Acid Rain and  Related Programs
2009 Environmental Results
      The Acid Rain Program (ARP), established under Ti-
      tle IV of the 1990 Clean Air Act (CAA) Amendments,
      requires major emission reductions of sulfur diox-
ide (SC>2) and nitrogen oxides (NOX), the primary precur-
sors 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 contiguous United States. 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 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 the fol-
lowing page shows the regional changes in key air quality
and atmospheric deposition measurements  linked to the
ARP's SC>2 and NOX emission reductions.

During 2010, EPA is releasing a series of reports summa-
rizing progress under the ARP. This third report compares
changes in emissions to changes in air quality, acid depo-
sition, 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 im-
proved air quality. Based on data from EPA's latest air emis-
sion trends report, the national composite average of SC>2
annual mean ambient concentrations decreased 76 per-
cent between 1980 and 2009, as shown in Figure 1 (based
on state, local, and  EPA monitoring sites located primarily
            United States
            Environmental Protection
           At a Glance: ARP Results in 2009

  Air Quality: Between 1989-1991 and 2007-2009, aver-
  age ambient sulfate concentrations have decreased by 44
  percent in the Mid-Atlantic, 47 percent in the Midwest, 49
  percent in the Northeast, and 41 percent in the Southeast.
  Acid Deposition: Between the 1989-1991 and 2007-
  2009 observation periods, regional decreases in wet
  deposition of sulfate across the Eastern United States
  averaged 43 percent.
  Surface Water Chemistry: Levels of Acid Neutralizing
  Capacity (ANC), the ability of a water body to neutralize
  acid deposition, have increased significantly from 1990 to
  2008 in lake and stream long-term monitoring sites in the
  Adirondack Mountains and the Northern Appalachian Pla-
  teau. These increasing ANC levels indicate trends toward
  recovery from acidification.
in urban areas). The largest single-year reduction (20 per-
cent) occurred in the first year of the ARP, between 1994
and  1995. The  second largest single-year reduction (16
percent) occurred most recently between 2008 and 2009.
These trends are consistent with the regional ambient air
quality trends observed in the Clean Air Status and Trends
Network (CASTNET).

Figure 1: National S02 Air Quality, 1980-2009
          • National Ambient Air
           Quality Standard
           90% of sites have
           concentrations below this line
 Average Concentration

•10% of sites have
 concentrations below this line
                                                        Source: EPA, 2010
                                         October 2010

Acid Rain and Related Programs: 2009 Environmental Results
Table 1: Regional Changes in Air Quality and Deposition of Sulfur and Nitrogen Compounds, 1989-1991 versus 2007-2009, from
Rural Monitoring Networks
Measurement Region Average, 1989-1 991 Average, 2007-2009 PercentChange Number of Sites
Ambient S02 Concentra-
tion (ug/m3)
Ambient Sulfate Concen-
tration (ug/m3)
Wet Sulfate Deposition
Dry Sulfur Deposition
Total Sulfur Deposition
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)
• 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 and values in bold indicates that differences were statistically significant at the 95 percent confidence level. Changes that are
  not statistically significant may be unduly influenced by measurements at only a few locations or large variability in measurements.
Source: EPA, 2010

                                                 Acid Rain and Related Programs: 2009 Environmental Results
Figure 2: Annual Mean Ambient S02 Concentration
Figure 3: Annual Mean Ambient Sulfate Concentration
• 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
Source: CASTNET, 2010
Dramatic regional improvements in SC>2 and ambient sul-
fate concentrations were observed following implementa-
tion of Phase I of the ARP during the late 1990s at CAST-
NET sites throughout the eastern United States, and these
improvements continue today. Analyses of regional moni-
toring 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 2007 to 2009 in both tabular form
and graphically in maps (see Table 1 and Figures 2 and 3).

The maps in Figure 2 show that the average annual ambi-
ent concentrations of SC>2 from 1989 to 1991 were highest
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.

Acid Rain and Related Programs: 2009 Environmental Results
Figure 4: Annual Mean Ambient Total Nitrate Concentration

                                   Total NO3
• 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, 2010
Like SC>2 concentrations, the highest average annual am-
bient sulfate concentrations from 1989 to 1991 were ob-
served in western Pennsylvania and along the Ohio River
Valley. Most of the eastern United States experienced an-
nual ambient sulfate concentrations greater than 5 micro-
grams per cubic meter (ug/m3).

Ambient sulfate concentrations have also decreased since
the program was implemented, with average concentra-
tions decreasing from 41 to  49 percent in regions of the
East (see Table 1). Both the magnitude and spatial extent
of the highest  concentrations have dramatically declined,
with the largest decreases observed along the Ohio River
Valley (see Figure 3).

