from serious health effects. See 71 FR at 61177.
    Despite the advances in knowledge about the effects of
PM2.5 on human health, the 2005 SP noted the continued
difficulty of being able to establish a dose-response relationship
between PM2.5 concentrations and specific health-related
effects. ``The available toxicologic studies have generally not been
designed to quantify dose-response relationships* * *. Among the
studies reviewed [in the 2004 CD] are some that report no evidence of a
dose-response relationship gradient, (CD, p. 7-152), while some do (CD,
p. 7-155), and the CD draws no overall conclusions regarding dose-
response relationships from toxicologic studies. Therefore, while
epidemiologic studies provide clear indication of increasing response
with increasing concentration, no conclusions can be drawn from
toxicologic evidence.'' 2005 SP at 3-30.
(2) Welfare Effects
    Ambient PM alone, and in combination with other pollutants, can
have a variety of effects on public welfare. While visibility
impairment is the most noticeable effect of fine particles present in
the atmosphere, both fine and coarse particles can have other
significant welfare-related effects, including effects on vegetation
and ecosystems, materials (e.g., soiling and corrosion), and climate
change processes. In 1997, EPA established a suite of secondary PM
standards, including annual and 24-hour PM2.5 standards and
annual and 24-hour PM10 standards, to address visibility
impairment associated with fine particles, and materials damage and
soiling related to both fine and coarse particles. See 62 FR 38683. In
2006, EPA considered the then-currently available evidence and decided
to revise the current suite of PM2.5 secondary standards by
making them identical in all respects to the revised suite of primary
PM2.5 standards, retain the current 24-hour PM10
secondary

standard, and revoke the current annual PM10 secondary
standard.
    In reaching our decision in 2006 to revise the suite of PM
secondary standards, EPA factored in several key conclusions from the
scientific and technical information contained in the 2004 CD and 2005
SP. These conclusions included the following: (1) PM-related visibility
impairment is principally related to fine particle levels, and most
directly related to instantaneous levels of visual air quality
associated with short-term averaging periods; (2) PM2.5
concentrations can be used as a general surrogate for visibility
impairment in urban areas; (3) any secondary NAAQS for visibility
protection should be considered in conjunction with the regional haze
program as a means of achieving appropriate levels of protection
against PM-related visibility impairment in urban, non-urban, and Class
I areas nationwide; (4) the available evidence is not sufficient to
support distinct secondary standards for fine or coarse particles for
any non-visibility related welfare effects; and (5) the secondary
standards should be considered in conjunction with protection afforded
by other programs intended to address various aspects of air pollution
effects on ecosystems and vegetation, such as the acid deposition
program and other regional approaches to reducing pollutants linked to
nitrate or acidic deposition.
    Notwithstanding the conclusions reached in setting the NAAQS for
PM, EPA has reviewed the scientific and technical information
concerning welfare related effects considered in the 2004 CD and 2005
SP to determine whether there is any basis for modifying the safe
harbor increments developed for PM2.5 to satisfy the
criteria under sections 166(c) and 160 of the Act. The EPA's review
began with visibility impairment, followed by effects on vegetation and
other ecosystem components, materials and soiling, and climate changes.
    (a) Visibility impairment.
    The EPA has long recognized that impairment of visibility is an
important effect of PM on public welfare. Visibility can be defined as
the degree to which the atmosphere is transparent to visible light.
Visibility conditions are determined by the scattering and absorption
of light by particles and gases from both natural and anthropogenic
sources. The classes of fine particles principally responsible for
visibility impairment are sulfates, nitrates, organic matter, elemental
carbon, and soil dust.
    Visibility impairment can occur in two principal ways: as local
visibility impairment (e.g., localized plumes) and as regional haze.
Local-scale impairment is generally the result of the plume from a
single source or small group of local sources, rather than from long-
range transport from more distant sources. With this type of
impairment, a band or layer of discoloration can be observed well above
the terrain, obscuring the sky or horizon relatively near the source,
or sources, which cause it. Such visibility problems in urban areas are
often dominated by local sources, which may include stationary, mobile
and area sources. Visibility impairment from the combined effects of
urban sources have been studied in several major cities because of
concerns about fine particles and their significant impacts on
residents of large metropolitan areas.
    The second type of impairment, regional haze, generally results
from pollutant emissions from a multitude of sources located across a
broad geographic region. Regional haze can impair visibility in every
direction over a relatively large area, in some cases over multi-state
regions. Regional haze is principally responsible for impairment in
national parks and wilderness areas (Class I areas) across the country
where scenic views are considered an important attribute. Fine
particles transported from urban and industrialized areas may, in some
cases, be significant contributors to regional-scale impairment in
Class I and other rural areas.
    Annual average visibility conditions vary regionally across the
United States. Higher visibility impairment tends to occur more in the
East, and is due to generally higher concentrations of anthropogenic
fine particles and higher relative humidity conditions. In addition,
the rural East generally has higher levels of impairment than remote
sites in the West. For Class I areas, visibility levels on the 20
percent haziest days in the West are about equal to levels on the 20
percent best days in the East. For urban areas, however, East/West
visibility differences from fine particles are substantially smaller
than they are in rural areas.
    The EPA's latest PM NAAQS review focused on visibility impairment
primarily in urban areas for the following reasons: (1) The efforts now
underway to address all human-caused visibility impairment in Class I
areas through regional strategies under the regional haze program (65
FR 35713, July 1, 1999), and (2) new information from visibility and
fine particle monitoring networks since the last PM NAAQS review that
has allowed for updated characterizations of visibility trends and
current levels in urban areas. Given the strong link between visibility
impairment and short-term PM2.5 concentrations, EPA gave
significant consideration to the question of whether visibility
impairment in urban areas allowed by the original 24-hour secondary
NAAQS for PM2.5 could be considered adverse to public
welfare.
    New data available on PM2.5, primarily in urban areas,
enabled EPA to better characterize urban visibility than was previously
possible. Such data includes Federal Reference Method (FRM)
measurements of PM2.5 mass, continuous measurements of
hourly PM2.5 mass, and PM2.5 chemical speciation
measurements. Using the new data EPA sought to explore the factors that
historically complicated efforts to address visibility impairment
nationally, including regional differences related to levels of
primarily fine particles and relative humidity. Using the most recent
monitoring information and analyses, as well as photographic
representations of visibility impairment in several urban areas to help
inform judgments about the acceptability of varying levels of visual
air quality in urban areas, EPA observed that:
    (1) At concentrations at or near the level of the 24-hour
PM2.5 standard (65 [mu]g/m\3\), which equates to visual
ranges roughly around 10 kilometers (6 miles), scenic views around and
within the urban areas, are significantly obscured from view.
    (2) Appreciable improvement in the visual clarity of the scenic
views occurs at PM2.5 concentrations below 35 to 40 [mu]g/
m\3\, which equates to visual ranges generally above 20 kilometers for
the urban areas considered.
    (3) Visual air quality appears to be good at PM2.5
concentrations generally below 20 [mu]g/m\3\, corresponding to visual
ranges of approximately 25 to 35 kilometers.
    While being mindful of the limitations in using visual
representations from a small number of areas as a basis for considering
national visibility-based secondary standards, EPA concluded that the
observations noted earlier supported consideration of revising the then
current PM2.5 secondary standards to enhance visual air
quality, particularly with a focus on urban areas. This led to the
evaluation of information related to indicator, averaging time, level,
and form to identify a range of alternative PM standards that would
protect visual air quality, primarily in urban areas. Notwithstanding
the selection process used in selecting the primary and

secondary NAAQS for PM, for this PM increment proposal we are examining
the same information to determine whether it might justify modifying
the safe harbor increments for PM2.5, which follow the
indicator, averaging times, and form of the NAAQS for PM2.5,
as described earlier as option 1.
    PM indicator. While both fine and coarse particles contribute to
visibility impairment, visibility impairment is the most noticeable
effect of fine particles present in the atmosphere. Analyses of hourly
PM2.5 measurements and other information demonstrate that
fine particles contribute to visibility impairment directly in
proportion to their concentration in the ambient air. Moreover,
hygroscopic components of fine particles, in particular sulfates and
nitrates, contribute disproportionately to visibility impairment under
high humidity conditions, when such components reach particle diameters
up to and even above 2.5 [mu]m. The EPA's analyses of how well
PM2.5 concentrations correlated with visibility in urban
locations across the United States lead to the conclusion that the
observed correlations were strong enough to support the use of
PM2.5 as the indicator for standards to address visibility
impairment in urban areas, especially when the indicator is defined for
a relatively short period of daylight hours.
    Averaging time. While EPA selected the 24-hour averaging time for
the PM2.5 secondary standard to address visibility
impairment primarily in urban areas, a range of shorter term (sub-
daily) daylight averaging times were also considered. Strong
correlations between visibility and PM2.5 concentrations
were found to occur at the 24-hour averaging time, but the strongest
correlations were found to occur at the sub-daily daylight averaging
times, e.g., 4-to 8-hour daylight averaging times. In fact, the
correlation was greatest in the 4-hour time period between 12 and 4
p.m. At the sub-daily daylight averaging times, correlations between
PM2.5 concentrations and light extinction were less
influenced by relative humidity and more consistent across regions.
    A number of different daylight time periods was selected to compare
correlations between visibility and hourly PM2.5
concentrations in urban areas across the United States and in eastern
and western regions. Ultimately, EPA staff recommended consideration of
a short-term averaging time, within the range of 4 to 8 hours, within a
daylight time period between approximately 10 a.m. to 6 p.m., to target
the driest part of the day. Most CASAC Panel members supported the SP
recommendation of a sub-daily averaging time.
    Following careful consideration of the various sets of data and
evidence concerning visibility impairment, the Administrator proposed
to revise the secondary 24-hour standard for PM2.5 to make
it identical to the proposed revised primary PM2.5 standard
(based on a 24-hour averaging time for the short-term standard).
Consistent with recommendations to consider a sub-daily averaging time,
the Administrator also solicited comment on 4-to 8-hour averaging time
for the secondary PM2.5 standard. In reaching his final
decision to rely on the 24-hour averaging period to set the secondary
standard for PM2.5, the Administrator concluded that the
relative protection against adverse effects on public welfare provided
by the proposed primary standards was equivalent or more protective
than several of the 4-hour secondary standard alternatives in the range
recommended by CASAC and the SP. He also believed that caution was
warranted in establishing a distinct secondary standard for visibility
impairment primarily in urban areas, given the limitations in the
underlying studies and the subjective nature of the judgment required.
    Level of increment. In evaluating the adequacy of the levels of the
contingent safe harbor increments for PM2.5, we examined the
range of PM2.5 concentrations considered in setting a
national visibility standard primarily for urban areas. We had
established that range of concentrations by using the results of public
perception and attitude surveys conducted in the United States and
Canada, State and local visibility standards within the United States,
and visual inspection of photographic representations of several urban
areas across the United States. These approaches are detailed in the
2005 SP (pp. 6-18 to 6-23.)
    The public perception and attitude studies were used to gain an
understanding of what the public regarded as an acceptable visible
range. In some urban areas, poor visibility has led to more localized
efforts to better characterize, as well as improve, urban visibility
conditions. Public perception surveys used in Denver, Phoenix, and
British Columbia studies yielded reasonably consistent results, with
each study indicating that a majority of citizens find value in
protecting local visibility to with a visual range of about 40 to 60
km. Visibility standards for the Lake Tahoe area in California and for
areas within the State of Vermont are both targeted at a visual range
of about 50 km. In contrast, California's longstanding general state-
wide visibility standard is a visual range of approximately 16 km.
    Aided by photographic representations of varying levels of visual
air quality developed for several cities across the United States, EPA
staff reached the conclusion that a national visibility standard in the
PM2.5 concentration range of 30 to 20 [mu]g/m\3\ should be
considered. Further analyses to characterize the distributions of
PM2.5 concentrations, 4-hour averages in the 12 to 4 p.m.
time frame, by region, that correspond to various visual range target
levels, resulted in a finding that concentrations of 30, 25, and 20
[mu]g/m\3\ correspond to the target visual ranges of approximately 25,
30 and 35 km, respectively. Thus, it was determined that a standard set
within the range of 30 to 20 [mu]g/m\3\ could be expected to correspond
generally to media visual range levels of approximately 25 to 35 km in
urban areas across the United States. This range was generally
consistent with a national target visual range below 40 km, the level
suggested by the public perception surveys and the local visibility
standards and goals. Nevertheless, EPA staff noted that a standard set
at any specific PM2.5concentration will necessarily result
in visual ranges that vary somewhat in urban areas across the country,
reflecting in part the less-than-perfect correlation between
PM2.5 concentrations and reconstructed light extinction.
2005 SP at page 7-8.
    Form of increment. In considering a reasonable range of forms for a
PM2.5 standard within the range of PM2.5
concentration levels being considered, EPA staff took into account the
same general factors that were taken into account in considering an
appropriate form for the primary PM2.5 standard. In that
case, EPA staff concluded that a concentration-based form should be
considered because of its advantages over the previously used expected-
exceedance form.\14\ For visibility, the advantages are that the
concentration-based form (1) Would give proportionally greater weight
to days when the PM-related visibility impairment is substantially
higher than to days just above the standard, and (2) has greater
stability. 2005 SP at 7-11. To identify a range of concentration
percentiles that would be appropriate for consideration, it was
concluded that

the upper end of the range of consideration should be the 98th to 99th
percentile, consistent with the forms being considered for the 24-hour
primary PM2.5 standard. For the lower end of the range, EPA
staff used the 92nd percentile because it represented the mean of the
distribution of the 20 percent worst days, consistent with the fact
that the regional haze program targets the 20 percent most impaired
days for improvements in visual air quality in Class I areas. 2005 SP
at 7-12.
---------------------------------------------------------------------------

