&EPA
United States
Environmental Protection
Agency
Latest Findings on National
Air Quality
STATUS AND TRENDS THROUGH 2006
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Printed on 100% recycled/recyclable process chlorine-free paper with 100% post-consumer fiber using vegetable-oil-based ink.
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Latest Findings on National
Air Quality
STATUS AND TRENDS THROUGH 2006
Contract No. EP-D-05-004
Work Assignment No. 3-4
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Air Quality Assessment Division
Research Triangle Park, North Carolina
EPA-454/R-07-007
January 2008
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TABLE OF CONTENTS
Highlights ... 1
Six Principal Pollutants ... 3
Ground-Level Ozone ... 8
Particle Pollution ...14
Nitrogen Dioxide ... 21
Carbon Monoxide ... 22
Sulfur Dioxide ... 23
Lead ... 24
Toxic Air Pollutants ... 25
Acid Rain ... 29
Visibility in Scenic Areas ... 30
Climate Change ... 32
Terminology ... 34
Web Sites ,. 35
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I-
HIGHLIGHTS
For more than 35 years, EPA has been working to
reduce pollution and make the nation's air cleaner and
healthier to breathe. This summary report highlights
the agency's most recent evaluation of status and trends
in our nation's air quality.
LEVELS OF SIX PRINCIPAL POLLUTANTS
CONTINUE TO DECLINE
Cleaner cars, industries, and consumer products
have contributed to cleaner air for much of the
United States. Since 1980, nationwide air quality,
measured at more than a thousand locations across
the country, has improved significantly for all six
principal pollutants. These common pollutants are
ground-level ozone, particle pollution, nitrogen
dioxide, carbon monoxide, sulfur dioxide, and lead.
• Despite this progress, ground-level ozone and
fine particle pollution (PM2 5) continue to present
challenges in many areas of the country. Ozone and
fine particle levels are continuing to decline. In 2006,
8-hour ozone concentrations were 9 percent lower
than in 1990, and annual PM2 5 concentrations were
14 percent lower than in the year 2000. But that same
year, more than 100 million people lived in counties
that exceeded national air quality standards for
ozone or PM2 5.
AIR TOXICS: MONITORING EXPANDS,
BENZENE LEVELS DROP
Twenty-three National Air Toxics Trends Stations
(NATTS) are now fully operational, adding a
consistent national network to the existing state and
local monitors for toxic air pollutants.
Benzene, a primary contributor to the cancer risk
associated with air toxics exposure, is one of the
most routinely and accurately monitored air toxics
across the country. Benzene levels in the outdoor air
declined about 17 percent between 2000 and 2005.
• Control programs for mobile sources and facilities
such as chemical plants, dry cleaners, coke ovens,
and incinerators were primarily responsible for
reductions of roughly 35 percent in air toxics
emissions between 1990 and 2002.
ACID RAIN AND HAZE DECLINING
EPA's Acid Rain Program continues to contribute
to significant improvements in air quality and
environmental health. The program's reductions
in sulfur dioxide and nitrogen oxides have led to
significant decreases in acid rain, contributing to
improved water quality in lakes and streams. For
example, between the 1989-1991 and 2004-2006 time
periods, sulf ate deposition decreased more than
30 percent in the Northeast and Midwest.
CO -
Lead —
NO, -
Ozone (8-hour) -
PM1D -
PM25 (annual and daily)
S0; (annual and daily) -
AnyNAAQS -
0.7
0.2
0.0
14.7
0.3
77,3
66.9
105.6
: i
20 40 60 80
Millions of People
100
120
Number of people living in counties with air quality concentrations above the level of the primary National Ambient Air
Quality Standards (NAAQS) in 2006.
Note: Multiple years of data are generally used to determine if an area attains the national standards. The chart above is for one year only.
LATEST FINDINGS ON NATIONAL AIR QUALITY
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Visibility in scenic areas, which can be impaired
by particles and gases in the air, has improved
throughout the country. Only one location—
Petrified Forest, Ariz. — shows degradation in
visibility for the worst visibility days between 1996
and 2005.
CLIMATE AND INTERNATIONAL TRANSPORT:
IMPROVING OUR UNDERSTANDING
Research is under way to examine and improve our
understanding of the links between air quality and
climate: how air quality affects climate and how a
warming climate could affect air quality.
• Researchers also are improving our understanding
about how pollution moves - not just within North
America, but also between continents.
MORE IMPROVEMENTS ANTICIPATED
• EPA expects the air quality to continue to improve
as recent regulations are fully implemented and
states work to meet national ambient air quality
standards. Among these rules are: the Clean Air
Interstate Rule, the Clean Air Mercury Rule, the
Tier II Vehicle and Gasoline Sulfur Program, the
Heavy-Duty Highway Diesel Rule, the Clean Air
Nonroad Diesel Rule, and the Mobile Source Air
Toxics Rule.
Air Pollution Pathways
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SIX PRINCIPAL POLLUTANTS
To protect public health and the environment, EPA has
established, and regularly reviews, national ambient
air quality standards (NAAQS) for six principal air
pollutants: ground-level ozone (O3), particulate matter
(PM), nitrogen dioxide (NO2), carbon monoxide (CO),
sulfur dioxide (SO2), and lead (Pb). Some of these
pollutants (CO, SO2, and lead) are emitted directly
from a variety of sources. Ozone is not directly emitted,
but is formed when nitrogen oxides (NOx) and volatile
organic compounds (VOCs) react in the presence of
sunlight. NO2 is formed in the air through the oxidation
of nitric oxide (NO). PM, also known as particle
pollution, can be directly emitted or formed when
gaseous emissions react in the atmosphere. Particle
pollution is regulated as PM2 5, or "fine particles" with
diameters less than or equal to 2.5 micrometers (|am),
and PM10, which includes all particles with diameters
less than or equal to 10 |jm.
This section discusses the six principal pollutants
and shows how air quality and emissions have
changed over time. Summary information across all
six pollutants is presented for the time period 1980 to
40%
20% -
National Standard
-20% -
-40% -
-60% -
-80% -
-100%
2006. Several approaches are used here to look at the
pollutants over time, including changes in (1) national
air quality concentrations, (2) Air Quality Index
"unhealthy" days, (3) air quality in nonattainment
areas, and (4) national emissions.
NATIONAL AIR QUALITY CONCENTRATIONS
Figure 1 shows national trends in the principal
pollutants relative to their air quality standards, as
measured by monitors located across the country.
Most pollutants show a steady decline throughout the
time period with a couple of exceptions. For example,
lead declined in the 1980s and remained low for the
remainder of the time period. Ozone declined in the
1980s, leveled off in the 1990s, and showed a notable
decline after 2002. Most of the pollutants show a
smooth, gradual trend from year to year, while ozone
and PM2 5 trends are not smooth and show year-to-year
influences of weather conditions which contribute to
their formation in the air.
All of the six principal pollutants show improvement
over the 27-year period. While progress has been made
nationally, there are still areas that
have local air quality problems caused
by one or more pollutants. Ozone
and fine particle pollution continue to
present air quality concerns throughout
much of the U.S., with many monitors
measuring concentrations above, or
close to, national air quality standards.
Ozone. 275 sites (4"1 maximum 8-hour average)
PM2S, 752 sites (annual average)
PMIO, 391 sites (2nd maximum 24-hour average)
NO,, 87 sites (annual average)
CO. 144 sites (2nd maximum 8-hour average)
SO2,154 sites {annual average)
Lead. 15 sites (maximum quarterly average)
80 82 84 86 88 90 92 94 96 98 00 02 04 06
Figure 1. Comparison of national levels of the six principal pollutants to national ambient airauality standards, 1980-2006.
National levels are averages across all sites with complete data for the time period.
Note: Air quality data for PMM and PM25 start in 1990 and 1999, respectively.
LATEST FINDINGS ON NATIONAL AIR QUALITY
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SIX PRINCIPAL POLLUTANTS
AIR QUALITY INDEX "UNHEALTHY" DAYS
The Air Quality Index (AQI) relates daily air pollution
concentrations for ozone, PM25, NO2, CO, and SO2 to
health concerns for the general public. Daily forecasts
allow people to take steps to protect their health when
air pollution reaches levels that are unhealthy for
them. Figure 2 shows how selected metropolitan areas
fared in 2006 relative to previous years. Most areas
had fewer unhealthy days in 2006 compared with the
previous five years. Notable exceptions were Atlanta
and Kansas City, with six more unhealthy days each in
2006 than the average of the previous five years. Most
of the unhealthy days are due to ozone and/or particle
pollution. Later in this report, the number of unhealthy
days for ozone and particle pollution are shown
separately.
EPA's Air Quality Index (AQI)
EPA provides an AQI based on five of the six
principal pollutants: ozone, PM25, NO2, CO, and
SO2. The AQI indicates a level of health concern.
A value of 100 generally corresponds to the
national air quality standard
for each pollutant. Values
below 100 are generally
thought of as satisfactory.
Values above 100 are
considered to be unhealthy—
at first for certain sensitive
groups of people, then
for everyone as the AQI
values increase. For more
information about the AQI,
visit http://www.airnow.gov.
DD
Sacramento
San Francisco
29
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Salt Lake City
Los Angeles
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1010
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Las Vegas
1615 13U
•an • an
San Diego Phoenix
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Nashville
20 17
DD
Charlotte
2325
DD
Dallas-^
Fort Worth
3630
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Memphis
24
30
2519
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2720
In
Baltimore
2223
DD
Washington DC
Atlanta
DD
Houston
New Orleans
3 £
Tampa
2.1
Orlando
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• Miami
5 Year Average (2001-2005)
2006
Figure 2. Number of days reaching Unhealthy for Sensitive Groups on the Air Quality Index for five of the six principal
pollutants for 2001-2005 (average) vs. 2006.
