2001

PROGRESS

REPORT

EPA 

ACID RAIN 

PROGRAM

November 2002
This Progress Report replaces the Compliance Report and the Emissions Scorecard used in previous years to report on the results of the Acid Rain Program. All data and results from the Acid

Rain Program are now compiled in a single document that reports information on:

S emission levels

S compliance with the SO2 and NOx components of the program

S SO2 allowance prices

S emissions monitoring

S air quality and deposition monitoring

S environmental and human health effects and benefits 

This Progress Report will be published annually by EPA to update the public on the status of

implementation of the Acid Rain Program and our progress towards achieving our environmental

goals. Detailed unit-level emissions data are available on our website at http://www.epa.gov/airmarkets/emissions/index. Our new query tool that provides access to a variety of EPA emissions

data is available at http://cfpub.epa.gov/gdm. For more information on the Acid Rain Program,

including information on SO 2 and NOx emissions, acid deposition monitoring, and the environmental effects of acid deposition, you can visit our website at http://www.epa.gov/airmarkets.

Photo credits (clockwise, from upper left): USDA; SoftKey International; VTweb.com; Rick Webb; SoftKey International; SoftKey International; Photodisc.com (center) 

EPA Acid Rain Program

2001 Progress Report

EPA-430-R-02-009

Clean Air Markets Program

Office of Air and Radiation

U.S. Environmental Protection Agency

November, 2002
Table of Contents

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

An Innovative Cap and Trade Program for SO2 ...................................... 2

The NOx Program.............................................................................. 2

Why Worry about Acid Rain?.............................................................3

The SO2 Program......................................................................................... 5

Emissions........................................................................................... 5

Compliance........................................................................................ 7

Geographic Trends in SO2 Emissions............................................... 9

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

The NOx Program......................................................................................... 14

Emissions........................................................................................... 14

Emission Limits.................................................................................. 16

Compliance........................................................................................ 18

Geographic Trends in NOx Emissions............................................... 18

Monitoring Results........................................................................................ 22 

Emission Monitoring.......................................................................... 22

Air Quality and Deposition in 2001.................................................... 23

Clean Air Mapping and Analysis Program (C-MAP).......................... 27

Freshwater Monitoring....................................................................... 27

Environmental Improvement and Trends..................................................... 29

Improved Air Quality and Reduced Acid Deposition......................... 29

Visibility.............................................................................................. 34

Human Health Benefits...................................................................... 35

Ecological Effects of Reduced Acid Deposition................................. 35

Freshwater...............................................................................35

Forests.................................................................................... 37

Coastal Waters........................................................................37

Materials and Structures.................................................................... 38

Summary.......................................................................................................39

For Further Information................................................................................. 41

T

he Acid Rain Program was established under Title IV of the 1990 Clean Air

Act Amendments. The program requires major reductions of sulfur dioxide

(SO2) and nitrogen oxides (NOx) emissions, the pollutants that cause acid

rain. Using an innovative market-based or “cap and trade” approach to environmental protection, the program sets a permanent cap on the total amount of SO2

that may be emitted by electric power plants nationwide. The cap is set at about

one half of the amount of SO2 emitted in 1980, and the trading component allows

flexibility for individual fossil fuel-fired combustion units to select their own methods of compliance. The program also sets NOx emission limitations (in pounds

per million British thermal units or lb/mmBtu) for certain coal-fired electric utility

boilers, representing about a 27% reduction from 1990 levels. The Acid Rain Program was implemented in two phases. Phase I applied primarily to the largest

coal-fired sources from 1995 through 1999 for SO2 and from 1996 through 1999

for NOx. Phase II for both pollutants began in 2000 and applies to thousands of

combustion units (see Figure 1). In 2001, there were 2,792 units affected by the

SO2 provisions of the Acid Rain Program. Additionally, 1,046 of these units were

required to meet a NOx emissions limit in 2001 under the Acid Rain Program provisions1

. The Acid Rain Program has significantly reduced emissions of SO2 and

page 1

I

ntroduction

Figure 1. Sources

Affected by the

SO2 Requirements of Title IV

(the Acid Rain

Program) of the

1990 Clean Air

Act, 2001

Source: EPA

1 Sources affected by the Acid Rain Program are tracked for compliance purposes at the unit level. A single source (power plant) may have

many units (combustion devices). 

Most Phase I

sources (the

largest, highestemitting sources)

were in the Midwest; in Phase II

sources nationwide are affected

by the Acid Rain

Program.
Acid Rain Program Annual Progress Report, 2001

page 2

An Innovative Cap and Trade Program for SO2

The SO2 component of the Acid Rain Program represents a dramatic departure

from traditional command and control regulatory approaches that establish

source-specific emissions limitations. Instead, the program uses an overall emissions cap for SO2

that ensures emissions reductions are achieved and main -

tained and a trading system that facilitates lowest-cost emissions reductions. The

program features tradeable SO2 emissions allowances, where one allowance is a

limited authorization to emit one ton of SO2. A fixed number of allowances are

issued by the government, and they may be bought, sold, or banked for future use

by utilities, brokers, or anyone else interested in holding them. Existing units are

allocated allowances for each year; new units do not receive allowances and must

buy them. At the end of the year all participants in the program are obliged to surrender to EPA the number of allowances that correspond to their annual SO2

emissions.

Affected sources must demonstrate compliance with the SO2 provisions of the

Acid Rain Program at the end of each year. Sources are granted a 60-day grace

period during which additional SO2 allowances may be purchased, if necessary,

to cover each unit's emissions for the year. At the end of the grace period (the

Allowance Transfer Deadline), the allowances a unit holds in its Allowance Tracking System (ATS) account must equal or exceed the unit's annual SO 2 emissions

for the previous year. The Acid Rain Program requires affected sources to monitor emissions continuously and to report their emissions regularly. Failure to surrender sufficient allowances results in significant automatic penalties that include

fines as well as a reduction in the number of allowances allocated in the following

year. Any remaining SO2 allowances may be sold and/or banked for future use. 

The NOx Program

The NOx component of the Acid Rain Program, using a more traditional regulatory approach, establishes an emission rate limit for certain types of coal-fired boilers. However, sources are provided a degree of flexibility through emissions averaging provisions, whereby a company can meet the standard emission limitations

by averaging the emissions rates of two or more boilers. This allows sources to

over-control at units where it is technically easier to control emissions, thereby

achieving emissions reductions at a lower cost. Additionally, certain Phase II units

elected to become subject to Phase I limits beginning in 1997. These early election units are not subject to the more stringent Phase II limits until 2008.

Sources affected by the NOx portion of the Acid Rain Program must also demonstrate that they have complied with the NOx provisions at the end of the year.

Sources demonstrate compliance with the NOx program by achieving an annual

emission rate at or below mandated levels.

NOxfrom electric power plants and resulted in substantial environmental and

human health benefits.
Acid Rain Program Annual Progress Report, 2001

page 3

Why Worry about Acid Rain?

Acid deposition, more commonly known as acid rain, occurs when emissions of

SO2 and NOxreact in the atmosphere (with water, oxygen, and oxidants) to form

various acidic compounds. These acidic compounds then fall to earth in either a

wet form (rain, snow, and fog) or a dry form (gases and particles). Prevailing

winds transport the acidic compounds hundreds of miles, often across state and

national borders. The acidic compounds (including small particles such as sulfates and nitrates) cause many negative environmental effects. These pollutants

impair air quality and damage public health, acidify lakes and streams, harm sensitive forest and coastal ecosystems, degrade visibility, and accelerate the decay

of building materials, paints, and cultural artifacts such as buildings, statues, and

sculptures nationwide.

Some places and people are more susceptible or sensitive to these impacts than

others. Areas where acid deposition damages ecosystems or gases and particles

impair visibility are called "sensitive receptors." Sensitive ecological receptors

include lakes and streams throughout the Appalachian Mountains; forests in the

Appalachian Mountains, the Colorado Front Range, and West Coast coastal

mountain ranges; and many East and Gulf coast estuaries and coastal waters.

