black cherry was projected in North Carolina, Tennessee, Indiana, Ohio,
Pennsylvania, Arizona, Michigan, New York, New Jersey, Maryland and
Delaware.  For ponderosa pine, an important tree species in the western
U.S., biomass loss was projected to be above 10 percent in much of its
range in California.  Biomass loss still occurred in many tree species
when O3 air quality was adjusted to meet the then current 8-hour
standard of 0.084 ppm.  For instance, black cherry, ponderosa pine,
eastern white pine, and aspen had estimated median seedling biomass
losses over portions of their growing range as high as 24, 11, 6, and 6
percent, respectively, when O3 air quality was rolled back to just meet
thea current0.084 ppm, 8-hour standard.  The 2007 Staff Paper noted that
these results are for tree seedlings and that mature trees of the same
species may have more or less of a response to O3 exposure.  Due to the
potential for compounding effects over multiple years, experts at a
consensus workshop on O3 vegetation effects and secondary standards,
hereinafter referred to as the 1996 Consensus Workshop, reported in a
subsequent 1997 Workshop Report, that a biomass loss greater than 2
percent annually can be significant (Heck and Cowling, 1997).. Decreased
seedling root growth and survivability could affect overall stand health
and composition in the long term.

In addition to the estimation of O3 effects on seedling growth, recent
work available in the 2008 rulemaking has enhanced our understanding of
risks beyond the seedling stage.  In order to better characterize the
potential O3 effects on mature tree growth, a tree growth model (TREGRO)
was used as a tool to evaluate the effect of changing O3 air quality
under just meet scenarios for selected alternative O3 standards on the
growth of mature trees.  TREGRO is a process-based, individual tree
growth simulation model (Weinstein et al, 1991).  This model has been
used to evaluate the effects of a variety of O3 exposure scenarios on
several species of trees by incorporating concurrent climate data in
different regions of the U.S. to account for O3 and climate/meteorology
interactions (Tingey et al., 2001; Weinstein et al., 1991; Retzlaff et
al., 2000; Laurence et al., 1993; Laurence et al., 2001; Weinstein et
al., 2005).  The model provides an analytical framework that accounts
for the nonlinear relationship between O3 exposure and response.  The
interactions between O3 exposure, precipitation and temperature are
integrated as they affect vegetation, thus providing an internal
consistency for comparing effects in trees under different exposure
scenarios and climatic conditions.  An earlier assessment of the
effectiveness of national ambient air quality standards in place since
the early 1970s took advantage of 40 years of air quality and climate
data for the Crestline site in the San Bernardino Mountains of
California to simulate ponderosa pine growth over time with the
improving air quality using TREGRO (Tingey et al., 2004).

	The TREGRO model was used to assess growth of Ponderosa pine in the San
Bernardino Mountains of California (Crestline) and the growth of yellow
poplar and red maple in the Appalachian mountains of Virginia and North
Carolina, Shenandoah National Park (Big Meadows) and Linville Gorge
Wilderness Area (Cranberry), respectively.  Total tree growth associated
with ‘as is’ air quality, and air quality adjusted to just meet
alternative O3 standards was assessed.  Ponderosa pine is one of the
most widely distributed pines in western North America, a major source
of timber, important as wildlife habitat, and valued for aesthetics
(Burns and Honkala, 1990).  Red maple is one of the most abundant
species in the eastern U.S. and is important for its brilliant fall
foliage and highly desirable wildlife browse food (Burns and Honkala,
1990).  Yellow poplar is an abundant species in the southern Appalachian
forest.  It is 10 percent of the cove hardwood stands in southern
Appalachians which are widely viewed as some of the country’s most
treasured forests because the protected, rich, moist set of conditions
permit trees to grow the largest in the eastern U.S.  The wood has high
commercial value because of its versatility and as a substitute for
increasingly scarce softwoods in furniture and framing construction.
Yellow poplar is also valued as a honey tree, a source of wildlife food,
and a shade tree for large areas (Burns and Honkala, 1990).

	The 2007 Staff Paper analyses found that just meeting a 0.084 ppm
standard would likely continue to allow O3-related reductions in annual
net biomass gain in these species.  This is based on model outputs that
estimate that as O3 levels are reduced below those of a 0.084 ppm
standard, significant improvements in growth would occur.  For instance,
estimated growth in red maple increased by 4 and 3 percent at Big
Meadows and Cranberry sites, respectively, when air quality was rolled
back to just met a W126 value of 13 ppm-hour.  Yellow poplar was
projected to have a growth increase between 0.6 and 8 percent under the
same scenario at the two eastern sites.

	Though there is uncertainty associated with the above analyses, this
information should be given careful consideration in light of several
other pieces of evidence.  Specifically, new evidence from experimental
studies that go beyond the seedling growth stage continues to show
decreased growth under elevated O3 (King et al. 2005).  Some mature
trees such as red oak have shown an even greater sensitivity of
photosynthesis to O3 than seedlings of the same species (Hanson et al.,
1994).  As indicated above, smaller growth loss increments may be
significant for perennial species.  The potential for cumulative
“carry over” effects as well as compounding also must be considered.
 The accumulation of such “carry-over” effects over time may affect
long-term survival and reproduction of individuals and ultimately the
abundance of sensitive tree species in forest stands.

c.	 Crops

	As discussed in the 2007 Staff Paper, risk of O3 exposure and
associated monetized benefits were estimated for commodity crops, fruits
and vegetables.  Similar to the tree seedling analysis, this analysis
combined C-R information on crops, crop growing regions and interpolated
exposures during each crop growing season.  NCLAN crop functions were
used for commodity crops.  According to USDA National Agricultural
Statistical Survey (NASS) data, the 9 commodity crop species (e.g.,
cotton, field corn, grain sorghum, peanut, soybean, winter wheat,
lettuce, kidney bean, potato) included in the 2007 Staff Paper analysis
accounted for 69 percent of 2004 principal crop acreage planted in the
U.S. in 2004.  The C-R functions for six fruit and vegetable species
(tomatoes-processing, grapes, onions, rice, cantaloupes, Valencia
oranges) were identified from the California fruit and vegetable
analysis from the 1997 review (Abt Associates Inc, 1995).  The 2007
Staff Paper noted that fruit and vegetable studies were not part of the
NCLAN program and C-R functions were available only in terms of seasonal
7-hour or 12-hour mean index.  This index form is considered less
effective in predicting plant response for a given change in air quality
than the cumulative form used with other crops.  Therefore, the fruit
and vegetable C-R functions were considered more uncertain than those
for commodity crops.

Analyses in the 2007 Staff Paper showed that some of the most important
commodity crops such as soybean, winter wheat and cotton had some
projected losses under the 2001 base year air quality.  Soybean yield
losses were projected to be 2-4 percent in parts of Pennsylvania, New
Jersey, Maryland and Texas.  Winter wheat was projected to have yield
losses of 2-6 percent in parts of California.  Additionally, cotton was
projected to have yield losses of above 6 percent in parts of
California, Texas and North Carolina in 2001.  The risk assessment
estimated that just meeting the then current 0.084 ppm, 8-hour standard
would still allow O3–related yield loss to occur in some commodity
crop species and fruit and vegetable species currently grown in the U.S.
 For example, based on median C-R function response, in counties with
the highest O3 levels, potatoes and cotton had estimated yield losses of
9-15 percent and 5-10 percent, respectively, when O3 air quality just
met the level of a 0.084 ppm, 8-hour standard.  Estimated yield improved
in these counties when the alternative W126 standard levels were met.
The very important soybean crop had generally small yield losses
throughout the country under just meeting the then current standard (0-4
percent).

	The 2007 Staff Paper also presented estimates of monetized benefits for
crops associated with a 0.084 ppm, 8-hour and alternative standards.
The Agriculture Simulation Model (AGSIM) (Taylor, 1994; Taylor, 1993)
was used to calculate annual average changes in total undiscounted
economic surplus for commodity crops and fruits and vegetables when the
then current and alternative standard levels were met.  Meeting the
various alternative standards did show some significant benefits beyond
a 0.084 ppm, 8-hour standard.  However, the 2007 Staff Paper recognized
that the AGSIM economic benefits estimates also incorporate several
sources of uncertainty, including:  (1) estimates of economic benefits
derived from use of the more uncertain C-R relationships for fruits and
vegetables; (2) uncertain assumptions about the treatment and effect of
government farm payment programs; and (3) uncertain assumptions about
near-term changes in the agriculture sector due to the increased use of
crops as biofuels.  Although the AGSIM model results provided a relative
comparison of agricultural benefits between alternative standards, these
uncertainties limited the utility of the absolute numbers.

D.	Reconsideration of Secondary Standard

	As discussed above at the beginning of section IV, this reconsideration
of the secondary O3 standard set in the 2008 rulemaking focuses on
reconsidering certain elements of the standard, the form, averaging
times, and level.  The general approach for setting a secondary O3
standard used in the 2008 rulemaking, and in the previous 1997
rulemaking in 1997, was to consider two basic policy options:  setting a
distinct secondary standard with a biologically relevant form and
averaging times, or setting a secondary standard identical to the
primary standard.  In the 2007 proposed rule, both such options were
evaluated, commented on by CASAC and the public, and proposed, as
discussed below in sections IV.D.1 and IV.D.2, respectively.  In the
2008 final rule, EPA decided to set the secondary standard identical to
the revised 8-hour primary standard, as discussed below in section
IV.D.3.  Section IV.D.4 summarizes comments received from CASAC
following the 2008 decision.  The Administrator’s proposed conclusions
based on this reconsideration are presented in section IV.D.5.

1.	Considerations Regarding the 2007 Proposed Cumulative Seasonal
Standard

a.	Form

The 2006 Criteria Document and 2007 Staff Paper concluded that the
recent vegetation effects literature evaluated in the 2008 rulemaking
strengthened and reaffirmed conclusions made in the 1997 review that the
use of a cumulative exposure index that differentially weights ambient
concentrations is best able to relate ambient exposures to vegetation
response at this time (EPA, 2006a, b; see also discussion in IV.B
above).  The 1997 review focused in particular on two of these
cumulative forms, the SUM06 and W126.  In the 2008 rulemaking, the 2007
Staff Paper again evaluated these two forms in light of two key pieces
of then recent information: estimates of PRB that were lower than in the
1997 review, and continued lack of evidence within the vegetation
effects literature of a biological threshold for vegetation exposures of
concern.  On the basis of those policy and science-related
considerations, the 2007 Staff Paper concluded that the W126 form was
more appropriate in the context of the 2008 rulemaking.  Specifically,
the W126 form, by its incorporation of a sigmoidal weighting scheme,
does not create an artificially imposed concentration threshold, gives
proportionally more weight to the higher and typically more biologically
potent concentrations, and is not significantly influenced by O3
concentrations within the range of estimated PRB. The Staff Paper
further concluded that “it is not appropriate to continue to use an
8-hour averaging time for the secondary standard” and that “the
8-hour average form should be replaced with a cumulative, seasonal,
concentration weighted form” (EPA, 2007b; pg.8-25).

