The Particle Pollution Report

Current Understanding of Air Quality and Emissions through 2003

United States 

Environmental Protection 

Agency

99 00 01 02 03

30

20

10

0

Concentration, μg/m3

1999–2003: -10%

National Standard

Average

PM2.5 Concentrations are Declining
Printed on 100% recycled/recyclable process chlorine-free paper with 100% post-consumer fiber using vegetable-oil-based ink.
EPA 454-R-04-002

December 2004

The Particle Pollution Report

Current Understanding of Air Quality and Emissions through 2003

Contract No. 68-D-02-065

Work Assignment No. 2-01

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Emissions, Monitoring, and Analysis Division

Research Triangle Park, North Carolina
Contents

Introduction ii

Major Findings 1

Understanding Particle Pollution 2

Particle Pollution Is... 2

Complex 2

A Continuum of Sizes 2

Made Up of Many Species 3

Seasonal 4

Both Local and Regional 8

Particle Pollution in 2003 10

PM10 – PM2.5 12

Looking at Trends 13

PM10 National and Regional Trends 13

PM2.5 National and Regional Trends 14

25-year PM2.5 Trends 16

Rural Sulfate Trends 18

Explaining the Trends 20

Regional PM2.5 Trends in Three 

Regions (1999-2003) 20

Effect of Meteorology 21

The PM2.5 Remainder 22

Control Programs 23

The Future 25

Upcoming PM2.5 Designations 26

PM2.5 and Other Pollutants 26

Introduction

From the black puff of smoke from an old diesel bus to the

haze that obscures the view in our national parks, particle

pollution affects us all. This complex pollutant is present

year-round, both in our cities and in the countryside, and 

it can cause health problems for millions of Americans.

EPA’s national air quality standards for particle pollution 

are designed to protect public health and the environment.

As this report shows, we are seeing progress: levels of particle

pollution are decreasing on a national scale. Yet millions 

of people still live in areas of the country where particle

pollution levels exceed national air quality standards. This

harmful pollution affects not only people, but also visibility,

ecosystems, and man-made materials.

EPA considers fine particle pollution its most pressing air

quality problem, and the Agency is taking a number of steps

that will reduce particle emissions and formation. These

efforts range from EPA’s Acid Rain program and regulations

reducing emissions from fuels and diesel engines, to implementation of the Agency’s first fine particle standards and 

a proposed rule to reduce particle-forming emissions from

power plants.

In this report, EPA

■ Explores characteristics of particle pollution in the

United States

■ Analyzes particle pollution for 2003 (the most recent

year of data)

■ Summarizes recent and long-term trends

■ Investigates the relationship between air quality and

emissions

■ Reviews some current programs and future prospects for

reducing particle pollution levels.

In addition, text boxes in this report present information 

on more specialized areas of interest, such as the PM

Supersite project, episodic events, satellite monitoring, and

the relationship of particle pollution to other air pollutants.

ii
1

Major Findings

Air Quality Improvements

■ Particulate matter (PM) air quality has been

improving nationwide, both for PM2.5 and PM10.

■ PM2.5 concentrations 

– in 2003 were the lowest since nationwide monitoring began in 1999 

– have decreased 10% since 1999

– are about 30% lower than EPA estimates they

were 25 years ago.

■ PM10 concentrations

– in 2003 were the second lowest since nationwide

monitoring began in 1988

– have declined 7% since 1999

– have declined 31% since 1988.

■ In 2003, 62 million people lived in 97 U.S. counties with monitors showing particle pollution levels

higher than the PM2.5 air quality standards, the

PM10 standards, or both.

■ Monitored levels of both PM2.5 and PM10 generally

decreased the most in areas with the highest

concentrations. For example, PM2.5 levels decreased

20% in the Southeast from 1999 to 2003. The

Northwest showed a 39% decrease in PM10 levels

from 1988 to 2003.

Sources and Emissions

■ Sulfates, nitrates, and carbon compounds are the

major constituents of fine particle pollution.

Sulfates and nitrates form from atmospheric transformation of sulfur dioxide and nitrogen oxide

gases. Carbon compounds can be directly emitted,

or they can form in the atmosphere from organic

vapors.

■ Approximately one-third of the PM2.5 improvement

observed in the eastern half of the country can be

attributed to reduced sulfates; a large portion of the

remaining PM2.5 improvement is attributable to

reductions in carbon-containing particles, especially

in the Industrial Midwest and the Southeast.

■ Power plant emissions of sulfur dioxide dropped

33% from 1990 to 2003, largely as a result of EPA’s

Acid Rain program. These reductions yielded

significant regional reductions in sulfate concentrations, reducing acid deposition and improving 

visibility.

■ Nationwide, reductions in industrial and highway

vehicle emissions of fine particles and volatile

organic compounds appear to have contributed to

the improvement in PM2.5.

■ In the eastern half of the country

– regional pollution accounts for more than half of

the measured PM2.5. This regional pollution

comes from a variety of sources, including power

plants, and can be transported hundreds of miles.

– sulfates account for 25% to 55% of PM2.5 levels.

Sulfate levels are similar in urban and nearby rural

areas. Power plants are the largest contributor to

this sulfate formation.

■ In the Industrial Midwest, Northeast, and southern

California, nitrates make up a large portion of

PM2.5, especially in winter. Average nitrate concentrations in urban areas are generally higher than

nearby rural levels. Power plants and highway

vehicle emissions are large contributors to nitrate

formation.

■ EPA and states have put in place a number of

control programs that will continue to reduce

particle-forming emissions. EPA’s 2004 Clean Air

Nonroad Diesel Rule will significantly reduce

emissions from nonroad diesel equipment across the

country. EPA’s proposed Clean Air Interstate Rule

(proposed December 2003) will reduce PMforming emissions from power plants in the eastern

United States.
Complex

Perhaps no other pollutant is as complex as

particle pollution. Also called particulate matter

or PM, particle pollution is a mixture of solid

particles and liquid droplets found in the air.

Some particles, such as dust, dirt, soot, or smoke,

are large or dark enough to be seen with the

naked eye. Others are so small, they can only be

detected using an electron microscope.

These tiny particles come in many sizes and

shapes and can be made up of hundreds of

different chemicals. Some particles are emitted

directly from a source, while others form in

complicated chemical reactions in the atmosphere. And some can change back and forth

from gas to particle form. Particle pollution also

varies by time of year and by location and is

affected by several aspects of weather, such as

temperature, humidity, and wind.

Understanding Particle Pollution

Figure 1. Comparison of PM sizes.

Note: In this report, particle size or diameter refers to a normalized measure called aerodynamic 

diameter, which accounts for the irregular shape and varying density of most particles.

2

Particle Pollution Is...

A Continuum of Sizes

In general, particle pollution consists of a mixture

of larger materials, called “coarse particles,” and

smaller particles, called “fine particles.” Coarse

particles have diameters ranging from about 

2.5 micrometers (μm) to more than 40 μm, while

fine particles, also known as known as PM2.5,

include particles with diameters equal to or

smaller than 2.5 μm. EPA also monitors and

regulates PM10, which refers to particles less than

or equal to 10 μm in diameter. PM10 includes

coarse particles that are “inhalable” — particles

ranging in size from 2.5 to 10 μm that can 

penetrate the upper regions of the body’s respiratory defense mechanisms. “Ultrafine” particles

are a subset of PM2.5, measuring less than 0.1 μm

in diameter.
3

Made Up of Many Species

Particles are made up of different chemical

components. The major components, or species,

are carbon, sulfate and nitrate compounds, and

crustal materials such as soil and ash. The

different components that make up particle

pollution come from specific sources and are

often formed in the atmosphere (see “Sources

and Transport of Particle Pollution” on page 6).

The chemical makeup of particles varies across

Sulfates

Nitrates

Carbon

Crustal

Northwest

Southern

California

Southeast

Industrial

Midwest

Northeast

Southwest

Upper

Midwest

Circle size corresponds

to PM2.5 concentration.

