending the year in the $130/ton 

range. Prices stabilized back to historical averages in 2002 after the more stringent limits 

in Phase II resulted in higher average prices 

in 2001. Some market observers believe lower than expected allowance prices during the 

first several years of the program were due 

primarily to lower than expected compliance 

costs (due mostly to increases in the 

availability of low-sulfur coal) and larger than 

expected emission reductions, which 

increased the supply of allowances and put 

downward pressure on prices.5 Market 

observers believe that technological innovation that resulted in lower than expected 

costs to operate scrubbers has also 

contributed to the lower allowance prices.6 

The level of activity in the allowance market 

created under Title IV increased fairly steadily through 2000 to over 30 million transfers 

and then leveled off somewhat in 2001 and 

* All allowance prices in this report are reported in current year dollars for the year in which they occurred, unless otherwise indicated. 
2002, the second and third years of Phase II 

(see Figure 6). The number of transfers between economically unrelated 

organizations—i.e., trades other than those 

between units or subsidiaries belonging to a 

single company—also rose fairly steadily 

from 1994 through 2000 before leveling out 

in Phase II. In most years, trades between 

economically unrelated organizations 

accounted for half or more of all trades. 

In 2002, over 5,700 allowance transfers that 

affected over 21 million allowances (of past, 

current, and future vintages) were recorded 

in the Allowance Transfer System, the 

accounting system developed to track holdings of allowances. Of the allowances transferred, 11.6 million, or 54%, were transferred 

in economically significant transactions (i.e., 

between economically unrelated parties). 

The majority of the allowances transferred in 

economically significant transactions were 

acquired by utilities. In December 2001, 

trading parties began to use the On-line 

Allowance Tracking System (OATS). By the 

end of 2002, OATS recorded 79% of all 

transfers electronically over the Internet. All official 

allowance transactions, as well as data on account 

balances and ownership, are posted and updated daily at 

www.epa.gov/airmarkets in order to better inform trading 

participants and the public of the status of the market. 

Cumulative market statistics and additional analyses are 

also available. It should be noted that some allowance 

transfers are not included in this data because they are not 

recorded or publicly declared until the allowances are used 

for compliance purposes. 

Program Costs 

The cap and trade mechanism in Title IV for reducing SO2 

emissions has led to lower than expected costs as well as to 

cost savings compared to conventional regulatory approaches. 

Emissions trading gives electricity generators the flexibility to 

choose between reducing emissions at their own units and 

purchasing allowances from other units that can reduce emissions at lower cost. Thus, with efficient trading, each unit faces 

the same marginal cost of compliance. In addition, there are no 

technological or permitting restrictions on the means by which 

SO2 emissions are reduced. Units under Title IV have many 

compliance options, including switching to a different type of 

fuel, switching the grade of fuel, or installing equipment that 

removes SO2 after it is created (e.g., scrubbers). 
PHASE I COSTS 

Early projections of annual Phase I compliance costs ranged 

from just under $678 million7 to $1,511 million8 ($2000). 

These cost estimates took into consideration the allowance 

trading provisions of Title IV, unlike earlier higher cost 

estimates for the same level of SO2 reductions under more 

restrictive regulatory options. The most recent ex post study 

of Phase I costs found that in 1995, Phase I units reduced 

emissions of SO2 by 3.9 million tons at an estimated cost of 

$814 million ($2000). The cost of this emission reduction 

was approximately 50% less than it would have been under a 

command and control program.4 An earlier study produced 

similar results, estimating total compliance costs to be $940 

million ($2000), although the costs could have been lower 

still with fully efficient trading.9 Both estimates of actual 

Phase I costs account for the “over-compliance” that 

occurred in Phase I while building up a bank of unused 

allowances (used to reduce costs of compliance in Phase II) 

and are at the lower end of the range of expected results. 

PHASE II COSTS 

Early proposals to reduce SO2 emissions that mandated 

scrubbers or other command and control options were estimated to cost as much as $7.5 billion/year ($2000).4 The 

first EPA estimate (1990) for annual Phase II costs was 

approximately $6 billion. As the approach of Phase I 

neared, the estimates for Phase II costs 

declined with the General Accounting 

Office (GAO) revising the estimate to 

approximately $2.5 billion per year in 2010 

($2000)10 (see Figure 7). 

Current estimates of the cost of full 

implementation of Title IV SO2 emission 

reductions are significantly lower than 

originally estimated. Independent analyses 

have estimated costs in the range of $1 to $2 

billion per year in 2010 ($2000) when the program is nearly fully implemented, substantially less than was predicted in 1990. The most 

recent estimates provided by Ellerman and by 

Carlson are $1.3 to $1.5 billion per year 

($2000) and $1.1 billion per year ($2000) by 

2010, respectively.

4,9 A recent Office of 

Management and Budget (OMB) analysis estimated costs of the SO2 program between 

$1.1 and $1.8 billion ($2000). 

NOx program costs will add to total program 

costs. EPA expects cost for NOx reductions to 

be no more than $1 billion ($2000) annually, 

and based on the limited analysis that has 

been done, likely to be substantially less. 
Several factors are responsible for the 

relatively low cost SO2 reductions that have 

been realized under Title IV. 

Low Sulfur Fuel: Switching to low-sulfur 

coal was the most widely used means of reducing SO2 emissions under Phase I and is 

continuing under Phase II.5 Historically, lowsulfur sub-bituminous coal from the Powder 

River Basin of Wyoming and Montana has 

been relatively inexpensive to mine but costly to transport long distances. In the 1980s, 

railroad deregulation increased competition 

and opened the possibility for lower industry 

freight rates, which would make low-sulfur 

western coal competitive for units in the 

Midwest and East. The demand for low-sulfur coal coupled with deregulation helped 

drive rates down during Phase I. Title IV facilities have taken advantage of reduced 

costs in complying with the SO2 limits under 

both Phase I and Phase II.9 Units have also 

taken advantage of mid-sulfur bituminous 

coal from the Midwest and Central 

Appalachia because, despite its higher sulfur 

content than western coal, mid-sulfur coal 

requires fewer modifications to boilers and 

operating procedures.5 

Scrubber Costs. The costs of scrubbing 

SO2 are lower than anticipated.5 Prior to 

Title IV, the initial capital investment for 

installing scrubbers was expected to be prohibitively high. As Phase I continued, these 

initial capital costs decreased, as did the 

marginal cost of scrubbing an additional ton 

of SO2 once the technology was installed. It 

was estimated that scrubbing SO2 from existing units would cost over $400/ton 

($1994) SO2.

5 The actual price per ton of 

SO2 scrubbers in 1995 was on the order of 

$280 ($1994); i.e., 40% less than predicted. 

Banking. Another aspect to the Title IV program that has led to cost savings in comparison with other regulatory approaches is the 

option to save or “bank” unused allowances 

for future use. Over-compliance in Phase I, 

particularly for those units that installed 

scrubbers, enabled banking of a large 

number of allowances forward to Phase II. 

This is expected to lower compliance costs during Phase II 

despite the more stringent national cap on overall SO2 emissions. According to recent research, the overall amount of 

banking during Phase I was roughly efficient over time given 

the current and projected costs of compliance.11 

Technological Innovation. The technological improvements 

that facilitated coal switching and the speed at which they 

emerged were also not anticipated.9 By experimenting with 

blends of low- and high-sulfur coal that did not lower generating capacity, operators were able to take advantage of 

the coincidental drop in freight costs for western coal and 

increased production of mid-sulfur coal in the Midwest. 

Industry also came up with innovative ways to lower operation and maintenance costs for scrubbers, and ways to 

increase their utilization rate and efficiency.

4 

Allowance Market. To be fully realized, the anticipated 

cost-saving benefits of the flexibility provided under Title IV 

required the creation of an efficient high-volume market for 

allowances. This has indeed been the case, as discussed 

above. Since the inception of the program trading has been 

very active, and trading between distinct organizations has 

increased in recent years. 

ADMINISTRATIVE COSTS 

The costs to the government of administering the Title IV 

SO2 program are less than in conventional regulatory 

programs. For example, Title IV’s performance-based 

approach eliminates the need to devise source-specific 

emission limits, review control technologies, and prepare 

and approve detailed compliance schedules and permits. In 

addition, eliminating case-by-case review and approval of 

each trade (including determining the “useful life” of equipment, the intent of the sources regarding future emission 

and activity levels, and “real” emission reductions expected 

to be achieved) greatly reduces the administrative and 

transaction costs associated with emissions trading 

programs.12 During Phase I, costs of the program were 

roughly $12 million/year or approximately $1.50/ton of pollution reduced.13 Most of the functions in the allowance 

market are carried out by private entities; the government’s 

role is primarily to ensure that units are monitoring 

emissions throughout the year and to ensure that each unit 

has enough allowances to cover emissions at the end of the 

year. In 2003, EPA estimates that sources, as a group, 

spent approximately $250 million on administrative 

functions of the program, including allowance transfers, 

permitting, emissions monitoring, compliance, and other 

administrative costs. 
Program Assessment 

EMISSIONS MONITORING 

Title IV includes strict emission monitoring requirements to 

guarantee compliance with both the cap and trade and 

emission rate limit components of the program. Most 

affected sources installed required monitoring equipment 

on time; the few that did not were subject to stringent 

emission estimation procedures. The monitoring regulations 

that implement Title IV include strict procedures to 

estimate emissions when approved methods are not used. 

These procedures, which overestimate total emissions, provide a strong incentive to install and maintain accurate 

monitoring equipment, minimize emission monitor downtime, and ensure that emissions are not under-reported. All 

monitors are required to meet stringent initial and ongoing 

performance standards to demonstrate the accuracy, precision, and timeliness of their measurement capabilities. The 

monitors used by sources to comply with Title IV have 

achieved an unparalleled level of performance with respect 

to all of these criteria. 

Sources have now completed installation of all continuous 

emission monitors (CEMs) required under the Acid Rain 

Program. Coal-fired units must use CEMs to measure 

concentrations of SO2 and NOx, as well as volumetric flow, 

to determine hourly mass emissions of SO2, hourly NOx 

emission rates, and hourly heat input. Coal-fired units may 

estimate their CO2 emissions, but virtually all have chosen 

to use CEMs to measure them instead. Oil- and gas-fired 

steam units may either use CEMs or they may use certified 

fuel flow meters and frequent fuel sampling analyses to determine the hourly SO2 and CO2 mass emissions and hourly 

heat input. Most oil- and gas-fired steam units are required 

to use CEMs to determine hourly NOx emission rates. 

However, for an oil- or gas-fired peaking unit, the owner or 

operator may develop a correlation curve between heat 

input and NOx emission rate, in lieu of installing a CEM. If 

this option is selected, the hourly heat input is monitored using a certified fuel flow meter, and the correlation curve is 

used to estimate the hourly NOx emission rate.14 

One measure of the accuracy of a CEM is the relative accuracy test audit (RATA), which is required for initial certification 

of a CEM and on at least an annual basis, thereafter. The 

RATA ensures that the installed monitor measures the “true” 

value of a pollutant by comparing the monitor to a reference 

method, which simultaneously measures the stack gas pollutant. All monitoring systems must meet a relative accuracy standard allowing no more than 

10% deviation from the true value in order to 

continue to be used for emissions reporting. 

Further, if the CEM is biased low compared to 

the true value, a bias adjustment factor must 

be applied to all future data from that monitoring system to ensure there is no underreporting. This “self-correcting” provision, coupled 

with daily quality assurance testing 

requirements, creates a strong incentive to 

maintain monitor performance (since failure to 

meet the accuracy requirements can result in 

sources being required to surrender more 

allowances at year’s end) and helps insure 

that each allowance is truly equivalent to a 

single ton of SO2. 

AIR QUALITY, DEPOSITION, AND 

ECOLOGICAL MONITORING 

Air quality and deposition monitoring are also 

important components of the overall 

implementation of Title IV. Several monitoring 

networks designed to measure changes in air 

quality and acid deposition as a result of 

emission reductions are currently in 

operation. There is also a surface water 

monitoring network in acid-sensitive areas in 

the Eastern U.S. that measures changes in 

lake and stream chemistry as a result of 

changes in emissions and atmospheric deposition. Finally, many researchers continue to 

conduct studies to assess the impacts of 

emission reductions on lakes, streams, 

forests, and coastal ecosystems. Together, 

this information allows policymakers to accurately assess the impact of Title IV and other 

air quality policies and to determine if the environmental goals are being achieved. 

Additional and more detailed information 

about the monitoring networks is presented 

in Chapter 2. In addition, there is ongoing 

work in the U.S., Canada, and Mexico to further evaluate the air quality and deposition 

impacts of pollutants that cross over international boundaries. 
Developing the Infrastructure for 

Long-Term Monitoring 

There are two primary national monitoring 

networks that measure regional trends in acid 

deposition and air quality. The National 

Atmospheric Deposition Program (NADP) measures acid and other chemical concentrations and 

annual deposition in precipitation, and the Clean 

Air Status and Trends Network (CASTNET) 

measures air quality concentrations and dry deposition of nitrogen and sulfur compounds. The 

data from these monitoring networks are the primary way for researchers and policy analysts to 

assess the environmental benefits of Title IV and 

any other regional or national emission reduction 

programs such as the NOx State Implementation 

Plan (SIP) call. 

PLANNING FOR AN AGING NETWORK: 

NADP’S REFURBISHMENT 

The original NADP deposition monitoring 

stations were installed in 1978, with a large 

portion of the network in place by 1984. The 

current sampling equipment has been operating 

continuously in the field for an average of 17 

years without substantial upgrade or refurbishment. A major refurbishment and/or 

replacement program is needed to prevent the 

aging equipment from limiting the network’s 

ability to collect high quality data. In order to 

begin the process of upgrading the network, 

testing of off-the-shelf modern precipitation 

recording gauges is now underway. The design 

of a new deposition sampler began in 2000. 

The overall objective is to develop and deploy, 

on a network-wide basis, a modern and more 

flexible deposition sampling system, one that 

would not only support all current NADP sampling needs, but also provide the capability to 

sample additional pollutants as the need arises. 

ADVANCING CASTNET’S MONITORING CAPACITY 

The original CASTNET monitoring stations were 

established in 1987. CASTNET plays a vital role 

in providing regional concentrations of important 

particle and gaseous chemical species, 

deposition estimates of important acidic species, 

rural ozone, and regional transport. CASTNET 

continues to provide accountability and performance measure-related information for the Acid 

Rain Program and is in position to provide deposition and air quality baselines to support future 

accountability and program evaluation needs 

(whether regulatory or legislative). As such, 

both near- and long-term program goals involve 

advancing the monitoring and assessment capabilities of the network. Specifically, plans for 

CASTNET include a multi-agency effort between 

EPA, NOAA, The National Park Service and the 

United States Forest Service to add high-resolution, continuous measurements of gases and 

particles to the suite of measurements conducted at each site. These additional measurements 

are important in expanding our understanding of 

fine particulate formation, transport, and air 

quality model evaluation. This change will also 

provide critical information for implementation of 

an integrated monitoring strategy for key monitoring programs in the U.S. (e.g., IMPROVE, 

NADP) and Canada. 
Chapter 2: Results of the Acid Rain 

Program: Status and Trends of Emissions 

and Environmental Impacts (1990–2002) 

Both SO2 and NOx emissions from power generation sources have significantly declined under Title IV. In 2002, SO2 emissions from Title 

IV-affected sources totaled 10.2 million tons 

and NOx emissions from all Title IV-affected 

sources totaled 4.5 million tons, down 35% and 

33% respectively from 1990 levels. Sources in 

states with the highest emissions continue to reduce their emissions the most, and there have 

been no significant geographic shifts in 

emissions. The benefits of these emission reductions include improvements in air quality 

(which are expected to lead to significant 

human health benefits), broad-scale reductions 

in sulfate deposition, and improvements in visibility. While surface waters in some areas have 

begun to show signs of recovery from acidification, acidification is still occurring in many 

areas. Although there have been no broad-scale 

regional reductions in nitrogen deposition, nitrogen deposition has declined in some areas, benefiting some nitrogen-sensitive forests and coastal 

waters and acid-sensitive lakes and streams. 

Emissions 

SO2 EMISSIONS 

SO2 emissions declined significantly in 

1995, the first year Title IV took effect. 

Emissions climbed slightly from 1995 levels 

during the late 1990s but remained far 

below the number of allowances allocated. 

These “extra” reductions allowed sources to 

build up a large bank of unused allowances 

that could be used in the future when the 

emission cap decreased during Phase II. 

Beginning in 2000, the first year of Phase II, 

sources complied with the lower cap by 

decreasing emissions significantly as well as by using some 

of their banked or stored allowances. In 2002, the 3,208 

sources in the SO2 program had reduced their SO2 emissions by 41% (7.1 million tons) compared to 1980 levels 

and 35% (5.5 million tons) compared to 1990 levels. 

Figure 8 shows the trend in SO2 emissions since 1980 for 

all affected sources. 

The electric utility industry is by far the largest single 

source of SO2 emissions, accounting for approximately 69% 

of total SO2 emissions nationwide in 200115 

(www.epa.gov/ttn/chief/trends/index.html). In addition to 

the significant reductions from the electric power 

generation sector, reductions in SO2 emissions from other 

sources, including smelters and sulfuric acid manufacturing 

plants, and use of cleaner fuels in residential and commercial burners, have contributed to the 39% decline of SO2 

emissions from all sources since 1980 (EPA Web site, 

2003). Figure 9 shows the trend in total SO2 emissions 

from all sources since 1980. 

NOX EMISSIONS 

NOx emissions from all Title IV sources dropped approximately 1.6 million tons between 1990 and 1996, when 

Phase I of the Acid Rain NOx program was implemented. 

Subsequently, NOx emissions remained fairly steady 

through 1998 before dropping significantly in 1999 and 

again in 2000 to comply with the stricter emission limits 

under Phase II. By 2002, emissions from the 1,048 NOx 

program affected sources were 27% (1.5 million tons) 

lower than they had been in 1990. NOx emissions for all 

3,208 Title IV units were 33% (2.2 million tons) lower than 

they had been in 1990. 

Title IV of the 1990 Clean Air Act requires the NOx program 

to achieve a 2 million ton reduction in emissions from 

projected 2000 NOx emission levels without Title IV. Total 

NOx emissions from all Title IV affected units surpassed 

that goal by 1 million tons in 2000 (see Figure 10). 

These reductions have been achieved by reducing the 

amount of NOx emitted per unit of heat input and despite 

the fact that the amount of fuel burned to produce electricity in power plants has increased 30% since 1990. As 

illustrated in Figure 11, many states with increasing 


electricity production had lower total NOx 

emissions in 2002 as compared to 1990 levels. Without further reductions in emissions 

rates or institution of a cap on NOx emissions, 

however, NOx emissions from power plants 

could rise as more fossil fuels are used to 

create energy in most areas of the country. 

NOx emissions come from a wide variety of 

sources including those affected by Title IV. 

