\119\ The E-31 Committee develops and maintains standards for
measurement of emissions from aircraft engines. (See https://www.sae.org/works/committeeHome.do?comtID=TEAE31, last accessed
October 31, 2022).
---------------------------------------------------------------------------

    These procedures build off the existing ICAO Annex 16, Volume II
aircraft engine measurement procedures for gaseous pollutants. As
described in the Annex 16, at least three engine tests need to be
conducted to determine the emissions rates. These tests can be
conducted on a single engine or multiple engines.\120\ A representative
sample of the engine exhaust is sampled at the engine exhaust exit. The
exhaust then travels through a heated sample line where it is diluted
and kept at a constant temperature prior to reaching the measurement
instruments.
---------------------------------------------------------------------------

    \120\ For example, all three tests could be conducted on a
single engine. Or two tests could be conducted on one engine and one
test on a second engine. Or three separate engines could each be
tested a single time.
---------------------------------------------------------------------------

    The methodology for measuring PM from aircraft engines differs from
certain other EPA test procedures for mobile source PM2.5
standards in two ways. First, as discussed in the introductory
paragraphs of Section IV, the procedure is designed to measure only the
non-volatile component of PM. The measurement of volatile PM is very
dependent on the environment where it is measured. The practical
development of a standardized method of measuring volatile PM from
aircraft engines has proved challenging. Therefore, the development of
a procedure for measuring nvPM was prioritized by ICAO and SAE E-31and
the result is adopted in this final rule.
    Second, the sample is measured continuously rather than being
collected on a filter and measured after the test. This approach was
taken primarily for the practical reasons that, due to high dilution
rates leading to relatively low concentrations of PM in the sample,
collecting enough particulate on a filter to analyze has the potential
to take hours. Given the high fuel flow rates of these engines, such
lengthy test modes would be very expensive. Additionally, because of
the high volume of air required to run a jet engine and the extreme
engine exhaust temperatures, it is not possible to collect the full
exhaust stream in a controlled manner as is done for other mobile
source PM2.5 measurements.
    Included in the procedures now incorporated by reference by the EPA
are measurement system specifications and requirements, instrument
specifications and calibration requirements, fuel specifications, and
corrections for fuel composition, dilution, and thermophoretic losses
in the collection part of the sampling system.
    To create a uniform sampling system design that works across gas
turbine engine testing facilities, the test procedure calls for a 35-
meter sample line. This results in a significant portion of the PM
being lost in the sample lines, on the order of 50 percent for PM mass
and 90 percent for PM number. These particle losses in the sampling
system are not corrected for in the standards. Compliance with the
standard is based on the measurement at the instruments rather than the
exit plane of the engine (instruments are 35 meters from engine exit).
This is due to the lack of robustness of the sampling system particle
loss correction methodology and that a more stringent standard at the
instrument will lead to a reduction in the nvPM emissions at the engine
exit plane. A correction methodology has been developed to better
estimate the actual PM emitted into the atmosphere. This correction is
described in Section V.A.2.
3. Test Duty Cycles
    Mass and number PM emissions are measured over the LTO cycle shown
in Table IV-1. This is the same duty cycle used to measure gaseous
emissions from aircraft engines and is intended to represent operations
and flight under an altitude of 3,000 feet near an airport. Emissions
rates for each mode can be calculated by testing the engine(s) over a
sufficient range of thrust settings such that the emission rates at
each condition in Table IV-1 can be determined.

 Table IV-1--Landing and Take-Off Cycle Thrust Settings and Time in Mode
                                  \121\
------------------------------------------------------------------------
                                                              Time in
           LTO operating mode             Thrust setting  operating mode
                                            percent rO        minutes
------------------------------------------------------------------------
Take-off................................             100             0.7
Climb...................................              85             2.2
Approach................................              30             4.0
Taxi/ground idle........................               7            26.0
------------------------------------------------------------------------

    The previous smoke number standard was adopted to reduce the
visible smoke emitted from aircraft engines. Smoke number has been
determined by measuring the visibility or opacity of a filter after
soot has been collected upon it during the test procedure. Another
means of measuring this visibility is by direct measurement of the
particulate matter mass concentration. By measuring visibility based on
mass concentration rather than smoke number, the number of tests needed
can be reduced, and mass concentration data can be collected
concurrently with other PM measurements. Like the previous smoke
standard, the PM mass concentration standard is be based on the maximum
value at any thrust setting. The engine(s) will be tested over a
sufficient range of thrust settings that the maximum can be determined.
This maximum could be at any thrust setting and is not limited to the
LTO thrust points in Table IV-1.
---------------------------------------------------------------------------

    \121\ ICAO, 2017: Aircraft Engine Emissions, International
Standards and Recommended Practices, Environmental Protection, Annex
16, Volume II, Fourth Edition, July 2017, III-4-2. The ICAO Annex
16, Volume II, Fourth Edition, includes Amendment 10 of January 1,
2021.
---------------------------------------------------------------------------

    The EPA is incorporating by reference ICAO's Annex 16 to the
Convention on International Civil Aviation, Environmental Protection,
Volume II--Aircraft Engine Emissions, Fourth Edition, July 2017.
4. Characteristic Level
    EPA is incorporating by reference Appendix 6 to ICAO Annex 16,
Volume II--International Standards and Recommended Practices for
correcting engine measurements to characteristic value. Like existing
gaseous standards, compliance with the PM standards adopted in this
action is based on the characteristic level of the engine. The
characteristic level is a statistical

method of accounting for engine-to-engine variation in the measurement
based on the number of engines tested. A minimum of three engine
emissions tests is needed to determine the engine type's emissions
rates for compliance with emission standards. The more engines that are
used for testing increases the confidence that the emissions rate
measured is from a typical engine rather than a high or low engine.
    Table IV-2 is reproduced from Annex 16 Volume II Appendix 6 Table
A6-1 and shows how these factors change based on the number of engines
tested.\122\ As the number of engines tested increases, the factor also
increases resulting in a smaller adjustment and reflecting the
increased confidence that the emissions rate is reflective of the
average engine off the production line. In this way, there is an
incentive to test more engines to reduce the characteristic adjustment
while also increasing confidence that the measured emissions rate is
representative of the typical production engine.
---------------------------------------------------------------------------

    \122\ ICAO, 2017: Aircraft Engine Emissions, International
Standards and Recommended Practices, Environmental Protection, Annex
16, Volume II, Fourth Edition, July 2017, App 6-2pp. The ICAO Annex
16, Volume II, Fourth Edition, includes Amendment 10 of January 1,
2021.
[GRAPHIC] [TIFF OMITTED] TR23NO22.177

    For PM mass and PM number, the characteristic level is based on the
mean of all engines tested, and appropriately corrected, divided by the
factor corresponding to the number of engine tests performed in Table
IV-1. For PM mass concentration, the characteristic level is based on
the mean of the maximum values of all engines tested, and appropriately
corrected, divided by the factor corresponding to the number of engine
tests performed in Table IV-2.
    For example, an engine type where three measurements were obtained
from the same engine has an nvPM mass metric value of 100 mg/kN (mean
metric value of all engine tests). The nvPM LTO mass factor (or nvPM
mass characteristic factor) from Table IV-2 for three engines is
0.7194. The metric value, with applicable corrections applied, is then
divided by the factor to obtain the characteristic level of the engine.
Therefore, the resulting characteristic level for this engine type, to
determine compliance with the nvPM mass standard is 139.005 mg/kN. If
instead three engines are each tested once, the characteristic factor
would be 0.8858 and the nvPM mass characteristic level to determine
compliance with the standard would be 112.892 mg/kN.
    An engine type's characteristic level can also be further improved
by testing additional engines. For example, if 10 separate engines were
tested of the same type, the nvPM mass characteristic factor becomes
0.9375. The resulting characteristic level (assuming the average nvPM
mass metric value remains 100 mg/kN) would be 106.667 mg/kN. This
approach could be used if an engine exceeds the standard at the time it
is initially tested or there is a desire to increase the margin to the
standard for whatever reason. Table IV-3 shows these three different
examples for nvPM LTO Mass.

              Table IV-3--Impact of the Number of Engines Tested on Resulting Characteristic Level
----------------------------------------------------------------------------------------------------------------
                                                     Number of     Measured nvPM
            Number of engines tested                 tests per     LTO Mass (mg/  Characteristic  Characteristic
                                                      engine            kN)           factor       level (mg/kN)
----------------------------------------------------------------------------------------------------------------
1...............................................               3             100          0.7194         139.005
3...............................................               1             100          0.8858         112.892
10..............................................               1             100          0.9375         106.667
----------------------------------------------------------------------------------------------------------------

5. Derivative Engines for Emissions Certification Purposes
    Aircraft engine types can remain in production for many years and
be subject to numerous modifications during their production life. As
part of the certification process for any change, the type certificate
applicant will need to show if the change will have an impact the
engine emissions. While some of these changes could impact engine
emissions rates, many of them will not. To simplify the certification
process and reduce burden on both type certificate applicant and
certification authorities, ICAO developed criteria to determine whether
there has been an emissions change that requires new testing. Such
criteria already exist at ICAO and in the EPA regulations for gaseous
and smoke standards.
    ICAO recommends \123\ that if the characteristic level for an
engine was type certificated at a level that is at or above 80 percent
of the PM mass, PM number, or PM mass concentration standard, the type
certificate applicant would be required to test the proposed derivative
engine. If the engine is below 80 percent of the standard, engineering
analysis can be used to determine new emission rates for the proposed
derivative engines. The EPA is implementing these ICAO recommended
practices in this final rule as the regulatory standard in the United
States.
---------------------------------------------------------------------------

    \123\ ICAO, 2020, Environmental Technical Manual, Doc 9501,
Volume II--Procedures for the Emissions Certification of aircraft
Engines, Fourth Edition, Section 2, Part III, Chapter 2.
---------------------------------------------------------------------------

    ICAO evaluated the measurement uncertainty to develop criteria for
determining if a proposed derivative engine's emissions are similar to
the previously certificated engine's emissions. The EPA is adopting
these ICAO criteria in this final rule.\124\
---------------------------------------------------------------------------

    \124\ Id.
---------------------------------------------------------------------------

    For PM mass measurements described in Section IV.A, the following
values apply:
     80 mg/kN if the characteristic level for
nvPMmass emissions is below 400 mg/kN.
     20% of the characteristic level if the
characteristic level for nvPMmass emissions is greater than
or equal to 400 mg/kN.
    For PM number measurements, described in Section IV.B, the
following values apply:
     4x10\14\ particles/kN if the characteristic level for
nvPMnum emissions is below 2x10\15\ particles/kN.
     20% of the characteristic level if the
characteristic level for nvPMnum emissions is greater than
or equal to 2x10\15\ particles/kN.
    For PM mass concentration measurements described in Section IV.C,
the following values apply:
     200 [mu]g/m\3\ if the characteristic level of
maximum nvPM mass concentration is below 1,000 [mu]g/m\3\.
     20% of the characteristic level if the
characteristic level for maximum nvPM mass concentration is at or above
1,000 [mu]g/m\3\.
    If a type certificate applicant can demonstrate that the engine's
emissions are within these ranges, then new emissions rates will not
need to be developed and the proposed derivative engine for emissions
certification purposes will keep the existing emissions rates.
    If the engine is not determined to be a derivative engine for
emissions certification purposes, the type certificate applicant will
need to certify the new emission rates for the engine.

