other welfare effects (e.g., mature tree biomass loss), a 3-year
averaging period may also be appropriate. Thus, the Staff Paper
concludes that it is appropriate to consider both an annual and a 3-
year averaging period. Further, the Staff Paper concludes that should a
3-year average of the 12-hour W126 form be selected, a potentially
lower level should be considered to reduce the potential of adverse
impacts to annual species from a single high O3 year that
could still occur while attaining a standard on average over 3-years.
    The CASAC, in considering what seasonal and diurnal time periods
are most appropriate when combined with a cumulative, concentration-
weighted form to protect vegetation from exposures of concern, agreed
that the Staff Paper conclusion regarding the 3-month seasonal period
and 12-hour daylight window was appropriate, with the distinction that
both time designations likely represents the minimum time periods of
importance. In particular, one O3 Panel member commented
that for some species, additional O3 exposures of importance
were occurring outside the 3-month seasonal and 12-hour diurnal
windows. Further, the CASAC concluded that multi-year averaging to
promote a ``stable'' secondary standard is less appropriate for a
cumulative, seasonal secondary standard than for a primary standard
based on maximum 8-hour concentrations. CASAC further concluded that if
multi-year averaging is employed to increase the stability of the
secondary standard, the level of the standard should be revised
downward to assure that the desired degree of protection is not
exceeded in individual years.
    The Administrator, in determining which seasonal and diurnal time
periods are most appropriate to propose, took into account Staff Paper
and CASAC views. The Administrator, in being careful to consider what
is needed to provide the requisite degree of protection, no more and no
less, proposes that the 3-month seasonal period and 12-hour daylight
period are appropriate. Based on the Staff Paper conclusions discussed
above, the Administrator is mindful that there is the potential for
under-protection with a 12-hour diurnal window in areas with
sufficiently elevated nighttime levels of O3 where sensitive
species with a high degree of nocturnal stomatal conductance occur. On
the other hand, the Administrator also recognizes that a longer diurnal
window (e.g., 24-hour) has the possibility of over-protecting
vegetation in areas where nighttime O3 levels remain
relatively high but where no species having significant nocturnal
uptake exist. In weighing these considerations, the Administrator
agrees with the Staff Paper conclusion that until additional
information is available about the extent to which this co-occurrence
of sensitive species and elevated nocturnal O3 exposures
exists, and what levels of nighttime uptake are adverse to affected
species, this information does not provide a basis for reaching a
different conclusion at this time. The Administrator also considered to
what extent the 3-month period within the O3 season was
appropriate, recognizing that many species of vegetation have longer
growing seasons. The Administrator further proposes that the maximum 3-
month period is sufficient and appropriate to characterize
O3 exposure levels associated with known levels of plant
response. Therefore, the Administrator proposes that the most
appropriate exposure periods for a cumulative, seasonal form is the
daytime 12-hour window (8 a.m. to 8 p.m.) during the consecutive 3-
month period within the O3 monitoring season with the
maximum W126 index value.
    The Administrator also proposes an annual rather than a multi-year
cumulative, seasonal standard. In proposing this alternative, the
Administrator also believes that it is appropriate to consider the
benefits to the public welfare that would accrue from establishing a 3-
year average secondary standard, and solicits comment on this
alternative. In so doing, the Administrator also agrees with Staff
Paper and CASAC conclusions that should a 3-year standard be finalized,
the level of the standard should be set so as to provide the requisite
degree of protection for those vegetation effects judged to be adverse
to the public welfare within a single annual period.

c. Level
    The Staff Paper, in identifying a range of levels for a 3-month,
12-hour W126 annual form appropriate to protect the public welfare from
adverse impacts to vegetation from O3 exposures, considers
what information from the array of vegetation effects evidence and
exposure and risk assessment results was most useful. In regards to the
vegetation effects evidence, the Staff Paper finds stronger support
than what was available at the time of the last review for an increased
level of protection for trees and ecosystems. Specifically, this
expanded body of support includes: (1) Additional field based data from
free air, gradient and biomonitoring surveys demonstrating adverse
levels of O3-induced above and/or below-ground growth
reductions on trees at the seedling, sapling and mature growth stages
and incidence of visible foliar injury occurring at biomonitoring sites
in the field at ambient levels of exposure; (2) qualitative support
from free air (e.g., AspenFACE) and gradient studies on a limited
number of tree species for the continued appropriateness of using OTC-
derived C-R functions to predict tree seedling response in the field;
(3) studies that continue to document below-ground effects on root
growth and ``carry-over'' effects occurring in subsequent years from
O3 exposures; and (4) increased recognition and
understanding of the structure and function of ecosystems and the
complex linkages through which O3, and other stressors,
acting at the organism and species level can influence higher levels
within the ecosystem hierarchy and disrupt essential ecological
attributes critical to the maintenance of ecosystem goods and services
important to the public welfare.
    Based on the above observations and on the vegetation effects and
the results of the exposure and impact assessment summarized above, the
Staff Paper concludes that just meeting the current standard would
still allow adverse levels of tree seedling biomass loss in sensitive
commercially and ecologically important tree species in many regions of
the country. Seedling risk assessment results showed that some tree
seedling species are extremely sensitive (e.g., cottonwood, black
cherry and aspen), with annual biomass losses occurring in the field of
the same or greater magnitude that that of annual crops. Such
information from the tree seedling risk assessment suggests that air
quality levels would need to be substantially reduced to protect
sensitive tree seedlings like black cherry from growth and foliar
injury effects.
    In addition to the currently quantifiable risks to trees from
ambient exposures, the Staff Paper also considers the more subtle
impacts of O3 acting in synergy with other natural and man-
made stressors to adversely affect individual plants, populations and
whole systems. By disrupting the photosynthetic process, decreasing
carbon storage in the roots, increasing early senescence of leaves and
affecting water use efficiency in trees, O3 exposures could
potentially disrupt or change the nutrient and water flow of an entire
system. Weakened trees can become more susceptible to other
environmental stresses such as pest and pathogen outbreaks or harsh
weather conditions. Though it is not possible to quantify all the
ecological and societal benefits associated with varying levels of
alternative secondary standards, the Staff Paper concludes that this
information should be weighed in considering the extent to which a
secondary standard should be set so as to provide potential protection
against effects that are anticipated to occur.
    In addition, the Staff Paper also recognizes that in the last
review, the Administrator took into account the results of a 1996
consensus-building workshop as described in a January 1997 report (Heck
and Cowling, 1997). At this workshop, a group of independent scientists
expressed their judgments on what standard form(s) and level(s) would
provide vegetation with adequate protection from O3-related
adverse effects. Consensus was reached with respect to selecting
appropriate ranges of levels in terms of a cumulative, seasonal 3-
month, 12-hr SUM06 standard for a number of vegetation effects
endpoints. These ranges are identified below, with the estimated
approximate equivalent W126 standard values shown in parentheses. For
growth effects to tree seedlings in natural forest stands, a consensus
was reached that a range of 10 to 15 (7 to 13) ppm-hours would be
protective. For growth effects to tree seedlings and saplings in
plantations, the consensus range was 12 to 16 (9 to 14) ppm-hours. For
visible foliar injury to natural ecosystems, the consensus range was 8
to 12 (5 to 9) ppm-hours (Heck and Cowling, 1997).
    Taking these consensus statements into account, the Administrator
stated in the final rule (62 FR 38856) that ``the report lends
important support to the view that the current secondary standard is
not adequately protective of vegetation * * * [and] * * * foreshadows
the direction of future scientific research in this area, the results
of which could be important in future reviews of the O3
secondary standard'' (62 FR 38856).
    Given the importance the Administrator put on the consensus report
in the last review, the Staff Paper considered to what extent new
research provided empirical support for the ranges of levels identified
by the experts as protective of different types of O3-
induced effects. On the basis of new field-based tree seedling growth
loss and foliar injury data, and including both the above quantitative
and qualitative information regarding O3-induced effects on
sensitive trees and forested ecosystems, the Staff Paper concludes that
it is appropriate to consider a range for a 3-month, 12-hour, W126
standard that includes the consensus recommendations for growth effects
in tree seedlings in natural forest stands.
    In considering the newly available information on O3-
related effects on crops in this review, the Staff Paper observes the
following regarding the strength of the underlying crop science: (1)
Nothing in the recent literature points to a change in the relationship
between O3 exposure and crop response across the range of
species and/or cultivars of commodity crops currently grown in the U.S.
that could be construed to make less appropriate the use of commodity
crop C-R functions developed in the NCLAN program; (2) new field-based
studies (e.g., SoyFACE) provide qualitative support in a few limited
cases for the appropriateness of using OTC-derived C-R functions to
predict crop response in the field; and (3) refinements in the
exposure, risk and benefits assessments in this review reduce some of
the uncertainties present in 1996. On the basis of these observations,
the Staff Paper concludes that nothing in the newly assessed
information calls into question the strength of the underlying science
upon which the Administrator based her proposed decision in the last
review to select a level of a cumulative, seasonal form associated with
protecting 50 percent of crop cases from no more than 10 percent yield
loss as providing the requisite degree of protection for commodity
crops.
    The Staff Paper then considered whether any additional information
is available to inform judgments as to the adversity of various
O3-induced levels of crop yield loss to the public welfare.
As noted above, the Staff Paper observes that agricultural systems are
heavily managed, and that in addition to stress from O3, the
annual productivity of agricultural systems is vulnerable to

