O3 chemistry in city centers where NOx is abundant. Such
limitations impede our ability to quantify the impact of incremental
changes in O3 concentrations in the U.S. on radiative
forcing and subsequent climate effects.
    With regard to tree mortality (the evidence for which the 2013 ISA
did not assess with regard to its support for inference of a causal
relationship with O3 exposure), the evidence available in
the last several reviews included field studies of pollution gradients
that concluded O3 damage to be an important contributor to
tree mortality although several confounding factors such as drought,
insect outbreak and forest management were identified as potential
contributors (2013 ISA, section <IP_ADDRESS>). Although three newly
available studies contribute to the ISA conclusion of sufficient
evidence to infer a likely causal relationship for O3 with
tree mortality (ISA, Appendix 8, section 8.4), there is only limited
experimental evidence that isolates the effect of O3 on tree
mortality and might be informative regarding O3
concentrations of interest in the review. This evidence, primarily from
an Aspen FACE study of aspen survival, involves cumulative seasonal
exposure to W126 index levels above 30 ppm-hrs during the first half of
the 11-year study period (ISA, Appendix 8, Tables 8-8 and 8-9).
Evidence is lacking regarding exposure conditions closer to those
occurring under the current standard and any contribution to tree
mortality.
    With regard to the two categories of welfare effects involving
insects (for which there are new causal determinations in this review),
there are multiple limitations and uncertainties regarding
characterization of exposure conditions that might elicit effects and
the comprehensive characterization of the effects (ISA, p. IS-91,
Appendix 8, section 8.6.3). For example, with regard to alteration of
herbivore growth and reproduction, although ``[t]here are multiple
studies demonstrating ozone effects on fecundity and growth in insects
that feed on ozone-exposed vegetation'', ``no consistent directionality
of response is observed across studies and uncertainties remain in
regard to different plant consumption methods across species and the
exposure conditions associated with particular severities of effects ''
(ISA, pp. ES-18). The ISA also notes the variation in study designs and
endpoints used to assess O3 response (ISA, IS.6.2.1 and
Appendix 8, section 8.6). Thus, while the evidence describes changes in
nutrient content and leaf chemistry following O3 exposure
(ISA, p. IS-73), the effect of these changes on herbivores consuming
the leaves is not well characterized, and factors such as identified
here preclude broader characterization, as well as quantitative
analysis related to air quality conditions meeting the O3
standard.
    The evidence for the second category, alteration of plant-insect
signaling, draws on new research that has provided clear evidence of
O3 modification of VPSCs and behavioral responses of insects
to these modified chemical signals (ISA, section IS.6.2.1). The
available evidence involves a relatively small number of plant species
and plant-insect associations. While the evidence documents effects on
plant production of signaling chemicals and on the atmospheric
persistence of signaling chemicals, as well as on the behaviors of
signal-responsive insects, it is limited with regard to
characterization of mechanisms and the consequences of any modification
of VPSCs by O3 (ISA, p. ES-18; sections ES.5.1.3 and
IS.6.2.1). Further, the available studies vary with regard to the
experimental exposure circumstances in which the different types of
effects have been reported (most of the studies have been carried out
in laboratory conditions rather than in natural environments), and many
of the studies involve quite short controlled exposures (hours to days)
to elevated concentrations, posing limitations for our purposes of
considering the potential for impacts associated with the studied
effects to be elicited by air quality conditions that meet the current
standard (ISA, section IS.6.2.1 and Appendix 8, section 8.7).
    With regard to previously recognized categories of vegetation-
related effects, other than growth and visible foliar injury, such as
reduced plant reproduction, reduced productivity in terrestrial
ecosystems, alteration of terrestrial community composition and
alteration of below-ground biogeochemical cycles, the newly available
evidence includes a variety of studies, as identified in the ISA (ISA,
Appendix 8, sections 8.4, 8.8 and 8.10). Across the studies, a variety
of metrics (including AOT40, 4- to 12-hour mean concentrations, and
others) are used to quantify exposure over varying durations and
various countries. The ISA additionally describes publications that
summarize previously published studies in several ways. For example, a
meta-analysis of reproduction studies categorized the reported
O3 exposures into bins of differing magnitude,

grouping differing concentration metrics and exposure durations
together, and performed statistical analyses to reach conclusions
regarding the presence of an O3-related effect (ISA,
Appendix 8, section 8.4.1). While such studies continue to support
conclusions of the ecological hazards of O3, they do not
improve capabilities for characterizing the likelihood of such effects
under varying patterns of environmental O3 concentrations
that occur with air quality conditions that meet the current standard.
    As at the time of the last review, growth impacts, most
specifically as evaluated by RBL for tree seedlings and RYL for crops,
remain the type of vegetation-related effects for which we have the
best understanding of exposure conditions likely to elicit them. Thus,
as was the case in the decision for the last review, the quantitative
analyses of exposures occurring under air quality that meets the
current standard, summarized below, are focused primarily on the W126
index, given its established relationship with growth effects.

C. Summary of Air Quality and Exposure Information

    The air quality and exposure analyses developed in this review,
like those in the last review, are of two types: (1) W126-based
cumulative exposure estimates in Class I areas; and (2) analyses of
W126-based exposures and their relationship with the current standard
for all U.S. monitoring locations (PA, Appendix 4D). As summarized in
the IRP, we identified these analyses to be updated in this review in
recognition of the relatively reduced uncertainty associated with the
use of these types of analyses (compared to the national or regional-
scale modeling analyses performed in the last review) to inform a
characterization of cumulative O3 exposure (in terms of the
W126 index) associated with air quality just meeting the current
standard (IRP, section 5.2.2). As in the last review, the lesser
uncertainty of these air quality monitoring-based analyses contributes
to their value in informing the current review. The sections below
present findings of the updated analyses that have been performed in
the current review using recently available information.
    As in the last review, the analyses focus on both the most recent
3-year period (2016 to 2018) for which data were available when the
analyses were performed, and also across the full historical period
back to 2000, which is now expanded from that available in the last
review.\165\ Design values (3-year average annual fourth-highest 8-hour
daily maximum concentration, also termed ``4th max metric'' in this
analysis) and W126 index values (in terms of the 3-year average) were
calculated at each site where sufficient data were available.\166\
Across the seventeen 3-year periods from 2000-2002 to 2016-2018, the
number of monitoring sites with sufficient data for calculation of
valid design values and W126 index values (across the 3-year design
value period) ranged from a low of 992 in 2000-2002 to a high of 1119
in 2015-2017. The specific monitoring sites differed somewhat across
the 19 years. There were 1,557 sites with sufficient data for
calculation of valid design values and W126 index values for at least
one 3-year period between 2000 and 2018, and 543 sites had such data
for all seventeen 3-year periods. Analyses in the current review are
based on the expanded set of air monitoring data now available \167\
(PA, Appendix 4D, section 4D.2.2).
---------------------------------------------------------------------------

    \165\ In the last review, the dataset analyzed included data
from 2000 through 2013, with the most recent period being 2011 to
2013 (Wells, 2015).
    \166\ Data adequacy requirements and methods for these
calculations are described in Appendix 4D, section 4D.2 of the PA.
    \167\ In addition to being expanded with regard to data for more
recent time periods than were available during the last review, the
current dataset also includes a small amount of newly available
older data for some rural monitoring sites that are now available in
the AQS.
---------------------------------------------------------------------------

    These analyses are based primarily on the hourly air monitoring
data that were reported to EPA from O3 monitoring sites
nationwide. In the recent and historical datasets, the O3
monitors (more than 1000 in the most recent period) are distributed
across the U.S., covering all nine NOAA climate regions and all 50
states (PA, Figure 4-6 and Appendix 4D, Table 4D-1). Some geographical
areas within these regions and states are more densely covered and well
represented by monitoring sites, while others may have sparse or no
data. Given that there has been a longstanding emphasis on urban areas
in the EPA's monitoring regulations, urban areas are generally well
represented in the U.S. dataset, with the effect being that the current
dataset is more representative of locations where people live than of
complete spatial coverage for all areas in the U.S., (i.e., the current
dataset is more population weighted than geographically weighted). As
O3 precursor sources are also generally more associated with
urban areas, one impact of this may be a greater representation of
relatively higher concentration sites (PA, section 4.4.3 and Appendix
4D, section 4D.4).
    With regard to Class I areas, of the 158 mandated federal Class I
areas, 65 (just over 40%) have or have had O3 monitors
within 15 km with valid design values, thus allowing inclusion in the
Class I area analysis. Even so, the Class I areas dataset includes
monitoring sites in 27 states distributed across all nine NOAA climatic
regions across the contiguous U.S, as well as Hawaii and Alaska. Some
NOAA regions have far fewer numbers of Class I areas with monitors than
others. For instance, the Central, Northeast, East North Central, and
South regions all have three or fewer Class I areas in the dataset.
However, these areas also have appreciably fewer Class I areas in
general when compared to the Southwest, Southeast, West, and West North
Central regions, which are more well represented in the dataset. The
West and Southwest regions are identified as having the largest number
of Class I areas, and they have approximately one third of those areas
represented with monitors, which include locations where W126 index
values are generally higher, thus playing a prominent role in the
analysis (PA, section 4.4.3 and Appendix 4D, section 4D.4).
    These updated air quality analyses, and what they indicate
regarding environmental exposures of interest in this review, are
summarized in the following two subsections which differ in their areas
of focus. The first subsection (section III.C.1) summarizes information
regarding relationships between air quality in terms of the form and
averaging time of the current standard and environmental exposures in
terms of the W126 index. The second subsection (section III.C.2)
summarizes findings of the analyses of the currently available
monitoring data with regard to the magnitude of environmental
exposures, in terms of the W126 index, in areas across the U.S., and
particularly in Class I areas, during periods in which air quality met
the current standard.
1. Influence of Form and Averaging Time of Current Standard on
Environmental Exposure
    In revising the standard in 2015 to the now-current standard, the
Administrator concluded that, with revision of the standard level, the
existing form and averaging time provided the control of cumulative
seasonal exposure circumstances needed for the public welfare
protection desired (80 FR 65408, October 26, 2015). The focus on
cumulative seasonal exposure as the type of exposure metric of interest
primarily reflects the

evidence on E-R relationships for plant growth (summarized in section
III.B.3 above). The 2015 conclusion was based on the air quality data
analyzed at that time (80 FR 65408, October 26, 2015). Analyses in the
current review of the now expanded set of air monitoring data, which
now span 19 years and 17 3-year periods, document similar findings as
from the analysis of data from 2000-2013 described in the last review
(PA, Appendix 4D, section 4D.2.2).
    Among the analyses performed is an evaluation of the variability in
the annual W126 index values across a 3-year period (PA, Appendix 4D,
section 4D.3.1.2). This evaluation was performed for all U.S.
monitoring sites with sufficient data available in the most recent 3-
year period, 2016 to 2018. This analysis indicates the extent to which
the three single-year W126 index values within a 3-year period deviate
from the average for the period. Across the full set of sites,
regardless of W126 index magnitude (or whether or not the current
standard is met), single-year W126 index values differ less than 15
ppm-hrs from the average for the 3-year period (PA, Appendix 4D, Figure
4D-6). Focusing on the approximately 850 sites meeting the current
standard (i.e., sites with a design value at or below 70 ppb), over 99%
of single-year W126 index values in this subset differ from the 3-year
average by no more than 5 ppm-hrs, and 87% by no more than 2 ppm-hrs
(PA, Appendix 4D, Figure 4D-7).
    Another air quality analysis performed for the current review
documents the positive nonlinear relationship that is observed between
cumulative seasonal exposure, quantified using the W126 index, and
design values, based on the form and averaging time of the current
standard. This relationship is shown for both the average W126 index
across the 3-year design value period and for W126 index values for
individual years within the period (PA, Figure 4-7). From this
presentation, it is clear that cumulative seasonal exposures, assessed
in terms of W126 index (in a year or averaged across years), are lower
at monitoring sites with lower design values. This is seen both for
design values above the level of the current standard (70 ppb), where
the slope is steeper (due to the sigmoidal weighting of higher
concentrations by the W126 index function), as well as for lower design
values that meet the current standard (PA, Figure 4-7). This
presentation also indicates some regional differences in the
relationship. For example, for the 2016-2018 period, at sites meeting
the current standard in the regions outside of the West and Southwest
regions, all 3-year average W126 index values are at or below 12 ppm-
hrs and all single-year values are at or below 16 ppm-hrs (PA, Figures
4-6 and 4-7). The W126 index values are generally higher in the West
and Southwest regions. However, the positive relationship between the
W126 index and the design value is evident in all nine regions (PA,
Figure 4-7).
    An additional analysis assesses the relationship between long-term
changes in design value and long-term changes in the W126 index. This
analysis is presented in detail in the PA and focuses on the
relationship between changes (at each monitoring site) in the 3-year
design value across the 16 design value periods from 2000-2002 to 2016-
2018 and changes in the W126 index over the same period (PA, Appendix
4D, section 4D.3.2.3).\168\ This analysis, performed using either the
3-year average W126 index or values for individual years, shows there
to be a positive, linear relationship between the changes in the W126
index and the changes in the design value at monitoring sites across
the U.S. (PA, Appendix 4D, Figure 4D-11). The existence of this
relationship means that a change in the design value at a monitoring
site was generally accompanied by a similar change in the W126 index.
Nationally, the W126 index (in terms of 3-year average) decreased by
approximately 0.62 ppm-hrs per ppb decrease in design value over the
full period from 2000 to 2018 (PA, Appendix 4D, Table 4D-12). This
relationship varies across the NOAA climate regions, with the greatest
change in the W126 index per unit change in design value observed in
the Southwest and West regions. Thus, the regions which had the highest
W126 index values at sites meeting the current standard (PA, Figure 4D-
6) also showed the greatest improvement in the W126 index per unit
decrease in their design values over the past 19 years (PA, Appendix
4D, Table 4D-12 and Figure 4D-14).
---------------------------------------------------------------------------

