standards.\52\ As outlined in this Executive Summary and discussed in
Sections III and IV, we are proposing to significantly lower
NOX emission standards and to significantly increase the
regulatory useful life for heavy-duty on highway engines, which would
require manufactures to develop and produce additional engine and
aftertreatment technology. Due to the combination of lower (more
stringent) numeric standards and longer useful periods included in our
proposal, we are requesting comment on whether

EPA should finalize in-use standards that are 40 to 100 percent higher
than the proposed Option 1 standards for MY 2027 to MY 2033 engines.
---------------------------------------------------------------------------

    \52\ See 81 FR 23414 (April 28, 2014).
---------------------------------------------------------------------------

    In Section IV we detail our requests for comment on a number of
topics related to our proposed lengthened useful life and warranty
periods, as well as other compliance provisions and flexibilities. For
instance, we are requesting stakeholder input on our proposed useful
life and warranty periods, as well as the range of options covered by
the proposed Options 1 and 2, or other alternatives outside of that
range. In addition to the proposed warranty periods, we request comment
on other approaches to warranty, such as graduated warranty phases,
that may similarly ensure long-term in-use emission performance with a
smaller impact on the purchase price. We further request comment on our
proposed provisions to increase the likelihood that emission controls
will be maintained properly through more of the service life of heavy-
duty engines (e.g., revise inducement strategies, improve
serviceability). In addition, we are interested in stakeholder input on
our proposed approaches for the durability demonstration that
manufacturers are required to include their application for
certification (see Section IV.F for details). We are also interested in
stakeholder input on our proposed requirements for manufacturers
choosing to generate NOX emission credits from BEVs or
FCEVs, as well as whether EPA should consider for this final rule, or
other future rules, restrictions for NOX emission credits in
the longer term (e.g., beyond MY 2031) (See Section IV.I for details).
    Throughout Sections III and IV, we discuss areas where our proposal
differs from the California Air Resources Board (CARB) Heavy-Duty
Omnibus Rulemaking, and request comment on our proposal, including
whether it is appropriate to harmonize the federal and CARB regulatory
programs more in light of the authority and requirements of CAA section
202, and the benefits or challenges if EPA were to finalize particular
aspects of its program that are or are not fully aligned with the
Omnibus.
    There are also several topics that we are requesting comment on
that relate to the analyses that support our proposal. For instance, we
are interested in stakeholder input on our approach for estimating
emission reductions from lengthening useful life and warranty periods
(see Section VI for details). We are also interested in comments on our
estimate of repair costs for emission control system components (see
Section V for details). We request comment on the method we outline to
estimate potential impacts of a proposed regulation on heavy-duty
vehicle sales; we also request comment on approaches to estimate
employment impacts attributable to the proposed rule (see Section X for
details).
    We are also interested in input from environmental justice
stakeholders and underserved and overburdened communities, including
children's health stakeholders, regarding the need for revised
standards and how heavy-duty vehicles affect communities (see Section
II); the air quality improvements we project from this proposal and how
they are distributed (see Section VII); and ways the proposal could be
improved to advance environmental protection for all people, including
people of color, low-income communities, and those who live near
highways or in heavily trafficked areas with frequent truck congestion
and idling, such as ports.
    In Section XI, we request comment in a number of areas related to
the proposed updates to the HD GHG Phase 2 program for certain heavy-
duty vehicles that are shifting to zero-emission vehicles. We are
considering whether it would be appropriate in the final rule to
increase the stringency of the standards even more than what we
propose. Therefore, we request information on heavy-duty electric
vehicle sales projections, including for what HD vehicle types, to help
inform our HD electric vehicle sales projections in the MY 2024 through
MY 2029 timeframe. We also are considering whether to establish more
stringent standards beyond MY 2027, specifically in MY 2028 and MY 2029
using the methodology described in Section XI.C.1. We request comment
on appropriate stringency and supporting data for each of those model
years.
    We are also interested in stakeholder input that supports changes
to the advanced technology credit multiplier approach under
consideration. In addition, we request comment under this proposal on
how EPA can best consider the potential for ZEV technology to
significantly reduce air pollution from the heavy-duty vehicle sector,
including whether and how to consider including specific sales
requirements for HD ZEVs.
    For these and all requests for comment detailed throughout the
proposal, stakeholders are encouraged to provide their rationale and
any available data that supports to their perspectives.

I. Introduction

A. Brief Overview of the Heavy-Duty Truck Industry

    Heavy-duty highway vehicles (also referred to as ``trucks'' in this
preamble) range from commercial pickup trucks to vocational vehicles
that support local and regional transportation, construction, refuse
collection, and delivery work, to line-haul tractor-trailers that move
freight cross-country. This diverse array of vehicles is categorized
into weight classes based on gross vehicle weight ratings (GVWR). These
weight classes span Class 2b pickup trucks and vans from 8,500 to
10,000 lbs GVWR through Class 8 line-haul tractors and other commercial
vehicles that exceed 33,000 lbs GVWR.53 54
---------------------------------------------------------------------------

    \53\ This proposed rulemaking includes revised criteria
pollutants standards for engine-certified Class 2b through 8 heavy-
duty engines and vehicles; this proposal also includes revised GHG
standards for Class 4 through 8 vehicles. Class 2b and 3 vehicles
with GVWR between 8,500 and 14,000 pounds are primarily commercial
pickup trucks and vans and are sometimes referred to as ``medium-
duty vehicles''. The majority of Class 2b and 3 vehicles are
chassis-certified vehicles and will be included in a future combined
light-duty and medium-duty rulemaking action, consistent with E.O.
14037, Section 2a. Heavy-duty engines and vehicles are also used in
nonroad applications, such as construction equipment; nonroad heavy-
duty engines and vehicles are not the focus of this proposal. See
Section I for more discussion on the spectrum of heavy-duty vehicles
and how they relate to the proposed rule. See Sections I.B and III
for more discussion on the spectrum of heavy-duty vehicles and how
they relate to the proposed rule.
    \54\ The focus of this proposal is on highway heavy-duty engines
and vehicles. However, we are also proposing limited amendments to
regulations that implement our air pollutant emission standards for
other sectors, including light-duty vehicles, light-duty trucks,
marine diesel engines, locomotives, and various types of nonroad
engines, vehicles, and equipment (see Section XII).
---------------------------------------------------------------------------

    Heavy-duty highway vehicles are primarily powered by diesel-fueled,
compression-ignition (CI) engines. However, gasoline-fueled, spark-
ignition (SI) engines are common in the lighter weight classes, and
smaller numbers of alternative fuel engines (e.g., liquified petroleum
gas, compressed natural gas) are found in the heavy-duty fleet.
Vehicles powered by electricity, either in the form of battery electric
vehicles (BEVs) or fuel cell electric vehicles (FCEVs) are also
increasingly entering the heavy-duty fleet. The operational
characteristics of some commercial applications (e.g., delivery
vehicles) can be similar across several vehicle weight classes,
allowing a single engine, or electric power source in the case of BEVs
and FCEVs, to be installed in a variety of vehicles. For instance,
engine specifications needed for a Class 4 parcel delivery vehicle may
be similar

to the needs of a Class 5 mixed freight delivery vehicle or a Class 6
beverage truck. Any performance differences needed to operate across
this range of vehicles can be achieved through adjustments to chassis-
based systems (i.e., transmission, cooling system) external to the
engine.
    The industry that designs and manufactures these heavy-duty
vehicles is composed of three primary segments: Vehicle manufacturers,
engine manufacturers and other major component manufacturers, and
secondary manufacturers (i.e., body builders). Some vehicle
manufacturers are vertically integrated, designing, developing, and
testing their engines in-house for use in their vehicles, while others
purchase some or all of their engines from independent engine
suppliers. Today, only one major independent engine manufacturer
supports the heavy-duty truck industry, though some vehicle
manufactures sell their engines or ``incomplete vehicles'' (i.e.,
chassis that include their engines, the frame, and a transmission) to
body builders who design and assemble the final vehicle. Each of these
subindustries is often supported by common suppliers for subsystems
such as transmissions, axles, engine controls, and emission controls.
    In addition to the manufacturers and suppliers responsible for
producing highway heavy-duty vehicles, an extended network of
dealerships, repair and service facilities, and rebuilding facilities
contribute to the sale, maintenance, and extended life of these
vehicles and engines. Heavy-duty vehicle dealerships offer customers a
place to order vehicles from a specific manufacturer and include
service facilities for those vehicles and engines. Dealership service
technicians are trained to perform regular maintenance and make
repairs, which generally include repairs under warranty and in response
to manufacturer recalls. Some trucking fleets, businesses, and large
municipalities benefit from hiring their own technicians to service
their vehicles in their own facilities. Many refueling centers along
major trucking routes have also expanded their facilities to include
roadside assistance and service stations to diagnose and repair common
problems.
    Heavy-duty CI engines installed in the larger weight classes of
vehicles are designed to be rebuilt. Dealerships and other service
facilities are generally equipped to replace common components, such as
pistons and bearings that wear over time. However, large-scale (i.e.,
``out-of-frame'') engine overhauls that replace most of the engine
components require a more sophisticated process that only a limited
number of facilities provide. Some heavy-duty engine manufacturers have
established their own rebuilding facilities as a separate branch of
their operations and others work with independent rebuilding factories
that are affiliated with multiple engine manufacturers. Rebuilding
allows owners to extend the life of their engines at a lower cost than
purchasing a replacement vehicle, which has made the practice common
for some heavy-duty engines.
    The end-users for highway heavy-duty vehicles are as diverse as the
applications for which these vehicles are purchased. Smaller weight
class heavy-duty vehicles are commonly purchased by delivery services,
contractors, and municipalities. The middle weight class vehicles tend
to be commercial vehicles for businesses and municipal work that
transport people and goods locally and regionally or provide services
such as utilities. Vehicles in the heaviest weight classes are
generally purchased by businesses with high load demands, such as
construction, towing or refuse collection, or freight delivery fleets
and owner-operators with both load and speed demands for regional and
long-haul goods movement. The competitive nature of the businesses and
owner-operators that purchase and operate highway heavy-duty vehicles
means that any time the vehicle is unable to operate due to maintenance
or repair (i.e., downtime) can lead to a loss in income. This need for
reliability drives much of the truck and engine manufacturers'
innovation and research to meet the needs of their customers.

B. History of Emission Standards for Heavy-Duty Engines and Vehicles

    Emission standards for heavy-duty highway engines in the U.S. were
first issued by the Department of Health, Education, and Welfare in the
1960s. These standards and the corresponding certification and testing
procedures were codified at 45 CFR part 1201. In 1972, shortly after
EPA was created as a federal agency and given responsibility for
regulating heavy-duty engines, EPA published new standards and updated
procedures while migrating the regulations to 40 CFR part 85 as part of
the effort to consolidate all EPA regulations in a single location.\55\
EPA created 40 CFR part 86 in 1976 to reorganize emission standards and
certification requirements for light-duty vehicles and heavy-duty
highway engines. In 1985, EPA promulgated new standards for heavy-duty
highway engines, codifying the standards in 40 CFR part 86, subpart A.
Since then, EPA has promulgated several rules for highway heavy-duty
engines and vehicles to set new and more stringent emission standards
for criteria pollutants and precursors,\56\ to set requirements for
controlling evaporative and refueling emissions,\57\ to establish
emission control programs for greenhouse gases (GHGs), and to add or
revise certification procedures.\58\
---------------------------------------------------------------------------

    \55\ See Section I.G for additional discussion on EPA's
Statutory Authority relevant to this proposal.
    \56\ For example, oxides of nitrogen (NOX),
hydrocarbons (HC), particulate matter (PM) and carbon monoxide (CO).
    \57\ See Section III.E for more discussion on controlling
evaporative and refueling emissions from light- and heavy-duty
vehicles.
    \58\ U.S. Environmental Protection Agency. ``EPA Emission
Standards for Heavy-Duty Highway Engines and Vehicles,'' Available
online: https://www.epa.gov/emission-standards-reference-guide/epa-emission-standards-heavy-duty-highway-engines-and-vehicles. (last
accessed June 25, 2021).
---------------------------------------------------------------------------

    EPA's criteria pollutant regulatory programs for the heavy-duty
highway industry apply to engines.\59\ Our regulations require that
engine manufacturers identify the ``primary intended service class''
for each engine by considering the vehicles for which they design and
market their engines. Heavy-duty CI engines are specified as light
heavy-duty engine (Light HDE), medium heavy-duty engine (Medium HDE),
or heavy heavy-duty engine (Heavy HDE) based largely on the weight
class of the vehicles in which the engines are expected to be installed
and the potential for rebuild. SI heavy-duty engines are generally
specified as a single spark-ignition HDE service class unless they are
designed or intended for use in the largest heavy-duty vehicles, and
therefore considered heavy HDEs.\60\ EPA sets emission standards and
other regulatory provisions, including regulatory useful life and
emissions warranty periods, that are targeted for the operational
characteristics of each primary intended service class.
---------------------------------------------------------------------------

