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Guidance on the Preparation of Clean Air Act
Section 179B Demonstrations for
Nonattainment Areas Affected by International
Transport of Emissions
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The contents of the guidance document do not have the force and effect of law. It does
not bind the public in any way and intends only to provide clarity to the public
regarding existing requirements under the law or Agency policies, except as authorized
by law or as incorporated into a contract.
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EPA-457/P-20-001F
December 2020
Guidance on the Preparation of Clean Air Act Section 179B Demonstrations for Nonattainment
Areas Affected by International Transport of Emissions
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Air Quality Policy Division
Research Triangle Park, NC
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TABLE OF CONTENTS
Acronyms ii
1. INTRODUCTION 1
1.1. Purpose 1
1.2. Statutory and Regulatory Framework 2
1.3. Scope and Definition of International Emissions 4
1.4. Analytical Considerations 5
2. INTERNATIONAL TRANSPORT OF POLLUTION 7
2.1. Near-border Transport 7
2.2. Long-range Transport 8
3. EVALUATING THE SECTION 179B RELATIONSHIP TO OTHER RELATED
PROVISIONS 10
3.1. Extension of Attainment Date 10
3.2. Exceptional Events 11
3.3. Sections 110(a)(2)(D) and 126 - Interstate Transport 11
4. THE DEMONSTRATION DEVELOPMENT PROCESS UNDER SECTION 179B 12
4.1. Early Engagement with EPA Regional Offices 12
4.2. Prospective Demonstrations under Section 179B(a) 12
4.2.1. Section 179B(a) Process and Key Questions 13
4.2.2. Section 179B(a) Demonstration Submission Schedule 15
4.2.3. Flow Chart for Section 179B(a) Prospective Demonstrations (SIP approval) 15
4.3. Retrospective Demonstrations under Sections 179B(b)-(d) 15
4.3.1. Marginal Nonattainment Areas for the O3 NAAQS 16
4.3.2. Schedule for Retrospective Demonstrations for O3 Nonattainment Areas 17
4.3.3. Schedule for Retrospective Demonstrations for PM10, PM2.5, and CO
Nonattainment Areas 17
4.3.4. Flow Chart for Section 179B(b)-(d) Retrospective Demonstrations (potential
reclassification) 18
5. CONCEPTUAL MODEL OF INTERNATIONAL INFLUENCE 18
6. TECHNICAL ANALYSES IN SUPPORT OF THE SECTION 179B DEMONSTRATION
23
6.1. Determining the Appropriate Analytic Approach for the Section 179B Demonstration23
6.2. Analyses to Consider in Weight-of-Evidence Demonstrations 26
6.3. Analyses to Demonstrate International Contribution 28
6.3.1. Measured Air Quality Data Analysis 29
6.3.2. Comprehensive Emission Analysis 39
6.3.3. Modeling to Quantify International Contribution 40
6.3.4. Receptor Modeling Analysis 44
6.4. Example Conclusion Statement in the Demonstration 46
7. Public Comment Process 47
8. References 48
Appendix A. Statutory Text 52
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Acronyms
AQS Air Quality System
CAA Clean Air Act
CFR Code of Federal Regulations
CO Carbon monoxide
CTM Chemical Transport Model
ENSO El Nino / Southern Oscillation
EPA Environmental Protection Agency
FR Federal Register
FT Free troposphere
HTAP Hemispheric Transport of Air Pollutants
HYSPLIT HYbrid Single-Particle Lagrangian Integrated Trajectory
K Kelvin
km Kilometer
LRTAP The Convention on Long-range Transboundary Air Pollution
mb Millibar
NAAQS National ambient air quality standard or standards
NASA National Aeronautics and Space Administration
NNSR Nonattainment New Source Review
NOAA National Oceanic and Atmospheric Administration
NO Nitric oxide
NOx Nitrogen oxides
NPES Normalized Potential Emissions Sensitivity
NPSC Normalized Potential Source Contribution
O3 Ozone
PA Policy Assessment
PBL Planetary boundary layer
PES Potential Emissions Sensitivity
PM Particulate matter
ppb Parts per billion
PSC Potential Source Contribution
PT Potential temperature
RACM Reasonably Available Control Measures
RACT Reasonably Available Control Technology
RFP Reasonable Further Progress
SIP State Implementation Plan
WAQS Western Air Quality Study
UN United Nations
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1. INTRODUCTION
1.1. Purpose
The purpose of this guidance document1 is to assist air agencies2 that are considering the
development of a demonstration, under section 179B of the Clean Air Act (CAA), that a
nonattainment area would be able to attain and maintain, or would have attained, the relevant
National Ambient Air Quality Standard (NAAQS) but for emissions emanating from outside the
United States (U.S.). To help air agencies better understand how to satisfy the requirements of
section 179B, the guidance describes and provides examples of the kinds of information and
analyses that the U.S. Environmental Protection Agency (EPA) recommends air agencies
consider including in a section 179B demonstration. The guidance also describes a weight-of-
evidence approach that EPA intends to use when evaluating section 179B demonstrations. This
non-binding guidance is intended to assist air agencies in the preparation of demonstrations but
does not limit the types of information and analysis that could be used to develop such
demonstrations under the CAA.
An air agency has the authority under section 179B to develop and submit to EPA a
demonstration that its state implementation plan (SIP) would be adequate to attain and maintain
the NAAQS in a designated nonattainment area, or that the nonattainment area would have
attained the NAAQS, but for emissions emanating from outside the U.S. EPA has the authority
under section 179B to assess such an international transport demonstration when evaluating a
SIP revision submitted in response to a nonattainment designation or reclassification of an area,
or when EPA determines whether a nonattainment area has failed to attain the standard by the
attainment date and thus becomes subject to additional CAA requirements. If upon such
assessment, the demonstration is to the Administrator's satisfaction, EPA will provide specified
regulatory relief as indicated in section 179B.
In addition to describing the kinds of information and analyses that may be helpful to include in
a section 179B demonstration, this guidance provides:
• A review of the existing regulatory framework for considering section 179B
demonstrations;
1 This document is intended only to provide clarity to the public regarding existing requirements under the law or
agency policies. This document is not a rule or regulation, and the guidance it contains may not apply to a particular
situation based upon the individual facts and circumstances. This guidance does not change or substitute for any law,
regulation, or other legally binding requirement and is not legally enforceable. The use of non-mandatory language
such as "guidance," "recommend," "may," "should," and "can" is intended to describe EPA's policies and
recommendations. The use of mandatory terminology such as "must" and "required" is intended to describe
controlling legal requirements under the terms of the CAA and of EPA regulations. Neither such language nor
anything else in this document is intended to or does establish legally binding requirements in and of itself. None of
the recommendations in this guidance are binding or enforceable against any person, and neither any part of the
guidance nor the guidance as a whole constitutes final agency action that affects the rights and obligations of any
person or represents the consummation of agency decision making. Only final actions taken to approve or
disapprove state implementation plan (SIP) submissions or final findings by the Administrator under section
179B(b)-(d) that implement any of the recommendations in this guidance would be final actions for purposes of
CAA section 307(b).
2 References to "air agencies" include state, local, and tribal air agencies.
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• A review of other existing regulatory mechanisms that may be more appropriate
alternatives to section 179B in certain situations;
• Recommended timeframes for the section 179B demonstration development process; and
• Background on the nature of intracontinental and intercontinental transport of air
pollution.
1.2. Statutory and Regulatory Framework
After EPA promulgates a new or revised NAAQS, the CAA requires EPA to designate all areas
of the country as either Nonattainment, Attainment, or Unclassifiable, with respect to that
NAAQS. The process for these initial area designations is outlined in CAA section 107(d)(1).
Under the CAA, air agencies are required to develop and submit SIPs to EPA that provide for the
implementation, attainment, maintenance, and enforcement of the NAAQS through control
programs directed at various sources of emissions. When designated as nonattainment, areas for
the ozone (O3), particulate matter equal to or less than 2.5 microns in diameter (PM2.5),
particulate matter equal to or less than 10 microns in diameter (PM10), and carbon monoxide
(CO) NAAQS are each assigned a classification that identifies the latest allowable attainment
date and associated requirements to be addressed in the SIP. The core statutory requirements3 to
be addressed in the SIP may include the following: an accurate inventory of current emissions
for all sources within the nonattainment area; a Nonattainment New Source Review (NNSR)
permit program; regulations providing for implementation of Reasonably Available Control
Measures (RACM), including Reasonably Available Control Technology (RACT); a
demonstration that the plan provides for Reasonable Further Progress (RFP) toward attainment; a
demonstration of attainment by the attainment date, commonly in the form of air quality
modeling analyses; and contingency measures to be implemented in certain circumstances
should the area fail to attain by the attainment date.4 These obligations are detailed in various
CAA sections, including sections 110 and 171 through 193.
After the attainment date has passed, the air agency and EPA evaluate ambient air quality data
for the nonattainment area to determine whether each area has attained by the attainment date.
Once EPA has determined that an O3, PM2.5, PM10, or CO nonattainment area has failed to attain
the NAAQS by the attainment date, that area by operation of law must be reclassified to a higher
classification, which will trigger additional planning obligations for the area, potentially resulting
in further emissions control requirements.5
3 Specific statutory SIP due dates and requirements for SIPs vary depending on the specific NAAQS and an area's
classification. Related implementation requirements and deadlines associated with these statutory requirements may
be established by EPA regulations. The requirements listed here only refer to the core statutory requirements
generally applicable to most nonattainment areas.
4 Note that O3 nonattainment areas classified as Marginal are not subject to most of the requirements that apply to
higher classifications. Compare CAA section 182(a) with sections 182(b)-(e).
5 Reclassification to a higher classification for failure to attain the NAAQS by the attainment date do not apply to
areas that are already classified at the highest classification and, in the case of ozone, a classification of Severe
(which is the second-highest level of classification for ozone). Although failure to attain the SO2, NO2, or Pb
NAAQS does not result in reclassification, it does also result in additional planning and attainment demonstration
requirements.
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Section 179B6 provides EPA with authority to consider whether state demonstrations of the
impacts from international emissions are satisfactory in two contexts: (1) at the implementation
plan review stage, when EPA determines prospectively whether a SIP adequately demonstrates
that a nonattainment area will attain the NAAQS by its future attainment date (in section
179B(a)); and (2) after each applicable attainment date, when EPA determines retrospectively
whether a nonattainment area has attained the NAAQS by that attainment date (in section
179B(b)-(d)).
In the first context, section 179B(a) provides that, "[Notwithstanding any other provision of law,
an implementation plan or plan revision required under this chapter shall be approved by the
Administrator if (1) such plan or revision meets all the requirements applicable to it.. .other than
a requirement that such plan or revision demonstrate attainment and maintenance of the relevant
national ambient air quality standards by the attainment date specified under the applicable
provision of this chapter, or in a regulation promulgated under such provision, and (2) the
submitting state establishes to the satisfaction of the Administrator that the implementation plan
of such state would be adequate to attain and maintain the relevant national ambient air quality
standards by the attainment date ... but for emissions emanating from outside of the United
States." (Emphasis added). For the purpose of this guidance, we refer to such section 179B
demonstrations as section 179B(a) or "prospective" demonstrations because they typically
involve as assessment of future air quality. Thus, an EPA-approved prospective demonstration
provides relief from demonstrating future attainment.
In the second context, sections 179B(b), (c), and (d) provide that, for O3, CO, and PM,
respectively, "[notwithstanding any other provision of law, any State that establishes to the
satisfaction of the Administrator that ... such State would have attained the national ambient air
quality standard ... by the applicable attainment date but for emissions emanating from outside
of the United States" shall not be subject to reclassification to a higher classification category by
operation of law, as otherwise required in CAA sections 181(b)(2), 186(b)(2), and 188(b)(2),
respectively.7 (Emphasis added.) For the purpose of this guidance, we refer to such section 179B
demonstrations as section 179B(b)-(d) or "retrospective" demonstrations because they involve
analysis of past air quality. Thus, an EPA-approved retrospective demonstration provides relief
from reclassification that would have resulted from EPA determining that the area failed to attain
the NAAQS by the relevant attainment date.
While section 179B can be an important tool for providing specified regulatory relief8 for air
agencies, EPA's approval of a section 179B demonstration does not relieve air agencies with
6 All references to CAA section 179B are to 42 U.S.C. § 7509a. International border areas, as added Pub. L. No.
101-549, title VIII, § 818, 104 Stat. 2697 (Nov. 15, 1990). See Appendix A for full statutory text.
7 EPA's longstanding view is that CAA section 179B(b) contains an erroneous reference to section 181(a)(2), and
that Congress actually intended to refer here to section 181(b)(2). See "State Implementation Plans; General
Preamble for the Implementation of Title I of the Clean Air Act Amendments of 1990," 57 Fed. Reg. 13,498, 13,569
n.41 (Apr. 16, 1992).
8 The regulatory relief gained if the state's section 179B(a) prospective demonstration is to the satisfaction of the
Administrator and approved by EPA is the ability for a state to submit an approvable attainment plan that does not
demonstrate attainment and maintenance of the relevant NAAQS. If the state's section 179B(b) - (d) retrospective
demonstration for an O3, CO, PM2 5, or PM10 area that does not attain the standard by the attainment date is to the
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nonattainment areas of having to meet the remaining applicable planning or emission reduction
requirements in the CAA. It also does not provide a basis for either excluding air monitoring data
influenced by international transport from regulatory determinations related to attainment and
nonattainment, or redesignating an area to attainment.
If an air agency is contemplating a section 179B demonstration in either the section 179B(a)
"prospective" context or the section 179B(b)-(d) "retrospective" context, EPA encourages early
consultation and communication throughout the demonstration development and submission
process, along the lines of these basic steps:
Step 4 - Air
agency submits
final section 179B
demonstration and
EPA makes an
approval
determination.
Step 2 - Air
agency begins
gathering
information and
developing
analyses for a
demonstration.
Step 1 - Air
agency contacts
its EPA Regional
office to discuss
section 179B
regulatory
interests and
conceptual model.
Step 3 - Air
agency submits
draft section 179B
demonstration to
EPA Regional
office for review
and discussion.
More detailed, context-specific recommended process diagrams can be found in Sections 4.2.3
and 4.3.4 of this guidance.
1.3. Scope and Definition of International Emissions
This guidance provides examples of recommended technical analyses that an air agency can
consider demonstrating either that:
1) its SIP would be adequate to show attainment and maintenance of the NAAQS by the
attainment date but for international emissions; or
2) the area would have attained the NAAQS by the area's attainment date but for
international emissions.
EPA expects section 179B demonstrations to be developed in a manner consistent with the CAA
principles and practices used in attainment plans. The overall plan requirements for
nonattainment areas, as identified in sections 171-193 of Part D of the Act9, call for plans that:
(i) include a comprehensive inventory of actual emissions from all sources; (ii) identify and
quantify the allowable emissions from major new or modified stationary sources, and require
permits for major stationary sources; (iii) provide for implementation of reasonably available
control measures (including reasonably available control technology); (iv) provide for reasonable
further progress toward attainment; (v) include emission limitations and such other control
measures as may be necessary or appropriate to provide for attainment; (vi) a demonstration of
satisfaction of the Administrator and is approved by EPA, EPA will not reclassify the area to the next higher
classification. A state with an approved section 179B(b) retrospective demonstration for a Severe or Extreme ozone
nonattainment area that fails to attain the standard by the attainment date also will not be subject to CAA section 185
fees.