WEST EAST

Figure 2. Average PM2.5 composition in urban areas by region, 2003.

Note: In this report, the term“sulfates” refers to ammonium sulfate and “nitrates” refers to ammonium nitrate.“Carbon” refers to

total carbonaceous mass, which is the sum of estimated organic carbon mass and elemental carbon.“Crustal” is estimated using the

IMPROVE equation for fine soil at vista.cira.colostate.edu/improve.

This report summarizes analysis results using the geographic areas shown in this map. The area definitions correspond to the regions 

used in EPA’s 1996 PM Criteria Document (www.epa.gov/ttn/naaqs).

In this report,“East” includes three regions: the Northeast, the Industrial Midwest, and the Southeast.

the United States (see Figure 2). For example,

fine particles in the eastern half of the United

States contain more sulfates than those in the

West, while fine particles in southern California

contain more nitrates than other areas of the

country. Carbon is a substantial component of

fine particles everywhere. (For information on

the composition of ultrafine and coarse particles

in Los Angeles, see page 7.)

Health Effects

Exposure to particles can lead to a variety of serious health

effects. The largest particles do not get very far into the lungs,

so they tend to cause fewer harmful health effects. Coarse 

and fine particles pose the greatest problems because they

can get deep into the lungs, and some may even get into the

bloodstream. Scientific studies show links between these small

particles and numerous adverse health effects. Long-term

exposures to PM, such as those experienced by people living

for many years in areas with high particle levels, are associated with problems such as decreased lung function, development of chronic bronchitis, and premature death. Short-term

exposures to particle pollution (hours or days) are associated

with a range of effects, including decreased lung function,

increased respiratory symptoms, cardiac arrythmias (heartbeat

irregularities), heart attacks, hospital admissions or emergency

room visits for heart or lung disease, and premature death.

Sensitive groups at greatest risk include people with heart or

lung disease, older adults, and children.

Environmental Effects

Fine particles are the major source of haze that reduces

visibility in many parts of the United States, including our

national parks. PM affects vegetation and ecosystems by

settling on soil and water, upsetting delicate nutrient and

chemical balances. PM also causes soiling and erosion damage

to structures, including culturally important objects such as

monuments and statues.

Health and Environmental Effects of Particulate Matter

Particle Pollution Is...
4

Seasonal

Fine particles often have a seasonal pattern. PM2.5

values in the eastern half of the United States are

typically higher in the third calendar quarter

(July-September) when sulfates are more readily

formed from sulfur dioxide (SO2) emissions from

power plants in that region. Fine particle concentrations tend to be higher in the fourth calendar

quarter in many areas of the West, in part because

fine particle nitrates are more readily formed in

cooler weather, and wood stove and fireplace use

produces more carbon.

The time of year also influences daily fine particle

patterns. Unlike daily ozone levels, which are

usually elevated in the summer, daily PM2.5 values

at some locations can be high at any time of the

year. Figure 4 shows 2003 PM2.5 levels for Fresno

and Baltimore. The colors in the background of

these charts correspond to the colors of the Air

Quality Index (AQI), EPA’s tool for informing

the public about air pollution levels in their

communities. As the Fresno graphic illustrates,

fine particles can be elevated in the fall and

winter in some areas, while ozone is elevated

only in the summer. Contrast the Fresno graphic

with the Baltimore graphic, which shows PM

elevated year-round. Note: Elevated levels on 

the AQI do not indicate that an area is violating

EPA’s national air quality standards for any 

particular pollutant. The AQI is designed to 

help people reduce their individual exposure 

to pollution.

0

5

10

15

20

25

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5

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25 0

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25

Oct-Dec

Jul-Sep

Apr-Jun

Jan-Mar

Northwest

Southern

California

Upper

Midwest

Southeast

Industrial

Midwest

Northeast

Southwest PM 2.5 Mass, μg/m3

PM 2.5 Mass, μg/m3

PM 2.5 Mass, μg/m3

PM 2.5 Mass, μg/m3

PM 2.5 Mass, μg/m3

PM 2.5 Mass, μg/m3

PM 2.5 Mass, μg/m3

Figure 3. Seasonal averages of PM2.5 concentration by region, 1999–2003.

Particle Pollution Is...
The AQI is an index for reporting daily air

quality. It tells how clean or polluted the

air is and what associated health effects

might be a concern. The AQI focuses on

health effects people may experience

within a few hours or days after

breathing polluted air. EPA calculates the

AQI for five major pollutants regulated

by the Clean Air Act: particulate matter,

ozone, carbon monoxide, sulfur dioxide,

and nitrogen dioxide. The AQI values for

particulate matter are shown here.

Air Quality Index (AQI) - Particulate Matter

5

PM2.5 PM10

AQI (μg/m3) (μg/m3) Air Quality Descriptor

0–50 0.0–15.4 0–54 Good

51–100 15.5–40.4 55–154 Moderate

101–150 40.5–65.4 155–254 Unhealthy for Sensitive Groups

151–200 65.5–150.4 255–354 Unhealthy

201–300 150.5–250.4 355–424 Very unhealthy

Ozone PM2.5

0

20

40

60

80

100

120

140

160

180

200

220

240

260

280

300

Date

JAN FEB MAR APR MAY JUN JUL AUG S EP OCT NOV DEC JAN

Very Unhealthy

Unhealthy

Unhealthy for Sensitive Groups

Moderate

Good

Air Quality Index (AQI)

0

20

40

60

80

100

120

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300

Date

JAN FEB MAR APR MAY JUN JUL AUG S EP OCT NOV DEC JAN

Very Unhealthy

Unhealthy

Unhealthy for Sensitive Groups

Moderate

Good

Fresno, CA

Air Quality Index (AQI)

Baltimore, MD

Note:These graphs represent data from Federal Reference Method monitors. They do not show data from all

monitors that report the Air Quality Index.

Figure 4. Daily PM2.5 and ozone AQI values, 2003.
Visibility

6

Sources

Particulate matter includes both “primary” PM, which is

directly emitted into the air, and “secondary” PM, which forms

indirectly from fuel combustion and other sources. Generally,

coarse PM is made up of primary particles, while 

fine PM is dominated by secondary particles.

Primary PM consists of carbon (soot) — emitted from cars,

trucks, heavy equipment, forest fires, and burning waste —

and crustal material from unpaved roads, stone crushing,

construction sites, and metallurgical operations.

Secondary PM forms in the atmosphere from gases. Some of

these reactions require sunlight and/or water vapor.

Secondary PM includes

• Sulfates formed from sulfur dioxide emissions from 

power plants and industrial facilities

• Nitrates formed from nitrogen oxide emissions from cars,

trucks, and power plants

• Carbon formed from reactive organic gas emissions from

cars, trucks, industrial facilities, forest fires, and biogenic

sources such as trees.

Cars, trucks, heavy equipment,

wild fires, waste burning,

and biogenics

Power

generation

Suspended soil

and metallurgical

operations

Cars, trucks, and

power generation

Carbon

Crustal

Nitrates Sulfates

Automobiles, Power Generation, and Other

Sources Contribute to Fine Particle Levels

Transport

In the atmosphere, coarse and fine particles behave in

different ways. Larger coarse particles may settle out from the

air more rapidly than fine particles and usually will be found

relatively close to their emission sources. Fine particles,

however, can be transported long distances by wind and

weather and can be found in the air thousands of miles from

where they were formed.

Sources and Transport of Particle Pollution

Note:Ammonia from sources such as fertilizer and animal feed

operations contributes to the formation of sulfates and nitrates

that exist in the atmosphere as ammonium sulfate and 

ammonium nitrate.

