standard. Exposure- and risk-based considerations draw upon EPA
assessments of risk of key welfare effects, including O3
effects on forest growth, productivity, carbon storage, crop yield and
visible foliar injury, expected to occur in model-based scenarios for
the current standard, with appropriate consideration of associated
uncertainties.
    The following sections describe consideration of the evidence and
the exposure/risk information in the PA and advice received from CASAC,
as well as the comments received from various parties, and the
Administrator's proposed conclusions regarding the adequacy of the
current secondary standard.
1. Evidence- and Exposure/Risk-Based Considerations in the Policy
Assessment
    Staff assessments in the PA focus on the policy-relevant aspects of
the assessment and integrative synthesis of the currently available
welfare effects evidence in the ISA, analyses of air quality
relationships with exposure metrics of interest, the exposure and risk
assessments in the WREA, comments and advice of CASAC and public
comment on drafts of the PA, ISA and WREA. The PA describes evidence-
and exposure/risk-based considerations and presents staff conclusions
for the Administrator to consider in reaching her proposed decision on
the current standard. The focus of the initial PA conclusions is
consideration of the question: Does the currently available scientific
evidence and exposure/risk information, as reflected in the ISA and
WREA, support or call into question the adequacy and/or appropriateness
of the protection afforded by the current secondary O3
standard?
    The PA's general approach to informing judgments by the
Administrator recognizes that the available welfare effects evidence
demonstrates a range of O3 sensitivity across studied plant
species and documents an array of O3-induced effects that
extend from lower to higher levels of biological organization. These
effects range from those affecting cell processes and individual plant
leaves to effects on the physiology of whole plants, as well as the
range from species effects and effects on plant communities to effects
on related ecosystem processes and services. Given this evidence, the
PA notes that it is not possible to generalize across all studied
species regarding which cumulative exposures are of greatest concern,
as this can vary by situation due to differences in exposed species
sensitivity, the importance of the observed or predicted O3-
induced effect, the role that the species plays in the ecosystem, the
intended use of the affected species and its associated ecosystem and
services, the presence of other co-occurring predisposing or mitigating
factors, and associated uncertainties and limitations. Therefore, in
developing conclusions in the PA, staff takes note of the complexity of
judgments to be made by the Administrator regarding the adversity of
known and anticipated effects to the public welfare and are mindful
that the Administrator's ultimate judgments on the secondary standard
will most appropriately reflect an interpretation of the available
scientific evidence and exposure/risk information that neither
overstates nor understates the strengths and limitations of that
evidence and information (U.S. EPA, 2014c, section 5.7).
    In considering the estimates of exposures and risks for air quality
scenarios assessed in the WREA, the PA: (1) Evaluates the weight of the
scientific evidence concerning vegetation effects associated with those
O3 exposures; (2) considers the importance, from a public
welfare perspective, of the O3-induced effects on sensitive
vegetation and associated ecosystem services that are known or
anticipated to occur as a result of exposures in the assessed air
quality scenarios; and, (3) recognizes that predictions of effects
associated with any given O3 exposure may be mitigated or
exacerbated by actual conditions in the environment (i.e., co-occurring
modifying environmental and genetic factors). When considering WREA
analyses that involve discrete exposure levels or varying levels of
severity of effects, the PA's approach recognizes that the available
welfare effects evidence demonstrates a wide range in O3
sensitivities across studied plant species. The PA additionally
considers the uncertainties associated with this information.
    As an initial matter, the PA recognizes that the CAA does not
require that a secondary standard be protective of all effects
associated with a pollutant in the ambient air, but rather those
considered adverse to the public welfare (as described in section
IV.B.2 above). In considering the extent to which it may be appropriate
to consider particular welfare effects adverse, the PA applies a
paradigm used in past reviews. As discussed in section IV.B.2 above,
this paradigm recognizes that the significance to the public welfare of
O3-induced effects on sensitive vegetation growing within
the U.S. can vary depending on the nature of the effect, the intended
use of the sensitive plants or ecosystems, and the types of
environments in which the sensitive vegetation and ecosystems are
located. Accordingly, any given O3-related effect on
vegetation and ecosystems (e.g., biomass loss, crop yield loss, visible
foliar injury) may be judged to have a different degree of impact on
the public welfare depending, for example, on whether that effect
occurs in a Class I area, a city park, or commercial cropland. In the
last review, the Administrator took note 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 for the enjoyment of future generations
(73 FR 16497, March 27, 2008). Such public lands that are protected
areas of national interest include national parks and forests, wildlife
refuges, and wilderness areas (73 FR 16497, March 27, 2008). The PA
notes that effects occurring in such areas would likely have the
highest potential for being classified as adverse to the public
welfare, given the expectation of preserving these areas to ensure
their intended use is met (U.S. EPA, 2014c, section 5.1). In
considering uses of vegetation and forested lands, the paradigm also
includes consideration of impacts to ecosystem goods and services. In
summary, the paradigm considered in the PA, consistent with the
discussion in section IV.B.2 above, integrates the concepts of: (1)
Variability in public welfare significance given intended use and value
of the affected entity, such as individual species; (2) relevance of
associated ecosystem services to public welfare; and (3) variability in
spatial, temporal, and social distribution of ecosystem services
associated with known and anticipated welfare effects. Further, the PA
recognizes that there is no bright-line rule delineating the set of
conditions or scales at which known or anticipated effects become
adverse to public welfare.
    With respect to the scientific evidence, the PA takes note of the
longstanding evidence base that demonstrates O3-induced
effects that occur across a range of biological and ecological scales
of organization, as described in the ISA and summarized in section
IV.B.1 above (U.S. EPA, 2013a, p. 1-8). Many of the recent studies
evaluated in this review have focused on and further increased our
understanding of the molecular, biochemical and physiological
mechanisms that explain how plants are affected by O3 in the
absence of other stressors (U.S. EPA, 2013a, section 9.3). These recent
studies, in combination with the extensive and long-standing evidence,
have further strengthened the coherence and consistency of the entire
body of research since the last review. Consistent with conclusions in
the 2006 AQCD, the ISA determined that a causal relationship exists
between O3 exposure and visible foliar injury on sensitive
vegetation, reduced plant growth, reduced productivity in terrestrial
ecosystems, reduced yield and quality of agricultural crops and
alteration of below-ground biogeochemical cycles (U.S. EPA, 2013a,
Table 1-2 and section 2.6). The relationship between O3
exposures and reduced carbon sequestration in terrestrial ecosystems,
alteration of terrestrial ecosystem water cycling and alteration of
terrestrial community composition was concluded to be likely causal
(U.S. EPA, 2013a, Table 1-2).
    The PA recognizes that consistent with conclusions drawn in the
last review, the currently available evidence base also strongly
supports that effects on vegetation are attributable to cumulative
seasonal O3 exposures. Moreover, on the basis of the entire
body of evidence in this regard, the ISA concludes that ``quantifying
exposure with indices that cumulate hourly O3 concentrations
and preferentially weight the higher concentrations improves the
explanatory power of exposure/response models for growth and yield,
over using indices based on mean and peak exposure values'' (U.S. EPA,
2013a, p. 2-44). Accordingly, as in other recent reviews, the evidence
continues to provide a strong basis for concluding that it is
appropriate to judge impacts of O3 on vegetation, related
effects and services, and the level of public welfare protection
achieved, using a cumulative, seasonal exposure metric, such as the
W126-based metric. In this review, as in the last review, the CASAC
concurs with this conclusion (Frey, 2014c, p. iii). Thus, based on the
consistent and well-established evidence described above, the PA
concludes that the most appropriate and biologically relevant way to
relate O3 exposure to plant growth, and to determine what
would be adequate protection for public welfare effects attributable to
the presence of O3 in the ambient air is to characterize
exposures in terms of a cumulative seasonal form, and in particular the
W126 metric.
    In considering the current standard with regard to protection from
the array of O3-related effects recognized in this review,
the PA first considers effects related to forest tree growth,
productivity and carbon storage, effects for which the ISA concludes
the evidence supports a causal or likely causal relationship with
exposures to O3 in ambient air (U.S. EPA, 2014c, sections
5.2 and 5.7). In so doing, the PA notes that while changes in biomass
affect individual tree species, the overall effect on forest ecosystem
productivity depends on the composition of forest stands and the
relative sensitivity of trees within those stands. In considering the
evidence for these effects and the extent to which they might be
expected to occur under conditions that meet the current secondary
standard, the PA focused particularly on RBL estimates for the 11
species for which robust E-R functions have been developed. The PA
recognized that recent studies, such as multiple-year exposures of
aspen and birch, have provided additional evidence on tree biomass or
growth effects associated with multiple year exposures in the field,
including the potential for cumulative and carry-over effects. For
example, findings from these studies indicate that effects of
O3 on birch seeds (reduced weight, germination, and starch
levels) could lead to a negative impact on species regeneration in
subsequent years and may have the potential to alter carbon metabolism
of overwintering buds, potentially affecting growth in the following
year. Other studies have reported that multiple-year exposures reduced
tree size parameters in an aspen community, and increased the rate of
conversion from a mixed aspen-birch community to a community dominated
by the more tolerant birch, such that elevated O3 may alter
intra- and inter-species competition within a forest stand (U.S. EPA,
2013a, section 9.4.3; U.S. EPA, 2014c, section 5.2). In giving
particular attention to tree seedling biomass loss estimates, the PA
notes that 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'' (Frey, 2014c, p. 10).
    In evaluating the current evidence and exposure/risk information
associated with tree growth, productivity and carbon storage, with
regard to the adequacy of public welfare protection afforded by the
current standard, the PA considers the evidence of vegetation and
welfare impacts in areas of the U.S. likely to have met the current
standard. With regard to O3 effects on tree growth,
productivity and carbon storage and associated ecosystems and services,
the PA focuses on relative biomass loss estimates based on the OTC-
based E-R functions, noting that analyses newly performed in this
review have reduced the uncertainty associated with using OTC E-R
functions to predict tree growth effects in the field (U.S. EPA, 2014c,
section 5.2.1; U.S. EPA, 2013a, section <IP_ADDRESS>).
    In focusing on RBL estimates, the PA recognized that comparison to
an array of benchmarks would be informative to considerations of
significance to public welfare. Included in this array were RBL values
of 2% and 6% given emphasis by CASAC (Frey, 2014c). In considering the
RBL estimates for different O3 conditions associated with
the current standard, the PA focused first on the median of the
species-specific (composite) E-R functions. In so doing,

the PA takes note of CASAC's comments that a 6% median RBL is
``unacceptably high'', and that the 2% median RBL is an important
benchmark to consider (Frey, 2014c).\214\ Based on the summary of RBL
estimates in the PA, the PA notes that the median species RBL estimate
is at or below 2% for W126 exposure index values less than or equal to
7 ppm-hrs (U.S. EPA, 2014c, Table 6-1 and Appendix 5C). The median
species RBL is at or above 6% for W126 index values of 19 ppm-hrs and
higher.
---------------------------------------------------------------------------

    \214\ The CASAC provided several comments related to 2% RBL for
tree seedlings both with regard to its use in summarizing WREA
results and with regard to consideration of the potential
significance of vegetation effects, as summarized in sections IV.D.2
and IV.E.3. In identifying 2% as an important benchmark, CASAC
referenced the 1996 workshop sponsored by the Southern Oxidants
Study group at which, as noted in section IV.B.2 above, participants
identified annual percentages of tree seedling growth reduction and
crop yield loss they considered important to their judgments on a
secondary standard. The workshop report provides no explicit
rationale for the percentages identified or specification with
regard to number or proportion of species for which such percentages
should be met (Heck and Cowling, 1997).
---------------------------------------------------------------------------

    In recognition of the significance of welfare effects in Class I
areas, the PA gives appreciable weight to consideration of the
occurrence of O3 concentrations associated with the
potential for RBL estimates above benchmarks of interest in Class I
areas that meet the current standard. Based on air quality data for the
period from 1998 to 2012, the PA focused consideration on 22 Class I
areas, in which during one or more three-year periods the air quality
met the current standard and the three-year average W126 index value
was at or above 15 ppm-hrs (see Table 7 below, drawn from U.S. EPA,
2014c, Table 5-2). Across these 22 Class I areas, the highest single-
year W126 index values for these three-year periods ranged from 17.4 to
29.0 ppm-hrs. In 20 of the areas, distributed across eight states (AZ,
CA, CO, KY, NM, SD, UT, WY) and four regions (West, Southwest, West
North Central and Central), this range was 19.1 to 29.0 ppm-hrs,
exposure values for which the corresponding median species RBL
estimates equal or exceed 6%, which CASAC has termed ``unacceptably
high''. Recognizing that in any given year, other environmental factors
can influence the extent to which O3 may have the impact
predicted by the E-R functions, the PA looked beyond single year
occurrences of such magnitudes of W126 index values. For example,
focusing on the highest three-year periods that include these highest
annual values for 21 areas, the PA notes that in 10 areas (across five
states in the West and Southwest regions), the three-year average W126
values (for the highest three-year period that includes these annual
values) are at or above 19 ppm-hrs, ranging up to 22.5 ppm-hrs (for
which the median species RBL estimate is above 7%). This indicates that
the W126 value above 19 ppm-hrs is not simply a single year in a period
of lower years, but that in these cases there were sustained higher
values that contributed to a three-year W126 also above 19 ppm-hrs. In
terms of the highest three-year values observed (regardless of single-
year values), the PA additionally notes that the highest three-year
average W126 index value (during periods meeting the current standard)
was at or above 19 (ranging up to 22.5 ppm-hrs) in 11 areas,
distributed among five states in the West and Southwest regions (U.S.
EPA, 2014c, Table 5-2, Appendix 5B).

