visibility impairment. As a result, the EPA proposed a new, distinct
secondary standard that was designed to address these deficiencies.
    The EPA notes that in critiquing the proposed secondary standard,
commenters generally did not advocate that the form of the existing
mass-based PM2.5 standards was better suited scientifically
to the task of protecting against visibility impairment. Rather, the
commenters' position that a distinct secondary standard was not needed
for purposes of protecting visibility was based almost entirely on the
relative degree of protection likely to be afforded by the existing
standards (in particular, the existing 24-hour PM2.5
standard) as compared to the proposed visibility index, along with the
relatively large uncertainties associated with the latter. Thus, for
all the reasons discussed in the proposal with regard to the scientific
appropriateness of an indicator that takes into account both species
composition and relative humidity, the EPA continues to conclude that
the proposed standard based on a visibility index would be appropriate
scientifically to provide targeted protection of visibility, since it
would provide a measure of PM-related light extinction that directly
takes into account the factors (i.e., species composition and relative
humidity) that influence the relationship between PM2.5 in
the ambient air and PM-related visibility impairment.
    Furthermore, the EPA disagrees with commenters who stated that
implementation concerns, in particular the additional resource burden
associated with implementing a distinct secondary standard, should
alter the Agency's decision making with regard to a standard to protect
visibility. The EPA may not take the costs of implementation into
account in setting or revising the NAAQS.
    However, in light of the results of the Kelly et al. (2012a)
analysis and the views expressed by commenters on the implications of
this analysis for conclusions regarding the adequacy of the current
secondary 24-hour PM2.5 standard, the EPA has reconsidered
some of the conclusions drawn at the time of proposal, in particular
with regard to the degree of protection that would be provided by the
current secondary standard. Based on a review of comments related to
indicator, averaging time, form and level, the Agency has concluded
that (as described in sections VI.C.1b-e above) a standard defined in
terms of a PM2.5 visibility index (based on speciated
PM2.5 mass concentrations and relative humidity data to
calculate PM2.5 light extinction), a 24-hour averaging time,
and a 90th percentile form, averaged over 3 years, and a level of 30
dv, would provide sufficient but not more than necessary protection of
the public welfare with regard to visual air quality. Having identified
this target level of protection, the EPA is now in a position to
compare it specifically to the existing secondary 24-hour
PM2.5 standard of 35 [mu]g/m\3\ for purposes of determining
whether it would provide more, the same, or less protection from
visibility impairment. The EPA must consider both whether the existing
secondary 24-hour PM2.5 standard of 35 [mu]g/m\3\ is
sufficient (i.e. not under-protective) and whether it is more stringent
than necessary (i.e. over-protective).
    With regard to the degree to which the existing secondary 24-hour
PM2.5 standard provides sufficient but not more than
necessary protection for visibility, the EPA first notes that the kind
of area-specific analysis conducted in Kelly et al. (2012a) is
essential for addressing the court remand of the 2006 secondary
standards. In the case of the 2006 secondary standards, the EPA had
argued that the 35 [mu]g/m\3\ 24-hour PM2.5 standard was
requisite because one part of the proposed range for a distinct
secondary standard the Agency had considered would affect the
attainment status of a somewhat fewer counties than the 35 [mu]g/m\3\
24-hour PM2.5 standard. The court rejected this kind of
rough balancing, finding that the EPA's equivalency analysis based on
percentages of counties demonstrated nothing about the relative
protection offered by the different standards. Based on this, an area-
by-area evaluation of the relative degree of protection offered by
different standards should be conducted to the extent air quality data
is available.
    Kelly et al. (2012a) performed such an evaluation. Based on 2008-
2010 data, there are no areas that would have exceeded a 30 dv, 24-hour
visibility index standard that would not also have exceeded a 24-hour
PM2.5 standard of 35 [mu]g/m\3\. Stated another way, all
areas that met the 24-hour PM2.5 standard of 35 [mu]g/m\3\
would have had visual air quality at least as good as 30 dv (24-hour
average, based on 90th percentile form averaged over 3 years). The
Kelly (2012a) analysis also showed that for some areas, particularly in
the West, areas that would have met a 24-hour PM2.5 standard
of 35 [mu]g/m\3\ would have had visual air quality better than 30 dv
for the PM2.5 visibility index standard, and that at sites
that violated both the 24-hour level and the visibility index 30 dv
level, the visibility index level of 30 dv would likely be attained if
PM2.5 concentrations were reduced such that the 24-hour
PM2.5 level of 35 [mu]g/m\3\ was attained.
    The EPA has conducted a reanalysis (Kelly et al., 2012b) to update
the area-by-area analysis in the original Kelly et al. (2012a) analysis
in three respects. First, noting that the original Kelly at al. (2012a)
analysis used a 1.4 multiplier to convert OC to OM at those monitors
not using the new CSN monitoring protocol, the EPA recalculated the
visibility index design values for 2008-2010 using a higher multiplier
for converting OC to OM at monitors not already using the new CSN
monitoring protocol SANDWICH approach, consistent with the Agency's
view that it is more appropriate to use a multiplier of 1.6 at such
monitors as compared to 1.4, as described in section VI.C.1.a.ii,

above.\197\ The recomputed visibility design index values for 2008-2010
show the same overall relationship to 24-hour PM2.5 design
values as presented in Kelly et al., 2012a.
---------------------------------------------------------------------------

    \197\ Some of the OC measurements were produced with CSN's newer
monitoring protocol and did not require a change in the computed OM.
---------------------------------------------------------------------------

    Second, the EPA repeated the calculations comparing visibility
index design values with 24-hour PM2.5 design values using
2009-2011 data, the most recent three years of air quality information
currently available.\198\ Third, the EPA modified the area-by-area
evaluation to ensure consistency with the data completeness criteria of
40 CFR part 50, Appendix N, including the removal of data approved by
EPA as exceptional events, for the current 24-hour PM2.5
standard and the proposed visibility index standard.
---------------------------------------------------------------------------

    \198\ The 2011 air quality data were not yet available at the
time of proposal.
---------------------------------------------------------------------------

    The results of this reanalysis, as presented in Kelly et al.
(2012b), show a similar pattern to that described in the original Kelly
memo. Specifically, the analysis indicates that there were no areas
with visibility impairment above 30 dv that did not also exceed the 24-
hour PM2.5 standard of 35 [mu]g/m\3\. The updated memo
concludes that the results for 2009-2011 corroborate the findings for
2008-2010.
    Based on these analyses (Kelly et al., 2012a; 2012b), the EPA
concludes with a high degree of confidence that having air quality that
meets the 24-hour PM2.5 standard of 35 [mu]g/m\3\ would be
sufficient to ensure areas would not exceed 30 dv. The EPA notes that
this conclusion from Kelly et al. (2012a) is supported by two analyses
submitted by industry commenters (API, Attachments 2 and 3).
    At the time of proposal, the EPA had reached a different
conclusion, specifically that the 35 [mu]g/m\3\ 24-hour
PM2.5 standard was not sufficiently protective. This
conclusion was based, in part, on the analyses conducted for the
Visibility Assessment and Policy Assessment regarding 1- to 4-hour peak
light extinction values based on 2007-2009 data. For the reasons
outlined above in sections VI.B.1.c and VI.C.1.c, the EPA originally
focused on hourly or sub-daily timeframes for evaluating visibility
conditions. However, data quality concerns effectively precluded
adoption of a 1-hour or sub-daily averaging time in this review, and
ultimately the EPA has concluded that a 24-hour averaging time can
serve as an appropriate surrogate. In reaching this conclusion, the EPA
has recognized that adopting a 24-hour averaging time will likely
smooth out some of the hour-by-hour variability in visibility index
values, and will effectively reduce peak values by averaging them
together with other hours. In concluding it is appropriate to adopt a
24-hour averaging time, which limits the impact of hour-specific
influences, the Agency is now placing less weight on the results of the
1-hour and 4-hour analyses presented in the Visibility Assessment and
the Policy Assessment which focused on identifying the percent of days
with peak hourly light extinction above various CPLs. In light of the
Agency's conclusion that a 24-hour averaging time would be appropriate,
the Agency has determined to place more weight on analyses of
visibility conditions over a 24-hour time period, especially the
results in Kelly et al. (2012a and 2012b). In addition, the EPA notes
that the Kelly et al. analyses reflects updated air quality information
from more recent years of data (2008-2010 for Kelly et al., 2012a;
2009-2011 for Kelly et al. 2012b) as compared to the air quality
information used in the Visibility Assessment and Policy Assessment.
    In light of all of these considerations, including the results of
the Kelly et al. (2012a; 2012b) analyses, and the supporting comments
provided by a broad range of public commenters, the EPA now concludes
that the 24-hour PM2.5 standard of 35 [mu]g/m\3\ provides
sufficient protection in all areas against the effects of visibility
impairment. The EPA concludes that the existing 24-hour
PM2.5 standard would provide at least the target level of
protection for visual air quality defined by a visibility index set at
30 dv, as described above, which the EPA judges appropriate.
    However, the EPA also recognizes that it is important to evaluate
whether such a standard would be over-protective (i.e. more stringent
than necessary to protect public welfare). The analyses presented in
Kelly et al. (2012a; 2012b) indicates that the 24-hour PM2.5
standard of 35 [mu]g/m\3\ would achieve more than the target level of
protection of visual air quality (30 dv) in some areas. That is, when
meeting a mass-based standard of 35 [mu]g/m\3\, some areas would have
levels of PM-related visibility impairment far below 30 dv. Thus, when
considered by itself and without consideration of the secondary
standards adopted for purposes of non-visibility welfare effects, the
24-hour PM2.5 standard of 35 [mu]g/m\3\ would be over-
protective of visibility in some areas. However, it is important to
note that as long as the current secondary 24-hour PM2.5
standard of 35 [mu]g/m\3\ remains in effect, this overprotection for
visibility would occur, regardless of whether a distinct secondary
standard based on a visibility index set at 30 dv were adopted. These
issues are discussed more fully in section VI.D, which outlines the
Administrator's final conclusions on the secondary PM standards, below.
g. Legal Issues
    Some commenters opposed the proposal to establish a distinct
secondary standard that would be defined in terms of a PM2.5
visibility index. The proposed standard would use measured
PM2.5 mass concentration, in combination with speciated
PM2.5 mass concentration and relative humidity data, to
calculate PM2.5 light extinction, translated to the deciview
(dv) scale. The standard would also be defined in terms of a specified
averaging time and form, and a level for the PM2.5
visibility index set at one of two options--either 30 dv or 28 dv. The
commenters argued that the entire approach proposed by the EPA is
inconsistent with the requirements of CAA section 109(b). They pointed
to a number of different aspects of the proposal which in their view
made it incompatible with the CAA. For example, the Utility Air
Resources Group (UARG) stated:

    In the past, EPA has always used a measure of PM mass as the
indicator for both primary and secondary PM NAAQS. Such a standard
is, as a general matter, consistent with the directive in the CAA
that the NAAQS ``specify a level of air quality'' and targets for
control the listed criteria air pollutant. CAA Sec.  109(b)(2). The
standard contained in EPA's proposed rule does neither of these
things. Instead, it would (1) regulate relative humidity, which is
not a criteria pollutant; (2) fail to ``specify a level of air
quality'' as required by section 109(b)(2) of the CAA; and (3)
result in a standard necessitating nationally variable PM
concentrations instead of a standard establishing a nationally
uniform, minimally acceptable PM concentration. (UARG, p. 22-23)

    Other commenters raised similar or related issues, arguing that the
EPA improperly set a visibility standard, and not a PM2.5
standard, and that NAAQS can only be set in terms of a level or
concentration of the air pollutant. Commenters also argued that an
endangerment finding and air quality criteria would be needed before
the EPA could set a standard based on PM components. Each of these
comments is discussed below.
    As an initial matter, the commenters argued that the proposed
standard is unlawful because it is ``not a PM2.5 standard at
all, but rather a visibility standard, and visibility is neither an air
pollutant nor a criteria pollutant for which a NAAQS may be
promulgated''

