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.\145\ For the sites that exceed the
current 24-hour PM2.5 standard, three of the 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 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.
---------------------------------------------------------------------------

    \145\ As noted above in section V.1.C.b, 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).
---------------------------------------------------------------------------

    In 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 PM10-2.5 is often a minor
contributor to visibility impairment (U.S. EPA,

2010a), though it may make a larger contribution in some areas in the
desert southwestern region of the U.S. However, at the time of the 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, as assessed and presented in the 2020 PA (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).
    In summary, 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 when light
extinction is calculated using the original and revised IMPROVE
equations, and generally at or below 28 dv when using the Lowenthal and
Kumar (2016) equation to estimate light extinction. Small differences
in the 3-year visibility metric were observed between the variations of
the IMPROVE equation. When light extinction is calculated using the
revised IMPROVE equation, there is a generally 1-2 dv at
the study locations compared to light extinction calculated using the
original IMPROVE equation (U.S. EPA, 2022b, Appendix D, Table D-3).
When light extinction is calculated using the Lowenthal and Kumar
(2016) equation, the difference compared to using either the original
or revised IMPROVE equation generally ranges from no difference to up
to 4 dv greater in areas that meet the current secondary 24-hour
PM2.5 standard (U.S. EPA, 2022b, Appendix D, Table D-3). As
noted in previous reviews, a change of 1 to 2 dv in light extinction
under many viewing conditions will be perceived as a small, but
noticeable, change in the appearance of a scene, regardless of the
initial amount of visibility impairment (U.S. EPA, 2004a; U.S. EPA,
2010a). Given that there is more variability when estimating light
extinction using the Lowenthal and Kumar (2016) IMPROVE equation
compared to the original or revised IMPROVE equations, it is important
to recognize that the PA notes that the Lowenthal and Kumar (2016)
equation may not be appropriate for all locations and source types. For
example, the larger multiplier used in the Lowenthal and Kumar (2016)
may be more appropriate for estimating light extinction in more remote
areas where there is more aged and oxygenated organic PM compared to in
urban areas. As such, the PA recognizes that one version of the IMPROVE
equation is not necessarily more accurate or precise in estimating
light extinction, and that differences in locations may support the
selection of inputs to the IMPROVE equation or of the appropriate
IMPROVE equation to estimate light extinction on a regional basis
rather than on a national basis. Overall, regardless of the IMPROVE
equation that is used to estimate light extinction, in areas that meet
the current 24-hour PM2.5 standards, the 3-year visibility
metric is at or below 28 dv, which is in the upper range of levels for
the target level of protection identified from the public preference
studies (i.e., 20 to 30 dv). In fact, even in areas that exceed the
secondary 24-hour PM2.5 standard, and regardless of the
IMPROVE equation that is used to calculate light extinction, all study
locations have 3-year visibility index values at or below 30 dv, which
is the upper end of the range of target levels of protection.
    With regard to PM-related climate effects, the PA recognizes that
while the evidence base has expanded since the completion of the 2009
ISA, the recent evidence has not appreciably improved the 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). Despite continuing research, there are still significant
limitations in quantifying the contributions of PM and PM components to
the direct and indirect effects on climate forcing (e.g., changes to
the pattern of rainfall, changes to wind patterns, effects on vertical
mixing in the atmosphere) (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and
5.5). In addition, while a number of improvements and refinements have
been made to climate models since the 2012 review, these models
continue to exhibit variability in estimates of the PM-related climate
effects on regional scales (e.g., ~100 km) compared to simulations at
the global scale (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). While
recent research has added to the understanding of climate forcing on a
global scale, there remain significant limitations to quantifying
potential adverse effects from PM on climate in the U.S. and how they
would vary in response to incremental changes in PM concentrations in
the U.S. Overall, the PA recognizes that while new research is
available on climate forcing on a global scale, the remaining
uncertainties and limitations are significant, and the new global scale
research does not translate directly to use at regional spatial scales.
Thus, the evidence does not provide a clear understanding at the
spatial scales needed for the NAAQS of a quantitative relationship
between concentrations of PM mass in ambient air and the associated
climate-related effects (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5).
The PA concludes that the evidence does not call into question the
adequacy of the current secondary PM standards for climate effects.
    With regard to materials effects, the PA notes the availability of
recent evidence in this reconsideration related to the soiling process
and the types of materials that are affected. Such evidence provides
some limited information to inform dose-response relationships and
damage functions associated with PM, though most recent studies have
been conducted outside the U.S. in areas where PM concentrations in
ambient air are higher than those observed in the U.S. (U.S. EPA,
2022b, section <IP_ADDRESS>.2; U.S. EPA, 2019a, section 13.4). The recent
evidence includes studies examining PM-related effects on the energy
efficiency of solar panels and passive cooling building materials,
though there remains insufficient evidence to establish quantitative
relationships between PM in ambient air and these or other materials
effects (U.S. EPA, 2022b, section <IP_ADDRESS>.2). While recent research has
expanded the body of evidence for PM-related materials effects, the PA
recognizes the lack of information to inform quantitative analyses
assessing

materials effects or the potential public welfare implications of such
effects (U.S. EPA, 2022b, section <IP_ADDRESS>.2). Thus, the PA concludes
that the evidence does not call into question the adequacy of the
current secondary PM standards for materials effects.
    Overall, the PA recognizes that the newly available welfare effects
evidence, critically assessed in the 2019 ISA as part of the full body
of evidence, and visibility effects evidence, assessed in the ISA
Supplement, reaffirms the conclusions on the visibility, climate, and
materials effects of PM as recognized in the 2012 and 2020 reviews
(U.S. EPA, 2022b, sections <IP_ADDRESS>, <IP_ADDRESS>, and 5.5). Further, there
is a general consistency of the currently available evidence with the
evidence that was available in previous reviews, including with regard
to key aspects of the decision to retain the standards in the 2012 and
2020 reviews (U.S. EPA, 2022b, sections <IP_ADDRESS>, <IP_ADDRESS>, and 5.5). The
quantitative analyses for visibility impairment for recent air quality
conditions indicate that estimated light extinction in areas meeting
the current secondary 24-hour PM2.5 standards have a 3-year
visibility index at or below 30 dv (i.e., the upper end of the range of
target levels of protection identified in the 2012 and 2020 reviews)
and most areas have 3-year visibility index values at or below the
midpoint of the range of target levels of protection (i.e., 25 dv)
(U.S. EPA, 2022b, sections <IP_ADDRESS> and 5.5). Collectively, the PA finds
that the evidence and quantitative information-based considerations
support consideration of retaining the current secondary PM standards,
without revision (U.S. EPA, 2022b, section 5.5).
3. Administrator's Proposed Decision on the Current Secondary PM
Standards
    This section summarizes the Administrator's considerations and
conclusions related to the current secondary PM2.5 and
PM10 standards and presents his proposed decision that no
change is required for those standards at this time. The CAA 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 final decision on the
adequacy of the current secondary standards is a public welfare policy
judgment to be made by the Administrator. The decision should 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.
    Given these requirements, the Administrator's final decision in
this reconsideration will be a public welfare policy judgment that
draws upon the scientific and technical information examining PM-
related visibility impairment, climate effects and materials effects,
including how to consider the range and magnitude of uncertainties
inherent in that information. The Administrator recognizes that his
final decision will be based on an interpretation of the scientific
evidence and technical analyses that neither overstates nor understates
their strengths and limitations, nor the appropriate inferences to be
drawn.
    As an initial matter in considering the secondary standards, the
Administrator notes the longstanding body of evidence for PM-related
visibility impairment. As in previous reviews, this evidence continues
to demonstrate a causal relationship between ambient PM and effects on
visibility (U.S. EPA, 2019a, section 13.2). The Administrator
recognizes that visibility impairment can have implications for
people's enjoyment of daily activities and for their overall sense of
well-being. Therefore, as in previous reviews, he considers the degree
to which the current secondary standards protect against PM-related
visibility impairment. In so doing, and consistent with previous
reviews, the Administrator considers the protection provided by the
current secondary standards against PM-related visibility impairment in
conjunction with the Regional Haze Program as a means of achieving
appropriate levels of protection against PM-related visibility
impairment in urban, suburban, rural, and Federal Class I areas across
the country. Programs implemented to meet the secondary PM NAAQS, along
with the requirements of the Regional Haze Program established for
protecting against visibility impairment in Class I areas, would be
expected to improve visual air quality across all areas.
    In addition, the Administrator notes that the Regional Haze Program
was established by Congress specifically to achieve ``the prevention of
any future, and the remedying of existing, impairment of visibility in
mandatory Class I areas, which impairment results from man-made air
pollution,'' and that Congress established a long-term program to
achieve that goal (CAA section 169A). The Administrator finds that in
adopting section 169A, Congress set a goal of eliminating anthropogenic
visibility impairment at Class I areas, as well as a framework for
achieving that goal which extends well beyond the planning process and
timeframe for attaining secondary NAAQS. Thus, recognizing that the
Regional Haze Program will continue to contribute to reductions in
visibility impairment in Class I areas, the Administrator proposes to
conclude that addressing visibility impairment in Class I areas is
beyond the scope of the secondary PM NAAQS and that setting the
secondary PM NAAQS at a level that would remedy visibility impairment
in Class I areas would result in standards that are more stringent than
is requisite.
    In further considering what standards are requisite to protect
against adverse public welfare effects from visibility impairment, the
Administrator adopts an approach consistent with the approach used in
previous reviews (section V.A.1.b). That is, he first identifies an
appropriate target level of protection in terms of a PM visibility
index that accounts for the factors that influence the relationship
between particles in the ambient air and visibility (i.e., size
fraction, species composition, and relative humidity). He then
considers air quality analyses examining the relationship between this
PM visibility index and the current secondary 24-hour PM2.5
standard in locations meeting the current 24-hour PM2.5 and
PM10 standards (U.S. EPA, 2022b, section <IP_ADDRESS>).
    To identify a target level of protection, the Administrator first
considers the characteristics of the visibility index and defines its
elements (indicator, averaging time, form, and level). With regard to
the indicator for the visibility index, the Administrator recognizes
that there is a lack of availability of methods

and an established network for directly measuring light extinction,
consistent with the conclusions reached in the PA (U.S. EPA, 2022b,
section <IP_ADDRESS>) and with the CASAC's recommendation for additional
research on direct measurement methods for light extinction (Sheppard,
2022a, p. 21 of consensus responses). He notes that in the 2012 and
2020 reviews, given the lack of such monitoring data, the EPA used an
index based on estimates of light extinction by PM2.5
components calculated using an adjusted version of the original IMPROVE
algorithm. As described above (sections V.B.1.a and V.D.2), this
algorithm allows the estimation of light extinction using routinely
monitored components of PM2.5 and PM10-2.5,\146\
along with estimates of relative humidity. While revisions have been
made to the IMPROVE algorithm since the 2012 review (U.S. EPA, 2022b,
section <IP_ADDRESS>), the Administrator recognizes that our fundamental
understanding of the relationship between ambient PM and light
extinction has changed little since the 2012 review. He further
recognizes that the results of the quantitative analyses in the PA that
examined three versions of the IMPROVE equation indicate that there are
very small differences in estimates of light extinction between the
equations, and that it is not always clear that one version of the
IMPROVE equation is more appropriate for estimating light extinction
across the U.S. than other versions of the IMPROVE equation. He does,
however, recognize that the PA suggests 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 (U.S. EPA, 2022b, section <IP_ADDRESS>), and he further
notes the CASAC's recognition that PM-visibility relationships are
region specific (Sheppard, 2022a, p. 21 of consensus responses). In the
absence of a robust monitoring network to directly measure light
extinction (sections V.B.1.a and V.D.2), he preliminarily judges that
estimated light extinction, as calculated using one or more versions of
the IMPROVE algorithms, continues to be the most appropriate indicator
for the visibility index in this reconsideration.
---------------------------------------------------------------------------

    \146\ In the 2012 review, the focus was on PM2.5
components given their prominent role in PM-related visibility
impairment in urban areas and the limited data available for
PM10-2.5 (77 FR 38980, June 29, 2012; U.S. EPA, 2022b,
section <IP_ADDRESS>).
---------------------------------------------------------------------------