9 CAA sections 171-193, 42 U.S.C. §§ 7501-7515.
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attainment by the attainment date, commonly in the form of air quality modeling analyses; and
(vii) contingency measures to be implemented in certain circumstances should the area fail to
attain by the attainment date. Each one of these mandatory planning requirements is linked to
anthropogenic emissions. To promote consistency between the nonattainment planning
requirements and the corresponding section 179B provisions for relief from certain elements of
those requirements, EPA recommends that section 179B demonstrations focus on the
contribution to ambient concentrations attributable to international anthropogenic emissions.
Since states are required to evaluate and adopt controls on domestic anthropogenic sources as
necessary to fulfill their nonattainment planning requirements, consistent treatment in section
179B demonstrations would focus on contributions from non-U.S. anthropogenic sources as
opposed to nonanthropogenic sources.10
For purposes of this guidance, the terms "international sources" and "international emissions"
therefore mean, respectively, anthropogenic sources located outside of the U.S, and
anthropogenic emissions emanating from sources located outside of the U.S. Emissions from
offshore areas where U.S. federal laws govern emission sources are not considered to be
international emissions for purposes of this guidance.11
1.4. Analytical Considerations
Despite the title of section 179B ("International Border Areas"), EPA has twice affirmed in
recent years its interpretation that this provision is not restricted to areas adjoining international
borders.12 As explained in these instances, and as further detailed later in this document,
domestic O3 air quality can be affected by sources of emissions located across U.S. borders in
Canada and Mexico, and under certain circumstances, from sources in other continents.
Additionally, in his April 12, 2018, Memorandum for the Administrator of the Environmental
Protection Agency ("Promoting Domestic Manufacturing and Job Creation - Policies and
Procedures Relating to Implementation of Air Quality Standards"), the President directed EPA to
"not limit its consideration of demonstrations or petitions to those submitted by States located on
the borders of the United States with Mexico or Canada, but rather consider[] section 179B
demonstrations or petitions submitted by any State, including but not limited to those located in
10 EPA believes anthropogenic emissions should be the focus of section 179B demonstrations for the reasons stated
here. A state's weight-of-evidence presentation in a section 179B demonstration may refer to nonanthropogenic
emissions. For example, certain analyses described herein, e.g., back trajectories, may not be able to distinguish the
source of emissions. Therefore, it may be useful to identify days with contributions primarily from natural sources to
contrast with days where ambient concentrations may be enhanced by contributions from anthropogenic sources.
11 For example, emissions within United States portion of the North American Emission Control Area, as is defined
in Annex VI of MARPOL, are regulated under the Act to Prevent Pollution from Ships. 33 U.S.C. §§ 1905-1915;
see 40 C.F.R. Part 1043. Annex VI of MARPOL is available at
http://www.imo.org/en/OurWork/Environment/PollutionPrevention/AirPollution/Pages/Air-Pollution.aspx. Air
emissions from sources located on the Outer Continental Shelf are also subject to federal regulation. 42 U.S.C. §
328, 43 U.S.C. § 1334(a)(8). States considering offshore sources as part of their section 179B demonstration should
consult with their EPA Regional office.
12 Although section 179B is titled "International Border Areas," EPA indicated in the preambles to the
implementation rules for both the 2015 and 2008 O3 NAAQS that the statutory language of section 179B does not
prohibit air agencies from submitting such demonstrations for nonattainment areas not located on the border with
another country. 83 FR 63010 (December 6, 2018); see also 80 FR 12294 (March 6, 2015). To date, EPA has not
acted on a section 179B demonstration for any nonattainment area not located on the Mexican border.
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the Western United States." As explained in this document, technical demonstrations for non-
border areas may involve additional technical rigor and resources compared to demonstrations
for border areas.
This guidance provides examples of information and analyses that are recommended for
inclusion in section 179B demonstrations. Air agencies may also use other well-documented,
appropriately applied, and technically sound information and analyses not identified in this
guidance. EPA recommends that the air agency consult with its EPA Regional office as early as
possible to reach a common understanding of the types of information and analyses that would
be most appropriate for a section 179B demonstration for a particular area. EPA is hopeful that
early consultation will help air agencies develop high-quality technical analyses and enable EPA
to conduct expeditious reviews of section 179B demonstrations.
EPA recommends that section 179B demonstrations include a conceptual model that describes
the conditions causing the exceedance(s)13 at the ambient monitor(s), discusses how emissions
from international anthropogenic sources led to the exceedances at the affected monitor(s), and
identifies the specific EPA regulatory decision (SIP approval or determination of attainment) that
is intended to be addressed by the demonstration. The conceptual model would generally appear
at or near the beginning of a demonstration to help readers and the reviewing EPA staff
understand the role of international transport before more detailed technical evidence is
presented. It would include much of the information that the air agency provided or discussed
with EPA during initial consultations regarding a possible demonstration. Section 4 of this
guidance describes the recommended scope of a conceptual model in more detail.
As described in Section 1.2 of this guidance, CAA section 179B contemplates two different
types of demonstrations that could be developed by an air agency at different points in the air
quality management process. If the air agency intends to submit a section 179B(a) prospective
demonstration (i.e., one intended to support approval of a SIP submission by showing that the
plan would be adequate to attain and maintain the standard by the attainment date but for
international emissions), it should submit the demonstration prior to or as part of the overall
nonattainment area SIP submission. A retrospective demonstration pursuant to sections 179B(b)-
(d) (i.e., one intended to avoid a reclassification by showing that an area would have attained the
standard but for international emissions) should illustrate that air quality was influenced by
international emissions on specific days during the years that contribute to the design value
calculation for the area. This retrospective demonstration would be submitted after air quality
data collected pursuant to federal reference or equivalent monitoring methods and indicating that
the area failed to attain by the attainment date are certified. As noted in Section 4.3 of this
guidance, the submittal of a retrospective demonstration in some cases could occur before the
attainment date.14
13 For purposes of this guidance, unless otherwise specified, the term "exceedance" also refers to non-exceedance-
level concentrations that contribute to a design value that violates certain NAAQS (e.g., one year's 4th highest daily
maximum 8-hour average value could be below the level of the O3 NAAQS, but the 3-year design value is above the
NAAQS).
14 Note that an area that has submitted a retrospective demonstration but that ultimately attains the NAAQS by the
relevant attainment date would not be subject to reclassification and, therefore, relief offered under section 179B(b)
- (d) would no longer be applicable.
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Given the extensive number of technical factors and meteorological conditions that can affect
international transport of air pollution, EPA believes that section 179B demonstrations should be
evaluated based on the weight of evidence15 of all information and analyses provided by the air
agency. The appropriate level of supporting documentation will vary on a case-by-case basis
depending on the nature and severity of international influence. EPA will consider and
qualitatively weigh all evidence based on its relevance to section 179B and the nature of
international contributions as described in the demonstration's conceptual model. Every
demonstration should include fact-specific analyses tailored to the nonattainment area in
question. When a section 179B demonstration shows that international contributions are larger
than domestic contributions, the weight of evidence will be more compelling than if the
demonstration shows domestic contributions exceeding international contributions. Section 6
includes additional discussion of weight of evidence considerations.
The demonstration should also consider and examine any contradictory evidence that may
indicate that influences other than international emissions caused or contributed to the subject
NAAQS exceedances or violations. Such evidence, for example, may include data indicating that
NAAQS exceedances or violations could be predominantly attributed to local, intrastate, or
interstate U.S. sources which may otherwise be addressed by other CAA authorities. EPA will
weigh the body of available evidence to determine whether it collectively indicates that the SIP
would be adequate to attain and maintain the NAAQS, or the area would have attained the
NAAQS, but for emissions emanating from outside of the U.S.
2. INTERNATIONAL TRANSPORT OF POLLUTION
The inclusion of section 179B in the 1990 CAA Amendments was an acknowledgement of the
long-standing recognition of international transport of pollution to the U.S. In the legislative
history to the 1990 Amendments (US Senate, 1993), the Senate committee on Environment and
Public Works recognized that EPA and air agencies would need to develop technical analyses
that attempted to quantify the impact of international transport of pollution (US Senate, 1993, p.
5742 and 10110). EPA has actively engaged in various efforts since that time to quantify and
understand the impacts to the U.S. of both nearby and more distant international emissions from
around the world. This section provides an overview of the type of transport that occurs at
intracontinental and intercontinental scales.
2.1. Near-border Transport
15 Throughout this guidance document, the term "weight of evidence" is used to describe the collective analysis by
which we evaluate CAA section 179B demonstrations. The guidance describes certain recommended analyses and
other supplemental analyses that can be provided as part of a section 179B demonstration. Because each
nonattainment area is unique, area-specific factors may affect the types of analyses that would be appropriate for any
particular area. If the state provides multiple analyses as part of a section 179B demonstration, EPA recommends the
state describe the analyses performed, databases used, key assumptions and outcomes of each analysis, and why an
air agency believes that the evidence, viewed as a whole, supports a conclusion that the area would not attain, or
would have attained, but for international emissions. The EPA will consider the weight of this evidence in
considering whether to approve any section 179B demonstration.
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Near-border transport to the U.S. from Canada and Mexico is more easily observed and
documented than intercontinental transport. Pollutants from near-border international emissions
sources, such as industrial facilities and motor vehicles, are transported on a scale comparable to
the distance extending across large metropolitan areas. For example, international near-border
emissions from Juarez, Mexico, have been demonstrated to have an impact on El Paso, Texas,
located directly across the border (TNRCC, 1994; EPA, 2009). Conceptually, analyses for near-
border areas can be similar to analyses performed to assess interstate transport under separate
CAA authorities and requirements.16 In addition, North American intracontinental transport can
also occur over greater distances, and it can potentially affect interior locations in the U.S. to a
lesser degree than border areas.
EPA has previously estimated the level of O3 and haze pollution contributed to U.S. air quality
monitors from near-border Canadian sources, Mexican sources, and international shipping
emissions from outside offshore areas governed by federal law. For example, air quality
modeling for the 2015 O3 NAAQS Policy Assessment (PA) estimated Canadian and Mexican
contributions to U.S. pollution for the calendar year 2007 (EPA, 2014). In addition, modeling to
support the Cross-State Air Pollution Rule and the Regional Haze Rule quantified projected 2017
O3 contributions and 2028 particulate matter contributions from an approximately 300,000
square mile region of northern Mexico and an approximately 600,000 square mile region of
southern Canada, respectively (EPA, 2016a; EPA, 2018b). These two studies use two techniques
(i.e.: (1) "zero-out"; and (2) source apportionment, both discussed in more detail in Section
6.3.3.1 of this guidance) that provide quantitative estimates of international transport.
2.2. Long-range Transport
Long-range international transport of air pollution has been recognized for decades, as illustrated
by Byrne (2015). Particularly with O3, modeling analyses have typically included boundary
conditions that implicitly included "background" O3. "Background" is a generic term that has
been used to refer to O3 formed from any non-local or regional source (EPA, 2015), but recent
publications have refined the term to focus on uncontrollable sources (Jaffe et al., 2018).
"Background" or "Uncontrollable sources," as used in discussions of domestic air pollution,
include natural17 sources globally (i.e., domestic and international), plus international
anthropogenic sources. EPA has been aware of global natural and international anthropogenic
contributions while setting prior O3 NAAQS, as illustrated by the use of boundary conditions in
modeling.18
The characterization of international transport of pollution has significantly advanced in the last
decade with the development of improved international emissions inventories and global-scale
chemical transport models (CTMs). While global scale CTM simulations alone have been used
to estimate international transport to the U.S., global-scale models typically have coarse spatial
resolution and perform poorly for estimating concentrations in U.S. urban areas (Jaffe, et al.,
16 For example, CAA sections 110(a)(2)(D) and 126.
17 Natural sources are inclusive of all non-anthropogenic sources, which include biogenic, geogenic, oceanic etc.
Some natural sources have complex relationships with human activity, such that some anthropogenic enhancement
of natural sources will be considered anthropogenic.
18 Boundary conditions accounted for all sources that were not explicitly accounted for within the modeling domain.
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2018). To better estimate impacts of international transport of emissions to U.S. urban areas,
recent modeling studies have used higher spatial resolution models of the North American
"region" with boundary conditions derived from global-scale CTMs.
EPA has also endeavored to characterize and quantify contributions from more distant
international sources. EPA has been actively engaged in the task force on Hemispheric Transport
of Air Pollutants (HTAP) as part of the Convention on Long-range Transboundary Air Pollution
(LRTAP). The HTAP effort has included two phases that focused on characterizing transported
air pollution for the years 2000 (Dentener, Keating, & Akimoto, 2010) and 2010. These studies
quantify the sensitivity of air pollution levels in the U.S. to anthropogenic emissions from other
regions, with an emphasis on long-range transport from Europe, Russia, and Asia. EPA also
supported the National Research Council's 2010 report "Global Sources of Local Pollution: An
Assessment of Long-Range Transport of Key Air Pollutants to and from the United States."
(NRC, 2010)
Synthesis of the literature, including citations above and other studies, show that contributions
from international emissions to U.S. O3 concentrations come from a combination of diffuse
background and identifiable plumes from intercontinental transport. Intercontinental transport
plumes occur more efficiently between certain locations on the globe than others (Dentener,
Keating, & Akimoto, 2010; NRC, 2010). For example, semi-permanent pressure systems can set
up an atmospheric "conveyor belt" between the Asian east coast and the U.S. west coast. Off the
coast of China, a semi-permanent low-pressure system lofts air and associated pollutants to the
middle and upper free troposphere. In this region of the atmosphere, fast winds can move
pollution eastward toward the U.S. Pacific coast over the course of days to weeks. During
transport in the upper troposphere, O3 has a long chemical lifetime because the low temperatures
in that part of the atmosphere are not conducive to O3 destruction.
The U.S. Pacific coast has a semi-permanent high-pressure system that draws down air from the
upper troposphere. During vertical mixing of air from the upper troposphere with air closer to the
surface, O3 transported in the upper troposphere is diluted and chemically destroyed (NRC,
2010). The amount of international O3 that is mixed down from the upper troposphere with air
near the surface depends on location and local meteorology. Generally, the further a location is
from an international source the less O3 will be available to mix down due to dispersion and
chemical lifetime. Local meteorology, however, creates exceptions to this rule due to the
dynamics of the planetary boundary layer (PBL), which is the lowest layer of the troposphere,
and topography. Areas with exceptionally deep PBLs can more rapidly transport free
tropospheric air to the surface. Complex topography can include mountain peaks that are
routinely exposed to free tropospheric air. The peaks can also create winds that enhance O3
mixing down mountain slopes (Zaveri, Saylor, Peters, McNider, & Song, 1995). As a result,
high-altitude locations with complex topography may experience greater impacts from
intercontinental transport of O3 as compared to locations at lower elevations.
There is a large body of research exploring the role of international transport and its evolution.
