Acid Rain and Related Programs:
2008 Environmental Results
The Acid Rain Program (ARP), established under Title
IV of the 1990 Clean Air Act Amendments, requires
major emission reductions of sulfur dioxide (S02)
and nitrogen oxide (NOX), the primary precursors of acid
rain, from the electric power industry. The SC>2 program
sets a permanent cap on the total amount of SC>2 that may
be emitted by electric generating units (EGUs) in the con-
tiguous United States, and includes provisions for trading
and banking allowances. The program is phased in, with
the final 2010 S02 cap set at 8.95 million tons, a level of
about one-half of the emissions from the power sector in
1980. NOX reductions under the ARP are achieved through
a program that applies to a subset of coal-fired EGUs and is
closer to a more traditional, rate-based regulatory system.
The emission reductions achieved under the ARP have led
to important environmental and public health benefits.
These include improvements in air quality with significant
benefits to human health; reductions in acid deposition;
the beginnings of recovery from acidification in fresh water
lakes and streams; improvements in visibility; and reduced
risk to forests, materials, and structures. Table 1 on page
2 shows the regional changes in key air quality and atmo-
spheric deposition measurements linked to the ARP's SC>2
and NOX emission reductions.
During 2009, EPA is releasing a series of reports
summarizing progress under the ARP. This third report
compares changes in emissions to changes in air quality,
acid deposition, and surface water chemistry. For more
information on the ARP, please visit: .
Air Quality
Sulfur Dioxide
Data collected from monitoring networks show that the
decline in SC>2 emissions from the power industry has
improved air quality. Based on EPA's latest air emission
trends data located at . the national composite average of SC>2 annual mean
ambient concentrations decreased 71 percent between
1980 and 2008, as shown in Figure 1 on page 3 (based on
At a Glance: ARP Results in 2008
Air Quality: Emission reductions achieved under the
ARP have led to improvements in air quality with signifi-
cant benefits to human health.
• Between 1989-1991 and 2006-2008 average ambi-
ent sulfate concentrations have decreased by 38 per-
cent in the Mid-Atlantic, 44 percent in the Midwest,
43 percent in the Northeast, and 28 percent in the
Southeast.
Acid Deposition: Monitoring data show significant im-
provements in the primary acid deposition indicator.
• Between the 1989 to 1991 and 2006 to 2008 obser-
vation periods, average decreases in wet deposition
of sulfate averaged more than 30 percent for the
eastern United States.
• Reduction of total sulfur deposition [wet plus dry
deposition) during the 1989 to 1991 and 2006 to
2008 observation periods has been even more dra-
matic, with average reductions of about 40 percent.
Surface Water Chemistry: Long-term surface water
monitoring programs indicate trends toward recovery
from acidification.
• Levels of Acid Neutralizing Capacity (ANC), the
ability of a water body to neutralize acid deposition,
have increased significantly from 1990 to 2007 in
lake and stream long-term monitoring sites in New
England, the Adirondack Mountains, and the North-
ern Appalachian Plateau.
• Although water quality has improved, many lakes
and streams still have acidic conditions harmful
to their biota and further emission reductions are
needed for full ecosystem protection and recovery of
sensitive aquatic systems.
state, local, and EPA monitoring sites located primarily
in urban areas). The largest single-year reduction (20
percent) occurred in the first year of the ARP, between 1994
and 1995. These trends are consistent with the regional
ambient air quality trends observed in the Clean Air Status
and Trends Network (CASTNET).
United States
Environmental Protection
Agency
October 2009
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Acid Rain and Related Programs: 2008 Environmental Results
Table 1: Regional Changes in Air Quality and Deposition of Sulfur and Nitrogen Compounds, 1989-1991 versus 2006-2008, from Rural
Monitoring Networks
Measurement Region Average, 1989-1 991 Average, 2006-2008 PercentChange Number of Sites
Ambient S02 Concentra-
tion (ug/m3)
Ambient Sulfate Concen-
tration (ug/m3)
Wet Sulfate Deposition
(kg-S/ha)
Dry Sulfur Deposition
(kg-S/ha)
Total Sulfur Deposition
(kg-S/ha)
Total Ambient Nitrate
Concentration (Nitrate +
Nitric Acid) (ug/m3)
Wet Inorganic Nitrogen
Deposition (kg-N/ha)
Dry Inorganic Nitrogen
Deposition (kg-N/ha)
Total Inorganic Nitrogen
Deposition (kg-N/ha)
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
Mid-Atlantic
Midwest
Northeast
Southeast
13
11
5.5
5.3
6.4
5.9
3.5
5.3
9.2
7.1
7.5
6.1
6.3
7
4
1.2
16
16
11
8
3.2
4.6
1.7
2.2
6.2
5.8
5.6
4.4
2.4
2.7
1.8
0.88
8.5
9.3
6.6
5.9
6
5
2.1
2.9
4
3.3
2
3.8
6.3
4.5
5
3.9
3.3
3.4
1.5
0.8
10
9
6
5.3
2.2
3.3
1
1.7
4.9
5.2
4.4
3.5
1.6
2
0.9
0.96
6.4
7.5
4.8
4.9
-54
-55
-62
-45
-38
-44
-43
-28
-32
-37
-33
-36
-48
-51
-63
-33
-38
-44
-45
-34
-31
-28
-41
-23
-21
-10
-21
-20
-33
-26
-50
9
-25
-19
-27
-17
12
10
3
9
12
10
3
9
11
27
17
23
8
9
2
2
8
9
2
2
12
10
3
9
11
27
17
23
9
9
2
2
9
9
2
2
Notes:
• Averages are the arithmetic mean of all sites in a region that were present and met the completeness criteria in both averaging periods. Thus,
average concentrations for 1989-1991 may differ from past reports.
• Total deposition is estimated from raw measurement data, not rounded, and may not equal the sum of dry and wet deposition.
• Percent change in bold indicates that differences were statistically significant at the 95 percent confidence level. Changes that are not statis-
tically significant may be unduly influenced by measurements at only a few locations or large variability in measurements.
