Guidance on New R-LINE Additions
to AERMOD 19191 for Refined
Transportation Project Analyses
A	United States
Environmental ProlGtlion
hI	Agency

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Guidance on New R-LINE Additions
to AERMOD 19191 for Refined
Transportation Project Analyses
Transportation and Climate Division
Office of Transportation and Air Quality
Office of Air and Radiation
and
Air Quality Assessment Division
Office of Air Quality Planning and Standards
Office of Air and Radiation
and
Computational Exposure Division
National Exposure Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
SEPA
United States	EPA-420-B-19-042
EnvironmBntBl ProlocHon	„ , .
Agency	September 2019

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Contents
1	Introduction	1
1.1	Purpose	1
1.2	Background on Transportation Conformity	1
1.3	EPA's AERMOD Model: Background and Recent History	2
1.4	Finding More Information about AERMOD	3
1.5	EPA's R-LINE Model: Background	3
1.6	Other Purposes for this Guidance	3
1.7	EPA Contact Information	4
1.8	Guidance and Existing Requirements	4
2	New R-LINE Features in the AERMOD 19191 Release	5
2.1	What features of R-LINE are included in AERMOD 19191, and what is their status?	5
2.2	What is the process for obtaining approval of RLINE as an alternative model?	6
2.3	If RLINE is approved through the alternative model process for use in a transportation conformity
determination, what regulations and guidance apply to the transportation conformity determination?
	7
2.4	How are RLINE sources included in an AERMOD input file?	7
2.4.1	Control Pathway:	7
2.4.2	Source Pathway	8
2.5	When would RLEMCONV be used with the RLINE source type?	9
2.6	What is MOVES2AERMOD?	10
3	ALPHA Options: RLINEXT, RBARRIER, and RDEPRESS	12
3.1	Can RLINEXT and its associated barrier and depressed roadway features be used through the
alternative model process?	12
3.2	How are RLINEXT sources included in an AERMOD input file?	12
3.2.1	Control Pathway:	12
3.2.2	Source Pathway	13
3.3	How would a solid barrier be modeled with RBARRIER?	15
3.4	What are some examples of how barrier could be modeled?	16
3.5	How would a depressed road be modeled with RDEPRESS?	19
3.6	Can RLEMCONV be used with the RLINEXT source type?	20

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1 Introduction
1.1	Purpose
On August 21, 2019, EPA released a new version of its preferred dispersion model, AERMOD
19191. AERMOD 19191 builds upon the previous version of AERMOD, AERMOD 18081, and
adds new features from EPA's R-LINE model. Information about AERMOD and this latest
release can be found on EPA's website at: https://www.epa.gov/scram/air-quality-dispersion-
m odeling-preferred-and-recommended-m odel s.
This guidance is part of the resources EPA is providing with the release of AERMOD 19191. It
addresses the use of the new R-LINE features in AERMOD 19191 for dispersion modeling of
transportation sources, which are described in Section 2 of this guidance.
This guidance applies when using the R-LINE features in AERMOD for refined modeling of
transportation projects, as appropriate, for transportation conformity and general conformity
purposes, and other purposes as well. While some of the features can be used for regulatory
purposes such as transportation conformity with EPA approval, others cannot be used for
regulatory purposes at this time (see Section 2.1). For those features that can be used with
approval, this guidance describes the alternative model approval process that would be used (see
Section 2.2).
EPA has coordinated with the Federal Highway Administration and the Federal Transit
Administration during the development of this guidance.
1.2	Background on Transportation Conformity
Transportation conformity is required under Clean Air Act (CAA) section 176(c) (42 U.S.C.
7506(c)) to ensure that federally supported highway and transit project activities are consistent
with (conform to) the purpose of a state air quality implementation plan (SIP). Conformity to the
purpose of the SIP means that transportation activities will not cause or contribute to new air
quality violations, worsen existing violations, or delay timely attainment of the relevant national
ambient air quality standards (NAAQS) or required interim milestones.
EPA's transportation conformity rule (40 CFR 51.390 and Part 93) establishes the criteria and
procedures for determining whether transportation activities conform to the SIP.1 Criteria and
procedures for transportation conformity hot-spot analyses are found specifically in 40 CFR
93.116 and 93.123. EPA also has guidance for PM hot-spot analyses and CO hot-spot analyses.
EPA's PM Hot-spot Guidance provides more details for conducting PM hot-spot analyses,
including information and guidance about air quality dispersion modeling.2 EPA hot-spot
guidance for PM and CO analyses as well as training materials and other helpful information can
1	EPA has provided the complete transportation conformity regulations in a document found at:
https://nepis.epa. gov/Exe/ZvPDF.cgi/P100E7CS.PDF?Dockev=P100E7CS.PDF.
2	"Transportation Conformity Guidance for Quantitative Hot-spot Analyses in PM2 5 and PMio Nonattainment and
Maintenance Areas, EPA-420-B-15-084, November 2015. This guidance is referred to as the "PM Hot-spot
Guidance" and is available on EPA's website at www.epa.gov/state~and~tocat~tfansportatlon/proiect~tevel~
co nfo rmi fv -a nd-ho t~spo t ¦~a nalv ses#p meuida nee.
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be found on EPA's website at www.epa.gov/state~and~local~transport.ation/proiect~level~
conformitv~and~hot~spot~analvses.
