1 6560-50-P ENVIRONMENTAL PR OTECTION AGENCY 40 CFR Parts 50 and 58 [EPA-HQ-OAR-2006-0922; FRL-] Primary National Ambient Air Qua lity Standard for Nitrogen Dioxide

AGENCY: Environmental Protection Agency (EPA). ACTION: Final Rule SUMMARY: Based on its review of the air quality criteria for oxides of nitrogen and the primary national ambient air quality standard (NAAQS) for oxides of nitrogen as measured by nitrogen dioxide (NO 2 ), EPA is making revisi ons to the primary NO 2 NAAQS in order to provide requisite protection of public health. Specifically, EP A is establishing a new 1-hour standard at a level of 100 ppb, base d on the 3-year average of the 98 th percentile of the yearly distribution of 1-hour daily maximum concentr ations, to supplement the existing annual standard. EPA is also esta blishing requirements for an NO 2 monitoring network that will include monitors at locations where maximum NO 2 concentrations are exp ected to occur, including within 50 meters of major roadways, as well as monitors sited to measure the area-wide NO 2 concentrations that occur more broadly across communities.

DATES: This final rule is effective on [insert date 60 days after da te of publication in the Federal Register ]

. ADDRESSES EPA has established a docket for th is action under Docket ID No. EPA
- HQOAR-2006-0922. All documents in the docket are listed on the www.regulations.gov website.2 Although listed in the index, some information is not publicly availabl e, e.g., confidential business information or other information whose disclo sure is restricted by statute. Certain other material, such as copyrighted material, will be publicly available only in hard copy form. Publicly available docket materials are available either el ectronically through www.regulations.gov or in hard copy at the Air and Ra diation Docket and Information Center, EPA/DC, EPA West, Room 3334, 1301 Constituti on Ave., NW, Washington, DC. The Public Reading Room is open from 8:30 a.m. to 4: 30 p.m., Monday through Friday, excluding legal holidays. The telephone number for the Public Reading Room is (202) 566-1744 and the telephone number for the Air and Radiation Do cket and Information Center is (202) 566-1742. FOR FURTHER INFORMATION CONTACT: Dr. Scott Jenkins, Health and Environmental Impacts Divisi on, Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency, Mail code C504-06, Research Triangle Park, NC 27711; telephone: 919-541-1167; fax: 919-541-0237; email: jenkins.scott@epa.gov

. SUPPLEMENTARY INFORMATION: Table of Contents The following topics are disc ussed in this preamble: I. Background A. Summary of revisions to the NO 2 primary NAAQS B. Legislative requirements C. Related NO 2 control programs D. Review of the Air Quality Criteria and Standards for Oxides of Nitrogen E. Summary of proposedrevisions to the NO 2 primary NAAQS F. Organization and approach to final NO 2 primary NAAQS decisions II. Rationale for Final Decisions on the NO 2 Primary Standard A. Characterization of NO 2 air quality 1. Current patterns of NO 2 air quality 2. NO 2 air quality and gradients around roadways B. Health effects information 1. Adverse respiratory effects and short-term exposure to NO 23 2. Other effects with short-term exposure to NO 2 a. Mortality b. Cardiovascular effects 3. Health effects with long-term exposure to NO 2 a. Respiratory morbidity b. Mortality c. Carcinogenic, cardiovascular , and reproductive/developmental effects 4. NO 2 -related impacts on public health C. Human exposure and hea lth risk characterization D. Approach for reviewing the need to retain or revise the current standard E. Adequacy of the current standard 1. Rationale for proposed decision 2. Comments on the adequacy of the current standard a. Comments on EPA’s interpretation of the epidemiologic evidence b. Comments on EPA’s interpretation of the controlled human exposure evidence c. Comments on EPA’s characterization of NO 2 -associated exposures and health risks 3. Conclusions on the adequacy of the current standard F. Elements of a new short-term standard 1. Indicator a. Rationale for proposed decision b. Comments on indicator c. Conclusions regarding indicator 2. Averaging time a. Rationale for proposed decision b. Comments on averaging time c. Conclusions on averaging time 3. Form a. Rationale for proposed decision b. CASAC and public comments on form c. Conclusions on form 4. Level a. Rationale for proposed d ecisions on approach and level b. Rationale for alternative deci sions on approach and level c. Comments on approach and level i. CASAC comments on the appro ach to setting the standard ii. Public comments on the appr oach to setting the standard iii. CASAC comments on standard level iv. Public comments on standard level d. Conclusions on approach and standard level G. Annual standard H. Summary of final decisions on the primary NO 2 standard III. Ambient Monitoring and Reporting Requirements4 A. Monitoring Methods 1. Chemiluminescence FRM and Alternative Methods 2. Allowable FRM and FEMs for comparison to the NAAQS a. Proposed changes to FRM and FEMs that may be compared to the NAAQS b. Comments c. Decisions on allowable FRM and FEMs for comparison to the NAAQS B. Network Design 1. Two-tiered Network Design a. Proposed Two-tier Network Design b. Comments c. Conclusions Regarding th e Two-tier Network Design 2. First Tier (Near-road Mon itoring Component) of the NO 2 Network Design a. Proposed First Tier (Near-road Monitoring Component) of the Network Design b. Comments c. Conclusions Regarding the First Tier (Near-road Monitoring Component) of the Network Design 3. Second Tier (Area-wide Monitoring Component) of the Network Design a. Proposed Second Tier (Area-wi de Monitoring Component) of the Network Design b. Comments c. Conclusions on the Second Tier (Area-wide Monitoring Component) of the Network Design 4. Regional Administrator Authority a. Proposed Regional Administrator Authority b. Comments c. Conclusions on Regional Administrator Authority 5. Monitoring Network Implementation a. Proposed Monitoring Networ k Implementation Approach b. Comments c. Conclusions on Monitoring Network Implementation 6. Near-road Site Selection a. Proposed Near-road Site Selection Criterion b. Comments c. Conclusions on Near-road Site Selection 7. Near-road Siting Criteria a. Proposed Near-road Siting Criteria b. Comments c. Conclusions on Near-road Siting Criteria 8. Area-wide Monitor Site Se lection and Siting Criteria a. Proposed Area-wide Monitor Si te Selection and Siting Criteria b. Comments c. Conclusions on Area-wide Mon itor Site Selection and Siting Criteria5 9. Meteorological Measurements a. Proposed Meteorological Measurements b. Comments c. Conclusions on Meteorological Measurements C. Data Reporting 1. Proposed Data Quality Objectives and Measurement Uncertainty 2. Comments 3. Conclusions on Data Quality Objectives and Measurement Uncertainty IV. Appendix S--Interpretation of the Primary NAAQS for Oxides of Nitrogen and Revisions to the Exceptional Events Rule A. Interpretation of the primary NAAQ S for oxides of nitrogen for the annual primary standard 1. Proposed interpretation of the annual standard 2. Comments on interpretati on of the annual standard 3. Conclusions on interpreta tion of the annual standard B. Interpretation of the primary NAAQ S for oxides of nitrogen 1-hour primary standard 1. Proposed interpretation of the 1-hour standard 2. Comments on interpretation of the 1-hour standard 3. Conclu sions on interpretation of the 1-hour standard C. Exceptional Events Information Submission Schedule V. Designation of Areas A. Proposed process B. Public comments C. Final designations process VI. Clean Air Act Implementation Requirements A. Classifications 1. Proposal 2. Public comments 3. Final B. Attainment Dates 1. Attaining the NAAQS a. Proposal b. Final 2. Consequences of Failing to Attain by the Statutory Attainment Date a. Proposal b. Final C. Section 110(a)(2) NAAQS Infr astructure Requirements 1. Proposal 2. Final D. Attainment Planning Requirements 1. Nonattainment Area SIPs6 a. Proposal b. Public comments c. Final 2. New Source Review and Preventi on of Significant Deterioration Requirements a. Proposal b. Public comments c. Final 3. General Conformity a. Proposal 4. Transportation Conformity a. Proposal b. Public comments c. Final VII. Communication of Public Health Information VIII. Statutory and Executive Order Reviews References

I. Background A. Summary of revisions to the NO 2 primary NAAQS Based on its review of the ai r quality criteria for oxides of nitrogen and the primary national ambient air quality standa rd (NAAQS) for oxides of nitrog en as measured by nitrogen dioxide (NO 2 ), EPA is making revisions to the primary NO 2 NAAQS in order to provide requisite protection of public he alth as appropriate under secti on 109 of the Clean Air Act (Act or CAA). Specifically, EPA is supplemen ting the existing annual standard for NO 2 of 53 parts per billion (ppb) by establishing a new short-term standard based on the 3-year average of the
98 th percentile of the yearly distribution of 1-hour daily maximum concentrations. EPA is setting the level of this new standard at 100 ppb. EPA is making changes in data handling conventions for NO 2 by adding provisions for this new 1-hour pr imary standard. EPA is also establishing requirements for an NO 2 monitoring network. These new provisions require monitors at7 locations where maximum NO 2 concentrations are expected to occur, including w ithin 50 meters of major roadways, as well as monitors sited to measure the area-wide NO 2 concentrations that occur more broadly across communities. EPA is making conforming changes to the air quality index (AQI). B. Legislative Requirements Two sections of the CAA govern the establis hment and revision of the NAAQS. Section 108 of the Act directs the Administrator to iden tify and list air pollutants that meet certain criteria, including that the air po llutant “in [her] judgment, caus e[s] or contribute[s] to air pollution which may reasonably be anticipated to endanger public health and welfare” and “the presence of which in the ambient air results fr om numerous or diverse mobile or stationary sources.” 42 U.S.C. 21 7408(a)(1)(A) & (B). Fo r those air pollutants li sted, section 108 requires the Administrator to issue air quality criteria that “accurately reflect the latest scientific knowledge useful in indicating the kind and extent of all identifiable eff ects on public health or welfare which may be expected from the presence of [a] pollutant in ambient air . . .” 42 U.S.C. 7408(2). Section 109(a) of the Act directs the Admi nistrator to promulgate “primary” and “secondary” NAAQS for pollutants for which air quali ty criteria have been issued. 42 U.S.C. 7409(1). Section 109(b)(1) define s a primary standard as one “t he attainment and maintenance of which in the judgment of the Administrator, based on [the air quality] criteria and allowing an adequate margin of safety, are requi site to protect the public health.” 1 42 U.S.C. 7409(b)(1). A

1 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 “r eference should be made to a representative sample of persons comprising the sensitive group ra ther than to a single person in such a group.” S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970)8 secondary standard, in turn, must “specify a le vel of air quality the attainment and maintenance of which, in the judgment of the Administrator, ba sed on [the air quality] crite ria, is requisite to protect the public welfare from any known or anticipated advers e effects associated with the presence of such polluta nt in the ambient air.” 2 42 U.S.C. 7409(b)(2). The requirement that primary standards include an adequate margin of safety is intended to address uncertainties associated with inc onclusive scientific and technical information available at the time of standard setting. It is also intended to provide a reasonable degree of protection against hazards that research has not yet identified. Lead Industries Association v.
EPA , 647 F.2d 1130, 1154 (D.C. Cir 1980), cert. denied , 449 U.S. 1042 (1980); American Petroleum Institute v. Costle , 665 F.2d 1176, 1186 (D.C. Cir. 1981), cert. denied , 455 U.S. 1034 (1982). Both kinds of uncertainties are components of the risk asso ciated 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 th at 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 leve ls that may pose an unacceptable risk of harm, even if the risk is not precisely identified as to nature or degree. In addressing the requirement for a margin of safety, EPA considers such factors as the nature and severity of the heal th effects involved, the size of the at-risk population(s), and the kind and degree of the uncertainties that must be addressed. The selection of any particular approach to providing an adequate margin of safety is a policy choice left specifically to the Administrator’s judgment. Lead Industries Association v. EPA , supra , 647 F.2d at 1161-62.

2 EPA is currently conducting a sepa rate review of the secondary NO 2 NAAQS jointly with a review of the secondary SO 2 NAAQS.9 In setting standards that are “requisite” to protect public health a nd welfare, as provided in section 109(b), EPA’s task is to establish standards that are neither more nor less stringent than necessary for these purposes. In so doing, EPA may not consider th e costs of implementing the standards. Whitman v. American Trucking Associations , 531 U.S. 457, 471, 475-76 (2001). Section 109(d)(1) of the Act requires the Ad ministrator to peri odically undertake a thorough review of the air quali ty criteria published under section 108 and the NAAQS and to revise the criteria and standards as may be appropriate. 42 U. S.C. 7409(d)(1). The Act also requires the Administrator to a ppoint an independent scientific review committee 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, to review the air quality criteria and NAAQS and to “recommend to the Administrator any new ... standards and revisions of existing criteria and standards as may be appropriate under section 108 and subsection (b) of this section." 42 U.S.C. 7409 (d)(2). This independe nt review function is performed by the Clean Air Scientific A dvisory Committee (CASAC) of EPA’s Science Advisory Board.
C. Related NO 2 Control Programs States are primarily responsible for ensuring attainment and maintenance of ambient air quality standards once EPA has established them. Under section 110 of the Act, 42 U.S.C. 7410, and related provisions, States ar e to submit, for EPA approval, St ate implementation plans (SIPs) that provide for the attainment and maintenan ce of such standards through control programs directed to sources of the po llutants involved. The States, in conjunction with EPA, also administer the prevention of signi ficant deterioration program that covers these pollutants. See 42 U.S.C. 7470-7479. In addition, Federal prog rams provide for nationwide reductions in10 emissions of these and other air pollutants und er Title II of the Act, 42 U.S.C. 7521–7574, which involves controls for automobile, truck, bus, mo torcycle, nonroad engine and equipment, and aircraft emissions; the new source performance sta ndards under section 111 of the Act, 42 U.S.C. 7411; and the national emission standards for ha zardous air pollutants u nder section 112 of the Act, 42 U.S.C. 7412. Currently there are no areas in the United Stat es that are designated as nonattainment of the NO 2 NAAQS. With the revisions to the NO 2 NAAQS that result from this review, however, some areas could be classified as non-attainment. Certain States will be required to develop SIPs that identify and implement specific air po llution control measures to reduce ambient NO 2 concentrations to attain and maintain the revised NO 2 NAAQS, most likely by requiring air pollution controls on sources th at emit oxides of nitrogen (NO X 3 ). While NO X is emitted from a wide variety of source types, the top three categories of sources of NO X emissions are on-road mobile sources, electricity generating units, and non-road mobile sources. EPA anticipates that NO X emissions will decrease substantially over the next 20 years as a result of the ongoing implementation of mobile source emissions standards. In particular, Tier 2 NO X emission standards for light-duty vehi cle emissions began phasing into the fleet beginning with model year 2004, in combina tion with low-sulfur gasoline fuel standards. For heavy-duty engines, new NO X standards are phasing in between the 2007 and 2010 model

3 In this document, the terms “oxides of nitrogen” and “nitrogen oxides” (NO X ) refer to all forms of oxidized nitrogen ( N ) compounds, including NO, NO 2 , and all other oxidized N- containing compounds formed from NO and NO 2 . This follows usage in th e Clean Air Act Section 108(c): “Such criteria [for oxides of nitrogen] shall incl ude a discussion of nitr ic and nitrous acids, nitrites, nitrates, nitrosamines, and other carcinogenic and potentia lly carcinogenic derivatives of oxides of nitrogen.” By contrast, within the ai r pollution research and control communities, the terms “oxides of nitrogen” and “nitrogen oxides” are restricted to refer only to the sum of NO and NO 2 , and this sum is commonly abbreviated as NO X . The category label used by this community for the sum of all forms of oxidized nitrogen compounds in cluding those listed in Section 108(c) is NO Y .11 years, following the introduc tion of ultra-low sulfur diesel fuel. Lower NO X standards for nonroad diesel engines, locomotives, and certa in marine engines ar e becoming effective throughout the next decade. In future decades, these lower- NO X vehicles and engines will become an increasingly large fraction of in-use mobile sources, effecting large NO X emission reductions. D. Review of the Air Quality Criteria and Standards for Oxides of Nitrogen On April 30, 1971, EPA promulgated identi cal primary and secondary NAAQS for NO 2 under section 109 of the Act. The standards we re set at 0.053 parts per million (ppm) (53 ppb), annual average (36 FR 8186). EPA completed re views of the air qual ity criteria and NO 2 standards in 1985 and 1996 with decisions to re tain the standard (50 FR 25532, June 19, 1985; 61 FR 52852, October 8, 1996). EPA initiated the current review of the air quality criteria for oxides of nitrogen and the
NO 2 primary NAAQS on December 9, 2005 (70 FR 73236) with a general call for information. EPA’s draft Integrated Review Plan for the Primary National Ambient Air Quality Standard for Nitrogen Dioxide (EPA, 2007a) was made availa ble in February, 2007 for public comment and was discussed by the CASAC via a publicly acc essible teleconference on May 11, 2007. As noted in that plan, NO X includes multiple gaseous (e.g., NO 2 , NO) and particulate (e.g., nitrate) species. Because the health effects asso ciated with particulate species of NO X have been considered within the context of the health effect s of ambient particles in the Agency’s review of the NAAQS for particulate matter (PM), th e current review of the primary NO 2 NAAQS is focused on the gaseous species of NO X and is not intended to address health effects directly associated with particulate species.12 The first draft of the Integrated Science Assessment for Oxides of Nitrogen-Health Criteria (ISA) and the Nitroge n Dioxide Health Assessment Plan: Scope and Methods for Exposure and Risk Assessment (EPA, 2007b) were reviewed by CASAC at a public meeting held on October 24-25, 2007. Based on comments re ceived from CASAC and the public, EPA developed the second draft of the ISA and the firs t draft of the Risk and Exposure Assessment to Support the Review of the NO 2 Primary National Ambient Air Quality Standard (Risk and Exposure Assessment (REA)). These documents were reviewed by CASAC at a public meeting held on May 1-2, 2008. Based on comments receiv ed from CASAC and the public at this meeting, EPA released the final ISA in July of 2008 (EPA, 2008a). In addition, comments received were considered in developing the s econd draft of the REA, which was released for public review and comment in two parts. The firs t part of this document, containing chapters 1- 7, 9 and appendices A and C as well as part of appendix B, was released in August 2008. The second part of this document, containing chapte r 8 (describing the Atla nta exposure assessment) and a completed appendix B, was released in Oc tober of 2008. This document was the subject of CASAC reviews at public meetings on Septem ber 9 and 10, 2008 (for the first part) and on October 22, 2008 (for the second part). In preparing the final REA (EPA, 2008b), EPA considered comments received from the CASAC and the public at those meetings. In the course of reviewing the second draf t REA, CASAC expresse d the view that the document would be incomplete without the additi on of a policy assessment chapter presenting an integration of evidence-based considerations and risk and exposure assessment results. CASAC stated that such a chapter would be “critic al for considering options for the NAAQS for NO 2 ” (Samet, 2008a). In addition, within the period of CASAC’s review of the second draft REA, EPA’s Deputy Administrator indicated in a lette r to the chair of CASAC, addressing earlier13 CASAC comments on the NAAQS review process (H enderson, 2008), that the risk and exposure assessment will include “a broader discussion of the science and how uncertainties may effect decisions on the standard” and “all analyses a nd approaches for considering the level of the standard under review, including risk assessm ent and weight of ev idence methodologies” (Peacock, 2008, p.3; September 8, 2008). Accordingly, the final REA included a new policy assessment chapter. This policy assessment chapter considered th e scientific eviden ce in the ISA and the exposure and risk characterization results presented in other chapters of the REA as they relate to the adequacy of the current NO 2 primary NAAQS and potential alternative primary NO 2 standards. In considering the current and potenti al alternative standards, the policy assessment chapter of the final REA focused on the information that is most pertinent to evaluating the basic elements of national ambient air quality standards: indicator, averaging time, form 4 , and level. These elements, which together serve to define each standard, must be considered collectively in evaluating the health protection afforded. CASAC discussed the final version of the REA, with an emphasis on the policy assessment chapter, during a public teleconference held on December 5, 2008. Following that teleconference, CASA C offered comments and advice on the NO 2 primary NAAQS in a letter to th e Administrator (Samet, 2008b). The schedule for completion of this review is governed by a judici al order resolving a lawsuit filed in September 2005, concerning the ti ming of the current review. The order that now governs this review, entered by the cour t in August 2007 and amended in December 2008, provides that the Administrator will sign, fo r publication, notices of proposed and final rulemaking concerning the review of the primary NO 2 NAAQS no later than June 26, 2009 and

4 The “form” of a standard defines the air quality st atistic that is to be compared to the level of the standard in determining whethe r an area attains the standard.14 January 22, 2010, respectively. In accordance with this schedule, the Administrator signed a notice of proposed rulemaking on June 26, 2009 (FR 74 34404). This action presents the Administrator’s final de cisions on the primary NO 2 standard. E. Summary of proposed revisions to the NO 2 primary NAAQS For the reasons discussed in the pr eamble of the proposal for the NO 2 primary NAAQS (74 FR 34404), EPA proposed to make revisions to the primary NO 2 NAAQS and to make related revisions for NO 2 data handling conventions in order to provide requisite protection of public health. EPA also proposed to ma ke corresponding changes to the AQI for NO 2 . Specifically, EPA proposed to supplement the cu rrent annual standard by establishing a new short-term NO 2 standard that would refl ect the maximum allowable NO 2 concentration anywhere in an area. EPA proposed that this new shor t-term standard would be based on the 3-year average of the 99 th percentile (or 4 thhighest) of the yearly distri bution of 1-hour daily maximum
NO 2 concentrations and solicited comment on using the 3-year average of the 98 th percentile (or
7 th or 8 th highest) of the yearly distri bution of 1-hour daily maximum NO 2 concentrations. EPA proposed to set the leve l of this new 1-hour standard with in the range of 80 to 100 ppb and solicited comment on standard levels as low as 65 ppb and as high as 150 ppb. EPA proposed to specify the level of the standard to the nearest ppb. EPA also proposed to establish requirements for an NO 2 monitoring network at locations where maximum NO 2 concentrations are expected to occur, including monitors within 50 meters of major roadways, as well as area-wide monitors sited to measure the NO 2 concentrations that can occur mo re broadly across communities. EPA also solicited comment on the alte rnative approach of setting a 1-hour standard that would reflect the allowable area-wide NO 2 concentration. F. Organization and approach to final NO 2 primary NAAQS decisions15 This action presents the Administrator’s final decisions regarding the need to revise the
current NO 2 primary NAAQS. Revisions to the primary NAAQS for NO 2 , and the rationale supporting those revisions, are described belo w in section II. Requirements for the NO 2 ambient monitoring network are described insection III. Related requirements for data completeness, data handling, data reporting, r ounding conventions, and exceptional events are described in section IV. Implementation of the revised NO 2 primary NAAQS is discussed in sections V and VI. Communication of public hea lth information through the AQI is discussed in section VII and a discussion of statutory and executive or der reviews is provided in section VIII. Today's final decisions are based on a t horough review in the ISA of scientific information on known and potential human health effects associated with exposure to NO 2 in the air. These final decisions also take into account: (1) assessments in the REA of the most policyrelevant information in the ISA as well as quant itative exposure and risk analyses based on that information; (2) CASAC Panel advice and recomme ndations, as reflected in its letters to the Administrator and its public discussions of the ISA, the REA, and the notice of proposed rulemaking; (3) public comments received during the development of ISA and REA; and (4) public comments received on the proposed rulemaking. Some commenters have referred to and discussed individual scientific analyses on the health effects of NO 2 that were not included in the IS A (EPA, 2008a) (“new studies”). In considering and responding to comments for whic h such “new studies” were cited in support, EPA has provisionally considered th e cited studies in the context of the findings of the ISA. As in prior NAAQS reviews, EPA is basing its decision in this review on studies and related information included in the ISA and staff’s policy assessment, which have undergone CASAC and public review. In this NO 2 NAAQS review, staff’s policy assessment was16 presented in the form of a po licy assessment chapter of the REA (EPA, 2008b). The studies assessed in the ISA and REA, and the integration of the scientific evidence presented in them, have undergone extensive critical review by EPA, CASAC, an d the public. The rigor of that review makes these studies, a nd their integrative assessment, the most reliable source of scientific information on which to base deci sions on the NAAQS, decisi ons that all parties recognize as of great import. NAAQS decisions can have profound impacts on public health and welfare, and NAAQS decisions shou ld be based on studies that ha ve been rigorous ly assessed in an integrative manner not only by EPA but also by the statutorily mandated independent advisory committee, as well as the public revi ew that accompanies this process. EPA’s provisional consideration of “new studies” did not and could not provide that kind of in-depth critical review. This decision is consistent with EPA’ s practice in prior NAAQS reviews and its interpretation of the requiremen ts of the CAA. Since the 1970 amendments, the EPA has taken the view that NAAQS decisions are to be based on scientific studies and related information that have been assessed as a part of the pertinent air quality criteria, and has consistently followed this approach. This longstanding interpre tation was strengthened by new legislative requirements enacted in 1977, which added sect ion 109(d)(2) of the Act concerning CASAC review of air quality criteri a. See 71 FR 61144, 61148 (Octob er 17, 2006) (final decision on review of PM NAAQS) for a detailed discussion of this issue and EPA’s past practice. As discussed in EPA’s 1993 decision not to revise the NAAQS for ozone (O 3 ), ‘‘new studies” may sometimes be of such significance th at it is appropriate to delay a decision on revision of a NAAQS and to supplement the pertin ent air quality criteria so the studies can be taken into account (58 FR at 13013–13014, March 9, 1993). In the present case, EPA’s17 provisional consideratio n of ‘‘new studies” concludes that , taken in context, the ‘‘new’’ information and findings do not materially chan ge any of the broad sc ientific conclusions regarding the health effects of NO 2 made in the air quality crite ria. For this reason, reopening the air quality criteria review would not be warr anted even if there were time to do so under the court order governing the sche dule for this rulemaking. Accordingly, EPA is basing the final decisions in this review on the studies and related information included in the NO 2 air quality criteria that have undergone CASAC and public review. EPA will consider the “new studies” for purposes of decision-making in the next periodic review of the NO 2 NAAQS, which will provide the oppor tunity to fully assess these studies through a more rigorous review process involving EPA, CASAC, and the public. Further discussion of these ‘‘new studies” can be found be low, in section II.E, and in the Response to Comments document. II. Rationale for Final Decisions on the NO 2 Primary Standard This section presents the rationale for the Ad ministrator’s decision to revise the existing
NO 2 primary standard by supplementing the current annual standard with a new 1-hour standard. In developing this rationale, EPA has drawn upon an integrative synthesis of the entire body of evidence on human health effects asso ciated with the presence of NO 2 in the air. As summarized below in section II.B, this body of evidence ad dresses a broad range of health endpoints associated with exposure to NO 2 . In considering this entire body of evidence, EPA focuses in particular on those health endpoints for which the ISA finds associations with NO 2 to be causal or likely causal. This rationa le also draws upon the results of quantitative exposure and risk assessments, summarized below in section II.C.18 As discussed below, a substa ntial amount of new research has been conducted since the last review of the NO 2 NAAQS, with important new information coming from epidemiologic studies in particular. The newl y available research studies eval uated in the ISA have undergone intensive scrutiny through multiple layers of p eer review and opportunitie s for public review and comment. While important uncertainties rema in in the qualitative and quantitative characterizations of health effects at tributable to exposure to ambient NO 2 , the review of this information has been extensive and deliberate. The remainder of this section provides background information that informed the Administrator’s decisions on the primary standard and discusses the rationale for those decisions. Section II.A presents a discussion of NO 2 air quality. Section II.B includes an overview of the scientific evidence related to h ealth effects associated with NO 2 exposure. This overview includes discussion of the healthendpoints and at-risk populations considered in the ISA. Section II.C discusses the appr oaches taken by EPA to assess exposures and health risks associated with NO 2 , including a discussion of key resu lts. Section II.D summarizes the approach that was used in the current review of the NO 2 NAAQS with regard to consideration of the scientific evidence and expos ure-/risk-based result s related to the ade quacy of the current standard and potential alternative standards. Sections II.E-II.G disc uss the Administrator’s decisions regarding the adequacy of the current standard, elements of a new 1-hour standard, and retention of the current annual standard, respectively, taking in to consideration public comments on the proposed decisions. Secti on II.H summarizes the Administra tor’s decisions with regard to the NO 2 primary NAAQS. A. Characterization of NO 2 air quality 1. Current patterns of NO 2 air quality19 The size of the State and local NO 2 monitoring network has remained relatively stable since the early 1980s, and currently has approximately 400 monito rs reporting data to EPA’s Air Quality System (AQS) database. 5 At present, there are no minimum monitoring requirements for
NO 2 in 40 CFR part 58 Appendix D, other than a requirement for EPA Regional Administrator approval before removing any existin g monitors, and that any ongoing NO 2 monitoring must have at least one monitor sited to m easure the maximum concentration of NO 2 in that area (though, as discussed below monitors in the curren t network do not measure peak concentrations associated with on-road mobile sources that can occur near major roadways because the network was not designed for this purpose). EPA removed the specific minimum monitoring requirements for NO 2 of two monitoring sites per area w ith a population of 1,000,000 or more in the 2006 monitoring rule revisions (71 FR 61236) , based on the fact that there were no NO 2 nonattainment areas at that time, coupled with trends evidence showing an increasing gap between national average NO 2 concentrations and the current annual standard. Additionally, the minimum requirements were removed to provide Stat e, local, and tribal air monitoring agencies flexibility in meeting higher priority monitoring needs for pollutants such as O 3 and PM 2.5 , or implementing the new multi-pollutant sites (NCo re network) required by the 2006 rule revisions, by allowing them to discontinue lower priority monitoring. There are requirements in 40 CFR part 58 Appendix D for NO 2 monitoring as part of the P hotochemical Assessment Monitoring Stations (PAMS) network. Howe ver, of the approximately 400 NO 2 monitors currently in

5 It should be noted that the ISA (section 2.4.1) re ferences a different number of active monitors in the NO 2 network. The discrepancy between the ISA nu mbers and the number presented here is due to differing metrics used in pulling data from AQS. The ISA only references SLAMS, NAMS, and PAMS sites with defined monito ring objectives, while Watkins and Thompson (2008) considered all NO 2 sites reporting data at any poi nt during the year. Based on this approach, Watkins and Thompson (2008) also noted that the size of the NO 2 monitoring network has remained relatively stab le since the early 1980s.20 operation, only about 10 percent may be due to the PAMS requirements. An analysis of the approximately 400 monitors comprising the current NO 2 monitoring network (Watkins and Thompson, 2008) indicates that the current NO 2 network has largely remained unchanged in terms of size and targ et monitor objective categories since it was introduced in the May 10, 1979 mo nitoring rule (44 FR 27571). The review of the current network found that the assessment of concentr ations for general population exposure and maximum concentrations at neighborhood and larger scales were the top obj ectives. A review of the distribution of listed spatial scales of re presentation shows that only approximately 3 monitors are described as microscale, representi ng an area on the order of several meters to 100 meters, and approximately 23 monitors are described as middle scale, which represents an area on the order of 100 to 500 meters. This low percenta ge of smaller spatiall y representative scale sites within the network of approximately 400 m onitoring sites indicates that the majority of monitors have, in fact, been sited to assess area-wide expos ures on the neighborhood, urban, and regional scales, as would be expected for a network sited to suppor t the current annual NO 2 standard and PAMS objectives. The current ne twork does not include monitors placed near major roadways and, therefore, monitors in th e current network do not necessarily measure the maximum concentrations that can occur on a localiz ed scale near these roadways (as discussed in the next section). It shouldbe noted that the network not only accommodates NAAQS related monitoring but also serves other monitoring objectives, such as support for photochemistry analysis, O 3 modeling and forecasting, and part iculate matter precursor tracking.
2. NO 2 air quality and gradients around roadways On-road and non-road mobile sources account for approximately 60% of NO X emissions (ISA, table 2.2-1) and traffic-related expos ures can dominate personal exposures to NO 2 (ISA21 section 2.5.4). While driving, pers onal exposure concentrations in the cabin of a vehicle could be substantially higher than ambient concentra tions measured nearby (ISA, section 2.5.4). For example, estimates presented in the REA suggest that on/near roadway NO 2 concentrations could be approximately 80% (REA, section 7.3.2) higher on average acr oss locations than concentrations away from roadways and that roadway-associated environments could be responsible for the majority of 1-hour peak NO 2 exposures (REA, Figur es 8-17 and 8-18). Because monitors in the current network are no t sited to measure peak roadway-associated NO 2 concentrations, individuals w ho spend time on and/or near major roadways could experience
NO 2 concentrations that are consid erably higher than indicated by monitors in the current areawide NO 2 monitoring network. Research suggests that the concentrations of on-road mobile source pollutants such as
NO X , carbon monoxide (CO), directly emitted air t oxics, and certain size distributions of particulate matter (PM), such as ultrafine PM, typical ly display peak concentrations on or immediately adjacent to roads (ISA, section 2.5). This situation typically produces a gradient in pollutant concentrations, with concentrations de creasing with increasing distance from the road, and concentrations generally decreasing to near area-wide ambient leve ls, or typical upwind urban background levels, within a few hundred me ters downwind. While such a concentration gradient is present on almost all roads, the charac teristics of the gradient , including the distance from the road that a mobile source pollutant signature can be differentiated from background concentrations, are heavily dependent on fact ors such as traffic volumes, local topography, roadside features, meteorology, and photochemical reactivity conditions (Baldauf, et al., 2009; Beckerman et al., 2008; Clements et al., 2008; Ha gler et al., 2009; Jansse n et al., 2001; Rodes and Holland, 1981; Roorda-Knape et al., 1998; Singer et al., 2004; Zhou and Levy, 2007).22 Because NO 2 in the ambient air is due largely to the atmospheric oxidation of NO emitted from combustion sources (ISA, section 2.2.1), elevated NO 2 concentrations can extend farther away from roadways than the primary pollutant s also emitted by on-road mobile sources. More specifically, review of the techni cal literature suggests that NO 2 concentrations may return to area-wide or typical urban backgr ound concentrations within distan ces up to 500 meters of roads, though the actual distance will vary with topography, roadside features, meteorology, and photochemical reactivity conditions (Baldauf et al., 2009; Beckerman et al., 2008; Clements et al., 2008; Gilbert et al. 2003; Rodes and Holla nd, 1981; Singer et al ., 2004; Zhou and Levy, 2007). Efforts to quantify the exte nt and slope of the concentrati on gradient that may exist from peak near-road concentrations to the typical urban background conc entrations must consider the variability that exists across locations and for a given location over time. As a result, we have identified a range of concentration gradients in the technical literature which indicate that, on average, peak NO 2 concentrations on or immediately adj acent to roads may typically be between 30 and 100 percent greater than con centrations monitored in the same area but farther away from the road (ISA, Section 2.5.4; Beckerman et al., 2008; Gilbert et al., 2003; Rodes and Holland, 1981; Roorda-Knape et al., 1998; Singer et al., 2004). This ra nge of concentration gradients has implications for revising the NO 2 primary standard and for the NO 2 monitoring network (discussed in sections II.F.4 and III).

B. Health effects information In the last review of the NO 2 NAAQS, the 1993 NO X Air Quality Criteria Document (1993 AQCD) (EPA, 1993) concluded th at there were two key health effects of greatest concern at ambient or near-ambient concentrations of NO 2 (ISA, section 5.3.1). The first was increased airway responsiveness in asthma tic individuals after short-term exposures. The second was23 increased respiratory illness among children asso ciated with longer-te rm exposures to NO 2 . Evidence also was found for increased risk of emphysema, but this appeared to be of major concern only with exposures to NO 2 at levels much higher than th en current ambient levels (ISA, section 5.3.1). Controlled human exposure and an imal toxicological stud ies provided qualitative evidence for airway hyperresponsiveness and lung function changes while epidemiologic studies provided evidence for increased respirator y symptoms with increased indoor NO 2 exposures. Animal toxicological findings of l ung host defense system changes with NO 2 exposure provided a biologically-plausible basis for the epidem iologic results. Subpopulations considered potentially more susceptib le to the effects of NO 2 exposure included persons with preexisting respiratory disease, children, and the elderly. Th e epidemiologic evidence for respiratory health effects was limited, and no studies had consid ered endpoints such as hospital admissions, emergency department visits, or mortality (ISA, section 5.3.1). As summarized below and disc ussed more fully in section II.B of the proposal notice, evidence published since the last review genera lly has confirmed and extended the conclusions articulated in the 1993 AQCD (ISA, section 5.3. 2). The epidemiologic evidence has grown substantially with the addition of field and panel studies, interven tion studies, time-series studies of endpoints such as hospital admissions, and a substantial number of studies evaluating mortality risk associat ed with short-term NO 2 exposures. While not as marked as the growth in the epidemiologic literature, a number of recen t toxicological and controlled human exposure studies also provide insights into relationships between NO 2 exposure and health effects. This body of evidence focuses the current review on NO 2 -related respiratory effects at lower ambient and exposure concentrations than consid ered in the previous review. 1. Adverse respiratory effects and short-term exposure to NO 224 The ISA concluded that the findings of epid emiologic, controlled human exposure, and animal toxicological studies provide evidence that is sufficient to infer a likely causal relationship for respiratory e ffects following short-term NO 2 exposure (ISA, sections 3.1.7 and 5.3.2.1). The ISA (section 5.4) concluded that the strongest evidence for an association between
NO 2 exposure and adverse human health effects come s from epidemiologic studies of respiratory symptoms, emergency department visits, and ho spital admissions. These studies include panel and field studies, studies that control for the effects of co-occurring pollutants, and studies conducted in areas where the whole dist ribution of ambient 24-hour average NO 2 concentrations was below the current NAAQS level of 53 ppb (annual average). With regard to this evidence, the ISA concluded that NO 2 epidemiologic studies provide “l ittle evidence of any effect threshold” (ISA, section 5.3.2.9, p. 5-15). In studie s that have evaluated concentration-response relationships, they appear linear within the observed range of da ta (ISA, section 5.3.2.9). Overall, the epidemiologic evidence for respir atory effects has been characterized in the ISA as consistent, in that associations are re ported in studies conducted in numerous locations with a variety of methodological approaches, and cohe rent, in that the studie s report associations with respiratory health outcomes that are logi cally linked together. In addition, a number of these associations are statisticallysignificant, particularly the more precise effect estimates (ISA, section 5.3.2.1). These epidemiologic studies ar e supported by evidence from toxicological and controlled human exposure studies, particularly those that eval uated airway hyperresponsiveness in asthmatic individuals (ISA, s ection 5.4). The ISA concluded th at together, the epidemiologic and experimental data sets form a plausible, consistent, and cohere nt description of a relationship between NO 2 exposures and an array of adverse respir atory health effects that range from the onset of respiratory symptoms to hospital admissions.25 In considering the uncertainties associated with the epidemiologic evidence, the ISA (section 5.4) noted that it is difficult to determine “the extent to which NO 2 is independently associated with respiratory effects or if NO 2 is a marker for the effects of another traffic-related pollutant or mix of pollutants.” On-road vehicl e exhaust emissions are a widespread source of combustion pollutant mixtures that include NO X and are an important contributor to NO 2 levels in near-road locations. Although th e presence of other pollutants from vehicle exhaust emissions complicates efforts to quantify specific NO 2 -related health effects, a number of epidemiologic studies have evaluated associations with NO 2 in models that also in clude co-occurring pollutants such as PM, O 3 , CO, and/or SO 2 . The evidence summarized in the ISA indicates that NO 2 associations generally remain r obust in these multi-pollu tant models and suppor ts a direct effect of short-term NO 2 exposure on respiratory morbidity (see ISA Figures 3.1-7, 3.1-10, 3.1-11). The plausibility and coherence of these effects are also supported by epidemiologic studies of indoor NO 2 as well as experimental (i.e., toxicologi cal and controlled human exposure) studies that have evaluated host defens e and immune system changes, airway inflammation, and airway responsiveness (see subsequent sections of this proposal and the ISA, section 5.3.2.1). The ISA (section 5.4) concluded that the robustness of epidemiologic findings to adjustment for copollutants, coupled with data from animal and human experimental studies, support a determination that the relationship between NO 2 and respiratory morbidity is likely causal, while still recognizing the relationship between NO 2 and other traffic related pollutants. The epidemiologic and experimental studies encompass a number of respiratory-related health endpoints, including em ergency department visits a nd hospitalizations, respiratory symptoms, airway hyperresponsiveness, airway inflammation, and lung f unction. The findings relevant to these endpoints, whic h provide the rationale to support the judgment of a likely causal26 relationship, are described in more deta il in section II.B.1 of the proposal. 2. Other effects with short-term exposure to NO 2 a. Mortality The ISA concluded that the epidemiologic evid ence is suggestive, but not sufficient, to infer a causal relationship between short-term exposure to NO 2 and all-cause and cardiopulmonary-related mortality (ISA, secti on 5.3.2.3). Results from several large United States and European multicity studies and a meta -analysis study indicate positive associations between ambient NO 2 concentrations and the risk of all- cause (nonaccidental) mortality, with effect estimates ranging from 0.5 to 3.6% excess risk in mortality per standardized increment (20 ppb for 24-hour averaging time, 30 ppb for 1-hour averaging time) (ISA, section 3.3.1, Figure 3.3-2, section 5.3.2.3). In general, the ISA concluded that NO 2 effect estimates were robust to adjustment for co-pollutants. Both cardiovascular and respiratory mortality have been associated with increased NO 2 concentrations in epidemiologic studies (ISA, Figure 3.3-3); however, similar associations were observed for other pollutants, including PM and SO 2 . The range of risk estimates for excess mortality is generally smaller than that for other pollutants such as PM. In addition, while NO 2 exposure, alone or in conjunction wi th other pollutants, may contribute to increased mortality, evaluation of the specificity of this effect is difficult. Clinical studies showing hematologic effects and animal toxico logical studies showing biochemical, lung host defense, permeability, and inflammation changes with short-term exposures to NO 2 provide limited evidence of plausible pathways by which risks of mortality may be increased, but no coherent picture is evident at this time (ISA, section 5.3.2.3). b. Cardiovascular effects The ISA concluded that the available eviden ce on cardiovascular health effects following27 short-term exposure to NO 2 is inadequate to infer the presence or absence of a causal relationship at this time (ISA, section 5.3.2.2). Evidence from epidemiologic studies of heart rate variability, repolarization changes, and cardi ac rhythm disorders among heart pa tients with ischemic cardiac disease are inconsistent (ISA, s ection 5.3.2.2). In most studies, a ssociations with PM were found to be similar or stronger than associations with NO 2 . Generally positive associations between ambient NO 2 concentrations and hospital admissions or emergency department visits for cardiovascular disease have been reported in single-pollutant models (ISA, section 5.3.2.2); however, most of these effect estimate values were diminished in multi-pollutant models that also contained CO and PM indices (ISA, sec tion 5.3.2.2). Mechanistic evidence of a role for
NO 2 in the development of cardiovascular diseases from studies of biomarkers of inflammation, cell adhesion, coagulation, and th rombosis is lacking (ISA, sect ion 5.3.2.2). Furthermore, the effects of NO 2 on various hematological parameters in animals are inconsistent and, thus, provide little biological plau sibility for effects of NO 2 on the cardiovascular system (ISA, section 5.3.2.2). 3. Health effects with long-term exposure to NO 2 a. Respiratory morbidity The ISA concluded that overall, the epidem iologic and experimental evidence is suggestive, but not sufficient, to infer a causal relationship between long-term NO 2 exposure and respiratory morbidity (ISA, section 5.3.2.4). The available database evaluating the relationship between respiratory illn ess in children and long-term exposures to NO 2 has increased since the 1996 review of the NO 2 NAAQS (see section II.B.3 of th e proposal for a more detailed discussion). A number of epidemiologic stud ies have examined the effects of long-term exposure to NO 2 and reported positive asso ciations with decremen ts in lung function and28 partially irreversible decremen ts in lung function growth (ISA, section 3.4.1, Figures 3.4-1 and 3.4-2). While animal toxicological studies may pr ovide biological plausi bility for the chronic effects of NO 2 that have been observed in epidemiologic studies (ISA, sections 3.4.5 and 5.3.2.4), the high correlation among traffic-related pollutants in epidemiologic studies makes it difficult to accurately estimate indepe ndent effects (ISA, section 5.3.2.4). b. Mortality The ISA concluded that the epidemiologic eviden ce is inadequate to infer the presence or absence of a causal relationship between long-term exposure to NO 2 and mortality (ISA, section 5.3.2.6). In the United States and European coho rt studies examining th e relationship between long-term exposure to NO 2 and mortality, results have been inconsistent (ISA, section 5.3.2.6). Further, when associations were sugg ested, they were not specific to NO 2 but also implicated PM and other traffic indicators. The relati vely high correlations reported between NO 2 and PM indices make it difficult to interpret these obser ved associations at this time (ISA, section 5.3.2.6). c. Carcinogenic, cardiovascular, and reproductive/developmental effects The ISA concluded that the available epid emiologic and toxicological evidence is inadequate to infer the presence or absence of a causal relationship for carcinogenic, cardiovascular, and reproductive and devel opmental effects related to long-term NO 2 exposure (ISA, section 5.3.2.5). Epidemiologic studies conduc ted in Europe have shown an association between long-term NO 2 exposure and increased inciden ce of cancer (ISA, section 5.3.2.5). However, the animal toxicological studies have provided no clear evidence that NO 2 acts as a carcinogen (ISA, section 5.3.2.5). The very limite d epidemiologic and toxicological evidence do not suggest that long -term exposure to NO 2 has cardiovascular effects (ISA, section 5.3.2.5).29 The epidemiologic evidence is not consistent for associations between NO 2 exposure and fetal growth retardation; however, some evidence is accumulating for effects on preterm delivery (ISA, section 5.3.2.5). Scant animal eviden ce supports a weak association between NO 2 exposure and adverse birth outcomes and provides little mechanistic information or biological plausibility for the epidemiologic findings.

