(U.S. EPA, 2020, section 4.2). These uncertainties contribute to the
determinations in the 2019 ISA that the evidence for key
PM10-2.5-related health effects is ``suggestive of, but not
sufficient to infer'' causal relationships (U.S. EPA, 2019). In light
of his emphasis on evidence supporting ``causal'' and ``likely to be
causal'' relationships (II.A.2, III.A.2), the Administrator judges that
the PM10-2.5-related health effects evidence provides an
uncertain scientific foundation for making standard-setting decisions.
He further judges that, as in the last review, limitations in this
evidence raise questions as to whether additional public health
improvements would be achieved by revising the existing PM10
standard.
    In reaching conclusions on the primary PM10 standard,
the Administrator also considers advice from the CASAC. As noted above,
the CASAC recognizes the uncertainties in the evidence for
PM10-2.5-related health effects, stating that ``key
uncertainties identified in the last review remain'' (Cox, 2019a, p. 13
of consensus responses). Given these uncertainties, the CASAC agrees
with the PA conclusion that the evidence ``does not call into question
the adequacy of the public health protection afforded by the current
primary PM10 standard'' (Cox, 2019a, p. 3 of letter). The
CASAC further recommends that this evidence ``supports consideration of
retaining the current standard in this review'' (Cox, 2019a, p. 3 of
letter).
    When the above information is taken together, the Administrator
proposes to conclude that the available scientific evidence continues
to support a PM10 standard to provide some measure of
protection against PM10-2.5 exposures. This conclusion
reflects the expanded evidence for PM10-2.5-related health
effects in the current review. However, important limitations in the
evidence remain. Consistent with the decision in the last review, the
Administrator proposes to conclude that these limitations lead to
considerable uncertainty regarding the potential public health
implications of revising the existing PM10 standard. Given
this uncertainty, and consistent with the CASAC's advice, the
Administrator proposes to conclude that the available evidence does not
call into question the adequacy of the public health protection
afforded by the current primary PM10 standard. Therefore, he
proposes to retain the primary PM10 standard, without
revision, in the current review. The Administrator solicits comment on
this proposed decision and on the supporting rationale described above.

IV. Rationale for Proposed Decisions on the Secondary PM Standards

    This section presents the rationale for the Administrator's
proposed decision to retain the current secondary PM standards, without
revision. This rationale is based on a thorough review of the latest
scientific information generally published through December 2017,\45\
as presented in the ISA, on non-ecological public welfare effects

associated with PM and pertaining to the presence of PM in ambient air.
The Administrator's rationale also takes into account the PA's
evaluation of the policy-relevant information in the ISA and
quantitative analyses of air quality related to visibility impairment
and the CASAC's advice and recommendations, as reflected in discussions
of the drafts of the ISA and PA at public meetings and in the CASAC's
letters to the Administrator.
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    \45\ In addition to the review's opening ``call for
information'' (79 FR 71764, December 3, 2014), ``the current ISA
identified and evaluated studies and reports 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). References that are cited in the ISA, the references that were
considered for inclusion but not cited, and electronic links to
bibliographic information and abstracts can be found at: https://hero.epa.gov/hero/particulate-matter.
---------------------------------------------------------------------------

    In presenting the rationale for the Administrator's proposed
decision and its foundations, section IV.A provides background on the
general approach for review of the secondary PM standards, including a
summary of the approach used in the last review (section IV.A.1) and
the general approach for the current review (section IV.A.2). Section
IV.B summarizes the currently available evidence for PM-related
visibility impairment and section IV.C summarizes the available
information for other PM-related welfare effects. Section IV.D presents
the Administrator's proposed conclusions on the current secondary PM
standards.

A. General Approach

    In the last review of the PM NAAQS, completed in 2012, the EPA
retained the secondary 24-hour PM2.5 standard, with its
level of 35 [mu]g/m\3\, and the 24-hour PM10 standard, with
its level of 150 [micro]g/m\3\ (78 FR 3228, January 15, 2013). The EPA
also retained the level, set at 15 [micro]g/m\3\, and averaging time of
the secondary annual PM2.5 standard, while revising the
form. With regard to the form of the annual PM2.5 standard,
the EPA removed the option for spatial averaging (78 FR 3228, January
15, 2013). Key aspects of the Administrator's decisions on the
secondary PM standards for non-visibility effects and visibility
effects are described below in section IV.A.1.
1. Approach Used in the Last Review
    The 2012 decision on the adequacy of the secondary PM standards was
based on consideration of the protection provided by those standards
for visibility and for the non-visibility effects of materials damage,
climate effects and ecological effects. As noted earlier, the current
review of the public welfare protection provided by the secondary PM
standards against ecological effects is occurring in the separate, on-
going review of the secondary NAAQS for oxides of nitrogen and oxides
of sulfur (U.S. EPA, 2016, Chapter 1, section 5.2; U.S. EPA, 2020,
Chapter 1, section 5.1.1). Thus, the consideration of ecological
effects in the 2012 review is not discussed here. Rather, the sections
below focus on the prior Administrator's consideration of climate and
materials effects (section IV.A.1.a) and visibility effects (section
IV.A.1.b).
a. Non-Visibility Effects
    With regard to the role of PM in climate, the prior Administrator
considered whether it was appropriate to establish any distinct
secondary PM standards to address welfare effects associated with
climate impacts. In considering the scientific evidence, she noted the
2009 ISA conclusion ``that a causal relationship exists between PM and
effects on climate'' and that aerosols \46\ alter climate processes
directly through radiative forcing and by indirect effects on cloud
brightness, changes in precipitation, and possible changes in cloud
lifetimes (U.S. EPA, 2009c, section 9.3.10). Additionally, the major
aerosol components with the potential to affect climate processes
(i.e., black carbon (BC), organic carbon (OC), sulfates, nitrates and
mineral dusts) vary in their reflectivity, forcing efficiencies, and
direction of climate forcing (U.S. EPA, 2009c, section 9.3.10).
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    \46\ In the climate sciences research community, PM is
encompassed by what is typically referred to as aerosol. An aerosol
is defined as a solid or liquid suspended in a gas, but PM refers to
the solid or liquid phase of an aerosol. In this review of the
secondary PM NAAQS the discussion on climate effects of PM uses the
term PM throughout for consistency with the ISA (U.S. EPA, 2019) as
well as to emphasize that the climate processes altered by aerosols
are generally altered by the PM portion of the aerosol. Exceptions
to this practice include the discussion of climate effects in the
last review, when aerosol was used when discussing suspending
aerosol particles, and for certain acronyms that are widely used by
the climate community that include the term aerosol (e.g., aerosol
optical depth, or AOD).
---------------------------------------------------------------------------

    Noting the strong evidence indicating that aerosols affect climate,
the prior Administrator further considered what the available
information indicated regarding the adequacy of protection provided by
the secondary PM standards. She noted that a number of uncertainties in
the scientific information affected our ability to quantitatively
evaluate the standards in this regard. For example, the ISA and PA
noted the spatial and temporal heterogeneity of PM components that
contribute to climate forcing, uncertainties in the measurement of
aerosol components, inadequate consideration of aerosol impacts in
climate modeling, insufficient data on local and regional microclimate
variations and heterogeneity of cloud formations. In light of these
uncertainties and the lack of sufficient data, the 2011 PA concluded
that it was not feasible in the last review ``to conduct a quantitative
analysis for the purpose of informing revisions [to the secondary PM
NAAQS] based on climate'' (U.S. EPA, 2011, pp. 5-11 to 5-12) and that
there was insufficient information available to base a national ambient
air quality standard on climate impacts associated with ambient air
concentrations of PM or its constituents (U.S. EPA, 2011, section
5.2.3). The prior Administrator agreed with this conclusion (78 FR
3225-3226, January 15, 2013).
    With regard to materials effects, the she also considered effects
associated with the deposition of PM (i.e., dry and wet deposition),
including both physical damage (materials effects) and aesthetic
qualities (soiling effects). The deposition of PM can physically affect
materials, adding to the effects of natural weathering processes, by
promoting or accelerating the corrosion of metals; by degrading paints;
and by deteriorating building materials such as stone, concrete, and
marble (U.S. EPA, 2009c, section 9.5). Additionally, the deposition of
PM from ambient air can reduce the aesthetic appeal of buildings and
objects through soiling. The ISA concluded that evidence was
``sufficient to conclude that a causal relationship exists between PM
and effects on materials'' (U.S. EPA, 2009c, sections 2.5.4 and 9.5.4).
However, the 2011 PA noted that quantitative relationships were lacking
between particle size, concentrations, and frequency of repainting and
repair of surfaces and that considerable uncertainty exists in the
contributions of co-occurring pollutants to materials damage and
soiling processes (U.S. EPA, 2011, p. 5-29). The 2011 PA concluded that
none of the evidence available in the last review called into question
the adequacy of the existing secondary PM standards to protect against
material effects (U.S. EPA, 2011, p. 5-29). The prior Administrator
agreed with this conclusion (78 FR 3225-3226, January 15, 2013).
    In considering non-visibility welfare effects in the last review,
as discussed above, the prior Administrator concluded that, while it is
important to maintain an appropriate degree of control of fine and
coarse particles to address non-visibility welfare effects, ``[i]n the
absence of information that would support any different standards . . .
it is appropriate to retain the existing suite of secondary standards''
(78 FR 3225-3226, January 15, 2013). Her decision was consistent with
the CASAC advice related to non-visibility effects. Specifically, the
CASAC agreed with the 2011 PA conclusions that, while these effects are
important, ``there is not currently a strong technical basis

