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## Cover Sheet for Transmitting Do cuments under E.O. 12866

Information about the Regulatory Ac tion Related to This Document

Title of the Regulatory Action : Primary National Ambient Air Quality Standard for Nitrogen Dioxide Regulatory Identifier Number (RIN) : 2060-AO19 Stage of Action : Proposed rule

Information about This Document

Document Creation Date : May 22, 2009

Document Type : ( X ) Draft preamble and regulatory text (i.e., this is the regulatory action) ( ) Draft Techni cal Support Document ( ) Draft Regulatory Impact Analyses ( ) Draft Informa tion Collection Request ( ) Other: _________________________________________________

Document Version : ( X ) Version sent to OMB for E.O. 12866 review ( ) Version substantively changed as a result of E.O. 12866 review2 6560-50-P ENVIRONMENTAL PR OTECTION AGENCY 40 CFR Parts 50, 53, 58 [EPA-HQ-OAR-2006-0922; FRL- ] Primary National Ambient Air Qua lity Standard for Nitrogen Dioxide

AGENCY: Environmental Protection Agency (EPA). ACTION: Proposed Rule SUMMARY: Based on its review of th e air quality criteria for ox ides of nitrogen and the primary national ambient air quali ty standard (NAAQS) for oxides of nitrogen as measured by nitrogen dioxide (NO 2 ), EPA proposes to make re visions to the primary NO 2 NAAQS in order to provide requisite protection of public health. Specifically, EPA proposes to supplement the current annual standard by es tablishing a new short-term NO 2 standard based on the 3-year average of the 99 th percentile (or 4 th highest) of 1-hour daily ma ximum concentrations. EPA proposes to set the level of th is new standard within the ra nge of 80 to 100 ppb and solicits comment on standard levels as low as 65 ppb and as high as 125 ppb. EPA also proposes to establish requirements for an NO 2 monitoring network that will include monitors near major roadways. Consistent with the terms of a consen t decree, the Administrator will sign a notice of final rulemaking by January 22, 2010.

DATES: Comments must be received on or before [insert date 60 days after date of publication in the Federal Register] . Under the Paperwork Reducti on Act, comments on the information collection provi sions must be received by OMB on or before {insert date thirty3 days after date of publication in the Federal Register.} Public Hearings: EPA intends to hold public hearings on this proposed rule in [August 2009 in Los Angeles, California and Ar lington, VA. These will be a nnounced in a separate Federal Register notice that provides de tails, including specific times and addresses, for these hearings. ADDRESSES: Submit your comments, id entified by Docket ID No. EPA- HQ-OAR-2006-0922 by one of the following methods:

• www.regulations.gov : Follow the on-line instructions for submitting comments.

• Email: a-and-r-Docket@epa.gov

• Fax: 202-566-9744

• Mail: Docket No. EPA-HQ-OAR-2006-0922, E nvironmental Protection Agency, Mail code 6102T, 1200 Pennsylvania Ave., NW., Wa shington, DC 20460. Please include a total of two copies.

• Hand Delivery: Docket No. EPA-HQ-OAR-2006- 0922, Environmental Protection Agency, EPA West, Room 3334, 1301 Constitu tion Ave., NW, Washington, DC. Such deliveries are only accepted during the Docket ’s normal hours of ope ration, and special arrangements should be made for de liveries of boxed information.
Instructions: Direct your comments to Docket ID No. EPA-HQ-OAR-2006-0922. EPA's policy is that all comments received w ill be included in the public dock et without change and may be made available online at www.regulations.gov , including any personal information provided, unless the comment includes information claimed to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Do not submit information that you consider to be CBI or othe rwise protected through www.re gulations.gov or e-mail. The www.regulations.gov website is an “anonymous access” system, which means EPA will not4 know your identity or contact information unless yo u provide it in the body of your comment. If you send an e-mail comment di rectly to EPA without going through www.regulations.gov your e-mail address will be automatically captured and included as part of the comment that is placed in the public docket and made available on the Inte rnet. If you submit an electronic comment, EPA recommends that you include your name and other contact informati on in the body of your comment and with any disk or CD-ROM you subm it. If EPA cannot read your comment due to technical difficulties and cannot contact you for clarification, EPA may not be able to consider your comment. Electronic files should avoid the use of special char acters, any form of encryption, and be free of any defects or viruses. For additional information about EPA’s public docket visit the EPA Docket Center homepage at http://www.epa.gov/epahome/dockets.htm

. Docket: All documents in the docket are listed in the www.regulations.gov index. Although listed in the index, some inform ation is not publicly available, e.g., CBI or other information whose disclosure is restricted by statute. Certain ot her material, such as copyrighted material, will be publicly available only in hard copy. P ublicly available docket materials are available either electronically in www.regulations.gov or in hard copy at the Ai r and Radiation Docket and Information Center, EPA/DC, EPA West , Room 3334, 1301 Constitution 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 holid ays. The telephone number for the Public Reading Room is (202) 566-1744 and the telephone nu mber for the Air and Radia tion Docket and Information Center is (202) 566-1742. FOR FURTHER INFORMATION CONTACT: Dr. Scott Jenkins, Health and Environmental Impacts Divisi on, Office of Air Quality Plan ning and Standards, U.S. Environmental Protection Agen cy, Mail code C504-06, Resear ch Triangle Park, NC 27711;5 telephone: 919-541-1167; fa x: 919-541-0237; email: jenkins.scott@epa.gov

. SUPPLEMENTARY INFORMATION: General Information What Should I Consider as I Prepare My Comments for EPA?
1. Submitting CBI. Do not submit this information to EPA through www.regulations.gov or email. Clearly mark the part or all of the information that you claim to be CBI. For CBI information in a disk or CD ROM that you mail to EPA, mark the outside of the disk or CD ROM as CBI and then identify electronically within the disk or CD ROM the specific information that is claimed as CBI. In addition to one complete version of the comment that includes information claime d as CBI, a copy of the comm ent that does not contain the information claimed as CBI must be submitted for inclusion in the public docket. Information so marked will not be disclosed except in accordance with procedures set forth in 40 CFR part 2.
2. Tips for Preparing Your Comments. When submitting comments, remember to:

• Identify the rulemaking by docket number and other iden tifying information (subject heading, Federal Register date and page number).

• Follow directions – the agency may ask you to respond to specific questions or organize comments by referencing a Code of Federal Regulations (CFR) part or section number.

• Explain why you agree or disa gree, suggest alternatives, and substitute language for your requested changes.

• Describe any assumptions and provide any technical informa tion and/or data that you used.

• Provide specific examples to illustrate your concerns, and suggest alternatives.6 • Explain your views as clearly as possible, avoiding the use of profanity or personal threats.

• Make sure to submit your comments by the comment period deadline identified. Availability of Related Information A number of the documents that are relevant to this rulemaking are available through EPA’s Office of Air Quality Pl anning and Standards (OAQPS) Technology Transfer Network (TTN) web site at h ttp://www.epa.gov/ttn/naaqs/standa rds/nox/s_nox_index.html. These documents include the Integrated Review Plan and the Health Assessment Plan, available at http://www.epa.gov/ttn/naaqs/ standards/nox/s_nox_cr_pd.html , the Integrated Science Assessment (ISA), available at http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=194645 , and the Risk and Exposure Assessment (REA), available at http://www.epa.gov/ttn/naaqs/st andards/nox/s_nox_cr_rea.html . These and other related documents are also available for inspection and co pying in the EPA docket identified above. Table of Contents The following topics are discussed in this preamble: I. Background A. Legislative requirements B. Related NO 2 control programs C. Review of the Air Qualit y Criteria and Standards for NO 2 II. Rationale for Proposed Decisi ons on the Primary Standard A. Characterization of NO 2 air quality 1. Current patterns of NO 2 air quality 2. NO 2 air quality and grad ients around roadways B. Health effects information 1. Adverse respiratory effects and short-term exposure to NO 2 a. Emergency department visits and hospital admissions b. Respiratory symptoms c. Impaired host defense d. Airway response e. Airway inflammation f. Lung function7 g. Conclusions from the ISA 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 a. Pre-existing disease b. Age c. Genetics d. Gender e. Proximity to roadways f. Socioeconomic status g. Size of the at-risk population C. Human exposure and health risk characterization 1. Evidence base for the risk characterization 2. Overview of approaches 3. Key limitations and uncertainties D. Considerations in Review of the Standard 1. Background on the current standard 2. Approach for reviewing the need to re tain or revise the current standard E. Adequacy of the current standard 1. Evidence-based considerations 2. Exposure- and risk-based considerations 3. Summary of considerations from the REA 4. CASAC views 5. Administrator’s conclusions regardi ng adequacy of the current standard F. Conclusions on the elements of a new s hort-term standard and an annual standard 1. Indicator 2. Averaging time a. Short-term averaging time b. Long-term averaging time c. CASAC views d. Administrator’s conclusions on averaging time 3. Form 4. Level a. Evidence-based considerations b. Exposure- and risk-based considerations c. Summary of considerations from the REA d. CASAC views e. Administrator’s conclusions on level G. Summary of proposed decisions on the primary standard III. Proposed Amendments to Ambient Monitoring and Reporting Requirements8 A. Monitoring methods B. Network design 1. Background 2. Proposed changes a. Monitoring in areas of expect ed maximum concentrations near major roads b. Area-wide monitoring at neighborhood and larger spatial scales 3. Solicitation for comment on an alternative network design C. Data reporting IV. Proposed Appendix S--Inter pretation of the Primary NAAQS for Oxides of Nitrogen and Proposed Revisions to the Exceptional Events Rule A. Background B. Interpretation of the NAAQS for NO 2 1. Annual primary standard 2. 1-hour primary standard based on the 99 th percentile value form 3. 1-hour standard based on the annual 4 th highest daily value form C. Exceptional Events Information Submission Schedule V. Clean Air Act Implemen tation Requirements for NO 2 NAAQS A. Designations for the NO 2 NAAQS B. Classifications C. Attainment Dates 1. Attaining the NO 2 NAAQS 2. Consequences of failing to attainby the Statutory Attainment Date D. Section 110(a)(2) NO 2 NAAQS Infrastructure Requirements E. Attainment Pla nning Requirements for NO 2 1. NO 2 Nonattainment Area SIPs 2. New Source Review and Prevention of Significant Deterioration Requirements 3. General Conformity 4. Transportation Conformity 5. Tran sition from the Existing NO 2 NAAQS to a Revised NO 2 NAAQS VI. Communication of Public Health Information VII. Statutory and Executive Order Reviews References

I. Background A. Legislative Requirements Two sections of the Clean Ai r Act (Act or CAA) govern the establishment and revision of the NAAQS. Section 108 of the Act directs the Administra tor to identify and list air pollutants that meet certain criter ia, including that the air pollutant “in his judgment, cause[s] or9 contribute[s] to air pollution wh ich may reasonably be anticipated to endanger public health and welfare” and “the presence of whic h in the ambient air results from numerous or diverse mobile or stationary sources.” 42 U.S. C. 21 7408(a)(1)(A) & (B). For t hose air pollutants listed, section 108 requires the Administrator to issue air qualitycriteria that “accurate ly reflect the latest scientific knowledge useful in indicating the kind and extent of all identifiable effects on public health or welfare which may be expected from the presence of [a] pollutant in ambient air . . .” 42 U.S.C. 7408(2). Section 109(a) of the Act di rects the Administrator to promulgate “primary” and “secondary” NAAQS for pollutants for which air qua lity criteria have been issued. 42 U.S.C. 7409(1). Section 109(b)(1) defines a primary standard as one “the 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 secondary standard, in turn, must “specify a le vel of air quality the a ttainment and maintenance of which, in the judgment of the Administrator, based on [the air qua lity] criteria, is requisite to protect the public welfare from any known or anticipated advers e effects associated with the presence of such pollutant 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 inconc lusive 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

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 an y [sensitive] group of the population,” and that for this purpose “reference should be made to a representative samp le of persons comprising the sensitive group rather than to a single person in such a group.” S. Rep. No. 91-1196, 91st Cong., 2d Sess. 10 (1970) 2 EPA is currently conducting a separate review of the secondary NO 2 NAAQS jointly with a review of the secondary SO 2 NAAQS.10 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 pr imary standards that include an adequate margin of safety, the Administrator is seeking not only to prevent polluti on levels that have been demonstrated to be harmful but also to prevent lowe r pollutant levels that may pose an unacceptable risk of harm, even if the risk is not precisely id entified 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 health effects involve d, the size of the at-ris k population(s), and the kind and degree of the uncertaintie s that must be addressed. Th e selection of any particular approach to providing an adequate margin of safe ty is a policy choice left specifically to the Administrator’s judgment. Lead Industries Association v. EPA , supra , 647 F.2d at 1161-62. In setting standards that are “requisite” to protect public health and welfare, as provided in section 109(b), EPA’s task is to establish st andards 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 Administrator to periodically undertake a thorough review of the air quali ty criteria published under secti on 108 and the NAAQS and to revise the criteria and standards as may be appr opriate. 42 U.S.C. 7409 (d)(1). The Act also requires the Administrator to appoint an indepe ndent 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 and11 revisions of existing criteria and standards as may be a ppropriate under section 108 and subsection (b) of this section." 42 U.S.C. 7409(d)(2). This independent review function is performed by the Clean Air Scientific A dvisory Committee (CASAC) of EPA’s Science Advisory Board.
B. Related NO 2 Control Programs States are primarily responsible for ensuring attainment and mainte nance 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 are to submit, for EPA approval, St ate implementati on plans (SIPs) that provide for the attainment and maintenance of such standards through control programs directed to sources of the po llutants involved. The States, inconjunction with EPA, also administer the prevention of significant deteriora tion program that covers these pollutants. See 42 U.S.C. 7470-7479. In additi on, Federal programs provide fo r nationwide reductions in emissions of these and other air pollutants unde r Title II of the Act, 42 U.S.C. 7521–7574, which involves controls for au tomobile, truck, bus, motorcycle, nonroad engine and equipment, and aircraft emissions; the new source performance standards 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 designate d as nonattainment of the NO 2 NAAQS. If the NO 2 NAAQS is revised as a result of this review; however, some areas could be classified as non-attain ment. Certain States would then be required to develop SIPs that identify and implement specific air polluti on control measures to reduce ambient NO 2 concentrations to attain and maintain the revised NO 2 NAAQS, most likel y by requiring air12 pollution controls on sources that emit oxides of nitrogen (NO X 3 ). While NO X is emitted from a wide variety of s ource 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 about the next 20 years as a result of the ongoing implementati on of mobile source emissions standards. In particular, Tier 2 NO X emission standards for light-duty ve hicle emissions began phasing into the fleet beginning with model year 2004 , in combination with low-sulfur gasoline fuel standards. For heavy-duty engines, new NO X standards are phasing in between the 2007 and 2010 model years, following the introduction of ultr a-low sulfur diesel fuel. Lower NO X standards for nonroad diesel engines, locomotives, and cer tain marine engines are 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. C. Review of the Air Quality Criteria and Standards for NO 2 On April 30, 1971, EPA promulgated identic al 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 reviews of th e air quality 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).

3 In this document, the terms “oxides of nitrogen” and “nitrogen oxides” (NOX) 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 the Clean Air Act Section 108(c): “Such criteria [for oxides of nitrogen] shall include a discussion of nitric and nitrous acids, nitrites, nitrat es, nitrosamines, and other carcinogenic and potentially carcinogenic derivatives of oxides of nitrogen.” By contrast, within th e air pollution research and control communities, the terms “oxides of nitrog en” 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 including those listed in Section 108(c) is NO Y .13 EPA initiated the current review of the air quality criteria for oxide s of nitrogen and the
NO 2 primary NAAQS on December 9, 2005 (70 FR 73236) with a gene ral call for information. EPA’s draft Integrated Review Plan for the Primary Nationa l Ambient Air Quality Standard for Nitrogen Dioxide (EPA, 2007a) wa s made available in Februa ry, 2007 for public comment and was discussed by the CASAC via a publicly accessible teleconferen ce on May 11, 2007. As noted in that plan, NO x includes multiple gaseous (e.g., NO 2 , NO) and particulat e (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 effects of ambient part icles in the Agency’s review of the NAAQS for particulate matter (PM), the current review of the primary NO 2 NAAQS is focused on the gaseous species of NO x and does not consider health effects directly associated with particulate species. The first draft of the Integr ated Science Assessment for Oxides of Nitrogen-Health Criteria (ISA) and the Nitrogen Dioxide Hea lth 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 received 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 we re reviewed by CASAC at a public meeting held on May 1-2, 2008. Based on comments received from CASA C 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 sec ond draft of the REA, which was released for public review and comment in two parts. The first 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. The14 second part of this document, containing chapte r 8 (describing the Atlanta exposure assessment) and a completed appendix B, was released in Oct ober of 2008. This document was the subject of CASAC reviews at public meetings on Sept ember 9 and 10, 2008 (for th e first part) and on October 22, 2008 (for the second part). In preparing th e final REA (EPA, 2008b), EPA considered comments recei ved from the CASAC and the public at those meetings. In the course of reviewing the second draf t REA, CASAC expressed the view that the document would be incomplete wi thout the addition of a policy assessment chapter presenting an integration of evidence-based considerations and risk and exposure asse ssment results. CASAC stated that such a chapter would be “critical 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 lett er to the chair of CA SAC, addressing earlier CASAC comments on the NAAQS re view process (Henderson, 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 and approaches for considering the level of the standard under review, including risk assessm ent and weight of evidence methodologies” (Peacock, 2008, p.3; September 8, 2008). Accordingly, the final REA included a new policy assessment chapter. This policy assessment chapter considered the scientific evidence in the ISA and the exposure and risk characterization results presented inother chapters of the REA as they relate to the adequacy of the current NO 2 primary NAAQS and potentia l alternative primary NO 2 standards. In considering the current and poten tial alternative standards, the final REA document focused on the information that is most pe rtinent to evaluating the basic elements of national ambient air15 quality standards: indicator, averaging time, form 4 , and level. These elements, which together serve to define each standard, must be considered collectively in evaluati ng the health protection afforded. CASAC discussed the final version of the REA, with an emphasis on the policy assessment chapter, during a public teleconfer ence held on December 5, 2008. Following that teleconference, CASAC offered comments and advice on the NO 2 primary NAAQS in a letter to the Administrator (Samet, 2008b). The schedule for completion of this review is governed by a judicial order resolving a lawsuit filed in September 2005, concerning the ti ming of the current revi ew. The order that now governs this review, entere d by the court 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 re view of the primary NO 2 NAAQS no later than June 26, 2009 and January 22, 2010, respectively. This action presents the Administrator’s proposed decisions on the current primary NO 2 standard. Throughout this preamb le a number of conclusions, findings, and determinations proposed by the Administrator are noted. While they identify the reasoni ng that supports this proposal, they are not intended to be final or co nclusive in nature. The EPA invites general, specific, and/or tec hnical comments on all issues involved wi th this proposal, including all such proposed judgments, conclusions, findings, and de terminations. Further, EPA invites specific comments from CASAC on the proposed appr oach of establishing a new 1-hour NO 2 standard in conjunction with a revised monito ring network that includes a s ubstantial number of monitors placed near major roads. In addition to requesting comment on th e overall approach, EPA invites specific comment on the level, or range of levels, appropriate for such a standard, as well

4 The “form” of a standard defines the ai r quality statistic that is to be compared to the level of the standard in determining whether an area attains the standard.16 as on the rationale that would support th at level or range of levels. II. Rationale for Proposed Deci sions on the Primary Standard This section presents the rationale for the Ad ministrator’s proposed decision to revise the existing NO 2 primary standard by supplementing th e current annual standard with a 1-hour standard and to specify the standards to the near est parts per billion (ppb) . As discussed more fully below, this rationale take s into account: (1) judgments and c onclusions presented in the ISA and the REA; (2) CASAC advice and recommendations, as reflected in discussions of drafts of the ISA and REA at public meetings, in separate written comments, and in CASAC’s letter to the Administrator (Samet, 2008b); a nd (3) public comments received at CASAC meetings during the development of the ISA and the REA. In developing this rationale, EPA has drawn upon an integrative synthesis of the entire body of evidence on human health effect s associated with the presence of NO 2 in the air. As discussed below, this body of evidence addresse s 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 th e ISA finds associations with NO 2 to be causal or likely causal (see section II.B below). This rationa le also draws upon the results of quantitative exposure and risk assessments. As discussed below, a substantial amount of new research has been conducted since the last review of the NO 2 NAAQS, with important new info rmation coming from epidemiologic studies in particular. The newly available res earch studies evaluated inthe ISA have undergone intensive scrutiny through multiple layers of peer review and opportunities for public review and comment. While important uncertainties re main in the qualitative and quantitative17 characterizations of hea lth effects attributable to exposure to ambient NO 2 , the review of this information has been extensive and deliberate. The remainder of this section discusses th e rationale for the Ad ministrator’s proposed decisions on the primary standard. Sec tion II.A presents a discussion of NO 2 air quality, including discussion of the NO 2 concentration gradients that can exist around roadways, and the
current NO 2 monitoring network. Section II.B includes an overview of the scientific evidence related to health effects associated with NO 2 exposure. This overview includes discussion of the health endpoints and at-risk populations considered in the ISA. Section II.C discusses the approaches takenby EPA to assess exposures and health risks associated with NO 2 , including a discussion of key uncertainties asso ciated with the analyses. Sec tion II.D presents the approach that is being used in the current review of the NO 2 NAAQS with regard to consideration of the scientific evidence and exposure-/risk-based resu lts related to the ade quacy of the current standard and potential alternative standards. Sec tions II.E and II.F discuss the scientific evidence and the exposure-/risk-based results specifically as they relate to the current and potential alternative standards, including discussion of the Administrator’s proposed decisions on the standard. Section II.G summarizes the Administ rator’s proposed decisions with regard to the
NO 2 primary NAAQS. A. Characterization of NO 2 air quality
1. Current NO 2 monitoring network The size of the State and local NO 2 monitoring network has re mained relatively stable since the early 1980s, and currently has approxima tely 400 monitors report ing data to EPA’s Air Quality System (AQS) database. 5 At present, there are no minim um monitoring requirements for

5 It should be noted that the ISA Section 2.4.1references a different number of active monitors in the NO 2 network. The discrepancy between the ISA numbers and the number pres ented here is due to differing metrics used in pulling18 NO 2 in 40 CFR part 58 Appendix D, other than a requirement for EPA Regional Administrator approval before removing any exis ting monitors, and that any ongoing NO 2 monitoring must have at least one monito r sited to measure the ma ximum concentration of NO 2 in that area (though, as discussed below monitors in the current 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 revisi ons (71 FR 61236), based on the fact that there were no NO 2 nonattainment areas at that ti me, coupled with trends evidence showing an increasing gap between national average NO 2 concentrations and the current a nnual standard. Additionally, the minimum requirements were removed to provide St ate, local, and tribal air monitoring agencies flexibility in mee ting higher priority moni toring needs for pollutants such as ozone 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. However, of the approximately 400 NO 2 monitors currently in operation, only about 10 percent may be due to the PAMS requirements. An analysis of the approximately 40 0 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 ta rget monitor objective categories since it was introduced in the May 10, 1979 mo nitoring rule (44 FR 27571). The review of the current

data from AQS. The ISA only references SLAMS, NAMS , and PAMS sites with defined monitoring objectives, while the Watkins and Thompson, 2008 value represents all NO 2 sites reporting data at any point during the year. These differences in number s of active monitors per year also explain why the Watkins and Thompson 2008 document characterized the NO2 network size as relatively stable since the early 1980s.19 network found that the assessment of concen trations for general population exposure and maximum concentrations at neighborhood and larger scales were the top objectives. A review of the distribution of listed spatial scales of representation s hows that only approximately 3 monitors are described as microscale, representin g an area on the order of several meters to 100 meters, and approximately 23 monito rs are described as middle scale, which represents an area on the order of 100 to 500 meters. This low percen tage of smaller spatially representative scale sites within the network of approximately 400 m onitoring sites indicates that the majority of monitors have, in fact, been sited to asse ss area-wide exposures on the neighborhood, urban, and regional scales, as would be expected for a network sited to support the current annual NO 2 standard and PAMS objectives. The current network does not include monitors placed near major roadways and, therefore, monitors in the current network do not necessarily measure the maximum concentrations that can occur on a locali zed scale near these roadways (as discussed in the next section). It should be noted that the network not only accommodates NAAQS related monitoring, but also serves other monitoring objectives suchas support for photochemistry analysis, ozone modeling and forecasting, and particulate matter precursor tracking.
2. NO 2 air quality and exposure gradients around roadways On-road and non-road mobile source s account for approximately 60% of NO x emissions (ISA, table 2.2-1) and traffi c-related exposures can dominate personal exposures to NO 2 (ISA section 2.5.4). While driving, personal exposure c oncentrations in the cabin of a vehicle could be substantially higher than ambient concentrat ions measured nearby (ISA, section 2.5.4). For example, mean in-vehicle NO 2 concentrations have been reporte d to be 2 to 3 times higher than non-traffic ambient concentrations (ISA, s ections 2.5.4 and 4.3.6). In addition, estimates presented in the REA suggest that on/near roadway NO 2 concentrations could be approximately20 40% (REA, compare Tables 7-11 and 7-13) or 80% (REA, secti on 7.3.2) higher on average than concentrations away from roadways and that roadway-associ ated environments could be responsible for the large ma jority of 1-hour peak NO 2 exposures (REA, Figur es 8-17 and 8-18). Because monitors in the current network are not sited to meas ure peak roadway-associated NO 2 concentrations, individuals who spend time on a nd/or near major roadwa ys 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 toxics, and certain size di stributions of particulate matter (PM), such as ultrafine PM, typically display peak concentrations on or immediately adjacent to roads (ISA, section 2.5). This situation typically produces a gradient in pollutant concentrations, with concentrations decreasing with increasing distance from the road, and concentrations generally decreasing back to near area-wide ambient le vels, or typical upwind urban background levels, within se veral hundred meters downwind. Wh ile this general concept is applicable to almost all roads, the actual character istics of the gradient and the distance that the mobile source pollutant signature from an individual road can be differentiated from background or upwind concentrations are heavily depende nt on factors including traffic volumes, local topography, roadside features, me teorology, and photoche mical reactivity conditions (Baldauf, et al., 2009; Beckerman et al., 2008; Clements et al ., 2008; Hagler et al., 2009 ; Janssen et al., 2001; Rodes and Holland, 1980; Roorda-Knape et al., 1998; Singer et al., 2004; Zhou and Levy, 2007).
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 canextend farther away from roadways than the primary pollutants al so emitted by on-road mobile sources. More21 specifically, review of the tec hnical 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 featur es, meteorology, and photochemical reactivity conditions (Baldauf et al., 2009; Beckerman et al., 2008; Clements et al., 2008; Gilbert et al. 2003; Rodes and Ho lland, 1980; 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 ur ban background concentrations 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 concentrations m onitored in the same ar ea but farther away from the road (ISA, Section 2.5.4; Beckerman et al ., 2008; Gilbert et al ., 2003; Rodes and Holland, 1980; Roorda-Knape et al., 1998; Sing er et al., 2004). This range of concentration gradients has implications for revising the NO 2 primary standard and for the NO 2 monitoring network (sees 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 that 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 asthmatic individuals after short-term exposures. The second was increased respiratory illness among children asso ciated with longer-term exposures to NO 2 . Evidence also was found for increased risk of em physema, 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,22 section 5.3.1). Controlled human exposure and an imal toxicological studies provided qualitative evidence for airway hyperresponsiveness and lung function changes while epidemiologic studies provided evidence for increased respirat ory symptoms with increased indoor NO 2 exposures. Animal toxicological findings of lung host defe nse system changes with NO 2 exposure provided a biologically-plausible basis for the epidemiologic results. Subpopulations considered potentially more susceptible to the effects of NO 2 exposure included persons with preexisting respiratory disease, children, and the elderly. The epidemiologic evidence for respiratory health effects was limited, and no studies had consider ed endpoints such as hospital admissions, emergency department visits, or mortality (ISA, section 5.3.1). As discussed below, evidence published since the last review generally has confirmed and extended the conclusions articulated inthe 1993 AQCD (ISA, section 5.3.2). The epidemiologic evidence has grown substantially with the addition of fi eld and panel studies, intervention studies, time-series studies of endpoints such as hospital admissions, and a substantial number of studies evaluating mort ality risk associated with short-term NO 2 exposures. While not as marked as the growth in the epidemiologic lit erature, a number of recent toxicological and contro lled human exposure studies also provide insights into relationships between NO 2 exposure and health effects. The body of evidence that has become available since the last review focuses the current review on NO 2 -related respiratory effects at lower ambient and exposure concentrations. The ISA, along with its associated annexe s, provides a comprehensive review and assessment of the scientific evidence related to the health effects associated with NO 2 exposures. For these health effects, the IS A characterized judgments about causality with a hierarchy that contains five levels (ISA, secti on 1.3): sufficient to infer a causal relationship, sufficient to infer23 a likely causal relationship (i.e., more likely than not), suggestive but not sufficient to infer a causal relationship, inadequate to infer the presence or absence of a causal relationship, and suggestive of no causal relationship. Judgments a bout causality were informed by a series of aspects that are based on those se t forth by Sir Austin Bradford Hill in 19 65 (ISA, Table 1.3-1). These aspects include strength of the observed asso ciation, availability of experimental evidence, consistency of the observed asso ciation, biological pl ausibility, coherence of the evidence, temporal relationship of the obs erved association, and the pr esence of an exposure-response relationship. A summary of each of the five levels of the hierar chy is provided in Table 1.3-2 of the ISA. Judgments made in the ISA about the extent to which relationships between various health endpoints and exposure to NO 2 are likely causal have been informed by several factors. As discussed in the ISA in section 1.3, these factors include the nature of the evidence ( i.e. , controlled human exposure, epid emiological, and/or t oxicological studies) and the weight of evidence. The weight of evidence takes into account such considerations as biological plausibility, coherence of the evid ence, strength of associations, a nd consistency of the evidence. Controlled human exposure studies provide dir ectly applicable inform ation for determining causality because these studies are not limited by differences in dosimetry and species sensitivity, which would need to be addressed in extrapolating animal toxicology data to human health effects, and because they provide da ta relating health effects specifically to NO 2 exposures, in the absence of the co-occurring pollut ants present in ambient air. Epidemiologic studies provide evidence of associations between NO 2 concentrations and more serious health endpoints ( e.g. , hospital admissions and emergency departme nt visits) that cannot be assessed in controlled human exposure studies . For these studies the degree of uncertainty introduced by24 confounding variables (e.g., other pol lutants) affects the level of confidence that the health effects being investigated are attributable to NO 2 exposures alone and/or in combination with cooccurring pollutants. In using a weight of evidence approach to inform judgments about the degree of confidence that various health effects ar e likely to be caused by exposure to NO 2 , confidence increases with the number of st udies consistently reporting a particular health endpoint, with increasing support for the biological plausibility of the health eff ects, and with the strength and coherence of the evidence. Conclusions regard ing biological plausibi lity, consistency, and coherence of evidence of NO 2 -related health effects are dr awn from the integration of epidemiologic studies with controlled human e xposure studies and with mechanistic information from animal toxicological studies. As discussed below, the weight of evidence is strongest for respiratory morbidity endpoints (e.g., respirator y symptoms, hospital admissions, and emergency department visits) associated with short-term (e.g., 1 to 24 hours) NO 2 exposures. For epidemiologic studies, strength of association refers to the magnitude of the association and its statistical stre ngth, which includes assessment of both effect estimate size and precision. In general, when associ ations yield large relative risk es timates, it is less likely that the association could be completely accounted for by a potential confounder or some other bias. Consistency refers to th e persistent finding of an associati on between exposure and outcome in multiple studies of adequate power in differen t persons, places, circumstances and times. Based on the information presented in the ISA and su mmarized below in sections II.B.1–II.B.3, this section discusses judgments concerning the extent to which relationships between various health endpoints and ambient NO 2 exposures have been judged in the ISA to be likely causal. As noted above, this section is devoted to di scussion of health effects associated with25 NO 2 exposure, as assessed in the ISA. Secti on II.B.1 below discusses respiratory morbidity associated with shor t-term exposure to NO 2 . The specific endpoints c onsidered in this section are respiratory-related emergency department visits and hospital admissions, respiratory symptoms, lung host defense and immunity, airway responsivene ss, airway inflammation, and lung function. Section II.B.2 discusses mortality and cardiovascular ef fects associated with short-term exposures. Section II.B.3 discusses eff ects that have been asso ciated with long-term
NO 2 exposures including re spiratory morbidity, mortality, cancer, cardiovascular effects, and reproductive/developmental effects. Section II.B.4 discusses the potential NO 2 -related impacts on public health. 1. Adverse respiratory effects and short-term exposure to NO 2 The ISA concluded that, taken together, recent st udies provide scientific evidence that is sufficient to infer a likely causal relationship between short-term NO 2 exposure and adverse effects on the respiratory system (ISA, sec tion 5.3.2.1). This determination was based on consideration of the broad array of relevant sc ientific evidence, as well as the uncertainties associated with that evidence. Specifically, this determination is supported by the large body of recent epidemiologic evidence as we ll as findings from hu man and animal experimental studies. 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 respir atory effect s or if NO 2 is a marker for the effect s of another traffic-related pollutant or mix of pollutants. ” On-road vehicle exhaust emi ssions are a nearly ubiquitous source of combustion pollutant mixtures that include NO x and can be an important contributor to
NO 2 levels in near-road locations . Although this complicates e fforts to quantify specific NO 2 - related health effects, a number of epidemiologi c studies have evaluate d associations with NO 226 in models that also include co-o ccurring pollutants such as PM, O 3 , CO, and/or SO 2 . The evidence summarized in the ISA indicates that NO 2 associations generally remain robust in these multi-pollutant models and supports a direct effect of short-term NO 2 exposure on respiratory morbidity. The plausibility a nd coherence of these effects are also supported by epidemiologic studies of indoor NO 2 as well as experimental (i.e., toxi cologic and controlled human exposure) studies that have evaluated host defense and immune system changes, airway inflammation, and airway responsiveness (see subsequent sections of this proposal and th e ISA, section 5.3.2.1). The ISA (section 5.4) concluded that the robustness of epidemiologic findi ngs to adjustment for co-pollutants, coupled with data from animal and human e xperimental 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 re lated pollutants. The epidemiologic and experime ntal studies encompass a num ber of endpoints, including emergency department visits and hospita lizations, respiratory symptoms, airway hyperresponsiveness, airway inflammation, a nd lung function. E ffect estimates from epidemiologic studies conducted in the United St ates and Canada generally indicate a 2-20% 6 increase in risks for emergency department visi ts and hospital admissions and higher risks for respiratory symptoms (ISA, section 5.4). The findings relevant to these endpoints, which provide the rationale to support the judgment of a likely causal relationship, are described in more detail below. a. Emergency department visi ts and hospital admissions Epidemiologic evidence exists for positive associations of short-term ambient NO 2 concentrations below the current NAAQS with increased numbers of emergency department

