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Integrated Review Plan for the Primary National
Ambient Air Quality Standards for Oxides of
Nitrogen.
Volume 1: Background Document
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EP A-452/R-24-01 Oa
March 2024
Integrated Review Plan for the Primary
National Ambient Air Quality Standards for
Oxides of Nitrogen.
Volume 1: Background Document
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Health and Environmental Impacts Division
and
Center for Public Health and Environmental Assessment
Office of Research and Development
Research Triangle Park, NC
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DISCLAIMER
This document serves as a public information document and as a management tool for the
U.S. Environmental Protection Agency's (EPA's) Center for Public Health and Environmental
Assessment and the Office of Air Quality Planning and Standards in conducting the review of
the health-based air quality criteria and the primary national ambient air quality standards for
oxides of nitrogen. It does not represent and should not be construed to represent an Agency
determination or policy. Mention of trade names or commercial products does not constitute
endorsement or recommendation for use.
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TABLE OF CONTENTS
1 Legislative Requirements 1-1
2 NAAQS Review Process and Documents 2-1
2.1 Integrated Science Assessment 2-4
2.2 Policy Assessment 2-5
2.3 Regulatory Decision Making 2-7
3 Background on the Primary NAAQS for Oxides of Nitrogen 3-1
3.1 History of the Health-Based Air Quality Criteria and Standards for Oxides of Nitrogen...
3-1
3.2 The Primary Standards 3-4
4 The Current Primary NO2 NAAQS Review 4-1
5 References 5-1
Appendix A Ambient Air Monitoring and Data Handling
A. 1 Ambient Air Monitoring Requirements and Network Requirements A-l
A.2 Data Handling Conventions and Computations for Determining Whether the Standards
are Met A-4
A.3 NO2 Concentrations Measured at Ambient Air Monitoring Sites Across the U.S A-5
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PREFACE
The planning phase of the U.S. Environmental Protection Agency's (EPA's) reviews of
the air quality criteria and national ambient air quality standards (NAAQS) includes the
development of an integrated review plan (IRP) which is made available for public comment and
provided to the Clean Air Scientific Advisory Committee (CASAC) for consultation. As a result
of recent efforts to improve the efficiency and timeliness of planning materials and receipt of
input from the CASAC and the public, the IRP for the current review of the primary NAAQS for
oxides of nitrogen is comprised of three volumes. Volume 1 (this document) provides
background information on the health-based air quality criteria and primary standards for NOx
and may serve as a reference for the public and the CASAC in their consideration of the
subsequent two volumes. Volume 2 addresses the general approach for the review of the primary
NAAQS for oxides of nitrogen and planning for the integrated science assessment (ISA), and
will be the subject of a consultation with the CASAC. Volume 2 identifies policy-relevant issues
in the review and describes key considerations in the EPA's development of the ISA. Volume 3
is the planning document for quantitative analyses to be considered in the policy assessment
(PA), including exposure and risk analyses as warranted. It will describe key considerations in
the EPA's planning with regard to any quantitative exposure/risk analyses to inform the review.
Given that the availability of new scientific evidence in the review can inform the plans for any
quantitative exposure/risk analyses, the development and public availability of Volume 3 will
generally coincide with the availability of the draft ISA, and it will be the subject of a
consultation with the CASAC at that time.
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1 LEGISLATIVE REQUIREMENTS
Two sections of the Clean Air Act (CAA) govern the establishment and revision of the
national ambient air quality standards (NAAQS). Section 108 (42 U.S.C. 7408) directs the
Administrator to identify and list certain air pollutants and then issue air quality criteria for those
pollutants. The Administrator is to list those pollutants "emissions of which, in his judgment,
cause or contribute to air pollution which may reasonably be anticipated to endanger public
health or welfare"; "the presence of which in the ambient air results from numerous or diverse
mobile or stationary sources"; and for which he "plans to issue air quality criteria...." (42 U.S.C.
7408(a)(1)). Air quality criteria are intended to "accurately reflect the latest scientific knowledge
useful in indicating the kind and extent of all identifiable effects on public health or welfare
which may be expected from the presence of [a] pollutant in the ambient air...." (42 U.S.C.
7408(a)(2)).
Section 109 (42 U.S.C. 7409) directs the Administrator to propose and promulgate
"primary" and "secondary" NAAQS1 for pollutants for which air quality criteria are issued (42
U.S.C. 7409(a)). Section 109(b)(1) defines primary standards as ones "the attainment and
maintenance of which in the judgment of the Administrator, based on such criteria and allowing
an adequate margin of safety, are requisite to protect the public health."2
In setting primary standards that are "requisite" to protect public health, as provided in
section 109(b), the EPA's task is to establish standards that are neither more nor less stringent
than necessary. In so doing, the EPA may not consider the costs of implementing the standards.
See, Whitman v. American Trucking Ass 'ns, 531 U.S. 457, 465-472, 475-76 (2001). Likewise,
"[attainability and technological feasibility are not relevant considerations in the promulgation
of national ambient air quality standards." Sqq American Petroleum Institute v. Costle, 665 F.2d
1176, 1185 (D.C. Cir. 1981); accord Murray Energy Corp. v. EPA, 936 F.3d 597, 623-24 (D.C.
Cir. 2019). At the same time, courts have clarified that the EPA may consider "relative proximity
to peak background ... concentrations" as a factor in deciding how to revise the NAAQS in the
context of considering standard levels within the range of reasonable values supported by the air
1 This document focuses on health effects associated with gaseous oxides of nitrogen and the protection afforded by
the primary NO2 standards. The EPA is separately reviewing the ecological welfare effects associated with and
the secondary standards for oxides of nitrogen, oxides of sulfur, and PM. Additional information on the currently
ongoing and prior reviews of the secondary NAAQS for oxides of nitrogen, oxides of sulfur, and PM is available
at: https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards.
2 The legislative history of section 109 indicates that a primary standard is to be set at "the maximum permissible
ambient air level ... which will protect the health of any [sensitive] group of the population," and that for this
purpose "reference should be made to a representative sample of persons comprising the sensitive group rather
than to a single person in such a group." S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970).
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quality criteria and judgments of the Administrator. See American Trucking Ass'ns, v. EPA, 283
F.3d 355, 379 (D.C. Cir. 2002), hereafter referred to as "ATA III."
The requirement that primary standards provide an adequate margin of safety was
intended to address uncertainties associated with inconclusive scientific and technical
information available at the time of standard setting. It was also intended to provide a reasonable
degree of protection against hazards that research still needs to identify. See Lead Industries
Ass 'n v. EPA, 647 F.2d 1130, 1154 (D.C. Cir. 1980); American Petroleum Institute v. Costle, 665
F.2d at 1186; Coalition of Battery Recyclers Ass'n v. EPA, 604 F.3d 613, 617-18 (D.C. Cir.
2010); Mississippi v. EPA, 744 F.3d 1334, 1353 (D.C. Cir. 2013). Both uncertainties are
components of the risk associated with pollution at levels below those at which human health
effects can be said to occur with reasonable scientific certainty. Thus, in selecting primary
standards that include an adequate margin of safety, the Administrator is seeking not only to
prevent pollution levels that have been demonstrated to be harmful but also to prevent lower
pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely
identified as to nature or degree. The CAA does not require the Administrator to establish a
primary NAAQS at a zero-risk level or at background concentration levels (see Lead Industries
Ass'n v. EPA, 647 F.2d at 1156 n.51, Mississippi v. EPA, 744 F.3dat 1351), but rather at a level
that reduces risk sufficiently to protect public health with an adequate margin of safety.
In addressing the requirement for an adequate margin of safety, the EPA considers such
factors as the nature and severity of the health effects involved, the size of the sensitive
population(s),3 and the kind and degree of uncertainties. Selecting any particular approach to
providing an adequate margin of safety is a policy choice left specifically to the Administrator's
judgment. See Lead Industries Ass 'n v. EPA, 647 F.2d at 1161-62; Mississippi v. EPA, 744 F.3d
at 1353.
Section 109(d)(1) of the Act requires periodic review and, if appropriate, revision of
existing air quality criteria to reflect advances in scientific knowledge concerning the effects of
the pollutant on public health and welfare. Under the same provision, the EPA is also to
periodically review and, if appropriate, revise the NAAQS based on the revised air quality
criteria.4
3 As used here and similarly throughout this document, the term population (or group) refers to persons having a
quality or characteristic in common, such as a specific pre-existing illness or a specific age or life stage.
Identification of such sensitive groups (called at-risk groups or at-risk populations) involves consideration of
susceptibility and vulnerability.
4 This section of the Act requires the Administrator to complete these reviews and make any revisions that may be
appropriate "at five-year intervals" and also provides that the Administrator "may review and revise criteria or
promulgate new standards earlier or more frequently than required under this paragraph."
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Section 109(d)(2) addresses the appointment and advisory functions of an independent
scientific review committee. Section 109(d)(2)(A) requires the Administrator to appoint this
committee, which is to be composed of "seven members including at least one member of the
National Academy of Sciences, one physician, and one person representing State air pollution
control agencies." Section 109(d)(2)(B) provides that the independent scientific review
committee "shall complete a review of the criteria... and the national primary and secondary
ambient air quality standards... and shall recommend to the Administrator any new... standards
and revisions of existing criteria and standards as may be appropriate ..." Since the early 1980s,
this independent review function has been performed by the Clean Air Scientific Advisory
Committee (CAS AC) of the EPA's Science Advisory Board. Several other advisory functions
are also identified for the committee by section 109(d)(2)(C), which reads:
Such committee shall also (i) advise the Administrator of areas in which
additional knowledge is required to appraise the adequacy and basis of existing,
new, or revised national ambient air quality standards, (ii) describe the research
efforts necessary to provide the required information, (iii) advise the
Administrator on the relative contribution to air pollution concentrations of
natural as well as anthropogenic activity, and (iv) advise the Administrator of any
adverse public health, welfare, social, economic, or energy effects which may
result from various strategies for attainment and maintenance of such national
ambient air quality standards.
As previously noted, the Supreme Court has held that section 109(b) "unambiguously
bars cost considerations from the NAAQS-setting process" in Whitman v. American Trucking
Ass'ns, 531 U.S. 457, 471 (2001). Accordingly, while some of the issues listed in section
109(d)(2)(C), such as those on which Congress has directed the CAS AC to advise the
Administrator, are relevant to the standard-setting process, others are not. Issues that are not
relevant to standard setting may be relevant to implementing the NAAQS once they are
established.5
5 Because some of these issues are not relevant to standard setting, some aspects of CASAC advice may not be
relevant to EPA's process of setting primary and secondary standards that are requisite to protect public health
and welfare. Indeed, were the EPA to consider costs of implementation when reviewing and revising the
standards "it would be grounds for vacating the NAAQS." Whitman v. American Trucking Ass 'ns, 531 U.S. 457,
471 n.4 (2001). At the same time, the CAA directs CASAC to provide advice on "any adverse public health,
welfare, social, economic, or energy effects which may result from various strategies for attainment and
maintenance" of the NAAQS to the Administrator under section 109(d)(2)(C)(iv). In Whitman, the Court clarified
that most of that advice would be relevant to implementation but not standard setting, as it "enable [s] the
Administrator to assist the States in carrying out their statutory role as primary implementers of the NAAQS" (id.
at 470 [emphasis in original]). However, the Court also noted that CASAC's "advice concerning certain aspects
of 'adverse public health... effects' from various attainment strategies is unquestionably pertinent" to the NAAQS
rulemaking record and relevant to the standard setting process (id. at 470 n.2).
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2 NAAQS REVIEW PROCESS AND DOCUMENTS
This section provides general information about how the NAAQS review process
typically proceeds and documents that are typically prepared over the course of the review. Each
review of ambient air quality criteria and standards begins with a Call for Information for the
Agency to consider in the review. This Call for Information, published in the Federal Register,
generally is focused on scientific information pertinent to the criteria review but may also solicit
comments from the public on policy-relevant issues important to address in the criteria and/or
standards reviews. The Call for Information kicks off the first of the three types of phases in
NAAQS reviews, the planning phase. The other two types of phases are assessment and
regulatory decision making (Figure 2-1). The documents prepared in these three phases,
summarized below, are available to the public on an Agency website maintained for this purpose
(,https://www. epa.gov/naaqs).
