Acid Rain and Related Programs

2006 PROGRESS REPORT

Acid Rain and Related Programs: 2006 Progress Report i

Contents

Introduction .....................................................................1

Summary ..........................................................................3

Origins of the Acid Rain Program ..................................7

SO2

 Emission Reductions ................................................8

SO2 Program Compliance ..........................................................11

SO2 Allowance Market ...............................................................12

NOX Emission Reductions and Compliance .................13

Emission Monitoring and Reporting ...........................15

Status and Trends in Air Quality, Acid Deposition, 

and Ecological Effects ...................................................19

Understanding the Monitoring Networks ...................................19

Enhancing Mercury Monitoring Capabilities ...............................23

Air Quality .................................................................................24

Acid Deposition .........................................................................30

Improvements in Surface Water .................................................33

Understanding Clean Air Rules ....................................37

Rule Background .......................................................................37

How the New Trading Programs Work .......................................39

CAIR Allowance Market and State Activity .................................41

CAMR .......................................................................................43

The Mercury Trading Program and State Activity ........................43

CAVR ........................................................................................44

Analyzing the Impacts of Rule Implementation ..........................46

Online Information, Data, and Resources ...................51

Endnotes ........................................................................53

Acid Rain and Related Programs

2006 PROGRESS REPORT
The Acid Rain Program was 

designed to reduce the adverse 

effects of acid deposition 

through reductions in annual 

emissions of SO2

 and NOX.

Introduction
Acid Rain and Related Programs: 2006 Progress Report 1

The U.S. Environmental Protection Agency 

(EPA) publishes an annual report to update 

the public on compliance with the Acid Rain 

Program (ARP), its status of implementation, and 

progress toward achieving environmental goals.

The Acid Rain and Related Programs 2006 Progress 

Report updates data reported in previous years, 

specifi cally:

• Sulfur dioxide (SO2) emissions, allowance market 

information, and program compliance. 

• Nitrogen oxides (NOX) emissions and program 

compliance.

• Status and trends in acid deposition, air quality, 

and ecological effects. 

• New programs, such as the Clean Air Interstate 

Rule (CAIR), that are building on the ARP to further 

improve environmental quality. 

In this year’s report, EPA incorporates early CAIR 

compliance planning into the fi ndings associated 

with the ARP, including contributions to a signifi cant 

SO2

 emission decrease in 2006, and other relationships between the ARP, CAIR, and other new air 

quality rules.

For more information on the ARP, CAIR, and related 

programs, including additional information on SO2

and NOX emissions, acid deposition monitoring, 

environmental effects of acid deposition, and 

detailed unit-level emission data, please visit EPA’s 

Clean Air Markets Web site at 

<www.epa.gov/airmarkets>. 

Introduction

Key Findings

• Total SO2 emissions fell below 10 million 

tons for the fi rst time under ARP.

• NOX emissions in 2006 were 3.3 million 

tons below 1990 levels.

• Acid deposition has declined signifi cantly 

from levels measured before ARP, 

with corresponding water quality 

improvements in lakes and streams.

• Estimated public health benefi ts from ARP 

emission reductions exceed program costs 

by a margin of more than 40 to 1.
Summary
Acid Rain and Related Programs: 2006 Progress Report 3

Summary

Sulfur dioxide (SO2) and nitrogen oxides (NOX) are the 

key pollutants involved in the formation of acid rain. 

These pollutants also contribute to the formation of 

fi ne particles (sulfates and nitrates) that are associated 

with signifi cant human health effects and regional 

haze. Sulfates and nitrates are transported and 

deposited at levels harmful to sensitive ecosystems in 

many areas of the country. Additionally, NOX combines 

with volatile organic compounds (VOCs) to form 

ground-level ozone (smog). The U.S. electric power 

industry accounts for approximately 70 percent of 

total U.S. SO2

 emissions and 20 percent of total U.S. 

NOX emissions from man-made sources.1

The Acid Rain Program (ARP) was created under Title 

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

reduce the adverse effects of acid deposition through 

reductions in annual emissions of SO2

 and NOX. The 

Act calls for SO2

 reductions of 10 million tons from 

1980 emission levels, largely achieved through a 

market-based cap and trade program, which utilizes 

emission caps to permanently limit SO2

 emissions 

from power plants. NOX reductions under the ARP 

are achieved through a program closer to a more 

traditional, rate-based regulatory system. The NOX

program is designed to limit NOX emission levels to 

2 million tons less than those projected for the year 

2000 without implementation of Title IV. 

Since the start of the ARP in 1995, reductions in 

SO2

 and NOX emissions from the power sector have 

contributed to signifi cant improvements in air quality 

and environmental and human health. The SO2

program affected 3,520 electric generating units 

(EGUs) in 2006 (with most emissions produced by 

1,062 coal-fi red units). The NOX program applied to a 

subset of 982 operating coal-fi red units in 2006. 

The 2006 compliance year marked the 12th year of 

the program. During this period, the ARP has: 

• Reduced SO2 emissions by more than 6.3 million tons 

from 1990 levels, or about 40 percent of total power 

sector emissions. 

• SO2 emissions from ARP units fell sharply, declining 

830,000 tons from 2005 levels. Reduced energy 

demand, decreased oil use because of fuel 

prices, and early Clean Air Interstate Rule (CAIR) 

compliance all appear to be factors in this decline. 

• Total SO2 emissions fell below 10 million tons 

for the fi rst time under the ARP. Sources emitted 

approximately 9.4 million tons of SO2

 in 2006, 

below the emission cap of 9.5 million tons. 

Summary
4 Acid Rain and Related Programs: 2006 Progress Report

• With nearly 6.1 million unused (banked) 

allowances from prior years, SO2 emissions were 

40 percent below the total 2006 allowable SO2

emissions of 15.7 million tons.

• Cut NOX emissions by 3.3 million tons from 1990 

levels, so that emissions in 2006 were less than half 

the level anticipated without the program. Other 

efforts, such as the NOX Budget Trading Program 

(NBP) in the eastern United States, also contributed 

to this reduction. 

• Led to signifi cant decreases in acid deposition. For 

example, between the 1989-1991 and 2004-2006 

observation periods, wet sulfate deposition 

decreased 35 percent in the Northeast and 33 

percent in the Midwest. These reductions have 

resulted in positive changes in environmental 

indicators, including improved water quality in 

lakes and streams. 

• Provided the most complete and accurate emission 

data ever developed and made those data available 

through comprehensive electronic data reporting 

and Web-based tools for agencies, researchers, 

affected sources, and the public.

• Delivered pioneering e-government results, 

automating administrative processes, reducing 

paper use, and providing online systems for doing 

business with EPA. 

• Achieved extremely high compliance levels, with 

100 percent compliance with the allowance 

holding requirements for SO2

 in 2006, and a single 

unit out of compliance for NOX. 

• Reduced implementation costs by allowing sources 

to choose cost-effective compliance strategies. 

After 12 years of implementation, monitoring, and 

assessment, the ARP has proven to be an effective and 

effi cient means of meeting emission reduction goals 

under the Clean Air Act (CAA). A 2005 study estimated 

the program’s benefi ts at $122 billion annually in 2010, 

while cost estimates are around $3 billion annually (in 

2000$).2

 Despite the program’s historic and projected 

benefi ts, EPA analyses of recent studies of human 

health, data from long-term monitoring networks, 

and ecological assessments have revealed the need for 

additional emission reductions to protect human health 

and continue ecological recovery and protection. EPA 

recognized the need for further SO2

 and NOX controls 

on the power industry to address pollutant transport 

problems many states face in efforts to attain National 

Ambient Air Quality Standards (NAAQS) for ozone and 

fi ne particles. The success of the acid rain trading and 

NOX emission reduction programs, along with the need 

for further reductions, provided the impetus for a suite 

of new rules promulgated in 2005: CAIR, the Clean Air 

Visibility Rule (CAVR), and the Clean Air Mercury Rule 

(CAMR). 

Building on the ARP model, EPA promulgated CAIR 

in the spring of 2005 to address transport of fi ne 
Acid Rain and Related Programs: 2006 Progress Report 5

particles and ozone in the eastern United States, 

CAVR to improve visibility in national parks and 

wilderness areas, and CAMR to reduce nationwide 

mercury emissions from coal-fi red power plants. 

Starting in 2009 and 2010, CAIR establishes regional 

caps on SO2

 and NOX emissions. Annual SO2 emissions 

for affected eastern states are capped at 3.7 million 

tons in 2010 and 2.6 million tons in 2015. Annual 

NOX emissions for affected eastern states are capped 

at 1.5 million tons in 2009 and 1.3 million tons in 

2015. CAIR will operate concurrently with the ARP. 

CAVR addresses SO2

 and NOX power sector emissions 

from non-CAIR states located in the West and parts 

of New England. Affected sources under CAVR must 

reduce SO2

 and NOX emissions that impair visibility 

in national parks and wilderness areas. Notably, EPA 

allows states to establish additional regional cap and 

trade programs to accomplish these reductions from 

power plants and other stationary sources. 

CAMR establishes a national cap on mercury emissions 

beginning in 2010 and utilizes a market-based cap 

and trade program. Additionally, new coal-fi red 

power plants will be required to meet standards of 

performance that limit mercury emissions. These 

programs will serve as a key component of strategies 

to protect human health and the environment across 

the United States into the next decade.
From 2005 to 2006 ARP sources 

reduced SO

2

 emissions below 

10 million tons for the fi rst 

time under the program, and 

NO

X emissions fell to 3.4 million 

tons, a decrease of nearly 50 

percent from 1990 levels.

Emission Reductions
Acid Rain and Related Programs: 2006 Progress Report 7

Acid deposition, more commonly known as acid 

rain, occurs when emissions of sulfur dioxide (SO2

) 

and nitrogen oxides (NOX) react with water, oxygen, 

and oxidants in the atmosphere to form various 

acidic compounds. Prevailing winds transport these 

compounds hundreds of miles, often across state 

borders, where they impair air quality and damage 

public health, acidify lakes and streams, harm 

sensitive forests and coastal ecosystems, degrade 

visibility, and accelerate the decay of building 

materials.

The Acid Rain Program (ARP), established under 

Title IV of the 1990 Clean Air Act Amendments 

(CAAA), requires major reductions of SO2

 and NOX

emissions from the electric power industry. The SO2

program sets a permanent cap on the total amount 

of SO2

 that may be emitted by electric generating 

units (EGUs) in the contiguous United States. The 

program is phased in, with the fi nal 2010 SO2

 cap set 

at 8.95 million tons, a level of about one-half of the 

emissions from the power sector in 1980.

As seen in Figure 1, emissions of both SO2

 and 

NOX have decreased markedly under the ARP while 

combustion of fossil fuels, measured as “heat input,” 

for electricity generation has increased signifi cantly. 

Using a market-based cap and trade mechanism to 

reduce SO2

 emissions allows fl exibility for individual 

combustion units to select their own methods of 

compliance. Currently, one allowance provides a 

-60%

-40%

-20%

0%

20%

40%

60%

Year

Percent Change

Electricity Generation* SO2

 Emissions

NOX Average Retail Price of Electricity Emissions

1990 1995 1996 1998 2000 2002 2004 2006

**

Heat Input

Figure 1: Trends in Electricity Generation, Fossil Energy Use, Prices, and Emissions 

from the Electric Power Industry

* Generation from fossil fuel-fi red plants.

** Constant year 2000 dollars adjusted for infl ation. 

Source: Energy Information Administration (electricity generation, retail price); EPA (heat input and emissions, representing all affected ARP units), 2007

 Origins of the Acid Rain Program
8 Acid Rain and Related Programs: 2006 Progress Report

regulated unit limited authorization to emit one ton 

of SO2

. The Clean Air Act (CAA) allocates allowances 

to regulated units based on historic fuel consumption 

and specifi c emission rates prior to the start of the 

program.3

 The total allowances allocated for each year 

equal the SO2

 emission cap. The program encourages 

early reductions by allowing sources to bank unused 

allowances in one year and use them in a later year. 

The ARP adopts a more traditional approach to 

achieve NOX emission reductions. Rate-based limits 

apply to most of the coal-fi red electric utility boilers 

subject to the ARP. An owner can meet these 

NOX limits on an individual unit basis or through 

averaging plans involving groups of its units.

The ARP is composed of two phases for SO2

 and 

NOX. Phase I applied primarily to the largest coalfi red electric generation sources from 1995-1999 for 

SO2

 and from 1996-1999 for NOX. Phase II for both 

pollutants began in 2000. In 2006, the SO2

 Phase II 

requirements applied to 3,520 units, with most of 

the emissions produced by 1,062 coal-fi red units. The 

Phase II NOX requirements applied to 982 of those units 

that have a generation capacity of 25 megawatts (MW) 

or more and burned coal between 1990 and 1995.

SO2 Emission Reductions

Electric power generation is by far the largest 

single source of SO2

 emissions in the United States, 

accounting for approximately 70 percent of total SO2

emissions nationwide.4

As shown in Figure 2, ARP sources have reduced 

annual SO2

 emissions by 46 percent compared to 

1980 levels and 40 percent compared to 1990 levels. 

