noted the occurrence of exposures for which the associated median
estimates of growth effects across the species with E-R functions
extend above a magnitude considered to be ``unacceptably high'' by the
CASAC.\112\ This analysis estimated cumulative exposures, in terms of
3-year average W126 index values, at and above 19 ppm-hrs, occurring
under the then-current standard for nearly a dozen areas, distributed
across two NOAA climatic regions of the U.S. (80 FR 65385-86, October
26, 2015). The Administrator gave particular weight to this analysis
because of its focus on exposures in Class I areas, which are lands
that Congress set aside for specific uses 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. This 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, was consistent with a similar emphasis in the 2008 review of the
standard (73 FR 16485, March 27, 2008). The Administrator additionally
recognized 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 (80 FR 65390, October 26, 2015).
---------------------------------------------------------------------------

    \110\ Areas designated as Class I include all international
parks, national wilderness areas which exceed 5,000 acres in size,
national memorial parks which exceed 5,000 acres in size, and
national parks which exceed 6,000 acres in size, provided the park
or wilderness area was in existence on August 7, 1977. Other areas
may also be Class I if designated as Class I consistent with the
CAA.
    \111\ In specifically evaluating exposure levels in terms of the
W126 index as to potential for impacts on vegetation, the
Administrator focused on the median RBL estimate across the eleven
tree species for which robust established E-R functions were
available. The presentation of these E-R functions for growth
effects on tree seedlings (and crops) included estimates of RBL (and
relative yield loss [RYL]) at a range of W126-based exposure levels
(2014 PA, Tables 5C-1 and 5C-2). The median tree species RBL or crop
RYL was presented for each W126 level (2014 PA, Table 5C-3; 80 FR
65391 [Table 4], October 26, 2015). The Administrator focused on RBL
as a surrogate or proxy for the broader array of vegetation-related
effects of potential public welfare significance, which include
effects on growth of individual sensitive species and extend to
ecosystem-level effects, such as community composition in natural
forests, particularly in protected public lands, as well as forest
productivity (80 FR 65406, October 26, 2015).
    \112\ In the CASAC's consideration of RBL estimates presented in
the 2014 draft PA, it characterized an estimate of 6% RBL in the
median studied species as being ``unacceptably high,'' (Frey,
2014b).
---------------------------------------------------------------------------

    As noted across past reviews of O3 secondary standards,
the Administrator's judgments regarding effects that are adverse to
public welfare consider the intended use of the ecological receptors,
resources and ecosystems affected (80 FR 65389, October 26, 2015; 73 FR
16496, March 27, 2008). Thus, in the 2015 review, the Administrator
utilized the median RBL estimate for the studied species as a
quantitative tool within a larger framework of considerations
pertaining to the public welfare significance of O3 effects.
She recognized such considerations to include effects that are
associated with effects on growth and that the 2013 ISA determined to
be causally or likely causally related to O3 in ambient air,
yet for which there are greater uncertainties affecting estimates of
impacts on public welfare. These other effects included reduced
productivity in terrestrial ecosystems, reduced carbon sequestration in
terrestrial ecosystems, alteration of terrestrial community
composition, alteration of below-ground biogeochemical cycles, and
alteration of terrestrial ecosystem water cycles. Thus, in giving
attention to the 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 the field and in
mature trees, was also mindful of related, broader, ecosystem-level
effects for which the available tools for quantitative estimates are
more uncertain and those for which the policy foundation for
consideration of public welfare impacts is less well established. As a
result, the Administrator considered tree growth effects of
O3, in terms of RBL ``as a surrogate for the broader array
of O3 effects at the plant and ecosystem levels'' (80 FR
65389, October 26, 2015).
    Based on all of these considerations, and taking into consideration
CASAC advice and public comment, the Administrator concluded that the
protection afforded by the then-current standard was not sufficient and
that the standard needed 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, and also in other areas set aside by
states, tribes and public interest groups to provide similar benefits
to the public welfare for residents on those lands, as well as for
visitors to those areas. In so doing, she further noted that a revised
standard would provide increased protection for other growth-related
effects, including relative yield loss (RYL) of crops, reduced carbon
storage, and types of effects for which it is more difficult to
determine public welfare significance, as well as other welfare effects
of O3, such as visible foliar injury (80 FR 65390, October
26, 2015).
    Consistent with the approach employed for considering the adequacy
of the then-current secondary standard, the approach for considering
revisions

that would result in a standard providing the requisite protection
under the Act also focused on growth-related effects of O3,
using RBL as a surrogate for the broader array of vegetation-related
effects and included judgments on the magnitude of such effects that
would contribute to public welfare impacts of concern. In considering
the adequacy of potential alternative standards to provide protection
from such effects, the approach also focused on considering the
cumulative seasonal O3 exposures likely to occur with
different alternative standards.
    In light of the judicial remand of the 2008 secondary O3
standard referenced above, the 2015 decision on selection of a revised
secondary standard first considered the available evidence and
quantitative analyses in the context of an approach for considering and
identifying public welfare objectives for such a standard (80 FR 65403-
65408, October 26, 2015). In light of the extensive evidence base of
O3 effects on vegetation and associated terrestrial
ecosystems, the Administrator focused on protection against adverse
public welfare effects of O3-related effects on vegetation,
giving particular attention to such effects in natural ecosystems, such
as those in areas with protection designated by Congress for current
and future generations, as well as areas similarly set aside by states,
tribes and public interest groups with the intention of providing
similar benefits to the public welfare. The Administrator additionally
recognized that providing protection for this purpose will also provide
a level of protection for other vegetation that is used by the public
and potentially affected by O3 including timber, produce
grown for consumption and horticultural plants used for landscaping (80
FR 65403, October 26, 2015).
    As mentioned above, the Administrator considered the use of a
cumulative seasonal exposure index (the W126 index) for purposes of
assessing potential public welfare risks, and similarly, for assessing
potential protection achieved against such risks on a national scale.
In consideration of conclusions of the 2013 ISA and 2014 PA, as well as
advice from the CASAC and public comments, this W126 index was defined
as a maximum, seasonal (3-month), 12-hour index (80 FR 65404, October
26, 2015).\113\ While recognizing that no one definition of an exposure
metric used for the assessment of protection for multiple effects at a
national scale will be exactly tailored to every species or each
vegetation type, ecosystem and region of the country, the Administrator
judged that on balance, a W126 index derived in this way, and averaged
over three years would be appropriate for such purposes (80 FR 65403,
October 26, 2015).
---------------------------------------------------------------------------

    \113\ As also described in section III.B.3.a below, this index
is defined by the 3-consecutive-month period within the
O3 season with the maximum sum of W126-weighted hourly
O3 concentrations during the period from 8:00 a.m. to
8:00 p.m. each day.
---------------------------------------------------------------------------

    Based on a number of considerations, the Administrator recognized
greater confidence in judgments related to public welfare impacts based
on a 3-year average metric than a single-year metric, and consequently
concluded it to be appropriate to use a seasonal W126 index averaged
across three years for judging public welfare protection afforded by a
revised secondary standard (80 FR 65404, October 26, 2015). For
example, the Administrator was mindful of both the strengths and
limitations of the evidence and of the information on which to base her
judgments with regard to adversity of effects on the public
welfare.\114\ While the Administrator recognized the scientific
information and interpretations, as well as CASAC advice, with regard
to a single-year exposure index, she also took note of uncertainties
associated with judging the degree of vegetation impacts for single-
year effects that would be adverse to public welfare. The Administrator
was also mindful of the variability in ambient air O3
concentrations from year to year, as well as year-to-year variability
in environmental factors, including rainfall and other meteorological
factors, that influence the occurrence and magnitude of O3-
related effects in any year, and contribute uncertainties to
interpretation of the potential for harm to public welfare over the
longer term (80 FR 65404, October 26, 2015).
---------------------------------------------------------------------------

    \114\ In this regard, she recognized uncertainties associated
with interpretation of the public welfare significance of effects
resulting from a single-year exposure, and that the public welfare
significance of effects associated with multiple years of critical
exposures are potentially greater than those associated with a
single year of such exposure. The Administrator concluded that use
of a 3-year average metric could address the potential for adverse
effects to public welfare that may relate to shorter exposure
periods, including a single year (80 FR 65404, October 26, 2015).
---------------------------------------------------------------------------

    In reaching a conclusion on the amount of public welfare protection
from the presence of O3 in ambient air that is appropriate
to be afforded by a revised secondary standard, the Administrator gave
particular consideration to the following: (1) The nature and degree of
effects of O3 on vegetation, including her judgments as to
what constitutes an adverse effect to the public welfare; (2) the
strengths and limitations of the available and relevant information;
(3) comments from the public on the Administrator's proposed decision,
including comments related to identification of a target level of
protection; and (4) the CASAC's views regarding the strength of the
evidence and its adequacy to inform judgments on public welfare
protection. The Administrator recognized that such judgments should
neither overstate nor understate the strengths and limitations of the
evidence and information nor the appropriate inferences to be drawn as
to risks to public welfare, and that the choice of the appropriate
level of protection is a public welfare policy judgment entrusted to
the Administrator under the CAA taking into account both the available
evidence and the uncertainties (80 FR 65404-05, October 26, 2015).\115\
---------------------------------------------------------------------------

    \115\ The CAA does not require that a secondary standard be
protective of all effects associated with a pollutant in the ambient
air but rather those known or anticipated effects judged ``adverse
to the public welfare'' (CAA section 109).
---------------------------------------------------------------------------

    With regard to the extensive evidence of welfare effects of
O3, including visible foliar injury and crop RYL, the
information available for tree species was judged to be more useful in
informing judgments regarding the nature and severity of effects
associated with different air quality conditions and associated public
welfare significance. Accordingly, the Administrator gave particular
attention to the effects related to native tree growth and
productivity, including forest and forest community composition,
recognizing the relationship of tree growth and productivity to a range
of ecosystem services, (80 FR 65405-06, October 26, 2015). In making
this judgment, the Administrator recognized that among the broad array
of O3-induced vegetation effects were the occurrence of
visible foliar injury and growth and/or yield loss in O3-
sensitive species, including crops and other commercial species (80 FR
65405, October 26, 2015). In regard to visible foliar injury, the
Administrator recognized the potential for this effect to affect the
public welfare in the context of affecting value ascribed to natural
forests, particularly those afforded special government protection,
with the significance of O3-induced visible foliar injury
depending on the extent and severity of the injury (80 FR 65407,
October 26, 2015). In so doing, however, the Administrator also took
note of limitations in the available visible foliar injury information,
including the lack of established E-R functions that would allow
prediction of

visible foliar injury severity and incidence under varying air quality
and environmental conditions, a lack of consistent quantitative
relationships linking visible foliar injury with other O3-
induced vegetation effects, such as growth or related ecosystem
effects, and a lack of established criteria or objectives that might
inform consideration of potential public welfare impacts related to
this vegetation effect (80 FR 65407, October 26, 2015). Similarly,
while O3-related growth effects on agricultural and
commodity crops had been extensively studied and robust E-R functions
developed for a number of species, the Administrator found this
information less useful in informing her judgments regarding an
appropriate level of public welfare protection (80 FR 65405, October
26, 2015).\116\
---------------------------------------------------------------------------

    \116\ With respect to commercial production of commodities, the
Administrator noted that judgments about the extent to which
O3-related effects on commercially managed vegetation are
adverse from a public welfare perspective are particularly difficult
to reach, given that the extensive management of such vegetation
(which, as the CASAC noted, may reduce yield variability) may also
to some degree mitigate potential O3-related effects. The
management practices used on such vegetation are highly variable and
are designed to achieve optimal yields, taking into consideration
various environmental conditions. In addition, changes in yield of
commercial crops and commercial commodities, such as timber, may
affect producers and consumers differently, further complicating the
question of assessing overall public welfare impacts (80 FR 65405,
October 26, 2015).
---------------------------------------------------------------------------

