Myhre et al., 2013). A positive forcing indicates net energy trapped
in the Earth system and suggests warming of the Earth's surface,
whereas a negative forcing indicates net loss of energy and suggests
cooling (U.S. EPA, 2019, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    As described in the proposal (85 FR 24133, April 30, 2020), PM has
a very heterogeneous distribution globally and patterns of forcing tend
to correlate with PM loading, with the greatest forcings centralized
over continental regions. The climate response to this PM forcing,
however, is more complicated since the perturbation to one climate
variable (e.g., temperature, cloud cover, precipitation) can lead to a
cascade of effects on other variables. While the initial PM radiative
forcing may be concentrated regionally, the eventual climate response
can be much broader spatially or be concentrated in remote regions, and
may be quite complex, affecting multiple climate variable with possible
differences in the direction of the forcing in different regions or for
different variables (U.S. EPA, 2019, section 13.3.6). The complex
climate system interactions lead to variation among climate models,
which have suggested a range of factors which can influence large-scale
meteorological processes and may affect temperature, including local
feedback effects involving soil moisture and cloud cover, changes in
the hygroscopicity of the PM, and interactions with clouds (U.S. EPA,
2019, section 13.3.7). Further research is needed to better
characterize the effects of PM on regional climate in the U.S. before
PM climate effects can be quantified.
iii. Materials
    Consistent with the last review, the current evidence continues to
support the conclusion that there is a causal relationship between PM
deposition and materials effects. Effects of deposited PM, particularly
sulfates and nitrates, to materials include both physical damage and
impaired aesthetic qualities, generally involving soiling and/or
corrosion (U.S. EPA, 2019, section 13.4.2; 85 FR 24133, April 30,
2020). Because of their electrolytic, hygroscopic, and acidic
properties and their ability to sorb corrosive gases, particles
contribute to materials damage by adding to the effects of natural
weathering processes, by potentially promoting or accelerating the
corrosion of metals, degradation of painted surfaces, deterioration of
building materials, and weakening of material components.\76\ There is
a limited amount of new data for consideration in this review from
studies primarily conducted outside of the U.S. on buildings and other
items of cultural heritage. However, these studies involved
concentrations PM in ambient air greater than those typically observed
in the U.S. (U.S. EPA, 2019, section 13.4).
---------------------------------------------------------------------------

    \76\ As discussed in the ISA (U.S. EPA, 2019, section 13.4.1),
corrosion typically involves reactions of acidic PM (i.e., acidic
sulfate or nitrate) with material surfaces, but gases like
SO2 and nitric acid (HNO3) also contribute.
Because ``the impacts of gaseous and particulate N and S wet
deposition cannot be clearly distinguished'' (U.S. EPA, 2019, p. 13-
1), the assessment of the evidence in the ISA considers the combined
impacts.
---------------------------------------------------------------------------

    Building on the evidence available in the 2009 ISA, and as
described in detail in the proposal (85 FR 24134, April 30, 2020) and
in the 2019 ISA (U.S. EPA, 2019, section 13.4), research has progressed
on: (1) The theoretical understanding of soiling of items of cultural
heritage; (2) the quantification of degradation rates and further
characterization of factors that influence damage of stone materials;
(3) materials damage from PM components besides sulfate and black
carbon and atmospheric gases besides SO2; (4) methods for
evaluating soiling of materials by PM mixtures; (5) PM-attributable
damage to other materials, including glass and photovoltaic panels; (6)
development of dose-response relationships for soiling of building
materials; and (7) damage functions to quantify material decay as a
function of pollutant type and load. While the evidence of PM-related
materials effects has expanded somewhat since the last review, there
remains insufficient evidence to relate soiling or damage to specific
PM levels in ambient air or to establish a quantitative relationship
between PM and materials degradation. The current evidence is generally
similar to the evidence available in the last review, including
associated limitations and uncertainties and a lack of evidence to
inform quantitative relationships between PM and materials effects,
therefore leading to similar conclusions about the PM-related effects
on materials.
3. Overview of Air Quality and Quantitative Information
a. Visibility Effects
    In the current review, quantitative analyses were conducted to
further our understanding of the relationship between recent air
quality and calculated light extinction. As at the time of the last
review, these analyses explored this relationship as an estimate of
visibility impairment in terms of the 24-hour PM2.5 standard
and the visibility index. Generally, the results of the updated
analyses are similar to those based on the data available at the time
of the last review (U.S. EPA, 2020, section <IP_ADDRESS>). Compared to the
last review, updated analyses incorporate several refinements,
including: (1) The evaluation of three versions of the IMPROVE equation
\77\ to calculate light extinction (U.S. EPA, 2020, Appendix D,
Equations D-1 through D-3) in order to better understand the influence
of variability in equation inputs; \78\ (2) the

use of 24-hour relative humidity data, rather than monthly average
relative humidity as was used in the last review (U.S. EPA, 2020,
section <IP_ADDRESS>, Appendix D); and (3) the inclusion of the coarse
fraction in the estimation of light extinction in the subset of areas
with PM10-2.5 monitoring data available for the time period
of interest (U.S. EPA, 2020, section <IP_ADDRESS>, Appendix D). The analyses
in the current review are updated from the last review and include 67
monitoring sites that measure PM2.5, including 20 sites that
measure both PM10 and PM2.5, that are
geographically distributed across the U.S. in both urban and rural
areas (U.S. EPA, 2020, Appendix D, Figure D-1).
---------------------------------------------------------------------------

    \77\ Given the lack of new information to inform a different
visibility metric, the metric used in the updated analyses is that
defined by the EPA in the last review as the target level of
protection for visibility (discussed above in section IV.A.1): A
PM2.5 visibility index with a 24-hour averaging time, a
90th percentile form averaged over 3 years, and a level of 30 dv
(U.S. EPA, 2020, section <IP_ADDRESS>).
    \78\ While the PM2.5 monitoring network has an
increasing number of continuous FEM monitors reporting hourly
PM2.5 mass concentrations, there continue to be data
quality uncertainties associated with providing hourly
PM2.5 mass and component measurements that could be input
into IMPROVE equation calculations for sub-daily visibility
impairment estimates. As detailed in the PA, there are uncertainties
associated with the precision and bias of 24-hour PM2.5
measurements (U.S. EPA, 2020, p. 2-18), as well as to the fractional
uncertainty associated with 24-hour PM component measurements (U.S.
EPA, 2020, p. 2-21). Given the uncertainties present when evaluating
data quality on a 24-hour basis, the uncertainty associated with
sub-daily measurements may be even greater. Therefore, the inputs to
these light extinction calculations are based on 24-hour average
measurements of PM2.5 mass and components, rather than
sub-daily information.
---------------------------------------------------------------------------

    In areas that meet the current 24-hour PM2.5 standard
for the 2015-2017 time period, all sites have light extinction
estimates at or below 27 dv using the original and revised IMPROVE
equations (and most areas are below 25 dv; U.S. EPA, 2020, section
<IP_ADDRESS>). In the one location that exceeds the current 24-hour
PM2.5 standard, light extinction estimates are at or below
27 dv (U.S. EPA, 2020, Figure 5-3). These findings are consistent with
the findings of the analysis in the last review with older air quality
data (Kelly et al., 2012b; 78 FR 3201, January 15, 2013).
    Using the recently modified IMPROVE equation from Lowenthal and
Kumar (2016), new in this review, the resulting 3-year visibility index
is slightly higher at all of the sites compared to the original and
revised IMPROVE equation estimates (U.S. EPA, 2020, Figure 5-4). These
higher estimates are to be expected, given the higher OC multiplier
included in the IMPROVE equation from Lowenthal and Kumar (2016), which
reflects the use of data from remote areas with higher concentrations
of organic PM when validating the equation. As such, it is important to
note that the Lowenthal and Kumar (2016) version of the equation may
overestimate light extinction in non-remote areas, including the urban
areas in the updated analyses in this review.
    Nevertheless, when light extinction is calculated using the
Lowenthal and Kumar (2016) equation for those sites that meet the
current 24-hour PM2.5 standard, the 3-year visibility metric
is generally at or below 30 dv. The one exception to this is a site in
Fairbanks, Alaska that just meets the current 24-hour PM2.5
standard in 2015-2017 and has a 3-year visibility index value just
above 30 dv, rounding to 31 dv (compared to 27 dv when light extinction
is calculated with the original IMPROVE equations) (U.S. EPA, 2020,
Appendix D, Table D-3). The unique conditions at this urban site (e.g.,
higher OC concentrations, much lower temperatures, and the complete
lack of sunlight for long periods) that affect quantitative
relationships between OC, OM and visibility (e.g., Hand et al., 2012;
Hand et al., 2013) may differ considerably from those under which the
Lowenthal and Kumar (2016) equation has been evaluated, making the most
appropriate approach for characterizing light extinction in this area
unclear.
    At the time of the last review, the EPA noted that PM2.5
is the size fraction of PM responsible for most of the visibility
impairment in urban areas (77 FR 38980, June 29, 2012). Data available
at the time of the last review suggested that PM10-2.5 was a
minor contributor to visibility impairment (U.S. EPA, 2010b), although
this fraction may be responsible for a larger contribution in some
areas in the desert southwestern region of the U.S. However, at the
time of the last review, there was very little data available from
PM10-2.5 monitors to quantify the contribution of coarse PM
to calculated light extinction.
    Since the last review, the expansion of PM10-2.5
monitoring efforts has increased the availability of data for use in
estimating light extinction. As such, both PM2.5 and
PM10-2.5 concentrations can be included as inputs in the
equations in the updated analyses in this review. For 2015-2017, 20 of
the 67 PM2.5 sites analyzed have collocated
PM10-2.5 monitoring data available. These 20 sites meet both
the 24-hour PM2.5 and 24-hour PM10 standards. All
of these sites have 3-year visibility metrics at or below 30 dv
regardless of whether light extinction is calculated with or without
the coarse fraction, and for all three versions of the IMPROVE
equation. Generally, the coarse fraction contribution to light
extinction is minimal, contributing less than 1 dv to the 3-year
visibility metric. The 20 locations with collocated PM2.5
and PM10-2.5 monitoring data available in this review would
be expected to have relatively low concentrations of coarse PM. In
areas with higher concentrations of coarse PM, such as the southwestern
U.S., the coarse fraction may be a more important contributor to light
extinction and visibility impairment than in the locations included in
the updated analyses in this review.
    Overall, the results of the updated analyses in this review are
consistent with those in the last review. The 3-year visibility metric
is generally at or below 27 dv in areas that meet the current secondary
standards, with only small differences observed for the three versions
of the IMPROVE equation. Though such differences are modest, the
IMPROVE equation from Lowenthal and Kumar (2016) results in higher
light extinction values, which were expected given the higher OC
multiplier in the equation and its validation using data from remote
areas far away from emission sources. There are only small differences
in estimates of light extinction when the coarse fraction is included
in the equation, although a somewhat larger coarse fraction
contribution to light extinction would be expected in areas with higher
concentrations of coarse PM. Overall, the updated analyses indicate
that the current secondary PM standards provide a degree of protection
against visibility impairment similar to the target level of protection
identified in the last review, in terms of a 3-year visibility index.
b. Non-Visibility Effects
    Consistent with the evidence available at the time of the last
review, and as described in detail in the PA (U.S. EPA, 2020, section
<IP_ADDRESS>), the data remain insufficient to conduct quantitative analyses
for PM effects on climate and materials. For PM-related climate
effects, as explained in more detail in the proposal (85 FR 24131-
24133, 24136, April 30, 2020), our understanding of PM-related climate
effects is still limited by significant key uncertainties. The newly
available evidence does not appreciably improve our understanding of
the spatial and temporal heterogeneity of PM components that contribute
to climate forcing (U.S. EPA, 2020, sections <IP_ADDRESS>.1 and 5.4).
Significant uncertainties also persist related to quantifying the
contributions of PM and PM components to the direct and indirect
effects on climate forcing, such as changes to the pattern of rainfall,
changes to wind patterns, and effects on vertical mixing in the
atmosphere (U.S. EPA, 2020, sections <IP_ADDRESS>.1 and 5.4). Additionally,
while improvements have been made to climate models since the time of
the last review, the models continue to exhibit variability in
estimates of the PM-related climate effects on regional scales (e.g.,
~100 km) compared to simulations at the global scale (U.S. EPA, 2020,
sections <IP_ADDRESS>.1 and 5.4). While our understanding of

