The EPA solicited comment on all aspects of this proposed decision,
as discussed in the following section.

D. Comments on Alternative Secondary Standards

    In this section, comments received on the proposal related to an
AAI-based standard are discussed in section III.D.1 and comments
related to the proposed decision to set 1-hour NO2 and
SO2 secondary standards are discussed in section III.D.2.
1. Comments Related to an AAI-Based Standard
    General comments that either supported or opposed the proposed
decision not to set an AAI-based standard in this review are addressed
in this section. Two groups of commenters offered sharply divergent
views on whether it is appropriate for the EPA to set or even consider
an AAI-based standard to protect against the effects in aquatic
ecosystems from acidifying deposition associated with ambient
concentrations of oxides of nitrogen and sulfur. These groups provided
strongly contrasting views on the strength and limitations in the
underlying scientific information upon which such a standard could be
based, as well as on the legal authority and requirements in the CAA
for the EPA to set such a standard. These comments are discussed below
in section III.D.1.a, and build in part on the overarching issue raised
by some commenters as to the EPA's authority under the CAA to include
deposition-related effects within the scope of a NAAQS review, which is
discussed above in section I.E. Some commenters also expressed views
about specific aspects of an AAI-based approach, as discussed below in
section III.D.1.b. More technical comments on specific elements and
factors of the AAI are discussed in the Response to Comments document.
General comments based on implementation-related factors that are not a
permissible basis for considering an alternative standard are noted in
the Response to Comments document.
a. Comments on Consideration of an AAI-Based Standard
    The first group of commenters, including several industry groups
(e.g., EPRI, UARG, and API), individual companies (e.g., East Kentucky
Power Cooperative), and two states (TX, SD), strongly supported the
EPA's proposed decision not to set an AAI-based

standard in this review. These commenters generally focused on the
limitations and uncertainties in the scientific evidence used by the
EPA as a basis for its consideration of an AAI-based standard,
expressing the view that these limitations and uncertainties were so
great as to preclude setting such a standard at this time. Several
industry commenters felt the uncertainties were of sufficient magnitude
as to invalidate the AAI approach for use in the NAAQS, while others
agreed with the EPA's finding that further information and analysis is
needed, and further noted that this work should be completed before the
EPA could propose a new multi-pollutant standard. More fundamentally,
some commenters in this group expressed the view that any consideration
of such a standard is inconsistent with various provisions of the CAA
and thus unlawful.
    With regard to their views on the underlying scientific
information, many of these commenters focused on what they asserted
were areas of substantial uncertainty in the AAI approach including
uncertainties in the individual F factors of the AAI, air deposition
modeling, critical loads modeling, and available water quality and
watershed data. Several commenters felt a more rigorous uncertainty and
variability analysis of the AAI, beyond the analyses that the EPA
presented in the PA, would be needed if the EPA were to consider such a
standard in the future.
    Some commenters expressed concerns with specific aspects of the
AAI, such as the adequacy of the Omernik ecoregion approach as a method
of waterbody aggregation for critical load calculations and whether ANC
was an appropriate ecological indicator. The commenters asserted that
the EPA needed to explore different methods for calculating critical
loads, collect essential data, and employ mechanistic water chemistry
models. The commenters also felt that the EPA was arbitrary in choosing
its criteria for sensitive ecoregions and percent waterbodies, and that
there was a bias in the field data toward sensitive areas. Several
commenters felt a more comprehensive research program was needed to
improve characterization of the biogeochemical and deposition processes
incorporated into the AAI.
    Some industry groups commented on uncertainties in the CMAQ
modeling, including high levels of uncertainty surrounding measurement
and modeling of chemically reduced forms of nitrogen (NHx).
Other commenters were also critical of the reliance of the AAI on
modeling, and expressed the view that CMAQ would require intensive
deposition-focused evaluation.
    A second group of commenters, including several environmental
groups (e.g., Center for Biological Diversity, Earthjustice, and
Adirondack Council), the U.S. Department of Interior and the National
Park Service, the New York Department of Environmental Conservation,
and two tribes (Fond du Lac Band and Potawatomi) strongly disagreed
with the EPA's proposed decision not to set an AAI-based standard in
this review. These commenters generally focused on the strengths of the
evidence of deposition-related effects, the extent to which analyses
presented in the PA addressed uncertainties and limitations in the
evidence, and on information regarding the adversity of such effects as
a basis for their views that such a standard was warranted at this
time. Many of these commenters pointed to CASAC's review of the
underlying scientific evidence and its support for moving forward with
an AAI-based standard at this time as support for their views.
    In general, the environmental group commenters expressed the view
that the current standards are clearly not adequate and that a combined
NOX/SOX standard that links ambient air quality
to an ecosystem indicator is appropriate, founded in science, and
necessary for protection of public welfare. The commenters stated the
current standards are neither sufficiently protective nor appropriate
to address deposition-related effects. They also noted that the EPA has
worked for decades to solve the acid deposition problem and that in
their view the AAI represents an elegant solution to that problem.
    With regard to their views on the underlying scientific
information, these commenters generally agreed with the EPA's proposed
conclusions that there are well-established water quality and
biological indicators of aquatic deposition and well-established models
that address air deposition, water quality impacts, and effects on
biota. Many of these commenters expressed the view that the
uncertainties and limitations in the scientific evidence were
adequately addressed in the PA, which was reviewed by CASAC. Many of
these commenters pointed to CASAC's support for adopting an AAI-based
standard in this review while concurrently conducting additional field
monitoring and longer-term research that might reduce uncertainties in
future reviews of secondary NAAQS for oxides of nitrogen and sulfur.
    Some governmental agency commenters were strongly supportive of an
AAI-based standard and clearly felt such a standard should be adopted
now. They also noted that the current ambient concentrations of
NOX and SOX are causing adverse ecological
impacts and they believe that ongoing damage due to acidic deposition
and the risks to ecosystems far outweigh the risk of setting an AAI-
based standard while some uncertainties remain. They assert that
NOX and SOX deposition is causing adversity to
public welfare and that the scientific uncertainties do not preclude
setting an AAI-based standard, and point to CASAC as generally
supporting this view. The commenters believe that the EPA has ample
evidence to support a new ecologically based standard and that the AAI
is reasonable and scientifically defensible. NY specifically
recommended an AAI of 50 with some flexibility built into the F
factors.
    Some of these agency and environmental group commenters also
referenced CASAC's support for specific elements of the AAI-based
standard developed in the PA, including (1) The use of ANC as an
appropriate ecological indicator for such a standard, (2) the use of
NOy and SOX as well-justified indicators of
atmospheric concentrations of oxides of nitrogen and sulfur, (3) the
use of Omernik Level III ecoregions, (4) the division of ecoregions
into sensitive and non-sensitive categories, (5) the use of a 3 to 5
year averaging time, and (6) the appropriateness of an AAI level
between 20 to 75 [mu]eq/L.
    With regard to their views on the requirements of the CAA, several
environmental group commenters stated that given the large body of
evidence supporting significant ongoing harm to the public welfare and
the EPA's finding the current standards are neither sufficiently
protective nor appropriate to address deposition-related effects, the
EPA's reliance on uncertainty as grounds for failing to propose
protective standards is irrational, arbitrary, and legally flawed. They
believe that the EPA cannot lawfully reject a new AAI-based standard
while continuing to rely solely on a form of the standard that is
inadequate and allows serious harms to the public welfare to continue.
When confronted with scientific uncertainties and incomplete data, they
feel the EPA must act in a precautionary manner that errs toward
stronger protections. Further, they believe that the EPA's reliance on
scientific uncertainty as a basis for its inaction is unsupportable in
light of CASAC's advice and the EPA staff's conclusions in the ISA, REA
and PA.
    In addition to the two broad groups of commenters discussed above,
a few other commenters offered more general

views on an AAI-based standard. For example, some state commenters (NC
and PA) expressed support for the concept of developing a multi-
pollutant, AAI-based standard, but felt that it would be important to
gather additional information before proposing any such standard. One
state organization (NESCAUM) expressed concern that the EPA was not
following CASAC's recommendation to propose an ecologically relevant
level and form for this NAAQS.
    The EPA has carefully considered these comments on whether or not
an AAI-based secondary standard for oxides of nitrogen and sulfur is
appropriate at this time. The EPA agrees with the second group of
commenters and CASAC's advice (outlined in section III.B) that there is
a strong scientific basis for development of the structure of such a
standard, specifically with regard to a standard that would provide
protection from deposition-related aquatic acidification in sensitive
ecosystems across the country. As discussed in section II.A and
supported by several commenters, the available scientific evidence is
sufficient to infer a causal relationship between acidifying deposition
of nitrogen and sulfur and potential adverse effects to aquatic
ecosystems, and that the deposition of oxides of nitrogen and sulfur
both cause such acidification under current conditions that are allowed
by the current secondary standards (U.S. EPA, 2008, chapter 3). The EPA
agrees with commenters that there are well-established water quality
and biological indicators of aquatic acidification as well as well-
established models that address deposition, water quality, and effects
on ecosystem biota, and that ecosystem sensitivity to acidification
varies across the country (U.S. EPA, 2011, chapter 7).
    The EPA also agrees with the second group of commenters and CASAC
that ANC would be an appropriate ecological indicator, reflecting the
acidifying effects of deposition of nitrogen and sulfur (U.S. EPA,
2011, chapter 7.2 and Russell and Samet, 2011a). Further, the EPA
agrees that the structure of an AAI-based standard is well-grounded in
science and would address the combined effects of deposition from
oxides of nitrogen and sulfur by characterizing the linkages between
ambient concentrations, deposition, and aquatic acidification, and that
the structure of the standard takes into account relevant variations in
these linkages across the country (section III.B. above and U.S. EPA,
2011, chapter 7).
    The EPA disagrees with the first group of commenters that the use
of Omernik ecoregions would be inadequate. A full explanation of the
EPA's rationale for selecting the Omernik ecoregion scheme for spatial
aggregation is found in section 7.2.5 of the PA. Omernik ecoregions
include consideration of geology, physiology, vegetation, climate,
soils, land use, wildlife, and hydrology. These factors also relate
well to sensitivity to acidification. The EPA also evaluated the
National Ecological Observatory Network (NEON) and Bailey's ecoregions
developed for the U.S. Forest Service and concluded that the Omernik
ecoregion classification would be the most appropriate for an AAI-based
standard. It offers several levels of spatial delineation, has
undergone extensive scientific peer review, and has explicitly been
applied to delineating acid sensitive areas of the U.S.
    Nonetheless, the EPA agrees with the first group of commenters that
there are important and significant remaining scientific uncertainties
within the derivation of the AAI, with the data used to specify the
factors within the AAI equation, and with the models themselves. These
uncertainties are more fully discussed in Appendix F and G of the PA
and in section III.A.5 above. These uncertainties have been reviewed by
CASAC, and the EPA recognizes that further research would help to
reduce the uncertainties. In general, the EPA also recognizes that the
AAI would depend on atmospheric and ecological modeling, with inherent
uncertainties, to specify the terms of an AAI equation that incorporate
the linkages between ambient concentrations, deposition, and aquatic
acidification.
    The EPA agrees with the first group of commenters that there are
several important limitations in the available data upon which elements
of the AAI are based (U.S. EPA, 2011, Chapter 7). For example, existing
monitors for NOy are generally not located in areas that are
representative of sensitive aquatic ecosystems, and there is relatively
sparse water quality data coverage in sensitive mountainous western
areas. Further, even in areas where relevant data are available, small
sample sizes impede efforts to characterize the representativeness of
the available data for some ecoregions, which was noted by CASAC as
being of particular concern (Russell and Samet, 2011a). Also,
measurements of reduced forms of nitrogen are available from only a
small number of monitoring sites, and emission inventories for reduced
forms of nitrogen used in atmospheric modeling are subject to a high
degree of uncertainty.
    The EPA agrees with the first group of commenters that
uncertainties related to the use of ecological and atmospheric models
are difficult to evaluate due to a lack of relevant observational data.
For example, relatively large uncertainties are introduced by a lack of
data with regard to pre-industrial environmental conditions and other
parameters that are necessary inputs to critical load models that are
the basis for factor F1 in the AAI equation. Also, observational data
are not generally available to evaluate the modeled relationships
between nitrogen and sulfur in the ambient air and associated
deposition, which are the basis for the other factors (i.e., F2, F3,
and F4) in the AAI equation. The EPA recognizes that, in contrast, such
model-related uncertainties are not relevant in the consideration of
other NAAQS since those NAAQS are not defined in terms of factors based
on such models.
    The EPA agrees that these data limitations and model uncertainties
create a number of inherent uncertainties and complexities in the
quantification of the F factors of the AAI and the representativeness
of the F factors at an ecoregion scale (U.S. EPA, 2011, Appendix F).
These uncertainties and complexities currently lead to a high degree of
uncertainty in characterizing the degree of protectiveness that would
be afforded by an AAI-based standard with quantified F factors derived
as discussed above, within the ranges of levels and forms identified in
section III.A above.
    The EPA disagrees with the first set of commenters that the
selection of sensitive ecoregions and percentile waterbodies would be
arbitrary. The EPA fully discussed its rationale and selection of
sensitive ecoregions and the range of percentiles used in section 7.2.5
of the PA. The EPA relied on available alkalinity and ANC data to draw
distinctions between sensitive and non-sensitive ecoregions. The EPA
used its judgment in selecting the range of percentiles for sensitive
and non-sensitive ecoregions, attempting to be neither over-protective
nor under-protective of the set of waterbodies in each ecoregion.
    In general, the first set of commenters tends to treat all aspects
of the AAI as subject to a high to very high degree of uncertainty. The
EPA disagrees with this view, and instead views some parts of the AAI
as based on more certain scientific information than others. For
example, the EPA believes there is a solid scientific basis for the
general