The CASAC, based on its assessment of the same vegetation effects
science, agreed with the 2006 Criteria Document and 2007 Staff Paper and
unanimously concluded that it is not appropriate to try to protect
vegetation from the known or anticipated adverse effects of ambient O3
by continuing to promulgate identical primary and secondary standards
for O3.  Moreover, the members of the CASAC and a substantial majority
of the CASAC O3 Panel agreed with 2007 Staff Paper conclusions and
encouraged EPA to establish an alternative cumulative secondary standard
for O3 and related photochemical oxidants that is distinctly different
in averaging time, form and level from the current or potentially
revised 8-hour primary standard.  The CASAC also stated that “the
recommended metric for the secondary ozone standard is the
(sigmoidally-weighted) W126 index” (Henderson, 2007).

The EPA agreed with the conclusions drawn in the 2006 Criteria Document,
2007 Staff Paper and by CASAC that the scientific evidence available in
the 2008 rulemaking continued to demonstrate the cumulative nature of
O3-induced plant effects and the need to give greater weight to higher
concentrations.  Thus, EPA concluded that a cumulative exposure index
that differentially weights O3 concentrations represents a reasonable
policy choice for a seasonal secondary standard to protect against the
effects of O3 on vegetation.  The EPA further agreed with both the 2007
Staff Paper and CASAC that the most appropriate cumulative,
concentration-weighted form to consider in the 2008 rulemaking was the
sigmoidally weighted W126 form, due to EPA’s recognition that there is
no evidence in the literature for an exposure threshold that would be
appropriate across all O3-sensitive vegetation and that this form is
unlikely to be significantly influenced by O3 air quality within the
range of PRB levels identified in this rulemaking.  Thus, in 2007 EPA
proposed as one option to replace the then current 0.084 ppm, 8-hour
average secondary standard with a standard defined in terms of the
cumulative, seasonal W126 form.  The EPA also proposed the option of
making the secondary identical to the proposed revised primary standard.

b.	Averaging Times

	The 2007 Staff Paper, in addition to form, also considered what
exposure periods or durations are most relevant for vegetation, which,
unlike people, is exposed to ambient air continuously throughout its
lifespan.  For annual species, this lifespan encompasses a period of
only one year or less; while for perennials, lifespans can range from a
few years to decades or centuries.  However, because O3 levels are not
continuously elevated and plants are not equally sensitive to O3 over
the course of a day, season or lifetime, it becomes necessary to
identify periods of exposure that have the most relevance for plant
response.  The 2007 Staff Paper discussed exposure periods relevant for
vegetation in terms of a seasonal window and a diurnal window, and it
also discussed defining the standard in terms of an annual index value
versus a 3-year average of annual index values.  The numbered paragraphs
below present the 2007 Staff Paper discussions on these exposure
periods, and the annual versus 3-year average index value, followed by a
discussion of CASAC views and EPA proposed conclusions.

(1)  In considering an appropriate seasonal window, the 2007 Staff Paper
recognized that, in general, many annual crops are grown for periods of
a few months before being harvested.  In contrast, other annual and
perennial species may be photosynthetically active longer, and for some
species and locations, throughout the entire year.  In general, the
period of maximum physiological activity and thus, maximum potential O3
uptake for annual crops, herbaceous species, and deciduous trees and
shrubs coincides with some or all of the intra-annual period defined as
the O3 season, which varies on a state-by-state basis.  This is because
the high temperature and high light conditions that promote the
formation of tropospheric O3 also promote physiological activity in
vegetation.

The 2007 Staff Paper noted that the selection of any single seasonal
exposure period for a national standard would represent a compromise,
given the significant variability in growth patterns and lengths of
growing seasons among the wide range of vegetation species occurring
within the U.S. that may experience adverse effects associated with O3
exposures.  However, the 2007 Staff Paper further concluded that the
consecutive 3-month period within the O3 season with the highest W126
index value (e.g., maximum 3 month period) would, in most cases, likely
coincide with the period of greatest plant sensitivity on an annual
basis.  Therefore, the 2007 Staff Paper again concluded, as it did in
the 1997 review, that the annual maximum consecutive 3-month period is a
reasonable seasonal time period, when combined with a cumulative,
concentration weighted form, for protection of sensitive vegetation.

	(2)  In considering an appropriate diurnal window, the Staff Paper
recognized that over the course of the 24-hour diurnal period, plant
stomatal conductance varies in response to changes in light level, soil
moisture and other environmentally and genetically controlled factors.
In general, stomata are most open during daylight hours in order to
allow sufficient CO2 uptake for use in carbohydrate production through
the light-driven process of photosynthesis.  At most locations, O3
concentrations are also highest during the daytime, and thus, most
likely to coincide with maximum stomatal uptake.  It is also known
however, that in some species, stomata may remain open sufficiently at
night to allow for some nocturnal uptake to occur.  In addition, at some
rural, high elevation sites, the O3 concentrations remain relatively
flat over the course of the day, often at levels above estimated PRB.
At these sites, nighttime W126 values can be of similar magnitude as
daytime values, though the significance of these exposures is much less
certain.  This is because O3 uptake during daylight hours is known to
impair the light-driven process of photosynthesis, which can then lead
to impacts on carbohydrate production, plant growth, reproduction
(yield) and root function.  It is less clear at this time to what extent
and by what mechanisms O3 uptake at night adversely impacts plant
function.  In addition, many species have not been shown to take up O3
at night and/or do not occur in areas with elevated nighttime O3
concentrations.

In reviewing the information on this topic that became available after
the 1997 review, the 2007 Staff Paper considered the information
compiled in a summary report by Musselman and Minnick (2000).  This work
reported that some species take up O3 at night, but that the degree of
nocturnal stomatal conductance varies widely between species and its
relevance to overall O3-induced vegetation effects remain unclear.  In
considering this information, the 2007 Staff Paper concluded that for
the vast majority of studied species, daytime exposures represent the
majority of diurnal plant O3 uptake and are responsible for inducing the
plant response of most significance to the health and productivity of
the plant (e.g., reduced carbohydrate production).  Until additional
information is available about the extent to which co-occurrence of
sensitive species and elevated nocturnal O3 exposures exists, and what
levels of nighttime uptake are adverse to affected species, the 2007
Staff Paper concluded that this information continues to be preliminary,
and does not provide a basis for reaching a different conclusion
regarding the diurnal window at this time.  The 2007 Staff Paper further
noted that additional research is needed to address the degree to which
a 12-hour diurnal window may be under-protective in areas where elevated
nighttime levels of O3 co-occur with sensitive species with a high
degree of nocturnal stomatal conductance.  Thus, as in the 1997 review,
the 2007 Staff Paper again concluded that based on the available
science, the daytime 12-hour window (8:00 a.m. to 8:00 p.m.) is the most
appropriate period over which to cumulate diurnal O3 exposures,
specifically those most relevant to plant growth and yield responses.

(3)  In considering whether the standard should be defined in terms of
an annual index value or a 3-year average of annual index values, the
2007 Staff Paper recognized that though most cumulative seasonal
exposure levels of concern for vegetation have been expressed in terms
of the annual timeframe, it may be appropriate to consider a 3-year
average for purposes of standard stability.  However, the 2007 Staff
Paper noted that for certain welfare effects of concern (e.g., foliar
injury, yield loss for annual crops, growth effects on other annual
vegetation and potentially tree seedlings), an annual time frame may be
a more appropriate period in which to assess what level would provide
the requisite degree of protection, while for other welfare effects
(e.g., mature tree biomass loss), a 3-year average may also be
appropriate.  Thus, the 2007 Staff Paper concluded that it is
appropriate to consider both an annual and a 3-year average.  Further,
the 2007 Staff Paper concluded that should a 3-year average of the
12-hour W126 form be selected, a lower standard 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, diurnal, and annual or
multiyear time periods are most appropriate when combined with a
cumulative, seasonal form to protect vegetation from exposures of
concern, agreed that the 2007 Staff Paper conclusion regarding the
3-month seasonal period and 12-hour daylight window was appropriate,
with the distinction that both of these time periods 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 daily maximum 8-hour concentrations.  The CASAC further
concluded that if multi-year averaging is employed to afford greater
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 EPA, in determining which seasonal and diurnal time periods are most
appropriate to propose, took into account the 2007 Staff Paper and CASAC
views.  In being careful to consider what is needed to provide the
requisite degree of protection, no more and no less, in 2007 EPA
proposed that the 3-month seasonal period and 12-hour daylight period
are appropriate.  Based on the 2007 Staff Paper conclusions discussed
above, EPA was 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, EPA also
recognized 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, EPA agreed
with the 2007 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 EPA 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 EPA further proposed that the maximum
3-month period is sufficient and appropriate to characterize O3 exposure
levels associated with known levels of plant response.  Therefore, EPA
proposed that the most appropriate exposure periods for a cumulative,
seasonal form is the daytime 12-hour window (8:00 a.m. to 8:00 p.m.)
during the consecutive 3-month period within the O3 monitoring season
with the maximum W126 index value.

The EPA also proposed an annual rather than a multi-year cumulative,
seasonal standard.  In proposing this option, EPA also believed that it
was appropriate to consider the benefits to the public welfare that
would accrue from establishing a 3-year average secondary standard, and
solicited comment on this alternative.  In so doing,  EPA also agreed
with 2007 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 2007 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, considered what
information from the array of vegetation effects evidence and exposure
and risk assessment results was most useful.  Regarding the vegetation
effects evidence, the 2007 Staff Paper found stronger support than what
was available at the time of the 1997 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 2007
Staff Paper concluded that just meeting the then 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 O3 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 2007 Staff Paper also considered 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 2007 Staff
Paper concluded 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 2007 Staff Paper also recognized that in the 1997
review, EPA took into account the results of a 1996 Consensus Workshop.
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 SUM06 range of 10 to 15 (W126
range of 7 to 13) ppm-hour would be protective.  For growth effects to
tree seedlings and saplings in plantations, the consensus SUM06 range
was 12 to 16 (W126 range of 9 to 14) ppm-hour.  For visible foliar
injury to natural ecosystems, the consensus SUM06 range was 8 to 12
(W126 range of 5 to 9) ppm-hour.

