National Air Quality 

STATUS AND TRENDS THROUGH 2007
 
Printed on 100% recycled/recyclable process chlorine-free paper with 100% post-consumer fiber using vegetable-oil-based ink. 
National Air Quality
 STATUS AND TRENDS THROUGH 2007
 

U.S. Environmental Protection Agency
	

Office of Air Quality Planning and Standards
	

Air Quality Assessment Division
	

Research Triangle Park, North Carolina
	

EPA-454/R-08-006
	

November 2008
	
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table of Contents
 

Highlights ... 1
	

Air Pollution ... 4
	

Six Common Pollutants ... 10

 Ground-Level Ozone ... 14

 Particle Pollution ... 19

 Lead ... 26

 Nitrogen Dioxide ... 27

 Carbon Monoxide ... 27

 Sulfur Dioxide ... 27
	

Toxic Air Pollutants ... 28
	

Atmospheric Deposition ... 32
	

Visibility in Scenic Areas ... 34
	

Climate Change and Air Quality ... 36
	

International Transport of Air Pollution ... 38
	

Terminology ... 40
	

Web Sites ... 41
	
 

HIGHLIGHTS 

This summary report highlights EPA’s most recent 

evaluation of the status and trends in our nation’s air 

quality. 

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 for six air 

pollutants for which there are national standards 

has improved significantly. These air pollutants are 

ground-level ozone (O3), particle pollution (PM2.5 

and PM10), lead (Pb), nitrogen dioxide (NO2

), carbon 

monoxide (CO), and sulfur dioxide (SO2). Nationally, 

air pollution was lower in 2007 than 1990 for: 

– 8-hour ozone, by 9 percent 

– annual PM2.5 (since 2000), by 11 percent 

– PM10, by 28 percent 

– Lead, by 80 percent 

– NO2 , by 35 percent 

– 8-hour CO, by 67 percent 

– SO2, by 54 percent 

• Despite clean air progress, in 2007, 158.5 million 

people lived in counties that exceeded any national 

ambient air quality standard (NAAQS). Groundlevel ozone and particle pollution still present 

challenges in many areas of the country. 

• Though PM2.5 concentrations were higher in 2007 

than in 2006, partly due to weather conditions, 

annual PM2.5 concentrations were nine percent 

lower in 2007 than in 2001.
 

• 8-hour ozone concentrations were fi ve percent 

lower in 2007 than in 2001. Ozone levels did not 

improve in much of the East until 2002, after 

which there was a significant decline. This decline 

is largely due to reductions in oxides of nitrogen 

(NOx

) emissions required by EPA’s rule to reduce 

ozone in the East, the NOx SIP Call. EPA tracks 

progress toward meeting these reductions through 

its NOx Budget Trading Program. 

LEVELS OF MANY TOXIC AIR POLLUTANTS 

HAVE DECLINED 

• In 2007, 27 National Air Toxics Trends Stations 

(NATTS) were fully operational, providing a 

consistent long-term national network operated by 

state and local agencies with coordination provided 

by EPA. 

Number of people living in counties with air quality concentrations above the level of the primary (health-based) National 

Ambient Air Quality Standards (NAAQS) in 2007. 

Note: In 2008, EPA strengthened the national standard for 8-hour ozone to 0.075 ppm and the national standard for lead to 0.15 μg/m3. This figure 

includes people living in counties that monitored ozone and lead concentrations above the new levels. PM2.5 are particles less than or equal to 

2.5 micrometers (μm) in diameter. PM10 are particles less than or equal to 10 μm in diameter. 

NATIONAL AIR QUALITY STATUS AND TRENDS 1 
 

HIGHLIGHTS 

Population density (2005 population per square mile) in counties with air quality concentrations above the 

level of any of the primary NAAQS in 2007. 

Note: This figure includes counties that monitored ozone and lead concentrations above the new levels set in 2008. 

• Toxic hydrocarbons such as benzene, 1,3-butadiene, 

styrene, xylenes, and toluene decreased by 

5 percent or more per year between 2000 and 

2005 at more than half of ambient monitoring 

sites. Other key contributors to cancer risk, such 

as carbon tetrachloride, tetrachloroethylene, and 

1,4-dichlorobenzene, declined at most sites. 

• Control programs for mobile sources and facilities 

such as chemical plants, dry cleaners, coke ovens, 

and incinerators are primarily responsible for 

reductions in toxic air pollutant emissions between 

2000 and 2005. These emissions reductions have 

contributed to reductions in cancer risk as well as 

reductions in the hydrocarbon contribution to ozone 

concentrations. 

ACID RAIN AND HAZE ARE DECLINING 

• EPA’s NO x SIP Call and Acid Rain Program have 

contributed to significant improvements in air 

quality and environmental health. The required 

reductions in sulfur dioxide and nitrogen oxides 

have led to significant decreases in atmospheric 

deposition, contributing to improved water quality 

in lakes and streams. For example, between the 

1989-1991 and 2005-2007 time periods, wet sulfate 

deposition and wet nitrate deposition decreased 

more than 30 percent in parts of the East. 

• Between 1996 and 2006, visibility in scenic areas 

has improved throughout the country. Five 

areas—Mt. Rainier National Park, Wash.; Great 

Smoky Mountains National Park, Tenn.; Great Gulf 

Wilderness, N.H.; Canyonlands National Park, 

Utah; and Snoqualmie Pass, Wash.—show notable 

improvement on days with the worst visibility. 

CLIMATE CHANGE AND INTERNATIONAL 

TRANSPORT: IMPROVING OUR 

UNDERSTANDING 

• The U.N. Intergovernmental Panel on Climate 

Change concluded climate change is evident from 

observations of increases in global average air and 

ocean temperatures, widespread melting of snow 

and ice, and rising global average sea level. 

• Research is under way to examine and improve our 

understanding of the links between air quality and 

climate: how a warming climate could aff ect air 

quality and how air quality could aff ect climate. 

• Researchers also are improving our understanding 

about how pollution moves between countries and 

continents. 

2 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

Sources-to-Effects Continuum 

EMISSIONS MONITORING DOSAGE 

ATMOSPHERIC HEALTH EFFECTS & 

CHEMISTRY/ POPULATION ENVIRONMENTAL

TRANSPORT EXPOSURE IMPACTS

Because air pollution harms human health and damages the environment, EPA tracks pollutant emissions. Air pollutants are 

emitted from a variety of sources including stationary fuel combustion, industrial processes, highway vehicles, and non-road 

sources. These pollutants react in and are transported through the atmosphere. EPA, other federal agencies, and state, local, 

and tribal agencies monitor air quality at locations throughout the U.S. Data collected through ambient monitoring are used 

in models to estimate population exposure. Personal health monitoring is conducted via special studies to better understand 

actual dosage of pollutants. EPA uses monitoring data, population exposure estimates, and personal dosage data to better 

understand health effects and environmental impacts of air pollutants. 

MORE IMPROVEMENTS ANTICIPATED 

• EPA expects air quality to continue to improve 

as recent regulations are fully implemented and 

states work to meet national standards. Among 

these regulations are: the Locomotive Engines and 

Marine Compression - Ignition Engines Rule, the 

Tier II Vehicle and Gasoline Sulfur Rule, the HeavyDuty Highway Diesel Rule, the Clean Air Non-road 

Diesel Rule, and the Mobile Source Air Toxics Rule. 

NATIONAL AIR QUALITY STATUS AND TRENDS 3 
 

 

AIR POLLUTION 

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 (as indicated 

by increased emergency department visits and 

hospital admissions); (2) decreased lung function 

and increased frequency and severity of respiratory 

symptoms such as difficulty breathing and coughing; 

(3) increased susceptibility to respiratory infections; 

(4) effects on the nervous system, including the brain, 

such as IQ loss and impacts on learning, memory, and 

behavior; (5) cancer; and (6) 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., asthma, 

emphysema, and chronic bronchitis), diabetics, older 

adults, and children. In 2007, 158.5 million people lived 

in counties that exceeded national air quality standards. 

Air pollution also damages our environment. Ozone 

can damage vegetation including adversely impacting 

the growth of trees and reducing crop yields. Visibility 

is reduced by particle pollutants that scatter and absorb 

light. Typical visual range in the eastern U.S. is 15 to 

30 miles, approximately one-third of what it would 

be without man-made air pollution. In the West, the 

typical visual range is about 60 to 90 miles, or about 

one-half of the visual range under natural conditions. 

Pollution in the form of acids and acid-forming 

compounds (such as sulfur dioxide [SO2

] and oxides of 

nitrogen [NOx]) can deposit from the atmosphere to the 

Earth’s surface. This is called acid deposition and can 

be either dry or wet. Wet deposition is more commonly 

known as acid rain. Acid rain can occur anywhere 

and, in some areas, rain can be 100 times more acidic 

than natural precipitation. Acid deposition can be a 

very serious regional problem, particularly in areas 

downwind from high SO2

 and NOx emitt ing sources, 

e.g., coal burning power plants, smelters, and factories. 

Acid deposition can have many harmful ecological 

effects in both land and water systems. While acid 

deposition can damage tree foliage directly, it more 

commonly stresses trees by changing the chemical 

and physical characteristics of the soil. In lakes, acid 

deposition can kill fish and other aquatic life. 

The burning of fossil fuels, such as coal and oil, and 

deforestation can cause concentrations of heat-trapping 

“greenhouse gases” to increase significantly in our 

atmosphere. These gases prevent heat from escaping to 

space, somewhat like the glass panels of a greenhouse. 

Greenhouse gases are necessary to life as we know it, 

because they keep the planet’s surface warmer than 

it would otherwise be. But, as the concentrations of 

these gases continue to increase in the atmosphere, 

the Earth’s temperature is climbing above past levels. 

Studies show that growth in greenhouse gases and 

associated changes in weather conditions may increase 

current air pollution levels. 

Air Pollution and Health/Welfare Effects – Improving Our Knowledge 

Air pollution continues to have adverse impacts on the human and environmental health of the United States, despite clear 

evidence that overall air quality has improved. EPA’s research program is evolving with growing emphasis on the development 

of a multi-pollutant approach for assessing the impacts of air pollution. Critical components of this research will inform 

our understanding of how pollutants from sources impact ambient air concentrations, how these concentrations relate to 

exposures, and, in turn, how exposures relate to health and welfare outcomes. Some highlights of current air pollution research 

activities include: 

• EPA-funded Particulate Matter Research Centers are conducting cutting-edge research to improve our understanding 

of how particle pollution affects human health and the sources of particles most responsible for these effects. Research 

grants focus high-priority issues including human susceptibility, mechanisms of health effects, exposure-response 

relationships, and the cross-cutting issue of linking health effects with particle pollution sources and components. 

