``4th max'' metric) and a three-year average, W126-based metric. The
first part of the analyses focus on the air quality values for the most
recent three-year period, 2011-2013. Based on this information, 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 up to 23 ppm-hrs (Figure 5a). At
sites with a three-year average fourth-highest daily maximum 8-hour
average concentration at or below a level of 70 ppb (566 monitors
distributed across all regions of the U.S.), three-year W126 index
values were above 17 ppm-hrs at no monitors, above 15 ppm-hrs at 4
monitors, and above 13 ppm-hrs at 16 monitors (1% of the monitors in
full dataset and less than 3% in this group). These 16 monitors are
located in the Southwest (15 monitors) and West North Central NOAA
climate regions and include the states of Arizona, Colorado, New
Mexico, Utah and Wyoming. At sites with a three-year average fourth-
highest daily maximum 8-hour average concentration at or below a level
of 65 ppb (220 monitors distributed across all regions of the
U.S.\225\), three-year W126 index values were above 11 ppm-hrs at no
monitors, above 7 ppm-hrs at 15 monitors. These 15 monitoring sites are
predominantly located in the West North Central and Southwest regions.
At all sites with a three-year average fourth-highest daily maximum 8-
hour average concentration at or below a level of 60 ppb, three-year
W126 index values were at or below 7 ppm-hrs (Wells, 2014, Figure 5b).
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    \225\ This memo utilizes the same regional specifications as are
used in the PA and WREA (e.g., U.S. EPA, 2014c, Appendix 2B, Figure
2B-1).
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    Further analysis in the technical memorandum focused on a
comparison of monitors with a three-year average fourth-highest daily
maximum 8-hour average concentration at or below a level of 70 ppb and
a three-year W126 index values above 13 ppm-hrs for sets of three-year
periods between 2001-2003 and 2011-2013 (Wells, 2014, Figure 8). This
analysis found that the number of sites meeting 70 ppb while exceeding
13 ppm-hrs has remained relatively constant over the past decade, with
these sites consistently being limited to a small number in the West
and Southwest. In addition, the number of sites meeting both 70 ppb and
13 ppm-hrs has increased over time, while the number of sites exceeding
both 70 ppb and 13 ppm-hrs has decreased by a similar amount.
    The second part of the analysis in the technical memorandum focused
on trends in the relationships between O3 levels in terms of
the 4th high metric and a three-year average W126 metric, starting with
the 2001-2003 period and ending with the 2011-2013 period. Based on
analysis of 729 monitors, trends in both the 4th high metric and the
three-year average, W126 metric showed decreasing values between 2001-
2003 and 2011-2013. In addition, the amount of year-to-year variability
in the two metrics tended to decrease over time with decreasing
O3 concentrations, especially for the W126 metric. Most
sites in the eastern U.S. and California saw large, widespread
decreases in both the 4th high metric and the three-year average W126
metrics over the past decade as a result of regional NOX
control programs. In the inter-mountain west, where control programs
have been more localized, the decreases observed in the 4th high metric
and three-year average W126 metrics were typically much smaller in
magnitude, with a small number of sites showing significant increases.
    As part of this analysis, regional comparisons were included on the
relative changes in the relationships between O3 levels in
terms of the 4th high metric and a three-year average W126 metric
between the periods of 2001-2003 and 2011-2013. Figure 12 in the
technical memorandum shows that a positive, linear relationship
persists within each region between the changes in 4th high and three-
year average W126 metrics. Nationally, the three-year average W126
metric decreased by approximately 0.7 ppm-hrs per unit ppb decrease in
the 4th high metric. In addition, the Southwest and West regions, which
have the greatest potential for sites to measure elevated cumulative,
seasonal O3 exposures without the occurrence of elevated
daily maximum 8-hour average O3 concentrations, exhibited
the greatest

response in W126 value change per unit change in 4th high metric
(Wells, 2014, Table 6).
    The technical memorandum concludes that the 4th high metric and a
three-year average W126 metric are highly correlated, as are the
relative changes in these two metrics over the past decade. In this
way, the technical memorandum concludes that that future control
programs designed to help meet a revised primary O3 standard
based on the three-year average of the 4th highest daily maximum 8-hour
concentration are expected to also result in decreases in the values of
a three-year average W126 metric.
    The above information suggests that depending on the level for a
standard of the current averaging time and form, the current form and
averaging time of the secondary standard can be expected to achieve
control of cumulative seasonal O3 exposures, providing air
quality that may meet specific three-year average W126 index values. As
discussed above, we recognize limitations in the dataset and associated
analyses, including those related to monitor coverage, which may
contribute uncertainties to conclusions related to the relationships
described. With respect to monitor coverage, the current O3
monitoring network is urban focused, with fewer monitors in some parts
of the country, particular rural areas of the southwestern and western
U.S. Because of this, there are potential uncertainties in the extent
to which the monitoring information discussed above represents air
quality patterns and relationships that would occur in areas without
monitors. There is some information suggesting that there is a
potential for inconsistencies in the relationship between W126 measures
of seasonal O3 concentrations and the fourth highest peak
O3 concentrations assessed by the current standard averaging
time and form, but the available data suggest that air quality in areas
meeting a standard of the current form and averaging time with a level
in the range of 65 to 70 ppb would also meet a three-year W126 index
value falling in the range of 13 to 17 ppm-hrs, and that to the extent
areas need to take action to attain a primary standard in the range of
65 to 70 ppb, those actions would also improve air quality as measured
by the W126 metric.\226\ To the extent to which the monitoring data can
be expected to describe future relationships in air quality, we
acknowledge potential uncertainties in specifying future air quality
but note that these uncertainties are limited by the fact that the data
analysis includes over a decade of O3 measurements, with
similar patterns and trends observed in air quality over this period of
time.
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    \226\ EPA notes that areas can be expected to have air quality
at least as good as that specified by the primary standard, so to
the extent there are inconsistencies between fourth highest peak
concentrations and W126 values such that some areas meeting a
standard of 0.065 to 0.070 ppm might be well below the range of 13
to 17 ppm-hours, those inconsistencies are less relevant to
consideration of the appropriate form and level for the secondary
standard.
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5. Administrator's Proposed Conclusions
    In considering what revisions to the secondary standard are
appropriate, the Administrator has drawn on the ISA conclusions
regarding the weight of the evidence for a range of welfare effects
associated with O3 in ambient air, and associated areas of
uncertainty; quantitative risk and exposure analyses in the WREA for
different adjusted air quality scenarios and associated limitations and
uncertainties; staff evaluations of the evidence, exposure/risk
information and air quality information in the PA; additional air
quality analyses of relationships between air quality metrics based on
form and averaging time of the current standards and a cumulative
seasonal exposure index; and CASAC advice; and, public comments
received thus far in the review.
    As described in section IV.E.1 above, the Administrator concludes
it is appropriate to continue to use O3 as the indicator for
a secondary standard intended to address adverse effects to public
welfare associated with exposure to O3 alone and in
combination with related photochemical oxidants. In this review, no
alternatives to O3 have been advanced as being a more
appropriate surrogate for ambient photochemical oxidants. Thus, as is
the case for the primary standard (discussed above in section II.E.1),
the Administrator proposes to continue to use O3 as the
indicator for a 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
would also be expected to reduce exposures to other photochemical
oxidants.
    The Administrator has next considered the array of information with
regard to identifying policy options for a revised secondary standard
for O3 that in her judgment would provide appropriate
protection for public welfare effects associated with O3 in
ambient air. This information includes ISA conclusions, WREA analysis
findings, staff considerations and conclusions in the PA and CASAC
advice, as well as the Administrator's conclusions in the last review,
with regard to a biologically relevant exposure metric for
O3 vegetation-related effects. The information also includes
PA conclusions and CASAC advice with regard to key aspects of the
definition of such a metric, as summarized in section IV.E.2 and IV.E.3
above. Additionally, the Administrator has considered findings of staff
evaluations in the PA with regard to potential impacts on vegetation
and forested ecosystems associated with a range of values for such a
metric and identified uncertainties and limitations of such
information, as summarized in section IV.E.2 above. Additionally
important to her deliberations here are findings of air quality
analyses of relationships between the W126-based exposure metric and
levels of a standard of the same form and averaging time as the current
standards, as described in section IV.E.4 above. Based on consideration
of this array of information, as described below, the Administrator has
drawn conclusions with regard to policy options for a revised secondary
standard. In drawing conclusions on such options, she recognizes that
the Act does not require that NAAQS be set at zero-risk or background
levels, but rather at levels that reduce risk sufficiently to protect
public welfare from adverse effects.
    As an initial matter, the Administrator recognizes the longstanding
evidence, described in the ISA, of O3 effects on vegetation
and associated terrestrial ecosystems. Further, in reaching a proposed
conclusion on the appropriate form and averaging time for a revised
secondary standard that would provide increased protection against
vegetation-related effects on public welfare, the Administrator takes
note of the conclusions drawn in the ISA, the PA and by CASAC in this
review that the scientific evidence continues to demonstrate the
cumulative nature of O3-induced plant effects and the need
to give greater weight to higher concentrations, as summarized in
sections IV.B.1, IV.D.1, IV.D.2, IV.E.2.a and IV.E.3 above. Based on
these considerations, the Administrator concurs with the CASAC that a
cumulative, seasonal, concentration-weighted exposure-based form and
averaging time provides the most direct link between O3 in
ambient air and O3-related effects on vegetation. The
Administrator further concludes that in judging the extent of public
welfare protection that might be afforded by a revised standard, it is
appropriate to use

a cumulative, seasonal concentration-weighted metric.
    In identifying a cumulative, seasonal, concentration-weighted
metric for use in judging public welfare protection, the Administrator
gives weight to the PA conclusions regarding consideration of a revised
secondary standard in terms of the cumulative, seasonal, concentration-
weighted form, the W126 index. As described in section IV.B.1 above,
the ISA has recognized the strength of the W126 index in its weighting
of potentially damaging O3 concentrations that contributes
to the advantages it offers over other weighted cumulative indices. The
Administrator notes the PA conclusions regarding the W126 metric,
specifically use of the three consecutive month period within the
O3 season with the maximum index value as the seasonal
period over which to cumulate hourly O3 exposures and the
cumulation of daily exposures for the 12-hour period from 8:00 a.m. to
8:00 p.m. The Administrator additionally takes note of CASAC support
for consideration of the W126 index defined in this way and concludes
it is appropriate to use the cumulative seasonal W126-based metric
derived in this way.
    In further considering the PA conclusions regarding a revised
secondary standard in terms of the W126 index, the Administrator takes
note of considerations in the PA of a three-year or single-year
evaluation period. Such considerations include the variability in
ambient air O3 concentrations from year to year, as well as
variability and uncertainties related to environmental factors that
influence the occurrence and magnitude of O3-related
effects. The Administrator additionally notes the PA observation of
greater significance for effects associated with multiple-year
exposures. Based on these and related considerations described in the
PA (and summarized in section IV.E.2 above), the Administrator, in
identifying a metric for use in judging public welfare protection
afforded, agrees with the PA conclusion that it is appropriate to
consider a form that averages W126 index values across three
consecutive years, and to do so in conjunction with identification of
levels for such a form that might be judged to provide the appropriate
degree of public welfare protection from O3 effects across
multiple years. In so doing, the Administrator takes note of the ISA
conclusions regarding the role of environmental factors in variability
associated with effects of ambient air O3 and the year-to-
year variability commonly observed in such environmental factors.
Further, the Administrator also recognizes uncertainties associated
with determining the degree of vegetation impacts for annual effects
that would be adverse to public welfare. Even in the case of annual
crops, the assessment of public welfare significance is unclear for the
reasons discussed below related to agricultural practices. The
considerations identified here lead the Administrator to conclude it is
appropriate to use an index averaged across three years.
    In reaching this conclusion regarding a three-year average metric,
the Administrator has considered CASAC comments that it favors a W126-
based secondary standard with a single year form and that its
recommended range of levels relates to such a form. The Administrator
concurs with CASAC that it is important to consider impacts associated
with a single year that may be of a magnitude concluded to represent an
adverse effect on public welfare. The Administrator further concludes
that such an occurrence can be addressed through use of a three-year
average metric, chosen with consideration of the relevant factors. As
noted above, the Administrator gives consideration to the
variabilities, as well as the uncertainties, associated with single
year and multiple year impacts. Based on all of these considerations,
the Administrator recognizes greater confidence in judgments related to
public welfare impacts based on a three-year average metric.
    Thus, based on all of the above, the Administrator proposes, for
purposes of judging the extent of public welfare protection that might
be afforded by a revised standard and whether it meets the appropriate
level of protection, to use the average W126 index value across three
years, with each year's value identified as that for the three-month
period yielding the highest seasonal value and with daily O3
exposures within a three-month period cumulated for the 12-hour period
from 8:00 a.m. to 8:00 p.m.
    In reaching a conclusion on the appropriate range of W126 index
values that describe the O3 conditions expected to provide
the requisite protection of public welfare, the Administrator has given
careful consideration to the following: (1) The nature and degree of
effects of O3 to the public welfare, including what
constitutes an adverse effect; (2) the strengths and limitations of the
evidence that is available regarding known or anticipated adverse
effects from cumulative, seasonal exposures, and its usefulness in
informing selection of a proposed range; and (3) CASAC's views
regarding a range of W126 levels appropriate to consider, as well as on
the strength of the evidence and its adequacy to inform a range of
levels. In this consideration, the Administrator recognizes that the
choice of a range of W126 index values (and the form of the W126 index)
that might be expected to provide protection of the public welfare from
any known or anticipated adverse effects requires judgments about the
interpretation of the evidence and other information, such as the
quantitative analyses of air quality monitoring, exposure and risk,
that neither overstates nor understates the strengths and limitations
of the evidence and information nor the appropriate inferences to be
drawn as to risks to public welfare. 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). The
Administrator additionally recognizes that there is not a bright line
clearly directing the choice of a range of W126 index values and that
the choice of what is appropriate is a public welfare policy judgment
entrusted to the Administrator.
    In determining the range of three-year average W126 index values
that might be expected to provide the appropriate level of public
welfare protection, the Administrator first considers the nature and
degree of effects of O3 on the public welfare. The
Administrator 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. Any given O3-related effect on vegetation and
ecosystems (e.g., biomass loss, visible foliar injury), therefore, may
be judged to have a different degree of impact on the public depending,
for example, on whether that effect occurs in a Class I area, or a
residential or commercial setting. The Administrator notes that such a
distinction is supported by CASAC advice in this review. In her
judgment, like those of the Administrator in the last review, it is
appropriate that this variation in the significance of O3-
related vegetation effects should be taken into consideration in making
judgments with regard to the level of ambient O3
concentrations that is requisite to protect the public welfare from any
known or anticipated adverse effects. As a result, the Administrator
concludes that of those known and anticipated O3-related
vegetation and ecosystem effects