Note: For more information about the apportionment of

fine particles to their sources, go to www.epa.gov/oar/

oaqps/pm25/docs.html

One of the most obvious effects of air pollution occurs both 

in urban areas and at the country’s best-known and mosttreasured national parks and wilderness areas. Visibility 

impairment occurs when fine particles scatter and absorb light,

creating a haze that limits the distance we can see and that

degrades the color, clarity, and contrast of the view. The 

particles that cause haze are the same particles that contribute

to serious health problems and environmental damage.

Visibility impairment—and the concentration of particles that

cause it—generally is worse in the eastern United States than it

is in the West. Humidity can significantly increase visibility

impairment by causing some particles to become more efficient

at scattering light. Average relative humidity levels are higher

in the East (70% to 80%) than in the West (50% to 60%).

In the East, reduced visibility is mainly attributable to sulfates,

organic carbon, and nitrates. Poor summertime visibility is

primarily the result of high sulfate concentrations, combined

with high humidity. Sulfates, which dominate the composition

of these visibility-impairing particles, have been found to

contribute even more to light extinction than they do to fine

particle concentrations. In the West, organic carbon, nitrates,

and crustal material make up a larger portion of total particle

concentrations than they do in the East.

Through its 1999 regional haze rule, EPA, states, and other

federal agencies are working to improve visibility in 156

national parks and wilderness areas such as the Grand Canyon,

Yosemite, the Great Smokies, and Shenandoah. Five multistate

regional planning organizations are working together to

develop and implement regional haze reduction plans. For

more information, see www.epa.gov/airtrends/vis.html.

Shenandoah National Park (Virginia) under bad and

good visibility conditions.Visual range is 25 km in the left

photo and 180 km in the right photo.

Yosemite National Park (California) under bad and good 

visibility conditions.Visual range is 111 kilometers (km) in

the left photo and greater than 208 km in the right photo.
7

After issuing the nation’s first PM2.5 standards in 1997, EPA

developed the PM Supersites project, a monitoring research

program, to address a number of scientific issues associated

with particulate matter. Program goals focus on obtaining

atmospheric measurements to

• Characterize PM, its constituents, atmospheric transport, and source categories that affect PM in any

region

• Compare and evaluate different PM measurement

methods (e.g., emerging sampling methods, routine

monitoring techniques)

• Support exposure and health effects research

concerning the relationships between sources, ambient

PM concentrations, and human exposures and health

effects and the biological basis for these relationships.

EPA selected eight locations for Supersites, including Los

Angeles. Atmospheric measurements taken at the Los

Angeles site between October 2002 and September 2003

show that ultrafine particles make up a small portion of the

PM concentration compared to inhalable coarse and fine

particles. However, the number of ultrafine particles is significantly larger than the number of coarse or fine particles. EPA

is studying this from a health perspective.

The Los Angeles data also show that coarse, fine, and ultrafine PM have different compositions. For each type of PM,

there is a difference in the relative amounts of nitrates,

sulfates, crustal materials, and carbon. Carbon, shown here 

as organic and elemental carbon, makes up a large fraction 

of ultrafine and fine PM in Los Angeles.

For more information, see

www.epa.gov/ttn/amtic/supersites.html and

www.epa.gov/ttn/amtic/laprog.html

PM Supersites

Los Angeles, CA

(University of Southern California Site)

Oct

 02

Nov

02

Dec

 02

Jan

 03

Feb

 03

Mar

 03

Apr

 03

May

 03

Jun

03

Jul

03

Aug

03

Sep

03

0

10

20

30

40

50

60

Mass, μg/m3

Ultrafine Fine Coarse

Ultrafine Particles (<0.1 μm)

Fine Particles (<2.5 μm)

Particles 2.5 to 10 μm

Nitrates

Sulfates

Metals

Organic Carbon

Elemental Carbon

Note:“Crustal materials” include windblown soil, industrial process emissions, sea salt, and flyash from combustion.
8

Both Local and Regional

Both local and regional sources contribute to

particle pollution. Figure 5 shows how much of

the PM2.5 mass can be attributed to local versus

regional sources for 13 selected urban areas

(arranged west to east). In each of these urban

areas, monitoring sites were paired with nearby

rural sites. When the average rural concentration

is subtracted from the measured urban concentration, the estimated local and regional contributions become apparent.

In the East, regional pollution contributes more

than half of total PM2.5 concentrations. Rural

background PM2.5 concentrations are high in the

East and are somewhat uniform over large

geographic areas. These regional concentrations

come from emission sources such as power

plants, natural sources, and urban pollution and

can be transported hundreds of miles.

Measured PM2.5 Concentration

Missoula

Salt Lake City

Tulsa

St. Louis

Birmingham

Indianapolis

Atlanta

Cleveland

Charlotte

Richmond

Baltimore

New York City

0 5 10 15 20 25 30

Annual Average PM2.5 Concentration, μg/m3

Fresno

WEST

EAST

Regional

Contribution

Local

Contribution

Figure 5. Local and regional contribution to urban PM2.5.

For the cities shown in Figure 5, local contributions range from 2 to 20 micrograms per cubic

meter (μg/m3), with the West generally showing

larger local contributions than the East. In the

East, local contributions are generally greatest in

cities with the highest annual average PM2.5

concentrations.

Figure 6 shows the local and regional contributions for the major chemical components that

make up urban PM2.5: sulfates, carbon, and

nitrates. In the eastern United States, the local

contribution of sulfates is generally small. Most

sulfates in the East are converted from regional

SO2 emissions and are transported long distances

from their sources.

Carbon has the largest local contribution of the

three major chemical components. These local

emissions come from a combination of mobile

and stationary combustion sources. The regional

Note: Urban and nearby rural PM2.5

concentrations suggest substantial

regional contributions to fine particles 

in the East.The measured PM2.5

concentration is not necessarily the

maximum for each urban area.

Particle Pollution Is...

Regional concentrations are derived

from the rural IMPROVE monitoring

network,

http://vista.cira.colostate.edu/improve.
9

Figure 6. Local and regional contribution of major PM2.5 chemical components.

contribution, which varies from 30% to 60% of

the total carbon at urban locations, is from rural

emission sources such as vegetation and wildfires,

as well as region-wide sources such as cars and

trucks.

Missoula

Salt Lake City

Tulsa

St. Louis

Birmingham

Indianapolis

Atlanta

Cleveland

Charlotte

Richmond

Baltimore

New York City

0 6 10 12

Fresno

WEST

EAST

Regional

Contribution

Local

Contribution

2 8 4

Sulfates

Annual Average Concentration

of Sulfates, μg/m3

Carbon

Missoula

Salt Lake City

Tulsa

St. Louis

Birmingham

Indianapolis

Atlanta

Cleveland

Charlotte

Richmond

Baltimore

New York City

0 6 10 12

Fresno

WEST

EAST

Regional

Contribution

Local

Contribution

2 8 4

Annual Average Concentration

of Carbon, μg/m3

Missoula

Salt Lake City

Tulsa

St. Louis

Birmingham

Indianapolis

Atlanta

Cleveland

Charlotte

Richmond

Baltimore

New York City

0 6 10 12

Fresno

WEST

EAST

Regional

Contribution

Local

Contribution

2 8 4

Nitrates

Annual Average Concentration

of Nitrates, μg/m3

Nitrates represent only about 10% to 30% of

annual average PM2.5,and urban concentrations

are higher than the nearby regional levels. This is

likely due to local nitrogen sources such as cars,

trucks, and small stationary combustion sources.

Note: Regional concentrations are

derived from the rural IMPROVE

monitoring network,

http://vista.cira.colostate.edu/improve.
≤ 10

10.1 - 15

15.1 - 20

> 20

PM2.5, 2003

Concentration range (μg/m3)

10

concentrations greater than the PM10 or PM2.5

national air quality standards. Thirty-seven

counties (21 million people) measured concentrations in excess of the national PM10 standards,

and 72 counties (53 million people) exceeded the

national PM2.5 standards. These numbers do not

include other areas outside of these counties that

might contribute to levels above the standards.

