recent 3-year design value greater than or equal to 85% of any
PM2.5 NAAQS are required to operate at least three
PM2.5 monitoring stations. Since one of these sites would be
the site in the area of expected maximum concentration, which most
often will be the design value site, and the other the near-road site,
only the third location would not address either of those two
requirements.
    The requirement for a third monitoring station in a MSA, where it
exists, would take on the revised network design requirement to address
at-risk communities near sources of concern. Many existing sites in the
area of expected maximum concentration or near-road sites that are
located in at-risk communities. Thus, having multiple sites located in
at-risk communities may be appropriate so long as each siting criteria
is achieved. Also, while we are proposing this modification to our
network design criteria, we recognize that the number of monitors to
support key monitoring objectives, including addressing at-risk
communities, could go well beyond what is currently minimally required.
Many monitoring agencies already operate more monitoring sites than are
minimally required and we expect this to continue in considering siting
monitors in at-risk communities. Thus, the existing and robust network
of almost 1,000 PM2.5 sites nationally will continue to
protect all populations at the level of the NAAQS discussed in section
II of this proposal, by always having at least one site in the area of
expected maximum concentration for each CBSA where monitoring is
required. Many existing and a few new sites will form an important sub-
component of the PM2.5 network by characterizing air quality
in at-risk communities, particularly with respect to sources of
concern.
    Monitoring requirements applicable at the state level include
measuring regional background and regional transport (40 CFR part 58,
appendix D, section 4.7.3). These required sites at the state level are
largely located in rural areas and may include use of IMPROVE samplers
or continuous PM2.5 monitors. The sites required at the
state level complement sites required at the MSA level. Together the
sites already required at the state level combined with existing siting
requirements at the MSA level as well as the proposed revisions
described herein to address at-risk communities will achieve several
monitoring objectives, including comparison to the NAAQS and AQI. The
availability of data from regional background and regional transport
sites compared to data from design value sites already allow for
calculating incremental exposure in communities with the highest design
value location. With the proposed addition of a siting requirement for
at-risk communities and the use of data from these sites compared to
select regional background and regional transport sites as well as
other sites in the same MSA, we can

assess the incremental burden of exposure from local emissions to at-
risk communities.
    In addition to using data from the robust network of almost 1,000
PM2.5 sites for NAAQS and AQI purposes, having a stable
network of long-term sites is especially valuable for trends and as an
input to long term health and epidemiology studies that support reviews
of the PM NAAQS. Therefore, while we are proposing to add a
PM2.5 network design criteria to address at-risk
communities, many sites are likely already in valuable locations
meeting one of the existing network design criteria (i.e., being in an
area-wide area of expected maximum concentration or collocated with
near-road sites) and supporting multiple monitoring objectives. Also,
in many communities there may already be sites meeting the network
design criteria we are proposing for at-risk communities. Thus,
acknowledging the value of having long-term data from a consistent set
of network sites, on balance the EPA believes that the movement of
sites should be minimized, especially in MSA's with a small number of
sites. However, a small number of new sites \168\ are expected to be
required due to the existing minimum monitoring requirements (Table D-5
of appendix D to part 58) and the revised primary annual
PM2.5 NAAQS proposed in section II of this proposal. Also,
sites do on occasion need to move due to loss of leases, no longer
meeting siting criteria, or other reasons. For any of these cases, we
believe it is appropriate to include prioritizing establishing sites in
at-risk communities near sources of concern, should new sites be
established, or existing locations be lost, and replacement sites need
to be identified. Therefore, the EPA proposes that annual monitoring
network plans [40 CFR 58.10(a)(1)] that include the few newly required
sites and five-year assessments [40 CFR 58.10(d)] include a provision
to examine the ability of existing and proposed sites to support air
quality characterization for areas with at-risk populations in the
community and the objective discussed herein.
---------------------------------------------------------------------------

    \168\ Gantt, B. (2022). Analyses of Minimally Required
PM2.5 Sites Under Alternative NAAQS. 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.
---------------------------------------------------------------------------

    Assessing and prioritizing at-risk communities for monitoring can
be accomplished through several approaches. The most critical aspect of
prioritizing which communities to monitor is their representation of
the at-risk populations described earlier in this section. The other
major consideration is whether the community is near source(s) of
concern. While many CBSA's have one or more sources of concern
described above, some CBSA's will not have the level of emissions from
sources of concern that result in an elevated level of measured
PM2.5 concentrations in surrounding communities. Since one
of our other siting criteria to ``. . . be in the area of expected
concentration'' [Sec.  58.1 and appendix D, section 4.7.1(b)(1)]
ensures there is a monitoring site in the community with the highest
exposure in each CBSA with a monitoring requirement, on balance the EPA
believes we should include being in an at-risk community for CBSAs with
a third site requirement when there are no sources of concern
identified in a CBSA or such sources do exist but are not expected to
lead to elevated levels of measured PM2.5 concentrations.
    To identify at-risk communities to consider for the proposed
monitoring requirement, tools such as the EPA's EJSCREEN \169\ are
available. The EPA solicits comment on other tools and/or datasets that
can could be utilized to identify the at-risk communities described
above. With information on at-risk communities, monitoring agencies
need data that can best inform where there may be elevated levels of
exposures from sources of concern. While we use FRMs and FEMs to
determine compliance with the NAAQS, there are several additional
datasets available that may be useful in evaluating the potential for
elevated levels of exposure to communities near sources of concern.
Potential datasets include non-regulatory data (CSN, IMPROVE, and AQI
non-regulatory PM2.5 continuous monitors), modelling data--
which utilizes emission inventory and meteorological data, emerging
sensor networks such as used in the EPA's AirNow fire and smoke
map,\170\ and satellites--which measure radiance and with computational
algorithms are then used to estimate PM2.5 from aerosol
optical depth (AOD). The 2019 ISA and PA (U.S. EPA, 2019a; U.S. EPA,
2022b) include details on each these, except for the AirNow fire and
smoke map, which first became operational in 2020. Each of these
datasets have advantages and disadvantages, especially when attempting
to determine exposure concentrations for the averaging times of the
PM2.5 NAAQS described in section II (i.e., annual NAAQS and
24-hour NAAQS). The EPA solicits comment on datasets most useful to
identify communities with high exposures for PM2.5 NAAQS
(i.e., annual or 24-hour), including any discussion on limitations or
advantages of the dataset of interest. The EPA is soliciting comment on
the use of these datasets for the purpose of identifying communities
where the proposed monitoring requirement would apply and not for the
purpose of satisfying the proposed monitoring requirement.
---------------------------------------------------------------------------

    \169\ See: https://www.epa.gov/ejscreen.
    \170\ See: https://fire.airnow.gov/.
---------------------------------------------------------------------------

    The monitoring methods appropriate for use at these proposed sites
are FRMs and automated continuous FEMs. These are the methods that are
eligible to compare to the PM2.5 NAAQS, which will be the
primary objective for collecting this data. There are several other
monitoring objectives that would benefit from use of automated
continuous FEMs. For example, having hourly data available from
automated continuous FEMs would allow sites to provide data in near-
real time to support forecasting and near real-time reporting of the
AQI. Automated continuous methods are also useful to support evaluation
of other methods such as low-cost sensors. When used in combination
with on-site wind speed and wind direction measurements, automated FEMs
can provide useful pollution roses indicating the origin of emissions
that affect a community. Additionally, when collocated with continuous
carbon methods such as an aethalometer, automated FEMs can help
identify potential local carbon sources contributing to increased
exposure in the community. The EPA and the CASAC worked collaboratively
in 2010 (Russell and Samet, 2010) to define a list of measurements that
would be useful to implement in the near-road environment, and a subset
of these measurements may additionally be of value to characterize the
exposure in at-risk communities. While either FRMs or automated FEMs
may be used at a site for comparison to the PM2.5 NAAQS, the
EPA encourages use of automated continuous FEMs at sites in at-risk
communities.
    Although there are only a few new sites required,\171\ plus any
potentially moved sites in cases where a site lease is lost, EPA
believes we should build upon our existing regulatory process for
selecting and approving these sites (40

CFR 58.10). For example, the timeline to implement the proposed
PM2.5 sites in at-risk communities should allow monitoring
agencies enough time for communities and other interested parties to
provide their input regarding moving or adding new sites, while also
minimally disrupting ongoing operations of monitoring agency programs.
Another important factor is to ensure all existing PM2.5
sites have data available for comparison to a revised PM2.5
NAAQS, which is discussed in section II of this proposal. With a final
rule from this proposal expected in 2023, we believe it would be
appropriate to provide at least 12 months from the effective date of a
final rule for monitoring agencies to initiate planning to implement
these measures by seeking input from communities and other interested
parties, and to consider revisions to their PM2.5 networks
or explain how the existing network meets the objectives of this
proposed modification. Thus, the EPA proposes that monitoring agencies
identify their initial approach to the question of whether any new or
moved sites are needed and to identify the potential communities in
which the agencies are considering adding monitoring, if applicable, as
well as identifying how they intend to meet the proposed revised
criteria for PM2.5 network design to address at-risk
communities. These aspects that will potentially affect the siting of
new and moved sites should be addressed in the agencies' annual
monitoring network plans due to each applicable EPA Regional office no
later than July 1, 2024 (40 CFR 58.10). Specifics on the resulting
proposed new or moved sites for PM2.5 network design to
address at-risk communities would need to be detailed in the annual
monitoring network plans due to each applicable EPA Regional office no
later than July 1, 2025 (40 CFR 58.10). We are proposing that any new
or moved sites would be required to be implemented and fully
operational no later than 24 months from the date of approval of a plan
or January 1, 2027, whichever comes first, but the EPA solicits comment
on whether less time is needed (e.g., 12 months from plan approval and/
or January 1, 2026).
---------------------------------------------------------------------------

    \171\ Gantt, B. (2022). Analyses of Minimally Required
PM2.5 Sites Under Alternative NAAQS. 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.
---------------------------------------------------------------------------

    In summary, the EPA is proposing to modify our PM2.5
network design criteria to include an environmental justice factor to
address at-risk communities with a focus on exposures from sources of
concern. While this proposal would require that sites be located in at-
risk communities, particularly those whose air quality is potentially
affected by local sources of concern, such sites should still meet the
requirement for being considered ``area-wide'' air quality. Specific
areas of interest we seek comment on include how to identify at-risk
communities, the sources of concern important to consider, the datasets
to identify communities with high exposures, and the most useful
measurements to collocate with PM2.5 in at-risk communities.
The EPA seeks comment on these areas of interest as well as the
proposed modification of our PM2.5 network design objectives
and implementation as described herein.
5. Proposed Revisions to Probe and Monitoring Path Siting Criteria
    The EPA is proposing changes to monitoring requirements in the
Appendix E--Probe and Monitoring Path Siting Criteria for Ambient Air
Quality Monitoring. Since 2006, multiple rule revisions were made to
establish siting requirements for PM10-2.5 and O3
monitoring sites (71 FR 2748, January 17, 2006), Near-Road
NO2 monitoring sites (75 FR 6535, February 9, 2010), Near-
Road CO monitoring sites (76 FR 54342, August 31, 2011), and Near-Road
PM2.5 monitoring sites (78 FR 3285, January 15, 2013).
Through these multiple revisions to the regulatory text, some
requirements were inadvertently omitted, and, over time, the clarity of
this appendix was reduced through these omissions that, in a few
instances, led to unintended and conflicting regulatory requirements.
The EPA proposes to reinstate portions of previous Probe and Monitoring
Path Siting Criteria Requirements from previous rulemaking where
appropriate to restore the original intent. The proposed changes that
affect the overall appendix follow, while those specific to the various
sections of the appendix will be addressed under a specific section
heading. The EPA notes that appendix E is being reprinted in its
entirety with this proposal because this section is being reorganized
for clarity in addition to being selectively revised as described in
detail below. The EPA is soliciting comment on the specific provisions
of appendix E proposed for revision. However, there are a number of
provisions that are being reprinted solely for clarity to assist the
public in understanding the changes being proposed and reconciling
requirements between different portions of the text; the EPA is not
soliciting comment on those provisions and considers changes to those
provisions to be beyond the scope of this proposed rulemaking.
a. Providing Separate Section for Open Path Monitoring Requirements
    The current appendix E regulation combines open path monitor siting
requirements with requirements for siting samplers and monitors that
utilize probe inlets. While this approach allowed the EPA to promulgate
an abbreviated regulation for probe-siting requirements, the EPA now
has determined that the clarity of the requirements for each monitoring
method type has been diminished by this combination. As such, the EPA
is proposing to relocate all open path monitor siting criteria
requirements to a separate section in this appendix. Providing separate
sections for these distinct monitoring method types will allow the EPA
to more clearly articulate minimum technical siting requirements for
each. Further rationale for creating these separate sections is that
the regulatory monitoring community has not submitted to AQS
measurement results from open path monitors since 2009. Because these
open path monitoring methods are rarely used for monitoring to compare
to the NAAQS, the EPA believes that moving the open path siting
criteria to their own section will make clearer the probe siting
criteria for the ambient air monitoring methods that are now most
commonly utilized by monitoring organizations.
b. Amending Distance Precision for Spacing Offsets
    The EPA proposes to require that when rounding is performed to
assess compliance with these siting requirements, the distance
measurements will be rounded such as to retain at least two significant
figures. The EPA proposes to communicate this rounding requirement in
the regulatory text using footnotes in Table E-1, Table E-2, and Table
E-3 of the current regulation.
c. Clarifying Summary Table of Probe Siting Criteria
    To provide additional specificity and flexibility to the summary
table for probe siting criteria (see current Table E-4 in appendix E),
the EPA proposes to change the ``>'' (greater than) symbols to ``>=''
(greater than or equal to) symbols. This minor revision will more
clearly express the EPA's intent that the distance offsets provided in
the current Table E-4 in appendix E are acceptable for NAAQS compliance
monitoring.
d. Adding Flexibility for the Spacing From Minor Sources
    Current requirements for the spacing of probe inlets and monitoring
paths from minor sources of SO2 and NO2