disruption from many other stressors (e.g., weather, insects, disease),
whose impact in any given year can greatly outweigh the direct
reduction in annual productivity resulting from elevated O3
exposures. On the other hand, O3 can also more subtly impact
crop and forage nutritive quality and indirectly exacerbate the
severity of the impact from other stressors. Though these latter
effects currently cannot be quantified, they should be considered in
judging to what extent a level of protection selected to protect
commodity crops should be precautionary.
    Based on the above considerations, the Staff Paper concludes that
the level of protection judged requisite in the last review to protect
the public welfare from adverse levels of O3-induced
reductions in crop yields, as provided by a W126 level of 21 ppm-hours,
remains appropriate for consideration as an upper bound of a range of
appropriate levels.
    Thus, the Staff Paper concludes, based on all the above
considerations, that an appropriate range of 3-month, 12-hour W126
levels is 7 to 21 ppm-hours, recognizing that the level selected is
largely a policy judgment as to the requisite level of protection
needed. In determining the requisite level of protection for crops and
trees, the Staff Paper recognizes that it is appropriate to weigh the
importance of the predicted risks of these effects in the overall
context of public welfare protection, along with a determination as to
the appropriate weight to place on the associated uncertainties and
limitations of this information.
    The CASAC, in its final letter to the Administrator (Henderson,
2007), agreed with the Staff Paper recommendations that the lower bound
of the range within which a seasonal W126 welfare-based (secondary)
O3 standard should be considered is approximately 7 ppm-
hours; however, it did not agree with Staff's recommendation that the
upper bound of the range should be as high as 21 ppm-hours. Rather,
CASAC recommended that the upper bound of the range considered should
be no higher than 15 ppm-hours, which the Panel estimates is
approximately equivalent to a seasonal 12-hour SUM06 level of 20 ppm-
hours. The lower end of this range (7 ppm-hours) is the same as the
lower end of the range identified in the 1997 Consensus Workshop as
protective of tree seedlings in natural forest stands from growth
effects (Heck and Cowling, 1997).
    The Administrator, taking Staff Paper and CASAC views into account,
proposes a range of levels for a cumulative, seasonal secondary
standard as expressed in terms of the maximum 3 month, 12-hour W126
form, in the range of 7 to 21 ppm-hours. This range encompasses the
range of levels recommended by CASAC, and also includes a higher level
as recommended in the Staff Paper. Given the uncertainty in determining
the risk attributable to various levels of exposure to O3,
the Administrator believes as a public welfare policy judgment that
this is a reasonable range to propose.
    In taking into account the uncertainty associated with the above,
the Administrator has also considered an alternative approach to
establishing a secondary standard(s). This alternative approach would
establish a cumulative, seasonal standard(s) that would afford
differing degrees of protection for O3-related impacts on
different types of vegetation with different intended uses.
    The Administrator recognizes that known O3-sensitive
plant species growing within the U.S experience a variety of
O3-induced effects, including visible foliar injury, biomass
loss and yield loss, and that the public welfare significance of each
of these effects can vary significantly, depending on the nature of the
effect, the intended use of the plant, and/or the type of environment
or location in which the plant grows. Any given O3-related
effect on vegetation (e.g., biomass loss, or foliar injury) may be
judged to have a different degree of impact on public welfare
depending, for example, on whether that effect occurs in a Class I
area, commercial cropland, or a city park. This variation in the
significance of O3-related vegetation effects from a public
welfare perspective across type of effect, intended plant use, and area
grown means that the level of ambient O3 that is requisite
to protect the public welfare may also vary. The level of ambient
O3 that is requisite in a federally designated Class I area
may be lower than the level that is requisite in a cropland area. EPA
is therefore considering and soliciting comment on an alternative
approach for the secondary O3 standard, with the aim of
reasonably reflecting these variations.
    Specifically, the Administrator seeks comment on an alternative
approach that would establish a suite of secondary standards. The suite
of standards would contain different ambient levels, with each standard
at a level that is requisite to protect public welfare for that
variation in plant effect, use, and/or location. For example, a
secondary standard intended to provide protection to natural systems
valued for their aesthetic beauty and/or important ecological functions
they might serve could be set at a lower, more protective level to
provide the requisite degree of protection against a broad array of
O3-related effects on important sensitive species in such
areas. In contrast, while negative impacts on yield production in
sensitive agricultural crops is also an important public welfare
effect, O3-related reductions in yield may be considered
less significant or adverse to the public welfare, depending on the
degree of impact, since the intended use of such land is to produce
optimum yields and croplands are already heavily managed to achieve
that goal. Thus, a secondary standard set to provide the requisite
degree of crop protection for such an area could be set at a higher
level.
    The Administrator recognizes that variation in vegetation type and
location, intended use, and impacts related to O3 exposure
can be diverse, and believes that it is appropriate to consider whether
it is appropriate and feasible to establish a suite of standards that
accounts more broadly for such variation. EPA recognizes that this
approach is unique with regard to secondary standards and will pose
unique challenges, including how to classify areas according to
intended use. Some geographic areas have already been identified for
specific uses, such as Federal Class I areas,\65\ which are intended to
conserve unimpaired natural ecosystems and their associated species for
the enjoyment of future generations. Likewise, the USDA has classified
cultivated areas in the U.S. into certain categories of intended use
(such as cropland, rangeland, timberland) that could help inform the
setting of a suite of standards.
---------------------------------------------------------------------------

