# An Integrated Assessment

**20 0**  **5** 

# Na t i o n al A c i d Pr e c i p i t a t i o n As s e ss m e nt

Pr o g r a m Re p o rt to Co n g r e s s :This document contains links leaving the EP A.gov domain. These links pr ovide additional information that may be useful or inter esting and ar e being pr ovided consistent with the intended purpose of the EP A W eb site. However , EP A cannot attest to the accuracy of information pr ovided by these links or any other link ed site. P r oviding links to a non-EP A W eb site does not constitute an endorsement by EP A or any of its employees of the sponsors of these sites or the information or pr oducts pr esented on these sites. Also, be awar e that the privacy pr otection pr ovided on the EP A.gov domain (see P rivacy and Security Notice) may not be available at these e xternal links.

### Member s of Cong ress:

I am pleased to transmit the National Acid Pr ecipitation Assessment Pr og r am Re por t to Cong r ess: An Integ r ated Assessment

. This repor t presents the latest scientific inf or mation and anal ysis concer ning the costs, benefits, and envir onmental ef f ectiv eness of the Acid Rain Pr og ram. This Pr og ra m was mandated b y Title IV of the 1990 Clean Air Act Amendments to

reduce sulfur dio xide (SO 2 

### ) and nitr og en o xides (NO x 

### ) emissions fr om electr ic g enerating
sources. The SO 2 

### pr og ram inc ludes the use of an inno v ativ e emissions cap and trade pr og ram. Title IX of the 1990 Clean Air Act Amendments requires the National Acid Precipitation Assessment Pr og ram (N AP AP) to repor t to Cong ress the costs, benefits, and ef f ectiv eness of the Acid Rain Pr og ram and c haracter ize what deposition reductions would be necessary to prev ent adv er se ecolog ical ef f ects in acid sensitiv e ecosystems. This repor t fulfills the requirements of Title IX.

NAP AP coordinates f ederal acid rain researc hand monitor ing under the auspices of the National Science and T e c h nology Council (NSTC) Committee on Envir onment and Natural Resources (CENR). The N AP AP member a g encies are the U .S. Environmental Protection Ag ency , the U .S. Depar tment of Energ y , the U .S. Depar tment of Inter ior/U .S. Geolog ical Sur ve y , the U. S

. Depar tment of Inter ior/National Pa r k

Service, the National Aer onautics and Space Administration, and the National Oceanic and Atmospher ic Administration. This assessment inv olv ed man y individuals fr om acr oss the f ederal a g encies representing numer ous scientific disciplines. N AP AP sought re vie w fr om sev e ral academic per spectiv es as well, and the results

repor ted here we re subjected to extensiv e peer re vie w

.
The N AP AP Repor t anal yzes the results of the Acid Rain Pr og ram thr ough 2002. The repor t uses quantitativ e and qualitativ e indicator s to assess the ef f ectiv eness of the cap and trade appr oac h to reduce emissions, impr o v e air quality and reduce acid deposition while minimizing compliance costs. It c losel y trac ks emission reductions since implementation of Title IV in 1995 and inc ludes anal ysis of emission reductions, compliance, the operation of the sulfur dio xide (SO 2 

### ) allo wance mar ket and pr og ram costs. The repor t also identifies emerging areas of acid rain researc h , long-ter m envir onmental monitor ing and assessment and urg es continued suppor t f or integ rated assessments.

The repor t also contains the status and trends of emission reductions in SO 2 

### and NO x 

### and associated envir onmental impacts. The repor t conc ludes that Title IV has been successful in impr o ving air quality , visibility and ac hieving br oad-scale reductions in sulf ate deposition. Dur ing Phase I of the pr og r am, SO 2 

### emissions fr om po w er plants w e r e 25–40% belo w

### allo w a b l e l e v els, resulting in larg er human health and envir onmental benefits dur ing those year s. Suc h emissions will continue to dec line to appr o ximatel y 9.86 million tons in 2010. While there hav e not been similar br oad-scale reductions in nitr og en deposition, the repor t indicates that nitr og en deposition has dec lined in some ar eas with r esulting benefits. Over all

a 40% reduction in NO x 

### emissions fr om po wer plants, compared with 1990 lev els, is expected by 2010 thr ough Title IV and the NO x 

### SIP Call.

As emissions of SO 2 

### appr oac h the cap leve l esta b lished in Title IV and compliance occur s under Title IV and the NO x 

### SIP Call, air quality is expected to sho w continued impr o vement and ther e will be some additional r eduction in deposition. Some lakes and str eams, par ticularl y in the Nor theast, ar e beg inning to sho w signs of r eco ve r y fr om acidification. The r epor t, ho we ve r , also indicates that emission r eductions be yond Title IV alone ar e not suf ficient to allo w sensitive f o r ests and aquatic systems to r ecove r

. In this r egar d, the r epor t anal yzed thr ee sce nar ios that inc luded r eductions be y ond those dir ectl y r equir ed b y T itle IV

. While the scen a r ios we r e a b le to inc lude emission r eductions fr om the NO x 

### SIP Call and cer tain state pr og r ams, the y did not, ho we ve r , inc lude other existing Clean Air Act pr og r ams, inc luding mobile sour ce r eductions.

Since the anal ysis f or this repor t was conc luded, EP A finalized sev e ral r ules that will result in fur ther substantial reductions of SO 2 

### and NO x 

### , inc luding the Clean Air Inter state Rule, the Non-Road Diesel Rule and the Best A v

aila b le Retr o fit T e c h nology guidance f or re gional haze. States will also reduce SO 2 

### and NO x 

### as necessary to attain or maintain the ne w fine par ticulate matter and ozone National Ambient Air Quality Standards (N AA QS). As par t of ong oing
wo rk , N AP AP intends to fur ther anal yze the impr o vements in air quality and the r esulting ben efits f or air quality and the envir onment.

J ohn H. Marburg e r , III Director

# Acknowledgements

The NAP AP Office of the Director would lik e to thank all those who wrote, edited, and reviewed this NAP AP R eport to Congress. Special thanks are e xtended to the e xtramural peer reviewers, who, in addition to providing a technical review , were ask ed to consider the relationship of this report to public policy . Their comments and suggestions mak e this a significantly better report.



# P eer Reviewers

Dallas Burtraw , Resour ces for the F utur e Thomas Butler , Cornell University
Kenneth Colburn, Northeast States for Coor dinated Air Use Management (NESCA UM) Charles Driscoll, Syracuse University David Grigal, University of Minnesota


# P rimar y Authors and Contributors

I NTRODUCTION AND E XECUTIVE S UMMARY T

amara Saltman, U.S. Envir onmental P r otection Agency

C HAPTER 1: P ROGRAM E LEMENTS AND I MPLEMENT A

TION U.S. Envir onmental P r otection Agency/Clean Air Mark ets Division

C HAPTER 2: R ESUL TS OF THE ACID R AIN P ROGRAM : S TA TUS AND T RENDS OF E MISSIONS AND E NVIRONMENT AL I MP A CTS (1990–2002) Robert Cook, U.S. Department of Ener gy Oak Ridge National Laborator y Chris Eagar , U.S. Department of Agricultur e F o r est Ser vice
Mark F enn, U.S. Department of Agricultur e F o r est Ser vice
Richar d Haeuber , U.S. Envir onmental P r otection Agency
T

om Huntington, U.S. Department of Interior/Geological Sur vey Sandy McLaughlin, U.S. Department of Ener gy Oak Ridge National Laborator y P eter Mur doch, U.S. Department of Interior/U.S. Geological Sur vey T

amara Saltman, U.S. Envir onmental P r otection Agency David Schmeltz, U.S. Envir onmental P r otection Agency
Mar y Striegel, U.S. Department of Interior/National P ark Ser vice Carl T r ettin, U.S. Department of Agricultur e F o r est Ser viceC HAPTER 3: ASSESSING ACID D EPOSITION : A DV A N CES I N T H E S TAT E O F THE S CIENCE Br yan Bloomer , U.S. Envir onmental P r otection Agency Robert Cook, U.S. Department of Ener gy Oak Ridge National Laborator y Chris Eagar , U.S. Department of Agricultur e F o r est Ser vice
Mark F enn, U.S. Department of Agricultur e F o r est Ser vice
Richar d Haeuber , U.S. Envir onmental P r otection Agency
T

om Huntington, U.S. Department of Interior/Geological Sur vey Sandy McLaughlin, U.S. Department of Ener gy Oak Ridge National Laborator y Mar y Ross, U.S. Envir onmental P r otection Agency David Schmeltz, U.S. Envir onmental P r otection Agency
Mar y Striegel, U.S. Department of Interior/National P ark Ser vice Carl T r ettin, U.S. Department of Agricultur e F o r est Ser vice

C HAPTER 4: E XPECT A

TIONS FOR F ULL I MPLEMENT A

TION OF T ITLE IV IN 2010 Br yan Bloomer , U.S. Envir onmental P r otection Agency
B.J. Cosby , University of V i r ginia
T

amara Saltman, U.S. Envir onmental P r otection Agency
Ann W atkins, U.S. Envir onmental P r otection Agency

C HAPTER 5: B EY OND T ITLE IV : E COLOGIC A L I MP A CTS OF F URTHER E MISSION R EDUCTIONS B.J. Cosby , University of V i r ginia
Richar d Haeuber , U.S. Envir onmental P r otection Agency
T

amara Saltman, U.S. Envir onmental P r otection Agency 

# About the National Acid P r ecipitation Assessment P r ogram

The National Acid P r ecipitation Assessment and to periodically assess the costs, benefits, and Pr ogram (NAP AP) is a cooperative federal effectiveness of T itle IV . The NAP AP member agencies ar e program first authorized in 1980 to coordinate the U.S. Envir onmental P r otection Agency , the U.S. acid rain r esear ch and r eport the findings to Department of Ener gy , the U.S. Department of Agricultur e, Congr ess. The r esear ch, monitoring, and the U.S. Department of Interior , the National Aeronautics
as sessment efforts by NAP AP and others in and Space Administration, a n d the National Oceanic and the 1980s culminated in T itle IV of the 1990 A tmospheric Administration. This r eport is the thir d Clean Air Act Amendments, also known as published by NAP AP since 1990 assessing T itle IV

. the Acid Deposition Contr ol P r ogram. In a bold new appr oach to envir onmental pr otec- In 1997 NAP AP began to operate under the auspices of the Committee on Envir onment and Natural Resour ces tion, T i tle IV includes a mark et-based pr ogram that pr ovides economic incentives (CENR) of the National Science and T echnology Council. for contr olling emissions of sulfur dio xide NAP AP’s goal continues to be pr oviding cr edible technical
fr om electricity generating facilities. T itle IX findings on acid deposition and its effects t o i n f o r m t h e public decision-making process. T o ensure that this goal is of the CAAA r eauthorized NAP AP to conduct acid rain r esear ch and monitoring met, NAP AP coordinates its activities through the Air Quality Resear ch Subcommittee of the CENR.

# T able of Contents

Executive Summar y

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
L ong-T erm Envir onmental Monitoring

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Key Components of a Model P r ogram

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


### Intr oduction

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
NAP AP Mission

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Structur e of the Report

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 

### Chapter 1. 

# Pr ogram Elements and Implementation

Emission Reductions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Pr ogram Design

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 SO 2 Pr ogram

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 NO x Pr ogram

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 T itle IV A ffected Sour ces

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

SO 2 Pr ogram

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 NO x Pr ogram

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 SO 2 Allowance Mark et

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Pr ogram Costs

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Phase I Costs

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Administrative Costs

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Pr ogram Assessment

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

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

. . . . . . . . . . . . . . . . . . . . . . . 15 

### Chapter 2. 

# Results of the Acid Rain P r ogram: Status and T r ends of Emissions and Envir onmental Impacts (1990–2002)

Emissions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 SO 2 Emissions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 NO x Emissions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Geographic Distribution of Emissions fr om P ower Generation

. . . . . . . . . . . . . . . . 20 Geographic T r ends in SO 2 Emissions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Geographic T r ends in NO x Emissions

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Air Quality , Deposition, and V isibility

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Air Quality: Sulfur

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Air Quality: Nitr ogen

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Acid Deposition: Sulfur in P r ecipitation

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Acid Deposition: Nitr ogen in P r ecipitation

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 V isibility

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36Ecological Recover y

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Fr eshwater Acid-Sensitive Ecosystems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Fo r est Ecosystems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Coastal Ecosystems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Materials and Structur es

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 

### Chapter 3. 

# Assessing Acid Deposition: Advances in the State of the Science Air Quality , Deposition, and V isibility

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Aquatic and F o r est Ecosystems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Fr eshwater Acid-Sensitive Ecosystems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Fo r est Ecosystems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Coastal Ecosystems

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Materials and Structur es

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Human Health

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Modeling

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

A tmospheric Modeling

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Nonlinear Responses to Decr eases in SO 2 and NO x Emissions

. . . . . . . . . . . . 53 W

ater/W atershed Modeling

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Health Effects Modeling

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 

### Chapter 4. 

# Expectations for F ull Implementation of T itle IV in 2010

Emissions

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 Acidification of F r eshwater Lak es and Str eams

. . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Human Health

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Other Benefits of T itle IV

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 

### Chapter 5. 

# Beyond T itle IV : Ecological Impacts of F urther Emission Reductions Analysis of the Envir onmental Impact of F urther Air Emission Reductions

. . . . . . . 67 Scenarios Analyzed

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Changes in Acid Deposition

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

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Changes in Nitr ogen Deposition

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 W

ater Quality

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 Summar y

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73


### Refer ences

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

# List of F igur es and T ables for NAP AP Report

F igur e 1. Sensitive ecological r eceptors in the United States
F igur e 2. T itle IV affected sour ces—2002
F igur e 3. P e r cent of SO 2 emission r eductions by compliance option for Acid Rain P r ogram sour ces—2001
F igur e 4. SO 2 allocations for electric generating units, 1995–2002
F igur e 5. Nominal SO 2 allowance prices, 1994–2002
F igur e 6. SO 2 allowances transferr ed, 1994–2002
F igure 7: Estimated costs of implementation of T i tle IV S O 2 emission r eductions i n 2010, 1989–1998
F igur e 8. SO 2 emissions fr om all affected sour ces (electric generating units), 1980–2002
F igur e 9. SO 2 emissions fr om all sour ces, 1980–2001
F igur e 10. NO x emissions fr om all affected sour ces (electric generating units), 1990–2002
F igur e 11. Ener gy use and NO x emissions by state, 1990–2002
F igur e 12. NO x emissions fr om all sour ces, 1990–2001
F igur e 13. SO 2 emission tr ends fr om power generation facilities by state, 1990–2002
F igur e 14. NO x emission tr ends fr om power generation facilities by state, 1990–2002
F igur e 15. L ocation of NADP/NTN, CASTNET , and IMPRO VE monitoring sites and pollutants measur ed by major networks
F igur e 16. Comparison of annual ambient sulfur dio xide (SO 2 ) concentrations in the rural Eastern United States fr om CASTNET monitoring data, 1989–1991 vs. 2000–2002
F igur e 17. Comparison of annual ambient sulfate (SO 4 2- ) concentrations in the rural Eastern United States
F igur e 18. Comparison of annual ambient nitric acid (HNO 3 ) concentrations in the rural Eastern United States
F igur e 19. Comparison of annual ambient nitrate (NO 3 - ) concentrations in rural Eastern United States
F igur e 20. Comparison of annual ambient ammonium (NH 4 + ) concentrations in rural Eastern United States
F igur e 21. Comparison of annual wet sulfate (SO 4 2- ) deposition in the United States
F igur e 22. Comparison of annual total (wet plus dr y) sulfur deposition in the United States
F igur e 23. Comparison of annual wet nitrate (NO 3 - ) deposition in the United States
F igur e 24. Comparison of annual concentrations of nitrate (NO 3 - ) in wet deposition in the United States
F igur e 25. Comparison of annual total (wet plus dry) nitr ogen deposition in the United States
F igur e 26. T r ends in visibility in Class 1 ar eas, 1992–2001
F igur e 27. Relationship between acid neutralizing capacity (ANC) and the number of species pr esent in aquatic fr eshwater ecosystems in Shenandoah National P ark
F igur e 28. Map of the T emporally Integrated Monitoring of Ecosystems (TIME) monitoring networkF igur e 29. T r end in acidification of eastern lak es and str eams, 1990–2000
F igur e 30. P r ojected change in wet sulfate deposition in 2010 with T itle IV compar ed to conditions in 1989–1991 and 2000–2002
F igur e 31. P r ojected change in inor ganic wet nitr ogen deposition in 2010 with T itle IV compar ed to conditions in 1989–1991 and 2000–2002
F igure 32. P rojected change in the number of chronically acidic lakes and streams in 2030 with and without T itle IV
F igur e 33. Stages of r ecover y for acidic lak es and str eams F igur e 34. P r ojected per cent change in annual sulfur deposition fr om base case in 2020 with additional SO 2 and NO x emission r eductions
F igur e 35. P r ojected per cent change in annual nitr ogen deposition fr om base case in 2020 with additional SO 2 and NO x emission r eductions
F igur e 36. Change in the number of chronically acidic lakes and streams i n 2030 with additional SO 2 and NO x emission r eductions

T

ABLES T

able 1. Nitr ogen contribution to estuaries in the United States fr om atmospheric deposition (dir ect and indir ect)

# Executive Summar y

Acid deposition, mor e commonly known as acid rain, occurs when emissions of sulfur dio xide (S O 2 ) and nitr ogen
o xides (NO x ) r eact in the atmospher e (with water, o xygen, and o xidants) to form various acidic compounds. These acidic compounds then fall to earth in either a wet form (rain, snow , and fog) or a dr y form (gases, aer osols, and particles). P r evailing winds transport the acidic compounds hundr eds of miles, of ten acr oss state and national bor ders. At certain levels the acidic compounds, including small part i

cles such as sulfates and nitrates can cause many negative human health and envi r onmental effects. While ecosystems
ar e subject to many str esses, 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,

• Acidif y lak es and str eams,

• Harm sensitive for ests,

• Harm sensitive coastal ecosystems, and

• Accelerate the decay of building materials, paints, and

cultural artifacts such as buildings, statues, and sculptur es. T itle IV was passed by Congr ess as part of the 1990 Clean Air Act Amendments to r educe emissions of SO 2 and NO x fr om fossil fuel-burning power plants in or der to pr otect ecosystems suffering damage fr om acid deposition and to impr ove air quality . A t the same time, the National Acid Pr ecipitation Assessment P r ogram (NAP AP) was ask ed to periodically assess and r eport to Congr ess on the implementation of the Acid Rain P r ogram, r ecent scientific knowledge surr ounding acid deposition and its effects, and the r eduction in acid deposition necessar y to pr event adverse ecological effects. This NAPAP Report focuses primarily on emission r eductions fr om power plants, both in terms of assessing past r eductions under the Acid Rain Pr ogram and in pr ojecting the ecological effects of additional r eductions of SO 2 and NO x . It should be noted that power generation

curr ently contributes appr o ximately 69% of the SO 2 emissions and 22% of the NO x emissions nationwide. This contribution is decr easing as emissions fr om power generation continue to decr ease, making the other

sour ces of these pollutants mor e pr ominent. Modeling suggests that even if SO 2 emissions fr om power generation wer e
r educed to zer o, some lak es and str eams would r emain acidic due to acid deposition. However , ther e ar e several other r egulations that r educe emissions of SO 2 and NO x fr om these non-power generation sour ces, such as the T ier II mobile sour ce standar ds, the Heavy Duty Diesel standar ds, and the Clean Air Non-Road Diesel Rule, that have also been pr omulgated since 1990. These r egulations, primarily designed to bring counties into attainment with fine particle and ozone air quality standar ds, also incidentally r educe emissions that contribute to acid deposition. Implementation of T itle IV has successfully and substantially r educed emissions of SO 2 and NO x fr om power generation at a significantly lower cost than e xpected:

• In 2002, SO 2 emissions wer e 10.2 million

tons, 35% lower than 1990 emissions and 40% lower than 1980 emissions.*

• In 2002, NO x emissions wer e 4.5 million

tons, 33% lower than 1990 emissions. In addition, SO 2 emissions fr om all sour ces have decr eased by 32% since 1990 and emissions of NO x fr om all sour ces have decr eased by 12% since 1990. P ower generating sour ces continue to close in on the goal of r educing power plant SO 2 emissions fr om 1980 levels by 50% (to 8.95 million tons) as r equir ed by the 1990 Clean Air Act. P ower generating sour ces have also e x ceed­

* 2002 was the latest year for which emissions data wer e available when this r eport was written and r eviewed. Mor e r ecent emiss ions data ar e available on

the EP A W eb site at www .epa.gov/airmark ets/emissions

.ed the goal of a two million ton r eduction in NO x emissions fr om pr ojected 2000 levels
as r equir ed by the 1990 Clean Air Act.
These emission r eductions have contributed to measurable impr ovements in air quality

, r eductions in acid deposition, and the beginnings of r ecover y of acid-sensitive waters in

some ar eas:

• S O 2 concentrations in the atmospher e (a

precursor to fine particles and acid deposition) have decreased since 1990. Average annual S O 2 concentrations in the Northeast in 2000–2002 wer e 40% lower than they were in 1989–1991, concentrations in the mid-A tlantic were 30% lower

, concentrations in the Southeast wer e 35% lower , and concentrations in the Midwest

were 45% lower

.
• Sulfate concentrations in the atmospher e (a major component of fine particles, especially in the East) have decr eased since 1990 as well. Average annual sulfate concentrations in the Northeast and Midwest in 2000–2002 wer e appr o ximately 30% lower than they wer e in 1989–1991, and concentrations in the mid-A tlantic and Southeast wer e 25% lower

.
• W et sulfate deposition, a major component of acid rain, has also decr eased 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-A tlantic and Midwest was 35% lower , and deposition in the Southeast was 25% lower

.
• W et nitrate deposition has not decreased regionally from historical levels because of the relatively moderate NO x reduction from power plants and the continuing lar ge contribution (over 50% of total NO x emissions) fr om other

sources of NO x such as vehicles and nonr oad vehicles.

• Although visibility has begun to impr ove in some parts

of the U.S., ther e is still significant impairment of visibility in many national parks and other Class I ar eas thr oughout the U.S.

• Acid neutralizing capacity is beginning to rise in some

surface waters in the Northeast, including lak es in the Adir ondack Mountains (see graphic below). This is an indication that r ecover y fr om acidification is occurring in those ar eas.• Reductions in fine particles due to r eductions in emissions of SO 2 and NO x ar e e xpected to continue to benefit human health by substantially lowering the incidence of r espirator y and car diovascular illnesses. Sour ces in both the mor e conventional emission rate-based NO x pr ogram and the cap and trade pr ogram for SO 2 have demonstrated a high level of compliance, and their efforts have achieved measurable envir onmental r esults. The fle xibility for sour ces to choose how to contr ol their emissions inher ent in the cap and trade appr oach for SO 2 has been successful at r educing compliance costs to a fraction of the cost estimated in 1990. It has not r esulted in any significant geographic shif ts in SO 2 emissions. The annual cost of the SO 2 trading pr ogram by 2010 is e xpected to be appr o ximately $1 to $2 billion per year ($2000), far lower than the estimated $6 billion annual cost in 1990 ($2000).
Emissions of S O 2 are e xpected to continue to decline as the bank of allowances that was built up in Phase I of the Acid Rain P rogram (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 T itle IV in 2010 indicates that the benefits are substantial, including thousands fewer premature deaths and hospital admissions and emergency
room visits. Despite anticipated envir onmental impr ovements, r esear ch over the last few years indicates that recover y from the effects of acidification is not possible for many areas that continue to be e xposed to acid deposition. Many published articles, as well as the modeling presented in this report, show that the S O 2 and NO x emission reductions achieved under T itle IV from power plants are now recognized as insufficient to achieve recover y or to prevent further acidification in some regions. Additional S O 2 and NO x 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 e xisting or future regulations to address transport of ozone and fine particles and

mercur y deposition, including the NO x 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

mercur y from power plants. 