Source: EPA, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
During the late 1990s, following implementation of
Phase I of the ARP, dramatic regional improvements in
SC>2 and ambient sulfate concentrations were observed at
CASTNET sites throughout the eastern United States, and
these improvements continue today. Analyses of regional
monitoring data from CASTNET show the geographic
pattern of SC>2 and airborne sulfate in the eastern United
States. Three-year mean annual concentrations of SC>2
and sulfate from CASTNET long-term monitoring sites are
compared from 1989 to 1991 and 2006 to 2008 in both
tabular form and graphically in maps (see Table 1 on
page 2, Figure 2 on page 3, and Figure 3 on page 4).
The maps in Figure 2 show that from 1989 to 1991, prior
to implementation of Phase I of the ARP, the highest annual
ambient concentrations of SC>2 in the East were observed
in western Pennsylvania and along the Ohio River Valley.
The maps indicate a significant decline in those concentra-
tions in nearly all affected areas after implementation of
the ARP and other programs.
Before the ARP, in 1989-1991, the highest annual ambient
sulfate concentrations were observed in western Pennsyl-
vania, along the Ohio River Valley, and in northern Alabama
at levels greater than 11 micrograms per cubic meter (ug/
m3). Most of the eastern United States experienced annual
ambient sulfate concentrations greater than 5 ug/m3.
Figure 1: National S02 Air Quality, 1980-2008
0.04
National Ambient Air
Quality Standard
— 90% of sites have
concentrations below this line
Source: EPA, 2009
Average Concentration
• 10% of sites have
concentrations below this line
Figure 2: Annual Mean Ambient S02 Concentration
1989-1991
2006-2008
Notes:
• For maps depicting these trends for the entire continental United
States, visit .
• Dots on all maps represent monitoring sites. Lack of shading for
southern Florida indicates lack of monitoring coverage in the
1989-1991 period.
Source: CASTNET, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
Figure 3: Annual Mean Ambient Sulfate Concentration
1989-1991
Figure 4: Annual Mean Ambient Total Nitrate Concentration
1989-1991
2006-2008
2006-2008
Notes:
• For maps depicting these trends for the entire continental United States, visit .
• Dots on all maps represent monitoring sites. Lack of shading for southern Florida indicates lack of monitoring coverage in the 1989-1991
period.
Source: CASTNET, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
Like SC>2 concentrations, ambient sulfate concentrations
have decreased since the program was implemented, with
average concentrations decreasing from 28 to 44 percent in
regions of the East (see Table 1 on page 2). Both the magni-
tude and spatial extent of the highest concentrations have
dramatically declined, with the largest decreases observed
along the Ohio River Valley (see Figure 3 on page 4).
Nitrogen Oxides
Although the ARP has met its NOX emission reduction tar-
gets, emissions from other sources (such as motor vehicles
and agriculture) contribute to ambient nitrate concen-
trations in many areas. NOX levels can also be affected by
emissions transported via air currents over wide regions.
From 2006 to 2008, reductions in NOX emissions during
the ozone season from power plants under the NOX SIP
Call have continued to result in significant region-specific
improvements in ambient total nitrate (NOs" plus HNOs)
concentrations. For instance, annual mean ambient total
nitrate concentrations for 2006 to 2008 in the Mid-Atlantic
region were 31 percent less than the annual mean concen-
tration in 1989 to 1991 (see Table 1 on page 2 and Figure
4 on page 4). While these improvements might be partly
attributed to added NOX controls installed for compliance
with the NOX SIP Call, the findings at this time are not con-
clusive.
Acid Deposition
National Atmospheric Deposition Program/National Trends
Network (NADP/NTN) monitoring data show significant
improvements in the primary acid deposition indicators.
For example, wet sulfate deposition (sulfate that falls to the
earth through rain, snow, and fog) has decreased since the
implementation of the ARP in much of the Ohio River Val-
ley and northeastern United States. Some of the greatest
reductions have occurred in the mid-Appalachian region,
including Maryland, New York, West Virginia, Virginia, and
most of Pennsylvania. Other less dramatic reductions have
been observed across much of New England, portions of
the southern Appalachian Mountains, and some areas of
the Midwest. Between the 1989 to 1991 and 2006 to 2008
observation periods, average decreases in wet deposition
of sulfate averaged more than 30 percent for the eastern
United States (see Table 1 on page 2 and Figure 5). Along
with wet sulfate deposition, wet sulfate concentrations
have also decreased by similar percentages. A strong cor-
relation between large-scale S02 emission reductions and
large reductions in sulfate concentrations in precipitation
has been noted in the Northeast, one of the areas most af-
fected by acid deposition. The reduction in dry and total
sulfur deposition (wet plus dry) has been even more dra-
matic than that of wet deposition in the Mid-Atlantic and
Midwest, with reductions of 38 and 44 percent, respec-
Figure 5: Annual Mean Wet Sulfate Deposition
1989-1991
2006-2008
Source: NADP, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
About Long-term Ambient and Deposition Monitoring Networks
To evaluate the impact of emission reductions on the en-
vironment, scientists and policymakers use data collect-
tal monitoring. Information and data from CASTNET
are available at .
ed from long-term national monitoring networks such
as CASTNET and the NADP/NTN. These complementary,
long-term monitoring networks provide information on
a variety of indicators necessary for tracking temporal
and spatial trends in regional air quality and acid depo-
sition (see Table 2).