1.3 EPA's AERMOD Model: Background and Recent History
The American Meteorological Society/EPA Regulatory Model (AERMOD) is EPA's
recommended near-field dispersion model for many regulatory applications. A dispersion model
is used to determine the concentration of air pollutants in the atmosphere, based on the emission
sources in the area and meteorology, as well as other inputs. AERMOD is a steady-state plume
model that incorporates air dispersion based on planetary boundary layer turbulence structure
and scaling concepts. AERMOD includes options for modeling concentrations from emissions
that come from area, volume, line, and point sources and can therefore model impacts of many
different source types, including highway and transit transportation projects. To date, AERMOD
has been used to model air quality near roadways, other transportation sources, and other ground-
level sources for regulatory applications by EPA and other federal and state agencies, including
transportation conformity hot-spot analyses (see Section 1.2).
EPA initially adopted AERMOD as a preferred model in a November 9, 2005 final rule (70 FR
68218) that amended EPA's "Guideline on Air Quality Models" ("Guideline") after more than
ten years of development and peer review that resulted in substantial improvements and
enhancements.3
When quantitative PM hot-spot analyses for certain transportation projects were initially required
in December of 2010, AERMOD was one of two dispersion models recommended for use. The
other model was CAL3QHCR, which is based on the CALINE3 series of models. EPA's PM
Hot-spot Guidance provides more details for conducting PM hot-spot analyses; refer to Section
1.2 for more information.
On January 17, 2017, EPA promulgated a final rule to amend the "Guideline" (82 FR 5182).4 In
this action, EPA included enhancements to the AERMOD modeling system, and established
AERMOD as the preferred model for refined dispersion modeling for most regulatory
applications. This final rule also removed the CALINE3 model from the Guideline. For refined
modeling for hot-spot analyses done for transportation conformity, EPA provided a three-year
grace period to fully transition from CALINE3 to AERMOD. This grace period ends January
17, 2020. For PM2.5 and PM10 hot-spot analyses, no new analyses can be started with
CAL3QHCR after January 17, 2020. New analyses begun after January 17, 2020 will need to
use the current version of AERMOD. However, PM hot-spot analyses that were begun before
this date with CAL3QCHR can be completed with CAL3QHCR.5
Note that as a practical matter, the end of the CALINE3 grace period is relevant only for refined
analyses: CAL3QHC can continue to be used for CO hot-spot screening analyses for
3	This 2005 final rule can be found at: www.epa.gov/scram001/giiidance/giiide/appw 05.pdf. Extensive
documentation is available on EPA's SCRAM website describing the various components of AERMOD, including
user guides, model formulation, and evaluation papers. See: www.epa.gov/scram/air~qiialitv-dispersion~modeling-
preferred-and-recommended-models#aermod.
4	This final rule can be found at: https://www3.epa.gov/ttti/scram/guidance/guide/appw 17.pdf.
5	For questions regarding whether an analysis has begun before the end of the grace period, please contact the
transportation conformity contact at the appropriate EPA Regional Office (see Section 1.7).
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transportation conformity under the 2017 revisions to Appendix W. Specifically, EPA stated in
the preamble to the January 17, 2017 final rule that EPA's 1992 CO guidance that employs
CAL3QHC will remain in place as the recommended approach for CO screening for
transportation projects until such time that EPA develops a new CO screening approach based on
AERMOD or another appropriate model and updates the regulation to include this new screening
approach.6
1.4	Finding More Information about AERMOD
For more information about AERMOD, see EPA's website at: www.epa.gov/scram/air-qualitv-
dispersion-modeling-preferred-and-recommended-modets#aermod. This website includes
extensive documentation on the various components of AERMOD, such as user guides, model
formulation, and evaluation papers. It also includes model change bulletins that describe how
each version of the model differs from its predecessor.
The AERMOD User's Guide, referred to in this guidance, can be accessed on EPA's website
directly from: https://www3.epa.gOv/ttn./scram./m.odels/aermod/aerm.od userguide.pdf.
1.5	EPA's R-LINE Model: Background
EPA has developed a separate line source model for research and development purposes called
the Research LINE source model (R-LINE). R-LINE is a dispersion model developed by EPA's
Office of Research and Development (ORD) for near-roadway assessments. This research
model is based on wind tunnel and field studies that EPA designed and conducted to evaluate
pollutant transport and dispersion of near-surface releases. These studies provided new and
expanded databases for development and evaluation of line source algorithms. The R-LINE
model is based upon a steady-state Gaussian formulation and is designed to simulate line type
source emissions (e.g. mobile sources along roadways) by numerically integrating point source
emissions.
The R-LINE model also includes algorithms to account for how concentrations of air pollutants
are affected by solid barriers, such as noise barriers (also referred to as sound walls), along line
sources. It also includes algorithms that allow modeling of depressed roadway segments, such as
a highway that is below grade. These algorithms continue to be developed and refined by EPA
ORD.
In this document, "R-LINE" refers to the original R-LINE model developed by EPA, whereas
"RLINE" refers to a specific feature, the RLINE source type, that is now included within
AERMOD 19191. (Source types are further described in Section 2.1.)
1.6	Other Purposes for this Guidance
The technical information included in this guidance is relevant for any refined project-level
analysis, regardless of whether its purpose is regulatory or research. Information about how
these new R-LINE features are described in an AERMOD input file and how the parameters are
used to represent transportation projects applies to all users of these AERMOD features. More
6 "Guideline for Modeling Carbon Monoxide from Roadway Intersections," EPA-454/R-92-005, November 1992;
available online at: www.epa.gov/scram001/guidance/guide/coguide.pdf.
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information about these features and the appropriate cases for their use is found in Section 2.1 of
this guidance.