    \59\ EPA's regulations address heavy-duty engines and vehicles
separately from light-duty vehicles. Vehicles with GVWR above 8,500
pounds (Class 2b and above) are classified in the regulations as
heavy-duty. For criteria pollutants EPA's standards generally apply
to the engine rather than the vehicle for heavy-duty. However, most
of the Class 2b and 3 pickup trucks and vans (vehicles with a GVWR
between 8,500 and 14,000 pounds) are chassis-certified heavy-duty
vehicles and covered by standards in EPA's Tier 3 program (79 FR
23414, April 28, 2014; 80 FR 0978, February 19, 2015). As noted in
Section III, there are a small number of Class 2b and 3 engines
(e.g., trucks with dual rear wheels that are sold with a cab and
chassis only), which are the subject of this proposed rulemaking.
    \60\ See 40 CFR 1036.140(a)(3).
---------------------------------------------------------------------------

    In the 1990s, EPA issued increasingly stringent standards for
NOX, CO, HC,

and PM. These exhaust standards were derived from engine-based emission
control strategies and manufacturers generally certified their engines'
emission performance over defined duty cycles on an engine dynamometer
(i.e., ``engine certification''). In 1997, EPA finalized standards for
heavy-duty highway diesels (62 FR 54693, October 21, 1997), effective
beginning with the 2004 model year, including a combined non-methane
hydrocarbon (NMHC) and NOX standard that represented a
reduction of NOX emissions by 50 percent. These
NOX reductions also resulted in significant reductions in
secondary nitrate PM.
    In early 2001, EPA finalized the 2007 Heavy-Duty Engine and Vehicle
Rule (66 FR 5002, January 18, 2001) to continue addressing
NOX and PM emissions from both diesel and gasoline-fueled
highway heavy-duty engines. This rule established a comprehensive
national program that regulated a heavy-duty engine and its fuel as a
single system, with emission standards taking effect beginning with
model year (MY) 2007 and fully phasing in by MY 2010 (EPA 2010
standards). Prior to 2007, emission standards were based on controlling
the emissions formed during the combustion process (i.e., engine-out
emissions), and there was no further control of emissions between the
engine and the truck's tailpipe. But with promulgation of the 2007
final rule, emission standards were, for the first time, based on the
use of technologies to capture, convert, and reduce harmful engine-out
emissions, resulting in tailpipe emissions that were cleaner than
engine-out emissions. By and large, the industry met these new
standards through the use of exhaust aftertreatment technologies,
namely, diesel oxidation catalysts, particulate filters, and high-
efficiency catalytic exhaust emission control devices. Consistent with
previous criteria pollutant regulatory programs, the program also
offered flexibility to manufacturers through the use of various
emission credits averaging, banking, and trading (ABT) programs.
    To ensure proper functioning of these aftertreatment technologies,
which could be damaged by sulfur, EPA also reduced the allowable level
of sulfur in highway diesel fuel by 97 percent by mid-2006. Together,
the use of exhaust aftertreatment technologies and lower-sulfur fuel
resulted in diesel-fueled trucks that emitted PM and NOX
tailpipe emissions at levels 90 percent and 95 percent below emission
levels from then-current highway heavy-duty engines, respectively. The
PM standard for new highway heavy-duty engines was set at 0.01 grams
(10 milligrams, or 10 mg) per horsepower-hour (mg/hp-hr) by MY 2007 and
the NOX and NMHC standards of 200 mg/hp-hr and 140 mg/hp-hr,
respectively, were set to phase in between model years 2007 and
2010.\61\ In finalizing that rule, EPA estimated that the emission
reductions would achieve significant health and environmental impacts,
and that the total monetized PM2.5 and ozone-related
benefits of the program would exceed $70 billion, versus program costs
of $4 billion (1999$).
---------------------------------------------------------------------------

    \61\ Heavy-duty engine emission standards are defined in work
specific units (i.e., milligrams per horsepower-hour) because the
standards cover a large range of engine ratings, and thus time
specific standards would not provide equal stringency across all
engines.
---------------------------------------------------------------------------

    In 2005, EPA finalized a manufacturer-run, in-use testing program
that uses portable emission measurement systems to measure HC, CO,
NOX, and PM emissions from the exhaust of in-use heavy-duty
diesel trucks (70 FR 34594, June 14, 2005). The fully enforceable
program began in 2007. This effort was a significant advancement in
helping to ensure that the benefits of more stringent emission
standards are realized under real-world driving conditions.
    In 2009, as advanced emissions control systems were being
introduced to meet the MY 2007/2010 standards, EPA promulgated a final
rule to require that these advanced emissions control systems be
monitored for malfunctions via an onboard diagnostic (OBD) system (74
FR 8310, February 24, 2009). The rule, which has been fully phased in,
required engine manufacturers to install OBD systems that monitor the
functioning of emission control components on new engines and alert the
vehicle operator to any detected need for emission-related repair. It
also required that manufacturers make available to the service and
repair industry information necessary to perform repair and maintenance
service on OBD systems and other emission related engine components. In
addition, EPA published a series of documents that provided guidance to
manufacturers on potential methods and measures to ensure that trucks
equipped with Selective Catalytic Reduction (SCR) technology would be
refilled with the specified quantity and quality of a urea-water
mixture (also known as diesel exhaust fluid, or DEF) necessary for the
proper functioning of this NOX-reducing technology. These
guidance documents describe potential approaches that included
progressive levels of alerts and warnings communicated to the driver of
the truck, which would allow adequate time to refill the DEF tank, but
ultimately, if DEF is not added, or if it is determined to be of
insufficient quality, a vehicle speed-limiting ``inducement'' would be
triggered, requiring the DEF tank to be refilled or the system to be
repaired.
    Also in 2009, EPA and Department of Transportation's National
Highway Traffic Safety Administration (NHTSA) began working on a joint
regulatory program to reduce GHG emissions and fuel consumption from
heavy-duty vehicles and engines.\62\ By utilizing regulatory approaches
recommended by the National Academy of Sciences, the first phase
(``Phase 1'') of the GHG and fuel efficiency program was finalized in
2011 (76 FR 57106, September 15, 2011).\63\ The Phase 1 program,
spanning implementation from MY 2014 to 2018, included separate
standards for highway heavy-duty vehicles and heavy-duty engines. The
program offered flexibility allowing manufacturers to attain these
standards through a mix of technologies and the option to participate
in an emissions credit ABT program. In the Phase 1 rulemaking EPA also
revised the heavy-duty vehicle and engine regulations to make them
consistent with the light-duty vehicle approach, such that all criteria
pollutant and GHG standards would apply regardless of fuel type,
including all-electric vehicles (EVs).
---------------------------------------------------------------------------

    \62\ Greenhouse gas emissions from heavy-duty engines are
primarily carbon dioxide (CO2), but also include methane
(CH4) and nitrous oxide (N2O). Because
CO2 is formed from the combustion of fuel, it is directly
related to fuel consumption.
    \63\ National Research Council; Transportation Research Board.
The National Academies' Committee to Assess Fuel Economy
Technologies for Medium- and Heavy-Duty Vehicles; ``Technologies and
Approaches to Reducing the Fuel Consumption of Medium- and Heavy-
Duty Vehicles.'' 2010. Available online: https://www.nap.edu/catalog/12845/technologies-and-approaches-to-reducing-the-fuel-consumption-of-medium-and-heavy-duty-vehicles.
---------------------------------------------------------------------------

    In 2016, EPA and NHTSA finalized the Heavy-Duty Phase 2 GHG and
fuel efficiency program (``HD GHG Phase 2'') (81 FR 73478, October 25,
2016). HD GHG Phase 2 includes technology-advancing performance-based
standards for highway heavy-duty vehicles and heavy-duty engines that
will phase in over the long term, with initial standards for most
vehicles and engines commencing in MY 2021, increasing in stringency in
MY 2024, and culminating in MY 2027 standards. HD GHG Phase 2 built
upon the Phase 1 program and set standards based not only on currently
available technologies, but also on technologies that were still under
development or not yet widely deployed. To ensure adequate time for

technology development, HD GHG Phase 2 provided up to 10 years lead
time to allow for the development and phase-in of these control
technologies. EPA recently finalized technical amendments to the HD GHG
Phase 2 rulemaking (``HD Technical Amendments'') that included changes
to the test procedures for heavy-duty engines and vehicles to improve
accuracy and reduce testing burden.\64\
---------------------------------------------------------------------------

    \64\ 86 FR 34308, June 29, 2021.
---------------------------------------------------------------------------

C. Petitions to EPA for Additional NOX Emissions Control

    In the summer of 2016 more than 20 organizations, including state
and local air agencies from across the country, petitioned EPA to
develop more stringent NOX emission standards for on-road
heavy-duty engines.\65\ Among the reasons stated by the petitioners for
such an EPA rulemaking was the need for NOX emission
reductions to reduce adverse health and welfare impacts and to help
areas attain the NAAQS. EPA subsequently met with a wide range of
stakeholders in listening sessions, during which certain themes were
consistent across those stakeholders.\66\ For example, it became clear
that there is broad support for federal action in collaboration with
the California Air Resources Board (CARB). So-called ``50-state''
standards would enable technology suppliers and manufacturers to
efficiently produce a single set of reliable and compliant products.
There was also broad acknowledgement of the value of aligning
implementation of new NOX standards with existing MY 2021,
2024, and 2027 milestones for HD Phase 2 GHG and fuel efficiency
standards. Stakeholders thought that such alignment would ensure that
the GHG and fuel consumption reductions achieved under HD GHG Phase 2
are maintained and allow the regulated industry to implement GHG- and
NOX-reducing technologies into their products at the same
time.\67\
---------------------------------------------------------------------------

    \65\ Brakora, Jessica. ``Petitions to EPA for Revised
NOX Standards for Heavy-Duty Engines'' Memorandum to
Docket EPA-HQ-OAR-2019-0055. December 4, 2019.
    \66\ Stakeholders included: Emissions control technology
suppliers; engine and vehicle manufacturers; a labor union that
represents heavy-duty engine, parts, and vehicle manufacturing
workers; a heavy-duty trucking fleet trade association; an owner-
operator driver association; a truck dealers trade association;
environmental, non-governmental organizations; states and regional
air quality districts; Tribal interests; California Air Resources
Board (CARB); and the petitioners.
    \67\ U.S. EPA. 2016. Memorandum in Response to Petition for
Rulemaking to Adopt Ultra-Low NOX Standards for On-
Highway Heavy-Duty Trucks and Engines. Available at https://19january2017snapshot.epa.gov/sites/production/files/2016-12/documents/nox-memorandum-nox-petition-response-2016-12-20.pdf.
---------------------------------------------------------------------------

    EPA responded to the petitions on December 20, 2016, noting that an
opportunity exists to develop a new, harmonized national NOX
reduction strategy for heavy-duty highway engines.\68\ EPA emphasized
the importance of scientific and technological information when
determining the appropriate level and form of a future low
NOX standard and highlighted the following potential
components of the action:
---------------------------------------------------------------------------

    \68\ U.S. EPA. 2016. Memorandum in Response to Petition for
Rulemaking to Adopt Ultra-Low NOX Standards for On-
Highway Heavy-Duty Trucks and Engines. Available at https://19january2017snapshot.epa.gov/sites/production/files/2016-12/documents/nox-memorandum-nox-petition-response-2016-12-20.pdf.

 Lower NOX emission standards
 Improvements to test procedures and test cycles to ensure
emission reductions occur in the real world, not only over the
currently applicable certification test cycles
 Updated certification and in-use testing protocols
 Longer periods of mandatory emission-related component
warranties
 Consideration of longer regulatory useful life, reflecting
actual in-use activity
 Consideration of rebuilding
 Incentives to encourage the transition to current- and next-
generation cleaner technologies as soon as possible

    As outlined in the Executive Summary and detailed in the sections
that follow, this proposed rulemaking considered these components.