stipulate that the probe inlets and monitoring paths must be away from
these minor sources (see current section 3(b) in appendix E). The EPA
proposes to clarify and provide flexibility by changing this
requirement to a goal. The EPA proposes to replace the ``must'' in this
regulation with a ``should''. As stated in section 1(c) of the current
rule, a ``must'' defines a requirement while a ``should'' specifies a
goal. Since the current rule does not specify how far the probe must be
spaced from such minor sources, the EPA proposes that a ``should'' in
this regulation is more appropriate. Minor sources can have adverse
impacts on the representativeness of the ambient pollutant
concentrations sampled by the probe inlet. As such, the EPA recommends
that sites with these minor sources be avoided whenever practicable and
probe inlets spaced as far from these minor sources as possible when
alternative monitoring stations are not suitable.
e. Amendments and Clarification for the Spacing From Obstructions and
Trees
    The EPA proposes to clarify and redefine that the minimum arc
required to be free of obstructions for a probe inlet or monitoring
path is 270 degrees. Currently this portion of the regulation (see
current section 4(b) of appendix E) specifies 180 degrees as this
minimum arc. However, this requirement is inconsistent with the
requirement found in footnote 5 of Table E-4 in appendix E that
specifies the probe inlet or monitoring path must have unrestricted
airflow of 270 degrees around the probe and 180 degrees for the arc is
only allowed if the probe is on the side of a building or a wall. These
inconsistent regulatory requirements were introduced in the 2006
rulemaking when the 270-degree requirement was omitted from the text of
section 4(b) (see 71 FR 61236, October 17, 2006).
    There are also inconsistent requirements in the current regulation
regarding the spacing of probe inlets from the driplines of trees.
Section 5(a) of appendix E requires the probe inlet must be no closer
than 10 meters to the driplines of any trees, while footnote 3 of Table
E-4 of the appendix E qualifies that this minimum 10-meter offset is
only required when the tree also acts as an obstruction.
f. Reinstating Minimum 270-Degree Arc and Clarifying 180-Degree Arc in
Regulatory Text
    The EPA proposes to correct identified inconsistencies in this
regulation by reinstating the 270-degree requirement in section 4(b) of
appendix E. Additionally, the EPA proposes to further clarify this
regulation by stating that the continuous 180-degree minimum arc of
unrestricted airflow provision is reserved for monitors sited on the
side of a building or a wall to comply with network design criteria
requirements specified in appendix D of part 58. Examples include CO
monitoring in urbanized areas that relies on monitoring in street
canyons and near-road monitoring where a continuous arc of 270 degrees
of unrestricted airflow is not routinely possible given limited monitor
siting options.
g. Clarification on Obstacles That Act as an Obstruction
    The EPA proposes to clarify the definitions of ``obstructions'' and
``obstacles'' in the regulatory text (see section 4 of the current
appendix E). While obstacles should be avoided as much as is
practicable, logistical constraints may dictate that some obstacles are
present within the vicinity of the monitoring probe inlet. Obstructions
to the air flow of the probe inlet are those obstacles that are
horizontally closer than twice the vertical distance the obstacle
protrudes above the probe inlet and can be reasonably thought to
scavenge reactive gases or to restrict the airflow for any pollutant.
The EPA does not generally consider objects or obstacles such as flag
poles or site towers for NOy convertors or towers for
meteorological sensors, etc. to be obstructions.
h. Amending and Clarifying the 10-Meter Tree Dripline Requirement
    The EPA proposes to reconcile the conflicting requirements in
section 5(a) and Table E-4, footnote 3 of the current regulation by
deleting the qualification in footnote 3 of Table E-4 to require that
the probe inlet must always be no closer than 10 meters to the tree
dripline. The EPA also proposes to reinstate the goal that was omitted
from section 5(a) during previous rule revisions, that monitor probe
inlets should be at least 20 meters from the driplines of trees.
Additionally, the EPA proposes to clarify section 5(a) of the current
regulation by adding that when the tree or group of trees is considered
an obstruction, then the regulatory requirements of section 4(a) apply.
i. Amending Spacing Requirement for Microscale Monitoring
    To obtain representative ambient air monitoring measurements for
source-oriented and microscale air monitoring stations, it is important
to have unobstructed airflow between the monitor's probe inlet and the
source under investigation. This reasoning was used by the EPA when
near-road NO2 monitoring stations were required to have an
unobstructed airflow between the monitor probe and the outside nearest
edge of the traffic lane (see current section 4(d) of this regulation).
To assist in further clarifying the monitoring siting criteria for the
spacing from obstructions and spacing from trees, the EPA proposes to
change from a goal to a requirement that microscale sites for any
pollutant shall have no trees or shrubs blocking the line-of-sight
fetch between the monitor's probe inlet and the source under
investigation. The EPA proposes to communicate this requirement by
changing the ``should'' to a ``shall'' in the regulatory text of
section 5(c). The EPA does not consider small obstacles such as shrubs
that are below this fetch to adversely impact the representativeness of
the air quality measurements results. This proposed revision of section
4(d) will bring more consistency to appendix E.
j. Amending Waiver Provisions
    The EPA believes the effects of any requirements in this proposal
that may be considered to be new are minor. While we are attempting to
clarify probe and siting criteria as part of our monitoring
regulations, the Agency fully intends to maintain waiver provisions
that exist in the regulation for these siting criteria (see current
section 10). For cases where long-term trend sites or monitors that
determine the design value for their area cannot reasonably meet these
regulatory siting requirements, the EPA encourages monitoring
organizations to work with their respective EPA Regional Offices to
determine if a waiver from these siting criteria is appropriate.
    Even though the current regulation adequately and clearly
identifies which monitoring situations are eligible for the EPA to
consider waiving the requirements for probe-siting criteria (see
current section 10), these waiver provisions are silent regarding how
long an approved waiver remains in force and effect. Environmental
conditions (e.g., airflow due to changes in growth of trees, shrubs,
construction of buildings or other obstructions) around monitoring
stations are prone to change over time. As such, the EPA has identified
that previously approved waivers should be periodically reevaluated to
ensure that the conditions upon which the original waiver was approved
still exist and that the siting conditions have not degraded to an
unacceptable level. The EPA proposes to modify section 10.3 of the

current regulation to state that waivers from the probe-siting criteria
must be renewed minimally every 5 years. Ideally, sites needing a
waiver renewal should be inspected by the EPA such as during a
Technical Systems Audit (TSA) typically conducted at a subset of sites
within each Primary Quality Assurance Organization (PQAO) every three
years. However, virtual inspections may also be acceptable using
documentation such as photos and traffic counts. Dates for the most
recent approval of a waiver must then be included in the applicable
network assessment and annual monitoring network plan. The EPA proposes
to revise Sec.  58.10(b)(10) of the regulation to maintain consistency
in the text for probe siting criteria requirements and annual
monitoring network plans. This proposal leverages the existing annual
assessment requirements found in Sec.  58.10(a)(1) and (d).
k. Broadening of Acceptable Probe Materials
    The current regulatory specifications for acceptable probe
materials for sampling reactive gases are limited to borosilicate
glass, fluorinated ethylene propylene (FEP) Teflon[supreg], or their
equivalent (see section 9 of the regulation). The EPA's selection of
``or its equivalent'' in the current regulatory text was intended to
allow flexibility to monitoring organizations when selecting suitable
sampling train materials. In practice, however, this text has resulted
in potentially suitable materials not being used for sampling trains
due to concerns that the material may not meet these regulatory
requirements. The current requirements for acceptable probe materials
were promulgated in 1979. Since 1979, several potential alternatives to
borosilicate glass and FEP were developed and are commercially
available.
    Because some of these alternative materials have advantages over
the currently approved materials (e.g., cost and durability), the EPA
has received numerous inquiries from monitoring organizations regarding
the regulatory suitability of these materials. Monitoring organizations
have expressed particular interest in the potential use of PVDF
(polyvinylidene fluoride) which is marketed under the registered
tradename of Kynar[supreg] by Arkema Inc. (Colombes, France). In
response to these inquiries, the EPA's Office of Research and
Development (ORD) recently designed and conducted a laboratory study to
determine the transport efficiency of O3, SO2,
NO2, and CO through several candidate tubing materials
(Johnson, 2022). Based on these tests results, the EPA is proposing to
revise Section 9 of the current regulation to add polyvinylidene
fluoride (PVDF), polytetrafluoroethylene (PTFE), and perfluoroalkoxy
(PFA) to the list of approved materials for efficiently transporting
gaseous criteria pollutants. The EPA also proposes to clarify that the
residence-time criteria for sampling reactive gas through these
approved materials applies to all O3, SO2, and
NO2 monitors. In conjunction with the previously approved
borosilicate glass and FEP materials, including these three new
materials would provide monitoring organizations with a wider variety
of efficient sampling and transport materials needed for conducting
NAAQS compliance monitoring.
    The EPA has also studied and approved the use of Nafion\TM\
upstream of ozone analyzers to minimize measurement bias associated
with high ambient RH levels (U.S. EPA, 2020b). Minimal loss of ozone
occurred in these systems as long as the Nafion\TM\ system was
conditioned beforehand. Nafion\TM\ is composed primarily of PTFE and
can be considered equivalent to PTFE. It has been shown in ORD's recent
tests described above to exhibit virtually no loss of ozone at 20
second residence times.

D. Taking Comment on Incorporating Data From Next Generation
Technologies

1. Background on Use of FRM and FEM Monitors
    The EPA approves FRM and FEM monitors for criteria pollutant
measurements in the Federal Register after careful review of
applications describing extensive testing of the methods operation and
performance. The siting of these monitors across State, local, and
Tribal networks is subject to detailed requirements for network design
detailed in appendix D to 40 CFR part 58 with probe and siting criteria
described in appendix E for 40 CFR part 58. The operation of these
monitors is subject to extensive quality assurance requirements
detailed in appendix A to 40 CFR part 58, which ensures data quality
statistics are produced to inform the quality of the data needed to
ensure regulatory grade decisions are made with data of known quality.
The EPA believes these requirements are important for ensuring the
degree of accurate and precise data which is appropriate for regulatory
decision-making, particularly decisions about attainment or
nonattainment of the NAAQS. However, the EPA also recognizes that the
capital and operating costs of these monitors is substantial, which
requires the EPA and states to prioritize where monitors should be
deployed. The EPA recognizes that making use of broader air quality
data sets which are less expensive can provide important benefits, even
if the EPA does not consider those datasets suitable for all regulatory
purposes. In some circumstances in the past, for example, the EPA has
used non-FRM monitoring to inform decisions about the boundaries of a
nonattainment area, although the data was not sufficient to support a
finding that an area was in nonattainment. Likewise, the EPA has
incorporated sensor data into its fire and smoke map for the purpose of
informing the public of potential imminent health risks, even though
that data would not be comparable to the NAAQS for purposes of
determining attainment. There are multiple uses of air quality data and
the EPA believes there may be additional opportunities to develop
broader air quality datasets which provide benefits to the EPA and the
public even where the data is not from FRM/FEM monitors and is not
suitable for comparison to the NAAQS.
2. Next Generation Technologies: Data Considerations
    The EPA and our State, local, and Tribal partners in cooperation
with other Federal agencies have made great strides in integrating data
from routine air monitoring methods with data from next generation
technologies to address emerging air quality issues. For example, the
EPA and U.S. Forest Service (USFS), in consultation with other
partners, launched the publicly available AirNow Fire and Smoke
Map,\172\ which has received over 26 million page views since its
release in July 2020. This fire and smoke map has been an invaluable
tool for the public, providing refined spatial information on current
Air Quality Index (AQI) conditions, fire and smoke plumes locations,
actions for communities to take based on local air quality, and links
to Smoke Forecast Outlooks developed by specially trained air resource
advisors. Data are brought together from multiple systems including
permanent and temporary PM2.5 continuous monitoring sites,
sensors, and satellite derived fire and smoke data. With the success of
the fire and smoke map and a robust and growing network of
PM2.5 continuous FEMs and sensor network data, as well as
existing and future satellites products, the EPA is interested in
considering further enhancements to

the evolution of data products to meet new and emerging non-regulatory
air quality data needs. Below we describe each of the major data sets,
their advantages, and any challenges to their use. We then solicit
input on additional approaches and/or products to incorporating data
from next generation technologies that can help address important non-
regulatory air quality data needs.
---------------------------------------------------------------------------