to support revisions of the current standards to protect against these
other welfare effects'' (Samet, 2010a, p. 5). Thus, the prior
Administrator concluded that it was appropriate to retain all aspects
of the existing 24-hour PM2.5 and PM10 secondary
standards. With regard to the secondary annual PM2.5
standard, she concluded that it was appropriate to retain a level of
15.0 [mu]g/m\3\ while revising only the form of the standard to remove
the option for spatial averaging (78 FR 3225-3226, January 15, 2013).
b. Visibility Effects
    Having reached the conclusion to retain the existing secondary PM
standards to protect against non-visibility welfare effects, the prior
Administrator next considered the level of protection that would be
requisite to protect public welfare against PM-related visibility
impairment and whether to adopt a distinct secondary standard to
achieve this level of protection. In reaching her final decision that
the existing 24-hour PM2.5 standard provides sufficient
protection against PM-related visibility impairment (78 FR 3228,
January 15, 2013), she considered the evidence assessed in the 2009 ISA
(U.S. EPA, 2009c) and the analyses included in the Urban-Focused
Visibility Assessment (2010 UFVA; U.S. EPA, 2010b) and the 2011 PA
(U.S. EPA, 2011). She also considered the degree of protection for
visibility that would be provided by the existing secondary standard,
focusing specifically on the secondary 24-hour PM2.5
standard with its level of 35 [micro]g/m\3\. These considerations, and
the prior Administrator's conclusions regarding visibility are
discussed in more detail below.
    In the last review, the ISA concluded that, ``collectively, the
evidence is sufficient to conclude that a causal relationship exists
between PM and visibility impairment'' (U.S. EPA, 2009c, p. 2-28).
Visibility impairment is caused by light scattering and absorption by
suspended particles and gases, including water content of aerosols.\47\
The available evidence in the last review indicated that specific
components of PM have been shown to contribute to visibility
impairment. For example, at sufficiently high relative humidity values,
sulfate and nitrate are the PM components that scatter more light and
thus contribute most efficiently to visibility impairment. Elemental
carbon (EC) and organic carbon (OC) are also important contributors,
especially in the northwestern U.S. where their contribution to
PM2.5 mass is higher. Crustal materials can be significant
contributors to visibility impairment, particularly for remote areas in
the arid southwestern U.S. (U.S. EPA, 2009c, section 2.5.1).
---------------------------------------------------------------------------

    \47\ All particles scatter light and, although a larger particle
scatters more light than a similarly shaped smaller particle of the
same composition, the light scattered per unit of mass is greatest
for particles with diameters from ~0.3-1.0 [micro]m (U.S. EPA,
2009c, section 2.5.1). Particles with hygroscopic components (e.g.,
particulate sulfate and nitrate) contribute more to light extinction
at higher relative humidity than at lower relative humidity because
they change size in the atmosphere in response to relative humidity.
---------------------------------------------------------------------------

    Visibility impairment can have implications for people's enjoyment
of daily activities and for their overall sense of well-being (U.S.
EPA, 2009c, section 9.2). In consideration of the potential public
welfare implication of various degrees of PM-related visibility
impairment, the prior Administrator considered the available visibility
preference studies that were part of the overall body of evidence in
the 2009 ISA and reviewed as a part of the 2010 UFVA. These preference
studies provided information about the potential public welfare
implications of visibility impairment from surveys in which
participants were asked questions about their preferences or the values
they placed on various visibility conditions, as displayed to them in
scenic photographs or in images with a range of known light extinction
levels.\48\
---------------------------------------------------------------------------

    \48\ Preference studies were available in four urban areas in
the last review. Three western preference studies were available,
including one in Denver, Colorado (Ely et al., 1991), one in the
lower Fraser River valley near Vancouver, British Columbia, Canada
(Pryor, 1996), and one in Phoenix, Arizona (BBC Research &
Consulting, 2003). A pilot focus group study was also conducted for
Washington, DC (Abt Associates, 2001), and a replicate study with 26
participants was also conducted for Washington, DC (Smith and
Howell, 2009). More details about these studies are available in
Appendix D of the PA.
---------------------------------------------------------------------------

    In noting the relationship between PM concentrations and PM-related
light extinction, the prior Administrator focused on identifying an
adequate level of protection against visibility-related welfare
effects. She first concluded that a standard in terms of a
PM2.5 visibility index would provide a measure of protection
against PM-related light extinction that directly takes into account
the factors (i.e., species composition and relative humidity) that
influence the relationship between PM2.5 in ambient air and
PM-related visibility impairment. A PM2.5 visibility index
standard would afford a relatively high degree of uniformity of visual
air quality protection in areas across the country by directly
incorporating the effects of differences of PM2.5
composition and relative humidity. In defining a target level of
protection in terms of a PM2.5 visibility index, as
discussed below, she considered specific elements of the index,
including the basis for its derivation, as well as an appropriate
averaging time, level, and form.
    With regard to the basis for derivation of a visibility index, the
prior Administrator concluded that it was appropriate to use an
adjusted version of the original IMPROVE algorithm,\49\ in conjunction
with monthly average relative humidity data based on long-term
climatological means. In so concluding, she noted the CASAC conclusion
on the reasonableness of reliance on a PM2.5 light
extinction indicator calculated from PM2.5 chemical
composition and relative humidity. In considering alternative
approaches for a focus on visibility, she recognized that the available
mass monitoring methods did not include measurement of the full water
content of ambient PM2.5, nor did they provide information
on the composition of PM2.5, both of which contribute to
visibility impacts (77 FR 38980, June 29, 2012). In addition, at the
time of the proposal, she recognized that suitable equipment and
performance-based verification procedures did not then exist for direct
measurement of light extinction and could not be developed within the
time frame of the review (77 FR 38980-38981, June 29, 2012).
---------------------------------------------------------------------------

    \49\ The revised IMPROVE algorithm (Pitchford et al., 2007) uses
major PM chemical composition measurements and relative humidity
estimates to calculate light extinction. For more information about
the derivation of and input data required for the original and
revised IMPROVE algorithms, see 78 FR 3168-3177, January 15, 2013.
---------------------------------------------------------------------------

    With regard to the averaging time of the index, the prior
Administrator concluded that a 24-hour averaging time would be
appropriate for a visibility index (78 FR 3226, January 15, 2013).
Although she recognized that hourly or sub-daily (4- to 6-hour)
averaging times, within daylight hours and excluding hours with
relatively high humidity, are more directly related to the short-term
nature of the perception of PM-related visibility impairment and
relevant exposure periods for segments of the viewing public than a 24-
hour averaging time, she also noted that there were data quality
uncertainties associated with the instruments used to provide the
hourly PM2.5 mass measurements required for an averaging
time shorter than 24 hours. She also considered the results of analyses
that compared 24-hour and 4-hour averaging times for calculating the
index. These analyses showed good correlation between 24-hour and 4-
hour

average PM2.5 light extinction, as evidenced by reasonably
high city-specific and pooled R-squared values, generally in the range
of over 0.6 to over 0.8. Based on these analyses and the 2011 PA
conclusions regarding them, the prior Administrator concluded that a
24-hour averaging time would be a reasonable and appropriate surrogate
for a sub-daily averaging time.
    With regard to the statistical form of the index, the prior
Administrator settled on a 3-year average of annual 90th percentile
values. In so doing, she noted that a 3-year average form provided
stability from the occasional effect of inter-annual meteorological
variability that can result in unusually high pollution levels for a
particular year (78 FR 3198, January 15, 2013; U.S. EPA, 2011, p. 4-
58).\50\ Regarding the annual statistic to be averaged, the 2010 UFVA
evaluated three different statistics: 90th, 95th, and 98th percentiles
(U.S. EPA, 2010b, chapter 4). In considering these alternative
percentiles, 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. Lastly, the 2011 PA recognized that the
available studies on people's preferences 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 that for Class I
areas (U.S. EPA, 2011, p. 4-59). These considerations led the prior
Administrator to conclude that 90th percentile form was the most
appropriate annual statistic to be averaged across three years (78 FR
3226, January 15, 2013).
---------------------------------------------------------------------------

    \50\ The EPA recognized that a percentile form averaged over
multiple years offers greater stability to the air quality
management process by reducing the possibility that statistically
unusual indicator values will lead to transient violations of the
standard, thus reducing the potential for disruption of programs
implementing the standard and reducing the potential for disruption
of the protections provided by those programs.
---------------------------------------------------------------------------

    With regard to the level of the index, she considered the
visibility preferences studies conducted in four urban areas (U.S. EPA,
2011, p. 4-61). Based on these studies, the PA identified a range of
levels from 20 to 30 deciviews (dv) \51\ as being a reasonable range of
``candidate protection levels'' (CPLs).\52\ In considering this range
of CPLs, she noted the 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. She 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 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 3226-3227, January 15, 2013).
---------------------------------------------------------------------------

    \51\ Deciview (dv) refers to a scale for characterizing
visibility that is defined directly in terms of light extinction.
The deciview scale is frequently used in the scientific and
regulatory literature on visibility.
    \52\ For comparison, 20 dv, 25 dv, and 30 dv are equivalent to
64, 112, and 191 megameters (Mm-\.1\), respectively.
---------------------------------------------------------------------------