Taking these consensus statements into account, EPA stated in the 1997
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 EPA put on the consensus report in the 1997 review,
the 2007 Staff Paper considered to what extent research published after
1997 provided empirical support for the ranges of levels identified by
the experts as protective of different types of O3-induced effects.
With regard to O3-induced biomass loss in sensitive tree
seedlings/saplings growing in natural forest stands, the information
discussed in the 2007 Staff Paper, including the evidence from free air
and gradient studies, provides additional direct support for the
conclusion that the 1996 Consensus Workshop approximate W126 range of
7-13 ppm-hour was an appropriate range to consider in selecting a
protective level.  With regard to visible foliar injury, the available
evidence, including the 2007 Staff Paper analysis of incidence in
counties with FIA monitoring sites and air quality data, showed
significant levels of county-level visible foliar injury incidence at
the W126 level of 13 ppm-hour.  However, because this analysis did not
address risks of this effect at lower levels of O3 air quality, and
because there is a significant uncertainty in predicting the degree of
visible foliar injury symptoms expected for lower levels of O3 air
quality, the evidence provides less certain but qualitative directional
support for the 1996 Consensus Workshop range of 5 to 9 ppm-hour to
protect against this effect.  With regard to O3-induced effects on
plantation trees, there is far less direct information available.
Though some forest plantation trees are O3-sensitive, the monoculture
nature of these stands makes uncertain the degree to which competition
for resources might play a role and to what degree the variety of
management practices applied would be expected to mitigate the
O3-induced effects.  Thus, it is difficult to distinguish a protective
range of levels for plantation trees from a range of levels that would
be protective of O3-sensitive tree seedlings and saplings in natural
forest stands.  Therefore, on the basis of the strength of the evidence
available, the 2007 Staff Paper concluded that it was appropriate to
consider a range for a 3-month, 12-hour, W126 standard that included the
1996 consensus recommendations for growth effects in tree seedlings in
natural forest stands (i.e., 7-13 ppm-hour in terms of a W126 form).

	In considering the available information on O3-related effects on crops
in the 2008 rulemaking, the 2007 Staff Paper observed 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 the 1997 review.  On the basis of these observations, the
2007 Staff Paper concluded that nothing in the newly assessed
information called into question the strength of the underlying science
upon which EPA based its proposed decision in the 1997 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 2007 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 2007 Staff Paper observed 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 2007 Staff Paper concluded that
the level of protection (no more than 10% yield or biomass loss in 50%
of studied cases) judged requisite in the 1997 review to protect the
public welfare from adverse levels of O3–-induced reductions in crop
yields and tree seedling biomass loss, as provided by a W126 level of 21
ppm-hour, remains appropriate for consideration as an upper bound of a
range of appropriate levels.

Thus, the 2007 Staff Paper concluded, based on all the above
considerations, that an appropriate range of 3-month, 12-hour W126
levels was 7 to 21 ppm-hour, 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, and indirectly, ecosystems, the 2007 Staff Paper recognized 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 2007 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-hour; however, it
did not agree with staff’s recommendation that the upper bound of the
range for consideration should be as high as 21 ppm-hour.  Rather, CASAC
recommended that the upper bound of the range considered should be no
higher than 15 ppm-hour, which is just above the upper ends of the
ranges identified in the 1996 Consensus Workshop as being protective of
tree seedlings and saplings grown in natural forest stands and in
plantations.  The lower end of this range (7 ppm-hour) is the same as
the lower end of the range identified in the 1996 Consensus Workshop as
protective of tree seedlings in natural forest stands from growth
effects.

In the 2007 proposed rule, taking 2007 Staff Paper and CASAC views into
account, EPA proposed 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-hour.  This range encompasses the
range of levels recommended by CASAC, and also includes a higher level
as recommended for consideration in the 2007 Staff Paper.  Given the
uncertainty in determining the risk attributable to various levels of
exposure to O3, EPA believed, as a public welfare policy judgment, that
this was a reasonable range to propose.

2.	Considerations Regarding the 2007 Proposed 8-Hour Standard

In the 1997 review, the 1996 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 2007 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 then
recent air quality.  Based on the results, the 2007 Staff Paper
concluded that the degree to which the then 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 2007 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 urged 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 was due to the concern that the analysis
in the 2007 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 2007 Staff Paper
concluded that it remained 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).

	In considering the appropriateness of proposing a revised secondary
standard that would be identical to the proposed primary standard, EPA
took into account the approach used by the Agency in the 1997 review,
the conclusions of the 2007 Staff Paper, CASAC advice, and the views of
public commenters.  The EPA first considered the 2007 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, EPA concluded 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 EPA also
recognized that lack of rural monitoring data made 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 using an 8-hour standard was clear, the number and size
of areas at issue and the degree of risk was hard to determine.  On the
other hand, EPA also considered at that time that there was a potential
risk of over-protection with a cumulative, seasonal standard given the
inherent uncertainties associated with moving to a new form for the
secondary standard, in particular those associated with predicting
exposure and risk patterns based on a limited rural monitoring network.

The EPA also considered the views and recommendations of CASAC, and
agreed 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, EPA also recognized that there
remained 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,
EPA also believed it was appropriate to consider the degree of
protection that would be afforded by a secondary standard that was
identical to the then proposed primary standard.  Based on its
consideration of the full range of views as described above, and in the
2007 proposed rule, EPA proposed as a second option to revise the
secondary standard to be identical in every way to the then proposed
primary standard.

3.	Basis for 2008 Decision on the Secondary Standard

In the 2008 final rule, EPA noted that deciding on the appropriate
secondary standard involved making a choice between two possible
alternatives, each with their strengths and weaknesses.  The 2008 final
rule reported that within the Administration at that time there had been
a robust discussion of the same strengths and weaknesses associated with
each option that were identified earlier.  The process by which EPA
reached its final conclusion is described in the final rule (73 FR
16497).  The rationale for the 2008 decision presented in the final rule
(73 FR 16499 – 16500) is described below.

In considering the appropriateness of establishing a new standard
defined in terms of a cumulative, seasonal form, or revising the then
current secondary standard by making it identical to the revised primary
standard, EPA took into account the approach used by the Agency in the
1997 review, the conclusions of the 2007 Staff Paper, CASAC advice, and
the views of public commenters.  In giving consideration to the approach
taken in the 1997 review, EPA first considered the 2007 Staff Paper
analysis of the projected degree of overlap between counties with air
quality expected to meet the revised 8-hour primary standard, set at a
level of 0.075 ppm, and alternative levels of a W126 standard based on
currently monitored air quality data. This analysis showed significant
overlap between the revised 8-hour primary standard and selected levels
of the W126 standard form being considered, with the degree of overlap
between these alternative standards depending greatly on the W126 level
selected and the distribution of hourly O3 concentrations within the
annual and/or 3-year average period.  On this basis, as an initial
matter, EPA concluded 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
then current 0.084 ppm secondary standard.  In further considering the
significant uncertainties that remain in the available body of evidence
of O3-related vegetation effects and in the exposure and risk analyses
conducted for the 2008 rulemaking, and the difficulty in determining at
what point various types of vegetation effects become adverse for
sensitive vegetation and ecosystems, EPA focused its consideration on a
level for an alternative W126 standard at the upper end of the proposed
range (i.e., 21 ppm-hour). The 2007 Staff Paper analysis showed that at
that W126 standard level, there would be essentially no counties with
air quality that would be expected both to exceed such an alternative
W126 standard and to meet the revised 8-hour primary standard – that
is, based on this analysis of currently monitored counties, a W126
standard would be unlikely to provide additional protection in any
monitored areas beyond that likely to be provided by the revised primary
standard.

The EPA also recognized that the general lack of rural monitoring data
made uncertain the degree to which the revised 8-hour standard or an
alternative W126 standard would be protective in those areas, 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
using an 8-hour standard was clear, the number and size of areas at
issue and the degree of risk was hard to determine. However, EPA
concluded at that time that an 8-hour standard would also tend to avoid
the potential for providing more protection than is necessary, a risk
that EPA concluded 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 EPA also considered the views and recommendations of CASAC, and
agreed that a cumulative, seasonal standard was the most biologically
relevant way to relate exposure to plant growth response.  However, as
reflected in some public comments, EPA also judged that there remained
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 these significant
uncertainties, EPA concluded at that time that establishing a new
secondary standard with a cumulative, seasonal form would result in
uncertain benefits beyond those afforded by the revised primary standard
and therefore may be more than necessary to provide the requisite degree
of protection.

Based on its consideration of the views discussed above, EPA judged in
the 2008 rulemaking that the appropriate balance to be drawn was to
revise the secondary standard to be identical in every way to the
revised primary standard.  The EPA believed that such a standard would
be sufficient to protect public welfare from known or anticipated
adverse effects, and did not believe that an alternative cumulative,
seasonal standard was needed to provide this degree of protection. The
EPA believed that this judgment appropriately considered the requirement
for a standard that is neither more nor less stringent than necessary
for this purpose.

For the reasons discussed above, and taking into account information and
assessments presented in the 2006 Criteria Document and 2007 Staff
Paper, the advice and recommendations of the CASAC Panel, and the public
comments to date, EPA decided to revise the existing 8-hour secondary
standard.  Specifically, EPA revised the then current 8-hour average
0.084 ppm secondary standard by making it identical to the revised
8-hour primary standard set at a level of 0.075 ppm.