In 2001, NOx emissions from electric power 

generation accounted for approximately 22% 

of NOx emissions from all sources (4.9 million 

tons). NOx emissions from transportation 

sources were 56% of NOx emissions from all 

sources (12.4 million tons). Nationally, NOx 

emissions have decreased 12% (3.2 million 

tons) between 1990 and 2001. This occurred 

in spite of an increase of 10% in NOx emissions from non-road vehicles since 1990 that 

has been more than offset by the emission 

decreases from electric power generation 

and other fuel combustion sources. This is 

due to a variety of federal and state emission reduction programs (including Title IV, 

the Ozone Transport Commission NOx 

Budget Trading Program, and anticipation of 

the NOx SIP call) and reductions from on-road vehicles 

(EPA Web site, 2003). In the past few years, EPA has promulgated several rules to reduce SO2 and NOx emissions 

from mobile sources, including the Heavy Duty Diesel Rule 

and the Non-Road Rule. As these rules take affect they will 

significantly reduce emissions from trucks and other diesel 

and off-road vehicles (see Figure 12). In addition, new rules 

for stationary sources (the Clean Air Interstate Rule, the 

Clean Air Mercury Rule, and the Clean Air Visibility Rule), as 

well as State plans to attain the fine particle and ozone 

NAAQS, will also significantly reduce future NOx emissions. 

Geographic Distribution of Emissions 

from Power Generation 

GEOGRAPHIC TRENDS IN SO2 EMISSIONS 

Total SO2 emissions from power generation have decreased 

approximately 35% under the cap and trade approach since 

1990. Emission reductions are taking place throughout the 

country; the geographic distribution of SO2 emissions has 

not changed significantly since 1990.16 Figure 13 compares 

state SO2 emission trends from power generation before 

implementation of Title IV (1990), during Phase I of Title 

IV (1995–1999 average), and in Phase II to date 

(2000–2002 average). 
Several geographic trends are evident. Although most SO2 

emissions still occur in the midwestern U.S., it is important 

to note that, over time, this same region has also seen the 

most significant decrease in SO2 emissions in the country 

(see Figure 13). This confirms an important principle of the 

cap and trade approach: reducing emissions is cheapest for 

the largest sources. These sources will, therefore, reduce 

emissions before others in the industry and take advantage 

of their ability to sell excess allowances to other sources 

where emission reductions are more expensive.17 The 

highest SO2 emitting states in 1990 (Ohio, Indiana, and 

Pennsylvania) reduced emissions an average of 39% in 

Phase 1 of the program (48%, 46%, and 24%, respectively) 

compared to 1990 levels. Other states in the region show 

similar trends since 1990. SO2 emissions decreased 57% in 

Illinois, 41% in Kentucky, 70% in Missouri, 53% in Tennessee, 

and 45% in West Virginia. The 27 shaded states represent 

states where SO2 emissions in Phase II (2000–2002) were 

lower than both 1990 levels and the 1995–1999 Phase I average. Most SO2 emission reductions during Phase I 

occurred in approximately a dozen states in the Eastern U.S. 

(Phase I affected the larger, higher emitting utilities in the 

eastern half of the country). Unlike the SO2 emission reductions achieved during Phase I, Phase II reductions are 

geographically more widespread, occurring in a larger number of southeastern and some western states. 

In 20 mostly western and southern states, 

average SO2 emissions in Phase I were higher than they had been in 1990. This is due to 

the large number of Phase II sources in 

these states that were not required to 

control for SO2 until 2000. In the 2000–2002 

period (Phase II), emissions in these states 

declined as their sources became subject to 

the provisions of the Acid Rain Program. For 

example, SO2 emissions in Texas increased 

from 1990–1999 because no Texas sources 

were affected by Title IV at that time. SO2 

emissions in the state decreased more then 

100,000 tons in Phase II when Title IV 

requirements took effect for Texas sources. 

In fact, sources in all the states where average emissions in Phase I were higher than 

emissions in 1990 collectively reduced their 

emissions approximately 500,000 tons in 

Phase II (2000–2002). 

In addition, some sources increased their 

emissions since 1990 due to increasing 

utilization of units. However, over half these 

units actually decreased their emission rate, 

indicating that increases in demand for elec­
tricity caused those emission increases. 

These emission increases, while not insignificant, were more than offset by the emission 

reductions that have occurred since 1990. In 

the 16 states where state-wide emissions increased, the emission increases totaled 

430,000 tons of SO2 between 1990 and 

2002; over that same period of time, the remaining states reduced SO2 emissions more 

than 13 times that much, by 5.5 million tons. 

GEOGRAPHIC TRENDS IN NOX EMISSIONS 

Total NOx emissions from NOx program 

affected sources (coal-fired) have decreased 

approximately 27% since Title IV was authorized by Congress in 1990. The geographic 

distribution of NOx emissions has not 

changed significantly since 1990.16 Even 

though most emission reductions have taken 

place in the areas of the country with the 

highest NOx emissions, these reductions have 

not resulted in a large change in the pattern 

of NOx emissions in the U.S. NOx emission 

reductions since 1999 are due in part to 

implementation of the OTC NOx Budget 

Program, the NOx SIP call, and several state 

reduction programs as well as Phase II of Title IV. Figure 14 

displays bar graphs illustrating relative state NOx emission 

trends from power generation sources affected by the NOx 

program before Title IV (1990), during Phase I (1996–1999 

average), and in Phase II (2000–2002 average). 

Several geographic trends are evident. NOx emissions were 

lower in 34 states in Phase II compared to 1990 levels, with 

the greatest reduction occurring in the Eastern United States. 

In 1990, the highest NOx emissions occurred in the 

midwestern and southern regions of the U.S. (see Figure 14). 

By 2002, emissions in many of these states had been 

significantly reduced from 1990 levels. The states with the 

highest emissions in 1990 (Ohio, Texas, and Pennsylvania), 

achieved an average reduction of 38% (33%, 30%, and 52%, 

respectively) in Phase II compared to 1990. Other states in 

the region show similar trends since 1990. NOx emissions 

decreased 27% in Indiana, 23% in Illinois, 35% in Kentucky, 

34% in Tennessee, and 31% in West Virginia. The 32 shaded 

states had lower average NOx emissions in Phase II 

compared to both 1990 levels and the 1996–1999 Phase I 

average. NOx reductions occurred predominantly in the 

northeastern U.S. during Phase I. In Phase II, NOx 

reductions are geographically more extensive and occur in a 

larger number of southern and midwestern states. 
In several states, average NOx emissions during Phase I 

were higher than they had been in 1990. This is due to the 

large number of Phase II sources in these states that were 

not required to control NOx emissions until 2000. In the 

2000–2002 period (Phase II), emissions in many of these 

states have declined. There are also several states where 

average Phase II NOx emissions were higher than emissions 

in 1990 and/or the Phase I average. This is because the 

rate-based Title IV NOx limits on coal-fired power plants do 

not limit total emissions of NOx in the way that total emissions of SO2 are limited. Since heat input (or fuel use) 

increased in those states due to additional power 

generation, overall NOx emissions also increased. 

Air Quality, Deposition, 

and Visibility 

Many government agencies, states, and universities work together to monitor the effects of emissions changes on air 

quality and acid deposition. A mosaic of national air quality 

and deposition monitoring networks has evolved to provide 

scientists and policymakers robust data for assessing the effectiveness of emission reduction programs and to determine 

environmental effects, particularly those caused by regional 

sources of emissions for which long range 

transport plays an important role. The 

information derived from these networks is 

used to determine how well existing air pollution control strategies are working and 

whether additional emission reductions are 

required to improve public health and 

facilitate ecosystem recovery. Measurements 

from these networks are also important for 

understanding non-ecological impacts of air 

pollution such as visibility impairment and 

damage to materials (e.g., historical sites and 

cultural monuments). Figure 15 shows the 

monitoring station locations of the primary 

long-term air quality and atmospheric deposition networks: The National Atmospheric 

Deposition Program/National Trends Network 

(NADP/NTN), CASTNET, and the 

Interagency Monitoring of Protected Visual 

Environment (IMPROVE) program. Figure 15 

also includes a table listing the major chemical species measured by these networks as 

well as the other networks discussed below. 

Primary Air Quality and 

Atmospheric Deposition Monitoring 

Networks 

A mosaic of national air quality and deposition 

monitoring networks has evolved to provide 

scientists and policymakers robust data on the 

fate and transport of regional sources of 

emissions, especially those for which long range 

transport plays an important role. Although it 

may appear that some networks perform redundant measurements, this overlap in fact provides 

important quality assurance information that is 

used to identify and account for biases in each 

of the measurement networks. The information 

derived from these networks is used to 

determine how well existing air pollution control 

strategies are working and whether additional 

emissions reductions are required to improve 

public health, improve visibility, and facilitate 

ecosystem recovery. 

WET DEPOSITION MONITORING 

NADP/NTN is a nationwide network of predominantly rural precipitation monitoring stations. 

Operating since 1978, the network collects 

weekly wet acid deposition data to determine

geographic patterns and temporal long-term

trends. NADP/NTN is responsible for measuring 

the wet deposition component of total pollution

loads across the U.S. The network is a 

collaborative effort between many different 

organizations, including U.S. Geological Survey 

(USGS), U.S. Environmental Protection Agency 

(USEPA), National Park Service (NPS), U.S. 

Department of Agriculture (USDA), National 

Oceanic and Atmospheric Administration 

(NOAA), State Agricultural Experiment Stations

(SAES), tribes, universities, and industry. 

NADP/NTN consists of over 230 monitoring

stations spanning the continental United States,

Alaska, Canada, Puerto Rico, and the Virgin 

Islands. The Atmospheric Integrated Research 

Monitoring Network (AIRMoN), a researchbased subnetwork of NADP, consists of nine 

monitoring stations that collect daily 

precipitation samples. The daily samples are analyzed for the same pollutants as NADP/NTN 

and are primarily used to identify source­
receptor relationships in atmospheric models. 

Quality assurance procedures are followed for all 

aspects of NADP field and laboratory operations. 

Quality assured data are available from the 

NADP Web site, http://nadp.sws.uiuc.edu/. 

AMBIENT AND DRY DEPOSITION MONITORING 

CASTNET is a long-term atmospheric deposition 

monitoring network that measures changes in 

ambient air quality and atmospheric deposition 

over broad regions of the U.S. Operating since 

1987, CASTNET has evolved into a robust monitoring program which now comprises over 80 

monitoring stations nationwide. Sites are located 

predominantly in rural locations. EPA operates 

most of the monitoring stations; however, NPS 

operates 30 stations in cooperation with EPA. 

CASTNET measures weekly average atmospheric concentrations of the major pollutants 

contributing to acid deposition, hourly concentrations of ambient ozone levels, and meteorological conditions. CASTNET estimates dry 

deposition or flux based on measured ambient 

air concentrations and modeled deposition 

velocity. Deposition velocity is inferred through 

the multilayer model (MLM) which accounts for 

the influence of meteorological conditions and 

vegetation on dry deposition processes. CASTNET data measured at point locations are used 

in conjunction with the NADP/NTN data to infer 

geographic patterns and long-term temporal 

trends in total (dry plus wet) atmospheric deposition. Quality assurance procedures are 

followed for all aspects of CASTNET field and 

laboratory operations. Quality assured data are 

available at www.epa.gov/castnet. 

VISIBILITY MONITORING 

IMPROVE is a long-term monitoring network 

that measures current visibility conditions, 

tracks changes in visibility and determines the 

causes of visibility impairment in national parks 

and wilderness areas. IMPROVE was 

established in 1985 to aid the development of 

federal and state implementation plans to 

protect visibility in Class I areas as stipulated in 

the 1977 amendments to the Clean Air Act. 

IMPROVE began collecting data in 1988 at 20 

Class I areas. The network expanded to monitor 

the impacts of the Regional Haze Rule and now 

consists of 167 sites nationwide. IMPROVE is a 

collaborative monitoring effort directed by a 

steering committee comprised of 

representatives from NPS, USEPA, USFS, FWS, 

NOAA, Bureau of Land Management, and state 

and regional air organizations. Every IMPROVE 

site deploys an aerosol sampler to measure speciated fine aerosols and PM10 mass. Select sites 

also deploy transmissometers and 

nephelometers to measure light extinction and 

scattering respectively, as well as automated 

camera systems to measure the “scene.” Light 

extinction coefficients and the deciview indices 

used to report visibility data are calculated from 

relative humidity and the measurements of the 

five key species: sulfates, nitrates, organic carbon, elemental carbon, and crustal material at 

each site. Data and additional information about 

IMPROVE are available at 

http://vista.cira.colostate.edu/improve/. 

AIR QUALITY MONITORING 

State and Local Air Monitoring Stations (SLAMS) 

and National Air Monitoring Stations (NAMS) 

are designed to judge compliance with ambient 

air quality standards; activate emergency control 

procedures that prevent or alleviate air pollution 

episodes; and observe pollution trends throughout the region. The information collected also 

provides a database for research on urban, land 

use, and transportation planning; development 

and evaluation of abatement strategies; and development and validation of dispersion models. 

The SLAMS/NAMS network performs measurements primarily in urban areas where air quality 

is influenced primarily by local sources. Some 

sites are located away from urban areas to characterize regional air quality. SLAMS consists of 

approximately 4,000 monitoring stations whose 

size and distribution are largely determined by 

the needs of state and local air pollution control 

agencies to meet their respective SIP 

requirements. NAMS (1,080 stations) are a subset of the SLAMS network with emphasis on areas of maximum concentrations and high 

population density. Additional information on 

SLAMS/NAMS is available at www.epa.gov/ 

oar/oaqps/qa/monprog.html. 

AIR QUALITY: SULFUR 

Analyses of regional monitoring data from 

CASTNET show the geographic pattern of 

SO2 and airborne sulfate in the Eastern 

United States, where concentrations from 

power plants are highest. Three-year mean 

annual concentrations of SO2 and sulfate 

from a set of 34 CASTNET long-term monitoring sites were compared from 1989–1991 

and 2000–2002. In 1989–1991, prior to 

implementation of Phase I of Title IV, the 

highest ambient concentrations of SO2 in the 

East were observed in western Pennsylvania, 

and along the Ohio River Valley in the vicinity of Chicago, Illinois, and Gary, Indiana. The 

highest ambient sulfate concentrations, 

greater than 7 micrograms per cubic meter 

(μg/m3), were observed in this area and in 

northern Alabama. Most of the Eastern U.S. 

experienced annual ambient sulfate concentrations greater than 5μg/m3. 

During the late 1990s following implementation of Phase I, dramatic regional 

improvements in SO2 and sulfate air quality 

were observed at CASTNET sites throughout the Eastern 

U.S. Ambient concentrations of SO2 in 2000–2002 

decreased 45% in the Midwest, 40% in the Northeast and 

30% in the mid-Atlantic compared to concentrations in the 

early 1990s (see Figure 16). Other researchers, using 

different statistical methods, have also reported reductions 

in SO2 concentrations of 30–40% in the northern midAtlantic and Midwest.18 The largest decreases in ambient 

SO2 concentrations were noted in high emissions and concentration areas (e.g., vicinity of Chicago and throughout 

Indiana, Ohio, Pennsylvania, Kentucky, and West Virginia). 

The highest SO2 concentrations observed in the rural parts 

of the United States are now concentrated in eastern Ohio 

and southwestern Pennsylvania. 

For ambient sulfate, both the size of the affected region 

and magnitude of the highest concentrations were dramatically reduced following implementation of Title IV. Sulfate 

concentrations decreased up to 30% in the Midwest and 

Northeast and 25% in the mid-Atlantic compared to concentrations observed in the early 1990s (see Figure 17). The 

largest decreases were observed along the Ohio River 

Valley in Illinois and Pennsylvania. These reductions have 

also been confirmed by other researchers using different 

statistical methods.18 
The urban monitoring networks (SLAMS/NAMS) also 

detected the environmental signal from Title IV SO2 

emission reductions. Analyses of urban monitoring data 

from 480 monitoring stations show a decreasing trend in 

SO2 in the atmosphere following implementation of Title IV. 

Between 1992 and 2001, annual arithmetic mean 

concentrations of atmospheric SO2 decreased 35%.16 

AIR QUALITY: NITROGEN 

Emissions of NOx are transformed in the atmosphere into a 

series of pollutants, including nitric acid vapor (HNO3) and 

nitrates (NO3). From the perspective of acid deposition, nitric acid is a key chemical species, for which the rural air 

quality and dry deposition monitoring networks now have 

an extensive record. While there is strong evidence of statistically significant declining trends in the concentrations 

of SO2 and sulfate, trends in nitrogen concentrations have 

not been as pronounced. 

Atmospheric nitrogen concentrations are a result of 

emissions of several different forms of nitrogen from many 

different types of sources; therefore, they tend to exhibit a 

more complicated pattern than sulfur concentrations. Nitric 

acid concentrations, for example, are highest in eastern 

Ohio and south central Pennsylvania/north central 

Maryland, areas with high emissions from power plants. 

Ammonium concentrations, in contrast, are 

highest from Ohio west to Missouri and Iowa 

and in northern Alabama as well as central 

Pennsylvania. Nitrate concentrations are 

highest even further west, in southern 

Wisconsin/northern Illinois and westward 

into the Plains, areas with large amounts of 

agricultural activity. 

Scientists have not reported any evidence of 

a statistically significant decreasing trend in 

nitric acid concentrations over the past 15 

years, but a recent analysis of CASTNET 

data does indicate that nitric acid concentrations may be lower than they were in the 

1989–1991 timeframe. CASTNET data indicate reductions of nitric acid concentrations 

between 1989–1991 and 2000–2002 of 

approximately 10% in the Northeast, midAtlantic, Southeast, and Midwest, and 

approximately 25% in the North Central and 

South Central Plains states (see Figure 18). 

In contrast, the only statistically significant 

published results seem to suggest nitric acid 

is increasing over time, particularly in the 

Northeast.19 Any increase in airborne nitric 
acid concentrations is of concern because of 

the potential for corresponding increases in 

deposition. It is not clear whether the cause 

is associated with changes in nitrogenous 

fertilizers, difficulties measuring nitric acid 

vapor, the length of the record, or some other reason not associated with upwind pointsource emission measurements. Scientists 

continue to monitor concentrations of nitric 

acid, as well as all the components of atmospheric deposition discussed here, and it is 

hoped that additional data will clarify the 

trends in nitric acid concentrations. 

Analyses of mean annual total ambient 

nitrate concentration data have not revealed 

any statistically significant changes from 

1989 through 2002. Although there are some 

indications that ambient nitrate concentrations 

are increasing, particularly in the Midwest, 

nitrate changes at individual CASTNET monitoring stations have been minimal over this 

period (see Figure 19). The highest nitrate 

concentrations in the East were recorded in 

Illinois, Wisconsin, and Iowa (>4 μg/m3). 

Ambient ammonia (NH3) concentrations, 

which might also indicate changes in emissions in states 

with significant agricultural activity, are not routinely measured by either CASTNET or AIRMoN. 

Ambient airborne concentrations of ammonium (NH4) are 

typically associated with nitrate and sulfate compounds. 

The highest annual mean ammonium concentrations have 

been observed in the Midwest and in northern Alabama. 

Between 1989 and 2002, decreases in ammonium concentrations in the atmosphere were observed in some parts of 

the U.S. (see Figure 20). The largest decreases of approximately 0.5 μg/m3 were noted in the Midwest. The decrease 

in ambient ammonium concentrations between 1989–1991 

and 2000–2002 appears to be associated with reductions in 

ambient levels of sulfate which bind to ammonium to form 

ammonium sulfate. Agricultural ammonia emission 

themselves have not decreased over the same time period. 

In fact, ammonia emissions from crops and livestock have 

increased 14% between 1990 and 2001.20 

ACID DEPOSITION: SULFUR IN PRECIPITATION 

Scientists have documented that both wet and dry sulfur 

deposition (and the acidity associated with sulfur 

deposition) have declined with reductions of SO2 emissions 

over a large portion of the Eastern U.S. following implementation of Title IV.

