from across the study cities and not placing as much emphasis on the
city-to-city variability as a basis for determining an appropriate
degree of protection on a national scale.
    The information available for the Administrator to consider when
setting the secondary PM standard raises a number of uncertainties.
While CASAC supported moving forward with a new standard on the basis
of the available information, CASAC also recognized these
uncertainties, referencing the discussion of key uncertainties and
areas for future research in the second draft of the Policy Assessment.
In discussing areas of future research, CASAC stated that: ``The range
of 50% acceptability values discussed as possible standards are based
on just four studies (Figure 4-2), which, given the large spread in
values, provide only limited confidence that the benchmark candidate
protection levels cover the appropriate range of preference values.
Studies using a range of urban scenes (including, but not limited to,
iconic scenes--``valued scenic elements'' such as those in the
Washington, DC study), should also be considered'' (Samet, 2010d, p.
12). The EPA solicited comment on how the Administrator should weigh
those uncertainties as well as any additional comments and information
to inform her consideration of these uncertainties.
    In addition, the EPA solicited comment on a number of other issues
related to the level of the standard, including:

    (1) Both of the proposed levels and the various approaches to
identifying generally equivalent levels upon which the alternative
proposed levels are based.
    (2) A broader range of levels down to 25 dv in conjunction with
a 24-hour averaging time.
    (3) A range of alternative levels from 30 to 25 dv in
conjunction with a sub-daily (e.g., 4-hour) averaging time.
    (4) The strengths and limitations associated with the public
preference studies and the use of these studies to inform the
selection of a range of levels that could be used to provide an
appropriate degree of public welfare protection when combined with
the other elements of the standard (i.e. indicator, form and
averaging time).
    (5) Specific aspects of the public preference studies, including
the extent to which the 50 percent acceptability criterion is an
appropriate basis for establishing target protection levels in the
context of establishing a distinct secondary NAAQS to address PM-
related visibility impairment in urban areas; how the variability
among preference studies in the extent to which study participants
may be representative of the broader study area population should be
weighed in the context of considering these studies in reaching
proposed conclusions on a distinct secondary NAAQS; and the extent
to which the ranges of VAQ levels presented to participants in each
of the studies may have influenced study results and on how this
aspect of the study designs should appropriately be weighed in the
context of considering these studies in the context of this review.
vi. Administrator's Proposed Conclusions Regarding PM Standards To
Protect Visibility
    At the time of proposal, based on the considerations described
above, the Administrator proposed to revise the suite of secondary PM
standards by adding a distinct standard for PM2.5 to address
PM-related visibility impairment, focused primarily on visibility in
urban areas. This proposed visibility standard was to be defined in
terms of a PM2.5 visibility index, which would use measured
PM2.5 mass, combined with PM2.5 speciation data
and relative humidity data, to calculate PM2.5 light
extinction, translated into the deciview (dv) scale; a 24-hour
averaging time; a 90th percentile form, averaged over 3 years; and a
level of 28-30 dv.
vii. Related Technical Analysis
    At the time of proposal, the EPA conducted a two-pronged technical
analysis of the relationships between the proposed PM2.5
visibility index standard and the current 24-hour PM2.5
mass-based standard (Kelly, et al., 2012a). This analysis was designed
to provide technical information to inform key issues related to
implementing a distinct secondary standard for visibility as proposed.
Specifically, the EPA recognized that significant technical issues were
likely to arise for new or modified emissions sources conducting air
quality analyses for purposes of demonstrating that they would not
cause or contribute to a violation of the visibility standard under the
Prevention of Significant Deterioration (PSD) program. Such a
demonstration for the proposed secondary PM2.5 visibility
index standard could require each PSD applicant to predict, via air
quality modeling, the increase in visibility impairment, in terms of
the proposed PM2.5 visibility index, that would result from
the proposed source's emissions in conjunction with an assessment of
existing air quality (visibility impairment) conditions in terms of the
proposed PM2.5 visibility index. The EPA noted that if this
demonstration were to be attempted using the six-step procedure that
the EPA proposed to use for calculating PM2.5 visibility
index design values from monitored air concentrations of
PM2.5 components, significant technical issues with the
modeling procedures could arise.
    To address these technical issues, the EPA sought to explore
whether sources that met the requirements pertaining to the 24-hour
mass-based standard of 35 [micro]g/m\3\ would also meet the
requirements pertaining to the proposed visibility index standard. As
described in Kelly et al. (2012a), the first prong of the analysis
addressed aspects of a PSD significant impact analysis by evaluating
whether an individual source's impact resulting in a small increase in
the ambient PM2.5 concentration would produce a comparably
small increase in visibility impairment. This analysis included
estimates of PM2.5 speciation profiles based on direct
PM2.5 emission profiles for a broad range of source
categories and for theoretical upper and lower bound scenarios.
    The second prong of the analysis addressed aspects of a PSD
cumulative impact analysis by exploring the relationship between the
three-year design values for the existing 24-hour PM2.5
standard and coincident design values for the proposed PM2.5
visibility index standard based on recent air quality data. This aspect
of the analysis indicated that increases in 24-hour PM2.5
design values generally correspond to increases in visibility index
design values, and vice-versa. The analysis further explored the
appropriateness of using a demonstration that a source does not cause
or contribute to a violation of the 24-hour PM2.5 standard
as a surrogate for a demonstration that a source does not cause or
contribute to a violation of the proposed secondary PM2.5
visibility index standard. This analysis was based on 2008 to 2010 air
quality data, and compared the proposed level of 35 [mu]g/m\3\ for the
24-hour PM2.5 standard and for illustrative purposes an
alternative standard level of 12 [micro]g/m\3\ for the annual
PM2.5 standard with the

proposed levels of 28 or 30 dv for the secondary PM2.5
visibility index standard with a 24-hour averaging time and a 90th
percentile form. The results indicated that all (for the 30 dv level)
or nearly all (for the 28 dv level) areas in attainment of the 24-hour
PM2.5 standard would also have been in attainment of the
proposed secondary PM2.5 visibility index standard.
    Based on this technical analysis, the EPA proposed that there is
sufficient evidence that a demonstration that a source does not cause
or contribute to a violation of the mass-based 24-hour PM2.5
standard serves as a suitable surrogate for demonstrating that a source
does not cause or contribute to a violation of the proposed secondary
24-hour PM2.5 visibility index standard under the PSD
program. As such, the EPA proposed to conclude that many or all sources
undergoing PSD review for PM2.5 could rely upon their
analysis for demonstrating that they do not cause or contribute to a
violation of the mass-based 24-hour PM2.5 standard to also
show that they do not cause or contribute to a violation of the
proposed secondary PM2.5 visibility index standard, if a
distinct visibility standard were finalized.
    Although this proposed ``surrogacy policy'' was designed to address
an implementation-related issue, the second prong of the technical
analysis addresses the broader technical question of the relationship
between the existing 24-hour PM2.5 standard and the proposed
PM2.5 visibility index standard in terms of the degree of
protection likely to be afforded by each standard. Specifically, the
analysis indicated that depending on the level of the proposed
PM2.5 visibility index standard, the existing 24-hour
PM2.5 mass-based standard would be as protective or in some
areas more protective of visibility than a distinct secondary standard
set within the range of levels proposed. Commenters on the proposed
PM2.5 visibility index explored the implications of this
analysis at length, as discussed further below in section VI.C.1.f. For
this reason, the analysis is described in some detail here.
    Kelly et al. (2012a) noted that the relationship between design
values for the 24-hour PM2.5 standard and the proposed
secondary visibility index standard is not obvious a priori because of
differences in design value calculations for the standards. However,
closer examination of this relationship indicated that increases or
decreases in 24-hour PM2.5 design values correspond,
respectively, to increases or decreases in visibility index values.
Specifically, based on measurements from 102 sites with complete data
from 2008-2010, Kelly et al. (2012a) found linear correlations between
the 24-hour PM2.5 design values and the visibility index
design values with r\2\ values ranging from 0.65 to 0.98 across these
sites, with an average r\2\ value of 0.75 across all U.S. sites.
Moreover, the data indicated that no design value existed where the
visibility index design value exceeded 30 dv, but the 24-hour
PM2.5 standard level of 35 [micro]g/m\3\ was attained.
Visibility index design values for certain sites in the Industrial
Midwest were shown to exceed 28 dv despite the fact that the 24-hour
PM2.5 design values for these sites were below 35 [micro]g/
m\3\. This was attributed to the combination of high nitrate and
sulfate fractions, substantial RH adjustment factors, and
PM2.5 distribution characteristics that led to relatively
high visibility index design values for a given 24-hour
PM2.5 design value for counties in the Industrial
Midwest.\180\ Kelly et al. (2012a) concluded that the ``overall, design
values based on 2008-2010 data suggest that counties that attain 24-
hour PM2.5 NAAQS level of 35 [micro]g/m\3\ would attain the
proposed secondary PM2.5 visibility index NAAQS level of 30
dv and generally attain the level of 28 dv'' (pp. 17-18). In addition,
the Kelly et al. analysis indicated that at sites that violated both
the 24-hour PM2.5 level and the proposed visibility index 30
dv level, the proposed level of 30 dv would likely be attained if
PM2.5 concentrations were reduced such that the 24-hour
PM2.5 level of 35 [micro]g/m\3\ was attained (Kelly et al.,
2012a, p.15).\181\ A key implication of this analysis, therefore, was
that within the range of levels proposed by the EPA for a visibility
index standard (28-30 dv), the 24-hour PM2.5 standard of 35
[micro]g/m\3\ would be controlling in almost all (at 28 dv) or all (at
30 dv) instances.
---------------------------------------------------------------------------

    \180\ Kelly et al. (2012a) also noted that ``Regional reductions
in sulfate PM2.5 due to emission controls planned as part
of national rules as well as emission reductions associated with
potential annual standard violations are expected to improve
visibility in this region'' (p. 17).
    \181\ The analysis also showed that attaining the 24-hour
PM2.5 standard level of 35 [micro]g/m\3\ would result in
achieving a lower PM2.5 visibility index level in certain
areas of the country, largely western areas, than would be achieved
in other areas of the country. This is due to differences in the
composition of ambient PM2.5 and the lower relative
humidity in those areas.
---------------------------------------------------------------------------

2. Other (Non-Visibility) PM-related Welfare Effects
    In the 2006 review, the EPA concluded that there was insufficient
information to consider a distinct secondary standard based on PM-
related impacts to ecosystems, materials damage and soiling, and
climatic and radiative processes (71 FR 61144, October 17, 2006).
Specifically, there was a lack of evidence linking various non-
visibility welfare effects to specific levels of ambient PM. In that
review, to provide a level of protection for these welfare-related
effects, the secondary standards were set equal to the revised primary
standards to directionally improve the level of protection afforded
vegetation, ecosystems, and materials (71 FR 61210, October 17, 2006).
    This section briefly outlines key conclusions discussed more fully
in section VI.E of the proposal regarding the non-visibility welfare
effects of PM. These conclusions relate to the climate, ecological
(including effects on plants, soil and nutrient cycling, wildlife and
water) and materials damage effects of PM. For all of these effects,
the Policy Assessment concluded that there is insufficient information
at this time to revise the current suite of secondary standards. It is
important to note that the Policy Assessment explicitly excluded
discussion of the effects associated with deposited particulate matter
components of NOX and SOx and their
transformation products which are addressed fully in the joint review
of the secondary NO2 and SO2 NAAQS.
a. Evidence of Other Welfare Effects Related to PM
    With regard to the role of PM in climate, the proposal noted that
there is considerable ongoing research focused on understanding aerosol
contributions to changes in global mean temperature and precipitation
patterns. 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
lifetimes'' (U.S. EPA, 2009a, section 9.3.10). These effects are
discussed in more detail in section VI.E.1 of the proposal, which
provides information on the major aerosol components of interest for
climate processes, including black carbon (BC), organic carbon (OC),
sulfates, nitrates, and mineral dusts, and the nature, magnitude, and
direction (e.g., cooling vs. warming) of various aerosol impacts on
climate.\182\ The Policy Assessment concluded that aerosols alter
climate processes directly through radiative forcing and by indirect
effects on cloud brightness, changes in precipitation, and

possible changes in cloud lifetimes (U.S. EPA, 2011a, p. 5-10).
Further, the Policy Assessment noted that the major aerosol components
that contribute to climate processes (i.e. BC, OC, sulfate, nitrate and
mineral dusts) vary in their reflectivity, forcing efficiencies and
even in the direction of climate forcing, though there is an overall
net climate cooling associated with aerosols in the global atmosphere
(U.S. EPA, 2009a, section 9.2.10). The Policy Assessment concluded that
the current mass-based PM2.5 and PM10 secondary
standards were 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). In addition, in
light of the significant uncertainties in current scientific
information and the lack of sufficient data, the Policy Assessment
concluded 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 (U.S. EPA, 2011a, p. 5-11).
Overall the Policy Assessment concluded 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 its
constituents (U.S. EPA, 2011a, p. 5-11, -12).\183\
---------------------------------------------------------------------------