Many national parks and wilderness areas, including Great Smoky National Park,

Acadia National Park, and Grand Canyon National Park have impaired visibility

due in part to emissions of SO2 and NOxfrom power generation sources. Many

people (especially children, the elderly and those with existing respiratory or cardiovascular conditions) are also adversely impacted by fine particles formed from

Figure 2. Sensitive Ecological

Receptors

Areas sensitive

to acid deposition, nitrogenoverenrichment,

and visibility

impairment

occur nationwide.

Class 1 areas such as national parks and wilderness areas

Acidic surface waters in surveyed regions
Acid Rain Program Annual Progress Report, 2001

page 4

SO2 and NOx emissions and ozone formed from NOx emissions from power generation sources. 

The pollutants that cause acid rain often cause human health and environmental

impacts hundreds of miles from where they are emitted. This long-range transport

makes it critical to reduce all emissions that cause acid rain, even those that occur

far from sensitive receptors or population centers. The Acid Rain Program's ultimate objective is to protect the environment and improve human health by reducing SO2 and NOx emissions from power generation sources. These emission

reductions benefit the nation by: 

S Improving air quality and protecting public health 

S Restoring acidified lakes and streams so they can once again support 

fish and other aquatic life 

S Improving visibility, especially at scenic vistas in national parks 

S Reducing the damage to sensitive forests, such as those along the

Appalachian Mountains and in the Colorado Front Range

S Reducing the damage to nitrogen-sensitive coastal waters along the East

and Gulf Coasts

S Protecting our historic buildings and monuments from degradation
page 5

T

here were 2,792 units2 used to produce electric power that were subject to

the SO2 provisions of the Acid Rain Program in 2001 (that is, they operated, submitted emissions data for SO2

, and were subject to annual reconciliation of allowable emissions with actual emissions in 2001). Acid Rain Program

sources reduced their combined SO2 emissions in 2001 by 39% from 1980 levels

(33% from 1990 levels). All but two of the 2,792 units complied with the requirement to hold sufficient allowances. There were no significant geographic shifts in

emissions. The price of an SO2 allowance ranged from $135 to $210/ton in 2001,

a price range that is comparable to allowance prices in previous years. 

Emissions

In 2001, the second year of Phase II, Acid Rain Program sources achieved a total

reduction in SO2 emissions of about 39% compared to 1980 levels (33% compared to 1990 levels). Compared to 2000 levels, these sources reduced their SO2

emissions by 5% or 569,000 tons. Figure 3 shows the trend in SO2 emissions

T

he SO2 Program

Figure 3. SO2

Emissions from

Acid Rain

Sources, 1980-

2001

The large

decrease in SO2

emissions from

power plants

since 1990 is

due to implementation of the

Acid Rain Program.

Source: EPA

2 In this report, the term “unit” means a fossil-fuel fired combustor that serves a generator that provides electricity for sale. The vast majority

of emissions affected by the program come from coal-fired generation units, but oil and natural gas units are also included in the program.
Acid Rain Program Annual Progress Report, 2001

page 6

Figure 4. Allocated, Used, and

Banked SO2

Allowances

The bank was

built up during

Phase I with

early reductions

and is now

being drawn

down under

Phase II.

since 1980 for all affected sources.

The electric utility industry is by far the largest single source of sulfur dioxide

emissions, accounting for approximately 65% of total SO2 emissions nationwide.

In addition to the significant reductions from the electric power generation sector,

reductions in SO2 emissions from other sources, including smelters and sulfuric

acid manufacturing plants, and use of cleaner fuels in residential and commercial

burners, have also contributed to the 50% decline of SO2 emissions from all

sources since 1980 (National Air Quality and Emissions Trends Report, 1999).

There were 2,792 units that underwent annual reconciliation for SO2in 2001.

(These units, as well as an additional 273 units which were retired or not yet operating, are listed in Appendix A of this Report. Appendix A is available on our website at www.epa.gov/airmarkets/cmprpt/arp01/index.html). 

The allowances (i.e., authorizations to emit SO2) allocated in a particular year to

each source are determined by several provisions of the Clean Air Act. For the

year 2001, a total of 9.55 million allowances were granted. Adding these 9.55 million allowances to the unused allowances carried over (or banked) from prior

years, a total of 19.93 million allowances were available for use in 2001. Sources

emitted 10.63 million tons in 2001, 1.08 million tons more than the allowances

granted in 2001 but far less than the allowable level. For the second year in a row

the number of allowances in the bank declined. As shown in Figure 4, the bank

shrank by 1.08 million allowances in 2001. Over time the bank is expected to continue to be depleted as sources use these banked allowances to comply with the

Source: EPA
Acid Rain Program Annual Progress Report, 2001

page 7

Type of Allowance

Allocation

Number of

Allowances

Explanation of Allowance Allocation

Type

Initial Allocation 9,190,9223Initial Allocation is the number of

allowances granted to units based on the

product of their historic utilization and

emissions rates (performance standards)

specified in the Clean Air Act.

Allowances for

Substitution Units

13,547 A lawsuit settlement allowed for a small

amount of allowances to be allocated for

Substitution Units in 2001 instead of an

earlier year during Phase I.

Allowance Auctions 250,000 Allowance Auctions provide allowances to

the market that were set aside in a Special

Allowance Reserve when the initial

allowance allocation was made.

Opt-in Allowances 99,188 Opt-in Allowances are provided to units

entering the program voluntarily. There

were 11 opt-in units in 2001.

TOTAL 2001

ALLOCATION

9,553,657

Banked Allowances 10,376,426 Banked Allowances are those held over

from 1995 through 2000 which can be used

for compliance in 2001 or any future year.

Conservation and

Renewable Energy

Allowances

3,528 These allowances come from a special

reserve set aside when the initial allowance

allocation was made. They are awarded to

utilities that undertake efficiency and

renewable energy measures. These are

year 1999 allowances that were allocated in

year 2001.

TOTAL 2001

ALLOWABLE

19,933,611

1

 The total year 2001 initial allocation was 9,191,897. 54 allowances were deducted as

offsets during year 2000 reconciliation, and 921 allowances were surrendered as part of

an enforcement action prior to the 2001 reconciliation.

Figure 5. Origin

of 2001 Allowable Emissions

Level

There were 

9.55 million

allowances allocated in 2001;

an additional

10.38 million

allowances had

been banked in

previous years

and were available for use.

3 The total year 2001 initial allocation was 9,191,897. Fifty-four allowances were deducted as offsets during year 2000 reconciliation, and 921 allowances were surrendered as part of an enforcement action

prior to the 2001 reconciliation.

Source: EPA

stringent Phase II requirements. Figure 5 explains in more detail the origin of the

allowances available for use in 2001. 

Compliance

A total of 10.6 million allowances were deducted from sources’ accounts in 2001.

Two units were short a total of 11 allowances to cover their emissions for the 2001

compliance year. Eleven year 2002 allowances were taken from these units as
Acid Rain Program Annual Progress Report, 2001

page 8

"offsets" and are included in the total number of used allowances for 2001. In

addition to the offsets, the operators of these units were assessed an automatic

monetary penalty totaling $30,514.4 Figure 6 displays these allowance deductions, as well as the remaining bank of 1995 through 2001 allowances.

During the compliance process, the number of allowances surrendered at an individual unit is equal to the number of tons emitted at the unit, except where the unit

shared a common stack with other units. For the purposes of surrendering

allowances for emissions at a common stack, the source was allowed to choose

the proportion of allowances deducted from each unit sharing the stack, as long

as enough allowances were surrendered to cover the total number of tons emitted. If no such apportionment was made, EPA deducted allowances equally

among the units sharing the stack to cover total emissions reported by the stack. 

The deductions for emissions at each unit after the common stack apportionment

was made can be found in Appendix A of this Report. Appendix A is available on

our website at www.epa.gov/airmarkets/cmprpt/arp01/index.html. Units sharing a

common stack are listed directly under the entry for their common stack.

Total Allowances Held in Accounts as of 3/1/2002

(1995 through 2001 Vintages)5

19,933,611

 Unit Accounts 14,749,028

 General Accounts65,184,583

Allowances Deducted for Emissions (1995 through

2001)

10,633,035

2002 Penalty Allowances Deducted 11

Banked Allowances 9,300,576

 Unit Accounts 4,115,993

 General Accounts65,184,583

Figure 6. SO2

Allowance Reconciliation Summary, 2001

5 The number of allowances held in the Allowance Tracking System (ATS) accounts equals the number of

2001 allowances allocated (see Figure 5) plus the number of banked allowances. March 1, 2002 represents the Allowance Transfer Deadline, the point in time at which unit accounts are frozen and after

which no transfers of 1995 through 2001 allowances will be recorded. The freeze on these accounts is

removed when annual reconciliation is complete.

