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DRAFT Guidance on the Preparation of Clean
Air Act Section 179B Demonstrations for
Nonattainment Areas Affected by International
Transport of Emissions
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EPA-457/P-20-001
January 2020
DRAFT Guidance on the Preparation of Clean Air Act Section 179B Demonstrations for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 7
3. EVALUATING THE SECTION 179B RELATIONSI IIP TO OTHERRELATED
PROVISIONS 9
3.1. Extension of Attainment Date 9
3.2. Exceptional Events 10
3.3. Section 110(a)(2)(D) - Interstate Transport 10
4. THE DEMONSTRATION DEVELOPMI AT PROCESS UN 1)1 -!R SI XTTON 179B 11
4.1. Early Engagement with EPA Regional Offices 11
4.2. Prospective Demonstrations under Section 179B(a) 11
4.2.1. Section 179B(a) Process and Key Questions 12
4.2.2. Section 179Ii(a) Demonstration Submission Schedule 13
4.2.3. Flow Chart for Section I 7lMi(a) Prospecti\ e Demonstrations (SIP approval) 13
4.3. Retrospective Demonstrations under Sections l7lMJ(b)-(d) 13
4.3.1. Marginal Nonallainmenl Areas for the O; NAAQS 14
4.3.2. Schedule lor Reliospccli\ e Demonstrations for O3 Nonattainment Areas 15
4.3.3 Schedule lor Retrospect^ e Demonstrations forPMio, PM2.5, and CO
Nonattainment Areas 15
4 3 4 I-'low Chart for Section I 7lMJ(b)-(d) Retrospective Demonstrations (potential
reclassification).... 16
5. COM I PTl AI. MODI I. ()l INTERNATIONAL INFLUENCE 16
6. TECHNICAL ANALYSES IN SUPPORT OF THE SECTION 179B DEMONSTRATION
20
6.1. Determining the Appropriate Analytic Approach for the Section 179B Demonstration20
6.2. Analyses to Consider in Weight-of-Evidence Demonstrations 24
6.3. Analyses to Demonstrate International Contribution 25
6.3.1. Measured Air Quality Data Analysis 27
6.3.2. Comprehensive Emission Analysis 36
6.3.3. Modeling to Quantify International Contribution 37
6.3.4. Receptor Modeling Analysi s 40
6.4. Example Conclusion Statement in the Demonstration 42
7. Public Comment Process 42
8. References 44
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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
Long-Range Transport of Air Pollutants
mb
Millibar
NAAQS
National ambient air quality standard or standards
NASA
National Aeronautics and Space Administration
NNSR
Nonattainment New Source Re\ ieu
NO A A
National Oceanic and Atmospheric Administration
NO
Nitric oxide
NOx
Nitrogen oxides
NPES
Normalized Potential Emissions Sensili\ily
NPSC
Normalized Potential Source Contribution
03
Ozone
PA
Policy Assessment
PBL
Planetary boundary layer
PES
Potential Emissions Scnsili\ily
PM
Particulate matter
ppb
Parts per billion
PSC
Potential Source Contribution
PT
Potential temperature
RACM
Reasonably A\ ailable Control Measures
RACT
Reasonably A\ailable Control Technology
RFP
Reasonable I'urther 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 nuclides2 that are considering the
development of a demonstration, under section 179B of ilie Clean Air Act (CAA), that a
nonattainment area would be able to attain, or would have attained, the relevant National
Ambient Air Quality Standard (NAAQS) but lor emissions emanating from outside the 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 inlbimation 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 in the preparation of demonstrations but does not limit the types of information
and analysis that could be used to de\ clop such demonstrations under the CAA.
An air agency has the authority under section I 7lMi to de\ clop and submit to EPA a
demonstration that its state implementation plan (SIP) would be adequate to attain the NAAQS,
or the area would ha\ e attained the N AAQS. but for emissions emanating from outside the U.S.
EPA has the authority under section I 7lMi to assess such an international transport demonstration
when e\ aluating a SIP submitted in response to a nonattainment designation or reclassification of
an area, or u hen EPA determines u hellier 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 laid out 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 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 could injure or otherwise affect the
rights and obligations of any person or represent the consummation of agency decision making. Only final actions
taken to approve or disapprove 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). Therefore, this guidance is not judicially reviewable. 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. The contents of this document do not have the force and effect of law and are
not meant to bind the public in any way. This document is intended only to provide clarity to the public regarding
existing requirements under the law or agency policies.
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 NA AQS. the ( A A requires I.IW 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 lo develop and sulnnil 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 (I'M 10), and carbon monoxide
(CO)NAAQS are each assigned a classification which identities 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 lor implementation of Reasonably Available Control
Measures (RACM), including Reasonably A\ailahle Control Technology (RACT); a
demonstration that the plan provides for Reasonable Further Progress (RFP) toward attainment; a
demonstration of attainment by the attainment date; 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 nonattainmenl 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 must, by operation of law, 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 include 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 179B 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 the area will attain by its future attainment date (see Section 179B(a)); and (2) after each
applicable attainment date, when EPA determines retrospectively whether an area has attained
the NAAQS by that attainment date (see 179B(b)-(d)).