Reductions in SO2

 emissions from other sources 

not affected by the ARP (including industrial and 

commercial boilers and the metals and refi ning 

industries) and use of cleaner fuels in residential and 

commercial burners have contributed to a similar 

overall decline (47 percent) in annual SO2

 emissions 

from all sources since 1980. National SO2

 emissions 

from all sources have fallen from nearly 26 million 

tons in 1980 to less than 14 million tons in 2006 (see 

<www.epa.gov/ttn/chief/trends>). 

Figure 2: SO2 Emissions from Acid Rain Program Sources

Source: EPA, 2007

9.4 9.3 8.7

5.3 5.4 5.5 5.3 4.9

9.4

17.3

16.1 15.7

11.9 12.5 13.0 13.1 12.5

11.2 10.6 10.2

10.6 10.3 10.2

8.7 8.3

7.1 7.0 7.0

9.6 9.5 9.5 9.5 9.5 9.5 10.0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

18.0

20.0

1980 1985 1990 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

SO2 Emissions (million tons)

All Affected Sources

Phase II (2000 on) Sources

Phase I (1995-1999) Sources

Allowances Allocated 

Year
Acid Rain and Related Programs: 2006 Progress Report 9

For 2006, EPA allocated over 9.5 million SO2

allowances under the ARP. Together with more 

than 6.1 million unused allowances carried over (or 

banked) from prior years, there were 15.7 million 

allowances available for use in 2006. Sources emitted 

approximately 9.4 million tons of SO2

 in 2006, less 

than the allowances allocated for the year, and far less 

than the total allowances available (see Figure 3).5

The number of banked allowances grew, from 6.1 

million available for 2006 compliance to 6.3 million 

available for 2007 and future years. In the next several 

years, industry anticipation of stringent emission 

requirements under CAIR is expected to encourage 

sources to pursue additional reductions. While 

these reductions will result in an increase in banked 

allowances, tighter retirement ratios under CAIR (that 

SO2 (million tons)

15.7

16.4 17.1

18.2 18.8

19.9

21.6

16.6

15.0

13.4

11.7

8.7

5.3 5.4 5.5 5.3 4.9

11.2 10.6 10.2 10.6 10.3 10.2

9.4

0

5

10

15

20

25

1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

Allowances allocated

Unused allowances from previous years

Actual emissions from affected sources

Figure 3: SO2 Emissions and the Allowance Bank, 1995-2006

Source: EPA, 2007

SO2 Emissions in 1990

SO2 Emissions in 2006

SO2

 Emissions in 1995

SO2 Emissions in 2000

Scale: Largest bar equals 

2.2 million tons of SO2

emissions in Ohio, 1990

Figure 4: State-by-State SO2 Emission Levels, 1990-2006

Source: EPA, 2007
10 Acid Rain and Related Programs: 2006 Progress Report

in effect lower the SO2 emission cap) will lead to 

depletion of the bank and further reduce emissions. 

In 2010, the total number of Title IV allowances 

allocated annually will drop to 8.95 million and remain 

statutorily fi xed at that annual level. Because of the 

retirement ratios in the CAIR region, EPA projects that 

emissions will be signifi cantly lower than this statutory 

cap. Table 1 explains in more detail the origin of the 

allowances that were available for use in 2006, and 

Table 3 shows how those allowances were used. 

From 2005 to 2006, reductions in SO2

 emissions from 

ARP units in 35 states and the District of Columbia 

totaled 873,000 tons. Modest increases in 13 states 

totaled 43,000 tons, resulting in a net national 

decrease of 830,000 tons, or more than 8 percent, 

for the year. Among the states with large reductions, 

12 states (Florida, Illinois, Indiana, Kansas, Kentucky, 

Massachusetts, North Carolina, Nevada, New York, 

Ohio, Pennsylvania, and Virginia) decreased emissions 

by more than 25,000 tons each. The largest reduction 

was in Ohio, where ARP units reduced emissions by 

more than 123,000 tons from 2005 levels. 

The states with the highest emitting sources in 1990 

have seen the greatest SO2

 reductions during the 

ARP (see Figure 4 on page 9). Most of these states 

are upwind of the areas the ARP was designed to 

protect, and reductions have resulted in important 

environmental and health benefi ts over a large region. 

For the 32 states and the District of Columbia that 

reduced annual SO2

 emissions from 1990 to 2006, total 

annual SO2

 emissions were approximately 6.7 million 

tons lower in 2006 than they were in 1990. For the 16 

states where annual emissions increased from 1990 to 

2006, total emissions were up by only about 328,000 

tons from 1990 levels. In contrast, the 2006 emissions 

were more than 100,000 tons less than 1990 levels in 

each of 13 states: Florida, Georgia, Illinois, Indiana, 

Kentucky, Massachusetts, Missouri, New York, Ohio, 

Pennsylvania, Tennessee, West Virginia, and Wisconsin. 

The six states with the greatest annual reductions 

include Ohio, which decreased emissions by 1.3 million 

tons, and Illinois, Indiana, Missouri, Tennessee, and 

West Virginia, each of which reduced emissions by 

more than 500,000 tons per year. 

Type of 

Allowance 

Allocation

Number 

of SO2

Allowances

Explanation 

of Allowance 

Allocation Type

Initial 

Allocation

9,191,897 The initial allocation of 

allowances is granted to 

units* based on the product 

of their historical utilization 

and emission rates specifi ed 

in the CAA.

Allowance 

Auction

250,000 The allowance auction 

provides allowances to the 

market that were set aside in 

a Special Allowance Reserve 

when the initial allowance 

allocation was made.

Opt-in 

Allowances

97,678 Opt-in allowances are provided 

to units entering the program 

voluntarily. There were eight 

opt-in units in 2006.

Total 2006 

Allocation

9,539,575

Total Banked 

Allowances**

6,116,062 Banked allowances accrue 

in accounts from previous 

years. These allowances were 

available for compliance in 

2006 or any future year.

Total 2006 

Allowable 

Emissions

15,655,637

Source: EPA, 2007

Table 1: Origin of 2006 Allowable 

SO2

 Emission Levels

* In this report, the term “unit” means a fossil fuel-fi red combustor that 

serves a generator that provides electricity for sale. The vast majority of 

SO2

 emissions under the program result from coal-fi red generation units, 

but oil and natural gas units are also included in the program.

** Total banked allowances are adjusted from the 2005 Progress Report

to account for additional deductions made for electronic data reporting 

(EDR) resubmissions after 2005 reconciliation was completed.
Acid Rain and Related Programs: 2006 Progress Report 11

SO2 Program Compliance

Approximately 9.4 million allowances were 

deducted from sources’ accounts in 2006 to 

cover emissions. Table 3 displays these allowance 

deductions, as well as the remaining banked 

allowances from 1995 through 2006. In 2006, 

all ARP facilities were in compliance with the 

SO2

 allowance holding requirements. Title IV set 

a penalty of $2,000 per ton in 1990, which is 

adjusted annually for infl ation. The 2006 penalty 

level was set at $3,152 per excess ton. The ARP’s 

cap and trade approach offers emission sources 

the fl exibility to comply with regulations using 

their choice of the most cost-effective strategies 

available. Since the program’s inception, 

the compliance rate has consistently been 

extraordinarily high.

Fuel

Type 2004 2005 2006

SO2 NOX

 HI SO2 NOX HI SO2 NOX HI

Coal 9,839 3,484 20.48 9,836 3,356 20.77 9,243 3,208 20.44

Oil 378 139 1.00 350 130 1.00 135 64 0.58

Gas 37 133 4.84 34 142 5.34 7 130 5.70

Other 3 6 0.03 3 6 0.03 7 7 0.05

Total 10,256 3,762 26.35 10,223 3,633 27.14 9,392 3,409 26.78

Notes: All emission data are in thousand tons and all heat input data are in 

billion mmBtu. Totals may not refl ect individual rows due to rounding. Fuel type 

represents primary fuel type, and many units may combust more than one fuel. 

Source: EPA, 2007

Table 2: SO2, NOX, and Heat Input Trends in 

Acid Rain Program Units, by Fuel Type

Total Allowances Held (1995-2006 Vintages)* 15,655,637

Facility Accounts** 12,483,262

General Accounts*** 3,172,375

Allowances Deducted for Emissions**** 9,392,922

Penalty Allowance Deductions (2007 Vintage) 0

Banked Allowances 6,262,715

Facility Accounts 3,090,340

General Accounts 3,172,375

Table 3: SO2 Allowance Reconciliation 

Summary, 2006

* The allowance transfer deadline is March 1 of the year following 

the compliance year. At this point, facility accounts are frozen, 

and no further transfers of allowances are recorded. The freeze on 

accounts is removed when the annual reconciliation is complete.

** From 1995 through 2005, EPA reconciled emissions and 

allowances for compliance under the ARP separately

for each unit. In 2006, EPA began reconciling emissions

and allowances at the facility level for compliance purposes. 

*** General accounts that are not subject to reconciliation can 

be established in the Allowance Tracking System (ATS) by any 

utility, individual, or other organization. 

**** Includes 489 allowances deducted from opt-in sources for 

reduced utilization. 

Source: EPA, 2007 

Why SO2 Emissions Decreased 

Sharply in 2006

For the fi rst time under the ARP, SO2 emissions in 2006 fell below 10 

million tons. Overall reductions from 2005 were over 830,000 tons 

(see Table 2). This decrease stemmed from a number of factors: 

• Heat input (measured in million British thermal units (mmBtu)) 

dropped for oil-fi red units, with a comparable increase in heat 

input from gas-fi red units as well as much less oil use at dual fuel 

units classifi ed as gas-fi red units. Switching from oil to gas reduces 

SO2

 because oil is higher in sulfur content than natural gas. Fuel 

switching resulted in about a 243,000-ton SO2

 reduction from oil 

and gas units combined. 

• Emissions from coal-fi red units decreased by about 593,000 tons. 

Reductions came from both uncontrolled units and those with 

installed fl ue gas desulfurization equipment (scrubbers). 

• Reductions in heat input and the SO2

 emission rate of units 

without scrubbers resulted in a decrease of about 412,000 tons 

of SO2

 emissions. Overall heat input for these units was down 

2.1 percent, but the overall emission rate was down even more, 

about 3.5 percent. The emission rate decline may be partially 

attributable to early CAIR compliance planning. 

• Units equipped with scrubbers (either in both 2005 and 2006, or 

just 2006) caused a decrease in SO2

 emissions of about 182,000 

tons. Heat input to these units declined by less than 1 percent, 

but their emission rate dropped nearly 11 percent, refl ecting the 

addition of several scrubbers on previously uncontrolled units. 

Some of the scrubber installations were expected as a result of 

existing state or federal actions. Others appear to be part of an 

early compliance response to CAIR. 

• Overall, about 650 coal units had at least some decrease in mass 

emissions due to reduced heat input, reduced emission rate, or both. 
12 Acid Rain and Related Programs: 2006 Progress Report

0

50

100

150

200

250

300

350

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

Allowances (millions)

EPA Transfers to Account Holders

Private Transactions

Figure 5: Cumulative SO2 Allowances Transferred through 2006

Source: EPA, 2007 

SO2 Allowance Market

Figure 5 shows the cumulative volume of SO2

allowances transferred under the ARP. The fi gure 

differentiates between allowances transferred in 

private transactions and those annually allocated 

and transferred to sources’ accounts by EPA. Private 

transactions are indicative of both market interest 

and use of allowances as a compliance strategy. 

Of the nearly 330 million allowances transferred 

since 1994, about 68 percent were traded in private 

transactions. In December 2001, parties began to use 

a system developed by EPA to allow online allowance 

transfers. In 2006, account holders registered about 

94 percent of all private allowance transfers through 

EPA’s online transfer system.6

In 2006, nearly 6,400 private allowance transfers 

(moving roughly 22.4 million allowances of past, 

current, and future vintages) were recorded in the 

EPA Allowance Tracking System (ATS). About 9.5 

million (42 percent) were transferred in economically 

signifi cant transactions (i.e., between economically 

unrelated parties). Transfers between economically 

unrelated parties are “arm’s length” transactions 

and are considered a better indicator of an active, 

functioning market than are transactions among the 

various units of a given company. 

In the majority of all private transfers, allowances 

were acquired by power companies. Figure 6 shows 

the annual volume of SO2

 allowances transferred 

under the ARP (excluding allocations, retirements, 

and other transfers by EPA) since offi cial recording 

of transfers began in 1994. Note that the volume 

of private transfers recorded in 2006 rose for the 

second straight year and returned to levels not seen 

since 2000-2001. Market liquidity had declined due 

to an overall contraction in the related electricity 

markets following disruptions precipitated by events 

such as the collapse of Enron in late 2001. 
Acid Rain and Related Programs: 2006 Progress Report 13

9.2

16.7

8.2

15.2

13.5

18.7

25.0

22.5 21.4

16.5 15.3

19.9

22.4

0.0

5.0

10.0

15.0

20.0

25.0

30.0

1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

Trades Between Distinct Entities (Significant Transfers) Trades Between Related Entities

Allowances (millions)

Figure 6: SO2 Allowances Transferred under the Acid Rain Program

NOX Emission Reductions 

and Compliance 

Title IV requires NOX emission reductions for certain 

coal-fi red EGUs. Unlike the SO2

 program, Congress 

applied rate-based emission limits based on a unit’s 

boiler type to achieve NOX reductions (see Table 4). 