CASTNET provides atmospheric data on the dry depo-
sition component of total acid deposition,
ground-level
ozone, and other forms of atmospheric pollution. Es-
tablished in 1987, CASTNET now consists
of more than
80 sites across the United States. EPA's Office of Air and
Radiation operates 50 of the monitoring
National Park Service (NFS) funds and
stations; the
operates ap-
proximately 30 stations in cooperation with EPA. Many
CASTNET sites have a continuous 20-year data record,
reflecting EPA's commitment to long-term
environmen-
NADP/NTN is a nationwide, long-term network
tracking the chemistry of precipitation. NADP/NTN
provides concentration and wet deposition data on
hydrogen ion (acidity as pH), sulfate, nitrate, ammo-
nium, chloride, and base cations. The network is a co-
operative effort involving many groups, including the
State Agricultural Experiment Stations, U.S. Geological
Survey (USGS), U.S. Department of Agriculture, EPA,
NFS, the National Oceanic and Atmospheric Adminis-
tration (NOAA), and other governmental and private
entities. NADP/NTN has grown from 22 stations at
the end of 1978 to more than 250 sites spanning the
continental United States, Alaska, Puerto Rico, and
the Virgin Islands. Information and data from NADP/
NTN are available at .
Table 2: Air Quality and Acid Deposition Measures
Chemical Name
Sulfur Dioxide
Sulfate Ion
Nitrate Ion
Nitric Acid
Ammonium Ion
Ionic Hydrogen
Calcium
Magnesium
Potassium
Sodium
Chemical Measured in:
Symbol Ambient Air
S02
S042-
N03-
HN03
NH4+
H+
Ca2+
Mg2+
K+
Na+
X
X
X
X
X
X
X
X
X
Source: EPA, 2009
Wet Deposition
X
X
X
X
X
X
X
X
Why are these measured by the networks? •
Primary precursor of wet and dry acid deposition; primary precursor to fine particles in
many regions.
Major contributor to wet acid deposition; major component of fine particles in the
Midwest and East; can be transported over large distances; formed from reaction of S02
in the atmosphere.
Contributor to acid and nitrogen wet deposition; major component of fine particles in
urban areas; formed from reaction of NOX in the atmosphere.
Strong acid and major component of dry nitrogen deposition; formed as a secondary
product from NOX in the atmosphere.
Contributor to wet and dry nitrogen deposition; major component of fine particles;
provides neutralizing role for acidic compounds; formed from ammonia gas in the
atmosphere.
Indicator of acidity in precipitation; formed from the reaction of sulfate and nitrate in
water.
These base cations neutralize acidic compounds in precipitation and the environment;
also play a major role in plant nutrition and soil productivity.
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Acid Rain and Related Programs: 2008 Environmental Results
Figure 6: Annual Mean Wet Inorganic Nitrogen Deposition
1989-1991
2006-2008
Source: NADP, 2009
lively (see Table 1 on page 2). Because continuous data re-
cords are available from only a few sites in the Northeast
and Southeast, it is unclear if the observed reductions in
total deposition are representative for those regions.
A principal reason for reduced sulfate deposition in the
Northeast is a reduction in the long-range transport of sul-
fate from emission sources located in the Ohio River Val-
ley. The reductions in sulfate documented in the Northeast,
particularly across New England and portions of New York,
were also affected by SC>2 emission reductions in eastern
Canada. NADP data indicate that similar reductions in pre-
cipitation acidity, expressed as hydrogen ion (H+) concen-
trations, occurred concurrently with sulfate reductions,
with reductions of 30 to 40 percent over much of the East.
Reductions in nitrogen deposition recorded since the early
1990s have been less pronounced than those for sulfur. As
noted earlier, emission trends from source categories oth-
er than ARP sources significantly affect air concentrations
and deposition of nitrogen. Inorganic nitrogen in wet depo-
sition decreased commensurately in the Mid-Atlantic and
Northeast (see Figure 6). Decreases in dry and total inor-
ganic nitrogen deposition at CASTNET sites have generally
been greater than that of wet deposition, with a 25 and 19
percent decrease in total nitrogen deposition for the Mid-
Atlantic and Midwest, respectively (see Table 1 on page 2).
Other source sectors and pollutants, particularly agricul-
ture and ammonium respectively, affect nitrogen transport
and deposition.
Ambient Mercury Monitoring
The NADP membership of federal agencies, states, tribes,
academic institutions, industry, and other organizations
have established a new network to measure atmospheric
concentrations of mercury throughout the U.S. The focus
of this effort is to develop national capacity to monitor the
three ambient mercury species—gaseous oxidized mer-
cury (COM), particulate-bound mercury (PBM2.s), and gas-
eous elemental mercury (GEM). Datasets generated from
this network are used to estimate mercury dry deposition,
assess mercury source/receptor relationships, evaluate at-
mospheric models, and determine long-term trends. Cur-
rently 20 sites provide high-resolution, high-quality atmo-
spheric data. In 2009, seven monitoring sites were added
to the network, some of which were co-sponsored by EPA,
including sites operated by the New Jersey Department of
Environmental Protection, University of Utah, University
of New Hampshire, and others. High quality speciated data
are available through the NADP Atmospheric Mercury Net-
work webpage. NADP also began supporting a site liaison
to oversee quality assurance of network data by manually
examining raw instrument data and performing annual site
visits. NADP worked with site operators and the broader
mercury scientific community to create a field standard op-
erating procedure (SOP) for monitoring atmospheric mer-
cury species in a network mode to ensure cross-network
data comparability. Additionally, NADP has developed a
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Acid Rain and Related Programs: 2008 Environmental Results
data management SOP to provide routine automated qual-
ity assured data and display network data products on the
web.
The NADP Atmospheric Mercury Networkis one component
of a broader effort to establish a more comprehensive,
integrated mercury monitoring network called MercNet.
A May 2008 National Mercury Monitoring Workshop was
an important step in building further broad support for
MercNet. The workshop included participants from federal
agencies (EPA, USGS, NOAA, U.S. Fish and Wildlife Service,
NFS), state and tribal agencies, the NADP, industry, and
academic and private research institutions. Workshop
participants agreed on the overall goal of a mercury
monitoring network: "To establish an integrated, national
network to systematically monitor, assess, and report
on policy-relevant indicators of atmospheric mercury
concentrations and deposition, and mercury levels in land,
water, and biota in terrestrial, freshwater, and coastal
ecosystems in response to changing mercury emissions
over time." Workshop scientists considered the conceptual
framework for MercNet to include national distribution of
sites to understand the sources, consequences, and changes
in U.S. mercury pollution. The design elements of this
effort include a national distribution of sites; monitoring
mercury concentrations within air, water, fish, sediments
and wildlife at each site; and a network that builds on
existing monitoring efforts, where possible, to maximize
information, benefits, coordination, and efficiency.