    \172\ See: https://fire.airnow.gov.
---------------------------------------------------------------------------

3. PM2.5 Continuous FEMs
    As described in the PA, State, local, and Tribal monitoring
agencies are using an increasing number of PM2.5 continuous
FEMs. These methods are primarily deployed to meet two monitoring
objectives: first, to compare to the NAAQS, and second, to report and
support forecasting of the AQI. PM2.5 continuous FEMs have
some key advantages over FRMs, most notably that they provide automated
hourly measurement of PM2.5 available in near real time. The
continuous PM2.5 data are reported as soon as practicable
after the end of each hour, usually within 5-10 minutes, and are used
in multiple applications of real-time data such as such as by State,
local, and Tribal websites,173 174 the EPA's AirNow website,
and national media outlets. Recent improvements in the availability and
exchange of near real-time data through a dedicated AirNow Application
Programming Interface (API) allow for efficient exchange of data
between the EPA, other Federal agencies, and commercial data providers
such as low-cost sensor networks. The efficient exchange of data
through the AirNow API was a key advancement in the successful
implementation of the EPA AirNow's fire and smoke map. The
PM2.5 continuous FEM data are critical to ``ground
truthing'' other datasets such as sensors and satellites for two
important reasons. First, PM2.5 continuous FEMs are subject
to extensive regulatory-grade quality assurance and quality control as
required by appendix A to 40 CFR part 58. Second, PM2.5
continuous FEMs are located in accordance with strict siting criteria
according to appendix E to 40 CFR part 58. The siting criteria assure
that measured data represent ambient air at ground level where people
are breathing and are thus exposed to particle pollution. The EPA and
State, local, and Tribal agencies are working to upgrade many existing
FRM-only sites with PM2.5 continuous FEMs through use of
American Rescue Plan funds.\175\ Despite these investments, there are
major challenges to monitoring agencies' ability to have enough trained
and available staff to support their regulatory monitoring networks,
especially in remote locations, and to have the capital resources to
implement new monitoring stations. So, while there may be some
improvements to the existing network of almost 1,000 PM2.5
regulatory-grade monitoring stations, regulatory instruments will not
produce data everywhere that it is desired. Thus, the integration of
PM2.5 continuous FEMs with other datasets is an important
opportunity to address existing and emerging air quality data needs for
non-regulatory purposes.
---------------------------------------------------------------------------

    \173\ See: https://www.airnow.gov/partners/state-and-local-partners/.
    \174\ See: https://www.airnow.gov/partners/tribal-partners/.
    \175\ See: https://www.epa.gov/arp/enhanced-air-quality-monitoring-funding-under-arp.
---------------------------------------------------------------------------

4. PM2.5 Satellite Products
    Satellite-based instruments provide measurements of radiance that
can be used to calculate the aerosol optical depth (AOD) of the
atmosphere. For over a decade, satellite AOD values have been used in
models that incorporate multiple datasets to predict surface level
PM2.5 concentrations over the U.S. (hereafter, satellite-
PM2.5). Despite some heterogeneity in performance under
varying conditions, the satellite-PM2.5 datasets have
significantly advanced in terms of accuracy in recent years (Di et al.,
2019; van Donkelaar et al., 2019; Zhang and Kondragunta, 2021). The EPA
is using satellite-PM2.5 datasets in a variety of contexts.
Satellite-PM2.5 data was included in a comparative analysis
of hybrid modeling methods in the PA (U.S. EPA, 2022b). The EPA is also
working with the National Aeronautics and Space Administration (NASA)
and National Oceanic and Atmospheric Administration (NOAA) to use
satellite-PM2.5 in the AirNow system.\176\ The EPA also uses
satellite AOD and many other satellite data products in the development
of our photochemical modeling platforms that are used in regulatory and
policy assessments both by the EPA and by our State and local partners.
---------------------------------------------------------------------------

    \176\ The EPA provided an update on the Health and Air Quality
Applied Scientist Team (HAQAST) AirNow Project at the NASA HAQAST
meeting in Texas in June 2022. For more information, see: https://haqast.org/haqast-houston-june-1-2/.
---------------------------------------------------------------------------

    Each satellite data product has its own strengths and limitations.
One strength is the spatial coverage, which can be once-a-day globally
for polar orbiting satellites or over a fixed field of view
continuously for geostationary satellites. Satellite-PM2.5
data has the limitation that it is not a direct measurement of
PM2.5 concentrations, but rather is derived through a model
that connects the total column AOD to surface PM2.5. In
addition, the satellite products are only capable of making daytime
measurements because they rely on sunlight. In fact, most satellite-
PM2.5 data products use the surface monitor network as an
input. As such, the satellite-PM2.5 data does not substitute
for a ground-based monitor; rather it complements the monitor network.
The EPA continues to explore ways to use the wealth of data from
satellites to address important air quality questions consistent with
their strengths and limitations.
5. Use of Air Sensors
    The term ``air sensor'' is a simplified way of referring to a class
of technology that has expanded on the market in recent years and has
common traits of directly reading a pollutant in the air, being smaller
in size, and often sold at lower prices that support a wider number of
monitoring locations than possible in the past. As explained on the
EPA's Air Sensor Toolbox website,\177\ air sensor monitors that are
lower in cost, portable, and generally easier to operate than
regulatory-grade monitors are widely used in the United States to
understand air quality conditions. Many refer to this class of
technology as ``low-cost air sensors,'' ``air sensor devices,'' or
``air quality sensors.'' Potential uses for these non-regulatory air
sensor technologies include, but are not limited to, science education,
supplementing regulatory air quality measurements, conducting research,
measuring local air quality to better understand sources of pollution,
locating leaks at industrial facilities, and emergency response.
---------------------------------------------------------------------------

    \177\ See: https://www.epa.gov/air-sensor-toolbox.
---------------------------------------------------------------------------

    The growth in use of sensors included in the EPA's fire and smoke
map provides a platform to build upon. There are thousands of PM
sensors whose data are coordinated and overlaid with routine and
temporary PM2.5 continuous monitors as well as satellite-
derived data on fires and smoke. Sensors offer an opportunity to
supplement higher-cost regulatory monitoring to provide data for the
non-regulatory uses as described above. However, there are several
challenges to using sensors. Each commercially available PM sensor
appears to have its own data quality challenges depending on season,
aerosol encountered, and meteorological conditions (typically
temperature and relative humidity). The EPA has gone to considerable
length to ensure the PM2.5 sensor data on the fire and smoke
map have a correction available with collocated FRMs and

FEMs.\178\ This was possible due to the large number of air sensors
that are the same make and model located across the country. Thus, an
important challenge for the use of sensors is the spatial richness in
sensor networks needed to make integrating the dataset with other
monitoring data viable. Even with corrected sensor data in hand,
publicly shared sensor data lacks reliability and accountability for
ensuring that basic siting criteria are met. Sensors are often
installed by members of the public who share data to the sensor
network, which is generally understood as implicitly representing that
the sensor is located in ambient air although, in fact, the sensor may
be located inside a home or next to a highly localized source of
emissions such as the flue of a home heating system. In areas with many
reporting sensors, these concerns about siting may be lessened through
site-to site comparison of data; however, the absence of any confirmed
information about siting presents challenges for use of sensor data.
---------------------------------------------------------------------------

    \178\ See: https://www.epa.gov/research-states/airnow-fire-and-smoke-map-extension-us-wide-correction-purpleair-pm25-sensors.
---------------------------------------------------------------------------

6. Summary
    The near real-time integration of data from PM2.5
continuous monitors, sensors, and satellites has been proven through
use of the EPA's fire and smoke map. This mapping product is possible
though the use of API's where data sets are automatically shared on
pre-specified computer servers. Given the success of the fire and smoke
map, the EPA is interested in pursuing additional approaches and/or
products that can help address important non-regulatory air quality
data needs. Therefore, the EPA solicits comment on the most important
data uses and data sets to consider in future products. Such approaches
and/or products could utilize historical or near real-time data. For
example, what are the advantages and disadvantages of using existing
data and tools to identify PM hot spots across an area of interest?
Could satellite data or a combined surface layer (PM2.5 FRM
and FEM data, sensor data, and satellite data) be useful in siting
regulatory monitors? Could combined surfaces layers be useful in
determining the boundaries of nonattainment areas? Could combined
surface layers be useful in exploring potential emission sources to
consider in SIP planning? To what extent would requirements for data
formats, units, or timescales of interest need to evolve to best
address these needs? What other datasets should the EPA consider
merging with the data sets listed above to help better inform air
quality management, including prioritizing network investments for
potential new sites such as in at risk communities described elsewhere
in this proposal? The EPA seeks input and prioritization on each of
these questions to help improve the utility of data to better support
air quality management to improve public health and the environment.

VIII. Clean Air Act Implementation Requirements for the PM NAAQS

    The proposed revision to the primary annual PM2.5 NAAQS
discussed in section II above, if finalized, would trigger a process
under which states \179\ will make recommendations to the Administrator
regarding area designations. States also will be required to review
their existing section 110 infrastructure state implementation plans
and modify them if necessary to implement a revised NAAQS. A revised
primary annual PM2.5 NAAQS will need to be incorporated into
the implementation of applicable air permitting requirements and the
transportation conformity and general conformity processes, and states
will need to review existing regulations for these programs that
already cover PM2.5 to determine the extent to which any
changes are needed. This section provides background information for
understanding the possible implications of the proposed NAAQS changes
and describes the EPA's plans for providing states guidance needed to
assist their implementation efforts. This section also describes
existing EPA interpretations of CAA requirements and other EPA guidance
relevant to implementation of a revised PM2.5 NAAQS. Given
the strong scientific evidence for disparities in PM2.5
exposures and PM2.5-related health risk among certain
populations (as discussed in section II of this document), the EPA
included in its 2016 PM2.5 State Implementation Plan (SIP)
Requirements Rule (81 FR 58010, August 24, 2016) (which was written to
be applicable for any future NAAQS revisions) included a number of key
recommendations for states to advance environmental justice through
their attainment planning process. In addition, as discussed throughout
this section, environmental justice considerations are evaluated with
regard to the several specific program elements of the overall
implementation process. State and local air agencies have a critically
important role in implementing the NAAQS, including this proposed
PM2.5 NAAQS, should it become finalized. Given the
information provided in this proposed rulemaking, state and local air
agencies are encouraged to begin to consider how they might develop
implementation plans that encourage early emission reductions as well
as emission reductions that facilitate or amplify reductions affecting
overburdened communities. The public is encouraged to share information
on this important topic and although this rulemaking is not requesting
comment specifically on this topic, information on this topic may be
submitted for informational purposes to the docket for this proposed
rulemaking. The EPA may consider whether additional guidance on the
topic of environmental justice and PM2.5 implementation is
appropriate, beyond what is already included in the existing
PM2.5 SIP Requirements Rule. The EPA encourages air agencies
and other stakeholders to review the existing PM2.5 SIP
Requirements Rule and the information provided therein regarding
environmental justice considerations in PM2.5 air planning.
To be clear, nothing in the above text should be interpreted as seeking
comment in this proposal on any aspect of the 2016 PM2.5 SIP
Requirements Rule.
---------------------------------------------------------------------------

    \179\ This and all subsequent references to ``state'' are meant
to include State, local, and Tribal agencies responsible for the
implementation of a PM2.5 control program.
---------------------------------------------------------------------------

    With respect to the topics covered in this section, the EPA
welcomes the public to provide input to the Agency through comments.
However, because these issues are not relevant to the establishment of
a revised primary annual PM2.5 NAAQS, and because no
specific revisions are proposed for the regulations implementing the
PM2.5 NAAQS (i.e., 40 CFR part 51, subpart Z), the EPA does
not expect to respond to these comments in the final action on this
proposal (nor is it required to do so).