• The Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air) is investigating the impact of air pollution on 

the progression of cardiovascular disease among more than 7,000 participants with diverse backgrounds from nine 

locations. The study will help evaluate if cardiac disease is accelerated by exposure to particle pollution in combination 

with gaseous pollutants and if some ethnic populations are more susceptible to effects associated with these exposures. 

• The Health Effects Institute’s National Particle Components Toxicity (NPACT) Initiative will build upon the existing scientific 

foundation for particles to improve our understanding of the toxicity of specific components and characteristics of 

particle pollution (and ultimately sources of these components). 

4 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

Air Pollution Sources, Health Effects, and Environmental Effects 

Pollutant Sources Health Effects Environmental Effects 

Ozone (O3) 

Secondary pollutant formed by 

chemical reaction of VOCs and 

NO x in the presence of sunlight. 

Aggravation of respiratory and 

cardiovascular disease, decreased 

lung function and increased 

respiratory symptoms, increased 

susceptibility to respiratory infection, 

and premature death. 

Damage to vegetation such as 

impacts on tree growth and reduced 

crop yields. 

Particles 

Emitted or formed through 

chemical reactions (e.g., NOx , 

SO2 , NH3); fuel combustion (e.g., 

burning coal, wood, diesel); 

industrial processes; agriculture 

(plowing, field burning); and 

unpaved roads. 

Aggravation of respiratory and 

cardiovascular disease, reduced 

lung function, increased respiratory 

symptoms, and premature death. 

Impairment of visibility, effects 

on climate, and damage and/ 

or discoloration of structures and 

property. 

Lead 

Smelters (metal refineries) 

and other metal industries; 

combustion of leaded gasoline 

in piston engine aircraft; waste 

incinerators; and battery 

manufacturing. 

Damage to developing nervous 

system, resulting in IQ loss and 

impacts on learning, memory, and 

behavior in children. Cardiovascular 

and kidney effects in adults and early 

effects related to anemia. 

Harm to environment and wildlife. 

Sulfur Dioxide 

(SO2

) 

Fuel combustion (especially highsulfur coal); electric utilities and 

industrial processes; and natural 

sources such as volcanoes. 

Aggravation of asthma and 

increased respiratory symptoms. 

Contributes to particle formation with 

associated health effects. 

Contributes to the acidifi cation of 

soil and surface water and mercury 

methylation in wetland areas. 

Contributes to particle formation with 

associated environmental effects. 

Oxides of 

Nitrogen 

(NOx

) 

Fuel combustion (e.g., electric 

utilities, industrial boilers, and 

vehicles) and wood burning. 

Aggravation of respiratory disease 

and increased susceptibility to 

respiratory infections. Contributes to 

ozone and particle formation with 

associated health effects. 

Contributes to the acidifi cation and 

nutrient enrichment (eutrophication, 

nitrogen saturation) of soil and surface 

water. Contributes to ozone and 

particle formation with associated 

environmental effects. 

Carbon 

Monoxide 

(CO) 

Fuel combustion (especially 

vehicles). 

Reduces the ability of blood to carry 

oxygen to body tissues including 

vital organs. Aggravation of 

cardiovascular disease. 

None known. 

Ammonia 

(NH3

) 

Livestock agriculture (i.e., raising/ 

maintaining livestock for milk, 

meat, and egg production); 

fertilizer application. 

Contributes to particle formation with 

associated health effects. 

Contributes to eutrophication 

of surface water and nitrate 

contamination of ground water. 

Contributes to particle formation with 

associated environmental effects. 

Volatile 

Organic 

Compounds 

(VOCs) 

Fuel combustion and 

evaporation (especially vehicles); 

solvents; paint; and natural 

sources such as trees and 

vegetation. 

Cancer (from some toxic air 

pollutants) and other serious health 

problems. Contributes to ozone 

formation with associated health 

effects. 

Contributes to ozone formation with 

associated environmental effects. 

Mercury 

Fuel combustion (especially 

coal-fired power plants); waste 

disposal; industrial processes; 

mining; and natural sources 

(volcanoes and evaporation 

from enriched soil, wetlands, and 

oceans). 

Liver, kidney, and brain damage; and 

neurological and developmental 

damage. 

Deposition into rivers, lakes, and 

oceans accumulates in fish resulting in 

exposure to humans and wildlife. 

Other Toxic 

Air Pollutants 

Fuel combustion (including 

particle and gaseous emissions); 

vehicles; industrial processes; 

building materials; and solvents. 

Cancer, immune system damage, 

neurological, reproductive, 

developmental, respiratory, and 

other health problems. Some toxic air 

pollutants contribute to ozone and 

particle pollution with associated 

health effects. 

Harmful to wildlife and livestock. 

Some toxic air pollutants accumulate 

in the food chain. Some toxic air 

pollutants contribute to ozone and 

particle pollution with associated 

environmental effects. 

NATIONAL AIR QUALITY STATUS AND TRENDS 5 
 

AIR POLLUTION 

SOURCES OF AIR POLLUTION 

Air pollution consists of gas and particle contaminants 

(PM2.5 and PM10) that are present in the atmosphere. 

Gaseous pollutants include SO2 , NOx , ozone (O3), 

carbon monoxide (CO), volatile organic compounds 

(VOCs), certain toxic air pollutants, and some gaseous 

forms of metals. Particle pollution includes a mixture 

of compounds. The majority of these compounds can 

be grouped into four categories: sulfates, nitrates, 

elemental carbon, organic carbon, and “crustal”

material. 

Some pollutants are compounds that are released 

directly into the atmosphere. These include gases 

such as SO2

 and some particles, such as soil and soot. 

Other pollutants are formed in the air. Ground-level 

ozone forms when emissions of NO and VOCs react x

in the presence of sunlight. Similarly, some particles 

are formed. For example, particle sulfates are the 

product of SO2

 and ammonia (NH3) gases reacting in 

the atmosphere. Weather plays an important role in 

the formation of air pollutants, as discussed later in the 

ozone and particle pollution sections. 

EPA tracks direct emissions of air pollutants and 

emissions that contribute to air pollution formation, 

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. 

Emissions, in general, are emitted from large stationary 

fuel combustion sources (such as electric utilities and 

industrial boilers), industrial and other processes (such 

as metal smelters, petroleum refi neries, manufacturing 

facilities, and solvent utilization), and mobile sources 

including highway vehicles and non-road sources 

(such as mobile equipment, marine vessels, aircraft, 

and locomotives). Sources emit diff erent combinations 

of pollutants. For example, electric utilities release SO2 , 

NOx , and particles. Figure 1 shows the distribution of 

national total emissions estimates by source category 

for specific pollutants for 2007. Highway vehicles 

and non-road mobile sources together contribute 

approximately three-fourths of national CO emissions. 

Electric utilities contribute about 70 percent of 

national SO2

 emissions. Agricultural operations (other 

processes) contribute nearly 80 percent of national 

NH3

 emissions. Almost 50 percent of the national VOC 

emissions are coming from highway vehicles and 

solvent use (other processes). Pollutant levels differ 

across regions of the country and within local areas, 

both urban and rural, depending on the size and type 

of sources present. 

The Clean Air Act and EPA have established a 

list of 187 air toxics (also known as hazardous air 

pollutants—HAPs). These pollutants are known 

or are suspected of causing serious health effects, 

such as cancer, birth defects, or reproductive effects. 

Many of the VOCs (e.g., benzene, 1,3-butadiene, and 

chloroform) and particles (e.g., arsenic, lead, and 

manganese) are toxic air pollutants, as shown in Figure 

2. 

A number of sources (e.g., stationary fuel combustion, 

industrial processes, mobile sources) emit both particle 

and gaseous toxic air pollutants that contribute to 

both ozone and particle formation. For example, diesel 

exhaust contains particles as well as VOCs, some of 

which are toxic. 

Figure 1. Distribution of national total emissions by source category for specific pollutants, 2007. 

6 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

Most toxic air pollutants come from a variety of 

source types. For example, though most benzene 

emissions are from highway vehicles, benzene is 

also emitted by some stationary fuel combustion, 

industrial, and non-road mobile sources. 

Control programs that target specifi c source 

types can provide multiple benefi ts. For 

example, lowering VOC emissions from vehicle 

sources reduces toxic air pollutant levels and 

also reduces VOCs that contribute to ozone 

formation. Lowering NOx

 emissions from 

electric utilities and industrial boilers reduces 

the NO contribution to both ozone and nitrate x

particle formation, both of which contribute to 

smog and reduced visibility. 

Energy production and transportation sources 

contribute to CO2

, VOC, SO2 , and NOx emissions 

which affect greenhouse gases and the formation 

of ozone and particle pollution. Reducing energy 

consumption and vehicle use, or converting 

to alternative or more effi cient energy sources 

will improve health protection and reduce 

environmental effects. 

Figure 2. Distribution of national total emissions by 

source category for individual urban toxic air pollutants 

and diesel particle pollution, 2005. 

Note: Contributions of aldehyde emissions (formaldehyde 

and acetaldehyde) are for primary direct emissions and do not 

include secondary aldehydes formed via photochemical reactions. 

Contributions from fires are not included. In 2005, fi res contributed 

roughly 35 percent of the polycyclic organic matter, 15 percent 

of the benzene, 37 percent of the 1,3-butadiene, 50 percent of the 

formaldehyde, 67 percent of the acrolein, and 24 percent of the 

acetaldehyde. 

Emissions Included in this Report 

• PM emissions are directly emitted particles only. They do not 

include gaseous emissions that condense in cooler air (i.e., 

condensibles) or emissions from fires and resuspended dust. 

• SO2 , NOx , VOC, CO, and lead emissions are from human 

activity sources only. 

• NH3

 emissions are primarily from animal livestock operations 

and are estimated using population data (e.g., cattle, cows, 

pigs, poultry) and management practices. 

• 2007 emissions were derived from the 2005 emissions 

inventory, except for SO2

 and NO x emissions, which were 

derived from measured data from electric utilities. 

• Highway vehicle emissions were based on emission 

measurements from vehicle testing programs. 

• Emissions data were compiled using the best methods and 

measurements available at the time. 

NATIONAL AIR QUALITY STATUS AND TRENDS 7 
 

AIR POLLUTION 

TRACKING POLLUTANT 

EMISSIONS Table 1. Change in annual national emissions 

per source category (1990 vs. 2007) (thousand tons). Since 1990, air pollutant 

emissions have declined, with the 

greatest percentage drop in lead 

emissions. The removal of lead 

from gasoline used in highway 

vehicles is primarily responsible 

for the 72 percent decrease in lead 

emissions. NH3

 shows the least 

percentage drop, four percent. 

