the current SO2 and NO2 secondary standards were
not exceeded at that time (U.S. EPA, 2011, Figures 6-1 and 6-2) in
locations where negative ecological effects have been observed. In many
locations, SO2 and NO2 concentrations are
substantially below the levels of the secondary standards. This pattern
suggests that levels of deposition and any negative effects on
ecosystems due to deposition of oxides of nitrogen and sulfur under
recent conditions are occurring even though areas meet or are below
current standards. In addition, based on conclusions in the REA, these
levels will not decline in the future to levels below which it is
reasonable to anticipate effects.
    In determining the adequacy of the current secondary standards for
oxides of nitrogen and sulfur the PA considered the extent to which
ambient deposition contributes to loadings in ecosystems. Since the
last review of the secondary standard for oxides of nitrogen, a great
deal of information on the contribution of atmospheric deposition
associated with ambient oxides of nitrogen has become available. The
REA presents a thorough assessment of the contribution of oxidized
nitrogen relative to total nitrogen deposition throughout the United
States, and the relative contributions of ambient oxidized and reduced
forms of nitrogen. The REA concludes that based on that analysis,
ambient oxides of nitrogen are a significant component of atmospheric
nitrogen deposition, even in areas with relatively high rates of
reduced nitrogen deposition. In addition, atmospheric deposition of
oxidized nitrogen contributes significantly to total nitrogen loadings
in nitrogen sensitive ecosystems.
    The ISA summarizes the available studies of relative nitrogen
contribution and finds that in much of the United States, oxides of
nitrogen contribute from 50 to 75 percent of total atmospheric
deposition relative to total reactive nitrogen, which includes oxidized
and reduced nitrogen species (U.S. EPA, 2008, section 2.8.4). Although
the proportion of total nitrogen loadings associated with atmospheric
deposition of nitrogen varies across locations, the ISA indicates that
atmospheric nitrogen deposition is the main source of new anthropogenic
nitrogen to most headwater streams, high elevation lakes, and low-order
streams. Atmospheric nitrogen deposition contributes to the total
nitrogen load in terrestrial, wetland, freshwater and estuarine
ecosystems that receive nitrogen through multiple pathways. In several
large estuarine systems, including the Chesapeake Bay, atmospheric
deposition accounts for between 10 and 40 percent of total nitrogen
loadings (U.S. EPA, 2008).
    Atmospheric concentrations of oxides of sulfur account for nearly
all sulfur deposition in the U.S. For the period 2004-2006, mean sulfur
deposition in the United States was greatest east of the Mississippi
River with the highest deposition amount, 21.3 kg S/ha-yr, in the Ohio
River Valley where most recording stations reported 3-year averages >10
kg S/ha-yr. Numerous other stations in the East reported S deposition
>5 kg S/ha-yr. Total sulfur deposition in the United States west of

the 100th meridian was relatively low, with all recording stations
reporting <2 kg S/ha-yr and many reporting <1 kg S/ha-yr. Sulfur was
primarily deposited in the form of wet SO42-
followed in decreasing order by a smaller proportion of dry
SO2 and a much smaller proportion of deposition as dry
SO42-.
    As discussed throughout the REA (U.S. EPA, 2009 and section II.B
above), there are several key areas of risk that are associated with
ambient concentrations of oxides of nitrogen and sulfur. As noted
earlier, in previous reviews of the secondary standards for oxides of
nitrogen and sulfur, the standards were designed to protect against
direct exposure of plants to ambient concentrations of the pollutants.
A significant shift in understanding of the effects of oxides of
nitrogen and sulfur has occurred since the last reviews, reflecting the
large amount of research that has been conducted on the effects of
deposition of nitrogen and sulfur to ecosystems. The most significant
current risks of adverse effects to public welfare are those related to
deposition of oxides of nitrogen and sulfur to both terrestrial and
aquatic ecosystems. These risks fall into two categories, acidification
and nutrient enrichment, which were emphasized in the REA as most
relevant to evaluating the adequacy of the existing standards in
protecting public welfare from adverse ecological effects.
(a) Aquatic Acidification
    The focus of the REA case studies was to determine whether
deposition of sulfur and oxidized nitrogen in locations where ambient
oxides of nitrogen and sulfur were at or below the current standards
resulted in acidification and related effects, including episodic
acidification and mercury methylation. Based on the case studies
conducted for lakes in the Adirondacks and streams in Shenandoah
National Park (case studies are discussed more fully in section II.B
and U.S. EPA, 2009), there is significant risk to acid sensitive
aquatic ecosystems at atmospheric concentrations of oxides of nitrogen
and sulfur at or below the current standards. The REA also strongly
supports a relationship between atmospheric deposition of oxides of
nitrogen and sulfur and loss of ANC in sensitive ecosystems and
indicates that ANC is an excellent indicator of aquatic acidification.
The REA also concludes that at levels of deposition associated with
oxides of nitrogen and sulfur concentrations at or below the current
standards, ANC levels are expected to be below benchmark values that
are associated with significant losses in fish species richness.
    Significant portions of the United States are acid sensitive, and
current deposition levels exceed those that would allow recovery of the
most acid sensitive lakes in the Adirondacks (U.S. EPA, 2008, Executive
Summary). In addition, because of past loadings, areas of the
Shenandoah are sensitive to current deposition levels (U.S. EPA, 2008,
Executive Summary). Parts of the West are naturally less sensitive to
acidification and subjected to lower deposition (particularly oxides of
sulfur) levels relative to the eastern United States, and as such, less
focus in the ISA is placed on the adequacy of the existing standards in
these areas, with the exception of the mountainous areas of the West,
which experience episodic acidification due to deposition.
    In describing the effects of acidification in the two case study
areas the REA uses the approach of describing benchmarks in terms of
ANC values. Many locations in sensitive areas of the United States have
ANC levels below benchmark levels for ANC classified as severe,
elevated, or moderate concern (U.S. EPA, 2011, Figure 2-1). The average
current ANC levels across 44 lakes in the Adirondack case study area is
62.1 [mu]eq/L (moderate concern). However, 44 percent of lakes had
deposition levels exceeding the critical load for an ANC of 50 [mu]eq/L
(elevated), and 28 percent of lakes had deposition levels exceeding the
(higher) critical load for an ANC of 20 [mu]eq/L (severe) (U.S. EPA,
2009, section <IP_ADDRESS>). This information indicates that almost half of
the 44 lakes in the Adirondacks case study area are at an elevated
concern level, and almost a third are at a severe concern level. These
levels are associated with greatly diminished fish species diversity,
and losses in the health and reproductive capacity of remaining
populations. Based on assessments of the relationship between number of
fish species and ANC level in both the Adirondacks and Shenandoah
areas, the number of fish species is decreased by over half at an ANC
level of 20 [mu]eq/L relative to an ANC level at 100 [mu]eq/L (U.S.
EPA, 2009, Figure 4.2-1). When extrapolated to the full population of
lakes in the Adirondacks area using weights based on the EMAP
probability survey (U.S. EPA, 2009, section <IP_ADDRESS>), 36 percent of
lakes exceeded the critical load for an ANC of 50 [mu]eq/L and 13
percent of lakes exceeded the critical load for an ANC of 20 [mu]eq/L.
    Many streams in the Shenandoah case study area also have levels of
deposition that are associated with ANC levels classified as severe,
elevated, or moderate concern. The average ANC under recent conditions
for the 60 streams evaluated in the Shenandoah case study area is 57.9
[mu]eq/L, indicating moderate concern. However, 85 percent of these
streams had recent deposition exceeding the critical load for an ANC of
50 [mu]eq/L, and 72 percent exceeded the critical load for an ANC of 20
[mu]eq/L. As with the Adirondacks area, this information suggests that
ANC levels may decline in the future and significant numbers of
sensitive streams in the Shenandoah area are at risk of adverse impacts
on fish populations if recent conditions persist. Many other streams in
the Shenandoah area are also likely to experience conditions of
elevated to severe concern based on the prevalence in the area of
bedrock geology associated with increased sensitivity to acidification
suggesting that effects due to stream acidification could be widespread
in the Shenandoah area (U.S. EPA, 2009, section <IP_ADDRESS>).
    In addition to these chronic acidification effects, the ISA notes
that ``consideration of episodic acidification greatly increases the
extent and degree of estimated effects for acidifying deposition on
surface waters'' (U.S. EPA, 2008, section <IP_ADDRESS>). Some studies show
that the number of lakes that could be classified as acid-impacted
based on episodic acidification is 2 to 3 times the number of lakes
classified as acid-impacted based on chronic ANC. These episodic
acidification events can have long-term effects on fish populations
(U.S. EPA, 2008, section <IP_ADDRESS>). Under recent conditions, episodic
acidification has been observed in locations in the eastern United
States and in the mountainous western United States (U.S. EPA, 2008,
section <IP_ADDRESS>).
    The ISA, REA and PA all conclude that the current standards are not
adequate to protect against the adverse impacts of aquatic
acidification on sensitive ecosystems. A recent survey, as reported in
the ISA, found sensitive streams in many locations in the United
States, including the Appalachian Mountains, the Coastal Plain, and the
Mountainous West (U.S. EPA, 2008, section <IP_ADDRESS>). In these sensitive
areas, between 1 and 6 percent of stream kilometers are chronically
acidified. The REA further concludes that both the Adirondack and
Shenandoah case study areas are currently receiving deposition from
ambient oxides of nitrogen and sulfur in excess of their ability to
neutralize such inputs. In addition, based on the current emission
scenarios, forecast modeling out to the year 2020 as well as 2050
indicates a large number of streams in these areas will still be

