United States 

Environmental 

Protection Agency

Our Nation’s Air

STATUS AND TRENDS THROUGH 2010

U.S. Environmental Protection Agency

Office of Air Quality Planning and Standards

Research Triangle Park, North Carolina

Our Nation’s Air

STATUS AND TRENDS THROUGH 2010

EPA-454/R-12-001

February 2012
Highlights ... 1

Air Pollution ... 3

Six Common Pollutants ... 6

 Ozone ... 9

 Particle Pollution ... 12

 Lead ... 16

 NO2 , CO, and SO2 ... 17

Toxic Air Pollutants ... 19

Climate Change and Air Quality ... 22

Appendix ... 25

Table of Contents

More information and discussion on additional air quality topics are available at 

http://www.epa.gov/airtrends/2010.
Our Nation’s Air 1

Figure 1. Number of 

people (in millions) 

living in counties 

with air quality 

concentrations above 

the level of the primary 

(health-based) National 

Ambient Air Quality 

Standards (NAAQS) in 

2010. 

Note: Projected population 

data for 2009 (U.S. Census 

Bureau, 2009). Ozone (8-hour) 

is based on the 2008 revised 

ozone NAAQS of 0.075 ppm. The 

revised 1-hour standards for NO2

and SO2

 are not included.

Highlights

Improving public health by reducing air pollution and 

improving air quality is one of the U.S. Environmental 

Protection Agency’s (EPA’s) top priorities. This 

summary report presents EPA’s most recent evaluation 

of our nation’s air quality status and trends through 

2010.

Levels of Six Common Pollutants 

Continue to Decline

•	 Cleaner cars, industries, and consumer products 

have contributed to cleaner air for much of the U. S.

•	 Since 1990, nationwide air quality has improved 

significantly for the six common air pollutants. 

These six pollutants are ground-level ozone, particle 

pollution [particles 2.5 micrometers in diameter 

and smaller (PM2.5) and particles 10 micrometers 

and smaller (PM10)], lead, nitrogen dioxide (NO2

), 

carbon monoxide (CO), and sulfur dioxide (SO2

). 

Nationally, air pollution was lower in 2010 than in 

1990 for:

– 8-hour ozone, by 17 percent

– 24-hour PM10 , by 38 percent

– 3-month average lead, by 83 percent

– annual NO2 , by 45 percent

– 8-hour CO, by 73 percent

– annual SO2 , by 75 percent

•	 Nationally, annual PM2.5 concentrations were 24 

percent lower in 2010 compared to 2001. 24-hour 

PM2.5 concentrations were 28 percent lower in 

2010 compared to 2001.

•	 Ozone levels did not improve in much of the East 

until 2002, after which there was a significant 

decline. 8-hour ozone concentrations were 13 

percent lower in 2010 than in 2001. This decline 

is largely due to reductions in oxides of nitrogen 

(NOx) emissions required by EPA rules including 

the NOx State Implementation Plan (SIP) Call, 

preliminary implementation of the Clean Air 

Interstate Rule (CAIR), and Tier 2 Light Duty 

Vehicle Emissions Standards.

•	 Despite clean air progress, approximately 124 

million people lived in counties that exceeded 

one or more national ambient air quality standard 

(NAAQS) in 2010, as shown in Figure 1. Groundlevel ozone and particle pollution still present 

challenges in many areas of the country.

Levels of Many Toxic Air Pollutants 

Have Declined

•	 Total emissions of toxic air pollutants have 

decreased by approximately 42 percent between 

1990 and 2005. Control programs for mobile 

sources and facilities such as chemical plants, dry 

cleaners, coke ovens, and incinerators are primarily 

responsible for these reductions. 
2 Our Nation’s Air

•	 Monitored concentrations of toxic pollutants 

such as benzene, 1,3-butadiene, ethylbenzene, 

and toluene decreased by 5 percent or more per 

year between 2003 and 2010 at more than half 

of ambient monitoring sites. Other toxic air 

pollutants of concern to public health such as 

carbon tetrachloride, formaldehyde, and several 

metals, declined at most sites. 

Air Quality and Greenhouse Gases

•	 EPA has concluded that there is compelling 

evidence that many fundamental measures of 

climate in the United States (e.g., air temperature) 

are changing, and many of these changes are linked 

to the accumulation of greenhouse gases (GHGs) 

in the atmosphere. GHG emissions from the U.S. 

have increased by approximately 7 percent since 

1990 and global GHG emissions are increasing at 

an even greater rate. Among other impacts, climate 

change also contributes to worsening air quality 

that can endanger public health.

•	 While reductions in emissions of long-lived 

GHGs like CO2

 will be essential for addressing 

climate change in the long term, there are also 

climate benefits associated with reductions in 

certain short lived pollutants. In addition to known 

health benefits, reductions in black carbon particle 

pollution and ozone are also likely to lead to 

climate benefits.

More Improvements Anticipated 

EPA expects air quality to continue to improve as 

recently adopted regulations are fully implemented 

and states work to meet current and recently revised 

national air quality standards. Key regulations 

include the Locomotive Engines and Marine 

Compression-Ignition Engines Rule, the Tier 2 

Vehicle and Gasoline Sulfur Rule, the Heavy-Duty 

Highway Diesel Rule, the Clean Air Non-Road Diesel 

Rule, the Mobile Source Air Toxics Rule, the Cross 

State Air Pollution Rule and the Mercury and Air 

Toxics Standards.

Highlights
Our Nation’s Air 3

Health and Environmental Impacts

Air pollution can affect our health in many ways. 

Numerous scientific studies have linked air pollution to 

a variety of health problems including: (1) aggravation 

of respiratory and cardiovascular disease; (2) decreased 

lung function; (3) increased frequency and severity of 

respiratory symptoms such as difficulty breathing and 

coughing; (4) increased susceptibility to respiratory 

infections; (5) effects on the nervous system, including 

the brain, such as IQ loss and impacts on learning, 

memory, and behavior; (6) cancer; and (7) premature 

death. Some sensitive individuals appear to be at greater 

risk for air pollution-related health effects, for example, 

those with pre-existing heart and lung diseases 

(e.g., heart failure/ischemic heart disease, asthma, 

emphysema, and chronic bronchitis), diabetics, older 

adults, and children. 

Air pollution also damages our environment. For 

example, ozone can damage vegetation, adversely 

impacting the growth of plants and trees. These impacts 

can reduce the ability of plants to uptake carbon 

dioxide (CO2

) from the atmosphere and indirectly 

affect entire ecosystems. 