    \168\ At each site, the trend in values of a metric (W126 or
design value), in terms of a per-year change in metric value, is
calculated using the Theil-Sen estimator, a type of linear
regression method that chooses the median slope among all lines
through pairs of sample points. For example, if applying this method
to a dataset with metric values for four consecutive years (e.g.,
W1261, W1262, W1263,
W1264), the trend would be the median of the different
per-year changes observed in the six possible pairs of values
([W1264-W1263]/1, [W1263-
W1262]/1, [W1262-W1261]/1,
[W1264-W1262]/2, [W1263-
W1261]/2, [W1264-W1261]/3).
---------------------------------------------------------------------------

    The trends analyses indicate that going forward as design values
are reduced in areas that are presently not meeting the current
standard, the W126 index in those areas would also be expected to
decline (PA, Appendix 4D, section 4D.3.2.3 and 4D.5). The overall trend
showing reductions in the W126 index concurrent with reductions in the
design value metric for the current standard is positive whether the
W126 index is expressed in terms of the average across the 3-year
design value period or the annual value (PA, Appendix 4D, section
4D.3.2.3). This similarity is consistent with the strong positive
relationship that exists between the W126 index and the design value
metric for the current standard summarized above.
    With regard to the control of the current form and averaging time
on vegetation exposures of potential concern, the PA also describes air
quality information pertinent to the evidence discussed in section
III.B.3 above regarding the potential for days with particularly high
O3 concentrations to play a contributing role in visible
foliar injury. In so doing, the PA notes that the current standard's
form and averaging time, by their very definition, limit occurrences of
such concentrations. For example, the peak 8-hour average
concentrations are lower at sites with lower design values, as
illustrated by the declining trends in annual fourth highest MDA8
concentrations that accompany the declining trend in design values (PA,
Figure 2-11). Additionally, the frequency of elevated 1-hour
concentrations, including concentrations at or above 100 ppb, decrease
with decreasing design values (PA, Appendix 2A, section 2A.2). For
example, in the most recent design value period (2016-2018) across all
sites with adequate data to derive design values, the mean number of
daily maximum 1-hour observations per site at or above 100 ppb was well
below one (0.19) for sites that meet the current standard, compared to
well above one (8.09) for sites not meeting the current (PA, Appendix
2A, Table 2A-2).
    In summary, monitoring sites with lower O3
concentrations as measured by the design value metric (based on the
current form and averaging time of the secondary standard) have lower
cumulative seasonal exposures, as quantified by the W126 index, as well
as lower short-term peak concentrations. As the form and averaging time
of the secondary standard have not changed since 1997, the analyses
performed have been able to assess the amount of control exerted by
these aspects of the standard, in combination with reductions in the
standard level (i.e., from 0.08 ppm in 1997 to 0.075 ppm in 2008 to
0.070 ppm in 2015) on

cumulative seasonal exposures in terms of W126 index (and on the
magnitude of short-term peak concentrations). The analyses have found
that the long-term reductions in the design values, presumably
associated with implementation of the revised standards, have been
accompanied by reductions in cumulative seasonal exposures in terms of
W126 index, as well as reductions in short-term peak concentrations.
2. Environmental Exposures in Terms of W126 Index
    The following presentation is framed by the question: What are the
nature and magnitude of vegetation exposures associated with conditions
meeting the current standard at sites across the U.S., particularly in
specially protected areas, such as Class I areas, and what do they
indicate regarding the potential for O3-related vegetation
impacts? Given the evidence indicating the W126 index to be strongly
related to growth effects and its use in the E-R functions for tree
seedling RBL (as summarized in section III.B above), exposure is
quantified using the W126 metric. The potential for impacts of interest
is assessed through considering the magnitude of estimated exposure, in
light of current information and, in comparison to levels given
particular focus in the 2015 decision on the current standard (80 FR
65292; October 26, 2015). The updated analyses summarized here, while
including assessment of all monitoring sites nationally, include a
particular focus on monitoring sites in or near Class I areas \169\, in
light of the greater public welfare significance of many O3
related impacts in such areas, as described in section III.B.2 above.
---------------------------------------------------------------------------

    \169\ This includes monitors sited within Class I areas or the
closest monitoring site within 15 km of the area boundary.
---------------------------------------------------------------------------

    The analyses summarized here consider both recent air quality
(2016-2018) and air quality since 2000 (PA, Appendix 4D). These air
quality analyses of cumulative seasonal exposures associated with
conditions meeting the current standard nationally provide conclusions
generally similar to those based on the data available at the time of
the last review when the current standard was set, when the most recent
data were available for 2011 to 2013 (Wells, 2015). Such conclusions
are with regard to regional differences as well as the rarity of W126
index values at or above 19 ppm-hrs in areas with air quality meeting
the current standard.\170\
---------------------------------------------------------------------------

    \170\ Rounding conventions are described in detail in the PA,
Appendix 4D, section 4D.2.2.
---------------------------------------------------------------------------

    Cumulative exposures vary across the U.S. with the highest W126
index values for sites that met the current standard being located
exclusively in Southwest and West climate regions (PA, Figure 4-6). At
sites meeting the current standard in all other NOAA climate regions,
W126 index values, averaged over the 3-year design value period are at
or below 13 ppm-hrs (PA, Figure 4-6 and Appendix 4D, Figure 4D-2). At
Southwest and West region sites that met the current standard, W126
index values, averaged across the 3-year design value period, are at or
below 17 ppm-hrs in virtually all cases in the most recent 3-year
period and across all of the seventeen 3-year periods in the full
dataset evaluated (i.e., all but one site out of 147 for recent period
and all but eight out of over 1,800 cases across full dataset). Across
all U.S. sites with valid design values at or below 70 ppb in the full
2000 to 2018 dataset, the W126 index, averaged over three years, was at
or below 17 ppm-hrs on 99.9% of all occasions, and at or below 13 ppm-
hrs on 97% of all occasions. All but one of the eight occasions when
the 3-year W126 index was above 17 ppm-hrs (including the highest
occasion at 19 ppm-hrs) occurred in the Southwest region during a
period before 2011. The most recent occasion occurred in 2018 at a site
in the West region when the 3-year average W126 index value was 18 ppm-
hrs (PA, Appendix 4D, section 4D.3.2).
    In summary, among sites meeting the current standard in the most
recent period of 2016 to 2018, there are none with a W126 index, based
on the 3-year average, above 19 ppm-hrs, and just one with such a value
above 17 ppm-hrs (Table 5). Additionally, the full historical dataset
includes no occurrences of a 3-year average W126 index above 19 ppm-hrs
for sites meeting the current standard, and just eight occurrences of a
W126 index above 17 ppm-hrs, with the highest such occurrence just
equaling 19 ppm-hrs (Table 5; PA, Appendix 4D, section 4D.3.2.1).
    With regard to Class I areas, the updated air quality analyses
include data at sites in or near 65 Class I areas. The findings for
these sites, which are distributed across all nine NOAA climate regions
in the contiguous U.S., as well as Alaska and Hawaii, mirror the
findings for the analysis of all U.S. sites. Among the Class I area
sites meeting the current standard (i.e., having a design value at or
below 70 ppb) in the most recent period of 2016 to 2018, there are none
with a W126 index (as average over design value period) above 17 ppm-
hrs (Table 5). The historical dataset includes just seven occurrences
(all dating from the 2000-2010 period) of a Class I area site meeting
the current standard and having a 3-year average W126 index above 17
ppm-hrs, and no such occurrences above 19 ppm-hrs (Table 5).
    The W126 exposures at sites with design values above 70 ppb range
up to approximately 60 ppm-hrs (Table 5). Among all sites across the
U.S. that do not meet the current standard in the 2016 to 2018 period,
more than a quarter have average W126 index values above 19 ppm-hrs and
a third exceed 17 ppm-hrs (Table 5). A similar situation exists for
Class I area sites (Table 5). Thus, as was the case in the last review,
the currently available quantitative information continues to indicate
appreciable control of seasonal W126 index-based cumulative exposure at
all sites with air quality meeting the current standard.

 Table 5--Distribution of 3-Yr Average Seasonal W126 Index for Sites in Class I Areas and Across U.S. That Meet the Current Standard and for Those That
                                                                         Do Not
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                Number of occurrences or site-DVs \A\
                                                 -------------------------------------------------------------------------------------------------------
                                                                   In Class I areas                      Across all monitoring sites (urban and rural)
                 3-year periods                  -------------------------------------------------------------------------------------------------------
                                                                           W126 (ppm-hrs)                                      W126 (ppm-hrs)
                                                     Total    ---------------------------------------    Total    --------------------------------------
                                                                   >19          >17          <=17                      >19          >17          <=17
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                        At Sites That Meet the Current Standard (Design Value at or Below 70 ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2016-2018.......................................           47            0            0           47          849            0            1          848
All from 2000 to 2018...........................          498            0            7          491        8,292            0            8        8,284
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                          At Sites That Exceed the Current Standard (Design Value Above 70 ppb)
--------------------------------------------------------------------------------------------------------------------------------------------------------
2016-2018.......................................           11            8            9            2          273           78           91          182
All from 2000 to 2018...........................          362          159          197          165       10,695        2,317        3,174        7,521
--------------------------------------------------------------------------------------------------------------------------------------------------------
\A\ Counts presented here are drawn from the PA, Appendix D, Tables 4D-1, 4D-4, 4D-5, 4D-6, 4D-9, 4D-10 and 4D-13 through 16.

    As summarized above, the information available in this review
continues to indicate that average cumulative seasonal exposure levels
at virtually all sites and 3-year periods with air quality meeting the
current standard fall at or below the level of 17 ppm-hrs that was
identified when the current standard was established (80 FR 65393;
October 26, 2015). Additionally, the full dataset indicates that at
sites meeting the current standard, annual W126 index values were less
than or equal to 19 ppm-hrs well over 99% of the time (PA, Appendix 4D,
section 4D.3.2.1). Additionally, the average W126 index in Class I
areas that meet the current standard for the most recent 3-year period
is below 17 ppm-hrs at all areas which have a monitor within or near
their borders (PA, Appendix 4D, Table 4D-16). Further, with the
exception of seven values that occurred prior to 2011, cumulative
seasonal exposures, in terms of average 3-year W126, in all Class I
areas during periods that met the current standard were no higher than
17 ppm-hrs. This contrasts with the occurrence of much higher W126
index values at sites when the current standard was not met. For
example, out of the 11 Class I area sites with design values above 70
ppb during the most recent period, eight sites had a 3-year average
W126 index above 19 ppm-hrs (ranging up to 47 ppm-hrs) and for nine, it
was above 17 ppm-hrs (Table 5; PA, Appendix 4D, Table 4D-17).