In the first context, section 179B(a) provides that, "[IS |oi\\ ithstandi rig 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 pro\ ision. 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 rele\ ant 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 modeling future air quality. Thus, an EPA-approved prospective demonstration may
provide relief from demonstrating future attainment and from the resulting imposition of
additional (i.e., beyond R ACM RACT, et al.) controls on domestic emission sources.
In the second context, sections 17l)B(b), (c), and (d) provide that, for O3, CO, and PM,
respectively, "[nlotwilhslanding any other pro\ ision of law, any State that establishes to the
satisfaction of the Administrator that such State would have attained the national ambient air
quality standard ... In the applicable attainment date but for emissions emanating from outside
of the I nited 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 " (I -niphasis added ) I-'or the purpose of this guidance, we refer to such section 179B
demonstrations as section 17lMi(h )-(d) or "retrospective" demonstrations because they involve
analysis of past ai r quality Thus, an EPA-approved retrospective demonstration may provide
relief from reclassillcation 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 relief7 for air
agencies, EPA's approval of a 179B demonstration does not relieve air agencies with
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
6 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 FR 13498, 13569 n.41
(April 16, 1992).
7 That "specified regulatory relief," gained if a state's 179B demonstration is to the satisfaction of the Administrator,
is what is discussed above: in 179B(a), the ability for a state to submit a potentially approvable attainment plan that
does not include a demonstration of attainment and maintenance of the relevant NAAQS, and in 179B(b)-(d), the
ability for an area that has not attained by the attainment date to avoid reclassification.
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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)
context or the section 179B(b)-(d) 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 he adequate to show attainment but for international emissions; or
2) the area would ha\ e attained the NAAOS by the area's attainment date but for
international emissions
EPA expects section I 7lMJ 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 identi lied i n Subpart D of the Act, call for plans to include provisions
that: (i) provide for implementation of reasonably available control measures, (ii) require
reasonable further progress, (iii) include a comprehensive inventory of actual emissions from all
sources, (iv) identify and quantify the allowable emissions from major new or modified
stationary sources, (v) require permits for new of modified major stationary sources, and (vi)
include emission limitations and such other control measures as may be necessary or appropriate
to provide for attainment. 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.
Specifically, states are required to evaluate and adopt controls on domestic anthropogenic
sources as necessary to fulfill their nonattainment planning requirements. To be consistent, 179B
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demonstrations should focus on contributions from non-U.S. anthropogenic sources as opposed
to nonanthropogenic sources on either side of the border.8
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 (extending 200 nautical miles from U.S. shores) where U.S. federal laws govern
emission sources are not considered to be international emissions lor purposes of this guidance.9
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.10 As explained in these instances, and as further detailed later in this document,
domestic ozone air quality can be affected In sources of emissions located across United States
borders in Canada and Mexico, and under certain circumstances, from sources in other
continents. Additionally, in his April 12. 2d IS. 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 holders of the I nited States with Mexico or Canada, but rather consider[]
section 179B demonstrations or petitions submitted hy any State, including but not limited to
those located in the Western United States "" Nevertheless, EPA recognizes that technical
demonstrations for non-border areas may necessitate additional technical rigor and resources, as
explained below.
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.
8 EPA believes anthropogenic emissions should be the focus of section 179B demonstrations for the reasons stated
here. References to nonanthropogenic emissions may be included as part of the weight-of-evidence in a section
179B demonstration.
9 As defined by Part V, Article 57 of the UN Convention on the Law of the Sea, available at
https://www.un.org/depts/los/convention agreements/texts/unclos/unclos e.pdf
10 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
received a section 179B demonstration for any nonattainment area not located on the Mexican border.
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EPA recommends that section 179B demonstrations include a conceptual model that describes
the conditions causing the exceedance(s)11 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, 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 appro\ al of a SIP submission In 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 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 impacted by international emissions
on specific days during the years that contribute to the design value calculation for the area.
Typically, this retrospecti\ e demonstration would be submitted after air quality data collected
pursuant to federal reference or equivalent monitoring methods are available indicating that the
area failed to attain In the attainment date (and as noted in Section 4.3, such indication and
associated submittal, could occur before the attainment date).
Given the e\tensi\ e number of technical factors and meteorological conditions that can affect
international transport of air pollution. EPA hel ie\ es that section 179B demonstrations should be
evaluated based on the weight of evidence of all information and analyses provided by the air
agency. The appropriate le\ el of supporting documentation will vary on a case-by-case basis
depending on the nature and se\ erity 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 I act-specific analyses tailored to the nonattainment area in
question.
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. EPA will weigh the body of available evidence to determine whether it
collectively indicates that the SIP would be adequate to attain the NAAQS, or the area would
have attained the NAAQS, but for emissions emanating from outside of the U.S.
11 For purposes of this guidance, unless otherwise specified, the term "exceedance" also includes non-exceedance-
level concentrations that contribute to a violation due to the way design values are calculated for certain NAAQS
(e.g., 3-year design value for the O3 NAAQS).
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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
Near-border transport to the U.S. from Canada and Mexico is more easily observed and
documented than intercontinental transport. Pollulanls 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 I-'or example, international near-border
emissions from Juarez, Mexico, ha\e been demonstrated to impact El Paso, Texas, located
directly across the border (TNRCC. I-P.V 2<)i)1)) Conceptually, analyses for near-border
areas can be similar to analyses performed for interstate transport programs. In addition, North
American intracontinental transport can also occur o\ er greater distances, and it can potentially
affect interior locations in the I S to a lesser degree than border areas.