    Thus, and in light of the extensive evidence base in this regard,
the Administrator focused on trees and associated ecosystems in
identifying the appropriate level of protection for the secondary
standard. Accordingly, the Administrator found the estimates of tree
seedling growth impacts (in terms of RBL) associated with a range of
W126-based index values developed from the E-R functions for 11 tree
species (referenced above) to be appropriate and useful for considering
the appropriate public welfare protection objective for a revised
standard (80 FR 65391-92, Table 4, October 26, 2015). The Administrator
also incorporated into her considerations the broader evidence base
associated with forest tree seedling biomass loss, including other less
quantifiable effects of potentially greater public welfare
significance. That is, in drawing on these RBL estimates, the
Administrator recognized she was not simply making judgments about a
specific magnitude of growth effect in seedlings that would be
acceptable or unacceptable in the natural environment. Rather, though
mindful of associated uncertainties, the Administrator used the RBL
estimates as a surrogate or proxy for consideration of the broader
array of related vegetation and ecosystem effects of potential public
welfare significance that include effects on growth of individual
sensitive species and extend to ecosystem-level effects, such as
community composition in natural forests, particularly in protected
public lands, as well as forest productivity (80 FR 65406, October 26,
2015). This broader array of vegetation-related effects included those
for which public welfare implications are more significant but for
which the tools for quantitative estimates were more uncertain.
    In using the RBL estimates as a proxy, and in consideration of
CASAC advice; strengths, limitations and uncertainties in the evidence;
and the linkages of growth effects to larger population, community and
ecosystem impacts, the Administrator considered it appropriate to focus
on a standard that would generally limit cumulative exposures to those
for which the median RBL estimate for seedlings of the 11 species with
robust and established E-R functions would be somewhat below 6% (80 FR
65406-07, October 26, 2015). In focusing on cumulative exposures
associated with a median RBL estimate somewhat below 6%, the
Administrator considered the relationships between W126-based exposure
and RBL in the studied species (presented in the final PA and proposal
document), noting that the median RBL estimate was 6% for a cumulative
seasonal W126 exposure index of 19 ppm-hrs (80 FR 65391-92, Table 4,
October 26, 2015).\117\ Given the information on median RBL at
different W126 exposure levels, using a 3-year cumulative exposure
index for assessing vegetation effects, the potential for single-season
effects of concern, and CASAC comments on the appropriateness of a
lower value for a 3-year average W126 index, the Administrator
concluded it was appropriate to identify a standard that would restrict
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances (80 FR 65407, October 26,
2015). Based on such information, available at that time, to inform
consideration of vegetation effects and their potential adversity to
public welfare, the Administrator additionally judged that the RBL
estimates associated with marginally higher exposures in isolated, rare
instances are not indicative of effects that would be adverse to the
public welfare, particularly in light of variability in the array of
environmental factors that can influence O3 effects in
different systems and uncertainties associated with estimates of
effects associated with this magnitude of cumulative exposure in the
natural environment (80 FR 65407, October 26, 2015).
---------------------------------------------------------------------------

    \117\ When stated to the first decimal place, the median RBL was
6.0% for a cumulative seasonal W126 exposure index of 19 ppm-hrs.
For 18 ppm-hrs, the median RBL estimate was 5.7%, which rounds to
6%, and for 17 ppm-hrs, the median RBL estimate was 5.3%, which
rounds to 5% (80 FR 65407, October 26, 2015).
---------------------------------------------------------------------------

    The Administrator's decisions regarding the revisions to the then-
current standard that would appropriately achieve these public welfare
protection objectives were based on extensive air quality analyses that
extended from the then most recently available data (monitoring year
2013) back more than a decade (80 FR 65408, October 26, 2015; Wells,
2015). These analyses evaluated the cumulative seasonal exposure levels
in locations meeting different alternative levels for a standard of the
existing form and averaging time, indicating reductions in cumulative
exposures associated with air quality meeting lower levels of a
standard of the existing form and averaging time. Based on these
analyses, the Administrator judged that the desired level of public
welfare protection could be achieved with a secondary standard having a
revised level in combination with the existing form and averaging time
(80 FR 65408, October 26, 2015).
    The air quality analyses described the occurrences of 3-year W126
index values of various magnitudes at monitor locations where
O3 concentrations met potential alternative standards; the
alternative standards were different levels for the current form and
averaging time (annual fourth-highest daily maximum 8-hour average
concentration, averaged over three consecutive years) (Wells, 2015). In
the then-most recent period, 2011-2013, across the more than 800
monitor locations meeting the then-current standard (with a level of 75
ppb), the 3-year W126 index values were above 17 ppm-hrs in 25 sites
distributed across different NOAA climatic regions, and above 19 ppm-
hrs at nearly half of these sites, with some well above. In comparison,
among sites meeting an alternative standard of 70 ppb, there were no
occurrences of a W126 value above 17 ppm-hrs and fewer than a handful
of occurrences that equaled 17 ppm-hrs.\118\ For the longer

time period (extending back to 2001), among the nearly 4000 instances
where a monitoring site met a standard level of 70 ppb, the
Administrator noted that there was only ``a handful of isolated
occurrences'' of 3-year W126 index values above 17 ppm-hrs, ``all but
one of which were below 19 ppm-hrs'' (80 FR 65409, October 26, 2015).
The Administrator concluded that that single value of 19.1 ppm-hrs
(just equaling 19, when rounded), observed at a monitor for the 3-year
period of 2006-2008, was reasonably regarded as an extremely rare and
isolated occurrence, and, as such, it was unclear whether it would
recur, particularly as areas across the U.S. took further steps to
reduce O3 to meet revised primary and secondary standards.
Further, based on all of the then available information, as noted
above, the Administrator did not judge RBL estimates associated with
marginally higher exposures in isolated, rare instances to be
indicative of adverse effects to the public welfare. The Administrator
concluded that a standard with a level of 70 ppb and the existing form
and averaging time would be expected to limit cumulative exposures, in
terms of a 3-year average W126 exposure index, to values at or below 17
ppm-hrs, in nearly all instances, and accordingly, to eliminate or
virtually eliminate cumulative exposures associated with a median RBL
of 6% or greater (80 FR 65409, October 26, 2015). Thus, using RBL as a
proxy in judging effects to public welfare, the Administrator judged
that such a standard with a level of 70 ppb would provide the requisite
protection from adverse effects to public welfare by limiting
cumulative seasonal exposures to 17 ppm-hrs or lower, in terms of a 3-
year W126 index, in nearly all instances.
---------------------------------------------------------------------------

    \118\ The more than 500 monitors that would meet an alternative
standard of 70 ppb during the 2011-2013 period were distributed
across all nine NOAA climatic regions and 46 of the 50 states
(Wells, 2015 and associated dataset in the docket [document
identifier, EPA-HQ-OAR-2008-0699-4325]).
---------------------------------------------------------------------------

    In summary, the Administrator judged that the revised standard
would protect natural forests in Class I and other similarly protected
areas against an array of adverse vegetation effects, most notably
including those related to effects on growth and productivity in
sensitive tree species. The Administrator additionally judged that the
revised standard would be sufficient to protect public welfare from
known or anticipated adverse effects. This judgment by the
Administrator appropriately recognized that the CAA does not require
that standards be set at a zero-risk level, but rather at a level that
reduces risk sufficiently so as to protect the public welfare from
known or anticipated adverse effects. Thus, based on the conclusions
drawn from the air quality analyses which demonstrated a strong,
positive relationship between the 8-hour and W126 metrics and the
findings that indicated the significant amount of control provided by
the fourth-high metric, the evidence base of O3 effects on
vegetation and her public welfare policy judgments, as well as public
comments and CASAC advice, the Administrator decided to retain the
existing form and averaging time and revise the level to 0.070 ppm,
judging that such a standard would provide the requisite protection to
the public welfare from any known or anticipated adverse effects
associated with the presence of O3 in ambient air (80 FR
65409-10, October 26, 2015).
2. Approach for the Current Review
    To evaluate whether it is appropriate to consider retaining the now
current secondary O3 standard, or whether consideration of
revision is appropriate, the EPA has adopted an approach in this review
that builds upon the general approach used in the last review and
reflects the body of evidence and information now available.
Accordingly the approach in this review takes into consideration the
approach used in the last review, including the substantial assessments
and evaluations performed over the course of that review, and also
taking into account the more recent scientific information and air
quality data now available to inform understanding of the key policy-
relevant issues in the current review. As summarized above, the
Administrator's decisions in the prior review were based on an
integration of O3 welfare effects information with judgments
on the public welfare significance of key effects, policy judgments as
to when the standard is requisite, consideration of CASAC advice, and
consideration of public comments.
    Similarly, in this review we draw on the current evidence and
quantitative analyses of air quality and exposure pertaining to the
welfare effects of O3 in ambient air. In so doing, we
consider both the information available at the time of the last review
and information more recently available, including that which has been
critically analyzed and characterized in the current ISA. The
evaluations in the PA, of the potential implications of various aspects
of the scientific evidence assessed in the ISA (building on prior such
assessments), augmented by the quantitative air quality, exposure or
risk-based information, are also considered along with the associated
uncertainties and limitations.
    This review of the secondary O3 standard also considers
the August 2019 decision by the D.C. Circuit on the secondary standard
established in 2015 and issues raised by the court in its remand of
that standard to the EPA such that the decision in this review will
incorporate the EPA's response to this remand. The opinion issued by
the court concluded, in relevant part, that EPA had not provided a
sufficient rationale for aspects of its decision on the 2015 secondary
standard. See Murray Energy Corp. v. EPA, 936 F.3d 597 (D.C. Cir.
2019). Accordingly, the court remanded the secondary standard to EPA
for further justification or reconsideration, particularly in relation
to its decision to focus on a 3-year average for consideration of the
cumulative exposure, in terms of W126, identified as providing
requisite public welfare protection, and its decision to not identify a
specific level of air quality related to visible foliar injury.\119\
Thus, in addition to considering the currently available welfare
effects evidence and quantitative air quality, exposure and risk
information, this proposed decision on the secondary standard that was
established in 2015, and the associated proposed conclusions and
judgments, also consider the court's remand. In so doing, we have, for
example, expanded certain analyses in this review compared with those
conducted in the last review, included discussion on issues raised in
the remand, and provided additional explanation of rationales for
proposed conclusions on these points in this review. Together, the
information, evaluations and considerations recognized here inform the
Administrator's public welfare policy judgments and conclusions,
including his decision as to whether to retain or revise this standard.
---------------------------------------------------------------------------

    \119\ The EPA's decision not to use a seasonal W126 index as the
form and averaging time of the secondary standard was also
challenged in this case, but the court did not reach that issue,
concluding that it lacked a basis to assess the EPA's rationale on
this point because the EPA had not yet fully explained its focus on
a 3-year average W126 in its consideration of the standard. See
Murray Energy Corp. v. EPA, 936 F.3d 597, 618 (D.C. Cir. 2019).
---------------------------------------------------------------------------

B. Welfare Effects Information

    The information summarized here is based on our scientific
assessment of the welfare effects evidence available in this review;
this assessment is documented in the ISA \120\ and its policy

implications are further discussed in the PA. In this review, as in
past reviews, the health effects evidence evaluated in the ISA for
O3 and related photochemical oxidants is focused on
O3 (ISA, p. IS-3). Ozone is concluded to be the most
prevalent photochemical oxidant present in the atmosphere and the one
for which there is a very large, well-established evidence base of its
health and welfare effects. Further, ``the primary literature
evaluating the health and ecological effects of photochemical oxidants
includes ozone almost exclusively as an indicator of photochemical
oxidants'' (ISA, section IS.1.1). Thus, the current welfare effects
evidence and the Agency's review of the evidence, including the
evidence newly available in this review, continues to focus on
O3.
---------------------------------------------------------------------------

    \120\ The ISA builds on evidence and conclusions from previous
assessments, focusing on synthesizing and integrating the newly
available evidence (ISA, section IS.1.1). Past assessments are cited
when providing further details not repeated in newer assessments.
---------------------------------------------------------------------------