# L ong-T erm Envir onmental Monitoring
Emissions, air quality , deposition, and ecological monitoring ar e critical components of T itle IV . These monitoring efforts allow

r esear chers and policymak ers to assess the effectiveness of the T itle IV pr ogram. Emissions monitoring is conducted by affected sour ces; the other types of monitoring ar e conducted by a wide variety of federal and state agencies, universities, and other or ganizations. The agencies of the National Acid P r ecipitation Assessment Pr ogram (NAP AP) continue to have a str ong commitment to the r esear ch and monitoring that mak es assessments lik e this NAP AP Report to Congr ess possible.


# Key Components of a Model P r ogram
T itle IV ser ves as a model structure of an air pollution control program because of its significant and measurable progress towar ds 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 lar gely responsible for its effectiveness. The cap is the core pr er equisite of an effective and efficient trading program that guarantees the environmental goals are achieved and sustained. Accountability, including re liable emissions, air quality, atmospheric deposition, and ecological monitoring and regular assessment of pr ogr ess towar ds the goals, promotes both public confidence in the environmental integrity of the pr ogram and business confidence in the financial integrity of the allowance mark et. The simple, clear

, straightfor war d natur e of T itle IV has r esulted in high levels of compliance, low administrative costs, and timely achievement of its envir onmental goals.

# Intr oduction

NAP AP Mission

The National Acid P r ecipitation Assessment P r ogram ( N A P A P ) is comprised of the U.S. Envir onmental Pr otection Agency , the U.S. Department of Ener gy , the U.S. Department of Interior/U.S. Geological Sur vey , the U.S. Department of Interior/National P ark Ser vice, and the National Oceanic and A tmospheric Administration. Under T itle IX of the 1990 Clean Air Act Amendments, Congr ess r eauthorized NAP AP to continue

coor dinating acid rain r esear ch and monitoring, as it had done during the pr evious decade, and to pr ovide Congr ess with periodic r eports. In particular , Congr ess ask ed NAP AP to assess all available data and information to answer two questions:

• What ar e the costs, benefits, and

effectiveness of T itle IV? This question addr esses the costs and economic impacts of complying with the Acid Rain P r ogram as well as benefit analyses associated with the various human health and welfar e effects, including r educed visibility

, damages to materials and cultural

r esour ces, and effects on ecosystems.

• What reductions in deposition rates are

needed to prevent adverse ecological effects? This comple x question addresses ecological systems and the deposition levels at which they e xperience harmful effects.
The r esults of the assessment of the effects of T itle IV and of the r elationship between acid deposition rates and ecological effects

wer e to be r eported to Congr ess quadr ennially , beginning with the 1996 r eport to Congr ess. The objective of this Report is to addr ess the two main questions posed by Congr ess and fully communicate the results of the assessment to decision-makers. Given the primar y audience, most of this r eport is not written as a technical document, although information supporting the conclusions is pr ovided along with r efer ences. 

# Impacts of SO 2 

# and NO x 

# Emissions

Ti tle IV of the 1990 Clean Air Act Amendments (CAAA)

re quir es significant r eductions of sulfur dio xide (SO 2 ) and nitr ogen o xide (NO x ) emissions. These emissions contribute to acid deposition, ultimately leading to a wide range of

en vir onmental 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 S O 2 and NO x emissions than others. Some people (especially children, the elderly, and those with ex isting respirator y or cardiovascular conditions) are more likely to be harmed by fine particles formed from S O 2 and NO x emissions and ozone formed from NO x emissions.
Areas where acid deposition damages ecosystems or where gases and particles impair visibility ar e called “sensitive ecological receptors.” Sensitive ecological receptors include lakes and streams throughout the Appalachian Mountains; forests in the Appalachian Mountains, the Colorado F ront Range, and western coastal mountain ranges; and many east and gulf coast estuaries and coastal waters (see F igur e 1). M any national parks and wilderness ar eas, including Shenandoah National P ark, Great Smoky Mountains National P ark, Acadia National P ark, and Grand Canyon National P ark have impaired visibility due in part to emissions of S O 2 and NO x from power generation sour ces. Adirondack P ark has a lar ge number of lakes that have been damaged by acidic deposition. Many of these health and envir onmental impacts can occur hundr eds of miles fr om wher e the pollutants ar e emitted. This long-range transport mak es it critical to r educeemissions that contribute to acid deposition and other envir onmental impacts, even those that occur far fr om sensitive waters, for ests or population centers. The purpose of T itle IV is to r educe the ecological impacts of acid deposition by

r educing emissions of SO 2 and NO x fr om power generation
sour ces. These emission r eductions have and will continue to benefit the nation by:

• Impr oving air quality and pr otecting public health

• Restoring acidified lak es and str eams so they can once

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

• Reducing the damage to sensitive for ests, such as those

along the Appalachian Mountains and in the Colorado Fr ont Range

• Reducing the damage to nitr ogen-sensitive coastal

waters along the east and gulf coasts

• P r otecting our historic buildings and monuments fr om

degradation 

# Structur e of the Report

This report is directed to Congress but provides valuable economic and scientific information to all public officials who ar e
responsible for determining or evaluating air quality policy . Its goal is to pr esent highly technical information pertinent to

cur r ent 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 T itle IV by EPA and the affected utilities at the end of 2002 is pr esented in the first chapter. This is followed by an analysis of the obser ved changes, both past and pr esent, in emissions of acid rain pr ecursors, air pollutant concentrations, deposition of acidic species, and various ecological endpoints, particularly surface water quality , in the second chapter

. The thir d chapter covers advances in the state of the science r egar ding atmospheric deposition monitoring, the impacts of acid deposition on aquatic and terr estrial ecosyst e m s , v isibility , 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 T itle IV in 2010. The fif th chapter addresses the question posed by Congress in the 1990 CAAA r egar ding the ecological impacts of further emission r eductions and the uncertainties in the estimated

r elationships between emission r eductions and the r esulting ecological effects.

# Chapter 1: P r ogram Elements and Implementation

Title IV has been successfully implemented according to the rules and guidance established by the Acid R ain Program. Emissions have decreased significantly since 1990, and source compliance has been e xtremely high since the program began. The mark et-based system for reducing SO 2 

### emissions is functioning well; there is a significant amount of mark et activity and SO 2 

### allowance prices, which have varied from $65–$210 per ton, are significantly lower than had been predicted in 1990.



# Emission Reductions

T itle IV has successfully r educed emissions of SO 2 and NO x fr om power generation
sour ces. In 2002, ther e wer e appr o x imately 3,200 electric power units subject to the SO 2 pr ovisions of T itle IV .* By 2002, these

sour ces had r educed their combined SO 2 emissions by appr o ximately 40% fr om 1980 levels and 35% fr om 1990 levels. Although not all sour ces in the SO 2 trading pr ogram
ar e also cover ed by the NO x pr ogram, the

sour ces in the SO 2 pr ogram r educed their NO x emissions by 33%. The appr o ximately 1,000 sour ces affected by T itle IV NO x r equir ements r educed their NO x emissions by appr o ximately 27% fr om 1990 levels in 2002. The emission r eductions achieved under T i tle IV ar e discussed in mor e detail in Chapter 2. 

# Pr ogram Design

SO 2 P ROGRAM The S O 2 component of T itle IV represents a dramatic departure from traditional command and control regulator y approaches that establish source-specific emission limitations. Instead, the program uses an overall emissions cap for S O 2 that ensures emission reductions are achieved and maintained as well as a trading system that facilitates lowest-cost emission reductions. The program features tradable S O 2 emission allowances, where one allowance is a limited authorization to emit one ton of S O 2 . A fix ed 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 S O 2 allowance market indicates that this is not a substantial barrier to entr y; 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 S O 2 emissions (one allowance for each ton of SO 2 ).
T itle IV requires a two-phased tightening of allowances (or SO 2 emission allocations) to fossil fuel-fired power plants in
or der to re ach 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 appro ximately half the amount of SO 2 emitted by these power plants in 1980. Phase I of the SO 2 program (1995–1999) affected 263 of the lar g er (>100 megawatt (MW)), higher emitting units which are located primarily in the Eastern United States. Phase I SO 2 allowance allocations were distributed to each source based on the following formula: the product of an emission rate of 2.5 lb SO 2 /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 wer e r etir ed or not yet operating, ar e listed in Appendix A of the annu al Acid Rain P r ogram

Pr ogr ess Report available at www .epa.gov/airmark ets/cmprpt

. **The number of sour ces 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 wer e not identified in the 1990 Clean Air Act as Phase I sour ces, enter ed the SO 2 or NO x pr ograms early in or der to

r educe costs or tak e advantage of the SO 2 allowance mark et.electric utility units ser ving generators lar ger than 25 MW and all new fossil fuel-fired generation units throughout the countr y

. This has br ought the total number of units covered by the SO 2 program to appro ximately 3,200 sources (see F igure 2). In Phase II, all Phase I and Phase II SO 2 affected

sources are allocated allowances equivalent to an amount no greater than the product of 1.2 lb SO 2 /mm BTU and their average heat input for 1985–1987.
A ffected sources must demonstrate compliance with the SO 2 provisions of T itle IV at the end of each year . Sources are granted a 60-day grace period during which additional SO 2 allowances may be purchased, if necessar y, to cover each unit’s emissions for the year . A t the end of the grace period (the Allowance T ransfer Deadline), the allowances a unit holds in its Allowance T racking System (A TS) account must equal or exceed the unit’s annual S O 2 emissions for the pr evious year . T itle IV r equires affected sour ces to monitor emissions continuously and to r eport their emissions quarterly . F ailure to surrender sufficient allowances results in two significant automatic penalties: a fine and a r eduction in the number of allowances allocated in the following year

. Any remaining S O 2 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. NO X P ROGRAM Ther e ar e no tradable emission allowances in the NO x r eduction portion of the Acid Rain Pr ogram. Instead, sour ces contr ol how much NO x is emitted fr om coal-fir ed boilers based on the use of cost-effective contr ol technologies for each unit of fuel consumed (lb NO x /mm BTU). Ther e wer e also two phases to the NO x contr ol pr ogram: Phase I beganin 1996 (delayed one year because of litigation) and Phase II in 2000. During Phase I, which applied to specific coal-fir ed boilers statutorily affected by Phase I SO 2 r equir ements, the NO x emissions rate was set at 0.50 lbs/mm BTU for dr y bottom wall-fir ed units and 0.45 lbs/mm BTU for tangentially

fir ed units. Beginning in 2000, Phase II plants wer e r equir ed to meet emissions rates between 0.40 lbs/mm BTU and 0.86 lbs/mm BTU depending on the type of boiler

. Although the NO x pr ogram does not include emission trading, sour ces ar e pr ovided a degr ee of fle xibility thr ough emission averaging pr ovisions, wher eby a company can meet its emission limitations by averaging theemission rates of two or mor e boilers. This allows sour ces to r educe their emissions at lower cost by allowing them to over -contr ol at units wher e it is technically easier to contr ol emissions. In addition, specific Phase II units elected to comply with Phase I limits beginning in 1997. These “early election units” ar e not subject to the mor e stringent Phase II limits until 2008. In 2002 ther e wer e over 1,000 units r equir ed to meet a NO x emission limit under T itle IV pr ovisions. Since NO x emission r e ductions ar e achieved by limiting the rate of NO x output per unit of heat input and not thr ough an emission cap, emissions could incr ease thr ough a combination of incr eased utilization of e xisting units and/or construction of new units.
Sources affected by the NO x portion of T itleIV must also demonstrate that they have complied with the NO x provisions at the end of the year . Sources demonstrate compliance with the NO x program by achieving an annual emission rate at or below mandated levels as outlined in their EP A-approved compliance plans.


# T itle IV A ffected Sour ces Generally, boilers or combustion turbines that burn fossil fuel, ser ve generators with nameplate capacity greater than 25 MW , and produce electricity for sale are subject to the Ti t l e IV SO 2 requirements. Ther e are several types of units, however, that meet these criteria that are not affected by the program:

• Simple combustion turbines , r egar dless

of their nameplate capacity , if they began to pr oduce electricity for sale befor e November 15, 1990.

• Cogeneration units whose annual electrical sales r emain below the thr eshold established by r egulation (one-thir d of operating capacity or 219,000 MW per hour

, whichever is gr eater).

• Specific qualif ying facilities and

independent power pr oducers that ar e contractually bound to sell electricity at a price that was established befor e November 15, 1990. Despite these e x ceptions, almost all non-cogeneration units built in the futur e that ser ve generators with total nameplate capacity gr eater than 25 MW and pr oduce electricity for sale will be affected units that must participate in the Acid Rain P r ogram. Of the units subject to the SO 2 portion of T itle IV , some ar e also cover ed by the T itle IV NO x Pr ogram r equir ements. All units wher e coal accounted for

mor e than 50% of its heat input for at least one year during the 1990 thr ough 1995 time period and that ar e configur ed for a specific type of boiler (cell burner , cyclone, dr y bottom wall-fir ed, tangentially -fir ed, vertically -fir ed, or wet bottom)

ar e affected by the T itle IV NO x Pr ogram. F or mor e details on Acid Rain P r ogram applicability criteria, see 40 CFR 72.6, the Acid Rain P r ogram applicability r egulations established under T itle IV

. 

# Compliance

SO 2 P ROGRAM The T i tle IV SO 2 pr ogram has seen a near -perfect compliance r ecor d since the pr ogram took effect in 1995. During the compliance pr ocess, the number of allowances

surr ender ed 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 thebeginning of Phase II a small number of units (1–6, depending on the year) wer e short allowances to cover their emissions. F or e xample, in 2002, due to an administrative err or 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 -thr ee year 2003 allowances wer e tak en fr om this unit as an “offset.” In addition to the offset,the operator of this unit was assessed an automatic monetar y penalty totaling over $90,000 ($2002) as pr escribed in the Clean Air Act.
The fle xibility pr ovided by T itle IV enables the sour ces subject to the T i tle IV SO 2 pr ogram to pursue a variety of compliance options, some of which, such as pr ocess innovation, might have been discouraged under other types of contr ol pr ograms. 2 Sour ces meet their SO 2 r eduction obligations by installing scrubbers, running e xisting scrubbers mor e efficiently , switching fuels, changing practices or pr ocedur es to impr ove ener gy efficiency , and buying allowances. Most sour ces ar e complying with the SO 2 r equir ements in T itle IV by blending or switching fuels. In 2001 this accounted for appr o ximately two-thir ds of the SO 2 emission r eductions (see F igur e 3). Scrubbers installed on 30 Phase I units accounted for almost all of the r emaining emission r eductions in 2001. 1

* The annual deductions for emissions at each unit af ter the common stack apportionment was made can be found in Appendix A of the most r ecent annual

Acid Rain P r ogram P r ogr ess Report available at www .epa.gov/airmark ets/cmprpt . Units sharing a common stack ar e listed dir ectly under the entr y for their common stack.NO X P ROGRAM Instead of using allowance trading to facilitate NO x emission r eductions, T itle IV establishes limits on NO x emission rates (in lb NO x /mm BTU) for coal-fir ed electric generating units. The various ways a sour ce can meet its emission limit ar e described below

.
• Standar d Limitation . A unit with a standar d limit simply

meets the applicable individual NO x limit pr escribed for its boiler (Gr oup 1: dr y bottom wall-fir ed and tangentially

fir ed boilers; and Gr oup 2: cell burner , cyclone, vertically

fir ed, and wet bottom boilers) under the T itle IV NO x r egulation (40 CFR part 76).

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

Gr oup 1 NO x affected unit meets a less stringent Phase I NO x limit beginning in 1997, thr ee years befor e it would normally be subject to an Acid Rain NO x limit. In r eturn for accepting a NO x limit thr ee years earlier than would normally be r equir ed, an early election unit does not become subject to the mor e stringent Phase II NO x limit until 2008.

• Emissions Averaging . A company can meet its NO x emissions r eduction r equir ements by choosing to mak e a gr oup of NO x pr ogram affected boilers subject to a gr oup NO x limit, rather than meeting individual NO x limits for each unit. The gr oup limit is established at the end of each calendar year , and the gr oup 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 NO x limit.

• Alternative Emission Limitation (AEL) . A

utility can petition for a less stringent AEL if it pr operly installs and operates the NO x emissions r eduction technology pr escribed for that boiler but is unable to meet its standar d limit. EP A determines whether an AEL is warranted based on analyses of emissions data and information about the NO x contr ol equipment.The NO x pr ogram has also had a high rate of compliance. F or e xample, in 2002, 1,047 units met their NO x emissions limits thr ough compliance with their r espective NO x compliance plans. Only one unit failed to meet its NO x emissions limit in 2002. That unit had

ex cess NO x emissions of 47 tons and was assessed a monetar y penalty totaling over $133,000 ($2002)*. Emissions averagingwas the most widely chosen compliance option for NO x affected units in 2002; mor e than 50 emission averaging compliance plans involving over 600 units wer e in place in 2002. 

# SO 2 

# Allowance Market
The number of allowances (authorizations to emit SO 2 ) allocated to each sour ce in any given year is determined by the Clean Air Act. The pr esence of the allowance mark et has given some sour ces the incentive to r e

duce their SO 2 emissions below the level of their allowance allocation in or der to sell their allowances to other sour ces or bank them for use in futur e years. Other sour ces have been able to postpone or r educe e xpenditur es for pollution contr ol by pur chasing allowances fr om sour ces that contr olled beyond their allowance allocation level. The SO 2 allowance mark et is generally thought of as r elatively wellfunctioning due to the “clear, consistent rules that emphasize transpar ency , fungibility , and mark et performance [that] have been the k e y factors in cr eating the investor certainty ...” 3
Under T itle IV , the “bank” or stor e of unused allowances gr ew thr oughout Phase I (1995–1999) as sour ces r educed emissions mor e than r equir ed. These “early r eductions” r e

duced the amount of fine particles and acid deposition

r eaching sensitive ecological r eceptors in the early years of the acid rain pr ogram, incr easing the human health and ecological benefits of the pr ogram in those years. Beginning in 2000, the tighter emission cap in Phase II took effect, and sour ces began to use pr eviously bank ed allowances in addition to allocations fr om the curr ent year in or der to comply with T itle IV

.

* Detailed annual compliance information by unit can be found in Appendices B1 and B2 of the most r ecent annual Acid Rain P r ogr am P r ogr ess Report

available at www .epa.gov/airmark ets/cmprpt

.In 2002, a total of 9.5 million allowances wer e granted to

sour ces as pr escribed in T itle IV . Adding these 2002 vintage allowances to the unused allowances bank ed fr om prior years, a total of 18.8 million allowances wer e available for use in 2002. Sour ces emitted 10.2 million tons in 2002, 650,000 tons mor e than the allowances granted in 2002, but far less than the allowable level. F igur e 4 shows the changing size of the allowance bank fr om 1995 thr ough 2002. In a well-functioning mark et, the price of an allowance should equal the e xpected mar gi nal cost of compliance, the cost of r educing the next ton of S O 2 emitted from the utility sector . Emission reductions continue to cost less than anticipated when T itle IV was enacted, and this is reflected in the price of an S O 2 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 F igure 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 downwar d trend that started in the second half of 2001. P r ices 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. P rices stabilized back to historical averages in 2002 af ter the more stringent limits in Phase II resulted in higher average prices in 2001. Some market obser vers 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 lar ger than

expected emission reductions, which increased the supply of allowances and put downwar d pr essur e on prices. 5 Market obser vers 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 mark et
cr eated under T itle IV incr eased fairly steadily thr ough 2000 to over 30 million transfers and then leveled off somewhat in 2001 and

* All allowance prices in this r eport ar e r eported in curr ent year dollars for the year in which they occurr ed, unless other wis e indicated.2002, the second and thir d years of Phase II (see F igur e 6). The number of transfers between economically unr elated

or ganizations—i.e., trades other than those between units or subsidiaries belonging to a single company—also r ose fairly steadily fr om 1994 thr ough 2000 befor e leveling out in Phase II. In most years, trades between economically unr elated or ganizations accounted for half or mor e of all trades. In 2002, over 5,700 allowance transfers that affected over 21 million allowances (of past,

curr ent, and futur e vintages) wer e r ecor ded in the Allowance T ransfer System, theaccounting system developed to track holdings of allowances. Of the allowances transferr ed, 11.6 million, or 54%, wer e transferr ed in economically significant transactions (i.e., between economically unr elated parties). The majority of the allowances transferr ed in economically significant transactions wer e acquir ed by utilities. In December 2001, trading parties began to use the On-line Allowance T racking System (OA TS). By the end of 2002, OA TS r ecor ded 79% of all transfers electr onically over the Internet. All official allowance transactions, as well as data on account balances and ownership, ar e posted and updated daily at

www .epa.gov/airmark ets in or der to better inform trading participants and the public of the status of the mark et. Cumulative mark et statistics and additional analyses ar e also available. It should be noted that some allowance transfers ar e not included in this data because they ar e not
r ecor ded or publicly declar ed until the allowances ar e used for compliance purposes.


# Pr ogram Costs
The cap and trade mechanism in T itle IV for r educing SO 2 emissions has led to lower than e xpected costs as well as to cost savings compar ed to conventional r egulator y appr oaches. Emissions trading gives electricity generators the fle xibility to choose between r educing emissions at their own units and pur chasing allowances fr om other units that can reduce emissions at lower cost. Thus, with efficient trading, each unit faces the same mar ginal cost of compliance. In addition, ther e ar e no technological or permitting r estrictions on the means by which SO 2 emissions ar e r educed. Units under T itle IV have many compliance options, including switching to a differ ent type of fuel, switching the grade of fuel, or installing equipment that

r emoves SO 2 af ter it is cr eated (e.g., scrubbers).P HASE I C OSTS Early projections of annual Phase I compliance costs ranged from just under $678 million 7 to $1,511 million 8 ($2000). These cost estimates took into consideration the allowance trading provisions of T itle IV , unlike earlier higher cost estimates for the same level of SO 2 reductions under mor e
restrictive regulator y options. The most recent ex post study of Phase I costs found that in 1995, Phase I units reduced emissions of SO 2 by 3.9 million tons at an estimated cost of $814 million ($2000). The cost of this emission reduction was appro ximately 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.

