epidemiologic study conducted by DeFlorio-Barker et al. (2019).
    Lastly, the EPA is also proposing revisions to the 500 value of the
AQI. The 500 value of the AQI is within the ``hazardous'' category but
is specified and used to calculate the slope of the AQI values in the
``hazardous category'' above and below AQI values of 500. In the past,
this breakpoint had a very prominent role in determining the current
upper AQI values given that it was used as part of the linear
relationship with the concentration at the AQI value of 100 to
determine the AQI values of 200 and 300 in 1999 (64 FR 42530, August 4,
1999).
    As discussed above, the current breakpoint concentration for the
500 value of the AQI was set in 1999 at a 24-hour average
PM2.5 concentration of 500 [mu]g/m\3\ and was based on
studies conducted in London using the British Smoke method, which used
a different particle size cutpoint and likely overestimated the
PM2.5 concentration. In looking to improve upon that
approach, the EPA considers several recent controlled human exposure
studies that observe health effects which are clearly associated with
more severe cardiovascular outcomes and note that these seem to follow
exposures to high PM2.5 concentrations that are well above
those typically observed in ambient air. In controlled human exposure
studies, Vieira et al. (2016a) and Vieira et al. (2016b) exposed
healthy subjects and subjects with heart failure to diesel exhaust (DE)
with a mean PM2.5 concentration of 325 [micro]g/m\3\ for 21
minutes and reported decreased stroke volume, and increased arterial
stiffness (an indicator of endothelial dysfunction) in both healthy and
heart failure subjects.\114\ Also as discussed above, Lucking et al.
(2011) exposed healthy subjects to DE with a mean PM2.5
concentration of 320 [micro]g/m\3\ for 1 hour.\115\ The types of
cardiovascular effects observed in these controlled human exposure
studies have been linked with the exacerbation of ischemic heart
disease (IHD) and heart failure as well as myocardial infarction (MI)
and stroke.
---------------------------------------------------------------------------

    \114\ These effects were attenuated when the DE was filtered, to
reduce PM2.5 concentrations, indicating the effects were
likely associated with PM2.5 exposure.
    \115\ When applying a particle trap, PM2.5
concentrations were reduced, and effects associated with
cardiovascular function including impaired vascular function, as
measured by vasodilatation and thrombus formation were attenuated
indicating associations with PM2.5.
---------------------------------------------------------------------------

    In addition to the controlled human exposure studies discussed
above, recent epidemiologic studies examining the relationship between
wildfire smoke and respiratory health can also inform proposed
decisions on the concentration for the AQI value of 500. As noted
earlier in this section, Hutchinson et al. (2018) reported increases in
a number of respiratory-related outcomes particularly ED visits for
asthma, upper respiratory infection, respiratory symptoms, acute
bronchitis, and all respiratory-related visits during the initial 5-day
period of the 2007 San Diego fire. During the initial 5-day window,
PM2.5 concentrations were found to be at their highest with
the 95th percentile of 24-hour average concentrations of 333 [micro]g/
m\3\.
    Although studies of short-term (i.e., daily) exposures to wildfire
smoke are more informative in considering alternative level for the AQI
value of 500 since they mirror the 24-hour exposure timeframe,
additional information from epidemiologic studies of longer-term
exposures (i.e., over many weeks) during wildfire events can provide
supporting information. For example, Orr et al. (2020) conducted a
longitudinal study that examined whether exposure to wildfire smoke
from a multi-month fire resulted in respiratory effects in subsequent
years. The authors conducted respiratory health assessments of adults
living in Seeley Lake and Thompson Falls, MT, during the 3-month summer
wildfire event that occurred in 2017 as well as follow-up visits in
each of the two years following the wildfire (Orr et al., 2020). During
the 2017 wildfire event (August 1 to September 19, 2017), Orr et al.
(2020) reported that many days during the multi-month fire had
PM2.5 concentrations above 300 [micro]g/m\3\, resulting in a
daily average PM2.5 concentration of 220.9 [mu]g/m\3\ with a
maximum PM2.5 concentration of 638 [micro]g/m\3\. This study
included full spirometry tests for all study participants during the
initial 2017 visit and again in 2018 and 2019 to assess lung function
and reported that the average FEV1/FVC (forced expiratory
volume in 1 second/forced vital capacity) decreased significantly in
2018 (71.6% observed; 77.35% predicted) and 2019 (73.4% observed;
76.52% predicted) (Orr et al., 2020). This study suggests that exposure
to high PM2.5 concentrations during a multi-week fire event
may lead to long-term health consequences in the future, such as
declines in lung function.
    The controlled human exposure studies provide biological
plausibility for increases in respiratory-related health care events
during the wildfires documented in epidemiologic studies. The
collective evidence from controlled human exposure and epidemiologic
studies, which includes decreases in stroke volume, increased arterial
stiffness, impaired vascular function and respiratory-related
healthcare encounters provide health-based evidence to inform proposed
decisions on the level of the AQI value of 500. Given the
concentrations observed in these studies, the Agency proposes to revise
the AQI value of 500 to a level set at a daily (i.e., 24-hour average)
PM2.5 concentration of 325 [mu]g/m\3\. This concentration is
at or below the lowest concentrations observed in the controlled human
exposure studies associated with more severe effects discussed above
and also at the low end of the daily concentrations observed in the
epidemiologic studies conducted by Hutchinson et al. (2018) and Orr et
al. (2020).
3. Summary
    Table 1 below summarizes the proposed breakpoints for the
PM2.5 sub-index.

                                Table 1--Proposed Breakpoints for PM2.5 Sub-Index
----------------------------------------------------------------------------------------------------------------
                                                                                Current
                                                                                breakpoints        Proposed
                        AQI category                           Index values    ([mu]g/m\3\,       breakpoints
                                                                                24- hour      ([mu]g/m\3\, 24-
                                                                                average)        hour average)
----------------------------------------------------------------------------------------------------------------
Good........................................................            0-50        0.0-12.0      0.0-(9.0-10.0)

Moderate....................................................          51-100       12.1-35.4     (9.1-10.1)-35.4
Unhealthy for Sensitive Groups..............................         101-150       35.5-55.4           35.5-55.4
Unhealthy...................................................         151-200      55.5-150.4          55.5-125.4
Very Unhealthy..............................................         201-300     150.5-250.4         125.5-225.4
Hazardous \1\...............................................            301+           250.5               225.5
----------------------------------------------------------------------------------------------------------------
\1\ AQI values between breakpoints are calculated using equation 1 in appendix G. For AQI values in the
  hazardous category, AQI values greater than 500 should be calculated using equation 1 and the PM2.5
  concentration specified for the AQI value of 500.

    As discussed above, the EPA recognizes that the health effects
evidence associated with PM2.5 exposure has greatly expanded
in recent years and concludes that the body of scientific evidence
supports the need to revise many of the AQI breakpoints. This is
particularly true of the AQI values of 200 and above, where the EPA
concludes that the available controlled human exposure and
epidemiologic studies support offering advisories to the public for
reducing exposures at lower concentrations than recommended with the
current AQI breakpoints. However, the EPA also recognizes that there
are interpretations and judgments that must be applied in making the
determinations of these breakpoints. Thus, the EPA is soliciting
comment on the proposed revisions to the AQI described above. In
particular, for the AQI values of 50, 100 and 150, the EPA is
soliciting comment on the proposed decision to continue to use the
approach used in AQI revisions in 2012 (77 FR 38890, June 29, 2012) of
setting the lower breakpoints (50, 100, and 150) to be consistent with
the levels of the primary annual and 24-hour PM2.5 standards
and proposed decision to revise the lower breakpoints to be consistent
with any changes to the primary PM2.5 standards that are
part of this reconsideration. With respect to the AQI values of 200 and
above, the EPA is soliciting comment on the proposed decision to revise
those AQI values, as well as comment on the approach being applied, the
health studies viewed as most relevant in these proposed decisions, and
the proposed AQI breakpoint concentrations. The EPA also notes that
while the newer studies do provide more refined information about the
concentration range in which health effects might occur, the evidence
continues to support a continuum of effects in concentration exposures
in the range of those defined by the upper AQI values, with increasing
PM2.5 concentrations being associated with increasingly
larger numbers of people likely experiencing serious health effects.
Given this, the EPA is also soliciting comment on maintaining the
linear relationship approach used to set the upper AQI values in 1999
but using a different linear relationship (64 FR 42530, August 4,
1999). For example, the EPA could set the AQI value of 150 based on the
primary NAAQS and the AQI value of 300 (which is the breakpoint that
identifies the starting concentration for the highest AQI category)
based on the considerations discussed above and using those values to
develop a linear relationship for the AQI values for 200 and 500. Under
this approach, if the AQI breakpoint for 150 is set at 55.4 [mu]g/m\3\
and the AQI breakpoint for 300 is set at 225.4 [mu]g/m\3\, the AQI
breakpoint for 200 would be 112.4 [mu]g/m\3\ and the AQI breakpoint for
500 would be 452.4 [mu]g/m\3\. The EPA solicits comments on whether to
use a linear approach for higher breakpoints, the appropriate
breakpoints to use for such an approach, and the appropriate values for
breakpoints under other approaches, falling within the range of the
current breakpoints and the breakpoints identified by these various
approaches, as well as to retain and not change the existing
breakpoints at this time.

C. Air Quality Index Category Breakpoints for PM10

    The EPA proposes to retain the PM10 sub-index of the AQI
consistent with the proposed decision to retain the primary
PM10 standard, and consistent with the health effects
information that supports this proposed decision, as discussed in
section III.D above.

D. Air Quality Index Reporting

    With respect to the reporting requirements for the AQI, there have
been many technological advances in air quality monitoring and data
reporting since the appendix G to 40 CFR part 58 was last revised in
1999. Federal, state, local, and Tribal agencies have used these
changes to make health information and air quality data more readily
available and easier to access. Given this, it is useful to update the
reporting requirements and recommendations to match current practices
and ensure the public has the most useful and timely information to
take health-protective behaviors.
    Currently, appendix G defines daily reporting as five days per
week. When this reporting requirement was originated in 1999 the
technology available at that time was not sufficient to calculate and
report the AQI more than five days per week without requiring
additional staffing on the weekends. Since that time, advances in
technology have allowed for reporting seven days per week automatically
without expending additional resources on weekends. As a result, most
state, local, and Tribal air agencies now report the AQI seven days per
a week. Given these technological advances and noting that reporting
agencies currently report the AQI seven days per week, the EPA is
proposing that state, local, and Tribal agencies that report the AQI be
required to report it seven days a week, ensuring that the public
continues to have access to daily air quality and health information
that they can use to take steps to protect their health.
    Improvements in monitoring networks and modeling capabilities have
also enabled the ability to report the AQI in near real-time. This
allows state, local, and Tribal air agencies to provide timely air
quality information to the public for making health-protective
decisions and to help satisfy AQI reporting requirements. The
availability of near real-time AQI data also allows for more timely
responses by the public when air quality conditions are changing
rapidly, such as during wildfire smoke events. Sub-daily reporting of
the AQI can be critical when there are rapidly change conditions and/or
high pollution events so that the public is able to make informed
decisions to protect their health. Many state, local, and Tribal air
agencies currently report the AQI hourly to ensure that the public has
access to

