year average (e.g., UARG, p. 76). These commenters further point to the
WREA modeling, noting that those estimates project that attainment of
the current standard would result in only 5 sites above 15 ppm-hrs.
Based on these statements, these commenters state that the current
standard is likely to provide conditions with no site having a monitor
over 17 ppm-hrs and a ``minimal number'' likely exceeding 13 ppm-hrs
(e.g., UARG, p. 77). We disagree with commenters' interpretation of the
modeling information from the two different assessments. As we
summarized in section IV.C.1 of the proposal with regard to the WREA
modeling, the modeling estimates are each based on a single set of
precursor emissions reductions that are estimated to achieve the
desired target conditions, which is also the case for the RIA
modeling\194\ (U.S. EPA, 2014c, pp. 5-40 to 5-41; see also section
1.2.2 of the 2014 RIA).
---------------------------------------------------------------------------

    \194\ Although commenters cite to both analyses as if providing
the same information, there are many differences in specific aspects
of the RIA approach from that of the WREA, which derive, at least in
part, from their very different purposes. The RIA is not developed
for consideration in the NAAQS review. Rather, it is intended to
provide insights and analysis of an illustrative control strategy
that states might adopt to meet the revised standard. The EPA does
not consider this analysis informative to consideration of the
protection provided by the current standard, and the results of the
RIA have not been considered in the EPA's decisions on the
O3 standards.
---------------------------------------------------------------------------

    As noted in section IV.A.2 above, and in the proposal, the model-
adjusted air quality in the WREA scenario for the current standard does
not represent an optimized control scenario that just meets the current
standard, but rather characterizes one potential distribution of air
quality across a region when all monitor locations meet the standard
(79 FR 75322; U.S. EPA, 2014b, section <IP_ADDRESS>). Alternate precursor
emissions reductions would be expected to produce different patterns of
O3 concentrations and associated differences in W126 index
values. Specifically, the precursor emissions reductions scenarios
examined in the WREA focuses on regional reductions over broad areas
rather than localized cuts that may focus more narrowly on areas
violating the current standard (U.S. EPA, 2014b, p. 4-35). The
assumption of regionally determined across-the-board emissions
reductions is a source of potential uncertainty with the potential to
overestimate W126 scenario benefits (U.S. EPA, 2014b, Table 4-5 [row
G]). The application of emissions reductions to all locations in each
region to bring down the highest monitor in the region to meet the
current standard could potentially lead to W126 index underestimates at
some locations, as noted in the WREA: ``[w]hile the scenarios
implemented in this analysis show that [] bringing down the highest
monitor in a region would lead to reductions below the targeted level
through the rest of the region, to the extent that the regional
reductions from on-the-books controls are supplemented with more local
controls the additional benefit may be overestimated'' (U.S. EPA,
2014b, p. 4-36; U.S. EPA, 2014c, pp. 5-40 to 5-41). This point was
emphasized by CASAC in their comments on the 2nd draft WREA. CASAC
noted that, ``[m]eeting a target level at the highest monitor requires
substantial reductions below the targeted level through the rest of the
region'' and stated that ``[t]his artificial simulation does not
represent an actual control strategy and may conflate differences in
control strategies required to meet different standards'' (Frey, 2014b,
p. 2).
    Due to the uncertainty about what actual future emissions control
strategies might be and their associated emissions reductions, and the
impact such uncertainty might have on modeling estimates involving
reductions from recent conditions, we believe it is important to place
weight on ambient air monitoring data for recent conditions in drawing
conclusions regarding W126 index values that would be expected in areas
that meet the current standard. The analysis of air quality data for
Class I areas described in the proposal, and updated in Table 3 above
(1998-2013), indicates the occurrence of 3-year W126 exposure index
values well above 19 ppm-hrs, a cumulative exposure value for which
CASAC termed the associated median RBL estimate ``unacceptably high,''
in multiple Class I areas that meet the current standard (79 FR 75312,
December 17, 2014, Table 7; updated in Table 3 above). Additionally,
analysis of recent air quality data (2011-2013) for all locations
across the U.S. indicates 10 monitor locations distributed across two
NOAA climatic regions that meet the current standard and at which 3-
year W126 index values are above 19 ppm-hrs, with the highest values
extending up to 23 ppm-hrs (Wells, 2015b).
    In support of their view that the EPA's modeling supports the
conclusion that W126 index values of interest are achieved under the
current secondary standard, some commenters additionally state that the
W126 values in the WREA are overestimated in unmonitored rural areas
due to the much greater prevalence of urban monitors across the U.S.
The EPA

disagrees with this conclusion. In order to estimate O3
concentrations in grid cells across a national-scale spatial surface,
the WREA applied the VNA spatial interpolation technique after applying
the HDDM technique to adjust O3 concentrations at monitoring
sites based on the emissions reductions necessary to just meet the
current standard. In estimating concentrations in unmonitored areas,
the VNA method considers only the ``neighboring'' monitors, using an
inverse distance squared weighting formula, which assigns the greatest
influence to the nearest neighboring monitor (U.S. EPA, 2014b, p. 4A-
6). By this approach, monitors in less-densely monitored areas
contribute to the concentration estimates over much larger areas than
do monitors in more-densely monitored areas. In an urban area,
neighboring monitors may be quite close to one another, such that any
one monitor may only be influencing concentration estimates for a
handful of spatial grid cells in the immediate vicinity. By contrast,
monitors in rural areas may influence hundreds of grid cells. A
specific example of this is the monitor in Great Basin National Park in
eastern Nevada. The VNA algorithm assigns very high weights to this
monitor for all of the grid cells covering a 100 km radius around it,
simply because there are no other monitors in that area and it is the
closest. On the other hand, a monitor near downtown Las Vegas may only
get a high weight for, and thus exert influence on the concentration
estimate in, the one grid cell containing it. We agree with the
commenter that urban monitors may influence the spatial surface for
some distance away from the urban areas, although the influence wanes
with increasing distance from that area and decreasing distance to the
next closest monitor. As we lack data for the intervening locations,
however, we have no reason to conclude that the VNA surface is
overestimating the W126 index values. Further, as was summarized in
section IV.A.2 above, and in the WREA, the PA and the proposal (U.S.
EPA, 2014b, Table 6-27, section 8.5; U.S. EPA, 2014c, p. 5-49; 79 FR
75323, December 17, 2014), the VNA approach results in a lowering of
the highest W126 index values at monitoring sites, which contributes to
underestimates of the highest W126 index values in each region.
    In support of their view that the current standard is adequate,
some industry commenters additionally cite WREA analyses for the
current standard scenario, including the W126 index estimates in
national parks, as showing that the current standard provides more than
adequate protection, with alternative scenarios providing only marginal
and increasingly uncertain benefits. As we noted in the proposal and
section IV.A.2 above, there are an array of uncertainties associated
with the W126 index estimates, in the current standard scenario and in
the other scenarios, which, as they are inputs to the vegetation risk
analyses, are propagated into those analyses (79 FR 75323; December 17,
2014). As a result, consistent with the approach in the proposal, the
Administrator has not based her decision with regard to adequacy of the
current standard in this review on these air quality scenario analyses.
    In support of their view that the current standard provides
adequate protection and should not be revised, some commenters
described their concerns with any consideration of visible foliar
injury in the decision regarding the secondary standard. These
commenters variously stated that visible foliar injury cannot be
reliably evaluated for adversity given lack of available information,
is not an adverse effect on public welfare that must be addressed
through a secondary standard, and is not directly relatable to growth
suppression (and the EPA's use of RBL captures that effect anyway).
Additionally, some state that any associated ecosystem services effects
are not quantifiable. In sum, the view of these commenters is that it
is not appropriate for the Administrator to place any weight on this
O3 effect in determining the adequacy of the current
standard. As an initial matter, the EPA agrees with the comment that
the current evidence does not include an approach for relating visible
foliar injury to growth suppression,\195\ as recognized in section
IV.A.1.b above. Further, we note that, similar to decisions in past
O3 reviews, the Administrator's proposed decision in this
review recognized the ``complexities and limitations in the evidence
base regarding characterizing air quality conditions with respect to
the magnitude and extent of risk for visible foliar injury'' and the
``challenges of associated judgments with regard to adversity of such
effects to public welfare'' (79 FR 75336; December 17, 2014). Contrary
to the implications of the commenters, although the Administrator took
into consideration the potential for adverse effects on public welfare
from visible foliar injury, she placed weight primarily on growth-
related effects of O3, both in her proposed decision on
adequacy and with regard to proposed judgments on what revisions would
be appropriate. Although visible foliar injury may impact the public
welfare and accordingly has the potential to be adverse to the public
welfare (as noted in section IV.B.2 of the proposal), the Administrator
placed less weight on visible foliar injury considerations in
identifying what revisions to the standard would be appropriate to
propose. In considering these effects for this purpose, she recognized
``significant challenges'' in light of ``the variability and the lack
of clear quantitative relationship with other effects on vegetation, as
well as the lack of established criteria or objectives that might
inform consideration of potential public welfare impacts related to
this vegetation effect'' (79 FR 75349; December 17, 2014). As
summarized in section IV.A.1.a above, the evidence demonstrates a
causal relationship of O3 with visible foliar injury.
Accordingly, we note that the uncertainty associated with visible
foliar injury is not with regard to whether O3 causes
visible foliar injury. Rather, the uncertainty is, as discussed in
sections IV.A.1.b and IV.A.3 above, with the lack of established,
quantitative exposure-response functions that document visible foliar
injury severity and incidence under varying air quality and
environmental conditions and information to support associated
judgments on the significance of such responses with regard to
associated public welfare impacts. As with the Administrator's proposed
decisions on the standard, such considerations also informed her final
decisions, described in sections IV.B.3 and IV.C.3 below.
---------------------------------------------------------------------------

    \195\ The current evidence indicates that``[t]he significance of
O3 injury at the leaf and whole plant levels depends on
how much of the total leaf area of the plant has been affected, as
well as the plant's age, size, developmental stage, and degree of
functional redundancy among the existing leaf area'' and ``in some
cases, visible foliar symptoms have been correlated with decreased
vegetative growth . . . and with impaired reproductive function''
(U.S. EPA, 2013, p. 9-39). The ISA concludes, however, ``it is not
presently possible to determine, with consistency across species and
environments, what degree of injury at the leaf level has
significance to the vigor of the whole plant'' (U.S. EPA, 2013, p.
9-39).
---------------------------------------------------------------------------