 Table 7--O3 Concentrations in Class I Areas During Period From 1998 to 2012 that Met the Current Standard and Where Three-year Average W126 Index Value
                                                               was at or Above 15 ppm-hrs
--------------------------------------------------------------------------------------------------------------------------------------------------------
                                                                                3-year Average W126 (ppm-
             Class I Area                       State/county           Design value    hrs)* (# >=19 ppm-hrs,    Annual W126 (ppm-hrs)*    Number of 3-
                                                                          (ppb)*               range)            (# >=19 ppm-hrs, range)   year periods
--------------------------------------------------------------------------------------------------------------------------------------------------------
Bandelier Wilderness Area QA, DF, PP.  NM/Sandoval..................           70-74  15.8-20.8 (2, 20.0-20.8)  12.1-25.3 (4, 19.2-25.3)               8
Bridger Wilderness Area QA, DF.......  WY/Sublette..................           69-72  15.1-17.4...............  9.9-19.2 (1, 19.2)......               5
Canyonlands National Park QA, DF, PP.  UT/San Juan..................           69-73  15.0-20.5 (2, 19.8-20.5)  9.9-24.8 (5, 19.3-24.8).               9
Carlsbad Caverns National Park PP....  NM/Eddy......................              69  15.0-15.3...............  8.6-26.7 (1, 26.7)......               3
Chiricahua National Monument DF, PP..  AZ/Cochise...................           69-73  15.7-18.0...............  13.2-21.6 (2, 19.3-21.6)               7
Grand Canyon National Park QA, DF, PP  AZ/Coconino..................           68-74  15.3-22.2 (7, 19.2-22.2)  11.3-26.7 (7, 19.8-26.7)              12
John Muir Wilderness Area QA, DF, PP.  CA/Inyo......................           71-72  16.5-18.6...............  10.1-25.8 (2, 23.9-25.8)               3
Lassen Volcanic National Park DF, PP.  CA/Shasta....................              75  15.3....................  13.6-18.7...............               1
Mammoth Cave National Park BC, C, LP,  KY/Edmonson..................              74  15.9....................  12.5-22.5 (1, 22.5).....               1
 RM, SM, VP, YP.
Mesa Verde National Park DF..........  CO/Montezuma.................           67-73  15.5-21.0 (2, 19.0-21.0)  10.7-23.6 (4, 19.7-23.6)              10
Mokelumne Wilderness Area DF, PP.....  CA/Amador....................              74  17.6....................  14.8-22.6 (1, 22.6).....               1
Petrified Forest National Park.......  AZ/Navajo....................              70  15.7....................  12.9-19.2 (1, 19.2).....               1
Pinnacles National Monument..........  CA/San Benito................              74  15.1....................  13.1-17.4...............               1
Rocky Mountain National Park QA, DF,   CO/Boulder...................           73-75  15.1-19.3 (1, 19.3).....  9.5-25.1 (5, 20.7-25.1).               6
 PP.
                                       CO/Larimer...................              74  15.0-18.3...............  8.1-25.8 (3, 19.1-25.8).               3
Saguaro National Park DF, PP.........  AZ/Pima......................           69-74  15.4-18.9...............  11.0-23.1 (3, 20.0-23.1)               6
Sierra Ancha Wilderness Area DF, PP..  AZ/Gila......................           72-75  17.9-22.4 (3, 20.2-22.4)  14.8-27.5 (4, 20.3-27.5)               4

Superstition Wilderness Area PP......
                                       AZ/Maricopa..................              75  22.4 (1, 22.4)..........  14.5-28.6 (2, 27.4-28.6)               1
                                       AZ/Pinal.....................           73-75  18.7-22.5 (2, 20.8-22.5)  14.8-29.0 (3, 22.6-29.0)               3
Weminuche Wilderness Area QA, DF, PP.  CO/La Plata..................           70-74  15.0-19.1 (1, 19.1).....  10.9-21.0 (2, 20.8-21.0)               5
West Elk Wilderness Area QA, DF......  CO/Gunnison..................           68-73  15.6-20.1 (1, 20.1).....  12.9-23.9 (3, 21.1-23.9)               8
Wind Cave National Park QA, PP.......  SD/Custer....................              70  15.4....................  12.2-20.6 (1, 20.6).....               1
Yosemite National Park QA, DF, PP....  CA/Tuolumne..................           73-74  20.7-20.8 (2, 20.7-20.8)  19.7-22.1 (4, 19.7-22.1)               2
Zion National Park QA, DF, PP........  UT/Washington................           70-73  17.8-21.1 (2, 20.3-21.1)  14.9-24.2 (5, 19.3-24.2)               4
--------------------------------------------------------------------------------------------------------------------------------------------------------
* Based on data from http://www.epa.gov/ttn/airs/airsaqs/detaildata/downloadaqsdata.htm. W126 values are truncated after first decimal place.
  Superscript letters refer to species present for which E-R functions have been developed. QA=Quaking Aspen, BC=Black Cherry, C=Cottonwood, DF=Douglas
  Fir, LP=Loblolly Pine, PP=Ponderosa Pine, RM=Red Maple, SM=Sugar Maple, VP=Virginia Pine, YP=Yellow (Tulip) Poplar. Sources include USDA-NRCS
  (2014,http://plants.usda.gov), USDA-FS (2014, http://www.fs.fed.us/foresthealth/technology/nidrm2012.shtml) UM-CFCWI (2014, http://www.wilderness.net/printFactSheet.cfm?WID=583) and Phillips and Comus (2000).

    In considering the data analysis for 22 Class I areas described
above, the PA additionally considers the species-specific RBL estimates
for quaking aspen and ponderosa pine, two tree species that are found
in many of these 22 areas and have a sensitivity to O3
exposure that places them near the middle of the group for which E-R
functions have been established (U.S. EPA, 2014c, sections 5.2 and
5.7). In the Class I areas where ponderosa pine is present, the highest
single year W126 index values ranged from 18.7 to 29.0 ppm-hrs and the
highest three-year average W126 values in which these single year
values were represented ranged from 15 to 22.5 ppm-hrs, with these
three-year values above 19 ppm-hrs in eight areas across five states.
The ponderosa pine RBL estimates for 29 and 22.5 ppm-hrs are
approximately 12% and 9%, respectively (U.S. EPA, 2014c, Appendix 5C).
In Class I areas where quaking aspen is present, the highest single
year W126 index values ranged from 19.2 to 26.7 ppm-hrs and the highest
three-year average W126 values in which these single year values were
represented ranged from 15.0 to 22.2 ppm-hrs, with these three-year
values above 19 ppm-hrs in eight areas across five states. The quaking
aspen RBL estimates for 26.7 and 22.2 ppm-hrs are approximately 16% and
13%, respectively (U.S. EPA, 2014c, Appendix 5C).
    The PA describes the above observations, particularly in light of
advice from CASAC, summarized in section IV.D.2 below, as evidence of
the occurrence in Class I areas during periods where the current
standard is met of cumulative seasonal O3 exposures of a
magnitude for which the tree growth impacts indicated by the estimated
median species RBL might reasonably be concluded to be important to
public welfare (U.S. EPA, 2014c, sections 5.2.1 and 5.7).
    In considering the WREA analyses of effects on tree growth and
associated ecosystem services in the air quality scenario for the
current standard, the PA first takes note of the potential for the
interpolation method used in creating the national surface of
O3 concentrations for the air quality scenarios to
underestimate the higher W126 values such that W126-based exposures
would be expected to be somewhat higher than those included in each
scenario (U.S. EPA, 2014b, pp. 5-31 to 5-32). While recognizing this,
the PA considers results of the WREA analyses for the current standard
scenario and the 11 species of trees, for which robust E-R functions
are available. These results indicate that O3 can impact
growth of these species across the U.S., as well as an array of
associated ecosystem services provided by forests, including timber
production, carbon storage and air pollution removal (U.S. EPA, 2014b,
sections 6.2-6.8; U.S. EPA, 2014c, section 5.2).
    With regard to WREA analyses of ecosystem services, the PA notes
that the national-scale analysis of O3 impacts on carbon
storage indicates appreciably more storage in the air quality scenario
for the current standard (approximately 11,000 MMtCO2e, over
30 years) compared to the scenario for recent, higher O3
conditions (U.S. EPA, 2014b, Appendix 6B, Table B-10). The PA
additionally considers the WREA estimates of tree growth and ecosystem
services provided by urban trees over a 25-year period for five urban
areas based on case-study scale analyses that quantified the effects of
biomass loss on carbon storage and pollution removal (U.S. EPA, 2014b,
sections 6.6.2 and 6.7; U.S. EPA, 2014c, sections 5.2 and 5.7). The
urban areas included in this analysis represent diverse geography in
the Northeast, Southeast, and Central regions, although they do not
include an urban area in the western U.S. Estimates of the effects of
O3-related biomass loss on carbon sequestration indicate the
potential for an increase of somewhat more than a MMtCO2e
for the current standard scenario as compared to the recent conditions
scenario (U.S. EPA, 2014b, section 6.6.2 and Appendix 6D; U.S. EPA,
2014c, sections 5.2 and 5.7). The PA also notes the WREA estimates of
increased pollution removal in the current standard scenario as
compared to the scenario for recent conditions (U.S. EPA, 2014b,
section 6.6.2; U.S. EPA, 2014c, section 5.2.2).
    In considering the significance of these WREA analyses of risks for
the associated ecosystem services for timber production, air pollution
removal, and carbon sequestration, the PA takes note of the large
uncertainties associated with these analyses (see U.S. EPA, 2014b,
Table 6-27), and the potential for these findings to underestimate the
response at the national scale. While noting the potential usefulness
of considering predicted and anticipated impacts to these services in
assessing the extent to which the current information supports or calls
into question the adequacy of the protection afforded by the current
standard, the PA also notes that staff places limited

weight on the absolute magnitude of the risk results for these
ecosystem service endpoints due to the identification of significant
associated uncertainties (U.S. EPA, 2014c, sections 5.2 and 5.7).
    In reaching conclusions regarding support for the adequacy of the
current secondary standard provided by the currently available
information on O3-induced effects on trees and associated
services, the PA takes note of: (1) the robust evidence supporting the
causal relationship between cumulative O3 exposures and
effects on tree growth and productivity, and information from model
simulations supporting the determination of a likely causal
relationships for carbon storage in terrestrial ecosystems (U.S. EPA,
2013a, sections <IP_ADDRESS> and 9.4.3); (2) the tree seedling E-R functions
evidence, which has been strengthened and demonstrates variability in
sensitivity to O3 across species; (3) estimates of median
species RBL at or above 6% associated with W126-based exposure levels
in several areas when O3 concentrations were at or below the
current standard; (4) growth effects estimates associated with exposure
concentrations in several Class I areas based on O3
concentrations from 1998-2012 that were at or below the current
standard; (5) evidence that impacts from single year exposures can
carry over to the subsequent year and/or cumulate over multiple years
with repeated annual exposures; (6) evidence from recent mechanistic
studies and field based studies that support earlier findings from OTC
studies; and (7) WREA analyses indicating that O3-induced
biomass loss can impact ecosystem services provided by forests,
including timber production, carbon storage, and air pollution removal,
even when air quality is adjusted to just meet the current standard.
Given the above, and noting CASAC views (described in section IV.D.2
below), the PA concludes that the current evidence and exposure/risk
information call into question the adequacy of public welfare
protection afforded by the current standard from the known and
anticipated adverse effects associated with O3-induced
impacts on tree growth, productivity and carbon storage, including the
associated ecosystem services assessed in this review. Therefore, the
PA concludes that it is appropriate to consider revision of the
secondary standard to provide increased protection.
    With respect to crops, the PA takes note of the extensive and long-
standing evidence on the detrimental effect of O3 on crop
production, which continues to be confirmed by newly available evidence
(U.S. EPA, 2013a, section 9.4.4; U.S. EPA, 2014c, sections 5.3 and
5.7). The PA additionally notes that recent studies have highlighted
the effects of O3 on crop quality, such as through decreases
in the nutritive quality of grasses, and in the macro- and micro-
nutrient concentrations in fruits and vegetable crops (U.S. EPA, 2013a,
section 9.4.4; U.S. EPA, 2014c, section 5.3). Further, the PA notes
that there has been little published evidence that crops are becoming
more tolerant of O3, taking note particularly of the ISA
analyses of data from cultivars used in NCLAN studies, and yield data
for modern cultivars from SoyFACE which confirm that the average
response of soybean yield to O3 exposure has not changed in
current cultivars (U.S. EPA, 2006a; U.S. EPA, 2013a, section 9.6.3;
U.S. EPA, 2014c, section 5.3). In consideration of the currently
available evidence for O3 effects on crops, the PA concludes
that the recently available evidence, as assessed in the ISA, continues
to support the conclusions of the 1996 and 2006 CDs that ambient
O3 concentrations can reduce the yield of major commodity
crops in the U.S, and that the currently available evidence continues
to support the use of the E-R functions developed for 10 crops from OTC
experiment data. Further, the PA recognizes that important
uncertainties have been reduced regarding the exposure-response
functions for crop yield loss, especially for soybean, the second-most
planted field crop in the U.S.,\215\ with the ISA generally reporting
consistent results across exposure techniques and across crop varieties
(U.S. EPA, 2013a, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    \215\ See http://www.ers.usda.gov/topics/crops/soybeans-oil-crops/background.aspx
---------------------------------------------------------------------------