    More than 1600 studies are newly available and considered in the
ISA, including more than 500 studies on welfare effects (ISA, Appendix
10, Figure 10-2). While expanding the evidence for some effect
categories, studies on growth-related effects, a key group of effects
from the last review, are largely consistent with the evidence that was
previously available. Policy implications of the currently available
evidence are discussed in the PA (as summarized in section III.D.1
below). The subsections below briefly summarize the following aspects
of the evidence: The nature of O3-related welfare effects
(section III.B.1), the potential public welfare implications (section
III.B.2), and exposure concentrations associated with effects (section
III.B.3).
1. Nature of Effects
    The welfare effects evidence base available in the current review
includes more than fifty years of extensive research on the phytotoxic
effects of O3, conducted both in and outside of the U.S.,
that documents the impacts of O3 on plants and their
associated ecosystems (1978 AQCD, 1986 AQCD, 1996 AQCD, 2006 AQCD, 2013
ISA, 2020 ISA). As was established in prior reviews, O3 can
interfere with carbon gain (photosynthesis) and allocation of carbon
within the plant, making fewer carbohydrates available for plant
growth, reproduction, and/or yield (1996 AQCD, pp. 5-28 and 5-29). For
seed-bearing plants, reproductive effects can include reduced seed or
fruit production or yield. The strongest evidence for effects from
O3 exposure on vegetation was recognized at the time of the
last review to be from controlled exposure studies, which ``have
clearly shown that exposure to O3 is causally linked to
visible foliar injury, decreased photosynthesis, changes in
reproduction, and decreased growth'' in many species of vegetation
(2013 ISA, p. 1-15). Such effects at the plant scale can also be linked
to an array of effects at larger spatial scales (and higher levels of
biological organization), with the evidence available in the last
review indicating that ``O3 exposures can affect ecosystem
productivity, crop yield, water cycling, and ecosystem community
composition'' (2013 ISA, p. 1-15, Chapter 9, section 9.4). Beyond its
effects on plants, the evidence in the last review also recognized
O3 in the troposphere as a major greenhouse gas (ranking
behind carbon dioxide and methane in importance), with associated
radiative forcing and effects on climate, and recognized the
accompanying ``large uncertainties in the magnitude of the radiative
forcing estimate . . . making the impact of tropospheric O3
on climate more uncertain than the effect of the longer-lived
greenhouse gases'' (2013 ISA, sections 10.3.4 and 10.5.1 [p. 10-30]).
    The evidence newly available in this review supports, sharpens and
expands somewhat on the conclusions reached in the last review (ISA,
Appendices 8 and 9). Consistent with the evidence in the last review,
the currently available evidence describes an array of O3
effects on vegetation and related ecosystem effects, as well as the
role of O3 in radiative forcing and subsequent climate-
related effects. Evidence newly available in this review augments more
limited previously available evidence related to insect interactions
with vegetation, contributing to conclusions regarding O3
effects on plant-insect signaling (ISA, Appendix 8, section 8.7) and on
insect herbivores (ISA, Appendix 8, section 8.6), as well as for ozone
effects on tree mortality (Appendix 8, section 8.4). Thus, conclusions
reached in the last review are supported by the current evidence base
and conclusions are also reached in a few new areas based on the now
expanded evidence.
    The current evidence base, including a wealth of longstanding
evidence, supports the conclusion of causal relationships between
O3 and visible foliar injury, reduced vegetation growth and
reduced plant reproduction,\121\ as well as reduced yield and quality
of agricultural crops, reduced productivity in terrestrial ecosystems,
alteration of terrestrial community composition,\122\ and alteration of
belowground biogeochemical cycles (ISA, section IS.5). Based on the
current evidence base, the ISA also concluded there likely to be a
causal relationship between O3 and alteration of ecosystem
water cycling, reduced carbon sequestration in terrestrial ecosystems,
and with increased tree mortality (ISA, section IS.5). Additional
evidence newly available in this review is concluded by the ISA to
support conclusions on two additional plant-related effects: The body
of evidence is concluded to be sufficient to infer that there is likely
to be a causal relationship between O3 exposure and
alteration of plant-insect signaling, and to infer that there is likely
to be a causal relationship between O3 exposure and altered
insect herbivore growth and reproduction (ISA, Table IS-12).
---------------------------------------------------------------------------

    \121\ The 2013 ISA did not include a separate causality
determination for reduced plant reproduction. Rather, it was
included with the conclusion of a causal relationship with reduced
vegetation growth (ISA, Table IS-12).
    \122\ The 2013 ISA concluded alteration of terrestrial community
composition to be likely causally related to O3 based on
the then available information (ISA, Table IS-12).
---------------------------------------------------------------------------

    As in the last review, the strongest evidence and the associated
findings of causal or likely causal relationships with O3 in
ambient air, and the quantitative characterizations of relationships
between O3 exposure and occurrence and magnitude of effects
are for vegetation effects. The scales of these effects range from the
individual plant scale to the ecosystem scale, with potential for
impacts on the public welfare (as discussed in section III.B.2 below).
The following summary addresses the identified vegetation-related
effects of O3 across these scales.
    The current evidence, consistent with the decades of previously
available evidence, documents and characterizes visible foliar injury
in many tree, shrub, herbaceous, and crop species as an effect of
exposure to O3 (ISA, Appendix 8, section 8.2; 2013 ISA,
section 9.4.2; 2006 AQCD, 1996 AQCD, 1986 AQCD, 1978 AQCD). As was also
stated in the last scientific assessment, ``[r]ecent experimental
evidence continues to show a consistent association between visible
injury and ozone exposure'' (ISA, Appendix 8, section 8.2, p. 8-13;
2013 ISA, section 9.4.2, p. 9-41). Ozone-induced visible foliar injury
symptoms on certain tree and herbaceous species, such as black cherry,
yellow-poplar and common milkweed, have long been considered diagnostic
of exposure to elevated O3 based on the consistent
association established with experimental evidence (ISA, Appendix 8,
section 8.2; 2013 ISA, p. 1-10).\123\
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    \123\ As described in the ISA, ``[t]ypical types of visible
injury to broadleaf plants include stippling, flecking, surface
bleaching, bifacial necrosis, pigmentation (e.g., bronzing), and
chlorosis or premature senescence'' and ``[t]ypical visible injury
symptoms for conifers include chlorotic banding, tip burn, flecking,
chlorotic mottling, and premature senescence of needles'' (ISA,
Appendix 8, p. 8-13).

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

    The currently available evidence, consistent with that in past
reviews, indicates that ``visible foliar injury usually occurs when
sensitive plants are exposed to elevated ozone concentrations in a
predisposing environment,'' with a major factor for such an environment
being the amount of soil moisture available to the plant (ISA, Appendix
8, p. 8-23; 2013 ISA, section 9.4.2). Further, the significance of
O3 injury at the leaf and whole plant levels also depends on
an array of factors that include the amount of total leaf area
affected, age of plant, size, developmental stage, and degree of
functional redundancy among the existing leaf area (ISA, Appendix 8,
section 8.2; 2013 ISA, section 9.4.2). In this review, as in the past,
such modifying factors contribute to the difficulty in quantitatively
relating visible foliar injury to other vegetation effects (e.g.,
individual tree growth, or effects at population or ecosystem levels),
such that visible foliar injury ``is not always a reliable indicator of
other negative effects on vegetation'' (ISA, Appendix 8, section 8.2,
p. 8-24; 2013 ISA, p. 9-39).\124\
---------------------------------------------------------------------------

    \124\ Similar to the 2013 ISA, the ISA for the current review
states the following (ISA, pp. 8-24).
    Although visible injury is a valuable indicator of the presence
of phytotoxic concentrations of ozone in ambient air, it is not
always a reliable indicator of other negative effects on vegetation
[e.g., growth, reproduction; U.S. EPA (2013)]. The significance of
ozone 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 (U.S. EPA, 2013). Previous
ozone AQCDs have noted the difficulty in relating visible foliar
injury symptoms to other vegetation effects, such as individual
plant growth, stand growth, or ecosystem characteristics (U.S. EPA,
2006, 1996). Thus, 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.
---------------------------------------------------------------------------

    Consistent with conclusions in past reviews, the evidence,
extending back several decades, continues to document the detrimental
effects of O3 on plant growth and reproduction (ISA,
Appendix 8, sections 8.3 and 8.4; 2013 ISA, p. 9-42). The available
studies come from a variety of different study types that cover an
array of different species, effects endpoints, and exposure methods and
durations. In addition to studies on scores of plant species that have
found O3 to reduce plant growth, the evidence accumulated
over the past several decades documents O3 alteration of
allocation of biomass within the plant and plant reproduction (ISA,
Appendix 8, sections 8.3 and 8.4; 2013 ISA, p. 1-10). The biological
mechanisms underlying the effect of O3 on plant reproduction
include ``both direct negative effects on reproductive tissues and
indirect negative effects that result from decreased photosynthesis and
other whole plant physiological changes'' (ISA, p. IS-71). A newly
available meta-analysis of more than 100 studies published between 1968
and 2010 summarizes effects of O3 on multiple measures of
reproduction (ISA, Appendix 8, section 8.4.1).
    Studies involving experimental field sites have also reported
effects on measures of plant reproduction, such as effects on seeds
(reduced weight, germination, and starch levels) that could lead to a
negative impact on species regeneration in subsequent years, and bud
size that might relate to a delay in spring leaf development (ISA,
Appendix 8, section 8.4; 2013 ISA, section 9.4.3; Darbah et al., 2007,
Darbah et al., 2008). A more recent laboratory study reported 6-hour
daily O3 exposures of flowering mustard plants to 100 ppb
during different developmental stages to have mixed effects on
reproductive metrics. While flowers exposed early versus later in
development produced shorter fruits, the number of mature seeds per
fruit was not significantly affected by flower developmental stage of
exposure (ISA, Appendix 8, section 8.4.1; Black et al., 2012). Another
study assessed seed viability for a flowering plant in laboratory and
field conditions, finding effects on seed viability of O3
exposures (90 and 120 ppb) under laboratory conditions but less clear
effects under more field-like conditions (ISA, Appendix 8, section
8.4.1; Landesmann et al., 2013).
    With regard to agricultural crops, the current evidence base, as in
the last review, is sufficient to infer a causal relationship between
O3 exposure and reduced yield and quality (ISA, section
IS.5.1.2). The current evidence is augmented by new research in a
number of areas, including studies on soybean, wheat and other nonsoy
legumes. The new information assessed in the ISA remains consistent
with the conclusions reached in the 2013 ISA (ISA, section IS.5.1.2).
    The evidence base for trees includes a number of studies conducted
at the Aspen free-air carbon-dioxide and ozone enrichment (FACE)
experiment site in Wisconsin (that operated from 1998 through 2011) and
also available in the last review (ISA, IS.5.1 and Appendix 8, section
<IP_ADDRESS>; 2013 ISA, section 9.2.4). These studies, which occurred in a
field setting (more similar to natural forest stands than open-top-
chamber studies), reported reduced tree growth when grown in single or
three species stands within 30-m diameter rings and exposed over one or
more years to elevated O3 concentrations (hourly
concentrations 1.5 times concentrations in ambient air at the site)
compared to unadjusted ambient air concentrations (2013 ISA, section
9.4.3; Kubiske et al., 2006, Kubiske et al., 2007).\125\
---------------------------------------------------------------------------

    \125\ Seasonal (90-day) W126 index values for unadjusted
O3 concentrations over six years of the Aspen FACE
experiments ranged from 2 to 3 ppm-hrs, while the elevated exposure
concentrations (reflecting addition of O3 to ambient air
concentrations) ranged from somewhat above 20 to somewhat above 35
ppm-hrs (ISA, Appendix 8, Figure 8-17).
---------------------------------------------------------------------------

    With regard to tree mortality, the 2013 ISA did not include a
determination of causality (ISA, Appendix 8, section 8.4). While the
then-available evidence included studies identifying ozone as a
contributor to tree mortality, which contributed to the 2013 conclusion
regarding O3 and alteration of community composition (2013
ISA, section <IP_ADDRESS>), a separate causality determination regarding
O3 and tree mortality was not assessed (ISA, Appendix 8,
section 8.4; 2013 ISA, Table 9-19). The evidence assessed in the 2013
ISA (and 2006 AQCD) was largely observational, including studies that
reported declines in conifer forests for which elevated O3
was identified as contributor but in which a variety of environmental
factors may have also played a role (2013 ISA, section <IP_ADDRESS>; 2006
AQCD, sections AX<IP_ADDRESS>, AX<IP_ADDRESS>, AX<IP_ADDRESS>, AX<IP_ADDRESS> and
AX<IP_ADDRESS>). Since the last review, three additional studies are
available (ISA, Appendix 8, Table 8-9). Two of these are analyses of
field observations, one of which is set in the Spanish Pyrenees.\126\ A
second study is a large-scale empirical statistical analysis of factors
potentially contributing to tree mortality in eastern and central U.S.
forests during the 1971-2005 period, which reported O3
(county-level 11-year [1996-2006] average 8 hour metric) \127\ to be
ninth among the 13 potential factors assessed \128\ and to have a

significant positive correlation with tree mortality (ISA, section
IS.5.1.2, Appendix 8, section 8.4.3; Dietze and Moorcroft, 2011). A
newly available experimental study also reported increased mortality in
two of five aspen genotypes grown in mixed stands under elevated
O3 concentrations (ISA, section IS.5.1.2; Moran and Kubiske,
2013). Coupled with the plant-level evidence of phytotoxicity discussed
above, as well as consideration of community composition effects, this
evidence was concluded to indicate the potential for elevated
O3 concentrations to contribute to tree mortality (ISA,
section IS.5.1.2 and Appendix 8, sections 8.4.3 and 8.4.4). Based on
the current evidence, the ISA concludes there is likely to be a causal
relationship between O3 and increased tree mortality (ISA,
Table IS-2, Appendix 8, section 8.4.4). A variety of factors in natural
environments can either mitigate or exacerbate predicted O3-
plant interactions and are recognized sources of uncertainty and
variability. Such factors at the plant level include multiple
genetically influenced determinants of O3 sensitivity,
changing sensitivity to O3 across vegetative growth stages,
co-occurring stressors and/or modifying environmental factors (ISA,
Appendix 8, section 8.12).
---------------------------------------------------------------------------