For example, it is well known that Asian NOx emissions increased quickly in the 1990s and
2000s (van der A, et al., 2017) and that transport of O3 also increased (Lin, Horowitz, Payton,
Fiore, & Tonnesen, 2017; Verstraeten, et al., 2015). Several recent studies (Huang, et al., 2017;
9
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Nopmongcol, Liu, Stoeckenius, & Yarwood, 2017) have quantified interhemispheric transport in
2008 or 2010 near the peak contribution (circa 2011). There is broad consistency in the literature
that transpacific transport increased and has since either flattened or decreased (Jaffe, et al.,
2018).
3. EVALUATING THE SECTION 179B RELATIONSHIP TO OTHER
RELATED PROVISIONS
As discussed in this document, section 179B addresses sources of emissions originating outside
of the U.S. and provides qualifying nonattainment areas with specified regulatory relief from
certain otherwise applicable planning and emissions control requirements. This section discusses
limitations to section 179B applicability and identifies related regulatory mechanisms that air
agencies may also find useful for addressing their obligations.
Section 179B applies only to nonattainment areas, and only provides relief from (1) the
attainment demonstration requirement and (2) the requirement that EPA determine whether an
area failed to attain by the attainment date (and associated reclassification as appropriate). It does
not provide EPA with the authority to do any of the following:
Exclude monitoring data influenced by international transport from regulatory
determinations related to an area's designation as attainment or nonattainment (however,
if an exceedance or violation is event-related, it may be able to qualify as an exceptional
event, as described in Section 3.2 of this guidance);
Classify an area with a lower classification than indicated by actual air quality;
Relax any mandatory control measures associated with the area's classification;
Redesignate a nonattainment area to attainment without meeting the other attainment plan
requirements of CAA section 107(d)(3); or
Determine that a state satisfied its interstate transport SIP obligations under CAA section
110(a)(2)(D)(i), the "good neighbor" provision.
Air agencies should also consider whether the regulatory mechanisms discussed below would be
a more appropriate mechanism before initiating a section 179B demonstration. Air agencies with
questions regarding these separate mechanisms are encouraged to consult their EPA Regional
office.
3.1. Extension of Attainment Date
A nonattainment area that fails to attain the O3, CO, PM2.5, or PM10 NAAQS by its attainment
date but has achieved a threshold level of air quality and has met all requirements and
commitments pertaining to the area in the applicable implementation plan may be eligible for a
1-year extension of the attainment date. For example, an area that fails to attain the 2015 O3
NAAQS by its attainment date may be eligible to request a 1-year extension if it has complied
10
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with all requirements in its SIP and if, for the attainment year, the area's fourth highest daily
maximum 8-hour average is at or below the level of the standard.19 A 1-year extension is also
possible for nonattainment areas for CO, PM2.5, and PM10.20 In addition, up to 5-year extensions
are also possible under certain conditions for PM2.5 and PM10 areas classified as Serious.21
As an attainment date approaches, an air agency may consider whether to develop a retrospective
section 179B demonstration while waiting to see whether certified air quality data will ultimately
qualify the area for an attainment date extension. For an area with a high likelihood of attaining a
NAAQS with the additional time provided by an extension, the relief provided by section 179B
may be unnecessary. However, due to the lead time to prepare a retrospective section 179B
demonstration, the air agency should consider beginning development of a retrospective
demonstration well before the attainment year air quality data are certified, in the event that the
area does not ultimately qualify for the extension.
3.2. Exceptional Events
Section 319(b) of the CAA recognizes that, when making certain NAAQS-related regulatory
determinations, including determinations of attainment by the attainment date, it may be
appropriate to exclude ambient monitoring data that are influenced by exceptional events. The
2016 Exceptional Events Rule provides the regulatory mechanism for this purpose.22
When exceptional events influence monitoring data and cause exceedances or violations of the
NAAQS, air agencies can develop and submit a technical demonstration to request the exclusion
of certain event-influenced data. If the demonstration satisfies the Exceptional Events Rule
criteria, EPA can exclude these data from the data set used for certain regulatory decisions.
Specifically, transported pollution, which may include pollution from outside the U.S., must be
event-related and be either natural or caused by a human activity unlikely to recur at a particular
location (see 40 CFR 50.14(c)(3)(iv)(E)).
Exceptional events from natural causes may include wildfires, stratospheric O3 intrusions, high
wind dust events, and volcanic activity. Exceptional events that are human activities unlikely to
recur at a particular location may include prescribed fires on wildland, chemical spills, industrial
accidents, or terrorist activities. In accordance with the CAA, routine emissions generated by and
transported from anthropogenic sources, foreign or domestic, are not exceptional events.23
3.3. Sections 110(a)(2)(D) and 126 - Interstate Transport
19 See CAA section 181(a)(5). More information regarding extending attainment dates for the O3 NAAQS is
available in Implementation of the 2015 National Ambient Air Quality Standards for Ozone: Nonattainment Area
State Implementation Plan Requirements, 83 FR 62998 (December 6, 2018).
20 See CAA section 186(a)(4) for CO, and CAA section 188(d) for Moderate PM2.5 and PM10 areas.
21 See CAA section 188(e).
22 81 FR 68216 (October 3, 2016).
23 An example of routine emissions generated by and transported from anthropogenic sources might include
emissions of O3 precursors or directly emitted particulate matter (or PM precursors) from one state or foreign
country's power plants transported into another state or the U.S.
11
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For some nonattainment areas, the contribution to poor air quality from interstate pollution
transport may be more substantial than contributions from international sources. The "good
neighbor" provisions in CAA sections 110(a)(2)(D) and 126 may apply in addressing out of state
pollution sources that may be interfering with attainment and maintenance of a NAAQS. CAA
section 110(a)(2)(D) requires all states, to have SIPs containing adequate provisions prohibiting
any source or other type of emissions activity within the state from emitting any air pollutant in
amounts that will contribute significantly to nonattainment in, or interfere with maintenance by,
any other state with respect to any NAAQS. CAA section 126 provides a mechanism for states to
petition EPA for a finding that any out-of-state major source or group of stationary sources emits
or would emit any air pollutant in violation of the prohibition of section 110(a)(2)(D).
EPA notes that an upwind state that is considering or has an approved section 179B
demonstration must still meet its obligations for contributions to downwind states under CAA
section 110(a)(2)(D). Likewise, an upwind state that contributes to a downwind state that is
considering or has an approved section 179B demonstration must still meet its obligations for
contributions to that downwind state, per CAA section 110(a)(2)(D). Policy governing whether
an air agency can take into account emissions emanating from outside the U.S. in developing a
plan to meet its interstate transport obligation24 for any NAAQS is outside the scope of this
guidance.
4. THE DEMONSTRATION DEVELOPMENT PROCESS UNDER
SECTION 179B
This section describes the recommended section 179B demonstration development and
submission process, including additional details about prospective and retrospective
demonstrations. The Clean Air Act timelines for consequential SIP obligations and regulatory
determinations associated with nonattainment areas give rise to different submission schedules
for each type of demonstration.
4.1. Early Engagement with EPA Regional Offices
EPA recommends that an air agency engage with its EPA Regional office as early as possible
when considering development of a demonstration under section 179B. The air agency and EPA
should discuss the conceptual model for characterizing the international impacts, identify the
types of analyses that would be most appropriate for the demonstration, and establish
expectations for timing and other considerations. An air agency also may choose to share an
early engagement draft demonstration with its EPA Regional office for review and feedback
prior to submitting a final demonstration to EPA. This early engagement draft may include
preliminary air quality data but a final retrospective demonstration submitted for EPA action
should rely on final certified air quality data including the attainment year as further explained in
section 4.3 of this guidance.
4.2. Prospective Demonstrations under Section 179B(a)
24 See CAA section 110(a)(2)(D)(i) (aka the "good neighbor" provision) for SIP requirements addressing interstate
transport.
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As discussed in Section 1.2 of this document, section 179B(a) states that a required
implementation plan or plan revision shall be approved by the Administrator if such plan or
revision meets all the requirements applicable to it, other than a requirement that such plan or
revision demonstrate attainment and maintenance of the relevant NAAQS by the attainment date,
if the air agency establishes to the satisfaction of the Administrator that the SIP would be
adequate to attain and maintain the relevant NAAQS by the applicable attainment date, but for
emissions emanating from outside of the U.S. The wording in paragraph (a) of section 179B (the
air agency must show that the SIP "would be" adequate to attain and maintain the NAAQS but
for international emissions) indicates that this demonstration is to be forward-looking or
prospective in nature. For this reason, we refer to such demonstrations as "prospective
demonstrations" under section 179B(a).
Section 179B(a) does not include language specific to only certain NAAQS, and thus an air
agency theoretically could develop such a demonstration for any NAAQS. If an air agency is
required to submit a SIP demonstrating attainment of the NAAQS by the relevant attainment date
and is seeking to submit a SIP that shows that the area would attain and maintain the NAAQS by
the attainment date but for international emissions, then the air agency should submit a
demonstration under the process and schedule described in this section.
With regard to O3 nonattainment areas classified Marginal, 25 a prospective attainment
demonstration is not a required SIP obligation for a Marginal area, and therefore there is no
prospective relief to be granted by EPA under section 179B(a). Accordingly, section 179B(a)
prospective demonstrations are not relevant for Marginal O3 areas, and EPA does not intend to
evaluate requests for prospective relief in these instances.
4.2.1. Section 179B(a) Process and Key Questions
A SIP that includes a prospective demonstration under section 179B(a) must meet all
requirements of the CAA Chapter applicable to it other than a demonstration that the area will
attain the NAAQS by the attainment date.26 In general, the applicable requirements for any
nonattainment area SIP include an accurate inventory of current emissions for all sources within
the nonattainment area; a Nonattainment New Source Review (NNSR) permit program;
regulations providing for implementation of Reasonably Available Control Measures (RACM),
including Reasonably Available Control Technology (RACT); a demonstration that the plan
provides for Reasonable Further Progress (RFP) toward attainment; a demonstration of
attainment by the attainment date, commonly in the form of air quality modeling analyses; and
contingency measures to be implemented in certain circumstances should the area fail to attain
by the attainment date. Marginal O3 nonattainment areas are the exception: applicable
requirements are limited to an emissions inventory, source emission statements, transportation
and general conformity programs, and a nonattainment new source review program.
26 See CAA section 179B(a).
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Requirements stated elsewhere in the CAA chapter 85 (Air Pollution Prevention and Control) but
that are not applicable to specific nonattainment area SIP submissions include infrastructure SIP
requirements such as the interstate transport program under section 110(a)(2)(D), which as
indicated above are referred to as "good neighbor" SIPs. These "good neighbor" SIP obligations
apply to all states and are due 3 years after a NAAQS is newly established or revised.
Section 179B(a)(2) defines the demonstration in terms of attainment "but for" international
emissions. If the plan, in meeting all the otherwise applicable requirements of the CAA, results
in attainment, it would be attaining "despite" international emissions rather than "but for"
international emissions. Thus, to support the 179B(a) demonstration, an air agency should show
that even after fulfilling other relevant obligations, such as imposition of required controls, the
area is still projected not to attain the NAAQS. The air agency should therefore first evaluate
whether the area can attain the standard by the attainment date based on required domestic
emission reductions only.
An air agency should follow the steps below when considering and developing a demonstration
under CAA section 179B(a). Step 1 is a required exercise as part of the attainment SIP
development process. Step 2 is conditional upon the outcome of the analyses for Step 1.
Step 1. SIP Attainment Modeling for Potential Domestic Reductions: For the subject
nonattainment area in the relevant SIP, the air agency should first model projected
air quality concentrations for the attainment year based on on-the-books domestic net
emission reductions and growth by the outermost attainment date (e.g., including
mobile source standards, interstate transport programs, rules already adopted by the
state for other nonattainment plans, etc.), and potential reductions associated with
required controls for that particular NAAQS and classification that can be
implemented by the attainment date.27'28
Go to Step 2 only if the Step 1 modeling shows the area could not attain with on-the-
books measures and potential reductions associated with required controls for that
particular NAAQS and classification (e.g., RACM/RACT) that can be implemented
by the attainment date.
Step 2. Section 179B(a) Analyses for International Anthropogenic Emissions: The air
agency may consider developing a demonstration of the impact of international
emissions using the analyses identified in Section 6 of this document.
An air agency should include documentation of completing the analyses for Step 1 as part of its
section 179B(a) demonstration.
27 For further information on how to conduct these analyses, see Modeling Guidance for Demonstrating Air Quality
Goals for Ozone, PM25, and Regional Haze, November 29, 2018. Available at:
https ://www3. epa. gov/ttn/scram/guidance/guide/03 -PM-RH-Modeling Guidance-2018 .pdf.
28 Note that details on the RACM/RACT analytical process for a particular NAAQS can be found in other
implementation rules and guidance. See, e.g., PM2 5 NAAQS implementation rule at 81 FR 58129-58132; O3
NAAQS implementation rules at 70 FR 71659, 80 FR 12264, and 83 FR 62998; General Preamble for
Implementation of Title I of the Clean Air Act Amendments of 1990 at 57 FR 13498.
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4.2.2. Section 179B(a) Demonstration Submission Schedule
EPA considers a section 179B(a) demonstration as part of an attainment SIP. Therefore, the
demonstration should be submitted to EPA no later than the date when all other SIP elements are
submitted. An air agency may also elect to submit a section 179B(a) demonstration before the
bulk of the attainment SIP is completed. EPA encourages air agencies to consult with their EPA
Regional office to discuss timing associated with development and submission of a section
179B(a) demonstration to avoid potentially missing CAA deadlines.
4.2.3. Flow Chart for Section 179B(a) Prospective Demonstrations (SIP approval)
Yes
Yes
No
No
Yes
The air agency should not
develop a section 179B(a)
demonstration.
An area has been
designated as
nonattainment for a
NAAQS.
Is the area an ozone
nonattainment area
classified as Marginal?
STOP: A Marginal ozone area
does not require an attainment
demonstration, therefore a
section 179B(a) demonstration
is not necessary.
Can the area attain the standard by the
attainment date based on existing and
potential new domestic emission
reductions, including the imple-
mentation of required controls (Step 1
in Section 4.2.1 of this guidance)?
Air agency provides the initial notification to its EPA
Regional office to begin discussing concept, scope,
implications, and timing of a possible section 179B(a)
demonstration. The air agency and EPA Regional office
work collaboratively to determine the appropriate scope of
demonstration, consistent with the technical analyses
provided in Section 6 of this document (Step 2 in Section
4.2.1). If the air agency can develop a successful technical
demonstration, the air agency should submit it no later than
when it submits all other parts of the attainment SIP for EPA
review.
4.3. Retrospective Demonstrations under Sections 179B(b)-(d)
As discussed in Section 1 of this guidance, sections 179B(b)-(d) (relating to nonattainment areas
for O3, CO, and PM, respectively) provide that "any State that establishes to the satisfaction of
the Administrator that such State would have attained the national ambient air quality standard
by the applicable attainment date, but for emissions emanating from outside of the
United States, shall not be subject to a mandatory reclassification to the next highest
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classification by operation of law."29 Because the wording in each of these section 179B
paragraphs is presented in the past tense, EPA believes that such demonstrations should be
retrospective in nature. In other words, the demonstration should include analyses showing that
the air quality data on specific days in the past were affected by international emissions to an
extent that prevented the area from attaining the standard by the attainment date.
A nonattainment area that may be eligible to submit a section 179B(b)-(d) retrospective
demonstration would have previously been required under the CAA to submit an attainment SIP.