The Agency's plans for the review are presented to the public in an Integrated Review
Plan (IRP).6 The IRP is prepared jointly by the EPA's Center for Public Health and
Environmental Assessment (CPHEA) within the Office of Research and Development (ORD)
and the EPA's Office of Air Quality Planning and Standards (OAQPS) within the Office of Air
and Radiation (OAR). In general, the IRP contains background material, including information
that is generic across reviews (e.g., presentation of legislative requirements) and specific to the
pollutant for the review (e.g., history of existing criteria and standards, monitoring methods and
network, and review timeline), as well as key scientific, technical or policy aspects of plans for
the new review. The IRP also presents the current plan and specifies the intended schedule and
process for conducting the review and the key policy-relevant science issues that will guide the
review. The IRP is made available to the public, and the critical aspects of plans for the new
review are the subject of consultation with the CAS AC.
As a result of recent efforts to improve the efficiency of the planning phase and to
facilitate the receipt of timely input from the CASAC and the public on key aspects of the
review, the IRP for the current review of the primary NAAQS for oxides of nitrogen is
comprised of three volumes. Volume 1 (this document) provides background information and
may serve as a reference for the public and the CASAC in their consideration of the subsequent
two volumes. Volume 1 includes introductory or background information on the legislative
requirements for reviews of the NAAQS, an overview of the review process, background
information on prior reviews of the health-based air quality criteria and primary standards for
6 Development of the IRP for some NAAQS reviews may be informed by a science policy workshop to help the
Agency identify issues and questions to frame the review.
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oxides of nitrogen and a summary of key aspects of the basis for the existing primary NO2
NAAQS, and a summary of the status and anticipated milestones for the current review. Volume
1 also includes an appendix that aspects of the ambient air monitoring program for oxides of
nitrogen, which includes NO2, the indicator for the primary NO2NAAQS. Volume 2 addresses
the general approach for the review, identifying policy-relevant issues in the review, and also
addresses planning for the integrated science assessment (ISA), including key considerations in
its development. Volume 2 will be the subject of a consultation with the CAS AC. Volume 3 is
the planning document for quantitative analyses to be considered in the policy assessment (PA),
including exposure and risk analyses as warranted. It will describe key considerations in the
EPA's planning with regard to any quantitative exposure/risk analyses to inform the review.
Given that the availability of new scientific evidence in the review can inform the plans for any
quantitative exposure/risk analyses, the development and public availability of Volume 3 will
generally coincide with the availability of the draft ISA, and it will be the subject of a
consultation with the CASAC at that time.
In the assessment phase, the EPA prepares an Integrated Science Assessment (ISA)7 and
any supplementary materials; quantitative air quality, exposure, and risk analyses, as warranted;
and a Policy Assessment (PA). The ISA, prepared by the CPHEA, provides a concise review,
synthesis, and evaluation of the most policy-relevant science, including key science judgments
that are important to the design and scope of air quality, exposure, and risk analyses, as well as
other aspects of the NAAQS review. The ISA and its supplementary materials provide a
comprehensive assessment of the current scientific literature about known and anticipated effects
on public health and welfare associated with the presence of the criteria pollutant in the ambient
air, emphasizing information that has become available since the last air quality criteria review to
reflect the current state of knowledge. In this way, the ISA forms the scientific foundation for
each NAAQS review. Section 2.1 summarizes key aspects of the ISA.
Based on the updated scientific information available in the review and considered in the
ISA, along with ISA conclusions, OAQPS staff considers the support provided for the
development of quantitative assessments of air quality, exposures, and/or risks of health and/or
welfare effects. As warranted in a given review, the EPA develops relevant quantitative analyses,
the details of which, in recent reviews, are presented in appendices to the PA. These appendices
concisely present methods, key results, observations, and related uncertainties.
The PA, like the OAQPS Staff Paper in earlier reviews, is a document that provides a
transparent OAQPS staff analysis and conclusions regarding the adequacy of the current
7 The ISA and its associated materials function in the NAAQS review process today, as the Air Quality Criteria
Document (AQCD) did in reviews of the past.
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standards and potential alternatives that are appropriate to consider before the issuance of
proposed and final decisions. This evaluation of policy implications is intended to help "bridge
the gap" between (1) the Agency's scientific and technical assessments (as presented in the ISA
and the quantitative exposure and risk analyses) and (2) the judgments required of the EPA
Administrator in determining whether it is appropriate to retain or revise the NAAQS. In this
way, the PA integrates and interprets the information from the ISA and quantitative exposure and
risk analyses to frame policy options for consideration by the Administrator. Development of the
PA is also intended to facilitate CASAC's advice to the Agency and recommendations to the
Administrator on the adequacy of the existing standards or revisions that may be appropriate to
consider, as provided for in the CAA. Section 2.2 summarizes key aspects of the PA.
In the last phase of the review process, which generally follows the issuance of the final
PA and consideration of conclusions presented therein, the Agency develops and publishes a
notice of proposed decision to communicate the Administrator's proposed decisions regarding
the standards review. To the extent the proposed decision is to revise the existing NAAQS or
establish a new NAAQS, the notice presents the proposed regulatory changes. Before publishing
a notice of proposed decision, it generally undergoes interagency review involving other federal
agencies coordinated by the Office of Management and Budget (OMB) per Executive Orders
12866 and 14094.8 Materials upon which the proposed decision is based, including the
documents described above, are available to the public in the docket for the review. A public
comment period, during which one or more public hearings are generally held, follows the
publication of the proposed decision. Considering comments received on the proposed decision,
the Agency develops a notice of final decision, including any regulatory revision, which
generally undergoes interagency review before publication to complete the regulatory decision-
making process. Section 2.3 summarizes the regulatory decision-making steps.
8 Where implementation of the proposed decision would necessitate implementing emissions controls to reduce
emissions to meet a revised standard, that may result in an estimated annual effect on the economy of $200
million or more, the EPA develops and releases a draft regulatory impact analysis (RIA) concurrent with the
notice of proposed rulemaking. The RIA is prepared in accordance with Executive Orders 12866 and 14094 and
is independent of and, by statute, is not considered in decisions regarding the review of the NAAQS.
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Planninq: based on new scientific information and policv-relevant issues
Call for Information
Workshop (if warranted)
Planning Documents
Clean Air
Scientific
~
Advisory
Committee
(CASAC)
review
Assessment: current scientific information assessment, includinq policy
implications particularly with regard to standards (indicator, averaging
time, form, level)
Scientific Assessment
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o"
o
o
3
3
Risk/Exposure Assessments (if warranted)
3
(/>
Policy Assessment
~
Rulemakinq: Aqencv decision making, interaqencv review and public
comments process
Proposed Decision
Final Decision
Figure 2-1. Overview of the NAAQS review process.
2.1 INTEGRATED SCIENCE ASSESSMENT
The purpose of the ISA is to draw upon the existing body of evidence to synthesize and
provide a critical evaluation of the current state of scientific knowledge on the most relevant
issues pertinent to the review of the NAAQS, identify changes in the scientific evidence bases
since the previous review, and describe remaining or newly identified uncertainties. The ISA
identifies, critically evaluates, and synthesizes the most policy-relevant current scientific
literature (e.g., epidemiology, controlled human exposure, animal toxicology, atmospheric
science, exposure science, environmental science, and ecology). In doing so, it presents a concise
policy-relevant evaluation of the current scientific information along with the EPA's conclusions
on the health and welfare effects of the criteria pollutant and associated key science findings that
are important to inform the development of risk and exposure analyses (as warranted) and the
PA, as well as other aspects of the NAAQS review process.
The ISA provides a focused assessment of the scientific evidence to address specific
scientific questions and inform the consideration of overall policy-relevant questions for the PA.
Through periodic reviews of the available scientific evidence, ISAs build on the data and
conclusions of previous assessments. The ISA for a NAAQS review identifies and evaluates
studies published since the cutoff date for studies in the prior ISA, synthesizing and integrating
the new evidence in the context of the conclusions from the previous review. Important older
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studies may be discussed to reinforce key concepts and conclusions. Older studies may also be
the primary focus in some subject areas or scientific disciplines where research efforts have
subsided and/or where these older studies remain the definitive works available in the literature.
More detail on the general ISA development process, as well as additional information on the
scientific and public review aspects addressed in the ISA for oxides of nitrogen for the current
review, is presented in Volume 2 of the IRP.
2.2 POLICY ASSESSMENT
The PA is a document that evaluates the currently available information regarding the
adequacy of the current standards and potential alternatives if any are appropriate to consider in
the current review. In so doing, the PA integrates and interprets the current scientific evidence
from the ISA and available information from quantitative exposure/risk analysis, together with
related limitations and uncertainties, to frame policy options for consideration by the
Administrator. This evaluation of policy implications is intended to "bridge the gap" between the
Agency's scientific assessments and the judgments required of the EPA Administrator in
determining whether it is appropriate to retain or revise the NAAQS.
Quantitative risk and exposure assessments (REAs), a term used in several past NAAQS
reviews, have generally referred to assessments presented in a stand-alone REA document. More
recently, we have also used this term or the phrase "REA analyses" to refer to the air quality,
exposure, and/or risk analyses that we intend to present in appendices or as supplemental
materials to the PA. These quantitative REAs are generally designed to assess human exposure
and health risks, as well as ecological exposures and risks to public welfare, for air quality
conditions associated with the existing standards and, as appropriate, for conditions associated
with potential alternative standards. The objective for such assessments is to provide quantitative
estimates of impacts that can inform the Administrator's judgments on the public health and
public welfare significance of exposures likely to occur under air quality conditions reflective of
the current NAAQS and, as appropriate, any alternative standards under consideration.
Accordingly, the assessments also provide a basis for judgments regarding the extent of public
health and public welfare protection afforded by such standards. The development of REAs in
each NAAQS review draws upon the currently available evidence characterized by the ISA and
current methods and tools. In considering whether new analyses are warranted for particular
types of assessments in each review, we evaluate the availability of new scientific evidence and
technical information, as well as improved methods and tools, that may provide support for
conducting updates to address key limitations or uncertainties in analyses from the last review or
to provide additional insight beyond those provided by the prior REA. Thus, we focus on
identifying new analyses that are warranted in consideration of factors such as those raised here,
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while also bearing in mind practical and logistical considerations, such as available resources and
timeline for the review. The details of any new analyses are documented within the PA (e.g., in
appendices or associated volumes), and the findings are presented and discussed within the main
body of the PA.
The PA includes pertinent background information, such as information on current air
quality and the decisions in the last NAAQS review, as well as a discussion of the currently
available health and welfare effects evidence and exposure/risk information. These discussions
focus on policy-relevant aspects important for the Agency to consider in reviewing the existing
standards. The policy evaluation in the PA of the current scientific evidence from the ISA and
the current exposure/risk information is generally framed by a consideration of a series of policy-
relevant questions, including the fundamental overarching questions associated with the
adequacy of the current standards and, as appropriate, consideration of alternative standards that
involve revision to any of the specific elements of the standards: indicator, averaging time, level,
and form.9 To the extent it is concluded to be appropriate to consider alternative standards, the
PA will also describe policy options for such revisions supported by the available information.
Key considerations in the development of the exposure/risk information will be discussed in
Volume 3 of the IRP.
The draft PA, including the current air quality, exposure, and risk information, whether
newly developed in this review or drawn from previously developed assessments, is distributed
to the CASAC for its consideration and released to the public for review and comment. Review
of the draft PA by the CASAC also facilitates CASAC's advice to the Agency and
recommendations to the Administrator on the adequacy of the existing standards or revisions that
may be appropriate to consider, as provided for in the CAA. The CASAC discusses its review of
the draft PA at public meetings that are announced in the Federal Register. Based on past
practice by the CASAC, the EPA expects that key advice and recommendations for revision
would be summarized by the CASAC in a letter to the EPA Administrator. In revising the draft
PA document, any such advice and recommendations are taken into account, and comments
received from the public are also considered. The final document is made available on an EPA
website, with its public availability announced in the Federal Register.