4. NO 2 -related impacts on public health Specific groups within the ge neral population are likely at increased risk for suffering adverse effects from NO 2 exposure. This could occur becaus e they are affected by lower levels of NO 2 than the general population or because they e xperience a larger health impact than the general population to a given le vel of exposure (suscep tibility) and/or because they are exposed to higher levels of NO 2 than the general population (vulne rability). The term susceptibility generally encompasses innate (e.g., genetic or developmental) and/or acquired (e.g., age or disease) factors that make individuals more likely to experience effects with exposure to pollutants. The severity of health effects experienced by a susceptible subgroup may be much greater than that experienced by the population at large. Factors that may influence susceptibility to the effects of air pollution include age (e.g., infa nts, children, elderly); gender; race/ethnicity; genetic factors; and pre-ex isting disease/condition (e.g., obesity, diabetes, respiratory disease, asthma, chronic obstructiv e pulmonary disease (COPD), cardiovascular disease, airway hyperresponsiveness, respiratory infection, adverse birth outcome) (ISA, sections 4.3.1, 4.3.5, and 5.3.2.8). In addition, certain groups may experience relatively high exposure to
NO 2 , thus forming a potentially vulnerable populati on (ISA, section 4.3.6). Factors that may influence susceptibility and vulnerability to ai r pollution include socioeconomic status (SES), education level, air conditioning use, proximity to roadways, geographic location, level of30 physical activity, and work environment (e.g., ind oor versus outdoor) (ISA, section 4.3.5). The ISA discussed factors that can c onfer susceptibility and/or vulnerability to air pollution with most of the discussion devoted to factors for which NO 2 -specific evidence exists (ISA, section 4.3). These factors include pre- existing disease (e.g., asthma), ag e (i.e., infants, children, older adults), genetic factors, gender, socioeconomic status, and proximity to roadways (see section II.B.4 in proposal for more detailed discussion of these factors). As discussed in more detail in the propos al (section II.B.4), th e population potentially affected by NO 2 is large. A considerable fraction of the population resides, works, or attends school near major roadways, and these individuals are likely to have in creased exposure to NO 2 (ISA, section 4.4). Based on data from the 2003 American Housing Survey, approximately 36 million individuals live within 300 feet (~90 meters) of a four-lane highway, railroad, or airport (ISA, section 4.4) 6 . Furthermore, in California, 2.3% of schools, with a total enrollment of more than 150,000 students were located within approxim ately 500 feet of high-traffic roads, with a higher proportion of non-white and economically disadvantaged students attending those schools (ISA, section 4.4). Of this population, asthma tics and members of other susceptible groups discussed above will have even greater risks of experiencing health effects related to NO 2 exposure. In the United States, approxima tely 10% of adults and 13% of children (approximately 22.2 million people in 2005) have been diagnosed with asthma, and 6% of adults

6 The most current American Housing Survey ( http://www.census.gov/hhe s/www/housing/ahs/ahs.html ) is from 2007 and lists a higher fraction of housing units within the 300 foot boundary than do prior surveys. According to Table 1A-6 from that report (http://www.census.gov/hhes/w ww/housing/ahs/ahs07/tab1a-6.pdf), out of 128,203,000 total housing units in the United States , 20,016,000 were reported by the surveyed occupant or landlord as being within 300 feet of a 4-or-more lane highway, railroad, or airport. That constitutes 15.613% of the total housing units in the U.S. Assuming equal distributions, with a current population of 306,330,199, that m eans that there would be 47.8 million people meeting the 300 foot criteria.31 have been diagnosed with COPD (ISA, section 4. 4). The prevalence and severity of asthma is higher among certain ethnic or racial groups such as Puerto Ricans, American Indians, Alaskan Natives, and African Americans (ISA, section 4.4). A higher prevalence of asthma among persons of lower SES and an excess burden of as thma hospitalizations and mortality in minority and inner-city communities have been observed (ISA, section 4.4). In addition, based on United States census data from 2000, about 72.3 million ( 26%) of the United States population are under 18 years of age, 18.3 million (7.4% ) are under 5 years of age, a nd 35 million (12%) are 65 years of age or older. Therefore, large portions of the United States population are in age groups that are likely at-risk for health effects associated with exposure to ambient NO 2 . The size of the potentially at-risk population sugge sts that exposure to ambient NO 2 could have a significant impact on public health in the United States. C. Human exposure and health risk characterization To put judgments about NO 2 -associated health effects in to a broader public health context, EPA has drawn upon the results of the quantitative exposure and risk assessments. Judgments reflecting the nature of the evidence and the overall weight of the evidence are taken into consideration in these quant itative exposure and risk assessm ents, discussed below. These assessments provide estimates of the likelihood that asthmatic individuals would experience exposures of potential concern and estimates of the incidence of NO 2 -associated respiratory emergency department visits under varying air quality scenarios ( e.g. , just meeting the current or alternative standards), as well as characteriza tions of the kind and de gree of uncertainties inherent in such estimates. As discussed more fully in section II.C of the proposal, this section summarizes the approach taken in the REA to characterize NO 2 -related exposures and health risks. Goals of the REA included estimating s hort-term exposures and potential human health32 risks associated with (1) recent levels of ambient NO 2 ; 2) NO 2 levels adjusted to simulate just meeting the current standard; and 3) NO 2 levels adjusted to simulate just meeting potential alternative standards. For purposes of the quantita tive characterization of NO 2 health risks, the REA determined that it was appropriate to focus on endpoints for which the ISA concluded that the available evidence is sufficient to infer either a causal or a likely causal relationship. This was generally consistent with judgments made in other recent NAAQS reviews (e.g., see EPA, 2005). As noted above in section II.A, the only heal th effect category for which the evidence was judged in the ISA to be sufficien t to infer either a causal or a likely causal relationship is respiratory morbidity following short-term NO 2 exposure. Therefore, for purposes of characterizing health risks associated with NO 2 , the REA focused on respiratory morbidity endpoints that have been asso ciated with short-term NO 2 exposures. In evaluating the appropriateness of specific endpoints for use in the NO 2 risk characterization, the REA considered both ep idemiologic and controlled human exposure studies. As described in more detail in the proposal (section II .C.1), the charac terization of NO 2 - associated health risks was based on an epid emiology study conducted in Atlanta, Georgia by Tolbert et al. (2007) and a meta-analysis of controlled human exposure studies of NO 2 and airway responsiveness in asthmatics (ISA, Table 3.1-3). 7 As noted above, the purpose of the assessments described in the REA was to characterize air quality, exposures, and hea lth risks associated with re cent ambient levels of NO 2 , with NO 2 levels that could be associated with just meeting the current NO 2 NAAQS, and with NO 2 levels

7 The study by Tolbert et al. (2007) reported positive associa tions between 1-hour ambient NO 2 concentrations and respiratory- related emergency department visits. The meta-analysis was included in the ISA and reported th at short-term exposures to NO 2 concentrations at or above 100 ppb increased airway responsiveness in most asthmatics.33 that could be associated with just meeting potentia l alternative standards. To characterize health risks, the REA employed three approaches. In th e first approach, for each air quality scenario,
NO 2 concentrations at fixed-site monitors and s imulated concentrations on/near roadways were compared to potential health effect benchmark values derived from the controlled human exposure literature. In the second approach, mode led estimates of exposures in asthmatics were compared to potential health effect benchmarks . In the third approac h, concentration-response relationships from an epidemiologic study were used in conjunction with ba seline incidence data and recent or simulated ambient concentrations to estimate health impacts. An overview of the approaches to characterizing health risks is provided in the proposal (section II.C.2) and each approach, along with its limitati ons and uncertainties (see propos al, section II.C.3) has been described in more detail in the REA (chapters 6 through 9). Chapters 7-9 of the REA estimated exposures a nd health risks associated with recent air quality and with air quality, as measured at mon itors in the current area-wide network, which had been adjusted to simulate just meeting the cu rrent and potential alternative standards. The specific standard levels evaluated, for an area-w ide standard based on the 3-year average of the
98 th and 99 thpercentile 1-hour daily maximum NO 2 concentrations, were 50, 100, 150, and 200 ppb. In interpreting these results within the context of the current revisions to the NO 2 primary NAAQS (see below), we note that simulation of diffe rent standard levels was based on adjusting
NO 2 concentrations at available area-wide monitors . Therefore, the standard levels referred to above reflect the allowable area-wide NO 2 concentrations, not the maximum allowable concentrations. As a consequence, the maximum c oncentrations in an area that just meets one of these standard levels would be expected to be high er than the standard leve l. For example, given that near-road NO 2 concentrations can be 30% to 100% higher than area-wide concentrations34 (see section II.E.2), an area-wi de concentration of 50 ppb could correspond to near-road concentrations from 65 to 100 ppb. Key results of the air quality, exposure, and risk analyses were presented in the policy assessment chapter of the REA and summarized in the proposal (Table 1 in proposal). In considering these results, the policy assessment chapter of the REA concluded that the risks estimated to be associated with just meeting th e current annual standard can be judged important from a public health perspective. The results fo r specific 1-hour standard levels estimate that limiting the 98 th /99 th percentile of th e distribution of 1-hour daily maximum NO 2 concentrations measured at area-wide monito rs to 50 or 100 ppb could subs tantially reduce exposures to ambient NO 2 and associated health risks (compared to just meeting the current standard). In contrast, limiting these area-wide NO 2 concentrations to 150 or 200 ppb is estimated to result in similar, or in some cases higher, NO 2 -associated exposures and health risks than just meeting the current standard. The pattern of results was si milar for standards just meeting either the 98 th or the 99 th percentile 1-hour daily maximum area-wide standards (REA, Chapters 7, 8, and 9). D. Approach for reviewing the need to retain or revise the current standard EPA notes that the final decision on reta ining or revising th e current primary NO 2 standard is a public health policy judgment to be made by the Admi nistrator. This judgment has been informed by a recognition th at the available health effect s evidence reflects a continuum consisting of ambient levels of NO 2 at which scientists generally agree that health effects are likely to occur, through lower levels at whic h the likelihood and magn itude of the response become increasingly uncertain. The Administ rator’s final decisions draw upon scientific information and analyses relate d to health effects, population exposures, and risks; judgments about the appropriate response to the range of uncertainti es that are inherent in the scientific35 evidence and analyses; and comments r eceived from CASAC and the public. To evaluate whether the current primary NO 2 standard is requisite or whether consideration of revisions is a ppropriate, EPA has used an appr oach in this review that was described in the policy assessment chapter of the REA. This approach builds upon those used in reviews of other criteria pollutants, incl uding the most recent reviews of the Pb, O 3 , and PM NAAQS (EPA, 2007d; EPA, 2007e; EPA, 2005), and reflects the body of evidence and information that is currently available. As in other recent reviews, EPA's considerations included the implications of placing more or less weight or emphasis on different aspects of the scientific evidence and the expos ure/risk-based information, rec ognizing that the weight to be given to various elements of the evidence and ex posure/risk information is part of the public health policy judgments that the Administrator w ill make in reaching decisions on the standard. A series of general questions framed this appr oach to considering the scientific evidence and exposure-/risk-based information. First, EPA’ s consideration of the scientific evidence and exposure/risk information with regard to the ade quacy of the current standard has been framed by the following questions:

• To what extent does evidence that has become available since the last review reinforce or call into question evidence for NO 2 -associated effects that were identified in the last review?

• To what extent has evidence for different health effects and/or sensitive populations become available since the last review?

• To what extent have uncertainti es identified in the last revi ew been reduced and/or have new uncertainties emerged?

• To what extent does evidence and exposure- /risk-based information that has become available since the last review reinforce or call into question any of the basic elements of the current standard?36 To the extent that the available evidence and exposure-/risk-based information suggests it may be appropriate to consider revision of the current standard, EPA considers that evidence and information with regard to its suppo rt for consideration of a standard that is either more or less protective than the current st andard. This evaluation has been framed by the following questions:

• Is there evidence that associations, especia lly causal or likely causa l associations, extend to ambient NO 2 concentrations as low as, or lower than, the concentrations that have previously been associated with health effect s? If so, what are the important uncertainties associated with that evidence?

• Are exposures above benchmark levels and/or he alth risks estimated to occur in areas that meet the current standard? If so, are the es timated exposures and health risks important from a public health perspective? What are the important uncertainties associated with the estimated risks?

To the extent that there is support for cons ideration of a revised standard, EPA then considers the specific elements of the standard (indicator, averagi ng time, form, and level) within the context of the currently ava ilable information. In so doing, the Agency has addressed the following questions:

• Does the evidence provide support for consid ering a different indi cator for gaseous NO X ?

• Does the evidence provide support for c onsidering different averaging times?

• What ranges of levels and forms of alternat ive standards are supported by the evidence, and what are the associated uncertainties and limitations?

• To what extent do specific averaging times, levels, and forms of alternative standards reduce the estimated exposures above benchm ark levels and risks attributable to NO 2 , and what are the uncertainties associated with the estimated exposure and risk reductions?
The questions outlined above have been addressed in the REA, the proposal, and in this final rulemaking. The following sections present the rationale for proposed decisions, discussion of37 public comments, and the Administrator’s conclusi ons on the adequacy of the current standard and potential alternative standard s in terms of indicator, averagi ng time, form, and level. E. Adequacy of the current standard This section discusses considerations relate d to the decision as to whether the current
NO 2 primary NAAQS is requisite to protect public health with an adequate margin of safety. Specifically, section II.E.1 provides an overview of the rationale supporti ng the Administrator’s conclusion in the proposal that the current standard alone does not provide adequate public health protection; section II.E.2 discusses co mments received on the adequacy of the current standard; and section II.E.3 disc usses the Administrator’s final decision on whether the current
NO 2 primary NAAQS is requisite to protect public health with an adequate margin of safety. 1. Rationale for proposed decision In reaching a conclusion regardi ng the adequacy of the current NO 2 NAAQS in the proposal (section II.E.5), the Admini strator considered the scientif ic evidence assessed in the ISA and the conclusions of the ISA, the exposure and risk information presented in the REA and the conclusions of the policy assessment chapter of the REA, and the views expressed by CASAC. These considerations are discussed in detail in the proposal (II.E.) and are summarized in this section. In the propos al, the Administrator noted th e following in considering the adequacy of the current standard:

• The ISA concluded that the results of epid emiologic and experimental studies form a plausible and coherent data set th at supports a relationship between NO 2 exposures and respiratory endpoints, includi ng respiratory symptoms and respiratory-related hospital admissions and emergency department visits, at ambient concentrations that are present in areas that meet the current NO 2 NAAQS (ISA, section 5.4).38 • The policy assessment chapter of the REA concl uded that risks estimated to be associated with air quality adjusted upward to simula te just meeting the current standard can reasonably be judged important from a public health perspective (REA, section 10.3.3).

• The policy assessment chapter of the REA c oncluded that exposure- and risk-based results reinforce the scientif ic evidence in supporting the co nclusion that consideration should be given to revising the current NO 2 NAAQS so as to provide increased public health protection, especially for at-risk groups, from NO 2 -related adverse health effects associated with short-term, and potential l ong-term, exposures (REA, section 10.3.3).

• CASAC agreed that the current annual standa rd alone is not sufficient to protect public health against the types of expos ures that could lead to these health effects. Specifically, in their letter to the Administrator on the fi nal REA, they stated that “CASAC concurs with EPA’s judgment that the current NAAQS does not protect the public’s health and that it should be revised” (Samet, 2008b). Based on these considerations (d iscussed in more detail in the proposal, section II.E), the Administrator concluded in the proposal that the current NO 2 primary NAAQS is not requisite to protect public health with an adequate margin of safety against adverse respiratory effects associated with short-term exposures. In consid ering approaches to revi sing the current standard, the Administrator concluded that it is appropriate to consider setting a new short-term standard in addition to retaining the current a nnual standard. The Administrato r noted that such a short-term standard could provide increased public health protection, especi ally for members of at-risk groups, from effects described in both epidemio logic and controlled hu man exposure studies to be associated with short-term exposures to NO 2 . 2. Comments on the adequacy of the current standard39 This section discusses comments received from CASAC and public commenters on the proposal that either supported or opposed the Administrator’s proposed decision to revise the
current NO 2 primary NAAQS. Comments on the adequacy of the current standard that focused on the scientific and/or the exposure/risk basis for the Administrator’s proposed conclusions are discussed in sections II.E.2.a-II.E.2.c. Commen ts on the epidemiologic evidence are considered in section II.E.2.a. Comments on the controlle d human exposure evidence are considered in section II.E.2.b. Comments on human exposure and h ealth risk assessments are considered in section II.E.2.c. To the extent these comments on the evidence and information are also used to justify commenters’ conclusions on decisions related to indicator, averaging time, level, or form, they are noted in the appropriate sections below (II.F.1-II.F.4). In their comments on the proposal (Samet, 2009), CASAC reiterated their support for the need to revise the current annual NO 2 NAAQS in order to increase public health protection. As noted above, in its letter to the Administrato r on the final REA (Samet, 2008b) CASAC stated that it “concurs with EPA’s judgment that th e current NAAQS does not protect the public’s health and that it should be revised.” In supporting adoption of a more stringent NAAQS for
NO 2 , CASAC considered the assessment of the scientific evidence presented in the ISA, the results of assessments presented in the REA, and the conclusions of the policy assessment chapter of the REA. As such, CASAC’s rati onale for revising the current standard was consistent with the Administrator’s rati onale as discussed in the proposal. Many public commenters agreed with CASAC that, based on the available information, the current NO 2 standard is not requisite to protect publ ic health with an adequate margin of safety and that revisions to the standard are appr opriate. Among those calling for revisions to the standard were environmental groups (e.g., Cl ean Air Council (CAC), Earth Justice (EJ),40 Environmental Defense Fund (EDF), Natura l Resources Defense Council (NRDC), Group Against Smog and Pollution (GASP)); medical/pub lic health organizations (e.g., American Lung Association (ALA), Amer ican Medical Associati on (AMA), American Thoracic Society (ATS), National Association for the Medical Directi on of Respiratory Care (NAMDRC), National Association of Cardiovascular and Pulmonary Rehabilitation (NACPR), American College of Chest Physicians (ACCP)); a larg e number of State agencies a nd organizations (e.g., National Association of Clean Air Agencies (NACAA), Northeast States for Coordinated Air Use Management (NESCAUM), and state or local ag encies in CA, IA, IL, MI, MO, NC, NM, NY, TX, VA, WI); Tribes (e.g., National Tribal Ai r Association (NTAA), Fond du Lac Band of Lake Superior Chippewa (Fond du Lac)), and a numbe r of individual commenters. These commenters concluded that the current NO 2 standard needs to be revised and that a more stringent standard is needed to protect the health of sensitive popul ation groups. In supporti ng the need to adopt a more stringent NAAQS for NO 2 , these commenters often refere nced the conclusions of CASAC and relied on the evidence and information pres ented in the proposal. As such, similar to CASAC, the rationale offered by these commenters was consistent with that presented in the proposal to support the Administrator’s pr oposed decision to revise the current NO 2 NAAQS. Some industry commenters (e.g., Alliance of Automobile Manufacturers (AAM), American Petroleum Institute (API), Interstate Natural Gas Association of America (INGAA), Utility Air Regulatory Group (UARG)) and one State commenter (IN Department of Environmental Management) expressed support for re taining the current annual standard alone. In supporting this view, these commenters genera lly concluded that the current standard is requisite to protect public health with an ade quate margin of safety and that the available evidence is not sufficient to s upport revision of the standard. For example, UARG stated that41 “EPA has failed to demonstrate that the present NO 2 NAAQS is no longer at the level requisite to protect public health with an adequate margin of safet y.” In addition, INGAA stated that “...EPA should be compelled to retain the curren t standard and defer a decision on a new shortterm standard until the science is more clearly defined.” In support of their views, these comme nters provided specific comments on the epidemiologic and controlled human exposure evidence as discussed below. In responding to these specific comments, we note that the Administrator relied in the proposal on the evidence, information and judgments contained in the IS A and the REA (including the policy assessment chapter) as well as on the advice of CASAC. In considering the evidence, information, and judgments of the ISA and the REA, the Agency not es that these documents have been reviewed extensively by CASAC and have been discusse d by CASAC at multiple public meetings (see section I.D). In their letter to the Admini strator regarding the second draft ISA (Henderson, 2008b), CASAC noted the following: Panel members concur with the primary conclusions reached in the ISA with regard to health risks that are associated with NO 2 exposure. In particular, the Panel agrees with the conclusion that the current scientific evidence is “sufficient to infer a likely causal relationship between short term NO 2 exposure and adverse effects on the respiratory system.” The strongest evidence in support of this conclusion comes from epidemiology st udies that show generally positive associations between NO 2 and respiratory symptoms, hospitalizations or emergency department visits, as summarized in Figure 5.3.1.” Similarly, in their letter to th e Administrator on the final REA (Samet, 2008b), CASAC noted the following: Overall, CASAC found this version of th e REA satisfactory in its approach to moving from the scientific foundation de veloped in the In tegrated Science Assessment (ISA) to setting out evid ence-based options for the NAAQS. The REA provides the needed bridge from th e evidence presented in the ISA to a characterization of the exposures and the a ssociated risks with different profiles of exposure. It draws on toxi cological and epidemiological evidence and addresses risk to an identified su sceptible population, people w ith asthmatic conditions.42 EPA has also systematically described uncertainties associated with the risk assessments. We commend EPA for de veloping a succinct and thoughtfully developed synthesis in Chapter 10. This summary chapter represents a longneeded and transparent model for linking a substantial body of scientific evidence to the four elements of the NAAQS. Therefore, in discussing comments on the in terpretation of the scientific evidence and exposure/risk information, we note that CASAC ha s endorsed the approaches and conclusions of the ISA and the REA. These approaches and co nclusions are discussed below in more detail, within the context of sp ecific public comments. a. Comments on EPA’s interpretation of the epidemiologic evidence Several industry groups (e.g., API, Nati onal Mining Association (NMA), American Chemistry Council (ACC), AAM, Annapolis Center for Science-Based Public Policy (ACSBPP), Engine Manufacturers Associ ation (EMA), ExxonMobil (Exxon) , National Association of Manufacturers (NAM)) commented that , given the presence of numerous co-pollutants in the air, epidemiologic studies do not support the contention that NO 2 itself is causing health effects. While EPA has recognized that multiple f actors can contribute to the etiology of respiratory disease and that more than one air pollutant could independently impact respiratory health, we continue to judge, as discussed in th e ISA, that the available evidence supports the conclusion that there is an independent effect of NO 2 on respiratory morbidity. In reaching this judgment, we recognize that a major methodological issue affecting NO 2 epidemiologic studies concerns the evaluation of the extent to whic h other air pollutants ma y confound or modify NO 2 - related effect estimates. The use of multipol lutant regression models is the most common approach for controlling potenti al confounding by copollutants in epidemiologic studies. The issues related to confounding and the evidence of potential confounding by copollutants has been thoroughly reviewed in the ISA (see Figures 3.1-10 and 3.1-11) and in previous assessments43 (e.g., EPA, 2004). NO 2 risk estimates for respiratory morbid ity endpoints, in general, were not sensitive to the inclusion of copollutants, incl uding particulate and ga seous pollutants. As observed in Figures 3.1-10 and 3.1-11 in the IS A, relative risks for hospital admissions or emergency department visits are generally unc hanged, nor is their interpretation modified, upon inclusion of PM or gaseous co-pollutants in th e models. Similarly, asso ciations between shortterm NO 2 exposure and asthma symptoms are generall y robust to adjustment for copollutants in multipollutant models, as shown in Figures 3.1 -5 and 3.1-7 of the ISA. These results, in conjunction with the results of a randomized in tervention study evaluating respiratory effects of indoor exposure to NO 2 (ISA, section 3.1.4.1), led to the co nclusion that the effect of NO 2 on respiratory health outcomes is robust and i ndependent of the effects of other ambient copollutants. In addition, experimental studies conducte d in animals and humans provide support for the plausibility of the associat ions reported in epidemiologic st udies. These controlled human exposure and animal toxicological st udies have reported effects of NO 2 on immune system function, lung host defense, airway inflammation, and airway responsiveness (ISA, section 5.4). These experimental study results sup port an independent contribution of NO 2 to the respiratory health effects reported in epidemiol ogic studies (ISA Section 5.4). In considering the entire body of evidence, including epidemiologic and experimental studies, the ISA (section 5.4, p. 5- 16) concluded the following: Although this [presence of co-pollutants] complicates the efforts to disentangle specific NO 2 -related health effects, the eviden ce summarized in this assessment indicates that NO 2 associations generally remain robust in multi-pollutant models and supports a direct e ffect of short-term NO 2 exposure on respiratory morbidity at ambient concentrations below the current NAAQS. The robustness of epidemiologic findings to adjustment for co-pollutants, coupled with data from animal and human experimental studie s, support a determination that the relationship between NO 2 and respiratory morbidity is likely causal, while still44 recognizing the relationship between NO 2 and other traffic-related pollutants. Comments on specific epidemiologic st udies are discussed below. The National Association of Ma nufacturers (NAM) commented that the final REA relied on an epidemiologic study (Delfino et al. 2002) not cr itically reviewed in the final ISA. Contrary to NAM’s contention, the study by Delfino et al. ( 2002) was critically reviewed by EPA staff and pertinent information was extracted from the study. The respiratory health effects of NO 2 on asthma reported in this study are included inFigure 5.3-1, Table 5.4-1, and Annex Table AX6.3- 2 of the ISA. While NAM comments on the narrative discussion of this study in the final ISA, their contention that EPA scientis ts did not critically analyze th e study while preparing the final ISA is incorrect. The inclusion of the study in the figur es and tables in this ISA, as well as inclusion in the 2004 PM AQCD, indicate critical analysis of the study that was implemented throughout the review process. The narrative discussion in the ISA focused on multicity studies (specifically those by Schwartz et al. 1994, Mort imer et al. 2002 and Sc hildcrout et al. 2006), which provide substantial epidemiologic eviden ce for the respiratory health effects of NO 2 on asthma among children. Additional comments from NAM contend that EPA’s interpretation of three individual epidemiologic studies (e.g. Krewsk i et al. 2000; Schildcrout et al. 2006; Mortimer et al. 2002) is inconsistent across different NAAQS reviews. The NAM comments on all three studies are discussed below. NAM stated the following regard ing the study by Krewski et al: In the Final ISA, EPA cites the Krewski, et al. (2000) study as evidence of a significant association between NO 2 exposure and mortality. Although EPA acknowledges that exposure to NO 2 was “highly correlated” with other pollutants, including PM 2.5 and SO 2 , EPA does not consider the analysis of the respective contributions of single pollutants in the same study that EPA included in its prior Staff Paper for Particulate Matter. In that document, EPA stated: “In single-45 pollutant models, none of the gaseous co- pollutants was significantly associated with mortality except SO 2 .” If EPA has not altered its scientific views concerning this study as expressed in the PM Staff Paper, it is entirely inappropriate for EPA to suggest that the Krewski, et al. (2000) study provides any evidence of an association between NO 2 exposure and mortality. In these comments, NAM fails to recognize that the Krewski et al. ( 2000) report contains a reanalysis of two cohort studies, the Harvard Si x Cities and the American Cancer Society (ACS) studies. NAM correctly quotes from the PM Sta ff Paper (2005) regarding the results of single pollutant models on associations with mortality. However, this statement in the PM Staff Paper is referring to the results of the ACS Study. In contrast, the characterization in the NO X ISA of the study by Krewski et al. ( 2000), referenced by NAM in their comments, refers to the reanalysis of the Harvard Six Cities Study. As stated in the NO X ISA (p. 3-74): Krewski et al. (2000) conducted a sensitivity analysis of the Harvard Six Cities study and examined associations between gaseous pollutants (i.e., O 3 , NO 2 , SO 2 , CO) and mortality. NO 2 showed risk estimates similar to those for PM 2.5 per “low to high” range increment with to tal (1.15 [95% CI: 1.04, 1.27] per 10-ppb increase), cardiopulmonary (1.17 [ 95% CI: 1.02, 1.34]), and lung cancer (1.09 [95% CI: 0.76, 1.57]) deaths; how ever, in this dataset NO 2 was highly correlated with PM 2.5 (r=0.78), SO 4 2– (r=0.78), and SO 2 (r=0.84). In the subsequent paragraph, the NO X ISA presents results from the Krewski et al. (2000) reanalysis of the ACS study, observing that “NO 2 showed no associations with mortality outcomes” (ISA, p.3-74). This is consistent w ith the quote drawn from the PM Staff Paper. Thus, there is no inconsistency in the interpreta tion of the results of the study by Krewski et al. (2000) in the PM Staff Paper (2005) and the NO X ISA (2008). NAM is confusing the conclusions on the results of the reanalysis of the Harvard Six Cities Study in the NO X ISA with conclusions regarding the r eanalysis of the ACS study in the PM Staff Paper. NAM also commented that EPA has relied on a study by Schildcrout et al. (2006) in the
NO X ISA but declined to rely on the same study for the pr evious review of the O 3 NAAQS.46 NAM made the following comment rega rding the study by Sc hildcrout et al: Another example of how EPA has reached di fferent scientific conclusions in the Final ISA than in prior NAAQS docume nts is provided by the Schildcrout, et al. (2006) study. In the Final ISA, EPA include s an extensive discussion of this study of asthmatic children and the relationsh ip purportedly found in this study between NO 2 and various respiratory symptoms. In c ontrast, as part of the NAAQS review for ozone, EPA expressly declined to rely on this same study because of specific limitations in the study design. Among the lim itations EPA cites were the fact that the Schildcrout, et al. (2006) study included “children in which the severity of their asthma was not clearly identified,” and the use of a study population that was “not comparable to other large multi- city studies.” EPA must explain why it chose to discount the value of the Schildcrout, et al. (2006) study when evaluating the effects of ozone, but has relied on it extensively in the Final ISA for NO 2 . The study by Schildcrout et al. (2006) appeared in the peer-review litera ture too late to be considered in the 2006 O 3 AQCD; however, this st udy was included in the O 3 Provisional Assessment. The purpose of the Provisional Assessment was to determine if new literature materially changed any of the br oad scientific conclu sions regarding the health effects of O 3 exposure as stated in the 2006 O 3 AQCD. EPA concluded that, taken in context, the “new” information and findings did not materially chan ge any of the broad sc ientific conclusions regarding the health effects of O 3 exposure made in the O 3 AQCD. Therefore, NAM’s contention that EPA “declined” to rely on the Schildcrout study for the O 3 review because of limitations in study design is not correct. The observations NAM draws from the O 3 Provisional Assessment regarding severity of asthma and the study population do not indicate lim itations that resulted in EPA “discounting” the study results. Rather, these observations were intended to put the st udy in perspective for purposes of interpreting the results within the contex t of the larger body of O 3 health effects evidence. These observations were drawn from comments submitted by Dr. Schildcrout regarding the interpreta tion of the results of his study in the decision to revise the ozone standards (see docket ID EPA-HQ-OAR-2005-0172-6991) . The results of this study will be47 fully considered in the upcoming review of the ozone NAAQS. Finally, NAM contends that EP A reached differing scientific conclusions on the use of self-reported peak expiratory flow (PEF) depending on regulatory context, particularly in the large multi-city trial by Mortimer et al. ( 2002). EPA consistently examines clinical measurements of lung function, which include PEF, forced expiratory flow in 1 second (FEV 1 ), forced vital capacity (FVC), maximal midexpirato ry flow (MMEF), maximal expiratory flow at 50% (MEF 50 ), maximal expiratory flow at 25% (MEF 25 ), and forced expiratory flow at 25 to 75% of FVC (FEF 25-75 ). Evidence for all of these clinical measurements is considered before drawing a conclusion related to th e association of lung function w ith a criteria pollutant. In different reviews, there may be more evidence from one of these clinic al measurements than another. In the previous NAA QS review of ozone, EPA identi fied statistically significant associations between increased ozone levels a nd morning PEF, which remained significant even when concentrations exceeding 0.08 ppm were ex cluded from the analysis (Mortimer et al. 2002). EPA used this as evidence, along with ev idence of other clinical measurements of lung function, in drawing conclusions on the rela tionship between ozone and lung function. Similarly, in the final NO X ISA, EPA weighed the all of the evidence pertinent to lung function, including studies that produced no statistically significant resu lts for PEF, and the summary section (3.1.5.3) of the NO 2 ISA states: In summary, epidemiologic studies usi ng data from supervised lung function measurements (spirometry or peak flow meters) report small decrements in lung function (Hoek and Brunekreef, 1995; Linn et al., 1996; Moshammer et al., 2006; Peacock et al., 2003; Schindl er et al., 2001). No signi ficant associations were reported in any studies using unsupervise d, self-administered peak flow [PEF] measurements with portable devices. This evaluation of the evidence is consistent with the way that the evidence from multiple clinical measures of lung function was used in making similar statements during the NAAQS48 review for ozone. b. Comments on EPA’s interpretation of the controlled human exposure evidence A number of industry groups (e.g., AAM, A CC, API, Dow Chemical Company (Dow), EMA, NAM, UARG) disagreed with EPA’s relia nce on a meta-analysis of controlled human exposure studies of airway responsiveness in as thmatics. Based on this meta-analysis (ISA, Table 3.1-3 for results), the ISA concluded that “small but significant increases in nonspecific airway hyperresponsiveness we re observed...at 0.1 ppm NO 2 for 60-min exposures in asthmatics” (ISA, p. 5-11). Industry groups raised a number of objections to this analysis and the way in which it has been us ed in the current review. Several of these industry groups concluded that, in relyi ng on this analysis, EPA has inappropriately relied on a new unpublished meta-analysis that ha s not been peer-reviewed, was not reviewed by CASAC, and was not conducted in a transparent manner. For example, as part of a Request for Correction submitted under EPA’s Information Quality Guidelines, NAM stated that “EPA’s substantial reliance on an unpublished assessment described as a “meta-analysis” of the relation between NO 2 exposure and changes in airway responsiveness violates EPA Guidelines requiring “transparenc y about data and methods.” EPA disagrees with this char acterization of the updated meta-analysis included in the final ISA. As described in the ISA (p. 3-16), th is meta-analysis is base d on an earlier analysis by Folinsbee (1992) that has been subject to peer-rev iew, that was published in a scientific journal ( Toxicol Ind Health . 8:1-11, 1992), and that was reviewed by CASAC as part of the previous review of the NO 2 NAAQS (EPA, 1993, Table 15-10). The updates to this earlier analysis did not include substantive changes to the approach. As discussed in the final ISA (p. 3-16), the changes made to the analysis were to remove the results of one allergen study and add results49 from a non-specific responsiveness study, which focused the meta-analysis on non-specific airway responsiveness, and to di scuss results for an additional exposure concentration (i.e., 100 ppb). The information needed to reproduce this meta-analysis is provided in the ISA (Tables 3.1-2 and 3.1-3, including footnotes). While the ISA meta-analysis reports findings on airway responsiveness in asthmatics following exposure to 100 ppb NO 2 , a concentration not specifically discussed in the findings of the original report by Folinsbee ( 1992), this does not constitute a substantive change to that original analysis. For exposures at rest, four of the studies incl uded in the analysis by Folinsbee evaluated the effects of exposure to 100 ppb NO 2 . In that original meta -analysis, these studies were grouped with another study that evaluated exposures to 140 ppb NO 2 . When analyzed together, exposures to NO 2 concentrations of 100 ppb and 140 ppb (grouped together in the manuscript and described as less than 0.2 ppm) increased airway res ponsiveness in 65% of resting asthmatics (p < 0.01). Ther efore, reporting results at 100 ppb NO 2 in the ISA metaanalysis reflects a change in the way the data are presented and does not reflect a substantive change to the study. This change in presentation allows specific consideration of the potential for exposures to 100 ppb NO 2 to increase airway responsiveness, rather than grouping results at 100 ppb with results at other exposure concentrations. In addition, the updated meta-analysis was c onsidered by CASAC during their review of the REA (REA, Table 4-5 reports the results of the updated meta-analysis), which based part of the assessment of NO 2 -associated health risks on the results of the meta-analysis. In their letter to the Administrator on the final REA (Samet , 2008b), CASAC stated that “[t]he evidence reviewed in the REA indicates th at adverse health effects have been documented in clinical studies of persons with asthma at 100 ppb” and that “CASAC firmly r ecommends that the upper50 end of the range [of standard levels] not exceed100 ppb, given the findings of the REA.” In addition, in their comments on the proposal, CASAC re iterated this advice in their statement that “the level of the one-hour NO 2 standard should be within th e range of 80-100 ppb and not above 100 ppb.” These statements indicate that CASAC di d specifically consider the results of the updated meta-analysis and that they used those results to inform their recommendations on the range of standard levels supported by the scientific evidence. In summary, we note the following:

• The original meta-analysis was published in a peer-reviewed journa l and was reviewed by CASAC in the previous review of the NO 2 NAAQS

• The updated meta-analysis does not include substantive changes to the methodology of this original analysis

• The changes that were made are clearly described in the ISA

• CASAC specifically reviewed and consid ered the ISA meta-analysis in making recommendations regarding the range of st andard levels sup ported by the science

Many of these same industry groups also referred in their comments to a recent metaanalysis of controlled human exposure studie s evaluating the airway response in asthmatics following NO 2 exposure (Goodman et al., 2009). Thes e groups generally recommended that EPA rely on this meta-analysis and on the authors’ conclusions with regard to NO 2 and airway responsiveness. Specific comments based on the manuscript by Goodman et al., as well as EPA’s responses, are discussed below in more detail. 8

8 EPA considers the Goodman study to be a “new study” on which, as discussed above in section 1.B, it would not be appr opriate to base a standard in the absence of thorough CASAC and public review of the study and its methodol ogy. However, as discussed below, EPA has51 Industry commenters generally claimed that the meta-analysis by Goodman et al. supports the conclusion that no adverse eff ects occur following exposures up to 600 ppb NO 2 . However, Table 4 of the Goodman study reports that 64% (95% Confidence Interval: 58%, 71%) of resting asthmatics exposed to NO 2 experienced an increase inairway responsiveness. Furthermore, Figure 2a of this manuscript repo rts that for exposures <0.2 ppm, the fraction affected is 0.61 (95% CI: 0.52, 0.70) while for ex posures of 0.2 ppm to <0.3 ppm, the fraction affected is 0.66 (95% CI: 0.59, 0.74). These findings are consistent with those reported in the meta-analysis by Folinsbee and in the updated meta-analy sis that was included in the final ISA. Also based on the meta-analysis by Goodman et al. (2009), several industry commenters concluded that NO 2 -induced airway hyperres ponsiveness is not advers e and, therefore, should not be considered in setting standards. The basi s for this comment appears to be the conclusions reached by Goodman et al. that there is no dose-response relationship for NO 2 and that the magnitude of any NO 2 effect on airway responsiveness is too small to be considered adverse. Due to differences in study protocols in the NO 2 -airway response literature (ISA, section 3.1.3), EPA disagrees with the approach taken in the Goodman study to use existing data to attempt to evaluate the presence of a dose-res ponse relationship and to determine the magnitude of the NO 2 response. Examples of differences in the study protocols include the NO 2 exposure method (i.e., mouthpiece versus chamber), subject ac tivity level (i.e., rest versus exercise) during
NO 2 exposure, choice of airway challenge agent, and physiological endpoi nt used to quantify airway responses. Goodman et al. (2009) also recognized heterogene ity among studies as a limitation in their analyses.

