United States
Environmental Protection
M * Agency
Office of Transportation and Air Quality
EPA-420-B-21-038
October 2021
PM Hot-Spot Guidance: Transportation Conformity Guidance for Quantitative
Hot-Spot Analyses in PIVh.sand PMi0 Nonattainment and Maintenance Areas
Guidance Change Bulletin: October 2021
Overall Guidance Changes:
Changes throughout the document include:
•	Updates to reflect M0VES3
•	Updates to eliminate references to outdated EMFAC model version
•	Updates to reflect that AERMOD is the required air quality dispersion model for PM hot-spot
analyses
•	Updated web pages and references
Specific Guidance Changes:
Part of Guidance
Change from 2015 Guidance to
2021 Guidance
Reason for Change
Section 1.2: Requirement for
Quantitative PM Hot-Spot
Analyses
Section abridged
Requirement for quantitative PM
hot-spot analyses has been in
effect since 2012
Section 1.5: Other Purposes
for this Guidance
Additional specificity about
other purposes, such as National
Environmental Policy Act or
assessing near-source air quality
in communities with
environmental justice concerns
Provides clarity to be helpful to
stakeholders
Section 2.3: Interagency
Consultation and Public
Participation Requirements
Deleted example to only model
one quarter
Added sentence about
importance of documentation
Example has not been used in
implementation to date
Reflects guidance implementation
and experience in the field
Section 2.4: Hot-Spot
Analyses Are Build/No-Build
Analyses
Added "design concentration"
terminology in addition to
design value; this new term is
used throughout the guidance
Term included in the last update
to the Appendix W regulation
published January 2017. Design
concentration is calculated based
on monitoring and modeled data
Section 2.8: Appropriate
Time Frame and Analysis
Years
Added example for selecting
both a near-term and a longer-
term analysis year
Provides a useful example
Section 2.9: Agency Roles
and Responsibilities
Added more description of
EPA's role (Section 2.9.3)
Clarifies all of EPA's
responsibilities to improve
implementation of guidance and
related requirements

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PM Hot-spot Guidance Change Bulletin, EPA-420-B-21-038
October 2021
Part of Guidance
Change from 2015 Guidance to
2021 Guidance
Reason for Change
Section 3.3: Determine
Approach, Models, and Data
(Step 2)
Added text about using
interagency consultation to help
identify the appropriate
receptor locations in the area
substantially affected by the
project (Section 3.3.2)
Adds useful concepts and detail to
assist in implementation

Deleted discussion of modeling
only one quarter of an analysis
year (Section 3.3.4)
Example has not been used in
implementation to date
Section 3.3.6: Determining
the models and methods to
be used
Added cite to EPA's Appendix W
Guideline
Deleted sentence about
selecting an air quality model
Appendix W is the regulation that
governs air quality modeling
AERMOD is now the only model
for PM hot-spot analyses, per the
Appendix W regulation published
January 2017
Section 3.6: Select Source
Types, Data Inputs, and
Receptors for AERMOD (Step
5)
Revised to clarify that AERMOD
is the required model
AERMOD is now the only model
for PM hot-spot analyses, per the
Appendix W regulation published
January 2017
Section 3.10: Document the
PM Hot-Spot Analysis
Added cite to transportation
conformity regulation
Provides clarity

Added to what should be
included in documentation,
including maps and diagrams,
explanation of naming
conventions if used for project
links, and step-by-step
explanation of how emissions
are used as inputs in air quality
modeling
Reflects guidance implementation
and experience in the field
Section 4: Estimating
Project-Level PM Emissions
Using MOVES
Comprehensively revised in its
entirety to reflect MOVES3
EPA released MOVES3 (86 Federal
Register 1106); this revised
guidance applies to minor
revisions to MOVES3 when issued
Section 4.2: Characterizing a
Project in Terms of Links
Updated text
Adds clarity
Section 4.3.: Determining
the Number of MOVES Runs
Changed guidance for number
of MOVES runs: for most
projects (those without gasoline
starts), four MOVES runs are
sufficient to capture variability
instead of 16
PM temperature effects have
changed with MOVES3; see EPA's
report Emission Adjustments for
Temperature¦, Humidity, Air
Conditioning, and Inspection and
Maintenance for On road Vehicles
2

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PM Hot-spot Guidance Change Bulletin, EPA-420-B-21-038
October 2021
Part of Guidance
Change from 2015 Guidance to
2021 Guidance
Reason for Change