D. California Heavy-Duty Highway Low NOX Program Development

    In this section, we present a summary of recent efforts by the
state of California to establish new, lower emission standards for
highway heavy-duty engines and vehicles.\69\ For the past several
decades, EPA and the California Air Resources Board (CARB) have worked
together to reduce air pollutants from highway heavy-duty engines and
vehicles by establishing harmonized emission standards for new engines
and vehicles. For much of this time, EPA has taken the lead in
establishing emission standards through notice and comment rulemaking,
after which CARB would adopt the same standards and test procedures.
For example, EPA promulgated the current heavy-duty engine
NOX and PM standards in a 2001 final rule, and CARB
subsequently adopted the same emission standards. EPA and CARB often
cooperate during the implementation of highway heavy-duty standards.
Thus, for many years, the regulated industry has been able to design a
single product line of engines and vehicles that can be certified to
both EPA and CARB emission standards (which have been the same) and
sold in all 50 states.
---------------------------------------------------------------------------

    \69\ California has long had the unique ability among states to
adopt its own separate new motor vehicle and engine standards per
Section 209 of the Clean Air Act. Although CAA section 209(a)
expressly preempts states from adopting and enforcing standards
relating to the control of emissions from new motor vehicles or new
motor vehicle engines (such as state controls for new heavy-duty
engines and vehicles), CAA section 209(b) directs EPA to waive this
preemption for California under certain conditions. Even with
California's ability under the CAA to establish its own emission
standards, EPA and the California Air Resources Board have worked
closely together over the past several decades to largely harmonize
new heavy-duty vehicle and engine criteria pollutant standard
programs.
---------------------------------------------------------------------------

    Given the significant ozone and PM air quality challenges in the
state of California, CARB has taken several steps since the EPA 2010
standards were implemented to encourage or establish standards and
requirements that go beyond EPA requirements, to further reduce
NOX emissions from heavy-duty vehicles and engines in its
state. CARB's optional (voluntary) low NOX program, which
started in 2013, was created to encourage heavy-duty engine
manufacturers to introduce technologies that emit NOX at
levels below the current EPA 2010 standards. Under this optional
program, manufacturers can certify engines to one of three levels of
stringency that are 50, 75, and 90 percent below the existing EPA 2010
standards with the lowest optional standard being 20 milligrams
NOX per horsepower-hour (mg/hp-h).\70\ To date, only natural
gas and liquefied petroleum gas engines have been certified to these
optional standards.\71\
---------------------------------------------------------------------------

    \70\ California Code of Regulations, Title 13, section 1956.8.
    \71\ California Air Resources Board. ``Optional Low
NOX Certified Heavy-Duty Engines''. February 2020.
Available online: https://ww3.arb.ca.gov/msprog/onroad/optionnox/optional_low_nox_certified_hd_engines.pdf.
---------------------------------------------------------------------------

    In May 2016, CARB published its Mobile Source Strategy that
outlined its approach to reduce in-state emissions from mobile sources
and meet its air quality targets.\72\ In November 2016, CARB held its
first Public Workshop on its plans to update its heavy-duty engine and
vehicle programs.\73\ CARB's 2016 Workshop kicked off a technology

demonstration program (the CARB ``Low NOX Demonstration
Program''), and announced plans to update emission standards,
laboratory-based and in-use test procedures, emissions warranty,
durability demonstration requirements, and regulatory useful life
provisions. The initiatives introduced in its 2016 Workshop have since
become components of CARB's Heavy-Duty ``Omnibus'' Rulemaking.\74\
---------------------------------------------------------------------------

    \72\ California Air Resources Board. ``Mobile Source Strategy''.
May 2016. Available online: https://ww3.arb.ca.gov/planning/sip/2016sip/2016mobsrc.pdf.
    \73\ California Air Resources Board. ``Heavy-Duty Low
NOX: Meetings & Workshops''. Available online: https://ww2.arb.ca.gov/our-work/programs/heavy-duty-low-nox/heavy-duty-low-nox-meetings-workshops.
    \74\ California Air Resources Board. Heavy-Duty Engine and
Vehicle Omnibus Regulation and Associated Amendments. Available
online: https://ww2.arb.ca.gov/rulemaking/2020/hdomnibuslownox.
---------------------------------------------------------------------------

    CARB's goal for its Low NOX Demonstration Program was to
investigate the feasibility of reducing NOX emissions to
levels significantly below today's EPA 2010 standards. Southwest
Research Institute (SwRI) was contracted to perform the work, which was
split into three ``Stages.'' \75\ In Stage 1 and 1b, SwRI demonstrated
an engine technology package capable of achieving a 90 percent
NOX emissions reduction on today's regulatory test cycles to
a useful life of 435,000 miles using an accelerated aging process.\76\
In Stage 2, SwRI developed and evaluated a new low load-focused engine
test cycle. In Stage 3, SwRI evaluated a new engine platform and
different technology package to ensure both criteria and GHG emission
performance. EPA has been closely observing CARB's Low NOX
Demonstration Program as a member of the Low NOX Advisory
Group for the technology development work, which includes
representatives from heavy-duty engine and aftertreatment industries,
as well as from federal, state, and local governmental agencies.\77\
---------------------------------------------------------------------------

    \75\ Southwest Research Institute. ``Update on Heavy-Duty Low
NOX Demonstration Programs at SwRI''. September 26, 2019.
Available online: https://ww3.arb.ca.gov/msprog/hdlownox/files/workgroup_20190926/guest/swri_hd_low_nox_demo_programs.pdf.
    \76\ Southwest Research Institute. ``Evaluating Technologies and
Methods to Lower Nitrogen Oxide Emissions from Heavy-Duty Vehicles:
Final Report''. April 2017. Available online: https://ww3.arb.ca.gov/research/apr/past/13-312.pdf.
    \77\ California Air Resources Board. ``Evaluating Technologies
and Methods to Lower Nitrogen Oxide Emissions from Heavy-Duty
Vehicles''. May 10, 2017. Available online: https://ww3.arb.ca.gov/research/veh-emissions/low-nox/low-nox.htm.
---------------------------------------------------------------------------

    CARB has published several updates related to its Omnibus
Rulemaking. In June 2018, CARB approved its ``Step 1'' update to
California's emission control system warranty regulations.\78\ Starting
in MY 2022, the existing 100,000-mile warranty for all diesel engines
will increase to 110,000 miles for engines certified as light heavy-
duty, 150,000 miles for medium heavy-duty engines, and 350,000 miles
for heavy heavy-duty engines. In November 2018, CARB approved revisions
to the OBD requirements that include implementation of real emissions
assessment logging (REAL) for heavy-duty engines and other
vehicles.\79\ In April 2019, CARB published a ``Staff White Paper'' to
present its staff's assessment of the technologies they believed were
feasible for medium and heavy heavy-duty diesel engines in the 2022-
2026 timeframe.\80\
---------------------------------------------------------------------------

    \78\ California Air Resources Board. ``HD Warranty 2018'' June
28, 2018. Available online: https://ww2.arb.ca.gov/rulemaking/2018/hd-warranty-2018.
    \79\ California Air Resources Board. ``Heavy-Duty OBD
Regulations and Rulemaking''. Available online: https://ww2.arb.ca.gov/resources/documents/heavy-duty-obd-regulations-and-rulemaking.
    \80\ California Air Resources Board. ``California Air Resources
Board Staff Current Assessment of the Technical Feasibility of Lower
NOX Standards and Associated Test Procedures for 2022 and
Subsequent Model Year Medium-Duty and Heavy-Duty Diesel Engines''.
April 18, 2019. Available online: https://ww3.arb.ca.gov/msprog/hdlownox/white_paper_04182019a.pdf.
---------------------------------------------------------------------------

    In August 2020, the CARB governing board approved the staff
proposal for the Omnibus rule and directed staff to initiate the
process of finalizing the provisions.81 82 The final Omnibus
rule was approved by the California Office of Administrative Law in
December 2021. The final rule includes updates to CARB engine
standards, duty-cycle test procedures, and heavy-duty off-cycle testing
program that would take effect in MY 2024, with additional updates to
warranty, durability, and useful life requirements and further
reductions in standards in MYs 2027 and 2031.\83\
---------------------------------------------------------------------------

    \81\ California Air Resources Board. ``Staff Report: Initial
Statement of Reasons-Public Hearing to Consider the Proposed Heavy-
Duty Engine and Vehicle Omnibus Regulation and Associated
Amendments''. June 23, 2020. Available online at: https://ww3.arb.ca.gov/regact/2020/hdomnibuslownox/isor.pdf.
    \82\ California Air Resources Board. Heavy-Duty Engine and
Vehicle Omnibus Regulation and Associated Amendments. Available
online: https://ww2.arb.ca.gov/rulemaking/2020/hdomnibuslownox.
    \83\ Throughout this proposal we use ``Omnibus'' to refer to the
engine standards, duty-cycle test procedures, heavy-duty off-cycle
testing program, useful life and warranty requirements included in
the final Omnibus.
---------------------------------------------------------------------------

    As described in Sections I.F and I.G, with details in Sections III
and IV, EPA is proposing new NOX, PM, HC, and CO emission
standards for heavy-duty engines that reflect the greatest degree of
emission reduction achievable through the application of technology
that we have determined would be available for the model years to which
the proposed standards would apply. In doing so we have given
appropriate consideration to additional factors, namely lead time,
cost, energy, and safety (see Sections I.F and I.G for more
discussion). Throughout the rulemaking process we will continue to
evaluate what standards are appropriate given the factors that we are
directed to consider under CAA section 202(a)(3). As noted at the start
of this Section I.D, EPA and CARB have historically worked together to
establish harmonized emission standards for new heavy-duty engines and
vehicles. We have received comments from different stakeholder groups
who have expressed perspectives on the alignment between the EPA and
CARB Omnibus standards they would like EPA to consider during the
rulemaking. For instance, in response to an Advance Notice of Proposed
Rulemaking (ANPR) for this rule, many stakeholders encouraged EPA to
develop a national program harmonized to the greatest extent possible
(see Section I.E).\84\ Following the ANPR, various stakeholders have
provided EPA with additional perspectives on the Omnibus rule and on
the extent to which EPA should align with the California program. For
example, organizations such as the National Association of Clean Air
Agencies,\85\ the National Tribal Air Association,\86\ as well as
multiple vehicle supplier trade associations \87\ have written letters
to EPA in support of strong federal standards that reflect both the
stringency and timeline of CARB's standards. In contrast, some engine
manufacturers have raised concerns about EPA harmonizing its national
program with California's rule because of their concerns with that
program's overall stringency, costs, and focus on near-term
NOX reductions over long-term CO2 emission
reductions. EPA has considered these harmonization comments in light of
the authority and requirements of CAA sections 202 and

207 in developing the proposed standards, regulatory useful life
periods, and emissions warranty periods and intends to continue to take
into consideration potential harmonization with the CARB Omnibus
program, as appropriate and consistent with CAA sections 202 and 207,
during the rulemaking. As described in Sections III and IV, a notable
difference between the proposed EPA program and the Omnibus rule is
that the first step of the Omnibus rule takes effect in MY 2024,
whereas the first step of the proposed EPA program is in MY 2027. EPA's
statutory authority requires a four-year lead time for any heavy-duty
engine or vehicle standard promulgated or revised under CAA section
202(a)(3) (see Section I.F). In Sections III and IV, we discuss areas
where our proposal aligns with or differs from the Omnibus rule and
request comment on issues related to harmonization between the federal
and CARB regulatory programs, including benefits or challenges if EPA
were to finalize particular aspects of its program that are not fully
aligned with the Omnibus rule.\88\
---------------------------------------------------------------------------

    \84\ The Agency published an ANPR on January 21, 2020 to present
EPA's early thinking on this rulemaking and solicit feedback from
stakeholders to inform this proposal (85 FR 3306).
    \85\ Letter to EPA Administrator Michael Regan from the National
Association of Clean Air Agencies. Re: The urgent need for federal
regulatory action to adopt more stringent NOX standards
for heavy-duty engines and vehicles, beginning immediately with
highway heavy-duty trucks. August 26, 2021.
    \86\ Letter to EPA Administrator Andrew Wheeler from the
National Tribal Air Association. Re: EPA's Advance Notice of
Proposed Rulemaking for Control of Air Pollution from New Motor
Vehicles: Heavy-Duty Engine Standards Docket ID EPA-HQ-OAR-2019-
0055. February 20, 2020.
    \87\ Letter to EPA Administrator Michael Regan from the Motor &
Equipment Manufacturers Association, Manufacturers of Emission
Controls Association, Advanced Engine Systems Institute, and
Alliance for Vehicle Efficiency. Re: Completion of EPA's Heavy-duty
Low-NOX Rulemaking. June 24, 2021.
    \88\ Draft RIA Chapter 5, Appendix 6 includes tables that
present the main elements (i.e., numeric level of standards, useful
life, emission warranty) of CARB Omnibus requirements and EPA
proposal.
---------------------------------------------------------------------------

    As discussed in the draft RIA, we analyzed the emission inventory
and air quality impacts for the proposed criteria pollutant standards
before the Omnibus Rule was finalized. We may incorporate the Omnibus
rule into our emission inventory and other analyses as appropriate for
the final rulemaking (FRM).89 90 We also may incorporate the
CARB Advanced Clean Truck (ACT) Regulation into our final rule
analyses. As further discussed in Sections IV, VI, and XI, the CARB ACT
Regulation requires a minimum percentage of each manufacturer's heavy-
duty vehicle sales in the state of California to be zero tailpipe
emission technologies starting in MY 2024.91 92
---------------------------------------------------------------------------

    \89\ See Section VI and draft RIA Chapter 5 for more information
on our emission inventory modeling for the proposal and plans to
incorporate other updates in our modeling for the final rule.
    \90\ EPA has received waiver requests under CAA section 209(b)
from California for the Omnibus or ACT rules; EPA is currently
reviewing the waiver requests for the CA Omnibus and ACT rules and
may consider including these rules in our analyses for the final
rule. See Section III.B for discussion on our proposed approach to a
voluntary standard based on one aspect of the Omnibus requirements.
    \91\ CARB. ``Notice of Decision: Advanced Clean Truck
Regulation.'' June 2020. Available online at: https://ww3.arb.ca.gov/regact/2019/act2019/nod.pdf.
    \92\ Buysse and Sharpe. (July 20, 2020) ``California's Advanced
Clean Trucks regulation: Sales requirements for zero-emission heavy-
duty trucks'', available online at: https://theicct.org/publications/california-hdv-ev-update-jul2020 (last accessed August
11, 2021).
---------------------------------------------------------------------------