    In further defining the characteristics of a visibility index based
on estimates of light extinction, the Administrator considers the
appropriate averaging time, form, and level of the index. With regard
to the averaging time and form, the Administrator notes that in
previous reviews, a 24-hour averaging time was selected and the form
was defined as the 3-year average of annual 90th percentile values. The
Administrator recognizes that the evidence available in this
reconsideration and described in the PA continue to provide support for
the short-term nature of PM-related visibility effects. In so doing, he
relies on analyses of 24-hour and sub-daily PM2.5 light
extinction to inform his conclusions on averaging time. The
Administrator notes that there are strong correlations between 24-hour
and sub-daily (i.e., 4-hour average) PM2.5 light
extinction), indicating that a 24-hour averaging time is an appropriate
surrogate for the sub-daily time periods relevant for visual perception
(U.S. EPA, 2011, Appendix G, section G.4). He further recognizes that
the longer averaging time may be less influenced by atypical conditions
and/or atypical instrument performance. Considering this information,
and noting that the CASAC did not provide advice or recommendations
with regard to the averaging time of the visibility index, the
Administrator preliminarily judges that it the 24-hour averaging time
continues to be appropriate for the visibility index.
    With regard to the form of the visibility index, the Administrator
notes that consistent with the approach taken in other NAAQS, including
the current secondary 24-hour PM2.5 NAAQS, a multi-year
percentile form offers greater stability to the air quality management
process by reducing the possibility that statistically unusual
indicator values will lead to transient violations of the standard.
Using a 3-year average provides stability from the occasional effects
of inter-annual meteorological variability that can result in unusually
high pollution levels for a particular year (U.S. EPA, 2011, p. 4-58).
In considering the percentile that would be appropriate with the 3-year
average, the Administrator first notes that the Regional Haze Program
targets the 20% most impaired days for improvements in visual air
quality in Class I areas.\147\ Based on analyses examining 90th, 95th,
and 98th percentile forms, the Administrator preliminarily judges that
a focus similar to the Regional Haze Program focused on improving the
20% most impaired days suggest that the 90th percentile, which
represents the median of the 20% most impaired days, such that 90% of
days have visual air quality that is at or below the target level of
protection of the visibility index, would be reasonably expected to
lead to improvements in visual air quality for the 20% most impaired
days (U.S. EPA, 2011, p. 4-59). In the analyses of percentiles, the
results suggest that a higher percentile value could have the effect of
limiting the occurrence of days with peak PM-related light extinction
in areas outside of Federal Class I areas to a greater degree. However,
the Administrator preliminarily concludes that it is appropriate to
balance concerns about focusing on the group of most impaired days with
concerns about focusing on the days with peak visibility impairment.
Additionally, the Administrator notes that the CASAC did not provide
advice or recommendations related to the form of the visibility index.
Therefore, the Administrator preliminarily judges that it remains
appropriate to define a visibility index in terms of a 24-hour
averaging time and a form based on the 3-year average of annual 90th
percentile values.
---------------------------------------------------------------------------

    \147\ As noted above, the Administrator views the Regional Haze
Program as a complement to the secondary PM NAAQS, and thus takes
into consideration its approach to improving visibility in
considering how to address visibility outside of Class I areas.
---------------------------------------------------------------------------

    With regard to the level of the visibility index, the Administrator
first notes that the information that is available regarding the range
of levels of visibility impairment judged to be acceptable by at least
50% of study participants in the visibility preference studies is
largely the same as was in previous reviews.\148\ As such, the
Administrator notes that the PA identifies a range of 20 to 30 dv as
appropriate for considering the level for the visibility index.
Furthermore, the Administrator notes that a level at the upper end of
the range (i.e., 30 dv) was selected for the 2012 and 2020 reviews,
given the uncertainties and limitations associated with the public
preference studies (U.S. EPA, 2022b, section <IP_ADDRESS>). In considering
the available public preference studies and the range of target levels
of protection derived from the studies, the Administrator notes that,
while methodologically similar, the studies have inherent differences
that impact the responses from the study participants. He notes that
the images used to evaluate public preferences differed significantly
depending on geographical location, and that public preferences for
visual air

quality can vary depending on the scenic elements depicted in the
images. He also recognizes that the older studies (i.e., those in
Denver, CO, and British Columbia, Canada) used photographs, paired with
ambient measurements of light extinction, as opposed to the computer-
generated images in more recent studies (i.e., those in Phoenix, AZ,
and Washington, DC), which introduces more variability in scene
appearance that can influence preferences. Furthermore, the distances
of objects depicted in the images can influence the perceived
visibility changes, as objects at a greater distance have more
sensitivity to changes in visibility impairment compared to those at
shorter distances. The Administrator recognizes that these differences,
and the uncertainties and limitations that result from them, are
important to consider when identifying a target level of protection for
the visibility index, particularly in identifying the appropriate level
of protection that would be neither more nor less stringent than
necessary for a national standard.
---------------------------------------------------------------------------

    \148\ For reasons stated above, the Administrator does not find
it appropriate to use the most recent preference study (Malm et al.,
2019) for purposes of identifying a target level of protection for
the visibility index.
---------------------------------------------------------------------------

    In addition to the methodological differences across the public
preferences studies, the Administrator takes note of the uncertainties
and limitations associated with the studies and discussed in the PA. In
particular, the Administrator notes that available studies may not
capture the full range of visibility preferences in the U.S.
population, particularly given the potential for preferences to vary
based on the visibility conditions commonly encountered and the types
of scenes being viewed and factors that are not captured by the methods
used in available preference studies may influence people's judgments
on acceptable visibility, including the duration of visibility
impairment, the time of day during which light extinction is greatest,
and the frequency of episodes of visibility impairment (U.S. EPA,
2022b, section <IP_ADDRESS>).
    In considering the appropriate target level of protection for the
visibility index, the Administrator also takes note of the CASAC's
advice. Specifically, he notes that the CASAC recognizes that such a
judgment is based on a limited number of visibility preference studies,
with studies conducted in the western U.S. reporting public preferences
for visibility impairment associated with the lower end of the range of
levels, while studies conducted in the eastern U.S. reporting public
preferences associated with the upper end of the range. While the CASAC
did not specifically recommend a level for the visibility index, they
did state that a visibility index of 30 deciviews ``needs to be
justified'' (Sheppard, 2022a, p. 21 of consensus responses). In
considering the available information and the CASAC's advice, the
Administrator notes that the public preference studies were conducted
in several geographical areas across the U.S., and while they provide
insight to regional preferences for visibility impairment, none of
these studies identify a specific level of visibility impairment that
would be perceived as ``acceptable'' or ``unacceptable'' across the
whole U.S. population. The Administrator notes that there have long
been significant questions about how to set a national standard for
visibility that is not overprotective for some areas of the U.S. In
establishing the Regional Haze Program to improve visibility in Class I
areas, Congress noted that ``as a matter of equity, the national
ambient air quality standards cannot be revised to adequately protect
visibility in all areas of the country.'' H.R. Rep. 95-294 at 205.
Similarly, in the 1997 review, the Administrator at that time noted
significant differences in visibility in the eastern U.S. compared to
the western U.S. due to background conditions, found that a standard
set to protect against visibility impairment nationwide would be
significantly overprotective and not justified for some parts of the
country, and concluded it was appropriate to rely on the Regional Haze
Program in conjunction with the secondary PM NAAQS to achieve the
requisite degree of protection from visibility impairment (62 FR 38652,
July 18, 1997). For the reasons noted above, the Administrator is not
seeking to set a standard that would eliminate visibility impairment in
Class I areas, but significant uncertainties remain regarding how to
judge visibility impairment across the entire range of daily outdoor
activities for Americans across the country. Thus, the Administrator
recognizes that there are substantial uncertainties and limitations in
the public preference studies that should be considered when selecting
a target level of protection for the visibility index. The
Administrator proposes to conclude that the uncertainties and
variability inherent in the public preference studies warrant setting a
higher target level of protection than if the underlying methods and
results from the public preference studies were more consistent. In so
doing, the Administrator first preliminarily judges that, consistent
with similar judgments in past reviews, it is appropriate to recognize
that the secondary 24-hour PM2.5 standard is intended to
address visibility impairment across a wide range of regions and
circumstances, and that the current standard works in conjunction with
the Regional Haze Program to improve visibility, and therefore, it is
appropriate to establish a target level of protection based on the
upper end of the range of levels. In considering the information
available in this reconsideration and the CASAC's advice, the
Administrator proposes to conclude that the protection provided by a
visibility index based on estimated light extinction, a 24-hour
averaging time, and a 90th percentile form, averaged over 3 years, set
at a level of 30 dv (the upper end of the range of levels) would be
requisite to protect public welfare with regard to visibility
impairment.
    Having provisionally concluded that it remains appropriate in this
reconsideration to define the target level of protection in terms of a
visibility index based on estimated light extinction as described above
(i.e., with a 24-hour averaging time; a 3-year, 90th percentile form;
and a level of 30 dv), the Administrator next considers the degree of
protection from visibility impairment afforded by the existing
secondary standards. He considers the updated analyses of PM-related
visibility impairment presented in the PA (U.S. EPA, 2022b, section
<IP_ADDRESS>), which reflect several improvements over the 2012 review.
Specifically, the updated analyses examine multiple versions of the
IMPROVE algorithm, including the version incorporating revisions since
the 2012 review (section V.B.1.a). This approach provides an improved
understanding of how variation in equation inputs impacts calculated
light extinction (U.S. EPA, 2022b, Appendix D). In addition, all of the
sites included in the analyses had PM10-2.5 data available,
which allows for better characterization of the influence of the coarse
fraction on light extinction (U.S. EPA, 2022b, section <IP_ADDRESS>).
    The Administrator notes that the results of these updated analyses
are consistent with the results from the 2012 and 2020 reviews.
Regardless of the IMPROVE equation used, these analyses demonstrate
that the 3-year visibility metric is at or below 28 dv in all areas
meeting the current 24-hour PM2.5 standard (section
V.C.1.b). Given the results of these analyses, the Administrator
concludes that the updated scientific evidence and technical
information support the adequacy of the current secondary
PM2.5 and PM10 standards to protect against PM-
related visibility impairment. While the inclusion of the coarse
fraction had a relatively modest impact on calculated