Collaboration and partnerships among existing mercury
science and monitoring programs are integral to MercNet.
For more information, visit the NADP MercNet website:
.
Figure 7: Ambient Mercury Monitoring Locations
Source: NADP, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
For a copy of the 2008 MercNet National Mercury Monitor-
ing Workshop Report, visit: .
Improvements in Surface Water Chemistry
Acid rain, resulting from SC>2 and NOX emissions, is one of
many large-scale anthropogenic effects that negatively af-
fect the health of lakes and streams in the United States.
Surface water chemistry provides direct indicators of the
potential effects of acidic deposition on the overall health
of aquatic ecosystems. Long-term surface water monitor-
ing networks provide information on the chemistry of
lakes and streams and on how water bodies are respond-
ing to changes in emissions. Since the implementation of
the ARP, scientists have measured changes in some lakes
and streams in the eastern United States and found signs
of recovery in many, but not all, of those areas (see Figures
8-10 on pages 9 and 10).
Two EPA-administered monitoring programs provide in-
formation on the effects of acid rain on aquatic systems:
the Temporally Integrated Monitoring of Ecosystems
(TIME) program and the Long-Term Monitoring (LTM) pro-
gram. These programs were designed to track the effect of
the 1990 Clean Air Act (CAA) Amendments in reducing the
acidity of surface waters in four regions: New England, the
Adirondack Mountains, the Northern Appalachian Plateau,
and the Ridge and Blue Ridge Province. The surface water
chemistry trend data in the four regions monitored by the
TIME and LTM programs are essential for tracking the eco-
logical response to ARP emission reductions.
The data presented here show regional trends in acidifi-
cation from 1990 to 2007 in lakes and streams sampled
through the LTM program (see Figures 8-10 on pages 9
and 10). Only sites that have a complete data record for the
time period are represented. Three indicators of acidity in
surface waters are presented—measured ions of sulfate
and nitrate and acid neutralizing capacity (ANC). These
indicators provide information regarding both sensitivity
to surface water acidification and the level of acidification
that has occurred today and in the past. Trends in these
chemical receptors allow for the determination of whether
the conditions of the water bodies are improving and head-
ing towards recovery or if the conditions are degrading.
Significant trends are statistically significant at the 95%
confidence interval (p<0.05).
Measurements of sulfate ion concentrations in surface
waters provide important information on the extent of
cation leaching in soils and how sulfate concentrations
relate to deposition and to the levels of ambient atmospheric
sulfur.
In June 2009, the regional TIME and LTM coopera-
tors met for a workshop at Penn State Univeristy.
The goals of the workshop were to exchange science
between regions; to discuss ways to improve pro-
gram management, relevance, and visibility; to clari-
fy current goals and objectives of the programs; and
to envision future goals and operations. For more
information on the TIME and LTM programs, visit:
.
Assessments of acidic deposition effects dating from the
1980s to the present have shown sulfate to be the primary
negatively charged ion in most acid-sensitive waters.1 Ni-
trate has the same potential as sulfate to acidify drainage
waters and leach acidic aluminum cations from watershed
soils. In most watersheds, however, nitrogen is a limiting
nutrient for plant growth, and therefore most nitrogen in-
puts through deposition are quickly incorporated into bio-
mass as organic nitrogen with little leaching of nitrate into
surface waters.
ANC is an important measure of the sensitivity and the de-
gree of surface water acidification or recovery that occurs
Figure 8: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2007, Sulfate Ion Concentration (ueq/L/yr)
1990-2007
Sulfate Ion Concentration
Increasing significant trend
0 Increasing non-significant trend
0 Decreasing non-significant trend
0 Decreasing significant trend
Source: EPA, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
Figure 9: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2007, Nitrate Ion Concentration (iieq/L/yr)
1990-2007
Nitrate Ion Concentration
Increasing significant trend
0 Increasing non-significant trend
0 Decreasing non-significant trend
Decreasing significant trend
Figure 10: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2007, ANC Levels (ueq/L/yr)
1990-2007
Acid Neutralizing Capacity
(ANC)
• Increasing significant trend
O Increasing non-significant trend
O Decreasing non-significant trend
• Decreasing significant trend
Source: EPA, 2009
Source: EPA, 2009
over time. Acidification results in the diminishing ability of
water in the lake or stream to neutralize strong acids that
enter aquatic ecosystems. Water bodies with ANC values
defined as less than or equal to 0 microequivalents2 per
liter (ueq/L) are acidic. Lakes and streams having spring-
time ANC values less than 50 ueq/L are generally consid-
ered "sensitive" to acidification. Lakes and streams with
ANC higher than 50 ueq/L are generally considered less
sensitive or insensitive to acidification. When ANC is low,
and especially when it is negative, stream water pH is also
low (less than 6), and there may be adverse impacts on fish
and other animals essential for a healthy aquatic ecosys-
tem. Movement toward recovery of an aquatic ecosystem is
indicated by positive trends in ANC and negative trends in
sulfate and nitrate.
Table 3 presents the aggregate sulfate, nitrate, and ANC
trends (ueq/L/yr) represented by the LTM sites shown in
Figures 8-10 for four acid sensitive regions of the eastern
U.S. The maps and summary results indicate that:
• Sulfate concentrations are declining at almost all sites
in the Northeast (New England, Adirondacks/Catskills
and Pennsylvania [Northern Appalachians]). However,
in the Southern Blue Ridge (Central Appalachians),
sulfate concentrations in many streams are increasing.