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 concentra-
tions in many areas. Ambient nitrate levels can also be af-
fected by emissions transported via air currents over wide
                                   Total NO3
From 2007 to 2009, reductions in NOX emissions during the
ozone season from power plants under the NOX SIP Call and
Clean Air Interstate Rule (CAIR) have continued to result in
significant region-specific improvements in ambient total ni-
trate (NOs- plus HNOs) concentrations. For instance, annual
mean ambient total nitrate concentrations for 2007 to 2009 in
the Mid-Atlantic region were 39 percent less than the annual
mean concentration in 1989 to 1991 (see Table 1 and Figure 4).
While these improvements might be partly attributed to added
NOX controls installed for compliance with the NOX SIP Call and
CAIR, the findings at this time are not conclusive.

Acid Deposition

National Atmospheric Deposition Program/National Deposi-
tion Trends Network (NADP/NTN) monitoring data show sig-
nificant improvements in the primary acid deposition indica-
tors. 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 Valley
and northeastern United States. Some of the greatest reduc-
tions have occurred in the mid-Appalachian region, including
Maryland, New York, West Virginia, Virginia, and most of Penn-
sylvania. Other less dramatic reductions have been observed
across much of New England, portions of the southern Appa-
lachian  Mountains, and some areas of the Midwest. Between
the 1989 to 1991 and 2007 to 2009 observation periods, aver-
age decreases in wet deposition of sulfate averaged more than

                                                  Acid Rain and Related Programs: 2009 Environmental Results
Figure 5: Annual Mean Wet Sulfate Deposition

Source: NADP, 2010
Figure 6: Annual Mean Wet Inorganic Nitrogen Deposition
 Source: NADP, 2010
43 percent for the eastern United States (see Table 1 and
Figure 5). Along with wet sulfate deposition, wet sulfate
concentrations have also decreased by similar percentag-
es. A strong correlation between large-scale SC>2 emission
reductions and large reductions in sulfate concentrations
in precipitation has been noted in the Northeast, one of the
areas most affected by acid deposition. The reduction in
total sulfur deposition (wet plus dry) has  been even more
dramatic than that of wet deposition in the Mid-Atlantic
and Midwest, with reductions of 50 and 53 percent, respec-
tively (see Table 1). Because continuous data records are
available from only a few sites in the Northeast and South-
east, it is unclear if the observed reductions in total deposi-
tion 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 31 and 23
percent decrease in total nitrogen deposition for the Mid-
Atlantic and Midwest, respectively (see Table 1).

Acid Rain and Related Programs: 2009 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-
   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 59 of the monitoring stations; the
   National Park Service (NFS) funds and operates ap-
   proximately 25  stations in cooperation with EPA. Many
   CASTNET sites have a continuous 20-year data record,
   reflecting EPA's commitment to long-term environmen-
   tal monitoring.

   NADP/NTN is a nationwide, long-term network track-
   ing the chemistry of precipitation. NADP/NTN provides

    Table 2: Air Quality and Acid Deposition Measures
concentration and wet deposition data on hydrogen
ion (acidity as pH), sulfate, nitrate, ammonium, chlo-
ride, and base cations. The network is a cooperative
effort involving many groups, including the State Ag-
ricultural Experiment Stations, U.S. Geological Survey
(USGS), U.S. Department of Agriculture (USDA), 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 the NADP's website.

NADP is running a pilot study to determine the fea-
sibility of operating a  long-term  passive  ammo-
nia (NHs)  monitoring network. The pilot network
(AMoN)  has been measuring 2-week samples of am-
bient NHs for over two years at more than 20 sites. It
will be the first nationwide network to measure NHs
routinely. More information on AMoN can be found
on the NADP website.
Chemical Measured in:
Chemical Name Symbol Ambient Air Wet Deposition Why are these measured by the networks?
Sulfur Dioxide
Sulfate Ion

Nitrate Ion

Nitric Acid

Ammonium Ion
Ionic Hydrogen










Primary precursor of wet and dry deposition; primary precursor of fine particles (PlVts).
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 SQi
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
Indicator of acidity in precipitation; formed from the reaction of sulfate and nitrate in
These base cations neutralize acidic compounds in precipitation and the environment;
also play a major role in plant nutrition and soil productivity.

