Some people have mostly intermittent opportunities on a daily basis
(e.g., during morning and/or afternoon commutes) to experience ambient
visibility conditions because they spend much of their time indoors
without access to windows. For such people a view of poor visual air
quality during their morning commute may provide their perception of
the day's visibility conditions until the next time they venture
outside during daylight hours later or perhaps the next day. Other
people have exposure to visibility conditions throughout the day,
conditions that may differ from hour to hour. A day with multiple hours
of visibility impairment would likely be judged as having a greater
impact on their wellbeing than a day with just one such hour followed
by clearer conditions.
    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 logical to conclude 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
judges 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.
This approach would recognize that at least some but not all of the
population of an area will actually be exposed to this worst hour and
that some of the people who are exposed to this worst hour may not have
an opportunity to observe clearer conditions in other hours if they
were to occur. Moreover, because visibility conditions and people's
daily activities on work/school days both tend to follow the same
diurnal pattern day after day, those who are exposed only to the worst
hour will tend to have this experience day after day.
    For another group of observers, those who have access to visibility
conditions often or continuously throughout the day, the impact of the
day's visibility conditions on their welfare may be based on the
varying visibility conditions they observe 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. Based on these considerations, the Policy Assessment judges
that it would also be appropriate to use a maximum multi-hour daylight
period for evaluating the adequacy of the current suite of secondary
standards (U.S. EPA, 2011a, p. 4-20).
    The above discussion is based on what people see, which is
determined by the extinction of light along the paths between observers
and the various objects they view. A related but separate issue is what
measurement period is relevant, if what will be measured is the light
extinction property or the PM concentration of the local air at a fixed
site. Light extinction conditions at a fixed site can change quickly
(i.e., in less than a minute). Sub-hourly variations in light
extinction determined at any point in the atmosphere are likely the
result of small-scale spatial pollution features (i.e., high
concentration plumes that have just been generated in the immediate
vicinity due to local sources or that have been transported by the wind
across that point). These small-scale pockets of air causing short
periods of higher light extinction at the fixed site likely do not
determine the visual effect for scenes with longer sight paths. In
contrast, atmospheric sight path-averaged light extinction which is
pertinent to visibility impacts generally changes more slowly (i.e.,
tens of minutes generally), because a larger air mass must be affected
by a broader set of emission sources or the larger air mass must be
replaced by a cleaner or dirtier air mass due to the wind operating
over time. At typical wind speeds found in U.S. cities, an hour
corresponds to a few tens of kilometers of air flowing past a point,
which is similar to sight path lengths of interest in urban areas.
Based on the above considerations, the Policy Assessment concludes
hourly average light extinction would generally be reasonably
representative of the net visibility effect of the spatial pattern of
light extinction levels, especially along site paths that generally
align with the wind direction (U.S. EPA, 2011a, p. 4-21).
2. Public Perception of Visibility Impairment
    As noted in the Integrated Science Assessment, 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 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.'' 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.\140\ The public perception studies are
described in detail below.
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    \140\ 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.\141\ 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.
---------------------------------------------------------------------------

    \141\ 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. Urban
visibility preference studies for four urban areas were reviewed in the
Visibility Assessment (U.S. EPA, 2010b, chapter 2) 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 includes 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 answered while
viewing a set of images of various urban visual air quality conditions.
    The approaches used in the four studies are similar and are 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 do images representing higher levels.
    Given the similarities in the approaches used, it is 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, 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: (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 the criterion as a useful index for
comparison between studies. The results of each individual 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.\142\ 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.\143\
---------------------------------------------------------------------------

    \142\ Only 47 percent of the British Columbia participants rated
a 19.2 dv photograph as acceptable.
    \143\ 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.
    \144\ 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] TP29JN12.004

    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. 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.
    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 by the Policy Assessment in a
range from 20 dv to 30 dv (75 to 200 Mm-1) \145\ 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). 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 least noisy preference results and the most
representative selection of participants. Based on the currently
available information, the Policy Assessment concludes 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).
---------------------------------------------------------------------------

    \145\ 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
considers 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 reflect
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% 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,\146\
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 continues to be used in
this 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 notes that the term CPL is meant only
to indicate target levels of visibility within a range that EPA staff
feels is appropriate for consideration that could, in conjunction with
other elements of the standard, including indicator, averaging time,
and form, provide an appropriate degree of visibility protection.
---------------------------------------------------------------------------

    \146\ 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 considers whether
they represent a true regional distinction in preferences for urban
visibility conditions between western and eastern U.S. There is 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.\147\ 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.
---------------------------------------------------------------------------

    \147\ 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 notes 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 and many areas in the eastern
U.S., and possibly some areas in the western U.S. as well. 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. Based on the
currently available information, the Policy Assessment concludes that
the high end of the CPL range (30 dv) is an appropriate level to
consider (U.S. EPA, 2011a, p. 4-27).
    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 concludes 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 concludes
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 supports this set of CPLs for
consideration by the EPA in this review. CASAC notes 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. CASAC concludes that this range of levels is
``adequately supported by the evidence presented'' (Samet, 2010d, p.
iii).

C. Adequacy of the Current Standards for PM-Related Visibility
Impairment

    As noted above, visibility impairment occurs during periods with
fog or precipitation irrespective of the presence or absence of PM.
While it is a popular notion that areas with many foggy or rainy days
are ``dreary'' places to live compared to areas with more sunny days
per year, the Policy Assessment has no basis for taking into account
how the occurrence of such days might modify the effect of pollution-
induced hazy days on public welfare. It is logical that periods with
naturally impaired visibility due to fog or precipitation should not be
treated as having PM-impaired visibility. Moreover, depending on the
specific indicator, averaging time, and measurement approach used for
the NAAQS, foggy conditions might result in measured or calculated
indicator values that are higher than the light extinction actually
caused by PM.\148\ Therefore, in order to avoid precipitation and fog
confounding estimates of PM visibility impairment, and as advised by
CASAC as part of its comments on the first draft Visibility Assessment,
the assessment of visibility conditions was restricted to daylight
hours with relative humidity less than or equal to 90 percent when
evaluating sub-daily alternative standards (U.S. EPA, 2010b, section
3.3.5, Appendix G).
---------------------------------------------------------------------------

    \148\ One example of an indicator and measurement approach for
which indicator values could be higher than true PM light extinction
as a result of fog would be a light extinction indicator measured in
part by an unheated nephelometer, which is an optical instrument for
measuring PM light scattering from an air sample as it flows through
a measurement chamber. Raindrops would be removed by the initial
size-selective inlet device, although some particles associated with
fog may be small enough that they might pass through the inlet and
enter the measurement chamber of the instrument. This would result
in a reported scattering coefficient that does not correspond to
true PM light extinction. Direct measurement of light extinction
using an open-path instrument would be even more affected by both
fog and precipitation.
---------------------------------------------------------------------------