E. Annual Reporting Requirement

    In 2012, the EPA adopted an annual reporting requirement as part of
a rulemaking to adopt updated aircraft engine NOX
standards.\125\ This provision, adopted into 40 CFR 87.42, requires the
manufacturers of covered engines to annually report data to the EPA
which includes information on engine identification and
characteristics, emissions data for all regulated pollutants, and
production volumes. In 2018, the EPA issued an information collection
request (ICR) which renewed the existing ICR and added PM information
to the list of required data.126 127 However,
that 2018 ICR was not part of a rulemaking effort, and the new PM
reporting requirements were not incorporated into the CFR at that time.
Further, that 2018 ICR is currently being renewed (in an action
separate from this rulemaking), and the EPA is including as part of
that effort some additional data elements to the ICR (specifically, the
emission indices for HC, CO, and NOX at each mode of the LTO
cycle).128 129 The EPA is now formally
incorporating all aspects of that ICR, as proposed to be renewed, into
40 CFR 1031.150. It is important to note that the incorporation of the
PM reporting requirements into the CFR will not create a new
requirement for the manufacturers of aircraft engines. Rather, it will
simply incorporate the existing reporting requirements (as proposed to
be amended and renewed in a separate action) into the CFR for ease of
use by having all the reporting requirements readily available in the
CFR.
---------------------------------------------------------------------------

    \125\ 77 FR 36342 (June 18, 2012).
    \126\ 83 FR 44621 (August 31, 2018).
    \127\ U.S. EPA, Aircraft Engines--Supplemental Information
Related to Exhaust Emissions (Renewal), OMB Control Number 2060-
0680, ICR Reference Number<PHONE_NUMBER>-08, December 17, 2018.
Available at https://www.reginfo.gov/public/do/PRAViewICR?ref_nbr=201809-2060-008, last accessed June 8, 2022.
    \128\ Proposed Information Collection Request; Comment Request;
Air Emissions Reporting Requirements (Renewal); EPA ICR No. 2170.08,
OMB Control No. 2060-0580, 86 FR 24614 (May 7, 2021).
    \129\ Documentation and Public comments are available at:
https://www.regulations.gov/docket/EPA-HQ-OAR-2016-0546, last
accessed June 8, 2022.
---------------------------------------------------------------------------

    The EPA uses the collection of information to help conduct
technology assessments, develop aircraft emission inventories (for
current and future inventories), and inform our policy decisions--
including future standard-setting actions. The information enables the
EPA to further understand the characteristics of aircraft engines that
are subject to emission standards--and engines subject to the PM
emission standards--and engines' impact on emission inventories. In
addition, the information helps the EPA set appropriate and achievable
emission standards and related requirements for aircraft engines.
Annually updated information helps in assessing technology trends and
their impacts on national emissions inventories. Also, it assists the
EPA to stay abreast of developments in the aircraft engine industry.
    As discussed in Section VII, the EPA is finalizing the proposal to
migrate the existing 40 CFR part 87 regulatory text to a new 40 CFR
part 1031. This effort includes clarifying portions of the regulatory
text for ease of use. In the old 40 CFR 87.42(c)(6), the regulatory
text did not specifically spell out some required data, but instead
relied on incorporation by reference of ICAO Annex 16, Volume II's data
reporting requirements and listed the data from this Annex that is not
required by the EPA's reporting requirement. For future ease of use, 40
CFR 1031.150 explicitly lists all the required items rather than
continuing the incorporation by reference approach in the existing
reporting regulations. Finally, the EPA is incorporating by reference
Appendix 8 of Annex 16, Volume II, which outlines procedures used to
estimate measurement system losses, which are a required element of the
reporting provisions.

F. Response to Key Comments

    The EPA received numerous comments on the proposed rulemaking

which are summarized in the Response to Comments document along with
the EPA's responses to those comments. Comments in their entirety are
available in the docket for this rulemaking action. The following
sections summarize the comments related to the stringency of the
standards and the EPA's response to these comments. Some adverse
comments are addressed more fully in the Response to Comments document.
1. Comments in Support of the Proposed Standards
    Comment summary: Some commenters stated that the proposed standards
adhere to the statutory requirements of CAA section 231. They say that
the proposed standards are well supported by an extensive
administrative record. The commenters point out that the D.C. Circuit
ruled in 2007 that CAA section 231 confers a broad degree of discretion
on the EPA in setting aircraft engine emission standards.\130\
---------------------------------------------------------------------------

    \130\ National Association of Clean Air Agencies v. EPA, 489
F.3d 1221, 1229-30 (D.C. Cir. 2007) (``When Congress enacted Sec.
231 providing that the Administrator could, `from time to time,' act
`in his judgment,' as `he deems appropriate,' it conferred broad
discretion to the Administrator to weigh various factors in arriving
at appropriate standards.'').
---------------------------------------------------------------------------

    Response: EPA is finalizing the standards as proposed. We agree
that the proposed standards, as well as the final standards, satisfy
our statutory obligations and are well-supported. The EPA acknowledges
that the D.C. Circuit recognized the EPA's broad authority in CAA
section 231 in National Association of Clean Air Agencies v. EPA, 489
F.3d 1221, 1229-30 (D.C. Cir. 2007) (NACAA).
    Comment summary: Several commenters expressed their support of the
EPA adopting PM standards that match the international PM standards
because doing so is vital to the competitiveness of U.S. industry and
regulatory certainty. They say it would protect U.S. jobs and
strengthen the U.S. aviation industry by ensuring the global acceptance
of U.S.-manufactured aircraft engines. They also say it will make sure
U.S.-manufactured aircraft engines are available to aircraft
manufacturers and U.S. airlines, while enabling U.S. airlines to obtain
aircraft and aircraft engines at market-driven, competitive prices.
    Response: The EPA agrees this rule has the benefit of helping to
ensure the acceptance of U.S.-manufactured aircraft engines by member
States, aircraft (airframe) manufacturers, and airlines around the
world. The EPA notes that under the terms of the Chicago Convention,
ICAO member States must recognize as valid certificates of
airworthiness issued by other ICAO member States, provided the
requirements under which such certificates were issued are as least as
stringent as the minimum ICAO standards.\131\
---------------------------------------------------------------------------

    \131\ ICAO, 2006: Convention on International Civil Aviation,
Article 33, Ninth Edition, Document 7300/9.
---------------------------------------------------------------------------

    Comment summary: Some commenters urged the EPA to promptly issue
the final rule with the standards matching the international standards.
They say that this EPA rulemaking and the subsequent FAA certification
rulemaking must be completed to start the certification process in the
United States. Thus, they believe that prompt EPA action is necessary
to provide sufficient time for FAA to promulgate their certification
rulemaking and U.S. aircraft engine manufacturers to conduct the
lengthy and expensive steps to demonstrate compliance with the
standards, for all aircraft engines that will be in-production in 2023.
They note that January 1, 2023, is the implementation date for the ICAO
standards.
    Response: The EPA acknowledges that the international effective
date for the ICAO mass concentration standards was January 1, 2020, and
that the international effective date for the mass and number standards
is January 1, 2023. The EPA also acknowledges that FAA will need to
conduct a separate, subsequent certification rulemaking process to
implement the EPA's PM standards finalized in this action.
    In this action, the EPA is aiming to minimize disruption by
finalizing this action before the January 1, 2023, the international
effective date of the PM mass and number standards.
    For comparison, the EPA notes the EPA finalized the domestic GHG
standards for airplanes on January 11, 2021, after the international
effective date for new type planes; \132\ however, disruption was
avoided in practice because no manufacturers applied to FAA for a type
certificate for a new type design airplane between January 1, 2020, and
January 11, 2021.
---------------------------------------------------------------------------

    \132\ CAEP/10 airplane CO2 standards apply to new
type design airplanes for which the application for a type
certificate was or will be submitted on or after January 1, 2020,
some modified in-production airplanes on or after January 1, 2023,
and all applicable in-production airplanes manufactured on or after
January 1, 2028.
---------------------------------------------------------------------------

    Comment summary: Some commenters state that the proposed standards
are identical to ICAO's aircraft engine PM standards and that adopting
them is consistent with the 1944 Chicago Convention treaty obligations.
They say that these standards continue the long collaborative tradition
between the EPA and ICAO. The commenters say that the objective of the
Chicago Convention is to foster global cooperation and encourage an
atmosphere where international civil aviation could be developed in a
safe and orderly manner, while being operated soundly and economically.
The commenters say that, with both the FAA and the EPA playing key
leadership roles, it was only after significant deliberation and
technical and economic analyses that CAEP agreed to the ICAO PM
standards. The commenters say that the EPA's adoption of standards that
align with ICAO standards supports international harmonization and
regulatory uniformity.
    Response: The EPA agrees adopting the PM standards in this action
satisfies the United States' treaty obligations under the Chicago
Convention. The EPA also agrees that the EPA and the FAA had key
leadership roles in the ICAO PM standard-setting process, and the EPA
recognizes the significant deliberations and economic analyses that
occurred in CAEP. The EPA agrees that this action promotes
international cooperation and harmonization.
    Comment summary: Some commenters say that the standards are
consistent with the CAEP terms of reference which provide that
standards be technologically feasible, economically reasonable,
environmentally beneficial, and balanced against interdependencies
(aircraft noise and competing emission reductions of other pollutants,
such as NOX). The commenters say that the CAEP terms of
reference align well with the considerations in CAA section 231, and
ICAO's assessment of each of the criteria of the terms of reference is
directly related to the decisions the EPA must make when issuing
aircraft engine emission standards. The commenters assert that CAA
section 231(b) requires that aircraft engine emission standards allow
sufficient lead time for the development of the necessary technology,
while giving consideration of the cost to comply within this time
period.
    Response: The EPA agrees that the final standards are consistent
with the CAEP terms of reference and that the standards also meet the
requirements of CAA section 231. The EPA would not adopt ICAO standards
domestically without exercising the Agency's own independent evaluation
of appropriate domestic standards under CAA section 231, which is what
the EPA has done in

this rulemaking. Any domestic aircraft engine standards adopted by the
EPA must comport with the requirements in CAA section 231.
    Comment summary: Some commenters say that CAA section
231(a)(2)(B)(ii) expressly prohibits changes in aircraft engine
emission standards that ``would significantly increase noise and
adversely affect safety.'' The commenters point out that, as the EPA
describes in the proposed rulemaking, ICAO/CAEP evaluates
``technological feasibility'' using the Technology Readiness Level
(``TRL'') scale and deems technologies that have attained TRL8 (defined
as the ``actual system completed and `flight qualified' through test
and demonstration'') to be ``technologically feasible.'' Therefore, the
commenters conclude, the use of TRL8 to evaluate ``technological
feasibility'' makes sure aircraft engine emission standards reflect
what technologies can safely deliver, instead of hypothetical
``technology forcing'' standards that could pose a potential threat to
air safety.\133\
---------------------------------------------------------------------------

    \133\ Any reference to technology-forcing standards in this
rulemaking is not based on the level of the final PM standards, but
it is intended to respond to comments.
---------------------------------------------------------------------------

    Response: The EPA agrees that TRL8 \134\ is an adequate and
appropriate criteria for identifying proven technologies that are
demonstrably safe and of an acceptable noise level for purposes of this
rulemaking. The EPA relies on TRL8 to support the PM standards
finalized in this rule because TRL8 was used to justify the PM
standards by ICAO, as described in Section VI.B. ICAO treats TRL8 as a
proxy for what is technologically feasible in the course of
establishing new international standards. This conservative approach
allows ICAO to ensure that all technology being considered is safe and
of acceptable noise level without having to conduct additional
evaluation of specific technologies. The EPA agrees this use of TRL8 is
a valid means for ICAO to develop standards that will, by definition,
be based on technologies that have been proven safe, of acceptable
noise level, and technologically feasible. The EPA also agrees that
ICAO's use of TRL8 means that technologies considered have been proven
safe and of an acceptable noise level, and therefore, that the final PM
standards do not adversely affect safety and do not significantly
increase noise. In setting the international standards, ICAO considered
the emissions performance of aircraft engines assumed to be in-
production on the implementation date for the PM mass and number
standards, January 1, 2023. Thus, the technology was already
demonstrated to be safe and of acceptable noise levels for these
standards, and ICAO did not view that a new safety and noise analysis
was necessary.
---------------------------------------------------------------------------

    \134\ As described in Section VI.B, TRL is a measure of
Technology Readiness Level. CAEP has defined TRL8 as the ``actual
system completed and `flight qualified' through test and
demonstration.'' TRL is a scale from 1 to 9, TRL1 is the conceptual
principle, and TRL9 is the ``actual system `flight proven' on
operational flight.'' The TRL scale was originally developed by
NASA. ICF International, CO2 Analysis of CO2-Reducing Technologies
for Aircraft, Final Report, EPA Contract Number EP-C-12-011, see
page 40, March 17, 2015.
---------------------------------------------------------------------------