    \14\ The form of the 1987 24-hour PM10 standard is
based on the expected number of day per year (averaged over 3 years)
on which the level of the standard is exceeded; thus, attainment
with the one-expected exceedance form is determined by comparing the
fourth-highest concentration in 3 years with the level of the
standard.
---------------------------------------------------------------------------

    While EPA staff regarded PM2.5 as the best indicator for
addressing visibility impairment in urban areas, they considered a
range of averaging times, levels, and forms for setting a
PM2.5 secondary standard. In summary, EPA staff recommended
that consideration be given to a short-term averaging time for a
PM2.5 standard, within the range of 4 to 8 hours, within a
daylight time period between approximately 10 a.m. to 6 p.m. In
addition, they recommended that consideration should be given to the
adoption of Federal equivalent methods for appropriate continuous
methods for measurement of short-term average PM2.5
concentrations to facilitate implementation of the standard. Within the
recommended 4- to 8-hour averaging time, the EPA staff recommended
consideration of a standard level within the range of 30 to 20 [mu]g/
m\3\, depending in part on the form of the standard selected. Finally,
staff recommended consideration of a percentile-based form, focusing on
a range from the 92nd percentile up to the 98th percentile of the
annual distribution of daily short-term PM2.5 concentrations
averaged over 3 years. 2005 SP at 7-13.
    (b) Vegetation and other ecosystem components.
    The 2004 CD found that then-current PM2.5 levels in the
United States ``[had] the potential to alter ecosystem structure and
function in ways that may reduce their ability to meet societal needs''
(CD, p. 4-153). However, studies show that vegetation and other
ecosystem components result predominantly from exposure to excess
amounts of specific chemical species than from particle source,
predominant form (particle, gas, or liquid) or size fraction. The 2004
CD discussed the effects of a number of different chemical species,
including dust, trace metals, and organics, found within ambient PM,
but ultimately focused on particulate nitrates and sulfates based on
the conclusion that these latter constituents of PM2.5 were
``of greatest and most widespread environmental significance.'' Thus,
the 2005 SP focused on the welfare effects of particulate nitrates and
sulfates, either individually, in combination, and/or as contributors
to total reactive nitrogen deposition and total deposition of
acidifying compounds on sensitive ecosystem components and essential
ecological attributes.
    Nitrogen and sulfur in varying amounts are necessary and beneficial
nutrients for most organisms that make up ecosystems. It is when
unintentional additions of atmospherically derived nutrient and
acidifying compounds containing nitrogen and sulfur force unintended
change on ecosystems, resulting in adverse impacts on essential
ecological attributes, that deposited particulate nitrate and sulfate
are termed ecosystem ``stressors.'' In order for any specific chemical
stressor present in ambient PM to impact ecosystems, it must first be
removed from the atmosphere through any of three different types of
deposition: wet (rain/frozen precipitation), dry, or occult (fog, mist,
or cloud). At the national scale, all types of deposition must be
considered in determining potential impacts to vegetation and
ecosystems because each type may dominate over specific intervals of
time or space.
    The most significant PM-related ecosystem-level effects result from
long-term cumulative deposition of a given chemical species (e.g.,
nitrate) or mix (e.g., acidic or acidifying deposition) that exceeds
the natural buffering or storage capacity of the ecosystem and/or
affects the nutrient status of the ecosystem. The 2005 SP examined the
environmental effects of both reactive nitrogen (of concern is the
reactive nitrogen resulting from the conversion of both atmospheric
nitrogen and fossil nitrogen during the combustion of fossil fuels) and
PM-related acidic and acidifying deposition on various ecosystems,
including vegetation, terrestrial ecosystems, threatened and endangered
species, and aquatic habitat.
    Vegetation. Various studies indicate that at current ambient
levels, risks to vegetation from short-term exposures to dry deposited
particulate nitrate or sulfate are low; however, when found in
acidifying deposition, such particles do have the potential to cause
direct foliar injury. The 2005 SP concluded on the basis of available
information that the risk of injury occurring from acid precipitation
in the eastern United States is high, noting that acid precipitation
with levels of acidity associated with adverse foliar effects exist in
some locations of the United States. Such adverse effects may include
damage to leaf surface structure; increased permeability of leaf
surface to toxic material, water, and disease agents; increased
leaching of nutrients from foliage; altered reproductive processes; and
overall weakening of trees making them more susceptible to other
stressors. Having said all this, the 2005 SP also found that the
contribution of particulate sulfates and nitrates to the total acidity
found in the acid precipitation impacting eastern vegetation is not
clear.
    Terrestrial ecosystems. The 2005 SP concluded that excess nitrogen
deposition is having a ``profound and adverse impact on the essential
ecological attributes associated with terrestrial ecosystems.''
Terrestrial ecosystems may be adversely impacted by (1) increased
nitrogen associated with atmospheric deposition, surface runoff, or
leaching from nitrogen saturated soils into ground or surface waters;
and (2) acidic and acidifying deposition.
    Long-term, chronic additions of reactive nitrogen (including
nitrate deposition and ammonium from ambient PM) can cause the nitrogen
input to plants to exceed the natural capacity of plants and soil
microorganisms to utilize and retain the nitrogen needed for normal
growth. As this excess occurs over time, a detrimental ecological
condition known as ``nitrogen saturation'' is said to exist.
    Nitrogen saturation does not occur at a specific point in time, but
reflects a set of gradually developing critical changes in the
ecosystem process. In addition, not all vegetation, organisms, or
ecosystems react in the same manner to increased nitrogen availability
from nitrogen deposition. Those plants that are predisposed to
capitalize on any increases in nitrogen availability gain an advantage
over those that are not as responsive to added nitrogen. Over time,
this shift in the competitive advantage may lead to shifts in overall
plant community composition. Whether this shift is considered adverse
would depend on the management context within which that ecosystem
falls and the ripple effects of this shift on other ecosystem
components, essential ecosystem attributes, and ecosystems.
    The addition of nitrogen on plant community succession patterns and
biodiversity has been studied in several long-term nitrogen
fertilization studies in both the United States and Europe. These
studies suggest that some forests receiving chronic inputs of nitrogen
may decline in productivity and experience greater mortality. Some of
the U.S. forests that are showing severe symptoms of nitrogen
saturation are: the northern hardwoods and mixed conifer

forests in the Adirondack and Catskill Mountains of New York; the red
spruce forests at Whitetop Mountain, Virginia, and Great Smoky
Mountains National Park, North Carolina; mixed hardwood watersheds at
Fernow Experimental Forest in West Virginia; American beech forests in
Great Smoky Mountains National Park, Tennessee; and mixed conifer
forests and chaparral watersheds in southern California and the
southwestern Sierra Nevada in Central California. 2005 SP at 6-31.
    Studies have shown that acid deposition has changed the chemical
composition of soils by depleting the content of available plant
nutrient cations (e.g., Ca\2+\, Mg\2+\, and K\+\) by increasing the
mobility of aluminum, and by increasing the sulfur and nitrogen
content. Effects of acidic deposition have been extensively documented,
as discussed in the 2004 CD and reports referenced therein. For
example, effects on some species of forest trees linked to acidic
deposition include increased permeability of leaf surfaces to toxic
materials, water, and disease agents; increased leaching of nutrients
from foliage; and altered reproductive processes; all of which serve to
weaken trees so that they are more susceptible to other stresses (e.g.,
extreme weather, pests, and pathogens). In particular, acidic
deposition has been implicated as a causal factor in the northeastern
high-elevation decline of red spruce. Although U.S. forest ecosystems
other than the high-elevation spruce-fir forests are not currently
manifesting symptoms of injury directly attributable to acid
deposition, less sensitive forests throughout the United States are
experiencing gradual losses of base cation nutrients, which in many
cases will reduce the quality of forest nutrition over the long term.
    Threatened and endangered species. The adverse ecological effects
of PM include those effects on rare and unique ecosystems, including
both plant and wildlife species. Nitrogen deposition, including
particulate nitrate, may have a direct adverse affect on some plant
species, while for others the harm results when added nitrogen serves
as a nutrient for some invasive species that eventually replace the
more sensitive, rare species.
    In some instances, as sensitive vegetation is harmed or lost,
wildlife species that depend on these plants are also adversely
affected. Several threatened or endangered species listed by the U.S.
Fish and Wildlife Service, such as the desert tortoise and checkerspot
butterfly have declined as a result of native food supplies being
replaced by invasive plant species whose productivity is enhanced in
part by nitrogen deposition.
    Aquatic habitat. Adverse effects of PM on aquatic systems (streams,
rivers, lakes, estuaries, and oceans) can be the result of either
elevated levels of reactive nitrogen input or acidification. In either
case, the nitrogen input contribution from PM may be the result of
atmospheric deposition directly into the water body or on terrestrial
ecosystems, reaching the water body via surface runoff or leaching from
nitrogen saturated soils into ground or surface waters. However, it is
not clear how much of the total nitrogen input to aquatic systems
results from atmospheric deposition rather than from other nitrogen
sources.
    Estuaries receive far greater nutrient inputs than other systems.
Excess nitrogen in estuaries results in eutrophic conditions whereupon
dissolved oxygen is significantly reduced; yielding an environment that
favors plant life over animal life. The 2005 SP describes research
being done in the Pamlico Sound in North Carolina, which is a key
fisheries nursery in the southeastern United States. Studies have shown
that direct nitrogen deposition onto waterways feeding into the Pamlico
Sound or onto the Sound itself and indirect nitrogen inputs via runoff
from the upstream watersheds contribute to conditions of severe water
oxygen depletion; formation of algae blooms in portions of the Pamlico
Sound estuarine complex; altered fish distributions, catches, and
physiological states; and increases in the incidence of disease. 2005
SP at p. 6-35.
    Other studies have shown that under extreme rainfall events,
massive influxes of reactive nitrogen (in combination with excess
loadings of metals or other nutrients) into watersheds and sounds can
lead to dramatic decreases of oxygen in water and the creation of
widespread ``dead zones'' and/or increases of algae blooms that can
cause extensive fish kills and damage to commercial fish and sea food
harvesting. 2005 SP at p. 6-35.
    The 2005 SP indicates that there is a clear link between acidic
water, which results from atmospheric deposition of strong acids, and
fish mortality. Studies have shown that inputs of acid deposition to
regions with base-poor soils have resulted in the acidification of soil
waters, shallow ground waters, streams, and lakes in a number of
locations with the United States. This can result in lower pH and
higher concentrations of inorganic monomeric aluminum, which causes
changes in chemical conditions that are toxic to fish and other aquatic
animals.
    (c) Materials damage and soiling.
    As part of the review for setting secondary standards for PM, the
2004 CD and 2005 SP considered the adverse effects that the deposition
of ambient PM can have on materials such as metals, paint finishes, and
building stone and concrete. Substantial evidence exists to show that
ambient PM plays a role in both physical damage and impaired aesthetic
qualities of materials. Physical damage to materials, including
corrosion, degradation, and deterioration, is known to result from
exposure to environmental factors such as sunlight, moisture, fungi,
and varying temperatures; however, to the extent that particles may
cause or contribute to physical damage of building materials, such
damage is primarily caused by chemically active--especially particulate
nitrates and sulfates--fine particles or hygroscopic coarse particles.
On the other hand, particles consisting of carbonaceous compounds are
responsible for soiling of commonly used building materials and
culturally important items (statues, works of art, etc.) Soiling or
exposure to PM can affect the aesthetic appeal of surfaces by giving
them a dirty appearance, resulting in an increased frequency of
cleaning. Nevertheless, while the role of ambient PM in specific
adverse effects is well documented in the available studies, the 2004
CD and 2005 SP also concluded that there remains insufficient evidence
to establish a quantitative relationship between ambient PM and any of
the various effects described.
    The EPA believes that these observations and the underlying
available evidence continue to support consideration of retaining an
appropriate degree of control on both fine and coarse particles.
Lacking any specific quantitative basis for establishing distinct
standards to protect against PM related to adverse effects on
materials, EPA believes that reductions in fine and coarse particles
likely to result from the current suite of secondary PM standards, or
the range of recommended revisions to the primary PM standards and to
the secondary PM2.5 standard to address visibility
impairment, would contribute to protection against PM-related soiling
and materials damage.
    (d) Climate and solar radiation effects.
    The effects of PM on climate result from either the scattering or
absorption of radiation by ambient particles, resulting in a cooling or
warming effect on climate, respectively. Studies suggest