4.	CASAC Views Following 2008 Decision

Following the 2008 decision on the O3 standards, serious questions were
raised as to whether the standards met the requirements of the CAA.  In
April 2008, the members of the CASAC Ozone Review Panel sent a letter to
EPA stating “In our most-recent letters to you on this subject - dated
October 2006 and March 2007 - … the Committee recommended an
alternative secondary standard of cumulative form that is substantially
different from the primary Ozone NAAQS in averaging time, level and form
— specifically, the W126 index within the range of 7 to 15 ppm-hour,
accumulated over at least the 12 “daylight” hours and the three
maximum ozone months of the summer growing season” (Henderson, 2008).
The letter continued: “The CASAC now wishes to convey, by means of
this letter, its additional, unsolicited advice with regard to the
primary and secondary Ozone NAAQS. In doing so, the participating
members of the CASAC Ozone Review Panel are unanimous in strongly urging
you or your successor as EPA Administrator to ensure that these
recommendations be considered during the next review cycle for the Ozone
NAAQS that will begin next year” (id.).  The letter further stated the
following views:

The CASAC was … greatly disappointed that you failed to change the
form of the secondary standard to make it different from the primary
standard. As stated in the preamble to the Final Rule, even in the
previous 1996 ozone review, “there was general agreement between the
EPA staff, CASAC, and the Administrator, … that a cumulative, seasonal
form was more biologically relevant than the previous 1-hour and new
8-hour average forms (61 FR 65716)” for the secondary standard.
Therefore, in both the previous review and in this review, the Agency
staff and its advisors agreed that a change in the form of the secondary
standard was scientifically well-justified.

:   :  :  :  ;The CASAC was pleased to see that the EPA Deputy
Administrator clearly articulated a robust scientific defense of this
position when he responded to Ms. Susan Dudley of the Office of
Management and Budget (OMB) in a memorandum dated March 7, 2008 that,
“In light of the available information, EPA believes that
ozone-related effects on vegetation are clearly linked to cumulative,
seasonal exposures and are not appropriately characterized by the use of
a short-term (8-hour) daily measure of ozone exposure.” However, the
Committee was disappointed and surprised that written correspondence
from OMB to the Agency apparently thwarted the opportunity to take a
major step forward in setting a separate secondary ozone standard that
is different in form from the primary standard. The CASAC is
particularly dismayed at the suggestion that setting a secondary NAAQS
that is different from the primary NAAQS is somehow against the law —
which is not only at odds with a plain-language reading of the Clean Air
Act but is also contrary to the Agency’s previous actions in setting a
separate secondary standard for the initial NAAQS for both particulate
matter and sulfur oxides, the latter of which (i.e., for SO2) remains in
effect.

Unfortunately, this scientifically-sound approach of using a cumulative
exposure index for welfare effects was not adopted, and the default
position of using the primary standard for the secondary standard was
once again instituted. Keeping the same form for the secondary Ozone
NAAQS as for the primary standard is not supported by current scientific
knowledge indicating that different indicator variables are needed to
protect vegetation compared to public health. The CASAC was further
disappointed that a secondary standard of the W126 form was not
considered from within the Committee’s previously-recommended range of
7 to 15 ppm-hour. The CASAC sincerely hopes that, in the next round of
Ozone NAAQS review, the Agency will be able to support and establish a
reasonable and scientifically-defensible cumulative form for the
secondary standard.  (Henderson, 2008)

5.	Administrator’s Proposed Conclusions

	For the reasons discussed below, the Administrator proposes to set a
cumulative seasonal standard expressed as an annual index of the sum of
weighted hourly concentrations (i.e., the W126 form), cumulated over 12
hours per day (8:00 am to 8:00 pm) during the consecutive 3-month period
within the O3 season with the maximum index value, set at a level within
the range of 7 to 15 ppm-hour.  This proposed decision takes into
account the information and assessments presented in the 2006 Criteria
Document and the 2007 Staff Paper and related technical support
documents, the advice and recommendations of CASAC both during and
following the 2008 rulemaking, and public comments received in
conjunction with review of drafts of these documents and on the 2007
proposed rule.

a.	Form

As discussed above in section IV.B, the 2006 Criteria Document and 2007
Staff Paper concluded that the recent vegetation effects literature
evaluated in the 2008 rulemaking strengthens and reaffirms conclusions
made in the 1997 review that the use of a cumulative exposure index that
differentially weights ambient concentrations is best able to relate
ambient exposures to vegetation response. The 1997 review focused in
particular on two of these cumulative forms, the SUM06 and W126 (EPA,
1996).  Given that the data available at that time were unable to
distinguish between these forms, the EPA, based on the policy
consideration of not including O3 concentrations considered to be within
the PRB, estimated at that time to be between 0.03 and 0.05 ppm,
concluded that the SUM06 form would be the more appropriate choice for a
cumulative, exposure index for a secondary standard.

In the 2008 rulemaking, the 2007 Staff Paper evaluated the continued
appropriateness of the SUM06 form in light of new estimates of PRB that
were lower than in the 1997 review, and the continued lack of evidence
within the vegetation effects literature of a biological threshold for
vegetation exposures of concern.  On the basis of these policy and
science-related considerations, the 2007 Staff Paper concluded that the
W126 form was the more appropriate cumulative, concentration-weighted
form.  Specifically, the W126, by its incorporation of a sigmoidal
weighting scheme, does not create an artificially imposed concentration
threshold, gives proportionally more weight to the higher and typically
more biologically potent concentrations, and is not significantly
influenced by O3 concentrations within the range of estimated PRB.

As discussed above, the CASAC, based on its assessment of the same
vegetation effects science, agreed with the 2006 Criteria Document and
2007 Staff Paper and unanimously concluded that protection of vegetation
from the known or anticipated adverse effects of ambient O3 “requires
a secondary standard that is substantially different from the primary
standard in averaging time, level, and form,” i.e. not identical to
the primary standard for O3 (Henderson, 2007).  Moreover, the members of
CASAC and a substantial majority of the other CASAC Panel members agreed
with 2007 Staff Paper conclusions and encouraged EPA to establish an
alternative cumulative secondary standard for O3 and related
photochemical oxidants that is distinctly different in averaging time,
form and level from the then current or potentially revised 8-hour
primary standard (Henderson, 2006c).  The CASAC Panel also stated that
“the recommended metric for the secondary ozone standard is the
(sigmoidally weighted) W126 index” (Henderson, 2007).

In reconsidering the 2008 final rule, the Administrator agrees with the
conclusions drawn in the 2006 Criteria Document, 2007 Staff Paper and by
CASAC that the scientific evidence available in the 2008 rulemaking
continues to demonstrate the cumulative nature of O3-induced plant
effects and the need to give greater weight to higher concentrations.
Thus, the Administrator concludes that a cumulative exposure index that
differentially weights O3 concentrations represents a reasonable policy
choice for a secondary standard to protect against the effects of O3 on
vegetation   The Administrator further agrees with both the 2007 Staff
Paper and CASAC that the most appropriate cumulative,
concentration-weighted form to consider is the sigmoidally weighted W126
form

The Administrator notes that in the 2007 proposed rule, EPA proposed a
second option of revising the then current 8-hour average secondary
standard by making it identical to the proposed 8-hour primary standard.
 The 2007 Staff Paper analyzed the degree of overlap expected between
alternative 8-hour and cumulative seasonal secondary standards using
recent air quality monitoring data.  Based on the results, the 2007
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.
 The 2007 Staff Paper also recognized that meeting the then current or
alternative levels of the 8-hour average standard could result in air
quality improvements that would potentially benefit vegetation in some
areas, but urged 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 was due to the concern that the analysis in the 2007 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, there are likely to be additional areas that
have similar air quality distributions that would lead to the same
disconnect between forms.  Thus, the 2007 Staff Paper concluded that it
remains problematic to determine the appropriate level of protection for
vegetation using an 8-hour average form.

The Administrator also notes that 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 form, averaging time, and level (Henderson, 2007).

In reaching her proposed decision in this reconsideration of the 2008
final rule, the Administrator has considered the comments received on
the 2007 proposed rule regarding revising the secondary standard either
to reflect a new, cumulative form or by remaining equal to a revised
primary standard.  The commenters generally fell into two groups.

One group of commenters, including environmental organizations, strongly
supported the proposed option of moving to a cumulative, seasonal
standard, generally based on the reasoning explained in the 2007
proposal.  Commenters in this group also expressed serious concerns with
the other proposed option of setting a secondary O3 standard in terms of
the same form and averaging time (i.e., daily maximum 8-hour average O3
concentration) as the primary standard.  These commenters expressed the
view that such a standard would fail to protect public welfare because
the maximum daily 8-hour average O3 concentration failed to adequately
characterize harmful O3 exposures to vegetation.  This view was
generally based on the observation that there is no consistent
relationship in areas across the U.S. between 8-hour peak O3
concentrations and the longer-term cumulative exposures aggregated over
a growing season that are biologically relevant in characterizing
O3-related effects on sensitive vegetation.  Thus, as EPA noted in the
2007 proposed rule, there is a lack of a rational connection between the
level of an 8-hour standard and the requisite degree of protection
required for a secondary O3 NAAQS.

Another group of commenters, including industry organizations, agreed
that a cumulative form of the standard may better match the underlying
data, but expressed the view that remaining uncertainties associated
with the vegetation effects evidence and/or EPA’s exposure, risk and
benefits assessments were so great that the available information did
not provide an adequate basis to adopt a standard with a level based on
a cumulative, seasonal form.  These commenters asserted that because of
the substantial uncertainties remaining at the time of the 2008
rulemaking, the benefits of changing to a W126 form were too uncertain
to warrant revising the form of the standard at that time.

The Administrator notes that in both the 1997 and the 2008 decisions,
EPA recognized that the risk to vegetation from O3 exposures comes from
cumulative exposures over a season or seasons.   The CASAC has fully
endorsed this view based on the available scientific evidence and
assessments, and there is no significant disagreement on this issue by
commenters.  Thus, it is clear that the purpose of the secondary O3
NAAQS should be to provide an appropriate degree of protection against
cumulative, seasonal exposures to O3 that are known or anticipated to
harm sensitive vegetation or ecosystems.  In reconsidering the 2008
final rule, the Administrator recognizes that the issue before the
Agency is what form of the standard is most appropriate to perform that
function.

Within this framework, the Administrator recognizes that it is clear
that a cumulative, seasonal form has a distinct advantage in protecting
against cumulative, seasonal exposures.  Such a form is specifically
designed to measure directly the kind of O3 exposures that can cause
harm to vegetation.  In contrast, an 8-hour standard does not measure
cumulative, seasonal exposures directly, and can only indirectly afford
some degree of protection against such exposures.  To the extent that
clear relationships exist between 8-hour daily peak O3 concentrations
and cumulative, seasonal exposures, the 8-hour form and averaging time
would have the potential to be effective as an indirect surrogate.
However, as discussed in the 2007 proposed rule and the 2008 final rule,
the evidence shows that there are known types of O3 air quality patterns
that can lead to high levels of cumulative, seasonal O3 exposures
without the occurrence of high daily 8-hour peak O3 concentrations.  An
8-hour form and averaging time is an indirect way to measure the
biologically relevant exposure patterns, is poorly correlated with such
exposure patterns, and therefore provides less certainty that the
standard willis less likely to identify and protect against the kind of
cumulative, seasonal exposure patterns that have been determined to be
harmful.

