The NOx Budget Trading Program (NBP) was a market-based cap and trade program created to reduce 

the regional transport of emissions of nitrogen oxides (NOx) from power plants and other large combustion 

sources that contribute to ozone nonattainment in the eastern United States. NOx is a major precursor to the formation 

of ground-level ozone, a pervasive air pollution problem in 

many areas in the eastern United States. The NBP was designed to reduce NOx emissions during the warm summer 

months, referred to as the ozone season, when ground-level ozone concentrations are highest. In 2009, the NBP was 

replaced by the Clean Air Interstate Rule (CAIR) NOx ozone 

season program, which started requiring emission reductions from affected sources in an expanded geographic 

area on May 1, 2009.

Throughout the summer, the U.S. EPA is releasing a series 

of reports summarizing progress under the NBP. This is 

the third report in the series and it contains 2008 data on 

environmental results as well as analyses of the effects of 

reduced NOx emissions on ozone and nitrate levels.

For more information on the NBP, visit: <www.epa.gov/

airmarkets/progsregs/nox/sip.html>. Detailed emission 

results and other facility and allowance data are also 

publicly available on EPA’s Data and Maps Web site at 

<camddataandmaps.epa.gov/gdm>. To view emission and 

other facility information in an interactive format using 

Google Earth or a similar three-dimensional platform, go to 

<www.epa.gov/airmarkets/progress/interactivemapping.

html>.

The NBP and Ozone

EPA has developed more than a dozen programs to reduce 

NOx and volatile organic compounds (VOCs) from mobile, 

industrial, and power sector sources since 1990. Between 

1999 and 2002, states in the Northeast reduced NOx emissions from electric generating units (EGUs) and industrial 

boilers through the OTC NOx Budget Program. In addition, 

mobile source programs such as the Tier 2 Vehicle and 

Gasoline Sulfur Program, the Clean Air Diesel Trucks and 

At a Glance: NBP Results in 2008

Ozone: Ground-level ozone has decreased since implementation of the NBP in 2003

•	 Analyses of ozone trends using various metrics show 

regionwide ozone reductions ranging from 10-14 

percent in the NBP region

•	 There is a strong association between areas with the 

greatest reductions in NOx emissions and downwind 

sites exhibiting the greatest improvements in ozone

Nonattainment Areas: Based on 2006-2008 air monitoring data, ozone air quality improved in almost all of the 

104 areas in the eastern U.S. designated to be in nonattainment for the 1997 8-hour National Ambient Air Quality Standards (NAAQS)

•	 88 areas (85 percent) now have ozone air quality that 

is better than the level of the 1997 standard

•	 13 areas continue to exceed the standard but have 

experienced an average of 10 percent improvement 

in ozone

•	 In total, over 103 million Americans in the East are 

living with cleaner air

Human Health Benefits: NOx reductions due to the NBP 

have led to improvements in ozone and PM2.5, saving an 

estimated 580-1,800 lives in 2008

Ecosystem Protection: Changes in ozone and nitrate concentrations due to the NBP have contributed to improvements in ecosystems in the East

•	 Decrease in areas with significant ozone damage to

seven ozone-sensitive tree species

•	 33 percent reduction in total nitrate concentrations

Buses Program (also known as the Heavy-Duty Highway 

Diesel Program), the Clean Air Nonroad Diesel program, 

and the recently finalized Locomotive and Marine Vessel

Compression-Engine Standards have started to reduce NOx

emissions from a variety of mobile source categories and 

will continue to do so into the future. These programs comThe NOx Budget Trading Program:

2008 Environmental Results

September 2009
plement state and local efforts to improve ozone air quality 

and meet national standards. 

While these programs, particularly the mobile source programs, have achieved dramatic decreases in NOx emissions 

in recent years, the majority of NOx reductions since 2003 

were achieved under the NBP. Accordingly, the NBP is the 

most significant contributor to the improvements in ozone

concentrations in the East.

To better understand how the NBP affected ozone formation 

in the atmosphere, this report examines changes in ozone 

concentrations before and after implementation of the NBP 

in the eastern United States. The report compares regional 

and geographic trends in ozone levels to changes in meteorological conditions (such as temperature) and NOx emissions 

from NBP sources. This report also explores changes in human health and forest ecosystems due to ground-level ozone 

effects as well as changes in nitrate concentration.

Measuring and Evaluating Changes in Ozone

Two long-term monitoring networks measure ozone levels as well as meteorological and other air quality data 

throughout the United States. Monitoring sites used for 

regulatory compliance are located mainly in urban areas 

and report data to EPA’s Air Quality System (AQS). Sites 

in EPA’s Clean Air Status and Trends Network (CASTNET) 

measure trends in ozone at rural sites. The changes in 

eastern ozone concentrations presented in this report depict data from AQS and CASTNET monitoring sites located 

within both NBP and adjacent states. These analyses show 

a range of ozone reductions based on the metric used and 

the years examined.