that global and regional climate changes could have both positive and
negative effects on human health and welfare, and the environment. Most
components of ambient PM, especially sulfates, scatter and reflect
incoming solar radiation back into space. However, some components of
ambient PM, especially black carbon, absorb incoming solar radiation or
outgoing terrestrial radiation. Sulfate particles indirectly affect
climate by serving as condensation nuclei which alter the size
distribution of cloud droplets (producing more droplets with smaller
sizes), causing the amount of solar radiation that clouds reflect back
to space to increase.
    While substantial qualitative information has shown the important
role that ambient PM plays in both global and regional climatic
processes, that role is presently poorly quantified. There are
considerable uncertainties and difficulties in projecting likely
climate change impacts. The 2005 SP indicates that ``any complete
assessment of the direct radiative effects of PM would require
computationally intensive calculations that incorporate the spatial and
temporal behavior of particles of varying composition that have been
emitted from, or formed by precursors emitted from, different
sources.'' 2005 SP at 6-55. In addition, calculations of indirect
physical effects of particles on climate are subject to much larger
uncertainties than those related to the direct radiative effects of
particles.
    Exposure to solar radiation may have direct effects on human health
and agricultural and ecological systems; indirect effects on human
health and ecosystems, and effects on materials. 2005 SP at 6-56ff.
Several studies cited in the 2004 CD reinforce the idea that particles
can play an important role in affecting the transmission of solar UV-B
radiation. However, none of these studies included measurements of
ambient PM concentrations, so that direct relationships between PM
levels and UV-B radiation transmission could not be determined. In
addition, the relationships between particles and UV-B radiation
transmission can vary considerably over location, conditions, and time.
2005 SP at 6-56. In summary, the EPA staff concluded that available
information is insufficient to project the extent to which, or even
whether, location-specific changes in ambient PM would indirectly
affect human health or the environment.
e. Fundamental Elements of Increments
    As we have previously noted, under the model established in the Act
and prior EPA regulations, the function of an increment is not like
that of the NAAQS in that an increment is not intended to set a uniform
ambient pollutant concentration ``ceiling'' across the United States.
See 70 FR 59600. That is, while both increments and NAAQS generally
serve to limit air pollution levels, increments are designed to allow a
uniform degree of pollutant concentration increase for each area in the
United States with a particular classification, with the allowable
increase measured against a baseline air quality level for a particular
area.\15\ Because the baseline air quality level varies from one
location to another, and is not established until a PSD permit is
submitted, it is not possible to determine what the maximum pollutant
concentration attainable is for a given area (to be used to determine
the protection afforded by an increment against potential adverse
environmental effects) until the specific baseline air quality level is
known.
---------------------------------------------------------------------------

    \15\ It should be noted, however, that an increment does not
allow air pollution levels in an area to increase beyond the ambient
concentration of a pollutant that would exceed the level allowed by
the NAAQS.
---------------------------------------------------------------------------

    For the reasons described in our increments rule for
NOX, our objective is to establish uniform increments that
allow the same level of deterioration for each area of the country
having the same classification. 70 FR 59601. Our goal is not to
establish increments to reduce existing air pollutant concentrations
below baseline levels in each area, but rather to define a level of
increase in pollutant concentrations above baseline levels that
represents ``significant'' deterioration for each area classification.
70 FR 59600.
f. Evaluation of the Safe Harbor Increments
    Mindful of the considerations we previously described about the
fundamental characteristics of the increments, we reviewed the
scientific and technical evidence available for the 2005 review of the
NAAQS for PM in order to determine whether, and to what extent, the
``safe harbor'' increments might need to be modified in order to
protect air quality values, health and welfare, and parks while
ensuring economic growth consistent with the preservation of clean air
resources in accordance with sections 166(c) and 160 of the Act. As we
did in our evaluation of the safe harbor NO2 increments, we
propose to rely on an approach that evaluates how protective the safe
harbor PM2.5 increments are by trying to compare the
marginal pollutant concentration increases allowed by the safe harbor
increment levels against the pollutant concentrations at which various
environmental responses occur. We analyzed the available evidence from
both a quantitative and qualitative perspective to reach a decision
about whether we should modify the contingent safe harbor
PM2.5 increments and whether we have sufficient information
to select a specific alternative level, averaging time, or pollutant
indicator for the increments.
(1) Non-Visibility Related Effects
    In quantitatively evaluating the adequacy of the contingent safe
harbor increments for PM2.5 for non-visibility related
welfare effects, we experienced difficulties with identifying the
appropriate indicator, as well as to the level of the increments. In
the most recent evaluation of the NAAQS for PM, EPA staff concluded
that ``sufficient information is not available at this time to
recommend consideration of either an ecologically based indicator or an
indicator based distinctly on soiling and materials damage, in terms of
specific chemical components of PM.'' 2005 SP at 7-15. For
consideration of the effects of ambient PM on vegetation and other
ecosystems, the available data indicate that the chemical species of PM
(especially particulate nitrate and sulfate) has more relevance than
the size fraction (coarse or fine). Acid precipitation, including
particulate sulfate, has been found to be particularly damaging to
foliage, and along with ambient SO2 contributes
significantly to materials damage and soiling.
    Determining the most effective levels for any indicator for PM from
the available data is difficult because the evidence is insufficient to
provide a quantitative relationship between ambient PM concentrations
and known and observed adverse ecological effects. Fundamental areas of
uncertainty preclude establishing predictable relationships between
ambient concentrations of particulate nitrogen and sulfur compounds and
associated ecosystem effects. One source of uncertainty hampering the
characterization of such relationships is the extreme complexity and
variability that exist in estimating particle deposition rates. These
rates are affected by numerous factors, including particle size and
composition, associated atmospheric conditions, and the properties of
the surfaces being impacted. A related source of uncertainty is
establishing the portion of the total nitrogen and sulfur deposition
occurring at a given site is attributable to ambient PM. Though several
national

deposition monitoring networks have been successfully measuring wet and
dry deposition for several decades, they often do not distinguish the
form (e.g., particle, wet, and dry gaseous) in which a given chemical
species is deposited. Further, it is not clear how well data from
monitoring sites may apply to non-monitored sites with different
surface cover, meteorology, or other deposition related factors.
    Another fundamental problem that makes it difficult to establish a
meaningful dose-response relationship between ambient PM levels and
specific adverse environmental effects is that ecosystems have
different sensitivities and capacities to buffer or assimilate
pollutants. Many of the documented ecosystem-level effects only became
evident after long-term, chronic exposures to total annual loads of
reactive nitrogen (Nr) or acidifying compounds that eventually exceeded
the natural buffering or assimilative capacity of the system. In most
cases, PM deposition is not the only contributor to the total load of
Nr or acidifying compounds entering the affected system. Since it is
difficult to predict the rate of PM deposition, and thus, the PM
contribution to total deposition at a given site, it is difficult to
predict the ambient concentration of PM that would likely lead to the
observed adverse effects within any particular ecosystem. Equally
difficult is the prediction of recovery rates for areas already
affected, if PM deposition rates of various chemical species were to be
reduced.
    In response to our 2005 proposal for NO2 increments,
some commenters expressed the opinion that a better way of identifying
acceptable pollutant loadings, particularly for protection against
ecological effects, is the use of a ``critical load'' concept.\16\ 70
FR 59612. At that time, EPA expressed support for the concept, but
indicated that our current knowledge about critical loads did not
``provide a sufficient basis for establishing a uniform, national
standard such as a PSD increment.''
---------------------------------------------------------------------------

    \16\ A ``critical load'' is a numerical estimate of the amount
of polloution that a sensitive ecosystem can absorb on a sustained
basis before it experiences a measurable amount of degradation. In
contrast to the units for increments, [mu]/m\3\, a critical load is
typicallly expressed as a loading rate in kilograms of a pollutant
per hectare per year.
---------------------------------------------------------------------------

    The critical load concept was once again reviewed in the 2005 SP
for PM. It was noted in that document that the ``[k]ey to the
establishment of a critical load is the selection of appropriate
ecological endpoints or indicators that are measurable characteristics
related to the structure, composition, or functioning of ecological
systems (i.e., indicators of condition).'' 2005 SP at 6-46. The EPA
recognized the value of using critical loads and acknowledged that a
number of different groups in the United States have begun to use or
develop critical loads. Nevertheless, while recognizing that current
activities ``hold promise,'' EPA concluded that ``widespread use of
[critical loads] in the U.S. is not yet possible.'' Among other things,
currently available data are insufficient to quantify the contribution
of ambient PM to total Nr or acid deposition, and it is not clear
whether a critical load could be developed just for the portion of the
total N or S input that is contributed by PM. SP at 4-49. Research, in
conjunction with the development of improved predictive models, could
help in future consideration within the United States of the critical
loads concept, and in determining how much of any given critical load
is contributed by different sources of pollutants.
    As explained earlier, the available scientific and technical data
do not yet enable us to adequately relate ambient concentrations of
PM2.5 to ecosystem responses. Without such key information,
it is difficult to quantitatively evaluate the effectiveness of the
``safe harbor'' increments for protecting air quality values, health
and welfare, and parks while ensuring economic growth consistent with
the preservation of clean air resources. Alternatively, we must make a
qualitative judgment as to whether the contingent safe harbor
increments for PM2.5 or some alternative increments meet the
applicable factors.
    In this situation, we believe that the determination of the
increment levels that satisfy the factors applicable under section
166(c) is ultimately a policy choice that the Administrator must make,
similar to the policy choice the Administrator must make in setting a
primary NAAQS ``with an adequate margin of safety.'' See Lead
Industries Ass'n v. EPA, 647 F.2d 1130, 1147 (D.C. Cir. 1980) (where
information is insufficient to permit fully informed factual
determinations, the Administrator's decisions rest largely on policy
judgments). Using a similar approach is warranted because both section
109 and section 160(1) direct the Administrator to use his or her
judgment in making choices regarding an adequate margin of safety or
protecting against effects that may still occur notwithstanding
compliance with the NAAQS--both areas of inquiry characterized by great
uncertainty. Thus, in the process for setting NAAQS, the Administrator
looks to factors such as the uncertainty of the science, the
seriousness of the health effects, and the magnitude of the
environmental problem (isolated or commonplace). E.g., 62 FR 38652
(July 18, 1997) (PM2.5 NAAQS).
    Bearing on this policy decision for increments are various
considerations, based on the available information and the factors
applicable under section 166(c). The factors establishing particular
environmental objectives (protecting air quality values, health and
welfare, and parks) might suggest that, in some areas, we permit little
or no increase in PM2.5 emissions or establish an increment
for another form of PM because there are data indicating that an effect
may be attributable to PM emissions. However, as explained in the
NOX rule, we do not believe that Congress intended for the
PSD program to eliminate all negative effects. Thus, rather than just
seeking to eliminate all negative effects, we must attempt to identify
a level of increase at which any additional effects beyond existing (or
baseline) levels would be ``significant'' and protect against those
``adverse'' effects. Furthermore, we need to ensure that our increments
provide room for some economic growth. Congress intended for EPA to
weigh these considerations carefully and establish regulations that
balance economic growth and environmental protection.
    Since we are unable to establish a direct, widely applicable,
quantitative relationship between particular levels of PM2.5
and specific negative effects, we give particular weight to the policy
judgment that Congress made when it set the statutory increments as a
percentage of the NAAQS and created increments for the same pollutant
form and time period that was reflected in the NAAQS. In section 166 of
the Act, Congress directed that EPA study the establishment of PSD
regulations for other pollutants for which Congress did not wish to set
increments at the time.
    Congress's own reluctance to set increments to prevent significant
deterioration of air quality due to emissions of NOX, and
the provisions ensuring time for Congressional review and action,
suggest that Congress intended for EPA to avoid speculative judgments
about the science where data are lacking. Thus, in the absence of
specific data showing that an increment level that of the ``safe
harbor'' level would better protect health, welfare, parks, and air
quality values, while simultaneously maximizing opportunities for
economic growth, we give weight in our qualitative analysis of the
factors applicable under section