framework of the AAI and for the relationship between ANC and effects
on aquatic life. There is a strong basis for selection of ANC as an
ecological indicator, for selection of NOy and
SOX as ambient air indicators, for selection of the annual
and 3- to 5-year averaging time frame, and for selection of the range
of ANC and percentile of water bodies for consideration. Likewise, the
EPA believes there is a solid scientific basis for selection of Omernik
ecoregions as the geographic basis for development of the AAI F
factors. The EPA believes that for many areas there is a strong basis
for determining whether an ecoregion is acid sensitive or not acid
sensitive, while recognizing there is some uncertainty in some areas as
to which category the area should fall in. The EPA's decision not to
adopt an AAI-based standard at this time is not driven by uncertainty
in these elements of the AAI, but instead in the elements needed to
derive the quantified F factors for ecoregions across the country and
our ability to evaluate the representativeness of those F factors for
an entire ecoregion. The greatest uncertainties concern the F1 and F2
factors, which relate to development of a single critical load to
represent a specified percentile of all of the waterbodies in an
ecoregion and development of the value for deposition of reduced
nitrogen. In addition, there are also important and significant
uncertainties related to development of the F3 and F4 factors, which
concern the quantified relationship between ambient levels of
NOy and SOX and deposition rates of nitrogen and
sulfur. The bases for these uncertainties are discussed in more detail
in sections III.A.5 above and are considered as well in section III.E
below. Thus, while the EPA agrees in part with the first group of
commenters, in general they paint with too broad a brush. The EPA's
decision is based instead on taking into account the areas where there
is less scientific uncertainty as well as the areas where there remain
significant scientific uncertainties.
    In general, the second set of commenters does not contest the
scientific evidence as discussed by the EPA or the scientific
conclusions the EPA draws. They do not contest the existence of
scientific uncertainty or the causes of it, and do not present
scientific or technical arguments to contest the nature or magnitude of
the uncertainty. Instead, they disagree with the conclusions or
judgments to draw from the uncertainty. In the view of these
commenters, the degree of uncertainty is low enough to warrant setting
an AAI standard at this time. They disagree with the Administrator's
policy judgment that the nature and magnitude of uncertainty is of such
significance that it warrants not setting an AAI standard at this time.
Their primary disagreement is with this judgment, not with the EPA's
underlying views on the science and its uncertainties. As discussed in
the proposal and below, however, the Administrator's reasoned judgment
is that it is not appropriate to establish an AAI-based secondary
standard at this time. The uncertainties discussed above prevent a
reasoned 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.
Therefore, the Administrator is unable to identify an appropriate
standard.
    The EPA recognizes that the AAI equation, with factors quantified
in the ranges discussed in section III.A above and described more fully
in chapter 7 of 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 (PA, chapter 7).
However, the various uncertainties discussed above are critical for
determining with any degree of confidence 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 with quantified F factors, and thus for
determining an appropriate AAI-based standard that meets the
requirements of Section 109 of the CAA. The EPA recognizes that these
limitations and uncertainties result in a high 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 a high degree of uncertainty
as to the actual degree of protectiveness that such a standard would
provide, especially for acid-sensitive ecoregions.
    With regard to comments that the EPA cannot lawfully reject a new
AAI-based standard, the EPA disagrees with the second group of
commenters that the Administrator is required to set an AAI-based
standard at this time. Although the Administrator has concluded that
the current secondary standards are neither appropriate nor adequate to
protect against potentially adverse deposition-related effects
associated with ambient concentrations of oxides of nitrogen and
sulfur, such a conclusion does not require the EPA to adopt a new NAAQS
where the Administrator cannot reasonably judge that it would meet the
criteria for a secondary NAAQS.
    The Administrator judges that the current limitations in relevant
data and the uncertainties associated with specifying the elements of a
new AAI-based NAAQS defined in terms of modeled factors are of such a
significant nature and degree as to prevent her from reaching a
reasoned decision 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. As a result, the Administrator has
determined that she cannot establish an AAI-based standard that is
requisite to protect public welfare. The Administrator has made a
similar judgment in deciding not to adopt new secondary NAAQS in the
form of 1-hour standards identical to the primary NO2 and
SO2 standards, as discussed below. No other NAAQS revisions
to address the effects of acid deposition associated with oxides of
nitrogen and sulfur in the ambient air have been suggested or
considered by the EPA, CASAC, or commenters in this review.\13\ As
such, all possible revisions to the secondary NAAQS to address the
effects of acid deposition would involve adoption of new secondary
standards that are judged by the Administrator to have such a high
degree of uncertainty that she cannot make a reasoned decision that a
new standard would satisfy the criteria of Section 109(b) of the CAA.
---------------------------------------------------------------------------

    \13\ No one has suggested that the EPA should revise the current
3-hour or annual secondary standards to address the effects of
acidifying deposition associated with oxides of nitrogen and sulfur
in the ambient air. All revisions under consideration have involved
adopting new secondary NAAQS.
---------------------------------------------------------------------------

    Commenters have pointed to the requirement in Section 109(b)(2) of
the CAA that any secondary NAAQS ``must specify a level of air quality
the attainment and maintenance of which * * * is requisite to protect
the public welfare from any know or anticipated adverse effects * * *''
in support of the argument that the EPA must adopt a new standard that
provides requisite protection, having concluded that the

current secondary standards are not sufficient to protect against
adverse effects. In considering this comment, the EPA has taken into
account the statutory language, as well as the bases for the EPA's
conclusion that the current standards for oxides of nitrogen and sulfur
are neither appropriate nor adequate to provide protection against
potentially adverse deposition-related effects and the data and model
uncertainties that limit our efforts to characterize the degree of
protectiveness that would be afforded by either an AAI-based standard
or a 1-hour standard. We have concluded that Section 109 of the CAA
does not require the EPA to adopt a new secondary standard where, as
here, in the reasoned judgment of the Administrator, the uncertainties
associated with such a standard are of such significance that they
prevent her from determining whether or not such a NAAQS is requisite
to protect public welfare. Section 109(b) of the CAA does not require
the EPA to set a new standard under circumstances where the
Administrator cannot reasonably judge that it would meet the criteria
for a secondary NAAQS.
    This is consistent with the decision by the Supreme Court in
Massachusetts v. EPA, 549 U.S. 497 (2007), which concerned the EPA's
authority under Section 202(a) of the CAA. There the Supreme Court
determined that scientific uncertainty that ``is so profound that it
precludes the EPA from making a reasoned judgment'' concerning
endangerment to public health and welfare from air pollution would
justify the EPA not making a finding on endangerment. Id at 534. The
Court noted that ``[t]he statutory question is whether sufficient
information exists to make'' an endangerment finding. Id. In this
review, the scientific uncertainty is of such a significant nature and
degree that sufficient information does not exist for the EPA to make a
reasoned judgment as to whether a new secondary standard addressing
aquatic acidification would satisfy the criteria of Section 109(b). As
such, adding a new AAI secondary standard at this time would not ``be
appropriate under [Section 109(b)].'' CAA Section 109(d)(1).
    The EPA recognizes and agrees with the comment from one
environmental group that the EPA is not ``foreclosed from setting a
standard unless it can identify * * * a `perfect' standard level that
is free from any noteworthy uncertainty.'' However, that is not the
situation in this rulemaking. The Agency has concluded that it would
not be appropriate to promulgate a standard to address the public
welfare effects of acidifying deposition where the remaining scientific
uncertainties are of such significance that they preclude the EPA from
making a reasoned determination of the degree of protectiveness that
would be afforded by such a standard. The EPA recognizes that as a
result of not setting a new secondary standard the current secondary
standards continue in place and continue to be neither appropriate nor
adequate to protect against potentially adverse deposition-related
effects associated with ambient concentrations of oxides of nitrogen
and sulfur. However, in the Administrator's view the proper response
under the current circumstances is to continue to develop the
scientific and technical basis for a future revision to the standards,
and not to adopt at this time a new secondary standard that she cannot
reasonably judge would comply with Section 109 of the CAA.
    Further, the EPA agrees with both groups of commenters and CASAC
that collecting further field data would be beneficial. A field pilot
program is discussed in detail in section IV below. However, the EPA
disagrees with the first group of commenters' assertions that these
uncertainties should invalidate or preclude the further development of
an AAI-based standard.
b. Comments on Specific Aspects of an AAI-Based Approach
    This section discusses comments on the following four specific
aspects of an AAI-based approach to setting a secondary standard for
oxides of nitrogen and sulfur: (1) The inclusion of chemically reduced
nitrogen (NHX), in addition to oxides of nitrogen, in the
AAI equation; (2) whether such a standard would be appropriately
construed as a national standard versus a regional standard; (3)
whether such a standard would be appropriately construed as an ambient
air quality standard versus a water quality standard, and (4) whether
the EPA has authority under the CAA to set a multi-pollutant NAAQS.
    (1) As described above in section III.A, the AAI equation contains
a separate factor that accounts for the acidifying potential of
NHX, in addition to the factor that accounts for the
acidifying potential of oxides of nitrogen. Several industry commenters
addressed this issue explicitly, with some expressing the view that
NHX should be treated the same as NOX in the AAI,
while others felt it should not be included at all in the AAI. Several
commenters expressed the view that accounting for NHX in the
AAI equation represents a de facto regulation of ammonia, which they
assert is unlawful since reduced nitrogen is not a listed air pollutant
under Section 108 of the CAA.
    Other commenters, including environmental groups and governmental
agency commenters, did not explicitly comment on the inclusion of
NHX in the AAI equation; however several commenters made
note of CASAC's advice on this issue. CASAC advised that it is
necessary to include a factor for NHX in the AAI equation,
even though it is not a listed pollutant, since aquatic ecosystems
respond to inputs of NHX to create acidity just like they do
with inputs of NOX and SOX.
    The EPA has included NHX deposition explicitly as part
of factor F2 in the AAI expression to account for the acidifying
potential afforded by ammonia gas and ammonium ion. Inclusion of
NHX deposition, in addition to deposition of oxides of
nitrogen, is necessary to account for potential effects of all reactive
nitrogen species which, in turn, allows for determining the
contributions of oxides of N and S to aquatic acidification. This
approach is consistent with the requirement in the CAA that where the
state of the science provides a basis for considering such effects, the
review of the air quality criteria for a pollutant should encompass the
ways in which other air pollutants may interact with the criteria
pollutant to produce adverse effects. See CAA Section 108(a)(2). In
effect, the inclusion of NHX deposition can be viewed as a
necessary component consistent with our scientific understanding that
links deposition of all nitrogen species to ecological effects.
    The EPA recognizes that the NAAQS is established to address the
pollutants oxides of nitrogen and oxides of sulfur. Consequently, the
ambient concentrations of oxides of sulfur (as SOX) and
nitrogen (as NOy) are accounted for separately from the
deposition of NHX in the AAI equation, thus defining the
standard specifically in terms of the acidifying potential of levels of
oxides of nitrogen and sulfur in the ambient air. More specifically,
compliance with an AAI-based standard would be based on using federal
reference or equivalent monitoring methods to measure ambient
concentrations of NOy and SOX to determine an
area's attainment status. Conversely, there would be no requirement to
measure concentrations of NHX to determine compliance with
an AAI-based standard. Rather, ecoregion-specific values of
NHX deposition would be determined by modeling and would be
specified by the EPA in conjunction with setting such a