    \126\ The concentration gradient with altitude in the Spanish
study, includes--at the highest site--annual average April-to-
September O3 concentrations for the 2004 to 2007 period
that range up to 74 ppb (Diaz-de-Quijano et al., 2016).
    \127\ Annual fourth highest daily maximum 8-hour O3
concentrations in these regions were above 80 ppb in the early 2000s
and median design values at national trend sites were nearly 85 ppb
(PA, Figures 2-11 and 2-12).
    \128\ This statistical analysis, which utilized datasets from
within the 1971-2005 period, included an examination of the
sensitivity of predicted mortality rate to 13 different covariates.
On average across the predictions for 10 groups of trees (based on
functional type and major representative species), the order of
mortality rate sensitivity to the covariates, from highest to
lowest, was: Sulfate deposition, tree diameter, nitrate deposition,
summer temperature, tree age, elevation, winter temperature,
precipitation, O3 concentration, tree basal area,
topographic moisture index, slope and topographic radiation index
(Dietze and Moorcroft, 2011).
---------------------------------------------------------------------------

    Ozone-induced effects at the scale of the whole plant have the
potential to translate to effects at the ecosystem scale, such as
reduced terrestrial productivity and carbon storage, and altered
terrestrial community composition, as well as impacts on ecosystem
functions, such as belowground biogeochemical cycles and ecosystem
water cycling. For example, under the relevant exposure conditions,
O3-related reduced tree growth and reproduction, as well as
increased mortality, could lead to reduced ecosystem productivity.
Recent studies from the Aspen FACE experiment and modeling simulations
indicate that O3-related negative effects on ecosystem
productivity may be temporary or may be limited in some systems (ISA,
Appendix 8, section 8.8.1). Previously available studies had reported
impacts on productivity in some forest types and locations, such as
ponderosa pine in southern California and other forest types in the
mid-Atlantic region (2013 ISA, section <IP_ADDRESS>). Through reductions in
sensitive species growth, and related ecosystem productivity,
O3 could lead to reduced ecosystem carbon storage (ISA,
IS.5.1.4; 2013 ISA, section 9.4.3). With regard to forest community
composition, available studies have reported changes in tree
communities composed of species with relatively greater and relatively
lesser sensitivity to O3 (ISA, section IS.<IP_ADDRESS>, Appendix
8, section 8.10; 2013 ISA, section 9.4.3; Kubiske et al., 2007). As the
ISA concludes, ``[t]he extent to which ozone affects terrestrial
productivity will depend on more than just community composition, but
other factors, which both directly influence [net primary productivity]
(i.e., availability of N and water) and modify the effect of ozone on
plant growth'' (ISA, Appendix 8, section 8.8.1). Thus, the magnitude of
O3 impact on ecosystem productivity, as on forest
composition, can vary among plant communities based on several factors,
including the type of stand or community in which the sensitive species
occurs (e.g., single species versus mixed canopy), the role or position
of the species in the stand (e.g., dominant, sub-dominant, canopy,
understory), and the sensitivity of co-occurring species and
environmental factors (e.g., drought and other factors).
    The effects of O3 on plants and plant populations have
implications for ecosystem functions. Two such functions, effects with
which O3 is concluded to be likely causally or causally
related, are ecosystem water cycling and belowground biogeochemical
cycles, respectively (ISA, Appendix 8, sections 8.11 and 8.9). With
regard to the former, the effects of O3 on plants (e.g., via
stomatal control, as well as leaf and root growth and changes in wood
anatomy associated with water transport) can affect ecosystem water
cycling through impacts on root uptake of soil moisture and groundwater
as well as transpiration through leaf stomata to the atmosphere (ISA,
Appendix 8, section 8.11.1). These ``impacts may in turn affect the
amount of water moving through the soil, running over land or through
groundwater and flowing through streams'' (ISA, Appendix 8, p. 8-161).
Evidence newly available in this review is supportive of previously
available evidence in this regard (ISA, Appendix 8, section 8.11.6).
The current evidence, including that newly available, indicates the
extent to which the effects of O3 on plant leaves and roots
(e.g., through effects on chemical composition and biomass) can impact
belowground biogeochemical cycles involving root growth, soil food web
structure, soil decomposer activities, soil microbial respiration, soil
carbon turnover, soil water cycling and soil nutrient cycling (ISA,
Appendix 8, section 8.9).
    Additional vegetation-related effects with implications beyond
individual plants include the effects of O3 on insect
herbivore growth and reproduction and plant-insect signaling (ISA,
Table IS-12, Appendix 8, sections 8.6 and 8.7). With regard to insect
herbivore growth and reproduction, the evidence includes multiple
effects in an array of insect species, although without a consistent
pattern of response for most endpoints (ISA, Appendix 8, Table 8-11).
As was also the case with the studies available at the time of the last
review,\129\ in the newly available studies individual-level responses
are highly context- and species-specific and not all species tested
showed a response (ISA, section IS.5.1.3 and Appendix 8, section 8.6).
Evidence on plant-insect signaling that is newly available in this
review comes from laboratory, greenhouse, open top chambers (OTC) and
FACE experiments (ISA, section IS.5.1.3 and Appendix 8, section 8.7).
The available evidence indicates a role for elevated O3 in
altering and degrading emissions of chemical signals from plants and
reducing detection of volatile plant signaling compounds (VPSCs) by
insects, including pollinators. Elevated O3 concentrations
degrade some VPSCs released by plants, potentially affecting ecological
processes including pollination and plant defenses against herbivory.
Further, the available studies report elevated O3 conditions
to be associated with plant VPSC emissions that may make a plant either
more attractive or more repellant to herbivorous insects, and to
predators and parasitoids that target phytophagous (plant-eating)
insects (ISA, section IS.5.1.3 and Appendix 8, section 8.7).
---------------------------------------------------------------------------

    \129\ During the last review, the 2013 ISA stated with regard to
O3 effects on insects and other wildlife that ``there is
no consensus on how these organisms respond to elevated
O3'' (2013 ISA, section <IP_ADDRESS>, p. 9-98).
---------------------------------------------------------------------------

    Ozone welfare effects also extend beyond effects on vegetation and
associated biota due to it being a major greenhouse gas and radiative
forcing agent.\130\ As in the last review, the

current evidence, augmented since the 2013 ISA, continues to support a
causal relationship between the global abundance of O3 in
the troposphere and radiative forcing, and a likely causal relationship
between the global abundance of O3 in the troposphere and
effects on temperature, precipitation, and related climate variables
\131\ (ISA, section IS.5.2 and Appendix 9; Myhre et al., 2013). As was
also true at the time of the last review, tropospheric O3
has been ranked third in importance for global radiative forcing, after
carbon dioxide and methane, with the radiative forcing of O3
since pre-industrial times estimated to be about 25 to 40% of the total
warming effects of anthropogenic carbon dioxide and about 75% of the
effects of anthropogenic methane (ISA, Appendix 9, section <IP_ADDRESS>).
Uncertainty in the magnitude of radiative forcing estimated to be
attributed to tropospheric O3 is a contributor to the
relatively greater uncertainty associated with climate effects of
tropospheric O3 compared to such effects of the well mixed
greenhouse gases, such as carbon dioxide and methane (ISA, section
IS.6.2.2).
---------------------------------------------------------------------------

    \130\ Radiative forcing is a metric used to quantify the change
in balance between radiation coming into and going out of the
atmosphere caused by the presence of a particular substance. The ISA
describes it more specifically as ``a perturbation in net radiative
flux at the tropopause (or top of the atmosphere) caused by a change
in radiatively active forcing agent(s) after stratospheric
temperatures have readjusted to radiative equilibrium
(stratospherically adjusted RF)'' (ISA, Appendix 9, section
<IP_ADDRESS>).
    \131\ Effects on temperature, precipitation, and related climate
variables were referred to as ``climate change'' or ``effects on
climate'' in the 2013 ISA (ISA, p. IS-82; 2013 ISA, pp. 1-14 and 10-
31).
---------------------------------------------------------------------------

    Lastly, the evidence regarding tropospheric O3 and UV-B
shielding (shielding of ultraviolet radiation at wavelengths of 280 to
320 nanometers) was evaluated in the 2013 ISA and determined to be
inadequate to draw a causal conclusion (2013 ISA, section 10.5.2). The
current ISA concludes there to be no new evidence since the 2013 ISA
relevant to the question of UV-B shielding by tropospheric
O3 (ISA, IS.1.2.1 and Appendix 9, section <IP_ADDRESS>).
2. Public Welfare Implications
    The secondary standard is to ``specify a level of air quality the
attainment and maintenance of which in the judgment of the
Administrator . . . 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'' (CAA, section 109(b)(2)). As
recognized in prior reviews, the secondary standard is not meant to
protect against all known or anticipated O3-related welfare
effects, but rather those that are judged to be adverse to the public
welfare, and a bright-line determination of adversity is not required
in judging what is requisite (78 FR 3212, January 15, 2013; 80 FR
65376, October 26, 2015; see also 73 FR 16496, March 27, 2008). Thus,
the level of protection from known or anticipated adverse effects to
public welfare that is requisite for the secondary standard is a public
welfare policy judgment to be made by the Administrator. In each
review, the Administrator's judgment regarding the currently available
information and adequacy of protection provided by the current standard
is generally informed by considerations in prior reviews and associated
conclusions.
    The categories of effects identified in the CAA to be included
among welfare effects are quite diverse,\132\ and among these
categories, any single category includes many different types of
effects that are of broadly varying specificity and level of
resolution. For example, effects on vegetation, is a category
identified in CAA section 302(h), and the ISA recognizes numerous
vegetation-related effects of O3 at the organism,
population, community and ecosystem level, as summarized in section
III.B.1 above (ISA, Appendix 8). The significance of each type of
vegetation-related effect with regard to potential effects on the
public welfare depends on the type and severity of effects, as well as
the extent of such effects on the affected environmental entity, and on
the societal use of the affected entity and the entity's significance
to the public welfare. Such factors are generally considered in light
of judgments and conclusions made in prior reviews regarding effects on
the public welfare. For example, a key consideration with regard to
public welfare implications in prior reviews of the O3
secondary standard was the intended use of the affected or sensitive
vegetation and the significance of the vegetation to the public welfare
(73 FR 16496, March 27, 2008; 80 FR 65292, October 26, 2015).
---------------------------------------------------------------------------

    \132\ Section 302(h) of the CAA states that language referring
to ``effects on welfare'' in the CAA ``includes, but is not limited
to, effects on soils, water, crops, vegetation, manmade materials,
animals, wildlife, weather, visibility, and climate, damage to and
deterioration of property, and hazards to transportation, as well as
effects on economic values and on personal comfort and well-being.''
---------------------------------------------------------------------------

    More specifically, judgments regarding public welfare significance
in the last two O3 NAAQS decisions gave particular attention
to O3 effects in areas with special federal protections, and
lands set aside by states, tribes and public interest groups to provide
similar benefits to the public welfare (73 FR 16496, March 27, 2008; 80
FR 65292, October 26, 2015). For example, in the decision to revise the
secondary standard in the 2008 review, the Administrator took note of
``a number of actions taken by Congress to establish public lands that
are set 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'' (73 FR 16496, March 27, 2008).\133\ Such areas
include Class I areas \134\ which are federally mandated to preserve
certain air quality related values. Additionally, as the Administrator
recognized, ``States, Tribes and public interest groups also set aside
areas that are intended to provide similar benefits to the public
welfare, for residents on State and Tribal lands, as well as for
visitors to those areas'' (73 FR 16496, March 27, 2008). The
Administrator took note of 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 lands contain
O3-sensitive species'' (73 FR 16496, March 27, 2008).
Similarly, in the 2015 review, the Administrator indicated particular
concern for O3-related effects on plant function and
productivity and associated ecosystem effects in natural ecosystems
``such as those in areas with protection designated by Congress for
current and future generations, as well

as areas similarly set aside by states, tribes and public interest
groups with the intention of providing similar benefits to the public
welfare'' (80 FR 65403, October 26, 2015).
---------------------------------------------------------------------------