Pollutant Sources Health Effects

Ozone (O3)

Secondary pollutant typically formed by chemical 

reaction of volatile organic compounds (VOCs) 

and NOx in the presence of sunlight.

Decreases lung function and causes respiratory symptoms, such 

as coughing and shortness of breath; aggravates asthma and 

other lung diseases leading to increased medication use, hospital 

admissions, emergency department (ED) visits, and premature 

mortality.

Particulate 

Matter (PM)

Emitted or formed through chemical reactions; fuel 

combustion (e.g., burning coal, wood, diesel); 

industrial processes; agriculture (plowing, field 

burning); and unpaved roads.

Short-term exposures can aggravate heart or lung diseases leading 

to respiratory symptoms, increased medication use, hospital 

admissions, ED visits, and premature mortality; long-term exposures 

can lead to the development of heart or lung disease and 

premature mortality.

Lead

Smelters (metal refineries) and other metal 

industries; combustion of leaded gasoline in piston 

engine aircraft; waste incinerators; and battery 

manufacturing.

Damages the developing nervous system, resulting in IQ loss 

and impacts on learning, memory, and behavior in children. 

Cardiovascular and renal effects in adults and early effects related 

to anemia.

Oxides of 

Nitrogen 

(NOx

)

Fuel combustion (e.g., electric utilities, industrial 

boilers, and vehicles) and wood burning.

Aggravate lung diseases leading to respiratory symptoms, hospital 

admissions, and ED visits; increased susceptibility to respiratory 

infection.

Carbon 

Monoxide 

(CO)

Fuel combustion (especially vehicles).

Reduces the amount of oxygen reaching the body’s organs and 

tissues; aggravates heart disease, resulting in chest pain and other 

symptoms leading to hospital admissions and ED visits.

Sulfur 

Dioxide 

(SO2

)

Fuel combustion (especially high-sulfur coal); 

electric utilities and industrial processes; and natural 

sources such as volcanoes.

Aggravates asthma and increased respiratory symptoms. 

Contributes to particle formation with associated health effects.

Sources and Health Effects of Air Pollution

Air Pollution
4 Our Nation’s Air

Sources of Air Pollution

Air pollution consists of gas and particle contaminants 

that are present in the atmosphere. Gaseous pollutants 

include SO2

 , NOx , ozone, CO, volatile organic 

compounds (VOCs), certain toxic air pollutants, and 

some gaseous forms of metals. Particle pollution (PM2.5

and PM10) includes a mixture of compounds. The 

majority of these compounds can be grouped into five 

categories: sulfate, nitrate, elemental (black) carbon, 

organic carbon, and crustal material. 

Some pollutants are released directly into the 

atmosphere. Other pollutants are formed in the air. 

Ground-level ozone forms when emissions of NOx

and VOCs react in the presence of sunlight. Similarly, 

some particles are formed from other directly emitted 

pollutants. For example, sulfate particles are formed 

from complex reactions in the atmosphere of SO2

emissions from power plants and industrial facilities. 

Weather plays an important role in the formation of 

secondarily formed air pollutants, as discussed later in 

the Ozone and Particle Pollution sections.

EPA and states track direct emissions of air pollutants 

and emissions that contribute to the formation of 

key pollutants, also known as precursor emissions. 

Emissions data are compiled from many different 

organizations, including industry and state, tribal, and 

local agencies. Some emissions data are based on actual 

measurements while others are estimates.

Generally, emissions come from large stationary fuel 

combustion sources (such as electric utilities and 

industrial boilers), industrial and other processes 

(such as metal smelters, petroleum refineries, cement 

kilns, manufacturing facilities, and solvent utilization), 

and mobile sources including highway vehicles and 

non-road sources (such as recreational and construction 

equipment, marine vessels, aircraft, and locomotives). 

Sources emit different combinations of pollutants. 

For example, electric utilities release SO2

 , NOx , and 

particles. 

Figure 2 shows the distribution of national total 

emissions estimates by source category for specific 

pollutants in 2010. Electric utilities contribute over 

60 percent of national SO2

 emissions. Agricultural 

operations (included in the “other processes” category) 

contribute over 80 percent of national NH3

 emissions. 

Almost 50 percent of the national VOC emissions 

originate from solvent use (included in the “other 

processes” category). Highway vehicles and non-road 

mobile sources together contribute approximately 60 

percent of national CO emissions. Pollutant levels 

differ across regions of the country and within local 

areas, depending on the size and type of sources 

present.

Figure 2. Distribution of national total emissions estimates by source category for specific pollutants, 2010.

Note: Lead emissions estimates are for 2008.

Air Pollution
Our Nation’s Air 5

Figure 3. Comparison of growth measures and emissions, 1990-2010.

Note: CO2 emissions estimates are from 1990 to 2009.

Tracking Pollutant Emissions

Since 1990, national annual air pollutant emissions 

have declined, with the greatest percentage drop in 

lead emissions. Direct PM2.5 emissions have declined 

by more than half; PM10 and SO2 emissions have 

declined by more than 60 percent, and NOx and VOC 

emissions have declined by more than 40 percent.The 

combined emissions of the six common pollutants 

and their precursors (PM2.5 and PM10 , SO2

 , NOx , 

VOCs, CO, and lead) dropped 59 percent on average 

since 1990, as shown in Figure 3. This progress has 

occurred while the U.S. economy continued to grow, 

Americans drove more miles, and population and 

energy use increased. These emissions reductions were 

achieved through regulations, voluntary measures taken 

by industry, partnerships between federal, state, local, 

and tribal governments; academia; industrial groups; 

and environmental organizations. This environmental 

progress has occurred while overall, the U.S. economy 

grew 65 percent, Americans drove 40 percent more 

miles, and population and energy use increased by 24 

and 15 percent respectively. There was a noticeable 

decline in Gross Domestic Product between 2008 and 

2009. There was also a notable reduction in vehicle 

miles traveled and energy consumed from 2007 to 

2009. Factors likely contributing to these reductions 

include the nationwide spike in gasoline prices during 

2008 and the economic recession that began in 

2008. These indicators showed an increase in 2010.

Figure 3 also shows total CO2

 emissions increasing by 

about 8 percent from 1990 to 2009 (http://epa.gov/

climatechange/emissions/usinventoryreport.html). 