While PM2.5 emissions have declined 

by over one half, PM10 , NOx , and 

VOC emissions have declined by 

around one third, and SO2 and CO 

emissions have declined by more Note: Lead (Pb) emission changes are from 1990 to 2002. 

than one-third, as shown in Table 1. 

Source Category PM2.5 PM10 NH3 SO2 NO x VOC CO Pb 

Stationary Fuel 

Combustion -693 -722 +40 -9036 -4894 +621 -207 -0.410 

Industrial and 

Other Processes -224 -43 -353 -844 +229 -2809 +8442 -2.621 

Highway Vehicles -223 -235 +152 -412 -4029 -5786 -68645 -0.421 

Non-road Mobile -49 -62 -28 -25 +383 -12 -2685 -0.153 

Total Change 

(thousand tons) -1189 -1062 -189 -10267 -8311 -7986 -63095 -3.604 

Percent Change 

(1990 vs. 2007) -51% -33% -4% -45% -33% -35% -44% -72% 

The combined emissions of the six common pollutants (PM2.5 , SO2 , NOx , VOCs, CO, and lead) dropped 41 percent 

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 resulted from a variety of 

control programs through regulations and through voluntary partnerships between federal, state, local, and tribal 

governments; academia; industrial groups; and environmental organizations. 

Figure 3. Comparison of growth measures and emissions, 1990-2007. 

Note: The U.S. Department of Transportation’s Federal Highway Administration reports that cumulative travel for January-April 

2008 is down by 2.1 percent compared to the same period in 2007. 

8 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

Emissions Where You Live 

To get emissions information at a state or local level, visit http://www.epa.gov/air/emissions/where.htm. Here you can find 

emissions totals for a state or county grouped by major source types, or select Google Earth to see nearby sources of emissions. 

Zoom to the area of interest, tilt the map to see emissions levels, select a site for facility information, or zoom closer for an aerial 

photo. 

NATIONAL AIR QUALITY STATUS AND TRENDS 9 
 

SIX COMMON 

POLLUTANTS 

To protect public health and the environment, EPA has 

established, and regularly reviews, national air quality 

standards for six common air pollutants also known 

as “criteria” pollutants: ground-level ozone, particle 

pollution (PM2.5 and PM10), lead, nitrogen dioxide 

(NO2), carbon monoxide (CO), and sulfur dioxide (SO2). 

TRENDS IN NATIONAL AIR QUALITY 

CONCENTRATIONS 

Air quality is measured by monitors located across 

the country. Monitored levels of the six common 

pollutants show improvement since the Clean Air Act 

was amended in 1990. Figure 4 shows national trends 

between 1990 and 2007 in the common pollutants 

relative to their air quality standards. Most pollutants 

show a steady decline throughout the time period. 

Lead declined in the 1990s as control programs were 

implemented to lower concentrations in areas above 

the national standard. In general, lead concentrations 

have remained low since 2002. Large year-to-year 

changes shown in lead concentrations refl ect the 

influence of emissions changes due to operating 

schedules or other facility activities, such as plant 

closings, on measurements at nearby monitors. Ozone 

and PM2.5 trends are not smooth and show year-to-year 

influences of weather conditions which contribute to 

their formation, dispersion, and removal from the air. 

Ozone was generally level in the 1990s, and showed 

a notable decline after 2002 mostly due to oxides of 

nitrogen (NOx

) emission reductions in the East. 

Many areas still have air quality problems caused by 

one or more pollutant. Ozone and particle pollution 

continue to present air quality challenges throughout 

much of the U.S., with many monitors measuring 

concentrations above, or close to, national air quality 

standards. 

Figure 4. Comparison of national levels of the six common pollutants to national ambient air quality standards, 

1990-2007. National levels are averages across all sites with complete data for the time period. 

Note: Air quality data for PM2.5 start in 1999. Trends from 2001 though 2007 (using the larger number of monitors operating since 

2001) are the focus of graphics in the following sections. 

Environmental Justice 

Environmental justice is the fair treatment and meaningful involvement of all people regardless 

of race, color, national origin, or income with respect to the development, implementation, 

and enforcement of environmental laws. EPA’s Office of Air and Radiation (OAR) is committed 

to promoting and supporting environmental justice. For more information about EPA OAR’s 

environmental justice program and air issues, visit http://www.epa.gov/air/ej/. 

10 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

TRENDS IN “UNHEALTHY” AIR QUALITY DAYS 

The Air Quality Index (AQI) relates daily air pollution 

concentrations for ozone, particles, NO2

 , CO, and SO2 

to health concerns for the general public. 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. 

Figure 5 shows the number of unhealthy days that 

selected metropolitan areas experienced in 2001-

2007. Most areas had fewer unhealthy days in 2007 

compared to 2001 or 2002. However, Los Angeles, 

Salt Lake City, and many cities in the east had more 

unhealthy days in 2007 than in 2006. Nearly all of 

EPA’s Air Quality Index (AQI) 

Air Quality Index

(AQI) Values 

Levels of Health Concern 

0 to 50 Good 

51-100 Moderate 

101-150 Unhealthy for Sensitive Groups 

151-200 Unhealthy 

201-300 Very Unhealthy 

301 to 500 Hazardous 

the increases in unhealthy days in the east are due to 

ozone and/or particle pollution. Weather conditions, 

as well as emissions, contribute to ozone and particle 

pollution formation. 

Figure 5. Number of days reaching Unhealthy for Sensitive Groups on the Air Quality Index for 2001-2007 at 

selected cities. 

Note: The AQI breakpoints reflect the new primary standard for 8-hour ozone set in 2008. 

NATIONAL AIR QUALITY STATUS AND TRENDS 11 
 

SIX COMMON POLLUTANTS 

Review of the National Ambient Air Quality Standards (NAAQS) 

The Clean Air Act requires EPA to set two types of NAAQS for the common 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 standards 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 “standards” and the science upon which they are based. The current 

standards and the status of each review are shown below. 

Pollutant Primary Standard(s) Secondary Standard(s) Status of Review 

Ozone 0.075 ppm (8-hour) Same as Primary Review completed 2008; the previous 0.08 ppm 

standard remains in effect 

Lead 0.15 μg/m3

 (3-month) Same as Primary Review completed 2008; the previous 1.5 μg/m3 

standard remains in effect 

NO2 0.053 ppm (annual) Same as Primary 

Primary standard review to be completed 2009; 

secondary standard review of SO2 and NO2 to be 

completed 2010 

SO2 

0.03 ppm (annual) 

0.14 ppm (24-hour) 

0.5 ppm (3-hour) 

Primary standard review to be completed 2010; 

secondary standard review of SO2

 and NO2 to be 

completed 2010 

PM2.5 

15 μg/m3 (annual) 

35 μg/m3 (24-hour) 

Same as Primary 

To be completed 2011 

PM10 150 μg/m3

 (24-hour) Same as Primary 

CO 

9 ppm (8-hour) 

35 ppm (1-hour) 

None; no evidence of 

adverse welfare effects at 

current ambient levels 

To be completed 2011 

Units of measure are parts per million (ppm) or micrograms per cubic meter of air (μg/m3

). For more information about 

the standards, visit http://www.epa.gov/air/criteria.html. 

The Air Quality Management Process 

Each time EPA establishes air quality goals necessary 

to protect public health and the environment, it sets in 

motion a chain of events. States and local agencies work 

with EPA to: 

• identify emissions reductions needed to achieve air

quality goals

 • develop emissions reduction programs 

• implement emissions reduction strategies and

enforcement activities

 • track and evaluate progress 

12 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

AIR QUALITY IN NONATTAINMENT AREAS 

Many areas of the country where air pollution levels 

have 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 plans to address air pollution 

in these areas. Each year, EPA tracks air quality 

progress in areas identified as nonatt ainment by 

reviewing changes in measured concentrations with 

respect to the standards. Figure 6 shows which of these 

areas are above or below one or more of the standards 

as of 2007. 

Air quality has improved in the areas that were 

designated nonattainment across all six common 

pollutants. All of the areas designated as 

nonattainment for CO, SO2

 , and NO2 had air quality 

levels below their respective standards as of December 

2007. Only one of the nonattainment areas was above 

the standard for lead (1.5 μg/m3

)—Herculaneum, 

Mo. For ozone, annual PM2.5 , and PM10 , a number of 

areas were above the standards: 51, 32, and 17 areas, 

respectively. Even though many areas were still above 

the standard in 2007, there have been improvements 

in the concentration levels in the nonatt ainment areas. 

For example, the ozone areas showed a 9 percent 

improvement, and the annual PM2.5 areas showed 

a 6 percent improvement between the time of 

designation and 2007. 

Common 

Pollutants 

Number of 

Nonattainment 

Areas 

Year 

Designated 

Nonattainment 

Areas Above 

NAAQS Level 

in 2007 

Ozone 126 2004 51 

PM2.5 39 2005 32 

PM10 87 1990 17 

Lead 13 1991 1 

CO 43 1991 0 

SO2 54 1990 0 

NO2 1 1990 0 

Figure 6. Status of original nonattainment areas for one or more standards (i.e., 8-hour ozone, maximum quarterly lead, 

annual PM2.5 , 24-hour PM10 , annual NO2 , 8-hour CO, and annual SO2

) as of 2007. 

Notes: Designations for the recently revised standards for ozone (2008), lead (2008), and 24-hour PM2.5 (2006) are to be determined. 

Depending on the form of the standard, a single year or an average of multiple years of data is compared with the level of the standard. 

For information about air quality standards, visit http://www.epa.gov/air criteria.html. For information about air trends design values, 

visit htt p://www.epa.gov/air/airtrends/values.html. 

NATIONAL AIR QUALITY STATUS AND TRENDS 13 
 

 

GROUND-LEVEL 

OZONE 

TRENDS IN OZONE 

CONCENTRATIONS 

Nationally, ozone concentrations were 

5 percent lower in 2007 than in 2001, as 

shown in Figure 7. The trend showed 

a notable decline after 2002. Though 

concentrations in 2007 were among the 

lowest since 2002, many areas measured 

concentrations above the 2008 national 

Figure 7. National 8-hour ozone air 

quality trend, 2001-2007 (average of 

annual fourth highest daily maximum 

8-hour concentrations). 

air quality standard for ozone 

(0.075 ppm). When comparing 

two 3-year periods, 2001-2003 and 

2005-2007, 97 percent of the sites 

show a decline or little change in 

ozone concentrations as shown in 

Figure 8. The sites that showed the 

greatest improvement were in or 

near the following metropolitan 

areas: Cleveland, Ohio; parts of 

Houston, Texas; Fresno, Calif.; 

and Chambersburg, Pa. However, 

other parts of Houston also 

showed a notable increase. 