(NMA/NCBA, p. 21). According to these commenters, the CAA requires that
NAAQS be established as limits on the concentration of an air pollutant
in ambient air, not limits on the ``identifiable effects'' caused by
that air pollutant. These commenters claimed that reduced visibility
due to light extinction is not an air pollutant but instead is an
effect, noting that ``the Act's definition of `air pollutant' speaks in
terms of specific substances or matter in the ambient air'' (NSSGA, p.
8). The commenters pointed to the use of the term ``air pollutant'' in
sections 109(a)(1)(A) and (b)(2) as support for their argument, as
these provisions refer to setting standards for the ``air pollutant''
to address the effects associated with the presence of the air
pollutant in the ambient air. They likewise pointed to section
108(a)(2)'s reference to the presence of the air pollutant in the
ambient air. Since reduced visibility is not an air pollutant, they
argue the EPA cannot set a NAAQS that is a standard for visibility.
They argue that the proposed secondary standard it is not a
PM2.5 standard as it does not limit the concentration of
PM2.5 or any other fraction of particulate matter in the
ambient air and therefore is not an ``ambient air quality standard''
for any pollutant.
    One commenter argued that the EPA is required to ``specify a level
of air quality'' under section 109(b)(2), which Congress intended as an
acceptable concentration level of the air pollutant in the ambient air,
noting that specification of acceptable visibility conditions is not
the same as an acceptable air pollution concentration level. Citing
American Farm Bureau v. EPA, 559 F.3d at 516, one commenter claimed
that the court had affirmed that ``the NAAQS--whether primary or
secondary--is a mass-based standard'' (Nevada DEP, p. 5). Commenters
also refer to the legislative history of the 1970 amendments, referring
to NAAQS as setting the ``maximum permissible ambient air level'' for
an air pollutant. The commenters argue that the proposed standard is
improper because it does not limit the concentration of
PM2.5 or any fraction of PM in ambient air, but improperly
sets a limit on visibility effects.
    With regard to humidity, these commenters argued that the proposed
standard improperly regulates relative humidity because it is included
in the calculation to determine the value of the visibility index.
According to these commenters, the CAA allows the EPA to control
criteria air pollutants through the NAAQS program, but not other
various substances. The commenters stated that the EPA recognized this
in the last review, treating humidity as a confounding factor and
considering addressing it by measuring PM2.5 mass-based
concentration over the midday hours, when humidity would have the least
effect. This would target the effects caused by PM, and not by
humidity. Referring to American Farm Bureau v. EPA, 559 F.3d 512, 528
(DC Cir. 2009) and 77 FR at 38979 n.153. UARG contested the proposed
calculated visibility index as it does not approach relative humidity
as a confounding factor but instead ``embraces it and treats it as if
it were a PM effect'' (UARG p. 24).
    The commenters also stated that the use of a calculated visibility
index, and the failure to exclude the effects of humidity, would result
in acceptable PM concentrations that vary across the nation. These
commenters claimed that such a standard is inconsistent with the
requirements of the CAA because the proposed approach fails to
establish a nationally uniform PM concentration standard. For example,
API argued that the proposed visibility index approach is ``essentially
specifying levels--not a level--of air quality'' (API, p. 29). UARG
agreed, and stated that the Act ``requires that criteria pollutant
concentrations throughout the nation reach, at the least, a single,
specified ambient concentration level'' (UARG, p. 25, emphasis in
original). The commenters argue that a PM2.5 visibility
index standard cannot provide equal protection nationwide due to
geographic variation in key factors such as relative humidity that
affect level of particles allowed in different areas. The commenters
noted that establishing a single national level for the
PM2.5 visibility index would necessarily result in unequal
acceptable PM2.5 levels in different areas of the country,
with lowest allowable PM2.5 levels in urban areas in the
Southeast and highest allowable levels in the arid West. UARG
recognized that under section 108 the air quality criteria are to
``address those variable factors (including atmospheric conditions)
which of themselves or in combination with other factors may alter the
effects on public health or welfare of such air pollutant,'' but stated
that while section 108 ``allows'' this, it has no bearing on this
issue. Instead, the commenter stated that the EPA may take such
information into account in setting a permissible concentration of the
pollutant that is uniform and national (UARG, p. 25).
    In addition, some commenters opposed to the proposed distinct
secondary standard argued that in order to base a standard on measured
levels of several speciated substances, the EPA must first make an
endangerment finding and issue air quality criteria for each of the
speciated substances included in the calculation of PM2.5
light extinction. According to these commenters, ``EPA cannot use NAAQS
to indirectly regulate multiple substances which are not criteria
pollutants under the guise of establishing a visibility standard''
(NMA/NCBA, p. 21). Noting that air quality criteria for particulate
matter were issued in 1969, NMA/NCBA argued that the 1969 Criteria
Document ``did not establish air quality criteria for individual
constituents that occur in particle form, instead it established
criteria for particulate matter as a whole'' (p. 27). In light of the
fact that criteria have never been issued for ``individual speciated
components of particulate matter,'' these commenters argued, ``if EPA
wishes to promulgate a rule such as its secondary visibility NAAQS, it
first must make a finding that the speciated components listed in
Appendix N endanger public health or welfare and then issue an air
quality criteria document for those components'' (NMA/NCBA, p. 29).
According to these commenters, the approach the EPA adopted in
promulgating a NAAQS for lead supports this view:

    When EPA promulgated a NAAQS for lead, an individual substance
in particle form, it did not assert that an endangerment finding or
criteria document for lead was unnecessary because lead was already
covered by the PM Criteria Document. Instead, EPA complied with the
Section 108 and 109 NAAQS prerequisites for lead, just as it must do
for Appendix N substances if it intends to promulgate a NAAQS for
those substances. * * * [In 1976], EPA listed lead as an air
pollutant that adversely affected public health or welfare, issued
an air quality criteria document for lead, and promulgated a NAAQS
for lead. 43 FR 46246 (Oct. 5, 1976). (NMA/NCBA, p. 29)

    Finally, UARG argued that the EPA has in the past recognized that
the secondary NAAQS is an inappropriate vehicle for regulating PM-
related visibility, referring to 62 FR at 38680, including fn 49. UARG
claimed the same situation continues, and the EPA has not provided a
valid basis for changing this conclusion.
    The EPA disagrees with the points raised by these commenters. While
the EPA is not adopting the proposed secondary standard, as explained
below, this decision is not based on concern over the EPA's authority
to adopt a secondary standard such as the one proposed.
    The proposed distinct secondary standard is a standard for
PM2.5, and is

not a ``visibility standard.'' The proposed secondary standard is based
on the mass concentration of PM2.5 in the ambient air. The
standard is defined in terms of calculated PM2.5 light
extinction which is based on the measurement of the mass concentration
of ambient PM2.5 over a 24-hour period. The measured mass
concentration is adjusted based on information on the speciated mass
components of the PM2.5 and the relative humidity, resulting
in a calculated visibility index. The level of the visibility index,
combined with the form of the standard and averaging time, identifies
whether a level of ambient mass concentration of PM2.5
achieves the standard or not. Given any specific mass concentration of
ambient PM2.5, combined with information on speciation and
relative humidity, it can be determined whether the specific mass
concentration of ambient PM2.5 achieved the NAAQS. Hence,
the proposed secondary NAAQS specifies acceptable levels of ambient
mass concentration of PM2.5.
    The combination of indicator, averaging time, form, and level of
the proposed NAAQS is designed to provide the appropriate degree of
protection from visibility impairment caused by ambient levels of
PM2.5. It does this by calculating the light extinction
associated with ambient concentrations of PM2.5 and
specifying the level of acceptable PM2.5 mass concentration
in terms of this calculation. However this does not change the fact
that the standard is for the air pollutant PM2.5, and
defines acceptable ambient levels of this air pollutant. It does not
transform the standard into a ``visibility standard'' and not a
standard for PM2.5. While the commenters had additional
concerns over the use of relative humidity in the calculation, and the
variation around the country of acceptable mass concentrations, those
issues are separate and do not change the fact that the proposed
standard defined in terms of calculated PM2.5 light
extinction is based on measurement of PM2.5 concentration in
the ambient air, and is a NAAQS for PM2.5.
    With regard to the contention that section 109(b) limits the EPA to
setting a standard that is based on the concentration of the pollutant
in the ambient air, we note that the term ``concentration'' typically
means some measure of relative content. For example, this would include
relative measures such as mass per unit of volume or parts per million.
The EPA has often used such metrics to define the NAAQS, largely
because the scientific evidence of health or welfare effects supporting
the NAAQS typically use such metrics in air pollution studies. For
example, the current secondary standards for PM are defined in terms of
the concentration of PM2.5 and PM10 in the
ambient air, measured as the dried mass of the particulate matter per
unit of air. However section 109(b) does not require that a NAAQS be
defined this way.
    Sections 109(a) and (b) both use the general term ``air quality''
when discussing the EPA's obligation to set NAAQS. The NAAQS are
clearly national ambient ``air quality'' standards under section
109(b), which specifies that the primary NAAQS ``shall be ambient air
quality standards'' and the secondary NAAQS ``shall specify a level of
air quality.'' Both the primary and secondary NAAQS are to be based on
the ``air quality criteria,'' which are to accurately reflect the
latest scientific knowledge on the effects on public health and welfare
associated with ``the presence of such air pollutant in the ambient
air, in varying quantities.'' Section 109(b), 108(a)(2). Congress spoke
in broad terms, tasking the EPA with assessing the latest scientific
knowledge about the public health and welfare associated with the
presence of the pollutant in the air, without limiting this to
consideration of only those effects associated with one or more
measures of concentration of the air pollutant. Congress referred to
any and all effects associated with the presence of the pollutant in
the ambient air, not just the effects associated with the concentration
of the pollutant in the ambient air. Based on this knowledge, the EPA
is required to set standards for the quality of the air that will
provide the appropriate degree of protection from these health and
welfare effects, without limitation on how to measure or define air
quality. While concentration in the air has typically been an
appropriate way to set a standard to achieve these requirements, the
more general terms used in section 108(a) and 109(b) do not limit the
EPA to using concentration as the only way to measure air quality for
purposes of setting a NAAQS. The EPA is charged with setting air
quality standards, and has the discretion under section 109(b) to
choose the metric for defining air quality that is appropriate to
address the health or welfare effect at issue.
    Congress did refer to ``concentration'' in certain situations. In
section 109(c) Congress required the EPA to set a primary NAAQS for
NO2 concentration over 3 hours. This addressed Congress'
concern over whether the then current NO2 standard, which
used concentration as a metric, provided adequate protection. Congress
also called on CASAC to advise the Administrator on the relative
contribution to ``air pollution concentrations'' of natural and
anthropogenic sources, under section 109(d)(2)(C)(iii). This
information is in addition to the advice CASAC is required to provide
concerning appropriate revisions to the ``air quality criteria'' and to
the NAAQS under section 109(d)(2)(B).\199\ While these provisions refer
to ambient concentrations of pollutants, this reflects the EPA's
standard practice to date in setting NAAQS, and none of them change or
limit the range of discretion provided under section 109(b) in setting
NAAQS. They do not change the fact that the EPA is to set ``air
quality'' standards, and is not limited to ``air concentration''
standards. The reference in the legislative history to a maximum
permissible ambient air level for the pollutant also does not limit the
EPA to a level of air pollutant concentration, as compared to a
different metric for specifying the level of air quality, if that is
judged to be appropriate.
---------------------------------------------------------------------------

    \199\ In a provision that is not part of the CAA, in 1990
Congress required EPA to request a report from the National Academy
of Sciences on the role of secondary national ambient air quality
standards, including information on the ``effects on welfare and the
environment which are caused by ambient concentrations of
pollutants'' listed under section 108, and the ``ambient
concentrations of each such pollutant which would be adequate to
protect welfare and the environment from such effects.'' Section
817(a) of the CAA Amendments of 1990, Pub. L. 101-549.
---------------------------------------------------------------------------

    The text of sections 108 and 109 does not support the limited
interpretation commenters suggest. Instead these provisions provide the
EPA with significant discretion in determining the metric for air
quality that is appropriate to achieve the required degree of
protection of public welfare. The commenters' interpretation would
improperly limit this discretion, interfering with achieving the goals
of section 109(b).
    For example, in this review the EPA considered whether it would be
appropriate to base a secondary NAAQS on direct measurement of the
light extinction caused by PM2.5. See 77 FR 38890, 38980-1
(June 29, 2012). There are several instrumental methods that directly
measure PM2.5 light extinction--the amount of light
extinction caused by the presence of PM2.5 in the ambient
air. This is not a measure of the concentration of PM2.5 in
the air, but a measure of the light extinction caused by
PM2.5. This is clearly an effect associated with the
presence of PM2.5 in the ambient air,

and this atmospheric property is directly related to visibility
effects. Unlike PM2.5 mass concentration, there is a close
scientific relationship between directly measured PM2.5
light extinction and visibility effects.
    It would appear straightforward to say that PM2.5 light
extinction is a quality of the ambient air, and a secondary NAAQS that
specified an acceptable level of PM2.5 based on directly
measured PM2.5 light extinction would be an ``ambient air
quality standard'' for the air pollutant that specifies a ``level of
air quality'' designed to provide protection against visibility
impairment. Unlike directly measured PM2.5 light extinction,
the mass concentration of PM2.5 does not have the same
direct relationship to light extinction, and specifying an acceptable
level of mass concentration of PM2.5 would be a more
indirect and less effective way to provide protection from visibility
impairment caused by the presence of PM2.5 in the ambient
air. Under the commenters' interpretation, the EPA would be precluded
from specifying a level of air quality in terms of directly measured
PM2.5 light extinction, the more scientifically appropriate
and direct measure of the effect PM2.5 has on visibility.
Instead the EPA would be limited to the more indirect and less
effective specification of a level of concentration of
PM2.5.
    The commenters also objected to the inclusion of relative humidity
as an adjustment factor in the calculation of PM2.5 light
extinction. Contrary to the claims of these commenters, the use of
calculated PM2.5 light extinction does not regulate relative
humidity. The proposed secondary standard would define acceptable
levels of ambient PM2.5, not acceptable levels of relative
humidity. In addition, section 108 explicitly requires that the air
quality criteria include information on the atmospheric conditions that
can alter the effects of the air pollutant on public health or welfare,
and relative humidity certainly has this kind of impact. Section 109(b)
requires that the standard be based on the air quality criteria,
indicating that this information can and should be taken into account
in setting the standard. Including relative humidity as an adjustment
factor in the calculation of PM2.5 light extinction is a
reasonable and straightforward way to use the scientific information in
the air quality criteria in establishing a standard to provide
protection from visibility impairment.\200\
---------------------------------------------------------------------------