E. Advance Notice of Proposed Rulemaking

    The ANPR provided background for the provisions proposed in this
rulemaking to address criteria pollutant emissions from heavy-duty
engines, including technologies we are evaluating, test programs we
have initiated, and compliance programs under consideration, as well as
requests for comments and data. The ANPR did not include discussion on
the potential stringency of standards, potential costs of the
standards, or a quantitative assessment of societal impacts (e.g., air
quality, economic, environmental health); these topics are presented in
this proposal.\93\
---------------------------------------------------------------------------

    \93\ The ANPR also did not include the proposed, targeted
revisions to the HD GHG Phase 2 program that are included in this
rulemaking (see Section I.G for a summary of these proposed
provisions and Section XI for details).
---------------------------------------------------------------------------

    EPA received over 300 comments on the ANPR from a wide range of
stakeholders, including: Government organizations (state, local, and
Tribal), environmental groups, trade associations, heavy-duty engine
manufacturers, independent owner-operators, suppliers, individual
fleets, and individual private citizens. We provide a brief overview of
the perspectives included in these comments in this subsection, with
more specific discussion of comments included in subsequent sections of
the proposal as relevant to individual comments or groups of comments.
    Comments from government organizations, including multiple state
and local air agencies, emphasized that reductions in NOX
emissions from heavy-duty engines are necessary for attainment and
maintenance of the NAAQS. States commented that they cannot control
heavy-duty engine emissions since they cross state borders and
controlling emissions from other sources would be economically
burdensome. Commenters stated that areas in nonattainment of the NAAQS
are having difficulty attaining, and some areas currently in attainment
are close to or exceeding the NAAQS. As further discussed in Section
II, commenters noted environmental justice and other public health
concerns, along with regional haze and ecosystem concerns. These
commenters requested stringent emission controls on heavy-duty engines
in as short a timeframe as possible (including early incentives) and
expressed widespread interest in ensuring control over the lifetime of
the engine, including addressing emissions from tampering and idling.
    Several environmental groups submitted comments that were similar
to several of the state and local agency comments; environmental groups
supported stringent emission controls and maintaining that level of
emission control for longer durations by lengthening useful life and
emission warranty periods. These commenters further supported
improvements to the in-use testing program for heavy-duty diesel
engines, and anti-tampering measures for all heavy-duty engines.
    Comments from the Truck and Engine Manufacturers Association (EMA),
a trade association for heavy-duty engine and truck manufacturers
emphasized broad support for a 50-state program and encouraged EPA to
conduct a thorough analysis of the costs and benefits of proposed
NOX emission standards. To emphasize their cost concerns,
EMA provided an industry-sponsored assessment of the cost to comply
with potential requirements discussed in the April 2019 CARB Staff
Whitepaper.\94\
---------------------------------------------------------------------------

    \94\ California Air Resources Board. ``California Air Resources
Board Staff Current Assessment of the Technical Feasibility of Lower
NOX Standards and Associated Test Procedures for 2022 and
Subsequent Model Year Medium-Duty and Heavy-Duty Diesel Engines''.
April 18, 2019. Available online: https://ww3.arb.ca.gov/msprog/hdlownox/white_paper_04182019a.pdf.
---------------------------------------------------------------------------

    Several truck owners, truck operators, fleets, and dealerships also
expressed general support for a national, harmonized low-NOX
program. Many commenters included their experiences with expensive
towing costs and downtime from emission system failures; they stated
that although the reliability of emission system controls has improved
since the 2010 timeframe, it remains an ongoing concern. ANPR
commenters also indicated that engine derates or ``inducements'' are a
significant source of operator frustration.\95\ In addition, commenters
urged EPA to conduct a thorough cost assessment, and noted that if the
initial purchase price, or operational costs for new trucks is too
high, then it may incentivize owners to keep older trucks on the road.
These commenters expressed varying views on lengthening emission
warranty requirements, with some urging a careful consideration of the
impacts of longer warranty requirements, while others expressed

support for longer warranty requirements.
---------------------------------------------------------------------------

    \95\ Engine derating is a control strategy that reduces engine
performance to protect the engine or induce an operator behavior,
such as maintaining appropriate levels of high-quality diesel
emission fluid (DEF) in their SCR-based aftertreatment systems.
Throughout this preamble we refer to engine derates that derive from
aftertreatment-related triggers as ``inducements''.
---------------------------------------------------------------------------

    Suppliers, supplier trade groups, and labor groups were all
generally supportive of more stringent NOX emission
standards. They also generally stated strong support for a 50-state,
harmonized EPA-CARB program. They also emphasized the importance of
providing industry with regulatory certainty. They noted that EPA must
balance emission reductions with technology costs, feasibility, lead-
time, and avoid market disruptions. Several suppliers and trade groups
provided detailed technical information on low NOX
technology. They also expressed support for longer useful life and
warranty requirements but cautioned EPA to carefully design longer
emissions warranty requirements and to consider a phase-in approach.
Several suppliers and trade groups also supported incentives for the
early introduction of low-NOX technology.
    All of the ANPR comments are part of the docket for the proposal
and have informed our thinking in developing the proposed provisions to
address criteria pollutant emissions from heavy-duty engines.

F. EPA Statutory Authority for the Proposal

    This section briefly summarizes the statutory authority for the
proposed rule. Title II of the Clean Air Act provides for comprehensive
regulation of mobile sources, authorizing EPA to regulate emissions of
air pollutants from all mobile source categories. Specific Title II
authorities for this proposal include: CAA sections 202, 203, 206, 207,
208, 213, 216, and 301 (42 U.S.C. 7521, 7522, 7525, 7541, 7542, 7547,
7550, and 7601). We discuss some key aspects of these sections in
relation to this proposed action immediately below (see also Section
XIV of this preamble), as well as in each of the relevant sections
later in this proposal. Regarding the confidentiality determinations
EPA is proposing to make through this notice and comment rulemaking for
much of the information collected by EPA for certification and
compliance under Title II, see Section XII.A. for discussion of
relevant statutory authority.
    Statutory authority for the proposed NOX, PM, HC, CO,
and GHG emission standards in this action comes from CAA section 202(a)
which states that ``the Administrator shall by regulation prescribe
(and from time to time revise) . . . standards applicable to the
emission of any air pollutant from any class or classes of new . . .
motor vehicle engines, which in his judgment cause, or contribute to,
air pollution which may reasonably be anticipated to endanger public
health or welfare.'' Standards under CAA section 202(a) 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 within such
period.'' Thus, in establishing or revising CAA section 202(a)
standards designed to reduce air pollution that endangers public health
and welfare, EPA also must consider issues of technological
feasibility, compliance cost, and lead time. EPA may consider other
factors and in previous engine and vehicle standards rulemakings has
considered the impacts of potential standards on the heavy-duty
industry, fuel savings, oil conservation, energy security and other
energy impacts, as well as other relevant considerations such as
safety.
1. Statutory Authority for Proposed Criteria Pollutant Program
    Section 202(a)(3) further addresses EPA authority to establish
standards for emissions of NOX, PM, HC, and CO from heavy-
duty engines and vehicles. Section 202(a)(3)(A) requires that such
standards ``reflect the greatest degree of emission reduction
achievable through the application of technology which the
Administrator determines will be available for the model year to which
such standards apply, giving appropriate consideration to cost, energy,
and safety factors associated with the application of such
technology.'' Section 202(a)(3)(B) allows EPA to take into account air
quality information in revising such standards. Section 202(a)(3)(C)
provides that standards shall apply for a period of no less than three
model years beginning no earlier than the model year commencing four
years after promulgation. CAA section 202(a)(3)(A) is a technology-
forcing provision and reflects Congress' intent that standards be based
on projections of future advances in pollution control capability,
considering costs and other statutory factors.96 97 CAA
section 202(a)(3) neither requires that EPA consider all the statutory
factors equally nor mandates a specific method of cost-analysis; rather
EPA has discretion in determining the appropriate consideration to give
such factors.\98\
---------------------------------------------------------------------------

    \96\ See National Petrochemical & Refiners Association v. EPA,
287 F.3d 1130, 1136 (D.C. Cir. 2002) (explaining that EPA is
authorized to adopt ``technology-forcing'' regulations under CAA
section 202(a)(3)); NRDC v. Thomas, 805 F.2d 410, 428 n.30 (D.C.
Cir. 1986) (explaining that such statutory language that ``seek[s]
to promote technological advances while also accounting for cost
does not detract from their categorization as technology-forcing
standards''); see also Husqvarna AB v. EPA, 254 F.3d 195 (D.C. Cir.
2001) (explaining that CAA sections 202 and 213 have similar
language and are technology-forcing standards).
    \97\ In this context, the term ``technology-forcing'' has a
specific legal meaning and is used to distinguish standards that may
require manufacturers to develop new technologies (or significantly
improve existing technologies) from standards that can be met using
off-the-shelf technology alone. Technology-forcing standards such as
those in this proposed rule do not require manufacturers to use
specific technologies.
    \98\ See, e.g., Sierra Club v. EPA, 325 F.3d 374, 378 (D.C. Cir.
2003) (explaining that similar technology-forcing language in CAA
section 202(1)(2) ``does not resolve how the Administrator should
weigh all [the statutory] factors in the process of finding the
`greatest emission reduction achievable' ''); Husqvarna AB v. EPA,
254 F.3d 195, 200 (D.C. Cir. 2001) (explaining that under CAA
section 213's similar technology-forcing authority that ``EPA did
not deviate from its statutory mandate or frustrate congressional
will by placing primary significance on the `greatest degree of
emission reduction achievable' '' or by considering cost and other
statutory factors as important but secondary).
---------------------------------------------------------------------------

    Section II, and Chapter 4 of the draft RIA, describe EPA's analysis
of information regarding heavy-duty engines' contribution to air
pollution and how that pollution adversely impacts public health and
welfare. Section I.G, with more detail in Section III and Chapter 4 of
the draft RIA, discusses our feasibility analysis of the standards and
useful life periods for both proposed Options. Our evaluation shows
that the standards and useful life periods in both steps of proposed
Option 1 are feasible and would result in the greatest emission
reductions achievable for the model years to which they are proposed to
apply, pursuant to CAA section 202(a)(3), giving appropriate
consideration to costs, lead time, and other factors. Our analysis
further shows that the standards and useful life periods in proposed
Option 2 are feasible in the 2027 model year, but would result in lower
levels of emission reductions compared to proposed Option 1. As
explained further in Section III and Chapter 3 of the draft RIA, we
expect that additional data from EPA's ongoing work to demonstrate the
performance of emission control technologies, as well as information
received in public comments, will allow us to refine our assessments
and consideration of the feasibility of the combination of the
standards and useful life periods, particularly for the largest CI
engines (HHDEs), in proposed Options 1 and 2, after consideration of
lead time, costs, and other factors. Therefore, we are co-proposing
Options 1 and 2 standards and useful life periods, and the range of
options in between them, as the options that may

potentially be appropriate to finalize pursuant to CAA section
202(a)(3) once EPA has considered that additional data and other
information. We considered costs and lead time in designing the
proposed program options, including in our analysis of how
manufacturers would adopt advanced emission control technologies to
meet the proposed standards for the applicable model years. For
example, the first step of proposed Option 1 allows manufacturers to
minimize costs by implementing a single redesign of heavy-duty engines
for MY 2027, which is when both the final step of the HD GHG Phase 2
standards and the first step of the proposed Option 1 standards would
start to apply. The second step of proposed Option 1 (MY 2031) would
provide manufacturers the time needed to ensure that emission control
components are durable enough for the proposed second step of revised
standards and longer useful life periods.99 100
---------------------------------------------------------------------------

    \99\ The second step of the proposed Option 1 standards in MY
2031 provides four years of stability following the first step of
the program.
    \100\ See Section III for details on our proposed test cycles
and standards, and Section IV for our proposed compliance
provisions.
---------------------------------------------------------------------------

    As described in Section III, we are proposing new test cycles for
both pre-production and post-certification testing. Manufacturers
demonstrate compliance over specified duty cycle test procedures during
pre-production testing, which is conducted by EPA or the manufacturer.
These data and other information submitted by the manufacturer as part
of their certification application are the basis on which EPA issues
certificates of conformity pursuant to CAA section 206. Under CAA
section 203, sales of new vehicles are prohibited unless the vehicle is
covered by a certificate of conformity. Compliance with standards is
required not only at certification but throughout the useful life
period of the engine and vehicle, based on post-certification testing.
Post-certification testing can include both specific duty cycle test
procedures and off-cycle test procedures that are conducted with
undefined duty cycles either on the road or in the laboratory (see
Sections III.A and IV.K for more discussion on for testing at various
stages in the life of an engine).
    As described in Section IV, we are proposing to lengthen regulatory
useful life and emission warranty periods to better reflect the
mileages and time periods over which heavy-duty engines are driven
today. CAA section 202(d) directs EPA to prescribe regulations under
which the useful life of vehicles and engines are determined and
establishes minimum values of 10 years or 100,000 miles, whichever
occurs first, unless EPA determines that a period of greater duration
or mileage is appropriate. EPA may apply adjustment factors to assure
compliance with requirements in use throughout useful life (CAA section
206(a)). CAA section 207(a) requires manufacturers to provide an
emissions warranty, which EPA last updated in its regulations for
heavy-duty engines in 1983 (see 40 CFR 86.085-2).\101\
---------------------------------------------------------------------------

    \101\ 48 FR 52170, November 16, 1983.
---------------------------------------------------------------------------

2. Statutory Authority for Targeted Revisions to the Heavy-Duty GHG
Phase 2 Program
    In addition, as discussed in Section XI, EPA is proposing a limited
set of revisions to MY 2027 Phase 2 GHG emissions standards under its
CAA section 202(a) authority described in this section (Section I.F).
We have developed an approach to propose targeted updates to HD GHG
Phase 2 standards that take into consideration the growing HD electric
vehicle market without fundamentally changing the HD GHG Phase 2
program as a whole. In addition, we are requesting comment on potential
changes to the advanced technology incentive program for electric
vehicles beginning in MY 2024.