166(c) to the method that Congress used to establish the statutory
increments.
    In making this qualitative judgment, we also consider the overall
regulatory framework that we have established in the PSD regulations
for PM2.5. This framework includes a case-by-case analysis
of each permit application to identify additional impacts (e.g., soils
and vegetation), a special review by the FLM and State reviewing
authority of potential adverse effects on air quality values in parks
and special areas, and a requirement that all new and modified sources
install BACT. In addition, the area classification system ensures that
there will be economic growth in particular areas that is consistent
with the values of each State and its individual communities. Based on
this qualitative analysis, we do not believe it is necessary to adopt
more stringent increments to satisfy section 166(c) of the Act with
respect to non-visibility related effects.
(2) Visibility Protection
    In the case of visibility protection, the available evidence was
strong enough to enable EPA to conclude that PM2.5 is the
appropriate indicator for measuring the effects of ambient PM on
visibility impairment. Accordingly, using PM2.5
concentrations as the basis for review, EPA evaluated a range of
PM2.5 ambient concentrations, averaging times (24 hours and
less), and a range of concentration percentiles (using a concentration-
based form for the standard) in order to establish a recommendation for
setting the secondary NAAQS for PM to address visibility impairment in
urban areas. As explained in the 2005 SP, EPA considered, as a lower
bound for setting the short-term secondary PM2.5 standard, a
PM2.5 concentration of either 20 or 25 [mu]g/m\3\, averaged
over a 4- to 8-hour averaging time within daylight hours, depending on
the percentile range considered for the form of the standard.
    The Class II, short-term safe harbor increment for PM2.5
is 9 [mu]g/m\3\. This level is well below the lower bound recommended
for setting the secondary PM2.5 standard, but is based on a
24-hour averaging time at the 98th percentile. The 2005 SP also notes
that the estimated 98th percentile values in distributions of daily
background levels are below 10 [mu]g/m\3\ in most areas. Thus, the
allowable deterioration from the safe harbor increment in addition to
the natural background level generally falls below the minimum values
recommended in the 2005 SP for the secondary short-term standard for
PM2.5.
    With regard to the Class I increments for PM2.5, we note
that Congress explicitly included visibility as an air quality related
value (AQRV), enabling Federal land managers to protect significant
attributes of Federal Class I areas. Act section 165(d)(2)(B). The FLM,
assigned the affirmative responsibility to protect Federal Class I
areas, are to use AQRVs which are separate and distinct from
increments, to address individual Class I areas and the unique
attributes identified for each Class I area. Congress recognized that
AQRVs and increments were not the same thing and established
independent procedures for the implementation of each. For example, the
Act authorizes FLM to evaluate the effects of pollutant increases using
AQRVs as the basis regardless of the effect of such pollutant increases
on the increments. In using the AQRV, FLM are not limited in their
evaluation by the maximum allowable pollutant increase set by the
increment and may identify adverse impacts on visibility pursuant to
AQRVs even when the pollution increase will not cause or contribute to
an exceedance of an increment. Instead, the pollutant increase is
evaluated against the AQRV which considers the specific conditions
existing in the Class I area of concern. Thus, regardless of the
increased amount of pollution that an increment may allow, the FLM may
determine that the visibility in the Class I area is adversely affected
by an amount of pollutant increase less than that allowed by the
increment.
    From a qualitative perspective, we believe that visibility
protection in Class I areas is more adequately provided by the AQRV
process, where each area can be addressed on the basis of the local
situation and the FLM's assessment of potential ambient impacts by a
particular source. Nevertheless, generally speaking an increment should
not be so large that it routinely results in substantially more
pollution in Class I areas than is generally acceptable under the AQRV
approach. The contingent safe harbor PM2.5 increments for
Class I areas are 1 [mu]g/m\3\ and 2 [mu]g/m\3\ for the annual and 24-
hour averaging periods, respectively.
    We believe the importance of using distinct PM2.5
increments to protect against visibility impairment is also lessened by
the fact that Congress, aware of the statutory requirements for
prevention of significant deterioration of air quality, established
several visibility programs that specifically target emissions
reductions to achieve the desired visibility benefits. Under the
regional haze regulations, promulgated by EPA in 1999, States are
required to establish goals for improving visibility on the 20 percent
most impaired days in each Class I area, and for allowing no
degradation on the 20 percent least impaired days. Each State must
adopt emission reduction strategies which, in combination with the
strategies of contributing States, assure that Class I area visibility
improvement goals are met. Five multi-state planning organizations are
evaluating the sources of PM2.5 contributing to Class I area
visibility impairment to lay the technical foundation for developing
strategies, coordinated among many States, in order to make reasonable
progress in Class I areas across the United States.
    We believe it is also important to consider the fact that some
State and local governments have also developed programs to improve
visual air quality in specific urban areas. These programs are
individually designed to focus on improving visibility to a visual
range defined by the specific area of concern. Such local programs can
more appropriately focus on the preferences of individual communities
where a uniform national increment for visibility protection generally
cannot.
    In setting the NAAQS for PM, EPA ultimately concluded that a
distinct secondary standard with a different averaging time or form was
not warranted at that time. Instead, we concluded that a set of
secondary PM2.5 standards set identical to the revised
primary PM2.5 standards a reasonable approach when
considered in conjunction with the regional haze program as a means of
achieving appropriate levels of visibility protection in urban, non-
urban, and Class I areas across the United States. With regard to
evaluating the safe harbor increments for PM2.5, we had to
consider how much weight to give to visibility protection as a function
of the increments. That is, whether the increments were the appropriate
means of providing the most effective protection against visibility
impairment in urban areas as well as in rural areas, including Federal
Class I areas. In light of the other more direct approaches being used
to address visibility problems across the United States, we believe
that the use of distinct PM increments for visibility protection is not
the most effective means of addressing the visibility problem. Thus, we
do not believe it is necessary to modify the safe harbor increments for
PM2.5 to further protect visibility.

3. Proposed Baseline Dates for PM2.5 Increments Under Option
1
    If we adopt option 1, we propose to require the implementation of
the PM2.5 increment system with new baseline areas, baseline
dates and trigger dates. Specifically, we are proposing that the major
source baseline date and trigger date, both fixed dates, will be
defined as the effective date of this rule after promulgation.
    In light of current and expected trends in PM2.5
concentrations, EPA's judgment is that starting with new baseline dates
on or after the effective date of this rule would make the new PSD
increments more protective. Under our proposed approach, any emissions
reductions occurring prior to the effective date of this rule would be
counted toward the baseline concentration rather than expanding the
PM2.5 increment. If a retroactive baseline date were to
apply, emissions reductions occurring prior to the effective date of
this rule would serve to expand the available increments, enabling more
pollution than would otherwise be allowed to occur.
    In addition, we believe starting with new baseline dates to
implement new increments for PM2.5 is appropriate under this
option because we would treat PM2.5 essentially as a ``new''
pollutant for purposes of PSD and section 166 of the Act. We believe
that establishing a new baseline overcomes significant implementation
concerns that would otherwise exist if the existing PM baseline were
maintained. If we were to require sources and reviewing authorities to
conduct PM2.5 increment analyses based on the minor source
baseline dates previously established under the TSP or PM10
program, they would have to attempt to recreate the PM2.5
emissions inventory as of the minor source baseline date in order to
determine the baseline PM2.5 concentration for the area. For
early minor source baseline dates in particular (e.g., 1976 in areas of
the United States), establishing the emissions inventory for PM2.5
would be extremely difficult, cumbersome and potentially inaccurate
because historic emissions inventories did not include PM2.5
emissions. For all of these reasons, we are proposing option 1 as our
preferred option and request comment on this contingent safe harbor
approach under option 1
4. Revocation of PM10 Annual Increments
    If we use option 1 to adopt additional increments for
PM2.5, we propose to revoke the annual increments for
PM10 based on the same technical evidence that led us to
revoke the annual PM10 NAAQS. As discussed earlier, we do
not believe EPA is precluded from adopting new particular matter
increments under section 166(a) of the Act because we promulgated a
NAAQS for PM2.5 after 1977. However, if we read section
166(f) to address PM10 alone, the interaction of sections
166(a) and 166(f) could suggest that pollutant-specific PSD regulations
for PM promulgated on the basis of section 166(a) must be limited to
regulations that address fine PM. However, this view would create
tension with language in section 166(a) that calls for us to conduct a
holistic evaluation to establish a system of PSD regulations (including
numerical and other measures) for each pollutant covered by this
provision. Since it would be preferable to develop a system of
regulation for PM generally and select the appropriate indicator for PM
increments based on a comprehensive review of the effects of all forms
of PM (as we did in the recent NAAQS rule), we do not believe Congress
could have intended to constrain EPA's discretion to consider the
potentially differing effects of coarse and fine particles when
developing pollutant-specific PSD regulations under section 166(a).
    Since EPA recently revised the NAAQS for PM10 and
eliminated the annual PM10 NAAQS, we believe it is
permissible for the Agency to interpret the phrase ``pollutants for
which national ambient air quality standards are promulgated after
August 7, 1977'' to apply to revisions to PM10 as well. In
our 1989 proposal to adopt PM10 increments, before the
addition of section 166(f) to the Act, we construed the language in
section 166(a) to be broad enough to support adoption of PM10
increments. Under a holistic approach, considering all forms of PM, we
do not believe the evidence supports retaining an annual increment for
PM10 under the PSD program. In our October 17, 2006 action
on the PM NAAQS, the Administrator concluded that an annual coarse
particle NAAQS was not warranted at this time. 71 FR 61198-99. The CD
concluded that the available evidence does not suggest an association
of adverse health effects with long-term exposure to coarse particles
and the SP concluded there is no quantitative evidence that directly
supports an annual standard. Id. at 61198. With respect to welfare
effects, the evidence indicated that a short-term PM2.5
standard was the best approach for addressing visibility. Id. at 61280.
For non-visibility welfare effects, the Administrator concluded that
the available evidence was not sufficient to support an additional
indicator, but that a secondary NAAQS identical to the primary NAAQS
directionally improves the level of protection afforded vegetation,
ecosystems, and materials. Id. at 61210.
    When the evidence described in the Criteria Document and Staff
Paper is considered in light of the legal criteria applicable under
section 166(c), we believe it supports the conclusion that an annual
PM10 increment is no longer needed under the PSD program. In
the absence of a clear association between long-term exposure to coarse
particles and adverse health effects, we do not see a justification for
an annual PM10 increment to protect public health,
notwithstanding compliance with the NAAQS. In addition, the new
increments for PM2.5 that we propose to adopt, in
combination with the existing 24-hour increment for PM10,
will address welfare effects, air quality related values, and air
quality in national parks and other special areas. As described
earlier, visibility impacts are principally attributable to short-term
fine particle concentrations and thus will be addressed by the new
short-term PM2.5 increment. The evidence indicates that the
non-visibility welfare effects of concern are primarily attributable to
deposition of sulfate and nitrate particles of any size. Thus, the
combination of the new PM2.5 increments and the existing 24-
hour PM10 increment will address non-visibility welfare
impact attributable to deposition. Since we propose to retain the 24-
hour PM10 increment and adopt new annual and 24-hour fine
particle increments that will target all of these effects, we do not
consider it warranted to require continued tracking of changes in
annual concentrations of PM10 under the PSD program.

