In commenting on the proposed decision, the CASAC requested that a
sub-daily standard to protect visibility ``be favorably reconsidered''
(Henderson, 2006a, p.6). The CASAC noted three cautions regarding the
proposed reliance on a secondary PM2.5 standard identical to
the proposed 24-hour primary PM2.5 standard: (1)
PM2.5 mass measurement is a better indicator of visibility
impairment during daylight hours, when relative humidity is generally
low; the sub-daily standard more clearly matches the nature of
visibility impairment, whose adverse effects are most evident during
the daylight hours; using a 24-hour PM2.5 standard as a
proxy introduces error and

uncertainty in protecting visibility; and sub-daily standards are used
for other NAAQS and should be the focus for visibility; (2) CASAC and
its monitoring subcommittees had repeatedly commended EPA's initiatives
promoting the introduction of continuous and near-continuous PM
monitoring and recognized that an expanded deployment of continuous
PM2.5 monitors would be consistent with setting a sub-daily
standard to protect visibility; and (3) the analysis showing a
similarity between percentages of counties not likely to meet what the
CASAC Panel considered to be a lenient 4- to 8-hour secondary standard
and a secondary standard identical to the proposed 24-hour primary
standard was a numerical coincidence that was not indicative of any
fundamental relationship between visibility and health. The CASAC Panel
further stated that ``visual air quality is substantially impaired at
PM2.5 concentrations of 35 [mu]g/m\3\'' and that ``[i]t is
not reasonable to have the visibility standard tied to the health
standard, which may change in ways that make it even less appropriate
for visibility concerns'' (Henderson, 2006a, pp. 5 to 6).
    In reaching a final decision, the Administrator focused on the
relative protection provided by the proposed primary standards based on
the above-mentioned similarities in percentages of counties meeting
alternative standards and on the limitations in the information
available concerning studies of public perception and attitudes
regarding the acceptability of various degrees of visibility impairment
in urban areas, as well as on the subjective nature of the judgment
required. In so doing, the Administrator concluded that caution was
warranted in establishing a distinct secondary standard for visibility
impairment and that the available information did not warrant adopting
a secondary standard that would provide either more or less protection
against visibility impairment in urban areas than would be provided by
secondary standards set equal to the proposed primary PM2.5
standards.
2. Remand of 2006 Secondary PM2.5 Standards
    As noted above in section II.B.2 above, several parties filed
petitions for review challenging EPA's decision to set the secondary
NAAQS for fine PM identical to the primary NAAQS. On judicial review,
the D.C. Circuit remanded to the EPA for reconsideration the secondary
NAAQS for fine PM because the Agency's decision was unreasonable and
contrary to the requirements of section 109(b)(2). American Farm Bureau
Federation v. EPA, 559 F. 3d 512 (D.C. Cir., 2009).
    The petitioners argued that the EPA's decision lacked a reasoned
basis. First, they asserted that the EPA never determined what level of
visibility was ``requisite to protect the public welfare.'' They argued
that the EPA unreasonably rejected the target level of protection
recommended by its staff, while failing to provide a target level of
its own. The court agreed, stating that ``the EPA's failure to identify
such a level when deciding where to set the level of air quality
required by the revised secondary fine PM NAAQS is contrary to the
statute and therefore unlawful. Furthermore, the failure to set any
target level of visibility protection deprived the EPA's decision-
making of a reasoned basis.'' 559 F. 3d at 530.
    Second, the petitioners challenged EPA's method of comparing the
protection expected from potential standards. They contended that the
EPA relied on a meaningless numerical comparison, ignored the effect of
humidity on the usefulness of a standard using a daily averaging time,
and unreasonably concluded that the primary standards would achieve a
level of visibility roughly equivalent to the level the EPA staff and
CASAC deemed ``requisite to protect the public welfare.'' The court
found that the EPA's equivalency analysis based on the percentages of
counties exceeding alternative standards ``failed on its own terms.''
The same table showing the percentages of counties exceeding
alternative secondary standards, used for comparison to the percentages
of counties exceeding alternative primary standards to show
equivalency, also included six other alternative secondary standards
within the recommended CASAC range that would be more ``protective''
under EPA's definition than the adopted primary standards. Two-thirds
of the potential secondary standards within the CASAC's recommended
range would be substantially more protective than the adopted primary
standards. The court found that the EPA failed to explain why it looked
only at one of the few potential secondary standards that would be less
protective, and only slightly less so, than the primary standards. More
fundamentally, however, the court found that the EPA's equivalency
analysis based on percentages of counties demonstrated nothing about
the relative protection offered by the different standards, and that
the tables offered no valid information about the relative visibility
protection provided by the standards. 559 F. 3d at 530-31.
    Finally, the Staff Paper had made clear that a visibility standard
using PM2.5 mass as the indicator in conjunction with a
daily averaging time would be confounded by regional differences in
humidity. The court noted that the EPA acknowledged this problem, yet
did not address this issue in concluding that the primary standards
would be sufficiently protective of visibility. 559 F. 3d at 530.
Therefore, the court granted the petition for review and remanded for
reconsideration the secondary PM2.5 NAAQS.
3. General Approach Used in the Policy Assessment for the Current
Review
    The approach used in this review broadened the general approaches
used in the last two PM NAAQS reviews by utilizing, to the extent
available, enhanced tools, methods, and data to more comprehensively
characterize visibility impacts. As such, the EPA took into account
considerations based on both the scientific evidence (``evidence-
based'') and a quantitative analysis of PM-related impacts on
visibility (``impact-based'') to inform conclusions related to the
adequacy of the current secondary PM2.5 standards and
alternative standards that were appropriate for consideration in this
review. As in past reviews, the EPA also considered that the secondary
NAAQS should address PM-related visibility impairment in conjunction
with the Regional Haze Program, such that the secondary NAAQS would
focus on protection from visibility impairment principally in urban
areas in conjunction with the Regional Haze Program that is focused on
improving visibility in Federal Class I areas. The EPA again recognized
that such an approach remains the most appropriate and effective means
of addressing the public welfare effects associated with visibility
impairment in areas across the country.
    The Policy Assessment drew from the qualitative evaluation of all
studies discussed in the Integrated Science Assessment (U.S. EPA,
2009a). Specifically, the Policy Assessment considered the extensive
new air quality and source apportionment information available from the
regional planning organizations, long-standing evidence of PM effects
on visibility, and limited public preference study information from
four urban areas (U.S. EPA, 2009a, chapter 9), as well as the
integration of evidence across disciplines (U.S. EPA, 2009a, chapter
2). In addition, limited information that had become available
regarding the characterization of public preferences in urban areas
provided

some new perspectives on the usefulness of this information in
informing the selection of target levels of urban visibility
protection. On these bases, the Policy Assessment again focused
assessments on visibility conditions in urban areas.
    The conclusions in the Policy Assessment reflected EPA staff's
understanding of both evidence-based and impact-based considerations to
inform two overarching questions related to (1) the adequacy of the
current suite of PM2.5 standards and (2) what potential
alternative standards, if any, should be considered in this review to
provide appropriate protection from PM-related visibility impairment.
In addressing these broad questions, the discussions in the Policy
Assessment were organized around a series of more specific questions
reflecting different aspects of each overarching question (U.S. EPA,
2011a, Figure 4-1). When evaluating the visibility protection afforded
by the current or any alternative standards considered, the Policy
Assessment took into account the four basic elements of the NAAQS:
indicator, averaging time, level, and form.

B. Proposed Decisions on Secondary PM Standards

    At the time of proposal, 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 standard was to
be defined in terms of a PM2.5 visibility index, which would
use measured PM2.5 mass concentration, in combination with
speciation 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. To address other non-visibility welfare effects, the
Administrator proposed 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 proposed change to the primary annual
PM2.5 standard. Each of these proposed decisions is
described in more detail in the proposal and below.
1. PM-Related Visibility Impairment
    As discussed in Section VI.B of the proposal, the Administrator's
proposed decision regarding a distinct secondary standard to provide
protection from visibility impairment reflected careful consideration
of the following: (1) The latest scientific information on visibility
effects associated with PM as described in the Integrated Science
Assessment (U.S. EPA, 2009a); (2) insights gained from assessments of
correlations between ambient PM2.5 and visibility impairment
prepared by EPA staff in the Visibility Assessment (U.S. EPA, 2010b);
and (3) specific conclusions regarding the need for revisions to the
current standards (i.e., indicator, averaging time, form, and level)
that, taken together, would be requisite to protect the public welfare
from adverse effects on visual air quality. This section summarizes key
information from the proposal regarding the nature of visibility
impairment, including the relationship between ambient PM and
visibility, temporal variations in light extinction, periods during the
day of interest for assessing visibility conditions, and exposure
durations of interest (section VI.B.1.a); limited public perceptions
and attitudes about visibility impairment and the impacts of visibility
impairment on public welfare (section VI.B.1.b); CASAC advice regarding
the need for, and design of, secondary standards to protect visibility
(section VI.B.1.c); and the Administrator's proposed conclusions
regarding setting a distinct standard to address visibility impairment
(section VI.B.1.d).
a. Nature of PM-Related Visibility Impairment
    As noted at the time of proposal, the fundamental science
characterizing the contribution of PM, especially fine particles, to
visibility impairment is well understood. This science provides the
basis for the Integrated Science Assessment designation of the
relationship between PM and visibility impairment as causal. New
research available in this review, discussed in chapter 9 of the
Integrated Science Assessment, continues to support and refine EPA's
understanding of the effect of PM on visibility and the source
contributions to that effect in rural and remote locations. This
research provides new insights regarding the regional source
contributions to urban visibility impairment and better
characterization of the increment in PM concentrations and visibility
impairment that occur in many cities (i.e., the urban excess) relative
to conditions in the surrounding rural areas (i.e., regional
background). Ongoing urban PM2.5 speciated and aggregated
mass monitoring has produced new information that has allowed for
updated characterization of current visibility levels in urban areas.
i. Relationship Between Ambient PM and Visibility
    Visibility impairment is caused by the scattering and absorption of
light by suspended particles and gases in the atmosphere. When PM is
present in the air, its contribution to light extinction typically
greatly exceeds that of gases. The combined effect of light scattering
and absorption by both particles and gases is characterized as light
extinction, i.e., the fraction of light that is scattered or absorbed
in the atmosphere. Light extinction can be quantified by a light
extinction coefficient with units of 1/distance, which is often
expressed as 1/(1 million meters) or inverse megameters (abbreviated
Mm-1) or in terms of an alternative scale known as the
deciview scale, defined by the following equation: \146\
---------------------------------------------------------------------------

    \146\ As used in the Regional Haze Program, the term
bext refers to light extinction due to PM2.5,
PM10-2.5, and ``clean'' atmospheric gases. In the Policy
Assessment, in focusing on light extinction due to PM2.5,
the deciview values include only the effects of PM2.5 and
the gases. The ``Rayleigh'' term associated with clean atmospheric
gases is represented by the constant value of 10 Mm-\1\.
Omission of the Rayleigh term would create the possibility of
negative deciview values when the PM2.5 concentration is
very low.
---------------------------------------------------------------------------

Deciview (dv) = 10 ln (bext/ 10 Mm-1)

The deciview scale is frequently used in the scientific literature on
visibility, as well as in the Regional Haze Program. In particular, the
deciview scale is used in the public perception studies that were
considered in the past and current reviews to inform judgments about an
appropriate degree of protection to be provided by a secondary NAAQS.
    The amount of light extinction contributed by PM depends on the
particle concentration as well as on the particle size distribution and
composition and also on the relative humidity. As described in detail
in section VI.B.1.a of the proposal, visibility scientists have
developed an algorithm, known as the IMPROVE algorithm,\147\ to
estimate light extinction using routinely monitored fine particle
(PM2.5) speciation and coarse particle mass
(PM10-2.5) data, as well as data on relative humidity. There
is both an original and a revised version of the IMPROVE algorithm
(Pitchford et al., 2007). The revised version was developed to address
observed biases in the predictions using the original algorithm under
very low and very high

light extinction conditions.\148\ These IMPROVE algorithms are
routinely used to calculate light extinction levels on a 24-hour basis
in Federal Class I areas under the Regional Haze Program.
---------------------------------------------------------------------------