    In support of their view that the current standard should be
retained, some commenters note the WREA finding for the current
standard scenario of no U.S. counties with RYL estimates at or above
5%, the RYL value emphasized by CASAC and state that policy reasons
provide support for not focusing on crops in the decision; other
commenters state that additional studies on crops and air quality are
needed. As

described previously in this section, and in section IV.A.2 above, an
aspect of uncertainties associated with the WREA air quality scenarios,
including the current standard scenario, is underestimation of the
highest W126 index values, contributing to underestimates in the
effects associated with the current standard scenario. The EPA agrees
with commenters that additional studies on crops and air quality will
be useful to future reviews. Additionally, however, as noted above, the
Administrator's proposed conclusion on adequacy of the current
standard, as well as her final decision described in section IV.B.3
below, gives less weight to consideration of effects on agricultural
crops in recognition of the complicating role of heavy management in
that area.
    Lastly, we note that many commenters cited the costs of compliance
as supporting their view that the standard should not be revised,
although as we have described in section I.B above, the EPA may not
consider the costs of compliance in determining what standard is
requisite to protect public welfare from known or anticipated adverse
effects.
3. Administrator's Conclusions on the Need for Revision
    Having carefully considered the advice from CASAC and public
comments, as discussed above, the Administrator believes that the
fundamental scientific conclusions on the welfare effects of
O3 in ambient air reached in the ISA and summarized in the
PA and in section IV.B of the proposal remain valid. Additionally, the
Administrator believes the judgments she reached in the proposal
(section IV.D.3) with regard to consideration of the evidence and
quantitative assessments and advice from CASAC remain appropriate.
Thus, as described below, the Administrator concludes that the current
secondary standard is not requisite to protect public welfare from
known and anticipated adverse effects associated with the presence of
O3 in the ambient air and that revision is needed to provide
additional protection.
    In considering the adequacy of the current secondary O3
standard, the Administrator has carefully considered the available
evidence, analyses and conclusions contained in the ISA, including
information newly available in this review; the information,
quantitative assessments, considerations and conclusions presented in
the PA; the advice and recommendations from CASAC; and public comments.
The Administrator gives primary consideration to the evidence of growth
effects in well-studied tree species and information, presented in the
PA and represented with a narrower focus in section IV.B.2 above, on
cumulative exposures occurring in Class I areas when the current
standard is met. This information indicates the occurrence of exposures
associated with Class I areas during periods when the current standard
is met for which associated estimates of growth effects, in terms of
the tree seedling RBL in the median species for which E-R functions
have been established, extend above a magnitude considered to be
``unacceptably high'' by CASAC. This analysis estimated such cumulative
exposures occurring under the current standard for nearly a dozen
areas, distributed across two NOAA climatic regions of the U.S. The
Administrator gives particular weight to this analysis, given its focus
in Class I areas. Such an emphasis on lands afforded special government
protections, such as national parks and forests, wildlife refuges, and
wilderness areas, some of which are designated Class I areas under the
CAA, is consistent with such emphasis in the 2008 revision of the
secondary standard (73 FR 16485, March 27, 2008). As noted in section
IV.A above, Congress has set such lands aside for specific uses that
are intended to provide benefits to the public welfare, including lands
that are to be protected so as to conserve the scenic value and the
natural vegetation and wildlife within such areas, and to leave them
unimpaired for the enjoyment of future generations. The Administrator
additionally recognizes that states, tribes and public interest groups
also set aside areas that are intended to provide similar benefits to
the public welfare for residents on those lands, as well as for
visitors to those areas.
    As noted in prior reviews, judgments regarding effects that are
adverse to public welfare consider the intended use of the ecological
receptors, resources and ecosystems affected. Thus, the Administrator
recognizes that the median RBL estimate for the studied species is a
quantitative tool within a larger framework of considerations
pertaining to the public welfare significance of O3 effects
on the public welfare. Such considerations include effects that are
associated with effects on growth and that the ISA has determined to be
causally or likely causally related to O3 in ambient air,
yet for which there are greater uncertainties affecting our estimates
of impacts on public welfare. These other effects include reduced
productivity in terrestrial ecosystems, reduced carbon sequestration in
terrestrial ecosystems, alteration of terrestrial community
composition, alteration of below-grown biogeochemical cycles, and
alteration of terrestrial ecosystem water cycles, as summarized in
section IV.A.1. Thus, in her attention to CASAC's characterization of a
6% estimate for tree seedling RBL in the median studied species as
``unacceptably high'', the Administrator, while mindful of
uncertainties with regard to the magnitude of growth impact that might
be expected in mature trees, is also mindful of related, broader,
ecosystem-level effects for which our tools for quantitative estimates
are more uncertain and those for which the policy foundation for
consideration of public welfare impacts is less well established. She
finds her consideration of tree growth effects consistent with CASAC
advice regarding consideration of O3-related biomass loss as
a surrogate for the broader array of O3 effects at the plant
and ecosystem levels.
    The Administrator also recognizes that O3-related
effects on sensitive vegetation can occur in other areas that have not
been afforded special federal protections, including effects on
vegetation growing in managed city parks and residential or commercial
settings, such as ornamentals used in urban/suburban landscaping or
vegetation grown in land use categories that are heavily managed for
commercial production of commodities such as timber. In her
consideration of the evidence and quantitative information of
O3 effects on crops, the Administrator recognizes the
complexity of considering adverse O3 impacts to public
welfare due to the heavy management common for achieving optimum yields
and market factors that influence associated services. In so doing, she
notes that her judgments that place emphasis on the protection of
forested ecosystems inherently also recognize a level of protection for
crops. Additionally, for vegetation used for residential or commercial
ornamental purposes, the Administrator believes that there is not
adequate information specific to vegetation used for those purposes,
but notes that a secondary standard revised to provide protection for
sensitive natural vegetation and ecosystems would likely also provide
some degree of protection for such vegetation.
    The Administrator also takes note of the long-established evidence
of consistent association of the presence of visible foliar injury with
O3 exposure and the currently available information that
indicates the occurrence of visible foliar injury in sensitive species
of

vegetation during recent air quality in public forests across the U.S.
She additionally notes the PA conclusions regarding difficulties in
quantitatively relating visible foliar injury symptoms to vegetation
effects such as growth or related ecosystem effects. As at the time of
the last review, the Administrator believes that the degree to which
such effects should be considered to be adverse depends on the intended
use of the vegetation and its significance. The Administrator also
believes that the significance of O3-induced visible foliar
injury depends on the extent and severity of the injury and takes note
of studies in the evidence base documenting increased severity and/or
prevalence with higher O3 exposures. However, the
Administrator takes note of limitations in the available information
with regard to judging the extent to which the extent and severity of
visible foliar injury occurrence associated with conditions allowed by
the current standard may be considered adverse to public welfare.
    Based on these considerations, and taking into consideration the
advice and recommendations of CASAC, the Administrator concludes that
the protection afforded by the current secondary O3 standard
is not sufficient and that the standard needs to be revised to provide
additional protection from known and anticipated adverse effects to
public welfare, related to effects on sensitive vegetation and
ecosystems, most particularly those occurring in Class I areas. The
Administrator additionally recognizes that states, tribes and public
interest groups also set aside areas that are intended to provide
similar benefits to the public welfare for residents on those lands, as
well as for visitors to those areas. Given the clear public interest in
and value of maintaining these areas in a condition that does not
impair their intended use, and the fact that many of these areas
contain O3-sensitive vegetation, the Administrator further
concludes that it is appropriate to revise the secondary standard in
part to provide increased protection against O3-caused
impairment to vegetation and ecosystems in such areas, which have been
specially protected to provide public welfare benefits. She further
notes that a revised standard would provide increased protection for
other growth-related effects, including for crop yield loss, reduced
carbon storage and for areas for which it is more difficult to
determine public welfare significance, as recognized in section IV.A.3
above, as well other welfare effects of O3, such as visible
foliar injury.

C. Conclusions on Revision of the Secondary Standard

    The elements of the standard--indicator, averaging time, form, and
level--serve to define the standard and are considered collectively in
evaluating the welfare protection afforded by the secondary standard.
Section IV.C.1 below summarizes the basis for the proposed revision.
Significant comments received from the public on the proposal are
discussed in section IV.C.2 and the Administrator's final decision on
revisions to the secondary standard is described in section IV.C.3.
1. Basis for Proposed Revision
    At the time of proposal, in considering what revisions to the
secondary standard would be appropriate, the Administrator considered
the ISA conclusions regarding the weight of the evidence for a range of
welfare effects associated with O3 in ambient air and
associated areas of uncertainty; quantitative risk and exposure
analyses in the WREA for different adjusted air quality scenarios and
associated limitations and uncertainties; staff evaluations of the
evidence, exposure/risk information and air quality information in the
PA; additional air quality analyses of relationships between air
quality metrics based on form and averaging time of the current
standards and a cumulative seasonal exposure index; CASAC advice; and
public comments received as of that date in the review. In the
paragraphs below, we summarize the proposal presentation with regard to
key aspects of the PA considerations, advice from the CASAC, air
quality analyses of different air quality metrics and the
Administrator's proposed conclusions, drawing from section IV.E of the
proposal.
a. Considerations and Conclusions in the PA
    As summarized in the proposal, in identifying alternative secondary
standards appropriate to consider in this review, the PA focused on
standards based on a cumulative, seasonal, concentration-weighted form
consistent with the CASAC advice in the current and last review. Based
on conclusions of the ISA, as also summarized in section IV.A above,
the PA considered a cumulative, seasonal, concentration-weighted
exposure index to provide the most scientifically defensible approach
for characterizing vegetation response to ambient O3 and
comparing study findings, as well as for defining indices for
vegetation protection, as summarized in the proposal section IV.E.2.a.
With regard to the appropriate index, the PA considered the evidence
for a number of different such indices, as described in the proposal,
and noted the ISA conclusion that the W126 index has some important
advantages over other similarly weighted indices. The PA additionally
considered the appropriate diurnal and seasonal exposure periods in a
given year by which to define the seasonal W126 index and based on the
evidence in the ISA and CASAC advice, as summarized in the proposal,
decided on the 12-hour daylight window (8:00 a.m. to 8:00 p.m.) and the
3-consecutive-month period providing the maximum W126 index value.
    Based on these considerations, the PA concluded it to be
appropriate to retain the current indicator of O3 and to
consider a secondary standard form that is an average of the seasonal
W126 index values (derived as described in section IV.A.1.c above)
across three consecutive years (U.S. EPA, 2014c, section 6.6). In so
doing, the PA recognized that there is limited information to discern
differences in the level of protection afforded for cumulative growth-
related effects by potential alternative W126-based standards of a
single-year form as compared to a 3-year form (U.S. EPA, 2014c, pp. 6-
30). The PA concluded a 3-year form to be appropriate for a standard
intended to provide the desired level of protection from longer-term
effects, including those associated with potential compounding, and
that such a form might be concluded to contribute to greater stability
in air quality management programs, and thus, greater effectiveness in
achieving the desired level of public welfare protection than might
result from a single-year form. (U.S. EPA, 2014c, section 6.6).
    As summarized in the proposal, the PA noted that, due to the
variability in the importance of the associated ecosystem services
provided by different species at different exposures and in different
locations, as well as differences in associated uncertainties and
limitations, it is essential to consider the species present and their
public welfare significance, together with the magnitude of the ambient
concentrations in drawing conclusions regarding the significance or
magnitude of public welfare impacts. Therefore, in development of the
PA conclusions, staff took note of the complexity of judgments to be
made by the Administrator regarding the adversity of known and
anticipated effects to the

public welfare and recognized that the Administrator's ultimate
judgments on the secondary standard will most appropriately reflect an
interpretation of the available scientific evidence and exposure/risk
information that neither overstates nor understates the strengths and
limitations of that evidence and information. In considering an
appropriate range of levels to consider for an alternative standard,
the PA primarily considered tree growth, crop yield loss, and visible
foliar injury, as well as impacts on the associated ecosystem services,
while noting key uncertainties and limitations.
    In specifically evaluating exposure levels, in terms of the W126
index, as to their appropriateness for consideration in this review
with regard to providing the desired level of vegetation protection for
a revised secondary standard, the PA focused particularly on RBL
estimates for the median across the 11 tree species for which robust E-
R functions are available. Table 4 below presents these estimates (U.S.
EPA, 2014c, Appendix 5C, Table 5C-3; also summarized in Table 8 of the
proposal). In so doing and recognizing the longstanding, strong
evidence base supporting these relationships, the PA also noted
uncertainties regarding inter-study variability for some species, as
well as with regard to the extent to which tree seedling E-R functions
can be used to represent mature trees. As summarized in the proposal,
the PA conclusions on a range of W126 levels appropriate to consider
are based on specific advice from CASAC with regard to median tree
seedling RBL estimates that might be considered unacceptably high (6%),
as well as its judgment on a RBL benchmark (2%) for identification of
the lower end of a W126 index value range for consideration that might
give more emphasis to the more sensitive tree seedlings (Frey, 2014c,
p. 14).\196\
---------------------------------------------------------------------------

    \196\ The CASAC provided several comments related to 2% RBL for
tree seedlings both with regard to its use in summarizing WREA
results and with regard to consideration of the potential
significance of vegetation effects, as summarized in sections IV.D.2
and IV.E.3 of the proposal.