Past arguments or reasons for not moving to a cumulative, seasonal form,
with appropriate exposure periods, have not been based on disagreement
over the biological relevance of the cumulative, seasonal form, or the
recognized disadvantages of an 8-hour standard in measuring and
identifying a specified cumulative, seasonal exposure pattern.  The
reasons for not moving to such a form have been based on concerns over
whether EPA has an adequate basis to identify the nature and magnitude
of cumulative, seasonal exposure patterns that the standard should be
designed to protect against, given the various uncertainties in the
evidence and the lack of rural O3 monitoring data.  This most directly
translates into a concern over whether EPA has an adequate basis to
determine an appropriate level for a cumulative, seasonal secondary
standard.

The Administrator has also considered issues associated with selection
of the W126 cumulative form, as reflected in the following assertions
made by some commenters on the 2007 proposed rule:  (1) the W126 form
lacks a biological basis, since it is merely a mathematical expression
of exposure that has been fit to specific responses in OTC studies, such
that its relevance for real world biological responses is unclear; (2) a
flux-based model would be a better choice than a cumulative metric
because it is an improvement over the many limitations and
simplifications associated with the cumulative form; however, there is
insufficient data to apply such a model at present; (3) the European
experience with cumulative O3 metrics has been disappointing and now
Europeans are working on their second level approach, which will be
flux-based; and (4) a second index that reflects the accumulation of
peaks at or above 0.10 ppm (called N100)  should be added to a W126
index to achieve appropriate protection.

With regard to whether the W126 index lacks a biological basis, the
Administrator finds no basis for reaching such a conclusion.  As
discussed above in section IV.B, the vegetation effects science is clear
that exposures of concern to plants are not based on one discrete 8-hour
period but on the repeated occurrence of elevated O3 levels throughout
the plant’s growing season.  The cumulative nature of the W126 is
supported by the basic biological understanding that plants in the U.S.
are generally most biologically active during the warm season and are
exposed to ambient O3 throughout this biologically active period.  In
addition, it has been shown in the scientific literature that all else
being equal, plants respond more to higher O3 concentrations, with no
evidence of an exposure threshold for vegetation effects.  The W126
sigmoidal weighting function reflects both of these understandings, by
not including a threshold below which concentrations are not included,
and by differentially weighting concentrations to give greater weight to
higher concentrations and less weight to lower ones.

With regard to whether a flux-based model would be a better choice, the
2007 Staff Paper acknowledged that flux models may produce a more
accurate calculation of dose to a specific plant species in a specific
area.  However, dose-response relationships have not been developed for
these flux calculations for plants growing in the U.S.  Further, flux
calculations require large amounts of data for the physiology of each
plant species and the local conditions for the growing range of each
plant species.  These exercises may be useful for limited small-scale
risk assessments, but do not provide an appropriate basis for a national
standard at this time.

With regard to dissatisfaction with the performance of a particular
cumulative index in use in Europe, and growing interest in development
of flux-based models, the 2007 Staff Paper (Appendix 7A) noted that
“because of a lack of flux-response data, a cumulative, cutoff
concentration based (e.g., AOT40) exposure index will remain in use in
Europe for the near future for most crops and for forests and
semi-natural herbaceous vegetation (Ashmore et al., 2004a).”  Further,
like the SUM06 index, the AOT40 index incorporates a threshold below
which concentrations are not considered.  Though the AOT40 threshold is
lower than the threshold value in SUM06, the 2007 Staff Paper concluded
that the vegetation effects information does not provide evidence of an
effects threshold that applies to all species.  Thus, the Administrator
concludes neither of these forms is as biologically relevant as the W126
form.

With regard to consideration of coupling a W126 form with a separate
N100 index, there was very little research on the N100 index or a
coupled approach to be evaluated in the 2008 rulemaking.  The CASAC,
after reviewing all the information in the 2006 Criteria Document and
the 2007 Staff Paper, did not recommend an additional N100 index for
consideration.  Therefore, there is no basis at this time to judge the
extent to which such a coupled W126-N100 form would be a better choice
than the proposed W126 form.  Further, the W126 form incorporates a
weighting scheme that places greater weight on increasing concentrations
and gives every concentration of 0.10 ppm and above an equal weight of
1, which is the highest weight in this sigmoidal weighting function.

	In summary, having considered the scientific information and assessment
results available in the 2008 rulemaking as discussed above in this
proposal notice, as well as the recommendations of the staff and CASAC,
and having taken into consideration issues raised in public comments
received as part of the 2008 rulemaking, and recognizing the
determinations made below in section IV.D.5.c on level, the
Administrator concludes that it is appropriate to set the secondary
standard using a cumulative, seasonal form.  The Administrator also
concludes that the W126 form is best suited to reflect the biological
impacts of O3 exposure on vegetation, and that there is adequate
certainty in the information available in the 2008 rulemaking to support
such a change in form.  Thus, the Administrator proposes to set the
secondary standard using a cumulative, seasonal W126 form.

b.	Averaging Times

The Administrator, in addition to reconsidering what form of a secondary
standard is most appropriate for protecting vegetation, is also
reconsidering what exposure periods (e.g., seasonal window, diurnal
window), and what standard index, in terms of an annual index value
versus a 3-year average of annual index values, are most appropriate
when used in conjunction with the W126 cumulative seasonal form.   Based
on the information set forth in the 2007 Staff Paper, as well as CASAC
views, as discussed above in section IV.D.1.b, the Administrator has
reached conclusions regarding exposure periods, and the annual versus
3-year average index, that have the most biological relevance for plant
response, as discussed below.

In considering an appropriate seasonal window, the Administrator notes
that the 2007 Staff Paper concluded that the consecutive 3-month period
within the O3 season with the highest W126 index value (e.g., maximum
3-month period) was a reasonable seasonal time period to consider.  The
Administrator further notes that the 2007 Staff Paper acknowledged that
the selection of any single seasonal exposure period for a national
standard would necessarily represent a compromise, given the significant
variability in growth patterns and lengths of growing seasons among the
wide range of sensitive vegetation species occurring within the U.S.
However, the Administrator also considered the Staff Paper conclusion
that the period of maximum potential plant uptake of O3 would also
likely coincide with the period of highest O3 occurring within the
intra-annual period defined as the O3 season, since the high temperature
and light conditions conducive to O3 formation are also conducive for
plant activity.  The Administrator also observes that the CASAC panel
was supportive of the Staff Paper views, while recognizing that 3 months
likely represented the minimum timeframe appropriate to consider.
Therefore, the Administrator concludes, on these bases, that the
consecutive 3-month period within the O3 season with the highest W126
index value (e.g., maximum 3-month period) remains an appropriate
seasonal window to propose for the protection of sensitive vegetation.

With regard to consideration of an appropriate diurnal window, the
Administrator has taken into account the 2007 Staff Paper conclusion
that for the vast majority of studied species, daytime exposures
represent the majority of diurnal plant O3 uptake and are responsible
for inducing the plant response of most significance to the health and
productivity of the plant (e.g., reduced carbohydrate production).   The
Administrator is also aware, based on discussions in the 2007 Staff
Paper that there are some number of species that show non-negligible
amounts of O3 uptake at night due to incomplete stomatal closure.   In
reaching her conclusion that the 2007 Staff Paper recommendation of a
12-hour daytime window (8:00 a.m. to 8:00 p.m.) remains the most
appropriate period over which to cumulate diurnal O3 exposures,
specifically those most relevant to plant growth and yield responses,
the Administrator places weight on the fact that the CASAC comments were
also supportive of this diurnal window, recognizing again that it likely
represents a minimum period over which plants can be vulnerable to O3
uptake.  Therefore, the Administrator is again proposing the 12-hour
daytime window (8:00 a.m. to 8:00 p.m.) as an appropriate diurnal window
to protect against O3-induced plant effects.

Lastly, in considering whether an annual or a 3-year average index is
more appropriate, the Administrator notes that in addition to the
available scientific evidence regarding plant effects that can be
brought to bear, there are also other public welfare considerations that
may be appropriate to consider.  In taking this view, the Administrator
notes that the 2007 Staff Paper recognized that though most cumulative
seasonal exposure levels of concern for vegetation have been expressed
in terms of the annual timeframe, it may be appropriate to consider a
3-year average for purposes of standard stability.  The Administrator
has considered that while the 2007 Staff Paper notes that for certain
welfare effects of concern (e.g., foliar injury, yield loss for annual
crops, growth effects on other annual vegetation and potentially tree
seedlings), an annual time frame may be a more appropriate period in
which to assess what level would provide the requisite degree of
protection, for other welfare effects (e.g., mature tree biomass loss),
it also points out that a 3-year average may also be appropriate.  The
Administrator further observes that in concluding that it was
appropriate to consider both an annual and a 3-year average, the 2007
Staff Paper also concluded that should a 3-year average of the 3-month,
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 Administrator also took note
that the CASAC Panel, in addressing this issue of annual versus 3-year
average 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, and 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 considering the merits of both the annual and 3-year
average, and taking into account both the 2007 Staff Paper and CASAC
views, concludes that it is important to place more weight on the public
welfare benefit in having a stable standard, and that appropriate
protection for vegetation can be achieved using a 3-year average form.
The Administrator is thus proposing a 3-year average.  However, given
the uncertain nature of the evidence and potential concerns with using a
3-year average form, the Administrator is proposing to take comment on
the appropriateness of the specific seasonal and diurnal exposure
periods proposed, as well as the use of a 3-year average, and, as
discussed below, the impact that selection of these proposed seasonal
and diurnal exposure periods would have, in conjunction with a 3-year
average form, on the appropriateness of the proposed range of levels.

c.	Level

i.	Considerations Regarding 2007 Proposed Range of Levels

The 2007 Staff Paper, in identifying a range of levels for a 3-month,
12-hour (daytime) W126 standard appropriate for the Administrator to
consider in protecting the public welfare from known or anticipated
adverse effects to vegetation from O3 exposures, considered what
information from the array of vegetation effects evidence and exposure
and risk assessment results was most useful.  With respect to the
vegetation effects evidence, the 2007 Staff Paper found stronger support
than what was available at the time of the 1997 review for an increased
level of protection for trees and forested ecosystems.  Specifically,
the expanded body of evidence included: (1) additional field based data
from free air, gradient and biomonitoring surveys demonstrating adverse
levels of O3-induced 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 continued 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 sources of vegetation effects information and the
results of the exposure and risk assessments summarized above, the 2007
Staff Paper concluded that just meeting the then current 0.084 ppm,
8-hour average standard would continue to allow adverse levels of
O3-induced effects to occur in sensitive commercially and ecologically
important tree species in many regions of the country.  The 2007 Staff
Paper further concluded that air quality levels would need to be
substantially reduced to protect sensitive tree seedlings, such as black
cherry, aspen, and cottonwood, from these growth and foliar injury
effects.