light extinction in the analyses presented in the PA, he nevertheless
recognizes the continued importance of the PM10 standard
given the potential for larger impacts in locations with higher coarse
particle concentrations, such as in the southwestern U.S., for which
only a few sites met the criteria for inclusion in the analyses in the
PA (U.S. EPA, 2019a, section <IP_ADDRESS>; U.S. EPA, 2022b, section
<IP_ADDRESS>).
    With regard to the adequacy of the secondary 24-hour
PM2.5 standard, the Administrator notes that the CASAC
stated that ``[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 [mu]g/m\3\ should
be considered'' (Sheppard, 2022a, p. 21 of consensus responses). The
Administrator recognizes that the CASAC recommended the Administrator
provide additional justification for a visibility index target of 30 dv
but did not specifically recommend that he choose an alternative level
for the visibility index. The Administrator has considered the CASAC's
advice, together with the available scientific evidence and
quantitative information in reaching his proposed conclusions. The
Administrator recognizes conclusions regarding the appropriate weight
to place on the scientific and technical information examining PM-
related visibility impairment including how to consider the range and
magnitude of uncertainties inherent in that information is a public
welfare policy judgment left to the Administrator. As such, the
Administrator notes his conclusion on the appropriate visibility index
(i.e., with a 24-hour averaging time; a 3-year, 90th percentile form;
and a level of 30 dv) and his conclusions regarding the quantitative
analyses of the relationship between the visibility index and the
current secondary 24-hour PM2.5 standard. In so doing, he
proposes to conclude that the current secondary standards provide
requisite protection against PM-related visibility effects. With
respect to non-visibility welfare effects, the Administrator considers
the evidence for PM-related impacts on climate and on materials and
concludes that it is generally appropriate to retain the existing
secondary standards and that it is not appropriate to establish any
distinct secondary PM standards to address non-visibility PM-related
welfare effects. With regard to climate, he recognizes that a number of
improvements and refinements have been made to climate models since the
time of the 2012 review. However, despite continuing research and the
strong evidence supporting a causal relationship with climate effects
(U.S. EPA, 2019a, section 13.3.9), the Administrator notes that there
are still significant limitations in quantifying the contributions of
the direct and indirect effects of PM and PM components on climate
forcing (U.S. EPA, 2022b, sections <IP_ADDRESS>.1 and 5.5). He also
recognizes that models continue to exhibit considerable variability in
estimates of PM-related climate impacts at regional scales (e.g., ~100
km), compared to simulations at the global scale (U.S. EPA, 2022b,
sections <IP_ADDRESS>.1 and 5.5). The resulting uncertainty leads the
Administrator to preliminarily conclude that the scientific information
available in this reconsideration remains insufficient to quantify,
with confidence, the impacts of ambient PM on climate in the U.S. (U.S.
EPA, 2022b, section <IP_ADDRESS>.1) and that there is insufficient
information at this time to base a national ambient standard on climate
impacts.
    With respect to materials effects, the Administrator notes that the
available evidence continues to support the conclusion that there is a
causal relationship with PM deposition (U.S. EPA, 2019a, section 13.4).
He recognizes that deposition of particles in the fine or coarse
fractions can result in physical damage and/or impaired aesthetic
qualities. Particles can contribute to materials damage by adding to
the effects of natural weathering processes and by promoting the
corrosion of metals, the degradation of painted surfaces, the
deterioration of building materials, and the weakening of material
components. While some recent evidence on materials effects of PM is
available in the 2019 ISA, the Administrator notes that this evidence
is primarily from studies 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, 2019a, section 13.4). Given the limited amount of
information on the quantitative relationships between PM and materials
effects in the U.S., and uncertainties in the degree to which those
effects could be adverse to the public welfare, the Administrator
preliminarily judges that the scientific information available in this
reconsideration remains insufficient to quantify, with confidence, the
public welfare impacts of ambient PM on materials and that there is
insufficient information at this time to support a distinct national
ambient standard based on materials impacts.
    Taken together, the Administrator proposes to conclude that the
scientific and technical information for PM-related visibility
impairment, climate impacts, and materials effects, with its attendant
uncertainties and limitations, supports the current level of protection
provided by the secondary PM standards as being requisite to protect
against known and anticipated adverse effects on public welfare. For
visibility impairment, this proposed conclusion reflects his
consideration of the evidence for PM-related light extinction, together
with his consideration of updated analyses of the protection provided
by the current secondary PM2.5 and PM10
standards. For climate and materials effects, this conclusion reflects
his preliminary judgment that, although it remains important to
maintain secondary PM2.5 and PM10 standards to
provide some degree of control over long- and short-term concentrations
of both fine and coarse particles, it is generally appropriate not to
change the existing secondary standards and that it is not appropriate
to establish any distinct secondary PM standards to address PM-related
climate and materials effects at this time. As such, the Administrator
recognizes that current suite of secondary standards (i.e., the 24-hour
PM2.5, 24-hour PM10, and annual PM2.5
standards) together provide such control for both fine and coarse
particles and long- and short-term visibility and non-visibility (e.g.,
climate and materials) \149\ effects related to PM in ambient air. His
proposed conclusions on the secondary standards are consistent with
advice from the CASAC, which noted substantial uncertainties remain in
the scientific evidence for climate and materials effects. Thus, based
on his consideration of the evidence and analyses for PM-related
welfare effects, as described above, and his consideration of CASAC
advice on the secondary standards, the Administrator proposes not to
change those standards (i.e., the current 24-hour and annual
PM2.5 standards, 24-hour PM10 standard) at this
time. The Administrator solicits comments on this proposed conclusion.
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    \149\ As noted earlier, 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.
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    The Administrator additionally recognizes that the available
evidence on visibility impairment generally reflects a continuum and
that the public preference studies did not identify a specific level of
visibility impairment that would be perceived as ``acceptable'' or
``unacceptable'' across the whole U.S. population. However, he notes a

judgment of a target level of protection, below 30 dv and down to 25
dv, could be supported if more weight was put on the public preference
study performed in the Phoenix, AZ, study (BBC Research & Consulting,
2003), which 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. While the Administrator notes
that CASAC did not recommend revising the level of the current 24-hour
PM2.5 standard, the Administrator recognizes that, should an
alternative level be considered for the visibility index, that the
CASAC recommends also considering revisions to the secondary 24-hour
PM2.5 standard (Sheppard, 2022a, p. 21 of consensus
responses). Thus, the Administrator solicits comment on the
appropriateness of a target level of protection for visibility below 30
dv and down as low as 25 dv, and of revising the level of the current
secondary 24-hour PM2.5 standard to a level as low as 25
[mu]g/m\3\. Any comments on such revisions should include an
explanation of the basis for the commenters' views.

E. Proposed Decisions on the Secondary PM Standards

    Taking the above considerations into account, upon reconsidering
the public welfare protection provided by the current secondary PM
standards for the known and anticipated adverse effects within the
scope of this reconsideration, in light of the currently available
scientific evidence and quantitative information, the Administrator
proposes not to change the current secondary PM standards at this time.
In the Administrator's preliminary judgment, such a suite of secondary
PM standards and the rationale supporting not revising the current
standards are reasonably judged to reflect the appropriate
consideration of the strength of the available evidence and other
information and their associated uncertainties and the advice of CASAC.
    The Administrator recognizes that the final suite of standards will
reflect his ultimate judgment in the final rulemaking, and in the on-
going review of the secondary NAAQS for oxides of nitrogen, oxides of
sulfur, and PM, as to the suite of secondary PM 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 final judgment to be made by the Administrator will
appropriately consider the requirement for standards that are neither
more nor less stringent than necessary and will recognize that the CAA
does not require that secondary 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 Administrator also solicits comment on whether it would be
appropriate to revise the current secondary 24-hour PM2.5
standard, in conjunction with considering a lower target level of
protection for the visibility index below 30 dv, and as low as 25 dv.
The Administrator takes note that, while the CASAC did not recommend
changes to the current level of 35 [mu]g/m\3\ for the secondary 24-hour
PM2.5 standard, they indicated that alternative levels
should be considered if a lower target level of protection (i.e., lower
than 30 dv) for the visibility index was judged to be appropriate.
Thus, the Administrator additionally solicits comment on the
appropriateness of revising the level of the current secondary 24-hour
PM2.5 standard to a level as low as 25 [mu]g/m\3\. Any
comments on such revisions should include an explanation of the basis
for the commenters' views.
    Having reached the proposed decision described here based on
interpretation of the welfare effects evidence for this
reconsideration, as assessed in the 2019 ISA and ISA Supplement, and
the quantitative analyses of visibility impairment in the PA; the
evaluation of policy-relevant aspects of the evidence and quantitative
analyses in the PA; the advice and recommendations from the CASAC;
public comments received to date in this reconsideration; and the
public welfare policy judgments described above, the Administrator
recognizes that other interpretations, assessments and judgments might
be possible. Therefore, the Administrator solicits comment on the array
of issues associated with reconsideration of the secondary PM
standards, including public welfare and science policy judgments
inherent in his proposed decision, as described above, and the
rationales upon which such views are based.

VI. Interpretation of the NAAQS for PM

A. Proposed Amendments to Appendix K: Interpretation of the NAAQS for
Particulate Matter

    The EPA proposes to revise appendix K to make the PM10
data handling procedures for the 24-hour PM10 standards
specified in 40 CFR 50.6 more consistent with those for other NAAQS
pollutants and to codify existing practices. The proposed revisions,
which describe site-level computations, site-to-site combinations, and
daily validity requirements are discussed in more detail below.
1. Updating Design Value Calculations To Be on a Site-Level Basis
    First, the EPA proposes to require PM10 design values be
calculated on a site-level basis. Past practice has been to calculate a
monitor-level design value for each individual PM10 monitor
when more than one monitor is located at a single site; however, this
practice is inconsistent with the data handling for PM2.5
and several other NAAQS pollutants. This inconsistency with
PM2.5 has led to public confusion about the applicable
PM10 design value and data completeness criteria at a site
because operators are more accustomed to site-level monitoring
requirements. To resolve this confusion, the EPA believes it would be
appropriate to identify a single design value for each site; the EPA is
proposing an analytic approach to combine data collected from multiple
PM10 monitors collocated at a site to obtain a single set of
daily PM10 concentration data for that site. This proposal
to move from monitor-level to site-level PM10 design values
is supported by the high level of consistency in the measurement data
obtained across the various Federal reference and equivalent
PM10 monitoring instruments currently in operation (U.S.
EPA, 2009a, section <IP_ADDRESS>).
    The proposed approach would provide for monitoring agencies to
designate in their annual network plan one monitor as the primary
monitor for each site.\150\ Once a primary monitor has been determined
for a site, missing daily PM10 concentrations for the
primary monitor would be substituted from any other monitors located at
the site. In the event of two or more monitors operating at the same
site, missing daily PM10 concentrations for the primary
monitor would be substituted with daily values averaged across the
other collocated monitors. The EPA notes that at the time of this
proposal, there were more than 100 sites nationwide with two or more
monitors operating simultaneously.
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    \150\ In the absence of a primary monitor designation, the
primary monitor would default to the monitor with the most complete
daily dataset in each year.
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    This proposed approach for combining data across collocated
monitors at a site is consistent with the existing approach described
in appendix N to part 50 for the current PM2.5 NAAQS. The
EPA invites public comment on the scientific validity of

combining data across PM10 monitors and the merits of the
proposed approach for combining data across multiple PM10
monitors collocated at a site.
2. Codifying Site Combinations To Maintain a Continuous Data Record
    Second, and complementary to the first proposed revision described
above, the EPA proposes to maintain the existing practice of combining
data from nearby monitoring sites to determine a valid design value,
known as a ``site combination.'' Site combinations typically involve
situations where one site closes and another begins monitoring a short
distance away within a few days, and the monitoring agency wishes to
combine the data from the two sites to maintain a continuous data
record. The EPA Regional offices have approved over ten site
combinations for PM10 since the promulgation of the 1987
PM10 NAAQS; these will be considered approved site
combinations if these revisions are promulgated.
    Relatedly, the EPA proposes to maintain the existing practice of
allowing monitoring agencies to submit site combination requests to the
appropriate Regional Administrator through the EPA's Air Quality System
(AQS) database. Site combinations may be approved by the Regional
Administrator after they determine that the measured air quality
concentrations do not differ substantially between the two sites. To
make this determination for a requested site combination, the Regional
Administrator may request additional information from the Agency
including detailed information on the locations and distance between
the two sites, levels of ambient concentrations measured at the two
sites, and local emissions or meteorology data. To improve
transparency, the EPA will make records of all approved site
combinations available in the AQS database and will update design value
calculations in AQS when approved site combinations are implemented.
The EPA invites public comment on the merits of the proposed process
for approving site combinations to obtain valid design values for the
PM10 NAAQS.
3. Clarifying Daily Validity Requirements for Continuous Monitors
    Third, the EPA proposes to maintain the existing practice of
considering daily averages to be valid if at least 75 percent of the
hourly averages (i.e., 18 hourly values) for the 24-hour period are
available unless a substitution test can show validity on days with
seven or more missing hours.