6 Effect estimates in the ISA were standardized to a 30 ppb increase in NO 2 for studies that evaluated 1-hour daily maximum NO 2 concentrations and to a 20 ppb increase for studies that evaluated 24-hour average concentrations.27 visits and hospital admissions fo r respiratory causes, especially asthma (ISA, section 5.3.2.1). Total respiratory causes for emergency department visits and hospitalizations typically include asthma, bronchitis and emphysema (collectively referred to as COPD), pneumonia, upper and lower respiratory infections, and other minor categories. Te mporal associations between respiratory emergency department visits or hospital admissions and ambient levels of NO 2 have been the subject of over 50 peer-reviewed res earch publications since the review of the NO 2 NAAQS that was completed in 1996 . These studies have examined morbidity in different age groups and have often utilized multi-pollutant models to eval uate potential confounding effects of co-pollutants. Associations are particularly consistent amo ng children (< 14 years) and older adults (> 65 years) when all re spiratory outcomes are analyzed t ogether (ISA, Figures 3.1-8 and 3.1-9) and among children and subjects of all ag es for asthma admissions (ISA, Figures 3.1-12 and 3.1-13). When examinedwith co-pollutant mode ls, associations of NO 2 with respiratory emergency department visits and hospital admissions were genera lly robust and independent of the effects of co-pollutants (i.e ., magnitude of effect estimates remained relatively unchanged) (ISA, Figures 3.1-10 and 3.1-11). Th e plausibility and coherence of these effects are supported by experimental (i.e., toxicol ogic and controlled human exposur e) studies that evaluate host defense and immune system ch anges, airway inflammation, an d airway responsiveness (see subsequent sections of this docum ent and ISA, section 5.3.2.1). Of the respiratory emergency department visit and hospital admission studies reviewed in the ISA, 6 key studies were conducted in the Un ited States (ISA, Tabl e 5.4-1). Of these 6 studies, 4 evaluated associations with NO 2 using multi-pollutant m odels (Peel et al., 2005 and updated in Tolbert et al., 2007 in Atlanta; New York Department of Health (NYDOH), 2006 and Ito et al., 2007 in New York City), while 2 stud ies evaluated only single pollutant models (Linn28 et al., 2000 in Los Angeles; Jaffe et al., 2003 in Cl eveland/Cincinnati, OH). In the study by Peel and colleagues, investigators evaluated respirator y emergency department visits among all ages in Atlanta, GA during the period from 1993 to 2000. Using single pollutant models, a 2.4% (95% CI: 0.9%, 4.1%) increase in respiratory emerge ncy department visits was associated with a 30-ppb increase in 1-hour maximum NO 2 concentrations. For asth ma visits, a 4.1% (95% CI: 0.8%, 7.6%) increase was estimated in individuals 2 to 18 years of age. Tolbert and colleagues reanalyzed these data with 4 additional years of information and found e ssentially similar results in single pollutant models (2.0% increase, 95% CI: 0.5%, 3.3%). This same study found that the associations were positive, but not statistically significant, in multi- pollutant models that included PM 10 or O 3 (Figure 2 in published manuscript). In the study conduc ted by the NYDOH, investigators evaluated asthma emergency department visits in Bronx and Manhattan, New York over the period of January 1999 to November 2000. In Bronx, a 6% (95% CI: 1%, 10%) increase in visits was estimated per 20 ppb in crease in 24-hour average concentrations of NO 2 and a 7% (95% CI: 2%, 12%) increase in visits was estimated per 30 ppb increase in daily 1-hour maximum concentrations. These effects were not statistically significant in 2-pollutant models that included PM 2.5 or SO 2 (Tables 4a and 9 in manuscript) . In Manhattan, the authors found non-significant decreases (3% for 24-hour and a 2% for daily 1-hour maximum) in asthmarelated emergency department visi ts associated with increasing NO 2 . In the study by Ito and colleagues (2007), investigators evaluated respirator y emergency department visits for asthma in New York City during the year s 1999 to 2002. A 12% (95% CI: 7%, 15%) increase in risk was estimated per 20 ppb increase in 24-hour ambient NO 2 . Risk estimates were robust and remained statistically significant in multi-po llutant models that included PM 2.5 , O 3 , CO, and SO 2 (figure 8 in manuscript). With regard to the studies that evaluated only single pollu tant models, Linn et al.29 (2000) detected a statistically si gnificant increase in respiratory hospital admissions and Jaffe et al. (2003) detected a positive, but not statistically significant, in crease in respiratory emergency department visits asso ciated with 24-hour NO 2 concentrations. b. Respiratory symptoms Evidence for associations between NO 2 and respiratory symptoms is derived primarily from the epidemiologic literature, although the e xperimental evidence for airway inflammation and immune system effects (described in th e ISA, section 3.1) does provide support for the plausibility and coherence fo r the epidemiologic results (IS A, section 5.3.2.1). Consistent evidence has been observed for an association of respiratory effects with indoor and personal
NO 2 exposures in children (ISA, sections 3.1.5.1 and 5.3.2.1) and with ambient levels of NO 2 , as measured by area-wide monitors (ISA, s ections 3.1.4.2 and 5.3.2.1, see Fi gure 3.1-6). In the results of multi-po llutant models, NO 2 associations in multicity studies are generally robust to adjustment for co-pollutants including O 3 , CO, and PM 10 (ISA, sections 3.1.4.3, 5.3.2.1 and Figure 3.1-7). Specific studi es of respiratory symptoms are disc ussed in more detail below. Epidemiologic studies using community am bient monitors have found associations between ambient NO 2 concentrations and respiratory symptoms (ISA, sections 3.1.4.2 and 5.3.2.1, Figure 3.1-6) in cities where the entire range of 24-hour average NO 2 concentrations were well below the level of the current NAAQS (0.053 ppm annual average). Several studies have been published since the last review in cluding single-city studies (e.g., Ostro et al., 2001; Delfino et al., 2002) and multicity studies in ur ban areas covering the continental United States and southern Ontario (Schwartz et al., 1994; Mort imer et al., 2002; Schild crout et al., 2006). Schwartz et al. (1994) studied 1,844 schoolchild ren, followed for 1 year, as part of the Six Cities Study that included the cities of Wa tertown, MA, St. Louis, MO, Kingston-Harriman,30 TN, Steubenville, OH, Topeka, KS , and Portage, WI. Respiratory symptoms were recorded daily. The authors reported a signifi cant association between 4-day mean NO 2 levels and incidence of cough among all children in single- pollutant models, with an odds ratio (OR) of 1.61 (95% CI: 1.08, 2.43) standardized to a 20-ppb increase in NO 2 . The incidence of cough increased up to approximately mean NO 2 levels (~13 ppb) (p = 0.01), after which no further increase was observed. The significant a ssociation between cough and 4-day mean NO 2 level remained unchanged in models that included O 3 but lost statistical sign ificance in two-pollutant models that included PM 10 (OR = 1.37 [95% CI: 0.88, 2.13]) or SO 2 (OR = 1.42 [95% CI: 0.90, 2.28]). Mortimer et al. (2002) studi ed the risk of asthma symptoms among 864 asthmatic children in New York City, NY, Washington, DC, Cleveland, OH, Detroit, MI, St Louis, MO, and Chicago, IL. Subjects were followed daily fo r four 2-week periods over the course of nine months with morning and eveni ng asthma symptoms and peak flow recorded. The greatest effect was observed for morning symptoms using a 6-day moving av erage, with a reported OR of 1.48 (95% CI: 1.02, 2.16) per 20 ppb increase in NO 2 . Although the magnitudes of effect estimates were generally robust in multi-pollutant models that included O 3 (OR for 20-ppb increase in NO 2 = 1.40 [95% CI: 0.93, 2.09]), O 3 and SO 2 (OR for NO 2 = 1.31 [95% CI: 0.87, 2.09]), or O 3 , SO 2 , and PM 10 (OR for NO 2 = 1.45 [95% CI: 0.63, 3.34 ]), they were not statistically significant. Schildcrout et al. (2006) investigated the association between ambient NO 2 and respiratory symptoms and rescue inhaler use as part of the Childhood Asthma Management Program (CAMP) study. The study reported on 990 asthmatic children living with in 50 miles of
an NO 2 monitor in Boston, MA, Ba ltimore, MD, Toronto, ON, St . Louis, MO, Denver, CO,31 Albuquerque, NM, or San Diego, CA. Symptoms a nd use of rescue medication were recorded daily, resulting in each subject having an averag e of approximately two months of data. The authors reported the strongest association between NO 2 and increased risk of cough for a 2-day lag, with an OR of 1.09 (95% CI: 1.03, 1.15) for each 20-ppb increase in NO 2 occurring 2 days before measurement. Multi-pollu tant models that included CO, PM 10 , or SO 2 produced similar results (ISA, Figure 3.1-5, panel A). Additionally, increased NO 2 exposure was associated with increased use of rescue medication, with the stro ngest association for a 2-day lag. In the singlepollutant model, the rela tive risk (RR) for increased inha ler usage was 1.05 (95% CI: 1.01, 1.09). Evidence supporting increased respiratory symptoms following NO 2 exposures is found in studies focused on indoor sources of NO 2 (ISA, section 3.1.4.1). These studies are not confounded by the same mix of co-pollutants presen t in the ambient air or by the contribution of
NO 2 to the formation of secondary particles or O 3 (ISA, section 3.1.4.1). Specifically, in a randomized intervention study in Australia (Pilotto et al., 200 4), asthmatic students attending schools that switched out unvented gas h eaters, a major source of indoor NO 2 , experienced a decrease in both levels of NO 2 and in respiratory symptoms (e.g., difficulty breathing, chest tightness, and asthma attacks) compared to st udents in schools that did not switch out unvented gas heaters (ISA, section 3.1.4.1). An earlier indoor study by Pilotto and colleagues (1997) also found that students in classroo ms with higher levels of NO 2 due primarily to indoor sources had higher rates of respiratory sympto ms (e.g., sore throat, cold) and absenteeism than students in classrooms with lower levels of NO 2 . This study detected a si gnificant concentration-response relationship, strengthening the argument that NO 2 is causally related to respiratory morbidity. A number of other indoor studies conducted in homes with gas app liances have also detected significant associations between indoor NO 2 and respiratory symptoms (ISA, section 3.1.4.1).32 c. Impaired host defense Impaired host-defense systems and increased risk of susceptibility to both viral and bacterial infections after NO 2 exposures have been observed in epidemiologic, controlled human exposure, and animal toxicological studi es (ISA, section 3.1.1 and 5.3.2.1). A recent epidemiologic study (Chauhan et al., 2003) provides evidence that increased personal exposure
to NO 2 worsened virus-associated symptoms and decreased lung function in children with asthma. The limited evidence from controlled human exposure studies indicates that NO 2 may increase susceptibility to lung inju ry by subsequent viral challenge at exposures of as low as 600 ppb for 3 hours in healthy adults (Frampton et al., 2002). Toxicol ogical studies have shown that lung host defenses, including mucociliary clear ance and immune cell function, are sensitive to
NO 2 exposure, with effects observed at concentra tions of less than 1000 ppb (ISA, section 3.1.7). When taken together, epidemiologic and experimental studies linking NO 2 exposure with viral illnesses provide coherent and consistent evidence that NO 2 exposure can result in lung host defense or immune system eff ects (ISA, sections 3.1.7 and 5.3.2.1). This group of outcomes also provides some plausibility for other respiratory sy stem effects. For exam ple, effects on ciliary action (clearance) or immune cell function (i.e. macrophage phagocyt osis) could be the basis for the effects observed in epidemiologic studi es, including increased respiratory illness or respiratory symptoms (ISA, section 5.3.2.1) . Proposed mechanisms by which NO 2 , in conjunction with viral in fections, may exacerbate airway sy mptoms are summarized in the ISA (Table 3.1-1). d. Airway response In acute exacerbations of asthma, bronchial sm ooth muscle contraction occurs quickly to narrow the airway in response to exposure to vari ous stimuli including alle rgens or irritants.33 Bronchoconstriction is the dominant physiological event leading to clinical symptoms and interference with airflow (Nati onal Heart, Lung, and Blood Institut e, 2007). Inhaled pollutants such as NO 2 may enhance the inherent responsiveness of the airway to a ch allenge by allergens and nonspecific agents (ISA, section 3.1.3). In the laboratory, airway resp onses can be measured by assessing changes in pulmonary function (e.g., decline in FEV1) or changes in the inflammatory response (e.g., using markers in br onchoalveolar lavage (B AL) fluid or induced sputum) (ISA, section 3.1.3). The ISA (section 5.3.2.1) drew two broad conclu sions regarding airway responsiveness in asthmatics following NO 2 exposure. First, the ISA concluded that NO 2 exposure may enhance the sensitivity to allergen-induced decrements inlung function and increase the allergen-induced airway inflammatory response at exposures as low as 260 ppb NO 2 for 30 minutes (ISA, section 5.3.2.1 and Figure 3.1-2). Second, exposure to NO 2 has been found to enhance the inherent responsiveness of the airway to subsequent nons pecific challenges in controlled human exposure studies (section 3.1.3.2). In gene ral, small but significant in creases in nonspecific airway responsiveness were observed inthe range of 200 to 300 ppb NO 2 for 30-minute exposures and at 100 ppb NO 2 for 60-minute exposures in asthmatics. These conclusions are consistent with results from animal toxicological studies which have detected 1) increased immune-mediated pulmonary inflammation in rats exposed to house dust mite allergen following exposure to 5000 ppb NO 2 for 3-h and 2) increased responsivene ss to non-specific challenges following subchronic (6-12 weeks) expos ure to 1000 to 4000 ppb NO 2 (ISA, section 5.3.2.1). Enhanced airway responsiveness could have important clinical implications for asthmatics since transient increases in airway responsiveness following NO 2 exposure have the potential to increase symptoms and worsen asthmacontrol (ISA, section 5.4). In addition, the34 ISA cited the controlled human exposure literature on the NO 2 airway response as being supportive of the epidemiologic evidence on resp iratory morbidity (ISA, section 5.4). Because studies on airway responsiveness have been used to identify potential health effect benchmark values and to inform the identification of potenti al alternative standards for evaluation (see REA, sections 4.5 and 5), more detail is provided below on the specific st udies that form the basis for the conclusions in the ISA regarding this endpoint. Folinsbee (1992) conducted a meta-analysis using individual level data from 19 NO 2 controlled human exposure studies measuring airway responsivene ss in asthmatics (ISA, section 3.1.3.2). These studies included NO 2 exposure levels between 100 and 1000 ppb and most of them used nonspecific bronchoconstricting agents su ch as methacholine, carbachol, histamine, or cold air. The largest effects were observed for asthmatics at rest. Am ong asthmatics exposed at rest, 76% experienced increased airway responsiveness following exposure to NO 2 levels between 200 and 300 ppb. Results from an update of this meta-analysis (results combined only from nonspecific responsiveness studies) are presen ted in the ISA (Table 3.1-3). When exposed at rest, 66% of asthmatics experienced an incr ease in airway responsiveness following exposure to 100 ppb NO 2 , 67% of asthmatics experienced an incr ease in airway responsiveness following exposure to NO 2 concentrations between 100 and 150 ppb (inclusively), 75% of subjects experienced an increase in airway responsiveness following exposure to NO 2 concentrations between 200 and 300 ppb (inclusively ), and 73% of subjects experi enced an increase in airway responsiveness following exposure to NO 2 concentrations above 300 ppb. Effects of NO 2 exposure on the direction of airway responsiveness were statistically significant at all of these levels. Because this meta-analysis evaluated only the direction of the change in airway responsiveness, it is not possible to discern th e magnitude of the change from these data.35 However, the results do suggest that short- term (i.e., 30-min to 3-h) exposures to NO 2 at nearambient levels (<300 ppb) can a lter airway responsiveness in pe ople with mild asthma (ISA, section 3.1.3.2). Several studies published since the 1996 revi ew evaluate the pot ential for low-level exposures to NO 2 to enhance the response to specific alle rgen challenge in mild asthmatics (ISA, section 3.1.3.1). These studies suggest that NO 2 may enhance the sensitivity to allergen-induced decrements in lung function and increase the alle rgen-induced airway inflammatory response. Strand et al. (1997) demons trated that single 30-min ute exposures to 260 ppb NO 2 increased the late phase response to allergen challenge 4 hours after exposure, as measured by changes in lung function. In a separate stud y (Strand et al., 1998), 4 daily repeated exposures to 260 ppb NO 2 for 30 minutes increased both the early and late-phase responses to allergen, as measured by changes in lung function. Barck et al. (2002) used the same exposure a nd challenge protocol in the earlier Strand study (260 ppb for 30 min, with a llergen challenge 4 hours after exposure), and performed BAL 19 hours after the alle rgen challenge to determine NO 2 effects on the allergeninduced inflammatory response. Compar ed with air follow ed by allergen, NO 2 followed by allergen caused an increase in the BAL r ecovery of polymorphonuclear (PMN) cells and eosinophil cationic protein (ECP) as well as a reduction in total BAL fluid volume and cell viability. ECP is released by degranulating eosinophils, is toxi c to respiratory epithelial cells, and is thought to play a role in the pathogenesis of airway injury in asth ma. Subsequently, Barck et al. (2005) exposed 18 mild asthmatics to air or 260 ppb NO 2 for 15 minutes on day 1, followed by two 15 minute exposures separated by 1 hour on day 2, with allergen challenge after exposures on both days 1 and 2. Sputum was induced before exposure on day 1 and after exposures (morning of da y 3). Compared to air plus allergen, NO 2 plus allergen resulted in36 increased levels of ECP in both sputum and bl ood and increased myeloperoxidase levels in blood. All exposures in these studies (Barck et al., 2002, 2005; Strand et al., 1997, 1998) used subjects at rest. They used an adequate numb er of subjects, included air control exposures, randomized exposure order, and separated exposures by at least 2 w eeks. Together, they indicate the possibility for effects on allergen respons iveness in some asthmatics following brief exposures to 260 ppb NO 2 . Other recent studies have failed to find effects using similar, but not identical, approaches (ISA, section 3.1.3.1). The differing findings may relate in part to differences in timing of the allergen challenge, the use of multiple versus single-dose allergen challenge, the use of BAL versus sputum inducti on, exercise versus rest during exposure, and differences in subject susceptib ility (ISA, section 3.1.3.1). e. Airway inflammation Effects of NO 2 on airway inflammation have beenobserved in controlled human exposure and animal toxicologi cal studies at higher than ambient levels (400-5000 ppb). Controlled human exposure studies provide evid ence for increased airway inflammation at NO 2 concentrations of <2000 ppb. The onset of inflam matory responses in healthy subjects appears to be between 100 and 200 ppm-minutes, i.e., 1000 ppb for 2 to 3 hours (ISA, Figure 3.1-1). Increases in biological marker s of inflammation were not obser ved consistently in healthy animals at levels of less than 5000 ppb; however , increased susceptibility (as indicated by biochemical markers of inflammation) to NO 2 concentrations of as low as 400 ppb was observed when lung vitamin C was reduced (by diet) to le vels that were <50% of normal. The few available epidemiologic studies were sugges tive of an association between ambient NO 2 concentrations and inflammatory response in the airway in ch ildren, though the associations37 were inconsistent in the adu lt populations examined (ISA, section 3.1.2 and 5.3.2.1). These data provide some evidence for biological plausibi lity and one potential mechanism for other respiratory effects, such as exacerbation of asthma sympto ms and increased emergency department visits for asth ma (ISA, section 5.3.2.1). f. Lung function Recent epidemiologic studies that examin ed the association between ambient NO 2 concentrations and lung function inchildren and adults have produ ced inconsistent results (ISA, sections 3.1.5.1 and 5.3.2.1). Controlle d human exposure studies gene rally did not find direct effects of NO 2 on lung function in healthy adults at levels as high as 4000 ppb (ISA, section 5.3.2.1). For asthmatics, the direct effects of NO 2 on lung function also have been inconsistent at exposure concentrations of less than 1000 ppb NO 2 . g. Conclusions from the ISA As noted previously, the ISA concluded that the findings of epidemiologic, controlled human exposure, and animal toxicological studies pr ovide evidence that is sufficient to infer a likely causal relationship for respiratory effects following short-term NO 2 exposure (ISA, sections 3.1.7 and 5.3.2.1). The ISA (section 5.4) c oncluded that the strongest evidence for an association between NO 2 exposure and adverse human health effects comes fro m epidemiologic studies of respiratory symptoms , emergency department visits, and hospital admissions. These studies include panel and field studies, studies that control for the e ffects of co-occurring pollutants, and studies conducted in areas wher e the whole distributi on of ambient 24-hour average NO 2 concentrations was below the current NAAQS level of 53 ppb (annual average). With regard to this eviden ce, the ISA concluded that NO 2 epidemiologic studi es provide “little evidence of any effect threshold” (ISA, section 5.3.2.9, p. 5-15). In studies that have evaluated38 concentration-response relationships, they appear linear within the observed range of data (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. Considering this large body of epidemiologic studies alone, the findings have also been char acterized as coherent inthat the studies report associations with respiratory health outcomes th at are logically linked together. In addition, a number of these associations ar e statistically significan t, particularly the more precise effect estimates (ISA, section 5.3.2.1). These epidemio logic studies are supported by evidence from toxicological and controlled human exposure studie s, particularly those that evaluated airway hyperresponsiveness in asthmatic individuals (ISA, section 5.4). The ISA concluded that together, the epidemiologic and expe rimental data sets form a plau sible, consistent, and coherent description of a relationship between NO 2 exposures and an array of adverse respiratory health effects that range from the ons et of respiratory symptoms to hospital admissions. 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, section 5.3.2.3). Results fr om several large U.S. and European multicity studies and a meta-analysi s study indicate positive associations between ambient NO 2 concentrations and the risk of all-caus e (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 3.3-2,39 section 5.3.2.3). In general, the NO 2 effect estimates were robust to adjustment for co-pollutants. Both cardiovascular and respiratory mortalit y have been associated with increased NO 2 concentrations in epidemiologic studies (ISA, Fi gure 3.3-3); however, similar associations were observed for other pollutant s, including PM and SO 2 . The range of risk estimates for excess mortality is generally smaller th an that for other pollutants such as PM. In addition, while NO 2 exposure, alone or in conjunction with other pollutants, may contri bute to increased mortality, evaluation of the specificity of th is effect is difficult. Clin ical studies showing hematologic effects and animal toxi cological studies showing biochemi cal, lung host defense, permeability, and inflammation changes with short-term exposures to NO 2 provide limited evidence of plausible pathways by which risks of mortalit y 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 following short-term exposure to NO 2 is inadequate to infer the presen ce or absence of a causal relationship at this time (ISA, section 5.3.2.2). Evidence from ep idemiologic studies of h eart rate variability, repolarization changes, and cardi ac rhythm disorders among heart pa tients with ischemic cardiac disease are inconsistent (ISA, section 5.3.2.2). In most studies, associati ons 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 insingle-pollutant mode ls (ISA, section 5.3.2.2); however, most of these effect es timate values were diminished inmulti-pollutant models that also contained CO and PM indi ces (ISA, section 5.3.2.2). Mechanistic evidence of a role for
NO 2 in the development of cardiova scular diseases from studies of biomarkers of inflammation,40 cell adhesion, coagulation, and th rombosis is lacking (ISA, s ection 5.3.2.2). Furthermore, the effects of NO 2 on various hematological parameters inanimals are inconsistent and, thus, provide little biologi cal plausibility 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 epidemiologic and experimental evidence is suggestive, but not sufficie nt, to infer a causal relationship between long-term NO 2 exposure and respiratory morbidity (ISA, sec tion 5.3.2.4). The available databa se evaluating the relationship between respiratory illness in children and long-term exposures to NO 2 has increased since the 1996 review of the NO 2 NAAQS. A number of epidemiologi c studies have examined the effects of long-term exposure to NO 2 and reported positive associations with decrements in lung function and partially irreversible decrements in lung function gr owth (ISA, section 3.4.1, Figures 3.4-1 and 3.4-2). Specifically, results from the California-bas ed Children’s Health Study, which evaluated NO 2 exposures in children over an 8-y ear period, demonstrated deficits in lung function growth (Gauderman et al., 2004). This effect has also been observed in Mexico City, Mexico (Rojas-Martinez et al., 2007a,b) and in Oslo, Norway (Oftedal et al., 2008), with decrements ranging from 1 to 17.5 ml per 20- ppb increase in annual NO 2 concentration. Similar associations have been found for PM, O 3 , and proximity to traffic (<500 m), though these studies did not report the results of co -pollutant models. The high co rrelation among traffic-related pollutants makes it di fficult to accurately estimate inde pendent effects in these long-term exposure studies (ISA, section 5.3.2.4). With regard to asth ma incidence and long-term NO 2 , two major cohort studies, the Children’s Health Study (Gauderman et al., 2005) and a birth41 cohort study in the Netherlands (Brauer et al., 2007), observed significant associations. However, several other studies failed to find consistent associations between long-term NO 2 exposure and asthma outcomes (ISA, section 5.3.2.4). Similarly, epidemiologic studies conducted in the United States and Europe reported inconsistent results re garding an association between long-term exposure to NO 2 and respiratory symptoms (ISA, sections 3.4.3 and 5.3.2.4). While some positive associations were note d, a large number of symptom outcomes were examined and the results across specific outco mes were inconsistent (ISA, section 5.3.2.4). Animal toxicological studies ma y provide biological plausibili ty for the chronic effects of
NO 2 that have been observed in epidemiologi c studies (ISA, secti ons 3.4.5 and 5.3.2.4). The main biochemical targets of NO 2 exposure appear to be antioxida nts, membrane polyunsaturated fatty acids, and thiol groups. NO 2 effects include changes in oxi dant/antioxidant homeostasis and chemical alterations of li pids and proteins. Lipid per oxidation has been observed at NO 2 exposures as low as 40 ppb for 9 months and at exposures of 1200 ppb for 1 week, suggesting lower effect thresholds with l onger durations of exposure. Othe r studies showed decreases in formation of key arachidonic acid metabolites in alveol ar macrophages following NO 2 exposures of 500 ppb. NO 2 has been shown to increase collagen synthesis rates at concentrations as low as 500 ppb. This could indicate increas ed total lung collagen, which is associated with pulmonary fibrosis, or increased collagen turnover, which is associated with remodeling of lung connective tissue. Morphological eff ects following chronic NO 2 exposures have been identified in animal studies that link to thes e increases in collagen synthesis and may provide plausibility for the deficits in lung function growth described in epidemiologic st udies of long-term exposure to NO 2 (ISA, section 3.4.5). b. Mortality42 The ISA concluded that the epidemiologic evid ence 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 cohort studies exa mining the relationship between long-term exposure to NO 2 and mortality, results have beeninconsistent (ISA, section 5.3.2.6). Further, when associations were su ggested, 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 inte rpret these observed 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 develo pmental 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 toxico logical 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 eff ects (ISA, section 5.3.2.5). The epidemiologic evidence is not cons istent for associations between NO 2 exposure and fetal growth retardation; however, some evidence is accumulating for effect s 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 inform ation or biological plausibility for the epidemiologic findings.

4. NO 2 -related impacts on public health43 Specific groups within the general population are likely at increased risk for suffering adverse effects from NO 2 exposure. This could occur because they are affected by lower levels of NO 2 than the general population (susceptibility), because they experience a larger health impact than the general population to a given level of exposure (susceptibility), and/or because they are exposed to higher levels of NO 2 than the general population (v ulnerability). The term susceptibility generally encompasses innate (e.g. , genetic or developmental) and/or acquired (e.g., age or disease) factors th at make individuals more like ly to experience effects with exposure to pollutants. The seve rity of health effects experien ced by a susceptible subgroup may be much greater than that experienced by the pop ulation at large. Factors that may influence susceptibility to the effects of air pollution incl ude age (e.g., infants, children, elderly); gender; race/ethnicity; genetic factors; and pre-exis ting disease/condition (e.g., obesity, diabetes, respiratory disease, asthma, chronic obstructi ve 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 population (ISA, sec tion 4.3.6). Factors that may influence exposures and/or susceptibility to ai r pollution include socio economic status (SES), education level, air conditioni ng use, proximity to roadways , geographic location, level of physical activity, and work environment (e.g., in door versus outdoor) (ISA, section 4.3.5). The ISA discussed factors that can c onfer susceptibility and/or vulne rability to air pollution with most of the discussion devot ed to factors for which NO 2 -specific evidence exists (ISA, section 4.3). These factors ar e discussed below. a. Pre-existing disease

A number of health conditions have been f ound to put individuals at greater risk for44 adverse events following exposure to air polluti on. In general, these include asthma, COPD, respiratory infection, cardiac conduction disorders, congestive heart failure (CHF), diabetes, past myocardial infarction (MI), obesity, coronary arte ry disease, low birth weight/prematurity, and hypertension (ISA, sections 4.3.1, 4.3.5, and 5.3.2.9). In addition to these conditions, epidemiologic evidence indicates that individuals with bronchial or airw ay hyperresponsiveness, as determined by methacholine provocation, ma y be at increased risk for experiencing respiratory symptoms (ISA, s ection 4.3.1). In considering NO 2 specifically, the ISA evaluated studies on asthmatics, individuals with cardiopulmonary disease, and diabetics (ISA, sections 4.3.1.1 and 4.3.1.2). These groups are discussed in more detail below. Epidemiologic and controlled human exposur e studies, supported by animal toxicology studies, have provided evidence for associations between NO 2 exposure and respiratory effects in asthmatics (ISA, section 4.3.1.1). The ISA found evidence from epidemiologic studies for an association between ambient NO 2 and children’s hospital admi ssions, emergency department visits, and calls to doctors for asthma. Long-term NO 2 exposure was associated with aggravation of asthma effects that include symptoms, medication use, and lung function. Time-series studies demonstrated a relationship in children between hospital admissions or emergency department visits for asthma and ambient NO 2 levels, even after adjusting fo r co-pollutants such as PM and CO (ISA, section 4.3.1.1). Impor tant evidence was available fro m epidemiologic studies of indoor NO 2 exposures. Recent studies have shown associ ations with asthma attacks and severity of virus-induced asthma (ISA, section 4.3.1.1). In addition, in controlled human exposure studies, airway hyperresponsiveness in asthma tics occurred following exposure to ambient or near-ambient NO 2 concentrations (ISA, sections 5.3.2.1- 5.3.2.6). Compared to asthma, less evidence is available to support cardiovascular disease as a mediator of susceptibility to NO 2 .45 However, recent epidemiologic studies report th at individuals with preexisting conditions (e.g., including diabetes, CHF, prior MI) may be at incr eased risk for adverse cardiac health events associated with ambient NO 2 concentrations (ISA, secti on 4.3.1.2). The small number of controlled human exposure and animal toxicologi cal studies that have evaluated cardiovascular endpoints provide only limited supporting evidence for susceptibility to NO 2 in persons with cardiovascular disease (ISA, section 4.3.1.2). b. Age The ISA identified infants, children (i.e., < 18 years of age), and older adults (i.e., >65 years of age) as groups that are potentially more susceptible than the general population to the health effects associated with ambient NO 2 concentrations (ISA, section 4.3.2). The ISA found evidence that associations of NO 2 with respiratory emergenc y department visits and hospitalizations were stronger among children and older adults, though not all studies had comparable findings on this issue (ISA, secti on 4.3.2). In addition, long-term exposure studies suggest effects in children that include impair ed lung function growth, increased respiratory symptoms and infections, and onset of asthma(ISA, section 3.4 and 4.3.2) . In some studies, associations between NO 2 and hospitalizations or emergency department visits for CVD have been observed in elderly populations. Among st udies that observed positive associations between NO 2 and mortality, a comparison indicated th at, in general, the elderly population was more susceptible than th e non-elderly population to NO 2 effects (ISA, section 4.3.2).

c. Genetics As noted in the ISA (section 4.3.4), genetic factors related to health outcomes and ambient pollutant exposures merit consideration. Several criteria should be satisfied in selecting and establishing useful links between polymorphis ms in candidate genes and adverse respiratory46 effects. First, the candidate gene must be significantly involved in the pathogenesis of the adverse effect of interest. Second, polymorphism s in the gene must produce a functional change in either the protein product or in the level of expression of the protein. Third, in epidemiologic studies, the issue of confounding by other environmental exposures must be carefully considered (ISA, section 4.3.4). Investigation of genetic susceptibility to NO 2 effects has focused on the glutathione S-tranferase (GST) gene. Several GST genes have common, functionally-important alleles that affect host defense in the lung (I SA, section 4.3.4). GST genes are inducible by electrophilic species (e.g., reactive oxygen species) and individuals with genotypes that result in enzymes with reduced or absent peroxidase activ ity are likely to have reduced defenses against oxidative insult. This could potentially result in increased susceptibility to inhaled oxidants and radicals. However, data on genetic susceptibility to NO 2 are only beginning to emerge and, while it remains plausible that there are genetic factors that can influence health responses to
NO 2 , the few available studies do not provide speci fic support for genetic susceptibility to NO 2 exposure (ISA, section 4.3.4).