Deleted discussion of modeling
only one quarter of an analysis
year
in MOVES3, November 2020, EPA-
420-R-20-013, available on EPA's
website at:
https://www. eDa.gov/moves/mov
es-onroad-technical-
reports#moves3
Example has not been used in
implementation to date
Section 5: Estimating
Project-Level PM Emissions
Using the EMFAC Model in
California
Sections 5.1 - 5.3 updated
Sections 5.4 - 5.9 deleted
Eliminates references to outdated
EMFAC version; provides EMFAC
general guidance
Sections were based on outdated
EMFAC model; EMFAC modelers
directed to California Air
Resources Board model
documentation, consistent with
approach in 2015 version of EPA's
guidance
Section 7: Estimating
Project-Level PM
Concentrations Using
AERMOD
Section updated throughout to
remove CAL3QHCR
AERMOD is now the only model
for PM hot-spot analyses, per the
Appendix W regulation published
January 2017
Section 7.3: Using AERMOD
Sentences added to describe
new AERMOD source types
(RLINE and RLINEXT)
Text added to describe that it is
useful to consider modeling
decisions in advance, for
example, with a modeling
protocol
New source types added to
AERMOD in version 19191
Reflects guidance implementation
and experience in the field
Section 7.6: Placing
Receptors
Added examples of where
maximum concentrations may
not be located closest to the
project
Clarified guidance for receptor
placement, including when
additional receptors are used for
communities in the project area
Reflects guidance implementation
and experience in the field
Reflects guidance implementation
and experiences in the field
3

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PM Hot-spot Guidance Change Bulletin, EPA-420-B-21-038
October 2021
Part of Guidance
Change from 2015 Guidance to
2021 Guidance
Reason for Change
Section 8.1: Introduction
Section 8.2: Nearby Sources
that Require Modeling
Clarified that nearby sources
would be included in air quality
modeling when they are not
reflected in background data, in
addition to when those sources
would be affected by the project
Important point to include,
consistent with Appendix W
requirements
Section 8.3: Options for
Background Concentrations
Removed option to adjust air
quality monitoring data using an
onroad mobile source
adjustment factor (previously,
Section 8.3.3)
This option was appropriate in
limited use cases; example has not
been used in implementation to
date
Section 9: Calculating PM
Design Concentrations and
Determining Conformity
Entire section updated with the
term "design concentration"
Adds clarity by distinguishing
design concentration, which is a
combination of modeled and
monitor values, from design value,
which is based solely on monitor
values
Section 9.3: Calculating
Design Concentrations and
Determining Conformity for
PM Hot-Spot Analyses
Moved paragraph about
alternative design concentration
methods from Section 9.3.4
(PM10 NAAQS) to introduction,
Section 9.3.1
Paragraph pertains to all PM
NAAQS

Eliminated calculation steps
pertaining to CAL3QHCR model
CAL3QHCR model removed in the
January 2017 Appendix W
regulation

Some steps amended to reflect
available website information
New information available on
EPA's website
Section 10: Mitigation and
Control Measures
Updated web links
Ensures document is current
Appendix A: Clearinghouse
of Websites, Guidance, and
Other Technical Resources
for PM Hot-Spot Analyses
Updated web links and
documents
Ensures document is current
Appendix B: Examples of
Projects of Local Air Quality
Concern
Added examples found in EPA's
PM Hot-spot FAQs
Ensures document is current and
includes all examples
Appendix D3: Option 2:
Using Link Drive Schedules
Updated graphic and discussion
Provides additional clarity about
this option; may make it easier to
implement
Appendix G: Example of
Using EMFAC2011 for a
Highway Project
Removed content but appendix
retained
Example pertained to a version of
EMFAC no longer in use
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PM Hot-spot Guidance Change Bulletin, EPA-420-B-21-038
October 2021
Part of Guidance
Change from 2015 Guidance to
2021 Guidance
Reason for Change
Appendix H: Example of
Using EMFAC2011to Develop
Emission Factors for a Transit
Project
Removed content but appendix
retained
Example pertained to a version of
EMFAC no longer in use
Appendix 1.3: Determining
When to Analyze Emissions
(Locomotives)
Deleted discussion of modeling
only one quarter of an analysis
year
Example has not been used in
implementation to date
Appendix J: Additional
Reference Information on
AERMOD
Appendix J updated throughout
to remove CAL3QHCR
References to AERMOD updated
Updated discussion of how to
match MOVES runs with periods
of time in AERMOD via
EMISFACT (Section J.3.3)
AERMOD is now the only model
for PM hot-spot analyses, per the
Appendix W regulation published
January 2017
Ensures document is current
Adds clarity and reflects guidance
change for number of MOVES runs
for certain cases
Appendix K: Examples of
Design Concentration
Calculations for PM Hot-Spot
Analysis
Updated years in examples
Ensures document is current
5

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