adversely impacted (section II.B). Based on these considerations, the
PA concludes that the current secondary NAAQS for oxides of nitrogen
and sulfur do not provide adequate protection of sensitive ecosystems
with regard to aquatic acidification.
(b) Terrestrial Acidification
    Based on the terrestrial acidification case studies, Kane
Experimental Forest in Pennsylvania and Hubbard Brook Experimental
Forest described in section II.B of sugar maple and red spruce habitat,
the REA concludes that there is significant risk to sensitive
terrestrial ecosystems from acidification at atmospheric concentrations
of NO2 and SO2 at or below the current standards.
The ecological indicator selected for terrestrial acidification is the
BC/Al, which has been linked to tree health and growth. The results of
the REA strongly support a relationship between atmospheric deposition
of oxides of nitrogen and sulfur and BC/Al, and that BC/Al is a good
indicator of terrestrial acidification. At levels of deposition
associated with oxides of nitrogen and sulfur concentrations at or
below the current standards, BC/Al levels are expected to be below
benchmark values that are associated with significant effects on tree
health and growth. Such degradation of terrestrial ecosystems could
affect ecosystem services such as habitat provisioning, endangered
species, goods production (timber, syrup, etc.) among others.
    Many locations in sensitive areas of the United States have BC/Al
levels below benchmark levels classified as providing low to
intermediate levels of protection to tree health. At a BC/Al ratio of
1.2 (intermediate level of protection), red spruce growth can be
reduced by 20 percent. At a BC/Al ratio of 0.6 (low level of
protection), sugar maple growth can be decreased by 20 percent. The REA
did not evaluate broad sensitive regions. However, in the sugar maple
case study area (Kane Experimental Forest), recent deposition levels
are associated with a BC/Al ratio below 1.2, indicating between
intermediate and low level of protection, which would indicate the
potential for a greater than 20 percent reduction in growth. In the red
spruce case study area (Hubbard Brook Experimental Forest), recent
deposition levels are associated with a BC/Al ratio slightly above 1.2,
indicating slightly better than an intermediate level of protection
(U.S. EPA, 2009, section <IP_ADDRESS>).
    Over the full range of sugar maple, 12 percent of evaluated forest
plots exceeded the critical loads for a BC/Al ratio of 1.2, and 3
percent exceeded the critical load for a BC/Al ratio of 0.6. However,
there was large variability across states. In New Jersey, 67 percent of
plots exceeded the critical load for a BC/Al ratio of 1.2, while in
several states on the outskirts of the range for sugar maple (e.g.
Arkansas, Illinois) no plots exceeded the critical load for a BC/Al
ratio of 1.2. For red spruce, overall 5 percent of plots exceeded the
critical load for a BC/Al ratio of 1.2, and 3 percent exceeded the
critical load for a BC/Al ratio of 0.6. In the major red spruce
producing states (Maine, New Hampshire, and Vermont), critical loads
for a BC/Al ratio of 1.2 were exceeded in 0.5, 38, and 6 percent of
plots, respectively.
    The ISA, REA and PA all conclude that the current standards are not
adequate to protect against the adverse impacts of terrestrial
acidification on sensitive ecosystems. As stated in the REA and PA, the
main drawback, with the understanding of terrestrial acidification lies
in the sparseness of available data by which we can predict critical
loads and that the data are based on laboratory responses rather than
field measurements. Other stressors that are present in the field but
that are not present in the laboratory may confound this relationship.
The REA does however, conclude that the case study results, when
extended to a 27 state region, show that nitrogen and sulfur acidifying
deposition in the sugar maple and red spruce forest areas caused the
calculated Bc/Al ratio to fall below 1.2 (the intermediate level of
protection) in 12 percent of the sugar maple plots and 5 percent of the
red spruce plots; however, results from individual states ranged from 0
to 67 percent of the plots for sugar maple and 0 to 100 percent of the
plots for red spruce.
(c) Terrestrial Nutrient Enrichment
    Nutrient enrichment effects are due to nitrogen loadings from both
atmospheric and non-atmospheric sources. Evaluation of nutrient
enrichment effects requires an understanding that nutrient inputs are
essential to ecosystem health and that specific long-term levels of
nutrients in a system affect the types of species that occur over long
periods of time. Short-term additions of nutrients can affect species
competition, and even small additions of nitrogen in areas that are
traditionally nutrient poor can have significant impacts on
productivity as well as species composition. Most ecosystems in the
United States are nitrogen-limited, so regional decreases in emissions
and deposition of airborne nitrogen compounds could lead to some
decrease in growth of the vegetation that surrounds the targeted
aquatic system but as discussed below evidence for this is mixed.
Whether these changes in plant growth are seen as beneficial or adverse
will depend on the nature of the ecosystem being assessed.
    Information on the effects of changes in nitrogen deposition on
forestlands and other terrestrial ecosystems is very limited. The
multiplicity of factors affecting forests, including other potential
stressors such as ozone, and limiting factors such as moisture and
other nutrients, confound assessments of marginal changes in any one
stressor or nutrient in forest ecosystems. The ISA notes that only a
fraction of the deposited nitrogen is taken up by the forests, most of
the nitrogen is retained in the soils (U.S. EPA, 2008, section
<IP_ADDRESS>). In addition, the ISA indicates that forest management
practices can significantly affect the nitrogen cycling within a forest
ecosystem, and as such, the response of managed forests to nitrogen
deposition will be variable depending on the forest management
practices employed in a given forest ecosystem (U.S. EPA, 2008, Annex C
C.6.3). Increases in the availability of nitrogen in nitrogen-limited
forests via atmospheric deposition could increase forest production
over large non-managed areas, but the evidence is mixed, with some
studies showing increased production and other showing little effect on
wood production (U.S. EPA, 2008, section 3.3.9). Because leaching of
nitrate can promote cation losses, which in some cases create nutrient
imbalances, slower growth and lessened disease and freezing tolerances
for forest trees, the net effect of increased N on forests in the
United States is uncertain (U.S. EPA, 2008, section 3.3.9).
    The scientific literature has many examples of the deleterious
effects caused by excessive nitrogen loadings to terrestrial systems.
Several studies have set benchmark values for levels of N deposition at
which scientifically adverse effects are known to occur. Large areas of
the country appear to be experiencing deposition above these
benchmarks. The ISA indicates studies that have found that at 3.1 kg N/
ha/yr, the community of lichens begins to change from acidophytic to
tolerant species; at 5.2 kg N/ha/yr, the typical dominance by
acidophytic species no longer occurs; and at 10.2 kg N/ha/yr,
acidophytic lichens are totally lost from the community. Additional
studies in the Colorado Front Range of the Rocky Mountain National Park
support these findings. These three values (3.1, 5.2,

and 10.2 kg/ha/yr) are one set of ecologically meaningful benchmarks
for the mixed conifer forest (MCF) of the pacific coast regions. Nearly
all of the known sensitive communities receive total nitrogen
deposition levels above the 3.1 N kg/ha/yr ecological benchmark
according to the 12 km, 2002 CMAQ/NADP data, with the exception of the
easternmost Sierra Nevadas. The MCFs in the southern portion of the
Sierra Nevada forests and nearly all MCF communities in the San
Bernardino forests receive total nitrogen deposition levels above the
5.2 N kg/ha/yr ecological benchmark.
    Coastal Sage Scrub communities are also known to be sensitive to
community shifts caused by excess nitrogen loadings. Studies have
investigated the amount of nitrogen utilized by healthy and degraded
CSS systems. In healthy stands, the authors estimated that 3.3 kg N/ha/
yr was used for CSS plant growth. It is assumed that 3.3 kg N/ha/yr is
near the point where nitrogen is no longer limiting in the CSS
community and above which level community changes occur, including
dominance by invasive species and loss of coastal sage scrub.
Therefore, this amount can be considered an ecological benchmark for
the CSS community. The majority of the known CSS range is currently
receiving deposition in excess of this benchmark. Thus, the REA
concludes that recent conditions where oxides of nitrogen ambient
concentrations are at or below the current oxides of nitrogen secondary
standards are not adequate to protect against anticipated adverse
impacts from N nutrient enrichment in sensitive ecosystems.
(d) Aquatic Nutrient Enrichment
    The REA aquatic nutrient enrichment case studies focused on coastal
estuaries and revealed that while current ambient loadings of
atmospheric oxides of nitrogen are contributing to the overall
depositional loading of coastal estuaries, other non-atmospheric
sources are contributing in far greater amounts in total, although
atmospheric contributions are as large as some other individual source
types. The ability of current data and models to characterize the
incremental adverse impacts of nitrogen deposition is limited, both by
the available ecological indicators, and by the inability to attribute
specific effects to atmospheric sources of nitrogen. The REA case
studies used ASSETS EI as the ecological indicator for aquatic nutrient
enrichment. This index is a six level index characterizing overall
eutrophication risk in a water body. This indictor is not sensitive to
changes in nitrogen deposition within a single level of the index. In
addition, this type of indicator does not reflect the impact of
nitrogen deposition in conjunction with other sources of nitrogen.
    Based on the above considerations, the REA concludes that the
ASSETS EI is not an appropriate ecological indicator for estuarine
aquatic eutrophication and that additional analysis is required to
develop an appropriate indicator for determining the appropriate levels
of protection from N nutrient enrichment effects in estuaries related
to deposition of oxides of nitrogen. As a result, the EPA is unable to
make a determination as to the adequacy of the existing secondary
oxides of nitrogen standard in protecting public welfare from nitrogen
nutrient enrichment effects in estuarine aquatic ecosystems.
    Additionally, nitrogen deposition can alter species composition and
cause eutrophication in freshwater systems. In the Rocky Mountains, for
example, deposition loads of 1.5 to 2 kg/ha/yr which are well within
current ambient levels are known to cause changes in species
composition in diatom communities indicating impaired water quality
(U.S. EPA, 2008, section <IP_ADDRESS>). This suggests that the existing
secondary standard for oxides of nitrogen does not protect such
ecosystems and their resulting services from impairment.
(e) Other Effects
    An important consideration in looking at the effects of deposition
of oxides of sulfur in aquatic ecosystems is the potential for
production of MeHg, a neurotoxic contaminant. The production of
meaningful amounts of MeHg requires the presence of
SO42- and mercury, and where mercury is present,
increased availability of SO42- results in
increased production of MeHg. There is increasing evidence on the
relationship between sulfur deposition and increased methylation of
mercury in aquatic environments; this effect occurs only where other
factors are present at levels within a range to allow methylation. The
production of MeHg requires the presence of SO42-
and mercury, but the amount of MeHg produced varies with oxygen
content, temperature, pH and supply of labile organic carbon (U.S. EPA,
2008, section 3.4). In watersheds where changes in sulfate deposition
did not produce an effect, one or several of those interacting factors
were not in the range required for meaningful methylation to occur
(U.S. EPA, 2008, section 3.4). Watersheds with conditions known to be
conducive to mercury methylation can be found in the northeastern
United States and southeastern Canada (U.S. EPA, 2009, section 6).
    With respect to sulfur deposition and mercury methylation, the
final ISA determined that ``[t]he evidence is sufficient to infer a
causal relationship between sulfur deposition and increased mercury
methylation in wetlands and aquatic environments.'' However, the EPA
did not conduct a quantitative assessment of the risks associated with
increased mercury methylation under current conditions. As such, the
EPA is unable to make a determination as to the adequacy of the
existing SO2 secondary standards in protecting against
welfare effects associated with increased mercury methylation.
c. Summary of Adequacy Considerations
    In summary, the PA concludes that currently available scientific
evidence and assessments clearly call into question the adequacy of the
current standards with regard to deposition-related effects on
sensitive aquatic and terrestrial ecosystems, including acidification
and nutrient enrichment. Further, the PA recognizes that the elements
of the current standards--indicator, averaging time, level and form--
are not ecologically relevant, and are thus not appropriate for
standards designed to provide such protection. Thus, the PA concludes
that consideration should be given to establishing a new ecologically
relevant multi-pollutant, multimedia standard to provide appropriate
protection from deposition-related ecological effects of oxides of
nitrogen and sulfur on sensitive ecosystems, with a focus on protecting
against adverse effects associated with acidifying deposition in
sensitive aquatic ecosystems.
2. CASAC Views
    In a letter to the Administrator (Russell and Samet 2011a), the
CASAC Oxides of Nitrogen and Oxides of Sulfur Panel, with full
endorsement of the chartered CASAC, unanimously concluded that:

    ``EPA staff has demonstrated through the Integrated Science
Assessment (ISA), Risk and Exposure Characterization (REA) and the
draft PA that ambient NOX and SOX can have,
and are having, adverse environmental impacts. The Panel views that
the current NOX and SOX secondary standards
should be retained to protect against direct adverse impacts to
vegetation from exposure to gas phase exposures of these two
families of air pollutants. Further, the ISA, REA and draft PA
demonstrate that adverse impacts to aquatic ecosystems are also
occurring due to deposition of NOX and SOX.
Those impacts

include acidification and undesirable levels of nutrient enrichment
in some aquatic ecosystems. The levels of the current NOX
and SOX secondary NAAQS are not sufficient, nor the forms
of those standards appropriate, to protect against adverse
depositional effects; thus a revised NAAQS is warranted.''

    In addition, with regard to the joint consideration of both oxides
of nitrogen and oxides of sulfur as well as the consideration of
deposition-related effects, CASAC concluded that the PA had developed a
credible methodology for considering such effects. The Panel stated
that ``the Policy Assessment develops a framework for a multi-
pollutant, multimedia standard that is ecologically relevant and
reflects the combined impacts of these two pollutants as they deposit
to sensitive aquatic ecosystems.''
3. Administrator's Proposed Conclusions
    Based on the above considerations and taking into account CASAC
advice, in the proposed rule the Administrator considered the adequacy
of the current NO2 and SO2 secondary standards
with regard to both direct effects on vegetation, as well as on
deposition-related effects on sensitive ecosystems. With regard to
direct phytotoxic effects on vegetation, the Administrator concluded
that the current secondary standards are adequately protective, and
thus proposed to retain the current NO2 and SO2
secondary standards for that purpose.
    With regard to deposition-related effects, the Administrator first
considered the appropriateness of the structure of the current
standards to address ecological effects of concern. Based on the
evidence as well as considering the advice given by CASAC, the
Administrator concluded that the elements of the current standards are
not ecologically relevant and thus are not appropriate to provide
protection of ecosystems. In considering the adequacy of protection
with regard to deposition-related effects, the Administrator considered
the full nature of ecological effects related to the deposition of
ambient oxides of nitrogen and sulfur into sensitive ecosystems across
the country. Based on the evidence and information evaluated in the
ISA, REA, and PA, and taking into account CASAC advice, the
Administrator concluded that current levels of oxides of nitrogen and
sulfur are sufficient to cause acidification of both aquatic and
terrestrial ecosystems, nutrient enrichment of terrestrial ecosystems
and contribute to nutrient enrichment effects in estuaries that could
be considered adverse, and that the current secondary standards do not
provide adequate protection from such effects.
    Having reached these conclusions, the Administrator determined that
it was appropriate to consider alternative standards that are
ecologically relevant. These considerations, as discussed below in
section III, supported the conclusion that the current secondary
standards are neither appropriate nor adequate to protect against
deposition-related effects.