Figure 7 shows the range of PM10 concentrations

across the country in 2003. The highest concentrations were recorded in Inyo and Mono counties, California; El Paso County, Texas; and Dona

Ana County, New Mexico. Figure 8 shows the

Nationally, fine particle concentrations in 2003

were the lowest since nationwide PM2.5 monitoring began in 1999. Compared with 2002, the

biggest decreases occurred in the Industrial

Midwest and parts of California — areas with

relatively high PM2.5 concentrations. PM10 concentrations were slightly higher in 2003 than the

previous year, but they are still the second lowest

since nationwide PM10 monitoring began in 1988.

Although average concentrations have declined

nationally, many areas still exceed the level of 

the PM standards. In 2003, monitors in 97 

counties (home to 62 million people) showed

Particle Pollution in 2003

Concentration range (μg/m3)

≤ 54

55 - 154

155 - 354

> 354

Figure 8. PM2.5 concentrations, 2003 (annual average).

Figure 7. PM10 concentrations, 2003 (second maximum 24-hour).

Note: Circle size corresponds

to PM2.5 concentration range.
11

range of 2003 PM2.5 annual averages across the

country. The highest annual averages occurred 

in southern California and Pittsburgh. High

levels are also seen in many urban areas in the

Southeast, Northeast, and Industrial Midwest. See

www.epa.gov/airtrends/pm.html for county-level

maps of PM.

PM2.5 concentrations can reach unhealthy levels

even in areas that meet the annual standard. In

2003, there were 277 counties with at least 

1 unhealthy day based on PM2.5 AQI values, as

shown in Figure 9. Nearly two-thirds of those

counties had annual averages below the level of

the standard. Figure 10 shows how several major

metropolitan areas fared in 2003 relative to

previous years. Most metropolitan areas had fewer

unhealthy PM2.5 days in 2003 compared to the

average from the previous 3 years, which reflects

the improvements observed in 2003.

Number of AQI

Days Above 100

None

1 - 5

6 - 10

11 - 30

> 30

Figure 9. PM2.5 AQI days above 100 (≥40.5 μg/m3) in 2003.

New York

Washington, DC Cincinnati

Chicago

St. Louis

Birmingham

Atlanta

Minneapolis

Charlotte

Dallas

Fresno

Sacramento

Salt Lake City

Phoenix

Denver

Missoula

Seattle

Number of days (2000-2002, average)

Number of days (2003)

7 2

2 1

14

3

6

3 1

10 13

0

3

1

21

1 1

0

13 6

8 4

13 7

21

6

11 9

2 2

12

5

9 6

Figure 10. Number of days with PM2.5 AQI levels above 100, 2003 versus average 2000–2002.

Note:This map represents data from Federal Reference Method monitors. It does not show data from all monitors that

report the Air Quality Index.As such, it may not provide a complete picture of days above the AQI in some cities.
EPA first established National Ambient Air Quality Standards

(NAAQS) for total suspended particulate (TSP) in 1971.

When the standards were revised in 1987, TSP was replaced

by PM10. In 1997, EPA revised the primary (health) and

secondary (welfare) PM NAAQS by adding standards for

PM2.5. EPA added PM2.5 standards because fine particles

are more closely associated with serious health effects.

The NAAQS for PM10 and PM2.5 include both short-term

(24-hour) and long-term (annual) standards:

NAAQS PM 2.5 PM10

Short-term 65 μg/m3 150 μg/m3

(24-hour average)

Long-term 15 μg/m3 50 μg/m3

(annual average)

12

PM10 – PM2.5

Particulate matter varies greatly in size. “Coarse”

particles can be as large as 40 micrometers (μm)

in diameter or even larger. EPA's National

Ambient Air Quality Standards (NAAQS) for

particulate matter, however, have focused on

particles that are 10 μm in diameter or smaller.

These particles are the most likely to be inhaled

and can penetrate into the lower respiratory tract.

EPA has had air quality standards for particles 

10 μm and smaller since 1987. In 1997, EPA also

established an NAAQS for fine particles — those

particles 2.5 μm in diameter or smaller. EPA is

now in the process of reviewing the PM

NAAQS.

As shown in Figure 11, the size distribution of

particles smaller than 10 μm but larger than 

2.5 μm varies by geographic location. Levels of

PM10-2.5 are generally higher in the West, particularly the Southwest. PM10-2.5 typically comprises

more than half of the PM10 mass in the West.

Data also suggest that concentrations of particles

between 2.5 and 10 μm in size are lower in the

mid-Atlantic and Southeast. Overall, while

directly emitted PM2.5 and its precursors can

come from both local and regional sources, the

larger particles that are part of PM10 tend to

come from local sources.

Figure 11. Percent of 2003 annual average concentration of particles smaller 

than 10 μm but larger than 2.5 μm, by region.

National Standards for Particulate Matter

Compliance

Each PM standard carries a separate threshold for 

compliance:

• For the long-term standards for both PM2.5 and PM10,

compliance is determined based on the average of three

consecutive annual average values.

• Compliance with the short-term PM2.5 standard is 

determined by the 3-year average of the annual 98th

percentile of 24-hour concentrations.

• The short-term standard for PM10 is not to be exceeded

more than once per year, averaged over 3 years.

EPA reviews the NAAQS on a regular basis. The standards

for PM10 and PM2.5 are currently under review, to be

completed in 2006.

NorthwestSouthern CASouthwestInd. MidwestUpper

Midwest

NortheastSoutheast

PM2.5 PM10-2.5

100%

90%

80%

70%

60%

50%

40%

30%

20%

10%

0%

WEST EAST

Note: μg/m3 = micrograms per cubic meter.
13

PM10 National and Regional Trends

Nationally, PM10 concentrations have decreased

31% since 1988, as shown in Figure 12.

Regionally, PM10 decreased most in areas with

historically higher concentrations — the

Northwest (39%), the Southwest (33%), and

southern California (35%).

Programs aimed at reducing direct emissions of

particles have played an important role in

reducing PM10 concentrations, particularly in

western areas. Some examples of PM10 controls

include paving unpaved roads, replacing wood

and coal with cleaner-burning fuels like natural

gas, and using best management practices for

agricultural sources of resuspended soil.

Additionally, EPA’s Acid Rain Program has

substantially reduced SO2 emissions from power

plants since 1995 in the eastern United States,

contributing to lower PM concentrations. Direct

emissions of PM10 have decreased approximately

25% nationally since 1988.

90 91 92 93 94 95 96 97 98 99 00 01

Concentration, μg/m3

NAAQS

10% of sites have concentrations below this line

90% of sites have concentrations below this line

1988–2003: 31% decrease

1999–2003: 7% decrease

02 03

434 Monitoring Sites

Average

0

10

20

30

40

50

60

88 89 0

1

2

94 96 98 00 02

Yearly Emissions, Million Tons

88 90 92

4

3

03

1988–2003: approximately 25% decrease

1999–2003: 3% decrease

Fuel Combustion

Transportation Industrial Processes

89 91 93 95 97 99 01

In 1996, EPA refined its

methods for estimating

emissions.

Northwest

Southern

California

Southeast

Industrial

Midwest

Northeast

Southwest

National Standard: 50 μg/m3

Regional Trend

39%

35.5 21.5

29%

34.2 24.3

29%

28.8 20.5

25%

32.9 33% 24.6

39.5 26.5 35%

42.3

27.6

Upper

Midwest

16%

35.8 29.9

Annual Average PM10 Concentrations, 1988–2003

National PM10 Air Quality

National Direct PM10 Emissions

Looking at Trends

Figure 12. Regional and national trends in annual average PM10 concentrations and emissions,

1988–2003.
14

PM2.5 National and Regional Trends

PM2.5 concentrations have decreased 10% nationally since 1999. Generally, PM2.5 has decreased

the most in regions with the highest concentrations — the Southeast (20%), southern California

(16%), and the Industrial Midwest (9%), as shown

in Figure 13. With the exception of the

Northeast, the remaining regions posted modest

declines in PM2.5 from 1999 to 2003.