         Table 4--Tree Seedling Biomass Loss and Crop Yield Loss Estimated for O3 Exposure Over a Season
----------------------------------------------------------------------------------------------------------------
                                       Tree seedling biomass loss A                  Crop yield loss B
  W126 index value for exposure  -------------------------------------------------------------------------------
             period                  Median value     Individual species     Median value     Individual species
----------------------------------------------------------------------------------------------------------------
23 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 4/10
                                   7.6% loss.          species.            8.8% loss.          species
                                                      <= 5% loss: 4/11                        >5,<10% loss: 1/10
                                                       species.                                species
                                                      <=10% loss: 8/11                        >10,<20% loss: 4/
                                                       species.                                10 species
                                                      <=15% loss: 10/11                       >20: 1/10 species
                                                       species.
                                                      >40% loss: 1/11
                                                       species.
22 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 4/10
                                   7.2% loss.          species.            8.2% loss.          species
                                                      <= 5% loss: 4/11                        >5,<10% loss: 1/10
                                                       species.                                species
                                                      <=10% loss: 7/11                        >10,<20% loss: 4/
                                                       species.                                10 species
                                                      <=15% loss: 10/11                       >20: 1/10 species
                                                       species.
                                                      >40% loss: 1/11
                                                       species.
21 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 4/10
                                   6.8% loss.          species.            7.7% loss.          species
                                                      <= 5% loss: 4/11                        >5,<10% loss: 3/10
                                                       species.                                species
                                                      <=10% loss: 7/11                        >10,<20% loss: 3/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >40% loss: 1/11
                                                       species.
20 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 5/10
                                   6.4% loss.          species.            7.1% loss.          species
                                                      <= 5% loss: 5/11                        >5,<10% loss: 3/10
                                                       species.                                species
                                                      <=10% loss: 7/11                        >10,<20% loss: 2/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >40% loss: 1/11
                                                       species.
19 ppm-hrs......................  Median species w.   <= 2% loss: 3/11    Median species w.   <= 5% loss: 5/10
                                   6.0% loss.          species.            6.4% loss.          species
                                                      <=5% loss: 5/11                         >5, <10% loss: 3/
                                                       species.                                10 species
                                                      <=10% loss: 7/11                        >10,<20% loss: 2/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
18 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 5/10
                                   5.7% loss.          species.            5.7% loss.          species
                                                      <= 5% loss: 5/11                        >5,<10% loss: 3/10
                                                       species.                                species
                                                      <=10% loss: 7/11                        >10,<20% loss: 2/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
17 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 5/10
                                   5.3% loss.          species.            5.1% loss.          species
                                                      <=5% loss: 5/11                         >5, <10% loss: 3/
                                                       species.                                10 species
                                                      <=10% loss: 9/11                        >10,<20% loss: 2/
                                                       species.                                10 species
                                                      <=15% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
16 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 5/10
                                   4.9% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 6/11                        >5,<10% loss: 4/10
                                                       species.                                species
                                                      <=10% loss: 10/11                       >10,<20% loss: 1/
                                                       species.                                10 species
                                                      >30% loss: 1/11
                                                       species.
15 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 6/10
                                   4.5% loss.          species.            <=5.0% loss.        species
                                                      <=5% loss: 6/11                         >5, <10% loss: 4/
                                                       species.                                10 species
                                                      <=10% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
14 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 6/10
                                   4.2% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 6/11                        >5,<10% loss: 4/10
                                                       species.                                species
                                                      <=10% loss: 10/11
                                                       species.
                                                      >30% loss: 1/11
                                                       species.
13 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 6/10
                                   3.8% loss.          species.            <=5.0% loss.        species
                                                      <5% loss: 7/11                          >5, <10% loss: 4/
                                                       species.                                10 species
                                                      <10% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.

12 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 8/10
                                   3.5% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 8/11                        >5,<10% loss: 2/10
                                                       species.                                species
                                                      <=10% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.
11 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 9/10
                                   3.1% loss.          species.            <=5.0% loss.        species
                                                      <=5% loss: 8/11                         >5, <10% loss: 1/
                                                       species.                                10 species
                                                      <=10% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.
10 ppm-hrs......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: 9/10
                                   2.8% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 9/11                        >5,<10% loss: 1/10
                                                       species.                                species
                                                      <10% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.
9 ppm-hrs.......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: all
                                   2.4% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 10/11
                                                       species.
                                                      >20% loss: 1/11
                                                       species.
8 ppm-hrs.......................  Median species w.   <= 2% loss: 5/11    Median species w.   <= 5% loss: all
                                   2.0% loss.          species.            <=5.0% loss.        species
                                                      <= 5% loss: 10/11
                                                       species.
                                                      >15% loss: 1/11
                                                       species.
7 ppm-hrs.......................  Median species w.   <= 2% loss: 7/11    Median species w.   <= 5% loss: all
                                   <2.0% loss.         species.            <=5.0% loss.        species
                                                      <=5% loss: 10/11
                                                       species.
                                                      >15% loss: 1/11
                                                       species.
----------------------------------------------------------------------------------------------------------------
\A\ Estimates here are based on the E-R functions for 11 species described in the WREA, section 6.2 and
  discussed in the PA, section 5.2.1. The cottonwood was excluded to address CASAC comments (Frey, 2014c; U.S.
  EPA, 2014b, U.S. EPA, 2014c, Appendix 6F). The median is the median of the 11 composite E-R functions (U.S.
  EPA, 2014c, Appendix 5C).
\B\ Estimates here are based on the 10 E-R functions for crops described in the WREA, section 6.2 and discussed
  in the PA, section 5.3.1. The median is the median of the 10 composite E-R functions (U.S. EPA, 2014b; U.S.
  EPA, 2014c, Appendix 5C).

    With regard to secondary standard revisions appropriate to consider
in this review, as summarized in the proposal, the PA concluded it to
be appropriate to consider a W126-based secondary standard with index
values within the range of 7 to 17 ppm-hrs and a form averaged over 3
years (U.S. EPA, 2014c, section 6.7). The PA additionally recognized
the role of policy judgments required of the Administrator with regard
to the public welfare significance of identified effects, the
appropriate weight to assign the range of uncertainties inherent in the
evidence and analyses, and ultimately, in identifying the requisite
protection for the secondary O3 standard.
    The PA additionally recognized that to the extent the Administrator
finds it useful to consider the public welfare protection that might be
afforded by revising the level of the current standard, this is
appropriately judged by evaluating the impact of associated
O3 exposures in terms of the cumulative seasonal W126-based
index, an exposure metric considered appropriate for evaluating impacts
on vegetation (U.S. EPA, 2014c, section 6.7). Accordingly, the PA
included several air quality data analyses that might inform such
consideration (U.S. EPA, 2014c, section 6.4). Additional air quality
analyses were performed subsequent to the PA, described in the proposal
and are summarized below.
b. CASAC Advice
    Advice received from the CASAC during the current review, similar
to that in the last review, recommended retaining O3 as the
indicator, while also recommending consideration of a secondary
standard with a revised form and averaging time based on the W126 index
(Frey, 2014c, p. iii). The CASAC concurred with the 12-hour period (8
a.m. to 8 p.m.) and 3-month summation period resulting in the maximum
W126 index value, as described in the PA, while recommending a somewhat
narrower range of levels from 7 ppm-hrs to 15 ppm-hrs. While the CASAC
recommended a W126 index limited to a single year, in contrast with the
PA's conclusion that it was appropriate to consider the W126 index
averaged across three years, it also noted that the Administrator may
prefer, as a policy matter, to base the secondary standard on a 3-year
averaging period. In such a case, the CASAC recommended revising
downward the level for such a metric to avoid a seasonal W126 index
value above a level in their recommended range in any given year of the
3-year period, indicating an upper end of 13 ppm-hrs as an example for
such a 3-year average W126 index range (Frey, 2014c, p. iii and iv).
c. Air Quality Analyses
    The proposal additionally summarized several analyses of air
quality that considered relationships between metrics based on a 3-year
W126 index and based on the form and averaging time of the current
standard, the ``fourth-high'' metric (U.S. EPA, 2014c, Chapter 2,
Appendix 2B and section 6.4; Wells, 2014a), as well as describing the
uncertainties and limitations associated with these analyses. The
proposal concluded that these analyses suggest that, depending on the
level, a standard of the current averaging time and form can be
expected to control cumulative seasonal O3 exposures to such
that they may meet specific 3-year average W126 index values. The
fourth-high and W126 metrics, and changes in the two metrics over the
past decade, were found to be highly correlated (U.S. EPA, 2014c,
section 6.4 and Appendix 2B; Wells, 2014a). From these analyses, it was
concluded that future control programs designed to help meet a standard
based on the fourth-high metric are also expected to result in
reductions in values of the W126 metric (Wells, 2014a). Further, the
second analysis also found that the Southwest and West NOAA climatic
regions, which showed the greatest potential for sites to measure
elevated cumulative, seasonal O3 exposures without the
occurrence of elevated daily maximum 8-hour average O3
concentrations, exhibited the greatest reduction in W126 metric value
per unit reduction in fourth-high metric (Wells, 2014a, Figures 5b and
12 and Table 6).

    Analyses of the most recent periods studied in the two analyses
(2009-2011 and 2011-2013) had similar findings regarding the highest
W126 metric values occurring at monitoring sites that meet alternative
levels of the fourth-high metric (U.S. EPA, 2014c, section 6.4; Wells,
2014a). In both analyses, the highest W126 metric values were in the
Southwest and West NOAA climatic regions. In both analyses, no
monitoring sites for which the fourth-high metric was at or below 70
ppb had a W126 metric value above 17 ppm-hrs (U.S. EPA, 2014c, Figure
2B-3b; Wells, 2014a, Table 4). All U.S. regions were represented in
these subsets. In the 2011-2013 subset of sites for which the fourth-
high metric was at or below a potential alternative primary standard
level of 65 ppb, no monitoring sites had W126 metric values above 11
ppm-hrs (Wells, 2014a, Table 4).
d. Administrator's Proposed Conclusions
    At the time of proposal, the Administrator concluded it to be
appropriate to continue to use O3 as the indicator for a
secondary standard that is intended to address effects associated with
exposure to O3 alone and in combination with related
photochemical oxidants. While the complex atmospheric chemistry in
which O3 plays a key role has been highlighted in this
review, no alternatives to O3 have been advanced as being a
more appropriate surrogate for ambient photochemical oxidants and their
effects on vegetation. The CASAC agreed that O3 should be
retained as the indicator for the standard (Frey, 2014c, p. iii). In
proposing to retain O3 as the indicator, the Administrator
recognized that measures leading to reductions in ecosystem exposures
to O3 would also be expected to reduce exposures to other
photochemical oxidants.
    The Administrator proposed to retain the current averaging time and
form and to revise the level of the current secondary standard to a
level within the range of 0.065 to 0.070 ppm. She based this proposal
on her provisional conclusions regarding the level of cumulative
seasonal O3 exposures that would provide the requisite
protection against known or anticipated adverse effects to the public
welfare and on a policy option that would provide this level of
protection. With regard to the former, the Administrator concluded that
in judging the extent of public welfare protection that might be
afforded by a revised standard and whether it meets the appropriate
level of protection, it is appropriate to use a cumulative, seasonal
concentration-weighted exposure metric. For this purpose, the
Administrator concluded it to be appropriate to use the W126 index
value, averaged across three years, with each year's value identified
as that for the 3-month period yielding the highest seasonal value and
with daily O3 exposures within a 3-month period cumulated
for the 12-hour period from 8:00 a.m. to 8:00 p.m.
    To identify the range of cumulative seasonal exposures, in terms of
the W126 index, expected to be associated with the appropriate degree
of public welfare protection, the Administrator gave primary
consideration to growth-related impacts, using tree seedling RBL
estimates for a range of W126 exposure index values and CASAC advice
regarding such estimates. Additionally taking into account judgments on
important uncertainties and limitations inherent in the current
available scientific evidence and quantitative assessments, and
judgments regarding the extent to which different RBL estimates might
be considered indicative of effects adverse to public welfare, the
Administrator proposed that ambient O3 concentrations
resulting in cumulative seasonal O3 exposures of a level
within the range from 13 ppm-hrs to 17 ppm-hrs, in terms of a W126
index averaged across three consecutive years, would provide the
requisite protection against known or anticipated adverse effects to
the public welfare. In identifying policy options for a revised
secondary standard that would control exposures to such an extent, the
Administrator considered the results of air quality analyses that
examined the responsiveness of cumulative exposures (in terms of the
W126 index) to O3 reductions in response to the current and
prior standard for which the form and averaging time are summarized as
a fourth-high metric, and also examined the extent to which cumulative
exposures (in terms of the W126 index) may be limited by alternative
levels of a metric based on the current standard averaging time and
form. Based on the results of these analyses, she proposed that
revision of the level of the current secondary standard to within the
range of 0.065 to 0.070 ppm would be expected to provide the requisite
public welfare protection, depending on final judgments concerning such
requisite protection.
2. Comments on Proposed Revision
    Significant comments from the public regarding revisions to the
secondary standard are addressed in the subsections below. We first
discuss comments related to our consideration of growth-related effects
and visible foliar injury in identifying appropriate revisions to the
standard (sections IV.C.2.a and IV.C.2.b). Next, we address comments
related to the use of the W126 metric in evaluating vegetation effects
and public welfare protection and comments related to the form and
averaging time for the revised standard (sections IV.C.2.c and
IV.C.2.d). Comments on revisions to the level of the standard are
described in section IV.C.2.e, and those related to the way in which
today's rulemaking addresses the 2013 court remand are addressed in
section IV.C.2.f. Other significant comments related to consideration
of a revised secondary standard, and that are based on relevant
factors, are addressed in the Response to Comments document.
a. Consideration of Growth-Related Effects
    In considering public comments received on the consideration of
growth-related effects of O3 in the context of the proposed
decision on a revised secondary standard, we first note related advice
and comments from the CASAC provided during development of the PA,
stating, as summarized in section IV.B.1.b above, that ``relative
biomass loss for tree species, crop yield loss, and visible foliar
injury are appropriate surrogates for a wide range of damage that is
adverse to public welfare'' (Frey, 2014c, p. 10). Additionally, in the
context of different standard levels they considered appropriate for
the EPA to consider, CASAC stated that it is appropriate to ``include[]
levels that aim for not greater than 2% RBL for the median tree
species'' and that a median tree species RBL of 6% is ``unacceptably
high'' (Frey, 2014c, p. 14).\197\ With respect to crop yield loss,
CASAC points to a benchmark of 5%, stating that a crop RYL for median
species over 5% is ``unacceptably high'' (Frey, 2014c, p. 13).
---------------------------------------------------------------------------