The NOX emission limit is expressed as pounds of NOX

per unit of heat input (lbs/mmBtu) for each boiler 

subject to a NOX limit. Owners can meet the NOX

limits for each individual unit or meet group NOX limits 

through averaging plans for groups of units that share 

a common owner and designated representative. 

In 2006, all but one unit met its NOX emission 

requirements under the ARP.

The ARP seeks to attain a 2 million ton annual 

reduction in NOX emissions from all ARP sources 

relative to the NOX emission levels that were 

projected to occur in 2000 absent the ARP (8.1 

million tons). This goal was fi rst achieved in 2000 

and has been met every year thereafter, including 

2006. Figure 7 on page 14 shows that NOX emissions 

from all ARP sources were 3.4 million tons in 2006. 

This level is 4.7 million tons less than the projected 

level in 2000 without the ARP, or more than double 

the Title IV NOX emission reduction objective. These 

reductions have been achieved even though the 

amount of fuel burned to produce electricity (as 

measured by heat input) at all ARP units in 2006 

has increased 37 percent since 1990. While the ARP 

Coal-Fired Boiler Type

Title IV Standard 

NOX Emission Limits (lb/mmBtu) # of Units

Phase 1 Group 1 Tangentially Fired 0.45 132

Phase 1 Group 1 Dry Bottom, Wall-Fired 0.50 113

Phase II Group 1 Tangentially Fired 0.40 301

Phase II Group 1 Dry Bottom, Wall-Fired 0.46 295

Cell Burners 0.68 37

Cyclones > 155 MW 0.86 54

Wet Bottom > 65 MW 0.84 24

Vertically Fired 0.80 26

Total All Units n/a 982

Table 4: NOX-Affected Title IV Units by Boiler Type and NOX Emission Limit

Source: EPA, 2007

Source: EPA, 2007 
14 Acid Rain and Related Programs: 2006 Progress Report

5.5 5.4 5.4 5.5 5.3

4.8 4.5

4.1 4.0 3.8

3.4 3.3 3.1

0

1

2

3

4

5

6

7

1990 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

NOx Emissions

(million tons)

NOx Program Affected Sources Title IV Sources Not Affected for NOx

6.7

6.1 5.9

5.5

6.0 6.0

5.1 4.7 4.5 4.2

3.8 3.6 3.4

Figure 7: NOX Emission Trends for Acid Rain Program Units, 1990-2006

Source: EPA, 2007 

Scale: Largest bar equals 500

thousand tons of NOx emissions

in Ohio, 1990

Note: NBP states shaded in gray 

NOx Emissions in 1990

NOx

 Emissions in 1995

NOx

 Emissions in 2000

NOx

 Emissions in 2006

Figure 8: State-by-State NOX Emission Levels for Acid Rain Program Sources, 1990-2006

Source: EPA, 2007 
Acid Rain and Related Programs: 2006 Progress Report 15

was responsible for a large portion of these annual 

NOX reductions, other programs—such as the Ozone 

Transport Commission (OTC), NOX Budget Trading 

Program (NBP) under EPA’s NOX State Implementation 

Plan (SIP) Call, and other regional NOX emission 

control programs—also contributed signifi cantly to 

the NOX reductions achieved by sources in 2006.

From 2005 to 2006, NOX emissions from ARP units 

dropped by 224,000 tons, a net decrease of more 

than 6 percent. Thirty-six states and the District of 

Columbia reduced 2006 NOX emissions by 247,000 

tons below 2005 levels. Of these states, Alabama, 

Florida, Iowa, Louisiana, Nevada, New York, and Ohio 

reduced their NOX emissions by more than 10,000 

tons each from 2005 levels. Twelve states had modest 

increases in NOX emissions in 2006, totaling 23,000 

tons above 2005 levels.

As with SO2

, the states with the highest NOX-emitting 

sources in 1990 tended to see the greatest power 

plant NOX emission reductions (see Figure 8). The 

sum of reductions in the 41 states and the District of 

Columbia that had lower annual NOX emissions in 

2006 than in 1990 was approximately 3 million tons, 

while the sum of increases in the seven states that had 

higher annual NOX emissions in 2006 than in 1990 

was much smaller, about 37,000 tons. Nine of the 

13 states with NOX emission decreases of more than 

100,000 tons were in the Ohio River Basin.

Emission Monitoring and 

Reporting

The ARP requires program participants to measure, 

record, and report emissions using continuous 

emission monitoring systems (CEMS) or an approved 

alternative measurement method. The vast majority 

of emissions are monitored with CEMS while the 

alternatives provide an effi cient means of monitoring 

emissions from the large universe of units with lower 

overall mass emissions. Figures 9 and 10 on page 

17 show the number of units with and without SO2

CEMS for various fuel types, as well as the amount of 

SO2

 emissions monitored using CEMS.

CEMS and approved alternatives are a cornerstone 

of the ARP’s accountability and transparency. Since 

the program’s inception in 1995, affected sources 

have met stringent monitor quality assurance 

and control requirements, and reported hourly 

emission data in quarterly electronic reports to EPA. 

Using automated software audits, EPA rigorously 

Role of Seasonal NOX

Control Programs in 

Reducing Annual Emissions

States subject to EPA’s 1998 NOX SIP Call have achieved 

signifi cant reductions in ozone season NOX emissions since 

the baseline years 1990 and 2000. All of these states have 

achieved reductions since 1990 as a result of programs 

implemented under the CAAA, with many of them 

reducing their emissions by more than half since 1990. A 

signifi cant portion of these decreases in NOX emissions has 

been achieved since 2000, largely as a result of reductions 

under the OTC program and NBP. With the CAIR ozone 

season NOX program taking effect in 2009, further 

emission declines will occur across the region through the 

year 2020. For NBP compliance reports, see:

<www.epa.gov/airmarkets/progress/progress-reports.html>. 
16 Acid Rain and Related Programs: 2006 Progress Report

Emissions Collection and 

Monitoring Plan System 

(ECMPS)

CAMD is reengineering the way the regulated community 

maintains, evaluates, and submits monitoring plans, quality 

assurance (QA) certifi cations, and quarterly emission data. 

An important tool in this effort is the Emissions Collection 

and Monitoring Plan System (ECMPS). ECMPS will replace 

the current processes and multiple software tools used for 

evaluating, submitting, and receiving the data. ECMPS will 

be available for use in 2008, but will be required for all 

sources beginning in 2009. 

CAMD’s goals for the ECMPS project include:

• Creating a single desktop tool for authorized users 

to import and evaluate their data and to submit it to 

CAMD.

• Creating a new data reporting format based on the 

fl exible XML (Extensible Markup Language) standard.

• Creating a centralized database at CAMD for receiving 

and maintaining submitted data. The desktop tool has 

direct access to this database.

• Providing users with the ability to assure the quality 

of data prior to submission and receive one set of 

evaluation results (feedback).

• Maintaining select data outside of the electronic data 

report. 

• Developing and implementing new security 

requirements.

checks the completeness, quality, and integrity of 

these data. All emission data are available to the 

public on the Data and Maps Web site maintained 

by EPA’s Clean Air Markets Division (CAMD) at 

<http://camddataandmaps.epa.gov/gdm/>. The 

site also provides access to other data associated 

with emission trading programs, including reports, 

queries, maps, charts, and fi le downloads covering 

source information, emissions, allowances, program 

compliance, and air quality. 

The emission monitoring requirements for the ARP are

found in 40 CFR Part 75. These provisions are also required for participation in the NBP. The Part 75 requirements will also be used in the future to implement 

CAIR and CAMR.
Acid Rain and Related Programs: 2006 Progress Report 17

Coal Units w / CEMS

Gas Units w / CEMS

Oil Units w / CEMS Other Fuel Units w / CEMS

Gas Units w /o CEMS

Oil Units w /o CEMS

Other Fuel Units w /o CEMS

1,062

168

46

2,158

8

1 13

Figure 9: SO2 Monitoring Methodology for the Acid Rain Program, Number of Units

Figure 10: Monitoring Methodology for the Acid Rain Program, Total SO2

 Mass

Note: “Other fuel units” include units that in 2006 combusted primarily wood, waste, or other non-fossil fuel. The total number of units in Figure 9 

excludes 64 affected units that did not operate in 2006. 

Source: EPA, 2007 

Note: Percentages do not add to 100 percent due to rounding.

Source: EPA, 2007

Coal Units w / CEMS All Other Units w / CEMS All Other Units w /o CEMS

98.41%

0.40%

1.18%
Emission reductions attributed 

to the ARP have helped reduce 

wet sulfate acid deposition by 

up to 35 percent since the late 

1980s in some eastern regions 

of the United States.

Environmental Results
Acid Rain and Related Programs: 2006 Progress Report 19

The emission reductions achieved under the ARP 

have led to important environmental and public 

health benefi ts. These include improvements in air 

quality with signifi cant benefi ts to human health; 

reductions in acid deposition; the beginnings of 

recovery from acidifi cation in fresh water lakes and 

streams; improvements in visibility; and reduced risk 

to forests, materials, and structures. Table 5 on page 

20 shows the regional changes in key air quality and 

atmospheric deposition measurements linked to 

the ARP’s SO2

 and NOX emission reductions.

Understanding the Monitoring 

Networks

To evaluate the impact of emission reductions on 

the environment, scientists and policymakers use 

data collected from long-term national monitoring 

networks such as the Clean Air Status and Trends 

Network (CASTNET) and the National Atmospheric 

Deposition Program/National Trends Network (NADP/

NTN). These complementary, long-term monitoring 

networks provide information on a variety of indicators 

necessary for tracking temporal and spatial trends in 

regional air quality and acid deposition (see Table 6 on 

page 21).

CASTNET provides atmospheric data on the dry 

deposition component of total acid deposition, 

ground-level ozone, and other forms of atmospheric 

pollution. Established in 1987, CASTNET now 

consists of 87 sites across the United States. EPA’s 

Offi ce of Air and Radiation operates most of the 

monitoring stations; the National Park Service (NPS) 

funds and operates approximately 30 stations in 

cooperation with EPA. Many CASTNET sites have 

a continuous 20-year data record, refl ecting EPA’s 

commitment to long-term environmental monitoring. 

Information and data from CASTNET are available at 

<www.epa.gov/castnet>.

NADP/NTN is a nationwide, long-term network 

tracking the chemistry of precipitation. NADP/NTN 

offers data on hydrogen (acidity as pH), sulfate, 

nitrate, ammonium, chloride, and base cations. 

The network is a cooperative effort involving many 

groups, including the State Agricultural Experiment 

Stations, U.S. Geological Survey, U.S. Department 

of Agriculture, EPA, NPS, the National Oceanic and 

Atmospheric Administration (NOAA), and other 

governmental and private entities. NADP/NTN has 

grown from 22 stations at the end of 1978 to more 

than 250 sites spanning the continental United 

States, Alaska, Puerto Rico, and the Virgin Islands. 

Information and data from NADP/NTN are available at 

<http://nadp.sws.uiuc.edu>.

Status and Trends in Air Quality, Acid Deposition,

and Ecological Eff ects
20 Acid Rain and Related Programs: 2006 Progress Report

Measurement Region

Average

1989-1991

Average

2004-2006 Percent Change

Ambient SO2

Concentration (μg/m3

)

Mid-Atlantic 12.2 7.5 -39

Midwest 10.0 5.1 -49

Northeast 6.7 2.8 -58

Southeast 5.2 3.3 -37

Wet Sulfate 

Concentration (mg/L)

Mid-Atlantic 2.3 1.6 -31

Midwest 2.2 1.5 -33

Northeast 1.9 1.1 -40

Southeast 1.3 1.1 -18

Ambient Sulfate 

Concentration (μg/m3

)

Mid-Atlantic 6.2 4.4 -29

Midwest 5.4 3.6 -34

Northeast 3.8 2.4 -37

Southeast 5.4 4.1 -24

Wet Sulfate

Deposition (kg/ha)

Mid-Atlantic 26.8 19.2 -28

Midwest 22.3 14.9 -33

Northeast 22.2 14.5 -35

Southeast 18.1 14.3 -21*

Wet Inorganic 

Nitrogen Deposition

(kg/ha)

Mid-Atlantic 5.9 5.0 -16*

Midwest 5.9 5.4 -9*

Northeast 5.5 4.1 -25*

Southeast 4.3 4.1 -5*

Wet Nitrate 

Concentration (mg/L)

Mid-Atlantic 1.5 1.0 -30

Midwest 1.5 1.2 -17

Northeast 1.4 0.8 -38

Southeast 0.8 0.7 -10

Ambient Nitrate 

Concentration (μg/m3

)

Mid-Atlantic 0.8 0.7 -1*

Midwest 2.1 1.7 -18

Northeast 0.4 0.4 3*

Southeast 0.6 0.7 17

Total Ambient Nitrate 

Concentration (Nitrate 

+ Nitric Acid) (μg/m3

)

Mid-Atlantic 3.2 2.5 -20*

Midwest 4.0 3.2 -20*

Northeast 1.9 1.4 -26*

Southeast 2.2 2.0 -7*

Table 5: Regional Changes in Air Quality and Deposition of Sulfur and Nitrogen, 

1989-1991 versus 2004-2006 (From Rural Monitoring Networks)

*Percent change is estimated from raw measurement data, not rounded; refi ned techniques for measuring and calculating percentages yield values that are at or below the sensitivity of the method may not be 

signifi cant due to the combination of margin of error and spatial averaging.