1.7	EPA Contact Information
For questions concerning transportation conformity in a specific nonattainment or maintenance
area, please contact the transportation conformity staff person responsible for your state at the
appropriate EPA Regional Office. Contact information for transportation conformity personnel
at EPA Regional Offices can be found at: https://www.epa.gov/state-and-local-
transportation/epa-regional-contacts-regarding-state-and-local-transportation.
For questions regarding air quality modeling in a specific nonattainment or maintenance area,
please contact the dispersion modeling staff person responsible for your state at the appropriate
EPA Regional Office. Contact information for dispersion modeling personnel can be found on
EPA's website at: https://www.epa.gov/scram/air-modeling-regional-contacts.
General questions about this guidance and its applicability for transportation conformity can be
directed to Laura Berry at EPA's Office of Transportation and Air Quality, berry.taura@epa. gov.
Questions or feedback about AERMOD 19191 and its new features and questions regarding the
Model Clearinghouse (discussed in Section 2.2) can be directed to Chris Owen at EPA's Office
of Air Quality Planning and Standards, owen. chris@epa. gov.
1.8	Guidance and Existing Requirements
This guidance does not create any new requirements. This guidance is based on CAA
requirements and existing associated regulations and does not create any new requirements. This
guidance describes the new R-LINE features in AERMOD and the circumstances in which they
can be used.
The CAA, EPA's transportation conformity regulations at 40 CFR part 51 subpart T and part 93
subpart A, EPA's general conformity regulations at 40 CFR part 51 subpart W and part 93
subpart B, and EPA's "Guideline on Air Quality Models" regulation at Appendix W of 40 CFR
part 51, contain legally binding requirements. This document is not a substitute for those
provisions or regulations, nor is it a regulation itself. Thus, it does not impose legally binding
requirements on EPA, the U.S. Department of Transportation (DOT), states, or the regulated
community, and may not apply to a particular situation based upon the circumstances. EPA
retains the discretion to adopt approaches on a case-by-case basis that may differ from this
guidance but still comply with the statute and applicable regulations. This guidance may be
revised periodically without public notice.
This guidance does not address FHWA's noise abatement program in any way. It does not
interpret or address procedures for noise studies and noise abatement measures, supply noise
abatement criteria, or establish requirements pursuant to 23 USC 109(i) or 23 CFR 772. This
guidance does not in any way interpret or address procedures for determining when a depressed
road is needed for engineering purposes or design requirements.
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2 New R-LINE Features in the AERMOD 19191 Release
2.1 What features of R-LINE are included in AERMOD 19191, and what is their
status?
AERMOD 19191 incorporates the R-LINE research model into the AERMOD framework. The
R-LINE model features are now available within the AERMOD model and can be accessed
within AERMOD using specific keywords and parameters in an AERMOD input file.
Each of these R-LINE features is either a "BETA" or "ALPHA" feature in AERMOD:
•	BETA features have been vetted through the scientific community and are waiting to be
promulgated as regulatory options. Until such features are included in the preferred
model via regulation, BETA options of AERMOD require alternative model approval for
use in regulatory applications, including transportation conformity. This process is
described in Section 2.2. 7
•	ALPHA features within AERMOD are research or experimental options that are provided
for review and evaluation by the user community. These options are ALPHA because
they need additional research and evaluation. They do not have the necessary level of
reliability or validity that the regulatory AERMOD model has, or the BETA options
have. ALPHA options are not intended for any regulatory purpose at this time - whether
such purposes are related to the Clean Air Act or other statutes.
BETA R-LINE Features:
•	The RLINE source type is a new BETA feature that can be used to model a line source
such as a road segment with the R-LINE algorithm.
ALPHA R-LINE Features:
•	The RLINEXT source type, which stands for "RLINE-extended," is a new ALPHA
feature. Like RLINE, RLINEXT can be used to model a line source. The RLINE and
RLINEXT sources use the same dispersion calculations, but the source characteristics are
defined in different ways for each source type (more details below). RLINEXT can be
used to model a source that includes a barrier extending to the ground, such as a noise
barrier next to the source, or model the source as a depressed roadway, whereas the
RLINE source cannot be used to model these additional roadway features.
o The RBARRTER keyword can be used with RLINEXT to describe a barrier the
extends to the ground next to the source,
o The RDEPRESS keyword can be used with RLINEXT to describe a depressed
roadway.
•	The URBAN option applied to either RLINE or RLINEXT source types is an ALPHA
feature. The URBAN option is a setting within AERMOD that affects dispersion. The
7 EPA believes all users of a BETA option for a refined project-level analysis should consult with EPA, regardless
of the purpose of the analysis, to ensure that the use of these features is appropriate in the specific situation and that
agencies use time and resources efficiently. See Section 1.7 for EPA contact information.
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original R-LINE model did not include a way to account for the "urban heat-island
effect," e.g., the URBAN option in AERMOD.8 EPA has added the URBAN option for
the new RLINE and RLINEXT sources in AERMOD.
Available under both options:
•	The keyword RLEMCONV, which stands for "RLINE Emission Conversion," can be
used with both RLINE and RLINEXT source types. This feature allows the user to input
emissions for RLINE or RLINEXT sources in terms of grams per link per hour (e.g.,
from MOVES).9
Both the RLINE and RLINEXT source types account for plume meander during low-wind
conditions.
Each of these features is discussed further below:
•	BETA features, that is, the RLINE source type and RLEMCONV, are covered further in
this section; and
•	ALPHA features, including the RLINEXT source type and associated keywords, are
covered in Section 3.