In addition to the currently quantifiable risks to trees from ambient
exposures, the 2007 Staff Paper also considered 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 2007 Staff
Paper concluded 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.

	The 2007 Staff Paper also recognized that in the 1997 review, EPA took
into account the results of a 1996 Consensus Workshop.  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 on protective 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 levels shown in
parentheses.  For growth effects to tree seedlings in natural forest
stands, a consensus was reached that a SUM06 range of 10 to 15 (W126
range of 7 to 13) ppm-hour would be protective.  For growth effects to
tree seedlings and saplings in plantations, the consensus SUM06 range
was 12 to 16 (W126 range of 9 to 14) ppm-hour.  For visible foliar
injury to natural ecosystems, the consensus SUM06 range was 8 to 12
(W126 range of 5 to 9) ppm-hour.

The 2007 Staff Paper then considered to what extent recent research
provided empirical support for the ranges of levels identified by the
experts as protective of different types of O3-induced effects.  As
discussed above in section IV.D.1.c, the 2007 Staff Paper concluded on
the basis of the available evidence that it was appropriate to consider
a range for a 3-month, 12-hour, W126 standard level that included the
1996 Consensus Workshop recommendations regarding a range of levels
protective against O3-induced growth effects in tree seedlings in
natural forest stands (i.e., 7-13 ppm-hour in terms of a W126 form).

	In considering the newly available information on O3-related effects on
crops in this review, the 2007 Staff Paper observed 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 2007 Staff
Paper concluded that nothing in the newly assessed information calls
into question the strength of the underlying science upon which EPA
based its 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 2007 Staff Paper then considered whether any additional information
was 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 2007 Staff Paper observed 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.
Since these latter effects could not be quantified at that time, they
could only be considered qualitatively in reaching judgments about an
appropriate degree of protection for commodity crops from O3–related
effects.

Based on the above considerations, the 2007 Staff Paper concluded that
the level of protection judged requisite in the 1997 review to protect
the public welfare from adverse levels of O3–induced reductions in
crop yields and tree seedling biomass loss, as approximately provided by
a W126 level of 21 ppm-hour, remained appropriate for consideration as
an upper bound of a range of appropriate levels.  The 2007 Staff Paper
also recognized that a standard set at this level would not protect the
most sensitive species or individuals within a species from all
potential effects related to O3 exposures and further, that this level
derives from the extensive and quantitative historic and recent crop
effects database, as well as current staff exposure and risk analyses
(EPA, 2007, pg. 8-22).

In identifying a lower bound for the range of alternative standard
levels appropriate for consideration, staff concluded that several lines
of evidence pointed to the need for greater protection for tree
seedlings, mature trees, and associated forested ecosystems. Staff
believed that tree growth was an important endpoint to consider because
it is related to other aspects of societal welfare such as sustainable
production of timber and related goods, recreation, and carbon (CO2)
sequestration.  Impacts on tree growth can also affect ecosystems
through shifts in species composition and the loss of genetic diversity
due to the loss of O3 sensitive individuals or species.  In selecting an
appropriate level of protection for trees, staff considered the results
of the 1996 Consensus Workshop which identified the SUM06 range of 10 to
15 (W126 of 7 to 13) ppm-hour for growth effects to tree seedlings in
natural forest stands.

Because staff believed that O3-related effects on forest tree species
are important public welfare effects of concern, it therefore concluded,
based on the above, that it was appropriate to include 7 ppm-hour as the
lower bound of the recommended range, the lower end of the approximate
range recommended by CASAC (Henderson, 2006c) and identified by the 1996
Consensus Workshop participants as protective of forest trees.  At this
lower end of the range, staff anticipated, based on its analyses of
risks of tree seedling biomass loss and mature tree growth reductions
and on the basis of the scientific effects literature, that adverse
effects of O3 on forested ecosystems would be substantially reduced.
Further, staff anticipated that the lower end of this range would
provide increased protection from 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. Staff also noted that
by disrupting the photosynthetic process, decreasing carbon storage in
the roots, increasing early senescence of leaves and affecting water use
efficiency in trees, O3 exposure could potentially disrupt or change the
nutrient and water flow of an entire system.  Such weakened trees can
become more susceptible to other environmental stresses such as pest and
pathogen outbreaks or harsh weather conditions. While recognizing that
it is not possible to quantify all the ecological and societal benefits
associated with varying levels of alternative secondary standards, staff
believed that this information should be weighed in considering the
extent to which a secondary standard should be precautionary in nature
in protecting against effects that have not yet been adequately studied
and evaluated.

Thus, the 2007 Staff Paper concluded, based on all the above
considerations, that an appropriate range of levels, for an annual
standard using a 3-month, 12-hour W126 form, for the Administrator to
consider was 7 to 21 ppm-hour, 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 2007 Staff Paper recognized that it was 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.

ii.	CASAC and Public Comments Prior to 2008 Decision

In considering the evidence described in both the 2006 Criteria Document
and 2006 draft Staff Paper, CASAC, in its October 24, 2006 letter to the
Administrator, expressed its view regarding the appropriate form and
range of levels for the Administrator to consider.  The CASAC preferred
a seasonal 3-month W126 standard in a range that is the approximate
equivalent of the SUM06 at 10 to 20 ppm-hour.  Following the 2007
proposal, EPA received additional CASAC and public comments regarding an
appropriate range of levels of a W126 form for the Administrator to
consider in finalizing a revised secondary NAAQS for O3.  The CASAC, in
its final letter to the Administrator (Henderson, 2007), agreed with the
2007 Staff Paper recommendations that the lower bound of the range
within which a seasonal W126 secondary O3 standard should be considered
is approximately 7 ppm-hour; however, it did not agree with staff’s
recommendation that the upper bound of the range should be as high as 21
ppm-hour.  Rather, as discussed above in section IV.D.1.c, the CASAC
Panel recommended that the upper bound of the range considered should be
no higher than a W126 of 15 ppm-hour for an annual standard.

The comments received from the public fell into two groups.  One group
of commenters supported the CASAC recommended range of 7- 15 ppm-hour
for a W126 standard.  Many of these same commenters further emphasized
the lower end of the proposed range as necessary to provide adequate
protection for sensitive species.  These commenters based their
recommendation primarily on four sources of information:  (1)
field-based evidence of foliar injury occurring on sensitive species at
air quality levels well below that of the current standard; (2) the 1996
Consensus Workshop recommendations for protective levels in terms of
cumulative exposures for different vegetation types; (3) CASAC advice
and recommendations;  and (4) studies published after the close of the
2006 Criteria Document that potentially strengthen the link between
species level impacts and ecosystem response.

	The other group of commenters did not support revising the current
secondary standard.  These commenters primarily focused on uncertainties
regarding the sources of information relied upon by the first group of
commenters as support for a level within the range of levels recommended
by CASAC.   These uncertainties included:  (1) potential confounders,
such as soil moisture, on visible foliar injury and the lack of a clear
relationship between visible foliar injury symptoms and other vegetation
effects; (2) lack of documentation of the basis for the recommendations
from the 1996 Consensus Workshop in selecting a range of levels,
indicating that these recommendations should be used with great caution;
(3) failure of CASAC and EPA to take into account the monitor height
measurement gradient when making their recommendations concerning the
level of the secondary standard; and  (4) inability to quantitatively
estimate ecosystem effects of O3 or to extrapolate meaningfully from
effects on individual plants to ecosystem effects due to inadequate
data.

iii.	Conclusions on Level

The Administrator is proposing to set a cumulative, seasonal standard
expressed in terms of the maximum 3-month, 12-hour W126 form, in the
range of 7 to 15 ppm-hour.  In reaching this proposed decision about an
appropriate range of levels for the secondary standard, the
Administrator has considered the following:  the evidence described in
the 2006 Criteria Document and the 2007 Staff Paper; the results of the
vegetation exposure and risk assessments discussed above and in the 2007
Staff Paper, giving weight to the assessments as judged appropriate; the
CASAC Panel’s advice and recommendations in the CASAC’s letters to
the Administrator; EPA staff recommendations; and public comments
received during the development of these documents, either in connection
with CASAC meetings or separately.  In considering what range of levels
of a cumulative 3-month standard to propose, the Administrator notes
that that this choice requires judgment as to what standard will protect
the public welfare from any known or anticipated adverse effects.  This
choice must be based on an interpretation of the evidence and other
information, such as the exposure and risk assessments, that neither
overstates nor understates the strength and limitations of the evidence
and information nor the appropriate inferences to be drawn.  In taking
all of the above into consideration, the Administrator also notes that
there is no bright line clearly directing the choice of level for any of
the effects of concern, and the choice of what is appropriate is clearly
a public welfare policy judgment entrusted to the Administrator.  This
judgment must include consideration of the strengths and limitations of
the evidence and the appropriate inferences to be drawn from the
evidence and the exposure and risk assessments.

In particular, 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) what are
CASAC’s views regarding the strength of the evidence and its adequacy
to inform a range of levels.  Each of these topics is discussed in turn
below.

In determining 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, 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, a city park, or commercial
cropland.  In her judgment, it is appropriate that this variation in the
significance of O3-related vegetation effects should be taken into
consideration in judging the level of ambient O3 that is requisite to
protect the public welfare from any known or anticipated adverse
effects.   In this regard, the Administrator agrees with the definition
of adversity as described above in section IV.A.3 and in the 2008
rulemaking.   As a result, the Administrator concludes that of those
known and anticipated O3-related vegetation and ecosystem effects
identified and discussed in this reconsideration, the highest priority
and significance should be given 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 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.