standard, and would not be a variable in the AAI equation as would
SOX and NOy. The contribution of reduced forms of
nitrogen to total nitrogen deposition would represent an ecosystem-
specific environmental factor that plays a necessary background role in
characterizing the relationship between the measured, variable levels
of the ambient air indicators of oxides of nitrogen and sulfur
(NOy and SOX) and the associated degree of
aquatic acidification. Section 108 requires the air quality criteria to
evaluate to the extent practicable the variable factors such as
atmospheric conditions that affect the impact of the ambient air
pollutant (in this case oxides of nitrogen and sulfur) on the public
welfare. In this review, such variable factors include the deposition
of reduced nitrogen in an ecoregion, as well as all of the other
elements reflected in the factors F1 to F4, and the designation of an
area as acid-sensitive or not acid-sensitive. Section 109 calls for the
EPA to base the NAAQS on the air quality criteria, and accounting for
the role of reduced nitrogen deposition in the AAI reflects this.
    In considering this aspect of an AAI-based standard, the EPA took
into account that in applying the AAI equation, all factors, including
NHX deposition, would be updated as appropriate as part of
the periodic reviews of the NAAQS, called for at five-year intervals by
the CAA, to account for changing environmental conditions and new data.
In determining an ecoregion's status with regard to meeting a
particular AAI-based standard, NHX deposition reflected in
the F2 factor would be treated just as all of the other environmental
terms--e.g. critical loads and transference ratios--which influence
factors F1, F3 and F4. To the extent that changes in NHX
deposition occur from one review to the next, the ecoregion-specific F2
factors would be updated to reflect such changes. To the extent that
NHX deposition decreased from one review to the next, an
AAI-based standard updated during a periodic review to reflect this
change would allow for potentially higher levels of NOy and
SOX that would meet a specific AAI-based standard;
conversely, increased levels of NHX deposition would allow
for potentially lower levels of NOy and SOX.
Meeting a specific AAI-based standard would only require that the
combined levels of NOy and SOX be such that a
calculated AAI value meet or exceed the AAI value set as the level of
the standard. Consequently, while the contribution of NHX
deposition would be accounted for, NHX emissions would not
be regulated through the implementation of an AAI-based standard.
NHX deposition would be treated as an ecologically relevant
background value that could be updated over time to reflect changes in
circumstances, but accounting for such changes would not be required
for purposes of determining compliance with an AAI-based standard.
Thus, the incorporation of NHX in the AAI equation would not
result in de facto regulation of NHX emissions.
    (2) Some commenters raised the issue of whether an AAI-based
standard would be a national standard, as required by Section 109 of
the CAA, or whether it is in essence a regional standard. One group of
commenters (the Center for Biological Diversity and the National Park
Service) generally expressed the view that an AAI-based standard would
be a national standard, whereas another group, including industry
commenters, asserted that an AAI-based standard would be a regional
standard and thus not consistent with the requirements of the CAA.
    The first group of commenters supported the use of a national ANC
indicator, recognizing that an AAI approach would account for regional
differences in sensitivity and relevant environmental factors while
providing a nationally consistent degree of protection across sensitive
ecoregions. For example, the National Park Service stated that the AAI
approach provides a uniform level of protection to sensitive ecosystems
while appropriately taking into account the variability in deposition,
meteorology, and other relevant environmental factors across
ecoregions.
    The second group of commenters noted that application of the AAI
equation in different areas of the country produced different allowable
concentrations of NOy and SOX, asserting as a
result that an AAI-based standard would be a regional standard. These
commenters asserted that the EPA lacks authority under the CAA to set
such a regional NAAQS. For example, UARG states that the AAI is applied
differently in different regions of the country (e.g., sensitive vs.
non-sensitive ecoregions). The Alliance of Automobile Manufacturers
commented that both the EPA and Congress historically have decided that
secondary national air quality standards are not an appropriate
approach to address regionally variable welfare effects.
    The EPA believes that a secondary NAAQS based on the AAI approach
could be a national standard, consistent with the CAA. An AAI-based
standard would apply all across the country. It would be defined in
part by a single level of the AAI--that is, every part of the country
would be expected to meet or exceed a specified AAI level. The
scientific basis for setting a national AAI level is rooted in the
similarity between AAI and acid neutralizing capacity (ANC), which is a
widely accepted ecological health indicator for aquatic acidification.
The rationale underlying the use of ANC is that the ecosystem health
reflected by an ANC value in one part of the country is generally
similar to that in another location, irrespective of regional
differences in biogeochemistry and atmospheric conditions. The EPA
recognizes that allowable concentrations of the ambient air pollutant
indicators for oxides of nitrogen and sulfur in the AAI equation can
vary from one location to another and result in the same calculated
AAI. The difference between an AAI-based standard and the existing
primary standards is that the level of the standard is defined directly
in terms of the measured ambient air pollutant indicator. That is, the
health-based indicator and the measured ambient air indicator are based
on the same chemical entity. In an AAI-based standard, the level of the
standard, reflecting a nationally consistent degree of protection,
would be defined in terms of an ecological indicator, ANC, and
compliance would be determined based on concentrations of the ambient
air indicators, NOy and SOX. From an ecosystem
health perspective, it is most relevant to use the ecological
indicator, ANC, to establish a single level that, in the context of an
AAI, leads to a similar degree of protection across the country. The
allowable levels of NOy and SOX could vary across
the country, while the specified AAI level and the corresponding degree
of protection, would not. This would facilitate ensuring that such a
NAAQS would provide sufficient protection, but not more than was
necessary. It should be noted that in the 2006 PM NAAQS decision the
EPA set a NAAQS that envisions variation in allowable ambient levels of
certain kinds of PM. The EPA set a PM10 standard with a
single numerical level, which then allowed varying levels of coarse PM,
a subset of PM10. The PM10 standard was designed
to allow lower levels of coarse PM in urban areas and higher levels of
coarse PM in non-urban, rural areas. The EPA's goal was to target
protection at urban areas, where the evidence showed coarse particles
presented a greater risk to public health. The single numerical
standard for PM10 allowed

variable levels of coarse PM, with higher allowable levels where there
was less evidence of risk and lower allowable levels where the evidence
of risk was greater. This approach was upheld in American Farm Bur.
Fed. v. EPA, 559 F.3d 512, 533-536 (D.C. Cir. 2009).
    In conjunction with consideration of an AAI-based standard, the EPA
has recognized that the nation includes some relatively acid-sensitive
and some relatively non-acid sensitive ecoregions. This delineation
allows for an appropriate application of the AAI equation that
increases its relevancy from a national perspective as it avoids
creating more than requisite protection in areas that are not acid
sensitive. The AAI equation and the selected level of such a standard
would be applicable everywhere; however, factors in the AAI equation
are appropriately dependent on the sensitive and non-sensitive
ecoregion classification. Therefore, the delineation of sensitive and
non-sensitive regions allows for a nationally consistent application of
the AAI equation as it targets protection on those areas most likely to
benefit from reductions in acidifying deposition of oxides of nitrogen
and sulfur, and avoids more than requisite protection in areas that
would not benefit from such reductions.
    (3) Some commenters expressed the view that an AAI-based standard
would essentially be a water quality standard, since it would use ANC,
a water quality property, as the ecological indicator. For example,
UARG expressed this view by noting that an AAI standard would be
defined in terms of a single water quality level with multiple
allowable air quality concentrations of oxides of nitrogen and sulfur.
    The EPA notes that the AAI relates aquatic acidification to ambient
air concentrations of oxides of nitrogen and sulfur. An AAI-based
standard would be set at a level such that ambient air concentrations
would not cause harmful acidification effects to water quality
resources, which is within the scope of welfare effects that secondary
NAAQS are to address (i.e., welfare effects include, but are not
limited to, ``effects on soils, water, * * *''). Accordingly, while an
AAI-based standard would address effects on water quality, it would do
so by defining the allowable ambient air concentrations of oxides of
nitrogen and sulfur that would provide appropriate protection against
such effects. Compliance with such a standard would be determined by
measuring ambient air concentrations of NOy and
SOX, not by measuring the water quality property of ANC. The
actual water quality of any body of water would not be used to
determine compliance with the air quality standard, and no body of
water would be considered in ``non-compliance'' with an AAI air quality
standard. Thus, an AAI-based standard is appropriately construed as an
air quality standard, not a water quality standard.
    (4) Some commenters questioned whether the EPA has the authority to
establish a NAAQS that jointly addresses ambient concentrations of
oxides of nitrogen and oxides of sulfur. Pointing to language in
Section 109(b)(2) that a NAAQS must address ``adverse effects
associated with the presence of such air pollutant in the ambient
air,'' these commenters took the position that the EPA may not allow
for tradeoffs between two pollutants in setting a NAAQS. See Section
109(b)(2) (emphasis added). These commenters suggest the NAAQS must be
set for ``such air pollutant'' only. The EPA disagrees that the phrase
``such air pollutant'' in Section 109(b)(2) would prohibit the Agency
from setting a multi-pollutant NAAQS in the form of an AAI. When the
Administrator sets a NAAQS, the standard must be ``requisite to protect
the public welfare from any known or anticipated adverse effects
associated with the presence of such air pollutant.'' CAA Section
109(b)(2). Oxides of nitrogen and sulfur, pollutants for which the EPA
has issued air quality criteria, both cause acidification of aquatic
ecosystems, effects that could be considered adverse to public welfare.
As such, acidifying deposition is a ``known or anticipated adverse
effect[ ] associated with the presence of [oxides of nitrogen] in the
ambient air.'' This known or anticipated adverse effect is also
associated with the presence of oxides of sulfur in the ambient air.
Given the scientific links between ambient air concentrations of oxides
of nitrogen and sulfur, the related deposition of nitrogen and sulfur,
and the associated ecological responses, the EPA appropriately
considered a multi-pollutant NAAQS in the form of an AAI to protect
against the effects of acidifying deposition to aquatic ecosystems that
took into account these linkages. Rather than limiting the EPA's
authority, the language cited by the commenters goes to the breadth of
the EPA's obligation and authority to set standards to protect against
``any known or anticipated adverse effects.'' In addition, the NAAQS
are to be based on the air quality criteria, which under Section
108(a)(2) are required to consider the kind of multi-pollutant linkage
evident in this review. The EPA does not read the language of Section
109(b) as prohibiting the Administrator from setting a multi-pollutant
NAAQS such as the AAI where such an approach would be judged as the
appropriate way to satisfy Section 109(b)'s requirements for each of
the pollutants involved.
2. Comments on 1-Hour NO2 and SO2 Secondary
Standards
    Comments received on the proposal related to setting new 1-hour
NO2 and SO2 secondary standards are addressed in
this section. Most generally, there was broad and strong opposition to
the EPA's proposed decision to set 1-hour NO2 and
SO2 secondary standards identical to the 1-hour
NO2 and SO2 primary standards. For example,
strong opposition to this proposed decision was expressed by a diverse
set of commenters, including some environmental groups (e.g.,
Environmental Justice, the Adirondack Council) and industry groups
(e.g., UARG, AAM, ASARCO, API, Portland Cement Association, Tri-State
Generation and Transmission Association, Louisiana Chemical
Association, East Kentucky Power Cooperative, FMMI, Rio Tinto), the
U.S. Department of the Interior, and some states (e.g., NY, PA, TX).
These commenters offered various arguments in support of their views
that the proposed decision is unlawful, arbitrary, and not supported by
the record of this rulemaking, as outlined below. One commenter (NC)
supported setting secondary standards identical to the 1-hour
NO2 and SO2 primary standards, while also
supporting the EPA's decision to take additional time to develop a
multi-pollutant AAI-based secondary standard. Another commenter (SD)
simply supported setting secondary standards that are no more stringent
than the primary standards.
    In proposing the 1-hour secondary standards, the EPA recognized
that such standards would not be ecologically relevant, but concluded
that they would nonetheless ``directionally provide some degree of
additional protection'' by reducing deposition to sensitive ecosystems.
The EPA also noted that this was 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.
    In arguing that the proposed decision to set 1-hour NO2
and SO2 secondary standards identical to the 1-hour
NO2 and SO2 primary standards is unlawful,
commenters asserted that the EPA's