D. Proposed Conclusions on the Secondary Standard

    In reaching proposed conclusions on the current secondary
O3 standard (presented in section III.D.3), the
Administrator has taken into account policy-relevant evidence-based and
air quality-, exposure- and risk-based considerations discussed in the
PA (summarized in section III.D.1), as well as advice from the CASAC,
and public comment on the standard received thus far in the review
(section III.D.2). In general, the role of the PA is to help ``bridge
the gap'' between the Agency's assessment of the current evidence and
quantitative analyses (of air quality, exposure and risk), and the
judgments required of the Administrator in determining whether it is
appropriate to retain or revise the NAAQS. Evidence-based
considerations draw upon the EPA's integrated assessment of the
scientific evidence of welfare effects related to O3
exposure presented in the ISA (summarized in section III.B above) to
address key policy-relevant questions in the review. Similarly, the air
quality-, exposure- and risk-based considerations draw upon our
assessment of air quality, exposure and associated risk (summarized in
section III.C above) in addressing policy-relevant questions focused on
the potential for O3 exposures associated with welfare
effects under air quality conditions meeting the current standard.
    This approach to reviewing the secondary standard is consistent
with requirements of the provisions of the CAA related to the review of
the NAAQS and with how the EPA and the courts have historically
interpreted the CAA. As discussed in section I.A above, these
provisions require the Administrator to establish secondary standards
that, in the Administrator's judgment, are requisite (i.e., neither
more nor less stringent than necessary) to protect the public welfare
from known or anticipated adverse effects associated with the presence
of the pollutant in the ambient air. Consistent with the Agency's
approach across all NAAQS reviews, the EPA's approach to informing
these judgments is based on a recognition that the available welfare
effects evidence generally reflects a continuum that includes ambient
air exposures for which scientists generally agree that effects are
likely to occur through lower levels at which the likelihood and
magnitude of response become increasingly uncertain. The CAA does not
require the Administrator to establish a secondary standard 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.
    The proposed decision on the adequacy of the current secondary
standard described below is a public welfare policy judgment by the
Administrator that draws upon the scientific evidence for welfare
effects, quantitative analyses of air quality, exposure and risks, as
available, and judgments about how to consider the uncertainties and
limitations that are inherent in the scientific evidence and
quantitative analyses. This proposed decision has additionally
considered the August 2019 remand of the secondary standard. The four
basic elements of the NAAQS (i.e., indicator, averaging time, form, and
level) have been considered collectively in evaluating the public
welfare protection afforded by the current standard. The
Administrator's final decision will additionally consider public
comments received on this proposed decision.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy
Assessment
    Based on its evaluation of the evidence and quantitative analyses
of

air quality, exposure and potential risk, the PA for this review
reaches the conclusion that consideration should be given to retaining
the current secondary standard, without revision (PA, section 4.5.3).
Accordingly, and in light of this conclusion that it is appropriate to
consider the current secondary standard to be adequate, the PA did not
identify any potential alternative secondary standards for
consideration in this review (PA, section 4.5.3). The PA additionally
recognized that, as is the case in NAAQS reviews in general, the extent
to which the Administrator judges the current secondary O3
standard to be adequate will depend on a variety of factors, including
science policy judgments and public welfare policy judgments. These
factors include public welfare policy judgments concerning the
appropriate benchmarks on which to place weight, as well as judgments
on the public welfare significance of the effects that have been
observed at the exposures evaluated in the welfare effects evidence.
The factors relevant to judging the adequacy of the standard also
include the interpretation of, and decisions as to the weight to place
on, different aspects of the quantitative analyses of air quality and
cumulative O3 exposure and any associated uncertainties.
Thus, the Administrator's conclusions regarding the adequacy of the
current standard will depend in part on public welfare policy
judgments, science policy judgments regarding aspects of the evidence
and exposure/risk estimates, as well as judgments about the level of
public welfare protection that is requisite under the Clean Air Act.
    The subsections below summarize key considerations and conclusions
from the PA. The main focus of the policy-relevant considerations in
the PA is the question: Does the currently available scientific
evidence- and exposure/risk-based information support or call into
question the adequacy of the protection afforded by the current
secondary O3 standard? In addressing this overarching
question, the PA focuses first on consideration of the evidence, as
evaluated in the ISA (and supported by the prior ISA and AQCDs),
including that newly available in this review, and the extent to which
it alters the EPA's overall conclusions regarding welfare effects
associated with photochemical oxidants, including O3, in
ambient air. The PA also considers questions related to the general
approach or framework in which to evaluate public welfare protection of
the standard. Additionally, the PA considers the currently available
quantitative information regarding environmental exposures likely to
occur in areas of the U.S. where the standard is met, including
associated limitations and uncertainties, and the significance of these
exposures with regard to the potential for O3-related
vegetation effects, their potential severity and any associated public
welfare implications and judgments about the uncertainties inherent in
the scientific evidence and quantitative analyses that are integral to
consideration of whether the currently available information supports
or calls into question the adequacy of the current secondary
O3 standard.
a. Welfare Effects Evidence
    With regard to the support in the current evidence for
O3 as the indicator for photochemical oxidants, no newly
available evidence has been identified in this review regarding the
importance of photochemical oxidants other than O3 with
regard to abundance in ambient air, and potential for welfare
effects.\171\ Data for photochemical oxidants other than O3
are generally derived from a few special field studies; such that
national-scale data for these other oxidants are scarce (ISA, Appendix
1, section 1.1; 2013 ISA, sections 3.1 and 3.6). Moreover, few studies
of the welfare effects of other photochemical oxidants beyond
O3 have been identified by literature searches conducted for
the 2013 ISA and prior AQCDs, such that ``the primary literature
evaluating the . . . ecological effects of photochemical oxidants
includes ozone almost exclusively as an indicator of photochemical
oxidants'' (ISA, section IS.1.1, Appendix 1, section 1.1). Thus, as was
the case for previous reviews, the PA finds that the evidence base for
welfare effects of photochemical oxidants does not indicate an
importance of any other photochemical oxidants such that O3
continues to be appropriately considered for the secondary standard's
indicator.
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    \171\ Close agreement between past ozone measurements and the
photochemical oxidant measurements upon which the early NAAQS (for
photochemical oxidants including O3) was based indicated
the very minor contribution of other oxidant species in comparison
to O3 (U.S. DHEW, 1970).
---------------------------------------------------------------------------

(i) Nature of Effects
    Across the full array of welfare effects, summarized in section
III.B.1 above, the evidence newly available in this review strengthens
previous conclusions, provides further mechanistic insights and
augments current understanding of varying effects of O3
among species, communities and ecosystems (ISA, sections IS.1.3.2, IS.5
and IS.6.2, and Appendices 8 and 9). The current evidence, including
the wealth of long-standing evidence, continues to support conclusions
of causal relationships between O3 and visible foliar
injury, reduced yield and quality of agricultural crops, reduced
vegetation growth and plant reproduction, reduced productivity in
terrestrial ecosystems, and alteration of belowground biogeochemical
cycles. The current evidence additionally continues to support
conclusions of likely causal relationships between O3 and
reduced carbon sequestration in terrestrial systems, and alteration of
terrestrial ecosystem water cycling (ISA, section IS.I.3.2). Also as in
the last review, the current ISA determines there to be a causal
relationship between tropospheric O3 and radiative forcing
and a likely causal relationship between tropospheric O3 and
temperature, precipitation and related climate variables (ISA, section
IS.1.3.3). The current evidence has led to an updated conclusion on the
relationship of O3 with alteration of terrestrial community
composition to causal (ISA, sections IS.I.3.2). Lastly, the current ISA
concludes the current evidence sufficient to infer likely causal
relationships of O3 with three additional categories of
effects (ISA, sections IS.I.3.2). For example, while previous
recognition of O3 as a contributor to tree mortality in a
number of field studies was a factor in the 2013 conclusion of a likely
causal relationship between O3 and alterations in community
composition, tree mortality has been separately assessed in this
review. Additionally, newly available evidence on two additional plant
related effects augments more limited previously available evidence
related to insect interactions with vegetation, contributing to
additional conclusions that the body of evidence is sufficient to infer
likely causal relationships between O3 and alterations of
plant-insect signaling and insect herbivore growth and reproduction
(ISA, Appendix 8, sections 8.6 and 8.7).\172\
---------------------------------------------------------------------------

    \172\ As in the last review, the ISA again concludes that the
evidence is inadequate to determine if a causal relationship exists
between changes in tropospheric ozone concentrations and UV-B
effects (ISA, Appendix 9, section <IP_ADDRESS>; 2013 ISA, section
10.5.2).
---------------------------------------------------------------------------

    As in the last review, the strongest evidence and the associated
findings of causal or likely causal relationships with O3 in
ambient air, and quantitative characterizations of relationships
between O3 exposure and occurrence and magnitude of effects
are for vegetation-related effects. With regard to uncertainties and
limitations associated with the current welfare effects

evidence, the PA recognized that the type of uncertainties for each
category of effects tends to vary, generally in relation to the
maturity of the associated evidence base, from those associated with
overarching characterizations of the effects to those associated with
quantification of the cause and effect relationships. For example,
given the longstanding nature of the evidence for many of the
vegetation effects identified in the ISA as causally or likely causally
related to O3 in ambient air, the key uncertainties and
limitations in our understanding of these effects relate largely to the
implications or specific aspects of the evidence, as well as to current
understanding of the quantitative relationships between O3
concentrations in the environment and the occurrence and severity (or
relative magnitude) of such effects or understanding of key influences
on these relationships. For more newly identified categories of
effects, the evidence may be less extensive, and accordingly, the areas
of uncertainty greater, thus precluding consideration of quantitative
details related to risk of such effects under varying air quality
conditions that would inform review of the current standard.
    The evidence bases for the three newly identified categories
provide examples of such gaps in relevant information. For example, the
evidence for increased tree mortality includes previously available
studies with field observations from locations and periods of
O3 concentrations higher than are common today and three
more recently available publications assessing O3 exposures
not expected under conditions meeting the current standard, as
summarized in section III.B.1 above. The information available
regarding the newly identified categories of plant-insect signaling and
insect herbivore growth and reproduction additionally does not provide
for a clear understanding of the specific environmental effects that
may occur in the natural environment under specific exposure
conditions, as summarized in sections III.B.1 and III.B.3 above (PA,
section <IP_ADDRESS>). Accordingly, the PA does not find the current
evidence for these newly identified categories to call into question
the adequacy of the current standard.
    With regard to tropospheric O3 as a greenhouse gas at
the global scale, and associated effects on climate, the PA notes that
while additional characterizations of tropospheric O3 and
climate have been completed since the last review, uncertainties and
limitations in the evidence that were also recognized in the last
review remain (PA, section <IP_ADDRESS>). As summarized in section III.B.3
above, there is appreciable uncertainty associated with understanding
quantitative relationships involving regional O3
concentrations near the earth's surface and climate effects of
tropospheric O3 on a global scale. Further, there are
limitations in our modeling tools and associated uncertainties in
interpretations related to capabilities for quantitatively estimating
effects of regional-scale lower tropospheric O3
concentrations on climate. These uncertainties and limitations affect
our ability to make a quantitative characterization of the potential
magnitude of climate response to changes in O3
concentrations in ambient air, particularly at regional (vs global)
scales, and thus our ability to assess the impact of changes in ambient
air O3 concentrations in regions of the U.S. on global
radiative forcing or temperature, precipitation and related climate
variables. Consequently, the PA finds that current evidence in this
area is not informative to consideration of the adequacy of public
welfare protection of the current standard (PA, section <IP_ADDRESS>).
(ii) E-R Information
    The category of O3 welfare effects for which current
understanding of quantitative relationships is strongest continues to
be reduced plant growth. While the ISA describes studies of welfare
effects associated with O3 exposures newly identified since
the last review, the established E-R functions for tree seedling growth
and crop yield that have been available in the last several reviews
continue to be the most robust descriptions of E-R relationships for
welfare effects. These well-established E-R functions for seedling
growth reduction in 11 tree species and yield loss in 10 crop species
are based on response information across multiple levels of cumulative
seasonal exposure (estimated from extensive records of hourly
O3 concentrations across the exposure periods). Studies of
some of the same species, conducted since the derivation of these
functions, provide supporting information (ISA, Appendix 8, section
8.13.2; 2013 ISA, sections <IP_ADDRESS> and <IP_ADDRESS>). The E-R functions
provide for estimation of the growth-related effect, RBL, for a range
of cumulative seasonal exposures.
    The evidence newly available in this review does not include
studies that assessed reductions in tree growth or crop yield responses
across multiple O3 exposures and for which sufficient data
are available for analyses of the shape of the E-R relationship across
a range of cumulative exposure levels (e.g., in terms of W126 index)
relevant to conditions associated with the current standard. While
there are several newly available studies that summarize previously
available studies or draw from them, such as for linear regression
analyses, these do not provide robust E-R functions or cumulative
seasonal exposure levels associated with important vegetation effects,
such as reduced growth, that define the associated exposure
circumstances in a consistent manner (as summarized in section III.B.3
above).\173\ This limits their usefulness for considering the potential
for occurrence of welfare effects in air quality conditions that meet
the current standard. Thus, the PA concludes that robust E-R functions
are not available for growth or yield effects on any additional tree
species or crops in this review.
---------------------------------------------------------------------------