accurate and timely information. In recognition of these advances, and
to continue to provide for near-real time AQI reporting that the public
has come to rely on, the EPA proposes to recommend that state, local,
and Tribal agencies report the AQI in near-real time. Like air quality
forecasting, which also allows the public to make health-protective,
near-real time AQI reporting is recommended but not required.
    In lieu of or along with reporting the near-real-time AQI directly
to the public, most state/local and Tribal agencies submit hourly air
quality data to the EPA. The EPA uses this near-real-time data in the
National, Interactive and Fire and Smoke maps on the AirNow website,
and to create products for use by weather service providers and the
media. Some state, local, and Tribal air quality agencies also use
these products on their own websites and in their own applications
(i.e., the California Air Resources Board uses the data in its
California Smoke Spotter application). To continue to ensure the
availability of the products that the public and many stakeholders rely
upon, the EPA is proposing to recommend that state, local, and Tribal
air quality agencies submit hourly data to the EPA's air quality
database. Submitting hourly data to the EPA for use on the AirNow
website and in other products also enables state, local, and Tribal air
quality agencies to meet the recommendation to report the AQI in near-
real-time.
    The Agency is updating the reporting requirements and near-real-
time reporting and data submission recommendations for the AQI. The
Agency is reformatting the question-and-answer format used in appendix
G to align with the current standard formatting used in the Code of
Federal Regulations. The EPA is not taking comment on or reopening the
language that has merely been moved or rearranged as there are no
substantive changes.
    Another change the EPA is proposing to make to appendix G is with
regard to Table 2- Breakpoints for the AQI for purposes of clarity. We
are proposing to collapse the two rows presented for the Hazardous
Category into one. The two rows in the current table specify pollutant
concentrations for two AQI ranges within the Hazardous category (301-
400 and 401-500), with an intermediate break at 400. This breakpoint of
400, along with those for 200 and 300, were defined and are the
historical basis for the Alert, Warning, and Emergency episode levels
included in 40 CFR part 51, appendix L, as part of the Prevention of
Air Pollution Emergency Episodes program (44 FR 92, May 10, 1979). The
400 breakpoint for all criteria pollutants in the current Table 2 is
set at the proportional pollutant concentration approximately halfway
between the index values of 300 and 500. In proposing updated AQI
breakpoints for PM2.5, the EPA considered adjusting the 400
breakpoint similarly. However, the EPA concluded that collapsing the
two rows into a single range (301-500) would provide a more transparent
and easy-to-follow presentation of the pollutant concentrations
corresponding to the AQI range for the Hazardous category. Moreover,
collapsing the Hazardous category into a single row in Table 2 has no
substantive effect on the Emergency Episode program in 40 CFR part 51,
appendix L. Thus, the EPA is proposing to remove the breakpoint of 400
from the table in appendix G but this change would not substantively
affect the derivation of the AQI for any pollutant.
    In addition, the EPA plans to move some information currently in
appendix G into the Technical Assistance Document for the Reporting of
Daily Air Quality, or TAD (U.S. EPA, 2018a), so that it can be updated
in a more timely manner to reflect current scientific and health
effects evidence and current communication methods, thereby assisting
state, local, and Tribal agencies in providing accurate and timely
information to the public. Information that will be moved from appendix
G to the TAD includes the definitions of the sensitive (at-risk)
populations for each pollutant. This definition is typically evaluated
and updated, as warranted, in most NAAQS reviews, even if the standard
is not revised. Generally, if the standard is not revised in a review
of the NAAQS, then appendix G is also not revised. Moving the
definitions of sensitive groups to the TAD allows them to be updated
even when a NAAQS is not revised to be consistent with the definitions
of the sensitive (at-risk) populations identified in the ISA for a
NAAQS review. Data calculations for non-required mathematical
equations, (i.e., the NowCast), are currently and will continue to be
included in the TAD. The EPA works with state, local, and Tribal air
agencies to modify these calculations as needed, which may not be
associated with a NAAQS review. Also, recognizing that the ways that
air quality and health information is supplied to the news media and
public changes regularly, information about suggested approaches will
be taken out of appendix G and discussed in the TAD.

V. Rationale for Proposed Decisions on the Secondary PM Standards

    This section presents the rationale for the Administrator's
proposed decision that no change to the current secondary PM standards
is required at this time to provide requisite protection against the
public welfare effects of PM within the scope of this reconsideration
(i.e., visibility, climate, and materials effects).\116\ This rationale
is based on a thorough review of the scientific evidence generally
published through December 2017,\117\ as presented in the 2019 ISA
(U.S. EPA, 2019a), on the non-ecological public welfare effects of PM
pertaining to the presence of PM in ambient air, specifically
visibility, climate, and materials effects. Additionally, this
rationale is based on a thorough evaluation of some studies that became
available after the literature cutoff date of the 2019 ISA that could
either further inform the adequacy of the current PM NAAQS or address
key scientific topics that have evolved since the literature cutoff
date for the 2019 ISA, generally through March 2021, as presented in
the ISA Supplement \118\ (U.S. EPA, 2022a). The selection of welfare
effects evaluated within the ISA Supplement was based on the causality
determinations reported in the 2019 ISA and the subsequent use of
scientific

evidence in the 2020 PA.\119\ Specifically, for welfare effects, the
focus within the ISA Supplement is on visibility effects. The ISA
Supplement does not include an evaluation of studies on climate or
materials effects. The Administrator's rationale also takes into
account: (1) the PA evaluation of the policy-relevant information in
the 2019 ISA and ISA Supplement and presentation of quantitative
analysis of air quality related to visibility impairment; (2) CASAC
advice and recommendations, as reflected in discussions of the drafts
of the ISA Supplement and PA at public meetings and in the CASAC's
letters to the Administrator; and (3) public comments received during
the development of these documents.
---------------------------------------------------------------------------

    \116\ Consistent with the 2016 Integrated Review Plan (U.S. EPA,
2016), other welfare effects of PM, such as ecological effects, are
being considered in the separate, on-going review of the secondary
NAAQS for oxides of nitrogen, oxides of sulfur and PM. Accordingly,
the public welfare protection provided by the secondary PM standards
against ecological effects such as those related to deposition of
nitrogen- and sulfur-containing compounds in vulnerable ecosystems
is being considered in that separate review. Thus, the
Administrator's conclusion in this reconsideration of the 2020 final
decision will be focused only and specifically on the adequacy of
public welfare protection provided by the secondary PM standards
from effects related to visibility, climate, and materials and
hereafter ``welfare effects'' refers to non-ecological welfare
effects (i.e., visibility, climate, and materials effects).
    \117\ In addition to the 2020 review's opening ``call for
information'' (79 FR 71764, December 3, 2014), the 2019 ISA
identified and evaluated studies and reports that have undergone
scientific peer review and were published or accepted for
publication between January 1, 2009 through approximately January
2018 (U.S. EPA, 2019a, p. ES-2). References that are cited in the
2019 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.
    \118\ As described in more detail in the ISA Supplement, ``the
scope of this Supplement provides specific criteria for the types of
studies considered for inclusion within the Supplement.
Specifically, studies must be peer reviewed and published between
approximately January 2018 and March 2021'' (U.S. EPA, 2022a,
section 1.2.2).
    \119\ As described in section 1.2.1 of the ISA Supplement, ``the
selection of welfare effects to evaluate within this Supplement is
based on the causality determinations reported in the 2019 PM ISA
and the subsequent use of scientific evidence in the 2020 PM PA. The
2019 PM ISA concluded a causal relationship for each of the welfare
effects categories evaluated (i.e., visibility, climate effects, and
materials effects). While the 2020 PM PA considered the broader set
of evidence for these effects, for climate effects and material
effects, it concluded that there remained `substantial uncertainties
with regard to the quantitative relationships with PM concentrations
and concentration patterns that limit[ed] [the] ability to
quantitatively assess the public welfare protection provided by the
standards from these effects (U.S. EPA, 2020a). Given these
uncertainties and limitations, the basis of the discussion on
conclusions regarding the secondary standards in the 2020 PM PA
primarily focused on visibility effects. Therefore, this Supplement
focuses only on visibility effects in evaluating newly available
scientific information and is limited to studies conducted in the
U.S. and Canada'' (U.S. EPA, 2022a, section 1.2.1).
---------------------------------------------------------------------------

    In presenting the rationale for the Administrator's proposed
decision and its foundations, section V.A provides background and
introductory information for this reconsideration of the secondary PM
standards. It includes background on the 2020 final decision to retain
the secondary PM standards (section V.A.1) and also describes the
general approach for this reconsideration (section V.A.2). Section V.B
summarizes the key aspects of the currently available evidence and
quantitative information for PM-related visibility impairment and
section V.C summarizes the available information for other PM-related
welfare effects. Section V.D presents the Administrator's proposed
conclusions on the current secondary PM standards (V.D.III), drawing on
both evidence- and quantitative information-based considerations
(section V.D.1) and advice from the CASAC (V.D.2).

A. General Approach

    This reconsideration of the 2020 final decision on the secondary PM
standards relies on the EPA's assessments of the current scientific
evidence and associated quantitative analyses to inform the
Administrator's judgments regarding secondary standards that are
requisite to protect the public welfare from known or anticipated
adverse effects associated with the pollutant's presence in the ambient
air. The EPA's assessments are primarily documented in the 2019 ISA,
ISA Supplement, and PA, all of which have received CASAC review and
public comment (83 FR 53471, October 23, 2018; 83 FR 55529, November 6,
2018; 85 FR 4655, January 27, 2020; 86 FR 52673, September 22, 2021; 86
FR 54186, September 30, 2021; 86 FR 56263, October 8, 2021; 87 FR 958,
January 7, 2022; 87 FR 22207, April 14, 2022; 87 FR 31965, May 26,
2022). In bridging the gap between the scientific assessments of the
2019 ISA and ISA Supplement and the judgments required of the
Administrator in determining whether the current standards provide the
requisite public welfare protection, the PA evaluates policy
implications of the evaluation of the current evidence in the 2019 ISA
and ISA Supplement, and the quantitative information documented in the
PA. In evaluating the public welfare protection afforded by the current
standards against PM-related effects within the scope of this
reconsideration, the four basic elements of the NAAQS (indicator,
averaging time, level, and form) are considered collectively.
    The final decision on the adequacy of the current secondary
standards is a public welfare policy judgment to be made by the
Administrator. In reaching conclusions with regard to the standard, the
decision will draw on the scientific information and analyses about
welfare effects, and associated public welfare significance, as well as
judgments about how to consider the range and magnitude of
uncertainties that are inherent in the scientific evidence and
analyses. This approach is based on the recognition that the available
evidence generally reflects a continuum that includes ambient air
exposures at which scientists agree that effects are likely to occur
through lower levels at which the likelihood and magnitude of responses
become increasingly uncertain. This approach is consistent with the
requirements of the provisions of the Clean Air Act related to the
review of NAAQS and with how the EPA and the courts have historically
interpreted the Act. These provisions require the Administrator to
establish secondary standards that, in the judgment of the
Administrator, are requisite to protect public welfare from known or
anticipated adverse effects associated with the presence of the
pollutant in the ambient air. In so doing, the Administrator seeks to
establish standards that are neither more nor less stringent than
necessary for this purpose. The Act does not require that standards be
set at a zero-risk level, but rather at a level that reduces risk
sufficiently so as to protect the public welfare from known or
anticipated adverse effects.
    The subsections below provide background and introductory
information. Background on the 2020 decision to retain the current
standards, including the rationale for that decision, for non-
visibility effects and visibility effects is summarized in sections
V.A.1.a and V.A.1.b below, respectively. This is followed, in section
V.A.2, by an overview of the general approach for the reconsideration
of the 2020 final decision. Following this introductory section and
subsections, the subsequent sections summarize current information and
analyses, including that newly available in this reconsideration. The
Administrator's proposed conclusions on the secondary PM standards,
based on the current information, are provided in section V.D.3.
1. Background on the Current Standards
    The current secondary PM standards were affirmed in 2020 based on
the scientific and technical information available at that time, as
well as the Administrator's judgments regarding the available welfare
effects evidence, the appropriate degree of public welfare protection
for the existing standards, and available air quality information on
visibility impairment that may be allowed by such a standard (85 FR
82684, December 18, 2020). With the 2020 decision, the Administrator
retained the secondary 24-hour PM2.5 standard, with its
level of 35 [micro]g/m\3\, the annual PM2.5 standard, with
its level of 15.0 [micro]g/m\3\, and the 24-hour PM10
standard, with its level of 150 [micro]g/m\3\. The subsections below
focus on the key considerations, and the Administrator's conclusions,
for climate and materials effects (section V.A.1.a) and visibility
effects (section V.A.2.b) in the 2020 review.
a. Non-Visibility Effects
    In light of the robust evidence base, the 2019 ISA concluded there
to be causal relationships between PM and climate effects and materials
effects (U.S. EPA, 2019a, sections 13.3.9 and 13.4.2). The 2020 final
decision was