EPA has previously estimated the le\ el 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 the Zoo-nautical-mile boundary. For example, air quality modeling for
the 2<)| 5 (); YV\()S Policy Assessment (PA) estimated Canadian and Mexican contributions to
U.S. pollution for the calendar year 2<><>7 (EPA, 2014). In addition, modeling to support the
Cross-State Air Pollution Rule and the Regional Haze Rule quantified projected 2017 O3
contributions and 2<~>28 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, respecti\cl\ (I-PA, 20 Km: 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 quanlilati\ e 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 always included boundary
conditions that implicitly included "background" O3. "Background" is a generic term that has in
the past been used to refer to O3 formed from any non-local or regional source, but recent
publications have refined the term to focus on uncontrollable sources (Jaffe et al., 2018).
Uncontrollable sources as used in discussion of domestic air pollution include global
contribution of natural and international anthropogenic sources. EPA has been aware of global
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natural and international anthropogenic contributions while setting prior O3 NAAQS, as
illustrated by the use of boundary conditions in modeling.12
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.,
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 United Nations (UN) Convention 011 I .ong-Range
Transport of Air Pollutants (LRTAP). The HTAP effort has included two phases that focused on
characterizing transported air pollution for the years 2i")i")f) (Dentener, Keati ng. & Akimoto, 2010)
and 2010. These studies quantify the sensitivity of air pollution levels in the I 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 I.ong-Range Transport of Key Air
Pollutants to and from the I Tnited States." (\ RC. 2<) I <))
Synthesis of the literature, including citations abo\ e 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). I or example, semi-permanent pressure systems can set
up an atmospheric "coin eyor 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 I 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. The further a location is from an
international source, in general, the less O3 will be available to mix down. Local meteorology,
however, creates exceptions to this rule due to planetary boundary layer (PBL), the lowest layer
of the troposphere, dynamics 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
12 Boundary conditions accounted for all sources that were not explicitly accounted for within the modeling domain.
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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;
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 Ul LATIONSIIIP TO OTHER
RELATED PROVISIONS
As discussed in this document, section 179B addresses sources of emissions originating outside
of the U.S. and provides qualifying nonattainmenl areas with specified regulatory relief from
otherwise-applicable additional planning requirements This section discusses limitations to
section 179B applicability and identities related regulatory mechanisms that air agencies may
also find useful for addressing their obligations
Section 179B relief is limited to nonattainmenl area requirements, and only speaks to 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 reclassify as appropriate). It does not
specifically address N\\ authority to do any of the following:
l-Aclude monitoring data influenced by international transport from regulatory
determinations related to an area's designation as attainment or nonattainment (however,
if an exceedance or \ iolation is event-related, it may be able to qualify as an exceptional
e\ent. as described in Section .1 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 nonattai 11 ment area to attainment without meeting the other attainment plan
requirements of CAA section 107(d)(3); or
Address interstate transport SIP obligations under CAA section 110(a)(2)(D)(i), the
"good neighbor" provision.
Where section 179B is not the most appropriate mechanism, air agencies should consider the
regulatory mechanisms discussed below.
3.1. Extension of Attainment Date
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In certain circumstances, a nonattainment area that fails to attain the 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 I -year extension if it has complied
with all requirements in its SIP and if, for the attainment year. I lie area's fourth highest daily
maximum 8-hour average is at or below the level of the standard.u A 1-year extension is also
possible for nonattainment areas for CO, PM2.5, and PM1.. 11 I11 addition, up to 5-year extensions
are also possible under certain conditions for P\l:.~ and I'Miu areas classified as Serious.15
Air agencies considering whether to develop a section 179B demonstration may also be eligible
for an attainment date extension. In such situations, the attainment date extension pathway may
offer different advantages relative to the 179B pathway Air agencies with questions regarding
these mechanisms are encouraged to consult their JilW Regional office.
3.2. Exceptional Events
Section 319(b) of the (\\.\ recognizes that, when making certain NAAQS-related regulatory
determinations, it may he appropriate to exclude ambient monitoring data that are influenced by
exceptional events. The 2<> I (¦> l-xceptional I a cuts Rule pro\ ides the regulatory mechanism for
this purpose.16
When exceptional e\ cuts influence monitoring data and cause exceedances or violations of the
NAAQS. air agencies can de\elop and submit a technical demonstration to request the exclusion
of certain e\ cut-influenced data. and. if (he demonstration satisfies the Exceptional Events Rule
criteria, N\\ can exclude these data from the data set used for certain regulatory decisions.
Specifically, transported pollution 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 \ olcanic 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 are not exceptional events.17
3.3. Section 110(a)(2)(D) - Interstate Transport
13 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).
14 See CAA section 186(a)(4) for CO, and CAA section 188(d) for Moderate PM2 5 and PM10 areas.
15 See CAA section 188(e).
16 81 FR 68216 (October 3, 2016).
17 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.
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CAA sections 110(a)(1) and 110(a)(2)(D)(i)(I) require all states, within 3 years of promulgation
of a new or revised NAAQS, to submit SIPs that contain 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 that NAAQS. Section 179B does not relieve a state of its
obligations to prohibit significant contribution under CAA section 110(a)(2)(D)(i) (aka the "good
neighbor" provision). Thus, this guidance does not address any 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 obligation18 for any NAAQS.