P HASE II C OSTS Early pr oposals to r educe SO 2 emissions that mandated scrubbers or other command and contr ol options wer e estimated to cost as much as $7.5 billion/year ($2000). 4 The first EP A estimate (1990) for annual Phase II costs was appr o ximately $6 billion. As the appr oach of Phase I near ed, the estimates for Phase II costs de clined with the General Accounting Office (GAO) r evising the estimate to appr o ximately $2.5 billion per year in 2010 ($2000) 10 (see F igur e 7).
Current estimates of the cost of full implementation of T itle IV SO 2 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 SO 2 program between $1.1 and $1.8 billion ($2000).
NO x program costs will add to total program costs. EP A e xpects cost for NO x 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 SO 2 reductions that have been realized under T itle IV

. Low Sulfur F uel : Switching to low-sulfur coal was the most widely used means of r e

ducing SO 2 emissions under Phase I and is continuing under Phase II. 5 Historically , lowsulfur sub-bituminous coal fr om the P owder River Basin of Wyoming and Montana has been r e latively ine xpensive to mine but costly to transport long distances. In the 1980s, railr oad der egulation incr eased competition and opened the possibility for lower industr y fr eight rates, which would mak e low-sulfur western coal competitive for units in theMidwest and East. The demand for low-sulfur coal coupled with der egulation helped drive rates down during Phase I. T itle IV facilities have tak en advantage of r educed costs in complying with the SO 2 limits under both Phase I and Phase II. 9 Units have also tak en advantage of mid-sulfur bituminous coal fr om the Midwest and Central Appalachia because, despite its higher sulfur content than western coal, mid-sulfur coal

r equir es fewer modifications to boilers and operating pr ocedur es. 5 Scrubber Costs . The costs of scrubbing SO 2 ar e lower than anticipated. 5 P rior to T itle IV , the initial capital investment for installing scrubbers was e xpected to be pr ohibitively high. As Phase I continued, these initial capital costs decr eased, as did the

mar ginal cost of scrubbing an additional ton of SO 2 once the technology was installed. It was estimated that scrubbing SO 2 fr om e x
isting units would cost over $400/ton ($1994) SO 2
. 5 The actual price per ton of SO 2 scrubbers in 1995 was on the or der of $280 ($1994); i.e., 40% less than pr edicted. Banking . Another aspect to the T itle IV program that has led to cost savings in comparison with other regulator y appr oaches 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 lar ge number of allowances for war d to Phase II. This is expected to lower compliance costs during Phase II despite the more stringent national cap on overall SO 2 emissions. Accor ding 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
T

echnological Innovation . The technological impr ovements that facilitated coal switching and the speed at which they

emer ged wer e also not anticipated. 9 By e xperimenting withblends of low- and high-sulfur coal that did not lower generating capacity , operators wer e able to tak e advantage of the coincidental dr op in fr eight costs for western coal and incr eased pr oduction of mid-sulfur coal in the Midwest. Industr yalso came up with innovative ways to lower operation and maintenance costs for scrubbers, and ways to incr ease their utilization rate and efficiency

. 4
Allowance Mark et . T o be fully r ealized, the anticipated cost-saving benefits of the fle xibility pr ovided under T itle IV

r equir ed the cr eation of an efficient high-volume mark et for allowances. This has indeed been the case, as discussed above. Since the inception of the pr ogram trading has been

ver y active, and trading between distinct or ganizations has incr eased in r ecent years.
A DMINISTRA TIVE C OSTS The costs to the government of administering the T itle IV SO 2 pr ogram ar e less than in conventional r egulator y pr ograms. F or e xample, T itle IV’s performance-based appr oach eliminates the need to devise sour ce-specific emission limits, r eview contr ol technologies, and pr epar e and appr ove detailed compliance schedules and permits. In addition, eliminating case-by -case r eview and appr oval ofeach trade (including determining the “useful life” of equipment, the intent of the sour ces r egar ding futur e emission and activity levels, and “r eal” emission r eductions e xpected to be achieved) gr eatly r educes the administrative and transaction costs associated with emissions trading pr ograms. 12 During Phase I, costs of the pr ogram wer e
r oughly $12 million/year or appr o ximately $1.50/ton of pollution r educed. 13 Most of the functions in the allowance mark et ar e carried out by private entities; the government’s

r ole is primarily to ensur e that units ar e monitoring emissions thr oughout the year and to ensur e that each unit has enough allowances to cover emissions at the end of the year . In 2003, EP A estimates that sour ces, as a gr oup, spent appr o ximately $250 million on administrative functions of the pr ogram, including allowance transfers, permitting, emissions monitoring, compliance, and other ad ministrative costs.

# Pr ogram Assessment

E MISSIONS M ONITORING T itle IV includes strict emission monitoring r equir ements to guarantee compliance with both the cap and trade and emission rate limit components of the pr ogram. Most affected sour ces installed r equir ed monitoring equipment on time; the few that did not wer e subject to stringent emission estimation pr ocedur es. The monitoring r egulations that implement T itle IV include strict pr ocedur es to estimate emissions when appr oved methods ar e not used. These pr ocedur es, which over estimate total emissions, pr ovide a str ong incentive to install and maintain accuratemonitoring equipment, minimize emission monitor downtime, and ensur e that emissions ar e not under -r eported. All monitors ar e r equir ed to meet stringent initial and ongoing performance standar ds to demonstrate the accuracy , pr ecision, and timeliness of their measur ement capabilities. The monitors used by sour ces to comply with T itle IV have achieved an unparalleled level of performance with r espect 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 S O 2 and NO x , as well as volumetric flow

, to determine hourly mass emissions of S O 2 , hourly NO x emission rates, and hourly heat input. Coal-fired units may estimate their CO 2 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 S O 2 and CO 2 mass emissions and hourly heat input. Most oil- and gas-fir ed steam units are required to use CEMs to determine hourly NO x emission rates. However , for an oil- or gas-fired peaking unit, the owner or operator may develop a correlation cur ve between heat input and NO x 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 cur ve is used to estimate the hourly NO x emission rate. 14 One measure of the accuracy of a CEM is the relative accuracy test audit (RA T

A), which is required for initial certification of a CEM and on at least an annual basis, thereaf ter . The RA T

A

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. Fu rther

, 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 S O 2
.

A IR Q U ALITY , D EPOSITION , AND E COLOGIC AL M ONITORING Air quality and deposition monitoring ar e also important components of the overall implementation of T itle IV . Several monitoring networks designed to measure changes in air quality and acid deposition as a r esult of emission reductions are currently in operation. Ther e is also a surface water monitoring network in acid-sensitive ar eas in the Eastern U.S. that measures changes in lake and stream chemistr y as a re sult ofchanges in emissions and atmospheric deposition. F inally, many researchers continue to conduct studies to assess the impacts of emission reductions on lakes, streams, forests, and coastal ecosystems. T ogether

, this information allows policymak ers to accurately assess the impact of T itle IV and otherair 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- T

erm Monitoring Ther e ar e two primar y national monitoring networks that measur e r egional tr ends in acid deposition and air quality . The National A tmospheric Deposition P r ogram (NADP) measur es acid and other chemical concentrations and annual deposition in pr ecipitation, and the Clean Air Status and T r ends Network (CASTNET)

measur es air quality concentrations and dr y deposition of nitr ogen and sulfur compounds. The data fr om these monitoring networks ar e the primar y way for r esear chers and policy analysts to assess the envir onmental benefits of T i tle IV and any other r egional or national emission r eduction pr ograms such as the NO x State Implementation Plan (SIP) call.

P LANNING FOR AN A GING N ETWORK : NADP ’ S R EFURBISHMENT The original NADP deposition monitoring stations wer e installed in 1978, with a lar ge portion of the network in place by 1984. The

curr ent sampling equipment has been operating continuously in the field for an average of 17 years without substantial upgrade or r efurbishment. A major r e furbishment and/or

r eplacement pr ogram is needed to pr event the aging equipment fr om limiting the network’s ability to collect high quality data. In or der to begin the pr ocess of upgrading the network, testing of off -the-shelf modern pr ecipitation
r ecor ding gauges is now under way . 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 mor e fle xible deposition sampling system, one that would not only support all curr ent NADP sampling needs, but also pr ovide the capability to sample additional pollutants as the need arises.
A DV ANCING C ASTNET’ S M ONITORING CAP A CITY The original CASTNET monitoring stations wer e established in 1987. CASTNET plays a vital r ole in pr oviding r egional concentrations of important particle and gaseous chemical species, deposition estimates of important acidic species, rural ozone, and r egional transport. CASTNET continues to pr ovide accountability and performance measur e-r elated information for the Acid Rain P r ogram and is in position to pr ovide deposition and air quality baselines to support futur e accountability and pr ogram evaluation needs (whether r egulator y or legislative). As such, both near - and long-term pr ogram goals involveadvancing the monitoring and assessment capabilities of the network. Specifically , plans for CASTNET include a multi-agency effort between EP A, NOAA, The National P ark Ser vice and the United States F o r est Ser vice to add high-r esolution, continuous measur ements of gases and particles to the suite of measur ements conducted at each site. These additional measur ements
ar e important in e xpanding our understanding of fine particulate formation, transport, and air quality model evaluation. This change will also pr ovide critical information for implementation of an integrated monitoring strategy for k ey monitoring pr ograms in the U.S. (e.g., IMPRO VE, NADP) and Canada.

# Chapter 2: Results of the Acid Rain Pr ogram: Status and T r ends of Emissions and Envir onmental Impacts (1990–2002)

Both SO 2 

### and NOx 

### emissions from power generation sources have significantly declined under Title IV . In 2002, SO 2 

### emissions from Title IV

-affected sources totaled 10.2 million tons and NO x 

### emissions from all Title IV -affected sources totaled 4.5 million tons, down 35% and 33% respectively from 1990 levels. Sources instates with the highest emissions continue to reduce their emissions the most, and there have been no significant geographic shifts inemissions. The benefits of these emission reductions include improvements in air quality (which are e xpected to lead to significant human health benefits), broad-scale reductionsin 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

ar

eas. Although there have been no broad-scaleregional 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

SO 2 E MISSIONS SO 2 emissions declined significantly in 1995, the first year T itle IV took effect. Emissions climbed slightly fr om 1995 levels during the late 1990s but r e mained far below the number of allowances allocated. These “e xtra” r eductions allowed sour ces to build up a lar ge bank of unused allowances that could be used in the futur e when the emission cap decr eased during Phase II. Beginning in 2000, the first year of Phase II,

sour ces complied with the lower cap by decr easing emissions significantly as well as by using some of their bank ed or stor ed allowances. In 2002, the 3,208

sour ces in the SO 2 pr ogram had r educed their SO 2 emissions by 41% (7.1 million tons) compar ed to 1980 levels and 35% (5.5 million tons) compar ed to 1990 levels. F igur e 8 shows the tr end in SO 2 emissions since 1980 for all affected sour ces. The electric utility industr y is by far the lar gest single

sour ce of SO 2 emissions, accounting for appr o ximately 69% of total SO 2 emissions nationwide in 2001 15 ( www .epa.gov/ttn/chief/trends/inde x.html ). In addition to the significant r eductions fr om the electric power generation sector , r eductions in SO 2 emissions fr om other

sour ces, including smelters and sulfuric acid manufacturing plants, and use of cleaner fuels in r esidential and commercial burners, have contributed to the 39% decline of SO 2 emissions fr om all sour ces since 1980 (EP A W eb site, 2003). F igur e 9 shows the tr end in total SO 2 emissions fr om all sour ces since 1980.
NO X E MISSIONS NO x emissions fr om all T itle IV sour ces dr opped appr o x imately 1.6 million tons between 1990 and 1996, when Phase I of the Acid Rain NO x pr ogram was implemented. Subsequently , NO x emissions r emained fairly steady thr ough 1998 befor e dr opping significantly in 1999 and again in 2000 to comply with the stricter emission limits under Phase II. By 2002, emissions fr om the 1,048 NO x pr ogram affected sour ces wer e 27% (1.5 million tons) lower than they had been in 1990. NO x emissions for all 3,208 T i tle IV units wer e 33% (2.2 million tons) lower than they had been in 1990.
T itle IV of the 1990 Clean Air Act r equir es the NO x pr ogram to achieve a 2 million ton r eduction in emissions fr om pr ojected 2000 NO x emission levels without T itle IV . T

otal NO x emissions fr om all T itle IV affected units surpassed that goal by 1 million tons in 2000 (see F igur e 1 0).
These r eductions have been achieved by r educing the amount of NO x emitted per unit of heat input and despite the fact that the amount of fuel burned to pr oduce electricity in power plants has incr eased 30% since 1990. As illustrated in F igur e 1 1, many states with incr easingelectricity pr oduction had lower total NO x emissions in 2002 as compar ed to 1990 levels. W ithout further r eductions in emissions rates or institution of a cap on NO x emissions, however , NO x emissions fr om power plants could rise as mor e fossil fuels ar e used to

cr eate ener gy in most ar eas of the countr y

. NO x emissions come from a wide variety of

sources including those affected by T itle IV

. In 2001, NO x emissions from electric power generation accounted for appro ximately 22% of NO x emissions from all sources (4.9 million tons). N O x emissions from transportation
sour ces wer e 56% of NO x emissions from all

sources (12.4 million tons). Nationally, NO x emissions have decreased 12% (3.2 million tons) between 1990 and 2001. This occurred in spite of an increase of 10% in NO x 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 sour ces. This is due to a variety o f federal and state emission r eduction pr o grams (including T itle IV

, the Ozone T ransport Commission NO x Budget T rading P rogram, and anticipation of the NO x SIP call) and reductions from on-road vehicles (EP A W eb site, 2003). In the past few years, EP A has promulgated several rules to reduce SO 2 and NO x 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 F igur e 12). In addition, new rules for stationar y sources (the Clean Air Interstate Rule, the Clean Air Mercur y Rule, and the Clean Air V isibility Rule), as well as State plans to attain the f i n e p a rticle and ozone NAAQS, will also significantly reduce future NO x emissions. 

# Geographic Distribution of Emissions fr om P ower Generation

G EOGRAPHIC T RENDS IN SO 2 E MISSIONS T

otal S O 2 emissions fr om power generation have decr eased appr o ximately 35% under the cap and trade appr oach since 1990. Emission r e ductions ar e taking place thr oughout the countr y; the geographic distribution of S O 2 emissions has not changed significantly since 1990. 16 F igur e 13 compar e s state S O 2 emission tr ends fr om power generation befor e implementation of T itle IV (1990), during Phase I of T itle IV (1995–1999 average), and in Phase II to date (2000–2002 average).Several geographic trends are evident. Although most SO 2 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 SO 2 emissions in the countr y (see F igure 13). This confirms an important principle of the cap and trade approach: reducing emissions is cheapest for the lar gest sour ces. These sour ces will, ther efore, reduce emissions before others in the industr y and take advantage of their ability to sell excess allowances to other sources where emission r eductions ar e more expensive. 17 The highest SO 2 emitting states in 1990 (Ohio, Indiana, and P ennsylvania) 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. SO 2 emissions decreased 57% in Illinois, 41% in Kentucky, 70% in Missouri, 53% in T ennessee, and 45% in W est V i r ginia. The 27 shaded states represent states wher e SO 2 emissions in Phase II (2000–2002) werelower than both 1990 levels and the 1995–1999 Phase I average. Most SO 2 emission reductions during Phase I

occurred in appro ximately a dozen states in the Eastern U.S. (Phase I affected the lar ger , higher emitting utilities in the eastern half of the countr y). Unlike the SO 2 emission r eductions achieved during Phase I, Phase II r eductions are geographically more widespread, occurring in a lar ger number of southeastern and some western states. In 20 mostly western and southern states, average SO 2 emissions in Phase I wer e higher than they had been in 1990. This is due to the lar ge number of Phase II sour ces in these states that wer e not r equir ed to contr ol for SO 2 until 2000. In the 2000–2002 period (Phase II), emissions in these states declined as their sour ces became subject to the pr ovisions of the Acid Rain P r ogram. F o r
e xample, SO 2 emissions in T e xas incr eased fr om 1990–1999 because no T e xas sour ces
wer e affected by T itle IV at that time. SO 2 emissions in the state decr eased mor e then 100,000 tons in Phase II when T itle IV

r equir ements took effect for T e xas sour ces. In fact, sour ces in all the states wher e average emissions in Phase I wer e higher than emissions in 1990 collectively r educed their emissions appr o ximately 500,000 tons in Phase II (2000–2002). In addition, some sour ces incr eased their emissions since 1990 due to incr easing utilization of units. However , over half these units actually decr eased their emission rate, indicating that incr eases in demand for elec­tricity caused those emission incr eases. These emission incr eases, while not insignificant, wer e mor e than offset by the emission

r eductions that have occurr ed since 1990. In the 16 states wher e state-wide emissions incr eased, the emission incr eases totaled 430,000 tons of SO 2 between 1990 and 2002; over that same period of time, the r e

maining states r educed SO 2 emissions mor e than 13 times that much, by 5.5 million tons.
G EOGRAPHIC T RENDS IN NO X E MISSIONS T

otal NO x emissions from NO x program affected sources (coal-fired) have decreased approximately 27% since T itle IV was authorized by Congress in 1990. The geographic distribution of NO x emissions has not changed significantly since 1990. 16 Even though most emission reductions have taken place in the areas of the countr y with the highest NO x emissions, these reductions have not resulted in a large change in the pattern of NO x emissions in the U.S. NO x emission
reductions since 1999 are due in part to implementation of the OT C NO x Budget Program, the NO x SIP call, and several state r eduction pr ograms as well as Phase II of T itle IV . F igur e 1 4 displays bar graphs illustrating relative state NO x emission trends from power generation sources affected by the NO x program before T itle IV (1990), during Phase I (1996–1999 average), and in Phase II (2000–2002 average). Several geographic tr ends ar e evident. NO x emissions wer e lower in 34 states in Phase II compar ed to 1990 levels, with the greatest reduction occurring in the Eastern United States. In 1990, the highest NO x emissions occurr ed in the midwestern and southern regions of the U.S. (see F igure 14). By 2002, emissions in many of these states had been significantly r educed fr om 1990 levels. The states with the highest emissions in 1990 (Ohio, T e xas, and P ennsylvania), achieved an average r eduction of 38% (33%, 30%, and 52%,

r espectively) in Phase II compar ed to 1990. Other states in the r egion show similar tr ends since 1990. NO x emissions decr eased 27% in Indiana, 23% in Illinois, 35% in Kentucky

, 34% in T ennessee, and 31% in W est V i r ginia. The 32 shaded states had lower average NO x emissions in Phase II compar ed to both 1990 levels and the 1996–1999 Phase I average. NO x r eductions occurr ed pr edominantly in the northeastern U.S. during Phase I. In Phase II, NO x r eductions ar e geographically mor e e xtensive and occur in a lar ger number of southern and midwestern states.In several states, average NO x 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 NO x 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 NO x emissions were higher than emissions in 1990 and/or the Phase I average. This is because the rate-based Title IV NO x limits on coal-fired power plants do not limit total emissions of NO x in the way that total emissions of SO 2 are limited. Since heat input (or fuel use) increased in those states due to additional power generation, overall NO x 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 providescientists 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 isused to determine how well existing air pollution control strategies are working and whether additional emission reductions are

re quired 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 primarylong-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 15also includes a table listing the major chemical species measured by these networks as well as the other networks discussed below.



## Primary Air Quality and A

tmospheric Deposition Monitoring Networks A mosaic of national air quality and deposition monitoring networks has evolved to provide sci entists 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 itmay 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 D EPOSITION M ONITORINGNADP/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 ResearchMonitoring Network (AIRMoN), a researchbased subnetwork of NADP, consists of nine monitoring stations that collect dailyprecipitation samples. The daily samples are analyzed for the same pollutants as NADP/NTN and are primarily used to identify source­r ecep tor r elationships in atmospheric models. Qua li ty assurance procedures are followed for all aspects of NADP field and laborator y operations. Quality assured data are available from the NADP W eb site, http://nadp.sws.uiuc.edu/

. AMBIENT AND D RY D EPOSITION M ONITORING CASTNET is a long-term atmospheric deposition monitoring network that measur es changes in ambient air quality and atmospheric deposition over br oad r egions of the U.S. Operating since 1987, CASTNET has evolved into a r obust monitoring pr ogram which now comprises over 80 monitoring stations nationwide. Sites ar e located pr edominantly in rural locations. EP A operates most of the monitoring stations; however , NPS operates 30 stations in cooperation with EP A. CASTNET measur es weekly average atmospheric concentrations of the major pollutantscontributing to acid deposition, hourly concentrations of ambient ozone levels, and meteor ological conditions. CASTNET estimates dr y deposition or flux based on measur ed ambient air concentrations and modeled deposition velocity . Deposition velocity is inferr ed thr ough the multilayer model (MLM) which accounts for the influence of meteor ological conditions and vegetation on dr y deposition pr ocesses. CASTNET data measur ed at point locations ar e used in conjunction with the NADP/NTN data to infer geographic patterns and long-term temporal tr ends in total (dr y plus wet) atmospheric deposition. Quality assurance pr ocedur es ar e followed for all aspects of CASTNET field and laborator y operations. Quality assur ed data ar e available at www .epa.gov/castnet

. V

ISIBILITY M ONITORING IMPRO VE is a long-term monitoring network that measur es curr ent visibility conditions, tracks changes in visibility and determines the causes of visibility impairment in national parks and wilderness ar eas. IMPRO VE was established in 1985 to aid the development of federal and state implementation plans to pr otect visibility in Class I ar eas as stipulated in the 1977 amendments to the Clean Air Act. IMPRO VE began collecting data in 1988 at 20 Class I ar eas. The network e xpanded to monitor the impacts of the Regional Haze Rule and now consists of 167 sites nationwide. IMPRO VE is a collaborative monitoring effort dir ected by a steering committee comprised of

r epr esentatives fr om NPS, USEP A, USF S, FWS, NOAA, Bur eau of Land Management, and state and r egional air or ganizations. Ever y IMPRO VE site deploys an aer osol sampler to measur e speciated fine aer osols and PM 10 mass. Select sites also deploy transmissometers and nephelometers to measur e light e xtinction and scattering r espectively , as well as automated camera systems to measur e the “scene.” Light

e xtinction coefficients and the deciview indices used to r eport visibility data ar e calculated fr om
r elative humidity and the measur ements of the five k ey species: sulfates, nitrates, or ganic carbon, elemental carbon, and crustal material at each site. Data and additional information about IMPRO VE ar e available at http://vista.cira.colostate.edu/impr ove/

. A IR Q U ALITY M ONITORING State and L ocal Air Monitoring Stations (SL AMS) and National Air Monitoring Stations (NAMS)

ar e designed to judge compliance with ambient air quality standar ds; activate emer gency contr ol pr ocedur es that pr event or alleviate air pollution episodes; and obser ve pollution tr ends thr oughout the r egion. The information collected also pr ovides a database for r esear ch on urban, land use, and transportation planning; developmentand evaluation of abatement strategies; and development and validation of dispersion models.
The SL AMS/NAMS network performs measur ements primarily in urban ar eas wher e air quality is influenced primarily by local sour ces. Some sites ar e located away fr om urban ar eas to characterize r egional air quality . SL AMS consists of appr o ximately 4,000 monitoring stations whose size and distribution ar e lar gely determined by the needs of state and local air pollution contr ol agencies to meet their r espective SIP

r equir ements. NAMS (1,080 stations) ar e a subset of the SL AMS network with emphasis on areas of maximum concentrations and high population density . Additional information on SL AMS/NAMS is available at www .epa.gov/ oar/oaqps/qa/monpr og.html