identified and discussed in this notice, particular significance should
be ascribed to those that occur on sensitive species that are known to
or are likely to occur in federally protected areas such as Class I
areas \227\ or on lands set aside by States, Tribes and public interest
groups to provide similar benefits to the public welfare, for residents
on those lands, as well as visitors to those areas.
---------------------------------------------------------------------------

    \227\ For example, the Wilderness Act of 1964 defines designated
``wilderness areas'' in part as areas ``protected and managed so as
to preserve [their] natural conditions'' and requires that these
areas ``shall be administered for the use and enjoyment of the
American people in such manner as will leave them unimpaired for
future use and enjoyment as wilderness, and so as to provide for the
protection of these areas [and] the preservation of their wilderness
character.''16 U.S.C. 1131(a).
---------------------------------------------------------------------------

    Likewise, the Administrator also notes that the same known or
anticipated O3-induced effects occurring in other areas may
call for less protection. For example, the maintenance of adequate
agricultural crop yields is extremely important to the public welfare
and is currently achieved through the application of intensive
management practices. With respect to commercial production of
commodities, the Administrator notes that judgments about the extent to
which O3-related effects on commercially managed vegetation
are adverse from a public welfare perspective are particularly
difficult to reach, given that the extensive management of such
vegetation (which, as CASAC noted, may reduce yield variability) may
also to some degree mitigate potential O3-related effects.
The management practices used on these lands are highly variable and
are designed to achieve optimal yields, taking into consideration
various environmental conditions. In addition, changes in yield of
commercial crops and timber may affect producers and consumers
differently, further complicating the question of assessing overall
public welfare impacts. Thus, the Administrator concludes that
agricultural crops do not have same need for additional protection from
the NAAQS as forested ecosystems and, while research on agricultural
crop species remains useful in illuminating mechanisms of action and
physiological processes, information from this sector on O3-
induced effects is considered less useful in informing judgments on
what level(s) would be sufficient but not more than necessary to
protect the public welfare. The CASAC identified a crop RYL benchmark
of 5% for the median species and indicated they found higher
percentages unacceptably high. Although the Administrator has not drawn
a conclusion with regard to this specific benchmark, the Administrator
finds the public welfare impacts associated with crop yield loss to be
a less important consideration in this review for the reasons discussed
here, including the extensive management of crop yields and the
dynamics of agricultural markets, and thus is not focusing on crop
yield loss in selecting a revised standard. She notes, however, the PA
finding that median species crop RYL estimates for W126 index values in
the PA identified range (17 to 7 ppm-hrs) fall below the 5% benchmark
emphasized by CASAC for this endpoint. The Administrator also notes
that a standard revised to increase protection for forested ecosystems
would also be expected to provide some increased protection for
agricultural crops.
    The Administrator also recognizes that O3-related
effects on sensitive vegetation can occur in other areas that have not
been afforded special federal protections, ranging from effects on
vegetation growing in managed city parks and residential or commercial
settings, such as ornamentals used in urban/suburban landscaping or
vegetation grown in land use categories that are heavily managed for
commercial production of commodities such as timber. For vegetation
used for residential or commercial ornamental purposes, the
Administrator believes that there is not adequate information at this
time to establish a secondary standard based specifically on impairment
of these categories of vegetation, but notes that a secondary standard
revised to provide protection for sensitive natural vegetation and
ecosystems would likely also provide some degree of protection for such
vegetation.
    Based on the above, the Administrator finds that the type of
information most useful in informing the selection of an appropriate
range of protective levels is appropriately focused on information
regarding exposures and responses of sensitive trees and other native
species known or anticipated to occur in 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, for residents
on those lands, as well as visitors to those areas.
    With regard to the available evidence, the Administrator finds the
coherence and strength of the weight of evidence from the large body of
available literature compelling. This evidence addresses a broad array
of O3-induced effects on a variety of tree species across a
range of growth stages (i.e., seedlings, saplings and mature trees)
using diverse field-based (e.g., free air, gradient and ambient) and
OTC exposure methods. The Administrator gives particular attention to
the effects related to native tree growth and productivity, recognizing
their relationship to a range of ecosystem services, including forest
and forest community composition.
    With regard to selection of the values for use with the W126 index
for the purpose of identifying a range of O3 conditions
expected to provide the appropriate level of protection from vegetation
effects of particular concern, the Administrator, as an initial matter,
takes note of the PA conclusion that, with regard to a target level of
protection for a revised standard, it is appropriate to give
consideration to a range of levels from 17 ppm-hrs to 7 ppm-hrs,
expressed in terms of the W126 index averaged across three consecutive
years. As summarized in section IV.E.2.b above, this PA conclusion
draws heavily on considerations related to estimates of tree seedling
growth impacts (in terms of relative biomass loss) associated with a
range of W126-based index values developed from the robust E-R
functions for 11 tree species. This conclusion also gives weight to
CASAC comments as to an unacceptably high magnitude of relative biomass
loss (6%) for the median species and a magnitude of median relative
biomass loss on which to focus considerations (2%). The Administrator
takes particular note of the CASAC view of a median species RBL of 6%
as unacceptably high.
    In considering the basis for the range of W126 index levels
identified by the PA, for which 17 ppm-hrs is the upper end, the
Administrator considers the CASAC advice, including their view that a
6% median tree seedling species RBL is unacceptably high, their
consideration of Table 6-1 in the second draft PA which indicated such
a RBL estimate for a W126 index value of 17 ppm-hrs, and their
consequent lack of support for levels higher than 15 ppm-hrs (Frey,
2014c, p. iii; U.S. EPA 2014j, Table 6-1). As noted in section IV.E.3
above, revisions to this table in the final PA, made in consideration
of CASAC comments have resulted in changes to the median species RBL
estimates such that the median species RBL estimate for a W126 index
value of 17 ppm-hrs in this table in the final PA (5.3%) is nearly
identical to the median species estimate for 15 ppm-hrs (the value
corresponding to the upper end of the CASAC-identified range) in the
second draft PA (5.2%) (U.S. EPA, 2014c, Table 6-1; U.S. EPA, 2014j,
Table 6-1).

    The Administrator additionally takes note of the PA observations
that the number and proportion of individual species with RBL estimates
at or below 2%, a benchmark given emphasis by CASAC, do not vary across
W126 index values from 17 ppm-hrs down to 9 ppm-hrs (as seen in Table 8
above), providing little distinction with regard to the significance of
growth impacts for exposures across this large portion of the PA range.
The Administrator also notes the CASAC recommendation regarding a
lowering of the level with consideration of a three-year average index;
however, the Administrator's judgments on a three-year average index,
as described above, focus on confidence in conclusions that might be
drawn with regard to single as compared to multiple year impacts. For
example, the Administrator, while recognizing the strength of the
evidence with regard to quantitative characterization of O3
effects on growth of tree seedlings and crops, in addition to noting
the additional difficulties for assessing welfare impacts of crops,
takes note of the uncertainty associated with drawing conclusions with
regard to the extent to which small percent reductions in annual growth
contribute to adverse effects on public welfare and the role of annual
variability in environmental factors that affect plant responses to
O3. Moreover, as explained above, the Administrator
concludes that concerns related to the possibility of a singly
unusually damaging year can be addressed through use of a three-year
average metric, chosen with consideration of the relevant factors.
Accordingly, she judges it appropriate to include 17 ppm-hrs, without
adjustment, in the range of three-year average W126 index values
appropriate to consider in determining what secondary standard will
provide air quality associated with the appropriate level of public
welfare protection. She thus judges it appropriate to focus on a range
for three-year average W126 levels with 17 ppm-hrs at the upper end. In
so doing, she additionally notes CASAC's recognition that, within a
scientifically appropriate range, the choice of levels is a public
policy judgment by the Administrator.
    In turning to consideration of the low end for the W126 index
range, the Administrator considers the full range of W126 levels
identified in the PA with regard to the evidence and exposure/risk-
based information, and associated uncertainties, identified in the PA,
as well as CASAC advice. The Administrator notes the CASAC policy view
regarding protection provided for trees and associated ecosystem
services from a W126 index value of 7 ppm-hrs, which is based on the
W126 index value for which the median species estimate falls below 2%
RBL. The Administrator recognizes, however, as noted above, the greater
uncertainty associated with the extent to which estimates of benefits
in terms of ecosystem services and reduced effects on vegetation at
lower O3 exposures might be judged significant to the public
welfare.
    The Administrator additionally notes the results of the EPA's
quantitative exposure and risk assessments for the air quality
scenarios for W126 levels at and below 11 ppm-hrs, including the
relatively small additional benefits and increased uncertainty with the
ecosystem services estimates in these lower W126 scenarios. With regard
to the PA evaluation of RBL estimates, the Administrator, while noting
the PA observations of similarity in the number of species with less
than 2% RBL across the W126 range from 17 to 9 ppm-hrs, as stated
above, additionally notes PA observations of a similar number of
studied species with RBL estimates below 5% for W126 index values of 13
and 11 ppm-hrs. Thus, to the extent that weight is given to the
importance of 5% RBL for individual species, both W126 index values are
observed to provide RBL estimates below this benchmark.
    With regard to considerations of O3 effects beyond
biomass loss in tree seedlings, the Administrator takes note of the
lack of new quantitative E-R relationships for larger trees growing in
the field that would help inform consideration of a standard level
within the lower part of PA range. Thus, the Administrator recognizes
that important uncertainties remain in interpreting the quantitative
O3-related growth effects for tree seedlings assessed in OTC
studies for the purpose of characterizing long-term growth effects, and
other more subtle but important effects on sensitive tree species,
natural forests, and forested ecosystems in the broader context of
protection of public welfare. Additionally, while the Administrator
notes that there is evidence that O3-related visible foliar
injury can occur at such lower levels (below a W126 index value of 13
ppm-hrs), she recognizes, as summarized in sections IV.C.3.c and IV.D.1
above, the significant challenges in judging the extent to which such
effects should be considered adverse to public welfare, in light of the
variability and the lack of clear quantitative relationship with other
effects on vegetation, as well as the lack of established criteria or
objectives that might inform consideration of potential public welfare
impacts related to this vegetation effect.
    Thus, in the Administrator's judgment, focus on a three-year
average W126 index value below 13 ppm-hrs would not give sufficient
attention to the important uncertainties and limitations inherent in
the currently available scientific evidence and in the quantitative
assessments conducted for the current review. Taking into account the
uncertainties that remain in interpreting the evidence, the
Administrator observes that the likelihood of obtaining benefits to
public welfare decreases with a standard set below a level of 13 ppm-
hrs, while the likelihood of requiring reductions in ambient
concentrations that go beyond those that are needed to reduce adverse
impacts to public welfare increases.
    Based on the above considerations and based on the entire body of
evidence and information currently available, the Administrator
identifies the range of three-year average W126 index values extending
from 13 to 17 ppm-hrs as appropriate to consider in identifying the
ambient O3 concentrations that would provide the appropriate
level of public welfare protection. In so doing, the Administrator
notes CASAC recognition that a three-year average W126 level of 13 ppm-
hrs may be appropriate depending on consideration of year-to-year
variability and policy considerations. Thus, based on the discussion
above, and with consideration of CASAC advice on these issues, the
Administrator proposes that ambient O3 concentrations
resulting in cumulative seasonal O3 exposures of a level
within the range from 13 ppm-hrs to 17 ppm-hrs, in terms of a W126
index averaged across three consecutive years, would provide the
requisite protection against known or anticipated adverse effects to
the public welfare. The EPA solicits comments on levels within this
range.
    The Administrator next turns to consideration of policy options for
a revised secondary standard that would provide this level of
protection. The Administrator takes note of staff conclusions that it
is appropriate to consider a revised secondary standard in terms of the
cumulative, seasonal, concentration-weighted form, the W126 index.
Further, she gives extensive consideration to CASAC advice to set such
a secondary standard. Such a standard, as mentioned above, would be
directly linked to O3 exposures to which vegetation are most
responsive and thus might be expected to provide some