C. Comments on Adequacy of the Current Standards

    The above sections outline the effects evidence and assessments
(section II.A) used by the Administrator to inform her proposed
judgments about the adequacy of the current secondary NO2
and SO2 standards with regard to both direct effects
associated with gas-phase oxides of nitrogen and sulfur (section
II.B.1) as well effects associated with deposition of oxides of
nitrogen and sulfur to sensitive aquatic and terrestrial ecosystems
(section II.B.2). This section discusses the comments received from the
public regarding the adequacy of the current secondary standards with
regard to both direct and deposition-related effects. Comments related
to the EPA's authority to address deposition-related effects through
the NAAQS are discussed above in section I.E. Comments related to the
EPA's proposed conclusions regarding alternative secondary standards
are discussed below in section III.D.
1. Adequacy of Current Secondary Standards To Address Direct Effects
    The current secondary NO2 and SO2 secondary
standards were set in 1971 to protect against direct effects of gaseous
oxides of nitrogen and sulfur. For oxides of nitrogen, the current
secondary NO2 standard is an annual standard set to protect
against adverse effects on vegetation from direct exposure to ambient
oxides of nitrogen. For oxides of sulfur, the current secondary
standard is a 3-hour standard intended to provide protection for plants
from the direct foliar damage associated with atmospheric
concentrations of SO2. As discussed above in section II.B.1,
the Administrator proposed to conclude that the current secondary
standards are adequate to protect against direct phytotoxic effects on
vegetation, and proposed to retain the current standards for that
purpose. Many commenters supported the EPA's proposed decision to
retain the current secondary standards for various reasons related to
their comments on alternative standards (as discussed below in section
III.D), a few commenters (Alliance of Automobile Manufacturers (AAM),
Pennsylvania Dept. of Environmental Protection) specifically expressed
the view that the current standards provide requisite protection from
the direct effects on vegetation from exposures to gaseous oxides of
nitrogen and sulfur, and no commenters opposed retention of the current
secondary standards.
2. Adequacy of Current Secondary Standards to Address Deposition-
Related Effects
    As discussed above in section II.B.2, with regard to deposition-
related effects, the Administrator proposed to conclude that the
elements of the current secondary standards are not ecologically
relevant, and thus not appropriate to provide protection of ecosystems,
and that they do not provide adequate protection from such
acidification and nutrient enrichment effects in both aquatic and
terrestrial ecosystems. Having reached these proposed conclusions, she
determined that it was appropriate to consider alternative standards
that are ecologically relevant.
    One group of commenters that addressed the adequacy of the current
standards with regard to deposition-related effects included
environmental organizations (Earthjustice, on behalf of the Appalachian
Mountain Club, National Parks Conservation Association, Sierra Club,
and Clean Air Council; the Center for Biological Diversity; the Nature
Conservancy; Adirondack Council; Chesapeake Bay Foundation), the U.S.
Department of the Interior, NESCAUM, New York Dept. of Environmental
Conservation, and two tribes. These commenters generally expressed the
view that the current secondary standards do not provide adequate
protection from deposition-related effects. More specifically, some of
these commenters stated that there was overwhelming evidence of
adversity to sensitive aquatic ecosystems from acidifying deposition.
These commenters cited a broad range of scientific evidence that
aquatic acidification was ongoing under current conditions allowed by
the current secondary standards, and that this acidification
represented an adverse effect on public welfare. Several commenters
noted that CASAC had agreed that deposition-related effects were
ongoing and harmful and that current standards were not adequate to
prevent these effects.
    Among these commenters, some also expressed the view that current
standards were not adequate to protect against terrestrial
acidification or nutrient enrichment. The Department of

the Interior as well as Earthjustice noted that the current standards
were not sufficient for these additional endpoints and cited ongoing
harm under current conditions. Two tribes and the Center for Biological
Diversity expressed the view that there was sufficient information to
judge that the current standards were not adequate to protect against
the adverse welfare effect of mercury methylation, contrary to the
EPA's proposed conclusion that the available evidence was not
sufficient to reach such a judgment. For example, The Forest County
Potawatomi Community provided several citations regarding the
relationships between aquatic acidification and mercury methylation and
stated that there was sufficient evidence to find that the current
standards were not adequate.
    With regard to the adequacy of the current secondary standards for
NO2 and SO2, the EPA concurs with commenters'
assertions that the current standards do not provide adequate
protection for ecosystems that are sensitive to aquatic acidification
and that effects to these ecosystems are ongoing from ambient
deposition of oxides of nitrogen and oxides of sulfur. The EPA also
agrees that there is sufficient evidence to conclude that ambient
deposition under the current secondary standards is causing or
contributing to terrestrial acidification as well as nutrient
enrichment in sensitive ecosystems. A complete discussion of
considerations with regard to adequacy can be found in section II.B
above. In short, the ISA has established that the major effects of
concern for this review of the oxides of nitrogen and sulfur standards
are associated with deposition of nitrogen and sulfur caused by
atmospheric concentrations of oxides of nitrogen and sulfur. The
current standards are not directed toward depositional effects, and
none of the elements of the current NAAQS--indicator, form, averaging
time, and level--are suited for addressing the effects of nitrogen and
sulfur deposition. Additionally, although the proportion of total
nitrogen loadings associated with atmospheric deposition of nitrogen
varies across locations, the ISA indicates that atmospheric nitrogen
deposition is the main source of new anthropogenic nitrogen to most
headwater streams, high elevation lakes, and low-order streams.
Atmospheric nitrogen deposition contributes to the total nitrogen load
in terrestrial, wetland, freshwater and estuarine ecosystems that
receive nitrogen through multiple pathways.
    There are expansive data to indicate that the levels of deposition
under the current standards are not sufficient to prevent adverse
effects in ecosystems. With regard to aquatic acidification, recent
data indicate that in the Adirondacks and Shenandoah areas, rates of
acidifying deposition of oxides of nitrogen and sulfur are still well
above pre-acidification (1860) conditions. Forty-four percent of
Adirondack lakes and 85 percent of Shenandoah streams evaluated exceed
the critical load for an ANC of 50 [mu]eq/L, and have suffered loss of
sensitive fish species. With regard to terrestrial acidification, the
REA evaluated a small number of sensitive areas as case studies and
showed the potential for reduced growth. When the methodology was
extended to a 27-state region, similar results were found to indicate
the potential for growth effects in sensitive forests. Nitrogen
deposition can alter species composition and cause eutrophication in
freshwater systems. In the Rocky Mountains, for example, current
deposition levels, which are within the range associated with ambient
nitrogen oxide levels meeting the current standard, are known to cause
changes in species composition in diatom communities indicating
impaired water quality. With regard to terrestrial nutrient enrichment,
most terrestrial ecosystems in the United States are nitrogen-limited,
and therefore they are sensitive to perturbation caused by nitrogen
additions. Under recent conditions, nearly all of the known sensitive
mixed conifer forest ecosystems receive total nitrogen deposition
levels above the ecological benchmark for changes in lichen species. In
addition, in Coastal Sage Scrub ecosystems in California, nitrogen
deposition exceeds the benchmark above which nitrogen is no longer a
limiting nutrient, leading to potential alterations in ecosystem
composition. Therefore, the EPA concludes that the current standards
are not adequate for these effects.
    The EPA, however, while agreeing that there is a causal effect
between deposition of sulfur and mercury methylation disagrees that
there is sufficient evidence to make the quantitative associations that
would be necessary to determine that the current standards were not
adequate to protect against mercury methylation. The ISA concluded that
evidence is sufficient to infer a casual relationship between sulfur
deposition and increased mercury methylation in wetlands and aquatic
environments. Since the rate of mercury methylation varies according to
several spatial and biogeochemical factors whose influence has not been
fully quantified, the correlation between sulfur deposition and
methylmercury could not be quantified for the purpose of interpolating
the association across waterbodies or regions. Therefore, since we are
unable to quantify the relationship between atmospherically deposited
oxides of sulfur and mercury methylation we cannot assess adequacy of
protection. This subject is discussed more fully in section 6.2 of the
REA (U.S. EPA, 2009).
    Another group of commenters, (e.g. Utility Air Regulatory Group
(UARG), Electric Power Research Institute (EPRI), American Petroleum
Institute (API), AAM, and American Road and Transportation Builders
Association (ARTBA)) generally took the position that the currently
available information was not sufficient to make informed judgments
about the adequacy of the current standards to address aquatic
acidification effects. These commenters generally based this view on
the complex nature of the interactions between pollutants and
ecosystems and uncertainties in the models and analyses considered in
this review. Several commenters asserted that there was not sufficient
data available to determine the relationship between acidifying
deposition of oxides of nitrogen and sulfur and adverse effects on
aquatic ecosystems, such that there was not sufficient information to
allow for the assessment of the adequacy of the current standards to
provide appropriate protection from this effect. For example, AAM noted
the uncertainties in models relating to dry deposition and questioned
the linkages between ambient concentrations of oxides of nitrogen and
sulfur and the amount of nitrogen and sulfur deposition. In addition to
commenting on data limitations, UARG also expressed the view that the
ecosystem services analyses included in the proposal were insufficient
to make judgments about adversity to aquatic ecosystems resulting from
acidifying deposition and that there is a lack of evidence
demonstrating that quantifiable changes in public welfare would result
from reductions in acidifying deposition. Many commenters within this
group did not directly comment on the adequacy of the current standards
to protect against aquatic acidification or other deposition-related
effects, but instead expressed the view that the EPA did not have the
authority to consider deposition-related effects in general or aquatic
acidification in particular through the NAAQS. This comment and

the EPA's response are discussed above in section I.E.
    With regard to the adequacy of the current standards to protect
against aquatic acidification, the EPA disagrees with commenters'
assertion that there is insufficient data to make linkages between
deposition from the atmosphere and aquatic acidification effects. To
the contrary, the EPA is confident that there is sufficient robust
science to conclude that aquatic acidification is ongoing in sensitive
ecosystems, that ambient deposition of oxides of nitrogen and oxides of
sulfur are causative in many ecosystems nationwide and that the current
standards are neither appropriate in form nor adequate in level to
protect against such effects. The ISA concluded that there was a causal
relationship between deposition of oxides of nitrogen and sulfur and
NHX and acidification of ecosystems. In addition, the ISA
found that effects of acidifying deposition on ecosystems have been
well studied over the past several decades, that vulnerable areas have
been identified for the United States and that the wealth of available
data has led to the development of robust ecological models used for
predicting soil and surface water acidification. With regard to the
scope of effects, the REA also concluded that the available data are
robust and considered high quality. There is high confidence about the
use of these data and their value for extrapolating to larger spatial
areas. The EPA TIME/LTM network represents a source of long-term,
representative sampling. Data on sulfate concentrations, nitrate
concentrations and ANC from 1990 to 2006 used for this analysis as well
as the EPA EMAP and Regional Environmental Monitoring and Assessment
Program (REMAP) surveys, provide considerable data on surface water
trends.
    The EPA also disagrees with commenters' assessment of limitations
in wet and dry deposition modeling. Further discussion of
characterizing deposition with models can be found in section IV.C.
Additionally, while the EPA recognizes that there are limitations
associated with modeled deposition values, the linkages between model
estimates of deposition and areas exhibiting aquatic acidification
effects are consistent and persuasive in considering adequacy of the
current standard. Section 2.3 of the PA and sections 2.8 and 2.10 of
the ISA provide additional detailed discussions of deposition modeling
and spatial resolution for deposition. CASAC concurred with the EPA's
conclusion on this matter and encouraged the EPA to move forward in
developing a new form of a standard which would address aquatic
acidification. Thus, while the EPA is fully mindful of the limitations
and uncertainties associated with the data and models, the EPA
concludes that the available evidence provides strong scientific
support for the view that harm from aquatic acidification is ongoing
and attributable in large part to atmospheric deposition of reactive
nitrogen and sulfur.
    With regard to the commenters' reliance on ecosystem services
analyses included in the proposal to make judgments about adversity and
public welfare, the EPA disagrees that comprehensive ecosystems
services analyses are necessary to determine adversity. Ecosystem
services analyses are used in this review to inform the decisions made
with regard to adequacy and as such are used in conjunction with other
considerations in the discussion of adversity to public welfare.
Section 4 of the PA further refines this discussion of adversity to
public welfare. Additionally, the paradigm of adversity to public
welfare as deriving from disruptions in ecosystem structure and
function has been used broadly by the EPA to categorize effects of
pollutants from the cellular to the ecosystem level. An evaluation of
adversity to public welfare might consider the likelihood, type,
magnitude, and spatial scale of the effect as well as the potential for
recovery and any uncertainties relating to these considerations. Within
this context, ecosystems services analyses are one of many tools used
in this review to help inform the Administrator's decision on
adversity. The EPA concludes that the analyses performed as part of
this review are sufficient to support the decisions made by the
Administrator with regard to the adequacy of the current standards.