rationale is not consistent with the requirements of Section 109 of the
CAA. Commenters argue that this rationale is not consistent with the
CAA requirement that the EPA set secondary NAAQS that are ``requisite
to protect public welfare;'' that is, a standard that is neither more
nor less stringent than necessary for this purpose. More specifically,
these commenters argue that a standard that is based solely on
``directionally'' improving the environment, without any evidence or
judgment that it would provide ``requisite'' protection, is not
consistent with the requirements of the CAA and is thus unlawful. Some
commenters also note that the CAA requires that the EPA revise
previously adopted NAAQS as ``appropriate'' to provide such protection.
These commenters assert that since the EPA's proposal concludes that
the 1-hour NO2 and SO2 standards are not
ecologically relevant to address deposition-related effects on
sensitive ecosystems, adding such standards cannot be considered to be
an appropriate revision to the NAAQS for the purpose of addressing
adverse ecological effects.
    Commenters also raised a number of issues in support of their views
that the proposed decision is arbitrary and unsupported by the
available information in the record of this rulemaking. Some commenters
noted that there is no evidence or analysis in the record that
addresses the degree of protection that would likely be afforded by 1-
hour NO2 and SO2 standards, and, further, that
the EPA does not claim otherwise. In the absence of such information,
commenters argue that the EPA cannot make a reasoned judgment as to
what levels of such 1-hour NO2 and SO2 standards
would be requisite to protect public welfare; in particular, some
commenters emphasized that the EPA cannot demonstrate that such
standards would not be more stringent than necessary to protect against
adverse deposition-related effects to sensitive ecosystems. Thus, in
the commenters' view, any such 1-hour standards would be arbitrary.
    One commenter also expressed the view that the EPA's proposed
decision to set new 1-hour NO2 and SO2 secondary
standards is inconsistent with the reasoning the EPA used as a basis
for proposing not to set a new ecologically relevant AAI-based
secondary standard at this time. As summarized above, the EPA based its
proposed decision not to set an AAI-based standard, which is expressly
designed to address important differences in ecosystem sensitivities,
in part on uncertainties and limitations in relevant information that
were of such nature and degree as to prevent the Administrator from
reaching a reasoned decision at this time as to what level and form of
such a standard would provide a particular degree of protection. This
commenter asserts that the proposed decision to set new 1-hour
NO2 and SO2 secondary standards completely
ignores such uncertainties inherent in 1-hour standards, which are not
even structured to account for differences in ecosystem sensitivities.
    Some commenters asserted not only that the EPA has failed to
provide any information on the degree of protection that would likely
be afforded by the proposed 1-hour NO2 and SO2
standards, but that such an analysis cannot be done since there is no
rational connection between any of the elements of the proposed 1-hour
secondary standards--including the averaging time and level--and the
ecological effects the proposed standards are intended to address. In
particular, commenters noted that EPA has not presented any rational
basis for concluding that standards designed to reduce human health
risks associated with short-term peak concentrations of NO2
and SO2 have any connection whatsoever to addressing long-
term deposition of oxides of nitrogen and sulfur and associated impacts
on sensitive ecosystems.
    Further, commenters argued that there is no evidence in the record
that demonstrates the proposed 1-hour secondary standards would provide
any environmental benefit. For example, commenters noted that such
standards do not take into account ecosystem sensitivity; they may not
result in reductions to long-term deposition that is the relevant time
frame for deposition-related effects on sensitive ecosystems; and they
would not provide any benefit beyond that which might accrue from the
identical primary standards that are already in effect. Some commenters
have also noted that many other environmental regulations are already
in place that will provide reductions in ambient oxides of nitrogen and
sulfur, and that the EPA has not demonstrated that any additional
reductions are needed to provide requisite protection.
    The EPA agrees that the Agency has not presented evidence or
analysis in the record that addresses the degree of protection that
would likely be afforded by secondary standards set identical to the
current 1-hour NO2 and SO2 primary standards. The
EPA further agrees that such an analysis cannot reasonably be done in
the absence of a demonstrable linkage between peak 1-hour average
concentrations of NO2 and SO2 in the ambient air
and the impact of deposition-related acidification associated with
oxides of nitrogen and sulfur on sensitive aquatic ecosystems that the
proposed standards were intended to address. As a result, the EPA
agrees that there is no factual basis to make a reasoned judgment as to
what levels of 1-hour NO2 and SO2 standards would
provide a desired degree of protection of the public welfare, such that
the EPA cannot demonstrate or judge that the proposed standards would
not be more or less stringent than necessary to provide the desired
degree of protection against potentially adverse deposition-related
effects to sensitive ecosystems.
    As to whether the proposed standards would provide any
environmental benefit, it is the EPA's view that it is reasonable to
conclude that any standard that would lead to reductions in
NO2 and SO2 emissions would likely result in some
environmental benefit for some acid-sensitive areas. Nonetheless, the
EPA recognizes that any such environmental benefit that would result
from reductions in NO2 and SO2 emissions
sufficient to attain the 1-hour standards cannot be specifically
quantified or linked to reductions in aquatic acidification in specific
ecoregions. In addition, unlike an AAI-based standard, the 1-hour
standards would tend to provide more protection than is warranted in
areas that are not acid-sensitive.
    Further, the EPA recognizes that any benefits that would accrue as
a result of actions taken to meet the 2010 1-hour NO2 and
SO2 primary standards will occur regardless of whether we
adopt identical secondary standards. Thus, there is no additional
environmental benefit to be gained by making the standards identical.
The EPA does not agree, however, that the Agency needs to consider
future reductions that may accrue from other environmental regulations
in the context of reaching a judgment as to what NAAQS is requisite to
protect public welfare.
    The EPA notes that the strongly held view of the commenters with
respect to the proposed 1-hour standards is that the EPA should reject
and not adopt a standard where there is not an adequate scientific or
technical basis for judging the degree of protection which such a
standard would provide. The EPA agrees with that general point.
According to commenters, the 1-hour standards should be rejected
because they do not have such a basis, and, as discussed below, the EPA
agrees. This is consistent with the reasoning that the EPA has applied
to consideration of an

AAI-based standard, as discussed above in response to comments related
to an AAI-based standard. As noted above, the limitations and
uncertainties in the scientific and technical basis for developing a
specific AAI-based standard result in a great degree of uncertainty as
to how well the quantified elements of the AAI 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 a high degree of uncertainty as to the actual
degree of protectiveness that such a standard would provide, especially
for acid-sensitive ecoregions. At this time, the Administrator judges
that the uncertainties are of such a significant nature and degree 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.\14\
---------------------------------------------------------------------------

    \14\ Thus, as discussed above, EPA's disagreement with
commenters concerning adoption of an AAI-based standard at this time
appears to stem from differing views on whether or not there is an
adequate scientific or technical basis for judging the degree of
protection which an AAI-based standard would afford. There does not
appear to be a disagreement with the view that EPA should not adopt
a standard absent such a scientific or technical basis.
---------------------------------------------------------------------------

    The EPA has also considered, in light of the public comments,
whether it is necessary or appropriate under Section 109 of the CAA to
make any revision to the current secondary standards for oxides of
nitrogen and sulfur, having concluded that the current standards are
neither adequate nor appropriate. As discussed above in section
III.D.1.a, with regard to comments on the EPA's proposed decision not
to set a new multi-pollutant AAI-based standard at this time, some
commenters argued that the EPA cannot lawfully use uncertainty as a
basis to decline to set an ecologically relevant standard, having
concluded that the current secondary standards are neither adequately
protective nor appropriate to provide protection to ecosystems. In
response, the EPA disagrees, stating that data limitations and
uncertainties in key elements of a standard, which are of such
significant nature and degree as to prevent the Administrator from
reaching a reasoned decision as to what specific standard would be
appropriate to provide requisite protection, are an appropriate basis
for deciding not to set such a standard, even one that is of an
ecologically relevant form. The EPA concludes that it is appropriate to
apply the same reasoning in reaching a decision as to whether to set
new 1-hour NO2 and SO2 secondary standards. In
this case, the uncertainties are arguably even greater than with an
AAI-based standard, since as noted above there is no demonstrable
linkage between the elements of such standards and impacts on sensitive
ecosystems that the standards would be intended to address.

E. Final Decisions on Alternative Secondary Standards for Oxides of
Nitrogen and Sulfur

    In considering the appropriateness of establishing a new multi-
pollutant AAI-based standard to provide protection against potentially
adverse deposition-related effects associated with oxides of nitrogen
and sulfur, or setting new secondary standards identical to the current
1-hour NO2 and SO2 primary standards, the
Administrator took into account the information and conclusions in the
ISA, REA, and PA, CASAC advice, and the views of public commenters.
This consideration follows from her conclusion, discussed above in
section II.D, that the existing NO2 and SO2
secondary standards are neither appropriate nor sufficiently protective
for this purpose.
    As an initial matter, the Administrator has again considered
whether it is appropriate at this time to set a new multi-pollutant
standard to provide protection against potentially adverse deposition-
related effects associated with oxides of nitrogen and sulfur, with a
structure that would better reflect the available science regarding
acidifying deposition. In considering this, she recognizes that such a
standard, for purposes of Section 109(b) and (d) of the CAA,\15\ 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 has focused on the new AAI-based standard
developed in the PA and reviewed by CASAC, as discussed above in
section III.A. In so doing, the Administrator has again considered the
extent to which there is a scientific basis for development of such a
standard, specifically with regard to a standard that would provide
protection from deposition-related aquatic acidification in sensitive
aquatic ecosystems in areas across the country.
---------------------------------------------------------------------------

    \15\ Section 109(d)(1) of the CAA requires that ``* * * the
Administrator shall complete a thorough review * * * and shall make
such revisions in such criteria and standards and promulgate such
new standards as may be appropriate under * * * subsection 109(b) of
this section.''
---------------------------------------------------------------------------