    However, in the EPA's view, ICAO's use of TRL8 to define
technological feasibility is not the only means to ensure a standard
does not adversely affect safety and does not significantly increase
noise. The EPA does not view TRL8 to represent the most stringent level
of technology that could be required in an EPA aircraft standard
setting rulemaking. Nor does the EPA agree with the premise that
standards based on technology below TRL8 would necessarily be
technology forcing or inherently have a negative effect on safety and
noise. In establishing U.S. aircraft engine emission standards, the EPA
is not constrained to ICAO's definition of technological feasibility in
assessing appropriate aircraft engine standards under CAA section
231(a). See NACAA, 489 F.3d at 1229-30. In fact, the EPA has adopted
technology-forcing standards under CAA section 231 in the past and
found them to be safe and not to significantly increase noise.\135\ In
the future, if the EPA were to consider setting emission standards
based on technology that was not yet at TRL8 or not expected to be at
TRL8 by the implementation date of the standards,\136\ the Agency, just
as it did in this action, in consultation with the FAA, would evaluate
the safety and noise impact (also lead time and cost) of such standards
before making a determination in this regard. CAA section 231(a)(2)(B)
and (a)(3). Any assessment of safety and noise (also lead time and
cost) in the context of hypothetical technology-forcing standards would
have to occur in the context of the specific standards under
consideration.
---------------------------------------------------------------------------

    \135\ See 38 FR 19088 (July 17, 1973); 41 FR 34722 (August 16,
1976).
    \136\ As described in Section VI.B, for the ICAO PM standard
setting, ICAO referred to technical feasibility as any technology
demonstrated to be safe and airworthy proven to Technology Readiness
Level 8 and available for application over a sufficient range of
newly certificated aircraft. This means that the ICAO analysis that
informed the international standard considered the emissions
performance of aircraft engines assumed to be in-production on the
ICAO implementation date for the PM mass and number standards,
January 1, 2023.
---------------------------------------------------------------------------

2. Comments in Support of More Stringent Standards
    Comment summary: Several commenters were dissatisfied with the
level of stringency of the PM standards. One commenter argued that CAA
section 231 requires the EPA to adopt technology-forcing standards.
Other comments argued CAA section 231 requires the EPA to set standards
according to expectations of the development of technology over time.
Some commenters say that, at a minimum, the EPA should establish
standards that reduce emissions based on available engine technology. A
number of commenters supported these arguments by pointing to the text
of the statute, the underlying legislative intent, legislative history,
and the purpose of the CAA.
    Response: The statutory-based arguments presented by commenters
that the level of stringency of the PM standards are not authorized by
CAA section 231 import requirements into the statute that do not exist.
    As described in Section II.A, CAA section 231(a)(2)(A) directs the
Administrator of the EPA to, from time to time, propose aircraft engine
emission standards applicable to the emission of any air pollutant from
classes of aircraft engines which in the Administrator's judgment
causes or contributes to air pollution that may reasonably be
anticipated to endanger public health or welfare. CAA section 231(a)(3)
provides that after the EPA proposes standards, the Administrator shall
issue such standards ``with such modifications as he deems
appropriate.'' CAA section 231(b) requires that any emission standards
``take effect after such period as the Administrator finds necessary .
. . to permit the development and application of the requisite
technology, giving appropriate consideration to the cost of compliance
during such period.'' The D.C. Circuit has held that the delegation of
authority in CAA section 231 ``is both explicit and extraordinarily
broad'' and that the text confers ``broad discretion . . . to weigh
various factors in arriving at appropriate standards.'' NACAA, 489 F.3d
1221, 1229-30.
    The statutory language of CAA section 231 is not identical to other
provisions in the CAA that direct the EPA to establish technology-based
standards. CAA section 231(a) states that the EPA must ``issue proposed
emission standards applicable to the emission of any air pollutant''
from aircraft engines

and to finalize ``such regulations'' with those modifications the EPA
``deems appropriate.'' CAA section 231(a)(2)(A) and (a)(3). This
language is in contrast to Congress' direction in other parts of the
Act, where it required the EPA to set standards that achieve a
particular degree of emission reduction or environmental or public
health protection. For example, in setting technology-based emission
standards for hazardous air pollutants under CAA section 112(d)(2) and
(3), the EPA must ``require the maximum degree of reduction . . . that
the Administrator . . . determines is achievable,'' taking into account
cost and non-air quality health and environmental impacts. CAA section
112(d)(2). Those standards also ``shall not be less stringent than''
explicitly prescribed levels. CAA section 112(d)(3). Health- and
environmental quality-based NAAQS under CAA section 109 must be set at
levels ``requisite to protect the public health'' and ``requisite to
protect the public welfare from any known or anticipated adverse
effects associated with the presence of [the] air pollutant in the
ambient air.'' CAA section 109(b)(1) and (2). When regulating certain
pollutants from motor vehicles and nonroad engine emissions under CAA
sections 202(a)(3) and 213(a)(3) and (5), the EPA's standards must
``reflect the greatest degree of emission reduction achievable . . . ,
giving appropriate consideration to cost, energy, and safety factors
associated with the application of such technology.'' CAA sections
202(a)(3) and 213(a)(3) and (5).
    CAA section 231 lacks comparable language requiring it to meet a
particular threshold of protectiveness, emission reduction, or
technological stringency, despite this clear evidence that Congress
knew how to impose such obligations when it wished. See generally CAA
section 231. ``Where Congress uses certain language in one part of a
statute and different language in another, it is generally presumed
that Congress acts intentionally.'' Nat'l Fed'n of Indep. Bus. v.
Sebelius, 567 U.S. 519, 544 (2012); Sosa v. Alvarez-Machain, 542 U.S.
692, 711 n.9 (2004) (citing a treatise on statutory construction and
calling this principle the ``usual rule'' of judicial interpretation).
In certain respects, the EPA's authority is broader than it is under
other CAA provisions, in that the EPA is not required in setting
aircraft emission standards to achieve a specified degree of emissions
reduction.
    Some commenters also presented a textual comparison of the House
and Senate bills to conclude that Congress intended for CAA section 231
to be based on a consideration of pollution impacts and technological
feasibility because the final CAA section 231(a)(1) required the EPA to
conduct a study within 90 days after December 31, 1970 of air
pollutants from aircraft to determine impact on air quality and
technological feasibility of controlling such pollutants. S. Rep. No.
91-1196, at 24, 1 Leg. Hist. at 424; H.R. Rep. No. 91-1783, at 55
(Conf. Rep.). One commenter alleged this means ``the necessary premise
[is] that such study should inform the standards themselves.'' \137\
However, the study requirement in CAA section 231(a)(1) does not
establish a requirement for aircraft engine standards to be forward-
looking technology-based regulation. That provision required EPA to
conduct a one-time ``study and investigation'' ``to determine'' the
extent of aircraft emissions' impacts on air quality and the
feasibility of controlling them ``[w]ithin 90 days after December 31,
1970.'' The single study required in CAA section 231(a)(1) is not a
continuing obligation that pertains to each exercise of the standard-
setting authority under CAA section 231(a)(2) and (3), which contain no
discussion of technological feasibility and under which standards are
set and may be revised ``from time to time.'' Cf. Sierra Club, 325 F.3d
374, 377 (D.C. Cir. 2003) (holding that a provision requiring EPA to
set standards ``based on'' such a study did not make the validity of
the standards dependent on their connection to that study).
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    \137\ Comments of California, Connecticut, Illinois, Maryland,
Massachusetts, New Jersey, New York, Oregon, Pennsylvania, Vermont,
Washington, and Wisconsin at 13. See also Comment of Sierra Club at
7-8.
---------------------------------------------------------------------------

    The commenters also quoted to a Senate report accompanying the CAA
1970 amendment Senate bill to suggest CAA section 231 requires
standards to be based on the degree of harm caused by aircraft
pollution and the technology that can be developed in the future to
reduce it. The statement cited by commenters from the Senate Report
does not constrain the EPA where the plain text of the statute does
not, and where Congress knew how, but declined, to make such
constraints mandatory on the Agency. ``Congress' authoritative
statement is the statutory text, not the legislative history.'' Chamber
of Com. Of U.S. v. Whiting, 563 U.S. 582, 599 (2011) (quoting Exxon
Mobil Corp. v. Allapattah Services, Inc., 545 U.S. 546, 568 (2005)
(internal quotation marks omitted). Further, the NACAA Court rejected
an argument that similar statements in the 1970 Senate Report
established Congress' intent that the EPA prioritize forward-looking
standards. NACAA, 489 F.3d at 1229-30; Sierra Club v. EPA, 325 F.3d
374, 379-380 (D.C. Cir. 2003).
    The EPA's interpretation of CAA section 231 is not categorically at
odds with the Clean Air Act's general protective purpose. The Act's
general goal of reducing air pollution does not, in itself, prescribe
regulatory factors for specific programs, nor does it restrict the
EPA's discretion as to how best effectuate that goal in a specific
action or in a regulatory program over time. Accordingly, while the
EPA's discretion under CAA section 231 would allow it to select more
stringent standards when appropriate, it does not mandate that the EPA
elevate pollution reduction over all relevant factors in the
consideration of any particular aircraft standard. See NACAA, 489 F.3d
at 1229-30.
    The final PM standards fall squarely within the EPA's statutory
authority under CAA section 231 to promulgate. As described in Section
I.B.2 and the introductory text of Section IV, in proposing and
adopting the final PM standards, the EPA considered the statutory
requirements of CAA section 231. The EPA also took into account the
need to control PM emissions, the importance of international
harmonization, avoiding adverse impacts that could result from delaying
adoption of PM standards at least as stringent as ICAO's PM standards,
and gaining experience from the novel approach to implementing PM
standards. Further, based on the EPA's independent view that technology
at the TRL8 has been demonstrated to be safe and of an acceptable
noise-level, the EPA is confident that the final standards will not
significantly increase noise or adversely affect safety. The EPA
reached the same conclusion as ICAO that a new noise and safety
analysis was not necessary. For the same reasons, the EPA believes
sufficient lead time has been provided since the technology has already
been developed. Costs information for the standards is described in
Section VI.D. Based on this assessment, the EPA concludes that it is
reasonable to finalize PM standards that match the international
standards in scope, stringency, and effective date.
    Additional legal issues raised by these comments are addressed in
the Response to Comments document.
    Comment summary: Some commenters claim the EPA has an obligation to
consider the feasibility, costs, and benefits of more stringent
standards, including technology-forcing standards, or at least explain
why it did