    \147\ The algorithm is referred to as the IMPROVE algorithm
because it was developed specifically to use the aerosol monitoring
data generated at network sites and with equipment specifically
designed to support the IMPROVE program and was evaluated using
IMPROVE optical measurements at the subset of sites that make those
measurements (Malm et al., 1994).
    \148\ These biases were detected by comparing light extinction
estimates generated from the IMPROVE algorithm to direct optical
measurements in a number of rural Federal Class I areas.
---------------------------------------------------------------------------

    In either version of the IMPROVE algorithm, the concentration of
each of the major aerosol components is multiplied by a dry extinction
efficiency value and, for the hygroscopic components (i.e., ammoniated
sulfate and ammonium nitrate), also multiplied by an additional factor
to account for the water growth to estimate these components'
contribution to light extinction. Summing the contribution of each
component gives the estimate of total light extinction per unit
distance denoted as the light extinction coefficient (bext), as shown
below for the original IMPROVE algorithm.

bext [ap] 3 x f(RH) x [Sulfate]
    + 3 x f(RH) x [Nitrate]
    + 4 x [Organic Mass]
    + 10 x [Elemental Carbon]
    + 1 x [Fine Soil]
    + 0.6 x [Coarse Mass]
    + 10

    Light extinction (bext) is in units of Mm-1,
the mass concentrations of the components indicated in brackets are in
units of [mu]g/m\3\, and f(RH) is the unitless water growth term that
depends on relative humidity. The final term of 10 Mm-1 is
known as the Rayleigh scattering term and accounts for light scattering
by the natural gases in unpolluted air. Despite the simplicity of this
algorithm, it performs reasonably well and permits the contributions to
light extinction from each of the major components (including the water
associated with the sulfate and nitrate compounds) to be separately
approximated. Inspection of the PM component-specific terms in the
simple original IMPROVE algorithm shows that most of the
PM2.5 components contribute 5 times or more light extinction
than a similar concentration of PM10-2.5.
    The f(RH) term in the original algorithm reflects the increase in
light scattering caused by particulate sulfate and nitrate under
conditions of high relative humidity. Particles with hygroscopic
components (e.g., particulate sulfate and nitrate) contribute more
light extinction at higher relative humidity than at lower relative
humidity because they change size in the atmosphere in response to
ambient relative humidity conditions. For relative humidity below 40
percent the f(RH) value is 1, but it increases to 2 at approximately 66
percent, 3 at approximately 83 percent, 4 at approximately 90 percent,
5 at approximately 93 percent, and 6 at approximately 95 percent
relative humidity. The result is that both particulate sulfate and
nitrate are more efficient per unit mass in light extinction than any
other aerosol component for relative humidity above approximately 85
percent where their total light extinction efficiency exceeds the 10
m\2\/g associated with elemental carbon (EC). PM containing elemental
or black carbon (BC) absorbs light as well as scattering it, making it
the component with the greatest light extinction contributions per unit
of mass concentration, except for the hygroscopic components under
these high relative humidity conditions.\149\
---------------------------------------------------------------------------

    \149\ The IMPROVE algorithm does not explicitly separate the
light-scattering and light-absorbing effects of elemental carbon.
---------------------------------------------------------------------------

    As noted above, subsequent to the development of the original
IMPROVE algorithm, an alternative algorithm (variously referred to as
the ``revised algorithm'' or the ``new algorithm'' in the literature)
was developed. The revised IMPROVE algorithm is different from the
original algorithm in several important ways. First, the revised
algorithm employs a more complex split-component mass extinction
efficiency to correct biases believed to be related to particle size
distributions.\150\ Specifically, the revised algorithm incorporates
terms to account for particles representing the different dry
extinction and water uptake from two size modes of sulfate, nitrate and
organic mass.\151\ Second, the revised algorithm uses a different
multiplier for organic carbon for purposes of estimating organic
carbonaceous material to better represent aged aerosol found in remote
areas.\152\ In addition, the revised algorithm includes a term for
hygroscopic sea salt that can be important for remote coastal areas,
and site-specific Rayleigh light scattering terms in place of a
universal Rayleigh light scattering value. As noted in section VI.B.1.a
of the proposal, the revised IMPROVE algorithm can yield higher
estimates of current light extinction levels in urban areas on days
with relatively poor visibility as compared to the original algorithm
(Pitchford, 2010). This difference is primarily attributable to the
split-component mass extinction efficiency treatment in the revised
algorithm. This revised algorithm was evaluated at 21 remote locations
and is generally used by RPOs and States for implementation of the
Regional Haze Rule.
---------------------------------------------------------------------------

    \150\ In either version of the IMPROVE algorithm, the
concentration of each of the major aerosol components is multiplied
by a dry extinction efficiency value and, for the hygroscopic
components (i.e., ammoniated sulfate and ammonium nitrate), also
multiplied by an additional factor to account for the water growth
to estimate these components' contribution to light extinction. Both
the dry extinction efficiency and water growth terms have been
developed by a combination of empirical assessment and theoretical
calculation using typical particle size distributions associated
with each of the major aerosol components.
    \151\ The relative contributions of sulfate, nitrate, and
organic mass concentrations to visibility impairment with the
revised algorithm are different than with the original algorithm due
to the combination of the dry extinction coefficient and f(RH)
functions for derived concentrations of small and large particles.
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 was empirically developed for
remote areas. The fraction of the fine particle component that is in
the large mode is estimated by dividing the total concentration of
the component by 20 [mu]g/m\3\. If the total concentration of a
component exceeds 20 [mu]g/m\3\, all of it is assumed to be in the
large mode.
    \152\ The revised IMPROVE algorithm uses a multiplier of 1.8 for
rural areas instead of 1.4 as used in the original algorithm for the
mean ratio of organic mass to organic carbon.
---------------------------------------------------------------------------

ii. Temporal Variations of Light Extinction
    Particulate matter concentrations and light extinction in urban
environments vary from hour to hour throughout the 24-hour day due to a
combination of diurnal changes in meteorological conditions and
systematic changes in emissions activity (e.g., rush hour traffic).
Various factors combine to make early morning the most likely time for
peak urban light extinction; although the net effects of the systematic
urban- and larger-scale variations mean that peak daytime PM light
extinction levels can occur any time of day, in many areas they occur
most often in early morning hours (U.S. EPA, 2010b, sections 3.4.2 and
3.4.3; Figures 3-9, 3-10, and 3-12). This temporal pattern in urban
areas contrasts with the general lack of a strong diurnal pattern in PM
concentrations and light extinction in most Federal Class I areas,
reflective of a relative lack of local sources as compared to urban
areas. The use in the Regional Haze Program of 24-hour average
concentrations in the IMPROVE algorithm is consistent with this general
lack of a strong diurnal pattern in Federal Class I areas.
iii. Periods During the Day of Interest for Assessment of Visibility
    As noted in sections VI.B.1.b and VI.B.1.c of the proposal, daytime
visibility has dominated the attention of

those who have studied the visibility effects of air pollution,
particularly in urban areas. The EPA recognizes, however, that
physically PM light extinction behaves the same at night as during the
day and can contribute to nighttime visibility effects by enhancing the
scattering of anthropogenic light, contributing to the ``skyglow''
within and over populated areas, adding to the total sky brightness,
and contributing to the reduction in contrast of stars against the
background. However, little research has been conducted on nighttime
visibility, and the state of the science is not comparable to that
associated with daytime visibility impairment, particularly in terms of
the impact on human welfare. The Policy Assessment notes that the
science is not available at this time to support adequate
characterization specifically of nighttime PM light extinction
conditions and the related effects on public welfare (U.S. EPA, 2011a,
p. 4-18). Therefore the EPA has focused its assessments of PM
visibility impacts in urban areas on daylight hours during this review.
iv. Exposure Durations of Interest
    As noted in section VI.B.1.d of the proposal, the roles that
exposure duration and variations in visual air quality within any given
exposure period play in determining the acceptability or
unacceptability of a given level of visual air quality have not been
investigated via preference studies. In the preference studies
available for this review, subjects were simply asked to rate the
acceptability or unacceptability of each image of a haze-obscured
scene, without being provided any suggestion of assumed duration or of
assumed conditions before or after the occurrence of the scene
presented. Preference and/or valuation studies show that atmospheric
visibility conditions can be quickly assessed and preferences
determined. The EPA is unaware of any studies that characterize the
extent to which different frequencies and durations of exposure to
visibility conditions contribute to the degree of public welfare impact
that occurs.
    The Policy Assessment considered a variety of circumstances that
are commonly expected to occur in evaluating the potential impact of
visibility impairment on the public welfare based on available
information (U.S. EPA, 2011a, pp. 4-19 to 4-20). In some circumstances,
such as infrequent visits to scenic vistas in natural or urban
environments, people are motivated specifically to take the opportunity
to view a valued scene and are likely to do so for many minutes to
hours to appreciate various aspects of the vista they choose to view.
However, the public has many more opportunities to notice visibility
conditions on a daily basis in settings associated with performing
daily routines (e.g., during commutes and while working, exercising, or
recreating outdoors). As noted in the Policy Assessment, information
regarding the fraction of the public that has only one or a few
opportunities to experience visibility during the day, or on the role
the duration of the observed visibility conditions has on wellbeing
effects associated with those visibility conditions, is not available
(U.S. EPA, 2011a, p. 4-20). However, it is possible that people with
limited opportunities to experience visibility conditions on a daily
basis would receive the entire impact of the day's visual air quality
based on the visibility conditions that occur during the short time
period when they can see it. Since this group could be affected on the
basis of observing visual air quality conditions for periods as short
as one hour or less, and because during each daylight hour there are
some people outdoors, commuting, or near windows, the Policy Assessment
judged that it would be appropriate to use the maximum hourly value of
PM light extinction during daylight hours for each day for purposes of
evaluating the adequacy of the current suite of secondary standards.
Other observers may have access to visibility conditions throughout the
day. For this group, it might be that an hour with poor or
``unacceptable'' visibility can be offset by one or more other hours
with clearer conditions. Therefore, the proposal acknowledged that it
might also be appropriate to consider a multi-hour daylight exposure
period.
v. Periods of Fog and Rain
    As discussed in section VI.C of the proposal, the EPA also
recognized that it is appropriate to give special treatment to periods
of fog and rain when considering whether current PM2.5
standards adequately protect public welfare from PM-related visibility
impairment. Visibility impairment occurs during periods with fog or
precipitation irrespective of the presence or absence of PM. Therefore,
it is logical that periods with naturally impaired visibility due to
fog or precipitation should not be treated as having PM-impaired
visibility. There are multiple ways to adjust visibility data to reduce
the effects of fog and precipitation. In the Visibility Assessment,
following the advice of CASAC, the EPA evaluated the effect of
excluding daylight hours for which relative humidity was greater than
90 percent from analyses in order to avoid precipitation and fog
confounding estimates of PM visibility impairment. For the 15 urban
areas included in the Visibility Assessment, the EPA found that a 90
percent relative humidity cutoff criterion was effective in that on
average less than 6 percent of the daylight hours were removed from
consideration, yet those hours had on average ten times the likelihood
of rain, six times the likelihood of snow/sleet, and 34 times the
likelihood of fog compared with hours with 90 percent or lower relative
humidity. In the Regional Haze program, the EPA utilizes monthly
average relative humidity values based on 10 years of climatological
data to reduce the effect of fog and precipitation. This approach
focuses on longer-term averages for each monitoring site and thereby
eliminates the effect of very high humidity conditions on visibility at
those locations.
b. Public Perception of Visibility Impairment
    As described in section VI.B.2 of the proposal, there are two main
types of studies that evaluate the public perception of urban
visibility impairment: urban visibility preference studies and urban
visibility valuation studies. As noted in the Integrated Science
Assessment, ``[b]oth types of studies are designed to evaluate
individuals' desire (or demand) for good visual air quality (VAQ) where
they live, using different metrics to evaluate demand. Urban visibility
preference studies examine individuals' demand by investigating what
amount of visibility degradation is unacceptable while economic studies
examine demand by investigating how much one would be willing to pay to
improve visibility'' (U.S. EPA, 2009a, p. 9-66). Because of the limited
number of new studies on urban visibility valuation, the Integrated
Science Assessment cites to the discussion in the 2004 Criteria
Document of the various methods one can use to determine the economic
valuation of changes in visibility, which include hedonic valuation,
contingent valuation and contingent choice, and travel cost.
    Contingent valuation studies are a type of stated preference study
that measures the strength of preferences and expresses that preference
in dollar values. Contingent valuation studies often include payment
vehicles that require respondents to consider implementation costs and
their ability to pay for visibility improvements in their responses.
This study design

aspect is critical because the EPA cannot consider implementations
costs in setting either primary or secondary NAAQS. Therefore in
considering the information available to help inform the standard-
setting process, the EPA has focused on the public perception studies
that do not embed consideration of implementation costs. Nonetheless,
the EPA recognizes that valuation studies do provide additional
evidence that the public is experiencing losses in welfare due to
visibility impairment.\153\ The public perception studies are described
in detail below.
---------------------------------------------------------------------------