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, including in some cases, genetic engineering.  These
management practices, in conjunction with market forces and government
programs, assure an appropriate balance is reached between costs of
production and market availability.  Thus, 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.  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 what is known about the relationship between O3 exposures and
agricultural crop yield response derives largely from data generated
almost 20 years ago.  The Administrator recognizes that there is
substantial uncertainty at this time as to whether these data remain
relevant to the majority of species and cultivars of crops being grown
in the field today.  In addition, the extensive management of such
vegetation may 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.  Thus, the Administrator concludes there is no
need for such additional protection for agricultural crops through the
NAAQS.

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
residential or commercial settings, such as ornamentals used in
urban/suburban landscaping, to 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, such as urban/suburban landscaping, the Administrator believes
that there is not adequate information at this time to establish a
secondary standard based specifically on impairment of urban/suburban
landscaping and other uses of ornamental 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 ornamental vegetation.

Based on the above, the Administrator finds that the types 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.  It demonstrates that significant numbers of forest tree
species are potentially experiencing O3-induced stress under levels of
ambient air quality, both at and below the level of the 1997 standard.

In particular, the Administrator notes the evidence from recent
field-based studies and a gradient study of eastern cottonwood saplings
(Gregg et al., 2003).  She observes that this study found that
cottonwood saplings grown in urban New York City grew faster than
saplings grown in downwind rural areas where cumulative O3 exposures
were higher, and the difference in biomass production between the urban
site with the lowest cumulative exposure and the rural site with the
highest cumulative exposure is

dramatic (Figure 7-17 in the 2007 Staff Paper).  The Administrator
further notes that cottonwood is one of the most sensitive tree species
studied to date and it is also important both from an ecological and
public welfare perspective, as discussed above in section IV.A.2.b and
in the 2007 Staff Paper.

The Administrator also notes the evidence related to the O3-induced
effect of visible foliar injury.  The Administrator observes that the
visible foliar injury database created from the ambient field-based
monitoring network managed by the Unites States Forest Service (USFS)
Forest Inventory and Analysis (FIA) Program has continued to expand
since the conclusion of the 1997 review.  In utilizing this dataset, EPA
staff collaborated with FIA staff to compare the incidence of visible
foliar injury at different levels of air quality by county throughout
the U.S. in counties with FIA monitoring sites.   In considering the
results of this analysis, depicted in Table 7-4 of the 2007 Staff Paper,
the Administrator notes that for the 2001-2004 period, the percent of
counties with documented foliar injury at a level approximately
equivalent to the W126 of 21 ppm-hour, was 26 to 49 percent, while at
the lower level approximately equivalent to a W126 of 13 ppm-hour,
incidence values ranged from 12 to 35 percent.  The Administrator
believes it likely that some sensitive species occurring in specially
protected areas would also exhibit visible foliar injury symptoms to a
similar degree at these exposure levels.  She further notes that while
direct links between O3 induced visible foliar injury symptoms and other
adverse effects (e.g., biomass loss) are not always found, visible
foliar injury in itself is considered by the National Park Service (NPS)
to affect adversely air quality related values (AQRV) in Class I areas.

The Administrator places significant weight on the judgments of CASAC.
In so doing, the Administrator has carefully considered its stated views
and the basis for the range of levels the CASAC O3 Panel recommended.
In its 2007 letter to the Administrator, the CASAC O3 Panel agreed with
EPA staff recommendations that the lower bound of the range within which
a seasonal W126 O3 standard should be considered is approximately 7
ppm-hour.  However, “it does not agree with Staff’s recommendations
that the upper bound of the range should be as high as 21 ppm-hour.
Rather, the Panel recommends that the upper bound of the range
considered should be no higher than 15 ppm-hour, which the Panel
estimates is approximately equivalent to a seasonal 12-hour SUM06 level
of 20 ppm-hour” (Henderson, 2007).   The Administrator notes that
CASAC views concerning an appropriate range of levels for the
Administrator to consider were presented after CASAC had considered the
entire body of evidence presented in both the 2006 Criteria Document and
2007 Staff Paper, and are generally consistent with the 1996 Consensus
Workshop recommendations.

In considering the issues raised by commenters on the 2007 proposed
rule, the Administrator noted that many public commenters supported the
range of levels recommended by CASAC.  The Administrator also considered
the views expressed by the NPS as to what range of levels it identified
as useful in helping it achieve its mandate to protect AQRVs in national
parks and wilderness areas and to provide a level of protection for its
resources in keeping with the Congressional mandate set forth in The
Wilderness Act of 1964.  In so doing, the Administrator notes that the
NPS supported the range recommended by CASAC, while emphasizing that the
lower end of the range was more appropriate.  The NPS notes that though
some visible foliar injury would still be expected to occur above the
lower end of the CASAC recommended range (i.e. 7 ppm-hour), the
potential for growth impacts at that level would be very low.  It
further notes that most of these parks contain aspen, black cherry, or
ponderosa pine, all sensitive species predicted to have significant
growth effects at current W126 levels.

The Administrator also considered those comments that highlighted
sources of uncertainty in the evidence and risk assessments (summarized
above in section IV.D.5.c.ii) to inform her judgments on how much weight
to place on these associated uncertainties, as discussed below.

With regard to the issue of possible confounders of foliar injury
information, the Administrator recognizes that visible foliar injury,
like other O3-induced plant effects, is moderated by environmental
factors other than O3 exposure.  However, the Administrator also notes
that the O3-related visible foliar injury effect persisted across a four
year period (2001-2004), despite year-to-year variability in meteorology
and other environmental factors (see Table 7-4 in the 2007 Staff Paper).
  She also notes that approximately 26 to 49 percent of counties had
visible foliar injury incidence at the approximate W126 level of 21
ppm-hour, while at a W126 level of 13 ppm-hour, this range of
percentages dropped to approximately 12 to 23 percent.  In an area such
as a national park, where visitors come in part for the aesthetic
quality of the landscape, the Administrator recognizes that visible
foliar injury incidence is an important welfare effect which should be
considered in determining an appropriately protective standard level.

With regard to the issues of what weight to place on the recommendations
from the 1996 Consensus Workshop in selecting a range of levels, as the
1997 Workshop Report did not clearly document the basis for its
recommendations, the Administrator recognizes that the absence of such
documentation does call for care in placing weight on such
recommendations.  However, the Administrator notes that the workshop
participants were asked to review both the 1996 O3 Criteria Document and
Staff Paper, representing the most up to date compilation of the state
of the science available at that time, in order to ensure that their
expert judgments made were also informed by the latest science.  She
also notes that another group of experts, the CASAC O3 Panel, reached a
similar consensus based upon an expanded body of scientific evidence.
In addition, the 2007 Staff Paper evaluated the same recommendations in
the context of subsequent empirical evidence, and reached similar views,
with the exception of the upper end of the recommended range, which in
the 2007 Staff Paper was based on effects on commercial crops that had
been considered in the 1997 review.  While it would always be more
useful to have documentation of the reasoning and basis for an
expert’s advice, in this case the Administrator judges that the 1996
Consensus Workshop recommendations should be given substantial weight.

With regard to other issues raised by some commenters related to
uncertainties in the technical evidence and analyses, the Administrator
notes that such issues had been addressed in the 2007 Staff Paper that
reflected CASAC’s advice on such issues.  For example, while the
Administrator recognizes that uncertainty remains as to what level of
annual tree seedling biomass loss when compounded over multiple years
should be judged adverse to the public welfare, she believes that the
potential for such anticipated effects should be considered in judging
to what degree a standard should be precautionary.

	In considering all of the issues discussed above, the Administrator has
decided to propose a range of 7-15 ppm-hour.  In selecting as an upper
bound a level of 15 ppm-hour, the Administrator notes that this level
was specifically supported by the CASAC O3 Panel and is just above the
range identified in the 1996 Consensus Workshop report as needed to
provide adequate protection for trees growing in natural areas.  In
addition, the NPS, along with many public commenters, were in support of
the CASAC range, including the upper bound of 15 ppm-hour, and
indicated that lower values within this range would be more protective
for sensitive trees in protected areas from biomass loss and visible
foliar injury symptoms.

While the upper end of this range is lower than the upper end of 21
ppm-hour recommended in the 2007 Staff Paper, this upper level of 21
ppm-hour was originally put forward in the 1997 review in terms of a
SUM06 of 25 ppm-hour (W126 of 21 ppm-hour) and was justified on the
basis that it was predicted to allow up to 10% biomass loss annually in
50% of studied commercial crops and tree seedling species.  Recognizing
the significant uncertainties that are associated with evaluating
effects on commercial crops from a public welfare perspective, the
Administrator now concludes that commercial crop data are no longer
useful for setting the upper level of the range for proposal.

	With regard to her selection of a proposed range, the Administrator has
considered that the direction from Congress to provide a high degree of
protection in Class I areas creates a clearer target for gauging what
types and magnitudes of effects would be known or anticipated to affect
the intended use of these and other similarly protected areas, that
would thus be considered adverse to the public welfare.  Such similar
areas include 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,  The Administrator
also believes that in order to preserve wilderness areas in an
unimpaired state for future generations, she must consider a level that
affords substantial protection from known adverse O3-related effects of
biomass loss and foliar injury on sensitive tree species, as well as a
level that takes into account potential  “anticipated” adverse
O3-related effects, including effects that result in continued
impairment in the year following O3 exposure (i.e., carry-over effects),
below ground impacts, ecosystem level impacts, and reduced CO2
sequestration

While the Administrator acknowledges that growth effects and visible
foliar injury can still occur in sensitive species at levels below the
upper bound of the proposed range, the Administrator also recognizes
that some significant uncertainties remain regarding the risk of these
effects, as discussed above.  For example, the Administrator concludes
that remaining uncertainties make it difficult to judge the point at
which visible foliar injury becomes adverse to the public welfare in
various types of specially protected areas.  Uncertainties associated
with monitoring ambient exposures must be considered in evaluating the
strength of predictions regarding the degree of tree seedling growth
impairment estimated to occur at varying ambient exposures.  These
uncertainties add to the challenge of judging which exposure levels are
expected be associated with levels of tree seedling growth effects
considered adverse to public welfare   The Administrator believes that
it is important to consider these uncertainties, and the weight to place
on such uncertainties, in selecting a range of standard levels to
propose.  Establishing 15 ppm-hour as the upper end of the proposed
range reflects her judgment regarding the appropriate weight to place on
these uncertainties in determining the degree of protection that is
warranted for known and anticipated adverse effects.