    \133\ For example, the fundamental purpose of parks in the
National Park System ``is to conserve the scenery, natural and
historic objects, and wild life in the System units and to provide
for the enjoyment of the scenery, natural and historic objects, and
wild life in such manner and by such means as will leave them
unimpaired for the enjoyment of future generations'' (54 U.S.C.
100101). Additionally, the Wilderness Act of 1964 defines designated
``wilderness areas'' in part as areas ``protected and managed so as
to preserve [their] natural conditions'' and requires that these
areas ``shall be administered for the use and enjoyment of the
American people in such manner as will leave them unimpaired for
future use and enjoyment as wilderness, and so as to provide for the
protection of these areas, [and] the preservation of their
wilderness character . . .'' (16 U.S.C. 1131 (a) and (c)). Other
lands that benefit the public welfare include national forests which
are managed for multiple uses including sustained yield management
in accordance with land management plans (see 16 U.S.C. 1600(1)-(3);
16 U.S.C. 1601(d)(1)).
    \134\ Areas designated as Class I include all international
parks, national wilderness areas which exceed 5,000 acres in size,
national memorial parks which exceed 5,000 acres in size, and
national parks which exceed 6,000 acres in size, provided the park
or wilderness area was in existence on August 7, 1977. Other areas
may also be Class I if designated as Class I consistent with the CAA
(as described in the PA, Appendix 4D, section 4D.2.4).
---------------------------------------------------------------------------

    The 2008 and 2015 decisions recognized that the degree to which
effects on vegetation in specially protected areas, such as those
identified above, may be judged adverse involves considerations from
the species level to the ecosystem level, such that judgments can
depend on the intended use for, or service (and value) of, the affected
vegetation, ecological receptors, ecosystems and resources and the
significance of that use to the public welfare (73 FR 16496, March 27,
2008; 80 FR 65377, October 26, 2015). Uses or services provided by
areas that have been afforded special protection can flow in part or
entirely from the vegetation that grows there. For example, ecosystem
services are the ``benefits that people derive from functioning
ecosystems'' (Costanza et al., 2017; ISA, section IS.5.1).\135\
Ecosystem services range from those directly related to the natural
functioning of the ecosystem to ecosystem uses for human recreation or
profit, such as through the production of lumber or fuel (Costanza et
al., 2017). Aesthetic value and outdoor recreation depend, at least in
part, on the perceived scenic beauty of the environment. Further, there
have been analyses that report the American public values--in monetary
as well as nonmonetary ways--the protection of forests from air
pollution damage (Haefele et al., 1991). In fact, public surveys have
indicated that Americans rank as very important the existence of
resources, the option or availability of the resource and the ability
to bequest or pass it on to future generations (Cordell et al., 2008).
The spatial, temporal and social dimensions of public welfare impacts
are also influenced by the type of service affected. For example, a
national park can provide direct recreational services to the thousands
of visitors that come each year, but also provide an indirect value to
the millions who may not visit but receive satisfaction from knowing
that it exists and is preserved for the future (80 FR 65377, October
26, 2015).
---------------------------------------------------------------------------

    \135\ Ecosystem services analyses were one of the tools used in
the last review of the secondary standards for oxides of nitrogen
and sulfur to inform the decisions made with regard to adequacy of
protection provided by the standards and as such, were used in
conjunction with other considerations in the discussion of adversity
to public welfare (77 FR 20241, April 3, 2012).
---------------------------------------------------------------------------

    The different types of effects on vegetation discussed in section
III.B.1 above differ with regard to aspects important to judging their
public welfare significance. In the case of crop yield loss, such
judgments depend on considerations related to the heavy management of
agriculture in the U.S. Judgments for other categories of effects may
generally relate to considerations regarding forested areas, including
specifically those forested areas that are not managed for harvest. For
example, effects on tree growth and reproduction, and also visible
foliar injury, have the potential to be significant to the public
welfare through impacts in Class I and other areas given special
protection in their natural/existing state, although they differ in how
they might be significant. Additionally, as described in section
III.B.1 above, O3 effects on tree growth and reproduction
could, depending on severity, extent and other factors, lead to effects
on a larger scale including reduced productivity, altered forest and
forest community (plant, insect and microbe) composition, reduced
carbon storage and altered ecosystem water cycling (ISA, section
IS.<IP_ADDRESS>; 2013 ISA, Figure 9-1, sections <IP_ADDRESS> and <IP_ADDRESS>). For
example, forest or forest community composition can be affected through
O3 effects on growth and reproductive success of sensitive
species in the community, with the extent of compositional changes
dependent on factors such as competitive interactions (ISA, section
IS.<IP_ADDRESS>; 2013 ISA, sections 9.4.3 and <IP_ADDRESS>). Impacts on some of
these characteristics (e.g., forest or forest community composition)
may be considered of greater public welfare significance when occurring
in Class I or other protected areas, due to value for particular
services that the public places on such areas.
    Depending on the type and location of the affected ecosystem,
however, a broader array of services benefitting the public can be
affected in a broader array of areas as well. For example, other
services valued by people that can be affected by reduced tree growth,
productivity and associated forest effects include aesthetic value,
food, fiber, timber, other forest products, habitat, recreational
opportunities, climate and water regulation, erosion control, air
pollution removal, and desired fire regimes (PA, Figure 4-2; ISA,
section IS.5.1; 2013 ISA, sections <IP_ADDRESS> and <IP_ADDRESS>). In considering
such services in past reviews, the Agency has given particular
attention to effects in natural ecosystems, indicating that a
protective standard, based on consideration of effects in natural
ecosystems in areas afforded special protection, would also ``provide a
level of protection for other vegetation that is used by the public and
potentially affected by O3 including timber, produce grown
for consumption and horticultural plants used for landscaping'' (80 FR
65403, October 26, 2015). For example, locations potentially vulnerable
to O3-related impacts might include forested lands, both
public and private, where trees are grown for timber production.
Forests in urbanized areas also provide a number of services that are
important to the public in those areas, such as air pollution removal,
cooling, and beautification. There are also many other tree species,
such as various ornamental and agricultural species (e.g., Christmas
trees, fruit and nut trees), that provide ecosystem services that may
be judged important to the public welfare.
    Depending on its severity and spatial extent, visible foliar
injury, which affects the physical appearance of the plant, also has
the potential to be significant to the public welfare through impacts
in Class I and other similarly protected areas. In cases of widespread
and severe injury during the growing season (particularly when
sustained across multiple years, and accompanied by obvious impacts on
the plant canopy), O3-induced visible foliar injury might be
expected to have the potential to impact the public welfare in scenic
and/or recreational areas, particularly in areas with special
protection, such as Class I areas.\136\ The ecosystem services most
likely to be affected by O3-induced visible foliar injury
(some of which are also recognized above for tree growth-related
effects) are cultural services, including aesthetic value and outdoor
recreation.
---------------------------------------------------------------------------

    \136\ For example, although analyses specific to visible foliar
injury are of limited availability, there have been analyses
developing estimates of recreation value damages of severe impacts
related to other types of forest effects, such as tree mortality due
to bark beetle outbreaks (e.g., Rosenberger et al., 2013). Such
analyses estimate reductions in recreational use when the damage is
severe (e.g., reductions in the density of live, robust trees). Such
damage would reasonably be expected to also reflect damage
indicative of injury with which a relationship with other plant
effects (e.g., growth and reproduction) would be also expected.
Similarly, a couple of studies from the 1970s and 1980s indicated
likelihood for reduced recreational use in areas with stands of pine
in which moderate to severe injury was apparent from 30 or 40 feet
(PA, section 4.3.2).
---------------------------------------------------------------------------

    The geographic extent of protected areas that may be vulnerable to
public welfare effects of O3, such as impacts to outdoor
recreation, is potentially appreciable. For example, biomonitoring
surveys that were routinely administered by the U.S.

Forest Service (USFS) as far back as 1994 in the eastern U.S. and 1998
in the western U.S. include many field sites at which there are plants
sensitive to O3-related visible foliar injury; there are 450
field sites across 24 states in the North East and North Central
regions (Smith, 2012).\137\ Since visible foliar injury is a visible
indication of O3 exposure in species sensitive to this
effect, a number of such species have been established as bioindicator
species, and such surveys have been used by federal land managers as
tools in assessing potential air quality impacts in Class I areas (U.S.
Forest Service, 2010). Additionally, the USFS has developed categories
for the scoring system that it uses for purposes of describing and
comparing injury severity at biomonitoring sites. The sites are termed
biosites and the scoring system involves deriving biosite index (BI)
scores that may be described with regard to one of several categories
ranging from little or no foliar injury to severe injury (e.g., Smith
et al., 2003; Campbell et al., 2007; Smith et al., 2007; Smith,
2012).\138\ As noted in section III.B.1 above, there is not an
established quantitative relationship between visible foliar injury and
other effects, such as reduced growth and productivity as visible
foliar injury ``is not always a reliable indicator of other negative
effects'' (ISA, Appendix 8, section 8.2).
---------------------------------------------------------------------------

    \137\ This aspect of the USFS biomonitoring surveys has
apparently been suspended, with the most recent surveys conducted in
2011 (USFS, 2013, USFS, 2017).
    \138\ Studies presenting USFS biomonitoring program data have
suggested what might be ``assumptions of risk'' related to scores in
these categories, e.g., none, low, moderate and high for BI scores
of zero to five, five to 15, 15 to 25 and above 25, respectively
(e.g., Smith et al., 2003; Smith et al., 2012).
---------------------------------------------------------------------------

    Public welfare implications associated with visible foliar injury
might further be considered to relate largely to effects on scenic and
aesthetic values. The available information does not yet address or
describe the relationships expected to exist between some level of
injury severity (e.g., little, low/light, moderate or severe) and/or
spatial extent affected and scenic or aesthetic values. This gap
impedes consideration of the public welfare implications of different
injury severities, and accordingly judgments on the potential for
public welfare significance. That notwithstanding, while minor spotting
on a few leaves of a plant may easily be concluded to be of little
public welfare significance, some level of severity and widespread
occurrence of visible foliar injury, particularly if occurring in
specially protected areas, such as Class I areas, where the public can
be expected to place value (e.g., for recreational uses), might
reasonably be concluded to impact the public welfare. Accordingly, key
considerations for public welfare significance of this endpoint would
relate to qualitative consideration of the potential for such effects
to affect the aesthetic value of plants in protected areas, such as
Class I areas (73 FR 16490, March 27, 2008).
    While, as noted above, public welfare benefits of forested lands
can be particular to the type of area in which the forest occurs, some
of the potential public welfare benefits associated with forest
ecosystems are not location dependent. A potentially extremely valuable
ecosystem service provided by forested lands is carbon sequestration or
storage (ISA, section IS.5.1.4 and Appendix 8, section 8.8.3; 2013 ISA,
section <IP_ADDRESS> and p. 9-37).\139\ As noted above, the EPA has
concluded that effects on this ecosystem service are likely causally
related to O3 in ambient air (ISA, Table IS-12). The
importance of carbon sequestration to the public welfare relates to its
role in counteracting the impact of greenhouse gases on radiative
forcing and related climate effects. As summarized in section III.B.1
above, O3 is also a greenhouse gas and O3
abundance in the troposphere is causally related to radiative forcing
and likely causally related to subsequent effects on temperature,
precipitation and related climate variables (ISA, section IS.6.2.2).
Accordingly, such effects also have important public welfare
implications, although their quantitative evaluation in response to
O3 concentrations in the U.S. is complicated by ``[c]urrent
limitations in climate modeling tools, variation across models, and the
need for more comprehensive observational data on these effects'' (ISA,
section IS.6.2.2). The service of carbon storage is of paramount
importance to the public welfare no matter in what location the trees
are growing or what their intended current or future use (e.g., 2013
ISA, section <IP_ADDRESS>). In other words, the benefit exists as long as
the trees are growing, regardless of what additional functions and
services it provides.
---------------------------------------------------------------------------

    \139\ While carbon sequestration or storage also occurs for
vegetated ecosystems other than forests, it is relatively larger in
forests given the relatively greater biomass for trees compared to
other plants.
---------------------------------------------------------------------------