21-25 Strong correlations, near linear, between 

large scale SO2 emission reductions and large reductions in 

sulfate concentrations in precipitation have 

been noted for the Northeast, one of the areas most affected by acid deposition.22 

A main reason for reduced concentrations of 

sulfate in precipitation in the Northeast is a 

reduction in the long-range transport of sulfate from emission sources located in the 

Ohio River Valley. The reductions in sulfate 

documented in the Northeast, particularly 

across New England and portions of New 

York State, were also affected by SO2 emission reductions in eastern Canada. 

Concurrent with these sulfate reductions 

were similar reductions in precipitation acidity, expressed as H+ concentrations. 

However, acidity may not have decreased as 

dramatically as sulfate concentrations due to 

a simultaneous decline in acid-neutralizing 

base cations which act to buffer acidity. 

Other factors influencing the decline of sulfate in precipitation may include climate variability (i.e., wet and dry summers) or 

temporal changes in base cations and 

ammonium concentrations.22 

Some of the greatest reductions in wet sulfate deposition occurred in the midAppalachian region, including Maryland, 

New York, West Virginia, Virginia, and most 

of Pennsylvania (see Figure 21). Wet sulfate 

deposition decreased 40% in the Northeast 

and 35% in the Midwest and mid-Atlantic 

since the early 1990s. Reductions of 25% 

have occurred in the Southeast. 

Figure 22 shows the total (wet plus dry) 

change in sulfur deposition in the 

conterminous U.S. between 1989–1991 and 

2000–2002. Total sulfur deposition is defined 

as the sum of the dry components (SO2 and 

particulate sulfate) and wet components (sulfate in precipitation) as sulfur. Wet sulfate 

and SO2 are the largest components of total 

sulfur deposition. In general, dry deposition is 

a larger percentage of total deposition in 

those areas nearest to SO2 emission sources. 

The highest rates of total sulfur deposition 

have been observed in the areas containing 

the highest SO2 emissions—the Midwest and 

the East. Most areas where total sulfur deposition has been calculated have had 

reductions in total sulfur deposition since 

1989–1991. 

Baumgardner et al. report that in the early 1990s, the mean 

annual total sulfur deposition at CASTNET sites in the Ohio 

River Valley was 15–22 kg/ha. Sites located in 

Pennsylvania, southern New York, and West Virginia also 

had total sulfur deposition greater than 15 kg/ha. From 

1997–2000, total sulfur deposition ranged from 12–17 

kg/ha for sites in the Ohio River Valley. Appreciable decreases in total sulfur deposition of 15–25% have occurred 

throughout the Eastern U.S. since the early 1990s.21 

ACID DEPOSITION: NITROGEN IN PRECIPITATION 

Nitrate concentrations in precipitation measured at 

NADP/NTN sites have generally remained the same or increased in some regions (e.g., the Southeast) over the past 

15 years. Unlike sulfate concentrations, sharp declines in 

nitrate concentrations have not been observed in the 

Northeast, but some decline has occurred.26 While wet 

nitrate deposition does appear to be substantially lower in the 

East in 2000–2002 (see Figure 23), nitrate concentrations in 

precipitation did not change (see Figure 24). Therefore, the 

lower nitrate deposition in 2000–2002 appears to be related to lower precipitation levels over those same years. 

Other researchers have also not found significant 

reductions in nitrogen deposition since 1990. Lynch, 

Bowersox, and Grimm found that most areas of the country 

were not substantially different in reduced or oxidized nitrogen in 1997–1999 compared to historic levels. These 

findings were not unexpected and are consistent with the 

NOx emissions that were not appreciably affected by Title 

IV in 1996 or 1997 on a broad, regional basis.27 The control 

of NOx emissions under Title IV was not required until 

1996. Moreover, any decreases in NOx emissions from power generation have been significantly offset by increases 

from the vehicle sector from 1991 to 2000.26 In addition, it 

is interesting to note that while air quality concentrations of 

ammonium have decreased in the Midwest, both concentrations of ammonium in precipitation and deposition of ammonium have increased in that region since 1985.28 

Figure 25 shows the difference in total (wet plus dry) nitrogen deposition in the conterminous U.S. between 

1989–1991 and 2000–2002. Total nitrogen deposition is 

the sum of the dry deposition components (nitric acid, particulate nitrate, particulate ammonium) and wet deposition 

components (nitrate and ammonium in precipitation). Wet 

nitrate, wet ammonium, and nitric acid are the largest contributors to nitrogen deposition. As is the case with sulfur, 

the dry nitrogen components comprise a larger part of total nitrogen deposition in those areas nearest to sources of 

NOx emissions. In some areas of southern California, for 

example, where NOx emissions are predominantly from 

mobile sources, the ratio of dry to wet deposition is 

approximately 4:1. 





Total nitrogen deposition is highest in the 

Eastern U.S., although several monitoring 

stations in the West observe high levels of 

total nitrogen deposition. There are no 

observable broad-scale reductions in total 

nitrogen deposition. 

Baumgardner et al. report that in the early 

1990s, the highest mean annual total 

nitrogen deposition noted was 8–11 kg/ha in 

the Midwest and portions of the Northeast. 

From 1997–2000, little change occurred from 

the nitrogen deposition values recorded in 

these areas earlier in the decade.21 

VISIBILITY 

SO2 and NOx gases are transformed in the 

atmosphere into fine particles of sulfates and 

nitrates. Sulfate and NO3

- particles scatter 

and absorb light, impairing visibility and contributing to regional haze. Sulfates are generally the largest contributor to visibility 

impairment in both the East and the West. 

The visual range under naturally-occurring 

conditions without pollution in the U.S. is 

approximately 75–150 km (45–90 miles) in 

the East and 200–300 km (120–180 miles) in 

the West. 

Data from the IMPROVE network, which 

monitors visibility trends in national parks, 

show that visibility has remained virtually 

unchanged since the early 1990s (see 

Figure 26). The level of visibility impairment 

on the worst visibility days in the West is 

similar to the levels seen on the best visibility days in the East. In 2001, mean visual 

range for the worst days in the East was 

only 29 km (18 miles) compared to 117 km 

(73 miles) for the best visibility. In the West, 

visibility impairment for the worst days 

remained relatively unchanged over the 10­

year period, with the mean visual range for 

2001 (103 km or 63 miles) nearly the same 

as the 1992 level (98 km or 61 miles). 

Visibility in the East is still significantly 

impaired in national parks and wilderness 

areas, especially on the haziest days. 

Ecological Recovery 

The atmospheric and environmental response to a reduction 

in pollutants is a complex and lengthy process.29, 30 It takes 

only a few hours for a reduction in emissions to improve air 

quality; it takes days to weeks for reductions in deposition 

to appear. Once the pollutants are deposited on the ecosystem, biogeochemical and hydrologic cycling processes drive 

the rate of recovery. Previous research tells us that, even 

after acid deposition decreases, ecosystems can 

experience lag times of several decades between chemical 

recovery (e.g., increased pH) and biological recovery (e.g., 

repopulation of fish or forest regrowth). A short “episodic” 

period of acidification can be eased in a few weeks to 

months; recovery from long-term “chronic” acidification can 

take years or even decades. Some soil processes can 

recover in a few weeks to months; building up the appropriate soil nutrient reserves can take centuries. Recovery of 

forest ecosystems generally takes years to decades. 

Therefore, while some ecological recovery processes occur 

quickly, others will take place more gradually over a generation or more. Every additional year of monitoring, research, 

and analysis improves our understanding of these recovery 

processes and allows us to better predict environmental response to emission reductions. 

FRESHWATER ACID-SENSITIVE ECOSYSTEMS 

Acid deposition can lead to acidification of surface waters. 

Acidification and low acid neutralizing capacity (ANC) can, 

in turn, lead to the loss of sensitive fish populations from 

those waters 31 (see Figure 27). In the 1980s, acid rain was 

found to be the dominant cause of acidification in 75% of 

acidic lakes and 50% of acidic streams.31 Broad areas of 

the United States still contain large numbers of lakes and 

streams with little ability to absorb acid deposition (a condition known as low acid neutralizing capacity or ANC). 

This includes portions of the Northeast (particularly Maine 

and the Adirondack and Catskill Mountains), the 

Appalachian Mountains, and areas of the Western United 

States (see Figure 1). High-elevation watersheds with 

steep topography, extensive areas of exposed bedrock, 

deep snowpack accumulation, and shallow, base-poor soils 

are very sensitive to episodic acidification.32 Such systems 

are common throughout the mountainous West and in portions of the Northeast and Appalachian Mountains. 

Freshwater monitoring in the Eastern United States is conducted by many programs, including the EPA’s Temporally 

Integrated Monitoring of Ecosystems (TIME) and Long-Term 

Monitoring (LTM) projects that were initiated in the early 

1980s by EPA’s Office of Research and Development (see 

Figure 28). The purpose of these networks is to detect 


trends in regional populations of lakes or streams to 

determine whether emission reductions have had the intended effect of reducing acidification of lakes and streams. 

TIME/LTM monitors a total of 145 lakes and 147 streams, 

representing all of the major acid-sensitive regions of the 

northern and eastern U.S. (upper Midwest, New England, 

Adirondack and Catskill Mountains, northern Appalachian 

Plateau, Ridge and Blueridge Provinces of Virginia). 

Additional sites in acid-sensitive regions of the Southeast, 

Midwest, and West would make more complete 

assessments possible. TIME/LTM measures a variety of important chemical characteristics, including ANC, pH, sulfate, 

nitrate, major cations (e.g., calcium and magnesium), and 

aluminum. While the representativeness of the TIME/LTM 

network is somewhat limited, the TIME program is the most 

coherent individual regional dataset for this kind of analysis. 

In addition, the U.S. Geological Survey has been measuring 

surface water quality at several research watersheds throughout the United States, where sample collection during hydrologic events and ancillary data on other watershed 

characteristics have been used to assess the watershed 

processes controlling acidification of surface waters.33–35 The 

NSF Long-Term Ecosystem Research (LTER) watershed at 

Hubbard Brook, New Hampshire, and university-sponsored 

watersheds throughout the East are also being monitored for 

long-term changes in water quality. Like the LTM program, 

these watersheds were selected to represent specific environmental conditions (alpine lake outlet, streams draining shallow 

acidic soils, etc.). The TIME program and the broader 

Environmental Monitoring and Assessment Program (EMAP) 

are the primary programs that provide probability-based 

data on surface water quality, and thus can be used to 

provide regional estimates of acidification recovery at base 

flow conditions. The current advantage of the TIME and LTM 

programs is that the data reside in a common database and 

common methodologies have been applied to all samples. 

A recent report using this TIME/LTM data concludes that 

measurable improvements in surface water chemistry (lower sulfate concentrations and decreases in acidity) have resulted from emissions regulations promulgated pursuant to 

the 1990 CAAA, primarily under the Acid Rain Program.36 

This conclusion was reached while acknowledging that other changes, including climate variability, forest maturation 

and recovery from disturbances, and land use history are 

also contributing to changes in the chemistry of lakes and 

streams in the Eastern United States. A summary of the 

TIME/LTM data is presented in Figure 29. A prediction of 

the long term response of lakes and streams to reductions 

in acid deposition is provided in Chapter 4. 

In three of the five areas studied by 

Stoddard et al.36 one-quarter to one-third of 

lakes and streams previously affected by 

acid rain are no longer acidic at base flow 

conditions, although they are still highly sensitive to future changes in deposition. In other areas, signs of recovery are not yet 

evident, suggesting that additional 

reductions are necessary for further ecosystem recovery. In general, trends in nitrate 

concentrations were much smaller than 

trends in sulfate concentrations, though 

lakes in the Adirondacks and streams in the 

northern Appalachian Plateau exhibited significant downward trends in nitrate in the 

1990s. It should be noted however, that 

nitrate increased during the 1980s and 

decreased dramatically in 1992 for reasons 

unrelated to changes in deposition. Analysis 

of stream nitrate concentrations based on 

the 20-year record indicates no significant or 

slightly decreasing trends.37 Increasing ANC, 

an indicator of aquatic ecosystem recovery, 

was evident in three of the regions 

(Adirondacks, northern Appalachian Plateau 

and upper Midwest) and was unchanged in 

New England and the Ridge/Blue Ridge region. Modest increases in ANC have 

reduced the number of acidic lakes and 

stream segments in some regions: 

• Eight percent of lakes in the Adirondacks 

are acidic, down from 13% in the early 

1990s. 

• In New England, 5.5% of lakes are acidic, 

an insignificant change from the early 

1990s when 5.6% of lakes in the region 

were acidic. 

• Fewer than 1% of lakes in the upper 

Midwest are acidic, down from 3% in the 

early 1980s. 

• Eight percent of the stream length in the 

northern Appalachian Plateau region is 

currently acidic during base-flow conditions, 

down from 12% in the early 1990s. 

• There has been no change in the number 

of acidic waters in the Ridge/Blue Ridge 

region in the past decade. 
Lake and stream recovery is due to 

reductions in acidic deposition, primarily 

sulfate deposition, achieved by Title IV.

36 

There have been decreases in sulfate 

concentrations in almost all Adirondack lakes 

that coincide with reductions in atmospheric 

concentrations of sulfur.

38 These reductions 

in sulfate, as well as reductions in nitrate 

concentrations that do not appear to be 

due to changes in atmospheric deposition, 

have resulted in increased pH and ANC as 

well as reductions in the amount of toxic inorganic aluminum in these lakes.38 In spite 

of declining sulfate concentrations, however, 

TIME/LTM data show that some lakes and 

streams have not begun to recover. 

Recovery was not evident in New England or 

in the Ridge and Blue Ridge Provinces 

(Virginia); the latter region is not expected 

to recover immediately, due to the nature of 

forest soils in the province. In part, declining 

levels of base cations (calcium, magnesium, 

and potassium) in lakes and streams 

throughout the East are believed to delay 

the onset of recovery.

36 In addition, it should be noted that, 

although chemical recovery (i.e., an increase in ANC and 

decrease in sulfate and nitrate concentrations) has begun 

in some lakes and streams, full biological recovery of fish 

and other aquatic organisms takes significantly longer.

30 

Throughout the Appalachians, many waterbodies are still 

acidic enough to damage sensitive fish and aquatic life. In 

Virginia, for example, over a decade of quarterly stream 

monitoring shows that the predominant trend, as indicated 

by declining ANC, has been continued acidification. The 

number of chronically acidic Virginia trout streams has 

increased, while the number of transitional and not-acidic 

streams has declined.39 The consequences for individual 

species have been well documented in the Shenandoah 

National Park by the Fish in Sensitive Habitats (FISH) 

Project. The FISH project demonstrated that acid-sensitive 

fish in both chronically and episodically acidic streams in 

Shenandoah National Park have lower body size and 

weight, both indications of chronic stress. 

Some lakes in the Colorado Front Range are showing indications of being affected by higher atmospheric nitrogen 

loads from urban, agricultural, and industrial sources.40 In 

the central and northern Rockies, sediment record studies 

strongly suggest that lakes began responding to nitrogen 
deposition levels in the region as early as the 1950s.40, 41 

Recent analyses demonstrate regional differences in lake 

surface water chemistry and show evidence of ecological 

change in the region. For example, studies of high-elevation 

lakes in the central Rockies found significantly higher 

nitrate concentrations in lakes on the east side of the Front 

Range, where emission sources are numerous, as compared 

to lakes on the west side.40, 42 An intensive sampling study 

projected that at least two sites in the Green Lakes Valley 

in the Colorado Front Range could become chronically 

acidic within the next decade, and a third site could become 

episodically acidified at present rates of ANC decrease.42 It 

has also been demonstrated that algal communities, soils, 

and subalpine forests on the east side of the Front Range 

are beginning to show signs consistent with receiving more 

nitrogen deposition from fixed, mobile, and agricultural 

nitrogen sources.40 With the accumulating weight of 

evidence, some researchers conclude that high-altitude watersheds in the Colorado Front Range show symptoms of 

nitrogen saturation.43, 44 

In some forest ecosystems, particularly in the Northeast, 

sulfur and nitrogen have accumulated so much that the 

soils are unable to retain them. This leads to leaching of 

important nutrients and higher nitrate and sulfate 

concentrations in some surface waters. The release of 

decades worth of previously accumulated atmospheric deposition may delay the recovery of these surface waters in 

response to emission reductions.30 Recovery of acidified 

waters is also slowed by continuing acid deposition, the 

presence of nitrate in surface waters, the contribution of 

naturally occurring acid sources, and the number of 

physical, chemical, and biological changes needed in the 

ecosystem before it can function as an ecosystem unaffected by acid deposition. Nitrogen export from recovering 

streams in the Catskill Mountains was reported to be 

greater during warm years than cool years, suggesting that 

a warmer climate could enhance acidification of surface waters if nitrogen deposition rates remained the same.37 

Throughout the United States, most streams and drainage 

lakes experience some decline in ANC during heavy 

rainstorms, snowmelt, or other episodic events. Current 

programs for monitoring trends in surface water chemistry 

either specifically avoid hydrologic ‘episodes’ (high flows 

during rain events or snowmelt) or sample with a set periodicity that ultimately samples random flow conditions. The 

trends presented are therefore representative of baseflow 

or median flow conditions over time and do not indicate 

trends in surface-water chemistry during high-runoff 

periods. In the Northeast and mid-Atlantic regions of the 

United States, the number of lakes and streams that experience episodic acidification are estimated to be much 

greater than the number of chronically acidic waterbodies.45 

Episodic acidification has been reported in 

the mid-Atlantic and Northeastern region in 

areas such as the Ridge and Valley Province 

of Pennsylvania, Virginia, and Maryland, the 

northern Appalachian Plateau of 

Pennsylvania, the Catskill and Adirondack 

Mountains of New York, and the Atlantic 

Coastal Plain of Maryland.45 Several recent 

studies also have demonstrated episodic 

acidification of streams in Shenandoah and 

Great Smoky Mountains National Parks.46–49 

A recent study of brook trout streams in 

Virginia estimated that 24% experience regular episodic acidification at levels harmful to 

brook trout and other aquatic species.46 Due 

to the difficulties and expense associated 

with collecting water samples during 

episodes of acidification, limited data on 

episodic acidification have been published 

since the 1988–1990 sampling, and few data 

collected after 1994 have been published. As 

a result, it is unknown whether the regional 

incidence of episodic acidification has 

decreased as a response to declines in acidic 

deposition in the 1990s. 

FOREST ECOSYSTEMS 

Acid deposition especially combined with 

other pollutants and natural stresses, can 

also damage forest ecosystems. Sulfates 

and nitrates from acid deposition leach nutrient base cations (calcium, magnesium, and 

potassium) from tree needles and leaves and 

from forest soils, reducing the forest’s 

capacity to buffer further acidification and 

removing elements essential for tree growth. 

Acidification also leads to the mobilization of 

naturally-occurring aluminum, which may interfere with nutrient uptake by roots in 

forest soils. In addition, exposure to 

tropospheric ozone (a product of NOx emissions interacting with volatile organic carbons 

(VOCs)) has both acute toxic effects on 

plant leaves and chronic toxic effects on the 

entire plant. The combined effects of depletion of foliar and soil nutrients, mobilization 

of aluminum and exposure to ozone make 

trees more susceptible to drought, temperature extremes, and diseases.50 Several 

forests throughout the U.S. are beginning to 

show signs of nitrogen saturation, a 

condition where the inputs of nitrogen 

exceed the forest’s need for them and 
excess nitrogen is leached into surrounding 

waterways.51 There are as yet no forests in 

the U.S. where research indicates recovery 

from acid deposition is occurring, as indicated by reversal of the soil acidification such 

as net accumulation of base cations and reductions in aluminum concentrations. 