    \182\ Atmospheric PM is referred to as aerosols in the remainder
of this section to be consistent with the Integrated Science
Assessment.
    \183\ This conclusion would apply for both the secondary
(welfare-based) and the primary (health-based) standards.
---------------------------------------------------------------------------

    With regard to ecological effects, the proposal noted that several
ecosystem components (e.g., plants, soils and nutrient cycling,
wildlife and water) are impacted by PM air pollution, which may alter
the services provided by affected ecosystems. Ecological effects
include both direct effects due to deposition (e.g., wet, dry or
occult) to vegetation surfaces and indirect effects occurring via
deposition to ecosystem soils or surface waters where the deposited
constituents of PM then interact with biological organisms. Some of the
ecological effects considered in this review include direct effects to
metabolic processes of plant foliage; contribution to total metal
loading resulting in alteration of soil biogeochemistry and
microbiology, and plant and animal growth and reproduction; and
contribution to total organics loading resulting in bioaccumulation and
biomagnification across trophic levels. Section VI.E.2 of the proposal
summarizes key findings related to:

    (1) Impacts on plants and the ecosystem services they provide
due to deposition of PM to vegetative surfaces, which alters the
radiation received by the plant, and uptake of deposited PM
components by plants from soil or foliage, which can lead to stress
and decreased photosynthesis;
    (2) Impacts on ecosystem support services such as nutrient
cycling, products such as crops and the regulation of flooding and
water quality;
    (3) Impacts on wildlife, especially due to biomagnification of
heavy metals (especially Hg) up the food chain and bioconcentration
of POPs and PBDEs; and
    (4) Impacts of deposited PM, especially metals and organics, on
the ecosystem services provided by water bodies, including primary
production, provision of fresh water, regulation of climate and
floods, recreational fishing and water purification.

    The proposal noted that the Integrated Science Assessment had
concluded that ecological evidence is sufficient to conclude that a
causal relationship is likely to exist between deposition of PM and a
variety of effects on individual organisms and ecosystems (U.S. EPA,
2009a, sections 2.5.3 and 9.4.7), and also noted that vegetation and
other ecosystem components are affected more by particulate chemistry
than size fraction. However, the proposal also pointed to the
Integrated Science Assessment conclusion that it is generally difficult
to characterize the nature and magnitude of effects and to quantify
relationships between ambient concentrations of PM and ecosystem
response due to significant data gaps and uncertainties as well as
considerable variability that exists in the components of PM and their
various ecological effects. There are few studies that link ambient PM
concentrations to observed effect. Most direct ecosystem effects
associated with particulate pollution occur in severely polluted areas
near industrial point sources (quarries, cement kilns, metal smelting)
(U.S. EPA, 2009a, sections 9.4.3 and <IP_ADDRESS>).
    Based on the evidence available at this time, the proposal noted
the following key conclusions in the Policy Assessment:

    (1) A number of significant environmental effects that either
have already occurred or are currently occurring are linked to
deposition of chemical constituents found in ambient PM.
    (2) Ecosystem services can be adversely impacted by PM in the
environment, including supporting, provisioning, regulating and
cultural services.
    (3) The lack of sufficient information to relate specific
ambient concentrations of particulate metals and organics to a
degree of impairment of a specific ecological endpoint hinders the
identification of a range of appropriate indicators, levels, forms
and averaging times of a distinct secondary standard to protect
against associated effects.
    (4) Data from regionally-based ecological studies can be used to
establish probable local, regional and/or global sources of
deposited PM components and their concurrent effects on ecological
receptors.

    The proposal noted that the Policy Assessment had concluded that
the currently available information is insufficient for purposes of
assessing the adequacy of the protection for ecosystems afforded by the
current suite of PM secondary standards or establishing a distinct
national standard for ambient PM based on ecosystem effects of
particulates not addressed in the NOX/SOX
secondary review (e.g., metals, organics) (U.S. EPA, 2011a, p. 5-24).
Furthermore, the Policy Assessment had concluded that in the absence of
information providing a basis for specific standards in terms of
particle composition, the observations continue to support retaining an
appropriate degree of control on both fine and coarse particles to help
address effects to ecosystems and ecosystem components associated with
PM (U.S. EPA, 2011a, p. 5-24).
    With regard to materials damage, the proposal discussed effects
associated with deposition of PM, including both physical damage
(materials damage effects) and impaired aesthetic qualities (soiling
effects). As with the other categories of welfare effects discussed
above, the Integrated Science Assessment concluded that evidence is
sufficient to support a causal relationship between PM and effects on
materials (U.S. EPA, 2009a, sections 2.5.4 and 9.5.4). The deposition
of PM can physically affect materials, adding to the effects of natural
weathering processes, by potentially promoting or accelerating the
corrosion of metals, by degrading paints and by deteriorating building
materials such as stone, concrete and marble (U.S. EPA, 2009a, section
9.5). In addition, the deposition of ambient PM can reduce the
aesthetic appeal of buildings and objects through soiling. The Policy
Assessment made the following observations:

    (1) Materials damage and soiling that occur through natural
weathering processes are enhanced by exposure to atmospheric
pollutants, most notably sulfur dioxide and particulate sulfates.
    (2) While ambient particles play a role in the corrosion of
metals and in the weathering of materials, no quantitative
relationships between ambient particle concentrations and rates of
damage have been established.
    (3) While soiling associated with fine and course particles can
result in increased cleaning frequency and repainting of surfaces,
no quantitative relationships between particle characteristics and
the frequency of cleaning or repainting have been established.
    (4) Limited new data on the role of microbial colonizers in
biodeterioration

processes and contributions of black crust to soiling are not
sufficient for quantitative analysis.
    (5) While several studies in the PM Integrated Science
Assessment and NOX/SOX Integrated Science
Assessment suggest that particles can promote corrosion of metals
there remains insufficient evidence to relate corrosive effects to
specific particulate levels or to establish a quantitative
relationship between ambient PM and metal degradation. With respect
to damage to calcareous stone, numerous studies suggest that wet or
dry deposition of particles and dry deposition of gypsum particles
can enhance natural weathering processes.

    The Policy Assessment concluded that none of the new evidence in
this review called into question the adequacy of the current standards
for protecting against material damage effects, that such effects could
play no quantitative role in determining whether revisions to the
secondary PM NAAQS are appropriate at this time, and that observations
continue to support retaining an appropriate degree of control on both
fine and coarse particles to help address materials damage and soiling
associated with PM (U.S. EPA, 2011a, p. 5-29).
b. CASAC Advice
    In advising the EPA regarding the non-visibility welfare effects,
CASAC stated that it ``concurs with the Policy Assessment's conclusions
that while these effects are important, and should be the focus of
future research efforts, there is not currently a strong technical
basis to support revisions of the current standards to protect against
these other welfare effects'' (Samet, 2010c). More specifically, with
regard to climate impacts, CASAC concluded that while there is
insufficient information on which to base a national standard, the
causal relationship is established and the risk of impacts is high, so
further research on a regional basis is urgently needed (Samet, 2010c,
p. 5). CASAC also noted that reducing certain aerosol components could
lead to increased radiative forcing and regional climate warming while
having a beneficial effect on PM-related visibility. As a consequence,
CASAC noted that a secondary standard directed toward reducing PM-
related visibility impairment has the potential to be accompanied by
regional warming if light scattering aerosols are preferentially
targeted.
    With regard to ecological effects, CASAC concluded that the
published literature is insufficient to support a national standard for
PM effects on ecosystem services (Samet, 2010c, p.23). CASAC noted that
the best-established effects are related to particles containing
nitrogen and sulfur, which are being considered in the EPA's ongoing
review of the secondary NAAQS for NOX/SOX. With
regard to PM-related effects on materials, CASAC concluded that the
published literature, including literature published since the last
review, is insufficient either to call into question the current level
of the standard or to support any specific national standard for PM
effects on materials (Samet, 2010c, p.23). Nonetheless, with regard to
both types of effects, CASAC noted the importance of maintaining an
appropriate degree of control of both fine and coarse particles to
address such effects, even in the current absence of sufficient
information to develop a standard.
c. Summary of Proposed Decisions Regarding Other Welfare Effects
    Based on the above considerations and the advice of CASAC, at the
time of proposal the Administrator provisionally concluded that it
would not be appropriate to establish any distinct secondary PM
standards to address other non-visibility PM-related welfare effects,
including ecological effects, effects on materials, and climate
impacts. Nonetheless, the Administrator concurred with the conclusions
of the Policy Assessment and CASAC advice that it is important to
maintain an appropriate degree of control of both fine and coarse
particles to address such effects. Noting that there is an absence of
information that would support any different standards, the
Administrator proposed generally to retain the current suite of
secondary PM standards \184\ to address non-visibility welfare effects.
Specifically, the Administrator proposed to retain all aspects of the
current secondary 24-hour PM2.5 and PM10
standards. With regard to the secondary annual PM2.5
standard, the Administrator proposed to retain the level of the current
standard and to revise the form of the standard by removing the option
for spatial averaging consistent with this change to the primary annual
PM2.5 standard.
---------------------------------------------------------------------------

    \184\ 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.
---------------------------------------------------------------------------

C. Public Comments on Proposed Decisions Regarding Secondary PM
Standards

    The EPA received a large number of comments on its proposed
decisions with regard to secondary PM standards, with the large
majority of those comments focusing on the proposal to set a distinct
standard to protect against visibility impairment, discussed below in
section VI.C.1. Very few commenters addressed the proposal to retain
the existing secondary standards for non-visibility welfare effects,
discussed below in section VI.C.2. As discussed in section VI.D. below,
the Administrator has decided 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. The Administrator has also decided, contrary
to what was proposed, not to establish a distinct secondary standard to
address PM-related visibility impairment. This section discusses EPA's
responses to the comments EPA received on its proposal, and the
rationale behind the Administrator's final decisions is discussed in
section VI.D. below.
1. Comments on Proposed Secondary Standard for Visibility Protection
a. Overview of Comments
    Among those commenting on the proposal to set a distinct secondary
PM2.5 visibility index standard, a large majority of
commenters, including more than 25 state and local agencies; regional
organizations such as NACAA, NESCAUM, and WESTAR; and industry
commenters, such as ACC, API, BP, EPRI, NCBA, NEDA-CAP, NMA, NSSGA, and
UARG, opposed setting a distinct secondary standard for visibility at
this time. Many commenters in this group expressed the view that such a
standard was not needed, primarily on the basis that adequate
protection was provided by the existing 24-hour secondary
PM2.5 standard. Some of these commenters also expressed
legal concerns with the nature of the proposed standard. Other
commenters in this group supported a distinct secondary standard for
visibility in concept, but expressed the view that it was premature to
set such a standard pending collection of additional visibility
preference study data and the resolution of a number of key technical
issues. Support for setting such a distinct secondary standard for
visibility at this time came from a second group of commenters,
including the Department of the Interior (National Park Service),
several states, the Mid-Atlantic/Northeast Visibility Union (MANE-VU),
the National Tribal Air Association (NTAA), environmental organizations
such as the Appalachian Mountain Club, National Parks Conservation
Association, Earthjustice (AMC, et al.) and the League of Women