6 General accounts can be established in the ATS by any utility, individual or other entity.

Source: EPA

4 A source that does not hold enough allowances in its unit account to cover its annual SO2 emissions has "excess emissions" and must pay a $2,000 per ton automatic penalty in 1990$. The

$2,000 penalty is adjusted annually for inflation, so the year 2001 penalty was $2,774.
Acid Rain Program Annual Progress Report, 2001

page 9

Geographic Trends in SO2 Emissions

Total sulfur dioxide emissions from power generation have decreased significantly since the Acid Rain Program was authorized by Congress in 1990, and they

continued to decline in 2001, the second year of Phase II. The geographic distribution of SO2 emissions did not change significantly between 1990 and 2001. Fig -

ure 7 displays bar graphs comparing state SO 2 emission trends from power generation before the Acid Rain Program (1990), during Phase I (1995-1999 average), and in Phase II to date (2000-2001 average).

Several geographic trends are evident:

S The bar graphs on the map in Figure 7 illustrate that the area with the

highest emissions--the Midwest--also had the largest reductions; 

S SO2 emission reductions during Phase I occurred predominantly in

approximately a dozen states in the Eastern U.S. (Phase I affected the

larger, higher emitting utilities in the Eastern half of the country);

S The 24 shaded states represent states where SO2 emissions in Phase

II (2000-2001) were lower than both 1990 levels and the 1995-1999

Phase I average. Unlike the SO2 emission reductions achieved during

Phase I, these Phase II reductions are geographically more widespread, occurring in a larger number of Southeastern and some Western states. 

In several states, average SO2 emissions during Phase 1 were higher than they

had been in 1990. This is due to the large number of Phase II sources in these

states that were not required to control for SO2 until 2000. In the 2000-2001 period (Phase II) these emissions declined to levels below what was emitted in 1990.

In general, because SO2 emissions are capped, there will be pressure to contin -

ue to reduce SO2 emissions by installing add-on controls. 

Figure 8 illustrates the geographic distribution by state of SO2 emissions from

power generation before implementation of the Acid Rain Program (1990), during

Phase I (1995-1999 average), and in Phase II (2000-2001 average). 

In 2001, Title IV sources achieved a 33% reduction from 1990 SO2levels nationwide. SO2 emissions in Texas did increase in Phase I; however, SO2 emissions in

the state decreased in Phase II when the Acid Rain Program requirements took

effect for Texas sources. Although most SO2 emissions still occur in the Midwestern U.S., it is important to note that, over time, this same region has also seen the

most significant decrease in SO2 emissions in the country. The highest SO2 emitting states in 1990 (Ohio, Indiana, and Pennsylvania), reduced emissions 40% in

2001 (49%, 47%, and 22%, respectively) compared to 1990 levels. Other states

in the region show similar trends since 1990. SO2 emissions decreased 59% in

Illinois, 41% in Kentucky, 70% in Missouri, 55% in Tennessee, and 49% in West

Virginia. 
Acid Rain Program Annual Progress Report, 2001

page 10

Figure 7. Average SO2 Emissions from Acid

Rain Sources by

State, 1990-2001 

Emissions in

the East, where

most Phase I

sources are

located,

decreased during Phase I.

Many additional

reductions during Phase II are

occurring in the

Southeast and

West. 
Acid Rain Program Annual Progress Report, 2001

page 11

Source: EPA

Power Plant SO2 Emissions

(thousand tons)

0 - 150

150 - 650

650 - 1150

1150 - 1650

> 1650

Figure 8. Geographic Distribution of Average

SO2 Emissions

from Acid Rain

Sources by

State, 1990-2001 

Since 1990 the

most significant

emissions

reductions have

taken place in

the highest

emitting states.

There have been

no significant

geographic

shifts in emissions since

1990.
Acid Rain Program Annual Progress Report, 2001

page 12

Figure 9. SO2

Allowance Price

Index

The cost of

allowances in

2001 did not

change substantially from the

previous few

years and

remains far lower

than estimated in

1990.

SO2 Allowance Market

The flexibility provided by the Acid Rain Program enabled the 2,792 units subject

to the SO2requirements in 2001 to pursue a variety of compliance options.

Sources met their SO2 reduction obligations by installing scrubbers, switching

fuels, changing practices or procedures to improve energy efficiency, and buying

allowances. The presence of the allowance market has given some sources the

incentive to reduce their SO 2 emissions below the level of their allowance allocation in order to bank their allowances for use in future years. Other sources have

been able to postpone or reduce expenditures for control by purchasing

allowances from sources that controlled below their allowance allocation level.

The flexibility in compliance options is possible because strict monitoring requirements for all affected units ensure one allowance is surrendered for every ton of

SO2 emitted. The program's flexibility significantly reduces the cost of achieving

these emissions reductions as compared to the cost of a technological mandate

or fixed emission rate.

The marginal cost of compliance--the cost of reducing the next ton of SO 2 emitted from the utility sector--is reflected in the price of an allowance. Emission

reductions continue to cost less than anticipated when the Clean Air Act Amendments were enacted and this is reflected in the price of allowances. The cost of

an allowance was initially estimated at $400-1,000/ton in 1990 dollars ($500-

1,200/ton in 2001 dollars). As shown in Figure 9, actual prices have been significantly lower than predicted. During 2001, SO2 allowances ranged in price from

$135-$210/ton. At the time of the annual allowance auction in April 2001,

allowances were approximately $170/ton. The price rose through the summer,

peaking at $210/ton in late August/early September. During the last quarter of

2001, prices dropped back to around $170/ton and then stabilized. Some market

observers believe lower-than-expected allowance prices during the first several

Source: Monthly price reports from Cantor Fitzgerald Environmental Brokerage Services 
Acid Rain Program Annual Progress Report, 2001

page 13

years of the program were due primarily to lower than expected compliance costs

and larger than expected emissions reductions, which increased the supply of

allowances and put downward pressure on prices. Additionally, the more stringent

limits in Phase II most likely resulted in higher average prices in 2001 than in 2000

as sources realized they would have to continue to withdraw from the bank and

employ further controls to comply in future years. 

The level of activity in the allowance market created under the Acid Rain Program

increased fairly steadily through 2000 and then dropped off somewhat in 2001,

the second year of Phase II. However, the number of official transfers in 2001 was

still higher than in any year of Phase I. 

In 2001, 4,900 allowance transfers that affected over 22 million allowances (of

past, current, and future vintages) were recorded in the Allowance Transfer System, the accounting system developed to track holdings of allowances. Of the

allowances transferred, 12.6 million, or 55%, were transferred in economically

significant transactions (i.e., between economically unrelated parties). Figure 10

shows the volume of SO2 allowances transferred under the Acid Rain Program

since official recording of transfers began in 1994. The majority of the allowances

transferred in economically significant transactions were acquired by utilities.

Trades between unrelated organizations accounted for approximately 12 million

allowances in 2001. In December 2001, trading parties began to use the On-line

Allowance Tracking System (OATS). By the end of 2001, OATS recorded 211

transfers electronically over the internet.

All official allowance transactions, as well as data on account balances and ownership, are posted and updated daily on the Clean Air Markets Division's website

(www.epa.gov/airmarkets) in order to better inform trading participants of the status of the market. Cumulative market statistics and analysis are also available.

Figure 10. SO2

Allowances

Transferred

under the Acid

Rain Program

The number of

official transfers

between economically unrelated organizations has

increased since

1994.

Between economically related organizations

Between economically unrelated organizations Source: EPA
page 14

T

he NOx Program

A

ll sources affected by the Acid Rain Program NOxrequirements reduced

their combined NOx emissions by 25% from 1990 levels in 2001. All but

one of the 1,046 NOx program affected units complied with their NOx

emission rate limitation. There were no significant shifts in the geographic distribution of emissions due to use of the emissions averaging compliance option.

