The N0X Budget Trading Program:
2008 Environmental Results
The NOx Budget Trading Program (NBP) was a mar-
ket-based cap and trade program created to reduce
the regional transport of emissions of nitrogen ox-
ides (NQX) from power plants and other large combustion
sources that contribute to ozone nonattainment in the east-
ern 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 de-
signed to reduce NOx emissions during the warm summer
months, referred to as the ozone season, when ground-lev-
el 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 reduc-
tions 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: . Detailed emission
results and other facility and allowance data are also
publicly available on EPA's Data and Maps Web site at
. To view emission and
other facility information in an interactive format using
Google Earth or a similar three-dimensional platform, go to
.
The NBP and Ozone
EPA has developed more than a dozen programs to reduce
N0X and volatile organic compounds (VOCs) from mobile,
industrial, and power sector sources since 1990. Between
1999 and 2002, states in the Northeast reduced N0X emis-
sions from electric generating units (EGUs) and industrial
boilers through the OTC N0X 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 imple-
mentation of the NBP in 2003
Analyses of ozone trends using various metrics show
regionwide ozone reductions ranging from 10-14
percent 111 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 monitor-
ing data, ozone air quality improved in almost all of the
104 areas in the eastern U.S. designated to be in nonat-
tainment for the 1997 8-hour National Ambient Air Qual-
ity 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 con-
centrations due to the NBP have contributed to improve-
ments 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 com-
rnA United States
Environmental Protection
Eh I Agency
September 2009
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The N0X Budget Trading Program: 2008 Environmental Results
plement state and local efforts to improve ozone air quality
and meet national standards.
While these programs, particularly the mobile source pro-
grams, 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 meteoro-
logical conditions (such as temperature) and NOx emissions
from NBP sources. This report also explores changes in hu-
man 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 lev-
els 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 de-
pict 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 met-
rics 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 three-
year average of the fourth highest daily maximum
8-hour ozone concentration is used to designate non-
attainment status in the United States.
CASTNET
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 three-
stage 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 lev-
els from CASTNET sites have been used to as-
sess 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 con-
centrations. These sites met data completeness
criteria and are located in NBP states or within
200 kilometers (km) of an NBP state's border.
Figure 1: Location of CASTNET Sites
Note: States in the NBP region are shaded.
Source: EPA, 2009
2
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The N0X Budget Trading Program: 2008 Environmental Results
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 statisti-
cally similar to the 4th highest daily maximum 8-hour
ozone concentrationthe ozone NAAQS. This metric
is representative of true, measured ozone concentra-
tions 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 implementa-
tion of the program in 2003. These results are further sup-
ported by several studies investigating the impact of NOx
emission reductions from NBP sources on ozone in the re-
gion.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 concen-
trations 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 concen-
trations, 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 (be-
fore 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 peri-
ods, with an average reduction in ozone concentrations in
NBP states of 11 percent.
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 im-
pacts 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, automo-
tive, and power generation sources) than urban measure-
ments. 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 con-
centrations can therefore be attributed to reductions in re-
gional 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 roll-
ing 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).
Ozone
99th Percentile
(% change)
| <-20
| -19 - -15
-9 - -5
l^-o
I I 1-5
I 16-10
¦ 16-20
3
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Regional Trends in Ozone
An Autoregressive Integrated Moving Average (ARIMA)
model was used to determine the trend in ozone concen-
trations 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 con-
centrations (the highest daily levels of ozone) measured
at CASTNET sites during the NBP ozone season were mod-
eled (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
1101i ii
100
I 40
<§ 30
20
10
0 -
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
o 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
Site-Specific Changes in Ozone
Similar trends in rural ozone concentrations were ob-
served at individual CASTNET monitoring sites. As ex-
pected, 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 Vir-
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 peri-
ods, 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
Figure 4: Percent Reduction in Rural 1-Hour Ozone during the
Ozone Season, 2000-2002 versus 2006-2008
0 Between -10% and -15%
O Between -5% and -10%
O Between -1 % and -5%
~ Statistically Significant Reductions
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 cri-
teria and are located in or adjacent to the NBP region.