LATEST FINDINGS ON NATIONAL AIR QUALITY
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AIR QUALITY IN NONATTAINMENT AREAS
Many areas of the country where air pollution
levels exceeded the NAAQS have been designated
"nonattainment." Under the Clean Air Act, EPA
and state, tribal, and local air quality planning
agencies work together to develop air
quality management plans to address the
air pollution in these areas. Each year, EPA
tracks air quality progress in areas identified
as nonattainment by reviewing changes in
measured concentrations with respect to the
standards. Figure 3 shows which of these
areas are above or below one or more of the
standards as of 2006, using the most recent
year(s) of data.
Air quality has improved in the areas that
were originally designated nonattainment
across all six principal pollutants. All of the
original areas designated as nonattainment
for NO2, CO, and SO2 had air quality levels
below their respective standards as of
December 2006. Only one area was above
the standard for lead, Herculaneum, Mo. For
ozone, annual PM25, and PM10, a number
of areas were above the standards: 35, 32,
and 41 areas, respectively. Even though many
areas were above the standard, there have been
improvements in the concentration levels in the
nonattainment areas. For example, the ozone
areas showed an 11 percent improvement, and the
annual PM2 5 areas showed a 6 percent improvement
between the time of designation and 2006.
Above NAAQS
I Below NAAQS
Puerto Rico
Figure 3. Status of original nonattainment areas for one or more
standards (i.e., ozone, annual PM25, PMW, NO2, CO, SO2, and lead)
as of 2006.
Notes: To determine NAAQS attainment, typically an average of multiple years
of data is required for comparison with the standard. For information about Air
Trends Design Values, visit http://www.epa.gov/air/airtrends/values.html.
Review of the National Ambient Air Quality Standards
The Clean Air Act requires EPA to set two types of NAAQS for the principal air pollutants:
• primary standards to protect public health with an adequate margin of safety, including the health of sensitive
populations such as asthmatics, children, and the elderly; and
• secondary sfandards to protect public welfare from adverse effects, including visibility impairment and effects on the
environment (e.g., vegetation, soils, water, and wildlife).
The Clean Air Act requires periodic review of the science upon which the standards are based and the standards themselves.
The current standards and the status of each review are shown below.
Pollutant
PM25
PM10
03
Pb
N02
S02
CO
Primary Standard(s)
15 MQ/m3 (annual)
35 MQ/m3 (daily)
150 u.g/m3 (daily)
0.08 ppm (8-hour)
1 .5 MQ/m3
0.053 ppm (annual)
0.03 ppm (annual)
0.14 ppm (daily)
9 ppm (8-hour)
35 ppm (1-hour)
Secondary Standard(s)
Same as Primary
Same as Primary
Same as Primary
Same as Primary
Same as Primary
0.5 ppm (3-hour)
None
Status of Review
Review completed 2006 (daily PM25 standard
strengthened and annual PM10 standard revoked)
Next review initiated 2007
Proposed tightening primary and secondary
standards July 2007; final decision March 2008
To be completed September 2008
To be completed 2010
To be completed 2010
Schedule under development
Units of measure are parts per million (ppm) or micrograms per cubic meter of air (u.g/m3). For more information
about the standards, visit http://www.epa.gov/ttn/naaqs/.
LATEST FINDINGS ON NATIONAL AIR QUALITY
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SIX PRINCIPAL POLLUTANTS
NATIONAL EMISSIONS
EPA tracks direct emissions of air pollutants and
emissions that contribute to air pollution formation.
Emissions data are compiled from many different
sources, including industry and state, tribal, and local
agencies. Some emissions data are based on actual
measurements, while others are estimates.
Since 1980, emissions of the six principal pollutants
have declined significantly, with the greatest drop in
lead, as shown in Figure 4. The removal of lead from
gasoline is primarily responsible for the 97 percent
decrease in lead emissions.
During that same time period, NOx emissions
have dropped by one third, and VOC, SO2, and
CO emissions have been cut by roughly one half.
Combined, the emissions of the six principal pollutants
dropped 49 percent since 1980, as shown in Figure 5.
All of this progress has occurred while the U.S.
economy continued to grow, Americans drove more
miles, and population and energy use increased. These
emission reductions resulted from a variety of control
programs, from regulations at the federal, state, local,
and regional level to voluntary partnerships between
federal, state, local, and tribal governments, academia,
industrial groups, and environmental organizations.
The following sections provide more information on
each pollutant, including where the pollutant comes
from, its health and environmental effects, and more
detailed trends in air quality and emissions between
1990 and 2006. The ozone and PM25 sections also show
how these two pollutants are affected by weather and
the extent to which they contribute to the number
of unhealthy days in selected cities. In addition, the
PM2 5 section includes regional trends for the different
components of PM2 5.
1980 Emissions
2006 Emissions
10
-33% -52%
-28%
PM25
-31%
Figure 4. Comparison of national annual emissions, 1980 vs. 2006.
Lead
-97%
Notes:
PM25 estimates are for 1990 vs. 2006.
PM, „ estimates are for 1985 vs. 2006.
Emissions Used in this Report
PM emissions are direct emissions only.
PM emissions do not include condensibles, fires, or dust sources.
VOC and NOx emissions are from anthropogenic (human activity) sources only.
In most cases, emission trends for major sources are shown.
Emissions were derived from 1996, 1999, and 2002 inventories, except for NOx and SO2 emissions from electric generating
units, which come from measured data.
Emissions inventories are compiled using the best methods and measurements available at the time.
LATEST FINDINGS ON NATIONAL AIR QUALITY
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80
\\ I I
90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05 06
Gross Domestic Product
Vehicle Miles Traveled
Population
Energy Consumption
Aggregate Emissions
(Six Principal Pollutants)
Figure 5. Comparison of growth measures and emissions, 1980-2006.
New National Monitoring Network
Location of candidate NCore sites.
The National Core Monitoring Network (NCore) will
provide a network of monitoring sites (owned and
operated by cities and states) that measure the
principal pollutants (ozone, particles, NO2, CO, SO2,
and lead), related gases (like VOCs and NOx), and
basic meteorology. NCore is primarily designed to
measure very low-level concentrations to support
air quality analyses and health effects studies. Sites
will be placed in urban (about 55 sites) and rural
(about 20 sites) locations throughout the country to
help characterize regional and urban air pollution.
Information provided by this network will improve our
understanding of the relationships among air quality
pollutants and meteorology. For information about
the NCore network, visit http://www.epa.gov/ttn/
am tic/files/ambient/monitorstrat/naamst rat2005.pdf.
LATEST FINDINGS ON NATIONAL AIR QUALITY
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I*
GROUND-LEVEL OZONE (O
NATURE AND SOURCES
Ozone can be helpful or harmful, depending on its
location. In the stratosphere - 10 to 30 miles above the
Earth - a layer of ozone provides protection by filtering
the sun's harmful rays. But at ground level, ozone can
harm both human health and the environment.
Ground-level ozone forms when emissions of nitrogen
oxides (NOJ and volatile organic compounds (VOCs)
react in the presence of sunlight. These ingredients
come from motor vehicle exhaust, power plant and
industrial emissions, gasoline vapors, chemical
solvents, and some natural sources. Because ground-
level ozone forms more readily in hot, sunny weather,
it is known as a summertime air pollutant. High ozone
levels can occur anywhere: wind can carry ozone and
the pollutants that form it hundreds of miles away from
their original sources. Changes in emissions, combined
with changing weather patterns, contribute to yearly
differences in ozone concentrations from region to
region.
HEALTH AND ENVIRONMENTAL EFFECTS
Breathing ground-level ozone can trigger a variety of
health problems including chest pain, coughing, throat
irritation, and congestion. It can aggravate bronchitis,
emphysema, and asthma. Ozone can also reduce lung
function and inflame the lining of the lungs. Repeated
exposure may permanently scar lung tissue. People
with lung disease, children, older adults, and people
who are active outdoors can be affected when ozone
levels are unhealthy.
Ground-level ozone can also have
detrimental effects on plants and
ecosystems. These effects include
(1) interfering with the ability of
sensitive plants to produce and
store food, (2) damaging the leaves
trees and other plants, and (3) reducing
crop yields and forest growth.
0.12
0.1 -
0.08 - -
o
'
Figure 6. National 8-hour ozone air quality
trend, 1990-2006 (average of annual
fourth highest daily maximum 8-hour
concentrations).
0.06 -
| 0.04 H
o
0 0.02 -
TRENDS IN OZONE CONCENTRATIONS
Nationally, ozone concentrations were 9 percent lower
in 2006 than in 1990, as shown in Figure 6. The trend
showed little change throughout the 1990s with a
notable decline after 2002. Concentrations in 2006 were
the second lowest over the 17-year period.
For each monitoring location, the map in Figure 7
shows whether ozone concentrations increased,
decreased, or stayed about the same over the trend
period. The sites that showed the greatest improvement
were the ones with the highest concentrations in 1990.
For example, southern California had some of the
highest ozone concentrations in the nation in 1990,
but showed more improvement than any other area
(a decline of over 0.040 ppm). Other sites in California,
plus the Northeast, Midwest, and Texas showed more
than 0.021 ppm improvement.
Eleven sites showed an increase of greater than
0.005 ppm. Of the 11 sites that showed an increase,
nine had air quality concentrations below the level of
the ozone standard (0.08 ppm) for the most recent year
of data; only Maricopa County, Ariz., and Clay County,
Mo., were above.