based on a thorough review in the 2019 ISA of the scientific
information on PM-induced climate and materials effects. The decision
also took into account: (1) assessments in the 2020 PA of the most
policy-relevant information in the 2019 ISA regarding evidence of
adverse effects of PM to climate and materials, (2) uncertainties in
the available evidence to inform a quantitative assessment of PM-
related climate and materials effects, (3) CASAC advice and
recommendations, and (4) public comments received during the
development of these documents and on the proposal document.
    Consistent with the general approach routinely employed in NAAQS
reviews, the initial consideration in the 2020 review of the secondary
standards was with regard to the adequacy of protection provided by the
existing standards. Key aspects of the consideration are summarized in
section V.A.1.a.i below.
i. Considerations Regarding Adequacy of the Existing Standards for Non-
Visibility Effects in the 2020 Review
    In considering non-visibility welfare effects in the 2020 review,
as discussed above, the 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, ``it is
generally appropriate to retain the existing standards and that there
is insufficient information to establish any distinct secondary PM
standards to address climate and materials effects of PM'' (85 FR
82744, December 18, 2020).
    With regard to climate, the Administrator recognized that there
were a number of improvements and refinements to climate models since
the 2012 review. However, while the evidence continued to support a
causal relationship between PM and climate effects, the Administrator
noted that significant limitations continued to exist related to
quantifying the contributions of direct and indirect effects of PM and
PM components on climate forcing (U.S. EPA, 2020a, sections <IP_ADDRESS>.1
and 5.4). He also recognized that the models continued to exhibit
considerable variability in estimates of PM-related climate impacts at
regional scales (e.g., ~100 km) as compared to simulations at global
scales. Therefore, the resulting uncertainty led the Administrator to
conclude that the available scientific information in the 2020 review
remained insufficient to quantify climate impacts associated with
particular concentrations of PM in ambient air (U.S. EPA, 2020a,
section <IP_ADDRESS>.1) or to evaluate or consider a level of PM air quality
in the U.S. to protect against climate effects and that there was
insufficient information available to base a national ambient standard
on climate impacts (85 FR 82744, December 18, 2020).
    With regard to materials effects, the Administrator noted that the
evidence available in the 2019 ISA continued to support a causal
relationship between materials effects and PM deposition (U.S. EPA,
2019a, section 13.4). He recognized that the deposition of fine and
coarse particles to materials can lead to physical damage and/or
impaired aesthetic qualities. Particles can contribute to materials
damage by adding to the natural weathering processes and by promoting
the corrosion of metals, the degradation of building materials, and the
weakening of material components. While some new information was
available in the 2019 ISA, the information was from studies primarily
conducted outside of the U.S. in areas where PM concentrations in
ambient air are higher than those observed in the U.S. (U.S. EPA,
2020a, section 13.4). Additionally, the information assessed in the
2019 ISA did not support quantitative analyses of PM-related materials
effects in the 2020 review (U.S. EPA, 2020a, section <IP_ADDRESS>.2). Given
the limited amount of information available and its inherent
uncertainties and limitations, the Administrator concluded that he was
unable to relate soiling or damage to specific levels of PM in ambient
air or to evaluate or consider a level of air quality to protect
against such materials effects, and that there was insufficient
information available to support a distinct national ambient standard
based on materials effects (85 FR 82744, December 18, 2020).
    In the 2020 review, the CASAC agreed with the 2020 PA conclusions
that, while these effects are important, ``the available evidence does
not call into question the protection afforded by the current secondary
PM standards'' and recommended that the secondary standards ``should be
retained'' (Cox, 2019b, p. 3 of letter). In reaching a final decision
in the 2020 review, for all of the reasons discussed above and
recognizing the CASAC conclusion that the evidence provided support for
retaining the current secondary PM standards, the Administrator
concluded that it was appropriate to retain the existing secondary PM
standards, without revision. For climate and materials effects, this
conclusion reflected his judgment that, although it remains important
to maintain secondary PM2.5 and PM10 standards to
provide some degree of control over long- and short-term concentrations
of both fine and coarse particles, there was insufficient information
to establish distinct secondary PM standards to address non-visibility
PM-related welfare effects (85 FR 82744, December 18, 2020).
b. Visibility Effects
    The 2019 ISA concluded that, ``the evidence is sufficient to
conclude that a causal relationship exists between PM and visibility
impairment'' (U.S. EPA, 2019a, section 13.2.6). The 2020 decision on
the adequacy of the secondary standards with regard to visibility
effects was a public welfare policy judgment made by the Administrator,
which drew upon the available scientific evidence for PM-related
visibility effects and on analyses of visibility impairment, as well as
judgments about the appropriate weight to place on the range of
uncertainties inherent in the evidence and analyses. The 2020 final
decision was based on a thorough review in the 2019 ISA of the
scientific information on PM-related visibility effects. The decision
also took into account: (1) assessments in the 2020 PA of the most
policy-relevant information in the 2019 ISA regarding evidence of
adverse effects of PM on visibility; (2) air quality analyses of the
PM2.5 visibility index and design values based on the form
and averaging time of the existing secondary 24-hour PM2.5
standard; (3) CASAC advice and recommendations; and (4) public comments
received during the development of these documents and on the 2020
proposal document.
    Consistent with the general approach routinely employed in NAAQS
reviews, the initial consideration in the 2020 review of the secondary
PM standards was with regard to the adequacy of the protection provided
by the then-existing standards. Key aspects of that consideration are
summarized in section V.A.1.b.i below.
i. Consideration Regarding the Adequacy of the Existing Standards for
Visibility Effects in the 2020 Review
    In considering the visibility effects in the 2020 review, the
Administrator noted the long-standing body of evidence for PM-related
visibility impairment. This evidence, which is based on the fundamental
relationship between light extinction and PM mass, demonstrated that
ambient PM can impair visibility in both urban and remote areas, and
had changed very little since the 2012 review (U.S. EPA, 2019a, section
13.1; U.S. EPA, 2009a, section 9.2.5). The evidence related to public
perception of visibility

impairment was from studies from four areas in North America.\120\
These studies provided information to inform our understanding of
levels of visibility impairment that the public judged to be
``acceptable'' (U.S. EPA, 2010a; 85 FR 24131, April 30, 2020). In
considering these public preference studies, the Administrator noted
that, as described in the 2019 ISA, no new visibility studies had been
conducted in the U.S. and there was little newly available information
with regard to acceptable levels of visibility impairment in the U.S.
The Administrator recognized that visibility impairment can have
implications for people's enjoyment of daily activities and their
overall well-being, and therefore, considered the degree to which the
current secondary standards protect against PM-related visibility
impairment.
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    \120\ Preference studies were available in four urban areas.
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 2022
PA (U.S. EPA, 2022b).
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    Consistent with the 2012 review, in the 2020 review, the
Administrator first concluded that a target level of protection for a
secondary PM standard is most appropriately defined in terms of a
visibility index that directly takes into account the factors (i.e.,
species composition and relative humidity) that influence the
relationship between PM2.5 in ambient air and PM-related
visibility impairment. In defining a target level of protection, the
Administrator considered the specific aspects of such an index,
including the appropriate indicator, averaging time, form and level (78
FR 82742-82744, December 18, 2020).
    First, with regard to indicator, the Administrator noted that in
the 2012 review, the EPA used an index based on estimates of light
extinction by PM2.5 components calculated using an adjusted
version of the IMPROVE algorithm, which allows the estimation of the
light extinction using routinely monitored components of
PM2.5 and PM10-2.5, along with estimates of
relative humidity. The Administrator recognized that, while there have
been some revisions to the IMPROVE algorithm since the time of the 2012
review, our fundamental understanding of the relationship between PM in
ambient air and light extinction had changed little and the various
IMPROVE algorithms appropriately reflected this relationship across the
U.S. In the absence of a monitoring network for direct measurement of
light extinction, he concluded that a calculated light extinction
indicator that utilizes the IMPROVE algorithms continued to provide a
reasonable basis for defining a target level of protection against PM-
related visibility impairment (78 FR 82742-82744, December 18, 2020).
    In further defining the characteristics of a visibility index, the
Administrator next considered the appropriate averaging time, form, and
level of the index. Given the available scientific information the
review, and in considering the CASAC's advice and public comments, the
Administrator concluded that, consistent with the decision in the 2012
review, a visibility index with a 24-hour averaging time and a form
based on the 3-year average of annual 90th percentile values remained
reasonable. With regard to the averaging time and form of such an
index, the Administrator noted analyses conducted in the last review
that demonstrated relatively strong correlations between 24-hour and
subdaily (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 subdaily time periods of the
perception of PM-related visibility impairment and the relevant
exposure periods for segments of the viewing public. This decision in
the 2020 review also recognized that a 24-hour averaging time may be
less influenced by atypical conditions and/or atypical instrument
performance (78 FR 3226, January 15, 2013). The Administrator
recognized that there was no new information to support updated
analyses of this nature, and therefore, he believed these analyses
continued to provide support for consideration of a 24-hour averaging
time for a visibility index in this review. With regard to the
statistical form of the index, the Administrator noted that, consistent
with the 2012 review: (1) a multi-year percentile form offers greater
stability from the occasional effect of interannual meteorological
variability (78 FR 3198, January 15, 2013; U.S. EPA, 2011, p. 4-58);
(2) a 90th percentile represents the median of the distribution of the
20 percent worst visibility days, which are targeted in Federal Class I
areas by the Regional Haze Program; and (3) public preference studies
did not provide information to identify a different target than that
identified for Federal Class I areas (U.S. EPA, 2011, p. 4-59).
Therefore, the Administrator judged that a visibility index based on
estimates of light extinction, with a 24-hour averaging time and a 90th
percentile form, averaged over three years, remained appropriate (78 FR
82742-82744, December 18, 2020).
    With regard to the level of a visibility index, consistent with the
2012 review, the Administrator judged that it was appropriate to
establish a target level of protection of 30 deciviews
(dv),121 122 reflecting the upper end of the range of
visibility impairment judged to be acceptable by at least 50% of study
participants in the available public preference studies (78 FR 3226,
January 15, 2013). The 2011 PA identified a range of levels from 20 to
30 dv based on the responses in the public preference studies available
at that time (U.S. EPA, 2011, section 4.3.4). At the time of the 2012
review, the Administrator noted a number of uncertainties and
limitations in public preference studies, including the small number of
stated preference studies available, the relatively small number of
study participants, the extent to which the study participants may not
be representative of the broader study area population in some of the
studies, and the variations in the specific materials and methods used
in each study. In considering the available preference studies, with
their inherent uncertainties and limitations, the prior Administrator
concluded that the substantial degree of variability and uncertainty in
the public preference studies should be reflected in a target level of
protection based on the upper end of the range of candidate protection
levels (CPLs).
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    \121\ 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.
    \122\ For comparison, 20 dv, 25 dv, and 30 dv are equivalent to
64, 112, and 191 megameters (Mm-1), respectively.
---------------------------------------------------------------------------