Source: EPA, 2009
4
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The N0X Budget Trading Program: 2008 Environmental Results
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 concentra-
tions 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 sea-
son) 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 no-
ticeable shift toward lower ozone concentration during the
ozone season (represented by the red arrow). As NOx emis-
sion controls were turned on at sources subject to the NBP,
there have been fewer days with high levels of ozone dur-
ing 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
Outside of the Ozone Season (January) During the Ozone Season (June)
Ozone concentration 2000-2002 Ozone concentration 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
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 10ppb
July
Down 9 ppb
August
Down 8 ppb
September
Down 8 ppb
October
Down 10ppb
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.
5
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The N0X Budget Trading Program: 2008 Environmental Results
Changes in 8-Hour Ozone Concentrations
Daily maximum 8-hour ozone concentration data were as-
sessed 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 con-
centration 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. Ru-
ral areas are represented by a single monitoring site. For more
information on AQS, visit .
For more information about CASTNET, visit .
Source: EPA, 2009
Ozone Changes after Adjusting for Meteorology
As noted earlier, weather plays an important role in deter-
mining 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 dif-
ferent 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 method-
ology and the subsequent ozone estimates are provided by
EPA's Office of Air Quality Planning and Standards (OAQPS),
Air Quality Assessment Division .
Figure 7 shows trends in the seasonal average 8-hour ozone
concentrations in the NBP region with and without consid-
ering the influence of weather.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 tem-
peratures in the NBP region during the 2004 ozone season
dampened ozone formation, while higher temperatures in
the 2007 ozone season increased ozone formation. Remov-
ing 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 condi-
Figure 7: Seasonal Average 8-Hour Ozone Concentrations in the
NBP Region, 1997-2008
70
0 1 1 1 1 1 1 1 1 1 1
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008
Adjusted for Weather - Unadjusted for Weather
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
6
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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 av-
erage 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 meteorologically-
adjusted ozone reveals a 14 percent reduction between
2002 and 2008.
Figure 8: Reductions in Seasonal N0 Emissions from NBP Sources and Changes in 8-Hour Ozone, 2002 versus 2008
© 2009]Eurqpa Technologies
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Percent Change in Meteorologically
Adjusted Seasonal 8-hr Ozone
2002 Versus 2008
Decrease Between ) 5*: and
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Oione Season NOm emissions,
2002 versus 2008 (tons)
H Inc Ttaw* Le» Than 30
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Decrease Between 50,000 and 75.000
Decu^iween 7iQP0and lOtJOO
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, includ-
ing unit-level emissions and controls, visit the Clean Air Market Division's interactive mapping site at .
Source: EPA, 2009
7
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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 be-
ing 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 emis-
sions from NBP sources and reductions in 8-hour average
ozone after implementation of the NBP. As indicated previ-
ously, 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 oc-
curred 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 gath-
ered 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 per-
cent of the original nonattainment areas in the East now
have ozone air quality that is better than the standard un-
der which they were originally designated nonattainment.
These improvements bring cleaner air to over 57 million
people. The majority of these areas have officially been re-
designated 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 ex-
ceed the level of the standard. In 13 of these areas, how-
ever, ozone concentrations have fallen by an average of 10
percent. Because of these reductions in ozone, over 46 mil-
lion Americans living in these areas are experiencing bet-
ter air quality.
Figure 9: Changes in Ozone Nonattainment Areas in the East,
2001-2003 (Original Designations) versus 2006-2008
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)
Note: States in the NBP region are shown inside the black bound-
ary line.
Source: EPA, 2009
Given that the majority of relevant NOx emission reduc-
tions 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 respira-
tory and cardiovascular effects as well as incidences of pre-
mature mortality. The analysis presented here is a screen-
ing-level estimate of the annual human health benefits (the
number of premature deaths avoided annually) from NOx
emission reductions achieved under the NBP.
8
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The N0X Budget Trading Program: 2008 Environmental Results
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 year-
ly measured heat input from 2003 through 2008.