climate forcing on a global scale is somewhat expanded since the last
review, significant limitations remain to quantifying potential adverse
PM-related climate effects in the U.S. and how they would vary in
response to incremental changes in PM concentrations across the U.S. As
such, while new research is available on climate forcing on a global
scale, the remaining limitations and uncertainties are significant, and
the new global scale research does not translate directly for use at
regional spatial scales. Therefore, the evidence does not provide a
clear understanding at the necessary spatial scales for quantifying the
relationship between PM mass in ambient air and the associated climate-
related effects in the U.S. that would be most relevant to informing
consideration of a national PM standard on climate in this review (U.S.
EPA, 2020, section <IP_ADDRESS>.1; U.S. EPA, 2019, section 13.3).
    For PM-related materials effects, as explained in more detail in
the proposal (85 FR 24133-24134, 24137, April 30, 2020), the available
evidence has been somewhat expanded to include additional information
about the soiling process and the types of materials impacted by PM.
This evidence provides some limited information to inform dose-response
relationships and damage functions associated with PM, although most of
these studies were conducted outside of the U.S. where PM
concentrations in ambient air are typically above those observed in the
U.S. (U.S. EPA, 2020, section <IP_ADDRESS>.2; U.S. EPA, 2019, section 13.4).
The evidence available in this review also includes studies examining
effects of PM on the energy efficiency of solar panels and passive
cooling building materials, although the evidence remains insufficient
to establish quantitative relationships between PM in ambient air and
these or other materials effects (U.S. EPA, 2020, section <IP_ADDRESS>.2).
While the available evidence is somewhat expanded since the time of the
last review, quantitative relationships have not been established for
PM-related soiling and corrosion and frequency of cleaning or repair
that would help inform our understanding of the public welfare
implications of materials effects (U.S. EPA, 2020, section <IP_ADDRESS>.2;
U.S. EPA, 2019, section 13.4). Therefore, there is insufficient
information to inform quantitative analyses assessing materials effects
to inform a consideration of a national PM standard on materials in
this review (U.S. EPA, 2020, section <IP_ADDRESS>.2; U.S. EPA, 2019, section
13.4).

B. Conclusions on the Secondary Standards

    In drawing conclusions on the adequacy of the current secondary PM
standards, in view of the advances in scientific knowledge and
additional information now available, the Administrator has considered
the evidence base, information, and policy judgments that were the
foundation of the last review and reflects upon the body of information
and evidence available in this review. In so doing, the Administrator
has taken into account both evidence-based and quantitative
information-based considerations, as well as advice from the CASAC and
public comments. Evidence-based considerations draw upon the EPA's
assessment and integrated synthesis of the scientific evidence from
studies evaluating welfare effects related to visibility, climate, and
materials associated with PM in ambient air as discussed in the PA
(summarized in sections IV..B, V.C, and IV.D.1 of the proposal, and
section IV.A.2 above). The quantitative information-based
considerations draw from the results of the quantitative analyses of
visibility impairment presented in the PA (as summarized in section
IV.D.1 of the proposal and section IV.A.3 above) and consideration of
these results in the PA.
    Consideration of the evidence and quantitative information in the
PA and by the Administrator is framed by consideration of a series of
policy-relevant questions. Section IV.B.2 below summarizes the
rationale for the Administrator's proposed decision, drawing from
section IV.D.3 of the proposal. The advice and recommendations of the
CASAC and public comments on the proposed decision are addressed below
in sections IV.D.2 and IV.D.3, respectively. The Administrator's
conclusions in this review regarding the adequacy of the secondary PM
standards and whether any revisions are appropriate are described in
section IV.D.4.
1. CASAC Advice in This Review
    In comments on the draft PA, the CASAC concurred with the staff's
overall preliminary conclusions that it is appropriate to consider
retaining the current secondary standards without revision (Cox,
2019a). The CASAC ``finds much of the information . . . on visibility
and materials effects of PM2.5 to be useful, while
recognizing that uncertainties and controversies remain about the best
ways to evaluate these effects'' (Cox, 2019a, p. 13 of consensus
responses). Regarding climate, while the CASAC agreed that research on
PM-related effects has expanded since the last review, it also
concluded that ``there are still significant uncertainties associated
with the accurate measurement of PM to the direct and indirect effects
of PM on climate'' (Cox, 2019a, pp. 13-14 of consensus responses). The
committee recommended that the EPA summarize the ``current scientific
knowledge and quantitative modeling results for effects of reducing
PM2.5'' on several climate-related outcomes (Cox, 2019a, p.
14 of consensus responses), while also recognizing that ``it is
appropriate to acknowledge uncertainties in climate change impacts and
resulting welfare impacts in the United States of reductions in
PM2.5 levels'' (Cox, 2019a, p. 14 of consensus responses).
When considering the overall body of scientific information for PM-
related effects on visibility, climate, and materials, the CASAC agreed
that ``the available evidence does not call into question the
protection afforded by the current secondary PM standards and concurs
that they should be retained'' (Cox, 2019a, p. 3 of letter).
2. Basis for the Proposed Decision
    At the time of the proposal, the Administrator carefully considered
the assessment of the current evidence and conclusions reached in the
ISA; the currently available quantitative information, including
associated limitations and uncertainties, described in detail and
characterized in the PA; considerations and staff conclusions and
associated rationales presented in the PA; and the advice and
recommendations from the CASAC (85 FR 24137, April 30, 2020).
    In reaching his proposed decision on the secondary PM standards,
the Administrator first recognized the longstanding body of evidence
for PM-related visibility impairment. The Administrator recognized that
visibility impairment can have implications for people's enjoyment of
daily activities and for their overall sense of well-being. In so
doing, and consistent with the approach used in the last review
(section IV.A.1 above), the Administrator first defined a target level
of protection in terms of a PM visibility index that accounts for the
factors that influence the relationship between PM in ambient air and
visibility (i.e., size fraction, species composition, and relative
humidity). He then considered air quality analyses examining the
relationship between this PM visibility index and the current 24-hour
PM2.5 and 24-hour PM10 standards in areas that

met data completeness criteria for inclusion in the analyses.\79\
---------------------------------------------------------------------------

    \79\ As described in detail in section IV.A.3.a above, the EPA's
updated quantitative analyses in this review included 67 areas that
met data completeness criteria for inclusion in the analyses (see
U.S. EPA, 2020, Appendix D for details of the criteria). Of those
monitoring locations that met the data completeness criteria, all
but one location met the current secondary PM2.5 standard
(U.S. EPA, 2020, Table D-7).
---------------------------------------------------------------------------

    To identify a target level of protection, the Administrator first
defined the specific characteristics of the visibility index, noting
that in the last review, the EPA used an index based on estimates of
light extinction by PM2.5 components calculated using the
IMPROVE algorithm. As described in section IV.A.2 above, the IMPROVE
algorithm estimates light extinction using routinely monitored
components of PM2.5 and PM10-2.5,\80\ along with
estimates of relative humidity. The Administrator recognized that,
despite revisions to the IMPROVE algorithm since the last review (U.S.
EPA, 2020, section <IP_ADDRESS>), our fundamental understanding of the
relationship between PM in ambient air and light extinction has changed
little and that the various IMPROVE algorithms can appropriately
reflect this relationship across the U.S. In the absence of a robust
monitoring network to measure light extinction (85 FR 24130, 24135,
April 30, 2020), the Administrator judged that estimated light
extinction, as calculated using the IMPROVE algorithms, continued to
provide a reasonable basis for defining a target level of protection
against PM-related visibility impairment in the current review.
---------------------------------------------------------------------------

    \80\ In the last review, the focus was on PM2.5
components given their prominent role in PM-related visibility
impairment in urban areas and the limited data available for
PM10-2.5 (77 FR 38980, June 29, 2010; U.S. EPA, 2020,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    In further defining the characteristics of a visibility index based
on estimates of light extinction, the Administrator considered the
appropriate averaging time, form, and level of the index. The
Administrator judged that the decisions made in the last review with
regard to averaging time and form remain reasonable. In the last
review, a 24-hour averaging time was judged to be an appropriate
surrogate for the sub-daily periods relevant for visual perception,\81\
recognizing the relatively strong correlations between 24-hour and sub-
daily (i.e., 4-hour) average PM2.5 light extinction and that
this longer averaging time may be less influenced by atypical
conditions and/or atypical instrument performance (78 FR 3226, January
15, 2013). In the decision to set the form as the 3-year average of
annual 90th percentile values in the last review, it was noted that:
(1) A 3-year average provided stability from the occasional effect of
interannual meteorological variability (78 FR 3198, January 15, 2013);
(2) the 90th percentile corresponds to the 20 percent worst days for
visibility, which are targeted in Class I areas by the Regional Haze
program; and (3) available studies on people's visibility preferences
did not identify a basis for a different target than that identified
for Class I areas (U.S. EPA, 2011, p. 4-59). Recognizing that the
information available in the current review is similar to that
available in the last review, at the time of proposal the Administrator
judged that these decisions remain reasonable, and it remains
appropriate to define a visibility index based on estimated light
extinction in terms of a 24-hour averaging time and a form based on the
3-year average of annual 90th percentile values.
---------------------------------------------------------------------------