    \173\ For example, among the newly available publications cited
in the ISA is a study that compiles EC10 values
(estimated concentration at which 10% lower biomass [compared to
zero O3] is predicted) derived for trees and grassland
species (including 17 native to the U.S. [ISA, Table 8-26]) using
linear regression of previously published data on plant growth
response and O3 concentration quantified as AOT40. The
data were from studies of various experimental designs, that
involved various durations ranging up from 21 days, and involving
various concentrations no higher than 100 ppb as a daily maximum
hourly concentration. More detailed analyses of exposure and
response information across a relevant range of seasonal exposure
levels (e.g., accompanied by detailed records of O3
concentrations) that would support derivation of robust E-R
functions for purposes discussed here are not available.
---------------------------------------------------------------------------

    In considering the E-R functions and their use in informing
judgments regarding such effects in areas with air quality of interest,
the PA additionally recognized a number of limitations, and associated
uncertainties, that remain in the current evidence base, and that
affect characterization of the magnitude of cumulative exposure
conditions eliciting growth reductions in U.S. forests (PA, section
4.3.4). For example, there are uncertainties in the extent to which the
11 tree species for which there are established E-R functions encompass
the range of O3 sensitive species in the U.S., and also the
extent to which they represent U.S. vegetation as a whole. These 11
species include both deciduous and coniferous trees with a wide range
of sensitivities and species native to every NOAA climate region across
the U.S. and in most cases are resident across multiple states and
regions. Thus, they may provide a range that encompasses species
without E-R

functions.\174\ The PA additionally recognizes important uncertainties
in the extent to which the E-R functions for reduced growth in tree
seedlings are also descriptive of such relationships during later
lifestages, for which there is a paucity of established E-R
relationships. Although such information is limited with regard to
mature trees, analyses in the 2013 ISA indicated that reported growth
response of young aspen over six years was similar to the reported
growth response of seedlings (ISA, Appendix 8, section 8.13.2; 2013
ISA, section <IP_ADDRESS>). Additionally, there are uncertainties with
regard to the extent to which various factors in natural environments
can either mitigate or exacerbate predicted O3-plant
interactions and contribute variability in vegetation-related effects,
including reduced growth. Such factors include multiple genetically
influenced determinants of O3 sensitivity, changing
sensitivity to O3 across vegetative growth stages, co-
occurring stressors and/or modifying environmental factors (PA, section
4.3.4).
---------------------------------------------------------------------------

    \174\ This was the view of the CASAC in the 2015 review (Frey,
2014b, p. 11).
---------------------------------------------------------------------------

    The PA additionally considered the quantitative information for
other long-recognized effects of O3 (PA, section 4.3.4). For
example, with regard to crop yield effects, as at the time of the last
review, the PA recognized the potential for greater uncertainty in
estimating the impacts of O3 exposure on agricultural crop
production than that associated with O3 impacts on
vegetation in natural forests. This relates to uncertainty in the
extent to which agricultural management methods influence potential for
O3-related effects and accordingly, the applicability of the
established E-R functions for RYL in current agricultural areas (PA,
section 4.3.4).
    With regard to visible foliar injury, the PA finds that, as in the
last review, there remains a lack of established E-R functions that
would quantitatively describe relationships between the occurrence and
severity of visible foliar injury and O3 exposure, as well
as factors influential in those relationships, such as soil moisture
conditions (PA, section <IP_ADDRESS>). While the currently available
information continues to include studies that document foliar injury in
sensitive plant species in response to specific O3
exposures, investigations of a quantitative relationship between
environmental O3 exposures and visible foliar injury
occurrence/severity have not yielded a predictive result. In addition
to experimental studies, the evidence includes multiple studies that
have analyzed data collected as part of the USFS biosite biomonitoring
program (e.g., Smith, 2012). These analyses continue to indicate the
limitations in capabilities for predicting the exposure circumstances
under which visible foliar injury would be expected to occur, as well
as the circumstances contributing to increased injury severity. As
noted in section III.B.3.b above, expanded summaries of the dataset
compiled in the 2015 review from several years of USFS biosite records
also does not clearly and consistently describe a relationship between
incidence of foliar injury or severity (based on individual site
scores) and W126 index estimates across the range of exposures.
Overall, however, the dataset indicates that the proportion of records
having different levels of severity score is generally highest in the
records at sites with the highest W126 index (e.g., greater than 25
ppm-hrs for the normal and dry soil moisture categories). This analysis
does not provide for identification of air quality conditions, in terms
of O3 concentrations associated with the relatively lower
environmental exposures most common in the USFS dataset that would
correspond to a specific magnitude of injury incidence or severity
scores across locations.
    As discussed in section III.B.3 above, a number of analyses of the
USFS biosite data (as well as several experimental studies), while
often using cumulative exposure metrics to quantify O3
exposures have additionally reported there to be a role for a metric
that quantifies the incidence of ``high'' O3 days (2013 ISA,
p. 9-10; Smith, 2012; Wang et al., 2012). Such analyses have not,
however, established specific air quality metrics and associated
quantitative functions for describing the influence of ambient air
O3 on incidence and severity of visible foliar injury. As a
result, the PA concludes that limitations recognized in the last review
remain in our ability to quantitatively estimate incidence and severity
of visible foliar injury likely to occur in areas across the U.S. under
different air quality conditions over a year, or over a multi-year
period.
    In looking across the full array of O3 welfare effects,
the PA recognizes that the E-R functions for growth-related effects
that were available in the last review continue to be the most robust
E-R information available. The currently available evidence for growth-
related effects, including that newly available in this review, does
not indicate the occurrence of growth-related responses attributable to
cumulative O3 exposures lower than was established at the
time of the last review. With regard to visible foliar injury, the
available information that would support estimates of occurrence and
severity across a range of air quality conditions continues to be
limited, affecting the nature of conclusions that may be reached
related to potential occurrence and/or severity for conditions. The
quantitative information for other effects is more limited, as
recognized earlier in this section and in section III.B.3 above. Thus,
the PA concludes that the newly available evidence does not appreciably
address key limitations or uncertainties as would be needed to expand
capabilities for estimating welfare impacts that might be expected as a
result of differing patterns of O3 concentrations in the
U.S.
(iii) W126 Index as Exposure Metric
    With regard to exposure metric the currently available evidence
continues to support a cumulative, seasonal exposure index as a
biologically relevant and appropriate metric for assessment of the
evidence of exposure/risk information for vegetation, most particularly
for growth-related effects. The most commonly used such metrics are the
SUM06, AOT40 (or AOT60) and W126 indices (ISA, section IS.3.2).\175\
The evidence for growth-related effects continues to support important
roles for cumulative exposure and for weighting higher concentrations
over lower concentrations. Thus, among the various such indices
considered in the literature, the cumulative, concentration-weighted
metric, defined by the W126 function, continues to be best supported
for purposes of relating O3 air quality to growth-related
effects. Accordingly, the PA continues to find the W126 index
appropriate for consideration of the potential for vegetation-related
effects to occur under air quality conditions (PA, section <IP_ADDRESS>).
The PA also recognizes, as recognized in the past, the lack of support
for E-R functions for incidence and severity of visible foliar injury
with W126 index as the descriptor of exposure, particularly in
environmental settings where exposures are below a

W126 index of 25 ppm-hrs. While the PA analysis of the dataset of USFS
biosite scores indicates appreciable increases in incidence and
severity at and above 25 ppm-hrs, a pattern is unclear at lower W126
index estimates across which the dataset does not support a predictive
relationship. As summarized in section III.3.b above, while the overall
evidence also indicates an important role for peak concentrations
(e.g., N100) in influencing the occurrence and severity of visible
foliar injury, the current evidence does not include an established
predictive relationship based on such an additional metric (PA, section
<IP_ADDRESS>).
---------------------------------------------------------------------------

    \175\ The evidence includes some studies reporting
O3-reduced soybean yield and perennial plant biomass loss
using AOT40 (as well as W126) as the exposure metric, however, no
newly available analyses are available that compare AOT40 to W126 in
terms of the strength of association with such responses. Nor are
studies available that provide analyses of E-R relationships for AOT
with reduced growth or RBL with such extensiveness as the analyses
supporting the established E-R functions for W126 with RBL and RYL.
---------------------------------------------------------------------------

b. General Approach for Considering Public Welfare Protection
    This section summarizes PA consideration of the current evidence
and air quality information with regard to key aspects of the general
approach and risk management framework for making judgments and
reaching conclusions regarding the adequacy of public welfare
protection provided by the secondary standard that was applied in 2015
(summarized in section III.A.1 above). Key aspects of the approach
include the use of RBL as a proxy for the broad array of O3
vegetation-related effects, E-R relationships for this endpoint with
the W126 index, and the focus on this index averaged across a 3-year
period.
(i) RBL as Proxy or Surrogate
    In the last review, the Administrator used RBL as a proxy or
surrogate for an array of adverse welfare effects based on
consideration of ecosystem services and potential for impacts to the
public, as well as conceptual relationships between vegetation growth
effects and ecosystem-scale effects. Such a use was supported by the
CASAC at that time (80 FR 65406, October 26, 2015; Frey, 2014b, pp.
iii, 9-10).\176\ In consideration of the broader evidence base and
public welfare implications, including associated strengths,
limitations and uncertainties, the Administrator focused on RBL, not
simply in making judgments specific to a magnitude of growth effect in
seedlings that would be acceptable or unacceptable in the natural
environment, but as a surrogate or proxy for consideration of the
broader array of vegetation-related effects of potential public welfare
significance, that included effects on growth of individual sensitive
species and extended to ecosystem-level effects, such as community
composition in natural forests, particularly in protected public lands
(80 FR 65406, October 26, 2015).
---------------------------------------------------------------------------

    \176\ The CASAC letter on the second draft PA in that review
stated the following (Frey, 2014b, p. 9-10):
    For example, CASAC concurs that trees are important from a
public welfare perspective because they provide valued services to
humans, including aesthetic value, food, fiber, timber, other forest
products, habitat, recreational opportunities, climate regulation,
erosion control, air pollution removal, and hydrologic and fire
regime stabilization. Damage effects to trees that are adverse to
public welfare occur in such locations as national parks, national
refuges, and other protected areas, as well as to timber for
commercial use. The CASAC concurs that biomass loss in trees is a
relevant surrogate for damage to tree growth that affects ecosystem
services such as habitat provision for wildlife, carbon storage,
provision of food and fiber, and pollution removal. Biomass loss may
also have indirect process-related effects such as on nutrient and
hydrologic cycles. Therefore, biomass loss is a scientifically valid
surrogate of a variety of adverse effects to public welfare.
---------------------------------------------------------------------------