B. Proposed Amendments to Appendix N: Interpretation of the NAAQS for
PM2.5

    The EPA proposes to revise appendix N by updating references to the
proposed revision(s) of the standards and changing data handling
provisions related to combining data from nearby monitoring sites to
codify existing practices that are currently being implemented as EPA
standard operating procedures.
1. Updating References to the Proposed Revision(s) of the Standards
    The EPA proposes to maintain the existing practice of combining
data from nearby monitoring sites to determine a valid design value,
known as a ``site combination.'' Site combinations typically involve
situations where one site closes and another begins monitoring a short
distance away within a few days, and the monitoring agency wishes to
combine the data from the two sites to maintain a continuous data
record. The EPA Regional offices have approved over 40 site
combinations for PM2.5 since the promulgation of the 1997
PM2.5 NAAQS; these will be considered approved site
combinations if these revisions are promulgated.
2. Codifying Site Combinations To Maintain a Continuous Data Record
    Relatedly, the EPA proposes to maintain the existing practice of
allowing monitoring agencies to submit site combination requests to the
appropriate Regional Administrator through the EPA's Air Quality System
(AQS) database. Site combinations may be approved by the Regional
Administrator after they determine that the measured air quality
concentrations do not differ substantially between the two sites. To
make this determination for a requested site combination, the Regional
Administrator may request additional information from the Agency
including detailed information on the locations and distance between
the two sites, levels of ambient concentrations measured at the two
sites, and local emissions or meteorology data. To improve
transparency, the EPA will make records of all approved site
combinations available in the AQS database and will update design value
calculations in AQS when approved site combinations are implemented.
The EPA invites public comment on the merits of the proposed process
for approving site combinations to obtain valid design values for the
PM2.5 NAAQS.

VII. Proposed Amendments to Ambient Monitoring and Quality Assurance
Requirements

    The EPA is proposing revisions to ambient air monitoring
requirements for PM to improve the usefulness of and appropriateness of
data used in regulatory decision making. These proposed changes focus
on ambient monitoring requirements found in 40 CFR parts 50 (appendix
L), 53, and 58 with associated appendices (A, B, C, D, and E). These
proposed changes include addressing updates in the approval of
reference and equivalent methods, updates in quality assurance
statistical calculations to account for lower concentration
measurements, updates to support improvements in PM methods, a revision
to the PM2.5 network design to account for at-risk
populations, and updates to the Probe and Monitoring Path Siting
Criteria for NAAQS pollutants.
    The EPA last completed revisions to PM ambient air monitoring
regulations as a part of the PM NAAQS review completed in 2012 (78 FR
3085, January 15, 2013). This final rulemaking included revisions to
ensure the suite of standards for PM provide requisite protection of
public health and welfare as well as corresponding revisions to the
data handling conventions for PM and to the ambient air monitoring,
reporting, and network design requirements. Other pollutant-specific
monitoring updates have occurred in conjunction with revisions to the
NAAQS. In such cases, the monitoring revisions were typically finalized
as part of the final rulemaking for the NAAQS.\151\ Specific proposed
changes are described below.
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    \151\ Links to the NAAQS final rules are available at: https://www.epa.gov/criteria-air-pollutants.
---------------------------------------------------------------------------

A. Proposed Amendment in 40 CFR Part 50 (Appendix L): Reference Method
for the Determination of Fine Particulate Matter as PM2.5 in the
Atmosphere--Addition of the Tisch Cyclone as an Approved Second Stage
Separator

    The EPA is proposing a technical change to appendix L to include
the addition of an alternative PM2.5 particle size separator
to that of the WINS and the VSCC size separators. The new separator is
the TE-PM2.5C cyclone manufactured by Tisch Environmental
Inc., Cleves, Ohio, and has been shown to have performance equivalent
to that of the originally specified WINS impactor with regards to
aerodynamic cutpoint and PM2.5 concentration measurement. In
addition, the new TE-PM2.5C has a service interval
comparable to the VSCC separator and is significantly longer than the
service

interval for the WINS. Generally, the TE-PM2.5C is also
physically interchangeable with the WINS and VSCC where both are
manufactured for the same sampler. The proposal would allow the WINS,
VSCC, or TE-PM2.5C to be used in a PM2.5 FRM
sampler. As is the case for the WINS and VSCC, the TE-PM2.5C
is now also an approved size separator for candidate PM2.5
FEMs. Currently, the EPA has designated one PM2.5 sampler
configured with TE-PM2.5C separator as a Class II
PM2.5 equivalent method and one as a PM10-2.5
equivalent method. Upon promulgation of this proposed change to
appendix L, these instruments would be redesignated as PM2.5
and PM10-2.5 FRMs, respectively. Owners of such samplers
would contact the sampler manufacturer to receive a new reference
method label for the samplers.

B. Proposed Amendments to Ambient Air Monitoring Reference and
Equivalent Methods in 40 CFR Part 53

    The EPA is proposing clarifications to the regulations associated
with submittal of candidate FRM and FEM applications for review by the
EPA. Revisions are also proposed in instances where current regulatory
specifications are no longer pertinent and require updating. In
addition, the EPA has compiled a list of noted minor errors to correct
in regulations associated with the testing requirements and acceptance
criteria for Federal reference methods (FRMs) and Federal equivalent
methods (FEMs) in part 53. These errors are typically not associated
with the content of Federal Register documents but often relate to
transcription errors and typographical errors in the electronic CFR
(eCFR) and printed versions of the CFR.
1. Update to Program Title and Delivery Address for FRM and FEM
Application and Modification Requests
    The EPA is proposing to update the name of the program and delivery
address for the EPA review of FRM and FEM Applications and Modification
Requests (Sec.  53.4). These revisions are due solely to organizational
changes and do not affect the structure or role of the Reference and
Equivalent Methods Designation Program in reviewing new FRM and FEM
application requests and requests to modify existing designated
instruments.
2. Requests for Delivery of a Candidate FRM or FEM Instrument
    As part of the current applicant review process, Sec.  53.4(d)
allows the EPA to request only candidate PM2.5 FRMs and
Class II or Class III equivalent methods for test purposes. The EPA
proposes to revise this section to allow the EPA to request any
candidate FRM, FEM, or a designated FRM or FEM associated with a
Modification Request, regardless of NAAQS pollutant type or metric.
3. Amendments to Requirements for Submission of Materials in Sec.
53.4(b)(7) for Language and Format
    The EPA proposes to amend Sec.  53.4(b)(7), which specifies the
format(s) in which all submissions must be received, to specify that
all written application materials must be submitted to the EPA in
English in MS Word format and that submitted data must be submitted in
MS Excel format.
4. Amendment to Designation of Reference and Equivalent Methods
    The EPA proposes to clarify the terms of new FRM and FEM methods
(Sec.  53.8(a)) to ensure that candidate samplers and analyzers are not
publicly announced, marketed, or sold as FRMs until the EPA's approval
has been formally announced in the Federal Register.
5. Amendment to One Test Field Campaign Requirement for Class III
PM2.5 FEMs
    Field comparability tests for candidate Class III PM2.5
FEMs include the requirement that a total of five field campaigns must
be conducted at four separate sites: A, B, C, and D. The site D
specifications of Sec.  53.35(b)(1)(ii)(D) require that the site ``. .
. shall be in a large city east of the Mississippi River, having
characteristically high sulfate concentrations and high humidity
levels.'' However, dramatic decreases in ambient sulfate concentration
make it difficult for applicants to routinely meet the high sulfate
concentration requirement. Therefore, the EPA proposes to revise the
site D specifications to read ``. . . shall be in a large city east of
the Mississippi River, having characteristically high humidity
levels.''
6. Amendment to Use of Monodisperse Aerosol Generator
    Wind tunnel evaluation of candidate PM10 inlets and
evaluation of candidate PM2.5 fractionators under static
conditions requires the generation and use of monodisperse calibration
aerosols of specified aerodynamic sizes. In the current regulations
(Sec.  53.61(g)), the TSI Incorporated Vibrating Orifice Aerosol
Generator (VOAG) is the only approved monodisperse generator for this
purpose. However, TSI Incorporated no longer manufacturers nor supports
the VOAG. Therefore, the EPA proposes to add a commercially available
monodisperse aerosol generator--the Model 1520 Flow-Focusing
Monodisperse Aerosol Generator, MSP Corporation, Shoreview, MN--to the
list of approved generators for this purpose.
7. Corrections to 40 CFR Part 53 (Reference and Equivalent Methods)
    Certain provisions of Sec.  53.14, Modification of a reference or
equivalent method, incorrectly state an EPA response deadline of 30
days for receipt of modification materials in response to an EPA
notice. Per a 2015 amendment (80 FR 65460, 65416; October 26, 2015),
all EPA response deadlines for modifications of reference or equivalent
methods are 90 days from day of receipt.
    The EPA proposes corrections to the following tables: Table A-1 to
Subpart A of Part 53--Summary of Applicable Requirements for Reference
and Equivalent Methods for Air Monitoring of Criteria Pollutants
identifies the applicable 40 CFR part 50 appendices and 40 CFR part 53
subparts for each criteria pollutant. The four rows in the section for
PM10-2.5 erroneously do not include the footnote instruction
that the aforementioned pollutant alternative Class III requirements
may be substituted in regard to Appendix O to Part 50--Reference Method
for the Determination of Coarse Particulate Matter as
PM10-2.5 in the Atmosphere.
    Table B-1 SO2 states the interference equivalent for
each interferent is 0.005 ppm for both the standard- and
lower-range limits, with the exception of nitric oxide (NO) for the
lower-range limit per note 4. When testing the lower range of
SO2, the limit for NO is 0.003 ppm, therefore an
incorrect lower limit (0.0003) is currently stated in note
4 for this exception to the SO2 lower-range limit.
    The EPA proposes corrections to the following figures: After the
EPA received an inquiry regarding the interaction of NO and
O3, the EPA investigated the interferent testing
requirements stated by 40 CFR part 53, subpart B. The EPA has
determined that during the 2011 SO2 amendment and subsequent
2015 O3 amendment, several typographical errors were
introduced into Table B-3, the most significant of which is the
omission of note 3, which instructs the applicant to not mix the
pollutant with the interferent. Additionally, appendix A to subpart B
of part 53 provides figures depicting optional forms for reporting test
results. Figure B-3 lists an incorrect formula: the lower detectible
limit section is missing the proper operator in the LDL

calculation formula and Figure B-5 lists an incorrect calculation
metric: there is a typesetting error in the calculation of the standard
deviation. The EPA proposes to correct the typesetting errors.
    The EPA proposes correcting typesetting errors in several formulas
provided throughout Sec.  53.43.

C. Proposed Changes to 40 CFR Part 58 (Ambient Air Quality
Surveillance)

1. Quality Assurance Requirements for Monitors Used in Evaluations for
National Ambient Air Quality Standards
    The EPA has evaluated the quality system as part of the PM NAAQS
reconsideration and identified several areas that could be improved in
light of lower average ambient PM2.5 concentrations across
the country and the proposed more revised primary annual
PM2.5 NAAQS described in section II above. Thus, we assessed
PM2.5 concentration data across a range of values to
determine if any changes were warranted to their use in the statistics
used to evaluate the data qualify in the PM2.5 network. This
section describes that work and any proposed changes as a result. Other
changes proposed in this section include clarifications and other
improvements that will better assist with the consistency and
operations of quality assurance programs.
a. Quality System Requirements
    The EPA has reconsidered the appendix A, section <IP_ADDRESS>, goal for
acceptable measurement uncertainty for automated and manual
PM2.5 methods currently stated as an upper 90 percent
confidence limit for the coefficient of variation (CV) of 10 percent
and 10 percent for total bias. The average PM2.5
concentrations across the nation have steadily declined since the
promulgation of the first PM2.5 standard (U.S. EPA, 2022,
section 2.3). As ambient concentrations decrease, the bias is inflated
using the current bias statistic in 4.2.5. The EPA has developed a new
bias statistic to minimize the effect of low PM2.5
concentrations on bias and is proposing to revise section 4.2.5 to
implement this new bias statistic. The EPA has concluded that with this
change to the bias statistic, the coefficient of variation (CV) of 10
percent and 10 percent for total bias is still an
acceptable goal for estimating total bias in the networks. The
technical justification and background for this change is documented in
a technical memorandum to the docket for this rulemaking titled ``Task
16 on PEP/NPAP Task Order: Bias and Precision DQOs for the
PM2.5 Ambient Air Monitoring Network.'' \152\
---------------------------------------------------------------------------