    Source: EPA, 2009

                                                 Acid Rain and Related Programs: 2009 Environmental Results
Ambient Mercury Monitoring

In addition to SC>2 and NOX, coal-fired power plants release
mercury into the atmosphere where it can be transported
and deposited locally, regionally, and globally. NADP recent-
ly launched  the Atmospheric Mercury Network (AMNet)
for monitoring three atmospheric mercury species:  gas-
eous elemental mercury (GEM), gaseous oxidized mercury
(COM), and particulate-bound mercury (PBM2.s). Data sets
generated from this network are used to estimate mer-
cury dry deposition, assess mercury source/receptor re-
lationships, evaluate atmospheric models, and determine
long-term trends. Currently, 20 sites in North America (as
shown in the map in Figure 7) participate in the network,
generating high-resolution, high-quality speciated atmo-
spheric  data. In 2010, the University of California - Santa
Cruz was the most recent partner to join AMNet, establish-
ing a new network site in Elkhorn Slough, California.

Figure 7: Ambient Mercury Monitoring  Locations
The AMNet Database

The AMNet database has received extensive quality assur-
ance (QA)/quality control (QC) review by a team of data
quality experts from EPA, USGS, NADP, and other institu-
tions. Quality control flags to determine whether datum
records are valid or invalid were developed based on peer-
reviewed criteria. The AMNet QA/QC review of data is a
robust, tiered approach,  including:  automatic  screening
of each raw datum and quality control flagging; additional
rigorous  automatic  screening, accounting for calibration
data and other checks (e.g., extensive 24-hour  trap bias,
etc.) and quality control flagging; and NADP site liaison
manual review of data, where automatically screened data
are reviewed against corresponding monthly site opera-
tor field report notes (e.g., glassware changes, site shelter
power failure, etc.).
                                                                                  Atmospheric Mercury Sites

                                                                                         AMNet Sites

                                                                                         Interested Speciated Sites
                                                                                         Other Atm. Mercury Sites
                                                                                         Ik (Canadian Atmospheric
                                                                                           Mercury Network)

Source: NADP/AMNet, 2010

Acid Rain and Related Programs: 2009 Environmental Results
At present, more than 56 site years of data have been qual-
ity assured. NADP now offers mercury speciation data
products available through the AMNet website (nadp.sws., including bi-hourly graphical  plots and
data tables for each site.

Beltsville, Maryland Case Study

Beltsville, MD is home to NADP/AMNet (site ID: MD99),
CASTNET (site  ID: BEL116),  NADP/Mercury Deposition
Network (site ID: MD99), and other monitoring sites where
mercury, meteorology data, and other ancillary data are re-
corded. The data plot shown below represents an impor-
tant example of the value of maintaining complementary,
collocated monitoring  network sites. In this example, col-
located AMNet bi-hourly gaseous oxidized mercury (COM)
and CASTNet hourly SC>2 values (Figure 8) from Beltsville
are plotted over the same time period.  COM is measured
in picograms per cubic meter (pg/m3).  SC>2 is a trace gas
that is often used as a signature of coal-fired power plants.
The plot shows both COM and SC>2 following a very similar
pattern over the same sample time period, which warrants
further analysis to determine if there is a relationship be-
tween the ambient data and emissions. The measurements
collected at AMNet can also be used with source-receptor
modeling and other tools to provide useful information on
source attributions and dry/total deposition estimates for
sensitive ecosystems.

Figure 8: Bi-Hourly Gaseous Oxidized Mercury Concentrations
and Hourly CASTNET S02 Gas Concentrations at the AMNet Site
     n .-J    K^v  ^ t  W^*-i    f-* \i     •MBhtfoJ: n
     U ~^^ ii|  n ir^ iiT i   I \ ^ ir^^    ^^^^^^^^^T~ U
     Sep22 Sep23 Sep 24  Sep 25 Sep 26  Sep 27 Sep 28 Sep 29

Source: NADP/AMNet, 2010
Improvements in Surface Water Chemistry

Acid rain resulting from SC>2 and NOX emissions is one of
many large-scale anthropogenic impacts 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

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 CAA Amendments in reducing the acidity of sur-
face waters in four regions: New England, the Adirondack
Mountains, the Northern Appalachian  Plateau, and the
Central Appalachians (the Ridge and Valley and Blue Ridge
Provinces). The surface water chemistry trend data in the
four regions monitored by the TIME and LTM programs are
essential for tracking the ecological response to ARP emis-
sion reductions (see Figure 9).