    The currently available evidence related to conceptual
relationships between plant growth impacts and the broader array of
vegetation effects (e.g., that supported the use of RBL as a surrogate
or proxy) is largely consistent with that available in the last review.
In fact, the ISA for the current review describes (or relies on) such
relationships in considering causality determinations for ecosystem-
scale effects such as altered terrestrial community composition and
reduced productivity, as well as reduced carbon sequestration, in
terrestrial ecosystems (ISA, Appendix 8, sections 8.8 and 8.10). Thus,
the PA concludes that the current evidence does not call into question
conceptual relationships between plant growth impacts and the broader
array of vegetation effects. Rather, the current evidence continues to
support the use of tree seedling RBL as a proxy for the broad array of
vegetation-related effects, most particularly those conceptually
related to growth (PA, sections <IP_ADDRESS> and 4.5.3).
    Beyond tree seedling growth, on which RBL is specifically based,
two other vegetation effect categories with extensive evidence bases,
crop yield and visible foliar injury, were also given attention in
considering the public welfare protection provided by the standard in
2015. Based on the available information for these endpoints, along
with associated limitations and uncertainties, the Administrator at
that time concluded there was not support for giving a primary focus,
in selecting a revised secondary standard, to these two types of
effects. With regard to crop yield, the Administrator recognized the
significant role of agricultural management practices in agricultural
productivity, as well as market variability, concluding that, in
describing her public welfare protection objectives, additional
attention to this endpoint was not necessary. The rough similarities in
estimated W126 levels of median crops and tree species are also
noteworthy. With regard to foliar injury, the lack of clear
quantitative relationships that would support predictive E-R functions
was recognized. In light of such considerations, the Administrator
focused on RBL estimates in identifying the requisite standard, and
judged that a standard set based on public welfare protection
objectives described in terms of cumulative exposures and relationships
with tree seedling RBL was an appropriate means to, and would, provide
appropriate protection for the array of vegetation-related effects.
With regard to the information available in the current review, the PA
concludes it does not call into question the basis for such judgments
and continues to be supportive of the use of tree seedling RBL as a
proxy for the broad array of vegetation-related effects (PA, section
<IP_ADDRESS>).
    In considering the magnitude of estimated RBL on which to focus in
its role as a surrogate or proxy for the full array of vegetation
effects in the last review, the Administrator endeavored to identify a
secondary standard that would limit 3-year average O3
exposures somewhat below W126 index values associated with a 6% RBL
median estimate from the established species-specific E-R functions.
This led to identification of a seasonal W126 index value of 17 ppm-hrs
that the Administrator concluded appropriate as a target at or below
which the new standard would generally restrict cumulative seasonal
exposures (80 FR 65407, October 26, 2015). In identifying this exposure
level as a target, the Administrator, recognizing limitations and
uncertainties in the evidence and variability in biota and ecosystems
in the natural environment, additionally judged that RBL estimates
associated with isolated rare instances of marginally higher cumulative
exposures (in terms of a 3-year average W126 index), e.g., those that
round to 19 ppm-hrs (which corresponds to 6% RBL as median from 11
established E-R functions), were not indicative of adverse effects to
the public welfare (80 FR 65409, October 26, 2015).
    The PA concludes that the information newly available in this
review does not differ from that available in the last review with
regard to a magnitude of RBL in the median species appropriately
considered a reference for judgments concerning

potential vegetation-related impacts to the public welfare (PA, section
<IP_ADDRESS>). The currently available evidence continues to indicate
conceptual relationships between reduced growth and the broader array
of vegetation-related effects, and limitations and uncertainties remain
with regard to quantitation. The PA notes that consideration of the
magnitude of tree growth effects that might cause or contribute to
adverse effects for trees, forests, forested ecosystems or the public
welfare is complicated by various uncertainties or limitations in the
evidence base, including those associated with relating magnitude of
tree seedling growth reduction to larger-scale forest ecosystem
impacts. Further, other factors can influence the degree to which
O3-induced growth effects in a sensitive species affect
forest and forest community composition and other ecosystem service
flows (e.g., productivity, belowground biogeochemical cycles and
terrestrial ecosystem water cycling) from forested ecosystems. These
include (1) the type of stand or community in which the sensitive
species is found (i.e., single species versus mixed canopy); (2) the
role or position the species has in the stand (i.e., dominant, sub-
dominant, canopy, understory); (3) the O3 sensitivity of the
other co-occurring species (O3 sensitive or tolerant); and
(4) environmental factors, such as soil moisture and others. The lack
of such established relationships with O3 complicates
consideration of the extent to which different estimates of impacts on
tree seedling growth would indicate significance to the public welfare.
Further, efforts to estimate O3 effects on carbon
sequestration are handicapped by the large uncertainties involved in
attempting to quantify the additional carbon uptake by plants as a
result of avoided O3-related growth reductions. Such
analyses require complex modeling of biological and ecological
processes with their associated sources of uncertainty.
    Quantitative representations of such relationships have been used
to study potential impacts of tree growth effects on such larger-scale
effects as community composition and productivity with the results
indicating the array of complexities involved (e.g., ISA, Appendix 8,
section 8.8.4). Given their purpose in exploring complex ecological
relationships and their responses to environmental variables, as well
as limitations of the information available for such work, these
analyses commonly utilize somewhat general representations. The PA
notes that this work indicates how established the existence of such
relationships is, while also identifying complexities inherent in
quantitative aspects of such relationships and interpretation of
estimated responses. Thus, the PA finds the currently available
evidence to be little changed from the last review with regard to
informing identification of an RBL reference point reflecting
ecosystem-scale effects with public welfare impacts elicited through
such linkages (PA, section <IP_ADDRESS>).
(ii) Focus on 3-Year Average W126 Index
    In setting the current standard, as described in section III.A.1
above, the Administrator focused on control of seasonal cumulative
exposures in terms of a 3-year average W126 index. The evaluations in
the PA for that review recognized there to be limited information to
discern differences in the level of protection afforded for cumulative
growth-related effects by a standard focused on a single-year W126
index as compared to a 3-year W126 index (80 FR 65390, October 26,
2015). Accordingly, 3-year average was identified for considering the
seasonal W126 index based on the recognition that there was year-to-
year variability not just in O3 concentrations, but also in
environmental factors, including rainfall and other meteorological
factors, that influence the occurrence and magnitude of O3-
related effects in any year (e.g., through changes in soil moisture),
contributing uncertainties to projections of the potential for harm to
public welfare (80 FR 65404 October 26, 2015). Given this recognition,
as well as other considerations, the Administrator expressed greater
confidence in judgments related to projections of public welfare
impacts based on seasonal W126 index estimated by a 3-year average and
accordingly, relied on that metric.
    A general area of uncertainty that remains in the current evidence
continues to affect interpretation of the potential for harm to public
welfare over multi-year periods of air quality that meet the current
standard (PA, section 4.3.4). As recognized in the last review, there
is 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 affect plant
growth and reproduction, such as through changes in soil moisture.
Accordingly, these variabilities contribute uncertainties to estimates
of the occurrence and magnitude of O3-related effects in any
year, and to such estimates over multi-year periods. The PA recognizes
that limitations in our ability to estimate the effects on growth over
tree lifetimes of year-to-year variation in O3
concentrations, particularly those associated with conditions meeting
the current standard, contribute uncertainty to estimates of cumulative
growth (biomass) effects over multi-year periods in the life of
individual trees and associated populations, as well as related effects
in associated communities and ecosystems (PA, section 4.3.4).
    As summarized in section III.B.3 above, the longstanding evidence
on O3 effects on plant growth includes the established and
robust E-R functions for 11 species of tree seedlings (ISA, Appendix 8,
Table 8-24; PA, Appendix 4A, Table 4A-1,). The PA recognized the
strength of these functions in describing tree seedling response across
a broad range of W126 index values, concluding that the evidence
continues to support their use in estimating the median RBL across
species in this review. In considering the appropriate representation
of seasonal W126 for use of these functions with air quality data, the
PA additionally considered the available information underlying the E-R
functions and the extent to which the information is specific to a
single seasonal exposure, e.g., as compared to providing representation
for an average W126 index across multiple seasons (PA, section
<IP_ADDRESS>). In so doing, the PA took note of aspects of the evidence that
reflect variability in organism response under different experimental
conditions and the extent to which this variability is represented in
the available data. This might indicate an appropriateness of assessing
environmental conditions using a mean across seasons in recognition of
the existence of such year-to-year variability in conditions and
responses. An additional aspect of the information underlying the E-R
functions that was identified as relevant to consider is the extent to
which the exposure conditions represented include those associated with
O3 concentrations that meet the current standard, and the
extent to which tree seedling growth responses to such conditions may
have been found to not be significantly different from responses to the
control (e.g., zero O3) conditions. The extent to which E-R
predictions are extrapolated beyond the tested exposure conditions also
contributes to uncertainty which the PA indicated may argue for a less
precise interpretation, such as an average across multiple seasons.
    The experiments from which the functions were derived vary in
duration

from periods of 82 to 140 days over a single year to periods of 180 to
555 days across two years, and in whether measurements were made
immediately following exposure period or in the subsequent season (PA,
section <IP_ADDRESS>, Appendix 4A, Table 4A-5; Lee and Hogsett, 1996). In
producing E-R functions of consistent duration across the experiments,
the E-R functions were derived first based on the exposure duration of
the experiment and then normalized to 3-month (seasonal) periods (see
Lee and Hogsett, 1996, section I.3; PA, Appendix 4A). Underlying the
adjustment is a simplifying assumption of uniform W126 distribution
across the exposure periods and of a linear relationship between
duration of cumulative exposure in terms of the W126 index and plant
growth response. Some functions for experiments that extended over two
seasons were derived by distributing responses observed at the end of
two seasons of varying exposures equally across the two seasons (e.g.,
essentially applying the average to both seasons).
    The PA additionally recognizes that the experiment-specific E-R
functions for both aspen and ponderosa pine illustrate appreciable
variability in response across experiments (PA, Appendix 4A, Figure 4A-
10). The PA suggested that reasons for this variability may relate to a
number of factors, including variability in seasonal response related
to variability in non-O3 related environmental influences on
growth, such as rainfall, temperature and other meteorological
variables, as well as biological variability across individual
seedlings, in addition to potentially variability in the pattern of
O3 concentrations contributing to similar cumulative
exposures (PA, section <IP_ADDRESS>). In recognition of some of the
variability in both seasonal environmental conditions in the studies
and the associated experimental data, the 11 species-specific E-R
functions are based on median responses (derived from experiment-
specific functions) across an array of W126 index values (PA, Appendix
4A; Lee and Hogsett, 1996).\177\ The number of experiments used in
deriving the E-R functions for each species varies. For example, there
are 7 experimental studies for wild aspen and 11 for ponderosa pine
(PA, Appendix 4A, Table 4A-5), and only two or three for the three
species (black cherry, sugar maple and tulip poplar) that exhibit
greater sensitivity than aspen and ponderosa pine (PA, Appendix 4A,
section 4A-2, Table 4A-5; 1996 AQCD, Table 5-28; Lee and Hogsett,
1996). Regarding the extent or strength of the database underlying the
E-R functions for cumulative exposure levels of interest in the current
review, the PA also notes that the data generally appear to be more
extensive for relatively higher (e.g., at/above a SUM06 of 30 ppm-hrs),
versus lower, seasonal exposures (PA, Appendix 4A, Table 4A-6).
Additionally, while the evidence is long-standing and robust for growth
effects of O3, the studies available for some species appear
to be somewhat limited in the extent to which they include cumulative
O3 exposures commonly occuring with air quality conditions
that meet the current standard (e.g., W126 index values below 20 ppm-
hrs).\178\ The PA concludes the factors identified here to contribute
to uncertainty or inexactitude in estimates based on the E-R functions.
---------------------------------------------------------------------------