9 The indicator defines the chemical species or mixture to be measured in the ambient air for the purpose of
determining whether an area attains the standard. The averaging time defines the period over which air quality
measurements are to be averaged or otherwise analyzed. The form of a standard defines the air quality statistic
that is to be compared to the level of the standard in determining whether an area attains the standard. For
example, the form of the annual NAAQS for fine particulate matter is the average of annual mean concentrations
for three consecutive years, while the form of the 8-hour NAAQS for carbon monoxide is the second-highest 8-
hour average in a year. The level of the standard defines the air quality concentration used for that purpose.
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2.3 REGULATORY DECISION MAKING
Following the issuance of the final PA, consideration of analyses and conclusions
presented therein, and taking into consideration CASAC advice and recommendations, the
Agency develops a notice of proposed decision. This notice conveys the Administrator's
proposed conclusions, reached in consideration of the analyses and conclusions in the documents
developed in the review (e.g., as described in the preceding sections) and advice and
recommendations from the CASAC regarding the adequacy of the current standards and any
revision(s) that may be appropriate. As appropriate, a draft notice of the proposed decision is
submitted to the Office of Management and Budget (OMB) for review and comment. In this
interagency review step, the OMB also provides other federal agencies the opportunity to review
and comment. After completing the interagency review, the notice of proposed decision is
published in the Federal Register.
At the time of publication of the notice of the proposed decision, all materials on which
the proposal is based are made available in the public docket for the review.10 Publication of the
proposal notice is followed by a public comment period, generally lasting 45 to 90 days, during
which the public is invited to submit comments on the proposal to the docket, and one or more
public hearings may be held. Taking into account comments received on the proposed decision,
the Agency then develops a notice of final decision, which communicates the Administrator's
decisions regarding this review and which may again undergo OMB-coordinated interagency
review before issuance by the EPA. At the time of the final decision, the Agency responds to all
significant comments on the proposal.11 Publication of the notice of the final decision in the
Federal Register will complete the review process.
The final decisions on the primary and secondary standards are largely public health or
welfare policy judgments by the Administrator. Final decisions must draw upon scientific
information and analyses about health or welfare effects and risks, as well as judgments about
how to deal with uncertainties inherent in scientific evidence and analyses. Consistent with the
Agency's approach across all NAAQS reviews, the approach of the PA to inform these
judgments is based on a recognition that the available evidence generally reflects continuums
that include ambient air exposures for which scientists generally agree that effects are likely to
10The docket for the current review of the primary NO2 standards is identified as EPA-HQ-OAR-2023-0317. This
docket has incorporated the ISA docket (EPA-HQ-ORD-2022-0831) by reference. Both dockets are publicly
accessible at www.regulations.gov.
11 For example, Agency responses to all substantive comments on the 2017 proposed decision notice in the last
review were provided in the final decision notice (83 FR 17226, April 18, 20218). In some reviews, responses are
additionally provided in a separate document (e.g., Responses to Significant Comments on the 2009 Proposed
Rule on the Primary National Ambient Air Quality Standards for Nitrogen Dioxide (July 15, 2009; 74 FR
34404)).
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occur through lower levels at which the likelihood and magnitude of response become
increasingly uncertain. This approach is consistent with the requirements of the NAAQS
provisions of the Act and how the EPA and the courts have historically interpreted the Act.
With regard to primary standards, these provisions require the Administrator to establish
standards that are requisite to protect public health with an adequate margin of safety. In so
doing, the Administrator seeks to establish standards that are neither more nor less stringent than
necessary for this purpose. The provisions do not require that standards be set at a zero-risk level,
but rather at a level that avoids unacceptable risks to public health, including the health of
sensitive groups.12
12 More than one population group may be identified as sensitive or at-risk in a NAAQS review. The decision in the
review will reflect consideration of the degree to which protection is provided for these sensitive population
groups. To the extent that any particular population group is not among the identified sensitive groups, a decision
that provides protection for the sensitive groups would be expected to also provide protection for other population
groups.
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3 BACKGROUND ON THE PRIMARY NAAQS FOR
OXIDES OF NITROGEN
3.1 HISTORY OF THE HEALTH-BASED AIR QUALITY CRITERIA
AND STANDARDS FOR OXIDES OF NITROGEN
In 1971, the EPA added oxides of nitrogen to the list of criteria pollutants under section
108(a)(1) of the CAA and issued the initial air quality criteria (36 FR 1515, January 30, 1971;
U.S. EPA, 1971). Based on these air quality criteria, the EPA promulgated NAAQS for oxides of
nitrogen using NO2 as the indicator (36 FR 8186, April 30, 1971). Both primary and secondary
standards were set at 100 micrograms per cubic meter (|ig/m3) (equal to 0.053 parts per million
[ppm]), as an annual average.
The EPA retained the primary and secondary NO2 standards, without revision, in
subsequent reviews completed in 1985 and 1996 (50 FR 25532, June 19, 1985; 61 FR 52852,
October 8, 1996). In the latter of the two decisions, the EPA concluded that "the existing annual
primary standard appears to be both adequate and necessary to protect human health against both
long- and short-term NO2 exposures" and that "retaining the existing annual standard is
consistent with the scientific data assessed in the Criteria Document (U.S. EPA, 1993) and the
Staff Paper (U.S. EPA, 1995) and with the advice and recommendations of the CAS AC" (61 FR
52854, October 8, 1996).
In 2005, the EPA again initiated the review of the health-based air quality criteria for
oxides of nitrogen and the primary NAAQS for oxides of nitrogen (70 FR 73236, December 9,
2005). The Agency's plan for conducting the review was contained in the Integrated Review
Plan for the Primary National Ambient Air Quality Standardfor Nitrogen Dioxide (2007 IRP;
U.S. EPA, 2007), which included consideration of comments received from CASAC
consultation as well as the public on a draft IRP. The scientific assessment for the review was
described in the 2008 Integrated Science Assessment for Oxides of Nitrogen - Health Criteria
(U.S. EPA, 2008b), multiple drafts of which received review by the CASAC and the public.
After consultation with the CASAC and public comment on a draft analysis plan, the EPA also
conducted quantitative human risk and exposure assessments. These technical analyses were
presented in the Risk and Exposure Assessment to Support the Review of the NO2 Primary
National Ambient Air Quality Standard (2008 REA; U.S. EPA, 2008a), multiple drafts of which
received CASAC and public review.
During the review initiated in 2005, the EPA was engaged in considering changes to the
NAAQS review process. An important change that was implemented was the discontinuation of
the Staff Paper (the prior term for PAs). To address this discontinuation, prior to the
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implementation of an alternative to serve its purpose of consideration of policy-relevant aspects
of the assessments and discussing policy options for the Administrator to consider, a policy
assessment chapter that considered the scientific evidence in the 2008 ISA and the exposure and
risk characterization results presented in other chapters of the 2008 REA as they related to the
adequacy of the then-current primary annual NO2 standard and potential alternative standards for
consideration was included in the final REA (U.S. EPA, 2008a, chapter 10). The CAS AC
discussed the final REA, emphasizing the policy assessment chapter during a public
teleconference on December 5, 2008 (73 FR 66895, November 12, 2008). Following that
teleconference, the CASAC offered comments and advice on the primary NO2 standard in a
letter to the Administrator (Samet, 2008).
After considering the body of evidence on human health effects associated with NO2
exposures in the ISA and the exposure and risk information in the REA, the Administrator
determined that the existing annual average primary NO2 NAAQS was not sufficient to protect
the public health from the array of effects that could occur following short-term exposures to
NO2 in ambient air. In so doing, the Administrator noted the potential for adverse health effects
to occur following exposures to elevated NO2 concentrations that can occur around major roads
(75 FR 6482, February 9, 2010). In a notice published in the Federal Register on July 15, 2009,
the EPA proposed to supplement the existing primary annual NO2 standard by establishing a new
short-term standard (74 FR 34404, July 15, 2009). In a notice published in the Federal Register
on February 9, 2010, the EPA finalized a new short-term NO2 standard with a level of 100 parts
per billion (ppb), based on the 3-year average of the 98th percentile of the yearly distribution of
1-hour daily maximum concentrations. The EPA also retained the existing primary annual NO2
standard of 53 ppb as an average annual average (75 FR 6474, February 9, 2010). The Agency's
final decision included consideration of the CASAC's advice and recommendations during the
review, as well as public comments on the proposed rule. The EPA's final rule was upheld
against challenges in a decision issued by the U.S. Court of Appeals for the District of Columbia
Circuit. APIv. EPA, 684 F.3d 1342 (D.C. Cir. 2012).
In addition to revisions to the NAAQS, revisions were also finalized related to the data
handling procedures, to the ambient air monitoring and reporting requirements and to the Air
Quality Index (AQI). The EPA also included new monitoring network requirements for States to
locate monitors near heavily trafficked roadways in large urban areas and in other locations
where maximum NO2 concentrations can occur. Subsequent to the 2010 rulemaking, the EPA
revised the deadlines by which the near-road monitors were to be operational to implement a
phased deployment approach (78 FR 16184, March 14, 2013). The bulk of the initial set of
required near-road NO2 monitors became operational between January 1, 2014, and January 1,
2017.
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In February 2012, the EPA again initiated a review of the health-based air quality criteria
and of the primary NAAQS for oxides of nitrogen and issued a Call for Information in the
Federal Register (77 FR 7149, February 10, 2012), with the review focused on health effects
associated with the gaseous species only. The gaseous oxides of nitrogen include NO2 and nitric
oxide (NO), as well as their gaseous reaction products. Total oxides of nitrogen include these
gaseous species and particulate species (e.g., nitrates).13'14
A wide range of external experts, as well as EPA staff representing a variety of areas of
expertise (e.g., epidemiology, human and animal toxicology, statistics, risk/exposure analysis,
atmospheric science, and biology), participated in a workshop held by the EPA on February 29
to March 1, 2012, in Research Triangle Park, NC. The workshop provided an opportunity for a
public discussion of the key policy-relevant issues associated with the review of the primary NO2
NAAQS and the most meaningful new science that would be available to inform our
understanding of these issues.
Based in part on the workshop discussions, the EPA developed a draft plan for the ISA
and a draft IRP outlining the schedule, process, and key policy-relevant questions that would
guide the evaluation of the health-based air quality criteria and the review of the primary
NAAQS for oxides of nitrogen. The draft plan for the ISA was released in May 2013 (U.S. EPA,
2013; 78 FR 26026, May 3, 2013) and was the subject of a consultation with the CASAC in June
2013 (78 FR 27234, May 9, 2013). Comments received from that consultation were considered
in preparing the first draft ISA, and subject matter experts reviewed preliminary drafts of key
ISA chapters at a public workshop hosted by the EPA's National Center for Environmental
Assessment (NCEA) in May 2013 (78 FR 27374, May 10, 2013). The first draft of ISA was
released in November 2013 (U.S. EPA, 2013; 78 FR 70040, November 22, 2013 ). During this
time, the draft IRP was also in preparation and was released in February 2014 (U.S. EPA, 2014;
79 FR 7184, February 06, 2014 ). The CASAC reviewed both the draft IRP and first draft ISA at
a public meeting held in March 2014 (79 FR 8701, February 13, 2014), and the first draft ISA
was further discussed at an additional teleconference held in May 2014 (79 FR 17538, March 28,
2014). The CASAC finalized its recommendations on the first draft ISA in a letter to the
Administrator in June 2014 (Frey, 2014).
13 Health effects associated with particulate oxides of nitrogen are addressed in the review of the PM NAAQS.
Additional information on the PM NAAQS reviews is available at: https://www.epa.gov/naaqs/particulate-
matter-pm-air-quality-standards.
14 The EPA is separately reviewing the ecological welfare effects associated with and the secondary standards for
oxides of nitrogen, oxides of sulfur, and PM. Additional information on the ongoing and previous review of the
secondary NAAQS for oxides of nitrogen, oxides of sulfur, and PM is available at:
https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-secondary-air-quality-standards.
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The EPA released a second draft ISA in January 2015 (U.S. EPA, 2015a; 80 FR 5110,
January 30, 2015) and the REA Planning Document in May 2015 (U.S. EPA, 2015b; 80 FR
27304, May 13, 2015). The CAS AC reviewed these documents at a public meeting held in June
2015 (80 FR 22993, April 24, 2015). A follow-up teleconference with the CAS AC was held in
August 2015 (80 FR 43085, July 21, 2015) to finalize recommendations on the second draft ISA.