A. Designation of Areas

    After the EPA establishes or revises a NAAQS, the CAA requires the
EPA and the states to take steps to ensure that the new or revised
NAAQS is met. The first step, known as the initial area designations,
involves identifying areas of the country that either meet or do not
meet the new or revised NAAQS, along with the nearby areas contributing
to the violations.
    Section 107(d)(1) of the CAA states that, ``By such date as the
Administrator may reasonably require, but not later than 1 year after
promulgation of a new or revised national ambient air quality standard
for any pollutant under section 109, the Governor of each state shall

. . . submit to the Administrator a list of all areas (or portions
thereof) in the State'' and that making recommendations for whether the
EPA should designate those areas as nonattainment, attainment, or
unclassifiable.\180\ The CAA provides the EPA discretion to require
states to submit their designations recommendations within a reasonable
amount of time not exceeding 1 year. The CAA also stipulates that ``the
Administrator may not require the Governor to submit the required list
sooner than 120 days after promulgating a new or revised national
ambient air quality standard.'' Section 107(d)(1)(B)(i) further
provides, ``Upon promulgation or revision of a NAAQS, the Administrator
shall promulgate the designations of all areas (or portions thereof) .
. . as expeditiously as practicable, but in no case later than 2 years
from the date of promulgation. Such period may be extended for up to
one year in the event the Administrator has insufficient information to
promulgate the designations.'' With respect to the NAAQS setting
process, courts have interpreted the term ``promulgation'' to be
signature and widespread dissemination of a final rule.\181\ One way
the EPA intends to account for environmental justice in the
implementation process is to promptly issue designations in accordance
with the statutory requirements to ensure expeditious public health
protections for all populations, including those currently experiencing
disparities in PM2.5 exposures and PM2.5-related
health risk.
---------------------------------------------------------------------------

    \180\ While the CAA says ``designating'' with respect to the
Governor's letter, in the full context of the CAA section it is
clear that the Governor actually makes a recommendation to which the
EPA must respond via a specified process if the EPA does not accept
it.
    \181\ API v. Costle, 609 F.2d 20 (D.C. Cir. 1979)
---------------------------------------------------------------------------

    If the EPA agrees with the designation recommendation of the state,
then it may proceed to promulgate the designations for such areas. If,
however, the EPA disagrees with the state's recommendation, then the
EPA may elect to make modifications to the recommended designations. By
no later than 120 days prior to promulgating the final designations,
the EPA is required to notify states of any intended modifications to
the designations of any areas or portions thereof, including the
boundaries of areas, as the EPA may deem necessary. States then have an
opportunity to comment on the EPA's tentative designation decision. If
a state elects not to provide designation recommendations, then the EPA
must timely promulgate the designation that it deems appropriate. While
section 107(d) of the CAA specifically addresses the designations
process for states, the EPA intends to follow the same process for
tribes to the extent practicable, pursuant to section 301(d) of the CAA
regarding Tribal authority, and the Tribal Authority Rule (63 FR 7254,
February 12, 1998). To provide clarity and consistency in doing so, the
EPA issued a guidance memorandum to our Regional Offices on working
with tribes during the designations process (Page, 2011a).
    Monitoring data are currently available from numerous existing
PM2.5 Federal Equivalent Methods (FEM) and Federal Reference
Methods (FRM) sites to determine compliance with the proposed revised
PM2.5 primary annual NAAQS. As discussed in section II
above, the EPA is proposing to: (1) revise the level of the primary
annual PM2.5 standard and retain the current primary 24-hour
PM2.5 standard (section II.D.3); and (2) not change the
current secondary annual and 24-hour PM2.5 standards at this
time (section V.D.3). Consistent with the process used in previous area
designations efforts, the EPA will evaluate each area on a case-by-case
basis considering the specific facts and circumstances unique to the
area \182\ to support area boundaries decisions for the revised
standard. Section 107(d) explicitly requires that the EPA designate as
nonattainment not only the area that is violating the pertinent
standard, but also those nearby areas that contribute to the violation
in the violating area. For the reason noted earlier, the EPA believes
it is important to consider environment justice within the framework of
this area-specific analysis. Consistent with past practice, the EPA
expects to address issues relevant to area designations more fully in a
separate designations-specific memorandum around the time of
promulgation of any revised PM2.5 NAAQS.\183\ Examples of
issues that may be included in the separate designations-specific
memorandum may include, but are not limited to, exceptional events
demonstrations for wildfire and/or prescribed fires on wildland,
factors to consider in identifying appropriate designations for areas
and boundaries, among other relevant topics. For informational
purposes, the public can comment on the process and schedule for the
initial area designations and nonattainment boundary setting effort
associated with a new or revised PM2.5 NAAQS. As noted
above, the EPA does not expect to respond to these comments in the
final regulatory action establishing the NAAQS.
---------------------------------------------------------------------------

    \182\ The EPA has historically used area-specific analyses to
support nonattainment area boundary recommendations and final
boundary determinations by evaluating factors such as air quality
data, emissions and emissions-related data (e.g., population density
and degree of urbanization, traffic and commuting patterns),
meteorology, geography/topography, and jurisdictional boundaries. We
expect to follow a similar process when establishing area
designations for any new or revised PM2.5 NAAQS.
    \183\ https://www3.epa.gov/pmdesignations/2012standards/docs/april2013guidance.pdf.
---------------------------------------------------------------------------

    As in past iterations of the PM2.5 NAAQS, the EPA
intends to make the designations for any revised NAAQS based on the
most recent 3 years of complete and valid air quality data.
Accordingly, the EPA recommends that states base their initial
designation recommendations on the most current available 3 years of
complete and valid air quality data. The EPA intends to use available
air quality data from the current PM2.5 mass and speciation
monitoring networks and other technical information. The EPA will then
base the final designations on 3 consecutive years of certified air
quality monitoring data, likely 2021-2023.\184\
---------------------------------------------------------------------------

    \184\ In certain circumstances in which the Administrator has
insufficient information to promulgate area designations within 2
years from the promulgation of a new or revised NAAQS, CAA section
107(d)(1)(B)(i) provides the EPA may extend the designations
schedule by up to 1 year.
---------------------------------------------------------------------------

    In some areas, State or Tribal air agencies may have flagged air
quality data for certain days in the Air Quality System due to
potential impacts from exceptional events (i.e., such as wildfires or
high wind dust storms). Air quality concentrations on such days may
affect the calculation of design values for regulatory air monitoring
sites in determining whether such sites may violate the revised
PM2.5 NAAQS, and therefore could influence the initial area
designations for this revised NAAQS. Under the 2016 Exceptional Events
Rule (see ``Treatment of Data Influenced by Exceptional Events; Final
Rule,'' 81 FR 68216, October 3, 2016), an air agency may submit to the
EPA a demonstration with supporting information and analyses for each
monitor and day the air agency claims should be excluded from design
value calculations for regulatory purposes. The EPA has provided a
number of tools to assist air agencies in preparing their
demonstrations \185\ and will continue to work with air agencies as
they identify, prepare and submit exceptional events demonstrations.
The EPA recognizes that some areas and stakeholders may be

concerned about wildfire and prescribed fire related impacts to
designations and/or other forthcoming actions of regulatory
significance for which a state may want to submit an exceptional events
demonstration. The EPA has already issued guidance addressing
development of exceptional events demonstrations for both wildfire and
prescribed fires on wildland. Existing guidance and other tools are
available on the EPA's website identified above. The air agency is
required to follow the exceptional events demonstration submission
deadlines that are identified in Table 2 to 40 CFR 50.14(c)(2)(vi)--
``Schedule for Initial Notification and Demonstration Submission for
Data Influenced by Exceptional Events for Use in Initial Area
Designations.'' Further, the EPA has notified states of areas subject
to mitigation plan provisions. Within 2 years of the notification, if
the air agency has not submitted a required mitigation plan, the EPA
will not concur with the air agency's request to exclude data until the
required plan is submitted and verified.
---------------------------------------------------------------------------

    \185\ See EPA's Exceptional Events homepage at https://www.epa.gov/air-quality-analysis/treatment-air-quality-data-influenced-exceptional-events-homepage-exceptional.
---------------------------------------------------------------------------

    As noted earlier, the EPA intends to provide designation guidance
to the states and tribes around the time of the promulgation of a
revised NAAQS, to assist in formulating these recommendations. With
regard to the area designations process, if, after evaluating the state
recommendations in light of the technical factors, the Administrator
intends to modify any state area recommendation, the EPA will notify
the appropriate state Governor no later than 120 days prior to making
final designations decisions. A state that believes the Administrator's
intended modification is inappropriate will have the opportunity to
demonstrate to the EPA why it believes its original recommendation (or
a revised recommendation) is more appropriate before final designations
are promulgated. The Administrator will take any additional input from
the state into account in making final designation decisions. If the
Administrator departs from the stated intentions in the initial 120-day
notification letter in a way that does not match the most recently
received recommendation from the Governor (or tribe) as of the date of
the final designation, the Administrator will provide an additional
120-day notification letter notifying the Governor of such
modifications. The EPA invites preliminary comment on all aspects of
the designation process at this time, which the Agency will consider in
developing any updated guidance.

B. Section 110(a)(1) and (2) Infrastructure SIP Requirements

    The CAA directs states to address basic SIP requirements to
implement, maintain, and enforce the NAAQS. Under CAA sections
110(a)(1) and (2), states are required to have state implementation
plans that provide the necessary air quality management infrastructure
including, among other things, enforceable emissions limitations, an
ambient monitoring program, an enforcement program, air quality
modeling capabilities, and adequate personnel, resources, and legal
authority. After the EPA promulgates a new or revised NAAQS, states are
required to make a new SIP submission to establish that they meet the
necessary structural requirements for such new or revised NAAQS or make
changes to do so. The EPA refers to this type of SIP submission as an
``infrastructure SIP submission.'' Under CAA sections 110(a)(1), all
states are required to make these infrastructure SIP submissions within
3 years after promulgation of a new or revised primary standard. While
the CAA authorizes the EPA to set a shorter time for states to make
these SIP submissions, the EPA does not currently intend to do so.
    Under CAA section 110(a)(1) and (2), states are required to make
SIP submissions that address a number of requirements pertaining to
implementation, maintenance, and enforcement of a new or revised NAAQS.
The specific subsections in CAA section 110(a)(2) require states to
address a number of requirements, as applicable: (A) Emissions limits
and other control measures, (B) Ambient air quality monitoring/data
system, (C) Programs for enforcement of control measures and for
construction or modification of stationary sources, (D)(i) Interstate
pollution transport; and (D)(ii) Interstate and international pollution
abatement, (E) Adequate resources and authority, conflict of interest,
and oversight of local governments and regional agencies, (F)
Stationary source monitoring and reporting, (G) Emergency episodes, (H)
SIP revisions, (I) Plan revisions for nonattainment areas, (J)
Consultation with government officials, public notification, PSD and
visibility protection, (K) Air quality modeling and submission of
modeling data, (L) Permitting fees, and (M) Consultation and
participation by affected local entities. These requirements apply to
all SIP submissions in general, but the EPA has provided specific
guidance to states concerning its interpretation of these requirements
in the specific context of infrastructure SIP submissions for a new or
revised NAAQS (Page, 2013).
    The EPA interprets the CAA such that two elements identified in
section 110(a)(2) are not subject to the 3-year submission deadline of
section 110(a)(1) and thus states are not required to address them in
the context of an infrastructure SIP submission. The elements pertain
to part D, in title I of the CAA, which addresses plan requirements for
nonattainment areas. Therefore, for the reasons explained below, the
following section 110(a)(2) elements are considered by the EPA to be
outside the scope of infrastructure SIP actions: (1) the portion of
section 110(a)(2)(C), programs for enforcement of control measures and
for construction or modification of stationary sources that applies to
permit programs applicable in designated nonattainment areas, (known as
``nonattainment new source review'') under part D; and (2) section
110(a)(2)(I), which requires a SIP submission pursuant to part D, in
its entirety. The EPA does not expect states to address the requirement
for a new or revised NAAQS in the infrastructure SIP submissions to
include regulations or emissions limits developed specifically for
attaining the relevant standard in areas designated nonattainment for
the proposed revised PM2.5 NAAQS. States will be required to
submit infrastructure SIP submissions for a revised PM2.5
NAAQS before they are required to submit nonattainment plan SIP
submissions to demonstrate attainment with the same NAAQS. States are
required to submit nonattainment plans to provide for attainment and
maintenance of a revised PM2.5 NAAQS within 18 months from
the effective date of nonattainment area designations as required under
CAA section 189(a)(2)(B). The EPA reviews and acts upon these later SIP
submissions through a separate process. For this reason, the EPA does
not expect states to address new nonattainment area emissions controls
per section 110(a)(2)(I) in their infrastructure SIP submissions.
    One of the required infrastructure SIP elements is that each
state's SIP must contain adequate provisions to prohibit, consistent
with the provisions of title I of the CAA, emissions from within the
state that will significantly contribute to nonattainment in, or
interfere with maintenance by, any other state of the primary or
secondary NAAQS.\186\ This element is often referred to as the ``good
neighbor'' or ``interstate transport''

provision.\187\ The provision has two prongs: significant contribution
to nonattainment (prong 1) and interference with maintenance (prong 2).
The EPA and states must give independent significance to prong 1 and
prong 2 when evaluating downwind air quality problems under CAA section
110(a)(2)(D)(i)(I).\188\ Further, case law has established that the EPA
and states must implement requirements to meet interstate transport
obligations in alignment with the applicable statutory attainment
schedule of the downwind areas impacted by upwind-state emissions.\189\
Thus, the EPA anticipates that states will need to address interstate
transport obligations associated with any revised PM NAAQS, if
finalized, in alignment with the provisions of subpart 4 of part D of
the CAA, as discussed in more detail in section VIII.C below.
Specifically, states must implement any measures required to address
interstate transport obligations as expeditiously as practicable and no
later than the next statutory attainment date, i.e., for this NAAQS
revision, if finalized, as expeditiously as practicable but no later
than the end of the sixth calendar year following nonattainment area
designations. See CAA section 188(c).
---------------------------------------------------------------------------