    Based on her considerations and conclusions summarized above, the
prior Administrator concluded that the protection provided by a
secondary standard based on a 3-year visibility metric, defined in
terms of 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, would be requisite to protect public welfare with regard to visual
air quality (78 FR 3227, January 15, 2013). Having reached this
conclusion, she next determined whether an additional distinct
secondary standard in terms of a visibility index was needed given the
degree of protection from visibility impairment afforded by the
existing secondary standards. Specifically, she noted 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 defined above (Kelly et al., 2012b, Kelly et al., 2012a). 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 prior Administrator judged 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
[she] judges appropriate'' (78 FR 3227, January 15, 2013). She further
judged that ``[s]ince sufficient protection from visibility impairment
would be provided for all areas of the country without adoption of a
distinct secondary standard, and adoption of a distinct secondary
standard will not change the degree of over-protection for some areas
of the country. . . adoption of such a distinct secondary standard is
not needed to provide requisite protection for both visibility and
nonvisibility related welfare effects'' (78 FR 3228, January 15, 2013).
2. Approach for the Current Review
    To evaluate whether it is appropriate to consider retaining the
current secondary PM standards, or whether consideration of revision is
appropriate, the EPA has adopted an approach in this review that builds
upon the general approach used in the last review and reflects the body
of evidence and information now available. As summarized above, past
approaches have been based most fundamentally on using information from
studies of PM-related visibility effects, quantitative analyses of PM-
related visibility impairment, information from studies of non-
visibility welfare effects, advice from the CASAC, and public comments
to inform the selection of secondary PM standards that, in the
Administrator's judgment, protect the public welfare from any known or
anticipated effects.
    Similarly, in this review, the EPA draws on the available evidence
and quantitative assessments pertaining to the public welfare impacts
of PM in ambient air. In considering the scientific and technical
information, the Agency considers both the information available at the
time of the last review and the information that is newly available in
this review. This includes information on PM-related visibility and
non-visibility effects. Consistent with the approach in the last
review, the quantitative air quality analyses for PM-related visibility
effects provide a context for interpreting the evidence of visibility
impairment and the potential public welfare significance of PM
concentrations in ambient air associated with recent air quality
conditions.

B. PM-Related Visibility Impairment

    The information summarized here is based on the EPA's scientific
assessment of the latest evidence on visibility effects associated with
PM; this assessment is documented in the ISA

and its policy implications are further discussed in the PA. In
considering the scientific and technical information, the PA reflects
upon both the information available in the last review and information
that is newly available since the last review. Policy implications of
the currently available evidence are discussed in the PA (as summarized
in section IV.D.1). The subsections below briefly summarize the
following aspects of the evidence: The nature of PM-related visibility
impairment (section IV.B.1), the relationship between ambient PM and
visibility (section IV.B.2), and public perception of visibility
impairment (section IV.B.3).
1. Nature of PM-Related Visibility Impairment
    Visibility refers to the visual quality of a human's view with
respect to color rendition and contrast definition. It is the ability
to perceive landscape form, colors, and textures. Visibility involves
optical and psychophysical properties involving human perception,
judgment, and interpretation. Light between the observer and the object
can be scattered into or out of the sight path and absorbed by PM or
gases in the sight path. The conclusions of the ISA that ``the evidence
is sufficient to conclude that a causal relationship exists between PM
and visibility impairment'' is consistent with conclusions of causality
in the last review (U.S. EPA, 2019, section 13.2.6). These conclusions
are based on strong and consistent evidence that ambient PM can impair
visibility in both urban and remote areas (U.S. EPA, 2019, section
13.1; U.S. EPA, 2009c, section 9.2.5).
2. Relationship Between Ambient PM and Visibility
    The fundamental relationship between light extinction and PM mass,
and the EPA's understanding of this relationship, has changed little
since the 2009 ISA (U.S. EPA, 2009c). The combined effect of light
scattering and absorption by particles and gases is characterized as
light extinction, i.e., the fraction of light that is scattered or
absorbed per unit of distance in the atmosphere. Light extinction is
measured in units of 1/distance, which is often expressed in the
technical literature as visibility per megameter (abbreviated
Mm-\1\). Higher values of light extinction (usually given in
units of Mm-\1\ or dv) correspond to lower visibility. When
PM is present in the air, its contribution to light extinction is
typically much greater than that of gases (U.S. EPA, 2019, section
13.2.1). The impact of PM on light scattering depends on particle size
and composition, as well as relative humidity. All particles scatter
light, as described by the Mie theory, which relates light scattering
to particle size, shape, and index of refraction (U.S. EPA, 2019,
section 13.2.3; Van de Hulst, 1981; Mie, 1908). Fine particles scatter
more light than coarse particles on a per unit mass basis and include
sulfates, nitrates, organics, light-absorbing carbon, and soil (Malm et
al., 1994). Hygroscopic particles like ammonium sulfate, ammonium
nitrate, and sea salt increase in size as relative humidity increases,
leading to increased light scattering (U.S. EPA, 2019, section 13.2.3).
    Direct measurements of PM light extinction, scattering, and
absorption are considered more accurate for quantifying visibility than
PM mass-based estimates because measurements do not depend on
assumptions about particle characteristics (e.g., size, shape, density,
component mixture, etc.) (U.S. EPA, 2019, section <IP_ADDRESS>).
Measurements of light extinction can be made with high time resolution,
allowing for characterization of subdaily temporal patterns of
visibility impairment. A variety of measurement methods have been used
(e.g., transmissometers, integrating nephelometers, teleradiometers,
telephotometers, and photography and photographic modeling), each with
its own strengths and limitations (U.S. EPA, 2019, Table 13-1).
However, there are no common performance-based criteria to evaluate
these methods and none have been deployed broadly across the U.S. for
routine measurement of visibility impairment.
    In the absence of a robust monitoring network for the routine
measurement of light extinction across the U.S., estimation of light
extinction based on existing PM monitoring can be used. A theoretical
relationship between light extinction and PM characteristics has been
derived from Mie theory (U.S. EPA, 2019, Equation 13.5) and can be used
to estimate light extinction by combining mass scattering efficiencies
of particles with particle concentrations (U.S. EPA, 2019, section
13.2.3; U.S. EPA, 2009c, sections <IP_ADDRESS> and <IP_ADDRESS>). However,
routine ambient air monitoring rarely includes measurements of particle
size and composition information with sufficient detail for these
calculations. Accordingly, a much simpler algorithm has been developed
to make estimating light extinction more practical.
    This algorithm, known as the IMPROVE algorithm,\53\ provides for
the estimation of light extinction (bext), in units of
Mm-\1\, using routinely monitored components of fine
(PM2.5) and coarse (PM10-2.5) PM. Relative
humidity data are also needed to estimate the contribution by liquid
water that is in solution with the hygroscopic components of PM. To
estimate each component's contribution to light extinction, their
concentrations are multiplied by extinction coefficients and are
additionally multiplied by a water growth factor that accounts for
their expansion with moisture. Both the extinction efficiency
coefficients and water growth factors of the IMPROVE algorithm have
been developed by a combination of empirical assessment and theoretical
calculation using particle size distributions associated with each of
the major aerosol components (U.S. EPA, 2019, section <IP_ADDRESS>, section
<IP_ADDRESS>).
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    \53\ The algorithm is referred to as the IMPROVE algorithm as it
was developed specifically to use monitoring data generated at
IMPROVE network sites and with equipment specifically designed to
support the IMPROVE program and was evaluated using IMPROVE optical
measurements at the subset of monitoring sites that make those
measurements (Malm et al., 1994).
---------------------------------------------------------------------------

    The original IMPROVE algorithm, so referenced here to distinguish
it from subsequent variations developed later, was found to
underestimate the highest light scattering values and overestimate the
lowest values at IMPROVE monitors throughout the U.S. (Malm and Hand,
2007; Ryan et al., 2005; Lowenthal and Kumar, 2004) and at sites in
China (U.S. EPA, 2019, section <IP_ADDRESS>). To resolve these biases, a
revised IMPROVE equation was developed (Pitchford et al., 2007). Since
the last review, Lowenthal and Kumar (2016) further offered a number of
modifications to the revised IMPROVE equation, with a focus of the
application of the IMPROVE equation in remote sites. In particular, one
of the modifications was to increase the multiplier to estimate the
concentration of organic matter, [OM], from the concentration of
organic carbon, [OC]. This modification was based on their evaluations
of monitoring data from remote IMPROVE sites, which showed that in
areas further away from PM sources, PM mass is often more oxygenated
and contains a larger amount of organic PM. (U.S. EPA, 2019, section
<IP_ADDRESS>). As discussed below in section IV.D.1, analyses conducted in
the current review estimate PM-related visibility impairment using each
of these versions of the IMPROVE equation.

3. Public Perception of Visibility Impairment
    In the last review, visibility preference studies were available
from four areas in North America.\54\ Study participants were queried
regarding multiple images that, depending on the study, were either
photographs of the same location and scenery that had been taken on
different days on which measured extinction data were available or
digitized photographs onto which a uniform ``haze'' had been
superimposed. Results of those studies indicated a wide range of
judgments on what study participants considered to be acceptable
visibility across the different study areas, depending on the setting
depicted in each photograph. As a part of the 2010 UFVA, each study was
evaluated separately, and figures were developed to display the
percentage of participants that rated the visual air quality depicted
as ``acceptable'' (U.S. EPA, 2010b). Based on the results of the
studies in the four cities, a range encompassing the PM2.5
visibility index values from images that were judged to be acceptable
by at least 50% of study participants across all four of the urban
preference studies was identified (U.S. EPA, 2010b, p. 4-24; PA, Figure
5-2). Much lower visibility (considerably more haze resulting in higher
values of light extinction) was considered acceptable in Washington,
DC, than was in Denver, and 30 dv reflected the highest degree of
visibility impairment judged to be acceptable by at least 50 percent of
study participants (78 FR 3226-3227, January 15, 2013).
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    \54\ Preference studies were available in four urban areas in
the last review: Denver, Colorado (Ely et al., 1991), Vancouver,
British Columbia, Canada (Pryor, 1996), Phoenix, Arizona (BBC
Research & Consulting, 2003), and Washington, DC (Abt Associates,
2001; Smith and Howell, 2009).
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    Since the time of the last review, no new visibility preference
studies have been conducted in the U.S. Similarly, there is little
newly available information regarding acceptable levels of visibility
impairment in the U.S.