    \153\ In the regulatory impact analysis (RIA) accompanying this
rulemaking, the EPA describes a revised approach to estimate urban
residential visibility benefits that applies the results of several
contingent valuation studies. The EPA is unable to apply the public
perception studies to estimate benefits because they do not provide
sufficient information on which to develop monetized benefits
estimates. Specifically, the public perception studies do not
provide preferences expressed in dollar values, even though they do
provide additional evidence that the benefits associated with
improving residential visibility are not zero. As previously noted
in this preamble, the RIA is done for informational purposes only,
and the proposed decisions on the NAAQS in this rulemaking are not
in any way based on consideration of the information or analyses in
the RIA.
---------------------------------------------------------------------------

    In order to identify levels of visibility impairment appropriate
for consideration in setting secondary PM NAAQS to protect the public
welfare, the Visibility Assessment comprehensively examined information
that was available in this review regarding people's stated preferences
regarding acceptable and unacceptable visual air quality.
    Light extinction is an atmospheric property that by itself does not
directly translate into a public welfare effect. Instead, light
extinction becomes meaningful in the context of the impact of
differences in visibility on the human observer. This has been studied
in terms of the acceptability or unacceptability expressed for the
visibility impact of a given level of light extinction by a human
observer. The perception of the visibility impact of a given level of
light extinction occurs in conjunction with the associated
characteristics and lighting conditions of the viewed scene.\154\ Thus,
a given level of light extinction may be perceived differently by
observers looking at different scenes or the same scene with different
lighting characteristics. Likewise, different observers looking at the
same scene with the same lighting may have different preferences
regarding the associated visual air quality. When scene and lighting
characteristics are held constant, the perceived appearance of a scene
(i.e., how well the scenic features can be seen and the amount of
visible haze) depends only on changes in light extinction. This has
been demonstrated using the WinHaze model (Molenar et al., 1994) that
uses image processing technology to apply user-specified changes in
light extinction values to the same base photograph with set scene and
lighting characteristics.
---------------------------------------------------------------------------

    \154\ By ``characteristics of the scene'' the EPA means the
distance(s) between the viewer and the object(s) of interest, the
shapes and colors of the objects, the contrast between objects and
the sky or other background, and the inherent interest of the
objects to the viewer. Distance is particularly important because at
a given value of light extinction, which is a property of air at a
given point(s) in space, more light is actually absorbed and
scattered when light passes through more air between the object and
the viewer.
---------------------------------------------------------------------------

    Much of what is known about the acceptability of levels of
visibility comes from survey studies in which participants were asked
questions about their preference or the value they place on various
visibility levels as displayed to them in scenic photographs and/or
WinHaze images with a range of known light extinction levels. The
Visibility Assessment (U.S. EPA, 2010b, chapter 2) reviewed the limited
number of urban visibility preference studies currently available
(i.e., four studies) to assess the light extinction levels judged by
the participant to have acceptable visibility for those particular
scenes.
    The reanalysis of urban preference studies conducted in the
Visibility Assessment for this review included three completed western
urban visibility preference survey studies plus a pair of smaller focus
studies designed to explore and further develop urban visibility survey
instruments. The three western studies included one in Denver, Colorado
(Ely et al., 1991), one in the lower Fraser River valley near
Vancouver, British Columbia (BC), Canada (Pryor, 1996), and one in
Phoenix, Arizona (BBC Research & Consulting, 2003). A pilot focus group
study was also conducted for Washington, DC (Abt Associates Inc.,
2001). In response to an EPA request for public comment on the Scope
and Methods Plan (74 FR 11580, March 18, 2009), comments were received
(Smith, 2009) about the results of a new focus group study of scenes
from Washington, DC, that had been conducted on subjects from both
Houston, Texas, and Washington, DC, using scenes, methods and
approaches similar to the method and approach employed in the EPA pilot
study (Smith and Howell, 2009). When taken together, these studies from
the four different urban areas included a total of 852 individuals,
with each individual responding to a series of questions while viewing
a set of images of various urban visual air quality conditions.
    The approaches used in the four studies were similar and were all
derived from the method first developed for the Denver urban visibility
study. In particular, the studies all used a similar group interview
type of survey to investigate the level of visibility impairment that
participants described as ``acceptable.'' In each preference study,
participants were initially given a set of ``warm up'' exercises to
familiarize them with how the scene in the photograph or image appears
under different VAQ conditions. The participants next were shown 25
randomly ordered photographs (images), and asked to rate each one based
on a scale of 1 (poor) to 7 (excellent). They were then shown the same
photographs or images again, in the same order, and asked to judge
whether each of the photographs (images) would violate what they would
consider to be an appropriate urban visibility standard (i.e. whether
the level of impairment was ``acceptable'' or ``unacceptable''). The
term ``acceptable'' was not defined, so that each person's response was
based on his/her own values and preferences for VAQ. However, when
answering this question, participants were instructed to consider the
following three factors: (1) The standard would be for their own urban
area, not a pristine national park area where the standards might be
stricter; (2) The level of an urban visibility standard violation
should be set at a VAQ level considered to be unreasonable,
objectionable, and unacceptable visually; and (3) Judgments of
standards violations should be based on visibility only, not on health
effects. While the results differed among the four urban areas, results
from a rating exercise show that within each preference study,
individual survey participants consistently distinguish between photos
or images representing different levels of light extinction, and that
more participants rate as acceptable images representing lower levels
of light extinction than they do images representing higher levels.
    Given the similarities in the approaches used, the EPA staff
concluded that it was reasonable to compare the results to identify
overall trends in the study findings and to conclude that this
comparison can usefully inform the selection of a range of levels for
use in further analyses. However, the staff also noted that variations
in the specific materials and methods used in each study introduce
uncertainties that should also be considered when interpreting the
results

of these comparisons. Key differences between the studies include the
following: (1) Scene characteristics; (2) image presentation methods
(e.g., projected slides of actual photos, projected images generated
using WinHaze (a significant technical advance in the method of
presenting visual air quality conditions), or use of a computer monitor
screen; (3) number of participants in each study; (4) participant
representativeness of the general population of the relevant
metropolitan area; and (5) specific wording used to frame the questions
used in the group interview process.
    In the Visibility Assessment, each study was evaluated separately
and figures developed to display the percentage of participants that
rated the visual air quality depicted in each photograph as
``acceptable.'' Ely et al. (1991) introduced a ``50% acceptability''
criterion analysis of the Denver preference study results. The 50
percent acceptability criterion is designed to identify the visual air
quality level (defined in terms of deciviews or light extinction) that
best divides the photographs into two groups: Those with a visual air
quality rated as acceptable by the majority of the participants, and
those rated not acceptable by the majority of participants. The
Visibility Assessment adopted this criterion as a useful index for
comparison between studies. The results of each analysis were then
combined graphically to allow for visual comparison. This information
was then carried forward into the Policy Assessment. Figure 5 presents
the graphical summary of the results of the studies in the four cities
and draws on results previously presented in Figures 2-3, 2-5, 2-7, and
2-11 of chapter 2 in the Visibility Assessment. Figure 5 also contains
lines at 20 dv and 30 dv that generally identify a range where the 50
percent acceptance criteria occur across all four of the urban
preference studies (U.S. EPA, 2011a, p. 4-24). Out of the 114 data
points shown in Figure 5, only one photograph (or image) with a visual
air quality below 20 dv was rated as acceptable by less than 50 percent
of the participants who rated that photograph.\155\ Similarly, only one
image with a visual air quality above 30 dv was rated acceptable by
more than 50 percent of the participants who viewed it.\156\
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    \155\ Only 47 percent of the British Columbia participants rated
a 19.2 dv photograph as acceptable.
    \156\ In the 2001 Washington, DC study, a 30.9 dv image was used
as a repeated slide. The first time it was shown 56 percent of the
participants rated it as acceptable, but only 11 percent rated it as
acceptable the second time it was shown. The same visual air quality
level was rated as acceptable by 4 percent of the participants in
the 2009 study (Test 1). All three points are shown in Figure 5.
    \157\ Top scale shows light extinction in inverse megameter
units; bottom scale in deciviews. Logit analysis estimated response
functions are shown as the color-coded curved lines for each of the
four urban areas.
[GRAPHIC] [TIFF OMITTED] TR15JA13.004

    As Figure 5 above shows, each urban area has a separate and unique
response curve that appears to indicate that it is distinct from the
others.\158\ These curves are the result of a logistical regression
analysis using a logit model of the greater than 19,000 ratings of haze
images as acceptable or unacceptable. The model results can be used to
estimate the visual air quality in terms of dv values where the
estimated response functions cross the 50 percent acceptability level,
as well as any alternative criteria levels. Selected examples of these
are shown in Table 4-

1 of the Policy Assessment (U.S. EPA, 2011a; U.S. EPA, 2010b, Table 2-
4). This table shows that the logit model results also support the
upper and lower ends of the range of 50th percentile acceptability
values (e.g., near 20 dv for Denver and near 30 dv for Washington, DC)
already identified in Figure 5.
---------------------------------------------------------------------------

    \158\ At present, data is only available for four urban areas,
as presented in Figure 5 and discussed throughout this section.
Additional research could help inform whether the range identified
by combining the results of the studies depicted in Figure 5 is more
broadly representative.
---------------------------------------------------------------------------

    Based on the composite results and the effective range of 50th
percentile acceptability across the four urban preference studies shown
in Figure 5 and Table 4-1 of the Policy Assessment, benchmark levels of
(total) light extinction were selected in a range from 20 dv to 30 dv
(75 to 200 Mm-1) \159\ for the purpose of provisionally
assessing whether visibility conditions would be considered acceptable
(i.e., less than the low end of the range), unacceptable (i.e., greater
than the high end of the range), or potentially acceptable (within the
range) based on the very limited public preference information. A
midpoint of 25 dv (120 Mm-1) was also selected for use in
the assessment. This level is also very near to the 50th percentile
criterion value from the Phoenix study (i.e., 24.2 dv), which is by far
the best of the four studies in terms of the fit of the data to the
response curve and the representativeness of study participants. Based
on the currently available information, the Policy Assessment concluded
that the use of 25 dv to represent the middle of the distribution of
results seemed well supported (U.S. EPA, 2011a, p. 4-25).
---------------------------------------------------------------------------

    \159\ These values were rounded from 74 Mm-1 and 201
Mm-1 to avoid an implication of greater precision than is
warranted. Note that the middle value of 25 dv when converted to
light extinction is 122 Mm-1 is rounded to 120
Mm-1 for the same reason. Assessments conducted for the
Visibility Assessment and the first and second drafts of the Policy
Assessment used the unrounded values. The Policy Assessment
considered the results of assessment using unrounded values to be
sufficiently representative of what would result if the rounded
values were used that it was unnecessary to redo the assessments.
That is why some tables and figures in the Policy Assessment
reflected the unrounded values.
---------------------------------------------------------------------------