The CAA requires an air agency to meet all SIP requirements applicable to the area during the
SIP development, submission, and implementation process. Section 179B does not relieve an air
agency of its planning or control obligations.
EPA acknowledges that for some NAAQS the timeline is challenging for air agencies to submit
retrospective demonstrations in advance of the CAA deadline for EPA to determine whether an
area attained by the attainment date. This is one reason why EPA recommends that the air
agency communicate with the EPA Regional office as soon as possible after determining that it
would like to explore the possibility of submitting a section 179B(b)- (d) retrospective
demonstration. Much of the relevant air quality data and other information for a retrospective
demonstration may be available to the air agency in advance of the attainment date. However, to
ensure the integrity of air quality data used in demonstrations, EPA recommends that the air
agency submit a final retrospective demonstration only after all air quality data used to calculate
the attainment year design value are certified, but before the date by which EPA is required to
make determinations of whether areas attained by the attainment date (i.e., 6 months after the
attainment date). As further described in Section 4.3.2 of this guidance, an air agency may
optionally pursue early certification of ambient data to help ensure timely submittal of a final
retrospective demonstration prior to the date by which EPA must make its determinations of
attainment. Provided that the attainment year air quality data are certified, in some cases air
agencies may be able to submit a retrospective demonstration prior to the relevant attainment
date.
As noted above, if EPA approves a section 179B(b)-(d) retrospective demonstration, EPA will
not be required to issue a determination regarding whether the area attained by the attainment
date pursuant to sections 181(b)(2), 186(b)(2), or 188(b)(2) of the CAA. Additionally, EPA does
not intend to take action on any retrospective section 179B demonstrations until after all air
quality data used to calculate the attainment year design value as of the attainment date are
certified.
4.3.1. Marginal Nonattainment Areas for the O3 NAAQS
As discussed in Section 4.2 of this guidance, the list of applicable requirements for O3
nonattainment areas classified as Marginal is limited to: emissions inventory, source emission
statements, transportation and general conformity programs, and a nonattainment new source
review program. As described in the 2015 O3 NAAQS Implementation Rule, section 182(a) of
the CAA does not require states to implement RACM/RACT in Marginal O3 nonattainment
29 See CAA sections 179B(b), (c), and (d). Any such state (or area) would also not be subject to a determination of
failure to attain by the attainment date.
16
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areas, and nothing in section 179B alters the statutory requirements with respect to
RACM/RACT obligations in subpart 2.30
However, an air agency with a Marginal O3 nonattainment area that is affected by international
emissions may wish to evaluate whether implementing emission reduction measures on domestic
sources in the nonattainment area can bring the area into attainment prior to the attainment date.
If an air agency submits a section 179B(b) demonstration that is approved, an area with air
quality violating a NAAQS will remain designated nonattainment and retain its current
classification, until such time as the area attains the standard and the air agency submits, and
EPA approves, a request for redesignation. The area will continue to be subject to NNSR and the
other requirements noted above until the area meets and EPA approves an air agency submission
addressing the redesignation criteria of CAA section 107(d)(3)(E).
4.3.2. Schedule for Retrospective Demonstrations for O3 Nonattainment Areas
Although EPA encourages an air agency to consult with its EPA Regional office regarding a
possible section 179B retrospective demonstration as soon as practicable, a complete section
179B retrospective demonstration should rely on certified air quality data for the entire period
evaluated. EPA recommends that the air agency submit such a demonstration for O3 as soon as
possible after the data from the attainment year are certified.
Air agencies are required to certify O3 air quality data for a given calendar year by May 1 of the
following year.31 An air agency may also elect to pursue early certification of the preceding
year's air quality data, which could improve the possibility of the air agency submitting a
complete a section 179B retrospective demonstration in advance of the area's attainment date.
Attainment dates for Marginal through Extreme O3 NAAQS nonattainment areas extend from 3
to 20 years from the effective date of area designations by EPA. For the nonattainment areas for
the 1997, 2008, and 2015 O3 NAAQS, attainment dates fall on June 15, July 20, and August 3
(respectively) of various years, depending on an area's classification. EPA is obligated to make
determinations of attainment within 6 months of each attainment date based on the area's design
value (CAA section 181(b)(2)(A)). An area's design value is based on air quality data for the 3
calendar years preceding the attainment date. Therefore, EPA recommends that an air agency
submit its section 179B retrospective demonstration as soon as practicable, and well ahead of
EPA's deadline to make a determination of attainment, because EPA approval of such a
demonstration would relieve the area from being subject to EPA's determination of whether the
area attained by its attainment date.
4.3.3. Schedule for Retrospective Demonstrations for PM10, PM2.5, and CO
Nonattainment Areas
Although EPA encourages the air agency to consult with its EPA Regional office regarding a
possible section 179B retrospective demonstration as soon as practicable, a complete section
179B retrospective demonstration should rely on certified air quality data for the entire period
evaluated. Thus, like the recommendation for O3, EPA recommends that the state submit such a
30 83 FR 63010 (December 6, 2018).
31 40 CFR § 58.15 - Annual air monitoring data certification.
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demonstration for PMio, PM2.5, and CO as soon as possible after the data from the most recent
year are certified.
Air agencies are required to certify air quality data for a given calendar year by May 1 of the
following year. An air agency may also elect to pursue early certification of the preceding year's
air quality data, which could improve the possibility of the air agency submitting a complete a
section 179B retrospective demonstration in advance of the area's attainment date. The
attainment date for PM10, PM2.5, and CO NAAQS nonattainment areas typically falls on
December 31 of a given year (although there are some exceptions). EPA is required to make
determinations of attainment within 6 months of the attainment date (i.e., by June 30th of the
year following the December 31 attainment date). See CAA section 186(b)(2)(A) for carbon
monoxide; CAA section 188(b)(2) for PM2.5 and PM10. Therefore, EPA recommends that the air
agency submit its section 179B retrospective demonstration as soon as practicable, and well
ahead of EPA's deadline to make a determination of attainment, because EPA approval of such a
demonstration would relieve the area from being subject to EPA's determination of whether the
area attained by its attainment date.
4.3.4. Flow Chart for Section 179B(b)-(d) Retrospective Demonstrations (potential
reclassification)
Certified data for the design
value period that precede the
relevant attainment date become
available.
The air agency contacts
its EPA Regional office
to discuss concept, scope,
implications, and timing
of a possible section
179B(b) - (d)
demonstration.
The air agency expects
the area to fail to attain
by the attainment date
and intends to explore
whether it would have
attained but for impacts
from international
emissions.
The air agency may choose to
begin developing a section
179B(b) - (d) demonstration
prior to the attainment date if air
quality data indicate the area is
unlikely to attain by the
attainment date (and if the air
agency has complied with other
applicable requirements).
If the relevant design value shows that the area has failed to
attain the relevant NAAQS, the state can submit its section
179B(b) - (d) demonstration for EPA review and action. EPA
will take final action on a section 179B(b) - (d)
demonstration via a public notice and comment rulemaking.
5. CONCEPTUAL MODEL OF INTERNATIONAL INFLUENCE
Section 179B demonstrations should include a conceptual model that is intended to frame the
"state of the knowledge" for air quality in the nonattainment area. The conceptual model
includes information regarding the influence of emissions, meteorology, transport, and/or other
relevant atmospheric processes on air quality in given nonattainment area (EPA, 2018c). A well-
constructed conceptual model of pollutant formation and transport for the area can assist in the
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determination of international transport impacts by highlighting the contrast between locally
formed pollutant days and the internationally influenced days in question. The conceptual model
should provide the context for reviewing the more detailed analyses provided by the state,
described in Section 3 of this guidance.
The conceptual model should clearly identify which type of transport and international
sources/source regions directly affect the nonattainment area and whether that transport type is
distinct from locally influenced high pollution conditions. Clearly identifying the conceptual
distinction between local and international contributions forms the basis for which analyses
should be included in the demonstration. The conceptual model should also identify which
regions and sources meaningfully contribute to the international portion of emissions that
influence ambient concentrations in the area of interest.
The information included in a conceptual model should help establish the context for the overall
analysis and should be consistent with more detailed evidence and analyses provided in the
overall demonstration32. To promote a shared understanding and interpretation of this
information, the list below provides examples of the kinds of information that would typically be
useful to include in a conceptual model.
• A summary of the affected area's NAAQS attainment and classification information.
• A description of the regulatory determination the state believes is influenced by the
international emissions.
• A map of the existing ambient monitors in the area, and a description of the sites (e.g.,
site ID, current measured design value, elevation, recent pollutant trends), and any other
relevant information.
• A list of the monitor(s) and days that the air agency has identified as influenced by
international anthropogenic emissions.
• A description of the key differences between the measured exceedances influenced by
international emissions concentrations and typical exceedances influenced by local, non-
international emissions. It would be helpful to include a table of the relevant monitor data
(e.g., date, hours, monitor values, and design value calculations with and without the
international emissions).
• A summary of the meteorological and atmospheric conditions that lead to high
concentrations at the monitor on days influenced by international anthropogenic
emissions and days not influenced by international anthropogenic emissions. The
contents of this summary will vary by area, but could include:
o the months in which high concentration days usually occur
o the diurnal evolution of a typical exceedance in the area
o the source sector(s) that contribute to the local and regional contribution
o the source sector(s) that contribute to the international contribution (if known)
o for PM, the pollutant species that contribute to typical exceedances in the area
o typical spatial patterns of exceedance days
32 All comments received on this guidance can be found at: https://www.regulations.gov/docket?D=EPA-HQ-OAR-
2019-0668.
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o the meteorological conditions associated with high concentration days influenced
by international emissions, including a description of the route traveled by
transported pollution, such as distance and altitude
o the meteorological conditions associated with high concentration days not
influenced by international emissions.
• Identification of specific international anthropogenic emissions sources (e.g., an
international emitting facility) or source regions (e.g., an international metropolitan area)
that predominantly impact the monitor location on internationally influenced days
• Where available, a description of how controls on the upwind international anthropogenic
sources differ from those required within the U.S. and how that difference could have
affected the regulatory determination.
• A synthesis of literature characterizing international transport contributions to the
geographic region of interest.
The California Air Resources Board's 2013 SIP for Imperial County for the 2006 24-Hour PM2.5
standard (CARB, 2014)33 contains useful examples for several components of a conceptual
model for a section 179B demonstration.
Figure 1 below, taken from the referenced Imperial County SIP, shows one example of an
overview map where the border with Mexico is clearly visible, as are major topographical
features, major roads, and cities. In addition, the lower left-hand inset presents the broader
context of the nonattainment area location within California. This type of graphic should also
show where area monitors are located relative to key international anthropogenic sources and
provide context for other geographical considerations.
33 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
20
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iNiland
ARIZONA
orland
Brawled
Anza Borrego Desert
ilexico
x
20
I
Cities
Major Roads
PM2.5 nonattainment area
Lakes
Figure 1. Example overview map, Imperial County I'M;.? nonattainment area overview map
(CARB, 2014)34
That same SIP also included a graphic that connected time and space trends for PM2.5
concentrations. Figure 2 below, also taken from the referenced Imperial County SIP, indicates
both that substantial air quality improvements have been made in the region and that the
southern-most monitor (Calexico) has the highest design values with the smallest reduction
between 2001 and 2012. This result would need to be corroborated with discussion of emissions
in a later section to establish that domestic emissions affecting Calexico have been reduced
commensurate with domestic emissions affecting El Centro and Brawley.
Other components of the conceptual model will be case-specific. In addition to the Imperial
County demonstration for the 2006 PM2.5 NAAQS referenced in Figures 1 and 2, components of
other section 179B demonstrations referenced in this document may provide helpful examples
for how elements can be addressed,35 Each situation regarding international contributions is
unique and eveiy section 179B demonstration will necessitate information and analyses to be
tailored accordingly.
34 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
35 This guidance includes these illustrative examples for the benefit of air agencies developing section 179B
demonstrations. Most of these examples were taken from demonstrations that have been submitted to EPA. As
previously mentioned, EPA has not yet acted on a demonstration for an area not adjoining an international border.
As additional demonstrations are submitted to EPA, more illustrative examples may become available.
21
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Figure 2. 2001 and 2012 24-hour Design Values for Brawley,
El Centro and Calexico
60
50
24-hour PM2.5 Standard
Brawley El Centro Calexico
¦ 2001 ¦ 2012
Figure 2. 2001 and 2012 24-hour PM2.5 Design Values for three Monitoring Sites in Imperial
County, California (CARB, 2014, pp. 15, Figure 2.5)-6
Identification of sources and how controls could affect attainment provides context to the
reviewer and informs potential international engagement. When international sources are
uncontrolled or under-controlled, this helps a demonstration reviewer understand why the source
is expected to have a large contribution. If potential controls of international sources could lead
to attainment, this information may highlight an area for EPA international engagement and/or
negotiation.
Finally, the conceptual model should be put in the context of historical and contemporary
literature. There have been multiple projects and analyses completed that focus on U.S.
background O3 or on international transport that may be relevant. The results from these projects
are often provided as annual averages or in metrics that are not specific to the NAAQS form, but
they still provide important insights and establish expectations. For example, a literature
synthesis should summarize international transport magnitudes, variability, and seasonality. The
synthesis should highlight where modeling results are consistent and where there is divergence
among models. Specific focus should be given to comparing and contrasting the literature with
the conceptual model.
36 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
22
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6. TECHNICAL ANALYSES IN SUPPORT OF THE SECTION 179B
DEMONSTRATION
6.1. Determining the Appropriate Analytic Approach for the Section 179B Demonstration
As noted in Section 1.3 of this guidance, EPA recognizes that the relationship between certain
NAAQS exceedances and associated international transport is clearer in some cases than in
others. Table 1 details analyses recommended for all demonstrations, and additional analyses for
demonstrations needing additional lines of evidence.
While each section 179B demonstration will involve a unique set of conditions, EPA believes
that the general characteristics listed below would suggest the need for a demonstration with
fewer lines of evidence to be appropriate:
• Affected monitors located near an international border.
• Large international emission sources located across the border near the affected monitors.
• Meteorology and international transport patterns connect emissions from identified
international sources to monitors on days with monitored exceedances.
• Exceedances do not occur on days with similar conditions when transport to monitors is
domestic in origin.
Conversely, the following characteristics would suggest the need for a more detailed
demonstration with additional lines of evidence:
• Affected monitors not located near an international border.
• Specific international sources and/or their contributing emissions are not identified or are
difficult to identify.
• Exceedances on internationally influenced days are in the range of typical exceedances
attributable to local sources.
• Exceedances occurred in association with other processes and sources of pollutants, or on
days where meteorological conditions were conducive to local pollutant formation (e.g., for
O3, clear skies and elevated temperatures).
Air quality in areas with proximity to borders and coasts often has clear and strong relationships
with nearby international anthropogenic sources. Figure 3 shows the combined O3 contributions
(the fourth highest 8-hour daily maximum value within the U.S.) from anthropogenic emissions
in Canada and Mexico based on source apportionment modeling results for 2023 (EPA, 2018b).
This modeling estimates future aggregate international contributions of emissions from Canada
and Mexico and is not necessarily representative of the international contributions that occur on
specific days (e.g., specific exceedances that may influence a SIP determination and be critical
for section 179B purposes) in a given area. Therefore, this type of analysis, by itself, would not
be sufficient to demonstrate international contributions for the purposes of section 179B, but it
may be helpful as part of a broader set of analyses used to support the weight of evidence in a
section 179B demonstration.