A variety of local and national programs have

resulted in a 5% decrease in estimated direct

emissions of PM2.5 over the past 5 years. In addition, programs that reduce the gaseous emissions

that can form particles in the atmosphere have

yielded additional reductions. National programs

that affect regional emissions — including

EPA’s Acid Rain Program — have contributed

to lower sulfate concentrations and, consequently, to lower PM2.5 concentrations, particularly in the Industrial Midwest and Southeast.

National ozone-reduction programs designed

to reduce emissions of volatile organic

compounds (VOCs) and nitrogen oxides

(NOx) also have helped reduce carbon and

nitrates, both of which are components of

PM2.5. Nationally, SO2, NOx, and VOC emissions decreased 9%, 9%, and 12%, respectively,

from 1999 to 2003. In eastern states affected

by the Acid Rain Program, sulfates decreased

7% over the same period.
15

Northwest

Southern

California

Upper

Midwest

Southeast

Industrial

Midwest

Northeast

Southwest

National Standard: 15 μg/m3

Regional Trend

10.5 9.0

15% 11.4 10.7

6% 14.1 15.6

9%

13.2 13.4

1%

12.6 15.7

20%

16.9 20.0

16% 8.7 8.1

7%

Figure 13. Regional and national trends in annual average PM2.5 concentrations and emissions

related to PM2.5 formation, 1999–2003.

Annual Average PM2.5 Concentrations, 1999–2003

99 00 01 02 03

30

20

10

0

Concentration, μg/m3

NAAQS

10% of sites have

concentrations

below this line

90% of sites have

concentrations

below this line

780 Monitoring Sites

Average

10% Decrease

Yearly Emissions, Thousand Tons

9%

5%

9%

12%

National PM2.5

Air Quality

National PM2.5 and Precursor Emissions

PM2.5

Fuel Combustion Industrial Processes Transportation

VOC

SO2 NOx

30

25

20

15

10

5

0

99 00 01 02 03

30

25

20

15

10

5

0

99 00 01 02 03

3

2

1

0

99 00 01 02 03

30

25

20

15

10

5

0

99 00 01 02 03

Only a subset of VOC

contributes to PM2.5

Note: Ammonia is a contributor to PM2.5 formation. However, because of uncertainty in ammonia 

emission estimates, its trends are not shown here.
16

years, average PM2.5 levels in 2003 were the

lowest on record. Low-sulfur gasoline use 

and ozone reduction programs designed to

control NOx and VOC emissions may have

contributed to the PM2.5 decrease observed 

in the 1990s.

■ Washington, DC: PM2.5 concentrations are

currently (2003) at their lowest levels. The

relatively large drop from 1994 to 1995

corresponds to decreases in sulfates (21%) and

organic carbon (30%). The decrease in

sulfates is attributable in part to the Acid

Rain Program, which substantially reduced

SO2 emissions from power plants during this

time. The decrease in organic carbon is

attributable in part to the use of reformulated

gasoline.

■ Chicago: PM2.5 concentrations at this site

have dropped substantially since the early

1980s, reaching their lowest levels in 2003.

For additional information on long-term trends,

see www.epa.gov/airtrends/pm.html.

25-Year PM2.5 Trends

Because EPA’s national PM2.5 monitoring

network is just 5 years old, we use data from

older PM2.5 monitoring networks to assess

longer-term trends. Although the earlier

networks are more limited in geographic scope

and years of coverage, their data do provide

historical perspective. The maps in Figure 14

show how PM2.5 concentrations 25 years ago

compare with PM2.5 concentrations today in 

39 major cities. Reductions vary among the 39

cities. Generally, the largest reductions occurred

in the areas with the highest concentrations. On

average, today’s levels are about 30% lower than

they were 25 years ago.

The following examples, illustrated in Figure 14,

show how PM2.5 concentrations have improved

over the past 25 years in three cities. Figure 14

also shows PM10 concentrations for comparison

(where available). PM2.5 accounts for more than

half of the PM10 levels in these areas.

■ Los Angeles: PM2.5 concentrations have

decreased substantially since 1980. Although

concentrations have leveled off in recent
17

Figure 14. Comparison of historical PM2.5 and PM10 annual average concentrations, 1979–2003.

Concentration range (μg/m3)

≤ 15 [5 Cities]

15.1 - 20 [13 Cities]

20.1 - 25 [14 Cities]

25.1 - 30 [5 Cities]

> 30 [2 Cities]

Average Annual PM2.5 Concentrations, 1979–1983

Source: Inhalable Particulate Network

Concentration range (μg/m3)

≤ 15 [24 Cities]

15.1 - 20 [13 Cities]

> 20 [2 Cities]

Average Annual PM2.5 Concentrations, 2001–2003

Source: Federal Reference Method Network

Note: The 1979–1983 data are from the Inhalable Particulate Network (IPN).The 1984–1999 data are from EPA’s Air

Quality System.The 1999–2003 data are from the Federal Reference Method (FRM) network.The 1993–2003 data for

Washington, DC, are from the IMPROVE network.

0

10

20

30

40

50

0

10

20

30

40

50

0

10

20

30

40

50

PM2.5 PM10

PM2.5 Annual Average Concentrations

Los Angeles, CA

Washington, DC

Chicago, IL

Concentration, μg/m3 Concentration, μg/m3 Concentration, μg/m3

79 81 83 85 87 89 91 93 95 97 99 01 03

Incomplete Data

Incomplete Data

79 81 83 85 87 89 91 93 95 97 99 01 03

79 81 83 85 87 89 91 93 95 97 99 01 03

PM2.5

NAAQS

Level

PM2.5

NAAQS

Level

PM2.5

NAAQS

Level

PM2.5 PM10

PM2.5 PM10
18

Episodic Events

Rural Sulfate Trends

In the eastern half of the United States, sulfates

account for 25% to 55% of PM2.5 annually. Power

plants are the largest contributor to sulfate formation

in the East, where they were responsible for more than

75% of sulfur dioxide emissions in 2003.

In the East, power plants reduced sulfur dioxide emissions 33% between 1990 and 2003. As Figure 15

shows, this downward trend matches well with the

trend in concentrations of rural sulfates (a 29%

decrease). Because sulfates have such a large regional

component (shown in Figure 6), these trends in sulfate

concentration can also be used to help explain urban

PM2.5 trends.

The reductions shown in Figure 15 are primarily

attributable to implementation of EPA’s Acid Rain

program. Phase I compliance began in 1995, affecting

large coal-burning power plants in 21 eastern and

midwestern states. Phase II implementation began in

2000, tightening emission limits on the Phase I power

plants and setting restrictions on smaller coal-, oil-, and

gas-fired plants. As the figure shows, sulfur dioxide

emissions and sulfate concentrations decreased

following implementation of both phases. A slight

increase in SO2 emissions in the latter half of the 1990s

was likely due to power plants not affected until Phase

II began in 2000. The small increase from 2002 to 2003

resulted from increased electricity production by coalfired and oil-fired units. These units emit much more

SO2 than natural gas units that generated less power in

2003. This annual variation does not affect the total

limit on SO2 emissions under the Acid Rain Program.

(For more information, see www.epa.gov/air/oap.html

and www.epa.gov/acidrainreport/.) 

15,000

10,000

5,000

0

90 91 92 93 94 95 96 97 98 99 00 01 03 02

Yearly Emissions,

Thousand Tons

6

5

4

3

2

1

0

Sulfate Concentrations,

μg/m3

Sulfate Concentrations

SO2 Emissions from Power Plants

Figure 15. Eastern annual trends of sulfur dioxide 

emissions from power plants and sulfate concentrations. Satellite photo of forest fires, southern

California, October 27, 2003.