Figure 8 shows a snapshot of ozone concentrations
in 2006. The highest ozone concentrations were
located in California and Texas. Overall, the greatest
improvements were in or near urban areas while the
greatest increases were in less populated or rural areas.
Increases in rural areas raise concerns about ozone's
detrimental effect on plants and ecosystems.
90 percent of sites are below this line. 588 sites
National Standard
10 percent of sites are below this line.
90
—i—
92
—i—
96
—i—
98
—i—
02
94 96 98 00
1990 to 2006: 9% decrease
—i—
04
06
LATEST FINDINGS ON NATIONAL AIR QUALITY
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Change in Concentration (ppm)
O Increase of 0.006 to 0.020 (11 Sites)
O Little change +- 0.005 (153 Sites)
O Decrease of 0.006 to 0.020 (205 Sites)
O Decrease of 0.021 to 0.040 (32 Sites)
9 Decrease of more than 0.040 (16 Sites)
Alaska
Puerto Rico
Figure 7. Change in ozone concentrations in ppm, 1990-1992 vs. 2004-2006 (3-year average of annual fourth highest daily
maximum 8-hour concentrations).
Concentration Range (ppm)
O 0.000-0.064(132 Sites)
• 0.065-0.084 (827 Sites)
O 0.085-0.104 (207 Sites)
• 0,105-0.125(18Sites)
Puerto Rico
Alaska
Figure 8. Ozone concentrations in ppm, 2006 (fourth highest daily maximum 8-hour concentrations).
LATEST FINDINGS ON NATIONAL AIR QUALITY
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GROUND-LEVEL OZONE (O3)
Figure 9 shows that all selected areas in the East had
fewer unhealthy ozone days in 2006 compared with the
average from the previous five years (2001-2005), with
the exception of Atlanta and Kansas City. In the West,
Los Angeles and Sacramento had the most unhealthy
ozone days in 2006 (over 40 days each), though Los
Angeles had fewer unhealthy ozone days in 2006 than
its average from the previous five years.
TRENDS IN OZONE-FORMING EMISSIONS
Ozone is formed by the reaction of VOCs and NOx in
the presence of sunlight. Because ozone is mostly a
summer-season pollutant, emissions are shown here
for the summer only (May-September). The year 1997
was selected as a base year for these ozone analyses
because of the change in methodology for VOC and
NOx emissions in 1996. Figure 10 shows that during
the period 1997 to 2006, summer emissions of VOCs
and NOx decreased 20 and 30 percent, respectively.
The majority of these emission reductions were from
transportation and fuel combustion sources. After 2002,
the largest reductions were in NOx emissions from fuel
combustion sources.
00 «
San Francisco
V
1919 Baltimore
ID
Washington DC
5 Year Average (2001-2005)
2006
Los Angeles
Figure 9. Number of days reaching Unhealthy for Sensitive Groups for ozone on the AQIfor 2001-2005 (average) vs. 2006.
m
5
6-
4 -
2-
0
Transportation
Industrial processes
12
2 10-
52 6-
Traosportation
Industrial processes
Fuel combustion
97 98 99 00 01 02 03 04
1997 to 2006: 20% decrease
05 06
96
99 00 01 02 03 04
1997 to 2006: 30% decrease
05 06
Figure 10. National trends in summertime ozone-forming emissions, 1997-2006.
Notes: Trends do not include miscellaneous emissions. Except for NOx emissions from electric generating units,
summertime emissions of VOC and NOx were estimated using 5/12 of annual emissions.
1 0
LATEST FINDINGS ON NATIONAL AIR QUALITY
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WEATHER INFLUENCE ON OZONE
Weather plays an important role in the formation of
ozone. A large number of hot, dry days can lead to
higher ozone levels in any given year, even if ozone-
forming emissions do not increase. Figure 11 shows
ozone trends for 1997 through 2006, before and after
adjusting for weather at selected sites. The hot, dry
summer of 2002 contributed to high concentrations of
ozone; after those levels were adjusted to remove the
influence of weather, ozone concentrations were much
lower. In 2004, the weather was cooler and more humid,
so ozone was less likely to form; removing the influence
of weather shows higher ozone concentrations that year.
Ozone concentrations decreased 3 percent from 1997
to 2006. When the influence of weather is removed, the
effect of changes in emissions on air quality is easier to
see, and ozone shows a 7 percent decrease from 1997 to
2006. Much of the improvement occurred in the East.
The average decrease among 79 sites in the East was
10 percent, while the average decrease among 54 sites in
the rest of the U.S. was 1 percent.
In Figure 11, both trend lines show a decline in ozone
concentrations between 2002 and 2004. This decline
is mostly due to reductions in fuel combustion NOx
emissions under the Acid Rain Program, which began
in 1995, and implementation of the NOx SIP Call rule,
which led to sustained reductions in the East beginning
in 2003 and 2004. The weather-adjusted trend line
confirms that the decrease in ozone concentrations
between 2002 and 2004 was caused by something other
than the weather. The weather-adjusted trend line
also shows lower ozone concentrations in 2005 and
2006, with concentrations similar to the 2004 levels.
Thus, ozone improvements achieved through emission
reductions in 2004 were maintained.
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0.055
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8
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Monitoring Sites ,
i Rural (CASTNET) '
. Urban IAO3)
Observed trend
- Adjusted trend
97 98 99 00
01
02
03
04
05 06
1997 to 2006: 3% decrease (observed)
1997 to 2006: 7% decrease (adjusted)
Figure 11. Trends in average summertime daily maximum 8-hour ozone concentrations, before and after
adjusting for weather, and the location of urban and rural monitoring sites used in the average.
Notes: Urban areas are represented by multiple monitoring sites. Rural areas are represented by a single monitoring site. For
more information about the Air Quality System (AQS), visit http://www.epa.gov/ttn/airs/airsaqs. For more information about the
Clean Air Status and Trends Network (CASTNET), visit http://www.epa.gov/castnet.
Air Quality, Emissions, and Weather
Ozone and some particles are formed by the reaction of emissions in the presence of sunlight, so both emissions and weather
conditions contribute to air pollution levels. As weather conditions vary from year to year, pollutant levels could be higher in
years with weather conditions conducive to their formation—even when emission control programs are working as expected.
To better understand how these pollutants are changing, EPA assesses both the changes in emissions as well as weather
conditions. EPA uses a statistical model to remove the influence of weather. This provides a clearer picture of the underlying
pollutant trend from year to year, making it easier to see the effect of changes in emissions on air quality.
For information on the statistical model, read "The effects of meteorology on ozone in urban areas and their use in assessing
ozone trends," by Louise Camalier, William Cox, and Pat Dolwick of the U.S. EPA. Atmospheric Environment, In Press, 2007.
LATEST FINDINGS ON NATIONAL AIR QUALITY
1 1
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GROUND-LEVEL OZONE (O3)
Figure 12 shows the effect of the NOx SIP Call in the
East, where the program was implemented. Weather-
adjusted average summertime ozone concentrations
were compared between the summers of 2000 and
2001 versus 2005 and 2006 (the years before and after
the largest NOx reductions). The large declines in
ozone occurred throughout the central portions of
the region, including North Carolina, Virginia, West
Virginia, Pennsylvania, and Ohio. On average, ozone
concentrations declined by 0.005 ppm (about 8 to
10 percent) over the region.
0.006
0.004
0.002
0.000
-0.002
-0.004
-0.006
Figure 12. Changes in summertime daily maximum 8-hour ozone concentrations (ppm) between 2000-2001 (average) and
2005-2006 (average). Concentrations have been adjusted using weather variables such as temperature and humidity. Estimated
changes for locations farther from monitoring sites (dots on map) have the largest uncertainty.
Future Air Programs Will Bring Cleaner Air to Many Areas
EPA's Clean Air Interstate Rule (CAIR) will help reduce particle pollution and ozone in the East by cutting emissions of SO2 by
70 percent and NOx by 60 percent over 2003 levels. The Clean Air Mercury Rule (CAMR) will build on CAIR to reduce utility
emissions of mercury by nearly 70 percent at full implementation. This rule makes the United States the first country to regulate
mercury emissions from utilities. In addition, recent national mobile source regulations will help reduce emissions of toxic air
pollutants, PM, NOx, and VOCs from new passenger vehicles, heavy-duty diesel engines, and other mobile sources. Together,
these programs create a strategy to reduce multiple air pollutants throughout the U.S.
1 2
LATEST FINDINGS ON NATIONAL AIR QUALITY
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Where You Live
Air quality trends can vary from one area to another. Local trends are available at individual monitoring locations for all
pollutants with enough historical data, http://www.epa.gov/airtrends/where.html. Trends in ozone adjusted for weather
conditions are also available, http://www.epa.gov/airtrends/weather.html.
Address = 106 Hope Well Road
County = Pickens
City = Greenville-Spartanburg-Anderson, S.C.
Site Code = 450770002
J.
Ozone Air Quality, 1990 - 2006
(Eased on Anrual 4th Maximum 8— hfcxir Average)
Greenville— Spartantajrg—ArKteisonSC
SITE= "150770002 POC= 1
Local Trends in Ozone Levels
Simple steps to obtain more information
1. Pick the state
2. Pick the site
3. Seethe trend
http://www.epa.gov/airtrends/ozone.html
LATEST FINDINGS ON NATIONAL AIR QUALITY
1 3
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I*
PARTICLE POLLUTION
NATURE AND SOURCES
Particle pollution is a general term used for a mixture
of solid particles and liquid droplets found in the
air. Some particles are large enough to be seen as
dust or dirt. Others are so small that they can only be
detected with an electron microscope. EPA regulates
particle pollution as PM2 5 (fine particles) and PM10 (all
particles 10 micrometers or less in diameter). The PM10
discussion follows the PM2 5 discussion in this section.