    The Administrator notes that the ISA concludes that the available
scientific evidence is sufficient to infer a causal relationship
between acidifying deposition of nitrogen and sulfur in aquatic
ecosystems, and that the deposition of oxides of nitrogen and sulfur
both cause such acidification under current conditions in the United
States. Further, the ISA concludes that there are well-established
water quality and biological indicators of aquatic acidification as
well as well-established models that address deposition, water quality,
and effects on ecosystem biota, and that ecosystem sensitivity to
acidification varies across the country according to present and
historic nitrogen and sulfur deposition as well as geologic, soil,
vegetative, and hydrologic factors. In considering public comments on
the relevant scientific evidence, the Administrator notes that some
commenters agree with these conclusions in the ISA, whereas other
commenters question the extent to which the scientific information
provides evidence of well-established water quality and biological
indicators of aquatic acidification and the extent to which relevant
models appropriately account for important factors or have been
adequately evaluated. The Administrator has carefully considered these
comments and the Agency's responses to these comments, as discussed
above in section III.D. The Administrator also has considered the views
of CASAC, including its general support for the conceptual framework of
the AAI-based standard developed in the PA based on the assessments of
the underlying scientific information in the ISA and REA.
    Based on these considerations, the Administrator again concludes
that the general structure of an AAI-based standard addresses the
combined effects of deposition from oxides of nitrogen and sulfur by
characterizing the linkages between ambient concentrations, deposition,
and aquatic acidification, and that it takes into account relevant
variations in these linkages across the country. She recognizes that
while such a standard clearly would be quite innovative and unique, the
general structure of such a standard is nonetheless 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. Based on these
considerations, the Administrator continues to agree with the
conclusion in the PA, and supported by CASAC, that there is a strong
scientific basis for continued development of a standard with the
general structure presented in the PA. Further, the Administrator
recognizes 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 directionally would lead to
higher calculated AAI values.
    Nonetheless, while the Administrator recognizes the strong
scientific foundation for the general structure of an AAI-based
standard, she also recognizes that a specific AAI-based standard would
depend to a great degree on atmospheric and ecological modeling, in
combination with appropriate data, to specify the quantified terms of
an equation that incorporates the linkages between ambient
concentrations, deposition, and aquatic acidification. This equation,
which defines an aquatic acidification index (AAI), has the effect of
translating spatially variable ambient concentrations and ecological
effects into a potential national standard.
    With respect to establishing the specific terms of this equation,
there are a number of important and significant uncertainties and
complexities that are critical to the question of whether it is
appropriate under Section 109 of the CAA to set a specific AAI-based
standard at this time, recognizing that such a standard must be one
that in the judgment of the Administrator is requisite to protect
public welfare without being either more or less stringent than
necessary for this purpose. As discussed above in section III.A, these
uncertainties and complexities generally relate not to the structure of
the standard, but to the quantification of the various elements of the
standard, i.e., the F factors, and their representativeness at an
ecoregion scale. These uncertainties and complexities, which are unique
to this NAAQS review, currently preclude the characterization of the
degree of protectiveness that would be afforded by such a standard,
within the ranges of levels and forms identified in the PA, and the
representativeness of F factors in the AAI equation described above and
in the PA. These uncertainties have been generally categorized as
limitations in available field data as well as uncertainties that are
related to reliance on the application of ecological and atmospheric
modeling at the ecoregion scale to specify the various elements of the
AAI.
    With regard to data limitations, the Administrator observes that
there are several key limitations in the available data upon which
elements of the AAI are based. For example, while ambient measurements
of NOy are made as part of a national monitoring network,
the monitors are not located in locations that have been determined to
be representative of sensitive aquatic ecosystems or individual
ecoregions. Further, while air and water quality data are generally
available in areas in the eastern United States, there is relatively
sparse coverage in mountainous western areas where a number of
sensitive aquatic ecosystems are located. Even in areas where relevant
data are available, small sample sizes in some areas impede efforts to
characterize the representativeness of the available data at an
ecoregion scale, which was noted by CASAC and some commenters as being
of particular concern. Also, measurements of reduced forms of nitrogen
are available from only a small number of monitoring sites, and
emission inventories for reduced forms of nitrogen used in atmospheric
modeling are subject to considerable uncertainty.
    With regard to uncertainties related to the use of ecological and
atmospheric modeling, the Administrator notes in particular that model
results are difficult to evaluate due to a lack of relevant
observational data. For example, large uncertainties are introduced by
a lack of data to inform the necessary inputs to critical load models
that are the basis for factor F1 in the AAI equation. Also,
observational data are not generally available to evaluate the modeled
relationships between nitrogen and sulfur in the ambient air and
associated deposition, which are the basis for the other factors (i.e.,
F2, F3, and F4) in the AAI equation.
    Taking into account the above considerations, the Administrator
recognizes that characterization of the uncertainties in the AAI
equation as a whole represents a unique challenge in this review
primarily as a result of the complexity in the structure of an AAI-
based standard. In this case, the very nature of some of the
uncertainties is fundamentally different than uncertainties that have
been relevant in other NAAQS reviews. She notes, for example, some of
the uncertainties uniquely associated with the quantification of
various elements of the AAI result from limitations in the extent to
which ecological and atmospheric models, which have not been used to
define other NAAQS, have been evaluated. Another important type of
uncertainty relates to limitations in the extent to which the
representativeness of various factors can be determined at an ecoregion
scale, which has not been a consideration in other NAAQS.
    In combination, these limitations and uncertainties are of such a
nature and degree as to result in a high 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, the EPA cannot reasonably characterize the actual
degree of protectiveness that such a standard would provide, especially
for acid-sensitive ecoregions. 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 of the CAA.
    In considering these uncertainties in light of CASAC's advice, the
Administrator notes 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. She also
notes that CASAC encouraged the Agency to engage in future monitoring
and model evaluation efforts to help inform the specification of model-
derived elements in the AAI equation. CASAC supported the view in the
PA that there was a scientific basis for consideration of an AAI, and
that is what the Administrator has done in that she has fully
considered an AAI-based standard. However, CASAC did not indicate that
there was such a degree of scientific support for quantifying the terms
of the AAI equation and setting a specific AAI-based standard at this
time that it would be inappropriate to consider not setting an AAI-
based standard in this review in light of the uncertainties that CASAC
itself recognized.
    Further, in considering these uncertainties in light of the public
comments discussed above, the Administrator notes that these

uncertainties and limitations have been highlighted by a number of
public commenters in support of their view that it would be
inappropriate to establish an AAI-based standard at this time. Other
commenters, however, noted that NAAQS decisions are always made in the
face of uncertainties, and expressed the view that the uncertainties in
this NAAQS review are not so great as to preclude establishing such a
standard at this time.
    The Administrator agrees with the commenters that note that NAAQS
decisions are always made in the face of uncertainties, since the
latest available scientific information upon which NAAQS are to be
based is often at the leading edge of research. Thus, the EPA
Administrator must always consider uncertainties in scientific and
other information in reaching decisions on whether to retain or revise
an existing NAAQS or to adopt a new NAAQS. As a result, it is clear
that the existence of scientific uncertainty does not preclude adoption
of a new or revised NAAQS. The issue here, however, is not whether
uncertainty exists, but whether it is of such a significant nature and
magnitude that it warrants not adopting an AAI-based standard at this
time. In that context, the Administrator recognizes that the AAI-based
standard considered in this review is by far the most complex form of a
NAAQS standard that the EPA has considered, to date, and that this is
the first review in which the scientific and technical details of an
AAI-based standard have been developed for consideration. This review
has served to bring into focus for the first time the nature and degree
of the uncertainties associated with quantifying the specific factors
in the equation that defines the AAI. Thus, in this review, the
Administrator must newly consider not only the scientific basis for the
conceptual framework of such a standard, but also the extent to which
the available data, models, and analyses provide a reasoned basis to
choose a specific AAI-based standard consistent with the requirements
of Section 109 of the CAA.
    The nature of the uncertainties present in this review, and the
implications of those uncertainties for reaching a reasoned decision as
to whether an AAI-based NAAQS could be set consistent with the
requirements of section 109(b), are in sharp contrast to the nature of
uncertainties present in other NAAQS reviews. In other NAAQS reviews,
studies are generally available directly linking ambient air
concentrations of the pollutant to evidence of effects on public health
or welfare. For example, in reviewing a health-based primary NAAQS the
EPA typically considers a wide range of clinical, epidemiologic,
toxicologic, and other studies that evaluate the relationship between
direct exposure to an ambient air pollutant and human health. The EPA
also often considers laboratory or field studies or surveys that
evaluate and characterize the relationship between ambient levels of an
air pollutant and welfare effects, such as effects of the ambient air
pollutant on the growth of plants or on injury to plants. These kinds
of scientific studies have provided a reasoned basis in other reviews
for the selection of an appropriate level and form of a standard, with
the EPA taking into account the nature and degree of uncertainties, for
example, in the relationships between varying ambient air
concentrations and the impact on human health or the environment.
    Further, the uncertainties present in the evidence available for
other NAAQS reviews have not been of such a significant nature that
they have precluded a reasoned assessment of the degree of
protectiveness that would likely be afforded by specific alternative
standards under consideration. In this case, however, unlike in other
NAAQS reviews, multi-pollutant and multi-media pathways of exposure
must be considered, and characterized in terms of an equation with
several factors, where the values of those factors vary from ecoregion
to ecoregion. The quantification of these factors must be based on the
use of ecological and atmospheric modeling at an ecoregion scale.
Further, the appropriateness of these factors depends upon analyses
that could be used to determine the representativeness of the data at
an ecoregion level. These circumstances, which are unique to this
review, result in such large uncertainties at this time that in the
aggregate they preclude the development of a reasoned assessment of the
degree of protectiveness that specific alternative AAI-based standards
would provide.
    Based on the above considerations, the Administrator has determined
that at this time it is not appropriate under Section 109 of the CAA to
set a new multi-pollutant standard to address deposition-related
effects of oxides of nitrogen and sulfur on aquatic acidification. As
the Administrator noted in the proposal, setting a NAAQS properly
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. As noted
above, 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, in the Administrator's judgment, the unique uncertainties
present in this review are of such significance that they preclude a
reasoned 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,
together with an AAI defined in terms of ecoregion-specific F factors.
The Administrator has considered whether these uncertainties could be
appropriately accounted for by choosing either a more or less
protective target ANC level and percentile of water bodies than would
otherwise be chosen if the uncertainties did not prevent a reasoned
judgment on the quantification of the AAI factors. However, in the
Administrator's judgment, the uncertainties are of such a significant
nature and degree that there is no reasoned way to choose such 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.
    Based on the above considerations, the Administrator judges that
the current limitations in relevant data and the uncertainties
associated with specifying the elements of the AAI are of such nature
and degree as to prevent her from reaching a reasoned judgment as to
what level and form (in terms of a selected percentile) of an AAI-based
standard would provide the degree of protection that the Administrator
determined was requisite. While acknowledging that CASAC supported
consideration of moving forward to establish the standard developed in
the PA at this time, the Administrator also observes that CASAC
supported conducting further field studies that would better inform the
continued development or modification of such a standard. Given the
current high degree of uncertainties and the large complexities
inherent in quantifying the elements of such a standard, largely
deriving from the nature of the standard under consideration for the
first time in this review, and having fully considered