In the 2023 modeling results discussed above, 24 of the 26 monitors with contributions greater
than 5 ppb were located within 30 miles of the Canada or Mexico international border, and all 26
were within 40 miles of the border. Additionally, the influence of international sources on near-
23
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border U.S. monitors appears to be substantially larger in locations with a large industrial source
or urban center directly on the international side of the border.
0 0 1.0 2.5 5.0 10.0 20 0
Color = CAN+MEX; Squares: 2023 Nonattainment
Figure 3. O3 (fourth highest maximum daily 8-hour average) contributions from in-domain
Mexican and Canadian anthropogenic emissions at AQS monitors in 2023 projections. Monitors
that are projected as nonattainment in 2023 are outlined squares and all other monitors are
smaller circles.
Consistent with the general pattern illustrated in Figure 3, Figure 4 shows a strong exponential
reduction of Mexico/Canada contribution as distance from the border increases. This modeling is
only able to quantify emissions from within the contiguous U.S. modeling domain (i.e., "in-
domain"), which extends from 23 to 52 degrees north. Outside the domain, international shipping
and transpacific international contributions could not be quantified in this modeling.
24
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logs = - Q.SltogcK + 3.07 !r=0.49)
y 10
100 2 00 300 400 500 600
Miles from Canada/Mexico Border (dx)
700
Figure 4. Mexico/Canada source contribution (S) in 2023 at monitors by distance from border
(dx).
Impacts from other international sources (not from Canada, Mexico, or shipping) are related to
longer-range transport efficiency, which is related to an area's elevation. Figure 5 shows the
influence of O3 and precursors from outside the modeling domain as represented by simulation
of initial and lateral boundary conditions as a function of elevation at monitors more than 100
miles from the boundary. The boundary conditions do not distinguish between international
anthropogenic and natural sources, but here serve as a proxy for efficiency of transport from the
boundaries to the monitor. Although the relationship is not trivial (r2=0.55), it is driven by a
cluster of low-elevation monitors and a scattering of high-elevation monitors. If the low-
elevation monitors (<500 m) are excluded, the relationship significantly weakens (r2=0.08). To
contrast, removing the monitors further from the border strengthens the relationship with
distance from border. Proximity to the boundary alone is not sufficient to explain the variability
in international contribution between sites. Instead, there is a complex relationship between
location, local topography, elevation, and season.
25
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w
mmmm 5 = O.Oldz + 17.63 (r2 = 0.55)
10 i
0
500
1000
1500
2000
2500
Meters above sea level (dz)
Figure 5. Initial and Boundary Conditions contribution (ICBC) to projected 2023 design values
as a function of elevation at AQS monitors that are farther than 100 miles from the border.
6.2. Analyses to Consider in Weight-of-Evidence Demonstrations
As with all intended section 179B demonstrations, the submitting air agency and its EPA
Regional office should discuss the appropriate level of evidence before developing a
demonstration. As a result of this discussion, EPA and the air agency can jointly identify the
appropriate analytical approach for the section 179B demonstration.
There is no rigid set of rules regarding which specific analytical elements will demonstrate (or
refute) the influence of international anthropogenic emissions. Each case is unique. Table 1
provides a checklist of recommended analyses that could support the demonstration of
international anthropogenic influence. Each analysis is described in more detail in Section 6.3 of
this guidance. In addition to these recommended analyses, other assessments not specifically
mentioned in this guidance may also be valuable for certain areas. An approvable demonstration
will generally contain a consistent analytical narrative that shows international anthropogenic
emissions meaningfully contributed to an exceedance at the monitor.
26
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Table 1. Recommended analyses to demonstrate international contribution for all
demonstrations, with specific analyses listedfor demonstrations needing additional lines of
evidence. All analyses should be selected in consultation with the EPA Regional office.
Type of Analysis
Description of Analyses
Conceptual Model
(Section 5)
Description and basic supporting information regarding the conditions
that lead to high pollutant concentrations in the area and what conditions
are conducive to international influence.
Ambient
Observational
Analysis
(Section 6.3.1)
• 5 years (or more) of peak concentration data with internationally
influenced days flagged.
• Table with percentile ranks of days for international influence.
• Relationship between high levels of pollution (i.e., criteria or
precursor pollutants) and meteorological/air quality conditions
characteristic of international transport.
• Back trajectories and/or backward dispersion analyses (or where
trajectory/dispersion analyses are not available, in-situ wind roses)
showing internationally influenced source-receptor relationships on
exceedance days
• Summary narrative including a comparison of locally and
internationally influenced days.
Demonstrations needing additional lines of evidence:
• Historical diurnal profile comparison on local and non-local
contribution days.
Comprehensive
Emissions Analysis
(Section 6.3.2)
• Develop a domestic emission inventory.
• Obtain or develop an international emission inventory for proximate
sources.
• Projections of both inventories to the relevant year (in some cases it
may be helpful to project prior-year inventories even for historical
analyses).
Demonstrations needing additional lines of evidence:
• Obtain or develop a broader international emission inventory.
27
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Type of Analysis
Description of Analyses
Modeling to
Quantify
International
Contribution
(Section 6.3.3)
Reference existing modeling that has been evaluated, typically for a
related analysis.
Demonstrations needing additional lines of evidence (choose one in
consultation with the Region):
• Dispersion modeling quantifying international contribution and
culpability (for inert pollutants such as direct PM2.5)
• Photochemical modeling quantifying contribution via sensitivity
modeling or source apportionment, (for chemically reactive pollutants
such as O3 or secondary PM2.5)
Chemical
Fingerprint Analysis
(aka "receptor
modeling" or "filter
analysis")
(Section 6.3.4)
• Where available and applicable (see section 6.3.4 of this guidance).
6.3. Analyses to Demonstrate International Contribution
After determining the appropriate analytical approach for a section 179B demonstration, the next
step in establishing a relationship between international emissions and a monitored exceedance is
to develop the analyses that describe how international emissions were transported to the monitor
in sufficient quantities to cause the exceedance. The analyses necessary to establish this
relationship should be tailored to the area and pollutant of interest, and should generally include
pollutant measurements, meteorological observations, emissions quantification, and air quality
modeling. The 1994 Addendum to the General Preamble of the Implementation of Title I of the
Clean Air Act Amendments of 1990 (59 FR 42001) suggested a set of five analyses that may
help support section 179B demonstrations (a, d) for PMio:
1. Place several ambient PM-10 monitors and a meteorological station;
measuring wind speed and direction, in the U.S. nonattainment area near the
international border.37 Evaluate and quantify any changes in monitored PM-
IO concentrations with a change in the predominant wind direction.
2. Comprehensively inventory PM-10 emissions within the U.S. in the vicinity
of the nonattainment area and demonstrate that the impact of those sources,
after application of reasonably available controls, does not cause the NAAQS
to be exceeded. This analysis must include an influx of background PM-10 in
the area. Background PM-10 levels could be based on concentrations
measured in a similar area not influenced by emissions from outside the U.S.
37 See 40 CFR part 58 for guidance on locating PM-10 monitors and "On-site Meteorological Program Guidance for
Regulatory Modeling Applications" (EPA, 1987) for guidance on locating meteorological stations.
28
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3. Analyze ambient sample filters for specific types ofparticles emanating from
across the border. (Although not required, characteristics of emissions from
sources may be helpful).
4. Inventory the sources on both sides of the border and compare the
magnitude ofPM-10 emissions originating within the U.S. to those emanating
from outside the U.S.
5. Perform air dispersion and/or receptor modeling to quantify the relative
impacts on the nonattainment area of sources located within the U.S., and of
foreign sources ofPMio emissions (this approach combines information
collectedfrom the international emission inventory and meteorological
stations, ambient monitoring network, and analysis of filters).
With the exception of item 3 above, these methods, with minor modifications, are also generally
applicable to other criteria pollutants, including carbon monoxide and O3 which are explicitly
addressed in section 179B(b)-(d). For O3 and potentially PM2.5, the type of modeling performed
will be chemical transport and not dispersion. Again, air agencies can describe the mechanics of
the international transport in a variety of ways. Although the specific analyses appropriate for a
section 179B demonstration will vary on a case-by-case basis, these methods are generally
aligned with the five factors considered in the present-day designations process: air quality data,
emissions and emissions-related data, meteorological data, geography/topography, and
jurisdictional boundaries.38
This guidance on analyses is structured into four types of supporting analyses. Section 6.3.1 of
this guidance discusses measured air quality data analyses. Section 6.3.2 of this guidance
describes comprehensive emissions analyses. Section 6.3.3 characterizes source-receptor
modeling analyses. Section 6.3.4 of this guidance highlights receptor modeling analyses. The
examples provided in each section focus on PM2.5 and O3, which EPA anticipates will be of
wider interest than CO and PM10. However, the examples may be applicable to other criteria
pollutants.
6.3.1. Measured Air Quality Data Analysis
The first component of establishing a relationship between international anthropogenic emissions
and the monitored exceedance is to prepare an analysis showing how the measured concentration
compares to the distribution or time series of historical concentrations measured at the same
monitor and/or at other monitors in the area. Air agencies can show the relationship between the
days with internationally influenced concentration(s) and historical concentrations across all days
in a variety of ways. Table 2 describes example analyses that could be completed to show
38 Memorandum to Regional Administrators entitled "Area Designations for the 2015 Ozone National Ambient Air
Quality Standards" February 2016 at: https://www.epa.gov/sites/production/files/2016-02/documents/ozone-
designations-guidance-2015.pdf.
29
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whether the international-influenced exceedances were outside the bounds of generally expected
pollutant levels.
Table 2. Suggested Analyses for Comparing Historical and Internationally Influenced
Concentrations
Historical
Concentration Evidence
Types of Analyses/Supporting Information
1. NAAQS concentration
data
Plot the NAAQS concentration matrix at the affected monitor(s)
for the most recent 5-year period39 that includes the international
transport influence(s) of regulatory significance. This can also be
supplemented with a table that briefly describes percentile ranks of
internationally influenced days with a comparison against
historical means and maxima.
2. Identify transport
influences
Distinguish any high concentrations associated with previously
approved international transport demonstrations, suspected
international transport, and other unusual occurrences (e.g.,
exceptional events) from other high pollution days that are
primarily due to normal domestic emissions (provide evidence to
support the identification when possible).
6.3.1.1. Demonstration Days and Historical Context
A brief overview of the measured concentration data and the transport patterns that governed the
suspected international influence should be provided near the beginning of a section 179B
demonstration as part of the conceptual model. For illustrative purposes, Figure 6 shows an
example time-series plot that highlights (in red) the days that are the focus of a hypothetical
demonstration and puts them in the context of other O3 days. In this case, 2015 and 2016, which
are the focus of the evaluation of international sources, do not fully cover the 5-year data period.
Therefore, there could be other historical days (i.e., a subset of the days in blue) that also have
international influence. Note that Figure 6 shows that days being evaluated for international
influence are not unlike other exceedance days in the historical record. An example of tabular
day identification is shown in Table 3 that could complement a figure like Figure 6.
39 Section 8.4.2.e of appendix W (82 FR 5182, January 17, 2017) recommends using 5 years of adequately
representative meteorology data from the National Weather Service (NWS) to ensure that worst-case meteorological
conditions are represented. Similarly, for international influence purposes, EPA believes that 5 years of ambient air
data better represent the range of "normal" air quality than do shorter periods.
30
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0.10
0.08
E
§; 0.06
QJ
C
o
N
° 0.04
0.02
0.00
0 50 100 150 200 250 300 350
Day of Year
Figure 6. Measured Os as a function of day-of-year with example days highlighted as influenced
by international sources marked in red and other days marked in green.
Table 3. Example tabular summary of exceedance days for internationally influenced Os data.
Transport Day
Concentration
(ppm)
5/30/2015
0.071
6/8/2015
0.076
3/1/2016
0.074
4/19/2016
0.072
5/13/2016
0.072
6/28/2016
0.072
7/5/2016
0.071
7/12/2016
0.075
6.3.1.2. Establishing an International Source-Receptor Relationship
Measured exceedances should be connected to international source emissions by meteorological
analysis. Fleming, Monks, and Manning (2012) published a particularly relevant review paper
highlighting in-situ wind direction, trajectory models, particle dispersion models, and chemical
transport models. The two most widely used analyses in section 179B demonstrations are in-situ
wind analyses and trajectory analyses. Fleming, Monks, and Manning (2012) highlight that more
sophisticated methods have largely replaced local wind direction as a method of identifying air
mass history. Further, they note that dispersion models provide a more useful field for composition
. 2012-2014 Days
. 2015-2016 Days
• •. 2015-2016 Demo Days
NAAQS
9 < •
m •• • * • •
. yaii1. t tff
31
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analysis. Trajectory analyses rely on the use of meteorological models and their interpretation of
observed conditions (aka reanalysis), which provides meteorological information over a broad
area, but introduces biases inherent in the meteorological model. For example, previous section
179B demonstration submissions have highlighted that reanalysis may be too coarse to properly
represent wind flow patterns in locations with complex topography. HYSPLIT now includes
several 12 km resolved products and a 3 km resolved product from the NOAA's High-Resolution
Rapid Refresh system (HRRR).40 The HRRR product and others like it will help future
demonstrations that require fine resolution for complex topography. Despite their respective
limitations, winds and trajectory analyses can provide useful information as part of the weight-of-
evidence in a section 179B demonstration.
In-situ Winds Analyses: In-situ winds analyses infer sources of pollution based on the speed and
direction from which the wind blows. The strength of the method is in its simplicity to perform,
but the simplicity is achieved by assuming that locally measured winds are representative of
wind directions between the source and monitor. One weakness of this approach is the known
existence of meandering or circular wind patterns. Wind analyses should only be used when
more sophisticated methods are precluded. If included in a demonstration, in-situ wind analyses
will hold less weight in the evidence assessment.
In-situ wind analysis was used in the section 179B demonstration for the Nogales, Arizona, PMio
nonattainment area (ADEQ, 2012). That demonstration suggested that local topography limited
the application of trajectory analyses. Each section 179B demonstration should review the
increasingly accurate and fine-scale meteorology data available at the time. When opting out of
trajectory or dispersion modeling, the air agency should still compare trajectory or dispersion
modeling to its proposed technique. In the case of Nogales, the demonstration instead included
analysis of measured local winds. Table 4 and Figure 7 highlight that wind direction on low
concentration days was disproportionately from the north and west, and that high concentration
days were disproportionately from the southerly direction with lower wind speeds. By itself, this
wind vector analysis is merely suggestive. It is possible that meteorology conducive for transport
from international sources is associated with southerly winds, or southerly winds may indicate a
larger-scale flow pattern that connects this monitor to other domestic sources. Therefore,
additional analyses are needed to help confirm conclusions from the wind vector analysis.
40 Although the web interface only has July 2019 forward, the University of Utah HRRR archive
(https://rapidrefresh.noaa.gov/hrrrA has been rebuilt going back as far as July 2016. HYSPLIT provides tools for
creating HYSPLIT compatible meteorological inputs from the available grib2 file
(https://www.readv.noaa.gov/documents/Tutorial/html/meteo cnvrt.htmT).