confidence that such exposures of concern would be controlled.
    In considering different policy options for a revised secondary
standard, the Administrator finds it useful to consider what can be
concluded from the available information with regard to relationships
between air quality characterized in terms of the current form and
averaging time and also in terms of the W126 metric. She has considered
particularly what such analyses and relationships indicate with regard
to the extent to which W126-based O3 concentrations may be
controlled by a revised secondary standard set identical to a revised
primary standard (in the range identified in section II.F above). In so
doing, she considers the air quality analyses in the PA and also the
analyses of more recent air quality data summarized in the EPA
technical memo (described in section IV.E.4 above), focusing
particularly on analyses examining the W126-based O3
exposure achieved in locations found to meet potential alternative
standards within the range of primary standards proposed in section
II.F above.
    Findings from these analyses of recent O3 measurements
and trends in the relationship between the current standard and the
W126 metrics were substantially similar for the various time periods
examined over the past decade. There is some information suggesting
that there is a potential for inconsistencies in the relationship
between W126 measures of seasonal O3 concentrations and the
fourth highest peak O3 concentrations assessed by the
current standard averaging time and form, but the available data
suggest that air quality in areas meeting a primary standard in the
range of 65 to 70 ppb would also meet a three-year W126 index value
falling in the range of 13 to 17 ppm-hrs, and that to the extent areas
need to take action to attain a standard in the range of 0.065 to 0.070
ppm, those actions would also improve air quality as measured by the
W126 metric. The Administrator also recognizes that the relatively
lesser density of monitors in rural areas, including in areas of the
West and Southwest NOAA climatic regions currently meeting the current
standard where O3 W126 index values are generally higher,
makes uncertain the degree to which a revised level for the current
standard would provide the appropriate degree of protection for
vegetation-related effects on public welfare in these areas. The
Administrator takes note of the PA finding, referenced in section
IV.D.3 above, 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. Thus, while
the potential for underprotection may exist, depending on the specific
levels chosen, the extent of such areas and of such a risk is not
clear.
    Based on the most recent period of monitoring data, the
Administrator notes that in all areas in which the O3
concentrations would have met a primary standard with a revised level
of 70 ppb (which includes over 500 monitors distributed across all
regions of U.S), the three-year average W126 index values are at or
below 17 ppm-hrs. In the same areas, only 16 monitors (or less than 3%
of all monitors in this group, all but one of which is located in the
Southwest region) had three-year average W126 index values above 13
ppm-hrs. She further notes that in all areas in which the O3
concentrations would have met a primary standard with a revised level
of 65 ppb (which includes 220 monitors distributed across all regions
of U.S), the three-year average W126 index values are at or below 13
ppm-hrs.
    In considering these findings regarding cumulative seasonal
O3 exposures in areas that would have met a primary standard
with a revised level within the proposed range, the Administrator also
takes note of the high correlation observed between the design value
for the current secondary (and primary) standard and values for the
three-year average, W126 metric, as well as the high correlation in the
relative changes in these two metrics based on air quality analyses of
O3 measurements from over the past decade. This finding
supports a conclusion that the air quality analyses indicate that
future control programs designed to reduce O3 concentrations
to help meet a revised primary O3 standard that retains the
current form and averaging time (three-year average of the 4th highest
daily maximum 8-hour concentration) would also be expected to result in
reductions in three-year average, W126 index values. Further, she notes
the conclusion from the air quality analysis that the Southwest and
West regions, which have the greatest potential for sites to measure
elevated cumulative, seasonal O3 exposures without the
occurrence of elevated daily maximum 8-hour average O3
concentrations, exhibited the greatest response in W126 index value
change per unit change in metric based on the current standard form and
averaging time. While recognizing the limitations of such analyses in
projections of future air quality patterns, the Administrator also
notes that the time period over which the analyses focused involved
emissions control programs to achieve O3 reductions such
that their findings would be expected to be informative of further
similar control activities, such as those to meet a revised standard
with a lower level, in the future.
    Based on the findings from these analyses, the Administrator finds
it appropriate to consider the policy option of retaining the form and
averaging time of the current secondary standard and revising the level
to within the range of 65 to 70 ppb. In such consideration, the
Administrator first notes her proposed conclusion that the requisite
protection from known or anticipated adverse effects to public welfare
may be achieved by cumulative, seasonal, concentration-weighted
O3 concentrations characterized in terms of a W126 index
value that falls within the range from 13 to 17 ppm-hrs. Her final
decision on the W126 index value in this range that affords the
requisite protection will be based on a series of judgments, as
described above. Given the focus on tree seedling growth effects in
identifying this range, such judgments will include the weight to give
the evidence of specific vegetation-related effects estimated to result
from W126 index values within this range, including the objectives for
consideration of tree species biomass loss estimates in relationship to
identified benchmarks (e.g., median species RBL of 2% and greater), the
weight to give associated uncertainties, including those related to the
variability in occurrence of such effects in forested areas, the
associated ecosystem services including those of particular public
welfare significance, and judgments on the extent to which such effects
in forested areas may be considered adverse to public welfare. This
final decision will also take into account judgments with regard to the
weight to give the evidence and quantitative analyses, and associated
uncertainties, related to other effects of O3 (summarized in
sections IV.C, IV.D.1 and IV.E.2 above), particularly including those
for which the ISA concludes causal or likely causal relationships with
O3 exposures. As noted above, a standard that provides the
appropriate level of protection for growth effects would also be
expected to provide additional protection for other effects including
visible foliar injury, crops and carbon storage.
    The Administrator notes that based on the above analyses, the
proposed range of levels for a revised primary standard provide air
quality, in terms of three-year average W126 index values, of a

range at or below the range which the Administrator has identified for
consideration with regard to the requisite public welfare protection.
Thus, depending on final judgments on revisions to the primary standard
and the requisite protection for the secondary standard, a revised
secondary standard identical to the revised primary standard may
provide sufficient protection for public welfare. Therefore, the
Administrator proposes to retain the current averaging time and form of
the secondary standard and revise the level to within the range of 65
to 70 ppb.
    In reaching such a conclusion, the Administrator recognizes that
such a strengthening of the secondary standard would be expected to
provide significant additional protection for public welfare, including
effects related to vegetation and associated ecosystem services (and
others discussed above), over that afforded by the current secondary
standard.
    Thus, based on her consideration of the full range of information
as described above, the Administrator judges that ambient O3
concentrations in terms of a three-year average W126 index value within
the range extending from 13 ppm-hrs to 17 ppm-hrs would provide
requisite public welfare protection. She further judges that it would
be appropriate to achieve that level of air quality by retaining the
existing averaging time and form, and revising the level to within the
range of 65 to 70 ppb. In recognition of CASAC's recommendation and the
PA conclusion with regard to a distinct secondary standard, the
Administrator additionally solicits comment on the policy option of
revising the form and averaging time for the secondary standard to a
W126 index value, averaged across three years, with each year's value
identified as that for the three-month period yielding the highest
seasonal value and with daily O3 exposures within a three-
month period cumulated for the 12-hour period from 8:00 a.m. to 8:00
p.m., and a level within the range from 13 ppm-hrs to 17 ppm-hrs.

F. Proposed Decision on the Secondary Standard

    The Administrator proposes to revise the level of the current
secondary standard within the range of 0.065 to 0.070 ppm. The EPA
solicits comments on this proposed revision of the secondary standard.
Further, the EPA solicits comments on the proposed conclusion that air
quality in terms of a W126 index value, averaged across three
consecutive years, within the range of 13 ppm-hrs to 17 ppm-hrs would
provide requisite protection against known or anticipated adverse
effects to the public welfare. Additionally, the EPA solicits comments
on alternative values for a three-year average W126 index for such a
purpose within the range extending below 13 ppm-hrs down to 7 ppm-hrs.
    The EPA also solicits comments on the alternative approach of
revising the secondary standard to a cumulative, seasonal,
concentration-weighted form, the W126 index based on the three
consecutive month period within the O3 season with the
maximum index value, with daily exposures cumulated for the 12-hour
period from 8:00am to 8:00pm and with a form that averages seasonal
W126 values across three consecutive years and a level within the range
of 13 to 17 ppm-hrs. The EPA additionally solicits comments on such a
distinct secondary standard with a level within the range extending
below 13 ppm-hrs down to 7 ppm-hrs. Further, the EPA solicits comments
on retaining the current secondary standard without revision, along
with the alternative views of the evidence that would support retaining
the current standard.

V. Appendix U: Interpretation of the Primary and Secondary NAAQS for
O3

A. Background

    The EPA is proposing to create Appendix U to 40 CFR part 50 to
reflect the proposed revisions to the primary and secondary NAAQS for
O3 discussed in previous sections of this preamble. The
proposed Appendix U explains the computations necessary for determining
when the proposed primary and secondary O3 NAAQS are met at
an ambient air quality monitoring site, similar to Appendix P to 40 CFR
part 50 which deals with interpretation of the O3 NAAQS
promulgated in 2008. Specifically, the proposed Appendix U addresses
data selection requirements (section V.B), data reporting and data
handling requirements (section V.C), and data completeness
requirements. The EPA is proposing to maintain the data completeness
requirements from the previous O3 NAAQS.
    Given that the EPA is soliciting public comment on a distinct
secondary standard based on the W126 metric, section V.D of this
preamble contains a discussion of additional data handling requirements
that would be adopted in Appendix U in the event that the Administrator
decides to set a distinct secondary standard based on public comments
received.
    The proposed Appendix U also provides specific requirements for the
handling of data affected by exceptional events in accordance with 40
CFR 50.14. Section V.E of this preamble addresses O3-
specific deadlines related to the flagging and submission of
demonstrations for exceptional event data for the proposed
O3 NAAQS.