    \81\ While the PM2.5 monitoring network has an
increasing number of continuous FEM monitors reporting hourly
PM2.5 mass concentrations, there continue to be data
quality uncertainties associated with providing hourly
PM2.5 mass and component measurements that could be input
into IMPROVE equation calculations for sub-daily visibility
impairment estimates. As detailed in the PA, there are uncertainties
associated with the precision and bias of 24-hour PM2.5
measurements (U.S. EPA, 2020, p. 2-18), as well as to the fractional
uncertainty associated with 24-hour PM component measurements (U.S.
EPA, 2020, p. 2-21). Given the uncertainties present when evaluating
data quality on a 24-hour basis, the uncertainty associated with
sub-daily measurements may be even greater. Therefore, the inputs to
these light extinction calculations are based on 24-hour average
measurements of PM2.5 mass and components, rather than
sub-daily information.
---------------------------------------------------------------------------

    At the time of the last review, the level of the visibility index
was set at 30 dv, based on the upper end of the range of levels of
visibility impairment judged to be acceptable by at least 50% of study
participants in the available visibility preference studies (U.S. EPA,
2020, section <IP_ADDRESS>). (78 FR 3226-27, January 15, 2013; 85 FR 24131
April 30, 2020).\82\ In the last review, the Administrator concluded
that the substantial degree of variability and uncertainty in the
public preference studies should be reflected in a target protection
level at the upper end of the 20 dv to 30 dv range of CPLs. Therefore,
she concluded that it was appropriate to set a target level of
protection in terms of a 24-hour PM2.5 visibility index at
30 dv (78 FR 3226-27, January 15, 2013).
---------------------------------------------------------------------------

    \82\ Based on the preference studies, the 2011 PA identified a
range of levels from 20 to 30 deciviews (dv) as being a reasonable
range of ``candidate protection levels'' or ``CPLs'' for a
visibility index (U.S. EPA, 2011, p. 4-61; U.S. EPA, 2020, section
<IP_ADDRESS>).
---------------------------------------------------------------------------

    In considering the preference studies in this review, the
Administrator first noted that, as a part of the last review, a range
of levels was identified for the PM2.5 visibility index
based on an aggregated evaluation of the results of these studies that
reflected variability in levels of visibility that were considered
acceptable in the four study areas (U.S. EPA, 2010b). Because no
visibility preference studies have been conducted in the U.S. since the
last review, and given the general lack of new preference studies over
the last several reviews, the Administrator proposed to conclude that
the range considered in the last review remained appropriate to
consider in the current review.
    The Administrator highlighted the following uncertainties and
limitations in the underlying public preference studies (U.S. EPA,
2020, section <IP_ADDRESS>), consistent with those identified in the last
review:
     The available studies may not capture the full range of
visibility preferences in the U.S. population, particularly given the
potential for preferences to vary based on the visibility conditions
commonly encountered and the types of scenes being viewed.
     The available preference studies were conducted 15 to 30
years ago and may not reflect visibility preferences in the U.S.
population today.
     The available preference studies have used a variety of
methods, potentially influencing responses as to what level of
visibility impairment is deemed acceptable.
     Factors that are not captured by the methods used in
available preference studies may influence people's judgments on
acceptable visibility, including the duration of visibility impairment,
the time of day during which light extinction is greatest, and the
frequency of episodes of visibility impairment.
    After considering these preference studies, along with their
inherent uncertainties and limitations, the Administrator judged in the
proposal that a level of 30 dv continued to be an appropriate target
level of protection for the visibility index in the current review.\83\
---------------------------------------------------------------------------

    \83\ As noted above, in the last review, the Administrator
explained that the current substantial degrees of variability and
uncertainty inherent in the public preference studies should be
reflected in a higher target protection level than would be
appropriate if the underlying information were more consistent and
certain (78 FR 3216, January 15, 2013).
---------------------------------------------------------------------------

    Having defined a target level of protection in terms of a
visibility index based on the elements described above, (i.e., with a
24-hour averaging time; a 3-year average of the annual 90th

percentile form; and a level of 30 dv), the Administrator next
considered the degree of protection from visibility impairment afforded
by the existing secondary standards. In so doing, he considered the
updated analyses of PM-related visibility impairment (U.S. EPA, 2020,
section <IP_ADDRESS>), specifically noting the improvements over the
analyses in the last review, in particular the use of multiple versions
of the IMPROVE algorithm, including the version incorporating revisions
since the last review (85 FR 24135-24136, April 30, 2020). The analyses
in this review expand upon our understanding of how variation in
equation inputs impacts calculated light extinction (U.S. EPA, 2020,
Appendix D) and also better characterizes the influence of the coarse
fraction on light extinction for the subset of sites with available
PM10-2.5 monitoring data (U.S. EPA, 2020, section <IP_ADDRESS>).
    The Administrator noted that the results of the updated analyses
are consistent with the results from the last review, regardless of the
IMPROVE equation used. The results of the analyses demonstrated that,
in areas meeting the 24-hour PM2.5 standard, the 3-year
visibility metric is at or below about 30 dv,\84\ and is below 25 dv in
most of the areas. In those locations with PM10-2.5
monitoring data available, which met both the current 24-hour
PM2.5 and 24-hour PM10 standards, 3-year
visibility metrics were at or below 30 dv regardless of if the coarse
fraction was included in the calculation (U.S. EPA, 2020, section
<IP_ADDRESS>). In considering these updated analyses, the Administrator
proposed to conclude that the scientific and quantitative information
available in this review support the adequacy of the current secondary
PM2.5 and PM10 standards to protect against PM-
related visibility impairment.
---------------------------------------------------------------------------

    \84\ As discussed above and in the PA (U.S. EPA, 2020, section
<IP_ADDRESS>), one site in Fairbanks, Alaska just meets the current 24-
hour PM2.5 standard and has a 3-year visibility index
value of 27 dv based on the original IMPROVE equation and 31 dv
based on the Lowenthal and Kumar (2016) equation. At this site, use
of the Lowenthal and Kumar (2016) equation may not be appropriate
given that PM composition and meteorological conditions may differ
considerably from those under which revisions to the equation have
been validated.
---------------------------------------------------------------------------

    With respect to non-visibility welfare effects, the Administrator
considered the evidence related to climate and materials effects and
proposed to conclude that it is generally appropriate to retain the
existing secondary standards and that it is not appropriate to
establish any distinct secondary PM standards to address non-visibility
PM-related welfare effects. With regard to climate, the Administrator
recognized that a number of improvements and refinements have been made
to climate models since the last review, while also noting that
significant limitations continue to exist in quantifying the
contributions of the direct and indirect effects of PM and PM
components on climate forcing (85 FR 24139, April 30, 2020; U.S. EPA,
2020, sections <IP_ADDRESS>.1 and 5.4). The Administrator also recognized
that climate models continue to exhibit considerable variability in
estimates of PM-related climate impacts at regional scales (e.g., ~100
km) compared to simulations at global scales (85 FR 24139, April 30,
2020; U.S. EPA, 2020, section <IP_ADDRESS>.1 and 5.4). In considering this
uncertainty, the Administrator proposed to conclude that the scientific
information available in the current review remains insufficient to
quantify the impacts of ambient PM on climate in the U.S. with
confidence (85 FR 34139, April 30, 2020; U.S. EPA, 2020, sections
<IP_ADDRESS>.1 and 5.4) and that there is insufficient information available
in this review to base a national ambient air quality standard on
climate impacts.
    With respect to materials effects, the Administrator recognized
that deposition of fine or coarse particles can result in physical
damage and/or impaired aesthetic qualities. Particles can contribute to
materials damage by adding to the effects of weathering processes and
by promoting the corrosion of metals, the degradation of painted
surfaces, the deterioration of building materials, and the weakening of
material components. The Administrator, while recognizing that some new
evidence of PM-related materials effects is available in this review,
noted that this evidence is primarily from studies conducted outside of
the U.S. with PM concentrations that are higher than those typically
observed in ambient air in the U.S. (U.S. EPA, 2019, section 13.4).
Consistent with the information available at the time of the last
review, the Administrator recognized a limited amount of information
available on the quantitative relationships between PM and materials
effects in the U.S., and uncertainties in the degree to which those
effects could be adverse to public welfare. Therefore, at the time of
proposal, the Administrator judged that the scientific information
available in this review remains insufficient to quantify the public
welfare impacts of PM in ambient air on materials with confidence and
that there is insufficient information available in this review to
support a distinct national ambient standard based on materials
effects.
    Thus, based on consideration of the scientific and quantitative
information available in this review, with its uncertainties and
limitations, and information that might inform his public welfare
judgments, as well as consideration of advice from the CASAC, including
their concurrence with the PA conclusions that the current evidence
does not support revision of the secondary PM standards (discussed in
section IV.B.1 above). The Administrator proposed to conclude that it
is appropriate to retain the current secondary PM standards without
revision based on his judgment that the current secondary PM standards
are requisite to protect against PM-related effects on visibility and
that there is insufficient information available in this review to base
a national ambient air quality standard for PM on climate and materials
impacts.
3. Comments on the Proposed Decision
    Of the public comments received on the proposal, very few were
specific to the secondary PM standards. Of those commenters who did
provide comments on the secondary PM standards, the majority support
the Administrator's proposed decision to retain the current standards.
Some commenters disagree with the Administrator's proposed conclusion
to retain the current secondary standards, primarily focusing their
comments on the need for a revised standard to protect against
visibility impairment. In addition to the comments addressed in this
notice, the EPA has prepared a Response to Comments document that
addresses other specific comments related to setting the secondary PM
standards. This document is available for review in the docket for this
rulemaking and through the EPA's NAAQS website (https://www.epa.gov/naaqs/particulate-matter-pm-air-quality-standards).
    Of the comments addressing the proposed decision, many of the
commenters support the Administrator's proposed decision to retain the
current secondary PM standards, without revision. This group includes
industries and industry groups and state and local governments and
organizations. All of these commenters generally note their agreement
with the rationale provided in the proposal and with the views
expressed by the CASAC that the current evidence does not support
revision to the standards. Most also cite the EPA and CASAC statements
that the scientific evidence and quantitative information in this
review has not substantially altered our previous