Voters of Texas. These commenters argued that the existing secondary
standards are not sufficiently protective of visual air quality, and
that a distinct secondary standard similar to the proposed visibility
index standard is both necessary and appropriate to ensure adequate
protection of visibility.
    Commenters in both groups expressed concerns about various aspects
of the proposed distinct secondary standard, including the indicator,
averaging time, level, and form. In addition, a large number of
commenters, including commenters from both groups, expressed concern
and/or confusion over the relationship between the Regional Haze
Program and the proposed distinct secondary standard for visibility,
raising issues such as analytical differences in methods between the
programs, monitoring issues, and other implementation challenges.
    A discussion of the significant comments outlined above, including
EPA's responses to the comments, is presented here, with more detailed
discussion in the Response to Comments document. Comments relating to
the specific elements of the proposed standard--indicator, averaging
time, form and level--are discussed in sections VI.C.1.b-e,
respectively. Comments related to the need for a distinct secondary
standard at this time are discussed in section VI.C.f. Legal issues
raised by commenters opposed to setting a secondary standard based on
the proposed visibility index are discussed in section VI.C.g. Finally,
comments related to the relationship between a distinct secondary
standard and the Regional Haze Program are discussed in section
VI.C.h.\185\ While the EPA concludes in section VI.D below to retain
the current suite of secondary PM2.5 standards, the
appropriateness of the protection that would be provided by the
proposed PM2.5 visibility index standard, and the
relationship between this degree of protection and that provided by the
current secondary 24-hour secondary PM2.5 standard, are key
elements in the Administrator's decision, and are discussed below.
---------------------------------------------------------------------------

    \185\ Comments pertaining to implementation issues, which the
Administrator may not consider in making decisions about setting
national ambient air quality standards, are discussed in the
Response to Comments document, as are comments regarding monitoring
issues related to the proposed distinct visibility index standard.
---------------------------------------------------------------------------

b. Indicator
    Numerous commenters, both those supporting a distinct secondary
standard and those opposed to setting such a standard, expressed views
on the suitability of utilizing a PM2.5 calculated light
extinction indicator for the standard as proposed. While these groups
of commenters differed in terms of their views on the appropriateness
of using calculated PM2.5 light extinction as the basis for
the indicator rather than relying on direct measurements of
PM2.5 light extinction, commenters from both groups
expressed concern over specific elements of the proposed method of
calculating PM2.5 light extinction. In particular,
commenters expressed differing views on which IMPROVE algorithm should
be utilized; whether it is appropriate to exclude coarse particles from
the indicator; and whether the proposed protocols for incorporating
data on relative humidity and PM2.5 species are
appropriate.\186\
---------------------------------------------------------------------------

    \186\ Some commenters expressed concern about the omission of
other contributors to visibility impairment from the visibility
index, as discussed in the Response to Comments document.
---------------------------------------------------------------------------

i. Comments on Calculated vs. Directly Measured Light Extinction
    The majority of commenters in both groups noted the uncertainties
associated with relying on a calculated light extinction indicator and
stated a preference for utilizing direct light extinction measurements.
However, recognizing the limitations on applying direct measurements at
present, commenters supporting the proposal to set a distinct standard
argued that relying on ``calculated light extinction is a reasonable
first approach'' (DOI, p. 2). These commenters pointed to the advice of
CASAC, which had acknowledged that it was not possible for the EPA to
develop an FRM for direct measurement of light extinction within the
time frame of this review and had concluded that relying on a
calculated PM2.5 light extinction indicator represented a
reasonable approach that could be implemented sooner than a directly
measured indicator. These commenters generally supported the proposal
to adopt a calculated PM2.5 light extinction indicator, at
least as an interim approach.
    Commenters opposed to setting a distinct standard generally argued
that it was inappropriate to rely on a calculated light extinction
indicator rather than direct measurements. Some of these commenters
argued that the proposed calculated light extinction indictor is ill
suited for a bright line standard because the method uses average
humidity and a reconstructed visibility measurement calculated from
PM2.5 speciation filter analysis, rather than measuring what
is actually observed by individuals. A number of commenters advocated
postponing setting a distinct standard until an approach based on
direct light extinction measurements can be adopted. Many of these
commenters stated that relying on direct light extinction measurements
would enable a standard to be based on a shorter averaging time, either
1-hour or sub-daily (4 to 6 hours), consistent with the more
instantaneous nature of perceptions of visual air quality and the
advice of CASAC in this review.
    The EPA generally agrees with commenters that an indicator based on
directly measured light extinction would provide the most direct link
between PM in the ambient air and PM-related light extinction. However,
as noted at the time of proposal and in accordance with the advice of
CASAC, the EPA has concluded that this is not an appropriate option in
this review because a suitable specification of currently available
equipment or performance-based verification procedures could not be
developed in the time frame of this review. Moreover, CASAC concluded
that relying on a calculated PM2.5 light extinction
indicator based on PM2.5 chemical speciation and relative
humidity data represented a reasonable approach. The inputs that are
necessary include measurements that are available through existing
monitoring networks and approved protocols. Thus, the EPA remains
confident that the available evidence demonstrates that a strong
correspondence exists between calculated PM2.5 light
extinction and PM-related visibility impairment. Furthermore, CASAC
agreed, noting that the proposed calculated PM2.5 light
extinction indicator based on the original IMPROVE algorithm ``appears
to be a reasonable approach for estimating hourly light extinction''
(Samet, 2010d, p. 11) and ``its reliance on procedures that have
already been implemented in the CSN and routinely collected continuous
PM2.5 data suggest that it could be implemented much sooner
than a directly measured indicator'' (Samet, 2010d, p. iii). Thus it
would not be appropriate to postpone setting a distinct secondary
standard until an approach based on direct light extinction
measurements could be adopted.
ii. Comments on Specific Aspects of Calculated Light Extinction
Indicator
    Some commenters, even those supporting the adoption of a calculated
light extinction indicator, also expressed concern over specific
aspects of the proposed indicator. First, a

number of commenters expressed concern over the proposal to use the
original IMPROVE algorithm as the basis for the calculated light
extinction indicator. These commenters noted that the original IMPROVE
algorithm has been shown to have consistent biases at both low and high
levels of light extinction. In particular, these commenters expressed
concern with the algorithm's bias at higher levels of light extinction,
which they pointed out were the conditions that might be encountered on
hazier days in urban areas.
    Some commenters supported use of the revised IMPROVE algorithm.
These commenters noted that the revised equation has been through a
peer review which confirmed that it is based on the best science and
corrects the biases inherent in the original algorithm. Commenters also
noted that this revised algorithm has been widely incorporated into
Regional Haze plans, and urged the EPA to use this same equation in the
visibility index for the sake of consistency: ``EPA approved this
approach for regional haze and does not dispute its greater accuracy.
Therefore, a national secondary ambient air quality standard based on
criteria that accurately reflect the latest scientific knowledge
logically should not revert to the original IMPROVE algorithm''
(Oklahoma DEQ, p. 2). Other commenters noted that both the original and
the revised IMPROVE algorithms were designed in support of the Regional
Haze Program which is focused on largely rural Class I areas, and that
neither algorithm is necessarily suitable for urban areas. Noting that
the EPA has not thoroughly evaluated the applicability of either
IMPROVE algorithm in urban areas, these commenters urged additional
research to evaluate the suitability of either algorithm (or an
alternative approach) in urban areas.
    Second, a number of commenters argued that exclusion of coarse PM
from the calculated light extinction indicator was inappropriate. These
commenters noted that coarse particulate matter is an important
contributor to visibility impairment in many areas, particularly in the
western U.S., and that the levels of ``acceptable'' visual air quality
derived from the visibility preference studies reflected total light
extinction due to the full mix of particles (including coarse PM) in
ambient air. A few commenters noted that due to the exclusion of coarse
particles, a ``deciview'' calculated for purposes of the proposed
PM2.5 visibility index is inconsistent with the unit as
conventionally defined under the Regional Haze Program. Other
commenters, however, supported the proposal to exclude coarse PM from
the calculated light extinction indicator, noting the important role
that PM2.5 plays in urban visibility and arguing it would be
more difficult to control the contribution of coarse particle sources
such as wind-blown dust to urban visibility impairment.
    Third, some commenters questioned why the EPA was proposing to rely
on monthly average relative humidity (f(RH)) values when hourly
humidity data are widely available, particularly in urban areas. One
commenter argued that the EPA's proposed approach involves ``guessing
relative humidity'' rather than relying on accurate, readily available
measurements (Oklahoma DEQ, p. 1). The commenter stated that since
relative humidity is highly variable and weather dependent, the
proposed approach ``effectively undermines the capacity of the
prescribed monitoring regime to identify periods when PM2.5
adversely affects visibility.'' Other commenters supported this view,
noting that relative humidity can vary substantially even within a 24-
hour period, and that light extinction can be very sensitive to these
changes. These commenters recommended that hourly or daily humidity
measurements should be utilized in place of the proposed monthly
average f(RH) values.
    Some commenters also recommended that the EPA should utilize a 90
percent relative humidity screen rather than 95 percent cap for
purposes of eliminating periods in which visibility impairment is due
to rain or fog. These commenters claimed that under a 95 percent cap,
both the average f(RH) values and the PM2.5 visibility index
values could be inflated in locations frequently affected by fog and/or
precipitation. These commenters preferred the approach of excluding
hours with relative humidity above 90 percent on the grounds that this
approach would eliminate foggy/rainy hours irrespective of the
frequency of occurrence.
    The EPA does not agree with commenters who advocated using the
revised IMPROVE algorithm. Both the original and the revised IMPROVE
algorithms have been evaluated by comparing the calculated estimates of
light extinction with coincident optical measurements. As discussed
above in section VI.B.1.a.i, the revised algorithm was developed to
address observed biases in the predictions using the original algorithm
under very low and very high light extinction conditions, with further
modifications and additions to better account for differences in
particle composition and aging in remote areas.\187\ However, the EPA
does not believe that these same modifications and additions would
necessarily be appropriate for calculating light extinction in urban
areas. Instead, the EPA considers the original algorithm to be suitable
for purposes of calculating urban light-extinction, although some
adjustments may be appropriate for urban environments as well. The
reasons why the original algorithm is suited to urban environments are
discussed further below, along with adjustments that the EPA believes
are likely appropriate based on the current (limited) state of
knowledge.
---------------------------------------------------------------------------

    \187\ Specifically, the revised IMPROVE algorithm incorporates
additional terms to account for particles representing the different
dry extinction and water uptake (f(RH)) from two size modes of
sulfate, nitrate and organic mass, as well as adding a term for
hygroscopic sea salt. There are also adjustments for the calculation
of OM as 1.8*OC compared to 1.4*OC in the original algorithm to
better account for the more aged PM organic components found in
remote areas.
---------------------------------------------------------------------------

    First, the EPA considers that the multiplier of 1.8 used to convert
OC to OM in the revised IMPROVE algorithm is too high for urban
environments. The EPA is aware that there has been considerable debate
within the research community about the appropriate multiplier to use
to best represent urban environments. As discussed in Appendix F of the
Policy Assessment (U.S. EPA, 2011a), the EPA used the SANDWICH mass
closure approach (Frank, 2006) in the Urban Focused Visibility
Assessment (U.S. EPA, 2010b) for purposes of calculating maximum
daylight hourly PM2.5 light extinction and evaluated which
multiplier would produce 24-hour results most similar to the SANDWICH
approach using 24-hour PM2.5 organic carbon derived from the
new Chemical Speciation Network (CSN) carbon monitoring protocol
established in 2007.\188\ Analyses presented in Appendix F of the
Policy Assessment indicate that a multiplier of 1.6 is most appropriate
for purposes of comparing the hourly PM2.5 light extinction
with calculated 24-hour extinction (see Appendix F, section F.6 for a
full explanation). The EPA also considers this higher multiplier to be
a better approach for urban CSN monitoring sites where the new
measurements of organic carbon tend to be lower than those produced by
the older NIOSH-type monitoring protocol

(Malm, 2011). A multiplier of 1.6 is now used to calculate OM from OC
measurements at CSN sites.
---------------------------------------------------------------------------