This region has highly weathered soils that can store
Table 3: Trend Slopes for LTM Sites in Four Eastern U.S. Regions,
1990-2007
Region Sulfate Slope Nitrate Slope ANC Slope
Adirondack Mountains
N.Appalachian Plateau
New England
Ridge and Blue Ridge Province
-2.225
-2.396
-1.58
0.07
-0.179
-0.206
0.009
-0.141
0.765
0.706
0.198
0.107
Notes:
• Bold values indicate significance at 95% confidence interval
(p<0.05). Confidence levels are used to express the reliability and
significance of the estimate.
• The slope or trend in a simple linear regression (SLR) model corre-
sponds to the change in the chemical variable over time. A negative
or positive slope indicates whether the chemical variable in the
regional distribution of water bodies is decreasing or increasing,
respectively.
• Adirondack 2007 data is only for a partial year from January
through May 1,2007.
• The table of values represents the average trend for all the sites in
each of the four regions.
Source: EPA, 2009
10
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Acid Rain and Related Programs: 2008 Environmental Results
large amounts of deposited sulfate. As long-term sulfate
deposition exhausts the soil's ability to store sulfate,
a decreasing proportion of the deposited sulfate is
retained in the soil and an increasing proportion is
exported to surface waters. Thus sulfate concentrations
in surface waters, mainly streams in this region, are
increasing despite reduced sulfate deposition.
• Nitrate concentrations are decreasing in three of the
four regions, but several lakes and streams indicate flat
or slightly increasing nitrate trends. This trend does
not appear to reflect changes in emissions or deposi-
tion in these areas and is likely a result of ecosystem
factors.
• ANC, as measured in surface waters, is on average
increasing in three of the four regions, which in part
can be attributed to declining sulfate deposition. The
site trends also indicate variation within each region.
Only two sites indicate a significant downward trend
in ANC.
The ANC of northeastern U.S. lakes monitored under the
TIME and LTM programs was also evaluated for 1992
to 1994 and 2004 to 2007 to assess the impacts of ARP
implementation. The analysis in Figure 11 compares
average ANC levels for the northeastern lakes that had data
in each time period. Thirty percent of lakes in 1992 to 1994
had three-year mean ANC levels below 0 ueq/L. These
lakes are categorized as "acute concern," in which a near
complete loss offish populations is expected, and planktonic
communities have low diversity and are dominated by
acid-tolerant forms (see Table 4). The percentage of lakes
in this category dropped to 18 percent in 2004 to 2007 (see
Figure 11). As a result, the three other categories (elevated,
moderate, or low concern) experienced slight increases.
These results point to a decrease in acidity, particularly for
the subset of lakes with low ANC.
Figure 11: Northeastern Lakes by ANC Status Category,
1992-1994 versus 2005-2007
Acute Concern
(<0 ueq/L]
Elevated Concern
(0-50 ueq/L]
• 1992-1994
Moderate Concern Low Concern
(50-100 ueq/L) (> 100 ueq/L]
2005-2007
Notes:
• Based on 156 TIME/LTM monitored sites.
• Adirondack 2007 data is only for a partial year from January
through May 1,2007.
• See Table 4 for descriptions of level of concern categories.
• It is important to note that the wide range of ANC values within
these categories makes it likely that substantial improvements
in ANC may occur without changing the categorization of a given
lake.
Source: EPA, 2009
Western Adirondack Stream Survey
The Adirondack Mountain region of New York has long
been a focal point for environmental concern over acid
deposition. Poor buffering capability of the soils in the
Adirondack region makes the surface waters particularly
susceptible to acidification. The effects of acid deposition
and recent improvements in surface water acidity have
been well documented for lakes in the region largely due to
Table 4: Aquatic Ecosystem Status Categories for the Adirondack Mountains
Category Label
Acute Concern
ANC Level
< 0 micro equivalent per Liter (ueq/L)
Expected Ecological Effects
Near complete loss offish populations is expected. Planktonic communities have extremely low diversity and are dominated
by acidophilic forms. The numbers of individuals in plankton species that are present are greatly reduced.
Elevated Concern
0-50 ueq/L
Fish species richness is greatly reduced (more than half of expected species are missing). On average, brook trout populations
experience sub-lethal effects, including loss of health and reproduction (fitness). During episodes of high acid deposition,
brook trout populations may experience lethal effects. Diversity and distribution of zooplankton communities declines.
Moderate Concern
Low Concern
50-100 ueq/L
> 100 ueq/L
Fish species richness begins to decline (sensitive species are lost from lakes). Brook trout populations are sensitive and vari-
able, with possible sub-lethal effects. Diversity and distribution of zooplankton communities begin to decline as species that
are sensitive to acid deposition are affected.
Fish species richness may be unaffected. Reproducing brook trout populations are expected where habitat is suitable.
Zooplankton communities are unaffected and exhibit expected diversity and distribution.
Source: EPA, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
comprehensive lake monitoring and assessment programs.3
In contrast, only a few stream surveys have been conducted
since the 1980s, which has provided little data to assess
the current status of Adirondack streams.
Although regional lake and stream chemistry are similar,
streams are often more prone to acidification than lakes
because they can receive much of their water from shal-
low soils that are often ineffective at neutralizing acidity.
Therefore, observed improvement in lake acidity over the
past two decades may not be occurring in streams. For
these reasons, the New York State Energy Research and
Development Authority (NYSERDA) sponsored the West-
ern Adirondack Stream Survey (WASS) from 2003 to 2005
to assess the current chemical and biological condition of
streams in the western section of the Adirondacks.4"5
Researchers from U.S. Geological Survey, New York State
Department of Environmental Conservation, Adirondack
Lakes Survey Corporation and the University of Texas at
Arlington assessed 565 streams representing 825 km2 of
the Black River and Oswegatchie River drainages within the
boundaries of the Adirondack Park (Figure 12). Streams
were sampled during base-flow, snowmelt and fall storm
events for water chemistry to determine if they were epi-
sodically or chronically acidic.6 The health of the biologi-
cal community (diatoms and macroinvertebrates) was also
measured in a subset of streams.