    \197\ The CASAC made this comment while focusing on Table 6-1 in
the second draft PA and the entry for 17 ppm-hrs (Frey, 2014c, p.
14). That table was revised for inclusion in the final PA in
consideration of CASAC comments on the E-R function for eastern
cottonwood, and after that revision, the median RBL estimate for 17
ppm-hrs in the final table (see Table 4 above) is below the value of
6% that CASAC described in this way.
---------------------------------------------------------------------------

    In addition, regarding consideration of RBL benchmarks for tree
seedlings, the CASAC stated that ``[a] 2% biomass loss is an
appropriate scientifically based value to consider as a benchmark of
adverse impact for long-lived perennial species such as trees, because
effects are cumulative over multiple

years'' (Frey, 2014c, p. 14).\198\ With regard to this benchmark, the
CASAC also commented that ``it is appropriate to identify a range of
levels of alternative W126-based standards that includes levels that
aim for not greater than 2% RBL for the median tree species'' in the PA
(Frey, 2014c, p. 14). The CASAC noted that the ``level of 7 ppm-hrs is
the only level analyzed for which the relative biomass loss for the
median tree species is less than or equal to 2 percent,'' indicating
that 7 ppm was appropriate as a lower bound for the recommended range
(Frey, 2014c, p. 14).\199\
---------------------------------------------------------------------------

    \198\ The CASAC provided several comments related to 2% RBL for
tree seedlings both with regard to its use in summarizing WREA
results and with regard to consideration of the potential
significance of vegetation effects, as summarized in sections IV.D.2
and IV.E.3 of the proposal.
    \199\ The CASAC made this comment while focusing on Table 6-1 in
the second draft PA, which included odd-numbered W126 index values
and in which the median RBL values were based on 12 species. That
table was revised for inclusion in the final PA in consideration of
CASAC comments on the E-R function for eastern cottonwood, such that
the median RBL species estimate for both 7 ppm-hrs and 8 ppm-hrs are
less than or equal to 2.0% in the final table (see Table 4 above and
Table 5C-3 of the final PA).
---------------------------------------------------------------------------

    With regard to consideration of effects on crops, in addition to
their comments regarding a median species RYL over 5% yield loss, noted
above (Frey, 2014c, p. 13), the CASAC further noted that ``[c]rop loss
appears to be less sensitive than these other indicators, largely
because of the CASAC judgment that a 5% yield loss represents an
adverse impact, and in part due to more opportunities to alter
management of annual crops'' (Frey, 2014c, p. 14).
    Comments from the public with regard to how the EPA considered
growth-related effects in the proposed decision on a revised secondary
standard varied. Generally, those commenters who recommended against
revision of the standard expressed the view that RBL estimates based on
the established E-R functions for the 11 studied species, and their
pertinence to mature trees, were too uncertain to serve as a basis for
judgments regarding public welfare protection afforded by the secondary
standard. The EPA generally disagrees with this view, as discussed in
section IV.B.2 above, and addressed in more detail in the Response to
Comments document.
    Some commenters also took note of the unclear basis for CASAC's 2%
benchmark, stating that the CASAC advice on this point is ``not wholly
scientific,'' given that it referenced the 1996 workshop, which
provided little specificity as to scientific basis for such a
benchmark; based on this, the commenters described this CASAC advice as
a policy judgment and described the important role of the EPA's
judgment in such instances. As noted in section IV.E.3 of the proposal,
we generally agree with these commenters regarding the unclear
scientific basis for the 2% value. Consistent with this advice from
CASAC, however, the range of levels for a revised secondary standard
that the PA concluded was appropriate for the Administrator to consider
did include a level for which the estimated median RBL across the 11
studied tree species would be 2%, as well as a level for which the
median RBL would be below 2% (U.S. EPA, 2014c, section 6.7 and Tables
6-1 and 5C-3), and, as described in the proposal, the Administrator
considered the conclusions of the PA in reaching her proposed decision
that it was appropriate to consider a range for the revised secondary
standard that did not focus on this benchmark. The Administrator has
further considered and explained any differences from CASAC's
recommendations on this point in her final decision, as described in
section IV.C.3 below.
    Some of the state and local environmental agencies and
organizations and environmental groups that supported the EPA's
proposed decision to revise the secondary standard additionally
indicated their view that the EPA should give more weight to growth-
related effects by setting the standard at a level for which the
estimated RBL would be at or below 2% in the median studied species. In
support of this recommendation, the commenters cited the CASAC advice
and stated that the EPA's rationale deviates from that advice with
regard to consideration of RBL. In so doing, the commenters implied
incorrectly that the EPA's proposal did not put the most weight on the
median RBL. In fact, in considering RBL as a metric for growth effects,
the Administrator's proposed conclusions focused solely on the median
RBL estimates, indicating that appreciable weight was given to growth-
related effects and on the median RBL. Additionally, the commenters
implied that the EPA misconstrued the CASAC comment on 6% RBL to
indicate that it was acceptable. Yet, the proposal notes CASAC's view
that a 6% RBL is ``unacceptably high'' nine times, and, in section
IV.B.3 above, the Administrator takes note of this view in reaching the
decision that the current standard should be revised. The EPA considers
this statement from CASAC, provided in the context of considering
effects related to different W126 index values, to be of a different
nature than CASAC advice discussed above that options for the EPA
consideration ``include'' a level that aims for median RBL at or below
2%.
    The comments that state that the standard should control cumulative
exposures to levels for which the estimated median species RBL is at or
below 2% provided little rationale beyond citing to CASAC advice. We
note, however, that the CASAC did not specify that the revised
secondary standard be set to limit cumulative exposures to that extent.
Nor, in identifying a range of alternatives for the EPA to consider,
did CASAC recommend that the EPA consider only W126 index levels
associated with median RBL estimates at or below 2%. Rather, the CASAC
stated that ``it is appropriate to identify a range of levels of
alternative W126-based standards that includes {emphasis added{time}
levels that aim for not greater than 2% RBL for the median tree
species'' (Frey, 2014c, p. 14) and seven of the nine levels in the
CASAC-recommended range of W126 index levels were associated with
higher RBL estimates (as shown in Table 4 above).
    In citing to CASAC advice, commenters quoted the CASAC
characterization of a 2% RBL as ``an appropriate scientifically based
value to consider as a benchmark of adverse impact for long-lived
perennial species such as trees, because effects are cumulative over
multiple years'' (Frey, 2014, p. 14). Presumably to indicate reasoning
for this statement, the subsequent sentence in the same CASAC letter
referenced findings for biomass loss in aspen exposed to elevated
O3 over seven years, citing Wittig et al., 2009. As noted in
the proposal, however, the way in which these findings would provide a
basis for CASAC's view with regard to 2% is unclear, as the original
publication that is the source for the 7-year biomass loss value (King,
et al., 2005) and which is cited in Wittig et al. (2009) indicates
yearly RBL values during this 7-year exposure that are each well above
2%, and, in fact, are all above 20% (King, et al., 2005). In the same
paragraph, the CASAC letter additionally referenced the report of the
1996 workshop sponsored by the Southern Oxidants Study group (Heck and
Cowling, 1997, noted in section IV.A.3 above). The workshop report
identified 1-2% per year growth reduction (based on a stated interest
in avoiding 2% cumulative effects) as an appropriate endpoint for
consideration of growth effects in trees, although an explicit
rationale for the identified percentages is not provided

(Frey, 2014c, p. 14).\200\ Like the 1996 workshop, the CASAC describes
2% RBL as providing the basis for consideration of 7 ppm-hrs, the lower
end of their recommended W126 range (Frey, 2014c, p. 14). As a result,
the specific scientific basis for judging a value of 2% RBL in the
median studied species as an appropriate benchmark of adverse impact
for trees and other long-lived perennials is not clear, which, as
described in the proposal, contributed to the Administrator noting the
greater uncertainty regarding the extent to which estimates of benefits
in terms of ecosystem services and reduced effects on vegetation at
O3 exposures below her identified range of 13 to 17 ppm-hrs
might be judged significant to the public welfare.
---------------------------------------------------------------------------

    \200\ The report of the 1996 workshop provides no more explicit
rationale for the percentages identified or specification with
regard to number or proportion of species for which such percentages
should be met (Heck and Cowling, 1997).
---------------------------------------------------------------------------