Thirty sites showed an increase 

of greater than 0.005 ppm. Of the 

30 sites that showed an increase, 

12 had air quality concentrations 

below the level of the 2008 ozone 

standard for the most recent year 

of data, 2007. The remaining 

Figure 8. Change in ozone concentrations in ppm, 2001-2003 vs. 2005-2007 (3-year 18 sites with concentrations 

average of annual fourth highest daily maximum 8-hour concentrations).
 

EPA Strengthens Ground-level Ozone Standards 

On March 12, 2008, EPA strengthened the primary and secondary National Ambient Air Quality Standards 

for 8-hour ozone to 0.075 ppm. The new standards are tighter than the previous level of 0.08 ppm (effectively 

0.084 ppm). The new standards will improve both public health protection and the protection of sensitive trees 

and plants. Improved health protection includes preventing cases of reduced lung function and respiratory 

symptoms, acute bronchitis, aggravated asthma, doctor visits, emergency department visits and hospital 

admissions for individuals with respiratory disease, and premature death in people with heart and lung disease. 

The Air Quality Index (AQI) breakpoints were changed to re ect the new primary standard. The new 100 AQI level 

for 8-hour ozone is 0.075 ppm. Information on the AQI can be found at http://www.airnow.gov. 

14 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

 

 

 

above the new ozone standard 

in 2007 were located in or near 

the following metropolitan areas: 

Birmingham, Ala.; El Centro, 

Calif.; Los Angeles, Calif.; 

Jacksonville, Fla.; Orlando, Fla.; 

Columbus, Ga.; Atlanta, Ga.; 

Baton Rouge, La.; New York, 

N.Y.; and Houston, Texas. Ozone 

trends can vary locally, as shown 

by the presence of increases and 

decreases at nearby sites. 

Figure 9 shows a snapshot 

of ozone concentrations in 

2007. The highest ozone 

concentrations were located in 

California, Connecticut, Georgia, 

Massachusetts, North Carolina, 

and Pennsylvania. Fift y-seven 

percent of the sites were above 

0.075 ppm, the level of the 2008 

standard. Figure 9. Ozone concentrations in ppm, 2007 (fourth highest daily maximum 

8-hour concentration). 

EPA Reviews Ozone Monitoring Requirements 

EPA is currently reviewing the requirements for ozone monitoring by state and local air agencies. At present, there are about 

1200 ozone monitors in operation, mostly in cities with population over 350,000. EPA is reviewing the following aspects of the 

ozone monitoring program: 

• The number of monitors required in smaller cities. 

• The number and location of monitors required in rural areas, especially near parks and protected areas. 

• The number of months of the year when ozone data must be collected and recorded. 

Required Ozone Monitoring Time Periods 

High concentrations of ozone 

typically occur during months 

with warm temperatures and 

strong sunlight. Therefore, 

year-round monitoring has 

not been required except in 

certain areas (see map). Some 

states monitor in additional 

months on a voluntary basis. 

EPA is considering extending 

the currently required 

monitoring seasons in light of 

the new ozone standard level 

of 0.075 ppm. Data collected 

during additional months may 

be necessary to alert the public 

of all unhealthy days and 

correctly identify nonattainment 

areas. For example, 26 of 

35 states that are not required 

to monitor ozone in March do 

so voluntarily, and in recent 

years they have measured 

ozone at unhealthy levels. 

Similar unhealthy levels may 

be happening in states not 

monitoring ozone in March. 

NATIONAL AIR QUALITY STATUS AND TRENDS 15 
 

GROUND-LEVEL OZONE 

WEATHER CONDITIONS INFLUENCE OZONE 

In addition to emissions, weather also 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. To better understand how ozone is 

changing, EPA assesses both the changes in emissions 

as well as weather conditions. EPA uses a statistical 

model to calculate a weather adjustment factor that 

estimates the influence of atypical weather on ozone 

formation. The adjustment factor is derived from using 

weather variables such as temperature and humidity. 

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. 

Geographic differences in the weather adjustment 

factor for 2007 are shown in Figure 10. In 2007, weather 

contributed to higher than expected ozone formation in 

the East, as indicated by values greater than 0.005 ppm. 

Figure 10. Difference between 2007 observed and adjusted ozone concentrations (average daily maximum 8-hour ozone for MaySeptember). The map shows areas where weather contributed to higher or lower ozone concentrations than expected. Estimated 

changes for locations farther from monitoring sites (dots on map) have the largest uncertainty. 

Note: 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 41, Pages 7127-7137, 2007. 

16 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

Figure 11 shows ozone trends for 2001 through 

2007, averaged across selected sites before and after 

adjusting for weather. At the national level, observed 

ozone levels show a very small decrease of one percent 

between 2001 and 2007 compared with a larger 

decrease of eight percent after removing the influence 

of weather. By examining the data separately for 

California vs. eastern U.S., it is clear that the majority 

of the ozone improvement, after adjusting for weather, 

occurs in the East (on the order of 10 percent). 

The largest change in observed and weather adjusted 

ozone in the East occurred during the period from 

2002 through 2004, and was especially noticeable 

between 2003 and 2004. This relatively abrupt 

change in ozone levels coincides with the large 

oxides of nitrogen (NOx

) emissions reductions 

brought about from implementation of the NOx 

SIP Call rule, which began in 2003 and 2004. This 

significant improvement in ozone continues into 

2007, i.e., weather-adjusted levels in 2007 are the 

lowest over the 7-year period. 

Figure 11. Trends in average summertime daily maximum 8-hour ozone concentrations (May-September), before and after 

adjusting for weather nationally, in California and in eastern states; and the location of urban and rural monitoring sites used 

in the averages. 

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 htt p://www.epa.gov/ttn/airs/airsaqs. For more information about the Clean Air Status and Trends Network 

(CASTNET), visit htt p://www.epa.gov/castnet/. 

NATIONAL AIR QUALITY STATUS AND TRENDS 17 
 

 

 

GROUND-LEVEL OZONE 

Air Quality Where You Live 

EPA has several Web sites to help answer frequently asked questions regarding local air quality. To see air quality trends for 

an individual area, visit the AirTrends Where You Live page at http://www.epa.gov/airtrends/where.html. Local trends are 

available at individual monitoring locations for pollutants monitored there. 

To get air quality information to compare different areas of the country, visit AirCompare at 

http://www.epa.gov/aircompare. Select up to 10 counties across the country and nd out how many days in each county 

the air was unhealthy last year for a speci c health concern (e.g., asthma). Also nd out which are the worst months. The 

example below shows a comparison of seven counties near Atlanta, Ga. 

18 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

 

 

PARTICLE 

POLLUTION 

Particle pollution refers to two 

classes of particles based in part 

on long-established information 

on differences in sources, 

properties, and atmospheric 

behavior. EPA has set national 

standards to protect against 

the health and welfare effects 

associated with exposures to fine 

and coarse particles. Fine particles 

are generally referred to as those 

particles less than or equal to 

2.5 micrometers (μm) in diameter, 

. PM10 PM (particles generally 2.5

less than or equal to 10 μm in 

diameter) 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 

PM concentrations declined by 2.5

9 and 10 percent, respectively,
 

between 2001 and 2007, as shown
 

in Figure 12.
 Figure 12. National PM2.5 air quality trends, 2001-2007 (annual 

and 24-hour average). 

Note: In 2006, EPA revised the 24-hour standard from 65 to 35 μg/m3. 

The Great American Woodstove Changeout 

Residential wood burning in the U.S. emits 420,000 tons of particle 

pollution each year. EPA has partnered with the Hearth, Patio, 

and Barbecue Association, the American Lung Association and 

state, tribal, and local air quality agencies in the Great American 

Wood Stove Changeout. This partnership program provides 

homeowners with information and nancial incentives to replace 

inef cient wood stoves with cleaner-burning gas, wood pellet, 

and EPA-certi ed wood stoves. This program can effectively 

reduce both particle pollution and toxic air pollutants, and help 

bring areas into attainment with the standards for PM2.5 . 

As of October 2008, 7600 wood stoves and replaces have 

been replaced nationwide, eliminating close to 200 tons in 

annual particle pollution emissions, and achieving an estimated 

$100 million/year of health bene ts. 

For more information on woodstoves and the Great American 

Woodstove Changeout, visit http://www.epa.gov/woodstoves/. 

NATIONAL AIR QUALITY STATUS AND TRENDS 19 
 

PARTICLE POLLUTION 

For each monitoring location, the maps in Figure 13 

show whether annual and 24-hour PM2.5 increased, 

decreased, or stayed about the same since the beginning 

of the decade. When comparing two 3-year periods, 

2001-2003 and 2005-2007, almost all of the sites show a 

decline or little change in PM2.5 concentrations. Several 

sites in California showed great improvement for both 

the 24-hour and annual PM2.5 standards. One site in 

Pennsylvania also showed great improvement in the 

24-hour PM2.5 concentrations. Eighteen of the 618 sites 

showed an increase in annual PM2.5 concentrations 

greater than 1 μg/m3. These sites were located in 

Montana, Arizona, Texas, Arkansas, Louisiana, Alabama, 

South Carolina, Illinois, and Wisconsin. Of the 18 sites 

that showed an increase in annual PM2.5 concentrations, 

only two (Birmingham and Houston) were above the 

level of the annual PM2.5 standard for the most 

recent year of data (2007). Fifty-eight sites showed 

an increase in 24-hour PM2.5 concentrations greater 

than 3 μg/m3

. Of the 58 sites that showed an 

increase, 39 were below the level of the 24-hour 

PM standard for the most recent year of data and 2.5

19 were above. The 19 sites above the standard were 

located in or near the following metropolitan areas: 

Birmingham, Ala.; Nogales, Ariz.; Chico, Calif.; 

Paducah, Ky.; Cincinnati, Ohio; Kalamath Falls, 

Ore.; Pittsburgh, Pa.; Clarksville, Tenn.; Provo, Utah; 

Green Bay, Wis.; Madison, Wis.; and Milwaukee, 

Wis. Due to the influence of local sources, it is 

possible for sites in the same general area to show 

opposite trends, as in the case of the Pitt sburgh area 

for the 24-hour standard. 

Figure 13. Change in PM2.5 concentrations in μg/m3, 2001-2003 vs. 2005-2007 (3-year average 

of annual and 24-hour average concentrations). 

20 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

In 2007, the highest annual average PM2.5 

concentrations were in California, Arizona, Alabama, 

and Pennsylvania, as shown in Figure 14. The highest 

24-hour PM2.5 concentrations were in California, Idaho, 

and Utah. Even though California and Pennsylvania 

showed the greatest improvement since the start of the 

decade, they had some of the highest concentrations in 

2007. 