    \65\ The Clean Air Act defines Class I areas as national parks
over 6,000 acres, national wilderness areas and national memorial
parks over 5,000 acres, and international parks. The National Park
Service was created in 1916 by Congress through the National Park
Service Organic Act in order to ``conserve the scenery and the
natural and historic objects and the wild life therein and to
provide for the enjoyment of the same in such manner and by such
means as will leave them unimpaired for the enjoyment of future
generations.''
---------------------------------------------------------------------------

    EPA is taking comment on all aspects of this alternative approach,
including whether it is appropriate to set a suite of secondary
standards that varies depending on use, location, and type of effect on
vegetation. EPA invites comment on the appropriateness of this
approach, from the scientific, legal, and policy perspectives, and on
other factors that should be considered in determining the
applicability of any one level within a suite of standards.

3. 8-Hour Average Standard
    The Administrator is also proposing to revise the current secondary
standard by making it identical to the proposed 8-hour primary
standard, which is proposed to be within the range of 0.070 to 0.075
ppm. For this option, EPA also solicits comment on a wider range of 8-
hour standard levels, including levels down to 0.060 ppm and up to the
current standard (i.e., effectively 0.084 ppm with the current rounding
convention).
    In the last review, the Staff Paper included an analysis to compare
the degree of overlap between areas that would be expected not to meet
the range of alternative 8-hour standards being considered for the
primary NAAQS and those expected not to meet the range of values
(expressed in terms of the seasonal SUM06 index) of concern for
vegetation. This result suggested that improvements in national air
quality expected to result from attaining an 8-hour primary standard
within the recommended range of levels would also be expected to reduce
levels of concern for vegetation in those same areas. In the 1997 final
rule, the decision was made, on the basis of both science and policy
considerations, to make the secondary identical to the primary
standard. It acknowledged, however, that uncertainties remained ``as to
the extent to which air quality improvements designed to reduce 8-hour
average O3 concentrations averaged over a 3-year period
would reduce O3 exposures measured by a seasonal SUM06
index'' (62 FR 38876).
    On the basis of that history, the current Staff Paper analyzed the
degree of overlap expected between alternative 8-hour and cumulative
seasonal secondary standards (as discussed above in section IV.C.1)
using recent air quality. Based on the results, the Staff Paper
concluded that the degree to which the current 8-hour standard form and
level would overlap with areas of concern for vegetation expressed in
terms of the 12-hour W126 standard is inconsistent from year to year
and would depend greatly on the level of the 12-hour W126 and 8-hour
standards selected and the distribution of hourly O3
concentrations within the annual and/or 3-year average period.
    Thus, though the Staff Paper recognized again that meeting the
current or alternative levels of the 8-hour average standard could
result in air quality improvements that would potentially benefit
vegetation in some areas, it urges caution be used in evaluating the
likely vegetation impacts associated with a given level of air quality
expressed in terms of the 8-hour average form in the absence of
parallel W126 information. This caution is due to the concern that the
analysis in the Staff Paper may not be an accurate reflection of the
true situation in non-monitored, rural counties due to the lack of more
complete monitor coverage in many rural areas. Further, of the counties
that did not show overlap between the two standard forms, most were
located in rural/remote high elevation areas which have O3
air quality patterns that are typically different from those associated
with urban and near urban sites at lower elevations. Because the
majority of such areas are currently not monitored, it is believed
there are likely to be additional areas that have similar air quality
distributions that would lead to the same disconnect between forms.
Thus, the Staff Paper concluded that it remains problematic to
determine the appropriate level of protection for vegetation using an
8-hour average form.
    The CASAC recognized that an important difference between the
effects of acute exposures to O3 on human health and the
effects of O3 exposures on welfare is that vegetation
effects are more dependent on the cumulative exposure to, and uptake
of, O3 over the course of the entire growing season
(Henderson, 2006c). The CASAC O3 Panel members were
unanimous in concluding the protection of natural terrestrial
ecosystems and managed agricultural crops requires a secondary
O3 standard that is substantially different from the primary
O3 standard in averaging time, level, and form (Henderson,
2007).
    A number of public commenters also presented views for the
Administrator's consideration regarding the adequacy of the current
standard and whether or not revisions to that standard were warranted.
These commenters did not support adopting an alternative, cumulative
form for the secondary standard. These commenters stated that ``though
directionally a cumulative form of the standard may better match the
underlying data,'' they believed further work is needed to determine
whether a cumulative exposure index for the form of the secondary
standard is necessary. These commenters identified a number of key
concerns regarding the available evidence that, in their view, make it
inappropriate to revise the secondary standard at this time. In
particular they assert that (1) The key uncertainties, cited by the
Administrator in the 1997 review as reasons for deciding it was not
appropriate to move forward with a seasonal secondary, have not been
materially reduced in the current review; and (2) the exposure
assessment is inaccurate and too uncertain due to the use of low
estimates of PRB, an arbitrary rollback method that is uninformed by
atmospheric chemistry from photochemical models, and the use of the
CMAQ model in the west, whose biases and uncertainties are
insufficiently characterized and evaluated.
    In considering the appropriateness of proposing a revised secondary
standard that would be identical to the proposed primary standard, the
Administrator took into account the approach used by the Agency in the
last review, the conclusions of the Staff Paper, CASAC advice, and the
views of public commenters. The Administrator first considered the
Staff Paper analysis of the projected degree of overlap between
counties with air quality expected to meet various alternative levels
of an 8-hour standard and alternative levels of a W126 standard based
on monitored air quality data. This analysis showed significant overlap
within the proposed range of the primary 8-hour form and selected
levels of the W126 standard form being considered, with the degree of
overlap between these two forms depending greatly on the levels
selected and the distribution of hourly O3 concentrations
within the annual and/or 3-year average period. On this basis, the
Administrator recognizes that a secondary standard set identical to the
proposed primary standard would provide a significant degree of
additional protection for vegetation as compared to that provided by
the current secondary standard. The Administrator also recognizes that
lack of rural monitoring data makes uncertain the degree to which the
proposed 8-hour or W126 alternatives would be protective, and that
there would be the potential for not providing the appropriate degree
of protection for vegetation in areas with air quality distributions
that result in a high cumulative, seasonal exposure but do not result
in high 8-hour average exposures. While this potential for under-
protection is clear, the number and size of areas at issue and the
degree of risk is hard to determine. However, such a standard would
also tend to avoid the potential for providing more protection than is
necessary, a risk that would arise from moving to a new form for the
secondary standard despite significant uncertainty in determining the
degree of risk for any exposure level and the appropriate level of
protection, as well as uncertainty in predicting exposure and risk
patterns.
    The Administrator also considered the views and recommendations of