2.2 What is the process for obtaining approval of RLINE as an alternative model?
As noted above, the use of the RLINE source type without the URBAN option is a BETA
feature, and therefore could be used in a regulatory application if it is approved through EPA's
alternative model process. EPA's Appendix W regulation of January 17, 2017 (82 FR 5182)
describes the use of alternative models in Section 3.2.
An alternative model can be used instead of a preferred model if approved by the EPA Regional
Office in consultation with EPA's Model Clearinghouse. This process provides a public record
of the application of the alternative model, so that future applications of the model can be
consistent with a previously approved application. Such approval is documented through a series
of memos between the EPA's Model Clearinghouse and the approving Regional Office. These
memoranda may inform other situations where transportation modelers would like to use the
RLINE source in AERMOD for other regulatory purposes. Therefore, once this process has
been completed for a specific project, subsequent alternative model approvals may be simplified
for projects where the circumstances are substantively similar. However, in situations that are
substantively different, the project sponsor would need to provide new or additional information
to justify the alternative model approval in their case. For more information about the Model
Clearinghouse, see EPA's webpage at https://www.epa. gov/scram/air-qualitv-m odel-
clearinghouse.
The process for obtaining approval of RLINE as an alternative model begins with making a
request to use RLINE in a regulatory application to the appropriate EPA Regional Office. Please
8	This effect is described in Section 7.5.5 of EPA's PM Hot-spot Guidance. See Section 1.2 for PM Hot-spot
Guidance reference information.
9	MOVES is EPA's emissions model for onroad and nonroad sources. For the latest version of MOVES that can be
downloaded as well as other documentation, please see EPA's website at https://www.epa.gov/moves.
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consult with the appropriate EPA Regional Office before any work begins on an analysis to
understand the requirements and time needed to receive alternative model approval. See the link
to the list of dispersion modeling contacts in Section 1.7. EPA Regional Office contacts can help
modelers begin the process. As the transportation conformity regulation also requires that
interagency consultation procedures include a specific process for "evaluating and choosing a
model (or models) and associated method and assumptions to be used in hot-spot analyses," (40
CFR 93.105(c)(l)(i)), consulting with the transportation conformity contact at the appropriate
EPA Regional Office would also be necessary. Section 1.7 includes a link to these contacts as
well.
2.3	If RLINE is approved through the alternative model process for use in a
transportation conformity determination, what regulations and guidance apply to
the transportation conformity determination?
EPA's transportation conformity regulations at 40 CFR Part 93 provide the criteria and
procedures for transportation conformity determinations, including those that apply to projects.
EPA has also issued guidance, "Transportation Conformity Guidance for Quantitative Hot-spot
Analyses in PM2.5 and PM10 Nonattainment and Maintenance Areas," known as the "PM Hot-
spot Guidance." EPA recommends that this guidance be followed when conducting a PM hot-
spot analysis for transportation conformity to ensure that the statutory and regulatory
requirements are met. This guidance can be found at EPA's web site at:
https://www.epa.gov/state-and4ocal-transport.ation/proiect-level-confoniiitv-and-hot-spot-
analvses#pmguidance. The guidance includes an overview of the regulatory requirements, as
well as technical guidance for modeling emissions, modeling concentrations, developing
background calculations, and calculating design values.
Please refer to EPA's web site at https://www.epa.gov/state-and-local-transportation/proiect-
level-confonnitv-and-hot-spot-analvses for additional information and resources for project-level
transportation conformity determinations.
2.4	How are RLINE sources included in an AERMOD input file?
This section provides an overview of the information the user would need to run the model with
the RLINE source type within AERMOD. However, it is not a substitute for the AERMOD
User's Guide, which provides the complete descriptions of keywords and parameters, and the
syntax necessary for their use in an AERMOD input file.10
2 4 1 Control Pathway:
To use the RLINE source type, the AERMOD input file has to include the BETA flag in the
MODELOPT keyword within the Control pathway; the model will not run without this flag.
This flag labels each page of the AERMOD output file with the word "BETA" to notify
reviewers that the output cannot be used for regulatory applications unless the use of the BETA
option, i.e., use of the RLINE source type, is approved through the alternative model process as
explained in Section 2.2.
10 See 1.4 for reference information.
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The exception to RLINE's BETA status is the use of RLINE with the URBAN option. This
combination is an ALPHA option and must be accompanied by the ALPHA flag in addition to
the BETA flag required for the RLINE source type (i.e., the ALPHA and BETA flag and options
would be used together). This means that RLINE with the URBAN option is not eligible for
possible regulatory application under the alternative model approval process.
As at least the BETA flag is required for RLINE source types, (and in the case of RLINE with
URBAN, both the ALPHA and BETA flags are required), RLINE source types cannot be used
with the regulatory default mode (the DFAULT option in the MODELOPT keyword).
In addition, the FLAT MODELOPT flag is also required if any RLINE sources are included in
the AERMOD run. R-LINE was formulated as a flat terrain model, and therefore RLINE source
types in AERMOD can only represent roads located on flat terrain. Using RLINE for any other
type of terrain would lead to erroneous results because the underlying algorithm does not take
terrain into account.
2 4 2 Source Pathway
The syntax for the RLINE source type in AERMOD is identical to the LINE source syntax in
AERMOD. To describe an RLINE source in an AERMOD input file, the user needs a
LOCATION statement and a source parameter (SRCPARAM) statement:
•	LOCATION: The LOCATION statement would include a source name (or source id,
abbreviated as Srcid) and the endpoints of the source in terms of (X,Y). Specifying
elevation, in a (Zs) term, is optional and would apply to the entire source. However,
since the RLINE sources will not process with terrain, this Zs term is not operational at
this point and if used, should be set to "0.0" or "FLAT." Since the FLAT MODELOPT
keyword is also required in the Control Pathway, any other input for elevation of an
RLINE source will produce an error and the model will not run. Please refer to the
AERMOD User's Guide for more information.11
•	SRCPARAM: The SRCPARAM statement would include the source name, its emissions
rate, release height, width, and optionally, the initial vertical dimension (or initial Sigma
z, abbreviated as "Szinit").