    These three benchmark values provide a low, middle, and high set of
light extinction conditions that are used to provisionally define
daylight hours with urban haze conditions that have been judged
unacceptable by at least 50 percent of the participants in one or more
of these preference studies. As discussed above, PM light extinction is
taken to be (total) light extinction minus the Rayleigh scatter,\160\
such that the low, middle, and high levels correspond to PM light
extinction levels of about 65 Mm-1, 110 Mm-1, and
190 Mm-1. In the Visibility Assessment, these three light
extinction levels were called Candidate Protection Levels (CPLs). This
term was also used in the Policy Assessment and in the proposal notice.
It is important to note, however, that the degree of protection
provided by a secondary NAAQS is not determined solely by any one
component of the standard but by all the components (i.e., indicator,
averaging time, form, and level) being applied together. Therefore, the
Policy Assessment noted that the term CPL is meant only to indicate
target levels of visibility within a range that the EPA staff felt
appropriate for consideration that could, in conjunction with other
elements of the standard, including indicator, averaging time, and
form, potentially provide an appropriate degree of visibility
protection.
---------------------------------------------------------------------------

    \160\ Rayleigh scatter is light scattering by atmospheric gases
which is on average about 10 Mm-1.
---------------------------------------------------------------------------

    In characterizing the Policy Assessment's confidence in each CPL
and across the range, a number of issues were considered (U.S. EPA,
2011a, p. 4-26). Looking first at the two studies that define the upper
and lower bounds of the range, the Policy Assessment considered whether
they represent a true regional distinction in preferences for urban
visibility conditions between western and eastern U.S. There was little
information available to help evaluate the possibility of a regional
distinction especially given that there have been preference studies in
only one eastern urban area. Smith and Howell (2009) found little
difference in preference response to Washington, DC, haze photographs
between the study participants from Washington, DC, and those from
Houston, Texas.\161\ This provides some limited evidence that the value
judgment of the public in different areas of the country may not be an
important factor in explaining the differences in these study results.
---------------------------------------------------------------------------

    \161\ The first preference study using WinHaze images of a
scenic vista from Washington, DC was conducted in 2001 using
subjects who were residents of Washington, DC. More recently, Smith
and Howell (2009) interviewed additional subjects using the same
images and interview procedure. The additional subjects included
some residents of the Washington, DC area and some residents of the
Houston, Texas area.
---------------------------------------------------------------------------

    In further considering what factors could explain the observed
differences in preferences across the four urban areas, the Policy
Assessment noted that the urban scenes used in each study had different
characteristics (U.S. EPA, 2011a, p. 4-26). For example, each of the
western urban visibility preference study scenes included mountains in
the background while the single eastern urban study did not. It is also
true that each of the western scenes included objects at greater
distances from the camera location than in the eastern study. There is
no question that objects at a greater distance have a greater
sensitivity to perceived visibility changes as light extinction is
changed compared to otherwise similar scenes with objects at a shorter
range. This alone might explain the difference between the results of
the eastern study and those from the western urban studies. Having
scenes with the object of greatest intrinsic value nearer and hence
less sensitive in the eastern urban area compared with more distant
objects of greatest intrinsic value in the western urban areas could
further explain the difference in preference results.
    Another question considered was whether the high CPL value that is
based on the eastern preference results is likely to be generally
representative of urban areas that do not have associated mountains or
other valued objects visible in the distant background. Such areas
would include the middle of the country, many areas in the eastern
U.S., and possibly some areas in the western U.S. as well.\162\ Based
on the currently available information, the Policy Assessment concluded
that the high end of the CPL range (30 dv) is an appropriate level to
consider (U.S. EPA, 2011a, p. 4-27).
---------------------------------------------------------------------------

    \162\ In order to examine this issue, an effort would have to be
made to see if scenes in such areas could be found that would be
generally comparable to the western scenes (e.g., scenes that
contain valued scenic elements at more sensitive distances than that
used in the eastern study). This is only one of a family of issues
concerning how exposure to urban scenes of varying sensitivity
affects public perception for which no preference study information
is currently available.
---------------------------------------------------------------------------

    With respect to the low end of the range, the Policy Assessment
considered factors that might further refine its understanding of the
robustness of this level. The Policy Assessment concluded that
additional urban preference studies, especially with a greater variety
in types of scenes, could help evaluate whether the lower CPL value of
20 dv is generally supportable (U.S. EPA, 2011a, p. 4-27). Further, the
reason for the noisiness in data points around the curves apparent in
both the Denver and British Columbia results compared to the smoother
curve fit of Phoenix study results could be explored. One possible
explanation discussed in the Policy Assessment is that these older
studies use photographs taken at different times of day and on
different days to capture the range of light extinction levels needed
for the preference studies. In contrast, the use of WinHaze in the
Phoenix (and Washington, DC) study reduced variations that affect scene
appearance preference rating and avoided the uncertainty inherent in
using ambient measurements to

represent sight path-averaged light extinction values. Reducing these
sources of noisiness and uncertainty in the results of future studies
of sensitive urban scenes could provide more confidence in the
selection of a low CPL value.
    Based on the above considerations, and recognizing the limitations
in the currently available information, the Policy Assessment concluded
that it is reasonable to consider a range of CPL values including a
high value of 30 dv, a mid-range value of 25 dv, and a low value of 20
dv (U.S. EPA, 2011a, p. 4-27). Based on its review of the second draft
Policy Assessment, CASAC also supported this set of CPLs for
consideration by the EPA in this review. CASAC noted that these CPL
values were based on all available visibility preference data and that
they bound the study results as represented by the 50 percent
acceptability criteria. While recommending that further visibility
preference studies be conducted to reduce remaining uncertainties,\163\
CASAC concluded that this range of levels was ``adequately supported by
the evidence presented'' (Samet, 2010d, p. iii).
---------------------------------------------------------------------------

    \163\ ``CASAC has also identified needs for the next review
cycle in terms of further research on a number of topics related to
urban visibility; * * *. In particular, there is a need for the
Agency to conduct additional urban visibility preference studies
over a broad range of urban areas and viewing conditions, to further
evaluate and refine the range of visibility levels considered to be
acceptable in the current assessment.'' (Samet, 2010a)
---------------------------------------------------------------------------

c. Summary of Proposed Conclusions
i. Adequacy of the Current Standards for PM-Related Visibility
Impairment
    At the time of proposal, the Administrator provisionally concluded
that the current suite of secondary PM standards is 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, with a
focus primarily in urban areas. This proposed conclusion was based on
the information presented in the proposal with regard to the nature of
PM-related visibility impairment, the results of public perception
surveys on the acceptability of varying degrees of visibility
impairment in urban areas, analyses of the number of days that are
estimated to exceed a range of candidate protection levels under
conditions simulated to just meet the current standards, and the advice
of CASAC. This section summarizes key points from section VI.C of the
proposal regarding visibility under current conditions, the degree of
protection afforded by the current standards, and CASAC's advice
regarding the adequacy of the current standards.
    As discussed in section VI.C.1 of the proposal, to evaluate
visibility under current conditions the Visibility Assessment and
Policy Assessment estimated PM-related light extinction\164\ levels for
15 urban areas\165\ in the United States. Consistent with the emphasis
in this review on the hourly or multi-hour time periods that might
reasonably characterize the visibility effects experienced by various
segments of the population, these analyses focused on using maximum 1-
hour and 4-hour values of PM light extinction during daylight hours for
purposes of evaluating the degree of visibility impairment. Hourly
average PM-related light extinction was analyzed in terms of both
PM10 and PM2.5 light extinction. For reasons
discussed above, hours with relative humidity greater than 90 percent
were excluded from consideration. Recent visibility conditions in these
urban areas were then compared to the CPLs identified above. The
Visibility Assessment, which focused on PM10 light
extinction in 14 of the 15 urban areas during the 2005 to 2007 time
period,\166\ found that all 14 areas had daily maximum hourly
PM10 light extinction values estimated to exceed even the
highest CPL some of the days. Except for the two Texas areas and the
non-California western urban areas, all of the other urban areas were
estimated to have maximum hourly PM10 concentrations that
exceeded the high CPL on about 20 percent to over 60 percent of the
days. All 14 of the urban areas were estimated to have maximum hourly
PM10 concentrations that exceeded the low CPL on about 40
percent to over 90 percent of the days. In general, areas in the East
and in California tend to have a higher frequency of hourly visibility
conditions estimated to be above the high CPL compared with those in
the western U.S.
---------------------------------------------------------------------------

    \164\ PM-related light extinction is used here to refer to the
light extinction caused by PM regardless of particle size;
PM10 light extinction refers to the contribution by
particles sampled through an inlet with a particle size 50 percent
cutpoint of 10 [mu]m diameter; and PM2.5 light extinction
refers to the contribution by particles sampled through an inlet
with a particle size 50 percent cutpoint of 2.5 [mu]m diameter.
    \165\ The 15 urban areas are Tacoma, Fresno, Los Angeles,
Phoenix, Salt Lake City, Dallas, Houston, St. Louis, Birmingham,
Atlanta, Detroit, Pittsburgh, Baltimore, Philadelphia, and New York.
    \166\ Comments on the second draft Visibility Assessment from
those familiar with the monitoring sites in St. Louis indicated that
the site selected to provide continuous PM10 monitoring,
although less than a mile from the site of the PM2.5
data, was not representative of the urban area and resulted in
unrealistically large PM10-2.5 values. The EPA staff
considered these comments credible and set aside the St. Louis
assessment results for PM10 light extinction. Thus,
results and statements in the Policy Assessment regarding
PM10 light extinction applied to only the other 14 areas.
However, results regarding PM2.5 light extinction in most
cases applied to all 15 study areas because the St. Louis estimates
for PM2.5 light extinction were not affected by the
PM10 monitoring issue.
---------------------------------------------------------------------------

    The Policy Assessment repeated the Visibility Assessment-type
modeling based on PM2.5 light extinction and data from the
more recent 2007 to 2009 time period for the same 15 study areas
(including St. Louis). While the estimates of the percentage of daily
maximum hourly PM2.5 light extinction values exceeding the
CPLs were somewhat lower than for PM10 light extinction, the
patterns of these estimates across the study areas was found to be
similar. More specifically, except for the two Texas and the non-
California western urban areas, all of the other urban areas were
estimated to have maximum hourly PM2.5 concentrations that
exceeded the high CPL on about 10 percent up to about 50 percent of the
days based on PM2.5 light extinction, while all 15 areas
were estimated to have maximum hourly PM2.5 concentrations
that exceeded the low CPL on over 10 percent to over 90 percent of the
days.
    To evaluate how PM-related visibility would be affected by just
meeting the current suite of PM2.5 secondary standards, the
Policy Assessment applied the proportional rollback approach described
in section VI.C.2 of the proposal to all the PM2.5
monitoring sites in each study area.\167\ After adjusting for
composition, the Policy Assessment applied the original IMPROVE
algorithm to calculate the PM10 light extinction, using
``rolled back'' PM2.5 component concentrations, the current
conditions PM10-2.5 concentration for the day and hour, and
relative humidity for the day and hour.
---------------------------------------------------------------------------

    \167\ Phoenix and Salt Lake City met the current
PM2.5 NAAQS under current conditions and required no
reduction.
---------------------------------------------------------------------------