There is no broad-scale assessment of forest 

health the way there has been for freshwater 

lakes and streams. However, both a series of 

individual studies in a variety of locations in 

areas sensitive to acidic deposition and 

regional studies across a gradient provide a 

snapshot of the status and trends in forest 

ecosystems. For example, mortality of red 

spruce in the mountains of the Northeast has 

increased, due in part to exposure to acidic 

cloudwater.

29 Acid deposition has led to 

reduced growth and/or vitality of red spruce 

across the high elevation portion of its range, 

thereby increasing the trees’ sensitivity to 

cold weather injuries and eventually resulting 

in a high rate of mortality.

52 

Forest soils are also adversely affected by 

acid deposition. Studies in New 

Hampshire,53, 54 the Catskill Mountains in 

New York,55 the Adirondacks in New York, 

West Virginia,56 North Carolina,57 South 

Carolina,58 Tennessee,59 and Ontario, 

Canada,60 have documented nutrient base 

cation (e.g. calcium, magnesium, 

potassium, and others) depletion in soils 

due to acid deposition and forest regrowth. 

Resampling of forest soils in northwestern 

Pennsylvania originally sampled in the 

1960s shows an increase in soil acidity.

61 In 

contrast to these studies,62 at one site in 

New Hampshire, Hamburg et al.,63 inferred 

(not direct re-measurement) no decline in 

calcium in the forest floor in a 

chronosequence study in New Hampshire. 

At sites where there may be appreciable 

amounts of dolomitic parent material64 that 

are accessible to trees, soil calcium may 

not be depleted to the extent it can be in 

nearby soils that lack a dolomitic source.59 

In these and other examples of calcium 

depletion in forest soils in Europe,65, 66 it has 

not been possible to quantify the component 

of depletion attributable to anthropogenic 

acidic deposition versus tree uptake. 

Similarly, in reported instances of forest decline it has not 

been possible to attribute the decline to calcium depletion 

directly. However, calcium depletion has been implicated indirectly in the decline in sugar maple in the U.S. and 

Canada65, and calcium depletion adversely affects many aspects of tree physiology and forest ecosystem health.67, 68 

The potential for weathering re-supply of exchangeable soil 

calcium remains an important uncertainty because 

estimates of weathering are based on indirect estimates.69 

Soil acidification may increase the rate of weathering, but 

the incremental increase in calcium release is likely to be 

small in soils that are already acidic. Studies that have 

attempted to quantify calcium weathering rates in acidic 

forest soils in the Eastern U.S. generally have found that 

rates are low53, 54, 70–72 in comparison with typical combined 

rates of tree uptake and soil leaching.69 

Acid deposition has also contributed to a regional decline in 

the availability of soil calcium and other base cations in 

many different types of forest ecosystems. In the high elevation and mid-elevation spruce-fir forests of New York, 

New England, and the southern Appalachians, the change 

in soil nutrient ratios (lower calcium/higher aluminum) can 

disrupt physiological processes that are important to maintaining forest health. These changes lower resistance to 

natural stresses, such as insects, disease and climatic 

extremes.67, 68 Acid deposition is also contributing to the depletion of base cations in many poorly buffered soils 

supporting southern pines. While this may slightly bolster 

growth of southern pines in the short-term, it is expected to 

have adverse effects on productivity over the long-term. 

Finally, the high mortality rates and reduced vigor reported 

for sugar maples experienced in northern Pennsylvania and 

Quebec, Canada over the last decade may be attributed in 

part to cation depletion from those soils.73–77 

Whereas the vast majority of forest ecosystems in the 

Western U.S. do not appear to be significantly affected by 

acid deposition, certain ecosystems in southern California 

and the Colorado Front Range are negatively affected, especially by high rates of nitrogen deposition. Nitrogen deposition in most affected areas of the Colorado Front Range, 

southern California, and in the Sierra Nevada is generally 

from a mix of mobile, industrial, and agricultural emissions 

sources.78 In some cases, ozone (formed from NOx 

emissions and other precursors) is also causing damage. 

Many of these sites are showing signs of nitrogen 

saturation. Nitrogen saturation occurs when the supply of 

nitrogen from deposition exceeds the nutritional demands 

of the ecosystem and nitrogen is gradually released or 

leached from the watershed into the surface waters as 

nitrate.79 In the Colorado Front Range, responses to chronic 

nitrogen deposition are evident in changes in the chemical 
composition of pine needles and soils. These results are interpreted as subtle signs of the effects of nitrogen deposition, suggesting that the trajectory towards nitrogen 

saturation may be in the incipient stages.40 Additional studies along air pollution gradients demonstrate that forest 

and chaparral sites in southern California and alpine 

ecosystems of the Front Range in Colorado are showing 

signs of nitrogen saturation.43, 80 The sites most affected are 

near intensive livestock operations and high population centers, although extremely high streamwater nitrate concentrations have been reported from a low-elevation chaparral 

watershed in Sequoia National Park in the Sierra Nevada 

Mountains in central California.81 

While forest ecosystems are clearly still suffering 

significant damage from acid deposition, the reduction in 

acid deposition due to Title IV is expected to be beneficial 

to these forest ecosystems in the long-term. Forests are 

generally thought to recover more slowly than aquatic 

ecosystems because of the complex series of processes 

that must take place. Leached nutrients must be restored 

through weathering of the bedrock or atmospheric deposition, and soilwater aluminum concentrations must be 

reduced. Even after soil chemistry is restored, recovery of 

sensitive forests is not expected to occur for decades 

because of the extensive recovery time of trees. 

COASTAL ECOSYSTEMS 

The nitrogen component of acid deposition is a significant 

source of nitrogen to many estuaries and coastal waters in 

the Eastern U.S. Excessive nitrogen loads from a variety of 

sources, including atmospheric deposition and runoff from 

agricultural practices, cause many of these estuaries and 

coastal waters to periodically become eutrophic.82 Eutrophic 

conditions include algal blooms (some of which may be 

harmful), low levels of dissolved oxygen in the water 

(hypoxia or anoxia) which can stress or kill fish and 

shellfish, and changes in the ecological structure of the 

ecosystem. There are currently 44 estuaries in the U.S. that 

are considered highly eutrophic, 37 of which are on the 

eastern or gulf coasts. Many of these are expected to 

become more eutrophic—to have worsening symptoms, including more algal blooms and fish kills—in the future.83 

Title IV has reduced nitrogen deposition in some coastal areas compared to what it would have been without Title IV 

(see Figures 23 and 25). However, in many sensitive coastal 

watersheds there has been little or no reduction in nitrogen 

deposition since 1990. 

Materials and Structures 

There is no monitoring network in place to assess the 

effects of reductions in acid deposition on materials and 

structures. However, based on our understanding of the 

processes involved, it is possible to predict 

likely changes that will take place. 

SO2, NOx, and many of the pollutants they 

form corrode materials, particularly those 

made of limestone or marble. Monuments 

and historic buildings, outdoor structures 

such as bridges and buildings, and automotive paints and finishes are all susceptible 

to damage by acidic pollutants. One set of 

studies indicated that several air pollutants 

from coal combustion are the main sources 

of soiling on a limestone building in 

Pittsburgh, Pennsylvania followed by erosion due to rainfall.84–86 In general, most 

damage from air pollutants appears to come 

from dry deposition. The rate of dry deposition is often accelerated when the surface is 

wet, so that structures affected by dewfall 

are expected to be especially vulnerable. 

Rain wets exposed structures, again modifying the dry deposition rates that would be 

expected if the same structures were dry. 

Hence, there is a strong interplay among dry 

deposition, wet deposition, and surface wetness that serves to complicate the understanding of causes of damage in specific 

locations.87 In addition, in rural areas and in 

areas where buildings and monuments 

remain wet for long periods of time, wet 

deposition can be a significant or primary 

cause of damage.88, 89 

Weathering due to acid deposition often 

harms cultural assets (e.g., statues and 

monuments) more than purely operational 

resources (e.g., bridges and buildings). This 

is because the appearance of cultural 

resources, where much of their value lies, is 

particularly vulnerable to damage. There are 

also historic and emotional values attached 

to cultural assets, which increase the value 

of their preservation. Title IV has reduced 

the risk of damage to sensitive buildings and 

materials by reducing the amount of SO2 

and NOx emitted into the atmosphere and 

the amount of dry sulfur deposition reaching 

sensitive structures. Therefore, ongoing 

monetary costs and cultural losses due to 

acid gases, particles, and deposition are also 

expected to be declining under Title IV. 
Chapter 3: Assessing Acid Deposition:


Advances in the State of the Science 


NAPAP has a long history of conducting 

research related to acid deposition. Throughout 

the 1980s NAPAP supported a large number of 

research projects that confirmed the link 

between SO2 and NOx emissions and acidic 

lakes and streams hundreds of miles away. 

Recent research is confirming the tight link between emissions of SO2 and the amount of several different forms of sulfur in the atmosphere 

and in precipitation, and improving our ability 

to measure acid deposition. Research has continued since 1990, albeit on a smaller scale, to 

increase our understanding of how acid deposition affects trees, soils, lakes, streams, coastal 

waters, and building materials such as stone 

and copper. Some of the areas where the most 

research has been done include: the importance 

of calcium in mediating soil and forest response 

to acidification; the role of nitrogen in acidification and recovery from acidification; and the 

role of nitrogen deposition in coastal 

ecosystems. In addition, substantial advances 

have been made in understanding the human 

health impacts of fine particles, including 

sulfates and nitrates, which are briefly 

mentioned here. 

Air Quality, Deposition, and 

Visibility 

Reductions of SO2 emissions from power 

generation under Title IV have resulted in 

significant declining trends in airborne concentrations of SO2 and sulfate throughout 

the Eastern U.S. Scientists have determined 

that the reduction in SO2 emissions has led 

to a comparable decrease in SO2 air 

concentrations, indicating that the relationship between emissions and air concentrations for SO2 is 

close to 1:1. For example, in the Midwest, which 

experienced some of the highest emission levels and the 

greatest annual change in SO2 emissions since 1990, the 

approximately 35% regional reduction of SO2 air concentrations corresponds well to the approximately 35% national 

reduction in SO2 emissions. Reductions in ambient airborne 

sulfate concentrations in the Midwest were not as linear; 

ambient sulfate concentrations declined by only 26%.90 The 

relationship between SO2 emission reductions and ambient 

sulfate concentrations is influenced by a number of chemical and meteorological factors that contribute to the nonlinear response between emission reductions and 

downwind concentrations.23 

Scientists have also recently documented that both wet 

and dry components of sulfur deposition (and the acidity 

associated with sulfur deposition) have declined in a near 

linear fashion with the decline in SO2 emissions impacting 

the Eastern U.S. 22–25 Strong correlations between largescale SO2 emission reductions and large reductions in 

sulfate concentrations in precipitation have been noted for 

the Northeast—one of the areas most affected by acid deposition.22 A main reason for reduced sulfate concentrations 

in precipitation in the Northeast is a reduction in the longrange transport of sulfate from emission sources located in 

the Ohio River Valley. The reductions in sulfate documented 

in the Northeast, particularly across New England and portions of New York State, were also affected by SO2 

emission reductions in eastern Canada. Factors influencing 

the decline of hydrogen ions (H+) in precipitation may include 

climate variability (i.e., wet or dry summers) or temporal 

changes in base cations and ammonium concentrations.22 

Concurrent with these sulfate reductions were similar 

reductions in precipitation acidity, expressed as H+ concentrations. However, acidity may not have decreased as dramatically as sulfate concentrations due to a simultaneous 

decline in deposition of acid-neutralizing base cations that 

act to buffer acidity. 

Emissions of NOx are transformed in the atmosphere into a 

series of pollutants, including nitric acid and nitrates. Nitric 

acid reacts with ammonia and primary particles such as sea 

spray, soil, and soot to form particle phase ammonium 

nitrate. From the perspective of acid deposition, nitric acid 
is an acidifying gas and a key chemical species for which 

the dry deposition monitoring networks now have an extensive record. While data from the monitoring networks 

provide strong evidence of statistically significant declining 

trends in the concentrations of sulfur species, trends in nitrogen concentrations have not been as pronounced over 

the past decade. 

Scientists are confident that the causes of acid rain and the 

response of deposition to reductions in emissions are well 

understood. Most of the recent advances in acid deposition 

research and monitoring, therefore, are related to improving 

the quality of data collected by the monitoring networks 

and developing new statistical methods of analyzing all existing data to better document any environmental change. 

Although several direct approaches to measuring dry deposition have been developed and applied over the past 

decade, inferential models that calculate dry deposition 

from atmospheric concentrations and other information 

have been the only practical way to quantify seasonal and 

annual dry deposition rates of acidic pollutants on the scale 

needed to operate a nationwide network.91 CASTNET and 

AIRMoN-Dry use this approach, which requires local measurements of ambient pollutant concentrations, observations 

of surface conditions, and standard surface meteorological 

conditions.92 Both these networks use the Multi-layer Model 

(MLM) to model air pollutant deposition velocity, a key 

component of dry deposition measurements. Scientists are 

working to improve the MLM and its effectiveness over a 

range of vegetation types and weather conditions. 

Research into deposition velocity modeling has led to innovations that will enable the development of a more 

accurate, next-generation-model.93 The Canadian Air and 

Precipitation Monitoring Network (CAPMON) and the 

European Monitoring and Evaluation Program (EMEP) have 

been able to “scale up” this approach from site-specific estimates to broader landscapes or regions. 

The air quality measurements required for using these inferential models are also being improved. Simultaneous sampling using colocated filter packs and duplicate annular 

denuder systems (ADS), revealed similar precision 

estimates for most of the measured species between the 

two approaches.94 Comparisons of weekly ADS results and 

composite weekly filter pack results generally show good 

agreement for sulfate and total nitrates. However, in some 

instances, filter pack nitric acid determinations may be 

biased high when substantial photochemical reactivity in 

the atmosphere is documented and filter packs may underestimate SO2 results by 12% to 20%. While perhaps significant, this amount of bias is not expected to substantially 

change the trend analyses in the Report. 

Researchers are also working to improve 

methods of measuring actual dry deposition 

that can provide higher quality data. Surface 

wash techniques such as throughfall (precipitation collected after it has passed through 

the tree canopy) and stemfall (also called 

stemflow; the process whereby precipitation 

drips down the branches and trunk of a tree 

or other rooted plant) that measure deposition quantities and chemical compositions 

are considered more reliable, especially for 

sulfur in complex mountainous terrain or locations where patchy forest conditions 

exist.95, 96 Although throughfall is not a widely 

used technique for routine atmospheric deposition monitoring, it is being evaluated by 

national dry deposition networks as an inexpensive method that produces high quality 

data representative of an area beyond a single point for a long period of time. At a minimum, throughfall is a cost-effective way to 

enhance inferential modeling results with actual field measurements. 

In recognition of the need to quantify total 

deposition rates across natural landscapes 

of mixed vegetation, sometimes in complex 

terrain, there is need to build upon the 

developments made in several other 

programs in EPA and in other agencies. For 

example, the aircraft flux systems developed 

within the NOAA AIRMoN program to 

address this issue offer a new opportunity to 

determine how surface complexity influences 

dry deposition rates. Moreover, the bounding 

techniques developed in the acid rain decade 

of the 1980s require new attention and verification, possibly using throughfall methods 

as well as micrometeorological models. 

The NADP/NTN, the largest wet deposition 

monitoring network in the U.S., is seeking 

new technologies to incorporate into their 

sample collection protocols to further 

improve the accuracy of the wet deposition 

monitoring. Improving wet deposition monitoring methods entails advancing sample collection techniques, the methods used for 

evaluating the data generated and the 

approaches for estimating site-specific and 

regional trends. 

A large amount of the research in the wet 

deposition field has involved developing new 
ways to analyze the monitoring data. To determine whether emission reductions have 

had their intended effect of reducing wet 

deposition, measurement data can be statistically modeled to characterize the 

magnitude and spatial distribution of reductions in wet deposition. The estimation of 

long-term trends with statistical models is 

affected by a number of factors such as the 

magnitude and shape of the underlying trend 

to be estimated and the length and variability of the data record. The advantage, however, is in deriving estimates of trend and 

uncertainty in data that have been adjusted 

for the sources of variability that may also 

influence the observed temporal pattern.97 

Aquatic and Forest 

Ecosystems 

Soils, forests, surface waters, and aquatic 

biota (e.g., fish and algae) are inextricably 

linked, sharing water, nutrients, and the other essential building blocks of a successful 

ecosystem. Therefore, to understand the full 

effects of acid deposition it is necessary to 

understand the interactions between all of 

these systems. For example, sulfur and nitrogen from acid deposition can be retained in 

watershed soils and released as sulfate and 

nitrate drainage into lakes and streams even 

after the levels of acid deposition decrease. 

The effect of acid deposition on an 

ecosystem depends largely upon the ecosystem’s ability to neutralize the acid.30 This is 

referred to as an ecosystem’s acid neutralizing capacity (ANC). 

Acid neutralization occurs when positively 

charged ions such as calcium, potassium, 

sodium, and magnesium, collectively known 

as base cations, are released. As water 

moves through a watershed, two important 

chemical processes act to neutralize acids. 

The first involves cation exchange in soils, a 

process by which hydrogen ions from the 

acid deposition displace other cations from 

the surface of soil particles, releasing these 

cations to soil and surface water. The second 

process is mineral weathering, where base 

cations bound in the mineral structure of 

rocks are released as the minerals gradually 

break down over long time periods. As the base cations are 

released by weathering, they neutralize acidity and increase 

the pH level in soil water and surface waters. Acid deposition, because it consists of acid anions (e.g., sulfate, 

nitrate), leaches some of the accumulated base cation 

reserves from the soils into drainage waters. The leaching 

rate of these base cations may accelerate to the point 

where it significantly exceeds the resupply via weathering 

and atmospheric deposition.30 

FRESHWATER ACID-SENSITIVE ECOSYSTEMS 

Acid deposition causes acidification of sensitive surface 

waters. These waters are sensitive due to their position in 

the landscape, geomorphology, and/or watershed soils. In 

the 1980s, acid rain was found to be the dominant cause of 

acidification in 75% of acidic lakes and 50% of acidic 

streams.31 Areas especially sensitive to acidification include 

portions of the Northeast (particularly Maine and the 

Adirondack and Catskill Mountains) and southeastern 

streams. Some high elevation western lakes, particularly in 

the Rocky Mountains, have become acidic, especially 

during snowmelt. However, although many western lakes 

and streams are sensitive to acidification, they are not subject to continuously high levels of acid deposition and so 

have not become chronically acidified.31 A significant 

amount of research conducted since 1990 has focused on 

western acid-sensitive ecosystems. 