The data presented here show regional trends in acidifi-
cation from 1990 to 2008 in lakes and streams sampled
through the LTM program (see Figures 10-12). Only sites
that have a complete data record for the time period are
represented. Three indicators of acidity in surface waters
Figure 9: Long-Term Monitoring Program Sites
                              LTM Sites by Region
                              • New England
                              o Adirondack Mountains
                              « Northern Appalachian Plateau
                              • Central Appalachians
                                 (Valley and Ridge and
                                 Blue Ridge Provinces)
                                                        Source: EPA, 2010

                                                    Acid Rain and Related Programs: 2009 Environmental Results
Figure 10: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2008 — Sulfate Ion Concentration (ueq/L/yr)
Figure 11: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2008 — Nitrate Ion Concentration (ueq/L/yr)
                            1990-2008 Sulfate Ion Concentration
                            • Increasing significant trend
                            • Increasing non-significant trend
                            O Decreasing non-significant trend
                              Decreasing significant trend
Source: EPA, 2010

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 heading towards re-
covery 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 ex-
tent of cation leaching in soils  and how sulfate concentra-
tions relate to  deposition and  to the levels of ambient at-
mospheric sulfur.

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
inputs from 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
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 li-
                               1990-2008 Nitrate Ion
                               • Increasing significant trend
                               • Increasing non-significant trend
                               o No change
                               O Decreasing non-significant trend
                               « Decreasing significant trend
Source: EPA, 2010

Figure 12: Trends in Lake and Stream Water Chemistry at LTM
Sites, 1990-2008 — ANC Levels (ueq/L/yr)
                                1990-2008 Acid Neutralizing
                                Capacity (ANC)
                                O Increasing significant trend
                                O Increasing non-significant trend

                                O Decreasing non-significant trend
                                  Decreasing significant trend
Source: EPA, 2010

ter (ueq/L) are of acute concern for acidification. Lakes and
streams having springtime ANC values less than 50 ueq/L
are generally considered of elevated concern for acidifica-
tion. Lakes and streams with ANC higher than  50 ueq/L
are generally considered of moderate to low concern for
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 es-
sential for  a healthy aquatic ecosystem. Movement toward
recovery of an aquatic ecosystem is indicated by increasing
trends in ANC and decreasing trends in sulfate and nitrate.

Acid Rain and Related Programs: 2009 Environmental Results
Table 3: Regional Trends in Sulfate, Nitrate, ANC, and DOC at Long-term Monitoring Sites, 1990-2008
% of Sites with % of Sites with % of Sites with % of Sites with
Improving Sulfate Improving Improving ANC Improving DOC
Region Waterbodies Covered Trend Nitrate Trend Trend Trend
Adirondack Mountains
Catskills/ N.Appalachian Plateau
New England
Central Appalachians
50 lakes in NY
9 streams in NY and PA
26 lakes in ME and VT
66 streams in VA
42% (26 sites)
29% (7 sites)
20% (10 sites)
• Trends are determined by multivariate Mann-Kendall tests.
• DOC was only examined in low-ANC waterbodies (ANC less than
  25 neq/L).
• DOC is not currently measured in Central Appalachian streams.
Source: EPA, 2010

 Dissolved organic carbon (DOC), essentially organic mate-
 rial, is derived from many sources, some of which include:
 atmospheric deposition,  decaying leaf litter, soil organic
 matter, aquatic sediments, and aquatic organisms. DOC is
 an important part of the acid-base chemistry of most low-
 alkalinity  freshwater  systems. A host of factors control
 DOC in surface water including the inputs from acidifying
 deposition, discharge, temperature, and nutrient enrich-
 ment. Recently, scientists have suggested  that increased
 concentrations of DOC are likely due to  declining sulfate
 content from atmospheric deposition, increasing seasonal
 temperatures,  or a combination of both.3'4 With increasing
 loading of acid deposition, soils release  lower quantities
 of organic acids, thereby causing DOC to decrease in sur-
 face water. This means that as surface waters recover from
 acidic deposition, DOC concentrations are returning to-
 ward pre-industrial levels. On the other hand, as tempera-
 tures warm, more organic acids in the soil break down and
 are released to surface waters and increase DOC concen-
 trations. These increasing DOC concentrations could be a
 possible sign of climate change.  Another mechanism that
 could cause increases in DOC is  a soil microbial response
 to nitrogen deposition that results in greater export of hu-
 mic material to surface waters.5

 DOC is an important water chemistry parameter and may
 be affected by acidification. Table 3 presents the aggregate
 sulfate, nitrate, ANC, and DOC trends represented by the
 LTM sites shown  in Figures 10-12 for four acid sensitive
 regions of the eastern United States, as well as DOC trends
 for low ANC (ANC<25 ueq/L) waterbodies.
             Acidification of Soils