.A IR Q U ALITY : S ULFUR Analyses of r egional monitoring data fr om CASTNET show the geographic pattern of SO 2 and airborne sulfate in the Eastern United States, wher e concentrations fr om power plants ar e highest. Thr ee-year mean annual concentrations of SO 2 and sulfate fr om a set of 34 CASTNET long-term monitoring sites wer e compar ed fr om 1989–1991 and 2000–2002. In 1989–1991, prior to implementation of Phase I of T itle IV , the highest ambient concentrations of SO 2 in the East were obser ved in western P ennsylvania, and along the Ohio River Valley in the vicinity of Chicago, Illinois, and Gar y , Indiana. The highest ambient sulfate concentrations, gr eater than 7 micr ograms per cubic meter (μg/m3), wer e obser ved in this ar ea and in northern Alabama. Most of the Eastern U.S.

e xperienced annual ambient sulfate concentrations gr eater than 5μg/m3.During the late 1990s following implementation of Phase I, dramatic r egional impr ovements in SO 2 and sulfate air quality wer e obser ved at CASTNET sites thr oughout the Eastern U.S. Ambient concentrations of SO 2 in 2000–2002 decr eased 45% in the Midwest, 40% in the Northeast and 30% in the mid-A tlantic compar ed to concentrations in the early 1990s (see F igur e 16). Other r esear chers, using differ ent statistical methods, have also r eported r eductions in SO 2 concentrations of 30–40% in the northern midA tlantic and Midwest. 18 The lar gest decr eases in ambient SO 2 concentrations wer e noted in high emissions and concentration ar eas (e.g., vicinity of Chicago and thr oughout Indiana, Ohio, P ennsylvania, Kentucky , and W est V i r ginia). The highest SO 2 concentrations obser ved in the rural parts of the United States ar e now concentrated in eastern Ohio and southwestern P ennsylvania.
F or ambient sulfate, both the size of the affected r egion and magnitude of the highest concentrations wer e dramatically r educed following implementation of T itle IV . Sulfate concentrations decr eased up to 30% in the Midwest and Northeast and 25% in the mid-A tlantic compar ed to concentrations obser ved in the early 1990s (see F igur e 17). The lar gest decr eases wer e obser ved along the Ohio River V

alley in Illinois and P ennsylvania. These r eductions have also been confirmed by other r esear chers using differ ent statistical methods. 18The urban monitoring networks (SL AMS/NAMS) also detected the envir onmental signal fr om T i tle IV SO 2 emission r eductions. Analyses of urban monitoring data fr om 480 monitoring stations show a decr easing tr end in SO 2 in the atmospher e following implementation of T itle IV

. Between 1992 and 2001, annual arithmetic mean concentrations of atmospheric SO 2 decr eased 35%. 16
A IR Q U ALITY : N ITROGEN Emissions of NO x ar e transformed in the atmospher e into a series of pollutants, including nitric acid vapor (HNO 3 ) and nitrates (NO 3 ). F r om the perspective of acid deposition, nitric acid is a k ey chemical species, for which the rural air quality and dr y deposition monitoring networks now have

an e xtensive r ecor d. While ther e is str ong evidence of statistically significant declining tr ends in the concentrations of SO 2 and sulfate, tr ends in nitr ogen concentrations have not been as pr onounced.
At mospheric nitr ogen concentrations ar e a r esult of emissions of several differ ent forms of nitr ogen fr om many differ ent types of sour ces; ther efor e, they tend to e xhibit a

mor e complicated pattern than sulfur concentrations. Nitric acid concentrations, for e xample, ar e highest in eastern Ohio and south central P ennsylvania/north central Mar yland, ar eas with high emissions fr om power plants. Ammonium concentrations, in contrast, ar e highest fr om Ohio west to Missouri and Iowa and in northern Alabama as well as central P ennsylvania. Nitrate concentrations ar e highest even further west, in southern Wi sconsin/northern Illinois and westwar d into the Plains, ar eas with lar ge amounts of agricultural activity

. Scientists have not r eported any evidence of a statistically significant decr easing tr end in nitric acid concentrations over the past 15 years, but a r ecent analysis of CASTNETdata does indicate that nitric acid concentrations may be lower than they wer e in the1989–1991 timeframe. CASTNET data indicate r eductions of nitric acid concentrations between 1989–1991 and 2000–2002 of appr o ximately 10% in the Northeast, midA tlantic, Southeast, and Midwest, and appr o ximately 25% in the North Central and South Central Plains states (see F igur e 18). In contrast, the only statistically significant published r esults seem to suggest nitric acid is incr easing over time, particularly in the Northeast. 19 Any incr ease in airborne nitricacid concentrations is of concern because of the potential for corr esponding incr eases in deposition. It is not clear whether the cause is associated with changes in nitr ogenous fertilizers, difficulties measuring nitric acid vapor , the length of the r ecor d, or some other r eason not associated with upwind pointsour ce emission measur ements. Scientists continue to monitor concentrations of nitricacid, as well as all the components of atmospheric deposition discussed her e, and it is hoped that additional data will clarif y the tr ends in nitric acid concentrations. Analyses of mean annual total ambient nitrate concentration data have not r evealed any statistically significant changes fr om 1989 thr ough 2002. Although ther e ar e some indications that ambient nitrate concentrations

ar e incr easing, particularly in the Midwest,nitrate changes at individual CASTNET monitoring stations have been minimal over this period (see F igure 19). The highest nitrate concentrations i n t h e E ast wer e r ecor ded in Illinois, W isconsin, and Iowa (>4 μg/m3). Ambient ammonia (NH 3 ) concentrations, which might also indicate changes in emissions in states with significant agricultural activity , ar e not r outinely measur ed by either CASTNET or AIRMoN. Ambient airborne concentrations of ammonium (NH 4 ) ar e typically associated with nitrate and sulfate compounds. The highest annual mean ammonium concentrations have been obser ved in the Midwest and in northern Alabama. Between 1989 and 2002, decr eases in ammonium concentrations in the atmospher e wer e obser ved in some parts of the U.S. (see F igur e 20). The lar gest decr eases of appr o x imately 0.5 μg/m3 wer e noted in the Midwest. The decr ease in ambient ammonium concentrations between 1989–1991 and 2000–2002 appears to be associated with r eductions in ambient levels of sulfate which bind to ammonium to form ammonium sulfate. Agricultural ammonia emission themselves have not decr eased over the same time period. In fact, ammonia emissions fr om cr ops and livestock have incr eased 14% between 1990 and 2001. 20
ACID D EPOSITION : S ULFUR IN P RECIPIT A

TION Scientists have documented that both wet and dr y sulfur deposition (and the acidity associated with sulfur deposition) have declined with reductions of SO 2 emissions over a lar ge portion of the Eastern U.S. following implementation of T itle IV

. 21-25 Strong correlations, near linear , between lar ge scale SO 2 emission reductions and lar ge reductions insulfate concentrations in precipitation havebeen noted for the Northeast, one of the areas most affected by acid deposition. 22
A main r eason for r educed concentrations of sulfate in pr ecipitation in the Northeast is a

r eduction in the long-range transport of sulfate fr om emission sour ces located in the Ohio River V alley . The r eductions in sulfate documented in the Northeast, particularly

acr oss New England and portions of New Y

ork State, wer e also affected by SO 2 emission r eductions in eastern Canada. Concurr ent with these sulfate r eductions

wer e similar r eductions in pr ecipitation acidity , e xpr essed as H + concentrations. However , acidity may not have decr eased 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 pr ecipitation may include climate variability (i.e., wet and dr y summers) or temporal changes in base cations and ammonium concentrations. 22 Some of the gr eatest r eductions in wet sulfate deposition occurr ed in the midAppalachian r egion, including Mar yland, New Y ork, W est V i r g inia, V i r ginia, and most of P ennsylvania (see F igur e 21). W et sulfate deposition decr eased 40% in the Northeast and 35% in the Midwest and mid-A tlantic since the early 1990s. Reductions of 25% have occurr ed in the Southeast.
F igure 22 shows the total (wet plus dr y) change in sulfur deposition in the conterminous U.S. between 1989–1991 and 2000–2002. T otal sulfur deposition is defined as the sum of the dr y components (SO 2 andparticulate sulfate) and wet components (sulfate in precipitation) as sulfur . W et sulfate and SO 2 are the lar gest components of total sulfur deposition. In general, dr y deposition is a lar ger percentage of total deposition in those areas nearest to SO 2 emission sour ces. The highest rates of total sulfur deposition have been obser ved in the areas containing the highest SO 2 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. Baumgar dner et al. r eport that in the early 1990s, the mean annual total sulfur deposition at CASTNET sites in the Ohio River V alley was 15–22 kg/ha. Sites located in P ennsylvania, southern New Y ork, and W est V i r g inia also had total sulfur deposition gr eater than 15 kg/ha. F r om 1997–2000, total sulfur deposition ranged fr om 12–17 kg/ha for sites in the Ohio River V alley. Appreciable decreases in total sulfur deposition of 15–25% have occurred throughout the Eastern U.S. since the early 1990s. 21
ACID D EPOSITION : N ITROGEN IN P RECIPIT A

TION Nitrate concentrations in pr ecipitation measur ed at NADP/NTN sites have generally r emained the same or incr eased in some r egions (e.g., the Southeast) over the past 15 years. Unlik e sulfate concentrations, sharp declines in nitrate concentrations have not been obser ved in the Northeast, but some decline has occurr ed. 26 While wet nitrate deposition does appear to be substantially lower in the East in 2000–2002 (see F igure 23), nitrate concentrations in precipitation did not change (see F igure 24). Ther efor e, the lower nitrate deposition in 2000–2002 appears to be r elated to lower pr ecipitation levels over those same years. Other r esear chers have also not found significant

r eductions in nitr ogen deposition since 1990. L ynch, Bowerso x, and Grimm found that most ar eas of the countr y wer e not substantially differ ent in r educed or o x idized nitr ogen in 1997–1999 compar ed to historic levels. These findings wer e not une xpected and ar e consistent with the NO x emissions that wer e not appr eciably affected by T itle IV in 1996 or 1997 on a br oad, r egional basis. 27 The contr ol of NO x emissions under T itle IV was not r equir ed until 1996. Mor eover , any decr eases in NO x emissions fr om power generation have been significantly offset by incr eases fr om the vehicle sector fr om 1991 to 2000. 26 In addition, it is inter esting to note that while air quality concentrations of ammonium have decr eased in the Midwest, both concentrations of ammonium in pr ecipitation and deposition of ammonium have incr eased in that r egion since 1985. 28
F igur e 25 shows the differ ence in total (wet plus dr y) nitr ogen deposition in the conterminous U.S. between 1989–1991 and 2000–2002. T otal nitr ogen deposition is the sum of the dr y deposition components (nitric acid, particulate nitrate, particulate ammonium) and wet deposition components (nitrate and ammonium in pr ecipitation). W e t nitrate, wet ammonium, and nitric acid ar e the lar gest contributors to nitr ogen deposition. As is the case with sulfur

, the dr y nitr ogen components comprise a lar ger part of total nitr ogen deposition in those ar eas near est to sour ces of NO x emissions. In some ar eas of southern California, for

e xample, wher e NO x emissions ar e pr edominantly fr om mobile sour ces, the ratio of dr y to wet deposition is appr o ximately 4:1.T otal nitr ogen deposition is highest in the Eastern U.S., although several monitoring stations in the W est obser ve high levels of total nitr ogen deposition. Ther e ar e no obser vable br oad-scale r eductions in total nitr ogen deposition. Baumgar dner et al. r eport that in the early 1990s, the highest mean annual total nitr ogen deposition noted was 8–11 kg/ha in the Midwest and portions of the Northeast. Fr om 1997–2000, little change occurr ed fr om the nitr ogen deposition values r ecor ded in these ar eas earlier in the decade. 21

V

ISIBILITY SO 2 and NO x gases are transformed in the atmosphere into fine particles of sulfates and nitrates. Sulfate and NO 3 - particles scatterand absorb light, impairing visibility and contributing to regional haze. Sulfates ar e generally the lar gest contributor to visibility impairment in both the East and the W est. The visual range under naturally -occurring conditions without pollution in the U.S. is appro ximately 75–150 km (45–90 miles) in the East and 200–300 km (120–180 miles) in the W est. Data fr om the IMPRO VE network, which monitors visibility tr ends in national parks, show that visibility has r emained virtually

un changed since the early 1990s (see F igur e 26). The level of visibility impairment on the worst visibility days in the W est issimilar 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) compar ed to 117 km (73 miles) for the best visibility . In the W est, visibility impairment for the worst days

r emained r elatively 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). V isibility in the East is still significantly impair ed in national parks and wilderness

ar eas, especially on the haziest days. 

# Ecological Recover y The atmospheric and envir onmental r esponse to a r eduction in pollutants is a comple x and lengthy pr ocess. 29, 30 It tak es only a few hours for a r eduction in emissions to impr ove air quality; it tak es days to weeks for r eductions in deposition to appear . Once the pollutants ar e deposited on the ecosystem, biogeochemical and hydr ologic cycling pr ocesses drive the rate of r ecover y

. P r evious r esear ch tells us that, even af ter acid deposition decr eases, ecosystems can
e xperience lag times of several decades between chemical

r ecover y (e.g., incr eased pH) and biological r ecover y (e.g.,

r epopulation of fish or for est r egr owth). A short “episodic” period of acidification can be eased in a few weeks to months; r ecover y fr om long-term “chr onic” acidification can tak e years or even decades. Some soil pr ocesses can
r ecover in a few weeks to months; building up the appr opriate soil nutrient r eser ves can tak e centuries. Recover y of for est ecosystems generally tak es years to decades. Ther efor e, while some ecological r ecover y pr ocesses occur quickly , others will tak e place mor e gradually over a generation or mor e. Ever y additional year of monitoring, r esear ch, and analysis impr oves our understanding of these r ecover y pr ocesses and allows us to better pr edict envir onmental r e

sponse to emission r eductions.

F RESHW A

TER ACID -S ENSITIVE E COSYSTEMS 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 fr om those waters 31 (see F igur e 27). In the 1980s, acid rain was found to be the dominant cause of acidification in 75% of acidic lak es and 50% of acidic str eams. 31 Br oad ar eas of the United States still contain lar ge numbers of lak es and str eams 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 Adir ondack and Catskill Mountains), the Appalachian Mountains, and ar eas of the W estern United States (see F igur e 1). High-elevation watersheds with steep topography , e xtensive ar eas of e xposed bedr ock, deep snowpack accumulation, and shallow , base-poor soils

ar e ver y sensitive to episodic acidification. 32 Such systems

ar e common thr oughout 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 EP A

’s T emporally Integrated Monitoring of Ecosystems (TIME) and L ong- T

e rm Monitoring (L TM) projects that were initiated in the early 1980s by EP A

’s Office of Research and Development (see F igure 28). The purpose of these networks is to detecttrends 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/L TM 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 P rovinces of V i r ginia). Additional sites in acid-sensitive regions of the Southeast, Midwest, and W est would make more complete assessments possible. TIME/L TM measur es a variety of important chemical characteristics, including ANC, pH, sulfate, nitrate, major cations (e.g., calcium and magnesium), and aluminum. While the repr esentativeness of the TIME/L TM 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 Sur vey has been measuring surface water quality at several research watersheds throughout the United States, where sample collection during hydrologic events and ancillar y data on other watershed characteristics have been used to assess the watershed processes controlling acidification of surface waters. 33–35 The NSF L ong- T

erm Ecosystem Research (L TER) 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 L TM 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 P rogram (EMAP)

ar e the primar y pr ograms that p rovide probability -based data on surface water quality , and thus can be used to provide regional estimates of acidification recover y at base flow conditions. The current advantage of the TIME and L T M programs is that the data reside in a common database and common methodologies have been applied to all samples.
A r e cent r eport using this TIME/L TM data concludes that measurable impr ovements in surface water chemistr y (lower sulfate concentrations and decr eases in acidity) have r e

sulted fr om emissions r egulations pr omulgated pursuant to the 1990 CAAA, primarily under the Acid Rain P r ogram. 36 This conclusion was r eached while acknowledging that o t h

er changes, including cli mate variab ility , for est maturation and r ecover y fr om disturbances, and land use histor y ar e also contributing to changes in the chemistr y of lak es and str eams in the Eastern United States. A summar y of the TIME/L TM data is pr esented in F igur e 29. A pr ediction of the long term r esponse of lak es and str eams to r eductions in acid deposition is pr ovided in Chapter 4. In thr ee of the five ar eas studied by Stoddar d et al. 36 one-quarter to one-thir d of lak es and str eams pr eviously affected by acid rain ar e no longer acidic at base flow conditions, although they ar e still highly sensitive to futur e changes in deposition. In other ar eas, signs of r ecover y ar e not yet evident, suggesting that additional

r eductions ar e necessar y for further ecosystem r ecover y

. In general, tr ends in nitrate concentrations wer e much smaller than tr ends in sulfate concentrations, though lak es in the Adir ondacks and str eams in the northern Appalachian Plateau e xhibited significant downwar d tr ends in nitrate in the 1990s. It should be noted however , that nitrate incr eased during the 1980s and decr eased dramatically in 1992 for r easons
unr elated to changes in deposition. Analysis of str eam nitrate concentrations based on the 20-year r ecor d indicates no significant or slightly decr easing tr ends. 37 Incr easing ANC, an indicator of aquatic ecosystem r ecover y , was evident in thr ee of the r egions (Adir ondacks, northern Appalachian Plateau and upper Midwest) and was unchanged in New England and the Ridge/Blue Ridge r e

gion. Modest incr eases in ANC have

r educed the number of acidic lak es and str eam segments in some r egions:

• Eight per cent of lak es in the Adir ondacks

ar e acidic, down fr om 13% in the early 1990s.

• In New England, 5.5% of lak es ar e acidic,

an insignificant change fr om the early 1990s when 5.6% of lak es in the r egion
wer e acidic.

• F e wer than 1% of lak es in the upper

Midwest ar e acidic, down fr om 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.

• Ther e has been no change in the number

of acidic waters in the Ridge/Blue Ridge

r egion in the past decade.Lak e and str eam r ecover y is due to

r eductions in acidic deposition, primarily sulfate deposition, achieved by T itle IV

. 36 Ther e have been decr eases in sulfate concentrations in almost all Adir ondack lak es that coincide with r eductions in atmospheric concentrations of sulfur

. 38 These r eductions in sulfate, as well as r eductions in nitrate concentrations that do not appear to be due to changes in atmospheric deposition, have r esulted in incr eased pH and ANC as well as r eductions in the amount of to xic inor ganic aluminum in these lak es. 38 In spite of declining sulfate concentrations, however

, TIME/L TM data show that some lak es and str eams have not begun to r ecover

. Recover y was not evident in New England or in the Ridge and Blue Ridge P r ovinces (V ir ginia); the latter r egion is not e xpected to r ecover immediately , due to the natur e of for est soils in the pr ovince. In part, declining levels of base cations (calcium, magnesium, and potassium) in lak es and str eams thr oughout the East ar e believed to delay the onset of r ecover y

. 36 In addition, it should be noted that, although chemical r ecover y (i.e., an incr ease in ANC and decr ease in sulfate and nitrate concentrations) has begun in some lak es and str eams, full biological r ecover y of fish and other aquatic or ganisms tak es significantly longer

. 30
Thr oughout the Appalachians, many waterbodies ar e still acidic enough to damage sensitive fish and aquatic life. In Vi r ginia, for e xample, over a decade of quarterly str eam monitoring shows that the pr edominant tr end, as indicated by declining ANC, has been continued acidification. The number of chr onically acidic V i r ginia tr out str eams has incr eased, while the number of transitional and not-acidic str eams has declined. 39 The consequences for individual species have been well documented in the Shenandoah National P ark by the F ish in Sensitive Habitats (FISH) Pr oject. The FISH pr oject demonstrated that acid-sensitive fish in both chr onically and episodically acidic str eams in Shenandoah National P ark have lower body size and weight, both indications of chr onic str ess. Some lak es in the Colorado F r ont Range ar e showing indications of being affected by higher atmospheric nitr ogen loads fr om urban, agricultural, and industrial sour ces. 40 In the central and northern Rockies, sediment r ecor d studies str ongly suggest that lak es began r esponding to nitr ogendeposition levels in the r egion as early as the 1950s. 40, 41 Recent analyses demonstrate r egional differ ences in lak e surface water chemistr y and show evidence of ecological change in the r egion. F or e xample, studies of high-elevation lak es in the central Rockies found significantly higher nitrate concentrations in lak es on the east side of the F r ont Range, wher e emission sour ces ar e numer ous, as compar ed to lak es on the west side. 40, 42 An intensive sampling study pr ojected that at least two sites in the Gr een Lak es V alley in the Colorado F r ont Range could become chr onically acidic within the ne xt decade, and a thir d site could become episodically acidified at pr esent rates of ANC decr ease. 42 It has also been demonstrated that algal communities, soils, and subalpine for ests on the east side of the F r ont Range

ar e beginning to show signs consistent with r eceiving mor e nitr ogen deposition fr om fix ed, mobile, and agricultural nitr ogen sour ces. 40 W ith the accumulating weight of evidence, some r esear chers conclude that high-altitude watersheds in the Colorado F r ont Range show symptoms of nitr ogen saturation. 43, 44 In some for est ecosystems, particularly in the Northeast, sulfur and nitr ogen have accumulated so much that the soils ar e unable to r etain them. This leads to leaching of im portant nutrients and higher nitrate and sulfate concentrations in some surface waters. The r elease of decades worth of pr eviously accumulated atmospheric deposition may delay the r ecover y of these surface waters in

r esponse to emission r eductions. 30 Recover y of acidified waters is also slowed by continuing acid deposition, the pr esence of nitrate in surface waters, the contribution of naturally occurring acid sour ces, and the number of physical, chemical, and biological changes needed in the ecosystem befor e it can function as an ecosystem unaffected by acid deposition. Nitr ogen e xport fr om r ecovering str eams in the Catskill Mountains was r eported to be gr eater during warm years than cool years, suggesting thata warmer climate could enhance acidification of surface waters if nitr ogen deposition rates r emained the same. 37
Thr oughout the United States, most str eams and drainage lak es e xperience some decline in ANC during heavy rainstorms, snowmelt, or other episodic events. Curr ent pr ograms for monitoring tr ends in surface water chemistr y either specifically avoid hydr ologic ‘episodes’ (high flowsduring rain events or snowmelt) or sample with a set periodicity that ultimately samples random flow conditions. The tr ends pr esented ar e ther efor e r epr esentative of baseflow or median flow conditions over time and do not indicate tr ends in surface-water chemistr y during high-runoff periods. In the Northeast and mid-A tlantic r egions of the United States, the number of lak es and str eams that e xperience episodic acidification ar e estimated to be much gr eater than the number of chr onically acidic waterbodies. 45 Episodic acidification has been r eported in the mid-A tlantic and Northeastern r egion in
ar eas such as the Ridge and V alley P r ovince of P ennsylvania, V i r ginia, and Mar yland, the northern Appalachian Plateau of P ennsylvania, the Catskill and Adir ondack Mountains of New Y ork, and the A tlantic Coastal Plain of Mar yland. 45 Several r ecent studies also have demonstrated episodic acidification of str eams in Shenandoah and Gr eat Smoky Mountains National P arks. 46–49 A r ecent study of br ook tr out str eams in Vi r ginia estimated that 24% e xperience r egular episodic acidification at levels harmful to br ook tr out and other aquatic species. 46 Due to the difficulties and e xpense 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 af ter 1994 have been published. As

a r esult, it is unknown whether the regional incidence of episodic acidification h a s decr eased as a r esponse to declines in acidic deposition in the 1990s.