D. Final Decisions on the Adequacy of the Current Standards

    Based on the considerations discussed above, including CASAC advice
and public comments, the Administrator believes that the conclusions
reached in the proposed rule with regard to the adequacy of the current
secondary standards for oxides of nitrogen and sulfur for direct and
deposition-related effects continue to be valid. The Administrator
recognizes that the purpose of the secondary standard is to protect
against ``adverse'' effects resulting from exposure to oxides of
nitrogen and sulfur, discussed above in section II.A. The Administrator
also recognizes the need for conclusions as to the adequacy of the
current standards for both direct and deposition-related effects as
well as conclusions as to the appropriateness and ecological relevance
of the current standards.
    In considering what constitutes an ecological effect that is also
adverse to the public welfare, the Administrator took into account the
ISA conclusions regarding the nature and strength of the effects
evidence, the risk and exposure assessment results, the degree to which
the associated uncertainties should be considered in interpreting the
results, the conclusions presented in the PA, and the views of CASAC
and members of the public. On these bases, the Administrator concludes
that the current secondary standards are adequate to protect against
direct phytotoxic effects on vegetation. Thus, the Administrator has
decided to retain the current secondary standards for oxides of
nitrogen at 53 ppb,\4\ annual average concentration, measured in the
ambient air as NO2, and the current secondary standard for
oxides of sulfur at 0.5 ppm, 3-hour average concentration, measured in
the ambient air as SO2.
---------------------------------------------------------------------------

    \4\ The annual secondary standard for oxides of nitrogen is
being specified in units of ppb to conform to the current version of
the annual primary standard, as specified in the final rule for the
most recent review of the NO2 primary NAAQS (75 FR 6531;
February 9, 2010).
---------------------------------------------------------------------------

    With regard to deposition-related effects, the Administrator first
considered the appropriateness of the structure of the current
secondary standards to address ecological effects of concern. Based on
the evidence as well as considering the advice given by CASAC and
public comments on this matter, the Administrator concludes that the
elements of the current standards are not ecologically relevant and
thus are not appropriate to provide protection of ecosystems. On the
subject of adequacy of protection with regard to deposition-related
effects, the Administrator considered the full nature of ecological
effects related to the deposition of ambient oxides of nitrogen and
sulfur into sensitive ecosystems across the country. Her conclusions
are based on the evidence presented in the ISA with regard to
acidification and nutrient enrichment effects, the findings of the REA
with regard to scope and severity of the current and likely future
effects of deposition, the synthesis of both the scientific evidence
and risk and exposure results in the PA as to the adequacy of the
current standards, and the advice of CASAC and public comments. After
such consideration, the Administrator concludes that current levels of
oxides of nitrogen and sulfur are sufficient to cause acidification of

both aquatic and terrestrial ecosystems, nutrient enrichment of
terrestrial ecosystems and contribute to nutrient enrichment effects in
estuaries that could be considered adverse, and the current secondary
standards do not provide adequate protection from such effects.
    Having reached these conclusions, the Administrator determined that
it was appropriate to consider alternative standards that are
ecologically relevant, as discussed below in section III. These
considerations further support her conclusion that the current
secondary standards for oxides of nitrogen and sulfur are neither
appropriate nor adequate to protect against deposition-related effects.

III. Rationale for Final Decisions on Alternative Secondary Standards

    This section presents the rationale for the Administrator's final
decisions regarding alternative secondary standards for oxides of
nitrogen and sulfur to address deposition-related effects. Section
III.A provides an overview of the aquatic acidification index (AAI)
approach presented in the PA to address such effects related to aquatic
acidification. Advice from CASAC on such a new approach is presented in
section III.B. The Administrator's proposed conclusions on an AAI-based
standard are presented in section III.C. Comments on an AAI-based
standard are discussed in section III.D as well as in the Response to
Comments document. The Administrator's final decisions regarding
alternative secondary standards are presented in section III.E.

A. Overview of AAI Approach

    Having reached the conclusion in the proposal that the current
NO2 and SO2 secondary standards are not adequate
to provide appropriate protection against potentially adverse
deposition-related effects associated with oxides of nitrogen and
sulfur, the Administrator then considered what new multi-pollutant
standard might be appropriate, at this time, to address such effects on
public welfare. The Administrator recognizes that the inherently
complex and variable linkages between ambient concentrations of
nitrogen and sulfur oxides, the related deposited forms of nitrogen and
sulfur, and the ecological responses that are associated with public
welfare effects call for consideration of a standard with an
ecologically relevant design that reflects these linkages. The
Administrator also recognizes that characterization of such complex and
variable linkages in this review requires consideration of information
and analyses that have important limitations and uncertainties.
    Despite its complexity, an ecologically relevant multi-pollutant
standard to address deposition-related effects would still
appropriately be defined in terms of the same basic elements that are
used to define any NAAQS--indicator, form, averaging time, and level.
The form would incorporate additional structural elements that reflect
relevant multi-pollutant and multimedia attributes. These structural
elements include the use of an ecological indicator, tied to the
ecological effect we are focused on, and other elements that account
for ecologically relevant factors other than ambient air
concentrations. All of these elements would be needed to enable a
linkage from ambient air indicators to the relevant ecological effect
to define an ecologically relevant standard. As a result, such a
standard would necessarily be more complex than the NAAQS that have
been set historically to address effects associated with ambient
concentrations of a single pollutant.
    More specifically, the Administrator considered an ecologically
relevant multi-pollutant standard to address effects associated with
acidifying deposition-related to ambient concentrations of oxides of
nitrogen and sulfur in sensitive aquatic ecosystems. This focus is
consistent with the information presented in the ISA, REA, and PA,
which highlighted the greater quantity and quality of the available
evidence and assessments associated with aquatic acidification relative
to the information and assessments available for other deposition-
related effects, including terrestrial acidification and aquatic and
terrestrial nutrient enrichment. Based on its review of these
documents, CASAC agreed that aquatic acidification should be the focus
for developing a new multi-pollutant standard in this review. In
reaching conclusions about an air quality standard designed to address
deposition-related aquatic acidification effects, the Administrator
also recognizes that such a standard may also provide some degree of
protection against other deposition-related effects.
    As discussed in chapter 7 of the PA, the development of a new
multi-pollutant ambient air quality standard to address deposition-
related aquatic acidification effects recognizes that it is appropriate
to consider a nationally applicable standard for protection against
adverse effects of aquatic acidification on public welfare. At the same
time, the PA recognizes the complex and heterogeneous interactions
between ambient air concentrations of nitrogen and sulfur oxides, the
related deposition of nitrogen and sulfur, and associated ecological
responses. The development of such a standard also needs to take into
account the limitations and uncertainties in the available information
and analyses upon which characterization of such interactions are
based. The approach used in the PA also recognizes that while such a
standard would be national in scope and coverage, the effects to public
welfare from aquatic acidification will not occur to the same extent in
all locations in the United States, given the inherent variability of
the responses of aquatic systems to the effects of acidifying
deposition. This contrasts with the relatively more homogeneous
relationships between ambient air concentrations of air pollutants and
the associated inhalation exposures and related public health responses
that are typically considered in setting primary NAAQS.
    As discussed above in section II-A, many locations in the United
States are naturally protected against acid deposition due to
underlying geological conditions. Likewise, some locations in the
United States, including lands managed for commercial agriculture and
forestry, are not likely to be negatively impacted by current levels of
nitrogen and sulfur deposition. As a result, while a new ecologically
relevant secondary standard would apply everywhere, it would be
structured to account for differences in the sensitivity of ecosystems
across the country. This would allow for appropriate protection of
sensitive aquatic ecosystems, which are relatively pristine and wild
and generally in rural areas, and the services provided by such
sensitive ecosystems, without requiring more protection than is needed
elsewhere.
    As discussed below, the multi-pollutant standard developed in the
PA would employ (1) Total reactive oxidized nitrogen (NOy)
and oxides of sulfur (SOX) as the atmospheric ambient air
indicators; (2) a form that takes into account variable factors, such
as atmospheric and ecosystem conditions that modify the amounts of
deposited nitrogen and sulfur; the distinction between oxidized and
reduced forms of nitrogen; effects of deposited nitrogen and sulfur on
aquatic ecosystems in terms of the ecological indicator ANC; and the
representativeness of water bodies within a defined spatial area; (3) a
multi-year averaging time, and (4) a standard level defined in terms of
a single, national target ANC value that, in the context of the above
form, identifies the various levels of

concentrations of NOy and SOX in the ambient air
that would meet the standard. The form of such a standard has been
defined by an index, AAI, which reflects the relationship between
ambient concentrations of NOy and SOX and aquatic
acidification effects that result from nitrogen and sulfur deposition-
related to these ambient concentrations.
    In summarizing the considerations associated with such an air
quality standard to address deposition-related aquatic acidification
effects, as discussed more fully in sections III.A-F of the proposal
and in the PA, the following sections focus on each element of the
standard, including ambient air indicators (section III.A.1), form
(section III.A.2), averaging time (section III.A.3), and level (section
III.A.4). Considerations related to important uncertainties inherent in
such an approach are discussed in section III.A.5.
1. Ambient Air Indicators
    The PA concludes that ambient air indicators other than
NO2 and SO2 should be considered as the
appropriate indicators of oxides of nitrogen and sulfur in the ambient
air for protection against the acidification effects associated with
deposition of the associated nitrogen and sulfur. This conclusion is
based on the recognition that all forms of nitrogen and sulfur in the
ambient air contribute to deposition and resulting acidification, and
as such, NO2 and SO2 are incomplete ambient air
indicators. In principle, the indicators should represent the species
that are associated with oxides of nitrogen and sulfur in the ambient
air and can contribute acidifying deposition. This includes both the
species of oxides of nitrogen and sulfur that are directly emitted as
well as species transformed in the atmosphere from oxides of nitrogen
and sulfur that retain the nitrogen and sulfur atoms from directly
emitted oxides of nitrogen and sulfur. All of these compounds are
associated with oxides of nitrogen and sulfur in the ambient air and
can contribute to acidifying deposition.
    The PA focuses in particular on the various compounds with nitrogen
or sulfur atoms that are associated with oxides of nitrogen and sulfur,
because the acidifying potential is specific to nitrogen and sulfur,
and not other atoms (e.g., H, C, O) whether derived from the original
source of oxides of nitrogen and sulfur emissions or from atmospheric
transformations. For example, the acidifying potential of each molecule
of NO2, NO, HNO3 or PAN is identical, as is the
potential for each molecule of SO2 or ion of particulate
sulfate (p-SO4). Each atom of sulfur affords twice the
acidifying potential of each atom of nitrogen.
a. Oxides of Sulfur
    As discussed in the PA (U.S. EPA, 2011, section 7.1.1), oxides of
sulfur include the gases sulfur monoxide (SO), SO2, sulfur
trioxide (SO3), disulfur monoxide (S2O), and
particulate-phase sulfur compounds (referred to as SO4) that
result from gas-phase sulfur oxides interacting with particles.
However, the sum of SO2 and SO4 does represent
virtually the entire ambient air mass of sulfur that contributes to
acidification. In addition to accounting for virtually all the
potential for acidification from oxidized sulfur in the ambient air,
there are reliable methods to monitor the concentrations of
SO2 and particulate SO4. The PA concludes that
the sum of SO2 and SO4, referred to as
SOX, are appropriate ambient air indicators of oxides of
sulfur because they represent virtually all of the acidification
potential of ambient air oxides of sulfur and there are reliable
methods suitable for measuring SO2 and SO4.
b. Oxides of Nitrogen
    As discussed in the PA (U.S. EPA, 2011, section 7.1.2),
NOy, as defined in chapter 2 of the PA, incorporates
basically all of the oxidized nitrogen species that have acidifying
potential and as such, NOy should be considered as an
appropriate indicator for oxides of nitrogen. Total reactive oxidized
nitrogen is an aggregate measure of NO and NO2 and all of
the reactive oxidized products of NO and NO2. That is,
NOy is a group of nitrogen compounds in which all of the
compounds are either an oxide of nitrogen or compounds in which the
nitrogen atoms came from oxides of nitrogen. Total reactive oxidized
nitrogen is especially relevant as an ambient indicator for
acidification in that it both relates to the oxides of nitrogen in the
ambient air and also represents the acidification potential of all
oxidized nitrogen species in the ambient air, whether an oxide of
nitrogen or derived from oxides of nitrogen. The merits of other
individual NOy species, particularly total nitrate, are
discussed in section 2 of the PA.
2. Form
    Based on the evidence of the aquatic acidification effects caused
by the deposition of NOy and SOX, the PA (U.S.
EPA, 2011, section 7.2) presents the development of a new form that is
ecologically relevant for addressing such effects. The conceptual
design for the form of such a standard includes three main components:
an ecological indicator, deposition metrics that relate to the
ecological indicator, and a function that relates ambient air
indicators to deposition metrics. Collectively, these three components
link the ecological indicator to ambient air indicators, as illustrated
below in Fig III-1.
[GRAPHIC] [TIFF OMITTED] TR03AP12.001