Emissions

Title IV of the 1990 Clean Air Act requires the Acid Rain Program NOx program

to achieve a 2 million ton reduction from projected NOx emissions levels in 2000.

Total NOx emissions from all Acid Rain Program affected units surpassed that

goal by 1 million tons in 2000 (see Figure 11). Emissions from those sources in

2001 were even less -- 3.4 million tons (over 40%) below projected 2000 emissions without the Acid Rain Program. For all 2,792 Title IV affected units, total

Figure 11. NOx

Emissions from

Acid Rain

Sources, 1990-

2001 

NOx emissions

have decreased

since 1990, particularly in

preparation for

and during

Phase II of the

Acid Rain Program. Reductions since 1999

are also due in

part to implementation of the

OTC NOx Budget Trading Program and the

NOx SIP call. Source: EPA
Acid Rain Program Annual Progress Report, 2001

page 15

Figure 12. Comparison of Heat

Input for Electricity Generation and NOx

Emissions from

Acid Rain

Sources, by

State, 1990-2001 

Heat input--a

measure of fossil fuel used to

generate electricity--increased

in almost every

state between

1990 and 2001

while nationwide

NOx emissions

decreased.
Acid Rain Program Annual Progress Report, 2001

page 16

NOx mass emissions in 2001 were 2 million tons lower than emissions in 1990.

Emissions from the 1,046 NOx program affected sources in 2001 were 1.4 million

tons lower than in 1990 and 8% lower than in 2000.

These reductions have been achieved while the amount of fuel burned to produce

electricity, as measured by heat input, increased 28% since 1990. As illustrated

in Figure 12, many states with increasing electricity production have also

decreased total NOx emissions in 2001, as compared to 1990 levels. Without further reductions in emissions rates or institution of a cap on NOx emissions, however, NOx emissions from power plants would have been expected to rise with

increased use of fossil fuels in most areas of the country.

NOx emissions come from a wide variety of sources including those affected by

the Acid Rain Program. NOx emissions from electric utilities account for approximately 20% of NOx emissions from all sources. NOx emissions from transportation sources are 55% of NOx emissions from all sources. Nationally, NOx emissions have increased 5% between 1990 and 1999. This is primarily due to an

increase of 17% in NOx emissions from transportation sources, particularly heavy

duty vehicles, since 1990. That increase has been offset to some extent by the

emissions decreases from electric utilities and other fuel combustion sources due

to a variety of federal and state emission reduction programs (including the Acid

Rain Program, the Ozone Trading Commission NOx Budget Trading Program,

and anticipation of the NOx SIP call) and federal enforcement actions (National

Air Quality and Emissions Trends Report, 1999).

Emission Limits

Instead of using allowance trading to facilitate NOx emissions reductions, the Acid

Rain Program establishes NOx emission limitations (lb/mmBtu NOx) for coal-fired

electric generation units.

The Acid Rain Program NOxregulation (40 CFR part 76), establishes NOxlimits

for Group 1 boilers (dry bottom wall-fired and tangentially fired boilers), as well as

Group 2 boilers (cell burner, cyclone, vertically-fired, and wet bottom boilers).

Figure 13 shows the number of NOx affected units by boiler type and the emissions limit for each boiler type. 

There were 1,046 units subject to NOx emissions limitations in 2001. The owners

and operators of a NOx affected unit must choose at least one NOx compliance

plan (described below) to indicate how the unit will comply with its NOxlimit:

S Standard Limitation. A unit with a standard limit simply meets the applicable individual NOx

limit prescribed for its boiler type under 40 CFR

76.5, 76.6, or 76.7.

S Early Election. Under this compliance option, a Phase II Group 1 NOx
Acid Rain Program Annual Progress Report, 2001

page 17

Coal-Fired Boiler Type7 Standard

Emission

Limit

(lb/mmBtu)

Number of

Units

Phase I Group1 Tangentially-fired 0.45 135

Phase I Group1 Dry Bottom Wall-fired 0.50 130

Phase II Group 1 Tangentially-fired 0.40 304

Phase II Group 1 Dry Bottom Wall-fired 0.46 312

Cell Burners 0.68 37

Cyclones > 155 MW 0.86 56

Wet Bottom > 65 MW 0.84 31

Vertically-fired 0.80 41

Total 1,046

Figure 13. Number of NOx

Affected Units

by Boiler Type

affected unit met a less stringent Phase I NOxlimit beginning in 1997,

three years before it would normally be subject to an Acid Rain NOxlimit.

In return for accepting a NOxlimit three years earlier than would normally

be required, an early election unit does not become subject to the more

stringent Phase II NOxlimit until 2008.

S Emissions Averaging. A company can meet its NOx emissions reduction requirements by choosing to make a group of NOx affected boilers

subject to a group NOxlimit, rather than meeting individual NOxlimits for

each unit. The group limit is established at the end of each calendar

year, and the group rate for the units must be less than or equal to the

Btu-weighted rate at which the units would have been limited had each

been subject to an individual NOxlimit. 

S Alternative Emission Limitation (AEL). A utility can petition for a less

stringent AEL if it properly installs and operates the NOx emissions

reduction technology prescribed for that boiler but is unable to meet its

standard limit. EPA determines whether an AEL is warranted based on

analyses of emissions data and information about the NOx control equip -

ment.

Source: EPA

7 All coverage for boilers > 25 MW unless otherwise noted.
Acid Rain Program Annual Progress Report, 2001

page 18

Geographic Trends in NOx Emissions

Total nitrogen oxide emissions from all NOx affected Acid Rain Program sources

have decreased 25% since 1990. Figure 15 displays bar graphs illustrating rela -

tive state NOx emission trends from power generation sources affected by the

NOx program before the Acid Rain Program (1990), during Phase I (1996-1999

average), and in Phase II (2000-2001 average). NOx emissions reductions since

1999 are due in part to implementation of the OTC NOx Budget Trading Program,

the NOx SIP call, and several state reduction programs as well as the Acid Rain

Program.

Several geographic trends are evident:

S The bar graphs illustrate that NOx emissions were lower in 35 states in

2001 compared to 1990 levels, with the greatest reduction occurring in

the Eastern United States; 

S The shaded states had lower NOx emissions in 2001 compared to both

1990 levels and the 1996-1999 Phase I average. NOxreductions

Compliance Option Number of Units

Standard Emission Limitation 140

Early Election 274

Emissions Averaging 638

Alternative Emission Limitation 27

TOTAL 1,0798

1

The total does not equal 1,046 because 28 units have both early election and

emissions averaging compliance plans, and 5 units have both AEL and emissions

averaging compliance plans.

Figure 14. Compliance Actions

in the NOx Program, 2001 

The primary

method of compliance with the

NOx program

was emissions

averaging.

8 The total does not equal 1,046 because 28 units have both early election and emissions averaging

compliance plans, and 5 units have both AELs and emissions averaging plans.

Source: EPA

Compliance

In 2001, 1,045 NOx units met their NOx emissions limits through compliance with

their respective NOx compliance plans. Only one unit failed to meet its NOx emissions limit in 2001. That unit had excess NOx emissions of 60 tons and was

assessed a monetary penalty of $166,440 (60 tons x $2,774 per ton penalty).

Detailed compliance information by unit can be found in Appendices B1 and B2.

These appendices are available on our website at www.epa.gov/airmarkets/

cmprpt/arp01/index. Figure 14 summarizes the compliance options chosen for

NOx affected units in 2001. Averaging was the most widely chosen compliance

option; 54 averaging plans involving 638 units were in place in 2001. 
Acid Rain Program Annual Progress Report, 2001

page 19

Figure 15. Average NOx Emissions from Acid

Rain Sources by

State, 1990-2001 

NOx emissions

in Phase II (2000

and 2001) continue to

decrease in

most states,

especially

among the highest emitting

states. 
Acid Rain Program Annual Progress Report, 2001

page 20

Figure 16. Geographic Distribution of Average

NOx Emissions

from Acid Rain

Sources by

State, 1990-2001 

Emissions

reductions in

Phase I took

place primarily

in the Northeast; in the first

two years of

Phase II additional emissions

reductions took

place in the

South and Mid -

west. There

have been no

significant geographic shifts in

emissions since

1990.