    The EPA recognizes that not all periods with relative humidity
above 90 percent have fog or precipitation. Removing those hours from
consideration for a secondary PM standard would involve a tradeoff
between the benefits of not including many of the hours with
meteorological causes of visibility impacts and the loss of public
welfare protection of not including some hours with high relative
humidity without fog or precipitation, where the growth of hygroscopic
PM into large solution droplets results in enhanced PM visibility
impacts. For the 15 urban areas included in the assessment for which
meteorological data were obtained to allow an examination of the co-
occurrence of high relative humidity and fog or precipitation, a 90
percent relative humidity cutoff criterion is effective in that on
average less than 6 percent of the daylight hours are removed from
consideration, yet those hours have 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. Based on these findings, the Policy Assessment concludes that
it is appropriate that a sub-daily standard intended to protect against
PM-related visibility impairment would be defined in such a way as to
exclude hours with relative humidity greater than approximately 90
percent, regardless of measured values of light extinction or PM (U.S.
EPA, 2011a, p. 4-29).
1. Visibility Under Current Conditions
    Recent visibility conditions have been characterized in the Policy
Assessment in terms of PM-related light extinction \149\ levels for the
15 urban areas \150\ that were selected for analysis in the Visibility
Assessment. Hourly average PM-related light extinction was analyzed in
terms of both PM10 and PM2.5 light extinction.
These recent visibility conditions were then compared to the CPLs
identified above. From Figure 4-3 and Table 4-2 in the Policy
Assessment (U.S. EPA, 2010b, Figure 3-8 and Table 3-7, respectively) it
can be seen that among these 14 urban areas, those in the East and in
California tend to have a higher frequency of visibility conditions
estimated to be above the high CPL compared with those in the western
U.S. Both the figure and table are based on data from the 2005 to 2007
time period and exclude hours with relative humidity greater than 90
percent. These displays indicate that all 14 urban areas have daily
maximum hourly PM10 light extinction values that are
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 are estimated to exceed the high CPL from about
20 percent to over 60 percent of the days. It is also noted that all 14
of the urban areas are estimated to exceed the low CPL from about 40
percent to over 90 percent of the days.
---------------------------------------------------------------------------

    \149\ 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% 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% cutpoint of 2.5 [mu]m diameter.
    \150\ 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.
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, is not representative of the urban area and resulted in
unrealistically large PM10-2.5 values. The EPA staff
considers these comments credible and has set aside the St. Louis
assessment results for PM10 light extinction. Thus,
results and statements in this Policy Assessment regarding
PM10 light extinction apply to only the other 14 areas.
However, results regarding PM2.5 light extinction in most
cases apply 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 repeats 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), as described in Policy Assessment Appendix F.
Figure 4-4 and Table 4-3 in the Policy Assessment present the same type
of information as do Figure 4-3 and Table

4-2, respectively. While the estimates of the percentage of daily
maximum hourly PM2.5 light extinction values exceeding the
CPLs are somewhat lower than for PM10 light extinction, the
patterns of these estimates across the study areas are similar. More
specifically, except for the two Texas and the non-California western
urban areas, all of the other urban areas are estimated to exceed the
high CPL from about 10 percent up to about 50 percent of the days based
on PM2.5 light extinction, while all 15 areas are estimated
to exceed the low CPL from over 10 percent to over 90 percent of the
days.
2. Protection Afforded by the Current Standards
    The Policy Assessment also conducted analyses to assess the
likelihood that PM-related visibility impairment would exceed the
various CPLs for a scenario based on simulating just meeting the
current suite of PM2.5 secondary standards: 15 [mu]g/m\3\
annual average PM2.5 concentration and 35 [mu]g/m\3\ 24-hour
average PM2.5 concentration with a 98th percentile form,
averaged over three years. As described in the Visibility Assessment,
the steps needed to model meeting the current NAAQS involve explicit
consideration of changes in PM2.5 components. First, the
Policy Assessment applied proportional rollback to all the
PM2.5 monitoring sites in each study area, taking into
account policy-relevant background PM2.5 mass, to ``just
meet'' the current NAAQS scenario for the area as a whole, not just at
the visibility assessment study site. The quantitative health risk
assessment document (U.S. EPA, 2010a) describes this air quality roll-
back procedure in detail. The degree of rollback (i.e., the percentage
reduction in non-policy-relevant background PM2.5 mass) is
controlled by the highest annual or 24-hour design value, which in most
study areas is from a site other than the site used in this visibility
assessment.\151\ The relevant result from this analysis is the
percentage reduction in non-policy-relevant background PM2.5
mass needed to ``just meet'' the current NAAQS, for each study area.
These percentage reductions are shown in Table 4-4 of the Visibility
Assessment. It was noted that Phoenix and Salt Lake City meet the
current PM2.5 NAAQS under current conditions and require no
reduction. PM2.5 levels in these two cities were not
``rolled up.'' Second, for each day and hour for each PM2.5
component, the Policy Assessment subtracted the policy-relevant
background concentration from the current conditions concentration to
determine the non- policy-relevant background portion of the current
conditions concentration. Third, the Policy Assessment applied the same
percentage reduction from the first step to the non- policy-relevant
background portion of each of the five PM2.5 components and
added back the policy-relevant background portion of the component.
Finally, the Policy Assessment applied the original IMPROVE algorithm,
using the reduced PM2.5 component concentrations, the
current conditions PM10-2.5 concentration for the day and
hour, and relative humidity for the day and hour to calculate the
PM10 light extinction.
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    \151\ The selection of the site used to assess visibility was
driven by the need for several types of PM data, and for most study
areas the site with the highest annual or 24-hour design value did
not have the needed types of data.
---------------------------------------------------------------------------