CASAC's advice and public comments, the Administrator concludes that it
would be premature and not appropriate to set a new, multi-pollutant
AAI-based secondary standard for oxides of nitrogen and sulfur at this
time.
    While the Administrator has concluded that it is not appropriate to
set such a multi-pollutant standard at this time, she has determined
that the Agency should undertake a field pilot program to gather
additional data, and that it is appropriate that such a program be
undertaken before, rather than after, reaching a decision to set such a
standard. As described below in section IV, the purpose of the program
is to collect and analyze data so as to enhance our understanding of
the degree of protectiveness that would likely be afforded by a
standard based on the AAI as developed in the PA. This will provide
additional information to aid the Agency in considering an appropriate
multi-pollutant standard in future reviews, specifically with respect
to the acidifying effects of deposition of oxides of nitrogen and
sulfur. Data generated by this field program will also support
development of an appropriate monitoring network that would work in
concert with such a standard to result in the intended degree of
protection. The information generated during the field program can also
be used to help state agencies and the EPA better understand how an
AAI-based standard would work in terms of the implementation of such a
standard.
    While not a basis for this decision, the Administrator also
recognizes, as she did at the time of the proposal, that a new,
innovative AAI-based standard would raise significant implementation
issues that would need to be addressed consistent with the CAA
requirements for implementation-related actions following the setting
of a new NAAQS. It will take time to address these issues, during which
the Agency will be conducting a field pilot program to gather relevant
data and the environment will benefit from reductions in oxides of
nitrogen and sulfur resulting from the new NO2 and
SO2 primary standards, as noted above, as well as reductions
expected to be achieved from the EPA's Cross-State Air Pollution Rule
and Mercury and Air Toxics standards. These implementation-related
issues are discussed in more detail below in section IV.A.5.
    The Administrator has also reconsidered whether it is appropriate
at this time to set new secondary standards identical to the current 1-
hour NO2 and SO2 primary standards. In the
proposal, the Administrator recognized that the new NO2 and
SO2 primary 1-hour standards set in 2010 were not
ecologically relevant for a secondary standard to address deposition-
related effects associated with oxides of nitrogen and sulfur.
Nonetheless, the Administrator proposed to set new secondary standards
identical to the 1-hour NO2 and SO2 primary
standards on the basis that they would directionally provide some
degree of additional protection. At that time, the Administrator
reasoned that setting such standards would be consistent with her
conclusions that the current NO2 and SO2
secondary standards are neither sufficiently protective nor appropriate
in form, and that it is not appropriate to set a new, ecologically
relevant multi-pollutant secondary standard at this time.
    In reconsidering this proposal, the Administrator first notes that
although the ISA, REA, and PA did not directly consider secondary
standards set identical to the 1-hour NO2 and SO2
primary standards, the information and conclusions in those documents
provide strong support for the judgment that such short-term, peak
standards are not ecologically relevant to address deposition-related
effects associated with long-term deposition from ambient
concentrations of oxides of nitrogen and sulfur. The Administrator
notes that commenters on this aspect of the proposal broadly and
strongly supported this view. The Administrator also recognizes that
the Agency has not presented in these documents or elsewhere any
analysis of the degree of protectiveness that would likely be afforded
by such standards with regard to deposition-related effects in general
or aquatic acidification effects in particular. She also recognizes, as
discussed above in response to comments on this issue, that such an
analysis cannot be done since there is no demonstrable linkage between
1-hour average concentrations of NO2 and SO2 in
the ambient air and the impact of longer-term deposition-related
acidification associated with oxides of nitrogen and sulfur on
sensitive aquatic ecosystems that the proposed standards were intended
to address. As a result, as in the case of an AAI-based standard as
discussed above, the Administrator concludes that there is no basis to
make a reasoned judgment as to what levels of 1-hour NO2 and
SO2 standards would be requisite to protect public welfare,
such that the EPA cannot demonstrate a reasoned basis for judging that
the proposed standards would be sufficient but not more stringent than
necessary to protect against adverse deposition-related effects to
sensitive ecosystems.
    With regard to considering the views of CASAC, the Administrator
notes that the PA did not discuss the alternative of setting secondary
standards that are identical to the 1-hour NO2 and
SO2 primary standards. As a consequence, this alternative
was not presented for consideration by CASAC and therefore CASAC has
not expressed its views on this alternative set of standards.
    In light of the above considerations, and taking into consideration
public comments, the Administrator has further considered whether it is
necessary or appropriate under Section 109 of the CAA to set such 1-
hour NO2 and SO2 secondary standards, having
concluded that the current NO2 and SO2 secondary
standards are neither adequate nor appropriate to address potentially
adverse deposition-related effects on sensitive ecosystems associated
with oxides of nitrogen and sulfur. In reaching this decision, the
Administrator concludes that it is appropriate to apply the same
reasoning as she did in reaching the decision that it is premature and
not appropriate under Section 109(b) to set a new AAI-based standard at
this time. In considering such 1-hour standards, the Administrator
judges that the uncertainties are likely even greater than with an AAI-
based standard, since as noted above there is no demonstrable linkage
between the elements of such standards and impacts on sensitive
ecosystems that the standards would be intended to address. In
addition, with respect to areas that are not acid sensitive, and unlike
an AAI standard, it is likely that the proposed 1-hour standards
directionally would provide more protection than is warranted.
Therefore, the Administrator now concludes that it is neither necessary
nor appropriate to set 1-hour NO2 and SO2
secondary standards, since in her judgment setting such standards
cannot reasonably be judged to provide requisite protection of public
welfare.
    In summary, for the reasons discussed above, and taking into
account information and assessments presented in the ISA, REA, and PA,
the advice and recommendations of CASAC, and the public comments on the
proposal, the Administrator has decided that it is not appropriate
under Section 109(b) to set any new secondary standards at this time to
address potentially adverse deposition-related effects associated with
oxides of nitrogen and sulfur. Further, as discussed above in section
II.D, she has also decided to retain the current NO2 and
SO2 secondary standards to address direct effects of

gaseous NO2 and SO2 on vegetation. Thus, taken
together, the Administrator has decided to retain and not revise the
current NO2 and SO2 secondary standards.
Specifically these secondary standards include an NO2
standard set at a level of 0.053 ppm, annual arithmetic average, and an
SO2 standard set at a level of 0.5 ppm, 3-hour average, not
to be exceeded more than once per year.

IV. Field Pilot Program and Ambient Monitoring

    This section discusses elements of a field pilot program and the
evaluation of monitoring methods for ambient air indicators of
NOy and SOX that could be conducted to implement
the Administrator's decision to undertake such a field monitoring
program in conjunction with her decision not to set a new multi-
pollutant secondary standard in this review, as discussed above in
section III.E. The PA included considerations related to monitoring
methods and network design that could support an AAI-based standard,
which were reviewed by the CASAC Ambient Monitoring Methods
Subcommittee (AMMS) (Russell and Samet, 2011b). As discussed below, the
CASAC AMMS supported the approach of basing a potential future air
monitoring network on the existing Clean Air Status and Trends Network
(CASTNET) program. In addition, the CASAC AMMS supported the use of the
CASTNET filter packs (CFPs) as appropriate methods to measure the
oxides of sulfur indictor, SOX, and the use of commercially
available NOy instruments to measure the oxides of nitrogen
indicator, NOy. CASAC AMMS also supported the inclusion of
complementary measurements in any future field monitoring program that
would support the evaluation of the monitoring methods and air quality
models upon which the AAI developed in the PA was based.
    Section IV.A below outlines the objectives, scope, and key elements
of the field pilot program as presented in the proposal and section
IV.B summarizes the EPA's proposed approach to evaluating monitoring
methods. These approaches reflect consideration of the advice of the
CASAC AMMS. Public comments on the field pilot program and evaluation
of monitor methods are discussed below in section IV.C. These comments
have been helpful in shaping the process that the EPA is now
undertaking to develop the field pilot program and monitoring methods
evaluation.
    The following sections provide insight into the EPA's current ideas
about what could be incorporated into the pilot program, but the EPA
has not made any final decisions on what will be included. These ideas
will be discussed further in a draft white paper to be made available
later this year for public comment. The draft white paper will present
more detailed plans for the field pilot program and monitoring methods
evaluation. The draft white paper is intended to serve as both a draft
work plan and a vehicle for continued input from outside interests.
Taking into consideration comments received on the draft white paper,
the EPA will prepare a final white paper that will serve as a program
management and communication document to facilitate engagement with
interested stakeholders and convey the EPA's final plans.

A. Overview of Proposed Field Pilot Program

    As discussed in the proposal, the primary goal of this field pilot
program, and the related monitoring methods evaluation summarized below
in section IV.B, is to enhance our understanding of the degree of
protectiveness that would likely be afforded by a standard based on the
AAI, as described above in section III.A. This program is intended to
aid the Agency in considering in future reviews an appropriate multi-
pollutant standard that would be requisite to protect public welfare
consistent with Section 109 of the CAA, through the following
objectives:
    (1) Evaluate measurement methods for the ambient air indicators of
NOy and SOX and consider designation of such
methods as Federal Reference Methods (FRMs);
    (2) Examine the variability and improve characterization of
concentration and deposition patterns of NOy and
SOX, as well as reduced forms of nitrogen, within and across
a number of sensitive ecoregions across the country;
    (3) Develop updated ecoregion-specific factors (i.e., F1 through
F4) for the AAI equation based in part on new observed air quality data
within the sample ecoregions as well as on updated nationwide air
quality model results and expanded critical load data bases, and
explore alternative approaches for developing such representative
factors;
    (4) Calculate ecoregion-specific AAI values using observed
NOy and SOX data and updated ecoregion-specific
factors to examine the extent to which the sample ecoregions would meet
a set of alternative AAI-based standards;
    (5) Develop air monitoring network design criteria for an AAI-based
standard;
    (6) Assess the use of total nitrate measurements as a potential
alternative indicator for NOy;
    (7) Support related longer-term research efforts, including
enhancements to and evaluation of modeled dry deposition algorithms;
and
    (8) Facilitate stakeholder engagement in addressing implementation
issues associated with possible future adoption of an AAI-based
standard.
    The EPA proposed to use CASTNET sites (Figure IV-1) in selected
acid-sensitive ecoregions to serve as the platform for this pilot
program, potentially starting in late 2012 and extending through 2018.
The CASTNET sites in three to five acid-sensitive ecoregions would
collect NOy and SOX (i.e., SO2 and p-
SO4) measurements over a 5-year period. The initial step in
developing a data base of observed ambient air indicators for oxides of
nitrogen and sulfur requires the addition of NOy samplers at
the pilot study sites so that a full complement of indicator
measurements are available to calculate AAI values. These CASTNET sites
would also be used to make supplemental observations useful for
evaluation of CMAQ's characterization of factors F2-F4 in the AAI
equation.
    The selected ecoregions would account for geographic variability by
including regions from across the United States, including the east,
upper midwest, and west. Each selected ecoregion would have at least
two existing CASTNET sites.

[GRAPHIC] [TIFF OMITTED] TR03AP12.006

    Over the course of this 5-year pilot program, the most current
national air quality modeling, based on the most current national
emissions inventory, would be used to develop an updated set of F2-F4
factors. A parallel multi-agency national critical load data base
development effort would be used as the basis for calculating updated
F1 factors. As discussed above in section III.A, these factors would be
based on average parameter values across an ecoregion. Using this new
set of F factors, observations of NOy and SOX
derived from the field pilot program, averaged across each ecoregion,
would be used to calculate AAI values in the sample ecoregions. The
data from the field pilot program would also be used to examine
alternative approaches to generating representative air quality values,
such as examining the appropriateness of spatial averaging in areas of
high spatial variability.
    Beyond this basic overview of the field pilot program, the
following sections highlight complementary measurements that may be
performed as part of the program (section IV.A.1), complementary areas
of related research (section IV.A.2), a discussion of implementation
challenges that would be addressed during the course of the field pilot
program (section IV.A.3), and plans for program development and
stakeholder participation (section IV.A.4).
1. Complementary Measurements
    Complementary measurements may be performed at some sites in the
pilot network to reduce uncertainties in the recommended methods for
measuring ambient oxides of nitrogen and sulfur and to better
characterize model performance and application to the AAI. The CASAC
AMMS advised the EPA that such supplemental measurements were of
critical importance in a field measurement program related to an AAI-
based standard (Russell and Samet, 2011b).
    Candidate complementary measurements to address sulfur, in addition
to those provided by CFPs, include trace gas continuous SO2
and speciated PM2.5 measurements. The co-located deployment
of a continuous SO2 analyzer with the CFP for SO2
will provide test data for determining suitability of continuous
SO2 measurements as a Federal Equivalent Method (FEM) for an
AAI-based standard, as well as producing valuable time-series data for
model evaluation purposes. The weekly averaging time provided by the
CFP adequately addresses the annual-average basis of an AAI-based
secondary standard, but would not be applicable to short-term (i.e., 1-
hour) averages associated with the primary SO2 standard.
Conversely, because of the relatively low SO2 concentrations
associated with many acid-sensitive ecoregions, existing SO2
FRMs designated for use in determining compliance with the primary
standard, which typically are used in higher concentration
environments, would not necessarily be appropriate for use in
conjunction with an AAI-based secondary standard.
    Co-locating the PM2.5 sampler used in the EPA Chemical
Speciation Network and the Interagency Monitoring of Protected Visual
Environments (IMPROVE) network at pilot network sites would allow for
characterizing the relationship between the CFP-derived p-
SO4 and the speciation samplers used throughout the state
and local air quality networks. The EPA notes that CASTNET already has
several co-