Figure 2-1 illustrates that a LINE source and an RLINE source are defined in the same manner.
In the figure, the endpoint at the top of both the LINE and RLINE sources is indicated as first
(i.e, "(Xsi, Ysi)") but they can be specified in either order.
11 See 1.4 for reference information.
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LINE Source:
Specified with endpoints
(Xsi, Ysi) and (Xs2, Ys2)
(Xs1# YSl)
(Xs„ Ys,)
RLINE Source:
Also specified with endpoints
(Xsi, Ysi) and (Xs2, Ys2)
(Xs1# YSl)
(Xs2, Ys2)
Key
Source
Endpoint of Source
Figure 2-1: LINE Source and RLINE Source
Because the syntax for the RLINE source is the same as the LINE source, the user can run an
existing AERMOD input file that uses the LINE source type with R-LINE dispersion
calculations just by using the BETA and FLAT model options in the Control Pathway, and
changing the source type from LINE to RLINE in the Source Pathway.
2.5 When would RLEMCONV be used with the RLINE source type?
The units that AERMOD uses for emissions for the RLINE source are the same as the AERMOD
AREA and LINE source types, grams per second per square meter, noted as g/s/m212 However,
emissions generated by an emissions model such as MOVES are not formatted in these units.
For example, a project scale MOVES run that uses the "Inventory" calculation type in the
MOVES model would result in emissions for each link over the hour, i.e., grams per link per
hour, and these units would need to be converted for use within AERMOD.
An additional keyword, RLEMCONV, which stands for "RLINE emissions conversion" can be
used with the RLINE source type. This additional keyword allows the user to define the source
emission rates in grams per link per hour, instead of g/s/m2. This keyword has no additional
inputs, but when present, emissions for all RLINE sources are assumed to be in grams per link
per hour. This feature is included to allow the user to include emission rates directly from an
emissions model without converting them.
RLEMCONV could be used with the MOVES results as-is if each AERMOD source is a unique
MOVES link. However, if one MOVES link represents more than one AERMOD source, the
12 In the R-LINE model, units are grains per second per meter. The RLINE source type in AERMOD is based on the
R-LINE model algorithm, but its units are the same as the LINE source type, grams per second per square meter.
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MOVES output cannot be used as-is, even with RLEMCONV. Instead, the user will need to
determine what portion of the link's total emissions belong to each source. One MOVES link
might need to be represented with more than one source if the MOVES link is curved. MOVES
can estimate the emissions over the link, but for AERMOD purposes, the curved link may need
to be broken into multiple straight segments to correspond to RLINE sources, and the emissions
apportioned to each source accordingly.
If MOVES has been run for different hours in different seasons of the year, more than one
MOVES emissions rate would be associated with each AERMOD source. These different rates
would need to be applied to each hour of the year appropriately within AERMOD, which is done
with the EMISFACT keyword.
EPA believes that using the MOVES2AERMOD script may be a more convenient method for
including emission rates from MOVES in an AERMOD input file at this time. When the
MOVES2AERMOD script is used, emission rates are converted into g/s/m2, the units AERMOD
uses for AREA, LINE, and RLINE sources. See Section 2.6 for more information.
2.6 What is MOVES2AERMOD?
MOVES2AERMOD is tool that EPA has provided, in the form of a MOVES script, that converts
emissions results from MOVES project-scale runs into a format for use within an AERMOD
input file. The AERMOD keyword EMISFACT is used to allow the emission factors of a source
to vary, and it is followed by a parameter indicating the number of factors. The parameter
SEASHR, or "season-hour," indicates that emissions vary for each hour of each of four seasons,
for a total of 96 factors.
The MOVES2AERMOD script uses output from MOVES runs to create a table of emission rates
in g/s/m2, the units AERMOD uses for AREA, LINE, and RLINE sources, for every source, for
each hour of four seasons. This table can then be used with the AERMOD keyword EMISFACT
with the SEASHR parameter, for any of these three source types.
To use the MOVES2AERMOD script, the user fills out three tables. The first is a list of how the
AERMOD sources relate to the MOVES links; the second is the area of each MOVES link in
square meters; and the third is a list of which MOVES run applies to each of the hour of each
season of the year. With these inputs, the MOVES2AERMOD script generates 96 emission
factors for each source, so the table it creates can be lengthy when there are multiple sources.
The MOVES2AERMOD script converts emission factors from MOVES into g/s/m2. The table
generated by MOVES2AERMOD can then be included in an AERMOD input file.
Within AERMOD, the season-hour factors in the EMISFACT keyword are multiplied by the
emissions rate for the source. Therefore, the emission rate for the source should be defined as
"1" in the source parameter statement because the EMISFACT table generated with the
MOVES2AERMOD script includes the specific emission rates for each hour.
More information about MOVES2AERMOD, including a ZIP file that includes directions for
how to "install" the tool (i.e., copy the script and place it in the right folder) and instructions on
using it for PM hot-spot analyses can be found on EPA's website at:
https://www.epa.gOv/moves/tools-develop-or-convert-m.oves-inpiits#em.isfact.