Air Pollution
6 Our Nation’s Air

The Clean Air Act requires EPA to set national air 

quality standards for specific pollutants to safeguard 

human health and the environment. These standards 

define the levels of air quality that EPA determines 

are necessary to protect against the adverse impacts 

of air pollution based on scientific evidence. EPA has 

established standards for six common air pollutants, 

which are referred to as “criteria” pollutants: ozone (O3

), 

particle pollution (PM), lead (Pb), nitrogen dioxide 

(NO2

), carbon monoxide (CO), and sulfur dioxide 

(SO2

). 

Trends in National Air Quality 

Concentrations

Air quality has improved continuously across the U.S. 

since the Clean Air Act was amended more than two 

decades ago. The downward trend in air pollution has 

been especially evident over the past several years as 

shown in Figure 4. The record-low air pollution levels 

observed in 2009 were primarily the result of numerous 

national and local regulations that have sharply 

reduced emissions. Also, meteorological conditions 

favorable to lower air pollution levels and the economic 

slowdown likely also contributed to the relatively clean 

conditions in 2009. This downward trend in air quality 

concentrations is expected to have had profound health 

benefits for the American people.

Figure 4 shows the national trend in lead and the 

national trends in the other five criteria pollutants 

between 1990 and 2010, relative to their respective 

Figure 4. Comparison of national levels of the six common pollutants 

to the most recent national ambient air quality standards, 1990-2010. 

National levels are averages across all monitors with complete data for 

the time period. Note: Air quality data for PM2.5 start in 1999. 

Six Common Pollutants
Our Nation’s Air 7

Air Quality in Nonattainment 

Areas

EPA works collaboratively with state, local, and tribal 

agencies to identify areas of the U.S. that do not meet 

the national ambient air quality standards (NAAQS). 

These areas, known as nonattainment areas, must 

develop plans to reduce air pollution and attain the 

NAAQS. EPA tracks the progress these areas make to 

assure air quality continues to improve in places where 

improvements are most needed.

Consistent with national averages, air quality in 

nonattainment areas has also improved. As of 2010, 

there were no violations of the annual standards for 

CO, NO2

, and SO2 . Figure 5 shows trends in average 

concentrations of ozone and particle pollution only in 

existing nonattainment areas with air quality exceeding 

one or more of these standards in 2010. Although 

many areas exceeded the level of the standard in 2010, 

there have been improvements in the levels of these 

pollutants in nonattainment areas since 2001. For 

example, between 2001 and 2010, ozone nonattainment 

areas showed a 9 percent improvement in ozone 

concentration levels. Figure 5 does not include all areas 

that are designated nonattainment for the pollutant 

shown. For more information on areas designated as 

nonattainment visit www.epa.gov/airquality/greenbook.

Despite these improvements, further reductions in air 

pollution are needed over parts of the country. EPA 

expects air quality to continue to improve as recent 

regulations are fully implemented and new measures 

are finalized. EPA periodically reviews and revises 

the national air quality standards as needed to protect 

public health and the environment. This means 

that although there is clear progress in reducing air 

pollution, and we expect that trend to continue, there 

may be a need to implement further control measures 

to meet new more protective air quality standards.

Figure 5. Air quality trends in nonattainment areas exceeding the ozone and particle pollution standards in 2010.

national ambient air quality standards. As noted above, 

most pollutants show a steady decline throughout that 

time period. For lead, there are significant year-toyear changes in lead concentrations largely driven by 

changes in lead concentrations at monitoring sites near 

stationary sources. These year-to-year changes reflect 

changes in operating schedules and plant closings. For 

ozone and particle pollution shown in Figure 4, the 

trends exhibit an even sharper decline over the past 

three to five years although meteorological conditions 

favorable to higher levels of ozone and particle 

pollution likely contributed to higher levels in 2010 

compared to 2009. 

Six Common Pollutants
8 Our Nation’s Air

Trends in Unhealthy Air Quality 

Days

The Air Quality Index (AQI) relates daily air 

pollution concentrations for ozone particle pollution, 

NO2, CO, and SO2 to health concerns for sensitive 

groups and for the general public. A value of 100 

generally corresponds to the national air quality 

standard for each pollutant. Values below 100 

are considered satisfactory. Values above 100 are 

considered unhealthy – first for certain sensitive 

groups of people, then for everyone as the AQI values 

increase.

Figure 6 shows the number of days on which the 

AQI exceeded 100 for each of the past nine years at 

35 select metropolitan areas. All areas experienced 

fewer unhealthy days in 2010 compared to 2002.

Ozone and particle pollution are the primary 

contributors to unhealthy AQI days. Weather 

conditions, as well as emissions, contribute to ozone 

Figure 6. Number of days on which AQI values were greater than 100 during 2002-2010 in selected cities.

and particle pollution formation. Some areas in the 

eastern U.S. experienced more unhealthy days in 2010 

compared to 2009, mostly due to weather conditions 

being more conducive to ozone formation in these areas 

in 2010.

http://www.airnow.gov

EPA’s Air Quality Index (AQI)

Six Common Pollutants
Our Nation’s Air 9

Figure 7. National 8-hour ozone air quality 

trend, 2001-2010 (average of annual fourth 

highest daily maximum 8 hour concentrations 

in ppm).

Trends in Ground-Level Ozone 

Concentrations

In March 2008, EPA strengthened the national 

standards for ground-level ozone, setting an 8-hour 

standard at 0.075 parts per million (ppm). Nationally, 

average ground-level ozone concentrations were 13 

percent lower in 2010 than in 2001, as shown in 

Figure 7. The trend showed a notable decline after 

2002. When comparing the three-year periods 2001-

2003 and 2008-2010, approximately 82 percent of 

the monitoring sites recorded a significant decline 

(> 0.005 ppm) in ozone concentrations. Sites that 

showed the greatest improvement were in or near the 

following metropolitan areas: South Bend, IN; Buffalo, 

NY; Chicago, IL; Milwaukee, WI; and Cleveland, 

OH. Ozone trends can vary locally. One site may 

show increases in ozone levels while nearby sites show 

decreases.

Ozone

Figure 8 shows a snapshot of ozone concentrations in 

2010. The highest ozone concentrations occurred in 

California. Note that the high concentration levels in 

Utah occurred in winter. Elevated wintertime ozone 

concentrations are most likely to occur when local 

sources of NOx and VOC emissions are trapped in a 

snow-covered valley on a clear day with light winds. 