d. Gender As reported in the ISA, a limited number of NO 2 studies have stratified results by gender. The results of these studies were mixed, and th e ISA did not draw conclusions regarding the potential for gender to confer sus ceptibility to the effects of NO 2 (ISA, section 4.3.3). e. Proximity to roadways Certain groups may experience relatively high exposure to NO 2 , thus forming a potentially vulnerable population. The ISA in cluded discussion of popul ations reported to experience increased NO 2 exposures on or near roadways (I SA, section 4.3.6). Large gradients in NO x concentrations near roadways may lead to increased exposures for individuals residing,47 working, traveling, or attending sc hool in the vicinity of roadwa ys. Many studies find that indoor, personal, and outdoor NO 2 levels are strongly associated w ith proximity to traffic or to traffic density (ISA, section 4.3.6). That adverse respiratory effects can be associ ated with proximity to roadways has been demonstrated in a number of studies. For ex ample, Gauderman and colleagues (2007) reported reduced lung function growth in children who liv ed within 500 m of a freeway compared to children who lived at least 1500 m from a freew ay. In a separate study, Gauderman and colleagues (2005) reported that the incidence of physician-diagnosed asthma increased with both increasing NO 2 concentrations outside the child’s resi dence and decreasing distance between the child’s residence and a major freeway. In addition to those who live near majo r roadways, individuals who spend time commuting on major roadways can also be exposed to relatively higher concentrations of NO 2 than the ones reported at monito rs away from the roads. Due to high air exchange rates, NO 2 concentrations inside a vehicle can rapidly a pproach ambient concentrations on the roadway during commuting (ISA, section 4.3.6). Mean in-vehicle NO 2 concentrations are often between 2 and 3 times higher than ambient levels measured at monitors located away from the road (ISA, section 4.3.6). Due to the potential for high p eak exposures while dr iving, total personal exposure could be underestimated if exposures wh ile commuting are not cons idered. Therefore, individuals with occupations that re quire them to be in traffic or close to traffic (e.g., bus and taxi drivers, highway patrol officer s, toll collectors) and individua ls with long commutes could be exposed to relatively high levels of NO 2 compared to the ambient leve ls measured at fixed-site monitors located away from the roadway. f. Socioeconomic status48 The ISA discussed evidence that SES modifies the effects of air pol lution (section 4.3.6). Many recent studies examined modification by SES indicators on the association between mortality and PM or other indices such as traffic density, distan ce to roadway, or a general air pollution index (ISA, section 4.3.6). SES modification of NO 2 associations has been examined in fewer studies. However, in a study conduc ted in Seoul, South Kor ea, community-level SES indicators modified th e association of air pollution with emer gency department vi sits for asthma. Of the five criteria air pollutants evaluated, NO 2 showed the strongest a ssociation in lower SES districts compared to high SES districts (Kim et al., 2007). In additi on, Clougherty et al. (2007) evaluated exposure to violence (a potential surrogat e for SES) as a modifi er of the effect of traffic-related air pollutants, including NO 2 , on childhood asthma. The authors reported an elevated risk of asthma with an increase in NO 2 exposure solely among children with abovemedian exposure to violence in their neighborho ods (ISA, section 4.3.6). Although these recent studies have evaluated the impact of SES on vulnerability to NO 2 , they are too few in number to draw definitive conclusions (ISA, section 5.3.2.8). g. Size of the at-risk population The population potentially affected by NO 2 is large. A considerable fraction of the population resides, works, or a ttends school near major roadwa ys, and these individuals are likely to have increased exposure to NO 2 (ISA, section 4.4). Ba sed on data from the 2003 American Housing Survey, approximately 36 million individuals live within 300 feet (~90 meters) of a four-lane highway, ra ilroad, or airport (ISA, section 4.4) 7 . Furthermore, in

7 The most current American Housing Survey ( http://www.census.gov/hhes/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 /www/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 units49 California, 2.3% of schools with a total enrollment of more than 150,000 students were located within approximately 500 feet of high-traffic roads, with a hi gher proportion of non-white and economically disadvantaged st udents attending those schools (IS A, section 4.4). Of this population, asthmatics and members of other susceptible groups di scussed above will have even greater risks of e xperiencing 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 been diagnosed with COPD (ISA, section 4.4). The preval ence and severity of asthma is higher among certain ethn ic or racial groups such as Pu erto 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 asthma hospitalizations and mortality in minority and inner-city communities have been observed (ISA, section 4.4). In addition, based on U.S. census data from 2000, about 72.3 million (26%) of the U.S. population are under 18 years of age, 18.3 million (7.4%) are under 5 years of age, and 35 million (12%) are 65 years of age or older. Therefore, large portions of the U.S. population are in age groups that are likely atrisk for health effects associat ed with exposure to ambient NO 2 . The size of the potentially atrisk population suggests 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 th e 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 quantitative exposure and risk assessments, discussed below. These

in the U.S. Assuming equal distributions, with a current population of 306,330,199, that means that there would be 47.8 million people meeting the 300 foot criteria.50 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 unde r varying air quality scenarios ( e.g. , just meeting the current or alternative standards), as well as characteri zations of the kind and degree of uncertainties inherent in such estimates. This section describes the approach taken in the REA to characterize NO 2 -related exposures and health risks. Goals of the RE A included estimating short-term exposures and potential human health risks associated with (1) recent leve ls of ambient NO 2 ; 2) NO 2 levels adjusted to simulate just meeti ng the current standard; and 3) NO 2 levels adjusted to simulate just meeting potential alternative standards. This section discusses the scientific evidence from the ISA that was used as the basis for the risk ch aracterization (II.C.1), th e approaches used in characterizing exposures and risks (II.C.2), and important uncertainties associated with these analyses (II.C.3). The results of the exposure and ri sk analyses, as they relate to the current and potential alternative standards, are discussed in subsequent sections of this proposal (sections II.E and II.F, respectively). 1. Evidence base for the risk characterization For purposes of the quantitat ive 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 like ly causal relationship. This was generally consistent with judgm ents made in other recent NAAQS reviews (e.g., see EPA, 2005). As noted above in section II.A, the only h ealth effect category for which the evidence was judged in the ISA to be sufficient to infer either a causal or a likely causal relationship is respiratory morbidity following short-term NO 2 exposure. Therefore, for purposes of51 characterizing health risks associated with NO 2 , the REA focused on respiratory morbidity endpoints that have been associated with short-term NO 2 exposures. Other health effects (e.g., those associated with long-term exposures) ar e considered as part of the evidence-based evaluation of potential alternative standard s (see section II.F.2). In evaluating the appropriateness of specific endpoints for use in the NO 2 risk characterization, the REA considered both epidemiologic and controlled human exposure studies. When evaluating epidemiologic studies as to th eir appropriateness for use as the basis for a quantitative risk assessment, the REA considered several factors. First, the REA concluded that studies conducted in the United States are preferable to those c onducted outside the United States given the potential for e ffect estimates to be impacted by factors such as the ambient pollutant mix, the placement of mo nitors, activity patterns of the population, and characteristics of the healthcare system. Second, the RE A concluded that studies of ambient NO 2 are preferable to those of indoor NO 2 , which focus on individuals exposed to NO 2 from indoor sources. These indoor sources can result in exposure patterns, NO 2 levels, and co-pollutants that are different from those typically associated with ambient NO 2 . Therefore, although indoor studies made important contributions to the evidence base fo r causality judgments in the ISA, the preferred approach for conducting a quantitative risk assessm ent based on the epidemio logic literature to inform decisions regarding an ambient NO 2 standard is to consid er studies of ambient NO 2 . Third, the REA concluded that it was appropriate to focus on st udies of emergency department visits and hospital admissions given the clear pub lic health significance of these endpoints and the availability of baseline incidence data. Fina lly, the REA concluded that it was appropriate to focus on studies that evaluated NO 2 health effect associations using both single- and multipollutant models. Taking thes e factors into consideration, the epidemiology-based risk52 assessment in the REA focused on the study conduc ted in Atlanta, Georgi a by Tolbert et al. (2007). This assessment is described in more detail in the REA (chapter 9). In identifying health endpoi nts from controlled human e xposure studies on which to focus the characterization of NO 2 health risks, the REA conclude d that it was appropriate to focus on endpoints that occur at or near ambient levels of NO 2 and endpoints that may be important from a public health pe rspective. Controlled human exposure studies have addressed the consequences of short-term (e .g., 30-minutes to several hours) NO 2 exposures for a number of health endpoints including ai rway responsiveness, host defe nse and immunity, inflammation, and lung function (ISA, section 3.1). With regard to the NO 2 levels at which different effects have been documented, the ISA concluded: (1) in asthmatics NO 2 may increase the allergeninduced airway inflammatory response at expos ures as low as 260 ppb for 30 min (ISA, Figure 3.1-2), and NO 2 exposures between 200 and 300 ppb for 30 minutes or 100 ppb for 60-minutes can result in small, but significan t, increases in nonspecific airw ay responsiveness (ISA, section 5.3.2.1); (2) limited eviden ce indicates that NO 2 may increase susceptib ility to injury by subsequent viral challenge following exposures of 600-1500 ppb for 3 hours; (3) evidence exists for increased airway inflammation at NO 2 concentrations less than 2000 ppb; and (4) the direct effects of NO 2 on lung function in asthmatics have been inconsistent at exposure concentrations below 1000 ppb (ISA, section 5.3.2.1). Therefor e, of the health effects caused by NO 2 in controlled human exposure studies, the only effect identified by the ISA to occur at or near ambient levels is increased airway responsiveness in asthmatics. The REA concluded that airway responsiv eness in the asthmatic population is an appropriate focus for the risk ch aracterization for several reasons. First, the ISA concluded that “persons with preexisting pulmonary conditions are likely at greater risk from ambient NO 253 exposures than the general public, with the most extensive evidence available for asthmatics as a potentially susceptible group” (ISA, section 5.3.2.8) . Second, when disc ussing the clinical significance of NO 2 -related airway hyperrespons iveness in asthmatics, the ISA concluded that “transient increases in airw ay responsiveness following NO 2 exposure have the potential to increase symptoms and worsen asthma control” (ISA, sections 3.1.3 and 5.4). That this effect could have public health implications is suggest ed by the large size of the asthmatic population in the United States (ISA, Table 4.4-1). Third, NO 2 effects on airway responsiveness in asthmatics are part of the body of experimental evidence that pr ovides plausibility and coherence for the effects observed on hospital admissions and emergency department visits in epidemiologic studies (ISA, section 5.3.2.1). As a result of these considerations, of the endpoints from controlled human exposur e studies, the REA focused on airway responsiveness in asthmatics for purposes of quantifyi ng risks associated with ambient NO 2 (see below). Because many of the studies of airway re sponsiveness evaluated only a single level of
NO 2 and because of methodological di fferences between the studies, th e data are not sufficient to derive an exposure-response relatio nship in the range of interest. Therefore, the REA concluded that the most appropriate a pproach to characterizing risks based on the controlled human exposure evidence for airway responsiv eness was to compare estimated NO 2 air quality and exposure levels with potential he alth effect benchmark levels. In this review, the term “exposures of potential concer n” is defined as personal e xposures to 1-hour ambient NO 2 concentrations at and above specific benchm ark levels. Benchmar k levels represent NO 2 exposure concentrations reported to increase ai rway responsiveness in most asthmatics, as discussed above in section II.B .1.d. Although the analysis of expos ures of potential concern was conducted using discrete benchmark levels ( i.e. , 100, 150, 200, 250, 300 ppb), EPA recognizes54 that there is no sharp breakpoint within the continuum ranging from at and above 300 ppb down to 100 ppb. In considering the concept of exposur es of potential concer n, it is important to balance concerns about the potential for health effects and their severity with the increasing uncertainty associated with ou r understanding of the likelihood of such effects at lower NO 2 levels. Within the context of this continuum, estimates of e xposures of potential concern at discrete benchmark levels provide some persp ective on the potential public health impacts of
NO 2 -related health effects that have been demonstr ated in controlled human exposure studies but cannot be evaluated in quantitative risk assessmen ts (i.e., increased airway responsiveness). They also help in understanding the extent to wh ich such impacts could change by just meeting the current and potential a lternative standards. The NO 2 -related increase in airway responsiv eness is plausibly linked to the NO 2 - associated morbidity reported in epidemiologic studies (e.g., increased respiratory symptoms, emergency department visits and hospital admissions). However, estimates of the number of asthmatics likely to experience exposures of poten tial concern cannot be translated directly into quantitative estimates of the number of people likely to experience specific health effects, since sufficient information to draw su ch comparisons is not available. Due to individual variability in responsiveness, only a subset of asthmatics expo sed at and above a specific benchmark level can be expected to experience health effects. The amount of weight to place on the estimates of exposures of potential concern at any of these benchmark levels depends in part on the weight of the scientific evidence concerning h ealth effects associated with NO 2 exposures at and above that benchmark level. It also depends on judgmen ts about the importance from a public health perspective of the health effects that are known or can reasonably be inferred to occur as a result of exposures at and above the benchmark leve l. Such public health policy judgments are55 embodied in the NAAQS sta ndard setting criteria ( i.e. , standards that, in the judgment of the Administrator, are requisite to protect public he alth with an adequate margin of safety). 2. Overview of approaches As noted above, the purpose of the assessments described in the REA was to characterize air quality, exposures, and health risks asso ciated with recent am bient 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 that could be associated with ju st meeting potential alternative st andards. To characterize health risks, we employed three approaches in the RE A. In the first approach, for each air quality scenario, NO 2 concentrations at fixed- site monitors and simulate d concentrations on/near roadways were compared to potential health effe ct benchmark values derived from the controlled human exposure literature. In the second appro ach, modeled estimates of actual exposures in asthmatics were compared to potential healtheffect benchmarks. In the third approach, concentration-response relationshi ps from an epidemiologic study were used in conjunction with baseline incidence data and recent or simulated ambient concentrations to estimate health impacts. An overview of the approaches to ch aracterizing health risk s is provided below and each approach has been described in more de tail in the REA (chapters 6 through 9). In the first approach, we compared ambient NO 2 concentrations with potential health effect benchmark levels for NO 2 . The ambient NO 2 concentrations used in these analyses were based on those measured at monitors in the current NO 2 monitoring network. These monitored concentrations were compared to benchmark leve ls directly and were also used, in conjunction with literature-derived ch aracterizations of the NO 2 concentration gradie nt around roadways, as the basis for estimating NO 2 concentrations on/n ear roadways. Scenario-driven air quality analyses were performed using ambient NO 2 concentrations for the years 1995 though 2006.56 With this approach, NO 2 air quality serves as a surrogate for exposure. All U.S. monitoring sites where NO 2 data have been collected, and that met completeness criteria (REA, chapter 7), were represented by this analysis. As such, th e results generated were considered a broad characterization of nation al air quality and human exposures th at might be associated with these concentrations. An advantage of this approach is its relative simplici ty; however, there is uncertainty associated w ith the assumption that NO 2 air quality can serve as an adequate surrogate for total e xposure to ambient NO 2 . Actual exposures might be influenced by factors not considered by this approach, including sm all scale spatial variability in ambient NO 2 concentrations (which mi ght not be captured by the network of fixed-site ambient monitors) and spatial/temporal variability in human activity patterns. In the second approach, we used an inhalati on exposure model to generate more realistic estimates of personal exposures in asthmatics (REA, chapte r 8 for more detail on this assessment). This analysis estimated temporally and spatially variable ambient NO 2 concentrations and simulated huma n contact with these pollutant concentrations. The approach was designed to incorporate exposures that are not necessarily captured by the existing ambient monitoring data, including those that occur on or near roadways. AERMOD, an EPA recommended dispersion model, was used to estimate 1-hour ambient NO 2 concentrations using emissions estimates from stationa ry and on-road mobile sources. 8 The Air Pollutants Exposure (APEX) model, an EPA human e xposure model, was then used to estimate population exposures using the hourly census block level NO 2 concentrations estimated by AERMOD. A probabilistic approach was used to model indi vidual exposures considering the time people spend in different microenvironments and the variable NO 2 concentrations that occur within these

8 Estimated emissions from Hartsfield In ternational Airport in Atlanta, a non-roa d mobile source, were also included in this analysis.57 microenvironments across time, space, and micro environment type. Estimates of personal
exposure were compar ed to potential NO 2 health benchmark levels. This approach to assessing exposures was more resource intensive than usi ng ambient levels as a surrogate for exposure; therefore, the final REA included the analysis of only one specific location in the U.S. (Atlanta MSA). Although the geographic scope of this an alysis was restricted, the approach provided estimates of NO 2 exposures in asthmatics in Atlanta, particularly those exposures associated with important emission sources of NO x , and the analysis served to complement the broad air quality characterization. For the characterization of ri sks in both the air quality analysis and the exposure modeling analysis described above, the REA used a range of short-term potential health effect benchmarks. As noted above, the levels of potential benchmarks are based on NO 2 exposure levels that have been associated with increased airway responsiveness in as thmatics in controlled human exposure studies (ISA, section 5.3.2.1). Benchmark values of 100, 150, 200, 250, and 300 ppb were compared to both NO 2 air quality levels and to estimates of NO 2 exposure in asthmatics. When NO 2 air quality was used as a surrogate for exposure, the output of the analysis was an estimate of the number of ti mes per year specific lo cations experience 1-hour levels of NO 2 that exceed a particular benchmark. When personal exposures were simulated, the output of the analysis was an estimate of the nu mber of asthmatics at ri sk for experiencing daily maximum 1-hour levels of NO 2 of ambient origin that exceed a particular benchmark. An advantage of using the benchmark approach to characterize health risk s is that the effects observed in controlled human expos ure studies clearly result from NO 2 exposure. A disadvantage of this approach is that the magnitude of the NO 2 effect on airway responsiveness can vary considerably from individual to individua l and not all asthmatics would be expected to58 respond to the same levels of NO 2 exposure. Theref ore, the public health impacts of NO 2 - induced airway hyperresponsiveness are difficult to quantify. In the third approach, we estimated respirator y emergency department visits as a function of ambient levels of NO 2 measured at a fixed-site monitor re presenting ambient air quality for an urban area. In this approach, concentration- response functions from an epidemiologic study (Tolbert et al., 2007) were used, in combination with baseline incidence data for respiratory emergency department visits in the Atlanta area and ambient NO 2 monitoring data , to estimate the impact on emergency department visits of ambient levels of NO 2 . Compared to the risk characterization based on the air qua lity and exposure analyses desc ribed above, this approach to characterizing health risks has several advantages. For example, the public health significance of respiratory emergency department visits is less ambiguous, in term s of its impact on individuals, than is an increase of unknown magnitude in the ai rway response. In addition, the concentrationresponse relationship reflects real-world levels of NO 2 and co-pollutants present in ambient air. However, as noted previously, a disadvantage of this approach is the am biguity and complexity associated with quantifying the contribution of NO 2 to emergency departme nt visits relative to the contributions of co -occurring pollutants. 3. Key limitations and uncertainties A number of key uncertainties should be consid ered when interpreting the results of these analyses. While the air quality, exposure, and qua ntitative risk analyses are each associated with unique uncertainties, they also share some uncertainties in common. Important uncertainties shared by these analyses, as well as uncertaintie s specifically associated with the air quality, exposure, and risk analyses, are discussed below. In order to simulate just meeting the current annual standard and ma ny of the alternative59 1-hour standards analyzed, an adjustment (eith er upward or downward) of recent ambient NO 2 concentrations was required. As noted in the REA, an upward adjustment does not reflect a judgment that levels of NO 2 are likely to increase across the c ountry or in any specific location under the current standard or any of the potentia l alternative standards. However, it does acknowledge that, under the current standard and so me of the alternative standards evaluated, an increase in NO 2 concentrations would be pe rmitted. The benefit of th ese air quality adjustments is that they can inform cons ideration of the current and a lternative standards by providing estimates of health risks that c ould be associated with ambient ai r quality levels that just meet these standards. In adjusting air quality to simula te just meeting these standards, the analyses in the REA assumed that the overall shape of the distribution of NO 2 concentrations in an area would not change. While the REA concluded that this is a reas onable assumption in the absence of evidence supporting a differen t distribution, and while available analyses support this approach (Rizzo, 2008), the REA re cognized this as an important uncertainty. It may be an especially important uncertainty for those scenarios where consider able adjustment is required to simulate just meeting one or more of the standards (REA, section 8.12). In addition, simulation of just meeting different alternativ e standards was achieved by adjusting NO 2 concentrations at mo nitors in the current area-wide network. Therefore, resulting estimates of the potential public health implicat ions of different decisi ons are most directly relevant to a standard focuse d specifically on the area-wide NO 2 concentrations that are the primary target of the current monitoring networ k. However, as discussed below (sections II.F.4.e and III), with this noti ce the Administrator is proposing to establish a standard focused specifically on the peak concentrations to which individuals can be expo sed from on-road mobile source emissions on or near majo r roadways and to support such a standard with a monitoring60 network that includes monitors placed near ma jor roadways. This proposed shift in the monitoring network introduces uncerta inty in the extent to which the exposure and risk analyses presented in the REA can directly info rm decisions on the proposed standard. In addition to the general uncertainties discu ssed above, some uncertainties are specific to the air quality analyses. In order to estimate ambient NO 2 concentrations on or near roadways in the air quality analyses, the REA used empiri cally-derived relationships between ambient concentrations measured at fixed- site monitors in the current NO 2 monitoring network and on/near-road concentrations. The data used to develop the relationships were likely collected under different conditions (e.g., different meteorol ogical conditions which can affect important parameters in this relationshi p, such as the production of NO 2 from NO). The REA noted that the extent to which these conditions are representative of the times and places included in our analyses is unknown. Therefore, there is uncertainty in the degree to which the relationships used to estimate on/near-road NO 2 concentrations reflect the actua l relationship in the locations and over the time periods of interest. Potential health benchmark levels used in th e air quality analyses were based largely on a meta-analysis (ISA, Table 3.1-3) of cont rolled human exposure studies of airway hyperresponsiveness. One important source of uncerta inty with regard to this approach is that controlled human exposure studies have typically involved volunteers with mild asthma. Data are lacking for more severely a ffected asthmatics, who may be more susceptible (ISA, section 3.1.3.2). As a result, the potential health eff ect benchmarks could underestimate risks in populations with greater susceptibi lity. While approaches to classi fying asthma severity differ, some estimates indicate that over half of asthmati cs could be classified as moderate or severe (Fuhlbrigge et al., 2002; Stout et al., 2006). A second important source of uncertainty with61 regard to this approach is that the meta-ana lysis showed increased airway responsiveness in asthmatics at the lowest NO 2 level for which data were av ailable (i.e. 100 ppb). Controlled human exposure studies have not evaluated the possibility of NO 2 effects on airway responsiveness in asthmatics at exposure c oncentrations below 100 ppb. A third important source of uncertainty associated with this approach is th at the meta-analysis provided information on the direction of the NO 2 -induced airway response, but not on the magnitude of the response. Therefore, although the ISA did co nclude that increased airway responsiveness associated with NO 2 exposure could increase symptoms and worsen asthma control (ISA, section 5.4), the full public health implications of benchmark exceedances are uncertain. The Atlanta exposure assessment was also asso ciated with a number of key uncertainties that should be considered when interpreting the re sults with regard to decisions on the standard. Some of these uncertainties, incl uding those associated with benc hmark levels, were shared with the air quality analyses. Additional uncertain ties associated specifical ly with the Atlanta exposure assessment are discussed briefly below. When compared to ambien t measurement data, predic ted upper percentile NO 2 concentrations may be 10-50% hi gher. Because these predicted c oncentrations are used as inputs for the exposure modeling, this s uggests the possibility that the exposure assessment is overpredicting upper percentile NO 2 exposures. Other approaches used to evaluate exposure results (i.e., comparison to personal exposure monito ring results and comparison of exposure-toambient concentration ratios with those identified in the ISA) have suggested that exposure estimates are reasonable. However, the possi bility cannot be rule d out that benchmark exceedances are over-predicted in the Atlanta exposure analysis. The exposure assessment was limited to Atlanta and the extent to which these results are62 representative of other locations in the U.S. is uncertain. The REA (section 8.11) concluded that the Atlanta exposure estimates are likely representative of other moderate to large urban areas. However, the REA also recognized that, given th e greater proximity of the population to mobile sources in large urban areas such as Los Ange les, New York, and Chi cago (see REA, Tables 8- 14 and 8-15), the estimates of benchmark exceedances in Atlanta may be sm aller than in these larger cities. A number of key uncertainties should also be c onsidered when interpreting the results of the Atlanta risk assessment with regard to deci sions on the standard. Some of these, including the appropriateness of generalizing results from Atlanta, are shared w ith the Atlanta exposure assessment. Additional uncertaintie s associated specifically with the Atlanta risk assessment are discussed briefly below. There is uncertainty about whether the association between NO 2 and emergency department visits actually reflects a causal rela tionship across the range of daily and hourly concentration levels in the epid emiologic studies. The ISA (sec tion 5.4, p. 5-15) noted that when interpreting the NO 2 epidemiologic results, “It is difficult to determine ...the extent to which NO 2 is independently associated withrespiratory e ffects or if NO 2 is a marker for the effects of another traffic-related pollutant or mix of pollutants (see section 5.2.2 for more details on exposure issues). A factor contributi ng to uncertainty in estimating the NO 2 -related effect from epidemiologic studies is that NO 2 is a component of a complex ai r pollution mixtur e from traffic related sources that include CO and various forms of PM.” This uncertainty should be considered when interpreting the quantitative NO 2 risk estimates based on the Atlanta epidemiologic study. However, in discussing these uncertainties, the ISA (section 5.4, p. 5-16) concluded that, “Although this complicates the efforts to di sentangle specific NO 2 -related health63 effects, the evidence summarized inthis assessment indicates that NO 2 associations generally remain robust in multi-pollutant models and supports a dir ect effect of short-term NO 2 exposure on respiratory morbidity at ambient concentratio ns below the current NAAQS. The robustness of epidemiologic findings to adjustment for co-p ollutants, coupled with data from animal and human experimental studies, support a dete rmination that the relationship between NO 2 and respiratory morbidity is likely causal, while still recognizing the relationship between NO 2 and other traffic-rela ted pollutants.” A related uncertainty is that a ssociated with the estimated NO 2 coefficient in the concentration-response function. Th is coefficient has been charact erized by confidence intervals reflecting sample size. However, these confidence intervals do not reflect all of the uncertainties related to the concentration-res ponse functions, such as whether or not the model used in the epidemiologic study is the correct model form. Concerning the possible role of co-pollutants in the Tolbert et al. (2007) study, single-pollutant models may produce overestimates of the NO 2 effects if some of thos e effects are really due in whole or part to one or more of the other pollutants. On the other hand, effect estimates based on multi-pollutant m odels can be uncertain, and can result in statistically non- significant estimates where a true relationship exists, if the copollutants included in the mode l are highly correlated with NO 2 . As a result of these considerations, we report risk estimates based on both the single - and multi-pollutant models from Tolbert et al. (2007). D. Considerations in Review of the Standard This section presents the integrative synthe sis of the evidence and information contained in the ISA and the REA with regard to the current and potential alternative standards. EPA notes that the final decision on retaining or revising the current primary NO 2 standard is a public health64 policy judgment to be made by the Administrator. This judg ment will be informed by a recognition that the available h ealth effects 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 which the likelihood and magn itude of the response become increasingly uncertain. The Administrator’s fi nal decision will draw upon scient ific information and analyses related to health effects, population exposures , and risks; judgments about the appropriate response to the range of uncertain ties that are inherent in the sc ientific evidence and analyses; and comments received in response to this proposal. 1. Background on the current standard The current standard, which is an annual av erage of 0.053 ppm (53 ppb), was retained by the Administrator in the most recent review in 1996 (61 FR 52854 (October 8, 1996)). The decision in that review to reta in the annual standard was base d on consideration of available scientific evidence for health effects associated with NO 2 and on air quality information. With regard to these considerations, the Administrator noted that “a 0.053 ppm annual standard would keep annual NO 2 concentrations considerably below th e long-term levels for which serious chronic effects have been observed in animals” and that “[r]etaining the existing standard would also provide protection against short-term peak NO 2 concentrations at the levels associated with mild changes in pulmonary function and airway responsiveness observed in controlled human studies” (60 FR 52874, 52880 (Oct. 11, 1995)). As a result, the Ad ministrator concluded that “the existing annual primary standard appears to be both adequate and necessary to protect human health against both long- and short-term NO 2 exposures” and that “retaining the existing annual standard is consistent w ith the scientific data assessed in the Criteria Document (EPA,65 1993) and the Staff Paper (EPA, 1995) and with the advice and recommendations of CASAC” (61 FR 52852 at 52854). As noted previously, the 1993 AQCD concluded th at there were two ke y health effects of greatest concern at ambient or near-ambient levels of NO 2 : increased airway responsiveness in asthmatic individuals after short-term exposures and increased occurrence of respiratory illness in children with longer-term exposures. Evid ence also was found for increased risk of emphysema, but this was of major concer n only with exposures to levels of NO 2 much higher than then-current ambient concentrations. Th e evidence regarding airway responsiveness was drawn largely from controlled human exposure studi es. The evidence for respiratory illness was drawn from epidemiologic studies that reported associations betw een respiratory symptoms and indoor exposures to NO 2 in people living in homes with gas st oves. The biologi cal plausibility of the epidemiologic results was supported by toxicological studies that detected changes in lung host defenses following NO 2 exposure. Subpopulations consider ed potentially more susceptible to the effects of NO 2 included individuals with preexisting respiratory disease, children, and the elderly. In that review, health risks were character ized by comparing ambient monitoring data, which were used as a surrogate for exposure, with pot ential health benchmark levels identified from controlled human exposure studies. At the tim e of the review, a meta-analysis of controlled human exposure studies i ndicated the possibility for adverse health effects due to short-term (e.g., 1-hour) exposures between 200 ppb and 300 ppb NO 2 . Therefore, the focus of the assessment was on the potential for short-term (i.e., 1-hour) exposures to NO 2 levels above potential health benchmarks in this range. Th e assessment used monito ring data from the years 1988-1992 and screened for sites with one or more hourly exceedances of potential short-term66 health effect benchmarks. Predictive models were then constructed to relate the frequency of hourly concentrations above short-term health e ffect benchmarks to a range of annual average concentrations, including the current standard. Based on the results of this analysis, both CASAC (Wolff, 1995) and the Administrator (60 FR 52874) concluded that the minimal occurrence of short-term peak concentrations at or above a potential health effect benchmark of 200 ppb (1-hour average) indicated that the exis ting annual standard would provide adequate health protection against short-term exposures. This conclusion, combined with the conclusion that the current annual standa rd would maintain annual aver age levels well-below those associated with serious effects in animal toxicological studies, form ed a large part of the basis for the decision in the 1996 review to reta in the existing annual standard. 2. Approach for reviewing the need to re tain or revise the current standard The decision in the present review on whether the current annual standa rd is requisite to protect public health with an adequate marginof safety will be in formed by a number of scientific studies and analyses that were not available in the 1996 review. Specifically, as discussed above (section II), a large number of epidemiologic studies have been published since the 1996 review. Many of these studie s evaluate associations between NO 2 and adverse respiratory endpoints (e.g., respiratory sympto ms, emergency department visits, hospital admissions) in locations where annual average NO 2 concentrations are well-below the level allowed by the current standard (53 ppb). In addition, the meta-analysi s of controlled human exposure studies has been updated for this review to include information on additional exposure concentrations. Finally, the REA described estimates of NO 2 -associated health risks that could be present in locations that just meet the current annual standard. These types of risk estimates were not available in the last review. The appr oach for considering this scientific evidence and67 exposure/risk information is discussed below. To evaluate whether the current primary NO 2 standard is adequate or whether consideration of revisions is appr opriate, EPA is using an approach in this review that has been described in chapter 10 of the REA. The approach outlined in the REA builds upon the approaches used in reviews of other criteria poll utants, including 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 inother recent reviews, EPA's considerations will include the implications of placing more or less weight or emphasis on different aspects of the scientific evidence and the exposure/risk-based information, recognizing that the weight to be given to various elements of the evidence and expo sure/risk information is part of the public health policy judgments that the Administrator will make in reaching decisions on the standard. A series of general questions frames 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 is 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 uncertain ties 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?68 To the extent that the available evidence and exposure-/risk-based information suggests it may be appropriate to consider revision of the cu rrent standard, EPA considers that evidence and information with regard to its s upport for consideration of a standard that is either more or less protective than the current standard. This eval uation is framed by th e following questions:

• Is there evidence that associations, especially 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 estimated exposures and health risks important from a public health perspectiv e? 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 available information. In so doing, the Agency addresses 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 differen t averaging times?