CASAC, and agrees that a cumulative, seasonal standard is the most
biologically relevant way to relate exposure to plant growth response.
However, as reflected in the public comments, the Administrator also
recognizes that there remain significant uncertainties in determining
or quantifying the degree of risk attributable to varying levels of
O3 exposure, the degree of protection that any specific
cumulative, seasonal standard would produce, and the associated
potential for error in determining the standard that will provide a
requisite degree of protection--i.e. sufficient but not more than what
is necessary. Given this uncertainty, the Administrator also believes
it is appropriate to consider the degree of protection that would be
afforded by a secondary standard that is identical to the proposed
primary standard. Based on his consideration of the full range of views
as described above, the Administrator proposes as a second option to
revise the secondary standard to be identical in every way to the
proposed primary standard.

F. Proposed Decision on the Secondary Standard

    The Administrator proposes to replace the current secondary
standard with one of two options. One option is a new cumulative,
seasonal standard expressed as an index of the annual sum of weighted
hourly concentrations (using the W126 form), set at a level in the
range of 7 to 21 ppm-hours. The index would be cumulated over the 12-
hour daylight period (8 a.m. to 8 p.m.) during the consecutive 3-month
period within the O3 season with the maximum index value.
The other option is to revise the current secondary standard by making
it identical to the proposed 8-hour primary standard, which is proposed
to be within the range of 0.070 to 0.075 ppm. For this option, EPA also
solicits comment on a wider range of 8-hour standard levels, including
levels down to 0.060 ppm and up to the current standard (i.e.,
effectively 0.084 ppm with the current rounding convention. The
Administrator is also soliciting comment on an alternative approach for
a setting cumulative, seasonal standard(s) that would afford differing
degrees of protection for O3-related impacts on different
types of vegetation with different intended uses.

V. Creation of Appendix P--Interpretation of the NAAQS for Ozone

    The EPA is proposing to create Appendix P to 40 CFR part 50 to
reflect the proposed revisions to the primary and secondary standards
discussed above. This Appendix would explain the computations necessary
for determining when the proposed primary and secondary standards are
met. More specifically, Appendix P addresses data completeness
requirements, data reporting, handling, and rounding conventions, and
example calculations. Although EPA is proposing two alternative
secondary standards, the proposed Appendix has been written to address
a seasonal secondary standard expressed in the W126 form. If EPA adopts
a secondary standard identical to the primary standard, Appendix P will
be modified accordingly. The proposed Appendix also reflects the final
rule promulgated on March 22, 2007 for the treatment of data influenced
by exceptional events (72 FR 13560).
Key elements of the proposed revisions to Appendix P are outlined
below.

A. Data Completeness

    The data completeness requirements in Appendix P proposed here for
the proposed 8-hr primary standard secondary standards are the same as
those in Appendix I to 40 CFR part 50 required for the current
standard. To satisfy the date completeness requirement, Appendix P
would require 90% data completeness, on average, for the 3-year period
at a monitoring site, with no single year within the period having less
than 75% data completeness. This data completeness requirement would
have to be satisfied in order to determine that the standard(s) have
been met at a monitoring site. A site could be found not to have met
the standard(s) with less than complete data. EPA concluded in adopting
these same data completeness requirements in Appendix I in 1997 that
these proposed requirements are reasonable based on its earlier
analysis of available air quality data that showed that 90% of all
monitoring sites that are operated on a continuous basis routinely meet
this objective. The EPA is seeking comment, however, on whether
meteorological data would provide an objective basis for determining,
on a day for which there is missing data, that the meteorological
conditions were not conducive to high O3 concentrations, and
therefore, that the day could be assumed to have an O3
concentration less than 0.070 to 0.075 ppm.
    We are proposing separate data completeness requirements for the
proposed seasonal secondary standard expressed in the W126 form. For
such a standard, Appendix P would require a site to have 75% data
completeness in a given month. Appendix P would also provide a
mechanism for adjusting for missing data. Because this alternative is a
seasonal cumulative index, representing a distribution of O3
values under a range of meteorological conditions, rather than a peak
statistic, the EPA is proposing a missing data procedure that would
require the monthly total index to be adjusted for incomplete data by
multiplying the unadjusted W126 value by the ratio of the number of
possible daylight hours (8:00 a.m. to 8:00 p.m.) to the number of hours
with valid ambient hourly concentrations. This adjustment is analogous
to calculating an estimated number of exceedances contained within part
50 Appendix I for the one hour O3 standard.

B. Data Handling and Rounding Conventions

    Almost all State agencies now report hourly O3
concentrations to three decimal places, in ppm, since the typical
incremental sensitivity of currently used O3 monitors is
0.001 ppm. Consistent with the current approach for computing 8-hr
averages, in calculating 8-hr average O3 concentrations from
such hourly data, any calculated digits past the third decimal place
would be truncated to preserve the number of significant digits in the
reported data. In calculating 3-year averages of the fourth highest
maximum 8-hr average concentrations, EPA is proposing to require the
result to be reported to the third decimal place with digits to the
right of the third decimal place truncated to preserve the number of
significant digits in the reported data, as prescribed by the current
standard. Analyses discussed in the Staff Paper demonstrated that
taking into account the precision and bias in 1-hour O3
measurements, the 8-hour design value had an uncertainty of
approximately 0.001 ppm. Thus, EPA considers any value less than 0.001
ppm to be highly uncertain and, therefore, proposes truncating both the
individual 8-hour averages used to determine the annual fourth maximum
as well as the 3-year average of the fourth maxima to the third decimal
place. Nevertheless, EPA solicits comment on the appropriateness of
rounding to the third decimal place as well as the policy reasons
behind either truncating or rounding the 3-year average to the third
decimal place (with 0.0005 and greater rounding up). EPA is also
seeking comment on the scientific validity of truncating the three year
average as opposed to rounding it as well as the policy reasons behind
either truncating or rounding the average to the third decimal place.