    \152\ Noah, G. (2022). Task 16 on PEP/NPAP Task Order: Bias and
Precision DQOs for the PM2.5 Ambient Air Monitoring
Network. Memorandum to the Rulemaking Docket for the Review of the
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
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    The EPA is proposing to update and clarify ambient air monitoring
requirements found in appendix A, section 2.6.1, pertaining to EPA
Protocol Gas standards used for ambient air monitoring and the Ambient
Air Protocol Gas Verification Program. Appendix A would be revised to
clarify that in order to participate in the Ambient Air Protocol Gas
Verification Program, producers of Protocol Gases must adhere to the
requirements of 40 CFR 75.21(g), and only regulatory ambient air
monitoring programs may submit cylinders for assay verification to the
EPA Ambient Air Protocol Gas Verification Program. The EPA is proposing
to include an allowable uncertainty of 2.0 percent for EPA
Protocol Gas standards used in ambient air monitoring. This allowable
uncertainty limit would match the existing limit set by the EPA's
continuous emission monitoring program found in part 75, appendix A,
section 5.1.4(b), and would make the EPA's regulations of quality
assurance of ambient air monitors more uniform and consistent.
b. Measurement Quality Check Requirements
    The EPA is proposing to remove section <IP_ADDRESS> from appendix A.
This provision in the quality assurance requirements for ambient air
monitoring allows for NO2 compressed gas standards to be
used to generate audit standards. However, NO2 compressed
gas standards are not currently designated by the EPA's Office of
Research and Development (ORD) as an EPA Protocol Gas Standard. As
such, this provision conflicts with section 2.6.1 of appendix A that
requires that any standard used for generating test atmospheres be an
EPA Protocol Gas Standard. The EPA is aware that there is a need for
NO2 compressed gas standards for direct read NO2
monitoring methods. If these NO2 compressed gas standards
can, in the future, be proven to be stable and approvable as EPA
Protocol Gas Standards, the EPA will consider restoring this provision
to appendix A.
    The EPA is proposing to revise the requirement in section <IP_ADDRESS>
pertaining to the validation of the gaseous cylinders used for the
National Performance Audit Program (NPAP). The EPA proposes to change
the requirement for annual verification to the ORD-recommended
certification periods for standards identified in Table 2-3 of the EPA
Traceability Protocol for Assay and Certification of Gaseous
Calibration Standards (appendix A, section 6.0(4)). These ORD-
recommended periods are based on the periods for which similar gas
mixtures over specific concentration ranges have been shown to be
stable, as documented in the peer-reviewed literature or in
concentration stability data submitted by the National Institute of
Standards and Technology (NIST) and specialty gas producers and
reviewed by the EPA. In effect, this would decrease the cost and burden
on the Protocol Gas Verification Program (PGVP), which performs these
verifications annually. The EPA anticipates this will also decrease the
delay in returning tanks back to the auditors. This would provide
auditors with longer periods with valid certifications to perform
audits without annual interruptions for the verification process.
    The EPA is proposing to adjust the minimum value required by
appendix A, section 3.2.4, to be considered valid sample pairs for the
PM2.5 Performance Evaluation Program (PEP) from 3 [mu]g/m\3\
to 2 [mu]g/m\3\. As discussed above, ambient PM2.5
concentrations have decreased, and many samples being collected now are
below the 3 [mu]g/m\3\ threshold and deemed invalid for purposes of a
valid audit sample. Therefore, decreasing this threshold from 3 [mu]g/
m\3\ to 2 [mu]g/m\3\ would increase the number of valid PEP sample
pairs collected, which would reduce the number of re-audits that need
to be performed to compensate for invalid sample pairs. Inclusion of
values down to 2 [mu]g/m\3\ would represent the concentrations
occurring in routine monitoring operations and are included in annual
mean concentrations of the networks. Reducing the number of re-audits
would reduce audit costs to monitoring organizations while better
representing the data in the networks. The technical justification and
background for this change is documented in a technical memorandum to
the docket for this rulemaking titled ``Task 16 on PEP/NPAP Task Order:
Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring
Network.'' \153\
---------------------------------------------------------------------------

    \153\ Noah, G. (2022). Task 16 on PEP/NPAP Task Order: Bias and
Precision DQOs for the PM2.5 Ambient Air Monitoring
Network. Memorandum to the Rulemaking Docket for the Review of the
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.

---------------------------------------------------------------------------

c. Calculations for Data Quality Assessments
    The EPA is proposing to update the appendix A, section 4.2.1,
Equations 6 and 7, for calculating the Collocated Quality Control
Sampler Precision Estimate for PM10, PM2.5 and Pb.
    The proposed changes are:

    [GRAPHIC] [TIFF OMITTED] TP27JA23.003

    These new statistics are designed to address the inflated precision
values that result from using these calculations to compare low
concentrations that are now observed in the networks. The current
precision estimate uses a relative percent difference (RPD) when
comparing two collocated samplers. As the two numbers used in the
comparison get smaller, the statistic generally produces a result that
is inflated. A precision statistic calculated for low-concentration
data may show poor agreement even if the nominal values are relatively
close to each other. By using the square root in the denominator in
these statistics, the variability is more constant across all
concentrations thereby reducing the inflated effect. The EPA believes
this proposed change would provide the correct context for considering
inflated RPDs when calculating the bias estimate. The technical
justification and background for this change is documented in a
technical memorandum to the docket for this rulemaking titled ``Task 16
on PEP/NPAP Task Order: Bias and Precision DQOs for the
PM2.5 Ambient Air Monitoring Network.'' \154\
---------------------------------------------------------------------------

    \154\ Noah, G. (2022). Task 16 on PEP/NPAP Task Order: Bias and
Precision DQOs for the PM2.5 Ambient Air Monitoring
Network. Memorandum to the Rulemaking Docket for the Review of the
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    The EPA is proposing to update the appendix A, section 4.2.5,
Equation 8, calculation for the Performance Evaluation Programs Bias
Estimate for PM2.5 from

[GRAPHIC] [TIFF OMITTED] TP27JA23.004

    Again, because the average ambient PM concentrations across the
nation have steadily declined since the promulgation of the
PM2.5 standard, the current method of calculation may not be
appropriate for determining bias for these lower ambient concentrations
and newer sampling methodologies. The current bias estimate uses a
percent difference (PD), referenced in appendix A, section 4.1.1, when
comparing an audit sampler against a routine sampler. As the two
numbers used in the comparison get smaller, the statistic generally
produces a result that is inflated. A bias statistic calculated for
low-concentration data may show poor agreement even if the nominal
values are relatively close to each other. This may be misleading when
trying to assess bias and summarizing data to be used in decision
making. The EPA believes this proposed change would provide the correct
context for considering inflated RPDs when calculating the bias
estimate. The technical justification and background for this change is
documented in a technical memorandum to the docket for this rulemaking
titled ``Task 16 on PEP/NPAP Task Order: Bias and Precision DQOs for
the PM2.5 Ambient Air Monitoring Network.'' \155\
---------------------------------------------------------------------------

    \155\ Noah, G. (2022). Task 16 on PEP/NPAP Task Order: Bias and
Precision DQOs for the PM2.5 Ambient Air Monitoring
Network. Memorandum to the Rulemaking Docket for the Review of the
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

d. References
    The EPA proposes to update the references and hyperlinks in
appendix A, section 6. Several of the reference documents have been
updated and the web locations have changed. This proposal provides
accuracy in identifying and locating essential supporting documentation
so that historical documents that do not represent current practices
are not used. The EPA believes that it is important that interested
parties--especially ambient air monitoring organizations and
stakeholders--have the most current materials that provide
clarifications and guidance on the interpretation of the regulations.
    The EPA is also proposing to add a footnote to Table A-1 of
Appendix A to Part 58--Minimum Data Assessment Requirements for NAAQS
Related Criteria Pollutant Monitors. The proposed footnote would
clarify the allowable time (i.e., every two weeks, once a month, once a
quarter, once every 6 months, or distributed over all 4 quarters
depending on the check) between checks and encourage monitoring
organizations to perform data assessments at regular intervals. The EPA
believes this proposal is appropriate because the current stipulation
is unclear regarding the specified interval for required verifications.
For example, under the current flow rate verification for
PM10 (low vol.), PM2.5, and Pb-PM10, a
flow check could be performed on April 1 and not checked again until
May 31, leaving approximately two months between checks. Following this
practice would leave large intervals of time between verifications, and
if a check fails using the described practice, an unacceptably large
data loss could result. Also, a check could be performed on the last
day of a quality control (QC) check interval and then on the first day
of the following interval, with only a day or two between checks. This
is not the intended practice for QC measures that are meant to ensure
equipment is continually operating properly over an operational period.
For this reason, the EPA is proposing to clarify the allowable time
between checks.
2. Quality Assurance Requirements for Prevention of Significant
Deterioration (PSD) Air Monitoring
    This section on Quality Assurance Requirements for Prevention of
Significant Deterioration (PSD) Air Monitoring was developed in
parallel to the proposed changes associated with appendix A. Thus, this
section includes similar detail and proposed changes for evaluating
quality system statistics for PM2.5, clarifications, and
other improvements that will better assist with the consistency and
operations of quality assurance programs for PSD.

a. Quality System Requirements
    The EPA has reconsidered the appendix A, section <IP_ADDRESS>, goal for
acceptable measurement uncertainty for automated and manual
PM2.5 methods currently stated as an upper 90 percent
confidence limit for the CV of 10 percent and 10 percent
for total bias. The average PM concentrations across the nation have
steadily declined since the promulgation of the first PM2.5
standard (U.S. EPA, 2022, section 2.3). As ambient concentrations
decrease, the bias is inflated using the current bias statistic in
section 4.2.5. Using a new statistic to replace the existing statistic
in section 4.2.5 developed to eliminate the effect of low
concentrations on bias, the EPA has concluded that the coefficient of
variation (CV) of 10 percent and 10 percent for total bias
is still an acceptable goal for estimating total bias in the networks.
The technical justification and background for this change is
documented in a technical memorandum to the docket for this rulemaking
titled ``Task 16 on PEP/NPAP Task Order: Bias and Precision DQOs for
the PM2.5 Ambient Air Monitoring Network.'' \156\
---------------------------------------------------------------------------

    \156\ Noah, G. (2022). Task 16 on PEP/NPAP Task Order: Bias and
Precision DQOs for the PM2.5 Ambient Air Monitoring
Network. Memorandum to the Rulemaking Docket for the Review of the
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

    The EPA is proposing to update and clarify ambient air monitoring
requirements found in appendix A, section 2.6.1, pertaining to EPA
Protocol Gas standards used for ambient air monitoring and the Ambient
Air Protocol Gas Verification Program. Appendix A would be revised to
clarify that in order to participate in the Ambient Air Protocol Gas
Verification Program, producers of Protocol Gases must adhere to the
requirements of 40 CFR 75.21(g), and only regulatory ambient air
monitoring programs may submit cylinders for assay verification to the
EPA Ambient Air Protocol Gas Verification Program. The EPA is proposing
to include an allowable uncertainty of 2.0 percent for EPA
Protocol Gas standards used in ambient air monitoring. This allowable
uncertainty limit would match the existing limit set by the EPA's
continuous emission monitoring program found in part 75, appendix A,
section 5.1.4(b), and would make the EPA's regulations more uniform and
consistent.
b. Measurement Quality Check Requirements
    The EPA is proposing to remove section <IP_ADDRESS> from appendix A.
This provision in the quality assurance requirements for ambient air
monitoring allows for NO2 compressed gas standards to be
used to generate audit standards. However, NO2 compressed
gas standards are not currently designated by the EPA's ORD as an EPA
Protocol Gas Standard. As such, this provision conflicts with section
2.6.1 of appendix A that requires that any standard used for generating
test atmospheres be an EPA Protocol Gas Standard. The EPA is aware that
there is a need for NO2 compressed gas standards for direct
read NO2 monitoring methods. If these NO2
compressed gas standards can, in the future, be proven to be stable and
approvable as EPA Protocol Gas Standards, the EPA will consider
restoring this provision to appendix A.
    The EPA is proposing to revise the requirement in section <IP_ADDRESS>
pertaining to the validation of the gaseous cylinders used for the
NPAP. The EPA proposes to change the requirement for annual
verification to the ORD-recommended certification periods for standards
identified in Table 2-3 of the EPA Traceability Protocol for Assay and
Certification of Gaseous Calibration Standards (appendix A, section
6.0(4)). These ORD-recommended periods are based on the periods for
which similar gas mixtures over specific concentration ranges have been
shown to be stable, as documented in the peer-reviewed literature or in
concentration stability data submitted by NIST and specialty gas
producers and reviewed by the EPA. In effect, this would decrease the
cost and burden on the PGVP, which performs these verifications
annually. The EPA anticipates this will also decrease the delay in
returning tanks back to the auditors. This would provide auditors with
longer periods with valid certifications to perform audits without
annual interruptions for the verification process.
    The EPA is proposing to adjust the minimum value required by
appendix A, section 3.2.4, to be considered valid sample pairs for the
PM2.5 Performance Evaluation Program (PEP) from 3 [mu]g/m\3\
to 2 [mu]g/m\3\. As discussed above, ambient PM2.5
concentrations have decreased, and many samples being collected now are
below the 3 [mu]g/m\3\ threshold and deemed invalid for purposes of a
valid audit sample. Therefore, decreasing this threshold from 3 [mu]g/
m\3\ to 2 [mu]g/m\3\ would increase the number of valid PEP sample
pairs collected, which would reduce the number of re-audits that need
to be performed to compensate for invalid sample pairs. Inclusion of
values down to 2 [mu]g/m\3\ would represent the concentrations
occurring in routine monitoring operations and are included in annual
mean concentrations of the networks. Reducing the number of re-audits
would reduce audit costs to monitoring organizations while better
representing the data in the networks. The technical justification and
background for this change is documented in a technical memorandum to
the docket for this rulemaking titled ``Task 16 on PEP/NPAP Task Order:
Bias and Precision DQOs for the PM2.5 Ambient Air Monitoring
Network.'' \157\
---------------------------------------------------------------------------