ANC, a key indicator of the ability of the water and watershed soil to neutralize the acid deposition it receives, depends largely on the watershed’s physical characteristics: 

geology, soils, and size. Waters that are sensitive to acidification tend to be located in small watersheds that have 

few alkaline minerals and shallow soils. Conversely, 

watersheds with high ANC tend to contain alkaline minerals, such as limestone. Larger watersheds with well developed riparian zones generally have a greater capacity to 

neutralize the acids being transported from their headwaters, but large forested watersheds have been shown to 

acidify during large rainfall and snowmelt episodes.98 

As acidity increases, aluminum leached from the soil flows 

into lakes and streams and can be toxic to aquatic 

species. The lower pH levels and higher aluminum levels 

that result from acidification make it difficult for some fish 

and other aquatic species to survive, grow, and reproduce. 

In some waters, the number of species of fish able to survive has been directly correlated to water acidity. 

Acidification can also decrease fish population density and 

individual fish size.30 

Recent watershed mass balance studies in the Northeast 

reveal that loss of sulfate from the watershed exceeds 

atmospheric sulfur deposition.99 This suggests that these 
soils have become saturated with sulfur, meaning that the 

supply of sulfur from deposition exceeds uptake by biotic 

and abiotic processes in the ecosystem. As a result, sulfur 

is gradually being released or leached from the watershed 

into the surface waters as sulfate. Scientists now expect 

that the release of sulfate that previously accumulated in 

watersheds will delay the recovery of surface waters in the 

Northeast that is anticipated in response to the recent SO2 

emission controls.99 

A recent study at a stream in the Catskill Mountains found 

that stream nitrate concentrations were positively correlated to mean annual air temperature but not to annual nitrogen deposition.33 This research suggests that, in soils with 

large amounts of nitrogen, microbial processes (N mineralization and nitrification), that are sensitive to changes in 

temperature and moisture, are the primary factors controlling nitrate leaching, rather than atmospheric deposition or 

vegetation uptake of nitrogen. Therefore, declines in nitrogen deposition in nitrogen-saturated soils may not immediately lead to improvements in streamwater chemistry.

33 

A major study of the ecological response to acidification is 

taking place in the Bear Brook watershed in Maine. 

Established in 1986 as part of the U.S. Environmental 

Protection Agency’s Watershed Manipulation Project, the 

project has found that experimental additions of sulfur and 

nitrogen to the watershed increased the concentrations of 

both sulfate and nitrate in the West Bear Brook stream. 

Streamwater concentrations of several other ions, including 

base cations, aluminum, and ANC changed substantially as 

well.100 During the first year of treatment, 94% of the nitrogen added experimentally to the Bear Brook watershed was 

retained, while the remainder leached into streams as 

nitrate. Nitrogen retention decreased to about 82% in subsequent years.101, 102 Although the forest ecosystem continued to accumulate nitrogen, nitrate leaching into the stream 

continued at elevated levels throughout the length of the 

experiment. This nitrate contributed to both episodic and 

chronic acidification of the stream. This and other similar 

studies have allowed scientists to quantify acidification 

processes in eastern watersheds in much more detail than 

was possible in 1990. 

The Appalachian Mountain region receives some of the 

highest rates of acid deposition in the United States.103 The 

acid-base status of streamwaters in forested upland 

watersheds in the Appalachian Mountains was extensively 

investigated in the early 1990s.103–106 A more recent assessment of the southern Appalachian region from West 

Virginia to Alabama identified watersheds that are 

sensitive to acid deposition using geologic bedrock and the 

associated buffering capacity of soils to neutralize acid. 

The assessment found that approximately 59% of all trout 

stream length in the region is in areas that 

are highly vulnerable to acidification and 

that 27% is in areas that are moderately vulnerable.107 Another study estimated that 

18% of potential brook trout streams in the 

mid-Appalachian Mountains are too acidic 

for brook trout survival.108 Perhaps the most 

important study of acid-base chemistry of 

streams in the Appalachian region in recent 

years has been the Virginia Trout Stream 

Sensitivity Study.

105 Trend analyses of these 

streams indicate that few long-term 

sampling sites are recovering from acidification, most are continuing to acidify, and the 

continuing acidification is at levels that are 

biologically significant for brook trout populations.39 

During the 1980s and 1990s, an integrated 

study of atmospheric deposition, terrestrial 

ecosystems, and aquatic ecosystems was 

conducted in several watersheds in the 

Sierra Nevada Mountains to determine if 

acid deposition was affecting these areas 

and to infer the implications of acidification 

on other surface waters in the region.109–111 

Nitrogen deposition in this region is due to a 

combination of sources, including power 

plant emissions, vehicle emissions, and 

emissions from agricultural activities. 

Chronic acidification of high elevation 

surface waters in the Sierra Nevada was not 

found, but episodic changes in streamwater 

chemistry did occur. In many of the 

watersheds studied, for example, pH 

decreased (waters became more acidic) with 

increasing runoff, reaching a minimum near 

peak snowmelt.111 In contrast, an accumulating weight of evidence has led some 

researchers to conclude that high-altitude 

watersheds in the Colorado Front Range 

show symptoms of nitrogen saturation (as 

opposed to acidification).43, 44 

Further west, levels of streamwater and 

groundwater nitrate in the San Gabriel and 

San Bernardino Mountains have been found 

to be strongly linked to the magnitude of nitrogen deposition in watersheds throughout 

the region. Streamwater nitrate 

concentrations at Devil’s Canyon in the 

San Bernardino Mountains112 and in chaparral watersheds with high smog exposure in 

the San Gabriel Mountains northeast of 
Los Angeles113, 114 are the highest in North 

America for forested watersheds. Chronic nitrogen deposition and nitrate export from 

these watersheds contribute to the groundwater nitrate problems in the eastern San 

Gabriel Basin where levels often exceed the 

federal drinking water standard.

114 

FOREST ECOSYSTEMS 

Current understanding of the effects of acid 

deposition on forest ecosystems has come 

to focus increasingly on the biogeochemical 

processes that affect plant uptake, retention, 

and cycling of nutrients within forested 

ecosystems. Research results from the 

1990s indicate that documented decreases 

in base cations (calcium, magnesium, potassium, and others) from soils in the 

Northeastern and Southeastern United 

States are at least partially attributable to 

acid deposition.65, 115 Base cation depletion is 

a cause for concern because of the role 

these ions play in acid neutralization and, in 

the case of calcium, magnesium and potassium, their importance as essential nutrients 

for tree growth. It has been known for some 

time that depletion of base cations from the 

soil interferes with the uptake of calcium by 

roots in forest soils.116 Recent research indicates it also leads to aluminum 

mobilization117 which can have harmful 

effects on fish.30 

A recent study using strontium isotopes to 

investigate base cation nutrition in acid deposition impacted forests raises new 

concerns about the capacity of mineral 

weathering to replenish exchangeable base 

cations. Unpolluted temperate forests can 

become nutritionally decoupled from deeper 

weathering processes, virtually functioning 

as atmospherically-fed ecosystems.118 One 

group of researchers showed that base 

cation turnover times are considerably more 

rapid than previously recognized in the plant 

available pool of soil.118 These results 

challenge the prevalent paradigm that plants 

largely feed on rock-derived cations and 

have important implications for understanding sensitivity of forests to air pollution. 

The plant physiological processes affected 

by reduced calcium availability include cell 

wall structure and growth, carbohydrate metabolism, stomatal regulation, resistance to plant 

pathogens, and tolerance of low temperatures.52 Soil structure, macro and micro fauna, decomposition rates, and 

nitrogen metabolism are also important processes that are 

significantly influenced by calcium levels in soils. The importance of calcium as an indicator of forest ecosystem 

function is due to its diverse physiological roles, coupled 

with the fact that calcium mobility in plants is very limited 

and can be further reduced by tree age, competition, and 

reduced soil water supply.

119 

A clear link has now been established in red spruce stands 

between acid deposition, calcium supply, and sensitivity to 

abiotic stress. Red spruce uptake and retention of calcium 

is impacted by acid deposition in two main ways: leaching 

of important stores of calcium from needles;52 and 

decreased root uptake of calcium due to calcium depletion 

from the soil and aluminum mobilization.115, 120, 121 Acid deposition leaches calcium from mesophyll cells of one-year old 

red spruce needles,122 which in turn reduces freezing tolerance.52 These changes increase the sensitivity of red spruce 

to winter injuries under normal winter conditions in the 

Northeast, result in the loss of needles, and impair the 

overall health of forest ecosystems.52 Red spruce must also 

expend more metabolic energy to acquire calcium from soils 

in areas with low calcium/aluminum ratios, resulting in 

slower tree growth.120 

Losses of calcium from forest soils and forested 

watersheds have now been documented as a sensitive early 

indicator of the soil response to acid deposition for a wide 

range of forest soils in the Northeastern U.S.53, 69 There is a 

strong relationship between acid deposition and leaching of 

base cations from soils in hardwood forests (e.g., maple, 

oak), as indicated by long-term data on watershed mass 

balances,53, 123 plot- and watershed-scale acidification experiments in the Adirondacks 124 and in Maine,125, 126 and studies 

of soil solution chemistry along an acid deposition gradient 

from Minnesota to Ohio.127 

Although sulfate is the primary cause of base cation leaching, nitrate is a significant contributor in watersheds that 

are nearly nitrogen saturated.128 Recent studies of the 

decline of sugar maples in the Northeast demonstrate a link 

between low base cation availability, high levels of 

aluminum and manganese in the soil, and increased levels 

of tree mortality due to native defoliating insects.74 The 

chemical composition of leaves and needles may also be altered by acid deposition, resulting in changes in organic 

matter turnover and nutrient cycling. The sensitivity of 

hardwood soils to acid deposition is largely controlled by inherent properties and land use; unfortunately, tools to 

assess present conditions or susceptibility to nutrient 
depletion are not readily available or widely applicable. 

However, recent studies have shown that forest harvesting 

has the potential to greatly accelerate calcium losses 

currently experienced due to leaching in low cation soils.129 

The last NAPAP assessment reported that significant 

impacts of acid deposition had not been detected over extensive forested areas in the southern pine and pinehardwood region. That understanding has not changed, and 

there is very little new information to report for this region. 

Acid deposition is contributing significantly to the depletion 

of base cations in many poorly buffered soils where southern pines grow. However, short-term positive effects on 

tree growth in some nitrogen deficient soils are also expected due to higher levels of nitrogen deposition.29 

A significant amount of research has been conducted since 

1990 on the effects of nitrogen deposition in the Los 

Angeles air basin in southern California. Nitrogen 

enrichment, in combination with ozone exposure, causes 

major changes in tree health by reducing fine root biomass 

and carbon allocation below ground and by greatly 

decreasing the life span of pine foliage.130, 131 Nitrogen 

enrichment results in greater leaf growth, while ozone 

causes premature leaf loss at the end of the growing season. The net result of these pollutants is significant litter 

accumulation on the forest floor.

132 Nitrogen cycling rates in 

soil are also stimulated by the high nitrogen inputs, resulting in large leachate losses of nitrate from these 

watersheds and elevated fluxes of nitric oxide gas from 

soil.51, 112, 132 In coastal sage ecosystems that occur in the 

low elevation sites, greenhouse and field studies indicate 

that nitrogen deposition may be one factor enhancing the 

invasion of exotic annual grasses.133–135 Recent controlled 

exposure studies demonstrate that nitric acid vapor 

(HNO3), a component of photochemical smog, injures 

leaves of wildland shrub and tree species at near-ambient 

doses in the Los Angeles air basin.136 

COASTAL ECOSYSTEMS 

Nitrogen is present in high enough quantities in many 

coastal ecosystems to be identified as a pollutant. There 

are many sources of nitrogen to estuaries and coastal 

waters, including point discharges from wastewater 

treatment plants, urban and agricultural runoff, and atmospheric deposition. Since 1990, a large amount of research 

has been conducted on the impact of nitrogen deposition to 

coastal waters. It is now known that nitrogen deposition is 

a significant source of nitrogen to many estuaries.137, 138 The 

amount of nitrogen entering estuaries due to atmospheric 

deposition varies widely, depending on the size and location 

of the estuarine watershed and other sources of nitrogen in 

the watershed. There are a handful of estuaries where 

atmospheric deposition of nitrogen 

contributes well over 40% of the total nitrogen load; however, in most estuaries where 

estimates exist, the contribution from atmospheric deposition ranges from 15–30% (see 

Table 1). The areas with the highest deposition rates stretch from Massachusetts to 

the Chesapeake Bay, and along the central 

gulf coast. 

Nitrogen is often the limiting nutrient in 

coastal ecosystems. Increasing the levels of 

nitrogen in coastal waters can cause significant changes to those ecosystems. 

Approximately 60% of estuaries in the U.S. 

(65% of the estuarine surface area) suffer 

from over-enrichment of nitrogen, a 

condition known as eutrophication.83 

Symptoms of eutrophication include 

changes in the dominant species of 

plankton (the primary food source for many 

kinds of marine life) that can cause algal 

blooms, low levels of oxygen in the water 

column, fish and shellfish kills, and cascading population changes up the food chain. 

In addition, increased levels of turbidity in 

the water due to large amounts of algae 

can kill off submerged aquatic vegetation 

(SAV), which is important habitat for many 

estuarine fish and shellfish species. Many 

of the most highly eutrophic estuaries are 

along the gulf and mid-Atlantic coasts, 

overlapping many of the areas with the 

highest nitrogen deposition, but there are 

eutrophic estuaries in every region of the 

conterminous U.S. coastline. 

Materials and Structures 

SO2 and NOx emissions, as well as many of 

the compounds they form in the atmosphere, are damaging certain materials, primarily cultural resources made of stone. 

Bronze, paint, and other surfaces and surface coatings are also susceptible to damage from acid pollutants and deposition. 

Weathering due to acid deposition often 

harms cultural assets (e.g., statues and 

monuments) more than purely operational 

resources (e.g., bridges and buildings). This 

is because the appearance of cultural 

resources, where much of their value lies, 
is particularly vulnerable to damage. There 

are also historic and emotional values 

attached to cultural assets that increase 

the value of their preservation. 

A recent review of the scientific literature 

on the contribution of acid deposition to 

stone deterioration indicates that dry deposition, which is mainly influenced by shortrange transport of pollutants from local 

sources, is the main source of damage.88 A 

secondary source of damage is wet deposition from long-range transport of 

pollutants. In areas where buildings and 

monuments remain wet for long periods of 

time, and in rural areas, wet deposition can 

be a major source of damage. If pollution is very low, this 

phenomenon becomes indistinguishable from the “normal” 

weathering of stone due to the dissolution effect from 

pure water.88 

Although the details of the mechanisms involved in 

pollutant-induced deterioration of materials are not fully understood, various kinds of research are expanding the base 

of knowledge available to address the problem. Damage to 

stone from acid deposition is a complex process that 

depends upon many factors. The type of stone, how it 

weathers, the porosity and texture of the stone, and even 

how it is cleaned, all contribute to the type and rate of damage. Geographic location and the natural environment of 

the stone also affect the extent and magnitude of damage. 

For example, stone degradation rates are negligible in dry 
areas such as the Southwestern United States. 

Stone weathering involves interactions between ever 

changing meteorological variables—rain, wind, 

temperature, humidity, and continually changing complex 

stone surfaces. The way water flows over a carved stone 

surface influences the mobilization of salts and particulate 

materials on the surface and can change the stone’s chemistry and porosity. A slight change in the overall shape of 

the surface, either due to weathering or by design, may 

change the way water flows over the surface and can radically influence the erosion and soiling patterns.29 

The porosity of the stone is another critical factor within 

the deposition process. Once sulfur dioxide gets into the 

pores, it is difficult to get out. Cleaning treatments have 

the potential to change the surface texture; therefore, some 

cleaning methods may result in increased pollutant deposition and subsequent damage to the building stone. 

Although scientists continue to study stone systems and 

describe the uptake of air pollution by stone, it is unlikely 

that research will define dose-response functions in the 

near future. Research to date leads scientists to believe 

that there is no pollution threshold below which no damage 

will occur; rather, any amount of SO2 will lead to some level of deterioration. In addition, the time it takes for stone 

to respond to pollution is extremely long; effects can 

appear 25, 50, or even 100 years after exposure.29 This understanding has led recent research efforts to focus more 

on treatments and mitigation for existing damage and 

methods to prevent damage to stone structures. However, 

preliminary attempts at preventing damage and restoring 

materials have met with limited success. 

Research is progressing on the development of new 

treatments to strengthen stone, including limestone and 

marble, that has been degraded by SO2.

139, 140 Chemical 

interactions between SO2 and stone leads to the formation 

of calcium sulfate, or gypsum. Because calcium sulfate is 

relatively water soluble, rain can dissolve and remove material, including the cementitious materials that bind the 

grains of stone together. The result is a weak, “sugaring” 

stone. Several methods are being investigated to strengthen these types of stone. One is ethyl silicate-based consolidants. The main limitation of using these products is their 

reported cracking behavior during drying. A more promising 

method under investigation is particle modified 

consolidants. These compounds consist of a silicate matrix 

plus colloidal oxide particles that are used to consolidate 

degraded stone. The presence of the particles physically 

limits the silicate network from shrinking under capillary 

pressure, and thereby reduces strength loss during drying. 

In addition, because the dried consolidants 

remain porous, the network maintains a 

higher permeability. 

In addition to this work on stone, additional 

research is focused on developing new coating systems to protect outdoor bronze.141 

Atmospheric pollutants such as SO2, NOx, 

CO2, and chlorides affect various materials 

including bronze, and cause greater 

corrosion. The research explores new coating 

technologies that would be appropriate for 

outdoor bronze sculptures. The coatings currently under investigation include a 

fluorocopolymer blended with various 

acrylics, conductive polymers, BTA pretreatments, and a very fine titanium dioxide. 

These coatings are being studied on both 

rolled bronze and satin-finish, cast monumental bronze substrates using electrochemical 

impedance spectroscopy. 

Human Health 

Scientific understanding of the human health 

effects of fine particles has changed dramatically since 1990. In 1996, EPA published Air 

Quality Criteria for Particulate Matter 

(“Criteria Document”), a comprehensive review and summary of the scientific 

literature.142 At that time, a large body of epidemiological evidence indicated that serious 

health effects were associated with PM at 

levels even below the then-current air quality 

standards. These health effects include: 

• Premature death and increased hospital 

admissions and emergency room visits, especially for heart and lung diseases, primarily in the elderly and individuals with 

pre-existing cardiopulmonary disease. 

• Increased respiratory symptoms and 

disease, primarily in children and individuals with cardiopulmonary disease such as 

asthma. 

• Decreased lung function, particularly in 

children and individuals with asthma. 

• Alterations in lung tissue and structure and 

in respiratory tract defense mechanisms. 
The epidemiology studies provided 

evidence for effects with both acute (e.g., 

hours or days) and long-term (e.g., months 

or years) exposures to PM. Important 

uncertainties remained, however, such as 

issues related to the interpretation of the 

role of gaseous co-pollutants in PM associations with health effects and the lack of 

accepted biological mechanisms that could 

explain observed effects. 