Soils are also affected by acidic deposition. As acidic
deposition enters the soil, it can cause base cations,
such as calcium (Ca2+), magnesium (Mg+), or potas-
sium (K+), to exchange with H+ and A13+ ions. This
causes base cations to be lost from the soil through
transport to surface water, which may lead to de-
clines in soil pH. As acidic deposition continues, more
base cations exchange, causing a further decline in
base cation adsorption to the soil surface (i.e., per-
cent base saturation). This is important because base
cations are important plant nutrients and their loss is
a potential threat to forest productivity and health.10
Additionally, the immobilization of A13+ by acidic
deposition can also be toxic to plants. While surface
waters have experienced some chemical recovery
(Table 3), soils are  still likely acidifying.11 In 2009,
researchers at Syracuse University published results
from a 2001 soil survey for soils in 139 watersheds
across the northeastern United States. Many of these
watersheds had previously been sampled as part of
the EPA's Direct/Delayed Response Project in 1984.
This comparative study showed that over the 17-yr
interval, median base saturation in the Oa-horizon
(organic surface soil layer) exhibited a statistically
significant decrease from 56% in 1984 to 33% in
2001. Soil pH also decreased from 3.05 to 2.95 pH
units over the same time period. These results are
consistent with other research, showing continued
soil acidification for this region.12  Soil acidification
is likely to continue until acidic deposition inputs to
soils decline to the point where soil base cation pools
are sufficient to neutralize them.

                                                  Acid Rain and Related Programs: 2009 Environmental Results
The maps and summary results indicate that:

• Sulfate concentrations are declining at most6 sites in
  the Northeast (New England, Adirondacks, Catskills/
  Northern Appalachian Plateau). However, in the Central
  Appalachians,  sulfate concentrations in some  streams
  (21%) are increasing. This region has highly weathered
  soils that can store 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 propor-
  tion is exported to surface waters. Thus sulfate concen-
  trations in surface waters, mainly streams in this region,
  are increasing despite reduced sulfate deposition.

• Nitrate concentrations are decreasing in some of the sites
  in all 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 deposition
  in these areas and is likely a result of ecosystem factors.
  In  2008, 45% of the Central Appalachian streams had a
  decreasing trend in nitrate, compared to  24% in 2007.
  This  increase in the number of sites with a decreasing
  nitrate trend may be due to continued recovery follow-
  ing gypsy moth  defoliation in the early  1990s. Gypsy
  moth defoliation has  been shown to increase nitrate ex-
  port  from affected forests to surface waters by as much
  as  50 times.7 While defoliation from gypsy moths may
  only occur over several months, impacts on nitrate trans-
  port and in-stream concentrations may be seen for many

• ANC, as measured in surface waters, is increasing in
  many of the sites in the Adirondack and Catskills/North-
  ern Appalachian Plateau regions, which in part can be at-
  tributed to declining  sulfate deposition. The site trends
  also indicate variation within each region. Only 12% of
  sites in New England  and the Central Appalachians have
  improving ANC trends, but overall, only seven sites in all
  regions have a  significant downward trend in ANC.

• DOC  is increasing in only about 20% to 42% of the low
  ANC  lakes and streams of the Adirondack Mountains,
  Catskills/Northern Appalachian Plateau, and New Eng-
  land. The Adirondack Mountains have the highest per-
  centage (42%) of lakes with  an  increasing DOC trend.
  These results suggest that the change of DOC in the LTM
  catchments is  complex, with  the majority of low ANC
  waterbodies not changing over the past 18 years. Of the
  lakes  and streams with increasing DOC, no single envi-
  ronmental factor is likely for the cause of the increase.
  Declines  in sulfate deposition (Figure  5) and warmer
  seasonal and annual temperatures may have contributed
  to the rise in surface DOC.9

The ANC of northeastern U.S. lakes monitored under the
TIME program was also evaluated for the  1991-1994 and
2006-2008 periods to  assess the impacts of ARP imple-
mentation.  The analysis in Figure 13 compares average
ANC levels for the northeastern lakes that had data in each
time period. From 1991 to 1994, 7.5 percent of lakes had
three-year mean ANC levels below 0 ueq/L. These lakes are
categorized as "acute concern," in which a near complete
loss of fish populations is expected, and planktonic com-
munities have low diversity and are dominated  by acid-
tolerant forms (see Table 4). The percentage of lakes in
this category dropped to 4.3 percent in 2006 to 2008 (see
Figure 13). Additionally, the percentage of elevated con-
cern lakes dropped from  13.8 percent for the 1991-1994
time period to  10.1  percent from  2006-2008, while the
percentage of lakes with in the moderate concern category
increased from 6.4 to 13.5 percent. These results point to a
decrease in acidity, particularly for the subset of lakes with
low ANC.
Figure 13: Northeastern Lakes by ANC Status Category,
1991-1994 versus 2006-2008

 a  40.