    \200\ UARG recognizes these provisions, but argues, as above,
that this is limited by the requirement that the EPA set a NAAQS
based solely on ambient concentration.
---------------------------------------------------------------------------

    Some commenters pointed to the EPA's position in the last review,
stating that the EPA properly treated relative humidity as a
confounding factor, and in this review improperly moves away from that
position. See 77 FR at 38979, 71 FR 61144, 61205 (October 17, 2006). In
the last review the EPA considered a distinct PM2.5 mass-
based secondary standard. In that context, limiting the measurement of
PM2.5 mass concentration to the mid-day hours when relative
humidity had the least impact would promote the correlation between
measured PM2.5 mass concentration and light extinction,
which would promote achievement of a relatively consistent degree of
visibility protection across the country. However in this rulemaking
the proposed calculated PM2.5 light extinction standard
achieves a consistent degree of visibility protection by directly
accounting for humidity, in a scientifically defensible manner. The
goal has not changed--achieving the desired degree of protection across
the country. What has changed is that calculated PM2.5 light
extinction is a more direct and scientifically appropriate way to
achieve that result.
    Finally, it should be made clear that water is not a separate
compound from PM2.5 that confounds the impact
PM2.5 has on light extinction. As described in the
Integrated Science Assessment, ``PM is the generic term for a broad
class of chemically and physically diverse substances that exist as
discrete particles (liquid droplets or solids) over a wide range of
sizes'' (U.S. EPA, 2009a, p. 1-4). ``Particles composed of water
soluble inorganic salts (i.e., ammoniated sulfate, ammonium nitrate,
sodium chloride, etc.) are hygroscopic in that they absorb water as a
function of relative humidity to form a liquid solution droplet. Aside
from the chemical consequences of this water growth, the droplets
become larger when relative humidity increases, resulting in increased
light scattering. Hence, the same PM dry concentration produces more
haze'' (U.S. EPA, 2009a, p. 9-6). Thus water is not a compound that is
separate and apart from the particle that acts as an extraneous
confounding factor.\201\ The effect of relative humidity occurs after
the water becomes part of the particle. Certain water soluble salts
absorb water and the resulting particle is larger in size and scatters
more light, increasing the visibility impact of the particle. But the
particle is still a PM2.5 particle. The fact that the PM
NAAQS traditionally uses a measurement of the dried mass of the
particles as the metric for the standard does not change the fact that
the particles in the air include liquid droplets and particles that
have increased in size because of absorption of water. These ambient
PM2.5 particles are what is in the air and impacting
visibility, not just the dried mass of PM2.5 that is
measured in the laboratory and is currently used as the indicator for
the PM NAAQS. Thus the commenters improperly claimed that the proposed
secondary standard regulates water or relative humidity, and not
PM2.5, when in fact the proposed secondary standard accounts
in a scientific manner for the fact that some PM2.5
particles are larger in size and have a greater impact on light
extinction when the relative humidity increases.
---------------------------------------------------------------------------

    \201\ According to the Integrated Science Assessment,
``Confounding is `* * * a confusion of effects. Specifically, the
apparent effect of the exposure of interest is distorted because the
effect of an extraneous factor is mistaken for or mixed with the
actual exposure effect (which may be null) ' (Rothman and Greenland,
1998, 086599)'' (U.S. EPA, 2009a, p. 1-16).
---------------------------------------------------------------------------

    The commenters also raised concerns that a standard based on
calculated PM2.5 light extinction, compared to a standard
based on just PM2.5 mass concentration, improperly results
in variable levels of acceptable PM2.5 mass concentrations
across the country. This stems from the adjustments in the calculation
for speciated components of PM2.5 and relative humidity.
According to commenters, this is improper as section 109(b) requires
that the NAAQS set a single, specified ambient concentration that is
nationally uniform across the country.
    As discussed above, the text of section 109(b) does not specify
this limitation of a single national acceptable concentration. Instead
the secondary NAAQS is to specify a level of air quality that achieves
the appropriate degree of protection. The proposed secondary standard
would do just that--specify a level of air quality, defined in terms of
calculated PM2.5 light extinction, that would achieve the
desired degree of protection. The fact that this results in varying
allowable levels of PM2.5 mass concentrations is not
inconsistent with the Act. The DC Circuit recently approved such a
result. In the last review of the PM10 primary NAAQS, the
court approved the EPA's choice of an indicator that was designed to
allow varying levels of acceptable coarse PM. The court stated that:

    The industry petitioners next argue that the 150 [mu]g/m\3\
standard for PM10 will result in arbitrarily varying
levels of coarse PM, and that the agency should instead have used a
PM10-2.5 indicator. The EPA does not dispute

that using the PM10 indicator will result in coarse PM
levels that vary within the limit of 150 [mu]g/m\3\. As the EPA
explains: ``Because the PM10 indicator includes both
coarse PM (PM10-2.5) and fine PM (PM2.5), the
concentration of PM10-2.5 allowed by a PM10
standard set at a single level declines as the concentration of
PM2.5 increases. Thus, the level of coarse particles
allowed varies depending on the level of fine particles present.''
Id. at 61,195.
    Although the EPA acknowledges that a PM10 indicator
will result in varying coarse PM levels, it does not agree that the
variance will be arbitrary. The EPA agrees with the industry
petitioners that protection from coarse particles should be targeted
at urban areas, where coarse particles have been shown to pose the
greatest danger. Id. at 61,194. But the agency argues that targeting
of urban areas is effectively accomplished by using an indicator
that permits the varying levels that the industry petitioners
challenge. * * * Id. at 61,195-96 (citations omitted). In other
words: ``The varying levels of coarse particles allowed by a
PM10 indicator will therefore target protection in urban
and industrial areas where the evidence of adverse health effects
associated with exposure to coarse particles is strongest.'' Id.
    The EPA also offers a further rationale for tying the stringency
of coarse PM regulation to increases in the level of
PM2.5.* * * EPA argues that it is ``logical to allow
lower levels of coarse particles when fine particle concentrations
are high.* * * [I]nclusion of PM2.5 in the
PM10 indicator for purposes of coarse particle protection
would appropriately reflect the contribution that contaminants
emitted in fine particle form can make to the overall health risk
posed by coarse particles.'' Id.
    In sum, we find that the EPA has provided a reasonable
explanation for its decision[ ] * * * to utilize a standard that
allows targeted variance in coarse PM levels in an inverse
relationship to the amount of fine PM in the air. American Farm
Bureau v. EPA, 559 F.3d 512, 534-5 (D.C. Cir. 2009).

    A similar result applies here. Under the proposed secondary
standard there would be a single level of air quality specified for the
NAAQS. The standard would apply across the nation; it would not be a
regional standard. The proposed standard would be the same standard
everywhere--the acceptable level of mass concentration of
PM2.5 would be defined the same way across the nation, using
the same method of calculating the allowable concentration of
PM2.5. The same degree of protection from visibility
impairment would apply across the country. While the allowable amount
of PM2.5 could vary, this would be a reasoned way to achieve
the desired degree of protection from visibility impairment. The
requirements of section 109(b) would be satisfied.
    Commenters also objected that the EPA could not set a NAAQS for the
separate components of PM2.5 without listing the components
of PM2.5 under section 108, based on an endangerment
finding, and issuing air quality criteria for these components. They
argued that the issuance of air quality criteria for particulate matter
starting in 1969 did not provide a lawful basis for a proposed
secondary standard that is based on components of PM, as the 1969 air
quality was for particulate matter ``as a whole,'' defining PM as
particles smaller than 500 micrometers (NMA/NCBA, p. 27). However, as
discussed above, the proposed standard sets the allowable limit on
ambient concentrations of PM2.5. Information on both the
speciated components of PM2.5 and the relative humidity
affect how much light extinction is associated with any specific level
of PM2.5, but the standard is for PM2.5. The D.C.
Circuit has made it clear that PM2.5, just like
PM10 and TSP before that, is an appropriate subset of PM for
the EPA to focus on in setting the NAAQS based on the scientific
evidence before the EPA. This focus of the NAAQS does not make the
subset a new pollutant that requires listing and new air quality
criteria under section 108 before setting a NAAQS. American Trucking
Association et al. v. EPA, 175 F.3d 1027, 1055 (D.C. Cir. 1999).
Commenters' interpretation would apply to PM2.5 as well as
to components of PM2.5, and is inconsistent with the ATA
decision. In addition, it is clear that the current air quality
criteria do address the scientific basis for calculating
PM2.5 light extinction as the EPA proposed (U.S. EPA, 2009a,
pp. 9-5 to 9-8).
    Finally, at least one commenter argued that the EPA has concluded
in prior reviews that the secondary NAAQS program is an inappropriate
vehicle for regulating PM related visibility impairment (UARG, p. 26).
UARG mischaracterized the EPA's past decision-making. In past reviews
the EPA has been clear that the EPA should take into account the
existence of the visibility program under section 169A, the regional
haze program, when considering a secondary NAAQS and should not treat
the secondary NAAQS as the sole mechanism to address visibility
impairment across the country. That is the approach the EPA has taken
in this and prior reviews. See 77 FR at 38990.
h. Relationship With Regional Haze Program
    A large number of commenters expressed confusion and concern over
differences between the proposed visibility index standard and the
Regional Haze Program. This included commenters who supported setting a
distinct secondary standard to protect visibility as well as those
opposed to setting such a standard. A number of these commenters noted
that visibility impairment would be assessed differently under the two
approaches due to differences in the way light extinction is
calculated, including different IMPROVE equations and differences in
the inclusion and weighting of specific species and components. The
commenters argued it would be inappropriate to have two different
regimes for managing visibility impairment in the exact same location.
These commenters claimed that since data from the IMPROVE monitoring
network would inform nonattainment designations, as well as an area's
obligations under the Regional Haze Program, there could be
considerable confusion over how to draw nonattainment boundaries and
what requirements would affect large sources in rural areas. These
commenters also noted the resource burden associated with maintaining
two different programs aimed at protecting visibility in the same
geographic area. Some commenters argued that a visibility NAAQS should
not apply to rural areas. The Department of the Interior requested that
the EPA clearly define the geographic area to which the visibility
index standard would be applicable, and suggested that Class I and
Class II areas should generally be excluded from the standard. As
discussed above, commenters questioned the need for a distinct
visibility standard, arguing that the existing primary PM standards
combined with the Regional Haze Program ensured adequate protection of
visibility, even in urban areas.
    In response to these comments relating to the overlap between the
Regional Haze program and a distinct secondary standard designed to
protect visibility principally in urban areas, the EPA notes that the
objectives of each program are distinct. While the Regional Haze
program is designed to eliminate man-made impairment of visibility in
Federal Class I areas over the course of several decades, a distinct
secondary standard for PM-related visibility impairment would be
focused on providing a nationally applicable level of protection for
all areas, particularly urban areas which do not receive targeted
protection under the Regional Haze Program. Moreover, the metric used
to assess visibility impairment differs between the two programs
precisely because each program is aimed at a different aspect of the
problem. Recognizing the importance of fresh emissions for urban
visibility, the