    \186\ CAA section 110(a)(2)(D)(i)(I).
    \187\ CAA section 110(a)(2)(D)(i)(II) also addresses certain
interstate effects that states must address and thus is also
sometimes referred to as relating to ``interstate transport.''
    \188\ See North Carolina v. EPA, 531 F.3d 896, 909-11 (D.C. Cir.
2008).
    \189\ See id. 911-13. See also Wisconsin v. EPA, 938 F.3d 303,
313-20 (D.C. Cir. 2019); Maryland v. EPA, 958 F.3d 1185, 1203-04
(D.C. Cir. 2020).
---------------------------------------------------------------------------

    The EPA anticipates developing further information and coordinating
with states with respect to the requirements of CAA section
110(a)(2)(D)(i)(I) for implementation of any revised PM NAAQS. We note
that states may elect to make SIP submissions that address certain
infrastructure SIP elements separately from the others. In recent
years, due in part to the complexity of addressing interstate transport
obligations, some states have found it efficient to make SIP
submissions to address the interstate transport provisions separately
from other infrastructure SIP elements.
    It is the responsibility of each state to review its air quality
management program's existing SIP provisions in light of each new or
revised NAAQS to determine if any revisions are necessary to implement
a new or revised NAAQS. Most states have revised and updated their SIPs
in recent years to address requirements associated with other revised
NAAQS. For some states, it may be the case that for a number of
infrastructure elements, the state may believe it already has adequate
state regulations already adopted and approved into the SIP to address
a particular requirement with respect to any revised PM2.5
NAAQS. For such portions of the state's infrastructure SIP submission,
the state may provide an explanation of how its existing SIP provisions
are adequate.
    If a state determines that existing SIP-approved provisions are
adequate in light of the revised PM2.5 NAAQS with respect to
a given infrastructure SIP element (or sub-element), then the state may
make a SIP submission ``certifying'' that the existing SIP contains
provisions that address those requirements of the specific section
110(a)(2) infrastructure elements.\190\ In the case of such a
certification submission, the state does not have to include a copy of
the relevant provision (e.g., rule or statute) itself. Rather, the
state in its infrastructure SIP submission may provide citations to the
SIP-approved state statutes, regulations, or non-regulatory measures,
as appropriate, which meet the relevant CAA requirement. Like any other
SIP submission, that state can make such a certification only after it
has provided reasonable notice and opportunity for public hearing. This
``reasonable notice and opportunity for public hearing'' requirement
for infrastructure SIP submissions is to meet the requirements of CAA
sections 110(a) and 110(l). Under the EPA's regulations at 40 CFR part
51, if a public hearing is held, an infrastructure SIP submittal must
include a certification by the state that the public hearing was held
in accordance with the EPA's procedural requirements for public
hearings. See 40 CFR part 51, appendix V, section 2.1(g), and see 40
CFR 51.102.
---------------------------------------------------------------------------

    \190\ A ``certification'' approach would not be appropriate for
the interstate pollution control requirements of CAA section
110(a)(2)(D)(i).
---------------------------------------------------------------------------

    In consultation with its EPA Regional office, a state should follow
all applicable EPA regulations governing infrastructure SIP submissions
in 40 CFR part 51--e.g., subpart I (Review of New Sources and
Modifications), subpart J (Ambient Air Quality Surveillance), subpart K
(Source Surveillance), subpart L (Legal Authority), subpart M
(Intergovernmental Consultation), subpart O (Miscellaneous Plan Content
Requirements), subpart P (Protection of Visibility), and subpart Q
(Reports). For the EPA's general criteria for infrastructure SIP
submissions, refer to 40 CFR part 51, appendix V, Criteria for
Determining the Completeness of Plan Submissions. The EPA recommends
that states electronically submit their infrastructure SIPs to the EPA
through the State Plan Electronic Collaboration System (SPeCS),\191\ an
online system available through the EPA's Central Data Exchange.
---------------------------------------------------------------------------

    \191\ https://cdx.epa.gov/.
---------------------------------------------------------------------------

C. Implementing Any Revised PM2.5 NAAQS in Nonattainment Areas

    Part D of the CAA describes the various program requirements that
apply to nonattainment areas for different NAAQS. Section 172 (found in
subpart 1 of part D) includes general SIP requirements, and sections
188-190 (found in subpart 4 of part D) include SIP requirements that
specifically govern implementation for the PM10 and
PM2.5 NAAQS. All PM2.5 nonattainment areas are
initially classified as Moderate per CAA section 188(a). Under section
189(a)(2), states are required to submit attainment plan SIP
submissions to the EPA within 18 months of the effective date of area
designations. These plans need to show how the nonattainment area will
attain the primary PM2.5 standards ``as expeditiously as
practicable,'' but presumptively by no later than the end of the 6th
calendar year after the effective date of designations. For example, if
the EPA finalizes nonattainment designations for a revised
PM2.5 NAAQS in 2024, then the outermost statutory Moderate
area attainment date would be December 31, 2030. If the state fails to
attain the standard by the end of the 6th calendar year after the
effective date of designations, the EPA is required to reclassify the
area to Serious, and the state then must attain the standard by the end
of the 10th calendar year after the effective date of designations
(e.g., December 31, 2034).
    On August 24, 2016, the EPA issued a detailed SIP Requirements Rule
for implementing the PM2.5 NAAQS (81 FR 58010, August 24,
2016) (PM2.5 SIP Requirements Rule). It provides guidance
and establishes additional regulatory requirements for states regarding
development of attainment plans for nonattainment areas for the 1997,
2006, and 2012 revisions of the PM2.5 NAAQS. The EPA also
intended this implementation rule to apply to nonattainment areas
designated pursuant to any future revisions of the PM2.5
NAAQS. The rule covers a number of SIP requirements for nonattainment
areas, including a nonattainment area emissions inventory, policies
regarding PM2.5 precursor pollutants (i.e., SO2,
NOX,

VOC, and ammonia), control strategies (such as reasonably available
control measures and reasonably available control technology), air
quality modeling, attainment demonstrations, reasonable further
progress requirements, quantitative milestones, and contingency
measures. Guidance provided in the PM2.5 SIP Requirements
Rule is supplemented by other EPA guidance documents, including
guidance on emissions inventory development (80 FR 8787, February 19,
2015; U.S. EPA, 2017), optional PM2.5 precursor
demonstrations (U.S. EPA, 2019b),\192\ and guidance on air quality
modeling for meeting air quality goals for the ozone and
PM2.5 NAAQS and regional haze program (U.S. EPA, 2018b).
---------------------------------------------------------------------------

    \192\ Provides guidance on developing demonstrations under
section 189(e) intended to show that a certain PM2.5
precursor in a particular nonattainment area does not significantly
contribute to PM2.5 concentrations that exceed the
standard.
---------------------------------------------------------------------------

    Under the basic approach outlined in the PM2.5 SIP
Requirements Rule, a state would first develop an updated emissions
inventory of sources and emissions activities in the nonattainment
area. It would then use air quality modeling or other tools to estimate
the air quality improvement that can be expected in the nonattainment
area by the attainment year due to enforceable and existing ``on the
books'' Federal, state, and local emissions reduction measures. The
state also would work with the regulated community and other
stakeholders to evaluate potential control measures for emissions
sources and activities in the nonattainment area, and identify the
additional reasonably available control measures (RACM) and reasonably
available control technology (RACT) that can be implemented by these
sources in order to attain the standard as expeditiously as
practicable, but no later than by the end of the 6th calendar year
after the effective date of designations.
    The evaluation of air quality improvement associated with potential
future emissions reductions is commonly performed with sophisticated
air quality modeling tools. Given that fine particle concentrations are
affected both by regionally-transported pollutants (e.g.,
SO2 and NOX emissions from power plants) and
emissions of direct PM2.5 and other pollutants from local
sources in the nonattainment area (e.g., steel mills, rail yards,
highway mobile sources), the EPA recommends the use of regional
photochemical models (such as CMAQ and CAMx), in combination with
source-oriented dispersion models (such as the American Meteorological
Society/Environmental Protection Agency Regulatory Model (AERMOD)), as
needed, to develop PM2.5 attainment strategies for any
revised PM2.5 NAAQS. The EPA SIP modeling guidance provides
details on the development of attainment demonstrations, and the EPA
will continue to assist air agencies in modeling and technical analyses
(80 FR 8787, February 19, 2015; U.S. EPA, 2017).
    The PM2.5 SIP Requirements Rule provides recommendations
to states regarding when and how to consider environmental justice in
the context of PM2.5 attainment planning. Some of the
considerations for states include: (1) identifying areas with
overburdened communities where more ambient monitoring may be
warranted; (2) targeting emissions reductions that may be needed to
attain the PM2.5 NAAQS; and (3) increasing opportunities for
meaningful involvement for overburdened populations (80 FR 58010,
58136, August 25, 2016). The EPA expects states to consider these and
other factors as part of their SIP development process.
    The PM2.5 SIP Requirements Rule outlines some examples
of how states can implement these recommendations.\193\ For instance,
states can use modeling and screening tools to better understand where
sources of PM2.5 or PM2.5 precursor emissions are
located and identify areas that may be candidates for additional
ambient monitoring. Furthermore, once these target areas are
identified, states can prioritize direct PM2.5 or
PM2.5 precursor control measures and enforcement strategies
in these areas to reduce ambient PM2.5 and achieve the
NAAQS. The EPA recognizes that states have flexibility under the CAA to
concentrate state resources on controlling sources of PM2.5
emissions that directly and adversely affect certain populations
currently experiencing disparities in PM2.5 exposures and
PM2.5-related health risk, thereby maximizing health
benefits for those populations. Moreover, states can establish
opportunities to bolster meaningful involvement in a number of ways,
such as communicating with communities with disparities in exposures
and risks in appropriate languages and developing enhanced notice-and-
comment opportunities for those communities.
---------------------------------------------------------------------------

    \193\ For more information on the EPA's recommendations and
examples, see 81 FR 58010, 58137, August 24, 2016.
---------------------------------------------------------------------------

    As previously mentioned, the 2016 PM2.5 SIP Requirements
Rule is structured in such a way that it provides guidance and
regulatory requirements for remaining nonattainment areas for the 1997,
2006, and 2012 revisions of the PM2.5 NAAQS, as well as for
nonattainment areas designated pursuant to any future revisions of the
PM2.5 NAAQS. Thus, the EPA is not proposing changes to the
current PM2.5 SIP Requirements Rule in this proposed
rulemaking, and therefore is not requesting comment on that rule.

D. Implementing the Primary and Secondary PM10 NAAQS

    As summarized in sections III.C.3 and III.D.3 above, the EPA is
proposing to retain the current primary and secondary 24-hour
PM10 standards to protect against the health effects
associated with short-term exposures to thoracic coarse particles and
against the welfare effects considered in this reconsideration (i.e.,
visibility, climate, and materials effects). The EPA intends to retain
the existing implementation strategy for meeting the CAA requirements
for the PM10 NAAQS. States and emissions sources should
continue to follow the existing guidance and regulations for
implementing the current standards.