F OREST E COSYSTEMS Acid deposition especially combined with other pollutants and natural str esses, can also damage for est ecosystems. Sulfates and nitrates fr om acid deposition leach nutrient base cations (calcium, magnesium, and potassium) fr om tr ee needles and leaves and fr om for est soils, r educing the for est’s capacity to buffer further acidification and

r emoving elements essential for tr ee gr owth. Acidification also leads to the mobilization of naturally -occurring aluminum, which may interfer e with nutrient uptak e by r oots in for est soils. In add ition, exposure to tropospheric ozone (a product of NOx emissions interacting with volatile or ganic carbons (VOCs)) has both acute to xic effects on plant leaves and chronic to xic effects on the entire plant. The combined effects of depletion of foliar and soil nutrients, mobilization of aluminum and e xposur e to ozone mak e tr ees mor e susceptible to dr ought, temperatur e e xtr emes, and diseases. 50 Several for ests thr oughout the U.S. ar e beginning to show signs of nitr ogen saturation, a condition wher e the inputs of nitr ogen
ex ceed the for est’s need for them andex cess nitr ogen is leached into surr ounding water ways. 51 Ther e ar e as yet no for ests in the U.S. wher e r esear ch indicates r ecover y fr om acid deposition is occurring, as indicated by r eversal of the soil acidification such as net accumulation of base cations and r e

ductions 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

ar eas sensitive to acidic deposition and

regional studies across a gradient provide a snapshot of the status and trends in for est ecosystems. F or example, mortality of re d 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
Fo r est soils ar e also adversely affected by acid deposition. Studies in New Hampshir e, 53, 54 the Catskill Mountains in New Y ork, 55 the Adir ondacks in New Y ork, West V i r ginia, 56 North Car olina, 57 South Car olina, 58 T ennessee, 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 for est r egr owth. Resampling of for est soils in northwestern P ennsylvania originally sampled in the 1960s shows an incr ease in soil acidity

. 61 In contrast to these studies, 62 at one site in New Hampshir e, Hambur g et al., 63 inferr ed (not dir ect r e-measur ement) no decline in calcium in the for est floor in a chr onosequence study in New Hampshir e. At sites wher e ther e may be appr eciable amounts of dolomitic par ent material 64 that

ar e accessible to tr ees, soil calcium may not be depleted to the e xtent it can be in nearby soils that lack a dolomitic sour ce. 59
In these and other e xamples of calcium depletion in for est soils in Eur ope, 65, 66 it has not been possible to quantif y the component of depletion attributable to anthr opogenic acidic deposition versus tr ee uptak e. Similarly , in r eported instances of for est decline it has not been possible to attribute the decline to calcium depletion dir ectly . However, calcium depletion has been implicated indir ectly in the decline in sugar maple in the U.S. and Canada 65 , and calcium depletion adversely affects many aspects of tr ee physiology and for est ecosystem health. 67, 68 The potential for weathering r e-supply of e x changeable soil calcium r emains an important uncertainty because estimates of weathering ar e based on indir ect estimates. 69 Soil acidification may incr ease the rate of weathering, but the incr emental incr ease in calcium r elease is lik ely to be small in soils that ar e alr eady acidic. Studies that have attempted to quantif y calcium weathering rates in acidic for est soils in the Eastern U.S. generally have found that rates ar e low 53, 54, 70–72 in comparison with typical combined rates of tr ee uptak e and soil leaching. 69 Acid deposition has also contributed to a r egional decline in the availability of soil calcium and other base cations in many differ ent types of for est ecosystems. In the high elevation and mid-elevation spruce-fir for ests of New Y ork, New England, and the southern Appalachians, the change in soil nutrient ratios (lower calcium/higher aluminum) can disrupt physiological pr ocesses that ar e important to maintaining for est health. These changes lower r esistance to natural str esses, such as insects, disease and climatic

e xtr emes. 67, 68 Acid deposition is also contributing to the depletion of base cations in many poorly buffer ed soils supporting southern pines. While this may slightly bolster gr owth of southern pines in the short-term, it is e xpected to have adverse effects on pr oductivity over the long-term. F inally , the high mortality rates and r educed vigor r eported for sugar maples e xperienced in northern P ennsylvania and Quebec, Canada over the last decade may be attributed in part to cation depletion fr om those soils. 73–77
Wher eas the vast majority of for est ecosystems in the W

estern U.S. do not appear to be significantly affected by acid deposition, certain ecosystems in southern California and the Colorado F r ont Range ar e negatively affected, especially by high rates of nitr ogen deposition. Nitr ogen deposition in most affected ar eas of the Colorado F r ont Range, southern California, and in the Sierra Nevada is generally fr om a mix of mobile, industrial, and agricultural emissions

sour ces. 78 In some cases, ozone (formed fr om NO x emissions and other pr ecursors) is also causing damage. Many of these sites ar e showing signs of nitr ogen saturation. Nitr ogen saturation occurs when the supply of nitr ogen fr om deposition e x ceeds the nutritional demands of the ecosystem and nitr ogen is gradually r eleased or leached fr om the watershed into the surface waters as nitrate. 79 In the Colorado F r ont Range, r esponses to chr onic nitr ogen deposition ar e evident in changes in the chemicalcomposition of pine needles and soils. These r esults ar e interpr eted as subtle signs of the effects of nitr ogen deposition, suggesting that the trajector y towar ds nitr ogen saturation may be in the incipient stages. 40 Additional studies along air pollution gradients demonstrate that for est and chaparral sites in southern California and alpine ecosystems of the F r ont Range in Colorado ar e showing signs of nitr ogen saturation. 43, 80 The sites most affected ar enear intensive livestock operations and high population centers, although e xtr emely high str eamwater nitrate concentrations have been r eported fr om a low-elevation chaparral watershed in Sequoia National P ark in the Sierra Nevada Mountains in central California. 81 While for est ecosystems ar e clearly still suffering significant damage fr om acid deposition, the r eduction in acid deposition due to T itle IV is e xpected to be beneficial to these for est ecosystems in the long-term. F o r ests ar e generally thought to r ecover mor e slowly than aquatic ecosystems because of the comple x series of pr ocesses that must tak e place. L eached nutrients must be r estor ed thr ough weathering of the bedr ock or atmospheric deposition, and soilwater aluminum concentrations must be

r educed. Even af ter soil chemistr y is r estor ed, r ecover y of sensitive for ests is not e xpected to occur for decades because of the e xtensive r ecover y time of tr ees.
C OAST AL E COSYSTEMS The nitr ogen component of acid deposition is a significant

sour ce of nitr ogen to many estuaries and coastal waters in the Eastern U.S. Ex cessive nitr ogen loads fr om a variety of

sour ces, including atmospheric deposition and runoff fr om agricultural practices, cause many of these estuaries and coastal waters to periodically become eutr ophic. 82 Eutr ophic conditions include algal blooms (some of which may be harmful), low levels of dissolved o xygen in the water (hypo xia or ano xia) which can str ess or kill fish and shellfish, and changes in the ecological structur e of the ecosystem. Ther e ar e curr ently 44 estuaries in the U.S. that

ar e consider ed highly eutr ophic, 37 of which ar e on the eastern or gulf coasts. Many of these ar e e xpected to become mor e eutr ophic—to have worsening symptoms, including mor e algal blooms and fish kills—in the futur e. 83 T itle IV has r educed nitr ogen deposition in some coastal areas compar ed to what it would have been without T itle IV (see F igur es 23 and 25). However , in many sensitive coastal watersheds ther e has been little or no r eduction in nitr ogen deposition since 1990. 

# Materials and Structur es
Ther e is no monitoring network in place to assess the effects of r eductions in acid deposition on materials and structur es. However , based on our understanding of the pr ocesses involved, it is possible to pr edict lik ely changes that will tak e place. SO 2 , NO x , and many of the pollutants they form corr ode materials, particularly those made of limestone or marble. Monuments and historic buildings, outdoor structur essuch as bridges and buildings, and automotive paints and finishes ar e all susceptible to damage by acidic pollutants. One set of studies indicated that several air pollutants fr om coal combustion ar e the main sour ces of soiling on a limestone building in P ittsbur gh, P ennsylvania followed by er osion due to rainfall. 84–86 In general, most damage fr om air pollutants appears to come fr om dr y deposition. The rate of dr y deposition is of ten accelerated when the surface is wet, so that structur es affected by dewfall

ar e e xpected to be especially vulnerable. Rain wets e xposed structur es, again modif ying the dr y deposition rates that would be
e xpected if the same structur es wer e dr y

. Hence, ther e is a str ong interplay among dr ydeposition, wet deposition, and surface wetness that ser ves to complicate the understanding of causes of damage in specific locations. 87 In addition, in rural ar eas and in
ar eas wher e buildings and monuments

r emain wet for long periods of time, wet deposition can be a significant or primar y cause of damage. 88, 89
W

eathering due to acid deposition of ten harms cultural assets (e.g., statues and monuments) mor e than pur ely operational

r esour ces (e.g., bridges and buildings). This is because the appearance of cultural

r esour ces, wher e much of their value lies, is particularly vulnerable to damage. Ther e ar e also historic and emotional values attached to cultural assets, which incr ease the value of their pr eser vation. T itle IV has r educed the risk of damage to sensitive buildings and materials by r educing the amount of SO 2 and NO x emitted into the atmospher e and the amount of dr y sulfur deposition r eaching sensitive structur es. Ther efor e, ongoing monetar y costs and cultural losses due to acid gases, particles, and deposition ar e also
e xpected to be declining under T itle IV

.

# Chapter 3: Assessing Acid Deposition: Advances in the State of the Science

NAP AP has a long histor y of conducting research related to acid deposition. Throughout the 1980s NAP AP supported a lar ge number of research projects that confirmed the link between SO 2 

### and NOx 

### emissions and acidic lak es and streams hundreds of miles away

. R ecent research is confirming the tight link between emissions of SO 2 

### and the amount of several different forms of sulfur in the atmosphere and in precipitation, and improving our ability to measure acid deposition. R esearch has continued since 1990, albeit on a smaller scale, toincrease our understanding of how acid deposition affects trees, soils, lak es, 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 responseto acidification; the role of nitrogen in acidification and recover y 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 V isibility

Reductions of S O 2 emissions fr om power generation under T itle IV have r esulted in significant declining tr ends in airborne concentrations of S O 2 and sulfate thr oughout the Eastern U.S. Scientists have determined that the r eduction in S O 2 emissions has led to a comparable decr ease in S O 2 air concentrations, indicating that the r elationship between emissions and air concentrations for S O 2 is close to 1:1. F or e xample, in the Midwest, which

e xperienced some of the highest emission levels and the gr eatest annual change in S O 2 emissions since 1990, t h e appr o ximately 35% r egional r eduction of SO 2 air concentrations corr esponds well to the appr o ximately 35% national

r eduction in SO 2 emissions. Reductions in ambient airborne sulfate concentrations in the Midwest wer e not as linear; ambient sulfate concentrations declined by only 26%. 90 The

r elationship between S O 2 emission r eductions and ambientsulfate concentrations is influenced by a number of chemical and meteor ological factors that contribute to the nonlinear r esponse between emission r eductions and downwind concentrations. 23 Scientists have also r ecently documented that both wet and dr y components of sulfur deposition (and the acidity associated with sulfur deposition) have declined in a near linear fashion with the decline in SO 2 emissions impacting the Eastern U.S. 22–25 Str ong corr elations between lar gescale SO 2 emission r eductions and lar ge r eductions in sulfate concentrations in pr ecipitation have been noted for the Northeast—one of the ar eas most affected by acid deposition. 22 A main r eason for r educed sulfate concentrations in pr ecipitation in the Northeast is a r eduction in the longrange transport of sulfate fr om emission sour ces located in the Ohio River V alley . The r eductions in sulfate documented in the Northeast, particularly acr oss New England and portions of New Y ork State, wer e also affected by SO 2 emission r eductions in eastern Canada. F actors influencing the decline of hydrogen ions (H + ) in precipitation may include climate variability (i.e., wet or dr y summers) or temporal changes in base cations and ammonium concentrations. 22 Concurr ent with these sulfate r eductions wer e similar

reductions in precipitation acidity , e

xpressed as H + concentratio ns. However, acidity may not have decr eased as dramatically as sulfate concentrations due to a simultaneous decline in deposition of acid-neutralizing base cations that act to buffer acidity

. Emissions of NO x ar e transformed in the atmospher e into a series of pollutants, including nitric acid and nitrates. Nitric acid r eacts with ammonia and primar y particles such as sea spray , soil, and soot to form particle phase ammonium nitrate. F r om the perspective of acid deposition, nitric acidis an acidif ying gas and a k ey chemical species for which the dr y deposition monitoring networks now have an e xtensive r ecor d. While data fr om the monitoring networks pr ovide str ong evidence of statistically significant declining tr ends in the concentrations of sulfur species, tr ends in nitr ogen concentrations have not been as pr onounced over the past decade. Scientists ar e confident that the causes of acid rain and the

r esponse of deposition to r eductions in emissions ar e well understood. Most of the r ecent advances in acid deposition

r esear ch and monitoring, ther efor e, ar e r elated to impr oving the quality of data collected by the monitoring networks and developing new statistical methods of analyzing all e x
isting data to better document any envir onmental change. Although several dir ect appr oaches to measuring dr y deposition have been developed and applied over the past decade, infer ential models that calculate dr y deposition fr om atmospheric concentrations and other information have been the only practical way to quantif y seasonal and annual dr y deposition rates of acidic pollutants on the scale needed to operate a nationwide network. 91 CASTNET and AIRMoN-Dr y use this appr oach, which r equir es local measur ements of ambient pollutant concentrations, obser vations of surface conditions, and standar d surface meteor ological conditions. 92 Both these networks use the Multi-layer Model (MLM) to model air pollutant deposition velocity , a k e y component of dr y deposition measur ements. Scientists ar e working to impr ove the MLM and its effectiveness over a range of vegetation types and weather conditions. Resear ch into deposition velocity modeling has led to innovations that will enable the development of a mor e accurate, ne xt-generation-model. 93 The Canadian Air and Pr ecipitation Monitoring Network (CAPMON) and the Eur opean Monitoring and Evaluation P r ogram (EMEP) have been able to “scale up” this appr oach fr om site-specific estimates to br oader landscapes or r egions. The air quality measur ements r equir ed for using these inferential models ar e also being impr oved. Simultaneous sampling using colocated filter packs and duplicate annular denuder systems (ADS), r evealed similar pr ecision estimates for most of the measur ed species between the two appr oaches. 94 Comparisons of weekly ADS r esults and composite weekly filter pack r esults generally show good agr eement for sulfate and total nitrates. However , in some instances, filter pack nitric acid determinations may be biased high when substantial photochemical r eactivity in the atmospher e is documented and filter packs may underestimate SO 2 r esults by 12% to 20%. While perhaps significant, this amount of bias is not e x pected to substantially change the tr end analyses in the Report. Resear chers ar e also working to impr ove methods of measuring actual dr y deposition that can pr ovide higher quality data. Surface wash techniques such as thr oughfall (pr ecipitation collected af ter it has passed thr ough the tr ee canopy) and stemfall (also called stemflow; the pr ocess wher eby pr ecipitation drips down the branches and trunk of a tr ee or other r ooted plant) that measur e deposition quantities and chemical compositions

ar e consider ed mor e r eliable, especially for sulfur in comple x mountainous terrain or locations wher e patchy for est conditions

e xist. 95, 96 Although thr oughfall is not a widely used technique for r outine atmospheric deposition monitoring, it is being evaluated by national dr y deposition networks as an ine xpensive method that pr oduces high quality data r epr esentative of an ar ea beyond a single point for a long period of time. A t a minimum, thr oughfall is a cost-effective way to enhance infer ential modeling r esults with actual field measur ements.
In r ecognition of the need to quantif y total deposition rates acr oss natural landscapes of mix ed vegetation, sometimes in comple x terrain, ther e is need to build upon the developments made in several other pr ograms in EP A and in other agencies. F o r
e xample, the air craf t flux systems developed within the NOAA AIRMoN pr ogram to addr ess this issue offer a new opportunity to determine how surface comple xity influences dr y deposition rates. Mor eover , the bounding techniques developed in the acid rain decade of the 1980s r equir e new attention and verification, possibly using thr oughfall methods as well as micr ometeor ological models. The NADP/NTN, the lar gest wet deposition monitoring network in the U.S., is seeking new technologies to incorporate into their sample collection pr otocols to further impr ove the accuracy of the wet deposition monitoring. Impr oving wet deposition monitoring methods entails advancing sample collection techniques, the methods used for evaluating the data generated and the appr oaches for estimating site-specific and

r egional tr ends.
A lar ge amount of the r esear ch in the wet deposition field has involved developing newways to analyze the monitoring data. T o determine whether emission r eductions have had their intended effect of r educing wet deposition, measur ement data can be statistically modeled to characterize the magnitude and spatial distribution of r eductions in wet deposition. The estimation of long-term tr ends with statistical models is affected by a number of factors such as the magnitude and shape of the underlying tr endto be estimated and the length and variability of the data r ecor d. The advantage, however

, is in deriving estimates of tr end and uncertainty in data that have been adjusted for the sour ces of variability that may also influence the obser ved temporal pattern. 97 

# Aquatic and F o r est Ecosystems
Soils, for ests, surface waters, and aquatic biota (e.g., fish and algae) ar e ine xtricably link ed, sharing water, nutrients, and the other essential building blocks of a successful ecosystem. Ther efor e, to understand the full effects of acid deposition it is necessar y to understand the interactions between all of these systems. F or e xample, sulfur and nitr ogen fr om acid deposition can be r e tained in watershed soils and r eleased as sulfate and nitrate drainage into lak es and str eams even af ter the levels of acid deposition decr ease. The effect of acid deposition on an ecosystem depends lar gely upon the ecosystem’s ability to neutralize the acid. 30 This is

r eferr ed to as an ecosystem’s acid neutralizing capacity (ANC). Acid neutralization occurs when positively char ged ions such as calcium, potassium, sodium, and magnesium, collectively known as base cations, ar e r eleased. As water moves thr ough a watershed, two important chemical pr ocesses act to neutralize acids. The first involves cation e x change in soils, a pr ocess by which hydr ogen ions fr om the acid deposition displace other cations fr om the surface of soil particles, r eleasing these cations to soil and surface water . The second pr ocess is mineral weathering, wher e base cations bound in the mineral structur e of

r ocks ar e r e leased as the minerals gradually br eak down over long time periods. As the base cations ar e
r eleased by weathering, they neutralize acidity and incr easethe 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

r eser ves fr om the soils into drainage waters. The leaching rate of these base cations may accelerate to the point wher e it significantly e x ceeds the r esupply via weathering and atmospheric deposition. 30

F RESHW A

TER ACID -S ENSITIVE E COSYSTEMS Acid deposition causes acidification of sensitive surface waters. These waters ar e 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 lak es and 50% of acidic str eams. 31 Ar eas especially sensitive to acidification include portions of the Northeast (particularly Maine and the Adir ondack and Catskill Mountains) and southeastern str eams. Some high elevation western lak es, particularly in the Rocky Mountains, have become acidic, especially during snowmelt. However , although many western lak es and str eams ar e sensitive to acidification, they ar e not subject to continuously high levels of acid deposition and so have not become chr onically acidified. 31 A significant amount of r esear ch conducted since 1990 has focused on western acid-sensitive ecosystems. ANC, a k ey indicator of the ability of the water and watershed soil to neutralize the acid deposition it r eceives, depends lar gely on the watershed’s physical characteristics: geology , soils, and size. Waters that ar e 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. Lar ger watersheds with well developed riparian zones generally have a gr eater capacity to neutralize the acids being transported fr om their headwaters, but lar ge for ested watersheds have been shown to acidif y during lar ge rainfall and snowmelt episodes. 98 As acidity incr eases, aluminum leached fr om the soil flows into lak es and str eams and can be to xic to aquatic species. The lower pH levels and higher aluminum levels that r esult fr om acidification mak e it difficult for some fish and other aquatic species to sur vive, gr ow , and r epr oduce.In some waters, the number of species of fish able to survive has been dir ectly corr elated to water acidity

. Acidification can also decr ease fish population density and individual fish size. 30
Recent watershed mass balance studies in the Northeast

r eveal that loss of sulfate fr om the watershed e x ceeds atmospheric sulfur deposition. 99 This suggests that thesesoils have become saturated with sulfur , meaning that the supply of sulfur fr om deposition e x ceeds uptak e by biotic and abiotic pr ocesses in the ecosystem. As a r esult, sulfur is gradually being r eleased or leached fr om the watershed into the surface waters as sulfate. Scientists now e xpect that the r elease of sulfate that pr eviously accumulated in watersheds will delay the r ecover y of surface waters in the Northeast that is anticipated in r esponse to the r ecent SO 2 emission contr ols. 99
A r ecent study at a str eam in the Catskill Mountains found that str eam nitrate concentrations wer e positively corr elated to mean annual air temperatur e but not to annual nitr ogen deposition. 33 This r esear ch suggests that, in soils with lar ge amounts of nitr ogen, micr obial pr ocesses (N mineralization and nitrification), that ar e sensitive to changes in temperatur e and moistur e, ar e the primar y factors contr olling nitrate leaching, rather than atmospheric deposition or vegetation uptak e of nitr ogen. Ther efor e, declines in nitr ogen deposition in nitr ogen-saturated soils may not immediately lead to impr ovements in str eamwater chemistr y

. 33 A major study of the ecological r esponse to acidification is taking place in the Bear Br ook watershed in Maine. Established in 1986 as part of the U.S. Envir onmental Pr otection Agency’s W atershed Manipulation P r oject, the pr oject has found that e x perimental additions of sulfur and nitr ogen to the watershed incr eased the concentrations of both sulfate and nitrate in the W est Bear Br ook str eam. Str eamwater concentrations of several other ions, including base cations, aluminum, and ANC changed substantially as well. 100 During the first year of tr eatment, 94% of the nitr ogen added e xperimentally to the Bear Br ook watershed was

r etained, while the r emainder leached into str eams as nitrate. Nitr ogen r etention decr eased to about 82% in subsequent years. 101, 102 Although the for est ecosystem continued to accumulate nitr ogen, nitrate leaching into the str eam continued at elevated levels thr oughout the length of the

e xperiment. This nitrate contributed to both episodic and chr onic acidification of the str eam. This and other similar studies have allowed scientists to quantif y acidification pr ocesses in eastern watersheds in much mor e detail than was possible in 1990. The Appalachian Mountain r egion r eceives some of the highest rates of acid deposition in the United States. 103 The acid-base status of str eamwaters in for ested upland watersheds in the Appalachian Mountains was e xtensively investigated in the early 1990s. 103–106 A mor e r ecent assessment of the southern Appalachian r egion fr om W est Vi r ginia to Alabama identified watersheds that ar e sensitive to acid deposition using geologic bedr ock and the associated buffering capacity of soils to neutralize acid. The assessment found that appr o ximately 59% of all tr out str eam length in the r egion is in ar eas that

ar e highly vulnerable to acidification and that 27% is in ar eas that ar e moderately vulnerable. 107 Another study estimated that 18% of potential br ook tr out str eams in the mid-Appalachian Mountains ar e too acidic for br ook tr out sur vival. 108 P erhaps the most important study of acid-base chemistr y of str eams in the Appalachian r egion in r ecent years has been the V i r ginia T r out Str eam Sensitivity Study