not do so. A few commenters proposed suggestions to alternative PM
controls such as de-rated takeoff, accelerated implementation of
Optimized Profile Descents, reduced power during taxiing, improved taxi
time, and reduced usage of auxiliary power units (APUs).
    Response: The focused scope of the EPA's proposed PM standards was
informed by the January 1, 2023, international effective date for the
mass and number PM standards, as well as the other considerations
identified elsewhere throughout this preamble. The EPA does not believe
it would be feasible to repropose more stringent PM standards and also
meet the international effective date of the new mass and number
standards. Should the United States miss the January 1, 2023, deadline,
U.S. airplane and engine manufacturers could be forced to seek PM
emissions certification from an aviation certification of another
country to market and operate their airplanes and engines
internationally. The United States would also miss its obligations
under the Chicago Convention.
    The EPA believes that the limited scope of the proposal is
permissible under CAA section 231 and, based on the plain language of
the statute, disagrees with the premise that the statute requires the
Agency to propose multiple levels of stringency of standards. To the
extent commenters identified specific alternative levels of stringency
they would prefer, the comments did not provide sufficient information
about safety, noise, lead time, and costs of those alternatives to
support the EPA finalizing more stringent standards in this rulemaking.
In light of the reasons the EPA has provided for adopting the PM
standards as proposed, the EPA does not view these ``modifications''
requested by commenters to be ``appropriate'' to incorporate into the
PM standards adopted in this rulemaking. See CAA section 231(a)(3). The
EPA's current and intended future work related to addressing PM
emissions from aircraft engines is described in Section I.C.
    A number of commenters also provided suggested ideas for
alternative methods to regulating PM emissions (e.g., de-rated takeoff,
reduced power during taxiing, and improved taxi time). The EPA has
carefully reviewed the alternatives raised by the commenters, but has
decided not to adopt them in this final rulemaking. The EPA does not
believe it would be feasible to assess the legal, technical, and policy
issues raised by suggested alternatives put forward by commenters;
repropose standards; take public comment; and meet the international
effective date of January 1, 2023. More specific comments related to
suggested alternative PM controls are addressed in the Response to
Comments document.
    Comment summary: According to some commenters, the EPA
impermissibly factored international harmonization, adverse impacts on
U.S. industry, or other non-statutory considerations into its rationale
supporting the PM standards.
    Response: The EPA's past practice and the D.C. Circuit's holding in
NACAA that the EPA's historical approach of taking international
harmonization into account in setting domestic standards as not
``manifestly contrary to the statute'', NACAA, 489 F.3d at 1230, affirm
that the EPA's broad discretion includes the ability to weigh
considerations such as international harmonization and the competitive
effects of the EPA's standards on international aviation. Nothing in
CAA section 231 precludes such considerations. Aircraft and their
engines are manufactured and sold around the world, and routinely
operate in international airspace. Furthermore, CAA section 231 does
not list or dictate the EPA's consideration of particular factors and
enables the EPA to identify and apply relevant considerations in
determining what standards are ``appropriate''. CAA section 231(a)(3).
The D.C. Circuit rejected an argument similar to the commenters' in
NACAA: ``Finding nothing in the text or structure of the statute to
indicate that the Congress intended to preclude the EPA from
considering `[factors other than air quality],' we refused to infer
from congressional silence an intention to preclude the agency from
considering factors other than those listed in a statute.'' 489 F.3d at
1230 (quoting George E. Warren Corp. v. EPA, 159 F.3d 61, 623-24 (D.C.
Cir. 1998)). Moreover, the Chicago Convention, ratified by the United
States, has the force of Federal law, and therefore, the EPA acts
appropriately in implementing our Clean Air Act authorities in a manner
that is harmonious and consistent with the Chicago Convention and the
United States' international obligations under the treaty.
    Having invested significant effort and resources, working with the
FAA and the Department of State, to gain international consensus within
ICAO to adopt the international PM standards for aircraft engines, the
EPA believes that meeting the United States' obligations under the
Chicago Convention by aligning domestic standards with the ICAO
standards, rather than adopting more stringent standards, will have
substantial benefits for future international cooperation on aircraft
engine emission standards, and such cooperation is the key for
achieving worldwide emission reductions. Deviating from the
international PM standards could undermine future efforts by the United
States to seek international consensus on aircraft emission standards
in general, including more stringent future standards for PM. Reaching
this conclusion is not tantamount to a determination that it would
never be appropriate for the EPA to adopt more stringent PM standards
than ICAO's standards. However, at this time, the EPA finds it
appropriate to finalize the standards as proposed.
    In addition, the ICAO applicability date of the mass and number
standards of January 1, 2023, is fast approaching. The U.S. aircraft
engine manufacturers, aircraft manufacturers, and airlines are urging
the EPA to promptly promulgate this final rulemaking to adopt ICAO's
standards, which were adopted back in 2017 and 2020, so they can build
(and sell) or have access to U.S. engines to remain competitive in the
global marketplace. Furthermore, the EPA understands that U.S. aircraft
engine manufacturers need time to certify their products, after the
subsequent FAA rulemaking to enforce the standards, to ensure the
aircraft engines comply with standards. Also, the EPA did not conduct
the analyses needed to support more stringent standards in the proposed
rulemaking, or otherwise develop a sufficient record for more stringent
standards, that would be necessary to support finalizing such standards
in this final rule. We do not believe we could finalize more stringent
standards without conducting significant additional analyses and
undertaking a new round of notice and comment, which would certainly
cause a significant delay in meeting the United States' obligations
under the Chicago Convention. We have decided that the most appropriate
course, under CAA section 231, is to adopt aircraft engine PM standards
that are harmonized with the standards adopted by ICAO in 2017 and
2020.
    In determining what final PM standards are appropriate under CAA
section 231 and after consultation with FAA, the EPA considered the
level of standards that could be met with the application of requisite
technology within the necessary period of time that would allow the
United States to meet its obligations under the Chicago Convention to
at least match the ICAO standards, and gave appropriate consideration
to the cost of compliance within this period. This determination also
took into account the requirement

that EPA's revised standards not significantly increase noise and
adversely affect safety.
    Comment summary: Some commenters argued that the EPA's position
that it would be appropriate to gain experience from implementation of
the novel approach to implementing PM standards before considering
whether to adopt more stringent regulations is arbitrary and
capricious.
    Response: As described the introductory paragraphs of Section IV,
these final standards change the approach to regulating aircraft engine
PM emissions from past smoke measurements to the measurement of mass
and number for the first time for U.S. manufacturers, and international
regulatory uniformity and certainty are key elements for these
manufacturers as they become familiar with adhering to these standards
and test procedures. Further, some manufacturers are still adapting to
how best control aircraft engine PM since they designed recent in-
production engines to optimize NOX control, as explained in
the succeeding paragraphs.\138\ We think that considering the novelty
of these approaches and the industry's response to them falls well
within our discretion. Moreover, they also pertain to the statutory
directive to consider the lead time necessary for the development and
application of the requisite technology. See CAA section 231(b).
---------------------------------------------------------------------------

    \138\ ICAO, 2019: Independent Expert Integrated Technology Goals
Assessment and Review for Engines and Aircraft, Document 10127. It
is found on page 34 of the English Edition of the ICAO Products &
Services 2022 Catalog and is copyright protected; Order No. 10127.
---------------------------------------------------------------------------

    Comment summary: Some commenters say that proposed standards are
far less stringent than PM emission levels that existing aircraft
engine technologies already achieve. Some commenters assert that more
stringent PM standards compared to the proposed standards are feasible
for in-production and new type design aircraft engines. Some commenters
argue that the proposed PM standards are not anti-backsliding. These
comments say that all in-production engines already meet the proposed
standards for in-production engines and most meet the proposed
standards for new type design engines by a considerable margin;
therefore, no backsliding could reasonably happen absent these
standards.
    Response: While it may be true that more stringent PM standards
compared to the final standards are feasible for some in-production and
new type design aircraft engines, for the reasons explained in the
proposal and again in this final rule the EPA does not consider more
stringent standards than those adopted in this action, applicable to
all in-production and new type design engines, to be appropriate at
this time. Additionally, the EPA did not propose more stringent
standards, and the existing record that has been developed does not
support finalizing more stringent standards absent significant
additional analyses.
    The EPA disagrees that the standards are not anti-backsliding.
Although the PM mass concentration standard is replacing the smoke
standard for some engines, the PM mass and number standards are the
first of their kind. In that regard, PM mass and number are currently
unregulated from aircraft engines and the standards finalized in this
action represent a new regulatory backstop of those two forms of
previously uncontrolled PM emissions. Further, all three PM standards
will prevent backsliding by ensuring that all new type design and in-
production aircraft engines will not exceed those regulatory levels in
the future.
    CAEP meets triennially, and in the future, we anticipate ICAO/CAEP
considering more stringent aircraft engine PM standards. The U.S.
Interagency Group on International Aviation (IGIA) facilitates
coordinated recommendations to the Secretary of State on issues
pertaining to international aviation (and ICAO/CAEP), and the FAA is
the chair of IGIA. Representatives of domestic states, non-governmental
organizations, and industry can participate in IGIA to provide input
into future standards for ICAO/CAEP. U.S. manufacturers will be better
prepared for any future standard change due to their experience with
measuring nvPM mass and number for the first time for these final
standards. The PM standards adopted in this rulemaking, within the
larger context of international aircraft standard-setting, send an
important signal that PM emissions is a factor that manufacturers need
to consider when building aircraft engines now and going forward--with
the anticipation that ICAO/CAEP will consider more stringent PM
standards in the future.
    In response to the comments that the standards are far less
stringent than PM emission levels of existing aircraft engine
technologies, the EPA notes that there is a wide range of PM levels for
in-production aircraft engines. As described in Section VI.C, for some
manufacturers new technologies aimed at reducing aircraft engine
NOX, which were implemented for in-production engines that
were recently built, also resulted in an order of magnitude reduction
in PM in comparison to most in-service engines. Specifically, the
current lean-burn engines and some advanced Rich-Quench-Lean (RQL)
engines developed for the purpose of achieving low NOX
emissions coincidentally provided order of magnitude reductions in PM
emissions in comparison to existing RQL engines.\139\ Other
manufacturers did not develop or implement such technologies that
resulted in such PM reduction, and thus, their recent in-production
aircraft engines are not achieving similar PM control. The final PM
standards are anti-backsliding for these aircraft engines by ensuring
that they will not exceed the final standards in the future. Further,
this information shows that available engine technology includes a wide
range of technologies, and the EPA's final standards are standards that
can be met by all engines expected to be in production by the
implementation date of the PM mass and number standards, January 1,
2023.
---------------------------------------------------------------------------

    \139\ ICAO, 2019: Independent Expert Integrated Technology Goals
Assessment and Review for Engines and Aircraft, Document 10127. It
is found on page 34 of the English Edition of the ICAO Products &
Services 2022 Catalog and is copyright protected; Order No. 10127.
---------------------------------------------------------------------------

    Comment summary:  Some commenters argued that the EPA is not bound
by the Chicago Convention to adopt standards equivalent to ICAO's
standards, and relatedly some commenters asserted the EPA is not
prohibited from adopting standards more stringent than ICAO's
standards. Some comments argued that the EPA cannot allow international
agreements to dictate its domestic regulation of PM from aircraft
engines.
    Response: As explained in the introductory text of Section IV and
in Section VI, and reiterated throughout the responses to comments, the
EPA conducted its independent assessment of the appropriateness of the
ICAO standards for domestic application in the United States and finds
it appropriate to adopt domestic PM standards aligned with the
international PM standards in this action. The EPA agrees that the
United States could adopt standards at a different stringency than
ICAO's, even more stringent standards. Under the terms of the Chicago
Convention, ICAO member States must recognize as valid certificates of
airworthiness issued by other ICAO member States, provided the
requirements under which such certificates were issued are as least as

stringent as the minimum ICAO standards.\140\
---------------------------------------------------------------------------

    \140\ ICAO, 2006: Convention on International Civil Aviation,
Article 33, Ninth Edition, Document 7300/9.
---------------------------------------------------------------------------

    The need for direct cooperation between countries gave rise to
ICAO, an active regulatory body that sets and revises standards. As
described in Section II.B, ICAO's work on the environment focuses
primarily on those problems that benefit most from a common and
coordinated approach on a worldwide basis, namely aircraft noise and
engine emissions. Compliance with ICAO's standards, including its
emission standards, is essential to ensure acceptance by other
countries as people, aircraft, and cargo move in international
commerce. The EPA recognizes nations have authority to vary from ICAO
standards, provided they give the required notice. Also, the EPA has
not concluded that the unique features of the aviation industry
necessitate a policy to never adopt more stringent emission standards
compared to ICAO standards. However, adopting more stringent PM
standards than ICAO's PM standards, which change the approach to
regulating aircraft engine PM emissions, would risk disruption to
international cooperation. The EPA considered the timing of the ICAO PM
mass and number standards for new type design and in-production
engines, which have a January 1, 2023 implementation date. Given the
limited time frame and potential implications of the EPA not adopting a
standard, the EPA has acted reasonably in this rulemaking by giving
significant weight to the value of international harmonization and to
the fact that, in the EPA's judgment, international harmonization would
promote ongoing cooperation to control global pollution of PM.
    Comment summary: Some commenters urged the EPA to withdraw the
proposed rule and issue a proposed rule that would assess the full
range of feasible stringency options and propose emission standards
that reduce aircraft PM emissions.
    Response: The EPA is finalizing the PM standards as proposed.
However, as explained in Section I.C, the EPA remains committed to
analyzing this issue and will continue to work with the United States'
international partners to revisit these standards in the future. We do
not believe it would be appropriate to withdraw the proposed rule and
issue a new proposal for the reasons stated in the preceding
paragraphs.