understanding of the effects of PM on visibility, climate, and
materials beyond what was previously examined and does not call into
question the adequacy of the current standards. They all find the
proposed decision to retain the current standards to be well supported
and a reasonable exercise of the Administrator's public welfare policy
judgment under the CAA. The EPA agrees with these comments and with the
CASAC advice regarding the adequacy of the current secondary PM
standards and the lack of support for revision of these standards.
    Of the commenters who disagree with the proposal to retain the
current standards, nearly all of these commenters recommend more
stringent standards, primarily to protect against visibility
impairment. These comments were submitted primarily by national public
health, medical, and environmental nongovernmental organizations, and
some individuals. The commenters who recommend strengthening the
standards state their support for revisions to provide greater public
welfare protection, generally claiming that the current standards are
inadequate and do not provide the requisite protection against known or
anticipated welfare effects. Additionally, some of the commenters who
disagree with the proposal did not specifically recommend revising the
current standards, but instead recommend additional research to address
key uncertainties and limitations in the available scientific and
quantitative information that would inform decisions regarding a
national standard to protect against PM-related non-visibility and
visibility effects.
    The EPA received relatively few comments on the proposed decision
that it is not appropriate to establish any distinct secondary PM
standards to address PM-related climate effects. The majority of the
comments that were received agree with the EPA that the currently
available information is not sufficient for supporting quantitative
analyses for the climate effects of PM in ambient air. These commenters
support the Administrator's proposed decision not to set a distinct
standard for climate. Several commenters note, however, that the EPA
should frequently reconsider the available evidence and quantitative
information and should revise the standard as necessary to provide
requisite protection against PM-related climate effects. The EPA agrees
with the commenters that quantitative analyses of the relationship
between PM and climate effects are not supported by the available
information in this review, and new information about PM-related
welfare effects, including climate, will be assessed consistent with
CAA requirements in the next review of the PM NAAQS.
    There were also very few commenters who commented on the proposed
decision that it is not appropriate to establish any distinct secondary
PM standards to address PM-related material effects. As with comments
on climate effects, commenters generally agree with the EPA that the
evidence is not sufficient to support quantitative analyses for PM-
related materials effects. However, some commenters contend that the
EPA failed to consider the following information: (1) Studies conducted
outside of the U.S. on the cost of soiling of materials that are also
found in the U.S.; (2) recent work related to soiling of photovoltaic
modules and other surfaces, and; (3) quantitative relationships between
PM in ambient air and materials effects used in several studies. These
commenters further assert that the EPA failed to specify a level of air
quality that protects against adverse effects of PM on materials and
failed to propose a standard that provides requisite protection against
materials effects attributable to PM.
    We disagree with the commenters that the EPA failed to consider the
relevant scientific information about materials effects available in
this review. As an initial matter, the ISA considered and included
studies related to materials effects of PM, including studies conducted
in and outside of the U.S., on newly studied materials including
photovoltaic modules that were published prior to the cutoff date for
the literature search.\85\ These include the Besson et al. (2017) study
referenced by the commenters (U.S. EPA, 2019, section 13.4.2). The
Gr[oslash]ntoft et al. (2019) study referenced by the same commenters
was published after the cutoff date for the literature search. However,
the EPA has provisionally considered new studies, including the new
studies highlighted by the commenters, in the context of the findings
of the ISA (see Appendix in Response to Comments document).\86\ Based
on this provisional consideration, the EPA concludes that the new
studies are not sufficient to alter the conclusions reached in the ISA
regarding PM and materials effects.
---------------------------------------------------------------------------

    \85\ As noted earlier in section IV, ``the current ISA
identified and evaluated studies and reports that that have
undergone scientific peer review and were published or accepted for
publication between January 1, 2009 and March 31, 2017. A limited
literature update identified some additional studies that were
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p.
A-3).
    \86\ As discussed in section I.D, the EPA has provisionally
considered studies that were highlighted by commenters and that were
published after the ISA. These studies are generally consistent with
the evidence assessed in the ISA, and they do not materially alter
our understanding of the scientific evidence or the Agency's
conclusions based on that evidence or warrant reopening of the air
quality criteria.
---------------------------------------------------------------------------

    Moreover, we disagree with the commenters that the EPA failed to
consider quantitative information from studies available in this
review. As detailed in section <IP_ADDRESS>.2 of the PA, a number of new
studies are available that apply new methods to characterize PM-related
effects on previously studied materials; however, the evidence remains
insufficient to relate soiling or damage to specific levels of PM in
ambient air or to establish quantitative relationships between PM and
materials degradation. The uncertainties in the evidence identified in
the last review persist in the evidence in the current review, with
significant uncertainties and limitations to establishing quantitative
relationships between particle size, concentration, chemical
components, and frequency of painting or repair of materials. While
some new evidence is available in this review, overall, the data are
insufficient to conduct quantitative analyses for PM-related materials
effects. Quantitative relationships have not been established between
characteristics of PM and frequency of repainting or cleaning of
materials, including photovoltaic panels and other energy-efficient
materials, that would help inform our understanding of the public
welfare implications of soiling (U.S. EPA, 2020, section <IP_ADDRESS>.2;
U.S. EPA, 2019, section 13.4). Similarly, the information does not
support quantitative analyses between microbial deterioration of
surfaces and the contribution of carbonaceous PM to the formation of
black crusts that contribute to soiling (U.S. EPA, 2020, section
<IP_ADDRESS>.2; U.S. EPA, 2019, section 13.4). We also note that
quantitative relationships are difficult to assess, in particular those
characterized using damage functions as these approaches depend on
human perception of the level of soiling deemed to be acceptable and
evidence in this area remains limited in the current review (U.S. EPA,
2020, section <IP_ADDRESS>.2). Additionally, we note the CASAC's concurrence
with conclusions in the PA that uncertainties remain about the best way
to evaluate materials effects of PM in ambient air (Cox, 2019a, p. 13
of consensus responses). Further, no new studies are available in this
review to link human perception of reduced aesthetic appeal of
buildings

and other objects to materials effects and PM in ambient air. Finally,
uncertainties remain about deposition rates of PM in ambient air to
surfaces and the interaction of PM with copollutants on these surfaces
(U.S. EPA, 2020, p. 5-34).
    As summarized above and in the proposal, the evidence in this
review for PM effects on materials is not substantively changed from
that in the last review. There continues to be a lack of evidence
related to materials effects that establishes quantitative
relationships and supports quantitative analyses of PM-related
materials soiling or damage. While the information available in this
review continues to support a causal relationship between PM in ambient
air and materials effects (U.S. EPA, 2019, section 13.4), the EPA is
unable to relate soiling or damage to specific levels of PM in ambient
air and is unable to evaluate or consider a level of air quality to
protect against such materials effects. Although the EPA did not
propose a distinct level of air quality or a national standard based on
air quality impacts (85 FR 24139, April 30, 2020), we did identify data
gaps that prevented us from doing so. The EPA identified a number of
key uncertainties and areas of future research (U.S. EPA, 2020, p. 5-
42) that may inform consideration of the materials effects of PM in
ambient air in future reviews of the PM NAAQS.
    Commenters who disagreed with the Administrator's proposed decision
to retain the current secondary PM standards provided a number of
comments on the scientific evidence and quantitative analyses for
visibility impairment. These commenters criticize various aspects of
the EPA's proposal to retain the standards, including specific aspects
of the visibility index, the target level of protection identified by
the Administrator, and the appropriateness of a single national
standard for purposes of protecting against PM-related visibility
impairment. In general, these comments indicated support for a more
stringent standard for visibility impairment, although the commenters
did not necessarily specify the alternative standard that would, in
their judgment, address the concerns raised. Rather, most of these
commenters focused on particular aspects of the visibility metric
underlying the current secondary standard, including the form,
averaging time, and target level of protection necessary to protect
against visibility impairment.
    Several commenters argue that the evidence does not support a
single level of ``acceptable'' visibility. Commenters expressed the
view that the public preference studies present important evidence
related to the importance of visibility, but that they do not provide
enough information to set a national standard for visibility impairment
because the results show that visibility preferences vary regionally
and/or locally for a variety of reasons. Commenters additionally state
that the EPA failed to explain and analyze the uncertainties associated
with the public preference studies, including: (1) The different
methods used in the studies and their influence on the responses; (2)
the impact of different scenes being viewed on the full range of public
preferences; and (3) factors that were not considered in the study
methods that could impact judgments in the studies. These commenters
suggest that the secondary standards should account for regional
variability, although they did not provide specific recommendations
regarding how to accomplish this.
    The EPA agrees with commenters that the available scientific
evidence indicates that public preferences for ``acceptable''
visibility and air quality depends in large part on the characteristics
of the scene being viewed. The EPA understands that there is a wide
range of urban and rural scenes within the U.S. and included in the
public preference studies, including natural vistas such as the Rocky
Mountains in Colorado and man-made urban structures such as the
Washington Monument. However, the EPA disagrees with commenters that
the available evidence cannot support a national standard to protect
against PM-related visibility impairment. As at the time of the last
review, the EPA believes that the scenes presented in the public
preference studies include important types of valued scenic views, and
therefore, when considered together, can inform consideration of an
acceptable level of visual air quality at the national scale, taking
into account variation across the U.S. as evidenced in the studies.
    With regard to the comments that these studies do not provide
enough information to account for regional variability that is
important to consider when setting a national standard for visibility
protection, the EPA recognizes that there may be regional variability
in the available evidence but believes that these studies provide
significant information that is useful for the Administrator to
consider in his judgments on the public welfare implications of PM-
related visibility effects. While the EPA acknowledges that there may
be regional differences in the stated preferences for visibility, the
EPA finds there is not enough information available at this time to
take such regional differences into account. The commenter did not
provide specific recommendations for the EPA's consideration of such
information even if such information were available, and the EPA finds
the question of how, or if, to account for regional preferences in
setting a national standard is a substantial question that should be
addressed when it is presented by the available information.
    With regard to the commenters' assertion that the current secondary
standards are inadequate to protect the public welfare from PM-related
visibility impairment, the EPA disagrees that the currently available
information is sufficient to suggest that a more stringent standard is
warranted. The EPA identified and addressed in great detail the
limitations and uncertainties associated with the public preference
studies as a part of the last review (78 FR 3210, January 15, 2013).
Given that the evidence related to public preferences is the same in
this review as it was at the time of the last review, the EPA
reiterated the limitations and uncertainties inherent in this evidence
as a part of the PA (U.S. EPA, 2020, section 5.5). The PA highlights
key uncertainties associated with public perception of visibility
impairment and identifies areas for future research to inform future PM
NAAQS reviews, including those raised by the commenters (U.S. EPA,
2020, p. 5-41). For example, the PA notes the critical need for
information to further our understanding of human perception of
visibility impairment in public preference studies in order to address
uncertainties and limitations in the evidence, including an expansion
of the number and geographic coverage of preference studies in urban,
rural, and Class I areas to account for the potential for people to
have different preferences based on the conditions that they commonly
encounter and potential differences in preferences based on the scene
types (U.S. EPA, 2020, p. 5-41).
    These same commenters further argue that the EPA omitted recent
studies that could further inform our understanding of the public
welfare implications of visibility impairment. Commenters specifically
point to a recent meta-analysis of available preference studies (Malm
et al., 2019) and also cites to several related studies (Malm et al.,
2011; Malm, 2013, 2016; Molenar and Malm, 2012). Commenters
additionally contend that studies of the economic effects of impaired
visibility were omitted from the ISA and PA and were