G. Basis of the Proposed Standards

    Our approach to further reduce air pollution from highway heavy-
duty engines and vehicles through the proposed program features several
key provisions. The primary provisions address criteria pollutant
emissions from heavy-duty engines. In addition, this proposal would
make targeted updates to the existing Heavy-Duty Greenhouse Gas
Emissions Phase 2 program, proposing that further GHG reductions in the
MY 2027 timeframe are appropriate considering lead time, costs, and
other factors, including market shifts to zero-emission technologies in
certain segments of the heavy-duty vehicle sector.
1. Basis of the Proposed Criteria Pollutant Standards
    Heavy-duty engines across the U.S. emit NOX, PM, VOCs,
and CO that contribute to ambient levels of ozone, PM, NOX,
and CO; these pollutants are linked to premature death, respiratory
illness (including childhood asthma), cardiovascular problems, and
other adverse health impacts. In addition, these pollutants reduce
visibility and negatively impact ecosystems. Data show that
NOX emissions from heavy-duty engines are important
contributors to concentrations of ozone and PM2.5 and their
resulting threat to public health.102 103 As discussed in
Section II, we estimate that heavy-duty engines will continue to be one
of the largest contributors to mobile source NOX emissions
nationwide in the future, representing 32 percent of the mobile source
and 89 percent of the onroad NOX emission inventories in
calendar year 2045.104 105 For the reasons summarized here
and explained further in those sections, EPA concludes that revised
standards are warranted to address the emissions of these pollutants
and their contribution to national air pollution.
---------------------------------------------------------------------------

    \102\ Zawacki et al, 2018. Mobile source contributions to
ambient ozone and particulate matter in 2025. Atmospheric
Environment, Vol 188, pg 129-141. Available online: https://doi.org/10.1016/j.atmosenv.2018.04.057.
    \103\ Davidson et al, 2020. The recent and future health burden
of the U.S. mobile sector apportioned by source. Environmental
Research Letters. Available online: https://doi.org/10.1088/1748-9326/ab83a8.
    \104\ U.S. Environmental Protection Agency (2021). 2016v1
Platform. https://www.epa.gov/air-emissions-modeling/2016v1-platform.
    \105\ Han, Jaehoon. Memorandum to the Docket EPA-HQ-OAR-2019-
0055: ``MOVES Modeling-Related Data Files (MOVES Code, Input
Databases and Runspecs) for the Proposed Heavy-Duty 2027
Standards''. February 2022.
---------------------------------------------------------------------------

    As required by CAA section 202(a)(3), EPA is proposing new
NOX, PM, HC, and CO emission standards for heavy-duty
engines that reflect the greatest degree of emission reduction
achievable through the application of technology that we have
determined would be available for the model years to which the proposed
standards would apply. In doing so we have given appropriate
consideration to additional factors, namely lead time, cost, energy,
and safety. Our technical assessments are primarily based on results
from diesel engine demonstration testing conducted by CARB at Southwest
Research Institute,\106\ heavy-duty gasoline and diesel engines testing
conducted at EPA's National Vehicle and Fuel Emissions Laboratory
(NVFEL), heavy-duty engine certification data submitted to EPA by
manufacturers, ANPR comments, and other data submitted by industry
stakeholders or studies conducted by EPA, as more specifically
identified in the sections that follow. We expect that additional data
from EPA's ongoing work to demonstrate the performance of emission
control technologies will allow us to refine our assessments and
consideration of the feasibility of the combination of

standards and useful life periods in proposed Options 1 and 2, after
consideration of lead time, costs, and other factors. Therefore, we are
co-proposing Options 1 and 2 to illustrate a broader range of potential
options. We also present an alternative (the Alternative) that we
considered in the development of this proposal but for which we
currently lack information to conclude would be feasible throughout the
useful periods included in this alternative and in the model year in
which the standards would begin. As outlined in this section and
detailed in Sections III and IV, we solicit comment on the proposed
Options 1 and 2, the Alternative presented, or other alternatives
within and outside the range of options.
---------------------------------------------------------------------------

    \106\ See Section III.B and draft RIA Chapter 3.1 for more
details and discussion on data from diesel engine demonstration
testing.
---------------------------------------------------------------------------

    As noted in the Executive Summary and discussed in Section III, the
proposed Options 1 and 2 standards and the Alternative would each begin
to apply in MY 2027. We selected this model year for two reasons.
First, as explained in Section I.F, the CAA requires EPA to provide at
least four years of lead time from the promulgation of a final rule. We
expect to finalize this rulemaking in 2022, such that MY 2027 would be
the earliest model year the new requirements could apply. Second, the
timing of the final stage of the HD GHG Phase 2 program in MY 2027
leads us to believe that MY 2027 is the appropriate time for the
proposed standards to begin since this would allow manufacturers to
design a single engine platform that complies with both HD GHG Phase 2
and the criteria pollutant requirements. We expect that a single engine
design for both rulemakings would minimize costs and improve
reliability of the emission control components by integrating design
changes for both rules (see Section III.A for more discussion on MY
2027 as the first implementation year for the proposed program).
    The MY 2031 standards in proposed Option 1 would begin four model
years after the MY 2027 standards in proposed Option 1, which is an
additional year beyond the CAA requirement for at least three years of
stability.\107\ Both steps of the proposed Option 1 standards reflect
the greatest degree of emission reductions achievable in each model
year when combined with the proposed longer useful life periods, new
test cycles, and other compliance provisions that start in each model
year. We expect that the changes to useful life in proposed Options 1
and 2 would improve component durability, but additional increases in
useful life, such as those associated with the proposed MY 2031
standards in proposed Option 1, may take manufacturers more time to
develop (see Section IV for more discussion). Therefore, proposed
Option 1 includes a two-step approach to allow additional lead time for
manufacturers to develop emission control components durable enough for
the proposed longer useful life periods. In Section III.A we request
comment on the two-step approach in proposed Option 1.
---------------------------------------------------------------------------

    \107\ The two alternative sets of standards that we present
would each be implemented in a single step beginning in MY 2027.
---------------------------------------------------------------------------

    In Sections III and IV, we present the details of the two-step
proposed Option 1 (MYs 2027 and 2031) and the proposed Option 2 that
would occur in a single step (MY 2027). We also present details of the
Alternative, which would also occur in a single step (MY 2027).
Overall, proposed Option 2 is less stringent than the MY 2031 standards
in proposed Option 1 due to higher numeric levels of the NOX
emission standards and shorter useful life periods in proposed Option
2. For our proposed Options 1 and 2 standards, we project that the
emission control technologies used in MY 2027 would build on those used
in light- and heavy-duty engines today. For heavy-duty CI engines,
under both the proposed Option 1 MY 2031 standards and the proposed
Option 2 standards, we project the use of the valvetrain engine
technology combined with updates to the SCR system configuration that
builds on what is used in current light-duty trucks and heavy-duty
engines. For heavy-duty SI engines, the technologies we are evaluating
that would achieve the standards in the proposed Options 1 and 2
largely build on the three-way catalyst-based emission control
strategies used in heavy-duty SI chassis certified engine products.
    The Alternative we considered includes lower (more stringent)
numeric NOX emission levels for Heavy HDEs, and lower HC
emission levels for all CI and SI engine classes, combined with longer
useful life periods and shorter lead time compared to the MY 2031
standards in proposed Option 1. The test data we currently have from
our engine demonstration program is not sufficient to conclude that the
Alternative standards would be feasible in the MY2027 timeframe; we
would need additional data before we could project that the Alternative
is feasible for the MY 2027 timeframe.
    We continue to believe it is appropriate for SI and CI engines to
have numerically identical standards for the criteria pollutants. As
described in Section III, the proposed standards for each pollutant are
primarily based on the engine type (CI and SI) for which the particular
emission standard is most challenging to achieve. The NOX
standards in proposed Options 1 and 2 are based primarily on emission
test data from CI engine demonstration work, while the HC and CO
standards in the proposed Options 1 and 2 are based on the SI engine
demonstration program. Currently available engine demonstration test
data show that the heavy-duty CI engine technologies we are evaluating
can achieve a 75 to 90 percent reduction from current NOX
standards. These data indicate that the NOX standards for MY
2027 in proposed Options 1 and 2 are achievable for a useful life
period of 600,000 miles, which encompasses the proposed Option 2 useful
life periods for Light HDE and Medium HDEs. Our evaluation of the
current data suggests that the proposed Option 2 standards would also
be feasible out to the proposed Option 2 Heavy HDE useful life; we are
continuing to collect data to confirm our extrapolation of data out to
the longer HDE useful life mileage. As discussed in Section IV.A,
useful life mileages for proposed Option 2 are higher than the MY 2027
useful life values in proposed Option 1, but lower than the MY 2031
useful life values in proposed Option 1. The useful life mileages
included in the proposed Options 1 and 2 are based on the operational
life of engines in the field today. Data show that heavy-duty engines
are operating in the real world well beyond the useful life periods in
our existing regulations, and thus we are proposing longer useful life
periods to ensure that emission control systems are durable for an
appropriate portion of their use in the real world (see Section IV for
details). For the Alternative, data suggest that to meet the
combination of numeric levels of the Alternative NOX
emission standards and useful life periods for Light HDEs and Medium
HDEs, it may be appropriate for EPA to consider providing manufacturers
with additional lead time, beyond the MY 2027 implementation date of
the Alternative. For Heavy HDEs, our evaluation of current data
suggests that wholly different emission control technologies than we
have evaluated to date (i.e., not based on CDA and a dual SCR) would be
needed to meet the Alternative NOX standards for Heavy HDEs;
we request comment on this conclusion and on the availability, or
potential development and timeline, of such additional technologies.
    Our demonstration test data do show that CI engines can achieve the
PM, HC, and CO standards in proposed Options

1 and 2, each of which would result in at least a 50 percent reduction
from current emission standards for PM, HC, and CO. The HC and CO
standards in the proposed Options 1 and 2, are based on SI engine
demonstration data with a catalyst aged beyond the useful life of those
scenarios. Available data indicate that the combination of
NOX, HC, and CO emission levels over the longer useful life
period reflected in the SI standards of the Alternative would be very
challenging to meet in the MY 2027 timeframe. In contrast, we believe
the additional lead time provided by the second step of the MY 2031
standards in proposed Option 1, combined with the higher numeric
standard for HC and the shorter useful life mileage, results in the MY
2031 standards in proposed Option 1 being both feasible and technology
forcing.
    We are also proposing to require onboard refueling vapor recovery
(ORVR) for incomplete vehicles over 14,000 lb GVWR fueled by gasoline
and other volatile fuels. Currently, hydrocarbon vapors from those
vehicles are uncontrolled during refueling events, despite technology
to control these emissions being widely adopted in vehicles in lower
weight classes for almost 20 years. Recent data show this lack of
emission control technology can result in refueling emissions that are
more than 10 times current light-duty refueling standards (see Section
III.D.2 for more discussion). We included ORVR in the analysis of both
proposed Options 1 and 2, as well as the Alternative.
    Our PM standards are based on certification test data that show the
proposed 50 percent reduction in the current PM standard is achievable
in CI and SI heavy-duty engines being certified today; the same
reduction in PM standard is included in both proposed Options 1 and 2,
as well as the Alternative. We believe lowering the PM standard to a
level currently achievable through the use of emission control
technology used in new engines being sold today is appropriate. EPA is
not aware of any technology that is feasible to adopt in the 2027
timeframe that would reduce PM emissions further, and variability in PM
measurement starts to increase at PM levels lower than the proposed
standard. Nevertheless, we request comment on if there are technologies
that EPA could consider that would enable a PM standard lower than 5
mg/hp-hr.
    The proposed Options 1 and 2 generally represent the range of
options, including the NOX, HC, and CO standards, useful
life periods and lead time that we are currently considering in this
rule; we expect we may receive additional information through public
comments or data we continue to collect on the feasibility, costs, and
other impacts of the proposed Options 1 and 2.\108\ In order to
consider adopting the Alternative in the final rule, we would need
additional information to be able to conclude that the Alternative is
feasible in the MY 2027 timeframe. We request comment on all aspects of
the proposal, including the revised emission standards and useful life
and warranty periods, one and two-step approaches, model years of
implementation in proposed Options 1 and 2, or other alternatives
roughly within the range of options covered by the proposed Options 1
and 2, as well as other provisions described in this proposal. We also
request comment, including relevant data and other information, related
to the feasibility of the implementation model year, numeric levels of
the emission standards, and useful life and warranty periods included
in the Alternative, or other alternatives outside the range of options
covered by the proposed Options 1 and 2.
---------------------------------------------------------------------------