B. Option 2--Equivalent Substitution Approach for Annual Increments--
Section 166(f)

    Under this option, we would recognize PM2.5 as a new
indicator for PM for NSR purposes, and develop annual PM2.5
increments to replace the annual PM10 increments using the
equivalent substitution approach under the authority of section 166(f)
of the Act.
    The approach proposed under this option in this proposed rule would
be similar to the one we used in 1993, and discussed earlier to convert
from TSP increments to PM10 increments, to avoid having to
implement increments based on standards that no longer existed. On
October 17, 2006, EPA revoked the primary and secondary annual
PM10 standards and retained the primary and

secondary 24-hour PM10 standards. 71 FR 61144.
    In this case, we therefore are developing annual PM2.5
increments to replace the annual PM10 increments. Also,
consistent with our prior action in 1993, we are proposing to eliminate
or revoke the PM10 increments in this notice. However, for
developing the 24-hour PM2.5 increments, we are proposing to
use the increment values derived under the contingent safe harbor
approach explained in option 1. We seek comment not only on the levels
of the proposed ``equivalent'' increments, but also on our use of this
equivalent increment option for only the annual PM2.5
increments. In addition, we seek comment on whether we should rely on
section 166(f) to also propose the 24-hour PM2.5 increments,
even though the primary and secondary 24-hour PM10 NAAQS are
not being revoked.
1. Development of Equivalent Increments
    To establish equivalent PM10 increments in the 1993
rule, EPA compared the TSP and PM10 impacts of each of the
249 major sources subject to major NSR in our NSR database. EPA
observed that, in principle, for any source the equivalent PM10
increments was simply the product of the TSP increment to the source's
PM10/PM emissions ratio. 58 FR 31627.
    In this rulemaking, EPA proposes to apply the same type of ratio
approach to establish equivalent increments for PM2.5 under
section 166(f) of the Act. Unlike the 1993 analysis where we evaluated
a database of 249 major sources, for this rulemaking EPA relied on a
more comprehensive analysis of the ``2001 National Emissions Estimates
by Source Categories'' for PM10 and PM2.5.\17\
From the 2001 National Emissions Inventory, the ratio of emission
estimates from utilities and industrial point source categories were
used to find the PM2.5 to the PM10 emissions
ratio. For purposes of deriving the ratio, area sources and non-road
and mobile sources were not included on the basis that for NSR
permitting virtually all of the permitted sources fall within the
utility and industrial point source categories.
---------------------------------------------------------------------------

    \17\ 2001 National Emissions Estimates by Source Categories.
---------------------------------------------------------------------------

    Utilities and industrial point source emission estimates were
combined and a ratio of 0.8 was calculated as the ratio of emissions of
PM2.5 to PM10. Hence, the annual increments
developed for PM2.5 would be equal to 0.8 multiplied by the
increment value for PM10. Although we believe that this
approach is based on a permissible interpretation of the statute, we
believe it results in increment values for PM2.5 that are
much higher than the values Congress envisioned when it established the
original increments for PM and SO2 based on percentages of
the then existing NAAQS. For example, an annual PM2.5
increment in Class II areas using this approach would be 13 [mu]g/m
\3\, which is 87 percent of the annual PM2.5 NAAQS of 15
[mu]g/m \3\. In contrast, Congress established the Class II Increments
for PM and SO2 to represent 25 percent of the NAAQS. To
avoid such an unreasonable outcome for PM2.5, we rejected
this approach and instead are proposing two variations (options 2A &
2B) of the equivalent increment approach as the second and third
option.
2. Proposed Annual Increments for PM2.5
a. Option 2A
    In addition to an emissions ratio to reflect the shift in the
indicator from PM10 to PM2.5, we have also
considered the shift in the stringency of the NAAQS that resulted when
we changed the pollutant indicator from PM10 to
PM2.5. Accordingly, the ratio of emissions (0.8) that we
previously calculated would be multiplied by the ratio of the
PM2.5 NAAQS over the PM10 NAAQS (15/50 = 0.3
[mu]g/m \3\ for the annual standard) to derive an adjustment factor
(0.24 for the annual NAAQS) for calculating the Class I, II, and III
annual PM2.5 increments. Hence, multiplying the Class I, II,
and III annual PM10 increments, 4, 17, and 34 [mu]g/m \3\,
respectively, by the new adjustment factor yields the following
proposed increment values (rounded to the nearest whole number) under
option 2A:

------------------------------------------------------------------------
                                NAAQS        Increments ([mu]g/m \3\)
      Averaging period         [mu]g/m  --------------------------------
                                 \3\)     Class I    Class II  Class III
------------------------------------------------------------------------
Annual......................         15          1          4          8
24-hr.......................         35          2          9         18
------------------------------------------------------------------------

    Coincidentally, this new adjustment based on the PM2.5-
to-PM10 NAAQS ratio results in annual PM2.5
increment values identical to the values derived using option 1, the
percentage-of-NAAQS approach. As stated earlier, because the 24-hour
PM10 NAAQS have not been revoked, we do not consider section
166(f) to be the best fit for the development of the 24-hour PM2.5
increments. Thus, for new 24-hour PM2.5 increments, we are
proposing to rely on the authority of section 166(a) to derive 24-hour
increments as proposed under option 1.
b. Option 2B
    This option represents another variation on the section 166(f)
equivalent increment approach. Under this option 2B, we are proposing
to develop annual PM2.5 increments based solely on the ratio
of the annual PM2.5 NAAQS to the primary annual
PM10 NAAQS (15/50 = 0.3 [mu]g/m \3\ for the annual NAAQS).
The values for the annual PM2.5 increments derived by
multiplying the Class I, II, and III annual PM10 increments,
4, 17, and 34 [mu]g/m \3\, respectively, by this adjustment ratio
yields the following proposed increment levels (rounded to the nearest
whole number) under option 2B:

------------------------------------------------------------------------
                                NAAQS       Increments  ([mu]g/m \3\)
      Averaging period         ([mu]g/m --------------------------------
                                 \3\)     Class I    Class II  Class III
------------------------------------------------------------------------
Annual......................         15          1          5         10

24-hr.......................         35          2          9         18
------------------------------------------------------------------------

    As with option 2A, for the 24-hour PM2.5 increments, we
are proposing to use increment values developed via the contingent safe
harbor approach as described in option 1.
3. Baseline Dates
    Under these options (2A and 2B), since we will be replacing annual
PM10 increments with annual PM2.5 increments, we
propose to retain the existing TSP/PM10 baseline and trigger
dates and baseline areas for the PM2.5 program. Section
166(f) does not address how EPA should handle baseline dates for a
substituted increment. In 1993, we decided to retain the existing
baseline dates for TSP when we replaced the section 163 increment with
PM10 increments. We propose the same approach under this
option in this rulemaking because the continuation of the historic TSP/
PM10 baseline dates would ensure that no past case of
increment consumption is abandoned and serve as the closest measure of
a substitute. However, as discussed earlier, given PM2.5
emissions trends, our judgment is that establishing baseline dates for
PM2.5 after the effective date of this rule may be more
effective at preventing significant deterioration because the baseline
concentrations will reflect emissions reductions. We request comment on
whether this would provide sufficient justification for EPA to
establish new baseline dates under the section 166(f) substitution
approach.
    However, in conjunction with the annual PM2.5 increments
discussed above, we are proposing to use option 1 increment levels for
24-hour PM2.5 increments which would use new baseline areas,
trigger and baseline dates. Thus, assuming the baseline date for the
PM10 increments has already been triggered, this results in
different baseline dates for the annual and 24-hour PM2.5
increments. This would also require a PSD applicant to develop two
separate emissions inventories of increment-consuming sources for
evaluating a new source's cumulative PM2.5 impacts in the
area of concern. We seek comment on this issue of multiple inventories
under the equivalent increments approach.

VI. Significant Impact Levels (SILs)

A. EPA's Guidance on SILs in the PSD Program

    Significant Impact Levels or SILs are numeric values derived by EPA
that may be used to evaluate the impact a proposed major source or
modification may have on the NAAQS or PSD increment. The SILs currently
appear in EPA's regulations in 40 CFR 51.165(b), which are the
provisions that require States to operate a preconstruction review
permit program for major stationary sources that wish to locate in an
attainment or unclassifiable area but would cause or contribute to a
violation of the NAAQS. The SILs in that regulation are the level of
ambient impact that is considered to represent a ``significant
contribution'' to nonattainment.
    Although 40 CFR 51.165 is the regulation that establishes the
minimum requirements for nonattainment NSR programs in SIPs, the
provisions of 40 CFR 51.165(b) are actually applicable to sources
located in attainment and unclassifiable areas. See 40 CFR
51.165(b)(4). Where a PSD source located in such areas may have an
impact on an adjacent non-attainment area, the PSD source must still
demonstrate that it will not cause or contribute to a violation of the
NAAQS in the adjacent area. This demonstration may be made by showing
that the emissions from the PSD source alone are below the significant
impact levels set forth in 40 CFR 51.165(b)(2). However, where
emissions from a proposed PSD source or modification would have an
ambient impact in a non-attainment area that would exceed the SILs, the
source is considered to cause or contribute to a violation of the NAAQS
and may not be issued a PSD permit without obtaining emissions
reductions to compensate for its impact. 40 CFR 51.165(b)(2)-(3).
    The EPA has also applied SILs in other analogous circumstances
under the PSD program. Based on EPA interpretations and guidance, SILs
have also been widely used in the PSD program as a screening tool for
determining when a new major source or major modification that wishes
to locate in an attainment or unclassifiable area must conduct a more
extensive air quality analysis to demonstrate that it will not cause or
contribute to a violation of the NAAQS or PSD increment in the
attainment or unclassifiable area. SILs are also used to define the
extent of the Significant Impact Area (SIA) where a cumulative air
quality analysis accounting for emissions changes from all sources in
the SIA is performed.
    The EPA's historical application of SILs to the analysis of major
source impacts on attainment and unclassifiable areas under the PSD
program has largely been based on interpretations reflected in EPA
guidance memorandum. The EPA has not previously incorporated the
concept of a SIL into our PSD regulations at 40 CFR 51.166 and 40 CFR
52.21. Nevertheless, EPA has long considered the ``significant
contribution'' test set forth in 40 CFR 51.165(b)(2) to apply to the
impact of PSD sources on attainment areas as well, since that provision
applies to major new sources and major modifications located in
attainment and unclassifiable areas. Thus, EPA has also supported the
use of SILs as screening mechanism when analyzing whether a source
located in a PSD area will cause or contribute to a violation of the
NAAQS or PSD increment in attainment or unclassifiable areas.
    Although EPA's current PSD regulations do not contain SILs, EPA
initially developed SILs for TSP and other pollutants under the PSD
program in 1978. 43 FR 26380 (June 19, 1978). In the preamble to our
1978 regulations, EPA described SILs as a screening technique to
alleviate resource burdens (the costs and time involved in
sophisticated computer modeling of ambient air impacts) where there was
little or no threat to the PSD increments or NAAQS. 45 FR 26398.
However, as the threat to the increments increased, EPA intended for
more sophisticated techniques to be used. Id. Since EPA's analysis
indicated that the air quality impact of many sources fell off rapidly
to insignificant levels, the Agency did not intend to analyze the
impacts beyond the geographic point where the concentrations from the
source fell below certain levels derived from the class I increments.
Id. These levels were interpreted by EPA as representing the minimum
amount of ambient impact that is significant and hence came to be

known as the significant impact levels or SILs. Id.
    When EPA substantially revised our PSD regulations in 1980 to
include significant emissions rates and significant monitoring
concentrations, EPA did not include the SILs in our PSD regulations. At
that time, EPA felt that there was no need for a separate table of SILs
because of the adoption of ``a de minimis exclusion for monitoring''
otherwise known as SMCs (described later). 45 FR 52707. In addition,
EPA saw little value in retaining SILs as an exemption from the air
quality analysis because the demonstration necessary to qualify for the
exemption was itself an air quality analysis. 45 FR 52707.
    Subsequently, in draft guidance for permit writers, EPA advised
that SILs may be used to determine whether a source needs to conduct a
cumulative or ``full'' impact analysis to demonstrate that in
conjunction with all other increment consuming sources, it will not
cause or contribute to violation of the NAAQS or PSD increment in an
attainment or unclassifiable areas. New Source Review Workshop Manual,
at C.24-C.25 (Draft 1990); See also 40 CFR 51.166(k); 40 CFR 52.21(k).
Permitting authorities followed this guidance, and this approach
remains an accepted aspect of PSD program implementation. If based on a
preliminary impact analysis, a source can show that its emissions alone
will not increase ambient concentrations by more than the SILs, EPA
considers this to be a sufficient demonstration that a source will not
cause or contribute to a violation of the NAAQS or increment.
    In light of the unique air quality considerations in Class I areas,
EPA has drawn a distinction between the use of SILs in Class II areas
and Class I areas. The EPA's draft 1990 guidance only identified SILs
to be used in Class II areas under the PSD program. Workshop Manual at
C.28. However, in 1991, EPA advised the State of Virginia that the
concept of a SIL might be applied to Class I areas if the levels were
determined in a reasonable manner. Memorandum from John Calcagni, Air
Quality Management Division, to Thomas J. Maslany, Air, Radiation, and
Toxics Divisions (Sept. 10, 1991). The EPA did not support the use of
SILs to determine whether a source should conduct an analysis of its
impact on air-quality related values (AQRVs). Since there are currently
no Class III areas for PSD in the United States, there has been no need
for EPA to apply SILs in these areas.