    In these analyses, the Policy Assessment estimated both
PM2.5 and PM10 light extinction in terms of both
daily maximum 1-hour average values and multi-hour (i.e., 4-hour)
average values for daylight hours. Figure 4-7 and Table 4-6 of the
Policy Assessment displayed the results of the rollback procedures as a
box and whisker plot of daily maximum daylight 1-hour PM2.5
light extinction and the percentage of daily maximum hourly
PM2.5 light extinction values estimated to exceed the CPLs
when just meeting the current

suite of PM2.5 secondary standards for all 15 areas
considered in the Visibility Assessment (including St. Louis)
(excluding hours with relative humidity greater than 90 percent). These
displays showed that the daily maximum 1-hour average PM2.5
light extinction values in all of the study areas other than the three
western non-California areas were estimated to exceed the high CPL on
about 8 percent up to over 30 percent of the days and to exceed the
middle CPL on about 30 percent up to about 70 percent of the days,
while all areas except Phoenix were estimated to have daily maximum 1-
hour average PM2.5 light extinction values that exceeded the
low CPL on over 15 percent to about 90 percent of the days. Figure 4-8
and Table 4-7 of the Policy Assessment present results based on daily
maximum 4-hour average values. These displays show that the daily
maximum 4-hour average PM2.5 light extinction values in all
of the study areas other than the three western non-California areas
and the two areas in Texas were estimated to exceed the high CPL on
about 4 percent up to over 15 percent of the days and to exceed the
middle CPL on about 15 percent up to about 45 percent of the days,
while all areas except Phoenix were estimated to have daily maximum 4-
hour average PM2.5 light extinction values that exceeded the
low CPL on over 10 percent to about 75 percent of the days. A similar
set of figures and tables were developed in terms of PM10
light extinction (U.S. EPA, 2011a, Figures 4-5 and 4-6, Tables 4-4 and
4-5).
    Taking the results of these analyses focusing on 1-hour and 4-hour
maximum light extinction values into account, the Policy Assessment
concluded that the available information in this review clearly called
into question the adequacy of the current suite of PM2.5
standards in the context of public welfare protection from visibility
impairment, primarily in urban areas, and supported consideration of
alternative standards to provide appropriate protection (U.S. EPA,
2011a, p. 4-39). This conclusion was based in part on the large
percentage of days, in many urban areas, that were estimated to have
maximum 1-hour or 4-hour light extinction values that exceed the range
of CPLs identified for consideration under simulations of conditions
that would just meet the current suite of PM2.5 secondary
standards. In particular, for air quality that was simulated to just
meet the current PM2.5 standards, greater than 10 percent of
the days were estimated to have peak light extinction values that
exceed the highest, least protective CPL of 30 dv in terms of
PM2.5 light extinction for 9 of the 15 urban areas, based on
1-hour average values, and would thus likely fail to meet a 90th
percentile-based standard at that level. For these areas, the percent
of days estimated to have maximum 1-hour values that exceed the highest
CPL ranged from over 10 percent to over 30 percent. Similarly, when the
middle CPL of 25 dv was considered, greater than 30 percent up to
approximately 70 percent of the days were estimated to have peak light
extinction that exceeded that CPL in terms of PM2.5 light
extinction, for 11 of the 15 urban areas, based on 1-hour average
values. Based on a 4-hour averaging time, 5 of the areas were estimated
to have at least 10 percent of the days with peak light extinction
exceeding the highest CPL in terms of PM2.5 light
extinction, and 8 of the areas were estimated to have at least 30
percent of the days with peak light extinction exceeding the middle CPL
in terms of PM2.5 light extinction. For the lowest CPL of 20
dv, the percentages of days with 4-hour maximum light extinction
estimated to exceed that CPL are even higher for all cases considered.
Based on all of the above, the Policy Assessment concluded that PM
light extinction estimated to be associated with just meeting the
current suite of PM2.5 secondary standards in many areas
across the country exceeded levels and percentages of days that could
reasonably be considered to be important from a public welfare
perspective (U.S. EPA, 2011a, p. 4-40).
    Further, the Policy Assessment concluded that use of the current
indicator of PM2.5 mass, in conjunction with the current 24-
hour and annual averaging times, is clearly called into question for a
national standard intended to protect public welfare from PM-related
visibility impairment (U.S. EPA, 2011a, p. 4-40). This is because such
a standard is inherently variable in the degree of protection provided
because of regional differences in relative humidity and species
composition of PM2.5, which are critical factors in the
relationship between the mix of fine particles in the ambient air and
the associated impairment of visibility. The Policy Assessment noted
that this concern was one of the important elements in the court's
decision to remand the PM2.5 secondary standards set in 2006
to the Agency.
    Thus, in addition to concluding that the available information
clearly calls into question the adequacy of the protection against PM-
related visibility impairment afforded by the current suite of
PM2.5 standards, the Policy Assessment also concluded that
it clearly calls into question the appropriateness of each of the
current standard elements: indicator, averaging time, form, and level
(U.S. EPA, 2011a, p. 4-40).
    After reviewing the information and analysis in the second draft
Policy Assessment, CASAC concluded that the ``currently available
information clearly calls into question the adequacy of the current
standards and that consideration should be given to revising the suite
of standards to provide increased public welfare protection'' (Samet,
2010d, p. iii). CASAC noted that the detailed estimates of hourly PM
light extinction associated with just meeting the current standards
``clearly demonstrate that current standards do not protect against
levels of visual air quality which have been judged to be unacceptable
in all of the available urban visibility preference studies.'' Further,
CASAC stated, with respect to the current suite of secondary
PM2.5 standards, that ``[T]he levels are too high, the
averaging times are too long, and the PM2.5 mass indicator
could be improved to correspond more closely to the light scattering
and absorption properties of suspended particles in the ambient air''
(Samet, 2010d, p. 9).
    After considering the available evidence and the advice of CASAC,
the Administrator concluded at the time of proposal that such
information did provide an appropriate basis to inform a conclusion as
to whether the current standards afford adequate protection against PM-
related visibility impairment in urban areas. The Administrator took
into account the information discussed above with regard to the nature
of PM-related visibility impairment, the results of public perception
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 are estimated to exceed a
range of candidate protection levels under conditions simulated to just
meet the current standards, and the advice of CASAC. She noted the
clear causal relationship between PM in the ambient air and impairment
of visibility, the evidence from the visibility preference studies, and
the rationale for determining a range of candidate protection levels
based on those studies. She also noted the relatively large number of
days when maximum 1-hour or 4-hour light extinction values were
estimated to exceed the three candidate protection levels, including
the highest level of 30 dv, under the current standards. While
recognizing the limitations in the available information on public

perceptions of the acceptability of varying degree of visibility
impairment and the information on the number of days estimated to
exceed the CPLs, she concluded that such information provided an
appropriate basis to inform a conclusion as to whether the current
standards provide adequate protection against PM-related visibility
impairment in urban areas. Based on these considerations, and placing
great importance on the advice of CASAC, the Administrator
provisionally concluded that the current standards are 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, with a
focus primarily in urban areas.
    Having reached this conclusion, the Administrator also stated at
the time of proposal that the current indicator of PM2.5
mass, in conjunction with the current 24-hour and annual averaging
times, is not well suited for a national standard intended to protect
public welfare from PM-related visibility impairment. As noted in the
proposal, the current standards do not incorporate information on the
concentrations of various species within the mix of ambient particles,
nor do they incorporate information on relative humidity, both of which
play a central role in determining the relationship between the mix of
PM in the ambient air and impairment of visibility. Such considerations
were reflected both in CASAC's advice to set a distinct secondary
standard that would more directly reflect the relationship between
ambient PM and visibility impairment and in the court's remand of the
current secondary PM2.5 standards. Based on the above
considerations, at the time of proposal the Administrator provisionally
concluded that the current secondary PM2.5 standards, taken
together, are neither sufficiently protective nor suitably structured
to provide an appropriate degree of public welfare protection from PM-
related visibility impairment, primarily in urban areas. This led the
EPA to consider alternative standards by looking at each of the
elements of the standards--indicator, averaging time, form, and level--
as discussed below.
ii. Indicator
    At the time of proposal, the EPA considered three alternative
indicators for a PM2.5 standard designed to protect against
visibility impairment: The current PM2.5 mass indicator;
directly measured PM2.5 light extinction; and calculated
PM2.5 light extinction. Directly measured PM2.5
light extinction is a measurement (or combination of measurements) of
the light absorption and scattering caused by PM2.5 under
ambient conditions. Calculated PM2.5 light extinction uses
the IMPROVE algorithm to calculate PM2.5 light extinction
using measured PM2.5 mass, speciated PM2.5 mass,
and measured relative humidity. The Policy Assessment evaluated each of
these alternatives, finally concluding that consideration should be
given to establishing a new calculated PM2.5 light
extinction indicator (U.S. EPA, 2011a, p. 4-51).
    As discussed in section VI.D.1 of the proposal, the Policy
Assessment concluded that consideration of the use of either directly
measured PM2.5 light extinction or calculated
PM2.5 light extinction as an indicator is justified because
light extinction is a physically meaningful measure of the
characteristic of ambient PM2.5 that is most relevant and
directly related to PM-related visibility effects (U.S. EPA, 2011a, p.
4-41). Further, as noted above, PM2.5 is the component of PM
responsible for most of the visibility impairment in most urban areas.
In these areas, the contribution of PM10-2.5 is a minor
contributor to visibility impairment most of the time. The Policy
Assessment also indicated that the available evidence demonstrated a
strong correspondence between calculated PM2.5 light
extinction and PM-related visibility impairment, as well as the
significant degree of variability in visibility protection across the
U.S. allowed by a PM2.5 mass indicator. The Policy
Assessment recognized that while in the future it would be appropriate
to consider a direct measurement of PM2.5 light extinction
it was not an appropriate option in this review because a suitable
specification of the equipment and associated performance verification
procedures cannot be developed in the time frame for this review.
(a) PM2.5 Mass
    In terms of utilizing a PM2.5 mass indicator, the
proposal noted that PM2.5 mass monitoring methods are in
widespread use, including the FRM involving the collection of periodic
(usually 1-day-in-6 or 1-day-in-3) 24-hour filter samples. However,
these routine monitoring activities do not include measurement of the
full water content of the ambient PM2.5 that contributes,
often significantly, to visibility impacts. Further, the
PM2.5 mass concentration monitors do not provide information
on the composition of the ambient PM2.5, which plays a
central role in the relationship between PM-related visibility
impairment and ambient PM2.5 mass concentrations. Additional
analyses discussed in the proposal that looked at the contribution of
PM2.5 to total PM-related light extinction (defined in terms
of hourly PM10 calculated light extinction) indicate that
there is a poor correlation between hourly PM10 light
extinction and hourly PM2.5 mass principally due to the
impact of the water content of the particles on light extinction, which
depends on both the composition of the PM2.5 and the ambient
relative humidity. Both composition and especially relative humidity
vary during a single day, as well as from day-to-day, at any site and
time of year. Also, there are systematic regional and seasonal
differences in the distribution of ambient humidity and
PM2.5 composition conditions that make it impossible to
select a PM2.5 concentration that generally would correspond
to the same PM-related light extinction levels across all areas of the
nation. Analyses discussed in the proposal quantify the projected
uneven protection that would result from the use of 1-hour average
PM2.5 mass as the indicator.
(b) Directly Measured PM2.5 Light Extinction
    PM light extinction has a nearly one-to-one relationship to light
extinction, unlike PM2.5 mass concentration. As explained
above, PM2.5 is the component responsible for the large
majority of PM light extinction in most places and times.
PM2.5 light extinction can be directly measured using
several instrumental methods, some of which have been used for decades
to routinely monitor the two components of PM2.5 light
extinction (light scattering and absorption) or to jointly measure both
as total light extinction (from which Rayleigh scattering is subtracted
to get PM2.5 light extinction). As noted at the time of
proposal, there are a number of advantages to direct measurements of
light extinction for use in a secondary standard relative to estimates
of PM2.5 light extinction calculated using PM2.5
mass and speciation data. These include greater accuracy of direct
measurements with shorter averaging times and overall greater
simplicity when compared to the need for measurements of multiple
parameters to calculate PM light extinction.
    In evaluating whether direct measurement of PM2.5 or
PM10 light extinction is appropriate to consider in the
context of this PM NAAQS review, the EPA solicited comment from the
Ambient Air Monitoring and Methods