considered the study in the context of respondi ng to public comments on the proposal and has concluded it does not provide a basis to materially change any of the broa d scientific conclusions regarding the health effects of NO 2 made in the air quality criteria.52 As a result of these differences, EPA judged it appropriate in the ISA meta-analysis to assess only the fraction of asthmatics experiencing increased or decreased airway responsiveness following NO 2 exposure. We have acknowledged in the REA, the proposal, and in this final rulemaking that there is uncertainty with regard to the magnitude and the clinical-significance of
NO 2 -induced increases in airway responsiveness (s ee sections II.C.3 and II.F.4.a in the proposed rulemaking as well as II.F.3 in this final rulemaking). The RE A stated the following (p. 302): [O]ne of the important uncertainties associated with these [NO 2 -induced airway hyperresponsiveness] results is that, becau se the meta-analysis evaluated only the direction of the change in airway respons iveness, it is not pos sible to discern the magnitude of the change from these data . This limitation makes it particularly difficult to quantify the public health implications of these results. While we acknowledge this uncertainty, EPA di sagrees with the conclusion that the NO 2 - induced increase in airway responsiv eness in asthmatics exposed to NO 2 concentrations up to 600 ppb is not adverse and should not be considered in setting standards. Specifically, we note that the ISA concluded that “[ t]ransient increases in airw ay responsiveness following NO 2 exposure have the potential to increase symptoms and worsen as thma control” (ISA, section 5.4). The uncertainty over the adversity of the respon se reported in controlle d human exposure studies does not mean that the NO 2 -induced increase in airway responsiv eness is not adverse. Rather, it means that there is a risk of adversity, especially for asthmatics with more than mild asthma, but that this risk cannot be fully characterized based on existi ng studies. The studies of NO 2 and airway responsiveness included in the meta-analy sis have generally evaluated mild asthmatics, rather than more severely affected asthma tics who could be more susceptible to the NO 2 -induced increase in airway responsivene ss (ISA, section 3.1.3.2). Given that this is the case, and given the large percentage of asth matics that experienced an NO 2 -induced increase in airway responsiveness in the studies and the large size of the asthmatic population in the United States,53 the REA concluded that it is appropriate to consider NO 2 -induced airway hype rresponsiveness in characterizing NO 2 -associated health risk s (REA, section 10.3.2). As noted above, CASAC endorsed this conclusion in their letters to the Administrator on the final REA and on the proposal (Samet, 2008b; Samet, 2009). c. Comments on EPA’s characterization of NO 2 -associated exposures and health risks Several commenters discussed the analyses of NO 2 -associated exposures and health risks presented in the REA. As in past re views (EPA 2005, 2007d, 2007e), EPA has estimated allowable risks associated with the current standard a nd potential alternative standards to inform judgments on the public health risks that could exist under different stan dard options. Some industry commenters (e.g., API, NMA) conclude d that the Administrator should consider modeled exposures and risks associated with actual NO 2 air quality rather than with NO 2 concentrations adjusted to simulate just m eeting the current annual standard or potential alternative 1-hour standards. These commenters pointed out that such simulations require large adjustments to air quality and are highly uncer tain and that NAAQS are intended to address actual, rather than highly improba ble, risks to health. We disagree with these commenters that expos ure- and risk-related considerations in the NAAQS review should rely only on unadjusted air quality. In c onsidering whether the current standard is requisite to protect public health wi th an adequate margin of safety, air quality adjustments allow estimates of NO 2 -related exposures and health ri sks that could exist in areas that just meet that standard. That is, these adjustments allow consideration of exposures and risks that would be permissible under the current standard. Therefore, such adjustments are clearly useful to inform a decision on the issue before EPA (i.e., the adequacy of the level of public health protection asso ciated with allowable NO 2 air quality under the standard).54 Similarly, air quality adjustments to simulate di fferent potential altern ative standards provide information on exposures and risks that woul d be permissible under these alternatives. 9 As noted above, in their letter to the Admini strator on the final REA (Samet, 2008b), CASAC concluded that “The REA provides the needed bri dge from the evidence presented in the ISA to a characterization of the exposures and the associat ed risks with different profiles of exposure.” We agree that there are uncertainties inhe rent in air quality adjustments. These uncertainties are discussed thoroughly in the REA (sections 7.4, 8.12, 9.6, and 10.3.2.1) and in the proposed rule (section II.C.3). For exampl e, the policy assessment chapter of the REA (section 10.3.2.1) noted the following regarding adjustment of NO 2 concentrations: In order to simulate just meeting the current annual standard and many of the alternative 1-h standards analyzed, an upward adjustment of recent ambient NO 2 concentrations was required. We note th at this adjustment does not reflect a judgment that levels of NO 2 are likely to increase unde r the current standard or any of the potential alternative standa rds under consideration. Rather, these adjustments reflect the fact that the curre nt standard, as well as some of the alternatives under consideration, could al low for such increases in ambient NO 2 concentrations. In adjusting air quality to simulate just meeting these standards, we have assumed that the overall shape of the distribution of NO 2 concentrations would not change. While we believe th is is a reasonable assumption in the absence of evidence supporting a different distribution and we note that available analyses support this approach (Rizzo, 2008), we recognize this as an important uncertainty. It may be an especially important uncertainty for those scenarios where considerable upward adjustment is re quired to simulate just meeting one or more of the standards. These air quality adjustments are not meant to imply an expectation that NO 2 concentrations will increase broadly across the United States or in any given area (REA, section 10.3.2.1). Rather, as noted above, they are meant to estimate NO 2 -related exposures and heal th risks that would be permitted under the current and potential alternat ive standards. Such estimates can inform

9 Once EPA determines whether to retain or revi se the current standard, the actual air quality levels in various areas of the country are clearly relevant under the NAAQS implementation provisions for the Act, such as the provision fo r designation of areas based on whether or not they attain the required NAAQS.55 decisions on whether the current standard, or particular potential alternative sta ndards, provide the requisite protection of public health. 3. Conclusions regarding the adequ acy of the current standard In considering the adequacy of the current st andard, the Administrator has considered the scientific evidence assessed in the ISA, the exposure and risk re sults presented in the REA, the conclusions of the policy assessment chapter of the REA, and comments from CASAC and the public. These considerations are described below. In considering the scientific evidence as it re lates to the adequacy of the current standard, the Administrator notes that the epidemiologic evidence has grown substantially since the last review with the addition of fiel d and panel studies, intervention studies, and time-series studies of effects such as emergency department visits and hospital admissions associated with shortterm NO 2 exposures. No epidemiologic studies we re available in 1993 assessing relationships between NO 2 and outcomes such as hospital admissions or emergency department visits. In contrast, dozens of epidemiologic studies on su ch outcomes, conducted at recent and current ambient NO 2 concentrations, are now included in this evaluation (ISA, chapter 3). As an initial consideration with regard to the adequacy of the current standard, the Administrator notes that the evidence relating long-term (weeks to years) NO 2 exposures at current ambient concentrations to adverse healtheffects was judged in the ISA to be either “suggestive but not sufficient to infer a causa l relationship” (respiratory morbidity) or “inadequate to infer the presence or absen ce of a causal relationship” (mortality, cancer, cardiovascular effects, reproductive/developmen tal effects) (ISA, sections 5.3.2.4-5.3.2.6). In contrast, the evidence relating sh ort-term (minutes to hours) NO 2 exposures to respiratory morbidity was judged to be “sufficient to in fer a likely causal relationship” (ISA, section56 5.3.2.1). This conclusion was supported primar ily by a large body of r ecent epidemiologic studies that evaluated asso ciations of short-term NO 2 concentrations with respiratory symptoms, emergency department visits, and hospital admissi ons. Given these conclusions from the ISA, the Administrator judges that, at a minimum, cons ideration of the adequacy of the current annual standard should take into account the extent to which that standard pr ovides protection against respiratory effects associ ated with short-term NO 2 exposures. In considering the NO 2 epidemiologic studies as they rela te to the adequacy of the current standard, the Administrator notes that annual average NO 2 concentrations were below the level of the current annual NO 2 NAAQS in many of the locations where positive, and often statistically significant, associ ations with respiratory morbid ity endpoints have been reported (ISA, section 5.4). As discussed previously, the ISA characterized that evidence for respiratory effects as consistent and coherent. The evidence is consistent in that asso ciations are reported in studies conducted in numerous locations and wi th a variety of methodol ogical approaches (ISA, section 5.3.2.1). It is coherent in the sense that the studies report associations with respiratory health outcomes that are logi cally linked together (ISA, section 5.3.2.1). The ISA noted that when the epidemiologic literature is consider ed as a whole, there are generally positive associations between NO 2 and respiratory symptoms, hospital admissions, and emergency department visits. A number of these associations are statistically signif icant, particularly the more precise effect estimates (ISA, section 5.3.2.1). As discussed in the proposal (II.E.1) and above, the Administrator acknowledges that the interpretation of these NO 2 epidemiologic studies is complicated by the fact that on-road vehicle exhaust emissions are a nearly ubiquitous source of combustion pollutant mixtures that include
NO 2 . She notes that, in order to provide some perspective on the uncertainty related to the57 presence of co-pollutants the ISA evaluated epid emiologic studies that employed multi-pollutant models, epidemiologic studies of indoor NO 2 exposure, and experimental studies. Specifically, the ISA noted that a number of NO 2 epidemiologic studies have attempted to disentangle the effects of NO 2 from those of co-occurring pollutants by employing multi-pollutant models. When evaluated as a whole, NO 2 effect estimates in these m odels generally remained robust when co-pollutants were included. Therefore, de spite uncertainties associated with separating the effects of NO 2 from those of co-occurring pollutants, the ISA (section 5.4, p. 5-16) concluded that “the evidence summarized in this assessment indicates that NO 2 associations generally remain robust in multi-pollutant models and s upports a direct effect of short-term NO 2 exposure on respiratory morbidity at ambient concentratio ns below the current NAAQS.” With regard to indoor studies, the ISA noted that these studies can test hypotheses related to NO 2 specifically (ISA, section 3.1.4.1). Although confounding by indoor combustion sources is a concern, indoor studies are not confounded by the same mix of co- pollutants present in th e ambient air or by the contribution of NO 2 to the formation of s econdary particles or O 3 (ISA, section 3.1.4.1). The ISA noted that the findings of indoor NO 2 studies are consistent w ith those of studies using ambient concentrations from central site monito rs and concluded that indoor studies provide evidence of coherence for respirat ory effects (ISA, section 3.1.4.1). With regard to experimental studies, the REA noted that they have the adva ntage of providing information on health effects that are specifically associ ated with exposure to NO 2 in the absence of co -pollutants. The ISA concluded that the NO 2 epidemiologic literature is suppor ted by 1) evidence from controlled human exposure studies of airw ay hyperresponsiveness in asthmatics, 2) controlled human exposure and animal toxicological studies of impa ired host-defense systems and increased risk of susceptibility to viral and bacterial infection, and 3) controlled human exposure and animal58 toxicological studies of airw ay inflammation (ISA, section 5.3.2.1 and 5.4). Given the above consideration of the evidence, particular ly the epidemiologic studies reporting NO 2 -associated health effects in locations that meet the curr ent standard, the Administ rator agrees with the conclusion in the policy assessment chapter of th e REA that the scientific evidence calls into question the adequacy of the current standard to protect public health. In addition to the evidence-based considera tions described above, the Administrator has considered the extent to which exposure- and risk-based information can inform decisions regarding the adequacy of the current annual NO 2 standard. While she acknowledges the uncertainties associated with adju sting air quality in these analyses , she judges that such analyses are appropriate for considerati on in this review of the NO 2 primary NAAQS. In reaching this conclusion she notes the considerations disc ussed above, particular ly the endorsement by CASAC of the REA and its characterization of NO 2 -associated exposures and health risks. In considering the exposure- and risk-based in formation with regard to the adequacy of the current annual NO 2 standard to protect the public health, the Administrator notes the conclusion in the policy assessment chapter of the RE A that risks estimated to be associated with air quality adjusted upward to simulate just m eeting the current standard can reasonably be concluded to be important from a public health perspective. In particular, a large percentage (8- 9%) of respiratory-related ED visits in Atla nta could be associated with short-term NO 2 exposures, most asthmatics in Atlanta could be exposed on multiple days per year to NO 2 concentrations at or above 300 ppb, and most loca tions evaluated could e xperience on-/near-road
NO 2 concentrations above 100 ppb on more than half of the days in a given year. Therefore, after considering the re sults of the exposure and risk analyses presented in the REA the Administrator agrees with the conclusion of th e policy assessment chapter of the REA that59 exposure- and risk-based results reinforce the scientific evidence in supporting the conclusion that consideration should be given to revising th e current standard so as to provide increased public health protection, especi ally for at-risk groups, from NO 2 -related adverse health effects associated with short-term, and potential long-term, exposures. In reaching a conclusion on the adequacy of the current standard, the Administrator has also considered advice received from CASAC. In their comments on the final REA, CASAC agreed that the primary concern in this review is to protect against health effects that have been associated with short-term NO 2 exposures. CASAC also agreed that the current annual standard is not sufficient to protect public health against the types of exposures th at could lead to these health effects. As noted in their letter to the EPA Administrator, “CASAC concurs with EPA’s judgment that the current NAAQS does not protect the public’s health and that it should be revised” (Samet, 2008b). Based on the considerations discussed above , the Administrator concludes that the
current NO 2 primary NAAQS alone is not requisite to protect public health with an adequate margin of safety. Accordi ngly, she concludes that the NO 2 primary standard should be revised in order to provide increased public health protecti on against respiratory effects associated with short-term exposures, particularly for suscepti ble populations such as asthmatics, children, and older adults. In considering approaches to revising the current standard, the Administrator concludes that it is appropriate to consider se tting a new short-term standard (see below). The Administrator notes that such a short-term st andard could provide increased public health protection, especially for members of at-ris k groups, from effects described in both epidemiologic and controlled human exposure studies to be associated with short-term exposures
to NO 2 .60 F. Elements of a new short-term standard In considering a revised NO 2 primary NAAQS, the Administrato r notes the need to protect at-risk individuals from s hort-term exposures to NO 2 air quality that coul d cause the types of respiratory morbidity effects reported in epidem iologic studies and the need to protect at-risk individuals from short-term exposure to NO 2 concentrations reported in controlled human exposure studies to increase airway responsiv eness in asthmatics. The Administrator’s considerations with regard to her decisions are discussed in the following sections in terms of indicator (II.F.1), averaging time (II.F.2) , level (II.F.3), and form (II.F.4).

1. Indicator a. Rationale for proposed decision In past reviews, EPA has focused on NO 2 as the most appropriate indicator for ambient
NO X . In making a decision in the current revi ew on the most appropr iate indicator, the Administrator considered the conclusions of th e ISA and the policy assessment chapter of the REA as well as the view expressed by CASAC. The policy assessment chapter of the REA noted that, while the presence of NO X species other than NO 2 has been recogni zed, no alternative
to NO 2 has been advanced as being a more appropr iate surrogate. Controlled human exposure studies and animal toxicology stud ies assessed in the ISA provide specific evidence for health effects following exposure to NO 2 . Epidemiologic studies also typically report levels of NO 2 though the degree to which monitored NO 2 reflects actual NO 2 levels, as opposed to NO 2 plus other gaseous NO X , can vary (REA, section 2.2.3). In a ddition, because emissions that lead to the formation of NO 2 generally also lead to the formation of other NO X oxidation products, measures leading to reductions in population exposures to NO 2 can generally be expected to lead to reductions in population exposures to other gaseous NO X . Therefore, an NO 2 standard can61 also be expected to provide some degree of protec tion against potential hea lth effects that may be independently associated with other gaseous NO X even though such effects are not discernable from currently availabl e studies indexed by NO 2 alone. Given these key points, the policy assessment chapter of the REA concluded that the evidence supports retaining NO 2 as the indicator. Consistent with this conclusion, th e CASAC Panel stated in its letter to the EPA Administrator that it “conc urs with retention of NO 2 as the indicator” (Samet, 2008b). In light of the above considerations, the Admi nistrator proposed to retain NO 2 as the indicator in the current review.
b. Comments on indicator A relatively small number of comments direct ly addressed the issue of the indicator for the standard (CASAC, Dow, API, AAM, and the Missouri Department of Natural Resources Air Pollution Control Program (MODNR)). All of these commenters endorsed the proposal to continue to use NO 2 as the indicator for ambient NO X. c. Conclusions on indicator Based on the available information discussed above, and consistent with the views of CASAC and other commenters, the Administrator conc ludes that it is appropr iate to continue to
use NO 2 as the indicator for a standard that is intended to address effects associated with exposure to NO 2 , alone or in combination with other gaseous NO X . In so doing, the Administrator recognizes that measures lead ing to reductions in popul ation exposures to NO 2 will also reduce exposures to other nitrogen oxides. 2. Averaging time62 This section discusses considerations related to the averaging time of the NO 2 primary NAAQS. Specifically, this secti on summarizes the ra tionale for the Administrator’s proposed decision regarding averaging time (II.F.2.a; see s ection II.F.2 of the proposal for more detail), discusses comments related to av eraging time (II.F.2.b), and presen ts the Administrator’s final conclusions regarding averaging time (II.F.2.c). a. Rationale for proposed decision In considering the most appropriate averaging time for the NO 2 primary NAAQS, the Administrator noted in the pr oposal the conclusions and judg ments made in the ISA about available scientific evidence, air quality correla tions discussed in the REA, conclusions of the policy assessment chapter of the REA, and CASAC recommendations (section II.F.2 in the proposal). Specifically, sh e noted the following:

• Experimental studies in humans and animals have reported respiratory effects following
NO 2 exposures lasting from less than 1-hour up to several hours. Epidemiologic studies have reported associations between resp iratory effects and both 1-hour and 24-hour NO 2 concentrations. Therefore, the experimental evidence provides s upport for an averaging time of shorter duration than 24 hours (e.g., 1- hour) while the epidemiologic evidence provides support for both 1-hour and 24-hour averaging times. At a minimum, this suggests that a primary concern with regard to averaging time is the level of protection provided against 1-hour NO 2 concentrations.

• Air quality correlations presented in the po licy assessment chapter of the REA illustrated the relatively high degree of variability in the ratios of annual av erage to short-term NO 2 concentrations (REA, Table 10-2). This va riability suggests that a standard based on annual average NO 2 concentrations would not likely be an effective or efficient approach63 to focus protection on short-term exposures.

• These air quality correlations (REA, Table 10-1) suggested that a standard based on 1- hour daily maximum NO 2 concentrations could also be e ffective at protecting against 24- hour NO 2 concentrations.

• The policy assessment chapter of the REA concluded that the scientific evidence, combined with the air quality correlations , support the appropriateness of a standard based on 1-hour daily maximum NO 2 concentrations to protect against health effects associated with short-term exposures.

• CASAC concurred “with having a short-term NAAQS primary standard for oxides of nitrogen and using the one-hour maximum NO 2 value” (Samet, 2008b). Based on these considerations, the Administrator proposed to set a new standard based on 1-hour daily maximum NO 2 concentrations. b. Comments on averaging time As discussed above, CASAC endorsed the establ ishment of a new standard with a 1-hour averaging time. CASAC stated the following in their comment s on the proposal (Samet, 2009): In reviewing the REA, CASAC supported a short-term standard for NO 2 and in reviewing the proposal, CASA C supports the proposed one-hour averaging time in EPA's proposed rule. The supporting rationale offered by CASAC in su pport of a new 1-hour standard was generally the same as that put forward in the final RE A and the proposal. Speci fically, that rationale considered the available sc ientific evidence, which s upports a link between 1-hour NO 2 concentrations and adverse respiratory effects, a nd air quality information presented in the REA, which suggests that a 1-hour standard can pr otect against effects lin ked to short-term NO 2 exposures while an annual standardwould not be an effective or efficient approach to protecting64 against these effects. A large number of public commenters also endo rsed the establishment of a new standard with a 1-hour averaging time. These included a number of State agenci es and organizations (e.g., NACAA, NESCAUM and agencies in CA, IL , NM, TX, VA); environmental, medical, and public health organizations (e.g., ACCP, ALA, AMA, ATS, CAC, EDF, EJ, GASP, NACPR, NAMDRC, NRDC); and most individual commen ters. The supporting rationales offered by these commenters often acknowledg ed the recommendations of CASAC and the Administrator’s rationale as discussed in the proposal. Though many industry commenters recomme nded not revising the current annual standard (as discussed above in section II.E.2), severa l of these groups did conclude that if a short-term standard were to be set, a 1-hour averaging time woul d be appropriate (e.g., Colorado Petroleum Association (CPA), Dow, NAM, Petroleum Associati on of Wyoming (PAW), Utah Petroleum Association (UPA)). As discussed above, industry commenters who disagreed with setting a new 1-hour standard generally based th is conclusion on their interpretation of the scientific evidence and their conclusion that this evidence does not support the need to revise the current annual standard. These comments, and EPA’s responses, are discussed in more detail above (section II.E) and in the Response to Comments document. c. Conclusions on averaging time In considering the most appropriate averaging time for the NO 2 primary NAAQS, the Administrator notes the available scientific evid ence as assessed in the ISA, the air quality analyses presented in the REA, the conclusions of the policy assessment chapter of the REA, CASAC recommendations, and public comments rece ived. These considerations are described below.65 When considering averaging time, the Admi nistrator notes that the evidence relating short-term (minutes to hours) NO 2 exposures to respiratory morb idity was judged in the ISA to be “sufficient to infer a likely causal relati onship” (ISA, section 5.3.2.1) while the evidence relating long-term (w eeks to years) NO 2 exposures to adverse health effects was judged to be either “suggestive but not sufficient to infer a cau sal relationship” (respi ratory morbidity) or “inadequate to infer the presence or absen ce of a causal relationship” (mortality, cancer, cardiovascular effects, reproductive/developm ental effects) (ISA, sections 5.3.2.4-5.3.2.6). Thus, the Administrator concludes that these judgments most direc tly support an averaging time that focuses protection on short-term exposures to NO 2 . As in past reviews of the NO 2 NAAQS, the Administrator notes that it is instructive to evaluate the potential for a sta ndard based on annual average NO 2 concentrations, as is the current standard, to provide pr otection against short-term NO 2 exposures. To this end, the Administrator notes that Table 10-1 in the REA reported the ratios of short-term to annual average NO 2 concentrations. Ratios of 1-hour daily maximum concentrations (98 th and 99 th percentile 10 ) to annual average concentrations across 14 locations ranged from 2.5 to 8.7 while ratios of 24-hour average concentrations to annu al average concentrations ranged from 1.6 to 3.8 (see Thompson, 2008 for more details). The policy assessment chapter of the REA concluded that the variability in these ratios across locatio ns, particularly those for 1-hour concentrations, suggested that a standard based on annual average NO 2 concentrations would not likely be an effective or efficient approach to focus protection on short-term NO 2 exposures. For example, in

10 As discussed below, 98 th and 99 th percentile forms were evaluated in the REA. A 99 th percentile form corresponds approximately to the 4 th highest 1-hour concentr ation in a year while a 98 th percentile form corresponds approximately to the 7 th or 8 th highest 1-hour concentration in a year. A 4 th highest concentration form has been used previously in the O 3 NAAQS while a 98 th percentile form has been us ed previously in the PM 2.5 NAAQS.66 an area with a relatively high ratio (e.g., 8), the current annua l standard (53 ppb) would be expected to allow 1- hour daily maximum NO 2 concentrations of about 400 ppb. In contrast, in an area with a relatively low rati o (e.g., 3), the current standard would be expected to allow 1- hour daily maximum NO 2 concentrations of about 150 ppb. Thus, for purposes of protecting against the range of 1-hour NO 2 exposures, the REA noted thata standard based on annual average concentrations would likely require more control than necessary in some areas and less control than necessary in others, depe nding on the standard level selected. In considering the level of support available for specific short-term averaging times, the Administrator notes that the pol icy assessment chapter of the REA considered evidence from both experimental and epidemiologic studies. Controlled human exposure studies and animal toxicological studies provide evidence that NO 2 exposures from less than 1-hour up to 3-hours can result in respiratory effects such as increas ed airway responsivene ss and inflammation (ISA, section 5.3.2.7). Specifically, the ISA concluded that NO 2 exposures of 100 ppb for 1-hour (or 200 ppb to 300 ppb for 30-min) can result in small but significant increases in nonspecific airway responsiveness (ISA, sec tion 5.3.2.1). In contrast, the epidem iologic literature provides support for short-term averaging times ranging from ap proximately 1-hour up to 24-hours (ISA, section 5.3.2.7). A number of epidemiologic studies ha ve detected positive associations between respiratory morbidity and 1-hour (d aily maximum) and/or 24-hour NO 2 concentrations. A few epidemiologic studies have considered both 1-hour and 24-hour averaging times, allowing comparisons to be made. The ISA reported that such comparisons in studies that evaluate asthma emergency department visits failed to re veal differences between effect estimates based on a 1-hour averaging time and those based on a 24-hour averaging time (ISA, section 5.3.2.7). Therefore, the ISA concluded that it is not possible, from the available epidemiologic evidence,67 to discern whether effects observed are attr ibutable to average daily (or multi-day) concentrations (24-hour averag e) or high, peak exposures (1 -hour maximum) (ISA, section 5.3.2.7). As noted in the policy assessment chapter of the REA, given the above conclusions, the experimental evidence provides support for an averaging time of shorter duration than 24 hours (e.g., 1-h) while the epidemiologic evidence provides support for both 1-hour and 24-hour averaging times. The Administra tor concludes that, at a minimum, this suggests that a primary concern with regard to averaging time is th e level of protection provided against 1-hour NO 2 concentrations. However, she also notes that it is important to consider the ability of a 1-hour averaging time to protectagainst 24-hour average NO 2 concentrations. To this end, the Administrator notes that Tabl e 10-2 in the REA presented co rrelations between 1-hour daily maximum NO 2 concentrations and 24-hour average NO 2 concentrations (98 th and 99 th percentile) across 14 locations (see Thompson, 2008 for more de tail). Typical ratios ranged from 1.5 to 2.0, though one ratio (Las Vegas) was 3.1 . These ratios were far less variable than those discussed above for annual average concentrations, sugge sting that a standard based on 1-hour daily maximum NO 2 concentrations could also be effe ctive at protecting against 24-hour NO 2 concentrations. The REA concluded that the scie ntific evidence, combined with the air quality correlations described above, support the appropri ateness of a standard based on 1-hour daily maximum NO 2 concentrations to protect against hea lth effects associated with short-term
exposures. Based on these considerations, the Administrato r concludes that a standard with a 1-hour averaging time can effectively lim it short-term (i.e., 1- to 24-hour s) exposures that have been linked to adverse respiratory e ffects. This conclusion is ba sed on the observations summarized68 above and in more detail in the proposal, partic ularly that: 1) the 1-hour averaging time has been directly associated with respir atory effects in both epidemiologi c and experimental studies and that 2) results from air quality analyses suggest that a 1-hour standard could also effectively control 24-hour NO 2 concentrations. In addition, the Ad ministrator notes the support provided for a 1-hour averaging time in comments fr om CASAC, States, environmental groups, and medical/public health groups. The Administrator notes that arguments offered by some industry groups against setting a 1-hour NO 2 standard generally focus on commenters’ conclusions regarding uncertainties in the sc ientific evidence. As discusse d in more detail above (section II.E.2), the Administrator disagr ees with the conclusions of th ese commenters regarding the appropriate interpretation of the scientific evidence and associated uncertainties. Given these considerations, the Administ rator judges that it is appropriate to set a new NO 2 standard with a 1- hour averaging time.
3. Form This section discusses co nsiderations related to the form of the 1-hour NO 2 primary NAAQS. Specifically, this secti on summarizes the ra tionale for the Administrator’s proposed decision regarding form (II.F.4.a; see section II.F. 3 of the proposal for more detail), discusses comments related to form (II.F.4.b), and presents the Administrator’s final conclusions regarding form (II.F.4.c). a. Rationale for proposed decision When considering alternative forms in the proposal, the Administrator noted the conclusions in the policy assessment chapter of th e REA. Specifically, she noted the conclusion that the adequacy of the public health protection provided by the combination of standard level and form should be the foremost considerati on. With regard to this, she noted that69 concentration-based forms can better reflect pollu tant-associated health risks than forms based on expected exceedances. This is the cas e because concentration-based forms give proportionally greater weight to years when polluta nt concentrations are well above the level of the standard than to years when the concentrations are just above the standard, while an expected exceedance form would give the sa me weight to years with concen trations that just exceed the standard as to years when concentrations grea tly exceed the standard. The Administrator also recognized the conclusion in the policy assessment chapter of the REA that it is desirable from a public health perspective to have a form that is reasonably stable and insulated from the impacts of extreme meteorological events. With regard to this, she noted that a form that calls for averaging concentrations over thr ee years would provide greater re gulatory stability than a form based on a single year of concentrations. Theref ore, consistent with recent reviews of the O 3 and PM NAAQS, the proposal focused on concentratio n-based forms averaged over 3 years, as evaluated in the REA. In considering specific concentration-based forms, the REA focused on 98 th and 99 th percentile concentrations averaged over 3 year s. This focus on the upper percentiles of the distribution is appropriate give n the reliance, in part, on NO 2 health evidence from experimental studies, which provide information on specific exposure concentrations that are linked to specific health effects. The REA noted that a 99 th percentile form for a 1- hour daily maximum standard would correspond approximately to the 4 th highest daily maximum c oncentration in a year (which is the form of the current O 3 NAAQS) while a 98 th percentile form (which is the form of the current short-term PM 2.5 NAAQS) would correspond approximately to the 7 th or 8 th highest daily maximum concentration in a year (REA, Table 10-4; see Thompson, 2008 for methods). Consideration in the REA of an appropriate form for a 1-hour standard was based on70 analyses of standard levels that reflected the allowable area-wide NO 2 concentration, not the maximum allowable concentration. Therefore, in their review of the final REA, CASAC did not have the opportunity to comment on the appropriate ness of specific forms in conjunction with a standard level that reflects the maximum allowable NO 2 concentration anywhere in an area. Given this, when considering a lternative forms for the 1-hour standard in the proposal, the Administrator judged that it was appropriate to consider both fo rms evaluated in the REA (i.e.,
98 th and 99 thpercentiles). Therefore, sh e proposed to adopt either a 99 th percentile or a 4 th highest form, averaged over 3 year s, and she solicited comment on both 98 th percentile and 7 th or 8 th highest forms. b. CASAC and public comments on form In their letter to the Administrator, CASAC di scussed the issue of form within the context of the proposed approach of setting a 1-hour standa rd level that reflects the maximum allowable
NO 2 concentration anywhere in an area. CASA C recommended that, for such a standard, EPA adopt a form based on the 3-year average of the 98 th percentile of the distribution of 1-hour daily maximum NO 2 concentrations. Specifically, they stat ed the following in their comments on the proposal (Samet, 2009): The 98th percentile is preferred by CA SAC for the form, given the likely instability of measurements at the upper range and the absence of data from the proposed two-tier approach. As indicated in their letter, CASAC concluded th at the potential instabil ity in higher percentile
NO 2 concentrations near major roads argues for a 98 th , rather than a 99 th , percentile form. Several State organizations and agencies (e .g., NESCAUM and agencies in IN, NC, SD, VA) and industry groups (e.g., AAM, ACC, API, AirQ uality Research and Logistics (AQRL), CPA, Dow, ExxonMobil, IPAMS, PAW, UPA) also recommended a 98 th percentile form in order to71 provide regulatory stability. In contrast, a small number of State and local agencies (e.g., in MO and TX), several environmental organiza tions (e.g., EDF, EJ, GASP, NRDC), and medical/public health organizations (e .g., ALA, ATS) recommended either a 99 th percentile form or a more stringent form (e.g., no exceedan ce) to further limit the occurrence of NO 2 concentrations that exceed the standard level in locations that attain the standard. c. Conclusions on form The Administrator recognizes that there is not a clear health ba sis for selecting one specific form over another. She also recognizes th at the analyses of different forms in the REA are most directly relevant to a standard that reflects NO 2 concentrations permitted to occur broadly across a community, rather than the maxi mum concentration that can occur anywhere in the area. In contrast, as discussed below (s ection II.F.4.c), the Administrator has judged it appropriate to set a new 1-hour standard that reflects the maximum allowable NO 2 concentration anywhere in an area. In light of this, th e Administrator places particular emphasis on the comments received on form from CASAC relating to a 1-hour standard level that reflects the maximum allowable NO 2 concentration anywhere in an area. In particular, the Administrator notes that CASAC recommended a 98 th percentile form averaged over 3 years for such a standard, given the potential for in stability in the higher percenti le concentrations around major roadways. In considering this recommendation, the Admini strator recognizes th at the public health protection provided by the 1-hour NO 2 standard is based on the appro ach used to set the standard and the level of the standard (see below), in c onjunction with the form of the standard. Given that the Administrator is setting a standa rd that reflects the maximum allowable NO 2 concentration anywhere in an area, rather than a standard that reflects the allowable area-wide72 NO 2 concentration, she agrees with CASAC that an appropriate consideration with regard to form is the extent to which specific statistics could be unstable at locations where maximum NO 2 concentrations are expected, such as near major roads. When considering alternative forms for the standard, the Administrator notes that an unst able form could result in areas shifting in and out of attainment, potentially disrupting ongoing air quality planning w ithout achieving public health goals. Given the limited availabl e information on the variability in peak NO 2 concentrations near important sources of NO 2 such as major roadways, and given the recommendation from CASAC that the potential for instability in the 99 th percentile concentration is cause for supporting a 98 th percentile form, the Administrator judges it appropriate to set the form base d on the 3-year average of the 98 th percentile of the annual distribution of 1-hour daily maximum NO 2 concentrations.

4. Level

As discussed below and in more detail in th e proposal (section II.F .4), the Administrator has considered two different approaches to setting the 1-hour NO 2 primary NAAQS. In the proposal, each of these approaches was linked wi th a different range of standard levels. Specifically, the Administrator proposed to set a 1-hour standard reflecting the maximum allowable NO 2 concentration anywhere in an area and to set the level of such a standard from 80 to 100 ppb. The Administrator also solicited comme nt on the alternative a pproach of setting a standard that reflects the allowable area-wide NO 2 concentration and set ting the standard level from 50 to 75 ppb. This section summarizes th e rationale for the Administrator’s proposed approach and range of standard levels (II.F.3.a), describes the alternative approach and range of standard levels (II.F.3.b), discusses comments re lated to each approach and range of standard levels (II.F.3.c), and presents the Administrato r’s final conclusions regarding the approach and73 level (II.F.3.d). a. Rationale for proposed deci sions on approach and level In assessing the most appropriate approach to setting the 1-hour standard and the most appropriate range of standard le vels to propose, the Administra tor considered the broad body of scientific evidence assessed in the ISA, including epidemiologi c and controlled human exposure studies, as well as the results of exposure/risk an alyses presented in the REA. In light of the body of available evidence and analyses, as descri bed above, the Administrator concluded in the proposal that it is necessary to provide increased public health pr otection for at-r isk individuals against an array of adverse resp iratory health effects linked with short-term (i.e., 30 minutes to 24 hours) exposures to NO 2 . Such health effects have been associated with exposure to the distribution of short-term ambient NO 2 concentrations across an area, including higher shortterm (i.e., peak) exposure concentrations, such as those that can occur on or near major roadways and near other sources of NO 2 , as well as the lower short-term exposure concentrations that can occur in areas not near major ro adways or other sources of NO 2 . The Administrator’s proposed decisions on approach and level, as discussed in detail in the propos al (section II.F.4), are outlined below. In considering a standard-setting approach, th e Administrator was mindful in the proposal that the available evidence and analyses fr om the ISA and REA support the public health importance of roadway-associated NO 2 exposures. The exposure assessment described in the REA estimated that roadway-associated exposures account for the majority of exposures to peak
NO 2 concentrations (REA, Figures 8-17, 8-18). The ISA concluded (section 4.3.6) that NO 2 concentrations in heavy traffic or on freeway s “can be twice the residential outdoor or residential/arterial road le vel.” In considering the potential variability in the NO 2 concentration74 gradient, the proposal noted that availa ble monitoring studie s suggest that NO 2 concentrations could be 30 to 100% higher than those in the same area but away from the road. 11
The Administrator also considered that millions of people in the United States live, work, and/or attend school near important sources of NO 2 such as major roadways (ISA, section 4.4), and that ambient NO 2 concentrations in these locations vary depending on the distance from major roads (i.e., the closer to a major road, the higher the NO 2 concentration) (ISA, section 2.5.4). Therefore, these populations, which lik ely include a dispr oportionate number of individuals in groups with hi gher prevalence of asthma and higher hospitalization rates for asthma (e.g. ethnic or racial minorities and indi viduals of low socioeconomic status ) (ISA, section 4.4), are likely exposed to NO 2 concentrations that are hi gher than those occurring away from major roadways. Given the above considerations, the Administra tor proposed an appro ach to setting the 1- hour NO 2 primary NAAQS whereby the standard would reflect the maximum allowable NO 2 concentration anywhere in an area. In many locations, this concen tration is likely to occur on or near a major roadway. EPA proposed to set the le vel of the standard such that, when available information regarding the concen tration gradient around roads is considered, appropriate public health protection would be provided by lim iting the higher short-term peak exposure concentrations expected to occur on and near ma jor roadways, as well as the lower short-term exposure concentrations expected to occur away from those roadways. The Administrator concluded that this appro ach to setting the 1-hour NO 2 NAAQS would be expected to protect

11 In addition, the air quality analyses pres ented in the REA estimated that on-road NO 2 concentrations are about 80% hi gher on average than concentrati ons away from the road (REA, section 7.3.2) and that NO 2 monitors within 20 m of roads measure NO 2 concentrations that are, on average across locations, 40% higher than con centrations measured by monitors at least 100 m from the road (REA, compar e Tables 7-11 and 7-13).75 public health against exposure to the distribution of short-term NO 2 concentrations across an area and would provide a relatively high degree of confidence regarding the protection provided against peak exposures to higher NO 2 concentrations, such as thos e that can occur around major roadways. The remainder of this section discusse s the proposed range of standard levels. In considering the appropriate range of levels to propose for a standa rd that reflects the maximum allowable NO 2 concentration anywhere in an area, the Administrator considered the broad body of scientific evidence an d exposure/risk information as well as available information on the relationship between NO 2 concentrations near roads and those away from roads. Specifically, she considered the extent to which a variety of levels would be expected to protect at-risk individuals against increased airwayresponsiveness, resp iratory symptoms, and respiratory-related emergency departme nt visits and hospital admissions. After considering the scientif ic evidence and the exposure/ri sk information (see sections II.B, II.C, and II.F.4.a.1 through II.F.4.a.3 in the pr oposal), as well as the available information on the NO 2 concentration gradient around roadways (s ection II.A.2 above and in the proposal), the Administrator concluded that the strongest support is for a st andard level at or somewhat below 100 ppb. The Administrator’s rationale in reaching this proposed conclusion is provided below. The Administrator noted that a standard level at or somewhat below 100 ppb in conjunction with the proposed approach w ould be expected to limit short-term NO 2 exposures to concentrations that have been reported to increas e airway responsiveness in asthmatics (i.e., at or above 100 ppb). While she acknowledged that exposure to NO 2 concentrations below 100 ppb could potentially increase airwayresponsiveness in some asthma tics, the Administrator also noted uncertainties regarding the magnitude and the clinical significance of the NO 2 -induced76 increase in airway responsiveness, as discus sed in the policy assessment chapter of the REA (section 10.3.2.1, discussed in sectio n II.F.4.e in the proposal). Given these uncertainties, the Administrator concluded in the proposal that controlled human exposure studies provide support for limiting exposures at or somewhat below 100 ppb NO 2 . The Administrator also noted that a standard level at or somewhat below 100 ppb in conjunction with the proposed approach would be expected to maintain peak area-wide NO 2 concentrations considerably below those measur ed in locations where key U.S. epidemiologic studies have reported associations with more serious respiratory effects, as indicated by increased emergency department visits and hosp ital admissions. Specifically, the Administrator noted that 5 key U.S. studies provide evidence fo r such associations in locations where the 99 th percentile of the distributi on of 1-hour daily maximum NO 2 concentrations measured at areawide monitors ranged from 93 to 112 ppb (Ito et al ., 2007; Jaffe et al., 2003; Peel et al., 2005; Tolbert et al., 2007; and a study by the New York State Department of Health, 2006). 12 The Administrator concluded that th ese studies provide support for a 1-hour standard that limits the
99 th percentile of the distribution of 1-hour daily maximum area-wide NO 2 concentrations to below 90 ppb (corresponds to a 98 th percentile concentration of85 ppb), and that limiting areawide concentrations to consider ably below 90 ppb would be appropr iate in order to provide an adequate margin of safety. The Administrator noted that, based on available information about the NO 2 concentration gradient around roads, a sta ndard level at or somewhat below 100 ppb set in conjunction with the proposed approach would be expected to accomplish this. Specifically, she noted that given availa ble information regarding NO 2 concentration gradients around roads (see section II.A.2), a standard level at or below 100 ppb (with either a 99 th or 98 th percentile

12 The 98 th percentile concentrations in these study locations ra nged from 85 to 94 ppb.77 form) would be expected to limit peak area-wide NO 2 concentrations to approximately 75 ppb or below. 13 Therefore, the Administrator concluded that a standard level at or somewhat below 100 ppb under the proposed approach would be expected to maintain peak area-wide NO 2 concentrations well below 90 ppb across location s despite the expected variation in the NO 2 concentration gradient that canexist around roadways in differe nt locations and over time. The Administrator also noted that a study by Delfino provides mixed evidence for effects in a location with area-wide 98 th and 99 th percentile 1-hour daily maximum NO 2 concentrations of 50 and 53 ppb, respectively. In that study, NO 2 effect estimates were positive, but some reported 95% confidence limits for the odds ratio (OR) that included values less than 1.00. Given the mixed results of the Delfino study, th e Administrator conclude d that it may not be necessary to maintain area-wide NO 2 concentrations at or belo w 50 ppb to provide protection against the effects reported in epidemiologic studies. In addition to these eviden ce-based considerations, the Administrator noted that a standard level at or somewhat below 100 ppb under the proposed approach would be consistent with the results of the exposure and risk analyses presented in the REA. As discussed in section II.C of the proposal, the results of these analyses provide support fo r setting a standard that limits 1-hour area-wide NO 2 concentrations to between 50 a nd 100 ppb. As described above, a standard level of 100 ppb that re flects the maximum allowable NO 2 concentration would be expected to maintain area-wide NO 2 concentrations at or below approximately 75 ppb. Given all of these considerations, the Admi nistrator concluded in the proposal that a standard level at or somewhat below 100 ppb (with a 99 th percentile form), in conjunction with the proposed

13 For a standard of 100 ppb, area-wide concentr ations would be expected to range from approximately 50 ppb (assuming near-road concen trations are 100% higher than area-wide concentrations) to 75 ppb (assuming near-road co ncentrations are 30% higher than area-wide concentrations).78 approach, would be requisite to protect public hea lth with an adequate margin of safety against the array of NO 2 -associated health effects. In addition to the considerat ions discussed above, which s upport setting a standard level at or somewhat below 100 ppb, the Administrator al so considered the extent to which available evidence could support standard levels below 100 ppb. The Administrator concluded that the evidence could support setting the standard level below 100 ppb to the extent the following were emphasized:

• The possibility that an NO 2 -induced increase in airway re sponsiveness could occur in asthmatics following exposures to concentra tions below 100 ppb and/or the possibility that such an increase could be clinically significant

• The mixed results reported in the study by Delfi no et al. (2002) of an association between respiratory symptoms and the relatively low ambient NO 2 concentrations measured in the study area Specifically, she noted that a standard level of 80 ppb (99 th percentile form), in conjunction with the proposed approach, could limit area-wide NO 2 concentrations to 50 ppb 14 and would be expected to limit exposure concentrations to be low those that have been reported to increase airway responsiveness in asthmatics. For the reasons stated above, th e Administrator proposed to set the level of a new 1-hour standard between 80 ppb and 100 ppb. b. Rationale for the alternative approach and range of levels

14 This conclusion assumes that near-road NO 2 concentrations are 65% higher than area-wide concentrations, reflecting the mid-point in the range of 30 to 100%. Based on available information suggesting that near-road concentrat ions can be 30 to 100% higher than area-wide concentrations, a standard level of 80 ppb coul d limit area-wide concentrations to between 40 and 60 ppb.79 As described above, the Administra tor proposed to set a 1-hour NO 2 NAAQS reflecting the maximum allowable NO 2 concentration anywhere in an area and to set the level of such a standard from 80 to 100 ppb. However, prior to the proposal, the approach of setting a 1-hour
NO 2 NAAQS that reflects the maximum allowable NO 2 concentration anywhere in an area had not been discussed by EPA in the REA or considered by CASAC. Rather, the potential alternative standards discussed in the REA, and reviewed by CASAC, reflected allowable areawide NO 2 concentrations (i.e., concentrations that occur broadly across communities). Given this, the Administrator noted in th e proposal that comments received on the approach to setting the 1-hour standard (i.e., from CASAC and from me mbers of the public) could provide important new information for cons ideration. Therefore, the Administrator also solicited comment on the alternative approach of setting a 1-hour NO 2 primary NAAQS that would reflect the allowable area-wide NO 2 concentration, analogous to the standards evaluated in the REA, and with a level se t within the range of 50 to 75 ppb. In discussing this alternative approach with a standard level from 50 to 75 ppb, the Administrator noted the following in the proposal:

• Such a standard would be expected to maintain area-wide NO 2 concentrations below peak 1-hour area-wide concentrations measured in locations where key U.S. epidemiologic studies have reported associations with resp iratory-related emergenc y department visits and hospital admissions.

• Standard levels from the lower end of th e range would be expe cted to limit roadwayassociated exposures to NO 2 concentrations that have been reported in controlled human exposure studies to increase airway respons iveness in asthmatics. Specifically, a standard level of 50 ppb under this approach could limit near-road concentrations to80 between approximately 65 and 100 ppb, dependi ng on the relationship between near-road
NO 2 concentrations and area -wide concentrations.