Nationally, approximately 24 percent of all sites 

measured concentrations above the standard of 0.075 

ppm on four or more days in 2010.

Ozone

Over the years, EPA has adopted a number of 

regulations that helped reduce ozone levels 

nationwide. Other recently adopted regulations 

will help to continue to make progress toward 

lower, healthier ozone levels. These regulations 

include:

•	 Coordinated steps to reduce power plant 

pollution 

» NOx State Implementation Plan (SIP) Call 

» Acid Rain Program

» Cross-State Air Pollution Rule (CSAPR) 

•	 Requiring other stationary sources to reduce 

pollution

» Aerosol, architectural, autobody, and 

miscellaneous coatings

» Consumer products 

» Regional haze requirements 

•	 Limiting emissions from mobile sources

» Light Duty Tier 2 Rule - new cars, SUVs, 

trucks and vans 

» Heavy-Duty Diesel Rule on and nonroad

» Requirements for marine vehicles, and 

locomotives

•	 On December 30, 2011, the D.C. Circuit Court 

stayed the CSAPR rule pending judicial review. 

This decision delays implementation of CSAPR 

and leaves the Clean Air Interstate Rule in place 

pending the court’s decision.
Our Nation’s Air 1 0

Ozone

Figure 8. Ozone concentrations in ppm, 2010 (fourth highest daily maximum 8-hour concentration). 
1 1 Our Nation’s Air

Weather Influences Ozone

In addition to precursor emissions, 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 

remain unchanged. To better evaluate the progress and 

effectiveness of ozone precursor emission reduction 

programs, EPA uses a statistical model to estimate the 

influence of weather on ozone formation. 

Figure 9 shows trends in average seasonal ozone levels 

from 2001 through 2010 across 180 selected sites, 

before and after adjusting for weather-related effects. 

For example, the summer of 2009 was characterized 

by cooler than normal conditions across much of 

the Eastern U.S., which contributed to less ozone 

formation and resulted in an upward adjustment to the 

ozone trend. By contrast, hot and dry conditions in the 

Ozone

Figure 9. Trends in average summertime daily maximum 8-hour ozone concentrations in ppm (May-September), before 

and after adjusting for weather nationally, in western states, and in eastern states, (and the location of monitoring sites 

used in the averages). 

Eastern U.S. during the summer of 2010 contributed 

to more ozone formation, resulting in a downward 

adjustment to the ozone trend.

Both the observed and adjusted ozone trends are 

characterized by a large decrease in ozone in the 

Eastern U.S. between 2002 and 2004. This abrupt 

decline in ozone levels coincides with the large 

reduction in NOx emissions brought about by EPA’s 

NOx SIP Call program which began in 2003 and was 

fully implemented in 2004. Removing the effects of 

weather confirms that ozone levels have continued to 

improve across the U.S. in recent years due to emission 

reduction programs.
Our Nation’s Air 1 2

EPA has set national standards to protect against the 

health and welfare effects associated with exposure to 

fine and coarse particles. Fine particles are generally 

considered to be less than or equal to 2.5 micrometers 

(μm) in aerodynamic diameter, or PM2.5 . Coarse 

particles are those between 2.5 and 10 μm in diameter. 

PM10 is the indicator used for the coarse particle 

standard.

Trends in PM2.5 Concentrations

There are two national air quality standards for 

PM2.5 : an annual standard (15 μg/m3

) and a 24-hour 

standard (35 μg/m3

). Nationally, annual and 24-hour 

PM2.5 concentrations declined by 24 and 28 percent, 

respectively, between 2001 and 2010, as shown in 

Figure 10.

Figure 10. National PM2.5 air quality trends, 2001-2010 (annual average concentration 

and 98th percentile of 24-hour concentration in μg/m3).

Particle Pollution
1 3 Our Nation’s Air

In 2010, the highest annual average PM2.5

concentrations were in California, Indiana, 

Pennsylvania and Hawaii, as shown in Figure 11. 

The highest 24-hour PM2.5 concentrations were in 

California and Alaska. 

Some sites showed high 24-hour PM2.5 concentrations 

but low annual PM2.5 concentrations. Sites that show 

high 24-hour concentrations but low or moderate 

annual concentrations exhibit substantial variability 

from season to season. For example, sites in the 

Northwest generally show low concentrations in warm 

Figure 11. Annual average and 

24‐hour (98th percentile of 24-hour 

concentrations) PM2.5 concentrations in 

μg/m3, 2010.

months but are prone to much higher concentrations 

in the winter. Factors that contribute to the higher 

levels in the winter are extensive woodstove use 

coupled with prevalent cold temperature inversions 

that trap pollution near the ground. Nationally, more 

sites exceeded the level of the 24-hour PM2.5 standard 

than the annual PM2.5 standard, as indicated by 

yellow and red dots on the maps below. Of the 6 sites 

that exceeded the annual standard and 43 sites that 

exceeded the 24-hour standard, 4 sites exceeded both.

Particle Pollution
Our Nation’s Air 1 4

Figure 12 shows trends in PM2.5 from 2001 to 2010, 

averaged across 145 selected sites before and after 

adjusting for weather. The warm months trend is 

characterized by a large decrease in average PM2.5

between 2008 and 2010, while the cool months trend 

shows a slow but steady decrease in PM2.5 over the past 

decade. Overall, average PM2.5 concentrations in the 

U.S. have declined steadily since 2005 after removing 

the effects due to weather indicating improvement 

based on recently enacted emissions reduction 

programs.

Weather Influences PM2.5

In addition to emissions, weather plays an important 

role in the formation of PM2.5. PM2.5 tends to be 

dominated by different components at different times 

of the year (e.g. sulfates in the summer and nitrates in 

the winter), so the statistical model adjusting the PM2.5 

trend for weather is split into a ‘warm months’ trend 

running from May to September and a ‘cool months’ 

trend encompassing the remaining months of the year. 

The two trends were combined to form the annual 

trend using a weighted average.

Figure 12. Trends in annual, cool-month (October–April) and warm-month (May–September) average PM2.5 concentrations 

in μg/m3 (before and after adjusting for weather), and the location of monitoring sites used in the average.

Particle Pollution
1 5 Our Nation’s Air

Figure 13. National PM10 air quality trend, 

2001-2010 (second maximum 24-hour 

concentration in μg/m3).

Figure 14. PM10 concentrations in 

μg/m3, 2010 (second maximum 

24-hour concentration).