 |  30-

             I 2006-2008
      Acute Concern Elevated Concern Moderate Concern  Low Concern
       (<0^eq/L)   (0-50 Meq/L)    (50-100 jjeq/L)   (>100 ^eq/L)
• Based on 305 EMAP/TIME monitoring sites.
• 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
Source: EPA, 2010

Acid Rain and Related Programs: 2009 Environmental Results
Table 4: Aquatic Ecosystem Status Categories for Northeastern Lakes
Category Label 1 ANC Level 1 Expected Ecological Effects
Acute Concern
Elevated Concern
Moderate Concern
Low Concern
< 0 micro equivalent per Liter (ueq/L)
0-50 ueq/L
50-100 ueq/L
> 100 ueq/L
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.
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.
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, 2010
Critical Loads and Exceedances

The Northeast and Mid-Appalachian Highlands in the east-
ern United States has been strongly affected by acidic depo-
sition. Many of the small forested watersheds, particularly
along the Appalachian Mountain spine, have soils and sur-
face waters that are unable to buffer the acidity from acidic
deposition, causing the ecosystem to acidify. As a result the
health of some tree species and aquatic biota have declined
or species, such as brook trout, are no longer present in the

Since the early 1980s, acidic deposition has acidified many
lakes and many miles of streams in the Northeast and Mid-
Appalachian Highlands.13 However, with the implementa-
tion  of Title IV and other emission reduction programs,
acidic deposition has declined  throughout  the eastern
United States as emissions of NOX and SC>2 have declined
(see Acid Rain and Related Programs: 2009 Emission, Com-
pliance, and Market Analyses report  available at < www.>).   Surface
waters across the region have also shown signs  of recov-
ery as indicated by declining sulfate concentrations and
increasing ANC levels (see Table 3). In the 2007 and 2008
Acid Rain Progress Reports,  critical  loads were used to
gauge the extent to which the acid sensitive  areas of the
Adirondack Mountains and Central  Appalachian Moun-
tains have  recovered from acid deposition. Results from
these regions indicate that 10-15% of monitored lakes and
streams now receive levels of acid deposition that are gen-
erally low enough that aquatic ecosystems are protected in
comparison to deposition levels in the early 1990s.

The critical load approach is an assessment tool that  can
be used  to determine the degree to  which air pollution
may be affecting ecological health. A critical load is a quan-
titative estimate of exposure to one or more pollutants
below which significant harmful effects on specific sensi-
tive elements of the environment do not occur according
to present knowledge.14 This approach provides a useful
lens through which to assess the results of current poli-
cies and programs and to evaluate the potential value of
proposed policy options in terms of ecosystem protection.
The critical loads approach has been employed routinely
as an assessment tool for many years in the countries of
the European Union and Canada.  Building on past criti-
cal load studies, this analysis explores the extent to which
lakes in the Northeast and streams in the Mid-Appalachian
Highlands in Virginia and West Virginia are protected from
acidifying nitrogen and sulfur deposition as a result of re-
cent emission reductions.

The critical load for a lake or stream provides a benchmark
against which to assess the extent to which a waterbody is
potentially  at risk due to current acidic deposition levels.
The analysis focuses on the combined load of sulfur and
nitrogen deposition below which the ANC level would still
support healthy aquatic ecosystems.  If pollutant exposure
is less than the critical load,  adverse  ecological effects
(e.g., reduced reproductive success,  stunted growth, loss
of biological diversity) are not anticipated, and recovery is
expected over time if an ecosystem has been damaged by
past exposure. A critical load exceedance is the measure
of pollutant exposure above the critical load. This means
pollutant exposure is higher than, or "exceeds," the critical
load and the ecosystem continues to be exposed to damag-
ing levels of pollutants.