• This alternative approach would provide rela tively more confidence regarding the degree to which a specific standard level would limit area-wide NO 2 concentrations and less confidence regarding the degree to which a spec ific standard level would limit the peak
NO 2 concentrations likely to o ccur near major roadways. c. Comments on approach and level In the proposal, each approach to setting the 1-hour standard, and each range of standard levels, was linked to different re quirements for the design of the NO 2 monitoring network. Specifically, in conjunction with the proposed ap proach (i.e., standard reflects the maximum allowable NO 2 concentration anywhere in an area and th e level is set within the range of 80 to 100 ppb), the Administrator proposed to establis h a 2-tiered monitoring network that would include monitors sited to measure the maximum NO 2 concentrations anywhere in an area, including near major roadways, and monitors sited to measure maximum area-wide NO 2 concentrations. In conjunction with the alterna tive approach (i.e., standa rd reflects the allowable area-wide NO 2 concentration and the level is set within the range of 50 to 75 ppb), the Administrator solicited comment on a monitori ng network that would only include area-wide
NO 2 monitors. Because of thes e linkages in the proposal, most commenters combined their comments on the approach to setting a 1-hour sta ndard and on the standard level with their comments on the monitoring requirements. In th is section, we discu ss comments from CASAC and public commenters on the appro ach to setting a 1-hour standard and on the standard level. Comments on the monitoring network are also di scussed in this section to the extent they indicate a preference for either the proposed or alternative approach to setting the 1-hour81 standard. More specific comments on monito r placement and network design are discussed below in section III.B.2 and in the Response to Comments document. EPA responses to technical comments on the scie ntific evidence and the expos ure/response information are discussed above in section II. E.2 and in the Response to Comments document. The Administrator’s response to commenters’ views on the approach to settin g the 1-hour standard and on the standard level is embodied in the discussed in section II.F.4.d. i. CASAC comments on the approach to setting the standard A majority of CASAC and CASAC Panel members 15 favored the proposed approach of setting a 1-hour standard that reflects the maximum allowable NO 2 concentration anywhere in an area and linking such a standard with a 2-tier ed monitoring network that would include both near-road and area-wide monito rs, though CASAC did not reach consensus on this approach. Specifically, in their letter to the Administra tor (Samet, 2009), CASAC stated the following: There was a split view on the two ap proaches among both CASAC and CASAC panel members with a majority of each favoring the Agency's proposed two-tiered monitoring network because they thought th is approach would be more effective in limiting near-roadway exposures that ma y reach levels in the range at which some individuals with asthma may be adversely affected. Other members acknowledged the need for research a nd development of near-road monitoring data for criteria pollutants ingeneral but favored reten tion of EPA's current areawide monitoring for NO 2 regulatory purposes, due to the lack of epidemiological data based on near-roadway exposure measurements and issues related to implementing a near-road monitoring system for NO 2 . Thus, the recommendation of the majority of CASAC Panel members was based on their conclusion that the proposed approach would be more effective than the alternative at limiting near-roadway exposures to NO 2 concentrations that could adve rsely affect asthmatics. In

15 CASAC members were also part of the CASAC Panel for the NO 2 NAAQS review (i.e., the Oxides of Nitrogen Primary National Ambient Air Quality Standards Panel). Therefore, references to the CASAC Panel include both CASAC members and Panel members.82 addition, these CASAC Panel members noted impor tant uncertainties with the alternative approach. Specifically, they st ated the following (Samet, 2009): Panel members also supported the propos ed two-tiered approach because basing regulations on area-wide m onitoring alone was problematic. Such an approach would require EPA to embed uncertainties and assumptions about the relationship between area-wide and road-side mon itoring into the area-wide standard. A minority of CASAC Panel members expresse d support for the alternative approach of setting a 1-hour standard that reflects the allowable area-wide NO 2 concentration. These CASAC Panel members concluded that there woul d be important uncertainties associated with the proposed approach. Specifically, they noted that the key U.S. NO 2 epidemiologic studies relied upon area-wide NO 2 concentrations. In their view, the use of area-wide concentrations in these studies introduces uncertainty into the selection of a standard level for a standard that reflects the maximum allowable NO 2 concentration anywhere in an area and that is linked with a requirement to place monitors near major roads. As a result of this uncertainty, CASAC Panel members who favored the alternative approach note d that “it would be bette r to set the standard on the same area-wide monitoring basis as empl oyed in the epidemiologic studies upon which it [the standard] now relies” (Samet, 2009). Thes e CASAC Panel members also strongly supported obtaining monitoring data near major roads, wh ile recognizing uncertainties associated with identifying appropriate monitoring sites near roads (see section III.B.2 and the Response to Comments document for more discussi on of CASAC’s monitoring comments). ii. Public comments on the appro ach to setting the standard Consistent with the views expressed by the majority of CASAC members, a number of commenters concluded that the mostappropriate approach would be to set a 1-hour standard that reflects the maximum allowable NO 2 concentration anywhere in an area and to couple that standard with a requirement that monitors be placed in locations where maximum concentrations83 are expected, including near major roads. This view was expressed by some State and local agencies (e.g., in CA, IA, NY, TX, WA, WI), by a numbe r of environmental organizations (e.g., CAC, EDF, EJ, GASP, NRDC), by the ALA, and individual commenters. Several additional medical and public health organizations (ACCP, AMA, ATS, NAD RC, NACPR) did not explicitly express a recommendation regarding the appro ach though these organizations did recommend that, in setting a 1-hour standard, part icular attention should be paid to NO X concentrations around major roadways. In s upport of their recommendation to adopt the proposed approach and to focus monitoring ar ound major roads, thes e commenters generally concluded that a primary consideration should be the extent to which the NO 2 NAAQS protects at-risk populations that live and/or attend school near important sources of NO 2 such as major roads. As such, these comments supported the rationale in the proposal for setting a 1-hour standard that reflects the maximum allowable NO 2 concentration anywhere in an area. A number of State commenter s expressed the view that area-wide monitors should be used for attainment/non-attainment determin ations (e.g., NACAA, NESCAUM and agencies in IL, IN, MI, MS, NC, NM, SC). One State comm enter (NESCAUM) agreed with EPA concerns about near-road exposures but c oncluded that it is premature to establish a large near-road monitoring network at this time due to uncertain ty regarding the relati onship between near-road and area-wide NO 2 concentrations and the variability in that relationship. NESCAUM recommended that EPA work with States to esta blish a targeted monito ring program in select urban areas to gather data that would inform fu ture modifications to th e monitoring network, but that “The existing area-wide mon itoring network should be used to identify initial nonattainment areas.” Other State commenters also concluded th at the most appropriate approach would be to base non-attainment determinations only on ar ea-wide monitors. Based on their monitoring84 comments, many of these commenters appeared to support setting a 1-hour st andard that reflects the allowable area-wide NO 2 concentration. State concerns w ith the proposed approach often included uncertainties associated with identifyi ng and accessing appropriate monitor sites near major roads, as well as concerns related to im plementation and cost to States (as discussed further in the Response to Comments document, the Administrator may not consider cost of implementation in decisions on a NAAQS). One commenter (AAM) concluded that th e focus of the proposed approach on NO 2 concentrations around major roadways is not justified because the REA and the proposal overstate the extent to which NO 2 concentrations near roads are higher than NO 2 concentrations farther away from the road. This conclusi on is based on an analysis of 42 existing NO 2 monitors in 6 locations. Comparing NO 2 concentrations measured by these monitors, some of which are close to roads and other of whic h are farther from roads, led AA M to conclude that “roadside monitors are not measuring high NO 2 concentrations.” We agree that there is uncertainty associat ed with estimates of roadway-associated NO 2 concentrations (see REA, sections 7.4.6 and 8.4.8.3 fo r detailed discussion of these uncertainties) and in identifying locations where maximum concentr ations are expected to occur. However, we note that the Administrator’s conclusions regarding the relationship between NO 2 concentrations near roads and those away from roads rely on multiple lines of scientific evidence and information. Specifically, the administrator relie d in the proposal on the following in drawing conclusions regarding the distribution of NO 2 concentrations across areas:

• Monitoring studies discussed in the ISA and REA that were designed to characterize the
NO 2 concentration gradient around ro ads, which indicated that NO 2 concentrations near85 roads can be approximately 30 to 100% higher than concentrations in the same area but away from the road

• Air quality and exposure analyses presen ted in the REA which estimate that NO 2 concentrations on roads could be 80% higher than away from the road, on average across locations, and that roadway-associated exposur es account for the majority of exposures to
NO 2 concentrations at or above 100 ppb In contrast, while the analysis subm itted by AAM does provide information on NO 2 concentrations at different dist ances from roads, the existing NO 2 monitoring network was not designed to characterize the spatial gradients in NO 2 concentrations surrounding roadways. Rather, concentrations of NO 2 measured by existing monitors are likely to reflect contributions from a combination of mobile and stationary sources, with one or the other dominating depending on the proximity of thes e sources to the monitors. Ther efore, we conclude that the analysis submitted by AAM, which does not consider other relevant lines of evidence and information, does not appropriately ch aracterize the relationship between NO 2 concentrations near roads and those away from roads. (See th e Response to Comments document for a more detailed discussion of AAM comments). In addition, we note that, a lthough the Administrator conclu ded in the proposal that maximum NO 2 concentrations in many areas are likel y to occur around major roads, she also allowed for situations where this is not the ca se. Specifically, she proposed to set a 1-hour NO 2 standard that reflects the maximum allowable NO 2 concentration anywhere in an area, regardless of where that maximum concentration occurs. 16 Therefore, the proposed approach to setting the

16 To measure maximum concentrations, the Admini strator proposed monito ring provisions that would require monitors within 50 meters of majo r roads and to allow the Regional Administrator to require additional monitors in situations wher e maximum concentrations would be expected to86 standard would be expected to limit the maximum NO 2 concentrations anywhe re in an area even if in some areas, as is co ntended by AAM, those maximum NO 2 concentrations do not occur near roads. iii. CASAC comments on standard level In commenting on the proposal, CASAC discu ssed both the proposed range of standard levels (i.e., 80-100 ppb) and the a lternative range of standard levels (i.e., 50-75 ppb). CASAC did express the consensus conclusi on that if the Agency finalizes a 1-hour standard in accordance with the proposed approach (i.e., standa rd level reflects the maximum allowable NO 2 concentration anywhere in an ar ea), then it is appropriate to consider the proposed range of standard levels from 80 to 100 ppb. Specifically, the CASAC letter to the Administrator on the proposal (Samet, 2009) stated th e following with regard to the proposed approach: [T]he level of the one-hour NO 2 standard should be within the range of 80-100 ppb and not above 100 ppb. In its letter of December 2, 2008, CASAC strongly voiced a consensus view that the upper end of the range should not exceed 100 ppb, based on evidence of risk at that concentration. The lower limit of 80 ppb was viewed as reasonable by CASAC; selection of a valu e lower than 80ppb would represent a policy judgment based on uncertainty and the degree of public health protection sought, given the limited health-based evidence at concentrations below 100 ppb. CASAC also recommended that th is level be employed with a 98 th percentile form, in order to promote the stability of the standard (see above for discussion of form). iv. Public comments on standard level A number of State and local agencies and organizations e xpressed support for setting the level of the 1-hour NO 2 standard within the proposed range of 80 to 100 ppb. While some State and local agencies (e.g., in CA, IA, MI, NY, TX) made this recommendation in conjunction with

occur in locations other than near major road s (e.g., due to the influence of multiple smaller roads and/or stationary sources).87 a recommendation to focus monitoring near ma jor roads and other important sources of NO 2 , a number of State commenters (e.g., NACAA, NESCAUM and agencies in IL, NC, NM, TX, VA) recommended a standard level from 80 to 100 ppb in conjunction with a recommendation that only area-wide monitors be deployed for purposes of determining attainment with the standard. Based on these monitoring comments, these State commenters appear to favor an approach where a standard level from 80 to 100 ppb w ould reflect the allowable area-wide NO 2 concentration. As discussed above (and in more detail in section III.B.2 and the Response to Comments document), State commenters often ba sed these recommendations on uncertainties in designing an appropriate national near-road monitoring network. A number of environmental organizations (e.g., CAC, EDF, EJ, GASP, NRDC) and medical/public health organizations (e.g., ACCP, ALA, AMA, ATS, NACPR, NAMDRC) supported setting a standard level below 80 ppb for a standard that reflects the maximum allowable NO 2 concentration anywhere in an area. Several of these groups recommended a standard level of 50 ppb. This recommendati on was typically based on the commenters’ interpretation of the epidemiol ogic and controlled human exposure ev idence, as described below. Some of these commenters noted that the 98 th percentile area-wide NO 2 concentration was below 80 ppb in the location of a single key U.S. epidemiologic study (i.e., 50 ppb in study by Delfino). Given this, comment ers concluded that the standard level should be set at 50 ppb. Their comments on the monitoring network generall y favored a requirement to place monitors near major roads and, therefore, these commenters appeared to favor a standard level as low as 50 ppb and to recommend that such a standa rd level reflect the maximum allowable NO 2 concentration anywhere in an area. In their co mments, the ALA, EDF, EJ, and NRDC stated the following:88 Considering the Delfino study alone on EP A’s terms, that is, focusing on the 98 th percentile of the 1-hour daily maxi mum concentrations, EPA reports a concentration of 50 ppb where asthma sy mptoms were observed. Based primarily on this study, EPA concluded in the REA th at it was appropriate to set the lower end of the range at 50 ppb, which corr esponded to the lowest-observed effects level of airway hyperrespons iveness in asthmatics. To provide the strongest public health protection, we therefore urge the level of the standard be set at 50 ppb. In some cases, the same commenters also appear ed to recommend setting a standard level below 50 ppb because mean area-wide NO 2 concentrations reported in locations of key U.S. epidemiologic studies are below this concentratio n. Specifically, with regard to the key U.S. epidemiologic studies, these co mmenters (e.g., ALA, EDF, EJ, NR DC) stated the following: These studies clearly identify adverse h ealth effects such as emergency room visits and hospital admissions for respirat ory causes at concentrations currently occurring in the United States. Mean c oncentrations for all but two of these studies are about or below 50 ppb, suggesting that the standard must be set below this level to allow for a margin of safety. The Administrator’s consideration of the De lfino study as it relates to a decision on standard level is discussed be low (section II.F.4.d). Regardi ng the recommendation to set the level below 50 ppb based on mean area-wide NO 2 concentrations in epidemiologic study locations, we note that the Administrator proposed to set a standard that reflects the maximum allowable NO 2 concentration anywhere in an area and to set the form of that standard at the upper end of the distribution of 1-hour daily maximum NO 2 concentrations. 17 As described in the proposal, such a standard, with a level fr om the proposed range of 80 to 100 ppb, would be expected to maintain peak area-wide NO 2 concentrations below the peak area-wide concentrations measured in locations where ke y U.S. epidemiologic studies have reported associations with respiratory-related emergenc y department visits and hospital admissions.

17 As discussed above, the Admi nistrator has selected the 98 th percentile as the form for the new 1-hour NO 2 standard.89 Because reducing NO X emissions to meet a 98 th percentile NO 2 standard should lower the distribution of NO 2 concentrations, including the mea n, a standard that limits the 98 th percentile of the distribution of 1-hour daily maximum concen trations would also be expected to limit mean concentrations. Therefore, although we acknowledge that the relationship between peak and
mean NO 2 concentrations will likely vary across locations and over time, if peak area-wide NO 2 concentrations are maintained below those in key epidemiologic study lo cations, mean area-wide
NO 2 concentrations would also be expected to be maintained below the mean area-wide concentrations in those locations (see ISA, figure 2.4-13 for information on the relationship between peak and mean NO 2 concentrations). As discussed above (section, II.E.2), a number of industry groups di d not support setting a new 1-hour NO 2 standard. However, several of these groups (e.g., AAM, Dow, NAM, NPRA) also concluded that, if EPA does choose to set a new 1-hour standard, the le vel of that standard should be above 100 ppb. As a basis for th is recommendation, these groups emphasized uncertainties in the scientific evidence. Specifi cally, as discussed in more detail above (section II.E.2), these commenters typical ly concluded that available epidemiologic studies do not support the conclusion that NO 2 causes reported health effects. This was based on their assertion that the presence of co-pollutants in the ambien t air precludes the identification of a specific NO 2 contribution to reported effects. As a result , these commenters recommended that a 1-hour standard should be based on the controlled hum an exposure evidence and that, in considering that evidence, EPA should rely on the meta-analysis of NO 2 airway responsiveness studies conducted by Goodman et al., (2009) rather than the meta-analysis in cluded in the final ISA. As described above, they concluded that in re lying on the ISA meta-analysis, EPA has inappropriately relied on a new unpublished meta-analysis that ha s not been peer-reviewed, was90 not reviewed by CASAC, and was not conducted in a transparent manner. EPA recognizes the uncertainties in the scientific evidence th at are discussed by thes e industry commenters; however, we strongly disagree with their conclu sions regarding the implications of these uncertainties for decisions on the NO 2 NAAQS. These comments, and EPA’s responses, are discussed in detail above (sect ion II.E.2) and in the Response to Comments document and are summarized briefly below. As noted in section II.E.2, we agree that the presence of co-pollutants in the ambient air complicates the interpretation of epidemiologi c studies; however, our conclusions regarding causality are based on consideration of the broa d body of epidemiologic studies (including those employing multi-pollutant models) as well as an imal toxicological and controlled human exposure studies. The ISA concluded that this body of evidence “supports a direct effect of short-term NO 2 exposure on respiratory morbidity at am bient concentrations below the current NAAQS level” (ISA, p. 5-16). In addition, the ISA (p. 5-15) concluded the following: [T]he strongest evidence for an association between NO 2 exposure and adverse human health effects comes from epidemio logic studies of re spiratory symptoms and ED visits and hospital admission s. These new findings were based on numerous studies, including panel and field studies, mu ltipollutant studies that control for the effects of other pollutant s, and studies conducted in areas where the whole distribution of ambient 24-h avg NO 2 concentrations was below the current NAAQS level of 0.053 ppm (53 ppb) (annual average). Given that epidemiologic studies provide the strongest support for an association between
NO 2 and respiratory morbidity, and that a num ber of these studies controlled for the presence of other pollutants with multi-pollutant models (in which NO 2 effect estimates remained robust), we disagree that NO 2 epidemiologic studies should not be used to inform a decision on the level of the 1-hour NO 2 standard.91 In addition, we agree that un certainty exists regarding the extent to which the NO 2 - induced increase in airway re sponsiveness is adverse (REA , section 10.3.2.1); however, as discussed in detail above (sect ion II.E.2), we disagree with the conclusion by many industry commenters that this effect is not adverse in asthmatics following exposures from 100 to 600 ppb
NO 2 . Specifically, we do not agree that the a pproach taken in the study by Goodman et al. (2009), which was used by many industry comme nters to support th eir conclusions, was appropriate. The authors of the Goodm an study used data from existing NO 2 studies to characterize the dose-res ponse relationship of NO 2 and airway responsiven ess and to calculate the magnitude of the NO 2 effect. Given the protocol diffe rences in existing studies of NO 2 and airway responsiveness, we do not agree that it is appropriate to base such an analysis on these studies. The Administrator’s consideration of these uncertainties, within the context of setting a standard level, is discusse d in the next section. d. Conclusions on approach and standard level Having carefully considered the public commen ts on the appropriate approach and level for a 1-hour NO 2 standard, as discussed above, the Ad ministrator believes the fundamental conclusions reached in the ISA and REA remain valid. In considering the approach, the Administrator continues to place primary em phasis on the conclusions of the ISA and the analyses of the REA, both of which focus attention on the importa nce of roadways in contributing to peak NO 2 exposures, given that roadway-a ssociated exposures can dominate personal exposures to NO 2 . In considering the level at which the 1-hour primary NO 2 standard should be set, the Administrator continues to place primary emphasis on the body of scientific evidence assessed in the ISA, as summarized above in section II.B, while viewing the results of92 exposure and risk analyses, discussed above in s ection II.C, as providing information in support of her decision. With regard to her decision on the appro ach to setting the 1- hour standard, the Administrator continues to judge it appropriate to provide increas ed public health protection for at-risk individuals against an ar ray of adverse respiratory healtheffects linked with short-term exposures to NO 2, where such health effects have beenassociated with exposure to the distribution of short-term ambient NO 2 concentrations across an area . In protecting public health against exposure to the di stribution of short-term NO 2 concentrations across an area, the Administrator is placing emphasi s on providing a relatively high degr ee of confidence regarding the protection provided against exposur es to peak concentrations of NO 2 , such as those that can occur around major roadways. Available evidence and information suggest that roadways account for the majority of exposures to peak NO 2 concentrations and, therefore, are important contributors to NO 2 -associated public health risks. In reaching this conclusion, the Administrator notes the following:

• Mobile sources account for the majority of NO X emissions (ISA, Table 2.2-1).

• The ISA stated that NO 2 concentrations in heavy traffic or on freeways “can be twice the residential outdoor or resident ial/arterial road level,” th at “exposure in traffic can dominate personal exposure to NO 2 ,” and that “NO 2 levels are strongly associated with distance from major roads (i.e., the clos er to a major roa d, the higher the NO 2 concentration)” (ISA, sections 2.5.4, 4.3.6).

• The exposure assessment presented in the REA estimated that roadway-associated exposures account for the majority of exposures to peak NO 2 concentrations (REA, Figures 8-17, 8-18).93 • Monitoring studies suggest that NO 2 concentrations near roads can be considerably higher than those in the same area but awayfrom the road (e.g., by 30-100%, see section II.A.2).

• In their comments on the approach to setting the 1-hour NO 2 standard, the majority of CASAC Panel members emphasized the importance of setting a standard that limits roadway-associated exposures to NO 2 concentrations that could adversely affect asthmatics. These CASAC Panel members favo red the proposed approach, including its focus on roads. In addition, the Administrator notes that a c onsiderable fraction of the population resides, works, or attends school near major roadways and that these populations are likely to have increased exposure to NO 2 (ISA, section 4.4). Based on data from the 2003 American Housing Survey, approximately 36 million individuals live within 300 feet (~90 meters) of a four-lane highway, railroad, or ai rport (ISA, section 4.4). 18 Furthermore, in California, 2.3% of schools with a total enrollment of more than 150,000 st udents were located within approximately 500 feet of high-traffic roads (IS A, section 4.4). Of this popul ation, which likely includes a disproportionate number of individuals in group s with higher prevalence of asthma and higher hospitalization rates for asthma (e.g. ethnic or racial minorities and individuals of low socioeconomic status) (ISA, section 4.4), asthma tics and members of other susceptible groups

18 The most current American Housing Survey ( http://www.census.gov/hhe s/www/housing/ahs/ahs.html ) is from 2007 and lists a higher fraction of housing units within the 300 foot boundary. According to Table 1A-6 from that report (http://www.census.gov/hhes/www/housing/ahs/ah s07/tab1a-6.pdf), out of 128.2 million total housing units in the United States, about 20 million were reported by the surveyed occupant or landlord as being within 300 feet of a 4-or-more lane highway, railroad, or airport. That constitutes 15.6% of the total housing units in the U.S. Assu ming equal distributions, with a current population of 306.3 million, that means that there would be 47.8 million people meeting the 300 foot criteria.94 (e.g., children, elderly) will have the greatest risk s of experiencing health effects related to NO 2 exposure. In the United States, approximately 10% of adults and 13% of children have been diagnosed with asthma, and 6% of adults have be en diagnosed with COPD (ISA, section 4.4). In considering the approach to setting the 1-hour standard, the Administrator also notes that concerns with the proposed approach e xpressed by the minority of CASAC Panel members included concern with the uncertainty in the relationship between near-road and area-wide NO 2 concentrations, given that U.S. epidemiologi c studies have been based on concentrations measured at area-wide monitors. However, as discussed by the majority of CASAC Panel members, a similar uncertainty would be involve d in setting a standard with the alternative approach (Samet, 2009). The Administrator agre es with the majority of CASAC members and concludes that uncertainty in the relati onship between near-road and area-wide NO 2 concentrations should be considered regardless of the approach sele cted to set the standard. She recognizes that this uncertainty can and should be taken into cons ideration when considering the level of the standard. In drawing conclusions on the approach, the Administrator has considered the extent to which each approach, in conjunction with the ranges of standard levels discussed in the proposal, would be expected to limit the distribution of NO 2 concentrations across an area and, therefore, would be expected to protect ag ainst risks associated with NO 2 exposures. Specifically, she has considered the extent to which a standard set with each approach would be expected to limit maximum NO 2 concentrations and area-wide NO 2 concentrations. With regard to expected maximum concentra tions, the Administrator notes the following:

• A standard reflecting the maximum allowable NO 2 concentration anywhere in an area would provide a relatively high degree of c onfidence regarding the level of protection95 provided against peak exposures, such as those that can occur on or near major roadways. A standard level from anywhere within the proposed range (i.e., 80 to 100 ppb) would be expected to limit exposures to NO 2 concentrations reported to increase airway responsiveness in asthmatics.

• A standard reflecting th e allowable area-wide NO 2 concentration would not provide a high degree of confidence regardi ng the extent to which maximum NO 2 concentrations would be limited. Maximum NO 2 concentrations would be e xpected to be controlled to varying degrees across locations depending on the NO 2 concentration gradient around roads. Most standard levels within the range cons idered in the proposal with this option (i.e., 50 to 75 ppb) would not be expected to limit the occurrence, across locations, of
NO 2 concentrations that have been reporte d to increase airway responsiveness in asthmatics. With regard to expected area-wide concentr ations, the Administrator notes the following:

• The extent to which a standard reflecting the maximum allowable NO 2 concentration anywhere in an area would be expected to limit area-wide NO 2 concentrations would vary across locations, e.g., depending on the NO 2 concentration gradient around roads. However, in conjunction with a standard le vel from anywhere within the proposed range (i.e., 80-100 ppb), such an approach would be expected to maintain area-wide NO 2 concentrations below those measured in loca tions where key U.S. epidemiologic studies have reported associa tions between ambient NO 2 and respiratory-related hospital admissions and emergency department visits (based on available information regarding the NO 2 concentration gradient around roads as discussed below).96 • A standard reflecting the ma ximum allowable area-wide NO 2 concentration would provide a relatively high degree of certainty regarding the ex tent to which area-wide NO 2 concentrations are limited. In conjunction wi th a standard level from anywhere within the range of levels discusse d in the proposal (i.e., 50-75 ppb) with this alternative approach, such a standard would be expected to maintain area-wide NO 2 concentrations below those measured in locations where key U.S. epidemiologic studies have reported associations between ambient NO 2 and respiratory-related hospital admissions and emergency department visits. Given the above considerations, the Admini strator notes that both approaches, in conjunction with appropriate sta ndard levels, would be expected to maintain area-wide NO 2 concentrations below those measured in locati ons where key U.S. epidemiologic studies have reported associations between ambient NO 2 and respiratory-related hospital admissions and emergency department visits. In contrast, onl y a standard reflecting the maximum allowable
NO 2 concentration anywhere in an area would be e xpected to effectively a nd efficiently (i.e., the standard would be neither more nor less string ent than necessary) limit peak exposures to NO 2 at concentrations that have been reported to increas e airway responsiveness in asthmatics. Only the very lowest part of the range considered for the area-wide approach would be expected to effectively limit such peak exposures. After c onsidering the evidence and uncertainties, and the advice of the CASAC Panel, the Administrator judges that the mo st appropriate approach to setting a 1-hour standard to protect ag ainst the distribution of short-term NO 2 concentrations across an area, including the higher concentrat ions that can occur ar ound roads and result in elevated exposure concentrations, is to set a standard that reflects the maximum allowable NO 2 concentration anywhere in an area.97 In considering the level of a 1-hour NO 2 standard that reflects the maximum allowable
NO 2 concentration anywhere in an area, the Admi nistrator notes that there is no bright line clearly directing the choice of level. Rather, the choice of what is appropriate is a public health policy judgment entrusted to the Administrator. This judgment must include consideration of the strengths and limitations of the evidence and the appropriate inferences to be drawn from the evidence and the exposure and risk assessments . Specifically, the Administrator notes the following:

• Controlled human exposure studies have reported that various NO 2 exposure concentrations increased airway responsiven ess in mostly mild asthmatics (section II above and II.B.1.d in proposal). These studies can inform an evaluation of the risks associated with exposure to specific NO 2 concentrations, regardless of where those exposures occur in an area. Because con centrations evaluated in controlled human exposure studies are at the high end of the distribution of ambient NO 2 concentrations (ISA, section 5.3.2.1), these stud ies most directly inform consideration of the risks associated with exposure to peak short-term NO 2 concentrations.

• Epidemiologic studies (section II.B.1.a and b) conducted in the United States have reported associations between ambient NO 2 concentrations measured at area-wide monitors in the current network and increased respiratory symptoms, emergency department visits, and hospital admissions. Area-wide monitors in the urban areas in which these epidemiologic studies were c onducted are not sited in locations where localized peak concentrations are likely to occur. Thus, they do not measure the full range of ambient NO 2 concentrations across the ar ea. Rather, the area-wide NO 2 concentrations measured by these monitors are used as surrogates fo r the distribution of98 ambient NO 2 concentrations across the area, a distribution that includes NO 2 concentrations both higher th an (e.g., around major roadways ) and lower than the areawide concentrations measured in study locat ions. Epidemiologic st udies then evaluate whether area-wide NO 2 concentrations are associated with the risk of respiratory morbidity. Therefore, in considering these epidemiologic studies within the context of the level of a standard that reflects the maximum allowable NO 2 concentration anywhere in an area, available inform ation on the distribution of NO 2 concentrations across areas (e.g., concentration gradients around roads) shou ld be considered. Considering available information on the relationship between maximum and area-wide NO 2 concentrations allows consideration of the extent to which a particular standard level would be expected to protect the public health against risk s associated with the distribution of NO 2 concentrations across an area, as indicated by area-wide concentrations measured in epidemiologic study locations.

• The risk and exposure analyses presente d in the REA provide information on the potential public health imp lications of setting standard s that limit area-wide NO 2 concentrations to specific levels. While the Administrator acknowledges the uncertainties associated with these analyses which, as discussed in the REA, could result in either over- or underestimates of NO 2 -associated health risks, she judges that these analyses are informative for considering the re lative levels of public health protection that could be provided by different standards. The Administrator’s consideration of the cont rolled human exposure evidence, epidemiologic evidence, and exposure/risk information are disc ussed below specifically with regard to a99 decision on the level of a standard th at reflects the maximum allowable NO 2 concentration anywhere in an area. In considering the potential for controlled human exposure studies of NO 2 and airway responsiveness to inform a decisi on on standard level, the Admi nistrator notes the following:

• NO 2 -induced increases in airway responsiven ess, as reported in controlled human exposure studies, are logically linked to the adverse respiratory effects that have been reported in NO 2 epidemiologic studies.

• The meta-analysis of controlled human e xposure data in the ISA reported increased airway responsiveness in a larg e percentage of asthmatics at rest following exposures at and above 100 ppb NO 2 , the lowest NO 2 concentration for whic h airway responsiveness data are available in humans.

• This meta-analysis does not provide any evid ence of a threshold below which effects do not occur. The studies included in the meta -analysis evaluated primarily mild asthmatics while more severely affected individuals could respond to lower concentrations. Therefore, it is possibl e that exposure to NO 2 concentrations below 100 ppb could increase airway responsiveness in some asthmatics. In considering the evidence, the Ad ministrator recognizes that the NO 2 -induced increases in airway responsiveness reported for exposures to NO 2 concentrations at or above 100 ppb could be adverse for some asthmatics. However, she al so notes that important uncertainties exist with regard to the extent to which NO 2 -induced increases in airway responsiveness are adverse. Specifically, she notes the following w ith regard to these uncertainties:100 • The magnitude of the NO 2 -induced increase in airway res ponsiveness, and the extent to which it is adverse, cannot be quantified from the ISA meta-analysis (REA, section 10.3.2.1).

• The NO 2 -induced increase in airway responsiven ess in resting asthmatics was typically not accompanied by increased respiratory symptoms, even following exposures to NO 2 concentrations well above 100 ppb (ISA, section 3.1.3.3).

• The increase in airway responsiveness that was reported for res ting asthmatics was not present in exercising asth matics (ISA, Table 3.1-3). Taking into consideration all of the above, the Administrator c oncludes that existing evidence supports the conclusion that the NO 2 -induced increase in airway re sponsiveness at or above 100 ppb presents a risk of adverse effects for some asthmatics, especially those with more serious (i.e., more than mild) asthma. The Administrator notes that the risks associated with increased airway responsiveness cannot be fu lly characterized by these studies, and thus she is not able to determine whether the increased airway responsiveness experi enced by asthmatics in these studies is an adverse health effect. Howeve r, based on these studies , the Administrator is concluding that asthmatics, particularly thos e suffering from more severe asthma, warrant protection from the risk of advers e effects associated with the NO 2 -induced increase in airway responsiveness. Therefore, the Administrator concludes that the controlled human exposure evidence supports sett ing a standard level no higher than 100 ppb to reflect a cautious approach to the uncertainty regarding the a dversity of the effect. However, those uncertainties lead her to also conclude that this eviden ce does not support setting a standa rd level lower than 100 ppb. In considering the more serious health effects reported in NO 2 epidemiologic studies, as they relate to the level of a standard that reflects the maximum allowable NO 2 concentration101 anywhere in an area, the Admi nistrator notes the following:

• A cluster of 5 key U.S. epidemiologic studies (Ito et al., 2007; Jaffe et al., 2003; Peel et al., 2005; Tolbert et al., 2007; and a study by th e New York State Department of Health, 2006) provide evidence for associations between NO 2 and respiratory-related emergency department visits and hospital admissions in locations where 98 th percentile 1-hour daily maximum NO 2 concentrations measured at area-wide monitors ranged from 85 to 94 ppb. The Administrator judges it appropriate to place substantial weight on this cluster of key U.S. epidemiologic studies in selecting a sta ndard level, as they are a group of studies that reported positive, and often statistically significant, associations between NO 2 and respiratory morbidity in multiple cities across the United States. 19

• A single study (Delfino et al., 2002 ) provides mixed evidence for NO 2 effects (i.e., respiratory symptoms) in a location with a 98 th percentile 1-hour daily maximum NO 2 concentration, as measured by an area-wide m onitor, of 50 ppb. In that study, most of the reported NO 2 effect estimates were positive, but not statistically significant. Given the variability in the NO 2 effect estimates in this study, as well as the lack of studies in other locations with similarly low NO 2 concentrations, the Administrator judges it appropriate to place limited weight on this study, compared to the cluster of 5 studies as noted above. Given these considerations, th e Administrator concludes that the epidemiologic evidence provides strong support for setting a standard that limits the 98 th percentile of the distribution of 1-hour daily maxi mum area-wide NO 2 concentrations to below 85 ppb. This judgment takes into account the determinations in the ISA, based on a much broader body of evidence, that there is a likely causal association between exposure to NO 2 and the types of respiratory morbidity effects

19 Some of these studies also included suscep tible and vulnerable populat ions (e.g., children in Peel et al. (2005); poor and minority populations in Ito et al., 2007).102 reported in these studies. Given the consider ations discussed above, the Administrator judges that it is not necessary, based on existing evidence, to set a standard that maintains peak areawide NO 2 concentrations to below 50 ppb. In considering specific standard levels supported by the epidemiologic evidence, the Administrator notes that a level of 100 ppb, for a standard reflecting the maximum allowable
NO 2 concentration anywhere in the area, woul d be expected to maintain area-wide NO 2 concentrations well below 85 ppb, which is the lowest 98 th percentile concentration in the cluster of 5 studies. With regard to this, she specifically notes the following:

• If NO 2 concentrations near roads are 100% hi gher than concentrations away from roads, a standard level of 100 ppb woul d limit area-wide c oncentrations to approximately 50 ppb.

• If NO 2 concentrations near roads are 30% hi gher than concentrations away from roads, a standard level of 100 ppb woul d limit area-wide c oncentrations to approximately 75 ppb. The Administrator has also considered the NO 2 exposure and risk information within the context of the above conclusions on standard level. Specifically, she notes that the results of exposure and risk analyses were interpreted as providing support for limiting area-wide NO 2 concentrations to no higher than 100 ppb. Specifi cally, these analyses estim ated that a standard that limits area-wide NO 2 concentrations to approximately 100 ppb or below would be expected to result in important reductions in respiratory risks, relative to the level of risk permitted by the current annual standard alone. As discussed a bove, a standard level of 100 ppb, for a standard that reflects the maximum allowable NO 2 concentration, would be expected to maintain areawide NO 2 concentrations to within a range of approximately 50 to 75 ppb. Given this, the103 Administrator concludes that a standard level of 100 ppb, for a standard that reflects the maximum allowable NO 2 concentration anywhere in an area, is consistent with conclusions based on the NO 2 exposure and risk information. Finally, the Administrator notes that a standard level of 100 ppb for a standard that reflects the maximum allowable NO 2 concentration anywhere in an area is consistent with the consensus recommendation of CASAC. Given the above considerations and the comments received on the proposal, the Administrator determines that the appropriate judgment, ba sed on the entire body of evidence and information available in this review, is a standard that reflects the maximum allowable NO 2 concentration anywhere in an area and with a level of 100 ppb. Specifically, she concludes that such a standard, with the averaging time and form discussed above, will provide a significant increase in public health prot ection compared to that provided by the current annual standard alone and would be expected to protect against th e respiratory effects that have been linked with
NO 2 exposures in both controlled human exposur e and epidemiologic studies. This includes focusing on and limiting exposures at and above 100 ppb for the vast majority of people, including those in at-risk groups , and maintainin g area-wide NO 2 concentrations well below those in locations where key U.S. epidemio logic studies have reported that ambient NO 2 is associated with clearly adverse respiratory heal th effects, as indica ted by increased hospital admissions and emergency department visits. In setting the standard level at 100 ppb rather than a lo wer level, the Administrator also acknowledges the uncertainties associated with th e scientific evidence. She notes that a 1-hour standard with a level lower than 100 ppb would only result in significant further public health protection if, in fact, there is a continuum of serious, adverse h ealth risks caused by exposure to104 NO 2 concentrations below 100 ppb and/or associated with area-wide NO 2 concentrations wellbelow those in locations where key U.S. epidemio logic studies have reported associations with respiratory-related emergency department visits and hospital admissions. Based on the available evidence, the Administrator does not believe that such assumptions are warranted. Taking into account the uncertainties that remain in inte rpreting the evidence from available controlled human exposure and epidemiologic studies, the Administrator notes th at the likelihood of obtaining benefits to public health with a standard set below 100 ppb decreases, while the likelihood of requiring reductions in ambient concen trations that go beyond those that are needed to protect public health increases. Therefore, the Administrator judges that a standard reflecting the maximum allowable
NO 2 concentration anywhere in an area set at 100 ppb would be sufficient to protect public health with an adequate margin of safety, in cluding the health of at-risk populations, from adverse respiratory effects th at have been linked with short-term exposures to NO 2 and for which the evidence has been judged sufficient to support a likely causal relationship with NO 2 exposures. The Administrator does not believe that a lower standard level is needed to provide this degree of protection. These conclusions by the Administrator appropriately consider the requirement for a standard that is neither more nor less stringent than ne cessary for this purpose and recognizes that the CAA does no t require that primary standards be set at a zero-risk level or to protect the most sensitive individual, but rather at a level that reduces risk sufficiently so as to protect the public health with an adeq uate margin of safety.
G. Annual Standard In the proposal, the Administ rator noted that some evid ence supports a link between long-term exposures to NO 2 and adverse respiratory effects and that CASAC recommended in105 their comments prior to the proposal that, in add ition to setting a new 1-hour standard to increase public health protection, the current annual st andard be retained. CASAC’s recommendation was based on the scientific evidence and on thei r conclusion that a 1-hour standard might not provide adequate protection ag ainst exposure to long-term NO 2 concentrations (Samet, 2008b). With regard to an annual standard, CASA C and a large number of public commenters (e.g., NACAA, NESCAUM; agencies from States including CA, IN, MO, NC, NY, SC, TX, VA; tribal organizations including Fon du Lac and the National Tribal Air Organization; environmental/medical/public health groups in cluding ACCP, ALA, AMA, ATS, CAC, EDF, EJ, GASP, NACPR, NAMDRC, NRDC ) agreed with the proposed decision to maintain an annual standard, though their recommendations with re gard to the level of that annual standard differed (see below). As noted above, CASAC recommended “retai ning the current standard based on the annual average” based on the “limited evidence related to potential l ong-term effects of NO 2 exposure and the lack of strong evidence of no effect” and that “the findings of the REA do not provide assurance that a short-term standard based on the one-hour maximum will necessarily protect the population from long-te rm exposures at levels potentia lly leading to adverse health effects” (Samet, 2008b). A number of State ag encies and organizati ons also recommended maintaining the current level of the annual st andard (i.e., 53 ppb). This recommendation was based on the conclusion that, while some ev idence supports a link between long-term NO 2 exposures and adverse respiratory effects, that evidence is not sufficient to support a standard level either higher or lower than the current le vel. In addition, a number of industry groups (e.g., AAM, API, Dow, INGAA, UARG) recommended re taining the level of the current annual106 standard but, as described above, did so within the context of a recomm endation that EPA should not set a new 1-hour standard. In contrast, some environmental organizations and medical/public health organizations as well as a small number of States (e.g., ALA, EDF, EJ, NRDC, and organizations in CA) recommended setting a lower level for the annu al standard. These commenters generally supported their recommendation by pointing to the State of Calif ornia’s annual standard of 30 ppb and to studies where long-term ambient NO 2 concentrations have been associated with adverse respiratory effects such as imp airments in lung function growth. As discussed above (II.B.3), th e evidence relating long-term NO 2 exposures to adverse health effects was judged in the ISA to be either “suggestive but not sufficient to infer a causal relationship” (respiratory morbidity ) or “inadequate to infer the presence or absence of a causal relationship” (mortality, cancer, cardiovascular effects, reproductive/developmental effects) (ISA, sections 5.3.2.4-5.3.2.6). Inthe case of respiratory mo rbidity, the ISA (section 5.3.2.4) concluded that “The high correlation among tr affic-related pollutants made it difficult to accurately estimate the independent effects in these long-term e xposure studies.” Given these uncertainties associated with the role of long-term NO 2 exposures in causing the reported effects, the Administrator concluded in the proposal th at, consistent with the CASAC recommendation, existing evidence is not sufficient to justify setting an annual standard with either a higher or lower level than the current standard. Commenters have not submitted any new analyses or information that would change this conclusion. Th erefore, the Administrator does not agree with the commenters who recommended a lowe r level for the annual standard. The Administrator judges that her conclusions in the pr oposal regarding the annual standard remain appropriate. Specifically, she continues to agree with the conclusion that,107 though some evidence does support the need to limit long-term exposures to NO 2 , the existing evidence for adverse health effects following long-term NO 2 exposures does not support either increasing or decreasing the level of the annual st andard. In light of th is and considering the recommendation from CASAC to retain the current level of the annual standard, the Administrator judges it appropriate to maintain the level of the annual standard at 53 ppb. H. Summary of final decisions on the primary NO 2 standard For the reasons discussed above, and taking into account information and assessments presented in the ISA and REA, the advice and recommendations of the CASAC, and public comments, the Administrator has decide d to revise the existing primary NO 2 standard. Specifically, the Administrator has determined that the current annual stan dard by itself is not requisite to protect public health with an adequa te margin of safety. In order to provide protection for asthmatics and othe r at-risk populations ag ainst an array of adverse respiratory health effects related to short-term NO 2 exposure, the Administrator is establishing a short-term
NO 2 standard defined by the 3-year average of the 98 th percentile of the yearly distribution of 1- hour daily maximum NO 2 concentrations. She is setting th e level of this standard at 100 ppb, which is to reflect the maximum allowable NO 2 concentration anywhere in an area. In addition to setting a new 1-hour standar d, the Administrator retains the current annual standard with a level of 53 ppb. The new 1-hour standard, in comb ination with the annual standard, will provide protection for susceptible groups ag ainst adverse respiratory health effects associated with shortterm exposures to NO 2 and effects potentially associated with long-term exposures to NO 2 . III. Ambient Monitoring and Reporting Requirements The EPA is finalizing several changes to the ambient air monitoring, reporting, and network design requirements for the NO 2 NAAQS. This section discusses the changes we are108 finalizing which are intended to support the proposed 1-hour NAAQS and retention of the current annual NAAQS as discus sed in Section II. Ambient NO 2 monitoring data are used to determine whether an area is in violation of the NO 2 NAAQS. Ambient NO 2 monitoring data are collected by state, local, and Tr ibal monitoring agencies (“monito ring agencies”) in accordance with the monitoring requirements c ontained in 40 CFR parts 50, 53, and 58 A. Monitoring Methods We are finalizing the proposed changes regarding the NO 2 Federal Reference Method (FRM) or Federal Equivalent Method (FEM) analyzer s. Specifically, we are continuing to use the NO 2 chemiluminescence FRM and are finalizing the requirement that any NO 2 FRM or FEM used for making primary NAAQS decisions must be capable of providing hourly averaged concentration data. The following paragra phs provide background and rationale for the continued use of the chemiluminescence FRM and th e decision to finalize the proposed changes. 1. Chemiluminescence FRM and Alternative Methods The current monitoring method in use by most St ate and local monitoring agencies is the gas-phase chemiluminescence FRM (40 CFR Part 50, Appendix F), which was implemented into the NO 2 monitoring network in the early 1980s. EP A did not propose to discontinue using the chemiluminescence FRM, although we received comme nt raising concerns about using a method that is subject to known interf erences from certain species of oxides of nitrogen known as NO Z . Important components of ambient NO Z include nitrous acid (HNO 2 ), nitric acid (HNO 3 ), and the peroxyacetyl nitrates (PANs). The issue of concern in public comme nts is that the reduction of NO 2 to NO on the
MoO X converter substrate used in chemilu minescent FRMs is not specific to NO 2 ; hence, chemiluminescence method analyzers are subject to varying interferen ces produced by the109 presence in the air sample of the NO Z species listed above and othe rs occurring in trace amounts in ambient air. This interference is often termed a “positive artifact” in the reported NO 2 concentration since the presence of NO Z results in an over-estimate in the reported measurement of the actual ambient NO 2 concentration. This interference by NO Z compounds has long been known and evaluated (Fehsenfeld et al., 1987; Nunnermacker et al., 1998; Parrish and Fehsenfeld, 2000; McClenny et al., 2002; U.S. Environmental Protection Agency, 1993, 2006a). Further, as noted in the ISA, it appears that interference by NO Z on chemiluminescence FRMs is not more than 10 percent of the reported NO 2 concentration during most or all of the day during winter (cold temperatures), butlarger interference ranging up to 70 percent can be found during summer (warm temperatures) in the afternoon at sites away and downwind from strong emission