Visibility Assessment focused on visibility impairment as measured by
the original IMPROVE equation because ``the original version is
considered more representative of urban situations when emissions are
still fresh rather than aged as at remote IMPROVE sites'' (U.S. EPA,
2010b, p. 3-19). The Regional Haze Program, on the other hand, has
shifted to a revised IMPROVE algorithm more suited to remote locations.
While this difference is discussed in more detail in section VI.C.1.b
above, the result is that each program would appropriately measure
those aspects of visibility impairment most closely related to the
problem the program is trying to prevent. Since the same data can be
used to calculate both visibility impairment under the Regional Haze
approach and the proposed visibility index, the additional calculation
burden for state and local agencies would be light. Also, to the extent
that there is any difference in terms of the emissions control
obligations the two different programs would impose upon state and
local areas, this is likely appropriate given the extent and nature of
visibility impairment in those areas. The EPA notes that in general,
there is likely to be substantial overlap in the control strategies a
state or local area would pursue under either program. Thus, the EPA
disagrees with commenters who stated that a distinct visibility
standard as proposed would inherently conflict with the Regional Haze
Program or that it would be appropriate to draw geographical
distinctions that would explicitly exclude some areas (e.g., Class I
areas) from the NAAQS. The EPA notes that the CAA requires that NAAQS
be national in scope, and that the specific requirements laid out in
the proposal for the distinct secondary standard would ensure that the
protection it afforded would be appropriately targeted toward urban
areas so that it could work in conjunction with--not be in conflict
with--the Regional Haze Program under sections 169A and 169B of the
CAA.
2. Comments on the Proposed Decision Regarding Non-Visibility Welfare
Effects
    Relatively few commenters addressed the proposal to retain the
existing suite of secondary PM standards to address non-visibility
welfare effects. A couple of states, including Mississippi and South
Dakota, offered brief endorsements of the proposal. A few other
commenters offered more extensive comments on the proposal to retain
the existing secondary standards, and these commenters opposed this
aspect of the proposal for one of two reasons. First, some commenters
opposed the proposal to retain the current secondary annual
PM2.5 standard of 15 [mu]g/m\3\ in light of the proposal to
revise the level of the primary annual PM2.5 standard to a
level between 12-13 [mu]g/m\3\. Expressing concern over the
implications of this decision for the air quality planning obligations
of states, these commenters argued that the EPA should revise the
secondary PM2.5 standards to be equivalent in all respects
to the primary PM2.5 standards. For example, the American
Association of State Highway and Transportation Officials (AASHTO)
supported ``retaining secondary standards that are consistent with the
primary standards in order to reduce the complexity of the
transportation and air quality planning processes, as well as the
transportation conformity process'' (AASHTO, p. 3). Thus, if the EPA
were to adopt a lower level for the primary annual PM2.5
standard, the commenters recommended that the EPA adopt this same lower
level for the primary secondary PM2.5 standard as well.
    In response to these comments, the EPA notes that the Agency lacks
an appropriate scientific basis for revising the level of the secondary
annual PM2.5 standard. As noted above in section VI.B.2,
there is an absence of information that would support any different
secondary standards for PM. Comments related to the implementation
challenges associated with distinct primary and secondary standards are
not relevant to the Administrator's final decisions regarding what
standards are requisite to protect the public welfare. Therefore, the
EPA continues to conclude that it would be appropriate to retain the
current suite of secondary PM standards \202\ to address non-visibility
welfare effects, while revising only the form of the secondary annual
PM2.5 standard to remove the option for spatial averaging
consistent with this change to the primary annual PM2.5
standard, as proposed.
---------------------------------------------------------------------------

    \202\ As summarized in section VI.A and Table 1 above, the
current suite of secondary PM standards includes annual and 24-hour
PM2.5 standards and a 24-hour PM10 standard.
---------------------------------------------------------------------------

    Other commenters focused on the impacts of particulate matter on
climate. One commenter cited a number of recent studies that considered
mobile source black carbon emissions and associated climate impacts,
and urged the EPA to protect the public welfare by setting ``higher
standards for gasoline quality'' (Urban Air Initiative, p. 4). This
commenter did not, however, advocate specific secondary NAAQS to
address climate impacts of PM. More extensive comments on this same
subject were provided by the Center for Biological Diversity (CBD),
which urged the EPA to ``set a separate limit for black carbon within
the overall PM2.5 standard'' to ensure that public welfare
is fully protected ``from the serious climate impacts of black carbon''
(CBD, p. 2). This commenter argued that ``[p]recaution is required for
secondary NAAQS,'' citing American Trucking Associations, Inc. v. EPA,
283 F.3d 355, 369 (D.C. Cir. 2002):

    [N]othing in the Clean Air Act requires EPA to wait until it has
perfect information before adopting a protective secondary NAAQS.
Rather, the Act mandates promulgation of secondary standards
requisite to protect public welfare from any ``anticipated adverse
effects associated with'' regulated pollutants, 42 U.S.C. 7409(b)(2)
(emphasis added), suggesting that EPA must act as soon as it has
enough information (even if crude) to ``anticipate[]'' such
effects[.]

The commenter stressed the growing scientific evidence regarding the
impacts of black carbon on climate, and argued that the EPA's proposal
ignores important research studies published within the last five years
which provide improved estimates of the radiative forcing associated
with black carbon, and the effects of black carbon on snow and ice, the
Arctic climate, water availability and climate ``tipping points.'' The
commenter also noted that reductions in cooling aerosol species,
particularly sulfate, due to pollution control programs are leading to
an ``unmasking'' of the true extent of warming due to the accumulation
of greenhouse gases in the atmosphere. The commenter argued that this
unmasking effect can be offset by ensuring ``that sufficient black
carbon reductions accompany reductions in overall aerosol pollution''
(CBD, p. 10). The commenter also argued that the EPA did not consider
the negative impacts of climate change on public health adequately in
the proposal.
    The commenter stated that the EPA had an obligation to address the
impacts of black carbon in the PM NAAQS, despite the remaining
uncertainties. The commenter pointed to the EPA's report to Congress on
Black Carbon (U.S. EPA, 2012c), stating that the ``report shows that
EPA is aware of the climate science and public health information that
point to the importance of addressing black carbon pollution. EPA must
use this information in its relevant decisionmaking'' (CBD, p. 13). The
commenter also noted that the U.S. participates in a number of
international forums that have recognized the need to take action on
black carbon, and argued

that the U.S. has ``an obligation under the Gothenburg Protocol to
address black carbon pollution.'' The commenter challenged the
uncertainties cited by EPA with regard to the climate impacts of
aerosols generally, arguing that they ``do not apply to the regulation
of black carbon'' (CBD, p. 14). Specifically, the commenter stated that
``there are significant anthropogenic sources of black carbon that
contribute a large proportion of total black carbon emissions''; that
``there is enough information related to black carbon's impact to know
that global temperatures will rise due to black carbon emissions'';
that spatial and temporal heterogeneity in black carbon emissions do
not matter for estimating likely climate effects; that ``[b]lack
carbon's negative climate impacts do not depend upon details of cloud
interactions with aerosols''; and that the EPA does not need to be able
to quantify the health or climate benefits precisely to know that it is
appropriate to control black carbon as a specific component of PM under
the CAA (CBD, pp. 14-15).
    As a result, the commenter concluded that the current size-based PM
mass standard ``is insufficient to fully protect health and welfare,''
and that the EPA was obligated to establish a specific limit on black
carbon as a component of PM. The commenter argued that ``Black carbon
must be regulated separately and in addition to PM2.5
because absent separate standards sulfates and nitrates may be more
likely to be mitigated than the black carbon component of PM'' (CBD, p.
17). To support this point, the commenter cited the conclusion in the
Policy Assessment that:

    The current standards that are defined in terms of aggregate
size mass cannot be expected to appropriately target controls on
components of fine and coarse particles that are related to climate
forcing effects. Thus, the current mass-based PM2.5 and
PM10 secondary standards are not an appropriate or
effective means of focusing protection against PM-associated climate
effects due to these differences in components. (U.S. EPA, 2011a, p.
5-11)

    The commenter also noted that existing regulations on diesel
engines, which are the largest source of black carbon in the United
States, do not affect existing engines and vehicles, and stated that
``The NAAQS program is one of the few opportunities to reduce black
carbon from existing engines, industrial and biofuel sources within the
United States and rapidly reduce emissions from this pollutant'' (CBD,
p. 18).
    The EPA agrees with the commenters' assertion that the scientific
information about the impacts of aerosol species on climate is
developing rapidly, and that understanding of the magnitude of aerosol
effects on climate and the contribution of individual aerosol
components to those effects has improved substantially over the past
decade. The EPA also agrees that certain species, in particular black
carbon, play a significant role in multiple aspects of climate. The
Policy Assessment recognized that ``Aerosols can impact glaciers,
snowpack, regional water supplies, precipitation and climate
patterns,'' and may contribute to the melting of ice and snow, a
decrease in surface albedo, and climate impacts in the Arctic and other
locations (U.S. EPA, 2011a, p. 5-9). The contribution of black carbon
to these effects is discussed in detail in the EPA's recent Report to
Congress on Black Carbon (U.S. EPA, 2012c). In particular, black carbon
plays an important role in heating the lower atmosphere by absorbing
incoming solar radiation and outgoing terrestrial radiation, i.e. via
``direct'' radiative forcing.
    However, the EPA disagrees that there is sufficient information
available at this time to establish a NAAQS to protect against the
climate impacts associated with current ambient concentrations of black
carbon or other PM constituents. While the Integrated Science
Assessment concluded that ``a causal relationship exists between PM and
effects on climate, including both direct effects on radiative forcing
and indirect effects that involve cloud feedbacks that influence
precipitation formation and cloud lifetime'' (U.S. EPA, 2009a, section
9.3.10), it also identified substantial remaining uncertainties with
regard to the contribution of individual aerosol species to these
climate effects. The contribution of individual aerosol components to
total aerosol direct radiative forcing is more uncertain than the
global average (U.S. EPA, 2009a, section <IP_ADDRESS>), and the indirect
effects of aerosols and aerosol components remain highly uncertain, in
particular with regard to their complex interactions with clouds.
    With regard to black carbon, for example, the EPA disagrees with
CBD's claims that ``black carbon's negative climate impacts do not
depend upon details of cloud interactions with aerosols'' and that the
uncertainties associated with climate impacts of aerosols generally do
not apply to black carbon. In fact, the EPA has pointed to cloud
interactions as the area of greatest uncertainty with regard to black
carbon: recognizing that black carbon affects cloud reflectivity
(albedo), lifetime, and stability as well as precipitation, the Report
to Congress on Black Carbon noted that ``few quantitative estimates of
these effects are available, and significant uncertainty remains. Due
to all of the remaining gaps in scientific knowledge, it is difficult
to place quantitative bounds on the forcing attributable to [black
carbon] impacts on clouds at present'' (U.S. EPA, 2012c, p. 4). The
Report acknowledged that ``most estimates of the forcing from aerosol
indirect effects are based on all aerosol species (e.g. total PM) and
are not estimated for individual species (e.g, BC alone)'' (U.S. EPA,
2012c, p. 40). The Report concluded that it remains unclear the extent
to which black carbon contributes to the overall aerosol indirect
effect, and did not assign any central estimate or even a range of
possible values to the role of black carbon in the overall aerosol
indirect effect. With regard to black carbon's net contribution to
climate, therefore, the Report concluded:

    The direct and snow/ice albedo effects of BC are widely
understood to lead to climate warming. However, the globally
averaged net climate effect of BC also includes the effects
associated with cloud interactions, which are not well quantified
and may cause either warming or cooling. Therefore, though most
estimates indicate that BC has a net warming influence, a net
cooling effect cannot be ruled out. It is also important to note
that the net radiative effect of all aerosols combined (including
sulfates, nitrates, BC and OC) is widely understood to be negative
(cooling) on a global average basis. (U.S. EPA, 2012c, p. 3)

    Given the remaining uncertainties about the impact of aerosols on
climate, there is even greater uncertainty with regard to how aerosol-
induced climate change will affect public health. At this time, it is
not possible to estimate the extent to which aerosols in general, let
alone particular aerosol components, contribute to the occurrence or
exacerbation of adverse health outcomes due to climate change. The EPA
therefore disagrees with CBD's claim that the EPA should pursue black
carbon reductions for purposes of reducing the impacts of climate
change on public health.
    The Report to Congress on Black Carbon also stressed the importance
of considering co-emitted PM species, such as SO2 and
NOX, in evaluating the benefits of black carbon mitigation
options. Noting that many of these co-emitted particles and gases have
a cooling influence on climate, the Report noted the difficulty of
estimating the net effect of various mitigation measures on net
radiative forcing or other climate variables. The EPA concluded that
the location and timing of emissions reductions would be critically
important for achieving climate benefits, and that