    With regard to agriculture-related effects, the EPA has recognized
other complexities related to areas and plant species that are heavily
managed to obtain a particular output (such as commodity crops or
commercial timber production). For example, the EPA has recognized that
the degree to which O3 impacts on vegetation that could
occur in such areas and on such species would impair the intended use
at a level that might be judged adverse to the public welfare has been
less clear (80 FR 65379, October 26, 2015; 73 FR 16497, March 27,
2008). While having sufficient crop yields is of high public welfare
value, important commodity crops are typically heavily managed to
produce optimum yields. Moreover, based on the economic theory of
supply and demand, increases in crop yields would be expected to result
in lower prices for affected crops and their associated goods, which
would primarily benefit consumers. These competing impacts on producers
and consumers complicate consideration of these effects in terms of
potential adversity to the public welfare (2014 WREA, sections 5.3.2
and 5.7). When agricultural impacts or vegetation effects in other
areas are contrasted with the emphasis on ecosystem effects in Class I
and similarly protected areas, the EPA most recently has judged the
significance to the public welfare of O3-induced effects on
sensitive vegetation growing within the U.S. to differ depending on the
nature of the effect, the intended use of the sensitive plants or
ecosystems, and the types of environments in which the sensitive
vegetation and ecosystems are located, with greater significance
ascribed to areas identified for specific uses and benefits to the
public welfare, such as Class I areas, than to areas for which such
uses have not been established (80 FR 65292, October 26, 2015; FR 73
16496-16497, March 27, 2008).
    Categories of effects newly identified as likely causally related
to O3 in ambient air, such as alteration of plant-insect
signaling and insect herbivore growth and reproduction, also have
potential public welfare implications. For example, given the role of
plant-insect signaling in such important ecological processes as insect
herbivore growth and reproduction. The potential to contribute to
adverse effects to the public welfare, e.g., given the role of the
plant-insect signaling process in pollination and seed dispersal, as
well as natural plant defenses against predation and parasitism,
particular effects on particular signaling processes can be seen to
have the potential for adverse effects on the public welfare (ISA,
section IS.5.1.3). However, uncertainties and limitations in the
current evidence (e.g., summarized in sections III.B.3 and III.D.1
below) preclude an assessment of the extent

and magnitude of O3 effects on these endpoints, which thus
also precludes an evaluation of the potential for associated public
welfare implications, particularly under exposure conditions expected
to occur in areas meeting the current standard.
    In summary, several considerations are recognized as important to
judgments on the public welfare significance of the array of welfare
effects of different O3 exposure conditions. There are
uncertainties and limitations associated with the consideration of the
magnitude of key welfare effects that might be concluded to be adverse
to ecosystems and associated services. There are numerous locations
where the presence of O3-sensitive tree species may
contribute to a vulnerability to impacts from O3 on tree
growth, productivity and carbon storage and their associated ecosystems
and services. Exposures that may elicit effects and the significance of
the effects in specific situations can vary due to differences in
exposed species sensitivity, the severity and associated significance
of the observed or predicted O3-induced effect, the role
that the species plays in the ecosystem, the intended use of the
affected species and its associated ecosystem and services, the
presence of other co-occurring predisposing or mitigating factors, and
associated uncertainties and limitations.
3. Exposures Associated With Effects
    The welfare effects identified in section III.B.1 above vary widely
with regard to the extent and level of detail of the available
information that describes the O3 exposure circumstances
that may elicit them. As recognized in the 2013 ISA and in the ISA for
this review, such information is most advanced for growth-related
effects such as growth and yield. For example, the information on
exposure metric and E-R relationships for these effects is long-
standing, having been first described in the 1997 review. The current
information regarding exposure metrics and relationships between
exposure and the occurrence and severity of visible foliar injury,
summarized in section III.B.3.b below, is much less advanced or well
established. The evidence base for other categories of effects is still
more lacking in information that might support characterization of
potential impacts related to these effects of changes in O3
concentrations.
a. Growth-Related Effects
(i) Exposure Metric
    The long-standing body of vegetation effects evidence includes a
wealth of information on aspects of O3 exposure that are
important in influencing effects on plant growth and yield that has
been described in the scientific assessments across the last several
decades (1996 AQCD; 2006 AQCD; 2013 ISA; 2020 ISA). A variety of
factors have been investigated, including ``concentration, time of day,
respite time, frequency of peak occurrence, plant phenology,
predisposition, etc.'' (2013 ISA, section 9.5.2), and the importance of
the duration of the exposure as well as the relatively greater
importance of higher concentrations over lower concentrations have been
consistently well documented (2013 ISA, section 9.5.3). Based on the
associated improved understanding of the biological basis for plant
response to O3 exposure, a number of mathematical approaches
have been developed for summarizing O3 exposure for the
purpose of assessing effects on vegetation, including those that
cumulate exposures over some specified period while weighting higher
concentrations more than lower (2013 ISA, sections 9.5.2 and 9.5.3;
ISA, Appendix 8, section <IP_ADDRESS>).
    In the last several reviews, based on the then-available evidence,
as well as advice from the CASAC, the EPA's scientific assessments have
focused on the use of a cumulative, seasonal \140\ concentration-
weighted index for considering the growth-related effects evidence and
in quantitative exposure analyses for purposes of reaching conclusions
on the secondary standard. More specifically, the Agency used the W126-
based cumulative, seasonal metric (80 FR 65404, October 26, 2015; ISA,
section IS.3.2, Appendix 8, section 8.13). This metric, commonly called
the W126 index, is a non-threshold approach described as the
sigmoidally weighted sum of all hourly O3 concentrations
observed during a specified daily and seasonal time window, where each
hourly O3 concentration is given a weight that increases
from zero to one with increasing concentration (2013 ISA, pp. 9-101, 9-
104).
---------------------------------------------------------------------------

    \140\ The ``seasonal'' descriptor refers to the duration of the
period quantified (3 months) rather than a specific season of the
year.
---------------------------------------------------------------------------

    Across the last several decades, several different exposure metrics
have been evaluated, primarily for their ability to summarize ambient
air O3 concentrations into a metric that best describes
quantitatively the relationship of O3 in ambient air with
the occurrence and/or extent of effects on vegetation, particularly
growth-related effects. More specifically, an important objective has
been to identify the metric that summarizes O3 exposure in a
way that is most predictive of the effect of interest (e.g., reduced
growth). Along with the continuous weighted, W126 index, the two other
cumulative indices that have received greatest attention across the
past several O3 NAAQS reviews are the threshold weighted
indices, AOT60 \141\ and SUM06.\142\ Accordingly, some studies of
O3 vegetation effects have reported exposures using these
metrics. Alternative methods for characterizing O3 exposure
to predict various plant responses (particularly those related to
photosynthesis, growth and productivity) have, in recent years, also
included flux models (models that are based on the amount of
O3 that enters the leaf). However, as was the case in the
last review, there remain a variety of complications, limitations and
uncertainties associated with this approach. For example, ``[w]hile
some efforts have been made in the U.S. to calculate ozone flux into
leaves and canopies, little information has been published relating
these fluxes to effects on vegetation'' (ISA, section IS.3.2). Further,
as flux of O3 into the plant under different conditions of
O3 in ambient air is affected by several factors including
temperature, vapor pressure deficit, light, soil moisture, and plant
growth stage, use of this approach to quantify the vegetation impact of
O3 would require information on these various types of
factors (ISA, section IS.3.2). In addition to these data requirements,
each species has different amounts of internal detoxification potential
that may protect species to differing degrees. The lack of detailed
species- and site-specific data required for flux modeling in the U.S.
and the lack of understanding of detoxification

processes continues to make this technique less viable for use in risk
assessments in the U.S. (ISA, section IS.3.2).
---------------------------------------------------------------------------

    \141\ The AOT60 index is the seasonal sum of the difference
between an hourly concentration above 60 ppb, minus 60 ppb (2006
AQCD, p. AX9-161). More recently, some studies have also reported
O3 exposures in terms of AOT40, which is conceptually
similar but with 40 substituted for 60 in its derivation (ISA,
Appendix 8, section 8.13.1).
    \142\ The SUM06 index is the seasonal sum of hourly
concentrations at or above 0.06 ppm during a specified daily time
window (2006 AQCD, p. AX9-161; 2013 ISA, section 9.5.2). This may
sometimes be referred to as SUM60, e.g., when concentrations are in
terms of ppb. There are also variations on this metric that utilize
alternative reference points above which hourly concentrations are
summed. For example, SUM08 is the seasonal sum of hourly
concentrations at or above 0.08 ppm and SUM0 is the seasonal sum of
all hourly concentrations.
---------------------------------------------------------------------------

    Based on extensive review of the published literature on different
types of E-R metrics, including comparisons between metrics, the EPA
has generally focused on cumulative, concentration-weighted indices of
exposure, recognizing them as the most appropriate biologically based
metrics to consider in this context (1996 AQCD; 2006 AQCD; 2013 ISA).
Quantifying exposure in this way has been found to improve the
explanatory power of E-R models for growth and yield over using indices
based only on mean and peak exposure values (2013 ISA, section <IP_ADDRESS>,
p. 2-44). The most well-analyzed datasets in such evaluations are two
detailed datasets established two decades ago, one for seedlings of 11
tree species and one for 10 crops, described further in section
III.B.3.a(ii) below (e.g., Lee and Hogsett, 1996, Hogsett et al.,
1997). These datasets, which include species-specific seedling growth
and crop yield response information across multiple seasonal cumulative
exposures, were used to develop robust quantitative E-R functions to
predict growth reduction relative to a zero-O3 setting
(termed relative biomass loss or RBL) in seedlings of the tree species
and E-R functions for RYL for a set of common crops (ISA, Appendix 8,
section 8.13.2; 2013 ISA, section 9.6.2).
    Among the studies newly available in this review, no new exposure
indices for assessing effects on vegetation growth or other
physiological process parameters have been identified. The SUM06, AOTx
(e.g., AOT60) and W126 exposure metrics remain the cumulative metrics
that are most commonly discussed (ISA, Appendix 8, section 8.13.1). The
ISA notes that ``[c]umulative indices of exposure that differentially
weight hourly concentrations [which would include the W126 index] have
been found to be best suited to characterize vegetation exposure to
ozone with regard to reductions in vegetation growth and yield'' (ISA,
section ES.3). Accordingly, in this review, as in the last two reviews,
the seasonal W126-based cumulative, concentration-weighted metric
receives primary attention in considering the effects evidence and
exposure analyses, particularly related to growth effects (e.g., in
sections III.C and III.D below).
    The first step in calculating the seasonal W126 index for a
specific year, as described and considered in this review, is to sum
the weighted hourly O3 concentrations in ambient air during
daylight hours (defined as 8:00 a.m. to 8:00 p.m. local standard time)
within each calendar month, resulting in monthly index values. The
monthly W126 index values are calculated from hourly O3
concentrations as follows.\143\
---------------------------------------------------------------------------

    \143\ In situations where data are missing, an adjustment is
factored into the monthly index (PA, Appendix 4D, section 4D.2.2).
[GRAPHIC] [TIFF OMITTED] TP14AU20.001

---------------------------------------------------------------------------
where,

N is the number of days in the month
d is the day of the month (d = 1, 2, . . ., N)
h is the hour of the day (h = 0, 1, . . ., 23)
Cdh is the hourly O3 concentration observed on
day d, hour h, in parts per million

    The W126 index value for a specific year is the maximum sum of the
monthly index values for three consecutive months within a calendar
year (i.e., January to March, February to April, . . . October to
December). Three-year average W126 index values are calculated by
taking the average of seasonal W126 index values for three consecutive
years (e.g., as described in the PA, Appendix 4D, section 4D.2.2).
(ii) Relationships Between Exposure Levels and Effects
    Across the array of O3-related welfare effects,
consistent and systematically evaluated information on E-R
relationships across multiple exposure levels is limited. Most
prominent is the information on E-R relationships for growth effects on
tree seedlings and crops,\144\ which has been available for the past
several reviews. The information on which these functions are based
comes primarily from the U.S. EPA's National Crop Loss Assessment
Network (NCLAN) \145\ project for crops and the NHEERL-WED project for
tree seedlings, projects implemented primarily to define E-R
relationships for major agricultural crops and tree species, thus
advancing understanding of responses to O3 exposures (ISA,
Appendix 8, section 8.13.2). These projects included a series of
experiments that used OTCs to investigate tree seedling growth response
and crop yield over a growing season under a variety of O3
exposures and growing conditions (2013 ISA, section 9.6.2; Lee and
Hogsett, 1996). These experiments have produced multiple studies that
document O3 effects on tree seedling growth and crop yield
across multiple levels of exposure. Importantly, the information on
exposure includes hourly concentrations across the season-long (or
longer) exposure period which can then be summarized in terms of the
various seasonal metrics.\146\ In the initial analyses of these data,
exposure was characterized in terms of several metrics, including
seasonal SUM06 and W126 indices (Lee and Hogsett, 1996; 1997 Staff
Paper, sections IV.D.2 and IV.D.3; 2007 Staff Paper, section 7.6),
while use of these functions more recently has focused on their
implementation in terms of seasonal W126 index (2013 ISA, section 9.6;
80 FR 65391-92, October 26, 2015).
---------------------------------------------------------------------------

    \144\ The E-R functions estimate O3-related reduction
in a year's tree seedling growth or crop yield as a percentage of
that expected in the absence of O3 (ISA, Appendix 8,
section 8.13.2).
    \145\ The NCLAN program, which was undertaken in the early to
mid-1980s, assessed multiple U.S. crops, locations, and
O3 exposure levels, using consistent methods, to provide
the largest, most uniform database on the effects of O3
on agricultural crop yields (1996 AQCD, 2006 AQCD, 2013 ISA,
sections 9.2, 9.4, and 9.6; ISA, Appendix 8, section 8.13.2).
    \146\ This underlying database for the exposure is a key
characteristic that sets this set of studies (and their associated
E-R analyses) apart from other available studies.
---------------------------------------------------------------------------