B. Data Selection Requirements

    The EPA is proposing to clarify which data are to be used in
comparisons with the NAAQS. First, the EPA proposes to maintain the
existing regulatory requirements that only O3 data collected
by a federal reference method specified in Appendix D to 40 CFR part
50, or an equivalent method designated in accordance with 40 CFR part
53, and meeting all applicable monitoring requirements listed in 40 CFR
part 58, are eligible for comparison to the proposed O3
NAAQS.
    Second, the EPA is proposing in Appendix U that O3
design values are to be calculated on a site-level basis. Past practice
has been to calculate a design value for each individual O3
monitor. However, this practice could be viewed as inconsistent with
the stated purpose of the previous O3 data handling
appendix, which is to determine ``whether the national 8-hour primary
and secondary ambient air quality standards for ozone (O3)
specified in Sec.  50.15 are met at an ambient O3 air
quality monitoring site.'' (40 CFR part 50, Appendix P, section 1
(emphasis added)). Given the level of consistency in the measurement
data obtained across the various federal reference and equivalent
O3 monitoring instruments currently in operation (U.S. EPA,
2013a, section <IP_ADDRESS>), the EPA believes that it would be appropriate
to combine data across O3 monitors operating at the same
site. Therefore, the EPA is proposing an analytic approach for
combining data collected from multiple O3 monitors at a site
in order to obtain a single set of hourly O3 concentration
data for each site.
    The proposed approach allows the monitoring agencies to designate
one monitor as the ``primary monitor'' for each site. In the absence of
a primary monitor designation, the primary monitor would default to the
monitor with the most complete hourly dataset in each year. Once a
primary monitor has been determined for the site, missing hourly
O3 concentrations for the primary monitor would be
substituted from any other monitors at the site. In the event of three
or more monitors operating at the same site, missing hourly
O3 concentrations for the primary monitor would be
substituted with hourly values averaged across the other monitors. The
EPA notes that at the time of this proposal, there were approximately
20 sites operating two

monitors simultaneously, and no O3 sites operating three or
more monitors simultaneously. This proposed approach for combining data
across monitors at a site is consistent with the existing approach
described in Appendix N to Part 50 for the PM2.5 NAAQS. The
EPA invites public comment on the scientific validity of combining data
across O3 monitors, and the merits of the proposed approach
for combining data across multiple O3 monitors at a site.
    Third, the EPA proposes to maintain the existing practice of
combining data from nearby monitoring sites in order to determine a
valid design value, known as a ``site combination''. Site combinations
typically involve situations where sites have been replaced or
relocated a short distance away, and the monitoring agency wishes to
combine the data from the two sites in order to maintain a continuous
data record. The EPA regional offices have approved over 100 site
combinations for O3 since the promulgation of the 1997
O3 NAAQS. The EPA has maintained records of approved site
combinations, but these records are not easily accessible by the
public.
    The EPA proposes to replace the current procedure for approving
O3 site combinations with a more formal procedure in
Appendix U, which would allow states to submit site combination
requests to the appropriate Regional Administrator. Site combinations
may be approved by the Regional Administrator, after he or she has
determined that the measured air quality concentrations do not differ
substantially between the two sites. In order to make this
determination, the Regional Administrator may request additional
information from the states, including detailed information on the
locations and distance between the two sites, levels of ambient
concentrations measured at the two sites, and local emissions or
meteorology data.
    In order to improve transparency, the EPA will make records of all
approved site combinations available in their Air Quality System (AQS)
database, and will update design value calculations in AQS so that
approved site combinations are implemented. The EPA invites public
comment on the merits of the proposed process for approving site
combinations in order to obtain valid design values for the
O3 NAAQS.

C. Data Reporting and Data Handling Requirements

    The EPA is proposing to maintain the requirement that hourly
O3 concentration data be reported in parts per million (ppm)
to three decimal places. Any decimal digits reported beyond three
decimal digits will be truncated, consistent with past practice (40 CFR
part 50, Appendix P, section 2.1) and the typical measurement
uncertainty associated with most O3 monitoring instruments.
The proposed Appendix U clarifies that hourly O3
concentrations are to be reported in Local Standard Time (LST),
consistent with how the values are currently stored in AQS.
    The EPA is proposing to maintain the existing procedures for
calculating moving 8-hour averages from the hourly O3 data
(40 CFR part 50, Appendix P, section 2.1), with one minor exception. In
instances where fewer than six hourly O3 concentrations are
available during an 8-hour period (i.e. less than 75% completeness),
the EPA is proposing to substitute zero (i.e. 0.000 ppm) instead of one
half of the O3 monitoring instrument's minimum detectable
limit (MDL) for the missing concentration values to determine if the
resulting 8-hour average is greater than the level of the NAAQS. The
purpose of this ``data substitution test'' is to identify any 8-hour
periods that do not meet the requirements for a valid 8-hour average,
but have reported concentrations that are so high that the NAAQS is
exceeded even when substituting low values for the missing
concentrations. The EPA believes that a constant substitution value of
zero is preferable to 1/2 MDL, which may vary across O3
monitoring instruments. The MDL value for most O3 monitoring
instruments is 0.005 ppm, and the 1/2 MDL value is 0.002 ppm (with
truncation); thus, in practice, the difference is slight. The EPA notes
that a value of zero micrograms per cubic meter (ug/m\3\) is used in
data substitution tests for 24-hour average PM2.5
concentrations, as specified in Appendix N to 40 CFR part 50. The EPA
invites public comment on the merits of using zero instead of 1/2 MDL
for the 8-hour average data substitution test.
    The EPA is proposing new procedures for determining daily maximum
8-hour average O3 concentrations. Past practice allows for
daily maximum 8-hour average O3 concentrations from two
consecutive days to have some hours in common (40 CFR part 50, Appendix
P, section 2.1). One implication of this is that an O3 site
may be counted as having exceeded the NAAQS on two distinct days based
on two 8-hour periods having up to 7 hours in common. Theoretically,
this could result in an annual fourth-highest value greater than the
NAAQS based on high overnight O3 concentrations occurring
only twice during the year.
    The EPA performed an analysis based on ambient O3
concentration data from 2004 to 2013 (Wells, 2014b), which showed that
at least one instance of overlapping daily maximum 8-hour averages
occurred at 99.5% of O3 sites during that time period.
Overlapping daily maximum 8-hour averages were infrequent at most
sites, but in some cases, these values occurred quite regularly (up to
60 times per year). Overlapping daily maximum 8-hour averages
contributed to additional exceedances of the proposed O3
NAAQS at 14% of sites for a level of 0.070 ppm, and at 23% of sites for
a level of 0.065 ppm. In addition, 8% of sites had overlapping daily
maximum 8-hour averages which contributed to a higher annual fourth-
highest daily maximum value in one or more years. Finally, the analysis
showed that O3 sites located in non-urban areas affected by
long-range transport, especially those sites at higher elevations, were
most likely to have additional exceedances of the proposed
O3 NAAQS due to the occurrence of overlapping daily maximum
8-hour averages.
    Based on this analysis, the EPA initially concludes that
overlapping daily maximum 8-hour averages are more likely to contribute
to additional exceedances of the O3 NAAQS as the level of
the standard is lowered. Therefore, the EPA is proposing a new
procedure for determining daily maximum 8-hour average O3
concentrations for the proposed NAAQS that is based on 17 consecutive
8-hour periods in each day, beginning with the 8-hour period from 7:00
a.m. to 3:00 p.m., and ending with the 8-hour period from 11:00 p.m. to
7:00 a.m. Given that 8-hour averages are stored in the beginning hour
of each period, this corresponds to the 8-hour averages from 7:00 a.m.
to 11:00 p.m.
    The rationale for the proposed approach is twofold. First, it
avoids any possibility of ``double counting'' exceedances of the NAAQS
based on 8-hour periods with one or more hours in common, while
continuing to make use of all of the hourly concentration data, and
keeping the calculations simple and straightforward. Second, it is more
consistent with the physical processes involved in the formation and
transport of ground-level O3. Specifically, the chemical
reactions involved in the formation of new ground-level O3
require sunlight. Therefore, it is appropriate to begin the
``O3 day'' at sunrise, which for simplicity is assumed to be
7:00 a.m. LST. Similarly, any daily maximum 8-hour averages occurring
after sunset are assumed to be caused by transport of O3
molecules which

originated before sunset. Therefore, it is appropriate to end the
``O3 day'' with the 8-hour period beginning at 11:00 p.m.
and ending at 7:00 a.m.
    In order to accommodate the above proposed approach to the hours
considered in an ``O3 day'', the EPA is also proposing to
modify the requirement for determining whether a daily maximum 8-hour
average O3 concentration is valid for assessing compliance
with the NAAQS (40 CFR part 50, Appendix P, section 2.1). The proposed
Appendix U requires valid 8-hour averages for 13 of the 17 8-hour
periods in a day in order to determine a valid daily maximum value. The
requirement of 13 valid 8-hour averages was chosen because 13/17 is the
smallest ratio greater than 75%, which is consistent with the long
standing requirement of 75% data completeness for daily and annual
NAAQS-related statistics. In addition, the EPA is proposing to maintain
the existing provision allowing daily maximum 8-hour averages greater
than the level of the NAAQS to be considered valid (40 CFR part 50,
Appendix P, section 2.1). The EPA invites public comment on the merits
of the proposed procedure for determining daily maximum 8-hour average
O3 concentrations, and the merits of the proposed daily
validity criteria.
    Finally, the EPA has included additional language in the proposed
Appendix U codifying existing data handling procedures for the previous
O3 NAAQS. First, the proposed Appendix U maintains the
provision that hourly O3 concentrations approved under 40
CFR 50.14 as having been affected by exceptional events are to be
counted as missing or unavailable when calculating 8-hour averages, and
that these concentrations are to be included when determining whether
the daily validity criteria have been met for a given day. Effectively,
this means that it is possible for an 8-hour period affected by
exceptional events to lack sufficient data to determine an 8-hour
average, yet the 8-hour period may still be counted toward meeting the
daily validity criteria. Second, the proposed Appendix U maintains the
existing practice of including monitored days outside of the
O3 monitoring season when determining the annual fourth-
highest daily maximum value. Finally, the proposed Appendix U maintains
the existing practice of using only daily maximum 8-hour average values
for days where the daily validity criteria have been met when
determining the annual fourth-highest daily maximum value.

D. Considerations for the Possibility of a Distinct Secondary Standard

    Given that the EPA is soliciting public comment on setting a
distinct secondary O3 NAAQS based on the W126 index, the EPA
is including a discussion on the data handling requirements for a
distinct secondary standard. In the event that the Administrator
decides to set a distinct secondary O3 standard based on the
W126 index, the EPA will adopt data handling requirements for the
secondary standard similar to those proposed during the reconsideration
of the 2008 O3 NAAQS in 2010 (see 75 FR 3049-3052, January
19, 2010).
    Two changes would need to be made to the data handling provisions
for the secondary standard proposed in 2010 in order to provide
consistency with what the EPA is proposing for the primary standard in
Appendix U. First, the secondary standard design value (i.e. the 3-year
average of the annual W126 index) would be truncated after the decimal
point, instead of being rounded to the nearest whole number. Second,
paragraph 4(c)(ii) would be modified to read:
    ``If one or more months during the ozone monitoring seasons of
three consecutive years has less than 75% data completeness, the three
years shall nevertheless be used in the computation of a valid design
value for the site, if, after adjusting the monthly W126 index values
for the months with less than 75% data completeness by a factor of 4/3,
the resulting design value is greater than the level of the standard.''