C. Other PM-Related Welfare Effects

    The information summarized here is based on the EPA's scientific
assessment of the latest evidence on the non-visibility welfare effects
associated with PM. This assessment is documented in the ISA and its
policy implications are further discussed in the PA. In considering the
scientific and technical information, the PA reflects consideration of
both the information available in the last review and information that
is newly available since the last review. The subsections below briefly
summarize the evidence related to climate effects (section IV.C.1) and
materials effects (section IV.C.2).
1. Climate
    In this review, as in the last review, the ISA concludes that
``overall the evidence is sufficient to conclude that a causal
relationship exists between PM and climate effects'' (U.S. EPA, 2019,
section 13.3.9). Since the last review, climate impacts have been
extensively studied and recent research reinforces and strengthens the
evidence evaluated in the 2009 ISA. New evidence provides greater
specificity about the details of radiative forcing effects \55\ and
increases the understanding of additional climate impacts driven by PM
radiative effects. The Intergovernmental Panel on Climate Change (IPCC)
assesses the role of anthropogenic activity in past and future climate
change, and since the last review, has issued the Fifth IPCC Assessment
Report (AR5; IPCC, 2013) which summarizes any key scientific advances
in understanding the climate effects of PM since the previous report.
As in the last review, the ISA draws substantially on the IPCC report
to summarize climate effects. As discussed in more detail below, the
general conclusions are similar between the IPCC AR4 and AR5 reports
with regard to effects of PM on global climate.
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    \55\ Radiative forcing (RF) for a given atmospheric constituent
is defined as the perturbation in net radiative flux, at the
tropopause (or the top of the atmosphere) caused by that
constituent, in watts per square meter (Wm-2), after allowing for
temperatures in the stratosphere to adjust to the perturbation but
holding all other climate responses constant, including surface and
tropospheric temperatures (Fiore et al., 2015; 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>).
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    Atmospheric PM has the potential to affect climate in multiple
ways, including absorbing and scattering of incoming solar radiation,
alterations in terrestrial radiation, effects on the hydrological
cycle, and changes in cloud properties (U.S. EPA, 2019, section
13.3.1). Atmospheric PM interacts with incoming solar radiation. Many
species of PM (e.g., sulfate and nitrate) efficiently scatter solar
energy. By enhancing reflection of solar energy back to space,
scattering PM exerts a cooling effects on the surface below. Certain
species of PM such as black carbon (BC), brown carbon (BrC), or dust
can also absorb incoming sunlight. A recent study found that whether
absorbing PM warms or cools the underlying surface depends on several
factors, including the altitude of the PM layer relative to cloud cover
and the albedo (i.e., reflectance) of the surface (Ban-Weiss et al.,
2014). PM also perturbs incoming solar radiation by influencing cloud
cover and cloud lifetime. For example, PM provides nuclei upon which
water vapor condenses, forming cloud droplets. Finally, absorbing PM
deposited on snow and ice can diminish surface albedo and lead to
regional warming (U.S. EPA, 2019, section 13.3.2).
    PM has direct and indirect effects on climate processes. PM
interactions with solar radiation through scattering and absorption,
collectively referred to as aerosol-radiation interactions (ARI), are
also known as the direct effects on climate, as opposed to the indirect
effects that involve aerosol-cloud interactions (ACI). The direct
effects of PM on climate result primarily from particles scattering
light away from Earth and sending a fraction of solar energy back into
space, decreasing the transmission of visible radiation to the surface
of the Earth and resulting in a decrease in the heating rate of the
surface and the lower atmosphere. The IPCC AR5, taking into account
both model simulations and satellite observations, reports a radiative
forcing from aerosol-radiation interactions (RFari) from anthropogenic
PM of -0.35  0.5 watts per square meter (Wm-2)
(Boucher, 2013), which is comparable to AR4 (-0.5  0.4
Wm-2). Estimates of effective radiative forcing \56\ from
aerosol-radiation interactions (ERFari), which include the rapid
feedback effects of temperature and cloud cover, rely mainly on model
simulations, as this forcing is complex and difficult to observe (U.S.
EPA, 2019, section <IP_ADDRESS>). The IPCC AR5 best estimate for ERFari is
-0.45  0.5 Wm-2, which reflects this uncertainty
(Boucher, 2013).
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    \56\ Effective radiative forcing (ERF), new in the IPCC AR5,
takes into account not just the instantaneous forcing but also a set
of climate feedbacks, involving atmospheric temperature, cloud
cover, and water vapor, that occur naturally in response to the
initial radiative perturbation (U.S. EPA, 2019, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    By providing cloud condensation nuclei, PM increases cloud droplet
number, thereby increasing cloud droplet surface area and albedo
(Twomey, 1977). The climate effects of these perturbations are more
difficult to quantify than the direct effects of aerosols with RF but
likely enhance the cooling influence of clouds by increasing cloud
reflectivity (traditionally referred to as the first indirect effect)
and lengthening cloud lifetime (second indirect effect). These effects
are reported as the radiative

forcing from aerosol-cloud interaction (ERFaci) (U.S. EPA, 2019,
section <IP_ADDRESS>).\57\ IPCC AR5 estimates ERFaci at -0.45
Wm-2, with a 90% confidence interval of -1.2 to 0
Wm-2 (U.S. EPA, 2019, section <IP_ADDRESS>). Studies have also
calculated the combined effective radiative forcing from aerosol-
radiation and aerosol-cloud interactions (ERFari+aci) (U.S. EPA, 2019,
section <IP_ADDRESS>). IPCC AR5 reports a best estimate of ERFari+aci of -
0.90 (-1.9 to -0.1) Wm-2, consistent with these estimates
(Boucher, 2013).
---------------------------------------------------------------------------

    \57\ While the ISA includes estimates of RFaci and ERFaci from a
number of studies (U.S. EPA, 2019, sections <IP_ADDRESS>, <IP_ADDRESS>,
<IP_ADDRESS>), this discussion focuses on the single best estimate with
a range of uncertainty, as reported in the IPCC AR5 (Boucher, 2013).
---------------------------------------------------------------------------

    PM can also strongly reflect incoming solar radiation in areas of
high albedo, such as snow- and ice-covered surfaces. The transport and
subsequent deposition of absorbing PM such as BC to snow- and ice-
covered regions can decrease the local surface albedo, leading to
surface heating. The absorbed energy can then melt the snow and ice
cover and further depress the albedo, resulting in a positive feedback
loop (U.S. EPA, 2019, section <IP_ADDRESS>; Bond et al., 2013; U.S. EPA,
2012b). Deposition of absorbing PM, such as BC, may also affect surface
temperatures over glacial regions (U.S. EPA, 2019, section <IP_ADDRESS>).
The IPCC AR5 best estimate of RF from the albedo effects is +0.04
Wm-2, with an uncertainty range of +0.02 to +0.09
Wm-2 (Boucher, 2013).
    A number of new studies are available since the last review that
have exampled the individual climate effects associated with key PM
components, including sulfate, nitrate, OC, BC, and dust, along with
updated quantitative estimate of the radiative forcing with the
individual species. Sulfate particles form through oxidation of
SO2 by OH in the gas phase and in the aqueous phase by a
number of pathways, including in particular those involving ozone and
H2O2 (U.S. EPA, 2019, section <IP_ADDRESS>). The main
source of anthropogenic sulfate is from coal-fired power plants, and
global trends in the anthropogenic SO2 emissions are
estimated to have increased dramatically during the 20th and early 21st
centuries, although the recent implementation of more stringent air
pollution controls on sources has led to a reversal in such trends in
many places (U.S. EPA, 2019, section <IP_ADDRESS>; U.S. EPA, 2020, section
2.3.1). Sulfate particles are highly reflective. Consistent with other
recent estimates (Takemura, 2012, Zelinka et al., 2014, Adams et al.,
2001, described below), on a global scale, the IPCC AR5 estimates that
sulfate contributes more than other PM types to RF, with RFari of -0.4
(-0.6 to -0.2) Wm-2, where the 5% and 95% uncertainty range
is represented by the numbers in the parentheses (Myhre et al., 2013),
which is the same estimate from AR4. Sulfate is also a major
contributor to the influence of PM on clouds (Takemura, 2012). A total
effective radiative forcing (ERFari+aci) for anthropogenic sulfate has
been estimated to be nearly -1.0 Wm-2 (Zelinka et al., 2014,
Adams et al., 2001).
    Nitrate particles form through the oxidation of nitrogen oxides and
occur mainly in the form of ammonium nitrate. Ammonium preferentially
associates with sulfate rather than nitrate, leading to formation of
ammonium sulfate at the expense of ammonium nitrate (Adams et al.,
2001). As anthropogenic emissions of SO2 decline, more
ammonium will be available to react with nitrate, potentially leading
to future increases in ammonium nitrate particles in the atmosphere
(U.S. EPA, 2019, section <IP_ADDRESS>; Hauglustaine et al., 2014; Lee et
al., 2013; Shindell et al., 2013). Warmer global temperatures, however,
may decrease nitrate abundance given that it is highly volatile at
higher temperatures (Tai et al., 2010). The IPCC AR5 estimates RFari of
nitrate of -0.11 (-0.3 to -0.03) Wm-2 (Boucher, 2013), which
is one-fourth of the RFari of sulfate.
    Primary organic carbonaceous PM, including BrC, are emitted from
wildfires, agricultural fires, and fossil fuel and biofuel combustion.
SOA form when anthropogenic or biogenic nonmethane hydrocarbons are
oxidized in the atmosphere, leading to less volatile products that may
partition into PM (U.S. EPA, 2019, section <IP_ADDRESS>). Organic particles
are generally reflective, but in the case of BrC, a portion is
significantly absorbing at shorter wavelengths (<400 nm). The IPCC AR5
estimates an RFari for primary organic PM from fossil fuel combustion
and biofuel use of -0.09 (-0.16 to -0.03) Wm-\2\ and an
RFari estimate for SOA from these sources of -0.03 (-0.27 to +0.20)
Wm-\2\ (Myhre et al., 2013). Changes in the RFari estimates
for individual PM components since AR4 have generally been modest, with
one exception for the estimate for primary organic PM from fossil fuel
combustion and biofuel use (Myhre et al., 2013).\58\ The wide range in
these estimates, including inconsistent signs for forcing, reflect
uncertainties in the optical properties of organic PM and its
atmospheric budgets, including the production pathways of anthropogenic
SOA (Scott et al., 2014; Myhre et al., 2013; McNeill et al., 2012;
Heald et al., 2010). The IPCC AR5 also estimates an RFari of -0.2
Wm-\2\ for primary organic PM arising from biomass burning
(Boucher, 2013).
---------------------------------------------------------------------------