The extent of stream acidification in the western
Adirondack Mountain region remains high. The streams
sampled in this study showed that 66 percent or 718 km of
streams are prone to acidification and likely have levels of
acidity harmful to their biota. Of the 66 percent of streams
found to be prone to acidification, about 50 percent were
likely to be chronically acidified, with the other 50 percent
episodically acidified.
The impacts of acidification on the health of the aquatic
communities also remain pronounced. The percentage of
streams determined to be moderately to severely impacted
on the basis of their diatom community ranged from 66
to 80 percent over the different surveys. WASS showed
clear evidence that over half of the assessed streams had
macroinvertebrate communities that were moderately
to severely impacted.7 The survey also showed that two
thirds of streams sampled have poor water quality during
some point during the year that can be toxic to fish such as
brook trout.
Figure 12: Acidification Categories of Sites Sampled in the
March 2004 Survey
• non acidified
prone to acidification
• acidified
•fr village
Note: Non acidified sites had base cation surplus (BCS) values > 25
eq L-l; sites prone to acidification had BCS values > 0 eq L-l but <
25 eq L-l; acidified sites had BCS values < 0 eq L-l.
Source: NYSERDA, 2008
These results also indicate that the recovery from acidi-
fication in these western Adirondack streams has been
minimal. Comparison with historical water chemistry data
available for 12 streams showed that less than half of the
streams had higher acid neutralizing capacity (ANC) in
2003 to 2005 than in the early 1980s. The overall increase
for these streams was 13 ueq/L over the 23 year period.
Further reductions in acid deposition in the Adirondacks
are necessary for greater recovery of these sensitive aquat-
ic systems.
12
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Acid Rain and Related Programs: 2008 Environmental Results
Critical Loads Case Study: Central Appalachian
Mountain Streams
The central Appalachian Mountain region of Virginia and
West Virginia that includes the Shenandoah National Park
is known to be sensitive to acidic deposition. Poor soils and
low weathering rates of the bedrock beneath the mountain
terrain in the central Appalachian Mountain region make
the streams particularly susceptible to acidification. As a
result, acid deposition has impacted many miles of streams
in the region, greatly reducing the diversity of aquatic life
including important recreational fish species such as brook
trout.
In this case study, critical loads of deposition were calculated
for 92 streams in the central Appalachian Mountain region.
The critical load is the maximum exposure to pollutant
deposition below which significant harmful effects to
ecological health of the environment do not occur according
to present knowledge. If the actual pollutant deposition
to a lake or stream is greater than its critical load—if the
critical load is exceeded—then that water body is at risk
for continuing ecological damage. If pollutant deposition is
less than the critical load, adverse ecological effects are not
anticipated (and if the water body has been damaged by
past deposition, recovery is expected).
There are numerous peer-reviewed scientific methods and
models that can be used to calculate critical loads for surface
water acidity for streams. Drawing on recent scientific
studies8 in the eastern United States, this case study uses
the Steady-State Water Chemistry (SSWC) and the Model
of Acidification of Groundwater In Catchment (MAGIC]
models to calculate the critical load. The analysis uses
water chemistry data from the TIME/LTM, Virginia Trout
Stream Sensitivity Study (VTSSS), and other programs that
are part of the Environmental Monitoring and Assessment
Program (see discussion of surface water trends on pages
9-11). The focus of this case study is on the combined load
of sulfur and nitrogen deposition below which the ANC level
would still support healthy aquatic ecosystems. Research
studies have shown that surface water with ANC values
greater than 50 ueq/L tends to protect most fish (e.g.,
brook trout, others) and other aquatic organisms (Table 4
on page 11). In this case, the critical load represents the
combined deposition load of sulfur and nitrogen to which
a stream and its watershed could be subjected and still
have a surface water concentration ANC of 50 ueq/L on
an annual basis. Critical loads of combined total sulfur and
nitrogen are expressed in terms of ionic charge balance as
milliequivalent9 per square meter per year (meq/m2/yr).
In the United States, the critical loads approach is not
an officially accepted approach to ecosystem protec-
tion. For example, language specifically requiring a
critical loads approach does not exist in the Clean
Air Act. However, recent activities within federal and
state agencies, as well as the research community,
indicate that critical loads are emerging as a useful
ecosystem protection and program assessment tool.
In June 2008, a report was released by the Nature
Conservancy and the Gary Institute of Ecosystem
Studies that called on Congress, federal and state
agencies, conservation groups, and scientists to
work together to establish critical loads to protect
sensitive ecosystems. The report, Threats from
Above: Air Pollution Impacts on Ecosystems and
Biological Diversity in the Eastern United States,w
is based on the results of an expert workshop to
evaluate air pollution effects in the Northeast and
Mid-Atlantic and identify conservation implications.
The report recommended that Congress direct the
EPA to develop and implement a deposition-based
air quality standard, like a critical load, for sulfur
and nitrogen pollution in sensitive ecosystems that
receive high deposition levels.
Figure 13: Annual Average Wet Deposition of Sulfate and
Nitrate in the Shenandoah National Park and Surrounding
Areas, 1990-2008
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
— SO4 Average — NOs Average
Note: NADP Sites: VA28, VA27, VAOO, VA99, WV04, WV18
Source: NADP, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
Figure 14: Annual Average Air Concentration of S02, Oxidized
Nitrogen, S04, and Reduced N intheShenandoah National Park
and Surrounding Areas, 1990-2008
1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
^— SO2 ^~ SO4 — Oxidized N — NHU (reduced nitrogen)
Note: CASTNETSite: SHN418
Source: CASTNET, 2009
Figure 15: Annual Average Surface Water Sulfate and Nitrate
Concentration in the Central Appalachian Mountain Region,
1990-2006, Compared with ANC
0.0
1990 1992 1994 1996 1998 2000 2002 2004 2006
SO4 NO3 ^— ANC
Source: EPA, 2009
Based on environmental monitoring data collected since
the 1990s in the central Appalachian Mountain region,
emission reductions from the ARP and other programs
have resulted in substantial decreases in atmospheric con-
centration and deposition of sulfur and nitrogen. Between
1990 and 2008, annual average wet deposition of sulfur
and nitrogen has decreased by approximately 47 percent
and 41 percent, respectively, as shown in Figure 13 on page
13. These deposition reductions parallel the decreases in
the air concentrations of these pollutants over the same
period as shown in Figure 14.