The simplified flow diagram in Figure III-1 compresses the various
atmospheric, biological, and geochemical processes associated with
acidifying deposition to aquatic ecosystems into a simplified
conceptual picture. The ecological indicator (left box) is related to
atmospheric deposition through biogeochemical ecosystem models (middle
box), which associate a target deposition load to a target ecological
indicator. Once a target deposition is established, associated
allowable air concentrations are determined (right box) through the
relationships between ambient air concentration and deposition that are
embodied in air quality models such as CMAQ. The PA describes the
development and rationale for each of these components, as well the
integration of these components into the full expression of the form of
the standard using the concept of a national AAI that represents a
target ANC level as a function of ambient air concentrations.
    The AAI was designed to be an ecologically relevant form of the
standard that determines the levels of NOy and
SOX in the ambient air that would achieve a target ANC limit
for the United States. The intent of the AAI is to weight atmospheric
concentrations of oxides of nitrogen and sulfur by their propensity to
contribute to acidification through deposition, given the fundamental
acidifying potential of each pollutant, and to take into account the
ecological factors that govern acid sensitivity in different
ecosystems. The index also accounts for the contribution of reduced
nitrogen to acidification. Thus, the AAI encompasses those attributes
of specific relevance to protecting ecosystems from the acidifying
potential of ambient air concentrations of NOy and
SOX.
a. Ecological Indicator
    This section summarizes the rationale in the PA for selecting ANC
as the appropriate ecological indicator for consideration. Recognizing
that ANC is not itself the causative or toxic agent for adverse aquatic
acidification effects, the rationale for using ANC as the relevant
ecological indicator is based on the following:
    (1) The ANC is directly associated with the causative agents, pH
and dissolved Al, both through empirical evidence and mechanistic
relationships;
    (2) Empirical evidence shows very clear and strong relationships
between adverse effects and ANC;
    (3) The ANC is a more reliable indicator from a modeling
perspective, allowing use of a body of studies and technical analyses
related to ANC and acidification to inform the development of the
standard; and
    (4) The ANC embodies the concept of acidification as posed by the
basic principles of acid base chemistry and the measurement method used
to estimate ANC and, therefore, serves as a direct index to protect
against acidification.
    Because ANC clearly links both to biological effects of aquatic
acidification as well as to acidifying inputs of NOy and
SOX deposition, the PA concludes that ANC is an appropriate
ecological indicator for relating adverse aquatic ecosystem effects to
acidifying atmospheric deposition of SOX and NOy,
and is preferred to other potential indicators. In reaching this
conclusion, the PA notes that in its review of the first draft PA,
CASAC concluded that ``information on levels of ANC protective to fish
and other aquatic biota has been well developed and presents probably
the lowest level of uncertainty in the entire methodology'' (Russell
and Samet, 2010a). In its more recent review of the second draft PA,
CASAC agreed ``that acid neutralizing capacity is an appropriate
ecological measure for reflecting the effects of aquatic
acidification'' (Russell and Samet, 2010b; p. 4).
b. Linking ANC to Deposition
    There is evidence to support a quantified relationship between
deposition of nitrogen and sulfur and ANC. This relationship was
analyzed in the REA for two case study areas, the Adirondack and
Shenandoah Mountains, based on time-series modeling and observed
trends. In the REA analysis, long-term trends in surface water nitrate,
sulfate and ANC were modeled using MAGIC for the two case study areas.
These data were used to compare recent surface water conditions in 2006
with preindustrial conditions (i.e., preacidification 1860). The
results showed a marked increase in the number of lakes affected by
acidifying deposition, characterized as a decrease in ANC levels, since
the onset of anthropogenic nitrogen and sulfur deposition, as discussed
in chapter 2 of the PA.
    In the REA, the quantified relationship between deposition and ANC
was investigated using ecosystem acidification models, also referred to
as acid balance models or critical loads models (U.S. EPA, 2011,
section 2 and U.S. EPA, 2009, section 4 and Appendix 4). These models
quantify the relationship between deposition of nitrogen and sulfur and
the resulting ANC in surface waters based on an ecosystem's inherent
generation of ANC and ability to neutralize nitrogen deposition through
biological and physical processes. A critical load is defined as the
amount of acidifying atmospheric deposition of nitrogen and sulfur
beyond which a target ANC is not reached. Relatively high critical load
values imply that an ecosystem can accommodate greater deposition
levels than lower critical loads for a specific target ANC level.
Ecosystem models that calculate critical loads form the basis for
linking deposition to ANC.
    As discussed in chapter 2 of the PA, both dynamic and steady-state
models calculate ANC as a function of ecosystem attributes and
atmospheric nitrogen and sulfur deposition, and can be used to
calculate critical loads. Steady-state models are time invariant and
reflect the long-term consequences associated with an ecosystem
reaching equilibrium under a constant level of atmospheric deposition.
Dynamic models are time variant and take into account the time
dependencies inherent in ecosystem hydrology, soil and biological
processes. Dynamic models like MAGIC can provide the time-series
response of ANC to deposition whereas steady-state models provide a
single ANC relationship to any fixed deposition level. Dynamic models
naturally are more complex than steady-state models as they attempt to
capture as much of the fundamental biogeochemical processes as
practicable, whereas steady-state models depend on far greater
parameterization and generalization of processes that is afforded, to
some degree, by not having to account for temporal variability.
    In the PA, a steady-state model is used to define the relevant
critical load, which is the amount of atmospheric deposition of
nitrogen (N) and sulfur (S) beyond which a target ANC is not achieved
and sustained.\5\ It is expressed as:
---------------------------------------------------------------------------

    \5\ This section discusses the linkages between deposition of
nitrogen and sulfur and ANC. Section III.A.2.c then discusses the
linkages between atmospheric concentrations of NOy and
SOX and deposition of nitrogen and sulfur.

---------------------------------------------------------------------------

[GRAPHIC] [TIFF OMITTED] TR03AP12.002

Where:

CLANClim(N + S) is the critical load of deposition, with
units of equivalent charge/(area-time);
[BC]0* is the natural contribution of base cations from
weathering, soil processes and preindustrial deposition, with units
of equivalent charge/volume;
[ANClim] is the target ANC value, with units of
equivalent charge/volume;
Q is the catchment level runoff rate governed by water mass balance
and dominated by precipitation, with units of distance/time; and
Neco is the amount of nitrogen deposition that is effectively
neutralized by a variety of biological (e.g., nutrient uptake) and
physical processes, with units of equivalent charge/(area-time).

    Equation III-1 is a modified expression that adopts the basic
formulation of the steady-state models that are described in chapter 2
of the PA. More detailed discussion of the rationale, assumptions and
derivation of equation III-1, as well as all of the equations in this
section, are included in Appendix B of the PA. The equation simply
reflects the amount of deposition of nitrogen and sulfur from the
atmosphere, CLANClim(N + S), that is associated with a
sustainable long-term ANC target, [ANClim], given the
capacity of the natural system to generate ANC, [BC]0*, and
the capacity of the natural system to neutralize nitrogen deposition,
Neco. This expression of critical load is valid when nitrogen
deposition is greater than Neco.\6\ The runoff rate, Q, allows for
balancing mass in the two environmental mediums--atmosphere and
catchment. This critical load expression can be focused on a single
water system or more broadly. To extend applicability of the critical
load expression (equation III-1) from the catchment level to broader
spatial areas, the terms Qr and CLr, are used,
which are the runoff rate and critical load, respectively, of the
region over which all the atmospheric terms in the equation are
defined.
---------------------------------------------------------------------------

    \6\ Because Neco is only relevant to nitrogen deposition, in
rare cases where Neco is greater than the total nitrogen deposition,
the critical load would be defined only in terms of acidifying
deposition of sulfur and the Neco term in equation III-1 would be
set to zero.
---------------------------------------------------------------------------

    As presented above, the terms S and N in the CLANClim (N
+ S) term broadly represent all species of sulfur or nitrogen that can
contribute to acidifying deposition. This follows conventions used in
the scientific literature that addresses critical loads, and it
reflects all possible acidifying contributions from any sulfur or
nitrogen species. For all practical purposes, S reflects SOX
as described above, the sum of sulfur dioxide gas and particulate
sulfate. However, N in equation III-1 includes both oxidized forms,
consistent with the ambient indicator, NOy, in addition to
the reduced nitrogen species, ammonia and ammonium ion, referred to as
NHX. The NHX is included in the critical load
formulation because it contributes to potentially acidifying nitrogen
deposition. Consequently, from a mass balance or modeling perspective,
the form of the standard needs to account for NHX, as
described below.
c. Linking Deposition to Ambient Air Indicators
    The last major component of the form illustrated in Figure III-1
addresses the linkage between deposition of nitrogen and sulfur and
concentrations of the ambient air indicators, NOy and
SOX. To link ambient air concentrations with deposition, the
PA defines a transference ratio, T, as the ratio of total wet and dry
deposition to ambient concentration, consistent with the area and time
period over which the standard is defined. To express deposition of
NOy and SOX in terms of NOy and
SOX ambient concentrations, two transference ratios were
defined, where TSOx equals the ratio of the combined dry and
wet deposition of SOX to the ambient air concentration of
SOX, and TNOy equals the ratio of the combined
dry and wet deposition of NOy to the ambient air
concentration of NOy.
    As described in chapter 7 of the PA, reduced forms of nitrogen
(NHX) are included in total nitrogen in the critical load
equation, III-1. Reduced forms of nitrogen are treated separately, as
are NOy and SOX, and the transference ratios are
applied. This results in the following critical load expression that is
defined explicitly in terms of the indicators NOy and
SOX:
[GRAPHIC] [TIFF OMITTED] TR03AP12.003

This is the same equation as III-1, with the deposition associated with
the critical load translated to deposition from ambient air
concentrations via transference ratios. In addition, deposition of
reduced nitrogen, oxidized nitrogen and oxidized sulfur are treated
separately.

    Transference ratios are a modeled construct, and therefore cannot
be compared directly to measurable quantities. Section III.B.3 of the
proposal discusses approaches to quantifying these ratios that consider
blending observational data and models. The PA more fully discusses the
rationale underlying transference ratios, as well as analyses
illustrating the relative stability and variability of these ratios.
d. Aquatic Acidification Index
    Having established the transference ratios that translate
atmospheric concentrations to deposition of nitrogen and sulfur and the
various expressions that link atmospheric deposition of nitrogen and
sulfur to ANC, the PA derived the following expression of these
linkages, which separates reduced forms of nitrogen, NHx, from oxidized
forms:
[GRAPHIC] [TIFF OMITTED] TR03AP12.004

    Equation III-3 is the basic expression of the form of a standard
that translates the conceptual framework into an explicit expression
that defines ANC as a function of the ambient air indicators,
NOy and SOX, reduced nitrogen deposition,\7\ and
the critical load necessary to achieve a target ANC level. This
equation calculates an expected ANC value based on ambient
concentrations of NOy and SOX. The calculated ANC
will differ from the target ANC (ANClim) depending on how much the
nitrogen and sulfur deposition associated with NOy,
SOX, and NHX differs from the critical load
associated with just achieving the target ANC.
---------------------------------------------------------------------------

    \7\ Because NHX is characterized directly as
deposition, not as an ambient concentration in this equation, no
transference ratio is needed for this term.
---------------------------------------------------------------------------

    Based on equation III-3, the PA defines an AAI that is more simply
stated using terms that highlight the ambient air indicators:
[GRAPHIC] [TIFF OMITTED] TR03AP12.005

where the AAI represents the long-term (or steady-state) ANC level
associated with ambient air concentrations of NOy and
SOX. The factors F1 through F4 convey three attributes: a
relative measure of the ecosystem's ability to neutralize acids
(F1), the acidifying potential of reduced nitrogen deposition (F2),
and the deposition-to-concentration translators for NOy
(F3) and SOX (F4).
Specifically:
F1 = ANClim + CLr/Qr ;
F2 = NHX/Qr = NHX deposition
divided by Qr;
F3 = TNOy/Qr ; TNOy is the
transference ratio that converts ambient air concentrations of
NOy to deposition of NOy; and
F4 = TSOx/Qr ; TSOx is the
transference ratio that converts ambient air concentrations of
SOX to deposition of SOX.