    With regard to consideration of the quantitative impacts of
O3 on crop yield, the PA considers RYL estimates for
O3 conditions associated with the current standard. As in
the case of the PA considerations of RBL estimates for tree seedlings,
the PA recognized CASAC comments, which described greater than 5% RYL
for the median crop species as ``unacceptably high'' and 5% RYL for the
median crop species as adverse, while noting the opportunities to alter
management of annual crops (Frey, 2014c, pp. iii and 14). The PA notes
that staff analyses of recent monitoring data (2009-2011) indicate that
O3 concentrations in multiple agricultural areas in the U.S.
that meet the current standard correspond to W126 index levels above 12
ppm-hrs, a value for which soybean RYL estimates are greater than 5%.
In particular, the PA notes that while the design values for two
counties in the Midwest met the current standard in 2009-2011, both had
a maximum annual W126 of 19 ppm-hrs (in 2011) for which the soybean
annual RYL estimate, based on the E-R function, is 9%.\216\
---------------------------------------------------------------------------

    \216\ The monitoring data reflect observations in locations that
meet the current standard. The WREA analysis that assessed crop
yield loss used a model-developed air quality scenario to reflect
air quality associated with the current standard (as described in
section IV.C.1 above). In so doing, adjustments are made to create
air quality that meets the standard and when the highest monitor in
an area is adjusted downward to meet the standard, concentrations at
nearby monitors that already meet the standard are also reduced.
---------------------------------------------------------------------------

    In considering the evidence and exposure/risk-based information for
effects on crops, the PA notes the CASAC comments regarding the use of
crop yields as a surrogate for consideration of public welfare impacts,
in which it noted that ``[c]rops provide food and fiber services to
humans'' and that ``[e]valuation of market-based welfare effects of
O3 exposure in forestry and agricultural sectors is an
appropriate approach to take into account damage that is adverse to
public welfare'' (Frey, 2014c, p. 10; U.S. EPA, 2014c, section 5.7).
The PA additionally notes, however, as recognized in section IV.B.2
above that the determination of the point at which O3-
induced crop yield loss becomes adverse to the public welfare is still
unclear, given that crops are heavily managed with additional inputs
that have their own associated markets and that benefits can be
unevenly distributed between producers and consumers. The PA further
notes that to the extent protection is provided by the current standard
with regard to impacts on trees, protection may also be provided for
commodity crops (U.S. EPA, 2014c, sections 5.3 and 5.7).
    In reaching conclusions regarding support provided for the adequacy
of the current secondary standard by the currently available
information on O3-related crop effects, the PA notes: (1)
the support for a causal relationship between cumulative O3
exposures and effects on crop yields and quality (U.S. EPA, 2013a,
section 9.4.4); (2) the evidence supporting E-R functions for 10 crops,
which has been strengthened in this review and which demonstrates
variability in sensitivity to O3 across species; (3)
evidence from recent mechanistic studies and field based studies
supporting earlier findings from OTC studies; (4) evidence that crops,
and in particular soybean, have not become more tolerant of
O3 (U.S. EPA, 2013a, section 9.6.3, <IP_ADDRESS>); and, (5) WREA
analysis results indicating that O3-induced crop yield loss
can impact producer and consumer surpluses and

the interaction between agriculture and timber production.
    With regard to visible foliar injury, the PA recognizes the long-
standing evidence that has established that O3 causes
diagnostic visible injury symptoms on studied bioindicator species and
that soil moisture is a major confounding effect that can decrease the
incidence and severity of visible foliar injury under dry conditions
and vice versa (U.S. EPA, 2014c, sections 5.4 and 5.7). As at the time
of the last review, the most extensive dataset regarding visible foliar
injury incidence across the U.S. is that collected by the U.S. Forest
Service (USFS) Forest Health Monitoring/Forest Inventory and Analysis
(FHM/FIA) Program, which has documented incidence of visible foliar
injury in both the eastern and western U.S. Evidence available in the
current review includes studies using controlled exposures as well as
multi-year field surveys. In addition to supporting prior conclusions,
the newly available studies also address some uncertainties identified
in the last review, such as the influence of soil moisture on visible
injury development (U.S. EPA, 2013a, section 9.4.2). As stated in the
ISA, ``many studies have shown that dry periods in local areas tend to
decrease the incidence and severity of O3-induced visible
foliar injury; therefore, the incidence of visible foliar injury is not
always higher in years and areas with higher O3, especially
with co-occurring drought'' (U.S. EPA, 2013a, p. 9-39). The ISA
additionally concludes, however, that ``the nature of the response is
largely species-specific and will depend to some extent upon the
sequence in which the stressors occur'' (U.S. EPA, 2013a, p. 9-87). As
recognized in the PA, this area of uncertainty complicates
characterization of the potential for visible foliar injury and its
severity or extent of occurrence for any given air quality conditions
and thus complicates identification of air quality conditions that
might be expected to provide a specific level of protection from this
effect (U.S. EPA, 2014c, sections 5.4 and 5.7).
    Information available in this review indicates the occurrence of
visible foliar injury in some Class I areas during times when
O3 concentrations met or would be expected to meet the
current standard (U.S. EPA, 2014c, sections 5.4.1 and 5.7). In noting
this occurrence in Class I areas, the PA notes it has particular public
welfare significance in light of direction from Congress that these
areas merit a high level of protection (U.S. EPA, 2014c, sections 5.1,
5.4.1 and 5.7). The PA also notes that visible foliar injury surveys
are used by the federal land managers to assess potential O3
impacts in Class I areas (USFS, NPS, FWS, 2010). Given this focus on
visible foliar injury, the PA concludes that such O3-induced
impacts have the potential to impact the public welfare in scenic and/
or recreational areas on an annual basis. Visible foliar injury is
associated with important cultural and recreational ecosystem services
to the public, such as scenic viewing, wildlife-watching, hiking, and
camping, that are of significance to the public welfare and enjoyed by
millions of Americans every year, generating millions of dollars in
economic value (U.S. EPA, 2014b, section 7.1). In addition, several
tribes have indicated that many of the O3-sensitive species
(including bioindicator species) are culturally significant (U.S. EPA,
2014c, Table 5-1). With respect to agricultural species, such visible
effects of O3 exposure can affect the market value of
certain crops and ornamentals for which leaves are the product, such as
spinach (U.S. EPA, 2006a, p. AX-9-189). The PA additionally notes CASAC
comments that ``visible foliar injury can impact public welfare by
damaging or impairing the intended use or service of a resource'',
including through ``visible damage to ornamental or leafy crops that
affects their economic value, yield, or usability; visible damage to
plants with special cultural significance; and visible damage to
species occurring in natural settings valued for scenic beauty or
recreational appeal'' (Frey, 2014c, p. 10).
    With regard to the exposure and risk-based information, the PA
takes note of the WREA analyses of the nationwide dataset (2006-2010)
for USFS/FHM biosites, including the observation that the proportion of
biosites with injury varies with soil moisture conditions and
O3 W126 index values (U.S. EPA, 2014b, Chapter 7, Figure 7-
10; U.S. EPA, 2014c, section 5.4.2). These analyses indicate that the
proportion of biosites showing visible foliar injury incidence
increases steeply with W126 index values up to approximately 10 ppm-
hrs, with little difference in incidence across higher W126 index
levels. The screening-level assessment of national parks indicated that
risk of visible foliar injury is likely to be lower in most national
parks after simulating just meeting the current standard, although
visible foliar injury would likely continue to occur at lower
O3 exposures, including some sensitive species growing in
National Parks and other Class I areas that may provide important
cultural ecosystem services to the public. The PA also notes the WREA
recognition that many of the outdoor recreational activities which
directly depend on the scenic value of the area are of significant
importance to public welfare as they are enjoyed by millions of
Americans every year and generate millions of dollars in economic value
(U.S. EPA, 2014b, Chapter 5, Chapter 7).
    In reaching conclusions regarding support for the adequacy of the
current secondary standard provided by the currently available
information on O3-induced visible foliar injury, the PA took
note of: (1) The evidence for many species of native plants, including
trees, that have been observed to have visible foliar injury symptoms
in both OTC and field settings, some of which have also been identified
as bioindicators of O3 exposure by the USFS; (2) the finding
that visible foliar injury incidence can occur at very low cumulative
exposures, but due to confounding by soil moisture and other factors,
it is difficult to predictively relate a given O3 exposure
to plant response; (3) information indicating the occurrence of visible
foliar injury in some Class I areas under air quality conditions
expected to meet the current standard; and, (4) WREA analyses, based on
USFS biosite data, indicating a relationship of the proportion of
biosites showing visible foliar injury incidence with W126 index values
below approximately 10 ppm-hrs (U.S. EPA, 2014c, section 5.7).
    The PA additionally recognizes a lack of guidance for federal land
managers regarding what spatial scale or degree of severity of visible
foliar injury is considered sufficient to trigger protective action for
O3 sensitive AQRVs. Further, there does not appear to be any
consensus in the literature in this regard, and CASAC, while
identifying benchmarks to consider for percent biomass loss and yield
loss for tree seedlings and commodity crops, respectively, did not
provide a similar recommendation for this endpoint. Likewise, as in
previous reviews, the ISA notes the difficulty in relating visible
foliar injury symptoms to other vegetation effects such as individual
plant growth, stand growth, or ecosystem characteristics (U.S. EPA,
2013a, section 9.4.2, p. 9-39) and further notes that the full body of
evidence indicates that there is wide variability in this endpoint,
such that although evidence shows visible foliar injury can occur under
very low cumulative O3 concentrations, ``. . .the degree and
extent of visible foliar injury development varies from year to year
and site to site . . ., even among co-members of a population exposed
to similar O3 levels, due to the influence