    \157\ Noah, G. (2022). Task 16 on PEP/NPAP Task Order: Bias and
Precision DQOs for the PM2.5 Ambient Air Monitoring
Network. Memorandum to the Rulemaking Docket for the Review of the
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

c. Calculations for Data Quality Assessments
    The EPA is proposing to update the appendix A, section 4.2.5,
Equation 8, calculation for the Performance Evaluation Programs Bias
Estimate for PM2.5 from

[GRAPHIC] [TIFF OMITTED] TP27JA23.005

    Again, because the average ambient PM concentrations across the
nation have steadily declined since the promulgation of the
PM2.5 standard, the current method of calculation may not be
appropriate for determining bias for these lower ambient concentrations
and newer sampling methodologies. The current bias estimate uses a PD,
referenced in appendix A, section 4.1.1, when comparing an audit
sampler against a routine sampler. As the two numbers used in the
comparison get smaller, the statistic generally produces a result that
is inflated. A bias statistic calculated for low-concentration data may
show poor agreement even if the nominal values are relatively close to
each other. This may be misleading when trying to assess bias and
summarizing data to be used in making decisions. The EPA believes this
proposed change would provide the correct context for considering
inflated RPDs when calculating the bias estimate. The technical
justification and background for this change is documented in a
technical memorandum to the docket for this rulemaking titled ``Task 16
on PEP/NPAP Task Order: Bias and Precision DQOs for the
PM2.5 Ambient Air Monitoring Network.'' \158\
---------------------------------------------------------------------------

    \158\ Noah, G. (2022). Task 16 on PEP/NPAP Task Order: Bias and
Precision DQOs for the PM2.5 Ambient Air Monitoring
Network. Memorandum to the Rulemaking Docket for the Review of the
National Ambient Air Quality Standards for Particulate Matter (EPA-
HQ-OAR-2015-0072). Available at: https://www.regulations.gov/docket/EPA-HQ-OAR-2015-0072.
---------------------------------------------------------------------------

d. References
    The EPA proposes to update the references and hyperlinks in
appendix A, section 6. Several of the reference documents have been
updated and the web locations have changed. This proposal provides
accuracy in identifying and locating essential supporting documentation
so that historical documents that do not represent current practices
are not used. The EPA believes that it is important that interested
parties--especially ambient air monitoring organizations and
stakeholders--have the most current materials that provide
clarifications and guidance on the interpretation of the regulations.
    The EPA is also proposing to add a footnote to Table A-1 of
Appendix A to Part 58--Minimum Data Assessment Requirements for NAAQS
Related Criteria Pollutant Monitors. The proposed footnote would
clarify the allowable time (i.e., every two weeks, once a month, once a
quarter, once every six months, or distributed over all four quarters
depending on the check) between checks and encourage monitoring
organizations to perform data assessments at regular intervals. The EPA
believes this proposal is appropriate because the current stipulation
is unclear regarding the specified interval for required verifications.
For example, under the current flow rate verification for
PM10 (low vol.), PM2.5, and Pb-PM10, a
flow check could be performed on April 1 and not checked again until
May 31, leaving approximately two months between checks. Following this
practice would leave large intervals of time between verifications, and
if a check fails using the described practice, an unacceptably large
data loss could result. Also, a check could be performed on the last
day of a QC check interval and then on the first day of the following
interval, with only a day or two between checks. This is not the
intended practice for quality control measures that are meant to ensure
equipment is continually operating properly over an operational period.
For this reason, the EPA is proposing to clarify the allowable time
between checks.
3. Proposed Amendments to PM Ambient Air Quality Methodology
a. Proposal To Revoke Approved Regional Methods (ARMs)
    The EPA is proposing to remove provisions for approval and use of
Approved Regional Methods (ARMs) throughout parts 50 and 58 of the CFR.
ARMs are continuous PM2.5 methods that have been approved
specifically within a State or local air agency monitoring network for
purposes of comparison to the NAAQS and to meet other monitoring
objectives. However, at this time, there are no approved ARMs, nor does
the EPA anticipate any will be requested. There are, however, more than
a dozen approved FEMs for PM2.5. These approved FEMs are
eligible for comparison to the NAAQS and to meet other monitoring
objectives.
    The EPA first proposed a process to approve and use ARMs in January
of 2006 (71 FR 2709, January 17, 2006). At that time, there were no
approved continuous PM2.5 methods available to compare to
the NAAQS. The hope was that approved ARMs would quickly start the use
of PM2.5 continuous methods that worked well in monitoring
agency networks, since the benefits of regulatory-grade automated
methods were not available at that time to air agency programs. It was
hoped that the benefits of automated PM2.5 methods--
including real-time data reporting of PM2.5 to support
forecasting and reporting of the AQI while also providing a regulatory
dataset eligible for comparison to the PM2.5 NAAQS--would
encourage the development of ARMs. The idea to encourage ARMs was
conceived following review of data across the country demonstrating
that some agencies were achieving acceptable data comparability with
their PM2.5 methods compared to collocated FRMs; however,
those methods did not necessarily provide consistent data across the
country. At that time, there were no approved PM2.5
continuous FEMs and it was unclear how soon any might be approved.
However, by March 2008, the EPA's Reference and Equivalent Methods
program had approved the first PM2.5 continuous FEM (73 FR
13224, March 12, 2008). Over the next eight years, an additional 12
PM2.5 continuous FEMs were approved. With many commercially
available PM2.5 continuous FEMs available to air agencies,
almost all agencies soon began implementing one or more
PM2.5 FEMs in their network. By 2020, monitoring agencies
were

reporting PM2.5 continuous FEM data from 660 sites across
the country (U.S. EPA, 2022, section <IP_ADDRESS>). Therefore, with a large
and growing network of PM2.5 continuous FEMs and no approved
applications for ARMs in the 16 years that this provision has been
available, the EPA is proposing to remove this provision, including any
related language, and to instead rely on the existing network of
approved PM2.5 FEMs and future approved FEMs. The EPA notes
that although references to ARMs occur across part 50 and part 58, the
EPA is not reopening the substance of the provisions where these
references occur and is only proposing regulatory text for these
provisions for the purpose of removing the reference to ARMs.
b. Proposal for Calibration of PM Federal Equivalent Methods (FEMs)
    The EPA is proposing to modify its specifications for PM FEMs
described in appendix C to part 58. Specifically, the EPA is proposing
that valid State, local, and Tribal air monitoring data generated in
routine networks and submitted to the EPA may be used to improve the PM
concentration measurement performance of approved FEMs. This approach,
initiated by instrument manufacturers, would be implemented as a
national solution in factory calibrations of approved FEMs through a
firmware update. This would apply to any PM FEM methods (i.e.,
PM10, PM2.5, and PM10-2.5). The EPA is
proposing this modification because there are some approved PM FEMs
that are not currently meeting measurement quality objectives (MQOs)
when evaluating data nationally (U.S. EPA, 2022, section <IP_ADDRESS>)
meaning that an update to a factory calibration may be appropriate;
however, there is not a clearly defined process to update the
calibration of an FEM. While there are several types of data available
to use as the reference for such updates (e.g., routinely operated
FRMs, audit program FRMs, and chemical speciation sampler data), we are
proposing to use routinely operated State, local, and Tribal FRMs as
the basis of comparison upon which to calibrate FEMs. The goal of
updating factory calibrations would be to increase the number of
routinely operating FEMs meeting MQOs across the networks in which they
are operated. The EPA has received input from CASAC (Sheppard, 2022, p.
2 of consensus responses) and State, local, and Tribal agencies
(National Association of Clean Air Agencies (NACAA) Monitoring
Committee 01/20/22; Association of Air Pollution Control Agencies
(AAPCA) Ambient Monitoring Committee 01/26/2022; Tribal air quality
professionals call on 02/17/22), all of which expressed strong interest
in improving FEM data comparability to collocated FRMs. While there are
other approaches that could improve data comparability between PM FEMs
and collocated FRMs, The EPA believes that this approach represents the
most reliable approach to update FEM factory calibrations, since the
existing FRM network data that meets MQOs would be used to set updated
factory calibrations. While the Agency is proposing to add this
language to more expressly define a process to update factory
calibrations of approved PM FEMs, the EPA believes that the existing
rules for updating approved FRMs and FEMs found at 40 CFR 53.14 may
also continue to be utilized for this purpose as appropriate. This
section allows instrument manufactures to submit to the EPA a
``Modification of a reference or equivalent method.'' Submitting a
modification request may be appropriate to ensure an approved FEM
continues to meet the 40 CFR 53.9, ``Conditions of designation''.
Specifically, 40 CFR 53.9(c) requires that, ``Any analyzer,
PM10 sampler, PM2.5 sampler, or
PM10-2.5 sampler offered for sale as part of an FRM or FEM
shall function within the limits of the performance specifications
referred to in Sec.  53.20(a), Sec.  53.30(a), Sec.  53.35, Sec.
53.50, or Sec.  53.60, as applicable, for at least 1 year after
delivery and acceptance when maintained and operated in accordance with
the manual referred to in Sec.  53.4(b)(3).'' Thus, instrument
manufactures are encouraged to seek improvements to their approved FEM
methods as needed to continue to meet data quality needs as operated
across the network. Instrument manufactures have an option to pursue
that now and may have an additional option in the future should we
finalize this proposal for calibration of PM FEMs.
    In the PA (U.S. EPA, 2022b, section <IP_ADDRESS>), the EPA analyzed the
quality of data from FRM samplers and continuous PM2.5 FEM
monitors operating in routine networks to determine whether they meet
the MQOs for PM2.5 FRMs and FEMs (40 CFR part 58, appendix
A, section <IP_ADDRESS>): ``Measurement Uncertainty for Automated and Manual
PM2.5 Methods. The goal for acceptable measurement
uncertainty is defined for precision as an upper 90 percent confidence
limit for the coefficient of variation (CV) of 10 percent and 10 percent for total bias.'' When aggregating data across the
country, all PM2.5 FRMs meet the MQOs for these methods. But
of PM2.5 continuous FEMs aggregated across the country, some
meet the MQOs, and others do not.
    One of the major challenges to ensuring uniform data from PM
methods is that there are no accepted standards against which to
calibrate PM methods. This was discussed in the 2004 Air Quality
Criteria for Particulate Matter (U.S. EPA, 2004b). PM reference methods
typically include the design and performance requirements set forth in
the 40 CFR part 50. This is a contrast to FRMs and FEMs for gaseous
NAAQS pollutants for which there are accepted calibration standards; in
the case of ozone, there is even a standard reference photometer that
can be used to calibrate approved methods in the field or laboratory.
For PM monitoring methods, in the absence of accepted calibration
standards, acceptable data quality is determined by comparing to other
PM FRMs. One challenge to comparing to other PM FRMs during the initial
field testing for purposes of FEM approval is that the dataset will in
almost all cases be substantially more limited than what's available in
routine networks once deployed. Thus, we seek to encourage instrument
manufacturers of approved FEMs to evaluate data in routine networks and
consider improvements to their FEM calibration, as needed.
    The EPA is proposing to use routine and collocated FRM data
operated by State, local, and Tribal agencies as the basis to update
factory calibrations. Routine State, local, and Tribal agency FRM data
form the largest portion of the monitored air quality data used in
epidemiologic studies that are being used to inform proposed decisions
regarding the adequacy of the public health protection afforded by the
primary PM2.5 NAAQS, as discussed in section II above. While
the EPA is proposing to use routine FRM data, there are other reference
datasets that could be considered. For example, the agency has an FRM
audit program\159\ operated by independent operators and laboratories.
This program is highly valuable to the success of the PM2.5
monitoring program by providing independent data to assess the quality
of routinely operated FRMs and FEMs. If we used the audit program data
as the basis for calibrating continuous monitors, we would lose the
ability to collect independent data from audit monitors to assess the
operation of routine monitors. Therefore, by using routinely operated
FRMs to calibrate continuous FEMs, the Agency will continue to maintain
the independence