Generally, coarse particles are directly emitted, while
fine particles are mostly formed in the atmosphere.
Directly emitted particles come from sources such as
construction sites, unpaved roads, fields, smokestacks
(combustion sources), or fires. Other particles form
when gases react in the atmosphere. These are sulfur
dioxide (SO2), nitrogen oxides (NOJ, and volatile
organic compounds (VOCs) emitted mostly from power
plants, industries, and automobiles; and ammonia
(NH3), mostly from agriculture. Particles formed in the
atmosphere make up most of the fine particle pollution
in the U.S. The chemical composition of particles
depends on location, time of year, and weather. In
addition to changes in emissions, weather patterns also
contribute to yearly differences in PM2 5 concentrations
from region to region.
HEALTH AND ENVIRONMENTAL EFFECTS
Particle pollution—especially fine particles—contains
microscopic solids or liquid droplets that are so small
they can get deep into the lungs and cause serious
health problems. Numerous scientific studies have
linked particle pollution exposure to a variety of
health problems including (1) increases in respiratory
symptoms such as irritation of the airways, coughing,
or difficulty breathing; (2) decreased lung function;
(3) aggravated asthma; (4) development of chronic
bronchitis; (5) irregular heartbeat; (6) heart attacks; and
(7) premature death. People with heart or lung disease,
the elderly, and children are at the highest risk from
exposure to particles. In addition to health problems,
particle pollution is the major cause of reduced
visibility and ecosystem damage in many parts of the
U.S., including national parks and wilderness areas.
TRENDS IN PM25 CONCENTRATIONS
There are two standards for PM25: an annual standard
(15 |ag/m3) and a daily standard (35 |jg/m3). The national
monitoring network for PM,, 5 began in 1999 and was
fully implemented in 2000. Nationally, annual PM25
concentrations declined by 14 percent between 2000 and
2006, as shown in Figure 13. Daily PM25 concentrations
have a similar trend with a 15 percent decline.
20
18
90 percent of sites are below this line. 721 sites
c u
* r-
o 6
o 4
O 4
2
10 percent of sites are below this line.
00 01 02 03 04 05
2000 to 2006: 14% decrease
06
figure 13. National PM25 air quality trend, 2000-2006 (annual average).
Note: Roughly 10 percent of sites are still above the standard in 2006.
14
LATEST FINDINGS ON NATIONAL AIR QUALITY
-------
The national trend for PM25 shows a steady dedine since
2000 with the exception of a temporary increase in 2005,
which is discussed on pages 18 and 19.
For each monitoring location, the map in Figure 14
shows whether PM2 5 increased, decreased, or stayed
about the same between 2000 and 2006. Almost all of
the sites show a dedine in PM2 5 during this period.
The areas that showed the greatest improvement were
the ones that had the highest concentrations in 2000,
induding Southern California. Eight sites showed an
increase greater than 1 |jg/m3 (Juno and Anchorage,
Alaska; Nogales, Ariz.; Klamath Falls, Ore.; New
Orleans, La.; El Paso and Houston, Texas; Vilas County,
Wis.). Of the eight areas that showed an increase, four
were below the level of the annual PM2 5 standard for
the most recent year of data and four were above. The
four areas above were New Orleans, Nogales, El Paso,
and Houston.
Change in Concentration (|jg/m3)
O Increase of 1.1 to 4 (8 Sites)
O Little change +-1 (147 Sites)
O Decrease of 1.1 to 4 (447 Sites)
O Decrease of 4.1 to S (34 Sites)
A Decrease of more than 6 (9 Sites)
Puerto Rico
Alaska
Figure 14. Change in PM25 concentrations in ug/m3, 2000 vs. 2006 (annual average).
Note: The national monitoring network for PM25 began in 1999 and was fully implemented in 2000. Three years of data are used
to determine if an area meets the annual PM25 national standard. The map above shows the difference between individual years.
LATEST FINDINGS ON NATIONAL AIR QUALITY
15
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PARTICLE POLLUTION
In 2006, annual and daily PM2 5 concentrations
were generally the lowest of the seven-year period.
As shown in Figure 15, the highest annual PM2 5
concentrations were in Alabama, Pennsylvania, and
California. The highest daily PM2 5 concentrations were
in California, Arizona, and Pennsylvania. Some sites
had high daily PM2 5 concentrations but low annual
PM2 5 concentrations, and vice versa.
Most of the metropolitan areas displayed in Figure 16
had fewer unhealthy AQI days due to particle
pollution in 2006 compared with the average from the
previous five years (2001-2005). Los Angeles, Salt Lake
City, and Cleveland had the largest decreases in the
number of unhealthy days.
Annual
Daily
Concentration Range (jjg/m3)
O 0-12(496 Sites)
• 12.1 -15 (333 Sites)
O 15,1-18 (57 Sites)
• 18.1 -21 (9 Sites)
Puerto Rico
Alaska
Concentration Range (MS/m
O 0 - 15 (46 Sites)
O 16-35 (723 Sites)
O 36-55 (120 Sites)
• 56 - 65 (6 Sites)
Puerto Rico
Alaska
Figure 15. Annual average and daily (98th percentile 24-hour concentrations) PM25 concentrations in ug/m3, 2006.
Note: In 2006, EPA revised daily PM25 standards from 65 to 35 jag/m3.
16
LATEST FINDINGS ON NATIONAL AIR QUALITY
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Di •
San Francisco
Dallas
Fort Worth
Atlanta
rill
2 1
Houston
New Orleans
Orlando
,--•
Tampa
_
Miami
5 Year Average (2001-2005)
2006
Figure 16. Number of days reaching Unhealthy for Sensitive Groups for PM25 on the AQI for 2001-2005 (average) vs. 2006.
20%
» 00 01 02 03 04 05 06
in
I PM
"25
J1.6 -
1.2 -
0.8 -
0.4 -
20
00 01 02 03 04 05 06
voc
16 -
I":
11% 8 -
4 -
8%
0 -T^^^^^^^^^^^^^^^^ 0
00 01 02 03 04 05 06 00 01 02 03 04 05 06
• Fuel Combustion!"" ~~] Industrial Processes! iTiansportation
TRENDS IN PM25-FORMING
EMISSIONS
Nationally, between 2000 and 2006, SO2,
NOx, VOC, and directly emitted PM25
emissions decreased by 16, 20, 8, and
11 percent, respectively, as shown in
Figure 17. The contribution of wildfires is
not shown here. In fire-conducive years,
up to 20 percent of direct PM2 5 emissions
may be from wildfires; normally wildfire
emissions are closer to 4 percent.
Figure 17. National trends in annual direct PM25
and PM25-farming emissions, 2000-2006.
LATEST FINDINGS ON NATIONAL AIR QUALITY
17
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PARTICLE POLLUTION
WEATHER INFLUENCE ON PM25
Weather plays an important role in the formation
of PM25 (see "Seasonal Influences" below). Figure
18 shows PM2 5 trends before and after adjusting
for weather at selected sites. PM2 5 concentrations
decreased 16 percent from 2000 through 2006. When
the influence of weather is removed, the effect of
changes in emissions on air quality is easier to see, and
PM25 shows an 11 percent decrease from 2000 through
2006. The observed PM25 levels in 2005 are lower
after removing the influence of weather. Without the
influence of weather, the underlying national trend in
PM2 5 shows a moderate decline over the past several
years and is more consistent with national trends in
emissions.
o
to
fe
8
§
o
in
[N
14
12-
10
Observed trend
•••••- Adjusted trend
00
01
02
03
04
05
06
2000 to 2006: 16% decrease (observed)
2000 to 2006: 11% decrease (adjusted)
Figure 18. Trends in annual average PM25 concentrations, before and after adjusting for weather, and the location of urban
monitoring sites used in the average.
Note: Meteorological adjustment is done on a site-by-site basis, with each of the 72 selected sites shown in this map representing an urban area.
Seasonal Influences on PM_
Emissions sources and the composition of PM25 differ by season. For example, in cool months the greater demand for home or
office heating (e.g., use of wood stoves or oil furnaces) creates more direct PM25 emissions, while in the warm months, weather
conditions more conducive to PM25 formation create more secondary PM25. To better understand weather influences on annual
PM25 concentrations, the data were partitioned into "warm" and "cool" seasons. A statistical model was used to remove the
influence of weather, as shown here for the eastern U.S. between 2000 and 2006. For the warm season, PM25 concentrations
generally decreased (shown in blue) in the East except for modest increases (shown by yellow, orange, or red) in Houston,
Texas, West Virginia, and South Carolina. During the cool season, noticeable decreases occurred across much of the East.
Warm Season
Cool Season
Note: Two-year averages
were used to mitigate
uncertainty in individual
year estimates. Estimated
changes for locations that
are not near monitoring
sites (dots on map) have
the largest uncertainty. For
PM25 speciation by season,
visit http://epa.gov/ttnnaaqs/
standards/pm/data/
pmstaffpaper_20051221.pdf
(see Figures 2-23 and 2-24).
Change in warm (April-September) and cool season (October-March) PMi5 concentrations in
ug/m3 after removing the influence of weather, 2000-2001 (average) vs. 2005-2006 (average).
18
LATEST FINDINGS ON NATIONAL AIR QUALITY
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TRENDS IN PM25 COMPOSITION 2002-2006
PM2 5 is made up of several different chemical components.