Some sites had high 24-hour PM2.5 concentrations 

but low annual PM2.5 concentrations, and vice versa. 

Sites that have high 24-hour concentrations but low 

or moderate annual concentrations exhibit substantial 

variability from season to season. For example, sites 

in the Northwest generally have low concentrations 

in warm 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 

PM standard than exceeded the level of the annual 2.5

PM standard, as indicated by yellow and red dots 2.5

on the maps below. About one-third of the sites that 

exceeded either standard exceeded both standards. 

Figure 14. Annual average and 24-hour (98th percentile 24-hour concentrations) PM2.5 concentrations in μg/m3, 2007. 

NATIONAL AIR QUALITY STATUS AND TRENDS 21 
 

PARTICLE POLLUTION 

WEATHER CONDITIONS INFLUENCE PM2.5 

As for ozone, in addition to emissions, weather plays 

an important role in the formation of PM2.5. Figure 15 

shows trends in PM2.5 from 2001 through 2007, before 

and after adjusting for weather. PM2.5 levels are 

monitored throughout the year, separate graphs are 

shown for the warm and cool months. Aft er adjusting 

for weather, PM2.5 concentrations have decreased by 

approximately 11 percent in both the warm and the 

cool season between 2001 and 2007. Weather influences 

during the warm season are generally larger than 

for the cool season, which is consistent with seasonal 

changes in emissions and temperature effects on the 

formation of secondary particle pollutants. 

Figure 15. Trends in annual, cool months (October–April), and warm months (May–September) average PM2.5 concentrations, 

before and after adjusting for weather, and the location of urban monitoring sites used in the average. 

TRENDS IN PM2.5 COMPOSITION 2002-2007 

The mixture of different chemical components which 

make up PM2.5 varies by season and location. This 

is true because of the differences in emissions and 

weather conditions that contribute to the formation 

and transport of PM2.5. In general, PM2.5 is primarily 

composed of sulfate, nitrate, organic carbon, and, to a 

lesser degree, elemental carbon and crustal material. 

Figure 16 shows regional trends in the composition of 

PM from 2002 to 2007 for warm and cool months. 2.5

Sulfate levels are generally higher in the warm months 

and can account for the largest chemical component of 

PM mass. Sulfate concentrations are their lowest in 2.5

the Northwest. Also, the sulfate portion of PM2.5 mass is 

lower in the Northwest than in any other region. Slight 

declines in sulfate levels are shown in the Northeast 

and Southeast during the cooler months. The highest 

sulfate concentrations appeared in the Southeast, 

Northeast, and Midwest during warm months of 2005, 

partly due to atypical weather conditions. The largest 

sources of sulfate in the eastern U.S. are SO2

 emissions 

from electric utilities and industrial boilers. In southern 

California and port cities in the Northwest, sulfates 

likely come from marine vessels. 

Organic carbon is also a major component of PM2.5 

throughout the year in all regions. Organic carbon 

concentrations are highest in southern California and 

the Southeast. Organic carbon levels are the largest 

component of PM2.5 in southern California and the 

Northwest during the cool months. Declines are shown 

22 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

year-round for southern California and during the 

warm months in the Northeast. The largest sources of 

organic carbon are VOCs and direct carbon emissions 

from highway vehicles, non-road mobile, waste 

burning, wildfires, and vegetation. In the western U.S., 

fireplaces and woodstoves are important contributors 

to organic carbon. 

Nitrate concentrations are higher in the cool months 

than in the warm months. The lowest nitrate levels 

are in the Northeast and the Southeast. Nitrate levels 

have declined substantially in southern California and 

slightly in all the other regions, except the Northwest, 

which shows no discernible trend. The largest sources 

of nitrates are NOx emissions from highway vehicles, 

non-road mobile, electric utilities, and industrial 

boilers. 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. 

The remaining two components, elemental carbon 

and crustal material, are comparatively small but also 

exhibit some seasonal variability. 

Figure 16. Regional and seasonal trends in annual 

PM2.5 composition in μg/m3

, 2002-2007. 

Note: This figure is based on 42 monitoring locations with the most complete data from the national chemical speciation network for 2002-2007. 

There were no sites with complete data in the Southwest. For related information, read “Retained nitrate, hydrated sulfates, and carbonaceous 

mass in federal reference method fine particulate matter for six eastern U.S. cities,” by N. H. Frank, J. Air & Waste Manage. Assoc. 56, Pages 500-511, 

2006. 

NATIONAL AIR QUALITY STATUS AND TRENDS 23 
 

PARTICLE POLLUTION 

TRENDS IN PM10 CONCENTRATIONS 

Nationally, 24-hour PM10 concentrations 

declined by 21 percent between 2001 and 2007 

as shown in Figure 17. 

When comparing two 3-year periods, 2001-

2003 and 2005-2007, most of the sites (nearly 

90 percent) showed a decline or litt le change 

in PM10 as shown in Figure 18. Twenty sites 

located in the Southwest, South Carolina, 

Missouri, and Wyoming showed a greater 

than 50 μg/m3 decline. Seventy-four sites 

showed an increase of greater than 10 μg/m3 

over the trend period. Four of these sites 

(Houston, Texas; Rock Springs, Wyo.; Albany, 

Ga.; and Las Cruces, N.M.) showed large 

increases of 50 μg/m3 or more. 

Figure 17. National PM10 air quality trend, 2001-2007 (second 

maximum 24-hour concentration). 

Figure 18. Change in PM10 concentrations in μg/m3, 2001-2003 vs. 2005-2007 (3-year average of 

annual average concentrations). 

24 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

 

 

 

Figure 19 shows that in 2007, the 

highest PM10 concentrations were 

located in California, Nevada, 

Arizona, and New Mexico. This 

is also where some of the sites 

showed a greater than 50 μg/m3 

decline. Highest concentrations 

are largely located in dry and/or 

industrial areas with high coarse 

particle sources. 

Figure 19. PM10 concentrations 

in μg/m3

, 2007 (second maximum 

24-hour concentration). 

Sustainable Skylines Initiative 

For more information about Dallas, visit 

http://www.sustainableskylines.org/Dallas/. 

For more information about Kansas City, visit 

http://www.epa.gov/region7/citizens/ssi.htm. 

EPA’s Sustainable Skylines Initiative (SSI) is an innovative approach to achieve sustainable 

air quality and other environmental improvements including reducing the six common air 

pollutants, toxic air pollutants, and greenhouse gases. Participating cities may integrate 

transportation, energy, land use, and air quality planning efforts to achieve measurable 

emissions reductions within three years. 

Each program is locally-driven, provides for collaboration 

among multiple stakeholders, identi es and leverages resources 

among public and private partners, and utilizes a consensusbased approach. Initiatives to encourage use of sustainable 

practices to help the air quality are already underway in Dallas, 

Texas; Kansas City, Kan.; and Missouri. EPA plans to have 10 cities 

in the program by the end of 2010. 

Sustainable skyline projects include: 

• Linking green building techniques with affordable housing 

initiatives. 

• Decreasing the amount of heated surfaces within the 

central city. 

• Increasing permeability of surfaces within the central city. 

• Conducting pollution prevention audits for small businesses 

to reduce energy consumption and environmental impacts. 

• Reducing landscape equipment emissions through 

sustainable lawn irrigation and turf management. 

• Lowering vehicle emissions by increasing public 

transportation and reducing vehicle miles traveled. 

• Converting parking lots to parks. 

• Reducing engine idling and applying retro ts to diesel 

engines. 

• Retro tting or replacing small off-road equipment to reduce 

emissions. 

NATIONAL AIR QUALITY STATUS AND TRENDS 25 
 

LEAD 

TRENDS IN LEAD CONCENTRATIONS 

Nationally, concentrations of lead decreased 

56 percent between 2001 and 2007, as shown in 

Figure 20. The national average concentrations 

shown are for 25 sites near large stationary 

sources and 78 sites that are not near stationary 

sources, 103 sites total. The typical average 

concentration near a stationary source (e.g., 

metals processors, battery manufacturers, and 

mining operations) is approximately 7 times the 

typical concentration at a site that is not near a 

stationary source. There are signifi cant yearto-year changes in lead concentrations at sites 

near stationary sources; these refl ect changes 

in emissions due to changes in operating 

schedules and plant closings. For example, lead 

concentrations declined between 2001 and 2002 

mostly due to lower lead concentrations at sites 

in Herculaneum, Mo. 

Figure 20. National lead air quality trend, 2001-2007 (maximum 

3-month average). 

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

Figure 21 shows lead concentrations in 2007. Of 

the 109 sites shown, 25 exceeded the new lead 

standard (0.15 μg/m3

). These sites are located

 in Alabama, Florida, Illinois, Indiana, 

Minnesota, Missouri, Ohio, Pennsylvania, 

Tennessee, and Texas. All of these sites are 

located near stationary lead sources. New 

requirements for monitoring near stationary 

lead sources will be implemented in 2010. 

Approximately 250 new locations will be 

monitoring lead concentrations. 

2007 (maximum 3-month averages). 

Figure 21. Lead concentrations in μg/m3, 

EPA Strengthens the National Ambient Air Quality Standards for Lead 

On October 15, 2008, EPA strengthened the National Ambient Air Quality Standards for lead. The level for the previous lead 

standards was 1.5 μg/m3, not to be exceeded as an average for a calendar quarter, based on an indicator of lead in total 

suspended particles (TSP). The new standards, also in terms of lead in TSP, have a level of 0.15 μg/m3, not to be exceeded as an 

average for any three-month period within three years. 

In conjunction with the revision of the lead standard, EPA also modified the lead air quality monitoring rules. Ambient lead 

monitoring is now required near lead emissions sources emitting 1 or more tons per year, and also in urban areas with a 

population equal to or greater than half a million people. Monitoring sites are required to sample every sixth day. 

26 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

NO2, CO, AND SO2 

TRENDS IN NO2, CO, AND SO2

CONCENTRATIONS 

Nationally, concentrations of nitrogen 

dioxide (NO2) decreased 20 percent 

between 2001 and 2007, as shown in 

Figure 22. In 2007, NO2 concentrations 

were the lowest of the seven year 

period. All recorded concentrations 

were well below the level of the annual 

standard (0.053 ppm). 

Figure 22. National NO2 

air quality trend, 2001-2007 

(annual average). 

Nationally, concentrations of 8-hour carbon 

monoxide (CO) decreased 39 percent 

between 2001 and 2007, as shown in 

Figure 23. In 2007, CO concentrations 

were the lowest in the past seven years. 