4. THE DEMONSTRATION DEVELOPMKM PROCESS UNDER
SECTION 179B
This section describes the recommended section I 7lMi demonstration de\ elopment and
submission process. It also provides additional details about prospecli\ e and retrospective
demonstrations under section 179B. The Clean Air Act timelines for nonaltainnient 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 N\\ Regional office as early as possible
when considering de\ elopmeni of a demonstration under section I 79B. The air agency and EPA
should discuss the conceptual model lor characterizing the international impacts, identify the
types of analyses that would he most appropriate for the demonstration, and establish
expectations for timing and other considerations
4.2 hosnecti\e Demonstrations under Section l7^U(a).
As discussed in Section I 2 of this document, section 179B(a) states that an implementation plan
or plan re\ ision 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 \ A AOS In 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.
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 plan demonstrating attainment of the NAAQS by the relevant attainment
18 See CAA section 110(a)(2)(D)(i) (aka the "good neighbor" provision) for information on addressing interstate
transport SIP obligations.
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date and is seeking to submit a SIP that shows that the area would attain but for international
emissions, then the air agency should submit a demonstration under the process and schedule
described in this section. The air agency is not required to develop an attainment demonstration
for an O3 nonattainment area classified as Marginal, and therefore EPA does not expect to
receive section 179B(a) prospective demonstrations for such areas.
4.2.1. Section 179B(a) Process and Key Questions
A SIP that includes a prospective demonstration under section 17lMi(a) must meet all applicable
requirements of the CAA other than a demonstration that the area will attain the NAAQS by the
attainment date.19 In general, the applicable requirements for any (); nonattainment area include
an emissions inventory requirement, the conformity requirements, and the requirement for a
nonattainment new source review program. Additional applicable requirements for a
nonattainment area other than a Marginal O3 nonattainment area include RA( \l (including
RACT) measures to show the area is making reasonable further progress toward attainment and
contingency measures. 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 I 7lMJ(a) demonstration, an air agency
should show that e\ en after fullilling other rele\ ant 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 In 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 17(Mi(a) Step I 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 Altainmenl 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 that can be implemented by the attainment date. 20'21
19 See CAA section 179B(a).
20 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/Q3-PM-RH-Modeling Guidance-2018.pdf.
21 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., PM25NAAQS 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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Go to Step 2 only if the Step 1 modeling shows the area could not attain with on-the-
books measures and RACM/RACT that can be implemented by the attainment date.
Step 2. 179B(a) Analyses for International Anthropogenic Emissions: The air agency
may consider developing a demonstration of the impact of international emissions
pursuant to 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.
4.2.2. Section 179B(a) Demonstration Submission Schedule
EPA considers a section 179B(a) demonstration as pai l of an attainment SIP. Therefore, the
demonstration should be submitted to EPA no later than the date u hen 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. The air agency should consult with its EPA Regional
office to determine a mutually agreeable timeline lor the development and submission of a
section 179B(a) demonstration to avoid potentially missing ( WA deadlines
4.2.3. Flow Chart for Section I 7lMi(a) hospeclix e Demonstrations (SIP approval)
Yes
Yes
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 need to submit an
attainment plan, and therefore
a section 179B(a)
demonstration would not be
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 SIP for EPA review.
4.3. Retrospective Demonstrations under Sections 179B(b)-(d)
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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
classification by operation of law."22 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 impacted hv international emissions to an
extent that prevented the area from attaining the standard l\\ the attainment date.
A nonattainment area that may be eligible to submit a section 179B(h) - (d) retrospective
demonstration already would have been required under the CAA to submit a SIP. The CAA
requires an air agency to meet all SIP requirements applicable to the area dining the SIP
development, submission, and implementation process Section 179B does not relieve an air
agency of its planning or control obligations
EPA recommends that the air agency communicate with the F.PA Regional office as soon as
possible after determining that they 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 retrospect^ e demonstration may be a\ ailable to the air agency in advance of
the attainment date. I lo\\e\er. to ensure the integrity of air quality data used in demonstrations,
EPA recommends 1 hi.il the air agency submit a retrospective demonstration only after all air
quality data used to calculate the attainment year design value are certified, but before the date
by which F.PA is required to make determinations of whether areas attained by the attainment
date (i.e.. (•> 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 have more
time to develop a demonstration prior to the date by which EPA must make its determinations of
attainment. Moreover, EPA does not believe a state is precluded from submitting its
demonstration prior to the attainment date if the appropriate attainment year data are certified.
As noted above, if EPA appro\ es a section 179B(b) - (d) retrospective demonstration, EPA will
not be required to issue a determination of attainment by the attainment date pursuant to sections
181(b)(2), 186(b)(2), or I KK(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, O3 nonattainment areas classified as Marginal are
only subject to new source review permitting, conformity, and emission inventory and source
emission statement requirements. As described in the 2015 O3 NAAQS Implementation Rule,
22 See CAA section 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.
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section 182(a) of the CAA does not require states to implement RACM/RACT in Marginal O3
nonattainment areas, and nothing in section 179B alters the statutory requirements with respect
to RACM/RACT obligations in subpart 2.23
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 the area instead submits a section 179B(b) demonstration that is approved, it will likely remain
designated nonattainment and retain its current classification. in general, until such time as the
area attains the standard and the air agency submits, and N\\ approves, a request for
redesignation. The area will continue to be subject to WSR 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)! I -).