32
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Table 4. Nogales, Arizona Hourly ambient PM10 concentrations sorted by concentration and
wind direction, 2007 2009 exceedance days. (ADEO, 2012, p. Appendix A Table ll)41
Range of Ambient Concentration Values (mierogram/m3)
Wind
Direction
Quadrant
< 150
150-250
250 - 350
350 - 450
450 - 550
>= 550
Share of All
Wind Direction
Observations
Northerly
NWtoNNE
27%
6%
3%
3%
3%
0%
17%
Easterly
NE to ESE
15%
16%
16%
11%
3%
8%
14%
Southerly
SE to WSW
41%
71%
72%
84%
92%
92%
57%
Westerly
SW to WNW
18%
6%
8%
3%
3%
0%
12%
Total
100%
100%
100%
100%
100%
100%
100%
200 T-2009 Wind Rose for Nogales Post Office (t xcluding Exceedance Days)
(a) Non-Exceedance Days
200 7 2019 Wind Rose (or All Hoqales Post Office Exceedsnces
(b) Exceedance Days
WIND SPEED
(rote)
~
>= 10 7
-10 7
5.4 - 70
¦
33- 5.4
EZ
\5- 33
~
02- 15
C*ms' 2 44t
Figure 7. Nogales, Arizona PM10 nonattainment area: wind roses for non-exceedance and
exceedance days (ADEO, 2012, pp. Appendix A, Figure 9)
Trajectory Analyses: Backward trajectory analyses promote a fuller understanding of transport
between sources and measured concentrations. A trajectory analysis follows an air mass
represented by a single point as it moves through space due to atmospheric forces, and a
backward trajectory retraces the path a particle would have taken to arrive at a point. The
location of trajectories can be paired with additional meteorological data, geopolitical
boundaries, and emission inventories to help distinguish between the influence of domestic and
international emissions.
41 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
33
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The examples shown in Figure 8 and Figure 9 present results of HYSPLIT trajectories to assess
the potential international and domestic impacts in the Imperial County, California, area.42
Trajectories were initialized every 2 hours through each 8-hour maximum O3 daily value (from
start,
-------
Jf=0.074 ppm; n = 264; USAPBL=45; MEXPBL=28
Time Since Initialization
(2015-04-10, 2016-07-14 03)
(a) All Exceedance Days
X=0.077 ppm; n = 12; USAPBL=68; MEXPBL=20 X=0.071 ppm; n = 12; U5APBL=12; MEXPBL=43
8
7
_ 6
1 4
-3
i
0
0
-10 -20 -30 -40 -50 -60 -70 -80
Time Since Initialization
8
7
6
1 4
^ 2
1
0
0
-10 -20 -30 -40 -50 -60 -70 -80
Time Since Initialization
(2015-04-15, 2015-04-19 01) (2015-05-27, 2015-05-31 02)
(b) Example US-related Exceedance (c) Example Mexico-related Exceedance
Figure 8. E1 Centra 72-hour back trajectories for (a) all exceedance days, (b) an example
primarily domestically influenced trajectory, and (c) an example more internationally influenced
exceedance day.
Figure 9 shows back trajectory-based NPES (a,b) and NPSC (c,d). The percent attributed to
various regions (NPES: U.S., Mexico, or Ocean; NPSC: U.S., Mexico, or Natural) is shown in
the figure panel titles. Comparing NPES panels (a) and (b) reveals that exceedance day
trajectories spent relatively more time in the Mexican PBL than non-exceedance days (24
35
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percent vs. 17 percent), but also show a density of PBL time along the U.S. Pacific coast. This
identifies both international and domestic sources on exceedance days. Comparing NPSC panels
(c,d) to NPES (a,b) shows that emissions across the border play a bigger role than suggested by
just looking at trajectories. Like NPES, the NPSC attributed to Mexico is greater on exceedance
days (52 percent) than all days (39 percent).
One challenge associated with using backward trajectories is that a large number of trajectories is
necessary to create a PES or PSC. The NPES and NPSC in Figure 9 are relatively sparse and
currently include 23 days with three altitudes and five releases (345 trajectories). Further, the
trajectory could pass near a source and the narrow nature of the trajectory would not include the
source. For this reason, trajectory-based NPES and NPSC can only provide supporting evidence.
When exceedance days show larger fractions of NPSC from international anthropogenic sources,
this adds to the weight of evidence that international anthropogenic sources contribute to
exceedances.
: It)"3
(a) All Day NPES
(b) Exceedance Day NPES
(c) All Day NPSC
(d) Exceedance Day NPSC
Points below PBL - NAT: 7%; US: 54%; MEX: 39
Points below PBL - NAT: 26%; US: 55%; MEX: 17%;
Points below PBL - NAT: 26%; US: 50%; MEX: 24%;
Points below PBL - NAT: 5%; US: 43%; MEX: 52
%;
%;
Figure 9. Imperial County, California Os nonattainment area: El Centra 72-hour back trajectory
Normalized Potential Emission Sensitivity maps (a,b) and Normalized Potential Source
36
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Contribution maps (c,d) for all days (a,c) and exceedance days (b,d). The frequency is shown by
the color and identifies the fraction of hourly locations from all trajectories that are in each cell.
The USA, MEX, and OCN (ocean) labels identify the percentage ofNPES within each region.
The USA, MEX and NAT labels identify the percentage ofNPSC from regional sources.
For areas farther from international sources, the HYSPLIT analysis will provide less direct
evidence and can be used only to assess general transport patterns. Back trajectory analysis can
provide information about the transport patterns, including the time spent in the local boundary
layer, the extent of vertical transport, and the transport time to upwind areas. The time spent in
the boundary layer will often be inversely related to international impact, while the vertical
transport connects the parcel to regions of the troposphere with efficient transport. This type of
information can be provided to support a section 179B demonstration. Fleming, Monks, and
Manning (2012) discusses several applications to long-range transport that include cluster
analysis. Cluster analysis may be useful for demonstrating similarity between types of days and,
therefore, to distinguish between days of varying international influence.
Backward Dispersion: Backward dispersion analyses follow plumes rather than single-point air
masses in backward trajectories. Backward dispersion results can improve upon trajectory
analyses in several ways. First, backward dispersion includes the effects of turbulent motion.
Each trajectory is no longer defined by just its average pathway, but instead is described by a
probability distribution of particles. Second, the backward dispersion model uses a continuous
release so that the backward trajectory includes many different initial conditions. The backward
dispersion creates a "retroplume" that is proportional to residence time (Seibert and Frank 2004;
10.5194/acp-4-51-2004). The retroplume does not account for all processes and is subject to
certain assumptions (Lin et al. 2003; 10.1175/BAMS-D-14-00110.1). When assumptions are
satisfied, the retroplume can be converted to a NPES and NPSC (similar residence time from
back trajectories).
The example shown in Figure 10 is based on HYSPLIT backward dispersion simulations.
Similar to the back-trajectory section (above), simulations were configured based on maximum
daily 8-hour O3 measurements on 587 days at the El Centro monitor. For backward dispersion,
source locations were configured to release a vertical line-source of a generic gas tracer between
100 and 1000 m over the monitor. The release lasted for the full 8-hour period. Compared to a
series of backward trajectories, the 8 hours of emissions provide a more continuous view of
possible sources, the dispersion model accounts for turbulent motion, and explicitly weights
closer sources more heavily. In the case of secondarily-formed O3 and PM2.5, the weighting
toward closer sources may overestimate their influence. In an ideal case, these receptor-oriented
backward dispersion results could be combined with source-oriented forward dispersion results
to produce a more complete source-receptor relationship.43
Figure 10 shows the backward dispersion NPES and NPSC. Compared to backward trajectories
(see Figure 9), the backward dispersion NPES and NPSC have a smoother distribution that
results from turbulent dispersion. The percent attributed to various regions (NPES: U.S., Mexico,
or Ocean; NPSC: U.S., Mexico, or Natural) is also similar to backward trajectories. A key
difference is that the dispersion-based continuous surface lends itself to creating day-specific
43 https://ready.arl.noaa.gov/documents/Tutorial/html/src_recp.html.
37
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NPES and NPSC in a way that trajectories do not. Figure 10 also shows two days that illustrate
primarily U.S.-influenced (Figure lOe) and primarily Mexico-influenced (Figure lOf)
exceedances. Despite the quantitative results, there are many uncertainties and the results depend
upon the configuration and choices made in the processing. When a large majority of NPSC are
from a source region (here Mexico), these results could be part of a weight of evidence that these
days have international influence. When the fraction of NPSC is substantially larger on
exceedance days than typical days, this strengthens the weight of evidence.
(c) Ml days NPSC (d) Exceedance days NPSC
(a) All days NPES
(b) Exceedance days NPES
Points below 1000m - NAT: 24%; US: 54%: MEX: 20%;
ICJ
c $
10"5 u
Points below 1000m - NAT: 24%; US: 46%; MEX: 30%;
Points below 1000m - NAT: 7%; US: 51%; MEX: 42%;
Points below 1000m - NAT: 5%; US: 39%; MEX: 56%;
38
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Points below 1000m - NAT: 5%; US: 75%: MEX: 20%;
Points below 1000m - NAT: 6%: US: 9%; MEX: 85%:
(e) Primarily U.S. Influenced (j) Primarily Mexico Influenced
Exceedance Exceedance
Figure 10. El Centro 72-hour back dispersion-based NPES (a,b) and NPSC (c-j) for all days
(a,c), exceedance days (b,d), a primarily U.S. influenced exceedance (e) and a primarily Mexico
influenced exceedance (f).
6.3.2. Comprehensive Emission Analysis
A comprehensive emissions analysis is an important component of a section 179B
demonstration. The emissions analysis provides most compelling direct evidence when the
international emissions are immediately abutting the nonattainment area. In this case, the
emissions analysis, including domestic and international emissions, will cover a geographic
region of similar size and proximity to a metropolitan area. Emissions within the region would be
separated into international and domestic components. A comprehensive emissions analysis
should consider:
• What emission sources currently exist, and what is the magnitude of domestic versus
international emissions?
• What controls are in place currently for the international sources (where available)?
• What change in emissions is expected in the foreseeable future (where available)?
• Are there international agreements that are already addressing these emissions?
Domestic and international emissions inventories should be developed in a manner consistent
with EPA's emission inventory guidance (EPA, 2017). Domestic emissions should have been
developed by the state during the SIP development process and should conform to that guidance.
Not all emission guidance that is relevant to the domestic emissions is practical for international
sources. The international emissions database may either be developed as a part of the section
179B demonstration or leveraged from a pre-existing public database. In either case, the process
for developing the database should be based on the same principles of emission inventory
development as are included in the guidance for domestic emissions. When using a pre-existing
database, the provenance and methodology used to build the international emissions database
should be well documented. Ideally, the database should also be compared to well-established
databases (e.g., EDGAR, EDGAR-HTAP, CEDS).
39
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A prospective attainment demonstration is based on future emissions estimates for international
and domestic sources. Thus, on-the-books emissions controls should be considered when
projecting future air quality. In some cases, it may be difficult to forecast international emission
changes. The section 179B demonstration should document a good faith effort to account for
expected international emissions growth and emission reduction measures. If there are significant
uncertainties in international emissions estimates, it may be useful to provide a range of
estimates.
ANogales, Arizona, PMio section 179B demonstration (ADEQ, 2012)44 illustrates the value of
comparing domestic and international emissions for metropolitan areas. Nogales is the name of
two cities (one in Arizona and one in Mexico) that share a border. The Nogales nonattainment
area included only the U.S. side. The majority of the population and PMio emissions, however,
occur on the Mexican side of the border. Table 5 shows an example PMio emissions from the
Nogales nonattainment area, a range of emissions estimates from the cross-border municipality,
and a range of percent contribution to the combined airshed emissions. ADEQ quantified
emissions bounding the years (2008-2011) of exceedances being evaluated. The results in Table
5 are based on aggregated estimates for several broad source categories (not shown), including
point, agricultural, residential wood combustion, waste burning, construction, onroad mobile,
and nonroad sources.
Table 5. Annual Emission Inventories for Nogales, Arizona, and Nogales, Sonora, Mexico.
Adaptedfrom ADEQ (2012) Appendix A Clean Air Act Section 179B Attainment Determination
for the Nogales, Arizona, PMio Nonattainment Area; Tables 6-9.
20C
8
20]
LI
PMio (tons)
Percent
(%)
PMio (tons)
Percent (%)
Nogales NAA, Arizona
1,531
18-36.1
1,528
17.1-35.7
Nogales Municipality, Mexico
[2,713-6,987]
[63.9-82]
[2,757-7,420]
[64.3-82.9]
Total
4,244-8,518
100
4,285-8,948
100
When the emissions inventory shows large emissions in a nearby international metropolitan area
(large both in total and relative to local emissions), this supports a weight of evidence that
international emissions are contributing to exceedances.
6.3.3. Modeling to Quantify International Contribution
Using air pollution modeling techniques—such as chemical transport models or dispersion
models—is the most complete way to estimate the contribution of international emissions to
monitors exceeding the NAAQS. In some cases, sufficient modeling may be readily available
from the relevant SIP or past EPA analyses. The key factors for determining which modeling
technique to use include: proximity of the nonattainment area to the emissions source or source
region, and whether the NAAQS exceedances at the monitor are the result of emissions that react
in the atmosphere (such as O3 or secondarily formed PM2.5) or primary emissions (such as direct
44 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
40
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emissions of PM2.5). Air agencies should consult with their EPA Regional office to determine the
need for and applicability of modeling techniques.
6.3.3.1. Chemical Transport Modeling
Chemical Transport Modeling (CTM) is the preferred approach for quantifying international
contribution for pollutants with a secondary component (such as O3 and PM2.5, which are
formed, at least in part, as a result of photochemical reactions of precursor gases in the
atmosphere). CTMs are necessary for quantifying long-range and secondary pollutant
international contributions, and Baker & Kelly (2014) showed that CTMs can also reliably
estimate single-source impacts relatively near the source. Thus, CTM application is not limited
to regional-scale or national-scale international source contribution estimates. For more
information on modeling single sources of secondarily formed pollutants see EPA (2016b)
section 2.2. EPA has released guidance on performing CTM simulations (EPA, 2018c), which
will be referred to hereafter as the modeling guidance. The applications described in the
modeling guidance focus on SIP demonstration modeling, which is most directly applicable to
the section 179B(a) demonstration. For a section 179B(a) demonstration, the observations and
modeling should be consistent with the recommendations for SIP modeling. For a section
179B(b)-(d) demonstration, the observations and modeling would be from the attainment period
rather than the designation period used in SIP modeling. The attainment period is the design
value period used to determine whether the area attained by the applicable attainment date, while
the designation period is the design value period used to designate the area as nonattainment. For
either type of demonstration, the technical approach is similar.
Simulations to support section 179B demonstrations benefit from day-specific and aggregate
analyses. Model performance evaluation often focuses on aggregate statistics, but may also
include day-specific evaluation and analysis. This is particularly useful for section 179B
demonstrations where the high international days may not align seasonally with other high
concentration days. Ideally, the modeling episode would cover all the relevant observations
(designation or attainment periods), but can use a representative modeling year to evaluate
sensitivity to sources (domestic and international). When using a surrogate year, a demonstration
should include an analysis to examine the impact of year-specific meteorology and transport
patterns. The analysis to establish year-specific similarity will depend on the specific situation,
but may include modeling, back trajectory cluster analyses, or other non-modeling analyses. The
choice of modeling episodes to perform will depend upon the conceptual model and should be
determined in coordination with the EPA Regional office.