In urban areas, PM2 5 is primarily composed of sulf ate,
nitrate, organic carbon (OC), and, to a lesser degree,
elemental carbon (EC) and crustal material. Figure 19
shows regional trends in the composition of PM2 5 from
2002 to 2006. Decreasing concentrations in southern
California from 2002 to 2006 were largely the result of
decreasing levels of nitrate; OC levels remained relatively
unchanged and have been the largest component of PM2 5
in the region. The Southeast had little change in PM2 5 and
its two major components—sulf ate and OC—over the
five-year period. The industrial Midwest and the Northeast
showed decreasing concentrations, except for an increased
amount of PM2 5 in 2005. In 2005, the industrial Midwest
had a temporary increase in PM2 5 concentrations, mostly
due to more nitrate and sulfate, which was caused by
a colder-than-normal winter and a hotter-than-normal
summer. The former conditions were more conducive to
nitrate formation, while the latter conditions were more
conducive to sulfate formation and also caused higher SO2
emissions due to higher electrical demand.
Sources of Particle Pollution
Component
Sources
Sulfates
Nitrates
Elemental and
organic carbon
Crustal
Power generation
Cars, trucks, and
power generation
Cars, trucks, heavy equipment,
wildfires, waste burning, and
vegetation
Suspended soil and metallurgical
operations
Note: Ammonia from sources such as fertilizer and animal
feed operations contributes to the formation of sulfates
and nitrates that exist in the air as ammonium sulfate and
ammonium nitrate. For more information about fine particle
sources, visit
http://www.epa.gov/air/airtrends/aqtrnd04/pm.html.
20-
116-
112-
1
"ra
en
w>
03
5
20-
16-
12-
8-
4-
n -
Northwest
-
_
-
-
P
Upper Midwest
02 03 04 05 06
" Industrial Midwest
Northeast
02 03 04 05 06
Southern CA
02 03 04 05 06
02 03 04 05 06
Sulfate
I Nitrate
Elemental carbon
Organic carbon
I Crustal
Figure 19. Regional trends in annual PM25 composition inug/m3, 2002-2006.
Note: This figure is based on 41 monitoring locations with the most complete data from the national chemical speciation network for 2002-
2006. There were no sites with complete data in the Southwest. These components are presented in terms of their mass as they might have been
measured by the PM25 Federal Reference Method (FRM). To characterize these trends, ambient nitrate measurements, and associated ammonium,
were adjusted to reflect the lower amount retained on FRM filters. Particle-bound water was included as a mass enhancement to measured sulfate,
ammonium, and adjusted nitrate. Organic carbon mass was derived by material balance between measured PM25 and the other components.
LATEST FINDINGS ON NATIONAL AIR QUALITY
19
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PARTICLE POLLUTION
TRENDS IN PM10
CONCENTRATIONS
Between 1990 and 2006, PM10
concentrations decreased 30 percent,
as shown in Figure 20. The largest
decreases were in Spokane, Wash., and
Klamath Falls, Ore. Forty-three sites had
an increase of more than 5 |ag/m3. The
largest increases were in Houston, Texas;
Las Cruces, N.M.; Nogales, Ariz.; Salt
Lake City, Utah, and areas of Colorado.
Figure 21 shows that in 2006 the highest
concentrations were located in Illinois
and the Southwest, including parts
of California, Nevada, Arizona, New
Mexico, and western Texas.
I
o
O
160
140
120
100
80
60
40
20
0
National Standard 391 sites
90 percent of sites are below this line.
10 percent of sites are below this line.
90 92 94 96 98 00 02
1990 to 2006: 30% decrease
04
06
Figure 20. National PMW air quality trend, 1990-2006 (second
maximum 24-hour concentration).
Figure 21. PMW concentrations
in ftg/m3, 2006 (second maximum
24-hour concentration).
Concentration Range djg.'
O 0-54 (479 Siles)
• 55-154(389 Sites)
O <55 - 255 (24 Sites!
• -"255 (12 Sites)
Puerto Rico
90
In 1996 and 2002,
EPA improved its method'
| for estimating emissions |
I Transportation J
~
I Industrial processes
I _ I
96 98 00 02
1990 to 2006: 20% decrease
Figure 22. National trends in direct PMW emissions, 1990-2006.
TRENDS IN PM10 EMISSIONS
Between 1990 and 2006, emissions
of directly emitted PM10 decreased
20 percent, as shown in Figure 22.
Changes in how EPA compiled the
national inventory over time may account
for some differences.
20
LATEST FINDINGS ON NATIONAL AIR QUALITY
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I*
NITROGEN DIOXIDE (NO2)
0.06
--0.05
Q.
3=0.04
NATURE AND SOURCES
Nitrogen dioxide (NO2) is a member of the nitrogen
oxide (NOJ family of gases. It is formed in the air
through the oxidation of nitric oxide (NO) emitted
when fuel is burned at a high temperature. The
monitoring network measures concentrations of
NO2 in the air to compare with national air quality
standards, and EPA tracks national emissions of NOx.
The major sources of NOx emissions are automobiles,
power plants, and any other industrial, commercial, or
residential source that burns fuel.
HEALTH AND
ENVIRONMENTAL EFFECTS
Exposure to NO2 has been associated with an
increased risk of respiratory illness in
children. Short-term exposures (e.g., less
than three hours) to low levels of NO2
may decrease lung function in individuals
with pre-existing respiratory illnesses.
Long-term exposures well above ambient
NO2 levels may cause irreversible changes
in lung structure.
NOx contributes to other air quality
problems that cause a variety of health
and environmental impacts. For example,
ground-level ozone forms when NOx and
VOCs react in the presence of sunlight.
NOx reacts with ammonia and moisture
to form nitric acid and particle nitrates.
NOx reacts with organic chemicals or
ozone to form a variety of toxic products
including nitrate radicals, nitroarenes, and
nitrosamines. NOx also contributes to
nutrient overloading that deteriorates
water quality and plays a major role in
visibility impairment and acid rain.
TRENDS IN NO2
CONCENTRATIONS
Nationally, concentrations of NO2
decreased 30 percent between 1990 and
2006, as shown in Figure 23. In 2006,
NO2 concentrations were generally the
lowest of the 17-year period. All recorded
concentrations were well below the level
of the national standard (0.053 ppm).
TRENDS IN NOX EMISSIONS
Between 1990 and 2006, NOx emissions decreased
29 percent, as shown in Figure 24. Most NOx emissions
come from transportation and fuel combustion sources,
which decreased by 21 and 41 percent, respectively.
Overall, NOx emissions did not change much between
1990 and 1998. After 1998, NOx emissions showed a
decrease similar to the decrease in NO2 concentrations
shown in Figure 23. NOx emissions from transportation
sources decreased 17 percent, and fuel combustion
sources decreased 38 percent between 1998 and 2006.
Most of the fuel combustion NOx emission reductions
were due to the Acid Rain Program, which began in
1995, and implementation of the NOx SIP Call, which
led to sustained reductions beginning in 2003 and 2004.
170 sites
National Standard
90 percent of sites are below this line.
10 percent of sites are below this line.
90 92 94 96 98 00 02 04
1990 to 2006: 30% decrease
06
Figure 23. National NO2 air quality trend, 1990-2006 (annual average).
30
.2
1
en
O
E
UJ
O
20-
15-
10
5
0
I
Transportation
Industrial processes
Fuel combustion
90
92
04
94 96 98 00 02
1990 to 2006: 29% decrease
Figure 24. National trends in annual NOx emissions, 1990-2006.
06
LATEST FINDINGS ON NATIONAL AIR QUALITY
21
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I*
CARBON MONOXIDE (CO)
NATURE AND SOURCES
Carbon monoxide (CO) is a colorless and odorless gas
formed when carbon in fuel is not burned completely.
It is a component of on-road vehicle exhaust and
other non-road engines and vehicles (such as aircraft,
locomotives, and construction equipment). Higher
concentrations of CO generally occur in areas with
heavy traffic congestion. In cities, as much as 95 percent
of all CO emissions may come from motor vehicle
exhaust. Other sources of CO emissions include
industrial processes (such as metal processing and
chemical manufacturing), residential wood burning,
and natural sources such as forest fires. The highest
levels of CO typically occur during the colder months
of the year when inversion conditions (in which air
pollutants are trapped near the ground
beneath a layer of warm air) are more
frequent.
HEALTH EFFECTS
CO enters the bloodstream through the
lungs and reduces oxygen delivery to the
body's organs and other tissues. Higher
levels of CO are most serious for those
suffering from heart disease such as
angina, clogged arteries, or congestive
heart failure. For a person with heart
disease, a single exposure to CO at high
levels may cause chest pain and reduce
the person's ability to exercise; repeated
exposures may contribute to other
cardiovascular effects. People who breathe
high levels of CO can develop vision problems,
reduced ability to work, reduced manual
dexterity, and difficulty performing
complex tasks. At even higher levels, CO
can cause death.
TRENDS IN CO EMISSIONS
Nationally, CO emissions (excluding wildfires and
prescribed burning) decreased 38 percent between 1990
and 2006, as shown in Figure 26. Emission reductions
from transportation sources, a major contributor to
ambient CO concentrations, were responsible for most
of this decrease. CO emissions from transportation
sources were reduced by more than 52 million tons (or
about 40 percent) over the 17-year period.
These improvements in CO concentrations and
emissions since 1990 occurred despite a 43 percent
increase in vehicle miles traveled during the same
17-year period. Cleaner cars have contributed to
cleaner air for much of the U.S.
90 percent of sites are below this line.
percent of sites are below this line
94 96 98 00 02 04
1990 to 2006: 62% decrease
Figure 25. National CO air quality trend, 1990-2006 (second
maximum 8-hour average).