All of these factors include representative Qr to maintain
unit (and mass) consistency between the AAI and the terms on the right
side of equation III-4.
    The F1 factor is the target ANC level plus the amount of deposition
(critical load) the ecosystem can receive and still achieve the target
level. It incorporates an ecosystem's ability to generate acid
neutralizing capacity through base cation supply ([BC]*0)
and to neutralize acidifying nitrogen deposition through Neco, both of
which are incorporated in the CL term. As noted above, because Neco can
only neutralize nitrogen deposition (oxidized or reduced) there may be
rare cases where Neco exceeds the combination of reduced and oxidized
nitrogen deposition. Consequently, to ensure that the AAI equation is
applicable in all cases that may occur, equation III-4 is conditional
on total nitrogen deposition, {NHX +
F3[NOy]{time} , being greater than Neco. In rare cases where
Neco is greater than {NHX + F3[NOy]{time} , F2,
F3, and Neco would be set equal to 0 in the AAI equation. The
consequence of setting F2 and F3 to zero is simply to constrain the AAI
calculation just to SOX, as nitrogen would have no bearing
on acidifying contributions in this case.
    The PA concludes that equation III-4 (U.S. EPA, 2011, equation 7-
12), which defines the AAI, is ecologically relevant and appropriate
for use as the form of a national standard designed to provide
protection for aquatic ecosystems from the effects of acidifying
deposition associated with concentrations of oxides of nitrogen and
sulfur in the ambient air. This AAI equation does not, however, in
itself, define the spatial areas over which the terms of the equation
would apply. To specify values for factors F1 through F4, it is
necessary to define spatial areas over which these factors are
determined. Thus, it is necessary to identify an approach for spatially
aggregating water bodies into ecologically meaningful regions across
the United States, as discussed below.
e. Spatial Aggregation
    As discussed in the PA, one of the unique aspects of this form is
the need to consider the spatial areas over which values for the F
factors in the AAI equation are quantified. Ecosystems across the
United States exhibit a wide range of geological, hydrological and
vegetation characteristics that influence greatly the ecosystem
parameters, Q, BC0* and Neco that are incorporated in the
AAI. Variations in ecosystem attributes naturally lead to wide
variability in the sensitivities of water bodies in the United States
to acidification, as well as in the responsiveness of water bodies to
changes in acidifying deposition. Consequently, variations in ecosystem
sensitivity, and the uncertainties inherent in characterizing these
variations, must be taken into account in developing a national
standard. In developing a secondary NAAQS to protect public welfare,
the focus of the PA is on protecting sensitive populations of water
bodies, not on each individual water body, which is consistent with the
Agency's approach to protecting public health through primary NAAQS
that focus on susceptible populations, not on each individual.
    The approach used for defining ecologically relevant regions across
the United States, along with approaches to characterizing each region
as acid sensitive or relatively non-acid sensitive is discussed in
detail in the PA (U.S. EPA, 2011, section 7.2.5). This characterization
facilitates a more detailed analysis and focus on those regions that
are relatively more acid sensitive, as well as avoiding over-protection
in relatively non-acid sensitive regions that would receive limited
benefit from reductions in the deposition of oxides of nitrogen and
sulfur with respect to aquatic acidification effects.
    Based on considering available classification schemes for spatial
aggregation, the PA concludes that Omernik's ecoregion classification
(as described at http://www.epa.gov/wed/pages/ecoregions) is the most
appropriate method to consider for the purposes of this review. The PA
concludes that ecoregion level III (Figure IV-1) resolution, with 84
defined ecoregions in the contiguous United States,\8\ is the most
appropriate level to consider for this purpose. The PA notes that the
use of ecoregions is an appropriate spatial aggregation scheme for an
AAI-based standard focused on deposition-related aquatic acidification
effects, while many of the same ecoregion attributes may be applicable
in subsequent NAAQS reviews that may address other deposition-related
aquatic and terrestrial ecological effects. Because atmospheric
deposition is modified by ecosystem attributes, the types of
vegetation, soils, bedrock geology, and topographic features that are
the basis of this ecoregion classification approach also will likely be
key attributes for other deposition-related effects (e.g., terrestrial
acidification, nutrient enrichment) that link atmospheric
concentrations to an aquatic or terrestrial ecological indicator.
---------------------------------------------------------------------------

    \8\ We note that an 85th area within Omernik's Ecoregion Level
III is currently being developed for California.
---------------------------------------------------------------------------

    The PA used Omernik's original alkalinity data (U.S. EPA, 2011,
section

2) and more recent ANC data to delineate two broad groupings of
ecoregions: acid-sensitive and relatively non-acid sensitive
ecoregions. This delineation was made to facilitate greater focus on
those ecoregions with water bodies that generally have greater acid
sensitivity and to avoid over-protection in regions with generally less
sensitive water bodies. The approach used to delineate acid-sensitive
and relatively non-acid sensitive regions included an initial
numerical-based sorting scheme using ANC data, which categorized
ecoregions with relatively high ANC values as being relatively non-acid
sensitive. This initial delineation resulted in 29 of the 84 Omernik
ecoregions being categorized as acid sensitive. Subsequently, land use
data based on the 2006 National Land Cover Data base (NLCD, http://www.epa.gov/mrlc/nlcd-2006.html) were also considered to determine to
what extent an ecoregion is of a relatively pristine and rural nature
by quantifying the degree to which active management practices related
to development and agriculture occur in each ecoregion, resulting in 22
relatively acid-sensitive ecoregions (Table III-1).

              Table III-1--List of 22 Acid-Sensitive Areas
------------------------------------------------------------------------
                                                               Ecoregion
                       Ecoregion name                           number
------------------------------------------------------------------------
Ridge and Valley............................................       8.4.1
Northern Appalachian Plateau and Uplands....................       8.1.3
Piedmont....................................................       8.3.4
Western Allegheny Plateau...................................       8.4.3
Southwestern Appalachians...................................       8.4.9
Boston Mountains............................................       8.4.6
Blue Ridge..................................................       8.4.4
Ouachita Mountains..........................................       8.4.8
Central Appalachians........................................       8.4.2
Northern Lakes and Forests..................................       5.2.1
Maine/New Brunswick Plains and Hills........................       8.1.8
North Central Appalachians..................................       5.3.3
Northern Appalachian and Atlantic Maritime Highlands........       5.3.1
Columbia Mountains/Northern Rockies.........................       6.2.3
Middle Rockies..............................................      6.2.10
Wasatch and Uinta Mountains.................................      6.2.13
North Cascades..............................................       6.2.5
Cascades....................................................       6.2.7
Southern Rockies............................................      6.2.14
Sierra Nevada...............................................      6.2.12
Idaho Batholith.............................................      6.2.15
Canadian Rockies............................................       6.2.4
------------------------------------------------------------------------

    Consideration was also given to the use of naturally acidic
conditions in defining relatively non-acid sensitive areas. For
example, several of the ecoregions located in plains near the coast
exhibit elevated dissolved organic carbon (DOC) levels, which is
associated with naturally acidic conditions. The DOC in surface waters
is derived from a variety of weak organic acid compounds generated from
the natural availability and decomposition of organic matter from
biota. Consequently, high DOC is associated with ``natural'' acidity,
with the implication that a standard intended to protect against
atmospheric contributions to acidity is not an area of focus. The
evidence suggests that several of the more highly managed ecoregions in
coastal or near coastal transition zones are associated with relatively
high DOC values, typically exceeding on average 5 milligrams per liter,
compared to other acid sensitive areas. Although there is sound logic
to interpret naturally acidic areas as relatively non-acid sensitive,
natural acidity indicators were not explicitly included in defining
relatively non-acid sensitive areas as there does not exist a generally
accepted quantifiable scientific definition of natural acidity.
Approaches to explicitly define natural acidity likely will be pursued
in future reviews of the standard.
    Having concluded that the Omernik level III ecoregions are an
appropriate approach to spatial aggregation for the purpose of a
standard to address deposition-related aquatic acidification effects,
the PA uses those ecoregions to define each of the factors in the AAI
equation. As discussed below, factors F1 through F4 in equation III-4
are defined for each ecoregion by specifying ecoregion-specific values
for each factor based on measured and modeled data.
i. Factor F1
    As discussed above, factor F1 reflects a relative measure of an
ecosystem's ability to neutralize acidifying deposition, and is defined
as: F1 = ANClim + CLr/Qr. The value of F1 for
each ecoregion would be based on a calculated critical load used to
represent the ecoregion (CLr) associated with a single
national target ANC level (ANClim, discussed below in section III.D),
as well as on a runoff rate (Qr) to represent the region. To
specify ecoregion-specific values for the term Qr, the PA
used the median value of the distribution of Q values that are
available for water bodies within each ecoregion. To specify ecoregion-
specific values for the term CLr in factor F1, a
distribution \9\ of calculated critical loads was created for the water
bodies in each ecoregion for which sufficient water quality and
hydrology data are available.\10\ The specified critical load was then
defined to be a specific percentile of the distribution of critical
loads in the ecoregion. Thus, for example, using the 90th percentile
means that within an ecoregion, the goal would be for 90 percent of the
water bodies to have higher calculated critical loads than the
specified critical load. That is, if the specified critical load were
to occur across the ecoregion, the goal would be for 90 percent of the
water bodies to achieve the national ANC target or better.
---------------------------------------------------------------------------

    \9\ The distribution of critical loads was based on CL values
calculated with Neco at the lake level. Consideration could also be
given to using a distribution of CLs without Neco and adding the
ecoregion average Neco value to the nth percentile critical load.
This would avoid cases where the lake-level Neco value potentially
could be greater than total nitrogen deposition. The CL at the lake
level represents the CL for the lake to achieve the specified
national target ANC value.
    \10\ The PA judged the data to be sufficient for this purpose if
data are available from more than 10 water bodies in an ecoregion.
---------------------------------------------------------------------------

    The specific percentile selected as part of the definition of F1 is
an important parameter that directly impacts the critical load
specified to represent each ecoregion, and therefore the degree of
protectiveness of the standard. A higher percentile corresponds to a
lower critical load and, therefore, to lower allowable ambient air
concentrations of NOy and SOX and related
deposition to achieve a target AAI level. In conjunction with the other
terms in the AAI equation, alternative forms can be appropriately
characterized in part by identifying a range of alternative
percentiles. The choice of an appropriate range of percentiles to
consider for acid-sensitive and relatively non-acid sensitive
ecoregions, respectively, is discussed below.
    For relatively acid-sensitive ecoregions, the PA concludes it is
appropriate to consider percentiles in the range of the 70th to the
90th percentile (of sensitivity). This conclusion is based on the
judgment that it would not be appropriate to represent an ecoregion
with the lowest or near lowest critical load, so as to avoid potential
extreme outliers that can be seen to exist at the extreme end of the
data distributions, which would not be representative of the population
of acid sensitive water bodies within the ecoregion and could lead to
an overly protective standard. At the same time, in considering
ecoregions that are inherently acid sensitive, it is judged to be
appropriate to limit the lower end of the range for consideration to
the 70th percentile, a value well above the median of the distribution,
so that a substantial majority of acid-sensitive water bodies are
protected. Since the percentile value influences the relative