An unprecedented number of new studies 

containing further evidence of serious health 

effects have been published since the 1996 

Criteria Document, with important new 

information coming from epidemiological, 

toxicological, controlled human exposure, 

and dosimetry studies. For example, important new epidemiological studies include 

multi-city studies that use uniform methodologies to investigate the effects of PM on 

health with data from multiple locations with 

varying climate and air pollution mixes.143 

The multi-city studies have increased confidence that the associations found with PM 

are not an artifact of associations with 

potential confounders such as the gaseous 

co-pollutants. In addition, a comprehensive 

validation and reanalysis study has used 

data from two cohort studies (the “six 

cities” and American Cancer Society 

cohorts); the results have generally supported the findings of the earlier studies showing associations between premature 

mortality and long-term PM exposure.144 

Many epidemiology studies have used measurements of PM10 or other PM indicators. As 

stated previously, the full body of scientific 

studies provided consistent and coherent evidence that adverse health effects were associated with ambient PM. A number of the 

new studies used measured fine particle 

concentrations (e.g., PM2.5) or other fine 

particle indicators (e.g., sulfates, black 

smoke). This group of studies indicated that 

fine particles are likely important 

contributors to the observed PM-associated 

effects. A number of new studies have evaluated independent associations between effects and fine- and coarse-fraction particles, 

and the results support the earlier studies’ 

findings of associations between serious health effects and 

ambient fine particles. Other organizations have reviewed 

the health evidence and drawn similar conclusions; for 

example, Health Canada and the World Health Organization 

have recently published scientific reviews, finding that it is 

largely the fine fraction of particulate matter that is 

involved in exacerbations of cardiorespiratory disease.145, 146 

In addition, new toxicological and controlled human 

exposure studies have provided insight into potential mechanisms for PM-related effects. These studies include animal 

and controlled human exposure studies using concentrated 

ambient particles, new indicators of response (e.g., heart 

rate variability), as well as animal models representing sensitive subpopulations. Some potential mechanisms and the 

available evidence regarding those mechanisms have been 

summarized in recent review articles.147, 148 These potential 

mechanisms include effects of oxidative stress in the respiratory tract, effects on the autonomic nervous system, 

changes in the blood system to increase coagulability of the 

blood, or lung injury and inflammation. While evidence is 

not available to fully support any of these mechanisms, it is 

likely that there will be multiple mechanisms underlying the 

range of PM-related health effects. 

For additional information on the latest research regarding the 

health impacts of fine particles, see “Air Quality Criteria for 

Particulate Matter” (October 2004) volumes I & II, which 

can be found at http://cfpub2.epa.gov/ncea/cfm/ 

recordisplay.cfm?deid=87903 

Modeling 

Researchers have used models to help them understand 

and predict atmospheric, environmental, and human health 

responses to acid deposition for well over 20 years. Since 

1990, there have been significant improvements in 

atmospheric modeling capabilities due to better 

understanding of the atmospheric chemistry and physics 

that play a role in transport and deposition of acid 

compounds, as well as significant improvements in computing capabilities. New models have also been developed that 

allow researchers to model sulfur and nitrogen transport 

and deposition nationwide (previously used models for NAPAP covered only the Eastern U.S.). Watershed modeling 

capabilities have also improved as researchers are continuing to refine and expand models that project acidification of 

waterbodies. A significant amount of research published 

since 1990 detailing how fine particle concentrations 

adversely impact human health has made human health 

benefits modeling an integral part of the assessment of the 

benefits of Title IV. 
ATMOSPHERIC MODELING 

Model developments have continued, especially for intended 

application in other parts of the world. In Europe, the Multilayer Acid Deposition model for Europe (MADE-50) has 

been developed to address the deposition of a range of sulfur 

and nitrogen species.149 For application in Asia, the Sulphur 

Transport Eulerian Model 2 (STEM-II) has been expanded to 

address a number of chemical species, including photochemical oxidants. Many other models have been developed, 

some addressing seasonal and annual averages and others 

targeting shorter periods of intensive investigation.150 

Nonlinear Responses to Decreases in SO2	

and NOx Emissions 

The relationships between reductions in SO2 and NOx 

emissions and changes in sulfate and nitrate formation 

involve a complex group of gas- and aqueous-phase 

chemical reactions between acid deposition and aerosol 

precursors and oxidants. These reactions can produce 

nonlinear responses to emission reductions.151 For example, reducing NOx while leaving SO2 unchanged can lead 

to an increase in sulfate formation under certain 

conditions. Reducing NOx emissions could increase the 

concentrations of the oxidant hydrogen peroxide (H2O2). 

Similarly, reducing SO2 but not NOx could, under certain 

conditions, lead to increased nitrate formation. In gasphase/aqueous-phase chemistry, ammonia 

preferentially neutralizes sulfate over nitrate 

to form ammonium sulfate aerosols. If SO2 is 

reduced, the excess ammonia that becomes 

available can neutralize nitric acid and form 

ammonium nitrate aerosols. These aerosols 

absorb gas-phase nitric acid, which increases 

the formation of particle nitrate. 

This non-linear chemistry makes it important 

to reduce emissions of SO2 and NOx

simultaneously whenever possible to get the 

full health and environmental benefits of reduced particle formation. 

WATER/WATERSHED MODELING 

The Model of Acidification of Groundwater 

in Catchments (MAGIC) is a prominent 

model developed to estimate acidification of

lakes and streams in response to sulfur deposition.152–154 MAGIC was the principal model 

used by NAPAP to estimate future damage 

to lakes and streams in the Eastern United 

States in the 1990 Integrated Assessment. 

MAGIC is a lumped-parameter model of intermediate complexity, developed to predict 

the long-term effects of acidic deposition on 

SPOTLIGHT: ADVANCES IN ATMOSPHERIC MODELING 

Throughout the 1980s, the scientific community 

dedicated significant resources to developing 

acid deposition modeling capabilities. The 

NAPAP 1990 Integrated Assessment primarily 

relied on a then state-of-the-art model called the	

Regional Acid Deposition Model (RADM). 

RADM was developed beginning in 1985 under 

the auspices of NAPAP to address policy and 

technical issues associated with acid	

deposition.31 The model was designed to provide 

a scientific basis for predicting changes in deposition resulting from changes in sulfur and nitrogen emissions, to predict the influence of	

emission sources in one region on acid 

deposition in other regions, and to predict the 

levels of acid deposition in certain sensitive 

receptor regions.	

The basic framework for acid deposition models 

has not changed since 1990. No major overhauls 

have been made; rather, the understanding of 

particular chemical and physical interactions has 

improved, and it has been possible to 

incorporate more complex aspects of 

atmospheric modeling physics and chemistry 

into acid deposition models. In addition, 

improvements in inventories make the data fed 

into the models more accurate, which improves 

overall model accuracy and credibility. Finally, 

improvements in computing power allow 

computers to do more calculations in less time, 

improving the speed at which models can operate, broadening the types of computers on which 

models can be run, and expanding the practical 

limit of complexity a model can achieve. 

There are a large number of atmospheric and 

deposition models that have been used to 

assess the relationship between emissions and 
acid deposition. The following are general 

descriptions of the most commonly used models 

in the U.S. 

• Regional Acid Deposition Model (RADM). 

RADM has been improved over the past 

decade by upgrading various modules to 

better represent the governing physical and 

chemical atmospheric processes. The most recent version is RADM 2.61. The Regional 

Particulate Model (RPM) is an extension of 

RADM that includes the chemistry and dynamics of atmospheric aerosol particles.155 The fullscale version of RADM is resource-intensive 

and has typically been applied on 

supercomputers. While the model is primarily 

designed for episodic applications, it has also 

been used to understand long-term 

relationships between changing emissions patterns and pollutant concentrations and deposition using an episode aggregation technique. 

RADM and RPM have been applied primarily 

to eastern North America; An international 

version of RADM, EURAD, has been applied in 

Europe.155, 156 RADM has also been used to 

identify emission source-regions affecting the 

Chesapeake Bay watershed.157 

• Regulatory Emissions Modeling System for 

Aerosols and Deposition (REMSAD). 

REMSAD was developed by ICF Consulting 

for the EPA. It is based on an Eulerian (grid) 

approach and may be applied at scales ranging 

from a single metropolitan region to a 

continent containing multiple urban areas. It 

was designed to be capable of simulating the 

complex long-range transport and deposition 

of atmospheric pollutants to aquatic 

environments and to assess the relative 

impacts of alternative control strategies. 

Although initially developed to study the transport and removal of airborne toxics, the interdependence of the processes that also control 

the formation and removal of particles was 

recognized, and therefore the model was 

designed for both toxics and particulate matter 

applications. Annual simulations of the entire 

U.S. are practical. 

• Community Multi-scale Air Quality (CMAQ). 

Models 3-CMAQ was developed by EPA to 

evaluate the impact of multiple pollutants at 

multiple temporal and spatial scales. The 

Models-3 framework provides tools to prepare 

emissions and meteorological inputs, define 

emissions control strategies, project future 

emissions inventories, execute meteorological 

models, delineate a geographic domain, select 

alternative atmospheric chemical reaction 

mechanisms, set vertical and horizontal grid 

resolutions, and manage a series of air quality 

model runs. CMAQ is applied through the 

Models-3 system. It contains state-of-thescience simulations of atmospheric transport 

processes, atmospheric chemistry, aerosol 

dynamics and chemistry, cloud chemistry and 

dynamics, and deposition processes. A key 

aspect of the Model-3/CMAQ system’s structure is its flexibility to incorporate scientific and 

modeling advances, to test alternative modeling approaches, and to link with human and 

ecosystem exposure models. In addition to 

combining point and area source emissions 

into a single three-dimensional hourly file as in 

RADM, CMAQ provides the option of treating 

large point sources separately using a plumein-grid approach that has a gridded structure 

that expands as the plume moves downwind. 

• Acid Deposition and Oxidant Model 

(ADOM). The ADOM is a comprehensive 

three-dimensional Eulerian episodic model developed in the 1980s in parallel with RADM. 

RADM and ADOM are the two comprehensive 

models that were used for the NAPAP 1990 

Integrated Assessment. The development of 

ADOM was sponsored by the Ontario Ministry 

of the Environment, the Atmospheric 

Environment Service of Environment Canada, 

the German Umweltbundesamt, and the 

Electric Power Research Institute. ADOM is 

primarily intended for episodic applications because of its computational resource 

requirements. An episode aggregation 

technique, however, based on the approach 

developed for RADM during NAPAP, has been 

developed for predicting changes in annual 

deposition resulting from changes in 
emissions.158 Like RADM, ADOM has been primarily applied to eastern North America. The 

reduced form, regional haze version of the 

model (VISHWA) has been applied to the 

western part of North America (including 

southern Canada and northern Mexico), while 

the mercury version of the model (TEAM) has 

been applied to the entire contiguous United 

States. ADOM has not been significantly 

updated since 1992. Many improvements are 

currently being made by Environment Canada, 

which is building the successor to ADOM, referred to as the AES Unified Regional Air 

Quality Modeling System (AURAMS), using 

existing and new modules as appropriate. 

• Advanced Statistical Trajectory Regional Air 

Pollution (ASTRAP). The ASTRAP model, developed at Argonne National Laboratory in the 

early 1980s, is a Lagrangian model designed to 

simulate long-term (annual or seasonal) 

average concentrations and deposition fluxes. 

ASTRAP was used as the atmospheric 

pathways module of the Tracking and Analysis 

Framework (TAF) for the 1998 NAPAP report.29 

The treatments of chemistry and dry and wet 

removal in ASTRAP are relatively simple compared to the Eulerian models described in this 

section. Because of the model’s simplicity, ASTRAP can be applied for long-term simulations 

and with relatively little computer cost. While 

much of the basic chemistry is well-represented in the model, it fails to treat aerosols or the 

non-linearity of the aqueous-phase SO2/sulfate 

chemistry. Therefore, a key use of the model is 

as a screening tool to identify scenarios to be 

run in more complex models. 

• Urban and Regional Multiscale Model 

(URM). The URM model is a three-dimensional 

Eulerian grid model designed primarily for

episodic simulations. The model evolved from 

the CIT (Carnegie Mellon/California Institute

of Technology) airshed model developed and 

upgraded over a 20-year period159–161 and is

now available in the public domain from the 

Georgia Institute of Technology. URM can be 

run on a workstation but the model is 

moderately difficult to use, and users must 

receive initial guidance and training. URM was 

recently extensively updated as part of the 

southern Appalachian Mountains Initiative 

(SAMI) program to incorporate cloud and 

aerosol processes. SAMI is also evaluating the 

new version of the model by comparing model 

predictions of ozone, particulate sulfate, 

nitrate, ammonium, soil, organic aerosols, elemental carbon, PM2.5 and PM10 concentrations 

and sulfate, nitrate, ammonium, and hydrogen 

ion wet deposition with corresponding 

measurements from 22 sites. 

surface water chemistry. The model simulates soil solution 

chemistry and surface water chemistry to predict the 

monthly and annual average concentrations of the major 

ions in waterbodies. At the heart of MAGIC is the size of 

the pool of exchangeable base cations in the soil. As the 

fluxes to and from the pool change over time due to 

changes in atmospheric deposition, the chemical equilibria 

between soil and soil solution shift to give changes in surface water chemistry. 

The original data collected on acidification in lakes and 

streams, the National Surface Water Survey (NSWS), used 

statistically representative methods to sample approximately 28,000 lakes and 56,000 stream reaches in the U.S.36 The 

MAGIC model predicts changes in lakes and streams that 

are a statistically representative subset of the waterbodies 

from the NSWS survey and, therefore, of 

the region being modeled. Therefore, within 

the limitations of the NSWS survey (primarily the limitation that it only sampled lakes 

larger than 4 hectares), the MAGIC model 

can estimate the level of acidification in waterbodies throughout the entire Adirondack, 

northeastern, and southeastern Appalachian 

regions of the U.S. Although there are some 

uncertainties with regard to the model, particularly concerning watershed nitrogen dynamics, MAGIC provides a generally 

accurate, well-tested, and widely accepted 

tool for modeling the response of surface 

water chemistry to sulfur deposition. 
PnET-BGC is an integrated biogeochemical 

model that simulates the concentrations and 

transport of major elements, including nitrogen, in forest vegetation, soil, and water. The 

model enables simultaneous simulation of 

major element cycles in forest and interconnected aquatic ecosystems.162 PnET-BGC 

uses measured and estimated data on meteorology and atmospheric deposition to simulate estimates of changes in atmospheric 

deposition from 1850 to the present day. 

Future scenarios of changes in atmospheric 

deposition are simulated using projections 

provided from air quality modeling based on 

model runs of air emission control scenarios. 

PnET-BGC has primarily been applied to forest watersheds in the Northeastern U.S. 

HEALTH EFFECTS MODELING 

When the Clean Air Act Amendments were 

passed in 1990, the health effects of fine 

particles were just beginning to be 

understood. In the years since, a series of 

studies have demonstrated the relationship 

between exposure to fine particles and a series of health impacts, including premature 

mortality and asthma attacks.163 As a result 

of this new understanding, NAPAP now 

includes the estimated health benefits of 

SO2 and NOx emission reductions and the 

monetary impact of those benefits as part of 

its analysis of the impacts of Title IV. 

EPA developed a new methodology for quantifying and 

monetizing the health benefits of improved air quality as 

part of EPA’s Report to Congress in 1999 entitled The 

Benefits and Costs of the Clean Air Act 1990–2010.164 

Each phase of the Report’s preparation, which spanned 

nearly three years, was thoroughly reviewed by academic 

and industry experts serving on EPA’s Science Advisory 

Board Council on Clean Air Compliance Analysis. 

Currently, health benefits are calculated using the same 

concentration-response functions and economic valuation 

functions that EPA uses in analysis of major rules that address fine particles and ozone. 

The air quality modeling data serve as inputs to both the 

Benefits Model for Air Quality (BenMod) and the Benefits 

Mapping and Analysis Program (BenMAP), two different 

modeling systems that translate air quality changes to 

changes in health outcomes (e.g., premature mortality, 

emergency room visits, cases of chronic bronchitis, asthma 

attacks) through the use of concentration-response 

functions. Peer-reviewed, published scientific literature on 

the health effects of air pollutants provides the source of 

these concentration-response functions. Health benefits are 

related to the change in air pollutant exposure experienced 

by individuals. Because the expected changes in pollutant 

concentrations vary from location to location, individuals in 

different parts of the country may not experience the same 

level of health benefits. Benefits are apportioned among individuals by matching the change in air pollutant concentration change with the size of the population that experiences 

that change. Using the expected population exposure, the 

concentration-response functions are applied to derive estimates of the number of incidences of the health outcomes 

that are associated with change in exposure to ambient 

particulate matter and ozone. 
Chapter 4: Expectations for Full 

Implementation of Title IV in 2010 

By 2010, emissions of SO2 are projected to be approximately 

9.86 million tons, less than a million tons above the cap of 8.95 

million tons, and approximately three million allowances are 

projected to be in the allowance bank. Emissions are expected 

to reach the level of the cap after 2010 once the power sector 

has depleted the allowance bank. Air quality in 2000–2002 

reflects the significant emission reductions that have already 

occurred since Title IV was passed. Air quality is expected to 

continue to improve as emissions approach the level of the cap. 

Some lakes and streams, particularly in the Northeast, are expected to begin showing signs of recovery from acidification; 

however, recovery is expected to continue for decades after 2010 

due to the fact that soils and biological communities recover 

more slowly than air quality or deposition improvements 

occur. A recent estimate of the human health benefits of 

Title IV indicates that the benefits are substantial. 

This analysis of full implementation of Title IV in 2010 

includes emissions modeling, air quality modeling, and 

freshwater quality modeling. It also includes an estimate of 

the health benefits of Title IV reported in the literature. 

Estimates of emissions, air quality, and acid deposition 

were projected for 2010 and compared to averaged 

monitored data from 1989–1991, before Title IV was 

passed. Although the SO2 allowance bank will still be in use 

in 2010, analysts have used that date, twenty years after 

Title IV was passed and ten years after Phase II began, as 

the marker for the time when emissions are close to the 

level of the 2010 cap and most of the emission reductions 

are expected to have taken place. The freshwater quality 

modeling projects conditions in 2030 and compares them to 

conditions in 1984 or 1985. The health benefits estimate relies on estimates of air quality changes and the annual benefits per ton of reducing emissions of SO2. 

Emissions modeling from power plants was conducted 

using the 2003 version of the Integrated Planning Model 

(IPM). IPM is an economic model that predicts emissions 

from the power sector due to changes in environmental regulations and the costs of various emission control options. 

To provide a more comprehensive understanding of the state 

of the environment in 2010, the 2010 projections of 

emissions from power plants are based on implementation of 

Title IV and the NOx SIP call and several state 

regulations that were finalized before April 

2003. (See text box “What Will Air Quality 

be Like in 2010?” on page 58.) They do not 

include any co-benefits that might be 

achieved from implementation of mercury 

controls on power plants, any possible New 

Source Review determinations or other 

administrative or legal agreements, reductions 

undertaken in order to meet the National 

Ambient Air Quality Standards (NAAQS) for 

which states are required to submit 

implementation plans by 2007, the Clean Air 

Interstate Rule (CAIR), the BART Rule, any 

additional actions taken by Congress, or any 

further reductions from sources in Canada. 

Mobile source rules will also result in NOx reductions as they are implemented. The 2010 

emissions projections were compared to 1990 

emissions data collected by the EPA. 

The air quality and deposition modeling 

analyses were conducted using REMSAD 

(see page 54 for a description of REMSAD). 

The scenario used was a 2010 Title IV implementation scenario that includes the SO2 

and NOx emissions in 2010 with Title IV as 

projected by IPM (the “with Title IV” run) 

The scenario also includes NOx emission reductions from power plants and industrial 

boilers due to the NOx SIP call and finalized 

state emission reduction programs. 