    \58\ The estimate of RFari for SOA is new in AR5 and was not
included in AR4 (Myhre et al., 2013).
---------------------------------------------------------------------------

    Black carbon (BC) particles occur as a result of inefficient
combustion of carbon-containing fuels. Like directly emitted organic
PM, BC is emitted from biofuel and fossil fuel combustion and by
biomass burning. BC is absorbing at all wavelengths and likely has a
large impact on the Earth's energy budget (Bond et al., 2013). The IPCC
AR5 estimates a RFari from anthropogenic fossil fuel and biofuel use of
+0.4 (+0.5 to +0.8) Wm-\2\ (Myhre et al., 2013). Biomass
burning contributes an additional +0.2 (+0.03 to +0.4)
Wm-\2\ to BC RFari, while the albedo effect of BC on snow
and ice adds another +0.04 (+0.02 to +0.09) Wm-\2\ (Myhre et
al., 2013; U.S. EPA, 2019, section <IP_ADDRESS>, section <IP_ADDRESS>).
    Dust, or mineral dust, is mobilized from dry or disturbed soils as
a result of both meteorological and anthropogenic activities. Dust has
traditionally been classified as scattering, but a recent study found
that dust may be substantially coarser than currently represented in
climate models, and thus more light-absorbing (Kok et al., 2017). The
IPCC AR5 estimates RFari as -0.1  0.2 Wm-\2\
(Boucher, 2013), although the results of the study by Kok et al. (2017)
would suggest that in some regions dust may have led to warming, not
cooling (U.S. EPA, 2019, section <IP_ADDRESS>).
    The new research available in this review expands upon the evidence
available at the time of the last review. Consistent with the evidence
available in the last review, the key PM components, including sulfate,
nitrate, OC, BC, and dust, that contribute to climate processes vary in
their reflectivity, forcing efficiencies, and direction of forcing.
    Radiative forcing due to PM elicits a number of responses in the
climate system that can lead to significant effects on weather and
climate over a range of spatial and temporal scales, mediated by a
number of feedbacks that link PM and climate. Since the last review,
the evidence base has expanded with respect to the mechanisms of
climate responses and feedbacks to PM radiative forcing. However, the
new literature published since the last review does not reduce the
considerable

uncertainties that continue to exist related to these mechanisms.
    Unlike well-mixed, long-lived greenhouse gases in the atmosphere,
PM has a very heterogenous distribution across the Earth. As such,
patterns of RFari and RFaci 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 variables with possible differences in the sign of the
response 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, with some studies showing relatively
close correlation between forcing and surface response temperatures
(e.g., Leibensperger et al., 2012), while other studies show much less
correlation (e.g., Levy et al., 2013). Many studies have examined
observed trends in PM and temperature in the U.S. Climate models 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 alone (U.S.
EPA, 2019, section 13.3.7). While evidence in this review suggests that
PM influenced temperature trends across the southern and eastern U.S.
in the 20th century, this evidence is not conclusive and significant
uncertainties continue to exist. 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.
    While expanded since the last review, the evidence of PM-related
climate effects is still limited by significant uncertainties,
particularly for understanding effects at regional scales. Large
spatial and temporal heterogeneities in direct and indirect PM
radiative forcing, and associated climate effects, can occur for a
number of reasons, including the frequency and distribution of
emissions of key PM components contributing to climate forcing, the
chemical and microphysical processing that occurs in the atmosphere,
and the atmospheric lifetime of PM relative to other pollutants
contributing to radiative forcing (U.S. EPA, 2019, section 13.3). In
addition to the uncertainty in characterizing radiative forcing, large
uncertainty exists in quantifying changes in specific climate variables
associated with PM-related radiative forcing. Moreover, studies have
shown that predicting climate variables for regions within the U.S.
(which is of particular interest for the review of the PM NAAQS) is
more uncertain than predicting climate variables globally due to
natural climate variability (e.g., Deser et al., 2012) and
uncertainties in the representation of key atmospheric processes in
state-of-the-art climate models. Furthermore, quantifying the influence
of incremental changes in U.S. anthropogenic emissions on regional
climate is subject to even greater uncertainty because the signal of
U.S. anthropogenic emissions is relatively small compared with the
global emissions considered in the studies cited above. Overall, these
limitations and uncertainties make it difficult to quantify how
incremental changes in the level of PM mass in ambient air in the U.S.
would result in changes to climate in the U.S. Thus, as in the last
review, the PA concludes that the data remain insufficient to conduct
quantitative analyses for PM effects on climate in the current review
(U.S. EPA, 2020, section <IP_ADDRESS>.1).
2. Materials
    In considering the evidence available in the current review of PM-
related materials effects, the current evidence continues to support
the conclusion from the last review 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. 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.\59\ The newly available evidence on materials effects of PM
in this review are primarily from studies conducted outside of the U.S.
on buildings and other items of cultural heritage and at concentrations
greater than those typically observed in the U.S.; however, they
provide limited new data for consideration in this review (U.S. EPA,
2019, section 13.4).
---------------------------------------------------------------------------

    \59\ 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.
---------------------------------------------------------------------------

    Materials damage from PM generally involves one or both of two
processes: soiling and corrosion (U.S. EPA, 2019, section 13.4.2).
Soiling and corrosion are complex, interdependent processes, typically
beginning with deposition of atmospheric PM or SO2 to
exposed surfaces. Constituents of deposited PM can interact directly
with materials or undergo further chemical and/or physical
transformation to cause soiling, corrosion, and physical damage.
Weathering, including exposure to moisture, ultraviolet (UV) radiation
and temperature fluctuations, affects the rate and degree of damage
(U.S. EPA, 2019, section 13.4.2).
    Soiling is the result of PM accumulation on an object that alters
its optical characteristics or appearance. These soiling effects can
impact the aesthetic value of a structure or result in reversible or
irreversible damage to the surface. The presence of air pollution can
increase the frequency and duration of cleaning and can enhance
biodeterioration processes on the surface of materials. For example,
deposition of carbonaceous components of PM can lead to the formation
of black crusts on surfaces, and the buildup of microbial biofilms \60\
can discolor surfaces by trapping PM more efficiently (U.S. EPA, 2009c,
p. 9-195; U.S. EPA, 2019, section 13.4.2). The presence of PM may alter
light transmission or change the reflectivity of a surface.
Additionally, the organic and nutrient content of deposited PM may
enhance microbial growth on surfaces.
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    \60\ Microbial biofilms are communities of microorganisms, which
may include bacteria, algae, fungi and lichens, that colonize an
inert surface. Microbial biofilms can contribute to biodeterioration
of materials via modification of the chemical environment.
---------------------------------------------------------------------------

    Since the last review, very little new evidence has become
available related to deposition of SO2 to materials such as
limestone, granite, and metal. Deposition of SO2 onto
limestone can transform the limestone into gypsum, resulting in a
rougher surface, which allows for increased surface area for
accumulation of deposited PM (Camuffo and Bernardi, 1993; U.S. EPA,
2019, section 13.4.2). Oxidation of deposited SO2 that
contributes to the transformation of limestone to gypsum can be
enhanced by the formation of surface coatings from deposited