4.3.2. Schedule for Retrospective Demon si rations for O3 Nonallainment Areas
Although EPA encourages an air agency to consult with its N\\ 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 most recent year arc certified
Air agencies are required to certify O3 air quality data lor a gi\ en calendar year by May 1 of the
following year.24 A11 air agency may also elect to pursue early certification of the preceding
year's air quality data, u liich could provide the ai r agency with more time to complete a section
179B retrospective demonstration in advance of the area's attainment date. Attainment dates for
Marginal through Extreme (); YV\()S nonattainment areas extend from 3 to 20 years from the
effective date of area designations In I-PA I-'or the nonattainment areas for the 1997, 2008, and
2015 (); Y\AQS, attainment dales fall 011 June 15, July 20, and August 3 (respectively) of
various years, depending 011 an area's classification. EPA is obligated to make determinations of
attainmenl within 6 months of each attainment date based on the area's design value (CAA
section 181(h)(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 retrospectse 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
23 83 FR 63010 (December 6, 2018).
24 40 CFR § 58.15 - Annual air monitoring data certification.
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evaluated. Thus, like the recommendation for O3, EPA recommends that the state submit such a
demonstration for PM10, 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 provide the state with more time to complete a section 179B
retrospective demonstration in advance of the area's attainment date. The attainment date for the
PM10, PM2.5, and CO NAAQS nonattainment areas falls on December 31 of a given year. EPA is
required to make determinations of attainment within 6 months of the December 31 attainment
date (i.e., by June 30th of the year following the attainment date). See CAA section 186(b)(2)(A)
for carbon monoxide; CAA section 188(b)(2) for PM2.5 and PMin 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 IV0111 being subject to I - IWs determination of
whether the area attained by its attainment dale
4.3.4. Flow Chart for Section 179B(h)-(ci) Retrospectix e 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 will
not attain by the attainment date
(and if state 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 typical 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 e\ idence and analyses provided in the
overall demonstration. 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 \ \.\QS 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 \ alLie. ele\ation. 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-
inteinalional emissions It would be helpful to include a table of the relevant monitor data
(e.g., date, hours, monitor \ allies, 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 inlluenced 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
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
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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.
The California Air Resources Board's 2013 SIP for Imperial County for the 2006 24-Hour PM2.5
standard (CARB, 2014)25 contains useful examples lor se\ era I 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 relati\ e to key international anthropogenic sources and
provide context for other geographical considerations
25 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
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Draft for Informal Review and Comment
jNiland
ARIZONA
We«
orland
Bra w lei
Anza Borrego Desert
El Centi
ilexico
~ Cities
Major Roads
PM2.5 nonattainment area
Lakes
Figure 1. Example overview map, Imperial County PM2.5 nonattainment area overview map
(CARB, 2014)26
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 shows 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. Each situation regarding international contributions is unique
and every section 179B demonstration will necessitate information and analyses to be tailored
accordingly.
S
26 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
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Draft for Informal Review and Comment
Figure 2. 2001 and 2012 24-hour Design Values for Brawley,
El Centro and Calexico
60 -i—
50
24-hour PM2.5 Standard
Brawley El Ceritro 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)27
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.
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.
27 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
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• 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 conduci\ e lo local pollutant formation (e.g., for
O3, clear skies and elevated temperatures).
Proximity to borders and coasts have 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 lor 2<~>23 (I-1\\. 2018b). This modeling
estimates future aggregate international contributions of emissions from Canada and Mexico and
is not necessarily representative of 1 lie 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 lype 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 2o monitors with contributions greater
than 5 ppb were located within 30 miles of the Canada or Mexico international border, and all 26
were within 4<) miles of the border. Additionally, the influence of international sources on near-
border I 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.
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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 AOS 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
decay of Mexico/Canada contribution with distance from the border. This modeling is only able
to quantify emissions from within the contiguous U.S. modeling domain (aka 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.
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_ 20 -
ja
CL
CL
VI
ro 15-
¦o
10
c
<0
U
8 io -
£
s
c
«J
E 5 -
o
¦o
t
0 -
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 Cband 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 away 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.
logS= - 0.61logdx + 3.07 (r2=0,49)
—i—
100
200 300 400 500
Miles from Canada/Mexico Border (dx)
600
700
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60 -
• •
• •
50 -
••• •
A
o. 40 -
Q.
u
S 30-
20 -
S = 0.01dz +17.63 (r2 = 0.55)
10 -1
0
500
1000
1500
2000
2500
300 C
Meters above sea level (dz)
Figure 5 Initial and Boundary Conditions contribution (ICBC) to design values in 2023 as a
function of elevation at AOS monitors that are farther than 100 miles fi'om 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 should 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 possible analyses that could support the demonstration of international
anthropogenic influence. Each analysis is described in more detail in Section 6.3 of this
guidance. 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.
Table 1. Potential analyses to demonstrate international contribution
Type of Analysis
Description of analyses
Conceptual Model
Description and basic supporting information regarding the conditions
that lead to high O3 in the area and what conditions are conducive to
internati onal -influence.