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Note: When the M0VES2AERM0D script is used to generate an EMISFACT table for the
AERMOD input file, the input file should not include RLEMCONV, because the conversion of
emissions into the correct units for AERMOD LINE, AREA, or RLINE sources, which is
grams/second/meter2, has already occurred within the MOVES2AERMOD script.
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3 ALPHA Options: RLINEXT, RBARRIER, and RDEPRESS
3.1	Can RLINEXT and its associated barrier and depressed roadway features be
used through the alternative model process?
As noted above, RLINEXT with RBARRIER and RDEPRESS are ALPHA features and
therefore are not intended to be used for regulatory applications. EPA has labeled these features
ALPHA instead of BETA because they are not as far along in the model development process
(e.g., there may be less scientific certainty about the appropriateness of the formulation). EPA
has provided them so that the user community can review and evaluate them. EPA would value
user input on these features and be interested in data gathered by the user community.
Even though any use of these ALPHA options would not be for regulatory purposes, EPA
recommends that any experimentation and evaluation of these sources be conducted in
consultation with the EPA Regional Office dispersion modeling contact person and EPA
Headquarters to address any usability issues that may not otherwise be addressed here. See EPA
contact information in Section 1.7.
As more research is conducted with these features, EPA will be evaluating available data to
determine how well model algorithms are performing and whether additional algorithm
development is needed.
3.2	How are RLINEXT sources included in an AERMOD input file?
This section provides an overview of the information the user would need to run the model with
the RLINEXT source type within AERMOD. However, it is not a substitute for the AERMOD
User's Guide, which provides the complete descriptions of keywords and parameters, and the
syntax necessary for their use in an AERMOD input file.13
3 2 1 Control Pathway:
To use the RLINEXT source type and its other keywords, RBARRIER and RDEPRESSED, the
input file has to include the ALPHA flag in the MODELOPT keyword within the Control
pathway. As in the case with the BETA flag, the ALPHA flag labels each page of the AERMOD
output file with the word "ALPHA" as a reminder of the status of the output, i.e., that the output
is for research and experimental purposes only.
As in the case with RLINE sources, the URBAN option can also be used with RLINEXT source
types in AERMOD. This is also an ALPHA option.
Because the ALPHA flag is required for RLINEXT source types, they cannot be used with the
regulatory default mode (the DFAULT option in the MODELOPT keyword).
Just like with RLINE sources, the FLAT MODELOPT flag is also required if any RLINEXT
sources are included in the AERMOD run. R-LINE was formulated as a flat terrain model, and
therefore RLINEXT source types in AERMOD can only represent roads located on flat terrain.
13 See 1.4 for reference information.
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Using RLINEXT for any other type of terrain would lead to erroneous results because the
underlying algorithm does not take terrain into account.
3 2 2 Source Pathway
While the RLINE and RLINEXT source types have the same underlying algorithm, the syntax
for each of these source types differs. The RLINE source type was created to be identical to the
LINE source type in terms of syntax of LOCATION and SRCPARAM statements. While
LOCATION and SRCPARAM statements are also needed for RLINEXT sources, additional
parameters are needed to describe an RLINEXT source type:
•	LOCATION: As in the RLINE source LOCATION statement, the RLINEXT
LOCATION statement would include a source name (or source id, abbreviated as
"Srcid") and specify the endpoints of the source. However, for an RLINEXT source,
endpoints must be provided in terms of X, Y, and Z. This is illustrated below in Figure
3-1, where the endpoints of the RLINEXT source on the right side of the figure are
labeled as (Xsi, Ysi, Zsi) and (Xs2, Ys2, ZS2). The endpoint Z terms represent release
heights and could potentially be different. In addition, the LOCATION statement can
also include an optional "Zs" term that applies to the entire source, which represents
elevation. However, since the RLINEXT sources will not process with terrain, this Zs
term is not operational at this point and if used, should be set to "0.0" or "FLAT." Since
the FLAT MODELOPT keyword is also required in the Control Pathway, any other input
for elevation of an RLINEXT source will produce an error and the model will not run.
Please refer to the AERMOD User's Guide for more information.14
•	SRCPARAM: The SRCPARM statement for an RLINEXT source type would include
the source name, its emissions rate, the distance from the center line (abbreviated as
"DCL" and explained further below), the source width, and the initial vertical dimension
(Szinit). All these parameters are required in the input file, but the user can enter "0" for
any parameter that is not needed.
14 See 1.4 for reference information.
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RLINE Source:
Specified with endpoints
(Xsi, Ysi) and (Xs2, Ys2)
(Optional Zterm not shown)
(Xsp YSj)
(Xs„ Ys,)
RUNEXT Source:
Specified with endpoints
(Xsi, Ysi, Zsi) and (Xs2, Ys2, Zs2)
(XSj, Ys1( ZsJ
Key
Source
(Xs2, Ys2, Zs2)
Endpoint of Source
Figure 3-1: RLINE and RUNEXT Source Types
The DCL parameter is optional for defining RLINEXT sources. It can be defined as "0" if it is
not used to describe the source. However, when using RLINEXT with the RDEPRESS keyword
to define a depressed road, all RLINEXT sources need to have coordinates that reflect the
roadway centerline, rather than coordinates representing an individual lane. Thus for modeling a
depressed road, the DCL term would be necessary, and would define the distance of each source
to a common location.
The DCL parameter is useful for describing multiple sources that are parallel and of the same
length. Figure 3-2 illustrates two different ways that parallel sources can be defined.