The CASAC's advice and recommendations on the second draft ISA were provided in a letter to
the Administrator in September 2015 (Diez Roux and Frey, 2015a). The final ISA was released
in January 2016 (U.S. EPA, 2016; 81 FR 4910, January 28, 2016). The CAS AC's
recommendations on the draft REA Planning Document were included in a letter provided to the
EPA in September 2015 (Diez Roux and Frey, 2015b). The EPA considered the CAS AC's
advice and public comments on the draft REA Planning Document in developing and performing
the quantitative analyses for the review, which were included as a part of the draft PA.
The EPA prepared a draft PA, which was released in September 2016 (U.S. EPA, 2016b;
81 FR 65353, September 22, 2016). The CASAC reviewed the draft PA at a public meeting held
on November 9-10, 2016 (81 FR 68414, October 4, 2016), and a follow-up teleconference was
held on January 24, 2017 (81 FR 95137, December 27, 2016). The CASAC's recommendations,
based on its review of the draft PA, were provided in a letter to the Administrator dated March 7,
2017 (Diez Roux and Sheppard, 2017). The EPA staff considered these recommendations and
public comments on the draft PA when developing the final PA, which was released in April
2017 (U.S. EPA, 2017; 82 FR 17947, April 14, 2017).
In July 2017, the Administrator proposed to retain the existing primary NO2 standards
without revision (82 FR 34792, July 2017). The Administrator solicited comments on his
proposed conclusion regarding the public health protection provided by the primary NO2
standards and on his proposal to retain the standard. In May 2018, after considering the available
scientific evidence, the results of quantitative analyses, the CASAC advice, and public
comments, the Administrator concluded that the current 1-hour and annual NO2 primary
standards, together, were requisite to protect public health with an adequate margin of safety.
Therefore, the EPA retained the 1-hour and annual NO2 primary standards, without revision
(83FR17226, May 2018). The rationale for the final decision is described in more detail in
section 3.2 below.
3.2 THE PRIMARY STANDARDS
Ambient air concentrations of NO2 are influenced by both direct NO2 emissions and by
emissions of nitric oxides (NO), with the subsequent conversation of NO to NO2 primarily
through reaction with ozone (O3). A large number of oxidized nitrogen species in the atmosphere
are formed from the oxidation of NO and NO2. These include nitrate radicals (NO3), nitrous acid
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(HONO), nitric acid (HNO3), dinitrogen pentoxide (N2O5), nitryl chloride (CINO2), peroxynitric
acid (HNO4), peroxyacetyl nitrate and its homologues (PANs), other organic nitrates, such as
alkyl nitrates (including isoprene nitrates), and particulate nitrate (pNCb). The sum of these
reactive oxidation products and NO plus NO2 comprise the oxides of nitrogen.15' 16
There are currently two primary standards for oxides of nitrogen. NO2 is the component
of oxides of nitrogen of greatest health concern and is the indicator for the primary NAAQS. The
two primary NO2 standards are: a 1-hour standard established in 2010 (75 FR 6502, February 9,
2010) at a level of 100 ppb and based on the 98th percentile of the annual distribution of daily
maximum 1-hour NO2 concentrations, averaged over 3 years; and an annual standard, originally
set in 1971, at a level of 53 ppb and based on annual average NO2 concentrations (36 FR 8186,
April 30, 1971).
Consistent with the review completed in 2010, the 2018 review focused on health effects
associated with gaseous oxides of nitrogen17 and the protection afforded by the primary NO2
standards. The gaseous oxides of nitrogen include NO2 and NO, as well as their gaseous reaction
products. Total oxides of nitrogen include these gaseous species as well as particulate species
(e.g., nitrates). Health effects and non-ecological welfare effects associated with the particulate
species are addressed in the review of the NAAQS for particulate matter (PM).18 The EPA is
separately reviewing the ecological welfare effects associated with and the secondary standards
for oxides of nitrogen, oxides of sulfur, and PM.19The 2018 review evaluated whether it was
appropriate to consider retaining or revising both of these primary NO2 standards (83 FR 17226,
April 18, 2018). The Administrator's review of these standards in 2018 concluded that they
provided the requisite protection of public health, with an adequate margin of safety, and should
be retained without revision. These conclusions were informed by careful consideration of the
full body of evidence available in the 2018 review, giving particular weight to the assessment of
15 The focus is on NO2 in this document, as this is in the indicator for the current standards and is most relevant to
the evaluation of health evidence.
16 Section 108(c) of the CAA specifies that: "Such criteria [for oxides of nitrogen] shall include a discussion of nitric
and nitrous acids, nitrites, nitrates, nitrosamines, and other carcinogenic and potentially carcinogenic derivatives
of oxides of nitrogen." By contrast, within air pollution research and control communities, the terms "nitrogen
oxides" and NOx are often restricted to refer to only to the sum of NO and NO2.
17 These gaseous oxides of nitrogen can also be referred to as "nitrogen oxides" and include a broad category of
gaseous oxides of nitrogen (i.e., oxidized nitrogen compounds), including NO2, NO, and their various reaction
products.
18 Additional information on the PM NAAQS is available at: https://www.epa.gov/naaqs/particulate-matter-pm-air-
quality-standards.
19 Additional information on the ongoing and previous review of the secondary NAAQS for oxides of nitrogen,
oxides of sulfur, and PM is available at: https://www.epa.gov/naaqs/nitrogen-dioxide-no2-and-sulfur-dioxide-so2-
secondary-air-quality-standards.
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the scientific evidence in the 2016 ISA, analyses in the 2017 PA comparing NO2 air quality with
health-based benchmarks, consideration of the evidence and analyses in the 2017 PA, and the
advice and recommendations from the CAS AC (83 FR 17226, April 18, 2018).
With regard to the short-term NO2 exposures, in the last review, the most robust evidence
came from studies examining respiratory effects. The strongest support for this relationship came
from controlled human exposure studies demonstrating NCh-induced increases in airway
responsiveness in individuals with asthma. Most of the controlled human exposure studies
assessed in the 2016 ISA were available in the 2010 review, with the addition in the 2018 review
of an updated meta-analysis that synthesized data from these studies. These studies provided an
important part of the body of evidence supporting the decision in the 2010 review to establish the
1-hour NO2 standard with its level of 100 ppb. Beyond the controlled human exposure studies,
additional supporting evidence came from epidemiologic studies reporting associations with a
range of asthma-related respiratory effects, including effects serious enough to result in
emergency room visits or hospital admissions. While there was some new evidence in the 2018
review from such epidemiologic studies of short-term NO2 exposures, the results of these newer
studies were generally consistent with the epidemiologic studies that were available in the 2010
review.
With regard to long-term NO2 exposures, the Administrator noted that although the
evidence supporting associations with asthma development in children was stronger in the 2018
review than it was in the 2010 review, uncertainties remained regarding the degree to which
estimates of long-term NO2 concentrations in these studies were serving primarily as surrogates
for exposures to the broader mixture of traffic-related pollutants. Supporting evidence also
included studies indicating a potential role for repeated short-term NO2 exposures in the
development of asthma (U.S. EPA, 2016a, p. 6-64 and p. 6-65).
In addition, the Administrator acknowledged that the evidence for some non-respiratory
effects had strengthened since the 2010 review. In particular, based on the assessment of the
evidence in the 2016 ISA, he noted stronger evidence for NCh-associated cardiovascular effects
(short- and long-term exposures), premature mortality (long-term exposures), and certain
reproductive effects (long-term exposures). As detailed in the 2016 ISA, while this evidence was
generally strengthened since the 2010 review, it remained subject to greater uncertainty than the
evidence of asthma-related respiratory effects (U.S. EPA, 2016a).
In the 2018 review, the Administrator's consideration of potential at-risk populations
drew from the assessment of the evidence in the 2016 ISA (U.S. EPA, 2016a, Chapter 7). Based
on the systematic approach to evaluating factors that may increase risks in a particular population
or during a particular life stage in the 2016 ISA, the Administrator was most concerned about the
potential effects of NO2 exposures in people with asthma, children, and older adults (U.S. EPA,
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2016a, Table 7-27). Support for potentially higher risks in these populations was based primarily
on evidence for asthma exacerbation or asthma development. Evidence for other health effects
was subject to greater uncertainty (U.S. EPA, 2017, Section 3.4).
The Administrator further used the scientific evidence, described in detail in the 2016
ISA (U.S. EPA, 2016a), to directly inform his consideration of the adequacy of the public health
protection provided by the primary NO2 standards. Consistent with the approach in the 2017 PA
(U.S. EPA, 2017), and with the CASAC's advice (Diez Roux and Sheppard, 2017), the
Administrator specifically considered the evidence within the context of the degree of public
health protection provided by the current 1-hour and annual standards together, including the
combination of all elements of these standards (i.e., indicator, averaging times, forms, levels).
In doing so, the Administrator focused on the results of controlled human exposure
studies of airway responsiveness in people with asthma and on the results of U.S. and Canadian
epidemiologic studies of asthma-related hospital admissions, asthma-related emergency
department visits, and asthma development in children. He particularly emphasized the results of
controlled human exposure studies, which were identified in the 2016 ISA as providing "[t]he
key evidence thatNCh exposure can independently exacerbate asthma" (U.S. EPA, 2016a , p. 1-
18). The Administrator's decision to focus on these studies was consistent with the CASAC's
advice that the strongest evidence was for an increase in airway responsiveness based on
controlled human exposure studies, with supporting evidence from epidemiologic studies.
In considering the controlled human exposure studies of airway responsiveness, the
Administrator focused on the results of an updated meta-analysis of data from these studies and
the consistency of findings across individual studies. As discussed above, and consistent with the
evidence in the 2010 review, the meta-analysis indicated that most study volunteers, generally
with mild asthma, experienced increased airway responsiveness following 30-minute to 1-hour
resting exposures to NO2 concentrations from 100 to 530 ppb. Based on these results, the
Administrator noted the potential for people with asthma to experience NCh-induced respiratory
effects following exposures in this range and that people with more severe asthma could
experience more serious effects. The Administrator further noted that individual studies
consistently reported statistically significant increases in airway responsiveness following
exposures to NO2 concentrations at or above 250 ppb, with less consistent results across studies
conducted at lower exposure concentrations, particularly 100 ppb.
Therefore, the Administrator judged that it was appropriate to consider the degree of
protection provided against exposures to NO2 concentrations at and above 100 ppb, though his
concern was greater for exposures to higher concentrations. In particular, based on the results of
the meta-analysis and on the consistent results across individual studies, the Administrator was
most concerned about the potential for people with asthma to experience adverse respiratory
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effects following NO2 exposures at or above 250 ppb. Because results were less consistent across
individual studies that evaluated lower exposure concentrations, the Administrator was
increasingly concerned about uncertainties in the evidence as he considered the potential
implications of such exposures. While taking these uncertainties into consideration, the
Administrator remained concerned about the potential for respiratory effects following exposures
to NO2 concentrations as low as 100 ppb, particularly in people with more severe cases of
asthma than have generally been evaluated in the available NCh-controlled human exposure
studies. Thus, when the evidence and uncertainties were considered together, the Administrator
judged that it was appropriate to consider the degree of protection provided against potential
exposures to NO2 concentrations at or above 100 ppb, with the most emphasis on the potential
for exposures at or above 250 ppb.
In further considering the potential public health implications of the controlled human
exposure studies, the Administrator considered the results of quantitative comparisons between
NO2 air quality and health-based benchmarks. As discussed in the 2017 PA, these comparisons
helped to place the results of the controlled human exposure studies, which provided the basis for
the benchmark concentrations, into a broader public health context. In considering the results of
the analyses comparing NO2 air quality to specific health-based benchmarks, the Administrator
first recognized that all areas of the U.S. met the current primary NO2 standards. When based on
the unadjusted NO2 air quality, these analyses estimated almost no days with the potential for 1-
hour exposures to NO2 concentrations at or above health-based benchmarks, including the lowest
benchmark examined (i.e., 100 ppb).