. 105 Tr end analyses of these str eams indicate that few long-term sampling sites ar e r ecovering fr om acidification, most ar e continuing to acidif y , and the continuing acidification is at levels that ar e biologically significant for br ook tr out populations. 39 During the 1980s and 1990s, an integrated study of atmospheric deposition, terr estrial ecosystems, and aquatic ecosystems was conducted in several watersheds in the Sierra Nevada Mountains to determine if acid deposition was affecting these ar eas and to infer the implications of acidification on other surface waters in the r egion. 109–111 Nitr ogen deposition in this r e gion is due to a combination of sour ces, including power plant emissions, vehicle emissions, and emissions fr om agricultural activities. Chr onic acidification of high elevation surface waters in the Sierra Nevada was not found, but episodic changes in str eamwater chemistr y did occur . In many of the watersheds studied, for e xample, pH decr eased (waters became mor e acidic) with incr easing runoff , r eaching a minimum near peak snowmelt. 111 In contrast, an accumulating weight of evidence has led some

r esear chers to conclude that high-altitude watersheds in the Colorado F r ont Range show symptoms of nitr ogen saturation (as opposed to acidification). 43, 44
F urther west, levels of str eamwater and gr oundwater nitrate in the San Gabriel and San Bernar dino Mountains have been found to be str ongly link ed to the magnitude of nitr ogen deposition in watersheds thr oughout the r e gion. Str eamwater nitrate concentrations at Devil’s Canyon in the San Bernar dino Mountains 112 and in chaparral watersheds with high smog e xposur e in the San Gabriel Mountains northeast ofLo s Angeles 113, 114 ar e the highest in North America for for ested watersheds. Chr onic nitr ogen deposition and nitrate e xport fr om these watersheds contribute to the gr oundwater nitrate pr oblems in the eastern San Gabriel Basin wher e levels of ten e x ceed the federal drinking water standar d

. 114

F OREST E COSYSTEMS Curr ent understanding of the effects of acid deposition on for est ecosystems has come to focus incr easingly on the biogeochemical processes that affect plant uptake, retention, and cycling of nutrients within for ested ecosystems. Resear ch r esults fr om the 1990s indicate that documented decr easesin base cations (calcium, magnesium, potassium, and others) fr om soils in the Northeastern and Southeastern United States ar e at least partially attributable to acid deposition. 65, 115 Base cation depletion is a cause for concern because of the r ole these ions play in acid neutralization and, inthe case of calcium, magnesium and potassium, their importance as essential nutrients for tr ee gr owth. It has been known for some time that depletion of base cations fr om the soil interfer es with the uptak e of calcium by

r oots in for est soils. 116 Recent r esear ch indicates it also leads to aluminum mobilization 117 which can have harmful effects on fish. 30
A r ecent study using str ontium isotopes toinvestigate base cation nutrition in acid deposition impacted for ests raises new concerns about the capacity of mineral weathering to r eplenish e x changeable base cations. Unpolluted temperate for ests can become nutritionally decoupled fr om deeper weathering pr ocesses, virtually functioning as atmospherically -fed ecosystems. 118 One gr oup of r esear chers showed that base cation turnover times ar e considerably mor e rapid than pr eviously r ecognized in the plant available pool of soil. 118 These r esults challenge the pr evalent paradigm that plants lar gely feed on r ock-derived cations andhave important implications for understanding sensitivity of for ests to air pollution. The plant physiological pr ocesses affected by r educed calcium availability include cell wall structur e and gr owth, carbohydrate metabolism, stomatal r e gulation, r esistance to plant pathogens, and tolerance of low temperatur es. 52 Soil structur e, macr o and micr o fauna, decomposition rates, and nitr ogen metabolism ar e also important pr ocesses that ar esignificantly influenced by calcium levels in soils. The importance of calcium as an indicator of for est ecosystem function is due to its diverse physiological r oles, coupled with the fact that calcium mobility in plants is ver y limited and can be further r educed by tr ee age, competition, and

r educed soil water supply

. 119
A clear link has now been established in r ed spruce stands between acid deposition, calcium supply , and sensitivity to abiotic str ess. Red spruce uptak e and r etention of calcium is impacted by acid deposition in two main ways: leaching of important stor es of calcium fr om needles; 52 and decr eased r oot uptak e of calcium due to calcium depletion fr om the soil and aluminum mobilization. 115, 120, 121 Acid deposition leaches calcium fr om mesophyll cells of one-year old

r ed spruce needles, 122 which in turn r educes fr eezing tolerance. 52 These changes incr ease the sensitivity of r ed spruce to winter injuries under normal winter conditions in the Northeast, r esult in the loss of needles, and impair the overall health of for est ecosystems. 52 Red spruce must also

e xpend mor e metabolic ener gy to acquir e calcium fr om soils in ar eas with low calcium/aluminum ratios, r esulting in slower tr ee gr owth. 120
L osses of calcium fr om for est soils and for ested watersheds have now been documented as a sensitive early indicator of the soil r esponse to acid deposition for a wide range of for est soils in the Northeastern U.S. 53, 69 Ther e is a str ong r elationship between acid deposition and leaching of base cations fr om soils in har dwood for ests (e.g., maple, oak), as indicated by long-term data on watershed mass balances, 53, 123 plot- and watershed-scale acidification e xperiments in the Adir ondacks 124 and in Maine, 125, 126 and studies of soil solution chemistr y along an acid deposition gradient fr om Minnesota to Ohio. 127 Although sulfate is the primar y cause of base cation leaching, nitrate is a significant contributor in watersheds that

ar e nearly nitr ogen 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 incr eased levels of tr ee mortality due to native defoliating insects. 74 Thechemical composition of leaves and needles may also be alter ed by acid deposition, r esulting in changes in or ganic matter turnover and nutrient cycling. The sensitivity of har dwood soils to acid deposition is lar gely contr olled by inher ent pr operties and land use; unfortunately , tools to assess pr esent conditions or susceptibility to nutrientdepletion ar e not r eadily available or widely applicable. However , r ecent studies have shown that for est har vesting has the potential to gr eatly accelerate calcium losses

curr ently e xperienced due to leaching in low cation soils. 129
The last NAP AP assessment r eported that significant impacts of acid deposition had not been detected over e x
tensive for ested ar eas in the southern pine and pinehar dwood r egion. That understanding has not changed, and ther e is ver y little new information to r eport for this r egion. Acid deposition is contributing significantly to the depletion of base cations in many poorly buffer ed soils wher e southern pines gr ow . However , short-term positive effects on tr ee gr owth in some nitr ogen deficient soils ar e also e xpected due to higher levels of nitr ogen deposition. 29 A significant amount of r esear ch has been conducted since 1990 on the effects of nitr ogen deposition in the L o s Angeles air basin in southern California. Nitr ogen enrichment, in combination with ozone e xposur e, causes major changes in tr ee health by r educing fine r oot biomass and carbon allocation below gr ound and by gr eatly decr easing the life span of pine foliage. 130, 131 Nitr ogen enrichment r esults in gr eater leaf gr owth, while ozone causes pr ematur e leaf loss at the end of the gr owing season. The net r esult of these pollutants is significant litter accumulation on the for est floor

. 132 Nitr ogen cycling rates in soil ar e also stimulated by the high nitr ogen inputs, r esulting in lar ge leachate losses of nitrate fr om these watersheds and elevated flux es of nitric o xide gas fr om soil. 51, 112, 132 In coastal sage ecosystems that occur in the low elevation sites, gr eenhouse and field studies indicate that nitr ogen deposition may be one factor enhancing the invasion of e x otic annual grasses. 133–135 Recent contr olled

e xposur e stud ies demonstrate that nitric acid vapor (HNO 3 ), a component o f photochemical smog, injur es leaves of wildland shrub and tr ee species at near -ambient doses in the L os Angeles air basin. 136
C OAST AL E COSYSTEMS Nitr ogen is pr esent in high enough quantities in many coastal ecosystems to be identified as a pollutant. Ther e
ar e many sour ces of nitr ogen to estuaries and coastal waters, including point dischar ges fr om wastewater tr eatment plants, urban and agricultural runoff , and atmospheric deposition. Since 1990, a lar ge amount of r esear ch has been conducted on the impact of nitr ogen deposition to coastal waters. It is now known that nitr ogen deposition is a significant sour ce of nitr ogen to many estuaries. 137, 138 The amount of nitr ogen entering estuaries due to atmospheric deposition varies widely , depending on the size and location of the estuarine watershed and other sour ces of nitr ogen in the watershed. Ther e ar e a handful of estuaries wher e atmospheric deposition of nitr ogen contributes well over 40% of the total nitr ogen load; however , in most estuaries wher e estimates e xist, the contribution fr om atmospheric deposition ranges fr om 15–30% (see T

able 1). The ar eas with the highest deposition rates str etch fr om Massachusetts to the Chesapeak e Bay , and along the central gulf coast.
Nitr ogen is of ten the limiting nutrient in coastal ecosystems. Incr easing the levels of nitr ogen in coastal waters can cause significant changes to those ecosystems. Appr o x imately 60% of estuaries in the U.S. (65% of the estuarine surface ar ea) suffer fr om over -enrichment of nitr ogen, a condition known as eutr ophication. 83 Symptoms of eutr ophication include changes in the dominant species of plankton (the primar y food sour ce for many kinds of marine life) that can cause algal blooms, low levels of o xygen in the watercolumn, fish and shellfish kills, and cascading population changes up the food chain. In addition, incr eased levels of turbidity in the water due to lar ge amounts of algae can kill off submer ged aquatic vegetation (SA V), which is important habitat for many estuarine fish and shellfish species. Many of the most highly eutr ophic estuaries ar e along the gulf and mid-A tlantic coasts, overlapping many of the ar eas with the highest nitr ogen deposition, but ther e ar e eutr ophic estuaries in ever y r egion of the conterminous U.S. coastline. 

# Materials and Structur es
SO 2 and NO x emissions, as well as many ofthe compounds they form in the atmospher e, ar e damaging certain materials, primarily cultural r esour ces made of stone. Br onze, paint, and other surfaces and surface coatings ar e also susceptible to damage fr om acid pollutants and deposition. Weathering due to acid deposition of ten harms cultural assets (e.g., statues and monuments) mor e than pur ely operational

r esour ces (e.g., bridges and buildings). This is because the appearance of cultural

r esour ces, wher e 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 tostone deterioration indicates that dry deposition, which is mainly influenced by shortrange transport of pollutants from local sources, is the main source of damage. 88 Asecondary 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 inpollutant-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 evenhow 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 dryar eas such as the Southwestern United States. Stone weathering involves interactions between ever changing meteor ological variables—rain, wind, temperatur e, humidity , and continually changing comple x stone surfaces. The way water flows over a car ved stone surface influences the mobilization of salts and particulatematerials on the surface and can change the stone’s chemistr y and por osity . A slight change in the overall shape of the surface, either due to weathering or by design, maychange the way water flows over the surface and can radically influence the er osion and soiling patterns. 29 The por osity of the stone is another critical factor within the deposition pr ocess. Once sulfur dio xide gets into the por es, it is difficult to get out. Cleaning tr eatments have the potential to change the surface te xtur e; ther efor e, some cleaning methods may r esult in incr eased pollutant deposition and subsequent damage to the building stone. Although scientists continue to study stone systems and describe the uptak e of air pollution by stone, it is unlik ely that r esear ch will define dose-r esponse functions in the near futur e. Resear ch to date leads scientists to believe that ther e is no pollution thr eshold below which no damage will occur; rather, any amount of S O 2 will lead to some level of deterioration. In addition, the time it tak es for stone to r espond to pollution is e xtr emely long; effects can appear 25, 50, or even 100 years af ter e xposur e. 29 This understanding has led r ecent r esear ch efforts to focus mor e on tr eatments and mitigation for e xisting damage and methods to pr event damage to stone structur es. However

, pr eliminar y attempts at pr eventing damage and r estoring materials have met with limited success.
Resear ch is pr ogr essing on the development of new tr eatments to str engthen stone, including limestone and marble, that has been degraded by SO 2
. 139, 140 Chemical interactions between SO 2 and stone leads to the formation of calcium sulfate, or gypsum. Because calcium sulfate is

r elatively water soluble, rain can dissolve and r emove material, including the cementitious materials that bind the grains of stone together . The r esult is a weak, “sugaring” stone. Several methods ar e being investigated to str engthen these types of stone. One is ethyl silicate-based consolidants. The main limitation of using these pr oducts is their

r eported cracking behavior during dr ying. A mor e pr omising method under investigation is particle modified consolidants. These compounds consist of a silicate matrix plus colloidal o xide particles that ar e used to consolidate degraded stone. The pr esence of the particles physically limits the silicate network fr om shrinking under capillar y pr essur e, and ther eby r educes str ength loss during dr ying. In addition, because the dried consolidants

r emain por ous, 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 A tmospheric pollutants such as SO 2 , NO x , CO 2 , and chlorides affect various materials including br onze, and cause gr eater
corr osion. The r esear ch e xplor es new coating technologies that would be appropriate for outdoor br onze sculptur es. The coatings currently under investigation include a fluorocopolymer blended with various

acr ylics, conductive polymers, BT A pretreatments, and a ver y fine titanium dio xide. 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 healtheffects of fine particles has changed dramatically since 1990. In 1996, EP A published Air Quality Criteria for P articulate Matter (“Criteria Document”), a compr ehensive r e

view and summar y of the scientific literatur e. 142 At that time, a lar ge body of epidemiological evidence indicated that serious health effects wer e associated with PM at levels even below the then-curr ent air quality standar ds. These health effects include:

• P r ematur e death and incr eased hospital

admissions and emer gency r oom visits, especially for heart and lung diseases, primarily in the elderly and individuals with pr e-e xisting car diopulmonar y disease.

• Incr eased r espirator y symptoms and

disease, primarily in childr en and individuals with car diopulmonar y disease such as asthma.

• Decr eased lung function, particularly in

childr en and individuals with asthma.

• Alterations in lung tissue and structur e and

in r espirator y tract defense mechanisms.The epidemiology studies pr ovided evidence for effects with both acute (e.g., hours or days) and long-term (e.g., months or years) e x posur es to PM. Important uncertainties r emained, however, such as issues r elated to the interpr etation of the

r ole of gaseous co-pollutants in PM associations with health effects and the lack of accepted biological mechanisms that could

e xplain obser ved effects. An unpr ecedented number of new studies containing further evidence of serious health effects have been published since the 1996 Criteria Document, with important new information coming fr om epidemiological, to xicological, contr olled human e xposur e, and dosimetr y studies. F or e xample, important new epidemiological studies includemulti-city studies that use uniform methodologies to investigate the effects of PM on health with data fr om multiple locations with

var ying climate and air pollution mix es. 143 The multi-city studies have incr eased confidence that the associations found with PM

ar e not an artifact of associations with potential confounders such as the gaseous co-pollutants. In addition, a compr ehensive validation and r eanalysis study has used data fr om two cohort studies (the “six cities” and American Cancer Society cohorts); the r esults have generally supported the findings of the earlier studies showing associations between pr ematur e mortality and long-term PM e xposur e. 144Many epidemiology studies have used measur ements of PM 10 or other PM indicators. As stated pr eviously , the full body of scientific studies pr ovided consistent and coher ent evidence that adverse health effects wer e associated with ambient PM. A number of the new studies used measur ed fine particle concentrations (e.g., PM 2.5 ) or other fine particle indicators (e.g., sulfates, black smok e). This gr oup of studies indicated that fine particles ar e lik ely important contributors to the obser ved PM-associatedeffects. A number of new studies have evaluated independent associations between effects and fine- and coarse-fraction particles, and the r esults support the earlier studies’ findings of associations between serious health effects and ambient fine particles. Other or ganizations have r eviewed the health evidence and drawn similar conclusions; for

e xample, Health Canada and the W orld Health Or ganization have r ecently published scientific r eviews, finding that it is lar gely the fine fraction of particulate matter that is involved in e xacerbations of car dior espirator y disease. 145, 146 In addition, new to xicological and contr olled human

e xposur e studies have pr ovided insight into potential mechanisms for PM-r elated effects. These studies include animal and contr olled human e xposur e studies using concentrated ambient particles, new indicators of r esponse (e.g., heart rate variability), as well as animal models r epr esenting sensitive subpopulations. Some potential mechanisms and the available evidence r egar ding those mechanisms have been summarized in r e cent r eview articles. 147, 148 These potential mechanisms include effects of o xidative str ess in the r espirator y tract, effects on the autonomic ner vous system, changes in the blood system to incr ease coagulability of the blood, or lung injur y and inflammation. While evidence is not available to fully support any of these mechanisms, it is lik ely that ther e will be multiple mechanisms underlying the range of PM-r elated health effects.
For additional information on the latest r esear ch r egar ding the health impacts of fine particles, see “ Air Quality Criteria for P articulate Matter ” (October 2004) volumes I & II, which can be found at http://cfpub2.epa.gov/ncea/cfm/ recordisplay .cfm?deid=87903 

# Modeling

Resear chers have used models to help them understand and pr edict atmospheric, envir onmental, and human health

r esponses to acid deposition for well over 20 years. Since 1990, ther e have been significant impr ovements in atmospheric modeling capabilities due to better understanding of the atmospheric chemistr yand physics that play a r ole in transport and deposition of acid compounds, as well as significant impr ovements in computing capabilities. New models have also been developed that allow r esear chers to model sulfur and nitr ogen transport and deposition nationwide (pr eviously used models for NAP

AP cover ed only the Eastern U.S.). W atershed modeling capabilities have also impr oved as r esear chers ar e continuing to r efine and e xpand models that pr oject acidification of waterbodies. A significant amount of r esear ch 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 T itle IV

.A TMOSPHERIC M ODELING Model developments have continued, especially for intendedapplication 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 F or application in Asia, the Sulphur T

ransport Eulerian Model 2 (STEM-II) has been expanded toaddress 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 SO 2 and NO x Emissions The relationships between reductions in SO 2 and NO x 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 F or example, reducing NO x while leaving SO 2 unchanged can lead to an increase in sulfate formation under certain conditions. Reducing NO x emissions could increase the concentrations of the oxidant hydrogen peroxide (H 2 O 2 ). Similarly, reducing SO 2 but not NO x could, under certainconditions, lead to increased nitrate formation. In gasphase/aqueous-phase chemistry, ammonia preferentially neutralizes sulfate over nitrate to form ammonium sulfate aerosols. If SO 2 is
r educed, 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 SO 2 and NO x simultaneously whenever possible to get thefull health and environmental benefits of reduced particle formation.

W

A

TER /W ATER SHED M ODELING The Model of Acidification of Groundwater in Catchments (MAGIC) is a prominent model developed to estimate acidification oflakes 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

S POTLIGHT : A DV ANCES IN A TMOSPHERIC M ODELING Throughout the 1980s, the scientific community dedicated significant resources to developing acid deposition modeling capabilities. The NAPAP 1990 Integrated Assessment primarily

r elied 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 providea 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

r eceptor 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 andacid deposition. The following ar e general descriptions of the most commonly used models in the U.S.

• R egional Acid Deposition Model (R ADM)

.

RADM has been impr oved over the past decade by upgrading various modules to better r epr esent the governing physical and chemical atmospheric pr ocesses. The most r e

cent version is RADM 2.61. The Regional P articulate Model (RPM) is an e xtension of RADM that includes the chemistr y and dynamics of atmospheric aer osol particles. 155 The fullscale version of RADM is r e sour ce-intensive and has typically been applied on super computers. While the model is primarily designed for episodic applications, it has also been used to understand long-term

r elationships between changing emissions patterns and pollutant concentrations and deposition using an episode aggr egation technique. RADM and RPM have been applied primarily to eastern North America; An international version of RADM, EURAD, has been applied in Eur ope. 155, 156 RADM has also been used to identif y emission sour ce-r egions affecting the Chesapeak e Bay watershed. 157

• R egulator y Emissions Modeling System for

Aer osols and Deposition (REMSAD)

. REMSAD was developed by ICF Consulting for the EP A. It is based on an Eulerian (grid) appr oach and may be applied at scales ranging fr om a single metr opolitan r egion to a continent containing multiple urban ar eas. It was designed to be capable of simulating the comple x long-range transport and deposition of atmospheric pollutants to aquatic envir onments and to assess the r e lative impacts of alternative contr ol strategies.Although initially developed to study the transport and r emoval of airborne to xics, the interdependence of the pr ocesses that also contr ol the formation and r emoval of particles was

r ecognized, and ther efor e the model was designed for both to xics and particulate matter applications. Annual simulations of the entir e U.S. ar e practical. • Community Multi-scale Air Quality (CMA Q)

. Models 3-CMAQ was developed by EP A to evaluate the impact of multiple pollutants at multiple temporal and spatial scales. The Models-3 framework pr ovides tools to pr epar e emissions and meteor ological inputs, define emissions contr ol strategies, pr oject futur e emissions inventories, e x ecute meteor ological models, delineate a geographic domain, select alternative atmospheric chemical r eaction mechanisms, set vertical and horizontal grid

r esolutions, and manage a series of air quality model runs. CMAQ is applied thr ough the Models-3 system. It contains state-of -thesci ence simulations of atmospheric transport pr ocesses, atmospheric chemistr y , aer osol dy namics and chemistr y , cloud chemistr y and dy namics, and deposition pr ocesses. A k e yaspect of the Model-3/CMAQ system’s structure is its flexibility to incorporate scientific a n dmodeling advances, to test alternative modeling appr oaches, and to link with human and ecosystem e xposur e models. In addition to combining point and ar ea sour ce emissions into a single thr ee-dimensional hourly file as in RADM, CMAQ pr ovides the option of tr eating lar ge point sour ces separately using a plumein-grid appr oach that has a gridded structur e that e xpands as the plume moves downwind.

• Acid Deposition and Oxidant Model

(ADOM) . The ADOM is a compr ehensive thr ee-dimensional Eulerian episodic model developed in the 1980s in parallel with RADM. RADM and ADOM ar e the two compr ehensive models that wer e used for the NAP AP 1990 Integrated Assessment . The development of ADOM was sponsor ed by the Ontario Ministr y of the Envir onment, the A tmospheric Envir onment Ser vice of Envir onment Canada, the German Umweltbundesamt, and the Electric P ower Resear ch Institute. ADOM isprimarily intended for episodic applications because of its computational r esour ce
r equir ements. An episode aggr egation technique, however , based on the appr oach developed for RADM during NAP AP , has been developed for pr edicting changes in annual deposition r esulting fr om changes inemissions. 158 Like RADM, ADOM has been primarily applied to eastern North America. The

r educed 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

e xisting and new modules as appropriate.