•	On the left side of this figure, the endpoints for Source A (southbound) are (Xsi.a, Ysi.a)
and (Xs2.a, Ysza). The endpoints for Source B (northbound), (Xsi.b, Ysi.b) and (Xszb,
Ys2.b), are different from Source A. The DCL for both of these sources would be defined
as "0."
•	On the right side of the figure, these same two sources are defined with the same
endpoints: (Xsi, Ysi) and (Xs2, YS2) are the same for both Source A and Source B. The
DCL for Source A would be a negative value and the DCL for Source B would be a
positive value. This example shows two sources; additional parallel sources could be
defined in a similar manner.
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RLINEXT sources A and B
specified with individual
endpoints (DCL specified as
"0" for each)
(Xsi,A'Ysi,a) 
VX/T' *"1
1
(Xs, ,, Ys, A) (Xs, R, Ys, R)
RLINEXT sources A and B
specified with common
endpoints and a unique DCL
for each source
(XSl,YSl
1
i
i
1
!XS2'YS2'A&B
Key
Source B
• Endpoint of Source
Median
— ¦ — Center of Median
Figure 3-2: Different Methods to Specify RLINEXT Sources
In Figure 3-2, each direction of traffic is modeled as a source. Another option would be to model
each lane as a unique source. Where each lane is a unique source, each source would have a
DCL reflecting the distance from the midpoint of the median to the middle of the individual lane
of traffic. Note that sources in the figure are shown oriented north to south. If sources are
oriented east to west and DCL is used, parallel sources north of the centerline should have
positive DCL values and parallel sources south of the centerline should have negative values.15
The AERMOD User's Guide further explains the meaning of the distance from the roadway
centerline with examples of how it is applied in different cases, including when the source is
modeled as a depressed roadway.
3.3 How would a solid barrier be modeled with RBARRIER?
An RLINEXT source can optionally include a solid barrier that extends to the ground (such as a
noise barrier) with the keyword RBARRIER. The barrier defined with this keyword is
associated with a specific source. For example, if a roadway with a solid barrier is made up of
several sources, each one of the RLINEXT sources would have an RBARRIER statement. The
length of the barrier will be the length of the RLINEXT source it is associated with. Therefore,
an RLINEXT source with an RBARRIER statement should be defined as no longer than the
barrier. If the road extends beyond the barrier, then more than one source would be used, one or
more with an RBARRIER statement, and one or more without.
15 For any questions about road orientation and whether the DCL should be positive or negative, please consult EPA
(see Section 1.7 for contact information). As mentioned in Section 3.1, EPA recommends that the use of ALPHA
features be conducted in consultation with EPA.
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To describe the barrier, the user would need to include the source name (Srcid), the height of the
barrier (abbreviated as Htwall), and the distance from the centerline of the source to the barrier
(abbreviated as DCLwall). There is no limit to the Htwall parameter, but an error message is
issued if the height of the barrier is negative.
Note: A barrier defined in AERMOD with RBARRIER affects the dispersion of emissions only
from the RLINEXT source it is associated with. It does not affect the modeled dispersion of
emissions from any other source in the vicinity; dispersion will be modeledfor these other
sources as if the barrier is not present.
Note that other regulations and highway design processes, such as the Federal Highway
Administration's (FHWA's) Noise Regulation and Guidance, would dictate whether a federally
funded transportation project would include new barriers.
FHWA has resources related to noise available on their website at
https://www.fhwa.dot.eov/environment/noise/. including information about solid barriers and
their use for noise abatement. The Federal Transit Administration (FTA) also has resources
related to noise available on their website at www.transit.dot.gov/regulations-and-
guidance/environmental-programs/noise-and-vibration.
3.4 What are some examples of how barrier could be modeled?
Example: Divided Arterial with a Barrier on One Side
Figure 3-3 shows two different ways to define a divided arterial with a barrier on one side of the
arterial only. In both cases, there are two sources, Source A and Source B. On the left side of
the figure, the two sources have the same endpoints, but different DCL terms as shown in the
figure. DCLa would be negative, and DCLb would be positive. The modeler would include an
RBARRIER statement for each source to define a barrier, with the DCLwall term representing
the distance from the source endpoints to the barrier. That is, the DCLwall term would be the
same in both RBARRIER statements, for both Source A and Source B. The height of the barrier
is not shown in this two-dimensional figure, but it is needed in both RBARRIER statements and
would be defined the same in each. The barrier would affect dispersion of emissions from travel
in both directions because each source includes the RBARRIER statement.
Alternatively, the two sources could be modeled with their own unique endpoints defined, as
shown on the right side of the figure. To ensure that the barrier has an effect on the emissions
from both directions of travel, the modeler would include an RBARRIER statement for each
source and define each of them to be in the same location. In other words, the two RBARRIER
statements would locate the barrier in the identical place with the identical height. As shown in
the figure, the two RBARRIER statements for Source A and Source B would have different
DCLwall terms, because this term is the distance from the center of each source to the barrier.
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RLINEXT sources specified
with the same endpoints.
DCLwall, which represents
distance from endpoints to
barrier, is the same for both
sources
(Xs1# YSj),
i
(Xs„, YsJa&b
DCLa 1 DCLb
	DCLwallA8,B.