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

• To what extent do specific av eraging 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?69 The questions outlined above have been addre ssed in the REA. The following sections present considerations regarding the adequacy of the current standard and potential alternative standards, as discussed in chapter 10 of the REA, in terms of indicator, averaging time, form, and level. E. Adequacy of the current standard In considering the adequacy of the current standard, the policy assessment chapter of the REA considered the scientific evidence assessed in the ISA and the quantitative exposure- and risk-based information presented in the REA. A summary of this evidence and information as well as CASAC recommendations and the Administr ator’s conclusions regarding the adequacy of the current standard are presented below. 1. Evidence-based considerations As discussed in chapter 10 of the REA, evidence published since the last review generally has confirmed and extended the conc lusions articulated in the 1993 AQCD (ISA, section 5.3.2). The epidemiologic evidence has gr own substantially with the addition of field and panel studies, intervention st udies, time-series studies of effects such as emergency department visits and hospita l admissions, and a substantial number of studies evaluating mortality risk associat ed with short-term NO 2 exposures. As noted above, no epidemiologic studies were available in 1993 that assessed relationships between NO 2 and outcomes such as hospital admissions, emergency de partment visits, or mortalit y. In contrast, dozens of epidemiologic studies on such outcomes, conducted at recent and current ambient NO 2 concentrations, are now included inthis evaluation (ISA, chapter 3) . While not as marked as the growth in the epidemiologic li terature, a number of recent toxicological and human clinical studies also provide insights into relationships between NO 2 exposure and health effects.70 As an initial consideration with regard to th e adequacy of the current standard, the REA noted that the evidence relating long-term (weeks to years) NO 2 exposures at current ambient concentrations to adverse health effects was judg ed in the ISA to be either “suggestive but not sufficient to infer a cau sal relationship” (respiratory morbid ity) or “inadequate to infer the presence or absence of a causal relationship” (mortality, cancer, car diovascular effects, reproductive/developmental eff ects) (ISA, sections 5.3.2.4-5.3.2.6). In contrast, the evidence relating short-term (minutes to hours) NO 2 exposures to respiratory mo rbidity was judged to be “sufficient to infer a likely causal relationship” (ISA, sec tion 5.3.2.1). This judgment was supported primarily by a large body of recent epid emiologic evidence that evaluated associations of short-term NO 2 concentrations with respiratory symp toms, emergency department visits, and hospital admissions. These conclusions from the ISA suggest that, at a minimum, consideration of the adequacy of the current annual standard sh ould take into account the extent to which that standard provides protection against respiratory effects a ssociated with short-term NO 2 exposures. As noted in the REA, such an em phasis on health endpoints for which evidence has been judged to be sufficient to infer a likely causal relationship would be consistent with other recent NAAQS reviews (e.g., EPA, 2005; EPA, 2007d; EPA, 2007e). In considering the NO 2 epidemiologic studies as they rela te to the adequacy of the current standard, the REA noted that annual average NO 2 concentrations were below the level of the current annual NO 2 NAAQS in many of the locations wh ere positive, and often statisticallysignificant, associations with respiratory morb idity endpoints have beenreported (ISA, section 5.4). As discussed previously, the ISA character ized that evidence for respiratory effects as consistent and coherent. The evidence is consiste nt in that associations are reported in studies conducted in numerous locations and with a variety of methodol ogical approaches (ISA, section71 5.3.2.1). It is coherent in the sense that the stud ies report associations with respiratory health outcomes that are logically linke d together (ISA, section 5.3.2.1). The ISA noted that when the epidemiologic literature is considered as a whole, there are genera lly positive associations between NO 2 and respiratory symptoms, hospital admissi ons, and emergency department visits. A number of these associations are statistically si gnificant, particularly the more precise effect estimates (ISA, section 5.3.2.1). As discussed previously, the interpretation of these NO 2 epidemiologic studies is complicated by the fact that on-road vehicle exha ust emissions are a nearly ubiquitous source of combustion pollutant mixtures that include NO 2 . In order to provide some perspective on the uncertainty related to the presence of co-pollutant s, the ISA evaluated epidemiologic studies that employed multi-pollutant m odels, epidemiologic studies of indoor and personal NO 2 exposure, and experimental studies. Specifical ly, the ISA noted that a number of NO 2 epidemiologic studies have attempted to di sentangle the effects of NO 2 from those of co-o ccurring pollutants by employing multi-pollutant models. When evaluated as a whole, NO 2 effect estimates in these models generally remained robust when co-po llutants were included. Therefore, despite 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 “t he evidence summarized in this assessment indicates that NO 2 associations generally remain robust inmulti-pollutant m odels and supports a direct effect of short-term NO 2 exposure on respiratory morbidity at ambient concentrations 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 st udies are not confounded by the same mix of co-pollutants present in the ambien t air or by the contribution of NO 2 to the formation of72 secondary particles or O 3 (ISA, section 3.1.4.1). The ISA noted that the findings of indoor NO 2 studies are consistent with those of studies us ing ambient concentrati ons from central site monitors and concluded that indoo r studies provide evidence of c oherence for respiratory effects (ISA, section 3.1.4.1). With regard to experimental studies, the REA noted that they have the advantage of providing informat ion on health effects that are specifically associated with exposure to NO 2 in the absence of co-pollutants. The ISA concluded that the NO 2 epidemiologic literature is supported by 1) evidence from controlled huma n exposure studies of airway hyperresponsiveness in asthmatics, 2) controll ed human exposure and animal toxicological studies of impaired host-defens e systems and increased risk of susceptibility to viral and bacterial infection, and 3) controlled human exposure and animal toxicological st udies of airway inflammation (ISA, section 5.3.2.1 and 5.4). In drawing broad conclusions regarding the evidence, the ISA considered the epidemiologic and experimental evidence as well as the uncertainties associated with that evidence. When this evidence and its associat ed uncertainties are ta ken together, the ISA concluded that the results of epidemiologic a nd experimental studies form a plausible and coherent data set that supports a relationship between NO 2 exposures and respiratory endpoints, including respiratory symptoms and emergency depart ment visits, at ambient concentrations that are present in areas th at meet the current NO 2 NAAQS. Thus, taking into consideration the evidence discussed above, particularly the epidemiologic studies reporting NO 2 -associated health effects in locations that meet th e current standard, the REA conclude d that the scientific evidence calls into question the adequa cy of the current standard to protect public health. 2. Exposure- and risk-based considerations In addition to the evidence-ba sed considerations described above, the REA considered73 the extent to which exposure- and risk-based information can inform decisions regarding the adequacy of the current annual NO 2 standard, taking into account key uncertainties associated with the estimated exposures and risks. As noted above, NO 2 -associated health risks were characterized with three appr oaches. In the first, NO 2 air quality from locations across the country was used as a surrogate for exposure. In the second, e xposures were estimated for all asthmatics and for asthmatic chil dren considering time spent in different microenvironments in one urban area, Atlanta, GA. For both of these analyses, health risks were characterized by comparing estimates of air quality or exposure to potential health benchmark levels. Benchmark levels spanned the range of NO 2 concentrations that have been reported to increase airway responsiveness in asthmatics (i.e., 100-300 ppb). In the third approach to characterizing NO 2 - related health risks, occurrences of NO 2 -related respiratory emergenc y department visits were estimated for Atlanta. This quantit ative risk assessment was based on NO 2 concentrationresponse relationships identified in an epidemiologic study of air pollution-related emergency department visits in Atlanta. The results of each of these analyses are discussed in this section, specifically as they relate to the current standard. When considering the Atlanta risk assessment results as they relate to the adequacy of the current standard, the REA noted that central estimates of incidence of NO 2 -related respiratory emergency department visits in Atlanta ranged fro m approximately 8 to 9% of total respiratoryrelated emergency department visits per year (or 9,800-10,900 NO 2 -related incidences) based on single pollutant models when air quality is adjust ed upward to simulate a situation where Atlanta just meets the current standard. Ce ntral estimates of incidence of NO 2 -related respiratory emergency department visits ranged from 2.9-7.7% of total re spiratory-related emergency department visits per year (or 3,600-9,400 NO 2 -related incidences) based on two-pollutant74 models. Inclusion of O 3 and/or PM 10 in multi-pollutant models re sulted in the inclusion of an estimate of zero NO 2 -related respiratory emergency department visits within the 95% confidence intervals. When considering the Atlanta exposure results as they relate to the adequacy of the current standard, the REA noted the number of days per year asthmatics could experience exposure to NO 2 concentrations greater than or equal to potential health benchmark levels, given air quality that is adjusted upward to simula te just meeting the current standard. If NO 2 concentrations were such that the Atlanta ar ea just meets the current standard, nearly all asthmatics in Atlanta (>97%) woul d be estimated to experience six or more days per year with 1- hour NO 2 exposure concentrations greater than or equal to our highest benchmark level (300 ppb) (REA, Figure 8-22). Six days per year was the largest number of days specifically considered in the REA, but these results suggest that some asthmatics could experience 1-hour
NO 2 exposure concentrations greater than or equal to 300 ppb on more than si x days per year. In addition, more frequent exceedan ces would be expected for the lower benchmark levels. When considering the air quality-based results as they relate to the adequacy of the current standard, the REA noted the number of benchmark exceedances estimated to occur in different locations given air quality that just meets that standar d. In situations where annual NO 2 concentrations were adjusted upward to simulate just meeting the curre nt standard, 1-hour NO 2 concentrations measured at fi xed-site monitors in locations across the U.S. could exceed benchmark levels. Most locations were estimated to experience at least 50 days per year with 1- hour ambient NO 2 concentrations at fixed-si te monitors in the current network greater than or equal to 100 ppb (Figures 7-2 and 7-3 in the RE A) under this hypothetical scenario. Far fewer ambient exceedances were predic ted for the higher benchmark levels. For example, only 5 areas75 were estimated to experience an y days with 1-hour ambient NO 2 concentrations at fixed-site monitors greater than or equal to 300 ppb, and none of those locations were estimated to experience more than 2 such days per year, on average (REA, Appendix A). However, on-road NO 2 concentrations were estimated in this analysis to be an average of 80% higher than concentrations at fixed-site m onitors (though this relationship will vary across locations and with time). In th e majority of locations evalua ted, roadway exceedances of the 100 ppb benchmark level could occur on most days of the year when ai r quality is adjusted upward to simulate just meeting th e current standard (Figure 7-6 in th e REA). Even for higher benchmark levels, most locations were esti mated to have exceedances on road ways. All locations evaluated except one (Boston) were estimated to experience on-road NO 2 concentrations greater than or equal to 300 ppb (REA, A ppendix A). Four of these locations were estimated to experience an average of greater than 20 da ys per year with on-road NO 2 concentrations great er than or equal to 300 ppb (REA, Appendix A). 3. Summary of considerations from the REA As noted above, the policy assessment chapte r of the REA considered the scientific evidence with regard to the current standard. This included consideration of causality judgments made in the ISA regarding the le vel of support for effects associ ated with short-term and longterm exposures, the epidemiologic evidence described in the ISA including associated uncertainties, the conclusions inthe ISA regarding the robustne ss of this evidence, and the support provided for epidemiologi c findings by experimental studi es. The REA concluded that, given these considerations, particularly the evidence for NO 2 -associated effects in locations that meet the current standard, the adequacy of the cu rrent standard to protect the public health is clearly called into question. This evidence provides support for consideration of an NO 276 standard that would provide increased health protection for at-risk groups, including asthmatics and individuals who spend time on or near major roadways, agai nst health effects associated with short-term exposures ranging from increas ed asthma symptoms to respiratory-related emergency department visits and hospital admissions, in addition to potential effects associated with long-term exposures. In examining the exposure- and risk-based info rmation with regard to the adequacy of the current annual NO 2 standard to protect the public healt h, the REA noted that estimated risks associated with air quality adjusted upward to simulate just meeting th e current standard can reasonably be concluded to be important from a pub lic health perspective. In particular, a large percentage (8-9%) of respiratory -related ED visits in Atlanta coul d be associated with short-term
NO 2 exposures, most asthmatics in Atlanta could be exposed on mu ltiple days per year to NO 2 concentrations at or above the highest benchmar k evaluated, and most locations evaluated could experience on-/near-road NO 2 concentrations above benchmark le vels on more than half of the days in a given year. Therefore, the REA noted that exposure- and risk-based results reinforce the scientific evidence in supporting the conclu sion that consideration should be given to revising the current standard so as to provide in creased public health protection, especially for atrisk groups, from NO 2 -related adverse health e ffects associated with short-term, and potential long-term, exposures. 4. CASAC views77 With regard to the adequacy of the current standard, CASAC conclusions were consistent with the views expressed in the policy assessment chapter of the REA. CASAC agreed that the primary concern in this review is to protect against hea lth 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 expos ures that could lead to these health effects. Given these considerations, and as noted in th eir 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). CASAC’s views on how the standard should be revised are provided below within the context of discussions on the elements (i.e., indicator, averaging time, form, level) of a new short-term standard. 5. Administrator’s conclusions regardi ng adequacy of the current standard In considering the adequacy of the current NO 2 NAAQS, the Administrator has considered the conclusions of the ISA, the conc lusions of the policy asse ssment chapter of the REA, and the views expressed by CASAC. In pa rticular, the ISA conclude d that the results of epidemiologic and experimental studies form a pl ausible and coherent data set that supports a likely-causal relationship between short-term NO 2 exposures and adverse respiratory effects at ambient NO 2 concentrations that are present in locations meeting the current NO 2 NAAQS. With regard to the exposure and risk results, the REA concludes that central risk estimates suggest that the current standard could allow important adverse public health impacts. Based on her consideration of these conclusi ons, as well as consid eration of CASAC’s conclusion that the current NO 2 NAAQS does not protect the public ’s health, the Administrator concludes that the current NO 2 standard does not pr ovide the requisite de gree of protection for public health against adverse effects associated with short-term exposures. In considering78 approaches to revising the current standard, the Administrator conc ludes that it is appropriate to consider setting a new short-term standard to supplement the current annual standard. The Administrator notes that such a short-term st andard could provide increased public health protection, especially for me mbers of at-risk groups, fro m effects described in both epidemiologic and controlled human exposure studies to be associated with short-term exposures
to NO 2 . F. Conclusions on the elements of a new short-term standard and an annual standard In considering alternative NO 2 primary NAAQS, the Adminis trator notes the need to protect at-risk individuals fro m short-term exposures to NO 2 air quality that could cause the types of respiratory morbidity e ffects reported in epidemiologic st udies and the need to protect at-risk individuals from short-term exposure to NO 2 concentrations report ed in controlled human exposure studies to increase airway responsiveness in asthmatics. Considerations with regard to potential alternative standards and the specific options being proposed are discussed in the following sections in terms of i ndicator, averaging time , form, and level (sec tions II.F.1-II.F.4).

1. Indicator In past reviews, EPA has focused on NO 2 as the most appropriat e indicator for ambient
NO x . In making a decision in the current revi ew on the most appropr iate indicator, the Administrator has considered the conclusions of the ISA and REA as well as the view expressed by CASAC. The REA noted th at, while the presence of NO xspecies other than NO 2 has been recognized, no alternative to NO 2 has been advanced as being a more appropriate surrogate. Controlled human exposure studies and animal toxicology studie s 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 279 plus other gaseous NO x , can vary (REA, section 2.2.3). In addition, 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 can also be expected to provide some degree of protection ag ainst potential health effects that may be independently associatedwith other gaseous NO x even though such effect s are not discernable from currently available studies indexed by NO 2 alone. Given these key points, the REA concluded that the evidence supports retaining NO 2 as the indicator. Consistent with this conclusion, the CASAC Panel recommended in its letter to the EPA Administrator that it “concurs with retention of NO 2 as the indicator” (Samet, 2008 b). In light of the above considerations, the Administrator proposes to retain NO 2 as the indicator in the current review. 2. Averaging time The current annual averaging time for the NO 2 NAAQS was original ly set in 1971, based on epidemiologic studies that supported a link betw een adverse respiratory effects and long-term exposure to low-levels of NO 2 . As noted above, that annual sta ndard was retained in subsequent reviews in part because an ai r quality assessment conducted by EPA concluded that areas that meet the annual standard would be unlikely to experience short-term ambient peaks above concentrations that had been reported in a meta -analysis of controlled human exposure studies to increase airway responsiveness in asthmatics. In the current review, additional scientific evidence is available to inform a decision on averag ing time. This includes the availability of a number of epidemiologic studies that have ev aluated endpoints includin g respiratory symptoms, emergency department visits, and hospital admissions as well as an updated meta-analysis of controlled human exposure studies of airw ay responsiveness in asthmatics.80 In order to inform conclusions with regard to averaging time in this review, the REA considered judgments on the evidence from the ISA, results from experimental and epidemiologic studies, and an analysis of correl ations between short- and long-term ambient NO 2 concentrations. These considerations ar e described in more detail below. a. Short-term averaging time As described previously, the evidence re lating short-term (minutes to hours) NO 2 exposures to respiratory morbidit y was judged in the ISA to be “s ufficient to infer a likely causal relationship” (ISA, section 5.3.2.1) while the ev idence relating long-term (weeks to years) NO 2 exposures to adverse health effects was judged to be either “suggestive but not sufficient to infer a causal relationship” (respiratory morbidity) or “i nadequate to infer the presence or absence of a causal relationship” (mortality, cancer, cardiova scular effects, repr oductive/developmental effects) (ISA, secti ons 5.3.2.4-5.3.2.6). Thus, the REA concl uded that these judgments most directly support an averaging time that focu ses protection on short- term exposures to NO 2 . As in past reviews of the NO 2 NAAQS, it is instructive to evaluate the potential for a standard based on annual average NO 2 concentrations, as is the current standard, to provide protection against short-term NO 2 exposures. To this end, Tabl e 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 9 ) to annual average concentrations across 14 locations ranged from 2.5 to 8.7 while ratios of 24-hour av erage concentrations to annual average concentrations ranged from 1.6 to 3.8 (see Thompson, 2008 for more details). The REA concluded that the variability in these ratios across locations, particul arly those for 1-hour

9 As discussed below, 98 thand 99 thpercentile forms were evaluated in the REA. A 99 thpercentile form corresponds approximately to the 4 thhighest 1-hour concentration in a year while a 98 thpercentile form corresponds approximately to the 7 thor 8 thhighest 1-hour concentration in a year. A 4 th highest concentration form has been used previously in the O 3 NAAQS while a 98 thpercentile form has been used previously in the PM 2.5 NAAQS.81 concentrations, suggested that a standard based on annual average NO 2 concentrations would not likely be an effective or efficient approach to focu s protection on short-term NO 2 exposures. For example, in an area with a relatively high ra tio (e.g., 8), the current annual 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 relati vely low ratio (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 that a standard based on annual average concentrations would likely require more control than necessary in some areas and less control than necessary in others, depending on the standard level selected. In considering the level of support available for specific s hort-term averaging times, the policy assessment chapter of th e REA noted evidence from both e xperimental and epidemiologic studies. Controlled human exposur e studies and animal toxicol ogical studies provide evidence that NO 2 exposures from less than 1-hour up to 3-hours can re sult in respiratory effects such as increased airway responsiveness and inflammati on (ISA, section 5.3.2.7). Specifically, the ISA concluded that NO 2 exposures of 100 ppb for 1-hour (o r 200 ppb to 300 ppb for 30-min) can result in small but sign ificant increases in nonspecific ai rway responsiveness (ISA, section 5.3.2.1). In contrast, the epidemiolo gic literature provides support for short-term averaging times ranging from approximately 1-hour up to 24- hours (ISA, section 5.3.2.7). A number of epidemiologic studies have detected positive a ssociations between respiratory morbidity and 1- hour (daily maximum) and/or 24-hour NO 2 concentrations. A few ep idemiologic studies have considered both 1-hour and 24-hour averaging tim es, allowing comparisons to be made. The ISA reported that such comparisons in studies that evaluate asth ma emergency department visits failed to reveal differences be tween effect estimates based on a 1-hour averaging time and those82 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, to dis cern whether effects observed are attributable to average daily (or multi-day) co ncentrations (24-hour av erage) 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 shorte r duration than 24 hours (e.g., 1-h) while the epidemiologic eviden ce provides support for both 1-hour and 24-hour averaging times. At a minimum, th is suggests that a primary con cern with regard to averaging time is the level of protection prov ided against 1-hour daily maximum NO 2 concentrations. However, it is also important to consider the ability of a 1-hour (daily maximum) averaging time to protect against 24-hour average NO 2 concentrations. To this end, Table 10-2 in the REA presented correlations 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 detail). Typical ratios range d from 1.5 to 2.0, though one rati o (Las Vegas) was 3.1. These ratios were far less variable than those disc ussed above for annual average concentrations, suggesting that a standard based on 1-hour daily maximum NO 2 concentrations could also be effective at protecti ng against 24-hour NO 2 concentrations. The REA concluded that the scientific evidence, combined with the air qua lity correlations described above, 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. b. Long-term averaging time While the REA concluded that the combinati on of the scientific evidence from the ISA and air quality analyses most di rectly support an averaging time that focuses protection on short-83 term exposures to NO 2 , some evidence does support the need to also consider health effects potentially associated with long-term exposures. As noted above, the ISA judged the evidence relating long-term (weeks to years) NO 2 exposures to respiratory morb idity to be “suggestive but not sufficient to infer a causal relationship.” The available database supporting the relationship between respiratory illness in children and long-term exposures to NO 2 has increased since the 1996 review of the NO 2 NAAQS. Results from several studies, including the California-based Children’s Health Study, have repo rted deficits in lung function growth (Gauderman et al., 2004) in association with long-term exposure to NO 2 . In addition, some studies have reported associations between asthma incidence and long-term NO 2 . The plausibility of these associations is supported by some animal toxi cological studies. Sp ecifically, morphological effects following chronic NO 2 exposures have been identified inanimal studies that link to these increases in collagen synthesis and may provide plausibility for the deficits in lung function growth described in epidemiologic st udies of long-term exposure to NO 2 (ISA, section 3.4.5). Therefore, though the evidence provides strong support for the need to protect against health effects associated with short-term NO 2 exposures, it may also be appropriate to consider the extent to which the NO 2 standard could protect against pote ntial effects associ ated with longterm exposures. To address this issue, the REA estimated annual average NO 2 concentrations assuming different 1-hour standard s were just met. For the locations evaluated, a 1-hour areawide standard with a level at or below 100 ppb was estimated to be associated with annual average NO 2 concentrations below the level of th e current annual standard (53 ppb) (REA, section 10.4.2). Therefore, it is possible that a 1-hour standard could al so provide protection against potential effect associated with long -term exposures, depending on the level of the standard.84 c. CASAC views CASAC agreed with the conclusions of the policy assessment chapter of the REA that a primary consideration of the NO 2 NAAQS should be the protec tion provided against health effects associated with short-term exposures. In their letter to the EPA Administrator, CASAC stated that they concur “withhaving a short-term NAAQS primary standard for oxides of nitrogen and using the one-hour maximum NO 2 value.” In addition, the letter noted that “CASAC also recommends reta ining the current standard based on the annual average.” CASAC based this recommendation on the “limit ed evidence related to potential long-term effects of NO 2 exposure and the lack of strong eviden ce of no effect.” In addition, CASAC concluded that “the findings of the REA do not provide assuran ce that a short-term standard based on the one-hour maximum will necessari ly protect the popul ation from long-term exposures at levels potentially leading to adverse health effects” (Samet, 2008b). d. Administrator’s conclusions on averaging time In considering the most appropria te averaging tim e(s) for the NO 2 primary NAAQS, the Administrator notes the conclusions and judgments made in the ISA about available scientific evidence, conclusions from the REA, and CASAC recommendations discussed above. Based on these considerations, the Administrator proposes to set a new standard based on 1-hour daily maximum NO 2 concentrations. In addition, the Admin istrator notes that CASAC recommended retaining the current annual standard to account for the fact that some evidence suggests that long-term NO 2 exposures could cause adverse effects on respiratory health. Taking into account these considerations, in add ition to proposing a new 1-hour NO 2 primary NAAQS to provide increased protection against effects associated w ith short-term exposures, the Administrator also proposes to retain an annual standard.85 3. Form When evaluating alternativ e forms in conjunction withspecific levels, the REA considered the adequacy of the public health prot ection provided by the co mbination of level and form to be the foremost consideration. In addi tion, the REA recognized that it is desirable to have a form that is reasonably stable and insu lated from the impacts of extreme meteorological events. As noted in the review of the O 3 NAAQS (EPA, 2007e), forms that call for averaging of concentrations over three years be tter reflect pollutant- associated health risks than forms based on expected exceedances. This is because such “concentration-based” fo rms give proportionally greater weight to periods of time when pollutant concentrations are well above the level of the standard than to times when the concentrations are just above the standard, while an expected exceedance form would give the sa me weight to periods of time with concentrations that just exceed the standard as to times when concentr ations greatly exceed the standard. Averaging concentrations over three years also provides grea ter regulatory stability than a form based on allowing only a single expected exce edance in a year. Therefore, c onsistent with recent reviews of the O 3 and PM NAAQS, the REA focused on c oncentration-based forms averaged over 3
years. In considering specific concentration-based forms, the REA focused on 98 th and 99 th percentile concentrations averaged over 3 years. Withregard to these alternative forms, the REA noted that a 99 th percentile form for a 1-hour dail y maximum standard would correspond approximately to the 4 th highest daily maximum concentration 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 (Table 10-4 in the REA; see Thompson, 2008 for methods). The REA86 concluded that either of these forms could provide an appropriate balance between limiting peak
NO 2 concentrations and pr oviding sufficient regula tory stability. This is consistent with judgments made in the 2006 review of the PM NAAQS (EPA, 2005). When considering the extent to which exposur e and risk analyses inform judgments on the form of the standard, the REA noted that a 99 th percentile form could be appreciably more protective than a 98 th percentile form (for the same standard level) in some locations, as shown by the results of air quality analyses. For example, a 99 th percentile standard of 200 ppb was estimated to decrease the number of be nchmark exceedances, relative to a 98 th percentile form, by approximately 50-70% in Boston, Philadelphi a, and Washington, D.C. (Table 10-5 in the REA). However, a 99 th percentile form was estimated to decrease the number of benchmark exceedances by only approximately 10% in St. Louis, Detroit, and Las Vegas (Table 10-5 in the REA). For most locations analyzed, the differ ence was estimated to be between approximately 10 and 50% (Table 10-5 in the RE A). With regard to the A tlanta exposure assessment, a 99 th percentile form was estimated to decrease th e number of days with 6 or more benchmark exceedances (for 300 ppb) , relative to a 98 th percentile form, by 5-35% depending on the standard level selected (REA Appendix B, table B-48). With regard to the Atlanta risk assessment, a 99 th percentile form was estima ted to be associated with approximately 6% to 8% fewer NO 2 -related emergency department visits than a 98 th percentile form, across the levels of the potential 1-hour standards examined. When considering these results as they relate to the form of the standard, the REA noted that a decision on form must be ma de in conjunction with selection of a particular standard level. The primary emphasis in such a decision will be on the degr ee of public health protection provided by the combination of form and level.87 CASAC agreed with the importance of consid ering the public health protection provided by the combination of form and level. In its le tter to the EPA Administrat or with regard to the final REA, the CASAC panel st ated that it “advises that EPA choose a health protective percentile appropriate for the level chosen for the one-hour standard.” CASAC went on to recommend that a 98th percentile form would be appropriate for a standard level at the lower boundary of the range evaluated (50 ppb, see belo w) but that a higher percentile should be considered for higher levels (Samet, 2008b). When considering alternative forms, the Administrator note s the views expressed in the REA and the recommendations from CASAC, as described above. In particular, she notes that a
99 th percentile (or 4 th highest) form could be appreciably mo re protective in some locations than
a 98 th (or 7 th or 8 th highest) form. Given these considerati ons, and in light of the specific range proposed for level below, the Administrator proposes to adopt either a 99 th percentile or a 4 th highest form, averaged over 3 years. In addition, the Administrator notes that a 98 th percentile form could be appropriate, particul arly for standard levels at the low end of the range considered in the REA. Therefore, she also solicits comment on both 98 th percentile and 7 th or 8 th highest forms. 4. Level Monitoring studies show that a gradient in NO 2 concentrations exists around roadways with maximum concentrations occurring on or near majo r roadways and decreasing relatively quickly with distance from the road. These monitoring studies suggest that NO 2 concentrations within 50 m of a major roadway can be approximately 30 to 100% higher than concentrations at areawide monitors located away from major roadwa ys. Similarly, the REA used data comparison and modeling approaches to estimate that NO 2 concentrations on or near roads could be88 approximately 40% or 80% higher than concen trations away from ro ads, on average across multiple locations (see section II.A.2 a bove). In addition, the gradient of NO 2 concentrations around roadways would not be expected to ch ange substantially even as the absolute concentrations of NO 2 continue to decrease due to ongoing implementation of mobile source standards (see section I.B). That is, as NO 2 concentrations around roadways change, approximately proportional changes would also be expected farthe r away from those roadways. In identifying an appropriate range of standard levels to propose for a 1-hour standard, the Administrator notes the need to protect at-ris k individuals against the distribution of exposure concentrations across an area, including both the higher peak exposure concentrations that can occur on or near major roadways and the lower exposure concentrat ions that can occur generally across an area away from roadways, which can also reflect exposures related to stationary source emissions. In considering how to address the h ealth risks associated with exposures to these different concentrations, our evaluation has been informed by information drawn from the different types of sc ientific studies. Specifically, controlled human exposure studi es have reported associations between various levels of NO 2 exposures and increased airway re sponsiveness in asthmatics. These studies can inform an evaluation of the ri sks associated with exposure to specific NO 2 concentrations, regardless of where they occur in an area. They most directly inform consideration of the risks asso ciated with exposure to the higher concentrations of NO 2 expected to occur on or near roadways, which could includ e concentrations within the range evaluated in the human exposure studies. These human expos ure studies are less informative in evaluating the risks associated with exposures to the lower concentrations of NO 2 expected away from roadways, as these concentrations are typically lower than those evaluated in the studies.89 In addition, epidemiologic studies have repor ted associations between the patterns of NO 2 air quality across an area, as indexed by the lower concentrations measured at area-wide monitors in the current network, and respirator y symptoms, emergency department visits, and hospital admissions. The health effects reporte d in the epidemiologic studies are reasonably inferred to be associated with exposur e to both the higher short-term peak NO 2 concentrations that could occur on or near majo r roadways and the lower short-te rm concentrations that could occur away from roadways. Thus the epidemiologic studies can info rm an evaluation of the risks associated with the full range of expos ures likely to occur across an area. The Administrator has considered two appr oaches to setting the level of a 1-hour NO 2 primary NAAQS in order to pr ovide appropriate protection fro m the adverse health effects associated with short-termexposures to ambient NO 2 . In one approach (h ereafter referred to as the near-road approach), the level of the NO 2 primary NAAQS would reflect the maximum allowable NO 2 concentration anywhere in the area, a concentration that would typically be expected to occur on or near major roadways. 10 The level of a standard under the near-road approach would be set such that, when ava ilable information regard ing the concentration gradient around roadways is considered, appropr iate protection would be provided against the higher exposure concentrations expected to occur on and near major roadways, as well as the lower exposure concentrations expected to o ccur away from those roadways (i.e., area-wide concentrations as defined below). In the other approach (hereaft er referred to as the area-wide approach), the level of the NO 2 primary NAAQS would reflect a maximum allowable area-wide
NO 2 concentration. For convenience, the term “area-wide concentrati on” will be used to refer to ambient concentrations that o ccur away from major roadways, su ch as those concentrations

10 For ease of discussion,, the phrase “maximum allowable concentration near major roadways” will be used to refer to the maximum allowable concentration anywhere in an area, given that we would expect maximum concentrations across an area to typically occur near major roadways.90 measured by monitors sited for the purpose of monitoring area-wide concentrations as in the current monitoring netw ork (see sections II.A.1, II.F.4.e and III.B.3). Given the NO 2 concentration gradients that exist around roadways, a range of higher near-roadway concentrations would likely be a ssociated with a given area-wide concentration. The level of a standard under the area-wide approach would be set such that, when available information regarding the concentration gr adient around roadways is cons idered, appropriate protection would be provided against exposures to area-wide NO 2 concentrations as well as the higher NO 2 concentrations expected to occur on and near major roadways. In considering these two approaches, the Admin istrator notes that, inorder to achieve a given degree of public health protection, the a ppropriate standard leve l under the area-wide approach would be lower than the appropriate standard level under the near-road approach. 11 The magnitude of the difference between the levels would depend on the NO 2 concentration gradient around roadways. This issue is discu ssed in more detail below (section II.E.4.e). The remainder of this section describes the c onsiderations relevant to the Administrator’s proposed decision on standard leve l. Specifically, evidence-based considerations drawn from the ISA and discussed in the policy-assessment chapter of the REA are discusse d in section II.E.4.a. Exposure- and risk-based considerations drawn fr om the analyses in the REA and discussed in the policy assessment chapter are discussed in se ction II.E.4.b. A summary of the considerations from the policy assessment chap ter of the REA is presented in section II.E.4.c and CASAC views expressed in the context of their comme nts on the final REA are presented in section II.E.4.d. Finally, the Administr ator’s proposed conclusions re garding standard level are presented in section II.E.4.e.