not considered in the EPA's approach for evaluating visibility.
    The EPA disagrees with the commenters that studies related to
visibility were inappropriately omitted from the ISA in this review. As
an initial matter, the ISA considered and included studies related to
PM-related visibility impairment and public preferences that were
published prior to the cutoff date for the literature search.\87\ As
described in the Preamble to the ISA, ``studies and reports that have
undergone scientific peer review and have been published (or accepted
for publication) are considered for inclusion in the ISA'' (U.S. EPA,
2015, p. 6). The meta-analysis by Malm et al. (2019) was published
after the cutoff date for the literature search for the ISA, and
therefore, was not included in the ISA. Malm et al. (2019) was
provisionally considered, along with other studies published after the
cut-off date, and the EPA concluded that these studies did not
materially change the broad scientific conclusions of the ISA regarding
welfare effects, including visibility impairment. Moreover, the other
citations provided by the commenters (Malm et al., 2011; Malm, 2013,
2016; Molenar and Malm, 2012) are not peer-reviewed publications and as
such do not meet the criteria for inclusion in the ISA. With regard to
studies of economic effects, these studies were not considered to be
within the scope of the ISA, and therefore were not included in this
review (U.S. EPA, 2019, p. P-16). The studies submitted by the
commenters, together with other new evidence, will be assessed
consistent with CAA requirements in the next review of the PM NAAQS.
---------------------------------------------------------------------------

    \87\ As noted earlier in section IV, ``the current ISA
identified and evaluated studies and reports that that have
undergone scientific peer review and were published or accepted for
publication between January 1, 2009 and March 31, 2017. A limited
literature update identified some additional studies that were
published before December 31, 2017'' (U.S. EPA, 2019, Appendix, p.
A-3).
---------------------------------------------------------------------------

    Some commenters contend that the EPA's visibility analyses only
focused on locations that met the current standards. These commenters
argue that the EPA concluded at the beginning of the analysis that the
current standards do not need to be revised and that the EPA's approach
ignores information available since the last review, leading to the
Administrator to propose no revisions to the standards based on this
flawed approach.
    We disagree with commenters that the updated analyses of visibility
impairment in this review only considered air quality in areas that
meet the current standards. As described in detail in the PA, locations
included in the analyses were those that met specific data completeness
criteria for the monitoring data required as inputs to the IMPROVE
equations for estimating light extinction (U.S. EPA, 2020, Appendix D).
The data set used for the updated analyses is comprised of sites with
data for the 2015-2017 period that supported a valid 24-hour
PM2.5 design value and met strict criteria for PM species.
For PM2.5 concentrations, data were screened so that all
days either had a valid filter-based 24-hour concentration measurement
or at least 18 valid hourly concentration measurements (U.S. EPA, 2020,
section D.2.1.2).\88\ For coarse PM concentrations, data were included
for sites with >=11 valid days for each quarter of 2015-2017. For
PM2.5 component concentrations, data were included for days
with valid data for all chemical components listed in Table D-1 in the
PA and for sites with >=11 valid days for each quarter of 2015-
2017.\89\ Of all of the PM monitoring locations in the U.S., 67
monitoring sites met the data completeness criteria and light
extinction was calculated without the coarse fraction in the IMPROVE
equations. Of these 67 monitoring sites, 20 locations met the data
completeness criteria for coarse PM, and as such, light extinction was
also estimated with the coarse fraction as an input to the IMPROVE
equation at these sites (U.S. EPA, 2020, section <IP_ADDRESS>, Appendix D).
For the sites that met the data completeness criteria for inclusion in
the analyses, all of the sites met the annual PM2.5 and 24-
hour PM10 standards, and all but one site (located in
southern California) met the 24-hour PM2.5 standard.
Therefore, we disagree with the commenters that the analysis was
designed to consider only locations that met the current standards and
did not consider locations that did not meet the current secondary PM
standards. Moreover, the EPA notes that data from areas exceeding the
current standard are generally of limited use in deciding whether to
retain the standard, or lower it, because it is not representative or
informative of circumstances and effects that would be expected to be
seen upon attainment of the standard.
---------------------------------------------------------------------------

    \88\ A valid filter-based 24-hour concentration measurement is
one collected via FRM, and that has undergone laboratory
equilibration (at least 24 hours at standardized conditions of 20-23
[deg]C and 30-40% relative humidity) prior to analysis (see Appendix
L of 40 CFR part 50 for the 2012 NAAQS for PM).
    \89\ For coarse PM and PM2.5 components, data
completeness criteria were selected for the quantitative analyses
consistent with those in Appendix N of 40 CFR part 50 for the 2012
NAAQS for PM.
---------------------------------------------------------------------------

    Furthermore, it is unclear what additional information the
commenters contend that the EPA omitted from its consideration in this
review. All scientific information available in this review has been
considered and integrated as a part of the ISA. The Administrator, in
considering the adequacy of the current secondary PM standards,
considered the available scientific evidence and quantitative
information in this review, along with CASAC advice and public
comments, and concluded that the current secondary PM standards provide
requisite protection against visibility impairment.
    Some commenters additionally contend that the EPA's evaluation of
public welfare effects of PM in the proposal solely focuses on fine PM
and ignores coarse PM. These commenters assert that trends data show
that coarse PM is increasing, which they believe to be a concern to
public welfare.
    We disagree with the commenters that the EPA's proposal failed to
consider the public welfare implications of coarse PM. First, we note
that there is limited new scientific evidence available in this review
on climate- and materials-related effects of coarse PM beyond that of
the last review (85 FR 24131, April 30, 2020). With regard to the
contribution of coarse PM to visibility impairment, we first note that
at the time of the last review, the EPA noted that PM2.5 is
the size fraction of PM responsible for most of the visibility
impairment in urban areas (U.S. EPA, 2020, p. 5-22). Data available for
PM10-2.5 was very limited in the last review and was not
used in quantitative analyses of estimated PM2.5 light
extinction (U.S. EPA, 2020, Appendix D, section D-1). Since the time of
the last review, an expansion of PM10-2.5 monitoring efforts
has increased the availability of data for use in estimating light
extinction with both fine and coarse fractions of PM. As described in
the PA, the analyses of visibility impairment were updated in this
review to include consideration of the coarse fraction of PM in
estimating light extinction in the subset of areas with
PM10-2.5 monitoring data available for the time period of
interest (U.S. EPA, 2020, section <IP_ADDRESS>, Appendix D). The updated
analyses in this review included 20 sites that measured both
PM10 and PM2.5 (U.S. EPA, 2020, section <IP_ADDRESS>,
Appendix D), all of which meet the current 24-hour PM2.5 and
PM10 standards. All of these sites have 3-year visibility at
or below 30 dv regardless of whether light extinction is calculated