    Given that there were no new preference studies available in the
2020 review, the Administrator's judgments were based on the same
studies, with the same range of levels, available in the 2012 review.
As identified in the 2020 PA (U.S. EPA, 2020a, section 5.5), there were
a number of limitations and uncertainties associated with these
studies, including the following:
     Available studies may not represent the full range of
preferences for visibility in the U.S. population, particularly given
the potential variability in preferences based on the conditions
commonly encountered and the scenes being viewed.
     Available preference studies were conducted 15 to 30 years
ago and may

not accurately represent the current day preferences of people in the
U.S.
     The variety of methods used in the preference studies may
potentially influence the responses as to what level of impairment is
deemed acceptable.
     Factors that are not captured in the methods of the
preference studies, such as the time of day when light extinction is
the greatest or the frequency of impairment episodes, may influence
people's judgment on acceptable visibility (U.S. EPA, 2020a, section
<IP_ADDRESS>).
    Therefore, in considering the scientific information, with its
uncertainties and limitations, as well as public comments on the level
of the target level of protection against visibility impairment, the
Administrator concluded that it was appropriate to again use a level of
30 dv for the visibility index (78 FR 82742-82744, December 18, 2020).
    Having concluded that the protection provided by a standard defined
in terms of a PM2.5 visibility index, with a 24-hour
averaging time, and a 90th percentile form, averaged over 3 years, set
at a level of 30 dv, was requisite to protect public welfare with
regard to visual air quality, the Administrator next considered the
degree of protection from visibility impairment afforded by the
existing suite of secondary PM standards.
    In this context, the Administrator considered the updated analyses
of visibility impairment presented in the 2020 PA (U.S. EPA, 2020a,
section <IP_ADDRESS>), which reflected a number of improvements since the
2012 review. Specifically, the updated analyses examined multiple
versions of the IMPROVE equation, including the version incorporating
revisions since the time of the 2012 review. These updated analyses
provided a further understanding of how variation in the inputs to the
algorithms affect the estimates of light extinction (U.S. EPA, 2020a,
Appendix D). Additionally, for a subset of monitoring sites with
available PM10-2.5 data, the updated analyses better
characterized the influence of coarse PM on light extinction than in
the 2012 review (U.S. EPA, 2020a, section <IP_ADDRESS>).
    The results of the updated analyses in the 2020 PA were consistent
with those from the 2012 review. Regardless of which version of the
IMPROVE equation was used, the analyses demonstrated that, based on
2015-2017 data, the 3-year visibility metric was at or below about 30
dv in all areas meeting the current 24-hour PM2.5 standard,
and below 25 dv in most of those areas. In locations with available
PM10-2.5 monitoring, which met both the current 24-hour
secondary PM2.5 and PM10 standards, 3-year
visibility index metrics were at or below 30 dv regardless of whether
the coarse fraction was included as an input to the algorithm for
estimating light extinction (U.S. EPA, 2020a, section <IP_ADDRESS>). While
the inclusion of the coarse fraction had a relatively modest impact on
the estimates of light extinction, the Administrator recognized the
continued importance of the PM10 standard given the
potential for larger impacts on light extinction in areas with higher
coarse particle concentrations, which were not included in the analyses
in the 2020 PA due to a lack of available data (U.S. EPA, 2019a,
section <IP_ADDRESS>; U.S. EPA, 2020a, section <IP_ADDRESS>). He 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. 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 PM2.5 standard.
Thus, the Administrator judged that the 24-hour PM2.5
standard provided sufficient protection in all areas against the
effects of visibility impairment, i.e., that the existing 24-hour
PM2.5 standard would provide at least the target level of
protection for visual air quality of 30 dv which he judged appropriate
(78 FR 82742-82744, December 18, 2020).
2. General Approach and Key Issues in This Reconsideration of the 2020
Final Decision
    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 reconsideration
that builds upon the general approach used in past reviews and reflects
the body of evidence and information now available. Accordingly, the
approach in this reconsideration takes into consideration the
approaches used in past reviews, including the substantial assessments
and evaluations performed in those reviews, and also takes into account
the more recent scientific information and air quality data now
available to inform understanding of the key policy-relevant issues in
the reconsideration. As summarized above, the Administrator's decisions
in the 2020 review were based on an integration of PM welfare effects
information with the judgments on the public welfare significance of
key effects, policy judgments as to when the standard is requisite,
consideration of CASAC advice, and consideration of public comments.
    Similarly, in this reconsideration, we draw on the current
information from studies of PM-related visibility effects, quantitative
analyses of PM-related visibility impairment, and information from
studies of non-visibility welfare effects. In so doing, we consider
both the information available at the time of the 2012 and 2020 reviews
and information more recently available, including that which has been
critically analyzed and characterized in the 2019 ISA and ISA
Supplement \123\ for visibility, climate, and materials effects. The
evaluations in the PA, of the potential implications of various aspects
of the scientific evidence in the 2019 ISA and ISA Supplement (building
on prior such assessments), augmented by the quantitative air quality,
exposure or risk-based information, are also considered along with the
associated uncertainties and limitations.
---------------------------------------------------------------------------

    \123\ As noted above and described in detail in section 1.4.2 of
the PA, the ISA Supplement focuses on a thorough evaluation of some
studies that became available after the literature cutoff date of
the 2019 ISA that could either further inform the adequacy of the
current PM NAAQS or address key scientific topics that have evolved
since the literature cutoff date for the 2019 ISA. The selection of
the welfare effects to evaluate within the ISA Supplement were based
on the causality determinations reported in the 2019 ISA and the
subsequent use of scientific evidence in the 2020 PA. Specifically,
for welfare effects, the focus within the ISA Supplement is on
visibility effects. The ISA Supplement does not include an
evaluation of studies on climate or materials effects.
---------------------------------------------------------------------------

B. Overview of Welfare Effects Evidence

    The information summarized here is based on the scientific
assessment of the welfare effects evidence available in this
reconsideration; this assessment is documented in the 2019 ISA and ISA
Supplement and its policy implications are further discussed in the PA.
While the 2019 ISA provides the broad scientific foundation for this
reconsideration, we recognize that additional literature has become
available since the cutoff date of the 2019 ISA that expands the body
of evidence related to visibility effects that can inform the
Administrator's judgment on the adequacy of the current secondary PM
standards. As such, the ISA Supplement builds on the information in the
2019 ISA with a targeted identification and evaluation of new
scientific information regarding visibility effects. As described in
the ISA Supplement and the PA, the selection of welfare effects to
evaluate

within the ISA Supplement were based on the causality determinations
reported in the 2019 ISA and the subsequent use of scientific evidence
in the 2020 PA (U.S. EPA, 2019a, section 1.2; U.S. EPA, 2022a, section
1.4.2). The ISA Supplement focuses on U.S. and Canadian studies that
provide new information on public preferences for visibility impairment
and/or developed new methodologies or conducted quantitative analyses
of light extinction (U.S. EPA, 2022a, section 1.2). Such studies of
visibility effects and quantitative relationships between visibility
impairment and PM in ambient air were considered to be of greatest
utility in informing the Administrator's conclusions on the adequacy of
the current secondary PM standards. The visibility effects evidence
presented within the 2019 ISA, along with the targeted identification
and evaluation of new scientific information in the ISA Supplement,
provides the scientific basis for the reconsideration of the 2020 final
decision on the secondary PM standards for visibility effects. For
climate and materials effects, the 2020 PA concluded that there were
substantial uncertainties associated with the quantitative
relationships with PM concentrations and the concentration patterns
that limited the ability to quantitatively assess the public welfare
protection provided by the standards from these effects. Therefore, the
evaluation of the information related to these effects draws heavily
from the 2019 ISA and 2020 PA. The subsections below briefly summarize
the nature of PM-related visibility (section V.B.1.a), climate (section
V.B.1.b), and materials (section V.B.1.c) effects.
1. Nature of Effects
    Visibility impairment can have implications for people's enjoyment
of daily activities and for their overall sense of well-being (U.S.
EPA, 2009a, section 9.2). The strongest evidence for PM-related
visibility impairment comes from the fundamental relationship between
light extinction and PM mass (U.S. EPA, 2009a), which confirms a well-
established ``causal relationship exists between PM and visibility
impairment'' (U.S. EPA, 2009a, p. 2-28). Beyond its effects on
visibility, the 2009 ISA also identified a causal relationship
``between PM and climate effects, including both direct effects of
radiative forcing and indirect effects that involve cloud and feedbacks
that influence precipitation formation and cloud lifetimes'' (U.S. EPA,
2009a, p. 2-29). The evidence also supports a causal relationship
between PM and effects on materials, including soiling effects and
materials damage (U.S. EPA, 2009a, p. 2-31).
    The evidence available in this reconsideration is consistent with
the evidence available at the time of the 2012 and 2020 reviews and
supports the conclusions of causal relationships between PM and
visibility, climate, and materials effects (U.S. EPA, 2019a, chapter
13). Evidence newly available in this reconsideration augments the
previously available evidence of the relationship between PM and
visibility impairment (U.S. EPA, 2019a, section 13.2; U.S. EPA, 2022a,
section 4), climate effects (U.S. EPA, 2019a, section 13.3), and
materials effects (U.S. EPA, 2019a, section 13.4).
a. Visibility
    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. Consistent with conclusions of causality in
the 2012 and 2020 reviews, the 2019 ISA concludes that ``the evidence
is sufficient to conclude that a causal relationship exists between PM
and visibility impairment'' (U.S. EPA, 2019a, section 13.2.6). These
conclusions are based on the strong and consistent evidence that
ambient PM can impair visibility in both urban and remote areas (U.S.
EPA, 2019a, section 13.1; U.S. EPA, 2009a, section 9.2.5).
    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, 2009a). 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.\124\ 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, 2019a, 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, 2019a,
section 13.2.3; Mie, 1908, Van de Hulst, 1981). 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, 2019a, section
13.2.3).
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    \124\ 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, 2009a, section 2.5.1; U.S. EPA, 2019a, section 13.2.1).
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.
---------------------------------------------------------------------------

    As at the time of the 2012 and 2020 reviews, direct measurements of
PM light extinction, scattering, and absorption continue to be
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, 2019a, 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 number of measurement methods have been used for
visibility impairment (e.g., transmissometers, integrating
nephelometers, teleradiometers, telephotometers, and photography and
photographic modeling), although each of these methods has its own
strengths and limitations (U.S. EPA, 2019a, Table 13-1). While some
recent research confirms and adds to the body of knowledge regarding
direct measurements as is described in the 2019 ISA and ISA Supplement,
no major new developments have been made with these measurement methods
since prior reviews (U.S. EPA, 2019a, section <IP_ADDRESS>; U.S. EPA,
2022a, section 4.2).
    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. The theoretical
relationship between light extinction and PM characteristics, as
derived from Mie theory (U.S. EPA, 2019a, Equation 13.5), can be used
to estimate light extinction by combining mass scattering efficiencies
of particles

with particle concentrations (U.S. EPA, 2019a, section 13.2.3; U.S.
EPA, 2009a, sections <IP_ADDRESS> and <IP_ADDRESS>). This estimation of light
extinction is consistent with the method used in previous reviews. The
algorithm used to estimate light extinction, known as the IMPROVE
algorithm,\125\ 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, 2019a, sections <IP_ADDRESS> and
<IP_ADDRESS>).
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    \125\ 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).
---------------------------------------------------------------------------