of co-occurring environmental and genetic factors'' (U.S. EPA, 2013a,
section 9.4.2, p. 9-38).
    Given the above, and taking note of CASAC views, the PA recognizes
visible foliar injury as an important O3 effect which,
depending on severity and spatial extent, may reasonably be concluded
to be of public welfare significance, especially when occurring in
nationally protected areas. While noting the uncertainties associated
with describing the potential for visible foliar injury and its
severity or extent of occurrence for any given air quality conditions,
the PA notes the occurrence of O3-induced visible foliar
injury in areas, including federally protected Class I areas that meet
the current standard, and suggests it may be appropriate to consider
revising the standard to achieve greater protection, while recognizing
that the degree to which O3-induced visible foliar injury
would be judged important and potentially adverse to public welfare is
uncertain (U.S. EPA, 2014c, section 5.7).
    With regard to other welfare effects, for which the ISA determined
a causal or likely causal relationships with O3 in ambient
air, such as alteration of ecosystem water cycling and changes in
climate, the PA concludes there are limitations in the available
information which affect our ability to consider potential impacts of
air quality conditions associated with the current standard.
    In reaching conclusions on options for the Administrator's
consideration, the PA indicates that the final decision to retain or
revise the current secondary O3 standard is a public welfare
policy judgment to be made by the Administrator, based on her judgment
as to what level of air quality would be requisite (i.e., neither more
nor less stringent than necessary) to protect the public welfare from
any known or anticipated adverse effects. This final decision will draw
upon the available scientific evidence for O3-attributable
welfare effects and on quantitative analyses of vegetation and
ecosystem exposures and associated risks to vegetation, ecosystems and
their associated services, and judgments about the appropriate weight
to place on the range of uncertainties inherent in the evidence and
analyses. In making this decision, the Administrator will need to weigh
the importance of these effects and their associated ecosystem services
in the overall context of public welfare protection.
    Based on the considerations described in the PA and summarized
here, the PA concludes that the currently available evidence and
exposure/risk information call into question the adequacy of the public
welfare protection provided by the current standard and provides
support for considering potential alternative standards to achieve
increased public welfare protection, especially for sensitive
vegetation and ecosystems in federally protected Class I and similarly
protected areas. In this conclusion, staff gives particular weight to
the evidence indicating the occurrence in Class I areas that meet the
current standard of cumulative seasonal O3 exposures
associated with estimates of tree growth impacts of a magnitude that
may reasonably be considered important to public welfare.
2. CASAC Advice
    Beyond the evidence- and exposure/risk-based considerations in the
PA discussed above, the EPA's consideration of the adequacy of the
current secondary standard also takes into account the advice and
recommendations of CASAC.
    In its advice offered in the current review, based on the updated
scientific and technical record since the 2008 rulemaking, the CASAC
stated that they ``support the conclusion in the Second Draft PA that
the current secondary standard is not adequate to protect against
current and anticipated welfare effects of ozone on vegetation'' (Frey,
2014c, p. iii) and that the PA ``clearly demonstrates that ozone-
induced injury may occur in areas that meet the current standard''
(Frey, 2014c, p. 12). The Panel further stated ``[w]e support EPA's
continued emphasis on Class I and other protected areas'' (Frey, 2014c,
p. 9). Additionally, the CASAC indicated support for the concept of
ecosystem services ``as part of the scope of characterizing damage that
is adverse to public welfare'' and ``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'' (Frey, 2014c, p. 9). Similar to comments from CASAC in
the last review, including comments on the proposed reconsideration,
the current CASAC also endorsed the PA discussions and conclusions on
biologically relevant exposure metrics and the focus on the W126 index
accumulated over a 12-hour period (8am-8pm) over the three-month
summation period of a year resulting in the maximum value (Frey, 2014c,
p. iii).
    In addition, CASAC stated that ``relative biomass loss for tree
species, crop yield loss, and visible foliar injury are appropriate
surrogates for a wide range of damage that is adverse to public
welfare'' (Frey, 2014c, p. 10). With respect to relative biomass loss
for tree species, CASAC states that it is appropriate to ``include
levels that aim for not greater than 2% RBL for the median tree
species'' and that a median tree species RBL of 6% is ``unacceptably
high.'' With respect to crop yield loss, CASAC points to a benchmark of
5%, stating that a crop RYL for median species over 5% is
``unacceptably high'' (Frey, 2014c, p. 13).
3. Administrator's Proposed Conclusions on Adequacy of the Current
Standard
    In considering the adequacy of the current secondary O3
standard, the Administrator has considered the assessment of the
current evidence in the ISA, findings of the WREA, including associated
limitations and uncertainties, considerations and staff conclusions and
associated rationales presented in the PA, views expressed by CASAC,
and public comments. In taking into account the information discussed
above with regard to the nature of O3-related effects on
vegetation, the Administrator has taken particular note of: the PA
analysis of the magnitude of tree seedling growth effects (biomass
loss) estimated for different cumulative, seasonal, concentration-
weighted exposures in terms of the W126 metric; the monitoring analysis
in the PA of W126 exposures occurring in locations where the current
standard is met, including those locations in Class I areas, and
associated estimates of tree seedling growth effects; the analyses in
the WREA illustrating the geographic distribution of tree species for
which E-R functions are available and relative differences estimated
for O3-related growth impacts across areas of the U.S. for
the air quality scenarios, taking into account the identified potential
for the WREA's scenario for the current standard to underestimate the
highest W126-based O3 values that would be expected to
occur.
    As an initial matter, the Administrator recognizes the
appropriateness and usefulness of the W126 metric, as described in
sections IV.B.1 and IV.D.1 above, in evaluating O3 exposures
of potential concern for vegetation effects. In so doing, the
Administrator additionally notes support conveyed by CASAC for such a
use for this metric.
    With regard to considering the adequacy of public welfare
protection provided by the current secondary standard, the
Administrator focuses first on welfare effects related to reduced

native plant growth and productivity in terrestrial ecosystems, taking
note of the ISA conclusion of a causal relationship between
O3 in the ambient air and these effects. In considering the
assessment of the information available in this review with regard to
O3 effects on vegetation growth and productivity, the
Administrator takes note of the evidence from OTC studies of the
effects of O3 exposure on tree seedling growth that support
robust E-R functions for 11 tree seedling species, and the
characterization of growth effects across these species for different
cumulative seasonal concentration-weighted exposures using the W126
metric. Reductions in growth of sensitive species, as recognized in
section IV.B above, have the potential to result in effects on
ecosystem productivity, as well as, on forest and forest community
composition. The Administrator takes particular note of the evidence,
described in section IV.D.1 above, of the occurrence in Class I areas
during periods where the current standard is met of cumulative seasonal
O3 exposures for which median species RBL estimates are of a
magnitude that CASAC has termed ``unacceptably high.'' In so doing, the
Administrator also takes note of a number of actions taken by Congress
to establish public lands that are set aside for specific uses intended
to provide benefits to the public welfare, including lands that are to
be protected so as to conserve the scenic value and the natural
vegetation and wildlife within such areas for the enjoyment of future
generations. Such public lands that are protected areas of national
interest include national parks and forests, wildlife refuges, and
wilderness areas (many of which have been designated Class I
areas).\217\
---------------------------------------------------------------------------

    \217\ As noted in section IV.A above, Congress has established
areas such as national parks and wilderness areas with specific
purposes including the preservation of the areas for future
generations, and has identified many of those areas as Class I
areas.
---------------------------------------------------------------------------

    While recognizing the variability in the various environmental
factors that can influence the occurrence and severity of the effect of
ambient O3 concentrations on vegetation in different
locations, the Administrator concludes that the information referenced
above including the currently available, extensive evidence base and
also factors affecting the significance of impacts to public welfare,
as well as WREA estimates regarding the potential for occurrence of
impacts important to public welfare, provides an appropriate basis to
inform a conclusion as to whether the current standards provide
adequate protection against O3-related vegetation effects on
public welfare. With regard to the results of the monitoring analysis,
the Administrator takes note of the PA conclusions that the impacts on
tree growth (and the potential for associated ecosystem effects)
estimated for W126 values found to occur in Class I areas when meeting
the current standard are reasonably concluded to be important from a
public welfare standpoint in terms of both the magnitude of the
vegetation effects and the significance to public welfare of such
effects in such areas, calling into question the adequacy of the
current secondary standard.
    The Administrator also recognizes the causal relationships between
O3 in the ambient air and visible foliar injury, reduced
yield and quality of agricultural crops and alteration of below-ground
biogeochemical cycles associated with effects on growth and
productivity. As to visible foliar injury, the Administrator takes note
of the complexities and limitations in the evidence base regarding
characterizing air quality conditions with respect to the magnitude and
extent of risk for visible foliar injury. She additionally recognizes
the challenges of associated judgments with regard to adversity of such
effects to public welfare. In taking note of the conclusions with
regard to crops, she recognizes the complexity of considering adverse
O3 impacts to public welfare due to the heavy management
common for achieving optimum yields and market factors that influence
associated services and additionally takes note of the PA conclusions
that placing emphasis on the protection afforded to trees inherently
also recognizes a level of protection afforded for crops.
    Based on her consideration of the conclusions in the PA, and with
particular weight given to PA findings pertaining to tree growth-
related effects, as well as with consideration of CASAC's conclusion
that the current standard is not adequate, the Administrator proposes
to conclude that the current standard is not requisite to protect
public welfare from known or anticipated effects and that revision is
needed to provide increased public welfare protection, especially for
sensitive vegetation and ecosystems in federally protected Class I
areas and in other areas providing similar public welfare benefits. The
Administrator further concludes that the scientific evidence and
quantitative analyses on tree growth-related effects provide strong
support for consideration of alternative standards that would provide
increased public welfare protection beyond that afforded by the current
O3 secondary standard. She further notes that a revised
standard would provide increased protection for other growth-related
effects, including for carbon storage and for areas for which it is
more difficult to determine public welfare significance, as recognized
in section IV.B.2 above, as well other welfare effects of
O3, including visible foliar injury and crop yield loss.
    In giving particular focus to tree growth-related effects of
O3 on public welfare, the Administrator additionally
recognizes that there are alternative approaches to viewing the
evidence and information, including alternative approaches to viewing,
evaluating, and weighing important uncertainties. In some cases, these
alternative approaches have been expressed by public commenters,
leading some public commenters to recommend retaining the current
standard. Given these alternative views, in addition to proposing to
revise the current secondary standard, the Administrator also solicits
comment on the option of retaining the standard without revision.

E. Consideration of Alternative Secondary Standards

    Given her proposed conclusion that the current secondary standard
is inadequate, the Administrator has then considered what revisions to
the standard may be appropriate, focusing on revisions to the key
standard elements of indicator, form, averaging time, and level. On the
basis of the strength and coherence of the vegetation effects evidence
indicating a cumulative, seasonal, concentration-weighted metric as the
most appropriate approach for judging potential impacts of and
protection from O3 in ambient air, the Administrator judges
that it is appropriate to consider revisions to the secondary standard
that reflect this understanding and to use such a metric in identifying
an appropriate level of protection and considering the protection
afforded by potential alternative standards. The Administrator also
judges that the current averaging time and form may also provide
protection to vegetation when set at an appropriate level. Therefore,
the Administrator considered whether revision to the level of the
current secondary standard might provide sufficient protection to also
achieve the level of air quality that is determined requisite to
protect the public welfare.
    The sections below address the indicator for the secondary standard
(section IV.E.1), consideration of a cumulative, seasonal exposure-
based

standard in the PA (section IV.E.2), CASAC advice and public input
(section IV.E.3), analyses of air quality in the PA and subsequent to
the PA (section IV.E.4) and the Administrator's proposed conclusions
regarding an alternative secondary standard (section IV.E.5)
1. Indicator
    In the last review of the air quality for O3 and other
photochemical oxidants and of the O3 standard, as in other
prior reviews, the EPA focused on a standard for O3 as the
most appropriate surrogate for ambient photochemical oxidants. Ozone is
a long-established surrogate for ambient photochemical oxidants, among
which it is by far the most widely studied with regard to effects on
welfare and specifically on vegetation. The information available in
this review adds to the understanding of the atmospheric chemistry for
photochemical oxidants and O3 in particular (as described in
the ISA, sections 3.2 and 3.6, and summarized in section 2.2 in the
PA). The 1996 Staff Paper noted that the database on vegetation effects
is generally considered to raise concern at levels found in the ambient
air for O3 and, therefore, control of ambient O3
levels has previously been concluded to provide the best means of
controlling other photochemical oxidants of potential welfare concern
(U.S. EPA, 1996b, p. 277). In the current review, while the complex
atmospheric chemistry in which O3 plays a key role has been
highlighted, no alternatives to O3 have been advanced as
being a more appropriate surrogate for ambient photochemical oxidants.
Ozone continues to be the only photochemical oxidant (other than
nitrogen dioxide) that is routinely monitored and for which a
comprehensive database exists (U.S. EPA, 2013a, section 3.6).
    Thus, the Administrator concludes that ozone is the appropriate
indicator and proposes to continue to use O3 as indicator
for a secondary standard that is intended to address effects associated
with exposure to O3, alone and in combination with related
photochemical oxidants. In so doing, the Administrator recognizes that
measures leading to reductions in ecosystem exposures to O3
will also reduce exposures to other photochemical oxidants.
2. Consideration of a Cumulative, Seasonal Exposure-based Standard in
the Policy Assessment
    In recognition of the extensive evidence supporting a cumulative,
seasonal exposure index as a biologically relevant metric for assessing
potential for O3 effects on vegetation, discussed in
sections IV.B.1 above, as well as advice from CASAC in the current and
last O3 NAAQS reviews, summarized in sections IV.D.3 above
and IV.E.3 below, the PA focused its consideration of alternative
standards on a revised secondary standard based on a cumulative,
seasonal, concentration-weighted form. The PA considered the currently
available information that has been critically analyzed and
characterized in the ISA, the risk and exposure information presented
in the WREA, and CASAC advice and public comment with regard to support
for consideration of options for alternative standards that might be
expected to provide increased protection from ambient O3
exposures over the current standard.
a. Form and Averaging Time
    In considering potential forms for a revised secondary standard,
the PA considers the characterization of the evidence in the ISA,
summarized in section IV.B.1 above, including the ISA conclusion that
exposure indices that cumulate and differentially weight the higher
hourly average concentrations over a season and also include the mid-
level values, such as the W126 index, offer the most scientifically
defensible approach for characterizing vegetation response to ambient
O3 and comparing study findings, as well as for defining
indices for vegetation protection (U.S. EPA, 2013a, section <IP_ADDRESS>).
The PA additionally considers CASAC advice in the current review, as
well as that from the last review, all of which provided support for
such a form. Thus, in considering alternative forms of a revised
standard, the PA concludes that it is reasonable and appropriate to
consider a cumulative, concentration-weighted form to provide
protection against cumulative, seasonal exposures to O3 that
are known or anticipated to harm sensitive vegetation or ecosystems.
The PA recognizes that such a metric is specifically designed to focus
on the kind of O3 exposures that have been shown to cause
harm to vegetation and states that it would have a distinct advantage
over the form of the current standard in characterizing air quality
conditions potentially of concern for vegetation and in more directly
demonstrating that the desired degree of protection against those
conditions was being achieved (U.S. EPA, 2014c, sections 6.2 and 6.6).
    With regard to the appropriate index for a cumulative seasonal
form, the PA considers the evidence and background for a number of
different cumulative concentration weighted indices that have been
developed and evaluated in the scientific literature and in past NAAQS
reviews in terms of their ability to predict vegetation response and
their usefulness in the NAAQS context (U.S. EPA, 2006a, pp. 9-11 to 9-
15 and pp. AX9-159 to AX9-187; U.S. EPA, 2007, pp. 7-15 to 7-16). While
these various forms have different strengths and limitations, the PA
notes the ISA conclusion that the W126 index, described in section
IV.B.1 above, has some important advantages over other non-sigmoidally
weighted cumulative indices, including its lack of a cut-off in its
weighting scheme which allows for cumulation of lower O3
concentrations (U.S. EPA, 2013a, section 9.5; U.S. EPA, 2014c, sections
6.2 and 6.6). Additionally, the W126 metric adds increasing weight to
hourly concentrations from about 40 ppb to about 100 ppb, which is an
important feature because ``as hourly concentrations become higher,
they become increasingly likely to overwhelm plant defenses and are
known to be more detrimental to vegetation'' (U.S. EPA, 2013a, p. 9-
104). The PA additionally takes note of CASAC advice in the current and
last review that concurred with a focus on the W126 form (Frey, 2014c,
p. iii; Henderson, 2006; Samet, 2010). Based on the considerations
summarized here, the PA concludes that the W126 index is the most
appropriate cumulative seasonal form to consider in the context of the
secondary O3 NAAQS review.
    The PA next considers the exposure periods--diurnal and seasonal--
over which the W126 index would be summed in any given year. The
currently available information continues to provide support for a
definition of the diurnal period of interest as the 12-hour period from
8:00 a.m. to 8:00 p.m., which the EPA identified in past reviews as
appropriately capturing the diurnal window with most relevance to the
photosynthetic process (U.S. EPA, 2013a, section 9.5.3; 72 FR 37900,
July 11, 2007). The CASAC has generally supported this 12-hour daylight
period as well (Frey, 2014c; Henderson, 2006, 2007). Based on these
considerations, the PA concludes that the 12-hour daylight window (8:00
a.m. to 8:00 p.m.) represents the portion of the diurnal exposure
period that is most relevant to predicting or inducing plant effects
related to photosynthesis and growth and thus is an appropriate diurnal
period to use in conjunction with a W126 cumulative metric (U.S. EPA,
2014c, sections 6.2 and 6.6). With regard