    \177\ This median-based approach is expected to guard against
statistical bias in parameter values.
    \178\ The evidence is unclear on the extent to which six of the
11 species include exposure treatments likely to correspond to W126
index values at or below 20 ppm-hrs (PA, Appendix 4A, Table 4A-5).
For five of the species in Table 4A-5 in Appendix 4A, SUM06 index
values below 25 ppm-hrs range from 12 to 21.7. In considering these
values, we note that an approach used in the 2007 Staff Paper on
specific temporal patterns of O3 concentrations concluded
that a SUM06 index value of 25 ppm-hrs would be estimated to
correspond to a W126 index value of approximately 21 ppm-hrs (U.S.
EPA, 2007, Appendix 7B, p. 7B-2). Accordingly, a SUM06 value of 21
ppm-hrs might be expected to correspond to a W126 index value below
20 ppm-hrs. The PA further notes that for one of the species for
which lower exposures were studied, black cherry, the findings for
at least one study reported statistical significance only for
effects observed for higher exposures (PA, section 4.3.4, Appendix
4A, Table 4A-6).
---------------------------------------------------------------------------

    The PA recognizes that the evidence that allows for specific
evaluation of the predictability of growth impacts from single-year
versus multiple-year average exposure estimates is quite limited. Such
evidence would include multi-year studies reporting results for each
year of the study, which are the most informative to the question of
plant annual and cumulative responses to individual years (high and
low) over multiple-year periods. The evidence is quite limited with
regard to studies of O3 effects that report seasonal
observations across multi-year periods and that also include detailed
hourly O3 concentration records (to allow for derivation of
exposure index values). Such a limitation contributes uncertainty and
accordingly a lack of precision to our understanding of the
quantitative impacts of seasonal O3 exposure, including its
year-to-year variability on tree growth and annual biomass accumulation
(PA, section 4.3.4). The PA finds this uncertainty to limit our
understanding of the extent to which tree biomass would be expected to
appreciably differ at the end of multi-year exposures for which the
overall average exposure is the same, yet for which the individual year
exposures varied in different ways (e.g., as analyzed in Appendix 4D of
the PA). Thus, the PA notes that the extent of any differences in tree
biomass for two multi-year scenarios with the same 3-year average W126
index but differing single-year indices is not clear, including for
exposures associated with O3 concentrations that would meet
the current standard (PA, section 4.3.4).\179\
---------------------------------------------------------------------------

    \179\ Variation in annual W126 index values indicates that for
the period, 2016-2018, the amount by which annual W126 index values
at a site differ from the 3-year average varies is generally below
10 ppm-hrs across all sites and generally below 5 ppm-hrs at sites
with design values at or below 70 ppb (PA, Appendix 4D, Figure 4D-
7).
---------------------------------------------------------------------------

    One such study, which tracked exposures across six years, is
available for aspen (King et al., 2005; 2013 ISA, section <IP_ADDRESS>; ISA,
Appendix 8, section 8.13.2).\180\ This study was used in a presentation
of the 2013 ISA that compared the observed growth response to that
predicted from the E-R function for aspen. Specifically, the observed
aboveground biomass (and RBL) after each of the six growing seasons was
compared to estimates derived from the aspen E-R function based on the
cumulative multiple-year average seasonal W126 index values for each
year \181\ (2013 ISA, section <IP_ADDRESS>). The conclusions reached were
that the agreement between the set of predictions and the Aspen FACE
observations were ``very close'' and that ``the function based on one
year of growth was shown to be applicable to subsequent years'' (2013
ISA, p. 9-135). The PA observes that such results indicate that when
considering O3 impacts on growing trees across multiple
years, a multi-year average index yields predictions close to observed
measurements across the multi-year time period (2013 ISA, section
<IP_ADDRESS> and Figure 9-20; PA, Appendix 4A, section 4.A.3). The PA also
includes example analyses that use biomass measurements from the multi-
year study (King et al., 2005) to estimate aboveground aspen biomass
over a multi-year period using the established

E-R function for aspen with a constant single-year W126 index, e.g., of
17 ppm-hrs, or with varying annual W126 index values (10, 17 and 24
ppm-hrs) for which the 3-year average is 17 ppm-hrs, and that yield
somewhat similar total biomass estimates after multiple years (PA,
Appendix 4A, section 4A.3).\182\
---------------------------------------------------------------------------

    \180\ A similar comparison is presented in the current ISA (ISA,
Appendix 8).
    \181\ Although not emphasized or explained in detail in the 2013
ISA, the W126 estimates used to generate the predicted growth
response were cumulative average. To clarify, the cumulative average
W126 for year 1 is simply the W126 index for that year (e.g., based
on highest 3 months). For year 2, it is the average of the year 1
seasonal W126 and year 2 seasonal W126, and so on. For year 6, it is
the average of each of the six year's seasonal W126 index values.
    \182\ This example, while simplistic in nature, and with
inherent uncertainties, including with regard to broad
interpretation given the reliance on data available for the single
study, quantitatively illustrates potential differences in growth
impacts of W126 index, as a 3-year average, for which individual
year values vary while still meeting the value specified for the
average, from such impacts from exposure controlled to the same W126
index value annually. The PA suggests that this example indicates
based on the magnitude of variation documented for annual W126 index
values occurring under the current standard, a quite small magnitude
of differences in tree biomass between single-year and multi-year
average approaches to controlling cumulative exposure (PA, Appendix
4A, section 4A.3).
---------------------------------------------------------------------------

    Thus, the PA finds that, while the E-R functions are based on
strong evidence of seasonal and cumulative seasonal O3
exposure reducing tree growth, and while they provide for quantitative
characterization of the extent of such effects across O3
exposure levels of appreciable magnitude, there is uncertainty
associated with the resulting RBL predictions. Further, the current
evidence does not indicate single-year seasonal exposure in combination
with the established E-R functions to be a better predictor of RBL than
a seasonal exposure based on a multi-year average, or vice versa
(Appendix 4A, section 4A.3.1). Rather, associated uncertainty
contributes or implies an imprecision or inexactitude in the resulting
predictions, particularly for the lower W126 index estimates of
interest in this review. In light of this, the current evidence does
not support concluding there to be an appreciable difference in the
effect of three years of exposure held at 17 ppm-hrs compared to a 3-
year exposure that averaged 17 ppm-hrs yet varied by 5 to 10 ppm (e.g.,
7 ppm-hrs) from 17 ppm-hrs in any of the three years for tree RBL over
such multiple-year periods. The PA considered all of the factors
identified here, the currently available evidence and recognized
limitations, variability and uncertainties, to contribute uncertainty
and resulting imprecision or inexactitude to RBL estimates of single-
year seasonal W126 index values. The PA found these considerations to
indicate there to be no lesser support for use of an average seasonal
W126 index derived from multiple years (with their representation of
variability in environmental factors), such as for a 3-year period, for
estimating median RBL using the established E-R functions than for use
of a single-year index.
(iii) Visible Foliar Injury
    In considering a public welfare protection approach related to
visible foliar injury, the PA first notes that some level of visible
foliar injury can impact public welfare and thus might reasonably be
judged adverse to public welfare.\183\ As summarized in section III.B.2
above, depending on its spatial extent and severity, there are many
situations or locations in which visible foliar injury can adversely
affect the public welfare. For example, significant, readily
perceivable and widespread injury in national parks and wilderness
areas can adversely affect the perceived scenic beauty of these areas,
harming the aesthetic experience for both outdoor enthusiasts and the
occasional park visitor. Such considerations have also been recognized
by the Agency in past reviews, in which decisions to revise the
O3 secondary standard emphasized protection of Class I
areas, which are areas such as national wilderness areas and national
parks given special protections by the Congress (e.g., 73 FR 16496,
March 27, 2008, ``the Administrator concludes it is appropriate to
revise the secondary standard, in part, to provide increased protection
against O3-caused impairment to such protected vegetation
and ecosystems'').\184\
---------------------------------------------------------------------------

    \183\ As stated in the 2015 decision notice: ``both tree growth-
related effects and visible foliar injury have the potential to be
significant to the public welfare'' (80 FR 65377, October 26, 2015);
``O3-induced visible foliar injury also has the potential
to be significant to the public welfare through impacts in Class I
and other similarly protected areas'' (80 FR 65378, October 26,
2015); ``[d]epending on the extent and severity, O3-
induced visible foliar injury might be expected to have the
potential to impact the public welfare in scenic and/or recreational
areas during the growing season, particularly in areas with special
protection, such as Class I areas. (80 FR 65379, October 26, 2015);
``[t]he Administrator also recognizes the potential for this effect
to affect the public welfare in the context of affecting values
pertaining to natural forests, particularly those afforded special
government protection (80 FR 65407, October 26, 2015).
    \184\ In the discussion of the need for revision of the 1997
secondary standard, the 2008 decision noted that ``[i]n considering
what constitutes a vegetation effect that is adverse from a public
welfare perspective, . . . the Administrator has taken 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). This passage of the 2008 decision notice clarified
that ``[s]uch public lands that are protected areas of national
interest include national parks and forests, wildlife refuges, and
wilderness areas'' (73 FR 16496, March 27, 2008).
---------------------------------------------------------------------------

    In establishing the current secondary standard and describing its
underlying public welfare protection objectives (as summarized in
section III.A.1, above), the Administrator at that time focused
primarily on RBL in tree seedlings as a proxy or surrogate for the full
array of vegetation related effects of O3, while
additionally concluding that the then-available information on visible
foliar injury provided some support for establishing a strengthened
standard. In so doing, she took note of the indication of the evidence
of the association between O3 and visible foliar injury, as
well as in the declines generally observed in USFS BI scores with
reductions in W126 index from well above 20 ppm-hrs to lower levels (80
FR 65407-65408, October 26, 2015). She recognized, however, that the
evidence was not conducive to use in identifying a quantitative public
welfare protection objective focused specifically on visible foliar
injury (based on judgment of the specific extent and severity at which
such effects should be considered adverse to the public welfare) due to
uncertainties and complexities associated with the available
information. In related manner, she specifically recognized significant
challenges posed by the lack of clear quantitative relationships
(including robust exposure-response functions that addressed the
variability observed in the available data, likely associated with the
variables creating a predisposing environment), that would allow
prediction of visible foliar injury severity and incidence under
varying air quality and environmental conditions, as well as the 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).
    The PA finds that these challenges are not addressed by the
information available in the current review. Beyond the lack of
established descriptive quantitative relationships for O3
concentrations or exposure metrics with incidence or severity of
visible foliar injury, summarized in sections III.D.1.a and III.B.3
above, there is a paucity of information clearly relating differing
levels of severity and extent of location affected to scenic or
aesthetic values (e.g., reflective of visitor enjoyment and likelihood
of frequenting such areas) that might inform judgments of public
welfare protection from adversity (PA, section 4.5.1). Thus, there
remain appreciable limitations of the current information for the
purpose of providing a foundation for judgments on public

welfare protection objectives specific to visible foliar injury.
    Notwithstanding these limitations with regard to a detailed
approach or framework for judging public welfare protection related to
impacts of visible foliar injury, the current evidence and analyses are
informative to such considerations. For example, the published studies
and EPA analyses of the USFS biosite data indicate that incidence and
severity of injury are increased at the highest exposures. With regard
to the dataset analyzed in the PA, while clear trends in incidence and
severity related to increasing W126 index are not evident across the
W126 bins below 25 ppm-hrs, the incidence of sites with the more severe
classification of injury (e.g., BI score above 15 [``moderate'' or
``severe''] or 5 [``light,'' ``moderate,'' or ``severe'']) is
appreciably lower at sites with W126 index values below 25 ppm-hrs than
at sites with higher values (e.g., PA, Appendix 4C, Figures 4C-5 and
4C-6 and Table 4C-5). This observation is based primarily on records
for the normal soil moisture category, for which is sufficient sample
size across the full range of W126 and the largest differences in
incidence and average score are observed.\185\ Based on these
observations and the full analysis, the PA concludes that the currently
available information does not support precise conclusions as to the
severity and extent of such injury associated with the lower values of
W126 index most common at USFS sites during the years of the dataset,
2006-2010.\186\ Based on the general pattern observed, however, the PA
suggests a reduced severity (average BI score below 5) and incidence of
visible foliar injury, as quantified by BI scores, to be expected under
conditions that maintain W126 index values below 25 ppm-hrs, (PA,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    \185\ Across W126 bins in which at least 1% of the wet soil
moisture records are represented, differences of highest bin from
lower bins for injury incidence or average score is less than a
factor of two (PA, section 4.3.3).
    \186\ Factors that may contribute to the observed variability in
BI scores and lack of a clear pattern with W126 index bin 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 potential role
of other aspects of O3 air quality not captured by the
W126 index.
---------------------------------------------------------------------------