    \188\ Starting in 2007, the CSN adopted the IMPROVE monitoring
protocol for the measurement of organic and elemental carbon using
the IMPROVE analytical method and an IMPROVE-like sampler. The
transition was completed in 2009. (See ``Modification of Carbon
Procedures in the Speciation Network,'' http://www.epa.gov/ttn/amtic/files/ambient/pm25/spec/faqcarbon.pdf.)
---------------------------------------------------------------------------

    At the time of proposal, the EPA proposed to use the original
IMPROVE algorithm with its 1.4 multiplier for converting OC to OM, but
requested comment on whether this value was appropriate. Comments
received by the Agency generally indicate that the OC-to-OM multiplier
of 1.4 used in the original IMPROVE algorithm is too low for urban
areas. Based on the analyses presented in Appendix F of the Policy
Assessment, the EPA agrees with these commenters. However, the EPA also
believes that it would be inappropriate to use a multiplier as high as
1.8 to convert OC to OM in urban areas. As noted by commenters, the
organic mass contribution to visibility impairment can be large, and
generally OM is significantly larger in urban areas compared to
surrounding rural areas.\189\ Because a large portion of the organic
component of urban PM results from nearby emissions sources, the total
OM mass is generally closer to the measured OC from which it is
derived. This means it is appropriate to use a smaller multiplier to
convert OC to OM in urban areas as compared to the value of 1.8 used in
the revised algorithm, which is tailored to remote areas. The CASAC
noted that urban OM includes fresh emissions and the EPA concluded in
the Visibility Assessment that ``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). Although the revised algorithm represents the best science of
estimating extinction in remote areas with its aged aerosol, the
commenters did not address how the EPA should modify the revised
algorithm to best represent the more complex and different urban
aerosol, particularly for OM. In light of all of these considerations,
in particular the analyses the EPA conducted for Appendix F of the
Policy Assessment and the fact that the monitoring method for organic
carbon has recently changed in the CSN network, the EPA judges that a
multiplier of 1.6 for urban areas would be most appropriate for
purposes of calculating PM2.5 light extinction in urban
areas.\190\ In formulating this judgment, the EPA recognizes that
neither the original nor the revised IMPROVE algorithm has been tested
for suitability in urban areas and that additional research is
necessary to reduce the uncertainties about the most appropriate value
for the OC to OM multiplier in urban environments. With regard to other
changes between the original and revised IMPROVE algorithms, the EPA
also does not believe that it would be appropriate to include a term
for hygroscopic sea salt for urban light extinction, or to
differentiate between different size modes of sulfate, nitrate, and
organic mass as empirically defined by the revised IMPROVE algorithm.
Unlike in some remote coastal locations, sea salt is not major
contributor to light extinction in urban areas. Moreover, urban sources
of salt include sanding of roads during the winter and those re-
entrained particles are mostly in the coarse size range.
---------------------------------------------------------------------------

    \189\ The difference between higher PM2.5 mass in
urban areas compared to surrounding regions, known as the urban
excess, is largely attributed to organic mass (U.S. EPA, 2004b).
    \190\ The implications of this shift to a 1.6 multiplier for OC
in urban areas for decisions about averaging time, level, and need
for a distinct secondary standard are discussed further below in
sections VI.C.1.c, VI.C.1.e, and VI.C.1.f, respectively.
---------------------------------------------------------------------------

    Like in remote areas, small and large size modes of sulfate,
nitrate and organic mass would exist in the urban environment. However,
the apportionment of the total fine particle concentration of each of
the three PM2.5 components into the concentrations of the
small and large size fractions would likely need a different approach
than that used for remote areas. This is because of the closer
proximity of urban sources to their emissions. This is a particular
concern not only for organic mass, which as explained previously has a
large contribution from nearby urban emission sources, but also for
PM2.5 nitrate whose concentrations are also higher in urban
areas compared to the surrounding regions. Thus, a higher portion of
the total urban concentration may be in the small mode compared to
remote areas and thus a different apportionment algorithm would be
needed.
    Finally, the EPA does not consider it necessary to employ site-
specific Rayleigh light scattering terms in place of a universal
Rayleigh light scattering value for purposes of calculating light
extinction in urban areas for purposes of calculating the 90th
percentile values. The site-specific Rayleigh value is most important
to accurately estimate extinction on the best visibility days which is
an essential metric for the regional haze program.
    For all of these reasons, the EPA considers the original IMPROVE
algorithm better suited to the task of calculating urban light
extinction than the revised IMPROVE algorithm. However, the EPA does
consider it appropriate to make certain adjustments to the original
algorithm for purposes of calculating urban light extinction. As
discussed above, the EPA believes it is appropriate to use a 1.6
multiplier to convert OC to OM in urban areas. In addition, the EPA
believes it is appropriate to exclude the term for coarse particles
from the equation. The EPA does not agree with commenters who suggested
that coarse particles should be included in the calculated light
extinction indicator. As noted in the proposal, PM2.5 is the
component of PM responsible for most of the visibility impairment in
most urban areas. Currently available data suggest that
PM10-2.5 is a minor contributor to visibility impairment
most of the time, although at some locations (U.S. EPA, 2010b, Figure
3-13 for Phoenix) PM10-2.5 can be a major contributor to
urban visibility effects. While it is reasonable to assume that other
urban areas in the desert southwestern region of the country may have
conditions similar to the conditions shown for Phoenix, in fact few
urban areas conduct continuous PM10-2.5 monitoring. This
significantly increases the difficulty of assessing the role of coarse
particles in urban visibility impairment. For example, among the 15
urban areas assessed in this review, only four areas had collocated
continuous PM10 data allowing calculation of hourly
PM10-2.5 data for 2005 to 2007. In addition,
PM10-2.5 is generally less homogenous in urban areas than
PM2.5 in that coarse particle concentrations exhibit greater
temporal variability and a steeper gradient across urban areas than
fine particles (U.S. EPA, 2009a, p. 3-72). This makes it more
challenging to select sites that would adequately represent urban
visibility conditions. Thus, while it would be possible to include a
PM10-2.5 light extinction term in a calculated light
extinction indicator, as was done in the Visibility Assessment, there
is insufficient information available at this time to assess the impact
and effectiveness of such a refinement in providing public welfare
protection in areas across the country (U.S. EPA, 2011a, pp. 4-41 to 4-
42). Therefore, the EPA concludes that it is not appropriate to set a
standard based on a calculated light extinction indicator that includes
coarse particles at this time, and the calculated indicator should be
based on PM2.5 light extinction.
    With regard to the suggestion by some commenters that the
calculated light extinction indicator should be calculated using hourly
humidity data,

the EPA disagrees that concurrent humidity measurements should be used.
The use of longer-term averages for each monitoring site adequately
captures the seasonal variability of relative humidity and its effects
of visibility impairment, and this approach focuses more on the
underlying aerosol contributions to visibility impairment and less on
the day-to-day variations in humidity. This provides a more stable
indicator for comparison to the NAAQS and one that is more directly
related to the underlying emissions that contribute to visibility
impairment.
    With regard to the comments advocating the use of a 90 percent
humidity screen as opposed to a 95 percent humidity cap, the EPA
believes that relying on monthly average relative humidity values based
on 10 years of climatological data appropriately reduces the effect of
fog and precipitation. Although the approach of using a 95 percent
humidity cap, as in the Regional Haze Program, includes some hours with
relative humidity between 90-95 percent, the general approach of using
a longer-term average for each monitoring site effectively eliminates
the effect of very high humidity conditions on visibility at those
locations.
    Therefore, taking all of the above considerations and CASAC advice
into account, the EPA continues to conclude that a calculated
PM2.5 light extinction indicator, similar to that used in
the Regional Haze Program (i.e., using an IMPROVE algorithm as
translated into the deciview scale), would be the most appropriate
indicator to replace the current PM2.5 mass indicator for a
distinct secondary standard. Moreover, the EPA continues to conclude
that this calculated indicator should based on 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 as
used in the Regional Haze Program. A PM2.5 visibility index
defined in this way would appropriately reflect the relationship
between ambient PM and PM-related light extinction, based on the
analyses discussed in the proposal and reflecting the aerosol and
relative humidity contributions to visibility impairment by
incorporation of factors based on measured PM2.5 speciation
concentrations and climatological average relative humidity data. In
addition, this type of indicator would address, in part, the issues
raised in the court's remand of the 2006 PM2.5 standards.
Such a PM2.5 visibility index 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.
c. Averaging Time
    Few commenters specifically addressed the issue of averaging time.
Those who did generally expressed the view that an hourly or sub-daily
averaging time would be the most appropriate approach, as supported by
CASAC and the EPA's own analyses in this review. These comments were
generally consistent with the emphasis among all commenters on the
desirability of adopting a directly measured light extinction indicator
that could be measured on an hourly or sub-daily time scale. Some
commenters noted that a standard based on a 4-6 hour averaging time
would better capture peak daily light extinction while allowing stable
signal quality; others urged EPA to adopt a 1-hour averaging time in
conjunction with direct measurements. Commenters pointed to significant
limitations associated with using a 24-hour averaging time, including
the uncertainties in translating hourly or sub-daily visibility index
values into 24-hour equivalent values. Some commenters criticized the
analysis presented in the Policy Assessment comparing the 24-hour
calculated light extinction values to the maximum daylight 4-hour
calculated light extinction values. These commenters stated that the
scatter plots and regressions presented in the Policy Assessment
indicate there is considerable variation in the 24-hour vs. 4-hour
relationship, and interpreted this to mean that 24-hour light
extinction values are a poor surrogate for 4-hour values. For example,
several industry commenters cited an analysis which noted that the
correlation coefficient between the 24-hour and 4-hour values was as
low as r\2\ = 0.42 in Houston, and stated that the EPA was being overly
``optimistic'' in concluding that city-specific and pooled r\2\ values
in the range of 0.6 to 0.8 showed good correlation (UARG, Attachment 2,
p. 27).
    In addition, some commenters expressed concern over potential bias
and greater uncertainty introduced by the inclusion of nighttime hours,
noting that because relative humidity tends to be higher at night,
inclusion of these hours could cause areas to ``record NAAQS
exceedances that have no corresponding visibility impairment value''
(UARG, p. 36). Commenters also emphasized the poor fit of a 24-hour
averaging time with the near instantaneous judgments about visibility
impairment reflected in the visibility preference studies. Commenters
also noted that there is greater hourly variation in PM concentrations
and resulting visibility conditions in urban areas than in Class I
areas; thus, while the Regional Haze Program uses 24-hour IMPROVE data,
the commenters stated that a shorter averaging time is needed for an
urban-focused PM2.5 visibility standard. Some commenters
objected to a 24-hour averaging time as unsupported by the record in
this review: ``Because the science the Administrator relies on for the
other elements of the proposed visibility standard is tied to short-
term exposures to visibility impairment, the EPA has no basis for
promulgating a standard that uses a 24-hour averaging time'' (API, p.
43). These commenters claimed that while the EPA may not have the
information or infrastructure in place to allow the Agency to set a
standard based on a 1-hour or other sub-daily averaging time, this does
not justify moving to a 24-hour averaging time.
    Among commenters supporting the proposed distinct secondary
standard for visibility, many commenters recognized the limitations on
monitoring methods and currently available data that led to the EPA's
proposal to adopt a standard based on a 24-hour averaging time. Most of
these commenters acknowledged that the lack of reliable hourly
speciation data means that a 24-hour averaging time is the only
workable approach for a standard based on calculated light extinction.
Commenters advocating a distinct secondary standard for visibility
therefore generally supported the proposal to adopt a 24-hour averaging
time, at least as an interim approach until a directly measured light
extinction indicator could be adopted in the future. This approach was
also supported by a few industry commenters who noted that since a
visibility index standard would be based on data from the IMPROVE and
CSN monitors, which operate on a 24-hour basis with 1-in-3 (or 1-in-6)
day sampling, ``it is imperative that EPA retain a 24-hour averaging
time if a secondary visibility standard is promulgated'' (API,
Attachment 2, p. 9).
    In response to comments supporting a 1-hour or sub-daily (4- to 6-
hour) averaging time in conjunction with a direct light extinction
measurements, the EPA notes that, as discussed above in the response to
comments on indicator, the Agency has concluded