The Administrator additionally recognized that, even when ambient NO2 concentrations
are adjusted upward just to meet the existing 1-hour standard, the analyses estimated no days
with the potential for exposures to the NO2 concentrations that have been shown most
consistently to increase airway responsiveness in people with asthma (i.e., above 250 ppb). Such
NO2 concentrations were not estimated to occur, even under worst-case conditions across various
study areas with among the highest NOx emissions in the U.S. and at monitoring sites adjacent to
some of the most heavily trafficked roadways in the U.S. In addition, analyses with adjusted air
quality indicated a limited number of days with the potential for exposures to 1-hour NO2
concentrations at or above 100 ppb, an exposure concentration with the potential to exacerbate
asthma-related respiratory effects, but where uncertainties in the evidence became increasingly
important.
As such, the Administrator concluded that evidence from controlled human exposure
studies of airway responsiveness and analyses comparing ambient air NO2 concentrations to
health-based benchmarks supported the degree of the public health protection provided by the
current primary NO2 NAAQS. In particular, he was concerned about exposures to NO2
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concentrations at and above 250 ppb, where the potential for NCh-induced respiratory effects
was supported by results of the meta-analysis and by consistent results reported across individual
studies. Regarding this, the Administrator noted that meeting the current standards was estimated
to allow no potential for exposure to 1-hour NO2 concentrations at or above 250 ppb.
Additionally, the Administrator was concerned about exposures to lower NO2 concentrations,
including concentrations as low as 100 ppb, though, as described above, he was also concerned
about the uncertainties in the evidence at such low exposure concentrations. In considering the
degree of protection provided against exposures to 100 ppb NO2, in light of uncertainties, the
Administrator judged that limiting such exposures was appropriate but that it was not necessary
to eliminate them. He noted that the current standard is estimated to allow limited potential for
exposures to NO2 concentrations at or above 100 ppb. Thus, given the substantial protection
provided against exposures to NO2 concentrations at and above 250 ppb and the protection
provided against exposures to concentrations as low as 100 ppb, the Administrator reached the
conclusion that the evidence, when considered in light of its uncertainties, supported the degree
of public health protection provided by the current primary NO2 NAAQS.
Although the epidemiologic evidence for NO2 is subject to greater uncertainty than the
controlled human exposure studies of NCh-induced changes in airway responsiveness, the
Administrator also considered what the available epidemiologic studies indicated with regard to
the adequacy of the public health protection provided by the current standards. In particular, he
considered analyses of NO2 air quality in the locations and during the time periods of available
U.S. and Canadian epidemiologic studies. These studies did not report associations in locations
meeting the current NO2 standards (i.e., associations were reported for NO2 concentrations that
exceeded the current standards). There was greater uncertainty regarding the potential for
reported effects to occur following the NO2 exposures that are associated with air quality
meeting those standards.
With regard to studies of short-term NO2 exposures, the Administrator noted that
epidemiologic studies provided consistent evidence for asthma-related emergency department
visits and hospital admissions associated with exposure to NO2 in locations likely to have
exceeded the current standards over at least parts of study periods (based on the presence of
relatively precise and generally statistically significant associations across several studies). These
studies have not consistently shown such NCh-associated outcomes in areas that would have
clearly met the current standards. In this regard, the Administrator recognized that the NO2
concentrations identified in these epidemiologic studies are based on a NO2 monitoring network
that, during the study periods, did not include monitors meeting the current near-road monitoring
requirements. This was particularly important given that NO2 concentrations near the most
heavily trafficked roadways were likely to have been higher than those reflected by the NO2
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concentrations measured at monitors in operation during study years. As such, the estimated
design values associated with the areas during the studies could have been higher had a near-road
monitoring network been in place. Thus, while these epidemiologic studies provide consistent
evidence for associations with asthma-related effects, the Administrator noted that studies
conducted in the U.S. and Canada did not support associations with asthma-related hospital
admissions or emergency department visits in locations that would have clearly met the current
standards.
With regard to studies of long-term NO2 exposures, the Administrator noted that the
preponderance of evidence for respiratory health effects comes from epidemiologic studies
evaluating asthma development in children. These studies report associations with long-term
average NO2 concentrations, while the broader body of evidence indicates the potential for
repeated short-term NO2 exposures to contribute to the development of asthma. Because of this,
and because air quality analyses indicate that meeting the current 1-hour standard can also limit
annual NO2 concentrations, when considering these studies of asthma development, the
Administrator considered the protection provided by the combination of both the annual and 1-
hour standards together. While available epidemiologic studies conducted in the U.S. and
Canada consistently report associations between long-term NO2 exposures and asthma
development in children in locations likely to have violated the current standards over at least
parts of study periods, those studies did not indicate such associations in locations that would
have clearly met the current annual and 1-hour standards. This was particularly the case given
that NO2 concentrations near the most heavily trafficked roadways were not likely reflected by
monitors operating during study years. Therefore, while recognizing the public health
significance of asthma development in children and recognizing that NO2 concentrations
exceeding the current standards was associated with asthma development, the Administrator
placed weight on the 2017 PA conclusion that the evidence did not provide support for NO2-
attributable asthma development in children in locations with NO2 concentrations that would
have clearly met both the annual and 1-hour standards.
Taking all of these considerations into account, the Administrator reached the conclusion
that the scientific evidence evaluated, in combination with the results of quantitative analyses
comparing NO2 air quality with health-based benchmarks, supported the degree of public health
protection provided by the current 1-hour and annual primary NO2 standards and did not call into
question any of the elements of those standards. He further concluded that the current 1-hour and
annual NO2 primary standards, together, were requisite to protect public health with an adequate
margin of safety.
In particular, with regard to short-term exposures and the current 1-hour standard, the
Administrator took note of the well-established body of scientific evidence supporting the
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occurrence of respiratory effects following short-term NO2 exposures. In reaching the conclusion
that the current standards provide requisite protection against these effects, the Administrator
noted that meeting the 1-hour NO2 standard provided a substantial margin of safety against
exposures to NO2 concentrations that have been shown most consistently to increase airway
responsiveness in people with asthma, even under worst-case conditions across a variety of study
areas with among the highest NOx emissions in the U.S. Such NO2 concentrations were not
estimated to occur, even at monitoring sites adjacent to some of the most heavily trafficked
roadways. Furthermore, the 1-hour standard limited exposure potential to 1-hour concentrations
at or above 100 ppb. Thus, the standard provided protection against NO2 exposures with the
potential to exacerbate symptoms in some people with asthma, but uncertainties in the evidence
became increasingly important, as discussed in more detail in the 2017 PA. Finally, the
Administrator noted that the 1-hour standard was expected to maintain ambient NO2
concentrations below those present in locations where key U.S. and Canadian epidemiologic
studies reported relatively precise and statistically significant associations between short-term
NO2 and asthma-related hospitalizations.
In addition, with regard to long-term NO2 exposures, the Administrator noted that the
evidence supporting associations with asthma development in children was strengthened since
the 2010 review, though important uncertainties remained. As discussed above, meeting the
current annual and 1-hour standards was expected to maintain ambient NO2 concentrations
below those present in locations where key U.S. and Canadian epidemiologic studies reported
such associations between long-term NO2 and asthma development. In considering the protection
provided against exposures that could contribute to asthma development, the Administrator
recognized the air quality relationship between the 1-hour standard and annual standard and that
analyses of historical ambient NO2 concentrations in the 2017 PA (U.S. EPA, 2017, Figure B3-1)
suggested that meeting the 1-hour standard with its level of 100 ppb would be expected to
maintain annual average NO2 concentrations well below the 53 ppb level of the annual standard,
and generally below 35 ppb . The Administrator judged that, as additional years of data become
available from the near-road NO2 monitors, it would be important in future reviews to evaluate
the degree to which this relationship is also observed in the near-road environment and the
degree to which the annual standard provides additional protection, beyond that provided by the
1-hour standard. Such an evaluation could inform future reviews of the primary NO2 NAAQS,
consistent with the CASAC advice that in the next review cycle for oxides of nitrogen, the EPA
should review the annual standard to determine if there is a need for revision or revocation (83
FR 17226, April 18, 2018).
Therefore, in the 2018 review, the Administrator retained the current primary NO2
standards without revision. The Administrator noted that his judgment to retain the current
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primary NO2 standards was consistent with CAS AC advice provided in its review of the 2016
draft PA. In their advice to retain the standard, the CASAC specifically focused its conclusions
on the degree of protection provided by combining the 1-hour and annual standards against
short- and long-term NO2 exposures. The CASAC stated that the suite of the current 1-hour and
annual standards, together, provide protection against adverse effects from exposure to NO2 (83
FR 17226, April 18, 2018)
Inherent in the Administrator's conclusions were public health policy judgments on the
public health implications of the available scientific evidence and analyses, including how to
weigh associated uncertainties. These public health policy judgments included those related to
the appropriate degree of public health protection that should be afforded against the risk of
respiratory morbidity in at-risk populations, such as the potential for worsened respiratory effects
in people with asthma, as well as judgments related to the appropriate weight to be given to
various aspects of the evidence and quantitative analyses, including how to consider their
associated uncertainties. Based on these considerations and the judgments identified here, the
Administrator concluded that the current standards provide the requisite protection of public
health with an adequate margin of safety, including protection of at-risk populations, such as
people with asthma.
The Administrator additionally recognized that the uncertainties and limitations
associated with the many aspects of the estimated relationships between respiratory morbidity
and NO2 exposures were amplified when considering progressively lower ambient NO2
concentrations. In his view, and consistent with the conclusions in the 2017 PA, there was
appreciable uncertainty in the extent to which reductions in asthma exacerbations or asthma
development would result from revising the primary NO2 NAAQS to be more stringent than the
current standards. Therefore, the Administrator also did not believe standards that are more
stringent than the current standards would be appropriate. With regard to this, the CASAC
advised that there was no scientific basis for a standard lower than the current 1-hour NO2
standard (83 FR 17226, April 18, 2018). The CASAC also did not advise setting the annual
standard level lower than the current level of 53 ppb, noting that the 1-hour standard can
generally maintain long-term NO2 concentrations below the annual standard (83 FR 17226, April
18, 2018). Thus, the Administrator concluded, based on the evidence, the public health policy
judgments summarized above, including weight given to uncertainties in the evidence, and
advice from the CASAC, that the 1-hour and annual standards were requisite and should be
retained, without revision (83 FR 17226, April 18, 2018).
3-12
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4 THE CURRENT PRIMARY N02 NAAQS REVIEW
In December 2022, the EPA announced the initiation of the current periodic review of the
health-based air quality criteria for oxides of nitrogen and the primary NO2 NAAQS and issued a
Call for Information in the Federal Register (87 FR 75625, December 9, 2022). The current
review of the primary NO2 standards builds on the substantial body of work completed during
prior reviews, represented in comprehensive science assessments (e.g., 2005 ISA and 2016 ISA)
and past quantitative exposure and risk analyses. These different types of information, evaluated
in policy assessments, provided the basis for decisions on the existing primary NO2 NAAQS.
The anticipated milestones for the current review are presented in Table 4-1. Concurrent
with the release of this background document (Volume 1 of the IRP), the EPA is releasing the
planning document for the review and the ISA, as Volume 2 of the IRP. Volume 2 identifies
policy-relevant science issues important to guiding the evaluation of the health-based air quality
criteria and review of the primary NAAQS for oxides of nitrogen. It will be the subject of a
consultation with the CASAC. Based on consideration of input received during this consultation,
the EPA will develop a draft ISA for external review by the CASAC and for public comment.
With consideration of the newly available evidence identified in the draft ISA, the EPA
will develop Volume 3 of the IRP, which is the planning document for the quantitative analyses,
including exposure/risk analyses, that might be warranted to inform decisions in the current
review. With consideration of the CASAC review of the draft ISA and consultation discussion
on Volume 3 of the IRP, the EPA will develop a draft of the PA (with associated policy
evaluations and quantitative analyses) for public and CASAC review.
In September 2023, the Center for Biological Diversity, Sierra Club, and Center for
Environmental Health filed a deadline suit regarding completion of the review of the health-
based air quality criteria and the primary NAAQS for oxides of nitrogen. That citizen suit has not
yet been resolved, and the EPA anticipates that resolution of those claims would inform the
schedule for completion of the review.
4-1
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Table 4-1. Milestones in the review of the health-based air quality criteria and primary
NAAQS for oxides of nitrogen.