150
TRENDS IN CO
CONCENTRATIONS
Nationally, CO concentrations declined
62 percent between 1990 and 2006,
as shown in Figure 25. In 2006, CO
concentrations were the lowest in the past
17 years. One site in Birmingham, Ala.,
showed concentrations above 9 ppm, the
level of the standard.
I 120-
o
1 90-
o
'
E
LU
O
o
60-
30-
Transportation
Fuel combustion
Industrial processes
90 92 94 96 98 00 02 04 06
1990 to 2006: 38% decrease
figure 26. National trends in annual CO emissions, 1990-2006.
22
LATEST FINDINGS ON NATIONAL AIR QUALITY
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SULFUR DIOXIDE (SOJ
NATURE AND SOURCES
Sulfur dioxide (SO2), a member of the sulfur oxide
(SOx) family of gases, is formed from burning fuels
containing sulfur (e.g., coal or oil), extracting gasoline
from oil, or extracting metals from ore. SO2 can
also dissolve in water vapor to form acid and can
interact with ammonia and particles to form sulfates
and other chemical products that can be harmful to
people and the environment. The monitoring network
measures concentrations of SO2 in the air to compare
with national air quality standards, and EPA tracks
national emissions of SO2. Eighty-seven percent of
the SO2 released into the air is attributable to fuel
combustion. Other sources of SO2 emissions include
industrial facilities such as petroleum refineries,
cement manufacturing facilities, and
metal processing facilities. Additionally,
locomotives, large ships, and some non-
road diesel equipment currently burn high
sulfur fuels that emit SO2.
HEALTH AND ENVIRONMENTAL
EFFECTS
SO2 causes a wide variety of health and
environmental impacts. Particularly
sensitive groups include asthmatics who
are active outdoors, children, the elderly,
and people of any age with heart or lung
disease. Longer-term exposures to high
levels of SO2 gases and related particles
have been shown to cause respiratory
illness and aggravate existing heart
disease. Sulfate particles can gather in the
lungs, causing respiratory symptoms
and disease, difficulty in breathing, and
premature death. Sulfate particles are the
major cause of reduced visibility in many
parts of the U.S., including national parks.
SO2 is also a major contributor to acid rain.
TRENDS IN SO2 CONCENTRATIONS
There are two standards for SO2: an
annual standard (0.03 ppm) and a daily
standard (0.14 ppm). The annual standard
is the focus in this report. Nationally,
concentrations of annual SO2 decreased
53 percent between 1990 and 2006, as
shown in Figure 27. In 2006, annual SO2
concentrations were generally the lowest of
the 17-year period. All concentrations were below the
level of the annual standard. One site in Northampton
County, Pa., showed concentrations above the level of
the daily standard in 2006.
TRENDS IN SO2 EMISSIONS
Since 1990, SO2 emissions have decreased 38 percent, as
shown in Figure 28. Emissions from fuel combustion,
industrial processes, and transportation sources
decreased 41, 40, and 30 percent, respectively.
The observed reductions in SO2 concentrations and
emissions since 1990 are mainly due to controls
implemented under EPA's Acid Rain Program, which
began in 1995.
u.uoo-
0,03-
E
£0.025-
o 0.02-
'•P
2
•g 0.01 5-
-------
I*
LEAD (Pb)
NATURE AND SOURCES
Automotive sources are no longer the
major contributors of lead emissions to the
atmosphere. As a result of EPA's regulatory
efforts to reduce the content of lead in
gasoline, lead emissions from the automotive
sector have greatly declined over the past
few decades. Today, industrial processes and
combustion of leaded fuel associated with
some small planes (piston-engine aircraft)
are the major sources of lead emissions to the
atmosphere.
HEALTH AND
ENVIRONMENTAL EFFECTS
People can be exposed to lead by inhaling it from the
air or by ingesting lead in contaminated drinking
water, lead-contaminated food, or lead-contaminated
soil and dust. Lead-based paint remains a major
exposure pathway in old houses. Depending on the
level of exposure, lead can adversely affect the nervous
system, kidneys, immune system, reproductive and
developmental systems, and the cardiovascular system.
Lead exposure also affects the oxygen carrying capacity
of the blood. The lead effects of greatest concern from
current exposures are neurological effects in children.
Infants and young children are especially sensitive
to even low levels of lead, which may contribute to
New Information on Lead Sources
1 .O -
.-,1.4-
rt
~ 1 -
c
l°-6:
o
i 0.4-
°n^
9
National Standard 44 sites
Averaqe ^ Percer^ °^ st^es are below this line.
0 92 94 96 98 00 02 04 Ol
1990 to 2006: 54% decrease
Figure 29. National lead air quality trend, 1990-2006 (maximum
quarterly average).
behavioral problems, learning deficits, and lower
intelligence quotients.
Lead is persistent in the environment and accumulates
in soils and sediments through deposition from air
sources, discharge of waste streams to water bodies,
mining, and erosion. Ecosystems near point sources
of lead demonstrate a wide range of adverse effects
including losses in biodiversity, changes in community
composition, decreased growth and reproductive
rates in plants and animals, and neurological effects in
vertebrates.
1.6
Number of sites
1.2-
f°.M
o
o
0.4-
47 127
National Standard
951-
751"
50"-
25";
Near Away from
sources sources
TRENDS IN LEAD CONCENTRATIONS
Large reductions in long-
term lead emissions from
transportation sources have
changed the nature of the
lead problem in the United
States. Unlike the early
1980s, most of the highest
lead concentrations in 2006
are near lead emissions
point sources. These point
sources include metals processors, battery manufacturers, waste incinerators, mining operations,
military installations, and facilities with large boilers (e.g., utility, industrial, and institutional).
Data for all lead monitoring sites with complete data in 2006 shows lead concentrations near
point sources are significantly higher than those not near point sources, as shown. The typical
concentration near a source is approximately 10 times the typical concentration for sites that are
not near a source.
Note: Concentrations shown are maximum quarterly averages using sites with complete data in 2006.
Because of the phase-out of leaded gasoline, lead
concentrations declined sharply during the 1980s and
early 1990s. Between 1980 and 2006, concentrations of
lead in the air decreased 95 percent, while emissions
of lead decreased 97 percent. From 1990 to 2006, lead
concentrations remained low, as shown in Figure 29. In
2006, only two sites had concentrations above the level
of the standard (1.5 |jg/m3); both are associated with
lead smelting operations in Herculaneum, Mo.
24
LATEST FINDINGS ON NATIONAL AIR QUALITY
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I*
TOXIC AIR POLLUTANTS
NATURE AND SOURCES
Hazardous air pollutants, or air toxics, are emitted
from thousands of sources across the nation, or they
are formed through atmospheric reactions of directly
emitted substances. Most air toxics originate from
man-made sources, including mobile sources (e.g.,
cars, trucks, construction equipment) and stationary
sources (e.g., factories, refineries, power plants), as well
as indoor sources (e.g., some building materials and
cleaning solvents). Some air toxics are also released
from natural sources such as volcanic eruptions and
forest fires. Examples of toxic air pollutants include
benzene, found in gasoline; tetracholorethylene (i.e.,
perchloroethylene), emitted from some dry cleaning
facilities; and dichloromethane (i.e., methylene
chloride), used as a solvent by a number of industries.
The Clean Air Act regulates 187 air toxics from various
sources. EPA has identified 21 pollutants as mobile
source air toxics, including diesel particulate matter and
diesel exhaust organic gases. In addition, EPA has listed
33 urban hazardous air pollutants that pose the greatest
threats to public health in urban areas.
HEALTH AND
ENVIRONMENTAL EFFECTS
People exposed to toxic
air pollutants at sufficient
concentrations may experience
various harmful health effects,
including cancer and damage
to the immune system, as well
as neurological, reproductive
(e.g., reduced fertility),
developmental, respiratory,
and other health problems.
In addition to exposure from
breathing air toxics, risks
are also associated with the
deposition of certain toxic
pollutants onto soils or
surface waters, where they
are taken up by plants and ingested by animals and
eventually magnified up through the food chain. Like
humans, animals and plants may be harmed by air
toxics exposure. Air toxics also may cause adverse
environmental and ecological effects.
TRENDS IN AIR TOXICS
CONCENTRATIONS AND EMISSIONS
The nation's monitoring network for air toxics is not
as extensive as that for many of the other pollutants
discussed in this report. Figure 30 shows ambient
monitoring locations for air toxics sites operating in
2005.
In 2003, working with its state and local partners,
EPA launched the implementation of the National Air
Toxics Trends Station (NATTS) program, a national
monitoring network for toxic air pollutants. The
central goal of the NATTS network is to assess trends
in high-risk air toxics such as benzene, formaldehyde,
1,3-butadiene, acrolein, and chromium. Fourteen of
the 23 sites began operation in 2003 and the remaining
nine were established in 2004.
Figure 30. Air toxics monitoring
sites operating in 2005 (by
monitoring program).
Monitoring Network
* NATTS
O UATMP
A Other
Puerto Rico
LATEST FINDINGS ON NATIONAL AIR QUALITY
25
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TOXIC AIR POLLUTANTS
In addition to the NATTS program, about 300 air toxics
monitoring sites are currently collecting data to help
air pollution control agencies track toxic air pollutant
levels in various locations around the country. State
and local air quality agencies operate these sites to
address specific concerns such as areas of elevated
concentrations or "hot spots," environmental justice
concerns, and/or public complaints. Some state and
local agencies use EPA sampling and analysis support
such as the Urban Air Toxics Monitoring Program
(UATMP).
Air toxics monitoring is generally most prevalent in
California, Texas, and the eastern U.S. and reflects
a tendency to monitor in densely populated areas.