with or without the coarse fraction, and for all three versions of the
IMPROVE equation used in this review. Generally, the contribution of
the coarse fraction of PM to light extinction in these locations was
minimal, contributing less than 1 dv to the 3-year visibility metric
(U.S. EPA, 2020, section <IP_ADDRESS>, Appendix D). While there were not
monitoring data available to evaluate the impact of coarse PM on
estimates of light extinction in locations expected to have higher
concentrations of coarse PM, the coarse fraction may be a more
important contributor to light extinction and visibility impairment
than in those areas included in the PA analyses in this review. As
additional information and monitoring data become available to further
evaluate the impact of coarse PM on estimates of light extinction in
more locations, including geographical locations expected to have high
concentrations of coarse PM, such information will be considered in a
future PM NAAQS review.
    Several commenters in support of revising the secondary PM
standards to protect against visibility impairment, generally recommend
revisions to elements of the secondary standard and visibility index
(indicator, averaging time, form, and level) consistent with those
supported by the CASAC and public comments in previous PM reviews. We
address comments on the elements of a visibility index and a revised
standard for visibility effects below.
    With regard to an indicator for the secondary standards to protect
against visibility impairment, a number of commenters suggest that the
EPA failed to explain why the current indicator is adequate and pointed
to recommendations from the CASAC in the PM reviews completed in 2012
and 2006 with regard to alternate indicators. As noted by the
commenters, in the 2012 review, the CASAC recommended three alternate
indicators for a secondary standard to protect against visibility
impairment: (1) Using direct, continuous measurement of PM light
extinction to support hourly or multi-hour daylight-only averaging
time(s); (2) using PM speciation data to calculate seasonal (or
monthly) regional species and relative humidity values to combine with
the denser continuous PM2.5 monitoring network to calculate
hourly PM light extinction; or, (3) using hourly PM2.5 as a
basis for a sub-daily (hourly or multi-hour) daylight-only indicator,
which would intentionally remove the variable influence of water from
the regulatory metric. In the 2006 review, as noted by the commenters,
the CASAC recommended a PM2.5 mass indicator, coupled with
revisions to the averaging time, form, and level of the standard, to
protect against visibility impairment.
    The EPA generally agrees with commenters that an indicator based on
directly measured light extinction would provide the most direct link
between PM in ambient air and PM-related visibility impairment.
However, as noted in the proposal (85 FR 24138, April 30, 2020,
sections IV.B.1 and IV.D.1), the Administrator concluded that in the
absence of a monitoring network to directly measure light extinction,
he judged that estimated light extinction, as calculated using the
IMPROVE algorithms, continues to provide a reasonable basis for
defining a target level of protection against PM-related visibility
impairment in the current review. There has been little progress in
development of such a monitoring network since the time of the last
review when CASAC concluded that, in the absence of such a monitoring
network, relying on a calculated PM2.5 light extinction
indicator based on PM2.5 components and relative humidity
represented a reasonable approach and that the inputs for calculating
light extinction were readily available through existing monitoring
networks and approved monitoring protocols (78 FR 3205, January 15,
2013). Further, in this review, the CASAC generally agreed with the EPA
that the available evidence does not call into question the protection
afforded by the current secondary PM standards and concurs that they
should be retained.
    With regard to the elements of the visibility index, in considering
the adequacy of the current secondary PM standards to protect against
visibility impairment, as described in the proposal (85 FR 24135, April
30, 2020), the Administrator first defined an appropriate target level
of protection in terms of a PM visibility index. In defining this
target level of protection, the Administrator first considered the
indicator of such an index. He noted that, given the lack of
availability of methods and an established network for directly
measuring light extinction, a visibility index based on estimates of
light extinction by PM2.5 components derived from an
adjusted version of the original IMPROVE algorithm would be most
appropriate, consistent with the last review. As described in the
proposal and above (section IV.A.2.a.i), the IMPROVE algorithm
estimates light extinction using routinely monitored components of
PM2.5 and PM10-2.5, along with estimates of
relative humidity. The Administrator, while recognizing that some
revisions to the IMPROVE algorithm have been made since the time of the
last review, noted that the fundamental relationship between ambient PM
and light extinction has changed very little and the different versions
of the IMPROVE algorithms can appropriately reflect this relationship
across the U.S. (85 FR 24138, April 30, 2020). As such, he judged that
defining a target level of protection in terms of estimated light
extinction continues to be a reasonable approach in the current review.
    With regard to averaging time, commenters were critical of the 24-
hour averaging time to protect against visibility impairment and argue
for a sub-daily averaging time. While some comments clearly focused on
the averaging time of the current secondary PM2.5 standard,
other comments were unclear as to whether they recommended a sub-daily
averaging time for the secondary PM2.5 standard or for the
visibility index used in defining a target level of the protection.
Nonetheless, all of these commenters contend that people do not
perceive visibility impairment over a 24-hour period, but rather their
perception of impairment ranges from minutes to multiday, and that
daylight hours are much more important in terms of visibility
impairment, particularly in urban areas. As with comments on the
indicator of the standard, some commenters also point to previous CASAC
advice on the need for a sub-daily standard.
    In defining the characteristics of a visibility index, the EPA
continues to believe that a 24-hour averaging time is reasonable. This
is in part based on analyses conducted in the last review that showed
relatively strong correlations between 24-hour and sub-daily (i.e., 4-
hour average) PM2.5 light extinction from the analyses
conducted in the last review (85 FR 24138, April 30, 2020; 78 FR 3226,
January 15, 2013), indicating that a 24-hour averaging time is an
appropriate surrogate for the sub-daily time periods relevant for
visual perception. The EPA believes that these analyses continue to
provide support for consideration of a 24-hour averaging time for the
visibility index in this review. The EPA also recognizes that the
longer averaging time may be less influenced by atypical conditions
and/or atypical instrument performance (85 FR 24138, April 30, 2020; 78
FR 3226, January 15, 2013). When taken together, the available
scientific information and updated analyses of calculated light
extinction available in this review continue to support that a 24-hour
averaging time is appropriate when defining a target level of
protection

against visibility impairment in terms of a visibility index.
    Moreover, the EPA disagrees with commenters that a secondary
PM2.5 standard with a 24-hour averaging time does not
provide requisite protection against the public welfare impacts of
visibility impairment. At the time of the last review, the EPA
recognized that hourly or sub-daily (i.e., 4- to 6-hour) averaging
times, within daylight hours and excluding hours with high relative
humidity, are more directly related to the short-term nature of
visibility impairment and the relevant viewing periods for segments of
the viewing public than a 24-hour averaging time. At that time, the EPA
agreed that a sub-daily averaging time would generally be preferable.
However, the Agency noted significant data quality uncertainties
associated with the instruments that would provide hourly
PM2.5 mass concentrations necessary to inform a sub-daily
averaging time. These uncertainties, as described in the last review,
included short-term variability in hourly data from available
continuous monitoring methods, which would prohibit establishing a sub-
daily averaging time (78 FR 3209, January 15, 2013). For all of these
reasons, the EPA continues to believe that a sub-daily averaging time
is not supported by the information available in this review.
    With regard to the form of the visibility index, many of the
commenters contend that the form used in evaluating visibility
impairment is not appropriate. First, commenters contend that a 90th
percentile form is too low and excludes too many days that could have
visibility impairment. These same commenters also suggest that a 3-year
average form is not justified and does not protect visibility and
public welfare. These commenters also argue that the EPA failed to
consider the 98th percentile form for the visibility index as a part of
the proposal. Second, some commenters recommend a form for the
visibility index within the range of 95th to 98th percentile, coupled
with a multi-hour sub-daily averaging time, consistent with the CASAC
advice in the 2006 review.
    The EPA disagrees with these commenters on both points. With regard
to the form of the visibility index, the EPA continues to conclude that
a 3-year average of annual 90th percentile values is appropriate. In so
doing, the EPA notes that a 3-year average form provides stability from
the occasional effect of inter-annual meteorological variability that
can result in unusually high pollution levels for a particular year,
consistent with the decision in the last review (78 FR 3198, January
15, 2013; U.S. EPA, 2011, p. 4-58). With regard to the annual
statistical form to be averaged over 3-years, the EPA considers the
evaluation in the 2010 UFVA of three different statistics: 90th, 95th,
and 98th percentiles (U.S. EPA, 2010b, chapter 4). In considering the
alternative statistical forms, the 2011 PA noted that the Regional Haze
Program targets the 20 percent most impaired days for improvements in
visual air quality in Federal Class I areas and that the median of the
distribution of these 20 percent worst days would be the 90th
percentile. The 2011 PA further noted that strategies that are
implemented so that 90 percent of days would 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. Finally, the 2011 PA recognized that the
public preference studies available at the time of the last review did
not address frequency of occurrence of different levels of visibility
and did not identify a basis for a different target for urban areas
than for Federal Class I areas (U.S. EPA, 2011, p. 4-59). The analyses
and considerations for the form of a visibility index from the 2011 PA
continue to provide support for a 90th percentile form, averaged across
three years, in defining the characteristics of a visibility index in
this review.
    Some commenters contend that the EPA's proposal to retain the level
of 30 dv for a visibility index is arbitrary, capricious, and not
technically sound. These commenters assert that the EPA failed to
consider recent research studies that provide a meta-analysis of
visibility preference studies that suggest that a level of 30 dv is
unacceptable to study participants included in the meta-analysis.
    As an initial matter, as described above, the studies cited by the
commenters in support of their rationale were either published after
the cutoff date for the literature search for the ISA (Malm et al.,
2019) or were not peer-reviewed studies that met the inclusion criteria
for the ISA (Malm et al., 2011; Malm, 2013, 2016; Molenar and Malm,
2012). The EPA provisionally considered the Malm et al. (2019) study
and concludes that this study does not sufficiently alter the
conclusions reached in the ISA regarding PM and visibility effects.
    With regard to a level of 30 dv for the visibility index, the EPA
believes that it is appropriate to establish a target level of
protection based on the upper end of the range of levels of visibility
impairment judged to be acceptable by at least 50% of study
participants in the available visibility preference studies (U.S. EPA,
2020, section <IP_ADDRESS>). The 2011 PA identified a range of levels from
20 to 30 dv based on the responses in the public preference studies
available at that time. Given the lack of new preferences studies
available in this review, the EPA again relies on the same studies and
the range of levels identified in those studies in the current review.
As described in detail in the PA (U.S. EPA, 2020, sections <IP_ADDRESS> and
5.5), there are a number of uncertainties and limitations associated
with the public preference studies, including those described in
section IV.B.2 above. Recognizing these uncertainties and limitations,
the EPA concludes that substantial degrees of variability and
uncertainty in the public preference studies should be reflected in a
target level of protection at the upper end of the range than if the
information was more consistent and certain. Therefore, the EPA
believes that 30 dv is an appropriate level for a visibility index in
this review.
    A number of commenters advocate for a more stringent standard,
recommending that the level of the secondary PM2.5 standards
be lowered. Some commenters reference the recommendations of previous
CASAC panels for revisions to the secondary 24-hour PM2.5
standard. Additionally, some commenters contend that the secondary
PM2.5 standards should be set equal to the primary
PM2.5 standards, with some of the commenters aligning their
support for their position with their recommendations for revisions to
the primary PM2.5 standards in this review.
    We disagree with the commenters that the secondary PM2.5
standard should be revised to provide additional public welfare
protection beyond that achieved under the current standard. Based on
the available scientific and quantitative information, and for the
reasons discussed above, the EPA concludes that it is appropriate to
define a target level of protection in terms of a visibility index
based on estimated light extinction with a 24-hour averaging time, a 3-
year 90th percentile form, and a level of 30 dv. In having concluded
that this visibility index is appropriate, the EPA then considers the
degree of protection from visibility impairment afforded by the
existing standard. In so doing, we consider results of updated analyses
of calculated light extinction that demonstrate that, in areas meeting
the current PM mass-based standards, the target level of protection in
terms of a visibility index is also achieved (85 FR 24135, April 30,
2020; U.S. EPA, 2020, section <IP_ADDRESS>). The results of these analyses
(as described in detail in