    In these analyses, the Policy Assessment has 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 display 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 show 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 are estimated
to exceed the high CPL from about 8 percent up to over 30 percent of
the days and the middle CPL from about 30 percent up to about 70
percent of the days, while all areas except Phoenix are estimated to
exceed the low CPL from 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 are estimated to exceed the
high CPL from about 4 percent up to over 15 percent of the days and the
middle CPL from about 15 percent up to about 45 percent of the days,
while all areas except Phoenix are estimated to exceed the low CPL from
over 10 percent to about 75 percent of the days. A similar set of
figures and tables have been developed in terms of PM10
light extinction (U.S. EPA, 2011a, Figures 4-5 and 4-6, Tables 4-4 and
4-5).
    Taking into account the above considerations, the Policy Assessment
concludes that the available information in this review, as described
above and in the Visibility Assessment and Integrated Science
Assessment, clearly calls 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
supports consideration of alternative standards to provide appropriate
protection (U.S. EPA, 2011a, p. 4-39).
    This conclusion is based in part on the large percentage of days,
in many urban areas, 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 is simulated to just meet the current
PM2.5 standards, greater than 10 percent of the days are
estimated to 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 exceed the highest CPL ranges from over 10 percent
to over 30 percent. Similarly, when the middle CPL of 25 dv is
considered, greater than 30 percent up to approximately 70 percent of
the days are estimated to exceed 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 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 exceeding the middle
CPL in terms of PM2.5 light extinction. For the lowest CPL
of 20 dv, the percentages of days estimated to exceed that CPL are even
higher for all cases considered. Based on all of the above, the Policy
Assessment concludes 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 exceeds 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 concludes 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 confounded by
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 notes 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, as
discussed above in section 4.1.2.
    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 concludes 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).
3. CASAC Advice
    Based on its review of the second draft Policy Assessment, CASAC
concludes 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 notes 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 states, 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).
4. Administrator's Proposed Conclusions on the Adequacy of Current
Standards for PM-Related Visibility Impairment
    In considering whether the current suite of secondary
PM2.5 standards is requisite to protect the public welfare
against PM-related visibility impairment primarily in urban areas, the
Administrator has taken 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 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. As an initial matter, the Administrator
recognizes the clear causal relationship between PM in the ambient air
and impairment of visibility. She takes note of the evidence from the
visibility preference studies, and the rationale for determining a
range of candidate protection levels based on those studies. She notes
the relatively large number of days 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, the Administrator
concludes that such information provides 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 concludes 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 recognizes
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. She recognizes that 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 plays a
central role in determining the relationship between the mix of PM in
the ambient air and impairment of visibility. The Administrator notes
that such considerations were reflected in CASAC's advice to set a
distinct secondary standard that would more directly reflect the
relationship between ambient PM and visibility impairment. The
Administrator also notes that such considerations were reflected in the
court's remand of the current secondary PM2.5 standards.
Based on the above considerations, the Administrator provisionally
concludes that the current secondary PM2.5 standards, taken
together, are neither sufficiently protective nor are they suitably
structured to provide an appropriate degree of public welfare
protection from PM-related visibility impairment, primarily in urban
areas. Thus, the Administrator has considered alternative standards by
looking at each of the elements of the standards--indicator, averaging
time, form, and level--as discussed below.

D. Consideration of Alternative Standards for Visibility Impairment

1. Indicator
a. Alternative Indicators Considered in the Policy Assessment
    As described below, the Policy Assessment considers three
indicators: The current PM2.5 mass indicator and two
alternative indicators, including directly measured PM2.5
light extinction and calculated PM2.5 light extinction (U.S.
EPA, 2011a, section <IP_ADDRESS>).\152\ 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 speciated PM2.5 mass and measured relative
humidity.\153\
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    \152\ In the second draft Policy Assessment, the calculated
PM2.5 light extinction indicator was referred to as
speciated PM2.5 mass calculated light extinction.
    \153\ In 2009, the D.C. Circuit remanded the secondary
PM2.5 standards to the Agency in part because the EPA did
not address the problem that a PM2.5 mass-based standard
using a daily averaging time would be confounded by regional
differences in relative humidity, although EPA had acknowledged this
problem. The EPA notes that the light extinction indicators
considered in the Policy Assessment explicitly took into account
differences in relative humidity in areas across the country (U.S.
EPA, 2011a, section 4.3.1).
---------------------------------------------------------------------------

    The Policy Assessment concludes 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 characteristic 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, although
at some locations (U.S. EPA, 2010b, Figure 3-13 for Phoenix)
PM10-2.5 can be a major contributor to urban visibility
effects. Few urban areas conduct continuous PM10-2.5
monitoring. For example, among the 15 urban areas assessed in this
review, only four areas had collocated continuous PM10 data
allowing calculation of hourly PM10-2.5 data for 2005 to
2007. In the absence of PM10-2.5 air quality information
from a much larger number of urban areas across the country, it is not
possible at this time to know in how many urban areas
PM10-2.5 is a major contributor to urban visibility effects,
though it is reasonable to assume that other urban areas in the desert
southwestern region of the country may have conditions similar to the
conditions shown for Phoenix. PM10-2.5 is generally less
homogenous in urban areas than PM2.5, making it more
challenging to select sites that would adequately represent urban
visibility conditions. While it would be possible to include a
PM10-2.5 light extinction term in a calculated light
extinction indicator, as was done in the Visibility Assessment, there
is insufficient information available at this time to assess the impact
and effectiveness of such a refinement in providing public welfare
protection in areas across the country (U.S. EPA, 2011a, pp. 4-41 to 4-
42).
    The basis for considering each of these three indicators is
discussed below. The discussion also addresses monitoring data
requirements for directly measured PM2.5 light extinction
and for calculated PM2.5 light extinction. The following
discussion also takes into consideration different averaging times
since the combination of indicator and averaging time is relevant to
understanding the monitoring data requirements. Consideration of
alternative averaging times is addressed more specifically in section
VI.D.2 on averaging time.
i. PM2.5 Mass
    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. Blank and loaded filters
are weighed to determine 24-hour PM2.5 mass. Continuous
PM2.5 monitoring produces hourly average mass concentrations
and is conducted at about 900 locations. About 180 of these locations
employ newer model continuous instruments that have been approved by
EPA as FEMs, although the Policy Assessment notes that FEM approval has
been based only on 24-hour average, not hourly, PM2.5 mass.
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.\154\ 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.\155\
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    \154\ FRM filters are stabilized in a laboratory at fixed
temperature and relative humidity levels, which alters whatever
water content was present on the filter when removed from the
sampler. FEM instruments are designed to meet performance criteria
compared to FRM measurements, and accordingly typically manage
temperature and/or humidity at the point of measurement to levels
that are not the same as ambient conditions.
    \155\ As discussed below, 24-hour average PM2.5
chemical component mass is measured at about 200 CSN sites.
---------------------------------------------------------------------------

    The overall performance of 1-hour average PM2.5 mass as
a predictor of PM-related visibility impairment as indicated by
PM10 calculated light extinction can be seen in scatter
plots shown in Figure 4-9 of the Policy Assessment for two illustrative
urban areas, Pittsburgh and Philadelphia (Similar plots for all 14
urban areas that have estimates of PM10 light extinction are
in Appendix D, Figure D-2 of U.S. EPA, 2010b). These illustrative
examples demonstrate the large variations in hourly PM10
light extinction corresponding to any specific level of hourly
PM2.5 mass concentration as well as differences in the
statistical average relationships (depicted as the best fit lines)
between cities. This poor correlation between hourly PM10
light extinction and hourly PM2.5 mass is not due to any
great extent to the contribution of PM10-2.5 to light
extinction, but rather is 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. This
contributes to the noisiness of the data on the relationship 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.
    As part of the Visibility Assessment, an assessment was conducted
that estimated PM10 light extinction levels that may prevail
if areas were simulated to just meet a range of alternative secondary
standards based on hourly PM2.5 mass as the indicator.
Appendix E of the Policy Assessment contains the results of this
rollback-based assessment. This assessment quantifies the projected
uneven protection, noted qualitatively above, that would result from
the use of 1-hour average PM2.5 mass as the indicator.
ii. Directly Measured PM2.5 Light Extinction
    PM light extinction is the major contributor to light extinction,
which is the property of the atmosphere that is most directly related
to visibility effects. It differs from light extinction by the nearly
constant contributions for Rayleigh (or clean air) light scattering and
the minor contributions by NO2 light absorption. The net
result is that 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. Direct
measurement of PM2.5 light extinction can be accomplished
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). 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.
    As part of the Visibility Assessment, an assessment was conducted
that estimated PM10 light extinction levels that may prevail
in 14 urban study areas if the areas were simulated to just meet a
secondary standard based on directly measured hourly PM10
light extinction as the indicator (U.S. EPA, 2010b,