Figure 14 shows that in 2010, the highest PM10

concentrations were located in California, Utah, 

Colorado and New Mexico. However, within these 

same states some sites showed a decline greater than 

50 μg/m3

. Highest concentrations are largely located 

in dry and/or industrial areas with a high number of 

coarse particle sources.

Particle Pollution

Trends in PM10 Concentrations

Nationally, 24-hour PM10 concentrations declined 

by 29 percent between 2001 and 2010, as shown in 

Figure 13. 
Our Nation’s Air 1 6

Figure 15. National lead air quality trend, 2001-2010 (maximum 

3-month average in μg/m3).

Note: 90 percent of sites are shown in the orange area.

Trends in Lead 

Concentrations 

Concentrations of lead decreased 

approximately 71 percent between 2001 

and 2010, as shown in Figure 15. Average 

concentrations are shown for 39 sites near 

large stationary sources and 63 sites that 

are not near stationary industrial sources. 

The typical average concentration near a 

stationary source (e.g., metals processors, 

battery manufacturers, and mining 

operations) is approximately eight times the 

typical concentration at a site that is not 

near a stationary industrial source. There 

are significant year-to-year changes in 

lead concentrations at sites near stationary 

sources; these reflect changes in emissions 

due to changes in operating schedules 

and plant closings. For example, national 

lead concentrations declined between 

2001 and 2002, mostly due to lower lead 

concentrations at sites in Herculaneum, 

MO.

Figure 16 shows lead concentrations in 

2010. Of the 196 sites shown, 34 sites 

exceeded the 2008 lead standard (0.15 μg/

m3). All of these sites are located near 

stationary lead sources. Also in 2010, EPA 

promulgated requirements for monitoring 

near additional stationary lead sources 

that are estimated to have 0.50 or more 

tons per year (tpy) lead emissions. Up to 

270 new locations will be monitoring lead 

concentrations by the end of 2011 as a result 

of changes to the monitoring requirements 

made in 2008 and 2010.

Figure 16. Lead concentrations in μg/m3, 2010 (maximum 3-month 

averages).

Note: The number of sites in Figure 15 (102) differs from the number of sites in Figure 16 (196) due 

to differences in the requirements for lead data to be considered complete for each figure

Lead
1 7 Our Nation’s Air

Trends in NO2 , CO, and SO2

Concentrations

Nationally, annual mean concentrations of 

NO2

 decreased 33 percent between 2001 

and 2010, as shown in Figure 17. In 2010, 

NO2

 concentrations were the lowest of the 

ten-year period. All recorded concentrations 

were well below the level of the annual 

standard (53 ppb).

Nationally, concentrations of 8-hour CO 

decreased 52 percent between 2001 and 

2010, as shown in Figure 18. In 2010, CO 

concentrations were the lowest in the past 

ten years. All concentrations were below 

the 8-hour standard (9 ppm) and 1-hour 

standard (35 ppm). 

Nationally, annual mean concentrations of 

SO2

 decreased 50 percent between 2001 

and 2010, as shown in Figure 19. In 2010, 

annual SO2

 concentrations were the lowest 

of the ten-year period. One site in Hawaii 

showed concentrations above the level of 

the annual standard (30 ppb) and four sites 

in Hawaii showed concentrations above the 

level of the 24-hour standard (140 ppb). 

These high measurements were probably 

caused by emissions from a nearby volcano.

Downward trends in annual NO2

 , CO, 

and SO2

 are the result of various national 

emissions control programs. Even though 

concentrations of these pollutants are low 

with respect to national annual standards, 

EPA continues to track these pollutants 

because of their contribution to other air 

pollutants (e.g., ozone and PM2.5) and 

reduced visibility. On August 12, 2011, 

EPA finalized the decision to retain existing 

primary CO standards.

Figure 18. National CO air quality trend, 2001-2010 

(second maximum 8-hour average in ppm).

Figure 19. National SO2 air quality trend, 2001-2010 

(annual average in ppm). 

Figure 17. National NO2 air quality trend, 2001-2010 

(annual average in ppm). 

NO2 , CO, SO2
Our Nation’s Air 1 8

2010 NO2 and SO2 Standards

On January 22, 2010, EPA strengthened the healthbased NAAQS for NO2 . This action did not impact the 

NO2 secondary standard, set to protect public welfare. 

EPA set the new 1-hour NO2

 standard at the level 

of 100 ppb. The form for the 1-hour NO2

 standard 

is the 3-year average of the 98th percentile of the 

annual distribution of daily maximum 1-hour average 

concentrations. EPA also retained, with no change, 

the current annual average NO2

 standard of 53 ppb. 

Although this new standard is a 3-year average, Figure 

20 shows a snapshot of the 98th percentile of the 

1-hour daily maximum NO2

 concentration for 2010 

only.

On June 2, 2010, EPA strengthened the health-based 

NAAQS for SO2

. This action did not impact the 

SO2

 secondary standard, set to protect public welfare, 

which is currently under review. EPA replaced the 

existing annual and 24-hour primary SO2

 standards 

with a new 1-hour SO2

 standard set at 75 ppb to better 

protect public health by reducing exposure to high 

short-term (5 minutes to 24 hours) concentrations 

of SO2

 . Although this new standard is based on a 

3-year average, Figure 21 shows a snapshot of the 

99th percentile of the daily 1-hour maximum SO2

 

concentration for 2010 only. Note that Figure 21 

shows that the highest daily 1-hour maximum SO2 

concentrations occurred at sites in the Upper Midwest 

and portions of the Northeastern U.S.

> 75 (57 Sites)

On July 12, 2011, EPA proposed action on the 

combined review of the secondary NAAQS for 

oxides of nitrogen (NOx

) and oxides of sulfur (SOx). 

EPA sets secondary standards to protect against 

environmental damage caused by certain air 

pollutants. Consistent with the scientific evidence

pointing to the interrelated impacts of NOx and SOx 

on plants, soils, lakes, and streams, EPA assessed the 

environmental effects of these pollutants together. 

Based on this scientific evidence, EPA is proposing

to retain the existing secondary standards for NOx

and SOx

. The existing secondary standards are:

NO2

: 53 ppb (parts per billion) averaged over 

a year; and

SO2

: 0.5 ppm averaged over three hours, not 

to be exceeded more than once per year.