    At the time of the 2012 review, two versions of the IMPROVE
algorithm were available in the literature--the original IMPROVE
algorithm (Lowenthal and Kumar, 2004, Malm and Hand, 2007, Ryan et al.,
2005) and the revised IMPROVE algorithm (Pitchford et al., 2007). As
described in detail in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>) and
the 2019 ISA (U.S. EPA, 2019a, section 13.2.3), the algorithm has been
further evaluated and refined since the time of the 2012 review
(Lowenthal and Kumar, 2016), particularly for PM characteristics and
relative humidity in remote areas. All three versions of the IMPROVE
algorithm were considered in evaluating visibility impairment in this
reconsideration.
    Consistent with the evidence available at the time of the 2012 and
2020 reviews, our understanding of public perception of visibility
impairment comes from visibility preference studies conducted in four
areas in North America.\126\ The detailed methodology for these studies
are described in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>), the 2019
ISA (U.S. EPA, 2019a), and the 2009 ISA (U.S. EPA, 2019a). In summary,
the study participants were queried regarding multiple images that 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 the 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. Based on the results of the four cities, a
range encompassing the PM2.5 visibility index values from
images that were judged to be acceptable by at least 50 percent of
study participants across all four of the urban preference studies was
identified (U.S. EPA, 2010a, p. 4-24; U.S. EPA, 2020a, 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 level of impairment
that was determined to be ``acceptable'' by at least 50 percent of
study participants (78 FR 3226-3227, January 15, 2013).
---------------------------------------------------------------------------

    \126\ 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).
---------------------------------------------------------------------------

    Since the completion of the 2009 and 2019 ISAs, there has been only
one public preference study that has become available in the U.S. This
study uses images of the Grand Canyon, AZ, described in the ISA
Supplement (U.S. EPA, 2022a). The Grand Canyon study, conducted by Malm
et al. (2019), has a similar study design to that used in the public
preference studies discussed above; however, there are several
important differences that make it difficult to directly compare the
results of the Malm et al. (2019) study with other public preference
studies. As an initial matter, the Grand Canyon study was conducted in
a Federal Class I area, as opposed to in an urban area, with a scene
depicted in the photographs that did not include urban features.\127\
We recognize that public preferences with respect to visibility in
Federal Class 1 areas may well differ from visibility preferences in
urban areas and other contexts, although there is currently a lack of
information to on such questions. Further, the Malm et al. (2019) study
also used a much lower range of superimposed ``haze'' than the
preference studies discussed above.\128\ It is unclear whether the
participant preferences are a function in part of the range of
potential values presented, such that the participant preferences for
the Grand Canyon were generally lower\129\ than the other preference
studies in part because of the lower range of superimposed ``haze'' for
the images in that study, or if their preferences would vary if
presented with images with a range of superimposed ``haze'' more
comparable to the levels used in the other studies (i.e., more ``haze''
superimposed on the images).
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    \127\ The Grand Canyon study used a single scene looking west
down the canyon with a small landscape feature of a 100-km-distant
mountain (Mount Trumbull), along with other closer landscape
features. The scenes presented in the previously available
visibility preference studies are presented in more detail in Table
D-9 in the PA (U.S. EPA, 2022b, Appendix D).
    \128\ The Grand Canyon study superimposed light extinction
ranging from 3 dv to 20 dv on the image slides shown to participants
compared to the previously available preference studies. In those
studies, the visibility ranges presented were as low as 9 dv and as
high as 45 dv. The visibility ranges presented in the previously
available visibility preference studies are described in more detail
in Table D-9 in the PA (U.S. EPA, 2022b, Appendix D).
    \129\ In the Grand Canyon study, the level of impairment that
was determined to be ``acceptable'' by at least 50 percent of study
participants was 7 dv (Malm et al., 2019).
---------------------------------------------------------------------------

    The Malm et al. (2019) study also explored alternate methods for
evaluating ``acceptable'' levels of visual air quality from the
preference studies, including the use of scene-specific visibility
indices as potential indicators of visibility levels as perceived by
the observer (Malm et al., 2019). In addition to measures of
atmospheric haze, such as atmospheric extinction, used in previously
available preference studies, other indices for visual air quality
include color and achromatic contrast of single landscape figures,
average and equivalent contrast of an entire scene, edge detection
algorithms such as the Sobel index, and just-noticeable difference or
change indexes. The results reported by Malm et al. (2019) suggest that
scene-dependent metrics, such as contrast, may be useful alternate
predictors of preference levels compared to universal metrics like
light extinction (U.S. EPA, 2022a, section 4.2.1). This is because
extinction alone is not a measure of ``haze,'' but of light attenuation
per unit distance, and visible ``haze'' is dependent on both light
extinction and distance to a landscape feature (U.S. EPA, 2022a,
section 4.2.1). However, there are very few studies available that use
scene-dependent metrics (i.e., contrast) to evaluate public preference
information, which makes it difficult to evaluate

them as an alternative to the light extinction approach.
b. Climate
    The available evidence continues to support the conclusion of a
causal relationship between PM and climate effects (U.S. EPA, 2019a,
section 13.3.9). Since the 2012 review, climate impacts have been
extensively studied and recent research reinforces and strengthens the
evidence evaluated in the 2009 ISA. Recent evidence provides greater
specificity about the details of radiative forcing effects \130\ 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 completion of the 2009 ISA, 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 2009 ISA, the 2019 ISA draws
substantially on the IPCC report to summarize climate effects. As
discussed in more detail in the PA (U.S. EPA, 2022b, section
<IP_ADDRESS>.1), the general conclusions are similar between the IPCC AR4
and AR5 reports with regard to effects of PM on global climate.
Consistent with the evidence available in the 2012 review, the key
components, including sulfate, nitrate, organic carbon (OC), black
carbon (BC), and dust, that contribute to climate processes vary in
their reflectivity, forcing efficiencies, and direction of forcing.
Since the completion of the 2009 ISA, the evidence base has expanded
with respect to the mechanisms of climate responses and feedbacks to PM
radiative forcing; however, the recently published literature assessed
in the 2019 ISA does not reduce the considerable uncertainties that
continue to exist related these mechanisms.
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    \130\ 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, 2019a, section <IP_ADDRESS>).
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    As described in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>.1), PM has
a very heterogeneous distribution globally and patterns of forcing tend
to correlate with PM loading, with the greatest forcings centralized
over continental regions. The climate response to this PM forcing,
however, is more complicated since the perturbation to one climate
variable (e.g., temperature, cloud cover, precipitation) can lead to a
cascade of effects on other variables. While the initial PM radiative
forcing may be concentrated regionally, the eventual climate response
can be much broader spatially or be concentrated in remote regions, and
may be quite complex, affecting multiple climate variables with
possible differences in the direction of the forcing in different
regions or for different variables (U.S. EPA, 2019a, section 13.3.6).
The complex climate system interactions lead to variation among climate
models, which have suggested a range of factors which can influence
large-scale meteorological processes and may affect temperature,
including local feedback effects involving soil moisture and cloud
cover, changes in the hygroscopicity of the PM, and interactions with
clouds (U.S. EPA, 2019a, section 13.3.7). However, there remains
insufficient evidence to related climate effects to specific PM levels
in ambient air or to establish a quantitative relationship between PM
and climate effects, particularly at a regional scale. 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.
c. Materials
    Consistent with the evidence assessed in the 2009 ISA, the
available evidence continues to support the conclusion that there is a
causal relationship between PM deposition and materials effects.
Effects of deposited PM, particularly sulfates and nitrates, to
materials include both physical damage and impaired aesthetic
qualities, generally involving soiling and/or corrosion (U.S. EPA,
2019a, section 13.4.2). 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.\131\ There is
a limited amount of recently available data for consideration in this
review from studies primarily conducted outside of the U.S. on
buildings and other items of cultural heritage. However, these studies
involved concentrations of PM in ambient air greater than those
typically observed in the U.S. (U.S. EPA, 2019a, section 13.4).
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    \131\ As discussed in the 2019 ISA (U.S. EPA, 2019a, 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, 2019a, p.
13-1), the assessment of the evidence in the 2019 ISA considers the
combined impacts.
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    Building on the evidence available in the 2009 ISA, and as
described in detail in the PA (U.S. EPA, 2022b, section <IP_ADDRESS>.2) and
in the 2019 ISA (U.S. EPA, 2019a, section 13.4), research has
progressed on (1) the theoretical understanding of soiling of items of
cultural heritage; (2) the quantification of degradation rates and
further characterization of factors that influence damage of stone
materials; (3) materials damage from PM components besides sulfate and
black carbon and atmospheric gases besides SO2; (4) methods
for evaluating soiling of materials by PM mixtures; (5) PM-attributable
damage to other materials, including glass and photovoltaic panels; (6)
development of dose-response relationships for soiling of building
materials; and (7) damage functions to quantify material decay as a
function of pollutant type and load. While the evidence of PM-related
materials effects has expanded somewhat since the completion of the
2009 ISA, there remains insufficient evidence to relate soiling or
damage to specific PM levels in ambient air or to establish a
quantitative relationship between PM and materials degradation. The
recent evidence assessed in the 2019 ISA is generally similar to the
evidence available in the 2009 ISA, including associated limitations
and uncertainties and a lack of evidence to inform quantitative
relationships between PM and materials effects, therefore leading to
similar conclusions about the PM-related effects on materials.

C. Summary of Air Quality and Quantitative Information

    Beyond the consideration of the scientific evidence, as discussed
in section V.B above, quantitative analyses of PM air quality, when
available, can also inform conclusions on the adequacy of the public
welfare protection provided by the current secondary PM standards.
1. Visibility Effects
    In the 2012 and 2020 reviews, quantitative analyses for PM-related
visibility effects focused on daily visibility impairment, given the
short-term nature of PM-related visibility effects. The evidence and
information available in this reconsideration