section 4.3).\156\ As would be expected, this assessment indicated that
a secondary standard based on a directly measured PM10 light
extinction indicator would provide the same percentage of days having
values above the level of the standard in each of the areas, with the
percentage being dependent on the statistical form of the standard. The
Policy Assessment considers this assessment reasonably informative for
a directly measured PM2.5 light extinction indicator as
well, because in most of the assessment study areas PM10
light extinction is dominated by PM2.5 light extinction.
---------------------------------------------------------------------------

    \156\ This assessment was conducted prior to staff's decision to
focus on PM2.5 light extinction indicators in the Policy
Assessment.
---------------------------------------------------------------------------

    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 produced a White Paper on
Particulate Matter (PM) Light Extinction Measurements (U.S. EPA,
2010g), and solicited comment on the White Paper from the Ambient Air
Monitoring and Methods Subcommittee (AAMMS) of CASAC. In its review of
the White Paper (Russell and Samet, 2010a), the CASAC AAMMS made the
recommendation that consideration of direct measurement should be
limited to PM2.5 light extinction as this can be
accomplished by a number of commercially available instruments and
because PM2.5 is generally responsible for most of the PM
visibility impairment in urban areas. The CASAC AAMMS indicated that it
is technically more challenging at this time to accurately measure the
PM10-2.5 component of light extinction.
    The CASAC AAMMS also commented on the capabilities of currently
available instruments, and expressed optimism regarding the near-term
development of even better instruments for such measurement than are
now commercially available. 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 EPA develop performance-based approval
criteria for direct measurement methods in order to put all approaches
on a level playing field. Such criteria would necessarily include
procedures and pass/fail requirements for demonstrating that the
performance criteria have been met. For example, instruments might be
required to demonstrate their performance in a wind tunnel, where the
concentration of PM2.5 components, and thus of
PM2.5 light extinction, could be controlled to known values.
It might also be possible to devise approval testing procedures based
on operation in ambient air, although knowing the true light extinction
level (without in effect treating some particular instrument as if it
were the FRM) would be more challenging. 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.
iii. Calculated PM2.5 Light Extinction
    As discussed above in section VI.B.1 above, PM2.5 light
extinction can be calculated from 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 also be
used for a NAAQS focused on protecting against PM-related visibility
impairment primarily in urban areas. This could use the type of data
that is routinely collected from the urban CSN \157\ in combination
with climatological relative humidity data as used in the Regional Haze
Program (U.S. EPA, 2011a, Appendix G, section G.2). This calculation
procedure, using the original IMPROVE light extinction equation
presented above in section VI.B.1 on a 24-hour basis (or the revised
IMPROVE equation), does not require PM2.5 mass concentration
measurements.
---------------------------------------------------------------------------

    \157\ 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).
---------------------------------------------------------------------------

    Alternatively, a conceptually similar approach could be applied in
urban areas on an hourly or multi-hour basis. Applying this conceptual
approach on a sub-daily basis would involve translating 24-hour
speciation data into hourly estimates of species concentrations, and
using 24-hour average species concentrations in conjunction with hourly
PM2.5 mass concentrations. This translation can be made
using more or less complex alternative approaches, as discussed below.
    The approach used to generate hourly PM10 light
extinction for the Visibility Assessment was a relatively more complex
method for implementing such a conceptual approach. It involved the use
of the original IMPROVE algorithm \158\ with estimates of hourly PM
2.5 components derived from day-specific 24-hour and hourly
measurements of PM 2.5 mass, 24-hour measurements of PM
2.5 composition, hourly measurements of PM 2.5
mass and (for some but not all study sites) hourly PM10-2.5
mass, along with hourly relative humidity information (U.S. EPA, 2010b,
section 3.3). The Visibility Assessment approach also involved the use
of output from a chemical transport modeling run to provide initial
estimates of diurnal profiles for PM2.5 components at
particular sites. The Visibility Assessment approach entailed numerous
and complex data processing steps to generate hourly PM2.5
composition information from these less time-resolved data, including
application of a mass-closure approach, referred to as the SANDWICH
approach \159\ (Frank, 2006), to adjust for nitrate retention
differences between FRM and CSN filters, which is a required step for
consistency with the IMPROVE algorithm and for estimating organic
carbonaceous material via mass balance.\160\ The EPA staff employed
complex custom software to do these data processing steps.
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    \158\ The original IMPROVE algorithm was selected for the
described analysis in the Visibility Assessment because of its
simplicity relative to the revised algorithm.
    \159\ Sulfate, adjusted nitrate, derived water, inferred
carbonaceous mass (SANDWICH) approach.
    \160\ Daily temperature data were also used as part of the
SANDWICH method.
---------------------------------------------------------------------------

    While the complexity of the approach used in the Visibility
Assessment was reasonable for assessment purposes at 15 urban areas,
the Policy Assessment recognizes that a relatively more simple approach
would be more straightforward and have greater transparency, and thus
should be considered for purposes of a national standard.\161\
Therefore, the Policy Assessment evaluated the degree to which simpler
approaches would correlate with the results of the highly complex
method used in the Visibility Assessment. This evaluation of two
specific simpler approaches (described briefly below and in more detail
in U.S. EPA, 2011a, Appendix F, especially Table F-1) demonstrated that
the PM2.5 portions of the PM10 light extinction

values developed for the Visibility Assessment can be well approximated
using the same IMPROVE algorithm applied to hourly PM2.5
composition values that were much more simply generated than with the
method used in the Visibility Assessment.
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    \161\ 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.
---------------------------------------------------------------------------

    The simplified approaches examined were aimed at calculating hourly
PM2.5 light extinction using the original IMPROVE algorithm
(see section VI.B.1.a. above) excluding the Rayleigh term for light
scattering by atmospheric gases and the term for
PM10-2.5.\162\ These approaches, including a description of
the sources of the data and steps required to determine calculated
PM2.5 light extinction for these simplified approaches, are
described in more detail in the Policy Assessment (U.S. EPA, 2011a, pp.
4-46 to 48, Appendix F, Table F-2). Also, Table F-1 of Appendix F of
the Policy Assessment compares and contrasts each of these approaches
with the Visibility Assessment approach and with each other.
---------------------------------------------------------------------------