Also, EPA is proposing to establish an additional set 

of secondary standards identical to the new healthbased primary standards the Agency set in 2010. 

The proposed new secondary standards would be:

For NO2

: 100 ppb (parts per billion) averaged 

over one hour; and

For SO2

: 75 ppb averaged over one hour.

For additional information on the proposed 

secondary standards visit www.epa.gov/air/

nitrogenoxides/actions.html.

Figure 20. NO2 concentrations in ppb, 2010 (98th 

percentile of daily 1-hr maximum). 

Note: Typically the 1-hour standard is determined as the three-year average of 

the 98th (NO2

) or 99th (SO2) percentile of the daily maximum 1-hour average; 

however, these maps only include one year (2010).

Figure 21. SO2 concentrations in ppb, 2010 (99th 

percentile of daily 1-hr maximum).

N O 2 , CO 2 , SO 2
1 9 Our Nation’s Air

Trends in Toxic Air Pollutant 

Concentrations

Under the Clean Air Act, EPA regulates 187 toxic air 

pollutants. Toxicity levels, or the potential for adverse 

effects on human health and the environment, vary 

from pollutant to pollutant. For example, a few pounds 

of a relatively toxic pollutant may have a greater health 

effect than several tons of emissions of a less toxic 

pollutant. EPA recommends a set of benchmark toxicity 

levels for estimating the effects of exposure to individual 

toxic air pollutants. For more information, visit http://

www.epa.gov/ttn/atw/toxsource/table1.pdf.

EPA frequently relies on modeling studies to 

supplement air toxic monitoring data and to better 

define trends in toxic air pollutants. One such modeling 

study, the National-Scale Air Toxic Assessment 

(NATA), is a nationwide study of ambient levels, 

inhalation exposures, and health risks associated 

with emissions of 177 toxic air pollutants plus diesel 

particulate (assessed for noncancer only). NATA 

examines individual pollutant effects as well as 

cumulative effects on human health. 

Figure 22 shows the estimated lifetime cancer risk 

across the continental U.S. by census tract based on 

2005 NATA model estimates. The national average 

cancer risk level in 2005 is 50 in a million. Many urban 

areas as well as transportation corridors show a risk 

above the national average. From a national perspective, 

formaldehyde and benzene are the most significant 

toxic air pollutants for which EPA could estimate 

cancer risk. These toxic air pollutants contributed 

nearly 60 percent of the average individual cancer risk 

identified in the 2005 assessment. In addition to the 

census tract level ambient concentrations predicted by 

the NATA 2005, EPA also used the model to compare 

with monitored air toxics concentrations at over 

1000 locations. When comparing modeling results 

to monitored data, a model-to-monitor ratio close to 

1 for a particular toxic pollutant at a monitoring site 

indicates a high level of confidence in the modeling 

results for that toxic pollutant and monitoring site. 

Good agreement was seen between the model and 

monitors for the following pollutants: acetaldehyde, 

arsenic (PM2.5), benzene, carbon tetrachloride, 

formaldehyde, methyl chloride and toluene. Results 

Figure 22. Estimated 

census-tract cancer 

risk from the 2005 

National-Scale Air 

Toxics Assessment 

(NATA2005). Darker 

colors show greater 

cancer risk associated 

with toxic air pollutants.

Toxic Air Pollutants
Our Nation’s Air 2 0

of this model-to-monitor comparison can be found at 

http://www.epa.gov/ttn/atw/nata2005/compare.html. 

Though not included in the figure below, exposure 

to diesel exhaust is also widespread. EPA has not 

adopted specific risk estimates for diesel exhaust but 

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 to human health. For more information on 

NATA visit http://www.epa.gov/ttn/atw/natamain.

Since 2003, EPA, working with state and local partners, 

has nationally monitored air toxic pollutants through 

the National Air Toxics Trends Station (NATTS) 

program. The principal objective of the NATTS 

network is to provide long-term monitoring data 

across representative areas of the country for NATA 

priority pollutants (e.g., benzene, formaldehyde, 

1,3-butadiene, hexavalent chromium, and polycyclic 

aromatic hydrocarbons [PAHs] such as napthalene) 

in order to establish overall trends. During 2010, data 

were collected every one in six days at 27 NATTS sites 

as shown in Figure 23 (20 urban and 7 rural) for PM10 

metals, VOCs, carbonyls, hexavalent chromium, and 

PAHs. In addition to the NATTS program, about 300 

Toxic Air Pollutants

monitoring sites—operated by state, local, and tribal 

agencies—are currently collecting data to help track 

toxic air pollutant levels across the country. For more 

information on NATTS visit http://www.epa.gov/ttn/

amtic/natts.html.

Figure 24 shows the trends from 2003 to 2010 in 

ambient monitoring levels for some of the important air 

toxic air pollutants. When the median percent change 

per year (marked by an x for each pollutant shown) 

is below zero, the majority of sites in the U.S. show 

a decrease in concentrations. Ambient monitoring 

data show that some of the toxic air pollutants of 

greatest widespread concern to public health, such 

as benzene, 1,3-butadiene, formaldehyde and several 

metals, are declining at most sites. Monitoring data 

shown in Figure 24 represent compilation of data from 

monitoring sites nationwide including data from the 

NATTS sites. Pollutants represented have at least 

a minimum of 40 valid trends sites with 35 percent 

of the data being measured at levels above monitor 

detection limits. Some pollutants which are more 

widely monitored such as lead and manganese may 

include data from several hundred sites which meet 

the 35 percent criteria. Some pollutants such as methyl 

Figure 23. National Air Toxics Trends Sites (NATTS)
2 1 Our Nation’s Air

Figure 24. Distribution of changes in ambient 

concentrations at U.S. toxic air pollutant 

monitoring sites, 2003-2010 (percent change in 

annual average concentrations). 

Toxic Air Pollutants

Assessing Outdoor Air Near Schools

In March 2009, EPA released a list of schools 

that would be part of an initiative to understand 

whether outdoor toxic air pollution poses health 

concerns to schoolchildren. The monitoring 

took place at 65 schools in 22 states and 2 tribal 

areas. EPA selected the schools using a number of 

factors, including results from computer modeling 

analyses – the 2002 NATA, results presented in 

a newspaper series on air toxics at schools, and 

in consultation with state and local air agencies. 