    Given the evidence regarding the role of peak O3
concentrations as an influence on occurrence of visible foliar injury
separate from that of the cumulative, concentration-weighted, W126
index (summarized in section III.B.3.b above), the PA additionally
finds that the conditions associated with visible foliar injury in
locations with sensitive species appear to relate to peak concentration
as well as cumulative exposure to generally higher concentrations over
the growing season (PA. section <IP_ADDRESS>). Accordingly, the PA also
considered the current information with regard to peak concentration
metrics. Such information includes the 2007 Staff Paper comparison
based on the less extensive USFS dataset of counties grouped by fourth
highest annual daily maximum 8-hour concentration. This analysis found
a smaller incidence of nonzero BI biosites in counties with a fourth-
high metric at or below 74 ppb as compared to counties limited to
metric values at or below 84 ppb (U.S. EPA 2007, pp. 7-63 to 7-64). The
indication of this finding that the averaging time and form of the
current standard, which emphasizes peak concentrations through a short
(8-hour) averaging time and a rare-occurrence form (annual fourth
highest daily maximum), exert some control on the incidence of sites
with visible foliar injury has a conceptual similarity to the finding
of the most extensive study of USFS data (1994-2009) that reductions in
peak 1-hour concentrations have influenced the declining trend observed
in visible foliar injury since 2002 (Smith, 2012).
(iv) Climate Effects
    In considering the currently available information for the effects
of the global tropospheric abundance of O3 on radiative
forcing, and temperature, precipitation and related climate variables,
the PA recognized there to be limitations and uncertainties in the
associated evidence bases with regard to assessing potential for
occurrence of climate-related effects as a result of varying
O3 concentrations in ambient air of locations in the U.S (as
summarized in III.B.3 above). The current evidence is limited with
regard to support for such quantitative analyses that might inform
considerations related to the current standard. For example, as stated
in the ISA, ``[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 9.3.1). These
are ``in addition to the key sources of uncertainty in quantifying
ozone RF changes, such as emissions over the time period of interest
and baseline ozone concentrations during preindustrial times'' (ISA,
section IS.9.3.1). Together such uncertainties limit development of
quantitative estimates of climate-related effects in response to earth
surface O3 concentrations at the regional scale, such as in
the U.S. While these complexities inhibit our ability to consider
tropospheric O3 effects, such as radiative forcing, we note
that our consideration of O3 growth-related impacts on trees
inherently encompasses consideration of the potential for O3
to reduce carbon sequestration in terrestrial ecosystems (e.g., through
reduced tree biomass as a result of reduced growth). That is, limiting
the extent of O3-related effects on growth would be expected
to also limit reductions in carbon sequestration, a process that can
reduce the tropospheric abundance of CO2, the greenhouse gas
ranked highest in importance as a greenhouse gas and radiative forcing
agent (section III.B.3 above; ISA, section 9.1.1).
c. Public Welfare Implications of Air Quality Under the Current
Standard
    In considering the potential for effects and related public welfare
implications of air quality conditions and associated exposures
indicated to occur under the current standard, the PA first looked to
the air quality analyses particular to cumulative O3
exposures, in terms of the W126 index, given its established
relationship with growth-related effects and specifically RBL as the
identified proxy or surrogate for the full array of such effects (PA,
section <IP_ADDRESS>, Appendix 4D). In that context, the PA gave relatively
greater emphasis to air quality in Class I areas in recognition of the
increased significance of effects in such areas that have been accorded
special protection, as discussed in section III.B.2 above. In
evaluating the extent and magnitude of O3 exposures, in
terms of W126, in such areas that meet the current standard, the PA
also considered year to year variability in the index, while
recognizing that, with regard to W126 index relationships with RBL,
there was uncertainty associated with RBL predictions from a single
year W126 estimate (PA, sections 4.3.4 and 4.5.1, Appendix 4A). As
discussed in section III.D.1.b above, the evidence does not indicate
estimates based on an average of seasonal W126 across three years to be
less, or more, predictive of RBL or resulting total plant biomass (PA,
sections 4.3.4 and <IP_ADDRESS>). The PA considered the magnitude of W126
index occurring in areas nationwide, and particularly in Class I areas,
that meet the current standard, as well as the frequency of the
relatively higher index values. Further, the PA evaluated the extent of
control of such index values exerted by the current standard, as

evidence by comparisons of sites with design values at or below the
current standard level and sites with higher design values (PA, section
4.4). Lastly, the PA also considered what the currently available
information indicated with regard to the incidence and severity of
visible foliar injury that might be expected to occur under air quality
conditions that meet the current standard, and the potential for
impacts on public welfare (PA, sections <IP_ADDRESS>, <IP_ADDRESS> and 4.5.3).
    The air quality analyses of monitoring data at sites across the
U.S. that meet the current standard in the most recent 3-year period
find that the seasonal W126 index, as assessed by the 3-year average,
is at or below 17 ppm-hrs, with just one exception, among 849
locations, where it equaled 18 ppm-hrs. No 3-year average W126 index
values exceeded 17 ppm-hrs in or near Class I areas. Further, such W126
exposures are generally well below 17 ppm-hrs across most of the U.S.
These findings for sites meeting the current standard, differ
dramatically from sites with higher design values. For example, a third
of all U.S. sites with design values above 70 ppb in the recent period,
and more than 80% of Class I area sites with design values above 70
ppb, have average W126 index values above 17 ppm-hrs. Looking back
across the 19 years covered by the full historical dataset, the
cumulative exposure estimates, averaged over the design value periods,
were virtually all at or below 17 ppm-hrs, with most of the W126 index
values below 13 ppm-hrs (PA, Appendix 4D, Table 4D-9).\187\
---------------------------------------------------------------------------

    \187\ Based on the established E-R functions for tree seedlings
of 11 species, the median RBL estimates for such W126 index values
are 3.8% or less (PA, Appendix 4A).
---------------------------------------------------------------------------

    The PA also considered the general occurrence and distribution of
relatively higher single-year W126 index values, finding a generally
similar pattern to that for averages over the design value period. For
example, fewer than two dozen of the 849 sites meeting the current
standard in the recent period had a single-year index above 17 ppm-hrs;
about a dozen of these sites fall above 19 ppm- hrs, the highest of
which just reaches 25 ppm-hrs in downtown Denver, CO.\188\ The
frequency of such occurrences is still lower for the Class I area
monitors. For example, during the most recent three years, when the
average seasonal W126 index is at or below 17 ppm-hrs in all Class I
areas meeting the current standard, there were just three single-year
W126 index values above 17 ppm-hrs and none above 19 ppm-hrs (PA,
Appendix 4D, Table 4D-15).\189\ The PA additionally notes that single-
year W126 index values in Class I areas over the 19-year dataset
evaluated were generally at or below 19 ppm-hrs, particularly in the
more recent years (PA, Appendix 4D, section 4D.3.2.3).
---------------------------------------------------------------------------

    \188\ These highest W126 index values occur in the South West
and West regions in which there are nearly 150 monitor locations
meeting the current standard (PA, Figure 4-6, Appendix 4D, Figure
4D-5, Table 4D-1). Across the full 19-year dataset, the downtown
Denver site value is just one of six instances in the more than
8,000 design value periods meeting the current standard of a single-
year W126 index value at or above 25 ppm-hrs. All but one of these
instances were equal to 25 ppm-hrs; the single higher occurrence was
equal to 26 ppm-hrs.
    \189\ Across the full 19-year dataset for Class I area monitors
meeting the current standard (58 monitors with at least one such
occurrence and approximately 500 total occurrences), there are no
more than 15 occurrences of single-year W126 index values above 19
ppm-hrs, all of which date prior to 2013 (PA, Appendix 4D, section
4D.3.2.4).
---------------------------------------------------------------------------

    In reflecting on the air quality analysis findings summarized here,
the PA additionally recognized limitations and uncertainties of the
underlying database, noting there to be inherent limitations in any air
monitoring network. The monitors for O3 are distributed
across the U.S., covering all NOAA regions and all states although some
geographical areas are more densely covered than others, which may have
sparse or no data. For example, only about 40% of all Federal Class I
Areas have or have had O3 monitors (with valid design
values) within 15 km, thus allowing inclusion in the Class I area
analysis. Even so, the dataset for that analysis includes sites in 27
states distributed across all nine NOAA climatic regions across the
contiguous U.S, as well as Hawaii and Alaska. While some NOAA regions
have far fewer numbers of Class I areas with monitors than others
(e.g., the Central, North East, East North Central, and South regions
versus other regions), these areas also have appreciably fewer Class 1
areas in general. Thus, the regions with relatively more Class I area
are also more well represented in the dataset. For example, the West
and Southwest regions (with the largest number of Class I areas) have
approximately a third of those areas represented with monitors, which
include locations where W126 index values are generally higher, thus
playing a prominent role in the analysis.
    Another inherent uncertainty is with regard to the extent to which
the results will prove to reflect conditions far out into the future as
air quality and patterns of O3 concentrations in ambient air
continue to change in response to changing circumstances, such as
changes in precursor emissions to meet the current standard across the
U.S. However, findings from these analyses in the current review are
largely consistent with those from analyses of the data available in
the last review. Further, the analysis of how changes in O3
patterns in the past have affected the relationship between W126 index
and the averaging time and form of the current standard finds a
positive, linear relationship between trends in design values and
trends in the W126 index (both in terms of single-year W126 index and
averages over 3-year design value period), as was also the case for
similar analyses conducted for the data available at the time of the
last review (Wells, 2015). While this relationship varied across NOAA
regions, the regions showing the greatest potential for exceeding W126
index values of interest (e.g., with 3-year average values above 17
and/or 19 ppm-hrs) also showed the greatest improvement in the W126
index per unit decrease in design value over the historical period
assessed (PA, Appendix 4D, section 4D.3.2.3). Thus, the available data
and this analysis appear to indicate that as design values are reduced
to meet the current standard in areas that presently do not, W126
values in those areas would also be expected to decline (PA, Appendix
4D, section 4D.4).
    In the last review, the Administrator focused on cumulative
exposure estimates derived as the average W126 index over the 3-year
design value period, concluding variations of single-year W126 index
from the average to be of little significance in assessing public
welfare protection. This focus generally reflected the judgment that
estimates based on the average adequately, and appropriately reflected
the precision of current understanding of O3-related growth
reductions, given the various limitations and uncertainties in such
predictions, that have been further evaluated in the current review (as
summarized in section III.D.1.b above). Based on the information
available in the current review, the PA concludes that, with the year-
to-year variation observed in areas meeting the current standard,\190\
differences in year-to-year tree growth in response to each year's
seasonal exposure from the tree growth estimated from the 3-year
average of the single-year values would, given the offsetting impacts
of seasonal exposures above and below the average, reasonably be
expected to generally be small over