Source: EPA

Power Plant NOx Emissions

(thousand tons)

0 - 50

50 - 150

150 - 250

250 - 350

350 - 450

> 450
Acid Rain Program Annual Progress Report, 2001

page 21

occurred predominantly in the Northeastern U.S. during Phase I; in

Phase II, NOxreductions are geographically more extensive and occur

in a larger number of Southern and Midwestern states. 

In several states, average NOx emissions during Phase I were higher than they

had been in 1990. This is due to the large number of Phase II sources in these

states that were not required to control NOx emissions until 2000. In the 2000-

2001 period (Phase II) emissions in these states have declined to levels below

what was emitted in 1990. There are also several states where average Phase II

NOx emissions were higher than emissions in 1990 and/or the Phase I average.

This is because while the Acid Rain Program limits the rate at which coal-fired

power plants may emit NOx, it does not limit total emissions of NOx the way total

emissions of SO2 are limited. Since heat input (or fuel use) increased in those

states, overall NOx emissions also increased. 

As illustrated in Figure 16, in 1990, the highest NOx emissions occurred in the

Midwestern and Southern regions of the U.S. By 2001, emissions in many of

these states had been significantly reduced from 1990 levels. The states with the

highest emissions in 1990 (Ohio, Texas, and Pennsylvania), achieved an average

reduction of 40% (38%, 31%, and 52%, respectively) in 2001. Other states in the

region are showing similar trends since 1990. NOx emissions decreased 27% in

Indiana, 33% in Kentucky, 34% in Tennessee, and 39% in West Virginia. 
page 22

T

he Acid Rain Program relies on several types of monitoring to implement

and assess the effectiveness of Title IV. Each affected source is required

to install and maintain Continuous Emissions Monitoring Systems (CEMS)

or approved compatible alternatives to accurately measure the amount of SO2

and NOx emitted. The Acid Rain Program also assesses the results of the emissions reductions by collaborating with other organizations to measure acid deposition nationwide. Wet acid deposition is monitored by the National Atmospheric

Deposition Program (NADP). Dry deposition is monitored by the Clean Air Status

and Trends Network (CASTNet). The impacts of acid deposition on lakes and

streams are monitored by the Long-Term Monitoring (LTM) and Temporally-Integrated Monitoring of Ecosystems (TIME) ecological monitoring programs.

Emissions Monitoring

Emissions monitoring is necessary in order to verify the reductions of SO2 and

NOx emissions mandated under the Act and to support the SO2 allowance trading program. A fundamental objective of the Acid Rain Program is to ensure consistent and accurate accounting of emissions from all affected boilers and turbines. To implement this objective, concentrations and mass emissions of SO2

and NOxfrom each affected unit are measured and recorded using Continuous

Emissions Monitoring Systems (CEMS) or an approved alternate measurement

method and reported to EPA on a quarterly basis. Daily, quarterly, and annual

quality assurance (QA) tests must be performed by each source to ensure that its

monitors continuously meet the high accuracy standards of the Acid Rain Program.

SO2 mass emissions are determined using CEMS to measure SO2 concentration

and stack gas flow rate. NOx mass emissions are determined by coupling NOx

concentration data with flow, diluent (i.e., CO2 or O2) concentrations, or fuel feed

rates. Whatever method is selected, all monitors are required to meet strict initial

and on-going performance standards to demonstrate the accuracy, precision, and

timeliness of their measurement capabilities. The monitors used in the Acid Rain

Program have achieved an unparalleled level of performance with respect to all

of these criteria. 

One measure of the accuracy of a CEMS is the relative accuracy test audit

(RATA), which is required for initial certification of a CEMS and on at least an

annual basis thereafter. The RATA ensures that the installed monitor measures

the "true" value of a pollutant by comparing the monitor to a reference method

which simultaneously measures the stack gas pollutant. All monitoring systems

M

onitoring Results
Acid Rain Program Annual Progress Report, 2001

page 23

Air Quality and Deposition in 2001

The Acid Rain Program also works with many partners to monitor the effects of

emissions changes on air quality, deposition of pollutants, and water quality. The

National Atmospheric Deposition Program (NADP) is a nationwide network of precipitation monitoring sites designed to measure regional levels of atmospheric

deposition. The network is a consortium of many different groups, including universities, state, local and federal government agencies, and other interested partners. The NADP National Trends Network (NADP-NTN) measures wet acid deposition (deposition that occurs in rain, snow, or sleet) weekly at about 250 monitoring stations throughout the U.S. The data are subject to strict quality assurance

Figure 19. Location of NADP/NTN

and CASTNet

Deposition Monitoring Sites, 2001

Source: NADP and CASTNet

must meet a relative accuracy standard allowing no more than ten percent deviation from the true value in order to continue to be used for emissions reporting.

Further, if the CEMS is biased low compared to the true value, a bias adjustment

factor must be applied to all future data from that monitoring system to ensure

there is no underreporting. This "self correcting" provision, coupled with daily

quality assurance testing requirements, creates a strong disincentive to allowing

any deterioration in monitor performance.

In 2001, data submitted on monitoring systems indicate that over 96% of the SO2

concentration monitors and 99% of all flow monitors met this relative accuracy

standard. In fact, most sources achieved much better results as the median relative accuracies for all of these monitors were 3% and 2.5%, respectively. 

NADP/NTN has

monitoring sites

throughout the

country; CASTNet

is concentrated in

the East and has

not yet completed

its expansion into

the Great Plains

and Western

NADP/NTN states.

CASTNet
Acid Rain Program Annual Progress Report, 2001

page 24

Figure 20. Eastern Regional Air

Quality, 2001:

Sulfate Concentrations

The highest

regional concentrations of sulfate (SO4

2-) in the

atmosphere are

in or downwind

of the areas

where SO2 emissions are highest.

and completeness screening in the field, in the laboratory, and during analysis. All

NADP data can be accessed online from the NADP website at

http://nadp.sws.uiuc.edu/. The Clean Air Status and Trends Network (CASTNet)

is a nationwide network of over 70 sites that measures ambient air concentrations

of pollutants, including ozone. CASTNet also measures dry deposition (the

process through which particles and gases are deposited in the absence of precipitation) of acidic compounds. CASTNet data are also subject to strict quality

assurance and completeness criteria. Figure 19 displays a map of the NADP-NTN

and CASTNet deposition monitoring sites (the dots on the maps in Figures 20, 21,

and 22 also indicate the location of monitoring sites). All CASTNet data can be

accessed online from the CASTNet website at http://www.epa.gov/castnet.

Figure 20 shows the sulfate (SO4

2-) concentrations (a primary component of fine

particles in the Eastern U.S.) in the atmosphere. Concentrations are highest in the

Midwest, mid-Atlantic, and parts of the South. Figure 21 shows the wet sulfate

and total (wet and dry) sulfur deposition in the continental U.S. during 2001. Wet

sulfate deposition is highest in the Midwest. Total sulfur deposition is highest in

the Eastern U.S. Most sites in the Eastern U.S. have a dry/wet ratio of about 1:1,

meaning that wet and dry deposition make up roughly equal portions of the total

deposition amount. In general, dry deposition is a larger percentage of total deposition in those areas nearest to SO2 emission sources.

Source: CASTNet

Notes: 

1) Data are presented for

the Eastern U.S. only

because there are not

enough. CASTNet monitoring sites in the West to

support this type of analysis.

2) The dots on the map

represent the locations of

monitoring sites.

3) Map colors represent

relative concentrations and

do not imply ecological or

human health status.
Acid Rain Program Annual Progress Report, 2001

page 25

Figure 21. Sulfur

Deposition,

2001: Wet Sulfate and Total

Sulfur Deposition 

The highest levels of wet sulfate (SO4

2-) deposition are in

the areas where

emissions are

highest and in

areas downwind.

Wet Sulfate Deposition

Total Sulfur Deposition

Wet and dry sulfur (S) deposition make up

roughly the

same percentage of total sulfur deposition in

the Midwest; in

most other

areas wet deposition makes up

a greater percentage of the

total.

Note: The size of the “pies” indicates the total magnitude of deposition; the colors indicate the percentage

of wet and dry deposition.