carbonaceous PM (both elemental and organic carbon) (McAlister et al.,
2008, Grossi et al., 2007). Ozga et al. (2011) characterized damage to
two concrete buildings in Poland and Italy. Gypsum was the main damage
product on surfaces of these buildings that were sheltered from rain
runoff, while PM embedded in the concrete, particularly carbonaceous
particles, were responsible for darkening of the building walls (Ozga
et al., 2011).
    Building on the evidence available in the 2009 ISA, research has
progressed on the theoretical understanding of soiling of cultural
heritage in a number of studies. Barca et al. (2010) developed and
tested a new methodological approach for characterizing trace elements
and heavy metals in black crusts on stone monuments to identify the
origin of the chemicals and the relationship between the concentrations
of elements in the black crusts and local environmental conditions.
Recent research has also used isotope tracers to distinguish between
contributions from local sources versus atmospheric pollution to black
crusts on historical monuments in France (Kloppmann et al., 2011). A
study in Portugal found that biological activity played a major role in
soiling, specifically in the development of colored layers and in the
detachment process (de Oliveira et al., 2011). Another study found
damage to cement renders, often used for restoration, consolidation,
and decorative purposes on buildings, following exposure to sulfuric
acid, resulting in the formation of gypsum (Lanzon and Garcia-Ruiz,
2010).
    Corrosion of stone and the decay of stone building materials by
acid deposition and sulfate salts were described in the 2009 ISA (U.S.
EPA, 2009c, section 9.5.3). Since that time, advances have been made on
the quantification of degradation rates and further characterization of
the factors that influence damage of stone materials (U.S. EPA, 2019,
section 13.4.2). Decay rates of marble grave stones were found to be
greater in heavily polluted areas compared to a relatively pristine
area (Mooers et al., 2016). The time of wetness and the number of
dissolution/crystallization cycles were identified as hazard indicators
for stone materials, with greater hazard during the spring and fall
when these indicators are relatively high (Casati et al., 2015).
    A study examining the corrosion of steel as a function of PM
composition and particle size found that changes in the composition of
resulting rust gradually changed with particle size (Lau et al., 2008).
In a study of damage to metal materials under in Hong Kong, which
generally has much higher PM concentrations than those observed in the
U.S., Liu et al. (2015) found that iron and steel were corroded by both
PM and gaseous pollutants (SO2 and NO2), while
copper and copper alloys were mainly corroded by gaseous pollutants
(SO2 and O3) and aluminum and aluminum alloy
corrosion was mainly attributed to PM and NO2.
    A number of studies have also found materials damage from PM
components besides sulfate and black carbon and atmospheric gases
besides SO2. Studies have characterized impacts of nitrates,
NOX, and organic compounds on direct materials damage or on
chemical reactions that enhance materials damage (U.S. EPA, 2019,
section 13.4.2). Other studies have found that soiling of building
materials can be attributed to enhanced biological processes and
colonization, including the development and thickening of biofilms,
resulting from the deposition of PM components and atmospheric gases
(U.S. EPA, 2019, section 13.4.2).
    Since the last review, other materials have been studied for damage
attributable to PM, including glass and photovoltaic panels. Soiling of
glass can impact its optical and thermal properties and can lead to
increased cleaning costs and frequency. The development of haze \61\ on
modern glass has been measured and modeled, with a strong correlation
between the size distribution of particles and the evolution of the
mass deposited on the surface of the glass. Measurements showed that,
under sheltered conditions, mass deposition accelerated regularly with
time in areas closest to sources of PM (i.e., near roadways) and coarse
mineral particles were more prevalent compared to other sites (Alfaro
et al., 2012). Model predictions were found to correctly simulate the
development of haze at site locations when compared with measurements
(Alfaro et al., 2012).
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    \61\ In this discussion of non-visibility welfare effects, haze
is used as it has been defined in the scientific literature on
soiling of glass, i.e., the ratio of diffuse transmitted light to
direct transmitted light (Lombardo et al., 2010). This differs from
the definition of haze as used in the discussion of visibility
welfare effects in section V.B above, where it is used as a
qualitative description of the blockage of sunlight by dust, smoke,
and pollution.
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    Soiling of photovoltaic panels can lead to decreased energy
efficiency. For example, soiling by carbonaceous PM decreased solar
efficiency by nearly 38%, while soil particles reduced efficiency by
almost 70% (Radonjic et al., 2017). The rate of photovoltaic power
output can also be degraded by soiling and has been found to be related
to the rate of dust accumulation. In five sites in the U.S.
representing different meteorological and climatological
conditions,\62\ photovoltaic module power transmission was reduced by
approximately 3% for every g/m\2\ of PM deposited on the cover plate of
the photovoltaic panel, independent of geographical location (Boyle et
al., 2017). Another study found that photovoltaic module power output
was reduced by 40% after 10 months of exposure without cleaning,
although a number of anti-reflective coatings can generally mitigate
power reduction resulting from dust deposition (Walwil et al., 2017).
Energy efficiency can also be impacted by the soiling of building
materials, such as light-colored marble panels on building exteriors,
that are used to reflect a large portion of solar radiation for passive
cooling and to counter the urban heat island effect. Exposure to acidic
pollutants in urban environments have been found to reduce the solar
reflectance of marble, decreasing the cooling effect (Rosso et al.,
2016). Highly reflective roofs, or cool roofs, have been designed and
constructed to increase reflectance from buildings in urban areas, to
both decrease air conditioning needs and urban heat island effects, but
these efforts can be impeded by soiling of materials used for
constructing cool roofs. Methods have been developed for accelerating
the aging process of roofing materials to better characterize the
impact of soiling and natural weather on materials used in constructing
cool roofs (Sleiman et al., 2014).
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    \62\ Of the five sites studied, three were in rural, suburban,
and urban areas representing a semi-arid environment (Front Range of
Colorado), one site represented a hot and humid environment (Cocoa,
Florida), and one represented a hot and arid environment
(Albuquerque, New Mexico) (U.S. EPA, 2019, section 13.4.2; Boyle et
al., 2017).
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    Some progress has been made since the last review in the
development of dose-response relationships for soiling of building
materials, yet some key relationships remain poorly characterized. The
first general dose-response relationships for soiling of materials were
generated by measuring contrast reflectance of a soiled surface to the
reflectance of the unsoiled substrate for different materials,
including acrylic house paint, cedar siding, concrete, brick,
limestone, asphalt shingles, and window glass with varying total
suspended particulate (TSP) concentrations (Beloin and Haynie, 1975;
U.S. EPA, 2019, section 13.4.3). Continued efforts to develop dose-
response curves for soiling have led to some advancements for modern
materials, but these relationships

remain poorly characterized for limestone. One study quantified the
dose-response relationships between PM10 and soiling for
painted steel, white plastic, and polycarbonate filter material, but
there was too much scatter in the data to produce a dose-response
relationship for limestone (Watt et al., 2008). A dose-response
relationship for silica-soda-lime window glass soiling by
PM10, NO2, and SO2 was quantified
based on 31 different locations (Lombardo et al., 2010; U.S. EPA, 2019,
section 13.4.3, Figure 13-32, Equation 13-8). The development of this
dose-response relationship required several years of observation time
and had inconsistent data reporting across the locations.
    Since the time of the last review, there has also been progress in
developing methods to more rapidly evaluate soiling of different
materials by PM mixtures. Modern buildings typically have simpler
lines, less detailed surfaces, and a greater use of glass, tile, and
metal, which are easier to clean than stone. There have also been major
changes in the types of materials used for buildings, including a
variety of polymers available for use as coatings and sealants. New
economic and environmental considerations beyond aesthetic appeal and
structural damage are emerging (U.S. EPA, 2019, section 13.4.3).
Changes in building materials and design, coupled with new approaches
in quantifying the dose-response relationship between PM and materials
effects, may reduce the amount of time needed for observations to
support the development of material-specific dose-response
relationships.
    In addition to dose-response functions, damage functions have also
been used to quantify material decay as a function of pollutant type
and load. Damage can be determined from sample surveys or inspection of
actual damage and a damage function can be developed to link the rate
of material damage to time of replacement or maintenance. A cost
function can then link the time for replacement and maintenance to a
monetary cost, and an economic function links cost to the dose of
pollution based on the dose-response relationship (U.S. EPA, 2019,
section 13.4.3). Damage functions are difficult to assess because it
depends on human perception of the level of soiling deemed to be
acceptable and evidence in this area remains limited in the current
review. Since the last review, damage functions for a wide range of
building materials (i.e., stone, aluminum, zinc, copper, plastic,
paint, rubber, stone) have been developed and reviewed (Brimblecombe
and Grossi, 2010). One study estimated long-term deterioration of
building materials and found that damage to durable building material
(such as limestone, iron, copper, and discoloration of stone) is no
longer controlled by pollution as was historically documented but
rather that natural weathering is a more important influence on these
materials in modern times (Brimblecombe and Grossi, 2009). Even as PM-
attributable damage to stone and metals has decreased over time, it has
been predicted that there will be potentially higher degradation rates
for polymeric materials, plastic, paint, and rubber due to increased
oxidant concentrations and solar radiation (Brimblecombe and Grossi,
2009).
    As at the time of the last review and described just above,
sufficient evidence is not available to conduct a quantitative
assessment of PM mass or component-related soiling and corrosion
effects. While soiling associated with PM can lead to increased
cleaning frequency and repainting of surfaces, no quantitative
relationships have been established between characteristics of PM or
the frequency of cleaning or repainting that would help to inform the
EPA's understanding of the public welfare implications of soiling (U.S.
EPA, 2019, section 13.4). Similarly, while some information is
available with regard to microbial deterioration of surfaces and the
contribution of carbonaceous PM to the formation of black crusts that
contribute to soiling, the available evidence does not support
quantitative analyses (U.S. EPA, 2019, section 13.4). While some new
evidence is available with respect to PM-attributable materials
effects, the data are insufficient to conduct quantitative analyses for
PM effects on materials in the current review.