sources. The EPA acknowledges that the NO Z interference in the reported NO 2 concentrations collected downwind of NO X source areas and in relatively remo te areas away from concentrated point, area, or mobile sources is significantly larger than the NO Z interference in NO 2 measurements taken in urban core s or other areas with fresh NO X emissions. To meet the primary objective of monitoring maximum NO 2 concentrations in an area the EPA is requiring
NO 2 monitors to be placed in locations of the expected highest concentrations, which will generally place monitors near fresh NO X sources or in areas of dense NO X emissions. Therefore, EPA believes that the positive artif act issue, although present, is small, relative to the actual NO 2 being measured.. As a result EPA believes the chemiluminescence FRM is suitable for continued use in the ambient NO 2 monitoring network, as the potential positive bias from NO Z species is not significant e nough to discontinue using the chemiluminescence FRM.110 EPA also received support th rough public comments in furthering the development of alternative methods in determining NO 2 concentrations. Such al ternative methods suggested included the photolytic-chemiluminescent met hod and cavity ring-down spectroscopy. As a result, EPA, both the Office of Research and Development and the Office of Air, will further efforts, working with commercial and industria l vendors, to identify and evaluate such new technologies. These efforts may include poten tially field testing in struments and possibly characterizing methods in a labo ratory setting of potential fu ture reference and equivalent methods that may more accuratelyand/or directly measure NO 2 . 2. Allowable FRM and FEMs for comparison to the NAAQS The current CFR language does not prohibit the use of any particular FRM or FEM to be used in comparison to the standard. There ar e designated wet chemical methods that are only able to report ambient concentration values averaged across multiple hours. With the establishment of a 1-hour NAAQS, any FRM or FEM which is a wet chemical based method would not be appropriate for use in determ ining compliance of the 1-hour NAAQS. EPA addressed this issue by propos ing that only those methods capable of providing 1-hour measurements be comparable to the NAAQS. a. Proposed changes to FRM and FEMs that may be compared to the NAAQS EPA proposed that only those FRMs or FEMs that are capable of providing hourly averaged concentration data may be used for comparison to the NAAQS. b. Comments EPA did not receive any substantive public comme nts that objected to only allowing those FRMs or FEMs that are capable of providing hourly averaged concentration data may be used for comparison to the NAAQS, and received so me comments that supported this concept.111 c. Decisions on allowable FRM and FEMs for comparison to the NAAQS As a result of the supporting public comments, and a lack of public objections, EPA is finalizing the proposed changes to 40 CFR Part 58 Appendix C to only allow data from FRM or FEMs that are capable of providing hourly data to be used for comparison to the NAAQS. B. Network Design With the establishment of a 1-hour NO 2 NAAQS intended to limit exposure to maximum concentrations that may occur anywhere in an area, EPA recognizes that the current NO 2 network is inadequate to assess compliance w ith the revised NAAQS. As a result, EPA is promulgating new NO 2 network design requirements. The following sections provide background, rationale, and details for the final changes to the NO 2 network design requirements. 1. Two-tiered Network Design The ISA and REA indicate that one of the larg est factors affecting ambient exposures to
NO 2 above health benchmarks concentrations ar e mobile source emissions, particularly at locations near major roads. Information in th e ISA and the REA shows that concentrations of mobile source polluta nts, including NO 2 , typically display peak concentrations on or immediately adjacent to roads, producing a gr adient in pollutant concentrations where concentrations decrease with increasing di stance from roads (ISA sections 2.5.4 and 4.3.6 and table 2.2-1; REA section 7.3.2 and Figures 8-17 a nd 8-18). Since the intent of the revised NAAQS is to limit exposure to NO2 concentrations that occur anywhere in an area, monitors intended to measure the maximum allowable NO 2 concentration in an area should include measurements of the peak concentrations that occur on and near roads due to on-road mobile sources. This need to measure maximum concentr ations is the basis for the first tier of the proposed network design, which focuses monitoring near highly trafficked roads in urban areas112 where peak NO 2 concentrations are expected to occur. The basis for the second tier of the network design is to utilize monitors sited to measure area-wide concentrations to characterize the wider area impact of NO 2 sources on urban populations. Area-wide monitoring of NO 2 also serves to maintain continuity in collecting data to inform l ong-term pollutant concentration trends analysis and health and scientific research. This section discusses the tw o-tier network design appro ach versus the alternative network design which was also presented for co mment in conjunction with a solicitation for comment on an alternative NAAQS. The details of the two-tier networ k design, including how many monitors are required, where they are to be located, and the related siting criteria are discussed in subsequent sections. a. Proposed Two-tier Network Design EPA proposed a two-tier network design composed of (1) near-road monitors which would be placed in locations of expected maximum 1-hour NO 2 concentrations near heavily trafficked roads in urban areas and (2) monitors located to characterize areas with the highest expected NO 2 concentrations at the nei ghborhood and larger spatial scal es (also referred to as “area-wide” monitors). As an a lternative, and in c onjunction with a solicitation for comment on an alternative NAAQS, EPA solicited comment on a network comprised of only area-wide monitors. b. Comments EPA received many comments on the overall two-tier network design, with commenters who made statements with a relatively clear position on the issue generally falling into four categories: (1) those who support the adoption of the proposed two-tier design approach, (2) ) those who support the adoption of the two-tier concept, but with modifications, (3) those who113 only supported the adoption of the alternative ne twork design, and (4) those who encourage EPA to commit to further research the near-road envi ronment by monitoring near-roads, but not to use near-road data for regulatory purposes, and ther efore supported the altern ative network design. Those commenters who generally support th e proposed two-tier network as proposed, which requires monitoring near-roads and area-wide monitoring, included the Clean Air Scientific Advisory Committee (although there was not a consensu s, there was a majority in support of the proposed network design), heal th groups (e.g. American Thoracic Society, American Medical Association, and the Nationa l Association for the Medical Direction of Respiratory Care, the American Association of Cardiovascular and Pulmonary Rehabilitation, and the American College of Chest Physicia ns), several state groups (e.g. the Central Pennsylvania Clean Air Board , the New York City Law Department and the Metropolitan Washington Air Quality Committee) , and some industry commenters (e.g. American Chemistry Council, The Clean Energy Group, and Dow Chemical). Those commenters who support the adoption of the two-tier network design concept, but suggest modifications to the actual design incl ude some health and environmental groups (e.g. the American Lung Association, Earthjustice, th e Environmental Defense Fund, and the National Resource Defense Council), some states (e.g. Ca lifornia, the Central Pennsylvania Clean Air Board, Harris County (Texas), Iowa, New York, San Joaquin Air Pollution Control District, Spokane Regional Clean Air Agency, the Texa s Commission on Environmental Quality, and Wisconsin), and some industry commenters, incl uding the American Petroleum Institute and the Utility Air Regulatory Group, who are cited by other industry commenters, which make statements in context that indicate the two- tier network is an acceptable approach.. The114 comments and suggestions made by this group of co mmenters are discussed in greater detail in the following sections. Those commenters who only supported the ad option of the alternative network design included state and industry groups (e.g. the stat e of Indiana, the New York Department of Transportation, Alliance of Automobile Ma nufacturers, and the Engine Manufacturers Association). These commenters typically ma de comments on the two-tier network design, but did not do so in a context that clea rly supported near-road research. EPA received comments from some states or state groups (e.g. National Association of Clean Air Agencies (NACAA), the Northeast St ates for Coordinated Air Use Management (NESCAUM), and 10 other individual states or state groups) and industry commenters (e.g. American Association of State Highway Trans portation Officials, Edison Electric, and the National Association of Manufactur ers) that encouraged EPA to further research the near-road environment, but not to require near-road monitor produced data for regulatory purposes, and therefore supported the adoption of the alternative network design for regulatory purposes. For example, with regard to implementing the tw o-tier network design that includes near-road regulatory monitoring, the National Association of Clean Air Agencies (NACAA) stated that “...a major new network – particularly one that is inherently complicat ed and untried – should not be rolled out without the benefit of an eff ective near-road monitoring research program that can address many of the relevant data questions , and inform the specifi c siting requirements of the rule.” The National Associ ation of Manufacturers stated th at “Conducting such a near road [research] monitoring program would allow EPA to collect necessary data that can be used to better understand the health impacts associated with short term NO 2 exposures.” EPA notes that the existing scientific research referenced in the proposal docume nt and this document show that115 there are on- and near-road peaks of NO 2 concentrations, relativ e to upwind or background levels, which exist due to on-road mobile so urce emissions. This research, as a body of evidence, also identifies the mu ltiple local factors that affect how, where, and when peak NO 2 concentrations occur on or near a particular road segment. Thes e factors include traffic volume, fleet mix, roadway design, congestion patterns, terrain, and meteorology. Although the nearroad environment may be complex with regard to mobile source polluta nt behavior, there is sufficient information from existing research on what factors to consider when identifying locations of expected maximum concentrations due to on-road mobile sources, and EPA and states have access to these data. Further, EP A notes that near-road monitoring is not a new concept for the ambient air monitoring community as near-road carbon monoxide monitoring has been a part of routine networks for nearly three decades and th erefore there is some experience within the whole of EPA and st ate and local agencies on conducting ambient monitoring nearroads. EPA believes that the existing research provides sufficient information with which we may require a near-road monitoring network and th e collective experience that exists in the ambient monitoring community will allow for succe ssful implementation of that network. EPA also believes that the two-tier network design will provid e a network that has a reasonable degree of similarity across th e country where the required near-r oad monitors are targeting the maximum NO 2 concentrations in an area pa rticularly attributable to on-road mobile sources. The process using the factors noted above in the se lection of a near-road monitoring site and the associated monitor siting criteria are explained in subsequent sections. Some industry commenters (e.g. Engine Manuf acturers Association, S.C. Chamber of Commerce, S.C. Manufacturers Alliance) who su pported the adoption of th e alternative network design suggested that monitoring in the near-ro ad environment would not be indicative of116exposure for general populations, and that EPA should not focus on the near-road microenvironment when requiring monitoring. For example, the South Carolina Chamber of Commerce and the South Carolina Manufacturers Alliance both state that “It appears the proposed monitoring network will re sult in a collection of microsca le data, which is not at all representative of air quality rele vant to population exposure.” EPA notes that the intent of nearroad monitoring is to support the NAAQS in prev enting exposure to NO2 concentrations that may occur anywhere in an area. EPA recognizes th at there is variability in the properties (such as traffic counts, fleet mix, and localized features ) among the road segments that may exist in an area, but on the whole, roads are ubiquitous, particularly in urba n environments, thus much of the public may be exposed to near-road environments. The US Department of Transportation (US DOT) Federal Highway Admi nistration (FHWA) “Status of the Nation’s Highways, Bridges, and Transit: 2006 Conditions and Performance” document ( http://www.fhwa.dot.gov/pol icy/2006cpr/es02h.htm ) states that “while urban mileage constitutes only 24.9 percent of total (US) mileage, these road s carried 64.1 percent of the 3 trillion vehicles miles (VMT) travelled in the Un ited States in 2004.” The document also states that “urban interstate highways made up only 0.4 percent of total (US) mileage but carried 15.5 percent of total VMT.” These statements indica te how much more traffic volume exists on roads in urban areas versus the more rural areas that have signifi cant amounts mileage of the total public road inventory. The 2007 American Housing Survey ( http://www.census.gov/hhes/www/ housing/ahs/ahs07/ahs07.html ) estimates that over 20 million housing units are within 300 feet (~91 meters) of a 4-lane highway, airport, or railroad. Although that survey includes airpor ts and railroads, roads are the most pervasive of the three, indicating that a significant numbe r of the general population live near roads. Resultantly, EPA117 believes that near-road monitors are indicative of exposure for a significant component of the general population. EPA received many comments from states a nd industry on the two-tier network design specifically concerning the funding to implement and operate the monitoring network. These comments on funding permeated almost all of th e comments form state and local monitoring agencies, regardless of their position on the netw ork design. EPA notes that it has historically funded part of the cost of the installation and operation of mon itors used to satisfy Federal monitoring requirements. EPA understands thes e concerns although the CAA requirements from which this final rule derives (Secion 103 (c)(2 ) of the CAA) are note contingent on EPA providing funding to states to as sist in meeting monitoring requi rements. However, EPA plans to work with state and local air agencies in identifying available State and Tribal Air Grant (STAG) funds and planning for increasing re source needs in future budgets to better accommodate the implementation costs that ne w minimum monitoring requirements such as those in this rule incur. c. Conclusions Regarding th e Two-tier Network Design As noted in Section II.F.3 above, the Ad ministrator is setting a new 1-hour NO 2 standard that reflects the maximum allowable NO 2 concentration anywhere in an area. After considering
NO 2 monitoring studies assessed in the ISA and discussed in the REA, as well as modeling simulations presented in the REA, the Administrator conclu ded that maximum NO 2 concentrations are expected to occur around ma jor roads in many areas (see section II.F.3 for more detail). Because of this, and because a co nsiderable fraction of the population lives, works, or attends school near major roadways, the Admi nistrator concludes that it is appropriate to require NO 2 monitors near major roads in urban areas. Monitoring NO 2 concentrations by major118 roadways will provide additional protection for asthmatics and members of other susceptible groups that may experience greater risk s of health effects related to NO 2 exposure. The Administrator notes that this decision makes progress in iden tifying and addre ssing the health burdens faced by communities di sproportionately impacted by po llution from major roadways. Although near-road monitoring is a new approach for NO 2 , it is a form of source-oriented monitoring, which is not a new concept with rega rd to monitoring to characterize stationary source impacts. The novel aspect of this NO2 network design is that the near-road monitoring component is targeting mobile sources instead of stationary sources. The required near-road NO 2 monitors are to be located in locations of exp ected maximum 1-hour concentrations (discussed in a subsequent section) and will likely represent the highest NO 2 concentrations in an area directly attributable to mobile sources or a group of sources that includ es mobile sources. Unlike a network with monitors focused primarily on stat ionary sources, if a near-road monitor shows data for a given area indicating concentrations below the NAAQS, it is likely that all near-road concentrations throughout the area are likely below the NAAQS. In addition, if a near-road monitor shows data that are above the NAAQS, it is probable that measures that may be taken to bring concentrations back down below the NAAQS will likely affect some or all of the mobile sources in the area, reducing mobile source emissions across an area. EPA also recognizes the need to have monitors away from roads that represent area-wide concentrations that serve multiple monitoring objectives including comparison to the NAAQS, photochemical pollutant assessment, aiding in oz one forecasting, and characterization of point and area source impacts. After considering the scientif ic data and the public comm ents regarding the proposed network design, EPA is selecting to implement th e two-tier network design composed of (1)119 near-road monitors which would be placed in locations of expected maximum 1-hour NO 2 concentrations near heavily trafficked roads in urban areas and (2) monitors located to characterize areas with the highest expected NO 2 concentrations at th e neighborhood and larger spatial scales (also referred to as “area-wide” monitors). 2. First Tier (Near-road Mon itoring Component) of the NO 2 Network Design The following paragraphs provide background, ra tionale, and details for the final changes to the first tier of the two-tier NO 2 network design. In particular, this section will focus on the thresholds that trigger monitoring requirements. Near-road site selection and siting criteria details will be di scussed in subsequent sections. a. Proposed First Tier (N ear-road Monitoring Component) of the Network Design EPA proposed that the first tier of the two-tier NO 2 monitoring network design focus monitors in locations of expected maximum 1- hour concentrations near major roads in urban areas. As noted in previous section, the ISA a nd REA indicate that one of the largest factors affecting ambient exposures to NO 2 above health benchmarks con centrations are mobile source emissions, particularly at locations near major ro ads. Since the combination of increased mobile source emissions and increased urban population de nsities can lead to increased exposures and associated risks, urban areas are the appropr iate areas to concentrate required near-road monitoring efforts. Therefore, we proposed that one near-road NO 2 monitor be required in CBSAs with a population greater than or equal to 350,000 persons. Based on 2008 Census Bureau statistics, this will result in approximately 143 sites in as many CBSAs. We also proposed that a second near-roa d monitor be required in CBSAs with a population greater than or equal to 2,500,000 persons , or in any CBSAs with one or more road segments with an Annual Average Daily Traffi c (AADT) count greater th an or equal to 250,000.120 Based on 2008 Census Bureau statistics and data from the 2007 Highway Performance Monitoring System (HPMS) maintained by the US Department of Transportation (DOT) Federal Highway Administration (FHWA) , this particular element of the minimum monitoring requirements will add approximately 24 sites to the approximate 143 near-road sites in CBSAs that already will have one n ear-road monitor required due to the 350,000 population threshold. Of the 24 additional sites, tw o sites are triggered due to the 250,000 AADT threshold and are attributed to the Las Vegas, Nevada and Sacramen to, California CBSAs. Overall, this first tier of the proposed network design is estimated to require 167 near-road sites in 143 CBSAs. b. Comments EPA received comments from some industry and public health groups supporting our proposed population thresholds which trigger mi nimum near-road monitoring requirements. For example, Dow Chemical Company stated that“Dow comments that the proposed population thresholds are reasonable for implementation of the new network design and that we don't see a need to establish a thre shold lower than 350,000 people for the lower bound.” EPA also received comments from some stat es and state groups suggesting that a combination of population and AADT counts or ju st AADT should be used to trigger minimum near-road monitoring requirements. For example, the San Joaquin Air Po llution Control District in California suggested that we modify minimu m monitoring requirements so that one near-road
NO 2 monitor be required for a ny CBSA with a population of 350,000 people and that had one or more road segments that had AADT counts of 125,000 or more. In another example, Harris County Public Health and Envir onmental Services suggested th at “...rather than specifying population limits for the monitoring, HCPHES su pports a metric like the Annual Average Daily Traffic (AADT) as a threshold for requiring a nea r-road monitor. An initial focus on an AADT in121 excess of 250,000 is acceptable as a starting point but EPA should revi sit that level and consider lowering it to 100,000 in five years.” EPA notes that, in general, roads with higher AADT counts have relatively higher amounts of mobile source emissions, which provide an increased potential to lead to relativelyhigher on-road and roadside NO 2 concentrations. This concept is supported, for example, by Gilbert et al. 2007, who state that the NO 2 concentrations analyzed in their study are significantly asso ciated with distance from the nearest highway and the traffic count on the nearest highway. EPA believes that pa rt of the intent of suggestions to include AADT as part of, or distinctly as the requirement threshold is to further increase th e focus of the near-road network to locations where NO 2 concentrations are expected to be highest. However, EPA chose the population threshold approach to trigger near-road monitoring in lieu of a population-AADT count combination or a distin ct AADT count threshold because of the uncertainty in defining which AADT counts would be appropriate to use in such an approach. The depth of knowledge and certainty in the understanding of what particular AADT count might be expected to cont ribute to some specific NO 2 concentration is not sufficient to translate into a specific, nationally applicable AADT count threshold that could be used as part of a population-AADT threshold approach to require n ear-road monitors. Therefore, EPA chose not to utilize a population-AADT or an AADT-onl y threshold to trigger near-road monitoring because of the lack of a quantit ative, nationally applicable re lationship between a certain AADT and an expected NO 2 concentration. A few commenters suggested focusing more near-road monitors only in the larger CBSAs. For example, the New York Departme nt of Environmental Conservation (NY DEC) suggested to require, at minimum, two near-ro ad monitors in any CBSA of 2,500,000 people or more, but none in CBSAs below that population th reshold. In their comme nts they point out the122 variety of near-road environments that exist in the larger CBSAs such as New York City. EPA concurs with NY DEC, and notes that the larger CBSAs, such as those with a population of 2,500,000 or more persons, are more likely to have a greater number of major roads across a potentially larger geographic area, and a corres ponding increase in potential for exposure in different settings. This is the reasoning behind the requirement for two monitors in CBSAs with more than 2,500,000 people. EPA also believes that having multiple monitors in the largest CBSAs will also allow better unders tanding of the differences that may exist between different roads in the same CBSAs due to fleet mix, conge stion patterns, terrain, or geographic locations within those individual CBSAs . However, EPA believes that a network with less overall monitors in a fewer number of CBSAs than that which was proposed would lead to an insufficiently sized network and would be an unbalanced approach for a regulatory network intended to support the revised NAAQS. Considering the potential variability that may exist from one CBSA to the next with regard to the number of roads, the populations that live near roads, the fleet mix, the terrain, and the local meteorology, EPA believes a network that has more monitors in a greater number of CBSAs is more appropriate to fulfill the intent of the revised NAAQS, where the public are to be protected from NO 2 concentrations exceeding the NAAQS anywhere in an area. EPA proposed to require a second monitor in CBSAs with a population greater than or equal to 2,500,000 persons, to allow for further char acterization of larger urban areas that are more likely to have a greater number of major ro ads across a potentially larger geographic area, and a corresponding increase in potential for expo sure. The additional provision for any CBSAs with one or more road segments with an A nnual Average Daily Traffic (AADT) count greater than or equal to 250,000 to have a second monito r if they do not alrea dy have two near-road123 monitors required due their population is to co ver situations where a re latively less populated area has a very highly trafficked road. In th is case, EPA believes th at because those road segments with 250,000 AADT have been identified by US DOT FHWA ( http://www.fhwa.dot.gov/polic yinformation/tables/02.cfm ) as being the top 0.03 percent of the most traveled public road segments, that they are the most heavily trafficked roads in the country. These roads segments may correspond to the greatest potential for high exposures directly connected to motor vehicle emissions in the entire country. Typi cally, these very highly trafficked roads are in the largest populated CBSAs, such as those with 2,500,000 people or more, and are somewhat atypical for CBSAs with less than 2,500,000 people. As a result, EPA believes it is appropriate to require a second monitor in a CBSA that has one or more road segments with 250,000 AADT counts or more if they do not already have two near-road monitors required due their population. EPA received a few comments requesting e xplanation of the reasoning behind the selection of the population thre sholds that trigger minimum monitoring requirements. For example, the New York Department of Transporta tion suggested that this final rule explain the basis for the 350,000 and 2,500,000 population thresholds th at will establish near-road monitors. In using the population threshold approach to trigger monitoring requirements, EPA proposed that those CBSAs with populations equal to or greater than 350,000 and 2,500,000 shall have one or two required near-road monito rs, respectively. The selecti on of the CBSA population level of 350,000 to trigger the requirement of one near-road monitor was chosen to require monitoring in CBSAs that will provide near-road monitoring data with a more diverse array of CBSAs in terms of population, potential fleet mixes, geographic extent, and geographic setting, from across the country relative to a network triggered by a higher population threshol d. EPA reviewed the124 spatial extent and the population included in CBS As of certain sizes when making selecting the population thresholds. There are 143 CBSAs w ith 350,000 people across th e country (including territories) which contain appr oximately 72% of th e total population acc ording to 2008 U.S. Census Bureau estimates (www.census.gov). This collection of CBSAs co llectively represents territory in 44 states, the Dist rict of Columbia, and Puerto Rico. For comparison, there are 52 CBSAs with 1,000,000 or more people, which co ntain approximately 55% of the total population. This particular collec tion of CBSAs collectively represen ts territory in only [SPACE
RESERVED FOR DATA – NEED TO VERIFY IN ArcGIS]. EPA believes that selecting 350,000 as a threshold to minimally require one n ear-road monitor provides a sufficiently sized network with good spatial representation across th e country in a diverse set of CBSAs that represent a significant porti on of the population. The se lection of the 2,500,000 population threshold to trigger a second near-road monitor, as noted earlier in this section, is based on the fact that the larger urban areas in the country are more likely to have a greater number of major roads across a potentially larger geographic area , and have a corresponding increase in potential for exposure in different settings. EPA received many state comments suggesti ng adjustments to the overall near-road network components’ size, withou t giving distinct sugge stions on how to do so. For example, the Regional Air Pollution Control Agency, who repr esents a portion of Ohio, stated “Given the fairly standard fleet of vehicles on the nation' s major highways, we urge EPA to consider the need for 142 near-roadway monitors. Perhaps a limited number of monitors across the country would suffice to sufficiently characterize near-ro adway NO2 levels.” This group of commenters had various reasons for why they suggested th e network be reduced, including funding concerns (discussed above), safety issues (discussed below) , the perceived need to do additional research125 monitoring (discussed above), problems with stat e implementation plans (discussed in ??), and designation issues (discussed in ??). EPA understands the issues and concerns presented, and believe that many of these issues are addressed throughout this document. However, in regard to reducing the size of the first tier of the network design, EPA believes that a significantly smaller network than what was proposed would be in sufficient in supporting the revised NAAQS by assuring that a significant amount of populations across the country are informed and ultimately protected by having near-road NO 2 data that can be compared to the NAAQS. The intent of the first tier of the network design is to require near-road monitori ng in CBSAs that will provide near-road monitoring data with a more diverse array of CBSAs in terms of population, potential fleet mixes, geographic extent, and geogr aphic setting, from across the country. c. Conclusions Regarding the First Tier (N ear-road Monitoring Com ponent) of the Network Design EPA notes that there is a significant lack of routine near-road NO 2 monitoring data from different locations around the country. If the proposed monitoring netw ork is to support the intent of the revised NAAQS, preventing exposures to NO 2 concentrations above the NAAQS anywhere in an area, and recognizing that mob ile sources have been identified as a major contributor to exposure risks, a sufficiently sized network includi ng near-road monitors, such as that proposed, is appropriate.. The number and distribution of the monitors in the proposed network will allow states and EPA to gather data for comparison to the NAAQS and ultimately inform the public on what NO 2 concentrations populations in a diverse array of urban areas, that contain over 72% of the total U.S. population (based on 2008 U.S. Census Bureau estimates), may routinely be exposed to on and near roads. Therefore, we are finaliz ing the thresholds that will trigger minimally required near-road NO 2 monitors as they were proposed, where one near-126 road NO 2 monitor is required in CBSAs with a population greater than or equal to 350,000 persons and a second near-road monitor is require d in CBSAs with a population greater than or equal to 2,500,000 persons, or in an y CBSAs with one or more road segments with an Annual Average Daily Traffic (AADT) count greater than or equal to 250,000. 3. Second Tier (Area-wide Monitoring Component) of the Network Design The following paragraphs provide background, ra tionale, and details for the final changes to the second tier of the two-tier NO 2 network design. In particular, this section will focus on the threshold that triggers monitoring requirements. Area-wide site sele ction and siting criteria details will be di scussed in a subsequent section. a. Proposed Second Tier (Ar ea-wide Monitoring Component ) of the Network Design As the second tier of the proposed two-ti er network design, EPA proposed to require monitors to characterize the highest expected NO 2 concentrations at th e neighborhood and larger (area-wide) spatial scales in a area. This component of the two-tier network design provides information on area-wide type exposures that may occur in an area due to an individual or a group of point, area, on-road, and/or non-road sour ces. Further, area-wide sites serve multiple monitoring objectives aside from NAAQS co mparison including photochemical pollutant assessment, aiding in ozone forecasting, aiding in particulate matter precursor analysis and particulate matter forecasting. We proposed to re quire one area-wide monitoring site in each CBSA with a population greater than or equal to 1,000,000. These area-wide sites are proposed to be sited to represent an area of highest c oncentration at the neighbor hood or larger spatial scales. Based on 2008 Census Bureau statistics , this minimum monitoring requirement is expected to trigger 52 area-wide monitors in as many CBSAs. EPA also proposed to allow any current photochemical assessment monitoring station (PAMS) sites that are situated to address127 the highest NO 2 concentrations in an urban area and sited at neighborhood or urban scales to satisfy this proposed area-w ide monitoring requirement. b. Comments Most commenters who made comments on area-wi de monitoring did so in regard to their support of the adoption of the alte rnative area-wide network design and not specifically about the area-wide monitoring component of the proposed two-tier networ k design. However, EPA did receive comments from health groups on area-w ide monitoring in context of the proposed network design. The health group commenters , including the American Lung Association, Earthjustice, the Environmental Defense Fund, and the National Resource Defense Council, stated they “oppose the proposed requirement to retain only 52 air monitors to measure areawide concentrations of NO2.” EPA understands the perceived con cern to be that we are actively reducing the number of requi red area-wide monitors. Prior to this rulemaking, the 2006 monito ring rule (REFERENCE) removed minimum monitoring requirements for NO 2 , however the rule has had a lim ited impact to date, evidenced by the fact that the size of the NO 2 network has remained relatively steady, with around 400 monitors, a majority of which are area-wide monitors, were operating in 2008 (Watkins and Thompson, 2008). This is due to multiple reasons, but in large part due to the fact that area-wide monitors serve multiple monitori ng objectives, as noted earlier, which includes photochemical pollutant assessment, aiding in pollutant forecasting, and in some cases in support to ongoing health research. Noting that states always have the right to conduc t monitoring above the minimum Federal requirements, and recognizi ng the objectives of programs such as the Photochemical Assessment Monitoring Stations (P AMS), EPA believes that the actual number of area-wide monitors that will ope rate in the NO2 network will be greater than the minimally128 required 52 sites. As a result, EPA does not be lieve that a modification to the two-tier network design that increases the number of required area-wide NO2 sites as was proposed is necessary. c. Conclusions on the Second Tier (Area-wi de Monitoring Component) of the Network Design EPA recognizes that significant portion of the existing NO2 mon itoring network can be characterized as area-wide monitors and that these monitoring sites serve multiple monitoring objectives, as noted above, even thought they were not previously required to operate. In order to ensure that a minimum number of area-wide monitors continue operating into the future, we are finalizing the proposed minimum monitori ng requirements for area-wide monitors, where one area-wide monitor is required in any CBSA with 1,000,000 people or more. Since there were no objections to allowing PAMS stations that meet siting criteria (discussed in a subsequent section) to satisfy minimum m onitoring requirements for area-wide monitors, we are finalizing that allowance as proposed. In addition, EPA en courages states to use the network assessment process to review existing area -wide NO2 sites to help determine what monitors might meet minimum monitoring requirements and whether or not other existing monitors warrant continued operation. EPA would expect that this process would occur gra dually over several years, but would greatly assist in streamlining the NO2 ne twork, as states move to meet new minimum monitoring requirements and modify the network to reflect the intent of the revised NAAQS and the needs of other ongoing monitoring objectives. 4. Regional Administrator Authority The following paragraphs provide background, ra tionale, and details for the final changes to Regional Administrator’s authority to us e their discretion in requiring additional NO2129 monitors in addition to those required in the mi nimum monitoring requirem ents of the two-tier network design. a. Proposed Regional Administrator Authority EPA proposed that the Regional Administrato r will have discretion to require monitoring above the minimum requirements as necessary to address situations where the required near-road monitors do not represent a lo cation or locations where th e expected maximum hourly NO 2 concentrations exist in a CBSA. We also proposed to allo w Regional Administrators the discretion to require additional near-road monitoring sites to a ddress situations where minimum monitoring requirements are not sufficient to meet monitoring objectives, such as a situation where there is a variety of exposur e potential in an area due to va riety in the amount or types of fleet mix, congestion patterns, te rrain, or geographic areas with in a CBSA. Finally, we proposed that EPA Regional Administrators have the discretion to require additional area-wide NO 2 monitoring sites above the mi nimum monitoring requirements where the minimum monitoring requirements for area-wide monitors are not sufficient to meet monitoring objectives. b. Comments EPA received comments from some state groups (e.g. the New York Department of Environmental Conservation, New York Departmentof Transportation, and the New York City Law Department) and an industry group (the Co uncil of Industrial Bo iler Operators) who requested greater clarification on how Regional Administrators may use their authority to require additional monitors above the minimum requireme nts. For example, the Council of Industrial Boiler Operators stated that “This [Regional Administrator Authority] unreasonably vests an unbounded amount of discretion in EPA to determine when "minimum monitoring requirements are not sufficient" and which neighborhoods ar e "uniquely affected," and impose additional130 monitoring requirements where al l applicable monitoring require ments are already met by the State and local agency.” EPA believes that although minimum monito ring requirement do provide a sufficiently sized and targeted monitoring network, a nationallyapplicable network design can not always be guaranteed to account for all the potential pollution expos ure situations that can exist in every area of the country. This is the basis for allowi ng Regional Administrators the authority to use their discretion to require additi onal monitors. The process by which an Regional Administrator would use this authority usually is part of a collaborative process with state agencies. Typically, Regional Administrators will work with state and local agencies to design and/or maintain the most appropriate NO 2 network to service the needs for a given area. For a ll the situations proposed where a Regional Administrator may re quire additional monitoring, EPA expects that Regional Administrators would work on a cas e-by-case basis in re quiring any additional monitors. One example might be a location imp acted by a stationary so urce where the required near-road NO 2 monitor site is not the location of maxi mum hourly concentra tion in a CBSA. As a result, that location could be th e location of expected maximum c oncentration in the area. This situation could arise due to impact s from one of or a combination of multiple sources that could include point, area, and non-ro ad source emissions in additio n to on-road mobile source emissions. Alternatively, in this example, if the sources responsible for this situation are producing more widespread impacts on a commun ity or area, the Regi onal Administrator may require an area-wide monitor to assess populatio n exposures, or support other monitor objectives served by area-wide monitors such as photoc hemical pollutant assessment or pollutant forecasting. Another example might be in a CBSA where a state or local agency is fulfilling its minimum monitoring requirements near-road monito rs, but an additional lo cation is identified131 where near-road population exposure exists, at concentrations near or above the NAAQS, that is unique, relative to the minimally required near-r oad monitor locations. For this example, the unique situation might be due to the particular populati on or community that is affected, or the differences in fleet mix, congestion patterns, terrain, or geographic area, relative to any minimally required monitoring site(s) in that area. EPA recognizes that a variety of exposures scenarios can occur in an area, and we believe that Regional Administrators shou ld have the discretion to require additional monitoring if such a unique situation is reco gnized in an area. In such situ ations, the actual locations where additional monitoring could be required would lik ely have already been identified by state or EPA Regional staff through data analysis, such as the evaluation of existing ambient data and/or emissions data, or through air quality modeli ng. Such information may indicate that an area might have ground-level NO 2 concentrations that may exceed the NAAQS, and that a there is potential for population exposure to those concentrations. c. Conclusions on Regional Administrator Authority Due to the fact that EPA recognizes that a nationally applied mon itoring network may not always provide the intended monitoring coverage due to case-by-case type situations, we are finalizing, as proposed, that (1) Regional Administrators shall have the discretion to require monitoring above the minimum requirements as necessary to address situations where the required near-road monitors do not represent a location or locations where the expected maximum hourly NO 2 concentrations exist in a CBSA. (2 ) Regional Administrators shall have the discretion to require additi onal near-road monitoring sites to address situations where minimum monitoring requirements are not sufficient to meet monitoring objectives, such as a situation where there is a variet y of exposure potential in an area due to variety in the amount or132 types of fleet mix, congestion patterns, terrain, or geographic areas within a CBSA. (3) Regional Administrators shall have the discre tion to require additional area-wide NO 2 monitoring sites above the minimum monitoring requirements wh ere the minimum monito ring requirements for area-wide monitors are not suffi cient to meet monitoring objec tives. In any case where a Regional Administrator might be in a position to use their authority to require additional monitoring, EPA expects that the state and the Re gional Administrator would work together to evaluate evidence that suggests an area may warrant additional monitoring. EPA also notes that if additional monitoring should be required, the state would present information on any potential new sites through its Annual M onitoring Network Plan, as re quired by 40 CFR §58.10, which includes a requirement for public inspection or comment, and approval by the EPA Regional Administrator. 5. Monitoring Network Implementation The following paragraphs provide background, ra tionale, and details for the final changes to the approach for the mon itoring network implementation. a. Proposed Monitoring Networ k Implementation Approach EPA proposed that state and, when appropria te, local air monitori ng agencies provide a plan for deploying monitors in accordance w ith the proposed network design by July 1, 2011. EPA also proposed that the proposed NO 2 network be physically established no later than January 1, 2013 b. Comments Most environmental and public health group commenters made comments that suggested EPA change the implementation date from the proposed January 1, 2013 to a date that would require the minimally required NO 2 network deployed sooner than proposed. Most states and133 state group commenters, along with industry grou p commenters, recommended that EPA keep the network implementation date as January 1, 2013, or move it later than proposed. Some of the commenters who suggested moving it later noted that issues with monitoring site identification, site development, and overall lack of experien ce working in the near-road environment would make implementation difficult under the proposed implementation deadline. EPA recognizes that a lot of work must occur to deploy th e two-tier network design by the January 1, 2013 date. We recogni ze the need for guidance and pl an to aid state agencies in the network implementation process, particular ly through guidance documentation that will be developed. EPA agrees with the sugges tion by NACAA that the CASAC Ambient Air Monitoring Methods subcommittee should be consu lted as part of any guidance development for near-road monitoring. Further, EPA believes that collaboration with the states and state groups in developing guidance will be highly beneficial to the implementation process. This would allow for those states that do have increased experience in near-road monitoring to lend to guidance development and provide a conduit for shar ing experiences amongst all stakeholders. In perspective, EPA believes that the approximate 2 years and 11 months between promulgation of this rulemaking and the manda ted January 1, 2013 network implementation date include extra time relative to what was trad itionally allowed for network implementation following rulemakings such as th is. Additionally, we note that are being cognizant of the time needed to collect complete data that would allow data from the two-tier network to be considered in the next NO2 NAAQS review. EPA would need to have data from 2013, 2014, and 2015 to have a chance to include data inthe next NAAQS review since it is intended to be performed on a 5-year cycle. Finally, delayi ng the installation of the minimally required monitors, particularly the near-road NO 2 sites, will also delay the acquisition of the three years of data that are needed134 to use in determining attainment, which, as note d in XX of this document, would not occur until 2017, at the earliest. c. Conclusions on Monitoring Network Implementation We are finalizing the date by which state and, when appropriate, local air monitoring agencies shall establish the required NO 2 monitoring network as January 1, 2013, as was proposed. We believe that the allotted time fo r implementation will allow for the development of guidance documentation, particularly allowi ng for interactions with CASAC and NACAA, and for the processes that will be involved in deploying this network, including the assessment of road segments in CBSAs, identifying and working with local transportation officials as needed on issues regarding access and safe ty, and the exchange of information and feedback on potential sites with EPA, prior to any commitment to a ny sites in an annual monitoring plan. However, based on feedback received through public comment s, and to allow for more time to process guidance information, to carry ou t the previously men tioned deployment processes, and to allow for information exchanges to occur, we ar e changing the date by which state and, when appropriate, local air monitoring agencies sha ll provide a plan for deploying monitors in accordance with required network design from July 1, 2011 to July 1, 2012. However, EPA encourages states and local air agencies to s upply as much information as possible on the NO 2 sites they may be considering, or have po ssibly selected, to sa tisfy the minimum NO 2 network monitoring requirements in their Annual Moni toring Network Plan submitted July 1, 2011. 6. Near-road Site Selection The following paragraphs provide background, ra tionale, and details for the final changes to the approach and criteria by which requi red near-road sites shall be selected. a. Proposed Near-road Site Selection Criterion135 EPA proposed that minimally required near-road NO 2 monitoring stations shall be selected by ranking all road segments within a CBSA by AADT and then identifying a location or locations adjacent to those highest ra nked road segments where maximum hourly NO 2 concentrations are expected to be highest and siting criteria (discussed below) can be met in accordance with that proposed for 40 CFR Part 58 Appendix E. Where a state or local air monitoring agency identifies multiple accepta ble candidate sites where maximum hourly NO 2 concentrations are expected to occur, the mon itoring agency should consider taking into account the potential for population exposure in the criteria utilized to sele ct the final site location. Where one CBSA is required to have two near-road NO 2 monitoring stations, we proposed that the sites shall be differentiated from each other by one or more of the following factors: fleet mix; congestion patterns; terrain; geog raphic area within the CBSA; or different route, interstate, or freeway designation. b. Comments EPA received many comments from CASAC, public health groups, states and state groups, and industry groups on the proposed pro cedure by which states will select near-road sites. CASAC, along with some health group and state commenters noted that interpreted proposed language to intend that stat es should select a site near th e road with the highest ranked AADT possible, without consider ing other factors. For example, one CASAC panel member noted that siting monitors based on traffic c ounts alone might miss locations where maximum NO2 concentrations would occur. They proceeded to recommend the use of modeling to assist in the site selection proce ss. In another example, the ALA, ED J, EJ, and NRDC, stated that “Nearroad monitor placement should be determined not only by the highest AADT volumes in a given CBSA, but also by the highest heavy-duty truck volumes.” In one more example, in NACAA136 statements they make a comment about “...basing monitor locations on th e annual average daily traffic (AADT) without regard to vehicle mix or dispersion characteristics...”. EPA does not intend for AADT counts to be the sole reason a near-road site is selected. As noted earlier in paragraph 2(b) of this section, there is a general relationship between AADT and mobile source pollution, where higher traffi c counts correspond to higher mobile source emissions. The use of AADT counts is intendedto be a mechanism by which the locations of expected maximum NO 2 concentrations due to mobile sour ces are identified. EPA notes that there other factors that can influence which road segment in a CBSA may be the actual location where the maximum NO 2 concentrations might occur. Thes e factors include vehicle fleet mix, roadway design, congestion patterns, terrain, and meteorology. When states identify their topranked road segments by AADT, EPA intends for states to evaluate all of th e factors listed above that influence where the locat ion of expected maximum NO 2 concentration may occur from the pool of candidate target road segments and monito ring sties. As a result of the comments on this interpretation of the proposal, EP A will specifically list the factor s that will be used considered by states in their site selection process once a state has identified the most heavily trafficked roads in a CBSA via AADT counts. In additi on, these same factors were proposed to be considered by states who are required to plac e two near-road monitors in one CBSA. EPA maintains that these factors will be key to differe ntiate the two monitoring sites from each other, providing further characterization of near-road environments in larg er urban areas that are more likely to have a greater number of major roads across a potentially larger geographic area, and a corresponding increase in potential for exposure in different settings. EPA received comments from some state a nd industry commenters (e.g. Iowa, NY DEC, Edison Electric, and Savannah Ri ver Nuclear Solutions) who sugge sted that population exposure137 should be a first-level consider ation in the near-road monitori ng site selection, instead of a second-level consideration which EPA proposed it to be. EPA notes that the intent of the revised primary NO 2 NAAQS is to protect against the maximum allowable NO 2 concentration anywhere in an area, which includes ambient air on and around roads, which would limit exposures to peak NO 2 concentrations, across locations and over time, with a relatively high degree of confidence. We also note the agency’s historical practice of locating monitors in locations of maximum concentra tion, at the appropriate spatial scale, so long as the monitor is characterizi ng ambient air. If EPA were to allow population, population density, or another popula tion weighted metric to influe nce where near-road monitors are located, it is possible that the required near -road monitors in a CBSA would not be where the location of expected maximum NO 2 concentration, due to mobile sources, would occur. By monitoring in the location of expected maximu m 1-hour concentrations , near-road monitoring sites will likely represent the highest NO 2 concentrations in an area directly attributable to mobile sources or a group of sources that includ es mobile sources. EPA agrees that population should not be ignored, however, which is reflected in the rule language as it was proposed that says that if a states identifies multiple acceptable candidate sites where maximum hourly NO 2 concentrations are expected to occur, the mon itoring agency should consider taking into account the potential for population exposure in the criteria utilized to sele ct the final site location. EPA received commenters from [NEED PUBLIC COMMENTER ID] who suggested that near-road monitoring site se lection should take into consider ation the location of other major mobile and point sources for NO 2 emissions, such as airports, seaports, and power plants. Further, some of these commenters also suggested that state and local ag encies have increased flexibility in determining the appropriate balance between loca ting near-road monitors along the138 busiest roadways, areas most likely to have the highest NO 2 concentrations, and diverse types of roadways. EPA agrees that other major NO 2 sources that have the pot ential to violate the NAAQS should be monitored. The issue is whether such monitoring should be ha ndled through a more inclusive set of minimum requirements or whethe r case-by-case treatment is appropriate. EPA believes that any of these mobile sources and any stationary source th at may be causing groundlevel NO 2 concentrations that are ne ar or above the NAAQS are able to be monitored through a requirement by the Regional Administrator (dis cussed above) or possibly through state and local initiative. The reason that Re gional Administrators have disc retion to require additional monitors is to provide the ability to deal with situations where other locations in an area might warrant monitoring, which EPA believes was the root of the respondent’s comment. One state commenter, the Wisconsin Department of Natural Resources, requested that the term “major road” be defined and also requested clarification on what “top-ranked” means with regard to AADT counts on road segments. The term “major road” is widely used in literature and can be found to be defined differently from on e scientific study to another. In this document and the proposal, EPA simply means for “major road” to be synonymous with a road that is relatively heavily trafficked. EPA also does not believe it is appropriate to define whether “topranked” means, for example, 5%, 10%, or 15%, of all road segments, as this would be arbitrary. Each CBSA will have a different distribution of total road segments and corresponding AADT counts on those segments. The intent of the requirement to rank all road segments by AADT counts and select a site, considering the other lo cal factors noted above, n ear a “top-ranked” road segment is to focus attention on the most heavily trafficked roads, around which there is higher potential for maximum NO 2 concentrations to occur.139 c. Conclusions on Near-road Site Selection We are finalizing the near-road site selecti on criteria as proposed, and are clarifying the original intent that the selection criteria include the consideration of localized factors. As a result, minimally required near-road NO 2 monitoring stations shall be selected by ranking all road segments within a CBSA by AADT and then identifyi ng a location or locations adjacent to those highest ranked road segments, considering fleet mix, roadway design, congestion patterns, terrain, and meteorology, where maximum hourly NO 2 concentrations are e xpected to be highest and siting criteria can be met in accordance with 40 CFR Part 58 Appendix E. As was noted in the network implementation secti on above, EPA will work with stat es to assist with the nearroad site selection process through the deve lopment of guidance material and through information exchanges amongst the air monitoring community. We are also finalizing the requirement, as proposed, that when one CBSA is required to have two near-road NO 2 monitoring stations, the sites shall be differentiated from each other by one or more of the following factors: fleet mix; congestion patterns; te rrain; geographic area within the CBSA; or different route, inters tate, or freeway desi gnation, as was proposed. 7. Near-road Siting Criteria The following paragraphs provide background, ra tionale, and details for the final changes to the siting criteria for require d near-road monitoring sites. a. Proposed Near-road Siting Criteria We proposed that near-road NO 2 monitoring stations must be sited so that the NO 2 monitor probe is no greater than 50 meters away, horizontally, from the outside nearest edge of the traffic lanes of the target ro ad segment, and shall have no obstructions in the fetch between the monitor probe and roadway traffic such as noise barriers or vegetation higher than the140 monitor probe height. We solicited comment on wh ether the near-road site s should be located on the predominantly downwind side of the target roadways. We proposed that the monitor probe be locate d within 2 to 7 meters above the ground, as is required for microscale PM 2.5 sites. We also proposed that monitor probe placement on noise barriers or buildings, where the inlet probe height is no less than 2 meters and no more than 7 meters above the target road, will be acceptable, so long as the inlet probe is at least 1 meter vertically or horizontally away (in the direction of the target road) from any supporting wall or structure, and the subsequent re sidence time of the pollutant in the sample line between the inlet probe and the analyzer does not exceed 20 seconds. b. Comments EPA received many comments state (e .g. LIST) and industry groups (e.g. LIST) indicating that the near -road network poses significant safe ty issues and a related need for increased logistical flexibility for instal ling a monitoring site. [INSERT QUOTE HERE]. EPA notes that in all instances of field work, safe ty is a top priority. EPA believes that there are various ways to install near-road sites while ensuring worker and traffic safety, and that safety is an important part of the logisti cal considerations that states should account for when selecting and installing near-road sites. An example of a step that could be taken to alleviate safety concerns might be purposefully placing a monito ring site behind existing barriers like guardrails and fencing, or possibly by installing a short distance of such ba rriers to protect the site workers, site infrastructure, and nearby tra ffic. In addition to intentional measures that can be taken to enhance safety, EPA notes that the 50m dist ance proposed is wide enough to accommodate a sites that would satisfy many setback provisions that exist for private or commercial building141 permits near roads, and may be viewed as a confirmation that our pr oposed siting criteria are safely attainable. Many commenters suggested that the allowable maximum distance a near-road monitoring probe can be from the target road be increased from 50 meters to something wider, such as 200 meters. Conversely, there were so me commenters who suggested that EPA reduce the allowable distance to as clos e as 30 meters to the target ro ad. EPA believes that increasing the allowable distance above 50 meters would comp romise the intent of near-road monitoring. As was noted in the proposal language, scientific literature indicates that on-road, mobile source derived NO 2 exhibits a peak concentration on or very near the source road, and those concentrations decay over a variable but relati vely short distance back to near area-wide or background (upwind of the target road) concentrations. Literature values indicate that the distance required for NO 2 concentrations to return to near area-wide or background concentrations away from major roadways can range up to 500 meters. The actual distance is variable, and highly dependent on topography, roadside features, meteorology, and the related photochemical reactivity conditions (Baldauf et al., 2008; Beckerman et al., 2007; Clements et al., 2008; Gilbert et al. 2003; Hagler et al., 2009; Rodes and Holland, 1980; Singer et al., 2003; Zhou and Levy, 2007). Therefore, monitor probe pl acement at increasing distances from a road, such as 200 meters, will correspondingly d ecrease the potential for sampling maximum concentrations of NO 2 due to the traffic on the target road. Baldauf et al. (2009) indicate that monitoring probes would ideally be situated be tween 10 and 20 meters from the nearest traffic lane for near-road pollutant monitoring. Having received comments suggesting wideni ng and shortening the allowable distance that an NO 2 monitor probe may be from the target road, EPA believes the proposed allowable142 distance a near-road NO 2 probe can be from the target road of 50 meters provides enough flexibility for site placement while not sacrific ing the potential to actually monitor the peak NO 2 concentrations. However, in light of the information provided here on how NO 2 peak concentrations can decay over relatively short di stances, EPA encourages states to place nearroad sites as close as safely possible to target roads to increase the prob ability of measuring the peak NO 2 concentrations that occur in the near-road environment, again noting that Baldauf et al. (2009) indicate that monitor probe s would ideally be situated be tween 10 and 20 meters from the nearest traffic lane for near-road pollutant monitoring. EPA received several comments on the solici tation for comment on requiring near-road monitoring sites to be placed on the downwind si de of the target road where the commenters encouraged such a requirement. Conversel y, other commenters suggested that such a requirement is unnecessary, with one commenting that siting monitors in downwind locations would not be feasible for all locations and th at EPA should allow upwind monitoring so long as upwind monitors are used in conjunction with air dispersion modeling. EPA noted in its proposal that research literature indicates that in certain cases , mobile source derived pollutant concentrations, including NO 2 , can be detected upwind of road s, above background levels, due to a phenomenon called upwind meanderi ng. Kalthoff et al. (2007) i ndicates that mobile source derived pollutants can meander upwind on the order of tens of meters, mainly due to vehicle induced turbulence, while Beckerma n et al. (2008) note that near -road pollutant concentrations on the predominantly upwind side of their study sites dropped off to near background levels within the first 50 meters, but were above bac kground in this short and variable upwind range, which could be due, at least in part, to vehicle induced turbulence. This upwind meandering characteristic of pollutants in the near-road envi ronment provides an additional basis for locating143 near-road sites within 50 meters of target road segments , but also reduces the absolute need to be downwind of the road. EPA believes that very few, if any, near-road sites would be able to be situated in a location that was always downwi nd. For example, a hypothetical site may have winds routinely out of several different cardi nal directions throughout the year, without one being a dominant direction. As a result, for some period of a year, and given the variability meteorology, some period of a day, or even an ho ur, the near-road site may not be downwind of the target road, no matter whic h side of the road its on. EPA is not going to finalize a requirement that near-road sites must be climat ologically downwind of the target road segment because of the additional limitations this introdu ces to finding potential site candidates in exchange for what may be a small increas e in the opportunity to monitor peak NO 2 concentrations. However, EPA encourages stat es to place monitors in the climatologically downwind direction whenever possible, inan attempt to measure the peak NO 2 concentrations more often than not. EPA proposed that required near-road NO 2 monitor probes be located within 2 to 7 meters above the ground, as is required for microscale PM 2.5 sites. Two commenters commented that monitor probes distributed across the range of 2 to 7 meters may generate variability in concentrations from site to site . One of those two went furthe r to suggest that the allowable height range be reduced to 2.5 to 3.5 meters above the ground. Another group of commenters commented that the 2 to 7 meter range may not be practical for many sites and suggested guidelines for alternate siting requirements. Finally, another group of commenters commented that “the lower end of the proposed height of 2 to 7 meters appears to capture the highest NO 2 concentrations, and more accurately represents human exposure at the breathing zone.” In the proposal, EPA noted that near-road monitoring sites will be adjacent to a variety of road types,144 where some target roads will be on an even plan e with the monitoring station, while others may be cut roads (i.e., below the plane of the monitoring station) or fill and open elevated roads (i.e., where the road plane is above the monitoring station). EPA noted in the proposal that it is appropriate to place the NO 2 monitor probe as close to the plan e of the target road segment as possible, while staying between 2 to 7 meters above the ground. An allowable range between 2 and 7 meters provides more flexibility in si te installation, which EPA considers important because of the variety of siting situations each state may have to deal with for each individual site. While EPA agrees that ti ghter allowable range such as 2. 5 to 3.5 meters would reduce site to site variability and keep probes nearer th e “breathing zone” as it was commented, the wider range of 2 to 7 meters is still appropriate. EP A encourages states, where possible, to have monitor probes closer to the suggested heights of 2.5 to 3.5 meters, while also attempting to keep monitor probes as close to the plan e of the target road as possible. In the proposal, EPA proposed in the siti ng criteria language that the subsequent residence time of the pollutant in the sample lin e between the inlet probe and the analyzer can not exceed 20 seconds. EPA received two co mments regarding guidelines for maximum allowable inlet length and sample residence time. EPA notes that in 40 CFR Part 58 Appendix E, paragraph (9)(c), it is written that samp le probes for reactive gas analyzers at NCore monitoring sites must have a sample residence tim e less than 20 seconds. EPA believes this rule is also appropriate for NO 2 monitors, particularly if a monito r inlet manifold is extended away from the main monitoring shelter. EPA also notes that this concept was proposed in the proposal preamble language, but was omitted from the proposed regulation text. The final version of the regulation text will have the re vised language for this requireme nt in 40 CFR Part 58 Appendix E, paragraph (9)(c).145 c. Conclusions on Near-road Siting Criteria We are finalizing the near-road NO 2 monitor siting criteria , as proposed, where (1) required near-road NO 2 monitor probes shall be as near as practicable to the outside nearest edge of the traffic lanes of the target road segment; but shall not be located at a distance greater than 50 meters, in the horizontal, from the outside neares t edge of the traffic lanes of the target road segment, (2) required near-road NO 2 monitor probes shall have an unobstructed air flow, where no obstacles exist at or above th e height of the monitor probe, be tween the monitor probe and the outside nearest edge of the traffic lanes of th e target road segment, (3) required near-road NO 2 monitors are required to have sampler inlets be tween 2 and 7 meters above ground level, and (4) residence time of NO 2 in the sample line between the in let probe and the analyzer does not exceed 20 seconds. 8. Area-wide Monitor Site Se lection and Siting Criteria The following paragraphs provide background, ra tionale, and details for the final changes to the site selection and mon itor siting criteria for required area-wide monitoring sites. a. Proposed Area-wide Monitor Si te Selection and Siting Criteria EPA proposed that sites requi red as part of the second tier of the NO2 monitoring network design, known as the area-wide monitori ng component, be sited to characterize the highest expected NO 2 concentrations at the neighborhood and larger (area-wide) spatial scales in
a CBSA. b. Comments While many commenters made comment suppor ting area-wide monitoring in regard to the adoption of the alternative area-wide network design in lieu of the proposed approach, only a relative few made comments that related to th e actual sites and siting criteria. Of those146 comments related to the siting of area-wide mon itors, one respondent noted that they should be located at least 1,000 meters away from any major roads or intersections to ensure that the concentration of NO 2 measured is representative of an ar ea-wide concentration instead of peak near-road concentrations. EPA notes that in order for an NO 2 monitoring site to be classified as a neighborhood (or larger) spatial scale site, it must meet the ro adway set-back requirements in Table E-1 of 40 CFR Part 58 Appendix E. EPA be lieves that this existing set-back table is appropriate to use to ensure that any NO 2 site that may be intended to be an area-wide site will be sufficiently distanced from any major road. c. Conclusions on Area-wide Monitor Site Selection and Siting Criteria We are finalizing the requirement that any si tes required as part of the second tier of the
NO 2 monitoring network design, known as the ar ea-wide monitoring component, be sited to characterize the highest expected NO 2 concentrations at the nei ghborhood and larger (area-wide) spatial scales in a CBSA. 9. Meteorological Measurements The following paragraphs provide background, ra tionale, and details for the final changes to the requirement of mete orological monitoring at near-road monitoring sites. a. Proposed Meteorological Measurements In further support of characterizing the peak NO 2 concentrations occurring in the nearroad environment, EPA proposed to require three-dimensional anemometry, providing wind vector data in the horizontal and vertical plan es, along with temperature and relative humidity measurements, at all required near-road monitoring sites. b. Comments147 EPA received comments from several states (e.g. LIST) that did not support the proposed meteorological measurement requirements. They noted issues with sitin g the probe nearer to structures and to the ground th an is typically done and averag ing periods were called into question. One respondent commented that the reco rding of air turbulence data at near-road monitoring stations ought to be encouraged but no t required. As a result, EPA will not require meteorological monitoring at near-road NO 2 monitoring stations, but encourages states to do some form of meteorological monitoring to be tter inform the conditions in which they are acquiring NO 2 data. At a minimum, basic anemometrydata would be useful in identifying whether the site is upwind, dow nwind, or otherwise oriented, relative to the target road. c. Conclusions on Meteorological Measurements We are not finalizing the pr oposal to require three-dimens ional anemometry, providing wind vector data in the horizo ntal and vertical planes, along with temperature and relative humidity measurements, at all requ ired near-road monitoring sites. C. Data Reporting The following paragraphs provide background, ra tionale, and details for the final changes to the data reporting requirements, data quality objectives, and measurement uncertainty. 1. Proposed Data Quality Objectives and Measurement Uncertainty In the proposal, EPA noted that state and loca l monitoring agencies are required to report hourly NO, NO 2 , and NO x data to AQS within 90 days of th e end of each calendar quarter. We also noted that many agencies also voluntarily re port their pre-validated da ta on an hourly basis to EPA’s real time AIRNow data system, where the data may be used by air quality forecasters to assist in ozone forecasting. We believe these data reporting procedures are appropriate to support the revised primary NO 2 NAAQS.148 EPA proposed to develop data quality objectives (DQOs) for the proposed NO 2 network. We proposed a goal for acceptable measurement uncertainty for NO 2 methods to be defined for precision as an upper 90 percent confidence limit for the coeffici ent of variation (CV) of 15 percent and for bias as an upper 95 percent confid ence limit for the absolute bias of 15 percent.
2. Comments EPA received no substantive comments that disagreed with the proposed language on data reporting, the data quality objective s, or goals for measurement uncertainty. 3. Conclusions on Data Quality Objectives and Measurement Uncertainty We are finalizing the approach to develop da ta quality objectives, and the proposed goal for measurement uncertainty, as proposed. IV. Appendix S--Interpretat ion of the Primary NAAQS for Oxides of Nitrogen and Revisions to the Exceptional Events Rule The EPA proposed to add Appendix S, Inte rpretation of the Primary National Ambient Air Quality Standards for Oxides of Nitrogen, to 40 CFR part 50 in or der to provide data handling procedures for the proposed NO 2 1-hour primary standard and for the existing NO 2 annual primary standard. The proposed Appendix S detailed the computations necessary for determining when the proposed 1-ho ur and existing annual primary NO 2 NAAQS are met. The proposed Appendix S also addressed data repor ting, data completeness considerations, and rounding conventions. Two versions of Appendix S were propos ed. The first applied to a 1-hour primary standard based on the annual 4 th high value form, while the se cond applied to a 1-hour primary standard based on the 99th perc entile daily value form. The final version of Appendix S is printed at the end of this notice and applies to an annual primary standard and a 1-hour primary st andard based on the 98th percentile daily value149 form. Appendix S is based on the near-roadway approach to the setting th e level of the 1-hour standard and to siting monitors. As such, thes e versions place no geogr aphical restrictions on which monitoring sites’ concentration data can and will be compared to the 1-hour standard when making nonattainment determin ations and other findings relate d to attainment or violation of the standard. The EPA is amending and moving the pr ovisions of 40 CFR 50.11 related to data completeness for the existing annual primary standard to the new Appendix S, and adding provisions for the proposed 1-hour primary st andard. Substantively, the data handling procedures for the annual primary standard in Appendix S are the same as the existing provisions in 40 CFR 50.11 for that standard, except for an addition of a cross-refe rence to the Exceptional Events Rule, the addition of Administrator discretion to consider otherwise incomplete data complete, and the addition of a provision addressing the possibility of there being multiple NO 2 monitors at one site. The procedures for th e 1-hour primary standard are entirely new. The EPA is also making NO 2 -specific changes to the deadlines, in 40 CFR 50.14, by which States must flag ambient air data that they believe have been affected by exceptional events and submit initial descriptions of those events, and the deadlines by which States must submit detailed justifications to support the excl usion of that data from EPA determinations of attainment or nonattainment with the NAAQS . The deadlines now contained in 40 CFR 50.14 are generic, and are not always appropriate for NO 2 given the anticipated schedule for the designations of areas under the final NO 2 NAAQS. The purpose of a data interpretation appendi x in general is to provide the practical details on how to make a comparison between mu lti-day and possibly multi-monitor ambient air concentration data and the level of the NAAQS, so that determinations of compliance and150 violation are as objective as possi ble. Data interpretation guidelin es also provide criteria for determining whether there are sufficient data to make a NAAQS level comparison at all. The regulatory language for the pre-existing annual NO 2 NAAQS, originally adopted in 1977, contained data interpretation inst ructions only for the issue of da ta completeness. This situation contrasts with the situations for ozone, PM 2.5 , PM 10 , and most recently Pb for which there are detailed data interpretation appendices in 40 CFR pa rt 50 addressing more issu es that can arise in comparing monitoring data to the NAAQS. A. Interpretation of the primary NAAQS for oxides of nitrogen for the annual primary standard The purpose of a data interpretation rule for the NO 2 NAAQS is to give effect to the form, level, averaging time, and indicator sp ecified in the regulatory text at 40 CFR 50.11, anticipating and resolving in adva nce various future situations that could occur. Appendix S provides common definitions and requirements that apply to bot h the annual and the 1-hour primary standards for NO 2 . The common requirements concern how ambient data are to be reported, what ambient data are to be consider ed (including the issue of which of multiple monitors’ data sets will be used when more th an one monitor has operate d at a site), and the applicability of the Exceptional Events Rule to the primary NO 2 NAAQS. The proposed Appendix S also addressed seve ral issues in ways which are specific to the individual primary NO 2 standards, as described below. 1. Proposed interpretation of the annual standard The proposed data interpretation provisions for the annual standard are consistent with the pre-existing instructions included along with the statement of the level and form of the standard in 40 CFR 50.11. These ar e the following: (1) At leas t 75% of the hours in the year must have reported concentration data. (2) The av ailable hourly data are arithmetically averaged,151 and then rounded (not truncated) to whole parts per billion. (3) Th e design value is this rounded annual average concentration. (4) The design valu e is compared with the level of the annual primary standard (expressed in parts per billion). In the proposal, EPA noted that it would be possible to introduce additional steps for the annual primary standard which in principl e could make the design value a more reliable indicator of actual annual average concentration in cases wher e some monitoring data have been lost. For example, averaging within a calendar quarter first and then av eraging across quarters could help compensate for uneven data capture across the year. For some aspects of the data interpretation procedures for some other polluta nts, the current data in terpretation appendices do contain such additional steps. The proposed pr ovisions for the proposed 1-hour NO2 standard also incorporated some such features. 2. Comments on interpretati on of the annual standard We received four comments, all from state ag encies, on data interpretation for the annual NO2 standard. Of the four commenters, two reco mmended the use of a weighted annual mean to appropriately implement the annual primary standard. Two other co mmenters asserted that there is no strong seasonality in NO2 concentrations, and that therefore there is no need to use a weighted annual mean or to require data completeness qua rter-by-quarter. 3. Conclusions on interpretati on of the annual standard Upon investigating the issue of NO 2 seasonality using data from AQS as part of considering the comments, we have found that th ere are notable variations in quarterly mean
NO 2 concentrations. It is therefore quite possi ble that an unweighted annual mean calculated without a quarter-by-quarter data completeness requirement might not represent the true annual mean as well as a weighted annual mean calcu lated with a quarter-b y-quarter completeness152 requirement. However, the current practice of re quiring 75% completeness of all of the hours in the year and calculating the annual mean without weighting has been retained in the final rule, because of its simplicity and because we be lieve it will not inte rfere with effective implementation of the annual NAAQS. No area pres ently is nonattainment for or comes close to violating the annual standard. Therefore, the ch oice between the two approaches can only have a practical effect, if any, on whether at some time in the future an area is determined to be newly violating the annual standard. If a monitor has a complete and valid design value below the standard using the unweighted mean appro ach (with only an annual data completeness requirement) but the design value would be c onsidered incomplete and invalid under a hypothetical weighted mean approach (with a qu arterly completeness requirement), the monitor would in either case be considered not to be vi olating and its data would not be the basis for a nonattainment designation. If a monitor has a design value above the standard using the unweighted annual mean approach but is incomple te with respect to a hypothetical quarterly completeness requirement, then the two approaches would have different implications for the determination of a violation. A quarterly co mpleteness requirement would make a finding of violation impossible, unless the Administrator ch ose to treat the data as if complete under another provision of the final rule. The unweighted annual mean approach would allow but not force a finding of violation, because the Administra tor will have discretion to make any such findings because there will be no mandatory round of designations for the annual standard given that the annual standard has not been revised in this review. Th e Administrator will be able to consider the representativeness of the unweighted annual mean when deciding whether to make a discretionary nonattainment redesignation. Given th at the annual standard requires only one year of monitoring data for the calculation of a design value, little time will be lost if the153 Administrator chooses to work with a state to obtain a new design value based on more complete and/or seasonally balanced monitoring data. B. Interpretation of the primary NAAQS for oxides of nitrogen 1-hour primary standard 1. Proposed interpretation of the 1-hour standard With regard to data completeness fo r the 1-hour primary standard with a 4 th highest daily value form, the proposed Appendix fo llowed past EPA practice for other NAAQS pollutants by requiring that in ge neral at least 75% of the mon itoring data that should have resulted from following the planned monitoring sc hedule in a period must be available for the key air quality statistic from that period to be considered valid. For the 1-hour primary NO 2 NAAQS, the key air quality statis tics are the daily maximum 1- hour concentrations in three successive years. It is important that samp ling within a day encompass the period when concentrations are likely to be highest and that all seasons of the year are well represented. Hence, the 75% requirement was proposed to be applied at the daily an d quarterly levels. Recognizing that there may be years with incomplete da ta, the proposed text provided that a design value derived from incomplete da ta would nevertheless be considered valid in either of two situations. First, if the design value cal culated from at least four da ys of monitoring observations in each of these years exceeds the level of the 1- hour primary standard, it would be valid. This situation could arise if monitoring was intermittent but high NO 2 levels were measured on enough hours and days for the mean of the three annual 4 th high values to exceed the standard. In this situation, more complete monitoring coul d not possibly have indicat ed that the standard was actually met. Second, we proposed a diagnostic data substi tution test which was intended to identify154 those cases with incomplete data in which it neve rtheless is very likely, if not virtually certain, that the daily 1-hour design value would have been observed to be below the level of the NAAQS if monitoring data had been minimally complete. It should be noted that one possible outco me of applying the proposed substitution test is that a year with incomplete data may nevert heless be determined to not have a valid design value and thus to be unusable in making 1-hour primary NAAQS compliance determinations for that 3-year period. Also, we proposed that the Administrator have general discretion to use incomplete data based on case-specific factors, either at th e request of a state or at her own initiative. Similar provisions exist al ready for some other NAAQS. The second version of the proposed Appe ndix S contained proposed interpretation procedures for a 1-hour primary standard based on the 99 th percentile daily value form. The 4 th high daily value form and the 99 th percentile daily value form would yield the same design value in a situation in which every hour and day of th e year has reported monitoring data, since the 99 th percentile of 365 daily values is the 4 th highest value. However, the two forms diverge if data completeness is 82% or less, because in that case the 99 th percentile value is the 3 rdhighest (or higher) value, to compensate for the lack of mo nitoring data on days when concentrations could also have been high. Logically, provisions to addr ess possible data incompleteness under the 99th percentile daily value form should be somewh at different from those for the 4 th highest form. With a 4 th highest form, incompleteness should not invalidate a design value that exceeds the standard, for reasons explained above. With the 99 th percentile form, however, a design value exceeding the standard stemming from incomplete data should not automatically be considered valid, because155 concentrations on the unmonitored days could have been relatively low, such that the actual 99 th percentile value for the year could have been lo wer, and the design value could have been below the standard. The second proposed version of Appendix S accordingly had somewhat different provisions for dealing with data incompleteness. One differen ce was the addition of another diagnostic test based on data subs titution, which in some cases can validate a design value based on incomplete data that exceeds the standard. The second version of the proposed Appendi x S provided a table for determining which day’s maximum 1-hour concentration will be used as the 99th percentile concentration for the year. The proposed table is similar to one used now for the 24-hour PM2.5 NAAQS, which is based on a 98th percentile form, but adjusted to reflect a 99th percen tile form for the 1-hour primary NO2 standard. The pr oposed Appendix S also provided instructions for rounding (not truncating) the average of three annual 99th percentile hourly concentrations before comparison to the level of the primary NAAQS. 2. Comments on interpretation of the 1-hour standard Three commenters expressed the view that the 75% completion per quarter requirement should apply with respect to the 1-hour standa rd. A fourth commenter recommended that the requirement be increased to 82%. Another pers on commented that the requirement of 75% of the hours in a day is too stringen t. The commenter noted that it would be inappropriate not to count the day if the maximum concentration obser ved in the hours measured is sufficiently high to make a difference with regard to complian ce with the NAAQS. A comment was received that the substitution test should not be included, on the grounds that nonattainment should not be declared without irrefutable proof. This commen ters also said that the same completeness requirement as used for nonattainment should be used for attainment. We received one comment156 that the computation of design values where multip le monitors are present at a site should be averaged and not taken from a designated primary monitor. 3. Conclusions on interpretati on of the 1-hour standard Consistent with the Administ rator’s decision to adopt a 98 th percentile form for the 1- hour NAAQS, the final version of Appendix S is based on that form. Table 1 has been revised from the version that was proposed, so th at it results in the selection of the 98 th percentile value rather than the 99 th percentile value. We agree with the three comments expressing the view that the requirement for 75% data completeness per quarter should apply with respec t to the 1-hour standa rd. A fourth comment recommended that the requirement be increased to 82%. We believe 82% is too stringent because of the number of monitors that would not achieve such a requirement and we believe that 75% captures the season. We agree that an incomplete day should be counted if the maximum concentration observed in the hours measur ed is sufficiently high to make a difference with regard to compliance with the NAAQS, and we have accounted for that in section 3.2.c.i by validating the design value if it is above the level of the prim ary 1-hour standard when at least 75 percent of the days in each quarter have at l east one reported hourly va lue. We agree that substitution should not be used for the estab lishment of attainment/nonattainment. The commenter who remarked on this issue appears not to have unders tood that the specific proposed substitution tests have essentially zero proba bility of making a clean area fail the NAAQS, or vice versa, because the substituted values are chosen to be conservative against such an outcome. As noted in section 3.2(c)(i), wh en substitution is used, the 3-year design value based on the data actually reported, not the ‘‘test design value’’, shall be used as the valid design value.157 In the course of considering the above comm ent regarding data subs titution tests to be used in cases of data incompleteness, EPA has r ealized that there could be some cases of data incompleteness in which the proposed procedur e for calculating the 1-hour design value might result in an in appropriately low design value. As proposed, only days with measurements for at least 75% of the hours in the day would be considered in any way when identifying the 99 th percentile value (99 th for purposes of the adopted NAAQS). However, there could be individual hours in other, incompletely monitored days that had measured concentrations higher than the identified 98 th percentile value from the complete days. It would be inappropriate not to consider those hours and days in some way. However, if a ll days with at least one hourly concentration were used to identify the 99 th percentile value without any rega rd to their incompleteness, this could also result in a design value that is biased low because the extra days could increase the number of “annual number of days with valid da ta” enough to affect which row of Table 1 of Appendix S is used. It could, for example, result in the 8 th highest ranked daily maximum concentration being identified as the 98 th percentile value (based on Table 1 of Appendix S) rather than a higher ranked con centration; this would also be inappropriate because days which were not monitored intensively enough to give a reasonable likelihood of catching the maximum hourly concentration would in eff ect be treated as if they had such a likelihood. For example, 50 days with only one hourly meas urement during a time of day with lower concentrations would “earn” the state the right to drop one notch lower in the ranking of days when identifying the 98 th percentile day, inappropriately. The final vers ion of Appendix S solves this problem by providing that two procedures be used to identifying the 98 th percentile value, the first based only on days with 75% data completene ss and the second based on all days with at least one hourly158 measurement. The final design value is the highe r of the two values that result from these two procedures. Lastly, with regard to situations with multip le monitors operating at one site, we think that designation of a primary monitor is preferable to averaging the data from multiple monitors based on administrative simplicity and transparency for the public , and is unbiased with respect to compliance outcome provided the state is able to make the designation only before any data has been collected. C. Exceptional Events Information Submission Schedule The Exceptional Events Rule at 40 CFR 50.14 c ontains generic deadlines for a state to submit to EPA specified information about exce ptional events and associated air pollutant concentration data. A state must initially notify EPA that data has been affected by an event by July 1 of the year after the da ta are collected; this is done by flagging the data in AQS and providing an initial event descri ption. The state must also, after notice and opportunity for public comment, submit a demonstration to justify any claim within 3 year s after the quarter in which the data were collected. However, if a re gulatory decision based on the data (for example, a designation action) is anticipated, the schedule to flag data in AQS and submit complete documentation to EPA for review is foreshorte ned, and all information must be submitted to EPA no later than one year before the decision is to be made. These generic deadlines are suitable for the period after initial de signations have been made under a NAAQS, when the decision that ma y depend on data exclusion is a redesignation from attainment to nonattainment or from nona ttainment to attainment. However, these deadlines present proble ms with respect to in itial designations under a newly revised NAAQS. One problem is that some of the deadlines, espe cially the deadlines for flagging some relevant159 data, may have already passed by the time the re vised NAAQS is promulgated. Until the level and form of the NAAQS have been promulgated a state does not know whether the criteria for excluding data (which are tied to the level and form of the NAAQS) were met on a given day. The only way a state could guard against this possibil ity is to flag all data that could possibly be eligible for exclusion under a future NAAQS. Th is could result in flagging far more data than will eventually be eligible for exclusion. EPA belie ves this is an ineffici ent use of state and EPA resources, and is potentially confusing and mi sleading to the public a nd regulated entities. Another problem is that it may not be feasible for information on some exceptional events that may affect final designations to be collected a nd submitted to EPA at least one year in advance of the final designation decision. This coul d have the unintended consequence of EPA designating an area nona ttainment as a result of uncontro llable natural or other qualified exceptional events. When Section 50.14 was revised in March 2007, EPA was mindful that designations were needed under th e recently revised PM 2.5 NAAQS, so exceptions to the generic deadline were included for PM 2.5 . The EPA was also mindful thatsimilar issues would arise for subsequent new or revised NAAQ S. The Exceptional Events Rule at section 50.14(c)(2)(v) indicates “when EPA sets a NAAQS for a new po llutant, or revises the NAAQS for an existing pollutant, it may revise or set a new schedule for flagging data for initial designation of areas for those NAAQS.” EPA proposed revised exceptional event data flagging and documentation deadlines in FR 34404 [Federal Regist er / Vol. 74, No. 134 / Wednesday, July 15, 2009 / Proposed Rules] and invited comments from the public. The Agency received no comments related to the revised proposed schedule for NO2 exceptional event data flagging and documentation deadlines.160 For the specific case of NO 2 , EPA anticipates that initial designations under the revised NAAQS may be made by January 22, 2012 based on air quality data from the years 2008-2010. (See Section VI below for more detailed discus sion of the designation schedule and what data EPA intends to use.) If final designations are made by January 22, 2012, all events to be considered during the designations process must be flagged and fully documented by states one year prior to designations, by January 22, 2011. This date also coincides with the Clean Air Act deadline for Governors to submit to EPA their re commendations for designating all areas of their