Source: NADP

Source: CASTNet

Notes: 

1) The dots on the map represent

the locations of monitoring sites. 

2) Map colors represent relative

concentrations and do not imply

ecological or human health status.
Acid Rain Program Annual Progress Report, 2001

page 26

Figure 22. 2001

Nitrogen Deposition: Wet and

Total Nitrogen

Deposition 

The highest levels of wet nitrogen (N) deposition are in the

Midwest and in

agricultural

areas in the

Great Plains.

Wet Nitrogen Deposition

Total Nitrogen Deposition

Wet deposition

makes up most

of the total deposition load at

most of the monitoring sites in

the Eastern U.S.;

in southern California dry deposition makes up

a greater percentage of the

total.

Note: The size of the “pies” indicates the total magnitude of deposition; the colors indicate the percentage

of wet and dry deposition.

Source: NADP

Source: CASTNet

Notes:

1) The dots on the map represent

the locations of monitoring sites.

2) Map colors represent relative

concentrations and do not imply

ecological or human health status.
Acid Rain Program Annual Progress Report, 2001

page 27

Figure 22 shows the wet and total (wet plus dry) nitrogen (N) deposition in the

continental U.S. during 2001. Wet nitrogen deposition is highest in the Midwest

and in heavily agricultural areas of the Plains. Total nitrogen deposition is highest

in the Eastern U.S., although several monitoring stations in the West also show

relatively high levels of total nitrogen deposition. As is the case with sulfur, dry

deposition of nitrogen makes up a larger part of the total amount of deposition in

those areas nearest to the sources of NOx emissions. In some areas of southern

California, for example, the ratio of dry to wet deposition is approximately 4:1.

Clean Air Mapping and Analysis Program (C-MAP)

EPA has developed a mapping and analysis tool that can help users conduct

assessments of regional and national environmental changes. C-MAP takes

advantage of geographic mapping techniques to assess the environmental benefits of sulfur dioxide and nitrogen oxide emission reduction programs, including

the Acid Rain Program. Using a Geographic Information System (GIS), C-MAP

allows users to view a series of national and regional maps in the “Map Gallery”

section, and then download the data used to generate the maps in the “GIS Data

Download” section. The maps display information showing how changes in

emissions result in changes in air quality indicators, acid deposition, and sensitive ecosystems. The GIS database provides an extensive inventory of national/regional level emissions, environmental effects, and demographic data available for download, including air quality, surface water quality, acid deposition,

forest health, and sensitive ecosystem data. The data behind many of the graphics in this Progress Report, as well as many of the graphics themselves, are

available for download and analysis at http://www.epa.gov/airmarkets/cmap.

Freshwater Monitoring

The Temporally Integrated Monitoring of Ecosystems (TIME) and Long-Term

Monitoring (LTM) projects were initiated in the early 1990s by EPA’s Office of

Research and Development to determine whether emissions reductions have had

the intended effect of reducing acidity in the environment. Currently all the

TIME/LTM sites are in the Northeast and mid-Atlantic (see Figure 23); additional

sites in acid sensitive regions of the Southeast and West would make more complete assessments possible. TIME/LTM measures a variety of important chemical

characteristics in a regional population of lakes and streams, including acid neutralizing capacity, pH, sulfate, nitrate, several cations (e.g., calcium and magnesium), and aluminum. Its central objectives are to detect trends of these characteristics in regional populations of lakes or streams. 

TIME/LTM utilizes a hybrid sampling design. Lakes or streams in the TIME network are measured annually; the results from these sites are used to infer regional changes in chronic acidification. The LTM sites are a non-random group of

lakes or streams sampled on a frequent schedule (8-16 times per year) in order

to characterize both long-term (over years) and short-term (over weeks) variation

in their acid-base chemistry. LTM sites have been chosen to represent the subpopulations of lakes and streams most sensitive to acidic deposition effects.
Acid Rain Program Annual Progress Report, 2001

page 28

Figure 23. Location of TIME/LTM

Surface Water

Monitoring Sites 

Long-term monitoring sites for

acid rain are

critical to

assess whether

lakes and

streams are

recovering from

acidification.

Researchers use these data to model the episodic behavior of the sites, so that

the models can be applied to TIME data. This approach allows the proportion of

lakes and streams that undergo episodic acidification (short-term highly acidic

pulses) to be estimated as an adjunct to the information on chronic acidification

provided by the TIME results. Data collected in this network is used to assess

trends in acidification and recovery as shown in Figure 29.

Source: EPA
page 29

T

he emission reductions achieved under the Acid Rain Program have led to

important environmental and public health benefits. These include

improvements in air quality with significant benefits to human health,

reductions in acid deposition, the beginnings of recovery in surface waters,

improvements in visibility, and less damage to forests, coastal waters, and materials and structures.

Improved Air Quality and Reduced Acid Deposition

To evaluate the impact of emissions reductions on the environment, scientists and

policymakers use data collected from long-term national monitoring networks

such as NADP and CASTNet. Deposition and air quality monitoring data from

these and other air quality monitoring networks, such as the Interagency Monitoring of PROtected Visual Environments (IMPROVE) and the State/Local/National Air Monitoring Stations, can be accessed on or through the CASTNet website

at http://www.epa.gov/castnet. 

Data collected from these networks show that the decline in SO2 emissions from

the power industry has decreased acidic deposition and improved air quality. The

decline in NOx emissions has not been as large and the environmental improvements are not as widespread.

Analyses of CASTNet data show that concentrations of SO2 in the atmosphere

have decreased up to 8 micrograms per cubic meter (ug/m3) in the Northeast and

mid-Atlantic from 10-20 ug/m3in 1990 (see Figure 24). These reductions are primarily due to the significant decrease in SO2 emissions from power plants under

the Acid Rain Program.

Sulfate concentrations in the atmosphere, a primary component of fine particulate

matter in the East, have also decreased significantly since 1990 (see Figure 25).

Sulfate concentrations have decreased up to 3 ug/m 3in most of the Eastern

U.S. from levels of 5-8 ug/m3in 1990. These reductions are also primarily due

to the significant decrease in SO2 emissions from power plants under the Acid

Rain Program.

Wet sulfate deposition has decreased more than 8 kilograms/hectare (kg/ha; 1

kg/ha is equivalent to 0.89 pounds/acre) from 30-40 kg/ha/year in 1990 in much

Environmental 

Improvement and Trends
Sulfur dioxide

(SO2) concentrations have

decreased substantially in

most of the Midwest and Northeast since 1989-

1991.

Source: CASTNet

Acid Rain Program Annual Progress Report, 2001

page 30

Figure 24.

Trends in Sulfur: Average

Yearly Sulfur

Dioxide Concentrations, 1989-91

vs. 1999-2001

Sulfur Dioxide Concentration

Average of 1989-1991

Notes:

1) Data is presented for

the Eastern U.S. only

because there are not

enough CASTNet monitoring sites in the West to

support this type of

analysis.

2) Map colors represent

relative concentrations

and do not imply ecological or human health status.

Sulfur Dioxide Concentration

Average of 1999-2001
Acid Rain Program Annual Progress Report, 2001

page 31

Source: CASTNet

Sulfate (SO4

2-)

concentrations

in air, a primary

component of

fine particles in

the East, have

decreased substantially in

most of the East

since 1989-1991.

Figure 25.

Trends in Sulfur: Average

Yearly Sulfate

Concentrations,

1989-91 vs.

1999-2001

Sulfate Concentration

Average of 1989-1991

Notes:

1) Data is presented for

the Eastern U.S. only

because there are not

enough CASTNet monitoring sites in the West to

support this type of

analysis.

2) Map colors represent

relative concentrations

and do not imply ecological or human health status.

Sulfate Concentration

Average of 1999-2001
Acid Rain Program Annual Progress Report, 2001

page 32

Figure 26.

Trends in Sulfur: Average

Yearly Wet Sulfate Deposition,

1989-91 vs.

1999-2001

Wet sulfate

(SO4

2-) deposition has

decreased substantially

throughout the

Midwest and

Northeast since

1989-1991.

Wet Sulfate Deposition

Average of 1989-1991 

Wet Sulfate Deposition

Average of 1999-2001

Source: NADP Note: Map colors represent relative concentrations and do not imply ecological or

human health status.
Acid Rain Program Annual Progress Report, 2001

page 33

Figure 27.