D. Proposed Conclusions on the Current Secondary PM Standards

    In reaching proposed conclusions on the current secondary PM
standards, the Administrator takes into account policy-relevant
evidence-based and quantitative information-based considerations, as
well as advice from the CASAC. Evidence-based considerations draw from
the EPA's assessment and integrated synthesis of the scientific
evidence of PM-related welfare effects in the ISA (U.S. EPA, 2019,
section 13.2). Quantitative information-based considerations draw from
the EPA's assessment of recent air quality and associated PM-related
visibility impairment in the PA (U.S. EPA, 2020, Chapter 5). Section
IV.D.1 below summarizes evidence- and quantitative information-based
considerations and the associated conclusions reached in the PA.
Section IV.D.2 describes advice received from the CASAC on the
secondary standards. Section IV.D.3 presents the Administrator's
proposed decision on the current secondary PM standards.
1. Evidence- and Quantitative Information-Based Considerations in the
Policy Assessment
    The PA considers the degree to which the available scientific
evidence and quantitative information supports or calls into question
the adequacy of the protection afforded by the current secondary PM
standards. In doing so, the PA considers the evidence assessed in the
ISA, including the extent to which the new evidence for PM-related
visibility impairment, climate effects, or materials effects alters key
conclusions from the last review. The PA also considers quantitative
analyses of visibility impairment and the extent to which they may
indicate different conclusions from those in the last review regarding
the degree of protection from adverse effects provided by the current
secondary standards.
    With regard to visibility impairment, the PA presents updated
analyses based on recent air quality information, with a focus on
locations meeting the current 24-hour PM2.5 and
PM10 standards. In the absence of advances in the monitoring
methods for directly measuring light extinction, and given the lack of
a robust monitoring network for the routine measurement of light
extinction across the U.S. (section IV.B.2), as in the last review, the
PA analyses use calculated light extinction to estimate PM-related
visibility impairment (U.S. EPA, 2020, section <IP_ADDRESS>). Compared to
the last review, updated analyses incorporate several refinements.
These include (1) the evaluation of three versions of the IMPROVE
equation \63\ 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; \64\ (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 PA's
updated analyses 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).\65\
---------------------------------------------------------------------------

    \63\ Given the lack of new information to inform a different
visibility metric, the metric used in the PA 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 year, and a level of 30 dv (U.S. EPA, 2020,
section <IP_ADDRESS>).
    \64\ 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. 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.
    \65\ These sites are those that have a valid 24-hour
PM2.5 design value for the 2015-2017 period and met
strict criteria for PM species for this analysis, based on 24-hour
average PM2.5 mass and component data that were available
from monitors in the IMPROVE network, CSN, and NCore Multipollutant
Monitoring Network (U.S. EPA, 2020, Appendix D). PM10-2.5
monitoring data is available for 20 of the 67 sites examined.
---------------------------------------------------------------------------

    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 addition, the one location that exceeds the current 24-
hour PM2.5 standard also has light extinction estimates 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 from 102 sites (Kelly et al., 2012b; 78 FR 3201,
January 15, 2013).
    When light extinction is calculated using the updated IMPROVE
equation from Lowenthal and Kumar (2016), the resulting 3-year
visibility metrics are slightly higher at all sites compared to light
extinction calculated using the IMPROVE equations used in previous
reviews (U.S. EPA, 2020, Figure 5-4). These results are consistent with
the higher OC multiplier included in the IMPROVE equation from
Lowenthal and Kumar (2016), reflecting the use of data from remote
areas with higher concentrations of organic PM when validating that
equation. As such, it is important to note that the Lowenthal and Kumar
(2016) version of the IMPROVE equation may overestimate light
extinction in non-remote areas, including in the urban areas in the
PA's analyses.
    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 2015-17 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 and revised IMPROVE equations) (U.S. EPA,
2020, Appendix D, Table D-3). However, the unique conditions at this
urban site (e.g., higher OC concentrations, much lower temperatures,
and the complete lack of sunlight for long periods) affect quantitative
relationships between OC, OM and visibility (e.g., Hand et al., 2012;
Hand et al., 2013), making the most appropriate approach for
characterizing light extinction in this area unclear.
    In 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 is often a minor
contributor to visibility impairment (U.S. EPA, 2010b), though it may
make 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
little data available from PM10-2.5 monitors to quantify the
contribution of coarse PM to calculated light extinction.
    Since 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 PM2.5 and
PM10-2.5 concentrations included as inputs in the equations.
For 2015-2017, 20 of the 67 PM2.5 sites analyzed in the PA
have collocated PM10-2.5 monitoring data available. These 20
sites meet both the 24-hour PM2.5 standard and 24-hour
PM10 standard. 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 contribution of the
coarse fraction to light extinction at these sites is minimal,
contributing less than 1 dv to the 3-year visibility metric. However,
these 20 locations would be expected to have relatively low
concentrations of coarse PM. If PM10 and PM10-2.5
data were available in locations with higher concentrations of coarse
PM, such as in the southwestern U.S., the coarse fraction may be a more
important contributor to light extinction and visibility impairment
than in the locations examined in the PA analyses.
    In summary, the findings of these updated quantitative analyses 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) always results in
higher light extinction values, which is expected given the higher OC
multiplier included in the equation and its validation using data from
remote areas far away from emissions sources. There is very little
difference in estimates of light extinction when PM10-2.5 is
included in the equation, although a somewhat larger coarse fraction
contribution to light extinction would be expected in areas with higher
coarse particle concentrations. Overall, the PA finds that updated
quantitative 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, defined
in terms of a PM visibility index.
    With regard to PM-related climate effects, the PA recognizes that
while the evidence base has expanded since the last review, the new
evidence has not appreciably improved the 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). Despite
continuing research, there are still significant limitations in
quantifying the contributions of PM and PM components to the direct and
indirect effects on climate forcing (e.g., changes to the pattern of
rainfall, changes to wind patterns, effects on vertical mixing in the
atmosphere) (U.S. EPA, 2020, sections <IP_ADDRESS>.1 and 5.4). In addition,
while a number of improvements and refinements have been made to
climate models since the last review, these 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 new research has
added to the understanding of climate forcing on a global scale, there
remain significant limitations to quantifying potential adverse effects
from PM on climate in the U.S. and how they would vary in response to
incremental changes in PM concentrations in the U.S. Overall, the PA
recognizes that while new research is available on climate forcing on a
global scale, the remaining uncertainties and limitations are
significant, and the new global scale

research does not translate directly to use at regional spatial scales.
Thus, the evidence does not provide a clear understanding at the
spatial scales needed for the NAAQS of a quantitative relationship
between concentrations of PM mass in ambient air and the associated
climate-related effects (U.S. EPA, 2020, sections <IP_ADDRESS>.1 and 5.4).
The PA concludes that the evidence does not call into question the
adequacy of the current secondary PM standards for climate effects.
    With regard to materials effects, the PA notes the availability of
new evidence in this review related to the soiling process and the
types of materials that are affected. Such evidence provides some
limited information to inform dose-response relationships and damage
functions associated with PM, though most recent studies have been
conducted outside the U.S. (U.S. EPA, 2020, section <IP_ADDRESS>.2; U.S.
EPA, 2019, section 13.4). The recent evidence includes studies
examining PM-related effects on the energy efficiency of solar panels
and passive cooling building materials, though there remains
insufficient evidence to establish quantitative relationships between
PM in ambient air and these or other materials effects (U.S. EPA, 2020,
section <IP_ADDRESS>.2). While new research has expanded the body of
evidence for PM-related materials effects, the PA recognizes the lack
of information to inform quantitative analyses assessing materials
effects or the potential public welfare implications of such effects.
Thus, the PA concludes that the evidence does not call into question
the adequacy of the current secondary PM standards for materials
effects.
    Overall, the PA recognizes that the newly available welfare effects
evidence, critically assessed in the ISA as part of the full body of
evidence, reaffirms the conclusions on the visibility, climate, and
materials effects of PM as recognized in the last review (U.S. EPA,
2020, sections <IP_ADDRESS>., <IP_ADDRESS>, and 5.4). Further, there is a general
consistency of the currently available evidence with the evidence that
was available in the last review, including with regard to key aspects
of the decision to retain the standards in the last review (U.S. EPA,
2020, sections <IP_ADDRESS>, <IP_ADDRESS>, and 5.4). The quantitative analyses
for visibility impairment for recent air quality conditions indicate a
similar level of protection against visibility effects considered to be
adverse in the last review (U.S. EPA, 2020, sections <IP_ADDRESS> and 5.4).
Collectively, the PA finds that the evidence and quantitative
information-based considerations support consideration of retaining the
current secondary PM standards, without revision (U.S. EPA, 2020,
section 5.4).
2. CASAC Advice
    As part of its review of the draft PA, the CASAC has provided
advice on the adequacy of the current secondary PM standards. In its
comments on the draft PA, the CASAC concurs with staff's overall
preliminary conclusions that it is appropriate to consider retaining
the current secondary PM 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 agrees that research on PM-related effects has
expanded since the last review, it also concludes that ``there are
still significant uncertainties associated with the accurate
measurement of PM contributions to the direct and indirect effects of
PM on climate'' (Cox, 2019a, pp. 13-14 of consensus responses). The
committee recommends 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, materials, and climate, the CASAC agrees
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).
3. Administrator's Proposed Decision on the Current Secondary PM
Standards
    This section summarizes the Administrator's considerations and
conclusions related to the current secondary PM2.5 and
PM10 standards and presents his proposed decision to retain
those standards, without revision. In establishing secondary standards
under the Act that are ``requisite'' to protect the public welfare from
any known or anticipated adverse effects, the Administrator is seeking
to establish standards that are neither more nor less stringent than
necessary for this purpose. He notes that secondary standards are not
meant to protect against all known or anticipated effects, but rather
those that are judged to be adverse to the public welfare. Consistent
with the primary standards discussed above (sections II.C.3 and
III.C.3), the Act does not require standards to be set at a zero-risk
level; but rather at a level that limits risk sufficiently so as to
protect the public welfare, but not more stringent than necessary to do
so.
    Given these requirements, the Administrator's final decision in
this review will be a public welfare policy judgment that draws upon
the scientific and technical information examining PM-related
visibility impairment, climate effects and materials effects, including
how to consider the range and magnitude of uncertainties inherent in
that information. The Administrator recognizes that his final decision
will be based on an interpretation of the scientific evidence and
technical analyses that neither overstates nor understates their
strengths and limitations, nor the appropriate inferences to be drawn.
    As an initial matter in considering the secondary standards, the
Administrator notes the longstanding body of evidence for PM-related
visibility impairment. As in the last review, this evidence continues
to demonstrate a causal relationship between ambient PM and effects on
visibility (U.S. EPA, 2019, section 13.2). The Administrator recognizes
that visibility impairment can have implications for people's enjoyment
of daily activities and for their overall sense of well-being.
Therefore, as in previous reviews, he considers the degree to which the
current secondary standards protect against PM-related visibility
impairment.
    In doing so, the Administrator adopts an approach consistent with
the approach used in the last review (section IV.A.1). That is, he
first defines an appropriate target level of protection in terms of a
PM visibility index that accounts for the factors that influence the
relationship between particles in the ambient air and visibility (i.e.,
size fraction, species composition, and relative humidity). He then
considers air quality analyses examining this PM visibility index in
locations meeting the current 24-hour PM2.5 and
PM10 standards (U.S. EPA, 2020, section <IP_ADDRESS>).
    To identify a target level of protection, the Administrator first
defines the specific characteristics of the visibility index. He 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 original IMPROVE
algorithm. As described above (sections IV.B and IV.D.1), this
algorithm allows the estimation of light extinction using routinely
monitored components of PM2.5 and PM10-2.5,\66\
along with estimates of relative humidity. While revisions have been
made to the IMPROVE algorithm since the last review (U.S. EPA, 2020,
section <IP_ADDRESS>), the Administrator recognizes that our fundamental
understanding of the relationship between ambient PM 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 directly measure light
extinction (sections IV.B.2 and IV.D.1), he judges 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.
---------------------------------------------------------------------------