E. Prevention of Significant Deterioration and Nonattainment New Source
Review Programs for the Proposed Revised Primary Annual PM2.5 NAAQS

    The CAA, at parts C and D of title I, contains preconstruction
review and permitting programs applicable to new major stationary
sources and major modifications of existing major sources. The
preconstruction review of each new major stationary source and major
modification applies on a pollutant-specific basis, and the
requirements that apply for each pollutant depend on whether the area
in which the source is situated is designated as attainment (or
unclassifiable) or nonattainment for that pollutant. In areas
designated attainment or unclassifiable for a pollutant, the Prevention
of Significant Deterioration (PSD) requirements under part C apply to
construction at major sources. In areas designated nonattainment for a
pollutant, the Nonattainment New Source Review (NNSR) requirements
under part D apply to major source construction. Collectively, those
two sets of permit requirements are commonly referred to as the ``major
New Source Review'' or ``major NSR'' programs.
    Until the EPA designates an area with respect to the proposed
revised PM2.5

NAAQS, the NSR provisions applicable under an area's designation for
the 1997, 2006, and 2012 PM2.5 NAAQS would continue to
apply. See 40 CFR 51.166(i)(2) and 52.21(i)(2). That is, for areas
designated as attainment/unclassifiable for the 1997, 2006, and 2012
PM2.5 NAAQS, PSD will apply to new major stationary sources
and major modifications that trigger major source permitting
requirements for PM2.5. For areas designated nonattainment
for the 1997, 2006, or 2012 PM2.5 NAAQS, NNSR requirements
will apply for new major stationary sources and major modifications
that trigger major source permitting requirements for PM2.5.
When the new designations for the proposed revised PM2.5
NAAQS, if finalized, become effective, those designations will further
inform whether PSD or NNSR applies to PM2.5 in a particular
area. New major sources and major modifications will be subject to the
PSD program requirements for PM2.5 if they are located in an
area that does not have a current nonattainment designation under CAA
section 107 for PM2.5.\194\
---------------------------------------------------------------------------

    \194\ 40 CFR 51.166(i)(2) and 52.21(i)(2)
---------------------------------------------------------------------------

    The EPA has assessed the proposed revision of the level of the
primary annual PM2.5 NAAQS and is not proposing any changes
to the NSR program regulations as part of this proposal to revise the
PM2.5 NAAQS. Sources and reviewing authorities will be able
to use existing NSR regulatory provisions. Under the PSD program, the
applicant must demonstrate that the new or modified source emissions
increase does not cause or contribute to a NAAQS violation. In 2017,
the EPA revised the Guideline on Air Quality Models (published as
appendix W to 40 CFR part 41) to address primary and secondary
PM2.5 impacts in making this demonstration and has since
provided associated technical guidance, models and tools, such as the
recent ``Final Guidance for Ozone and Fine Particulate Matter Permit
Modeling'' (July 29, 2022).\195\ The EPA will consider whether changes
or updates to PSD program guidance or associated tools are warranted as
a result of the proposed revision to the primary annual
PM2.5 NAAQS, should it be finalized, and would communicate
such changes through separate action(s) following promulgation of a
revised standard.
---------------------------------------------------------------------------

    \195\ On July 29, 2022, the EPA issued ``Final Guidance for
Ozone and Fine Particulate Matter Permit Modeling,'' available at
https://www.epa.gov/system/files/documents/2022-07/Guidance_for_O3_PM25_Permit_Modeling.pdf. This guidance provides the
EPA's recommendations for how a stationary source seeking a PSD
permit may demonstrate that it will not cause or contribute to a
violation of the National Ambient Air Quality Standards for Ozone
and PM2.5 and PSD increments for PM2.5, as
required under section 165(a)(3) of the Clean Air Act and 40 CFR
51.166(k) and 52.21(k). The EPA has also previously issued two
technical guidance documents for use in conducting these
demonstrations: ``Guidance on the Development of Modeled Emission
Rates for Precursors (MERPs) as a Tier 1 Demonstration Tool for
Ozone and PM2.5 under the PSD Permitting Program,''
available at https://www.epa.gov/sites/default/files/2020-09/documents/epa-454_r-19-003.pdf, and ``Guidance on the Use of Models
for Assessing the Impacts of Emissions from Single Sources on the
Secondarily Formed Pollutants: Ozone and PM2.5,''
available at https://www.epa.gov/sites/default/files/2020-09/documents/epa-454_r-16-005.pdf.
---------------------------------------------------------------------------

    The statutory requirements for a PSD permit program set forth under
part C of title I of the CAA (sections 160 through 169) are addressed
by the EPA's PSD regulations found at 40 CFR 51.166 (minimum
requirements for an approvable PSD SIP) and 40 CFR 52.21 (PSD
permitting program for permits issued under the EPA's Federal
permitting authority). These regulations already apply for
PM2.5 in areas that have been designated attainment or
unclassifiable for PM2.5 whenever a proposed new major
source or major modification triggers PSD requirements for
PM2.5.
    For PSD, a ``major stationary source'' is one with the potential to
emit 250 tons per year (tpy) or more of any regulated NSR pollutant,
unless the new or modified source is classified under a list of 28
source categories contained in the statutory definition of ``major
emitting facility'' in section 169(1) of the CAA. For those 28 source
categories, a ``major stationary source'' is one with the potential to
emit 100 tpy or more of any regulated NSR pollutant. A ``major
modification'' is a physical change or a change in the method of
operation of an existing major stationary source that results, first,
in a significant emissions increase of a regulated NSR pollutant and,
second, in a significant net emissions increase of that pollutant. See
40 CFR 51.166(b)(2)(i), 40 CFR 52.21(b)(2)(i). The EPA PSD regulations
define the term ``regulated NSR pollutant'' to include any pollutant
for which a NAAQS has been promulgated and any pollutant identified in
the EPA regulations as a constituent or precursor to such pollutant.
See 40 CFR 51.166(b)(49), 40 CFR 52.21(b)(50). These regulations
identify SO2 and NOX as precursors to
PM2.5 in all attainment and unclassifiable areas. See 40 CFR
51.166(b)(49)(i), 40 CFR 52.21(b)(50)(i). Thus, for PM2.5,
the PSD program currently requires the review and control of emissions
of direct PM2.5 emissions and SO2 and
NOX (as precursors to PM2.5), as applicable.\196\
Among other things, for each regulated NSR pollutant emitted or
increased in a significant amount, the PSD program requires a new major
stationary source or a major modification to apply the ``best available
control technology'' (BACT) and to conduct an air quality impact
analysis to demonstrate that the proposed major stationary source or
major modification will not cause or contribute to a violation of any
NAAQS or PSD increment.\197\ See CAA section 165(a)(3) and (4), 40 CFR
51.166(j) and (k), 40 CFR 52.21(j) and (k). The PSD requirements may
also include, in appropriate cases, an analysis of potential adverse
impacts on Class I areas. See CAA sections 162(a) and 165, 40 CFR
51.166(p); 40 CFR 52.21(p)).\198\ The EPA has developed the Guideline
on Air Quality Models and other documents to, among other things,
provide methods and guidance for demonstrating compliance with the
PM2.5 NAAQS and PSD increments for PM2.5.\199\
---------------------------------------------------------------------------

    \196\ Sulfur dioxide is a precursor to PM2.5 in all
attainment and unclassifiable areas. NOX is presumed to
be a precursor to PM2.5 in all attainment and
unclassifiable areas, unless a state or the EPA demonstrates that
emissions of NOX from sources in a specific area are not
a significant contributor to that area's ambient PM2.5
concentrations. VOC is presumed not to be a precursor to
PM2.5 in any attainment or unclassifiable area, unless a
state or the EPA demonstrates that emissions of VOC from sources in
a specific area are a significant contributor to that area's ambient
PM2.5 concentrations.
    \197\ By establishing the maximum allowable level of ambient
pollutant concentration increase in a particular area, an increment
defines ``significant deterioration'' of air quality in that area.
Increments are defined by the CAA as maximum allowable increases in
ambient air concentrations above a baseline concentration and are
specified in the PSD regulations by pollutant and area
classification (Class I, II and III). 40 CFR 51.166(c), 40 CFR
52.21(c); 75 FR 64864; October 20, 2010.
    \198\ Congress established certain Class I areas in section
162(a) of the CAA, including international parks, national
wilderness areas, and national parks that meet certain criteria.
Such Class I areas, known as mandatory Federal Class I areas, are
afforded special protection under the CAA. In addition, States and
Tribal governments may establish Class I areas within their own
political jurisdictions to provide similar special air quality
protection.
    \199\ See 40 CFR part 51, appendix W; 82 FR 5182, January 17,
2017; See also U.S. EPA, 2021c. The EPA provided an initial version
of the same guidance for public comment on February 10, 2020. Upon
consideration of the comments received, and consistent with
Executive Order 13990, the EPA revised the initial draft guidance
and posted the revised version for additional public comment.
---------------------------------------------------------------------------

    The EPA has historically interpreted the requirement for an air
quality impact analysis under CAA section 165(a)(3) and the
implementing regulations to include a requirement to demonstrate that
emissions from the proposed facility will not cause or contribute to a
violation of any NAAQS

that is in effect as of the date a PSD permit is issued, except to the
extent that a pending permit application was subject to grandfathering
provisions that the EPA had established through rulemaking. The EPA is
not proposing such provisions for this action. In past NAAQS revision
rules, including the 2012 PM2.5 NAAQS (78 FR 3086, January
15, 2013) and 2015 Ozone NAAQS (80 FR 65292, October 26, 2015), the EPA
included limited grandfathering provisions that exempted certain
pending PSD permit actions (those that had reached a particular stage
in the permitting process at the time the revised NAAQS was promulgated
or became effective) from the requirement to demonstrate that the
proposed emissions increases would not cause or contribute to a
violation of the revised NAAQS. In August 2019, the U.S. Court of
Appeals for the D.C. Circuit vacated the grandfathering provision in
the PSD rules applicable to the 2015 Ozone NAAQS, finding that the
provision contradicted ``Congress's `express policy choice' not to
allow construction which will `cause or contribute to' nonattainment of
`any' effective NAAQS, regardless of when they are adopted or when a
permit was completed.'' Murray Energy Corp. v. EPA, 936 F.3d 597, 627
(D.C. Cir. 2019).\200\ Based on that court decision, the EPA is not
proposing any grandfathering provision for this proposed
PM2.5 NAAQS revision, if finalized. Accordingly, PSD permits
issued on or after the effective date of any final revised
PM2.5 NAAQS would require a demonstration that the proposed
emissions increases would not cause or contribute to a violation of the
revised PM2.5 NAAQS.
---------------------------------------------------------------------------

    \200\ While the specifics of this case involved the 2015 ozone
NAAQS, the case was based upon an interpretation of CAA section
165(a) and therefore applies equally to any PSD grandfathering for a
new or revised NAAQS.
---------------------------------------------------------------------------

    The EPA anticipates that, if this rule is finalized as proposed,
the existing PM2.5 air quality in some areas will not be in
attainment of the new revised primary annual PM2.5 NAAQS,
and that these areas will be designated as ``nonattainment'' at a later
date, consistent with the designation process described in the
preceding sections. However, until such nonattainment designation
occurs, proposed new major sources and major modifications located in
any area currently designated attainment or unclassifiable for
PM2.5 will continue to be subject to the PSD program
requirements for PM2.5.\201\ This raises the question as to
how a source can be issued a PSD permit in light of known existing
ambient violations of the revised NAAQS. Section 165(a)(3)(B) of the
CAA states that a proposed source may not construct unless it
demonstrates that it will not cause or contribute to a violation of any
NAAQS. This statutory requirement is implemented through a provision
contained in the PSD regulations at 40 CFR 51.166(k) and 52.21(k).\202\
If a source cannot make this demonstration, or if its initial air
quality impact analysis shows that the source's impact would cause or
contribute to a violation, a PSD permit may not be issued unless the
permit applicant compensates for the adverse impact that would
otherwise cause or contribute to a violation of the NAAQS. While the
PSD regulations do not explicitly specify remedial actions that a
prospective source can take to address such a situation, the EPA has
historically recognized in regulations, and through other actions, that
sources applying for PSD permits may utilize offsets as part of the
required PSD demonstration under CAA section 165(a)(3)(B).\203\
---------------------------------------------------------------------------

    \201\ Any proposed major stationary source or major modification
triggering PSD requirements for PM2.5 that does not
receive its PSD permit by the effective date of a new nonattainment
designation for the area where the source would locate would then be
required to satisfy applicable NNSR preconstruction permit
requirements for PM2.5.
    \202\ 40 CFR 51.166(k) requires that SIPs shall provide that the
owner or operator of the proposed source or modification shall
demonstrate that allowable emission increases from the proposed
source or modification, in conjunction with all other applicable
emissions increases or reductions (including secondary emissions),
would not cause or contribute to air pollution in violation of: (i)
any national ambient air quality standard in any air quality control
region; or (ii) any applicable maximum allowable increase over the
baseline concentration in any area.
    \203\ See, e.g., Page, 2010; 44 FR 3274, 3278, January 16, 1979;
See also In re Interpower of New York, Inc., 5 E.A.D. 130, 141 (EAB
1994) (describing an EPA Region 2 PSD permit that relied in part on
offsets to demonstrate the source would not cause or contribute to a
violation of the NAAQS). 52 FR 24634, 24684, July 1, 1987; 78 FR
3085, 3261-62, Jan. 15, 2013. The EPA has recognized the ability of
sources to obtain offsets in the context of PSD though the PSD
provisions of the Act do not expressly reference offsets as the NNSR
provisions of the Act do. See 80 FR 65292, 65441, October 26, 2015.
---------------------------------------------------------------------------