All concentrations were below the 8-hour 

standard (9 ppm). One site near Salt Lake 

City, Utah, showed concentrations above the 

level of the 1-hour standard (35 ppm). 

Figure 23. National CO air quality trend, 

2001-2007 (second maximum 8-hour average). 

Nationally, concentrations of sulfur 

dioxide (SO2) decreased 24 percent 

between 2001 and 2007, as shown 

in Figure 24. In 2007, annual SO2 

concentrations were the lowest of the 

seven year period. All concentrations 

were below the level of the annual 

standard (0.03 ppm). One site in Hawaii 

showed concentrations above the level 

of the 24-hour standard (0.14 ppm), due 

to a nearby volcano. 

Figure 24. National SO2 air quality 

trend, 2001-2007 (annual average). 

Downward trends in 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 standards, EPA continues to track these gaseous 

pollutants because of their contribution to other air pollutants (e.g., ozone and PM2.5) and reduced visibility. National 

ambient air quality standards for these pollutants are under review. 

NATIONAL AIR QUALITY STATUS AND TRENDS 27 
 

TOXIC AIR 

POLLUTANTS 

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, vary from pollutant to 

pollutant. For example, a few pounds of a relatively 

toxic pollutant may have a greater health eff ect than 

several tons of emissions of a less toxic pollutant. These 

toxicity levels can vary by orders of magnitude between 

pollutants. EPA has a recommended set of benchmark 

toxicity levels for estimating the effects of exposure to 

individual toxic air pollutants. For more information, 

visit htt p://www.epa.gov/ttn/atw/toxsource/table1.pdf. 

Monitoring data are limited for most toxic air 

pollutants. Because ambient monitoring data is so 

limited for toxic air pollutants, EPA frequently relies 

on ambient modeling studies to bett er defi ne 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 (a subset of the Clean Air 

Act’s list of 187 toxic air pollutants). NATA examines 

individual pollutant effects as well as cumulative 

effects of many air pollutants on human health. 

Figure 25 shows the estimated lifetime cancer risk 

across the continental U.S. by county based on 2002 

NATA model estimates. The national average cancer 

risk level in 2002 is 36 in a million. Many urban areas 

as well as transportation corridors show a risk above 

the national average. From a national perspective, 

benzene is the most significant toxic air pollutant for 

which cancer risk could be estimated, contributing 

over 30 percent of the average individual cancer 

risk identified in the 2002 assessment. Though not 

included in the figure, exposure to diesel exhaust is 

also widespread. EPA has not adopted specifi c 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. 

Figure 26 shows the trends in ambient monitoring 

levels for some of the important toxic air pollutants 

Figure 25. Estimated county-level cancer risk from the 2002 National Air Toxics Assessment (NATA2002). 

Darker colors show greater cancer risk associated with toxic air pollutants. 

28 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

 

identified by NATA. When the median 

percent change per year (marked by an x 

for each pollutant shown) is below zero, the 

majority of the sites in the U.S. are showing 

a decrease in concentrations. Ambient 

monitoring data show that for some of the 

toxic air pollutants of greatest widespread 

concern to public health (shown in yellow), 

1,3-butadiene, benzene, tetrachloroethylene, 

and 1,4-dichlorobenzene concentration levels 

are declining at most sites. Concentrations 

of VOCs such as 1,3-butadiene, benzene, 

styrene, xylenes, and toluene decreased 

by approximately 5 percent or more per 

year at more than half of all monitoring 

sites. Concentrations of carbonyls such 

as formaldehyde, acetaldehyde, and 

propionaldehyde were equally likely to have 

increased or decreased. Chlorinated VOCs 

such as tetrachloroethylene, dichloromethane, 

and methyl chloroform decreased at more 

than half of all monitoring sites, but decreases 

among these species were much less 

consistent from site to site than among the 

other VOCs shown. Lead particles decreased 

in concentration at most monitoring sites; 

trends in other metals are less reliable due 

Figure 26. Distribution of changes in 

ambient concentrations at U.S. toxic 

air pollutant monitoring sites, 2000-

2005 (percent change in annual average 

concentrations). 

(Source: McCarthy M.C., Hafner H.R., 

Chinkin L.R., and Charrier J.G. [2007] 

Temporal variability of selected air toxics 

in the United States. Atmos. Environ. 41 

[34], 7180-7194) 

Notes: 10th and 90th percentiles are excluded 

if fewer than 10 monitoring sites were available 

for analyses. For chloroform and nickel, the 90th 

percentile percent changes per year are cut off

at 30. 

to the small number of sampling sites available for 

analysis. 

In 2003, in an effort to improve accuracy and 

geographic coverage of monitoring, EPA, working with 

its state and local partners, launched the National Air 

Toxics Trends Station (NATTS) program, a national 

monitoring network for toxic air pollutants. 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, acrolein, 

and hexavalent chromium) in order to establish 

overall trends. The initial 23 stations were established 

between 2003 and 2005, two stations were added in 

2007 and two more in 2008 for a total of 27 NATTS 

sites. In addition, the list of pollutants monitored was 

expanded to include polycyclic aromatic hydrocarbons 

(PAHs), of which naphthalene is the most prevalent. 

NATIONAL AIR QUALITY STATUS AND TRENDS 29 
 

TOXIC AIR POLLUTANTS 

In addition to the NATTS program, about 300 

monitoring sites are currently collecting data to help 

air pollution control agencies track toxic air pollutant 

levels in various locations around the country. State, 

local, and tribal 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. 

Figure 27 shows the locations of the toxic air pollutant 

monitoring sites. A majority of these sites are located 

in or near densely populated areas. Most sampling 

is conducted on a 1-in-6-day schedule for a 24-hour 

period. For more information about ambient air quality 

monitoring programs, visit htt p://www.epa.gov/ttn/ 

amtic/. 

Figure 27. Toxic air pollutant monitoring sites operating in 2007 (by monitoring program). 

Note: Some agencies use EPA-contracted sampling and laboratory analysis support services at the sites that are not NATTS program sites; these 

sites collectively are referred to as the Urban Air Toxics Monitoring Program (UATMP). At other monitoring sites, agencies perform their own 

laboratory analyses or use non-EPA contracted laboratories. 

30 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

Local Short-term Toxic Air Pollutant Monitoring Projects 

Due to the local nature of toxic air pollutant problems in 2004, EPA began funding local-scale monitoring projects. Typically 

these projects collect one to two years of monitoring data. To date, EPA has funded 51 projects and 25 have been completed. 

Woodson site for the Hopewell Urban Air Toxics special study currently 

in progress located at Carter G. Woodson Middle School in Hopewell, Va. 

Apartment complexes can be seen in the background. 

Detroit Exposure and Aerosol Research Study (DEARS) 

A research study that the U.S. Environmental Protection Agency conducted 

in Detroit, Michigan, named the Detroit Exposure and Aerosol Research Study 

(DEARS), will help develop data that improves our understanding of human 

exposure to various air pollutants in our environment. The primary objective of 

DEARS was to compare air pollutant concentrations measured at central or 

community air monitoring stations with those measured in various neighborhoods in 

the Detroit, Michigan, area. 

The study collected air quality samples over a three-year period (2004 through 

2007) involving roughly 120 adults, randomly selected from among seven 

neighborhoods. These neighborhoods were selected because they represent a 

variation of potential industrial and regional source influences, housing type/age, 

and proximity to mobile emissions sources. Sampling included personal, indoor, 

backyard, and community monitors. Data were collected on particle pollution and 

toxic air pollutants. 

These are the key questions to be addressed: 

• How do air pollutant concentrations measured at community sites relate to 

those from residential indoor, outdoor, and personal monitoring? 

• Can air pollutant concentrations monitored at community sites adequately 

represent estimates of what local residents are exposed to and the sources of 

these pollutants? 

Participants engaged in five days of summertime monitoring and five days of 

wintertime monitoring per year. The summer and winter data collections provide 

important information on seasonal influences on pollutant concentrations and 

personal exposures to various sources. 

Early findings indicate pollutant exposures may vary greatly among individuals 

living in the same area. The indoor air environment often highly infl uences individual 

exposures to some pollutant species, including those associated with volatile 

organic compounds and particle pollution. The movement of air into and out of the 

home was determined to be highly seasonal (nearly twice as high in the summer). 

This resulted in much higher exposures of individuals to particle pollution formed 

outside during the summer as compared to the winter. And, while the outdoor 

environment was a significant contributor of pollutants to local air quality outside 

homes close to major roadways, the impact of mobile-source related pollutants 

on air quality as a function of distance to the roadway was clearly evident. The 

impact of mobile-source related pollutants on air quality fell to near-background 

levels as distances from the roadway approached 300 meters. 

(Source: http://www.epa.gov/dears/, photos courtesy of EPA) 

Monitoring Methods 

Personal Monitoring 

Backyard Monitoring 

Indoor Monitoring 

Community Monitoring 

The goal of local monitoring is to provide more 

flexibility to address middle- and neighborhoodscale (0.5 km to 4 km) issues that are not 

handled well by national networks. Objectives 

for these projects include: 

• characterizing the degree and extent of 

local toxic air pollutant problems 

• identifying and profiling local toxic air 

pollutant sources 

• developing and assessing emerging 

measurement methods 

• verifying the success of toxic air pollutant 

reduction activities 

Results from these efforts are used to identify 

emission reduction options to be implemented 

at the local level. 

NATIONAL AIR QUALITY STATUS AND TRENDS 31 
 

ATMOSPHERIC 

DEPOSITION 

TRENDS IN ATMOSPHERIC DEPOSITION 

Pollution in the form of acids and acid-forming 

compounds (such as sulfur dioxide [SO2

] and oxides 

of nitrogen [NOx]) can deposit from the atmosphere to 

the Earth’s surface. Between the 1989-1991 and 2005­

2007 time periods, sulfate deposition decreased over 

30 percent in the Northeast and the Midwest, as shown 

in Figure 28. In addition, nitrate deposition decreased 

by about 30 percent in the Mid-Atlantic and Northeast, 

and 20 percent in the Midwest. These reductions have 

led to improving water quality in lakes and streams. 

Most of these improvements are due to reductions 

in SO2

 and NOx emissions from electric utilities and 

industrial boilers. The Acid Rain Program and the NOx 

SIP Call in the East have led to significant reductions in 

SO2 and NOx emissions. 

1989-1991 

• SO2

 emissions have been reduced by more 

than 6.7 million tons from 1990 levels, or about 

43 percent. Compared to 1980 levels, SO2 emissions 

from power plants have dropped by more than 

8 million tons, or about 48 percent. In 2007, annual 

SO2

 emissions fell by over 400,000 tons from 2006 

levels. 