PM concentrations can increase dramatically due to

human-caused or natural episodic events, such as biomass

burning, meteorological inversions, dust storms, and

volcanic and seismic activity. These events are rare,

affecting less than 1% of reported PM2.5 concentrations

between 2001 and 2003. Episodic events can affect

people’s short-term PM exposure, briefly pushing hourly

and daily PM levels into the unhealthy ranges of the Air

Quality Index. However, these events rarely have a significant effect on annual or longer averages of PM.

Biomass burning can be either a human-initiated event, as

in the burning of vegetation for land clearing or land use

change, or a natural event, as in the wild fires resulting

from lightning. Biomass burning can significantly increase

PM levels in local areas and sometimes more distant areas.

Air currents can carry smoke from forest fires half-way

around the earth. Organic carbon compounds usually

dominate the PM2.5 concentration profile during these 

fire episodes.

Topography and meteorological conditions make some

areas more susceptible to episodic events. In mountain

regions, temperature inversions sometimes trap polluted

air during the winter. Wintertime PM2.5 and PM10 can 

be more than three times higher than other seasonal 

averages. Woodstove smoke, containing large amounts 

Note: Sulfate concentrations are from EPA’s CASTNET

monitoring network, www.epa.gov/castnet
19

Dust storm, Phoenix, Arizona, August 19, 1999.

Temperature inversion, Salt Lake Valley, Utah,

January 13, 2004.

of organic carbon, is often identified as a significant source

of the elevated wintertime PM concentrations.

Arid desert conditions in the southwestern United States

make this region more vulnerable to wind-blown dust than

other regions of the nation. Most dust events are caused

by passage of weather fronts and troughs and downmixing

of upper-level winds. Cyclone development and thunderstorms result in the most dramatic dust clouds with the

lowest visibilities. Dust-related events are typically dominated by large, coarse particles, but fine particle levels also

increase.

The effects of dust storms can also be seen globally. Giant

sand storms originating in the Sahara Desert can blow

across the Atlantic to South America, the Caribbean, and

the southeastern United States, transporting several

hundred million tons of dust each year. Movement of dust

from Africa has increased since 1970 because of an increase

of dry weather in the Saharan region. Satellite pictures

also confirm that sandstorms originating in China’s Gobi

Desert occasionally cross the Pacific to the United States.

Transport from Africa typically occurs in the summer, and

transport from Asia typically occurs in the spring.

Satellite photo of giant cloud of dust originating

in North Africa moving westward.
20

Note: Sulfate and nitrate concentrations from the CASTNET monitoring network were adjusted to represent mass in

PM2.5 and include ammonium as well as water.Trends for crustal were not available so constant values based on

2002–2003 data for each region were used. See www.epa.gov/airtrends/pm.html for further details.

concentrations in the eastern United States match

well with trends in SO2 emissions from power

plants over the past 14 years (based on analyses

discussed in the previous section; see Figure 15).

Figure 17 shows that, on smaller subregional

scales, the relationship between sulfate concentrations and power plant SO2 emissions can vary

among the subregions. Although trends in sulfate

concentrations and SO2 emissions match best

overall in the Southeast (SO2 emissions down

15%, sulfate concentrations down 13%, from

1999 to 2003), the year-to-year comparisons for

the Industrial Midwest and the Northeast do not

show such a close match. Sulfur dioxide emissions in the Industrial Midwest declined 19%,

while sulfate concentrations declined 5%. In the

Northeast, sulfur dioxide emissions were down

6%, and sulfate concentrations were up 3%.

These subregional differences may be caused by

several factors, the most important of which is

likely to be transport. As Figure 17 shows, the

ratio of sulfate concentrations to SO2 emissions is

higher in the Northeast than in the other

regions. This suggests that transport of emissions

PM2.5 Trends in Three Regions 

(1999–2003)

To better understand ambient air quality, it is

helpful to examine trends and the factors that

contribute to those trends in specific regions. This

section explores, in detail, trends in three regions in

the eastern half of the country from 1999 to 2003.

Figure 16 shows the 5-year regional trends in

urban PM2.5 and its major chemical constituents.

In the Southeast, PM2.5 declined sharply from

1999 to 2002, with little further change to 2003.

Overall, the Southeast shows a 20% decrease in

PM2.5 from 1999 to 2003. In the Industrial

Midwest, there is a gradual downward PM2.5

trend from 1999 to 2001 and a more pronounced

decrease from 2001 to 2003. Overall, PM2.5

decreased 9% over the 5-year period. In the

Northeast, PM2.5 increased slightly from 1999

through 2001, then decreased through 2003,

for an overall increase of 1%. Trends in PM

components indicate that reductions in sulfates

appear to be responsible for approximately onethird of the reductions in PM2.5 in the Industrial

Midwest and the Southeast. Trends in sulfate

Explaining the Trends

Figure 16. Trends in PM2.5 and its chemical constituents, 1999–2003.

Industrial Midwest

Concentration, μg/m3

0

2

4

6

8

10

12

14

16

Year

1999 2000 2001 2002 2003

Crustal

Nitrate

PM2.5

Sulfate

PM2.5 Remainder

 (mostly carbon)

119 PM2.5 Monitoring Sites

-9%

-5%

-17%

-4%

Southeast

Concentration, μg/m3

0

2

4

6

8

10

12

14

16

Year

1999 2000 2001 2002 2003

Crustal

Nitrate

PM2.5

Sulfate

PM2.5 Remainder

 (mostly carbon)

115 PM2.5 Monitoring Sites

-20%

-13%

-32%

+18%

Northeast

Concentration, μg/m3

0

2

4

6

8

10

12

14

16

Year

1999 2000 2001 2002 2003

Crustal

Nitrate

PM2.5

Sulfate

PM2.5 Remainder

 (mostly carbon)

56 PM2.5 Monitoring Sites

+1%

+3%

0%

+2%
21

Effect of Meteorology

Weather plays a role both in the atmospheric

formation of PM2.5 and in the quantity of emissions that contribute to this pollution. For this

report, we examined the effect of meteorology

on sulfates, which are a major component of

PM2.5, especially in the eastern half of the United

States. To assess the effect of meteorology on

annual average sulfate concentrations, EPA has

conducted a preliminary analysis, adjusting sulfate

levels based on weather conditions. (The blue

line in Figure 17 represents measured sulfate

concentrations; the red line represents the meteorologically adjusted sulfate concentrations.) One

of the main parameters driving these preliminary

adjustments is temperature. In the eastern half of

the United States, 1999, 2001, and 2003 were

near-normal meteorological years, so only

minimal adjustments to sulfate concentrations

were needed. In 2000, however, a cool summer

may well have caused sulfur dioxide emissions to

be lower than average, resulting in lower amounts

of sulfates in the air. Adjusting for weather in

2000 raised estimated sulfate levels in all three

regions to the level expected during a year with

average weather conditions.

Conversely, the summer of 2002 in the eastern

United States was one of the hottest in recent

years. Sulfur dioxide emissions were higher that

year, likely due (at least in part) to increased

demand for electricity for cooling. The meteorological adjustment for 2002 reduces the amount

of sulfates in all three regions to levels expected

during a normal meteorological year.

In two of the three regions, the variations in

power plant SO2 emissions (illustrated by the

yellow bars in Figure 17) generally correlate more

closely with the meteorologically adjusted sulfates

(the red line) than the unadjusted sulfates (the

blue line). In the Industrial Midwest, however,

adjusting for weather causes the sulfate trend to

move farther away from the sulfur dioxide emission trend in 2002-2003. More refined meteorological analyses and emission inventories are

necessary to fully understand these results.