    \108\ The numeric level of the standards for PM are consistent
across the proposal and both alternatives since they are intended to
ensure that the level of PM emissions from current engines does not
increase as manufacturers make adjustments to further control
NOX, CO2 or other pollutants. See Section
III.B.2 for more discussion.
---------------------------------------------------------------------------

    As described in Section III, we are proposing new laboratory test
duty cycles and standards in response to data that show a current lack
of emission control under low-load conditions in CI heavy-duty engines,
and under high-load in SI heavy-duty engines. As noted in Section VI,
we project that without the proposed provisions, low- and high-load
engine operations would account for 28 and 36 percent, respectively, of
the heavy-duty NOX emission inventory in 2045.
    Proposed Option 1 includes requirements for lowering the numeric
level of the standard and lengthening useful life in two steps.
Consistent with our approach for useful life, proposed Option 1 would
lengthen emission warranty mileages in two steps, such that the
proposed MY 2031 warranty would cover an appropriate portion of the
proposed MY 2031 regulatory useful life (see Section IV.B for more
discussion). The proposed Option 2 would lengthen emission warranty
mileages in a single step, consistent with the proposed single step
increase in useful life in proposed Option 2. While warranty periods do
not directly impact the stringency of the proposed standards, we expect
the proposed lengthened warranty periods would improve air quality and
we included them in our inventory and cost analyses of the proposed
Option 1 and Option 2 standards.
    We are also proposing additional compliance provisions that would
begin in MY 2027, such as targeted provisions to help ensure that
owners can efficiently maintain emissions performance over the
operational life of the engine. We are proposing provisions to enhance
communication with operators, including updated diagnostic
requirements, a revised inducement policy for SCR-based aftertreatment
systems, and improved access to service information (see Section IV.B
for more discussion). We believe these proposed provisions could
decrease the likelihood that owners tamper with (i.e., remove or
otherwise disable) emission control systems.
    The emission reductions from the proposed program would increase
over time as more new, cleaner vehicles enter the fleet. For example,
by 2040 the proposed Option 1 would reduce heavy-duty NOX
emissions by more than 55 percent, compared to projected 2040 emissions
without the proposed rule. The proposed Option 2 would reduce heavy-
duty NOX emissions by 44 percent in 2040 (see Section VI for
details on projected emission reductions from proposed Option 1 or 2).
These emission reductions would lower ambient concentrations of
pollutants such as ozone and PM2.5. Our analysis shows that
the proposed Option 1 would provide more emission reductions than
proposed Option 2, and less reductions than the Alternative. Our air
quality modeling analysis of Option 1's projected emission reductions
shows widespread reductions in ambient concentrations of air pollutants
in 2045, which is a year by which most of the regulated fleet would
have turned over.\109\ Our analysis shows that these emission
reductions would result in significant improvements in ozone
concentrations; ambient PM2.5, NO2 and CO
concentrations would also improve in 2045 (see Section VII for
details). Based on our air quality analysis of PM2.5 and
ozone, we estimate that in 2045, the proposed Option 1 would result in
total annual monetized health benefits of $12 and $33 billion at a 3
percent discount rate and $10 and $30 billion at a 7 percent discount
rate (2017 dollars). We estimate that in 2045, the proposed Option 2
would result in total annual

monetized health benefits of $9 and $26 billion at a 3 percent discount
rate and $8 and $23 billion at a 7 percent discount rate (2017 dollars)
(see Section VIII for details).
---------------------------------------------------------------------------

    \109\ Due to resource constraints, we only conducted air quality
modeling for the proposed Option 1.
---------------------------------------------------------------------------

    In addition to projected health benefits, we considered several
other factors in developing the proposed standards, including cost,
energy, and safety. Our cost analysis, presented in Section V, accounts
for costs to manufacturers and to truck owners. Costs to manufacturers
include direct manufacturing costs (i.e., new hardware/technology) and
indirect costs (e.g., emission warranty, R&D), while costs to truck
owners include operating costs (e.g., fuel, diesel exhaust fluid,
emission control system repairs). Our analysis shows that direct
manufacturing costs are the same for proposed Options 1 and 2; however,
indirect costs result in total costs to manufacturers (i.e., total
technology costs) under the proposed Option 1 being slightly higher
than under the proposed Option 2. The operating costs associated with
the proposed Option 1 are estimated to be lower than those of proposed
Option 2. The lower operating costs in proposed Option 1 (largely from
lower repair costs) offset the higher technology costs (due to higher
warranty and R&D driven indirect costs) in proposed Option 1, which
results in a lower total cost of proposed Option 1 relative to proposed
Option 2 when costs are summed for 2027 through 2045. For the
Alternative, we have not determined the incremental direct
manufacturing costs of the technology needed to meet the standards, and
we would need additional data before we could project that the
Alternative is feasible for the MY 2027 timeframe.
    Section IX compares the benefits and costs of the proposed Options
1 and 2. Our analysis shows that while proposed Option 2 provides
higher emission reductions in the early years of the program, it has
lower net benefits than proposed Option 1 when considering the time
period of 2027 through 2045; this is a result of both higher costs and
lower emission reductions relative to proposed Option 1 in the later
years of the program. As noted throughout this section and discussed in
Sections III and IV, we do not currently have information to project
that the Alternative standards as currently formulated are feasible in
the MY 2027 timeframe with the emission control technologies we
evaluated to date, and thus we are not presenting an analysis of the
costs or benefits of the Alternative.
    Our current evaluation of available data shows that the standards
and useful life periods in both steps of proposed Option 1 are feasible
and that each step would result in the greatest degree of emission
reduction achievable for the model years to which they are proposed to
apply, pursuant to CAA section 202(a)(3), giving appropriate
consideration to cost, lead time, and other factors. Our analysis
further shows that the standards and useful life periods in proposed
Option 2 are feasible in the 2027 model year, but would result in lower
levels of emission reductions compared to proposed Option 1. Given the
analysis we present in this proposal, we currently believe that
proposed Option 1 may be a more appropriate level of stringency as it
would result in a greater level of achievable emission reduction for
the model years proposed, which is consistent with EPA's statutory
authority under Clean Air Act section 202(a)(3). However, as further
discussed in Section III and draft RIA Chapter 3, we expect that
additional data from EPA's ongoing work to demonstrate the performance
of emission control technologies, as well as information received in
public comments, will allow us to refine our assessments and
consideration of the feasibility of the combination of the standards
and useful life periods, particularly for the largest CI engines
(HHDEs), in proposed Options 1 and 2, after consideration of lead time,
costs, and other factors. Therefore, we are co-proposing Options 1 and
2 standards and useful life periods, and the range of options in
between them, as the options that may potentially be appropriate to
finalize pursuant to CAA section 202(a)(3) once EPA has considered that
additional data and other information.
    Our analysis further shows that the proposed Option 1 and 2
standards would have no negative impacts on energy; as discussed in
Section III, our evaluation of test engine data shows no change in
energy consumption (i.e., fuel) relative to a baseline engine.
Similarly, we anticipate no negative impacts on safety due to the
proposed program.
2. Basis of the Targeted Revisions to the HD GHG Phase 2 Program
    In addition to the proposed criteria pollutant program provisions,
we are proposing targeted updates to certain CO2 standards
for MY 2027 trucks, and we are requesting comment on updates to the
advanced technology incentive program for electric vehicles. The
transportation sector is the largest U.S. source of GHG emissions,
representing 29 percent of total GHG emissions.\110\ Within the
transportation sector, heavy-duty vehicles are the second largest
contributor, at 23 percent.\111\ GHG emissions have significant impacts
on public health and welfare as evidenced by the well-documented
scientific record and as set forth in EPA's Endangerment and Cause or
Contribute Findings under CAA section 202(a).\112\ Therefore, continued
emission reductions in the heavy-duty vehicle sector are appropriate.
---------------------------------------------------------------------------

    \110\ Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-2019 (EPA-430-R-21-005, published April 2021).
    \111\ Ibid.
    \112\ 74 FR 66496, December 15, 2009; 81 FR 54422, August 15,
2016.
---------------------------------------------------------------------------

    We are at the early stages of a significant transition in the
history of the heavy-duty on-highway sector--a shift to zero-emission
vehicle technologies. This change is underway and presents an
opportunity for significant reductions in heavy-duty GHG emissions.
Major trucking fleets, manufacturers and U.S. states have announced
plans to shift the heavy-duty fleet toward zero-emissions technology
beyond levels we accounted for in setting the existing HD GHG Phase 2
standards, as detailed in Section XI. Specifically, we set the existing
Phase 2 standards at levels that would require all conventional
vehicles to install varying combinations of emission-control
technologies (the degree and types of technology can differ, with some
vehicles that have less being offset by others with more, which would
lead to CO2 emissions reductions). As discussed in Section
XI, the rise in electrification beyond what we had anticipated when
finalizing the HD GHG Phase 2 program (e.g., the California Advanced
Clean Trucks rulemaking) would enable manufacturers to produce some
conventional vehicles without installing any of the GHG emission-
reducing technologies that we projected in the HD GHG Phase 2
rulemaking, absent the changes we are proposing in this
document.113 114
---------------------------------------------------------------------------

    \113\ CARB. ``Notice of Decision: Advanced Clean Truck
Regulation.'' June 2020. Available online at: https://ww3.arb.ca.gov/regact/2019/act2019/nod.pdf.
    For more information on this proposed rulemaking in California
see: https://ww2.arb.ca.gov/rulemaking/2019/advancedcleantrucks?utm_medium=email&utm_source=govdelivery.
    \114\ EPA is currently reviewing a waiver request under CAA
section 209(b) from California for the ACT rule.
---------------------------------------------------------------------------

    To address this issue, EPA is proposing under its authority in CAA
section 202(a) to revise GHG emissions standards for a subset of MY
2027 heavy-duty vehicles. Specifically, we

propose to adjust HD Phase 2 vehicle GHG emission standards by sales-
weighting the projected EV production levels of school buses, transit
buses, delivery trucks, and short-haul tractors and by lowering the
applicable GHG emission standards in MY 2027 accordingly. Our proposed
approach adjusts 17 of the 33 MY 2027 Phase 2 vocational vehicle and
tractor standards and does not change any MY 2021 or MY 2024 standards
or any of the Class 2b/3 pickup truck and van standards. In addition,
we are requesting comment on potential changes to the advanced
technology incentive program for electric vehicles beginning in MY
2024.
    Under CAA section 202(a), 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 within such
period.'' Thus, in establishing or revising CAA section 202(a)
standards, EPA must consider issues of technological feasibility,
compliance cost, and lead time. The proposed revised standards are
based on the same technology packages used to derive the current HD GHG
Phase 2 standards, which we applied to the subset of the vehicles that
would otherwise not require GHG-reducing technologies due to the higher
projection of HD electric vehicles in MY 2027 and beyond and the
incentive program. The HD GHG Phase 2 standards were based on adoption
rates for technologies in technology packages that EPA regards as
appropriate under CAA section 202(a) for the reasons given in the HD
GHG Phase 2 rulemaking in Section III.D.1 for tractors and Section
V.C.1 for vocational vehicles.\115\ We continue to believe these
technologies can be adopted at the estimated technology adoption rates
for these proposed revised standards within the lead time that would be
provided. The fleet-wide average cost per tractor projected to meet the
proposed revised MY 2027 standards is approximately $10,200 to $10,500.
The fleet-wide average cost per vocational vehicle to meet the proposed
revised MY 2027 standards ranges between $1,500 and $5,700. These
increased costs would be recovered in the form of fuel savings during
the first two years of ownership for tractors and first four years for
vocational vehicles, which we still consider to be reasonable.\116\ In
addition, manufacturers would retain leeway to develop alternative
compliance paths, increasing the likelihood of the proposed revised
standards' successful implementation. The targeted adjustments to the
select standards we are proposing would result in modest CO2
emissions reductions and climate-related benefits associated with these
emission reductions. As described in more detail in Section XI, we
believe this proposal considered feasibility, cost, lead time,
emissions impact, and other relevant factors, and therefore these
standards are appropriate under CAA section 202(a).
---------------------------------------------------------------------------

    \115\ 81 FR 73585 through 73613; 81 FR 73693 through 73719.
    \116\ 81 FR 73904.
---------------------------------------------------------------------------

    In addition to these proposed standard adjustments, we are
requesting comment on options to update the advanced technology
incentive program for electric and plug-in hybrid vehicles beginning in
MY 2024. These changes may be appropriate to reflect that such levels
of incentives for electrification may no longer be appropriate for
certain segments of the HD EV market. We are interested in trying to
balance providing incentivizes for the continued development of zero
and near-zero emission vehicles without inadvertently undermining the
GHG emission reductions expected from the existing HD GHG Phase 2
program with inappropriate incentives.