to a seasonal period of interest, the current evidence base continues
to provide support for a seasonal period with a minimum duration of
three months, as described more fully in the ISA and considered in the
PA (U.S. EPA, 2013a, section 9.5.3; U.S. EPA, 2014c, sections 6.2 and
6.6). The CASAC has also indicated support for such a three month
period (Frey, 2014c; Samet, 2010; Henderson, 2006). The PA thus
concludes that it is appropriate to identify the seasonal W126 index
value as that derived from the consecutive 3-month period within the
O3 season with the highest W126 index value.
    The PA additionally considers the period of time over which a
cumulative seasonal W126-based standard should be evaluated,
considering the support for both a single year form and a form averaged
over three years (U.S. EPA, 2014c, pp. 6-29 through 6-33). The PA
considers the evidence of effects associated with single year and
multiple year exposures as well as their potential public welfare
significance. The PA also considers comments from CASAC, including
their comment in the current review that ``[t]he CASAC does not
recommend the use of a three-year averaging period'' and that they
``favor a single-year period for determining the highest three-month
summation which will provide more protection for annual crops and for
the anticipated cumulative effects on perennial species'' (Frey, 2014c,
p. iii).
    The PA considered O3-induced effects that can occur with
a single year's exposure, including visible foliar injury, growth
reduction in annual and perennial species and yield loss in annual
crops (U.S. EPA, 2014c, section 6.3). While recognizing that there are
a number of O3-induced effects that have the potential for
public welfare significance within the annual timeframe, the PA also
notes the uncertainties associated with these effects that complicate
consideration of the level of appropriate protection on an annual basis
for such effects in order to protect the public welfare from known or
anticipated adverse effects, and thus recognizes the possibility that a
multiple-year form could be considered to provide a more consistent
target level of protection for certain effects (U.S. EPA, 2014c, pp. 6-
29 to 6-31). With regard to visible foliar injury, the ISA notes that
``the degree and extent of visible foliar injury development varies
from year to year and site to site . . . even among co-members of a
population exposed to similar O3 levels, due to the
influence of co-occurring environmental and genetic factors'' (U.S.
EPA, 2013a, p. 9-38; U.S. EPA, 2014c, p. 6-30). Additionally, the PA
takes note of the difficulty and complexity shown by the WREA analyses
with regard to identifying W126 index values that would provide
consistent protection on an annual basis given likely fluctuations in
annual O3 and soil moisture conditions (U.S. EPA, 2014c, p.
6-30).
    The PA additionally notes evidence of some O3 effects on
perennial species that may result from a single season's elevated
O3 exposures, such as reduced bud size or starch content,
which may have the potential for some ``carry over'' of effects on
plant growth or reproduction in the subsequent season. Another effect
where such potential for ``carry over'' has been noted with elevated
O3 exposure is reduction in below-ground carbohydrate
reserves which can impair growth in subsequent seasons (U.S. EPA,
2014c, pp. 6-30 to 6-31; U.S. EPA, 2013a, pp. 9-43 to 9-44 and p. 9-
86). The PA notes that the occurrence of such annual effects of
elevated O3 exposures over multiple years may contribute to
a potential to be compounded, increasing the potential for effects at
larger scales (e.g., population, ecosystem). In the PA, staff notes
that multiple consecutive years of critical O3 exposures
might be expected to result in larger impacts on forested areas than
intermittent occurrences of such exposures due to the potential for
compounding or carry-over effects on tree growth (U.S. EPA, 2014c, pp.
6-29 to 6-31).
    In light of the above summarized considerations for potential
compounding of carry-over effects, the PA concludes that the public
welfare significance of the effects that can occur as a result of
three-year O3 exposures are potentially greater than those
associated with a single year of such exposure. Thus, to the extent
that the focus for public welfare protection to be afforded by the
secondary O3 standard is on long-term effects that occur in
sensitive tree species in natural forested ecosystems, including
federally protected areas such as Class I areas or on lands set aside
by States, Tribes and public interest groups to provide similar
benefits to the public welfare, the PA concludes that a standard with a
form that evaluates the cumulative seasonal index across multiple years
might be considered to provide a more appropriate match to the nature
of O3-related effects on vegetation upon which the secondary
O3 standard is focused. In considering such forms, the PA
focuses on one that averages the W126 index values across three years
(U.S. EPA, 2014c, section 6.2).
    With regard to single-year and three-year forms, the PA considers a
WREA analysis that examined the extent to which cumulative RBL across a
three-year period might be underestimated when each year's RBL is
derived from the three-year average W126 index value versus each
single-year W126 index value for each of three years (in which no other
influence on plant growth is presumed to change). This analysis
indicates that use of the three-year average may lead to an
underestimation, although of relatively small magnitude (U.S. EPA,
2014b, section <IP_ADDRESS>). The PA notes that this limited analysis does
not account for moisture levels and other environmental factors that
could affect plant growth and that vary from year to year. When
considering an appropriate level for a form that averages W126 index
values across three years, the PA also recognizes the importance of
considering the extent to which the cumulative effect of different
average W126 exposures across the three-year period would be judged
adverse (U.S. EPA, 2014c, p. 6-31).
    Although single-year W126 index values were not separately analyzed
in the PA analysis of recent monitoring data, the data indicate
appreciable variation in cumulative, seasonal O3
concentrations among monitor locations meeting different levels of a
standard of the current form (U.S. EPA 2014c, section 6. Appendix
2B).Therefore, a standard with an annual form would have the cumulative
seasonal index values be at or lower than the level of the standard in
all years and, noting the inter-annual observed variability in seasonal
W126 index values, could be appreciably below the standard level in
some years. For a standard with a form that averages the cumulative
seasonal index values across three consecutive years, the annual
seasonal index value could be above the level in some years, but would
have to be below it in others within the same three-year period, thus
restricting the air quality for a given area to have no more than two
years out of three with a W126 index value above the standard level,
and depending on magnitude of each year's index, potentially having no
more than one.
    In its consideration of one year as compared to three year forms,
the PA also considers implications with regard to stability of air
quality programs that implement the NAAQS (U.S. EPA, 2014c, pp. 6-31 to
6-32). The PA notes that a standard based on a single year W126 index
would be expected to have a less stability relative to a standard based
on a form that averages seasonal indices across three consecutive
years, given the potential for large year-to-year variability in annual
W126 index values in areas across the country. Thus, a

three-year evaluation period can contribute to greater public welfare
protection by limiting year-to-year disruptions in ongoing control
programs that would occur if an area was frequently shifting in and out
of attainment due to extreme year-to-year variations in meteorological
conditions. This greater stability in air quality management programs
would thus facilitate achievement of the protection intended by a
standard. Such considerations of stability often receive particular
weight in NAAQS reviews, such as those resulting in selection of the
form for the current O3 primary and secondary standards (62
FR 38856, July 18, 1997), as well as the primary standards for nitrogen
dioxide (75 FR 6474, February 9, 2010) and sulfur dioxide (75 FR 35520,
June 22, 2010). See also ATA III, 283 F. 3d at 374-75 (recognizing
programmatic stability as a legitimate consideration in the NAAQS
standard-setting process).
    Thus, to the extent that emphasis continues to be placed on
protecting against effects associated with multi-year exposures and
maintaining more year-to-year stability of public welfare protection,
the PA concludes that it is appropriate to consider a secondary
standard form that is an average of the seasonal W126 index values
across three consecutive years. The PA concludes that such a form might
be appropriate for a standard intended to achieve the desired level of
protection from longer-term effects, including those associated with
potential compounding, and that such a form might be concluded to
contribute to greater stability in air quality management programs, and
thus, greater effectiveness in achieving the desired level of public
welfare protection, than that might result from a single year form
(U.S. EPA, 2014c, section 6.6).
    The PA additionally recognized that to the extent the Administrator
finds it useful to consider the public welfare protection that might be
afforded by a revised primary standard, this is appropriately judged by
evaluating the impact of attainment of such a revised primary standard
on O3 exposures in terms of the cumulative seasonal W126-
based exposure index.
b. Level
    In considering an appropriate range of levels to consider for a
W126-based standard, the PA notes that, due to the variability in the
importance of the associated ecosystem services provided by different
species at different exposures and in different locations, as well as
differences in associated uncertainties and limitations, both the
species present and their public welfare significance, in addition to
the magnitude of the ambient concentrations, are essential
considerations in drawing conclusions regarding the significance or
magnitude of public welfare impact. Therefore, in development of the PA
conclusions, staff took note of the complexity of judgments to be made
by the Administrator regarding the adversity of known and anticipated
effects to the public welfare and recognized that the Administrator's
ultimate judgments on the secondary standard will most appropriately
reflect an interpretation of the available scientific evidence and
exposure/risk information that neither overstates nor understates the
strengths and limitations of that evidence and information.
    As described in section IV.D.1 above, the PA employed a paradigm,
which has evolved over the course of the O3 and other
secondary NAAQS reviews, to assist in putting the available science and
exposure/risk information into the public welfare context (U.S. EPA,
2014c, section 5.1). This paradigm recognizes that the significance to
the public welfare of O3-induced effects on sensitive
vegetation growing within the U.S. can vary depending on the nature of
the effect, the intended use of the sensitive plants or ecosystems, and
the types of environments in which the sensitive vegetation and
ecosystems are located. Accordingly, any given O3-related
effect on vegetation and ecosystems (e.g., biomass loss, crop yield
loss, visible foliar injury) may be judged to have a different degree
of impact on or significance to the public welfare depending, for
example, on whether that effect occurs in a Class I area, a city park,
or commercial cropland. This approach also includes consideration of
impacts to ecosystem goods and services, which are an important
category of public welfare effects with an obvious relationship to
consideration of intended use (73 FR 16492, March 27, 2008).
    In considering potential levels for an alternative standard based
on the W126 metric, the PA focused primarily on impacts on tree growth,
crop yield loss, and visible foliar injury, as well as impacts on the
associated ecosystem services, while taking note of the uncertainties
and limitations associated with several key aspects of this
information. In addition to uncertainties related to the WREA air
quality scenarios and assessments summarized in section IV.C above, the
PA also recognized uncertainties associated with the evidence
underlying the tree seedling and crop E-R functions (U.S. EPA, 2014c,
section 6.3). These include uncertainties regarding intra-species
variability due to the different numbers of studies that exist for
different species so that the weight of evidence is not the same for
each species. Those species with more than one study show variability
in response and E-R functions. The potential variability in less well-
studied species is, however, unknown (U.S. EPA, 2013a, pp. 9-123 to 9-
125; U.S. EPA, 2014b, section <IP_ADDRESS>, and Table 6-27). The PA also
recognizes uncertainty regarding the extent to which tree seedling E-R
functions can be used to represent mature trees since seedling
sensitivity has been shown in some cases to not reflect mature tree
O3 sensitivity in the same species and uncertainty in the
relationship of O3 effects on tree seedlings (e.g., relative
biomass loss) in one or a few growing seasons to effects that might be
expected to accrue over the life of the trees extending into adulthood
(U.S. EPA, 2013a, section 9.6 and pp. 9-52 to 9-53; U.S. EPA, 2014b,
sections <IP_ADDRESS>, <IP_ADDRESS> and Tables 6-5 and 6-27).
    With respect to tree growth, the PA gave primary consideration to
relative biomass loss estimates derived from the E-R functions,
described in section IV.B.1.b above and in the PA, while also
considering WREA risk/exposure estimates related to this effect (U.S.
EPA, 2014c, section 6.4). The PA takes note of the different index
value estimates presented in Table 6-1 of PA (Table 8 below) with
regard to the number of studied species below different response
benchmarks, as well as with regard to the median response. The PA
additionally considers the WREA estimates regarding: (1) percent of
assessed geographic area exceeding 2% weighted relative biomass (U.S.
EPA, 2014c, Table 6-2); (2) number of assessed Class I areas with tree
seedling weighted relative biomass loss estimates below 2% (U.S. EPA,
2014c, Table 6-3); and (3) the percent median biomass loss across
counties for different air quality scenarios (U.S. EPA, 2014c, Table 5-
5). The PA further notes other WREA estimates for effects on ecosystem
services related to public welfare, such as carbon sequestration and
air pollutant removal. With respect to crop yield loss, the PA notes
the summary of RYL estimates for individual crop species and for the
median across species (Table 8), and the WREA risk/exposure estimates
(U.S. EPA, 2014b, Section 6.3). The PA also notes information available
on visible foliar damage to species occurring in areas preserved for
their natural character,