of the FRM audit program to assess the quality of routinely operated
FRM and continuous FEM data. The EPA also has chemical speciation data
available at sites where the Chemical Speciation Network (CSN) or
IMPROVE samplers are operated; however, these samplers use technologies
that operate at different flow rates and with different-size selective
devices than approved FRMs, and neither of these programs use FRMs as
the basis to collect samples. Therefore, while CSN and IMPROVE data can
be useful to help determine the aerosol chemistry of PM2.5
and may provide additional validation of collocated FRM or FEM data, by
themselves these data are not appropriate to update factory calibration
of continuous FEMs.
---------------------------------------------------------------------------

    \159\ See: https://www.epa.gov/amtic/national-pm25-performance-evaluation-program.
---------------------------------------------------------------------------

    The EPA proposes to direct instrument companies and other
interested stakeholders to the EPA's Air Quality System (AQS) database
\160\ to access the valid routine network data that the Agency proposes
to allow for use in updating factory calibration of continuous FEMs.
There are several ways to obtain data from the AQS database, and many
do not require registration. For example, daily processed datasets by
year are publicly available at the website of ``Pre-Generated Data
Fields.'' \161\ The data utilized would need to be valid PM FRM and FEM
data that are collocated and aligned to the same date. For example, for
PM2.5 mass concentrations, there are files by year for ``
PM2.5 FRM/FEM Mass'' identified with a parameter code of
88101. This information, already aggregated to daily data, represent
the time-period of midnight-to-midnight local standard time. While any
years of data may be considered, instrument companies should normally
use at least two years of recent data where we are past the
certification period for the previous-year data, which is May 1st of
each year. Including at least two years of data is intended to address
cases where one of the years may have high or low air quality
concentrations. Data in the current year and previous year when we are
not past the May 1st certification date can be considered to test data
with a correction established from a previous year or more than one
year. If multiple factors are included, any new statistical correction
or corrections should be based on one or more calendar years, with
independent testing of that data on another year or more that was not
used to develop the equation(s).
---------------------------------------------------------------------------

    \160\ See: https://www.epa.gov/aqs.
    \161\ See: https://aqs.epa.gov/aqsweb/airdata/download_files.html.
---------------------------------------------------------------------------

    The EPA also encourages instrument companies to consider and
implement all the ways to optimize PM2.5 FEMs. This may
include, but is not limited to, whether a method's data can be improved
by operating the FEM inside a heating, ventilation, and air-
conditioning (HVAC)-controlled shelter or outside with minimal or no
HVAC control; optimizing heating of the airstream to avoid condensation
while retaining semi-volatile PM captured on the FRM; and any
specialized guidance or training that may help monitoring agencies
optimize their data quality and comparability to collocated FRMs. Other
options might include updates to unique coefficients used in the
factory calibration such as the density of the aerosol, where
applicable. Such changes would normally need to be approved by the EPA
according to existing rules found at 40 CFR 53.14.
    Another challenge to consider is how to deal with potential
outliers that may exist in the validated State, local, and Tribal
agency network data available from AQS that would be used to establish
new factory calibrations. One of the reasons to use data from the AQS
database is that there are tens of thousands of collocated data pairs
available that include many of the approved continuous PM2.5
FEMs. Having a large data set will diminish the effect of any one or
more outliers. However, acknowledging that the goal of this proposed
change is to update factory calibrations to increase the number of
routinely operating FEMs meeting MQOs across the networks in which they
are operated, we propose that instrument companies may, but are not
required to, check for and exclude any potential outliers.
Additionally, we propose that the range of data may be limited to those
concentrations that are within the normal operating ranges of most
sites, but this is not required. This approach, for example, could
include 24-hour average PM2.5 concentrations up to the level
of the primary 24-hour PM2.5 NAAQS or some percentile above
that level (e.g., 125% of the 24-hour NAAQS). The rationale for this is
that there are very few sites with routine concentrations above the
level of the primary 24-hour PM2.5 NAAQS, and the
establishment of any equation with this data would need to be
constructed carefully to avoid having data below the primary annual
PM2.5 NAAQS drive the coefficients used above the level of
the primary 24-hour PM2.5 NAAQS.
    Ideally, the geographic coverage of the data used in establishing a
new factory calibration would be national in scope; however, instrument
companies can only use the data that is available. For widely used
PM2.5 FEMs, this will not be an issue, but for less-operated
PM2.5 FEMs, there may be limitations in the geographic scope
of data produced. Another challenge may be a large grouping of sites in
one part of the country that drives development of an equation used
across all networks. Instrument companies may limit the use of sites
with large groupings in one or more geographical area so that the data
are more geographically representative across the network so long as
there is a reasonable rationale as to why data from certain sites are
not being included. With a new factory calibration available,
instrument companies will need to test the performance of the updated
calibration across a variety of sites. Testing of an updated factory
calibration can be accomplished by utilizing a different year or years
other than the time-period used to establish the revised factory
calibration or a subset of data across all years. Testing should also
include the range of sites in which the method is used.
    Building off the geographic location of the sites in which an
updated factory calibration is tested with previously collected data,
the EPA considered what performance level should be acceptable.
Ideally, an updated factory calibration would work such that a
significantly larger number of, or all, individual sites operating with
the updated factory calibration would meet the MQOs. However, due to
several complicating factors such as seasonal changes in temperature
and humidity, elevation, differences in aerosol composition, and
differences in concentration between more polluted urban sites and
relatively cleaner rural sites (some of which read well below the
proposed revisions to the level of the primary annual PM2.5
NAAQS discussed in section II above), the EPA should not expect that
every site will necessarily meet the MQOs. Therefore, the goal of this
proposal is to increase the number of routinely operating FEMs meeting
MQOs across the networks in which they are operated, especially for
sites near the level of the NAAQS proposed elsewhere in this proposal.
Since there are multiple MQOs to consider, the EPA proposes to place
the most attention on improvements to the bias MQO goal because this
statistic will likely have the most influence on improving the
resultant data collected. In attempting to address this goal,
instrument companies may be interested in testing their original data
used in field studies of their candidate FEMs with an updated

factory calibration. While this could be a useful exercise to
understand the sensitivity of the original and any updated factory
calibration, the EPA proposes not to require meeting the performance
criteria of the original field testing as a condition of approving an
updated factory calibration.
    Regarding how frequently factory calibrations should be updated,
the EPA believes it would be most appropriate to not define a specific
time-period for updates. Rather, updates should be based on the
available of quality data being produced across the network. Monitoring
agencies routinely check their data comparability to collocated FRMs,
including as part of annual data certification where an AMP-256 report
describing data quality is included as part of the certification
package (Sec.  58.15(c)). In addition, monitoring agencies typically
provide a more thorough review of their networks and accompanying data
quality as part of the five-year assessments due to the EPA pursuant to
40 CFR 58.10(d).
    Another important aspect to implementing updated factory
calibrations is the treatment of data already collected under the
original factory calibration. There are two time periods to consider.
First, there is the time-period before the EPA approves an updated to a
factory calibration. We propose that data collected prior to an
approved update to a factory calibration be allowed to remain as
measured based on the factory calibration that was approved at the time
the data was collected. Second, there is the time-period between when
an updated factory calibration is approved by the EPA and when that
updated calibration is implemented in the field. While ideally, this
time-period would be short, there may be reasons why some agencies and
the sites they run cannot easily update the firmware with the updated
factory calibration. We solicit comment on how to handle these
situations and whether there should be an allowance to correct such
data.
    The EPA sought early input from State, local, and Tribal monitoring
agencies (NACAA Monitoring Committee 01/20/22; AAPCA Ambient Monitoring
Committee 01/26/2022; Tribal air quality professionals call on 02/17/
22) regarding how best to address the issue of some PM2.5
FEMs having bias issues. Many monitoring agencies identified that they
strongly favor a national solution that can be accomplished and
implemented through a firmware upgrade or similar resolution that is
consistent with the approach described above. One State suggested that
the EPA should consider and allow site-by-site corrections between FRM
and collocated FEMs with ongoing collocation at a 1:6 sample frequency
for FRMs. The rationale for site-by-site corrections was that there are
differences in the types of aerosol composition and concentration
between urban and rural locations and having site-by-site corrections
would ensure that each type of location is individually calibrated to a
collocated FRM rather than to a consistent factory calibration that may
average out any differences. In contrast, other monitoring agencies
expressed concern about the challenges of implementing a site-by-site
approach, especially for those agencies who stated that they would not
be able to redeploy the FRMs that would be necessary to perform the
site-by-site corrections in their networks for reasons including no
longer having FRMs, not having staff available to support and operate
the FRMs, and no longer have gravimetric laboratory capacity to support
a larger inventory of FRMs operating in their networks.
    The CASAC also provided input on the FEM bias issue. As part of
their review of the draft PA, the CASAC stated that ``the FEM bias
needs to be addressed to make the FRMs and FEMs more comparable''
(Sheppard, 2022a, p. 2 of consensus responses). The CASAC offered two
options for the EPA to consider. ``One option would be to allow states
to develop correction factors for co-located FRMs and FEMs. These
correction factors could be used to adjust FEM concentrations downward
(or upward) to be comparable to FRMs. Another option would be for the
EPA to revise the `equivalency box' (EB) criteria used to judge whether
the bias of a new continuous PM2.5 monitor relative to an
FRM is acceptable during field testing'' (Sheppard, 2022a, p. 2 of
consensus responses). The CASAC's first option is consistent with the
input received during early input described above. The EPA believes
that the second option should be considered in future reviews of the PM
NAAQS to help establish updated goals for data quality from
PM2.5 FEMs. The existing network of commercially available
PM2.5 FRMs and some of the continuous FEMs are already
meeting the MQOs at the existing concentrations, which are at or below
the proposed revisions to the level of the primary annual
PM2.5 NAAQS discussed in section II above. However, the EPA
recognizes that not all PM2.5 FEMs are meeting MQOs and,
therefore, the EPA intends to address improvements to existing FEMs
that are not meeting MQOs as described above.
    In attempting to address the comparability of PM2.5 FEMs
to collocated FRMs through our proposal to allow updates to factory
calibrations, the EPA recognizes that other potential solutions do not
need to be mutually exclusive. That is, there can be multiple
approaches to improve the comparability of PM2.5 FRMs to
continuous FEMs. Therefore, the EPA solicits comment on additional ways
to improve PM2.5 data comparability between PM2.5
FRMs and collocated continuous FEMs.
    The EPA encourages early dialogue with instrument companies
considering an update to any part (e.g., hardware, software, and/or
firmware revision) of an approved FEM designation. Dialogue with the
EPA as well as applications by instrument manufactures can be initiated
by contacting the EPA ORD's Reference and Equivalent (R&E) Methods
Designation program. The contact information for this can be found at
40 CFR 53.4, ``Applications for reference or equivalent method
determinations.''
    In summary, the EPA is proposing that valid State, local, and
Tribal air monitoring data generated in routine networks and submitted
to the EPA may be used to update factory calibrations included as part
of approved FEMs. This approach, initiated by instrument manufacturers,
subject to EPA approval, would be implemented as a national solution in
factory calibrations of approved FEMs through a firmware update. This
would apply to any PM FEM methods (i.e., PM10,
PM2.5, and PM10-2.5). As part of this process,
the EPA proposes that a range of data based on the most representative
concentrations up to all available concentrations may be used in
developing and testing a new factory calibration, that a representative
set of geographic locations can be used, that outliers may be included
or not included, that a new factory calibration should be developed
using data from at least two years and tested on a separate year(s) of
data, that updates to factory calibrations can occur as often as
needed, and should be evaluated by monitoring agencies as part of
routine data assessments such as during certification of data and five
year assessments, that the EPA recognizes only data from existing
operating sites is available, and that an updated factory calibration
does not have to work with the original field study data submitted that
led to the designation as an FEM. The EPA solicits input on this
approach and any alternatives that would lead to more sites meeting the
bias MQO with automated FEMs, especially for those sites that are near
the level of the primary annual PM2.5 NAAQS, as