Metrics for Assessing Ozone Concentrations

Two types of metrics are used to evaluate trends in ozone 

levels in this report. Both metrics indicate that ozone has 

decreased since implementation of the NBP. The two metrics are: 

• Meteorologically-adjusted daily maximum 8-hour ozone 

concentrations: This metric is the maximum 8-hour 

rolling average for each day. It shows progress toward 

meeting the primary (health-based) ozone National 

Ambient Air Quality Standards (NAAQS). The seasonal 

average indicates general changes in daily maximum 

8-hour concentrations in the NBP region. The threeyear average of the fourth highest daily maximum 

8-hour ozone concentration is used to designate nonattainment status in the United States.

The Clean Air Status and Trends Network 

(CASTNET) is a long-term monitoring network 

designed to assess trends in regional (rural) 

ozone levels and acidic dry deposition. Sites are 

equipped with an ozone analyzer and a threestage filter pack to collect total weekly gaseous

(i.e., nitric acid) and particulate (i.e., nitrate) 

concentrations. Many of the monitoring sites 

have been running continuously for over 15 

years, making the network ideal for long-term 

trends analyses. Regional trends in ozone levels from CASTNET sites have been used to assess emission reduction programs, such as the 

NBP and the Acid Rain Program (ARP). Figure 1 

shows the 43 CASTNET sites used in this report’s 

analysis of trends in rural ozone and nitrate concentrations. These sites met data completeness 

criteria and are located in NBP states or within 

200 kilometers (km) of an NBP state’s border.

Note: States in the NBP region are shaded.

Source: EPA, 2009

Figure 1: Location of CASTNET Sites

CASTNET

CASTNET Sites

The NOx Budget Trading Program: 2008 Environmental Results

2
Figure 2: Percent Change in 1-Hour Ozone Concentrations 

during the Ozone Season, 2000-2002 versus 2006-2008

Note: AQS and CASTNET monitoring sites used for this 

analysis are shown as black dots on this map.

Source: EPA, 2009

Changes in Rural Ozone Concentrations

Rural ozone measurements are useful in assessing the impacts on air quality resulting from regional NOx emission 

reductions because these monitoring sites are typically less 

affected by local sources of NOx (e.g., industrial, automotive, and power generation sources) than urban measurements. Consequently, the formation of ozone in these areas 

is particularly sensitive to changes in levels of regional NOx

emissions. The majority of reductions in rural ozone concentrations can therefore be attributed to reductions in regional NOx emissions and transported ozone.

While ozone levels vary by season and location, regional 

trends reveal notable drops in ambient ozone levels since 

the NBP began in 2003. EPA investigated trends in both rolling 8-hour and 1-hour ozone concentrations as measured 

at CASTNET monitoring sites within the NBP region and 

in adjacent states (states within 200 km of an NBP state’s 

borders). 

• Unadjusted 99th percentile of 1-hour and 8-hour ozone 

concentrations: This metric shows changes in the 

highest concentrations of ozone and provides a broad 

picture of ozone in the eastern United States. The 99th

percentile is used in this report because it is statistically similar to the 4th highest daily maximum 8-hour 

ozone concentration—the ozone NAAQS. This metric 

is representative of true, measured ozone concentrations without meteorological adjustments. 

Although they differ in the amount of ozone reduced, all 

of the analyses presented in this report show substantial 

overall improvements in the NBP region since implementation of the program in 2003. These results are further supported by several studies investigating the impact of NOx

emission reductions from NBP sources on ozone in the region.1

Weather

Weather plays an important role in determining ozone 

levels. EPA often uses a statistical model to account for 

the weather-related variability in seasonal ozone concentrations to provide a trend that is more representative of 

changes in emissions. Averaging ozone concentrations 

across multi-year periods is another way to account for the 

effects of weather on ozone formation. Both methods are 

used in the analyses that follow.

Changes in 1-Hour Ozone Concentrations in the 

East

EPA examined changes in regional 1-hour ozone concentrations, as measured at urban (AQS) and rural (CASTNET) 

sites. Results demonstrate how NOx emission reduction 

policies have affected ozone concentrations in the eastern 

United States. Figure 2 shows changes in the 99th percentile 

of 1-hour ozone concentrations between 2000–2002 (before implementation of the NBP) and 2006-2008 (under 

the NBP). Using this metric, an overall regional reduction 

in ozone levels was observed between these two time periods, with an average reduction in ozone concentrations in 

NBP states of 11 percent. 

The NOx Budget Trading Program: 2008 Environmental Results

3
Site-Specific Changes in Ozone

Similar trends in rural ozone concentrations were observed at individual CASTNET monitoring sites. As expected, there was variation across the region. The largest 

and most significant decreases in ozone were observed at

sites downwind of the Ohio River Valley (Ohio, West VirFigure 4: Percent Reduction in Rural 1-Hour Ozone during the 

Ozone Season, 2000-2002 versus 2006-2008

Notes: 

•	 States in the NBP region are shaded.

•	 The change in ozone concentration is the percent change of the

average of the 99th percentile of 1-hour ozone concentrations 

between each three-year period. 