The pollutants monitored varied by school. EPA 

identified pollutants to measure at each school

based on the best available information about the 

pollution sources, potential air concentrations, and 

risk in each area. Initial monitoring was completed 

for all schools in May 2010. EPA posted monitoring 

results after data was quality-assured and intends 

to post final reports for each monitoring location

as the information is analyzed. For the majority 

of schools, monitored concentrations have been 

lower than EPA’s models predicted. However, 

additional monitoring will be conducted for a few 

schools for various reasons. As a follow on to the 

schools program, EPA issued a request in 2011 

for proposals for grants for community-scale air 

toxics ambient monitoring projects. Through 

these grants, local air toxics concerns will be 

investigated by state and local agencies. For more 

information, visit http://www.epa.gov/schoolair/.

tert-butyl ether (MTBE) whose use was discontinued 

after 2006 are no longer being measured at ambient 

monitoring sites as the levels are very low. There are 

two chlorinated VOCs which appear to have increased 

slightly, dichloromethane (methylene chloride) which is 

commonly used as a solvent, and chloromethane which 

was once used as a refrigerant and is also naturally 

formed in the oceans.
2 2 Our Nation’s Air

Climate Change and GHG 

Emissions Trends

Climate change and air pollution are closely coupled. 

Just as air pollution can have adverse effects on human 

health and ecosystems, it can also impact the Earth’s 

climate. When energy from the sun reaches the Earth, 

the planet absorbs some of this energy and radiates 

the rest back to space as heat. The Earth’s surface 

temperature depends on this balance between incoming 

and outgoing energy. Atmospheric greenhouse gases 

(GHGs) like carbon dioxide (CO2

) and methane (CH4) 

can trap this energy and prevent the heat from escaping. 

In 2009, EPA issued a finding under the Clean Air 

Act that GHGs constitute air pollution that threatens 

public health and welfare. The science supporting 

that finding allowed EPA to conclude that warming 

of the climate system is unequivocal, and that most of 

the observed increase in global average temperatures 

since the mid-20th century is very likely due to the 

anthropogenic increase in GHG concentrations 

(EPA, 2009). EPA has further concluded that there is 

compelling evidence that many fundamental measures 

of climate in the United States are changing, and 

many of these changes are linked to the accumulation 

of GHGs in the atmosphere. Examples of these 

climate-driven effects include warmer air and ocean 

temperatures, more high-intensity rainfall events, and 

more frequent heat waves. 

In collaboration with other government agencies, 

EPA tracks both GHG emissions (EPA, 2011) and 

indicators of climate change (EPA, 2010). Figure 25 

shows trends in domestic GHG emissions over the 

past two decades. Total U.S. GHG emissions have 

increased 7.3 percent from 1990 to 2009. The majority 

of domestic GHG emissions result from electricity 

generation and transportation.

In January 2012, EPA released for the first time 

comprehensive greenhouse gas (GHG) emissions data 

reported directly from large facilities and suppliers 

across the country through the GHG Reporting 

Program. The 2010 GHG data includes public 

information from facilities in nine industry groups that 

directly emit large quantities of GHGs (e.g., power 

plants, petroleum refineries, landfills, etc.) as well 

as suppliers of certain fossil fuels. EPA’s online data 

publication tool allows users to view and sort GHG 

Climate Change & Air Quality

Figure 25. Domestic greenhouse 

gas emissions in teragrams of 

carbon dioxide equivalents 

(Tg CO2 eq), 1990-2009.

(EPA, 2011)

Notes: A teragram is equal to 1 million 

metric tons. Emissions in the figure include

fluorocarbons (HFCs, PFCs) and sulfur

hexafluoride (SF6). CO2 eq refers to the 

global warming potential (GWP) of each 

greenhouse gas (e.g., nitrous oxide) as 

compared to the GWP of CO2 (EPA, 2011)
Our Nation’s Air 2 3

data from more than 6,700 facilities in a variety of 

ways; including by facility, location, industrial sector, 

and type of GHG emitted. This information can 

be used by communities to identify nearby sources 

of greenhouse gas emissions, help businesses track 

emissions and find cost- and fuel-saving opportunities, 

and provide information to the finance and investment 

communities. For more information, visit http://epa.

gov/climatechange/emissions/ghgdata.

Climate Impacts of Air Pollution 

Conventional air pollutants such as ozone and particle 

pollution can also contribute to climate change. 

Because ozone and particle pollution stay in the 

atmosphere for only a few days or weeks, reducing these 

emissions can help reduce climate impacts in the nearterm.

Figure 26. Estimates of global average radiative forcing (W/m2) resulting from changes in key 

climate-related air pollutants between the pre-industrial era and 2005. Data source: Forster, et. al., 

2007. For additional information on the level of scientific understanding for key climate-related air 

pollutants see Forster, et.al., 2007.

Ozone is a significant contributor to climate warming, 

as shown in Figure 25. The climate impacts of ozone 

are greatest when the ozone is located in the upper 

part of the troposphere. Concentrations of ozone in 

this part of the Earth’s atmosphere, sometimes referred 

to as “global background ozone,” are determined by 

worldwide emissions of CH4

, CO, NOx , and VOCs; as 

well as by natural processes like lightning and transport 

from the stratosphere. While ground-level ozone 

concentrations over the U.S. are generally declining, 

there is evidence that global background ozone levels 

continue to rise (Cooper, 2010). 

Particle pollution can also have significant impacts 

on climate, both directly and indirectly. The direct 

effects come from particles’ ability to absorb and 

scatter light. Different types of particles have different 

impacts on climate: some warm (e.g., black carbon); 

others cool (e.g., sulfates and nitrates). Black carbon, 

a component of soot particles, contributes to global 

warming by absorbing sunlight, thereby heating the 

atmosphere. When black carbon is deposited on snow 

and ice, melting accelerates. Black carbon’s effects 

are particularly strong in the Arctic and other alpine 

regions. The direct effects of particles on climate are 

shown in Figure 25. Particle pollution can also have 

important indirect effects on climate. For example, 

particles can change the reflectivity of clouds and also 

indirectly influence cloud lifetime and precipitation. 

Climate Change & Air Quality
2 4 Our Nation’s Air

Climate Change & Air Quality

The net effect for all particles in the atmosphere is 

cooling, as scattering generally dominates, though 

effects can vary dramatically by region (Forster et. al., 

2007). While the health benefits of reducing all types 

of emissions contributing to particle pollution are 

relatively clear, the net climate impact of emissions 

reduction strategies will depend on the relative 

reductions in particles of different types.