Contributions from international sources can be estimated using various techniques described in
the modeling guidance. The modeling guidance describes "brute-force" sensitivity modeling
(i.e., rerunning the model simulation with emissions that have either been reduced or zeroed out
from sources of interest) and source apportionment modeling that can be used to identify
contributions from specific types of sources or source sectors, including international emissions.
It will be useful to provide day-specific results as both absolute model contributions and relative
contributions (Relative Contribution = Observation * Absolute Contribution / Total Prediction).
To summarize the results, the contribution estimates may be applied to the design value from the
designation or attainment period in a relative manner that is consistent with the Relative
41
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Reduction Factor approach (RRF). For an example, see the Imperial County section 179B(b)
demonstration for the 2008 O3 standard (CARB, 2018).45 This combination of contribution
estimates will provide context for variability and for the magnitude of corrections applied via
relative attribution.
Any modeling analysis for evaluating long-range impacts of international emissions will
typically include a global simulation and a regional simulation. The global component is often
used to supply boundary conditions to the regional simulation. To the extent practical, the two
scales should use consistent meteorology, vertical resolution, emissions and representation of
chemical species. The need for consistency is particularly true for the international emissions. It
is also recommended to use consistent gas-phase chemical mechanisms and aerosol
physics/chemistry components when conducting analyses with both global and regional
simulations. Maintaining this consistency is critical when conducting analyses that zero-out
anthropogenic emissions or rely on source apportionment modeling that targets sources present
in both modeling scales.
Before estimating source contributions, the base case simulation46 should be able to reasonably
reproduce historical exceedances and typical differences between internationally influenced days
and other days. For example, if internationally influenced days (identified by meteorology or
trajectory analysis) are observed to have a higher concentration than on other days, then the base
case should similarly predict higher concentrations on those days. This relationship is important
because international contribution is often considered in a fractional sense. As a fraction, the
denominator is total O3. If the total O3 on internationally influenced days is biased low, that
could be due to international or local sources. Predicting the right pattern related to the
international contribution does not guarantee accurate fractional prediction, but accuracy
improves confidence in model results. Thus, the air agency should conduct both a model
performance evaluation and diagnostic evaluation as described in the modeling guidance. These
evaluations will provide confidence that variability in international contributions is reasonably
well represented.
After the base case has been evaluated and shown capable of representing observations on
internationally influenced days, then quantification of the international sources can commence.
The choice of base case should match the application. For prospective demonstrations, the
modeling should use the same base case and future year, consistent with the SIP modeling. For
retrospective demonstrations, the modeling should focus on years used in the attainment
evaluation. Quantifying the impact from international sources, as previously stated, may be done
using a combination of sensitivity and/or source apportionment model runs. Depending on the
scale of the analysis, this work may require coordinated efforts between a global and regional
simulation. When this is the case, it is especially important to understand the consistency of
inventories developed at the different scales. Regardless of the technique used for quantification
of source contributions (e.g., sensitivity or source apportionment modeling), the most important
aspect is appropriate implementation, discussed next.
45 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
46 Base case is used to denote a simulation of a historical year that represents a baseline period (e.g., design value
period) before emission projections and hypothetical controls are applied to project a future year air quality state.
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The appropriate implementation of both sensitivity and source apportionment simulations begins
with identifying the sources to be quantified. The sources to be quantified should be consistent
with those described in the conceptual model. If the conceptual model links proximate
international sources (e.g., for near-border receptors), then sensitivity runs would include
perturbations to just these emissions and source apportionment should isolate the influence of
these emissions as a separate tag. When the emissions cross scales (global to regional), the
emissions should be consistent between the two scales and the sensitivity or source
apportionment modeling must also cross scales. For sensitivity analyses, the boundary conditions
for the regional perturbation simulation would be provided by a consistent perturbation in the
global model similar to what has been done for U.S. Background (Emery, et al., 2012; Dolwick,
et al., 2015; Zhang, et al., 2011). For international anthropogenic assessments that cross scales,
examples are available in HTAP/AQMEII (Im, et al., 2018), the updated modeling for regional
haze (EPA, 2019a), and EPA's most recent Ozone Policy Assessment (EPA, 2020). Most of
these examples use zero-out or perturbation at multiple scales, but the regional haze application
fused zero-out at hemispheric scale with source apportionment at regional scale. For either
method, the appropriate emission perturbations are a critical step. Documentation on the
hemispheric-scale perturbations for the two EPA applications is described in EPA (2019b).
In addition, the sensitivity or source apportionment modeling results should include an estimate
of contribution from the U.S. for comparison. It may also be useful to separate the U.S.
contribution into the nonattainment area's own state contribution and contributions from all other
U.S. states. The collection of modeled results can be used to estimate the contribution of
domestic and international anthropogenic sources.
Model contributions will be imperfect, and an estimate of a range should be considered and
discussed in the context of the demonstration. Particularly for sensitivity modeling, the order of
emission perturbations influences the result (zeroing the international source or the local source
give different answers). Thus, at least two estimates of international source contribution should
be developed and used to help characterize a range. Similarly, model attribution is often done
using a relative approach (i.e., the Relative Reduction Factor or RRF). Photochemical modeling
used in section 179B demonstrations use the RRF approach. Dolwick et al. (2015) showed that a
relative approach for U.S. Background O3 led to convergence in model results that had raw
biases of differing sign. Similar convergence is expected for international contributions. Relative
model application can mask problems when the model performance is poor due
disproportionately to specific sources (e.g., missing stratosphere or oil & gas contributions). As a
result, it is often useful to compare the raw results to the relative results because relative results
drawn from poor performing days may lead to contribution estimates that are unreasonable. The
range of results should demonstrate that international anthropogenic sources were large
contributors relative to U.S. contributions on exceedance days.
Simulation results may be available from analyses conducted for a related regulatory program in
lieu of developing modeling specific for the application. For example, EPA often performs
source apportionment analyses with its modeling platform. EPA's 2011 modeling platform and
the Western Air Quality Study ("WAQS") 2011 platform have both made source apportionment
43
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modeling publicly available (EPA, 2016a).47 These model results include an estimate of future-
year contribution from "in-domain" Mexico and Canada. If the conceptual model identifies near-
source Mexican or Canadian sources as key contributors to nonattainment area exceedances,
results from EPA modeling analyses may provide useful information in support of a prospective
demonstration. For example, EPA's source apportionment results can be used as an upper bound
estimate used to constrain adjustments proposed by trajectory analysis. EPA source
apportionment results could also supplement state developed estimates to help characterize the
credible range. In addition, the inputs for EPA's analysis provide a foundation for additional
analyses that could be developed for a demonstration.
When results show that international contributions are larger on exceedance days and
meaningfully larger than domestic contributions, the weight of evidence will be more
compelling. The appropriateness of using pre-existing modeling for a section 179B
demonstration should be discussed with the appropriate EPA Regional office.
6.3.3.2. Dispersion Modeling
Dispersion modeling is the preferred approach for quantifying contributions of near-monitor
international emission sources of primary pollutants. For situations where the international
contribution is from a single or group of industrial sources in close proximity (less than 50 km)
to the impacted monitor, EPA has established several preferred dispersion models that can be
used (The Guideline on Air Quality Models, i.e., Appendix W48), depending on the application.
In most applications, EPA's preferred near-field model, AERMOD will be used. When applied
in a section 179B demonstration, a dispersion model should be applied with actual emissions
from the international source(s) and modeled for the time period of the monitor design value
calculation. For these applications, EPA's Appendix W offers guidance on many modeling
inputs and procedures. Additionally, the SO2NAAQS Designations Modeling Technical
Assistance Document (EPA, 2016c), provides recommendations on modeling domain, receptor
placement, emissions inputs, meteorological data, and other inputs (EPA, 2018d). While the
TAD is for SO2, many of the recommendations would apply to other pollutants in section 179B
demonstrations.
6.3.4. Receptor Modeling Analysis
This section describes receptor modeling and chemical finger-print approaches that are only
applicable to identifying contributions from sources of particulate matter (or sources of
pollutants that correlate well with particulate matter). Many tools are available for receptor
modeling and discussed on the Support Center for Regulatory Atmospheric Modeling website
(https://www.epa.gov/scram/air-pollutant-receptor-modeling). In an ideal case, there may be a
unique tracer emission from a specific source across the border that can be used to identify
international contribution. In more complex conditions, a Chemical Mass Balance or Positive
47 Newer data can be made available upon request from the Ozone Policy Assessment. EPA: Policy Assessment for
the Review of the Ozone National Ambient Air Quality Standards (EPA-452/R-20-001). U.S. Environmental
Protection Agency, Research Triangle Park, NC, 2020b.
48 Appendix W to 40 CFR Part 51, Guideline on Air Quality Models
44
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Matrix Factorization may be used to identify international influence. With either approach, the
connection of the receptor modeling to specific international sources will be critical.
6.3.4.1. Unique Tracer Analysis
A unique tracer analysis will need to include evidence that the tracer was emitted from the
identified international source and that there are no sources of the tracer on the U.S. side of the
border. Further, the tracer will need to be measurable above the detection limit in measurements
made at or near the exceeding monitor.
The tracer choice will be extremely specific to the specific source and geographic area, but it
should have certain properties. It should have a proportional relationship to total emissions of the
pollutant and precursor potential from the source of interest. It should also be either chemically
inert or have a lifetime much longer than the time spent in transport from the source to the
monitor. If both of these properties are met, then the measured tracer concentration on exceeding
days should be proportional to the international source contribution on exceeding days.
An analysis of tracer and international source contribution should be done on both internationally
influenced exceedance days and days believed to be influenced by local sources (see Section
3.4.4 of this guidance). The contribution should be clearly larger on international transport days
to demonstrate that there are not local sources of the tracer. Then, the contribution on exceeding,
internationally influenced days would be subtracted from measured total to isolate the non-
international contributions. If the non-international contribution is below the level of the
NAAQS and the international (by difference) is large, then this analysis would support a weight
of evidence that international anthropogenic emissions caused the exceedance.
6.3.4.2. Chemical Mass Balance or Positive Matrix Factorization
Chemical mass balance (CMB) (Schauer & Cass, 2000; Schauer, et al., 1996; Watson, Chow, &
Mathai, 1989; Watson, et al., 2015) and positive matrix factorization (PMF) (Aiken, et al., 2009;
Lanz, et al., 2007; Larsen & Baker, 2003; Ulbrich, Canagaratna, Zhang, & Worsnop, 2009) are
two examples of receptor modeling methods which can be used to estimate source contributions
to particulate matter. Both techniques combine ambient observations of speciated particulate
matter with a set of minimization equations to produce an estimate of the source or factor (which
can be associated with a type of source) contribution to PMio or PM2.5.
Using these techniques in a section 179B demonstration necessitates a careful selection of source
profiles (for CMB) or interpretation of factors (for PMF) to properly attribute which sources are
international. For CMB, source profiles specific to or dominated by emission sources located
outside of the U.S. need to be used along with domestic source profiles for proper attribution of
the international contribution. For PMF, additional measurements such as wind direction may be
used as input to strengthen the confidence that one of the factors is international in origin.
Like the unique tracer analysis, CMB or PMF should be performed on both exceedance days and
days believed to be influenced primarily by local emissions. If a source or factor is identified that
is solely or mostly international in origin, the contribution should be clearly larger on the days
45
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identified to be dominated by international transport. The contribution on exceeding,
internationally influenced days would then be subtracted from measured total to isolate the non-
international contributions. If the non-international contribution is below the level of the
NAAQS and the international (by difference) is large, then this analysis would support a weight
of evidence that international anthropogenic emissions caused the exceedance.
PMF was applied for the Calexico-Ethel monitor in a 2014 section 179B demonstration. In their
analysis, they identified industrial sources, secondary nitrate/sulfate, motor vehicle sources,
airborne soil, and refuse burning. Refuse burning, which was attributed exclusively to Mexico,
was identified by a high organic carbon, elemental carbon and chlorine signature. Figure 11
shows the results of that analysis pooled for 2010 to 2012 and for just the days during that period
with high PM2.5 concentrations. The demonstration also showed that the individual days
identified as international transport-influenced had high refuse concentrations.
2010 - 2012 Average Source Contribution in
Calexico-Ethel
Industrial
3.9%
Refuse
hum inn _. 1
UUI IIIIIU —.
15.4% W
Airborne \
s \
24.4% \
Secondary
\ Motor /
16.5%
\ vehicle /
\ 20.2%/
Secondary—
Sulfate
19.5%
2010 - 2012 Average Source Contribution in
Calexico-Ethel when PM2.5 > 35 ug/m3
Industrial
1.0%
Airborne
soil
12.9%
Motor
vehicle
^6.7%
K/
/ Refuse
m \ Secondary
44.1% i
¦ 2.6%
(Secondary
nitrate
32.8%
Figure 11. Average source contributions between 2010 and2012 (CARB, 2014, p. Figure B2)49
Receptor modeling techniques are generally contrasted with dispersion/chemical transport
models that use pollutant emissions rate estimates, meteorological transport, and chemical
transformation mechanisms to estimate the contribution of each source to receptor
concentrations. Receptor and dispersion/chemical transport models can be complementary in a
section 179B demonstration, with each type having strengths that compensate for the weaknesses
of the other.
6.4. Example Conclusion Statement in the Demonstration
A section 179B demonstration should begin with a conceptual model and follow with a
demonstration that establishes a relationship between international anthropogenic emissions and
the monitored exceedance(s) based on the weight of evidence. The demonstration includes
49 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
46
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multiple lines of evidence and analyses such as those identified in Section 6 of this guidance and
should conclude with a statement similar to the language below:
Based on the evidence, including comparisons and analyses, provided in section(s) [reference
the relevant section(s)] of this demonstration with respect to ambient air concentrations
measured at the [name of monitor] on [dates], [Air Agency Name] has established that the
[area name] area [would attain (for section 179B(a) demonstrations) or would have attained
(for section 179B(b)-(d) demonstrations) the [name of NAAQS] NAAQS by the relevant
attainment date but for emissions emanating from outside the U.S.
7. Public Comment Process
In addition to providing a conceptual model and evidence of international anthropogenic
emissions transport to the subject area in a demonstration, EPA encourages air agencies to
conduct and document (in the demonstration) a public comment process for all section 179B
demonstrations prior to submitting the demonstration to EPA. In addition to coordinating with
their respective EPA Regional office throughout the development of any section 179B
demonstrations, EPA also recommends that air agencies notify their respective EPA Regional
office when the state public comment process begins. In the case of a section 179B(a)
"prospective" demonstration, the public comment process would be documented as part of
completeness requirements in the associated SIP. In the case of a section 179B(b)-(d)
"retrospective" demonstration, the air agency would likely need to conduct a demonstration-
specific public comment process to include in its stand-alone submission.
Documentation of a public comment process as part of a section 179B demonstration should
include information about how the public comment process was publicized, such as newspaper
listings, website postings, and/or places (e.g., library, agency office) where a hardcopy was
available. EPA also recommends that air agencies include any comments received and the
agency's responses to those public comments.