• Advanced Statistical Trajectory Regional Air

P ollution (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 F ramework (TAF) for the 1998 NAPAP report. 29 The treatments of chemistry and dry and wet

r emoval in ASTRAP are relatively simple compared to the Eulerian models described in thissection. 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 SO 2 /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 period 159–161 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

r eceive initial guidance and training. URM was

r ecently 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, PM 2.5 and PM 10 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 equilibriabetween 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), usedstatistically 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, withinthe limitations of the NSWS survey (primarily the limitation that it only sampled lakes larger than 4 hectares), the MAGIC modelcan estimate the level of acidification in waterbodies throughout the entire Adirondack, northeastern, and southeastern Appalachian

r egions of the U.S. Although there are someuncertainties 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.P nET-BGC is an integrated biogeochemical model that simulates the concentrations and transport of major elements, including nitr ogen, in for est vegetation, soil, and water . The model enables simultaneous simulation of major element cycles in for est and inter connected aquatic ecosystems. 162 P nET-BGC

uses measur ed and estimated data on meteor ology and atmospheric deposition to simulate estimates of changes in atmospheric deposition fr om 1850 to the pr esent day

. F utur e scenarios of changes in atmospheric deposition ar e simulated using pr ojections pr ovided fr om air quality modeling based on model runs of air emission contr ol scenarios. P nET-BGC has primarily been applied to forest watersheds in the Northeastern U.S.

H EAL TH E FFECTS M ODELING When the Clean Air Act Amendments wer e passed in 1990, the health effects of fine particles wer e just beginning to be understood. In the years since, a series of studies have demonstrated the r e lationship between e xposur e to fine particles and a series of health impacts, including pr ematur e mortality and asthma attacks. 163 As a r esult of this new understanding, NAP AP now includes the estimated health benefits of SO 2 and NO x emission r eductions and the monetar y impact of those benefits as part of its analysis of the impacts of T itle IV

. EP A developed a new methodology for quantif ying and monetizing the health benefits of impr oved air quality as part of EP A

’s Report to Congr ess in 1999 entitled The Benefits and Costs of the Clean Air Act 1990–2010. 164 Each phase of the Report’s pr eparation, which spanned nearly thr ee years, was thor oughly r eviewed by academic and industr y e xperts ser ving on EP A

’s Science Advisor y Boar d Council on Clean Air Compliance Analysis. Curr ently , health benefits ar e calculated using the same concentration-r esponse functions and economic valuation functions that EPA uses in analysis of major rules that addr ess fine particles and ozone. The air quality modeling data ser ve as inputs to both the Benefits Model for Air Quality (BenMod) and the Benefits Mapping and Analysis P r ogram (BenMAP), two differ ent modeling systems that translate air quality changes to changes in health outcomes (e.g., pr ematur e mortality

, emer gency r oom visits, cases of chr onic br onchitis, asthma attacks) thr ough the use of concentration-r esponse functions. P eer -r eviewed, published scientific literatur e on the health effects of air pollutants pr ovides the sour ce of these concentration-r esponse functions. Health benefits ar e
r elated to the change in air pollutant e xposur e e xperienced by individuals. Because the e xpected changes in pollutant concentrations var y fr om location to location, individuals in differ ent parts of the countr y may not e xperience the same level of health benefits. Benefits ar e apportioned among individuals by matching the change in air pollutant concentration change with the size of the population that e xperiences that change. Using the e xpected population e xposur e, the concentration-r esponse functions ar e applied to derive estimates of the number of incidences of the health outcomes that ar e associated with change in e xposur e to ambient particulate matter and ozone.

# Chapter 4: Expectations for F ull Implementation of T itle IV in 2010

By 2010, emissions of SO 2 

### are projected to be appro ximately 9.86 million tons, less than a million tons above the cap of 8.95 million tons, and appro ximately three million allowances are projected to be in the allowance bank. Emissions are e xpected 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 e xpected to continue to improve as emissions approach the level of the cap. Some lak es and streams, particularly in the Northeast, are e x

pected to begin showing signs of recover y from acidification; however , recover y is e xpected 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 indi cates that the benefits are substantial.

This analysis of full implementation of T itle IV in 2010 includes emissions modeling, air quality modeling, and fr eshwater quality modeling. It also includes an estimate of the health benefits of T itle IV r eported in the literatur e. Estimates of emissions, air quality , and acid deposition

wer e pr ojected for 2010 and compar ed to averaged monitor ed data fr om 1989–1991, befor e T itle IV was passed. Although the SO 2 allowance bank will still be in use in 2010, analysts have used that date, twenty years af ter T itle IV was passed and ten years af ter Phase II began, as the mark er for the time when emissions ar e close to the level of the 2010 cap and most of the emission r eductions

ar e e xpected to have tak en place. The fr eshwater quality modeling pr ojects conditions in 2030 and compar es them to conditions in 1984 or 1985. The health benefits estimate r e

lies on estimates of air quality changes and the annual benefits per ton of r educing emissions of SO 2
. Emissions modeling fr om power plants was conducted using the 2003 version of the Integrated Planning Model (IPM). IPM is an economic model that pr edicts emissions fr om the power sector due to changes in envir onmental r e gulations and the costs of various emission contr ol 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 T itle IV and the NO x SIP call and several state

regulations that were finalized before April 2003. (See te xt bo x “What W ill Air Quality be Lik e in 2010?” on page 58.) They do not include any co-benefits that might be achieved from implementation of mercur y 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 BAR T Rule, any addi tional actions taken by Congress, o r a n y further reductions from sources in Canada. Mobile source rules will also result in N O x re

ductions as they are implemented. The 2010 emissions projections were compared to 1990 emissions data collected by the EP A. The air quality and deposition modeling analyses wer e conducted using REMSAD (see page 54 for a description of REMSAD). The scenario used was a 2010 T itle IV implementation scenario that includes the SO 2 and NO x emissions in 2010 with T itle IV as pr ojected by IPM (the “with T itle IV” run) The scenario also includes NO x emission r e

ductions fr om power plants and industrial boilers due to the NO x SIP call and finalized state emission r eduction pr ograms. Stationar y and ar ea sour ces wer e modeled in the scenario as pr ojected emissions in 2010 as estimated based on the 1996 National Emissions Inventor y (NEI). The scenario does not r eflect futur e r eductions fr om mobile sour ce pr ograms. The r esults of the 20 10 T itle IV implementation scenario wer e scaled to monitoring data by comparing a 2001 baseline modeled scenario to 1999–2001 monitoring data. This allows a comparison of 2010 modeled pr ojections of full implementation of T itle IV with monitoring data fr om befor e the pr ogram took effect (1989–1991).The magnitude of emission reductions affects both the amount of recover y that takes place in a lake or stream and the rate at which that

recover y occurs. 30 Lake conditions in the Adirondacks and Northeast and stream conditions in the Southeast were modeled using the MA GIC model (see page 53 for a description of MAGIC). These r egions were chosen because they ar e among the most acid- sensitive ecosystems and are downwind of many o f t h e emission sources affected by the Acid Rain Program. SO 2 emissions under T itle 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). P rojected 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 ver y small portion of the recover y expected due to T itle IV , lake and stream conditions for 2030 are considered a more appropriate representation of the environmental benefits of T itle IV

. 

# Emissions

As a r esult of T itle IV , emissions of SO 2 ar e e xpected 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 e xpected that on Januar y 1, 2010, the bank of unused allowances will still contain appr o ximately 3 million allowances available for use. The SO 2 bank was cr eated when affected sour ces over complied with the emission r eduction r equir ements during the first years of the pr ogram. During Phase I, emissions wer e 25–40% below allowable levels, r e sulting in lar ger human health and envir onmental benefits during those years. NO x emissions fr om power plants ar e e xpected to be 3.94 million tons in 2010, appr o ximately 500,000 tons below wher e they ar e now . This 40% r eduction fr om the 1990 level of 6.7 million tons is due to r eductions under both T itle IV and the NO x SIP call. Note that implementation of CAIR is

e xpected to r educe emissions of SO 2 and NOx to 6.1 million tons and 2.4 million tons, r espectively , in 2010. 

# Acid Deposition

Air concentrations r espond ver y quickly , in a matter of hours to days, to changes in SO 2 and NO x emissions. Ther efor e, while NAP AP e xpects some small additional emission r eductions to tak e place af ter 2010, air quality in 2010 r eflects the vast majority of the impr ovement in air quality that is e xpected to occur due to T itle IV . The data pr esented in Chapter 2 on the status and tr ends of air concentrations of SO 2 indicate that T itle IV has substantially

r educed concentrations of SO 2 and sulfate. This r eduction in air concentrations has occurr ed in many heavily populated ar eas wher e concentrations of SO 2 and sulfate in the air

cause r espirator y , car diac, and other health pr oblems. This impr ovement in air quality , ther efor e, is e xpected to r esult in impr ovements in human health, as discussed below

. Acid deposition also r esponds quickly to r eductions in emissions of SO 2 and NO x . Ther efor e, as is the case with air quality , NAP AP believes that the acid deposition rates in 2010 reflect the vast majority of the reductions in deposition

e xpected due to T itle IV . Most of these r eductions in aciddeposition have alr eady tak en effect, but some additional

r eductions in both sulfur and nitr ogen deposition ar e
e xpected by 2010.
The lar gest r eductions in sulfur deposition ar e e xpected in the Ohio River V a lley and in downwind ar eas, including the acid-sensitive r egions of the Northeast and mid-A tlantic (see F igur e 30). Some additional r eductions in deposition in the Midwest and Northeast beyond what has occurr ed by 2000–2002 ar e e xpected as emissions continue to dr op to the level of the cap.
W

et sulfate deposition is e xpected to be significantly lower in 2010 than it was in 1989–1991. This r eduction in sulfate deposition has alr eady led to impr ovements in the health of some acid-sensitive lak es and str eams, particularly those in the Adir ondacks, northern Appalachian Mountains, and the upper Midwest, as discussed in Chapter 2.
W

et inor ganic 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 lar ge r e

ductions that occurr ed between 1989–1991 and 2000–2002 (see F igure 31). This is primarily due to implementation of the NO x SIP call, which r educes emissions of NO x from power plants, industrial boilers, and other sources in the Eastern U.S. 

# Acidification of F r eshwater Lakes and Str eams
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. F or example, some acidified lakes in the

WHY A RE THE ACIDIFIC A

TION N UMBERS D IFFERENT ? The e xtent of acidification as measur ed by the TIME/L TM monitoring sites in the Adir ondacks is differ ent than that r eported in the modeling assessment r eported in this analysis. This difference is due to differing types of Adir ondack lak es included in each assessment. Since the original acid rain sur veys wer e conducted in the 1980s, r esear chers have continued to r efine their understanding of how to best measur e changes in acidification due to anthr opogenic causes. Resear chers curr ently model a differ ent subset of lak es and str eams than
wer e modeled in the 1980s and early 1990s. This change in “model population” means that
curr ent modeled pr edictions ar e somewhat differ ent fr om the monitoring data r eported in the 1990 NAP AP Integrated Assessment or the 2003 EP A Response of Surface W ater Chemistr y to the CAAA r eport. The original NAPA P assessment, and the subsequent EP A

r eport, ar e based on data collected during the National Surface W ater Sur vey (NSWS). These waterbodies, known as NSWS lak es and str eams, wer e selected to be statistically r epr esentative of the r egion being sampled. The purpose of the NSWS was to assess lak es and str eams that wer e acidic due to anthr opogenic activities. In the Adir ondacks, lak es smaller than 4 hectar es (10 acr es) and gr eater than 2,000 hectar es (5,000 acr es) wer e not sampled because it was thought they wer e mor e str ongly affected by or ganic acidity

. The NSWS lak es and str eams wer e further classified and subdivided based on several characteristics. A pr oject to assess the rate of

r esponse of waterbodies to acidification (the Dir ect/Delayed Response P r oject, or DDRP), used a subset of NSWS lak es that had lower ANC values (<400 ueq/L), had depths gr eater than 1.5 m deep, wer e smaller than 3,000 ha, and wer e not anthr opogenically disturbed. It was determined that these lak e characteristics wer e necessar y to adequately assess the rate of change in acidification in sensitive northeastern lak es. The DDRP pr oject also assessed str eams in the mid-Appalachian and southern Blue Ridge

r egions; these waterbodies ar e also subsets of the lar ger NSWS set. The purpose of the MA GIC model is to calculate the rate and extent of recover y from acidification, so “DDRP waterbodies” are used to calibrate the model. The MA GIC estimate of acidification pr esented in this NAP AP Report for the Adir ondacks, Northeast, and Southeast

r epr esents a r easonable scientific estimate of the curr ent r egional e xtent of acidification in acid-sensitive lak es and str eams in the Eastern U.S. and the projected future conditions of those waterbodies on a regioinal scale.Adir ondacks and str eams in V i r ginia no longer support populations of fish. 30, 165 Unlik e the r esponse of air quality and deposition to changes in emissions, lak es and str eams tak e years to decades to fully r eflect r eductions in acid deposition. In some cases soil chemistr y has been significantly alter ed, and ions must either build up (i.e. base cations) or be leached out ( i.e. sulfate and/or nitrate) befor e the chemistr y can r eturn to its pr e-acidification status. In addition, algae, zooplankton, and fish communities tak e time to r ebuild their food webs, or , in some cases, to migrate back into ar eas wher e they havebecome completely absent due to acidification. Ther efor e, lak e and str eam conditions in 2010 reflect only a small part of the recover y that is expected due to T itle IV . The full impacts of T i tle IV are not expected to be seen in the Northeastern U.S., including in the Adirondacks, for several decades af ter emission reductions begin. 30 Streams in the Southeast, because of their soil chemistr y, are expected to take even longer

. 103 Lak e and str eam quality in 2010 would r epr esent only a small portion of the

r ecover y e xpected due to T itle IV . Ther efor e, lak e and str eam conditions ar e pr esented for 2030 and r eflect emission r eductions that occur ed between 1990 and 2010. Modeling conducted with the MA GIC model indicates that, by 2030, the per centage of chr onically acidic lak es in the Adir ondacks is e xpected to be 12%, down fr om 33% in 1984 (see F igur e 32). The per centage of non-acidic lak es is e xpected to r emain con stant at 36%. In the Northeast, the per centage of chr onically acidic lak es is e xpected to be 6%, down fr om 13% in 1984. The per centage of nonacidic lak es is e x pected to be 69%, up fr om 67% in 1984. The per centage of episodically acidic lak es in both the Adir ondacks and the Northeast as a whole actually gr owsin this stage of r ecover y

. This is because mor e chr onically acidic lak es become episodically acidic than episodically acidic lak es become non-acidic (see F igur e 33). While chr onically acidic lak es ar e most affected by acidification, episodically acidic lak es ar e also ecosystems that ar e significantly impair ed by acid deposition and biological endpoints (i.e. sensitive fish species) r emain susceptible. In the Southeast, because of the type of soils in the r egion, emission r eductions ar e e xpected only to slow the rate of acidification in acid-sensitive str eams. In 2030, the per centage of chr onically acidic str eams in the Southeast is

e xpected to be 17%, up fr om 8% in 1985 (the southeastern str eams wer e assessed the year af ter the northeastern lak es) (see F igur e 32). The per centage of non-acidic str eams is e xpected to be 56%, down fr om 73% in 1985. This is not because T itle IV has had no effect in the Southeast; on the contrar y , as indicated in F igur e 32, ther e would be many mor e c h r onically acidic str eams in 2030 if T itle IV wer e not being implemented. The unglaciated (of ten sand or clay) soils in the Southeast have absorbed a lar ge amount of acid deposition alr eady and, because of their composition, ar e e xpected to continue r eleasing it into waters in the r egion for many decades. This lag time, which is much longer than the lag time seen in northern glaciated (of ten granite) soils, means that the str eams ar e still showing the effects of historic emissions and will not show the full effects of emission r eductions for many decades. 103 

# Human Health

SO 2 and NO x emissions r eact in the atmospher e to form fine particles and ozone. These gases and fine particles ar e associated with a number of significant health effects in sensitive populations. While high SO 2 concentrations can r esult in temporar y br eathing impairments in sensitive people, including asthmatics and those who ar e active outdoors, most of the health effects

ar e due to fine particles.
A lar ge number of epidemiological studiesover the past 10 to 20 years show an association between ambient fine particle

concen trations (e.g. sulfates and nitrates formed in the atmospher e fr om SO 2 and NO x emissions) and health effects, such as incr eased numbers of hospital admissions and emergency room visits for heart and lung disease, increased incidences of respirator ydisease 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 T itle IV has reduced the amount of fine particles in the air (see F igures 16–20) by lowering S O 2 and NO x emissions, achieving significanthu man health benefits nationwide long before emissions drop to the level of the cap. It is ex
pected that T itle IV will achieve further benefits as S O 2 emissions continue to decrease to the level of the emissions cap.
NO x emissions r eact with volatile or ganic compound gases in the atmospher e in the pr esence of sunlight to form ozone. The scientific literatur e shows associations between ozone and a number of effects on the r espirator y system, including aggravation of asthma, incr eased susceptibility to

r espirator y illnesses lik e pneumonia and br onchitis, and permanent lung damage. Childr en, the elderly , and people with e xisting r espirator y pr oblems ar e most vulnerable to the health effects of ozone. Those e x ercising or working outside during the ozone season (May–September) tend to have the highest e xposur es to ozone. Health benefits have been achieved under T itle IV due to NO x r eductions not only because NO x contributes to the formation of fine particles but also because r educing NO x emissions
r educes ozone concentrations. The value of T itle IV’s health benefits has been estimated by several or ganizations and analysts since its passage in 1990. 166–171 In each case, these analyses found that the quantified health benefits of T i tle IV ar e
ver y substantial. F or e xample, in their most

r ecent analysis, OMB estimates that the annual health benefits of T itle IV in 2010 will be $79 billion to $83 billion ($2001). T itle IV’s S O 2 r eductions account for $78 to $79 billion of this total, and T itle IV’s Phase II NO x r eductions account for an additional $1 to $5 billion. 171 These benefits include only the benefits of avoiding pr ematur e mortality due to air pollution. T itle IV has pr ovided additional benefits as well, that ar e not included in this most r ecent monetized estimate fr om OMB, including r eductions in the number of non-fatal heart attacks, cases of acute and chr onic br onchitis, emer gency r oom visits, asthma attacks, and days lost fr om work and school. As with other benefits analyses, the benefits estimates ar e subject to uncertainties such as those associated with estimates of futur e-year emissions and air quality and uncertainties in the estimated

r elationships between changes in pollutant concentrations and r esulting changes in health effects. 

# Other Benefits of T itle IV

In addition to the r educed acidification and human health benefits of T itle IV , ther e ar e other, un-quantified, benefits of full implementation of T itle IV . These benefits fr om
r eductions in sulfur deposition and sulfate and nitrate concentrations include healthier for ests, less er osion of buildings, monuments, and other historic structur es, and impr oved visibility . These benefits occur thr oughout the Eastern U.S. in national parks and wilderness ar eas, cities and urban ar eas, and in rural ar eas. The r eductions in nitr ogen deposition, while not e xtensive, ar e e xpected to benefit nitr ogen-sensitive coastal waters along the eastern and gulf coasts as well. F or mor e information on the envir onment a l effects of acid deposition see Chapters 2 and 3. Acidification of lak es has been link ed in some ar eas to higher levels of mer cur y contamination in fish;

r eductions in the acidity of these lak es may help r educe levels of mer cur y contamination in fish. 172, 173 F inally , these envir onmental benefits can r esult in economic benefits by helping to pr eser ve natural r esour ces critical to industries such as for estr y and fishing and incr easing the amount of tourism, as well as impr oving the overall quality of life in

ar eas wher e ecosystems have suffer ed fr om acidification.

# Chapter 5: Beyond T itle IV : Ecological Impacts of F urther Emission Reductions

A broad consensus of scientists studying acid deposition and ecosystem recover y have published reports since publication of the last NAP AP report (1996) indicating that further emission reductions beyond those achieved by Title IV are necessar y to allow sensitive forests and aquatic ecosystems to recover from acidification. The SO 2 

### and NOx 

### emission reductions achievedunder Title IV from the power sector are now recognized as insufficient to achieve recover y or to prevent further acidification in some regions. Further reductions in SO 2 

### 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 lak es and streams; however, even elimination of SO 2 

### emissions from power plants will not fully protect all acid-sensitive ecosystems.

T itle IX of the 1990 Clean Air Act Amendments r equir es NAP AP to r eport quadr ennially on “the r eduction in deposition rates that must be achieved in or der to pr event adverse ecological effects” (P ublic Law 101-549-Nov. 15, 1990). Deposition levels that corr elate with a “thr eshold” of adversity are scientifically complex and difficult to establish. NAP AP’s 1990 State of Science and T echnology R eport No. 13 states that “biological r esponses to changes in acid-base chemistr y occur along a continuum. Ther e is not a single value or set of chemical concentrations that r epr esents a ‘thr eshold’ for ‘significant adverse biological effects’” 172 In addition, some lak es and str eams ar e naturally acidic and

e xperience “adverse ecological effects” in the form of

r educed fish populations or other impacts due to the pr esence of or ganic acids. Ther efor e, NAP AP determined that the logical path was to describe the ecosystem r esponses along a continuum, ther eby allowing decision mak ers to determine the level of acceptable risk. 174
NAP AP pr esented a working definition of “adverse ecological effects” in the 1996 NAP AP Report to Congr ess based on the intent of Congr ess, as e xpr essed in the 1990 Clean Air Act Amendments and shaped by other r elevant envir onmental statutes (Compr ehensive Envir onmental Response, Compensation and Liability Act and the Clean W ater Act) and associated r egulations. The definition is:


### A dverse 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 pr ocesses or ecosystem components include r eductions in acid neutralizing capacity and incr easing aluminum concentrations in a lak e or str eam, leading to loss of fish and other biota; loss of important nutrients such as calcium, fr om for est soils; and, incr eased susceptibility of tr ees to pests, disease, and winter temperatur es, leading to decr easing for est pr oductivity and for est dieback. Adverse ecological impacts also include the effects of nitr ogen saturation in for ests, coastal eutr ophication as a r esult of atmospheric deposition, and injur y to plants as a

r esult of ozone e xposur e. Other ar eas addr essed by NAPAP (i.e., materials, visibility , and human health) would follow the same definition, but ar e not consider ed her e as ecological effects. 29 This NAP AP Report is based on that definition of adverse ecological effects and uses the same appr oach of investigating ecosystem

re sponses along a continuum. The r eport focuses on “r ecover y” as indicated by changes in water chemistr y (i.e. chr onically acidic, episodically acidic, and non-acidic lak es and str eams).
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 F easibility Study concluded that, for the near term, sulfur deposition was likely to

remain the primar y source of acidification in most sensitive areas of eastern North America. However , as SO 2 emissions decrease, the importance of nitr ogen increases in relation to both short-term and longterm acidification. The Nitrogen Bounding Study (NBS), the primar y component of theAcid Deposition Standard F easibility Study

, demonstrated that, at certain times and under certain deposition scenarios, sulfur and nitrogen could have appro ximately 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, EP A