•	 Ozone data are from CASTNET sites that met completeness criteria and are located in or adjacent to the NBP region.

Source: EPA, 2009

Percent Change in the 99th

Percentile Ozone Concentration

Between -20% and -23%

Between -15% and -20%

Between -10% and -15%

Between -5% and -10%

Between -1% and -5%

Statistically Significant Reductions

Regional Trends in Ozone 

An Autoregressive Integrated Moving Average (ARIMA) 

model was used to determine the trend in ozone concentrations since the inception of the NBP. The ARIMA model 

is an advanced statistical analysis tool that can evaluate 

trends over time (time series analysis). The average of the 

99th percentile of the 8-hour daily maximum ozone concentrations (the highest daily levels of ozone) measured 

at CASTNET sites during the NBP ozone season were modeled (Figure 3). The ARIMA model shows that between 

1990 and 2003, the average of the 99th percentile of ozone 

concentration was 91 parts per billion (ppb). After 2004, a 

statistically significant shift occurred and a new trend was

established, with an average ozone level of 79 ppb. The 

ARIMA model shows a statistically significant, 13 percent

(12 ppb) decrease in ozone concentrations beginning at 

the start of the NBP, suggesting that this program is a major 

contributor to these regional improvements in ozone.

Figure 3: Shift in 8-Hour Seasonal Ozone Concentrations in the 

NBP Region, 1990-2008

70 

80 

90 

100 

Ozone Concentration (ppb)

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

60 

50 

40

30

20

10

0

110 

Actual Model Estimate 95% Confidence Band

Notes: Total ozone concentration data are from CASTNET sites that 

met completeness criteria and are located in and adjacent to the 

NBP region.

Source: EPA, 2009

ginia, and Pennsylvania), where NBP sources reduced NOx 

emissions most dramatically. Figure 4 displays the average 

percent reduction in the 99th percentile of 1-hour ozone 

concentrations at individual rural CASTNET sites between 

2000–2002 and 2006–2008. Between the two time periods, ozone levels fell by an average of 14 percent in the NBP 

region. A total of four CASTNET sites (in Connecticut, New 

York, Pennsylvania, and North Carolina) measured ozone 

reductions greater than 20 percent. The largest reduction 

The NOx Budget Trading Program: 2008 Environmental Results

4
in ozone (23 percent) was measured at the Abington site 

(ABT147) in Connecticut. Furthermore, at many sites in 

the NBP region, the decrease in ozone between the two 

time periods was significant and statistically different.

Changes in the Monthly Distribution of Ozone

An additional statistical analysis shows that the monthly 

distribution of 8-hour daily maximum ozone concentrations has shifted to lower concentrations. This shift has 

occurred in the NBP region during the ozone season since 

implementation of the program. Figure 5 depicts every 

8-hour daily maximum value measured in January and 

June (where January represents months outside the ozone 

season and June represents months within the ozone season) for two time periods. The blue lines represent ozone 

concentrations before implementation of the NPB (2000-

2002), while the red lines represents ozone concentrations 

after implementation of the NBP (2006-2008). The y-axis 

represents the percent of the total number of days in a 

month with measurements at a specific ozone ppb level.

As the right-hand graph in Figure 5 shows, there is a noticeable shift toward lower ozone concentration during the 

ozone season (represented by the red arrow). As NOx emission controls were turned on at sources subject to the NBP, 

there have been fewer days with high levels of ozone during the ozone season. In months outside the ozone season 

(represented by the month of January), there is little to no 

shift.

As Table 1 shows, the 99th percentile of 8-hour ozone 

dropped by 3-10 ppb during the ozone season, while there 

Figure 5: Monthly Distribution of 8-Hour Ozone Concentrations, 2000-2002 versus 2006-2008

Note: Ozone data are from CASTNET sites that met completeness criteria and are located in or adjacent to the NBP region.

Source: EPA, 2009

Outside of the Ozone Season (January) During the Ozone Season (June)

10 

60 

70 

80 

Percent

0 10 20 30 40 50 60 70 80 90

0 

20 

50 

40 

30 

100

Ozone (ppb)

10 

60 

70 

80 

Percent

0 10 20 30 40 50 60 70 80 90

0 

20 

50 

40 

30 

100

Ozone (ppb)

Ozone concentration 2000–2002 Ozone concentration 2006–2008

Table 1: Shift in 8-Hour Ozone Concentration by Month, 

2000-2002 versus 2006-2008

Month

Change in 99th Percentile 8-Hour 

Ozone Concentration

January No statistically significant shift

February No statistically significant shift

March No statistically significant shift

April No statistically significant shift

May Down 3 ppb

June Down 10 ppb

July Down 9 ppb

August Down 8 ppb

September Down 8 ppb

October Down 10 ppb

November No statistically significant shift

December No statistically significant shift

Note: Months within the ozone season are shaded.

Source: EPA, 2009

were no significant changes in the months outside of the

ozone season (except for October). 