    To determine whether the proposed standard is met, the calculated
value of the fourth highest maximum 8-hour average concentrations,
averaged over three years, would be compared to the level of the
standard. As discussed in section II, the EPA is proposing to issue an
8-hr standard extending to three decimal places, based on the staff's
analysis and conclusions discussed in the Staff paper that expressing
the proposed standard to the third decimal place is consistent with the
precision requirements of the current O3 monitoring
technology. Given that both the proposed standard and the calculated
value of the 3-year average of the fourth highest maximum 8-hr
O3 concentration are expressed to three decimal places, the
two values can be compared directly. This is different than the
approach for determining compliance with the current standard
O3 standard. In comparing the calculated 3-year average
(which is expressed to three decimal places) to the current standard
O3 standard (which is expressed to only two decimal places),
Appendix I requires the calculated 3-year average to be rounded to two
decimal places. This additional step would not be necessary for the
proposed standard given that the standard and the 3-year average are
each expressed to three decimal places.
    For the proposed seasonal secondary standard, the annual maximum 3-
month W126 value computed on a calendar year basis using the three
highest, consecutive monthly W126 values would be used as the summary
statistic. The resulting value would then be compared to the level of
the secondary O3 standard. The Agency is also interested in
receiving comments regarding a 3-year average form summary statistic.

VI. Ambient Monitoring Related to Proposed Revised O3
Standards

    The EPA is not proposing any specific changes to existing
requirements for monitoring of O3 in the ambient air.
However, we invite comment on a number of issues which naturally arise
in connection with the proposed revision of the O3 NAAQS.
The EPA may propose changes to some of the existing requirements at a
later date.
    Current requirements regarding EPA-approved measurement methods for
ambient O3 are stated in 40 CFR part 50 Appendix D,
Measurement Principle and Calibration Procedure for the Measurement of
Ozone in the Atmosphere, and in 40 CFR part 53, Ambient Air Monitoring
Reference and Equivalent Methods. The EPA does not intend to propose
any changes to these requirements, because we believe these
requirements would continue to be appropriate to support implementation
of a revised O3 NAAQS.
    Presently, States (including the District of Columbia, Puerto Rico,
and the Virgin Islands, and including local agencies when so delegated
by the State) are required to operate minimum numbers of EPA-approved
O3 monitors based on the population of each of their
Metropolitan Statistical Areas (MSA) and the most recently measured
O3 levels in each area. Each State (or in some cases
portions of a State) also has a required O3 monitoring
season based on historical experience on when O3 levels are
high enough to be of regulatory or public health concern. These
requirements are contained in 40 CFR part 58 Appendix D, Network Design
Criteria for Ambient Air Quality Monitoring. See section 4.1,
especially Tables D-2 and D-3. These requirements were last revised on
October 17, 2006 as part of a comprehensive review of ambient
monitoring requirements for all criteria pollutants. (71 FR 61236)
Certain deviations including minimum monitoring requirements and/or
monitoring season requirements may be approved by the EPA Regional
Administrator on a case-by-case basis.
    Required O3 monitoring seasons range from four to 12
months. The minimum number of monitors in an MSA ranges from zero (for
an area with population under 350,000 and no recent history of an
O3 design value greater than 85 percent of the NAAQS) to
four (for an area with population greater than 10 million and an
O3 design value greater than 85 percent of the NAAQS).
Because these requirements apply at the MSA level, large urban areas
consisting of multiple MSAs can require more than four monitors. For
example, the New York-Newark-Bristol NY-NJ-CT-PA combined statistical
area requires about 14 monitors. In total, about 400 monitors are
required in MSAs, but about 1100 are actually operating in MSAs because
most States operate more than the minimum required number of monitors.
    There are no EPA requirements for O3 monitoring in less
populated areas outside of MSA boundaries (e.g., Metropolitan
Statistical Areas) or in rural areas. However, there are about 250
O3 monitors in counties that are not part of MSAs. Some
required State monitors are placed downwind of the urban center of the
MSA of interest in locations that are in some cases in a county outside
the MSA itself; some States also operate a few rural monitors for
research purposes. The EPA operates a network of about 56 O3
monitors as part of its Clean Air Status and Trends Network (CASTNET).
The National Park Service (NPS) operates about 27 monitors at other
CASTNET sites. The NPS also has O3 monitoring stations in
parks that are not part of the CASTNET dry deposition monitoring effort
including multiple O3 stations in Great Smoky Mountains,
Sequoia, Yosemite, and Joshua Tree National Parks.
    Required quality assurance procedures for O3 monitoring
are given in 40 CFR Part 58 Appendix A, Quality Assurance Requirements
for State and local air monitoring stations (SLAMS), special purpose
monitors (SPM), and prevention of significant deterioration (PSD) Air
Monitoring. The EPA does not intend to propose any changes to these
quality assurance requirements, because we believe that the current
measurement uncertainty goals and related procedures for assessing
precision and bias as documented in paragraph <IP_ADDRESS> of Appendix A are
appropriate to support the implementation of a revised O3
NAAQS.
    States are required to report O3 data quarterly to EPA's
Air Quality System (AQS), and most also voluntarily report their pre-
validated O3 data on an hourly basis to EPA's real time
AirNow data system, where the data are used to forecast O3
concentrations and to provide public advisories. The National Park
Service and many other organizations also report their O3
data to AQS and/or AirNow. The locations of currently operating
O3 monitors which report data to EPA's Air Quality System
are available through the EPA AirData Web site http://www.epa.gov/air/data/index.html
.

    Data from O3 monitors at CASTNET stations are currently
kept in a separate national data base.\66\
---------------------------------------------------------------------------

    \66\ At present, not all ozone monitors at CASTNET sites are
operated in full compliance with the quality assurance requirements
of 40 CFR Part 58 Appendix D, as they have not been primarily
intended for regulatory use. The EPA is working towards such
compliance in the near future and towards making CASTNET ozone data
available through AQS.
---------------------------------------------------------------------------

    The EPA invites comments on O3 monitoring issues (other
than O3 monitoring methods and quality assurance
requirements), including the following:
    (1) Ozone monitoring network requirements in urban areas. Table D-2
of 40 CFR Part 58 Appendix D is based on the percentage of the
O3 NAAQS, with a break point at 85 percent of the NAAQS.
Therefore, a revision of the NAAQS would automatically increase the
required number of O3 monitors. For example, assuming a
final NAAQS of