E. Exceptional Events Information Submission Schedule

    States \228\ are responsible for identifying air quality data that
they believe warrant special consideration, including data affected by
exceptional events. States identify such data by flagging (making a
notation in a designated field in the electronic data record) specific
values in the AQS database. States flag the data and submit supporting
documentation showing that the data have been affected by exceptional
events if they wish the EPA to consider excluding the data in
regulatory decisions, including determining whether or not an area is
attaining the proposed revised O3 NAAQS, if a different
standard is finalized.
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    \228\ References to ``state'' are meant to include state, local
and tribal agencies responsible for preparing and submitting
exceptional event documentation as identified in the Exceptional
Events Rule (72 FR 13560, March 22, 2007).
---------------------------------------------------------------------------

    All states and areas of Indian country that include areas that
could exceed or contribute to an exceedance of any revised
O3 NAAQS in a nearby area and could therefore be designated
as nonattainment have the potential to be affected by this rulemaking.
Therefore, this action applies to all states; to local air quality
agencies to which a state has delegated relevant responsibilities for
air quality management including air quality monitoring and data
analysis; and to tribal air quality agencies, where appropriate.
    The ``Treatment of Data Influenced by Exceptional Events; Final
Rule'' (72 FR 13560, March 22, 2007), known as the Exceptional Events
Rule and codified at 40 CFR 50.1, 50.14 and 51.930, contains generic
deadlines for a state to submit to the EPA specified information about
exceptional events and associated air pollutant concentration data.
Under this generic flagging schedule in 40 CFR 50.14(c)(2)(iii), a
state must initially notify the EPA that data have been affected by an
event by July 1 of the calendar year following the year in which the
event occurred. This is done by flagging the data in AQS and providing
an initial event description. According to the generic demonstration
schedule in 40 CFR 50.14(c)(3)(i), the state must also, after notice
and opportunity for public comment, submit a demonstration to justify
any claim within 3 years after the quarter in which the data were
collected. This section of the regulation also states that if the EPA
must make a regulatory decision based on the data, the state must
submit all information to the EPA no later than 1 year before the
decision is to be made.
    These generic deadlines in the Exceptional Events Rule apply to
data influencing redesignation efforts or other regulatory decisions
made by the EPA after the EPA promulgates initial area designations for
a new or revised NAAQS. However, these same generic deadlines in the
Exceptional Events Rule may not work well with the timing of the
initial area designation process and schedule under a new or revised
NAAQS. Until the EPA promulgates the level and form of the NAAQS, a
state does not know whether the criteria for excluding data (which are
tied to the level and form of the NAAQS) were met for a given event. In
some cases, the generic deadlines, especially the deadlines for
flagging some relevant data, may have already passed by the time the
EPA promulgates the new or revised NAAQS. This scheduling constraint
could result in the EPA's being unable to consider whether an
exceptional event has affected the data relied on for initial area
designations and further result in an area being designated
nonattainment based on data

that might have been excluded as having been influenced by an
exceptional event if the EPA had been able to consider it during the
designation process. For this reason, the EPA has historically
undertaken rulemaking as part of the NAAQS promulgation process to
adjust the generic deadlines in sections 50.14(c)(2)(iii) and
50.14(c)(3)(i) of the Exceptional Events Rule to accommodate the
initial area designation process and schedule under a new or revised
NAAQS.
    The Exceptional Events Rule at section 50.14(c)(2)(vi) indicates
``when EPA sets a NAAQS for a new pollutant or revises the NAAQS for an
existing pollutant, it may revise or set a new schedule for flagging
exceptional event data, providing initial data descriptions and
providing detailed data documentation in AQS for the initial
designations of areas for those NAAQS.'' The EPA intends to issue its
final action promulgating a revised O3 NAAQS or determine
that it is not necessary to do so in October 2015.
    The CAA provides requirements regarding the schedule for initial
area designations. Section 107(d)(1) of the CAA states that, ``By such
date as the Administrator may reasonably require, but not later than 1
year after promulgation of a new or revised national ambient air
quality standard for any pollutant under section 109, the Governor of
each state shall . . . submit to the Administrator a list of all areas
(or portions thereof) in the State, designating . . . '' those areas as
nonattainment, attainment, or unclassifiable.\229\ No later than 120
days prior to promulgating designations, the EPA is required to notify
states of any intended modifications to their designation
recommendations as the EPA may deem necessary. Section 107(d)(1)(B)(i)
further provides, ``Upon promulgation or revision of a NAAQS, the
Administrator shall promulgate the designations of all areas (or
portions thereof) . . . as expeditiously as practicable, but in no case
later than 2 years from the date of promulgation. Such period may be
extended for up to one year in the event the Administrator has
insufficient information to promulgate the designations.'' As described
in more detail in section VII.C of this proposal, the EPA intends to
complete designations for any revised O3 NAAQS promulgated
in 2015 following the standard 2-year process. The EPA is required by
Court Order to take final action for this O3 NAAQS review no
later than October 1, 2015. The EPA does not intend to establish a date
earlier than the 1 year submission period provided in CAA section
107(d)(4); thus, state Governors (and tribes, if they choose) would be
required to submit their initial designation recommendations for any
revised NAAQS no later than 1 year after promulgation (i.e., by October
1, 2016, if the EPA promulgates a revised NAAQS on October 1, 2015).
State Governors (and tribes, if they choose) would likely use air
quality data from the years 2013 to 2015 as the basis for their
recommendations. The EPA would notify states and tribes of intended
modifications to their recommendations no later than June 2017 and the
EPA would promulgate initial designations for any revised NAAQS in
October 2017. We anticipate that the EPA's notification of intended
modifications and the final designations would be based on air quality
data from the years 2014 to 2016, because air quality data from 2016 is
required to be certified by the state no later than May 1, 2017, and
thus would be available for consideration for purposes of initial area
designations by October 2017.
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    \229\ While the CAA says ``designating'' with respect to the
Governor's letter, in the full context of the CAA section it is
clear that the Governor actually makes a recommendation.
---------------------------------------------------------------------------

    As indicated above, and as explained in additional detail in
section VII.C of this preamble, section 107(d)(1)(B) of the CAA allows
the Administrator to extend the designations schedule for up to 1 year
in the event the Administrator has insufficient information to
promulgate the designations for a newly promulgated NAAQS. If the EPA
were to determine that it is necessary to extend the schedule for
designating areas for a revised O3 NAAQS (promulgation in
October 2015) from 2017 to 2018, then it is possible that air quality
data from 2017 could be considered for designations. This could raise
concerns about whether influences from exceptional events in 2017 could
be investigated and submitted by the state and reviewed by the EPA in
sufficient time for consideration during the designation process.
    For purposes of initial designations, where the EPA considers the
most recent air quality monitoring data in a relatively quick
timeframe, the EPA is proposing revisions to the flagging and data
submission schedule in 40 CFR 50.14 applicable to the initial area
designations process. The proposed exceptional events schedule is based
on following a standard 2 year designation process. However, because
the CAA also provides for a 3-year process in the event the
Administrator has insufficient information to promulgate the
designations for a newly promulgated NAAQS within 2 years and provides
for the promulgation of designations as ``expeditiously as
practicable,'' which could include accelerating the designations
schedule ahead of the 2-year schedule, the proposed exceptional event
schedule also includes provisions for both an accelerated designations
process and a 3-year process. If the EPA were to pursue a designations
schedule other than a 2- or 3-year process, the EPA would notify the
state Governors of the intended date for final designations through
notification letters, guidance and/or Federal Register notices.
    These proposed revised exceptional event scheduling provisions
would, if promulgated, apply to submission of information supporting
claimed exceptional events affecting pollutant data for initial area
designations under any new or revised NAAQS, including any revised
O3 NAAQS promulgated in October 2015. The general data
flagging deadlines in the Exceptional Events Rule at 40 CFR
50.14(c)(2)(iii) and the general schedule for submission of
demonstrations at 40 CFR 50.14(c)(3)(i) would continue to apply to
regulatory decisions other than those related to the initial area
designations process under a new or revised NAAQS. The EPA believes
these proposed revisions to the exceptional events scheduling
provisions will provide adequate time for states to determine whether
data have been influenced by an exceptional event, to notify the EPA by
flagging the relevant data and providing an initial description in AQS,
and to submit documentation to support claims for exceptional events.
    Therefore, using the authority provided in CAA section 319(b)(2),
the EPA proposes to modify the schedule for data flagging and
submission of demonstrations for exceptional events data considered for
initial area designations by replacing the deadlines and information in
Table 1 in 40 CFR 50.14 with the deadlines and information presented in
Table 9. The EPA is also providing Table 10 to illustrate how the
proposed schedule might apply to the designations process for any
revised O3 NAAQS promulgated in October 2015 or to
designations processes for future new or revised NAAQS. The EPA invites
comment on these proposed changes, shown in Table 9, to the exceptional
event data flagging and documentation submission deadlines for future
new or revised

NAAQS, including any revised O3 NAAQS promulgated in 2015.

      Table 9--Proposed Schedule for Exceptional Event Flagging and
      Documentation Submission for Data To Be Used in Initial Area
                              Designations
------------------------------------------------------------------------
                                                      Exceptional event
        Exceptional event/regulatory action           deadline schedule
                                                             \d\
------------------------------------------------------------------------
Exceptional event data flagging and initial         If state and tribal
 description deadline for data years 1, 2 and 3      initial designation
 \a\.                                                recommendations for
                                                     the new/revised
                                                     NAAQS are due
                                                     August through
                                                     January, then the
                                                     flagging and
                                                     initial description
                                                     deadline will be
                                                     the July 1 prior to
                                                     the recommendation
                                                     deadline. If state
                                                     and tribal
                                                     recommendations for
                                                     the new/revised
                                                     NAAQS are due
                                                     February through
                                                     July, then the
                                                     flagging and
                                                     initial description
                                                     deadline will be
                                                     the January 1 prior
                                                     to the
                                                     recommendation
                                                     deadline.
Exceptional event demonstration submittal deadline  No later than the
 for data years 1, 2 and 3 \a\.                      date that state and
                                                     tribal
                                                     recommendations are
                                                     due to EPA.
Exceptional event data flagging, initial            By the last day of
 description, and exceptional event demonstration    the month that is 1
 submittal deadline for data year 4 \b\ and          year and 7 months
 potential data year 5 \c\.                          after promulgation
                                                     of a new or revised
                                                     NAAQS, unless
                                                     either option a or
                                                     b applies.
                                                    a. If the EPA
                                                     follows a 3-year
                                                     designation
                                                     schedule, the
                                                     deadline is 2 years
                                                     and 7 months after
                                                     promulgation of a
                                                     new or revised
                                                     NAAQS.
                                                    b. If the EPA
                                                     notifies the state/
                                                     tribe via Federal
                                                     Register notice,
                                                     letter or guidance
                                                     that it intends to
                                                     complete the
                                                     initial area
                                                     designations
                                                     process according
                                                     to a schedule other
                                                     than a 2-year or 3-
                                                     year timeline, the
                                                     deadline is 5
                                                     months prior to the
                                                     date specified for
                                                     final designations
                                                     decisions in such
                                                     EPA notification.
------------------------------------------------------------------------
\a\ Where data years 1, 2, and 3 are those years expected to be
  considered in state and tribal recommendations.
\b\ Where data year 4 is the additional year of data that the EPA may
  consider when it makes final area designations for the new/revised
  NAAQS under the standard designations schedule.
\c\ Where data year 5 is the additional year of data that the EPA may
  consider when it makes final area designations for the new/revised
  NAAQS under an extended designations schedule.
\d\ The date by which air agencies must certify their ambient air
  quality monitoring data in AQS is annually on May 1 of the year
  following the year of data collection. The EPA cannot require air
  agencies to certify data prior to this date. In some cases, however,
  air agencies may choose to certify a prior year's data in advance of
  May 1 of the following year, particularly if the EPA has indicated its
  intent to promulgate final designations in the months of May, June,
  July or August. Exceptional event flagging, initial description, and
  demonstration deadlines for ``early certified'' data will follow the
  deadlines for ``year 4'' and ``year 5'' data.

[GRAPHIC] [TIFF OMITTED] TP17DE14.000

[GRAPHIC] [TIFF OMITTED] TP17DE14.001

    Additionally, in conjunction with proposing exceptional event
schedules related to implementing any revised O3 standards,
the EPA is also proposing to remove obsolete regulatory language
associated with exceptional event

schedules for historical standards. The EPA expects to propose
additional revisions to the Exceptional Events Rule in a future notice
and comment rulemaking effort and will solicit public comment on other,
non-schedule related, aspects of the Exceptional Events Rule at that
time.