    Part D of title I of the CAA includes preconstruction review and
permitting requirements applicable to new major stationary sources and
major modifications located in areas designated nonattainment for a
pollutant for which a NAAQS has been established (i.e., a criteria
pollutant). The relevant part D requirements are typically referred to
as the NNSR program. The EPA's regulations for the NNSR programs are
contained in 40 CFR 51.165 and 52.24 and part 51, appendix S.
Specifically, the EPA has developed minimum program requirements for an
NNSR program that is approvable in a SIP, and those requirements, which
include requirements for PM2.5, are contained in 40 CFR
51.165. In addition, 40 CFR part 51, appendix S, contains requirements
constituting an interim NNSR program. This program enables NNSR
permitting in nonattainment areas by states that lack a SIP-approved
NNSR permitting program during the time between the date of the
relevant designation and the date that the EPA approves into the SIP a
NNSR program. See 40 CFR part 51, appendix S, section I; 40 CFR
52.24(k).
    For NNSR, ``major stationary source'' is generally defined as a
source with the potential to emit at least 100 tpy of the regulated NSR
pollutant for which the area is designated nonattainment. In some
cases, however, the CAA and the NNSR regulations define ``major
stationary source'' for NNSR in terms of a lower rate dependent on the
pollutant and degree of nonattainment in the area. For
PM2.5, in addition to the general threshold level of 100
tpy, a lower major source threshold of 70 tpy applies in Serious
PM2.5 nonattainment areas pursuant to subpart 4 of part D,
title I of the CAA. See 40 CFR 51.165(a)(1)(iv)(A)(1)(vii) and (viii);
40 CFR part 51, appendix S, II.A.4.(i)(a)(7) and (8).
    Under the NNSR program, direct PM2.5 emissions and
emissions of each PM2.5 precursor are reviewed separately in
accordance with the applicable major source threshold. For example, the
threshold for Serious PM2.5 nonattainment areas is 70 tpy of
direct PM2.5, as well as for the PM2.5 precursors
SO2, NOX, VOC, and ammonia.\204\ See 40 CFR
51.165(a)(1)(iv)(A)(1)(vii) and (viii); 40 CFR part 51, appendix S,
II.A.4.(i)(a)(7) and (8). For modifications, NNSR applies to proposed
physical changes or changes in the method of operation of an existing
stationary source where (1) the source is major for the nonattainment
pollutant (or a precursor for that pollutant) and (2) the physical
change or change in the method of operation of a major stationary
source results, first, in a significant emissions increase of a
regulated NSR pollutant and, second, in a significant net emissions
increase of that same

nonattainment pollutant (or same precursor for that pollutant). See 40
CFR 51.165(a)(1)(v)(A); 40 CFR part 51, appendix S, II.A.5.(i).
---------------------------------------------------------------------------

    \204\ All recognized precursors to PM2.5 are
regulated as precursors for NNSR. See 40 CFR
51.165(a)(1)(xxxvii)(C)(2). No significant emission rate is
established by the EPA for ammonia, and states are required to
define ``significant'' for ammonia for their respective areas unless
the state pursues the optional precursor demonstration to exclude
ammonia from planning requirements. See 40 CFR 51.165(a)(1)(x)(F);
40 CFR 51.165(a)(13).
---------------------------------------------------------------------------

    For example, to qualify as a major modification for SO2
(as a PM2.5 precursor) in a moderate PM2.5
nonattainment area, the existing source would have to have the
potential to emit 100 tpy or more of SO2, and the project
would have to result in an increase in SO2 emissions of 40
tpy or more. See 40 CFR 51.165(a)(1)(x)(A). New major stationary
sources and major modifications for PM2.5 subject to NNSR
must comply with the ``lowest achievable emission rate'' (LAER) as
defined in the CAA and NNSR rules, as well as performing other analyses
as required under section 173 of the CAA.
    Following the promulgation of any revised NAAQS for
PM2.5, some new nonattainment areas for PM2.5 may
result. Where a state does not have an NNSR program or where the
current NNSR program does not apply to PM2.5, that state
will be required to submit the necessary SIP revisions to ensure that
new major stationary sources and major modifications for
PM2.5 undergo preconstruction review pursuant to the NNSR
program. States are required to submit nonattainment plans to provide
for attainment and maintenance of a revised PM2.5 NAAQS
within 18 months from the effective date of nonattainment area
designations as required under CAA section 189(a)(2)(B). Therefore,
states whose existing NNSR program requirements, if any, cannot be
interpreted to apply to the revised primary annual PM2.5
NAAQS at that time will be allowed to issue the necessary permits in
accordance with the applicable nonattainment permitting requirements
contained in 40 CFR part 51, appendix S, which would apply to the
revised PM2.5 NAAQS upon its effective date. See 73 FR
28321, 28340, May 16, 2008.
    Finally, the EPA recommends that, where appropriate, PSD and NNSR
permitting authorities assess impacts to communities with environmental
justice concerns. For example, this may include conducting a
demographic analysis to inform development of a plan for community
outreach and engagement, conducting a cumulative emissions impact
analysis,\205\ or considering the environmental and social costs
imposed on the impacted community when conducting an alternative sites
analysis.\206\ Another option could be improving the understanding of
the potential impact of minor sources by generating an emissions
inventory for such minor sources, including sources that are not
currently required to report emissions, to generate options on how
emissions can be reduced in the target area. See 81 FR 58010, 58137.
The EPA anticipates developing further information and consulting with
permitting authorities on how to best address environmental justice in
the permitting process.
---------------------------------------------------------------------------

    \205\ The permitting authority may conduct a cumulative analysis
of the projected PM2.5 emissions from all emission units
at the proposed facility and PM2.5 emissions from nearby
facilities, to provide a more complete assessment of the ambient air
impacts of the proposed facility on affected communities. See 40 CFR
part 51, appendix W, section 9.2.3.
    \206\ Section 173(a)(5) of the CAA requires for an NNSR permit
``an analysis of alternative sites, sizes, production processes, and
environmental control techniques for such proposed source [that]
demonstrates that benefits of the proposed source significantly
outweigh the environmental and social costs imposed as a result of
its location, construction, or modification.'' This requirement is
referred to as the ``alternative sites analysis.''
---------------------------------------------------------------------------

F. Transportation Conformity Program

    Transportation conformity is required under CAA section 176(c) to
ensure that transportation plans, transportation improvement programs
(TIPs) and federally supported highway and transit projects will not
cause or contribute to any new air quality violation, increase the
frequency or severity of any existing violation, or delay timely
attainment or any required interim emissions reductions or other
milestones. Transportation conformity applies to areas that are
designated as nonattainment or nonattainment areas that have been
redesignated to attainment with an approved CAA section 175A
maintenance plan (i.e., maintenance areas) for transportation-related
criteria pollutants: carbon monoxide, ozone, NO2,
PM2.5, and PM10. Transportation conformity for
any new or revised NAAQS for PM2.5 does not apply until one
year after the effective date of the nonattainment designation for that
NAAQS. See CAA section 176(c)(6) and 40 CFR 93.102(d)). The EPA's
Transportation Conformity Rule \207\ establishes the criteria and
procedures for determining whether transportation activities conform to
the SIP. The EPA is not proposing changes to the transportation
conformity rule in this proposed rulemaking. The EPA notes that the
transportation conformity rule already addresses the PM2.5
and PM10 NAAQS. However, in the future, the EPA will review
the need to issue or revise guidance describing how the current
conformity rule applies in nonattainment and maintenance areas for any
new or revised primary or secondary PM NAAQS, as needed.
---------------------------------------------------------------------------

    \207\ 40 CFR part 93, subpart A
---------------------------------------------------------------------------

G. General Conformity Program

    The general conformity program implements CAA section 176(c) and
requires that Federal agencies do not adopt, accept, approve, or fund
activities that are not consistent with state air quality goals.
General conformity applies to any Federal action (e.g., funding,
licensing, permitting, or approving) if (1) the action takes place in a
nonattainment or maintenance area for any of the criteria pollutants
and (2) it is not a Federal Highway Administration (FHWA) or Federal
Transit Administration (FTA) project as defined in 40 CFR 93.101 (these
projects are covered under the transportation conformity program
described above).
    The EPA's General Conformity Rule \208\ establishes the criteria
and procedures for determining if a Federal action conforms to the
applicable attainment plan. General conformity for any revised
PM2.5 NAAQS does not apply until one year after the
effective date of the nonattainment designation for that NAAQS. The EPA
is not proposing changes to the General Conformity Rule in this
proposed rulemaking. The EPA notes that the General Conformity Rule
already addresses the PM2.5 and PM10 NAAQS.
---------------------------------------------------------------------------

    \208\ 40 CFR 93.150 through 93.165
---------------------------------------------------------------------------

IX. Statutory and Executive Order Reviews

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

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

    This action is an economically significant regulatory action that
was submitted to the Office of Management and Budget (OMB) for review.
Any changes made in response to OMB recommendations have been
documented in the docket. The EPA prepared an illustrative analysis of
the potential costs and benefits associated with this action. This
analysis is contained in the document ``Regulatory Impact Analysis for
the Proposed Reconsideration of the National Ambient Air Quality
Standards for Particulate Matter,'' which is available in the
Regulatory Impact Analysis (RIA) docket (EPA-HQ-OAR-2019-0587) and
briefly summarized below. The RIA estimates the costs and monetized
human health benefits in 2032, after

implementing existing and expected regulations and assessing emissions
reductions to meet the current annual and 24-hour particulate matter
NAAQS (12/35 [mu]g/m\3\), associated with applying national control
strategies for the proposed annual and 24-hour alternative standard
levels of 10/35 [mu]g/m\3\ and 9/35 [mu]g/m\3\, as well as the
following two more stringent alternative standard levels: (1) an
alternative annual standard level of 8 [mu]g/m\3\ in combination with
the current 24-hour standard (i.e., 8/35 [mu]g/m\3\), and (2) an
alternative 24-hour standard level of 30 [mu]g/m\3\ in combination with
the proposed annual standard level of 10 [mu]g/m\3\ (i.e., 10/30 [mu]g/
m\3\). Table 2 provides a summary of the estimated monetized benefits,
costs, and net benefits associated with applying national control
strategies toward reaching alternative standard levels. However, the
CAA and judicial decisions make clear that the economic and technical
feasibility of attaining ambient standards are not to be considered in
setting or revising NAAQS, although such factors may be considered in
the development of state plans to implement the standards. Accordingly,
although an RIA has been prepared, the results of the RIA have not been
considered in issuing this proposed rule.

  Table 2--Estimated Monetized Benefits, Costs, and Net Benefits of the Illustrative Control Strategies Applied Toward the Primary Alternative Annual and Daily Standard Levels of 10/35 [mu]g/
                                                        m\3\, 10/30 [mu]g/m\3\, 9/35 [mu]g/m\3\, and 8/35 [mu]g/m\3\ in 2032 for the U.S.
                                                                                [Millions of 2017$]
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
                                                      10/35                                  10/30                                   9/35                                   8/35
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Benefits \a\........................  $8,500 and $17,000...................  $9,600 and $20,000...................  $21,000 and $43,000..................  $46,000 and $95,000
Costs \b\...........................  $95..................................  $260.................................  $390.................................  $1,800
                                     -----------------------------------------------------------------------------------------------------------------------------------------------------------
    Net Benefits....................  $8,400 and $17,000...................  $9,300 and $19,000...................  $20,000 and $43,000..................  $44,000 and $93,000
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Notes: Rows may not appear to add correctly due to rounding. We focus results to provide a snapshot of costs and benefits in 2032, using the best available information to approximate social
  costs and social benefits recognizing uncertainties and limitations in those estimates. The estimated costs and monetized human health benefits associated with applying national control
  strategies do not fully account for all the emissions reductions needed to reach the proposed and more stringent alternative standard levels for some standard levels analyzed.
\a\ We assume that there is a cessation lag between the change in PM exposures and the total realization of changes in mortality effects. Specifically, we assume that some of the incidences of
  premature mortality related to PM2.5 exposures occur in a distributed fashion over the 20 years following exposure, which affects the valuation of mortality benefits at different discount
  rates. Similarly, we assume there is a cessation lag between the change in PM exposures and both the development and diagnosis of lung cancer. The benefits are associated with two point
  estimates from two different epidemiologic studies, and we present the benefits calculated at a real discount rate of 3 percent. The benefits exclude additional health and welfare benefits
  that could not be quantified.
\b\ The costs are annualized using a 7 percent interest rate.

B. Paperwork Reduction Act (PRA)

    This action does not impose an information collection burden under
the PRA. There are no information collection requirements directly
associated with a proposed decision to revise or retain a NAAQS under
section 109 of the CAA.

C. Regulatory Flexibility Act (RFA)

    I certify that this action will not have a significant economic
impact on a substantial number of small entities under the RFA. This
action will not impose any requirements on small entities. Rather, this
proposed rule establishes national standards for allowable
concentrations of PM in ambient air as required by section 109 of the
CAA. See also American Trucking Associations v. EPA, 175 F.3d 1027,
1044-45 (D.C. Cir. 1999) (NAAQS do not have significant impacts upon
small entities because NAAQS themselves impose no regulations upon
small entities), rev'd in part on other grounds, Whitman v. American
Trucking Associations, 531 U.S. 457 (2001).

D. Unfunded Mandates Reform Act (UMRA)

    This action does not contain any unfunded mandate as described in
the Unfunded Mandates Reform Act (UMRA), 2 U.S.C. 1531-1538, and does
not significantly or uniquely affect small governments. Furthermore, as
indicated previously, in setting a NAAQS the EPA cannot consider the
economic or technological feasibility of attaining ambient air quality
standards, although such factors may be considered to a degree in the
development of state plans to implement the standards. See also
American Trucking Associations v. EPA, 175 F. 3d at 1043 (noting that
because the EPA is precluded from considering costs of implementation
in establishing NAAQS, preparation of the RIA pursuant to the Unfunded
Mandates Reform Act would not furnish any information that the court
could consider in reviewing the NAAQS).
    The EPA acknowledges, however, that if corresponding revisions to
associated SIP requirements and air quality surveillance requirements
are proposed at a later time, those revisions might result in such
effects. Any such effects would be addressed as appropriate if and when
such revisions are proposed.