degree of protectiveness afforded by the AAI approach, the degree of
confidence in characterizing the representativeness of sampled water
bodies relative to all water bodies within an ecoregion is a critical
issue, and it is important to continually improve this confidence.
    For relatively non-acid sensitive ecoregions, the PA concludes it
is appropriate to consider the use of a range of percentiles that
extends lower than the range identified above for acid-sensitive
ecoregions. Consideration of a lower percentile would avoid
representing a relatively non-acid sensitive ecoregion by a critical
load associated with relatively more acid-sensitive water bodies. In
particular, the PA concludes it is appropriate to focus on the median
or 50th percentile of the distribution of critical loads so as to avoid
over-protection in such ecoregions.
ii. Factor F2, F3 and F4
    As discussed above, factor F2 is the amount of reduced nitrogen
deposition within an ecoregion, including the deposition of both
ammonia gas and ammonium ion, and is defined as: F2 = NHX/
Qr. The PA calculated the representative runoff rate,
Qr, using a similar approach as noted above for factor F1;
i.e., the median value of the distribution of Q values that are
available for water bodies within each ecoregion. In the PA, 2005 CMAQ
model simulations over 12-km grids are used to calculate an average
value of NHX for each ecoregion. The NHX term is
based on annual average model outputs for each grid cell, which are
spatially averaged across all the grid cells contained in each
ecoregion to calculate a representative annual average value for each
ecoregion. The PA concludes that this approach of using spatially
averaged values is appropriate for modeling, largely due to the
relatively rapid mixing of air masses that typically results in
relatively homogeneous air quality patterns for regionally dispersed
pollutants. In addition, there is greater confidence in using spatially
averaged modeled atmospheric fields than in using modeled point-
specific fields.
    This averaging approach is also used for the air concentration and
deposition terms in factors F3 and F4, which are the ratios that relate
ambient air concentrations of NOy and SOX to the
associated deposition, and are defined as follows: F3 =
TNOy/Qr and F4 = TSOx/Qr.
TNOy is the transference ratio that converts ambient air
concentrations of NOy to deposition of NOy and
TSOx is the transference ratio that converts ambient air
concentrations of SOX to deposition of SOX. The
transference ratios are based on the 2005 CMAQ simulations, using
average values for each ecoregion, as noted above for factor F2. More
specifically, the transference ratios are calculated as the annual
deposition of NOy or SOX spatially averaged
across the ecoregion and divided by the annual ambient air
concentration of NOy or SOX, respectively,
spatially averaged across the ecoregion.
f. Summary of the AAI Form
    The PA developed an ecologically relevant form of an ambient air
quality standard to address deposition-related aquatic acidification
effects using an equation to calculate an AAI value in terms of the
ambient air indicators of oxides of nitrogen and sulfur and the
relevant ecological and atmospheric factors that modify the
relationships between the ambient air indicators and ANC. Recognizing
the spatial variability of such factors across the United States, the
PA concludes it is appropriate to divide the country into ecologically
relevant regions, characterized as acid-sensitive or relatively non-
acid sensitive, and specify the value of each of the factors in the AAI
equation for each such region.
    Using the equation, a value of AAI can be calculated for any
measured values of ambient NOy and SOX. For such
a NAAQS, the Administrator would set a single, national value for the
level of the AAI used to determine achievement of the NAAQS, as
summarized below in section III.A.4. The ecoregion-specific values for
factors F1 through F4 would be specified by the EPA based on the most
recent data and CMAQ model simulations, and codified as part of such a
standard. These factors would be reviewed and updated as appropriate in
the context of each periodic review of the NAAQS.
3. Averaging Time
    Reflecting a focus on long-term effects of acidifying deposition,
the PA developed the AAI that links ambient air indicators to
deposition-related ecological effects, in terms of several factors, F1
through F4. As discussed above, these factors are all calculated as
annual average values, whether based on water quality and hydrology
data or on CMAQ model simulations. In the context of a standard defined
in terms of the AAI, the PA concludes that it is appropriate to
consider the same annual averaging time for the ambient air indicators
as is used for the factors in the AAI equation. As noted in chapter 3
of the ISA, protection against episodic acidity events can be achieved
by establishing a higher chronic ANC level.
    The PA also considered interannual variability in both ambient air
quality and in precipitation, which is directly related to the
deposition of oxides of nitrogen and sulfur from the ambient air. While
ambient air concentrations show year-to-year variability, often the
year-to-year variability in precipitation is considerably greater,
given the highly stochastic nature of precipitation. The use of
multiple years over which annual averages are determined would dampen
the effects of interannual variability in both air quality and
precipitation. Consequently, the PA concludes that an annual averaging
time based on the average of each year over a consecutive 3- to 5-year
period is appropriate to consider for the ambient air indicators
NOy and SOX.
4. Level
    The PA concludes that the level of a standard for aquatic
acidification based on the AAI would be defined in terms of a single,
national value of the AAI. Such a standard would be met at a monitoring
site when the multi-year average of the calculated annual values of the
AAI was equal to or above the specified level of the standard.\11\ The
annual values of the AAI would be calculated based on the AAI equation
using the assigned ecoregion-specific values for factors F1 through F4
and monitored annual average NOy and SOX
concentrations. Since the AAI equation is based on chronic ANC as the
ecological indicator, the level chosen for the standard would reflect a
target chronic ANC value. The combination of the form of the standard,
discussed above in section III.A.2, defined by the AAI equation and the
assigned values of the F factors in the equation, other elements of the
standard including the ambient air indicators (section III.A.1) and
their averaging time (section III.A.3), and the level of the standard
determines the allowable levels of NOy and SOX in
the ambient air within each ecoregion. All of the elements of the
standard together determine the degree of protection from adverse
aquatic acidification effects associated with oxides of nitrogen and
sulfur in the ambient air. The level of the standard plays a central
role in determining the degree of protection provided and is discussed
below.
---------------------------------------------------------------------------

    \11\ Unlike other NAAQS, where the standard is met when the
relevant value is at or below the level of the standard since a
lower standard level is more protective, in this case a higher
standard level is more protective.
---------------------------------------------------------------------------

    Based on associations between pH levels and target ANC levels and

between ANC levels and aquatic ecosystem effects, as well as
consideration of episodic acidity, ecosystem response time, precedent
uses of target ANC levels, and public welfare benefits, the PA
concludes that consideration should be given to a range of standard AAI
levels from 20 to 75 [mu]eq/L. The available evidence indicates that
target ANC levels below 20 [mu]eq/L would be inadequate to protect
against substantial ecological effects and potential catastrophic loss
of ecosystem function in some sensitive aquatic ecosystems. While
ecological effects occur at ANC levels below 50 [mu]eq/L in some
sensitive ecosystems, the degree and nature of those effects are less
significant than at levels below 20 [mu]eq/L. Levels at and above 50
[mu]eq/L would be expected to provide additional protection, although
uncertainties regarding the potential for additional protection from
adverse ecological effects are much larger for target ANC levels above
about 75 [mu]eq/L, as effects are generally appreciably less sensitive
to changes in ANC at such higher levels.
    The PA recognizes that the level of the standard together with the
other elements of the standard, including the ambient air indicators,
averaging time, and form, determine the overall protectiveness of the
standard. Thus, consideration of a standard level should reflect the
strengths and limitations of the evidence and assessments as well as
the inherent uncertainties in the development of each of the elements
of the standard. The implications of considering alternative standards,
defined in terms of alternative combinations of levels and percentile
values that are a critical component of factor F1 in the form of the
standard, are discussed in section III.E of the proposal and more fully
in the PA.
5. Characterization of Uncertainties
    The characterization of uncertainties is intended to address the
relative confidence associated with the linked atmospheric-ecological
effects system described above, and is described in detail in the PA
(U.S. EPA, 2011, section 7.6 and Appendices F and G) and summarized in
section III.F of the proposal. A brief overview of uncertainties is
presented here in the context of the major structural components
underlying the standard, as well as with regard to areas of relatively
high uncertainty.
    As discussed in the PA (U.S. EPA, 2011, Table 7-3), there is
relatively low uncertainty with regard to the conceptual formulation of
the overall structure of the AAI-based standard that incorporates the
major associations linking biological effects to air concentrations.
Based on the strength of the evidence that links species richness and
mortality to water quality, the associations are strongly causal and
without any obvious confounding influence. The strong association
between the ecosystem indicator (ANC) and the causative water chemistry
species (dissolved aluminum and hydrogen ion) reinforces the confidence
in the linkage between deposition of nitrogen and sulfur and effects.
This strong association between ANC and effects is supported by a sound
mechanistic foundation between deposition and ANC. The same mechanistic
strength holds true for the relationship between ambient air levels of
nitrogen and sulfur and deposition, which completes the linkage from
ambient air indicators through deposition to ecological effects.
    There are much higher uncertainties, however, in considering and
quantifying the specific elements within the structure of an AAI-based
standard, including the deposition of SOX, NOy,
and NHX as well as the critical load-related component, each
of which can vary within and across ecoregions. Overall system
uncertainty with an AAI approach relates not just to the uncertainty in
each element, but also to the combined uncertainties that result from
linking these elements together within the AAI-based structure and over
the defined spatial scale (i.e., ecoregions). Some of these elements--
including, for example, dry deposition, pre-industrial base cation
production, and reduced nitrogen deposition--are estimated with less
confidence than other elements (U.S. EPA, 2011, Table 7.3). The
uncertainties associated with all of these elements, and the
combination of these elements through the AAI equation and over the
ecoregion spatial scale, are summarized below.
    The lack of observed dry deposition data, which affects confidence
in the AAI on an ecoregion scale, is constrained in part by the lack of
efficient measurement technologies. Progress in reducing uncertainties
in dry deposition will depend on improved atmospheric concentration
data and direct deposition flux measurements of the relevant suite of
NOy and SOX species.
    Pre-industrial base cation productivity by definition is not
observable. Contemporary observations and inter-model comparisons are
useful tools that help reduce the uncertainty in estimates of pre-
industrial base cation productivity used in the AAI equation. In
characterizing contemporary base cation flux using basic water quality
measurements (i.e., major anion and cation species as defined in
equation 2.11 in the PA), it is reasonable to assume that a major
component of contemporary base cation flux is associated with pre-
industrial weathering rates. To the extent that multiple models
converge on similar solutions within and across ecoregions, greater
confidence in estimating pre-industrial base cation production within
the AAI and ecoregion frameworks would be achieved.
    While characterization of NHX deposition has been
evolving over the last decade, the high uncertainty in characterizing
NHX deposition is due to both the lack of field measurements
and the inherent complexity of characterizing NHX with
respect to source emissions and dry deposition.\12\ Because ammonia
emissions are generated through a combination of man-made and
biological activities, and ammonia is semi-volatile, the ability to
characterize spatial and temporal distributions of NHX
concentrations and deposition patterns is limited. While direct
measurement of NHX deposition is resource intensive because
of the diffuse nature of sources (i.e., area-wide and non-point
sources), there have been more frequent deposition flux studies,
relative to other nitrogen species, that enable the estimation of both
emissions and dry deposition. Also, while ammonia has a relatively high
deposition velocity and traditionally was thought to deposit close to
the emissions release areas, the semi-volatile nature of ammonia
results in re-entrainment back into the lower boundary layer of the
atmosphere resulting in a more dispersed concentration pattern
exhibiting transport characteristics similar to longer lived
atmospheric species. These inherent complexities in source
characterization and ambient concentration patterns significantly
increase the degree of uncertainty in NHX deposition in
general, and in the AAI equation applied on an ecoregion scale in
particular. However, the PA notes that progress is being made in
measuring ammonia with cost efficient samplers and anticipates the
gradual evolution of a spatially robust ammonia sampling network that
would help support analyses to reduce underlying uncertainties in
NHX deposition.
---------------------------------------------------------------------------

    \12\ Field measurements of NHX have been extremely
limited, but have begun to be enhanced through the NADP's passive
ammonia network (AMoN).
---------------------------------------------------------------------------

    In characterizing uncertainties with respect to available
measurement data and the use of ecological and

atmospheric models, as summarized in sections III.F.2-3 of the
proposal, the PA identified data gaps and model uncertainties in
relative terms by comparing, for example, the relative richness of data
between geographic areas or environmental media. As discussed in the
proposal and more fully in the PA, from an uncertainty perspective,
gaps in field measurement data increase uncertainties in modeled
processes and in the specific application of such models. As noted
above, processes that are embodied in an AAI-based standard are modeled
using the CMAQ atmospheric model and steady-state ecological models.
These models are characterized in the ISA as being well-established and
have undergone extensive peer review. Nonetheless, the application of
these models for purposes of specifying the factors in the AAI
equation, on an ecoregion scale, is a new application that introduces
uncertainties, especially in areas with limited observational data that
can be used to evaluate this specific application. Understanding
uncertainties in relevant modeled processes thus involves consideration
of the uncertainties associated with applying each model as well as the
combination of these uncertainties as the models are applied in
combination within the AAI framework applied on an ecoregion scale.
    Our confidence in improving critical load estimates can be
increased by expanding water quality data bases used as inputs and
evaluation metrics for critical load models. With regard to water
quality data, the PA notes that such data are typically limited
relative to air quality data sets, and are also relatively sparse in
the western United States. While there are several state and local
agency water quality data bases, it is unclear the extent to which
differences in sampling, chemical analysis and reporting protocols
would impact the use of such data for the purpose of better
understanding the degree of protectiveness that would be afforded by an
AAI-based standard within sensitive ecoregions across the country. In
addition, our understanding of water quality in Alaska and Hawaii and
the acid sensitivity of their ecoregions is particularly limited.
Expanding the water quality data bases would enable clearer delineation
of ecoregion representative critical loads in terms of the
nth percentile. This would provide more refined
characterization of the degree of protection afforded by a given
standard. Longer term, the availability of water quality trend data
(annual to monthly sampled) would support accountability assessments
that examine if an ecoregion's response to air management efforts is as
predicted by earlier model forecasting. The most obvious example is the
long-term response of water quality ANC change to changes in calculated
AAI, deposition, ambient NOY and SOX
concentrations, and emissions. In addition, water quality trends data
provide a basis for evaluating and improving the parameterizations of
processes in critical load models applied at the ecoregion scale
related to nitrogen retention and base cation supply. A better
understanding of soil processes, especially in the southern
Appalachians, would enhance efforts to examine the variability within
ecoregions of the soil-based adsorption and exchange processes which
moderate the supply of major cations and anions to surface waters and
strongly influence the response of surface water ANC to changes in
deposition of nitrogen and sulfur.
    Steady-state biogeochemical ecosystem modeling is used to develop
critical load estimates that are incorporated in the AAI equation
through factor F1. Consequently, the PA notes that an estimate of the
temporal response of surface water ANC to deposition and air
concentration changes is not directly available. Lacking a predicted
temporal response impairs the ability to conduct accountability
assessments down to the effects level. Accountability assessments would
examine the response of each step in the emissions source through air
concentration--deposition--surface water quality--biota continuum. The
steady-state assumption at the ecosystem level does not impair
accountability assessments through the air concentration/deposition
range of that continuum. However, in using steady-state ecosystem
modeling, several assumptions are made relative to the long-term
importance of processes related to soil adsorption of major ions and
ecosystem nitrogen dynamics. Because these models often were developed
and applied in glaciated areas with relatively thin and organically
rich soils, their applicability is relatively more uncertain in areas
such as those in the non-glaciated clay-based soil regions of the
central Appalachians. Consequently, it is desirable to develop the
information bases to drive simple dynamic ecosystem models that
incorporate more detailed treatment of subsurface processes, such as
adsorption and exchange processes and sulfate absorption.