    Some commenters recommended revision of the standard to 7 ppm-hrs
as a W126 form stating that such a change is needed to protect against
climate change. In so doing, one commenter expressed the view that the
relatively lesser weight the EPA placed on the WREA estimates of carbon
storage (in terms of CO2) in consideration of a proposed
revision to the secondary standard is inconsistent with the emphasis
that the EPA placed on CO2 emissions reductions estimated
for the proposed Clean Power Plan (79 FR 34830, 34931-33). As support
for this view of inconsistency, the commenter compared the WREA 30-year
estimate of the amount of CO2 removed from the air and
stored in vegetation with estimated reductions in CO2
emissions from power plants over a 4-year period. We note, however,
some key distinctions between the two types of estimates which
appropriately lead to different levels of emphasis by the EPA in the
two actions. First, we note that the lengths of time pertaining to the
two estimates that the commenter states to be ``roughly equal'' (e.g.,
ALA et al., p. 211) differ by more than a factor of seven (4 years
compared to 30). Second, the CPP estimates are for reductions in
CO2 produced and emitted from power plants, while the WREA
estimates are for amounts of CO2 removed from the air and
stored in vegetation as a result of plant photosynthesis occurring
across the U.S. This leads to two important differences. The first is
whether a ton of additional carbon uptake by plants is equal to a ton
of reduced emissions from fossil fuels. This is still an active area of
discussion due in part to the potentially transient nature of the
carbon storage in vegetation. The second is that there are much larger
uncertainties involved in attempting to quantify the additional carbon
uptake by plants which requires complex modeling of biological and
ecological processes and their associated sources of uncertainty.
Therefore, as summarized in section IV.C.3 below, the Administrator is
judging, as at the time of proposal, that the quantitative
uncertainties are too great to support identification of a revised
standard based specifically on the WREA quantitative estimates of
carbon storage benefits to climate. In so doing, she notes that a
revised standard, established primarily based on other effects for
which our quantitative estimates are less uncertain, can be expected to
also provide increased protection in terms of carbon storage.
b. Consideration of Visible Foliar Injury
    In considering public comments received on the EPA's consideration
of visible foliar injury in its decision on a revised secondary
standard, the EPA first notes related advice and comments from the
CASAC received during development of the PA. The CASAC stated that
``[w]ith respect to the secondary standard, the CASAC concurs with the
EPA's identification of adverse welfare effects related to . . . damage
to resource use from foliar injury'' (Frey, 2014, p. iii). In its
comments on levels of a W126-based standard, the CASAC, seemingly in
reference to the WREA visible foliar injury analyses, additionally
stated that ``[a] level below 10 ppm-hrs is required to reduce foliar
injury'' (Frey, 2014, pp. iii and 15), with ``W126 values below 10 ppm-
hr required to reduce the number of sites showing visible foliar
injury'' (Frey, 2014, p. 14).
    Public comments were generally split between two views, either that
visible foliar injury was not appropriate to consider in decisions
regarding the standard, based on variously identified reasons, or that
it should be considered and it would lead the EPA to focus on a W126
value below approximately 10 ppm-hrs. Comments of the former type are
discussed in section IV.B.2 above, with, in some cases, additional
detail in the Response to Comments document. Commenters expressing the
latter view variously cite CASAC advice and figures from the WREA
cumulative analysis of USFS biosite data with WREA W126 index value
estimates. The EPA disagrees that only a reduction in cumulative
exposures to W126 index values below 10 ppm-hrs will affect the
occurrence or extent of visible foliar injury. In so doing, we note
that the extensive evidence, which is summarized in the ISA (including
studies of the USFS biomonitoring program), analyses in the 2007 Staff
Paper and also observations based on the WREA dataset do not support
this conclusion.
    The evidence regarding visible foliar injury as an indicator of
O3 exposure is well established and generally documents a
greater extent and severity of visible foliar injury with higher
O3 exposures and a modifying role of soil moisture
conditions (U.S. EPA, 2013, section 9.4.2). As stated in the ISA,
``[v]isible foliar injury resulting from exposure to O3 has
been well characterized and documented over several decades of research
on many tree, shrub, herbaceous and crop species'' and ``[o]zone-
induced visible foliar injury symptoms on certain bioindicator plant
species are considered diagnostic as they have been verified
experimentally'' (U.S. EPA, 2013, p. 9-41). Further, a recent study
highlighted in the ISA, which analyzed trends in the incidence and
severity of foliar injury, reported a declining trend in the incidence
of foliar injury as peak O3 concentrations declined (U.S.
EPA, 2013, p. 9-40; Smith, 2012). Another study available in this
review that focused on O3-induced visible foliar injury in
forests of west coast states observed that both percentage of biosites
with injury and average biosite index were higher for sites with
average cumulative O3 concentrations above 25 ppm-hrs in
terms of SUM06 (may correspond to W126 of approximately 21 ppm-hrs
[U.S. EPA, 2007, p. 8-26, Appendix 7B]) as compared to groups of sites
with lower average cumulative exposure concentrations, with much less
clear differences between the two lower exposure groups (Campbell et
al., 2007, Figures 27 and 28 and p. 30). A similar finding was reported
in the 2007 Staff Paper which reported on an analysis that showed a
smaller percentage of injured sites among the group of sites with
O3 exposures below a SUM06 metric of 15 ppm-hrs or a fourth-
high metric of 74 ppb as compared to larger groups that also included
sites with SUM06 values up to 25 ppm-hrs or fourth-high metric up to 84
ppb, respectively (U.S. EPA 2007, pp. 7-63 to 7-64).
    With regard to the comments referencing the WREA cumulative
analysis of USFS FHM/FIA biosite data or related CASAC comments, we
note some clarification of this analysis. This analysis does not show,
as implied by the comments, that at W126 index values above 10 ppm-hrs,
there is little change with increasing W126 index in

the proportion of records with any visible foliar injury (biosite index
above 0). As the analysis is a cumulative analysis, each point graphed
in the analysis includes the records for the same and lower W126 index
values, so the analysis does not compare results for groups of records
with differing, non-overlapping W126 index values. Rather, the points
represent groups with records (and W126 index values) in common and the
number of records in the groups is greater for higher W126 index values
(U.S. EPA, 2014b, section 7.2). Additionally, we note that the pattern
observed in the cumulative analysis is substantially influenced by the
large number of records for which the W126 index estimates are at or
below 11 ppm-hrs, more than two thirds of the dataset (Smith and
Murphy, 2015, Table 1).
    To more fully address the comments related to this WREA analysis,
we have drawn several additional observations from the WREA dataset,
re-presenting the same data in a different format in a technical
memorandum to the docket (Smith and Murphy, 2015). Contrary to the
implication of the statements from the commenters and CASAC that no
reduction in the occurrence of visible foliar injury can be achieved
with exposures above 10 ppm-hrs, both the proportion of records with
injury and the average biosite index are lower for groups of records
with W126 index estimates at or below 17 ppm-hrs compared to the group
for the highest W126 index range. This is true when considered
regardless of soil moisture conditions (all records), as well as for
dry, normal and wet records, separately (Smith and Murphy, 2015, Table
2). The pattern of the two measures across record groups with lower
W126 index values differs with moisture level, with the wetter than
normal records generally showing decreasing proportions of injured
sites and decreasing average biosite index with lower W126 index
values, while little difference in these measures is seen among the
middle W126 values although they are lower than the highest W126 index
group and higher than the lowest W126 index group (Smith and Murphy,
2015, Table 2). In summary, the EPA disagrees with commenters, noting
that the available information, including additional observations from
the WREA dataset, indicate declines in the occurrence of visible foliar
injury across decreasing W126 index values that are higher than 10 ppm-
hrs.
c. Use of W126 Metric in Evaluating Vegetation Effects and Public
Welfare Protection
    In considering public comments received on the EPA's use of the
W126 exposure index in its decision on a revised secondary standard,
the EPA first notes related advice and comments from the CASAC received
during development of the PA. Although we recognize that CASAC's
comments on the W126 index were provided in the context of its
recommendation for a secondary standard of that form, we find them to
also relate to our use of the W126 metric in evaluating the magnitude
and extent of vegetation effects that might be expected and conversely
the level of protection that might be provided under different air
quality conditions. In comments on the first draft PA, the CASAC stated
that ``discussions and conclusions on biologically relevant exposure
metrics are clear and compelling and the focus on the W126 form is
appropriate'' (Frey and Samet, 2012a). With regard to specific aspects
of the W126 index, the CASAC concurred with the second draft PA focus
on ``the biologically-relevant W126 index accumulated over a 12-hour
period (8 a.m.-8 p.m.) over the 3-month summation period of a single
year resulting in the maximum value of W126'' (Frey, 2014c, p. iii).
    The CASAC advice on levels of the W126 index on which to focus for
public welfare protection recommended a level within the range of 7
ppm-hrs to 15 ppm-hrs (Frey, 2014c, p. iii). We note, however, as
summarized in section IV.E.3 of the proposal, that this advice was
provided in the context of the CASAC review of the second draft PA,
which concluded that a range from 7 to 17 ppm-hrs was appropriate to
consider. In considering the upper end of this range, the CASAC
consulted Table 6-1 of the second draft PA which indicated for a W126
index value of 17 ppm-hrs an RBL estimate of 6%, a magnitude that CASAC
described as ``unacceptably high'' and that contributed to a lack CASAC
support for W126 exposures values higher than 15 ppm-hrs (Frey, 2014c,
p. 14; U.S. EPA 2014d, Table 6-1). As noted in section IV.E.3 of the
proposal, revisions to the RBL estimate table in the final PA, which
were made in consideration of other CASAC comments, have resulted in
changes to the median species RBL estimate associated with each W126
index value, such that the median species RBL estimate for a W126 index
value of 17 ppm-hrs in this table in the final PA was 5.3%, rather than
the ``unacceptably high'' value of 6% (U.S. EPA, 2014c, Table 6-1; U.S.
EPA, 2014d, Table 6-1; Frey, 2014c, p. 14).\201\ Additionally, the
CASAC recognized that the Administrator may, as a policy matter, prefer
to use a 3-year average, and stated that in that case, the range of
levels should be revised downward (Frey, 2014c, p. iii-iv).
---------------------------------------------------------------------------

    \201\ We additionally note that the median species RBL estimate
for 17 ppm-hrs in the final PA is nearly identical to the estimate
for 15 ppm-hrs (the value corresponding to the upper end of the
CASAC-identified range) that was in the second draft PA (5.2%) which
was the subject of the CASAC review (U.S. EPA, 2014c, Table 6-1;
U.S. EPA, 2014d, Table 6-1).
---------------------------------------------------------------------------

    The majority of comments on the W126 index concurred with its use
for assessing O3 exposures, while some commenters
additionally expressed the view that this index should be used as the
form of the secondary standard (as discussed in section IV.C.2.d
below). Most submissions from state and local environmental agencies or
governments, as well as organizations of state agencies, that provided
comments on the magnitude of cumulative exposure, in terms of the W126
index, appropriate to consider for a revised secondary standard,
recommended that the EPA focus on an index value within the EPA's
proposed range of 13 to 17 ppm-hrs, as did the industry commenters.
These commenters variously noted their agreement with the rationale
provided by the EPA in the proposal or cited to CASAC comments,
including for a downward adjustment of its recommended values if a 3-
year average W126 was used rather than a single year index. Some other
commenters, including two groups of environmental organizations,
submitted comments recommending a focus on a W126 index level as low as
7 ppm-hrs based on reasons generally focused on consideration of
visible foliar injury.
    Some aspects of these comments have been addressed in sections
IV.C.2.a and IV.C.2.b above. In the Response to Comments document, we
have additionally addressed other comments that recommend a focus on
W126 index values for specific reasons other than generally citing the
CASAC recommended range. Further, in her consideration of a target
level of protection for the revised secondary standard in section
IV.C.3 below, the Administrator has considered comments from the CASAC
regarding the basis for their recommended range.
    An additional comment from an organization of western state air
quality managers indicated a concern with the use of W126 for
vegetation in arid and high altitude regions, such as those in the
western states, which the

commenter hypothesized may have reduced sensitivity. The commenters did
not provide evidence of this hypothesis, calling for further research
in order to characterize the sensitivity of vegetation in such areas.
The EPA agrees that additional research would be useful in more
completely characterizing the response of species in such areas, as
well as other less well studied areas, but does not find support in the
currently available evidence for the commenter's suggestion that
species in arid and high altitude regions may be less sensitive than
those in other areas.\202\
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    \202\ For example, we note that among the 11 species for which
robust E-R functions have been established for O3 effects
on tree seedling growth, the sensitivity of ponderosa pine, a
species occurring in arid and high altitude regions of the western
U.S., is similar to the median (U.S. EPA, 2014c, Table 5C-1).
---------------------------------------------------------------------------