    The 11 tree species for which robust and well-established E-R
functions for RBL are available are black cherry, Douglas fir, loblolly
pine, ponderosa pine, quaking aspen, red alder, red maple, sugar maple,
tulip poplar, Virginia pine, and white pine (PA, Appendix 4A; 2013 ISA,
section 9.6).\147\ While these 11 species represent only a small
fraction of the total number of native tree species in the contiguous
U.S., this small subset includes eastern and western species, deciduous
and coniferous species, and species that

grow in a variety of ecosystems and represent a range of tolerance to
O3 (PA, Appendix 4B; 2013 ISA, section 9.6.2). The
established E-R functions for most of the 11 species were derived using
data from multiple studies or experiments involving a wide range of
exposure and/or growing conditions. From the available data, separate
E-R functions were developed for each combination of species and
experiment (2013 ISA, section 9.6.1; Lee and Hogsett, 1996). From these
separate species-experiment-specific E-R functions, species-specific
composite E-R functions were developed (PA, Appendix 4A).
---------------------------------------------------------------------------

    \147\ A quantitative analysis of E-R information for an
additional species was considered in the 2014 WREA. But the
underlying study, rather than being a controlled exposure study,
involves exposure to ambient air along an existing gradient of
O3 concentrations in the New York City metropolitan area,
such that O3 and climate conditions were not controlled
(2013 ISA, section <IP_ADDRESS>). Based on recognition that this dataset
is not as strong as those for the 11 species for which E-R functions
are based on controlled ozone exposure, this study is not included
with the established E-R functions for the 11 species (PA, section
4.3.3).
---------------------------------------------------------------------------

    In total, the 11 species-specific composite E-R functions are based
on 51 tree seedling studies or experiments (PA, Appendix 4A, section
4A.1.1). For six of the 11 species, this function is based on just one
or two studies (e.g., red maple and black cherry), while for other
species there were as many as 11 studies available (e.g., ponderosa
pine). A stochastic analysis drawing on the experiment-specific
functions provides a sense of the variability and uncertainty
associated with the estimated E-R relationships among and within
species (PA, Appendix 4A, section 4A.1.1, Figure 4A-13). Based on the
species-specific E-R functions, growth of the studied tree species at
the seedling stage appears to vary widely in sensitivity to
O3 exposure (PA, Appendix 4A, section 4A.1.1). Since the
initial set of studies were completed, several additional studies,
focused on aspen, have been published based on the Aspen FACE
experiment in a planted forest in Wisconsin; the findings were
consistent with many of the earlier OTC studies (ISA, Appendix 8,
section 8.13.2).
    With regard to crops, established E-R functions are available for
10 crops: Barley, field corn, cotton, kidney bean, lettuce, peanut,
potato, grain sorghum, soybean and winter wheat (PA, Appendix 4A,
section 4A.1; ISA, Appendix 8, section 8.13.2). Studies available since
the last review for seven soybean cultivars support conclusions from
prior studies that of similarity of current soybean cultivar
sensitivity compared to the earlier genotypes from which the soybean E-
R functions were (ISA, Appendix 8, section 8.13.2).
    Newly available studies that investigated growth effects of
O3 exposures are also consistent with the existing evidence
base, and generally involve particular aspects of the effect rather
than expanding the conditions under which plant species, particularly
trees, have been assessed (ISA, section IS.5.1.2). These include a
compilation of previously available studies on plant biomass response
to O3 (in terms of AOT40); the compilation reports linear
regressions conducted on the associated varying datasets (ISA, Appendix
8, section 8.13.2; van Goethem et al., 2013). Based on these
regressions, this study describes distributions of sensitivity to
O3 effects on biomass across nearly 100 plant species (trees
and grasslands) including 17 species native to the U.S. and 65
additional species that have been introduced to the U.S. (ISA, Appendix
8, section 8.13.2; van Goethem et al., 2013). Additional information is
needed to more completely describe O3 exposure response
relationships for these species in the U.S.\148\
---------------------------------------------------------------------------

    \148\ The set of studies included in this compilation were
described as meeting a set of criteria, such as: Including
O3 only exposures in conditions described as ``close to
field'' exposures (which were expressed as AOT40); including at
least 21 days exposure above 40 ppb O3; and having a
maximum hourly concentration that was no higher than 100 ppb (van
Goethem et al., 2013). The publication does not report exposure
duration for each study or details of biomass response measurements,
making it less useful for the purpose of describing E-R
relationships that might provide for estimation of specific impacts
associated with air quality conditions meeting the current standard
(e.g., 2013 ISA, p. 9-118).
---------------------------------------------------------------------------

b. Visible Foliar Injury
    With regard to visible foliar injury, as with the evidence
available in the last review, the current evidence ``continues to show
a consistent association between visible injury and ozone exposure,''
while also recognizing the role of modifying factors such as soil
moisture and time of day (ISA, section IS.5.1.1). The current ISA, in
concluding that the newly available information is consistent with
conclusions of the 2013 ISA, also summarizes several recently available
studies that continue to document that O3 elicits visible
foliar injury in many plant species. These include a synthesis of
previously published studies that categorizes studied species (and
their associated taxonomic classifications) as to whether or not
O3-related foliar injury has been reported to occur in the
presence of elevated O3,\149\ while not providing
quantitative information regarding specific exposure conditions or
analyses of E-R relationships (ISA, Appendix 8, section 8.2). The
evidence in the current review, as was the case in the last review,
while documenting that elevated O3 conditions in ambient air
generally results in visible foliar injury in sensitive species (when
in a predisposing environment),\150\ does not include a quantitative
description of the relationship of incidence or severity of visible
foliar injury in sensitive species in natural locations in the U.S.
with specific metrics of O3 exposure.
---------------------------------------------------------------------------

    \149\ The publication identifies 245 species across 28 plant
genera, many native to the U.S., in which O3-related
visible foliar injury has been reported (ISA, Appendix 8, Table 8-
3).
    \150\ As noted in the 2013 ISA and the ISA for the current
review, visible foliar injury usually occurs when sensitive plants
are exposed to elevated ozone concentrations in a predisposing
environment, with a major modifying factor being the amount of soil
moisture available to a plant. Accordingly, dry periods are
concluded to decrease the incidence and severity of ozone-induced
visible foliar injury, such that the incidence of visible foliar
injury is not always higher in years and areas with higher ozone,
especially with co-occurring drought (ISA, Appendix 8, p. 8-23;
Smith, 2012; Smith et al., 2003).
---------------------------------------------------------------------------

    Several studies of the extensive USFS field-based dataset of
visible foliar injury incidence in forests across the U.S.\151\
illustrate the extent to which our current understanding of this
relationship is limited. For example, a study that was available in the
last review presents a trend analysis of these data for sites located
in 24 states of the northeast and north central U.S. for the 16-year
period from 1994 through 2009 that provides some insight into the
influence of changes in air quality and soil moisture on visible foliar
injury and the difficulty inherent in predicting foliar injury response
under different air quality and soil moisture scenarios (Smith, 2012,
Smith et al., 2012; ISA, Appendix 8, section 8.2). This study, like
prior analyses of such data, shows the dependence of foliar injury
incidence and severity on local site conditions for soil moisture
availability and O3 exposure. For example, while the authors
characterize the ambient air O3 concentrations to be the
``driving force'' behind incidence of injury and its severity, they
state that ``site moisture conditions are also a very strong influence
on the biomonitoring data'' (Smith et al., 2003). In general, the USFS
data analyses have found foliar injury prevalence and severity to be
higher during seasons and sites that have experienced the highest
O3 than during other periods (e.g., Campbell et al., 2007;
Smith, 2012).
---------------------------------------------------------------------------

    \151\ These data were collected as part of the U.S. Forest
Service Forest Health Monitoring/Forest Inventory and Analysis (USFS
FHM/FIA) biomonitoring network program (2013 ISA, section <IP_ADDRESS>;
Campbell et al., 2007, Smith et al., 2012).
---------------------------------------------------------------------------

    Although studies of the incidence of visible foliar injury in
national forests, wildlife refuges, and similar areas have often used
cumulative indices (e.g., SUM06) to investigate variations in incidence
of foliar injury, studies also suggest an additional role for metrics
focused on peak concentrations (ISA; 2013 ISA; 2006 AQCD; Hildebrand et
al., 1996; Smith, 2012). For example, a

study of six years of USFS biosite \152\ data (2000-2006) for three
western states found that the biosites with the highest O3
exposure (SUM06 at or above 25 ppm-hrs) had the highest percentage of
biosites with injury and the highest mean BI, with little discernable
difference among the lower exposure categories; this study also
identified ``better linkage between air levels and visible injury'' as
an O3 research need (Campbell et al., 2007).\153\ More
recent studies of the complete 16 years of data in 24 northeast and
north central states have suggested that a cumulative exposure index
alone may not completely describe the O3-related risk of
this effect at USFS sites (Smith et al., 2012; Smith, 2012). For
example, Smith (2012) observed there to be a declining trend in the 16-
year dataset, ``especially after 2002 when peak ozone concentrations
declined across the entire region'' thus suggesting a role for peak
concentrations.
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    \152\ As described in section III.B.2 above, biosites are
biomonitoring sites where the USFS applies a scoring system for
purposes of categorizing areas with regard to severity of visible
foliar injury occurrence (U.S. Forest Service, 2010).
    \153\ In considering their findings, the authors expressed the
view that ``[a]lthough the number of sites or species with injury is
informative, the average biosite injury index (which takes into
account both severity and amount of injury on multiple species at a
site) provides a more meaningful measure of injury'' for their
assessment at a statewide scale (Campbell et al., 2007).
---------------------------------------------------------------------------

    Some studies of visible foliar injury incidence data have
investigated the role of peak concentrations quantified by an
O3 exposure index that is a count of hourly concentrations
(e.g., in a growing season) above a threshold 1-hour concentration of
100 ppb, N100 (e.g., Smith, 2012; Smith et al., 2012). For example, the
study by Smith (2012) discussed injury patterns at biosites in 24
states in the Northeast and North Central regions in the context of the
SUM06 index and N100 metrics (although not via a statistical
model).\154\ That study of 16 years of biomonitoring data from these
sites suggested that there may be a threshold exposure needed for
injury to occur, and the number of hours of elevated O3
concentrations during the growing season (such as what is captured by a
metric like N100) may be more important than cumulative exposure in
determining the occurrence of foliar injury (Smith, 2012).\155\ The
study's authors noted this finding to be consistent with findings
reported by a study of statistical analyses of seven years of visible
foliar injury data from a wildlife refuge in the mid-Atlantic (Davis
and Orendovici, 2006, Smith et al., 2012). The latter study
investigated the fit of multiple models that included various metrics
of cumulative O3 (SUM06, SUM0, SUM08), alone and in
combination with some other variables (Davis and Orendovici, 2006).
Among the statistical models investigated (which did not include one
with either W126 index or N100 alone), the model with the best fit to
the visible foliar injury incidence data was found to be one that
included the cumulative metric, W126, and the N100 index, as well as
drought index (Davis and Orendovici, 2006).\156\
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    \154\ The current ISA, 2013 ISA and prior AQCDs have not
described extensive evaluation of specific peak-concentration
metrics such as the N100 that might assist in identifying the one
best suited for such purposes.
    \155\ In summarizing this study in the last review, the ISA
observed that ``[o]verall, there was a declining trend in the
incidence of foliar injury as peak O3 concentrations
declined'' (2013 ISA, p. 9-40).
    \156\ The models evaluated included several with cumulative
exposure indices alone. These included SUM60, SUM0, and SUM80, but
not W126. They did not include a model with W126 that did not also
include N100. Across all of the models evaluated, the model with the
best fit to the data was found to be the one that included N100 and
W126, along with the drought index (Davis and Orendovici, 2006).
---------------------------------------------------------------------------

    The established significant role of higher or peak O3
concentrations, as well as pattern of their occurrence, in plant
responses has been noted in prior ISAs or AQCDs. In identifying support
with regard to foliar injury as the response, the 2013 ISA and 2006
AQCD both cite studies that support the ``important role that peak
concentrations, as well as the pattern of occurrence, plays in plant
response to O3'' (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-
169). For example, a study of European white birch saplings reported
that peak concentrations and the duration of the exposure event were
important determinants of foliar injury (2013 ISA, section <IP_ADDRESS>;
Oksanen and Holopainen, 2001). This study also evaluated tree growth,
which was found to be more related to cumulative exposure (2013 ISA, p.
9-105).\157\ A second study that was cited by both assessments that
focused on aspen, reported that ``the variable peak exposures were
important in causing injury, and that the different exposure
treatments, although having the same SUM06, resulted in very different
patterns of foliar injury'' (2013 ISA, p. 9-105; 2006 AQCD, p. AX9-169;
Yun and Laurence, 1999). As noted in the 2006 AQCD, the cumulative
exposure indices (e.g., SUM06, W126) were ``originally developed and
tested using only growth/yield data, not foliar injury'' and ``[t]his
distinction is critical in comparing the efficacy of one index to
another'' (2006 AQCD, p. AX9-173). It is also recognized that where
cumulative indices are highly correlated with the frequency or
occurrence of higher hourly average concentrations, they could be good
predictors of such effects (2006 AQCD, section AX<IP_ADDRESS>).
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    \157\ The study authors concluded that ``high peak
concentrations were important for visible injuries and stomatal
conductance, but less important for determining growth responses''
(Oksanen and Holopainen, 2001).
---------------------------------------------------------------------------