B. CASAC Views

    The CASAC has advised the EPA concerning the ISA, the REA, and the
PA. The CASAC supported the EPA's interpretation of the science
embodied in the ISA and the assessment approaches and conclusions
incorporated in the REA.
    Most recently, CASAC considered the information in the final PA in
providing its recommendations on the review of the new multi-pollutant
standard developed in that document and discussed above (Russell and
Samet, 2011a). In so doing, CASAC expressed general support for the
conceptual framework of the standard based on the underlying scientific
information, as well as for the conclusions in the PA with regard to
indicators, averaging time, form and level of the standard that are
appropriate for consideration by the Agency in reaching decisions on
the review of the secondary NAAQS for oxides of nitrogen and sulfur:

    ``The final Policy Assessment clearly sets out the basis for the
recommended ranges for each of the four elements (indicator,
averaging time, level and form) of a potential NAAQS that uses
ambient air indicators to address the combined effects of oxides of
nitrogen and oxides of sulfur on aquatic ecosystems, primarily
streams and lakes. As requested in our previous letters, the Policy
Assessment also describes the implications of choosing specific
combinations of elements and provides numerous maps and tabular
estimates of the spatial extent and degree of severity of NAAQS
exceedances expected to result from possible combinations of the
elements of the standard.''
    ``We believe this final PA is appropriate for use in determining
a secondary standard to help protect aquatic ecosystems from
acidifying deposition of oxides of sulfur and nitrogen. The EPA
staff has done a commendable job developing the innovative Aquatic
Acidification Index (AAI), which provides a framework for a national
standard based on ambient concentrations that also takes into
account regional differences in sensitivities of ecosystems across
the country to effects of acidifying deposition.''

(Russell and Samet, 2011a).

    With respect to indicators, CASAC supported the use of
SOX and NOy as ambient air indicators (discussed
above in section III.A) and ANC as the ecological indicator (discussed
above in section III.B.1). With respect to averaging time (discussed
above in section III.C), CASAC agreed with the conclusions in the PA
that ``an averaging time of three to five years for the AAI parameters
is appropriate.'' CASAC noted that ``a longer averaging time would mask
possible trends of AAI, while a shorter averaging time would make the
AAI being more influenced by the conditions of the

particular years selected'' (Russell and Samet, 2011a).
    With respect to the form of the standard (discussed above in
section III.B), CASAC stated the following:

    ``EPA has developed the AAI, an innovative ``form'' of the NAAQS
itself that incorporates the multi-pollutant, multi-media,
environmentally modified, geographically variable nature of
SOX/NOy deposition-related aquatic
acidification effects. With the caveats noted below, CASAC believes
that this form of the NAAQS as described in the final Policy
Assessment is consistent with and directly reflective of current
scientific understanding of effects of acidifying deposition on
aquatic ecosystems.'' (Russell and Samet, 2011a)
    ``CASAC agrees that the spatial components of the form in the
Policy Assessment are reasonable and that use of Omernik's
ecoregions (Level III) is appropriate for a secondary NAAQS intended
to protect the aquatic environment from acidification * * *''

(Russell and Samet, 2011a).

    The caveats noted by CASAC include a recognition of the importance
of continuing to evaluate the performance of the CMAQ and ecological
models to account for model uncertainties and to make the model-
dependent factors in the AAI more transparent. In addition, CASAC noted
that the role of DOC and its effects on ANC would benefit from further
refinement and clarification (Russell and Samet, 2011a). While CASAC
expressed the view that the ``division of ecoregions into `sensitive'
and `non-sensitive' subsets, with a more protective percentile applied
to the sensitive areas, also seems reasonable'' (Russell and Samet,
2011a), CASAC also noted that there was the need for greater clarity in
specifying how appropriate screening criteria would be applied in
assigning ecoregions to these categories. Further, CASAC identified
potential biases in critical load calculations and in the regional
representativeness of available water chemistry data, leading to the
observation that a given percentile of the distribution of estimated
critical loads may be protective of a higher percentage of surface
waters in some regions (Russell and Samet, 2011a). Such potential
biases led CASAC to recommend that ``some attention be given to our
residual concern that the available data may reflect the more sensitive
water bodies and thus, the selection of the percentiles of waterbodies
to be protected could be conservatively biased'' (Russell and Samet,
2011a).
    With respect to level as well as the combination of level and form
as they are presented as alternative standards (discussed above in
sections III.D-E), CASAC agreed with the PA conclusions that
consideration should be given to standard levels within the range of 20
and 75 [mu]eq/L. CASAC also recognized that the level and the form of
any AAI-based standard are so closely linked that these two elements
should be considered together:

    ``When considered in isolation, it is difficult to evaluate the
logic or implications of selecting from percentiles (70th to 90th)
of the distribution of estimated critical loads for lakes in
sensitive ecoregions to determine an acceptable amount of deposition
for a given ecoregion. However, when these percentile ranges are
combined with alternative levels within the staff-recommended ANC
range of 20 to 75 microequivalents per liter ([mu]eq/L), the results
using the AAI point to the ecoregions across the country that would
be expected to require additional protection from acidifying
deposition. Reasonable choices were made in developing the form. The
number of acid sensitive regions not likely to meet the standard
will be affected both by choice of ANC level and the percentile of
the distribution of critical loads for lakes to meet alternative ANC
levels in each region. These combined recommendations provide the
Administrator with a broad but reasonable range of minimally to
substantially protective options for the standard.''

(Russell and Samet, 2011a).

    CASAC also commented on the EPA's uncertainty analysis, and
provided advice on areas requiring further clarification in the
proposed rule and future research. The CASAC found it ``difficult to
judge the adequacy of the uncertainty analysis performed by the EPA
because of lack of details on data inputs and the methodology used, and
lack of clarity in presentation'' (Russell and Samet, 2011a). In
particular, CASAC identified the need for more thorough model
evaluations of critical load and atmospheric modeling, recognizing the
important role of models as they are incorporated in the form of the
standard. In light of the innovative nature of the standard developed
in the PA, CASAC identified ``a number of areas that should be the
focus of further research'' (Russell and Samet, 2011a). While CASAC
recognized that the EPA staff was able to address some of the issues in
the PA, they also noted areas ``that would benefit from further study
or consideration in potential revisions or modifications to the form of
the standard.'' Such research areas include ``sulfur retention and
mobilization in the soils, aluminum availability, soil versus water
acidification and ecosystem recovery times.'' Further, CASAC encouraged
future efforts to monitor individual ambient nitrogen species, which
would help inform further CMAQ evaluations and the specification of
model-derived elements in the AAI equation (Russell and Samet, 2011a).

C. Proposed Conclusions on Alternative Secondary Standards

    As discussed in section III.H of the proposal, the Administrator
considered whether it is appropriate at this time to set a new multi-
pollutant standard to address deposition-related effects associated
with oxides of nitrogen and sulfur, with a structure that would better
reflect the available science regarding acidifying deposition to
sensitive aquatic ecosystems. In so doing, she recognized that such a
standard, for purposes of Section 109(b) and (d) of the CAA, must in
her judgment be requisite to protect public welfare, such that it would
be neither more nor less stringent than necessary for that purpose. In
particular, she focused on the AAI-based standard developed in the PA
and reviewed by CASAC, as discussed above. Based on consideration of
the scientific basis for such a standard and the conclusions reached in
the ISA, the Administrator agreed with the conclusion in the PA, and
supported by CASAC, that there is a strong scientific basis for
development of a standard with the general structure presented in the
PA. She recognized that while the standard is innovative and unique,
the structure of the standard is well-grounded in the science
underlying the relationships between ambient concentrations of oxides
of nitrogen and sulfur and the aquatic acidification related to
deposition of nitrogen and sulfur associated with such ambient
concentrations.
    Nonetheless, the Administrator also recognized that such a standard
would depend on atmospheric and ecological modeling, based on
appropriate data, to specify the terms of an equation that incorporates
the linkages between ambient concentrations, deposition, and aquatic
acidification, for each separate ecoregion, and that there are a number
of inherent uncertainties and complexities that are relevant to the
question of whether it is appropriate under Section 109 of the CAA to
set a specific AAI-based standard at this time. Based on her
consideration of these important uncertainties and limitations, the
Administrator recognized that in combination, these limitations and
uncertainties result in a considerable degree of uncertainty as to how
well the quantified elements of the AAI standard would predict the
actual relationship between varying ambient concentrations of oxides of
nitrogen and sulfur and steady-state ANC levels across the distribution
of water bodies within the

various ecoregions in the United States. Because of this, there is
considerable uncertainty as to the actual degree of protectiveness that
such a standard would provide, especially for acid-sensitive
ecoregions. The Administrator recognized that the AAI equation, with
factors quantified in the ranges discussed above and described more
fully in the PA, generally performs well in identifying areas of the
country that are sensitive to such acidifying deposition and indicates,
as expected, that lower ambient levels of oxides of nitrogen and sulfur
would lead to higher calculated AAI values. However, the uncertainties
discussed here are critical for determining the actual degree of
protection that would be afforded such areas by any specific target ANC
level and percentile of water bodies that would be chosen in setting a
new AAI-based standard, and thus for determining an appropriate AAI-
based standard that meets the requirements of Section 109.
    The Administrator noted that setting a NAAQS generally involves
consideration of the degree of uncertainties in the science and other
information, such as gaps in the relevant data and, in this case,
limitations in the evaluation of the application of relevant ecological
and atmospheric models at an ecoregion scale. She noted that the issue
here is not a question of uncertainties about the scientific soundness
of the structure of the AAI, but instead uncertainties in the
quantification and representativeness of the elements of the AAI as
they vary in ecoregions across the country. At present, these
uncertainties prevent an understanding of the degree of protectiveness
that would be afforded to various ecoregions across the country by a
new standard defined in terms of a specific nationwide target ANC level
and a specific percentile of water bodies for acid-sensitive ecoregions
and thus prevent identification of an appropriate standard.
    The Administrator judged that the uncertainties are of such nature
and magnitude that there is no reasoned way to choose a specific AAI-
based standard, in terms of a specific nationwide target ANC level or
percentile of water bodies that would appropriately account for the
uncertainties, since neither the direction nor the magnitude of change
from the target level and percentile that would otherwise be chosen can
reasonably be ascertained at this time. Further, she noted that CASAC
acknowledged that important uncertainties remain that would benefit
from further study and data collection efforts, which might lead to
potential revisions or modifications to the form of the standard
developed in the PA, and that CASAC encouraged the Agency to engage in
future monitoring and model evaluation efforts to help inform further
development of the elements of an AAI-based standard. Based on these
considerations the Administrator judged that the current limitations in
relevant data and the uncertainties associated with specifying the
elements of the AAI based on modeled factors are of such nature and
degree as to prevent her from reaching a reasoned decision such that
she is adequately confident as to what level and form (in terms of a
selected percentile) of such a standard would provide any particular
intended degree of protection of public welfare that the Administrator
determined satisfied the requirements to set an appropriate standard
under Section 109 of the CAA.
    Based on the above considerations, the Administrator provisionally
concluded that it is premature to set a new, multi-pollutant secondary
standard for oxides of nitrogen and sulfur at this time, and as such
she proposed not to set such a new secondary standard. Nonetheless,
while the Administrator concluded that it is premature to set such a
multi-pollutant standard at this time, she determined that the Agency
should undertake a field pilot program to gather additional data
(discussed below in section IV). She concluded that it is appropriate
that such a program be undertaken before, rather than after, reaching a
decision to set such a standard.
    In reaching her proposed decision not to set a new AAI-based
standard at this time, the Administrator recognized that the new
NO2 and SO2 primary 1-hour standards set in 2010,
while not ecologically relevant for a secondary standard, will
nonetheless result in reductions in oxides of nitrogen and sulfur that
will directionally benefit the environment by reducing NOy
and SOX deposition to sensitive ecosystems. The
Administrator proposed to revise the secondary standards by adding
secondary standards identical to the NO2 and SO2
primary 1-hour standards set in 2010, including a 1-hour secondary
NO2 standard set at a level of 100 ppb and a 1-hour
secondary SO2 standard set at a level of 75 ppb. The EPA
noted that while this will not add secondary standards of an
ecologically relevant form to address deposition-related effects, it
will provide additional protection for sensitive areas. The EPA further
noted that this proposed decision is consistent with the view that the
current secondary standards are neither sufficiently protective nor
appropriate in form, but that it is not appropriate to propose to set a
new, ecologically relevant multi-pollutant secondary standard at this
time, for the reasons summarized above.