that a directly measured light extinction indicator is not an
appropriate option in this review, independent of the decision on
averaging time. Having reached the conclusion that a calculated
PM2.5 light extinction indicator would be most appropriate,
the EPA has next considered what averaging time would be most desirable
for such an indicator. As noted in the proposal, the EPA has recognized
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. Thus, the
Agency agrees with commenters' general point that, as a starting
premise, a sub-daily averaging time would generally be preferable.
    However, as noted at the time of proposal and discussed above in
section VI.B.1.c, important data quality uncertainties 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. As a result, at this time the Agency has strong
technical reservations about a secondary standard that would be defined
in terms of a sub-daily averaging time. The data quality issues which
have been identified, including short-term variability in hourly data
from currently available continuous monitoring methods, effectively
preclude adoption of a 1-hour averaging time in this review, given the
sensitivity of a 1-hour averaging time to these data quality
limitations. Even with regard to multi-hour averaging times, the EPA
continues to conclude that the data quality concerns preclude adoption
of a sub-daily averaging time.
    Moreover, analyses conducted for the Policy Assessment 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. The
scatter plots comparing 24-hour and 4-hour calculated PM2.5
light extinction in the Policy Assessment (U.S. EPA, 2011a, Figures G-4
and G-5) do show some scatter around the regression line for each city.
This was to be expected, since the calculated 4-hour light extinction
includes day-specific and hour-specific influences that are not
captured by the simpler 24-hour approach. Overall, however, in the
EPA's view, both the city-specific and pooled 15-city 24-hour vs. 4-
hour comparisons show strong correlation between the two averaging
times. Moreover, the 90th percentile design values calculated for 4-
hour vs. 24-hour light extinction are much more closely correlated than
are the values for individual days in particular urban areas calculated
using these two approaches. Thus, while the EPA agrees with commenters
who pointed out the relatively low correlation between 4- and 24-hour
values in cities such as Houston, the Agency points out that the
correlations of 90th percentile values are much higher, particularly
when one considers the average values across urban areas. In general,
the 90th percentile values line up better and demonstrate closer to a
one-to-one relationship.
    The EPA has conducted a reanalysis (Frank et al., 2012b) of the
relationships between estimated 24-hour and 4-hour visibility
impairment based on the variety of metrics discussed in Appendix G of
the Policy Assessment that further supports this finding. The
reanalysis more appropriately considered the uncertainty of the
calculated 4-hour values. It also considered the effect of changing the
OC to OM multiplier used in urban areas with the new CSN monitoring
protocol from 1.4 to 1.6. The revised analysis shows that the 24-hour
values are generally closer to the 4-hour values than originally
estimated.
    Since conclusions in the proposal about the relationship between 4-
hour and 24-hour values were drawn not just on the basis of the city-
specific results but also on the more robust 90th percentile values,
the EPA disagrees with commenters who state that the Agency was overly
optimistic in considering 24-hour values an appropriate surrogate for
4-hour values. Also, it is appropriate to focus on the 90th percentile
design value comparison since the design values would determine
attainment status and the degree of improvement in air quality that
could be expected in areas instituting controls to meet the NAAQS.
Therefore the EPA does not agree with commenters who state that a 24-
hour averaging time cannot serve as an appropriate surrogate for sub-
daily periods of visibility impairment. On the contrary, the EPA
continues to conclude, on the basis of this analysis, that
PM2.5 light extinction calculated on a 24-hour basis is a
reasonable and appropriate surrogate for sub-daily PM2.5
light extinction calculated on a 4-hour basis.
    The EPA recognizes that the effect of adopting a 24-hour averaging
time may be to smooth out some of the hour-by-hour variability in
visibility index values. (Indeed, this is true if we compare a 4-hour
averaging time to a 1-hour averaging time as well.) Hour-specific
influences which would be evident if an hourly or sub-daily averaging
time were to be used will be masked to some extent when those hours are
averaged together with other hours. This means, in part, that a 24-hour
averaging time may effectively reduce peak values by means of averaging
them together with other hours, which may have lower values. However,
given the well documented variability in hourly visibility conditions,
especially in urban areas, as noted by commenters, it is reasonable to
assume that in some cases peak hours may be significantly influenced by
atypical conditions, making it appropriate to adopt an averaging time
that is sufficiently long to ensure that hour-specific influences are
balanced against more typical conditions. Perhaps even more important
is the concern that many peak hourly measurements may be significantly
influenced by atypical instrument performance; this reinforces the
conclusion that it is appropriate to adopt a longer averaging time, to
ensure that hour-specific uncertainties are balanced against more
robust measurements.
    Thus, in agreement with commenters who supported a daily averaging
time, the EPA concludes that a 24-hour averaging time would be
appropriate for a distinct secondary standard based on a calculated
PM2.5 light extinction indicator.
d. Form
    The EPA received very few comments with regard to the proposal to
adopt a 90th percentile form, averaged over 3-years, in conjunction
with a PM2.5 visibility index and a 24-hour averaging time.
One commenter stated that it was inappropriate to use a 90th percentile
form, noting that this would result in the exclusion of a minimum of 36
days of data annually. The commenter expressed particular concern that
this proposed approach, in combination with a 24-hour standard based on
an unadjusted CPL, would not capture the worst visibility impairment
and that this would undermine ``the intent of setting a meaningful
secondary visibility standard'' (AMC, et al., p. 2). Another commenter
argued that the EPA had provided no scientific basis for why the 90th
percentile form was suitable, and claimed that the Agency was making
``a somewhat arbitrary judgment that people's welfare would be affected
only if adverse urban visibility were to occur

more than 10 percent of the time'' (API, Attachment 2, p. 4).
    On other hand, a few commenters who appeared to generally support
the proposal to use a 90th percentile form advocated averaging the 90th
percentile values over longer time periods, arguing that averaging over
only 3 years would not provide a stable assessment of visual air
quality in the West because this time period is insufficient to
properly account for western drought and fire cycles. These commenters
pointed to the approach in the Regional Haze Program of averaging
visibility impairment over 5 years, and noted that even within this
longer time period data can be significantly influenced by high
emissions during significant fire years.
    The EPA disagrees with all of these comments. With regard to the
comment opposing the 90th percentile form as inappropriately excluding
the worst visibility days, the EPA notes that there is a significant
lack of information on, and a high degree of uncertainty regarding, the
impact on public welfare of the number of days with visibility
impairment over the course of a year. For example, the visibility
preference studies used to derive the range of CPLs considered in this
review offered no information regarding the frequency of time that
visibility levels should be below those values. Based on this
limitation, the EPA concluded in the Policy Assessment that it would
not be appropriate to consider eliminating all exposures above the
level of the standard and that it was reasonable to consider allowing
some number of days with reduced visibility. Recognizing that the
Regional Haze Program focuses attention on the 20 percent worst
visibility days (i.e., those at or above the 80th percentile of
visibility impairment), the EPA continues to believe, as noted in the
proposal, that a percentile well above the 80th percentile would be
appropriate to increase the likelihood that all days in this range
would be improved by control strategies intended to help areas attain
the standard. Focusing on the 90th percentile, which represents the
median of the distribution of the 20 percent worst visibility days,
could be reasonably expected to lead to improvements in visual air
quality on the 20 percent most impaired days. Thus, the EPA has made a
reasoned judgment based on a full consideration of the upper end of the
distribution of visibility impairment conditions and continues to
conclude that it is appropriate to focus on the 90th percentile of
visibility impairment values.
    With regard to comments requesting the EPA adopt a longer multi-
year averaging period for the 90th percentile values, the EPA disagrees
that it would be appropriate to average the 90th percentile values over
periods longer than 3 years. The EPA recognizes that 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 3-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. The Agency has
adopted this approach in other NAAQS, including the current secondary
24-hour PM2.5 NAAQS, which has a 98th percentile form
averaged over 3 years. However, adopting a multi-year averaging period
longer than 3 years would increase the number of days with visibility
impairment above the target level of protection and would therefore
reduce the protectiveness of the standard. Based on this the EPA does
not believe it would be appropriate to average 90th percentile values
over a period as long as five years. Therefore, the EPA continues to
conclude that a 90th percentile form, averaged over 3 years, would be
appropriate, in conjunction with a calculated PM2.5 light
extinction indicator and a 24-hour averaging time.
e. Level
    With regard to level, commenters focused on two main themes. First,
a large number of commenters addressed the information available from
the public preference studies with regard to the acceptability of
various levels of visual air quality. These comments, which are
discussed in subsection VI.C.1.e.i below, address the EPA's use of
visibility preference studies as the basis for the selection of a range
of appropriate levels for the Administrator to consider. Many
commenters challenged the use of these studies as the basis for setting
a distinct secondary standard, arguing that limitations in these
studies rendered them an unsuitable and insufficient basis on which to
establish such a standard. Second, commenters expressed different views
as to what level(s) of a distinct secondary standard would be
appropriate, if the EPA were to set such a standard. These comments
reflected consideration of the results of the public preference studies
as well as analyses conducted in the Visibility Assessment and the
Policy Assessment, as discussed in the proposal. Comments addressing
the appropriateness of specific levels are discussed in subsection
VI.C.1.e.ii below.
i. Comments on Visibility Preference Studies
    A majority of commenters expressed the view that the existing
preference studies provide an insufficient basis for selection by the
Administrator of an appropriate level of public welfare visibility
protection for a national standard. These commenters highlighted a
number of limitations and uncertainties (enumerated below) associated
with these studies as support for this view. In contrast, other
commenters felt that despite certain limitations, these studies do
provide a sufficient basis on which the Administrator can select an
appropriate level of a standard to provide national public welfare
visibility protection. The remainder of this section organizes and
discusses these comments under four broad topic areas, including: (a)
Limitations and uncertainties associated with the visibility preference
studies; (b) preference study methods and design; (c) use of preference
study results for determining adversity; (d) the appropriateness of
using regionally varying preference study results to select a single
level for a national standard.
(a) Preference Study Limitations and Uncertainties
    A large and diverse number of limitations and uncertainties
associated with the visibility preference studies have been identified
and discussed in the public comments. Many of these same limitations
and uncertainties were also identified and discussed by the EPA in the
various documents developed throughout this review. The most important
and fundamental limitations and uncertainties will be discussed here in
the preamble, while more specific, unique or detailed comments will be
addressed in the Response to Comments document.
    The primary or most frequent limitation cited by many commenters
relates to the small number of preference studies that are available in
this review. In particular, some commenters note that these preference
studies cover just four locations, only three of which occur in the
U.S., that the two studies conducted in Washington, DC were pilot
studies, not full preference studies, and/or that three of the
preference studies were conducted in the West, while only one was
conducted in the East, providing only limited geographic coverage.
Typically, these same commenters also pointed out that taken together,
these

limited studies only included a total of 852 participants, which they
claimed was too small a sample size and unrepresentative nationally.
These commenters thus concluded that there is insufficient information,
both geographically and demographically, upon which to select a
national level of a visibility index for purposes of visibility
protection.
    In contrast, several commenters stated support for using the
preference studies, concluding they provide an adequate basis, in spite
of their limited nature. In particular, AMC et al. state:

    We believe that these studies provide sufficient results to
inform setting a national visibility standard. While the number of
studies is small, they do incorporate spatial variation and, in the
case of Denver and Phoenix, varied populations* * *. EPA should have
confidence, rather than uncertainty, in the fact that these studies
used different methods and respondents and yield a range of 20-24
dv, with one outlier of 29. (AMC, et al., pp. 6-7)