    Among the small number of commenters recommending against using the
W126 metric to assess O3 exposure, a few expressed the view
that some other, not-yet-identified cumulative exposure metric should
be used. These commenters cited a variety of concerns that they state
are not addressed by the W126 index: that plant exposure to and uptake
of O3 are not always equivalent because of variations in
stomatal conductance and plant defenses and their respective diel
patterns, which will also influence plant response; that the duration
between harmful O3 exposures affects the plant's ability to
repair damage; and, that night-time exposures may be important. These
commenters do not identify an alternative to the W126 index that they
conclude to better represent exposures relevant to considering
O3 effects on vegetation and particularly for growth
effects. The EPA has considered the items raised by these commenters,
recognizing some as areas of uncertainty (U.S. EPA, 2013, pp. 9-109 to
9-113), yet has concluded that based on the information available at
this time, exposure indices that cumulate and differentially weight the
higher hourly average concentrations while also including the ``mid-
level'' values offer the most appropriate approach for use in
developing response functions and comparing studies of O3
effects on vegetation (U.S. EPA, 2013, p. 9-117). When considering the
response of vegetation to O3 exposures represented by the
threshold (e.g., SUM06) and non-threshold (e.g., W126) indices, the ISA
notes that ``the W126 metric does not have a cut-off in the weighting
scheme as does SUM06 and thus it includes consideration of potentially
damaging exposures below 60 ppb'' and that ``[t]he W126 metric also
adds increasing weight to hourly concentrations from about 40 ppb to
about 100 ppb'' (U.S. EPA, 2013, p. 9-104). This aspect of W126 is one
way it differs from cut-off metrics such as the SUM06 where all
concentrations above 60 ppb are treated equally and is identified by
the ISA as ``an important feature of the W126 since as hourly
concentrations become higher, they become increasingly likely to
overwhelm plant defenses and are known to be more detrimental to
vegetation'' (U.S. EPA, 2013, p. 9-104). Further, we note the
concurrence by CASAC with the EPA's focus on the W126 exposure index,
as noted above.
    Some commenters also raised concerns regarding the sensitivity of
vegetation in desert areas where plants take in ambient air during
nighttime rather than daylight hours, such that little exposure occurs
from 8 a.m. to 8 p.m., stating that the W126 index as defined by the
EPA to cumulate hourly O3 from 8 a.m. to 8 p.m. may result
in an overly stringent exposure level in areas with such vegetation.
The EPA recognizes that plants, such as cacti, that commonly occur in
desert systems exhibit a particular type of metabolism (referred to as
CAM photosynthesis) such that they only open their stomata at night
(U.S. EPA, 2013, p. 9-109). We note, however, that few if any
O3 exposure studies of these species are available \203\ to
further inform our characterization of these species' responses to
O3, and we have no basis on which to conclude that an
exposure level based on the studied species and a daylight exposure
metric would be overly or underly stringent in areas where only species
utilizing CAM photosynthesis occur. As summarized above, the CASAC
advice concurred with the use of an 8am to 8pm diurnal period for the
W126 exposure index. Thus, we conclude that for our purposes in this
review the focus on daylight hours is appropriate. Our use of the W126
index in this review has been for purposes of characterizing the
potential harm and conversely the potential protection that might be
afforded from the well-characterized effects of O3 on
vegetation, while recognizing associated uncertainties and limitations.
We note that different ecosystems across the U.S. will be expected to
be of varying sensitivities with regard to the effects of
O3. For example, large water bodies without vegetation
extending above the water's surface would be expected to be less
sensitive than forests of sensitive species. The EPA notes, however,
that the NAAQS are set with applicability to all ambient air in the
U.S., such that the secondary O3 standard provides
protection in areas across the U.S. regardless of site-specific aspects
of vegetation sensitivity to O3. In considering the evidence
on O3 and associated welfare effects, we recognize
variability in sensitivity that may relate to a number of factors, as
discussed in the ISA (U.S. EPA, 2013, section 9.4.8). This variability
is among the Administrator's considerations in setting the secondary
standard for O3 that is requisite to protect public welfare
against anticipated or known adverse effects.
---------------------------------------------------------------------------

    \203\ No O3 exposure studies on cacti or other
species that utilize CAM photosynthesis are reported in the ISA
(U.S. EPA, 2013).
---------------------------------------------------------------------------

    Further, some commenters who agreed with a focus on the W126
exposure index also stated that the EPA's definition of the index for
the daylight hours of 8 a.m. to 8 p.m. and a 3-month period was not
appropriate, stating that derivation of the W126 metric should involve
summing concentrations for all 24 hours in each day and all months in
each year to avoid underestimating O3 exposure that the
commenters viewed as pertinent. Support for the EPA's definition of the
W126 index, with which CASAC concurred (Frey, 2014c, p. iii), is based
on the assessment of the evidence in the ISA (U.S. 2013, section
<IP_ADDRESS>) and the context for use of the W126 index in relating
O3 exposure to magnitude and/or extent of O3
response. This context has a particular focus on growth effects for the
purposes of judging the potential for public welfare impacts, as well
as the level of protection, associated with different exposure
circumstances. We note that the ISA stated there is a lack of
information that would allow consideration of the extent to which
nocturnal exposures that may be of interest occur (U.S. EPA, 2013, p.
9-109). Additionally, in our use of the W126 index, we are relying on
E-R functions based on studies that were generally of 3-month duration
and involved controlled exposures during the daylight period.
Accordingly we have relied on the E-R function derived for 12-hour and
3-month W126 indices, as described in section IV.A.1 above. To apply
these E-R functions to the W126 estimates derived using 24 hours-per-
day index values would inaccurately represent the response observed in
the study (producing an overestimate). Similarly, with regard to the 3-
month duration, ``[d]espite the possibility that plants may be exposed
to ambient O3 longer than 3 months in some locations, there
is generally a lack of exposure experiments conducted for longer than

3 months'' (U.S. EPA, 2014c, p. 9-112). Thus, in consideration of the
lack of support in the current evidence for characterizing exposure for
purposes of estimating RBL based on cumulative exposures derived from a
combination of daytime and nighttime exposures and consideration of
year-round O3 concentrations across the U.S., we disagree
with the commenters' view of the appropriateness of using an exposure
index based on 24-hour, year-round O3 concentrations.
    The commenters supporting the use of the W126 exposure index were
divided with regard to whether the EPA should focus on an annual index
or one averaged over three years. Some of the commenters indicating
support for the EPA's proposed focus on a 3-year average W126 index
stated that this was appropriate in light of the wide variations in
W126 index values that can occur on a year-to-year basis as a result of
the natural variation of climatic conditions that have a direct impact
on O3 formation; in their view, these factors are mitigated
by use of a 3-yr average, which thus provides ``stability'' in the
assessment dampening out the natural variation of climatic conditions
that have a direct impact on O3 formation. Others noted that
use of a 3-year average may be supported as matter of policy. We
generally concur with the relevance of these points, among others, to a
focus on the 3-year average W126. Other commenters expressed the view
that the EPA should focus on an annual W126 index, generally making
these comments in the context of expressing their support for a
secondary standard with a W126 form. These commenters variously cited
CASAC advice and its rationale for preferring a single year W126 form,
stated that vegetation damage occurs on an annual basis, and/or
questioned the EPA's statements of greater confidence in conclusions as
to O3 impacts based on a 3-year average exposure metric.
    The EPA agrees with commenters that, as discussed in the PA and the
proposal, depending on the exposure conditions, O3 can
contribute to measurable effects on vegetation in a single year. We
additionally recognize that, as described in the PA and proposal, there
is generally a greater significance for effects associated with
multiple-year exposures. The proposal described a number of
considerations raised in the PA as influencing the Administrator's
decision to focus on a 3-year average W126 index (79 FR 75347, December
17, 2014). These included, among others, the observation of a greater
significance for effects associated with multiple-year exposures, and
the uncertainties associated with consideration of annual effects
relative to multiple-year effects.
    Further, we note that among the judgments contributing to the
Administrator's decision on the level of protection appropriate for the
secondary standard are judgments regarding the weight to place on the
evidence of specific vegetation-related effects estimated to result
across a range of cumulative seasonal concentration-weighted
O3 exposures and judgments on the extent to which such
effects in such areas may be considered adverse to public welfare (79
FR 75312, December 17, 2014). Thus, conclusions regarding the extent to
which the size and/or prevalence of effects on vegetation in a single
year and any ramifications for future years represent an adverse effect
to the public welfare, conclusions that are also inherently linked to
overall magnitudes of exposures, are dependent on the Administrator's
judgment. Accordingly, the decision regarding the need to focus on a 1-
year or 3-year W126 index value is also a judgment of the
Administrator, informed by the evidence, staff evaluations and advice
from CASAC, as described in section IV.C.3 below.
d. Form and Averaging Time
    In considering comments received on the proposed form for the
revised standard, the EPA first notes the advice and comments from the
CASAC, received in its review of the second draft PA. Similar to its
advice in the last review, the CASAC recommended ``establishing a
revised form of the secondary standard to be the biologically relevant
W126 index'' (Frey, 2014c, p. iii). With regard to its reasons for this
view, the CASAC cites the PA in stating that it ``concurs with the
justification in [section 5.7] that the form of the standard should be
changed from the current 8-hr form to the cumulative W126 index''
(Frey, 2014c, p. 12). In addressing specific aspects of this index, the
CASAC concurred with the EPA's focus on the 3-month period with the
highest index value and further states that ``[a]ccumulation over the
08:00 a.m.-08:00 p.m. daytime 12-hour period is a scientifically
acceptable and recommended means of generalizing across latitudes and
seasons'' (Frey, 2014c, p. 13). As section 5.7 of the PA discusses the
W126 index in the context of the support in the evidence for use of the
W126 exposure index for assessing impacts of O3 on
vegetation and the extent of protection from such impacts, we interpret
CASAC's statement on this point to indicate that the basis for CASAC's
view with regard to the form for the secondary standard relates to the
appropriateness of the W126 exposure index for those assessment
purposes.\204\ \205\
---------------------------------------------------------------------------

    \204\ Section 5.7 of the PA states that ``the evidence continues
to provide a strong basis for concluding that it is appropriate to
judge impacts of O3 on vegetation, related effects and
services, and the level of public welfare protection achieved, using
a cumulative, seasonal exposure metric, such as the W126-based
metric,'' references the support of CASAC for a W126-based secondary
standard, and then concludes that ``based on the consistent and
well-established evidence described above, . . . the most
appropriate and biologically relevant way to relate O3
exposure to plant growth, and to determine what would be adequate
protection for public welfare effects attributable to the presence
of O3 in the ambient air, is to characterize exposures in
terms of a cumulative seasonal form, and in particular the W126
metric'' (U.S. EPA, 2014c, p. 5-78).
    \205\ The CASAC also mentioned its support for revising the
secondary standard to a W126 index-based form in its review of
Chapter 6 of the second draft PA (Frey, 2014c, p. 13). Similar to
section 5.7, in that chapter of the PA staff concluded that
``specific features associated with the W126 index still make it the
most appropriate and biologically relevant cumulative concentration-
weighted form for use in the context of the secondary O3 NAAQS
review'' (U.S. EPA, 2014c, p. 6-5) and also concluded that ``it is
appropriate to consider a revised secondary standard in terms of the
cumulative, seasonal, concentration-weighted form, the W126 index''
(U.S. EPA, 2014c, p. 6-57).
---------------------------------------------------------------------------