    In a more recent study (by Wang et al. [2012]) that is cited in the
current ISA, a statistical modeling analysis was performed on a subset
of the years of data that were described in Smith (2012). This
analysis, which involved 5,940 data records from 1997 through 2007 from
the 24 northeast and north central states, tested a number of models
for their ability to predict the presence of visible foliar injury (a
nonzero biosite score), regardless of severity, and generally found
that the type of O3 exposure metric (e.g., SUM06 versus
N100) made only a small difference, although the models that included
both a cumulative index (SUM06) and N100 had a just slightly better fit
(Wang et al., 2012). Based on their investigation of 15 different
models, using differing combination of several types of potential
predictors, the study authors concluded that they were not able to
identify environmental conditions under which they ``could reliably
expect plants to be damaged'' (Wang et al., 2012). This is indicative
of the current state of knowledge, in which there remains a lack of
established quantitative functions describing E-R relationships that
would allow prediction of visible foliar injury severity and incidence
under varying air quality and environmental conditions.
    The available information related to O3 exposures
associated with visible foliar injury of varying severity also includes
the dataset developed by the EPA in the last review from USFS BI
scores, collected during the years 2006 through 2010 at locations in 37
states. In developing this dataset, the BI scores were combined with
estimates of soil moisture \158\ and estimates of seasonal cumulative
O3 exposure in terms of

W126 index \159\ (Smith and Murphy, 2015; PA, Appendix 4C). This
dataset includes more than 5,000 records of which more than 80 percent
have a BI score of zero (indicating a lack of visible foliar injury).
While the estimated W126 index assigned to records in this dataset
ranges from zero to somewhat above 50 ppm-hrs, more than a third of all
the records (and also of records with BI scores above zero or five)
\160\ are at sites with W126 index estimates below 7 ppm-hrs.
---------------------------------------------------------------------------

    \158\ Soil moisture categories (dry, wet or normal) were
assigned to each biosite record based on the NOAA Palmer Z drought
index values obtained from the NCDC website for the April-through-
August periods, averaged for the relevant year; details are provided
in the PA, Appendix 4C, section 4C.2. There are inherent
uncertainties in this assignment, including the substantial spatial
variation in soil moisture and large size of NOAA climate divisions
(hundreds of miles). This dataset, including associated
uncertainties and limitations, is described in the PA, Appendix 4C,
section 4C.5.
    \159\ The W126 index values assigned to the biosite locations
are estimates developed for 12 kilometer (km) by 12 km cells in a
national-scale spatial grid for each year. The grid cell estimates
were derived from applying a spatial interpolation technique to
annual W126 values derived from O3 measurements at
ambient air monitoring locations for the years corresponding to the
biosite surveys (details in the PA, Appendix 4C, sections 4.C.2 and
4C.5).
    \160\ One third (33%) of scores above 15 are at sites with W126
below 7 ppm-hrs (PA, Appendix 4C, Table 4C-3).
---------------------------------------------------------------------------

    In an extension of analyses of this dataset developed in the last
review, the presentation in the PA \161\ describes the BI scores for
the records in the dataset in relation to the W126 index estimate for
each record, using bins of increasing W126 index values. The PA
presentation utilizes the BI score breakpoints in the scheme used by
the USFS to categorize severity. The lowest USFS category encompasses
BI scores from zero to just below 5; scores of this magnitude are
described as ``little or no foliar injury'' (Smith et al., 2012). The
next highest category encompasses scores from five to just below 15 and
is described as ``light to moderate foliar injury,'' BI scores of 15 up
to 25 are described as ``moderate'' and above 25 is described as
``severe'' (Smith et al., 2012). The PA presentation indicates that
across the W126 bins, there is variation in both the incidence of
particular magnitude BI scores and in the average score per bin. In
general, however the greatest incidence of records with BI scores above
zero, five, or higher--and the highest average BI score--occurs with
the highest W126 bin, i.e., the bin for W126 index estimates greater
than 25 ppm-hrs (PA, Appendix 4C, Table 4C-6).
---------------------------------------------------------------------------

    \161\ Beyond the presentation of a statistical analysis
developed in the last review, the PA presentations are primarily
descriptive (as compared to statistical) in recognition of the
limitations and uncertainties of the dataset (PA, Appendix 4C,
section 4C.5).
---------------------------------------------------------------------------

    While recognizing limitations in the dataset,\162\ the PA makes
several observations, focusing particularly on records in the normal
soil category (PA, section 4.5.1). For records categorized as wet soil
moisture, the sample size for the W126 bins above 13 ppm-hrs is quite
small (including only 18 of the 1,189 records in that soil moisture
category), precluding meaningful interpretation.\163\ For the normal
soil category, the percentages of records in the greater than 25 ppm-
hrs bin that have BI scores above 15 (``moderate'' and ``severe''
injury) or above 5 (``little,'' ``moderate'' and ``severe'' injury) are
both more than three times greater than such percentages in any of the
lower W126 bins.\164\ For example, the proportion of records with BI
above five fluctuates between 5% and 13% across all but the highest
W126 bin (>25 ppm-hrs) for which the proportion is 41% (PA, Appendix
4C, Table 4C-6). The same pattern is observed for BI scores above 15 at
sites with normal and dry soil moisture conditions, albeit with lower
incidences. For example, the incidence of normal soil moisture records
with BI score above 15 in the bin for W126 index values above 25 ppm-
hrs was 20% but fluctuates between 1% and 4% in the bin for W126 index
values at or below 25 ppm-hrs (PA, Appendix 4C, Table 4C-6). The
average BI of 7.9 in the greater-then-25-ppm-hrs bin is more than three
times the next highest W126 bin average. The average BI in each of the
next two lower W126 bins is just slightly higher than average BIs for
the rest of the bins, and the average BI for all bins at or below 25
ppm-hrs are well below 5 (PA, Appendix 4C).
---------------------------------------------------------------------------

    \162\ For example, the majority of records have W126 index
estimates at or below 9 ppm-hrs, and fewer than 10% have W126
estimates above 15 ppm-hrs. Further, the BI scores are quite
variable across the range of W126 bins, with even the lowest W126
bin (estimates below 7 ppm-hrs) including BI scores well above 15
(PA, Appendix 4C, section 4C.4.2). The records for the wet soil
moisture category in the higher W126 bins are more limited that the
other categories, with nearly 90% of the wet soil moisture records
falling into the bins for W126 index at or below 9 ppm-hrs, limiting
interpretations for higher W126 bins (PA, Appendix 4C, Table 4C.4
and section 4C.6). Accordingly, the PA observations focused
primarily on the records for the normal or dry soil moisture
categories, for which W126 index above 13 ppm-hrs is better
represented.
    \163\ The full database includes only 18 records at sites in the
wet soil moisture category with estimated W126 index above 13 ppm-
hrs, with 9 or fewer (less than 1%) in each of the W126 bins above
13 ppm-hrs (PA, Appendix 4C, Table 4C-3). Among the bins for W126 at
or below 13 ppm-hrs, the average BI score is less than 2 (PA,
Appendix 4C, Table 4C-5).
    \164\ When scores characterized as ``little injury'' by the USFS
classification scheme are also included (i.e., when considering all
scores above zero), there is a suggestion of increased frequency of
records for the W126 bins above 19 or 17 ppm-hrs, although
difference from lower bins is less than a factor of two (PA,
Appendix 4C).
---------------------------------------------------------------------------

    Overall, the dataset described in the PA generally indicates the
risk of injury, and particularly injury considered at least light,
moderate or severe, to be higher at the highest W126 index values, with
appreciable variability in the data for the lower bins (PA, Appendix
4C). This appears to be consistent with the conclusions of the studies
of detailed quantitative analyses, summarized above, that the pattern
is stronger at higher O3 concentrations. A number of factors
may contribute to the observed variability in BI scores and lack of a
clear pattern with W126 index bin; among others, these may include
uncertainties in assignment of W126 estimates and soil moisture
categories to biosite locations, variability in biological response
among the sensitive species monitored, and the potential role of other
aspects of O3 air quality not captured by the W126 index.
Thus, the dataset has limitations affecting associated conclusions and
uncertainty remains regarding the tools for and the appropriate metric
(or metrics) for quantifying O3 exposures, as well as
perhaps soil moisture conditions, with regard to their influence on
extent and/or severity of injury in sensitive species in natural areas
(Davis and Orendovici, 2006, Smith et al., 2012; Wang et al., 2012).
    Dose modeling or flux models (referenced in section III.B.3.a(i)
above, have also been considered for quantifying O3 dose
that may be related to plant leaf injury. Among the newly available
evidence is a study examining relationships between short-term flux and
leaf injury on cotton plants that described a sensitivity parameter
that might characterize the influence on the flux-injury relationship
of diel and seasonal variability in plant defenses (among other
factors) and suggested additional research might provide for such a
sensitivity parameter to ``function well in combination with a
sigmoidal weighting of flux, analogous to the W126 weighting of
concentration'', and perhaps an additional parameter (Grantz et al.,
2013, p. 1710; ISA, Appendix 8, section 8.13.1). However, the ISA
recognizes there is ``much unknown'' with regard to the relationship
between O3 uptake and leaf injury, and relationships with
detoxification processes (ISA, Appendix 8, section 8.13.1 and p. 8-
184). These uncertainties have made this technique less viable for
assessments in the U.S., precluding use of a flux-based approach at
this time (ISA, Appendix 8, section 8.13.1 and p. 8-184).
c. Other Effects
    With regard to radiative forcing and subsequent climate effects
associated with the global tropospheric abundance of O3, the
newly available evidence in this review does not provide more detailed
quantitative information

regarding O3 concentrations at the national scale. For
example, tropospheric O3 continues to be recognized as
having a causal relationship with radiative forcing, although
``uncertainty in the magnitude of radiative forcing estimated to be
attributed to tropospheric ozone is a contributor to the relatively
greater uncertainty associated with climate effects of tropospheric
ozone compared to such effects of the well mixed greenhouse gases
(e.g., carbon dioxide and methane)'' (ISA, section IS.6.2.2).
    While tropospheric O3 also continues to be recognized as
having a likely causal relationship with subsequent effects on
temperature, precipitation and related climate variables, the non-
uniform distribution of O3 within the troposphere (spatially
and temporally) makes the development of quantitative relationships
between the magnitude of such effects and differing O3
concentrations in the U.S. challenging (ISA, Appendix 9). Additionally,
``the heterogeneous distribution of ozone in the troposphere
complicates the direct attribution of spatial patterns of temperature
change to ozone induced [radiative forcing]'' and there are ``ozone
climate feedbacks that further alter the relationship between ozone
[radiative forcing] and temperature (and other climate variables) in
complex ways'' (ISA, Appendix 9, section 9.3.1, p. 9-19). Thus, various
uncertainties ``render the precise magnitude of the overall effect of
tropospheric ozone on climate more uncertain than that of the well-
mixed GHGs'' and ``[c]urrent limitations in climate modeling tools,
variation across models, and the need for more comprehensive
observational data on these effects represent sources of uncertainty in
quantifying the precise magnitude of climate responses to ozone
changes, particularly at regional scales'' (ISA, section IS.6.2.2,
Appendix 9, section 9.3.3, p. 9-22). For example, current limitations
in modeling tools include ``uncertainties associated with simulating
trends in upper tropospheric ozone concentrations'' (ISA, section
9.3.1, p. 9-19), and uncertainties such as ``the magnitude of
[radiative forcing] estimated to be attributed to tropospheric ozone''
(ISA, section 9.3.3, p. 9-22). Further, ``precisely quantifying the
change in surface temperature (and other climate variables) due to
tropospheric ozone changes requires complex climate simulations that
include all relevant feedbacks and interactions'' (ISA, section 9.3.3,
p. 9-22). For example, an important limitation in current climate
modeling capabilities for O3 is representation of important
urban- or regional-scale physical and chemical processes, such as
O3 enhancement in high-temperature urban situations or