Most sampling is conducted on a l-in-6-day schedule
for a 24-hour period. For the latest information about
national air toxics monitoring, visit
http://www.epa.gov/ttn/amtic.
EPA compiles an air toxics inventory as part of the
National Emissions Inventory (NEI) to estimate and
track national emissions trends for the 187 toxic air
pollutants regulated under the Clean Air Act. Figure 31
shows the emissions of toxic air pollutants divided
among the four types of sources, based on 2002
estimates (the most recent year of data available).
Non-road
19%
Area/Other,
33%
On-road
29%
Major
19%
Figure 31. Percent contribution by source sector to national
air toxics emissions, 2002.
Note: Emission sectors are (1) major (large industrial) sources;
(2) area and other sources, which include smaller industrial sources
like small dry cleaners and gasoline stations, as well as natural
sources like wildfires; (3) on-road mobile sources, including
highway vehicles; and (4) non-road mobile sources, such as aircraft,
locomotives, and construction equipment.
Non-road
On-road
Area/Other
Major
1990-1993
2002
Figure 32. Trends in national air toxics emissions for
1990-1993 vs. 2002.
Nationwide, air toxics emissions decreased
approximately 35 percent between the 1990-1993
baseline and 2002 as shown in Figure 32. Major and
on-road mobile sources showed the greatest emission
reductions (67 and 47 percent, respectively), while
emissions from both area and non-road mobile
sources increased over this period (26 and 15 percent,
respectively).
Although changes in how EPA compiled the national
inventory over time may account for some differences,
EPA and state regulations, as well as voluntary
reductions by industry, have dearly achieved large
reductions in total toxic emissions.
26
LATEST FINDINGS ON NATIONAL AIR QUALITY
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Ambient trends in air toxics vary by pollutant. Benzene,
one of the most routinely and accurately monitored
air toxics, is also estimated to be the most important at
a national level with regard to the average individual
cancer risk it poses. Figure 33 shows a national average
trend in benzene levels at 107 monitoring sites across
the country. These sites are generally in urban areas
that have higher levels of benzene than other areas of
the country. Data from these sites suggest an average
improvement of almost 17 percent between 2000 and
2005.
Figure 34 shows the location and change in benzene
concentrations at individual sites used to compile the
national trend. While some sites show an increase over
the time period of interest, no site shows a significant
increase, and most sites indicate improvement from
1990 to 2005.
90 percent of sites are below this line
10 percent of sites are below this line
01 02 03 04
2000 to 2005: 17% decrease
Figure 33. National benzene air quality trend, 2000-2005 (annual average).
Percentage Change per Year
Decreasing Trend, Significant
4f 0 - 8% per year
^ 8 -25% per year
- 100% per year
Decreasing Trend, non-significant
3t 0 - 8% per year
^ 8 - 25% per year
-100% per year
Increasing Trend, non-significant
^ 0 - 8% per year
^ 8 - 25% per year
-100% per year
figure 34. Trends in annual average benzene concentrations at individual sites for any period of at least five years
within 1990-2005.
LATEST FINDINGS ON NATIONAL AIR QUALITY
27
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TOXIC AIR POLLUTANTS
RISK ASSESSMENT
EPA has developed health risk estimates for 177 toxic
air pollutants (a subset of the Clean Air Act's list of
187 air toxics plus diesel particulate matter). Figure 35
shows the estimated lifetime cancer risk across the
continental U.S. by county based on 1999 model
estimates. More than 270 million people live in census
tracts for which the lifetime cancer risk from these
compounds exceeds a 10-in-one-million risk. From a
national perspective, benzene is the most significant
air toxic for which cancer risk could be estimated,
contributing 25 percent of the average individual
cancer risk identified in the 1999 assessment.
Though not included in the figure, exposure to diesel
exhaust is widespread. EPA has concluded that diesel
exhaust is a likely human carcinogen and ranks
with the other substances that the national-scale
assessment suggests pose the greatest relative risk.
For more information about EPA's National Air Toxics
Assessment, visit
http://www.epa.gov/ttn/atw/natal999.
Median Risk Level
0-25 in a Million
25 - 50 in a Million
50 - 75 in a Million
^|75-100 in a Million
^•> 100 in a Million
Puerto Rico
Figure 35. Estimated county-level cancer risk from the 1999 National Air Toxics Assessment (NATA99).
28
LATEST FINDINGS ON NATIONAL AIR QUALITY
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I-
ACID RAIN
Acid rain occurs when emissions of SO2
and NOx react in the atmosphere to form
acidic gases. The acidic gases react with
water vapor to form acid droplets, which
fall to the ground as acid deposition (more
commonly known as acid rain), harming
sensitive ecosystems in many areas of the
country. Acid rain leads to the acidification
of lakes and streams, rendering some of
them incapable of supporting aquatic life.
The electric power industry accounts for
about 67 percent of SO2 emissions and
19 percent of NOx emissions in the U.S.
from man-made sources.
The 1990 Clean Air Act Amendments
established the Acid Rain Program to
reduce the harmful effects of acid rain
through reductions in emissions of SO2 and
NOx. SO2 reductions are achieved by a cap
and trade program, which lets sources buy
or sell fixed amounts of SO2 allowances
on the open market while a limit, or cap,
is set on the total amount of SO2 that
can be emitted from all power plants.
NOx reductions are achieved through an
emissions rate-based program.
Since the start of the Acid Rain Program in
1995, reductions of SO2 and NOx emissions
from the power industry have contributed
to significant improvements in air quality
and environmental and human health. As of
2006, the program had
• Reduced SO2 emissions by more than
6.3 million tons from 1990 levels, or
about 40 percent of total power industry
emissions. Compared to 1980 levels,
SO2 emissions from power plants have
dropped by more than 7.9 million tons,
or about 46 percent. In 2006, annual SO2
emissions fell by over 800,000 tons from
2005 levels.
• Cut NOx emissions by about 3 million
tons from 1990 levels, so that emissions
in 2006 were less than half the level
anticipated without the program. Other
efforts, such as the NOx SIP Call in the
East, also contributed to this reduction.
• Led to significant decreases in acid rain.
For example, between the 1989-1991
and 2004-2006 time periods, sulf ate deposition decreased
over 30 percent in the Northeast and the Midwest, as shown
in Figure 36. These reductions have led to improving water
quality in lakes and streams.
Reduced sulf ate concentration in the air by about 30 percent
in most regions of the East. Both the size of the affected
region and magnitude of the highest concentrations have
dramatically declined, with the largest decreases observed
along the Ohio River Valley.
1989-1991
[
2004-2006
2.0
2.5
>3.0
Figure 36. Three-year average precipitation of sulf ate concentrations (SO42~)
in 1989-1991 and 2004-2006. Dots show monitoring locations.
(Data source: National Atmospheric Deposition Program,
http://na.dp.sws.uiuc.edu/)
LATEST FINDINGS ON NATIONAL AIR QUALITY
29
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I-
VISIBILITY IN SCENIC AREAS
Air pollution can impair visibility—and not just in
cities. Reduced visibility affects many of our best
known and most treasured national parks and
wilderness areas, such as Grand Canyon, Yosemite,
Yellowstone, Mount Rainier, Shenandoah, and Great
Smoky Mountains national parks, and the Mount Hood
and Okefenokee wilderness areas, as well as urban
areas. Visibility impairment results from the scattering
and absorption of light by air pollution, including
particles and gases. This limits the distance we can see
and can also degrade the color, clarity, and contrast of
those views. The same fine particles that are linked to
serious health effects and premature death can also
significantly affect visibility.
Some particles that contribute to visibility impairment
are emitted directly into the atmosphere from their
sources, such as dust from roads or elemental carbon
(soot) from wood combustion. In other cases, particles
form in the atmosphere from primary gaseous
emissions such as sulfates (formed from SO2 emissions
from power plants and other industrial facilities) and
nitrates (formed from NOx emissions from power
plants, automobiles, and other types of combustion
sources).
EPA monitors visibility trends, defined by the Regional
Haze Rule, in 155 of the 156 mandatory Class I areas
(certain national parks and wilderness areas meeting
the criteria established in the 1977 Clean Air Act
amendments). The Regional Haze Rule requires states
to identify the most effective means of preserving
conditions in Class I areas when visibility is at its
best—based on the best 20 percent visibility days—
and to gradually improve visibility when it is most
impaired—based on the worst 20 percent visibility
days. Trends in visibility for the annual 20 percent
best and worst visibility days are shown in Figure 37.
Several locations showed improving visibility
(decreasing haze) for the best visibility days at eastern
national park and wilderness monitoring sites (Acadia,
Moosehorn, Lye Brook, Dolly Sods, and Shenandoah),
and one location showed improvement for the worst
visibility days (Great Smoky Mountains). The western
U.S., which has most of the Class I areas, showed
improvement at 24 locations for the best visibility
days. Mount Rainer and Redwoods also showed
improvement on the worst visibility days. Only one
location—Petrified Forest, Ariz.—showed a notable
degradation in visibility (increasing haze) for the worst
days.
In 2001, EPA promoted the establishment of five
Regional Planning Organizations (RPOs) to serve as
centers for conducting the coordinated Regional Haze
Rule technical assessments and policy development
required of states and tribes in concert with federal
land managers and other stakeholders in five regions
of the U.S. More detailed information concerning
measured visibility levels, as well as links to all five
RPOs, are available at the Visibility Information
Exchange Web Site (VIEWS) at
http://vista.cira.colostate.edu/views/.
For more information about EPA's Regional Haze
Program, visit http://www.epa.gov/visibility.
These photographs taken at Grand Canyon National Park show how visibility can differ. PM25 concentrations were 0.2 |ag/m3 (left) and
37.3 |ag/m3 (right).