11 Thus, the remainder of this section will distinguish be tween appropriate standard levels reflecting either a maximum allowable area-wide concentra tion or a maximum allowable concentration anywhere in a given area (e.g., likely to occur near major roads).91 a. Evidence-based considerations

Evidence-based considerations take into acc ount the full body of scientific evidence assessed in the ISA. When considering the extent to which this scientific evidence can inform a decision on standard level, the policy asse ssment chapter of the REA notes that NO 2 concentrations represent differ ent measures of exposure when drawn from experimental versus epidemiologic studies. NO 2 concentrations tested in experime ntal studies, such as controlled human exposure studies, represent exposure levels inthe breathing zone of the individual test subjects. In cases where controlled human e xposure studies report eff ects, those effects are caused directly by exposure to a specified concentration of NO 2 . In contrast, concentrations of
NO 2 drawn from epidemiologic studie s are often based on ambient mo nitoring data. In the case of key U.S. studies that have been specifically considered within the context of assessing the appropriate level for the standard, these monitors measure area-wide NO 2 concentrations that occur away from major roadways. NO 2 concentrations recorded at the ambient monitors used in these studies are used as surr ogates for the distribution of NO 2 exposures across the study area and over the time period of the study. These am bient monitors do not dir ectly measure the full range of concentrations in an area, instead they capture one part of the di stribution (the area-wide concentration) and this is used as a surrogate for the entire distributi on. Thus, the area-wide monitored NO 2 concentrations associated with the studi es would not be expected to directly identify the full range of personal exposur e levels that are responsible for the NO 2 -associated health effects reported in th e studies. As noted in the REA, the interpretation of NO 2 concentrations from diff erent types of studies is an impo rtant consideration for decisions on standard level. These implica tions are discussed in more deta il below in section II.E.4.e. In considering the epidemiologic evidence, the REA noted the ISA conclusion that92 epidemiologic studies provide the strongest support for the link be tween short-term NO 2 exposure and respiratory morbidity. In additi on, epidemiologic studies provide evidence for the most serious NO 2 -associated respiratory effects, in cluding respiratory-related hospital admissions and emergency departme nt visits. As noted above, these effects have been reported to be associated with area-wide NO 2 concentrations in key U.S. studies, and are reasonably inferred to be associated with exposur e to both the higher short-term peak NO 2 concentrations that could occur on or near roadways and the lo wer short-term concentrations that could occur away from roadways. The reporte d effects are logically linked to the types of peak exposures that can occur on or near major roadways and that have been reporte d in controlled human exposure studies to increase airway re sponsiveness in asthmatics. When evaluating the epidemiologi c literature for its potential to inform the selection of an appropriate range of standard levels , the REA noted the ISA conclusion that NO 2 epidemiologic studies provide “l ittle evidence of any effect threshold” (ISA, section 5.3.2.9, p. 5- 15). In studies that have evaluated concentr ation-response relationshi ps, they appear linear within the observed range of data (ISA, section 5.3.2.9). Given this lack of an apparent threshold below which effects do not occur, an important consideration with rega rd to providing an adequate margin of safety is the extent to wh ich it is appropriate fo r the range of proposed standard levels to extend below NO 2 concentrations that have been associated with health effects in these studies. For purposes of using the epidemiologic evidence to identify a range of standard levels for evaluation in the absence of an apparent threshold, the REA considered the range of NO 2 concentrations that have been monitore d in locations, and during time periods, of key U.S. epidemiologic studi es (ISA, Table 5.4-1). Figures 1 and 2 below (REA, Figures 5-1 and 5-2) show standardized effect estimates93 and the 99 th and 98 th percentiles of the 1-hour daily maximum NO 2 concentrations recorded at area-wide monitors in the locati ons, and during the time periods, of key U.S. studies. The peak
NO 2 concentrations to which indi viduals were exposed on roadways in these locations during the study periods could be substantia lly higher than the concentrations recorded at these area-wide monitors (e.g., up to 2-fold higher). Th e lowest area-wide 1-hour daily maximum concentrations, 53 (99 th percentile) and 50 (98 th percentile) ppb, were mo nitored in the location of the study by Delfino et al. (2002). This single study reported mixed re sults for respiratory symptoms with most reported NO 2 effect estimates being positiv e, and with some but not all positive effect estimates being st atistically-significant. A cluste r of 5 studies (Ito et al., 2007; Jaffe et al., 2003; NYDOH, 2006; P eel et al., 2005; Tolbert et al., 2007; ) were conducted in locations with area-wide 1-hour daily maximum NO 2 concentrations rang ing from 93 to 112 ppb (99 th percentile) and fro m 85 to 94 ppb (98 th percentile). These studies reported generally positive and often statistically-significant NO 2 effect estimates for resp iratory-related emergency department visits and hospital admissions in a variety of lo cations across the U.S. Two additional studies (Linn et al ., 2000; Ostro et al., 2001) report ed positive and statistically significant NO 2 effect estimates in locations with a ppreciably higher area-wide 1-hour daily maximum NO 2 concentrations (i.e., around 200 ppb).94 Figure 1. NO 2 effect estimates 12 (95% CI) for emergency department visits/hospital admissions and 1-hour daily maximum NO 2 concentrations (98 th and 99 th percentile values in boxes 13 )

12 Effect estimates presented in Figures 1 and 2 are from single pollutant models. 13 Authors of relevant U.S. and Canadian studies were c ontacted and, for each study, air quality statistics were requested from the monitor th at recorded the highest NO 2 concentrations. In cases where authors provided 1-hour daily maximum air quality statistics, this information is presented in Figures 1 and 2 (studies by Tolbert, Peel, NYDOH, Delfino). In four cases (studies by Ito, Jaffe, Linn, Ostro), we were not able to identify 1-hour NO 2 statistics from the information provided by the auth ors. In these cases, we evaluated monitored NO 2 concentrations reported to EPA’s Air Quality System (AQS) for the location and time of the study. Figures 1 and 2 present the highest 98 th/99 thpercentile 1-hour daily maximum NO 2 concentrations that corresp ond to each study location and time period. Prior to identif ying potential alternative standards, we did not receive air quality information from any of the Canadian authors contacted and we were unable to reconstruct the air quality data sets for the Canadian studies. Therefore, for purposes of identifying levels of potential alternative standards, our analysis was based on these key U.S. studies. Note that the NO 2 concentrations reported in the study by Jaffe are labeled as 24-hour concentrations, but the author indicated in a personal co mmunication (Jaffe, 2008) that they actually represent 1- hour daily maximum concentrations.95 Figure 2. NO 2 effect estimates for respiratory symp toms and associated 1-hour daily maximum NO 2 levels (98 th and 99 th percentile values in boxes)96 When evaluating the controlled human exposure literature for its potential to inform the selection of a range of appropria te standard levels for evaluation, the REA noted that available studies have addressed the consequences of short-term (e.g., 30-minutes to several hours) NO 2 exposures for a number of hea lth endpoints including airway responsiveness, host defense and immunity, inflammation, and lung function (ISA, section 3.1). In identifying health endpoints on which to focus for purposes of informing decisi ons about potential altern ative standard levels, the REA concluded that it was appropriate to fo cus on those endpoints that occur at or near ambient levels of NO 2 and endpoints that are of potential public health significance. As described above in more detail (s ection II.C.1), the only en dpoint to meet both of these criteria is increased airway responsiveness in as thmatics. The ISA concluded that NO 2 exposures between 200 and 300 ppb for 30 minutes and 100 ppb for 60-mi nutes can result in small but significant increases in nonspecific airway responsivene ss (ISA, section 5.3.2.1) and that “transient increases in airway responsiveness following NO 2 exposure have the potential to increase symptoms and worsen asthma control” (ISA, se ctions 3.1.3 and 5.4). This effect could have important public health implications due to the large si ze of the asthmatic population in the United States (ISA, Table 4.4-1). In addition, NO 2 effects on airway responsiveness in asthmatics are part of the body of experimental evidence that pr ovides plausibility and coherence for the observed NO 2 -related increase in hospital admissions and emergency department visits in epidemiologic studies (ISA, s ection 5.3.2.1). For all of these reasons, the REA focused on increased airway responsiveness in asthmatics when considering the controlled human exposure literature in terms of its ab ility to inform decisions on a lternative standard levels. With regard to controlled human exposure st udies of airway responsiveness, the ISA and the REA discussed a meta-analysis of individua l level data from 19 studi es involving asthmatic97 volunteers (ISA, Table 3.1- 3; REA, section 3.1.3.2 and Table 4- 5). This meta-analysis is an update of an analysis considered in the 1993 NO X AQCD and published by Folinsbee (1992). The studies included in the original meta-analysis by Folinsbee used NO 2 exposure levels between 100 and 1000 ppb. Most of the studies in cluded in the original meta-analysis used nonspecific bronchoconstricting agents such as methac holine, carbachol, hista mine, or cold air. Compared to asthmatics exposed to NO 2 while exercising, a larger percentage of asthmatics exposed during rest experienced an NO 2 -related increase in airway responsiveness. The reason for this is not known as the factors that predispose some asthmatics to NO 2 responsiveness are not understood (ISA, section 3.1.3.2). Among subjects exposed at rest, 76% experienced increased airway responsiveness following exposure to NO 2 levels between 200 and 300 ppb. The update of this meta-analysis presente d in the ISA (Table 3.1-3) included one additional study of non-specific responsiveness and removed an allergen responsiveness study that was included in the original (see ISA, section 3.1.3.2 for more discussion). It also reported results for an additional NO 2 concentration, 100 ppb. This update d analysis reported that, when exposed at rest, 66% of asthmatics experienced an increase in airway responsiveness following exposure to 100 ppb NO 2 , 67% of asthmatics experienced an increase in airway responsiveness following exposure to NO 2 concentrations from 100 to 150 p pb, 75% of asthmatics experienced an increase in airway responsiveness following exposure to NO 2 concentrations from 200 to 300 ppb, and 73% of asthmatics experienced an in crease in airway responsiveness following exposure to NO 2 concentrations above 300 ppb. The fr action of asthmatics experiencing an increase in airway responsiveness was sta tistically significant at each of these NO 2 concentrations. Based on this evidence, we have identified exposure to NO 2 at a level of 100 ppb to be98 the lowest level at which effects have been observed in controlled human exposure studies, noting that it is also the lowest level tested in the studies used in the me ta-analysis. There is no evidence from this meta-analysis, howe ver, of a threshold below which NO 2 -related effects do

not occur. b. Exposure- and risk-based considerations Chapters 7-9 of the REA estimated exposures and health risks associated with recent air quality and with air quality, as measured at moni tors in the current area-wide network, that had been adjusted to simulate just meeting the current and potential alternative standards. The specific standard levels evaluated, for an area-wi de 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. The results of the air quality, exposure, and ri sk analyses are presented below in Table 1. With regard to the air quality results, table 1 presents the numb er of days per year that NO 2 concentrations on/near roads were estimated to e qual or exceed the lowest and the highest health benchmarks evaluated (100 and 300 ppb). Compared to just meeting the cu rrent annual standard, exceedances estimated to be asso ciated with just meeting 99 th percentile 1-hour daily maximum area-wide standard levels of either 50 or 100 ppb were substantially lower. In contrast, exceedances estimated to be associated with 1- hour area-wide standards of 150 or 200 ppb were either similar to, or slightly higher than, those es timated for just meeting the current standard. Table 1 also presents the results of the Atlanta exposure and risk assessments. Similar to the air quality analyses, NO 2 exposures and risks estimated to be associated with ju st meeting 1-hour area-wide standard levels of either 50 or 100 ppb were substantially lower than those associated with just meeting the current annual standard. Exposures and risks estimated to be associated99 with 1-hour area-wide standard levels of 150 or 200 ppb were somewhat lower than, or similar to, those estimated for just meeti ng the current annual standard.100 Table 1. Summary of results of the exposure and risk analyses presented in the REA101 c. Summary of considerations from the REA The policy assessment chapter of the REA cons idered the scientific evidence and the exposure/risk information as they re late to the level of a 1-hour NO 2 standard. The REA did not explicitly address whether the range of st andard levels supported by the evidence and information would differ dependi ng on whether the standard is se t in conjunction with the areawide approach (maximum allo wable area-wide level) or in conjunction with a near-road approach (maximum allowable peak level). Howe ver, the analyses presented in the REA were premised on the use of area-wide monitors. That is, in simulating just meeting different standards, NO 2 concentrations were adjusted at moni tors in the current area-wide network. Therefore, the estimates of potenti al public health risks associated with each standard evaluated in the REA are applicable to th e area-wide approach. Further, while the REA did not explicitly address whether its conclusions were more appr opriately applied to an area-wide approach versus a near-road approach, they are more directly applicable to the former than the latter. The implications of the REA conclusions regarding st andard level, as they relate to each of the approaches considered by the Administrator (i.e ., area-wide and near-road), are discussed below in section II.B.4.e. When considering an appropr iate lower end of a range of levels supported by the scientific evidence, the REA noted the following:

• The epidemiologic study by De lfino et al., (2002) provides evidence for associations between short-term ambient NO 2 concentrations and respiratory morbidity in a location (Alpine, CA) where area-wide NO 2 concentrations were well below levels in other key U.S. epidemiol ogic studies. This stud y reported mostly positive associations, some of which were statisti cally significant, between 1-hour and 8-hour102 (only 8-hour associations were sta tistically significant) levels of NO 2 and asthma symptoms in a location where the 99 th and 98 th percentile 1-hour daily maximum NO 2 concentrations recorded at an area-wide monitor during the time period of the study were 53 and 50 ppb, respectively.

• The meta-analysis of controlled human expos ure studies reported increased airway responsiveness in asthmatics at the lowest NO 2 concentration for which data were available (i.e. 100 ppb). In identifying th e specific lower level that could be reasonably supported by this controlled humanexposure evidence, there were several reasons why it was appropriate to considerlevels below 100 ppb. First, the metaanalysis did not provide inform ation on the poten tial for an NO 2 -induced increase in airway responsiveness at concentrations below 100 ppb, leaving open the possibility for effects following exposures to lower c oncentrations. Second, the studies included in the meta-analysis did not evaluate se vere asthmatics and most of the subjects included in these studies were mild asthmati cs. Asthmatics characterized as having more severe asthma may be more susceptible th an mild asthmatics to the effects of
NO 2 exposure (ISA, section 3.1.3.2).

Thus, the REA concluded that it was appropria te to base the lower end of the range of standard levels on NO 2 concentrations in the location of th e epidemiologic study by Delfino and on providing increased protecti on relative to the lowest leve l at which increased airway responsiveness in asthmatics was reported in controlled human exposur e studies. Given the mixed results reported in the Delfino study, th e REA concluded that it was appropriate to consider area-wide standard levels equal to, rather than below, t hose measured in the location of103 the study. Given these considerations, the REA concluded that an appropr iate lower end of the range of area-wide standard levels supporte d by the scientific evidence is 50 ppb. When considering an appropr iate upper end of the range of 1-hour daily maximum standard levels supported by the scientific evidence, the REA not ed the following:

• Positive and statistically significant associa tions were observed in several key U.S. epidemiologic studies in locations with area-wide 98 th /99 th percentile 1-hour daily maximum NO 2 concentrations rang ing from 85 to 112 ppb (Peel et al., 2005; NYDOH, 2006; Ito et al., 2007; Tolbert et al., 2007) (see Figure 1 above). To the extent that uncertainties associated with ascribing e ffects reported in the Delfino study to NO 2 are emphasized, this cluster of studies provides stronger support for 1- hour standard levels close to 100 ppb.

• The meta-analysis of airway responsivene ss presented in the ISA reported increased airway responsiveness in most asthmatics following short-term exposures to 100 ppb
NO 2 , which was the lowest concentration for wh ich such data were available. Although some uncertainties associated with this evid ence, as described above, provide support for considering standard levels below 100 ppb, others (i.e., the magnitude and clinicalsignificance of the NO 2 -associated increase in airway re sponsiveness) provide support for considering higher standard levels.

Given these considerations, the REA conclude d that the scientific evidence provides support for an area-wide standard level up to 100 ppb. The REA also noted that, to the extent more emphasis is placed on the uncertainties associated with ascribing effects to NO 2 in the cluster of epidemiologic studies and on the magnitude and clinical-significance of the NO 2 -104 associated increase in airway res ponsiveness following exposure to NO 2 , standard levels higher than 100 ppb could be considered. However, th e strongest support was concluded to be for standard levels at or below 100 ppb.

In addition to these evidence-based consider ations, the REA compared the health risks estimated to be associated withjust meeting the current standard to those estimated to be associated with differ ent 1-hour area-wide standards. As noted above (sec tion II.C), the REA characterized NO 2 -associated health risks by estima ting air quality exceedances of NO 2 concentrations reported to incr ease airway responsiveness, exposures of asthmatics to NO 2 concentrations reported to increase airway responsiveness, and the incidence of NO 2 -associated emergency department visits. Given the RE A conclusion that the available evidence and information clearly call into ques tion the adequacy of the current standard, the adequacy of any 1-hour standards would also be called into quest ion if those standards were estimated to be associated with similar or higher risks. In cons idering the three analyses that characterized NO 2 - associated health risks, the REA noted that just meeting 1-hour area-wide standard levels of 150 and 200 ppb was estimated to be associated with risks ranging from somewh at lower to slightly higher than those estimated for the just meeting th e current standard. In contrast, just meeting 1- hour standard levels of 50 or 100 ppb, in c onjunction with the current area-wide monitoring network, was estimated to result in appreciably lower health risks than the current standard. Given this, the REA concluded that the exposure/ risk information reinfo rces the scientific evidence in supporting an area-wide standard level from 50 to 100 ppb.

d. CASAC views CASAC expressed their views ina letter to the EPA Admini strator (Samet, 2008b) within the context of their review of the final REA, a review which focused primarily on the policy105 assessment chapter. 14 In drawing conclusions regarding the level of a short-term standard, CASAC considered the scientific evidence evaluate d in the ISA, the exposure and risk results presented in the REA, and the evidence- and risk -based considerations presented in the policy assessment chapter of the REA. CASAC concurred with th e conclusion from the policy assessment chapter that the strongest support is for standard levels between 50 and 100 ppb. Their letter noted that, “CASAC firmly recommends that the upper end of the range not exceed 100 ppb.” In considering the impact of margin of safety on standard level, CASAC noted that “the intent of the Clean Air Act is to protect public health with anadequate margin of safety and consequently uncertainty should be considered as a reason to move towards the lower end of the range of levels and not to the upper.” In addition, with regard to the NO 2 concentration gradient around roadways, CASAC noted that “the highest exposures likely occur when individuals are near roadways.” As a result they recommended that the Agency cons ider the implications of this exposure issue when interpreting the evidence an d when considering the siting of regulatory monitors. CASAC comments were offered within the cont ext of the final REA, which analyzed the potential public health implicati ons of different area-wide standa rds and did not discuss potential differences between area-wide and near-road standard s. Thus, it is not clear the degree to which CASAC recommendations might diff er for a standard level that would be determined with respect to maximum allowable near-road NO 2 concentrations. As noted in section I.C above, we are specifically soliciting CASAC input on the appropr iate level, or range of levels, with respect to a standard set using the near-road a pproach, reflecting a maximum allowable NO 2 concentration near major roads.

14 Earlier CASAC letters focused on their review of the air quality, exposure, and risk analyses as presented in other chapters of the draft REA.106 In drawing conclusions regarding the leve l of an annual standard, CASAC noted the scientific evidence assessed in the ISA. Speci fically, CASAC concluded that while there is evidence supporting the li nk between long-term NO 2 exposure and adverse health effects, this evidence does not provide a strong quantitative basi s for changing the level of the current annual standard. Therefore, with regard to the annual standard, CASAC recommended “ retaining the current level, as evidence has not been cited that would lead to either an increase or decrease” (Samet, 2008b). e. Administrator’s conclusions on level In considering the appropriate level for an NO 2 standard based on the 3-year average of the 99 th percentile (or 4 th highest) 1-hour daily maximum NO 2 concentration, the Administrator notes the need to protect at-ris k individuals from an array of health effects that have been associated with NO 2 exposures. Specifically, the Adminis trator has considered the extent to which a variety of levels under two sepa rate approaches for setting a 1-hour NO 2 standard would be expected to protect at-risk i ndividuals against exposures to NO 2 concentrations reported in controlled human exposure studies to increase airway responsiveness in asthmatics as well as against the patterns of NO 2 air quality, as indicated by concen trations measured at monitors in the current area wide-oriented netw ork, reported in epidemiologic st udies to be associated with respiratory symptoms, emergency department vis its, and hospital admissions . The Administrator notes that these health endpoints are logically linked together in that the evidence for increased airway responsiveness is part of the body of experimental evid ence that the ISA recognized as supporting the plausibility of a ssociations reported in epidemio logic studies and given that NO 2 exposure patterns associated with respiratory morbidity in epidemiologic studies plausibly include short-term peak exposures on and/or near major roadways that could increase airway responsiveness in asthmatics. As discussed pr eviously, the Administrator has considered two107 approaches to setti ng a 1-hour primary NO 2 standard in order to pr ovide appropriate protection of public health against th is array of effects. In one approach (i.e., the area-wide approach), the NO 2 primary NAAQS level would reflect maximum allo wable area-wide NO 2 concentrations. The area-wide level under this approach would be chosen to protect against heal th effects associated with the overall pattern of
NO 2 air quality across an area, as i ndicated by concentrations at ar ea-wide monitors. In addition, the level of a standard under this approach woul d be set such that, when available information regarding the concentration gr adient around roadways is cons idered, appropriate protection would also be provided against the higher NO 2 exposures on and near major roadways. For purposes of determining compliance with the NAAQ S, it would be appropriate to compare the level of such an area-wide standard to area-wide NO 2 concentrations (see section III.B.2.c). Comparing a standard level set under this approach to NO 2 concentrations that occur near major roadways, where considerably higher concentrations would be expected, would have the effect of increasing the stringency of the area-wide standard re lative to what was determined to be requisite in setting such a standard. In the other approach (i.e. the near road approach), the NO 2 primary NAAQS level would reflect maximum allowable NO 2 concentrations anywhere in an area, which we would expect to occur near major roadways. Under this approach, the standard level would be chosen to protect against health effects asso ciated with the higher NO 2 exposures expected on and near major roadways. In addition, the level of a standard un der this approach would be set such that, when available information regarding the concentration gradient around roadways is considered, appropriate protection would also be provided against exposure to the overall distribution of NO 2 air quality, including the lower concentrations exp ected to occur away from major roadways.108 In considering these two approaches, th e Administrator notes that area-wide concentrations would be expected to be lower ina given area than near-r oad concentrations in that same area because of the NO 2 concentration gradient around ro adways. Therefore, in order to achieve a given degree of public health protect ion, the appropriate standard level would differ depending on the approach chosen. Specifically, in order to provide a comparable level of public health protection, the level of a standard reflecting maximum allowable area-wide NO 2 concentrations would be lower than the level of a standard reflecti ng maximum allowable NO 2 concentrations near major roadways. 15 As discussed above, the magnitude of the diff erence between the appropriate levels under these two approaches depends on the nature and magnitude of the NO 2 concentration gradient around roadways. In considering th is issue, the Administrator not es that (1) monitoring studies suggest that NO 2 concentrations within 50 m of a majo r roadway can be approximately 30 to 100% higher than concentrations away from major roadways and (2) estimates from the REA, which are based on monitor comparisons and model simulations, suggest that NO 2 concentrations on or near roads could be 40% or 80% higher than con centrations away from roads, on average across multiple locations. In considering this information, the Administrator notes that, in order to provide a comparable degree of public hea lth protection, a standard level reflecting maximum allowable NO 2 concentrations near major ro adways could appropriately be from 30 to 100% higher thana level reflecting maximu m allowable area-wide NO 2 concentrations. Further, the Administr ator notes that an assumption that NO 2 concentrations on or near roadways are approxima tely 65% higher than concentra tions away from roadways would

15 As noted above, for ease of discussion, the phrases maximum allowable peak concentration or maximum allowable concentration near roadways will be used to re fer to the maximum allowable concentration anywhere in an area, which we would expect to occur near major roadways.109 reflect a reasonable mid-poi nt based on the available informati on. The impact of the gradient on the proposed range of standard levels is discussed more specifically below. The remainder of this section describes th e following. First, the Administrator’s conclusions are described with regard to the emph asis placed on the different scientific evidence and exposure/risk information. Second, the Admin istrator’s conclusions are described with regard to the appropriate range of levels to consider in conjuncti on with the area-wide approach. Third, the Administrator’s conclusions are descri bed with regard to the appropriate range of levels to consider in conjunction with the n ear-road approach. Fourth, the Administrator’s conclusions with regard to the strengths and limitations of each approach are described followed by the Administrator’s proposed approach to setting a 1-hour primary NO 2 standard and her range of proposed levels. Finally, the Administra tor’s conclusions with regard to the level of the annual standard are described. To inform the appropriate range of leve ls to consider under each approach, the Administrator has considered the scientific evid ence and exposure/risk results. In considering the weight to place on different evidence and info rmation, the Administrator has considered the results of epidemiologic studies, controlled hu man exposure studies, and exposure/risk analyses as well as the uncertainties associated with this evidence and information. Specifically, she notes the following:

• The ISA concluded that epidemiologic studi es provide the strongest support for the relationship between short-term exposure to NO 2 and respiratory mo rbidity. Despite the possibility that associations between health effects and NO 2 in epidemiologic studies may be confounded by the presence of co-occurring pollutants, particularly other traffic-relate d pollutants, the IS A concluded that NO 2 effect estimates remain110 robust in multi-pollutant mode ls and that the evidence supports a direct effect of NO 2 exposures on respiratory morbidity, independe nt of associations with other trafficrelated pollutants. Give n this conclusion, along with conclusions from the ISA regarding the consistency and the coherence of results across the relatively large number of NO 2 epidemiologic studies (both i ndoor and outdoor) and the supporting evidence from experimental studies, the Admin istrator has determined it appropriate to place substantial weight on epidemiologic studies in identifying an appropriate range of levels to propose.

• Controlled human exposure studies repor t that short-term exposures to NO 2 can increase airway responsiveness in asthmatics . With regard to this evidence, the Administrator also has considered the uncer tainties associated with the magnitude and the clinical relevance of the NO 2 -associated increase in ai rway responsiveness, noting that this effect may or may not be clini cally-significant for any given asthmatic. However, given the potential public health sign ificance of this effect, due to the large size of the asthmatic popul ation in the U.S. and th e possibility that the NO 2 - associated increase in airway responsive ness could worsen asthma symptoms and decrease control of asthma, the Administrator concludes that it is appropriate to also place some weight on this evidence when identifying an appropriate range of levels to
propose.

• The results of the risk and exposure an alyses presented in the REA provide information on the potential public health im plications of setting the standard at different levels. The Admin istrator acknowledges the uncer tainties associated with these analyses which, as discussed in th e REA, could result in either over- or111 underestimates of NO 2 -associated health risks. Howe ver, she also notes that those uncertainties should be simila r across different air quality simulations within the air quality, exposure, and risk analyses. Ther efore, the Administrator concludes that these analyses are potentially useful for considering the relati ve levels of public health protection that could be provi ded by specific standard levels.

To inform the identification of an appropriate range of levels that would reflect maximum allowable area-wide NO 2 concentrations, the Administrator notes that it is appropriate to consider directly the NO 2 concentrations measured by monito rs in the current area-wide network in locations of key U.S. epid emiologic studies, without adjust ment to account for the roadway gradient. Similarly, it is appropriate to consider directly the health risks estimated in the REA to be associated with just meeting different 1-hour standards, as NO 2 air quality was simulated in these analyses by adjusting NO 2 concentrations at mo nitors in the current area-wide network. In contrast, because area-wide concentrations woul d be related to but lo wer than the maximum NO 2 concentrations likely to occur in a given area (i.e ., on or near major roadwa ys), the Administrator notes that it is appropriate to consider NO 2 concentrations reported to increase airway responsiveness in asthmatics w ithin the context of the NO 2 concentration gradient that exists around roadways. The Administrator’s consider ation of this evidence and information is discussed in more detail below. In considering the epidemiologic evidence as it relates to the level of a standard reflecting maximum allowable area-wide NO 2 concentrations, the Admini strator first notes the NO 2 concentrations that have been measured at ar ea-wide monitors in locations where key U.S. epidemiologic studies have reported associations with respiratory morbidity. Specifically, the112 Administrator notes that the study by Delfino provides mixed evidence fo r effects in a location with a 99 th percentile 1-hour daily maximum NO 2 concentration, as measured by an area-wide monitor, of 53 ppb. In that st udy, most of the reported NO 2 effect estimates were positive, but not statistically significant. In addition, she notes that 5 ke y U.S. studies provide stronger evidence for effects in locations where 99 th percentile 1-hou r daily maximum NO 2 concentrations measured at ar ea-wide monitors range d from 93 to 112 ppb. Gi ven that there is not an observable threshold concentration below which effects do not occur, the Administrator also notes the need to consider how far below th ese concentrations it is appropriate to set an area-wide standard level, taking into consideration the need to provide an adequate margin of safety. Focusing on the cluster of 5 epidemiologic studies, the Administrator notes that they provide support for a standardlevel below 90 ppb to provide protection against the reported effects. Given the mixed results of the Delfino study, the Administr ator concludes that it is not necessary to set the level below about 50 ppb to pr ovide protection against the effects reported in epidemiologic studies. Therefore, for an NO 2 primary standard le vel reflecting maximum allowable area-wide concentrations, the Administr ator concludes that th e epidemiologic evidence provides support for standard levels from 50 ppb to below about 90 ppb, though she notes that standard levels considerably below 90 ppb could be appropriate in order to provide an adequate margin of safety against these effects. In considering the controlled human exposure evidence as it relates to the level of a standard reflecting maximu m allowable area-wide NO 2 concentrations, the Administrator notes that 100 ppb is the lowest NO 2 concentration for which airway responsiveness data are available in humans. In considering the implications of this evidence, she also notes the following:113 • The meta-analysis of controlled human expos ure data in the ISA reported increased airway responsiveness in asthmati cs following exposure to 100 ppb NO 2 .

• This meta-analysis does not provide informat ion regarding effects at lower levels, nor does it provide any evidence of an effects thre shold below which effects would not occur. Further, the studies included in the meta-analysis evaluated primarily mild asthmatics while more severely affected individuals could respond to lower concentrations. Therefore, it is possi ble that exposure to NO 2 concentrations below 100 ppb could increase airway responsiveness in some asthmatics.

• The magnitude of the NO 2 -induced increase in airway responsiveness, and its clinical implications for a given asthmatic, cannot be quantified from the meta-analysis. As noted previously, the NO 2 -induced increase in airway res ponsiveness may or may not be clinically significant for a given asthmatic.

• The controlled human exposure studies of airway responsiveness evaluated effects following actual exposure to NO 2 . However, if the level of the standard is set with respect to maximum allowabl e area-wide concentrations, individuals are likely to experience higher exposure concentrations on or near major roadways. If NO 2 concentrations on or near major roads are 65% higher than those away from roads, a concentration of 60 ppb measured at an area-wide monitor could correspond approximately to 100 ppb on or near the road.

Given these considerations, for an NO 2 primary standard level reflecting maximum allowable area-wide concentrations, the Adminis trator concludes that the controlled human exposure evidence provides the greatest support for an area-wide standard level of about 60 ppb.114 She notes that an area-wide st andard level of 60 ppb would ge nerally be expected to limit exposures on or near road s to approximately 100 ppb NO 2 . However, given the variability in the
NO 2 concentration gradient around roadways across locations and time, the Administrator also notes that the controlled huma n exposure evidence could provide support for area-wide standard levels as high as 75 ppb, reflecting an assumption that concentrations on /near roadways are 30% higher than concentrations aw ay from roadways (100 ppb is approximately 30% higher than 75 ppb) and as low as 50 ppb, reflecting an assumpti on that concentrations on/near roadways are 100% higher than concentrations away from ro adways (100 ppb is 100% higher than 50 ppb). In considering the implications for standard level of estimates of NO 2 -associated exposures and risks, the Administrator notes the conclusions of the REA. Specifically, she notes that of the levels ex amined, an area-wide NO 2 primary NAAQS would need to have a level at or below approximately 100 ppb in order to achieve substantial risk reductions compared to the current annual standard. Given th at the Administrator has conclude d that the current standard is not requisite to protect public health with an ad equate margin of safety (see above), she notes that the exposure/risk results provide support for a standard le vel reflecting maximum allowable area-wide concentrations at or below 100 ppb. Given all of the above considerations, the Ad ministrator concludes that, for an area-wide standard, the greatest support is fo r a standard level of approximatel y 60 ppb. With regard to this level, the Administrator notes that 60 ppb is well-below peak 1-hour NO 2 concentrations in multiple U.S. locations where epidemiologic studies have reported positive, and often statistically significant, associations between NO 2 and emergency department visits and hospital admissions, recognizing the substant ial weight that is appropriat e to place on these studies. The Administrator also notes that while it is above the 50 ppb level reported in the Delfino study, the115 limited inferences to be drawn from the results of that one study, as discussed above, indicate that it could be appropriate to set a level significantly below 90 ppb but above 50 ppb. In addition, the Administrator notes that an area-wi de concentration of 60 ppb could correspond to
NO 2 concentrations on or near a major roadway of approximat ely 100 ppb (i.e., based on our evaluation that it is reasonable to estimate concentrations on or n ear roadways to be 65% higher than concentrations away from those roads), providi ng protection against roadway-associated exposures to NO 2 concentrations that have been reported to increase airway responsiveness in asthmatics. The Administrator al so notes that, compared to the current annual standard, a 1-hour area-wide standard with a level of 60 ppb would be estimated to substantially reduce allowable risks. Given these considerations, the Admini strator concludes that the evidence and other information provides the most suppo rt for an area-wide 1-hour standard with a level of 60 ppb as requisite to protect public h ealth against the array of NO 2 -associated health effects with an adequate margin of safety. However, the Administrator also notes that, to the extent a fina l decision on standard level emphasizes the results reported in the study by Delfino and colleagues, the possibility that
NO 2 -induced airway responsivene ss in asthmatics could occur following exposures below 100 ppb, or the possibility that NO 2 concentrations on/near roadways could be closer to 100% higher than area-wide concentrations, the evidence would support setting the level below 60 ppb. Specifically, she notes that a standard level of 50 ppb would provide so me protection against potential respiratory effects in locatio ns with relatively low area-wide NO 2 concentrations and that a level of 50 ppb would be expected to pr ovide protection against exposure concentrations below those that have been reported to increase airway responsiveness in asthmatics (i.e., an area-wide concentration of 50 ppb could corr espond to a concentration on/near roads of116 approximately 80 ppb based on our evaluation that it is reasonable to estimate concentrations on or near roadways to be 65% higher than concentr ations away from those roads). She also notes that, if on-/near-roadway concentrations are 100% higher than area-wide concentrations, an areawide standard level of 50 ppb could limit roadway-associated exposures to 100 ppb. In addition, the Administrator notes that, to the extent a final decision on standard level emphasizes the possibility that NO 2 concentrations on/near roadwa ys are closer to 30% higher than concentrations measured at area-wide monitors or the un certainty associated with the magnitude and the clinical significance of the NO 2 -induced increase in airway responsiveness, the evidence could support setting the le vel above 60 ppb. Specifically, if NO 2 concentrations on/near roads are 30% higher than those measured at area-wide monitors in the current network, then an area-wide standard level of 75 ppb would limit roadway-associated exposures to concentrations of 100 ppb. However, if the difference in concentrat ions is greater (e.g., closer to 65%), the Administrator notes that an area-wide NO 2 concentration of 75 ppb could correspond
to NO 2 concentrations greater than 100 ppb on or n ear roadways (i.e., 125 ppb if concentrations on or near roadways are 65% higher than concen trations away from those roads). In addition, she notes that a level of 75 ppb w ould still mainta in area-wide NO 2 concentrations considerably lower than those in the cluster of key U.S. studies that provide evidence for NO 2 -associated emergency department visits and hospital admissions. To inform the identification of a range of standard levels reflec ting maximum allowable
NO 2 concentrations near major roadways that w ould provide public healthprotection equivalent to that provided by an area-wide standard le vel of approximately 60 ppb, the Administrator has considered the NO 2 concentration gradient around roadwa ys. Specifically, she notes that an area-wide NO 2 concentration of 60 ppb could correspond to an NO 2 concentration on or near a117 roadway in the same area of approximately 100 ppb, based on our evaluati on that it is reasonable to estimate concentrations on or near roadways to be 65% higher than concentrations away from those roads. Therefore, a near-road standard level of 100 ppb could provide a level of public health protection that is equivale nt to that provided by an area- wide standard level of 60 ppb. In addition, if NO 2 concentrations on/near ma jor roads are 30% higher thanthose measured at areawide monitors in the current network, then a near-roadway standard level of 100 ppb could provide a level of public health protection that is equivalent to that pr ovided by an area-wide standard level of 75 ppb. However, if the differ ence in concentrations is greater (e.g., closer to 100%), the Administrator notes that a near-roadway standard level of 100 ppb could provide a level of public health protection eq uivalent to that provided by anarea-wide standard level of 50 ppb. The Administrator also notes that a near-ro ad standard with a level of 100 ppb could limit roadway-associated exposures to NO 2 that have been reported to increase airway responsiveness in asthmatics. Therefore, with regard to a standard level reflec ting maximum allowable NO 2 concentrations near major roadwa ys, the Administrator concludes th at the greatest support is for a level of 100 ppb. However, the Administrator notes that leve ls both below and above 100 ppb may also be appropriate to the extent that a different emphasis is placed on the various evidence and information and their associated uncertainties. Based on our evalua tion that it is reasonable to estimate concentrations on or near roadways to be 65% higher than c oncentrations away from those roads, the Administrator concludes that the available evidence and information could support a near-road standard level of 80 ppb (appr oximately 65% higher than 50 ppb) or a level as high as 125 ppb (approximately 65% higher than 75).118 In considering the area-wide and near -road approaches described above, the Administrator is mindful that there are strengths and limitations associated with each approach. In considering those strengths and limitati ons, the Administrator notes the following:

• The near-road approach would provide more certainty than the area-wide approach regarding the level of protection provided ag ainst peak exposures that can occur on or near major roadways. In contrast, the area -wide approach would provide more certainty than the near-road approach with regard to the level of protec tion provided against NO 2 exposures indicated by area- wide concentrations.

• The available scientific evidence and expos ure information support the importance of roadway-associated exposures for public h ealth. The ISA noted that in-vehicle NO 2 exposures could be 2-3 times higher than indi cated by ambient monitors in the current network. In addition, millions of people in the U.S. live, wo rk, and/or attend school near major roadways, which are important sources of NO 2 . The exposure assessment presented in the REA estimated that roadwa y-associated exposures account for the great majority of exposures to peak NO 2 concentrations.