``more research is needed on the benefits of individual control
measures in specific locations to support policy decisions made at the
national level'' (U.S. EPA, 2012c, p. 140). Thus, the EPA disagrees
with CBD's claim that spatial and temporal heterogeneity in black
carbon emissions do not matter for estimating likely climate effects,
and continues to believe that being able to quantify the climate
impacts of various aerosol species, alone and in combination, is
essential for informing any possible revisions to the current secondary
PM standards based on climate.
    Furthermore, while the EPA agrees with the commenter that a large
percentage of black carbon emissions come from anthropogenic sources,
including diesel engines and vehicles, the EPA notes that existing
regulations on mobile diesel engines are already reducing these
emissions substantially. Between 1990 and 2005, new engine requirements
resulted in a 32 percent reduction in black carbon emissions from
mobile sources, and a further 86 percent reduction from 2005 levels is
projected to occur by 2030 as vehicles and engines meeting existing
regulations are phased into the fleet (U.S. EPA, 2012c, p. 175). Long-
term historic data indicate that there has been a dramatic overall
decline in black carbon emissions over the past century, due to changes
in fuel use, more efficient combustion practices, and implementation of
PM controls. Therefore, the EPA disagrees with CBD's claim that a
distinct black carbon NAAQS is necessary to achieve reductions in black
carbon emissions. Clearly, U.S. emissions of black carbon are already
declining substantially, suggesting that the existing mass-based PM
standards, though not targeting black carbon specifically, have been
effective in achieving black carbon emissions reductions in practice.
As acknowledged in the Report to Congress on Black Carbon, ``While
[black carbon] is not the direct target of existing programs, it has
been reduced through controls aimed at reducing ambient
PM2.5 concentrations and/or direct particle emissions''
(U.S. EPA, 2012c, p. 161). The EPA has acknowledged the need to
encourage PM mitigation strategies that focus on reducing directly
emitted PM2.5 for purposes of reducing black carbon, and
this is reflected in U.S. commitments under the Gothenburg Protocol:
the new provisions in the Protocol pertaining to PM encourage parties
to develop national inventories and projections for black carbon, and
to ``give priority'' to black carbon when implementing measures to
control PM. However, the EPA notes that the U.S. has not yet ratified
the PM amendments to the Gothenburg Protocol, and furthermore, these
amendments do not require action specifically to reduce black carbon,
but rather encourage countries to take such actions voluntarily within
the context of their broader PM reduction strategies. Thus the EPA
disagrees with the commenter that the U.S. has an ``obligation'' to
reduce black carbon under the Gothenburg Protocol, or that it has
``agree[d] to choose mitigation options for particulate matter that
focus on black carbon reductions'' under the Protocol (CBD, p. 13).
    In sum, the EPA notes the substantial remaining the uncertainties
and gaps with regard to the climate impacts of PM components, including
black carbon. These include the uncertainties associated with the
spatial and temporal heterogeneity of PM components that contribute to
climate forcing; the uncertainties associated with measurement of
aerosol components; the inadequate consideration of aerosol impacts in
climate modeling; and the currently insufficient data on local and
regional microclimate variations and the heterogeneity of cloud
formations. As a result, the EPA continues to conclude that it is not
currently feasible to conduct a quantitative analysis for the purpose
of informing revisions of the current secondary PM standards based on
climate, and that there is insufficient information at this time to
base a national ambient standard on climate impacts associated with
current ambient concentrations of PM or any of its constituents.\203\
---------------------------------------------------------------------------

    \203\ This conclusion applies for both the secondary (welfare-
based) and the primary (health-based) standards.
---------------------------------------------------------------------------

D. Conclusions on Secondary PM Standards

    This section describes the Administrator's conclusions regarding
the secondary PM standards and the rationale leading to the
Administrator's final decision to retain the current suite of secondary
PM standards, including an annual PM2.5 standard of 15
[mu]g/m\3\ a 24-hour PM2.5 standard of 35 [mu]g/m\3\, and a
24-hour PM10 standard of 150 [mu]g/m\3\, to address PM-
related visibility impairment as well as other PM-related welfare
effects, including ecological effects, effects on materials, and
climate impacts. Specifically, this section explains the
Administrator's decision, consistent with the proposal, to retain the
current suite of secondary PM standards generally, while revising only
the form of the secondary annual PM2.5 standard to remove
the option for spatial averaging consistent with this change to the
primary annual PM2.5 standard. It also explains the
Administrator's decision, contrary to what was proposed, not to
establish a distinct standard to address PM-related visibility
impairment.
    In reaching conclusions regarding the need to revise the secondary
PM standards for both visibility and non-visibility welfare effects,
the Administrator has taken into account several key factors,
including: (1) The latest scientific information on both visibility and
non-visibility welfare effects associated with PM, as previously
described; (2) the advice of CASAC; and (3) the comments received
during the public comment period, as discussed above. Based on this
information, the Administrator has reached final conclusions about the
secondary PM standards and made final decisions about those standards,
as outlined below. Because the Administrator's final conclusions with
regard to the need to establish a distinct secondary standard to
protect against visibility impairment reflect, in part, her conclusions
on secondary PM standards for non-visibility welfare effects, section
VI.D.1 first outlines her conclusions regarding secondary PM standards
to address non-visibility welfare effects. This is followed by section
VI.D.2 which outlines her conclusions regarding a secondary PM standard
to address PM-related visibility impairment. Finally, section VI.D.3
summarizes the Administrator's final decisions with regard to the
secondary PM standards for both visibility and non-visibility welfare
effects.
1. Conclusions Regarding Secondary PM Standards To Address Non-
Visibility Welfare Effects
    With regard to the secondary PM standards to address non-visibility
welfare effects, the Administrator concludes that it is generally
appropriate to retain the existing secondary standards and that it is
not appropriate to establish any distinct secondary PM standards to
address non-visibility PM-related welfare effects. This conclusion is
based on the considerations discussed above in section VI.B.2,
including the latest scientific information and the advice of CASAC,
and the public comments received on the proposal, as discussed above in
section VI.C.2. The Administrator concurs with the advice of CASAC and
the conclusions expressed at the time of proposal that it is important
to maintain an appropriate

degree of control of both fine and coarse particles to address non-
visibility welfare effects, including ecological effects, effects on
materials, and climate impacts. In the absence of information that
would support any different standards the Administrator concludes that
it is appropriate to retain the existing suite of secondary standards
to address non-visibility welfare effects, as proposed. More
specifically, the Administrator concludes it is appropriate to retain
all aspects of the current 24-hour PM2.5 and PM10
standards. With regard to the secondary annual PM2.5
standard, the Administrator concludes that it is appropriate to retain
a level of 15.0 [mu]g/m\3\ for this standard while revising only the
form of the secondary annual PM2.5 standard to remove the
option for spatial averaging consistent with this change to the primary
annual PM2.5 standard. In reaching this conclusion, the
Administrator notes that no areas in the country are currently using
the option for spatial averaging to demonstrate attainment with the
secondary annual PM2.5 standard.
2. Conclusions Regarding Secondary PM Standards for Visibility
Protection
    Having reached the conclusion that it is generally appropriate to
retain the existing secondary standards to protect against non-
visibility welfare effects, the Administrator next considered the
target level of protection that would be requisite to protect public
welfare with regard to visual air quality. The Administrator then
determined whether to adopt a distinct secondary standard to achieve
this target level of protection. In making this decision, the
Administrator compared the degree of protection for visibility that
would be provided by such a distinct secondary standard to the degree
of protection provided by the existing secondary standards, focusing
specifically on the secondary 24-hour PM2.5 standard of 35
[mu]g/m\3\.\204\
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    \204\ This focus on the 24-hour PM2.5 standard
reflects the Administrator's judgments that PM-related visibility
impairment is principally related to fine particle concentrations
and that perception of visibility impairment is most directly
related to short-term levels of visual air quality.
---------------------------------------------------------------------------

    Based on the considerations discussed above in section VI.B and
VI.C, the Administrator first concludes that a target level of
protection for a secondary standard is most appropriately defined in
terms of a PM2.5 visibility index as proposed, since it
would provide a measure of PM-related light extinction that directly
takes into account the factors (i.e., species composition and relative
humidity) that influence the relationship between PM2.5 in
the ambient air and PM-related visibility impairment. Such a
PM2.5 visibility index standard would afford a relatively
high degree of uniformity of visual air quality protection in areas
across the country by virtue of directly incorporating the effects of
differences in PM2.5 composition and relative humidity
across the country.
    In defining a target level of protection based on a
PM2.5 visibility index, the Administrator has considered
specific aspects of such an index, including the appropriate indicator,
averaging time, level, and form. First, with regard to indicator, the
Administrator notes the conclusion of CASAC that relying on a
calculated PM2.5 light extinction indicator based on
PM2.5 chemical speciation and relative humidity data
represented a reasonable approach. Based on the analyses conducted in
support of this rulemaking, as described above, as well as the advice
of CASAC, the Administrator concludes that a calculated
PM2.5 light extinction indicator that utilizes the original
IMPROVE algorithm, adjusted to use a 1.6 OC multiplier and exclude the
term for coarse particles, in conjunction with monthly average relative
humidity data (i.e., f(RH) values) based on long-term climatological
means would be the most appropriate indicator for a PM2.5
visibility index standard.
    With regard to averaging time, the Administrator notes that both
CASAC and EPA staff have concluded that hourly or sub-daily (4- to 6-
hour) averaging times, within daylight hours and excluding hours with
high relative humidity, are more directly related than a 24-hour
averaging time to the short-term nature of the perception of PM-related
visibility impairment and the relevant exposure periods for segments of
the viewing public. However, in light of the important data quality
uncertainties that have recently been identified in association with
currently available instruments that would be used to provide the
hourly PM2.5 mass measurements that would be needed in
conjunction with an averaging time shorter than 24 hours, the
Administrator concludes it would not be appropriate at this time to set
a standard based on a sub-daily averaging time. Moreover, the
Administrator notes that analyses conducted by the EPA during this
review clearly indicate that PM2.5 light extinction
calculated on a 24-hour average basis would be a reasonable and
appropriate surrogate for PM2.5 light extinction calculated
on a 4-hour basis. Thus, the Administrator concludes that a 24-hour
averaging time would be appropriate for a PM2.5 visibility
index. The Administrator recognizes that a 24-hour averaging time would
effectively reduce the influence of peak hours of visibility impairment
on visibility index values, but concludes that in light of the concern
that peak hourly measurements may be significantly influenced by
atypical conditions and/or atypical instrument performance, it is
appropriate to adopt a longer averaging time to ensure that hour-
specific influences and uncertainties are balanced against more robust
measurements.
    With regard to form, the Administrator notes that consistent with
the approach taken in other NAAQS, including the current 24-hour
PM2.5 NAAQS, a multi-year percentile form offers greater
stability to the air quality management process by reducing the
possibility that statistically unusual indicator values will lead to
transient violations of the standard. Utilizing a three-year average
form provides stability from the occasional effects of inter-annual
meteorological variability that can result in unusually high pollution
levels for a particular year. Moreover, considering the lack of
information on and the high degree of uncertainty regarding the impact
on public welfare of the number of days with visibility impairment over
the course of a year, the Administrator considers it reasonable to
focus on the 90th percentile, which represents the median of the
distribution of the 20 percent worst visibility days, a key focus of
the Regional Haze program. The Administrator concludes that ensuring
that 90 percent of days have visual air quality that is at or below the
target level of protection could be reasonably expected to lead to
improvements in visual air quality on the 20 percent most impaired
days, and that the limited information available in this review
provides no basis for adopting a different form which would limit the
occurrence of days with peak PM-related light extinction in urban areas
to a greater degree. Therefore, the Administrator concludes that a 90th
percentile form, averaged over 3 years, is appropriate, for purposes of
establishing a target level of protection in terms of a 24-hour
PM2.5 visibility index.
    With regard to level, the Administrator concludes that in light of
the uncertainty associated with the high degree of variability in
visibility conditions and the potential variability in visibility
preferences across different parts of the country, it is appropriate to
establish a target level of protection based on the upper end of the
range of Candidate Protection Levels (CPLs)