Source: CASTNET and NADP/NTN, 2007

While CASTNET provides ambient air quality data, 

EPA also uses data from other ambient monitoring 

networks, including the State and Local Ambient 

Monitoring and National Ambient Monitoring 

Systems (SLAMS/NAMS). These networks are used to 

document National Ambient Air Quality Standards 

(NAAQS) attainment and show trends in ambient air 

quality over time.
Acid Rain and Related Programs: 2006 Progress Report 21

Chemical 

Name

Chemical 

Symbol

Measured In:

Why are these measured by the networks? Ambient 

Air

Wet 

Deposition

Sulfur Dioxide SO2 X Primary precursor of wet and dry acid deposition; primary precursor to 

fi ne particles in many regions.

Sulfate Ion SO4

2- X X

Major contributor to wet acid deposition; major component of fi ne 

particles in the Midwest and East; can be transported over large 

distances; formed from reaction of SO2

 in the atmosphere.

Nitrate Ion NO3

- X X Contributor to acid and nitrogen wet deposition; major component of fi ne 

particles in urban areas; formed from reaction of NOX

 in the atmosphere.

Nitric Acid HNO3 X Strong acid and major component of dry nitrogen deposition; formed as 

a secondary product from NOX

 in the atmosphere.

Ammonium 

Ion NH4

+ X X

Contributor to wet and dry nitrogen deposition; major component of 

fi ne particles; provides neutralizing role for acidic compounds; formed 

from ammonia gas in the atmosphere.

Ionic 

Hydrogen H+ X Indicator of acidity in precipitation; formed from the reaction of sulfate 

and nitrate in water.

Calcium 

Magnesium 

Potassium 

Sodium 

Ca2

+

Mg2

+

K+

Na+

X

X

X

X

X

X

X

X

These base cations neutralize acidic compounds in precipitation and 

the environment; also play a major role in plant nutrition and soil 

productivity.

Table 6: Air Quality and Acid Deposition Measures

Source: EPA, 2007
22 Acid Rain and Related Programs: 2006 Progress Report

Accountability and the ARP: Assessing Ecological Response

To determine the effectiveness of the ARP, EPA must be able 

to track, assess, and report on the trends and conditions in 

the environment as they respond to program implementation 

and emission reductions.

The data collected through air monitoring networks, such 

as CASTNET and NADP, provide a picture of changes in 

air quality and pollutant deposition as a result of emission 

reductions achieved by the ARP (see Figure 11). However, 

to assess the ecological impact of the ARP, EPA must go a 

step further and look at how changes in pollutant deposition 

correspond to changes in ecosystem conditions. This step 

requires examining the trends and conditions of ecosystems 

that are susceptible to changes in pollutant deposition and, 

specifi cally, studying a few key ecological indicators that can 

be used to represent ecosystem response and recovery.

EPA’s Temporally Integrated Monitoring of Ecosystems (TIME) 

and Long-Term Monitoring (LTM) programs are designed to 

detect trends in the chemistry of lakes or streams in regions 

sensitive to acid deposition. TIME/LTM monitors a total of 

145 lakes and 147 streams, representing all of the major acid 

sensitive regions of the northern and eastern United States 

(New England; Adirondack Mountains; northern Appalachian 

Plateau, including the Catskill Mountains; and the Ridge/Blue 

Ridge Provinces of Virginia) (see Figure 11). TIME/LTM measures 

a variety of important chemical characteristics, including acid 

neutralizing capacity (ANC), pH, sulfate, nitrate, major cations 

(e.g., calcium and magnesium), and aluminum. The TIME 

program is the most coherent individual regional dataset for 

this kind of analysis. In addition, the U.S. Geological Survey 

has been measuring surface water quality at several research 

watersheds throughout the United States, where sample 

collection during hydrologic events and ancillary data on 

other watershed characteristics have been used to assess the 

watershed processes controlling acidifi cation of surface waters.

To determine whether decreased emissions have had the 

intended effect of reducing impacts of acid deposition on 

ecosystems, EPA links emission trends with data from the 

CASTNET and NADP networks and the TIME/LTM programs. 

Combining these links in the “chain of accountability” 

allows EPA to determine whether emission reductions, and 

consequent reductions in pollutant deposition, translate into 

ecological response (i.e., changes in surface water quality 

necessary to protect fi sh and other aquatic organisms). This 

integration of data enables EPA to assess the effectiveness 

of the ARP in meeting its goal of protecting ecosystems by 

reducing the adverse effects of acid deposition.

Without long-term monitoring of atmospheric deposition and 

lake and stream chemistry, EPA would not be able to assess 

the ecological response to the emission reductions achieved 

by the ARP. Such monitoring networks are critical for tracking 

the progress made in restoring and/or protecting sensitive 

ecosystems under regulatory programs and informing future 

policy decisions.

Reports to Congress and Public 

 How do emission reductions correlate 

 with decreases in deposition? 

•

Are surface waters recovering?

•

•

 Are we meeting the goals?

• Are additional reductions necessary? 

 

ARP 

Development & Implementation 

Accountability/Monitoring

Total Acid Deposition

• Spatial Patterns

• Long-term Trends

Surface Water Monitoring National Deposition Monitoring 

CAAA Title IV Emission

Reductions

Figure 11: National Deposition and Surface Water Monitoring Sites

Source: EPA, 2007
Acid Rain and Related Programs: 2006 Progress Report 23

Enhancing Mercury Monitoring 

Capabilities

In March 2005, EPA promulgated CAMR to reduce 

mercury emissions from power plants by 2010. In 

conjunction with CAIR, mercury emission reductions 

under CAMR are expected to reduce atmospheric 

concentrations and deposition of mercury. Cutting 

mercury emissions would also translate to reductions 

in methylmercury contamination in fi sh, particularly 

in mercury-sensitive watersheds throughout many 

parts of the United States. This reduction would 

improve the health of people and wildlife that consume fi sh from these waters. These reductions will be 

achieved through implementation of a combination 

of independently operated state programs plus an 

interstate emission cap and trade program modeled 

after the ARP.

In order to assess the effi cacy of these emerging 

regulatory programs, scientifi c information is needed, 

including a more complete understanding of the fate 

of mercury emissions with respect to total (i.e., wet 

and dry) deposition, a quantifi able assessment of the 

sources contributing to mercury deposition (especially 

coal-fi red power plant emissions), and an assessment 

as to whether or not mercury “hotspots” exist or 

may develop over the course of implementing the 

mercury rules. At present, EPA lacks the ambient 

mercury concentration and deposition data, 

particularly on dry deposition in source-impacted 

areas, to adequately assess the atmospheric mercury 

changes anticipated from the regulatory programs. 

In addition, mercury atmospheric data are needed to 

evaluate and improve mercury model estimates and 

to facilitate source apportionment analyses. 

To meet some of these data needs, EPA is 

collaborating with NADP, as well as other federal 

agencies, states, tribes, academic institutions, 

industry, and other organizations to establish 

a new, coordinated network for monitoring 

atmospheric mercury species. The network will 

measure air concentrations of mercury in its gaseous 

and particulate forms, event-based mercury wet 

deposition, and meteorological and land-cover 

variables needed for estimating dry deposition fl uxes. 

When fully implemented, the network will serve 

many functions, including:

• Facilitate the calculation of wet, dry, and total 

mercury deposition.

• Provide data for evaluating predictive and 

diagnostic models and for assessing sourcereceptor relationships.

• Build a data set for analyzing spatial and temporal 

trends of ambient mercury concentrations and wet 

deposition in selected locations.

The network will consist of monitoring stations with 

a broad range of classifi cations, including rural, 

suburban, and urban; near-source/high-emissions; 

sensitive ecosystems; and regionally representative. 

Stations will adopt standard operational procedures 

based on methods developed from EPA and other 

research efforts. Data will be quality-assured and 

accessible online. For more information about this 

effort, please visit the NADP mercury initiative Web 

site at <http://nadp.sws.uiuc.edu/mtn/>.
24 Acid Rain and Related Programs: 2006 Progress Report

Emerging Issues: Remote 

Assessment Methods

Satellite observations and other remote sensing technologies 

are emerging as potentially useful new techniques for 

understanding atmospheric chemistry and analyzing changes 

in atmospheric pollutant concentrations. For example, the 

NASA EOS Aura orbiting satellite platform launched in 

2004 includes the Ozone Monitoring Instrument (OMI).7

Researchers have developed an algorithm using the output 

of this instrument to observe from space the signal of SO2

gases in the atmosphere near the ground. Using weekly, 

monthly, or annual average SO2

 concentrations observed by 

this instrument, maps of degassing and air pollution stationary 

sources can be generated. SO2

 emissions have been measured 

by OMI over known sources of air pollution, such as the Ohio 

River Valley (see Figure 12).

A wealth of data, such as information used in a recent 

NOAA report on NO2

 observations,8 as well as the 

emerging data in the scientifi c literature on a number 

of air pollutants, is currently available from satellite 

observation,9

 and the potential of these sources for 

analyzing atmospheric chemistry and changes in pollutant 

emissions provides exciting new opportunities for program 

assessments.

Figure 12: Two-Year Average SO2

Burdens over the Ohio River Valley

Air Quality 

SO2

Data collected from monitoring networks show 

that the decline in SO2

 emissions from the power 

industry has improved air quality.10 Based on EPA’s 

latest air emission trends data located at <www.

epa.gov/airtrends/index.html>, the national 

composite average of SO2

 annual mean ambient 

concentrations decreased 53 percent between 

1990 and 2006 as shown in Figure 13. The largest 

single-year reduction (21 percent) occurred in the 

fi rst year of the ARP, between 1994 and 1995. 

These trends are consistent with the ambient trends 

observed in the CASTNET network. During the 

late 1990s, following implementation of Phase I of 

the ARP, dramatic regional improvements in SO2

and ambient sulfate concentrations were observed 

at CASTNET sites throughout the eastern United 

States, due to the large reductions in SO2

 emissions 

from ARP sources. Analyses of regional monitoring 

data from CASTNET show the geographic pattern 

of SO2

 and airborne sulfate in the eastern United 

States. Three-year mean annual concentrations 

of SO2

 and sulfate from CASTNET long-term 

monitoring sites are compared from 1989-1991 

and 2004-2006 in both tabular form and graphically in maps (see Table 5 on page 20 and Figures 

17a through 18b on page 28). 

The map in Figure 17a shows that from 1989-1991, 

prior to implementation of Phase I of the ARP, 

the highest ambient concentrations of SO2

 in the 

East were observed in western Pennsylvania and 

along the Ohio River Valley. Figure 17b indicates a 

signifi cant decline in those concentrations in nearly 

all affected areas after implementation of the ARP 

and other programs.

Before the ARP, in 1989-1991, the highest ambient 

sulfate concentrations, greater than 7 micrograms 

per cubic meter (μg/m3

), were also observed in 
Acid Rain and Related Programs: 2006 Progress Report 25

western Pennsylvania, along the Ohio River Valley, 

and in northern Alabama. Most of the eastern 

United States experienced annual ambient sulfate 

concentrations greater than 5 μg/m3

. Like SO2

concentrations, ambient sulfate concentrations have 

decreased since the program was implemented, with 

average concentrations decreasing 35 percent in all 

regions of the East (see Table 7 on page 27). Both 

the size of the affected region and magnitude of the 

highest concentrations have dramatically declined, 

with the largest decreases observed along the Ohio 

River Valley (see Figures 18a and 18b on page 28).

1990

0.04

National Ambient Air Quality Standard

0.03

0.02

0.01

0.00

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

SO2 Ambient Concentration, ppm

10% of sites have concentrations below this line.

AVERAGE

10% of sites have concentrations above this line.

Figure 13: National SO2 Air Quality, 1990-2006 (Based on Annual Arithmetic 

Average)

Source: EPA, 2007
26 Acid Rain and Related Programs: 2006 Progress Report

Emerging Issues: Trends in Sulfate 

Concentrations

Since SO2 is a precursor to the formation of sulfate, reductions 

in SO2

 emissions under the ARP were expected to translate into 

similar reductions in sulfate. Although there is an observed 

downward trend in the ambient concentration of sulfate since 

the implementation of the ARP, these reductions have not been 

as dramatic as those observed for SO2

 emissions and ambient 

SO2

 concentrations. 

The ARP was established to reduce emissions of the two key 

contributors to the formation of acid deposition, SO2

 and NOX. 