After considering the strengths and limitations of both of the appro aches described above, the Administrator concludes that the near-roadway approach, in combination with the standard levels and ranges of levels discussed below, is the most appropriate appr oach to revising the NO 2 primary NAAQS in order to protect public health with an adequate margin of safety. This conclusion emphasizes the degree of certainty regarding the level of protection provided by the standard against peak exposures, such as thos e that can occur on or near major roadways. Available information and analys es clearly indicate th at roadway-associat ed exposures account119 for the great majority of exposures to peak NO 2 concentrations, and the near-road approach would therefore directly address the great majority of peak expos ures, which are expected to be the primary contributor to the h ealth effects discussed above. Incontrast, given the geographic variability in the NO 2 concentration gradient around roadwa ys, the level of protection against roadway-associated exposures would be more uncertain for any given location under the areawide approach. In addition, the range of near-r oadway standard levels described below would provide a reasonable degree of certainty that the accompanying area-wide levels could be approximately 60 ppb, with significantly more ce rtainty that the area-wide levels would be within a range of 50–75 ppb. Therefore, the Adminis trator proposes to set such a near-roadway standard. For the reasons stated above, the Administrato r proposes to set the level of a new 1-hour standard to between 80 ppb and 100 ppb, with 100 ppb appearing to have the greatest support. With regard to the standard level, the Admin istrator proposes to place emphasis on reported findings from both epidemiologic studies and from controlled hu man exposure studies. In order to protect against NO 2 -associated emergency department vi sits and hospital admissions reported in multiple key U.S. epidemiologic studies, and against observed NO 2 -induced increases in airway responsiveness, the Administrator proposes to set the standard level no higher than 100 ppb. In addition, if more weight is put on the risk of potential NO 2 -associated effects that could occur in locations with relatively low area-wide NO 2 concentrations, and the possibility that NO 2 could increase airway responsiveness in some as thmatics following exposur es to concentrations below 100 ppb, the Administrator also proposes to set a standard level within a range that includes 100 ppb but is no lower than 80 ppb.120 With regard to this range, the Administrator notes that the levels proposed are consistent with the recommendation by CASAC to set a standard level no higher than 100 ppb. However, CASAC did not explicitly address whether or how the standard le vel should differ with a nearroad approach for the standard, as described inthis proposal. Since a near-road standard level greater than 100 ppb could provide public health protection equivale nt to that provided by an area-wide standard level below 100 ppb, such as 75 ppb, the Administrator also solicits comment on near-road standard levels as high as 125 ppb. The Administrator notes that a near-road standard level as high as 125 ppb could be appropriat e to the extent that near-road concentrations are determined to be closer to 100% higher than area-wide concentrations, or to the extent more emphasis is placed on uncertainties a ssociated with the magnitude and clinical significance of the
NO 2 -induced increase in airway responsiveness inasthmatics, particularly at the lower concentrations evaluated in the controlle d human exposure studies. In addition, the Administrator notes that a near-road standard le vel lower than 80 ppb could be appropriate to the extent that near-road concentrations are determin ed to be closer to 30% higher than area-wide concentrations. Accordingly, the Administrator also solicits comment on standard levels as low as 65 ppb (30% higher than an area-wi de concentration of 50 ppb). For the reasons discussed above, the Adminis trator is proposing a level reflecting the maximum allowable near-road concentrations, an d is not proposing to set the level based on maximum allowable area-wide con centrations. The Administrator recognizes that her proposed approach results from a comprehe nsive evaluation of alternative approaches to determining the level of the NO 2 primary NAAQS, but that these approaches have not previously been presented to CASAC, or other stakeholders , for their evaluation and public di scussion. More specifically, the Administrator notes that much of the info rmation included in the policy assessment chapter121 of the REA, which formed the foundation for CASAC’s recommendations regarding standard level, was based on evaluation of data drawn from the current monitoring network. Further, the Administrator notes that CASAC did not explici tly discuss in their recommendations whether and how the standard level should differ if that level reflects the maximum allowable near-road
NO 2 concentration rather than th e maximum allowable area-wide NO 2 concentration. Given this, the Administrator recognizes the possibility that comments receive d on this proposal, particularly those received from CASAC, coul d provide important new info rmation for consideration. Therefore, EPA also solicits comm ent on the option of revising the NO 2 primary NAAQS such that the level would reflect th e maximum allowable area-wide NO 2 concentration, and on setting the level of such a standard w ithin the range of 50 to 75 ppb. In addition to soliciting comment generally on this area-wide approach, the Administrator specifically requests comment on the a ppropriate definiti on of area-wide NO 2 concentrations and how best to use data representing these concentr ations to determine co mpliance with an areawide standard. As described above, comparing NO 2 concentrations measured near major roadways to a level meant to reflect the maximum allowable area-wide NO 2 concentrations would have the effect of increas ing the stringency of the standard beyond that intended. With regard to this specific request for comment, the Administrator notes that the definition of areawide concentrations could incl ude a provision requiring that they be monitored at a distance greater than or equal to some prescribed distance from the neares t roadway. The Administrator notes that, while it is clear th at peak roadway-associated NO 2 concentrations occur within about 50 m of the major roads, the point at which these concentr ations return to area-wide concentrations comparable to the area-wide standard is less certain and ma y vary considerable by location. As discussed above (section II.A.2), the scientific litera ture suggests that122 concentrations can return to typical urban bac kground concentrations within distances of up to 500 meters from roads, though the actual distance will vary with topography, roadside features, meteorology, and photochemical reactivity conditi ons. The REA notes that studies suggest the return to urban background concentrations can o ccur from within distances of up to 200 to 500 m from the roads. Therefore, the Administrator requests comment on the degree to which these distances above (50m to 200m, and 50m to 500 m) serve to further de fine the distance from roadway that would represent concentrations co mparable to the area-wi de standard. Further, since roadways of various sizes and traffic volumes can affect nearby NO 2 concentrations and roadways are ubiquitous in urban areas, the Ad ministrator notes that defining representative area-wide concentrations could require more than a uniform a ssumption of a single specific distance from a class of roadway. The Administrator notes that the approach to defining representative area-wide distan ces could include considerati on of location-specific roadway traffic volume and location-speci fic roadway characteristics su ch as topography, presence of sound walls, vehicle mix, and traffic patterns, to adequately addre ss the variability. Given these considerations, the Administrator solicits comme nt on how to define th e minimum distance to the nearest major roadway such that measured c oncentrations at this distance (or farther) would represent area-wide NO 2 concentrations for comparis on to the area-wide standard. With regard to the annual standard, the Ad ministrator notes that CASAC recommended retaining an annual standard inorder to provide protection ag ainst potential health effects associated with long-term exposures. With regard to the level of an annual standard, CASAC recommended retaining the current level as the evidence considered did not suggest that either an increase or decrease would be appr opriate. Given these consider ations, and recognizing that a123 new 1-hour standard level as proposed would also provide some degree of protection from longterm exposures, the Administrator also proposes to re tain the current annual standard level. G. Summary of proposed decisions on the primary standard For the reasons discussed above, and taking into account information and assessments presented in the ISA and REA as well as th e advice and recommendations of CASAC, the Administrator proposes that the current annual standard is not requi site to protect public health with an adequate margin of sa fety. The Administrator proposes to establish a new short-term standard that will afford incr eased protection for asthmatics a nd other at-risk populations against an array of adverse respiratory hea lth effects related to short-term NO 2 exposure. These effects include increased asthma symptoms, worsened c ontrol of asthma, an increase in respiratory illnesses and symptoms, and re lated serious indicators of respiratory morbidity including emergency department visits and hosp ital admissions for respiratory causes. Specifically, the Administrat or proposes to set a ne w short-term primary NO 2 standard, with a1-hour (daily maximum) averaging time and a form defined as the 3-year average of the
99 th percentile or the 4 th highest daily maximum concentratio n The proposed range of levels, reflecting maximum allowable concen trations in an area, such as those that occur near roadways, is 80 to 100 ppb. In conjunction with this proposed near-road standard, the Administrator also solicits comment on levels as low as 65 ppb a nd as high as 125 ppb, and on alternative forms including the 3-year average of the 98 th percentile or the 7 th or 8 th highest daily maximum concentration. In addition, the Administrator solicits comment on using the area-wide approach to set a new 1-hour standard, and on a range of levels, reflecting maximu m allowable area-wide concentrations, from 50 to 75 ppb.124 With regard to the current annual standard, the Administrat or proposes to retain the current standard to supplement the new short-te rm standard, such that the combination would provide requisite protection for pub lic health with an adequate margin of safety against health effects potentially associated w ith long-term exposures to NO 2 . III. Proposed Amendments to Ambient Mo nitoring and Reporting Requirements The EPA is proposing changes to the ambi ent air monito ring, reporting, and network design requirements for the NO 2 NAAQS. This section discusse s the changes we are proposing which are intended to support the proposed 1-hour NAAQS and proposed retention of the current annual NAAQS, discussed above 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 requ irements contained in 40 CFR parts 50, 53, and 58 A. Monitoring Methods To be used in a determina tion of compliance with the NO 2 NAAQS, NO 2 data must be collected using a Federal Reference Method (F RM) or a Federal Equivalent Method (FEM) analyzer. The current monitoring method in use by most State a nd local monitoring agencies is the gas-phase chemiluminescen ce FRM (40 CFR Part 50, Appendi x F), which was implemented into the NO 2 monitoring network in the early 1980s. The current list of all approved FRMs and FEMs capable of pr oviding ambient NO 2 data for use in attainment designations may be found on the EPA website (http://www.epa.gov/ttn/amtic/f iles/ambient/criteria/re ference-equivalentmethods-list.pdf). It must be noted however, that due to the proposal of a ne w 1-hour NAAQS, wet chemical based FEMs would not be appropria te for use in determin ing compliance of the proposed 1-hour NAAQS, since such methods are in capable of providing hourly averaged data.125 Therefore, we propose that any NO 2 FRM or FEM used for maki ng primary NAAQS decisions must be capable of providing hourly averaged concentration data. We solicit comment on only allowing FRM of FEMs capable of providing hourly averaged concentration data to used to produce data for comparison to the NAAQS. The sum of nitric oxide (NO) and NO 2 is commonly referred to as NO X . Nitrogen oxides, technically the total reactive ni trogen oxide family, referred to as NO Y , is defined as the sum of NO, NO 2 , and the higher nitrogen oxides collectively termed NO Z . Important components of ambient NO Z include nitrous acid (HNO 2 ), nitric acid (HNO 3 ), and the peroxyacetyl nitrates (PANs). More specifically, NO 2 is the indicator of nitrogen oxides for purposes of the NAAQS, as noted in section II.F.1, as the specific health e ffects associated with exposure to NO 2 make it the component of greatest inte rest for the nitrogen oxides. In the ambient monitoring network, very nearly all measurements of NO 2 are collected by the chemiluminescence FRM. However, this technique directly measures only NO by the principle of gas-phase chemiluminescence induced by the reaction of NO with ozone (O 3 ) at low pressure.
NO 2 concentrations are determined i ndirectly by the analyzer in tw o steps: (1) by first measuring the ambient NO concentration, and (2) determining total NO X , including NO 2 , by measuring a second NO concentratio n after reducing the NO 2 in the sample air stream to NO (most often through the use of a molybdenum oxide (MoO X ) substrate heated to between 300°C and 400°C in the sample flow path). The difference be tween the second concentration (NO plus the NO 2 reduced to NO) and the first concentrati on (ambient NO only) is reported as the NO 2 concentration. One issue of note with the chemilumin escence FRM is that the reduction of NO 2 to NO on the MoO X converter substrate is not specific to NO 2 ; hence, chemiluminescence method126 analyzers are subject to varying interferences pr oduced by the presence in the air sample of the
NO Z species listed above and others occurring in trace amounts in am bient air. This interference is often termed a “positive artifact” in the reported NO 2 concentration sinc e the presence of NO Z results in an over-estimate in the report ed measurement of the actual ambient NO 2 concentration. This interference by NO Z compounds has long been known and tested (Fehsenfeld et al., 1987; Nunnermacker et al., 1998; Parrish and Fehs enfeld, 2000; McClenny et al., 2002; U.S. Environmental Protection Agency, 1993, 2006a). The sensitivity of the chemiluminescence FRM to potential interfer ence by individual NO Z compounds is variable and depends in part on characteristics of i ndividual monitors, such as the design of the instrument inlet, the temperature and composition of the reducing substrate, and th e interactions of atmosp heric species with the reducing substrate. Furthermor e, the concentrations of NO Z compounds in ambient air are variable with time and distan ce from the sources of NO and NO 2 , chiefly the point source and both on-road and non-road mobile source combustion of fossil fuels. Nearer to these sources, the potential interference is lower than it is farthe r away because more of the measured nitrogen oxides are present as the emitted NO and quickly formed NO 2 , rather than NO Z . This is because oxidation to the NO Z compounds from NO and NO 2 requires time and the presence of other atmospheric compounds like the hydroxyl radical. Overall, 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 (c old temperatures), but larger interfer ence ranging as high as 70 percent can be found during summer (warm temp eratures) in the afternoon at sites away and downwind from strong emission sources. In general, the NO Z interference in the reported NO 2 concentrations collected downwind of source areas and NO 2 concentrations collected in relatively remote areas127 away from concentrated point , area, or mobile sources is larger than the NO Z interference in NO 2 measurements taken in urban cores or other areas with fresh NO X emissions. The chemiluminescence FRM is well established and comprises a large majority of the current operating network. Many of the epidemio logic studies referenced in the REA as the health basis for the proposed primary NO 2 NAAQS utilized ambient NO 2 data obtained from chemiluminescence FRMs, and subsequently, the un certainties that may occur from the potential positive influence of NO Z species on NO 2 values provided by th e ambient FRM monitoring network are already reflected in those studies. Therefore, for purposes of comparing NO 2 monitoring data to the NO 2 NAAQS, the EPA believes that the chemiluminescence FRMs are appropriate for continued use under the current standard and under any of the options being considered for a new 1-hour averaged primary NO 2 NAAQS. EPA is aware of the more recent developmen t of an alternative method in determining
NO 2 concentrations by chemilu minescence, specifically through the use of a photolytic converter, which uses specific wavelengt hs of ultraviolet light to reduce NO 2 to NO in lieu of the
FRM’s MoO X substrate converter. The advantage of the photolytic-chemiluminescence method is that the photolytic conve rter is more specific to NO 2 , as compared to a MoO X substrate converter and does not reduce many NO Z species to NO (Ryerson et al., 2000), reducing the potential influence of NO Z concentrations on the reported NO 2 concentration. The photolyticchemiluminescence method is currently deployed w ithin certain research networks, but the EPA has not approved this method as an FRM or an FEM. If this technique is to be advanced to an FRM or FEM, the method may require additiona l research and development to ensure the stability of the photolytic converter rates in a variety of ambient conditions and monitor set-ups128 that might be experienced in th e field and a consistent method of mathematically correcting for the known converter efficiencies. EPA also recognizes that, although not wide ly used by state and local monitoring agencies, the existing FRM and FEM path-integra ted optical remote sensing techniques, also known as open-path and remote sensing methods, which use spectrometers to detect pollutant concentrations by light absorption over an optical path length, are suitabl e for continued use in the ambient monitoring networ k as they can provide NO 2 measurements with reduced influences of NO Z species on the reported NO 2 concentrations, relative to the chemiluminescence FRM. However, these methods do not provide point spec ific concentrations li ke those provided by chemiluminescence FRMs that are ty pically expected and seen inthe monitoring network, and may be one of the reasons these me thods are not more widely used. In recognition of the existence of alterna tive methods that may be useful in the measurement of NO 2 for NAAQS compliance purposes, as we ll as other objectives, EPA solicits comment on the advantages and disadvantages ofadvancing technology, such as the photolyticchemiluminescence method, or the use of existing open-path or remote sensing FRM and FEM technology, as alternative methods to s upplement the approved chemiluminescence FRMs already deployed across the U.S. at NO 2 monitoring sites.

B. Network Design 1. Background The basic objectives of an ambient mon itoring network, as noted in 40 CFR Part 58 Appendix D, include 1) pr oviding air pollution data to the gene ral public in a timely manner, 2) supporting compliance with ambient air quality standards and emi ssions strategy development,129 and 3) providing support for air pollution research. Sec tion II.A.1 notes that there are currently no minimum monitoring requirements for NO 2 in 40 CFR part 58 Appendix D, other than the requirement for EPA Regional Ad ministrator approval before re moving any existing monitors, and that any ongoing NO 2 monitoring must have at least one monitor sited to measure the maximum concentration of NO 2 in that area. As discussed in S ection II.A.2, an analysis of the approximately 400 16 monitors comprising the current NO 2 monitoring network (Watkins and Thompson, 2008) indicates that the most frequently stated monitor objectives for sites in the
current NO 2 network are for the assessm ent of concentrations fo r general population exposure and maximum (highest) concentra tions typically at the neighborhood and urban scales. Spatial scales are defined in 40 CF R Part 58 Appendix D, Secti on 1.2, where the scales of representativeness of most interest for the monitoring site types include: 1. Microscale – Defines the concentration in ai r volumes associated with area dimensions ranging from severa l meters up to about 100 meters. 2. Middle scale – Defines the con centration typical of areas up to several city blocks in size with dimensions ranging from about 100 meters to 0.5 kilometers. 3. Neighborhood scale – Defines concentrations within some extended area of the city that has relatively uniform land use with dime nsions in the 0.5 to 4.0 kilometers range. The neighborhood and urban scales liste d below have the potential to ove rlap in applications that concern secondarily formed or homoge neously distributed air pollutants.

16 It should be noted that the ISA Section 2.4.1 references a different number of active monitors in the NO 2 network. The difference between the ISA number and the number presen ted here is due to differing metrics used in pulling data from AQS. The ISA only references SLAMS, NAMS , and PAMS sites with defined monitoring objectives, while the Watkins and Thompson, 2008 value represents all NO 2 sites reporting data at any point during the year.130 4. Urban scale – Defines concentrations with in an area of city-like dimensions, on the order of 4 to 50 kilometers. W ithin a city, the geographic plac ement of sources may result in there being no single site that can be said to represent air quality on an urban scale. 5. Regional scale – Defines usually a rura l area of reasonably homogeneous geography without large sources, and extends from tens to hundreds of kilometers.

The ISA and REA indicate that one of the larg est factors affecting ambient exposures to
NO 2 above health benchmark concentrations ar e mobile source emissions, particularly at locations near major roads. Information in th e ISA and the REA shows th at concentrations of mobile source pollutants, 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 dist ance from the road (sec tion II.A.2 above, ISA sections 2.5.4 and 4.3.6 and table 2.2-1; REA secti on 7.3.2 and Figures 8-17 and 8-18). In the ambient environment, NO 2 is largely a secondary pollutant resulting from the reaction of NO with available ozone (O 3 ), the concentrations of which de pend on photochemical reactions of ambient hydrocarbons and prior (pre-cursory) NO X emissions. The ISA notes that the primary emission of NO 2 from mobile sources is estimated to be only a fe w percent of the total NO X emissions for light duty gaso line vehicles, and anywhere fro m less than 10 percent up to 70 percent of the total NO X emission from heavy duty diesel vehicles, depending on the engine, the use of emission control technologies such as catal yzed diesel particulat e filters (CDPFs), and mode of vehicle operation. 17 However, since the reaction rate s converting mobile source NO to

17 The ISA references studies of heavy- duty diesel vehicles retrofitted with a CDPF in describing the range of NO 2 to NO X ratios from diesel vehicles. These studies are base d on vehicles equipped with CDPFs prior to 2009. However, as of January 1, 2009, EPA’s National Clean Diesel Campaign requires that emission control devices included on its Verified Technologies List raise the fraction of NO 2 in exhaust NO X from an engine no more than131 NO 2 as described above are indicat ive of a fast process, on the order of a minute (ISA section 2.2.2), NO 2 behaves like a primary pollutant in the n ear-road environment. It exhibits peak concentrations on or closely adjacent to roads, although resulting from the secondary formation characteristic of NO 2 , its decay rate with increasing distance from a road can be slower than that of the other pollutants direct ly emitted from mobile sources including carbon monoxide (CO), ultrafine particulates, some air toxics, and bl ack carbon. Literature va lues 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, though this distance is 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; Si nger et al., 2003; Zhou and Levy, 2007). Nonetheless, any efforts to measure peak ambient NO 2 concentrations from on-road mobile sources, or other mobile source pollutants of interest noted above, would be best served by monitoring as near as practicab le to roadways of interest. 2. Proposed Changes In conjunction with the propos ed 1-hour NAAQS, based on the near-road approach that reflects maximum allowable NO 2 concentrations in an area suchas those near major roadways, and the proposed retention of the current annua l NAAQS, we propose a nu mber of changes to the NO 2 monitoring network. Since the current ne twork is not oriented to address peak concentrations in the on-road and near-road environment, the EPA is proposing a two-tier NO 2 network design to monitor am bient concentrations of NO 2 and assess compliance with the NO 2 NAAQS. The two tiers would provide data to be compared to both the 1-hour and annual