B. Legal Basis for SILs

    The concept of a significant impact level is grounded on the de
minimis principles described by the court in Alabama Power Co. v.
Costle, 636 F.2d 323, 360 (D.C. Cir. 1980). In this case reviewing
EPA's 1978 PSD regulations, the court recognized that ``there is likely
a basis for an implication of de minimis authority to provide exemption
when the burdens of regulation yield a gain of trivial or no value.''
636 F.2d at 360. Based on this de minimis principle from the court's
opinion, EPA developed significant emissions rates and significant
monitoring concentrations in our 1980s regulations for PSD. The
significant emission rates reflect levels below which EPA considers an
emissions increase to be de minimis and thus not a major modification
that requires a PSD permit or NA-NSR permit. 45 FR 52676, 52705-07. See
also 40 CFR 51.166(b)(23); 40 CFR 52.21(b)(23). As discussed further
later, the significant monitoring concentrations in EPA regulations
define a de minimis level of impact that EPA has concluded does not
justify collecting pre-construction monitoring data for purposes of an
air quality impact analysis. 45 FR 52710.
    Similarly, significant impact levels are intended to identify a
level of ambient impact on air quality concentrations that EPA regards
as de minimis. The EPA considers a source whose individual impact falls
below a SIL to have a de minimis impact on air quality concentrations.
Thus, a source that demonstrates its impact does not exceed a SIL at
the relevant location is not required to conduct more extensive air
quality analysis or modeling to demonstrate that its emissions, in
combination with the emissions of other sources in the vicinity, will
not cause or contribute to a violation of the NAAQS at that location.
In light of insignificance of the ambient impact from the source alone,
EPA considers the conduct of a cumulative air quality analysis and
modeling by such a source to yield information of trivial or no value
with respect to the impact of the proposed source or modification. The
EPA's Environment Appeals Board has recently reiterated and affirmed
EPA's interpretation of the Act to allow EPA to evaluate the
significance of a source's impact when determining whether it would
``cause or contribute'' to a NAAQS or increment violation under section
165(a)(3) of the Act. In Re: Prairie State Generating Company, PSD
Appeal No. 05-05, slip op. at 139-144 (Aug. 24, 2006).
    Thus, in developing SILs for this proposal, EPA sought to derive
SILs for PM2.5 utilizing methods that would identify levels
representing a de minimis or insignificant impact on ambient air
quality. In choosing among the options set forth later, EPA proposes to
select an option that reflects the degree of ambient impact on
PM2.5 concentrations that can be considered truly de minimis
and would justify no further analysis or modeling of the air quality
impact of a source in combination with other sources in the area
because the source would not cause or contribute to an exceedance of
the PM2.5 NAAQS or the PM2.5 increments
established elsewhere in this proposal.

C. Relationship of SILs to AQRVs

    We wish to emphasize that consistent with the original purpose of
the Class I SILs, the Class I SILs for PM2.5 we are
proposing are not intended to serve as thresholds for determining the
need for an AQRV analysis or whether an adverse impact on an AQRV will
occur. An adverse impact on an AQRV depends upon the sensitivity of the
particular AQRV. An ambient concentration that is considered
insignificant for purposes of increment consumption should not
automatically be considered inconsequential relative to the inherently
fact-specific demonstration upon which an adverse impact on an AQRV is
to be based. Accordingly, the fact that a source's predicted impact is
less than the SIL in a Class I area would neither relieve the source
from having to complete an analysis of impacts on AQRVs nor
automatically allow the reviewing authority to reject the FLM's
demonstration of adverse impact on an AQRV. See 61 FR at 38292.

D. Proposed Options for PM2.5 SILs (for PSD and NA-NSR)

    We are seeking comment on the relative merits of each of the
following options for setting PM2.5 SILs.
1. Option 1. Propose SILs Using the Approach We Proposed for
PM10 in 1996
    The first option that we are proposing utilizes the same approach
we proposed for PM10 in the 1996 NSR Reform proposal. For
Class I areas we would set the SIL to 4 percent of the Class I
PM2.5 increment. For Class II and Class III areas, we would
codify the SIL values of 1.0 [mu]g/m \3\ for the annual averaging
period and 5.0 [mu]g/m \3\ for the 24-hour averaging period, that
already exist for PM10 in 40 CFR 51.165(b)(2). If we adopt
this option, we would set the Class I SILs based on the Class I
increments

that we elect to adopt under the increment options. Based on the Class
I increment values proposed in the percent of NAAQS increment option 1,
the SILs under this option would be as follows:

------------------------------------------------------------------------
                               Class I         SILs  ([mu]g/m \3\)
                              increment --------------------------------
      Averaging period         ([mu]g/m
                                 \3\)     Class I    Class II  Class III
------------------------------------------------------------------------
Annual......................          1       0.04        1.0        1.0
24-hour.....................          2       0.08        5.0        5.0
------------------------------------------------------------------------

    As stated earlier, we had proposed this approach for setting
PM10 SILs in our 1996 NSR Reform proposal. Many commenters
supported this approach and believed that the proposed SIL values would
serve as appropriate de minimis values. In fact, EPA is aware that many
States have been using these proposed SILs for PM10 as
screening tools since 1996.
    Regarding the proposal to set the level of Class I SILs at 4
percent of the Class I increments, we believe that where a proposed
source contributes less than 4 percent to the Class I increment,
concentrations are sufficiently low so as not to warrant a detailed
analysis of the combined effects of the proposed source and all other
increment-consuming emissions. We previously used a similar rationale
to establish the significant emissions rates for PSD applicability
purposes, concluding in part that emissions rates that resulted in
ambient impacts less than 4 percent of the 24-hour standards for PM and
SO2 were sufficiently small so as to be considered de
minimis.
    The original SIL values of 1.0 and 5.0 [mu]g/m \3\ for TSP and PM10
were interpreted by EPA as representing the minimum amount of ambient
impact that is significant. This forms the basis of the proposed
PM2.5 SIL values of 1.0 and 5.0 [mu]g/m \3\ for the annual
and 24-hour standard for Class II and III areas.
2. Option 2. PM2.5 to PM10 Emissions Ratio
    In our second proposed option for SILs, we would multiply the
PM10 SILs (proposed in 1996) by the emissions ratio of
PM2.5 to PM10 for point sources in the 2001
extrapolation of the final 1999 NEI. This is very similar to option 2A
for developing increments, and would use the same PM2.5/
PM10 emissions ratio (0.8). The Class I PM10 SILs
that we proposed in 1996 were 0.2 [mu]g/m3 (annual) and 0.3
[mu]g/m \3\ (24-hour). For Class II and III PM10 SILs, we
proposed 1.0 [mu]g/m \3\ (annual) and 5.0 [mu]g/m \3\ (24-hour) levels.
The SIL values determined in this option are as follows:

------------------------------------------------------------------------
                                                SILs ([mu]g/m\3\)
            Averaging period            --------------------------------
                                          Class I    Class II  Class III
------------------------------------------------------------------------
Annual.................................       0.16        0.8        0.8
24-hour................................       0.24        4.0        4.0
------------------------------------------------------------------------

    The SILs derived under this option are slightly more stringent for
Class II & III areas than those in option 1. Since PM2.5
emissions are a subset of PM10 emissions, we believe that an
emissions ratio of the PM10 SILs would serve as an
appropriate de minimis SIL value and represent insignificant impact on
ambient air quality.
3. Option 3. PM2.5 to PM10 NAAQS Ratio
    Under the third option that we are proposing, we would multiply the
PM10 SILs by the ratio of the PM2.5 NAAQS to the
PM10 NAAQS. This is very similar to option 2B for developing
PM2.5 increments, and would use the same factors. We would
start with the same values for the PM10 SILs that we used
for option 2 above for SILs. The PM2.5 SILs determined using
this approach are as follows:

------------------------------------------------------------------------
                                                SILs ([mu]g/m\3\)
            Averaging period            --------------------------------
                                          Class I    Class II  Class III
------------------------------------------------------------------------
Annual.................................       0.06        0.3        0.3
24-hour................................       0.07        1.2        1.2
------------------------------------------------------------------------

    The SILs derived under this option are very stringent for Class II
and III areas compared to options 1 and 2. Nevertheless, we believe
that the NAAQS ratio approach is an appropriate alternative to
determine SILs, since it reflects the stringency in the NAAQS for
PM2.5 relative to that of PM10. We believe that
these SIL values would serve as appropriate de minimis values.

VII. Significant Monitoring Concentrations (SMCS)

A. Background on SMCs

1. Preconstruction Monitoring and Its Role in NSR Program
    Under the Act and EPA regulations, an applicant for a PSD permit is
required to gather preconstruction monitoring data in certain
circumstances. Section 165(a)(7) calls for ``such monitoring as may be
necessary to determine the effect which emissions from any such
facility may have, or is having, on air quality in any areas which may
be affected by emissions from such source.'' 42 U.S.C. 7475(a)(7). In
addition, section 165(e) requires an analysis of the air quality in
areas affected by a proposed major facility or major modification and
calls for gathering 1 year of monitoring data unless the reviewing
authority determines that a complete and adequate analysis may be
accomplished in a shorter period. 42 U.S.C. 7575(e)(3). These
requirements are codified in

EPA's PSD regulations at 40 CFR 51.166(m) and 40 CFR 52.21(m).
    In accordance with EPA's Guideline for Air Quality Modeling (40 CFR
part 51, Appendix W), the preconstruction monitoring data is primarily
used to determine background concentrations in modeling conducted to
demonstrate that the proposed source or modification will not cause or
contribute to a violation of the NAAQS. 40 CFR part 51, Appendix W,
section 9.2. For most areas where multiple sources of air pollution are
present, EPA's Guideline recommends using monitoring data to identify
the portion of background concentrations attributable to natural
background, minor sources, and distant major sources. 40 CFR part 51,
Appendix W, section 9.2.3.f. For nearby major sources, EPA recommends
explicitly modeling the emissions of such sources rather than relying
on monitored data as part of the NAAQS compliance demonstrations. As
described earlier, the compliance demonstration with respect to the PSD
increment compliance focuses on modeling the change in emissions from
sources in the Significant Impact Area.
2. History of SMC Rules Adopted by EPA
    In 1980, EPA adopted regulations that exempt sources from
preconstruction monitoring requirements for a pollutant if the source
can demonstrate that its ambient air impact is less than a value known
as the Significant Monitoring Concentration or SMC. The pollutant-
specific SMCs are codified at 40 CFR 51.166(i)(5)(i) and 40 CFR
52.21(i)(5)(i). The EPA developed SMCs as a screening tool for sources
to determine whether they should conduct site-specific preconstruction
ambient monitoring. At the time they were adopted, EPA described the
SMCs as ``air quality concentration de minimis level[s] for each
pollutant * * * for the purpose of providing a possible exemption from
monitoring requirements.'' 45 FR 52676, 52707 (Aug. 7, 1980). The EPA
explained that it believed there was ``little to be gained from
preconstruction monitoring'' where a source could show that its
projected impact on the affected area was below these de minimis
levels. 45 FR 52710.
    In 1980, EPA determined the SMCs based on the current capability of
providing a meaningful measure of the pollutants. The EPA promulgated
values that represented five times the lowest detectable concentration
in ambient air that could be measured by the instruments available for
monitoring the pollutants. 45 FR 52710. The EPA chose the factor of
five after reviewing test data for various methods and considering
instrument sensitivity, potential for sampling error, instrument
variability, and the capability to read recorded data. Id.
    For PM, EPA set the SMCs for TSP at five times the lowest
detectable ambient concentration for TSP (2.0 [mu]g/m \3\) using the
Reference Method 5 for ambient sampling at that time. Memorandum from
Rehme, K. A., EPA/EMSL/QAD/MSB, to Peters, W., EPA/OAQPS/CPDD, on PSD
Monitoring (May 20, 1980). We set a SMC only for the 24-hour averaging
period, at a level of 10 [mu]g/m \3\. We retained the same numerical
level when we replaced the TSP NAAQS and increments with the
PM10 NAAQS and increments.

B. Legal Basis for SMCs

    As with the SMCs adopted by EPA in 1980, the SMCs for
PM2.5 proposed in this action are supported by the de
minimis doctrine set forth in the Alabama Power v. Costle opinion. Like
the other pollutants for which EPA has promulgated SMCs, EPA believes
there is little to be gained from preconstruction monitoring of
PM2.5 concentrations when the increased emissions of
PM2.5 from a proposed source or modification has a de
minimis impact on ambient concentrations of PM2.5. If a
source can show through modeling of its emissions alone that its
impacts are less than the corresponding SMC, there is little to be
gained by requiring that source to collect additional monitoring data
on PM2.5 emissions to establish background concentrations
for further analysis.
    Therefore, in developing SMCs for this proposal, EPA sought to use
methods that would identify levels representing a de minimis or
insignificant impact on PM2.5 ambient air quality that makes
the collection of additional monitoring data extraneous. In choosing
among the options set forth later, EPA proposes to select an option
that reflects the degree of ambient impact on PM2.5
concentrations that can be considered truly de minimis and would not
justify the gathering of monitoring data to establish background
concentrations for a demonstration of compliance with the NAAQS.