The scientific research community has recently completed
and published many peer-reviewed scientific articles that
advance the tools for calculating critical loads in the United
States. Drawing on the methods from  the peer-reviewed
scientific literature,15 critical  loads  were  calculated for
over 1,300  lakes and streams using the Steady-State Wa-

                                                 Acid Rain and Related Programs: 2009 Environmental Results
Figure 14: Lake and Stream Exceedences of Estimated Critical
Loads (Sulfur + Nitrogen) for Total Nitrogen and Sulfur Deposi-
tion for the Period 1989-1991
 Critical Load Exceedances,
 •  Does Not Exceed Critical Load
 O  Within 10% of the Critical Load
 •  Exceeds Critical Load
Source: EPA, 2010

ter Chemistry (SSWC) model and the Model of Acidifica-
tion of Groundwater In Catchments (MAGIC) model. These
critical load estimates represent only lakes and streams
where surface water samples have been collected and do
not represent all types of lakes and streams in the study
region. Water quality chemistry include data collected by
EPA-administered surface  water  monitoring and assess-
ment programs, such as the National Surface Water Survey
(NSWS), Environmental Monitoring and Assessment Pro-
gram (EMAP), the TIME program, and the LTM program.
The lakes and streams associated with these programs
consist of a subset of lakes  and  streams that are located in
areas most affected by acid deposition and many sites pro-
vide long term records of surface acidification. The NSWS
and EMAP programs employ probability sampling; each
monitoring site was chosen statistically from a  predefined
target population. In New England and the Mid-Appala-
chian Highlands, the target populations include lakes and
streams likely to be responsive to changes in acidic deposi-
Figure 15: Lake and Stream Exceedences of Estimated Critical
Loads (Sulfur + Nitrogen) for Total Nitrogen and Sulfur Deposi-
tion for the Period 2007-2009
 Critical Load Exceedances,
 O Does Not Exceed Critical Load
 O Within 10% of the Critical Load
 • Exceeds Critical Load
Source: EPA, 2010

For this particular analysis, the critical load represents the
combined deposition loads of sulfur and nitrogen to which
a lake or stream could be subjected and still have a calcu-
lated ANC of 50 ueq/L or higher. While a critical load can be
calculated for any ANC level, this level was chosen because
it tends to protect most fish and other aquatic organisms,
although systems can become episodically acidic and some
sensitive species maybe lost. Critical loads of combined to-
tal sulfur and nitrogen deposition are expressed in terms of
ionic charge balance as milliequivalents per square meter
per year  (meq/m2/yr). When actual measured deposition
of nitrogen and sulfur  is greater than the critical load, the
critical load is "exceeded," meaning that  combined sulfur
and nitrogen deposition was greater than a lake  or stream
could sustain  and still maintain the calculated ANC level of
50 ueq/L or above. In  order to assess the extent to which
regional lake and stream ecosystems are  protected by the
emission reductions achieved by Title  IV, this case study
compares the amount of deposition systems can receive—
the critical  load—to measured deposition for the  period
before implementation of the ARP (1989  to 1991) and for
a recent period after ARP implementation (2007 to 2009).

Acid Rain and Related Programs: 2009 Environmental Results
Overall, this critical load analysis shows that emission re-
ductions achieved by the ARP have resulted in improved
environmental conditions and  increased ecosystem pro-
tection in the Northeast and Mid-Appalachian Highlands.
For the period from 2007 to 2009, 26% of the waterbodies
examined received levels of combined sulfur and nitrogen
deposition that exceeded the critical load (Figure 15). This
is an improvement when compared to the 1989-1991 pe-
riod before implementation of Title IV, during which 42%
of waterbodies exceeded the critical load (Figure 14). Thus,
during the 2007 to 2009 period, 37% of those waterbod-
ies exceeding their  critical  load in the previous period
were no longer receiving sulfur and nitrogen deposition
loads that threaten the health of these ecosystems. Areas
with the largest concentration  of lakes where acid depo-
sition currently is greater than — or exceeds — estimated
critical loads include the Adirondack  mountain region in
New York, southern  New Hampshire and Vermont, north-
ern Massachusetts, northeast Pennsylvania, and the cen-
tral Appalachian Mountains  of Virginia and West Virginia
(Figure 15).

Reductions  in acidic deposition have occurred over the
past  decade, as  demonstrated by the  deposition  maps in
Figures 5 and 6 on  page 5.  However, this comparison of
past  and current total  deposition estimates with critical
loads estimates  indicates that acid-sensitive ecosystems in
the Eastern United States are still at risk of acidification at
current deposition levels. As a result, additional reductions
in acidic deposition from current levels might be necessary
to fully protect these ecosystems.