20% above the baseline engine NO 2 to NO X ratio. Retrofit technologies sold after January 1, 2009 that do not meet the NO 2 emission limit may not be installed or sold as EPA verified technologies.132 standards, and would be compri sed of 1) monitoring near major roads in areas of expected maximum 1-hour concentrations and 2) monito ring to characterize areas with the highest expected NO 2 concentrations at the neighborhood and larg er spatial scales, or “area-wide” scales. Because the maximum NO 2 concentrations in an area are exp ected to be due to on-road mobile source and dominating stationary source emissions, the EPA believes that the first tier of the monitoring network should include a component requiring monitori ng near major roads, where higher NO 2 concentrations have been id entified and there are no signi ficant monitori ng efforts to address roadway exposures. The EPA recognizes th at requiring a component of the ambient NO 2 monitoring network to ch aracterize the peak NO 2 concentrations derive d from on-road mobile sources, using “near-road monitors,” will introdu ce new requirements for monitoring sites that, for a majority of the state and local monitoring networks, currently does not exist. However, the monitoring of maximum concentrations of NO 2 , including the near-road environment, is a critical network design component if the ambient monitoring networ k is to appropriately function as a mechanism to support the proposed 1-hour av eraged NAAQS and provi de determinations on violations of the NAAQS. In addition, the EPA r ecognizes that the establishment of near-road monitoring sites may have certain other advant ages, particularly by supporting future NAAQS reviews, allowing for the track ing of mobile source emission reductions progress, providing monitoring infrastructure that may be of use for other criteria po llutants in the multi-pollutant paradigm, providing a new data source for public hea lth studies, and support ing scientific studies of other criteria pollutants like CO and ultrafine particulate matter and non-criteria pollutants such as some air toxics and black carbon. The second tier of the proposed network desi gn, the area-wide monitoring component, is intended to characterize the hi ghest concentrations of NO 2 that typically representing133 neighborhood and larger spatial scales whic h can address the wide r area impact of NO 2 sources on populations. Further, a require ment for the continuation of area-wide m onitoring of NO 2 serves to maintain con tinuity in collecting area-wide data th at has served to inform long-term pollutant concentration trends analysis and h ealth and scientific research in the past. We propose that state and, when appropriate, lo cal air monitoring agen cies provide a plan for deploying monitors in accordance with th e following proposed network design by July 1, 2011. We also propose that the NO 2 network being proposed be ph ysically established no later than January 1, 2013. Considering the proposed tim eline and criteria presented in the network design, we solicit comment on whether state an d local monitoring agencies could deploy monitors sooner than January 1, 2013. a. Monitoring in Areas of Expected Maximum Concentrations Near Major Roads We are proposing to require monitoring in locations of expected maximum concentrations near major roads in larger ur ban areas, with minimum monitoring requirements triggered for metropol itan areas based on Core Based St atistical Area (CBSA) population thresholds and the traffic re lated metric annual average da ily traffic (AADT). The US Department of Transportation (US DOT) Federal Highway Admini stration’s Status of the Nation’s Highways, Bridges, and Trans it: 2006 Conditions and Performance document ( http://www.fhwa.dot.gov/policy/2006cpr/es02h.htm ) states that “wh ile urban mileage constitutes only 24.9 percent of tota l (US) mileage, these roads carried 64.1 percent of the 3 trillion vehicles miles (VMT) trav elled in the United 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 st atements indicate how much more traffic volume exists on roads in urban areas versus the more rural areas that have signifi cant amounts mileage of the total134 public road inventory. Because the combinati on of increased mobile source emissions and increased urban population densities can lead to in creased exposures and associated risks, urban areas are the appropriate areas to concentrate required near-road monitoring efforts. Therefore, we propose that one near-road NO 2 monitor be required in CBSAs with a population greater than or equal to 350,000 persons. This po pulation threshold is proposed because it will give the nearroad monitoring component of the network a greater spatial ex tent across the country than using a higher population thres hold, which may be important when we consider that this near-road monitoring approach is a new requirement for monitoring NO 2 , and there are very few data sets indicating what levels might be seen in any given area. We also note that this population threshold corresponds to the mini mum population level in which Ai r Quality Index (AQI) levels are required to be reported, as noted in 40 CFR Part 58 Subpart F. Based on 2007 Census Bureau statistics, this w ill result in approximately 142 sites in as many CBSAs. We also propose that a second near-road mon itor be required in CB SAs with a population greater than or equal to 2,500,000 million, or in any CBSAs with one or more road segments with an AADT count greater than or equal to 250,000. Based on 2007 Census Bureau statistics and data from the 2007 Highway Performance Monitoring System (HPMS) maintained by the US DOT Federal Highway Administration (FHWA) , this particular el ement of the minimum monitoring requirements will add approximately 23 sites to the approximate 142 near-road sites in CBSAs that already will have one near-road monitor required due to the 350,000 population threshold. Of the 23 additional sites, two site s are due to the 250,000 AADT threshold and are attributed to the Las Vegas, Nevada and Sacramento, Calif ornia CBSAs. The 2,500,000 population threshold is proposed as a second thres hold to allow for further characterization of larger urban areas that are likely to have an in creased number of large roads across a potentially135 larger geographic area, and a corresponding incr ease in potential for exposure. Of the approximate 1.66 million public road segments tracked in the HPMS, road segments of 250,000 AADT or greater make up the t op 0.03 percent of the most trav eled public road segments. The FHWA has also used this threshold on its website to give an indication of the most travelled urban highways in the country ( http://www.fhwa.dot.gov/policyin formation/tables/02.cfm ). We proposed to use HPMS-reported AADT as the traf fic volume metric because AADT appears to be the most widely used traffic me tric in the scientific literature, is widely available, and offers the most objective and consistent metric available to indicate tr affic volumes across the country. AADT data is typically available from local Metropolitan Planni ng Organizations (MPOs), state departments of transportation, and from the FHWA’s HPMS. FHWA al so provides national guidance on the appropriate measurement and estim ation of AADT for diff erent road types in their HPMS Field Manual ( http://www.fhwa.dot.gov/ohi m/hpmsmanl/hpms.cfm ). We are therefore proposing the 250,000 AADT threshold for requiring a near road monitor because that threshold represents the highe st traffic volume road segmen ts in the country, which may correspond to the greatest potential for high expos ures directly connected to motor vehicle emissions. The EPA recognizes that in certain cases, th ere can be an area or areas of expected maximum hourly concentration in a CBSA due to a major stationary source or to the combination of mult iple sources that could include point, area, non-road and on-road mobile sources emissions. Such locations might be identified through data analysis, such as the evaluation of existing ambient dataand/or emissions data. In thes e situations, we propose that the Regional Administrator will have discretion in re quiring monitoring to address individual cases,136 in cooperation with the State and local monitori ng agencies, and state and local agencies, of course, can choose to monito r additional locations beyond the proposed requirements. In summary, the combinati on of the above proposed minim um monitoring requirement thresholds for the near-road monitors as part of the ma ximum concentration monitoring component of the ambient NO 2 monitoring network are anticipated to require approximately 165 near-road sites in 142 CBSAs. We solicit comm ent on the proposed CBSA population threshold values (i.e., 350,000 and 2,500,000), the use of the traffic volume metr ic AADT, and the 250,000 AADT threshold in establishing the minimum number of required near-road sites for urban areas. In choosing these population and traffic related thresholds for the minimu m monitoring requirements, it should be noted that the U.S. Virgin Islands and seven states (Delaware, Montana, North Dakota, South Da kota, Vermont, West Virginia, and Wyoming) currently would not have required near-road mon itoring sites. Considering the relative lack of near-road monitoring data nationwide, the new level and averaging time of the NAAQS being proposed, and the desire to establish a spatially repres entative and protective ne twork, we solicit comment on the inclusion or exclusion of an additional or alternative moni toring requirement so that each state and territory would have at least one n ear-road monitoring site. The EPA also recognizes that while there are states that would have no required near-road sites with the current proposal, there are other states with larger numbers of required near-road sites relative to all the states that would have required near-road moni toring sites. Of the 42 states, the District of Columbia, and Puerto Rico, which are anticipated to have near-road sites under this proposal, we estimate that six states have seven or more CBSAs with required near road monitors: California has 16 CBSAs, Florida has 12 CBSAs, Texas has 10 CB SAs, Ohio has 8 CBSAs, Pennsylvania has 8 CBSAs, and North Carolina has 7 CBSAs. The EP A solicits comment on a cap of the number of137 CBSAs subject to near-road monitoring requirements per state, particularly for a cap of 6 CBSAs per state. The use of such a cap would be to allow for a balance between appropriate protection through the use of population and traffic-based triggered minimum monitoring requirements and the avoidance of overburdening or redundant monitoring requirem ents. Further, we solicit comment on the scenario where a CBSA cap is imp lemented, if the selection of the 6 CBSAs in states with 7 or more CBSAs meeting the popul ation or AADT thresholds proposed to trigger near-road monitoring sites should be based on po pulation, the amount of tr affic volume, or the amount of population estimated to live near major roads in those eligible CBSAs. The EPA also solicits comment on the propos al to allow Regional Administrators the ability require additional near road monitori ng sites to address situations where minimum monitoring requirements are not su fficient. An example of requir ing an additional near road monitor might be a case where a particular community or nei ghborhood is significantly affected by road emissions, but the site or area is not mo nitored even though the responsible State or local monitoring agency is fulfilling the minimum monitoring requirements. The new near-road monitoring sites that are to be part of the NO 2 ambient monitoring network will require specific site selection criteria to focus mo nitoring efforts on one or a few major roads in a given CBSA. We propose that monitoring agencies be required to select their near road monitoring site locatio n(s) to characterize the largest traffic volume segment(s) in the CBSA, determined by ranking all road segmen ts by AADT, and iden tifying a location or locations adjacent to those top ranked AADT se gments where motor vehi cle emission-derived
NO 2 concentrations are expected to be at a maximum. The EPA anticipates that these near-road monitoring sites will likely be best characterized as microscale, mobile source oriented sites. Specifically, near-road monitoring stati ons must be sited so that the NO 2 monitor probe is no138 greater than 50 meters away, horiz ontally, from the outside nearest e dge of the traffic lanes of the target road segment, and shall have no obstructi ons in the fetch between the monitor probe and roadway traffic such as noise barriers or ve getation higher than th e monitor probe height. Baldauf et al. (2009) indicate that the NO 2 probe would ideally be situated between 10 and 20 meters from the nearest traf fic lane. We are not propos ing that the near-road NO 2 monitor be on the predominantly downwind side of the targ et roadway, however, we solicit comment on whether this requirement is necessary to ensure near-road NO 2 sites capture maximum expected hourly concentrations. We propose th at the monitor probe be located within 2 to 7 meters above the ground, as is required for microscale PM 2.5 sites. EPA recognizes that these near-road monitoring sites will be adjacent to a variety of ro ad types, where some target roads will be on an even plane with the monitoring station, while othe rs may be cut roads, below the plane of the monitoring station, or fill and ope n elevated roads, where the road plane is above the monitoring station. In any given case, it is most appropriate to place the NO 2 monitor probe as close to the plane of the target road segmen t as possible, while staying between 2 to 7 meters above the ground. In addition, we propose that monitor pr obe placement on noise barriers or buildings, where the inlet probe height above the target road is no closer than 2 meters and does not exceed 7 meters, will be acceptable, so long as the inle t 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 residence time of the pollutant in the sample lin e between the inlet probe and the analyzer does not exceed 20 seconds. Although a wa ll-mounted or noise ba rrier-mounted nearroad monitor set-up is not ideal , it may allow for existing sites to be utilized as near-road monitoring stations if they also meet the site selection criter ion, described below, of being adjacent to a top ranked AADT road segment.139 During the near-road monitor site selection process, monitoring agencies may utilize forms of quantitative analysis, such as emissions and/or air quality mode ling, data analysis, or saturation studies, to gather better evaluate whic h of their top ranked AA DT road segments may exhibit the potential for creating the highest NO 2 concentrations that might be monitored in the CBSA. As an example, such an analysis migh t indicate that of the top ranked AADT road segments in a given area, those se gments that are part of or ad jacent to interchanges and toll plazas, that have higher ratios of heavy duty diesel traffic to light duty traffic, have a high fraction of rapidly-acceler ating or grade-climbing vehicles, or that ar e located in or near particular terrain or la nd features, may exhibit higher potential maximum NO 2 concentrations. In addition, top ranked AADT road segment analysis may allow the monitoring agencies to select a near-road monitoring site located in a more densely populated area or a location representing more vulnerable populations from a pool of otherwise similarly cate gorized site candidates. In CBSAs required to have two nea r-road monitoring sites, we pr opose that the second site be selected based on AADT ranking and expected ma ximum concentration, but differentiated from the first site by factors such as : selecting a road with a differ ent fleet mix, diff erent congestion patterns, different terrain, different geographic area within the CBSA, or at minimum, selecting a site along a different road, in an effort to avoid having the two s ites characterize the same traffic when there are potentially other av ailable road segments with diff erent traffic char acteristics that meet siting criteria for th e second near-road monitor. The EPA proposes the siting criterion for NO 2 monitor probe placement to be no greater than 50 meters away from the outside nearest edge of the traffic lanes of th e target road segment. Based on a review of the scientific literature, as discussed in section II.A and the background portion of this section, locations on or immediately adjacent to ro ads typically exhibit the peak140 concentrations for mobile source pollutants, th erefore monitoring at in creasing distances from a road will correspondingly decrease the potential for sampling maximum concentrations of NO 2 . In addition, monitor probe placemen t within 50 meters of a target road allows for increased probability of reading el evated concentrations from the mobi le source emissions even when wind conditions cause the near-road monitoring site to be upwind of the ta rget road. Research literature indicates that in certain cases, mobile source deri ved pollutant concentrations, including NO 2 , can be detected upwind of roads, a bove background levels, due to a phenomenon called upwind meandering. Kalthoff et al. (2007), i ndicates that mobile so urce derived pollutants can meander upwind on the order of tens of meters, mainly due to vehicle induced turbulence, while Beckerman et al. (2008) note that near-roa d pollutant concentrations on the predominantly upwind side of their study sites dropped off to near background leve ls within the first 50 meters, which could be due to, at least in part, vehi cle induced turbulence. This upwind meandering characteristic of pollutants in th e near-road environment provides an additional basis for locating near-road sites within 50 meters of target road segments because of the increased opportunity to monitor mobile source derived NO 2 concentrations that, although are not peak concentrations, are still elevated above background levels, in me teorological conditions wh ere the site is upwind of the target road. In situations where open-path mo nitors are used at near-road NO 2 sites, we have not identified an appropriate path length for this microscale monitori ng site. For the purpose of this proposal, we propose a path length range of 50 to 300 meters as an appropr iate path length range for open-path near-road NO 2 monitors. The high end of this proposed range coincides with path lengths identified for other pollu tants at the micro and middle-scal es. We solicit comment on the appropriate path length for a near-road NO 2 open-path monitor.141 In further support of ch aracterizing the peak NO 2 concentrations occurring in the nearroad environment, the EPA proposes 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. Due to the near-road NO 2 site being a somewhat specialized microscale site, we pr opose that the meteorol ogical measurement hardware would be required to be si tuated at the same height as the NO 2 monitor probe, as opposed to a standardized height, to aid in characterizing what NO 2 analyzers are measuring from the target road segments. Th e requirement of three-dimensiona l anemometry is to allow for the determination of the standard deviation of ve rtical wind velocities ( σ w ) and is of particular interest if meteorological data are to be collected at near-road monitoring sites, as Venkatram et al. (2007) notes that σ w is a key meteorological factor in governing the dispersion of on road pollutant emissions. Therefore, the measurement of three di mensional wind would serve to inform when the near-road site is relatively upw ind or downwind of the target road, provide a method to identify the magnitude of vehicle induced turbulence, calculating σ w in the near road environment and providing a better understanding of the mixing of mobile source pollutants at the monitoring site and how site characteristics influence mixing, and with the inclusion of temperature and relative humidi ty, would provide basic meteor ological data. We solicit comment on the proposed requireme nt for three-dimensional anem ometry, the placement of the meteorological equipment at the same height of the NO 2 monitor probe height, and the requirement of meteorological measurements in general at all require d near-road monitoring sites. b. Area-wide Monitoring at Nei ghborhood and Larger Spatial Scales142 As the second tier of the NO 2 ambient monitoring network, we are proposing a minimum number of monitors to characterize that area with highest expected NO 2 concentrations at the neighborhood and larger (area-wide ) spatial scales. We are propos ing to require one area-wide monitoring site in each CBSA with a population greater than or equal to 1,000,000, which shall be sited to represent an area of maximum con centration at the neighbor hood or larger spatial scales. This minimum monitoring requirement is expected to tr igger 52 monitoring sites in as many CBSAs. Many of these monitors are likely already in place as part of the approximately
400 NO 2 monitoring sites that are currently opera ting across the country. Further, the EPA proposes to allow any current photochemical assessment monitori ng station (PAMS) sites that are situated to a ddress the highest NO 2 concentrations in an urban area at neighborhood or urban scales to satisfy this proposed area-wide mon itoring requirement. While in many cases it may be found that these area-wide mon itors may show lower concen trations than the maximum concentration near-road NO 2 monitors, data from these larger spatially representative sites would provide information on area-wide e xposures from an individual or a group of point, area, on-road and/or non-road mobile sources. This area-wide monitoring data may also, when coupled with the near-road monitoring data, assist in th e determination of spatial variation of NO 2 concentrations across a given area, and assist inproviding insight to th e gradients that exist between local near road or st ationary source derived concentr ation maxima and the area-wide concentration levels. The EPA recognizes that the total number of NO 2 monitoring sites needed to fully support the multiple mon itoring objectives that neighborhood and larger scale sites can serve in addition to determining compliance with the NAAQ S may be more than the minimum monitors required. These monitoring objective include am bient photochemical pollutant assessment,143 aiding in ozone forecasting, aidi ng in PM precursor analysis and PM forecasting, and to characterize point and ar ea sources that may be impacting ce rtain communities. We propose that EPA Regional Administrators have the abili ty to require additional area-wide NO 2 monitoring sites above the minimum monitoring requirement s where the minimum m onitoring requirements for area-wide monitors or the current network ar e not sufficient to meet monitoring objectives. For example, the Regional Administrator may require additional NO 2 monitors in certain communities, both inside and outsi de of CBSAs, which are impac ted by an individual or group of sources that does not have an NO 2 monitor as part of the mi nimum monitoring requirements. It is expected that a Regional Administrator and the responsible State or local air monitoring agency will need to work together to design and/or maintain the most appropriate area-wide NO 2 network component to service the variet y of data needs in a given area. We solicit comment on the proposed minimum monitoring requirement of approximately 52 monitors to characterize ar eas with highest expected NO 2 concentrations at the area-wide (neighborhood and larger) spat ial scales in CBSAs with populations of 1,000,000 or more persons. We also solicit comment on the proposal that the Regional Administrator can require additional monitoring sites based on a case by cas e basis, to address situations where the minimum monitoring requirements for area-wid e monitoring sites are not sufficient. 3. Solicitation for Comment on an Alternative Network Design In conjunction with the option of setting a new short-term NO 2 primary NAAQS such that the level would reflect the maximum allowable area-wid e, rather than near-road, NO 2 concentrations that EPA is so liciting comment on but not pro posing, we solicit comment on an alternative network design where near road mo nitors are not specifi cally included in the minimum monitoring requirements. This alterna tive is similar in concept to the second tier of144 the two-tiered network design being proposed, relying on the continued use of primarily neighborhood and larger spatially scaled monitors to determine attainment of the NAAQS. We expect that in this alternative network desi gn, minimum monitoring requi rements would apply to CBSAs based on population threshol ds, where one monitor would be required in CBSAs with populations of 350,000 or more persons and a sec ond monitor would be re quired for CBSAs with populations of 1,000,000 or more persons. Based on 2007 US Census Bureau statistics, we estimate that these population thresholds would trigger requi ring approximately 194 monitoring sites in 142 CBSAs. The first mo nitor required in any CBSA woul d be expected to be sited at the neighborhood or larger scale to character ize that area with highest expected NO 2 concentrations. Any second monito r required in a CBSA would be expected to characterize a separate area within the same CBSA, also with expected high NO 2 concentrations . Such monitor site locations are anticipated to be in areas of higher population densitie s of CBSAs and in, or adjacent in the downwind direction of, urban cores. We would also allo w for middle- or microscale locations that represent ma ny such locations throughout a metropolitan area; in these cases, the smaller spatial scales may also represent air quality that exis ts in multiple other and similar such locations in a given CBSA. In areas where a significant fraction of the population is located near similar primary emi ssion sources, such middle- or micro-s cale monitors may be considered representative of area-wide expos ures. The EPA expects such an alternative network design to retain the characteristics of th e current proposal where the Regi onal Administrator can require monitoring above the minimum requirements, in particular to addre ss community impacts from the variety of NO 2 emission sources. If the alternative network design is followed, EPA expects that little or no progress would be made in th e development of long-t erm near-road monitoring capabilities due to the lack of specific network design requirements. Nevertheless, EPA seeks145 comment on this alternative area-wide approach to network design. C. Data Reporting
NO 2 chemiluminescence FRMs are continuous gas analyzers, producing updated data values on the order of every 20 seconds. Data values are typically aggregated into minute averages and then compiled into hourly aver ages for reporting purposes. State and local monitoring agencies are requi red to report hourly NO, NO 2 , and NO x data to AQS within 90 days of the end of each calendar quarter. Some agenci es also voluntarily report their pre-validated data 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. The EPA believes these data reporting procedures are appropriate to support the current NO 2 NAAQS and any options being considered for a revised primary NO 2 NAAQS. IV. Proposed Appendix S--Interpretatio n of the Primary NAAQS for Oxides of Nitrogen and Proposed Revisions to the Exceptional Events Rule The EPA is proposing to a dd Appendix S, Interpretati on of the Primary National Ambient Air Quality Standards for Oxides of Nitrogen (“NO 2 NAAQS”), to 40 CFR part 50 in order to provide data handling procedures for the proposed NO 2 1-hour primary standard and for the existing NO 2 annual primary standard. The pr oposed Appendix S would detail the computations necessary for determining when the proposed 1-hour and existing annual primary
NO 2 NAAQS are met. The proposed Appendix S also would a ddress data reporting, data completeness considerations, and rounding conventions. Two versions of the proposed Appendix S are printed at the end of this notice. The first applies to an annual primary standard a nd a 1-hour primary standard based on the 99 th percentile value form, while the second applies to an annual primary standard and a 1-hour primary146 standard based on the annual 4 th highest daily value form. The discussion here addresses the first of these versions, followed by a brief description of the differen ces found in the second version. Both versions of the proposed Appendix S are based on a near-roadway approach to the setting the level of the 1-hour standard and to si ting monitors. As such, these versions place no geographical restrictions on which monitoring sites’ concentration data can and will be compared to the standard when making nonattainment dete rminations and other findings related to attainment or violation of the st andard. If the final rule adopts the area-wide approach on which section II.F.4.e of this notice invites comment, provisions woul d be added to section 2 of Appendix S to specify geogr aphical criteria for determining wh ich monitoring sites’ data can and will be compared to the standard consistent withthe area-wide approach as described in that section. The EPA is not proposing any change in the provisions of 40 CFR 50.11 related to data completeness for the existing annual secondary standard. Substantively, the proposed data handling procedures for the annual primary standard are the same as these existing provisions for the existing annual secondary standard, except for a proposed addition of a cross-reference to the Exceptional Events Rule, a proposed addition of Administrator discretion to consider otherwise incomplete data complete, and a proposed provi sion addressing the possibility of there being multiple NO 2 monitors at one site. The EPA is also proposing 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 exclusion of that data from EP A determinations of attainment or nonattainment w ith the NAAQS. The deadlines now contained in 40 CFR 50.14147 are generic, and are not always appropriate for NO 2 given the anticipated schedule for the designations of areas under the proposed NO 2 NAAQS. A. Background 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 mult i-monitor ambient air concentration data and the leve l of the NAAQS, so that dete rminations of compliance and violation are as objective as possible. Data inte rpretation guidelines al so provide criteria for determining whether there are sufficient data to make a NAAQS level comparison at all. The regulatory language for the current NO 2 NAAQS, originally adopted in 1977, contains data interpretation inst ructions only for the issue of data completeness. This situation contrasts with the situations for ozone, PM 2.5 , PM 10 , and most recently Pb for which there are detailed data interpretati on appendices in 40 CFR part 50 addressi ng more issues that can arise in comparing monitoring data to the NAAQS. EPA has used its experience drafting and applying these other data interpretation appendices to develop the proposed text for Appendix S. An exceptional event is defined in 40 CFR 50.1 as an event that affects air quality, is not reasonably controllable or pr eventable, is an event caused by human activity that is unlikely to recur at a particular location or a natural ev ent, and is determined by the Administrator in accordance with 40 CFR 50.14 to be an exceptional even t. Air quality data th at is determined to have been affected by an exceptional event under the procedural steps and substantive criteria specified in section 50.14 may be excluded from consid eration when EPA makes a determination that an area is meeting or violating th e associated NAAQS. The key procedural deadlines in section 50.14 are that a State must notify EP A that data have been affected by an event, i.e., “flag” the data in the Air Qualit y Systems (AQS) database, and provide an initial148 description of the event by July 1 of the year after the data are collected, and that the State must submit the full justification for exclusion within 3 years after the quarter in which the data were collected. However, if a regulat ory decision based on the data, for example a designation action, is anticipated, the schedule is foreshortened and all information must be submitted to EPA no later than a year before the decision is to be ma de. This generic schedule presents problems when a NAAQS has been recently revised, as discussed below. The REA did not address data interpretati on details. However, the approach to data interpretation used in the REA, for example to report the number of counties which would violate possible 1-hour primary NAAQS, was gene rally consistent with the proposed data interpretation procedures. B. Interpretation of the Primary NAAQS for Nitrogen Dioxide The purpose of a data inte rpretation rule for the NO 2 NAAQS is to give effect to the form, level, averaging time, and indicator specified in the pro posed regulatory text at 40 CFR 50.11, anticipating and resolving in advance various future situations that could occur. The proposed Appendix S provides common definitions and requirements that apply to both the annual and the 1-hour primary standards for NO2. The common requirements concern how ambient data are to be reported, what ambient dataare to be considered (including the issue of which of multiple monito rs’ data sets will be used when more than one monitor has operated at a site), and the applicability of the Ex ceptional Events Rule to the primary NO 2 NAAQS. The proposed Appendix S also addresses seve ral issues in ways which are specific to the individual primary NO 2 standards, as described below. 1. Annual primary standard The proposed data interpretation provisions for the annual standard are consistent with149 the current instructions included al ong with the statement of the leve l and form of th e standard in 40 CFR 53.11. These are the followi ng: (1) At least 75% of the hours in the year must have reported concentration data. (2) The available hour ly data is arithmetically averaged, and then rounded (not truncated) to whole parts per billi on. (3) The design value is the annual average concentration. (4) The design value is compared with the level of the annual primary standard (expressed in parts per billion). It would be possible to introduce additional steps for the annual primary standard which in principle could make th e design value a more reliable indi cator of actual annual average concentration in cases where so me monitoring data have been lost. For example, averaging within a calendar quarter first and then averag ing across quarters could help compensate for uneven data capture across the year. For some asp ects of the data interpretation procedures for some other pollutants, the current data interpretation appendices do contain such additional steps. The proposed provisions for the pr oposed 1-hour NO2 standard (d escribed immediately below) also incorporate some such featur es. However, we believe that such complexity is not needed to appropriately implement the annual primary standard, especially since no area presently comes close to violating the standard. 2. 1-hour primary standard based on the 99 th percentile value form With regard to data completeness for the proposed 1-hour primary standard, the proposed Appendix follows past EPA practice for other NAAQS pollutants by requiring that in general at least 75% of the mon itoring data that should have re sulted from followi ng the planned monitoring schedule in a period must be available for the key air qua lity statistic from that period to be considered valid. For the proposed 1-hour primary NO 2 NAAQS, the key air quality statistics are the daily maximum 1- hour concentrations in three su ccessive years. It is important150 that sampling within a day encompass the period when concentrations are likely to be highest and that all seasons of the year are well represen ted. Hence, the 75% requirement is proposed to be applied at the daily and quarterly levels. EPA invites comment on th e proposed completeness requirements. Recognizing that there may be years with in complete data, the proposed text provides for two diagnostic data substitution tests which are intende d to identify those cases with incomplete data in which it nevert heless is very likely, if not vi rtually certain, that the daily 1- hour maximum concentration would have been observed to be either above or below the level of the NAAQS if monitoring data had been minim ally complete. Data substitution tests are currently used for ozone, PM 2.5 and Pb. The first test would be applied (regardless of data capture in each quarter) if the annual
99 th percentile daily maximum 1-hou r of the incomplete data is above the NAAQS level. The test would substitute a low hypothetical concentration for as much of the missing data as needed to meet the normal 75% requirement in each quarter. The value that is substituted for the missing values is the lowest daily maximum 1-hour obser ved in the same quarter, looking across all three years under evaluation. If the resu lting three-year design value is still above the NAAQS, it is virtually certain that the annual 99 th percentile daily maximum 1- hour of complete data would also have been above the NAAQS, so the origin al design value indicati ng a violation would be considered valid. The second test is similar but works in th e opposite direction. It would be applied only if there is at least 50% data capture in each quarter and if the annual 99 th percentile daily maximum 1-hour of the incomplete data is below the NAAQS level. The test would substitute a high hypothetical concentration fo r as much of the missing data as needed to meet the 100%151 requirement. The value that is substituted for th e missing values is the highest daily maximum 1- hour observed in the same quarter, looking acr oss all three years unde r evaluation. If the resulting three-year design value is below the NA AQS, it is virtually certain that the annual 99 th percentile daily maximum 1-hour of complete data would also have been below the NAAQS, so the original design value indicating compliance would be considered valid. It should be noted that one outcome of a pplying the substitution tests proposed is that a year with incomplete data may nevertheless be determined to not have a valid annual 99 th percentile daily maximum 1-hou r and thus to be unusable inmaking 1-hour primary NAAQS compliance determinations for any three-year pe riod that encompasses that year. EPA invites comment on incorporating into the fina l rule the proposed substitution tests. Two other proposed provisions also address data incomple teness. If a day has valid data for fewer than 75% of its hours but at least one hour ly value is above the level of the 1-hour standard or if treating th e day (and similarly incomplete days) as if it were complete results in a three-year design value above the standard, the day would be treated as if it met the 75% requirement. Also, we are proposing that the Administrator have general discretion to use incomplete data based on case-sp ecific factors, either at the request of a state or at her own initiative. Similar provisions exis t already for some other NAAQS. The proposed Appendix S provides a table for determining which day’s maximum 1- hour concentration will be used as the 99 th percentile concentration fo r the year. The proposed table is similar to one used now for the 24-hour PM 2.5 NAAQS which is based on a 98 th percentile form, but adjusted to reflect the 99 th percentile form of the proposed 1-hour primary NO2 standard. The proposed Appendix S al so provides instructions for rounding (not truncating) the averag e of three annual 99 th percentile hourly concentr ations before comparison152 to the level of the primary NAAQS. 3. 1-hour standard based on the annual 4 th highest daily value form The second version of the proposed Appendi x S appearing at the end of this notice contains proposed interpretati on procedures for a 1-hour primary standard based on the annual
4 th highest daily value form, rather than the 99 th percentile form. These two forms would yield the same design value in a situation in whic h every hour and day of the year has reported monitoring data. However, the provisions to logically address possible data incompleteness under the annual 4 th highest daily value form are necessari ly somewhat different. The second version of Appendix S is partially based on the existing Appendix P for the 8-hour ozone NAAQS adopted in 2008, wh ich employs the annual 4 th highest daily value form, with certain differences to maintain substantive consistency with the first version of Appendix S on aspects not tied to the form of the standard.
C. Exceptional Events Information Submission Schedule The Exceptional Events Rule at 40 CFR 50.14 co ntains generic deadlines for a state to submit to EPA specified information about ex ceptional 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 description. The st ate must also, after notice and opportunity for public comment, submit a demonstration to justif y 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 anticipat ed, the schedule to flag data in AQS and submit complete documentation to EPA for review is foreshortene d, and all information must be submitted to EPA no later than one year before the decision is to be made.153 These generic deadlines are suitable for the period after initial de signations have been made under a NAAQS, when the decision that may depend on data exclusion is a redesignation from attainment to nona ttainment or from nonattainment to attainment. However, these deadlines present problems with respect to init ial designations under a newly revised NAAQS. One problem is that some of th e deadlines, especially the deadlin es for flagging some relevant data, may have already passed by the time the revised NAAQS is promulgated. Until the level and form of the NAAQS have been promulgate d a state does not know whether the criteria for excluding data (which are tied to the level and fo rm of the NAAQS) were met on a given day. The only way a state could guard agai nst this possibility is to flag all data that could possibly be eligible for exclusion un der a future NAAQS. This could re sult in flagging far more data than will eventually be eligible for exclusion. EPA beli eves this is an inefficient use of state and EPA resources, and is potentially confusing and misle ading to the public and 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 and submitted to EPA at least one year in advance of the final designation decisi on. This could have the unintended consequence of EPA designating an area nonattainment as a result of uncontrollable natural or other qualified exceptional events. When Section 50.14 was revised in Marc h 2007, EPA was mindful that designations were needed under the recently revised PM 2.5 NAAQS, so exceptions to the generic deadline were included for PM 2.5 . The EPA was also mi ndful that similar i ssues would arise for subsequent new or revised N AAQS. The Exceptional Events Rule at section 50.14(c)(2)(v) indicates “when EPA sets a NAAQ S for a new pollutant, or revises the NAAQS for an existing pollutant, it may revise or set a new schedule for flagging data for initial designation of areas for154 those NAAQS.” For the specific case of NO 2 , EPA anticipates that initial designations under the revised NAAQS may be made by Janu ary 22, 2012 based on air quality data from the years 2008-2010. (See Section VI below for more detailed discu ssion of the designation schedule and what data EPA intends to use.) If fi nal designations are ma de by January 22, 2012, all events to be considered during the designations process must be flagged and fu lly documented by states one year prior to designations, by Ja nuary 22, 2011. This date also co incides with the Clean Air Act deadline for Governors to submit to EPA their recommendations fo r designating all areas of their

states. EPA is proposing revisions to 40 CFR 50.14 to change submission da tes for information supporting claimed excepti onal events affecting NO 2 data. The proposed rule text at the end of this notice shows the changes that would apply if a revised NO 2 NAAQS is promulgated by January 22, 2010, and designations are made two years after pr omulgation of a NO 2 NAAQS revision. For air quality data collected in 2008, we propose to extend the generic July 1, 2009 deadline for flagging data (and provi ding a brief initial description of the event) to July 1, 2010. EPA believes this extension provides adequate time for states to review th e impact of exceptional events from 2008 on the revised stan dard and notify EPA by flagging the relevant data in AQS. EPA is not proposing to change the generi c deadline of January 22, 2011 for submitting documentation to justify an NO 2 -related exceptional event from 2008. We believe the generic deadline provides adequate time for states to develop and submit proper documentation. For data collected in 2009, EPA does not belie ve it is necessary to change the generic deadline of July 1, 2010 for flagging data and pr oviding initial event descriptions. Similarly, EPA does not believe it is necessary to change the generic deadline of January 22, 2011 for states155 to submit documentation to justify an NO 2 -related exceptiona l event from 2009. For data collected in 2010, EPA believes the designations deadline of January 22, 2011 for flagging data and provi ding initial event desc riptions does not provide states with adequate time to review and indentify potential exceptional events that occur in calendar year 2010, especially events that might occur late in th e year. Therefore, EPA is proposing that states may flag and provide initial ev ent descriptions for 2010 data no later than April 1, 2011. This affords states more than 2 additional months than would be provided under the generic schedule to review and identify excep tional events affecting 2010 NO 2 data. Similarly, EPA believes the designations schedule that would require states to submit detailed documentation to justify 2010 events claims by January 22, 2011 is not reasonabl e, because it would potentially preclude states from completing the required public review of the documentation pr ior to submitting to EPA. Therefore, EPA is proposing to extend this dead line to July 1, 2011. This would afford states more than 5 additional months than provided by the generic schedule to complete the required public review and submit full supporting docum entation, yet would still allow EPA adequate time to review the documentation and develop its final plans for designations by January 22,
2012. Table 2 summarizes the proposed two year designation deadlines discussed in this section. If the promulga tion date for a revised NO 2 NAAQS will occur on a different date than January 22, 2010, EPA will revise the final NO 2 exceptional event flagging and documentation submission deadlines accordingly, consistent w ith this proposal, to provide states with reasonably adequate opportunity to review, iden tify, and document exceptional events that may affect an area designation under a revised N AAQS. EPA invites comment on these proposed changes in the exceptional event flagging and documentation submission deadlines.156 Table 2. Proposed Schedule for NO 2 Exceptional Event Flagging and Documentation Submission If Designations Promulgate d in Two Years from January 22, 2010

Air Quality Data Collected for Calendar Year Event Flagging Deadline Detailed Documentation Submission Deadline 2008 July 1, 2010* January 22, 2011 2009 July 1, 2010 January 22, 2011 2010 April 1, 2011* July 1, 2011* * Indicates proposed change from generic schedule in 40 CFR 50.14.

V. Clean Air Act Implementation Requirements for NO 2 NAAQS 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. EPA may pr ovide additional guidance in the future, as necessary, to assist states and emissi ons sources to comply with the CAA requirements for implementing new or revised NO 2 NAAQS. The CAA assigns important role s to EPA, states, and Tribal governments to achieve the NAAQS. States have the primary responsib ility for developing and implementing State Implementation Plans (SIPs) that contain state measur es necessary to achieve the air quality standards in each area. EPA pr ovides assistance to states and Tribes by pr oviding technical tools, assistance, and guidance, including inform ation on the potential control measures that may assist in helping areas attain the standards. States are primarily responsible for ensuring attainment and mainte nance of ambient air quality standards once they have been establishe d by EPA. Under sec tion 110 of the CAA, 42 U.S.C. § 7410, and related provision s, states are required to submit , for EPA approval, SIPs that provide for the attainment and ma intenance of such standards th rough control programs directed at sources of NO 2 emissions. If a state fails to adopt and implement the required SIPs by the time periods provided in the CAA, the EPA has re sponsibility under the CAA to adopt a Federal Implementation Plan (FIP) to assure that areas attain the NAAQS in an expeditious manner.157 The states, in conjunction with EPA, also administer the prevention of significant deterioration (PSD) program for NO 2 . See sections 160-169 of the C AA. In addition, Federal programs provide for nationwide re ductions in emissions of NO 2 and other air pollutants under Title II of the Act, 42 U.S.C. §§7521–7574, which involves controls for automobiles, trucks, buses, motorcycles, nonroad engines, and ai rcraft emissions; the new source performance standards (NSPS) for stationary sources under section 111 of the CAA, 42 § U.S.C. 7411; and the national emission standards for hazardous air pollutants for stationary sources under section 112 of the CAA, 42 U.S.C. § 7412. A. Designations for the NO 2 NAAQS After EPA establishes or revises a NAAQS, the CAA requires EPA and the states to begin taking steps to ensure that the new or revised N AAQS are met. The first step is to identify areas of the country that do not meet the new or revised NAAQS. The CAA defines EPA's authority to designate areas that do not meet a new or revised NA AQS. Section 107(d)(1) provides that, “By such date as the Administrat or may reasonably require , but not later than 1 year after promulgation of a ne w or revised NAAQS for any po llutant under se ction 109, the Governor of each state shall * * * submit to the Administrator a list of all areas (or portions thereof) in the state” that desi gnates those areas as nonattainment , attainment, or unclassifiable. Section 107(d)(1)(B)(i) further pr ovides, “Upon promulgation 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 la ter than 2 years from the date of promulgation. Such period may be extended for up to one year in the event the Administrator has insufficient information to promulgate the de signations. “The term “promulgation” has been interpreted by the courts to be signature and dissemination of a rule. By no later than 120 days prior to158 promulgating designations, EPA is required to notify states or Tribes of any intended modifications to their boundaries as EPA may deem necessary. States and Tribes then have an opportunity to comment on EPA's tentative decision. Whether or not a state or a Tribe provides a recommendation, EPA must promulgate th e designation that it deems appropriate. Thus, following promulgation of the revised NO2 NAAQS in January 2010, EPA must promulgate initial designations by January 2012 (2 years after promul gation of the revised NAAQS), or, by January 2013 in the event that the Administrator ha s insufficient information to promulgate initial designations within 2 years. In the case of the NO2 NAAQS, in today’s action EPA is proposing new NO2 monitor siting rules that focus on roadways. EPA anticipates that it will require up to 3 year s to get a new monitoring network in place, plus an additional 3 years of monitoring thereafter in order to determine compliance with the revised standard. This means that a full set of air quality data fro m the new network will not be available until approximately 2016. Since data from the new netw ork will not be available prior to the CAA designation deadlines even if EPA takes an add itional year, EPA intends to complete initial designations in 2012 using air qua lity data from the current NO 2 monitoring network in place, using NO 2 monitoring data fro m the years 2008-2010. Accordingly, Governors and Tribal leaders will be required to s ubmit their initial designation recommendations to EP A no later than January 2011. If the Administrator intends to modify any state or Trib al area recommendation, EPA will notify the Governor or Tribal leader no later than 120 days prior to in itial designations in January 2012. States or Tribes that believe the Administrator’s modification is inappropriate will have an opportunity to demonstrate why they believe their recommendati on is more appropriate before designations are promulgated in January 2012. As explained below in more detail , we intend to designate areas under the current159 NO2 monitoring network as "uncla ssifiable" or “nonattainment” based on the data set for 2008-
2010. We intend to designate areas that do not s how violations of the revised NO2 NAAQS as “unclassifiable” since the existi ng area-wide monitoring network doe s not fully satisfy the near roadway-oriented NO2 monitori ng requirements proposed in this notice. Because there are no monitors in the current NO2 network that me et the proposed definition of “near-roadway,” monitoring data that does not in dicate a violation of the NAAQS would not necessarily mean that an area is meeting the re vised NO2 NAAQS. Rather, an ar ea-wide monitor may not record concentrations that violate the revised NO2 NAAQS because it is not sited where concentrations in the area are highest. Thus, we do not believe the current monitoring network provides information that supports designa ting an area as “attain ment” with today’s proposed standards. Furthermore, as discussed in other parts of this action, the revised NO2 NAAQS that EPA is proposing is based, in part, on NO2 concentrations that cause health effects in controlled human exposure studies. These studies provide information on the rela tionship between exposure to specific concentrations of NO2 and specific health effects. In or der to protect and to notify the public of the health risks where NO2 levels have the greatest potential to be harmful to human health, a near-roadway network would need to be deployed and data collected. The EPA anticipates that areas designated as “unclassifiable” in January 2012 will remain so until a new NO2 monitoring network is deploy ed and 3 years of monitoring data have been collected. Once the NO2 monitors are placed in locations meeting the proposed near-roadway siting requirements and monitoring data become available, the Agency could subsequently redesignate areas as “nonattainment” or “attainment” under section 107(d)(3).160 In January 2012 we intend to designate as “ nonattainment” areas that show violations of the revised standard under the current monitoring ne twork. As discussed above, the current monitoring network may not r ecord NO2 concentrations near roadways where NO2 concentrations are highest. We thus anticipate that any area show ing violations of the revised NO2 standard based on the current monitoring netw ork will continue to show violations when monitors are placed in near-roadway locations. In summary, as required by section 107(d)(1)(A)(i)of the CAA, in January 2012 the EPA must designate as “nonattainment ” any areas with monitors with in the existing network that report violations of the revised NO2 NAAQS. All other areas not indicating a violation of the revised NO2 NAAQS will be designated as “unc lassifiable.” While the CAA provides the Agency an additional third year from promulgation of a NAAQS to complete designations in the event that there is insufficient information to make NAAQS complian ce determinations, we believe that there is litt le value in taking an additional year to collect more data from the existing NO2 network when we expect to initially designate most areas in the country as “unclassifiable.” Once a near-roadway network has been deployed and 3 years of air quality data has been collected, we anticipate redesignating areas to “attainment” or “nonattainment.” B. Classifications Section 172(a)(1)(A) of the CAA authorizes EPA to classify areas designated as nonattainment for the purpose of applying an attainment da te pursuant to section 172(a)(2), or for other reasons. In determini ng the appropriate classification, EPA may consider such factors as the severity of the nonattainment problem and 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 re quired to do so. Accordingly, the primary reason to establish161 classifications is to set different deadlines fo r each class of nonattainment area to complete the planning process and to provide for different attainment dates based upon the severity of the nonattainment problem for the affected area. However, the C AA separately establishes specific planning and attainment deadlin es in sections 191 and 192, a maximum 18 months for the submittal of an attainment planand a maximum of 5 years for ar eas to attain standard. These relatively short deadlines provide a limited oppo rtunity to establish even nearer deadlines through classifications. Th erefore, EPA is not proposing to esta blish classifications for a revised
NO 2 NAAQS.
C. Attainment Dates The maximum deadline date by which an area is required to attain the NO 2 NAAQS is determined from the effective date of the nonatta inment designation for th e affected area. For areas designated nonattainme nt for the revised NO 2 NAAQS, SIPs must pr ovide for attainment of the NAAQS as expeditiously as practicable, but no later than 5 years from the date of the nonattainment designation for the area (see section 192(a) of the CAA). The EPA will determine whether an area has demonstr ated attainment of the NO 2 NAAQS by evaluating air quality monitoring data consistent with the form of the NO 2 NAAQS if revised, which will be codified at 40 CFR part 50, Appendix F. 1. Attaining the NO 2 NAAQS

In order for an area to be rede signated as attainment, the stat e 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 enforceable162 reductions in emissions; - EPA has fully approved a main tenance plan for the area; - the state(s) containing th e area have met all applicable requirements under section 110 and part D. 2. Consequences of Failing to Attain by the Statutory Attainment Date Any NO 2 nonattainment area that fails to attain by its statutory attain ment date would be subject to the requirements of sections 179(c) an d (d) of the CAA. EPA is required to make a finding of failure to attain no later than 6 mo nths after the specified attainment date and publish a notice in the Federal Register . The state would be required to submit an implementation plan revision no later than one year following th e effective date of the Federal Register notice making the determination of the area’s fa ilure to attain which demonstrat es that the standard will be attained as expeditiously as practicable, 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 im plemented in the area in light of technological achievability, costs, and any nonair quality and other air quality-related h ealth and environmental impacts.” D. Section 110(a)(2) NO 2 NAAQS Infrastructure Requirements Section 110(a)(2) of the CAA requires all st ates to develop and maintain a solid air quality management infrastructu re, including enforceable emi ssion 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 stat e which contribute significantly to nonattainment or maintenance163 areas in any other state, or which interfere wi th programs under part C to prevent significant deterioration of air quality or to achieve reas onable progress toward the national visibility goal for Federal class I areas (nationa l parks and wilderness areas). Under section 110(a)(1) and (2) of the CAA, all states are requ ired to submit SIPs to EPA which demonstrate that basic pr ogram elements have been addr essed within 3 years of the promulgation of any ne w or revised NAAQS. Subsections (A) through (M) of section 110(a)(2) listed below, set forth the elements that a state’s program must contain in the SIP. 18 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 ambient air quality monitors, 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 regul ation and permitting of new/modified sources.

• Interstate transport: Sec tion 110(a)(2)(D) requires SIPs to include provisions prohibiting any source or other type of emissions activit y in the state from contributing significantly to nonattainment in another st ate or from interfering with measures required to prevent significant deterioration of air qual ity or to protect visibility.

• Adequate resources: Section 110(a)(2)(E) re quires states to provide assurances of adequate funding, personnel and legal aut hority for implementation of their SIPs.

18 Two elements identified in secti on 110(a)(2) are not listed below because , as EPA interprets the CAA, SIPs incorporating any necessary local nonattain ment 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 requires areas designated as nonattainment to meet the applicable nonattainment planning requirements of part D, title I of the CAA.164 • Stationary source monitoring sy stem: Section 110(a)(2)(F) re quires states to establish a 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 changes or findings of inadequacies: Section 110(a)(2)(H) requires states to provide for revisions of thei r SIPs in response to changes in the NAAQS, availability of improved methods for attainin g 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 exceedances: Se ction 110(a)(2)(J) requi res 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 may be necessary to prevent significant deterioration of air quality inattainment areas and protect visibility 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) requires that SIPs provide for performing air quality modeling for predicting effects on air quality 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,165 implementing and enforcing a permit.