(Source: http://vista.cira.colostate.edu/improve/Publications/GrayLit/NPSSpecialImagesAJpdated%20NPS%20Special%20Images.pdf)
30
LATEST FINDINGS ON NATIONAL AIR QUALITY
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20% Best Days
JT, Decreasing haze
0 Possible decreasing haze
ft Possible increasing haze
T Increasing haze
• No trend
20% Worst Days
I Decreasing haze
0 Possible decreasing haze
t Possible increasing haze
f Increasing haze
• No trend
figure 37. Trends in visibility (haze index in deciviews) on the 20 percent best and worst visibility days, 1996-2005.
(Source: http://www.nature.nps.gov/air)
Note: Visibility trends using a haze index for the annual average for the 20 percent best and worst visibility days are based on aerosol
measurements collected at Interagency Monitoring of Protected Visual Environments (IMPROVE) monitoring sites. The haze index is measured
in deciviews (dv), a visibility metric based on the light extinction coefficient that expresses incremental changes in perceived visibility. Sites
having at least six years of complete data were used.
LATEST FINDINGS ON NATIONAL AIR QUALITY
31
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I-
CLIMATE CHANGE
Climate and ground-level air quality are
closely coupled within the atmosphere and
Earth system. For example, ground-level
ozone is a greenhouse gas (GHG). Particle
pollution can influence global and regional
climate by scattering or absorbing incoming
solar radiation, and by changing cloud
formation processes and cloud cover. And
climate changes have effects on air quality.
For example, warming of the atmosphere
increases the formation of ground-level
ozone, while increases in cloud cover tend to
decrease ozone formation.
Figure 38 shows the trends in domestic
GHG emissions over time in the U.S. The
dominant gas emitted is carbon dioxide
(mostly from fossil fuel combustion).
Total U.S. GHG emissions increased
16 percent between 1990 and 2005. The
Intergovernmental Panel on Climate Change
has concluded that the Earth's climate will
continue to warm as global GHG emissions
increase.
Research is under way that will provide an
improved understanding of the connections
between air quality and climate change.
Using estimates from a computer model that
assumes continued growth in global GHG
emissions, Figure 39 shows
how ground-level ozone in the eastern U.S.
may increase from current levels given
future climate change. For particle pollution,
the interrelationships of climate and
concentrations are more complex.
For more information about emissions and
trends in GHGs, visit http://www.ipcc.ch/
and http://www.epa.gov/climatechange/
emissions/usinventoryreport.html. For
information about what EPA is doing to
address climate change, visit
http://www.epa.gov/climatechange/.
MFCs, PFCs, & SF6
Nitrous Oxide
Methane
Carbon Dioxide
8000
6000
CT
LLI
O 4000
2000
90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05
Figure 38. Domestic greenhouse gas emissions in teragrams of
carbon dioxide equivalents (Tg CO2 eq), 1990-2005. (Source:
http://epa.gov/climatechange/emissions/usinventoryreport.html)
Notes: A teragram is equal to 1 million metric tons. Emissions in the figure
include fluorocarbons (HFCs, PFCs) and sulfur hexafluoride (SF6).
Increase in Summer
Dally 8-Hr Max Ozone
• < 4%
• 4 to 8%
O 8 to 12%
A 12% +
Figure 39. Predicted increases in summertime daily maximum
8-hour ozone concentrations between the 1990s and 2050s.
(Source: Bell, M., et al. Climate change, ambient ozone, and health
in 50 U.S. cities. Climatic Change, Vol. 82, Numbers 1-2, May 2007,
pp. 61-76)
32
LATEST FINDINGS ON NATIONAL AIR QUALITY
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International Transport of Air Pollution
While domestic sources of emissions are the primary cause of air pollution in our country, the U.S. is both an importer and
exporter of air pollution. Air pollution flows across boundaries — not only between the U.S. and our closest neighbors, Canada
and Mexico, but also between North America, Europe, and Asia, and to some extent, between North America, Africa, and
Central and South America.
Economic growth, in conjunction with increased emissions of particle pollution, mercury, and ozone precursors in developing
countries, may increase background levels of these pollutants in the U.S. EPA and other agencies are working via treaties and
international cooperative efforts to address the international transport of air pollution. For information about the Task Force on
Hemispheric Transport of Air Pollution, an international panel on which EPA serves, visit http://www.htap.org/.
The figure below illustrates how pollution can move. In the summer of 2004, NASA researchers sampled a variety of trace
gases and aerosols (tiny particles suspended in the air) across North America. During this time, wildfires in western Canada
and eastern Alaska were burning more acres than at any time in the previous 50 years. Smoke from these fires traveled
eastward and southward, reaching as far as the U.S. Gulf Coast.
This figure shows, for July 12-26, 2004, the total amount of carbon monoxide, one of the pollutants emitted by wildfires,
as measured by the Measurements of Pollution in the Troposphere (MOPITT) instrument aboard NASA's Terra Satellite.
High levels of pollution are indicated by yellow and red colors, and blue indicates low pollution levels. (Source: National
Center for Atmospheric Research/NASA)
LATEST FINDINGS ON NATIONAL AIR QUALITY
33
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TERMINOLOGY
AQI Air Quality Index
AQS Air Quality System
CAA Clean Air Act
CAIR Clean Air Interstate Rule
CASTNET Clean Air Status and Trends Network
CO carbon monoxide
dv deciviews
EC elemental carbon
EPA U.S. Environmental Protection Agency
FRM Federal Reference Method
GHG greenhouse gas
HFCs hydrofluorocarbons
IMPROVE Interagency Monitoring of Protected Visual Environments
MOPITT Measurements of Pollution in the Troposphere
NAAQS National Ambient Air Quality Standards
NASA National Aeronautics and Space Administration
NATTS National Air Toxics Trends Stations
NCAR National Center for Atmospheric Research
NCore National Core Monitoring Network
NEI National Emissions Inventory
NH3 ammonia
NO nitric oxide
NOx oxides of nitrogen
NO2 nitrogen dioxide
O3 ground-level ozone
OCM organic carbon mass
Pb lead
PFCs perfluorinated compounds
PM particulate matter
PM25 particulate matter (fine) 2.5 |jm or less in size
PM10 particulate matter 10 |om or less in size
ppm parts per million
SF, sulfur hexafluoride
b
SIP state implementation plan
SO sulfur oxides
x
SO2 sulfur dioxide
UATMP Urban Air Toxics Monitoring Program
|om micrometers
|jg/m3 micrograms per cubic meter
VOC volatile organic compound
34
LATEST FINDINGS ON NATIONAL AIR QUALITY
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WEB SITES
1999 National-Scale Air Toxics Assessment: http://www.epa.gov/ttn/atw/natal999
Acid Rain Program: http://www.epa.gov/airmarkets/progsregs/arp/index.html
Acid Rain Program 2006 Progress Report: http://www.epa.gov/airmarket/progress/arp06.html
Air Quality Index: http://www.airnow.gov
Air Quality System: http://www.epa.gov/ttn/airs/airsaqs
Air Quality Trends: http://www.epa.gov/airtrends
Air Toxics Monitoring: http://www.epa.gov/ttn/amtic
Air Trends Design Values: http://www.epa.gov/air/airtrends/values.html
Clean Air Status and Trends Network (CASTNET): http://www.epa.gov/castnet
Clean Air Interstate Rule: http://www.epa.gov/cair
Climate change: http://www.epa.gov/climatechange/
Emissions: http://www.epa.gov/ttn/chief/
Emissions and trends in greenhouse gases:
http://www.ipcc.ch
http://www.epa.gov/climatechange/emissions/usinventoryreport.html
EPA Monitoring Network: http://www.epa.gov/ttn/amtic
Health and Ecological Effects: http://www.epa.gov/air/urbanair
Local air quality trends: http://www.epa.gov/airtrends/where.html
Local trends in ozone levels: http://www.epa.gov/airtrends/ozone.html
National Air Monitoring Strategy Information:
http://www.epa.gov/ttn/amtic/monstratdoc.html
National Ambient Air Quality Standards:
http://w w w. epa. gov/ttn/naaqs/
http://w w w. epa. gov/air/criteria.html
National Atmospheric Deposition Program: http://nadp.sws.uiuc.edu/
National Core Monitoring Network, "DRAFT National Ambient Air Monitoring Strategy":
http://w w w. epa. gov/ttn/amtic/files/ambient/monitor str at/naamstr at2005. pdf
National Park Service: http://www.nature.nps.gov/air
NOx Budget Program Under the NOx SIP Call: http://www.epa.gov/airmarkets/progsregs/nox/sip.html
Office of Air and Radiation: http://www.epa.gov/air
Office of Air Quality Planning and Standards: http://www.epa.gov/air/oaqps
Office of Atmospheric Progams: http://www.epa.gov/air/oap.html
Office of Transportation and Air Quality: http://www.epa.gov/otaq
Regional Haze Program: http://www.epa.gov/visibility
Review of the National Ambient Air Quality Standards for Particulate Matter:
http://epa.gov/ttnnaaqs/standards/pm/data/pmstaffpaper_20051221.pdf
Task Force on Hemispheric Transport of Air Pollution: http://www.htap.org
The Particle Pollution Report - Current Understanding of Air Quality and Emissions Through 2003:
http://www.epa.gov/air/airtrends/aqtrnd04/pm.html
Trends in ozone adjusted for weather conditions: http://www.epa.gov/airtrends/weather.html
Visibility Information Exchange Web Site (VIEWS): http://vista.cira.colostate.edu/views/
LATESTFINDINGSONNATIONALAIRQUALITY 35
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