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Type of Analysis
Description of analyses
Ambient
Observational
Analysis
• 5 years (or more) of peak concentration data with other internationally
influenced days flagged.
• Table with percentile ranks of days for section 179B.
• Coincidence between high pollution and meteorological/air quality
conditions characteristic of international transport.
• Wind roses and back trajectories indicating potential influence of
international sources on exceedances.
• Summary narrative.
Demonstrations needing additional lines of evidence:
• Historical diurnal profile com pari son on local and non-local
contribution days.
Comprehensive
Emission Analysis
• Develop a domestic emission inventory.
• Obtain or develop an international emission in\ eiitory 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 de\ clop a broader international emission inventory.
Modelinu lo
Quantify
International
Contribution
Optional:
If a\ ailaMe. reference existing modeling that has been evaluated, typically
lor a related analysis
Demonstrations needing additional lines of evidence (choose one in
consultation with the Region):
• Dispersion modeling quantifying international contribution and
culpability, (inert pollutants)
• Photochemical modeling quantifying contribution via sensitivity
modeling or source apportionment, (chemically reactive pollutants)
Chemical
Fingerprint Analysis
(aka "receptor
modeling" or "filter
analysis")
• Where available and applicable.
6.3. Analyses to Demonstrate International Contribution
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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 P\l|..
1. Place several ambient PM-10 monitors ami a meteorological station;
measuring wind speed and direction, in the U.S. nonattainment area near the
international border.28 Evaluate and quannfy any changes in monitored PM-
10 concentrations with a change in the preilominant wind direction.
2. Comprehensively inventory PM-10 emissions i\ iilim the U.S. in the vicinity
of the nonattainment area and demonstrate i/un 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 PI I-10 levels could be based on concentrations
measured in a similar area not influenced by emissions from outside the U.S.
3. Analyze ambient sample filters for specific types oj panicles emanating from
across the bonier. (. \ hhough not required, characteristics of emissions from
sources may be helpful).
4. Inventory the sources on both sides of the border and compare the
magniimle of l'.\ I-10 emissions originating within the U.S. to those emanating
from outside the I .S.
5. I'ei form air dispersion aiul or receptor modeling to quantify the relative
impacts on the nonaikiinmeni area oj sources located within the U.S., and of
foreign sources of l'.\ I in emissions (this approach combines information
collected from the international emission inventory and meteorological
stations, ambient mount/ring network, and analysis offilters).
With the exception of item 3 al">o\ e, these methods with minor modifications are also generally
applicable to other criteria pollutants, including carbon monoxide and O3 which are explicitly
noted in section 179B(b)-(d) I'or 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.29
28 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.
29 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.
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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 hcluccn international anthropogenic emissions
and the monitored exceedance is to prepare an analysis showing how (lie 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^) and historical concentrations across all days
in a variety of ways. Table 2 describes example analyses that could be completed to show
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 Kvidcncc
'I'ypcs of Analyses/Supporting Information
1. NAAOS concentration
data
Plot the N A AOS concentration matrix at the affected monitor(s)
lor the most recent 5-year period30 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
30 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.
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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.
0.10
0.08
E
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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, 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 pro\ ide a more useful field for
composition analysis. Trajectory analyses rely on the use of meteorological models and their
interpretation of observed conditions (aka reanalysis), which pro\ ides 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.
Despite their respective limitations, winds and trajectory analyses can pro\ ide 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 blow s 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 e\ idence assessment
In-situ wind analysis was used in the section I 7lMi 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 line-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 w i nds 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.
Table 4: Nogales, Arizona Hourly ambient PM10 concentrations sorted by concentration and
wind direction, 2007 - 2009 exceedance days. (ADEQ, 2012, p. Appendix A Table ll)31
31 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
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Draft for Informal Review and Comment
Range of Ambient Concentration Values (microgram nf)
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%
2007-2009 Wind Rose for Nogales Co* Office (Excluding ExceedanceOays>
(a) Non-Exceedance Days
2007-20*8 Wind Rose for All Hoqales Post Office ExcoeOances
(b) Exceedance Days
WIND SPEED
~
>= 10.7
?a -10 7
5.4 • 79
33- 5.4
!~
15* 33
n
02 * 15
Calms: 2 44*
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.
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.3"
Trajectories were initialized every 2 hours through each 8-hour maximum O3 daily value (from
start, dt=0,2,4,6,8 hours) from 2015-01-01 to 2016-08-24 using North American Mesoscale
! Trajectories were developed to support this guidance document and have not been used in a submission.
30
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Forecast System archived on the Sigma coordinate (NAMS) data downloaded from the
HYSPLIT archive. Each initialization uses three altitudes: 100, 500, and 1000 meters. The three
altitudes help to characterize air throughout the PBL. Ideally, each altitude release would be
paired with hourly trajectories (8 trajectories/day for O3, 24 trajectories/day for PM2.5) instead of
the five used here. The trajectories used in these examples were also configured to include PBL
height as an output along with coordinates. The altitude of the trajectory can be compared to the
PBL to identify when a trajectory is likely to interact with emissions. If the PBL is not known,
using a low assumed PBL may be appropriate. The residence time of trajectories within the PBL
or below a fixed level can qualitatively be considered a potential emissions sensitivity (PES, /i).