identified in the Policy Assessment (i.e., 20-30 dv) and generally
supported by CASAC. Thus, the Administrator concludes that it would be
appropriate to set a target level of protection in terms of a
PM2.5 visibility index with a 24-hour averaging time that
would provide protection equivalent to the protection afforded by a 4-
hour PM2.5 visibility index with a level of 30 dv.
Furthermore, the Administrator notes that the approaches used to
estimate generally equivalent levels for a 24-hour PM2.5
visibility index generated 90th percentile 24-hour values similar to
the 4-hour values and a range of city-specific estimates of generally
equivalent 24-hour levels that encompassed the range of levels
considered appropriate for 4-hour CPLs, including the CPL of 30 dv at
the upper end of that range. The Administrator thus concludes that it
would be appropriate to use an unadjusted 4-hour CPL for purposes of
establishing a target level of protection in terms of a 24-hour
PM2.5 visibility index.
    In considering the alternative levels proposed for a 24-hour
standard, either 28 dv or 30 dv, the Administrator concludes that the
current substantial degrees of variability and uncertainty inherent in
the public preference studies should be reflected in a higher target
protection level than would be appropriate if the underlying
information were more consistent and certain. In addition, she
concludes that, in light of the significant uncertainties, it is
appropriate to place less weight on the results of western visibility
preference studies and that the CPL value (30 dv) that is based on the
eastern preference study results is likely to be more representative of
urban areas that do not have associated mountains or other valued
objects visible in the distant background For all of these reasons, the
Administrator concludes that it is appropriate to set a target level of
protection in terms of a 24-hour PM2.5 visibility index at
30 dv.
    In summary, in light of all the information available in this
review, the Administrator concludes that the protection provided by a
standard defined in terms of a PM2.5 visibility index (based
on speciated PM2.5 mass concentrations and relative humidity
data to calculate PM2.5 light extinction), a 24-hour
averaging time, and a 90th percentile form, averaged over 3 years, set
at a level of 30 dv, would be requisite to protect public welfare with
regard to visual air quality.
    In reaching this conclusion, the Administrator notes that any
national ambient air quality standard to address PM-related visibility
impairment would be designed to work in conjunction with the Regional
Haze Program as a means of achieving appropriate levels of protection
against PM-related visibility impairment in all areas of the country,
including urban, non-urban, and Federal Class I areas. While the
Regional Haze Program is focused on improving visibility in Federal
Class I areas and a secondary NAAQS to address PM-related visibility
impairment would focus on protecting visual air quality principally in
urban areas, both programs could be expected to provide benefits in
surrounding areas. In addition, the development of local programs, such
as those in Denver and Phoenix, could continue to be an effective and
appropriate approach to provide additional protection, beyond that
afforded by a national standard, for unique scenic resources in and
around certain urban areas that are particularly highly valued by
people living in those areas.
    Having concluded that the protection provided by a standard defined
in terms of a PM2.5 visibility index, with a 24-hour
averaging time, and a 90th percentile form, averaged over 3 years, set
at a level of 30 dv, would be requisite to protect public welfare with
regard to visual air quality, the Administrator next has to determine
whether to adopt such a visibility index as a distinct secondary
standard. This determination requires considering such a secondary
standard not in isolation but in the context of the full suite of
secondary standards. As discussed above, the Administrator has
determined to retain the current suite of secondary PM standards to
address non-visibility welfare effects (except for the form of the
annual standard). A distinct secondary standard to address visibility
impairment is properly considered in a context where there is also a
24-hour PM2.5 standard of 35 [mu]g/m\3\.
    In this context, the Administrator has considered the degree of
protection from visibility impairment afforded by the existing
secondary PM2.5 standards. The Administrator has considered
both whether the existing 24-hour PM2.5 standard of 35
[mu]g/m\3\ is sufficient (i.e. not under-protective) and whether it is
not more stringent than necessary (i.e. not over-protective).
    As discussed above in section VI.C.1.f, the results of the Kelly et
al. (2012a; 2012b) analyses indicate that based on 2008-2010 and 2009-
2011 data, all areas meeting the 24-hour PM2.5 standard of
35 [mu]g/m\3\ had visual air quality at least as good as 30 dv (24-hour
average, based on 90th percentile form averaged over 3 years). This
means that it is highly likely that the secondary 24-hour
PM2.5 standard of 35 [mu]g/m\3\ would be controlling
relative to a 24-hour standard based on a PM2.5 visibility
index set at a level of 30 dv, and highly unlikely that areas would
exceed the target level of protection for visibility of 30 dv without
also exceeding the existing secondary 24-hour standard. On the basis of
this evidence, and the supporting public comments, the Administrator
judges that the 24-hour PM2.5 standard of 35 [mu]g/m\3\
provides sufficient protection in all areas against the effects of
visibility impairment--i.e., that the existing 24-hour PM2.5
standard would provide at least the target level of protection for
visual air quality of 30 dv which the Administrator judges appropriate.
    The Administrator also recognizes that the analyses presented in
Kelly et al. (2012a; 2012b) indicate that the 24-hour PM2.5
standard of 35 [mu]g/m\3\ also would likely achieve more than the
target level of protection of visual air quality (30 dv) in some areas.
That is, when meeting a mass-based standard of 35 [mu]g/m\3\, some
areas would have levels of PM-related visibility impairment below 30
dv. Thus, the 24-hour PM2.5 standard of 35 [mu]g/m\3\ would
be over-protective in some areas (i.e. more stringent than necessary)
relative to the target level of protection for visibility. This is not
surprising, as the current mass-based standard does not account for
variation in particle species and relative humidity. The 24-hour
PM2.5 standard of 35 [mu]g/m\3\ would provide more than the
necessary protection in the areas where this would be expected, for
example western areas with lower relative humidity.
    In light of the Administrator's conclusion that it is appropriate
to retain the current secondary 24-hour PM2.5 standard of 35
[mu]g/m\3\ for non-visibility welfare effects, the Administrator notes
that this standard will remain in place regardless of whether she
elects to set a distinct secondary standard in terms of a
PM2.5 visibility index. The issue is not whether to adopt a
PM2.5 visibility index standard when viewed in isolation,
but whether such a distinct secondary standard should be adopted in
addition to the current secondary 24-hour PM2.5 standard of
35 [mu]g/m\3\. The EPA notes that adoption of such a distinct secondary
standard is not needed to provide sufficient protection from visibility
impairment with respect to the target level of protection determined
above. In addition, adoption of such a distinct secondary standard
would not change the fact that the current secondary 24-hour
PM2.5 standard of 35 [mu]g/m\3\ would result in over-
protection

from visibility impairment in certain areas of the country. Such over-
protection will occur whether or not such a distinct secondary standard
is adopted. In effect, adopting such a distinct secondary standard
would have no impact on the degree of protection provided from
visibility impairment. Since sufficient protection from visibility
impairment would be provided for all areas of the country without
adoption of a distinct secondary standard, and adoption of a distinct
secondary standard will not change the degree of over-protection
provided for some areas of the country, the Administrator judges that
adoption of such a distinct secondary standard is not needed to provide
requisite protection for both visibility and non-visibility related
welfare effects.
    It is important to note that this conclusion is based on the
specific target level of protection determined above, and the specific
set of current secondary standards. The Administrator's conclusion with
regard to the sufficiency of the protection provided by the current
suite of secondary standards is based on comparing the a 30 dv target
level of protection for a PM2.5 visibility index standard
against the degree of protection provided by the current secondary 24-
hour PM2.5 standard of 35 [mu]g/m\3\. It is the combination
of the specific target level of protection and the current suite of
secondary standards that is the basis for the decision not to adopt a
distinct secondary standard in terms of a PM2.5 visibility
index at this time.
    The EPA recognizes that, as in the last review, the final decision
is to not adopt a distinct secondary standard to address visibility
impairment. While the DC Circuit remanded the decision on a secondary
standard in the last review, the EPA's decision in this review has
addressed the issues raised in the court's remand. Here the EPA has
clearly identified the target degree of protection (defined in terms of
a PM2.5 visibility index at a level of 30 dv based on a 24-
hour averaging time, and a 90th percentile form, averaged over 3 years)
that would be requisite to protect public welfare with regard to visual
air quality. The EPA has carefully compared this degree of protection
with that provided by the current secondary 24-hour PM2.5
standard of 35 [mu]g/m\3\, based on an area-specific analysis of recent
air quality data and concluded that the degree of protection from
visibility impairment provided by the current secondary standard is
sufficient to protect public welfare consistent with section 109(b)(2).
This provides a clear basis for judging that the current secondary 24-
hour PM2.5 standard of 35 [mu]g/m\3\ would provide
sufficient protection. The analysis also shows that the current
secondary 24-hour PM2.5 standard would provide more
protection than is needed in some areas, largely because it does not
take into account variable factors such as relative humidity. However,
the EPA has recognized that adoption of a distinct secondary standard
to address visibility, in addition to retaining the current secondary
standard, would not change this result. The EPA has therefore concluded
that adoption of such a distinct secondary standard, in addition to the
current suite of secondary PM standards, is not needed to provide
requisite protection for both visibility and non-visibility related
welfare effects. Thus the EPA's decision has carefully considered and
accounted for the views of the court in the remand of the 2006 NAAQS.

E. Administrator's Final Decisions on Secondary PM Standards

    To address PM-related welfare effects, including ecological
effects, effects on materials, climate impacts, and visibility
impairment, the Administrator is retaining the current suite of
secondary PM standards, except for a change to the form of the annual
standard. Specifically, to address PM-related non-visibility welfare
effects including ecological effects, effects on materials, and climate
impacts, the EPA is retaining the current secondary 24-hour
PM2.5 and PM10 standard and is revising only the
form of the secondary annual PM2.5 standard to remove the
option for spatial averaging consistent with this change to the primary
annual PM2.5 standard. With respect to PM-related visibility
impairment, the Administrator has identified a target degree of
protection, defined in terms of a PM2.5 visibility index
(based on speciated PM2.5 mass concentrations and relative
humidity data to calculate PM2.5 light extinction), a 24-
hour averaging time, and a 90th percentile form, averaged over 3 years,
and a level of 30 deciviews (dv), which she judges to be requisite to
protect public welfare with regard to visual air quality. The EPA's
analysis of monitoring data provides the basis for concluding that the
current secondary 24-hour PM2.5 standard would provide
sufficient protection, and in some areas greater protection, relative
to this target protection level. Adding a distinct secondary standard
to address PM-related visibility impairment would not affect this
protection. Since sufficient protection from visibility impairment will
be provided for all areas of the country without adoption of a distinct
secondary standard, and adoption of a distinct secondary standard will
not change the degree of over-protection of visual air quality provided
for some areas of the country by the secondary 24-hour PM2.5
standard, the Administrator judges that adoption of a distinct
secondary standard, in addition to the current suite of secondary
standards, is not needed to provide requisite protection for both
visibility and non-visibility related welfare effects.

VII. Interpretation of the NAAQS for PM

    This section discusses the EPA Administrator's final decisions on
the revisions proposed to the data handling procedures for the primary
and secondary PM2.5 standards. Appendix N to 40 CFR part 50
describes the computations necessary for determining when the
PM2.5 standards are met and also addresses which measurement
data are appropriate for comparison to the standards; as well, it
specifies associated data reporting protocols, data completeness
criteria, and rounding conventions. The EPA is modifying appendix N to
conform to the revised PM2.5 standards; most notably, the
EPA is amending the appendix N procedures by removing the option for
spatial averaging. In addition to making changes to appendix N that
correspond to the changes in the annual standard form and the revised
primary annual standard level, the EPA is also finalizing additional
proposed revisions to the appendix in order to codify existing
practices currently included in guidance documents or implemented as
EPA standard operating procedures; better align appendix N language and
requirements with changes in PM2.5 ambient monitoring and
reporting requirements; provide greater clarity and transparency in the
provisions; and enhance consistency with data handling protocols
utilized for other pollutants.

A. Revised Amendments to Appendix N: Interpretation of the NAAQS for
PM2.5

    As discussed in sections III and VI above, the EPA Administrator
has decided to: (1) Revise the form and level of the primary annual
PM2.5 standard, and retain the current primary 24-hour
PM2.5 standard (section III.F) and (2) retain the current
secondary 24-hour PM2.5 standard, and revise the form and
retain the level of the secondary annual PM2.5 standard (for
visibility and non-visibility-related welfare protection) (section
VI.E). Appendix N is being revised to conform to those changes to the
standards. In the proposal, the EPA