EPA's Response Surface Model (RSM) and Benefits Map-
ping and Analysis Program (BenMAP) were used to esti-
mate the human health benefits per ton of NOx emissions
reduced for both ozone9 and PM2510. 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 PM25), 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,
Figure 10: Annual Incidences of Premature Mortality Avoided,
2003-2008
1400
2003 2004 2005
Range of Ozone Benefits
2006 2007 2008
¦ 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
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 PM25 protected an
additional 450 to 1,200 lives in 2008. Therefore, as a result
of reductions in N0X 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 eco-
logical systems, leading to changes in the ecological com-
munity 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 impor-
tant 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 pro-
duce 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 re-
sources 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 aesthet-
ic 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 spe-
cies 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 "bio-
mass loss." In areas where certain species dominate the
forest community, the biomass loss from ozone can be sig-
nificant. In this analysis, biomass loss is used as an indica-
tor 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 (Liriodendrort tulipifera), sugar
maple [Acer saccharum), eastern white pine (Pinusstrobus),
Virginia pine (Pinus virginiana), red maple (Acer rubrum),
and quaking aspen (Populus trenuloides). To estimate the
9
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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
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
biomass loss for forest ecosystems across the eastern Unit-
ed States, the biomass loss for each of the seven tree spe-
cies 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 analy-
sis 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. Compar-
ing 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 east-
ern 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 concentra-
tion contributed to this environmental improvement.
10
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Changes In Nitrate
N0X is emitted from a source as nitric oxide (NO) and ni-
trogen dioxide (NO2). Once in the air, several chemical re-
actions occur, depending on meteorological conditions and
concentrations of other pollutants in the atmosphere. N0X
contributes to the formation of many secondary pollutants,
including particulate nitrate (NO3), nitric acid (HNO3),
ozone, and organic compounds. For example, ozone is pro-
duced when NO2, volatile organic compounds (VOCs), and
sunlight are present.
Generally, N0X 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 N0X in the NBP
region is removed from the atmosphere over a period of
four to nine days, nitrogen deposition from transported
N0X emissions may still affect areas that are considerable
distances from N0X emission sources.18
As facilities install and use control technologies, reducing
the amount of N0X emitted in the NBP region, the amount of
N0X 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 ng/m3.
After 2004, a statistically significant shift occurred and a
new trend was established with an average concentration
of 2 ng/m3. Similar to the shift observed for ozone concen-
trations, the ARIMA model shows a statistically significant,
33 percent (1 ng/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.
Figure 12: Shift in Seasonal Nitrate Concentrations in the NBP
Region, 1990-2008
o Actual Model Estimate ~ 95% Confidence Band
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
Endnotes
1 Gilliland, A.B., Hogrefe, C., Pinder, R.W., Godowitch, J.M., Foley, K.L., & Rao, ST. 2008. Dynamic evaluation of regional air quality models: As-
sessing changes in 03 stemming from changes in emissions and meteorology. Atmospheric Environment. 42:20, 5110-5123.
Godowitch, J., Gilliland, A.B., Draxter, R.R., & Rao, ST. 2008. Modeling assessment of point source NOx emission reduction on ozone air qual-
ity 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. Satellite-
observed 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 Envi-
ronment. 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. At-
mospheric 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 Gego, E.P., P.S. Porter, A. Gilliland, & ST. 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.
Gego, 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.
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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 Dockeiy, D.W. 2006. Reduction in Fine Particulate Air Pollution and Mortality: Extended follow-up of
the Harvard Six Cities Study. American Journal of Respiratoiy and Critical Care Medicine 173: 667-672.
03 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? Ameri can Journal of Respira-
toiy 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.
03 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.
03 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 Com-
puter 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 . 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 re-
ported 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 secondaiy ozone standard - an ecological perspective. Environmental
Management, January, 23-33.
18 Seinfeld, J.H. & Pandis, S.N. 1998. Atmospheric Chemistiy and Physics. John Wiley and Sons, Inc. New York.
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