states. The final rule text at the end of this noti ce shows the changes that will apply if a revised
NO 2 NAAQS is promulgated by January 22, 2010, a nd designations are made two years after promulgation of a NO 2 NAAQS revision. Table 1 below summarizes the data fla gging and documentation d eadlines corresponding to the two year designation schedul e discussed in this section. If the promulgation date for a revised NO 2 NAAQS occurs on a different date than January 22, 2010, EPA will revise the final
NO 2 exceptional event flagging and documentation submission deadlines accordingly to provide states with reasonably adequate opportunity to review, identif y, and document exceptional events that may affect an area designa tion under a revised NAAQS.161 Table 1. Schedule for Exceptional Event Flagging and Documentation Submission for Data to be Used in Designations Decisions for New or Revised NAAQS
NAAQS Pollutant/ Standard/(Leve l)/ Promulgation Date Air Quality Data Collected for Calendar Year Event Flagging & Initial Description Deadline Detailed Documentation Submission

Deadline

NO2/1-Hour Standard(100 PPB) 2008 July 1, 2010 a January 22, 2011 2009 July 1, 2010 January 22, 2011 2010 April 1, 2011 a July 1, 2011 a

a Indicates change from general schedule in 40 CFR 50.14. Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment.162 V. Designation of Areas A. Proposed process The CAA requires EPA and the states to take steps to ensure that the new or revised NAAQS are met following promulgation. The first step is to identify areas of the country that do not meet the new or revised NAAQS. Section 10 7(d)(1) provides that, “By such date as the Administrator may reasonably require, but not late r than 1 year after promulgation of a new or revised NAAQS for any pollutant under section 109, the Governor of each state shall * * * submit to the Administrator a list of all areas (or portions thereof) in the state” that should be designated as nonattainment, attainment, or unclassifiable for the new NAAQS. Section 107(d)(1)(B)(i) further provides, “Upon pr omulgation or revision of a NAAQS, the Administrator shall promulgate the designations of all areas (o r portions thereof) * * * as expeditiously as practicable, but in no case later than 2 years from the date of promulgation.” No later than 120 days prior to promulga ting designations, EPA is required to notify states of any intended modifications to their de signations as EPA may de em necessary. States then have an opportunity to comment on EPA' s tentative decision. Whether or not a state provides a recommendation, the EPA must promulga te the designation that it deems appropriate. Accordingly, Governors mu st submit their initial NO 2 designation recommendations to EPA no later than January 2011. If the Admi nistrator intends to modify any state’s recommendation, the EPA will notify the Governor no later than 120 days prior to designations in January 2012. States that believe the Admini strator’s modification is inappropriate will have an opportunity to demonstrate why they believe their recommendation is mo re appropriate before designations are finalized.163 B. Public comments Several industry commenters requested th at EPA slow the timeline for implementing a near-roadway monitoring network and designa ting roadway areas because they believe EPA lacks significant information about the implemen tation and performance of a national, nearroadway monitoring network. Two commenters also requested that if a near-roadway monitoring network is deployed, that 1-hour NO 2 standards be made more lenient until the next review period so that more information will be available about near-roadway NO 2 concentrations before a stringent standard is selected. A response to commenters’ requests that EPA slow the monitoring implementation schedule and the request thatEPA make the 1-hour NO 2 standard more lenient until the next review period are addressed in sectio ns III.B.5 and II.F.4.D, respectively. Section 110(d)(1)(B) requires the EPA to de signate areas no later than 2 years following promulgation of a new or revised NAAQS (i.e., by January 2012). While the CAA provides the Agency an additional third year from promulgati on of a NAAQS to complete designations in the event that there is insuffici ent information to make NAAQS compliance determinations, we anticipate that delaying designations for an additional year would not result in significant new data to inform the initial designations. A near-ro adway monitoring network is not expected to be fully deployed until January 2013 therefore, EPA mu st proceed with initia l designations using air quality data from the existing NO 2 monitoring network. Beca use none of the current NO 2 monitors are sited to measure near-roadway ambient air, we expect that most areas in the country with current NO 2 monitors will not violate the new NO 2 NAAQS. In the event that a current
NO 2 monitor indicates a violation of the revised st andards, EPA intends to designate such areas “nonattainment” no later than 2 years following promulgation of the revised standards. We164 intend to designate the rest of the country as “unclassifiable” for the revised NO2 NAAQS until sufficient air quality data is collected from a near-roadway monitoring network. Once the nearroadway network is fully deployed and 3 years of air quality data are available, the EPA has authority under the CAA to redesignate areas as appropriate from “unclassifiable” to “attainment” or “nonattainment.” We anticipate that sufficient data to conduct designations would be available after 2015. A number of commenters, largely from i ndustry groups, focused on the concern that a near-roadway monitoring network would lead to regional nonattainment on the basis of high
NO 2 concentrations found near roadways. These commenters requested that any future nonattainment areas be limited to the area direc tly surrounding roadways found to have abovestandard NO 2 concentrations. The CAA requires that any area that does not meet a NAAQS or that contributes to a violation in a nearby area that does not meet the NAAQS be designated “nonattainment.” States and EPA will need to determine which sources and activities contribute to a NAAQS violation in each area. Depending on the circumstances ineach area this may include sources and activities in areas beyond the area di rectly surrounding a major road way. EPA intends to issue nonattainment area boundary guida nce after additional informati on is gathered on the probable contributors to viola ting near-roadway NO 2 monitors. C. Final designations process The EPA intends to promulgate initial NO 2 designations by January 2012 (2 years after promulgation of the revised NAAQS). Along with today’s action EPA is also promulgating new monitoring rules that focus on roadways. As noted in section III, states must site required NO 2 near-roadway monitors and have them opera tional by January 1, 2013. States will need an165 additional 3 years thereafter to co llect air quality data in order to determine compliance with the revised NAAQS. This means that a full set of ai r quality data from the new network will not be available until after 2015. Since we anticipate th at data from the new network will not be available prior to the CAA designation deadlines discussed above, the EPA intends to complete initial NO 2 designations by January 2012 using the 3 mo st recent years of quality-assured air quality data from the current monitoring netw ork, which would be for the years 2008-2010. The EPA will designate as “nonattainment” any areas with NO 2 monitors recording violations of the revised NO 2 NAAQS. We intend to designate all other areas of the country as “unclassifiable” to indicate that there is insufficient data to dete rmine whether or not they are attaining the revised
NO 2 NAAQS. Once the NO 2 monitors are positioned in locations meeting the near-roadway siting requirements and monitoring data become availa ble, the Agency has au thority under section 107(d)(3) of the CAA to redesignat e areas as appropriate from “ unclassifiable” to “attainment” or “nonattainment.” The EPA intends to issue guid ance on the factors that st ates should consider when determining nonattainment boundaries after additional information is gathered on the probable contributors to violating near-roadway NO 2 monitors. VI. Clean Air Act Implem entation Requirements This section of the preamble discusses the Clean Air Act (CAA) requirements that states and emissions sources must address when implementing new or revised NO 2 NAAQS based on the structure outlined in the CAA and existing rules. 20 EPA may provide additional guidance in the future, as necessary, to assist states a nd emissions sources to comply with the CAA requirements for implementing new or revised NO 2 NAAQS.