recommended additional data handling procedures to appendix N for the
proposed distinct secondary standard to address PM2.5-
related visibility impairment. However, as discussed in section VI.E,
the Administrator has decided not to establish the proposed distinct
secondary standard to address visibility impairment, and therefore, the
associated proposed data handling procedures related to that proposed
standard are not included in the final revised appendix N.
    In addition to the changes to appendix N necessitated by the annual
NAAQS form and level revisions (discussed in depth in sections III and
VI above), the EPA is also finalizing additional revisions to appendix
N in order to: (1) Better align appendix N language and requirements
with changes in the PM2.5 ambient monitoring and reporting
requirements as discussed in section VIII below; (2) enhance
consistency with recently codified changes in data handling procedures
for other criteria pollutants; (3) codify existing practices currently
included in guidance documents or implemented as the EPA standard
operating procedures; and (4) provide enhanced clarity and consistency
in the articulation and application of appendix N provisions. Key
elements of the finalized revisions to appendix N are summarized in
sections VII.A.1 through VII.A.4 below which correspond to the
similarly numbered sections in appendix N. The proposed potential new
fifth section of appendix N dealt with the proposed distinct
PM2.5-related visibility secondary standard that was not
finalized by the Administrator and thus the proposed appendix N section
5 is not included in the final appendix N. Furthermore, proposed
changes to sections 1 through 4 of appendix N that also dealt with the
proposed secondary visibility index standard (e.g., term definitions,
rounding conventions, etc.) are also omitted from the final revised
appendix.
1. General
    As proposed, the EPA is finalizing modifications to section 1.0 of
appendix N to provide additional clarity regarding the scope and
interpretation of the PM2.5 NAAQS. This appendix section now
references the finalized revisions of the primary annual
PM2.5 standard (40 CFR 50.18) and the retained secondary
PM2.5 NAAQS. With regard to the appendix N term definitions
which are delineated in this initial section, the EPA has added,
modified, and eliminated term definitions, as appropriate, in
accordance with the final data handling rule revisions such as the
modification of terms that referenced spatial averaging. Additional
term definitions were also added to reference otherwise unchanged
appendix N content in an effort to streamline the appendix text,
enhance clarity and thus improve readability and understanding. In
particular, the definition of data substitution tests was shortened,
and a definition for ``test design value'' (TDV) was added for
completeness and for further clarity. This term was previously part of
the data substitution definition and now it is more explicitly defined.
The EPA notes that there were no substantive public comments received
with regard to this section.
2. Monitoring Considerations; Spatial Averaging
    As proposed, the EPA has finalized revisions to section 2.0 of
appendix N consistent with the concurrent modification of the form of
the primary annual PM2.5 standard that removes the option
for spatial averaging. As described in more detail in section III.E.3.a
above, the EPA decided to remove this option as part of the form of the
primary annual PM2.5 standard in light of analysis that
indicates that the existing constraints on spatial averaging, as
modified in 2006, may be inadequate to avoid substantially greater
exposures in some areas, potentially resulting in disproportionate
impacts on susceptible populations (Schmidt 2011a, Analysis A).
    With respect to the form of the secondary annual PM2.5
standard, as discussed in section VI.E above, the EPA has decided to
retain the current secondary annual PM2.5 standard to
provide protection for welfare effects. In the proposal, the EPA
believed it would be reasonable and appropriate to align the data
handling procedures for the primary and secondary annual
PM2.5 standards and remove the option for spatial averaging
for the secondary annual PM2.5 standard to be consistent
with the revised form of the primary annual PM2.5 standard
(FR 77 39000, June 29, 2012). The EPA noted that no areas in the
country are currently using the option for spatial averaging to
demonstrate attainment with the secondary annual PM2.5
standard. There were no comments on the proposed change and the EPA has
therefore concluded it appropriate to remove the option for spatial
averaging for the secondary annual PM2.5 standard from
Appendix N.
    Consistent with the revised form of the primary and secondary
annual PM2.5 standards, the levels of both standards will be
compared to measurements from each appropriate (i.e., ``eligible'')
monitoring site in an area, as specified in 40 CFR 58.30, with no
allowance for spatial averaging. Thus, for an area with multiple
eligible monitoring sites, the site with the highest design value would
determine the attainment status for that area. As a result of the
decision to eliminate the spatial averaging option for both the primary
and secondary annual standards, the EPA omitted all references to the
spatial averaging option in the finalized version of appendix N. See
section III.E.3.a above for a discussion of EPA's response to received
public comment on the issue of removal of the spatial averaging option.
3. Requirements for Data Use and Reporting for Comparisons With the
NAAQS for PM2.5
    In the proposal, the EPA suggested changes to section 3.0 of
appendix N to correspond to the proposed new secondary standard to
address PM-related visibility impairment. Since the EPA is not
finalizing the proposed distinct secondary standard to address
visibility impairment, none of these proposed changes are necessary and
are not being made. The EPA is, however, finalizing proposed changes to
improve consistency with procedures used for other NAAQS as well as to
improve consistency with current standard operating procedures.
Specifically, the EPA proposed revisions to this section regarding: (1)
Clarification of monitoring data appropriate to compare to the
PM2.5 NAAQS; (2) clarification of procedures for combining
monitoring data from collocated instruments into a single ``combined
site'' record; and (3) codification of the current standard operating
procedure whereby the EPA uses data for which the certification
deadline has passed but the monitoring agency has not requested
certification of the data to determine compliance with the
PM2.5 NAAQS provided the data are complete and accurate
(thus making appendix N consistent with data handling appendices for
other criteria pollutants). In the final revision to appendix N, the
EPA is incorporating all the above noted modifications to section 3 of
appendix N. Additional details describing the incorporated
modifications are provided below.
    With regard to clarification of which monitoring data are
appropriate for comparison to the PM2.5 NAAQS, the proposal
acknowledged important data quality concerns associated with the
PM2.5 measurements collected by continuous PM2.5
FEMs and referenced a subsequent preamble proposal section that
discussed the issue in more depth and put forward a solution to
mitigate the data quality concerns. The revised

monitoring rule, promulgated today in conjunction with the PM NAAQS
revision, includes, as proposed, language allowing monitoring agencies
to identify PM2.5 FEMs that are not providing data of
sufficient comparability to the FRM and, with EPA approval, to allow
such data to be deemed ineligible for comparisons with the
PM2.5 NAAQS \205\; see detailed discussion of this decision
in section VIII.A.1 below. Rule language for the definition of
``suitable monitors'' in section 1.0 of the finalized revised appendix
N accommodates and references this monitoring rule revision codified in
40 CFR 58.11.
---------------------------------------------------------------------------

    \205\ The EPA also allows use of alternative methods where
explicitly stated in the monitoring methodology requirements
(appendix C of 40 CFR part 58), such as PM2.5 Approved
Regional Methods (ARMs) which can be used to determine compliance
with the NAAQS. Monitoring agencies identifying ARMs that are not
providing data of sufficient quality will also be allowed to exclude
these data in making comparisons to the PM2.5 NAAQS.
Currently, there are no designated ARMs for PM2.5.
---------------------------------------------------------------------------

    With respect to the procedures for combining monitored data from
collocated instruments into a single ``combined site'' data record, the
EPA proposed to revise the current methodology in situations where an
FRM monitor operating on a non-daily schedule is collocated with a
continuous FEM monitor (that has acceptable comparability with an FRM).
As noted in the proposal, the EPA was not advocating a change to the
actual procedures for constructing a combined site record but rather a
modification to the subsequent evaluation of whether the specific
measurements were considered ``creditable'' or ``extra'' samples.\206\
The language clarification proposed is currently standard operating
procedure in Agency design value computations so the language
modification in appendix N merely proposed to modify actual
practices.\207\ The revised appendix N finalized in today's action
incorporates the modification as proposed. The EPA notes that there
were no substantive public comments received regarding this change.
---------------------------------------------------------------------------

    \206\ Data for a combined site record originates by default from
the designated ``primary'' monitor at the site location and is then
augmented with data from collocated FRM or FEM monitors whenever
valid data are not generated by the primary monitor. Samples in the
combined site record are deemed ``creditable'' or ``extra''
according to the required sampling frequency for a specific
monitoring site (i.e., ``site-level sampling frequency'') which, by
default, is defined to be the same as the sampling frequency
required of the primary monitor. Samples in the combined site data
record that correspond to scheduled days according to the site-level
sampling frequency are deemed ``creditable'' and, thus, are
considered for determining whether or not a specific monitoring site
meets data completeness requirements. These samples also determine
which daily value in the ranked list of daily values for a year
represents the annual 98th percentile concentration. Samples that
are not deemed ``creditable'' are classified as ``extra'' samples.
These samples do not count towards data completeness requirements
and do not affect which daily values represent the annual 98th
percentile concentration; ``extra'' samples, however, are candidates
for selection as the 98th percentile.
    \207\ Before the introduction of continuous FEMs, when two or
more samplers were collocated at the same site, monitoring agencies
typically identified the sampler that operated on the more frequent
sampling schedule as the ``primary'' monitor for developing a single
site record. However, due to concerns regarding the comparability of
FEMs to FRMs operated in some monitoring agency networks, and as
briefly discussed above and in more detail in section VIII.B.3.b.iii
below, many monitoring agencies have kept the FRM as the ``primary''
monitor and delegated the continuous FEM (which samples more
frequently, except in cases where the FRM operates on an ``every
day'' schedule) to be the ``supplemental'' (non-primary) collocated
monitor. In such cases, FEM measurements reported on the FRM ``off''
days were technically considered ``extra.'' In light of this
practice, EPA modified standing operating procedures whereby
supplemental collocated FEM samples reported on the FRM ``off'' days
would be considered ``scheduled'' and ``creditable.'' Thus,
collocated FEM samples would count towards data capture rates
(actually, increasing both the numerator and the denominator in the
capture rate equation), and also would count towards identifying
annual 98th percentile concentrations. Further, if data from a
supplemental collocated FEM are missing on an FRM ``off'' day (and
no unscheduled FRM data are reported that day), the EPA proposed not
to identify these as ``scheduled'' days consistent with current
practice, and thus, reported data generated from the supplemental
collocated continuous FEMs can only help increase data capture rates
(77 FR 39001, June 29, 2012)).
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4. Comparisons with the PM2.5 NAAQS
    Section 4.0 of appendix N specifies the procedures for comparing
monitored data to the PM2.5 standards. The EPA proposed
revisions to section 4.0 of appendix N to: (1) Provide consistency with
the proposed primary and secondary annual PM2.5 standards;
(2) expand the data completeness assessments to be consistent with
current guidance and standard operating procedures; and (3) simplify
the procedure for calculating annual 98th percentile concentrations
when using an approved seasonal sampling schedule.
    Consistent with the proposed decisions to revise the level of the
primary annual PM2.5 standard (section III.E.4.b.iii) and to
retain the current level of the secondary annual PM2.5
standard (section VI.B.1.c.vi), the EPA proposed to modify section
4.1(a) of appendix N to separately list the levels of the primary and
secondary annual PM2.5 standards. The final revised appendix
N incorporates this proposed change; this appendix N section now
references the revised primary annual standard level of 12.0 [micro]g/
m\3\ and the retained secondary annual standard level of 15.0 [micro]g/
m\3\. However, as discussed above with respect to the final decision to
not establish a distinct secondary standard to provide protection for
visibility impairment, the final appendix N now explicitly references
all PM2.5 secondary standard protection (that is, protection
from visibility impairment and non-visibility-related welfare effects)
to be provided by the revised annual standard with retained level of
15.0 [micro]g/m\3\ and the retained 24-hour standard with retained
level of 35 [micro]g/m\3\. Consistent with the final decisions to
remove the option for spatial averaging for the primary annual
PM2.5 standard (section III.F), as well as for the secondary
annual PM2.5 standard (section VII.A.2), the EPA amended
section 4.4 of appendix N to remove equations and associated
instructions relating to spatial averaging.
    With regard to assessments of data completeness, the EPA proposal
included two additional data substitution tests \208\ (making a total
of three data substitution tests) into appendix N for validating annual
and 24-hour PM2.5 design values otherwise deemed incomplete
(via the 75 percent and 11 creditable sample minimum quarterly data
completeness requirements). The EPA proposed to add these tests in
order to codify existing practices currently included in guidance
documents (U.S. EPA, 1999) and implemented as EPA standard operating
procedures, and further, to make the data handling procedures for
PM2.5 more consistent with the procedures used for other
NAAQS. While the need for data substitution will lessen as more
continuous PM2.5 monitors continue to be deployed in
PM2.5 networks, the EPA believes that these substitution
procedures are important to ensure that available data, if incomplete,
can be confidently used to make comparisons to the NAAQS. As noted in
the EPA proposal, data substitution tests are diagnostic in nature;
that is; they are only used in an illustrative manner to show that the
NAAQS status based on incomplete data is reasonable. As codified in
section 4 of Appendix N, data are substituted for missing data to
produce a ``test design value'' which is compared to the level of the
NAAQS. If the test design value passes the diagnostic test, the
``incomplete'' design value (without the data substitutions) is then
considered a valid design value. If an ``incomplete'' design value does
not pass any data substitution test, then the original