Stage of Review Major Milestone
Planning Federal Register Call for Information
Integrated Review Plan (IRP), Volumes 1 and 2
CASAC consultation on IRP, Volume 2
IRP, Volume 3
CASAC consultation on IRP, Volume 3
Science Assessment External review draft of ISA
CASAC public meeting for review of draft ISA
Final ISA
Quantitative External draft of PA (including quantitative air quality, exposure and/or risk
Exposure/Risk analyses, as warranted)
Analyses and Policy CASAC public meeting for review of draft PA
Assessment r. ,n.
Final PA
Regulatory Process Notice of proposed decision
Notice of final decision
4-2
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5 REFERENCES
Diez Roux, A and Frey, HC. (2015a). Letter from Drs. Ana Diez Roux, Chair and H. Christopher
Frey, Immediate Past Chair, Clean Air Scientific Advisory Committee to EPA
Administrator Gina McCarthy. CAS AC Review of the EPA's Integrated Science
Assessment for Oxides of Nitrogen- Health Criteria (Second External Review Draft).
September 9, 2015. EPA-CASAC-15-001. Office of the Administrator, Science Advisory
Board U.S. EPA HQ, Washington DC. Available at:
https://casac.epa.gov/ords/sab/rZsab apex/casac/activity?p 18 id=2347&clear=18&sessi
on=2247202833493.
Diez Roux, A and Frey, HC. (2015b). Letter from Drs. Ana Diez Roux, Chair and H. Christopher
Frey, Immediate Past Chair, Clean Air Scientific Advisory Committee to EPA
Administrator Gina McCarthy. CAS AC Review of the EPA's Review of the Primary
National Ambient Air Quality Standards for Nitrogen Dioxide: Risk and Exposure
Assessment Planning Document. September 9, 2015. EPA-CASAC-15-002. Office of the
Administrator, Science Advisory Board U.S. EPA HQ, Washington DC. Available at:
https://casac.epa.gov/ords/sab/rZsab apex/casac/activity?p 18 id=2384&clear=18&sessi
on=5675533147708.
Diez Roux, A and Sheppard, E. (2017). Letter form Dr. Elizabeth A. (Lianne) Sheppard, Chair,
Clean Air Scientific Advisory Committee to EPA Administrator E. Scott Pruitt. CASAC
Review of the EPA's Policy Assessment for the Review of the Primary National Ambient
Air Quality Standards for Nitrogen Dioxide (External Review Draft- September 2016).
March 7th, 2017. EPA-CASAC-17-001. Office of the Administrator, Science Advisory
Board U.S. EPA HQ, Washington DC. Available at:
https://casac.epa.gov/ords/sab/f?p=l13:0:13696634981754: APPLICATIONPROCESS
A GENCYREVIEW:: :AR ID:2441.
Frey, H. (2014). Letter from Dr. H. Christopher Frey, Clean Air Scientific Advisory Committee
to EPA Administrator Gina McCarthy. CASAC Review of the EPA's Integrated Science
Assessment for Oxides of Nitrogen- Health Criteria (First External Review Draft). June
10, 2014. EPA-CASAC-14-002. Office of the Administrator, Science Advisory Board
U.S. EPA HQ, Washington DC. Available at:
https://casac.epa.gov/ords/sab/rZsab apex/casac/activity?p 18 id=2262&clear=18&sessi
on=578021156055l#report.
Samet, J. (2008). Letter from Dr. Jonathan M. Samet, Chair, Clean Air Scientific Advisory
Committee, to EPA Administrator Stephen Johnson. Clean Air Scientific Advisory
Committee's (CASAC) Review comments on EPA's Risk and Exposure Assessment to
Support the Review of the N02 Primary National Ambient Air Quality Standard.
December 16, 2008. EPA-CASAC-09-003. Office of the Administrator, Science
Advisory Board U.S. EPA HQ, Washington DC. Available at:
http://yosemite. epa.gov/sab/sabproduct. nsf/264cbl22 7d55e02c85257402007446a4/9C4A
540D86BFB6 7A8525 752100 74A 7AE/$File/EPA-CASAC-09-003-unsigned. pdf.
5-1
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U.S. EPA (1993). Air Quality Criteria for Oxides of Nitrogen. Office of Health and
Environmental Assessment. Environmental Criteria and Assessment Office. Washington,
DC. U.S. EPA. EPA-600/8-91-049aF-cF. August 1993. Available at:
https://cfpub. epa.gov/ncea/risk/recordisplay. cfm ?deid=40179.
U.S. EPA (1995). Review of the National Ambient Air Quality Standards for Nitrogen Oxides:
Assessment of Scientific and Technical Information, OAQPS Staff Paper. Office of Air
Quality Planning and Standards. Research Triangle Park, NC. U.S. EPA. EPA-452/R-
95-005. September 1995. Available at:
http://www.epa.gov/ttn/naaqs/standards/nox/data/noxspl995.pdf.
U.S. EPA (2007). Integrated Review Plan for the Primary National Ambient Air Quality
Standard for Nitrogen Dioxide. National Center for Environmental Assessment and
Office of Air Quality Planning and Standards. Research Triangle Park, NC. Available at:
https://www.epa.gov/naaqs/nitrogen-dioxide-no2-primary-air-quality-standards-
documents-review-completed-2010.
U.S. EPA (2008a). Risk and Exposure Assessment to Support the Review of the N02 Primary
National Ambient Air Quality Standard. Office of Air Quality Planning and Standards.
Research Triangle Park, NC. US EPA. EPA 452/R-08-008a. Available at:
https://www.regulations.gov/document/EPA-HQ-OAR-2006-0922-0047.
U.S. EPA (2008b). Integrated Science Assessment for Oxides of Nitrogen - Health Criteria.
Office of Research and Development, National Center for Environmental Assessment.
Research Triangle Park, NC. U.S. EPA. EPA/600/R-08/071. July 2008. Available at:
http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=194645.
U.S. EPA (2013). Draft Plan for Development of the Integrated Science Assessment for Nitrogen
Oxides-Health Criteria. U.S. Environmental Protection Agency, Office of Research and
Development, National Center for Environmental Assessment. Research Triangle Park,
NC. 78 FR 26026. Available at:
https://www.federalregister.gOv/documents/2013/05/03/2013-10430/draft-plan-for-
development-of-the-integrated-science-assessment-for-nitrogen-oxides-health-criteria.
U.S. EPA (2014). Integrated Review Plan for the National Ambient Air Quality Standards for
Nitrogen Dioxide (Final). Office of Air Quality Planning and Standards. Research
Triangle Park, NC. U.S. EPA. EPA-452/R-14-003. June 2014. Available at:
https://www. epa. gov/sites/production/files/2020-
0 7/documents/201406finalirpprimaryno2.pdf.
U.S. EPA (2015a). Integrated Science Assessment for Oxides of Nitrogen-Health Criteria
(Second External Review Draft). Office of Research and Development, National Center
for Environmental Assessment. Research Triangle Park, NC. EPA/600/R-14/006.
Available at:
https://ordspub. epa.gov/ords/eims/eimscomm.getfile?p download id=521587.
U.S. EPA (2015b). Review of the Primary National Ambient Air Quality Standards for Nitrogen
Dioxide:Risk and Exposure Assessment Planning Document. Office of Air Quality
5-2
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Planning and Standards. Research Triangle Park, NC. U.S. EPA. EPA-452/D-15-001.
Available at: https://www.epa.gov/sites/default/files/2020-
07/documents/20150504reaplanning.pdf
U.S. EPA (2016a). Integrated Science Assessment for Oxides of Nitrogen-Health Criteria. U.S.
Environmental Protection Agency, Office of Research and Development, National Center
for Environmental Assessment. Research Triangle Park, NC. EPA/600/R-15/068.
Available at: https://cfpub. epa.gov/ncea/isa/recordisplay. cfm ?deid=310879.
U.S. EPA (2016b). Policy Assessment for the Review of the National Ambient Air Quality
Standards for Nitrogen Dioxide (External Review Draft). Office of Air Quality Planning
and Standards. Research Triangle Park, NC. U.S. EPA. EPA-452/P-16-001. September
2016. Available at: https://www.epa.gov/sites/default/files/2020-07/documents/20160927-
no2-pa-external-review-draft.pdf.
U.S. EPA (2017). Policy Assessment for the Review of the National Ambient Air Quality
Standards for Oxides of Nitrogen (Final). Office of Air Quality Planning and Standards.
Research Triangle Park, NC. U.S. EPA. EPA-452/R-17-003. April 2017. Available at:
https://www. epa.gov/sites/default/files/2017-
04/documents/policy assessment for the review of the no2 naaqs - final report.pdf \
5-3
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APPENDIX A AMBIENT AIR MONITORING AND
DATA HANDLING
In the course of NAAQS reviews, aspects of the methods for sampling and analysis of the
NAAQS pollutant and the current network of monitors, including their physical locations and
monitoring objectives, are reviewed. The methods for sampling and analysis of each NAAQS
pollutant are generally reviewed in conjunction with consideration of the indicator element20 for
that NAAQS. Consideration of the ambient air monitoring network generally informs the
interpretation of current data on ambient air concentrations and helps identify if the monitoring
network is adequate to determine compliance with the existing or, as appropriate, a potentially
revised NAAQS. This Appendix describes aspects of the ambient air monitoring program for
oxides of nitrogen, which includes NO2, the indicator for the primary NO2 NAAQS.
A. 1 AMBIENT AIR MONITORING REQUIREMENTS AND NETWORK
REQUIREMENTS
Ambient NO2 concentrations used to determine compliance with the NAAQS are
primarily measured by monitors operated by state, local, and Tribal air agencies (SLTs),
typically funded in part by the EPA. The EPA provides minimum monitoring requirements for
NO2 and other pollutants in 40 CFR Part 58; additionally, SLTs have the ability to conduct
monitoring above the minimum requirements to satisfy additional data needs. From 2022 to
2023, approximately 460 monitoring sites reported hourly NO2 concentration data to EPA's Air
Quality System (AQS). Approximately 96% of these sites are State/Local Air Monitoring
Stations (SLAMS), fewer than 2% are operated by industrial sources, and the remaining 2% are
sites operated by other federal agencies such as the National Park Service. The monitors used to
measure NO2 for NAAQS compliance are comprised of a chemiluminescent Federal Reference
Method (FRM) and Federal Equivalent Methods (FEM) that use either chemiluminescence or
direct measurement methods of NO2. Data produced by chemiluminescence method-based
analyzers include NO, NO2, and NOx (NO + NO2) concentrations, while direct methods typically
only report NO2, all of which are routinely logged by SLTs.
The NO2 monitoring network reflects the minimum monitoring requirements established
at 40 CFR Part 58, Appendix D, Section 4.3, plus any additional monitoring conducted by SLTs.
The first component of the network is monitors placed near major and highly trafficked
roadways in urban areas, that is called the near-road network. The near-road network was
20 The indicator defines the chemical species or mixture to be measured in the ambient air for the purpose of
determining whether an area attains the standard.
A-l
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introduced and promulgated as part of the 2010 NO2 NAAQS review in response to the fact that
on-road mobile exposures are a primary concern in the setting of the NAAQS. Near-road sites
are required in each Core Based Statistical Area (CBSA) with a population of 1 million or more
persons, with a second near-road site required in each CBSA with a population of 2.5 million or
more persons. Additionally, any CBSA with over 1 million persons that has one or more
roadway segments with an average daily traffic volume of 250,000 or more vehicles per day also
is required to have a second near-road site. As of the end of 2023, there are 75 near-road sites
with NO2 monitors in operation.
The second component of the NO2 monitoring network is monitors required at sites with
neighborhood or larger spatial scales of representation where maximum concentrations of NO2
may occur in CBS As with more than 1 million persons. Notably, there are many more of these
types of sites in both urban and rural locations across the country, operated by SLTs for a variety
of data needs. Some of these required monitors may also be fulling requirements for the
Photochemical Assessment Monitoring Stations (PAMS) program. PAMS monitoring is required
within a multipollutant monitoring site network called the National Core multipollutant
monitoring station (NCore) network. PAMS monitoring operations are to be conducted at those
NCore sites in CBS As with more than 1 million persons. PAMS measurement include NO, NO2,
NOy (total oxides of nitrogen), and other O3 precursors during the months of June, July and
August, although some precursor monitoring may be required for longer periods of time.21
A third component of the NO2 network includes the monitors required for inclusion or
identification of any type of NO2 monitoring site (whether within or above the minimum
monitoring requirements) that focuses on making measurements in areas with susceptible and
vulnerable populations (40 CFR Part 58, Appendix D, Section 4.3.4(a)). This requirement can be
satisfied by any type of NO2 monitor, so long as it is characterizing air quality in an area with
susceptible and vulnerable populations.