    \162\ The original IMPROVE algorithm was the basis for the
approaches considered in the Policy Assessment to maintain
comparability to the estimates developed in the Visibility
Assessment. This allowed the effects of other simplifications
relative to the Visibility Assessment approach to be better
discerned.
---------------------------------------------------------------------------

    The hourly PM2.5 light extinction values generated by
using either simplified approach are comparable to those developed for
use in the Visibility Assessment as indicated by the regression
statistics for scatter plots of the paired data (i.e., the slopes of
the regression equation and the R\2\ values are near 1 as shown in U.S.
EPA, 2011a, Appendix F, Tables F-3 and F-4). Appendix F notes that both
approaches underestimate PM2.5 light extinction on some days
in a few study areas, which the Policy Assessment attributes to the
occurrence of very high nitrate concentrations and the failure of the
FRM-correlated/adjusted FEM instrument to report the entire nitrate
mass. Nevertheless, the Policy Assessment concludes that each of these
simplified approaches provides reasonably good estimates of
PM2.5 light extinction and each is appropriate to consider
as the indicator for a distinct hourly or multi-hour secondary standard
(U.S. EPA, 2011a, p. 4-48).
    In addition, the Policy Assessment notes that there are variations
of these simplified approaches that may also be appropriate to
consider. For example, some variations that may improve the correlation
with actual ambient light extinction in certain areas of the country
include the use of the split-component mass extinction efficiency
approach from the revised IMPROVE algorithm,\163\ the use of more
refined value(s) for the organic carbon multiplier (see U.S. EPA,
2011a, Appendix F),\164\ and 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. Other variations may
serve to simplify the calculation of PM2.5 light extinction
values, such as those suggested by CASAC for consideration, including
the use of historical monthly or seasonal speciation averages as well
as speciation estimates on a regional basis (Samet, 2010d, p. 11). Some
of these variations would also be appropriate to consider in
conjunction with a 24-hour average calculated PM2.5 light
extinction indicator, including the use of the revised IMPROVE
algorithm, the use of an alternative value for the organic carbon
multiplier (e.g., 1.6), and the use of historical monthly or seasonal,
or regional, speciation averages.
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    \163\ 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.
    \164\ 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.
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    As mentioned above, as part of the Visibility Assessment, an
assessment was conducted of PM10 light extinction levels
that would prevail if areas met a standard based on directly measured
hourly PM10 light extinction as the indicator. This
assessment indicated that a standard based on a directly measured
PM10 light extinction indicator would provide the same
percentage of days having indicator values above the level of the
standard across areas, with the percentage being dependent on the
statistical form of the standard. This assessment was based on the more
complex Visibility Assessment approach to estimating PM10
light extinction, rather than the simpler approaches for estimating
PM2.5 light extinction. Nevertheless, the generally close
correspondence between design values for PM2.5 light
extinction developed consistent with the Visibility Assessment approach
and design values based on the simplified approaches (U.S. EPA, 2011a,
Appendix F, Figure F-5) suggest that the findings regarding the
protection offered by alternative PM10 light extinction
standards using directly measured light extinction would also hold
quite well for standards based on the simplified indicators.\165\ Thus,
the Policy Assessment concludes that the use of a calculated
PM2.5 light extinction indicator would provide a much higher
degree of uniformity in terms of the visibility levels across the
country than is possible using PM2.5 mass as the indicator
(U.S. EPA, 2011a, p. 4-49). This is due to the fact that the
PM2.5 mass indicator does not account for the effects of
humidity and PM2.5 composition differences between various
regions, while a calculated PM2.5 light extinction indicator
directly incorporates those effects.
---------------------------------------------------------------------------

    \165\ The degree of emission reduction needed to meet a standard
is tightly tied to the degree to which the design value exceeds the
level of the standard.
---------------------------------------------------------------------------

    The inputs that would be necessary to use either simplified
approach to calculate a sub-daily PM2.5 light extinction
indicator (e.g., 1- or 4-hour averaging time) include PM2.5
chemical speciation, relative humidity, and hourly PM2.5
mass measurements. In defining a standard in terms of calculated light
extinction, the criteria for allowable protocols for these calculations
would need to be specified. It would be appropriate to base these
criteria on the protocols utilized in the IMPROVE \166\ and CSN
networks, as well as sampling and analysis protocols for ambient
relative humidity sensors, and approved FEM mass monitors for
PM2.5. Any approach to approving methods for use in
calculating a light extinction indicator should take advantage of the
existing inventory of monitoring and analysis methods.
---------------------------------------------------------------------------

    \166\ Several monitoring agencies utilize IMPROVE in urban areas
to meet their chemical speciation monitoring needs. These sites are
known as IMPROVE-protocol stations.
---------------------------------------------------------------------------

    The CSN measurements have a strong history of being reviewed by
CASAC technical committees, both during their initial deployment about
ten years ago (Mauderly 1999a,b) and during the more recent transition
to carbon sampling that is consistent with the IMPROVE protocols
(Henderson, 2005c). Because the methods for the CSN are well documented
in a nationally implemented Quality Assurance Project Plan (QAPP) and
accompanying standard operating procedures (SOPs), are validated
through independent performance testing, and are used to meet multiple
data objectives (e.g., source apportionment, trends, and as an input to
health studies), consideration

should be given to an approach that utilizes the existing methods as
the basis for criteria for allowable sampling and analysis protocols
for purposes of a calculated light extinction indicator. Such an
approach of basing criteria on the current CSN and IMPROVE methods
provides a nationally consistent way to provide the chemical species
data used in the light extinction calculation, while preserving the
opportunity for improved methods for measuring the chemical species.
For relative humidity, in conjunction with either hourly, multi-hour,
or 24-hour average calculated PM2.5 light extinction,
consideration should be given to simply using criteria based on
available relative humidity sensors such as already utilized by the
National Oceanic and Atmospheric Administration (NOAA) at routine
weather stations. These relative humidity sensors are already widely
used by a number of monitoring agencies and can be easily compared to
other relative humidity measurements.\167\ Finally, the simplified
approaches for a sub-daily averaging period depend on having values of
hourly PM2.5 mass, as discussed below.
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    \167\ For the purposes of using relative humidity measurements
to derive multi-hour or 24-hour average PM2.5 calculated
light extinction, the non-linear f(RH) enhancement factor should be
developed separately for each hour and then averaged over the
desired multi-hour period. This averaging approach is consistent
with derivation of climatological f(RH) factors used by the IMPROVE
program and for the Regional Haze rule.
---------------------------------------------------------------------------