such as federal Class I areas, and the analyses in the WREA evaluating
biosite data and several benchmarks of injury (U.S. EPA, 2014b, section
5.4.2).

         Table 8--Tree Seedling Biomass Loss and Crop Yield Loss Estimated for O3 Exposure Over a Season
----------------------------------------------------------------------------------------------------------------
                                      Tree seedling RBL \a\                           Crop RYL \c\
 W126 value for exposure  --------------------------------------------------------------------------------------
          period               Median value       Individual species       Median value       Individual species
----------------------------------------------------------------------------------------------------------------
21 ppm-hrs...............  Median species w.     <= 2% loss: 3/11      Median species w.     <=5% loss: 4/10
                            6.8% loss \b\.        species.              7.7% loss \d\.        species.
                                                 <=5% loss: 5/11                             >5, <10% loss: 3/10
                                                  species.                                    species.
                                                 <=10% loss: 7/11                            >10, <20% loss: 3/
                                                  species.                                    10 species.
                                                 <=15% loss: 10/11
                                                  species.
                                                 >40% loss: 1/11
                                                  species.
19 ppm-hrs...............  Median species w.     <=2% loss: 3/11       Median species w.     <=5% loss: 5/10
                            6.0% loss \b\.        species.              6.4% loss \d\.        species.
                                                 <5% loss: 5/11                              >5, <10% loss: 3/10
                                                  species.                                    species.
                                                 <=10% loss: 7/11                            >10, <20% loss: 2/
                                                  species.                                    10 species.
                                                 <=15% loss: 10/11
                                                  species.
                                                 >30% loss: 1/11
                                                  species.
17 ppm-hrs...............  Median species w.     <=2% loss: 5/11       Median species w.     <=5% loss: 5/10
                            5.3% loss \b\.        species.              5.1% loss \d\.        species.
                                                 <5% loss: 5/11                              >5, <10% loss: 3/10
                                                  species.                                    species.
                                                 <=10% loss: 9/11                            >10, <20% loss: 2/
                                                  species.                                    10 species.
                                                 15% loss: 10/11
                                                  species.
                                                 >30% loss: 1/11
                                                  species.
15 ppm-hrs...............  Median species w.     <=2% loss: 5/11       Median species w.     <=5% loss: 6/10
                            4.5% loss \b\.        species.              <=5% loss \d\.        species.
                                                 <=5% loss: 6/11                             >5, <10% loss: 4/10
                                                  species.                                    species.
                                                 <=10% loss: 10/11
                                                  species.
                                                 >30% loss: 1/11
                                                  species.
13 ppm-hrs...............  Median species w.     <=2% loss: 5/11       Median species w.     <=5% loss: 6/10
                            3.8% loss \b\.        species.              <=5% loss \d\.        species.
                                                 <5% loss: 7/11                              >5, <10% loss: 4/10
                                                  species.                                    species.
                                                 <10% loss: 10/11
                                                  species.
                                                 >20% loss: 1/11
                                                  species.
11 ppm-hrs...............  Median species w.     <=2% loss: 5/11       Median species w.     <=5% loss: 9/10
                            3.1% loss \b\.        species.              <=5% loss \d\.        species.
                                                 <=5% loss: 8/11                             >5, <10% loss: 1/10
                                                  species.                                    species.
                                                 <=10% loss: 10/11
                                                  species.
                                                 >20% loss: 1/11
                                                  species.
9 ppm-hrs................  Median species w.     <=2% loss: 5/11       Median species w.     <=5% loss: all
                            2.4% loss \b\.        species.              <=5% loss \d\.        species.
                                                 <=5% loss: 10/11
                                                  species.
                                                 >20% loss: 1/11
                                                  species.
7 ppm-hrs................  Median species w.     <=2% loss: 7/11       Median species w.     <=5% loss: all
                            <=2% loss \b\.        species.              <=5% loss \d\.        species.
                                                 <=5% loss: 10/11
                                                  species.
                                                 >15% loss: 1/11
                                                  species.
----------------------------------------------------------------------------------------------------------------
\a\ Estimates are based on the 11 E-R functions for tree seedlings described in WREA, Appendix 6F and discussed
  in the PA, section 5.2.1, with the exclusion of cottonwood in consideration of CASAC comments on differences
  of that study from the other controlled E-R studies (Frey, 2014b, 2014c).
\b\ This is the median of the composite E-R functions for 11 tree species from the WREA, Appendix 6F (discussed
  in the PA, section 5.2.1).
\c\ Estimates here are based on the 10 E-R functions for crops (from the PA, Appendix 6F and section 5.3.1).
\d\ This median value is the median of the composite E-R functions for 10 crops from WREA, Appendix 6F (also
  discussed in the PA, section 5.3.1).

    Given the wide variation in sensitivity of studied tree species to
O3-induced relative biomass loss, the PA focused
consideration on both median species values and individual species
responses and RBL estimates for a given range of W126 index values. In
this consideration, the PA took note of CASAC's advice regarding RBL
levels, specifically their emphasis on a benchmark of median relative
tree biomass loss at or below 2% and their view that a 6% median
relative biomass loss is ``unacceptably high.'' The median tree species
RBL estimate is at or below 2% only at the lowest W126 level assessed,
7 ppm-hrs. At incrementally higher W126 index levels, the median RBL is
also incrementally higher, so that at W126 index values of 9, 11, 13,
15, 17 and 19 ppm-hrs, the median RBL increases to 2.4%, 3.1%, 3.8%,
4.5%, 5.3% and 6.0%, respectively. Thus, the median species biomass
loss is below 6%, the level characterized by the CASAC as unacceptably
high, across the W126 range of 7 to 17 ppm-hrs, for which it varies
from approximately 2% to approximately 5%. Given this finding, the PA
discussion of a range of levels appropriate to consider focuses on this
range. In focusing on this range, the PA considers the full array of
CASAC advice with regard to interpretation of the evidence and
exposure/risk information on vegetation-related effects of
O3, as well as the role of the Administrator's judgments in
identifying the level of air quality that is requisite to protect
public welfare from adverse effects, as noted in section IV.A
above.\218\
---------------------------------------------------------------------------

    \218\ In the context of the O3 standard, such
judgments include: The weight to give the evidence of specific
vegetation-related effects estimated to result across a range of
cumulative seasonal concentration-weighted O3 exposures;
the weight to give associated uncertainties, including those related
to the variability in occurrence of such effects in specific areas
of U.S., such as those of particular public welfare significance;
and, judgments on the extent to which such effects in such areas may
be considered adverse to public welfare.
---------------------------------------------------------------------------

    The PA recognizes that public welfare judgments may reasonably be
informed by a range of biomass loss benchmarks, in contexts of
considering both median RBL estimates and RBL estimates for individual
species. Accordingly, in considering individual tree species estimates,
the PA notes the value of additionally characterizing the RBL

estimates in comparison to higher loss levels such as 10% or 15%.
    For every W126 value over the full range from 7 to 17 ppm-hrs, the
RBLs for each of five species is less than 2% (Table 8), which is the
lower benchmark that CASAC identified for tree species. Accordingly,
the PA focused attention on the remaining six more sensitive studied
species (i.e., eastern white pine, aspen, tulip poplar, ponderosa pine,
red alder, and black cherry) to evaluate the protection against tree
seedling biomass loss at different W126 levels within the range from 17
to 7 ppm-hrs. At a W126 index value of 17 ppm-hrs, one of these six
species (red alder) has a RBL estimate below 6%, while at the W126
index value of 7 ppm-hrs, five of these six species have RBLs below 6%
(eastern white pine, aspen, tulip poplar, ponderosa pine, red alder).
Taken together with the more tolerant species, the proportion of the
studied tree species with RBLs below 6% are 6 of 11, 7 of 11, 8 of 11,
and 10 of 11 at W126 index values of 17, 15, 13, and 11 ppm-hrs,
respectively.
    With regard to other, higher, RBL benchmark levels and estimates
for all 11 species, the PA notes that 9 of 11 studied tree species have
a predicted RBL below 10% at the W126 level of 17 ppm-hrs, while 10 of
11 species have a predicted RBL below 10% for W126 levels of 15 to 7
ppm-hrs. In addition, 10 of 11 studied tree species have a predicted
RBL below 15% for W126 levels of 17 to 7 ppm-hrs. The PA notes that the
RBL estimates for black cherry, the most sensitive of the 11 species,
remain above 15% for W126 index values across the range from 17 to 7
ppm-hrs, making unclear the extent to which black cherry estimates
might inform consideration of different W126 exposures within this
range (U.S. EPA, 2014c, section 6.6 and Table 6-1; U.S. EPA, 2014b,
section 6.2 and Appendix 6A).
    While recognizing the limitations and uncertainties associated with
the WREA air quality scenarios with regard to their representation of
conditions just meeting different three-year average W126 index values
(as summarized in section IV.C.1 above), including the potential
underestimation of the highest O3 concentrations, the PA
additionally considers several WREA RBL analyses (U.S. EPA, 2014c,
section 6.3). In the WREA characterization of the number of counties
where the median RBLs were greater than 2%, 7% of the counties have
median RBLs greater than 2% in the 15 and 11 ppm-hrs W126 scenarios, as
compared to 8% for the current standard (U.S. EPA, 2014c, Table 5-5;
U.S. EPA, 2014b, Table 6-7). The percentage is 6% in the 7 ppm-hrs W126
scenario. Of the 221 counties (7% of counties) estimated to have a
median RBL above 2% for the 15 ppm-hrs scenario, 203 of those counties
have a RBL greater than 2% because of the presence of black cherry
(U.S. EPA, 2014c, section 6.3).
    In considering the potential magnitude of the ecosystem impact of
O3-related biomass effects on tree growth, the PA
additionally focused on the WREA estimates of weighted RBL for the W126
air quality scenarios (U.S. EPA, 2014b, section 6.8). For the current
standard and the three W126 scenarios, the percent of total assessed
area having weighted RBL greater than 2% was 0.2%, 0.2%, 0.1% and
<0.1%, respectively (U.S. EPA, 2014c, Table 6-2; U.S. EPA 2014b, Table
6-25). In giving particular attention to estimates for Class I areas,
the PA notes that for all four scenarios, the WREA estimates indicate
weighted RBL greater than 2% in one or two of the 145 assessed
nationally protected Class I areas (U.S. EPA, 2014c, sections 6.3 and
6.6).
    In considering potential impacts on ecosystem services related to
reductions in O3 effects on tree growth, the PA particularly
recognizes that impacts on climate regulation can reasonably be
concluded to be potentially significant from a public welfare
perspective. In additionally recognizing that carbon sequestration has
been identified as a potentially important tool for managing
anthropogenic impacts on climate, the PA considers the WREA estimates
of potential increases in forestry carbon storage for ambient
O3 reductions in the three W126 air quality scenarios (U.S.
EPA, 2014c, sections 6.3 and 6.6; U.S. EPA, 2014b, section 6.6.1). The
WREA estimates additional forestry carbon storage potential of 13, 593
and 1,600 MMtCO2e (over 30 years) for the W126 scenarios of
15, 11 and 7 ppm-hrs, respectively, as compared to the current standard
(U.S. EPA, 2014b, Table 6-18). Compared to the absolute estimate for
the current standard scenario (approximately 89,000 MMtCO2e,
over 30 years), these amounts represent additional storage of less than
0.1%, just under 1% and under 2% for the 15, 11 and 7 ppm-hrs
scenarios, respectively (U.S. EPA, 2014b, section 6.6.1 and Appendix
6B).
    The PA additionally considers the WREA estimates for five urban
areas of how reduced growth of O3-sensitive trees in urban
forests may affect air pollutant removal (U.S. EPA, 2014b, sections
6.6.2 and 6.7 and Appendix 6D). As with the national estimates,
estimates for all five case study areas indicate generally small
differences between the current standard and the three W126 scenarios
(U.S. EPA, 2014c, Table 6-5). The PA additionally notes significant
uncertainties and limitations associated with WREA estimates related to
carbon sequestration and air pollution removal (U.S. EPA, 2014b, Table
6-27; U.S. EPA, 2014c, sections 6.3 and 6.6), some of which are
summarized in section IV.C.2.b above. The PA recognizes that, as with
consideration of other pertinent evidence and exposure/risk
information, the Administrator's consideration of WREA estimates for
these ecosystem services will involve judgments regarding the
appropriate weight to place on such uncertainties as well as the
potential impacts to the public welfare of the estimates.
    The PA additionally considers the biomass effects of O3
on crops estimated for different W126 index values across the range
identified above. For this consideration, the PA focuses on the 10
crops for which robust E-R functions have been established, as
described in section IV.B.1 above: Barley, lettuce, field corn, grain
sorghum, peanut, winter wheat, field cotton, soybean, potato and kidney
bean (U.S. EPA, 2013a; U.S. EPA, 2014b, section 6.5 and Figure 6-3). In
evaluating this information, the PA takes note of CASAC's comment
regarding significance of 5% for median crop relative yield loss (RYL).
The PA finds that the median crop RYL is at or below 5% for all W126
index values from 7 to 17 ppm-hrs and observes that this finding makes
it unclear to what extent this information informs consideration of
levels within this range. The RBL estimates for half of the ten
individual species are below 5% RYL at 17 ppm-hrs. The number of the
ten individual crops with RYL below 5% is six for W126 values of 15 and
13 ppm-hrs, nine for a W126 value of 11 ppm-hrs and ten for W126 levels
of 9 and 7 ppm-hrs. Recognizing that different crops are likely to have
different values or importance to public welfare, the PA also considers
the RYL estimates across the W126 range for individual species.
    In considering these RYL estimates, the PA recognizes that they do
not reflect the influence of the heavy management of agricultural crops
that is common in the U.S. and so cannot be easily interpreted with
regard to potential public welfare significance. In light of the median
RYL estimates of approximately 5% or lower for W126 index values at and
below 17 ppm-hrs, the PA gives less emphasis to consideration of crop
RYL, while noting that this information indicates that a secondary
standard revised to provide