The sensitivity can then be combined with emission inventories 10 develop an expected potential
source contribution (PSC,fxEx). Because PES and PSC do not account for pollutant production
and loss processes, the results should not be interpreted as absolute contributions and may be
most useful in section 179B demonstrations when normalized, (l or example, NPESX = PESX /
YI'ESx; andNPSCX = I'SCx XAYG). The normalized \ alues show the expected relative
contributions.
Figure 8 simply compares trajectories from (a) all exceedance day trajectories; (b) days that are
primarily U.S. influenced; and (c) days that are primarily Mexican influenced. Trajectories are
rarely conclusive on their own because they are individually subject to model configuration
artifacts and represent probable trajectories When a large majority {e.g., 75 percent) of
trajectories pass over an international source region of interest with known emissions (here
Mexicali), this is part of a weight of evidence that an international contribution exists on that
day. When the international contribution is more pre\ alent 011 exceedance days than other days,
that reinforces the weight of e\ idence
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Draft for Informal Review and Comment
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
" 2
1
0
0
-10 -20 -30 -40 -50 -60 -70 -80
Time Since Initialization
8
7
_ 6
1 4
-3
i
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 El Centro 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). Comparing NPES panels (a)
and (b) reveals that exceedance day trajectories spent relatively more time in the Mexican PBL
than non-exceedance days (24 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
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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.
(a) All Day NPES
(b) Exceedance Day NPES
(c) All Day NPSC
(d) Exceedance Day NPSC
Points below PBL - NAT: 26%; US: 55%; MEX: 17%;
Points below PBL - NAT: 26%; US: 50%; MEX: 24%;
Points below PBL - NAT: 7%; US: 54%; MEX: 39%;
%;
Points below PBL - NAT: 5%; US: 43%; MEX: 52
Figure 9: Imperial County, California O3 nonattainment area: El Centra 72-hour back
trajectory Normalized Potential Emission Sensitivity maps (a,b) and Normalized Potential
Source 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
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Draft for Informal Review and Comment
each cell. The USA, MEX, and OCN (ocean) labels identify the percentage o/NPES within each
region. The USA, MEX and NAT labels identify the percentage o/NPSC from regional sources.
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 lo residence lime (Seibert and Frank 2004;
10.5194/acp-4-51-2004). The retroplume does not account lor all processes and is subject to
certain assumptions (Lin et al. 2003; 10.1175/BAMS-l)-14-<><»1 in I) When assumptions are
satisfied, the retroplume can be converted to aNPES and NPSC (similar residence time from
back trajectories).
The example shown in Figure 10 is based on 11YSPLIT backward dispersion simulations.
Similar to the back-trajectory section (above), simulations were configured hased 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 lull S-hour period. Compared to a
series of backward trajectories, the S hours of emissions pro\ ide a more continuous view of
possible sources, the dispersion model accounts for turbulent motion, and explicitly weights
closer sources more hea\ ily I11 the case of secondarily-formed O3 and PM2.5, the weighting
toward closer sources may o\ ereslimate their inlluence I11 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.33
Figure l<> shows the backward dispersion NPliS and NPSC. Compared to backward trajectories
(see Figure '¦>). 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-lxised continuous surface lends itself to creating day-specific
NPES and NPSC in a way that trajectories do not. Figure 10 also shows two days that illustrate
primarily U.S.-influenced (l iuuie l Oe) and primarily Mexico-influenced (Figure lOf)
exceedances. Despite the <.|iianlilative 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.
33 https://ready.arl.noaa.gov/documents/Tutorial/html/src_recp.html.
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Draft for Informal Review and Comment
Points below 1000m - NAT: 24%; US: 54%: MEX: 20%;
Points below 1000m - NAT: 24%; US: 46%; MEX: 30%;
V r
t
L
n
10-4
St
kvl
10"5 &j
(a) Alt days NPES
Points below 1000m - NAT: 7%; US: 51%; MEX: 42%;
5 a.
(b) Exceedance days NPES
Points below 1000m - NAT: 5%; US: 39%; MEX: 56%;
10"'
10~!
10-'
10_;
10"'
I 10~!
! 10":
(c) All days NPSC\
Points below 1000m - NAT: 5%; US: 75%; MEX: 20%;
10"4
1/1
ksl
10-5 K
(d) Exceedance days NPSC
Points below 1000m - NAT: 6%; US: 9%; MEX: 85%;
10-" 2
ICJ
IJ
10-5 £
(e) Primarily U.S. Influenced
Exceedance
(f) Primarily Mexico Influenced
Exceedance
Figure 10: El Centro 72-hour back dispersion-based NPES (a,b) and NPSC (c-f) for all days
(a,c), exceedance days (b,d), a primarily U.S. influenced exceedance (e) and a primarily Mexico
influenced exceedance (f).
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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 (u here available)?
• Are there international agreements that are already addressing these emissions?
Domestic and international emissions inventories should be de\ eloped in a manner consistent
with EPA's emission inventory guidance (EPA, 201 7) 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 de\ eloped as a part of the section
179B demonstration or le\ eraued 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 pro\ enance 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., NXiAR. l-l)(i.\R-l ITAP)
A prospecti\ e 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.
A Nogales, Arizona, PMio section 179B demonstration (ADEQ, 2012)34 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
34 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
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Draft for Informal Review and Comment
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, PMioNonattainment Area; Tables 6-9.