located IMPROVE chemical speciation samplers. Because the AAI equation
is based in part on the concentration of p-SO4, the original
motivation for capturing all particle size fractions is not as
important relative to simply capturing the concentration of total p-
SO4.
    Candidate measurements to complement oxidized nitrogen
measurements, in addition to the CFP, include a mix of continuous and
periodic sampling for the dominant NOy species, namely NO,
directly measured NO2, PAN, HNO3, and particulate
nitrate, p-NO3. The CASAC AMMS (Russell and Samet, 2011b)
recommended that the EPA consider the use of total nitrate (t-
NO3) obtained from CASTNET sampling as an indicator for
NOy, reasoning that t-NO3 is typically a
significant fraction of deposited oxidized nitrogen in rural
environments and CASTNET measurements are widely available. Collection
of these data would support further consideration of using the CFP for
t-NO3 as the indicator of oxides of nitrogen for use in an
AAI-based secondary standard.
    The CASAC AMMS also recommended that total NHX
(NH3 and particulate ammonium (p-NH4)) be
considered as a proxy for reduced nitrogen species, reasoning that the
subsequent partitioning to NH3 and p-NH4 may be
estimated using equilibrium chemistry calculations. Reduced nitrogen
measurements are used to evaluate air quality modeling that is used in
generating factor F2. Additional studies are needed to determine the
applicability of NHX measurements and calculated values of
NH3 and ammonium (NH4) to the AAI.
    The additional supplemental measurements of speciated
NOy, continuous SO2 and NHX will be
used in future air quality modeling evaluation efforts. Because there
often is significant lag in the availability of contemporary emissions
data to drive air quality modeling, the complete use of these data sets
will extend beyond the 5-year collection period of the pilot program.
Consequently, the immediate application of those data will address
instrument performance comparisons that explore the feasibility of
using continuous SO2 instruments in rural environments, and
using the speciated NOy data to assess NOy
instrument performance. Although contemporary air quality modeling will
lag behind measurement data availability, the observations can be used
in deposition models to compare observed transference ratios with the
previously calculated transference ratios to test temporal stability of
the ratios.
    An extended water quality sampling effort that would parallel the
air quality measurement program would help to address some of the
uncertainties related to factor F1 and the representativeness of the
nth percentile critical load, as discussed in section III.B.5.b.i of
the proposal. The objective of the water quality sampling would be to
develop a larger data base of critical loads in each of the pilot
ecoregions such that the n\th\ percentile can adequately be
characterized in terms of representing all water bodies. Opportunities
to leverage and perhaps enhance existing ecosystem modeling efforts
enabling more advanced critical load modeling and improved methods to
estimate base cation production could be pursued. For example, areas
with ongoing research studies producing data for dynamic critical load
modeling could be considered when selecting the pilot ecoregions.
2. Complementary Areas of Research
    The EPA recognizes that a source of uncertainty in an AAI-based
secondary standard that would not be directly addressed in the pilot
program stems from the uncertainty in the model used to link
atmospheric concentrations to dry deposition fluxes. Currently, there
are no ongoing direct dry deposition measurement studies at CASTNET
sites that can be used to evaluate modeled results. It was strongly
recommended by CASAC AMMS that a comprehensive sampling-intensive study
be conducted in at least one, preferably two sites in different
ecoregions to assess characterization of dry deposition of sulfur and
nitrogen. These sites would be the same as those for the complementary
measurements described above, but they would afford an opportunity to
also complement dry deposition process research that benefits from the
ambient air measurements collected in the pilot program. The concerns
regarding uncertainties underlying an AAI-based secondary standard
suggest that research that includes dry deposition measurements and
evaluation of dry deposition models would be a high priority.
    Similar leveraging could be pursued with respect to ecosystem
research activities. For example, studies that capture a suite of soil,
vegetation, hydrological, and water quality properties that can help
evaluate more advanced critical load models would complement the
atmospheric-based pilot program. In concept, such studies could provide
the infrastructure for true multi-pollutant, multimedia ``super'' sites
assuming the planning, coordination, and resource facets can be
aligned. While this discussion emphasizes the opportunity of leveraging
ongoing research efforts, consideration could be given to explicitly
including related research components directly in the pilot program.
3. Implementation Challenges
    The CAA requires that once a NAAQS is established, designation and
implementation must move forward. With a standard as innovative as the
AAI-based standard considered in this review, the Administrator
believes that should such a standard be adopted in the future, its
success would be greatly improved if, while additional data are being
collected to reduce the uncertainties discussed above, the implementing
agencies and other stakeholders have an opportunity to discuss and
thoroughly understand how such a standard would work. And since, as
noted above, emissions reductions that are directionally correct to
reduce aquatic acidification will be occurring as a result of other CAA
programs, the Administrator believes that this period of further
discussion will enable agencies to implement a multi-pollutant standard
to address aquatic acidification if one is adopted in a future review.
    Consideration of an AAI-based secondary standard for oxides of
nitrogen and sulfur would present significant implementation challenges
because it involves multiple, regionally-dispersed pollutants and
relatively complex compliance determinations based on regionally
variable levels of NOy and SOX concentrations
that would be necessary to achieve a national ANC target. The
anticipated implementation challenges fall into three main categories:
monitoring and compliance determinations for area designations, pre-
construction permit application analyses of individual source impacts,
and State Implementation Plan (SIP) development. Several overarching
implementation questions that we anticipate will be addressed in
parallel with the field pilot program's five-year data collection
period include:
    (1) What are the appropriate monitoring network density and siting
requirements to support a compliance system based on ecoregions?
    (2) Given the unique spatial nature of the secondary standard
(e.g., ecoregions), what are the appropriate parameters for
establishing nonattainment areas?
    (3) How can new or modified major sources of oxides of nitrogen and
oxides of sulfur emissions assess their ambient

impacts on the standard and demonstrate that they are not causing or
contributing to a violation of the NAAQS for preconstruction
permitting? To what extent does the fact that a single source may be
impacting multiple areas, with different acid sensitivities and
variable levels of NOy and SOX concentrations
that would be necessary to achieve a national ANC target, complicate
this assessment and how can these additional complexities best be
addressed?
    (4) What additional tools, information, and planning structures are
needed to assist states with SIP development, including the assessment
of interstate pollutant transport and deposition?
    (5) Would transportation conformity apply in nonattainment and
maintenance areas for this secondary standard, and, if it does, would
satisfying requirements that apply for related primary standards (e.g.,
ozone, PM2.5, and NO2) be demonstrated to satisfy
requirements for this secondary standard?
4. Monitoring Plan Development and Stakeholder Participation
    The existing CASTNET sampling site infrastructure provides an
effective means of quickly and efficiently deploying a monitoring
program to support potential implementation of an AAI-based secondary
standard, and also provides an additional opportunity for federally
managed networks to collaborate and support the states, local agencies
and tribes (SLT) in determining compliance with a secondary standard. A
collaborative effort would help to optimize limited federal and SLT
monitoring funds and would be beneficial to all involved. The CASTNET
is already a stakeholder-based program with over 20 participants and
contributors, including federal, state and tribal partners.
    The CASAC AMMS generally endorsed the technical approaches used in
CASTNET, but concerns were raised by individual representatives of
state agencies concerning the perception of the EPA-controlled
management aspects of CASTNET and data ownership. Potential approaches
to resolve these issues will be developed and evaluated in existing
National Association of Clean Air Agencies (NACAA)/EPA ambient air
monitoring and National Atmospheric Deposition Program (NADP) science
committees. The EPA Office of Air and Radiation (which includes the
Office of Air Quality Planning Standards, OAQPS; and the Office of
Atmospheric Program's Clean Air Markets Division, OAP-CAMD), and their
partners on the NACAA monitor steering committee will work to develop a
prioritized plan that identifies three to five ecoregions and specific
instrumentation to be deployed. Although this pilot program is focused
on data collection, the plan will also include data analysis approaches
as well as a process to facilitate engagement by those within the EPA
and the SLTs to foster progress on the implementation questions noted
above.

B. Summary of Proposed Evaluation of Monitoring Methods

    This section provides a brief overview of the EPA's plans for
evaluating monitoring methods of NOy and SOX, as
discussed in section IV.B of the proposal. The EPA generally relies on
monitoring methods that have been designated as FRMs or FEMs for the
purpose of determining the attainment status of areas with regard to
existing NAAQS. Such FRMs or FEMs are generally required to measure the
air quality indicators that are compared to the level of a standard to
assess compliance with a NAAQS. Prior to their designation by the EPA
as FRM/FEMs through a rulemaking process, these methods must be
determined to be applicable for routine field use and need to have been
experimentally validated by meeting or exceeding specific accuracy,
reproducibility, and reliability criteria established by the EPA for
this purpose. As discussed above in section III.A, the ambient air
indicators being considered for use in an AAI-based standard include
SO2, p-SO4, and NOy.
    The CASTNET provides a well-established infrastructure that would
meet the basic location and measurement requirements of an AAI-based
secondary standard given the rural placement of sites in acid sensitive
areas. In addition, CFPs currently provide very economical weekly,
integrated average concentration measurements of SO2, p-
SO4, NH4 and t-NO3, the sum of
HNO3 and p-NO3.
    While routinely operated instruments that measure SO2,
p-SO4, NOy and/or t-NO3 exist,
instruments that measure p-SO4, NOy, t-
NO3, or the CFP for SO2 have not been designated
by the EPA as FRMs or FEMs. The EPA's Office of Research and
Development has initiated work that will support future FRM
designations by the EPA for SO2 and p-SO4
measurements based on the CFP. Such a designation by the EPA could be
done for the purpose of facilitating consistent research related to an
AAI-based standard and/or in conjunction with setting and supporting an
AAI-based secondary standard.
    Based on extensive review of literature and available data, the EPA
has identified potential methods that appear suitable for measuring
each of the three components of the indicators. As discussed more fully
in section IV.B of the proposal, these methods are being considered as
new FRMs to be used for measuring the ambient concentrations of the
three components (SO2, p-SO4 and NOy)
that would be needed to determine compliance with an AAI-based
secondary standard.
    For the SO2 and p-SO4 measurements, the EPA
is considering the CFP method, which provides weekly average
concentration measurements for SO2 and p-SO4.
This method has been used in the EPA's CASTNET monitoring network for
15 years, and experience with this method strongly indicates that it
will meet the requirements for use as an FRM for the SO2 and
p-SO4 concentrations for an AAI-based secondary standard.
    Although the CFP method would provide measurements of both the
SO2 and p-SO4 components in a unified sampling
and analysis procedure, individual FRMs will be considered for each.
The EPA recognizes that an existing FRM to measure SO2
concentrations using ultra-violet fluorescence (UVF) exists (40 CFR
part 50, appendix A-1) for the purpose of monitoring compliance for the
primary SO2 NAAQS. However, several factors suggest that the
CFP method would be superior to the UVF FRM for monitoring compliance
with an AAI-based secondary standard.
    For monitoring the NOy component, a continuous analyzer
for measuring NOy is commercially available and is
considered by the EPA to be likely suitable for use as an FRM. This
method is similar in design to the existing NO2 FRM
(described in 40 CFR part 50, appendix F), which is based on the ozone
chemiluminescence measurement technique. The method is adapted to and
further optimized to measure all NOy. However, this
NOy method requires further evaluation before it can be
fully confirmed as a suitable FRM. The EPA is currently completing a
full scientific assessment of the NOy method to determine
whether it would be appropriate to consider for designation by the EPA
as an FRM.
    On February 16, 2011, the EPA presented this set of potential FRMs
to the CASAC AMMS for their consideration and comment. In response, the
CASAC AMMS stated that, overall, it believes that the EPA's planned
evaluation of methods for measuring NOy, SO2 and
p-SO4 as ambient air indicators is a suitable

approach in concept. On supporting the CFP method as a potential FRM
for SO2, CASAC stated that they felt that the CFP is
adequate for measuring long-term average SO2 gas
concentrations in rural areas with low levels (less than 5 parts per
billion by volume (ppbv)) and is therefore suitable for consideration
as an FRM. For p-SO4, CASAC AMMS generally supports the use
of the CFP as a potential FRM for measuring p-SO4 for an
AAI-based secondary standard. The method has been relatively well-
characterized and evaluated, and it has a documented, long-term track
record of successful use in a field network designed to assess spatial
patterns and long-term trends. On supporting the photometric
NOy method as a potential FRM, CASAC AMMS concluded that the
existing NOy method is generally an appropriate approach for
the indicator of an AAI-based standard. However, CASAC AMMS agreed that
additional characterization and research is needed to fully understand
the method in order to designate it as an FRM.