    Regarding the first group of commenters, the EPA notes that it is
well aware of the limited nature of the information, which it has
described in great detail in the Integrated Science Assessment,
Visibility Assessment, and Policy Assessment, as well as in section
VI.B.2 of the proposed rule (77 FR 38973). The EPA further notes,
however, that limited information does not preclude the Administrator
from making judgments based on the best available science, taking into
account the existing uncertainties and limitations associated with that
available science. Thus, in reaching judgments based on the science,
the Administrator appropriately weighs the associated uncertainties.
The CASAC supported this view and concluded that the available
information provided a sufficient basis on which the Administrator
could form a judgment about requisite PM-related public welfare
visibility protection. Specifically, CASAC stated ``[t]he 20-30
deciview range of levels chosen by EPA staff as `Candidate Protection
Levels' is adequately supported by the evidence presented'' (Samet,
2010b, p. iii). As discussed in the proposed rule (77 FR 38990), the
Administrator recognized and explicitly took into account the
uncertainties and limitations in the science in determining an
appropriate degree of protection when she proposed a level at the upper
end of the recommended range. As discussed below, the Administrator
continues to be mindful of these uncertainties and limitations in
reaching her final determination regarding what constitutes an
appropriate degree of protection with respect to PM-related visibility
impairment.
    With respect to the comments of AMC et al., the EPA agrees that
these studies provide a sufficient basis to inform the Administrator's
judgments regarding an appropriate level of protection from PM-related
visibility impairment, but she recognizes that these studies, which are
the only studies before her, are a limited source of information.
However, the EPA does not agree that the Washington, DC, results
represent an outlier, and thus the EPA believes these results are
appropriately included in the range identified for the Administrator to
consider.
    Some commenters made the point that the EPA relied on much of this
same evidence to reach the conclusion in 2006 that the information was
too limited to allow selection of a national standard. For example, API
stated:

    [T]he bulk of the VAQ preference studies were available during
the previous PM NAAQS review and were considered by the Agency in
its establishment of the 2006 p.m. secondary NAAQS * * *. The
Proposed Rule does not mention this fact and does not explain why
many of these same studies now compel EPA to propose this new
secondary NAAQS * * *. The Proposed Rule notes in passing that,
since the last review of the PM NAAQS, `limited information that has
become available regarding the characterization of public
preferences in urban areas has provided some new perspectives on the
usefulness of this information in informing the selection of target
levels of urban visibility protection.' 77 Fed. Reg. at 38969/2. It
is a serious oversight that the Proposed Rule makes no attempt to
explain what that information is or how it affects the
interpretation of the VAQ preference studies. This `limited
information' is an apparent reference to information provided by Dr.
Anne Smith. (API, p. 37)

    The EPA disagrees with these commenters. First, the EPA disagrees
that it failed to distinguish between studies that were available in
the previous review and the current review. The discussion in section
VI.A.1 of the proposal specifically identifies the studies from Denver,
Phoenix and British Columbia (77 FR 38967/2) as being considered in the
last review. The EPA further disagrees with the implication that it is
being circumspect about identifying the ``limited information that has
become available regarding the characterization of public preferences
in urban areas.'' Beginning in section VI.A.3 of the proposed rule (77
FR 38969), the EPA was clear about what information, both preexisting
and new, it relied upon in this review to inform its views and provide
the basis for its proposal. In section VI.B.2, the EPA elaborates on
the specific information, tools, methods and data which are considered
in relation to the public preference studies, including the new
information available since the last review.
    As noted above and in the proposal, in addition to the substantial
PM urban air quality information and analyses new to this review, there
are three other sources of information that have specifically
``provided some new perspectives on the usefulness of'' the preference
studies ``in informing the selection of target levels of urban
visibility protection'' (77 FR 38969). They include: (1) Results from
additional urban visibility preference study experiments conducted for
Washington, DC by Smith and Howell (2009) which added to the preference
data for that location and shed light on the role of location in
preference responses; (2) a review and reanalysis (Stratus Consulting,
2009) of the urban visibility public preference studies from the four
urban areas, including the newly available Smith and Howell (2009)
experiments which examined the similarities and differences between the
studies and evaluated the potential significance of those differences
on the study results; and (3) additional analyses, including most
importantly a logit analysis (Deck and Lawson, 2010, as discussed in
Chapter 2 and Appendix J of the Visibility Assessment), which was
requested and reviewed by CASAC, which showed that each city's
responses represented unique and statistically different curves. Taken
together, these sources contributed to the EPA's current knowledge and
understanding of each survey study's results, the appropriateness of
comparing each study's results to the others, and the key uncertainties
relevant to data interpretation. In addition, in the last review the
decision to not adopt a distinct secondary standard was remanded as
contrary to law and failing to provide a reasoned explanation for the
decision. As such it is not appropriate for purposes of comparison with
the Administrator's judgment and reasoning in this review.
(b) Preference Study Methods and Design
    In addition to the limitations and uncertainties noted above, many
comments also asserted the methodologies used in the preference studies
are fundamentally flawed. Many commenters cited some of the same issues
that have already been identified by the EPA as sources of uncertainty
and potential factors in producing the statistically different study
results (see section VI.B.1.b above). As noted above,

the EPA is well aware of the issues raised regarding the adequacy of
the preference studies to serve as a basis for a secondary NAAQS (see
77 FR 38975) and solicited comment on how these uncertainties should be
considered (see 77 FR 38990). Most of these same commenters also
pointed to an assessment of the preference studies methodology provided
by Smith and Howell (2009) as the basis for their views, as indicated
by the following comments:

    Smith and Howell (2009) show that VAQ preference study outcomes
are malleable and depend entirely on the design of the study.
Accordingly, such studies do not identify any meaningful threshold
of acceptable visibility conditions. Despite Smith and Howell's
conclusions, EPA continues to assert that the VAQ preference studies
can be used to identify minimally acceptable visibility conditions
even though the Agency has never provided any valid scientific basis
for discounting the Smith and Howell (2009) results. (API, p. 38)
    Well-controlled preference studies discussed by Anne Smith of
Charles River Associates at the March 2010 CASAC meeting
demonstrated that the judgment of panel members was affected by the
order in which photographs were presented and tendency to identify
the middle of the range of visibility degredation as a threshold of
acceptability. This points to a potential flaw in these studies and
that artifacts caused by these tendencies may have influenced study
results. Dismissing these inherent flaws in the existing preference
studies and then using these studies to set a secondary NAAQS is
arbitrary and capricious. (API, Attachment 2, p. 12)
    EPA also fails to acknowledge that the only study conducted
since the last review rebuts the validity of the VAQ preference
studies previously conducted. (UARG, Attachment 2, p. 28)

    As is explained in a more detailed discussion in the Response to
Comments document, the EPA disagrees that the study conducted by Smith
and Howell (2009) supports the conclusion that the preference study
methodologies were fundamentally flawed; however, the EPA notes that
their experiments do identify areas where additional research would be
useful to further inform our limited understanding of public
preferences in urban areas. The EPA views the Smith and Howell
experiments as increasing the EPA's knowledge and understanding of the
findings of the 2001 Washington, DC focus group pilot study (Abt, 2001)
in several important ways, although this information still remains
limited overall. Specifically, the Smith and Howell results suggest:
(1) The 2001 results, while based on a small sample size of 9, were
consistent with results from a larger sample of the general Washington,
DC population; (2) an individual's preferences for visibility in one
location may not depend on whether they live in that location; and (3)
presentation method (i.e., changing from slide projection to computer
monitor) did not appear to affect the reported preferences.
(c) Preference Study Results and Adversity
    A number of comments were received regarding the EPA's use of
preference study results to make the determination that adverse
PM2.5-related visibility effects on the public welfare are
occurring. In this context, several commenters questioned whether the
EPA had made the case that unacceptable levels of visual air quality
based on preference study results alone can be equated with an adverse
public welfare effect. These commenters suggested that unless
preference study information is linked to personal comfort and well-
being or other associated welfare effects, it cannot form the basis of
a determination of adversity. For example, Kennecott Utah Copper LLC
stated that:

    Thus, EPA seemingly was building the foundation for a
determination of what constitutes an adverse effect on visibility in
the context of public welfare. However * * * EPA subsequently veered
toward an oversimplified focus on public acceptance of visibility
conditions * * *. EPA's discussion of visibility in the Policy
Assessment and its proposed rule in the Federal Register focuses
entirely on ``acceptable'' and ``unacceptable'' visual air quality
and make no mention of an ``adverse effect'' in the context of
visibility. EPA's reliance on only 3 urban preference studies
represents a paucity of data and a wholesale abandonment of any
effort to seek a scientifically measurable adverse effect.
(Kennecott Utah Copper LLC, p. 26)

    In response, the EPA first notes that the definition of effects on
welfare included in section 302(h) of the CAA identifies both
visibility and the broader category of effects on personal comfort and
well-being as effects on welfare. In setting a secondary standard to
address visibility impairment, the EPA considers the effect on the
public from impairment of visibility as a separate and distinct welfare
effect in its own right. The EPA is not required to translate this into
terms of personal comfort and well-being, as visibility impairment is
designated explicitly by Congress as an effect on welfare. While there
may be a large degree of overlap among these different welfare effects,
the EPA properly focuses on evaluating all of the information before
the Agency on the effect visibility impairment has on the public,
whether or not this impairment would also be categorized as having an
adverse effect on personal comfort and well-being. It is in the context
of all of this information that the EPA makes the judgment as to the
appropriate degree of protection from known and anticipated adverse
effects on the public from visibility impairment. The EPA recognizes
that there is uncertainty about the degree of adversity to the public
welfare associated with PM-related visibility impairment. However a
secondary standard is designed to provide protection from ``known or
anticipated'' adverse effects, and a bright line determination of
adversity is not required in judging the requisite degree of protection
under section 109(b)(2). Furthermore, the EPA disagrees that it has
abandoned its consideration of visibility-related impacts on the
welfare effect of personal comfort and well-being, as is made clear in
the following quote:

    Research has demonstrated that people are emotionally affected
by low visual air quality, that perception of pollution is
correlated with stress, annoyance, and symptoms of depression, and
that visual air quality is deeply intertwined with a ``sense of
place,'' affecting people's sense of the desirability of a
neighborhood (U.S. EPA, 2009a, section 9.2.4). Though it is not
known to what extent these emotional effects are linked to different
periods of exposure to poor visual air quality, providing additional
protection against short-term exposures to levels of visual air
quality considered unacceptable by subjects in the context of the
preference studies would be expected to provide some degree of
protection against the risk of loss in the public's ``sense of well-
being.'' (77 FR 38973/1, emphasis added)

    The approach taken to address such qualitative, but policy-
relevant, information in this review is the same as in other NAAQS
reviews. The review is initiated with a comprehensive assessment of all
possible public health and welfare effects associated with PM in the
Integrated Science Assessment. Then policy-relevant effects for which
there is sufficient quantitative information to allow a determination
of the change in risks associated with incremental changes in air
quality are assessed (in this review, in the Visibility Assessment) and
used to provide a quantitative basis to inform the selection of an
appropriate range of levels for further consideration in the Policy
Assessment. In the Policy Assessment, the EPA considers all important
policy-relevant evidence and information, both quantitative and
qualitative, in making recommendations regarding the range of policy
options appropriate for the Administrator to consider. It is in the
context of all of this information that the Administrator

makes her final judgment as to the appropriate degree of protection
from known and anticipated adverse effects on the public from
visibility impairment.
    Another issue raised in the comments regarding adversity is the
EPA's decision to use the 50 percent acceptability criterion from the
public preference studies in determining candidate protection levels of
visibility impairment for the selection of a national level of
visibility protection. For example, AMC et al. recommended ``a 75%
acceptability criterion as a target that is in line with protecting the
broader public from the negative effects of visibility impairment''
(AMC, et al., p. 9).
    In the Visibility Assessment, the EPA noted that the use of the 50
percent acceptance level for urban visibility was first presented in
Ely et al. (1991) (U.S. EPA, 2010b, p. 2-5). Ely discussed the use of
the 50 percent acceptability criterion as a reasonable basis for
setting an urban visibility standard.