• NOx

 emissions have been reduced by about 

3 million tons from 1990 levels, so that emissions 

in 2007 were less than half the level anticipated 

without the Acid Rain and NO x SIP Call programs. 

Ongoing review of the NO2 and SO2 secondary 

standards, which is scheduled to be completed in 2010, 

is addressing residual atmospheric deposition. 

2005-2007 

1989-1991 2005-2007

Figure 28. Three-year average deposition of sulfate (wet SO4

2-) and nitrate (wet NO3-

) in 1989-1991 and 2005-2007. Dots show 

monitoring locations. (Data source: National Atmospheric Deposition Program, http://nadp.sws.uiuc.edu/) 

32 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

Mercury does not naturally occur as pure “quicksilver” but usually occurs as its principal ore cinnabar 

(HgS), one of 25 mercury-containing minerals that make up about 0.5 parts per million of the Earth’s 

crust. Mercury is used in industry, commerce, mining, metallurgy, manufacturing, medicine, and 

dentistry. Human sources of atmospheric mercury include by-products of coal combustion, municipal 

and medical waste incineration, mining of metals for industry, and many others. Natural sources of 

atmospheric mercury include out-gassing from volcanoes and geothermal vents, and evaporation 

from naturally enriched soils, wetlands, and oceans. Atmospheric mercury concentrations can vary 

greatly depending on the location. Away from sources, elemental mercury concentrations are 

normally about 1.4 to 1.6 ng/m3

 and reactive gaseous and particle-bound mercury concentrations 

are normally below 0.05 ng/m3. Close to sources, and in unique environments, concentrations can 

range widely, from 0.1 to over 100 ng/L in some outliers. Wet deposition could be responsible for 

50-90 percent of mercury loading to many inland water bodies. 

Mercury in the air is usually of little direct concern. But when mercury is washed from the air by 

precipitation into our streams and lakes, it is transformed into highly toxic methyl-mercury that can 

build up in fish. People are then exposed to mercury by eating fish. 

Tracking progress and results is a critical 

step in understanding mercury in the 

environment. Since 1996, the Mercury 

Deposition Network (MDN) provides 

measurements of the amount of mercury 

in rain; the network now has more than 100 

sites. In 2006 the highest concentrations 

of mercury wet-deposition are shown in 

the eastern U.S. Between 1996 and 2005, 

significant decreases in mercury wetdeposition concentrations were found 

at about half of 49 selected sites. Several 

sites in the mid-Atlantic and northeast 

show decreases greater than 1.5 percent. 

Technologies used to remove NOx , SOx 

, and particles also reduce mercury 

emissions (“Control of Mercury Emissions 

from Coal-fired Electric Utility Boilers: Interim 

Report”, EPA-600/R-01-109, April 2002). 

For more information about the MDN, visit 

http://nadp.sws.uiuc.edu/mdn/. 

Mercury in the Environment 

NATIONAL AIR QUALITY STATUS AND TRENDS 33 
 

 

VISIBILITY IN 

SCENIC AREAS 

TRENDS IN VISIBILITY 

EPA monitors visibility trends 

in 155 of the 156 National parks 

and wilderness areas meeting the 

criteria established in the 1977 Clean 

Air Act amendments. Long-term 

trends in visibility on the annual 

20 percent best and worst visibility 

days are shown in Figure 29. Most 

locations show improving visibility 

(decreasing haze) for the best 

visibility days, only Everglades 

National Park in Florida shows 

increasing haze. Five locations— 

Mt. Rainier National Park, Wash.; 

Great Smoky Mountains National 

Park, Tenn.; Great Gulf Wilderness, 

N.H.; Canyonlands National 

Park, Utah.; and Snoqualmie Pass, 

Wash.—show a notable decrease in 

haze for the worst days. 

The Regional Haze Rule requires 

states to identify the most eff ective 

means of preserving conditions 

in these 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. States are required to adopt 

progress goals for improving 

visibility, or visual range, from 

baseline conditions (represented 

by 2000 to 2004) to achieve natural 

background conditions within 

60 years (represented by 2064). 

States determine whether they are 

meeting their goals by comparing 

visibility conditions from one fi veyear average to another (e.g., 2000­

2004 to 2013-2017). The glide path 

to natural conditions in 2064 for the 

Shenandoah National Park is shown Figure 29. Trends in visibility on the 20 percent worst and best visibility 

in Figure 30. days, 1996-2006. 

(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 to compute the change in dv per year over the trend period and its statistical significance. 

34 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

Figure 30. Glide path to natural conditions in 2064 for Shenandoah (deciviews).
 

(Source: Visibility Improvement State and Tribal Association of the Southeast—VISTAS)
 

Notes: A change of one deciview is a change in visibility that is descernable. The figure shows a 

5-year rolling average for the 20 percent worst visibility days. 

Visibility at Great Smoky Mountains National Park 

Visibility at Great Smoky Mountains National Park for the 20 percent haziest days in the baseline period 2000-2004 

(30.3 deciviews) was among the poorest in the country. However, projected improvements in visibility in the Southern 

Appalachian Mountains, such as the Great Smoky Mountains, are among the largest in the country. Ammonium sulfate is the 

major contributor to haze in the southeastern U.S. There has been a small but significant reduction in sulfate and corresponding 

improvement in visibility at Great Smoky Mountains National Park between 1990 and 2004. These improvements are due 

primarily to SO2

 emissions reductions under the Acid Rain Title IV provisions of the 1990 Clean Air Act Amendments. 

Visibility Improvement State and Tribal Association of the Southeast (VISTAS) modeling projects that emissions reductions under 

existing state and federal regulations will significantly improve visibility by 2018. The uniform rate of progress for improving visibility 

between baseline conditions and natural background would mean visibility of 25.8 deciviews in 2018; modeling indicates that 

visibility in 2018 will be 23.5 deciviews, better than the uniform rate of progress, and is a 6.8 deciview improvement compared to 

baseline conditions (2000-2004). Natural visibility conditions on the 20 percent haziest days at Great Smoky Mountains National 

Park are projected to be 11.1 deciviews. Considerable additional progress is needed to achieve natural visibility. 

(Source: Images from WinHaze Visual Air Quality Model, Air Resource Specialists, Inc. and Jim Renfro, Great Smoky 

Mountains National Park) 

NATIONAL AIR QUALITY STATUS AND TRENDS 35 
 

CLIMATE CHANGE 

AND AIR QUALITY 

CLIMATE AND AIR QUALITY 

Climate and air pollution are closely coupled. 

Ground-level ozone absorbs solar radiation, and thus 

contributes to increases in global temperature. Particle 

pollution scatters or absorbs solar radiation and 

changes cloud formation processes and the amount 

of cloud cover. The net effect of particle pollution is 

cooling as scattering generally dominates. 

Changes in climate affect air quality. Warming of the 

atmosphere increases the formation of ground-level 

ozone, while the overall directional impact of climate 

change on particle pollution in the U.S. remains 

uncertain. 

Because of these links between climate and air quality, 

the National Academy of Sciences recommends that 

air pollution and climate change policies be developed 

through an integrated approach. A number of strategies 

being discussed for climate—energy effi ciency, 

renewable energy, and reducing the number of vehicles 

on the highway will provide reductions in emissions 

that contribute to multiple air quality concerns such 

as ozone and particle pollution, toxic air pollutants, 

atmospheric deposition, and visibility. 

TRENDS IN GREENHOUSE GAS EMISSIONS 

AND CLIMATE 

EPA, in collaboration with other government 

agencies, tracks both changes in climate and changes 

in greenhouse gas emissions. Figure 31 shows the 

trends in domestic greenhouse gas emissions over 

time in the U.S. The dominant gas emitted is carbon 

dioxide (mostly from fossil fuel combustion). Total 

U.S. greenhouse gas emissions increased 15 percent 

between 1990 and 2006. 

A number of EPA scientists participate on the 

Intergovernmental Panel on Climate Change (IPCC), 

an international scientific body that provides 

information about the causes of climate change and 

its potential effects on the environment. In a series of 

comprehensive reports completed in 2007, the IPCC 

concludes that “warming of the climate system is 

unequivocal, as is now evident from observations of 

increases in global average air and ocean temperatures, 

widespread melting of snow and ice, and rising global 

average sea level.” Average global temperatures have 

been rising and the warming is accelerating. 

Figure 31. Domestic greenhouse gas 

emissions in teragrams of carbon dioxide 

equivalents (Tg CO2

 eq), 1990-2006. 

(Source: http://epa.gov/climatechange/ 

emissions/usinventoryreport.html) 

Notes: A teragram is equal to 1 million 

metric tons. Emissions in the fi gure include 

fluorocarbons (HFCs, PFCs) and sulfur 

hexafl uoride (SF6

). 

36 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

 

CHANGES IN CLIMATE AFFECT AIR QUALITY 

Due to the warming, the IPCC projects with 

virtual certainty “declining air quality in cities.”

In summarizing the impact of climate change on 

ozone and particle pollution, the IPCC concludes 

that “future climate change may cause signifi cant air 

quality degradation by changing the dispersion rate 

of pollutants, the chemical environment for ozone 

and particle pollution generation, and the strength of 

emissions from the biosphere, fires, and dust.” Large 

uncertainties remain, limiting our ability to provide 

a quantitative description of the interactions between 

air quality and climate change. However, as noted in 

the following two examples, research is under way 

that will provide an improved understanding of these 

connections. 

Using estimates from a computer model that assumes 

continued growth in global GHG emissions, a study 

cited in the 2007 IPCC report shows how ground-level 

ozone in the New York metropolitan area may increase 

from current levels given future climate change. 

Figure 32 shows this study projects daily 1-hour ozone 

increases of 0.0003 to 0.0043 ppm across the region due 

to climate change alone in the 2050s compared to the 

1990s. 

Pollutants from forest fires can affect air quality for 

thousands of miles. The IPCC reported that in North 

America wildfires are increasing and in a warmer 

future are likely to intensify with drier soils and longer 

growing seasons. Figure 33 shows increases in the 

annual frequency of large (>100,000 hectares) western 

U.S. forest wildfires (bars) associated with the mean 

March through August temperature. In the last three 

decades, the wildfire season in the western U.S. has 

increased by 78 days in response to a spring-summer 

warming of 0.87°C. 

Figure 32. Estimated changes in 1-hour daily maximum 

ozone concentrations (ppm) in the 2050s compared with 

those in the 1990s for the New York metropolitan area, 

under scenario M1 in which climate change alone drives 

changes in air quality. 

(Source: Knowlton K., et al. [2004] Assessing ozonerelated health impacts under a changing climate. 

Environ. Health. Perspect., 112: 1557-1563) 

Figure 33. Frequency of Western U.S. forest wildfires compared to spring-summer 

temperature. 