0.070 ppm for purposes of illustration only, about 70 MSAs with current
O3 design values in the range of about 0.060 ppm (about 85
percent of the current NAAQS) to 0.070 ppm (about 85 percent of 0.070
ppm) would be affected, with most changing from no required monitors to
one, or from one required monitor to two. Because most of these areas
already are operating at least as many monitors as the possible new
requirement, the number of monitors which would need to be initiated
(or moved from a location of excess monitors) would be only about five
monitors. About 100 MSAs with populations less than 350,000 presently
are without any O3 monitors, and hence they do not have an
O3 design value for use with Table D-2. If for the purpose
of applying Table D-2, these areas are treated as if they have
O3 concentrations below 85 percent of the revised NAAQS,
then a NAAQS revision would not automatically result in a requirement
for O3 monitoring in these MSAs.\67\ EPA invites comments on
the appropriateness of the existing minimum monitoring requirements for
purposes of implementing the proposed revised NAAQS, including the
automatic changes to minimum monitoring requirements that would be
triggered by a NAAQS revision.
---------------------------------------------------------------------------

    \67\ EPA might instead treat one or more of these counties as
having a design value based on a monitor in a nearby monitored
county, in which case ozone monitoring might become required in
certain currently unmonitored MSAs and the number of new required
monitors would increase in the illustrative NAAQS example stated
above.
---------------------------------------------------------------------------

    (2) Ozone monitoring seasons. As mentioned, the currently required
O3 monitoring seasons range from four to 12 months of the
year. In some cases, O3 monitoring may start a couple of
weeks before and may end a couple of weeks after the required season.
With a lower O3 NAAQS, the issue arises of whether in some
areas the required O3 monitoring season should be made
longer. The EPA notes that under the existing regulations, the Regional
Administrator may approve State-requested deviations from the
established O3 monitoring season but EPA may not increase
the length of the season for an area at EPA's own initiative other than
by notice and comment rulemaking.
    (3) Monitoring to support implementation of a secondary
O3 NAAQS. It is fair to say that the existing O3
monitoring requirements and current State monitoring practices are
primarily oriented towards protecting against health effects in people,
i.e., towards implementation of the primary NAAQS. This accounts for
the focus on urban areas, which can combine large populations, large
emissions of O3-forming precursors, and O3
concentrations of concern. The purpose of the secondary NAAQS is to
protect against vegetation damage and other welfare effects, which can
occur in both urban and rural areas. States have largely been given
discretion on whether to add additional monitors aimed specifically at
achieving the objectives of the previous and current secondary NAAQS.
In urban areas, EPA in general believes that an O3
monitoring network (and monitoring season) appropriate to support
implementation of the primary NAAQS will also be appropriate for
implementing the secondary NAAQS. However, rural areas are presently
only sparsely monitored for O3 so violations of the
secondary NAAQS in areas with sensitive vegetation may occur
undetected, as a result of transport from urban areas with high
precursor emissions and/or O3 concentrations or from
formation of additional O3 from precursors emitted from
sources outside urban areas. It is conceivable that rural violations of
a secondary NAAQS could occur in areas with sensitive vegetation even
though urban monitoring networks are showing compliance with the
primary NAAQS, whether the forms and levels of the two standards are
the same or different. The EPA invites comment on the likelihood of
this occurring under the possible combinations of primary and secondary
standards proposed in this notice, and on whether, where, and how EPA
should require monitoring in rural areas specifically aimed at
implementation of the secondary NAAQS (and/or promote more voluntary
monitoring or conduct monitoring itself in rural areas).

VII. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under section 3(f)(1) of Executive Order (EO) 12866 (58 FR 51735,
October 4, 1993), the O3 NAAQS action is an ``economically
significant regulatory action'' because it is likely to have an annual
effect on the economy of $100 million or more. Accordingly, EPA
prepared this regulatory impact analysis (RIA) of the potential costs
and benefits associated with this action. The RIA estimates the costs
and monetized human health and welfare benefits of attaining three
alternative O3 NAAQS nationwide. Specifically, the RIA
examines the alternatives of 0.075 ppm, 0.070 ppm, and 0.065 ppm. The
RIA contains illustrative analyses that consider a limited number of
emissions control scenarios that States and Regional Planning
Organizations might implement to achieve these alternative
O3 NAAQS. However, the Clean Air Act (CAA) and judicial
decisions make clear that the economic and technical feasibility of
attaining ambient standards are not to be considered in setting or
revising NAAQS, although such factors may be considered in the
development of State plans to implement the standards. Accordingly,
although an RIA has been prepared, the results of the RIA have not been
considered in issuing this final rule.

B. Paperwork Reduction Act

    This action does not impose an information collection burden under
the provisions of the Paperwork Reduction Act, 44 U.S.C. 3501 et seq.
There are no information collection requirements directly associated
with the establishment of a NAAQS under section 109 of the CAA.
    Burden means the total time, effort, or financial resources
expended by persons to generate, maintain, retain, or disclose or
provide information to or for a Federal agency. This includes the time
needed to review instructions; develop, acquire, install, and utilize
technology and systems for the purposes of collecting, validating, and
verifying information, processing and maintaining information, and
disclosing and providing information; adjust the existing ways to
comply with any previously applicable instructions and requirements;
train personnel to be able to respond to a collection of information;
search data sources; complete and review the collection of information;
and transmit or otherwise disclose the information.
    An agency may not conduct or sponsor, and a person is not required
to respond to a collection of information unless it displays a
currently valid OMB control number. The OMB control numbers for EPA's
regulations in 40 CFR are listed in 40 CFR part 9.

C. Regulatory Flexibility Act

    The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.

    For purposes of assessing the impacts of today's rule on small
entities, small entity is defined as: (1) A small business that is a
small industrial entity as defined by the Small Business
Administration's (SBA) regulations at 13 CFR 121.201; (2) a small
governmental jurisdiction that is a government of a city, county, town,
school district or special district with a population of less than
50,000; and (3) a small organization that is any not-for-profit
enterprise which is independently owned and operated and is not
dominant in its field.
    After considering the economic impacts of today's proposed rule on
small entities, I certify that this action will not have a significant
economic impact on a substantial number of small entities. This
proposed rule will not impose any requirements on small entities.
Rather, this rule establishes national standards for allowable
concentrations of O3 in ambient air as required by section
109 of the CAA. See also American Trucking Associations v. EPA. 175 F.
3d at 1044-45 (NAAQS do not have significant impacts upon small
entities because NAAQS themselves impose no regulations upon small
entities). We continue to be interested in the potential impacts of the
proposed rule on small entities and welcome comments on issues related
to such impacts.