The downward shift in the monthly distribution of ozone 

levels in the NBP region is indicative of broader, substantial 

change in ozone concentrations due in significant part to

the program.

The NOx Budget Trading Program: 2008 Environmental Results

5
Ozone Changes after Adjusting for Meteorology 

As noted earlier, weather plays an important role in determining ozone levels. Accordingly, EPA uses a generalized 

linear model to describe the relationship between daily 

ozone and several meteorological parameters.2 The model 

also accounts for the variation in seasonal ozone across different years by correcting for meteorological fluctuations.

The most important meteorological parameters considered 

in this model are daily maximum 1-hour temperature and 

midday (10 a.m. to 4 p.m.) relative humidity. This methodology and the subsequent ozone estimates are provided by 

EPA’sOffice of AirQuality Planning and Standards (OAQPS),

Air Quality Assessment Division <www.epa.gov/airtrends/

weather.html>. 

Figure 7 shows trends in the seasonal average 8-hour ozone 

concentrations in the NBP region with and without considering the influenceofweather.3 It is important to account for 

meteorological variations when comparing two years with 

notably different weather conditions and ozone-forming 

potential (e.g., 2004 versus 2007). In general, lower temperatures in the NBP region during the 2004 ozone season 

dampened ozone formation, while higher temperatures in 

the 2007 ozone season increased ozone formation. Removing the effects of weather results in a higher-than-observed 

ozone estimate for 2004 and a lower-than-observed ozone 

estimate for 2007. Consequently, despite weather condiChanges in 8-Hour Ozone Concentrations

Daily maximum 8-hour ozone concentration data were assessed from 52 urban AQS areas and 28 rural CASTNET 

sites located in the NBP region. For a monitor or area to be 

included in this trend analysis, it had to provide complete 

and valid data for 50 percent of the ozone season days 

for each of the years from 1997–2008. In addition, urban 

AQS areas often include more than one monitoring site. In 

these cases, the site with the highest observed ozone concentration for each day was used. Figure 6 shows the AQS 

and CASTNET monitoring sites in the NBP region that met 

these completeness criteria.

Figure 6: Location of Urban and Rural Ozone Monitoring Sites

Notes: 

•	 States in the NBP region are shaded.

•	 Urban areas are represented by multiple monitoring sites. Rural areas are represented by a single monitoring site. For more 

information on AQS, visit <www.epa.gov/ttn/airs/airsaqs>. 

For more information about CASTNET, visit <www.epa.gov/

CASTNET>.

Source: EPA, 2009

Figure 7: Seasonal Average 8-Hour Ozone Concentrations in the 

NBP Region, 1997-2008

Note: Data presented in this figure are averages of 8-hour daily

maximum ozone concentrations during the ozone season for AQS 

and CASTNET sites within the NBP region.

Source: EPA, 2009 Ozone Concentration (ppb)

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008

Adjusted for Weather Unadjusted for Weather

0

10

20

30

40

50

60

70

Rural site (CASTNET)

Urban area (AQS)

The NOx Budget Trading Program: 2008 Environmental Results

6
tions conducive to ozone formation in 2008, average ozone 

concentrations in the NBP region were lower than in 2002, 

before implementation of the NBP. 

A closer look at the meteorologically-adjusted ozone 

trends since the start of the NBP in 2003 indicates that 

these reductions are substantive and sustainable. The average reduction in seasonal 8-hour ozone concentrations 

measured in the NBP region in the 2000-2002 and 2006-

2008 time periods was about 6 percent. After considering 

the influence of weather, the improvement in 8-hour ozone

concentrations between these three-year periods was 11 

percent. A comparison of single year meteorologicallyadjusted ozone reveals a 14 percent reduction between 

2002 and 2008. 

Figure 8: Reductions in Seasonal NOx Emissions from NBP Sources and Changes in 8-Hour Ozone, 2002 versus 2008

Notes:

•	 From 1999 to 2002, states in the Northeast reduced emissions from EGUs and industrial boilers under the OTC NOx Budget Program. OTC 

states include Connecticut, Delaware, the District of Columbia, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, 

Pennsylvania, Rhode Island, Vermont, and Virginia.

•	 Meteorologically-adjusted ozone data are from AQS and CASTNET sites that met completeness criteria.

•	 Google Earth was used to display the information shown in this figure. To access the data layers shown here as well as other data, including unit-level emissions and controls, visit the Clean Air Market Division’s interactive mapping site at <www.epa.gov/airmarkt/progress/

interactivemapping.html>.

Source: EPA, 2009

The NOx Budget Trading Program: 2008 Environmental Results

7
Furthermore, the pace of these reductions has increased 

since implementation of the NBP. Between 1997 and 2002, 

ozone fell by 3 percent, while between 2002 and 2008, 

ozone dropped by 14 percent. On average, across the NBP 

region, meteorologically-adjusted ozone levels have been 

fairly stable since 2004, indicating that the majority of the 

progress made in reducing ozone levels since 2003 is being maintained. This is also consistent with the downward 

trend in NOx emissions.