Ammonium Deposition in the Eastern United States
Ammonium (NH4+) forms when ammonia gas (NHs) reacts
with sulfur, nitrogen and other acidic compounds in the
atmosphere. Ammonium is of concern because it contrib-
utes to the formation of fine particles, which have negative
human health effects and can cause reduced visibility. Wet
deposition of NH4+ is measured at NTN sites across the US
while dry deposition of NH4+ is estimated from weekly fil-
ter pack concentrations  measured at CASTNET sites and
estimated deposition velocities. While total nitrate deposi-
tion fluxes have decreased 24% in the eastern US, wet de-
position has only decreased 4% since 1990. The 36 CAST-
NET sites used for this analysis are shown in Figure  16.
Figure 17 shows the wet and dry NH4+ deposition flux from
36 CASTNET sites between 1990 and 2009. The three year
average for wet and dry  NH4+ flux between 1990-1992 is
2.34 and 0.42 kg-N/ha, respectively. The most recent three
year average, 2007-2009, the wet NH4+ flux is 2.24 and the
dry flux is 0.31 kg-N/ha. While there  is a small decrease
in dry deposition (0.11 kg-N/ha), the 3 year average total
nitrogen deposition decreased from 7.62 to 5.80 kg-N/ha
(1.82 kg-N/ha), a larger reduction likely attributed to the
large reductions we have seen in particulate nitrate (NOy
) deposition fluxes. The NH4+ ion is the basic component
of PM2.5 formation, causing negative health effects and re-
duced visibility. In addition, NH4+ leads to eutrophication
of ecosystems. While NOX emissions under the Acid Rain
Program, the NOX Budget Trading Program, and CAIR have
resulted in significant improvements in air quality, gaseous
     emissions have been increasing. This will provide a
Figure 16: Eastern U.S. CASTNET Monitoring Locations Analyzed
for Ammonium Deposition
Source: EPA, 2010

Figure 17: Measured Wet and Estimated Dry Deposition of
Ammonium at Eastern CASTNET sites, 1990-2009

1  1.0

                                                 Acid Rain and Related Programs: 2009 Environmental Results
challenge in the future for understanding changes in atmo-
spheric chemistry and nitrogen deposition to forests, lakes,
streams and other sensitive areas. Therefore, it is impor-
tant to continue monitoring long-term trends in air qual-
ity and deposition to further understand pollutants which
contribute to poor air quality.

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 col-
lecting these data and assessing the effectiveness of cap and
trade programs, including the Acid Rain Program. CAMD
then makes these data available to the public in readily us-
able and interactive formats. The CAMD website provides
a public resource for general information on how market-

Figure 18: US S02 Emissions and Sulfate Concentrations, 1990
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.

To increase  data transparency, EPA has created supple-
mentary maps that allow the user to display air market
program data geospatially on an interactive 3D platform.
Figures 18 and 19 are examples of these 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 differ-
ent 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 pro-
grams 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 this tool, visit the
website at  .
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 2010	

 Acid Rain and Related Programs: 2009 Environmental Results
 In another effort to increase data transparency, EPA reg-
 ularly posts updates of quarterly SC>2 and NOX emissions
 data  from coal-fired power plants  controlled under the
 ARP and other programs to make it easy for the public to
 track changes in emissions from these sources (available
 at:   ).
 The data presented on the  quarterly emissions tracking
 website compare emissions, emission rates, and heat input
 from power plant units in the ARP. These files graphically
 and spatially compare quarterly emission data from the
 most recent completed quarter of 2010 with data for the
 same quarter from 2008.

 Interactive motion charts are a key feature on the quarterly
 tracking website. Figure 20 on page  17 shows examples of
 motion charts created  to show changes in ARP SC>2 emis-
 sions and SC>2 emission rates over time  (from 1990 to
 Figure 19: US S02 Emissions and Sulfate Concentrations, 2009
2009). These motion charts show, historically, how coal-
fired power plants have responded to the ARP. Each circle
on the motion chart represents a facility in the ARP with
one or more units that burn coal to create electricity. The
size and color of these circles  tell us something about the
facility. To the right of the motion chart you'll find two leg-
ends. The color spectrum at the top represents the emis-
sions generated per unit of fuel (also known as the SC>2
emission rate),  with warmer colors  (yellow through red)
representing a high emission rate and cooler colors (green
through blue) representing a  low emission rate. The size
of the circle on  the chart is proportional to the emissions
from that plant. On the interactive  mapping website,  the
user can watch this data move  through time by clicking the
play button.

For more  information or to utilize these and other tools,
visit the website at <>.
 Note: This example depicts 2009 S02 emissions from ARP sources along with 2009
 sulfate concentration data as measured by the CASTNET monitoring program.
 Source: EPA, 2010

                                                        Acid Rain and Related Programs: 2009 Environmental Results
Figure 20: Motion Charts of Annual ARP Coal-Fired Emissions,
S02 Emission Rates and Heat Input over Time, 1990 and 2009
Source: EPA, 2010

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