As discussed earlier, SO2

 and NOX emissions can react in the 

atmosphere to form fi ne particulates which are harmful to the 

human respiratory system and damaging to sensitive ecosystems. 

Sulfate particles are formed after gaseous SO2

 is emitted and 

oxidized by hydroxyl radical ions. Sulfate particles can then 

be deposited on the surface (dry deposition) or the particles 

can react with H2

02 or O3 in clouds or fog to form sulfuric acid 

(H2

SO4). Sulfuric acid in wet deposition is known as acid rain.

In order to assess environmental results of the ARP, air quality 

monitoring networks, such as CASTNET, were established to 

measure ambient concentrations of SO2

 and sulfate. These data, 

combined with SO2

 emission data from CEMS at ARP-affected 

sources, provide an idea of how emission reductions under the 

ARP are translating into reductions in acid deposition over time 

(see Figures 14 through 16 and Table 7). 

Pre CAAA: 1985

Pre-ARP: 1990

Phase I: 1996-1999

Phase II: 2003-2006

Regional SO2 Emissions (tons)

from CEMS Measurements

6,400 tons

Figure 14: Trends in Regional Annual SO2 

Emissions (Coal-fi red, Acid Rain Program Units)

Pre CAAA: 1987-1990

Pre-ARP: 1993-1994

Phase I: 1996-1999

Phase II: 2003-2006

Regional SO2

Concentrations (ug/m3)

from CASTNET Measurements

7 ug/m3

Figure 15: Trends in Regional SO2 Ambient 

Concentrations (CASTNET Sites)

Source: EPA, 2007 Source: EPA, 2007
Acid Rain and Related Programs: 2006 Progress Report 27

Further study is necessary to understand the cause of annual 

changes in ambient sulfate concentrations. A study of 

meteorological data and multiple species data, such as ozone 

and ammonia ion concentrations, might provide insight into 

the factors infl uencing the rate of sulfate formation and what, 

if any, additional sources or pollutants might be responsible for 

the trends in sulfate concentrations in air quality. 

Region

Change in SO2 Emissions Change in SO2 Concentration Change in Sulfate 

Concentration

1985*

versus

2003-2006

1987-1990

versus

2003-2006

1987-1990 

versus

2003-2006

Northeast -79% -64% -43%

Midwest -68% -50% -38%

Mid-Atlantic -58% -44% -33%

Southeast -66% -40% -29%

Total Change in East -67% -50% -35%

Table 7: Summary of Regional Trends Data (SO2 Emissions and Ambient SO2

and Sulfate Concentrations)

* Data are not available for 1987-1989; therefore, 1985 is used for this comparison. 

Source: EPA, 2007; CASTNET, 2007

Pre CAAA: 1987-1990

Pre-ARP: 1993-1994

Phase I: 1996-1999

Phase II: 2003-2006

Average Regional SO4

Concentrations (ug/m3)

from CASTNET Measurements

7 ug/m3

Figure 16: Trends in Regional Ambient 

Sulfate Concentrations (CASTNET Sites)

Source: EPA, 2007
28 Acid Rain and Related Programs: 2006 Progress Report

Source: CASTNET

Figure 17a: Annual Mean Ambient SO2

Concentration, 1989-1991 

Source: CASTNET

Figure 17b: Annual Mean Ambient SO2

Concentration, 2004-2006 

Source: CASTNET

Figure 18a: Annual Mean Ambient Sulfate 

Concentration, 1989-1991 

Source: CASTNET

Figure 18b: Annual Mean Ambient Sulfate 

Concentration, 2004-2006

Note: For maps depicting these trends for the entire continental United States, see maps available at <www.epa.gov/castnet/mapindex.html>.
Acid Rain and Related Programs: 2006 Progress Report 29

NOX

Although the ARP has met its NOX emission 

reduction targets, emissions from other sources 

(such as motor vehicles and agriculture) have led to 

increased ambient nitrate concentrations in some 

areas. NOX levels can also be affected by emissions 

transported via air currents over wide regions.11 From 

2004 through 2006, reduced ozone season NOX

emissions from power plants under the NOX SIP Call 

led to more signifi cant region-specifi c improvements 

in some indicators than have been seen in previous 

years. For instance, annual mean ambient nitrate 

concentrations for 2004-2006 decreased in the 

Midwest by nearly 20 percent from the annual 

mean concentration in 1989-1991 (see Figures 

19a and 19b). While these improvements may be 

partly attributed to added NOX controls installed for 

compliance with the NOX SIP Call, the fi ndings at this 

time are not conclusive.

Source: CASTNET

Figure 19a: Annual Mean Total Nitrate 

Ambient Concentration, 1989-1991 

Source: CASTNET

Figure 19b: Annual Mean Total Nitrate 

Ambient Concentration, 2004-2006 

Note: For maps depicting these trends for the entire continental United States, see maps available at <www.epa.gov/castnet/mapindex.html>.
30 Acid Rain and Related Programs: 2006 Progress Report

NADP/NTN monitoring data show signifi cant 

improvements in some deposition indicators. For 

example, wet sulfate deposition (sulfate that falls 

to the earth through rain, snow, and fog) has 

decreased since the implementation of the ARP in 

much of the Ohio River Valley and northeastern 

United States. Some of the greatest reductions 

have occurred in the mid-Appalachian region, 

including Maryland, New York, West Virginia, 

Virginia, and most of Pennsylvania. Other less 

dramatic reductions have been observed across 

much of New England, portions of the southern 

Appalachian Mountains, and some areas of the 

Midwest. Between the 1989-1991 and 2004-2006 

observation periods, average decreases in wet 

deposition of sulfate averaged around 30 percent 

for the eastern United States (see Table 5 on page 

20 and Figures 20a and 20b). Along with wet 

sulfate deposition, wet sulfate concentrations have 

also decreased signifi cantly. Since the 1989-1991 

period, average levels decreased 40 percent in the 

Northeast, 31 percent in the Mid-Atlantic, and 

33 percent in the Midwest. A strong correlation 

between large-scale SO2

 emission reductions 

and large reductions in sulfate concentrations in 

precipitation has been noted in the Northeast, one 

of the areas most affected by acid deposition.

Source: NADP

Figure 20a: Annual Mean Wet Sulfate 

Deposition, 1989-1991 

Source: NADP

Figure 20b: Annual Mean Wet Sulfate 

Deposition, 2004-2006

Acid Deposition

Note: For maps depicting these data for the entire continental United States, 

see maps available at <www.epa.gov/castnet/mapindex.html>.
Acid Rain and Related Programs: 2006 Progress Report 31

A principal reason for reduced concentrations of 

sulfate in precipitation in the Northeast is a reduction 

in the long-range transport of sulfate from emission 

sources located in the Ohio River Valley. The reductions 

in sulfate documented in the Northeast, particularly 

across New England and portions of New York, were 

also affected by SO2

 emission reductions in eastern 

Canada. NADP data indicate that similar reductions in 

precipitation acidity, expressed as hydrogen ion (H+) 

concentrations, occurred concurrently with sulfate 

reductions, but have not decreased as dramatically 

due to a simultaneous decline in acid-neutralizing 

base cations, which act to buffer acidity.

Reductions in nitrogen deposition recorded since the 

early 1990s have been less pronounced than those 

for sulfur. As noted earlier, emission trends from 

source categories other than ARP sources signifi cantly 

affect air concentrations and deposition of nitrogen. 

Inorganic nitrogen deposition decreased modestly in 

the Mid-Atlantic and Northeast but remained virtually 

unchanged in other regions (see Figures 21a and 21b).

Source: NADP

Figure 21a: Annual Mean Wet Inorganic 

Nitrogen Deposition, 1989-1991

Source: NADP

Figure 21b: Annual Mean Wet Inorganic 

Nitrogen Deposition, 2004-2006

Note: For maps depicting these data for the entire continental United States, 

see maps available at <www.epa.gov/castnet/mapindex.html>.
32 Acid Rain and Related Programs: 2006 Progress Report

Emerging Issues: Using Critical Loads to Assess Ecosystem Health

A critical load is a quantitative estimate of the exposure to 

one or more pollutants below which signifi cant harmful 

effects on specifi c sensitive elements of the environment do 

not occur according to present knowledge.

-From the 1988 United Nations Economic Commission 

for Europe (UNECE) Protocol Concerning the Control 

of Nitrogen Oxides or Their Transboundary Fluxes. 

Accepted by the United States in July 1989.

Recommendations in separate reports of the National 

Research Council (NRC) and the federal Clean Air Act 

Advisory Committee (CAAAC) urge EPA to expand its 

ecosystem protection capacities by exploring issues such 

as the use of critical loads in the development of 

secondary NAAQS. 

The NRC formed a Committee on Air Quality Management 

to examine the role of science and technology in the 

implementation of the CAA and to recommend ways in 

which the scientifi c and technical foundations for air quality 

management in the United States can be enhanced. In its 

fi ndings and recommendations to EPA, the NRC Committee 

pointed out the need for alternative air quality standards to 

protect ecosystems and recommended investigating the use of 

critical loads as a potential mechanism to address this need.12

CAAAC echoed the recommendation to examine critical loads 

as a useful tool for ecosystem protection in its 2005 report 

to EPA.13

Critical loads provide a science-based tool for managers 

and policymakers to assess the progress made by federal 

air emission reduction programs, evaluate the impact of 

potential new emission sources in federally protected areas, 

and manage sensitive natural resources where air pollution 

and other disturbances occur. Critical loads were fi rst 

developed and applied in Europe to address the impacts 

of acid deposition associated with SO2

 and NOX emissions. 

The UNECE Convention on Long-Range Transboundary Air 

Pollution was signed in 1979. Critical loads were adopted 

in 1988 as part of the protocol process to address the 

effects of air pollution on ecosystems, human health, 

and cultural resources.

In North America, the concept of critical loads was applied 

in the 1960s with the fi rst Great Lakes Water Quality 

Agreement, which set lake phosphorus loading limits to 

reduce eutrophication. Canada established the fi rst critical 

load for air pollution in the 1980s (for wet sulfate deposition) 

as part of a U.S.-Canada memorandum on transboundary 

air pollution. Although the United States was a signatory to 

the memorandum, critical loads were not used in the United 

States until 1989, when the U.S. Forest Service applied the 

critical loads concept as a screening tool to protect air quality 

in Class I areas. 

Over the past fi ve years, there has been renewed interest 

in critical loads in the United States. Recent critical loads 

initiatives include the Conference of New England Governors 

and Eastern Canadian Premiers project to map forest 

sensitivity to sulfur and nitrogen deposition; the Federal 

Land Managers Air Quality Report, which articulated a 

commitment to fostering the development of critical loads; 

a series of meetings known as the “Riverside Meetings” 

convened by the U.S. Forest Service; and a 2006 multiagency Critical Loads Workshop.

For more information on EPA’s assessment-related activities, 

go to <www.epa.gov/airmarkets>. See also <http://nadp.

sws.uiuc.edu/clad/>.
Acid Rain and Related Programs: 2006 Progress Report 33

Acid rain, resulting from SO2 and NOX emissions, is 

one of many large-scale anthropogenic effects that 

negatively affect the health of lakes and streams in the 

United States. Since the implementation of the ARP, 

scientists have observed measurable improvements 

in some lakes and streams in four regions of the 

eastern United States—New England, the Adirondack 

Mountains, the northern Appalachians (including the 

Catskill Mountains), and the southern Appalachians 

(including the Blue Ridge)—and found signs of recovery 

in many, but not all, of those areas (see Figure 22).14

The long-term monitoring networks that exist in 

these regions provide information on the chemistry 

of lakes and streams, and a look at how water bodies 

are responding to changes in emissions. The data 

presented here show regional trends in acidifi cation 

from 1990 to 2005 (see Figure 22). For each lake 

or stream in the network, measurements of various 

indicators of recovery from acidifi cation were taken. 

These measurements were plotted against time, 

and trends for the given lake or stream during the 

15-year period were then calculated as the change in 

each of the measurements per year (e.g., change in 

concentration of sulfate per year). Using the trends 

calculated for each water body, median regional 

changes were determined for each of the measures 

of recovery. A negative value of the “slope of the 

regional trend” means that the measure has been 

declining in the region, while a positive value means 

it has been increasing. The greater the value of the 

trend, the greater the yearly change. Movement 

Improvements in Surface Water

No. Appalachian Streams (n=9)

So. Appalachian Streams (n=69)

-2.5 -2.0 -1.5 -1.0

Slope of Regional Trend 

Sulfate (μeq/L/yr)

Nitrate (μeq/L/yr)

ANC (μeq/L/yr)

Hydrogen Ion (μeq/L/yr)

Base Cations (μeq/L/yr)

New England Lakes (n=31)

Adirondack Lakes (n=49)

-0.5 0.0 0.5 1.0

Figure 22: Regional Trends in Eastern Lakes and Streams, 1990-2005

Note: Bars show the magnitude of the regional trend for each variable in each region.