V. Aggregate PM Inventory Methodology and Impacts

    The PM emissions inventory is presented here to provide information
on the contribution of aircraft engine emissions to local inventories
as context for this regulatory effort. This PM emissions inventory is
from the aviation portion of the EPA's 2017 National Emissions
Inventory (NEI).141 142 143 The NEI contains comprehensive
emissions data for criteria pollutants and hazardous air pollutants for
mobile, point, and nonpoint sources covering both natural and
anthropogenic contribution to the overall national PM emissions
inventory. For this PM rulemaking, we updated the aviation portion of
the PM emissions inventory using newly available measured data reported
for most in-production engines and an improved approximation method for
engines without measurement data, as described in this section.
---------------------------------------------------------------------------

    \141\ 2017 National Emissions Inventory: Aviation Component,
Eastern Research Group, Inc., June 25, 2020, EPA Contract No. EP-C-
17-011, Work Order No. 2-19.
    \142\ See section 3.2 for airports and aircraft related
emissions in the Technical Supporting Document for the 2017 National
Emissions Inventory, January 2021 Updated Release.
    \143\ U.S. EPA, 2017 National Emissions Inventory (NEI) Data.
---------------------------------------------------------------------------

    The inventory is developed from using actual operations at
airports. The number of aircraft operations or landings and takeoffs
affects PM emissions that contribute to the local air quality near
airports. The landing and take-off (LTO) emissions are defined as
emissions between ground level and an altitude of about 3,000 feet.
These LTO emissions directly affect the ground level air quality at the
vicinity of the airport since they are within the local mixing height.
They are composed of emissions during departure operations (taxi-out
movement from gate to runway, aircraft take-off run and climb-out to
3,000 feet), and during arrival operations (approach at or below 3,000
feet down to landing on the ground and taxi-in from runway to gate).
Depending on the meteorological conditions, the emissions will be mixed
with ambient air down to ground level, dispersed, and transported to
areas downwind from the airport with elevated concentration
levels.\144\
---------------------------------------------------------------------------

    \144\ A local air quality ``emissions inventory for aircraft
focuses on the emission characteristics of this source relative to
the vertical column of air that ultimately affects ground level
pollutant concentrations. This portion of the atmosphere, which
begins at the earth's surface and is simulated in air quality
models, is often referred to as the mixing zone'' or mixing height.
(page 137.) The air in this mixing height is completely mixed and
pollutants emitted anywhere within it will be carried down to ground
level. (page 143.) ``The aircraft operations of interest within the
[mixing height] are defined as the [LTO] cycle.'' (page 137.) The
default mixing height in the U.S. is 3,000 feet. (EPA, 1992:
Procedures for Emission Inventory Preparation--Volume IV: Mobile
Sources, EPA420-R-92-009.
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    As described in Section III.A, aircraft PM emissions are composed
of both volatile and non-volatile PM (nvPM) components.\145\ With a
precisely controlled air-fuel mixture, a typical aircraft engine yields
combustion products on the order of 27.6 percent water
(H2O), 72 percent CO2, about 0.02 percent
SOX, and only about 0.4 percent incomplete residual
products. These incomplete residual products can be broken down to 84
percent NOX, 11.8 percent CO, 4 percent unburned
hydrocarbons (UHC), 0.1 percent PM, and trace amounts of other
products.\146\ Although the PM emissions are a small fraction of total
engine exhaust, the composition and morphology of PM are complex and
dynamic. While the emissions certification test procedures focus only
on measuring non-volatile PM (black carbon), our emissions inventory
includes estimates for volatile PM (organic, lubrication oil residues
and sulfuric acid) as well.
---------------------------------------------------------------------------

    \145\ ICAO: 2019, ICAO Environmental Report. A copy of this
document is available in the docket for this rulemaking under
document identification number EPA-HQ-OAR-2019-0660-0022. See pages
100 and 101 for a description of non-volatile PM and volatile PM.
    ``At the engine exhaust, particulate emissions mainly consist of
ultrafine soot or black carbon emissions. Such particles are called
`non-volatile' (nvPM). They are present at the high temperatures at
the engine exhaust and they do not change in mass or number as they
mix and dilute in the exhaust plume near the aircraft. The geometric
mean diameter of these particles is much smaller than
PM2.5 (geometric mean diameter of 2.5 Microns) and ranges
roughly from 15nm to 60nm (0.06 Microns). These are classified as
ultrafine particles (UFP).'' (See page 100.) ``The new ICAO standard
is a measure to control the ultrafine non-volatile particulate
matter emissions emitted at the engine exit[.]'' (See page 101.)
    ``Additionally, gaseous emissions from engines can also condense
to produce new particles (i.e., volatile particulate matter--vPM),
or coat the emitted soot particles. Gaseous emissions species react
chemically with ambient chemical constituents in the atmosphere to
produce the so called secondary particulate matter. Volatile
particulate matter is dependent on these gaseous precursor
emissions. While these precursors are controlled by gaseous
emissions certification and the fuel composition (e.g., sulfur
content) for aircraft gas turbine engines, the volatile particulate
matter is also dependent on the ambient air background
composition.'' (See pages 100 and 101.)
    \146\ European Monitoring and Evaluation Programme/European
Environment Agency, Air Pollutant Emission Inventory Guidebook 2019.
---------------------------------------------------------------------------

A. Aircraft Engine PM Emissions Modeling Methodologies

    This section describes the nvPM approximation method we used in the
proposed rulemaking, the use of newly available measured nvPM data, and

improvement to the nvPM approximation method for the final rulemaking.
1. PM Emission Indices Used in the Rulemaking
    Measured PM data were not available when the EPA first developed
the 2017 inventory. Thus, to calculate the baseline aircraft engine PM
emissions, we used the First Order Approximation Version 3.0 (FOA3)
method defined in the Society of Automotive Engineers (SAE) Aerospace
Information Report, AIR5715.\147\ For nvPM mass, the FOA3 method is
based on an empirical correlation of Smoke Number (SN) values and the
nvPM mass concentrations of aircraft engines. The nvPM mass
concentration (g/m\3\) derived from SN can then be converted into an
nvPM mass emission index (EI) in gram of nvPM per kg fuel using the
method developed by Wayson et al.\148\ based on a set of empirically
determined Air Fuel Ratios (AFR) and engine volumetric flow rates at
the four ICAO LTO thrust settings (see Table IV-1). Subsequently, the
nvPM mass EI can be used to calculate the nvPM mass for the four LTO
modes with engine fuel flow rate and time-in-mode information. As the
name suggests, the FOA3 method is a rough estimate, and it is only for
PM mass (not number).
---------------------------------------------------------------------------

    \147\ SAE Aerospace Information Report, AIR5715, Procedure for
the Calculation of Aircraft Emissions, 2009, SAE International.
    \148\ Wayson R.L., Fleming G.G., Iovinelli R. Methodology to
Estimate Particulate Matter Emissions from Certified Commercial
Aircraft Engines. J Air Waste Management Assoc. 2009 Jan 1; 59(1).
---------------------------------------------------------------------------

    In addition, as described in sections III.A and IV, volatile PM and
nvPM together make up total PM. The FOA3 method for volatile PM is
based on the jet fuel organics \149\ and sulfur content. Since the
total PM is the emission inventory we are estimating for this
rulemaking, we are including the volatile PM emission estimates from
the FOA3 method in our emission inventory.
---------------------------------------------------------------------------

    \149\ In this context, organics refers to hydrocarbons in the
exhaust that coat on existing particles or condense to form new
particles after the engine exit.
---------------------------------------------------------------------------

2. Measured nvPM Emission Indices for Inventory Modeling
    The measurement and reporting of engine EIs allows for improved
accuracy of engine emission inventories. As mentioned in Section
IV.D.2, the regulatory compliance level is based on the amount of
particulate that is directly measured by the instruments. The test
procedures specify a sampling line that can be up to 35 meters long.
This length results in significant particle loss in the measurement
system, on the order of 50 percent for nvPM mass and 90 percent for
nvPM number.\150\ Further the particle loss is size dependent, and thus
the losses will be dependent on the engine operating condition (e.g.,
idle vs take-off thrust), engine combustor design, and technology. To
assess the emissions contribution of aircraft engines for inventory and
modeling purposes, and subsequently for human health and environmental
effects, it is necessary to know the emissions rate at the engine exit.
Thus, the measured PM mass and PM number values must be corrected for
system losses to determine the engine exit emissions rate.
---------------------------------------------------------------------------

    \150\ Annex 16 Vol. II Appendix 8 Note 2.
---------------------------------------------------------------------------

    The EPA led the effort within the SAE E-31 committee to develop the
methodology to correct for system losses. The EPA led the development
of two SAE standards publications, AIR 6504 \151\ and Aerospace
Recommended Practice (ARP) 6481,\152\ describing this methodology to
correct for system losses. Also, the EPA funded and led test campaigns
that verified the methodology.\153\ ICAO has incorporated this same
procedure into Annex 16 Volume II Appendix 8.
---------------------------------------------------------------------------

    \151\ SAE International. 2017. Procedure for the Calculation of
non-volatile Particulate Matter Sampling and Measurement System
Penetration Functions and System Loss Correction Factors. Aerospace
Information Report 6504, Warrendale, PA, October 2017.
    \152\ SAE International. 2019. Procedure for the Calculation of
Non-Volatile Particulate Matter Sampling and Measurement System
Losses and System Loss Correction Factors. Aerospace Recommended
Practice 6481, Warrendale, PA, February 2019.
    \153\ D.B. Kittelson, et al., Experimental verification of
principal losses in a regulatory particulate matter emissions
sampling system for aircraft turbine engines, Aerosol Science &
Technology, 2022, 56, 1, 63-74.
---------------------------------------------------------------------------

    The engine exit emissions rate, which is corrected for system
losses, is specific to each measurement system and to each engine. The
calculation is an iterative function based upon the measured nvPM mass
and nvPM number values and the geometry of the measurement system.
Manufacturers provide the corrected emissions values to the ICAO EEDB
and to the EPA.
    When calculating emissions inventories, these corrected EIs are
used rather than the values used to show compliance with emission
standards as they are more reflective of what is emitted into the
atmosphere. These measured EIs are only for the non-volatile component
of PM, and an approximation method is still required for quantifying
the volatile PM inventory.
3. Improvements to Calculated Emission Indices
    As described in Section V.A, an improved approximation method has
also been developed since the EPA's 2017 NEI was first published. This
new approximation method is needed for modeling PM emissions of in-
service engines that do not have measured PM data. The new version of
the approximation method, known as FOA4, has been developed by CAEP to
improve nvPM mass estimation and to extend the methodology to nvPM
number based on the newly available PM measurement data.\154\ The
simultaneously collected data of nvPM mass concentration and smoke
number from test engines help define a better correlation between nvPM
mass concentration and smoke number.\155\ The FOA4 estimated nvPM mass
concentration tracks closely with FOA3's for some smoke numbers, but it
is much higher for other smoke numbers. Overall, we found that
fleetwide nvPM mass emissions using the new method (FOA4 and measured
data when available) increase by 27 percent over the nvPM mass
emissions reported in 2017 NEI using the FOA3 method. Note that the
data has significant variation at the individual airport level. For the
top airports modeled the effect on total PM ranges from a 3 percent
decrease to a 14 percent increase relative to the modeling in the
proposed rulemaking.
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    \154\ ICAO: Second edition, 2020: Doc 9889, Airport Air Quality
Manual. Order Number 9889. See Attachment D to Appendix 1 of Chapter
3. Doc 9889 can be ordered from ICAO. It is found on page 78 of the
English Edition of the ICAO Products & Services 2022 Catalog and is
copyright protected: Order No. 9889.
    \155\ Agarwal, A. et al., SCOPE11 Method for Estimating Aircraft
Black Carbon Mass and Particle Number Emissions, Environmental
Science & Technology, 2019, DOI: 10.1021/acs.est.8b04060.
---------------------------------------------------------------------------

    Recognizing that the development of the first order approximation
method is not static and continues to evolve, while more accurate
measurement data and better understanding of the underlying mechanisms
will certainly help to improve the estimate further, FOA4 represents
the state of the science today. It has been used to update the nvPM
baseline emission rates for this final rule.
    The calculation of volatile PM has not changed between FOA3 and
FOA4 because no improved data or method has become available to inform
improvements.