continues to provide support for the short-term (i.e., hourly or daily)
nature of PM-related visibility impairment. As such, the quantitative
analyses presented in the PA continue to focus on daily visibility
impairment and utilize a two-phase assessment approach for visibility
impairment, consistent with the approaches taken in past reviews.
First, the PA considers the appropriateness of the elements (indicator,
averaging time, form, and level) of the visibility index for providing
protection against PM-related visibility effects. Second, recent air
quality was used to evaluate the relationship between the current
secondary 24-hour PM2.5 standard and the visibility index.
The information available since the 2012 review includes an updated
equation for estimating light extinction, summarized in the PA (U.S.
EPA, 2022b, section <IP_ADDRESS>) and described in the 2019 ISA (U.S. EPA,
2019a, section <IP_ADDRESS>), as well as more recent air monitoring data,
that together allow for development of an updated assessment of PM-
related visibility impairment in study locations in the U.S.
a. Target Level of Protection in Terms of a PM2.5 Visibility
Index
    In evaluating the adequacy of the current secondary PM standards,
the PA first evaluates the appropriateness of the elements (indicator,
averaging time, form, and level) identified for a distinct secondary
standard to protect against visibility effects. In previous reviews,
the visibility index was set at a level of 30 dv, with estimated light
extinction as the indicator, a 24-hour averaging time, and a 90th
percentile form, averaged over three years.
    With regard to an indicator for the visibility index, the PA
recognizes the lack of availability of methods and an established
network for directly measuring light extinction (U.S. EPA, 2022b,
section <IP_ADDRESS>). Therefore, consistent with previous reviews, the PA
concludes that a visibility index based on estimates of light
extinction by PM2.5 components derived from an adjusted
version of the original IMPROVE algorithm to be the most appropriate
indicator for the visibility index in this reconsideration. As
described in section <IP_ADDRESS> of the PA, the IMPROVE algorithm estimates
light extinction using routinely monitored components of
PM2.5 and PM10-2.5, along with estimates of
relative humidity (U.S. EPA, 2022b, section <IP_ADDRESS>).
    With regard to averaging time, the PA notes that the evidence
continues to provide support for the short-term nature of PM-related
visibility effects. Given that there is no new information available
regarding the time periods during which visibility impairment occurs or
public preferences related to specific time periods for visibility
impairment, the PA concludes that it is appropriate to continue to
focus on daily visibility impairment. In so doing, the PA relies on
analyses that were conducted in the 2012 review that showed relatively
strong correlations between 24-hour and sub-daily (i.e., 4-hour
average) PM2.5 light extinction that indicated that a 24-
hour averaging time is an appropriate surrogate for the sub-daily time
periods relevant for visual perception (U.S. EPA, 2011, Figures G-4 and
G-5; Frank, 2012). These analyses continue to provide support for a 24-
hour averaging time for the visibility index in this reconsideration.
Consistent with previous reviews, the PA also notes that the 24-hour
averaging time may be less influenced by atypical conditions and/or
atypical instrument performance than a sub-daily averaging time (85 FR
82740, December 18, 2020; 78 FR 3226, January 15, 2013).
    With regard to the form for the visibility index, the available
information continues to provide support for a 3-year average of annual
90th percentile values. Given that there is no new information to
inform selection of an alternate form, as in previous reviews, the PA
notes that the 3-year average form provides stability from the
occasional effect of inter-annual meteorological variability that can
result in unusually high pollution levels for a particular year (85 FR
82741, December 18, 2020; 78 FR 3198, January 15, 2013; U.S. EPA, 2011,
p. 4-58). In so doing, the PA considers the evaluation in the 2010
Urban-Focused Visibility Assessment (UFVA) of three different
statistical forms: 90th, 95th, and 98th percentiles (U.S. EPA, 2010a,
Chapter 4). In considering this evaluation of statistical forms from
the 2010 UFVA, consistent with the 2011 PA, the PA notes that the
Regional Haze Program targets the 20 percent most impaired days for
visibility improvements in visual air quality in Federal Class I areas
and that the median of the distribution of these 20 percent most
impaired days would be the 90th percentile. The 2011 PA also 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 visibility
index would reasonably be expected to lead to improvements in visual
air quality for the 20 percent most impaired days. Additionally, as in
the 2011 PA, the PA recognizes that the available public preference
studies do not address frequency of occurrence of different levels of
visibility (U.S. EPA, 2022b, section <IP_ADDRESS>). Therefore, the analyses
and consideration for the form of a visibility index from the 2011 PA
continue to provide support for a 90th percentile form, averaged across
three years, in defining the characteristics of a visibility index in
this reconsideration.
    With regard to the level for the visibility index, the PA
recognizes that there is an additional public preference study (Malm et
al., 2019) available in this reconsideration. As noted above, however,
this study differs from the previously available public preference
studies in several ways which makes it difficult to integrate this
newly available study with the previously available studies. Most
significantly, this study was evaluated public preferences for
visibility in the Grand Canyon, perhaps the most notable Class I area
in the country for visibility purposes. Therefore, the PA concludes
that the Grand Canyon study is not directly comparable to the other
available preferences studies and public preferences of visibility
impairment in the Malm et al. (2019) are not appropriate to consider in
identifying a range of levels for the target level of protection
against visibility impairment for this reconsideration of the secondary
PM NAAQS.
    Therefore, the PA continues to rely on the same studies \132\ and
the range of 20 to 30 dv identified from those studies in previous
reviews. With regard to selecting the appropriate target level of
protection for visibility impairment within this range, the PA notes
that in previous reviews, a level at the upper end of the range (i.e.,
30 dv) was selected given the uncertainties and limitations associated
with the public preference studies (U.S. EPA, 2022b, section <IP_ADDRESS>).
However, the PA also recognizes that (1) the degree of protection
provided by a secondary PM NAAQS is not determined solely by any one
element of the standard but by all elements (i.e., indicator, averaging
time, form, and level) being considered together, and (2) decisions
regarding the adequacy of the current secondary standards is a public
welfare policy judgment to be made by the Administrator. As such, the
Administrator may judge that a target

level of protection below the upper end of the range (i.e., less than
30 dv) is appropriate, depending on his public welfare policy
judgments, which draw upon the available scientific evidence for PM-
related visibility effects and on analyses of visibility impairment, as
well as judgments about the appropriate weight to place on the range of
uncertainties inherent in the evidence and analyses.
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    \132\ As noted above, the available public preference studies
include those conducted in 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).
---------------------------------------------------------------------------

    In considering the available public preference studies, consistent
with past reviews, the PA concludes that it is reasonable to consider a
range of 20 to 30 dv for selecting a target level of protection,
including a high value of 30 dv, a midpoint value of 25 dv, and a low
value of 20 dv. A target level of protection at or in the upper end of
the range would focus on the Washington, DC, preference study results
(Abt Associates, 2001; Smith and Howell, 2009) which identified 30 dv
as the level of impairment that was determined to be ``acceptable'' by
at least 50 percent of study participants. The public preferences of
visibility impairment in the Washington, DC, study are likely to be
generally representative of urban areas that do not have valued scenic
elements (e.g., mountains) in the distant background. This would be
more representative of areas in the middle of the country and many
areas in the eastern U.S., as well as possibly some areas in the
western U.S.
    A target level of protection in the middle of the range would be
most closely associated with the level of impairment that was
determined to be ``acceptable'' by at least 50 percent of study
participants in the Phoenix, AZ, study (BBC Research & Consulting,
2003), which was 24.3 dv. This study, while methodologically similar to
the other public preference studies, included participants that were
selected as a representative sample of the Phoenix area population
\133\ and used computer-generated images to depict specific uniform
visibility impairment conditions. This study yielded the best results
of the four public preference studies in terms of the least noisy
preference results and the most representative selection of
participants. Therefore, based on this study, the use of 25 dv to
represent a midpoint within the range of target levels protection is
well supported.
---------------------------------------------------------------------------

    \133\ The other preference studies did not include populations
that were necessarily representative of the population in the area
for which the images being judged. For example, in the Denver, CO,
study, participants were from intact groups (i.e., those who were
meeting for other reasons) and were asked to provide a period of
time during a regularly scheduled meeting to participate in the
study (Ely et al., 1991). As another example, in the British
Columbia, Canada, study, participants were recruited from
undergraduate and graduate students enrolled in classes at the
University of British Columbia's Department of Geography (Pryor,
1996).
---------------------------------------------------------------------------

    A target level of protection at or just above the lower end of the
range would focus on the Denver, CO, study, but may not be as strongly
supported as higher levels within the range (Ely et al., 1991). Older
studies, such as those conducted in Denver, CO (Ely et al., 1991), and
British Columbia, Canada (Pryor, 1996), used photographs that were
taken at different times of the day and on different days to capture a
range of light extinction levels needed for the preference studies.
Compared to studies that used computer-generated images (i.e., those in
Phoenix, AZ, and Washington, DC) there was more variability in scene
appearance in these older studies that could affect preference rating
and includes uncertainties associated with using ambient measurements
to represent sight path-averaged light extinction values rather than
superimposing a computer-generated amount of haze onto the images. When
using photographs, the intrinsic appearance of the scene can change due
to meteorological conditions (i.e., shadow patterns and cloud
conditions) and spatial variations in ambient air quality that can
result in ambient light extinction measurement not being representative
of the sight-path-averaged light extinction. Computer-generated images,
such as those generated with WinHaze, do not introduce such
uncertainties, as the same base photograph is used (i.e., there is no
intrinsic change in scene appearance) and the modeled haze that is
superimposed on the photograph is determined based on uniform light
extinction throughout the scene.
    In addition to differences in preferences that may arise from
photographs versus computer-generated images, urban visibility
preference may differ by location, and such differences may arise from
differences in the cityscape scene that is depicted in the images.
These differences are related to the perceived value of objects and
scenes that are included in the image, as objects at a greater distance
have a greater sensitivity to perceived visibility changes as light
extinction is changed compared to similar scenes with objects at
shorter distances. For example, a person (regardless of their location)
evaluating visibility in an image with more scenic elements such as
mountains or natural views may value better visibility conditions in
these images compared to the same level of visibility impairment in an
image that only depicts urban features such as buildings and roads.
That is, if a person was shown the same level of visibility impairment
in two images depicting different scenes--one with mountains in the
background and urban features in the foreground and one with no
mountains in the background and nearby buildings in the image without
mountains in the distance--may find the amount of haze to be
unacceptable in the image with the mountains in the distance because of
a greater perceived value of viewing the mountains, while finding the
amount of haze to be acceptable in the image with the buildings because
of a lesser value of viewing the cityscape or an expectation that such
urban areas may generally have higher levels of haze in general. This
is consistent when comparing the differences between the Denver, CO,
study results (which found the 50% acceptance criteria occurred at the
best visual air quality levels among the four cities) and the
Washington, DC, results (which found the 50% acceptability criteria
occurred at the worst visual air quality levels among the four cities).
These results may occur because the most prominent and picturesque
feature of the cityscape of Denver is the visible snow-covered
mountains in the distance, while the prominent and picturesque features
of the Washington, DC, cityscape are buildings relatively nearby
without prominent and/or values scenic features that are more distant.
Given these variabilities in preferences it is unclear to what extent,
the available evidence provides strong support for a target level of
protection at the lower end of the range. Future studies that reduce
sources of noisiness and uncertainty in the results could provide more
information that would support selection of a target level of
protection at or just above the lower end of the range.
    Taken together, the PA concludes that available information
continues to support a visibility index with estimated light extinction
as the indicator, a 24-hour averaging time, and a 90th percentile form,
averaged over three years, with a level within the range of 20 to 30
dv.
b. Relationship Between the PM2.5 Visibility Index and the
Current Secondary 24-Hour PM2.5 Standard
    The PA presents quantitative analyses based on recent air quality
that evaluate the relationship between recent air quality and
calculated light extinction. As in previous reviews, these analyses
explored this relationship as an estimate of visibility impairment in
terms of the

24-hour PM2.5 standard and the visibility index. Generally,
the results of the updated analyses are similar to those based on the
data available at the time of the 2012 and 2020 reviews (U.S. EPA,
2022b, section <IP_ADDRESS>). As discussed in section V.C.1.a above, the PA
concludes that the available evidence continues to support a visibility
index with estimated light extinction as the indicator, a 24-hour
averaging time, and a 90th percentile form, averaged over three years,
with a level within the range of 20 to 30 dv. These analyses evaluate
visibility impairment in the U.S. under recent air quality conditions,
particularly those conditions that meet the current standards, and the
relative influence of various factors on light extinction. Given the
relationship of visibility with short-term PM, we focus particularly on
the short-term PM standards.\134\ Compared to the 2012 review, updated
analyses incorporate several refinements, including (1) the evaluation
of three versions of the IMPROVE equation to calculate light extinction
(U.S. EPA, 2022b, Appendix D, Equations D-1 through D-3) in order to
better understand the influence of variability in equation inputs;
\135\ (2) the use of 24-hour relative humidity data, rather than
monthly average relative humidity as was used in the 2012 review (U.S.
EPA, 2022b, section <IP_ADDRESS>, Appendix D); and (3) the inclusion of the
coarse fraction in the estimation of light extinction (U.S. EPA, 2022b,
section <IP_ADDRESS>, Appendix D). The analyses in the reconsideration are
updated from the 2012 and 2020 reviews and include 60 monitoring sites
that measure PM2.5 and PM10 and are
geographically distributed across the U.S. in both urban and rural
areas (U.S. EPA, 2022b, Appendix D, Figure D-1).
---------------------------------------------------------------------------