II. Need for Additional Emissions Control

    This proposal would reduce emissions from heavy-duty engines that
contribute to ambient levels of ozone, PM, NOX and CO, which
are all pollutants for which EPA has established health-based NAAQS.
These pollutants are linked to premature death, respiratory illness
(including childhood asthma), cardiovascular problems, and other
adverse health impacts. Many groups are at greater risk than healthy
people from these pollutants, including people with heart or lung
disease, outdoor workers, older adults and children. These pollutants
also reduce visibility and negatively impact ecosystems. This proposal
would also reduce emissions of air toxics from heavy-duty engines. A
more detailed discussion of the health and environmental effects
associated with the pollutants affected by this proposed rule is
included in Sections II.B and II.C and Chapter 4 of the draft RIA.
    As further described in Sections II.B.7 and II.B.8, populations who
live, work, or go to school near high-traffic roadways experience
higher rates of numerous adverse health effects, compared to
populations far away from major roads. In addition, there is
substantial evidence that people who live or attend school near major
roadways are more likely to be people of color, Hispanic ethnicity,
and/or low socioeconomic status.
    Across the U.S., NOX emissions from heavy-duty engines
are important contributors to concentrations of ozone and
PM2.5 and their resulting threat to public
health.117 118 The emissions modeling done for the proposed
rule \119\ (see Chapter 5 of the draft RIA) indicates that heavy-duty
engines will continue to be one of the largest contributors to mobile
source NOX emissions nationwide in the future, representing
32 percent of the mobile source NOX in calendar year
2045.\120\ Furthermore, it is estimated that heavy-duty engines will
represent 89 percent of the onroad NOX inventory in calendar
year 2045.\121\ The emission reductions that would occur from the
proposed rule are projected to reduce air pollution that is (and is
projected to continue to be) at levels that endanger public health and
welfare.
---------------------------------------------------------------------------

    \117\ Zawacki et al., 2018. Mobile source contributions to
ambient ozone and particulate matter in 2025. Atmospheric
Environment, Vol 188, pg 129-141. Available online: https://doi.org/10.1016/j.atmosenv.2018.04.057.
    \118\ Davidson et al., 2020. The recent and future health burden
of the U.S. mobile sector apportioned by source. Environmental
Research Letters. Available online: https://doi.org/10.1088/1748-9326/ab83a8.
    \119\ Sectors other than onroad were projected from 2016v1
Emissions Modeling Platform, http://views.cira.colostate.edu/wiki/wiki/10202.
    \120\ U.S. Environmental Protection Agency (2021). 2016v1
Platform. https://www.epa.gov/air-emissions-modeling/2016v1-platform.
    \121\ Han, Jaehoon. Memorandum to the Docket EPA-HQ-OAR-2019-
0055: ``MOVES Modeling-Related Data Files (MOVES Code, Input
Databases and Runspecs) for the Proposed Heavy-Duty 2027
Standards''. February 2022.
---------------------------------------------------------------------------

    Many state and local agencies across the country have asked the EPA
to further reduce NOX emissions, specifically from heavy-
duty engines, because such reductions will be a critical part of many
areas' strategies to attain and maintain the ozone and PM NAAQS. These
state and local agencies anticipate challenges in attaining the NAAQS,
maintaining the NAAQS in the future, and/or preventing nonattainment.
Some nonattainment areas have already been ``bumped up'' to higher
classifications because of challenges in attaining the NAAQS; others
say they are struggling to avoid nonattainment.\122\ Many state and
local agencies commented on the ANPR that heavy-duty vehicles are one
of their largest sources of NOX emissions. They

commented that without action to reduce emissions from heavy-duty
vehicles, they would have to adopt other potentially more burdensome
and costly measures to reduce emissions from other sources under their
state or local authority, such as local businesses. More information on
the projected emission reductions and air quality impacts that would
result from this proposed rule is provided in Sections VI and VII.
---------------------------------------------------------------------------

    \122\ For example, in September 2019 several 2008 ozone
nonattainment areas were reclassified from moderate to serious,
including Dallas, Chicago, Connecticut, New York/New Jersey and
Houston, and in January 2020, Denver. The 2008 NAAQS for ozone is an
8-hour standard with a level of 0.075 ppm, which the 2015 ozone
NAAQS lowered to 0.070 ppm.
---------------------------------------------------------------------------

    In their comments on the ANPR, environmental groups as well as
state, local, and Tribal agencies supported additional NOX
reductions from heavy-duty vehicles to address concerns about
environmental justice and ensuring that all communities benefit from
improvements in air quality. Commenters also supported additional
NOX reductions from heavy-duty vehicles in order to address
concerns about regional haze, and damage to terrestrial and aquatic
ecosystems. They mentioned the impacts of NOX emissions on
numerous locations, such as the Chesapeake Bay, Narragansett Bay, Long
Island Sound, Joshua Tree National Park and the surrounding Mojave
Desert, the Adirondacks, and other areas. Tribes and agencies commented
that NOX deposition into lakes is harmful to fish and other
aquatic life forms on which they depend for subsistence livelihoods.
They also commented that regional haze and increased rates of
weathering caused by pollution are of particular concern and can damage
culturally significant archeological sites.

A. Background on Pollutants Impacted by This Proposal

1. Ozone
    Ground-level ozone pollution forms in areas with high
concentrations of ambient NOX and VOCs when solar radiation
is strong. Major U.S. sources of NOX are highway and nonroad
motor vehicles, engines, power plants and other industrial sources,
with natural sources, such as soil, vegetation, and lightning, serving
as smaller sources. Vegetation is the dominant source of VOCs in the
U.S. Volatile consumer and commercial products, such as propellants and
solvents, highway and nonroad vehicles, engines, fires, and industrial
sources also contribute to the atmospheric burden of VOCs at ground-
level.
    The processes underlying ozone formation, transport, and
accumulation are complex. Ground-level ozone is produced and destroyed
by an interwoven network of free radical reactions involving the
hydroxyl radical (OH), NO, NO2, and complex reaction
intermediates derived from VOCs. Many of these reactions are sensitive
to temperature and available sunlight. High ozone events most often
occur when ambient temperatures and sunlight intensities remain high
for several days under stagnant conditions. Ozone and its precursors
can also be transported hundreds of miles downwind which can lead to
elevated ozone levels in areas with otherwise low VOC or NOX
emissions. As an air mass moves and is exposed to changing ambient
concentrations of NOX and VOCs, the ozone photochemical
regime (relative sensitivity of ozone formation to NOX and
VOC emissions) can change.
    When ambient VOC concentrations are high, comparatively small
amounts of NOX catalyze rapid ozone formation. Without
available NOX, ground-level ozone production is severely
limited, and VOC reductions would have little impact on ozone
concentrations. Photochemistry under these conditions is said to be
``NOX-limited.'' When NOX levels are sufficiently
high, faster NO2 oxidation consumes more radicals, dampening
ozone production. Under these ``VOC-limited'' conditions (also referred
to as '' NOX-saturated'' conditions), VOC reductions are
effective in reducing ozone, and NOX can react directly with
ozone resulting in suppressed ozone concentrations near NOX
emission sources. Under these NOX-saturated conditions,
NOX reductions can actually increase local ozone under
certain circumstances, but overall ozone production (considering
downwind formation) decreases and even in VOC-limited areas,
NOX reductions are not expected to increase ozone levels if
the NOX reductions are sufficiently large--large enough to
become NOX-limited.
    The primary NAAQS for ozone, established in 2015 and retained in
2020, is an 8-hour standard with a level of 0.07 ppm.\123\ EPA recently
announced that it will reconsider the previous administration's
decision to retain the ozone NAAQS.\124\ The EPA is also implementing
the previous 8-hour ozone primary standard, set in 2008, at a level of
0.075 ppm. As of May 31, 2021, there were 34 ozone nonattainment areas
for the 2008 ozone NAAQS, composed of 151 full or partial counties,
with a population of more than 99 million, and 50 ozone nonattainment
areas for the 2015 ozone NAAQS, composed of 205 full or partial
counties, with a population of more than 122 million. In total, there
are currently, as of May 31, 2021, 57 ozone nonattainment areas with a
population of more than 122 million people.\125\
---------------------------------------------------------------------------

    \123\ https://www.epa.gov/ground-level-ozone-pollution/ozone-national-ambient-air-quality-standards-naaqs.
    \124\ https://www.epa.gov/ground-level-ozone-pollution/epa-reconsider-previous-administrations-decision-retain-2015-ozone.
    \125\ The population total is calculated by summing, without
double counting, the 2008 and 2015 ozone nonattainment populations
contained in the Criteria Pollutant Nonattainment Summary report
(https://www.epa.gov/green-book/green-book-data-download).
---------------------------------------------------------------------------

    States with ozone nonattainment areas are required to take action
to bring those areas into attainment. The attainment date assigned to
an ozone nonattainment area is based on the area's classification. The
attainment dates for areas designated nonattainment for the 2008 8-hour
ozone NAAQS are in the 2015 to 2032 timeframe, depending on the
severity of the problem in each area. Attainment dates for areas
designated nonattainment for the 2015 ozone NAAQS will be in the 2021
to 2038 timeframe, again depending on the severity of the problem in
each area.\126\ The proposed rule would begin to take effect in MY 2027
and would assist areas with attaining the NAAQS and may relieve areas
with already stringent local regulations from some of the burden
associated with adopting additional local controls.\127\ The proposed
rule could also provide assistance to counties with ambient
concentrations near the level of the NAAQS who are working to ensure
long-term attainment or maintenance of the NAAQS.
---------------------------------------------------------------------------

    \126\ https://www.epa.gov/ground-level-ozone-pollution/ozone-naaqs-timelines.
    \127\ While not quantified in the air quality modeling analysis
for this proposed rule, the Early Adoption Incentives under the
proposed program could encourage manufacturers to introduce new
emission control technologies prior to the 2027 model year, which
may help to accelerate some benefits of the proposed program (See
Preamble Section IV.H for more details on the proposed Early
Adoption Incentives).
---------------------------------------------------------------------------

2. Particulate Matter
    Particulate matter (PM) is a complex mixture of solid particles and
liquid droplets distributed among numerous atmospheric gases which
interact with solid and liquid phases. Particles in the atmosphere
range in size from less than 0.01 to more than 10 micrometers ([mu]m)
in diameter.\128\ Atmospheric particles can be grouped into several
classes according to their aerodynamic diameter and physical sizes.
Generally, the three broad classes of particles include ultrafine
particles (UFPs, generally

considered as particles with a diameter less than or equal to 0.1 [mu]m
[typically based on physical size, thermal diffusivity or electrical
mobility]), ``fine'' particles (PM2.5; particles with a
nominal mean aerodynamic diameter less than or equal to 2.5 [mu]m), and
``thoracic'' particles (PM10; particles with a nominal mean
aerodynamic diameter less than or equal to 10 [mu]m). Particles that
fall within the size range between PM2.5 and
PM10, are referred to as ``thoracic coarse particles''
(PM10-2.5, particles with a nominal mean aerodynamic
diameter greater than 2.5 [mu]m and less than or equal to 10 [mu]m).
EPA currently has NAAQS for PM2.5 and PM10.\129\
---------------------------------------------------------------------------

    \128\ U.S. EPA. Policy Assessment (PA) for the Review of the
National Ambient Air Quality Standards for Particulate Matter (Final
Report, 2020). U.S. Environmental Protection Agency, Washington, DC,
EPA/452/R-20/002, 2020.
    \129\ Regulatory definitions of PM size fractions, and
information on reference and equivalent methods for measuring PM in
ambient air, are provided in 40 CFR parts 50, 53, and 58. With
regard to NAAQS which provide protection against health and welfare
effects, the 24-hour PM10 standard provides protection
against effects associated with short-term exposure to thoracic
coarse particles (i.e., PM10-2.5).
---------------------------------------------------------------------------

    Most particles are found in the lower troposphere, where they can
have residence times ranging from a few hours to weeks. Particles are
removed from the atmosphere by wet deposition, such as when they are
carried by rain or snow, or by dry deposition, when particles settle
out of suspension due to gravity. Atmospheric lifetimes are generally
longest for PM2.5, which often remains in the atmosphere for
days to weeks before being removed by wet or dry deposition.\130\ In
contrast, atmospheric lifetimes for UFP and PM10-2.5 are
shorter. Within hours, UFP can undergo coagulation and condensation
that lead to formation of larger particles in the accumulation mode, or
can be removed from the atmosphere by evaporation, deposition, or
reactions with other atmospheric components. PM10-2.5 are
also generally removed from the atmosphere within hours, through wet or
dry deposition.\131\
---------------------------------------------------------------------------

    \130\ U.S. EPA. Integrated Science Assessment (ISA) for
Particulate Matter (Final Report, 2019). U.S. Environmental
Protection Agency, Washington, DC, EPA/600/R-19/188, 2019. Table 2-
1.
    \131\ U.S. EPA. Integrated Science Assessment (ISA) for
Particulate Matter (Final Report, 2019). U.S. Environmental
Protection Agency, Washington, DC, EPA/600/R-19/188, 2019. Table 2-
1.
---------------------------------------------------------------------------

    Particulate matter consists of both primary and secondary
particles. Primary particles are emitted directly from sources, such as
combustion-related activities (e.g., industrial activities, motor
vehicle operation, biomass burning), while secondary particles are
formed through atmospheric chemical reactions of gaseous precursors
(e.g., sulfur oxides (SOX), nitrogen oxides (NOX)
and volatile organic compounds (VOCs)). From 2000 to 2017, national
annual average ambient PM2.5 concentrations have declined by
over 40 percent,\132\ largely reflecting reductions in emissions of
precursor gases.
---------------------------------------------------------------------------