The declines in air concentrations and deposition of sulfur
and nitrogen since the 1990s have resulted in only slight
signs of recovery from acid rain in the streams of the central
Appalachian Mountain region. Figure 15 shows trends in
sulfate, nitrate, and ANC for central Appalachian Mountain
streams monitored through the LTM program. Sulfate con-
centrations in these streams have remained level, nitrate
concentrations have dropped slightly, and the resulting
overall trend for ANC is a slight increase. These observed
trends indicate an important first step towards ecological
recovery.
It is difficult to determine whether central Appalachian
Mountain region aquatic ecosystems will recover and be
sufficiently protected from acid deposition based on envi-
ronmental monitoring data alone. The critical load provides
a benchmark to gauge whether deposition has decreased
enough to protect the ecological health of a lake or stream. In
Figure 16 on page 15, a critical load exceedance indicates
that the combined sulfur and nitrogen deposition was
greater than a stream could sustain and still maintain the
ANC level of 50 ueq/L or above. Exceedances were calculat-
ed from deposition for the period before implementation
of the ARP (1989-1991) and for a recent period after ARP
implementation (2006-2008).
For the period before ARP implementation, 90 percent of
streams received levels of combined sulfur and nitrogen
deposition that exceeded the critical load and could not be
neutralized by the environment. For the period from 2006
to 2008, 82 percent of the streams examined continued to
receive greater acid deposition than could be neutralized,
only an eight percent improvement from before ARP im-
plementation.
Figure 16 on page 15 also shows streams where deposition
was within 10 percent of the critical load. These streams
illustrate areas where ecosystem health has improved only
slightly over time.
This critical load analysis shows that emission reductions
achieved by the ARP have resulted in some improvement
in environmental conditions and increased ecosystem pro-
tection in the central Appalachian Mountain region. The
analysis also demonstrates that the central Appalachian
Mountain region remains at risk to acidification due to
14
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Acid Rain and Related Programs: 2008 Environmental Results
Figure 16: Comparison of Critical Load Exceedances in Central Appalachian Mountain Streams before and after Implementation of
the Acid Rain Program
1989-1991
2006-2008
Source: EPA, 2009
current acidic deposition levels and deeper reductions are
necessary for recovery of these sensitive aquatic systems
and full ecosystem protection.
Trends in Atmospheric Sulfur Concentrations
ARIMA Model
To help assess the trend in atmospheric sulfur concentra-
tions since inception of the ARP, an Autoregressive Inte-
grated Moving Average (ARIMA) model11 was used to plot
average sulfur concentrations captured by the 12 long-
term CASTNET sites in Ohio, Pennsylvania and West Vir-
ginia. The ARIMA model, an advanced statistical analysis
tool that can evaluate trends over time, shows that from
1994 to 1995 there was a step decrease in average atmo-
spheric sulfur concentrations that is statistically significant
at the 99.9 percent level. This step decrease is the result
of ARP pre-compliance and the start of phase I of the ARP
(Figure 17). From 1996 to 2008 the ARIMA model shows a
steady downward trend (a negative slope) in atmospheric
sulfur concentrations that is also statistically significant at
the 99.9 percent confidence level. The results of this ARIMA
assessment demonstrate the substantial impact of the ARP
on atmospheric sulfur concentrations over time.
Figure 17: Ambient Sulfur Concentrations at CASTNET Sites in
Ohio, Pennsylvania, and West Virginia
Sulfur Concentration (ug/m3)
o o o o o o o
i^criocriocriocri
5 O
„
— ,
o
^^
(1994)
\,.-PI
o
(1995
asel E
^-3--.
arly Cor
>-2-_
npliana
i— . 0
!-
\~
1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008
o Actual Model Estimate D 95% Confidence Band
Note: Sulfur concentrations include ambient S02 and sulfate.
Source: EPA, 2009
15
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Acid Rain and Related Programs: 2008 Environmental Results
Ammonium Sulfate
SC>2 and NOX emissions can react in the atmosphere to form
fine particles, which are harmful to humans and sensitive
ecosystems. Sulfate particles are formed after gaseous SC>2
is emitted and oxidized. When the oxidative potential of the
atmosphere is high, a large fraction of atmospheric sulfur
in the form of gaseous SC>2 is converted into particulate sul-
fate.
Some of the common particulate sulfate compounds
formed in the atmosphere include ammonium sulfate
((NH4)2SC>4), ammonium bisulfate (NH4HS04), and letovic-
ite ((NH4)3H(SC>4)2). Ammonium sulfate is a particularly
stable atmospheric compound, meaning that once it is
formed it will be transported and deposited in that form.
Ammonium sulfate makes up a significant fraction of fine
particulate matter (PM2.s) in the Northeast.12 When at
least two ammonium ions are present per one sulfate ion in
the atmosphere, ammonium sulfate is the dominant sulfate
particulate compound formed.
Particulate sulfate, including ammonium sulfate, is col-
lected by the CASTNET Teflon® filter. The relative average
annual 2007 proportions of ammonium and sulfate were
determined at each CASTNET site (Figure 18). When ex-
actly twice the concentration of ammonium to sulfate is
measured, it is implied that all of the particulate sulfur is in
the form of ammonium sulfate. This is signified by a ratio
of 1 in Figure 18 (0.5 moles of ammonium divided by moles
of sulfate) at the given CASTNET sites. Ratios larger than
one suggest that proportionally, at least two times more
ammonium ions are present than sulfate on the particulate
filter. Conversely, ratios less than one suggest proportion-
ally more sulfate is present than ammonium. For locations
with ratios equal to or greater than one, the chemical state
of the atmosphere is recognized to be "sulfur limited" with
respect to sulfate PM formation, meaning that the amount
of sulfate available in the atmosphere is controlling the for-
mation of ammonium sulfate. Thus it is expected that de-
creases in sulfur emissions in these areas would likely lead
to a more pronounced decrease in particulate sulfate for-
Figure 18: Ratios of Ammonium versus Sulfate, as Measured at CASTNET Locations in 2007
Note: Ratios near or larger than 1.00 suggest that most of the sulfate is in the form of (NH4)2S04.