design value is still considered incomplete (and not valid for NAAQS
comparisons). Previously, section 4.1(c) of appendix N specified only
one data substitution test for validating an otherwise incomplete
design value. That diagnostic test only applied to the primary and
secondary annual PM2.5 standard and only applies in
instances of a violation; this test is referred to as the ``minimum
quarterly value'' test and is used to determine if the NAAQS has not
been met. The two proposed additional data substitution tests were to
be applicable for making comparisons to the primary and secondary
annual and 24-hour PM2.5 standards, specifically to show
that the NAAQS had been met. One of these proposed tests uses
collocated PM10 data to fill in ``slightly incomplete''
\209\ data records, and the other uses quarter-specific maximum values
to fill in slightly incomplete data records; these two test are
referred to as the ``collocated PM10 test'' and the
``maximum quarterly value test'', respectively. Both tests are designed
to confirm that the PM2.5 design value is valid and is less
than the level of the NAAQS.
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    \208\ Data substitution tests are supplemental data completeness
assessments that use estimates of 24-hour average concentrations to
fill in for missing data (i.e., ``data substitution'').
    \209\ Slightly incomplete is defined as less than 75 percent but
at least 50 percent quarterly data capture.
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    The EPA received several comments on the proposed addition of the
two data substitution tests to determine that the NAAQS was met. The
majority of comments generally supported the proposed addition of data
substitution tests. However, one commenter questioned the general
philosophy of all appendix N data substitution tests (i.e., the
existing ``over NAAQS'' test and the two proposed ``under NAAQS''
tests) by suggesting that there were more appropriate techniques for
filling in for missing data that would result in better estimates of
true design value level. The EPA believes that the data substitution
tests provided in the finalized appendix N are all very conservative
approaches to verify that the NAAQS standards are either met or not
met, and that the test design values are not to be used as the best
estimators of the design value concentration.\210\
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    \210\ Appendix N states that when the data substitution tests
are satisfied, then the NAAQS design values derived from reported
PM2.5 data which otherwise would be considered to be
incomplete shall be considered valid for comparisons to the
PM2.5 NAAQS.
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    Another commenter questioned, and argued against, the use of
collocated PM10 data in PM2.5 data substitution
tests. The commenter stressed that this type of test is not consistent
with those established for other pollutants. The commenter further
argued that while PM10 and PM2.5 are both
measurements of particulate matter, they are essentially different
pollutants with different sources and different dispersion
characteristics, and further, that the ratio of PM2.5 to
PM10 varies spatially and temporally. In general, the
commenter claimed that the EPA had offered no explanation of why
PM10 data were valid for a PM2.5 data
substitution test. At the time of proposal, the EPA believed that
PM10 data would be appropriate for a PM2.5 data
substitution test. After consideration of public comments and
additional air quality analyses, the EPA has decided that a collocated
PM10 test is largely redundant with the maximum quarterly
value test and thus not necessary to include it in Appendix N. The EPA
has analyzed the most recent three years of PM2.5 and
PM10 data (2009-2011) and assessed the separate benefit of
the PM10 substitution routine compared to the maximum
quarterly value test (Schmidt, 2012b). In this assessment of 2009-2011
PM2.5 design values which did not meet the nominal data
completeness requirements, the EPA found that the collocated
PM10 test was almost entirely redundant with the maximum
quarterly value test. It was also very infrequently needed as a
separate test. For the annual NAAQS, the maximum quarter value test in
100 cases resulted in a test design value (TDVmax) less than
or equal to 12.0 [micro]g/m\3\. There were only two additional cases
(i.e. 2 percent) when TDVmax was greater than 12.0 [micro]g/
m\3\ but the TDV associated with the collocated PM10 test
was less than 12.0 [micro]g/m\3\. Similarly for the 24-hour NAAQS, the
maximum quarter value test in 116 cases resulted in a test design value
(TDVmax) less than or equal to 35 [micro]g/m\3\ and again
only 2 additional sites (less than 2 percent) passed the collocated
PM10 test but not the maximum quarterly value test.
Furthermore, the maximum quarterly value tests allowed the annual and
24-hour design value to be validated approximately 5 times more often
than through the use of the collocated PM10 test.
Accordingly, the EPA has decided to not include the collocated
PM10 data substitution tests in Appendix N, and thereby
further simplify the data handling procedures for making comparisons to
the annual and daily NAAQS.
    With regard to identifying annual 98th percentile concentrations
for comparison to the primary and secondary 24-hour PM2.5
standards, the EPA suggested in the proposal to simplify the procedures
used with an approved seasonal sampling schedule. Specifically, the EPA
proposed to eliminate the use of a special formula for calculating
annual 98th percentile concentrations with a seasonal sampling schedule
and thereby proposed to use only one method for calculating annual 98th
percentile concentrations for all sites (77 FR 39002, June 29, 2012).
    The proposal explained that with an approved seasonal sampling
schedule, a site is typically required to sample during periods of the
year when the highest concentrations are expected to occur, but less
frequently during periods of the year when lower concentrations are
expected to occur (77 FR 39002, June 29, 2012). This type of sampling
schedule generally leads to an unbalanced data record; that is, a data
record with proportionally more ambient measurements (with respect to
the total number of days in the sampling period) in the ``high'' season
and proportionally fewer ambient measurements in the ``low'' season. In
the last review, the EPA revised section 4.5 of appendix N to include a
special formula for computing annual 98th percentile values when a site
operates on an approved seasonal sampling schedule. This special
formula accounted for an unbalanced data record and was consistent with
guidance documentation (US EPA, 1999), and, where appropriate, with
official OAQPS design value calculations (71 FR 61211, October 17,
2006). In cases where there is a balanced \211\ (or near-balanced) data
record, the special formula yields the same result as the regular
procedure for calculating annual 98th percentile concentrations.
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    \211\ A balanced data record has the same proportion of ambient
measurements (with respect to the total number of days in the
sampling period) in the ``high'' season as in the ``low'' season.
---------------------------------------------------------------------------

    To qualify for a seasonal sampling schedule, monitoring agencies
are required to co-locate a continuous PM2.5 instrument with
the seasonal sampling FRM. Since the last review, there has been
considerable deployment of continuous PM2.5 FEM monitors. In
situations where a PM2.5 FRM monitor operating on a non-
daily periodic schedule (such as a 1-day-in-3 or a 1-day-in-6 schedule)
is collocated with a continuous PM2.5 FEM monitor, data are
combined based on procedures stated in section 3.0 of appendix N as
modified, as discussed in section VII.A.3 above. Combining collocated
FRM and FEM data effectively results in a site which samples everyday
and results in a balanced data record. In such a case, if a site used a
seasonal sampling schedule regime for the FRM monitor, these data would
be balanced by the every-day

FEM data and there would be no need for the special formula for
calculating annual 98th percentile concentrations on the combined site
data.
    As EPA noted in the proposal, there are very few PM2.5
FRM monitors that operated on an approved seasonal sampling schedule
(only 15 sites out of approximately 1,000 total sites in 2010) and that
for almost half of those sites, the collocated continuous instrument
was a PM2.5 FEM (77 FR 39002, June 29, 2012). The proposal
stated that for the 3-year period 2008 to 2010, the annual 98th
percentile concentrations calculated with the special formula at those
15 sites were approximately five percent lower than if the regular
procedure was used. The EPA also noted in the proposal that, in the
last review, the Agency modified the monitoring requirements for areas
with an FRM operating on a non-daily schedule such that, when the
design values were within five percent of the 24-hour PM2.5
NAAQS, those areas would be required to increase the frequency of
sampling to every day (40 CFR 58.12(d)(1); 71 FR 61165, October 17,
2006; 71 FR 61249, October 17, 2006). In consideration of these facts,
the EPA proposed to simplify the data handling procedures for sites
operating on a seasonal sampling schedule by eliminating the special
formula and all references to it for the following reasons: (1) The
small difference between 98th percentile concentrations calculated
using the special formula versus the regular procedure and the small
number of sites currently using the special formula; (2) the EPA
requires every day sampling in areas with design values that are within
five percent of the 24-hour PM2.5 NAAQS; and (3) FRMs
operating on an approved seasonal sampling schedule are required to be
collocated with a continuous PM2.5 instrument (and if that
instrument were an FEM, the resulting combined site record would tend
to be balanced over the year and thus the special formula would be
superfluous) (77 FR 39002, June 29, 2012). Thus, the EPA proposal
included only one method for calculating annual 98th percentile
concentrations, the ``regular'' table look-up method specified in
section 4.5(a)(1) of appendix N.
    In light of the rationale provided above and because EPA received
no significant negative comments regarding the proposal, the EPA
concludes it is appropriate to eliminate the special seasonal sampling
98th percentile identification procedure from appendix N. The final
revised appendix N specifies only one method for identifying annual
98th percentile concentrations; the table look-up method is now the
only permitted technique for identifying annual 98th percentile
concentrations.

B. Exceptional Events

    The EPA is finalizing primary annual PM2.5-specific
deadlines in 40 CFR 50.14 by which air agencies \212\ must flag ambient
air quality data that they believe have been affected by exceptional
events and submit initial descriptions of those events. The EPA is also
finalizing the deadlines by which air agencies must submit detailed
exceptional events documentation to support the exclusion of those data
from the EPA's monitoring-based determinations of attainment or
nonattainment with the revised primary annual PM2.5 NAAQS.
The final exceptional events-related schedule is aligned with the
designations schedule, discussed in greater detail in section IX, and
is promulgated as proposed and as supported by multiple commenters.
Without revisions to 40 CFR 50.14, an air agency may not be able to
flag and submit documentation for some relevant data either because the
generic deadlines may have already passed by the time the new or
revised NAAQS is promulgated or because the generic deadlines require
documentation submission at least 12 months prior to the date that the
EPA must make a regulatory decision.
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    \212\ References to ``air agencies'' are meant to include state,
local, and tribal air agencies responsible for implementing the
Exceptional Events Rule.
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    The EPA acknowledges the concern raised by a few commenters that
numerous wildfires occurred between 2010 and 2012 that air agencies may
determine influenced ambient air quality concentrations potentially
affecting compliance with the revised primary annual PM2.5
NAAQS, and that air agencies may want to submit detailed exceptional
events documentation associated with multiple wildfires. Commenters
further noted that 1 year to provide documentation of these potential
exceptional events may not be sufficient. The EPA believes that the
promulgated schedules provide sufficient time for air agencies to
submit information related to the annual standard and for the EPA to
fully consider and act on the submitted information during the initial
area designation process. The EPA recently released draft exceptional
events guidance that clarifies key provisions of the 2007 Exceptional
Events Rule, provides examples of best practices, and streamlines the
documentation development process. The guidance provides approaches
that are broadly applicable to all event/pollutant combinations and
would apply to many PM events, including wildfire/PM combinations.
Additionally, the EPA has posted several concurred upon wildfire/PM
exceptional event demonstration packages on its Web site at: http://www.epa.gov/ttn/analysis/exevents.htm. Considered together, the EPA
believes this guidance will help air agencies submit information in a
timely manner.\213\ The EPA notes that under the promulgated schedule,
except for events that occur in December 2012, air agencies will have
more than 1 year to provide documentation for these potential events.
The EPA intends to work with potentially affected areas to identify,
screen, and prioritize events potentially influencing compliance with
the primary annual PM2.5 NAAQS and associated area
designations.
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    \213\ The EPA released draft exceptional events guidance
documents (U.S. EPA, 2012e) for public comment via a Notice of
Availability in the Federal Register on July 6, 2012 (77 FR 39959).
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    Also in response to comments, the EPA is clarifying that this
preamble language and the associated promulgated exceptional events
schedules apply only to the NAAQS that the EPA is newly promulgating or
revising in this action, that is, the revised primary annual
PM2.5 NAAQS. The promulgated exceptional event schedule
revisions do not apply to the retained PM standards (i.e., secondary PM
standards, primary 24-hour PM10, primary 24-hour
PM2.5). Further, the revised/extended exceptional event
schedules apply only to those data the EPA will use to establish
initial area designations for the revised primary annual
PM2.5 NAAQS.
    The ``Treatment of Data Influenced by Exceptional Events; Final
Rule'' (72 FR 13560, March 22, 2007), known as the Exceptional Events
Rule and codified at 40 CFR 50.14, contains generic deadlines for an
air agency to submit to the EPA specified information about exceptional
events and associated air pollutant concentration data. As discussed in
the proposal, without revisions to 40 CFR 50.14, an air agency may not
be able to flag and submit documentation for some relevant data because
the generic deadlines may have already passed by the time the new or
revised NAAQS is promulgated. Similarly, revisions to 40 CFR 50.14 are
needed because air agencies may not be able to flag and submit
documentation for events that occurred in December of 2013 by 1 year
before the designations

are made in 2014 as is required by the existing generic schedule
requires.
    To support appropriate consideration of exceptional event data
influencing ambient air quality concentrations potentially affecting
compliance with the revised primary annual PM2.5 NAAQS, the
EPA is adopting revisions to 40 CFR 50.14 to change the submission
dates for claimed exceptional events information affecting
PM2.5 data considered during the initial area designations
process under the promulgated revised primary annual PM2.5
NAAQS. As proposed, for air quality data collected in 2010 or 2011, the
EPA is extending to July 1, 2013 the otherwise applicable generic
deadlines of July 1, 2011 and July 1, 2012, respectively, for flagging
data and providing an initial description of an event (40 CFR
50.14(c)(2)(iii)). The EPA is retaining the existing generic deadline
in the Exceptional Events Rule of July 1, 2013 for flagging data and
providing an initial description of events occurring in 2012.
Similarly, the EPA is revising to December 12, 2013, the deadline for
submitting documentation to justify exceptional events occurring in
2010 through 2012 and potentially influencing compliance with the
revised primary annual PM2.5 NAAQS. The EPA believes these
revisions/extensions will provide adequate time for air agencies to