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public
Law 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and Tribal
governments and the private sector. Under section 202 of the UMRA, EPA
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with ``Federal mandates'' that
may result in expenditures to State, local, and Tribal governments, in
the aggregate, or to the private sector, of $100 million or more in any
1 year. Before promulgating an EPA rule for which a written statement
is needed, section 205 of the UMRA generally requires EPA to identify
and consider a reasonable number of regulatory alternatives and to
adopt the least costly, most cost-effective or least burdensome
alternative that achieves the objectives of the rule. The provisions of
section 205 do not apply when they are inconsistent with applicable
law. Moreover, section 205 allows EPA to adopt an alternative other
than the least costly, most cost-effective or least burdensome
alternative if the Administrator publishes with the final rule an
explanation why that alternative was not adopted. Before EPA
establishes any regulatory requirements that may significantly or
uniquely affect small governments, including Tribal governments, it
must have developed under section 203 of the UMRA a small government
agency plan. The plan must provide for notifying potentially affected
small governments, enabling officials of affected small governments to
have meaningful and timely input in the development of EPA regulatory
proposals with significant Federal intergovernmental mandates, and
informing, educating, and advising small governments on compliance with
the regulatory requirements.
    Today's rule contains no Federal mandates (under the regulatory
provisions of Title II of the UMRA) for State, local, or Tribal
governments or the private sector. The rule imposes no new expenditure
or enforceable duty on any State, local or Tribal governments or the
private sector, and EPA has determined that this rule contains no
regulatory requirements that might significantly or uniquely affect
small governments. Furthermore, as indicated previously, in setting a
NAAQS EPA cannot consider the economic or technological feasibility of
attaining ambient air quality standards, although such factors may be
considered to a degree in the development of State plans to implement
the standards. See also American Trucking Associations v. EPA, 175 F.
3d at 1043 (noting that because EPA is precluded from considering costs
of implementation in establishing NAAQS, preparation of a Regulatory
Impact Analysis pursuant to the Unfunded Mandates Reform Act would not
furnish any information which the court could consider in reviewing the
NAAQS). Accordingly, EPA has determined that the provisions of sections
202, 203, and 205 of the UMRA do not apply to this proposed decision.
The EPA acknowledges, however, that any corresponding revisions to
associated SIP requirements and air quality surveillance requirements,
40 CFR part 51 and 40 CFR part 58, respectively, might result in such
effects. Accordingly, EPA will address, as appropriate, unfunded
mandates if and when it proposes any revisions to 40 CFR parts 51 or
58.

E. Executive Order 13132: Federalism

    Executive Order 13132, entitled ``Federalism'' (64 FR 43255, August
10, 1999), requires EPA to develop an accountable process to ensure
``meaningful and timely input by State and local officials in the
development of regulatory policies that have federalism implications.''
``Policies that have federalism implications'' is defined in the
Executive Order to include regulations that have ``substantial direct
effects on the States, on the relationship between the national
government and the States, or on the distribution of power and
responsibilities among the various levels of government.''
    This proposed rule does not have federalism implications. It will
not have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. The rule does not alter the
relationship between the Federal government and the States regarding
the establishment and implementation of air quality improvement
programs as codified in the CAA. Under section 109 of the CAA, EPA is
mandated to establish NAAQS; however, CAA section 116 preserves the
rights of States to establish more stringent requirements if deemed
necessary by a State. Furthermore, this rule does not impact CAA
section 107 which establishes that the States have primary
responsibility for implementation of the NAAQS. Finally, as noted in
section E (above) on UMRA, this rule does not impose significant costs
on State, local, or Tribal governments or the private sector. Thus,
Executive Order 13132 does not apply to this rule.
    However, as also noted in section E (above) on UMRA, EPA recognizes
that States will have a substantial interest in this rule and any
corresponding revisions to associated SIP requirements and air quality
surveillance requirements, 40 CFR part 51 and 40 CFR part 58,
respectively. Therefore, in the spirit of Executive Order 13132, and
consistent with EPA policy to promote communications between EPA and
State and local governments, EPA specifically solicits comment on this
proposed rule from State and local officials.

F. Executive Order 13175: Consultation and Coordination With Indian
Tribal Governments

    Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Governments'' (65 FR 67249, November 9, 2000),
requires EPA to develop an accountable process to ensure ``meaningful
and timely input by tribal officials in the development of regulatory
policies that have tribal implications.'' This rule concerns the
establishment of O3 NAAQS. The Tribal Authority Rule gives
Tribes the opportunity to develop and implement

CAA programs such as the O3 NAAQS, but it leaves to the
discretion of the Tribe whether to develop these programs and which
programs, or appropriate elements of a program, they will adopt.
    This proposed rule does not have Tribal implications, as specified
in Executive Order 13175. It does not have a substantial direct effect
on one or more Indian Tribes, since Tribes are not obligated to adopt
or implement any NAAQS. Thus, Executive Order 13175 does not apply to
this rule.
    Although Executive Order 13175 does not apply to this rule, EPA
contacted tribal environmental professionals during the development of
this rule. The EPA staff participated in the regularly scheduled Tribal
Air call sponsored by the National Tribal Air Association during the
spring of 2007 as this proposal was under development. EPA specifically
solicits additional comment on this proposed rule from Tribal
officials.

G. Executive Order 13045: Protection of Children From Environmental
Health & Safety Risks

    Executive Order 13045, ``Protection of Children from Environmental
Health Risks and Safety Risks'' (62 FR 19885, April 23, 1997) applies
to any rule that: (1) Is determined to be ``economically significant''
as defined under Executive Order 12866, and (2) concerns an
environmental health or safety risk that EPA has reason to believe may
have a disproportionate effect on children. If the regulatory action
meets both criteria, the Agency must evaluate the environmental health
or safety effects of the planned rule on children, and explain why the
planned regulation is preferable to other potentially effective and
reasonably feasible alternatives considered by the Agency.
    This proposed rule is subject to Executive Order 13045 because it
is an economically significant regulatory action as defined by
Executive Order 12866, and we believe that the environmental health
risk addressed by this action may have a disproportionate effect on
children. The proposed rule will establish uniform national ambient air
quality standards for O3; these standards are designed to
protect public health with an adequate margin of safety, as required by
CAA section 109. However, the protection offered by these standards may
be especially important for children because children, especially
children with asthma, along with other sensitive population subgroups
such as all people with lung disease and people active outdoors, are
potentially susceptible to health effects resulting from O3
exposure. Because children are considered a potentially susceptible
population, we have carefully evaluated the environmental health
effects of exposure to O3 pollution among children. These
effects and the size of the population affected are summarized in
section 8.7 of the Criteria Document and section 3.6 of the Staff
Paper, and the results of our evaluation of the effects of
O3 pollution on children are discussed in sections II.A-C of
this preamble.

H. Executive Order 13211: Actions That Significantly Affect Energy
Supply, Distribution or Use

    This proposed rule is not a ``significant energy action'' as
defined in Executive Order 13211, ``Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR 28355
(May 22, 2001)) because in the Agency's judgment it is not likely to
have a significant adverse effect on the supply, distribution, or use
of energy. The purpose of this rule is to establish revised NAAQS for
O3. The rule does not prescribe specific pollution control
strategies by which these ambient standards will be met. Such
strategies will be developed by States on a case-by-case basis, and EPA
cannot predict whether the control options selected by States will
include regulations on energy suppliers, distributors, or users. Thus,
EPA concludes that this rule is not likely to have any adverse energy
effects and does not constitute a significant energy action as defined
in Executive Order 13211.