    \134\ The analyses presented in the PA focus on the visibility
index and the current secondary 24-hour PM2.5 standard
with a level of 35 [micro]g/m\3\. However, we recognize that all
three secondary PM standards influence the PM concentrations
associated with the air quality distribution. As noted in section
V.A.1 above, the current secondary PM standards include the 24-hour
PM2.5 standard, with its level of 35 [micro]g/m\3\, the
annual PM2.5 standard, with its level of 15.0 [micro]g/
m\3\, and the 24-hour PM10 standard, with its level of
150 [micro]g/m\3\. With regard to the annual PM2.5
standard, we note that all 60 areas included in the analyses meet
the current secondary annual PM standard (U.S. EPA, 2022b, Table D-
7).
    \135\ While the PM2.5 monitoring network has an
increasing number of continuous FEM monitors reporting hourly
PM2.5 mass concentrations, there continue to be data
quality uncertainties associated with providing hourly
PM2.5 mass and component measurements that could be input
into IMPROVE equation calculations for sub-daily visibility
impairment estimates. As detailed in the PA, there are uncertainties
associated with the precision and bias of 24-hour PM2.5
measurements (U.S. EPA, 2022b, p. 2-18), as well as to the
fractional uncertainty associated with 24-hour PM component
measurements (U.S. EPA, 2022b, p. 2-21). Given the uncertainties
present when evaluating data quality on a 24-hour basis, the
uncertainty associated with sub-daily measurements may be even
greater. Therefore, the inputs to these light extinction
calculations are based on 24-hour average measurements of
PM2.5 mass and components, rather than sub-daily
information.
---------------------------------------------------------------------------

    When light extinction was calculated using the revised IMPROVE
equation, in areas that meet the current 24-hour PM2.5
standard for the 2017-2019 time period, all sites have light extinction
estimates at or below 26 dv (U.S. EPA, 2022b, Figure 5-3). For the four
locations that exceed the current 24-hour PM2.5 standard,
light extinction estimates range from 22 dv to 27 dv (U.S. EPA, 2022b,
Figure 5-3). These findings are consistent with the findings of the
analyses using the same IMPROVE equation in the 2012 review with data
from 102 sites with data from 2008-2010 and in the 2020 review with
data from 67 sites with data from 2015-2017. The analyses presented in
the PA indicate similar findings to those from the analyses in the 2012
and 2020 reviews, i.e., the updated quantitative analysis shows that
the 3-year visibility metric was no higher than 30 dv \136\ at sites
meeting the current secondary PM standards, and at most such sites the
3-year visibility index values are much lower (e.g., an average of 20
dv across the 60 sites).\137\
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    \136\ A 3-year visibility metric with a level of 30 dv would be
at the upper end of the range of levels identified from the public
preference studies.
    \137\ When light extinction is calculated using the original
IMPROVE equation, all 60 sites have 3-year visibility metrics below
30 dv, 58 sites are at or below 25 dv, and 26 sites are at or below
20 dv (see U.S. EPA, 2022b, Appendix D, Table D-3).
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    When light extinction was calculated using the revised IMPROVE
equation,\138\ the resulting 3-year visibility metrics are nearly
identical to light extinction estimates calculated using the original
IMPROVE equation (U.S. EPA, 2022b, Figure 5-4), but some sites are just
slightly higher. Using the revised IMPROVE equation, for those sites
that meet the current 24-hour PM2.5 standard, the 3-year
visibility metric is at or below 26 dv. For the four locations that
exceed the current 24-hour PM2.5 standard, light extinction
estimates range from 22 dv to 29 dv (U.S. EPA, 2022b, Figure 5-4).
These results are similar to those for light extinction calculated
using the original IMPROVE equation,\139\ and those from previous
reviews.
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    \138\ As described in more detail in the PA, the revised IMPROVE
equation divides PM components into smaller and larger sizes of
particles in PM2.5, with separate mass scattering
efficiencies and hygroscopic growth functions for each size category
(U.S. EPA, 2022b, section <IP_ADDRESS>).
    \139\ When light extinction is calculated using the revised
IMPROVE equation, all 60 sites have 3-year visibility metrics below
30 dv, 56 sites are at or below 25 dv, and 26 sites are at or below
20 dv (see U.S. EPA, 2022b, Appendix D, Table D-3).
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    When light extinction was calculated using the refined equation
from Lowenthal and Kumar (2016), the resulting 3-year visibility
metrics are slightly higher at all sites compared to light extinction
estimates calculated using the original IMPROVE equation (U.S. EPA,
2022b, Figure 5-5).\140\ These higher estimates are to be expected,
given the higher OC multiplier included in the IMPROVE equation from
Lowenthal and Kumar (2016), which reflects the use of data from remote
areas with higher concentrations of organic PM when validating the
equation. As such, it is important to note that the Lowenthal and Kumar
(2016) version of the equation may overestimate light extinction in
non-remote areas, including the urban areas in the updated analyses in
this reconsideration.
---------------------------------------------------------------------------

    \140\ When light extinction is calculated using the Lowenthal
and Kumar IMPROVE equation, 59 sites have 3-year visibility metrics
below 30 dv, 45 sites are at or below 25 dv, and 15 sites are at or
below 20 dv. The one site with a 3-year visibility metric of 32 dv
exceeds the secondary 24-hour PM2.5 standard, with a
design value of 56 [mu]g/m\3\ (see U.S. EPA, 2022b, Appendix D,
Table D-3).
---------------------------------------------------------------------------

    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 28 dv. For those sites that exceed the current
24-hour PM2.5 standard, three of these sites have a 3-year
visibility metric ranging between 26 dv and 30 dv, while one site in
Fresno, California that exceeds the current 24-hour PM2.5
standard and has a 3-year visibility index value of 32 dv (compared to
29 dv when light extinction is calculated with the original IMPROVE
equation) (see U.S. EPA, 2022b, Appendix D, Table D-3). At this site,
it is likely that the 3-year visibility metric using the Lowenthal and
Kumar (2016) equation would be below 30 dv if PM2.5
concentrations were reduced such that the 24-hour PM2.5
level of 35 [mu]g/m\3\ was attained.
    In considering visibility impairment under recent air quality
conditions, the PA recognizes that the differences in the inputs to
equations estimating light extinction can influence the resulting
values. For example, given the varying chemical composition of
emissions from different sources, the 2.1 multiplier in the Lowenthal
and Kumar (2016) equation may not be appropriate for all source types.
At the time of the 2012 review, the EPA judged that a 1.6 multiplier
for converting OC to organic matter (OM) was more appropriate, for

the purposes of estimating visibility index at sites across the U.S.,
than the 1.4 or 1.8 multipliers used in the original and revised
IMPROVE equations, respectively. A multiplier of 1.8 or 2.1 would
account for the more aged and oxygenated organic PM that tends to be
found in more remote regions than in urban regions, whereas a
multiplier of 1.4 may underestimate the contribution of organic PM
found in remote regions when estimating light extinction (78 FR 3206,
January 15, 2013; U.S. EPA, 2012, p. IV-5). The available scientific
information and results of the air quality analyses indicate that it
may be appropriate to select inputs to the IMPROVE equation (e.g., the
multiplier for OC to OM) on a regional basis rather than a national
basis when calculating light extinction. This is especially true when
comparing sites with localized PM sources (such as sites in urban or
industrial areas) to sites with PM derived largely from biogenic
precursor emissions (that contribute to widespread secondary organic
aerosol formation), such as those in the southeastern U.S. The PA
notes, however, that conditions involving PM from such different
sources have not been well studied in the context of applying a
multiplier to estimate light extinction, contributing uncertainty to
estimates of light extinction for such conditions.
    At the time of the 2012 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 2012 review suggested that, generally,
PM10-2.5 was a minor contributor to visibility impairment
most of the time (U.S. EPA, 2010a) although the coarse fraction may be
a major contributor in some areas in the desert southwestern region of
the U.S. Moreover, at the time of the 2012 review, there were few data
available from PM10-2.5 monitors to quantify the
contribution of coarse PM to calculated light extinction. Since that
time, an expansion in 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. The analysis in the
2020 review addressed light extinction at 20 of the 67 PM2.5
sites where collocated PM10-2.5 monitoring data were
available. Since the 2020 review, PM10-2.5 monitoring data
are available at more locations and the analyses presented in the PA
include those for light extinction estimated with coarse and fine PM at
all 60 sites. 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 (U.S. EPA, 2020a, section <IP_ADDRESS>).
However, the PA notes that in the updated quantitative analyses, only a
few sites were in locations that would be expected to have high
concentrations of coarse PM, such as the Southwest. These results are
consistent with those in the analyses in the 2019 ISA, which found that
mass scattering from PM10-2.5 was relatively small (less
than 10%) in the eastern and northwestern U.S., whereas mass scattering
was much larger in the Southwest (more than 20%) particularly in
southern Arizona and New Mexico (U.S. EPA, 2019a, section <IP_ADDRESS>, p.
13-36).
    Overall, the findings of these updated quantitative analyses are
generally consistent with those in the 2012 and 2020 reviews. The 3-
year visibility metric was generally below 26 dv in most areas that
meet the current 24-hour PM2.5 standard. Small differences
in the 3-year visibility metric were observed between the variations of
the IMPROVE equation, which may suggest that it may be more appropriate
to use one version over another in different regions of the U.S. based
on PM characteristics such as particle size and composition to more
accurately estimate light extinction.
2. Non-Visibility Effects
    Consistent with the evidence available at the time of the 2012 and
2020 reviews, and as described in detail in the PA (U.S. EPA, 2022b,
section <IP_ADDRESS>), the data remain insufficient to conduct quantitative
analyses for PM effects on climate and materials. For PM-related
climate effects, as explained in more detail in the PA (U.S. EPA,
2022b, section <IP_ADDRESS>.1), our understanding of PM-related climate
effects is still limited by significant key uncertainties. The recently
available evidence does not appreciably improve our understanding of
the spatial and temporal heterogeneity of PM components that contribute
to climate forcing (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5).
Significant uncertainties also persist related to quantifying the
contributions of PM and PM components to the direct and indirect
effects on climate forcing, such as changes to the pattern of rainfall,
changes to wind patterns, and effects on vertical mixing in the
atmosphere (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). Additionally,
while improvements have been made to climate models since the
completion of the 2009 ISA, the models continue to exhibit variability
in estimates of the PM-related climate effects on regional scales
(e.g., ~100 km) compared to simulations at the global scale (U.S. EPA,
2022b, sections <IP_ADDRESS>.1 and 5.5). While our understanding of climate
forcing on a global scale is somewhat expanded since the 2012 review,
significant limitations remain to quantifying potential adverse PM-
related climate effects in the U.S. and how they would vary in response
to incremental changes in PM concentrations across the U.S. As such,
while recent research is available on climate forcing on a global
scale, the remaining limitations and uncertainties are significant, and
the recent global scale research does not translate directly for use at
regional spatial scales. Therefore, the evidence does not provide a
clear understanding at the necessary spatial scales for quantifying the
relationship between PM mass in ambient air and the associated climate-
related effects in the U.S. that would be necessary for informing
consideration of a national PM standard on climate in this
reconsideration (U.S. EPA, 2022b, section <IP_ADDRESS>.1; U.S. EPA, 2019a,
section 13.3).
    For PM-related materials effects, as explained in more detail in
the PA (U.S. EPA, 2022b, section <IP_ADDRESS>.2), the available evidence has
been somewhat expanded to include additional information about the
soiling process and the types of materials impacted by PM. This
evidence provides some limited information to inform dose-response
relationships and damage functions associated with PM, although most of
these studies were conducted outside of the U.S. where PM
concentrations in ambient air are typically above those observed in the
U.S. (U.S. EPA, 2022b, section <IP_ADDRESS>.2; U.S. EPA, 2019a, section
13.4). The evidence on materials effects characterized in the 2019 ISA
also includes studies examining effects of PM on the energy efficiency
of solar panels and passive cooling building materials, although the
evidence remains insufficient to establish quantitative relationships
between PM in ambient air and these or other materials effects (U.S.
EPA, 2022b, section <IP_ADDRESS>.2). While the available evidence assessed
in the 2019 ISA is somewhat expanded since the time of the 2012 review,
quantitative relationships have not been established for PM-related
soiling and corrosion and frequency of cleaning or repair that further
the understanding of the public welfare implications of materials
effects (U.S. EPA, 2022b, section <IP_ADDRESS>.2; U.S. EPA, 2019a, section
13.4). Therefore, there is insufficient information to inform
quantitative analyses assessing

materials effects to inform consideration of a national PM standard on
materials in this reconsideration (U.S. EPA, 2022b, section <IP_ADDRESS>.2;
U.S. EPA, 2019a, section 13.4).