’s re port concluded that further reductions of 40–50% beyond T i tle IV SO 2 and NO x emissions may be necessar y to fully pr otect the most sensitive lakes and streams. 175
T

wo national e xpert workshops or ganized by the Ecological Society of America r eached similar conclusions r egar ding the effects of nitr ogen and sulfur deposition. A June 1997 workshop on atmospheric nitr ogen deposition to coastal watersheds found that incr easing supplies of nitr ogen have r esulted in a range of ecological impacts, including an incr eased incidence of harmful algae blooms and fish kills, r educing the number of acr es
cover ed with submer ged aquatic vegetation (SAV), and changes to the fundamental ecological structur e of some coastal waters. 176 A Mar ch 1999 workshop of acid rain e xperts focused on ecological r esponse and

r ecover y fr om sulfur and nitr ogen emissions and deposition. The workshop r eport highlighted the importance of nitr ogen deposition as a cause of acidification and found that ecological r ecover y has yet to occur in many r egions. The workshop concluded that further SO 2 emission r eductions beyond T itle IV may be necessar y to achieve ecological r ecover y in lak es and str eams affected by acid deposition. 177 In the period since the 1996 NAP AP r e port
r elease, various studies focused on specific

r egions of the countr y have shed new light on the r egional impacts of sulfur and nitr ogen deposition. These mor e r ecent analyses consider ed r eductions gr eater than those suggested in EP A

’s 1995 r eport. F o r
e xample, a r egional sur vey of str eams in the southern Appalachian r egion found that 65% of sampled locations demonstrate moderate to sever edepletion of fish communities, some portion of which is attributable to acidification. 107 Studies of tr out str eams in the Vi r ginia part of that r egion document the first loss of a fish population in the Shenandoah National P ark due to acidification. 46 Resear chers in the southern Appalachian r egion conclude that str eams in this r egion ar e still thr eatened by acid deposition. Based on model pr ojections they conclude that further r eductions of sulfate deposition beyond levels achieved by the T i tle IV SO 2 emission r eductions ar e necessar y to pr event further acidification of V i r g inia br ook tr out str eams. 46 A r ecent r egional assessment completed by theSouthern Appalachian Mountain Initiative (SAMI) concluded that local and r egional power generation sour ces must
r educe SO 2 emissions by at least 70% and NO x emissions by at least 67% fr om 2000 levels in or der to decr ease str eam acidification and r educe for est ecosystem damage due to nitr ogen deposition in high elevation for ests. 178
T

wo major assessments of acid deposition and its effects in the Northeast r eached similar conclusions. Driscoll et al. (2001) found that full implementation of the 1990 CAAA will not r esult in substantial r ecover y in northeastern lak es and str eams impacted by acid deposition. Based on model pr ojections, the study concluded that r eductions of SO 2 emissions fr om power generation of up to 80% beyond T itle IV r equir ements wer e necessar y for curr ently -acidic str eams in the Northeast to become non-acidic. 30 Driscoll et al. 2003 consider ed the impact of nitr ogen deposition on for ests, lak es, and str eams in the Northeast. The study found that r eduction in NO x emissions fr om power generation of 75%, combined with mobile sour ce r eductions, ar e
necessar y to arr est acidification of for est soils, lak es, and str eams in that r egion. 38
F inally , additional r ecent studies demonstrate symptoms of nitr ogen saturation in many ecosystems acr oss the countr y , 51 including several ar eas in the W estern U.S. 133 Fo r
e xample, high-altitude watersheds of the Colorado F r ont Range e xhibit symptoms of advanced stages of nitr ogen e x
cess. 44 A 1995 sur vey of 91 high-elevation lak es in the central Rockies found changes in water quality caused by incr easing amounts of inor ganic nitr ogen deposition, and intensive sampling at a smaller number of sites demonstrated that incr easing wet nitr ogen deposition is causing episodic acidification in headwater catchments of the Gr een Lak es V

alley in the Colorado F r ont Range. 42 These r esults ar e significant since incr eases in atmospheric nitr ogen deposition can lead to ecosystem changes in this r egion, 40 including lak e eutr ophication and alter ed ecosystem composition in both lak e and alpine meadow envir onments. 133 While gr eater uncertainty e xists, nitr ogen deposition in the W est also has been link ed to enhanced gr owth of invasive species, delete­rious impacts on thr eatened and endanger ed species, and alterations of the fir e cycle. 133
An additional e xamination of the literatur e surr ounding the effects of atmospheric deposition is included in the Regulator y Impact Analysis in support of the 2005 Clean Air Interstate Rule (CAIR). This analysis can be found on the internet at www .epa.gov/interstateair quality/pdfs/ finaltech08.pdf
In r esponse to the Congr essional r equest to identif y the deposition rates that would pr event any adverse ecological ef fects, NAP AP has analyzed several emission

r eduction scenarios that br oadly bound the range of r eductions pr esented in the post-1996 literatur e cited above. These r e sults pr ovide an indication of the envir onmental impr ovements that would be e xpected fr om additional emission r eductions fr om both T itle IV and non-T itle IV

sour ces. These envir onmental impr ovements do not necessarily constitute r ecover y of acid-sensitive for ests, lak es, and/or str eams that have been impair ed by acid deposition (see F igur e 1 for the location of acid-sensitive ar eas). They do, however , pr ovide an indica tion 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 fr om 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 Envir onmental Impact of F urther Air Emission Reductions

S CENARIOS A NAL YZED This modeling, conducted by EPA, analyzed several scenarios r epr esenting emission changes fr om stationar y sour ces. The r esults ar e pr esented for sulfur and nitr ogen deposition because these pollutants ar e the primar y components of acid deposition. This NAP AP analysis focuses on r eductions fr om power plants; other sour ces also emit pollutants that contribute to acid deposition. Notably , sour ces outside the power generation sector ar e pr ojected to emit appr o ximately 45% of the SO 2 and 75% of the NO x emitted in 2020 under the Base Case scenario. 179
Thr ee sensitivity scenarios estimating various levels of additional r eductions ar e compar ed to a Base Case pr ojecting emissions with full implementation of alr eady -pr omulgated Clean Air Act pr ograms in 2000 (T itle IV , the NO x SIP call, and T ier II and Heavy Duty Diesel SO 2 and NO x r eductions that ar e pr ojected to tak e effect by 2010). The scenarios

wer e modeled using a cap and trade mechanism for both SO 2 and NO x (r educing the SO 2 cap under T itle IV and imposing an annual cap for NO x ). The r eductions identified below for each scenario r epr esent cap levels for each pollutant in 2020. The pr ojected emissions used for the air quality modeling in 2020 ar e somewhat higher than the cap levels for all scenarios as a result of the early reductions and allowance banking pr edicted by the emissions model (IPM). Additional details on the scenarios ar e pr esented below

.
• Base Case (2020): This scenario does not

include rules that wer e finalized af ter the spring of 2001 (including CAIR, CAMR, and BAR T), new or anticipated actions under the Clean Air Act, including the NonRoad Diesel Rule, or other emission

r eductions that would be necessar y to attain and maintain the fine particle and ozone NAAQS for which states ar e
r equir ed to submit implementation plans by 2007, or to achieve r egional haze r eduction goals.

• Scenario A: This scenario includes an additional 60% r eduction in SO 2 emissions fr om the power generation sector beyond the cap of 8.95 million tons r equir ed under T itle IV (a cap of 3.58 million tons/year ofemissions in 2020). It also includes national annual NO x emission r eductions fr om the power generation sector of 68% beyond T itle IV levels (a cap of 1.87 millionton/year of emissions in 2020, or an emission rate of 0.15 lbs/mm BTU).

• Scenario B: This scenario includes an additional 75% r eduction in SO 2 emissions fr om the power generation sector beyond the cap of 8.95 million tons r equir ed under T itle IV (a cap of 2.25 million tons/year ofemissions in 2020). It also includes national annual NO x emission r eductions fr om the power generation sector of 80% beyond T itle IV levels (a cap of 1.25 millionton/year of emissions in 2020, or an emission rate of 0.10 lbs/mm BTU).• Scenario C: This scenario is r oughly equivalent, in terms of tons of SO 2 r educed, to elimination of SO 2 emissions fr om the power generation sector. It includes an additional 90% r eduction in SO 2 emissions fr om the power generation sector beyond the cap of 8.95 million tons r equir ed under Ti tle IV (a cap of 1 million tons/year of emissions in 2020) and a 50% r eduction in SO 2 emissions fr om non-power generation
sour ces (e.g., industrial boilers).* The scenario also includes national annual NO x emission r eductions of appr o ximately 80% beyond T itle IV levels (a cap of 1.25 millionton/year of emissions in 2020, or an emission rate of 0.10 lbs/mm BTU). The magnitude of emission r eductions influences both the amount of r ecover y fr om acidification that takes place and the rate at which recover y happens. The rate of recover y is also influenced by th e geological and ecological characteristics of the lak es and/or str eams in the ar ea. Lak e conditions in the Adir ondacks and Northeast and str eam conditions in the Southeast wer e modeled using the MA GIC model (see page 53 for a description of MA GIC). These r egions wer e chosen because they ar e among the most acid-sensitive ecosystems and ar e downwind of many of the emission sour ces affected by the Acid Rain P r ogram (see F igur e 1). Emissions under Scenarios A, B, and C wer e assumed to be held constant af ter 2020; emissions under the Base Case continued to decr ease slightly af ter 2020 until 2030. Pr ojected lak e and str eam conditions wer e compar ed to modeled estimates of lak e and str eam conditions in 1984/1985. Since lak e and str eam quality in 2020 would r epr esent only a small portion of the r ecover y e xpected due to the scenarios analyzed, lak e and str eam conditions ar e pr esented for 2030. Regional pr edictive models of the ecological

r esponses in for ests to changes in emissions or deposition ar e not curr ently available and

wer e not modeled in this assessment. This

r eport employs models and analytical tools that ar e widely used and peer -r eviewed. As with any detailed analysis of comple x scenarios, the r esults pr esented in this r eport ar e subject to uncertainties concerning emissions estimates, air quality modeling and deposition pr ojections, and the impact of emission

r eductions on ecological systems.
C HANGES IN ACID D EPOSITION Even with T itle IV in place, lak es and str eams in many parts of the U.S. ar e still e xperiencing acidification due to sulfur and nitr ogen deposition. This is especially pr evalent in the East, wher e sulfur deposition is curr ently appr o x imately 15–25 kg/ha/year . Sulfur deposition levels in the W e st ar e much lower

, generally less than 1 0 kg/ha/year . Inor ganic n i

trogen deposition is currently appro ximately 3

– 7 kg/ha/year in the East. Inor ganic nitr o gen deposition levels in the W est, although in some ar eas still high enough to cause ecological damage, ar e lower , on the or der of less than 4 kg/ha/year

. Acid deposition in sensitive ecosystems alters soils, str esses for est vegetation, acidifies lak es and str eams, and harms fish and other aquatic life. F or e xample, 24% of Adir ondack lak es with low pH and ANC levels sur veyed in

* The SO 2 emission r eductions included fr om non-power generation sour ces is appr o ximately 2 million tons, almost twice the r emaining SO 2 emissions fr om

the power generation sector once the level of the cap is r eached.the mid-1980s did not support fish, and 6% of V i r ginia tr out str eams studied in the late 1990s wer e unable to support br ook tr out or other fish species due to chr onic acidity

. 30, 165 While some Adir ondack lak es have always been acidic due to the pr esence of or ganic acids, data collected in the 1980s indicates that the vast majority of those lar ger than 4 hectar es and almost all str eams in the Southeast wer e acidic due to atmospheric deposition. 31 Recent r esear ch on for est ecosystems shows that acid deposition has contributed to the decline of r ed spruce thr oughout the Eastern U.S. and may have contributed to the decline of sugar maple tr ees in central and western P ennsylvania. 30Overall, the effects of acid deposition can interfer e with valuable ecosystem benefits, such as for est pr oductivity and water quality

. C HANGES IN S ULFUR D EPOSITION All additional r eduction scenarios modeled in this analysis ar e pr ojected to lead to significant r egional r eductions in sulfur deposition as compar ed to pr ojected conditions under the Base Case in 2010 (F igur e 34). Under Scenario A, r eductions in deposition of 10–30% would be found fr om the Plains states east to the A tlantic seaboar d, with r e

ductions of 30–60% in the southeastern states str etching up into P ennsylvania and southeastern Ohio. Most western states, fr om the Rocky Mountains west, would see

r eductions in deposition of up to 10%.
Scenario B e xtends the ar ea of lar gest
r eduction in sulfur deposition northwar ds and westwar ds. Scenario C would e xtend the ar ea with the gr eatest r eductions in sulfur deposition northwar d to Maine and

acr oss the Mississippi River into the eastern portion of the Plains states. F or e xample, under Scenario B, the Adir ondacks, midAppalachians, and southern Blue Ridge Regions would all e xperience r eductions in sulfur deposition of up to 60%. Scenario C would e xpand those r eductions into other acid sensitive ar eas, including Northern New England and the Upper Midwest. These

r eductions ar e e xpected to pr ovide ecological benefits t o these acid-sens itive r egions, but ar e not e xpected to solve all ecological pr oblems r elated to acid deposition in those ar eas.
C HANGES IN N ITROGEN D EPOSITION As mentioned above, acidification pr events lak es and str eams fr om supporting fish and other aquatic life. In addition, many estuaries and for ests ar e being affected by

ex cessive loading of nitr ogen, including atmospheric deposition of nitr ogen. Modeling of the Base Case indicates that, in 2020, T itle IV , the NO x SIP call, and the T ier II and Heavy Duty Diesel mobile sour ce rules ar e e xpected to achieve r eductions in nitr ogen deposition of gr eater than 30% fr om North Car olina north along the A tlantic seaboar d to Maine. Lar ge portions of the r est of the Eastern U.S. will

e xperience r eductions in nitr ogen deposition of 10–20% due to e xisting pr ograms. Lar ge portions of the W estern U.S. will also achieve significant r eductions in deposition of 10–20% or mor e due to e x isting mobile sour ce r eductions. These r eductions in deposition will tak e place in some of the ar eas e xperiencing the most damage fr om nitr ogen deposition, including the Adir ondacks, Appalachians, Colorado Fr ont Range, and the San Bernadino Mountains.
The emission r eductions fr om power plants in the scenarios modeled in this analysis ar e p r ojected to lead to additional

re gional r eductions in nitr ogen deposition as compar ed to pr ojected conditions in 2020 under the Base Case (F igur e 35). Under Scenario A, ther e would be additional

r eductions in nitr ogen deposition of 10–20% beyond the levels seen under the Base Case in patches thr oughout the Eastern U.S. Similar levels of emission r eductions would be found in the F our Corners r egion of the W est. The model suggests a slight incr ease in deposition in east T e xas, southern L ouisiana, and eastern North Car olina due to incr eased utilization of units (these units ar e meeting their NO x limits under T itle IV). The r est of the countr y would see additional r eductions in deposition of up to 10%.
Scenario B would e xpand the ar ea of 10–20% r eduction in deposition fr om the Base Case to cover mor e of the eastern seaboar d, Colorado, New Me xico, and Utah. It would r educe the ar ea r eceiving slightly incr eased levels of nitr ogen deposition in T e xas, but not eliminate it.
These emission r e ductions would also lead to significant additional r eductions in nitr ogen deposition compar ed to the Base Case in sensitive ecosystems still e xperiencing water quality and/or for est health pr oblems due to acidification. F or e xample, the Adir ondacks would r eceive a 10–20%

r eduction in deposition as compar ed to the Base Case under Scenario B and the Chesapeak e Bay watershed would

r eceive an appr o ximately 10% r eduction in deposition as compar ed to the Base Case under Scenario B. A sensitiveecosystem thought to be in danger of experiencing

ex cessive nitrogen loading, the Colorado Front Range, would receive approximately a 20% reduction in 2020, with

r eductions in some areas over 30%, as compared to the Base Case scenario under Scenario B. As is the case with

r eductions in sulfur deposition, these reductions are expected to provide ecological benefits in many sensitive ecosystems, but some problems due to nitrogen deposition are

e xpected to remain. NO x emissions are the same in Scenario B and C so nitrogen deposition was notmodeled for Scenario C. Although the nonlinearities involved in particulate chemistry indicate that there would be some change innitrogen deposition based on changes in sulfur emissions, Scenario C is expected to showfew significant differences in nitrogen deposition compared to Scenario B.W A

TER Q U ALITYAcid deposition affects water quality in sensitive ecosystems by acidif ying lakes and streams, changing water chemistr y , and altering soil chemistr y

. F or example, 24% of Adirondack lakes with low pH and ANC levels sur veyed in the mid-1980s did not support sensitive fish, and 6% of V i r ginia trout streams studied in the late 1990s were unable to support brook trout or other fish species due to chronic acidity

. 30, 165 Curr ently available modeling tools allow r esear chers to estimate the effects of r eductions in acid deposition on several aspects of the water chemistr y of lak es and str eams. The effects of acid deposition on the other ecosystem components, such as soil chemistr y , zooplankton or fish health, or for est vegetation cannot at this point be identified quantitatively on a r egional basis.
Watershed characteristics (e.g., soils, bedr ock type, geologic histor y) affect the rate of water chemistr y r esponse to acid deposition. “Dir ect r esponse” lak es or str eams manifest changes r elatively quickly in r esponse to changes in deposition, wher eas “delayed r esponse” lak es or str eams manifest changes over a longer period of time. The implementation of the Base Case is e x pected to significantly r educe the per centage of chr onically -acidic str eams in all thr ee ar eas beyond what would have

occurr ed without T itle IV (see Chapter 4 for a detailed discussion of the water quality improvements e xpected fr om full implementation of T itle IV and for a discussion of the population of lak es and str eams consider ed in this assessment). This r eduction in the number of acidic lak es and str eams, as well as r educti ons in the frequency and/orseverity of acidification in the r e ma ining acidic lak es and str eams, impr oves the health of fish populations and other acid-sensitive species. However , as shown in the base case modeling pr esented in F igur e 36, T itle IV will not eliminate the pr oblem of acidic waters. Chr onically acidic waters ar e pr ojected to r emain in both the Northeast and the Southeast and some waters in all thr ee of the acid-sensitive

r egions modeled in this analysis ar e e xpected to continue to

e xperience episodic acidification. The changes in water quality for the thr ee additional r eduction scenarios wer e also modeled in this analysis. While the effects of additional emission r eductions differ by r egion, the amount of r eductions mak es little differ ence in the

r esulting chr onic acidity of any one r egion. However , the lar ger the emission r eductions, the mor e “non-acidic” lak e s and str eams ar e pr ojected (See F igur e 22 for an

e xplanation of the stages of r ecover y fr om acidification). Modeling r esults indicate that the additional 60% r eduction in SO 2 emissions fr om power generation beyond T itle IV would r esult in the virtual elimination of chr onic acidity fr om lak es in the Northeast, including the Adir ondack Mountains, by 2030 (F igur e 36). Resear ch indicates that lar ger r eductions in deposition would speed the time to r e

cover y of those Northeastern lak es. 30 Only under Scenario C would the per centage of episodically acidic lak es also begin to decr ease. The stor y is somewhat differ ent for the Southeast (F igur e 36). Due to the unique geology of the ar ea, including its por ous sandy soils, southeastern str eams r espond ver y slowly to r eductions in acid deposition (characterized as “delayed r esponse”). As a r esult, no additional r ecover y is
e xpected in chr onically acidic str eams in the Southeast, even with r eductions beyond the base case included in Scenarios A and B. Accor ding to model pr ojections, a
r eduction of mor e than 75% in SO 2 emissions fr om power generation sour ces would not r educe the number of chr onically acidic str eams in the Southeast. This level of emission

r eductions fr om the power generation sector would slow the rate of acidification of these waters, but impr ovement (i.e., a r eduction in the number of chr onically acidic str eams) is pr ojected to begin only af ter lar ger r eductions and/or r eductions fr om other sour ce categories such as those associated with Scenario C occur

. 

# Summar y
T itle IV has been quite successful at effectively r educing emissions of SO 2 and NO x fr om power generation to the levels set by Congr ess. In fact, by 2020 emissions fr om power plants ar e e xpected to be less than half of the total SO 2 emissions in the U.S. As a r esult of these emission

r eductions, some acid-sensitive ar eas ar e alr eady beginning to show signs of r ecover y

. Fu ll implementation of T itle IV, and the passage of time needed for affected ecosystems to r espond to the new envir onmental conditions, ar e e xpected to allow mor e ar eas to r ecover . However , several scientific studies indicate that the emission

r eductions achieved by T i tle IV ar e not sufficient to allow r ecover y of acid-sensitive ecosystems. Estimates fr om the literatur e of the scope of additional emission r eductions that ar e necessar y in or der to pr otect acidsensitive ecosystems r ange fr om appr o ximately 40–80% beyond full implementation of T itle IV , with the most r e

cent studies focusing on the lar gest r eductions. This r eport analyzes the impacts of arange of levels that would achieve fuller envir onmental r ecover y fr om acid rain and gr eater avoidance of the adverse ecological effects associated with acid deposition.
The r esults of the modeling pr esented in this Report to Congr ess indicate that br oader r ecover y is not pr edicted without additional emission r eductions. Elimination of all SO 2 emissions fr om power plants, however , will not fully pr otect all acid-sensitive ecosystems affected by acid deposition; r e

ductions fr om other sour ce categories would be needed. The information r equested by Congr ess and pr esented her e r epr esents part of what is needed when determining appr opriate futur e actions; other information includes the costs and other impacts of emission r eductions and the value the public places on further impr ovements to the envir onment and human health.

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173 Driscoll, C.T ., K.M. Driscoll, M.J. Mitchell, and D.J. Raynal. 2003. Effects of acid deposition on for est and aquatic ecosystems in New Y ork State. Envir onmental Pollution 123: 327-336.

174 NAP AP . 1990b. Acid Deposition: State of Science and T echnology , V olume II Aquatic pr ocesses and Effects. National Acid P r ecipitation Assessment P r ogram Office of the Dir ector , W ashington, DC.

175 U.S. EP A. 1995b. Acid Deposition Standar d F e asibility Study Report to Congr ess. W ashington, DC. EP A 430-R-95-001a.

176 Ecological Society of America. 1997. A tmospheric Nitr ogen Deposition to Coastal W atersheds. (W ashington, DC: Ecological Society of America, www .esa.or g/sbi/sbi_publications/sbindep1.htm ).

177 Ecological Society of America. 1999. Acid Deposition: The Ecological Response. W ashington, DC: Ecological Society of America, ( www .esa.or g/sbi/sbi_publications/aciddep.htm ).

178 Southern Appalachians Mountains Initiative (SAMI). 2002. Southern Appalachians Mountain Initiative F inal Report.

179 U.S. EP A. 2003b. T echnical Support Document for the Clear Skies Act 2003 Air Quality Modeling Analyses. U.S. Envir onmental P r otection Agency Office of Air Quality Planning and Standar ds Emissions Analysis and Monitoring Division Resear ch T riangle P ark.To contact the NAP AP or ganization or to r equest copies of this r eport, contact National Acid P r ecipitation Assessment P r ogram
NOAA, MailCode R/W A Room 11216 1315 East- W est Highway Silver Spring, MD 20910

T

elephone: (301) 713–0460 x202 F ax: (301) 713–3515
e-mail: napap@noaa.gov

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ar e available at www .napap.noaa.gov/r eports