Subcommittee (AAMMS) of CASAC. The CASAC AAMMS recommended that
consideration of direct measurement should be limited to
PM2.5 light extinction, and that although instruments
suitable for this purpose are commercially available at present,
research is expected to produce even better instruments in the near
term. The CASAC AAMMS advised against choosing any currently available
commercial instrument, or even a general measurement approach, as an
FRM because to do so could discourage development of other potentially
superior approaches. Instead, the CASAC AAMMS recommended that the EPA
develop performance-based approval criteria for direct measurement
methods in order to put all approaches on a level playing field.
    At the present time, the EPA has not undertaken to develop and test
such performance-base approval criteria. The EPA anticipates that if an
effort were begun it would take at least several years before such
criteria would be ready for regulatory use. Thus, the Policy Assessment
concluded that while in the future it would be appropriate to consider
a direct measurement of PM2.5 light extinction, or the sum
of separate measurements of light scattering and light absorption, as
the indicator for the secondary PM2.5 standard, this is not
an appropriate option in this review because a suitable specification
of the equipment or appropriate performance-based verification
procedures cannot be developed in the time frame for this review (U.S.
EPA, 2011a, p. 4-51, -52).
(c) Calculated PM2.5 Light Extinction
    For the reasons discussed above, the Policy Assessment concluded
that a calculated PM2.5 light extinction indicator would be
the preferred approach. PM2.5 light extinction can be
calculated from PM2.5 mass, combined with speciated
PM2.5 mass concentration data plus relative humidity data,
as is presently routinely done on a 24-hour average basis under the
Regional Haze Program using data from the rural IMPROVE monitoring
network. This same calculation procedure, using a 24-hour average
basis, could be used for a NAAQS focused on protecting against PM-
related visibility impairment primarily in urban areas. This approach
would use the type of data that is routinely collected from the urban
CSN \168\ in combination with monthly average relative humidity data
based on long-term climatological means as used in the Regional Haze
Program (U.S. EPA, 2011a, Appendix G, section G.2). The proposal
discussed the complex approach utilized in the Visibility Assessment
for calculating hourly PM2.5 light extinction \169\ and
discussed various simplified approaches for calculating these hourly
values that were analyzed in the Policy Assessment. The Policy
Assessment concluded that each of these simplified approaches provided
reasonably good estimates of PM2.5 light extinction and each
would be appropriate to consider as the indicator for a distinct hourly
or multi-hour secondary standard (U.S. EPA, 2011a, p. 4-48). The
proposal also recognized that the Policy Assessment identified a number
of variations on these simplified approaches that it would be
appropriate to consider, including:
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    \168\ About 200 sites in the CSN routinely measure 24-hour
average PM2.5 chemical components using filter-based
samplers and chemical analysis in a laboratory, on either a 1-day-
in-3 or 1-day-in-6 schedule (U.S. EPA, 2011a, Appendix B, section
B.1.3).
    \169\ As noted at the time of proposal, the sheer size of the
ambient air quality, meteorological, and chemical transport modeling
data files involved with the Visibility Assessment approach would
make it very difficult for state agencies or any interested party to
consistently apply such an approach on a routine basis for the
purpose of implementing a national standard defined in terms of the
Visibility Assessment approach.

    (1) The use of the split-component mass extinction efficiency
approach from the revised IMPROVE algorithm\170\
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    \170\ If the revised IMPROVE algorithm were used to define the
calculated PM2.5 mass-based indicator, it would not be
possible to algebraically reduce the revised algorithm to a two-
factor version as described above and in Appendix F of the Policy
Assessment for the simplified approaches. Instead, five component
fractions would be determined from each day of speciated sampling,
and then either applied to hourly measurements of PM2.5
mass on the same day or averaged across a month and then applied to
measurements of PM2.5 mass on each day of the month.
---------------------------------------------------------------------------

    (2) The use of more refined value(s) for the organic carbon
multiplier \171\
---------------------------------------------------------------------------

    \171\ An organic carbon (OC)-to-organic mass (OM) multiplier of
1.6 was used for the assessment, which was found to produce a value
of OM comparable to the one derived with the original, albeit more
complex, Visibility Assessment method.
---------------------------------------------------------------------------

    (3) The use of the reconstructed 24-hour PM2.5 mass
(i.e., the sum of the five PM2.5 components from
speciated monitoring) as a normalization value for the hourly
measurements from the PM2.5 instrument as a way of better
reflecting ambient nitrate concentrations
    (4) The use of historical monthly or seasonal, or regional,
speciation averages

    Overall, the analyses conducted for the Visibility Assessment and
Policy Assessment indicated that the use of a calculated
PM2.5 light extinction indicator would provide a much higher
degree of uniformity in terms of the degree of protection from
visibility impairment across the country than a PM2.5 mass
indicator, because a calculated PM2.5 light extinction
indicator would directly incorporate the effects of humidity and
PM2.5 composition differences between various regions.
Further, the proposal noted that the Policy Assessment concluded that
consideration could be given to defining a calculated PM2.5
light extinction indicator on either a 24-hour or a sub-daily basis
(U.S. EPA, 2011a, p. 4-52). However, the Policy Assessment noted that
approval of continuous FEM monitors has been based only on 24-hour
average, not hourly, PM2.5 mass. In addition, there are
mixed results of data quality assessments on a 24-hour basis for these
monitors, as well as the near absence of performance data for sub-daily
averaging periods. Thus, while it is possible to utilize data from
PM2.5 continuous FEMs on a 1-hour or multi-hour (e.g., 4-
hour) basis, these factors increase the uncertainty of utilizing
continuous methods to support 1-hour or 4-hour PM2.5 mass
measurements as an input to the light extinction calculation.
Therefore, as noted at the time of proposal, until issues regarding the
comparability of 24-hour PM2.5 mass values derived from
continuous FEMs and filter-based FRMs \172\ are resolved, there is
reason to be cautious about relying on a calculation procedure that
uses hourly PM2.5 mass values reported by continuous FEMs in
combination with speciated PM2.5 mass values from 24-hour
filter-based samplers.
---------------------------------------------------------------------------

    \172\ Filter-based FRMs are designed to adequately quantify the
amount of PM2.5 collected over 24-hours. They cannot be
presumed to be appropriate for quantifying average concentrations
over 1-hour or 4-hour periods.
---------------------------------------------------------------------------

(d) CASAC Advice
    In reviewing the second draft Policy Assessment, CASAC stated that
it ``overwhelmingly * * * would prefer the direct measurement of light
extinction,'' recognizing it as the property of the atmosphere that
most directly relates to visibility effects (Samet, 2010d, p. iii).
CASAC noted that ``[I]t has the advantage of relating directly to the
demonstrated harmful welfare effect of ambient PM on human visual
perception.'' However, CASAC also concluded that the calculated
PM2.5 light extinction indicator ``appears to be a
reasonable approach for estimating hourly light extinction'' (Samet,
2010d, p. 11). Further, based on CASAC's understanding of the time that
would be required to develop an FRM for this indicator, CASAC agreed
with the staff preference presented in the second draft Policy
Assessment for a calculated PM2.5 light extinction
indicator. CASAC noted that ``[I]ts 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).\173\
---------------------------------------------------------------------------

    \173\ In commenting on the second draft Policy Assessment, CASAC
did not have an opportunity to review the assessment of continuous
PM2.5 FEMs compared to collocated FRMs (Hanley and Reff,
2011) as presented and discussed in the final Policy Assessment
(U.S. EPA, 2011a, p. 4-50).
---------------------------------------------------------------------------

(e) Administrator's Proposed Conclusions on Indicator
    At the time of proposal, while agreeing with CASAC that a directly
measured PM light extinction indicator would provide the most direct
link between PM in the ambient air and PM-related light extinction, the
Administrator provisionally concluded that this was not an appropriate
option in this review because a suitable specification of currently
available equipment or performance-based verification procedures cannot
be developed in the time frame of this review. Taking all of the above
considerations and CASAC advice into account, the Administrator
provisionally concluded that a new 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), was the appropriate indicator to replace the current
PM2.5 mass indicator. Such an indicator, referred to as a
PM2.5 visibility index, would appropriately reflect the
relationship between ambient PM and PM-related light extinction, based
on the analyses discussed in the proposal and incorporation of factors
based on measured PM2.5 speciation concentrations and
relative humidity data. In addition, selection of this type of
indicator would address, in part, the issues raised in the court's
remand of the 2006 p.m.2.5 standards. The Administrator also
noted that 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.
    Based on these above considerations, the Administrator proposed to
set a distinct secondary standard for PM2.5 defined in terms
of a PM2.5 visibility index (i.e., a calculated
PM2.5 light extinction indicator, translated into the
deciview scale) to protect against PM-related visibility impairment
primarily in urban areas. The Administrator proposed that such an index
be based on the original IMPROVE algorithm in conjunction with monthly
average relative humidity data based on long-term climatological means
as used in the Regional Haze Program. The EPA solicited comment on all
aspects of the proposed indicator, especially:

    (1) The proposed use of a PM2.5 visibility index
rather than a PM10 visibility index which would include
an additional term for coarse particles;
    (2) Using the revised IMPROVE algorithm rather than the original
IMPROVE algorithm;
    (3) The use of alternative values for the organic carbon
multiplier in conjunction with either the original or revised
IMPROVE algorithm;
    (4) The use of historical monthly, seasonal, or regional
speciation averages;
    (5) Alternative approaches to determining relative humidity; and
    (6) Simplified approaches to generating hourly PM2.5
light extinction values for purposes of calculating an hourly or
multi-hour indicator.

iii. Averaging Times
    In this review, as discussed in section VI.D.2 of the proposal,
consideration of appropriate averaging times for use in conjunction
with a PM2.5 visibility index was informed by information
related to the nature of PM visibility effects and the nature of inputs
to the calculation of PM2.5 light extinction, as discussed
above. The EPA considered both sub-daily (1- and 4-hour averaging
times) and 24-hour averaging times. In considering sub-daily averaging
times, the EPA has also considered what diurnal periods and ambient
relative humidity conditions would be appropriate to consider in
conjunction with such an averaging time.
    As an initial matter, the Policy Assessment considered sub-daily
averaging times. Taking into account what is known from available
studies concerning how quickly people experience and judge visibility
conditions, the possibility that some fraction of the public
experiences infrequent or short periods of exposure to ambient
visibility conditions, and the typical rate of change of the path-
averaged PM light extinction over urban areas, the initial analyses
conducted as part of the Visibility Assessment focused on a 1-hour
averaging time. In its review of the first draft Policy Assessment,
CASAC agreed that a 1-hour averaging time would be appropriate to
consider, noting that PM effects on visibility can vary widely and
rapidly over the course of a day and such changes are almost
instantaneously perceptible to human observers (Samet, 2010c, p. 19).
The Policy Assessment noted that this view related specifically to a
standard defined in terms of a directly measured PM light extinction
indicator, in that CASAC also noted that a 1-hour averaging time is
well within the instrument response times of the various currently
available and developing optical monitoring methods.
    However, CASAC also advised that if a PM2.5 mass
indicator were to be used, it would be appropriate to consider
``somewhat longer averaging times--2 to 4 hours--to assure a more
stable instrumental response'' (Samet, 2010c, p. 19). In considering
this advice, the Policy Assessment concluded that since a calculated
PM2.5 light extinction indicator relies in part on measured
PM2.5 mass, it would be appropriate to consider a multi-hour
averaging time on the order of a few hours (e.g. 4-hours). A multi-hour
averaging time might reasonably characterize the visibility effects
experienced by the segment of the population who have access to
visibility conditions often or continuously throughout the day. For
this segment of the population, it may be that their perception of
visual air quality reflects some degree of offsetting an hour with poor
visual air quality with one or more hours of clearer visual conditions.
Further, the Policy Assessment recognized that a multi-hour averaging
time would have the effect of averaging away peak hourly visibility
impairment, which can change significantly from one hour to the next
(U.S. EPA, 2011a, p. 4-53; U.S. EPA, 2010b, Figure 3-12).
    In considering either 1-hour or multi-hour averaging times, the
Policy Assessment recognized that no data are available with regard to
how the duration and variation of time a person spends outdoors during
the daytime impacts his or her judgment of the acceptability of
different degrees of visibility impairment. As a consequence, it is not
clear to what degree, if at all, the protection levels found to be
acceptable in the public preference studies would change for a multi-
hour averaging time as compared to a 1-hour averaging time. Thus, the
Policy Assessment concluded that it is appropriate to consider a 1-hour
or multi-hour (e.g., 4-hour) averaging time as the basis for a sub-
daily standard defined in terms of a calculated PM2.5 light
extinction indicator (U.S. EPA, 2011a, p. 4-53).
    In addition, as discussed above, some data quality uncertainties
have been observed with regard to hourly data collected by FEMs.
Specifically, as part of the review of data from all continuous FEM
PM2.5 instruments operating at state/local monitoring sites,
the Policy Assessment noted that the occurrence of questionable
outliers in 1-

hour data submitted to AQS from continuous FEM PM2.5
instruments had been observed at some of these sites (Evangelista,
2011). Some of these outliers were questionable simply by virtue of
their extreme magnitude, as high as 985 [mu]g/m\3\, whereas other
values were questionable because they were isolated to single hours
with much lower values before and after, a pattern that is much less
plausible than if the high concentrations were more sustained.\174\ The
Policy Assessment noted that any current data quality problems might be
resolved in the normal course of monitoring program evolution as
operators become more adept at instrument operation and maintenance and
data validation or by improving the approval criteria and testing
requirements for continuous instruments. Regardless, the Policy
Assessment noted that multi-hour averaging of FEM data could serve to
reduce the effects of such outliers relative to the use of a 1-hour
averaging time.
---------------------------------------------------------------------------