D. Proposed Conclusions on the Secondary PM Standards

    In reaching proposed conclusions on the current secondary PM
standards (presented in section IV.D.3), the Administrator has taken
into account policy-relevant evidence- and quantitative information-
based considerations discussed in the PA (summarized in section
IV.D.2), as well as advice from the CASAC and public comment on the
standards received thus far in the reconsideration (section IV.D.1). In
general, the role of the PA is to help ``bridge the gap'' between the
Agency's assessment of the current evidence and quantitative analyses,
and the judgments required of the Administrator in determining whether
it is appropriate to retain or revise the NAAQS. Evidence-based
considerations draw upon the EPA's integrated assessment of the
scientific evidence of PM-related welfare effects presented in the 2019
ISA and ISA Supplement (summarized in section V.B above) to address key
policy-relevant questions in the reconsideration. Similarly, the
quantitative information-based considerations (summarized in section
V.C above) focused on the potential for PM-related welfare effects
under recent air quality conditions for the purposes of addressing the
policy-relevant questions.
    This approach to reviewing the secondary standards is consistent
with the requirements of the provisions of the CAA related to the
review of the NAAQS and with how the EPA and the courts have
historically interpreted the CAA. As discussed in section I.A above,
these provisions require the Administrator to establish secondary
standards that, in the Administrator's judgment, are requisite (i.e.,
neither more nor less stringent than necessary) to protect the public
welfare from known or anticipated adverse effects associated with the
presence of the pollutant in ambient air. Consistent with the Agency's
approach across all NAAQS reviews, the EPA's approach to informing
these judgments is based on a recognition that the available welfare
effects evidence generally reflects a continuum that includes ambient
air exposures for which scientists generally agree that effects are
likely to occur through lower levels at which the likelihood and
magnitude of response become increasingly uncertain. The CAA does not
require the Administrator to establish secondary standards at a zero-
risk level, but rather at a level that reduces risk sufficiently so as
to protect the public welfare from known or anticipated adverse
effects.
    The proposed decision on the adequacy of the current secondary
standards described below is a public welfare policy judgment by the
Administrator that draws upon the scientific evidence for the relevant
welfare effects, quantitative analyses of air quality, as available,
and judgments about how to consider the uncertainties and limitations
that are inherent in the scientific evidence and quantitative analyses.
The four basic elements of the NAAQS (i.e., indicator, averaging time,
form, and level) have been considered collectively in evaluating the
public welfare protection afforded by the current standard against PM-
related visibility, climate and materials effects. The Administrator's
final decision will additionally consider public comments received on
this proposed decision.
1. CASAC Advice in This Reconsideration
    The CASAC provided its advice regarding the current secondary
standards in the context of its review of the draft PA (Sheppard,
2022a).\141\ In its comments on the draft PA, the CASAC first
recognized the scientific evidence is sufficient to support a causal
relationship between PM and visibility effects, climate effects and
materials effects.
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    \141\ A limited number of public comments have also been
received in this reconsideration to date, including comments focused
on the draft PA. Of those public comments that addressed the
adequacy of the secondary PM standards, the majority of commenters
support the preliminary conclusion that it is appropriate to
consider retaining the current secondary PM standards, without
revision. These commenters generally cite to a lack of newly
available evidence and information that would inform consideration
of alternative secondary PM standards to protect against PM-related
effects on visibility, climate, and materials. One commenter,
however, supported the revision of the secondary PM standards to
provide additional protection against PM-related visibility effects.
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    With regard to visibility effects, the CASAC recognized that the
identification of a target level of protection for the visibility index
is based on a limited number of studies and suggested that ``additional
region- and view-specific visibility preference studies and data
analyses are needed to support a more refined visibility target''
(Sheppard, 2022a, p. 21 of consensus responses). While the CASAC did
not recommend revising either the target level of protection for the
visibility index or the level of the current 24-hour PM2.5
standard, they did state that a visibility index of 30 deciviews
``needs to be justified'' and ``[i]f a value of 20-25 deciviews is
deemed to be an appropriate visibility target level of protection, then
a secondary 24-hour PM2.5 standard in the range of 25-35
[micro]g/m\3\ should be considered'' (Sheppard, 2022a, p. 21 of
consensus responses).
    The CASAC also recognized the limited availability of monitoring
methods and networks for directly measuring light extinction. As such,
they suggest that ``[a] more extensive technical evaluation of the
alternatives for visibility indicators and practical measurement
methods (including the necessity for a visibility FRM) is need for
future reviews'' (Sheppard, 2022a, p. 22 of consensus letter). The
majority of the CASAC ``recommend[ed] that an FRM for a directly
measured PM2.5 light extinction indicator be developed'' to
inform the consideration of the protection afforded by the secondary PM
standards against visibility impairment, the minority of the CASAC
``believe that a light extinction FRM is not necessary to set a
secondary standard protective of visibility'' (Sheppard, 2022a, p. 22
of consensus responses).
    With regard to climate and materials effects, the CASAC noted that
substantial uncertainties remain in the scientific evidence for these
effects. The CASAC suggested a number of areas for future research to
further inform our understanding of these effects, including more
climate-related research and research that would allow for quantitative
assessment of the relationship between materials effects and PM in
ambient air.
2. Evidence- and Quantitative Information-Based Considerations in the
Policy Assessment
    The secondary PM standards include the 24-hour PM2.5
standard, with its level of 35 [mu]g/m\3\ as the 98th percentile,
averaged over three years; the annual PM2.5 standard, with
its level of 15.0 [mu]g/m\3\ as the annual mean, averaged over three
years; and the 24-hour PM10 standard, with its level of 150
[mu]g/m\3\, not to be exceeded more than once per year on average over
three years. Together, these standards provide protection against both
long-term average and short-term peak PM concentrations. For example,
the 24-hour PM2.5 standard is most effective at limiting
peak 24-hour PM2.5 concentrations, but in doing so, also has
an effect on annual average PM2.5 concentrations.
Additionally, the annual standard is most effective in controlling
``typical'' or average PM2.5 concentrations, but also
provides some

measure of protection against peak exposures.
    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
2019 ISA and ISA Supplement, including the extent to which the evidence
for PM-related visibility impairment, climate effects, or materials
effects alters key conclusions from the 2020 review. The PA also
considers quantitative analyses of visibility impairment and the extent
to which they may indicate different conclusions from those in the 2020
review regarding the degree of protection from adverse effects provided
by the current secondary standards.
    Consistent with the approaches used in previous reviews, the
quantitative analyses in the PA utilized a two-phase assessment for
visibility impairment. First, the PA considered the appropriateness of
the elements (indicator, averaging time, form, and level) of the
visibility index for providing protection against PM-related visibility
effects. Second, the PA evaluated the relationship between the current
secondary 24-hour PM2.5 standard and the visibility index.
    With regard to the appropriateness of the visibility index and its
target level of protection against PM-related visibility effects, the
PA notes that there is limited information available in this
reconsideration beyond that available in previous reviews to inform
conclusions on the elements (indicator, averaging time, form, and
level) of the visibility index (described in more detail in section
V.C.1.a above). In considering the available information, the PA
concludes that the available information continues to support a
visibility index with estimated light extinction as the indicator, a
24-hour averaging time, and a 90th percentile form, averaged over three
years, with a level within the range of 20 to 30 dv.
    With regard to the relationship between the current secondary 24-
hour PM2.5 standard and the visibility index, the PA
presents updated analyses based on recent air quality information, with
a focus on locations meeting the current secondary 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
V.B.1.a), as in previous reviews, the PA analyses use calculated light
extinction to estimate PM-related visibility impairment (U.S. EPA,
2022b, section <IP_ADDRESS>). Compared to the 2012 review, updated analyses
incorporate several refinements. These include (1) the evaluation of
three versions of the IMPROVE equation to calculate light extinction
(U.S. EPA, 2022b, Appendix D, Equations D-1 through D-3) in order to
better understand the influence of variability in equation inputs;
\142\ (2) the use of 24-hour relative humidity data, rather than
monthly average relative humidity as was used in the 2012 review (U.S.
EPA, 2022b, section <IP_ADDRESS>, Appendix D); and (3) the inclusion of the
coarse fraction in the estimation of light extinction (U.S. EPA, 2022b,
section <IP_ADDRESS>, Appendix D). The PA's updated analyses include 60
monitoring sites that measure PM2.5 and PM10 that
are geographically distributed across the U.S. in both urban and rural
areas (U.S. EPA, 2022b, Appendix D, Figure D-1).\143\
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    \142\ 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.
    \143\ 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 and PM10-2.5 mass and component
data that were available from monitors in the IMPROVE network, CSN,
and NCore Multipollutant Monitoring Network (U.S. EPA, 2022b,
Appendix D).
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    In areas that meet the current 24-hour PM2.5 standard
for the 2017-2019 time period, all sites have light extinction
estimates at or below 26 dv using the original and revised IMPROVE
equations (U.S. EPA, 2022b, section <IP_ADDRESS>). In addition, the four
locations that exceeds the current 24-hour PM2.5 standard
have light extinction estimates that range from 22 to 27 dv when using
the original IMPROVE equation (U.S. EPA, 2022b, Figure 5-3) and from 22
to 29 dv when using the revised IMPROVE equation (U.S. EPA, 2022b,
Figure 5-4). The analyses presented in the PA indicate similar findings
to those from the analyses in the 2012 and 2020 reviews, i.e., the
updated quantitative analysis shows that the 3-year visibility metric
was no higher than 30 dv (the upper end of the range of target levels
of protection) at sites meeting the current secondary PM standards, and
at most such sites the 3-year visibility index values are much lower
(e.g., an average of 20 dv across the 60 sites).\144\
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    \144\ As noted above in section V.1.C.b, when light extinction
is calculated using the original IMPROVE equation, all 60 sites have
3-year visibility metrics below 30 dv, 58 sites are at or below 25
dv, and 26 sites are at or below 20 dv (see U.S. EPA, 2022b,
Appendix D, Table D-3). When light extinction is calculated using
the revised IMPROVE equation, all 60 sites have 3-year visibility
metrics below 30 dv, 56 sites are at or below 25 dv, and 26 sites
are at or below 20 dv (see U.S. EPA, 2022b, Appendix D, Table D-3).
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    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 original and revised IMPROVE equations
(U.S. EPA, 2022b, Figure 5-5). The slightly higher estimates of light
extinction 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 included in the analyses presented in the PA.
    Nevertheless, when light extinction is calculated using the