Linking Ozone and NOx Emissions

Figure 8 on the previous page is a Google Earth snapshot 

depicting the relationship between reductions in NOx emissions from NBP sources and reductions in 8-hour average 

ozone after implementation of the NBP. As indicated previously, between 2002 and 2008, ozone decreased across all 

NBP states (after adjusting for meteorology) by an average 

of 14 percent. The largest reductions occurred in New York, 

Ohio, Virginia, North Carolina, and Pennsylvania.

Generally, there is a strong association between areas with 

the greatest NOx emission reductions from NBP sources 

and downwind monitoring sites measuring the greatest 

improvements in ozone. This suggests that, as a result of 

the NBP, transported NOx emissions have been reduced in 

the East, contributing to ozone reductions that have occurred after 2002.4

Changes in Ozone Nonattainment Areas

In April 2004, EPA designated 126 areas as nonattainment 

for the 8-hour ozone standard adopted in 1997.5 These 

designations were made using data from 2001-2003. Of 

those areas, 104 are in the East (as shown in Figure 9) and 

are home to about 108 million people.6 Based on data gathered from 2006-2008, 88 of these original nonattainment 

areas show concentrations below the level of the 1997 

ozone standard (0.08 ppm), indicating improvements in 

ozone. Improvements in these 88 areas mean that 85 percent of the original nonattainment areas in the East now 

have ozone air quality that is better than the standard under which they were originally designated nonattainment. 

These improvements bring cleaner air to over 57 million 

people. The majority of these areas have officially been redesignated to attainment or maintenance, as described in 

Section 107 of the Clean Air Act. 

Fifteen of the original 104 areas in the East continue to exceed the level of the standard. In 13 of these areas, however, ozone concentrations have fallen by an average of 10 

percent. Because of these reductions in ozone, over 46 million Americans living in these areas are experiencing better air quality. 

Given that the majority of relevant NOx emission reductions occurring after 2003 are attributable to the NBP, it 

is clear that the NBP is the most significant contributor to

these improvements in ozone air quality.

Human Health Benefits from 

NBP Implementation

Epidemiological studies7 continue to show a significant

link between exposure to air pollution, particularly PM2.5

and ozone, and adverse health effects, including respiratory and cardiovascular effects as well as incidences of premature mortality. The analysis presented here is a screening-level estimate of the annual human health benefits (the

number of premature deaths avoided annually) from NOx

emission reductions achieved under the NBP.

Figure 9: Changes in Ozone Nonattainment Areas in the East, 

2001-2003 (Original Designations) versus 2006-2008

Note: States in the NBP region are shown inside the black boundary line.

Source: EPA, 2009

Areas Improved to be below the 

NAAQS (88 areas)

Areas above the NAAQS that Show 

Improvement (13 areas)

Areas above the NAAQS that Show 

No Change (1 area)

Areas above the NAAQS that Are 

Increasing (1 area)

Areas with Incomplete NAAQS 

Data (1 area)

The NOx Budget Trading Program: 2008 Environmental Results

8
In order to calculate the amount of NOx emissions reduced 

as a result of NBP implementation, EPA compared NOx 

emission levels under the program with estimates of what 

emissions would have been without the program. In this 

analysis, actual monitored NOx emission data were used 

for most years to represent NOx emissions as a result of the 

program.8 The amount of NOx emitted from affected units 

in the absence of the NBP was estimated using historical, 

unit-level NOx rates (from 2000, where available) and yearly measured heat input from 2003 through 2008.

EPA’s Response Surface Model (RSM) and Benefits Mapping and Analysis Program (BenMAP) were used to estimate the human health benefits per ton of NOx emissions 

reduced for both ozone9 and PM2.510. RSM scenarios were 

run to develop “benefit per ton” estimates, specifically for

NOx emission reductions in the power sector.

Figure 10 illustrates how many lives were estimated to 

be saved annually because of air quality improvements in 

ozone and PM2.5 as a result of the NBP. The results of this 

health benefits assessment are influenced by the amount of

NOx emissions reduced by NBP sources in a given year, the 

pollutant affected (ozone or PM2.5), and the health function 

that relates exposure to that pollutant with incidences of 

premature mortality. In addition, results are presented as a 

range in order to capture the variation across the 8 studies 

used in this analysis. 

This analysis shows that improvements in ozone air quality 

have led to fewer premature deaths annually. For example, 

in 2008 alone, between 130 and 600 lives were estimated 

to be saved because of decreases in ozone. In addition to 

these ozone benefits, improvements in PM2.5 protected an 

additional 450 to 1,200 lives in 2008. Therefore, as a result 

of reductions in NOx emissions due to the NBP, total human 

health benefits (for ozone and PM2.5) range from 580 to 

1,800 incidences of premature deaths avoided in 2008.