C. Proposed Options for PM2.5 SMC

1. Option 1. Lowest Detectable Concentration
    For this approach, we would use the same methodology originally
used in 1980 to set the SMC for TSP, i.e., determining the lowest
detectable concentration and multiplying this value by five. The lowest
detectable 24-hour average concentration for PM2.5 is 2.0
[mu]g/m \3\ (40 CFR 50 App L, section 3). Thus, applying this
methodology for PM2.5 yields an SMC of 10 [mu]g/m \3\ for
the 24-hour averaging period.
    As we indicated in 1980 when we originally used this methodology to
set the SMCs for TSP and the other PSD pollutants, the use of five
times the lowest detectable concentration was chosen to realistically
reflect pollutant levels at which low level concentrations or small
incremental changes in pollutant concentrations can reasonably be
determined. The factor of five takes into account the measurement
errors associated with the monitoring of these low pollutant levels or
small incremental changes in concentration. These measurement errors
arise from various sources, such as sample collection, analytical
measurement, calibration, and interferences (See Memorandum from Rehme,
K. A. mentioned earlier). We believe this is a reasonable approach,
since it has also been used for PM2.5 and TSP. We seek
comment on this approach.
2. Option 2. PM2.5 to PM2.5 Emissions Ratio
    Proposed option 2 establishes the SMC for PM2.5 by
multiplying the existing PM10 SMC (10 [mu]g/m \3\) by the
ratio of PM2.5 emissions to PM10 emissions in the
2001 extrapolation of the final 1999 NEI. This is the same methodology
used in Increments option 2A and SIL option 2, and uses the same
emissions ratio (0.8). This yields a SMC value of 8.0 [mu]g/m \3\ for
PM2.5 for the 24-hour averaging period.
    This approach gives a PM2.5 SMC value that is
equivalent, in terms of emissions, to the existing PM10 SMC.
We believe that this approach is consistent with the approach that
Congress set out for increments in section 166(f) of the Act and is, as
such, a reasonable approach for developing PM2.5 SMCs. We
seek comment on this approach.
3. Option 3. PM2.5 to PM10 NAAQS Ratio
    Under the third option, we propose to multiply the PM10
SMC by the ratio of the PM2.5 NAAQS to the PM10
NAAQS. This is the same approach proposed for Increment option 2B and
SIL option 3. Because the PM10 SMC is for the 24-hour
averaging period, we would use the ratio of the 24-hour NAAQS for
PM2.5 (35 [mu]g/m \3\) and PM10 (150 [mu]g/m
\3\). The resulting factor is 0.233. Thus, the PM2.5 SMC
developed using this option would be 2.3 [mu]g/m \3\, for the 24-hour
averaging period.

    The SMC developed using this approach is very stringent compared to
options 1 and 2, since it reflects the stringency of the 24-hour NAAQS
of PM2.5 relative to PM10. Nevertheless, we
believe this to be also a reasonable approach and seek comments on it.

D. Correction of Cross References

    In addition to exempting sources that have emissions increases
below the SMCs, EPA also exempts sources from preconstruction
monitoring where the source demonstrates that existing ambient
concentrations of a pollutant in the affected area are currently below
the SMCs. 40 CFR 51.166(i)(5)(ii); 52.21(i)(5)(ii). This aspect of the
monitoring exemption was also adopted in the 1980 rulemaking. 45 FR
52710.
    The EPA also proposes in this rulemaking to correct a cross
reference contained in these parts of the regulations. Paragraphs (ii)
and (iii) in 40 CFR 51.166(i)(5) and paragraph (ii) in 40 CFR
52.21(i)(5) each refer to concentrations listed in paragraphs
(i)(8)(i). However, there is no paragraph (i)(8)(i) in Sec.  51.166 and
no concentration values are contained in section (i)(8)(i) of Sec.
52.21. The cross references in these provisions were intended to
reference the SMCs in paragraph (i)(5)(i), but EPA failed to make this
change when the paragraphs were renumbered in a prior rulemaking. We
propose to correct that oversight in this rule.

VIII. Effective Date of the Final Rule, SIP Submittal/Approval
Deadlines and PM10 Revocation Deadline

    This section sets forth EPA's proposed effective dates for the
PM2.5 increments (under different options), SILs and SMC. In
addition, we are setting forth the proposed deadlines for States to
submit revisions to their SIPs incorporating these changes to the PSD
regulations, and for EPA to approve or disapprove the revised plans.
Finally, this section describes EPA's proposed schedule for revoking
the PM10 annual increments in conjunction with the
commencement of the PM2.5 increment system under the part 51
and part 52 PSD regulations and we request comment on establishing a
transition period for processing complete permit applications. Please
see Table 1 in the docket (Docket ID No. EPA-HQ-OAR-2006-0605) for a
summary of the proposed options and alternatives on which we seek
comment.

A. Option 1: Increments Promulgated Pursuant to Section 166(a) of the
Act

1. Effective Date of Final Rule
    As described in section III.E.2.a of this preamble, section 166(b)
of the Act specifies that new regulations for increments promulgated
pursuant to section 166(a) of the Act become effective 1 year after the
date of promulgation. Accordingly, if we promulgate the new PM2.5
increment under the authority of Section 166(a) following Option 1, we
propose a year's delay in the effective date.
    Alternatively, EPA seeks comments on whether we could make the new
increment regulations effective 60 days from promulgation. Considering
the various timeframes outlined in section 166, it is clear that
Congress envisioned that increments or other measures would become
effective within 3 years of the promulgation of a NAAQS. In the current
circumstance, due to prolonged litigation and other implementation
concerns, there has been an extended delay of over 10 years since we
established the PM2.5 NAAQS. Given this extended delay, we
believe that the overall Congressional intent reflected in section 166
may best be met by advancing the effective date of the proposed
regulations.
    States have to submit SIPs by April 5, 2008 to address the NSR
provisions of the final PM2.5 implementation rule after the
Federal NSR implementation rule is promulgated later this year. If EPA
decides to promulgate option 1 for increments and section 166(b)
timelines, the increments rule would not be implemented in SIP-approved
States until approximately January 2010 (assuming promulgation of this
rule in Spring 2008 and allowing 21 months for SIP submittal). Thus
from April 2008 to January 2010, PSD sources would be subject to a
PM2.5 applicability program, but would need to continue the
current PM10 air quality impacts analysis. Under these
circumstances, we expect that States, affected industry, and
environmental groups will see value in advancing the effective date of
the promulgated increments.
    Legislative history indicates that, when section 166(b) was first
enacted in 1977, Congress established the delayed effective date in
order to allow time for ``contrary Congressional action.'' H.R. Conf.
Rep. 95-564, at 151 (1977). The Congressional Review Act (CRA) provides
Congress with an expedited means of reviewing and potentially
disapproving final actions issued by Federal agencies. Under the CRA, a
member of Congress can introduce a joint resolution to disapprove a
particular rule and have that resolution considered using expedited
procedures if the resolution is introduced within the designated time
period (generally 60 days depending on the Congressional calendar).
Furthermore, an agency rule meeting the CRA definition of ``major''
cannot take effect for 60 days. We request comment on whether, given
these procedures under the CRA, a 60-day delay in the effective date of
the proposed rule could satisfy the Congressional intent reflected in
section 166(b).
2. State Program
    In this action, we propose to establish final PM2.5
increments as minimal program element for all State Programs.
Accordingly, States must submit revised SIPs for EPA's approval that
incorporate the final PM2.5 increments, or demonstrate that
an alternative approach is at least equivalent to this minimum program
element. Irrespective of whether we establish an effective date of the
final rule that falls 1 year after or 60 days after we promulgate the
final rule, we propose to require States to submit revised
implementation plans to EPA for approval within 21 months of
promulgation (9 months after the effective date of such regulations) in
accordance with the time frames specified in section 166(b) of the Act.
Section 166(b) also specifies that we must approve or disapprove these
revisions within 25 months of promulgation (4 months from the statutory
deadline for SIP submittal). We regard these statutory deadlines as
maximum allowed timeframes for action and do not believe that the Act
restricts our ability to approve SIP revisions requested by a State at
any time before these deadlines. We also propose to change the
regulatory provisions at 40 CFR 166(a)(6) to specifically articulate
these deadlines for these State SIP submittals.
3. Federal Program
    The EPA must also decide how to apply the procedures set forth in
section 166(b) of the Act to the new PM2.5 increments under
our 40 CFR part 52 PSD regulations, which apply for States without
approved PSD programs as well as for Indian lands. We propose to begin
to implement the Part 52 PSD program upon the effective date of the
final rule. Accordingly, if we delay the effective date for 1 year
after the date of promulgation in accordance with Section 166(b), then
the Part 52 PSD program would become effective and implemented in the
applicable areas, on this date. Alternatively, if we establish an
effective date 60 days after we promulgate the final rules, the Part 52
PSD program would become effective on this same date.
    Alternatively, we request comment on whether we should delay
implementation of the Federal Part 52

PSD program until 25 months after promulgation, or the outside date by
which EPA is required to approve State SIP revisions. This is the same
approach we took in 1988 to implement the then new NO2
increments. 53 FR at 40658. We are not offering this as our proposed
approach because of the significant delay that has already occurred
between the time we promulgated the PM2.5 NAAQS and the time
we will finalize this rule. However, we recognize that it may not be
equitable to begin implementation of the new program requirements in
those few areas where the Federal program applies before the majority
of States are required to implement the program. Nonetheless, we seek
comment on applying this alternative approach for the Federal Part 52
PSD program and specifically on the consequences of potential
inequities.

B. Option 2: Increments Promulgated Pursuant to Section 166(f) of the
Act

1. Effective Date of Final Rule
    In contrast to the proposed delay of the effective date of the new
PM2.5 increments under option 1, we propose to make the new
PM2.5 increments proposed under option 2 effective 60 days
from the date of promulgation, consistent with the CRA timeframe. We do
not interpret section 166(b) of the Act to apply to increments
promulgated under the authority of section 166(f) because the first
sentence in section 166(b) describes only ``[r]egulations referred to
in subsection (a).''
2. State Program
    We previously stated that we believe that it is appropriate to
establish a deadline for States to submit required SIP revisions
analogous to the deadline that applies to States when we promulgate or
revise a NAAQS. 67 FR 80241. We previously codified, conforming
regulatory text at 40 CFR 166(a)(6). Under Option 2 of this proposal,
we propose to follow the existing regulatory provisions that require a
State to adopt and submit for EPA approval its PM2.5 plan
revisions no later than 3 years after the date on which we promulgate
(i.e., publish in the Federal Register) the new regulations in the 40
CFR part 51 PSD regulations. Alternatively, we request comment on
whether we should require a timeframe shorter than 36 months, such as
the statutory maximum of 21 months required under Option 1. Given the
limited nature of the required changes, we believe that States
generally may not need more than 21 months to adopt and submit revised
plans to EPA for approval. If we select this alternative approach, we
propose to make conforming regulatory changes to 40 CFR 166(a)(6).
3. Federal Program
    For the Federal part 52 PSD regulations, we propose under Option 2
to make the new PM2.5 increments effective 60 days from the
date we promulgate the final rules. However, unlike the proposed 3-year
period being proposed for States to submit their plan revisions to EPA
for review and approval, we propose to implement the new increments
under the part 52 PSD regulations upon the effective date of the final
rules. Since it would be difficult to know when States are planning to
revise their own PSD programs consistent with the new increment
regulations, it is not possible to ensure a consistent implementation
date between approved State programs and programs being implemented
under the part 52 PSD regulations unless we delayed implementation for
a full 4 years (3 years for SIP submission and 1 year for EPA to
approve the revision). We believe that this delay is excessive and does
not accomplish the goal of expedient implementation of a PM2.5
PSD program. We request comment on this approach.

C. Revocation of the PM10 Increment

    While we believe it is appropriate to revoke the annual PM10
increment as explained earlier in this preamble, we propose to retain
the PM10 increments in both 40 CFR part 51 and part 52 PSD
regulations until the new PM2.5 increments are being
implemented either by a State through an approved SIP, or by EPA
through the Federal Part 52 PSD program. Accordingly, we propose to
approve the removal of the annual PM10 increments from any
SIP on or after the date we approve the new PM2.5 increments
in the same plan. We believe that States should request the removal of
the annual PM10 increments from their PSD programs at the
same time they submit plan revisions containing the new PM2.5
increments, allowing EPA to act on both actions simultaneously.
    Similarly, we propose to retain the annual PM10
increments in the Part 52 PSD regulations until the effective date of
the new PM2.5 increments.

D. Transition Period

    We believe that it is appropriate to establish a transition period
to clarify when PSD permit applications must contain an increment
analysis for the new PM2.5 increments following the date
they become effective and are approved as part of any State or Federal
PSD program. In the past, we have allowed for permit applications
submitted before the implementation date of new increment regulations
to continue to be processed under the existing rules, so long as the
reviewing authority has determined that the application is complete
before the implementation date. See e.g., existing 40 CFR
51.166(a)(i)(8) and (9). Consequently, we are also proposing a new
provision in both the 40 CFR parts 51 and 52 PSD regulations to provide
a transition process for initiating the requirement for analysis of the
new PM2.5 increments. Under the Part 51 regulations, we are
proposing that during the transition period, States have discretion to
continue the existing PM10 increment program or begin
implementing the new PM2.5 increment program. For the
federally administered programs under the Part 52 PSD regulations, the
provision would apply to each new PSD permit applicant upon the
effective date of the rule. However, we are also proposing a similar
transition period in these programs. See proposed 40 CFR 51.166(i)(10)
and 40 CFR 52.21(i)(11), respectively.