    The public comments on the form for a revised secondary standard
were divided. Most of the state and local environmental agencies or
governments, and all of the tribal agencies and organizations that
provided comments on the form for the secondary standard concurred with
the EPA's proposed decision, as did the industry commenters. These
commenters generally indicated agreement with the rationale provided in
the proposal that drew from the EPA analyses of recent air quality data
examining relationships at sites across the U.S. between values of the
fourth-high metric (the current design value) and values of a 3-year
average W126-based metric, stating that this analysis showed that a
standard in the form of the fourth-high metric, as proposed, can
provide air quality consistent with or below the range of 3-year W126
exposure index values identified in the proposal. Some commenters
additionally stated that the choice of form was a policy decision for
the EPA and that little or no additional protection of public welfare
would be gained by adopting a W126-based form. Some of these commenters
provided analyses of data for their state or region that further
supported this view. As

described in section IV.C.3 below, the EPA generally agrees with these
commenters.
    Some commenters, including a regional organization of state
agencies and two groups of environmental organizations, submitted
comments recommending revision of the standard to a cumulative,
seasonal form based on the W126 index. In support of their position,
these commenters generally cited CASAC advice, variously additionally
indicating their view that the standard form should be a metric
described as biologically relevant, and that the existing form, with a
level in the proposed range, would not provide adequate ecosystem
protection. Some commenters additionally suggested that the EPA cannot
lawfully retain the form and averaging time that were initially
established for purposes of the primary standard when the EPA has
identified the W126 index as a metric appropriate for judging
vegetation-related effects on public welfare. With regard to the EPA
air quality analyses, summarized in the proposal, of the W126 index
values at sites where O3 concentrations met different levels
of fourth-high metric, some of these commenters stated that the
analyses showed widespread variation in W126 values for each fourth-
high metric examined. Further, some commenters disagreed with the EPA
that the analyses indicated that a revised standard level within the
proposed range would be expected to limit W126 exposures in the future
to the extent suggested by the analyses of data from the past.
    We agree with public commenters and CASAC regarding the
appropriateness of the W126 index (the sum of hourly concentrations
over a specified period) as a biologically relevant metric for
assessing exposures of concern for vegetation-related public welfare
effects, as discussed in the proposal, PA and ISA. Accordingly, we
agree that this metric is appropriate for use in considering the
protection that might be expected to be afforded by potential
alternative secondary standards, as discussed in section IV.C.2.c
above. We disagree with commenters, however, that use of the W126
metric for this purpose dictates that we must establish a secondary
standard with a W126 index form.
    In support of this position, we note the common use, in assessments
conducted for NAAQS reviews, of exposure metrics that differ in a
variety of ways from the ambient air concentration metrics of those
standards.\206\ Across reviews for the various NAAQS pollutants, we
have used a variety of exposure metrics to evaluate the protection
afforded by the standards. These exposure metrics are based on the
health or welfare effects evidence for the specific pollutant and
commonly, in assessments for primary standards, on established
exposure-response relationships or health-based benchmarks (doses or
exposures of concern) for effects associated with specific exposure
circumstances. Some examples of exposure metrics used to evaluate
health impacts in primary standard reviews include the concentration of
lead in blood of young children and a 5-minute exposure concentration
for sulfur dioxide. In contrast, the health-based standards for these
two pollutants are the 3-month concentration of lead in total suspended
particles and the average across three years of the 99th percentile of
1-hour daily maximum concentration of sulfur dioxide in ambient air,
respectively (73 FR 66964, November 12, 2008; 75 FR 35520, June 22,
2010). In somewhat similar manner, in the 2012 PM review, the EPA
assessed the extent to which the existing 24-hour secondary standard
for PM2.5, expressed as a 24-hour concentration (of
PM2.5 mass per cubic meter of air) not to be exceeded more
than once per year on average over three years, could provide the
desired protection from effects on visibility in terms of the 90th
percentile, 24-hour average PM2.5 light extinction, averaged
over three years, based on speciated PM2.5 mass
concentrations and relative humidity data (79 FR 3086, January 15,
2013). Additionally, in the case of the screening-level risk analyses
in the 2008 review of the secondary standard for lead, concentrations
of lead in soil, surface water and sediment were evaluated to assess
the potential for welfare effects related to lead deposition from air,
while the standard is expressed in terms of the concentration of lead
in particles suspended in air (73 FR 67009, November 12, 2008).
---------------------------------------------------------------------------

    \206\ The term design value is commonly used to refer to the
metric for the standard. Consistent with the summary in section I.D
above, a design value is the statistic that describes the air
quality of a given location in terms of the indicator, form and
averaging time of the standard such that it can then be compared to
the level of the standard.
---------------------------------------------------------------------------

    Further, depending on the evidence base, some NAAQS reviews may
consider multiple exposure metrics in assessing risks associated with a
particular pollutant in ambient air in order to judge the adequacy of
an existing standard in providing the required level of protection. And
a standard with an averaging time of one duration may provide
protection against effects elicited by exposures of appreciably shorter
or longer durations. For example, in the current review of the primary
O3 standard, as described in section II above, we have
considered the potential for effects associated with both short- and
long-term exposures and concluded, based on a combination of air
quality and risk analyses and the health effects evidence, that the
existing standard with its short (8-hour) averaging time provides
control of both the long and short term exposures (e.g., from one hour
to months or years) that may be of concern to public health. Similarly,
during the 1996 review of the NO2 primary standard, while
health effects were recognized to result from both long-term and short-
term exposures to NO2, the primary standard, which was a
long-term (annual) standard, was concluded to provide the requisite
protection against both long- and short-term exposures (61 FR 52852,
Oct 8 1996). In the subsequent review of the NO2 primary
standard in which the available air quality information indicated that
the annual standard was not providing the needed control of the shorter
term exposures, an additional short-term standard was established (75
FR 6474, February 9, 2010).
    Thus, we note that different metrics may logically, reasonably, and
for technically sound reasons, be used in assessing exposures of
concern or characterizing risk as compared to the metric of the
standard which is used to control air quality to provide the desired
degree of protection. That is, exposure metrics are used to assess the
likely occurrence and/or frequency and extent of effects under
different air quality conditions, while the air quality standards are
intended to control air quality to the extent requisite to protect from
the occurrence of public health or welfare effects judged to be
adverse. In this review of the secondary standard for O3,
the EPA agrees that, for the reasons summarized in section IV.A.1 above
and described in the ISA, the W126 index--and not an 8-hour daily
maximum concentration that has relevance in human health risk
characterization, as described in section II above--is the appropriate
metric for assessing exposures of concern for vegetation,
characterizing risk to public welfare, and evaluating what air quality
conditions might provide the desired degree of public welfare
protection. We disagree, however, that the secondary standard must be
established using that same metric.
    Moreover, we note that the CAA does not require that the secondary
O3 standard be established in a specific

form. Section 109(b)(2) provides only that any secondary NAAQS ``shall
specify a level of air quality the attainment and maintenance of which
in the judgment of the Administrator, based on [the air quality]
criteria, is requisite to protect the public welfare from any known or
anticipated adverse effects associated with the presence of such air
pollutant in the ambient air. . . . [S]econdary standards may be
revised in the same manner as promulgated.'' The EPA interprets this
provision to leave it considerable discretion to determine whether a
particular form is appropriate, in combination with the other aspects
of the standard (averaging time, level and indicator), for specifying
the air quality that provides the requisite protection, and to
determine whether, once a standard has been established in a particular
form, that form must be revised. Moreover, nothing in the Act or the
relevant case law precludes the EPA from establishing a secondary
standard equivalent to the primary standard in some or all respects, as
long as the Agency has engaged in reasoned decision-making.\207\
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    \207\ In fact, the D.C. Circuit has upheld secondary NAAQS that
were identical to the corresponding primary standard for the
pollutant (e.g., ATA III, 283 F.3d at 375, 380 [D.C. Cir. 2002,
upholding secondary standards for PM2.5 and O3
that were identical to primary standards]).
---------------------------------------------------------------------------

    With regard to the commenter's emphasis on advice from CASAC on the
form of the secondary standard, the EPA agrees with the importance of
giving such advice careful consideration. The EPA further notes,
however, that the Administrator is not legally precluded from departing
from CASAC's recommendations, when she has provided an explanation of
the reasons for such differences.\208\ Accordingly, in reaching
conclusions on the revised secondary standard in this review, the
Administrator has given careful consideration to the CASAC advice in
this review and, when she has differed from CASAC recommendations, she
has fully explained the reasons and judgments that led her to a
different conclusion, as described in section IV.C.3 below.
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    \208\ See CAA sections 307(d)(3) and 307(d)(6)(A); see also
Mississippi v. EPA, 744 F.3d 1334, 1354 (D.C. Cir. 2013) (``Although
EPA is not bound by CASAC's recommendations, it must fully explain
its reasons for any departure from them'').
---------------------------------------------------------------------------

    In disagreeing with the EPA's conclusions drawn from analyses of
recent air quality data on the extent to which cumulative seasonal
exposures might be limited to within or below the identified 3-year
average W126 index values by controlling air quality using different
values for the fourth-high metric, one group of environmental
organizations emphasized the range of W126 index values that occur at
monitors with concentrations at or below specific values for the
fourth-high metric. For monitor observations for which the fourth-high
metric was at or below 70 ppb, this commenter group stated that some
sites have 3-year average W126 index values above 17 ppm-hrs and noted
a maximum 3-year W126 index value of 19.1 ppm-hrs, while additionally
noting occurrences of other W126 values above the CASAC range of 7 to
15 ppm-hrs. This commenter additionally stated that the air quality
data ``do not support a claim of congruence'' between the fourth-high
and W126 metrics (e.g., ALA et al., p. 196), that there is no basis for
concluding that there is some fundamental underlying relationship that
assures meeting the fourth-high metric will mean meeting any of the
W126 options, and that the relationship between the metrics is non-
linear with significant spread in the data (citing visual inspection of
a graph).
    The EPA does not agree with the commenter's statements regarding
the relationship between the two metrics.\209\ We have not, as stated
by the commenter, claimed there to be ``congruence'' between the two
metrics (e.g., ALA et al., p. 196), or that the two metrics coincide
exactly. Rather, at any location, values of both metrics are a
reflection of the temporal distribution of hourly O3
concentrations across the year and both vary in response to changes in
that distribution. While the EPA's air quality analysis shows that the
specific relationship differs among individual sites, it documents an
overall strong, positive, non-linear relationship between the two
metrics (Wells, 2014a, p. 6, Figures 5a and 5b; Wells, 2015b). Further,
this analysis finds the amount of year-to-year variability in the two
metrics tended to decrease over time with decreasing O3
concentrations, especially for the W126 metric, as described in section
IV.E.4 of the proposal (Wells, 2014a; Wells, 2015b).
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    \209\ The EPA additionally notes that commenters contradict
their own assertion when, after stating their view that no
relationship exists between the 4th high and W126 metrics, the
commenter then states that there is a nonlinear relationship and yet
then relies on a predicted linear relationship to estimate W126
values occurring when air quality meets different values for the 4th
high metric at 11 national parks.
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    With regard to the highest 3-year average W126 exposure index
values that might reasonably be expected in the future in areas where a
revised standard with a fourth-high form is met, we disagree with the
commenters as to the significance of the W126 index value of 19.1 ppm-
hrs in the 13-year dataset. This value, for a site during the period
2006-2008, is the only occurrence at or above 19 ppm-hrs in the nearly
4000 3-year W126 index values--across the 11 3-year periods extending
back in time from 2013--for which the fourth-high metric for the same
monitor location is at or below 70 ppb. This is clearly an isolated
occurrence.
    In considering this comment, we have expanded the technical
memorandum that was available at the time of proposal (Wells, 2014a).
The expanded memorandum describes the same air quality analyses for 3-
year periods from 2001 through 2013 as the 2014 memorandum, and
includes additional summary tables for all 3-year periods from 2001
through 2013 as well as tables for the most recent period, 2011-2013
(Wells, 2015b). After the 3-year W126 index value of 19 ppm-hrs, the
next three highest 3-year average W126 index values, which are the only
other such values above 17 ppm-hrs in the 13-year dataset, and which
also occur during periods in the past, round to 18 ppm-hrs (Wells,
2015b). Additionally, we note that reductions in the fourth-high metric
over the 13-year period analyzed are strongly associated with
reductions in the cumulative W126 index (Wells, 2014a, Figure 11, Table
6; Wells, 2015b). Specifically, the regression analysis of changes in
W126 index between the 2001-2003 period and the 2011-2013 period with
changes in the fourth-high metric across the same periods indicates a
fairly linear and positive relationship between reductions of the two
types of metrics, with, on average, a change of approximately 0.7 ppm-
hr in the W126 index per ppb change in the fourth-high metric value.
From this information we conclude that W126 exposures above 17 ppm-hrs
at sites for which the fourth-high metric is at or below 70 ppb would
be expected to continue to be rare in the future, particularly as steps
are taken to meet a 70 ppb standard.
    With regard to the comment that the relationship between the two
metrics varies across locations, the EPA agrees that there is variation
in cumulative seasonal O3 exposure (in terms of a 3-year
average W126 index) among locations that are at or below the same
fourth-high metric. As noted in the proposal, the analysis illustrates
this variation, with the locations in the West and Southwest NOAA
climatic regions tending to have the highest cumulative seasonal
exposures for the same fourth-high metric value. In considering
expectations for the future in light of this observation, however, we
note that