Trends in Nitrogen: Average

Yearly Wet

Nitrogen Deposition and Nitric

Acid Concentrations, 1989-91

vs. 1999-2001

Nitrogen (N)

deposition

decreased

slightly in areas

of the Eastern

U.S. since 1990;

increases

occurred in

areas with sig -

nificant agricultural activity

(e.g., the Plains

and coastal

North Carolina).

Wet Nitrogen Deposition

Average of 1989-1991

Wet Nitrogen Deposition

Average of 1999-2001

Source: NADP Note: Map colors represent relative concentrations and do not imply ecological or

human health status.
Acid Rain Program Annual Progress Report, 2001

page 34

of the Ohio River Valley and Northeastern U.S. (see Figure 26). The greatest

reductions have occurred in the mid-Appalachian region. Important reductions

have also occurred in the Northeast, portions of the southern Appalachians, and

the Midwest. 

There have been no dramatic regional changes in wet nitrate deposition (see Fig -

ure 27). This reflects the fact that total nitrogen emissions from sources other than

power plants (e.g., automobiles, trucks, non-road vehicles, and agricultural activities) have increased since 1990. Wet nitrate deposition has in fact increased up

to 3 kg/ha in many areas since 1990. These increases occurred in the Plains and

eastern North Carolina, where there is significant agricultural activity, and in

southern California where motor vehicles are the predominant source of NOx

emissions. 

Visibility

In the atmosphere, SO2 and NOx gases are transformed into fine particles of sulfates and nitrates. Sulfate and nitrate particles scatter and absorb light, impairing

visibility and contributing to haze. In the East, sulfate particles account for more

than 50 percent of visibility impairment. The haziest days in the East reduce the

visual range to 20-24 km (12-15 miles). The visual range under naturally-occurring conditions is 128-144 km (77-86 miles). 

The Interagency Monitoring of PROtected Visual Environments (IMPROVE) network monitors visibility in the nation's national parks and wilderness areas. From

1992-1999, visibility in the ten eastern Class I area trend sites improved 1.5

Figure 28. Trends

in Visibility in the

Eastern U.S.

Class 1 Areas

Although visibility has only

improved slightly

in the Eastern

U.S. since the

early 1990s, 

significant visibility benefits are

expected when

the Acid Rain

Program is fully

implemented.

Worst visibility range is 20-24 km 

Mid-range visibility is 44-50 km

Best visibility range is 83-94 km

Source: Latest Findings on National Air Quality: 2000 Status and Trends, EPA 2001
Acid Rain Program Annual Progress Report, 2001

page 35

deciviews since 1992 on the haziest days (see Figure 28). A deciview is a measure of human perception of visibility; an improvement of 1 deciview is a perceptible change. On typical days in the East, visibility improved 1 deciview since 1992.

Visibility in the East is still significantly impaired in national parks and wilderness

areas, especially on the haziest days. Further reductions in fine particle concentrations will be necessary to restore visibility to natural levels.

Human Health Benefits

SO2 and NOx emissions react in the atmosphere to form fine particles and ozone.

These gases and fine particles are associated with a number of significant health

effects in sensitive populations. High SO2 concentrations can result in temporary

breathing impairments in sensitive populations, including asthmatics and those

who are active outdoors. A large number of epidemiological studies over the past

10-20 years show an association between ambient fine particle concentrations

and health effects, such as increased numbers of hospital admissions and emergency room visits for heart and lung disease, increased incidences of respiratory

disease and symptoms (such as asthma), decreased lung function, and even premature death. Children, the elderly, and individuals with existing cardiovascular or

lung conditions, such as asthma, are especially vulnerable to the effects of particles. The Acid Rain Program has reduced the amount of fine particles in the air

(see Figure 25) by lowering SO2 and NOx emissions, achieving significant human

health benefits nationwide. It is expected that the Acid Rain Program will achieve

further benefits as SO2 emissions continue to decrease to the level of the cap.

NOx emissions react with volatile organic compound gases in the atmosphere in

the presence of sunlight to form ozone. The scientific literature shows associa -

tions between ozone and a number of effects on the respiratory system, including

aggravation of asthma, increased susceptibility to respiratory illnesses like pneumonia and bronchitis, and permanent lung damage. Children, the elderly, people

with existing respiratory problems, and those exercising or working outside during

the ozone season are most vulnerable to the health effects of ozone. Additional

health benefits have been achieved under the Acid Rain Program due to NOx

reductions that reduce ozone concentrations. 

Ecological Effects of Reduced Acid Deposition

Freshwater

Acid deposition causes acidification of surface waters. In the 1980s, acid rain was

found to be the dominant cause of acidification in 75% of the acidic lakes and

50% of acidic streams. Areas especially sensitive to acidification include portions

of the Northeast (particularly Maine and the Adirondack and Catskill Mountains)

and Southeastern streams. Some high elevation Western lakes, particularly in the

Rocky Mountains, have become acidic, especially during snowmelt. However,

although many Western lakes and streams are sensitive to acidification, they are

not subject to continuously high levels of acid deposition and so have not become

chronically acidified. 
Acid Rain Program Annual Progress Report, 2001

page 36

Whether surface waters can resist acidification depends on the ability of the water

and watershed soil to neutralize the acid deposition it receives. This quality, called

Acid Neutralizing Capacity (ANC), depends largely on the watershed's physical

characteristics such as geology, soils, and size. Waters that are sensitive to acid -

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

and shallow soils. Conversely, watersheds that contain alkaline minerals, such as

limestone, tend to have waters with a high ANC.

As acidity increases, aluminum leached from the soil flows into lakes and streams

and can be toxic to aquatic species. The lower pH levels and higher aluminum

levels that result from acidification can make it difficult for fish and other aquatic

species to grow, reproduce, and survive. In some waters, the number of species

of fish able to survive has been directly correlated to water acidity. Acidification

can also decrease fish population density and individual fish size. 

Figure 29 indicates that since the beginning of the Acid Rain Program, sulfate

concentrations in lakes and streams have declined significantly in all monitored

regions of the Eastern United States, except Virginia. The type of soils present in

Figure 29.

Trends in Acidity

of Sensitive

Waters, 1990-

2000

In Pennsylvania

and the Adirondacks, acid neutralizing capacity (ANC) has

begun to

increase, an

indication of the

beginning of

recovery. 

Source: Recovery of Surface Water Chemistry in the Northern and Eastern U.S.: Effectiveness of the

Clean Air Act Amendments of 1990, EPA/620/R-02/004

Slope of Trend

* = not significant

decreasing 

concentrations

increasing 

concentrations
Acid Rain Program Annual Progress Report, 2001

page 37

Virginia make decreases in sulfate concentrations there unlikely for some time.

Nitrate concentrations have decreased significantly in the Catskills, Adirondacks,

and Vermont since 1990. Recovery, as shown by increasing Acid Neutralizing

Capacity (ANC), is occurring, especially in the Adirondacks and Pennsylvania. In

several regions, including the Adirondacks, recovery has begun in the past few

years (circa 1995). However, levels of base cations, including calcium, magnesium, and potassium, are not increasing; in fact, they are decreasing. This reduction in base cation levels over the same period is believed to slow the onset of

recovery. The sulfate reductions achieved by the Acid Rain Program are projected to spur the recovery of lakes and streams in the East. 

In spite of declining sulfate concentrations, some lakes and streams have been

slow to recover. Their recovery is slowed by continuing acid deposition, the presence of nitrate in surface waters, the loss of the soil's ability to neutralize excess

acidity, the contribution of naturally occurring acid sources, and a lengthy lag time

between deposition reduction and ecosystem recovery. 

Full recovery of damaged watersheds will be a lengthy process, especially since

acidic deposition is still occurring, albeit to a lesser extent. Although conditions

would have been worse without the Acid Rain Program, full recovery of some surface waters requires additional reductions in sulfur and nitrogen emissions.