    Since 2008, EPA has approved several PM2.5 continuous
mass monitoring methods as FEMs.\168\ These methods have several
advantages over filter-based FRMs, such as producing hourly data and
the ability to report air quality information in near real-time.
However, initial assessments of the data quality as operated by state
and local monitoring agencies have had mixed results. A recent
assessment of continuous FEMs and collocated FRMs conducted by EPA
staff (Hanley and Reff, 2011) found some sites and continuous FEM
instruments to have an acceptable degree of comparability of 24-hour
average PM2.5 mass values derived from continuous FEMs and
filter-based FRMs, while others had poor data quality that would not
meet current data quality objectives. The EPA is working closely with
the monitoring committee of the National Association of Clean Air
Agencies (NACAA), instrument manufacturers, and monitoring agencies to
document and communicate best practices on these methods to improve
quality and consistency of resulting data. It should be noted that
performance testing submitted to EPA for purposes of designating the
PM2.5 continuous methods as FEMs, and the recent assessment
of collocated FRMs and continuous FEMs, are both based on 24-hour
sample periods. Therefore, the EPA does not have similar performance
data for continuous PM2.5 FEMs for 1-hour or 4-hour
averaging periods, nor is there an accepted practice to generate
performance standards for these time periods.\169\ Until issues
regarding the comparability of 24-hour PM2.5 mass values
derived from continuous FEMs and filter-based FRMs 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
and speciated PM2.5 mass values from 24-hour filter-based
samplers. Section <IP_ADDRESS> of the Policy Assessment discusses another
reason for such caution, based on a preliminary assessment of hourly
data from continuous FEMs (U.S. EPA, 2011a, pp. 4-52 to 4-54).
---------------------------------------------------------------------------

    \168\ The EPA maintains a list of designated Reference and
Equivalent Methods on its Web site at: http://www.epa.gov/ttn/amtic/files/ambient/criteria/reference-equivalent-methods-list.pdf.
    \169\ 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.
---------------------------------------------------------------------------

    This section has addressed the types of measurements that would be
necessary to support a calculated PM2.5 light extinction
indicator for either 24-hour or sub-daily (e.g., 1-hour and 4-hour)
averaging periods. Considerations related specifically to each of these
alternative averaging times, in conjunction with a standard defined in
terms of a calculated PM2.5 light extinction indicator, are
discussed further in section 4.3.2 of the Policy Assessment.
iv. Conclusions in the Policy Assessment
    Taking the above considerations and CASAC's advice into account,
the Policy Assessment concludes that consideration should be given to
establishing a new calculated PM2.5 light extinction
indicator (U.S. EPA, 2011a, p. 4-51). This conclusion takes into
consideration the available evidence that demonstrates 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. While a secondary standard that uses a
PM2.5 mass indicator could be set to provide additional
protection from PM2.5-related visibility impairment, the
Policy Assessment concludes that the advantages of using a calculated
PM2.5 light extinction indicator make it the preferred
choice (U.S. EPA, 2011a, p. 4-51). In addition, the Policy Assessment
recognizes 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, it concludes
that 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).
    Further, the Policy Assessment concludes 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). In either case, it would be appropriate to base criteria for
allowable monitoring and analysis protocols to obtain PM2.5
speciation measurements on the protocols utilized in the IMPROVE and
CSN networks. Further, in the case of a calculated PM2.5
light extinction indicator defined on a sub-daily basis, it would be
appropriate to consider using the simplified approaches described, or
some variations on these approaches. In reaching this conclusion, as
discussed above, the Policy Assessment notes that while it is possible
to utilize data from PM2.5 continuous FEMs on a 1-hour or
multi-hour (e.g., 4-hour) basis, the mixed results of data quality
assessments on a 24-hour basis, as well as the near absence of
performance data for sub-daily averaging periods, increases 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.
b. CASAC Advice
    Based on its review of 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 concludes 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).\170\
---------------------------------------------------------------------------

    \170\ 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).
---------------------------------------------------------------------------

c. Administrator's Proposed Conclusions on Indicator
    In reaching a proposed conclusion on the appropriate indicator for
a standard intended to protect against PM-related visibility
impairment, as an initial matter, the Administrator concurs 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. However, she also recognizes that while instruments
currently exist that can directly measure PM2.5 light
extinction, they are not an appropriate option in this review because a
suitable specification of the equipment or performance-based
verification procedures cannot be developed in the time frame of this
review.
    Taking the above considerations and CASAC advice into account, the
Administrator provisionally concludes 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), is an appropriate indicator to replace the current
PM2.5 mass indicator. Such an indicator, referred to as a
PM2.5 visibility index, appropriately reflects the
relationship between ambient PM and PM-related light extinction, based
on the analyses discussed above and incorporation of factors based on
measured PM2.5 speciation concentrations and relative
humidity data. In addition, this addresses, in part, the issues raised
in the court's remand of the 2006 PM2.5 standards. The
Administrator also notes 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 the above considerations, the Administrator proposes 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 proposes that such an index
be based on the original IMPROVE algorithm in conjunction with
climatological relative humidity data as used in the Regional Haze
Program. A more detailed discussion of the steps involved in the
calculation of PM2.5 visibility index values is presented in
section VII.A.5 below.
    The Administrator solicits comment on all aspects of the proposed
indicator. In particular, the Administrator solicits comment on the
proposed use of a PM2.5 visibility index rather than a
PM10 visibility index which would include an additional term
for coarse particles. The Administrator also solicits comment on
alternatively using the revised IMPROVE algorithm rather than the
original IMPROVE algorithm the use of alternative values for the
organic carbon multiplier in conjunction with either the original or
revised IMPROVE algorithm; the use of historical monthly, seasonal, or
regional speciation averages; and on alternative approaches to
determining relative humidity, as discussed above. Further, in
conjunction with an hourly or multi-hour indicator, comment is
solicited on variations on the simplified approaches discussed above
and on other approaches that may be appropriate to consider for such an
indicator.
2. Averaging Times
a. Alternative Averaging Times
    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, as discussed above in
section VI.B.1 and in section 4.2.1 of the Policy Assessment, and the
nature of inputs to the calculation of PM2.5 light
extinction, as discussed above in section VI.D.1 and in section 4.3.1
of the Policy Assessment. Based on this information, the Policy
Assessment considered both sub-daily (1- and 4-hour averaging times)
and 24-hour averaging times, as discussed below. In considering sub-
daily averaging times, the Policy Assessment also addressed what
diurnal periods and ambient relative humidity conditions would be
appropriate to consider in conjunction with such an averaging time.
i. Sub-daily
    As an initial matter, in considering sub-daily averaging times, the
Policy Assessment took 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. While perception of
change in visibility can occur in less than a minute, meaningful
changes to path-averaged light extinction occur more slowly. As
discussed above and in section 4.2.1 of the Policy Assessment, one hour
is a short enough averaging period to result in indicator values that
are close to the maximum one- or few-minute visibility impact that an
observer could be exposed to within the hour. Further, a 1-hour
averaging time could reasonably characterize the visibility effects
experienced by the segment of the population that experiences
infrequent short-term exposures during peak visibility impairment
periods in each area/site. Based on the above considerations, the
initial analyses conducted in the Policy Assessment as part of the
Visibility Assessment to support consideration of alternative standards
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 notes
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 concludes
that since a calculated PM2.5 light extinction indicator
relies in part on measured PM2.5 mass, as discussed above
and in section 4.3.1 of the Policy Assessment, it is also appropriate
to consider a multi-hour averaging time in