B. PM Emission Inventory

    As discussed in the introductory paragraphs of Section V, the PM

emissions inventory used for this rule is from the aviation portion of
the EPA's 2017 National Emissions Inventory
(NEI).156 157 158 The NEI is compiled by the EPA triennially
based on comprehensive emissions data for criteria pollutants and
hazardous air pollutants for mobile, point, and nonpoint sources. The
mobile sources in the NEI include aviation, marine, railroad, on-road
vehicles, and nonroad engines. As described in Section V.A, the
aircraft emission estimates in the EPA's 2017 NEI (or the baseline PM
emissions inventory) are based on the FOA3 method instead of the newly
developed FOA4 or measured PM emissions data. For the final rulemaking,
we have updated the baseline PM emissions inventory based on measured
data reported to the EPA or the European Union Aviation Safety Agency
(EASA) for most in-production engines and FOA4 for engines without
measurement data.
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    \156\ 2017 National Emissions Inventory: Aviation Component,
Eastern Research Group, Inc., June 25, 2020, EPA Contract No. EP-C-
17-011, Work Order No. 2-19.
    \157\ See section 3.2 for airports and aircraft related
emissions in the Technical Supporting Document for the 2017 National
Emissions Inventory, January 2021 Updated Release.
    \158\ U.S. EPA, 2017 National Emissions Inventory (NEI) Data.
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    The aviation emissions developed for the NEI include emissions
associated with airport activities in commercial aircraft, air taxi
aircraft,\159\ general aviation aircraft, military aircraft, auxiliary
power units, and ground support equipment. All emissions from aircraft
with gas turbine engines of rated output greater than 26.7 kN, except
military aircraft, are used in the emissions inventory for this final
rule (which is only a subset of the aviation emissions inventory in the
2017 NEI). To estimate emissions, 2017 activity data by states were
compiled and supplemented with publicly available FAA data. The FAA
activity data included 2017 T-100 \160\ dataset, 2014 Terminal Area
Forecast (TAF) \161\ data, 2014 Air Traffic Activity Data System
(ATADS) \162\ data, and 2014 Airport Master Record (form 5010) \163\
data.\164\ The NEI used the FAA's Aviation Environmental Design Tool
(AEDT) \165\ version 2d to estimate emissions for aircraft that were in
the AEDT database. The NEI used a more general estimation methodology
to account for emissions from aircraft types not available in AEDT by
multiplying the reported activities by fleet-wide average emission
factors of generic aircraft types (or by aircraft category, such as
general aviation or air taxi).\166\
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    \159\ Air taxis fly scheduled service carrying passengers and/or
freight, but they usually are smaller aircraft and operate on a more
limited basis compared to the commercial aircraft operated by
airlines.
    \160\ Title 14--Code of Federal Regulations--Part 241 Uniform
System of Accounts and Reports for Large Certificated Air Carriers.
T-100 Segment (All Carriers)--Published Online by Bureau of
Transportation Statistics.
    \161\ Federal Aviation Administration. Terminal Area Forecast
(TAF).
    \162\ Federal Aviation Administration. ATADS: Airport
Operations: Standard Report.
    \163\ Federal Aviation Administration. 2009. Airport Master
Record Form 5010. Published by GCR & Associates.
    \164\ The rationale for the use of multiple FAA activity
databases is described in the 2017 NEI report (2017 National
Emissions Inventory: Aviation Component, Eastern Research Group,
Inc., June 25, 2020, EPA Contract No. EP-C-17-011, Work Order No. 2-
19. See section 3.2 for airports and aircraft related emissions in
the Technical Supporting Document for the 2017 National Emissions
Inventory, January 2021 Updated Release).
    \165\ AEDT is a software system that models aircraft performance
in space and time to estimate fuel consumption, emissions, noise,
and air quality consequences.
    \166\ See section 4.1.2 of the 2017 National Emissions
Inventory: Aviation Component, Eastern Research Group, Inc., June
25, 2020, EPA Contract No. EP-C-17-011, Work Order No. 2-19.
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    For aircraft PM contribution in 2017 to total mobile PM emissions
in counties and MSAs for the top 25 airports (inventories for aircraft
with engines >26.7 kN), see Figure III-1 and Figure III-2 in Section
III.E.
    We respond to comments on the emissions inventory in Section 7 of
the Response to Comments document.

C. Projected Reductions in PM Emissions

    Due to the technology-following nature of the PM standards, the
final in-production and new type design standards will not result in
emission reductions below current levels of engine emissions. The in-
production standards for both PM mass and PM number, which are set at
levels where all in-production engines meet the standards, will not
affect any in-production engines as shown in Figure IV-1 and Figure IV-
2. Thus, the in-production standards are not expected to produce
emission reductions, beyond the business-as-usual fleet turn over that
would occur in the absence of the standards. The EPA projects that all
future new type design engines will meet the new type design standards.
There are a few in-production engines that do not meet the new type
design standards, but because in-production engines will not be subject
to these new type design standards, engine manufacturers will not be
required to make improvements to these engines to meet the standards.
Therefore, the EPA also does not anticipate emission reductions from
the new type design standards.
    Most of the in-production engines that do not meet the new type
design standards are older engines that already have replacement
engines that will meet the new type design standards. There is only one
newer in-production engine (an engine that recently started being
manufactured) that does not meet the new type design standards, and it
does not currently have a replacement engine. Since the new type design
standards will not apply to in-production engines, the manufacturer of
this engine could continue producing and selling its one in-production
engine that does not meet the new type design standards. Market forces
might drive the manufacturer of this in-production engine to make some
improvements to meet the new type design standards, or chose to bring
forward its next generation new type design engine to the market a few
years earlier than currently planned. The manufacturer has announced
plans to develop the next generation of engines to improve emission
levels compared to the previous generation of
engines.167 168 We expect that these next generation engines
from this manufacturer will meet the new type design standards. Further
details on market forces are provided in Section VI.A. In conclusion,
when considering the final new type design standards in the context of
the in-production engines that already have a replacement engine or the
one in-production engine that does not, the EPA expects no emission
reductions from the new type design standards.
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    \167\ https://www.rolls-royce.com/products-and-services/civil-aerospace/future-products.aspx#/; last accessed on October 31, 2022.
    \168\ Aviation Week, Rolls-Royce Considers UltraFan Development
Pause, Guy Norris, January 4, 2021.
---------------------------------------------------------------------------

    All website addresses for references cited in this section are
provided in a memorandum to the docket.\169\
---------------------------------------------------------------------------

    \169\ U.S. EPA, Yen, D. Memorandum to Docket EPA-HQ-OAR-2019-
0660, ``website addresses for references cited in Section V of the
Preamble for Control of Air Pollution from Aircraft Engines:
Emission Standards and Test Procedures; Final Rule,'' November 9,
2022.
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VI. Technological Feasibility and Economic Impacts

    As described in Section IV, we are adopting PM mass concentration,
PM mass, and PM number standards that match ICAO's standards. As
discussed in Section V.C, for in-production aircraft engines, the 2017
ICAO PM maximum mass concentration standard and the 2020 ICAO PM mass
and number

standards are set at emission levels where all in-production engines
meet these standards. Thus, there will not be costs or emission
reductions associated with the final standards for in-production
engines. For new type design engines, the 2020 ICAO PM mass and number
standards are set at more stringent emission levels compared to the PM
mass and number standards for in-production engines, but nearly all in-
production engines meet these new type design standards. In addition,
in-production engines will not be required to meet these new type
design standards. Only new type design engines will need to comply with
the new type design standards. The EPA projects that all new type
design engines entering into service into the future will meet these PM
mass and number standards. Thus, the EPA expects that there will not be
costs and emission reductions from the standards for new type design
engines, although the standards would likely prevent backsliding for
some new type design engines. In addition, following this final
rulemaking for the PM standards, the FAA will issue a rulemaking to
enforce compliance to these standards, and any anticipated
certification costs for the PM standards will be accounted for in the
FAA rulemaking.
    As described in Section I.B.2, when developing new emission
standards, ICAO/CAEP seeks to capture the technological advances made
in the control of emissions through the adoption of anti-backsliding
standards reflecting the current state of technology. The final
standards that match ICAO's standards are anti-backsliding standards
that prevent aircraft engine PM levels from increasing beyond their
current levels. As discussed in Section IV.F.2, in that regard, PM mass
and number are currently unregulated from aircraft engines and the
standards finalized in this action represent a new regulatory backstop
of those two new standards. Further, all three PM standards will
prevent backsliding by ensuring that all new type design and in-
production aircraft engines will not exceed those regulatory levels in
the future.
    As described in Section IV.F.2, for some manufacturers, new
technologies aimed at reducing aircraft engine NOX, which
were implemented for in-production engines that were recently built,
also resulted in significant PM reductions. Other manufacturers did not
develop or implement technologies that resulted in such PM reductions.
In either case, the final PM standards ensure that PM emissions do not
increase beyond the levels of these PM standards. In addition, the
final PM standards send an important signal to manufacturers that they
need to consider PM emissions when producing aircraft engines now and
going forward--with the anticipation that more stringent PM standards
will be adopted by ICAO/CAEP in the future.
    U.S. manufacturers could be at a significant disadvantage if the
United States fails to adopt standards by the international
implementation date, January 1, 2023. Also, given the short timeframe
from this final action and the international implementation date, there
would not be enough lead time for manufacturers to respond to more
stringent standards that would require them to develop and implement
new technologies.

A. Market Considerations

    Aircraft and aircraft engines are sold around the world, and
international aircraft emission standards help ensure the worldwide
acceptability of these products. Aircraft and aircraft engine
manufacturers make business decisions and respond to the international
market by designing and building products that conform to ICAO's
international standards. However, ICAO's standards need to be
implemented domestically for products to prove such conformity.
Domestic action through the EPA rulemaking and subsequent FAA
rulemaking enables U.S. manufacturers to obtain internationally
recognized U.S. certification, which for the final PM standards will
ensure type certification consistent with the requirements of the
international PM emission standards. This is important, as compliance
with the international standards (via U.S. type certification) is a
critical consideration in aircraft manufacturer and airlines'
purchasing decisions. By implementing the requirements in the United
States that align with ICAO standards, any question regarding the
compliance of aircraft engines certificated in the United States will
be removed. The rulemaking will help ensure the acceptance of U.S.
aircraft engines by member States, aircraft manufacturers, and airlines
around the world. Conversely, without this domestic action, U.S.
aircraft engine manufacturers would likely be at a competitive
disadvantage compared with their international competitors.
    In considering the aviation market, it is important to understand
that the international PM emission standards were predicated on
demonstrating ICAO's concept of technological feasibility; i.e., that
manufacturers have already developed or are developing improved
technology that meets the ICAO PM standards, and that the new
technology will be integrated in aircraft engines throughout the fleet
in the time frame provided before the standards' effective date.
Therefore, the EPA projects that these final standards will impose no
additional burden on manufacturers.

B. Conceptual Framework for Technology

    The long-established ICAO/CAEP terms of reference were taken into
account when deciding the international PM standards, principal among
these being technical feasibility. For the ICAO PM standard setting,
technical feasibility refers to any technology demonstrated to be safe
and airworthy proven to Technology Readiness Level \170\ (TRL) 8 and
available for application over a sufficient range of newly certificated
aircraft.\171\ This means that the analysis that informed the
international standard considered the emissions performance of aircraft
engines assumed to be in-production on the ICAO/CAEP implementation
date for the PM mass and number standards, January 1, 2023.\172\ The
analysis included the current in-production fleet and engines scheduled
for entry into the fleet by this date. (ICAO/CAEP's analysis was
completed in 2018 and considered at the February 2019 ICAO/CAEP
meeting.)
---------------------------------------------------------------------------

    \170\ TRL is a measure of Technology Readiness Level. CAEP has
defined TRL8 as the ``actual system completed and `flight qualified'
through test and demonstration.'' TRL is a scale from 1 to 9, TRL1
is the conceptual principle, and TRL9 is the ``actual system `flight
proven' on operational flight.'' The TRL scale was originally
developed by NASA. ICF International, CO2 Analysis of CO2-Reducing
Technologies for Aircraft, Final Report, EPA Contract Number EP-C-
12-011, see page 40, March 17, 2015.
    \171\ ICAO, 2019: Report of the Eleventh Meeting, Montreal, 4-15
February 2019, Committee on Aviation Environmental Protection,
Document 10126, CAEP/11. It is found on page 27 of the English
Edition of the ICAO Products & Services 2022 Catalog and is
copyright protected: Order No. 10126. The statement on technological
feasibility is located in Appendix C of Agenda Item 3 of this report
(see page 3C-4, paragraph 2.2).
    \172\ Id., starting on page 3C-1.
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C. Technological Feasibility

    The EPA and FAA participated in the ICAO analysis that informed the
adoption of the international aircraft engine PM emission standards. A
summary of that analysis was published in the report of ICAO/CAEP's
eleventh meeting (CAEP/11),\173\ which occurred in February 2019.
However, due to the commercial sensitivity of much of the data used in
the ICAO analysis, the publicly available, published version of the
ICAO report of the CAEP/11 meeting

only provides limited supporting data for the ICAO analysis. Separately
from this ICAO analysis and the CAEP/11 meeting report, information on
technology for the control of aircraft engine PM emissions is provided
in an Independent Expert Review document on technology goals for
engines and aircraft, which was published in 2019.\174\ Although this
ICAO document is primarily used for setting goals, and is not directly
related to ICAO's adoption of the PM emission standards, information
from the Independent Expert Review is helpful in understanding the
state of aircraft engine technology.
---------------------------------------------------------------------------