VI. Ambient Monitoring Related to Proposed O3 Standards

A. Background

    The EPA is proposing to: Revise the state-by-state O3
monitoring seasons; revise the PAMS monitoring requirements; revise the
FRM for measuring O3; and revise the FEM testing
requirements. The EPA is also proposing to make additional minor
changes to the FEM testing requirements for NO2 and
particulate matter in part 53 as discussed below.
    The EPA is proposing to extend the length of the required
O3 monitoring season in some states to be appropriate for
the O3 NAAQS revision finalized in 2008, as well as a final
revised O3 standard, if a revision is finalized in 2015.
    The EPA is proposing to make changes to the PAMS monitoring
requirements in 40 CFR part 58, Appendix D section 5. Section VI.C of
this preamble provides background on the current PAMS monitoring
requirements, recent efforts to re-evaluate the current PAMS
requirements, and a summary of the proposed PAMS requirement revisions.
    The EPA is proposing to revise the FRM to establish a new,
additional technique for measuring O3 in the ambient air.
This new technique is based on nitric oxide-chemiluminescence (NO-CL)
methodology. Because of the similarity of this new chemiluminescence
technique to the existing ethylene-chemiluminescence (ET-CL)
methodology, the EPA proposes that it be incorporated into the existing
O3 FRM, using the same calibration procedure. Appendix D of
40 CFR part 50 would be revised to include both the original ET-CL as
well as the new NO-CL methodology. A minor change is proposed to the
existing O3 FRM calibration procedure, which would be
applicable to both of the chemiluminescence FRM methodologies. The
proposed change in section <IP_ADDRESS> of the calibration procedure in
appendix D provides for more flexibility in the range of the linearity
test.
    The only substantial changes proposed to the requirements of 40 CFR
part 53 are in Tables B-1 and B-3 of subpart B. Table B-1 has been
updated in recent years with regard to FRM and FEM methods for
SO2 (74 FR 64877, December 8, 2009) and CO (76 FR 54294,
August 31, 2011) to be more consistent with current analyzer
performance capabilities. Similar changes to Table B-1 are proposed
here for methods for O3. Modest changes to Table B-3 would
add new interferent test concentrations specifically for NO-CL
analyzers, adding a test for NO2. Also, the table would
clarify that the existing test concentrations apply to ET-CL
O3 analyzers.
    In addition, the EPA is making minor additional changes to Part 53
including: conforming changes to the FEM testing requirements in Table
B-1 and Figure B-5 for NO2; extending the period of time for
the Administrator to take action on a request for modification of a FRM
or FEM from 30 days to 90 days; and removing an obsolete provision for
manufacturers to submit Product Manufacturing Checklists for certain PM
monitors.

B. Revisions to the Length of the Required O3 Monitoring
Seasons

    Unlike the ambient monitoring requirements for other criteria
pollutants that mandate year-round monitoring, O3 monitoring
is only required during the seasons of the year that are conducive to
O3 formation. These seasons vary in length from place to
place as the conditions conducive to the formation of O3
(i.e., seasonally-dependent factors such as ambient temperature,
strength of solar insolation, and length of day) differ by
location.\230\ In some locations, conditions conducive to O3
formation are limited to the summer months of the year. For example, in
states with colder climates such as Montana and South Dakota, the
currently required O3 monitoring season is four months long.
However, in other states with warmer climates such as California,
Nevada, and Arizona, the currently required O3 monitoring
season is year-round.\231\
---------------------------------------------------------------------------

    \230\ See 40 CFR part 58 Appendix D, section 4.1, Table D-3 for
a table of required O3 seasons.
    \231\ Certain states, such as California and Arizona, have
approved shorter seasons for a subset of O3 sites, based
on Regional Administrator review and approval (see 40 CFR part 58,
Appendix D, section 4.1(i) for the waiver authority).
---------------------------------------------------------------------------

    Based on the O3 NAAQS revision that was finalized in
2008, as well as the proposed NAAQS revisions discussed in this
rulemaking, the EPA has determined that lengthening the O3
monitoring seasons may be appropriate. Ambient O3
concentrations could approach or exceed the level of the 2008 NAAQS, as
well as the proposed NAAQS, more frequently and during more months of
the year. The EPA has done an analysis to address the issue of whether
extensions of currently required monitoring seasons are appropriate
(Rice, 2014). In this analysis, we determined the number of days where
one or more monitors had a daily maximum 8-hour O3 average
equal to or above 0.060 ppm in the months outside the currently-
required state O3 monitoring season using data from monitors
that collected O3 data year-round in 2010-2013.\232\ We find
that this level, taking into consideration reasonable uncertainty,
serves as an appropriate indicator of ambient conditions that may be
conducive to the formation of O3 concentrations that
approach or exceed the 2008 NAAQS or the proposed 8-hour average range
of 0.065 to 0.070 ppm. Although we refer to these days as ``exceedance
days'' in the analysis, this 0.060 ppm threshold is simply a
conservative benchmark that is below the levels proposed for the
revised NAAQS. Proposals for revising each state's required monitoring
season are based on the observed ``exceedance days'' where the 8-hour
average daily maximum was >=0.060 ppm in and surrounding the state. The
EPA considered a number of factors including out-of-season ``exceedance
days'' either before or after the current O3 monitoring
season, the pattern of ``exceedance days'' in the out-of-season months,
and regional consistency. We note that seasonal O3 patterns
vary year-to-year due primarily to highly variable meteorological
conditions conducive to the formation of early or late season elevated
O3 concentrations in some years and not others. The EPA
believes it is important that O3 monitors operate during all
periods when there is a reasonable possibility of ambient levels
approaching the level of the proposed NAAQS.
---------------------------------------------------------------------------

    \232\ Approximately 800 O3 monitors are currently
operated year-round, representing greater than 50% of the total
O3 monitoring network of about 1500 monitors. They
include monitors that are mandated to operate year-round due to the
required O3 season and other monitors that are
voluntarily operated year-round by states and other organizations
including EPA-operated monitors at Clean Air Status and Trends
Network (CASTNET) sites.
---------------------------------------------------------------------------

    The EPA reviewed the year-round, O3 data for 2010
through 2013. A year-round monitor was identified as ``year-round'' if
it had at least 20 daily observations in all 12 months, for at least 1
year of the 4 year period. During the 2010-2013 data period, all states
operated a portion of their monitoring network outside of their
required O3 monitoring season and reported the data to the
EPA Air Quality System (AQS).

The EPA's analysis found the frequency of observed ``exceedance days''
of daily maximum 8-hour average O3 readings of >=0.060 ppm
to be quite high in several states across the country in months outside
of the currently required monitoring season. A total of 43 states
experienced at least one ``exceedance day'' outside of their current
O3 season; 21 states had ``exceedance days'' only before the
required monitoring season; 4 states had ``exceedance days'' only after
the required monitoring season; and 18 states had ``exceedance days''
both before and after the required monitoring season. In some cases,
the frequency of ``exceedance days'' before the current O3
season was high, with four states (South Dakota, Colorado, Wyoming, and
Utah) experiencing between 31 and 230 out-of-season ``exceedance days''
from 2010 to 2013 at monitors operating year-round.
    Basing O3 monitoring season requirements on the goal of
ensuring monitoring when ambient O3 levels approach or
exceed the level of the proposed NAAQS supports established monitoring
network objectives described in Appendix D of Part 58, including the
requirement to provide air pollution data to the general public in a
timely manner \233\ and to support comparisons of an area's air
pollution levels against the NAAQS. The EPA believes that frequency of
``exceedance days'' in which daily maximum of 8-hour O3
levels are observed to be greater than or equal to a threshold level of
0.060 ppm in months outside the currently required O3
monitoring season supports the proposed lengthening of the
O3 monitoring season requirements for certain states.
---------------------------------------------------------------------------

    \233\ Public reporting requirements are detailed in 40 CFR part
58 Appendix G, Uniform Air Quality Index (AQI) and Daily Reporting.
---------------------------------------------------------------------------

    The operation of O3 monitors during periods of time when
ambient levels approach or exceed the level of the proposed NAAQS
ensures that persons unusually sensitive to O3 are alerted
to potential levels of health concern allowing them to take
precautionary measures. The majority of O3 monitors in the
U.S. report to AIRNOW,\234\ as well as to state-operated Web sites and
automated phone reporting systems. These programs support many
objectives including real-time air quality reporting to the public,
O3 forecasting programs, and the verification of real-time
air quality forecast models.
---------------------------------------------------------------------------

    \234\ See http://airnow.gov/.
---------------------------------------------------------------------------

    The specific proposed changes to the required state O3
monitoring seasons are detailed in the proposed changes to Table D-3 of
40 CFR part 58, Appendix D (O3 Monitoring Season by State).
Although 43 states had at least one exceedance day outside the current
monitoring season, changes are proposed for only 33 of those states.
These proposed changes would entail an increase of 1 month for 23
states (Connecticut, Delaware, District of Columbia, Idaho, Illinois,
Iowa, Kansas, Maryland, Massachusetts, Minnesota, Missouri, Nebraska,
New Hampshire, New Jersey, New York, North Carolina, Ohio,
Pennsylvania, Rhode Island, South Carolina, Northern Texas, Virginia,
and West Virginia), an increase of one and one half months for
Wisconsin, an increase of two months for four states (Indiana,
Michigan, Montana, and North Dakota), an increase of four months for
Florida and South Dakota, an increase of five months for Colorado, and
an increase of seven months for Utah. For Wyoming, we are proposing to
add three months at the beginning of the season and remove one month at
the end of the season, resulting in a net increase of two months. Ozone
season requirements are currently split by Air Quality Control Region
(AQCR) in Louisiana and Texas. Included in the state-by-state
accounting is the proposal to lengthen the required season in the
northern part of Texas (AQCR 022, 210, 211, 212, 215, 217, and 218) by
one month. Southern Texas O3 monitors in AQCRs 106, 153,
213, 214, and 216 would remain on a year-round schedule. In some states
with limited available data and few exceedance days outside the current
season, proposed changes were made by considering regional consistency
and using supporting information from the surrounding states; these
changes were all minor, involving the proposed addition of 1 month to
the current required season in Iowa, Missouri, and West Virginia.
    The EPA solicits comment on the proposed changes to the required
O3 monitoring seasons. We note that EPA Regional
Administrators have previously approved certain deviations from the
required O3 monitoring seasons through rulemakings (64 FR
3028, January 20, 1999; 67 FR 57332, September 10, 2002; and 69 FR
52836, August 30, 2004). The current ambient monitoring rule, in
paragraph 4.1(i) of 40 CFR part 58 Appendix D (71 FR 61319, October 17,
2006), allows the EPA Regional Administrators to approve changes to the
O3 monitoring season without rulemaking. The EPA is
retaining the rule language allowing such deviations from the required
O3 monitoring seasons in the proposed revision to paragraph
4.1(i) of 40 CFR part 58, Appendix D. The proposed changes to
O3 monitoring season requirements, if finalized, will revoke
previous Regional Administrator-granted waiver approvals. As
appropriate, monitoring agencies could seek new waivers. Post-final
rule requests submitted along with relevant supporting information by
states for monitoring season waivers from the revised requirements will
be reviewed by Regional Administrators using, at a minimum, occurrences
of the moderate AQI level, the frequency of out-of-season O3
NAAQS exceedances, and regional consistency. Any deviations based on
the Regional Administrator's waiver of requirements must be described
in the state's annual monitoring network plan and updated in the AQS.
    Current regulations permit O3 monitors located at NCore
multi-pollutant stations to be counted toward meeting minimum network
monitoring requirements. The NCore network requirements were
promulgated in the October 17, 2006 (71 FR 61317) revisions to ambient
monitoring regulations in order to build a long-term, nationwide
network that supports multiple objectives including air quality trends
analyses, model evaluation, ecosystem studies, and assessment of
transport between urban and rural areas. In the 2006 rulemaking, the
EPA did not propose a different O3 monitoring season for
NCore stations.
    NCore stations are required to operate a full suite of gaseous and
particulate matter monitors as well as basic meteorology to support the
objectives. Given the potential value of NCore data to support year-
round scientific studies, the EPA believes that it is appropriate to
require O3 monitors at NCore stations to be operated year-
round. Accordingly, the EPA proposes that the required monitoring
season for NCore stations be January through December regardless of the
length of the required O3 monitoring season for the
remainder of the SLAMS (State and Local Air Monitoring Stations)
monitors within a state.
    The EPA has estimated the cost of the proposed changes to the
O3 seasons. The results are detailed in the EPA ICR #2313.03
and summarized in Section VIII.B., ``Paperwork Reduction Act''. The
estimated cost is $1,668,433 which is about 7% of the total average
annual cost of $24,115,182 for the national O3 monitoring
network. This estimate is based on the current requirements in 40 CFR
part 58 and the proposed requirements in this rule. We note however,
that greater than 50% of the monitors are currently operated year-round
due to existing requirements, as

well as other monitors that are voluntarily operated year-round by the
states. Taking into consideration the number of year-round
O3 monitors that are operated due to existing requirements,
as well as on a discretionary basis by states, the incremental cost of
these proposed changes is reduced from $1,668,433 to approximately
$230,000, which is less than 1% of the total average annual cost of the
national O3 monitoring network.
    Considering the timing of this proposal and the final rulemaking
(court ordered deadline of October 1, 2015) and associated burden on
state/local monitoring agencies, we propose that implementation of the
revised O3 seasons become effective at SLAMS (including
NCore sites) on January 1, 2017. The EPA is proposing to add paragraph
58.13 (g) of 40 CFR part 58 to require that monitors operating under
the requirements of section 4.1 of 40 CFR part 58, Appendix D operate
on the applicable required O3 monitoring seasons effective
January 1, 2017 as listed in Table D-3 of appendix D to this part. We
solicit comment on whether the revised seasons could be implemented
beginning January 1, 2016 for all monitors or for a subset of monitors,
such as those currently operating year-round or on a schedule that
corresponds to the proposed O3 season. If we determine,
based on any such comments that implementation could occur earlier in
such cases, we could proceed to final action requiring earlier
implementation.