Finally, there can also be NO2 monitors that are installed and operated in a temporary
manner (at least initially) known as Special Purpose Monitors (SPMs). While SPMs are to be
operated just like routine SLAMS site monitors, SPMs do not count towards minimum
monitoring requirements and generally are not initially intended for use for regulatory purposes.
The purpose for operating SPMs can include collecting data for human health and welfare
studies, industry or facility air quality impact characterization, prevention of significant
deterioration information, and other purposes. If an SPM operates for more than 24 months, its
data can be eligible for comparison to the relevant NAAQS if it has met all applicable
21 The requirements for PAMS, which were most recently updated in 2015, is fully described in section 5 of
Appendix D to 40 CFR Part 58.
A-2
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operational requirements and quality assurance criteria established by 40 CFRPart 58, Appendix
D, 58.20.
Figure A-l. Map of U.S. NO2 monitoring sites reporting data to the EPA during the 2022-
2023 period. (Source: https: www.cpa.gov aqs)
To improve certainty in monitoring data and support assessment of data quality,
monitoring agencies must operate with a quality system, which requires the development and
adherence to Quality Assurance Project Plans (QAPPs), the use of Standard Operating
Procedures (SOPs), and the conduction of quality assurance (QA) activities. For example, for
NO2, SLTs are required to perform QA checks at least once every two weeks to derive estimates
of precision and bias for NO2 and the other gaseous criteria pollutant measurements. The data
quality goal for precision and bias is 15 percent or lower forNCh monitors. Further, SLTs are
also subject to routine audits under the National Performance Audit Program managed by the
EPA.
A-3
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Ambient air quality data and associated QA data are reported to the EPA via AQS.22 Data
are reported quarterly and must be submitted to AQS within 90 days after the end of each
calendar quarter (i.e., Jan/Feb/Mar, Apr/May/Jun, Jul/Aug/Sep, Oct/Nov/Dec). Additionally,
each monitoring agency is required to certify all FRM/FEM data that is submitted to AQS
annually, taking into consideration any QA findings, and a data certification letter must be sent
to the EPA Regional Administrator by May 1st of the following year.
A.2 DATA HANDLING CONVENTIONS AND COMPUTATIONS FOR
DETERMINING WHETHER THE STANDARDS ARE MET
To assess whether a monitoring site or geographic area (usually a county or urban area)
meets or exceeds a NAAQS, the monitoring data are analyzed consistent with the established
regulatory requirements for the handling of monitoring data for the purposes of deriving a design
value. A design value summarizes ambient air concentrations for an area in terms of the
indicator, averaging time and form for a given standard such that its comparison to the level of
the standard indicates whether the area meets or exceeds the standard. There are currently two
primary NO2 NAAQS in effect: the annual NAAQS (established in 1971) and the 1-hour
NAAQS (established in 2010). See 40 CFR 50.11. Under 40 CFR 50.11(e) and (f), the
procedures for calculating design values for both primary NO2 NAAQS are detailed in Appendix
S to 40 CFR Part 50 and are summarized below. For a more detailed description of these
requirements, Appendix S should be consulted.
Hourly NO2 measurement data collected at an ambient air monitoring site using FRMs or
FEMs, meeting all applicable requirements in 40 CFR Part 58 and reported to AQS in parts per
billion (ppb) with decimal digits after the first decimal place truncated are used in design value
calculations. If multiple monitors collect measurements at the same site, one monitor is
designated as the primary monitor. Measurement data collected with the primary monitor are
used to calculate the design value and may be supplemented with data from collocated monitors
only if (a) the primary monitor did not collect sufficient data to determine a valid design value,
or (b) the primary monitor has been discontinued and replaced by another monitor.
The design value for the primary annual NO2 NAAQS is simply the mean of all hourly
concentration values reported for a single year, rounded to the nearest integer in ppb. The annual
design value is considered valid when hourly concentrations are reported for at least 75% of the
hours in the year, or if the design value is greater than 53 ppb, the level of the NAAQS. The
22 Quality assurance requirements for monitors used in evaluations of the NAAQS are provided in Appendix A to 40
CFR Part 58. Annual summary reports of precision and bias can be obtained for each monitoring site at the EPA's
Air Data website.
A-4
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primary annual NO2 NAAQS is met at a site when the valid annual primary standard design
value is less than or equal to 53 ppb.
For the 1-hour NO2 NAAQS, the maximum hourly concentration is determined for each
day (i.e., the "daily maximum value") in a given 3-year period. For each year, the 98th percentile
of the daily maximum values is determined, as described in Appendix S, and the design value is
the average of the three consecutive annual 98th percentile values, rounded to the nearest integer
in ppb. The 1-hour NO2 NAAQS is met when the valid 1-hour primary standard design value is
less than or equal to 100 ppb, the level of the NAAQS.
In addition, the 1-hour design value must meet data completeness requirements in order
to be considered valid. Specifically, a sampling day is considered complete when at least 75%
(i.e., 18) hourly measurements are reported. For each calendar quarter, the quarter is considered
complete if at least 75% of the sampling days in the quarter have complete data. The 1-hour NO2
design value is considered complete when all 12 calendar quarters in the 3-year period have
complete data. In addition, there are two data substitution tests specified in Appendix S to 40
CFR Part 50 which may be used to yield a valid design value above or below the NAAQS,
respectively, in the event that a site falls short of the minimum data completeness requirement.
A.3 NO2 CONCENTRATIONS MEASURED AT AMBIENT AIR
MONITORING SITES ACROSS THE U.S.
Table A-l below presents summary statistics based on the two daily NO2 NAAQS
metrics: the daily maximum 1-hour (MDA1) metric and the daily 24-hour average (DA24)
metric. These statistics are presented for year-round and each season (winter=Dec/Jan/Feb,
spring=Mar/Apr/May, summer=Jun/Jul/Aug, autumn=Sep/Oct/Nov) based on data reported to
AQS for 2020-2022. Table A-2 presents the same summary statistics for the MDA1 and DA24
metrics for each NOAA Climate Region.23 Finally, Table A-3 presents the same set of summary
statistics for the two daily NO2 metrics based on three types of sites: near-road sites, urban
NCore and PAMS sites, and rural sites.
23 A map of the NOAA climate regions is available at:
https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/regional_monitoring/regions.shtml. For Table A-2,
monitoring sites in Alaska were assigned to the Northwest Region, monitoring sites in Hawaii were assigned to
the West region, and monitoring sites in Puerto Rico were assigned to the Southeast region.
A-5
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Table A-l. National distribution of NO2 concentrations in ppb by season for 2020-2022,24 (Source: https://www.epa.gov/aqs)
metric
season
N.sites
N.obs
mean
SD
min
P1
p5
p10
p25
p50
p75
p90
p95
p98
p99
max
max.site
MDA1
all
404
419482
16.5
12.3
-3.4
0.6
1.7
2.9
6.5
14.0
24.3
34.3
39.9
45.9
50.1
315.3
201950001
MDA1
winter
394
101627
20.3
13.3
-1.0
0.7
2.0
3.8
9.1
19.0
29.9
38.5
43.2
49.0
53.6
109.2
295100094
MDA1
spring
391
103552
15.2
11.8
-3.0
0.5
1.3
2.3
5.5
12.2
22.6
32.7
38.2
44.0
48.0
107.4
191770006
MDA1
summer
392
103040
12.9
9.8
-3.0
0.7
1.6
2.5
5.3
10.5
18.3
26.9
32.4
38.7
43.0
84.0
340390004
MDA1
autumn
393
102607
17.8
12.7
-3.4
0.7
1.8
3.0
7.5
15.7
25.8
35.5
41.2
47.8
52.6
315.3
201950001
DA24
all
404
419482
7.8
7.0
-4.5
0.1
0.7
1.2
2.7
5.8
11.0
17.4
22.0
27.5
31.4
64.1
060374008
DA24
winter
394
101627
10.4
8.3
-1.8
0.1
0.8
1.6
4.0
8.5
14.9
22.0
26.6
32.2
36.1
64.1
060374008
DA24
spring
391
103552
6.6
5.9
-3.9
0.0
0.5
1.0
2.3
4.8
9.1
14.7
18.4
23.3
26.5
57.7
060374008
DA24
summer
392
103040
5.9
5.1
-4.1
0.0
0.6
1.1
2.3
4.4
8.0
12.7
16.0
20.4
23.6
53.6
530330030
DA24
autumn
393
102607
8.6
7.3
-4.5
0.1
0.7
1.3
3.1
6.7
12.1
18.6
23.1
28.4
32.2
62.0
060374008
N.sites = number of sites; N.obs = number of observations; SD = standard deviation; min = minimum; p1, p5, p10, p25, p50, p90, p95, p98, p99 = 1st, 5th, 10th, 25th, 50th, 90th,
95th, 98th, 99th percentiles; max = maximum; max.site = AQS ID number for the monitoring site corresponding to the observation in the max column, winter =
December/January/February; spring = March/April/May; summer = June/July/August; autumn = September/October/November.
24 Negative concentration values may appear in AQS datasets down to the negative of the lower detection limit (LDL) to allow for normal instrument variability
at very low concentrations. Data that exceed the negative of the LDL are typically indicative of a malfunction or another issue that affects the data
defensibility.
A-6
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According to Table A-l, NO2 concentrations are generally higher during the autumn and
winter months and lower during the spring and summer months. This is at least partially due to
NOx budgeting programs such as CSAPR which are designed to reduce NOx emissions from
stationary sources during the spring and summer months when these emissions are most likely to
contribute to elevated ozone concentrations. Table A-2 shows that measured NO2 concentrations
are comparable across most regions of the U.S., except for the West North Central region, which
includes more rural states in the northern Rocky Mountains and Great Plains, where NO2
concentrations are significantly lower. Finally, Table A-3 shows that near-road NO2 monitoring
sites tend to measure slightly higher concentrations than typical urban NO2 sites, while NO2
concentrations measured at rural sites are typically much lower than those measured in urban
areas. The high maximum MDA1 concentration measured at a rural site in Kansas appears to be
an isolated occurrence whose cause is unknown.