20 Since EPA is retaining the annual standard w ithout revision, the disc ussion in this section relates to implementatio n of the proposed 1-hour standard, ra ther than the annual standard.166 The CAA assigns important roles to EPA, stat es, and, in specified circumstances, Tribal governments to achieve the NAAQS. States have the primary responsibility for developing and implementing State Implementation Plans (SIPs) that contain state measures necessary to achieve the air quality standard s in each area. EPA provides assistance to states by providing technical tools, assistance, and guidance, including information on the potential control measures that may help areas meet the standards. States are primarily responsible for ensuring atta inment and maintenance of ambient air quality standards once they have been established by EPA. Under section 110 of the CAA, 42 U.S.C. § 7410, and related provisions, states are required to submit, for EP A approval, SIPs that provide for the attainment and maintenance of such st andards through control programs directed at sources of NO 2 emissions. If a state fails to adopt a nd implement the required SIPs by the time periods provided in the CAA, the EPA has re sponsibility under the C AA to adopt a Federal Implementation Plan (FIP) to assure that areas attain the NAAQS in an expeditious manner. The states, in conjunction with EPA, also administer the prevention of significant deterioration (PSD) program for NO 2 and nonattainment new source re view (NSR). See sections 160-169 of the CAA. In addition, Federal pr ograms provide for nationwide reductions in emissions of NO 2 and other air pollutants under T itle II of the Act, 42 U.S.C. §§7521–7574, which involves controls for automobiles, truc ks, buses, motorcycles, nonroad engines, and aircraft emissions; the new source performance st andards (NSPS) for stationary sources under section 111 of the CAA, 42 § U.S.C. 7411. CAA Section 301(d) authorizes EPA to treat e ligible Indian tribes in the same manner as states (TAS) under the CAA and requires EPA to promulgate regulations specifying the provisions of the statute for which such treatmen t is appropriate. EPA has promulgated these regulations – known as the Tribal Authority Rule or TAR – at 40 C.F.R. Part 49. See 63 Fed. Reg. 7254 (February 12, 1998). The TAR establishes the process for Indian tribes to seek TAS eligibility and sets forth the CAA functions for which TAS will be available. Under the TAR,167 eligible tribes may seek approval for all CAA a nd regulatory purposes other than a small number of functions enumerated at section 49.4. Im plementation plans under section 110 are included within the scope of CAA functions for which eligible tribes may obtain approval. Section 110(o) also specifically describes tribal roles in submittin g implementation plans. Eligible Indian tribes may thus submit implementation plans covering th eir reservations and ot her areas under their jurisdiction. Under the CAA and TAR, tribes are not , however, required to apply for TAS or implement any CAA program. In promulgating th e TAR EPA explicitly determined that it was not appropriate to treat tribes similarly to st ates for purposes of, among other things, specific plan submittal and implementation deadlines for NAAQS-related requirements. 40 C.F.R. § 49.4(a). In addition, where tribes do seek a pproval of CAA programs, including section 110 implementation plans, the TAR provides flexibility and allows them to submit partial program elements, so long as such elements are reasonably severable – i.e. , “not integrally related to program elements that are not included in the pl an submittal, and are consistent with applicable statutory and regulatory re quirements.” 40 C.F.R. §49.7. To date, very few tribes have sought TAS for purposes of section 110 implementation plans. However, some tribes may be interest ed in pursuing such plans to implement today’s proposed standard. As noted above, such tribes may seek approval of partial, reasonably severable plan elements, or they may seek to im plement all relevant comp onents of an air quality program for purposes of meeting the requirements of the Act. In several sections of this preamble, EPA describes the various roles and re quirements states will address in implementing today’s proposed standard. Such references to states are generally intended to include eligible Indian tribes to the extent c onsistent with the flexibility pr ovided to tribes under the TAR.168 Where tribes do not seek TAS for section 110 implementation plans, EPA will promulgate Federal implementation plans as “necessary or appr opriate to protect air qua lity.” 40 C. F. R. §49.11(a). EPA also notes that some tribes opera te air quality monitoring networks in their areas. For such monitors to be used to m easure attainment with this primary NAAQS for NO 2 , the criteria and procedur es identified in this rule would apply. A. Classifications 1. Proposal Section 172(a)(1)(A) of the CAA authorizes EPA to classify areas designated as nonattainment for the purpose of applying an atta inment date pursuant to section 172(a)(2), or for other reasons. In determining the appropriate classification, EPA may consider such factors as the severity of the nonattainment problem a nd the availability and feasibility of pollution control measures (see section 172(a)(1)(A) of the CAA). The EPA may classify NO 2 nonattainment areas, but is not required to do so. The primary reason to es tablish classifications is to set different deadlines for each class of non attainment area to complete the planning process and to provide for different attainment date s based upon the severity of the nonattainment problem for the affected area. However, the C AA separately establishe s specific planning and attainment deadlines for certain pollutants including NO2 in sections 191 and 192: 18 months from nonattainment designation for the submittal of an attainment plan, and as expeditiously as possible, but no later than 5 years from nonatta inment designation for areas to attain the standard. In the proposal, EPA stated its belief th at classifications are unnecessary in light of these relatively short deadlines. 2. Public comments169One commenter stated that they disagree with EPA’s decision not to impose nonattainment classifications on areas with measured near-road NO 2 concentrations in excess of the
new NO 2 standard, and urged EPA to provide a gra duated non-attainment classification system for the new standard. According to the comm enter, “a classification system defining higher levels of non-attainment with increasingly string ent requirements at thos e levels is one that allows for finer calibration of air quality regulatory response defined at the federal level.” As stated in the proposed rule, Section 192( a), of part D, of the CAA specifically provides an attainment date for areas designated as nonattainment for the NO 2 NAAQS. Therefore, EPA has legal authority to classify NO 2 nonattainment areas, but the 5-year attainment date addressed under section 192( a) cannot be extended pursuant to section 172(a)(2)(D). Based on this limitation, EPA proposed not to establish clas sifications within the 5- year interval for atta ining any new or revised NO 2 NAAQS. It is also EPA’s belief that given the short deadlines that States have to deve lop and submit SIP’s and for areas to achieve emissions reductions in order to attain the st andard within the 5 y ear attainment period, a graduated classifications system would not be appropriate. Therefore, EPA is using it’s discretion under the CAA not to establish classifications. 3. Final EPA is not making any changes to the discus sion on classifications in the proposed rule. Therefore, there will be no cl assifications for the revised NO 2 NAAQS.
B. Attainment Dates The maximum deadline by which an ar ea is required to attain the NO 2 NAAQS is determined from the effective date of the nonatt ainment designation for the affected area. For areas designated nonattainment for the revised NO 2 NAAQS, SIPs must provide for attainment170 of the NAAQS as expeditiously as practicable, but no later than 5 years from the date of the nonattainment designation for the area (see secti on 192(a) of the CAA). The EPA will determine whether an area has demonstrated attainment of the NO 2 NAAQS by evaluating air quality monitoring data consistentwith the form of the NAAQS for NO 2 if revised, which will be codified at 40 CFR part 50, Appendix F. 1. Attaining the NAAQS
a. Proposal

In order for an area to be rede signated as attainment, the state must comply with the five requirements as provided under section 107(d)(3)(E) of the CAA. This section requires that: - EPA must have determined that the area has met the NO 2 NAAQS; - EPA has fully approved the state’s implementation plan; - the improvement in air quality in the affect ed area is due to permanent and enforceable reductions in emissions; - EPA has fully approved a maintenance plan for the area; and - the state(s) containing the area have met all applicable requirements under section 110 and part D. b. Final EPA did not receive any comments on this asp ect of the proposed rule and is not making any changes to the discussion on attaining the NAAQS in the proposed rule. 2. Consequences of Failing to Attain by the Statutory Attainment Date a. Proposal Any NO 2 nonattainment area that fails to attain by its statutory attainment date would be subject to the requirements of s ections 179(c) and (d) of the CAA. EPA is required to make a171 finding of failure to attain no later than 6 months after the specified atta inment date and publish a notice in the Federal Register . The state would be required to submit an implementation plan revision, no later than one y ear following the effective date of the Federal Register notice making the determination of the area’s failure to attai n, which demonstrates that the standard will be attained as expeditiously as pr acticable, but no later than 5 years from the effective date of EPA’s finding that the area failed to attain. In addition, secti on 179(d)(2) provides that the SIP revision must include any specific additional measures as may be reasonably prescribed by EPA, including “all measures that can be feasibly implemented in th e area in light of technological achievability, costs, and any nonair quality and other air quality-related h ealth and environmental impacts.” b. Final EPA did not receive any comments on this asp ect of the proposed rule and is not making any changes to the discussion on consequences of failing to attain by the statutory attainment date in the proposed rule. C. Section 110(a)(2) NAAQS Infr astructure Requirements 1. Proposal Section 110(a)(2) of the CAA requires all states to develop and maintain a solid air quality management infrastructure, including enforceable emission limitations, an ambient monitoring program, an enforcement program, ai r quality modeling, and adequate personnel, resources, and legal authority. Section 110(a) (2)(D) also requires state plans to prohibit emissions from within the state which contribut e significantly to nonattainment or maintenance areas in any other state, or wh ich interfere with programs under part C to prevent significant deterioration of air quality or to achieve reason able progress toward the national visibility goal172 for Federal class I areas (nationa l parks and wilderness areas). Under section 110(a)(1) and (2) of the CAA, all states ar e required to submit SIPs to EPA which demonstrate that basic program elements have been addressed within 3 years of the promulgation of any new or revised NAAQS. S ubsections (A) through (M) of section 110(a)(2) listed below, set forth the elements that a state’s program must contain in the SIP. 21 The list of section 110(a)(2) NAAQS implementation requirements are the following:

• Ambient air quality monitoring/data system : Section 110(a)(2)(B) requires SIPs to provide for setting up and operating ambien t air quality monito rs, collecting and analyzing data and making these da ta available to EPA upon request.

• Program for enforcement of control measures : Section 110(a)(2)(C) requires SIPs to include a program providing for enforcement of measures and regulation and permitting of new/modified sources.

• Interstate transport: Section 110(a)(2)(D) requires SIPs to include provisions prohibiting any source or other type of em issions activity in the state fr om contributing significantly to nonattainment in another state or from in terfering with measures required to prevent significant deterioration of air qua lity or to protect visibility.

• Adequate resources: Section 110(a)(2)(E) requires states to provide assurances of adequate funding, personnel and legal auth ority for implementation of their SIPs.

• Stationary source monitoring system: Secti on 110(a)(2)(F) requires st ates to establish a

21 Two elements identified in se ction 110(a)(2) are not listed belo w because, as EPA interprets the CAA, SIPs incorporating any necessary loca l nonattainment area controls would not be due within 3 years, but rather are due at the time the nonattainment area planning requirements are due. These elements are: (1) Emission limits and other control measures, section 110(a)(2)(A), and (2) Provisions for meeting part D, section 110(a)(2)(I), which requi res areas designated as nonattainment to meet the appli cable nonattainment planning require ments of part D, title I of the CAA.173 system to monitor emissions from stationary sources and to submit periodic emissions reports to EPA.

• Emergency power: Section 110(a) (2)(G) requires states to include contingency plans, and adequate authority to implement them , for emergency episodes in their SIPs.

• Provisions for SIP revision due to NAAQS change s or findings of inadequacies: Section 110(a)(2)(H) requires states to provide for revi sions of their SIPs in response to changes in the NAAQS, availability of improved methods for attaining the NAAQS, or in response to an EPA finding th at the SIP is inadequate.

• Consultation with local and Federal government officials: Section 110(a)(2)(J) requires states to meet applicable local and Fede ral government consultation requirements when developing SIP and reviewi ng preconstruction permits.

• Public notification of NAAQS ex ceedances: Section 110(a)(2)(J) requires states to adopt measures to notify the public of instances or areas in which a NAAQS is exceeded.

• PSD and visibility protection: Section 110(a)(2)(J) also requir es states to adopt emissions limitations, and such other measures, as ma y be necessary to prevent significant deterioration of air quality in attainment ar eas and protect visibil ity in Federal Class I areas in accordance with the requirements of CAA Title I, part C.

• Air quality modeling/data: Section 110(a)(2)(K) require s that SIPs provide for performing air quality modeling for predicting effects on air qua lity of emissions of any NAAQS pollutant and submission of data to EPA upon request.

• Permitting fees: Section 110(a)(2)(L) requires the SIP to include requirements for each major stationary source to pay permitting fees to cover the cost of reviewing, approving, implementing and enforcing a permit.174 • Consultation and participation by affected local government: Section 110(a)(2)(M) requires states to provide for consultation and participation by local political s ubdivisions affected by the SIP. 2. Final EPA did not receive any comments on this asp ect of the proposed rule and is not making any changes to the discussion on section 110(a) (2) NAAQS infrastructure requirements in the proposed rule. D. Attainment Planning Requirements
1. Nonattainment Area SIPs a. Proposal Any state containing an area designated as nonattainment with respect to the NO 2 NAAQS must develop for submission a SIP meeting the requirements of part D, Title I, of the CAA, providing for attainment by the applicable statutory attainment date (see sections 191(a) and 192(a) of the CAA). As indicated insection 191(a) all components of the NO 2 part D SIP must be submitted within 18 months of the effective date of an area’s designation as nonattainment. Section 172 of the CAA include s general requirements for all designated nonattainment areas. Section 172(c)(1) requires that each nonattainment area plan “provide for the implementation of all reasonably available control measures (RACM) as expeditiously as practicable (including such reductions in emission s from existing sources in the area as may be obtained through the adoption, at a minimum, of Reasonably Available Control Technology (RACT)), and shall provide for a ttainment of the nationa l primary ambient air quality standards.” States are required to implemen t RACM and RACT in order to attain “as expeditiously as practicable”.175 Section 172(c) requires states with nonatta inment areas to submit a SIP for these areas which contains an attainment de monstration that shows that the affected area will attain the standard by the applicable statutor y attainment date. The state mu st also show that the area will attain the standards as expediti ously as practicable, and it must include an analysis of whether implementation of reasonably available measures will advance the attainment date for the area. Part D SIPs must also pr ovide for reasonable further progress (RFP) (see section 172(c)(2) of the CAA). The CAA defines RFP as “such annual incremental reductions in emissions of the relevant air pollution as are re quired by part D, or ma y reasonably be required by the Administrator for the purpose of ensuri ng attainment of the applicable NAAQS by the applicable attainment date.” (See section 171 of the CAA) Historically, for some pollutants, RFP has been met by showing annual incremental emissi on reductions sufficient to maintain generally linear progress toward attainment by th e applicable attainment date. All NO 2 nonattainment area SIPs must include contingency measures which must be implemented in the event that an area fails to m eet RFP or fails to attain the standards by its attainment date. (See section 172( c)(9)) These contingency measures must be fully adopted rules or control measures that take e ffect without further action by the state or the Administrator. The EPA interprets this requirement to mean that the contingency measures must be implemented with only minimal further action by the state or the affected sour ces with no additional rulemaking actions such as public he arings or legislative review. Emission inventories are also critical for the efforts of state, local, and Federal agencies to attain and maintain the NAAQS that EPA ha s established for criteria pollutants including
NO 2 . Section 191(a) in conjuncti on with section 172(c) require s that areas designated as nonattainment for NO 2 submit an emission inventory to EPA no later than 18 months after176 designation as nonattainment. In the case of NO 2 , sections 191(a) and 172 (c) also require that states submit periodic emission inventories for nonattainment areas. The periodic inventory must include emissions of NO 2 for point, nonpoint, mobile (on -road and non-road), and area

sources. b. Public comments Several commenters indicated that EPA should take steps to ensure that states actually require mobile source emissions reduc tions in order to attain the NO 2 NAAQS as opposed to controlling point sources. Anothe r commenter went further and stat ed that states be required to control on-road emissions as opposed to emissions from stationary sources and in particular EGUs. This commenter also indicated that EPA should delay nonattainment designations until states had a cost effective means of reducing on-road emissions of NO 2 . EPA cannot require states to deve lop a SIP that only addresses one type of source, in this case on-road mobile sources. States may select appropriate control measures to attain the NAAQS and EPA must approve them if they otherwise meet all applicable requirements of the Act. See CAA § 116. EPA expects that states will evaluate a range of control measures that will reduce NO2 emissions within the time allowed to attain the standard. This would include the emissions reductions attributable to Federal controls on on-road and non-road mobile sources, and controls that they have put in place to reduce NO X emissions in order to attain the 8-hour ozone NAAQS and/or the PM 2.5 NAAQS. If these existing controls are not sufficient for an area to reach attainment with the NO 2 NAAQS, EPA would expect the state to implement additional control measures that would bring the area into attainment by the dead line. For a designation based on data from a near roadway monitor EPA would expect the states to give primary consideration to controlling emi ssions from on-road sources; however , it is likely that other types177 of sources contribute to the c oncentrations that are measured at a near roadway monitor and a state may decide to implement controls on these other contributing sources. The Clean Air Act requires that EPA finali ze designations within two years after a NAAQS is revised unless the available air quality data is insufficient to make designations by that time. In that case, EPA must finalize designations within three years after the NAAQS is revised. As discussed elsewhere in today’s final rule, EPA believes that it has sufficient data to make designations within two years and that most areas will be designate d as unclassifiable at that time. Taking the additional year provided by the CAA would not allow additional data from the new near roadway monitors to be factored into the designations process in any event. Therefore, it is EPA’s intention to designate areas within two year s as required by the Act. EPA intends to redesignate areas once it has suffici ent data from the new monitoring network to designate areas as clearly attaini ng or not attaining the standard. c. Final The EPA is not making any changes to the discussion on nonattainment area SIPs in the proposed rule. 2. New Source Review and Prevention of Significant Deteriorat ion Requirements a. Proposal The Prevention of Significant Deteriorat ion (PSD) and nonattainment New Source Review (NSR) programs contained in parts C and D of Title I of the CAA govern preconstruction review of any ne w or modified major stationa ry sources of air pollutants regulated under the CAA as well as any precursor s to the formation of that pollutant when178 identified for regulation by the Administrator. 22 The EPA rules addressing these programs can be found at 40 CFR 51.165, 51.166, 52.21, 52.24, and part 51, appendix S. States which have areas designated as nonattainment for the NO 2 NAAQS must submit, as a part of the SIP due 18 months after an area is designated as nonatta inment, provisions requiring permits for the construction and operation of new ormodified stationary sources anywhere in the nonattainment area. SIPs that address the PSD requirements rela ted to attainment areas are due no later than 3 years after the promulgation of a revised NAAQS for NO 2 . The NSR program is composed of three different permit programs:

• Prevention of Significant Deterioration (PSD)

• Nonattainment NSR (NA NSR)

• Minor NSR.
The PSD program applies when a major source, that is located in an area that is designated as attainment or unclassifiable for a ny criteria pollutant, is constructed, or undergoes a major modification. 23 The nonattainment NSR program app lies on a pollutant-specific basis when a major source constructs or modifies in an area that is designated as nonattainment for that pollutant. The minor source NSR program addresses both major and minor sources which undergo construction or modi fication activities that do not qualif y as major, and it applies, as

22 The terms “major” and “minor” define the size of a stationary source, for applicability purposes, in terms of an annual emissions rate (t ons per year, tpy) for a pollutant. Generally, a minor source is any source that is not “major .” “Major” is defined by the applicable regulations—PSD or nonattainment NSR.

23 In addition, the PSD program applies to non-crite ria pollutants subject to regulation under the Act, except those pollutants regu lated under section 112 and polluta nts subject to regulation only under section 211(o).179 necessary to assure attainment, regardless of the designation of the ar ea in which a source is located. The PSD requirements include but ar e not limited to the following:

• installation of Best Availabl e Control Technology (BACT);

• air quality monitoring and modeli ng analyses to ensure that a project’s emissions will not cause or contribute to a viol ation of any NAAQS or maximum allowable pollutant increase (PSD increment);

• notification of Federal Land Mana ger of nearby Class I areas; and

• public comment on permit.
Nonattainment NSR requirements in clude but are not limited to:

• installation of Lowest Achi evable Emissions Rate (LAER) control technology;

• offsetting new emissions with creditable emissions reductions;

• a certification that all major sources owned and operated in the state by the same owner are in compliance with all appli cable requirements under the CAA;

• an alternative siting analysis demonstrati ng that the benefits of a proposed source significantly outweigh the environmental and social costs imposed as a result of its location, construction, or modification; and

• public comment on the permit.
Minor NSR programs must meet the statutory requirements in section 110(a)(2)(C) of the180 CAA which requires “...regulation of the modificati on and construction of a ny stationary source ...as necessary to assure that the [NAAQS] are achieved.” Areas which are newly designated as nonattainment for the NO 2 NAAQS as a result of any changes made to the NAAQS will be required to adopt a nonattainment NSR pr ogram to address major sources of NO 2 where the program does not currently exist for the NO 2 NAAQS and may need to amend their minor source program as well. Prior to adoption of the SIP revision addressing major source nonattainment NSR for NO 2 nonattainment areas, the requirements of 40 CFR part 51, appendix S may apply. b. Public comments One commenter claimed that EPA’s setting of a more stringent standard, i.e., short-term
NO 2 NAAQS, could have important implications for NSR and PSD and title V permits. Another commenter indicated that the promulgation of a new 1-hr NO 2 short-term standard could create the need for a short-term PSD incremen t. Another commenter stated that a 1-hr NO 2 Significant Impact Level (SIL) should be developed. The EPA acknowledges that a decision to pr omulgate a new short-term NO2 NAAQS will clearly have implications for the air permitti ng process. The full extent of how a new shortterm NO 2 NAAAQS will affect the NSR process will need to be carefully evaluated. First, major new and modified sources applying for NSR/ PSD permits will initially be required to demonstrate that their proposed emissions increases of NO X will not cause or contribute to a violation of either th e annual or 1-hour NO 2 NAAQS and the annual PSD increment. In addition, we believe that section 166 of the C AA authorizes us to consider the need to promulgate a new 1-hour increment. Histori cally, EPA has developed increments for each applicable averaging period for which a NAAQS ha s been promulgated. However, increments for a particular pollutant do not necessarily need to match the averaging periods that have been181 established for NAAQS for the same pollutant. Environmental Defense Fund, Inc. v. EPA , 898 F.2d 183, 189-190 (D.C. Cir. 1990)(“... the ‘goals a nd purposes’ of the PSD program, set forth in § 160, are not identical to the criteria on whic h the ambient standards are based.”) Thus, we would need to evaluate the need for a new 1-hour NO 2 increment in association with the goals and purposes of the statutory PSD program requirements. We also believe that there may be a need to revise the screening tools currently used under the NSR/PSD program for completing NO 2 analyses. These screening tools include the significant impact levels (SILs), as mentioned by one commenter, bu t also include the significant emissions rate for emissions of NO X and the significant monitoring concentration (SMC) for
NO 2 . EPA intends to evaluate the need for possi ble changes or additions to each of these important screening tools for NO X /NO 2 due to the addition of a 1-hour NO 2 NAAQS. If changes or additions are deemed necessary, EPA will pr opose any such changes for public notice and comment in a separate action. c. Final The EPA is not making any changes to the discussion concerning the requirements for NSR and PSD as stated in the proposed rule. 3. General Conformity a. Proposal Section 176(c) of the CAA, as amended (42 U. S.C. 7401 et seq.), requires that all Federal actions conform to an applicable implementation plan developed pursuant to section 110 and part D of the CAA. The EPA rules, developed under the authority of section 176(c) of the CAA, prescribe the criteria and procedures for demo nstrating and assuring co nformity of Federal actions to a SIP. Each Federal agency must determine that any actions covered by the general182 conformity rule conform to the applicable SIP before the action is taken. The criteria and procedures for conformity apply only in nona ttainment areas and those areas redesignated attainment since 1990 (“maintenan ce areas”) with respec t to the criteria pollutants under the
CAA: 24 carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO 2 ), ozone (O3), particulate matter (PM 2.5 and PM 10 ), and sulfur dioxide (SO 2 ). The general conformity rules apply one year following the effective date of design ations for any new or revised NAAQS. The general conformity determination exam ines the impacts of direct and indirect emissions related to Federal actions. The genera l conformity rule provides several options to satisfy air quality criteria, such as modeling or offsets, and requires the Federal action to also meet any applicable SIP requirements and emissi ons milestones. The general conformity rule also requires that notic es of draft and final general conf ormity determinations be provided directly to air quality regulatory agencies and to the public by publicati on in a local newspaper. b. Final EPA did not receive any comments on this asp ect of the proposed rule and is not making any changes to the discussion concerning genera l conformity stated in the proposed rule. 4. Transportation Conformity a. Proposal Transportation conformity is required under CAA section 176(c) (42 U.S.C. 7506(c)) to ensure that transportation plans, transporta tion improvement programs (TIPs) and federally supported highway and transit projects will not ca use new air quality violations, worsen existing violations, or delay timely attainment of the relevant NAAQS or interim reductions and milestones. Transportation conformity applies to areas that are desi gnated nonattainment and

24 Criteria pollutants are those pollutants for wh ich EPA has established a NAAQS under section 109 of the CAA.183 maintenance for transportation-related criteria pollutants: carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO 2 ), and particulate matter (PM 2.5 and PM 10 ). Transportation conformity for a revised NO 2 NAAQS does not apply until one year afte r the effective date of a nonattainment designation. (See CAA section 176 (c)(6) and 40 CFR 93.102(d)). EPA’s Transportation Conformity Rule (40 CFR 51.390, and Part 93, Subpart A establishes the crit eria and procedures for dete rmining whether transportation activities conform to the SIP. The EPA is not making changes to the Transportation Conformity rule in this rulemaking. However, in the future, EPA will review the need to conduct a rulemaking to establish any new or revised tran sportation conformity te sts that would apply under a revision to the NO 2 NAAQS for transportation plans, TIPs, and applicable highway and transit projects. b. Public comments Several commenters stated that transpor tation conformity coul d stop the funding of highway and transit projects in NO 2 nonattainment areas. These commenters stated that if an area fails to demonstrate conformity it enters a conformity lapse and only certain types of projects can be funded during a lapse. Th e commenters further stated that the NO 2 NAAQS will require more areas to determine conformity for the first time. The commenters also expressed concern that the NO 2 NAAQS proposal did not c ontain sufficient information to understand to what extent revisions to the NAAQS, and the NO 2 monitoring requirements, will result in transportation conformity requirements for individua l transportation projects such as the need for a hot-spot analysis. The commenters further stated that hot-spot analyses could result in needless delays for transportation improvement projects. With regard to the comment that more areas will have to demonstrate conformity for the184 first time due to the revisions to the NO2 NAAQS, given that today’ s final rule is requiring that near roadway monitoring be carried out in urba n areas with populations greater than 350K, EPA believes that most areas with su ch populations that would be designated nonattainment for NO2 are already designated nonattainment or maintenan ce for one or more of the other transportationrelated criteria pollutants (ozone, PM2.5, PM10 and carbon monoxide). As such, these areas would have experience in making transportation c onformity determinations . If areas with no conformity experience are desi gnated nonattainment for the NO 2 NAAQS, EPA and US DOT would be available to assist areas in implementi ng the transportation conformity requirements. The commenter expressed concern that trans portation conformity could stop highway and transit funding because areas could experience a conformity lapse and in such cases only certain types of projects could be funded. A conformity lapse occurs when an area misses a deadline for a required conformity determination. A new no nattainment area must de monstrate conformity within one year after the effective date of its design ation. For any areas designated nonattainment for the revised NO2 NAAQS in early-2012, they would have to determine conformity within one year of the effective date of that desi gnation which would be in early2013. If that date was missed, a lapse would occu r and only projects exempt from conformity such as safety projects, transportation control measures in an approved SIP for the area and projects or project phases that were approved by US DOT before the lapse began can proceed during the lapse. EPA’s experience in impl ementing the 1997 ozone and PM2.5 NAAQS shows that nearly all areas make th eir initial conformity determin ations within the one-year grace period. Areas can also lapse if they fail to dete rmine conformity by an applicable deadline such as determining conformity within two years after motor vehicle em issions budgets are found adequate. However, areas that miss one of these conformity deadlines have a one-year grace185 period before the lapse goes into effect. Du ring the grace period, the area can continue to advance projects from the transportation plan and transportation improvement program. EPA’s experience is that areas generally are able to make a conformity determination before the end of the grace period. The commenter expressed concern that the NO2 NAAQS proposal did not contain sufficient detail concerning possi ble project-level requirements fo r transportation projects and that any requirements for hot-spot analyses coul d needlessly delay transportation projects. As EPA indicated in the NPRM, EPA is considering wh ether to revise the transportation conformity rule to establish requirements that would a pply to transportation plans, transportation improvement programs and/or transportation pr ojects in NO2 nonattainment and maintenance areas. If EPA concludes that the conformity ru le must be revised in light of the final NO2 NAAQS, we will conduct notice and comment rulema king to accomplish the revisions. At that time interested parties will have the opportunity to comment on any transportation conformity NPRM. This is the same course of action th at EPA has taken with respect to revising the transportation conformity rule for the ozone and PM2.5 NAAQS. With regard to the commenter’s assertion th at a requirement for hot-spot analyses for individual projects would needle ssly delay transporta tion projects, EPA disagrees. First, CAA section 176(c)(1)(B) requires th at transportation projects not cause new violations or make existing violations worse, or dela y timely attainment or cause an interim milestone to be missed. EPA would only impose a hot-spot requirement for projects in NO2 nonattainment and maintenance areas if they are necessary to comply with CAA conformity requirements and therefore are needed to protec t public health by reduc ing exposures to unhea lthy levels of NO2 that could be created by the implementation of a proposed highway or transit project. The public186 would be exposed to unhealthy le vels of NO2 if a highway or transit project caused a new violation of the NO2 NAAQS, made an existing violation worse, or delayed timely attainment or delayed achieving an interim emissions milestone. If any delay in the project did occur, it would not be viewed as needless as it occurred for the important purpose of protecting the exposed public’s health. Second, EPA does notagree that requiring a hot-spot analysis would needlessly delay projects in NO2 nonattainment areas. Su ch hot-spot analyses, if they are eventually required, generally would be done as part of the NEPA process, which these projects are already subject to; therefore, conducting an NO2 hot-spot analysis would not be introducing a new step to a project’s approval process, but rather w ould add one additional analysis which must be completed as part of an exis ting project approval process. c. Final EPA is not making any changes to the discussion concerning transp ortation conformity as stated in the proposed rule. VII. Communication of Public Health Information Information on the public health implications of ambient concentr ations of criteria pollutants is currently made available prim arily through EPA’s Air Quality Index (AQI) program. This section describes the conformi ng changes that were proposed, major comments received on these changes, EPA’s responses to these comments and final decisions on the AQI breakpoints. Recognizing the importance of revisi ng the AQI in a timely manner to be consistent with any revisions to the NAA QS, EPA proposed conforming changes to the AQI in connection with the final decision on the NO 2 NAAQS if revisions to th e primary standard were promulgated. Conforming changes would include setting the 100 level of the AQI at the same level as the revised primary NO 2 NAAQS and also setting the ot her AQI breakpoints at the lower187 end of the AQI scale ( i.e. , AQI values of 50 and 150). EPA did not propose to change breakpoints at the higher end of the AQI scale (from 200 to 500), which would apply to state contingency plans or the Significant Harm Leve l (40 CFR 51.16), because the information from this review does not inform decisions a bout breakpoints at t hose higher levels. With regard to an AQI value of 50, the breakpoint between the good and moderate categories, EPA proposed to set this value to be between 0.040 and 0.053 ppm NO 2 , 1-hour average. EPA proposed that the figure towards th e lower end of this range would be appropriate if the standard is set towa rds the lower end of the proposed range for the standard ( e.g. 80 ppb), while figures towards the higher end of the range would be more appropriate for standards set at the higher end of the range for the standard ( e.g. , 100 ppb). EPA noted that historically this value is set at the level of th e annual NAAQS, if there is one, orone-half the level of the shortterm NAAQS in the absence of an annual NAAQS, and solicited comments on this range for an AQI of 50 and the appropriate basis for sele cting an AQI of 50 within this range. With regard to an AQI value of 150, the br eakpoint between the unhealthy for sensitive groups and unhealthy categories, the range of 0.360 to 0.370 ppm NO 2 , 1-hour average, represents the midpoint between the proposed range for the short-term sta ndard and the level of an AQI value of 200 (0.64 ppm NO 2 , 1-hour average). Therefore, EPA proposed to set the AQI value of 150 to be between 0.360 and 0.370 ppm NO 2 , 1-hour average. EPA received comments from several state envi ronmental agencies and organizations of state and local agencies that generally expresse d the view that the AQI was designed to provide the public with information about regional ai r quality and therefore it should be based on community-wide monitors. These commenters went on to state that using near-road NO 2 monitors for the AQI would present problems because they would not represent regional NO 2188 concentrations and it would be difficult to comm unicate this type of information to the public using the AQI. Some expressed concern that NO 2 measured at near-roadway monitors could be the critical pollutant and could drive the AQI even though it may not represent air quality across the area. Other agencies expressed concern that there is currently no way to forecast ambient
NO 2 levels near roadways. One state agency commented that the AQI is intended to represent air quality where people live work and play. EPA agrees with commenters that the AQI s hould represent regional air quality, and that measurements that apply to a limited area should not be used to characterize air quality across the region. Community-wide NO 2 monitors should be used to characterize air quality across the region. However, the AQI reporting requirements en courage, but do not require, the reporting of index values of sub-areas of an MSA. We agree with the commenter that stated the view that the AQI is intended to represent air quality where pe ople live work and play. To the extent that near-roadway monitoring occurs in areas wher e people live, work or play, EPA encourages reporting of the AQI for that specific sub-ar ea of the MSA (64 FR 42548, August 4, 1999). We also agree that it may be difficult to communicate this type of information and we plan to work with state and local air agencies to figure out the best way to present this information to the public using the AQI. Air quality forecasting is recommended but not required (64 FR 42548, August 4, 1999). EPA will work with state agencies that want to develop a forecasting program. With regard to the proposed breakpoints, EPA received few comments. The National Association of Clean Air Agencies commented that it would be confusing to the public to have an AQI value of 50 set below the level of the annual NO 2 standard. No other comments were received on the basis for setting this value. We agree with this comment, and therefore have decided that it is appropriate to leave the AQI value of 50, th e breakpoint between the good and189 moderate ranges, set at the numerical level of the annual standard, 53 ppb NO 2 , 1-hour average. The AQI value of 100, the breakpoint between th e moderate and unhealthy for sensitive groups category, is set at 100 ppb, 1-hour aver age, the level of the primary NO 2 NAAQS. EPA is setting an AQI value of 150, the breakpoint betw een the unhealthy for sensitive groups and unhealthy categories, at 0.360 ppm NO 2 , 1-hour average. VIII. Statutory and Executive Order Reviews A. Executive Order 12866: Regulatory Planning and Review Under section 3(f)(1) of Executive Order 12866 (58 FR 51735, October 4, 1993), this action is an “economically significant regulatory action” because it is likely to have an annual effect on the economy of $100 million or more. Accordingl y, EPA submitted this action to the Office of Management and Budget (OMB) for review unde r EO 12866 and any changes made in response to OMB recommendations have been documented in the docket for this action. In addition, EPA prepared a Regulatory Impact Analysis (RIA) of the potential costs and benefits associated with this action. However, the CAA and judicial decisions make clear that the economic and technical feasibility of attaining ambient standards are no t to be considered in setting or revising NAAQS, although such factors may be considered in the development of State plans to implement the standards. Accordingly, although an RIA has b een prepared, the results of the RIA have not been considered in deve loping this final rule. B. Paperwork Reduction Act The information collection requirements in th is final rule have been submitted for approval to the Office of Management and B udget (OMB) under the Paperwork Reduction Act, 44 U.S.C. 3501 et seq. The Information Coll ection Request (ICR) document prepared by EPA for these revisions to part 58 ha s been assigned EPA ICR number 2358.02.190 The information collected under 40 CFR part 53 (e.g., test results, monitoring records, instruction manual, and other asso ciated information) is needed to determine whether a candidate method intended for use in determining attain ment of the National Ambient Air Quality Standards (NAAQS) in 40 CFR part 50 will meet the design, performance, and/or comparability requirements for designation as a Federal refere nce method (FRM) or Federal equivalent method (FEM). We do not expect the number of FRM or FEM determinations to increase over the number that is currently used to estimate burden associated with NO 2 FRM/FEM determinations provided in the current ICR for 40 CFR part 53 (EPA ICR numbers 2358.01). As such, no change in the burden estimate for 40 CFR part 53 has been made as part of this rulemaking. The information collected and reported under 40 CFR part 58 is needed to determine compliance with the NAAQS, to characterize air qua lity and associated health impacts, to develop emissions control strategies, and to meas ure progress for the air pollution program. The amendments would revise the technical requirements for NO 2 monitoring sites, require the siting and operation of additional NO 2 ambient air monitors, and the reporting of the collected ambient
NO 2 monitoring data to EPA’s Air Quality System(AQS). The annual average reporting burden for the collection under 40 CFR part 58 (average d over the first 3 years of this ICR) is $3,616,487. Burden is defined at 5 CFR 1320.3(b). Stat e, local, and tribal entities are eligible for State assistance grants provided by the Federal government under the CAA which can be used for monitors and related activities. An agency may not conduct or sponsor, a nd a person is not required to respond to, a collection of information unless it displays a currently valid OMB control number. The OMB control numbers for EPA's regulations in 40 CFR are listed in 40 CFR part 9.191 C. Regulatory Flexibility Act The Regulatory Flexibility Act (RFA) gene rally requires an agency to prepare a regulatory flexibility analysis of any rule subject to notice and comment rulemaking requirements under the Administrative Procedure Act or any other statute unless the agency certifies that the rule will not have a signifi cant economic impact on a substantial number of small entities. Small entities include sma ll businesses, small organizations, and small governmental jurisdictions. For purposes of assessing the impacts of this ru le on small entities, small entity is defined as: (1) a small business that is a small industrial entity as defined by the Small Business Administration’s (SBA) regulati ons at 13 CFR 121.201; (2) a sm all governmental jurisdiction that is a government of a city, c ounty, town, school distri ct or special district with a population of less than 50,000; and (3) a small organization that is any not-fo r-profit enterprise which is independently owned and operated and is not dominant in its field. After considering the economic impacts of this final rule on small entities, I certify that this action will not have a significant economic imp act on a substantial number of small entities. This final rule will not impose any requirements on small entities. Rather, this rule establishes national standards for allowa ble concentrations of NO 2 in ambient air as required by section 109 of the CAA. American Trucking Ass’ns v. EPA, 175 F. 3d 1027, 1044-45 (D.C. cir. 1999) (NAAQS do not have significant impacts upon small entities because NAAQS themselves impose no regulations upon small entities). Simila rly, the amendments to 40 CFR part 58 address the requirements for States to collect info rmation and report compliance with the NAAQS and will not impose any requirements on small entities.192 D. Unfunded Mandates Reform Act Title II of the Unfunded Mandates Reform Act of 1995 (UMRA), Public Law 104-4, establishes requirements for Federal agencies to assess the effects of th eir regulatory actions on State, local, and tribal governme nts and the private sector. Unless otherwise prohibited by law, under section 202 of the UMRA, EPA generally must prep are a written statement, including a cost-benefit analysis, for proposed and final rules with “Federal mandates” that may result in expenditures to State, local, and tribal governments, in the aggregate, or to the private sector, of $100 million or more in any one year. Before promulgating an EPA rule for which a written statement is required under sec tion 202, section 205 of the UM RA generally requires EPA to identify and consider a reasonable number of regul atory alternatives and to adopt the least costly, most cost-effective or least burdensome alternative that achieves the objectives of the rule. The provisions of section 205 do not apply when they are inconsistent with applicable law. Moreover, section 205 allows EPA to adopt an alternative other thanthe least costly, most costeffective or least burdensome alternative if the Administrator publishes with the final rule an explanation why that alternative was not adopt ed. Before EPA establishes any regulatory requirements that may significantly or uniquely affect small governments, including tribal governments, it must have developed under section 203 of the UMRA a small government agency plan. The plan must provide for no tifying potentially affected small governments, enabling officials of affected small governments to have meaningful and timely input in the development of EPA regulatory proposals with significant Federal intergovernmental mandates, and informing, educating, and advising small governments on compliance with the regulatory requirements.193 This action is not subject to the requireme nts of sections 202 and 205 of the UMRA. EPA has determined that this final rule does no t contain a Federal mandate that may result in expenditures of $100 million or more for State, local, and tribal governments, in the aggregate, or the private sector in any one year. The revisions to the NO 2 NAAQS impose no enforceable duty on any State, local or Trib al governments or the private sector. The expected costs associated with the monitoring requirements ar e described in EPA’s ICR document, but those costs are not expected to exceed $100 million in the aggregate for any year. Furthermore, as indicated previously, in sett ing a NAAQS EPA cannot consider the economic or technological feasibility of attaining ambient air quality stan dards. Because the Clean Air Act prohibits EPA from considering the types of estimates and a ssessments described in section 202 when setting the NAAQS, the UMRA does not require EPA to prepare a written statement under section 202 for the revisions to the NO 2 NAAQS. With regard to implementation guidance, th e CAA imposes the obligation for States to submit SIPs to implement the NO 2 NAAQS. In this final rule, EP A is merely providing an interpretation of those requirements. However, ev en if this rule did establish an independent obligation for States to submit SIPs, it is ques tionable whether an obligation to submit a SIP revision would constitute a Federal mandate in any case. The obligation for a State to submit a SIP that arises out of section 110 and section 191 of the CAA is not legally enforceable by a court of law, and at most is a condition for con tinued receipt of highway funds. Therefore, it is possible to view an action requiring such a submitta l as not creating any enforceable duty within the meaning of 2 U.S.C. 658 for purposes of the UMRA. Even if it did, the duty could be viewed as falling within the exception for a condition of Federa l assistance under 2 U.S.C. 658.194 EPA has determined that this final rule c ontains no regulatory re quirements that might significantly or uniquely affect small governme nts because it imposes no enforceable duty on any small governments. Therefore, this rule is not subject to the requirements of section 203 of the UMRA.
E. Executive Order 13132: Federalism Executive Order 13132, entitled “Federalism” (64 FR 43255, August 10, 1999), requires EPA to develop an accountable process to ensu re “meaningful and timely input by State and local officials in the development of regulatory policies that have federa lism implications.” “Policies that have federalism implications” is defined in the Executive Order to include regulations that have “substanti al direct effects on the States , on the relationship between the national government and the States, or on the di stribution of power and responsibilities among the various levels of government.” This final rule does not have federalism impli cations. It will not ha ve substantial direct effects on the States, on the relationship between the national government and the States, or on the distribution of power and responsibilities among the various levels of government, as specified in Executive Order 13132. The rule does not alter the relationshi p between the Federal government and the States regarding the esta blishment and implementation of air quality improvement programs as codified in the C AA. Under section 109 of the CAA, EPA is mandated to establish NAAQS; however, CAA s ection 116 preserves the rights of States to establish more stringent requirements if deemed necessary by a State. Furthermore, this rule does not impact CAA section 107 which establishes that the States have primary responsibility for implementation of the NAAQS. Finally, as noted in section E (above) on UMRA, this rule195 does not impose significant costs on State, local, or tribal governm ents or the private sector. Thus, Executive Order 13132 does not apply to this rule.
F. Executive Order 13175: Consultation and Coor dination with Indian Tribal Governments Executive Order 13175, entitled “Consultation a nd Coordination with Indian Tribal Governments” (65 FR 67249, November 9, 2000), requires EPA to develop an accountable process to ensure “meaningful and timely input by tribal officials in the development of regulatory policies that have tribal implications.” This final rule does not have tribal implications, as specified in Executive Order 13175. It does not have a subs tantial direct effect on one or more Indian tribes, on the relationship between the Federal government and Indian tribes, or on the distribution of power and re sponsibilities between the Federal government and tribes. The rule does not alter the relationshi p between the Federal government and tribes as established in the CAA and the TAR. Under section 109 of the CAA, EPA is mandated to establish NAAQS; however, this ru le does not infringe existing trib al authorities to regulate air quality under their own programs or under pr ograms submitted to EPA for approval. Furthermore, this rule does not affect the flexib ility afforded to tribes in seeking to implement CAA programs consistent with the TAR, nor do es it impose any new obligation on tribes to adopt or implement any NAAQS. Finally, as not ed in section E (above) on UMRA, this rule does not impose significant costs on tribal gove rnments. Thus, Executive Order 13175 does not apply to this rule. G. Executive Order 13045: Protection of Child ren from Environmen tal Health & Safety
Risks This action is subject to Executive Order (62 FR 19885, April 23, 1997) because it is an economically significant regulator y action as defined by Executive Order 12866, and we believe196 that the environmental health risk addressed by this action has a disp roportionate effect on children. The final rule will establish uniform national ambient air quality standards for NO 2 ; these standards are designed to protect public he alth with an adequate margin of safety, as required by CAA section 109. The protection offered by these st andards may be especially important for asthmatics, including asthmatic children, because respiratory effects in asthmatics are among the most sensitive health endpoints for NO 2 exposure. Because asthmatic children are considered a sensitive population, we have evalua ted the potential health effects of exposure to
NO 2 pollution among asthmatic children. These eff ects and the size of the population affected are discussed in chapters 3 and 4 of the ISA; ch apters 3, 4, and 8 of the REA, and sections II.A through II.E of this preamble. H. Executive Order 13211: Actions that Signific antly Affect Energy Supply, Distribution or
Use This rule is not a “significant energy action” as defined in Executive Order 13211, “Actions Concerning Regulations That Significantly Affect Ener gy Supply, Distribution, or Use” (66 FR 28355 (May 22, 2001)) because it is not likely to have a sign ificant adverse effect on the supply, distribution, or use of energy. The purpose of this rule is to esta blish revised NAAQS for
NO 2 . The rule does not prescribe specific control strategies by which these ambient standards will be met. Such strategies will be devel oped by States on a case-by-case basis, and EPA cannot predict whether the control options select ed by States will include regulations on energy suppliers, distributors, or users. Thus, EPA concludes that this rule is not likely to have any adverse energy effects.197 I. National Technology Transfer and Advancement Act Section 12(d) of the Nati onal Technology Transfer and Advancement Act of 1995 (NTTAA), Public Law 104-113, s ection 12(d) (15 U.S.C. 27 2 note) directs EPA to use voluntary consensus standards in its regul atory activities unless to do so would be inconsistent with applicable law or otherwise impractical. Volunt ary consensus standards are technical standards (e.g., materials specifications, test methods, samp ling procedures, and busi ness practices) that are developed or adopted by voluntary consensus standards bodies. The NTTAA directs EPA to provide Congress, through OMB, explanations whenthe Agency decides not to use available and applicable voluntary consensus standards. This final rulemaking involves technical standards with rega rd to ambient monitoring of
NO 2 . The use of this voluntary consensus standa rd would be impractical because the analysis method does not provide for the method detection limits necessary to adequately characterize ambient NO 2 concentrations for the purpose of determin ing compliance with the revisions to the
NO 2 NAAQS. J. Executive Order 12898: Federal Actions to Address Environmental Justice in Minority Populations and Low-Income Populations Executive Order 12898 (59 FR 7629; Feb. 16, 1994) establishes Federal executive policy on environmental justice. Its ma in provision directs Federal agen cies, to the greatest extent practicable and permitted by law, to make environmental justice part of their mission by identifying and addressing, as appropriate, disp roportionately high and adverse human health or environmental effects of their programs, policie s, and activities on minority populations and lowincome populations in the United States.198 EPA has determined that this final rule will not have disproportionately high and adverse human health or environmental effects on minority or low-income populations because it increases the level of environmental protection for a ll affected populations without having any disproportionately high and a dverse human health effects on any population, including any minority or low-income population. The final rule will establish uniform national standards for
NO 2 in ambient air. K. Congressional Review Act The Congressional Review Act, 5 U.S.C. 801 et seq., as added by the Small Business Regulatory Enforcement Fairness Act of 1996, genera lly provides that before a rule may take effect, the agency promulgating the rule must su bmit a rule report, which includes a copy of the rule, to each House of the Congress and to the Comptroller General of the United States. EPA will submit a report containing this rule and othe r required information to the U.S. Senate, the U.S. House of Representatives, and the Comptr oller General of the United States prior to publication of the rule in the Fede ral Register. A Major rule cannot take effect until 60 days after it is published in the Federal Register. This action is a “major rule” as defined by 5 U.S.C. 804(2). This rule will be effective on [insert date 60 days after da te of publication in the Federal Register ]

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40 CFR Part 50 Environmental protection, Air pollution c ontrol, Carbon monoxide, Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
40 CFR Part 58 Environmental protection, Admi nistrative practice and procedure, Air pollution control, Intergovernmental relations, Reporti ng and recordkeeping requirements

Dated: January 22, 2010.