Stationary and area sources were modeled 

in the scenario as projected emissions in 

2010 as estimated based on the 1996 

National Emissions Inventory (NEI). The scenario does not reflect future reductions from 

mobile source programs. The results of the 

2010 Title IV implementation scenario were 

scaled to monitoring data by comparing a 

2001 baseline modeled scenario to 

1999–2001 monitoring data. This allows a 

comparison of 2010 modeled projections of 

full implementation of Title IV with monitoring data from before the program took effect 

(1989–1991). 
The magnitude of emission reductions affects 

both the amount of recovery that takes place 

in a lake or stream and the rate at which that 

recovery occurs.30 Lake conditions in the 

Adirondacks and Northeast and stream conditions in the Southeast were modeled using 

the MAGIC model (see page 53 for a description of MAGIC). These regions were chosen because they are among the most acid-sensitive 

ecosystems and are downwind of many of the 

emission sources affected by the Acid Rain 

Program. SO2 emissions under Title IV were 

assumed to continue to decrease during the 

model run time frame; emissions from 

electric generating sources in 2030 were estimated to be approximately 9 million tons). Projected lake and stream conditions were compared to modeled estimates of lake and 

stream conditions in 1984 and 1985. Since lake and stream 

quality in 2010 would represent only a very small portion of 

the recovery expected due to Title IV, lake and stream conditions for 2030 are considered a more appropriate representation of the environmental benefits of Title IV. 

Emissions 

As a result of Title IV, emissions of SO2 are expected to be 

9.86 million tons in 2010, less than a million tons above the 

annual allocation (cap) of 8.95 million tons. Emissions can 

be above the cap because it is expected that on January 1, 

2010, the bank of unused allowances will still contain 

approximately 3 million allowances available for use. The 

SO2 bank was created when affected sources overcomplied 

with the emission reduction requirements during the first 

years of the program. During Phase I, emissions were 

25–40% below allowable levels, resulting in larger human 

health and environmental benefits during those years. NOx 

emissions from power plants are expected to be 3.94 

million tons in 2010, approximately 500,000 tons below 

where they are now. This 40% reduction from the 1990 level 

of 6.7 million tons is due to reductions under both Title IV 

and the NOx SIP call. Note that implementation of CAIR is 

expected to reduce emissions of SO2 and NOx to 6.1 million 

tons and 2.4 million tons, respectively, in 2010. 

Acid Deposition 

Air concentrations respond very quickly, in a matter of 

hours to days, to changes in SO2 and NOx emissions. 

Therefore, while NAPAP expects some small additional 

emission reductions to take place after 2010, air quality in 

2010 reflects the vast majority of the improvement in air 

quality that is expected to occur due to Title IV. The data 

presented in Chapter 2 on the status and trends of air concentrations of SO2 indicate that Title IV has substantially 

reduced concentrations of SO2 and sulfate. This reduction 

in air concentrations has occurred in many heavily populated areas where concentrations of SO2 and sulfate in the air 

cause respiratory, cardiac, and other health problems. This 

improvement in air quality, therefore, is expected to result 

in improvements in human health, as discussed below. 

Acid deposition also responds quickly to reductions in emissions of SO2 and NOx. Therefore, as is the case with air 

quality, NAPAP believes that the acid deposition rates in 

2010 reflect the vast majority of the reductions in deposition 

expected due to Title IV. Most of these reductions in acid 


deposition have already taken effect, but some additional 

reductions in both sulfur and nitrogen deposition are 

expected by 2010. 

The largest reductions in sulfur deposition are expected in 

the Ohio River Valley and in downwind areas, including the 

acid-sensitive regions of the Northeast and mid-Atlantic 

(see Figure 30). Some additional reductions in deposition in 

the Midwest and Northeast beyond what has occurred by 

2000–2002 are expected as emissions continue to drop to 

the level of the cap. 

Wet sulfate deposition is expected to be significantly lower 

in 2010 than it was in 1989–1991. This reduction in sulfate 

deposition has already led to improvements in the health of 

some acid-sensitive lakes and streams, particularly those in 

the Adirondacks, northern Appalachian Mountains, and the 

upper Midwest, as discussed in Chapter 2. 

Wet inorganic nitrogen deposition (deposition of nitrate and 

ammonium) is also expected to continue to 

decline substantially between 2000–2002 

and 2010. This is in addition to the large reductions that occurred between 1989–1991 

and 2000–2002 (see Figure 31). This is 

primarily due to implementation of the NOx 

SIP call, which reduces emissions of NOx 

from power plants, industrial boilers, and other sources in the Eastern U.S. 

Acidification of Freshwater 

Lakes and Streams 

Throughout acid-sensitive ecosystems in the 

Eastern U.S., acid deposition alters soils, 

stresses forest vegetation, acidifies lakes and 

streams, and harms fish and other aquatic 

life. For example, some acidified lakes in the 

WHY ARE THE ACIDIFICATION NUMBERS DIFFERENT? 

The extent of acidification as measured by the 

TIME/LTM monitoring sites in the Adirondacks 

is different than that reported in the modeling 

assessment reported in this analysis. This difference is due to differing types of Adirondack 

lakes included in each assessment. 

Since the original acid rain surveys were 

conducted in the 1980s, researchers have continued to refine their understanding of how to 

best measure changes in acidification due to anthropogenic causes. Researchers currently model a different subset of lakes and streams than 

were modeled in the 1980s and early 1990s. 

This change in “model population” means that 

current modeled predictions are somewhat different from the monitoring data reported in the 

1990 NAPAP Integrated Assessment or the 

2003 EPA Response of Surface Water 

Chemistry to the CAAA report. The original NAPAP assessment, and the subsequent EPA 

report, are based on data collected during the 

National Surface Water Survey (NSWS). These 

waterbodies, known as NSWS lakes and 

streams, were selected to be statistically representative of the region being sampled. The purpose of the NSWS was to assess lakes and 

streams that were acidic due to anthropogenic 

activities. In the Adirondacks, lakes smaller than 

4 hectares (10 acres) and greater than 2,000 

hectares (5,000 acres) were not sampled 

because it was thought they were more strongly 

affected by organic acidity. 

The NSWS lakes and streams were further classified and subdivided based on several 

characteristics. A project to assess the rate of 

response of waterbodies to acidification (the 

Direct/Delayed Response Project, or DDRP), 

used a subset of NSWS lakes that had lower 

ANC values (<400 ueq/L), had depths greater 

than 1.5 m deep, were smaller than 3,000 ha, 

and were not anthropogenically disturbed. It was 

determined that these lake characteristics were 

necessary to adequately assess the rate of 

change in acidification in sensitive northeastern 

lakes. The DDRP project also assessed streams 

in the mid-Appalachian and southern Blue Ridge 

regions; these waterbodies are also subsets of 

the larger NSWS set. 

The purpose of the MAGIC model is to calculate 

the rate and extent of recovery from acidification, 

so “DDRP waterbodies” are used to calibrate the 

model. The MAGIC estimate of acidification presented in this NAPAP Report for the 

Adirondacks, Northeast, and Southeast 

represents a reasonable scientific estimate of 

the current regional extent of acidification in 

acid-sensitive lakes and streams in the Eastern 

U.S. and the projected future conditions of those 

waterbodies on a regioinal scale. 
Adirondacks and streams in Virginia no 

longer support populations of fish.30, 165 Unlike 

the response of air quality and deposition to 

changes in emissions, lakes and streams 

take years to decades to fully reflect reductions in acid deposition. In some cases soil 

chemistry has been significantly altered, and 

ions must either build up (i.e. base cations) 

or be leached out ( i.e. sulfate and/or 

nitrate) before the chemistry can return to 

its pre-acidification status. In addition, algae, 

zooplankton, and fish communities take time 

to rebuild their food webs, or, in some cases, 

to migrate back into areas where they have 

become completely absent due to acidification. Therefore, lake and stream conditions in 

2010 reflect only a small part of the recovery 

that is expected due to Title IV. The full 

impacts of Title IV are not expected to be 

seen in the Northeastern U.S., including in the Adirondacks, 

for several decades after emission reductions begin.30 

Streams in the Southeast, because of their soil chemistry, 

are expected to take even longer.

103 Lake and stream quality 

in 2010 would represent only a small portion of the 

recovery expected due to Title IV. Therefore, lake and 

stream conditions are presented for 2030 and reflect emission reductions that occured between 1990 and 2010. 

Modeling conducted with the MAGIC model indicates 

that, by 2030, the percentage of chronically acidic lakes in 

the Adirondacks is expected to be 12%, down from 33% in 

1984 (see Figure 32). The percentage of non-acidic lakes 

is expected to remain constant at 36%. In the Northeast, 

the percentage of chronically acidic lakes is expected to 

be 6%, down from 13% in 1984. The percentage of nonacidic lakes is expected to be 69%, up from 67% in 1984. 

The percentage of episodically acidic lakes in both the 

Adirondacks and the Northeast as a whole actually grows 
in this stage of recovery. This is because more chronically 

acidic lakes become episodically acidic than episodically 

acidic lakes become non-acidic (see Figure 33). While 

chronically acidic lakes are most affected by acidification, 

episodically acidic lakes are also ecosystems that are significantly impaired by acid deposition and biological 

endpoints (i.e. sensitive fish species) remain susceptible. 

In the Southeast, because of the type of soils in the region, 

emission reductions are expected only to slow the rate of 

acidification in acid-sensitive streams. In 2030, the percentage of chronically acidic streams in the Southeast is 

expected to be 17%, up from 8% in 1985 (the southeastern 

streams were assessed the year after the northeastern 

lakes) (see Figure 32). The percentage of non-acidic 

streams is expected to be 56%, down from 73% in 1985. 

This is not because Title IV has had no effect in the 

Southeast; on the contrary, as indicated in Figure 32, there 

would be many more chronically acidic streams in 2030 if 

Title IV were not being implemented. The unglaciated (often 

sand or clay) soils in the Southeast have absorbed a large 

amount of acid deposition already and, because of their 

composition, are expected to continue releasing it into waters in the region for many decades. This lag time, which is 

much longer than the lag time seen in northern glaciated 

(often granite) soils, means that the streams are still showing the effects of historic emissions and will not show the 

full effects of emission reductions for many decades.103 

Human Health 

SO2 and NOx emissions react in the atmosphere to form fine particles and ozone. 

These gases and fine particles are associated with a number of significant health 

effects in sensitive populations. While high 

SO2 concentrations can result in temporary 

breathing impairments in sensitive people, 

including asthmatics and those who are 

active outdoors, most of the health effects 

are due to fine particles. 

A large number of epidemiological studies 

over the past 10 to 20 years show an association between ambient fine particle 

concentrations (e.g. sulfates and nitrates 

formed in the atmosphere from SO2 and NOx 

emissions) and health effects, such as 

increased numbers of hospital admissions 

and emergency room visits for heart and lung 

disease, increased incidences of respiratory 

disease and symptoms (such as asthma), decreased lung function, and even premature 

death. Children, the elderly, and individuals 

with existing cardiovascular or lung 

conditions, such as asthma, are especially vulnerable to the effects of particles.15 Title IV 

has reduced the amount of fine particles in 

the air (see Figures 16–20) by lowering SO2 

and NOx emissions, achieving significant 
human health benefits nationwide long before 

emissions drop to the level of the cap. It is expected that Title IV will achieve further benefits as SO2 emissions continue to decrease to 

the level of the emissions cap. 

NOx emissions react with volatile organic 

compound gases in the atmosphere in the 

presence of sunlight to form ozone. The scientific literature shows associations 

between ozone and a number of effects on 

the respiratory system, including aggravation of asthma, increased susceptibility to 

respiratory illnesses like pneumonia and 

bronchitis, and permanent lung damage. 

Children, the elderly, and people with existing respiratory problems are most vulnerable 

to the health effects of ozone. Those exercising or working outside during the ozone 

season (May–September) tend to have the 

highest exposures to ozone. Health benefits 

have been achieved under Title IV due to 

NOx reductions not only because NOx contributes to the formation of fine particles but 

also because reducing NOx emissions 

reduces ozone concentrations. 

The value of Title IV’s health benefits has 

been estimated by several organizations and 

analysts since its passage in 1990.166–171 In 

each case, these analyses found that the 

quantified health benefits of Title IV are 

very substantial. For example, in their most 

recent analysis, OMB estimates that the annual health benefits of Title IV in 2010 will 

be $79 billion to $83 billion ($2001). Title 

IV’s SO2 reductions account for $78 to $79 

billion of this total, and Title IV’s Phase II NOx reductions 

account for an additional $1 to $5 billion.171 These benefits 

include only the benefits of avoiding premature mortality 

due to air pollution. Title IV has provided additional benefits 

as well, that are not included in this most recent monetized 

estimate from OMB, including reductions in the number of 

non-fatal heart attacks, cases of acute and chronic bronchitis, emergency room visits, asthma attacks, and days lost 

from work and school. As with other benefits analyses, the 

benefits estimates are subject to uncertainties such as 

those associated with estimates of future-year emissions 

and air quality and uncertainties in the estimated 

relationships between changes in pollutant concentrations 

and resulting changes in health effects. 

Other Benefits of Title IV 

In addition to the reduced acidification and human health 

benefits of Title IV, there are other, un-quantified, benefits 

of full implementation of Title IV. These benefits from 

reductions in sulfur deposition and sulfate and nitrate concentrations include healthier forests, less erosion of buildings, monuments, and other historic structures, and 

improved visibility. These benefits occur throughout the 

Eastern U.S. in national parks and wilderness areas, cities 

and urban areas, and in rural areas. The reductions in nitrogen deposition, while not extensive, are expected to benefit nitrogen-sensitive coastal waters along the eastern and 

gulf coasts as well. For more information on the 

environmental effects of acid deposition see Chapters 2 

and 3. Acidification of lakes has been linked in some areas 

to higher levels of mercury contamination in fish; 

reductions in the acidity of these lakes may help reduce 

levels of mercury contamination in fish.172, 173 Finally, these 

environmental benefits can result in economic benefits by 

helping to preserve natural resources critical to industries 

such as forestry and fishing and increasing the amount of 

tourism, as well as improving the overall quality of life in 

areas where ecosystems have suffered from acidification. 
Chapter 5: Beyond Title IV: Ecological


Impacts of Further Emission Reductions


A broad consensus of scientists studying acid deposition and 

ecosystem recovery have published reports since publication of 

the last NAPAP report (1996) indicating that further emission 

reductions beyond those achieved by Title IV are necessary to 

allow sensitive forests and aquatic ecosystems to recover from 

acidification. The SO2 and NOx emission reductions achieved 

under Title IV from the power sector are now recognized as insufficient to achieve recovery or to prevent further acidification 

in some regions. Further reductions in SO2 and NOx emissions 

from the power generation sector would reduce the amount of 

acidic deposition in sensitive ecosystems and result in healthier 

forests and fewer acidic lakes and streams; however, even elimination of SO2 emissions from power plants will not fully protect 

all acid-sensitive ecosystems. 

Title IX of the 1990 Clean Air Act Amendments requires 

NAPAP to report quadrennially on “the reduction in deposition rates that must be achieved in order to prevent 

adverse ecological effects” (Public Law 101-549-Nov. 15, 

1990). Deposition levels that correlate with a “threshold” of 

adversity are scientifically complex and difficult to establish. 

NAPAP’s 1990 State of Science and Technology Report No. 13 

states that “biological responses to changes in acid-base 

chemistry occur along a continuum. There is not a single 

value or set of chemical concentrations that represents a 

‘threshold’ for ‘significant adverse biological effects’”172 In 

addition, some lakes and streams are naturally acidic and 

experience “adverse ecological effects” in the form of 

reduced fish populations or other impacts due to the presence of organic acids. Therefore, NAPAP determined that 

the logical path was to describe the ecosystem responses 

along a continuum, thereby allowing decision makers to determine the level of acceptable risk.174 

NAPAP presented a working definition of “adverse ecological effects” in the 1996 NAPAP Report to Congress based 

on the intent of Congress, as expressed in the 1990 Clean 

Air Act Amendments and shaped by other relevant environmental statutes (Comprehensive Environmental Response, 

Compensation and Liability Act and the Clean Water Act) 

and associated regulations. The definition is: 

Adverse ecological effects: any injury (i.e., loss of chemical or physical quality or viability), to any ecological or 

ecosystem component, up to and including at the regional level, over both longand short-terms. 

Adverse impacts to ecological processes or 

ecosystem components include reductions 

in acid neutralizing capacity and increasing 

aluminum concentrations in a lake or 

stream, leading to loss of fish and other biota; loss of important nutrients such as calcium, from forest soils; and, increased 

susceptibility of trees to pests, disease, and 

winter temperatures, leading to decreasing 

forest productivity and forest dieback. 

Adverse ecological impacts also include the 

effects of nitrogen saturation in forests, 

coastal eutrophication as a result of atmospheric deposition, and injury to plants as a 

result of ozone exposure. Other areas 

addressed by NAPAP (i.e., materials, visibility, and human health) would follow the 

same definition, but are not considered here 

as ecological effects.29 This NAPAP Report 

is based on that definition of adverse 

ecological effects and uses the same 

approach of investigating ecosystem 

responses along a continuum. The report focuses on “recovery” as indicated by 

changes in water chemistry (i.e. chronically 

acidic, episodically acidic, and non-acidic 

lakes and streams). 

While the definition of a “threshold” is complex, there has been a significant amount of 

research in the past decade indicating that 

ecosystems continue to be affected by acid 

deposition. The 1995 Acid Deposition 

Standard Feasibility Study concluded that, for 

the near term, sulfur deposition was likely to 

remain the primary source of acidification in 

most sensitive areas of eastern North 

America. However, as SO2 emissions 

decrease, the importance of nitrogen increases in relation to both short-term and longterm acidification. The Nitrogen Bounding 

Study (NBS), the primary component of the 
Acid Deposition Standard Feasibility Study, 

demonstrated that, at certain times and 

under certain deposition scenarios, sulfur and 

nitrogen could have approximately equal 

roles in surface water acidification. In 

addition, the NBS concluded that in regions 

where nitrogen deposition is or could become 

a more direct cause of chronically acidic lakes 

and streams, further limits on nitrogen deposition could produce dual benefits by 

reducing acid deposition rates and lengthening times to watershed nitrogen saturation. 

Based on model projections, EPA’s report 

concluded that further reductions of 40–50% 

beyond Title IV SO2 and NOx emissions may 

be necessary to fully protect the most sensitive lakes and streams.175 

Two national expert workshops organized by 

the Ecological Society of America reached 

similar conclusions regarding the effects of 

nitrogen and sulfur deposition. A June 1997 

workshop on atmospheric nitrogen 

deposition to coastal watersheds found that 

increasing supplies of nitrogen have resulted 

in a range of ecological impacts, including an 

increased incidence of harmful algae blooms 

and fish kills, reducing the number of acres 

covered with submerged aquatic vegetation 

(SAV), and changes to the fundamental ecological structure of some coastal waters.176 

A March 1999 workshop of acid rain experts 

focused on ecological response and 

recovery from sulfur and nitrogen emissions 

and deposition. The workshop report 

highlighted the importance of nitrogen deposition as a cause of acidification and found 

that ecological recovery has yet to occur in 

many regions. The workshop concluded that 

further SO2 emission reductions beyond 

Title IV may be necessary to achieve 

ecological recovery in lakes and streams 

affected by acid deposition.177 

In the period since the 1996 NAPAP report 

release, various studies focused on specific 

regions of the country have shed new light 

on the regional impacts of sulfur and 

nitrogen deposition. These more recent 

analyses considered reductions greater than 

those suggested in EPA’s 1995 report. For 

example, a regional survey of streams in the 

southern Appalachian region found that 65% 

of sampled locations demonstrate moderate to severe 

depletion of fish communities, some portion of which is attributable to acidification.107 Studies of trout streams in the 

Virginia part of that region document the first loss of a fish 

population in the Shenandoah National Park due to acidification.46 Researchers in the southern Appalachian region 

conclude that streams in this region are still threatened by 

acid deposition. Based on model projections they conclude 

that further reductions of sulfate deposition beyond levels 

achieved by the Title IV SO2 emission reductions are necessary to prevent further acidification of Virginia brook trout 

streams.46 A recent regional assessment completed by the 

Southern Appalachian Mountain Initiative (SAMI) concluded that local and regional power generation sources must 

reduce SO2 emissions by at least 70% and NOx emissions 

by at least 67% from 2000 levels in order to decrease 

stream acidification and reduce forest ecosystem damage 

due to nitrogen deposition in high elevation forests.178 

Two major assessments of acid deposition and its effects 

in the Northeast reached similar conclusions. Driscoll et al. 