With regard to her selection of a lower bound for the proposed range,
the Administrator believes that if weight is placed on taking a more
precautionary approach, recognizing that the real world impacts on trees
and ecosystems could, in some cases, be greater than predicted, then the
lower end of the range of 7 ppm-hour could be warranted. There is clear
evidence that higher cumulative exposures can occur in rural areas
downwind of urban areas and potentially in Class I areas.  Unmonitored
high elevation sites would also likely have higher cumulative exposures
than lower elevation sites that are currently monitored.  All of these
considerations lead the Administrator to propose 7 ppm-hour as the low
end of the proposed range.

As discussed above in section IV.D.5.a, the main opposition to changing
to a secondary standard with a cumulative, seasonal form has been the
view that EPA does not have an adequate basis to identify the kinds and
types of cumulative, seasonal exposure patterns that the standard should
be designed to protect against, given the various uncertainties in the
evidence, and whether EPA has an adequate basis to determine an
appropriate level for a cumulative, seasonal secondary standard.  While
EPA agreed with this position in the 1997 review, the Administrator
believes that the evidence before her appropriately supports a secondary
standard that is distinctly different in form and averaging time from
the 8-hour primary standard, and that such a standard is necessary to
provide sufficient protection from cumulative, seasonal exposures to O3.

While a different conclusion on this issue was reached in the 1997
review, the current conclusion that an exposure index that is cumulative
and seasonal in nature, and therefore that setting a standard based on
such a form is necessary and appropriate, is based on information newly
available in the 2008 rulemaking, which strengthens the information
available in the 1997 review and reduces remaining uncertainties.

Such newly available information includes quantitative information for a
broader array of vegetation effects (extending to sapling and mature
tree growth stages) obtained using a more diverse set of field-based
research study designs and improved analytical methods for assessing
O3-related exposures and risks as discussed above in sections IV.A-C.

.	These newly available studies also provide important support to the
quantitative estimates of impaired tree growth based on earlier studies
available in the 1997 review and address one of the key data gaps cited
in the 1997 review.  Additional qualitative information is also
available regarding improved understanding of linkages between
stress-related effects of O3 exposures at the species level and those at
higher levels within ecosystems.  Finally, this information includes the
use of new analytical methods, including a new multi-pollutant,
multi-scale air quality model used to characterize exposures of
O3-sensitive tree and crop species further address uncertainties in the
assessments done in the 1997 review.  In total, this newly available
information increases the Administrator’s confidence in important
aspects of this rulemaking

The decision in 2008 to set the secondary O3 standard identical to the
8-hour primary standard largely mirrored the decision in 1997, but
failed to account for this significant increase in the body of knowledge
available to support the 2008 rulemaking.  This body of knowledge, while
continuing to reflect significant uncertainties, provides an appropriate
basis for determining a level of a cumulative, seasonal standard that,
in the judgment of the Administrator, provides sufficient but not more
than necessary protection from cumulative, seasonal exposures to O3.
This is clearly so when compared to a standard that uses an indirect
form that is not biologically relevant, an 8-hour average standard aimed
at peak daily exposures.  This judgment is fully consistent with the
advice provided by CASAC.

After carefully taking the above considerations into account, and giving
significant weight to the views of CASAC, the Administrator has decided
to propose a range of levels of 7-15 ppm-hour for a cumulative, seasonal
secondary O3 standard expressed as an index of the annual sum of
weighted hourly concentrations (i.e., the W126 form), cumulated over 12
hours per day during the consecutive 3-month period within the O3 season
with the maximum index value, averaged over three years.  In the
Administrator's judgment, based on the information available in the 2008
rulemaking, a standard could be set within this range that would be
requisite to protect public welfare from known or anticipated adverse
effects to O3-sensitive vegetation and ecosystems.  In the
Administrator’s judgment, a standard set at a level below the lower
end of the range is not now supported by the weight of evidence and
would not give sufficient weight to the important uncertainties and
limitations inherent in the available scientific evidence and in the
quantitative assessments conducted for the 2008 rulemaking.  A standard
set at a level above the upper end of the range is also not now
supported by the weight of evidence and would not give sufficient weight
to the credible inferences that the Agency has drawn from the scientific
evidence nor to the quantitative assessments conducted for the 2008
rulemaking.  The Administrator judges that the appropriate balance to be
drawn, based on the entire body of evidence and information available in
the 2008 rulemaking, is a range between 7 and 15 ppm-hour.  On balance,
the Administrator believes that a standard could be set within this
range that would be sufficient but not more than necessary to protect
public welfare from known or anticipated adverse effects due to O3.

In reaching this proposed decision, as discussed above, the
Administrator has focused on the nature of the benefits associated with
setting a distinct secondary standard with a cumulative, seasonal form
relative to a standard with a peak daily 8-hour average form, as well as
on assessments that quantify the degree of protection likely to be
afforded by such standards.  In so doing, the Administrator has
acknowledged limitations in quantifying the expected benefits associated
with the proposed cumulative seasonal standard relative to the secondary
standard set in 2008.  Having considered the public comments received on
the 2007 proposed rule in reaching this proposed decision, the
Administrator is interested in again receiving public comment on the
benefits to public welfare associated with the proposed cumulative
seasonal standard set at specific levels within the proposed range
relative to the standard set in 2008.

E.	Proposed Decision on the Secondary O3 Standard

For the reasons discussed above, and taking into account information and
assessments presented in the 2006 Criteria Document and 2007 Staff
Paper, the advice and recommendations of CASAC, and the public comments
received in conjunction with the 2008 rulemaking, the Administrator has
decided to propose to set a new cumulative, seasonal secondary O3
standard with a form expressed as an index of the annual sum of weighted
hourly concentrations (i.e., the W126 form), cumulated over 12 hours per
day (8:00 am to 8:00 pm) during the consecutive 3-month period within
the O3 season with the maximum index value, averaged over three years,
set within a range of 7 to 15 ppm-hour.  The Administrator solicits
comment on the weight that is appropriately placed on the various types
of evidence and analyses upon which this proposed standard is based, as
well asand on the appropriate weight to be placed on the uncertainties
in this information, to help inform selection of a final standardas well
as on the benefits to public welfare associated with the proposed
standard relative to the benefits associated with the standard set in
2008.

Data handling conventions for the proposed new secondary O3 standard are
specified in the proposed addition of a new section to 40 CFR 50
Appendix P, as discussed in section V below.  Issues related to
monitoring requirements for the proposed new secondary O3 standard are
discussed below in section VI.

V.	Interpretation of the NAAQS for O3 and Proposed Revisions to the
Exceptional Events Rule

	Appendix P to 40 CFR part 50, Interpretation of the Primary and
Secondary National Ambient Air Quality Standards for Ozone, addresses
data completeness requirements, data reporting, handling, and rounding
conventions, and example calculations. The current Appendix P explains
the computations necessary for determining when the current identical
primary and secondary standards are met. The EPA is proposing to revise
Appendix P to reflect the proposed revisions to the primary and
secondary O3 NAAQS discussed above and to make other changes described
below.

	As discussed below, the proposed revisions to Appendix P include the
following:  the addition of data interpretation procedures applicable to
the proposed cumulative, seasonal secondary NAAQS (see section V.B);
clarification of certain language in the current provisions applicable
to the primary NAAQS to reduce potential confusion (section V.C);
revisions to the provisions regarding the use of incomplete data sets
for purposes of the primary NAAQS and the data completeness requirements
across three years (sections V.D and V.E); the addition of a provision
providing the Administrator discretion to use incomplete data as if it
were complete, for the purpose of the primary NAAQS (section V.F); a
change from truncation to rounding of multi-hour and multi-year average
O3 concentrations for the purposes of the primary standard (section
V.G); and the addition of provisions addressing data to be used in
making comparisons to the NAAQS (section V.H).  The proposed revisions
also include changes in organization for greater clarity and consistency
with other data interpretation appendices to 40 CFR part 50, which are
not further described in this preamble.

	The EPA is also proposing changes to the O3-specific deadlines, in 40
CFR 50.14, by which states must flag ambient air data that they believe
have been affected by exceptional events and submit initial descriptions
of those events, and the deadlines by which states must submit detailed
justifications to support the exclusion of that data from EPA
determinations of attainment or nonattainment with the NAAQS.  The
O3-specific deadlines in the current 40 CFR 50.14 would not be
appropriate given the anticipated schedule for the designations of areas
under the proposed revised O3 NAAQS.

A.	Background

	The purpose of a data interpretation appendix in general is to provide
the practical details on how to make a comparison between multi-day and
possibly multi-monitor ambient air concentration data and the level of
the NAAQS, so that determinations of compliance and violation are as
objective as possible.  Data interpretation guidelines also provide
criteria for determining whether there are sufficient data to make a
NAAQS level comparison at all.  Appendix P was promulgated in March 2008
along with the most recent revisions to the primary and secondary O3
NAAQS.  It is very similar to Appendix I, Interpretation of the 8-Hour
Primary and Secondary National Ambient Air Quality Standards for Ozone,
which was adopted in 1997 when the O3 NAAQS were first revised to have
an 8-hour averaging period rather than the earlier 1-hour averaging
period, along with other changes in form and level.  The only
substantive difference between Appendix I and the current version of
Appendix P is that Appendix P contains truncation procedures consistent
with the additional decimal digit used to express the level of the 2008
NAAQS in parts per million (0.075 ppm) compared to the 1997 NAAQS (0.08
ppm).  In July 2007, EPA had also proposed to include in Appendix P data
interpretation procedures for the proposed cumulative, seasonal
secondary O3 NAAQS, but these procedures were not finalized given that
the final secondary NAAQS was identical in all respects to the final
primary NAAQS.

	An exceptional event is defined in 40 CFR 50.1 as an event that affects
air quality, is not reasonably controllable or preventable, is an event
caused by human activity that is unlikely to recur at a particular
location or a natural event, and is determined by the Administrator in
accordance with 40 CFR 50.14 to be an exceptional event.  Air quality
data that are determined to have been affected by an exceptional event
under the procedural steps and substantive criteria specified in section
50.14 may be excluded from consideration when EPA makes a determination
that an area is meeting or violating the associated NAAQS.  The key
procedural deadlines in section 50.14 are that a state must notify EPA
that data have been affected by an event, i.e., “flag” the data in
the Air Quality Systems (AQS) database, and provide an initial
description of the event by July 1 of the year after the data are
collected, and that the State must submit the full justification for
exclusion within 3 years after the quarter in which the data were
collected.  However, if a regulatory decision based on the data, for
example a designation action, is anticipated, the schedule is shortened
and all information must be submitted to EPA no later than a year before