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8. References
ADEQ. (2012). 2012 State Implementation Plan. Nogales, AZ: Arizona Department of
Environmental Quality. Retrieved from https://www.regulations.gov/docket?D=EPA-
R09-OAR-2012-045 8
Aiken, A., Salcedo, D., Cubison, M., Huffman, J., DeCarlo, P., Ulbrich, I., . . . Laski. (2009).
Mexico City aerosol analysis during MILAGRO using high resolution aerosol mass
spectrometry at the urban supersite (TO) - Part 1: Fine particle composition and organic
source apportionment. Atmospheric Chemistry and Physics(9), 6633-6653.
Baker, K. R., & Kelly, J. T. (2014, 10). Single source impacts estimated with photochemical
model source sensitivity and apportionment approaches. Atmospheric Environment, 96,
266-274. doi:10.1016/j.atmosenv.2014.07.042
Byrne, A. (2015). The 1979 Convention onLong-Range Transboundary Air Pollution: Assessing
its Effectiveness as a Multilateral Environmental Regime after 35 Years. Translational
Environmental Law, 37-67. doi:10.1017/S2047102514000296
CARB. (2014). Imperial County 2013 State Implementation Plan for the 2006 24-Hour PM2.5
Moderate Nonattainment Area. California Air Resources Board.
CARB. (2018). Imperial County Clean Air Act Section 179B(b) Retrospective Analysis for the 75
ppb 8-Hour Ozone Standard. California Air Resources Board.
Dentener, F., Keating, T. J., & Akimoto, H. (2010). Hemispheric transport of air pollution. Part
A: Ozone and Particulate Matter. Geneva: Economic Commission For Europe, United
Nations.
Dolwick, P., Akhtar, F., Baker, K. R., Possiel, N., Simon, H., & Tonnesen, G. (2015).
Comparison of background ozone estimates over the western United States based on two
separate model methodologies. Atmospheric Environment, 109, 282-296.
doi:10.1016/j.atmosenv.2015.01.005
Emery, C., Jung, J., Downey, N., Johnson, J., Jimenez, M., Yarwood, G., & Morris, R. (2012).
Regional and global modeling estimates of policy relevant background ozone over the
United States. Atmospheric Environment, 47, 206-217.
doi: 10.1016/j.atmosenv. 2011.11.012
EPA. (1987). On-site Meteorological Program Guidance for Regulatory Modeling Applications.
RTP, NC: US EPA.
EPA. (2005). Guideline on Air Quality Models, 40 CFRPart 51, Appendix W. U.S.
Environmental Protection Agency, RTP, NC. Retrieved from
https://www3.epa.gov/ttn/scram/guidance/guide/appw_17.pdf
EPA. (2009, January 15). Approval and Promulgation of Air Quality Implementation Plans;
Texas; Approval of the Section 110(a)(1) Maintenance Plan for the 1997 8-Hour Ozone
Standard for El Paso County. Federal Register, 74, 2387-2392. Retrieved from
https://www.federalregister.gOv/d/E9-708
EPA. (2014). Policy Assessment for the Review of the Ozone National Ambient Air Quality
Standards. RTP, NC: US EPA. Retrieved May 2018, from
https://www.epa.gov/naaqs/ozone-o3-standards-policy-assessments-current-review
EPA. (2015). Implementation of the 2015 Primary Ozone NAAQS: Issues Associated with
Background Ozone, White Paper for Discussion. RTP, NC: U.S. EPA. Retrieved from
https://www.epa.gov/sites/production/files/2016-03/documents/whitepaper-bgo3-
final.pdf
48
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EPA. (2016a). Air Quality Modeling Technical Support Document for the 2015 Ozone NAAQS
Preliminary Interstate Transport Assessment. Office of Air Quality Planning and
Standards. RTP, NC: US EPA. Retrieved from
https://www.epa.gov/sites/production/files/2017-
0 l/documents/aq_modeling_tsd_2015_o3_naaqs_preliminary_interstate_transport_assess
men.pdf
EPA. (2016b). Final Guidance on the Preparation of Exceptional Events Demonstrations for
Wildfire Events that May Influence Ozone Concentrations. RTP, NC: US EPA. Retrieved
6 7, 2018, from https://www.epa.gov/air-quality-analysis/exceptional-events-rule-and-
gui dance
EPA. (2016b). Guidance on the Use of Models for Assessing the Impacts of Emissions from
Single Sources on the Secondarily Formed Pollutants: Ozone and PM2.5. RTP, NC: U.S.
Environmental Protection Agency.
EPA. (2016c). SO 2 NAAQS Designations Modeling. RTP, NC: U.S. Environmental Protection
Agency. Retrieved from https://www.epa.gov/sites/production/files/2016-
06/documents/so2modelingtad. pdf
EPA. (2017). Emissions Inventory Guidance for Implementation of Ozone and Particulate
Matter National Ambient Air Quality Standards (NAAQS) and Regional Haze
Regulations. RTP, NC: U.S. Environmental Protection Agency. Retrieved 6 7, 2018,
from https://www.epa.gov/air-emissions-inventories/air-emissions-inventory-guidance-
implementation-ozone-and-particulate
EPA. (2018a). Guidance on the Preparation of Exceptional Events Demonstrations for
Stratospheric Ozone Intrusions. RTP, NC: Environmental Protection Agency. Retrieved
4 22, 2019, from https://www.epa.gov/sites/production/files/2018-
1 l/documents/exceptional_events_soi_guidance_l l-8-2018.pdf
EPA. (2018b). Information on the Interstate Transport State Implementation Plan Submissions
for the 2015 Ozone Nationaal Ambient Air Quality Standards under Clean Air Act
Section 110(a)(2)(D)(i)(I). RTP, NC: Environmental Protection Agency.
doi:https://www.epa.gov/sites/production/files/2018-
03/documents/transport_memo_03_27_l 8_1 .pdf
EPA. (2018c). Modeling Guidance for Demonstrating Air Quality Goals for Ozone, PM2.5, and
Regional Haze. RTP, NC: Environmental Protection Agency. Retrieved 4 22, 2019, from
https://www3.epa.gov/ttn/scram/guidance/guide/03-PM-RH-Modeling_Guidance-
2018.pdf
EPA. (2018d). User's guide for the AMS/EPA Regulatory Model (AERMOD). U.S.
Environmental Protection Agency, RTP, NC. Retrieved from
https://www3.epa.gov/ttn/scram/models/aermod/aermod_userguide.pdf
EPA. (2019a). Technical Support Document for EPA 's Updated 2028 Regional Haze Modeling.
RTP, NC: United States Environmental Protection Agency. Retrieved from
https://www3.epa.gov/ttn/scram/reports/Updated_2028_Regional_Haze_Modeling-TSD-
2019.pdf
EPA. (2019b). Preparation of Emissions Inventories for the Version 7.1 2016 Hemispheric
Emissions Modeling Platform. Research Triangle Park, NC: U.S. Environmental
Protection Agency. Retrieved from https://www.epa.gov/sites/production/files/2019-
12/documents/2016fe_hemi spheri ctsd. pdf
49
-------
EPA. (2020). Policy Assessment for the Review of the Ozone National Ambient Air Quality
Standards. Research Triangle Park, NC: U.S. Environmental Protection Agency.
Fleming, Z. L., Monks, P. S., & Manning, A. J. (2012). Review: Untangling the influence of air-
mass history in interpreting observed atmospheric composition. Atmospheric
Research^ 104-105), 1-39. doi:10.1016/j.atmosres.2011.09.009
Huang, M., Carmichael, G. R., Pierce, R. B., Jo, D. S., Park, R. J., Flemming, J., . . . Payne, V.
H. (2017). Impact of intercontinental pollution transport on North American ozone air
pollution: an HTAP phase 2 multi-model study. Atmospheric Chemistry and Physics,
77(9), 1680-7316. doi:10.5194/acp-17-5721-2017
Im, U., Christensen, J., Geels, C., Hansen, K., Brandt, J., Solazzo, E., . . . Liu. (2018). Influence
of anthropogenic emissions and boundary conditions on multi-model simulations of
major air pollutants over Europe and North America in the framework of AQMEII3.
Atmospheric Chemistry and Physics, 18, 8929-8952. doi:10.5194/acp-18-8929-2018
Jaffe, D., Cooper, O., Fiore, A., Henderson, B., Tonneson, G., Russell, A., . . . Moore, T. (2018).
Scientific assessment of background ozone over the U.S.: Implications for air quality
management. Elementa Science of the Anthropocene(6), 56. doi:10.1525/elementa.309
Lanz, V., Alfarra, M., Baltensperger, U., Buchmann, B., Hueglin, C., & Pervot, A. (2007).
Source apportionment of submicron organic aerosols at an urban site by factor analytical
modelling of aerosol mass spectra. Atmospheric Chemistry andPhysics(7), 1503-1522.
Larsen, R., & Baker, J. (2003). Source apportionment of polycyclic aromatic hydrocarbons in the
urban atmosphere: A comparison of three models. Environmental Science &
'/ echnologyi37), 1873-1881.
Lin, M., Horowitz, L. W., Payton, R., Fiore, A. M., & Tonnesen, G. (2017). US surface ozone
trends and extremes from 1980 to 2014: quantifying the roles of rising Asian emissions,
domestic controls, wildfires, and climate. Atmospheric Chemistry and Physics, 77(4),
2943-2970. doi: 10.5194/acp-17-2943-2017
Nopmongcol, U., Liu, Z., Stoeckenius, T., & Yarwood, G. (2017, 08 24). Modeling
intercontinental transport of ozone in North America with CAMx for the Air Quality
Model Evaluation International Initiative (AQMEII) Phase 3. Atmospheric Chemistry and
Physics, 1680-7324. doi: 10.5194/acp-17-9931-2017
NRC. (2010). Global sources of local pollution: an assessment of long-range transport of key air
pollutants to andfrom the United States. (U. N. US National Research Council, Ed.)
Washington, D.C., 978-0-309-14401-8: National Academies Press.
Schauer, J., & Cass, G. (2000). Source apportionment of wintertime gas-phase and particle-phase
air pollutants using organic compounds as tracers. Environmental Science & Technology,
30, 1821-1832.
Schauer, J., Rogge, W., Hildemann, L., Maurek, M., Cass, G., & Simoneit, B. (1996). Source
apportionment of airborne particulate matter using organic compounds as tracers.
AtmosphericEnvironment(30), 3837-3855.
TNRCC. (1994). REVISIONS TO THE STATE IMPLEMENTATION PLAN (SIP) FOR THE
CONTROL OF OZONE AIR POLLUTION: SECTION 818 DEMONSTRATION FOR
THE EL PASO NONATTAINMENTAREA. Austin, TX: Texas Natural Resource
Conservation Commission. Retrieved from
https://www.tceq.texas.gov/assets/public/implementation/air/sip/sipdocs/1994-09-
ELP/sept94_818_el_paso.pdf
50
-------
Ulbrich, I., Canagaratna, M., Zhang, Q., & Worsnop, D. J. (2009). Interpretation of organic
components from Poistive Matrix Factorization of aerosol mass spectrometric data.
Atmospheric Chemistry andPhysics(9), 2891-2918.
US Senate. (1993). A Legislative history of the Clean Air Act Ammendments vl-6. Washington
D.C.: U.S. G.P.O. Retrieved from https://catalog.hathitrust.org/Record/006091060
van der A, R. J., Mijling, B., Ding, J., Koukouli, M., Liu, F., Li, Q., . . . Theys, N. (2017).
Cleaning up the air: effectiveness of air quality policy for S02 and NOx emissions in
China. Atmospheric Chemistry and Physics, 77(3), 1775-1789. doi:10.5194/acp-17-1775-
2017
Verstraeten, W. W., Neu, J. L., Williams, J. E., Bowman, K. W., Worden, J. R., & Folkert, B. K.
(2015). Rapid increases in tropospheric ozone production and export from China. Nature
Geoscience, 5(9), 690-695. doi:10.1038/ngeo2493
Watson, J., Chow, J., & Mathai, C. (1989). Receptor models in air resources management: A
summary of the APCA international specialty conference. JAPCA(39), 419-426.
Watson, J., Chow, J., Lowenthal, D., Antony Chen, L., Shaw, S., Edgerton, E., & Blanchard, C.
(2015). PM2.5 source apportionment with organic markers in the southeastern aerosol
research and characterization (SEARCH) study. Journal of the Air & Waste Management
Association(65), 1104-1118.
Zaveri, R. A., Saylor, R. D., Peters, L. K., McNider, R., & Song, A. (1995, May 1). A model
investigation of summertime diurnal ozone behavior in rural mountainous locations.
Atmospheric Environment, 29(9), 1043-1065. doi: 10.1016/1352-2310(94)00319-G
Zhang, L., Jacob, D., Downey, N., Wood, D., Blewitt, D., Carouge, C., . . . Wang, Y. (2011).
Improved estimate of the policy-relevant background ozone in the United States using the
GEOS-Chem global model with 1/2° x 2/3° horizontal resolution over North America.
Atmospheric Environment, 45, 6769-6776. doi: 10.1016/j.atmosenv.2011.07.054
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Appendix A. Statutory Text
§7509a. International border areas
(a) Implementation plans and revisions
Notwithstanding any other provision of law, an implementation plan or plan revision required
under this chapter shall be approved by the Administrator if—
(1) such plan or revision meets all the requirements applicable to it under the chapter other than a
requirement that such plan or revision demonstrate attainment and maintenance of the relevant
national ambient air quality standards by the attainment date specified under the applicable
provision of this chapter, or in a regulation promulgated under such provision, and
(2) the submitting State establishes to the satisfaction of the Administrator that the
implementation plan of such State would be adequate to attain and maintain the relevant national
ambient air quality standards by the attainment date specified under the applicable provision of
this chapter, or in a regulation promulgated under such provision, but for emissions emanating
from outside of the United States.
(b) Attainment of ozone levels
Notwithstanding any other provision of law, any State that establishes to the satisfaction of the
Administrator that, with respect to an ozone nonattainment area in such State, such State would
have attained the national ambient air quality standard for ozone by the applicable attainment
date, but for emissions emanating from outside of the United States, shall not be subject to the
provisions of section 7511(a)(2)50 or (5) of this title or section 751 Id of this title.
(c) Attainment of carbon monoxide levels
Notwithstanding any other provision of law, any State that establishes to the satisfaction of the
Administrator, with respect to a carbon monoxide nonattainment area in such State, that such
State has attained the national ambient air quality standard for carbon monoxide by the
applicable attainment date, but for emissions emanating from outside of the United States, shall
not be subject to the provisions of section 7512(b)(2) or (9) of this title.
(d) Attainment of PM-10 levels
Notwithstanding any other provision of law, any State that establishes to the satisfaction of the
Administrator that, with respect to a PM-10 nonattainment area in such State, such State would
have attained the national ambient air quality standard for carbon monoxide by the applicable
attainment date, but for emissions emanating from outside the United States, shall not be subject
to the provisions of section 7513(b)(2) of this title.
50 The statute contained an erroneous reference to section 7511(a)(2) instead of 7511(b)(2).
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United States Office of Air Quality Planning and Standards Publication No. EPA-457/P-20-00 IF
Environmental Protection Air Quality Policy Division December 2020
Agency Research Triangle Park, NC
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