proposed to be revised in section II above.
4. Proposed Amendment to the PM2.5 Monitoring Network Design
Criteria To Address At-Risk Communities
    To enhance protection of air quality in communities subject to
disproportionate air pollution risk, particularly in light of the
proposed range for a revised PM2.5 annual standard, the EPA
proposes to modify our PM2.5 monitoring network design
criteria to include an environmental justice factor that accounts for
proximity of populations at increased risk of adverse health effects
from PM2.5 exposures to sources of concern. Specifically,
the EPA proposes to modify our existing requirement (40 CFR part 58,
appendix D, section 4.7.1(b)(3)): ``For areas with additional required
SLAMS, a monitoring station is to be sited in an area of poor air
quality,'' to additionally address at-risk communities with a focus on
anticipated exposures from local sources of emissions. The scientific
evidence evaluated in the 2019 ISA and ISA Supplement indicates that
sub-populations at potentially greater risk from PM2.5
exposures include: children, lower socioeconomic status (SES) \162\
populations, minority populations (particularly Black populations), and
people with certain preexisting diseases (particularly cardiovascular
disease and asthma). The EPA is proposing that communities with
relatively higher proportions of sub-populations at greater risk from
PM2.5 exposure within the jurisdiction of a state or local
monitoring agency should be considered ``at-risk communities'' for
these purposes.
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    \162\ SES is a composite measure that includes metrics such as
income, occupation, and education, and can play a role in
populations' access to healthy environments and healthcare.
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    The PM2.5 network design criteria has led to a robust
national network of PM2.5 monitoring stations. These
monitoring stations are largely in Core-Based Statistical Areas (CBSAs)
\163\ across the country that include many PM2.5 monitor
sites in at-risk communities. Many of the epidemiologic studies
evaluated in the 2019 ISA and ISA Supplement, including those that
provide evidence of disparities in PM2.5 exposure and health
risk in minority populations and low SES populations, often use data
from these existing PM2.5 monitoring sites. However, we
anticipate that if the level of the annual NAAQS is lowered,
characterizing localized air quality issues may become even more
important around local emission sources. The EPA believes that adding a
network design requirement to specifically locate monitors in at-risk
communities will improve our characterization of exposures for at-risk
communities where localized air quality issues may exist. Requiring the
siting of PM2.5 monitoring stations in at-risk communities
allows other methods to be operated alongside PM2.5
measurements to support multiple monitoring objectives (40 CFR part 58,
appendix D, section 1.1). The EPA believes that it is appropriate to
formalize the monitoring network's characterization of PM2.5
concentrations in communities at increased risk, to provide these areas
with the level of protection intended with the PM2.5 NAAQS.
The addition of this requirement will also lead to enhanced local data
that will allow regulatory air quality agencies to assist communities
to reduce exposures and to help inform future implementation and
reviews of the NAAQS.
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    \163\ CBSAs--Metropolitan and Micropolitan Statistical Areas are
collectively referred to as Core-Based Statistical Areas.
Metropolitan statistical areas have at least one urbanized area of
50,000 or more population, plus adjacent territory that has a high
degree of social and economic integration with the core as measured
by commuting ties. Micropolitan statistical areas are a set of
statistical areas that have at least one urban cluster of at least
10,000 but less than 50,000 population, plus adjacent territory that
has a high degree of social and economic integration with the core
as measured by commuting ties.
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    As described in section II.B.2 above and in more detail in the PA
(U.S. EPA, 2022b, section 3.3.2), the public health implications of
health effects associated with PM2.5 in ambient air are
dependent on the type and severity of effects, as well as the size of
the population affected and whether there are populations and/or
lifestages at increased risk of a PM2.5-related health
effect. The 2019 ISA cites extensive evidence indicating that ``both
the general population as well as specific populations and lifestages
are at risk for PM2.5-related health effects'' (U.S. EPA,
2019, p. 12-1). Factors that may contribute to increased risk of
PM2.5-related health effects include lifestage, pre-existing
diseases (cardiovascular disease and respiratory disease), race/
ethnicity, and socioeconomic status. The increased risk faced by these
sub-populations raises environmental justice \164\ concerns. Section II
of this preamble, section 12.5 of the 2019 ISA (U.S. EPA, 2019a) and
section 3.3.3 of the ISA Supplement (U.S. EPA, 2022a) provide extensive
discussion on the evidence for disparities in PM2.5
exposures and PM2.5-related health risks of these sub-
populations.
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    \164\ The EPA defines environmental justice as the fair
treatment and meaningful involvement of all people regardless of
race, color, national origin, or income with respect to the
development, implementation, and enforcement of environmental laws,
regulations, and policies. The EPA further defines the term fair
treatment to mean that ``no group of people should bear a
disproportionate burden of environmental harms and risks, including
those resulting from the negative environmental consequences of
industrial, governmental, and commercial operations or programs and
policies.''
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    Consistent with the requirement of the Clean Air Act to protect
sensitive sub-populations, the EPA is particularly concerned with
protecting sub-populations identified as being at higher risk of
adverse health effects from PM2.5 exposure in the 2019 ISA,
ISA Supplement and PA (U.S. EPA, 2019a; U.S. EPA, 2022a; U.S. EPA,
2022b). The EPA finds it appropriate to better characterize the
localized air quality in communities with relatively higher proportions
of these sub-populations to ensure these sub-populations receive the
intended level of protection of a revised NAAQS proposed earlier in
section II. Thus, the EPA is proposing to modify the PM2.5
ambient monitoring network design criteria to add a provision
pertaining to sub-populations identified as at increased risk for
PM2.5 exposures and health risks associated with
PM2.5 (``at-risk communities'').
    An enhanced network should include representation of at-risk
communities who live near emission sources of concern such as, but not
limited to, major ports, rail yards, airports, industrial areas, or
major transportation corridors. The EPA finds it appropriate, in light
of the evidence of increased risk to these communities, to better
characterize exposures given proximity to local sources of concern. For
example, the EPA believes it is worthwhile to characterize localized
ambient concentrations occurring when there are emission sources
located in a part of a metropolitan area that are different than the
design value \165\ site of the same metropolitan area. Thus, while
there may be sites with higher overall maximum concentrations in
another part of the same metropolitan area, those sites are covered by
our long-standing existing requirement that monitors be placed ``. . .
in the area of expected maximum concentration'' [Sec.  58.1 and
appendix D, section 4.7.1(b)(1)].
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    \165\ Design value is defined in Sec.  58.1 as the calculated
concentration according to the applicable appendix of 40 CFR part 50
for the highest site in an attainment or nonattainment area.
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    PM2.5 concentrations have generally trended down when
averaged across all monitoring sites over the last two decades since
PM2.5 measurements

commenced nationally in 1999.\166\ This downward trend has resulted in
lower background concentrations being measured upwind of urban areas;
however, the impact of local emissions on PM2.5 may not be
known if there is not a requirement to monitor ambient air in these
areas. For example, the presence of new local sources of fine particle
air pollution proximate to at-risk communities, such as significant
increases in heavy duty truck traffic since monitors were originally
sited, should be taken into consideration. As explained in the PA (U.S.
EPA, 2022b), measured PM2.5 at near-road monitoring stations
include an increment relative to other sites in the same CBSA. The
near-road sites will complement any new or moved sites located to
specifically address at-risk communities near sources of concern. We
anticipate the significance of local emissions may increase if, as
proposed, the level of the annual PM2.5 NAAQS is lowered.
Thus, the EPA seeks to support communities with at-risk populations in
proximity to local sources of concern so that they have access to
PM2.5 NAAQS-comparable data to ensure compliance with the
PM2.5 NAAQS and for other data uses.
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    \166\ See: https://www.epa.gov/air-trends/particulate-matter-pm25-trends.
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    To successfully select and deploy an ambient air monitoring
station, monitoring agencies must comply with the requirements of the
EPA network design criteria (40 CFR part 58, appendix D, section 4.7),
consider input from the community and other interested stakeholders,
and then overlay the requirements and input with logistically available
options in the neighborhoods they intend to monitor. Often, monitoring
agencies partner with schools and other government agencies that have
access to property in a neighborhood so that the desired monitoring
stations can be sited, deployed, and maintained. Locating monitoring
stations in neighborhoods should be done in a way that provides a good
representation of the particulate matter exposures of the communities
in which they are located. Alternatively, monitoring stations can be
located directly next to emission sources of concern. However, these
locations, known as ``source-oriented'' sites, may not necessarily
represent the exposures in community or the effect of a multitude of
emissions that can impact a neighborhood.
    To ensure monitoring sites are appropriately representing exposure
in at-risk communities, we propose that sites represent ``area-wide''
air quality near local sources of concern. Sites representing ``area-
wide'' air quality are those monitors sited at neighborhood, urban, and
regional scales, as well as those monitors sited at either micro- or
middle-scale that are identified as being representative of many such
locations in the same Metropolitan Statistical Area (MSA).\167\ Most
existing as well as new or moved sites are expected to be neighborhood-
scale, which means that the monitoring stations would typically
represent conditions throughout some reasonably homogeneous urban sub-
region with dimensions of a few kilometers [part 58, appendix D,
section 4.7.1(c)(3)]. Additionally, as described in Sec.  58.30, sites
representing ``area-wide'' air quality have a long-standing
applicability to both the annual and 24-hour PM2.5 NAAQS.
Siting in a community representing ``area-wide'' air quality as
proposed is consistent with other network design objectives pursuant to
which we locate monitors where people live, work, and play.
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    \167\ MSA means a CBSA associated with at least one urbanized
area of 50,000 population or greater. The central-county, plus
adjacent counties with a high degree of integration, comprise the
area.
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    The types of sites that are minimally required as part of the
PM2.5 network design are associated with two geopolitical
levels: MSAs and states. The minimum number and type of sites that are
required within an MSA are a function of the population of the MSA,
based on the latest available information from the Census Bureau, and
the design value of the existing network of PM2.5 sites
reported for that MSA. MSAs with design values at or above 85% of any
PM2.5 NAAQS are required to operate one more site than those
MSAs with values that are less than 85% of any PM2.5 NAAQS
(40 CFR part 58, appendix D, Table D-5). Each MSA required to operate
at least one monitoring station is to site the monitor at neighborhood
or larger scale in an area of expected maximum concentration. MSAs with
a population of 1 million or more are required to operate a
PM2.5 monitor at a NO2 near-road station in the
same MSA. Thus, according to Table D-5 of appendix D to part 58, only
those MSAs with a population of greater than 1 million with the most