conjunction with such an indicator (U.S. EPA, 2011a, p. 4-53).
    Thus, the Policy Assessment has considered multi-hour averaging
times, on the order of a few hours as illustrated by a 4-hour averaging
time. Such averaging times 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 recognizes 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 recognizes 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 concludes 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).
    Additionally, as part of the review of data from all continuous FEM
PM2.5 instruments operating at state/local monitoring sites,
as discussed above, the Policy Assessment notes that the occurrence of
questionable outliers in 1-hour data submitted to AQS from continuous
FEM PM2.5 instruments has been observed at some of these
sites (Evangelista, 2011). Some of these outliers are questionable
simply by virtue of their extreme magnitude, as high as 985 [micro]g/
m\3\, whereas other values are questionable because they are 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.\171\ The nature and frequency of questionable 1-hour FEM
data collected in the past two years are being investigated. At this
time, the Policy Assessment notes 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 notes 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.
---------------------------------------------------------------------------

    \171\ 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.
---------------------------------------------------------------------------

    In considering an appropriate diurnal period for use in conjunction
with a sub-daily averaging time, the Policy Assessment recognizes that
nighttime visibility impacts, described in the Integrated Science
Assessment (U.S. EPA, 2009a, section 9.2.2) are significantly different
from daytime impacts and are not sufficiently well understood to be
included at this time. As a result, consistent with CASAC advice
(Samet, 2010c, p. 4), the Policy Assessment concludes that it would be
appropriate to define a sub-daily standard in terms of only daylight
hours at this time (U.S. EPA, 2011a, p. 4-54). In the Visibility
Assessment, daylight hours were defined to be those morning hours
having no minutes prior to local sunrise and afternoon hours having no
minutes after local sunset. This definition ensures the exclusion of
periods of time where the sun is not the primary outdoor source of
light to illuminate scenic features.
    In considering the well-known interaction of PM with ambient
relative humidity conditions, the Policy Assessment recognizes that PM
is not generally the primary source of visibility impairment during
periods with fog or precipitation. In order to reduce the probability
that hours with a high degree of visibility impairment caused by fog or
precipitation are unintentionally used for purposes of determining
compliance with a standard, the Policy Assessment determined that a
relative humidity screen that excludes daylight hours with average
relative humidity above approximately 90 percent is appropriate (U.S.
EPA, 2001, pp. 4-54 to 4-55; see also U.S. EPA, 2010b, section 3.3.5,
Appendix G). For example, for the 15 urban areas \172\ included in the
Visibility Assessment, a 90 percent relative humidity cutoff criterion
proved effective in that on average less than 6 percent of the daylight
hours were removed from consideration, yet those same hours had on
average 10 times the likelihood of rain, 6 times the likelihood of
snow/sleet, and 34 times the likelihood of fog compared with hours with
90 percent or lower relative humidity. However, not all periods with
relative humidity above 90 percent have fog or precipitation. The
Policy Assessment recognizes that removing those hours from
consideration involves a tradeoff between the benefits of avoiding many
of the hours with meteorological causes of visibility impacts and not
counting some hours without fog or precipitation in which high humidity
levels (e.g., greater than 90 percent) lead to the growth of
hygroscopic PM to large solution droplets resulting in larger PM
visibility impacts.
---------------------------------------------------------------------------

    \172\ The 90 percent relative humidity cap assessment was
conducted as part of the Visibility Assessment on all 15 of the
urban areas, including St. Louis.
---------------------------------------------------------------------------

ii. 24-Hour
    As discussed in section 4.3.1 of the Policy Assessment and below,
there are significant reasons to consider using PM2.5 light
extinction calculated on a 24-hour basis to reduce the various data
quality concerns over relying on continuous PM2.5 monitoring
data. However, the Policy Assessment recognizes 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, as
discussed above and in section <IP_ADDRESS> of the Policy Assessment. Thus,
consideration of a 24-hour averaging time depends 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, as discussed below in this
section. Further, since a 24-hour averaging time combines daytime and
nighttime periods, the Policy Assessment recognizes that the public
preference studies do not directly provide a basis for identifying an
appropriate level of protection, in terms of 24-hour average light
extinction, based on judgments of acceptable daytime visual air quality
obtained in

those studies. Thus, consideration of a 24-hour averaging time also
depends upon developing an approach to translate the candidate levels
of protection derived from the public preference studies, which the
Policy Assessment has interpreted on an hourly or multi-hour basis, to
a candidate level of protection defined in terms of a 24-hour average
calculated light extinction, as discussed in section.VI.D.4 below.
    To determine whether 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, the
Policy Assessment performed comparative analyses of 24-hour and 4-hour
averaging times in conjunction with a calculated PM2.5
indicator.\173\ These analyses are presented and discussed in Appendix
G, section G.4 of the Policy Assessment. 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 calculations used the original
IMPROVE algorithm and long-term (1988 to 1997) average relative
humidity conditions, to calculate monthly average values of the
relative humidity term in the IMPROVE algorithm, consistent with the
approach used for the Regional Haze Program. Similar to the approach
used to assess a sub-daily visibility index discussed in section
VI.2.a.i above, these 1988-1997 humidity data are similarly screened to
remove the effect of high hourly relative humidity. In this case, any
relative humidity value great than 95 percent was treated as 95
percent. Because 10-years of hourly data were used to produce a single
humidity term for each month, the EPA believes that the resulting
monthly average of the humidity term is sufficient and appropriate to
reduce the effects of fog or precipitation. Based on these analyses,
scatter plots comparing 24-hour and 4-hour calculated PM2.5
light extinction are shown 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). It can be seen, as expected, that there is
some scatter around the regression line for each city, because the
calculated 4-hour light extinction includes day-specific and hour-
specific influences that are not captured by the simpler 24-hour
approach. The Policy Assessment notes that this scatter could be
reduced by the use of same-day hourly relative humidity data to
calculate a 24-hour average value of the relative humidity term in the
IMPROVE algorithm. In the Policy Assessment, scatter plots are also
shown for the annual 90th percentile values, based on data from 2007 to
2009, for 4-hour and 24-hour calculated PM2.5 light
extinction across all 15 cities (U.S. EPA, 2011a, Figure G-7) and for
the 3-year design values across all 15 cities (U.S. EPA, 2011a, Figure
G-8). These analyses 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.\174\
---------------------------------------------------------------------------