Lisa P. Jackson, Administrator.207 For the reasons stated in the preamble, title 40, ch apter I of the Code of Federal Regulations is amended as follows:208 PART 50-NATIONAL PRIMARY AMBI ENT AIR QUALIT Y STANDARDS 1. The authority citation for part 50 continues to read as follows: Authority: 42 U.S.C. 7401, et seq. Subpart A—General Provisions

2. Section 50.11 is amended by redesignating paragr aphs (b) and (c) as pa ragraphs (c) and (d), respectively, and amending the other provisions as follows--

§ 50.11 National primary and secondary ambient air quality standards fo r oxides of nitrogen (nitrogen dioxide)

(a) The level of the national primary annual ambient air quality standard for oxides of nitrogen is 53 parts per billion (ppb, which is 1 part in 1,000,000,000), annual average concentration, measured in the ambient air as nitrogen dioxide.

(b) The level of the national primary 1-hour ambien t air quality standard for oxides of nitrogen is 100 ppb, 1-hour average concentration, measured in the ambient air as nitrogen dioxide.

(c) The level of the national s econdary ambient air quality sta ndard for nitrogen dioxide is 0.053 parts per million (100 micrograms per cubic mete r), annual arithmetic mean concentration.

(d) The levels of the standa rds shall be measured by:

(1) a reference method based on appendix F to this part, or

(2) by a Federal equivalent method (FEM) design ated in accordance with part 53 of this209 chapter.

(e) The annual primary standard is met when th e annual average concentration in a calendar year is less than or equal to 53 ppb, as determin ed in accordance with Appendix S for the annual standard.

(f) The 1-hour primary standard is met wh en the three-year average of the annual 98 th percentile of the daily maximum 1-hour average concentr ation is less than or equal to 100 ppb, as determined in accordance with Appendix S for the 1-hour standard.

(g) The secondary standard is attained when the annual arithmetic mean concentration in a calendar year is less than or equal to 0.053 ppm, rounded to three decimal places (fractional parts equal to or greater than 0.0005 ppm must be ro unded up). To demonstrate attainment, an annual mean must be based upon hourly data that are atleast 75 percent complete or upon data derived from manual methods that are at least 75 percen t complete for the scheduled sampling days in each calendar quarter.

3. Section 50.14 is amended by adding an entry to the end of table of paragraph (c)(2)(vi) to read as follows: §50.14 Treatment of air quality monitoring da ta influenced by exceptional events.
* * * * * (c) *** (2) *** (vi) ***210 Table 1. Schedule for Exceptional Event Flagging and Documentation Submission for Data to be Used in Designations Decisions for New or Revised NAAQS
NAAQS Pollutant/ Standard/(Leve l)/ Promulgation Date Air Quality Data Collected for Calendar Year Event Flagging & Initial Description Deadline Detailed Documentation Submission

Deadline

******* NO2/1-Hour Standard(100 PPB) 2008 July 1, 2010 a January 22, 2011 2009 July 1, 2010 January 22, 2011 2010 April 1, 2011 a July 1, 2011 a

a Indicates change from general schedule in 40 CFR 50.14. Note: EPA notes that the table of revised deadlines only applies to data EPA will use to establish the final initial designations for new or revised NAAQS. The general schedule applies for all other purposes, most notably, for data used by EPA for redesignations to attainment.
*****

4. Appendix S is added to read as follows: Appendix S to Part 50—Interpretation of th e Primary National Ambient Air Quality Standards for Oxides of Nitrogen (Nitrogen Dioxide) 1. General. (a) This appendix explains the data handling conventions and computations necessary for determining when the primary national ambient air quality standards for oxides of nitrogen as measured by nitrogen dioxide (“NO 2 NAAQS”) specified in § 50.11 ar e met. Nitrogen dioxide (NO 2 ) is measured in the ambient air by a Fede ral reference method (FRM) based on appendix F211 to this part or by a Federal equivalent method (FEM) designated in accordance with part 53 of this chapter. Data handling a nd computation procedures to be used in making comparisons between reported NO 2 concentrations and the levels of the NO 2 NAAQS are specified in the following sections.

(b) Whether to exclude, retain, or make adjustme nts to the data affected by exceptional events, including natural events, is determined by the re quirements and process dead lines specified in §§ 50.1, 50.14 and 51.930 of this chapter.

(c) The terms used in this appendix are defined as follows:

Annual mean refers to the annual average of all of the 1-hour concentr ation values as defined in section 5.1 of this appendix.

Daily maximum 1-hour values for NO 2 refers to the maximum 1-hour NO 2 concentration values measured from midnight to midnight (local standard time) that are used in NAAQS computations.

Design values are the metrics (i.e., statistics) that are compared to the NAAQS levels to determine compliance, calculated as specified in section 5 of this appendix. The design values for the primary NAAQS are:

(1) The annual mean value for a monitoring site for one year (referred to as the “annual primary standard design value”).212 (2) The 3-year average of annual 98 thpercentile daily maximum 1-hour values for a monitoring site (referred to as the “1-hour primary standard design value”).

98 th percentile daily maximum 1-hour value is the value below whic h nominally 98 percent of all daily maximum 1-hour concentration values fall, using the ranki ng and selection method specified in section 5.2 of this appendix.

Quarter refers to a calendar quarter.

Year refers to a calendar year.
2. Requirements for Data Used for Comparisons with the NO 2 NAAQS and Data Reporting Considerations.

(a) All valid FRM/FEM NO 2 hourly data required to be submitted to EPA’s Air Quality System (AQS), or otherwise available to EPA, meeting the requirements of part 58 of this chapter including appendices A, C, and E shall be used in design value calculati ons. Multi-hour average concentration values collected by wet chemistry methods shall not be used.

(b) When two or more NO 2 monitors are operated at a site, the state may in advance designate one of them as the primary monitor. If the st ate has not made this designation, the Administrator will make the designation, either in advance or retrospectively. Design values will be developed using only the data from the primary monitor, if this results in a valid design value. If data from the primary monitor do not allow th e development of a valid design value, data solely from the other monitor(s) will be used in tu rn to develop a valid design value, if this results in a valid213 design value. If there are three or more mon itors, the order for such comparison of the other monitors will be determined by the Administrato r. The Administrator may combine data from different monitors in different years for th e purpose of developing a valid 1-hour primary standard design value, if a valid design value can not be developed solely with the data from a single monitor. However, data from two or more monitors in the same year at the same site will not be combined in an attempt to meet data completeness requirements, except if one monitor has physically replaced another instrument perm anently, in which case the two instruments will be considered to be the same monitor, or if th e state has switched the designation of the primary monitor from one instrument to another during the year.

(c) Hourly NO 2 measurement data shall be reported to AQS in units of parts per billion (ppb), to at most one place after the decimal, with additio nal digits to the right being truncated with no further rounding.

3. Comparisons with the NO 2 NAAQS.

3.1 The Annual Primary NO 2 NAAQS.
(a) The annual primary NO 2 NAAQS is met at a site when th e valid annual primary standard design value is less than or equa l to 53 parts pe r billion (ppb).
(b) An annual primary standard design value is va lid when at least 75 perc ent of the hours in the year are reported.214 (c) An annual primary standard design value based on data that do not meet the completeness criteria stated in section 3.1(b) may also be c onsidered valid with the approval of, or at the initiative of, the Administrator, who may consider factors such as monitoring site closures/moves, monitoring diligence, the consis tency and levels of the valid concentration measurements that are available, and nearby conc entrations in determining whether to use such data.
(d) The procedures for calculating the annual prim ary standard design values are given in section 5.1 of this appendix.
3.2 The 1-hour Primary NO 2 NAAQS.
(a) The 1-hour primary NO 2 NAAQS is met at a site when th e valid 1-hour primary standard design value is less than or equa l to 100 parts pe r billion (ppb).
(b) An NO 2 1-hour primary standard desi gn value is valid if it en compasses three consecutive calendar years of complete data. A year meet s data completeness requirements when all 4 quarters are complete. A quarter is complete wh en at least 75 percent of the sampling days for each quarter have complete data. A sampling day has complete data if 75 percent of the hourly concentration values are reported.
(c) In the case of one, two, or three years that do not meet the completeness requirements of section 3.2(b) of this appendix and thus would normally not be useable for the calculation of a valid 3-year 1-hour primary stan dard design value, th e 3-year 1-hour primary standard design value shall nevertheless be cons idered valid if one of the fo llowing conditions is true.215 (i) At least 75 percent of the days in each quarter of each of three consecutive years have at least one reported hourly value, and the design value ca lculated according to th e procedures specified in section 5.2 is above the leve l of the primary 1-hour standard.
(ii) (A) A 1-hour primary standard design value that is below the level of the NAAQS can be validated if the substitution test in section 3.2(c)(ii)(B) results in a “test design value” that is below the level of the NAAQS. The test subst itutes actual ‘‘high’’ re ported daily maximum 1- hour values from the same site at about the sa me time of the year (sp ecifically, in the same calendar quarter) for unknown values that were not successfully measured. Note that the test is merely diagnostic in nature, intended to confir m that there is a very high likelihood that the original design value (the one with less than 75 percent data capture of hours by day and of days by quarter) reflects the true under-N AAQS-level status for that 3-ye ar period; the result of this data substitution test (the ‘‘test design value”, as defi ned in section 3.2 (c)(ii)(B)) is not considered the actual design value. For this test, substitution is permitted only if there are at least 200 days across the three matching quarters of the three years under consideration (which is about 75 percent of all possible daily values in those three quarters) for which 75 percent of the hours in the day have reported concentrations. Ho wever, maximum 1-hour values from days with less than 75 percent of the hours repo rted shall also be considered in identifying the high value to be used for substitution.
(B) The substitution test is as follows: Data subst itution will be performed in all quarter periods that have less than 75 percent data capture but at least 50 percent data cap ture; if any quarter has less than 50 percent data capture then this substi tution test cannot be used. Identify for each quarter (e.g., January-March) the highest reported daily maximum 1-hour value for that quarter,216 looking across those three months of all three years under consid eration. All daily maximum 1- hour values from all days in the quarter period sh all be considered when identifying this highest value, including days with less than 75 percent data capture. If after substituting the highest reported daily maximum 1-hour value for a quarter for as much of the missing daily data in the matching deficient quarter(s) as is needed to ma ke them 100 percent comp lete, the procedure in section 5.2 yields a recalculated 3-year 1-hour standard “test desi gn value” below the level of the standard, then the 1-hour primary standard design value is deemed to have passed the diagnostic test and is valid, and the level of the standard is deemed to have been met in that 3-year period. As noted in section 3.2(c)(i), in such a case, the 3-year design value based on the data actually reported, not the ‘‘test desi gn value’’, shall be used as the valid design value.
(iii) (A) A 1-hour primary standard design value that is above the leve l of the NAAQS can be validated if the substitution test in section 3.2(c)(iii)(B) results in a “test design value” that is above the level of the NAAQS. The test subst itutes actual ‘‘low” reported daily maximum 1- hour values from the same site at about the same time of the year (specifi cally, in the same three months of the calendar) for unknown values that were not successfully measured. Note that the test is merely diagnostic in nature, intended to confirm that there is a very high likelihood that the original design value (the one with less than 75 percent data capture of hours by day and of days by quarter) reflects the true above-NAAQS-level status for that 3-year period; the result of this data substitution test (the ‘‘test design value”, as defined in section 3.2(c)(iii)(B)) is not considered the actual design value. For this test , substitution is permitted only if there are a minimum number of available daily data points fr om which to identify the low quarter-specific daily maximum 1-hour values, specifically if there are at least 200 days across the three matching quarters of the three y ears under consideration (which is about 75 percent of all217 possible daily values in those three quarters) fo r which 75 percent of th e hours in the day have reported concentrations. Only days with at le ast 75 percent of the hours reported shall be considered in identifying the low value to be used for substitution.
(B) The substitution test is as follows: Data subst itution will be performed in all quarter periods that have less than 75 percent data capture. Identify for each quarter (e.g., January-March) the lowest reported daily maximum 1-hour value for that quarter, looking acro ss those three months of all three years under considera tion. All daily maximum 1-hour va lues from all days with at least 75 percent capture in the qua rter period shall be considered when identifying this lowest value. If after substituting the lowest reporte d daily maximum 1-hour valu e for a quarter for as much of the missing daily data in the matching defi cient quarter(s) as is needed to make them 75 percent complete, the procedure in section 5.2 yields a recalculated 3-year 1-hour standard “test design value” above the level of the standard, th en the 1-hour primary standard design value is deemed to have passed the diagnostic test and is va lid, and the level of the standard is deemed to have been exceeded in that 3-year period. As noted in section 3.2(c)(i), in such a case, the 3-year design value based on the data actually reported, not the ‘‘test design value’’, shall be used as the valid design value.
(d) A 1-hour primary standard design value ba sed on data that do not meet the completeness criteria stated in 3.2(b) and also do not satisfy se ction 3.2(c), may also be considered valid with the approval of, or at the init iative of, the Administrator, w ho may consider factors such as monitoring site closures/moves, monitoring diligence, the consiste ncy and levels of the valid concentration measurements that are availabl e, and nearby concentra tions in determining whether to use such data.218 (e) The procedures for calculati ng the 1-hour primary standard desi gn values are given in section 5.2 of this appendix.
4. Rounding Conventions.

4.1 Rounding Conventions for the Annual Primary NO 2 NAAQS.
(a) Hourly NO 2 measurement data shall be reported to AQS in units of parts per billion (ppb), to at most one place after the decimal, with additio nal digits to the right being truncated with no further rounding.
(b) The annual primary standard design value is calculated pursuant to section 5.1 and then rounded to the nearest whole number or 1 ppb (d ecimals 0.5 and greater are rounded up to the nearest whole number, and any decimal lower th an 0.5 is rounded down to the nearest whole number).
4.2 Rounding Conventions for the 1-hour Primary NO 2 NAAQS.
(a) Hourly NO 2 measurement data shall be reported to AQS in units of parts per billion (ppb), to at most one place after the decimal, with additio nal digits to the right being truncated with no further rounding.

(b) Daily maximum 1-hour values are not rounded.
(c) The 1-hour primary standard design value is calculated pursuant to section 5.2 and then rounded to the nearest whole number or 1 ppb (d ecimals 0.5 and greater are rounded up to the219 nearest whole number, and any decimal lower th an 0.5 is rounded down to the nearest whole number).
5. Calculation Procedures for the Primary NO 2 NAAQS.
5.1 Procedures for the Annual Primary NO 2 NAAQS.
(a) When the data for a site and year meet the data completeness requirements in section 3.1(b) of this appendix, or if the Admi nistrator exercises the discretiona ry authority in section 3.1(c), the annual mean is simply the arithmetic av erage of all of the reported 1-hour values.
(b) The annual primary standard design value for a site is the valid annual mean rounded according to the conventions in section 4.1.
5.2 Calculation Procedures for the 1-hour Primary NO 2 NAAQS.
(a) Procedure for identifying annual 98 th percentile values. When the data for a particular site and year meet the data completeness requirements in section 3.2(b), or if one of the conditions of section 3.2(c) is met, or if the Administrator exercises the discretionaryauthority in section 3.2(d), identification of annual 98th percen tile value is accomplished as follows.

(i) The annual 98th percen tile value for a year is the higher of the two va lues resulting from the following two procedures.
(1) Procedure 1. For the year, determine the num ber of days with at least 75 percent of the hourly values reported.220 (A) For the year, from only the days with at l east 75 percent of the hourly values reported, select from each day the maximum hourly value.
(B) Sort all these daily maximum hourly values from a particular site and year by descending value. (For example: (x[1], x[2], x[3], * * *, x[ n]). In this case, x[1] is the largest number and x[n] is the smallest value.) The 98 th percentile is determined from this sorted series of daily values which is ordered from the highest to the lowest number. Using the left column of Table 1, determine the appropriate range (i.e., row) for th e annual number of days with valid data for year y (cn y ). The corresponding “n” value in the right column identifies the rank of the annual 98 th percentile value in the descending sorted list of daily site values for year y. Thus, P 0.98, y = the nth largest value.
(2) Procedure 2. For the year, determine the num ber of days with at least one hourly value reported.
(A) For the year, from all the days with at leas t one hourly value reported, select from each day the maximum hourly value.
(B) Sort all these daily maximum values from a particular site and year by descending value. (For example: (x[1], x[2], x[3], * * *, x[n]). In this case, x[1] is the largest number and x[n] is the smallest value.) The 98 th percentile is determined from this sorted series of daily values which is ordered from the highest to the lowe st number. Using the left column of Table 1, determine the appropriate range (i.e., row) for th e annual number of days with valid data for year y (cn y ). The corresponding “n” value in the right column identifies the rank of the annual 98 th221 percentile value in the descending sorted list of daily site values for year y. Thus, P 0.98, y = the nth largest value.
(b) The 1-hour primary standard design value for a site is mean of the three annual 98 th percentile values, rounded according to the conventions in section 4.

Table 1

Annual number of da y s with valid data for year “y” (cn y ) P 0.98, y is the nth maximum value of the y ear, where n is the listed number 1–50 1

51-100 2

101-150 3

151-200 4

201-250. 5

251-300 6

301-350 7

351-366 8

PART 58--AMBIENT AIR QUALITY SURVEILLANCE222 The authority citation for part 58 continues to read as follows: Authority: 42 U.S.C. 7403, 7410, 7601(a), 7611, and 7619. Subpart A [AMENDED] Section 58.1, is amended by adding the following de finitions in alphabetical order to read as follows:
AADT means the annual aver age daily traffic.
*** Near-road NO 2 Monitor means any NO 2 monitor meeting the specifi cations in 4.3.2 of Appendix D and paragraphs 2, 4(b), 6.1, and 6.4 of Appendix E of this part.
* * * * * Subpart B [AMENDED]

Section 58.10, is amended by adding paragraph (a)(5 ) and adding paragraph (b)(12) to read as follows:
§ 58.10 Annual monitoring network pl an and periodic network assessment.

* * * * *

(a) * * *
(5) A plan for establishing NO 2 monitoring sites in accordance with the requirements of appendix D to this part shall be submitted to the Administrator by July 1, 2012. The plan shall provide for all required stations to be operational by January 1, 2013.

* * * * *223 (b) * * *
(12) The identific ation of required NO 2 monitors as either near-r oad or area-wide sites in accordance with Appendix D, S ection 4.3 of this part.

* * * * *
Section 58.13 is amended by adding pa ragraph (c) to read as follows:
§ 58.13 Monitoring network completion.

* * * * *
(c) The network of NO 2 monitors must be physically es tablished no later than January 1, 2013, and at that time, must be operating under all of the requirements of this part, including the requirements of appendices A, C, D, and E to this part.

* * * * *
Section 58.16 is amended by revising paragraph (a) to read as follows:

§ 58.16 Data submittal and archiving requirements.

* * * * *
(a) The State, or where appropriate, local ag ency, shall report to the Administrator, via AQS all ambient air quality data and associated quality assurance data for SO 2 ; CO; O 3 ; NO 2 ; NO; NO Y ; NO X ; Pb-TSP mass concentration; Pb-PM 10 mass concentration; PM 10 mass concentration; PM 2.5 mass concentration; for filter-based PM 2.5 FRM/FEM the field blank mass,224 sampler-generated average daily temperature, and sampler-generated average daily pressure; chemically speciated PM 2.5 mass concentration data; PM 10–2.5 mass concentration; chemically speciated PM 10–2.5 mass concentration data; meteorological data from NCore and PAMS sites; average daily temperature and average daily pressu re for Pb sites if not already reported from sampler generated records; and metadata reco rds and information specified by the AQS Data Coding Manual (http://www.epa.gov/ttn/airs/airsaqs/ manuals/manuals.htm ). The State, or where appropriate, local agency, may report site speci fic meteorological measurements generated by onsite equipment (meteorological in struments, or sampler generated) or measurements from the nearest airport reporting ambient pressure a nd temperature. Such air quality data and information must be submitted directly to the AQS via electronic transmission on the specified quarterly schedule described in pa ragraph (b) of this section.

* * * * *
Appendix A to Part 58 is amended as by a dding paragraph 2.3.1.5 to read as follows:

Appendix A to Part 58—Quality Assurance Re quirements for SLAMS, SPMs and PSD Air Monitoring 2.3.1.5 Measurement Uncertainty for NO 2 . The goal for acceptable measurement uncertainty is defined for precision as an upper 90 percent confidence limit for the coefficient of variation (CV) of 15 percent and for bias as an upper 95 percent confidence limit for the absolute bias of 15 percent.

* * * * *

Appendix C to Part 58 is amended as by a dding paragraph 2.1.1 to read as follows:225 Appendix C to Part 58—Ambient Ai r Quality Monitoring Methodology * * * * *

2.1.1 Any NO 2 FRM or FEM used for making primary NAAQS decisions must be capable of providing hourly averaged concentration data.

* * * * *
Appendix D to Part 58 is amended as by re placing paragraph 4.3 to read as follows:

Appendix D to Part 58—Network Design Crit eria for Ambient Air Quality Monitoring * * * * *

4.3 Nitrogen Dioxide (NO 2 ) Design Criteria.

4.3.1 General Requirement s. (a) State and, where appropriate, local agencies must operate a minimum number of required NO 2 monitoring sites as described below.
4.3.2 Requirement for Near-road NO 2 Monitors. (a) Within the NO 2 network, there must be one microscale near-road NO 2 monitoring station in each CBSA with a population of 350,000 or more persons to monitor a location of expect ed maximum hourly concen trations sited near a major road with high AADT counts as specified in paragraph 4.3.2(a)(1) of this appendix. An additional near-road NO 2 monitoring station is required for any CBSA with a population of 2,500,000 persons or more, or in any CBSA with a population of 350,000 or more persons that has one or more roadway segments with 250,000 or greater AADT counts to monitor a second location of expected maximum hourly concentrat ions. CBSA populations shall be based on the latest available census figures.226 (1) The near-road NO 2 monitoring stations shall be sele cted by ranking all road segments within a CBSA by AADT and then identifying a lo cation or locations adjacent to those highest ranked road segments, considering fleet mix, ro adway design, congestion patterns, terrain, and meteorology, where maximum hourly NO 2 concentrations are expected to be highest and siting criteria can be met in accordance with appendix E of this part. Where a state or local air monitoring agency identifies multiple accepta ble candidate sites where maximum hourly NO 2 concentrations are expected to occur, the monitoring agency shall consider the potential for population exposure in the criteria utilized to se lect the final site lo cation. Where one CBSA is required to have two near-road NO 2 monitoring stations, the sites shall be differentiated from each other by one or more of the following fact ors: fleet mix; congest ion patterns; terrain; geographic area within the CBSA; or different route, interstate, or freeway designation.
(b) Measurements at required near-road NO 2 monitor sites utilizing chemiluminescent FRMs must include at a minimum: NO, NO 2 , and NO X .. 4.3.3 Requirement for Area-wide NO 2 Monitoring. (a) Within the NO 2 network, there must be one monitoring station in each CBSA with a population of 1,000,000 or more persons to monitor a location of expected highest NO 2 concentrations representing the neighborhood or larger spatial scales. PAMS sites collecting NO 2 data that are situated in an area of expected high
NO 2 concentrations at the neighborhood or larger spatial scale may be used to satisfy this minimum monitoring requirement when the NO 2 monitor is operated year round. Emission inventories and meteorological an alysis should be used to iden tify the appropriate locations within a CBSA for locating required area-wide NO 2 monitoring stations. CBSA populations shall be based on the latest available census figures.227 4.3.4 Regional Administrator Required Mo nitoring. (a) The Regi onal Administrator may require additional NO 2 monitoring stations above the mi nimum requirements to monitor in locations away from roads, or sites that do not meet near-road NO 2 monitor siting criteria noted in appendix E of this part, where required ne ar-road monitors do not represent a location or locations where the expected maximum hourly NO 2 concentrations exist in a CBSA. The Regional Administrator may also require additional near-road NO 2 monitoring stations above the minimum required in situations where the mini mum monitoring requireme nts are not sufficient to meet monitoring objectives, and may consid er additional locations of expected high NO 2 concentrations and the variety of exposure potenti al due to increased variety in amount or types of fleet mix, congestion pattern s, terrain, or geographic areas within a CBSA. The Regional Administrator and the responsible State or local air monitoring ag ency should work together to design and/or maintain the most appropriate NO 2 network to service the variety of data needs for

an area.
(b) The Regional Administrator may require additional NO 2 monitoring stations for areawide NO 2 monitors at the neighborhood and larger spatial scales above the minimum monitoring requirements where the minimum monitoring require ments are not sufficient to meet monitoring objectives for an area, such as supporting pho tochemical pollutant assessment, air quality forecasting, PM precursor analysis, and characterizing impacts of NO 2 sources on certain communities. The Regional Administrator and th e responsible State or local air monitoring agency should work together to design and/or maintain the most appropriate NO 2 network to service the variety of data needs for an area.228 4.3.5 NO 2 Monitoring Spatial Scales. (a) The most important spatial scale for near-road
NO 2 monitoring stations to e ffectively characterize the maximum expected hourly NO 2 concentration due to mobile source emissions on major roadways is the microscale. The most important spatial scales for other monitoring st ations characterizing maximum expected hourly
NO 2 concentrations are the microscale and middle s cale. The most important spatial scale for area-wide monitoring of high NO 2 concentrations is the neighborhood scale.
(1) Microscale —This scale woul d typify areas in close prox imity to major roadways or point and area sources. Emissions from roadways result in high ground level NO 2 concentrations at the microscale, where concentration gradient s generally exhibit a marked decrease with increasing downwind distance from major roads. As noted in appendix E of this part, near-road
NO 2 monitoring stations are required to be within 50 meters of targ et road segments in order to measure expected peak concentrations. Emission s from stationary point and area sources, and non-road sources may, under certain plume conditions , result in high ground level concentrations at the microscale. The microscale typically re presents an area impacted by the plume with dimensions extending up to approximately 100 meters.
(2) Middle scale —This scale generally represen ts air quality levels in areas up to several city blocks in size with dimensions on the orde r of approximately 100 meters to 500 meters. The middle scale may include locations of expect ed maximum hourly concentrations due to proximity to major NO 2 point, area, and/or non-road sources.
(3) Neighborhood scale —The neighborhood scale would characterize air quality conditions throughout some relatively uniform la nd use areas with dimensions in the 0.5 to 4.0 kilometer range. Emissions from stationary point and area sources may, under certain plume229 conditions, result in high NO 2 concentrations at the nei ghborhood scale. Where a neighborhood site is located away from immediate NO 2 sources, the site may be us eful in representing typical air quality values for a larger residential area, and therefore suitable for population exposure and trends analyses.
(4) Urban scale – Measurements in this scal e would be used to estimate concentrations over large portions of an ur ban area with dimensions from 4 to 50 kilometers. Such measurements would be useful for assessing trends in area-wide air quality, and hence, the effectiveness of large scale air pollution control strategies. Urban scale sites may also support other monitoring objectives of the NO 2 monitoring network identifie d in paragraph 4.3.4 above.

4.3.6 NO y Monitoring. (a) NO/ NO y measurements are incl uded within the NCore multipollutant site requirements a nd the PAMS program. These NO/ NO y measurements will produce conservative estimates for NO 2 that can be used to ensure tracking continued compliance with the NO 2 NAAQS. NO/ NO y monitors are used at these sites because it is important to collect data on total reactive n itrogen species fo r understanding O 3 photochemistry.

* * * * *

Appendix E to Part 58 is ame nded by revising paragraph 2, 4( b), 6.1, adding paragraph 6.4 and revising paragraph 9(c) and Ta ble E-4 to read as follows:

Appendix E to Part 58—Probe and Monitori ng Path Siting Criteria for Ambient Air Quality Monitoring

* * * * *230 2. Horizontal and Vertical Placement
The probe or at least 80 percent of the monitoring path must be located between 2 and 15 meters above ground level for all ozone and sulfur di oxide monitoring sites, and for neighborhood or larger spatial scale Pb, PM 10 , PM 10-2.5 , PM 2.5 , NO 2 and carbon monoxide sites. Middle scale
PM 10-2.5 sites are required to have sampler inlets between 2 and 7 meters above ground level. Microscale Pb, PM 10 , PM 10-2.5 and PM 2.5 sites are required to have sa mpler inlets between 2 and 7 meters above ground level. Microscale near-road NO 2 monitoring sites are required to have sampler inlets between 2 and 7 meters above ground level. The inlet probes for microscale carbon monoxide monitors that are being used to measure concentrations near roadways must be
3+ 1⁄2 meters above ground level. The probe or at least 90 percent of the m onitoring path must be at least 1 meter vertical ly or horizontally away from any supporting structure, walls, parapets, penthouses, etc., and away from dusty or dirty areas . If the probe or a sign ificant portion of the monitoring path is located near th e side of a building or wall, th en it should be located on the windward side of the building relative to the prevailing wind direction during the season of highest concentration potential fo r the pollutant being measured.

* * * * *
4. Spacing from Obstructions

(a) * * *

(b) * * *

(c) * * *231 (d) For near-road NO 2 monitoring stations, the monitor probe shall have an unobstructed air flow, where no obstacles exis t at or above the height of the monitor probe, between the monitor probe and the outside nearest edge of th e traffic lanes of the target road segment.

* * * * *

6. Spacing from Roadways
6.1 Spacing for Ozone Probes and Mon itoring Paths. In siting an O 3 analyzer, it is important to minimize destructive interferences form sour ces of NO, since NO readily reacts with O 3 . Table E-1 of this appendix provides the required mi nimum separation distances between a roadway and a probe or, where applicable, at least 90 percent of a monitoring path for various ranges of daily roadway traffic. A sampling site having a point an alyzer probe located closer to a roadway than allowed by the Table E-1 requirements should be clas sified as microscale or middle scale, rather than neighborhood or urban scale, since the measur ements from such a site would more closely represent the middle scale. If an open path analyzer is used at a site, th e monitoring path(s) must not cross over a roadway with an average daily traffic count of 10,000 vehicles per day or more. For those situations where a monitoring path cr osses a roadway with fe wer than 10,000 vehicles per day, monitoring agencies must consider the en tire segment of the monitoring path in the area of potential atmospheric interfer ence from automobile emissions . Therefore, this calculation must include the length of the monitoring path over the roadway plus any segments of the monitoring path that lie in the area between th e roadway and minimum separation distance, as determined from the Table E-1 of this appendix. The sum of these distances must not be greater than 10 percent of the tota l monitoring path length.232 * * * * *
6.4 Spacing for Nitrogen Dioxide (NO 2 ) Probes and Monitoring Paths (a) In siting nearroad NO 2 monitors as required in pa ragraph 4.3.2 of appendix D of this part, the monitor probe shall be as near as practicable to the outside near est edge of the traffic lanes of the target road segment; but shall not be located at a distance greater than 50 me ters, in the horizontal, from the outside nearest edge of the traffic lanes of the target road segment.
(b) In siting NO 2 monitors for neighborhood and larger scale monitoring, it is important to minimize near-road influences. Table E-1 of this appendix provides the required minimum separation distances between a road way and a probe or, where applicab le, at least 90 percent of a monitoring path for various ranges of daily ro adway traffic. A sampling site having a point analyzer probe located closer to a roadway th an allowed by the Table E-1 requirements should be classified as microscale or middle scale rath er than neighborhood or urban scale. If an open path analyzer is used at a site, the monitoring path(s) must not cross over a roadway with an average daily traffic count of 10,000 vehicles pe r day or more. For those situations where a monitoring path crosses a roadwa y with fewer than 10,000 vehicles per day, monitoring agencies must consider the entire segm ent of the monitoring path in the area of potential atmospheric interference form automobile emi ssions. Therefore, this calculat ion must include the length of the monitoring path over the roadway plus any segm ents of the monitoring path that lie in the area between the roadway and minimum separation distance, as determined form the Table E-1 of this appendix. The sum of thes e distances must not be greate r than 10 percent of the total monitoring path length.

* * * * *233 9. Probe Material and Polluta nt Sample Residence Time

(a) * * *

(b) * * *
(c) No matter how nonreactive the sampling prob e material is initia lly, after a period of use reactive particulate matter is deposited on the probe walls. Th erefore, the time it takes the gas to transfer from the probe inlet to the sampli ng device is also critical. Ozone in the presence of nitrogen oxide (NO) will show significant losses even in the most inert probe material when the residence time exceeds 20 seconds. 26 Other studies 27-28 indicate that a 10 second or less residence time is easily achievable. Therefore, sampling probes for reactive gas monitors at NCore and at NO 2 sites must have a sample resi dence time less than 20 seconds.

* * * * *

11. Summary
Table E-4 of this appendix presents a summaryof the general requirements for probe and monitoring path siting criteria with respect to distances and heights. It is a pparent from Table E-4 that different elevation distances above the gr ound are shown for the various pollutants. The discussion in this appendix for each of the polluta nts describes reasons for elevating the monitor, probe, or monitoring path. The differences in th e specified range of he ights are based on the vertical concentration gradie nts. For CO and near-road NO 2 monitors, the gradients in the vertical direction are very large for the micros cale, so a small range of heights are used. The234 upper limit of 15 meters is specified for the cons istency between pollutants and to allow the use of a single manifold or monitoring path for monitoring more than one pollutant.
Table E-4 of Appendix E to Pa rt 58. Summary of Probe and Monitoring Path Siting Criteria

Pollutant Scale (maximum monitoring path length, meters) Height from ground to probe, inlet or 80% of monitoring path 1 Horizontal and vertical distance from supporting structures 2 to probe, inlet or 90% of monitoring path 1 (meters) Distance from trees to probe, inlet or 90% of monitoring path 1 (meters) Distance from roadways to probe, inlet or monitoring path 1 (meters)

SO 2 3,4,5,6 Middle (300 m) Neighborhood Urban, and Regional (1 km) 2-15 > 1 > 10 N/A

CO 4,5,7 Micro, middle (300 m), Neighborhood (1 km) 3

 1⁄2: 2-15 > 1 > 10 2-10; see Table E-2 of this appendix for middle and neighborhood scales.

O 3 3,4,5 Middle (300 m) Neighborhood, Urban, and Regional (1 km) 2-15 > 1 > 10 See Table E-1 of this appendix for all scales.

NO 2 3,4,5 Micro (Near-road [50-300]) Middle(300m) Neighborhood, Urban, and Regional (1 km) 2-7 (micro); 2-15(all other scales) > 1 > 10 < 50 meters for near-road microscale; See Table E-1 of this appendix for all other scales Ozone precursors (for PAMS) 3,4,5 Neighborhood and Urban (1 km) 2-15 > 1 > 10 See Table E-4 of this appendix for all scales.

PM,Pb 3,4,5,6,8 Micro: Middle, Neighborhood, Urban and Regional 2-7 (micro); 2-7 (middle PM 10-2.5 ); 2-15 (all other scales) > 2 (all scales, horizontal distance only) > 10 (all scales) 2-10 (micro); see Figure E-1 of this appendix for all other scales. N/A--Not applicable.235 1 Monitoring path for open path analyzers is applicable only to middle or neighborhood scale CO monitoring, middle, neighborhood, urban, and regional scale NO 2 monitoring, and all applicable scales for monitoring SO 2 ,O 3 , and O 3 precursors. 2 When probe is located on a rooftop, this separation di stance is in reference to walls, parapets, or penthouses located on roof. 3 Should be >20 meters from the dripline of tree(s) and must be 10 meters from the dripline when the tree(s) act as an obstruction. 4 Distance from sampler, probe, or 90% of monitoring path to obstacle, such as a building, must be at least twice the height the obstacle protrudes above the sampler, probe, or monitoring path. Sites not meeting this criterion may be classified as middle scale (see text). 5 Must have unrestricted airflow 270 degrees around the probe or sampler; 180 degrees if the probe is on the side of a building or a wall. 6 The probe, sampler, or monitoring pa th should be away from minor sources, such as furnace or in cineration flues. The separation distance is dependent on the height of the minor source's emission point (such as a flue), the type of fuel or waste burned, and the quality of the fuel (sulfur, as h, or lead content). This criterion is designed to avoid undue influences from minor sources. 7 For microscale CO monitoring sites, the probe must be >10 meters from a street intersection and preferably at a midblock location. 8 Collocated monitors must be within 4 meters of each other and at least 2 mete rs apart for flow rates greater than 200 liters/min or at least 1 meter apart for samplers havi ng flow rates less than 200 liters/min to preclude airflow interference. * * * * *
Appendix G to Part 58 is amended as by revising paragraph 9 and Table 2 to read as follows:

Appendix G to Part 58 - Uniform Air Qu ality Index (AQI) and Daily Reporting * * * * * 9. How Does the AQI Relate to Air Pollution Levels? For each pollutant, the AQI transforms ambient c oncentrations to a scale from 0 to 500. The AQI is keyed as appropriate to the national ambient air qua lity standards (NAAQS) for each pollutant. In most cases, the index value of 100 is associated with the numerical level of the short-term (i.e., averaging time of 24-hours or le ss) standard for each pollutant. The index value of 50 is associated with one of the following: the numerical leve l of the annual standard for a pollutant, if there is one; one-half the level of th e short-term standard for the pollutant; or the level at which it is appropriate to begin to provide guidance on cautionary language. Higher categories of the index are base d on increasingly serious health effects that affect increasing236 proportions of the population. An index value is calculated each day for each pollutant (as described in section 12 of this appendix), unle ss that pollutant is specifically excluded (see section 8 of this appendix). The pollutant with th e highest index value for the day is the “critical” pollutant, and must be included in the daily AQI report. As a result, the AQI for any given day is equal to the index value of th e critical pollutant for that day. For the purposes of reporting the AQI, the indexes for PM 10 and PM 2.5 are to be considered separately. * * * * *TABLE 2.—BREAKPOINTS FOR THE AQIThese breakpoints Equal these AQI’sO3(ppm)8-hourO3(ppm)1-hour1PM2.5(μg/m3)PM10(μg/m3)CO (ppm) SO2(ppm) NO2(ppm)1-hourAQI Category0.000-0.059 .................. 0.0-15.40-54 0.0-4.4 0.000-0.0340 –(0.040 – 0.053)0-50 Good.0.060-0.075 .................. 15.5-40.4 55-154 4.5-9.4 0.035-0.144(0.041 – 0.054) –(0.080 – 0.100)51-100 Moderate.0.076-0.095 0.125-0.164 40.5-65.4 155-254 9.5-12.4 0.145-0.224(0.081 – 0.101) –(0.360 – 0.370)101-150 Unhealthy forSensitive Groups.0.096-0.115 0.165-0.204365.5-150.4 255-354 12.5-15.4 0.225-0.304(0.361 – 0.371) –0.64151-200 Unhealthy.0.116-0.374 0.205-0.4043150.5-250.4 355-424 15.5-30.4 0.305-0.6040.65 – 1.24201-300 Very Unhealthy.(2)............... 0.405-0.5043250.5-350.4 425-504 30.5-40.4 0.605-0.8041.25 – 1.64301-400 Hazardous.(2)............... 0.505-0.6043350.5-500.4 505-604 40.5-50.4 0.805-1.0041.65 – 2.04401-500 Hazardous.1Areas are generally required toreport the AQI based on 8-hour ozone values. However, there are a small number of areas whereanAQI based on 1-hour ozone values would be more precautionary. Inthese cases, in addition to calculating the 8-hour ozone indexvalue, the 1-hour ozone index value may be calculated, and the maximum of the two values reported.28-hour O3values do not define higher AQI values (≥301). AQI values of 301 or greater are calculated with 1-hour O3concentrations.3If a different SHL for PM2.5is promulgated, these numbers will change accordingly.