    \173\ Id.
    \174\ ICAO, 2019: Independent Expert Integrated Technology Goals
Assessment and Review for Engines and Aircraft, Document 10127. It
is found on page 34 of the English Edition of the ICAO Products &
Services 2022 Catalog and is copyright protected; Order No. 10127.
---------------------------------------------------------------------------

    The 2019 ICAO Independent Expert Review document indicates that new
technologies aimed at reducing aircraft engine NOX also
resulted in an order of magnitude reduction in non-volatile PM (nvPM)
mass and nvPM number in comparison to most in-service engines.\175\ (As
described in Section IV.D.2, only nvPM emissions will be measured in
the final test procedure for the final standards.) Specifically, the
current lean-burn engines and some advanced Rich-Quench-Lean (RQL)
engines 176 177 developed for the purpose of achieving low
NOX emissions coincidentally provide order of magnitude
reductions in nvPM emissions in comparison to existing RQL
engines.\178\ However, achieving these levels of nvPM emissions is more
difficult for physically smaller-sized engines due to technical
constraints.\179\ In addition, some previous generation engines that
are in production meet the final new type design standards, which match
the ICAO standards, with considerable margin. When considering the nvPM
emission levels for current in-production engines and those engines
expected to be in production by the effective date of the ICAO
standard, January 1, 2023, the lean-burn, advanced RQL, and some
previous generation technologies (with relatively low levels of nvPM
emissions) of many of the engines demonstrate that the final standards,
which match ICAO standards, are technologically feasible.
---------------------------------------------------------------------------

    \175\ See id. at 8.
    \176\ See id. at 47 and 48. For lean-burn engines (or
combustors), enough air is introduced with the fuel from the
injector so it is never overall rich. For aviation combustors, the
fuel is not premixed and pre-vaporized, and in the microscopic
region around each droplet, the mixture can be near to
stoichiometric. Yet, the mixture remains lean throughout the
combustor, and the temperature does not approach the stoichiometric
value. For a lean-burn combustor, the peak temperatures are not as
high, and thus, the NOX is low.
    \177\ See id. at 47. For Rich-Quench-Lean (RQL) engines (or
combustors), the fuel first burns rich, and thus, there is little
oxygen free to form NOX. Dilution air is introduced to
take the mixture as quickly as possible through the stoichiometric
region (when it briefly becomes very hot) to a cooler, lean state.
    \178\ See id. at 57 and 58. From previous generation rich-burn
to lean-burn technology, an order of magnitude improvement in nvPM
mass and nvPM number is likely for the LTO cycle. Also, potentially,
an order of magnitude improvement in nvPM mass and nvPM number could
be achieved for the LTO cycle from previous generation rich-burn to
advanced rich-burn combustor technology.
    \179\ For example, the relatively small combustor space and
section height of these engines creates constraints on the use of
low NOX combustor concepts, which inherently require the
availability of greater flow path cross-sectional area than
conventional combustors. Also, fuel-staged combustors need more fuel
injectors, and this need is not compatible with the relatively
smaller total fuel flows of lower thrust engines. (Reductions in
fuel flow per nozzle are difficult to attain without having clogging
problems due to the small sizes of the fuel metering ports.) In
addition, lower thrust engine combustors have an inherently greater
liner surface-to combustion volume ratio, and this requires
increased wall cooling air flow. Thus, less air will be available to
obtain acceptable turbine inlet temperature distribution and for
emission control. See 77 FR 36342, 36353 (June 18, 2012).
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D. Costs Associated With the Rule

    The EPA does not anticipate new technology costs (non-recurring
costs) due to the final rule. As described in the introductory
paragraph of Section VI, since all in-production engines meet the in-
production standards and nearly all in-production engines meet these
new type design standards, we project there will not be costs, nor
emission reductions, from the final rule. Also, because current in-
production engines will not be required to make any changes under this
final rule, there will not be any adverse impact on noise and safety of
these engines. Likewise, the noise and safety of future type designs
should not be adversely impacted by compliance with these final new
type design standards since all manufacturers currently have engines
that meet that level.
    Following this final rulemaking for the PM standards, the FAA will
issue a rulemaking to enforce compliance to these standards, and any
anticipated certification costs for the PM standards will be estimated
by FAA.
    As described in Section VI.A, manufacturers have already developed
or are developing technologies to respond to ICAO standards that are
equivalent to the final standards, and they will comply with the ICAO
standards in the absence of U.S. regulations. Also, domestic
implementation of the ICAO standards will potentially provide for cost
savings to U.S. manufacturers since it will enable them to certify
their aircraft engine (via subsequent FAA rulemaking) domestically
instead of having to certificate with a foreign authority (which will
occur without this EPA rulemaking). If the final PM standards, which
match the ICAO standards, are not ultimately adopted in the United
States, U.S. civil aircraft engine manufacturers will have to certify
to the ICAO standards at higher costs because they will have to move
their entire certification program(s) to a non-U.S. certification
authority.\180\ Any potential costs or cost savings related to
certification will be estimated by FAA.
---------------------------------------------------------------------------

    \180\ In addition, European authorities charge fees to aircraft
engine manufacturers for the certification of their engines, but FAA
does not charge fees for certification.
---------------------------------------------------------------------------

    For the same reasons there will be no non-recurring and
certification costs for the rule, there also will be no recurring costs
(recurring operating and maintenance costs) for the rule. The elements
of recurring costs include additional maintenance, material, labor, and
tooling costs.
    As described in Section IV.E, the EPA is formally incorporating the
PM aspects of the existing information collection request (ICR) into
the CFR (or regulations) in 40 CFR 1031.150 and 1031.160. This action
will not create a new requirement for the manufacturers of aircraft
engines. Instead, it will simply incorporate the existing reporting
requirements into the CFR for ease of use by having all the reporting
requirements readily available in the CFR. Thus, this action will not
create new costs.

E. Summary of Benefits and Costs

    The final standards match the ICAO standards, and as discussed in
Section II.C and Section IV.F.1 of this preamble, ICAO intentionally
established its standards at a level which is technology following. The
final rule takes an appropriate step in controlling aircraft engine PM
emissions and prevents backsliding by ensuring that all in-production
and new type design engines have at least the PM emission levels of
today's aircraft engines. Additionally, this final rule maintains
consistency or harmonizes with the international standards and meets
the United States' treaty obligations under the Chicago Convention.
Also, it allows U.S. manufacturers of covered aircraft engines to
remain competitive in the global marketplace by ensuring the acceptance
of their engines worldwide (which benefits U.S. manufacturers and
consumers), provides uniformity and

certainty to U.S. manufacturers as they become familiar with the new
approach to adhering to these PM standards and test procedures,\181\
and prevents U.S. manufacturers from having to seek PM emissions
certification from an aviation certification authority of another
country (not the FAA) to market and operate their aircraft engines
internationally. All engines currently manufactured will meet the ICAO
in-production standards, and nearly all these same engines will meet
the new type design standards--even though these new type design
standards do not apply to in-production engines. Therefore, as further
described in the introductory paragraph of Section VI and in Section
VI.C, there will be no costs and no emission reductions from complying
with these final standards.
---------------------------------------------------------------------------

    \181\ The final standards change the approach to regulating
aircraft engine PM emissions from past smoke measurements to the
measurement of mass and number for the first time for U.S.
manufacturers.
---------------------------------------------------------------------------

VII. Technical Amendments

    In addition to the PM-related regulatory provisions discussed in
Section IV, the EPA is finalizing technical amendments to the
regulatory text that apply more broadly than to just the new PM
standards. First, the EPA is migrating the existing aircraft engine
emissions regulations from 40 CFR part 87 to a new 40 CFR part 1031.
Along with this migration, the EPA is restructuring the regulations to
allow for better ease of use and allow for more efficient future
updates. The EPA is also deleting some regulatory provisions and
definitions that are unnecessary, as well as making several other minor
technical amendments to the regulations. Finally, the EPA is also
revising 40 CFR part 87 to provide continuity during the transition of
40 CFR part 87 to 40 CFR part 1031. In this final rule, the EPA did not
reexamine or reopen the substantive provisions of 40 CFR part 87 that
were merely migrated to the new 40 CFR part 1031 and streamlined or the
substantive provisions of 40 CFR part 1030 and 40 CFR part 1031 beyond
those specially discussed in the proposed rule. Any comments we
received on the substance of the provisions migrated from 40 CFR part
87 to 40 CFR part 1031 provisions, as opposed to comments pointing out
typos or inadvertent impacts on substantive provisions caused by the
regulatory streamlining, are beyond the scope of this rulemaking.

A. Migration of Regulatory Text to New Part

    In the 1990s, the EPA began an effort to migrate all
transportation-related air emissions regulations to new parts, such
that all mobile source regulations are contained in a single group of
contiguous parts of the CFR. In addition to the migration, that effort
has included clarifications to regulations and improvements to the ease
of use through plain language updates and restructuring. To date, the
aircraft engine emission regulations contained in 40 CFR part 87 are
the only mobile source emission regulations which have not undergone
this migration and update process.
    The current 40 CFR part 87 was initially drafted in the early 1970s
and has seen numerous updates and revisions since then. This has led to
a set of aircraft engine emission regulations that is difficult to
navigate and contains numerous unnecessary provisions. Further, the
current structure of the regulations would make the adoption of the PM
standards finalized in this document, as well as any future standards
the EPA may adopt, difficult to incorporate.
    Therefore, the EPA is migrating the existing aircraft engine
regulations from 40 CFR part 87 to a new 40 CFR part 1031, directly
after the airplane GHG standards contained in 40 CFR part 1030. In the
process, the EPA is restructuring, streamlining, and clarifying the
regulatory provisions for ease of use and to facilitate more efficient
future updates. Finally, the EPA is deleting unnecessary regulatory
provisions, which are discussed in detail in the next section. This
regulatory migration and restructuring effort is not intended to change
any substantive provision of the existing regulatory provisions.
    As noted in the amendatory instructions in the regulations, the EPA
is making this transition effective on January 1, 2023. The new 40 CFR
part 1031 will become effective (i.e., be incorporated into the Code of
Federal Regulations) 30 days following the publication of this final
rule in the Federal Register. However, the applicability language in 40
CFR 1031.1 indicates that the new 40 CFR part 1031 will apply to
engines subject to the standards beginning January 1, 2023. Prior to
January 1, 2023, the existing 40 CFR part 87 will continue to apply. On
January 1, 2023, the existing 40 CFR part 87 will be replaced with a
significantly abbreviated version of 40 CFR part 87 whose sole purpose
will be to direct readers to the new 40 CFR part 1031. Additionally, a
reference in the current 40 CFR part 1030 to 40 CFR part 87 will be
updated to reference 40 CFR part 1031 at that time. The purpose of the
abbreviated 40 CFR part 87 is to accommodate any references to 40 CFR
part 87 that currently exist in the type certification documentation
and advisory circulars issued by the FAA, as well as any other
references to 40 CFR part 87 that currently exist elsewhere. Since it
would be extremely difficult to identify and update all such documents
prior to January 1, 2023, the EPA is instead adopting language in 40
CFR part 87 that simply states the provisions relating to a particular
section of 40 CFR part 87 apply as described in a corresponding section
of the new 40 CFR part 1031.
    The EPA received a comment regarding some existing equations being
incorrectly migrated from 40 CFR part 87 to the new 40 CFR part 1031.
Specifically, the equations in the proposed 40 CFR 1031.40(a)(1),
1031.50(a)(1), and 1031.90(a)(1), (b) and (c) contained terms that
should have been exponents but were instead expressed as multiplicative
terms. Given that the EPA's stated intent with the proposed migration
from 40 CFR part 87 to 40 CFR part 1031 was to move, restructure,
streamline and clarify the existing regulations without changing the
underlying regulatory requirements, the equations contained in the
paragraphs in 40 CFR part 1031 should have aligned with the
corresponding equations in 40 CFR part 87. Thus, these equations in 40
CFR part 1031 have been accordingly corrected in this final rule.