    \132\ See https://www.epa.gov/air-trends/particulate-matter-pm25-trends and https://www.epa.gov/air-trends/particulate-matter-pm25-trends#pmnat for more information.
---------------------------------------------------------------------------

    There are two primary NAAQS for PM2.5: An annual
standard (12.0 micrograms per cubic meter ([mu]g/m3)) and a 24-hour
standard (35 [mu]g/m3), and there are two secondary NAAQS for
PM2.5: An annual standard (15.0 [mu]g/m3) and a 24-hour
standard (35 [mu]g/m3). The initial PM2.5 standards were set
in 1997 and revisions to the standards were finalized in 2006 and in
December 2012 and then retained in 2020. On June 10, 2021, EPA
announced that it will reconsider the previous administration's
decision to retain the PM NAAQS.\133\
---------------------------------------------------------------------------

    \133\ https://www.epa.gov/pm-pollution/national-ambient-air-quality-standards-naaqs-pm.
---------------------------------------------------------------------------

    There are many areas of the country that are currently in
nonattainment for the annual and 24-hour primary PM2.5
NAAQS. As of May 31, 2021, more than 19 million people lived in the 4
areas that are designated as nonattainment for the 1997
PM2.5 NAAQS. Also, as of May 31, 2021, more than 31 million
people lived in the 14 areas that are designated as nonattainment for
the 2006 PM2.5 NAAQS and more than 20 million people lived
in the 6 areas designated as nonattainment for the 2012
PM2.5 NAAQS. In total, there are currently 17
PM2.5 nonattainment areas with a population of more than 32
million people.\134\ The proposed rule would take effect in MY 2027 and
would assist areas with attaining the NAAQS and may relieve areas with
already stringent local regulations from some of the burden associated
with adopting additional local controls.\135\ The proposed rule would
also assist counties with ambient concentrations near the level of the
NAAQS who are working to ensure long-term attainment or maintenance of
the PM2.5 NAAQS.
---------------------------------------------------------------------------

    \134\ The population total is calculated by summing, without
double counting, the 1997, 2006 and 2012 PM2.5
nonattainment populations contained in the Criteria Pollutant
Nonattainment Summary report (https://www.epa.gov/green-book/green-book-data-download).
    \135\ While not quantified in the air quality modeling analysis
for this proposed rule, the Early Adoption Incentives under the
proposed program could encourage manufacturers to introduce new
emission control technologies prior to the 2027 model year, which
may help to accelerate some benefits of the proposed program (See
Preamble Section IV.H for more details on the proposed Early
Adoption Incentives).
---------------------------------------------------------------------------

3. Nitrogen Oxides
    Oxides of nitrogen (NOX) refers to nitric oxide (NO) and
nitrogen dioxide (NO2). Most NO2 is formed in the
air through the oxidation of nitric oxide (NO) emitted when fuel is
burned at a high temperature. NOX is a criteria pollutant,
regulated for its adverse effects on public health and the environment,
and highway vehicles are an important contributor to NOX
emissions. NOX, along with VOCs, are the two major
precursors of ozone and NOX is also a major contributor to
secondary PM2.5 formation. There are two primary NAAQS for
NO2: An annual standard (53 ppb) and a 1-hour standard (100
ppb).\136\ In 2010, EPA established requirements for monitoring
NO2 near roadways expected to have the highest
concentrations within large cities. Monitoring within this near-roadway
network began in 2014, with additional sites deployed in the following
years. At present, there are no nonattainment areas for NO2.
---------------------------------------------------------------------------

    \136\ The statistical form of the 1-hour NAAQS for
NO2 is the 3-year average of the yearly distribution of
1-hour daily maximum concentrations.
---------------------------------------------------------------------------

4. Carbon Monoxide
    Carbon monoxide (CO) is a colorless, odorless gas emitted from
combustion processes. Nationally, particularly in urban areas, the
majority of CO emissions to ambient air come from mobile sources.\137\
There are two primary NAAQS for CO: An 8-hour standard (9 ppm) and a 1-
hour standard (35 ppm). There are currently no CO nonattainment areas;
as of September 27, 2010, all CO nonattainment areas have been
redesignated to attainment. The past designations were based on the
existing community-wide monitoring network. EPA made an addition to the
ambient air monitoring requirements for CO during the 2011 NAAQS
review. Those new requirements called for CO monitors to be operated
near roads in Core Based Statistical Areas (CBSAs) of 1 million or more
persons, in addition to the existing community-based network (76 FR
54294, August 31, 2011).
---------------------------------------------------------------------------

    \137\ U.S. EPA, (2010). Integrated Science Assessment for Carbon
Monoxide (Final Report). U.S. Environmental Protection Agency,
Washington, DC, EPA/600/R-09/019F, 2010. http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=218686. See Section 2.1.
---------------------------------------------------------------------------

5. Diesel Exhaust
    Diesel exhaust is a complex mixture composed of particulate matter,
carbon dioxide, oxygen, nitrogen, water vapor, carbon monoxide,
nitrogen compounds, sulfur compounds and numerous low-molecular-weight
hydrocarbons. A number of these gaseous hydrocarbon components are
individually known to be toxic, including aldehydes, benzene

and 1,3-butadiene. The diesel particulate matter present in diesel
exhaust consists mostly of fine particles (<2.5 [mu]m), of which a
significant fraction is ultrafine particles (<0.1 [mu]m). These
particles have a large surface area which makes them an excellent
medium for adsorbing organics and their small size makes them highly
respirable. Many of the organic compounds present in the gases and on
the particles, such as polycyclic organic matter, are individually
known to have mutagenic and carcinogenic properties.
    Diesel exhaust varies significantly in chemical composition and
particle sizes between different engine types (heavy-duty, light-duty),
engine operating conditions (idle, acceleration, deceleration), and
fuel formulations (high/low sulfur fuel). Also, there are emissions
differences between on-road and nonroad engines because the nonroad
engines are generally of older technology. After being emitted in the
engine exhaust, diesel exhaust undergoes dilution as well as chemical
and physical changes in the atmosphere. The lifetime of the components
present in diesel exhaust ranges from seconds to days.
    Because diesel particulate matter (DPM) is part of overall ambient
PM, varies considerably in composition, and lacks distinct chemical
markers that enable it to be easily distinguished from overall primary
PM, we do not have direct measurements of DPM in the ambient air.\138\
DPM concentrations are estimated using ambient air quality modeling
based on DPM emission inventories. DPM emission inventories are
computed as the exhaust PM emissions from mobile sources combusting
diesel or residual oil fuel. DPM concentrations were estimated as part
of the 2014 National Air Toxics Assessment (NATA).\139\ Areas with high
concentrations are clustered in the Northeast, Great Lake States,
California, and the Gulf Coast States, with the highest impacts
occurring in major urban cores, and are also distributed throughout the
rest of the U.S. Approximately half of average ambient DPM in the U.S.
can be attributed to heavy-duty diesel engines, with the remainder
attributable to nonroad engines.
---------------------------------------------------------------------------

    \138\ DPM in exhaust from a high-load, high-speed engine (e.g.,
heavy-duty truck engines) without aftertreatment such as a diesel
particle filter (DPM) is mostly made of ``soot,'' consisting of
elemental/black carbon (EC/BC), some organic material, and trace
elements. At low loads, DPM in high-speed engine exhaust is mostly
made of organic carbon (OC), with considerably less EC/BC. Low-speed
diesel engines' (e.g., large marine engines) exhaust PM is comprised
of more sulfate and less EC/BC, with OC contributing as well.
    \139\ U.S. EPA (2018) Technical Support Document EPA's 2014
National Air Toxics Assessment. https://www.epa.gov/national-air-toxics-assessment/2014-nata-assessment-results.
---------------------------------------------------------------------------

6. Air Toxics
    The most recent available data indicate that the majority of
Americans continue to be exposed to ambient concentrations of air
toxics at levels which have the potential to cause adverse health
effects.\140\ The levels of air toxics to which people are exposed vary
depending on where people live and work and the kinds of activities in
which they engage, as discussed in detail in EPA's 2007 Mobile Source
Air Toxics Rule.\141\ According to the National Air Toxic Assessment
(NATA) for 2014, mobile sources were responsible for over 40 percent of
outdoor anthropogenic toxic emissions and were the largest contributor
to national average cancer and noncancer risk from directly emitted
pollutants.142 143 Mobile sources are also significant
contributors to precursor emissions which react to form air
toxics.\144\ Formaldehyde is the largest contributor to cancer risk of
all 71 pollutants quantitatively assessed in the 2014 NATA. Mobile
sources were responsible for more than 25 percent of primary
anthropogenic emissions of this pollutant in 2014 and are significant
contributors to formaldehyde precursor emissions. Benzene is also a
large contributor to cancer risk, and mobile sources account for almost
70 percent of ambient exposure.
---------------------------------------------------------------------------

    \140\ U.S. EPA (2018) Technical Support Document EPA's 2014
National Air Toxics Assessment. https://www.epa.gov/national-air-toxics-assessment/2014-nata-assessment-results.
    \141\ U.S. Environmental Protection Agency (2007). Control of
Hazardous Air Pollutants from Mobile Sources; Final Rule. 72 FR
8434, February 26, 2007.
    \142\ U.S. EPA. (2018) 2014 NATA: Assessment Results. https://www.epa.gov/national-air-toxics-assessment/2014-nata-assessment-results.
    \143\ NATA also includes estimates of risk attributable to
background concentrations, which includes contributions from long-
range transport, persistent air toxics, and natural sources; as well
as secondary concentrations, where toxics are formed via secondary
formation. Mobile sources substantially contribute to long-range
transport and secondarily formed air toxics.
    \144\ Rich Cook, Sharon Phillips, Madeleine Strum, Alison Eyth &
James Thurman (2020): Contribution of mobile sources to secondary
formation of carbonyl compounds, Journal of the Air & Waste
Management Association, DOI: 10.1080/10962247.2020.1813839.
---------------------------------------------------------------------------

B. Health Effects Associated With Exposure to Pollutants Impacted by
This Proposal

    Heavy duty engines emit pollutants that contribute to ambient
concentrations of ozone, PM, NO2, CO, and air toxics. A
discussion of the health effects associated with exposure to these
pollutants, and a discussion on environmental justice, is included in
this section of the preamble. Additionally, children are recognized to
have increased vulnerability and susceptibility related to air
pollution and other environmental exposures; this is discussed further
in Section XIII of the Preamble. Information on emission reductions and
air quality impacts from this proposed rule are included in Section VI
and VII of this preamble.
1. Ozone
    This section provides a summary of the health effects associated
with exposure to ambient concentrations of ozone.\145\ The information
in this section is based on the information and conclusions in the
April 2020 Integrated Science Assessment for Ozone (Ozone ISA).\146\
The Ozone ISA concludes that human exposures to ambient concentrations
of ozone are associated with a number of adverse health effects and
characterizes the weight of evidence for these health effects.\147\ The
discussion below highlights the Ozone ISA's conclusions pertaining to
health effects associated with both short-term and long-term periods of
exposure to ozone.
---------------------------------------------------------------------------

    \145\ Human exposure to ozone varies over time due to changes in
ambient ozone concentration and because people move between
locations which have notably different ozone concentrations. Also,
the amount of ozone delivered to the lung is influenced not only by
the ambient concentrations but also by the breathing route and rate.
    \146\ U.S. EPA. Integrated Science Assessment (ISA) for Ozone
and Related Photochemical Oxidants (Final Report). U.S.
Environmental Protection Agency, Washington, DC, EPA/600/R-20/012,
2020.
    \147\ The ISA evaluates evidence and draws conclusions on the
causal relationship between relevant pollutant exposures and health
effects, assigning one of five ``weight of evidence''
determinations: Causal relationship, likely to be a causal
relationship, suggestive of a causal relationship, inadequate to
infer a causal relationship, and not likely to be a causal
relationship. For more information on these levels of evidence,
please refer to Table II in the Preamble of the ISA.
---------------------------------------------------------------------------

    For short-term exposure to ozone, the Ozone ISA concludes that
respiratory effects, including lung function decrements, pulmonary
inflammation, exacerbation of asthma, respiratory-related hospital
admissions, and mortality, are causally associated with ozone exposure.
It also concludes that metabolic effects, including metabolic syndrome
(i.e., changes in insulin or glucose levels, cholesterol levels,
obesity and blood pressure) and complications due to diabetes are
likely to be causally associated with short-term exposure to ozone and
that evidence is suggestive of a causal relationship between
cardiovascular

effects, central nervous system effects and total mortality and short-
term exposure to ozone.
    For long-term exposure to ozone, the Ozone ISA concludes that
respiratory effects, including new onset asthma, pulmonary inflammation
and injury, are likely to be causally related with ozone exposure. The
Ozone ISA characterizes the evidence as suggestive of a causal
relationship for associations between long-term ozone exposure and
cardiovascular effects, metabolic effects, reproductive and
developmental effects, central nervous system effects and total
mortality. The evidence is inadequate to infer a causal relationship
between chronic ozone exposure and increased risk of cancer.
    Finally, interindividual variation in human responses to ozone
exposure can result in some groups being at increased risk for
detrimental effects in response to exposure. In addition, some groups