Source: EPA, 2007
34 Acid Rain and Related Programs: 2006 Progress Report

toward recovery is indicated by positive trends in acid 

neutralizing capacity (ANC) and negative trends in 

sulfate, nitrate, hydrogen ion (measured in microequivalents per liter per year [μeq/L/yr]), and aluminum 

(measured in micrograms per liter per year [μg/L/

yr]). Negative trends in base cations (which are weak 

acid cations in soils, such as calcium, magnesium, 

and potassium) and positive trends in organic acids 

can balance out the decreasing trends in sulfate and 

nitrate and prevent ANC from increasing. The results 

of these regional trend analyses are shown in Figure 

22 on page 33 and Table 8.

Trends in surface water from 1990 to 2005 include:

• Sulfate concentrations are declining substantially 

in all but one of the regions. In the southern 

Appalachians, however, sulfate concentrations are 

increasing. This region is unusual because its soils 

can store large amounts of sulfate deposited from 

the atmosphere. Only after large amounts of sulfate 

have accumulated in the soils do stream water 

sulfate concentrations begin to increase, remaining 

elevated until the stored sulfur is depleted. This 

phenomenon is now being observed in the 

southern Appalachians, despite decreasing sulfate 

in atmospheric deposition. Still, due to inclusion 

of the latest data, the magnitude and direction of 

the trends in this region are substantially changed 

from the 2005 Acid Rain Program Progress Report.

Chemical Variable New England Lakes

(n = 31)

Adirondack Lakes 

(n = 49)

No. Appalachian 

Streams (n = 9)

So. Appalachian 

Streams (n = 69)

Sulfate (μeq/L/yr) -1.42 -2.07 -2.30 +0.09

Nitrate (μeq/L/yr) -0.03 -0.37 -0.31 -0.10

Acid Neutralizing Capacity 

(μeq/L/yr)

+0.15 +0.93 +0.80 +0.08

Base Cations (μeq/L/yr) -0.93 -1.19 -2.25 +0.17

Hydrogen Ion (μeq/L/yr) -0.01 -0.24 +0.01 -0.01

Organic Acids (μeq/L/yr) +0.04 +0.15 -0.06 insuffi cient data

Aluminum (μg/L/yr) insuffi cient data -4.72 insuffi cient data insuffi cient data

Table 8: Results of Regional Trend Analyses on Lakes and Streams, 1990-2005* 

* Values show the slope of the regional trend (the median value for the trends in all of the sites in the region). Regional trends that 

are statistically signifi cant are shown in bold.

Source: EPA, 2007
Acid Rain and Related Programs: 2006 Progress Report 35

Trend estimates using previous data (through the 

early 2000s) were heavily infl uenced by gypsy 

moth defoliation of trees in most of the region’s 

watersheds, particularly affecting trends in ANC, 

sulfate, and nitrate. As ecosystems have recovered 

from the impacts of this defoliation event, the 

extent of deposition impacts on this region appear 

less severe than in past years. 

• Nitrate concentrations are decreasing signifi cantly 

in all of the regions. This trend does not appear to 

refl ect changes in emissions or deposition in these 

areas and is likely a result of ecosystem factors.

• The acidity of lake and stream water, as indicated 

by ANC trends, is decreasing in three of the four 

regions as a result of declining sulfate (and to some 

extent nitrate).

• Base cations are decreasing in the northern 

Appalachians, Adirondack Lakes, and New England 

Lakes. This may be a concern because, although 

base cation concentrations in lakes and streams are 

expected to decrease when rates of atmospheric 

deposition decline, if they decrease too much, they 

limit recovery in pH and ANC. 

• Concentrations of organic acids, natural forms of 

acidity, are currently increasing in many parts of the 

world, but the cause is still being debated. Increases 

in organic acids over time can limit the amount 

of recovery observed. Only the New England and 

Adirondack regions are showing signifi cant increases 

in organic acids, which may be responsible for 10-15 

percent less recovery (in ANC) than expected.

• Aluminum is a critical element because it increases 

when lakes and streams acidify, and is very toxic to 

fi sh and other wildlife. The one region where good 

aluminum data exist (the Adirondacks) is showing 

strong declines in the most toxic form of aluminum 

(inorganic monomeric aluminum).
CAIR will reduce SO2

 emissions 

by approximately 70 percent and

NOX emissions by approximately 

60 percent from 2003 levels

Related Programs
Acid Rain and Related Programs: 2006 Progress Report 37

Rule Background 

Building on the ARP and NBP, EPA fi nalized CAIR 

in 2005, requiring further SO2

 and NOX emission 

reductions in 28 eastern states and the District of 

Columbia. CAIR will reduce region-wide SO2

 emissions 

by approximately 70 percent and NOX emissions 

by approximately 60 percent from 2003 levels to 

prevent signifi cant contribution to nonattainment in 

downwind ozone and PM2.5 nonattainment areas. 

CAIR includes emission budgets for each jurisdiction 

based on application of highly cost-effective controls 

to fossil fuel-fi red EGUs in model cap and trade 

programs with two phases of reductions. However, 

states have discretion in deciding which sources to 

control to meet the budget, and whether to participate in the federally run cap and trade programs 

delineated in the model rules.

Rules and Programs to Ensure Further Improvements

A combination of well-established, existing programs, and new 

regulations that will soon begin implementation, are in a position 

to address the interstate transport of ozone, fi ne particles, 

and mercury deposition. Together, these rules and programs 

will help ensure further improvements in human health and 

environmental protection. Along with the ARP, the NOX SIP Call 

in the eastern United States and the Tier 2 mobile source and 

diesel rules establish programs that will help states achieve the 

ozone and fi ne particle NAAQS. 

In the spring of 2005, EPA promulgated a suite of air quality 

rules designed to achieve additional reductions of SO2

, NOX, and 

mercury from power plants. These rules include CAIR, CAMR, 

and CAVR.15 See Figure 23 for a fl ow-chart showing how the 

power sector rules are connected. 

Figure 23: Key Clean Air Rules Related to Electric Power Industry

Source: EPA, 2007

Clean Air Act (CAA)

Title I Title IV

Section 110 - State

Implementation Plans

(SIPs)

Section 111(d)

(State Plans for Non-Criteria

Pollutants from Existing

Sources)

Sections 169A and

169B (Visibility SIPs)

CAVR

(2013/2014)

CAMR

(2010 – forward)

CAIR

(2009 – forward)

NOX Budget

Trading Program

(2003 – 2008)

Acid Rain Program

(1995 – forward)

Parts C and D

(New Source

Review)

Section 111 (New

Source Performance

Standards)

Understanding Clean Air Rules
38 Acid Rain and Related Programs: 2006 Progress Report

To address their contribution to unhealthy levels of 

fi ne particles in downwind states, CAIR requires 25 

eastern states and the District of Columbia to reduce 

and cap annual SO2

 and NOX emissions. 

In addition, CAIR requires 25 eastern states and the 

District of Columbia to reduce and cap ozone season 

(May through September) NOX emissions to address 

their contribution to unhealthy levels of 8-hour ozone. 

As shown in Figure 24, most states covered under 

CAIR are required to address contributions to both 

PM2.5 and ozone nonattainment, and therefore reduce 

annual SO2

 and NOX emissions as well as seasonal NOX

emissions. These reductions will help states attain the 

NAAQS for ozone and fi ne particles in most areas that 

were designated as being in nonattainment as of April 

2006 (see Figure 25). 

All the states covered under CAIR have chosen to 

participate in the trading programs for SO2

 and NOX. 

Some states also have direct control programs that 

complement the trading programs.

Generally, the CAIR model rules include fossil fuel-fi red 

boilers and combustion turbines serving an electric 

generator with a nameplate capacity greater than 

25 MW and producing electricity for sale. These are 

generally the same types of sources as covered under 

the ARP and NOX SIP Call. However, the universe 

of CAIR sources is somewhat more inclusive in two 

ways. First, CAIR affects some sources that either 

permanently (e.g., simple-cycle turbines and certain 

cogeneration units) or temporarily (e.g., independent 

power producers, or IPPs, with power-purchase 

agreements in effect) were exempt from the ARP. EPA 

included these units because they were designed and 

Figure 24: States Covered under CAIR/CAVR/CAMR for SO2, NOX, and Mercury

Source: EPA, 2007

All states, plus 2 tribes and the District of Columbia, are covered by CAMR.

CAIR Emission Caps*

(million tons)

2009/2010 2015

Annual SO2

 3.7 2.6

(2010)

Annual NOX 1.5 1.3

(2009)

Seasonal NOX 0.6 0.5

(2009)

 *for the affected regions

CAMR Annual Emission Caps*

(tons)

2010 2018

Mercury 38 15

CAVR

Outside of CAIR Region — BART
Acid Rain and Related Programs: 2006 Progress Report 39

operated to be in the business of producing electricity 

for sale and were part of the universe of sources that 

EPA demonstrated could reduce emissions in a highly 

cost-effective manner for purposes of CAIR.

Second, CAIR affects some power-generating sources 

that were not regulated under the NOX SIP Call because 

the CAIR defi nition of “fossil-fuel-fi red” is consistent 

with the defi nition used in the ARP (i.e., combusting 

any fossil fuel is considered “fossil fuel-fi red”). The NOX

SIP Call defi nition only considers a source to be “fossil 

fuel-fi red” if more than 50 percent of annual heat input 

results from combusting fossil fuels. 

The majority of the approximately 320 new sources 

expected to be affected under CAIR are simple-cycle 

combustion turbines outside the NOX SIP Call region 

that came online prior to 1991. Most of the others 

are IPP units or cogeneration units that were exempt 

from the ARP. Table 9 on page 40 delineates the 

expanding coverage of electricity generators from the 

ARP to CAIR and CAMR.

How the New Trading 

Programs Work

States had the choice of participating in the federal 

cap and trade programs to reduce SO2

 and NOX

emissions and all have elected to do so. The result is 

a larger seasonal NOX program beginning in 2009, a 

new annual NOX program beginning in 2009, and a 

new SO2

 program in the CAIR region with a tighter, 

regional cap in 2010. All three new CAIR programs 

require additional reductions in 2015 (See timeline in 

Figure 26 on page 41). States can either submit state 

plans for EPA approval or come under a federal plan 

that also serves as a backstop to enter the program. 

The state plans can use EPA’s model trading rules, with 

Legend

Both PM and Ozone 

Nonattainment 

PM Only 

Nonattainment 

Ozone Only 

Nonattainment 

Area 

Count

13

7

3

Nonattainment Areas 106

Projected to Attain

Figure 25: Projected Nonattainment Areas in 2020 After Reductions From CAIR, CAVR, 

and Clean Air Act Programs

Note: Figure 25 depicts 129 areas that, as of April 2006, were in nonattainment of the PM2.5 or ozone NAAQS (or both). As indicated in 

the legend, 106 of those areas are projected to attain the applicable NAAQS by 2020 as a result of existing programs, such as Title IV of 

the Clean Air Act, the NOX SIP Call, some existing state rules, and the addition of CAIR and CAVR. Note that the 23 areas that are forecast 

to remain in nonattainment may need to adopt additional local or regional controls to attain the NAAQS by the dates set pursuant to the 

CAA. These additional local or regional measures are not forecast in Figure 25, and therefore the fi gure overstates the extent of expected 

nonattainment in 2020.

Source: EPA, 2006
40 Acid Rain and Related Programs: 2006 Progress Report

state-specifi c approaches to allocating NOX allowances, 

allowing sources to opt-in, and including industrial 

sources that are subject to the NOX SIP Call trading 

program. The federal backstop program was published 

in May 2006 and went into effect in June 2006. 

Therefore, the regulated community has faced CAIR 

requirements that have been in effect since June 2006.

Under Title IV of the CAA, the ARP will continue to 

operate even after the new regional CAIR SO2

 trading 

program begins in 2010. (Title IV NOX requirements 

also remain unchanged under CAIR.) Sources will use 

Title IV SO2

 allowances to demonstrate compliance 

with annual CAIR requirements as well as with annual 

Title IV requirements. As a result, banked Title IV 

allowances can be used for CAIR compliance, and 

sources in all states subject to CAIR for SO2

 will be 

subject to two SO2

 trading programs that share the 

same currency. 

Under CAIR, however, one allowance does not always 

cover one ton of emissions. Instead, for purposes of 

CAIR, SO2

 allowances of vintage 2009 and earlier will 

each cover one ton of emissions; vintage 2010 - 2014 

allowances will authorize 0.50 tons of emissions; 

and vintage 2015 or later allowances will authorize 

0.35 tons of emissions. These ratios achieve the more 

stringent reductions required under CAIR, maintain the 

value of ARP allowances, and make ARP compliance a 

foregone conclusion with CAIR compliance. 

The NBP will cease to operate with the start of the 

seasonal NOX trading program under CAIR in 2009. 

Sources in most CAIR states will be subject to two 

separate CAIR NOX trading programs: an annual NOX

program for PM2.5 control and a seasonal NOX program 

for ozone control. However, these two programs will 

not share currency, as CAIR annual and ozone season 

NOX allowances are not interchangeable. 

EPA will provide NOX emission allowances to each state 

according to the state budget for each program. States 

covered by both programs will allocate both annual and 

seasonal allowances to sources (or other entities). 