additional protection for vegetation with attention to tree growth,
would be expected to also provide additional protection to crops over
that provided by the current standard (U.S. EPA, 2014c, section 6.6).
    The PA also considers the evidence and exposure/risk information
with regard to visible foliar injury and the extent to which that might
inform consideration of potential alternative secondary standards
appropriate for the Administrator to consider. Specifically, the PA
notes the findings of the WREA analyses of the nationwide USFS/FHM
biosite dataset (2006-2010) that while soil moisture conditions
influence the proportion of biosites with O3-related visible
foliar injury, as described in section IV.B.1.a above, the proportion
of such sites increases appreciably with increasing W126 index values
up to approximately 10 ppm-hrs, while relatively little or no change in
incidence of injury is seen with O3 exposures at higher W126
index values (U.S. EPA 2014b, Chapter 7, Figure 7-10). The PA
additionally notes that visible foliar injury has been identified by
the federal land managers as a diagnostic tool for informing
conclusions regarding potential O3 impacts on potentially
sensitive AQRVs (USFS, NPS, FWS, 2010), which the PA concludes
indicates that such O3-induced impacts might be considered
to have the potential to impact the public welfare in scenic and/or
recreational areas during years they occur.
    The PA was unable, however, to identify any guidance for federal
land managers regarding at what spatial scale or what degree of
severity visible foliar injury might be sufficient to trigger
protective action based on this potential impact on AQRVs. The PA
states that there does not appear to be consensus in the literature
regarding severity of visible foliar injury and risks to plant
functions or services, additionally noting that CASAC, while
identifying percent biomass loss and yield loss benchmarks for tree
seedlings and commodity crops, respectively, did not provide any
benchmark or criteria for consideration of O3 impacts
related to this endpoint. Further, as in previous reviews, the ISA
concludes visible foliar injury is not always a reliable indicator of
other negative effects on vegetation, making it difficult to relate
visible foliar injury symptoms to other vegetation effects such as
individual plant growth, stand growth, or ecosystem characteristics
(U.S. EPA, 2013a, section 9.4.2, p. 9-39). Additionally, although
evidence shows visible foliar injury can occur under very low
cumulative O3 exposures, ``. . . the degree and extent of
visible foliar injury development varies from year to year and site to
site . . ., even among co-members of a population exposed to similar
O3 levels, due to the influence of co-occurring
environmental and genetic factors'' (U.S. EPA 2013a, section 9.4.2, p.
9-38). Thus, while the PA recognizes visible foliar injury as an
important O3 effect which, depending on severity and spatial
extent may reasonably be concluded to be of public welfare
significance, most particularly in nationally protected areas such as
Class I areas, it additionally recognizes the appreciable variability
in this endpoint, which poses challenges to giving it primary emphasis
in identifying potential alternative standard levels.
    On the basis of all the considerations described above, including
the evidence and exposure/risk analyses, and advice from CASAC, the PA
concludes that a range of W126 index values appropriate for the
Administrator to consider in identifying a secondary standard that
might be expected to provide the requisite protection to the public
welfare from any known or anticipated adverse effects, extends from 7
to 17 ppm-hrs. The PA notes, however, the role of judgments by the
Administrator in such decisions, as recognized above. In selecting this
range, the PA primarily considers the evidence- and exposure/risk-based
information for cumulative seasonal O3 exposures represented
by W126 index values (including those represented by the WREA average
W126 scenarios) associated with biomass loss in studied tree species,
both in and outside areas that have been afforded special protections.
The PA recognizes that tree biomass loss can be an indicator of more
significant ecosystem-wide effects which might reasonably be concluded
to be significant to public welfare. For example, when biomass loss
occurs over multiple years at a sufficient magnitude, it is linked to
some level of effects on an array of ecosystem-level processes, such as
nutrient and water cycles, changes in above and below ground
communities, carbon storage and air pollution removal, that benefit the
public welfare (U.S. EPA, 2014c, Figure 5-1). In focusing on tree
biomass effects, the PA gave emphasis to CASAC's judgment that a 6%
median RBL is unacceptably high, and that the 2% median RBL is an
important benchmark to consider. The PA notes that for the lower W126
value of 7 ppm-hrs that the median tree species biomass loss is at or
below 2% and that for the upper value of 17 ppm-hrs that the median
tree biomass loss is below 6%.\219\
---------------------------------------------------------------------------

    \219\ We note that a W126 index value of 19 ppm-hrs is estimated
to result in a median RBL value of 6%, as shown in Table 2 above.
---------------------------------------------------------------------------

    In considering the stability and potential for associated greater
public welfare protection offered by a three-year form, as well as
based on the recognition that in any given year in the environment,
other environmental factors can influence the extent to which
O3 may have the impact predicted by the E-R functions on
which much of the range discussion above focuses, the PA gave careful
consideration to the support for consideration of potential alternative
W126 based standards with levels in the range identified above (17 ppm-
hrs to 7 ppm-hrs) with a three-year average form.
    Thus, the PA concludes that in staff's view, the evidence- and
exposure/risk-based information relevant to tree biomass loss and the
associated ecosystem services important to the public welfare support
consideration of a W126-based secondary standard with index values
within the range of 7 to 17 ppm-hrs, and a form averaged over three
years. In reaching this conclusion, the PA gave particular
consideration to the importance of considering the lasting or carry-
over effects that can derive from single year exposures of perennial
plants, recognizing the importance of considering the available
evidence and exposure/risk based information related to such effects,
as well as associated uncertainties. The PA additionally recognized
that there is limited information to discern differences in the level
of protection afforded for cumulative growth-related effects by
potential alternative W126-based standards of a single year form as
compared to a three-year average form. Lastly, the PA recognizes the
role of policy judgments required of the Administrator with regard to
the public welfare significance of identified effects, the appropriate
weight to assign the range of uncertainties inherent in the evidence
and analyses, and, ultimately, in identifying the requisite protection
for the secondary O3 standard. Examples of areas where the
Administrator's judgments would be expected include those stemming from
consideration of the effects associated with longer-term conditions and
the role that year-to-year exposure variability may play in associated
public welfare impacts, as well as the objectives for consideration of
tree species biomass loss estimates in relationship to identified
benchmarks (e.g., 2% or greater).

    The PA also concludes that, to the extent the Administrator finds
it useful to consider the public welfare protection that might be
afforded by a revised primary standard, this is appropriately judged
through the use of a cumulative seasonal W126-based exposure metric, a
metric considered appropriate for evaluating impacts on vegetation. For
example, comparison of the air quality conditions (expressed in terms
of W126 exposures) expected to result from a revised primary standard
to the W126-based exposures concluded to provide requisite public
welfare protection would thus inform a judgment of whether a secondary
standard set identical to a revised primary standard would be expected
to achieve the appropriate level of air quality. The PA notes that such
a comparison would be in terms of a metric considered appropriate for
evaluating impacts on vegetation which inform conclusions on public
welfare impacts. The PA further concludes that the drawing of
conclusions with regard to the public welfare protection afforded by
such a standard should entail consideration of the air quality
conditions likely to be achieved in terms of the cumulative seasonal
W126-based metric described above.
    Accordingly, the PA describes several analyses of air quality data
that might inform such consideration (U.S. EPA, 2014c, section 6.4),
and notes the importance of taking into account associated
uncertainties, including those associated with the limited monitor
coverage in many rural areas, such as those in the West and Southwest
regions and at high elevation sites. Additional such analyses, based on
more recent O3 monitoring data, have been developed since
the completion of the PA. All of these analyses are summarized in
section IV.E.4 below. In reaching conclusions on appropriate policy
options for a revised secondary standard the Administrator has
considered the findings of these analyses, as described in section
IV.E.5 below.
3. CASAC Advice
    Beyond the evidence- and exposure/risk-based considerations in the
PA discussed above, the EPA's consideration of a revised secondary
standard also takes into account the advice and recommendations of
CASAC. The EPA also considered public comments received to date, some
of which urged the consideration of a secondary standard with a
cumulative seasonal form using the W126 metric and a level within the
range of 7 to 15 ppm-hrs or in the low end of this range,\220\ while
others have urged retaining the existing form and averaging time due to
their view of a lack of new information to support a distinct secondary
standard.
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    \220\ Public comment received thus far in this review are in the
docket EPA-HQ-OAR-2008-0699, accessible at www.regulations.gov
---------------------------------------------------------------------------

    In advice offered on a revised secondary standard in the current
review, similar to advice in the last review, including advice offered
on the 2010 proposed reconsideration, the CASAC recommended ``retaining
the current indicator (ozone) but establishing a revised form of the
secondary standard to be the biologically relevant W126 index
accumulated over a 12-hour period (8 a.m.-8 p.m.) over the 3-month
summation period of a single year resulting in the maximum value of
W126'' (Frey, 2014c, p. iii). With regard to the level, the CASAC
recommended that ``that the level associated with this form be within
the range of 7 ppm-hrs to 15 ppm-hrs to protect against current and
anticipated welfare effects of ozone'' and that ``CASAC does not
support a level higher than 15 ppm-hrs'' (Frey, 2014c, p. iii). The
CASAC additionally stated that ``[i]n reaching its scientific judgment
regarding the indicator, form, summation time, and range of levels for
a revised secondary standard, the CASAC has focused on the scientific
evidence for the identification of the kind and extent of adverse
effects on public welfare,'' while also acknowledging ``that the choice
of a level within the range recommended based on scientific evidence is
a policy judgment under the statutory mandate of the Clean Air Act''
(Frey, 2014c, p. iii).
    In providing advice on a range for the secondary standard, the
CASAC noted a W126 index value for which the median tree species RBL
estimate was 6 percent, and the median crop species RBL estimate was
over 5 percent, stating that ``[t]hese levels are unacceptably high''
(Frey, 2014c, p. iii).\221\ In addition, regarding consideration of
relative biomass loss benchmarks for tree seedlings, the CASAC stated
that ``[a] 2% biomass loss is an appropriate scientifically based value
to consider as a benchmark of adverse impact for long-lived perennial
species such as trees, because effects are cumulative over multiple
years'' (Frey, 2014c, p. 14). In so stating, the CASAC referenced
findings for biomass loss in aspen exposed to elevated O3
over seven years, citing Wittig et al., 2009.\222\ The CASAC
additionally pointed to the report of the 1996 workshop sponsored by
the Southern Oxidants Study group (Heck and Cowling, 1997, noted in
section IV.B.2 above) which described a general consideration of 1-2%
per year growth reduction in making judgments the group identified as
appropriate for the endpoint of growth effects in trees, without
providing an explicit rationale for the identified percentages (Frey,
2014c, p. 14). The CASAC also commented that ``it is appropriate to
identify a range of levels of alternative W126-based standards that
includes levels that aim for not greater than 2% RBL for the median
tree species'' (Frey, 2014c, p. 14). The CASAC noted that the ``level
of 7 ppm-hrs is the only level analyzed for which the relative biomass
loss for the median tree species is less than or equal to 2 percent''
indicating that 7 ppm was appropriate lower bound for the recommended
range (Frey, 2014c, p. 14).
---------------------------------------------------------------------------