C. Revisions to the Photochemical Assessment Monitoring Stations (PAMS)

    Section 182 (c)(1) of the CAA required the EPA to promulgate rules
for enhanced monitoring of O3, oxides of nitrogen, and VOCs
for nonattainment areas classified as serious (or above) to obtain more
comprehensive and representative data on O3 air pollution.
In addition, Section 185B of the CAA required the EPA to work with the
National Academy of Sciences (NAS) to conduct a study on the role of
O3 precursors in tropospheric O3 formation and
control. In 1992, the NAS issued the report entitled, ``Rethinking the
Ozone Problem in Urban and Regional Air Pollution'', (NAS, 1991).
    In response to the CAA requirements and the recommendations of the
NAS report, on February 12, 1993 (58 FR 8452), the EPA revised the
ambient air quality surveillance regulations to require PAMS in each
O3 nonattainment area classified as serious, severe, or
extreme (``PAMS areas'').\235\ As noted in EPA's Technical Assistance
Document (TAD) for Sampling and Analysis of Ozone Precursors (U.S. EPA,
1998), the objectives of the PAMS program are to: (1) Provide a
speciated ambient air database which is both representative and useful
in evaluating control strategies and understanding the mechanisms of
pollutant transport by ascertaining ambient profiles and distinguishing
among various individual VOCs; (2) provide local, current
meteorological and ambient data to serve as initial and boundary
condition information for photochemical grid models; (3) provide a
representative, speciated ambient air database which is characteristic
of source emission impacts to be used in analyzing emissions inventory
issues and corroborating progress toward attainment; (4) provide
ambient data measurements which would allow later preparation of
unadjusted and adjusted pollutant trends reports; (5) provide
additional measurements of selected criteria pollutants for attainment/
nonattainment decisions and to construct NAAQS maintenance plans; and
(6) provide additional measurements of selected criteria and non-
criteria pollutants to be used for evaluating population exposure to
air toxics as well as criteria pollutants.
---------------------------------------------------------------------------

    \235\ Additional information on the O3 designation
process can be obtained at EPA's O3 designations Web page
at http://www.epa.gov/groundlevelozone/designations/.
---------------------------------------------------------------------------

    The original PAMS requirements called for two to five sites per
area depending on the area's population. Four types of PAMS sites were
identified including upwind (Type 1), maximum precursor emission rate
(Type 2), maximum O3 (Type 3), and extreme downwind (Type 4)
sites. Each PAMS site was required to measure O3, NO,
NO2, speciated VOCs, selected carbonyl compounds, and
selected meteorological parameters. In addition, upper air
meteorological monitoring was required at one site in each PAMS area.
    In the October 17, 2006 monitoring rule (71 FR 61267), the EPA
revised the PAMS requirements to only require two PAMS sites per PAMS
area.\236\ The intent of the revision was to ``allow PAMS monitoring to
be more customized to local data needs rather than meeting so many
specific requirements common to all subject O3 nonattainment
areas; the PAMS changes would also give states the flexibility to
reduce the overall size of their PAMS programs--within limits--and to
use the associated resources for other types of monitoring they
consider more useful.'' In addition to reducing the number of required
sites per PAMS area, the 2006 revisions also limited the requirement
for carbonyl measurements (specifically formaldehyde, acetaldehyde, and
acetone) to areas classified as serious or above for the 8-hour
O3 standard. This change was made in recognition of carbonyl
sampling issues which were believed to cause significant uncertainty in
the measured concentrations.
---------------------------------------------------------------------------

    \236\ One Type 2 site and either a Type 1 or a Type 3 site are
currently required.
---------------------------------------------------------------------------

    Twenty-two areas were classified as serious or above O3
nonattainment at the time the PAMS requirements were promulgated in
1993. On July 18, 1997 (62 FR 38856), the EPA revised the averaging
time of the O3 NAAQS from a 1-hour averaging period to an 8-
hour averaging period. On June 15, 2005 (70 FR 44470), the EPA revoked
the 1-hour standard in most areas of the country; however, PAMS
requirements were identified as requirements that had to be retained in
the anti-backsliding provisions\237\ included in that action.
Therefore, PAMS requirements continue to be applicable to areas that
were classified as serious or above nonattainment for the 1-hour
O3 standard as of June 15, 2004. Currently, 25 areas are
subject to the PAMS requirements with a total of 75 sites. As will be
discussed in detail later, the current PAMS sites are concentrated in
the North East and California with relatively limited coverage in the
rest of the country (Cavender, 2014).
---------------------------------------------------------------------------

    \237\ Refer to 40 CFR part 51.905
---------------------------------------------------------------------------

    As discussed above, the first PAMS sites began operation in 1994,
and have been in operation for over 20 years. Many changes have
occurred during that time that have changed the O3 problem
in the U.S. as well as our understanding of it. The O3
standard has been revised multiple times since the PAMS program was
first implemented. On July 18, 1997, the EPA revised the O3
NAAQS to a level of 0.08 ppm, with a form based on the 3-year average
of the annual fourth-highest daily maximum 8-hour average O3
concentration. On March 28, 2008 (73 FR 16436), the EPA revised the
O3 standard to a level of 0.075 ppm, with a form based on
the 3-year average of the annual fourth-highest daily maximum 8-hour
average O3 concentration. These changes in the level and
form of the O3 NAAQS, along with notable decreases in
O3 levels in most parts of the U.S., have changed the
landscape of the O3 problem in the U.S. At the time of the
first round of designations for the 8-hour standard

(June 15, 2005), only five areas were classified as serious or above
for the 8-hour standard as compared to 22 areas that were classified as
serious or above for the 1-hour standard.\238\ While the number of
serious and above areas decreased, the number of nonattainment areas
remained nearly the same. In addition, much of the equipment used at
PAMS sites is old and in need of replacement. New technologies have
been developed since the inception of the PAMS program that should be
considered for use in the network. For these reasons, the EPA
determined that it would be appropriate to re-evaluate the PAMS program
and associated requirements in light of current O3 issues.
---------------------------------------------------------------------------

    \238\ PAMS requirements continue to apply to all areas
classified as serious or above as of June 15, 2005 due to anti-
backsliding provisions of 40 CFR 51.905.
---------------------------------------------------------------------------

    In 2011 (U.S. EPA, 2011c), the EPA initiated an effort to re-
evaluate the PAMS requirements in light of changes in the needs of PAMS
data users and the improvements in monitoring technology. The EPA
consulted with CASAC's, Air Monitoring and Methods Subcommittee (AMMS)
to seek advice on potential revisions to the technical and regulatory
aspects of the PAMS program; including changes to required measurements
and associated network design requirements. The EPA also requested
advice on appropriate technology, sampling frequency, and overall
program objectives in the context of the most recently revised
O3 NAAQS and changes to atmospheric chemistry that have
occurred over the past 10-15 years in the significantly impacted areas.
The CASAC AMMS met on May 16 and May 17, 2011, and provided a report
with their advice on the PAMS program on September 28, 2011 (U.S. EPA,
2011c). In addition, the EPA met multiple times with the National
Association of Clean Air Agencies (NACAA) Monitoring Steering Committee
(MSC) to seek advice on the PAMS program. The MSC includes monitoring
experts from various state and local agencies actively engaged in
ambient air monitoring and many members of the MSC have direct
experience with running PAMS sites. As discussed in more detail in the
appropriate sections below, the EPA took into consideration advice from
the CASAC AMMS and the MSC in proposing changes to the PAMS
requirements.
    Based on the findings of the PAMS evaluation and the consultations
with the CASAC AMMS and NACAA MSC, the EPA is proposing to revise
several aspects of the PAMS monitoring requirements including changes
in 1) network design, 2) VOC sampling, 3) carbonyl sampling, 4)
nitrogen oxides sampling, and 5) upper air meteorology measurements.
The following paragraphs describe the changes being proposed including
the rationale for the proposed changes. Timing and other implementation
issues associated with these proposed changes are discussed at the end
of this section.
1. Network Design
    As discussed above, the current PAMS network design calls for two
sites (a Type 2, and a Type 1 or Type 3) per PAMS area. In their report
(U.S EPA, 2011c), the CASAC AMMS found ``that the existing uniform
national network design model for PAMS is outdated and too resource
intensive,'' and recommended ``that greater flexibility for network
design and implementation of the PAMS program be transferred to state
and local monitoring agencies to allow monitoring, research and data
analysis to be better tailored to the specific needs of each
O3 problem area.'' While stating that the current PAMS
objectives were appropriate, the AMMS report also stated that
``objectives may need to be revised to include both a national and
regional focus because national objectives may be different from
regional objectives.'' The NACAA MSC also advised the EPA that the
existing PAMS requirements were too prescriptive and may hinder state
efforts to collect other types of data that were more useful in
understanding their local O3 problems.
    The EPA agrees with CASAC that the PAMS objectives include both
local and national objectives, and believes that the current PAMS
network design is no longer suited for meeting either sets of
objectives. As part of the PAMS evaluation, it was determined that at
the national level the primary use of the PAMS data has been to
evaluate photochemical model performance. Due to the locations of the
current PAMS areas and the current network design, existing PAMS sites
are clustered along the northeast and west coasts leading to
significant redundancy in these areas and very limited coverage
throughout the remainder of the country (Cavender, 2014). The resulting
uneven spatial coverage greatly limits the value of the PAMS data for
evaluation of model performance. CASAC (U.S. EPA, 2011c) noted the
spatial coverage issue and advised that EPA should consider requiring
PAMS measurements in areas in addition to ``areas classified as serious
and above for the O3 NAAQS to improve spatial coverage.''
The EPA also agrees with CASAC and the NACAA that the PAMS requirements
should be revised to provide monitoring agencies greater flexibility in
meeting local objectives.
    The EPA is proposing changes to the network design requirements
that we believe will better serve both national and local objectives.
The EPA is proposing a two part network design. The first part of the
design includes a network of fixed sites (required PAMS sites) intended
to support O3 model development and the tracking of trends
of important O3 precursor concentrations. The second part of
the network design includes monitoring agency directed Enhanced
Monitoring Plans which allow monitoring agencies the needed flexibility
to implement additional monitoring capabilities to suit the needs of
their area.
    The EPA considered a number of options to revise the fixed site
portion of the network design (Cavender, 2014). An initial option
considered was to require all NCore sites to make PAMS measurements
regardless of O3 attainment status. This option would take
advantage of the existing NCore infrastructure and would result in a
relatively wide geographic distribution of sites. However, it was noted
that this option would place some PAMS measurements in areas with
relatively low O3 levels and would also result in a network
of approximately 80 required sites, which would strain existing
resources with a somewhat larger network than the current situation,
and could make it difficult to also implement the desired state-
directed Enhanced Monitoring Plans. The second option considered was to
require only NCore sites in O3 nonattainment areas to
collect PAMS measurements. This option would provide the benefits
discussed above for collecting PAMS measurements at existing NCore
sites. This option would also reduce the total number of sites required
and focus efforts in areas with higher, non-attaining, levels of
O3. The final option considered would add a population limit
in addition to the consideration of O3 attainment status at
NCore sites. An illustration of this example would be a PAMS
requirement that applied only to NCore sites in O3
nonattainment areas with a population greater than a given threshold,
for example, Core Based Statistical Areas with 1,000,000 people or
more. This approach would continue the current practice of focusing
PAMS resources in areas of elevated O3 readings with an
additional consideration that measurements in these larger population
areas would be