ARP NBP

CAIR 

CAMR

PM2.5 Ozone

Annual SO2 Annual NOX Summer NOX

Annual NOX

and SO2

Summer NOX Annual Mercury

Basic 

Applicability

>25 MW >25 MW 

coal-fi red 

utility boilers 

that burned 

coal between 

1990 – 95

>25 MW (but 

OTC states may 

be >15 MW) and 

certain non-EGUs 

greater than 250 

mmBtu/hr

>25 MW >25 MW and 

certain nonEGUs greater 

than 250 

mmBtu/hr*

>25 MW, coal-fi red

Exceptions Certain IPPs, 

cogens, qualifying facilities, 

and simple 

cycle turbines 

Same as SO2 

plus some 

boiler types 

Certain cogens, 

plus units that 

burn less than 

50% fossil fuel

Certain cogens 

(different than 

NBP) 

Certain cogens 

(different than 

NBP)

Certain cogens 

(same as CAIR)

Geographic 

coverage

48 contiguous 

states + DC 

48 contiguous 

states + DC 

20 states + DC 25 states + DC 25 states + DC 50 states + DC 

+ 2 tribes

Table 9: Overview of Fossil Fuel-Fired Electricity Generators Covered under EPA’s 

Cap and Trade Programs

* States in the NBP can expand their CAIR NOX ozone season program applicability to include non-EGUs in the NBP.

Source: EPA, 2007
Acid Rain and Related Programs: 2006 Progress Report 41

The CAIR seasonal NOX program allows the use of 

banked allowances from the NOX SIP Call, just as 

the CAIR SO2

 program allows the use of banked 

allowances from the ARP. The annual NOX trading 

program includes a limited compliance supplement 

pool of allowances to be awarded for early reductions 

in 2007 and 2008, or to address issues of reliability of 

electricity supply in 2009.

The structure of the CAIR programs and, in particular, 

the provisions allowing use of banked allowances from 

ARP and NBP, exemplify EPA’s effort to ensure an orderly 

transition to CAIR’s trading programs and strongly 

encourage early reductions. There is a substantial 

incentive for sources to begin complying with CAIR 

immediately, and emissions already have dropped 

as a result. 

CAIR Allowance Market and 

State Activity

Although there will be two distinct markets, EPA 

expects that the prices in both the annual and seasonal 

markets will be established by the cost of controls for 

annual compliance. There has been trading activity 

in the 2009 seasonal NOX market and limited annual 

NOX CAIR market trading. Observers expect that 

active trading will not occur until CAIR SIPs have been 

approved and NOX allowance accounts are populated 

later this year.

For both the SO2

 and NOX markets, it will take 

time for buyers and sellers to continue to assess 

the fundamentals of the changes introduced by 

CAIR, but this is secondary to the achievement of 

Phase I: CAIR NOx Programs 

(ozone-season and annual)

(09)

Phase I: CAIR SO2 Program 

(10)

States develop SPs

(18 months)

SIPs due

(Sep 06)

Early Emission Reduction Period 

(annual CAIR NOx program)

(07 and 08)

Phase II: CAIR NOx and 

SO2 Programs Begin

(15)

Early reductions for CAIR NOx ozone-season

program and CAIR SO2 program begin

immediately because NOx SIP Call and Title IV 

allowances can be banked into CAIR.

Note: Dotted lines indicate a range of time. 

05 06 07 08 09 10 12 13 14 11 15

CAMR 

signed

16 17 18

CAIR

CAMR and CAVR

CAIR

signed

SPs Due

(Nov 06)

Phase I: CAMR Hg Program 

(10)

Phase II: CAMR Hg Program 

Regional Haze (RH) (18)

SIPs Due (Dec 08) CAVR BART Controls Required 

(5 years after RH SIPs approved)

CAVR

signed

FIP (June 06)

Figure 26: Timeline for Implementation of CAIR/CAMR/CAVR (2005-2018)

Note: During the CAIR annual NOX program early emission reduction period, owners may earn additional allowances available through a 

compliance supplement pool established under EPA’s CAIR rulemaking.

Source: EPA, 2007
42 Acid Rain and Related Programs: 2006 Progress Report

the environmental accountability and results of the 

program. CAIR required covered states to submit SIPs to 

EPA by September 2006. The agency also promulgated 

a federal implementation plan (FIP) that implements the 

model trading rules for every CAIR state and offered to 

leave it in place for states not wishing to submit a SIP. 

EPA expects all 29 affected jurisdictions to participate in 

the EPA-run trading programs.

States have some fl exibility in participating in the 

trading programs, including determining NOX allowance 

allocations independently. Nearly all states submitting 

SIPs thus far have established their own allocation 

methodologies, often including special set-asides 

for new sources and for various state priorities, like 

renewable energy or add-on emission controls. In some 

cases, states roll any unclaimed set-aside allowances 

back into the main allowance pool; others hold them 

over for possible distribution in the future.

States may also choose to allow participation by nonEGUs from the NBP and can allow other units to opt-in 

using methodologies in the CAIR model rules. Of the 

19 states plus the District of Columbia that are subject 

to both the NOX SIP Call and CAIR (note that Rhode 

Island was included in the former, but not the latter), all 

but fi ve have indicated they will include the NBP’s nonEGUs in the CAIR NOX ozone season program. Most 

states thus far have chosen not to include the model 

rule provisions that allow sources to opt-in. 

Whether sources in a state are subject to a SIP or a 

FIP, there will be initiation of the allocation of NOX

allowances under CAIR by the end of this year (SO2

allowances already have been allocated under Title IV). 

Installed by

2006 Installed by

2006 Installed by

2006

0

50,000

100,000

150,000

200,000

250,000

300,000

Scrubbers SCRs Scrubbers + SCRs

Gigawatts

Projected 2010

Projected 2010

Projected 2010

Projected 2020

Projected 2020

Committed

2007-2010

Committed 

2007-2010

Committed

2007-2010 

Projected 2020

69 units

180 units

111 units

247 units

209 units

83 units

37 units

179 units

121 units

16 units

27 units

133 units

Figure 27: Current and Projected SO2 Scrubber and NOX SCR Controls on

Coal-fi red EGUs

Note: Existing, committed, and projected controls are due to existing programs as well as CAIR, CAMR, and CAVR.

Source: EPA, for IPM, 2007
Acid Rain and Related Programs: 2006 Progress Report 43

Figure 27 shows advanced SO2 and NOX controls 

already in place in the CAIR region, as well as those 

controls that facilities have already committed to 

install or that are projected under CAIR with additional 

consideration of CAMR and CAVR requirements. 

CAMR

CAMR requires all 50 states, the District of Columbia, 

and two tribes to regulate mercury emissions from 

coal-fi red EGUs. CAMR establishes “standards of 

performance” limiting mercury emissions from new 

and existing coal-fi red power plants and, like CAIR, 

creates a model cap and trade program with two 

phases of reductions. The fi rst phase cap is 38 tons, 

taking advantage of “co-benefi t” reductions—mercury 

reductions achieved by reducing SO2

 and NOX emissions 

under CAIR—to fulfi ll EPA’s requirement to act on 

mercury emissions. The second phase, beginning in 

2018, goes further to reduce emissions to 15 tons upon 

full implementation. CAMR sets an emission reduction 

requirement in the form of an annual budget for each 

state and two tribes in accordance with the two caps.

New coal-fi red power plants will have to meet new 

source performance standards in addition to being 

subject to the caps. EPA established annual budgets 

for each state, and states must ensure that current 

and future mercury emissions from coal-fi red EGUs do 

not exceed the annual state budget. Like CAIR, CAMR 

does not exempt the units that may be exempt under 

the ARP. The summary of applicability across programs 

in Table 9 on page 40 includes general CAMR 

applicability for comparison.

Furthermore, under CAMR, affected coal-fi red 

electric utility units will be required to continuously 

monitor mercury mass emissions for the fi rst 

time, regardless of whether or not they will be 

participating in the trading program. Monitoring 

technologies will be subject to rigorous certifi cation 

and quality assurance/quality control requirements 

under 40 CFR Part 75. Affected sources are required 

to install and certify continuous emissions or sorbent 

trap monitoring systems by January 1, 2009.

This new requirement is one of the primary areas 

of focus for EPA’s CAMR implementation efforts. 

Recent work by both EPA and industry has advanced 

mercury monitoring systems, reference testing 

methods, and calibration standards to a point that 

measuring capabilities that had limited feasibility 

a few years ago now are fully or nearly ready and 

even commercially available. Over the past year, the 

performance and reliability of mercury monitoring 

systems have substantially improved as a result of 

fi eld demonstrations and testing by EPA and industry. 

EPA continues to work closely with the regulated 

community, monitoring equipment and software 

vendors, academia, and other organizations to ensure 

timely implementation of a technically sound, effective 

CAMR mercury monitoring program. 

The Mercury Trading Program 

and State Activity 

The trading program under CAMR will work similarly 

to existing programs and the SO2

 and NOX programs 

under CAIR, with two notable differences between the 

CAMR and CAIR trading programs. First, there are no 

opt-in provisions included in CAMR; second, allowances 

under CAMR are measured in ounces rather than tons.

Even with some states choosing to control mercury 

emissions directly, EPA expects a robust trading 

program. In July 2006, EPA conducted limited modeling 

meant to be illustrative of a reduced market based on 

the states and tribes EPA projected would participate 

in the national trading program at the time. This 

more limited market represents states that allocated 

close to 69 percent of the initial budget of mercury 

allowances and comprises more than 700 units. These 

units represent more than 200 GW of capacity—nearly 

equivalent to the number of coal-fi red units in the 

successful NBP across a larger number of states. As with 

the NBP, EPA expects a viable market will result. 
44 Acid Rain and Related Programs: 2006 Progress Report

Based on this modeling, prices for mercury allowances 

are expected to be the same as or lower than prices 

in a full national market. This is because several states 

that would have required relatively large amounts 

of mercury allowances to comply with CAMR, such 

as Illinois, will not be participating. Overall, states 

opting to participate in the trading program generally 

are characterized by larger percentages of coal-fi red 

generation. Moreover, 21 states submitted state 

plans to EPA by the November 2006 CAMR deadline, 

and additional state plans have been received since. 

The remaining states are actively working on plans. 

Of all affected jurisdictions, 35 states and two tribes 

are planning on participating in the CAMR trading 

program. Twelve states have indicated they will not 

participate in the trading program, and at least one 

state is still undecided. Three states (Idaho, Vermont, 

and Rhode Island) and the District of Columbia do not 

have any coal-fi red EGUs and thus have zero budgets. 

States not participating in the trading program 

must ensure they meet their state budget with 

other methods. Alternatives often involve control 

requirements based on percent reduction provisions 

determined through an analysis of control options 

that states have evaluated as feasible. Some states 

have chosen to do this in phases like EPA has, though 

the start of the second phase is accelerated in some 

cases. Unlike a capped program, percent reduction 

programs do not necessarily guarantee emissions will 

remain below a state’s budget, often because of the 

uncertainty of new source growth over time. Therefore, 

states are often coupling these programs with caps to 

ensure the state’s budget will be maintained.

Where mercury trading programs are enacted, EPA 

expects emission controls to exceed requirements in 

2010. This is because sources are likely to optimize the 

controls installed for CAIR to reduce as much mercury 

as possible in anticipation of increasing prices for 

mercury allowances under the lower second phase cap. 

In December 2006, EPA proposed a CAMR federal 

plan to be fi nalized in states that either fail to submit 

a CAMR state plan or whose state plan is somehow 

defi cient. EPA is evaluating comments and plans to 

fi nalize the CAMR federal plan by the end of 2007 

and have it go into effect in the fi rst half of 2008. The 

plan puts into place a cap and trade program in any 

state in which the federal plan is fi nalized and contains 

provisions to create trading programs for states or 

regions that could emerge in the future.

CAVR

CAVR supplements the emission reductions of 

CAIR by requiring emission controls known as best 

available retrofi t technology (BART) for industrial 

facilities emitting air pollutants that reduce visibility 

by contributing to regional haze in national parks 

and wilderness areas. For the electric power industry, 

CAVR applies outside of the states covered by CAIR. 

For all other industries, it is a nationwide program. The 

pollutants include PM2.5 and its precursors, such as 

SO2

, NOX, volatile organic compounds, and ammonia. 

The BART requirements apply to facilities built 
Acid Rain and Related Programs: 2006 Progress Report 45

Source: EPA, 2007

Figure 28: Projected Coal-fi red EGU Retrofi ts with CAIR/CAMR/CAVR by 2020*

between 1962 and 1977 that have the potential to 

emit more than 250 tons a year of visibility-impairing 

pollution. The requirements cover 26 categories, 

including utility and industrial boilers and large 

industrial plants such as pulp mills, refi neries, 

and smelters. 

Many of these facilities have not been subject to federal 

pollution control requirements for these pollutants. 