90 91 92 93 94 95 96 97 98 99 00 01 02 03

0

1

2

3

4

5

6

Sulfate Concentration, μg/m3

0

1

2

3

4

5

6

7

8

9

Power plant SO2 Emissions, Million Tons

Northeast 

Year

90 91 92 93 94 95 96 97 98 99 00 01 02 03

0

1

2

3

4

5

6

Sulfate Concentration, μg/m3

0

1

2

3

4

5

6

7

8

9

Power plant SO2 Emissions, Million Tons

Southeast 

Year

Industrial Midwest

90 91 92 93 94 95 96 97 98 99 00 01 02 03

0

1

2

3

4

5

6

Sulfate Concentration, μg/m3

0

1

2

3

4

5

6

7

8

9

Power plant SO2 Emissions, Million Tons

SO2 emissions from power plants CASTNET sulfate concentrations

Meteorologically adjusted sulfate concentrations

Year

Figure 17. Meteorologically adjusted sulfate 

concentrations, 1999–2003. 

from other regions contributes to sulfate formation in the Northeast. Other factors that may

contribute to the subregional differences in these

trends include variations in meteorological conditions that are important to sulfate formation and

transport, contributions to the Northeast from

Canada, and subregional differences in the contributions of sources other than power plants.
22

The PM2.5 Remainder

Figure 16 (page 20) also shows the estimated

trend in the “PM2.5 remainder” for each of the

three regions. The remainder is estimated by

subtracting all known PM2.5 components from

the total PM2.5 mass. Some uncertainties exist in

our interpretations of these data; however, the

PM2.5 remainder appears to consist mostly of

carbon-containing particles. Some small contributions to the PM2.5 remainder trend shown in

Figure 16 include 

■ Trends in crustal material

■ Local contributions for nitrates and sulfates

(see the discussion on pages 8 and 9)

■ Any changes in data quality or the operation

of EPA’s PM2.5 Federal Reference Method

monitoring network during its first few years

of operation.

Despite the uncertainties, the reductions in the

PM2.5 remainder for the Industrial Midwest and

Southeast appear to be due, in large part, to

reductions in emissions that contribute to the

formation of carbon-containing particles. The

relative importance of various man-made emissions sources to these trends is uncertain and may

vary by region and urban area. Important sources

of carbon-containing particles in urban air

include direct emissions from sources such as

motor vehicles, fuel combustion, and fires and

atmospheric transformation of certain organic

gases, including both regional biogenic emissions

and some components of man-made VOCs.

It is interesting to note that, in Figure 18, the

decrease in the estimated PM2.5 remainder corresponds either to reductions in directly emitted

fine particles or reductions in man-made VOC

emissions. The Northeast region, however, shows

virtually no net change in PM2.5 or in any of its

estimated components. Yet both direct PM2.5

emissions and VOC emissions decreased from

1999 to 2003. EPA is continuing to conduct

research and analysis to better identify and 

quantify key direct emission sources in addition

to the relative contribution of man-made VOC

emissions to atmospheric formation of carboncontaining particles.

Figure 18. PM2.5 emission trends. 

96 97 98 99 00 01 02 03

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

Yearly Emissions, Thousand Tons

SO2 Emissions

96 97 98 99 00 01 02 03

0

100

200

300

400

500

600

Yearly Emissions, Thousand Tons

Direct PM2.5 Emissions

96 97 98 99 00 01 02 03

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

Yearly Emissions, Thousand Tons

NOx Emissions

96 97 98 99 00 01 02 03

0

100

200

300

400

500

600

Yearly Emissions, Thousand Tons

Industrial Midwest Northeast Southeast

VOC Emissions

Year

SO2

NOx

PM2.5

VOC

Northeast Southeast Industrial

Midwest

Percent Change in Emissions from

1999 to 2003

-12 -15 -9

-5 -5 -7

-15 -3 -9

-14 -10 -9
23

will be required to attain the National Ambient

Air Quality Standards for fine particles. EPA’s

proposed Clean Air Interstate Rule (proposed 

in December 2003) will help states meet those

requirements by reducing SO2 and NOx emissions in the eastern United States thus reducing

particle pollution transported across state boundaries. Another regulation, the Best Available

Retrofit Technology (BART) program, will

require the older, existing power plants to control

PM emissions with retrofit pollution control

equipment. Also, national mobile source rules are

in place to strengthen the emission requirements

for virtually all types of mobile sources. Many

localities also have pollution reduction requirements for diesel engine retrofits as well as sulfur

limits in diesel and gasoline engines.

For more details on the PM2.5 remainder, see

ww.epa.gov/airtrends/pm.html. For information

on EPA’s monitoring networks, see

www.epa.gov/ttn/amtic/.

Control Programs

Many programs have been put in place to reduce

levels of particulate matter. Table 1 lists the major

emission control programs that have contributed

to reductions in PM since 1995 and will

continue to reduce PM in the future. These

programs control direct PM emissions and/or the

emissions that contribute to PM formation, such

as SO2, NOx,and VOCs. The control programs

consist of a series of regulations that reduce emissions from many stationary and mobile source

sectors. For example, beginning in 2008, states

PM Precursors

Direct PMa SO2 NOx VOC Implementation

Program Sector Reductions Reductions Reductions Reductions Date

Clean Air Nonroad Mobile sources X X X 2004-2015

Diesel Rule

Clean Air Interstate Rule Electric Utilities X X X 2010-2015

(proposed December 2003) 

Acid Rain Program Electric Utilities X X 1995-2010

NOx SIP Call Electric Utilities X X 2004

Regional Haze Rule/ Electric Utilitiesb X X X 2013-2015

Best Available Retrofit 

Technology

PM2.5 Implementationc Stationary/Area/ X X X X 2008-2015

Mobile sources

PM10 SIPs Stationary/Area/ X X X X Ongoing

(e.g., San Joaquin Valley) Mobile sources

Maximum Achievable Stationary/Area X X 1996-2003

Control Technology 

(MACT) Standardsd

Various Mobile X X X X Ongoing

Source Programse

a Includes elemental and organic carbon, metals, and other direct emissions of PM.

b Also applies to industrial boiler and the other source categories also covered under Prevention of Significant Deterioration (PSD).

c Includes Reasonably Available Control Technology (RACT) and Reasonably Available Control Measures (RACM).

d Includes a variety of source categories such as Boilers and Process heaters, Pulp and Paper, Petroleum Refineries, various minerals and ores,

and others. While these standards are for hazardous air pollutants (HAPs) such as metals, measures to reduce HAPs in many cases also

reduce PM emissions.

e Includes such programs as onroad diesel and gasoline engines, nonroad gasoline engines, Low Sulfur Diesel and Gasoline Fuel Limits for

onroad and offroad engines, Motorcycles, Land-based recreational vehicles, and Marine diesel engines.

Table 1. A Selection of Emission Control Programs Contributing to PM Emission Reductions, 1995–2015 
24

The most direct way to obtain surface concentration data 

for particles is from the routine measurements made at surface 

monitoring stations across the United States. This approach has

some limitations, however, because large regions of the country

do not have surface monitors, and coastal regions are often influenced by polluted air approaching over water. In addition, pollution may be transported aloft, undetected by surface monitors,

and then descend to influence air at the ground. New work

being done through a collaborative partnership between EPA,

the National Aeronautics and Space Administration (NASA), and

the National Oceanic and Atmospheric Administration (NOAA) 

uses satellite observations to augment the surface network 

monitoring data with satellite data.

The NASA MODIS (Moderate Resolution Imaging Spectroradiometer) instruments on board the EOS (Earth Observing

System) satellites EOS-Terra and EOS-Aqua provide twice-daily

measurements of aerosol optical depth (AOD), a measure of how

much light airborne particles prevent from passing through a

column of atmosphere. Scientists use these measurements to 

estimate the relative amount of aerosols suspended in the 

atmosphere.

Initial research shows that MODIS-derived data are suitable for 

tracking air quality events on a regional scale and may be a good

surrogate for estimating the intensity of surface PM2.5 concentrations. More research and data are needed to help show how

aerosol loads are distributed vertically in the atmosphere so that

MODIS-derived AOD can be put into the proper context. For

more information on the MODIS-derived AOD and PM2.5 pollution events, go to the Cooperative Institute for Meteorological

Satellite Studies/Space Science and Engineering Center at the

University of Wisconsin-Madison website: http://idea.ssec.wisc.edu.