B. Deletion of Unnecessary Provisions

    As previously mentioned, the existing aircraft engine emission
regulations contain some unnecessary provisions which the EPA is
deleting. These deletions include transitional exemption provisions
that are no longer available, several definitions, and some unnecessary
language regarding the Secretary of the Department of Transportation,
as detailed in the following paragraphs.
    The EPA is not migrating the current 40 CFR 87.23(d)(1) and (3) to
the new 40 CFR part 1031. Both these paragraphs contain specific phase-
in provisions available for a short period after the Tier 6
NOX standards began to apply, and their availability as
compliance provisions ended on August 31, 2013. Thus, they are no
longer needed. It should be noted that while the EPA is effectively
deleting these provisions by not migrating them to the new 40 CFR part
1031, the underlying standards referred to in these provisions (i.e.,
the Tier 4 and 6 NOX standards) remain unchanged. Thus, the
underlying certification basis for any engines

certificated under these provisions will remain intact.
    The EPA is also deleting several definitions from the current 40
CFR part 87 as it is migrated to the new 40 CFR part 1031 for two
reasons. First, in the effort to streamline and clarify the
regulations, some of these definitions have effectively been
incorporated directly into the regulatory text where they are used,
making a stand-alone definition unnecessary. Second, some of these
definitions are simply not needed for any regulatory purpose and are
likely artifacts of previous revisions to the regulations (e.g., where
a regulatory provision was deleted but the associated definition was
not).
    The definitions that the EPA is deleting and the reasons for the
deletions are listed in Table VII-1.

  Table VII-1--List of Terms for Which Definitions Will Be Deleted From
                                 the CFR
------------------------------------------------------------------------
                  Term                         Reason for deletion
------------------------------------------------------------------------
Act....................................  Not used in the regulatory
                                          text.
Administrator..........................  No longer needed as not used in
                                          revised and streamlined
                                          regulatory text.
Class TP...............................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Class TF...............................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Class T3...............................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Class T8...............................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Class TSS..............................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Commercial aircraft....................  No longer needed as not used in
                                          revised and streamlined
                                          regulatory text.
Commercial aircraft gas turbine engine.  No longer needed as not used in
                                          revised and streamlined
                                          regulatory text.
Date of introduction...................  Unnecessary definition that is
                                          not used in existing
                                          regulatory text and not needed
                                          in revised regulatory text.
Engine.................................  For regulatory purposes,
                                          definition of engine not
                                          needed given existing
                                          definitions of Aircraft
                                          engine, Engine model, and
                                          Engine sub-model.
In-use aircraft gas turbine engine.....  No longer needed in light of
                                          deletion of unnecessary
                                          provisions and technical
                                          amendments to fuel venting
                                          requirements.
Military aircraft......................  Not needed as regulatory text
                                          applies to commercial engines.
Operator...............................  No longer needed as not used in
                                          revised and streamlined
                                          regulatory text.
Production cutoff or the date of         No longer needed with deletion
 production cutoff.                       of unnecessary exemption
                                          provisions and streamlining of
                                          exemption regulatory text.
Tier 0.................................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Tier 2.................................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Tier 4.................................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Tier 6.................................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
Tier 8.................................  No longer needed as definition
                                          was effectively incorporated
                                          into regulatory text during
                                          migration.
U.S.-registered aircraft...............  Unnecessary term that is not
                                          used in the regulatory text.
------------------------------------------------------------------------

    The EPA is also not migrating the current 40 CFR 87.3(b) to the new
40 CFR part 1031, which in effect results in its deletion. This
paragraph is simply a restatement of an obligation directly imposed
under the Clean Air Act that the Secretary shall issue regulations to
assure compliance with the regulations issued under the Act. This is
not a regulatory requirement related to the rest of the part, and as
such it is not needed in 40 CFR part 1031.

C. Other Technical Amendments and Minor Changes

    In addition to the migration of the regulations to a new part and
the removal of unnecessary provisions just discussed, the EPA is
adopting some minor technical amendments to the regulations.
    The EPA is adding definitions for ``Airplane'' and ``Emission
index.'' Both these terms are used in the current aircraft engine
emissions regulations, but they are currently undefined. The new
definitions will help provide clarity to the provisions that utilize
those terms.
    The EPA is modifying the definitions for ``Exception'' and
``Exemption.'' The current definitions of these terms in 40 CFR 87.1 go
beyond simply defining the terms and contain what could more accurately
be described as regulatory requirements stating what provisions an
excepted or exempted engine must meet. These portions of the
definitions, which are more accurately described as regulatory
requirements, are being moved to the introductory text in 1031.15 and
1031.20, as applicable. These changes are in no way intended to change
any regulatory requirement applicable to excepted or exempted engines.
Rather, they are intended simply to more clearly separate definitions
from the related regulatory requirements.
    The EPA is not migrating the existing 40 CFR 87.42(d) to the new 40
CFR part 1031, which in effect results in the deletion of this
provision. This paragraph related to the annual production report
regards the identification and treatment of confidential business
information (CBI) in manufacturers' annual production reports. The EPA
is instead relying on the existing CBI regulations in 40 CFR 1068.10
(as referenced in 40 CFR 1031.170). This change will have no impact on
the ability of manufacturers to make claims of CBI, or in the EPA's
handling of such claims. However, it will assure a more consistent
treatment of CBI across mobile source programs.
    The EPA is adopting a minor change to the existing emission
requirements

for spare engines, as found in the existing 40 CFR 87.50(c)(2). In the
regulatory text for 40 CFR 1031.20(a), the EPA is deleting the existing
provision that a spare engine is required to meet standards applicable
to Tier 4 or later engines (currently contained in 40 CFR 87.50(c)(2)).
The EPA is retaining and migrating to 40 CFR part 1031 the requirement
in 40 CFR 87.50(c)(3) such that a spare engine will need to be
certificated to emission standards equal to or lower than those of the
engines they are replacing, for all regulated pollutants. This deletion
of 40 CFR 87.50(c)(2) aligns with ICAO's current guidance on the
emissions of spare engines and is consistent with U.S. efforts to
secure the highest practicable degree of uniformity in aviation
regulations and standards. The EPA does not believe this change will
have any impact on current industry practices. Deleting the provision
currently in 40 CFR 87.50(c)(2) will leave in place the requirement
that any new engine manufactured as a spare will need to be at least as
clean as the engine it is replacing (as stated in the current 40 CFR
87.50(c)(3)), but with no requirement that it meet standards applicable
to Tier 4 or later engines. Thus, under this deletion a new spare
engine could, in theory, be manufactured that only met pre-Tier 4
standards. The Tier 4 standards became effective in 2004, so the
deletion will only impact spare engines manufactured to replace engines
manufactured roughly before 2004. It is extremely unlikely that a
manufacturer would build a new engine as a replacement for such an old
design as it would be very disruptive to the manufacturing of current
designs for new aircraft. Rather, it is common practice that spares for
use in replacing older engines would not be newly manufactured engines
of an old design, but engines that have been taken from similar
aircraft that have been retired. The EPA does not believe that any
engines would be manufactured to pre-Tier 4 designs for use as spare
engines given current practices. Thus, the EPA does not believe that
this effective deletion of 40 CFR 87.50(c)(2) for the purposes of
uniformity will have any practical impact on current industry
practices.
    The EPA is aligning the applicability of smoke number standards for
engines used in supersonic airplanes with ICAO's applicability. The EPA
adopted emission standards for engines used on supersonic airplanes in
2012.\182\ Those standards were equivalent to ICAO's existing standards
with one exception. ICAO's emission standards fully apply to all
engines to be used on supersonic airplanes, regardless of rated output.
In an apparent oversight, the EPA only applied the smoke number
standards to engines of greater than or equal to 26.7 kN rated output
in that 2012 action. Thus, the EPA is applying smoke number standards
to include engines below 26.7 kN rated output for use on supersonic
airplanes which are equivalent to ICAO's provisions. This change is
consistent with U.S. efforts to secure the highest practicable degree
of uniformity in aviation regulations and standards and will have no
practical impact on engine manufacturers. The EPA is currently unaware
of any engines in production which could be used on supersonic
airplanes, and those being developed for application to future
supersonic airplanes are expected to be well above 26.7 kN rated
output, and thus, they will be covered by the existing smoke number
standard. Throughout its regulations, the EPA is aligning with ICAO
regarding a common rated output threshold for emission regulations. The
applicability and stringency of several aircraft engine emission
standards can be different depending on whether an engine's rated
output is above or below 26.7 kN. In the ICAO regulations, the
threshold is consistently stated as either greater than, or less than
or equal to 26.7 kN. In the current 40 CFR part 87, the equal to
portion of the threshold is applied inconsistently. In some cases, it
is expressed as less than, and greater than or equal to. In other
cases, it is expressed as greater than, and less than or equal to. The
EPA is making all instances in the new 40 CFR part 1031 consistent with
ICAO, i.e., greater than, and less than or equal to. As there are no
current engines with a rated output of exactly at 26.7 kN, this change
will have no practical impact. However, it is consistent with U.S.
efforts to secure the highest practicable degree of uniformity in
aviation regulations and standards.
---------------------------------------------------------------------------

    \182\ 77 FR 36342 (June 18, 2012).
---------------------------------------------------------------------------

    The EPA is incorporating by reference Appendix 1 of ICAO Annex 16,
Volume II. This appendix deals with the determination of a test
engine's reference pressure ratio, and its exclusion from the U.S.
regulations was an oversight. Other Annex 16, Volume II appendices
which contain test procedures, fuel specifications, and other
compliance-related provisions have been incorporated by reference into
the U.S. regulations for many years, and it is important to correct
this oversight so the complete testing and compliance provisions are
clear.
    The EPA is streamlining, restructuring, and updating the exemption
provisions currently in 40 CFR 87.50. First, this section contains
provisions regarding exemptions, exceptions, and annual reporting
provisions relating to exempted and excepted engines. The EPA is
migrating the exceptions section concerning spare engines (40 CFR
87.50(c)) to 40 CFR 1031.20(a), with the changes discussed in the
preceding paragraphs. The provisions regarding the annual reporting of
exempted and excepted engines are being incorporated into the new
annual reporting 40 CFR 1031.150. These reporting provisions otherwise
remain unchanged. Section 87.50(a), regarding engines installed on new
aircraft, and 40 CFR 87.50(b), regarding temporary exemptions based on
flights for short durations at infrequent intervals, are being migrated
to a new 40 CFR 1031.15. The temporary exemptions provisions remain
unchanged, with the exception of adding ``of Transportation'' after
``Secretary'' in 40 CFR 1031.15(b)(4) to improve clarity. The changes
to the exemptions for engines installed on new aircraft are a bit more
extensive, as discussed in the next paragraph.
    In 2012, the EPA adopted new exemption provisions specifically to
provide flexibility during the transition to Tier 6 and Tier 8
NOX standards.\183\ These provisions were only available
through December 31, 2016, and they are being deleted in this action.
However, during the adoption of those transitional flexibilities, the
EPA inadvertently replaced the existing exemption provisions with the
new transitional provisions rather than appending the transitional
provisions to the existing ones. This left 40 CFR 87.50 with no general
exemption language, only those provisions specific to the newly adopted
NOX standards. Given that the transitional NOX
exemption provisions have expired and are now obsolete, the EPA is
deleting them rather than migrating them to the new 40 CFR 1031.15. The
EPA is further restoring the general exemption provisions that were
inadvertently removed in 2012. In a recent action which established GHG
standards for airplanes, the EPA adopted much more streamlined
exemption provisions for airplanes in consultation with the FAA.\184\
The EPA is adopting similarly streamlined general exemption provisions
for aircraft engines as well, as contained in 40 CFR 1031.15(a).
---------------------------------------------------------------------------

    \183\ 77 FR 36342 (June 18, 2012).
    \184\ 86 FR 2136 (January 11, 2021).
---------------------------------------------------------------------------

    The EPA is adopting some changes relative to the prohibition on
fuel venting. The fuel venting standard is

intended to prevent the discharge of fuel to the atmosphere following
engine shutdown, as explicitly stated in 40 CFR 87.11(a). The existing
definition for fuel venting emissions in 40 CFR 87.1 defines fuel
venting emissions as fuel discharge during all normal ground and flight
operations. As the standard section itself limits the applicability
only to venting that occurs following engine shutdown, consistent with
ICAO's fuel venting provisions, the EPA is deleting the definition for
fuel venting emissions as both unnecessary and contradictory to the
actual requirement.
    The EPA is adding the word `liquid' in front of the phrase ``fuel
emissions'' in 40 CFR 1031.30(b)(2). That phrase has been interpreted
internationally in significantly different ways. Some have interpreted
the word ``emissions'' to mean any emission of pollutants from the
combustion process. The EPA's rule that promulgated the requirement to
control fuel venting emissions, however, dates to 1973 and was intended
to address the issue of liquid fuel being released from an aircraft
engine after engine shutdown when no combustion processes are
occurring.\185\ This term addresses both liquid fuel that reaches the
ground, and liquid fuel released from the engine after shutdown that