Under the 1999 regional haze rule, states are required 

to set periodic goals for improving visibility in the 156 

“Class I” natural areas, including national parks. CAVR 

includes guidelines for states to use in determining 

which facilities must install controls and the type of 

controls the facilities must use. States must develop 

their implementation plans by December 2007, identify 

the facilities that will have to reduce emissions under 

BART, set emission limits for those facilities, and require 

installation of BART in 2014.
46 Acid Rain and Related Programs: 2006 Progress Report

Simplified Monthly Transfer Volume and Price, 2000-2007

$200

$400

$600

$800

$1,000

$1,200

$1,400

$1,600

$1,800

DecJun-2007

2006

Jun-2006

JunDec-2005

2005

Dec-2004

Dec-200

Jun-2002

Dec-2002

Jun-2003

Dec-2003

Jun-2004

1

Jun-2001

Jun-2000

Dec-2000

Date

Nominal Price ($/ton)

0.00

0.50

1.00

1.50

2.00

Transfer Volume (millions)

Price

Volume

Figure 29: SO2 Allowance Trading Volume and Prices from June 2000 to June 2007

Source: EPA, 2007

For CAIR-affected EGUs, participation in the CAIR 

programs meets federal source-specifi c BART 

requirements because CAIR was determined to be 

better than BART controls under CAVR in the CAIR 

region. Specifi cally, controls for EGUs subject to CAIR 

will result in more visibility improvement in natural 

areas than BART would have provided. States could, 

however, require additional reductions.

Analyzing the Impacts of Rule 

Implementation

Projected Controls

Although aspects of CAIR and CAMR are in 

litigation, implementation moves ahead. Having 

promulgated these environmental programs, EPA has 

gone on to work with states, which are now working 

aggressively to put implementing rules in place. 

The regulated community is going forward with 

installing equipment for CAIR, entering into contracts 

for construction of mercury controls, and putting 

monitoring systems in place. 

Sources have begun responding to the new requirements with investments and application of retrofi t 

technology. EPA estimates that in 2010, EGUs 

accounting for 60 percent of total capacity will have 

scrubbers, increasing to 73 percent by 2020. Modeling 

shows that the percentage of advanced controls will 

go up (with the amount of capacity with advanced 

controls projected to increase even faster) and the 

number of units without advanced controls will go 

down, especially for larger units (see Figure 28 on 

page 45).

As observed with previous programs, the regulated 

community responds with a sense of purpose and 

alacrity to cap and trade programs. EPA, using existing 

CAA authority, is moving to address interstate transport 
Acid Rain and Related Programs: 2006 Progress Report 47

Figure 30: Actual and Forecasted SO2 Allowance Prices

Source: EPA, 2007

$533

$505

$267

$370

$270

EPA Projected Allowance

Price, 2006 Dollars

2010-2014

Vintage

2006

Vintage

2010

Vintage

Up to 2010

Vintage

June 2007 Spot Market

Price Range

$705

of SO2 and NOX emissions and lower mercury emissions 

with CAIR, CAMR, and CAVR. EPA expects these 

programs to deliver signifi cant human health and 

environmental improvements in a cost-effective manner 

by harnessing market forces to achieve substantial 

required emission reductions.

CAIR and the SO2 Trading Market

In 2006 trading, prices began the year at nearly 

$1,500 per ton. As EPA discussed in the 2005 Acid 

Rain Program Progress Report, market observers 

characterized this high price as a result of uncertainty 

over the implementation of CAIR. However, by 

mid-2006, prices were lower and had stabilized, 

generally trading through the second half of 2006 in 

a band between $400 and $600 per ton. Prices have 

generally remained at this level through the end of 

June 2007. EPA also observed that, during the period 

of peak allowance prices in late-2005 and early-2006, 

transfer volumes were generally lower in the market, 

which indicates that many market participants were not 

trading during this period of high volatility. EPA expects 

that trade volumes will again increase as the market 

continues to stabilize in 2007. Figure 29 shows the 

variation in SO2

 allowance price and transfer volume 

from June 2000 though June 2007.

Current prices continue to compare favorably with 

EPA’s updated estimate of future SO2

 allowance prices 

under CAIR. As shown in Figure 30, EPA projected 

that pre-2010 vintage allowances would be worth 

$533 per allowance in 2010, and that 2010-2014 

vintage allowances would be worth $267 per 

allowance due to the 2:1 retirement ratio that applies 

to those vintage allowances in the CAIR region.
48 Acid Rain and Related Programs: 2006 Progress Report

Category Label ANC Levels* Expected Ecological Effects

Acute Concern < 0 micro 

equivalent

per Liter 

(μeq/L)

Complete loss of fi sh populations is expected. Planktonic communities have extremely low 

diversity and are dominated by acidophilic forms. The numbers of individuals in plankton 

species that are present are greatly reduced.

Elevated Concern 0 - 50 μeq/L Fish species richness is greatly reduced (more than half of expected species are missing). On 

average, brook trout populations experience sub-lethal effects, including loss of health and 

reproduction (fi tness). During episodes of high acid deposition, brook trout populations may 

experience lethal effects. Diversity and distribution of zooplankton communities declines.

Moderate 

Concern 

50 - 100 

μeq/L

Fish species richness begins to decline (sensitive species are lost from lakes). Brook trout 

populations are sensitive and variable, with possible sub-lethal effects. Diversity and 

distribution of zooplankton communities begin to decline as species that are sensitive to 

acid deposition are affected.

Low Concern > 100 μeq/L Fish species richness may be unaffected. Reproducing brook trout populations are expected 

where habitat is suitable. Zooplankton communities are unaffected and exhibit expected 

diversity and distribution.

Table 10: Aquatic Ecosystem Status Categories for the Adirondacks

* It is important to note that the wide range of ANC values within these categories makes it likely that substantial improvements in ANC may 

occur without changing the categorization of a given lake.

Source: EPA, 2007

June 2007 spot market prices show that the prices 

for the earlier vintages are trading for $505 to $705 

per ton, and that the later vintages (2010-2014) are 

trading for $270 to $370 per ton (see Figure 30 on 

page 47). These market prices compare favorably 

with, though slightly above, EPA’s estimate for the 

CAIR markets.

Predicting the Response of Acidifi ed Lakes 

and Streams Under CAIR

In addition to the improvements in lake and stream 

acidity resulting from implementation of the ARP, 

CAIR will further reduce SO2

 and NOX emissions, 

thereby reducing acid deposition and contributing 

to improvements in lake and stream conditions. EPA 

utilized a surface water chemistry model called the 

Model of Acidifi cation of Groundwater in Catchments 

(MAGIC) to estimate the response of acidifi ed lakes 

and streams to these reductions in acid deposition. 

MAGIC incorporates a small number of processes that 

are important in infl uencing the long-term response of 

surface waters to acidic deposition. 

The Adirondack region of New York was selected as the 

location for this evaluation. Aquatic ecosystem status 

categories have been defi ned to track recovery for this 

area and are presented in Table 10. This analysis uses 

projected acid deposition scenarios for 2010, 2015, 

and 2020 that depict acid deposition in the absence 

(baseline) and presence of CAIR for each year. Using the 

difference between the baseline and CAIR deposition 

data, MAGIC projects the response of indicators of 

stream and lake acidity (such as ANC) to reductions in 

acid deposition resulting from CAIR implementation.
Acid Rain and Related Programs: 2006 Progress Report 49

0

25

50

75

Acute

Concern

Elevated

Concern

Moderate

Concern

Low

Concern

2050 Lake Status

% of Lakes

-50 0

0

25

50

50 100 150 200 250

75

100

2050 Lake ANC (ueq/L)

Cumulative % of Lakes

-50

2020 2050

11

32 36

20

11

30 32 27

0

25

50

75

Acute

Concern

Elevated

Concern

Moderate

Concern

Low

Concern

2020 Lake Status

Base Scenario CAIR Scenario Base Scenario CAIR Scenario

11

32 32 32 36

20

7

30

0

0

25

50

50 100 150 200 250

75

100

2020 Lake ANC (ueq/L)

Cumulative % of Lakes % of Lakes

Figure 31: Simulated Responses in 2020 and 2050 of Adirondack Lakes to Changes 

in Acid Deposition (Baseline Conditions versus CAIR Scenarios)

Note: Baseline results are shown in red while CAIR results are shown in green - values rounded. The wide range of ANC values within these categories 

makes it likely that substantial improvements in ANC may occur without changing the categorization of a given lake.

Source: EPA, 2007

The improvements in ANC predicted for lakes in 

the Adirondacks are depicted in the cumulative 

distribution plots in Figure 31. The cumulative 

distribution plots provide information about the 

change in ANC for all lakes and show that ANC under 

CAIR is consistently higher than without CAIR. For 

example in 2020, 59 percent of lakes under CAIR 

and 56 percent of lakes without CAIR have ANC 

greater than 50. The bar graphs in Figure 31 show 

the change in ecosystem status categories for lakes 

in the Adirondacks between the baseline and CAIR 

conditions. On average, MAGIC projects that ANC 

will increase by 7.5 μeq/L in 2020 and 12 μeq/L in 

2050 with CAIR. This analysis clearly indicates that 

improvements in aquatic ecosystem status for the 

lakes in the Adirondacks should occur as a result of 

reductions in acid deposition attributable to CAIR 

emission reductions.

Acid Rain and Related Programs: 2006 Progress Report 51

The availability and transparency of data, from 

emission measurement to allowance trading to 

deposition monitoring, is a cornerstone of effective 

cap and trade programs. The Clean Air Markets 

Division (CAMD), in the Offi ce of Air and Radiation’s 

Offi ce of Atmospheric Programs, develops and 

manages programs for collecting these data 

and assessing the effectiveness of cap and trade 

programs, including the ARP. 

The CAMD Web site provides a public resource for 

general information on how market-based programs 

work and what they have accomplished, along with 

the processes, information, and tools necessary to 

participate in any of these market-based programs.

For information about EPA’s air emissions trading 

programs, see: <http://www.epa.gov/airmarkets>. 

For information about the ARP, see: 

<http://www.epa.gov/airmarkets/progsregs/arp/

index.html>.

Online Information, Data, and Resources

Acid Rain and Related Programs: 2006 Progress Report 53

1. See <www.epa.gov/ttn/chief/trends> (Total emissions are preliminary projections, based on 2002 National Emissions Inventory).

2. Chestnut, L. G., Mills, D. M. (2005, November). A fresh look at the benefi ts and costs of the U.S. Acid Rain Program. Journal 

of Environmental Management, Vol. 77, Issue 3, 252-256. 

3. For the statutory provisions on allowance allocations, see Section 403 of the CAA, as amended in 1990. 

See <www.epa.gov/air/caa/caa403.txt>. 

4. See <www.epa.gov/ttn/chief/trends>. 

5. Detailed emissions and allowance data for ARP sources are available on the Data and Maps portion of EPA’s Clean Air Markets 

Web site, see <http://camddataandmaps.epa.gov/gdm/>. 

6. Allowance transfers are posted and updated daily on <www.epa.gov/airmarkets>. 

7. See <http://aura.gsfc.nasa.gov/instruments/omi/index.html> for further information. The SO2

 data team includes researchers 

from a number of locations, including the Goddard Earth Sciences and Technology Center at the University of Maryland 

Baltimore County; the Joint Center for Earth Systems Technology at the University of Maryland Baltimore County; the National 

Aeronautics and Space Administration, Goddard Space Flight Center; and the Royal Netherlands Meteorological Institute 

(KNMI).

8. Kim, S.W., et al. “Satellite-observed U.S. power plant NOX emission reductions and their impact on air quality.” Geophysical 

Research Letters, Vol. 33, No. 22, L22812, doi: 10.1029/2006GL027749, 29 November 2006.

9. Borrell, P., Burrows, J., Platt, U., & Zehner, C. (2001). Determining tropospheric concentrations of trace gases from space. ESA 

Bulletin, 107, 72-81.

10. It should be noted that there has not been a violation of the SO2

 standard at any U.S. monitoring site since 2000.

11. See the EPA Offi ce of Transportation and Air Quality Web site < www.epa.gov/otaq> for information on recent rules 

to reduce NOX emissions from mobile sources. Additional NOX reductions are occurring as a result of the NBP. See EPA’s 

September 2007 report, NOX Budget Trading Program: 2006 Program Compliance and Environmental Results, at <www.epa.

gov/airmarkets/progress/nbp06.html>, which discusses these NOX reduction efforts.

12. National Research Council (2004). Air Quality Management in the United States. National Academies Press, Washington, DC. 

13. Clean Air Act Advisory Committee, Air Quality Management Work Group (2005). “Recommendations to the Clean Air Act 

Advisory Committee: Phase 1 and Next Steps.”

14. The data used to compute trends for the Southern Appalachians include a signifi cant update (over fi ve years of new data), 

resulting in a substantial change in the magnitude and direction of the trends shown in the 2005 Acid Rain Progress Report. 

The trends shown here should be regarded as more accurate estimates of long-term patterns for this region. 

15. CAIR (see <www.epa.gov/cair/index.html>), CAMR (see <www.epa.gov/air/mercuryrule>), CAVR (see <www.epa.gov/oar/

visibility/index.html>). 

Endnotes


United States 

Environmental Protection Agency

Offi ce of Air and Radiation

Clean Air Markets Division

1200 Pennsylvania Ave, NW (6204)

Washington, DC 20460 

EPA-430-R-07-011