section IV.A.3.a above and in section <IP_ADDRESS> of the PA) demonstrate
that the 3-year visibility metric is at or below about 30 dv in all
areas meeting the current PM2.5 standard, and below 25 dv in
most areas. For those areas with available PM10-2.5
monitoring data, which met both the current 24-hour PM2.5
and PM10 standards, 3-year visibility metrics were at or
below 30 dv regardless of if the coarse fraction was included in the
calculation (U.S. EPA, 2020, section <IP_ADDRESS>). Given the results of
these analyses, the Administrator concluded at the time of proposal
that the updated scientific evidence and quantitative information
support the adequacy of the current secondary PM2.5 and
PM10 standards to protect against PM-related visibility
impairment (85 FR 24138-24139, April 30, 2020).
    With regard to comments recommending to set the secondary
PM2.5 standards equal to the current primary
PM2.5 standards, these commenters do not provide a basis for
their recommendation, nor do they provide a rationale for revising the
secondary PM2.5 standards to their recommended revised
levels of the primary PM2.5 standards. However, we note that
the primary annual PM2.5 standard, with its lower level,
would be the controlling standard. The EPA disagrees that such
revisions would be appropriate, for all of the reasons discussed above.
4. Administrator's Conclusions
    In considering the adequacy of the current secondary PM standards
in this review, the Administrator has carefully considered the: (1)
Policy-relevant evidence and conclusions contained in the ISA; (2) the
quantitative information presented and assessed in the PA; (3) the
evaluation of this evidence, the quantitative information, and the
rationale and conclusions presented in the PA; (4) the advice and
recommendations from the CASAC; and (5) public comments, as addressed
in section IV.B.3 above. In the discussion below, the Administrator
gives weight to the PA conclusions, with which the CASAC concurred, as
summarized in section IV.D of the proposal, and takes note of key
aspects of the rationale for those conclusions that contribute to his
decision in this review. After giving careful consideration to all of
this information, the Administrator believes that the conclusions and
policy judgments supporting his proposed decision remain valid and the
secondary PM standards should be retained.
    In considering the PA evaluations and conclusions, the
Administrator specifically takes note of the overall conclusions that
the welfare effects evidence and quantitative information are generally
consistent with what was considered in the last review (U.S. EPA, 2020,
section 5.4). In so doing, he additionally notes that the CASAC
supports retaining the current standard agreeing with the EPA ``that
the available evidence does not call into question the protection
afforded by the current secondary PM standards'' (Cox, 2019a, p. 3 of
letter). As noted below, the newly available welfare effects evidence,
critically assessed in the ISA as part of the full body of current
evidence, reaffirms conclusions on the visibility, climate, and
materials effects recognized in the last review, including key
conclusions on which the current standard is based. Further, as
discussed in more detail above, the updated quantitative analyses of
visibility impairment for areas meeting the current standards support
the adequacy of the current secondary PM2.5 and
PM10 standards to protect against PM-related visibility
impairment. The Administrator also recognizes limitations and
uncertainties continue to be associated with the available information.
    With regard to the current evidence on visibility effects, as
summarized in the PA and discussed in detail in the ISA, the
Administrator takes note of the long-standing body of evidence for PM-
related visibility impairment. This evidence, which is based on the
fundamental relationship between light extinction and PM mass,
demonstrates that ambient PM can impair visibility in both urban and
remote areas, and has changed very little since the last review (U.S.
EPA, 2019, section 13.1; U.S. EPA, 2009a, section 9.2.5). The evidence
related to public perception of visibility impairment comes from
studies from four areas in North America. These studies provide
information to inform our understanding of levels of visibility
impairment that the public judged to be ``acceptable'' (U.S. EPA,
2010b; 85 FR 24131, April 30, 2020). In considering these public
preference studies, the Administrator notes that, as described in the
ISA, no new visibility studies have been conducted in the U.S. and
there is little newly available information with regard to acceptable
levels of visibility impairment in the U.S. The Administrator
recognizes that visibility impairment can have implications for
people's enjoyment of daily activities and their overall well-being,
and therefore, considers the degree to which the current secondary
standards protect against PM-related visibility impairment.
    Based on the considerations discussed above in sections IV.B.2 and
IV.B.3, the Administrator first concludes, consistent with the last
review, that a target level of protection for a secondary PM standard
is most appropriately defined in terms of a visibility index that
directly takes into account the factors (i.e., species composition and
relative humidity) that influence the relationship between
PM2.5 in ambient air and PM-related visibility impairment.
In defining a target level of protection, the Administrator has
considered the specific aspects of such an index, including the
appropriate indicator, averaging time, form, and level.
    First, with regard to indicator, the Administrator notes that in
the last review, the EPA used an index based on estimates of light
extinction by PM2.5 components calculated using an adjusted
version of the IMPROVE algorithm. As described above (section IV.A.3),
this algorithm allows the estimation of light extinction using
routinely monitored components of PM2.5 and
PM10-2.5, along with estimates of relative humidity. The
Administrator recognizes that, while there have been some revisions to
the IMPROVE algorithm since the time of the last review, our
fundamental understanding of the relationship between PM in ambient air
and light extinction has changed little and the various IMPROVE
algorithms can appropriately reflect this relationship across the U.S.
In the absence of a monitoring network for direct measurement of light
extinction (section IV.A.3), he concludes that calculated light
extinction indicator that utilizes the IMPROVE algorithms continues to
provide a reasonable basis for defining a target level of protection
against PM-related visibility impairment in the current review.
    In further defining the characteristics of a visibility index, the
Administrator next considers the appropriate averaging time, form, and
level of the index. Given the available scientific information in this
review, and in considering the CASAC's advice and public comments, the
Administrator concludes that, consistent with the decision in the last
review, a visibility index with a 24-hour averaging time and a form
based on the 3-year average of annual 90th percentile values remains
reasonable in this review. With regard to the averaging time and form
of such an index, the Administrator takes note of analyses conducted in
the last review that demonstrated relatively strong correlations
between 24-hour and sub-daily (i.e., 4-hour average) PM2.5
light extinction (78 FR 3226, January 15,

2013), indicating that a 24-hour averaging time is an appropriate
surrogate for the sub-daily time periods of the perception of PM-
related visibility impairment and the relevant exposure periods for
segments of the viewing public. This decision also recognized that a
24-hour averaging time may be less influenced by atypical conditions
and/or atypical instrument performance (78 FR 3226, January 15, 2013).
The Administrator recognizes that there is no new information in the
current review to support updated analyses of this nature, and
therefore, he believes these analyses continue to provide support for
consideration of a 24-hour averaging time for a visibility index in
this review. With regard to the statistical form of the index, the
Administrator notes that, consistent with the last review: (1) A multi-
year percentile form offers greater stability from the occasional
effect of inter-annual meteorological variability (78 FR 3198, January
15, 2013; U.S. EPA, 2011, p. 4-58); (2) a 90th percentile represents
the median of the distribution of the 20 percent worst visibility days,
which are targeted in Federal Class I areas by the Regional Haze
Program; and (3) public preference studies did not provide information
to identify a different target than that identified for Federal Class I
areas (U.S. EPA, 2011, p. 4-59). Therefore, the Administrator judges
that a visibility index based on estimates of light extinction, with a
24-hour averaging time and a 90th percentile form, averaged over three
years, remains appropriate.
    With regard to the level of a visibility index, the Administrator
judges that it is appropriate to establish a target level of protection
of 30 dv, reflecting the upper end of the range of visibility
impairment judged to be acceptable by at least 50% of study
participants in the available public preference studies (78 FR 3226,
January 15, 2013). The 2011 PA identified a range of levels from 20 to
30 dv based on the responses in the public preference studies available
at that time. At the time of the last review, the Administrator noted a
number of uncertainties and limitations in public preference studies,
including the small number of stated preference studies available, the
relatively small number of study participants and the extent to which
the study participants may not be representative of the broader study
area population in some of the studies, and the variations in the
specific materials and methods used in each study. In considering the
available preference studies, with their inherent uncertainties and
limitations, the prior Administrator concluded that the substantial
degree of variability and uncertainty in the public preference studies
should be reflected in a target level of protection based on the upper
end of the range of CPLs.
    Given that there are no new preference studies available in this
review, the Administrator notes that his judgments are based on the
same studies, with the same range of levels, available in the last
review. The Administrator recognizes a number of limitations and
uncertainties associated with these studies, as identified in the PA
(U.S. EPA, 2020, section 5.5), including the following: (1) Available
studies may not represent the full range of preferences for visibility
in the U.S. population, particularly given the potential variability in
preferences based on the conditions commonly encountered and the scenes
being viewed; (2) available preference studies were conducted 15 to 30
years ago and may not accurately represent the current day preferences
of people in the U.S.; (3) the variety of methods used in the
preference studies may potentially influence the responses as to what
level of impairment is deemed acceptable; and (4) factors that are not
captured in the methods of the preference studies, such as the time of
day when light extinction is the greatest or the frequency of
impairment episodes, may influence people's judgment on acceptable
visibility (U.S. EPA, 2020, section <IP_ADDRESS>). Therefore, in considering
the scientific information, with its uncertainties and limitations, as
well as public comments on the level of the target level of protection
against visibility impairment, the Administrator concludes that it is
appropriate to again use a level of 30 dv for the visibility index.
    Having concluded that the protection provided by a standard defined
in terms of a PM2.5 visibility index, with a 24-hour
averaging time, and a 90th percentile form, averaged over 3 years, set
at a level of 30 dv, is requisite to protect public welfare with regard
to visual air quality, the Administrator next considers the degree of
protection from visibility impairment afforded by the existing
secondary PM standards. This determination requires considering such
protection not in isolation but in the context of the full suite of
secondary standards.
    In this context, the Administrator has considered the degree of
protection from visibility afforded by the existing secondary
PM2.5 standards. The Administrator has considered both
whether the existing 24-hour PM2.5 standard of 35 [micro]g/
m\3\ is sufficient (i.e., not under-protective) and whether it is not
more stringent than necessary (i.e., not over-protective).
    As discussed in section IV.A.3 above, the Administrator considers
the updated analyses of visibility impairment presented in the PA (U.S.
EPA, 2020, section <IP_ADDRESS>), which reflect a number of improvements
since the last review. Specifically, the updated analyses examine
multiple versions of the IMPROVE equation, including the version
incorporating revisions since the time of the last review (section
IV.A.3.a above). These updated analyses provide a further understanding
of how variation in the inputs to the algorithms impact the estimates
of light extinction (U.S. EPA, 2020, Appendix D). Additionally, for a
subset of monitoring sites with available PM10-2.5 data, the
updated analyses better characterize the influence of coarse PM on
light extinction than in the last review (U.S. EPA, 2020, section
<IP_ADDRESS>).
    As discussed above in section IV.A.3.a, the results of the updated
analyses are consistent with those from the last review. Regardless of
which version of the IMPROVE equation is used, the analyses demonstrate
that, based on 2015-2017 data, the 3-year visibility metric is at or
below about 30 dv in all areas meeting the current 24-hour
PM2.5 standard, and below 25 dv in most of those areas. In
locations with available PM10-2.5 monitoring, which met both
the current 24-hour secondary PM2.5 and PM10
standards, 3-year visibility index metrics were at or below 30 dv
regardless of whether the coarse fraction was included as an input to
the algorithm for estimating light extinction (U.S. EPA, 2020, section
<IP_ADDRESS>). While the inclusion of the coarse fraction had a relatively
modest impact on the estimates of light extinction, as noted in
responding to comments in section IV.B.3 above, the Administrator
recognizes the continued importance of the PM10 standard
given the potential for larger impacts on light extinction in areas
with higher coarse particle concentrations, which were not included in
the PA's analyses due to a lack of available data (U.S. EPA, 2019,
section <IP_ADDRESS>; U.S. EPA, 2020, section <IP_ADDRESS>). He notes that the
air quality analyses showed that all areas meeting the existing 24-hour
PM2.5 standard, with its level of 35 [micro]g/m\3\, had
visual air quality at least as good as 30 dv, based on the visibility
index. Thus, the secondary 24-hour PM2.5 standard would
likely be controlling relative to a 24-hour visibility index set at a
level of 30 dv. Additionally, areas would be unlikely to exceed the
target level of protection for visibility of 30 dv without