    \174\ Similarly questionable hourly data were not observed in
the 2005 to 2007 continuous PM2.5 data used in the
Visibility Assessment, all of which came from early-generation
continuous instruments that had not been approved as FEMs. However,
only 15 sites and instruments were involved in the Visibility
Assessment analyses, versus about 180 currently operating FEM
instruments submitting data to AQS. Therefore, there were more
opportunities for very infrequent measurement errors to be observed
in the larger FEM data set.
---------------------------------------------------------------------------

    The Policy Assessment noted that there are significant reasons to
consider using PM2.5 light extinction calculated on a 24-
hour basis to reduce the various data quality concerns described above
with respect to relying on continuous PM2.5 monitoring data.
However, the Policy Assessment recognized that 24 hours is far longer
than the hourly or multi-hour time periods that might reasonably
characterize the visibility effects experienced by various segments of
the population, including both those who do and do not have access to
visibility conditions often or continuously throughout the day. Thus,
the Policy Assessment concluded that the appropriateness of considering
a 24-hour averaging time would depend upon the extent to which PM-
related light extinction calculated on a 24-hour average basis would be
a reasonable and appropriate surrogate for PM-related light extinction
calculated on a sub-daily basis.
    To examine this relationship, the EPA conducted comparative
analyses of 24-hour and 4-hour averaging times in conjunction with a
calculated PM2.5 indicator. For these analyses, 4-hour
average PM2.5 light extinction was calculated based on using
the Visibility Assessment approach. The 24-hour average
PM2.5 light extinction was calculated using the original
IMPROVE algorithm and long-term relative humidity conditions to
calculate PM2.5 light extinction. Based on these
analyses,\175\ which are presented and discussed in Appendix G of the
Policy Assessment, scatter plots comparing 24-hour and 4-hour
calculated PM2.5 light extinction were constructed for each
of the 15 cities included in the Visibility Assessment and for all 15
cities pooled together (U.S. EPA, 2011a, Figures G-4 and G-5). Though
there was some scatter around the regression line for each city because
the calculated 4-hour light extinction values included day-specific and
hour-specific influences that are not captured by the simpler 24-hour
approach, these analyses generally showed good correlation between 24-
hour and 4-hour average PM2.5 light extinction, as evidenced
by reasonably high city-specific and pooled R\2\ values, generally in
the range of over 0.6 to over 0.8.\176\ This suggested that
PM2.5 light extinction calculated on a 24-hour basis is a
reasonable and appropriate surrogate to PM2.5 light
extinction calculated on a sub-daily basis.
---------------------------------------------------------------------------

    \175\ These analyses are also based on the use of a 90th
percentile form, averaged over 3 years, as discussed below in
section VI.D.3 and in section 4.3.3 of the Policy Assessment (U.S.
EPA, 2011a).
    \176\ The EPA staff noted that the R\2\ value (0.44) for Houston
was notably lower than for the other cities.
---------------------------------------------------------------------------

    Taking the above considerations and CASAC's advice into account,
the Policy Assessment concluded that it would be appropriate to
consider a 24-hour averaging time, in conjunction with a calculated
PM2.5 light extinction indicator and an appropriately
specified standard level, as discussed below. By using site-specific
daily data on PM2.5 composition and site-specific long-term
relative humidity conditions, this 24-hour average indicator would
provide more consistent protection from PM2.5-related
visibility impairment than would a secondary PM2.5 NAAQS
based only on 24-hour or annual average PM2.5 mass. In
particular, this approach would account for the systematic difference
in humidity conditions between most eastern states and most western
states. The Policy Assessment also concluded that it would also be
appropriate to consider a multi-hour, sub-daily averaging time, for
example a period of 4 hours, in conjunction with a calculated
PM2.5 light extinction indicator and with further
consideration of the data quality issues discussed above. Such an
averaging time, to the extent that data quality issues can be
appropriately addressed, would be more directly related to the short-
term nature of the perception of visibility impairment, short-term
variability in PM-related visual air quality, and the short-term nature
(hourly to multiple hours) of relevant exposure periods for segments of
the viewing public. Such an averaging time would still result in an
indicator that is less sensitive than a 1-hour averaging time to short-
term instrument variability with respect to PM2.5 mass
measurement. In conjunction with consideration of a multi-hour, sub-
daily averaging time, the Policy Assessment concluded that
consideration should be given to including daylight hours only and to
applying a relative humidity screen of approximately 90 percent to
remove hours in which fog or precipitation is much more likely to
contribute to the observed visibility impairment (U.S. EPA, 2011a, p.
4-58). Recognizing that a 1-hour averaging time would be even more
sensitive to data quality issues, including short-term variability in
hourly data from currently available continuous monitoring methods, the
Policy Assessment concluded that it would not be appropriate to
consider a 1-hour averaging time in conjunction with a calculated
PM2.5 light extinction indicator in this review (U.S. EPA,
2011a, p. 4-58).
    As noted above, in its review of the first draft Policy Assessment,
CASAC concluded that PM effects on visibility can vary widely and
rapidly over the course of a day and such changes are almost
instantaneously perceptible to human observers (Samet, 2010c, p. 19).
Based in part on this consideration, CASAC agreed that a 1-hour
averaging time would be appropriate to consider in conjunction with a
directly measured PM light extinction indicator, noting that a 1-hour
averaging time is well within the instrument response times of the
various currently available and developing optical monitoring methods.
At that time, CASAC also advised that if a PM2.5 mass
indicator were to be used, it would be appropriate to consider
``somewhat longer averaging times--2- to 4-hours--to assure a more
stable instrumental response'' (Samet, 2010c, p. 19). Thus, CASAC's
advice on averaging times that would be appropriate for consideration
was predicated in part on the capabilities of monitoring methods that
were available for the alternative indicators discussed in the draft
Policy Assessment. CASAC's views on a multi-hour averaging time would
also apply to the calculated PM2.5 light extinction
indicator since hourly PM2.5 mass measurements are also
required for this

indicator when calculated on a sub-daily basis.
    It is important to note that at the time it provided advice on
suitable averaging times, CASAC did not have the benefit of EPA's
subsequent assessment of the data quality issues associated with the
use of continuous FEMs as the basis for hourly PM2.5 mass
measurements. Furthermore, since CASAC only commented on the first and
second drafts of the Policy Assessment, neither of which included
discussion of a calculated PM2.5 indicator based on a 24-
hour averaging time, CASAC did not have a basis to offer advice
regarding a 24-hour averaging time. In addition, the 24-hour averaging
time is not based on consideration of 24-hours as a relevant exposure
period, but rather as a surrogate for a sub-daily period of 4 hours,
which is consistent with CASAC's advice concerning an averaging time
associated with the use of a PM2.5 mass indicator.
    Taking into account the information discussed above with regard to
analyses and conclusions presented in the final Policy Assessment the
Administrator recognized that hourly or sub-daily, multi-hour averaging
times, within daylight hours and excluding hours with relative humidity
above approximately 90 percent, 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. On the other hand, she recognized that
data quality uncertainties have recently been associated 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, while the
Administrator recognized the desirability of a sub-daily averaging
time, she had strong reservations about proposing to set a standard at
this time in terms of a sub-daily averaging time.
    In considering the information and analyses related to
consideration of a 24-hour averaging time, the Administrator recognized
that the Policy Assessment concluded that PM2.5 light
extinction calculated on a 24-hour averaging basis is a reasonable and
appropriate surrogate for sub-daily PM2.5 light extinction
calculated on a 4-hour average basis. In light of this finding and the
views of CASAC based on its reviews of the first and second drafts of
the Policy Assessment, the Administrator proposed to set a distinct
secondary standard with a 24-hour averaging time in conjunction with a
PM2.5 visibility index.
iv. Form
    As discussed in section VI.D.3 of the proposal, the ``form'' of a
standard defines the air quality statistic that is to be compared to
the level of the standard in determining whether the standard is
achieved. The form of the current 24-hour PM2.5 NAAQS is
such that the level of the standard is compared to the 3-year average
of the annual 98th percentile value of the measured indicator. The
purpose in averaging for three years is to provide stability from the
occasional effects of inter-annual meteorological variability that can
result in unusually high pollution levels for a particular year. The
use of a multi-year percentile form, among other things, makes the
standard less subject to the possibility of transient violations caused
by statistically unusual indicator values, thereby providing more
stability to the air quality management process that may enhance the
practical effectiveness of efforts to implement the NAAQS. Also, a
percentile form can be used to take into account the number of times an
exposure might occur as part of the judgment on protectiveness in
setting a NAAQS. For all of these reasons, the Policy Assessment
concluded it would be appropriate to consider defining the form of a
distinct secondary standard in terms of a 3-year average of a specified
percentile air quality statistic (U.S. EPA, 2011a, p. 4-58).
    The urban visibility preference studies that provided results
leading to the range of CPLs being considered in this review offer no
information that addresses the frequency of time that visibility levels
should be below those values. Given this lack of information, and
recognizing that the nature of the public welfare effect is one of
aesthetics and/or feelings of well-being, the Policy Assessment
concluded that it would not be appropriate to consider eliminating all
exposures above the level of the standard and that allowing some number
of hours/days with reduced visibility can reasonably be considered
(U.S. EPA, 2011a, p. 4-59). In the Visibility Assessment, 90th, 95th,
and 98th percentile forms were assessed for alternative PM light
extinction standards (U.S. EPA, 2010b, section 4.3.3). In considering
these alternative percentiles, the Policy Assessment noted that the
Regional Haze Program targets the 20 percent most impaired days for
improvements in visual air quality in Federal Class I areas. If
improvement in the 20 percent most impaired days were similarly judged
to be appropriate for protecting visual air quality in urban areas, 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 attain the standard. A focus on
improving the 20 percent most impaired days suggests that the 90th
percentile, which represents the median of the distribution of the 20
percent worst days, would be an appropriate form to consider.
Strategies that are implemented so that 90 percent of days have visual
air quality that is at or below the level of the standard would
reasonably be expected to lead to improvements in visual air quality
for the 20 percent most impaired days. Higher percentile values within
the range assessed could have the effect of limiting the occurrence of
days with peak PM-related light extinction in urban areas to a greater
degree. In considering the limited information available from the
public preference studies, the Policy Assessment found no basis to
conclude that it would be appropriate to consider limiting the
occurrence of days with peak PM-related light extinction in urban areas
to a greater degree.
    Another aspect of the form discussed in the proposal for a sub-
daily averaging time was whether to include all daylight hours or only
the maximum daily daylight hour(s). The maximum daily daylight 1-hour
or multi-hour form would be most directly protective of the welfare of
people who have limited, infrequent or intermittent exposure to
visibility during the day (e.g., during commutes), but spend most of
their time without an outdoor view. For such people a view of poor
visibility during their morning commute may represent their perception
of the day's visibility conditions until the next time they venture
outside during daylight, which may be hours later or perhaps the next
day. Other people have exposure to visibility conditions throughout the
day. For those people, it might be more appropriate to include every
daylight hour in assessing compliance with a standard, since it is more
likely that each daylight hour could affect their welfare.
    The Policy Assessment did not have information regarding the
fraction of the public that has only one or a few opportunities to
experience visibility during the day, nor did it have information on
the role the duration of the observed visibility conditions has on
wellbeing effects associated with those visibility conditions. However,
it is logical to conclude that people with limited opportunities to
experience visibility conditions on a daily basis