    The standard was determined based on a 50% acceptability
criterion, that is, the standard was set at the level of extinction
that would divide the slides into two groups: those judged
acceptable and those judged unacceptable by a majority of the people
in the study. The criterion is politically reasonable because it
defines the point where a majority of the study participants begin
to judge slides as representing unacceptable visibility. It is also
consistent with psychological scaling theory which indicates that a
``true score'' exceeds a standard when more than 50% of the
``observed scores'' exceed that standard. (Ely et al., 1991, p. 11)

    As Ely described, the 50 percent acceptability criterion and the
preference study conducted by Ely were used as the basis for setting
the level of the Denver Visibility Standard in 1990. That same
criterion was judged appropriate and selected for use in the Phoenix
preference study (BBC research, 2003) and as the basis for setting the
level of the Phoenix Visibility Standard in 2003. Most recently, the 50
percent acceptability criterion has been recommended by the British
Columbia Visibility Coordinating Committee as the basis for the
visibility standard currently under consideration by British Columbia,
Canada. Furthermore, CASAC supported this approach, while recognizing
the uncertainty associated with this issue. Specifically, CASAC agreed
that ``the 50th percentile for the acceptability criteria is logical,
given the noted similarities in methodologies employed in the 4 study
areas. * * * In terms of choosing a specific percentile from the
preference studies, we note that there may not be a ``preferred'' one,
but in assessing preference studies to propose a PM secondary NAAQS,
the 50th percentile is sufficient, as it is the basis for existing
visibility indexes used in the Denver/Colorado Front Range and Phoenix
metropolitan areas'' (Samet, 2009c, pp. 8-9). Therefore, after
considering the information that served as the original basis for its
selection as described in Ely et al., 1991, and given its acceptance
and use in existing visibility programs, the EPA continues to conclude,
consistent with the advice of CASAC, that it is reasonable to use the
50 percent acceptability criterion in determining target levels of
protection from visibility impairment.
    (d) Appropriateness of using regionally varying preference study
results to select a single level for a national standard.
    A number of commenters raised concerns regarding the bases for and
implications of the differences observed in the preference study
results, concluding that these results were due to regionally varying
factors and thus could not be used to set a national standard. For
example, some commenters asserted that because the confidence intervals
around the four 50 percent acceptability levels do not overlap at all,
and because there are variations in preference study designs and
inherent differences in the visual setting among cities and panels, the
four preference curves and their associated 50 percent dv values are
city-specific and statistically different. The commenters concluded,
therefore, that it was inappropriate to aggregate the 50th percentile
dv values from multiple studies and that they should instead be
evaluated individually.
    Other commenters expressed the related view that the preference
study results cannot be used to set a national standard for visibility
impairment because the results show that visibility preferences vary
regionally. For example, API stated that:

    The `one-size-fits-all' approach * * * is not viable because it
does not account for regional and city-specific factors that have
been made evident in the disparity of preference study data * * *.
It is well known, for example, that the level of light extinction to
which people in different areas of the country are accustomed, as
well as the urban setting, are the primary factors that affect a
person's visual perception of an urban vista. Thus, the degree to
which extinction threshold can be related to human welfare is
inevitably regionally-dependent. (API, Attachment 2, p. 4)

Some of these commenters argued that because acceptable visual air
quality is regionally dependent, it would be more appropriate to
develop distinct visibility standards at the state or local level.
Others pointed out that areas which lack ``important visibility
vistas'' might not need standards at all, since flat areas without
significant terrain have a limited maximum visual range (NEDA/CAP, p.
3).
    Other commenters stated that due to regionally varying factors,
such as relative humidity, it is not possible to select a single level
for a national standard to protect visibility across the United States.
In particular, these commenters pointed to differences between Eastern
and Western areas, arguing that a single national standard could not
offer the appropriate degree of protection in locations with distinct
characteristics. For example:

    [T]he proposed method falls short because it is not temporally
or geographically representative enough to have any meaning * * *.
The uncertainty evidenced in these studies and the non-uniformity
between the western and eastern vistas makes it impossible at this
time to set an acceptable light extinction value that would
appropriately address visibility concerns in non-Class I areas. (New
York DOH/DEC, pp. 5-6)

    The EPA agrees that the preference curves and the 50 percent dv
levels are separate and distinct data points representing four
different VAQ preference curves for four unique urban scenes. However,
the EPA does not consider the fact that the four curves are distinct as
a weakness of the approach or a reason that the results cannot be
compared. In addition, the EPA does not agree that the study results
necessarily support a conclusion that preferences are regionally
dependent. In particular, the EPA notes that the results of Smith and
Howell (2009) which show that participants in Houston and Washington,
DC did not have significantly different views on acceptable air quality
in Washington, DC, provide limited support for the conclusion that
people's preferences differ less because of where they live and more
because of the scene they are viewing.
    On the other hand, the existing literature indicates that people's
preferences for VAQ depend in large part on the characteristics and
sensitivity of the scene being viewed. The EPA understands there is a
wide variety or range of urban scenes within the United States. These
sensitive urban scenes include those with natural vistas such as the
Colorado Rocky Mountains as well as those with iconic man-made urban
structures like the Washington Monument. The EPA believes that the
scenes presented in the four urban areas

include important types of sensitive valued urban scenes and therefore,
when considered together, can inform the selection of a level of
acceptable urban VAQ at the national scale, taking into account the
variation across the country evidenced in the studies. This is
discussed further in the next section, below.
    The EPA does agree with commenters that there are regionally
varying factors that are important to take into account when setting a
national standard for visibility protection. Section VI.A above
regarding the history of the secondary PM NAAQS review discusses the
evolution of the EPA's understanding regarding the regional differences
in PM concentrations, relative humidity and other factors. As a result,
the current review has gone to great lengths to address these factors,
leading to the EPA's proposal to use the IMPROVE algorithm to calculate
light extinction in order to take into account the varying effects of
relative humidity and speciated PM. While this approach does not result
in a uniform level of ambient PM2.5, it does ensure a
nationally uniform level of visibility protection. The EPA refers the
reader to other sections of the final rule, including sections
VI.B.1.a, VI.B.1.c, VI.C.1.b and VI.C.1.f, and the Response to Comments
document for a more detailed response as to how it is taking these
variables into account.
ii. Specific Comments on Level
    The EPA received relatively few comments endorsing a specific level
for a distinct secondary standard for visibility. In general,
commenters who opposed setting a distinct secondary standard at this
time did not address the question of what level would be appropriate if
the EPA were to set a distinct secondary standard for visibility;
similarly, commenters who supported adopting a distinct secondary
standard at this time generally did not recommend a specific level.
However, a few commenters did provide comments in support of a specific
level or range of levels, with some commenters advocating standards at
the upper end of the range of proposed levels (i.e., 30 dv), while
others supported levels below the lower end of the proposed range
(i.e., below 28 dv).
    As discussed above, a large number of commenters argued that the
currently available data are insufficient to determine what constitutes
a standard that would be neither more nor less protective than
necessary and that no standard should be set at this time. These
commenters pointed to the limitations and uncertainties in the
preference studies discussed above as the basis for this claim. These
commenters pointed to significant variation in the results of the
preference studies in support of their arguments that the studies
should not be used to derive a level for a distinct secondary standard
for visibility. For example, one consultant cited by several industry
commenters argued that the proposed level of 28 or 30 dv did not
reflect the substantial difference in visibility preferences between
the East and the West reflected in the preference studies (UARG,
Attachment 2, p. 11), and that it did not reflect the full range of
preferences (i.e., potential 50 percent acceptability levels) likely to
exist nationwide (UARG, Attachment 2, p. 19). This commenter further
objected to the EPA's proposal for a level of 28 or 30 dv on the
grounds that the EPA had inaccurately adjusted 4-hour values into 24-
hour values. Based on his analysis, the consultant concluded that ``a
range of adjusted values from 28 to 32 dv is needed'' to account for
the majority of the spread between the 4-hour vs. 24-hour equivalent
values at the upper end of the distribution of values.
    A number of commenters questioned whether the proposed range of
levels was appropriate. One industry commenter claimed that the EPA had
not explicitly justified why a standard within the proposed range was
requisite, stating that ``EPA makes no attempt to explain how the
proposed level of the standard is neither lower nor higher than
necessary to protect public welfare'' (NSSGA, p. 15). Arizona DEQ noted
that since the proposed calculated light extinction indicator excluded
coarse particles and Rayleigh scattering, the proposed levels of 28 or
30 dv were inconsistent with the visibility preference studies, which
considered total light extinction. Noting these perceived problems with
the proposed range of levels, a few commenters noted that if the EPA
were to set a distinct secondary standard, the level should be set no
lower than 30 dv, ``to account for inconsistent value judgments, a
great deal of spatial and temporal variability, and a very high level
of uncertainty'' (Texas CEQ, p. 7).
    In contrast, some commenters supporting the EPA's proposal for a
distinct secondary standard for visibility stated that the proposed
range of levels from 28-30 dv was insufficiently protective based on a
24-hour averaging time, and recommended a lower level for the
visibility index standard. These commenters expressed the view that the
proposed levels of 28 or 30 dv represented neither adequate surrogates
for equivalent 4-hour values, as the EPA claimed, nor sufficiently
protective levels based on recent air quality data. Several commenters
stated that the EPA's own analyses suggested that a standard set at a
level of 28 or 30 dv was insufficiently protective based on a 24-hour
averaging time. One commenter emphasized that the Policy Assessment had
indicated a level between 25-28 dv was appropriate for a standard
calculated on a 24-hour average, and encouraged the EPA to adopt a
standard level of 25 dv. Several environmental groups provided comments
stating that a 24-hour average would underestimate a 4-hour value by
13-42 percent and certain areas of the country--particularly the
Northeast--would be affected disproportionately. These commenters
suggested that a 24-hour PM2.5 visibility index standard
should be set at a level of 18.6-20 dv. The Department of the Interior
pointed to recent air quality data indicating that visibility on the
20% worst days in several large metropolitan areas, including
Birmingham, Fresno, New York City, Phoenix, and Washington, DC was
below 29 dv. While noting that these calculations were based on IMPROVE
calculations which include contributions from coarse PM mass, DOI
expressed the view that the proposed level of 28 to 30 dv would not
provide adequate visibility protection compared to the current 24-hour
PM2.5 standard of 35 [micro]g/m\3\ and recommended that the
standard be set at a level of 25 dv consistent with the results of the
Phoenix visibility preference study.
    In contrast, the states of Arizona and Colorado submitted comments
arguing that the visibility preference studies conducted in Phoenix and
Denver, respectively, were designed to address a specific local problem
and that the results of these studies were not an appropriate basis for
selecting the level of a national standard. For example, Arizona DEQ
noted:

    The cited studies were conducted considering total light
extinction; including extinction resulting from particulate matter
and Rayleigh scattering. Visibility impairment due to coarse
particulate matter can be an important contributor in Arizona,
specifically in the Phoenix area where ongoing measurements have
been made. Therefore, ADEQ believes that the proposed levels of the
secondary visibility standard are inconsistent with applicable urban
studies. (Arizona DEQ, p. 2)