Changes in Ozone and Nitrate Impacts on 

Ecosystems

Ozone Impacts on Forests

Air pollution can impact the environment and affect ecological systems, leading to changes in the ecological community and influencing the diversity, health, and vigor of

individual species. Ozone has been shown in numerous 

studies to have a strong effect on the health of many plants, 

including a variety of commercial and ecologically important forest tree species throughout the United States.13

When ozone is present in the air, it can enter a plant through 

pores in its leaves and cause significant cellular damage.

This damage can compromise the ability of the plant to produce energy during photosynthesis. The remaining energy 

resources of the plant are then depleted as leaves attempt 

to repair or replace damaged tissue. This loss of energy resources can lead to reduced growth and/or reproduction 

of plants. Ozone stress also increases the susceptibility of 

plants to disease, insects, fungus, and other environmental 

stresses (e.g., harsh weather). Because ozone damage can 

also cause visible injury to leaves, it can reduce the aesthetic value of ornamental vegetation and trees, and negatively 

affect scenic vistas in protected natural areas. 

Assessing the impact of ozone on forests in the eastern 

United States involves understanding the risk to tree species from ambient ozone concentrations and accounting 

for the prevalence of those species within the forest. As a 

way to quantify the risk to particular trees, scientists have 

developed concentration-response (C-R) functions which 

relate ozone exposure to tree response. Tree C-R functions 

are determined by exposing tree seedlings to different 

ozone levels and measuring reductions in growth as “biomass loss.” In areas where certain species dominate the

forest community, the biomass loss from ozone can be significant. In this analysis, biomass loss is used as an indicator for the effects of ozone on the forest ecosystem.

Some of the common tree species in the eastern United 

States that are sensitive to ozone are black cherry (Prunus 

serotina), yellow-poplar (Liriodendron tulipifera), sugar 

maple (Acer saccharum), eastern white pine (Pinus strobus), 

Virginia pine (Pinus virginiana), red maple (Acer rubrum), 

and quaking aspen (Populus trenuloides). To estimate the 

Figure 10: Annual Incidences of Premature Mortality Avoided, 

2003-2008 Deaths Avoided

2003 2004 2005 2006 2007 2008

0

200

400

600

800

1000

1200

1400

Range of Ozone Benefits Range of PM2.5 Benefits

Notes:

•	 Ozone benefits were calculated using Ito, Schwartz, Bell 2004,

Bell 2005, Levy, and Huang.11

•	 PM2.5 benefits were calculated using Laden and Pope.12

Source: EPA, 2009

The NOx Budget Trading Program: 2008 Environmental Results

9
biomass loss for forest ecosystems across the eastern United States, the biomass loss for each of the seven tree species was calculated using the three-month, 12-hour W126 

exposure metric15 at each location, along with each tree’s 

individual C-R functions. The W126 exposure metric was 

calculated using monitored ozone data from CASTNET and 

AQS sites, and a three-year average was used to mitigate 

the effect of variations in meteorological and soil moisture 

conditions. The biomass loss estimate for each species was 

then multiplied by its prevalence in the forest community 

using the U.S. Department of Agriculture (USDA) Forest 

Service IV index of tree abundance calculated from Forest 

Inventory and Analysis (FIA) measurements.16 This analysis compared two time periods: 2000-2002 (before the 

NBP) and 2006-2008 (under the NBP) and demonstrates 

the benefit to forest ecosystems from decreasing ozone

concentrations. 

Since implementation of the NBP, the number of areas with 

significant biomass loss17 due to ozone has decreased for 

all seven tree species across their range in the East (see 

Figure 11). Of these seven species, the black cherry and 

yellow poplar are the most sensitive to ozone. Comparing data from 2000-2002 versus 2006-2008, EPA found 

that the total land area in the Eastern U.S. with significant

biomass loss has decreased by 5 percent for black cherry 

and by 4 percent for yellow poplar. In addition, areas with 

significant biomass loss for the remaining five species (red

maple, sugar maple, quaking aspen, Virginia pine, and eastern white pine) now make up less than 1 percent of their 

total range. While this change in biomass loss cannot be 

exclusively attributed to the implementation of the NBP, it 

is likely that NOx emission reductions achieved under the 

NBP and the corresponding decreases in ozone concentration contributed to this environmental improvement.

Note: Sources of uncertainty include the ozone-exposure/plant-response functions, the tree abundance index, and other factors 

(e.g., soil moisture). Although these factors were not considered, they can affect ozone damage.14

Source: EPA, 2009

Figure 11: Estimated Black Cherry, Yellow Poplar, Sugar Maple, Eastern White Pine, Virginia Pine, Red Maple, and Quaking Aspen 

Biomass Loss due to Ozone Exposure, 2000-2002 versus 2006-2008

Pre-NBP Implementation Average Biomass Loss, 2000-2002 Post-NBP Implementation Average Biomass Loss, 2006-2008 Pre-NBP Implementation Average Biomass Loss, 2000-2002 Post-NBP Implementation Average Biomass Loss, 2006-2008 Pre-NBP Implementation Average Biomass Loss, 2000-2002 Post-NBP Implementation Average Biomass Loss, 2006-2008

The NOx Budget Trading Program: 2008 Environmental Results

10
Figure 12: Shift in Seasonal Nitrate Concentrations in the NBP 

Region, 1990-2008

Notes: Total nitrate concentration data are from CASTNET sites 

that met completeness criteria and are located in and adjacent to 

the NBP region.