C. Comments on Field Pilot Program and Monitoring Methods Evaluation

    Public comments on the EPA's proposed plans for a field pilot
program and related evaluation of monitoring methods generally fell
into the following four topic areas: (1) Goals, objectives, and scope;
(2) monitoring network and site selection; (3) complementary
measurements and instrumentation; and (4) collaboration and stakeholder
participation. An overview of these comments and the EPA's responses
are discussed below. In addition, many commenters generally requested
that the EPA provide clarification of its plans regarding the field
pilot program.
1. Goals, Objectives, and Scope of Field Pilot Program
    There was a mix of comments regarding the need for and the overall
purpose and scope of the field pilot program. In general commenters
that supported the AAI approach (e.g., DOI/National Park Service (NPS),
Nature Conservancy, Adirondack Council, NESCAUM, NY, PA, NC) also
supported the concept of deploying a field pilot program as well as the
proposed goals and objectives, while offering specific comments on the
scope of the proposed monitoring effort. Other commenters supporting
the AAI approach, including Earthjustice and the Center for Biological
Diversity, expressed the view that a field pilot program was not needed
to support adoption of such a standard in this review. A variety of
commenters expressed the view that a field pilot program in 3 to 5
ecoregions was too limited to adequately capture differences in
concentrations and deposition patterns across the nation.
    Commenters that did not support the adoption or future development
of an AAI-based secondary NAAQS (e.g., EPRI, UARG, AAM, NCBA, Aluminum
Association, and TX) expressed the view that a field pilot program was
therefore not needed. However, these commenters nonetheless expressed
the view that if the EPA intended to consider such a standard in future
reviews, the field pilot program would need to expand in coverage and
incorporate a much more comprehensive research program to address data
gaps and uncertainties inherent in such an approach. These commenters
suggested that the field pilot program should be more responsive to the
issues raised by the members of the CASAC review panel. One commenter
(API) expressed the view that even if the EPA intended to consider such
a standard in the future, a field study was not appropriate at this
time on the basis that the AAI-based approach was still only very
preliminary in nature.
    These commenters not supporting the AAI and the field pilot program
as proposed contended that the proposed program fails to address key
scientific uncertainties and data needs with regard to a methodology
based on the AAI, and cannot meaningfully reduce the uncertainties that
would be associated with such a standard. Some of these commenters
offered specific recommendations for areas of research, noted below,
that in their view would be necessary to support any further
consideration of such a standard. For example, these commenters
contended that it was necessary to conduct research in the following
areas before further consideration of an AAI-based standard: (1) The
effect of other sources, including wastewater pollution from permitted
or unpermitted sources and fertilization of farm lands, on aquatic
acidification; (2) relationships between measured air quality and
deposition rates and related model performance evaluations; (3)
improved methods for measuring dry deposition; and (4) characterization
of NHX concentrations that are representative of specific
ecoregions for all ecoregions based on a model performance evaluation.
    Additional views were expressed by various commenters in regard to
implementation, site selection and data availability. Many commenters
from State agencies and industry agreed with the EPA that
implementation challenges should be addressed during the course of the
field pilot program. For example, commenters expressed the view that
guidance should emerge for monitoring network design accounting for the
influence of variability of air concentration and deposition patterns
within specific ecoregions. Some commenters also noted that much of the
underlying information for the AAI was based on the Adirondacks and
Shenandoah regions which are relatively rich data sources and the field
pilot program should consider under-sampled areas in other parts of
country such as the mountainous West. Also, some commenters requested
that relatively non-acid sensitive areas be included in the field pilot
program in the interest of broader national applicability or, as one
state agency suggested, the availability of a rich data base in the
Chesapeake Bay region. Some commenters also expressed the view that
results from the field pilot program would not be available for the
next periodic review of the secondary standards for oxides of nitrogen
and sulfur.
    Having considered these comments contending that the scope of the
field pilot program is too limited spatially and not sufficiently
comprehensive, the EPA maintains that the purpose and scope of the
pilot studies program as presented in the proposal remain appropriate.
As summarized above in section IV.A, the primary goal of the field
pilot program is to collect and analyze data so as to enhance the
Agency's understanding of the degree of protectiveness that would
likely be afforded by an AAI-based standard. The EPA also intends that
data generated by this program would support development of an
appropriate monitoring network for such a standard. This field pilot
program is not intended to be a research program, but rather to be a
more targeted data collection and analysis effort, which will be done
in conjunction with ongoing research efforts that are better suited to
address some of the issues raised by commenters on the breadth of the
field pilot program.
    The EPA largely agrees that the scope of the field pilot program is
not adequate to address many of the issues raised by the commenters
regarding either the ability to adequately capture air quality and
deposition patterns in all ecoregions or fully addressing scientific
uncertainties related to numerous investigations into measurement
development methods and biogeochemical and atmospheric deposition
processes. However, as noted earlier, a field pilot program by
definition is limited in scope and

intended to guide future broader applications. Toward that end, the
field pilot program is intended to provide an intermediate link between
initial conceptual design and potential future development and adoption
of a standard, where the breadth and depth of spatial coverage would
explicitly be addressed through monitoring network rules and
implementation guidance.
    The relevant ongoing programs addressing underlying atmospheric
deposition uncertainties and development of critical load models
include the EPA's atmospheric deposition research program and the
multi-agency National Critical Load Data Base (NCLDB) program,
respectively. In addition, the NAAQS review process of iterative
science review and assessment provides a framework for evaluating newly
available information that may address current data gaps and scientific
uncertainties. These research programs are appropriate venues for
addressing comments, including relevant CASAC recommendations,
regarding desired improvements in the science underlying an AAI-based
standard. In light of these ongoing research programs, it is not
appropriate to duplicate these efforts through an expanded scope of the
field pilot program. Rather, the most efficient approach is to increase
the coordination between the field pilot program and these existing
efforts. For example, the EPA plans to explore co-locating planned dry
deposition studies at field pilot program sites that would result in
mutually beneficial data enhancements that support both pilot program
and research program objectives.
    With regard to views regarding the importance of water quality
monitoring, the EPA agrees with comments recommending increased
coordination with water quality sampling and critical load modeling
programs. In addition to working closely with the NCLDB, the EPA plans
to factor in availability of water quality monitoring data in selecting
field pilot program sites. The field pilot program has the potential to
spur increased water quality monitoring in under-sampled areas which
would improve confidence in generating ecoregion representative
critical loads, as well as enhancing longer-term assessment of
progress.
    In addressing the last group of comments concerning implementation,
site selection and data availability, the EPA offers the following
views. The field pilot program does provide an opportunity to assist in
answering a number of implementation challenges, including the design
of a future network that could support an AAI-based secondary standard.
Toward that end, the EPA plans to work closely with its state and local
agency partners in utilizing the field pilot program as a test case for
implementation-based issues. In optimizing the design of a field pilot
program, emphasis will be placed on relatively acid-sensitive areas
given that those are areas an AAI-based standard would be intended to
protect. Nevertheless, the EPA will consider ecoregions that may offer
advantages in having multiple deposition-based effects beyond aquatic
acidification that potentially could support future reviews that
consider multiple ecological effects. In addition, nearly all
ecoregions have a mix of acid-sensitive and non-acid sensitive water
bodies which will allow for assessing some of the AAI applicability to
different aquatic systems. The EPA also notes that the field pilot
program will provide data and analyses that will help inform
consideration of an AAI-based standard in the next review. For example,
data and analyses generated as part of the field pilot program will be
incorporated into the EPA's characterization of environmental factors
and evaluations of alternative approaches to specifying the terms of an
AAI that would be included in the exposure/risk assessment and policy
assessment prepared as part of the next review.
2. Network Design and Role of CASTNET
    Most commenters expressed the view that CASTNET was an appropriate
program to support the field pilot program and a potential AAI-based
standard. While government agencies generally supported the use of
CASTNET, some State organizations suggested that the NCore monitoring
network may be more efficient given that the costs of adding CASTNET
filter packs (CFPs) to NCore locations is less than that of adding NOy
instruments, which exist at NCore locations, to CASTNET locations.
Support also was expressed by New York State and NESCAUM for the use of
rural NCore monitoring stations, where appropriate, in combination with
CASTNET sites. Some states requested that access to the sampling
methods and laboratory analyses used in the program and all data
results be made through a national contract for States and local
agencies, a concern related to CASTNET operations being managed by the
EPA. Environmental groups also supported the use of CASTNET and
encouraged the EPA to adopt the multiple stakeholder process of the
NCLDB program and to align CASTNET sites with the Temporally Integrated
Monitoring of Ecosystems and Long-Term Monitoring (TIME/LTM) water
sampling programs. These water sampling programs should also be
extended to other under-sampled areas of the country that are acid
sensitive. Some industry commenters raised concerns regarding the CFPs
as they have measurement artifacts associated with both mass loss and
gain.
    Some state agencies commented that states should not be required to
fund or implement the pilot monitoring studies, and funding should
arise from sources other than State and Territorial Air Grant (STAG)
funds. Relatedly, the NPS and environmental groups encouraged the EPA
to make this effort a priority for funding.
    The EPA has considered all available monitoring networks in the
interest of locating the most suitable sites for a pilot study and to
effectively leverage resources. The CASTNET monitoring program offers
substantially more available platforms in acid-sensitive ecoregions
relative to rural NCore sites and CASTNET sites already include the CFP
method for measurements of key atmospheric species. Consequently, the
financial burden on states, tribes and local air monitoring agencies
would be less using this existing infrastructure instead of expanding
measurements at or relocating rural NCore sites. The CASTNET siting
design originally was intended to discern contributions of acidifying
deposition of NOX and SOX to sensitive
ecosystems, which is especially relevant for the AAI applications.
NCore was designed as a more generalized network to collect
measurements in a variety of geographical areas, with no specific focus
on acid-sensitive ecosystems. Moreover, CASTNET has established a track
record over the last two decades of providing quality measurements,
whereas NCore is a relatively new network that has been fully deployed
for less than two years and therefore not been subjected to review and
analysis commensurate with the CASTNET program. Nevertheless, as some
states suggested, this pilot program should afford an opportunity to
explore the use of existing rural NCore sites in acid-sensitive
ecoregions. The EPA welcomes the inclusion of rural NCore sites into
the pilot study in cases where there are clear advantages of using such
sites, and especially where such sites provide additional information
likely resulting in more conclusive data findings. The development of
site selection criteria and site selection will be conducted in
partnership with other federal, state and local agencies. Although
CASTNET is managed by the EPA, the agency has

aggressively supported the user community management approach adopted
in the NADP and views the field pilot program as an opportunity to
expand ownership of CASTNET analysis and data products, which currently
can be accessed by the public.
    While the field pilot program resources are focused on atmospheric
measurements, as noted above the EPA will try to leverage existing
water quality monitoring programs such as TIME/LTM in selecting field
pilot program site locations. The EPA would rely heavily on the NCLDB
critical load work for generating AAI values at monitoring locations as
part of the field pilot program. In regard to issues raised by
commenters regarding artifacts in the CFP, which would be the basis for
SOX data in the field pilot program, the EPA notes that
these methods have been extensively deployed and evaluated and have
exhibited generally excellent performance. As part of the CASAC review
on measurement methods, CASAC pointed out that the CFPs are preferred
methods for measuring SOX in rural, low concentration
environments due to the sensitivity of the CFP method.
3. Complementary Measurements and Instrumentation