Similarly, the Colorado Department of Public Health and the Environment
noted that the Denver visibility standard was designed to address
``brown clouds'', i.e., strong inversions that occur in the Denver
metropolitan area, and that this standard ``is based on a

specific view of Denver'' associated with particular sight paths and
direct measurement methods. The commenter stated that this standard
``is applicable only to this location,'' and that these limitations
make it potentially unsuitable for application as ``a national
secondary standard, particularly a proposed standard that does not use
a direct measurement method'' (Colorado DPHE, p. 2).
    While acknowledging the uncertainties and limitations associated
with the visibility preference studies as discussed above, the EPA
continues to conclude, as did CASAC, that the preference studies are
appropriate to use as the basis for selecting a target level of
protection from visibility impairment. However, the EPA agrees with
commenters who emphasize the high degree of variability in visibility
conditions and the potential variability in visibility preferences
across different parts of the country. In light of the associated
uncertainty, as noted in the proposal, the Administrator judged it
appropriate to establish a target level of protection equivalent to the
upper end of the range of Candidate Protection Levels (CPLs) identified
in the Policy Assessment and generally supported by CASAC. Thus, the
EPA proposed to set a 24-hour visibility index standard that would
provide protection equivalent to the protection afforded by a 4-hour
standard set at a level of 30 dv. In light of the comments received on
the proposal, in particular comments emphasizing the uncertainty and
variability in the results of the public preference studies, the EPA
continues to conclude that this approach is warranted, and that it is
appropriate to set a target level of protection equivalent to the
protection that would be afforded by a 4-hour, 30 dv visibility index
standard.
    Moreover, the EPA disagrees with commenters who argued that the
EPA's approach for translating 4-hour CPLs into equivalent 24-hour
values was inappropriate. In adjusting 4-hour values for purposes of
defining an appropriate level for a 24-hour standard, the EPA noted at
the time of proposal that there were multiple approaches for estimating
generally equivalent levels on a city-specific or national basis. While
expressing the view that it was appropriate to consider the two
approaches with the highest r\2\ values (Approaches A and B in Appendix
G of the Policy Assessment),\191\ which used regressions of 90th
percentile light extinction values, the EPA determined it would also be
appropriate to consider the city-specific estimates resulting from
Approaches C and E which showed greater variability than the aggregated
estimates. Approaches C and E generated 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. This information provided
support for using the same CPL for a 24-hour standard as for a 4-hour
standard, since no single approach could generate an equivalent 24-hour
standard level in each urban area for each CPL. The EPA continues to
conclude, as it did at the time of proposal, that using an unadjusted
4-hour CPL for purposes of establishing a target level of protection
for a 24-hour standard is appropriate because this approach places more
emphasis on the relatively high degree of spatial and temporal
variability in relative humidity and fine particle composition observed
in urban areas across the country, consistent with EPA's reanalysis
discussed below.
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    \191\ In particular, EPA staff expressed a preference for
Approach B in the Policy Assessment. However, in light of the
additional information provided by the other approaches explored in
Appendix G of the Policy Assessment and the reanalysis in Frank, et
al. (2012b), the EPA judges it more appropriate to consider the
range of values resulting from all five analytical approaches for
purposes of informing decisions about the equivalent level of a 24-
hour standard.
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    The EPA has conducted a reanalysis (Frank et al., 2012b) of the
relationships between estimated 24-hour and 4-hour visibility
impairment based on the variety of metrics discussed in Appendix G of
the Policy Assessment. The reanalysis has more appropriately considered
the uncertainty of the calculated 4-hour values. The revised analysis
shows that the 24-hour equivalent level is generally closer to the 4-
hour value at the upper end of the range of CPLs than originally
estimated, as can be seen in the results for Approaches B, C, and
D.\192\ For example, the reanalysis indicates that Approach B yields an
adjusted 24-hour CPL of 29 dv\193\ as generally being equivalent to a
4-hour CPL of 30 dv, while Approach C yields a 24-hour equivalent CPL
of 29 dv averaged across cities and a range of city-specific values
from 25-36 dv.194 195 Not only are the 90th percentile and
pooled average values closer to the 4-hour CPL of 30 dv, the range of
city-specific results shows a wider spread that clearly encompasses the
unadjusted 4-hour value of 30 dv near the midpoint of the city-specific
range. This provides support for concluding that the EPA's approach to
translating of 4-hour CPLs into equivalent 24-hour values was
appropriate, and that it is appropriate to use unadjusted 4-hour values
for purposes of selecting a level for a standard based on a 24-hour
averaging time.\196\
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    \192\ Approach E as presented in the Policy Assessment is based
on the median values for each city; these results are not affected
by the regression analyses. Therefore, Approach E was not included
in the reanalysis, and the results remain unchanged from those
reported in the corrected Table G-6 as reported in Frank, et al.,
2012b.
    \193\ In Appendix G of the Policy Assessment, a 24-hour adjusted
CPL of 28 dv was estimated to be equivalent to a 4-hour value of 30
dv under Approach B (annual 90th percentile values regression).
    \194\ In Appendix G of the Policy Assessment, under Approach C
(all-days city-specific regression), a 24-hour adjusted CPL of 27 dv
was estimated to be equivalent to a 4-hour CPL of 30 dv when
averaged across cities, while city-specific values were estimated to
range from 24-30 dv.
    \195\ In the reanalysis, Approach D (all days pooled regression)
generated results of 28 dv for the 24-hour CPL equivalent to a 4-
hour value of 30 dv as compared to a value of 27 dv in the original
analysis described in Appendix G.
    \196\ The analysis in Appendix G of the Policy Assessment used
the 4-hour light extinction value treated as the independent (x-
axis) variable in an ordinary least squares regression. The EPA now
concludes that this regression approach was not the most appropriate
approach because that variable has error and in fact may be more
uncertain than the calculated 24-hour extinction values. The Frank
et al. (2012b) reanalysis uses an orthogonal regression instead of
ordinary least squares regression and results in slopes closer to
the 1:1 line for all the results, particularly for Dallas, TX.
Furthermore, consistent with the EPA's conclusion that a higher
multiplier for converting OC to OM would be appropriate (see section
VI.C.1.b.ii above), the reanalysis substitutes a 1.6 multiplier for
converting OC to OM in the calculation of 24-hour values instead of
the value of 1.4 that was used in calculating 24-hour values for
Appendix G. The higher multiplier is more consistent with the
SANDWICH approach used to calculate the 4-hour values found in
Appendix G. See Frank et al. (2012b) for a more detailed
explanation.
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    Moreover, the EPA disagrees with commenters who argue that the
currently available evidence is sufficient to justify establishing a
target level of protection at 25 dv or below. The EPA recognizes that
25 dv represents the middle of the range of 50 percent acceptability
levels from the 4 cities studied, and represents the 50 percent
acceptability level from the Phoenix study, which the Agency has
acknowledged as the best of the four studies in terms of having the
least noise in the preference study results and the most representative
selection of participants. The EPA also notes the caveats discussed in
the proposal regarding whether it would be appropriate to interpret
results from the western studies as generally representative of a
broader range of scenic vistas in urban areas across the country. The
Policy Assessment noted significant differences in the

characteristics of the urban scenes used in each study, with western
urban visibility preference study scenes including mountains in the
background and objects at greater distances, while scenes in the
eastern study did not. Since objects at a greater distance have a
greater sensitivity to perceived visibility changes as light extinction
changes compared to otherwise similar scenes with objects at a shorter
range, this likely explains part of the difference between the results
of the eastern study and results of the western studies. In the
proposal, the EPA noted that the scenic vistas available on a daily
basis in many urban areas across the country generally do not have the
inherent visual interest or the distance between viewer and object of
greatest intrinsic value as in the Denver and Phoenix preference
studies. Also, the Agency takes note of the caution expressed by
Colorado and Arizona about using the results of the Denver and Phoenix
preference studies, which were aimed at addressing specific local
visibility problems, to inform the choice of level for a national
standard. Therefore, the Agency considers it reasonable to conclude,
especially in light of the significant uncertainties, that it is
appropriate to place less weight on the western preference results and
that the high CPL value (30 dv) that is based on the eastern preference
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. These areas would include the middle of the country
and many areas in the eastern U.S., as well as some western areas. As a
result, the EPA concludes that it is more appropriate to establish a
target level of protection at the upper end of the range of 24-hour
CPLs considered, recognizing that no one level will be ``correct'' for
every urban area in the country.
    In considering the upper end of this range, the EPA must identify a
target level of protection that is considered requisite to protect
public welfare from a national perspective, recognizing that the same
target level would apply in all locations. Making this judgment
requires a balancing of the risks to the public welfare and the
substantial uncertainties surrounding appropriate levels of visibility
protection. As acknowledged in the proposal, the EPA recognizes that
setting a target level of protection for a 24-hour standard at 30 dv
would reflect a judgment 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. Also, a 24-hour visibility index at a level of 30 dv would
reflect recognition that there is considerable spatial and temporal
variability in the key factors that determine the value of the
PM2.5 visibility index in any given urban area, such that
there is a relatively high degree of uncertainty as to the most
appropriate approach to use in selecting a 24-hour standard level that
would be generally equivalent to a specific 4-hour standard level. In
light of these uncertainties, the EPA continues to believe that it is
appropriate to establish a target level of protection for visual air
quality of 30 dv, averaged over 24-hours, with a form as discussed
above.
    In reaching this conclusion, the EPA notes that any national
ambient air quality standard for visibility 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
visibility index NAAQS 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, can 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. With regard to comments
from the Department of Interior noting that many large metropolitan
areas have 24-hour IMPROVE values below 30 dv on the worst 20 percent
of days already, the EPA notes that the purpose of establishing NAAQS
is to ensure adequate protection of public welfare everywhere, not to
mandate continuous improvements in areas that may already be relatively
clean. In fact, the evidence from the IMPROVE program that many urban
areas have total 24-hour PM-related light extinction below 29 dv on the
20 percent worst visibility days suggests that many areas have
relatively good visual air quality already.
f. Need for a Distinct Secondary Standard To Protect Visibility
    Numerous commenters questioned whether a distinct secondary
standard for visibility is necessary in light of the analysis described
in section VI.B.1.c.vii above (Kelly et al., 2012a) which indicated
that a 24-hour mass-based PM2.5 standard of 35 [mu]g/m\3\
would protect against visibility impacts exceeding the range of levels
considered in the proposal (28-30 dv). While this analysis was
conducted in support of proposed implementation requirements for a
distinct secondary standard (specifically, the modeling demonstrations
that would be required under the PSD program), the second prong of the
analysis showed that within the range of levels proposed by the EPA for
the visibility index NAAQS (28-30 dv), the 24-hour PM2.5
standard of 35 [mu]g/m\3\ would generally be controlling. Kelly et al.
(2012a) concluded that ``overall, design values based on 2008-2010 data
suggest that counties that attain 24-hour PM2.5 NAAQS level
of 35 [mu]g/m\3\ would attain the proposed secondary PM2.5
visibility index NAAQS level of 30 dv and generally attain the level of
28 dv'' (pp. 17-18).
    Citing this conclusion, many state and local agencies and industry
commenters argued that a visibility index standard in the range
proposed (28-30 dv) would provide no additional protection beyond that
afforded by the existing secondary 24-hour PM2.5 NAAQS, and
therefore no distinct visibility standard was necessary. These
commenters advocated retaining the current 24-hour PM2.5
mass-based standard to protect against visibility effects. ``Since the
24-hour PM2.5 standard already protects the welfare the 24-
hour PM2.5 visibility standard is designed to protect, the
new standard is duplicative and unnecessary'' (South Dakota DENR, p.
2). Furthermore, a number of state commenters objected to the
additional resource burden associated with implementing a standard
which had, in their view, no practical effect: ``If the 24-hour
PM2.5 mass standard has the same effect as the visibility
standard, crafting complex regulations to implement another standard
seems redundant'' (South Carolina DHEC, p. 3). Other states agreed: ``A
PM2.5-related Visibility Index appears redundant since the
benefits achieved from the current primary and secondary annual and 24-
hour PM2.5 standards already provide reductions that would
improve visibility. Establishing a new PM2.5 secondary
standard for visibility would be an additional complication and burden
to the states that is not warranted'' (Indiana DEM, p. 5).
    In addition, several commenters submitted additional analyses
supporting their position that a 35 [mu]g/m\3\ 24-hour PM2.5
standard would provide at least equivalent protection to

a distinct 24-hour visibility standard within the range of levels
proposed (API, Attachment 2, p. 8 and Attachment 3, p. 1).
    In responding to these comments stating that a distinct visibility
standard is not needed, the EPA notes as an initial matter that the
Administrator provisionally concluded at the time of proposal that the
current PM standards were not sufficiently protective of visual air
quality, and that consideration should be given to an alternative
secondary standard that would provide additional protection against PM-
related visibility impairment, especially in urban areas. This
provisional conclusion was based on the results of public preference
surveys on the acceptability of varying degrees of visibility
impairment in urban areas, analyses of the number of days on which peak
1-hour or 4-hour light extinction values were estimated to exceed a
range of CPLs under conditions simulated to just meet the current
standards, and the advice of CASAC. The Administrator also noted that
the current indicator of PM2.5 mass, in conjunction with the
current 24-hour and annual averaging times, was not well suited for
purposes of protecting visibility, since it does not incorporate
species composition or relative humidity, both of which play a central
role in determining the impact of ambient PM on visibility. Taking into
account the advice of CASAC and the court's remand of the current
secondary PM2.5 standards, the Administrator provisionally
concluded that the current secondary standards were neither
sufficiently protective nor suitably structured to provide an
appropriate degree of public welfare protection from PM-related