Forests

Acid deposition, especially combined with other pollutants and natural stresses,

can also damage forest ecosystems. Sulfates and nitrates from acid deposition

leach nutrients from forest soils, reducing the forest's capacity to buffer further

acidification and removing elements essential for tree growth. Acidification also

leads to the mobilization of naturally-occurring aluminum, which may interfere

with the uptake of calcium by roots in forest soils. In addition, exposure to tropospheric ozone (a product of NOx emissions) has direct toxic effects on plant

leaves. The combined effects of depletion of soil nutrients, mobilization of aluminum, and exposure to ozone make trees more susceptible to drought, temperature extremes, and diseases. 

There is currently less stress on forest ecosystems compared to what it would

have been without the Acid Rain Program. The timeframe for full recovery, however, is uncertain. Leached nutrients must first be restored through weathering of

the bedrock and soilwater aluminum concentrations must be reduced. Even after

soil chemistry is restored, full recovery of sensitive forests is not expected to

occur for decades because of the extensive recovery time of trees and the time

required to re-establish forest floor ecosystems (soil biota, microbes, and roots). 

Coastal Waters

The nitrogen component of acid deposition is a significant source of nitrogen to

many estuaries and coastal waters in the Eastern U.S. Excessive nitrogen loads

from a variety of sources, including atmospheric deposition, causes many of
Acid Rain Program Annual Progress Report, 2001

page 38

those estuaries and coastal waters to periodically become eutrophic. Eutrophic

conditions include algal blooms (some of which may be harmful) and low levels of

dissolved oxygen in the water (hypoxia or anoxia) which can stress or kill fish and

shellfish. 

The Acid Rain Program has reduced nitrogen deposition in some places compared to what it would have been without Title IV (see Figure 27). However, in

many sensitive coastal waters there has been little or no reduction in nitrogen

deposition since 1990. Additional reductions from the power generation sector, as

well as reductions from other atmospheric sources of nitrogen, such as automobiles and trucks, and other land-based sources such as septic systems and urban

runoff, may be needed before coastal waters can recover from eutrophication.

Materials and Structures

SO2, NOx, and many of the pollutants they form can also corrode materials, particularly those made of limestone or marble. Monuments and historic buildings,

outdoor structures such as bridges and buildings, and automotive paints and fin -

ishes are all susceptible to damage by acidic pollutants. Studies have shown that

air pollution has been responsible for more deterioration of carbonate buildings

and statues than other weathering processes. Structures made of limestone and

marble are particularly sensitive to acidic deposition. Most damage appears to

come from dry deposition. However, in rural areas and in areas where buildings

and monuments remain wet for long periods of time, wet deposition can be a sig -

nificant or primary cause of damage. 

Weathering due to acid deposition may harm cultural assets (e.g., statues and

monuments) more than purely operational resources (e.g., bridges and buildings).

This is because the appearance of cultural resources, where much of their value

lies, is particularly vulnerable to damage. There are also historic and emotional

values attached to cultural assets, which increase the value of their preservation.

The Acid Rain Program has reduced the risk of damage to sensitive buildings and

materials by reducing the amount of SO2 and NOx emitted into the atmosphere

and the amount of dry sulfur deposition reaching sensitive structures. Therefore,

on-going monetary costs and cultural losses due to acid gases, particles, and

deposition are also expected to be declining under Title IV.
page 39

S

ummary

The Acid Rain Program continued to be successful in substantially reducing emissions of SO2 and NOx

from electric power plants during the second year of Phase

II. Sources continue to close in on the goal of reducing power plant SO2 emissions

from 1980 levels by 50% (8.5 million tons) in 2010 as required by the 1990 Clean

Air Act. Sources have also exceeded the goal of a two million ton reduction in NOx

emissions from projected 2000 levels as required by the 1990 Clean Air Act. 

Sources in both the more conventional NOx program and the cap and trade

approach for SO2 have demonstrated a high level of compliance and their efforts

have achieved measurable environmental results. The flexibility for sources inherent in the cap and trade approach has been successful at reducing compliance

costs and has not resulted in any significant geographic shifts in SO2 emissions. 

The Acid Rain Program has:

Established and maintained a robust infrastructure to ensure compliance with

the program and expanded our ability to assess its environmental benefits,

including: 

S A sound compliance tracking system;

S A high quality emissions monitoring system at every source;

S An expanded national dry deposition monitoring network to complement the nationwide wet deposition monitoring network.

Reduced emissions of SO2 and NOx substantially from the power generation

sector at a significantly lower cost than expected:

S In 2001, SO2 emissions were 10.6 million tons, 33% lower than

1990 emissions and 5% lower than 2000 emissions.

S In 2001, NOx emissions were 4.10 million tons, 25% lower than

1990 emissions and 8% lower than 2000 emissions. 

Contributed to measurable improvements in air quality, reductions in deposition, and recovery of acid-sensitive waters:

S SO2 concentrations in the atmosphere (a precursor to fine particles

and acid deposition) have decreased since 1990. In 2001, concentrations in the Northeast and Mid-Atlantic were 8-12 ug/m3

, as

much as 8 ug/m3lower than in 1990. 

S Sulfate concentrations in the atmosphere (a major component of

fine particles, especially in the East) have decreased since 1990.

In 2001, concentrations in most of the East were 2-3 ug/m3, as

much as 3 ug/m3lower than in 1990. 
Acid Rain Program Annual Progress Report, 2001

page 40

S Wet sulfate deposition, a major component of acid rain, has also

decreased since 1990. In 2001, deposition in the Northeast and

Midwest was 20-30 kg/ha/yr, as much as 12 kg/ha/yr lower than it

was in 1990.

S Wet nitrate deposition has not decreased regionally because of the

relatively small NOx reduction from power plants and the large contribution from other sources of NOx

. 

S Visibility has improved in the Eastern U.S.

S Acid neutralizing capacity, a major indicator of recovery in acidified

lakes and streams, is beginning to rise in streams in the Northeast,

including the Adirondacks. This is an indicator that recovery from

acidification is beginning in those areas.

S Reductions in fine particles due to reductions in emissions of SO2

and NOx are expected to continue to benefit human health by

reducing the incidence of respiratory and cardiovascular illnesses.

For more information on the EPA Acid Rain Program, visit our website at

http://www.epa.gov/airmarkets. For additional detailed emissions data see

http://www.epa.gov/airmarkets/emissions/index.
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F

or Further Information

The following publications are suggested as a starting point for those who want

additional information on any of the topics discussed in this Progress Report.

Additional information on emissions, air quality and deposition trends can be

found at http://www.epa.gov/airtrends and http://www.epa.gov/castnet. Additional

published scientific literature on emissions trading, acid rain, and benefits

assessments can be found at http://www.epa.gov/airmarkets/articles/index.

Emissions and Air Quality

A Ten-Year Spatial and Temporal Trend of Sulfate Across the United States,

Malm, Schichtel, Ames and Gebhart, Journal of Geophysical Research-Atmospheres, in press

Latest Findings on National Air Quality: 2000 Status and Trends, EPA 2001

EPA 454/K-02-001

National Air Quality and Emissions Trends Report, 1999

EPA 454/R-01-004

Environmental Effects of Acid Rain

Acidic Deposition in the Northeastern United States, BioScience 51(3):180-198,

2001

National Acid Precipitation Assessment Program Biennial Report to Congress:

an Integrated Assessment, National Science and Technology Council

Committee on Environment and Natural Resources, 1998

Recovery of Surface Water Chemistry in the Northern and Eastern U.S.: Effectiveness of the Clean Air Act Amendments of 1990, Stoddard, J. L., J. S. Kahl,

F. A. Deviney, D. R. DeWalle, D. C.T., A. T. Herlihy, J. H. Kellogg, P. S. Murdoch, J. R. Webb, and K. E. Webster, in press

Health Benefits Assessment

Reanalysis of the Harvard Six Cities Study and the American Cancer Society

Study of Particulate Air Pollution and Mortality; Health Effects Institute, 2000

Air Quality Criteria for Particulate Matter, volumes I-III, EPA 1996

EPA /600/P-95/001af-cf

Human Health Benefits from Sulfate Reduction Under Title IV of the 1990 Clean

Air Act Amendments, EPA 1995

EPA 430-R-95-010


Office of Air and Radiation

Clean Air Market Programs

EPA-430-R-02-009

1200 Pennsylvania Ave, NW

(6204N)

Washington, DC 20460

www.epa.gov/airmarkets

November 2002