tree lifetimes (PA, section <IP_ADDRESS>). In so doing, the PA takes note of
limitations in aspects of the data underlying the E-R functions that
contribute to imprecision or inexactitude to estimates of growth
impacts associated with multi-year exposures in the relatively lower
W126 index values pertinent to air quality under the current standard.
The information newly available in the current review does not
appreciably address such limitations and uncertainties or improve the
certainty or precision in RBL estimates for such exposures (PA,
sections 4.3.4, 4.5.1).
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    \190\ The current air quality data indicates single-year W126
index values generally to vary by less than 5 ppm-hrs from the 3-
year average when the 3-year average is below 20 ppm-hrs, which is
the case for locations meeting the current standard (PA, Appendix
4D).
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    Combining the findings of W126 index values (averaged over design
value period) likely under the current standard with the established E-
R functions for reduced growth in 11 tree seedling species yields a
median species RBL for tree seedlings at or below 5.3% for the recent
period, with very few exceptions, with the highest estimates occurring
in areas not near or within Class I areas. This general pattern is
confirmed over the longer time period (2000-2018) for the vast majority
of the data, with virtually all RBL estimates below 6%.\191\ Further,
given the variability and uncertainty associated with the data
underlying the E-R functions (as summarized in section III.D.1.a
above), the few higher single-year occurrences are reasonably
considered to be of less significance than 3-year average values.
Judgments in the last review (in the context of the framework
summarized in section III.D.1.b above) concluded isolated rare
occurrences of exposures for which median RBL estimates might be at or
just above 6% to not be indicative of conditions adverse to the public
welfare, particularly considering the variability in the array of
environmental factors that can influence O3 effects in
different systems, and the uncertainties associated with estimates of
effects in the natural environment.
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    \191\ Although potential for effects on crop yield was not given
particular emphasis in the last review (for reasons similar to those
summarized earlier), we additionally note that combining the
exposure levels summarized for areas across the U.S. where the
current standard is met with the E-R functions established for 10
crop species indicates a median RYL across crops to be at or below
5.1%, on average, with very few exceptions. Further, estimates based
on W126 index at the great majority of the areas are below 5% (PA,
Appendices 4A and 4D).
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    With regard to visible foliar injury, the PA observes that the
available evidence does not include an approach for characterizing
natural areas experiencing some severity or extent injury (e.g., via
USFS BI score) with regard to public perception and potential impacts
on public enjoyment; nor does it address this in combination with
information on whether air quality conditions in sites with scores of a
particular severity level do or do not meet the current standard (PA,
section 4.5.1). As summarized in section III.B.2 above, public welfare
implications relate largely to effects on scenic and aesthetic values.
Accordingly, key considerations of this endpoint in past reviews have
generally related to qualitative consideration of potential impacts
related to the plant's aesthetic value in protected forested areas and
the somewhat general, nonspecific judgment that a more restrictive
standard is likely to provide increased protection. The currently
available information does not yet address or describe the
relationships expected to exist for some level of visible foliar injury
severity (below that at which broader physiological effects on plant
growth and survival might also be expected) and/or extent of location
or site injury (e.g., BI) scores with values held by the public and
associated impacts on public uses of the locations.\192\ Additionally,
no criteria have been established regarding a level or prevalence of
visible foliar injury considered to be adverse to the affected
vegetation as the current evidence does not provide for determination
of a degree of leaf injury that would have significance to the vigor of
the whole plant (ISA, Appendix 8, p. 8-24). Nevertheless, while minor
spotting on a few leaves of a plant may easily be concluded to be of
little public welfare significance, it is reasonable to conclude that
cases of widespread and relatively severe injury during the growing
season (particularly when sustained across multiple years, and
accompanied by obvious impacts on the plant canopy) would likely impact
the public welfare in scenic and/or recreational areas, particularly in
areas with special protection, such as Class I areas. However, the gaps
in our information and tools, as summarized in prior sections, restrict
our ability to identify air quality conditions that might be expected
to provide a specific level of protection from public welfare effects
of this endpoint.
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    \192\ Information with some broadly conceptual similarity to
this has been used for judging public welfare implications of
visibility effects of PM in setting the PM secondary standard (78 FR
3086, January 15, 2012).
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    Assessment of any public welfare implications of air quality
occurring under the current standard with regard to visible foliar
injury is further hampered by the lack of an established quantitative
description of the relationship between O3 concentrations
(or exposure metrics) and injury extent or incidence, as well as
severity, that would support estimates of potential injury for varying
air quality and environmental conditions (e.g., moisture), most
particularly for situations that meet the current standard. Although no
such relationship or pertinent metrics for describing exposure are
established, the available information, indicates a role for both a
cumulative metric of exposure as well as the occurrence of relatively
higher concentrations. More specifically, the PA notes the information
indicating potential for increased incidence and severity of injury in
locations with W126 index above 25 ppm-hrs and with increased
occurrence of peak (1-hour) concentrations such as above 100 ppb (PA,
section 4.5.1).
    The analyses of recent and historical air quality at monitoring
sites where the current standard is met do not indicate a tendency for
such occurrence of cumulative exposures or peak concentrations (PA,
sections 2.4.5 and 4.4, Appendices 2A and 4D). In these analyses, all
3-year average W126 index values are below 25 ppm-hrs, and values above
17 ppm-hrs are rare. In addition, all single-year, W126 index values at
Class I area locations meeting the current standard (and virtually all
sites across the U.S.) are at or below 25 ppm-hr; even, and values
above 19 ppm-hrs are rare, and mores so in more recent years (PA,
section 4.4.2, Appendix 4D). Accordingly, while the current evidence is
limited for the purposes of identifying public welfare protection
objectives related to visible foliar injury in terms of specific air
quality metrics, the PA notes that the current information indicates
that the occurrence of injury categorized as more severe than
``little'' by the USFS categorization (i.e., a BI scores above 5 or
above 15) would be expected to be infrequent in areas that meet the
current standard.
    In light of the evidence regarding a role for peak concentrations,
the PA additionally took note of the control of peak concentrations
exerted by the form and averaging time of the current standard. For
example, daily maximum 1-hour, as well as 8-hour average O3
concentrations have declined over the past 15 years, a period in which
there have been two revisions of the level of the secondary standard,
each providing greater stringency, while retaining the same averaging
time and form as the current standard (e.g., PA, Figures 2-10, 2-12 and
2-17). Further, during periods when the current standard is met, there
is less than one day per site, on average

with a maximum hourly concentration at or above 100 ppb. This compares
with roughly 40 times as many such days, on average, for sites with
design values above the current standard level (PA, Appendix 2A,
section 2A.2). The currently available information indicates that the
current standard provides appreciable control of peak 1-hour
concentrations, as well as W126 index values, and thus, to the extent
that such metrics play a role in the occurrence and severity of visible
foliar injury, the current standard also provides appreciable control
of these.
    Thus, although the current information does not establish a metric
or combination of metrics that well describes the relationship between
occurrence and severity of visible foliar injury across a broad range
of O3 concentration patterns from those more common in the
past to those in areas recently meeting the current standard, the PA
concludes that the currently available information does not indicate
that a situation of widespread and relatively severe visible foliar
injury, with apparent implications for the public welfare, is likely
associated with air quality that meets the current standard. Based on
the USFS dataset presentations as well as the air quality analyses of
W126 index values and frequency of 1-hour observations at or above 100
ppb, the prevalence of injury scores categorized as severe, or even
moderate, which, depending on spatial extent, might reasonably be
concluded to have potential to be adverse to the public welfare do not
appear likely to occur under air quality conditions that meet the
current standard. Thus, the PA finds, based on the current evidence and
currently available air quality information, that the exposure
conditions associated with air quality meeting the current standard are
not those that might reasonably be concluded to result in the
occurrence of significant foliar injury (with regard to severity and
extent).
    With regard to other vegetation-related effects, including those at
the ecosystem scale, such as alteration in community composition or
reduced productivity in terrestrial ecosystems, as recognized in
section III.D.1.a above, the available evidence is not clear with
regard to the risk of such impacts (and their magnitude or severity)
associated with the environmental O3 exposures estimated to
occur under air quality conditions meeting the current standard, which
primarily include W126 index at or below 17 ppm-hrs. In considering
effects on crop yield, the air quality analyses at monitoring locations
that meet the current standard indicate estimates of RYL for such
conditions to be at and below 5.1%, based on the median estimate
derived from the established E-R functions for 10 crops (PA, Appendix
4A, Table 4A-5). We additionally recognize there to be complexities
involved in interpreting the significance of such small RYL estimates
in light of the factors also recognized in the last review. These
included the extensive management of crops in agricultural areas that
may to some degree mitigate potential O3-related effects, as
well as the use of variable management practices to achieve optimal
yields, while taking into consideration various environmental
conditions. We also recognize that changes in yield of commercial crops
and commercial commodities may affect producers and consumers
differently, further complicating the question of assessing overall
public welfare impacts for such RYL estimates (80 FR 65405, October 26,
2015).
2. CASAC Advice
    The CASAC provided its advice regarding the current secondary
standard in the context of its review of the draft PA (Cox,
2020a).\193\ In so doing, the CASAC concurred with the PA conclusions,
stating that it ``finds, in agreement with the EPA, that the available
evidence does not reasonably call into question the adequacy of the
current secondary ozone standard and concurs that it should be
retained'' (Cox, 2020a, p. 1). The CASAC additionally stated that it
``commends the EPA for the thorough discussion and rationale for the
secondary standard'' (Cox, 2020, p. 2). The CASAC also provided
comments particular to the consideration of climate and growth-related
effects.
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    \193\ A limited number of public comments have been received in
this review to date, including comments focused on the draft IRP,
draft ISA or draft PA. Of the commenters that addressed adequacy of
the current secondary O3 standard, most expressed
agreement with staff conclusions in the draft PA, while some
expressed the view that the standard should be revised to a W126-
based form or that articulation of its rationale should more
explicitly address the protection the standard provides for public
welfare effects.
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    With regard to O3 effects on climate, the CASAC
recommended quantitative uncertainty and variability analyses, with
associated discussion (Cox, 2020a, pp. 2, 22).\194\ With regard to
growth-related effects and consideration of the evidence in
quantitative exposure analyses, it stated that the W126 index ``appears
reasonable and scientifically sound,'' ``particularly [as] related to
growth effects'' (Cox, 2020a, p. 16). Additionally, with regard to the
prior Administrator's expression of greater confidence in judgments
related to public welfare impacts based on a seasonal W126 index
estimated by a three-year average and accordingly relying on that
metric the CASAC expressed the view that this ``appears of reasonable
thought and scientifically sound'' (Cox, 2020, p. 19). Further, the
CASAC stated that ``RBL appears to be appropriately considered as a
surrogate for an array of adverse welfare effects and based on
consideration of ecosystem services and potential for impact to the
public as well as conceptual relationships between vegetation growth
effects and ecosystem scale effects'' and that it agrees ``that biomass
loss, as reported in RBL, is a scientifically-sound surrogate of a
variety of adverse effects that could be exerted to public welfare,''
concurring that this approach is not called into question by the
current evidence which continues to support ``the use of tree seedling
RBL as a proxy for the broader array of vegetation related effects,
most particularly those related to growth that could be impacted by
ozone'' (Cox, 2020a, p. 21). The CASAC additionally concurred that the
strategy of a secondary standard that generally limits 3-year average
W126 index values somewhat below those associated with a 6% RBL in the
median species is ``scientifically reasonable'' and that, accordingly,
a W126 index target value of 17 ppm-hrs for generally restricting
cumulative exposures ``is still effective in particularly protecting
the public welfare in light of vegetation impacts from ozone'' (Cox,
2020a, p 21.).
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    \194\ As recognized in the ISA, ``[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). These complexities impede our ability to consider specific
O3 concentrations in the U.S. with regard to specific
magnitudes of impact on radiative forcing and subsequent climate
effects.
---------------------------------------------------------------------------

    With regard to the court's remand of the 2015 secondary standard to
the 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''),
while the CASAC stated that it was not clear whether the draft PA had
fully addressed this concern (Cox, 2020a, p. 21), it described there to
be a solid

scientific foundation for the current secondary standard and also
commented on areas related to the remand. With regard to the focus on
the 3-year average W126 index, in addition to the comments summarized
above, the CASAC concluded, as noted above, that the EPA
Administrator's focus on the 3-year average and her judgments in doing
so ``appears of reasonable thought and scientifically sound'' (Cox,
2020a, p. 19). Further, while recognizing the existence of established
E-R functions that relate cumulative seasonal exposure of varying
magnitudes to various incremental reductions in expected tree seedling
growth (in terms of RBL) and in expected crop yield, the CASAC letter
also noted that while decades of research also recognizes visible
foliar injury as an effect of O3, ``uncertainties continue
to hamper efforts to quantitatively characterize the relationship of
its occurrence and relative severity with ozone exposures'' (Cox,
2020a, p 20). In summary, the CASAC stated that the approach described