E. Effective Date for SILs and SMCs

    Unlike the approach we propose for PM2.5 increments, we
are not proposing to make SILs and SMCs a minimum element of an
approved SIP. Accordingly, we are not proposing to establish specific
deadlines for submission of revisions to incorporate the final rules
into SIPs. We do not believe that SILs or SMCs are required elements of
an approvable State program because in the absence of these
requirements, States can satisfy the statutory requirements by
obtaining pre-construction monitoring data and conducting a cumulative
air quality analysis for every PSD permit application.
    Nonetheless, we believe that availability of SILs and SMCs greatly
improve program implementation by streamlining the permit process and
reducing the labor hours necessary to submit and review a complete
permit application where the projected impact of the proposed source is
de minimis in the relevant area. For these reasons, we request comment
on whether we have authority to establish these as minimum program
elements based on the improved efficiency of the permit process. If we
require States to incorporate SILs and SMCs as mandatory elements of an
approvable program, then we would apply the existing regulations at 40
CFR 166(a)(6) for establishing the SIP submittal deadline. Under either
approach, the

final rules would become effective 60 days after we promulgate the
final rules.

IX. Statutory and Executive Order Reviews

A. Executive Order 12866: Regulatory Planning and Review

    Under Executive Order 12866 (58 FR 51735, October 4, 1993), this
action is a significant regulatory action because it raises novel legal
or policy issues arising out of legal mandates, the President's
priorities, or the principle set forth in the EO. Accordingly, EPA
submitted this action to the Office of Management and Budget (OMB) for
review under EO 12866 and any changes made in response to OMB
recommendations have been documented in the docket for this action.

B. Paperwork Reduction Act

    The information collection requirements in this rule have been
submitted for approval to the OMB under the Paperwork Reduction Act, 44
U.S.C. 3501 et seq. The information collection requirements are not
enforceable until OMB approves them. The Information Collection Request
(ICR) document prepared by EPA has been assigned EPA ICR number
2276.01.
    Certain records and reports are necessary for the State or local
agency (or the EPA Administrator in non-delegated States), for example,
to: (1) Confirm the compliance of status of stationary sources,
identify any stationary sources not subject to the standards, and
identify stationary sources subject to the rules; and (2) ensure that
the stationary source control requirements are being achieved. The
information would be used by EPA or State enforcement personnel to (1)
Identify stationary sources subject to the rules, (2) ensure that
appropriate control technology is being properly applied, and (3)
ensure that the emission control devices are being properly operated
and maintained on a continuous basis.
    The proposed rule would increase the PSD permitting burden for
owners and operators of major stationary sources of PM2.5
emissions by adding PM2.5 to the list of regulated NSR
pollutants for which air quality impact analyses must be carried out to
track increment consumption and demonstrate compliance with the NAAQS.
At the same time, there would be a reduction in burden directly
associated with the revocation of the annual increment for
PM10, as proposed in this proposed rule. Over the 3-year
period covered by the ICR, we estimate an average annual burden
totaling about 14,000 hours and $920,000 for all industry entities that
would be affected by the proposed rule. For the same reasons, we also
expect the proposed rule (when fully implemented) to increase burden
for the State and local authorities reviewing PSD permit applications.
In addition, there would be additional burden for State and local
agencies to revise their SIPs to incorporate the proposed changes. Over
the 3-year period covered by the ICR, we estimate that the average
annual burden for all State and local reviewing authorities will total
about 4,150 hours and $180,000.
    Burden means the total time, effort, or financial resources
expended by persons to generate, maintain, retain, or disclose or
provide information to or for a Federal agency. This includes the time
needed to review instructions; develop, acquire, install, and utilize
technology and systems for the purposes of collecting, validating, and
verifying information, processing and maintaining information, and
disclosing and providing information; adjust the existing ways to
comply with any previously applicable instructions and requirements;
train personnel to be able to respond to a collection of information;
search data sources; complete and review the collection of information;
and transmit or otherwise disclose the information.
    Any agency may not conduct or sponsor, and a person is not required
to respond to a collection of information unless it displays a
currently valid OMB control number. The OMB control numbers for EPA's
regulations in 40 CFR are listed in 40 CFR part 9.
    To comment on the Agency's need for this information, the accuracy
of the provided burden estimates, and any suggested methods for
minimizing respondent burden, including the use of automated collection
techniques, EPA has established a public docket for this ICR under
Docket ID number EPA-HQ-OAR-2007-0628. Submit any comments related to
the ICR for this proposed rule to EPA and OMB. See `Addresses' section
at the beginning of this notice for where to submit comments to EPA.
Send comments to OMB at the Office of Information and Regulatory
Affairs, Office of Management and Budget, 725 17th Street, NW.,
Washington, DC 20503, Attention: Desk Office for EPA. Since OMB is
required to make a decision concerning the ICR between 30 and 60 days
after September 21, 2007, a comment to OMB is best assured of having
its full effect if OMB receives it by October 22, 2007. The final rule
will respond to any OMB or public comments on the information
collection requirements contained in this proposal.

C. Regulatory Flexibility Act

    The Regulatory Flexibility Act (RFA) generally requires an agency
to prepare a regulatory flexibility analysis of any rule subject to
notice and comment rulemaking requirements under the Administrative
Procedure Act or any other statute unless the agency certifies that the
rule will not have a significant economic impact on a substantial
number of small entities. Small entities include small businesses,
small organizations, and small governmental jurisdictions.
    For purposes of assessing the impacts of this proposed rule on
small entities, ``small entity'' is defined as: (1) A small business as
defined by the Small Business Administration's regulations at 13 CFR
121.201; (2) a small governmental jurisdiction that is a government or
a city, county, town, school district or special district with a
population of less than 50,000; and (3) a small organization that is
any not-for-profit enterprise which is independently owned and operated
and is not dominant in its field.
    After considering the economic impacts of this proposed rule on
small entities, I certify that this rule will not have a significant
economic impact on a substantial number of small entities. This
proposed rule will not impose any requirements on small entities. We
continue to be interested in the potential impacts of the proposed rule
on small entities and welcome comments on issues related to such
impacts.

D. Unfunded Mandates Reform Act

    Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public
Law 104-4, establishes requirements for Federal agencies to assess the
effects of their regulatory actions on State, local, and tribal
governments and the private sector. Under section 202 of the UMRA, we
generally must prepare a written statement, including a cost-benefit
analysis, for proposed and final rules with ``Federal mandates'' that
may result in expenditures to State, local, and tribal governments, in
aggregate, or to the private sector, of $100 million or more in any 1
year. Before promulgating an EPA rule for which a written statement is
needed, section 205 of the UMRA generally requires us to identify and
consider a reasonable number of regulatory alternatives and adopt the
least costly, most cost-effective, or least burdensome alternative that
achieves the objectives of the rule. The provisions of section 205 do
not apply when they are inconsistent with applicable law. Moreover,
section 205

allows us to adopt an alternative other than the least-costly, most
cost-effective, or least-burdensome alternative if the Administrator
publishes with the final rule an explanation why that alternative was
not adopted. Before we establish any regulatory requirements that may
significantly or uniquely affect small governments, including tribal
governments, we must have developed under section 203 of the UMRA a
small government agency plan. The plan must provide for notifying
potentially affected small governments, enabling officials of affected
small governments to have meaningful and timely input in the
development of our regulatory proposals with significant Federal
intergovernmental mandates, and informing, educating, and advising
small governments on compliance with the regulatory requirements.
    We have determined that this proposed rule does not contain a
Federal mandate that may result in expenditures of $100 million or more
for State, local, and tribal governments, in the aggregate, or the
private sector in any one year. The proposed rule adds only a
relatively small number of new requirements to the existing permit
requirements already in place under the PSD program, since States are
currently implementing a PM10 surrogate program pursuant to
EPA guidance. Thus, this proposed rulemaking is not subject to the
requirements of sections 202 and 205 of the UMRA. We have also
determined that this rule contains no regulatory requirements that
might significantly or uniquely affect small governments because this
rule applies only to new major stationary sources. Thus, this proposed
rulemaking is not subject to the requirements of section 203 of the
URMA.

E. Executive Order 13132: Federalism

    Executive Order 13132, entitled ``Federalism'' (64 FR 43255, August
10, 1999), requires us to develop an accountable process to ensure
``meaningful and timely input by State and local officials in the
development of regulatory policies that have federalism implications.''
``Policies that have federalism implications'' is defined in the
Executive Order to include regulations that have ``substantial direct
effects on the States, on the relationship between the national
government and the States, or on the distribution of power and
responsibilities among the various levels of government.''
    This proposed rule does not have federalism implications. It would
not have substantial direct effects on the States, on the relationship
between the national government and the States, or on the distribution
of power and responsibilities among the various levels of government,
as specified in Executive Order 13132. Pursuant to the terms of
Executive Order 13132, it has been determined that this proposed rule
does not have ``federalism implications'' because it does not meet the
necessary criteria. Thus, the requirements of section 6 of the
Executive Order do not apply to this proposed rule.
    In the spirit of Executive Order 13132, and consistent with our
policy to promote communications between us and State and local
governments, we specifically solicit comment on this proposed rule from
State and local officials.

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

    Executive Order 13175, entitled ``Consultation and Coordination
with Indian Tribal Government'' (65 FR 67249, November 6, 2000),
requires us to develop an accountable process to ensure ``meaningful
and timely input by tribal officials in the development of regulatory
policies that have tribal implications.''
    This proposed rule does not have tribal implications as defined in
Executive Order 13175. This rule provides the elements to implement a
PM2.5 PSD program in attainment areas. The CAA provides for
States to develop plans to regulate emissions of air pollutants within
their jurisdictions. The Tribal Air Rule (TAR) under the CAA gives
tribes the opportunity to develop and implement CAA programs such as
programs to attain and maintain the PM2.5 NAAQS, but it
leaves to the discretion of the Tribe the decision of whether to
develop these programs and which programs, or appropriate elements of a
program, they will adopt.
    Although Executive Order 13175 does not apply to this rule, EPA did
reach out to national tribal organizations in 2006 to provide a forum
for tribal professionals to provide input to the rulemaking. However,
not much participation or input was received. It will neither impose
substantial direct compliance costs on tribal governments, nor preempt
tribal law. The EPA specifically solicits additional comment on this
proposed rule from tribal officials.

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

    Executive Order 13045 ``Protection of Children from Environmental
Health Risks and Safety Risks'' (62 FR 19885, April 23, 1997) applies
to any rule that: (1) Is determined to be ``economically significant''
as defined under Executive Order 12866, and (2) concerns an
environmental health or safety risk that we have reason to believe may
have a disproportionate effect on children. If the regulatory action
meets both criteria, the Agency must evaluate the environmental health
or safety effects of the planned rule on children, and explain why the
planned regulation is preferable to other potentially effective and
reasonably feasible alternatives considered by the Agency.
    This proposed rule is not subject to the EO because it is not
economically significant as defined in EO 12866, and because the Agency
does not have a reason to believe the environmental health or safety
risks addressed by this action present a disproportionate risk to
children because one of the basic requirements of the PSD program is
that new and modified major stationary sources must demonstrate that
any new emissions do not cause or contribute to air quality in
violation of the national ambient air quality standards. The public is
invited to submit or identify peer-reviewed studies and data, of which
EPA may not be aware, that assessed resolutions of early life exposure
to ambient concentrations of fine particulate measured as
PM2.5.

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

    This rule is not a ``significant energy action'' as defined in
Executive Order 13211, ``Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution, or Use'' (66 FR 28355
(May 22, 2001)) because it is not likely to have a significant adverse
effect on the supply, distribution, or use of energy. Further, we have
concluded that this rule is not likely to have any adverse energy
effects.

I. National Technology Transfer and Advancement Act

    Section 12(d) of the National Technology Transfer and Advancement
Act of 1995 (NTTAA), Public Law No. 104-113, 12(d) (15 U.S.C. 272 note)
directs us to use voluntary consensus standards (VCS) in our regulatory
and procurement activities unless to do so would be inconsistent with
applicable law or otherwise impractical. The VCS are technical
standards (e.g., materials specifications, test methods, sampling
procedures, and business practices) developed or adopted by one or more
voluntary consensus bodies. The NTTAA directs us to provide Congress,
through annual reports to OMB, with explanations when we do not use

available and applicable VCS. This proposed rule does not involve
technical standards. Therefore, we are not considering the use of any
VCS.

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

    Executive Order 12898 (59 FR 7629 (February 16, 1994)) establishes
Federal executive policy on environmental justice. Its main provision
directs Federal agencies, to the greatest extent practicable and
permitted by law, to make environmental justice part of their mission
by identifying and addressing, as appropriate, disproportionately high