also exceeding the existing secondary 24-hour standard. Thus, the
Administrator judges that the 24-hour PM2.5 standard
provides sufficient protection in all areas against the effects of
visibility impairment--i.e., that the existing 24-hour PM2.5
standard would provide at least the target level of protection for
visual air quality of 30 dv which he judges appropriate.
    With respect to the non-visibility welfare effects of PM in ambient
air, the Administrator concludes that it is generally appropriate to
retain the existing standards and that there is insufficient
information to establish any distinct secondary PM standards to address
climate and materials effects of PM. With regard to climate, he
recognizes that there have been a number of improvements and
refinements to climate models since the last review. However, as
discussed in sections IV.A.3.b and IV.B.3 above, while the evidence
continues to support a causal relationship between PM and climate
effects (U.S. EPA, 2019, section 13.3.9), the Administrator notes that
significant limitations continue to exist related to quantifying the
contributions of direct and indirect effects of PM and PM components on
climate forcing (U.S. EPA, 2020, sections <IP_ADDRESS>.1 and 5.4). He also
recognizes that that models continue to exhibit considerable
variability in estimates of PM-related climate impacts at regional
scales (e.g., ~100 km) as compared to simulations at global scales.
Therefore, the resulting uncertainty leads the Administrator to
conclude that the available scientific information in this review
remains insufficient to quantify climate impacts associated with
particular concentrations of PM in ambient air (U.S. EPA, 2020, section
<IP_ADDRESS>.1) or to evaluate or consider a level of PM air quality in the
U.S. to protect against climate effects and that there is insufficient
information available at this time to base a national ambient standard
on climate impacts.
    With regard to materials effects, the Administrator notes that the
evidence available in this review continues to support a causal
relationship between materials effects and PM deposition (U.S. EPA,
2019, section 13.4). He recognizes that the deposition of fine and
coarse particles to materials can lead to physical damage and/or
impaired aesthetic qualities. Particles can contribute to materials
damage by adding to the natural weathering processes and by promoting
the corrosion of metals, the degradation of painted surfaces, the
deterioration of building materials, and the weakening of material
components. While some new information is available in this review, as
discussed in sections IV.A.3.b and IV.B.3 above, this information is
primarily conducted outside the U.S. in areas where PM concentrations
in ambient air are typically higher than those observed in the U.S.
(U.S. EPA, 2020, section 13.4). Additionally, the newly available
information in this review does not support quantitative analyses of
PM-related materials effects in this review (U.S. EPA, 2020, section
<IP_ADDRESS>.2). Given the limited amount of information available and its
inherent uncertainties and limitations, the Administrator concludes
that he is unable to relate soiling or damage to specific levels of PM
in ambient air or to evaluate or consider a level of air quality to
protect against such materials effects, and that there is insufficient
information available in this review to support a distinct national
ambient standard based on materials effects.
    With regard to the secondary PM standards, the Administrator
concludes that it is appropriate to retain the existing secondary PM
standards, without revision. This conclusion is based on the
considerations discussed above in sections IV.A.3.b and IV.B.2,
including the latest scientific information and the advice of the
CASAC, and the public comments received on the proposal, as discussed
above in section IV.B.3. For visibility effects, this decision also
reflects his consideration of the evidence for PM-related light
extinction, together with his consideration of the updated analyses of
the protection provided against visibility impairment by the current
secondary PM2.5 and PM10 standards. For climate
and materials effects, this conclusion reflects his judgment that,
although it remains important to maintain secondary PM2.5
and PM10 standards to provide some degree of control over
long- and short-term concentrations of both fine and coarse particles,
there is insufficient information to establish distinct secondary PM
standards to address non-visibility PM-related welfare effects. The
Administrator concurs with the advice of the CASAC, which agrees ``that
the available evidence does not call into question the protection
afforded by the current secondary PM standards'' and recommends that
the secondary standards ``should be retained'' (Cox, 2019a, p. 3 of
letter). This is also consistent with the conclusions at the time of
the proposal (IV.B.2) and with the majority of public comments received
on the proposed decision (section IV.B.3).
    In addition, the Administrator judges that, based on his review of
the science and his judgment that air quality should be maintained to
provide the target level of protection for visual air quality of 30 dv
(as discussed in more detail above), the degree of public welfare
protection provided by the current secondary standards is not greater
than warranted. This judgment, together with the fact that no CASAC
member expressed support for a less stringent standard, leads the
Administrator to conclude that standards less stringent than the
current secondary standards (e.g., with higher levels) are also not
supported.
    Thus, based on his consideration of the evidence and analyses for
welfare effects, his consideration of the CASAC's advice and public
comments on the secondary standards, and in the absence of information
that would support establishment of any different standards, the
Administrator concludes that it is appropriate to retain the current
24-hour and annual PM2.5 standards and the 24-hour
PM10 standard, without revision.

D. Decision on the Secondary PM Standards

    For the reasons discussed above and taking into account information
and assessments presented in the ISA and PA, advice from the CASAC, and
consideration of public comments, the Administrator concludes that the
current secondary PM standards are requisite to protect public welfare
from known or anticipated adverse effects and is retaining the
standards, without revision.

V. Statutory and Executive Order Reviews

    Additional information about these statutes and Executive Orders
can be found at http://www2.epa.gov/laws-regulations/laws-and-executive-orders.

A. Executive Order 12866: Regulatory Planning and Review and Executive
Order 13563: Improving Regulation and Regulatory Review

    The Office of Management and Budget (OMB) determined that this
action is a significant regulatory action and it was submitted to OMB
for review. Changes made during Executive Order 12866 review have been
documented in the docket. Because this action does not change the
existing PM NAAQS, it does not impose costs or benefits relative to the
baseline of continuing with the current NAAQS in effect. Thus, the EPA
has not prepared a Regulatory Impact Analysis for this action.

B. Executive Order 13771: Reducing Regulations and Controlling
Regulatory Costs

    This action is not an Executive Order 13771 regulatory action.
There are no costs or cost savings compared to the current baseline for
this action because EPA is retaining the current standards.

C. Paperwork Reduction Act (PRA)

    This action does not impose an information collection burden under
the PRA. There are no information collection requirements directly
associated with a decision to retain a NAAQS without any revision under
section 109 of the CAA and this action retains the current PM NAAQS
without any revisions.

D. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action will not impose any requirements on small entities. Rather, this
action retains, without revision, existing national standards for
allowable concentrations of PM in ambient air as required by section
109 of the CAA. See also American Trucking Associations v. EPA, 175
F.3d 1027, 1044-45 (D.C. Cir. 1999) (NAAQS do not have significant
impacts upon small entities because NAAQS themselves impose no
regulations upon small entities), reviewed in part on other grounds,
Whitman v. American Trucking Associations, 531 U.S. 457 (2001).

E. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in
the UMRA, 2 U.S.C. 1531-1538, and does not significantly or uniquely
affect small governments. This action imposes no enforceable duty on
any state, local, or tribal governments or the private sector.

F. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the national government and the states, or on the distribution of power
and responsibilities among the various levels of government.

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

    This action does not have tribal implications, as specified in
Executive Order 13175. It does not have a substantial direct effect on
one or more Indian Tribes. This action does not change existing
regulations; it retains the existing PM NAAQS, without revision.
Executive Order 13175 does not apply to this action.

H. Executive Order 13045: Protection of Children From Environmental
Health Risks and Safety Risks

    This action is not subject to Executive Order 13045 because it is
not economically significant as defined in Executive Order 12866. The
health effects evidence for this action, which includes evidence for
effects in children, is summarized in section II.B above and is
described in the ISA and PA, copies of which are in the public docket
for this action.

I. Executive Order 13211: Actions Concerning Regulations That
Significantly Affect Energy Supply, Distribution or Use

    This action is not a ``significant energy action'' as defined by
Executive Order 13211 (66 FR 28355, May 22, 2001) because it is not
likely to have a significant adverse effect on the supply,
distribution, or use of energy and has not otherwise been designated as
a significant energy action by the Administrator of the Office of
Information and Regulatory Affairs (OIRA).

J. National Technology Transfer and Advancement Act (NTTAA)

    This action does not involve technical standards.

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

    The EPA believes that this action does not have disproportionately
high and adverse human health or environmental effects on minority,
low-income populations and/or indigenous peoples, as specified in
Executive Order 12898 (59 FR 7629, February 16, 1994). The action
described in this document is to retain without revision the existing
PM NAAQS based on the Administrator's conclusions that the existing
primary standards protect public health, including the health of
sensitive groups, with an adequate margin of safety, and the existing
secondary standards protect public welfare from known or anticipated
adverse effects. As discussed in section II, the EPA expressly
considered the available information regarding health effects among at-
risk populations in reaching the decision that the existing standard is
requisite.

L. Determination Under Section 307(d)

    Section 307(d)(1)(V) of the CAA provides that the provisions of
section 307(d) apply to ``such other actions as the Administrator may
determine.'' Pursuant to section 307(d)(1)(V), the Administrator
determines that this action is subject to the provisions of section
307(d).

M. Congressional Review Act (CRA)

    This action is subject to the CRA, and the EPA will submit a rule
report to each House of the Congress and to the Comptroller General of
the United States. The Administrator of OIRA has not determined that
this action is a ``major rule'' as defined by 5 U.S.C. 804(2).

References

Abt Associates, Inc. (2001). Assessing public opinions on visibility
impairment due to air pollution: Summary report. Research Triangle
Park, NC, U.S. Environmental Protection Agency.
Abt Associates, Inc. (2005). Particulate matter health risk
assessment for selected urban areas: Draft report. Research Triangle
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