Source: EPA, 2009

1.5 

2.5

3.0 

3.5 

Nitrate Concentration (μg/m3)

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008

2.0

1.0 

0.5 

0.0 

Actual Model Estimate 95% Confidence Band

Changes In Nitrate

NOx is emitted from a source as nitric oxide (NO) and nitrogen dioxide (NO2). Once in the air, several chemical reactions occur, depending on meteorological conditions and 

concentrations of other pollutants in the atmosphere. NOx

contributes to the formation of many secondary pollutants, 

including particulate nitrate (NO3), nitric acid (HNO3), 

ozone, and organic compounds. For example, ozone is produced when NO2, volatile organic compounds (VOCs), and 

sunlight are present. 

Generally, NOx is removed directly from the atmosphere by 

dry deposition of nitric acid and particulate nitrates, and 

wet deposition of dissolved nitrates. Nitrate deposition can 

be harmful to sensitive ecosystems, vegetation, and water 

bodies by causing eutrophication, changes in biological 

communities, and an increased sensitivity to changes in 

the environment. Because the majority of NOx in the NBP 

region is removed from the atmosphere over a period of 

four to nine days, nitrogen deposition from transported 

NOx emissions may still affect areas that are considerable 

distances from NOx emission sources.18

As facilities install and use control technologies, reducing 

the amount of NOx emitted in the NBP region, the amount of 

NOx secondary pollutants also decreases. To determine the 

trend in total nitrate since the start of the NBP, an ARIMA 

model was used to assess changes in the average of median 

total nitrate concentrations as measured at CASTNET sites 

located in the NBP region during the ozone season (Figure 

12). The ARIMA model illustrates that between 1990 and 

2003, total nitrate concentrations averaged about 3 μg/m3. 

After 2004, a statistically significant shift occurred and a

new trend was established with an average concentration 

of 2 μg/m3. Similar to the shift observed for ozone concentrations, the ARIMA model shows a statistically significant,

33 percent (1 μg/m3) decrease in total nitrate since the 

start of the NBP, suggesting that the NBP was a significant

contributor to these improvements in total nitrate.

Endnotes

1	 Gilliland, A.B., Hogrefe, C., Pinder, R.W., Godowitch, J.M., Foley, K.L., & Rao, S.T. 2008. Dynamic evaluation of regional air quality models: Assessing changes in O3 stemming from changes in emissions and meteorology. Atmospheric Environment. 42:20, 5110-5123.

	 Godowitch, J., Gilliland, A.B., Draxter, R.R., & Rao, S.T. 2008. Modeling assessment of point source NOx emission reduction on ozone air quality in the eastern United States. Atmospheric Environment. 42:1, 87-100. 

	 Kim, S.W., Heckel, A., McKeen, S.A., Frost, G.J., Hsie, E.Y., Trainer, M.K., Richtec, A., Burrows, J.P., Peckham, S.E., & Grell, G.A. 2006. Satelliteobserved U.S. power plant NOx emission reductions and their impact on air quality. Geophysical Research Letters, 33, L22812.

2	 Cox, W. M. & Chu, S.H. 1996. Assessment of interannual ozone variation in urban areas from a climatological perspective. Atmospheric Environment. 30:16, 2615-2625. 

	 Camalier, L., Cox, W.M. & Dolwick, P. 2007. The effects of meteorology on ozone in urban areas and their use in assessing ozone trends. Atmospheric Environment. 41:33, 7127-7137. 

3 The seasonal average ozone concentration is calculated as the average of the daily maximum 8-hour ozone levels during the ozone season. 

These results provide a combined seasonal average for NBP states and do not show variations in ozone concentrations for specific urban or

rural areas.

4	 Gégo, E.P., P.S. Porter, A. Gilliland, & S.T. Rao. 2007. Observation-based assessment of the impact of nitrogen oxides emissions reductions

on ozone air quality over the eastern United States. Journal of Applied Meteorology and Climatology, special issue on the NOAA-EPA Golden 

Jubilee Symposium. 46(7):994-1008.

Gégo, E., et al. 2008. Modeling analyses of the effects of changes in nitrogen oxides emissions from the electric power sector on ozone air 

quality in the eastern United States. Journal of the Air & Waste Management Association, 58: 580-588.

The NOx Budget Trading Program: 2008 Environmental Results

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	 Godowitch, J., A.B. Gilliland, R.R. Draxler, & S.T. Rao. 2008. Modeling assessment of point source NOx emission reductions on ozone air quality 

in the eastern United States. Atmospheric Environment, 42: 87-100.

	 Godowitch, J.M., C. Hogfrefe, & S.T. Rao. 2008. Diagnostic analyses of a regional air quality model: Changes in modeled processes affecting

ozone and chemical-transport indicators from NO point source emission reductions. Journal of Geophysical Research, 113, D19303.