the regional regressions of reductions in W126 metric with reductions
in the fourth-high metric indicate that the Southwest and West regions,
which had the greatest potential for sites having 3-year W126 index
values greater than the various W126 values of interest when fourth-
high values are less than or equal to the various fourth-high metric
values of interest, also exhibited the greatest reduction in the W126
index values per unit reduction in the fourth-high values (Wells,
2015b). Thus, in considering the potential for occurrences of values
above 17 ppm-hrs in the future in areas that meet a fourth-high of 70
ppb, the EPA notes that the analysis indicates that those areas that
exhibited the greatest likelihood of occurrence of a 3-year W126 index
above a level of interest (e.g., the commenters' example in the
Southwest region of a value of 19.1 ppm-hrs [2006-2008] in comparison
to the W126 level of 17 ppm-hrs) also exhibit the greatest improvement
in W126 per unit decrease in fourth-high metric.\210\ It is expected
that future control programs designed to meet a standard with a fourth-
high form would provide similar improvements in terms of the W126
metric.
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    \210\ Additionally, O3 levels at any location are
influenced by upwind precursor emissions, and many rural areas,
including the site referenced by the commenter, are impacted by
precursor emissions from upwind urban areas, such that as emissions
are reduced to meet a revised standard in the upwind locations,
reductions in those upwind emissions will contribute to reductions
at the downwind sites (Wells, 2014a; ISA, pp. 3-129 to 3-133).
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    As part of their rationale in support of revising the current form
and averaging time, one commenter pointed to the regional variation in
the highest W126 index values expected at sites that just meet a
fourth-high metric of 70 ppb, based on the EPA's analysis of recent air
quality data available at the time of the proposal (Wells, 2014a). This
commenter observed that, while in some U.S. regions, locations that
meet a potential alternative standard with the current form and a level
of 70 ppb also have 3-year average W126 index values no higher than 17
ppm-hrs, the highest W126 index values in other parts of the country
are lower. As a result, the commenter concluded that such a standard
would result in regionally differing levels of welfare protection. The
commenter additionally states that, for extreme values, a W126 form for
the secondary standard would also offer different levels of protection,
although with the primary standard setting the upper boundary for such
values.
    The EPA recognizes that a standard with the current form might be
expected to result in regionally differing distributions of W126
exposure index values (including different maximum values) depending on
precursor sources, local meteorology, and patterns of O3
formation. Variation in exposures is to be expected with any standard
(secondary or primary) of any form. In fact, variation in exposures and
any associated variation in welfare or health risk is generally an
inherent aspect of the Administrator's judgment on a specific standard,
and any associated variation in welfare or health protection may play a
role in the Administrator's judgment with regard to public welfare or
public health protection objectives for a national standard. In
considering the comment, however, we have focused only on the extent to
which the commenter's conclusion that a secondary standard of the
current form and averaging time would provide regionally varying
welfare protection might indicate that the specified air quality is
more (or less) than necessary to achieve the purposes of the standard.
In so doing, we additionally respond to a separate comment that the EPA
needs to address how the revised secondary standard is neither more or
less than necessary to protect the public welfare.
    The CAA requirement in establishing a standard is that it be set at
a level of air quality that is requisite, meaning ``sufficient, but not
more than necessary'' (Whitman v. American Trucking Ass'ns, 531 U.S.
457, 473 [2001]). We note that the air quality that is specified by the
revised primary standard has been concluded to be ``necessary'' and it
may be reasonable and appropriate to consider the stringency of the
secondary standard in light of what is identified as ``necessary'' for
the primary standard. The EPA considered the stringency of the
O3 secondary standard in this way in the 1979 decision (44
FR 8211, February 8, 1979), which was upheld in subsequent litigation
(API v Costle, 665 F.2d 1176 [D.C. Cir. 1991]). We note that, in
similar manner, the commenter considered public welfare protection that
might be afforded by the primary standard in noting that the primary
standard would be expected to provide welfare protection from extreme
values.\211\
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    \211\ As described earlier in this section, the EPA has also
considered the air quality specified by one secondary standard in a
decision on the need for a second secondary standard. In the
decision not to adopt a second PM2.5 secondary standard
specific to visibility-related welfare effects, the Administrator,
after describing the public welfare protection objective related to
visibility effects, considered analyses that related air quality
associated with the existing secondary standard to that expected for
the proposed visibility-focused secondary standard. From these
analyses, she concluded sufficient protection against visibility
effects would be provided by the existing standard, and to the
extent that the existing standard would provide more protection than
had been her objective for such effects, adoption of a second
secondary standard focused on visibility would not change that
result (78 FR 3227-3228, January 15, 2013). This decision responded
to a court remand of the prior EPA decision that visibility
protection would be afforded by a secondary standard set equal to
the primary standard based on the court's conclusion that the EPA
had not adequately described the Administrator's objectives for
visibility-related public welfare protection under the standard
(American Farm Bureau, 559 F.3d at 530-531).
---------------------------------------------------------------------------

    In addressing the remand of the 2008 secondary standard in this
rulemaking, as discussed in section IV.C.2.e below, the EPA recognizes
that it must explain the basis for concluding that the standard
selected by the Administrator specifies air quality that will provide
the degree of public welfare protection needed from the secondary
standard (Mississippi v. EPA, 744 F.3d 1334, 1360-61 [D.C. Cir. 2013]).
In this review, the Administrator describes the degree or level of
public welfare protection needed from the secondary standard and fully
explains the basis for concluding that the standard selected specifies
air quality that will provide that degree of protection. If the
Administrator concludes that the level of air quality specified by the
primary standard would provide sufficient protection against known or
anticipated adverse public welfare effects, the EPA believes that a
secondary standard with that indicator, level, form and averaging time
could be considered to be requisite. If the level of air quality that
areas will need to achieve or maintain for purposes of the primary
standard also provides a level of air quality that is adequate to
provide the level of protection identified for the secondary standard,
there would be little purpose in requiring the EPA to establish a less
stringent secondary standard. For these reasons, the expectation of
regionally differing cumulative exposures under a secondary standard of
the current form and averaging time does not lead us to conclude that
the air quality specified by such a standard would be more (or less)
than necessary (and thus not requisite) for the desired level of public
welfare protection.
e. Revisions to the Standard Level
    Some comments specifically addressed the level for a revised
secondary standard of the current form and averaging time. Of the
comments that addressed this, some from states or industry groups
generally supported a level within the proposed range, frequently
specifying the upper end of the range (70 ppb), while comments

from tribes and tribal organizations, and a few others, recommended a
level no higher than 65 ppb. The Administrator has considered such
comments in reaching her decision on the appropriate revisions to the
standard, described in section IV.C.3. Detailed aspects of these
comments are discussed in the Response to Comments document.
f. 2013 Court Remand and Levels of Protection
    Both industry groups and a group of environmental advocacy
organizations submitted comments on the extent to which the proposal
addressed the July 2013 remand of the secondary standard by the U.S.
Court of Appeals for the D.C. Circuit. The former generally concluded
that the proposal had adequately addressed the remand, while the latter
expressed the view that the EPA had failed to comply with the court's
remand because it had failed to identify the target levels of
vegetation protection for which the proposed range of standards would
provide the requisite protection, claiming that the identified W126
index range of 13-17 ppm-hrs was not based on a proposed level of
protection against biomass loss, carbon storage loss, or foliar injury
that the EPA had identified as requisite for public welfare.
    We agree with the comments that state that we have addressed the
court's remand. More specifically, with this rulemaking, including
today's decision and the Administrator's conclusions described in
section IV.C.3 below, the EPA has fully addressed the remand of the
2008 secondary O3 standard. In Mississippi v. EPA, the D.C.
Circuit remanded the 2008 secondary O3 standard to the EPA
for reconsideration because it had not adequately explained why that
standard provided the requisite public welfare protection. 744 F.3d
1334, 1360-61 (D.C. Cir. 2013). In doing so, the court relied on the
language of CAA section 109(b)(2), and the court's prior decision,
American Farm Bureau Federation v. EPA, 559 F.3d 512, 528-32 (D.C. Cir.
2009), which came to the same conclusion for the 2006 secondary
PM2.5 standard. Both decisions recognize that the plain
language of section 109(b)(2) requires the EPA to ``specify a level of
air quality the maintenance of which . . . is requisite to protect the
public welfare from any known or anticipated adverse effects''
(Mississippi, 744 F.3d at 1360 [citing American Farm Bureau, 559 F.3d
at 530]). Further, explaining that it was insufficient for the EPA
``merely to compare the level of protection afforded by the primary
standard to possible secondary standards and to find the two roughly
equivalent'' (Mississippi, 744 F.3d at 1360), the court rejected the
EPA's justification for setting the secondary standard equivalent to
the primary standard because that justification was based on comparing
the protection from the primary standard to that expected from one
possible standard with a cumulative, seasonal form (21 ppm-hrs) without
stating that such a cumulative seasonal standard would be requisite to
protect welfare or explaining why that would be so. Because the EPA had
``failed to determine what level of protection was `requisite to
protect the public welfare'' (Mississippi, 744 F.3d at 1362), the court
found that the EPA's rationale failed to satisfy the requirements of
the Act.
    Today's rulemaking both satisfies the requirements of section
109(b)(2) of the Act and addresses the issues raised in the court's
remand. In this rulemaking, the Administrator has established a revised
secondary standard that replaces the remanded 2008 secondary standard.
In so doing, based on her consideration of the currently available
evidence and quantitative exposure and air quality information, as well
as advice from CASAC and input from public comments, the Administrator
has described the requisite public welfare protection for the secondary
standard and explained how the standard selected specifies air quality
that will provide that protection. As explained in detail in IV.C.3
below, in this review the Administrator is describing the public
welfare protection she finds requisite in terms of seedling RBL in the
median species, which serves as a surrogate for a broader array of
O3 effects at the plant and ecosystem levels. This
description of the desired protection sufficiently articulates the
standard that the Administrator is using to evaluate welfare
protection. Further, the Administrator has considered air quality
analyses in determining how to achieve the air quality conditions
associated with the desired protection. Based on these analyses, the
Administrator is determining that revising the level of the secondary
standard to 70 ppb, while retaining the current form, averaging time,
and indicator, specifies a level of air quality that will provide the
requisite public welfare protection.
    To the extent the comments suggest that the EPA is required in
establishing a standard to identify a precise and quantified level of
public welfare protection that is requisite with respect to every
potentially adverse public welfare impact (e.g., visible foliar injury,
crop yield loss) that is considered in establishing the standard, we
disagree. While the D.C. Circuit has required the EPA to
``qualitatively describe the standard governing its selection of
particular NAAQS,'' it has expressly ``rejected the notion that the
Agency must establish a measure of the risk to safety it considers
adequate to protect public health every time it establishes a NAAQS''
(ATA III, 283 F.3d at 369 [internal marks and citations omitted]). That
is, the EPA must ``engage in reasoned decision-making,'' but is not
required to ``definitively identify pollutant levels below which risks