    \66\ 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, 2012; U.S. EPA, 2020,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    In further defining the characteristics of a visibility index based
on estimates of light extinction, the Administrator considers the
appropriate averaging time, form, and level of the index. With regard
to the averaging time and form, the Administrator judges that the
decisions made in the last review remain reasonable. In that review, a
24-hour averaging time was selected and the form was defined as the 3-
year average of annual 90th percentile values. The decision on
averaging time recognized the 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 relevant for visual perception. This decision also recognized
that the longer averaging time may be less influenced by atypical
conditions and/or atypical instrument performance (78 FR 3226, January
15, 2013). The decision to set the form as the 3-year average of annual
90th percentile values noted that (1) a 3-year average provides
stability from the occasional effect of inter-annual meteorological
variability (78 FR 3198, January 15, 2013; U.S. EPA, 2011, p. 4-58);
(2) the 90th percentile corresponds to the median of the distribution
of the 20 percent worst days for visibility, which are targeted in
Class I areas by the Regional Haze Program; \67\ 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). Given the similar information available in the current
review, the Administrator judges that these decisions remain reasonable
and, therefore, that it remains appropriate to define a visibility
index in terms of a 24-hour averaging time and a form based on the 3-
year average of annual 90th percentile values.
---------------------------------------------------------------------------

    \67\ In the last review, 90th, 95th, and 98th percentile forms
were evaluated (U.S. EPA, 2010b, section 4.3.3; 78 FR 3198, January
15, 2013), and a standard with a 90th percentile form was reasonably
expected to limit the occurrence of days with peak PM-related light
extinction (78 FR 3198, January 15, 2013).
---------------------------------------------------------------------------

    The level of the index was set at 30 dv in the last review,
reflecting the highest degree of visibility impairment judged to be
acceptable by at least 50% of study participants in the available
visibility preference studies (78 FR 3226-3227, January 15, 2013). The
focus on 30 dv, rather than a lower level, was supported in light of
the important uncertainties and limitations in the underlying public
preference studies. Consistent with the last review, the Administrator
notes the following uncertainties and limitations in these studies
(U.S. EPA, 2020, section <IP_ADDRESS>):
     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 the visibility preferences of 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.
    Because no visibility preference studies have been conducted in the
U.S. since the last review, the Administrator recognizes that these
uncertainties and limitations persist. Therefore, in the current review
his consideration of the degree of visibility impairment constituting
an adverse public welfare impact is based on the same preference
studies, with the same uncertainties and limitations, that were
available in the last review. Drawing from this information, the
Administrator judges it appropriate to again use 30 dv as the level of
the visibility index.
    Having concluded that it remains appropriate in this review to
define the target level of protection in terms of a visibility index
based on estimated light extinction as described above (i.e., with a
24-hour averaging time; a 3-year, 90th percentile form; and a level of
30 dv), the Administrator next considers the degree of protection from
visibility impairment afforded by the existing secondary standards. He
considers the updated analyses of PM-related visibility impairment
presented in the PA (U.S. EPA, 2020, section <IP_ADDRESS>), which reflect
several improvements over the previous review. Specifically, the
updated analyses examine multiple versions of the IMPROVE algorithm,
including the version incorporating revisions since the last review
(section IV.D.1). This approach provides an improved understanding of
how variation in equation inputs impacts calculated light extinction
(U.S. EPA, 2020, Appendix D). In addition, for a subset of monitoring
sites with available PM10-2.5 data, updated analyses better
characterize the influence of the coarse fraction on light extinction
(U.S. EPA, 2020, section <IP_ADDRESS>).
    The Administrator notes that the results of these updated analyses
are consistent with the results from the last review. Regardless of the
IMPROVE equation used, they demonstrate that the 3-year visibility
metric is at or below about 30 dv in all areas meeting the current 24-
hour PM2.5 standard,\68\ and below 25 dv in most of those
areas (section IV.D.1). In the locations with available
PM10-2.5 monitoring, which met both the current 24-hour
PM2.5 and PM10 standards, 3-year visibility
metrics were at or below 30 dv regardless of whether the coarse
fraction was included in the calculation (U.S. EPA, 2020, section
<IP_ADDRESS>). Given the results of these analyses, the Administrator
concludes that the updated scientific

evidence and technical information support the adequacy of the current
secondary PM2.5 and PM10 standards to protect
against PM-related visibility impairment. While the inclusion of the
coarse fraction had a relatively modest impact on calculated light
extinction in these analyses, he nevertheless recognizes the continued
importance of the PM10 standard given the potential for
larger impacts in locations with higher coarse particle concentrations,
such as in the southwestern U.S., which were not included in the PA's
analyses due to insufficient coarse particle data (U.S. EPA, 2019,
section <IP_ADDRESS>; U.S. EPA, 2020, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    \68\ As discussed 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 (U.S. EPA, 2020, section <IP_ADDRESS>).
---------------------------------------------------------------------------

    With respect to non-visibility welfare effects, the Administrator
considers the evidence for PM-related impacts on climate and on
materials and concludes that it is generally appropriate to retain the
existing secondary standards and that it is not appropriate to
establish any distinct secondary PM standards to address non-visibility
PM-related welfare effects. With regard to climate, he recognizes that
a number of improvements and refinements have been made to climate
models since the time of the last review. However, despite continuing
research and the strong evidence supporting a causal relationship with
climate effects (U.S. EPA, 2019, section 13.3.9), the Administrator
notes that there are still significant limitations in quantifying the
contributions of the 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 models continue to exhibit considerable
variability in estimates of PM-related climate impacts at regional
scales (e.g., ~100 km), compared to simulations at the global scale
(U.S. EPA, 2020, sections <IP_ADDRESS>.1 and 5.4). The resulting uncertainty
leads the Administrator to conclude that the scientific information
available in the current review remains insufficient to quantify, with
confidence, the impacts of ambient PM on climate in the U.S. (U.S. EPA,
2020, section <IP_ADDRESS>.1) and that there is insufficient information at
this time to base a national ambient standard on climate impacts.
    With respect to materials effects, the Administrator notes that the
evidence available in the current review continues to support the
conclusion that there is a causal relationship with PM deposition (U.S.
EPA, 2019, section 13.4). He recognizes that deposition of particles in
the fine or coarse fractions can result in physical damage and/or
impaired aesthetic qualities. Particles can contribute to materials
damage by adding to the effects of 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 evidence on materials effects of PM is
available in this review, the Administrator notes that this evidence is
primarily from studies conducted outside of the U.S. (U.S. EPA, 2019,
section 13.4). Given the more limited amount of information 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 the public welfare, the Administrator judges that the
scientific information available in the current review remains
insufficient to quantify, with confidence, the public welfare impacts
of ambient PM on materials and that there is insufficient information
at this time to support a distinct national ambient standard based on
materials impacts.
    Taken together, the Administrator concludes that the scientific and
technical information for PM-related visibility impairment, climate
impacts, and materials effects, with its attendant uncertainties and
limitations, supports the current level of protection provided by the
secondary PM standards as being requisite to protect against known and
anticipated adverse effects on public welfare. For visibility
impairment, this conclusion reflects his consideration of the evidence
for PM-related light extinction, together with his consideration of
updated analyses of the protection provided 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, 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. His
conclusions on the secondary standards are consistent with advice from
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). Thus, based on his
consideration of the evidence and analyses for PM-related welfare
effects, as described above, and his consideration of CASAC advice on
the secondary standards, the Administrator proposes to retain those
standards (i.e., the current 24-hour and annual PM2.5
standards, 24-hour PM10 standard), 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. Any changes made in response to OMB recommendations have
been documented in the docket. Because this action does not propose to
change the existing NAAQS for PM, 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 expected to be an Executive Order 13771
regulatory action. There are no quantified cost estimates for this
proposed action because EPA is proposing to retain 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 proposes to retain 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 proposes to retain, 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), rev'd 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 proposes to retain the current primary NAAQS for PM,
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 subject to Executive Order 13211, because it is
not likely to have a significant adverse effect on the supply,
distribution, or use of energy. The purpose of this document is to
propose to retain the current PM NAAQS. This proposal does not change
existing requirements. Thus, the EPA concludes that this proposal does
not constitute a significant energy action as defined in Executive
Order 13211.

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
documentation related to this is contained in sections II through IV
above. The action proposed in this document is to retain, without
revision, the existing NAAQS for PM based on the Administrator's
conclusion that the existing standards protect public health, including
the health of sensitive groups, with an adequate margin of safety and
protect the public welfare. As discussed in section II, the EPA
expressly considered the available information regarding health effects
among at-risk populations in reaching the proposed 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).

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