E. Executive Order 13132: Federalism

    This action does not have federalism implications. It will not have
substantial direct effects on the states, on the relationship between
the National Government and the states, or on the distribution of power
and responsibilities among the various levels of government. However,
the EPA recognizes that states will have a substantial interest in this
action and any future revisions to associated requirements.

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

    This action does not have Tribal implications, as specified in
Executive Order 13175. It does not have a substantial direct effect on
one or more Indian Tribes as tribes are not obligated to adopt or
implement any NAAQS. In addition, tribes are not obligated to conduct
ambient monitoring for PM or to adopt the ambient monitoring
requirements of 40 CFR part 58. Thus, Executive Order 13175 does not
apply to this action. However, consistent with the EPA Policy on
Consultation and Coordination with Indian Tribes, the EPA will offer
government-to-government consultation with tribes as requested.

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

    This action is subject to Executive Order 13045 because it is an
economically significant regulatory action as defined by Executive
Order 12866, and the EPA believes that the environmental health or
safety risk addressed by this action may have a disproportionate effect
on children. The Policy on Children's Health also applies to this
action. Accordingly, we have evaluated the environmental health or
safety effects of PM exposures on children. The protection offered by
these standards may be especially important for children because
childhood represents a lifestage associated with increased
susceptibility to PM-related health effects. Because children have been
identified as a susceptible population, we have carefully evaluated the
environmental health effects of exposure to PM pollution among
children. Children make up a substantial fraction of the U.S.
population, and often have unique factors that contribute to their
increased risk of experiencing a health effect due to exposures to
ambient air pollutants because of their continuous growth and
development. As described in the 2019 Integrated Science Assessment,
children may be particularly at risk for health effects related to
ambient air PM2.5 exposures compared with adults because
they have (1) a developing respiratory system, (2) increased
ventilation rates relative to body mass compared with adults, and (3)
an increased proportion of oral breathing, particularly in boys,
relative to adults. More detailed information on the evaluation of the
scientific evidence and policy considerations pertaining to children,
including an explanation for why the Administrator judges the proposed
standards to be requisite to protect public health, including the
health of children, with an adequate margin of safety, are contained in
sections II.B and II.D of this preamble. Copies of all documents have
been placed in the public docket for this action.

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

    This action is not a ``significant energy action'' because it is
not likely to have a significant adverse effect on the supply,
distribution, or use of energy. The purpose of this action is to
propose to revise the primary annual PM2.5 NAAQS and to
retain the primary 24-hour PM2.5 NAAQS, primary
PM10 NAAQS, and secondary PM NAAQS. The action does not
prescribe specific pollution control strategies by which these ambient
standards and monitoring revisions will be met. Such strategies will be
developed by states on a case-by-case basis, and the EPA cannot predict
whether the control options selected by states will include regulations
on energy suppliers, distributors, or users. Thus, the EPA concludes
that this proposal does not constitute a significant energy action as
defined in Executive Order 13211.

I. National Technology Transfer and Advancement Act (NTTAA)

    This action involves technical standards. The EPA proposes to use
the current indicators for fine (PM2.5) and coarse
(PM10) particles. The indicator for fine particles is
measured using the Reference Method for the Determination of Fine
Particulate Matter as PM2.5 in the Atmosphere (appendix L to
40 CFR part 50), which is known as the PM2.5 FRM, and the
indicator for coarse particles is measured using the Reference Method
for the Determination of Particulate Matter as PM10 in the
Atmosphere (appendix J to 40 CFR part 50), which is known as the
PM10 FRM.
    To the extent feasible, the EPA employs a Performance-Based
Measurement System (PBMS), which does not require the use of specific,
prescribed analytic methods. The PBMS is defined as a set of processes
wherein the data quality needs, mandates or limitations of a program or
project are specified, and serve as criteria for selecting appropriate
methods to meet those needs in a cost-effective manner. It is intended
to be more flexible and cost effective for the regulated community; it
is also intended to encourage innovation in analytical technology and
improved data quality. Though the FRM defines the particular
specifications for ambient monitors, there is some variability with
regard to how monitors measure PM, depending on the type and size of PM
and environmental conditions. Therefore, it is not practically possible
to fully define the FRM in performance terms to account for this
variability. Nevertheless, our approach in the past has resulted in
multiple brands of monitors being approved as FRM for PM, and we expect
this to continue. Also, the FRMs described in 40 CFR part 50 and the
equivalency criteria described in 40 CFR part 53, constitute a
performance-based measurement system for PM, since methods that meet
the field testing and performance criteria can be approved as FEMs.
Since finalized in 2006 (71 FR 61236, October 17, 2006) the new field
and performance criteria for approval of PM2.5 continuous
FEMs has resulted in the approval of 13 approved FEMs. In summary, for
measurement of PM2.5 and PM10, the EPA relies on
both FRMs and FEMs, with FEMs relying on a PBMS approach for their
approval. The EPA is not precluding the use of any other method,
whether it constitutes a voluntary consensus standard or not, as long
as it meets the specified performance criteria.

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

    The EPA believes that this action does not have disproportionately
high and adverse human health or environmental effects on minority
populations, low-income populations and/or indigenous peoples, as
specified in Executive Order 12898 (59 FR 7629, February 16, 1994). The
documentation for this assessment is contained in sections II.B.2,
II.C.1, II.C.3, II.D.2, and II.D. of this preamble and also in the 2019
Integrated Science Assessment, Supplement to the 2019 Integrated
Science Assessment, and Policy Assessment. The EPA has carefully
evaluated the potential impacts on minority populations and low SES
populations as discussed in sections II.B.2, II.C.1, II.C.3, II.D.2,
and II.D.3 of this preamble. The Integrated Science Assessment,
Supplement to the Integrated Science Assessment, and Policy Assessment
contain the evaluation of the scientific evidence, quantitative risk
analyses and policy considerations that pertain to these populations.
These documents are available as described in this SUPPLEMENTARY
INFORMATION section and copies of all documents have been placed in the
public docket for this action.

References

Abt Associates, Inc. (2001). Assessing public opinions on visibility
impairment due to air pollution: Summary report. U.S. Environmental
Protection Agency. Research Triangle Park, NC.
Abt Associates, Inc. (2005). Particulate matter health risk
assessment for selected urban areas: Draft report. EPA Contract No.
68-D-03-002. U.S. Environmental Protection Agency. Research Triangle
Park, NC. Available at: http://www3.epa.gov/ttn/naaqs/standards/pm/data/PMrisk20051220.pdf.
ATS (2000). What Constitutes an Adverse Health Effect of Air
Pollution? American Journal of Respiratory and Critical Care
Medicine 161(2): 665-673.
BBC Research & Consulting (2003). Phoenix area visibility survey.
Denver, CO. http://

www.azdeq.gov/environ/air/download/vis_021903f.pdf.
Behbod, B, Urch, B, Speck, M, Scott, JA, Liu, L, Poon, R, Coull, B,
Schwartz, J, Koutrakis, P, Silverman, F and Gold, DR (2013).
Endotoxin in concentrated coarse and fine ambient particles induces
acute systemic inflammation in controlled human exposures.
Occupational and Environmental Medicine 70(11): 761-767.
Bellavia, A, Urch, B, Speck, M, Brook, RD, Scott, JA, Albetti, B,
Behbod, B, North, M, Valeri, L, Bertazzi, PA, Silverman, F, Gold, D
and Baccarelli, AA (2013). DNA hypomethylation, ambient particulate
matter, and increased blood pressure: Findings from controlled human
exposure experiments. Journal of the American Heart Association
2(3): e000212.
Bennett, JE, Tamura-Wicks, H, Parks, RM, Burnett, RT, Pope, CA,
Bechle, MJ, Marshall, JD, Danaei, G and Ezzati, M (2019).
Particulate matter air pollution and national and county life
expectancy loss in the USA: A spatiotemporal analysis. PLoS Medicine
16(7): e1002856.
Bourdrel, T, Annesi-Maesano, I, Alahmad, B, Maesano, CN and Bind, MA
(2021). The impact of outdoor air pollution on COVID-19: a review of
evidence from in vitro, animal, and human studies. 30(159).
Br[auml]uner, EV, M[oslash]ller, P, Barregard, L, Dragsted, LO,
Glasius, M, W[aring]hlin, P, Vinzents, P, Raaschou-Nielsen, O and
Loft, S (2008). Exposure to ambient concentrations of particulate
air pollution does not influence vascular function or inflammatory
pathways in young healthy individuals. Particle and Fibre Toxicology
5: 13.
Brook, RD, Urch, B, Dvonch, JT, Bard, RL, Speck, M, Keeler, G,
Morishita, M, Marsik, FJ, Kamal, AS, Kaciroti, N, Harkema, J, Corey,
P, Silverman, F, Gold, DR, Wellenius, G, Mittleman, MA, Rajagopalan,
S and Brook, JR (2009). Insights into the mechanisms and mediators
of the effects of air pollution exposure on blood pressure and
vascular function in healthy humans. Hypertension 54(3): 659-667.
Cangerana Pereira, FA, Lemos, M, Mauad, T, de Assuncao, JV and
Nascimento Saldiva, PH (2011). Urban, traffic-related particles and
lung tumors in urethane treated mice. Clinics 66(6): 1051-1054.
Chan, E. (2019). Email and attachment from Elizabeth Chan, EPA,
Question re: Request for additional PM2.5 information
from Di et al 2017. Available in PM NAAQS Review Docket (EPA-HQ-OAR-
2015-0072). December 3, 2019. Office of Air Quality Planning and
Standards Research Triangle Park, NC.
Chan, EAW, Gantt, B and McDow, S (2018). The reduction of summer
sulfate and switch from summertime to wintertime PM2.5
concentration maxima in the United States. Atmospheric Environment
175: 25-32.
Chen, H, Burnett, RT, Copes, R, Kwong, JC, Villeneuve, PJ, Goldberg,
MS, Brook, RD, van Donkelaar, A, Jerrett, M, Martin, RV, Brook, JR,
Kopp, A and Tu, JV (2016). Ambient fine particulate matter and
mortality among survivors of myocardial infarction: population-based
cohort study. Environmental Health Perspectives 124(9): 1421-1428.
Corrigan, AE, Becker, MM, Neas, LM, Cascio, WE and Rappold, AG
(2018). Fine particulate matters: The impact of air quality
standards on cardiovascular mortality. Environmental Research 161:
364-369.
Cox, LA. (2019a). Letter from Louis Anthony Cox, Jr., Chair, Clean
Air Scientific Advisory Committee, to Administrator Andrew R.
Wheeler. Re: CASAC Review of the EPA's Policy Assessment for the
Review of the National Ambient Air Quality Standards for Particulate
Matter (External Review Draft--September 2019). December 16, 2019.
EPA-CASAC-20-001. Office of the Administrator, Science Advisory
Board U.S. EPA HQ, Washington DC. Available at: https://www.epa.gov/sites/default/files/2019-09/documents/draft_policy_assessment_for_pm_naaqs_09-05-2019.pdf.
Cox, LA. (2019b). Letter from Louis Anthony Cox, Jr., Chair, Clean
Air Scientific Advisory Committee, to Administrator Andrew R.
Wheeler. Re: CASAC Review of the EPA's Integrated Science Assessment
for Particulate Matter (External Review Draft--October 2018). April
11, 2019. EPA-CASAC-19-002. Office of the Administrator, Science
Advisory Board U.S. EPA HQ, Washington DC. Available at: https://yosemite.epa.gov/sab/sabproduct.nsf/LookupWebReportsLastMonthCASAC/932D1DF8C2A9043F852581000048170D?OpenDocument&TableRow=2.3#2.
DeFlorio-Barker, S, Crooks, J, Reyes, J and Rappold, AG (2019).
Cardiopulmonary effects of fine particulate matter exposure among
older adults, during wildfire and non-wildfire periods, in the
United States 2008-2010. Environmental health perspectives 127(3):
037006.
deSouza, P, Braun, D, Parks, RM, Schwartz, J, Dominici, F and
Kioumourtzoglou, MA (2021). Nationwide Study of Short-term Exposure
to Fine Particulate Matter and Cardiovascular Hospitalizations Among
Medicaid Enrollees. Epidemiology 32(1): 6-13.
DHEW (1969). Air Quality Criteria for Particulate Matter. National
Air Pollution Control Administration. Washington, D.C. U.S.
Department of Health. January 1969.
Di, Q, Amini, H, Shi, L, Kloog, I, Silvern, R, Kelly, J, Sabath, MB,
Choirat, C, Koutrakis, P and Lyapustin, A (2019). An ensemble-based
model of PM2.5 concentration across the contiguous United