RLINEXT sources A and B specified
with unique endpoints (DCL is "0"
for each source). Each source
defines the barrier in the same
location with a unique DCLwall
term
(XSj B, YSj B)
¦ - •
/\

Key
1 1
Source A
— — —
Source B
l_ _ J

•
Endpoint of Source

Median
— . —
Center of Median

Soundwall
(Xs, a' Ys, .) (Xs , Ys )
DCLwall,
Figure 3-3: Two Methods to Define a Divided Arterial with a Barrier on One Side
In both of these approaches, the two directions of travel are unique sources, which is important in
representing emissions for a transportation facility. The emissions rates for each direction of
travel are likely to be different from each other over the course of the day. For example, one
direction could be congested during the morning peak and the other direction congested during
the evening peak. Because the purpose of the analysis is to determine the air quality effects of a
transportation project, accurately characterizing the quantity and location of emissions in
AERMOD over time is key. The analysis could be further refined if travel data was available by
lane. In such a case, separate sources could be defined for each lane of travel, using either the
method on the right or on the left of the figure to define the sources and barrier.
An additional variation that could be applied to either of these approaches would be to model
light-duty and heavy-duty emissions separately, that is, as separate sources located in the same
place. The sources are in effect, superimposed over one another. Appendix J of the PM Hot-
spot Guidance16 describes modeling different vehicle types as overlapping sources. If this were
to be done for the example above, each source would have its own RBARRIER statement.
16 See Section 1.2 for reference information.
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•	For the option on the left side of Figure 3-3, there would be four RLINEXT sources:
southbound light-duty, southbound heavy-duty, northbound light-duty, and northbound
heavy-duty, and all four would have an identical RBARRIER statement.
•	For the option on the right side of the figure, there would also be four RLINEXT sources,
and each of these would also have an RBARRIER statement with the same height. The
two RBARRIER statements associated with the southbound sources would both include
the distance of DCLwalU, and the two RBARRIER statements for the northbound
sources would both include the distance of DCLwallB.
Example: A Divided Arterial with Barriers on Both Sides
If a highway has solid barriers on both sides, one way to include both barriers would be to model
each direction of the highway as a separate RLINEXT source, so that a barrier can be defined for
each one. For example, if a north-south highway has barriers on both sides, the northbound side
can be defined as an RLINEXT source with a barrier to the east, and the southbound side can be
defined as a different RLINEXT source and include a barrier to the west, as shown in Figure 3-4.
In this example, there are two sources, A and B, each defining a different barrier. DCLwalU
would be a negative value, and DCLwallB would be a positive value.
Two RLINEXT sources; each
with a barrier
(Xsi,a> Ys!,a) (XslfB, Ys1jB)
X DCLwall
DCLwall W-

Key

Source A
l_ _ J
Source B
•
Endpoint of Source

Median
— . —
Center of Median

Soundwall Source A

¦ ¦ ¦ ¦
Soundwall Source B
Figure 3-4: Divided Arterial with Solid Barriers on Both Sides
Again, note that any barrier affects modeled dispersion of only the source it is associated with
and does not affect the modeled dispersion of emissions from any other source. Using the
example of the north-south highway above, the barrier associated with Source A (solid line),
would only affect the modeled dispersion of Source A and would have no effect on modeled
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dispersion from Source B or any other sources in the vicinity. Similarly, the barrier associated
with Source B (dashed line) would affect the modeled dispersion of Source B only, and not
Source A or any other sources in the vicinity.
3.5 How would a depressed road be modeled with RDEPRESS?
An RLINEXT source can also represent a depressed roadway with the addition of the keyword
RDEPRESS. With this keyword, the user would include the source name (Srcid), the depth of
the depression (a negative value, reflecting a lower elevation than the surrounding terrain), and
the width of both the top (Wtop) and bottom (Wbottom) of the depression that contains the
source.
Use of the RDEPRESS keyword requires that all RLINEXT sources have coordinates reflecting
the roadway centerline coordinates, and not coordinates representing the individual lane of
traffic. That is, when using RDEPRESS, the user would have to specify the centerline endpoints
and use a non-zero DCL for RLINEXT sources. The DCL is necessary in the depressed road
model computations, because the DCL tells the model the location of the source within the
depression, thus the amount of recirculation and dispersion of that source before its emissions are
emitted on the downwind edge of the depression.
The depth parameter must be negative; the user will get an error if the depth parameter is greater
than zero. Also, the user will get an error if Wtop or Wbottom parameters are less than zero, or
if the Wbottom is greater than Wtop for the source (i.e., the bottom width cannot be greater than
the top width).
Figure 3-5, which is a view of the cross-section of a depressed road, illustrates the parameters
necessary for RDEPRESS. In this figure, the roadway is made up of two sources, A and B. The
SRCPARAM statement for each of these sources would define a non-zero DCL. The
RDEPRESS statement for each of these sources would define the depth, Wtop, and Wbottom.
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Key:
—	Source A
—	— Source B
• Endpoint/location of median
—Wtop—
L Depth
(negative)
DCL
DCL
bottom
Figure 3-5: Depressed Roadway Parameters
3.6 Can RLEMCONV be used with the RLINEXT source type?
Yes. RLEMCONV, described in Section 2.5, can work with both RLINE and RLINEXT source
types, even though the units that AERMOD uses for these two source types are different. The
units that AERMOD uses for emissions for the RLINE source are the same as the AERMOD
AREA and LINE source types, grams per second per square meter, noted as g/s/m2 The
emission units for the RLINEXT source type is the same as the original R-LINE model, which is
grams per second per meter, noted as g/s/m.
The RLEMCONV keyword allows the user to define the RLINEXT source emission rates in
grams per link per hour, instead of grams per second per meter. This keyword has no additional
inputs, but when present, emissions for all RLINEXT sources are assumed to be in grams per
link per hour. This feature is included to allow the user to include emission rates directly from
an emissions model without converting them. See Section 2.4 for more information.
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