5	 40 CFR Part 81. Air quality designations and classification for the 8-hour ozone national ambient air quality standards (NAAQS).

6 U.S. Census. 2000.

7 PM2.5 Mortality, All Cause

	 Pope, C.A., III, Burnett, R.T., Thun, M.J., Calle, E.E., Krewski, D., Ito, K., and Thurston, G.D. 2002. Lung cancer, cardiopulmonary mortality, and

long-term exposure to fine particulate air pollution. JAMA 287 (9): 1132-41.

	 Laden, F., Schwartz, J., Speizer, F.E., and Dockery, D.W. 2006. Reduction in Fine Particulate Air Pollution and Mortality: Extended follow-up of

the Harvard Six Cities Study. American Journal of Respiratory and Critical Care Medicine 173: 667-672.

O3 Mortality, Non-Accidental

	 Ito, K., De Leon, S.F., and Lippmann, M. 2005. Associations between ozone and daily mortality: analysis and meta-analysis. Epidemiology

16(4): 446-57.

Schwartz, J. 2005. How sensitive is the association between ozone and daily deaths to control for temperature? American Journal of Respiratory and Critical Care Medicine 171 (6): 627-31.

	 Bell, M.L., McDermott, A., Zeger, S.L., Samet, J.M., and Dominici, F. 2004. Ozone and short-term mortality in 95 US urban communities, 1987-

2000. JAMA 292(19): 2372-8.

O3 Mortality, All Cause

	 Levy, J.I., Chemerynski, S.M., and Sarnat, J.A. 2005. Ozone exposure and mortality: an empiric bayes metaregression analysis. Epidemiology

16(4): 458-68.

	 Bell, M.L., Dominici, F., and Samet, J.M. 2005. A meta-analysis of time-series studies of ozone and mortality with comparison to the national

morbidity, mortality, and air pollution study. Epidemiology 16 (4): 436-45.

O3 Mortality, Cardiopulmonary

	 Huang, Y., Dominici, F., and Bell, M. L. 2005. Bayesian hierarchical distributed lag models for summer ozone exposure and cardio-respiratory

mortality. Environmetrics 16: 547–562.

8	 The NBP regional cap was used to represent emissions for 2007 and 2008 in order to reduce the influence of early reductions from CAIR.

9 Hubbell, B., Dolwick, P., Mooney, D., and Morara, M. 2005. Evaluating the Relative Effectiveness of Ozone Precursor Controls: Design of Computer Experiments Applied to the Comprehensive Air Quality Method with Extensions (CAMX). Proceedings of the Air Quality V Conference, 

Arlington, VA.

	 Abt Associates Inc. 2008. Environmental Benefits Mapping and Analysis Program (Version 3.0). Bethesda, MD. Prepared for Environmental

Protection Agency, Office of Air Quality Planning and Standards, Innovative Strategies and Economic Group. Research Triangle Park, NC.

10	 Fann, N., Fulcher, C.M., and Hubbell, B.J. 2009. The influence of location, source, and emission type in estimates of the human health benefits

of reducing a ton of air pollution. Air Quality, Atmosphere & Health. ISSN 1873-9318.

11	 Ito et al., Schwartz, Bell et al. 2004, Bell et al. 2005, Levy et al., and Huang.

12	 Laden et al. and Pope et al.

13	 U.S. EPA. 2007. Review of the National Ambient Air Quality Standards for Ozone, Policy Assessment of Scientific and Technical Information.

OAQPS Staff Paper. EPA-452/R-07-003. This document is available in Docket EPA-HQ-OAR-2003-0190 and online at <http://www.epa.gov/

ttn/naaqs/standards/ozone/s_o3_cr_sp.html>.

14	 Chappelka, A.H. and Samuelson, L.J. 1998. Ambient ozone effects on forest trees of the eastern United States: A review. New Phytologist 139: 

91-108.

15 The W126 exposure metric is a cumulative (not average) exposure index that is biologically based and places a greater weight on the higher 

hourly ozone concentrations.

16	 Prasad, A.M. and Iverson, L.R. 2003. Little’s range and FIA importance value database for 135 eastern U.S. tree species. <http://www.fs.fed.

us/ne/delaware/4153/global/littlefia/index.html>, Northeastern Research Station, USDA Forest Service.

17	 Areas with more than 2 percent biomass loss are defined here as significant based on a consensus workshop on ozone effects, which reported that a 2 percent annual biomass loss causes harm because of the potential for compounding effects over multiple years as short-term 

negative effects on seedlings affect long-term forest health. See:

	 Heck, W.W. &Cowling E.B. 1997. The need for a long term cumulative secondary ozone standard – an ecological perspective. Environmental 

Management, January, 23-33.

18	 Seinfeld, J.H. & Pandis, S.N. 1998. Atmospheric Chemistry and Physics. John Wiley and Sons, Inc. New York.

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