Air Quality Impacts of Climate 

Change

The close connection between climate and air quality 

is also reflected in the impacts of climate change on 

air pollution levels. As previously discussed, ozone and 

particle pollution are strongly influenced by shifts in 

the weather (e.g., heat waves or droughts). Based on 

projected future climate scenarios, and in the absence of 

additional emissions reductions, the Intergovernmental 

Panel on Climate Change (IPCC) projected “declining 

air quality in cities” into the future as a result of climate 

change. Further, EPA concluded in 2009 that GHG 

emissions “may reasonably be anticipated both to 

endanger public health and to endanger public welfare.” 

This endangerment finding was based, in part, on the 

potential for climate change to worsen air quality over 

the U.S. and the accompanying public health impacts 

that would result.

EPA has concluded (EPA, 2009) that climate change 

could have the following impacts on national air quality 

levels:

•	 Produce 2-8 ppb increases in summertime average 

ground-level ozone concentrations in many regions 

of the country.

•	 Further exacerbate ozone concentrations on days 

when weather is already conducive to high ozone 

concentrations

•	 Lengthen the ozone season

•	 Produce both increases and decreases in particle 

pollution over different regions of the U.S.

Because climate represents meteorological conditions 

over a long period of time, it is difficult to identify a 

climate fingerprint in the current trends in air quality 

discussed earlier in this report. Given the general 

improvement in air quality over the past decade, it 

appears that emissions reductions from air quality 

regulations are outpacing any climate-driven impacts.
Our Nation’s Air 2 5

Terminology

AQI Air Quality Index

AQS Air Quality System

BC black carbon

CASTNET Clean Air Status and Trends Network

CFCs chlorofluorocarbons

CH4 methane

CO carbon monoxide

CO2 carbon dioxide

EPA U.S. Environmental Protection Agency

GHG greenhouse gas

HFCs hydrofluorocarbons

IPCC Intergovernmental Panel on Climate 

 Change

NAAQS National Ambient Air Quality 

 Standards

NAS National Academy of Sciences

NATA National-Scale Air Toxic Assessment

NATTS National Air Toxics Trends Stations

NEI National Emissions Inventory

NH3 ammonia

NOx oxides of nitrogen

NO2 nitrogen dioxide

O3 ground-level ozone

PAHs polycyclic aromatic hydrocarbons

PFCs perfluorinated compounds

PM particulate matter (particle pollution)

PM2.5 particulate matter (fine) 2.5 μm or less 

 in size

PM10 particulate matter 10 μm or less in size

ppb parts per billion

ppm parts per million

SF6 sulfur hexafluoride

SIP state implementation plan

SO2 sulfur dioxide

μm micrometers (microns)

μg/m3 micrograms per cubic meter

VOCs volatile organic compounds

Appendix
2 6 Our Nation’s Air

Websites

Background/General Information

Air Quality Index: http://www.airnow.gov

Air Quality System: http://www.epa.gov/ttn/airs/airsaqs/

Air Quality System Detailed Data: http://www.epa.gov/ttn/airs/airsaqs/detaildata

Health and Ecological Effects: http://www.epa.gov/air/urbanair/

National Ambient Air Quality Standards: http://www.epa.gov/air/criteria.html

National Center for Environmental Assessment: http://cfpub.epa.gov/ncea/

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 Programs: http://www.epa.gov/air/oap.html 

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

Climate Change

Climate change: http://www.epa.gov/climatechange/

U.S. Climate Change Science Program: http://www.climatescience.gov

Emissions and trends in greenhouse gases: 

http://www.epa.gov/climatechange/emissions/usinventoryreport.html

Intergovernmental Panel on Climate Change: http://www.ipcc.ch

Emissions and Control Programs

Emissions: http://www.epa.gov/air/emissions/

NOx

 Budget Trading Program/NOx SIP Call: http://www.epa.gov/airmarkets/progsregs/nox/sip.html

Toxic Air Pollutants

2002 National-Scale Air Toxics Assessment: http://www.epa.gov/ttn/atw/nata2002/

Measurements and Trends

Air Quality Trends: http://www.epa.gov/airtrends/

Air Trends Design Values: http://www.epa.gov/air/airtrends/values.html

Clean Air Status and Trends Network: http://www.epa.gov/castnet/

EPA Monitoring Network: http://www.epa.gov/ttn/amtic/

Local air quality trends: http://www.epa.gov/airtrends/where.html

National Core Monitoring Network: http://www.epa.gov/ttn/amtic/ncore/index.html

Trends in ozone adjusted for weather conditions: http://www.epa.gov/airtrends/weather.html

Appendix
Our Nation’s Air 2 7

References

Highlights

U.S. Census Bureau, Population Division, Annual Estimates of the Resident Population by Selected Age Groups 

and Sex for Counties: April 1, 2000 to July 1, 2008, available on the Internet at http://www.census.gov/popest/

counties/asrh/CC-EST2008-agesex.html.

Climate Change and Air Quality

Cooper, O. R. et al., 2010. Nature, 463, 344-348.

Forster, P., V. Ramaswamy, P. Artaxo, T. Berntsen, R. Betts, D.W. Fahey, J. Haywood, J. Lean, D.C. Lowe, 

G. Myhre, J. Nganga, R. Prinn, G. Raga, M. Schulz and R. Van Dorland, 2007: Changes in Atmospheric 

Constituents and in Radiative Forcing. In: Climate Change 2007: The Physical Science Basis. Contribution of 

Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change 

[Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. 

Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA

National Academy of Sciences, National Research Council, 2005. Radiative Forcing of Climate Change: 

Expanding the Concept and Addressing Uncertainties, National Academies Press, Washington, D.C., October.

U.S. Environmental Protection Agency, 2009. Assessment of the Impacts of Global Change on Regional U.S. 

Air Quality: A Synthesis of Climate Change Impacts on Ground-Level Ozone, EPA 600-R-07-094F, Office of 

Research and Development, National Center for Environmental Assessment, Research Triangle Park, NC.

U.S. Environmental Protection Agency, 2010. Climate Change Indicators in the United States, EPA 430-R-10-007, 

Washington, D.C., 74pp.

U.S. Environmental Protection Agency, 2011. Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-

2009, EPA 430-R-11-005, Washington, D.C.
Protection Agency

Environmental United States Office of Air Quality Planning and Standards EPA Publication No. EPA-454/R-11-001

Research Triangle Park, NCAir Quality Assessment Division