(2001) found that full implementation of the 1990 CAAA 

will not result in substantial recovery in northeastern lakes 

and streams impacted by acid deposition. Based on model 

projections, the study concluded that reductions of SO2 

emissions from power generation of up to 80% beyond 

Title IV requirements were necessary for currently-acidic 

streams in the Northeast to become non-acidic.30 Driscoll et 

al. 2003 considered the impact of nitrogen deposition on 

forests, lakes, and streams in the Northeast. The study 

found that reduction in NOx emissions from power generation of 75%, combined with mobile source reductions, are 

necessary to arrest acidification of forest soils, lakes, and 

streams in that region.38 

Finally, additional recent studies demonstrate symptoms of 

nitrogen saturation in many ecosystems across the 

country,

51 including several areas in the Western U.S.133 For 

example, high-altitude watersheds of the Colorado Front 

Range exhibit symptoms of advanced stages of nitrogen excess.44 A 1995 survey of 91 high-elevation lakes in the central Rockies found changes in water quality caused by 

increasing amounts of inorganic nitrogen deposition, and intensive sampling at a smaller number of sites demonstrated 

that increasing wet nitrogen deposition is causing episodic 

acidification in headwater catchments of the Green Lakes 

Valley in the Colorado Front Range.42 These results are significant since increases in atmospheric nitrogen deposition 

can lead to ecosystem changes in this region,40 including 

lake eutrophication and altered ecosystem composition in 

both lake and alpine meadow environments.133 While greater 

uncertainty exists, nitrogen deposition in the West also has 

been linked to enhanced growth of invasive species, delete­
rious impacts on threatened and endangered species, and 

alterations of the fire cycle.133 

An additional examination of the literature surrounding the 

effects of atmospheric deposition is included in the 

Regulatory Impact Analysis in support of the 2005 Clean 

Air Interstate Rule (CAIR). This analysis can be found on 

the internet at www.epa.gov/interstateairquality/pdfs/ 

finaltech08.pdf 

In response to the Congressional request to identify the 

deposition rates that would prevent any adverse ecological effects, NAPAP has analyzed several emission 

reduction scenarios that broadly bound the range of reductions presented in the post-1996 literature cited above. 

These results provide an indication of the environmental improvements that would be expected from additional 

emission reductions from both Title IV and non-Title IV 

sources. These environmental improvements do not necessarily constitute recovery of acid-sensitive forests, lakes, 

and/or streams that have been impaired by acid deposition 

(see Figure 1 for the location of acid-sensitive areas). They 

do, however, provide an indication of the scope and magnitude of the impact of emission reductions on deposition levels and impact on acid-sensitive ecosystems. Other sensitive 

ecosystems, such as estuaries and coastal waters, would 

also benefit from reductions in nitrogen deposition, but are 

not analyzed here. The information requested by Congress 

and presented here represents part of what is needed when 

determining appropriate future actions; other information includes the costs and other impacts of emission reductions 

from the power sector or any other sectors and the value 

the public places on further improvements to the 

environment and human health. 

Analysis of the Environmental Impact 

of Further Air Emission Reductions 

SCENARIOS ANALYZED 

This modeling, conducted by EPA, analyzed several scenarios representing emission changes from stationary sources. 

The results are presented for sulfur and nitrogen deposition 

because these pollutants are the primary components of 

acid deposition. This NAPAP analysis focuses on reductions 

from power plants; other sources also emit pollutants that 

contribute to acid deposition. Notably, sources outside the 

power generation sector are projected to emit approximately 45% of the SO2 and 75% of the NOx emitted in 2020 under the Base Case scenario.179 

Three sensitivity scenarios estimating various levels of additional reductions are compared to a Base Case projecting 

emissions with full implementation of already-promulgated 

Clean Air Act programs in 2000 (Title IV, the 

NOx SIP call, and Tier II and Heavy Duty 

Diesel SO2 and NOx reductions that are projected to take effect by 2010). The scenarios 

were modeled using a cap and trade mechanism for both SO2 and NOx (reducing the 

SO2 cap under Title IV and imposing an 

annual cap for NOx). The reductions 

identified below for each scenario represent 

cap levels for each pollutant in 2020. The 

projected emissions used for the air quality 

modeling in 2020 are somewhat higher than 

the cap levels for all scenarios as a result of 

the early reductions and allowance banking 

predicted by the emissions model (IPM). 

Additional details on the scenarios are 

presented below. 

• Base Case (2020): This scenario does not 

include rules that were finalized after the 

spring of 2001 (including CAIR, CAMR, 

and BART), new or anticipated actions under the Clean Air Act, including the NonRoad Diesel Rule, or other emission 

reductions that would be necessary to attain and maintain the fine particle and 

ozone NAAQS for which states are 

required to submit implementation plans 

by 2007, or to achieve regional haze reduction goals. 

• Scenario A: This scenario includes an additional 60% reduction in SO2 emissions 

from the power generation sector beyond 

the cap of 8.95 million tons required under 

Title IV (a cap of 3.58 million tons/year of 

emissions in 2020). It also includes national annual NOx emission reductions from 

the power generation sector of 68% 

beyond Title IV levels (a cap of 1.87 million 

ton/year of emissions in 2020, or an emission rate of 0.15 lbs/mm BTU). 

• Scenario B: This scenario includes an additional 75% reduction in SO2 emissions 

from the power generation sector beyond 

the cap of 8.95 million tons required under 

Title IV (a cap of 2.25 million tons/year of 

emissions in 2020). It also includes national annual NOx emission reductions from 

the power generation sector of 80% 

beyond Title IV levels (a cap of 1.25 million 

ton/year of emissions in 2020, or an emission rate of 0.10 lbs/mm BTU). 
• Scenario C: This scenario is roughly equivalent, in terms of tons of SO2 reduced, to 

elimination of SO2 emissions from the 

power generation sector. It includes an additional 90% reduction in SO2 emissions 

from the power generation sector beyond 

the cap of 8.95 million tons required under 

Title IV (a cap of 1 million tons/year of 

emissions in 2020) and a 50% reduction in 

SO2 emissions from non-power generation 

sources (e.g., industrial boilers).* The scenario also includes national annual NOx 

emission reductions of approximately 80% 

beyond Title IV levels (a cap of 1.25 million 

ton/year of emissions in 2020, or an emission rate of 0.10 lbs/mm BTU). 

The magnitude of emission reductions influences both the amount of recovery from 

acidification that takes place and the rate at 

which recovery happens. The rate of recovery 

is also influenced by the geological and ecological characteristics of the lakes and/or 

streams in the area. Lake conditions in the 

Adirondacks and Northeast and stream conditions in the Southeast were modeled using 

the MAGIC model (see page 53 for a 

description of MAGIC). These regions were 

chosen because they are among the most 

acid-sensitive ecosystems and are downwind 

of many of the emission sources affected by 

the Acid Rain Program (see Figure 1). 

Emissions under Scenarios A, B, and C were 

assumed to be held constant after 2020; 

emissions under the Base Case continued to 

decrease slightly after 2020 until 2030. 

Projected lake and stream conditions were 

compared to modeled estimates of lake and 

stream conditions in 1984/1985. Since lake 

and stream quality in 2020 would represent 

only a small portion of the recovery expected 

due to the scenarios analyzed, lake and 

stream conditions are presented for 2030. 

Regional predictive models of the ecological 

responses in forests to changes in emissions 

or deposition are not currently available and 

were not modeled in this assessment. This 

report employs models and analytical tools 

that are widely used and peer-reviewed. As 

with any detailed analysis of complex scenarios, the results 

presented in this report are subject to uncertainties 

concerning emissions estimates, air quality modeling and 

deposition projections, and the impact of emission 

reductions on ecological systems. 

CHANGES IN ACID DEPOSITION 

Even with Title IV in place, lakes and streams in many parts 

of the U.S. are still experiencing acidification due to sulfur 

and nitrogen deposition. This is especially prevalent in the 

East, where sulfur deposition is currently approximately 

15–25 kg/ha/year. Sulfur deposition levels in the West are 

much lower, generally less than 10 kg/ha/year. Inorganic nitrogen deposition is currently approximately 3–7 

kg/ha/year in the East. Inorganic nitrogen deposition levels in the West, although in some areas still high enough to 

cause ecological damage, are lower, on the order of less 

than 4 kg/ha/year. 

Acid deposition in sensitive ecosystems alters soils, stresses forest vegetation, acidifies lakes and streams, and 

harms fish and other aquatic life. For example, 24% of 

Adirondack lakes with low pH and ANC levels surveyed in 

* The SO2 emission reductions included from non-power generation sources is approximately 2 million tons, almost twice the remaining SO2 emissions from 

the power generation sector once the level of the cap is reached. 

the mid-1980s did not support fish, and 6% 

of Virginia trout streams studied in the late 

1990s were unable to support brook trout or 

other fish species due to chronic acidity.

30, 165 

While some Adirondack lakes have always 

been acidic due to the presence of organic 

acids, data collected in the 1980s indicates 

that the vast majority of those larger than 4 

hectares and almost all streams in the 

Southeast were acidic due to atmospheric 

deposition.31 Recent research on forest 

ecosystems shows that acid deposition has 

contributed to the decline of red spruce 

throughout the Eastern U.S. and may have 

contributed to the decline of sugar maple 

trees in central and western Pennsylvania.30 

Overall, the effects of acid deposition can interfere with valuable ecosystem benefits, 

such as forest productivity and water quality. 

CHANGES IN SULFUR DEPOSITION 

All additional reduction scenarios modeled in 

this analysis are projected to lead to significant regional reductions in sulfur deposition 

as compared to projected conditions under 

the Base Case in 2010 (Figure 34). Under 

Scenario A, reductions in deposition of 

10–30% would be found from the Plains 

states east to the Atlantic seaboard, with reductions of 30–60% in the southeastern 

states stretching up into Pennsylvania and 

southeastern Ohio. Most western states, 

from the Rocky Mountains west, would see 

reductions in deposition of up to 10%. 

Scenario B extends the area of largest 

reduction in sulfur deposition northwards 

and westwards. Scenario C would extend 

the area with the greatest reductions in sulfur deposition northward to Maine and 

across the Mississippi River into the eastern 

portion of the Plains states. For example, under Scenario B, the Adirondacks, midAppalachians, and southern Blue Ridge 

Regions would all experience reductions in 

sulfur deposition of up to 60%. Scenario C 

would expand those reductions into other 

acid sensitive areas, including Northern New 

England and the Upper Midwest. These 

reductions are expected to provide 

ecological benefits to these acid-sensitive 

regions, but are not expected to solve all ecological 

problems related to acid deposition in those areas. 

CHANGES IN NITROGEN DEPOSITION 

As mentioned above, acidification prevents lakes and 

streams from supporting fish and other aquatic life. In addition, many estuaries and forests are being affected by 

excessive loading of nitrogen, including atmospheric deposition of nitrogen. Modeling of the Base Case indicates 

that, in 2020, Title IV, the NOx SIP call, and the Tier II and 

Heavy Duty Diesel mobile source rules are expected to 

achieve reductions in nitrogen deposition of greater than 

30% from North Carolina north along the Atlantic seaboard 

to Maine. Large portions of the rest of the Eastern U.S. will 

experience reductions in nitrogen deposition of 10–20% due 

to existing programs. Large portions of the Western U.S. 

will also achieve significant reductions in deposition of 

10–20% or more due to existing mobile source reductions. 

These reductions in deposition will take place in some of 

the areas experiencing the most damage from nitrogen deposition, including the Adirondacks, Appalachians, Colorado 

Front Range, and the San Bernadino Mountains. 

The emission reductions from power plants in the scenarios 

modeled in this analysis are projected to lead to additional 

regional reductions in nitrogen deposition as compared to 

projected conditions in 2020 under the Base Case (Figure 

35). Under Scenario A, there would be additional 

reductions in nitrogen deposition of 10–20% beyond the levels seen under the Base Case in patches throughout the 

Eastern U.S. Similar levels of emission reductions would be 

found in the Four Corners region of the West. The model 

suggests a slight increase in deposition in east Texas, 

southern Louisiana, and eastern North Carolina due to 

increased utilization of units (these units are meeting their 

NOx limits under Title IV). The rest of the country would 

see additional reductions in deposition of up to 10%. 

Scenario B would expand the area of 10–20% reduction in 

deposition from the Base Case to cover more of the 

eastern seaboard, Colorado, New Mexico, and Utah. It 

would reduce the area receiving slightly increased levels of 

nitrogen deposition in Texas, but not eliminate it. 

These emission reductions would also lead to significant additional reductions in nitrogen deposition compared to the 

Base Case in sensitive ecosystems still experiencing water 

quality and/or forest health problems due to acidification. 

For example, the Adirondacks would receive a 10–20% 

reduction in deposition as compared to the Base Case under Scenario B and the Chesapeake Bay watershed would 

receive an approximately 10% reduction in deposition as 

compared to the Base Case under Scenario B. A sensitive 
ecosystem thought to be in danger of experiencing 

excessive nitrogen loading, the Colorado Front Range, 

would receive approximately a 20% reduction in 2020, with 

reductions in some areas over 30%, as compared to the 

Base Case scenario under Scenario B. As is the case with 

reductions in sulfur deposition, these reductions are expected to provide ecological benefits in many sensitive ecosystems, but some problems due to nitrogen deposition are 

expected to remain. 

NOx emissions are the same in Scenario B 

and C so nitrogen deposition was not 

modeled for Scenario C. Although the nonlinearities involved in particulate chemistry 

indicate that there would be some change in 

nitrogen deposition based on changes in sulfur emissions, Scenario C is expected to show 

few significant differences in nitrogen deposition compared to Scenario B. 
WATER QUALITY 

Acid deposition affects water quality in sensitive ecosystems by acidifying lakes and 

streams, changing water chemistry, and 

altering soil chemistry. For example, 24% of 

Adirondack lakes with low pH and ANC 

levels surveyed in the mid-1980s did not support sensitive fish, and 6% of Virginia trout 

streams studied in the late 1990s were 

unable to support brook trout or other fish 

species due to chronic acidity.

30, 165 Currently 

available modeling tools allow researchers 

to estimate the effects of reductions in acid 

deposition on several aspects of the water 

chemistry of lakes and streams. The effects 

of acid deposition on the other ecosystem 

components, such as soil chemistry, 

zooplankton or fish health, or forest vegetation cannot at 

this point be identified quantitatively on a regional basis. 

Watershed characteristics (e.g., soils, bedrock type, geologic history) affect the rate of water chemistry response 

to acid deposition. “Direct response” lakes or streams 

manifest changes relatively quickly in response to 

changes in deposition, whereas “delayed response” lakes 

or streams manifest changes over a longer period of time. 

The implementation of the Base Case is expected to significantly reduce the percentage of chronically-acidic 

streams in all three areas beyond what would have 

occurred without Title IV (see Chapter 4 for a detailed discussion of the water quality improvements expected from 

full implementation of Title IV and for a discussion of the 

population of lakes and streams considered in this assessment). This reduction in the number of acidic lakes and 

streams, as well as reductions in the frequency and/or 
severity of acidification in the remaining acidic lakes and 

streams, improves the health of fish populations and other 

acid-sensitive species. However, as shown in the base case 

modeling presented in Figure 36, Title IV will not eliminate 

the problem of acidic waters. Chronically acidic waters are 

projected to remain in both the Northeast and the 

Southeast and some waters in all three of the acid-sensitive 

regions modeled in this analysis are expected to continue to 

experience episodic acidification. 

The changes in water quality for the three additional reduction scenarios were also modeled in this analysis. While the 

effects of additional emission reductions differ by region, 

the amount of reductions makes little difference in the 

resulting chronic acidity of any one region. However, the 

larger the emission reductions, the more “non-acidic” lakes 

and streams are projected (See Figure 22 for an 

explanation of the stages of recovery from acidification). 

Modeling results indicate that the additional 60% reduction 

in SO2 emissions from power generation beyond Title IV 

would result in the virtual elimination of chronic acidity 

from lakes in the Northeast, including the Adirondack 

Mountains, by 2030 (Figure 36). Research indicates that 

larger reductions in deposition would speed the time to recovery of those Northeastern lakes.30 Only under Scenario 

C would the percentage of episodically acidic lakes also 

begin to decrease. 

The story is somewhat different for the Southeast (Figure 

36). Due to the unique geology of the area, including its 

porous sandy soils, southeastern streams respond very 

slowly to reductions in acid deposition (characterized as 

“delayed response”). As a result, no additional recovery is 

expected in chronically acidic streams in the Southeast, 

even with reductions beyond the base case included in 

Scenarios A and B. According to model projections, a 

reduction of more than 75% in SO2 emissions from power 

generation sources would not reduce the number of chronically acidic streams in the Southeast. This level of emission 

reductions from the power generation sector would slow 

the rate of acidification of these waters, but improvement 

(i.e., a reduction in the number of chronically acidic 

streams) is projected to begin only after larger reductions 

and/or reductions from other source categories such as 

those associated with Scenario C occur. 

Summary 

Title IV has been quite successful at 

effectively reducing emissions of SO2 and 

NOx from power generation to the levels set 

by Congress. In fact, by 2020 emissions 

from power plants are expected to be less 

than half of the total SO2 emissions in the 

U.S. As a result of these emission 

reductions, some acid-sensitive areas are already beginning to show signs of recovery. 

Full implementation of Title IV, and the passage of time needed for affected 

ecosystems to respond to the new 

environmental conditions, are expected to allow more areas to recover. However, several 

scientific studies indicate that the emission 

reductions achieved by Title IV are not sufficient to allow recovery of acid-sensitive 

ecosystems. Estimates from the literature of 

the scope of additional emission reductions 

that are necessary in order to protect acidsensitive ecosystems range from 

approximately 40–80% beyond full 

implementation of Title IV, with the most recent studies focusing on the largest reductions. This report analyzes the impacts of a 

range of levels that would achieve fuller environmental recovery from acid rain and 

greater avoidance of the adverse ecological 

effects associated with acid deposition. 

The results of the modeling presented in 

this Report to Congress indicate that 

broader recovery is not predicted without 

additional emission reductions. Elimination 

of all SO2 emissions from power plants, however, will not fully protect all acid-sensitive 

ecosystems affected by acid deposition; reductions from other source categories would 

be needed. The information requested by 

Congress and presented here represents 

part of what is needed when determining 

appropriate future actions; other information 

includes the costs and other impacts of 

emission reductions and the value the public 

places on further improvements to the environment and human health. 

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