would experience the entire impact associated with visibility based on
their short-term exposure. The impact of visibility for those who have
access to visibility conditions often or continuously during the day
may be based on varying conditions throughout the day.
    In light of these considerations, the analyses conducted as part of
the Visibility Assessment analyses included both the maximum daily hour
and the all daylight hours forms. The Policy Assessment noted that
there is a close correspondence between the level of protection
afforded for all 15 urban areas by a maximum daily daylight 1-hour
approach using the 90th percentile form and an all daylight hours
approach combined with the 98th percentile form (U.S. EPA, 2010b,
section 4.1.4). This suggested that reductions in visibility impairment
required to meet either form of the standard would provide protection
to both fractions of the public (i.e., those with limited opportunities
and those with greater opportunities to view PM-related visibility
conditions). CASAC generally supported consideration of both types of
forms without expressing a preference based on its review of
information presented in the second draft Policy Assessment (Samet,
2010d, p. 11).
    In conjunction with a calculated PM2.5 light extinction
indicator and alternative 24-hour or sub-daily (e.g., 4-hour) averaging
times, based on the above considerations, and given the lack of
information on and the high degree of uncertainty over the impact on
public welfare of the number of days with visibility impairment over a
year, the Policy Assessment concluded that it would be appropriate to
give primary consideration to a 90th percentile form, averaged over
three years (U.S. EPA, 2011a, p. 4-60). Further, in the case of a
multi-hour, sub-daily alternative standard, the Policy Assessment
concluded that it would be appropriate to give primary consideration to
a form based on the maximum daily multi-hour period in conjunction with
the 90th percentile form (U.S. EPA, 2011a, p. 4-60). This sub-daily
form would be expected to provide appropriate protection for various
segments of the population, including those with limited opportunities
during a day and those with more extended opportunities over the
daylight hours to experience PM-related visual air quality.
    Though CASAC did not provide advice as to a specific form that
would be appropriate, it took note of the alternative forms considered
in that document and encouraged further analyses in the final Policy
Assessment that might help to clarify a basis for selecting from within
the range of forms identified. In considering the available information
and the conclusions in the final Policy Assessment in light of CASAC's
comments, at the time of proposal the Administrator concluded that a
90th percentile form, averaged over 3 years, is appropriate, and
proposed such a form in conjunction with a PM2.5 visibility
index and a 24-hour averaging time.
v. Level
    As discussed in section VI.D.4 of the proposal, in considering
appropriate levels for a 24-hour standard defined in terms of a
PM2.5 visibility index and an 90th percentile form, averaged
over 3 years, the Policy Assessment took into account the evidence- and
impact-based considerations discussed above, with a focus on the
results of public perception and attitude surveys related to the
acceptability of various levels of visual air quality and on the
important limitations in the design and scope of such available
studies. The Policy Assessment considered a variety of approaches for
identifying appropriate levels for such a standard, including utilizing
both adjusted and unadjusted CPLs derived from the visibility
preference studies.
    The Policy Assessment interpreted the results from the visibility
preferences studies conducted in four urban areas to define a range of
low, middle, and high CPLs for a sub-daily standard (e.g., 1- to 4-hour
averaging time) of 20, 25, and 30 dv, which are approximately
equivalent to PM2.5 light extinction of values of 65, 110,
and 190 Mm-1. The CASAC generally supported this approach,
noting that the ``EPA staff's approach for translating and presenting
the technical evidence and assessment results is logically conceived
and clearly presented. The 20-30 deciview range of levels chosen by EPA
staff as `Candidate Protection Levels' is adequately supported by the
evidence presented'' (Samet, 2010d, p. 11).\177\ The Policy Assessment
also recognized that to define a range of alternative levels that would
be appropriate to consider for a 24-hour calculated PM2.5
light extinction standard, it would be appropriate to consider whether
some adjustment to these CPLs is warranted since these preference
studies cannot be directly interpreted as applying to a 24-hour
exposure period (as noted above and in Policy Assessment section
4.3.1). Considerations related to such adjustments are more
specifically discussed below.
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    \177\ In 2009, the DC Circuit remanded the secondary
PM2.5 standards to the EPA in part because the Agency
failed to identify a target level of protection, even though EPA
staff and CASAC had identified a range of target levels of
protection that were appropriate for consideration. The court
determined that the Agency's failure to identify a target level of
protection as part of its final decision was contrary to the statute
and therefore unlawful, and that it deprived EPA's decision-making
of a reasoned basis. See 559F. 3d at 528-31; see also section VI.A.2
above and the Policy Assessment, section 4.1.2.
---------------------------------------------------------------------------

    In considering alternative levels for a sub-daily standard based
directly on the four preference study results, the Policy Assessment
noted that the individual low and high CPLs are in fact generally
reflective of the results from the Denver and Washington, DC studies
respectively, and the middle CPL is very near to the 50th percentile
criteria result from the Phoenix study, which was by far the best of
the studies, providing somewhat more support for the middle CPL.
    In considering the results from the four visibility preference
studies, the Policy Assessment recognized that currently available
studies are limited in that they were conducted in only four areas,
three in the U.S. and one in Canada. Further, the Policy Assessment
recognized that available studies provide no information on how the
duration and variation of time a person spends outdoors during the
daytime may impact their judgment of the acceptability of different
degrees of visibility impairment. As such, there is a relatively high
degree of uncertainty associated with using the results of these
studies to inform consideration of a national standard for any specific
averaging time. Nonetheless, the Policy Assessment concluded, as did
CASAC, that these studies are appropriate to use for this purpose (U.S.
EPA, 2011a, p. 4-61).
    Using approaches described in section VI.C.4 of the proposal, the
Policy Assessment explored various approaches to adjusting the CPLs
derived from the preference studies to inform alternative levels for a
24-hour standard. These various approaches, based on analyses of 2007-
2009 data from the 15 urban areas assessed in the Visibility
Assessment, focused on estimating CPLs for a 24-hour standard that
would provide generally equivalent protection as that provided by a 4-
hour standard with CPLs of 20, 25, and 30 dv. In conducting these
analyses, staff initially expected that the values of 24-hour average
PM2.5 light extinction and daily maximum daylight 4-hour
average PM2.5 light extinction would differ on any given
day, with the shorter term peak value generally being larger. This
would mean that, in concept, the level of a 24-hour standard should
include a

downward adjustment compared to the level of a 4-hour standard to
provide generally equivalent protection. As discussed more fully in
section G.5 of Appendix G and summarized below, this initial
expectation was not found to be the case across the range of CPLs
considered. In fact, as shown in Tables G-7 and G-8 of Appendix G and
in the corrected version of Table G-6 found in Frank et al.
(2012b),\178\ in considering estimates aggregated or averaged over all
15 cities as well as the range of city-specific estimates for the
various approaches considered, these analyses indicated that the
generally equivalent 24-hour levels ranged from somewhat below the 4-
hour level to just above the 4-hour level for each of the CPLs.\179\ In
all cases, the range of city-specific estimates of generally equivalent
24-hour levels included the 4-hour level for each of the CPLs of 20,
25, and 30 dv. As noted in the proposal, looking more broadly at these
results could support consideration of using the same CPL for a 24-hour
standard as for a 4-hour standard, recognizing that there is no one
approach that can most closely identify a generally equivalent 24-hour
standard level in each urban area for each CPL. The use of such an
unadjusted CPL for a 24-hour standard would place 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, so as to reduce the potential of setting a 24-hour
standard level that would require more than the intended degree of
protection in some areas.
---------------------------------------------------------------------------

    \178\ Note that the city-specific ranges shown in Table G-6,
Appendix G of the Policy Assessment are incorrectly stated for
Approaches C and E. Drawing from the more detailed and correct
results for Approaches C and E presented in Tables G-7 and G-8,
respectively, the city-specific ranges in Table G-6 for Approach C
should be 17-21 dv for the CPL of 20 dv; 21-25 dv for the CPL of 25
dv; and 24-30 dv for the CPL of 30 dv; the city-specific ranges in
Table G-6 for Approach E should be 17-21 dv for the CPL of 20 dv;
21-26 dv for the CPL of 25 dv; and 25-31 dv for the CPL of 30 dv. In
the EPA's reanalysis comparing 4- vs. 24-hour values, Frank et al.
(2012b) recreated Table G-6 using the correct values from Tables G-7
and G-8.
    \179\ As discussed in more detail in Appendix G of the Policy
Assessment, some days have higher values for 24-hour average light
extinction than for daily maximum 4-hour daylight light extinction,
and consequently an adjusted ``equivalent'' 24-hour CPL can be
greater than the original 4-hour CPL. This can happen for two
reasons. First, the use of monthly average historical RH data will
lead to cases in which the f(RH) values used for the calculation of
24-hour average light extinction are higher than all or some of the
four hourly values of f(RH) used to determine daily maximum 4-hour
daylight light extinction on the same day. Second, PM2.5
concentrations may be greater during non-daylight periods than
during daylight hours.
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    In considering the appropriate level of a secondary standard
focused on protection from PM-related urban visibility impairment based
on either a 24-hour or a multi-hour, sub-daily (e.g., 4-hour) averaging
time, the EPA has been mindful of the important limitations in the
available evidence from public preference studies. These uncertainties
and limitations are due in part to the small number of stated
preference studies available for this review; the relatively small
number of study participants and the extent to which the study
participants may not be representative of the broader study area
population in some of the studies; and the variations in the specific
materials and methods used in each study such as scene characteristics,
the range of VAQ levels presented to study participants, image
presentation methods and specific wording used to frame the questions
used in the group interviews. In addition the EPA has noted that the
scenic vistas available on a daily basis in many urban areas across the
country generally may 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, and that there is the
possibility that there could be regional differences in individual
preferences for VAQ.
    It is also important to note that as in past reviews, the EPA is
considering a national visibility standard in conjunction with the
Regional Haze Program as a means of achieving appropriate levels of
protection against PM-related visibility impairment in urban, non-
urban, and Federal Class I areas across the country. The EPA recognizes
that programs implemented to meet a national standard focused primarily
on the visibility problems in urban areas can be expected to improve
visual air quality in surrounding non-urban areas as well, as would
programs now being developed to address the requirements of the
Regional Haze Program established for protection of visual air quality
in Federal Class I areas. The EPA also believes that 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.
    The Policy Assessment concluded that it is appropriate to give
primary consideration to alternative standard levels toward the upper
end of the ranges identified above for 24-hour and sub-daily standards,
respectively (U.S. EPA, 2011a, p. 4-63). Thus, the Policy Assessment
concluded it is appropriate to consider the following alternative
levels: A level of 28 dv or somewhat below, down to 25 dv, for a
standard defined in terms of a calculated PM2.5 light
extinction indicator, a 90th percentile form, and a 24-hour averaging
time; and a standard level of 30 dv or somewhat below, down to 25 dv,
for a similar standard but with a 4-hour averaging time (U.S. EPA,
2011a, p. 4-63). The Policy Assessment judged that such standards would
provide appropriate protection against PM-related visibility impairment
primarily in urban areas. The Policy Assessment noted that CASAC
generally supported consideration of the 20-30 dv range as CPLs and,
more specifically, that support for consideration of the upper part of
the range of the CPLs derived from the public preference studies was
expressed by some CASAC Panel members during the public meeting on the
second draft Policy Assessment. The Policy Assessment concluded that
such a standard would be appropriate in conjunction with the Regional
Haze Program to achieve appropriate levels of protection against PM-
related visibility impairment in areas across the country (U.S. EPA,
2011a, p. 4-63).
    Based on the considerations discussed above and in section VI.D.4
of the proposal, and taking into account the advice of CASAC, at the
time of proposal the Administrator concluded that it would be
appropriate to establish a target level of protection--for a standard
defined in terms of a PM2.5 visibility index; a 90th
percentile form averaged over 3 years; and a 24-hour averaging time--
equivalent to the protection afforded by such a sub-daily (i.e., 4-
hour) standard at a level of 30 dv, which is the upper end of the range
of CPLs identified in the Policy Assessment and generally supported by
CASAC. More specifically, the Administrator provisionally concluded
that a 24-hour level of either 30 dv or 28 dv could be construed as
providing such a degree of protection, and that either level was
supported by the available information and was generally consistent
with the advice of CASAC. Thus, the EPA proposed two options for the
level of a new 24-hour standard (defined in terms of a PM2.5
visibility index and a 90th percentile form, averaged over 3 years) to
provide appropriate protection from PM-related visibility impairment:
Either 30 dv or 28 dv. As noted in the proposal, the option of setting
such a 24-hour standard 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.
Selecting a 24-hour standard level of 30 dv would reflect a judgment
that such substantial degrees of variability and uncertainty should be
reflected in a higher standard level than would be appropriate if the
underlying information were more consistent and certain. Alternatively,
the option of setting such a 24-hour standard at a level of 28 dv would
reflect placing more weight on statistical analyses of aggregated data