(Source: Westerling A.L., et al. [2006] Warming and earlier spring increase western 

U.S. forest wildfi re activity. Science, 313: 940-943) 

NATIONAL AIR QUALITY STATUS AND TRENDS 37 
 

 

INTERNATIONAL 

TRANSPORT OF AIR 

POLLUTION 

INTERNATIONAL TRANSPORT 

OF AIR POLLUTION 

While domestic sources of emissions are the primary 

cause of most air pollution in our country, the U.S. is 

both a source of pollution and a receiver of pollution 

from other countries. Air pollution fl ows 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. International flow of air pollutants 

into the U.S. contributes to observed concentrations 

of ozone and particle pollution and deposition of 

mercury, persistent organic pollutants (POPs), and acid 

deposition. 

TRANSPORT OF AIR POLLUTION 

AFFECTS THE U.S. 

The impact that international transport of air pollution 

has on our ability to attain air quality standards or 

other environmental objectives in the U.S. has yet to 

be characterized (except in areas that are downwind 

of cities or sources in Mexico or Canada). Figure 34 

illustrates major intercontinental transport pathways. 

Estimates based on the available evidence are highly 

uncertain, but suggest that the current contributions 

of international transport to observed concentrations, 

acid deposition and deposition of mercury are small. 

Increased emissions of particle pollution, mercury, and 

ozone precursors in developing countries associated 

with economic growth may increase background levels 

of these pollutants in the U.S. 

For ozone and particle pollution, increased background 

levels of these pollutants could potentially make it 

more difficult for local and regional areas to achieve 

the National Ambient Air Quality Standards and longterm visibility improvement goals. Transported ozone 

and particle pollution also contribute to radiative 

forcing and global and regional climate change. For 

mercury and POPs, international fl ows contribute 

to deposition, and eventual human and ecosystem 

chemical exposures. In some locations, especially in 

Alaska, international sources are the dominant source 

of contamination for these toxic air pollutants. 

Figure 34. Major intercontinental transport pathways of CO emissions in the Northern Hemisphere. The colored boxes indicate 

the four source and receptor regions used in the Task Force on Hemispheric Transport of Air Pollution’s (HTAP) on-going model 

intercomparison study. The arrows approximate the magnitude of main transport pathways in summer (June, July, August) and 

winter (December, January, February), based on modelled average CO transport over 8–10 day periods. Light arrows indicate 

transport generally near ground level (less than 3 km above the surface) and dark arrows indicate transport higher in the 

atmosphere (more than 3 km above the surface). 

(Source: Figure E-1, HTAP 2007. Adapted from Figure 2 of Stohl and Eckhardt [2004], with kind permission of Springer Science 

and Business Media) 

38 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

 

International Efforts to 

Address Air Pollution Transport
 

EPA is involved in a number of international efforts to 

address air pollution transport, including:
 

• Reducing transborder air pollution transport, visit http:// 

www.epa.gov/airmarkets/progsregs/usca/index.htm 

• Understanding intercontinental transport in the 

northern hemisphere, visit http://www.htap.org
 

• Addressing global scale transport, visit http://chm. 

pops.int and http://www.chem.unep.ch/mercury/ 

new_partnership.htm 

• Building cooperative relationships to improve air 

quality and reduce long-range transport of air 

pollution in key countries, visit http://www.epa.gov/ 

oia/regions/ 

EFFORTS TO BETTER UNDERSTAND 

TRANSPORT OF AIR POLLUTION 

EPA and other agencies are working via treaties 

and international cooperative efforts to address the 

international transport of air pollution. Since 2001, 

EPA has led collaborative efforts among many of the 

leading U.S. researchers in the global atmospheric 

chemistry community to improve our understanding 

of trans-Pacific and trans-Atlantic transport. EPA and 

the European Commission co-chair the Task Force on 

Hemispheric Transport of Air Pollution, a multinational 

Shipping and aviation are two of the fastest growing 

sources of emissions globally, with important 

consequences for air quality. Emissions from both sectors 

have received increased attention and the International 

Maritime Organization recently acted to strengthen 

emission controls on ocean-going ships. 

Shipping and Aviation Emissions 

effort to better understand the soures, transport, and 

impacts of air pollution in the northern hemisphere. 

In 2008, EPA, with contributions from NOAA, NASA, 

and the National Science Foundation (NSF), has 

sponsored a National Academy of Sciences study to 

examine the significance of the international transport 

of air pollutants for air quality, atmospheric deposition, 

and climate change. 

Tracking Pollutant

Transport with Satellites 

During the 2004 summer, the largest Alaskan 

wild fire event on record occurred in late 

June-July and consumed 2.72 million hectares 

of boreal forest. The figure shows aerosol 

optical depth (AOD) data from the Moderate 

Resolution Imaging Spectroradiometer (MODIS) 

instrument aboard the Terra satellite for a 

series of days in July 2004. The MODIS AOD is 

plotted over the MODIS Terra true color image 

for each day. This series of days shows high 

aerosol concentrations (in red) associated with 

long-range transport of the Alaskan wild fire 

plume as it crosses over the northern border of 

the U.S. on July 16. This aerosol plume travelled 

south-eastward behind the cold front (evident 

in the clouds captured in the MODIS true color 

image) over the following days, eventually 

affecting surface PM2.5 levels along the Eastern 

U.S. 

Aerosol optical depth (AOD) measurements 

for a series of days in July 2004. 

(Image provided by J. Szykman, EPA, and 

C. Kitt aka, SSAI-NASA/LaRC) 

NATIONAL AIR QUALITY STATUS AND TRENDS 39 
 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TERMINOLOGY 

TERMINOLOGY
 

AQI Air Quality Index 

AQS Air Quality System 

AOD aerosol optical depth 

CAA Clean Air Act 

CASTNET Clean Air Status and Trends Network 

CCSP U.S. Climate Change Science Program 

CO carbon monoxide 

dv deciviews 

EC elemental carbon 

EPA U.S. Environmental Protection Agency 

FRM Federal Reference Method 

GHG greenhouse gas 

HFCs hydrofluorocarbons 

HTAP Hemispheric Transport of Air Pollution 

IMPROVE Interagency Monitoring of Protected Visual Environments 

MODIS Moderate Resolution Imaging Spectroradiometer 

NAAQS National Ambient Air Quality Standards 

NASA National Aeronautics and Space Administration 

NATTS National Air Toxics Trends Stations 

NEI National Emissions Inventory 

NH3 ammonia 

NO nitric oxide 

NO x oxides of nitrogen 

NO2 nitrogen dioxide 

NSF National Science Foundation 

O3 ground-level ozone 

Pb lead 

PFCs perfl uorinated compounds 

PM particulate matter 

PM2.5 particulate matt er (fi ne) 2.5 μm or less in size 

PM10 particulate matt er 10 μm or less in size 

POP persistent organic pollutants 

ppm parts per million 

SF6 sulfur hexafluoride 

SIP state implementation plan 

SO x sulfur oxides 

SO2 sulfur dioxide 

UATMP Urban Air Toxics Monitoring Program 

μm micrometers 

μg/m3 micrograms per cubic meter 

VOC volatile organic compound 

40 NATIONAL AIR QUALITY STATUS AND TRENDS 
 

 

 

 

 

 

 

WEB SITES 

Atmospheric Deposition 

Acid Rain Program: htt p://www.epa.gov/airmarkets/progsregs/arp/index.html
 

Acid Rain Program 2006 Progress Report: htt p://www.epa.gov/airmarket/progress/arp06.html
 

National Atmospheric Deposition Program: http://nadp.sws.uiuc.edu/
 

Background/General Information 

Air Quality Index: htt p://www.airnow.gov
 

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

EPA’s Clean Air Research Program: htt p://www.epa.gov/ord/npd/cleanair-research-intro.htm
 

EPA-Funded Particulate Matter Research Centers: 

http://cfpub.epa.gov/ncer_abstracts/index.cfm/fuseaction/outlinks.centers#19
 

Framework for Assessing the Public Health Impacts of Risk Management Decisions:

 htt p://www.epa.gov/ORD/npd/hhrp/fi les/hhrp-framework.pdf 

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

Multi-Ethnic Study of Atherosclerosis and Air Pollution (MESA Air): htt p://depts.washington.edu/mesaair/ 

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

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

National Particle Components Toxicity (NPACT) Initiative: htt p://www.healtheff ects.org/Pubs/NPACT.pdf 

Office of Air and Radiation: htt p://www.epa.gov/air/ 

Office of Air Quality Planning and Standards: htt p://www.epa.gov/air/oaqps/ 

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

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

Climate Change 

Climate change: htt p://www.epa.gov/climatechange/ 

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

Emissions and trends in greenhouse gases: 

htt p://www.epa.gov/climatechange/emissions/usinventoryreport.html
 

Green Car Congress: htt p://www.greencarcongress.com/2008/06/us-vehicle-mile.html
 

Intergovernmental Panel on Climate Change: htt p://www.ipcc.ch
 

Traffic Volume Trends: htt p://www.fh wa.dot.gov/ohim/tvtw/tvtpage.cfm
 

Emissions and Control Programs 

Emissions: htt p://www.epa.gov/air/emissions/ 

NOx

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

Toxic Air Pollutants 

1999 National-Scale Air Toxics Assessment: htt p://www.epa.gov/ttn/atw/nata1999/ 

Measurements and Trends 

Air Quality Trends: htt p://www.epa.gov/airtrends/ 

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

Clean Air Status and Trends Network (CASTNET): htt p://www.epa.gov/castnet/ 

EPA Monitoring Network: htt p://www.epa.gov/ttn/amtic/ 

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

National Air Monitoring Strategy Information: htt p://www.epa.gov/ttn/amtic/monstratdoc.html 

NATIONAL AIR QUALITY STATUS AND TRENDS 41 
 

WEB SITES 

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

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

Visibility 

National Park Service: htt p://www.nature.nps.gov/air/
 

Regional Haze Program: htt p://www.epa.gov/visibility
 

Visibility Information Exchange Web System (VIEWS): http://vista.cira.colostate.edu/views/
 

International Transport 

International Maritime Organization: htt p://www.imo.org 

FAA’s Aviation Climate Change Research Initiative (ACCRI): 

htt p://www.faa.gov/about/office_org/headquarters_offices/aep/aviation_climate/ 

Task Force on Hemispheric Transport of Air Pollution: htt p://www.htap.org 

42 NATIONAL AIR QUALITY STATUS AND TRENDS 

Office of Air Quality Planning and Standards EPA Publication No. EPA-454/R-08-006 

Research Triangle Park, NC Air Quality Assessment Division