Composites of MODIS-derived AOD (color) and cloud optical thickness (black-white) from September 5 to 8, 2003. The

majority of the high AOD seen in the images (yellow-red) was the result of several very large wildfires in western North

America from British Columbia to Oregon. MODIS-derived AOD tracked the movement of the plume, which eventually

affected surface PM2.5 concentrations throughout the midwestern United States.

IDEA (Infusing satellite Data into Environmental

Applications) is a partnership between EPA, NASA, and

NOAA. These agencies are working to improve air

quality assessment, management, and prediction by

infusing satellite measurements from NASA into EPA

and NOAA analyses for public benefit.

1 2

3 4

Using Satellites to Track Particulate Matter
25

National and regional regulations will make

major reductions in ambient PM2.5 levels over the

next 10 to 20 years. In particular, the proposed

Clean Air Interstate Rule (CAIR) and the

existing NOx SIP Call, will reduce SO2 and NOx

emissions from certain electric generating units

and industrial boilers across the eastern half of the

United States. Regulations to implement the

ambient air quality standards for PM2.5 will

require direct PM2.5 and PM2.5 precursor controls

in nonattainment areas. New national mobile

source regulations affecting heavy-duty diesel

engines, highway vehicles, and other mobile

sources will reduce emissions of NOx, direct

PM2.5, SO2,and VOCs.

EPA estimates that current and proposed regulations for stationary and mobile sources will cut

SO2 emissions by 6 million tons annually in 2015

from 2001 levels. NOx emissions will be cut 

9 million tons annually in 2015 from 2001 levels.

VOC emissions will drop by 3 million tons, and

direct PM2.5 emissions will be cut by 200,000

tons in 2015, compared to 2001 levels. Figure 19

shows anticipated emission reductions. Most of

the SO2 reductions are associated with electric

generating sources, while NOx and VOC reductions for mobile sources are associated with

continuing improvements in onroad and nonroad

vehicles.

Models predicting the effect of these emission

reductions on air quality show that all areas in 

the eastern United States will have lower PM2.5

concentrations in 2015 relative to present-day

conditions. In most cases, the predicted improvement in PM2.5 ranges from 10% to 20%. EPA

estimates that the proposed CAIR combined

with existing regulations will bring the majority

of the counties in the East into attainment for the

PM2.5 standards. As Figure 20 shows, 99 eastern

counties are estimated to have exceeded the

annual PM2.5 standard in the 1999–2002 period,

but only 13 of those counties are projected to

exceed the PM2.5 standard by 2015. More information on CAIR can be found at:

www.epa.gov/interstateairquality/.

0

5

10

15

20

25

2001 2015

Direct PM2.5

SO2

NOx

VOC

Tons (millions)

Figure 19. Projected emission reductions by 2015.

Figure 20. Estimated reduction in number of counties exceeding PM2.5 standards from 2001 (99) to

2015 (13), based on current programs plus the Clean

Air Interstate Rule as proposed in December 2003.

Ambient PM2.5, 1999–2002

99 Counties Exceeding PM2.5 Standards

Estimated PM2.5, 2015

13 Counties Exceeding PM2.5 Standards

The Future
26

Upcoming PM2.5 Designations

EPA designates areas as attaining or not attaining

the National Ambient Air Quality Standards for

fine particulate matter (PM2.5). EPA designates an

area as “nonattainment” if it has violated the annual

or 24-hour national PM2.5 standard (assessed over 

a 3-year period) or if it has contributed to a violation of one of the standards. Once designated,

nonattainment areas must take actions to improve

their PM2.5 air quality on a certain timeline.

Designations are a crucial first step in the efforts 

of states, tribes, and local governments to reduce

harmful levels of fine particles. For more details on

PM2.5 designations, visit www.epa.gov/

pmdesignations.

Note: Designations are based on 3 years of data,

and the boundaries of defined nonattainment areas

may differ from the county boundaries used in 

this report.

PM2.5 and Other Pollutants

Areas that experience PM2.5 concentrations that

exceed the National Ambient Air Quality

Standards can also have air quality problems associated with other pollutants. This association in the

presence of different pollutants is not unexpected.

A 2004 report by the National Academies of

Sciences (Air Quality Management in the United

States) indicates that air pollutants “often share

similar precursors and similar chemical reactions

once in the atmosphere.” For example, nitrogen

oxides, which contribute to PM2.5 formation, are

also a key ingredient in ground-level ozone.

Pollutants may also be emitted from the same types

of sources. Industries that emit air toxics may also

emit chemicals that contribute to ozone or PM

formation. Data indicate that millions of people

likely live in areas where particle pollution levels

are elevated along with ozone and/or air toxics.

EPA will continue to analyze this information as

we work to protect public health across the

country.
27

For Further Information

Web Sites

Additional technical information: www. epa.gov/airtrends/pm.html

Air Quality Index (AQI): www.epa.gov/airnow

Clean Air Interstate Rule: www.epa.gov/interstateairquality/

CASTNET: www.epa.gov/castnet/

Emissions: www.epa.gov/ttn/chief/

EPA 1996 PM Criteria Document: www.epa.gov/ttn/naaqs

EPA Monitoring Networks: www.epa.gov/ttn/amtic/

Health and Ecological Effects: www.epa.gov/air/urbanair/pm/index.html

IMPROVE: vista.cira.colostate.edu/improve

National Academies: www4.nationalacademies.org/nas/nashome.nsf

Office of Air and Radiation: www.epa.gov/oar

Office of Air Quality Planning and Standards: www.epa.gov/oar/oaqps

Office of Atmospheric Programs: www.epa.gov/air/oap.html

Office of Transportation and Air Quality: www.epa.gov/otaq

Online Air Quality Data: www.epa.gov/air/data/index.html

PM Supersites: www.epa.gov/ttn/amtic/supersites.html and www.epa.gov/ttn/amtic/laprog.html

PM2.5 Designations: www.epa.gov/pmdesignations

Report on Acid Rain: www.epa.gov/acidrainreport/

Satellite information: idea.ssec.wisc.edu

Source apportionment: www.epa.gov/oar/oaqps/pm25/docs.html

Visibility: www.epa.gov/airtrends/vis.html

AOD aerosol optical depth

AQI Air Quality Index

BART best available retrofit technology

CAA Clean Air Act

CAIR Clean Air Interstate Rule

CASTNET Clean Air Status and Trends

Network

EOS Earth Observing System

EPA U.S. Environmental Protection

Agency

FRM Federal Reference Method

HAP hazardous air pollutants

IDEA Infusing satellite Data into

Environmental Applications

IMPROVE Interagency Monitoring of

Protected Visual Environments

IPN Inhalable Particulate Network

km kilometer

MACT maximum achievable control 

technology

MODIS moderate resolution imaging 

spectroradiometer

NAAQS National Ambient Air Quality

Standards

NASA National Aeronautics and Space

Administration

NOAA National Oceanic and Atmospheric

Administration

NOx oxides of nitrogen

PM particulate matter

PM10 particulate matter 10 μm or less in

size

PM2.5 particulate matter (fine) 2.5 μm or

less in size

PM10–2.5 particulate matter (coarse) between

10 and 2.5 μm in size

PSD prevention of significant 

deterioation

RACM reasonably available control 

measures

RACT reasonably available control 

technology

SIP State Implementation Plan

SO2 sulfur dioxide

TCM total carbonaceous mass

TSP total suspended particulate

μg/m3 micrograms per cubic meter

μm micrometers

VOC volatile organic compound

Acronyms
Agency

Environmental ProtectionUnited States

Research Triangle Park, NCEmissions, Monitoring, and Analysis DivisionOffice of Air Quality Planning and Standards EPA Publication No. EPA 454-R-04-002