I. National Technology Transfer and Advancement Act

    Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (NTTAA), Public Law No. 104-113, Sec.  12(d) (15 U.S.C. 272
note) directs EPA to use voluntary consensus standards in its
regulatory activities unless to do so would be inconsistent with
applicable law or otherwise impractical. Voluntary consensus standards
are technical standards (e.g., materials specifications, test methods,
sampling procedures, and business practices) that are developed or
adopted by voluntary consensus standards bodies. The NTTAA directs EPA
to provide Congress, through OMB, explanations when the Agency decides
not to use available and applicable voluntary consensus standards.
    This proposed rulemaking does not involve technical standards.
Therefore, EPA is not considering the use of any voluntary consensus
standards.

J. Executive Order 12898: Federal Actions To Address Environmental
Justice in Minority Populations and Low-Income Populations

    Executive Order 12898 (59 FR 7629 (Feb. 16, 1994)) establishes
federal executive policy on environmental justice. Its main provision
directs federal agencies, to the greatest extent practicable and
permitted by law, to make environmental justice part of their mission
by identifying and addressing, as appropriate, disproportionately high
and adverse human health or environmental effects of their programs,
policies, and activities on minority populations and low-income
populations in the United States.
    EPA has determined that this proposed rule will not have
disproportionately high and adverse human health or environmental
effects on minority or low-income populations because it increases the
level of environmental protection for all affected populations without
having any disproportionately high and adverse human health or
environmental effects on any population, including any minority or low-
income population. The proposed rule will establish uniform national
standards for O3 air pollution.

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List of Subjects in 40 CFR Part 50

    Environmental protection, Air pollution control, Carbon monoxide,
Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.

    Dated: June 20, 2007.
Stephen L. Johnson,
Administrator.
    For the reasons stated in the preamble, title 40, chapter I of the
code of Federal regulations is proposed to be amended as follows:

PART 50--NATIONAL PRIMARY AND SECONDARY AMBIENT AIR QUALITY
STANDARDS

    1. The authority citation for part 50 continues to read as follows:

    Authority: 42 U.S.C. 7401 et seq.

    2. Section 50.15 is added to read as follows:

Sec.  50.15  National primary and secondary ambient air quality
standards for ozone.

    (a) The level of the national 8-hour primary ambient air quality
standard for O3 is (0.070-0.075) parts per million (ppm),
daily maximum 8-hour average, measured by a reference method based on
Appendix D to this part and designated in accordance with part 53 of
this chapter.
    (b) The 8-hour primary O3 ambient air quality standard
is met at an ambient air quality monitoring site when the average of
the annual fourth-highest daily maximum 8-hour average O3
concentration is less than or equal to (0.070-0.075) ppm, as determined
in accordance with appendix P to this part.
    (c) The level of the national secondary ambient air quality
standard for O3 is a cumulative index value of (7-21) ppm-
hours, measured by a reference method based on Appendix D to this part
and designated in accordance with part 53 of this chapter.
    (d) The secondary O3 ambient air quality standard is a
seasonal standard expressed as a sum of weighted hourly concentrations,
cumulated over the 12 hour daylight period from 8 a.m. to 8 p.m. local
standard time, during the consecutive 3-month period within the
O3 monitoring season with the maximum index value. The
secondary O3 standard is met at an ambient air quality
monitoring site when the annual maximum consecutive 3-month cumulative
index value (W126) is less than or equal to (7-21) ppm-hours, as

[[Continued on page 37917]]

]

[[pp. 37917-37919]] National Ambient Air Quality Standards for Ozone

[[Continued from page 37916]]

determined in accordance with appendix P to this part.
    3. Appendix P is added to read as follows:

Appendix P to Part 50--Interpretation of the Primary and Secondary
National Ambient Air Quality Standards for Ozone

1. General

    (a) This appendix explains the data handling conventions and
computations necessary for determining whether the national 8-hour
primary and secondary ambient air quality standards for
O3 specified in Sec.  50.14 are met at an ambient
O3 air quality monitoring site. Ozone is measured in the
ambient air by a Federal reference method (FRM) based on appendix D
of this part, as applicable, and designated in accordance with part
53 of this chapter, or by a Federal equivalent method (FEM)
designated in accordance with part 53 of this chapter, or by an
Approved Regional Method (ARM) designated in accordance with part 58
of this chapter. Data reporting, data handling, and computation
procedures to be used in making comparisons between reported
O3 concentrations and the level of the O3
standard are specified in the following sections. Whether to
exclude, retain, or make adjustments to the data affected by
exceptional events, including stratospheric O3 intrusion
and other natural events, is subject to the requirements under Sec.
50.1, Sec.  50.14 and Sec.  51.930.
    (b) The terms used in this appendix are defined as follows:
    8-hour average is the rolling average of hourly O3
concentrations as explained in section 2 of this appendix.
    Annual fourth highest daily maximum refers to the fourth highest
value measured at a monitoring location during the O3
season for a particular year.
    Daily maximum 8-hour average concentration refers to the maximum
calculated 8 hour average for a particular day as explained in
section 2 of this appendix.
    Design values are the metrics (i.e., statistics) that are
compared to the NAAQS levels to determine compliance, calculated as
shown in sections 3 and 4 of this appendix.
    Ozone monitoring season refers to the span of time within a
calendar year when individual States are required to measure ambient
O3 concentrations as listed in part 58 appendix D to this
chapter.
    W126 is the weighted hourly O3 concentrations based
on seasonal measurements as explained in section 4 of this appendix.
    Year refers to calendar year.

2. Primary Ambient Air Quality Standard for Ozone

2.1 Data Reporting and Handling Conventions

    Computing 8-hour averages. Hourly average concentrations shall
be reported in parts per million (ppm) to the third decimal place,
with additional digits to the right being truncated. Running 8-hour
averages shall be computed from the hourly O3
concentration data for each hour of the year and the result shall be
stored in the first, or start, hour of the 8-hour period. An 8-hour
average shall be considered valid if at least 75% of the hourly
averages for the 8-hour period are available. In the event that only
6 (or 7) hourly averages are available, the 8-hour average shall be
computed on the basis of the hours available using 6 (or 7) as the
divisor (8-hour periods with three or more missing hours shall not
be ignored if, after substituting one-half the minimum detectable
limit for the missing hourly concentrations, the 8-hour average
concentration is greater than the level of the standard). The
computed 8-hour average O3 concentrations shall be
reported to three decimal places (the insignificant digits to the
right of the third decimal place are truncated, consistent with the
data handling procedures for the reported data).
    Daily maximum 8-hour average concentrations. (a) There are 24
possible running 8-hour average O3 concentrations for
each calendar day during the O3 monitoring season. The
daily maximum 8-hour concentration for a given calendar day is the
highest of the 24 possible 8-hour average concentrations computed
for that day. This process is repeated, yielding a daily maximum 8-