    \221\ The CASAC made this comment while focusing on Table 6-1 in
the second draft PA and the entry for 17 ppm-hrs. That table was
revised for inclusion in the final PA in consideration of CASAC
comments on the E-R function for eastern cottonwood, such that the
RBL estimates for 17 ppm-hrs in the final table (see Table 2 above)
are below the values CASAC viewed as ``unacceptably high''.
    \222\ The way in which the statement pointing to the aspen
seven-year biomass loss value from Wittig et al (2009) relates to
CASAC's view with regard to 2%, however, is unclear as the original
source for this finding (cited in Wittig et al., 2009) indicates
yearly relative biomass loss values during this seven year exposure
that are each well above 2%, and, in fact, are all above 20% (King,
et al., 2005).
---------------------------------------------------------------------------

    With regard to consideration of effects on crops, the CASAC, as
noted above, described median species RYL over 5% yield loss as
``unacceptably high.'' The CASAC further noted that ``[c]rop loss
appears to be less sensitive than these other indicators, largely
because of the CASAC judgment that a 5% yield loss represents an
adverse impact, and in part due to more opportunities to alter
management of annual crops'' (Frey, 2014c, p. 14).
    The CASAC acknowledged that ``the choice of a level within the
range recommended based on scientific evidence is a policy judgment
under the statutory mandate of the Clean Air Act'', while further
providing its own policy recommendations, including the following
(Frey, 2014c, p. iii).

    [T]he CASAC advises that a level of 15 ppm-hrs for the highest
3-month sum in a single year is requisite to protect crop yield
loss, but that lower levels provide additional protection against
crop yield loss. Furthermore, there are specific economically
significant crops, such as soybeans, that may not be protected at 15
ppm-hrs but would be protected at lower levels. A level below 10
ppm-hrs is required to reduce foliar injury. A level of 7 ppm-hrs is
protective of relative biomass loss for trees and offers additional

protection against crop yield loss and foliar injury. Therefore, 7
ppm-hrs is protective of ecosystem services. Thus, lower levels
within the recommended range offer a greater degree of protection of
more endpoints than do higher levels within the range.

    Additionally, in regard to consideration of form, the CASAC noted
that ``[i]f, as a policy matter, the Administrator prefers to base the
secondary standard on a three-year averaging period for the purpose of
program stability, then the level of the standard should be revised
downward such that the level for the highest three-month summation in
any given year of the three-year period would not exceed the
scientifically recommended range of 7 ppm-hrs to 15 ppm-hrs'' (Frey,
2014c, pp. iii and iv). In related manner, the CASAC noted that a
three-year average W126 level of 13 ppm-hrs may be appropriate
depending on consideration of year-to-year variability and such policy
considerations (Frey, 2014c, p. iv).
    Lastly, in comments recognizing uncertainties associated with the
evidence and exposure and risk analyses, the CASAC stated that ``there
is sufficient scientific evidence, and sufficient confidence in the
available research results, to support the advice we have given above
for this review cycle of the primary and secondary standards'' (Frey,
2014c, p. iv).
4. Air Quality Analyses
    As described in section II.D. above, the PA concludes with regard
to the primary standard that it is appropriate for the Administrator to
consider revision of the level to within the range of 60 to 70 ppb. In
consideration of this conclusion for the primary standard, although the
PA also concludes it is appropriate to consider a revised secondary
standard with a cumulative, seasonal, concentration-weighted form, the
PA recognized that, it may be practical to consider the extent to which
a revised secondary standard in the form of the current secondary
standard might be expected to also reduce and provide protection from
cumulative seasonal exposures of concern, noting that, for example, if
a clear and robust relationship was found to exist between 8-hour daily
peak O3 concentrations and cumulative, seasonal exposures,
the averaging time and form of the current standard might be concluded
to have the potential to be effective as a surrogate (U.S. EPA, 2014c,
section 6.4).
    Therefore, the PA evaluated what the available information
indicated with regard to control of cumulative O3 exposures
that might be afforded by alternative secondary standards with the
averaging time and form of the current standard (a three-year average
of 4th highest 8-hour average concentrations). The available
information addressing this point includes a ``focus study'' in the
ISA, and several air quality analyses described in the PA, chapters 2,
5 and 6 and Appendix 2b.\223\ Additionally, a similar air quality
analysis performed with more recent monitoring data is now available
and is also described here.
---------------------------------------------------------------------------

    \223\ This information and analyses were included in the second
draft PA (U.S. EPA, 2014j), reviewed by CASAC in early 2014, and
drafts of the ISA, reviewed by CASAC earlier in the review.
---------------------------------------------------------------------------

    The focus study described in the ISA examined the diel variability
in O3 concentrations in six rural areas between 2007 and
2009 (U.S. EPA, 2013a, pp. 3-131 to 3-133). The ISA reported that
``[t]here was considerable variability in the diel patterns observed in
the six rural focus areas'' with the three mountainous eastern sites
exhibiting a ``generally flat profile with little hourly variability in
the median concentration and the upper percentiles,'' while the three
western rural areas demonstrated a ``clear diel pattern to the hourly
O3 data with a peak in concentration in the afternoon
similar to those seen in the urban areas,'' which was especially
obvious at the San Bernardino National Forest site, 90 km east of Los
Angeles at an elevation of 1,384 meters (U.S. EPA, 2013a, p. 3-132).
Thus, while the western sites that are influenced by upwind urban
plumes may have increased cumulative seasonal values coincident with
increased daily 8-hour peak O3 concentrations, this analysis
indicates that, in sites without such an urban influence (the eastern
sites in this analysis), such a relationship does not occur (U.S. EPA,
2013a, section <IP_ADDRESS>). Thus, the lack of such a relationship
indicates that in some locations, O3 air quality patterns
can lead to elevated cumulative, seasonal O3 exposures
without the occurrence of elevated daily maximum 8-hour average
O3 concentrations (U.S. EPA, 2013a, section <IP_ADDRESS>).
Further, staff notes that the prevalence and geographic extent of such
locations is unclear, since as in the last review, there continue to be
relatively fewer monitors in the western U.S., including in high
elevation remote sites. In considering the findings of this analysis,
the PA additionally recognized, however, that the cumulative seasonal
values for the eastern rural sites, where cumulative seasonal
O3 concentrations appear to be relatively less related to
daily maximum 8-hour concentrations, are lower in general than those of
the western, urban-influenced sites.
    In addition to the focus study described in the ISA (U.S. EPA,
2013a, section <IP_ADDRESS>), the PA considers additional analyses of air
quality monitoring data. For example, Chapter 2 of the PA characterized
recent monitoring data of O3 air quality in rural areas.
While approximately 80 percent of the O3 monitoring network
is urban focused, about 120 rural monitors are divided among CASTNET,
NCore, and portable O3 monitors (POMs) sites (U.S. EPA,
2014c, Chapter 2, pp. 2-2 to 2-3, Figure 2.1). Specifically, as stated
in Chapter 2 of the PA, ``[a]lthough rural monitoring sites tend to be
less directly affected by anthropogenic pollution sources than urban
sites, rural sites can be affected by transport of O3 or
O3 precursors from upwind urban areas and by local
anthropogenic sources such as motor vehicles, power generation, biomass
combustion, or oil and gas operations'' (U.S. EPA, 2013a, section
<IP_ADDRESS>). In addition, O3 tends to persist longer in rural
than in urban areas due to lower rates of chemical scavenging in non-
urban environments. At higher elevations, increased O3
concentrations can also result from stratospheric intrusions (U.S. EPA,
2013a, sections 3.4, <IP_ADDRESS>). As a result, O3
concentrations measured in some rural sites can be higher than those
measured in nearby urban areas (U.S. EPA, 2013a, section <IP_ADDRESS>) and
the ISA concludes that ``cumulative exposures for humans and vegetation
in rural areas can be substantial, and often higher than cumulative
exposures in urban areas'' (U.S. EPA, 2013a, p. 3-120). These known
differences between urban and rural sites suggest that there is the
potential for an inconsistent relationship between 8-hour daily peak
O3 concentrations and cumulative, seasonal exposures in
those areas. However, the PA also notes that reductions in NOx
emissions that occur in urban areas to attain primary standards would
also have the effect of reducing downwind, rural concentrations over
the season (U.S. EPA, 2014c, section 6.4).
    In addition, as was done in both the 1997 and 2008 reviews, the PA
analyzed relationships between O3 levels in terms of the
current averaging time and form and a W126 cumulative form, based on
recent air quality data. One analysis in the PA describes the W126
index values and current standard design values at each monitor for two
periods: 2001-2003 and 2009-2011 (e.g., U.S. EPA, 2014c, Appendix 2B,
Figures 2B-2 and 2B-3). This shows that between the two periods, during
which broad scale O3

precursor emission reductions occurred, O3 concentrations in
terms of both metrics were reduced. There is a fairly strong, positive
degree of correlation between the two metrics (U.S. EPA, 2014c,
Appendix 2B).\224\ Focusing only on the latter dataset (2009-2011), it
can be seen that at monitors just meeting the current standard (three-
year average fourth-highest daily maximum 8-hour average concentration
equal to 0.075 ppm), W126 index values (in this case three-year
averages) varied from less than 3 ppm-hrs to approximately 20 ppm-hrs
(U.S. EPA, 2014c, Appendix 2B, Figure 2B-3b). At sites with a three-
year average fourth-highest daily maximum 8-hour average concentration
at or below a potential alternative primary standard level of 70 ppb,
three-year W126 index values were above 17 ppm-hrs at no monitors,
above 15 ppm-hrs at one monitor, and above 13 ppm-hrs at 8 monitors in
the West and Southwest NOAA climate regions. At sites with a three-year
average fourth-highest daily maximum 8-hour average concentration at or
below a potential alternative primary standard level of 65 ppb, three-
year W126 index values were above 11 ppm-hrs at no monitors, above 9
ppm-hrs at three monitors, and above 7 ppm-hrs at 9 monitors
(distributed across five regions). The majority of these nine
monitoring sites are located in the West and Southwest regions and
include the states of Arizona, California, Colorado, Nevada, New
Mexico, and Utah. At sites with a three-year average fourth-highest
daily maximum 8-hour average concentration at or below a potential
alternative primary standard level of 60 ppb, three-year W126 index
values were at or below 7 ppm-hrs at all monitors (U.S. EPA, 2014c,
Figure 2B-3b).
---------------------------------------------------------------------------

    \224\ Appendix 2B in the PA additionally observes that the
program implemented for reducing precursor emissions, especially
NOx, appears to have been an effective strategy for lowering both
design values and W126 index values.
---------------------------------------------------------------------------

    Another analysis in Chapter 2 of the PA presents the data for sets
of recent three-year periods back to 2006-2008 and indicates that among
the counties with O3 concentrations that met the current
standard, the number of counties with three-year W126 index values
above 15 ppm-hrs ranges from fewer than 10 to 24 (U.S. EPA, 2014c,
Appendix 2B, Figure 2B-9). In general during this longer period, W126
index values above 15 ppm-hrs and meeting the current standard were
predominantly in Southwest region. As the first analysis in Appendix 2B
of the PA (for the 2001-2003 and 2009-2011 periods) indicates, monitors
in the West and Southwest tend to have higher W126 index values
relative to their design values than do monitors in other regions. This
pattern is noteworthy because the Southwest region has a less dense
monitoring network than regions in the eastern U.S. (see U.S. EPA,
2014c, Figure 2-1), so that the extent to which this pattern occurs
throughout these regions is uncertain.
    An additional air quality analysis was performed for this review
that is documented in a technical memorandum (Wells, 2014). This
analysis examines the relationships between O3 levels in
terms of the form and averaging time for the current standard (the