sufficient to characterize O3 formation on a national basis.
    After considering the above options as well as the comments of
CASAC and NACAA, the EPA believes that an approach focused primarily on
the use of the existing NCore sites in O3 nonattainment
areas provides an appropriate balance to the consideration of
O3 levels as well as population, noting that a majority of
NCore sites are already located in the larger urban areas of each
state. Accordingly, the EPA is proposing to require PAMS measurements
at any existing NCore site in an O3 nonattainment area
(either based on the 2008 O3 NAAQS or the 2015 O3
NAAQS if finalized) in lieu of the current PAMS network design
requirements.\239\ The NCore network is a multi-pollutant monitoring
network consisting of 80 sites (63 urban, 17 rural) and is intended to
support multiple air quality objectives including the development and
model evaluation of photochemical models (including both
PM2.5 and O3 models), and the tracking of
regional precursor trends. NCore sites are sited in typical
neighborhood scale locations which are more suitable than source
impacted locations for evaluation of grid models typical of current
photochemical models and tracking of trends in pre-cursor
concentrations. The EPA believes NCore sites are well suited for
O3 model development and evaluation.
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    \239\ Although enhanced monitoring for O3, oxides of
nitrogen, and VOCs is specifically required for areas classified at
least serious for the O3 NAAQS by section 182(c)(1) of
the CAA, the EPA has concluded that requiring enhanced monitoring
for all O3 nonattainment areas is appropriate for the
purposes of monitoring ambient air quality and better understanding
O3 pollution.
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    The proposal to require PAMS measurements at existing NCore sites
in O3 nonattainment areas would replace the existing PAMS
network design.\240\ This change would keep roughly the same number of
required PAMS sites while improving spatial coverage (Cavender, 2014).
Based on the range of the O3 NAAQS being proposed today and
current O3 design value estimates (based on 2011-2013 air
quality data), the number of required sites is estimated to be between
48 and 65, which compares to 50 currently required sites, and 75
currently operating sites. Potential redundancy in the existing network
would be reduced while important network coverage in the Southeast and
Midwest would be added. The improved spatial coverage will also improve
the EPA's ability to track trends in precursor concentrations
regionally. The EPA notes that in limited situations, an O3
nonattainment area may not have an NCore site and in those cases, the
area would only be subject to the requirement for an Enhanced
Monitoring Plan as discussed in more detail below. The EPA believes
that the network coverage provided by existing NCore sites in
O3 nonattainment areas would be adequate for the national
PAMS objectives discussed above, and that requiring PAMS sites, in
addition to Enhanced Monitoring Plans, in those O3
nonattainment areas without NCore sites would not substantially improve
the network coverage.
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    \240\ While the EPA is proposing to replace the multi-site
design, monitoring agencies would be encouraged to identify the type
of PAMS site the NCore site represents. In most cases, NCore sites
would likely be classified as either a Type 2 or Type 3 site. In
limited situations, rural NCore sites might be subject to these
proposed requirements, in which case, these sites would likely be
either Type 1 or Type 4 sites.
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    The EPA notes that the proposed network design change would provide
significant cost efficiencies. By adding PAMS measurements to existing
NCore sites, the PAMS network would be taking advantage of existing
infrastructure and measurements currently being collected at NCore
sites. NCore sites already have the larger, climate-controlled shelters
that are necessary to operate the automated gas chromatographs (``auto-
GCs'') used to collect speciated VOCs. In addition, existing NCore
sites currently collect data on many of the required PAMS measurements
including O3, CO, total reactive nitrogen (NOy),
and meteorological measurements including wind speed and direction,
temperature, and relative humidity.
    While the EPA believes these proposed changes will result in fixed
network cost savings for the overall network on a national basis,
individual monitoring agencies may see either an increase or a decrease
in burden as a result of these proposed changes. Monitoring agencies in
O3 nonattainment areas who are not currently affected by the
existing PAMS requirements would be required to add PAMS measurements
to their existing NCore sites, while several monitoring agencies with
existing PAMS sites would not be required to continue PAMS monitoring
if these proposed requirements are promulgated.\241\ As discussed later
in this preamble, the EPA is proposing a staggered compliance schedule
for the proposed PAMS requirements in recognition of the need for
capital investment and staff training at these sites.
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    \241\ Monitoring agencies would be able to seek approval to shut
down non-required PAMS sites at their discretion pursuant to the
requirements detailed in 40 CFR 58.14.
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    The EPA recognizes that in limited situations, existing NCore sites
may not be the most appropriate locations for making PAMS measurements.
For example, an existing PAMS site in an O3 nonattainment
area may be sited at a different location than the existing NCore site.
In this case, it may be appropriate to continue monitoring at the
existing PAMS site to support ongoing research and to maintain trends
information. To account for these situations, the EPA is also proposing
to provide the EPA Regional Administrator the authority to approve an
alternative location for a required PAMS sites where appropriate.
    The EPA seeks comment on the network design revision proposed
above, the requirement for PAMS measurements at NCore sites in
O3 nonattainment areas, and the removal of current multi-
site PAMS network design requirements. The EPA also solicits comment on
whether, instead of requiring PAMS measurements at all NCore sites in
nonattainment areas, we should instead adopt one of the other options
discussed above, for example, using both attainment status and
population thresholds, that may result in a fixed PAMS monitoring
network that is either smaller or larger than what will result from the
proposed requirement.
    The second part of the proposed PAMS network design includes
monitoring agency directed enhanced O3 monitoring activities
intended to provide data needed to understand an area's specific
O3 issues. To implement this part of the PAMS network
design, the EPA is proposing to add a requirement for states with
O3 nonattainment areas to develop an ``Enhanced Monitoring
Plan.'' These Enhanced Monitoring Plans, which are to be submitted as
part of their required Annual Monitoring Network Plan (40 CFR 58.10),
would be reviewed and approved by the EPA Regional Administrator as
part of the annual plan review process. The purpose of the Enhanced
Monitoring Plan is to improve monitoring for ambient concentrations of
O3, NOX/NOy, VOCs, and meteorology.
The goal of the Enhanced Monitoring Plan is to allow monitoring
agencies flexibility in determining and collecting the data they need
to understand their O3 problems, consistent with this
purpose and the advice obtained from the CASAC AMMS and the NACAA MSC.
Types of activities that might be included in the Enhanced Monitoring
Plan include (but are not limited to) additional PAMS

sites (e.g., upwind or downwind sites), additional O3 and
NOX monitoring, ozonesondes or other aloft measurements,
rural measurements, mobile PAMS sites, additional meteorological
measurements, and episodic or intensive studies. The savings from a
smaller less costly fixed network of required PAMS sites would be
available for re-investment in the development and implementation of
the proposed Enhanced Monitoring Plans.
2. Speciated VOC Measurements
    Measurement of speciated VOCs important to O3 formation
is a key aspect of the PAMS program. Currently, the existing PAMS
requirements allow for a number of options in measuring speciated VOCs
at PAMS sites which include 1) hourly measurements using an auto-GC, 2)
eight 3-hour samples daily using canisters, or 3) one morning and one
afternoon sample with a 3-hour or less averaging time daily using
canisters plus continuous total non-methane hydrocarbon (TNMHC)
measurements.
    The EPA believes that the options provided for VOC measurements
limit the comparative value of the data being collected, and is
proposing to require instead that all required PAMS sites measure and
report hourly speciated VOCs using an auto GC. More complete and
consistent speciated VOC data nationally would better help meet certain
objectives of the PAMS program described above (e.g., a speciated
ambient air database useful in evaluating control strategies, analyzing
emissions inventory issues, corroborating progress toward attainment,
and evaluating population exposure to air toxics). Furthermore, as
noted by the CASAC AMMS, hourly VOC data are ``particularly useful in
evaluating air quality models and performing diagnostic emission
attribution studies. These data can be provided on a near real-time
basis and presented along with other precursor species (e.g., oxides of
nitrogen and carbon monoxide) collected over similar averaging times.''
Longer time-averaged data are of significantly lower value for model
evaluation.\242\ In addition, creating consistent monitoring
requirements across the network will provide better data for analyzing
regional trends and spatial patterns.
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    \242\ Data of longer than a 1-hour average are often not used in
model evaluations due to the complexity of trying to accommodate
non-hourly averaged data.
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    At the time the original PAMS requirements were promulgated, the
canister options were included because the EPA recognized that the
technologies necessary to measure hourly average speciated VOCs
concentrations were relatively new and may not have been suitable for
broad network use. At that time, gas chromatographs designed for
laboratory use were equipped with auto-samplers designed to ``trap''
the VOC compounds from a gas sample, and then ``purge'' the compounds
onto the GC column. The EPA did not believe that auto-GCs were
universally appropriate due to the technical skill and effort necessary
at that time to properly operate an auto-GC.
    While the basic principles of auto-GC technology have not changed,
the hardware and software of modern auto-GCs are greatly improved over
that available at the time of the original PAMS requirements. Based on
advice from the CASAC AMMS, the EPA has initiated an evaluation of
current auto-GCs potentially suitable for use in the PAMS network.
Based on the preliminary results, the EPA believes that typical NCore
site operators, with appropriate training, will have the skill
necessary to operate a modern auto-GC successfully. Considering the
advances in auto-GC technology, the added value obtained from hourly
data, and the proposed move of PAMS measurements to NCore sites in
O3 nonattainment areas, the EPA is proposing to require
hourly speciated VOC sampling at all PAMS sites. The EPA notes that
this proposed requirement would effectively prevent the use of
canisters to collect speciated VOCs at the required PAMS sites.
However, canister sampling may continue to be an appropriate method for
collecting speciated VOCs at other locations as part of the proposed
Enhanced Monitoring Plans.
    While the EPA believes that the proposed transition to hourly
speciated VOC sampling is the appropriate strategy to take advantage of
improved technology and to broaden the utility of collected data, we
are also mindful of the additional rigidity that the proposed mandatory
use of auto-GCs may have for monitoring agencies, especially those that
have experience with and have established effective and reliable
canister sampling programs. Therefore, the EPA is requesting comment on
the proposed requirement for hourly VOC sampling as well as the range
of alternatives that might be appropriate in lieu of a strict
requirement. Such alternatives could range from a more formal process
where monitoring agencies could request a Regional Administrator-
granted waiver from the hourly VOC requirements through the Annual
Monitoring Network Plan process to collect canister-based speciated VOC
data, to a more flexible set of alternatives where canister sampling
could be retained based on each monitoring agency's evaluation of
programmatic needs as well as their own logistical and technical
capabilities.
3. Carbonyl Sampling
    Carbonyls include a number of compounds important to O3
formation that cannot currently be measured using the auto-GCs or
canisters used at PAMS sites to measure speciated VOCs.\243\ The
current method for measuring carbonyls in the PAMS program is
Compendium Method TO-11A (U.S. EPA, 1999). In this method, carbonyl
compounds are adsorbed and converted into stable hydrazones using
dinitrophenylhydrazine (DNPH) cartridges. These cartridges are then
analyzed for the individual carbonyl compounds using liquid
chromatography (LC) techniques. Three carbonyls (formaldehyde,
acetaldehyde, and acetone) are currently required to be measured in the
PAMS program.
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    \243\ Carbonyls compounds including formaldehyde and
acetaldehyde are difficult to analyze by GC with Flame Ionization
Detectors (FID). Both of these compounds in their free state, do not
respond well to FID detectors. GC analysis is difficult due to the
chemical composition of these compounds, increased polarity and
their inherently low boiling points.
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    In 2006, the EPA revised the PAMS requirements such that carbonyl
sampling was only required in areas classified as serious or above
nonattainment for O3 under the 8-hour O3 standard
which effectively reduced the applicability of carbonyl sampling to a
few areas in California. This change was made in recognition that there
were a number of issues with Method TO-11A that raised concerns with
the uncertainty in the carbonyl data being collected. These issues
include interferences (humidity and O3) and breakthrough