20C
8
20]
LI
PM10
Percent
PM10
Percent
Nogales NAA, Arizona
1,531
IS-30 1°,.
1,528
17.1-35.7%
Nogales Municipality, Mexico
[2,713-6,9871
| (->3
\2,757-7,4201
r64.3-82.91%
Total
4,244-8,518
1 < )<)"..
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), lliis supports a weight of evidence that
international emissions are contributing to exceedances.
6.3.3. Modeling to Quantify International ('onirihniion
Using air pollution modeling techniques such as chemical tmnsport models or dispersion
models - can be a good way to eslimate the contribution of international emissions to monitors
exceeding the NAAQS. In some cases, sufficient modeling may he readily available from the
relevant SIP or past FPA analyses The key factors lor 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 (); or secondarily formed I'M:.-) or primary emissions (such as direct
emissions of PM2.5). Air agencies should consult with their EPA Regional office to determine the
need for and applicability of modeling techniques
(•> 3 3 I Chemical Transport Modeling
Chemical Transport Modeling (( I'M) 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. 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 179B(a) demonstration. For a
179B(b)-(d) demonstration, the observations and modeling would be from the attainment period
rather than the designation period. 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. Both benefit from day-specific evaluation and
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analysis but can use a representative modeling year to evaluate sensitivity to controls (either
domestically or internationally). When using a surrogate year, a demonstration should examine
the impact of year-specific meteorology and transport patterns. Contributions from international
sources can use various techniques described in the modeling guidance. The modeling guidance
document 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.
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 glolxil 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 simulation ° should be able to reasonably
reproduce historical exceedances and gradients between internationally influenced days and
other days. For example, if internationally inlluenced 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 (); 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
impro\ es confidence in model results Thus, the air agency should conduct both a model
performance e\ ill nation and diagnostic e\ aluation 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 e\ aluated 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
35 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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of source contributions (e.g., sensitivity or source apportionment modeling), the most important
aspect is appropriate implementation, discussed next.
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 sensili\ ily analyses, the boundary conditions
for the regional perturbation simulation would be pro\ ided In a consistent perturbation in the
global model.
In addition, the sensitivity or source apportionment modeling results should include an estimate
of contribution from the U.S. for comparison ll 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 lor sensitivity modeling, the order of
emission perturbations influences the result (zeroing the international source or the local source
give different answers) Thus, two estimates of international source contribution should be
developed and used to help characterize a range 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 a\ ailable from analyses conducted for a related regulatory program in
lieu of de\ eloping modeling specilic lor the application. For example, EPA often performs
source apportionment analyses with its modeling platform. EPA 2011 modeling platform and
Western Air Quality Study ("W AOS") 2011 platform have both made source apportionment
modeling publicly a\ ailable (I- l\V 2016a).36 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
36 Henderson, B. H. et. al. 2019 "Global Sources of North American Ozone" 18th Annual CMAS, Chapel Hill, North
Carolina.
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compelling. The appropriateness of using preexisting 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 W ' ), 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 SO2 NAAOS Designations Modeling Technical
Assistance Document (TAD), provides recommendations on modeling domain, receptor
placement, emissions inputs, meteorological data, and oilier inputs (EPA, 2d I Sd). While the
TAD is for SO2, many of the recommendations would apply 10 other pollutants in section 179B
demonstrations.
6.3.4. Receptor Modeling Analysis
This section describes receptor modeling and chemical linuei-piint 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 011 the Support Center lor Regulatory Atmospheric Modeling website
(https:/ www cna eov/scram aii-nolluiani-recentoi-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 I11 more complex conditions, a Chemical Mass Balance or Positive
Matrix I'actori/ation 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 I 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.
37 Appendix W to 40 CFR Part 51, Guideline on Air Quality Models
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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 NAAQS, then this
analysis would support a weight of evidence that international anthropogenic emissions caused
the exceedance.
6.3.4.2. Chemical Mass Balance or Posili\ c Matrix factorization
Chemical mass balance (CMB) (Schauer & Cass, 2
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Draft for Informal Review and Comment
2010 - 2012 Average Source Contribution in
Calexico-Ethel
2010 - 2012 Average Source Contribution in
Calexico-Ethel when PM2.5 > 35 ug/m3
Industrial
3.9%
Airborne
soil
Secondary
nitrate
16.5%
Refuse
burnim
15.4°/l
Secc j
Sulfate
19.5%
Airborne
Motor
vehicle
20.2%,
\
Industrial
1.0%
Motor
vehicle
_6.7%
Refuse
burning
44.1%
iSecondary
Secondary
Sulfate
2.6%
nitrate
32.8%
Figure 11: Average source contributions between 2010 and2012 (CARB, 2014, p. Figure B2)-8
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
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 [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 179B(a) demonstrations) or would have attained for
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
38 The citation of specific sections of a plan or demonstration as useful examples is not intended as an endorsement
of the entire document.
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Draft for Informal Review and Comment
demonstrations prior to submitting the demonstration to EPA. 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 tiny comments received and the
agency's responses to those public comments.
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United States Office of Air Quality Planning and Standards Publication No. EPA-457/P-20-001
Environmental Protection Air Quality Policy Division January 2020
Agency Research Triangle Park, NC
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