Source: EPA, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
mation and potentially a decrease in PM2.5 in areas where
ammonium sulfate is a significant fraction of the PM.
Online Information, Data, and Resources
The availability and transparency of data, from emission
measurement to allowance trading to deposition monitor-
ing, is a cornerstone of effective cap and trade programs.
CAMD, in the Office of Air and Radiation's Office of Atmo-
spheric Programs, develops and manages programs for
collecting these data and assessing the effectiveness of cap
and trade programs, including the ARP.
The CAMD Web site provides a public resource for general
information on how market-based programs work and
what they have accomplished, along with the processes,
information, and tools necessary to participate in any
of these market-based programs. For information about
EPA's air emission trading programs, see . For information about the ARP, see .
To increase data transparency, EPA has created
supplementary maps that allow the user to display air
market program data geospatially on an interactive
Figure 19: U.S. Sulfur Dioxide Emissions from ARP Sources and Ambient Sulfate Concentrations, 1990
Note: This example depicts 1990 S02 emissions from ARP sources along with 1990 sulfate concentration data as measured by the
CASTNET monitoring program.
Source: EPA, 2009
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Acid Rain and Related Programs: 2008 Environmental Results
Figure 20: U.S. Sulfur Dioxide Emissions from ARP Sources and Ambient Sulfate Concentrations, 2008
Note: This example depicts 2008 S02 emissions from ARP sources along with 2007 sulfate concentration data as measured by the
CASTNET monitoring program.
Source: EPA, 2009
3D platform. Figures 19 and 20 are examples of these
interactive maps. The maps come in the form of a KMZ
file (a compressed KML file) that is downloaded directly
to the user's computer. Data can be explored in new and
meaningful ways by turning different layers on and off,
overlaying data points and satellite imagery, and using
navigation tools to change the view of the Earth's surface.
KMZ/KML files are supported by programs such as Google
Earth, ESRI Arc Explorer, and NASA WorldWind View. These
interactive mapping applications provide a unique way to
identify environmental trends and track the progress of
various EPA programs, such as the ARP.
For more information or to utilize the program, visit the Web
site at .
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Acid Rain and Related Programs: 2008 Environmental Results
Endnotes
1 Driscoll, C.T., Lawrence, G., Bulger, A., Butler, T, Cronan, C, Eagar, C, Lambert, K.F., Likens, G.E., Stoddard, ]., and Weathers, K. 2001. Acid
Deposition in the Northeastern U.S.: Sources and Inputs, Ecosystem Effects, and Management Strategies. Bioscience, 51:180-198.
2 An equivalent is a measure of a substance's ability to combine with other substances. The equivalent is formally defined as the amount of a
substance, in moles, that will react with one mole of electrons. A microequivalent is one millionth of an equivalent.
3 Temporally Integrated Monitoring of Ecosystems (TIME) program. Long Term Monitoring (LTM) program, and Adirondack Long-Term
Monitoring (ALTM) program.
4 Results from the 2003-2005 Western Adirondack Stream Survey Final Report: 08-22 New York State Energy Research and Development
Authority 2008.
5 Lawrence, G. B., Roy, K. M., Baldigo, B. P., Simonin, H. A., Capone, S. B., Sutherland, J. W., Nierzwicki-Bauer, S. A., Boylen, C. W. 2008. Chronic
and Episodic Acidification of Adirondack Streams from Acid Rain in 2003-2005. Journal of Environmental Quality 37: 2264-2274.
6 Acidification categories of streams: Nonacidified streams had base cation surplus (BCS) values > 25 |ieq/L, sites prone to acidification had
BCS values betwee 0-2 5 |ieq/L, and acidified sites had BCS values < 0 |ieq/L. Base-cation surplus (BCS) is an adjusted measure of Acid
Neutralizing Capacity (ANC) that includes the concentration of strong organic anions.
7 Baldigo, R B., Lawrence G. B., Bode R. W., Simonin H. A., Roy K. M., Smith A. J., 2009. Impacts of acidification on macroinvertebrate commu-
nities in streams of the western Adirondack Mountains, New York, USA, Ecological Indicators, Volume 9(2), Pages 226-239.
8 These studies reflect research completed by E&S Environmental and the University of Virginia in partnerships with US EPA, NFS, and
USDA Forest Service:
Sullivan, T. J., B. J. Cosby, J. R. Webb, R. L. Dennis, A. J. Bulger, F. A. Deviney Jr. 2008. Streamwater acid-base chemistry and critical loads of
atmospheric sulfur deposition in Shenandoah National Park, Virginia. Environmental Monitoring and Assessment (2008) 137:85-99.
Dupont, J., Clair, T.A., Gagnon, C., Jeffries, D.S, Kahl, J.S., Nelson, S.J., and Peckenham, J.M. 2005. Estimation of Critical Loads of Acidity for
Lakes in Northeastern United States and Eastern Canada, Environmental Monitoring and Assessment, 109:275-291.
9 A milliequivalent is one thousandth of an equivalent.
10 Lovett, G.M., and T.H. Tear. 2008. Threats from Above: Air Pollution Impacts on Ecosystems and Biological Diversity in the Eastern United
States. The Nature Conservancy and the Gary Institute of Ecosystem Studies.
11 Statistical Analysis with SAS/STAT® Software.
12 Review of the National Ambient Air Quality Standards for Particulate Matter: Policy Assessment of Scientific and Technical Information,
OAQPS Staff Paper,
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