A-7
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Table A-2. National distribution of NO2 concentrations in ppb by climate region for 2020-2022.25 (Source:
https://www. epa.gov/aqs)
metric
region
season
N.sites
N.obs
mean
SD
min
P1
p5
p10
p25
p50
p75
p90
p95
p98
p99
max
max.site
MDA1
all
all
404
419482
16.5
12.3
-3.4
0.6
1.7
2.9
6.5
14.0
24.3
34.3
39.9
45.9
50.1
315.3
201950001
MDA1
C
all
32
33617
18.7
11.2
-0.6
1.6
3.5
5.1
9.7
17.0
26.3
34.2
39.0
44.6
48.0
109.2
295100094
MDA1
E-N-C
all
16
16554
16.5
10.8
0.1
0.9
1.7
2.8
7.9
15.3
23.7
31.3
36.0
41.3
44.2
107.4
191770006
MDA1
NE
all
62
63993
18.4
11.6
-3.0
1.0
3.0
4.2
9.0
17.0
26.1
34.5
39.3
44.9
48.8
97.1
230050029
MDA1
NW
all
6
6420
21.3
9.7
1.6
3.9
6.6
9.3
14.2
20.7
27.5
33.7
38.1
43.5
47.7
78.0
530330030
MDA1
S
all
67
67833
14.1
10.9
-1.8
0.7
2.0
3.0
5.7
11.1
20.0
30.1
35.8
41.8
46.2
315.3
201950001
MDA1
SE
all
37
38279
16.1
10.3
-1.0
0.9
2.2
3.9
7.8
14.7
23.0
30.5
35.0
40.0
43.5
94.8
120110035
MDA1
SW
all
54
56427
17.5
13.7
-3.4
0.6
1.5
2.8
6.3
13.8
26.6
38.5
43.7
49.5
53.9
86.5
080310002
MDA1
W
all
97
102331
19.4
13.2
-2.0
1.0
2.4
4.1
8.6
16.9
28.2
38.2
43.4
49.7
54.6
101.6
060710027
MDA1
W-N-C
all
33
34028
5.2
5.7
-1.8
0.0
0.6
0.9
1.5
3.1
6.6
12.6
17.4
23.1
27.6
79.9
300310017
DA24
all
all
404
419482
7.8
7.0
-4.5
0.1
0.7
1.2
2.7
5.8
11.0
17.4
22.0
27.5
31.4
64.1
060374008
DA24
C
all
32
33617
8.9
6.2
-1.3
0.8
1.5
2.3
4.2
7.5
12.2
17.6
21.2
25.1
27.9
44.0
170310076
DA24
E-N-C
all
16
16554
8.1
5.7
-0.1
0.4
0.9
1.5
3.6
7.0
11.6
16.0
19.0
22.4
24.5
44.6
261630100
DA24
NE
all
62
63993
8.9
6.7
-4.1
0.4
1.3
2.0
3.8
7.3
12.5
18.1
22.1
26.8
30.5
55.8
340130003
DA24
NW
all
6
6420
10.8
5.6
1.0
2.0
3.0
4.1
6.5
10.1
14.3
18.1
20.6
23.8
26.3
53.6
530330030
DA24
S
all
67
67833
6.1
5.2
-2.3
-0.2
0.7
1.2
2.4
4.5
8.2
13.2
16.8
21.1
23.9
47.5
482011052
DA24
SE
all
37
38279
7.6
5.6
-1.6
0.5
1.0
1.7
3.2
6.2
10.6
15.5
18.7
22.5
25.2
39.7
510130020
DA24
SW
all
54
56427
8.1
8.1
-4.5
0.1
0.6
1.1
2.4
5.3
11.1
19.9
25.7
32.0
36.3
60.5
080310028
DA24
W
all
97
102331
9.7
8.0
-2.0
0.1
1.0
1.9
3.7
7.3
13.5
21.2
26.0
31.5
35.4
64.1
060374008
DA24
W-N-C
all
33
34028
1.9
2.0
-2.2
-0.1
0.1
0.3
0.7
1.3
2.4
4.2
5.6
7.9
10.0
26.4
560070009
N.sites = number of sites; N.obs = number of observations; SD = standard deviation; min = minimum; p1, p5, p10, p25, p50, p90, p95, p98, p99 = 1st, 5th, 10th, 25th,
50th, 90th, 95th, 98th, 99th percentiles; max = maximum; max.site = AQS ID number for the monitoring site corresponding to the observation in the max column.
Central (C) = Illinois, Indiana, Kentucky, Missouri, Ohio, Tennessee, West Virginia; East North Central (E-N-C) = Iowa, Minnesota, Michigan, Wisconsin; Northeast
(NE) = Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont; Northwest (NW) =
25 Negative concentration values may appear in AQS datasets down to the negative of the lower detection limit (LDL) to allow for normal instrument variability
at very low concentrations. Data that exceed the negative of the LDL are typically indicative of a malfunction or another issue that affects the data
defensibility.
A-8
-------
Alaska, Idaho, Oregon, Washington; South (S) = Arkansas, Kansas, Louisiana, Mississippi, Oklahoma, Texas; Southeast (SE) = Alabama, Florida, Georgia, North
Carolina, South Carolina, Virginia; Southwest (SW) = Arizona, Colorado, New Mexico, Utah; West (W) = California, Hawaii, Nevada; West North Central (W-N-C) =
Montana, Nebraska, North Dakota, South Dakota, Wyoming.
Table A-3. National distribution of NO2 concentrations in ppb by site type for 2020-2022.26 (Source: https://www.epa.gov/aqs)
metric
site.type
season
N.sites
N.obs
mean
SD
Min
P1
p5
p10
p25
p50
p75
p90
p95
p98
p99
max
max.site
MDA1
All Sites
all
404
419482
16.5
12.3
-3.4
0.6
1.7
2.9
6.5
14.0
24.3
34.3
39.9
45.9
50.1
315.3
201950001
MDA1
Near Road
all
64
66554
24.9
12.0
-0.3
4.1
8.0
10.6
16.0
23.4
32.2
41.0
46.3
53.1
58.2
109.2
295100094
MDA1
NCore/PAMS
all
57
59041
16.8
11.2
-3.0
1.0
3.0
4.5
8.0
14.1
23.5
33.2
38.5
44.2
48.4
92.3
481410044
MDA1
Rural Sites
all
39
39999
4.9
6.2
-0.9
0.0
0.7
1.0
1.5
2.9
5.7
11.2
17.0
25.0
30.0
315.3
201950001
DA24
All Sites
all
404
419482
7.8
7.0
-4.5
0.1
0.7
1.2
2.7
5.8
11.0
17.4
22.0
27.5
31.4
64.1
060374008
DA24
Near Road
all
64
66554
13.7
8.0
-1.2
1.8
3.7
5.0
7.9
12.1
17.8
24.5
29.2
35.1
39.2
64.1
060374008
DA24
NCore/PAMS
all
57
59041
7.4
5.7
-4.1
0.1
1.2
1.9
3.4
5.9
9.8
15.0
18.7
23.9
27.1
55.8
340130003
DA24
Rural Sites
all
39
39999
2.0
2.4
-1.1
0.0
0.2
0.3
0.7
1.3
2.3
4.4
6.5
9.9
12.7
35.2
081230013
N.sites = number of sites; N.obs = number of observations; SD = standard deviation; min = minimum; p1, p5, p10, p25, p50, p90, p95, p98, p99 = 1st, 5th, 10th, 25th, 50th,
90th, 95th, 98th, 99th percentiles; max = maximum; max.site = AOS ID number for the monitoring site corresponding to the observation in the max column.
26 Negative concentration values may appear in AQS datasets down to the negative of the lower detection limit (LDL) to allow for normal instrument variability
at very low concentrations. Data that exceed the negative of the LDL are typically indicative of a malfunction or another issue that affects the data
defensibility.
A-9
-------
Figure A-2 below shows a map of the annual NO2 design values at U.S. ambient air
monitoring sites based on data from 2022 and Figure A-3 shows a map of the 1-hour NO2 design
values based on data from the 2020-2022 period. There were no sites with design values
exceeding either NAAQS. The maximum annual design value was 29 ppb, while the maximum
1-hour design value was 79 ppb. Both of these maximum design values occurred at near-road
sites in the Los Angeles, CA metropolitan area.
• 0 -10 ppb (289 sites) O 11 - 20 ppb (105 sites) O 21 - 30 ppb (8 sites)
Figure A-2. Annual NO2 design values in ppb based on data from 2022. (Source:
https: uw w.epa.gov aqs)
A-10
-------
• 3 - 25 ppb (70 sites) O 26 -50 ppb (229 sites) O 51 - 75 ppb (39 sites) €> 76 - 100 ppb (1 site)
Figure A-3. 1-hour NO2 design values in ppb for the 2020-2022 period. (Source:
https: uw w.epa.gov aqs)
Figure A-4 below shows a map of the site-level trends in the annual NO2 design values at
U.S. monitoring sites having valid design values in at least 18 years from 2000 through 2022.
Figure A-5 shows a map of the site-level trends in the 1-hour NO2 design values at U.S.
monitoring sites having valid design values in at least 16 of the 21 3-year periods from 2000
through 2022. The trends were computed using the Thiel-Sen estimator, and tests for
significance (p-value < 0.05) were computed using the Mann-Kendall test. From these figures it
is apparent that NO2 concentrations have been decreasing at nearly all sites in the U.S., which is
in part due to federal and state programs designed to reduce NOx emissions from electricity
generation, industrial and mobile sources. Two sites in North Dakota showed an increasing trend
in the annual design value (one of these sites also had an increasing trend in the 1-hour design
value), which is likely due to an increase in NOx emissions from oil and gas extraction activity
in the region.
A-ll
-------
~ Decreasing > 0.5 ppb/yr (68 sites) ° No Significant Trend (5 sites)
v Decreasing < 0.5 ppb/yr (134 sites) A Increasing < 0.5 ppb/yr (2 sites)
Figure A-4. Site-level trends in annual NO2 design values based on data from 2000
through 2022. (Source: Imps: uw w.epa.gov aqs. trends computed using R
statistical software)
A-12
-------
~ Decreasing > 1 ppb/yr (84 sites) ° No Significant Trend (4 sites)
v Decreasing < 1 ppb/yr (46 sites) A Increasing < 1 ppb/yr (1 sites)
Figure A-5. Site-level trends in 1-hour NO2 design values based on data from 2000
through 2022. (Source: Imps: uw w.epa.gov aqs. trends computed using R
statistical software)
Figure A-6 below shows the national trends in the annual and 1-hour NO2 design values
based on the 209 sites shown in Figure A-4 and the 135 sites shown in Figure A-5, respectively.
The national median of the annual design values has decreased by 54% from about 15.7 ppb in
2000 to about 7.3 ppb in 2022. The national median of the 1-hour design values has decreased by
38% from 60 ppb in 2000 to 37 ppb in 2022.
A-13
-------
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concentrations that exceeded the level of the annual NO2 NAAQS since 1991. The size of the
NO2 monitoring network increased from 1980 through the early 2000s, decreased slightly
between 2002 and 2011, then increased again over the next few years as the near-road network
was implemented. Over the past decade, annual mean and 98th percentile MDA1 NO2
concentrations have been relatively constant at levels well below the NAAQS. This is likely due
to higher concentrations measured at near-road sites offsetting continued reductions in NO2
concentrations associated with reductions in NOx emissions.
270
240
210
180
x 150
Q 120
^in(DN(0(J)OT-CM(OtlOlDNCO(J)0'-CM(O^U)lDNCO(J)O^MCOtlOlDNCOO)OT-CM
OOOOOOCOOOOOCr)Cr>CT)CT>CT>CT)CT)CT)CT)OOOOOOOOOOT-T-T-T-T-T-T-T-T-T-CMC\JC\J
OCDCDCDCDCDOCDCDOOCDCDCDCDiDOOOOOOOOOOOOOOOOOOOOOOO
t— t— t— t— t— t— t— t— t— t— t— t— t— t— t— CMCMCMCMCMC\ICMCvJCvJCMCMCMCNJCNJCMCN(MCMCMCMCMCvJCNJ
Distribution of annual 98th percentile MDA1 NO2 concentrations measured
at U.S. monitoring sites, 1980 to 2022. Boxes represent the median and
interquartile range, whiskers extend to the 1st and 99th percentiles, and values
outside this range are shown as circles. The red line shows the number of NO2
monitoring sites reporting data to the EPA in each year. (Source:
https: uw w.epa.gov aqs)
150
100
50
Figure A-7.
Number of N02 Sites
N02 NAAQS Level
- 450
400
350
250 .
200
A-15
-------
i 25J
500
- 450
- 400
- 350
300
- 250 .
E
z
- 200
150
- 100
— - 50
OT-wn^if)(ONcooiot-wn^iocoNcomo^Mrt'jm(DN(omot-
(OoococcicocococococDtncnOTOTcnaTdicncnOTOOOOOOOOOOT-T-
cr>a>a>a)cna>cna>a>cr>o>cr>aiOTa>oooooooooooo
^^^^>*t-<-^T-T-T-r-r^t-r-r-^t-T-N«CMNN«(NNWWN(MNNMNNN«(NWNN
o o o o o o o
Figure A-8. Distribution of annual mean NO2 concentrations measured at U.S.
monitoring sites, 1980 to 2022. Boxes represent the median and interquartile
range, whiskers extend to the 1st and 99th percentiles, and values outside this
range are shown as circles. The red line shows the number of NO2 monitoring
sites reporting data to the EPA in each year. (Source: https://www.epa.gov/aqs)
A-16
-------
United States Office of Air Quality Planning and Standards EPA-452/R-24-010a
Environmental Protection Health and Environmental Impacts Division March 2024
Agency Research Triangle Park, NC
------- |