    \173\ 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).
    \174\ The EPA staff note that the R\2\ value (0.44) for Houston
was notably lower than for the other cities.
---------------------------------------------------------------------------

iii. Conclusions in the Policy Assessment
    Taking the above considerations and CASAC's advice into account,
the Policy Assessment concludes that it is appropriate to consider in
this review a 24-hour averaging time, in conjunction with a calculated
PM2.5 light extinction indicator and an appropriately
specified standard level (U.S. EPA, 2011a, p. 4-57). This conclusion
reflects the judgment that PM2.5 light extinction calculated
on a 24-hour basis is a reasonable and appropriate surrogate for sub-
daily PM2.5 light extinction calculated on a 4-hour average
basis. This conclusion is also predicated on consideration of a 24-hour
average standard level, as discussed below and in section 4.3.4 of the
Policy Assessment, that is appropriately translated from the CPLs
derived from the public preference studies, which the Policy Assessment
has interpreted as providing information on the acceptability of
daytime visual air quality over an hourly or multi-hour exposure
period.
    A 24-hour average calculated PM2.5 light extinction
indicator would avoid data quality uncertainties that have recently
been associated with currently available instruments for measurement of
hourly PM2.5 mass. The particular 24-hour indicator
considered by the Policy Assessment uses the original IMPROVE algorithm
and long-term relative humidity conditions to calculate
PM2.5 light extinction. 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.
    Further, the Policy Assessment concludes 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 that have been raised by the
recent EPA study of continuous FEMs (U.S. EPA, 2011a, p. 4-58). 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 concludes 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 concludes 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).
b. CASAC Advice
    As noted above, in its review of the first draft Policy Assessment,
CASAC concludes 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.
    In considering this advice, the Policy Assessment first notes that
CASAC did not have the benefit of EPA's recent assessment of the data
quality issues associated with the use of continuous FEMs as the basis
for hourly PM2.5 mass measurements. The Policy Assessment
also notes that since earlier drafts of this Policy Assessment did not
include 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.
c. Administrator's Proposed Conclusions on Averaging Time
    In reaching a proposed conclusion on the appropriate averaging time
for a standard intended to protect against PM-related visibility
impairment, the Administrator has taken into account the information
discussed above with regard to analyses and conclusions presented in
the final Policy Assessment as well as the views of CASAC based on its
reviews of the first and second drafts of the Policy Assessment. As an
initial matter, the Administrator recognizes 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 recognizes 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 recognizes the desirability of a sub-
daily averaging time, she has 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 recognizes
that the Policy Assessment concludes 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 (U.S. EPA, 2011a, p. 4-57). In
light of this finding, the Administrator proposes to set a distinct
secondary standard with a 24-hour averaging time in conjunction with a
PM2.5 visibility index.
    Further, in light of the desirability of a sub-daily averaging
time, the Administrator solicits comment on a sub-daily (e.g., 4-hour)
averaging time and related data quality issues associated with
currently available monitoring instrumentation. In so doing, the
Administrator notes that CASAC's advice on averaging times was
predicated in part on the capabilities of available monitoring
instrumentation as CASAC understood them when it provided its advice.
3. Form
    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 concludes it is 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
concludes 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 notes 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 finds 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 that was considered in the Visibility
Assessment for a sub-daily (i.e., 1-hour) averaging time is whether to
include all daylight hours or only the maximum daily daylight hour.
This consideration would also be relevant for a multi-hour (e.g., 4-
hour) averaging time, although such an analysis was not included in the
Visibility Assessment. The maximum daily daylight 1-hour or multi-hour
form is 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 does not have information regarding the
fraction of the public that has only one or a few opportunities to
experience visibility during the day, nor does 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 Visibility Assessment
analyses included both the maximum daily hour and the all daylight
hours forms. The Policy Assessment observed a close correspondence
between the level of protection afforded for all 15 urban areas in the
assessment by the maximum daily daylight 1-hour approach using the 90th
percentile form and the all daylight hours approach combined with the
98th percentile form (U.S. EPA, 2010b, section 4.1.4). On this basis,
the Policy Assessment notes that the 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). The Policy Assessment also notes that 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 concludes that it is 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 concludes that it
is 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.
    Based on its review of the second draft Policy Assessment, CASAC
did not provide advice as to a specific form that would be appropriate,
but 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, the Administrator provisionally concludes that a 90th
percentile form, averaged over 3 years, is appropriate, and proposes
such a form in conjunction with a PM2.5 visibility index and
a 24-hour averaging time.
4. Level
    In considering alternative levels for a new standard that would
provide requisite protection against PM-related visibility impairment
primarily in urban areas, the Policy Assessment has taken into account
the evidence- and impact-based considerations discussed above and in
section 4.2.1 of the Policy Assessment, 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 this information in the context of a standard
defined in terms of a calculated PM2.5 light extinction
indicator, discussed above and in the Policy Assessment section 4.3.1;
with alternative averaging times of 24-hours or multi-hour, sub-daily
periods (e.g., 4-hours), discussed above and in Policy Assessment
section 4.3.2; and a 90th percentile-based form, discussed above and in
section 4.3.3 of the Policy Assessment.
    As part of the Policy Assessment's assessment of the adequacy of
the current standards, summarized in section VI.B. above and in Policy
Assessment section 4.2.1, it 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 Policy Assessment notes that
CASAC agreed that this was an appropriate range of levels to consider
for such a standard (Samet, 2010d, p. 11).\175\ The Policy Assessment
also recognizes 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 is 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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    \175\ In 2009, the D.C. 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.
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    As an initial matter, in considering alternative levels for a sub-
daily standard based directly on the four preference study results, the
Policy Assessment notes 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. As discussed above and in section 4.2.1 of the Policy
Assessment, the Phoenix study was by far the best of the studies,
providing somewhat more support for the middle CPL. In considering the
results from these studies, the Policy Assessment recognizes that the
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 recognizes 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 concludes, as did
CASAC, that these studies are appropriate to use for this purpose (U.S.
EPA, 2011a, p. 4-61).
    In considering potential alternative levels for a 24-hour standard,
the Policy Assessment explores various approaches to adjusting the CPLs
derived directly from the preference studies, as presented and
discussed in Appendix G of the Policy Assessment, especially section G-
5. 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 so doing, staff recognized that there are
multiple approaches for estimating generally equivalent levels on a
city-specific or national basis, and that the inherent spatial and
temporal variability in relative humidity and fine particle composition
across cities leads to a set of alternative estimates of levels that
may be construed as being generally equivalent on a national basis.
    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 Table G-6 of Appendix G,\176\
in considering estimates aggregated or averaged over all 15 cities as
well as the range of city-specific estimates for the various approaches
considered, 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.\177\
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    \176\ 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.
    \177\ 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