• Consultation/participa tion by affected local government : Section 110(a )(2)(M) requires states to provide for consultation and participa tion by local political subdivisions affected by the SIP.
E. Attainment Pl anning Requirements for NO 2 1. NO 2 Nonattainment AreaSIP Requirements Any state containing an ar ea 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 includes general requ irements for all de signated 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 reduc tions in emissions 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 at tainment of the national primary ambient air quality standards.” States are required to implement RACM and RACT in order to attain “as expeditiously as practicable”. However, under the Tribal Authority Rule (TAR), requi rements for RACT and RACM may be considered to be severable elem ents of implementation plan requirements for Tribes. Section 172(c) requires states with nonattainment areas to submit a SIP for these areas166 which contain an attainment demonstration which shows that the affected area will attain the standard by the applicable statut ory attainment date. The state must also show that the area will attain the standards as expeditiously as practicab le, and it must include an analysis of whether implementation of reasonabl y available measures will advance th e attainment date for the area. Part D SIPs must also pr ovide for 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 required by part D, or may reasonably be re quired by the Administrat or for the purpose of ensuring attainment of the applicable NAAQS by the applicable attainment date.” (See section 171 of the CAA) Historically, for some pollu tants, RFP has been met by showing annual incremental emission reductions sufficient to maintain generally linear progress toward attainment by the applicab le 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 ac tion by the state or th e Administrator. The EPA interprets this requi rement to mean that the continge ncy measures must be implemented with only minimal further acti on by the state or the affect ed sources with no additional rulemaking actions such as public h earings or legisl ative 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 criter ia pollutants including
NO 2 . Section 191(a) in conjunction with secti on 172(c) requires that areas designated as nonattainment for NO 2 submit an emission inventory to EPA no later than 18 months after designation as nonattainment. In the case of NO 2 , sections 191(a) and 172(c) also require that167 states submit periodic emissi on inventories for nona ttainment areas. The periodic inventory must include e missions of NO 2 for point, nonpoint, mobile ( on-road and non-road), and area

sources. 2. New Source Review and Pr evention of Significant De terioration Requirements The Prevention of Si gnificant Deterioration (PSD) and nonattainment New Source Review (NSR) programs contained in part s C and D of Title I of the CAA govern preconstruction review of any ne w or modified major stationary sources of air pollutants regulated under the CAA as well as any precursor s to the formation of that pollutant when identified for regulati on by the Administrator. 19 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 nona ttainment 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 re quiring permits for the construction and operation of new or modified stationary sources anywhere in the nonattainment area. SIPs that address the PSD requirements related 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

19 The terms “major” and “minor” define the size of a stationary source, for applicability purposes, in terms of an annual emissions rate (tons 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.168 designated as attainment or uncla ssifiable for any crite ria pollutant, is constructed, or undergoes a major modification. 20 The nonattainment NSR program a pplies when a major source of a criteria pollutant that is located in an area that is designated as nonattainment for th at pollutant is constructed or undergoes a majo r modification. The minor NSR program addresses both major and minor sources that undergo construction or modification activ ities that do not qualify as major, and it applies regardless of the designation of the area 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 emissi ons will not cause or contribute to a viol ation of any NA AQS or maximum allowable pollutant increase (PSD increment);

• notification of Federal Land Manage r of nearby Class I areas; and

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

• installation of Lowest Achievable Emi ssions Rate (LAER) control technology;

• offsetting new emissions with creditable emissions reductions;

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

20 In addition, the PSD program applies to non-criteria pollu tants subject to regulation under the Act, except those pollutants regulated under section 112 and pollutants subject to regulation only under section 211(o).169 • 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 st atutory requirements in s ection 110(a)(2)(C) of the CAA which requires “...regulation of the modifica tion and construction of any 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 change s 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. Prior to adop tion of the SIP revision addressing NSR for NO 2 nonattainment areas, the requireme nts of 40 CFR part 51, appendix S will apply. 3. General Conformity 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 secti on 176(c) of the CAA, prescribe the criteria and proce dures for demonstrating and a ssuring conformity of Federal actions to a SIP. Each Federal agency must de termine that any actions covered by the general conformity rule conform to the applicable SI P before the action is taken. The criteria and procedures for conformity apply only in nonatta inment areas and those areas redesignated attainment since 1990 (“maintenanc e areas”) with respect to th e criteria pollutants under the170 CAA: 21 carbon monoxide (CO), 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 designati ons for any new or revised NO 2 NAAQS. The general conformity dete rmination examines the impact s of direct and indirect emissions related to Federal acti ons. The general conformity ru le 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 gene ral conformity rule also requires that notices of draft and final general conformity dete rminations be provided directly to air quality regulatory agencies and to the public by publication in a local newspaper. 4. Transportation Conformity 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 cause new air quality violations, worsen existing violations, or delay timely a ttainment of the relevant NA AQS or interim reductions and milestones. Transportation conf ormity applies to areas that are designated nonattainment and 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 Conformi ty Rule (40 CFR Part 51, Subpart T, and Part 93, Subpart A establishes the criteria and proce dures for determining whether transportation activities conform to the SIP. The EPA is not proposing ch anges to the Transportation Conformity rule in this proposed rulemaking. Howe ver, in the future, EPA will review the need

21 Criteria pollutants are those pollutants for which EPA ha s established a NAAQS under section 109 of the CAA.171 to conduct a rulemaking to establish any new or revised transportation conformity tests that would apply under a revision to the NO 2 NAAQS for transportation pl ans, TIPs, and applicable highway and transit projects. 5. Transition from the Existing NO 2 NAAQS to a Revised NO 2 NAAQS As stated in section I of this proposed rule making, EPA is proposing to retain the current annual mean NO 2 NAAQS (0.053 ppm, or 100 μg/m 3 ) and is also proposi ng to promulgate a short-term 1-hour NO 2 NAAQS. The EPA is taking comme nt on both of the aforementioned actions in this rulemaking. After considering public comments, if EPA decides to promulgate only a short-term NO 2 NAAQS and revoke the current annual mean standard, EPA would need to promulgate anti-backsliding provisions. Th e CAA establishes anti-backsliding requirements where EPA relaxes a NAAQS. He re, however, replacing the annual standard with a short term standard would be a tighteni ng of the standard and 172(e) would not directly apply. Because section 172(e) does not directly apply for a transi tion to a more stringent NAAQS, EPA has previously l ooked to other provisions of the CAA to determine how to address anti-backsliding. EPA has recognized that there are a number of CAA provisions that indicate Congress’s intent to not allow states to alter or remo ve provisions from implementation plans if the plan revision w ould jeopardize the air quality pr otection being provided by the existing plan when EPA tightens a NAAQS. For example, section 110(l) provides that EPA may not approve a SIP revision if it interferes with any applicable requirement concerning attainment and RFP, or any other applicable requirement under the CAA. In addition, section 193 of the CAA prohibits the modificati on of a control, or a cont rol requirement, in eff ect or required to be adopted as of November 15, 1990 (i.e., prior to the promulgation of the Clean Air Act Amendments (CAAA) of 1990), unless such a modi fication would ensure e quivalent or greater172 emissions reductions. Further, s ection 172(e) of the CAA specifie s that if EPA revises a NAAQS to make it less stringent than a previous NAAQS, stat es may not relax control obligations that apply in nonattainment area SIPs, or avoid adopt ing those controls that have not yet been adopted as required. Looking to these provisions, EPA determined for the transition from the 1- hour ozone NAAQS to the 1997 8-hour ozone NAAQS, that it was consistent with Congressional intent to require areas to continue to meet control obligat ions for the revoked 1- hour NAAQS until the area attained the revised 8-hour NAAQS. This interpretation was upheld in South Coast Air Quality Management Dist. v. EPA , 472 F.3d 882 (D.C. Cir. 2006). If EPA revises the NO 2 NAAQS by revoking the current standa rd and replacing it with a shortterm standard, EPA is proposing to apply the princi ples in section 172(e) inorder to ensure that the health protection provided under the existing NO 2 NAAQS continues to be achieved and maintained as states transition to implementation of the revised NAAQS. EPA is proposing that the current NO 2 NAAQS would remain in ef fect for one year followi ng effective date of the initial designations for the revised NO 2 NAAQS before the current NAAQS is revoked. This timeframe would ensure that there is no gap in coverage of PSD requireme nts for areas attaining the current annual NO 2 standard. It would also ensure that there is no gap in conformity requirements for the Los Angeles South Coast Ai r Basin area, which is the only maintenance area in the country. Pursuant to CAA sections 110(l) a nd 193, any proposed SIP revision being considered by EPA after the effective date of the revised NO2 NAAQS would be evaluated for its potential to interfere with attainment or maintenance of th e new standard. EPA believes any area attaining the revised NO2 NAAQS would also attain the existing NO2 NAAQS, and thus reviewing proposed SIP provisions for interference with the new standard will be sufficient to prevent backsliding.173 VI. Communication of Pub lic Health Information Information on the public health implicati ons of ambient concen trations of criteria pollutants is currently made av ailable primarily through EPA's Ai r Quality Index (AQI) program. The current Air Quality Index has been in us e since its inception in 1999 (64 FR 42530). It provides accurate, timely, and easily understandable information about daily levels of pollution (40 CFR 58.50). The AQI establishes a nationall y uniform system of indexing pollution levels for NO 2 , carbon monoxide, ozone, particulate matter and sulfur dioxide. The AQI converts pollutant concentrations in a community's air to a number on a scale from 0 to 500. Reported AQI values enable the public to know whether ai r pollution levels in a particular location are characterized as good (0 - 50), moderate (51 - 100), unhealthy for sensitive groups (101 - 150), unhealthy (151 - 200), very unhealthy (201 - 300), or hazardous (300 - 500). The AQI index value of 100 typically corresponds to the level of the short-term NAAQS for each pollutant. An AQI value greater than 100 means that a pollutant is in one of the unhealthy categories (i.e., unhealthy for sensitive groups, unhealthy, very unh ealthy, or hazardous) on a given day; an AQI value at or below 100 means that a pollutant concentration is in one of th e satisfactory categories (i.e., moderate or good). Decisions about the pollutant concentrati ons at which to set the various AQI breakpoints, that delineate th e various AQI categor ies, draw directly from the underlying health information that supports the NAAQS review. The Agency recognizes the importance of re vising the AQI in a timely manner to be consistent with any revisions to the NAAQS. Ther efore EPA proposes to finalize conforming changes to the AQI, in connection with the Agency's final decision on the NO 2 NAAQS if revisions to the primary standa rd are promulgated. Currently , no AQI breakpoints are identified below an AQI value of 200 since there is no short-term NO 2 NAAQS. Therefore, if a short-term174 NO 2 NAAQS is 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 other AQI breakpoints at the lower end of the range (i .e., AQI values of 50 and 150). EPA does not propose to change breakpoints at the higher e nd of the range (from 200 to 500), which would apply to state contingency plans or the Signi ficant Harm Level (40 CFR 51.16), because the information from this review does not inform decisions about breakpoints at those higher levels. With regard to an AQI value of 50, th e breakpoint between the good and moderate categories, historically this value is set at the level of the annual NAAQS, if there is one, or onehalf the level of the short-term NAAQS in th e absence of an annual NAAQS (63 FR 67823, Dec. 12, 1998). If a short-term NO 2 NAAQS is promulgated from the upper end of the proposed range, then it is likely that an AQI value of 50 would be set at the level of the annual NAAQS (53 ppb NO 2 , 1-hour average). If a short-term NO 2 NAAQS is promulgate d from the lower end of the proposed range, then it is likely that an AQ I value of 50 would be se t at one-half the level of the short-term NO 2 standard. EPA is soliciting comment on this approach for selecting an AQI value of 50; specifically at what point the Agency should sw itch from using the level of the annual NAAQS to using one-half the level of the short-term NAAQS, or some other level, for an AQI value of 50. With regard to an AQI value of 150, the breakpoint between the unhealthy for sensitive groups and unhealthy categories, hist orically values between the s hort-term standard and an AQI value of 500 are set at levels that are approxi mately equidistant between the AQI values of 100 and 500 unless there is health evidence that sugge sts a specific level woul d be appropriate (63 FR 67829, Dec. 12, 1998). For an AQI value of 150, the level of 304 ppb NO 2 , 1-hour average, is approximately equidistant between the proposed range for the short-term standard and the175 level of an AQI value of 200 (640 ppb NO 2 , 1-hour average). This level also served as a benchmark level in the REA, as discussed above (section II.B.1). The level was selected based on a meta-analysis of indivi dual level data from 19 NO 2 controlled human exposure studies measuring airway responsiveness in asthmatics. Th e meta-analysis indicates that at the level of 300 ppb NO 2 , most people with asthma, who compromise a sensitive group for NO 2 , are likely to experience increased airway responsiveness. This health evidence supports using the level of 300 ppb NO 2 , 1-hour average, as the upper bound of the unhealthy for sensitive groups category. VII. Statutory and Executive Order Reviews A. Executive Order 12866: Regulatory Planning and Review Under section 3(f)(1) of Ex ecutive Order 12866 (58 FR 51735, October 4, 1993), this action is an “economically significant regulatory action” b ecause it is likely to have an annual effect on the economy of $100 million or more. Accordingly, EPA submitted this acti on to the Office of Management and Budget (OMB) for review under EO 12866 and an y changes made in response to OMB recommendations have been documented inthe docket for this action. In addition, EPA prepared a Regulatory Impact Analysis (RIA) of th e 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 not to be considered in se tting or revi sing 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 developing this proposed rule. B. Paperwork Reduction Act The information collection requirements in th is proposed rule have been submitted for approval to the Office of Management and B udget (OMB) under the Paperwork Reduction Act,176 44 U.S.C. 3501 et seq. The Information Collect ion Request (ICR) docum ent prepared by EPA for these proposed revisions to part 58 has been assigned EPA ICR number 2358.01. 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 dete rmining attainment 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 FR M 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 quality and associated health impacts, to develop emissions control strategies, and to meas ure progress for the air pollution program. The proposed amendments would revise the technical requirements for NO 2 monitoring sites, require the siting and operati on of additional NO 2 ambient air monitors, a nd the reporting of the collected ambient NO 2 monitoring data to EPA’s Air Qualit y System (AQS). Because this rulemaking includes a range of pr oposals for the level and form, it is not possible accurately predict the size of the final netw ork, and its associated burden. Rather we have estimated the upper range of burden possible based on the regul atory options being proposed which would result in a higher reporting burde n (i.e., a final level for the standard of 50 ppb with a 99 th percentile form). Based on these assumpti ons, the annual average reporting burden for the collection under 40 CFR part 58 (averaged over th e first 3 years of this ICR) 7,068,755. Burden177 is defined at 5 CFR 1320.3(b). State, local, and tr ibal entities are eligible for State assistance grants provided by the Federa l 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 re gulations in 40 CFR are listed in 40 CFR part 9. To comment on the Agency's need for this information, the accuracy of the provided burden estimates, and any s uggested methods for minimizi ng respondent burden, EPA has established a public docke t for this rule, which includes this ICR, under Docket ID number EPAHQ-OAR-2006-0922. Submit any comments related to the ICR to EPA and OMB. See ADDRESSES section at the beginning of this no tice for where to submit comments to EPA. Send comments to OMB at the Office of Info rmation and Regulatory Affairs, Office of Management and Budget, 725 17th Street, NW , Washington, DC 20503, Attention: Desk Office for EPA. Since OMB is required to make a d ecision concerning the ICR between 30 and 60 days after [Insert date of publication in the Federal Register] , a comment to OMB is best assured of having its full effect if OMB receives it by [Insert date 30 days after publication in the Federal Register.] . The final rule will respond to any OMB or public comments on the information collection requirement s contained in this proposal. 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 significant economic imp act on a substantial number of178 small entities. Small entiti es include small businesses, small organizations, and small governmental jurisdictions. For purposes of assessing the impacts of this ru le on small entities, sm all entity is defined as: (1) a small business that is a small indus trial entity as defined by the Small Business Administration’s (SBA) regulations at 13 CFR 121.201; (2) a small governmental jurisdiction that is a government of a city, county, town, school district or special distri ct with a population of less than 50,000; and (3) a small organization that is any not-for-p rofit enterprise which is independently owned and operated and is not dominant in its field. After considering the economic impacts of this proposed rule on small entities, I certify that this action will not have a significant econo mic impact on a substantial number of small entities. This proposed rule w ill not impose any requirements on sma ll entities. Rather, this rule establishes national standards for allowable 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 signifi cant impacts upon small entities because NAAQS themselves impose no regulations upon small entities). Similarl y, the proposed amendments to 40 CFR part 58 address the requirements for States to coll ect information and repor t compliance with the NAAQS and will not impose any requirements on small en tities. We continue to be interested in the potential impacts of the proposed rule on small entities and welcome comments on issues related to such impacts. D. Unfunded Mandates Reform Act Title II of the Unfunded Mandates Refo rm Act of 1995 (UMRA), Public Law 104-4, establishes requirements for Federa l agencies to assess the effect s of their regulatory actions on State, local, and tribal governments and the priv ate sector. Unless otherwise prohibited by law,179 under section 202 of the UMRA, EPA generally must pr epare a written statement, including a cost-benefit analysis, for proposed and final rule s 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 UMRA generally requires EPA to identify and consider a reasonable number of regula tory alternatives and to adopt the least costly, most cost-effective or least burdensome alternativ e 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 alte rnative other than the least costly, most costeffective or least burdensome alte rnative if the Administrator pub lishes 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 governme nts, including tribal governments, it must have de veloped under section 203 of th e UMRA a small government agency plan. The plan must provide for not ifying potentially affected small governments, enabling officials of affected small government s to have meaningful and timely input in the development of EPA regulatory proposals with significant Federal intergovernmental mandates, and informing, educating, and advising small go vernments on compliance with the regulatory requirements. This action is not subject to the requirem ents of sections 202 and 205 of the UMRA. EPA has determined that this proposed rule does not contain a Federal manda te 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 Tribal governments or the priv ate sector. The expected costs180 associated with the monitoring requirements are 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 setti ng a NAAQS EPA cannot consider th e economic or technological feasibility of attaining ambient air quality sta ndards. 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 proposed rule , EPA is merely providing an interpretation of those requirement s. However, even if this rule did establish an independent obligation for States to submit SIPs, it is questionable whether an obligation to submit a SIP revision would constitute a Federal mandate in any case. The obligation fo r a State to submit a SIP that arises out of secti on 110 and section 191 of the CAA is not legally enforceable by a court of law, and at most is a condition for continued receipt of highway funds. Therefore, it is possible to view an action requiring such a submitt al as not creating any en forceable duty within the meaning of 2 U.S.C. 658 for purposes of the UMRA. Ev en if it did, the duty could be viewed as falling within the exception for a condition of Federal assistance under 2 U.S.C. 658. EPA has determined that this proposed rule contains no regulatory requirements that might significantly or uniquely affect small governments because it imposes no enforceable duty on any small governments. Therefor e, this rule is not subject to the requirements of section 203 of the UMRA.181 E. Executive Order 13132: Federalism Executive Order 13132, entitled “Federalism” (64 FR 43255, Augus t 10, 1999), requires EPA to develop an accountable process to ensure “meaningful and timely input by State and local officials in the development of regulatory policies that have fede ralism implications.” “Policies that have federalism implications” is defined in the Executive Order to include regulations that have “substantial direct effect s 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 proposed rule does not have federalism im plications. It will not have 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 sec tion 116 preserves the rights of States to establish more stringent requireme nts if deemed necessary by a St ate. 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 sect ion E (above) on UMRA, this rule does not impose significant costs on State, local, or trib al governments or th e private sector. Thus, Executive Order 13132 does not apply to this rule. However, EPA recognizes that States will have a substantial interest in this rule and any corresponding revisions to associated air quality surveillance requirements, 40 CFR part 58. Therefore, in the spirit of Executive Order 13132, and consistent with EPA policy to promote182 communications between EPA and State and lo cal governments, EPA specifically solicits comment on this proposed rule from State and local officials.
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 proposed rule does not have tribal implications, as specified in Executive Order 13175. It does not have a substantial direct effect on one or more Indian Tribes, si nce Tribes are not obligated to adopt or implemen t any NAAQS. Thus, Executive Order 13175 does not apply to this rule. However, EPA specifically solicits additional comment on this proposedrule from tribal officials. 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 regulatory action as defined by Executive Order 12866, and we believe that the environmental health risk addressed by this action has a disproportionate effect on children. The proposed rule will establish unifo rm national ambient air quality standards for
NO 2 ; these standards are designed to protect public health with an ad equate margin of safety, as required by CAA section 109. Th e protection offered by these standards may be especially important for asthma tics, 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 effects and the size of the population affected183 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 Energy Supply, Distribution, or Use” (66 FR 28355 (May 22, 2001)) because it is not likely to have a significant adverse effect on the supply, distribution, or use of ener gy. The purpose of this rule is to establish 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 developed by States on a case-by-case basis, and EPA cannot predict whether the control options selected by States will include regulations on energy suppliers, distributors, or users. Thus, EPA conc ludes that this rule is not likely to have any adverse energy effects. 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, section 12(d) (15 U.S.C. 27 2 note) directs EPA to use voluntary consensus standards in its regulatory 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, sampli ng procedures, and business practices) that are developed or adopted by voluntary consensus standards bodies. The NTTAA directs EPA to provide Congress, through OMB, ex planations when the Agency de cides not to use available and applicable voluntary consensus standards.184 This proposed rulemaking involves technical standards with regard to ambient monitoring of NO 2 . The use of this voluntar y consensus standard woul d be impractical because the analysis method does not provide for the meth od detection limits necessary to adequately characterize ambient NO 2 concentrations for the purpose of determining compliance with the proposed revisions to the NO 2 NAAQS. EPA welcomes comments on this aspect of the proposed rule, and specifically invites the public to identify potentially a pplicable voluntary consensus standards and to explain why such standards should be used in the regulation.
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 agencies, to the greatest extent practicable and permitted by law, to make e nvironmental justice part of their mission by identifying and addressing, as appropriate, dispro portionately high and adve rse human health or environmental effects of their programs, policies, and activitie s on minority populations and lowincome populations in the United States. EPA has determined that this proposed rule will not have disproportionately high and adverse human health or environmental effects on minority or low-inco me populations because it increases the level of environm ental protection for all affected populations without having any disproportionately high and a dverse human health effects on any population, including any minority or low-income population. The proposed rule will estab lish uniform national standards for NO 2 in ambient air. EPA solicits comment on e nvironmental justice issues related to the proposed revision of the NO 2 NAAQS.185 References Baldauf, R, Watkins N, Heist D, Bailey C, Rowl ey P, Shores R. (2009). Near-road air quality monitoring: Factors aff ecting network design and interpreation of data. Air Qual. Atmos. Health . 2:1-9. Barck, C, Sandström T, Lundahl J, Halldén G, Svartengren M, Strand V, Rak S, Bylin G. (2002). Ambient level of NO 2 augments the inflammatory res ponse to inhaled allergen in asthmatics. Respir Med . 96:907-917. Barck, C, Lundahl J, Halldén G, Bylin G. (2005). Brief exposures to NO 2 augment the allergic inflammation in asthmatics. Environ Res . 97:58-66.
Beckerman, B, Jerrett M, Brook JR, Verma DK, Arain MA, Finkelstein MM. (2008). Correlation of nitrogen dioxide with other traffic pollutants near a major expressway. Atmos Environ . 42:275-290. Brauer, M, Hoek G, Smit HA, De Jongste JC, Ge rritsen J, Postma DS, Kerkhof M, Brunekreef B. (2007). Air pollution and deve lopment of asthma, allergy and infections in a birth cohort. Eur. Respir. J . 29:879-888. Chauhan, AJ, Inskip HM, Linaker CH, Smith S, Schreiber J, Johnston SL, Holgate ST (2003). Personal exposure to nitrogen dioxide (NO2) and the severity of virus-induced asthma in children. Lancet 361:1939-1944. Clements, A, Yuling J, Fraser MP, Yifang Z, Pu dota J, DenBleyker A, Michel E, Collins DR, McDonald-Buller E, Allen DT. (2008). Ai r Pollutant Concentrations near Texas Roadways: Chemical Transformation of Pollutants. Proceedings of the 101st Air & Waste Management Annual Conference , Portland, OR. Clougherty, JE, Levy JI, Kubzansky LD, Ryan PB, Suglia SF, Canner MJ, and Wright RJ. (2007). Synergistic effects of traffic-rela ted air pollution and exposure to violence on urban asthma etiology. Environ. Health Perspect. 115:1140-1146. Delfino, RJ, Zeiger RS, Seltzer JM, Street DH, McLaren CE. ( 2002). Association of asthma symptoms with peak particulate air po llution and effect m odification by antiinflammatory medication use. Environ. Health Perspect. 110:A607-A617. EPA. (1993). Air Quality Crite ria Document for the Oxides of Nitrogen. National Center for Environmental Assessment, Research Tr iangle Park, NC. EPA-600/8-91/049F. Available at: http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=40179

.186 EPA. (1995). Review of the National Ambi ent Air Quality Standards for Nitrogen Dioxide: Assessment of Scientific and Technical In formation. OAQPS Staff Paper. Office of Air Quality Planning and Standa rds, Research Triangle Park, NC. EPA-452/R-95-005. September 1995. EPA (2005). Review of the National Ambient Ai r Quality Standards for Particulate Matter: Policy Assessment of Scientific and Technical Information, OAQPS St aff Paper. Office of Air Quality Planning and Standards, Re search Triangle Park, NC. Available at: http://www.epa.gov/ttn/naaqs/standa rds/pm/data/pmstaffpaper_20051221.pdf

. EPA. (2007a). Plan for Revi ew of the Primary National Ambi ent Air Quality Standard for Nitrogen Dioxide. Available at: http://www.epa.gov/ttn/naaqs/ standards/nox/s_nox_cr_pd.html

. EPA. (2007b). Nitrogen Dioxi de Health Assessment Plan: Scope and Methods for Exposure and Risk Assessment. Office of Air Quality Planning and Standards, Research Triangle Park, NC. Available at: http://www. epa.gov/ttn/naaqs/standards/nox/s_nox_cr_pd.html

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40 CFR Part 50 Environmental protection, Air pollution c ontrol, Carbon monoxi de, Lead, Nitrogen dioxide, Ozone, Particulate matter, Sulfur oxides.
40 CFR Part 58 Environmental protection, Administrative pract ice and procedure, Ai r pollution control, Intergovernmental relations, Reporti ng and recordkeeping requirements193 Dated:

Lisa Jackson Administrator For the reasons stated in the preamble, title 40, ch apter I of the Code of Federal Regulations is proposed to be amended as follows:194 Part 50-NATIONAL PRIMARY AMBIEN T AIR QUALITY STANDARDS Section 50.11 is amended by redesignating paragraphs (b) and (c) as paragraphs (c) and (d), respectively, and amending the other provisions as follows--

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

(a) The level of the national pr imary annual ambient air quality st andard for oxides of nitrogen is 53 parts per billion (ppb, wh ich 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 fo r oxides of nitrogen is [80-100] ppb, 1-hour average concen tration, measured in the ambi ent air as nitrogen dioxide.

(c) The level of the national secondary ambient air quality standard for nitrogen dioxide is 0.053 parts per million (100 microgram s per cubic meter), annual arith metic mean concentration.

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

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

(2) by a Federal equivalent method (FEM) de signated in accordance with part 53 of this chapter.

(e) The annual primary standard is met when the annual average concentration in a calendar year is less than or equal to 53 ppb, as determined in accordance with Appendix S for the annual195 standard.

(f) The 1-hour primary standard is met when the three-year average of the annual [99 th percentile][fourth highest] of the daily maximu m 1-hour average concentr ation is less than or equal to [80-100] ppb, as determined in accord ance 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 thre e decimal places (fractional parts equal to or greater than 0.0005 pp m must be rounded up). To dem onstrate attainment, an annual mean must be based upon hourly da ta that are at least 75 percen t complete or upon data derived from manual methods that are at least 75 percent complete for th e scheduled sampling days in each calendar quarter.

(h) The annual primary standard se t forth in this section and the implementation plan obligations set forth in sections 110, 191, and 192 of the Clean Air Act associated with that standard, notwithstanding the promulgation of the 1-hour primary standard, w ill remain applicable to all areas for one year after the eff ective date of the initial design ation of the areas for the 1-hour standard, pursuant to section 107 (d)(1) of the Clean Air Act.

Appendix S to Part 50—Interpretation of th e Primary National Ambient Air Quality Standards for Oxides of Nitrogen (Nitrogen Dioxide) [1-hour primary standard based on the 99 th percentile form] 1. General.196 (a) This appendix explains the data handling conventions and computations necessary for determining when the primary national ambient ai r 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 F to this part or by a Federal equivalent method (FEM) designate d in accordance with part 53 of this chapter. Data handling and computation pr ocedures 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 adjustment s to the data affected by exceptional events, including natural events, is determined by the re quirements and process deadlines specified in §§ 50.1, 50.14 and 51.930 of this chapter.

(c) The terms used in this appe ndix 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 between 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 to197 determine compliance, calculated as speci fied in section 5 of this appendix:

(1) The annual mean value for a monitoring site for one year (referred to as the “annual primary standard design value”).
(2) The 3-year average of annual 99 th percentile daily maximum 1- hour values for a monitoring site (referred to as the “1-hour primary standard design value”).

99 th percentile daily maximum 1-hour value is the value below which nominally 99 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 data required to be s ubmitted to EPA’s Air Quality System (AQS), or otherwise available to EPA, meeting the re quirements of part 58 of this chapter including appendices A, C, and E shall be used in design value calculations.

(b) When two or more NO 2 monitors are operated at a site during a year, the state may in advance designate one of them as the primary monitor. If the stat e has not made this designation, the Administrator will ma ke the designation, either in advance or retrospectively. If198 data from the primary m onitor for a year are co mplete under the requirements of section 3 and can be used (in the case of the 1-hour primary standard, along with data fro m two other years) to develop a valid design value for either or both of the primary standards, or if the Administrator chooses to consider the design value resulting fro m the primary monitor’s data to be complete under section 3.1(c) or 3.2(d), data for the same ye ar from other monitor(s) at the site will not be used in the development of the design value(s) fo r the site. If data fro m the primary monitor are not complete and are not considered by the Admini strator as complete, data solely from the other monitor(s) will be used in turn to develop va lid design values if suchdata are sufficiently complete. If there are three or more monitors, the order fo r such comparison of the other monitors will be determined by the Administrat or. If at its turn a m onitor is found to have sufficient completeness for the year to allow calcu lation of a valid design va lue or is considered complete, data from other monito rs will not be considered. H ourly and daily data from two or more monitors in the same year at the same site will not be combin ed in an attempt to meet data completeness requirements, except when one m onitor has physically re placed another monitor permanently in which case the two monitors will be considered to be the same monitor.

(c) Hourly NO 2 measurement data shall be reported to AQS in units of parts per billion (ppb) to one place after the decimal, with additiona l digits to the righ t being truncated.

3. Comparisons with the NO 2 NAAQS.

3.1 The Annual Primary NO 2 NAAQS.199 (a) The annual primary NO 2 NAAQS is met at a site when the valid annual primary standard design value is less than or equa l to 53 parts per billion (ppb).

(b) An annual mean is valid when at least 75 pe rcent of the hours in the year are reported.

(c) Years that do not meet the completeness criteria stated in 3.1(c) and the associated annual primary standard design values ma y also be considered complete and 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 concentrations 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 equal to [80-100] parts per billion (ppb).
(b) An NO 2 1-hour primary standard design value is valid if it encompasses 3 consecutive calendar years of complete data . A year meets data comple teness requirements when all 4 quarters are complete. A quarter is complete when at least 75 percent of the sampling days for each quarter have complete data. A day is complete if 75 percent of the ho urs are reported; or if at least one reported hourly concentration exceeds the level of the 1-hour NAAQS; or if there is200 at least one reported hourly concentration and th e three-year design valu e calculated by ignoring the 75 percent daily completeness requirement, for all days at once, exceeds the 1-hour NAAQS.
(c)(i) In the case of one, two, or three years that do not meet the comple teness requirements of section 3.2(b) of this appendix and thus would normally not be useable for the calculation of a valid three-year 1-hour primary standard design value, the three-year 1-hour primary standard design value shall nevertheless be considered valid if either of the following two ‘‘data substitution’’ tests is passed. The first test is for validating an above-NAAQS-level (i.e., violating) 1-hour primary standard design value, by substituting actual ‘‘low’’ reported values from the same site at about the same time of th e year (i.e., in the same quarter) looking across three years. The second test is for validati ng a below-NAAQS-level 1-hour primary standard design value, by substituting actual ‘‘high’’ values reported for the same site at about the same time of the year (i.e., in the same quarter) looking across three years. Note that both tests are merely diagnostic in nature, intended to confir m that there is a very high likelihood if not certainty that the original desi gn value (the one with less than 75 percent data capture) reflects the true over/under-NAAQS-level stat us for that 3-year period; th e result of one of these data substitution tests (i.e., a ‘‘test desi gn value”, as defined in section 3(c)(ii) or 3(c) (iii)) is not considered the actual design value. For both types of data substitution, substitution is permitted only if there are sufficient available data points from which to identify the high or low quarterspecific daily maximum 1-hour valu es, specifically if there are at least 200 days across the three matching quarters of the three years under consid eration (which is about 75 percent of all possible daily values in those three quarters) fo r which 75 percent of th e hours in the day have reported concentrations.201 (ii) The ‘‘above NAAQS level’’ te st is as follows: Data subs titution will be performed in all quarter periods that have less than 75 percent datacapture. If by substitu ting the lowest reported daily maximum 1-hour valu e for that quarter (year non-specifi c; e.g., for January-March over the three years under consideration) ) for as much of the missing dail y data in the deficient quarters as is needed to make them 75 percent complete , the computation yields a recalculated 3-year 1- hour standard “test design value” above the le vel of the standard, th en the 1-hour primary standard design value is deemed to have passed the diagnostic test and the le vel of the standard is deemed to have been exceeded in that 3-year peri od. As noted in section 3(c)(i), in such a case, the 3-year design value based on the data actuall y reported, not the ‘‘test design value’’ including the low values, shall be used as the design value.
(iii) The ‘‘below NAAQS level’’ te st is as follows: Data subs titution will be performed in all quarter periods that have less than 75 percent data capture but at least 50 percent data capture; if any quarter has less than 50 percent data capture th en the quarter can not utilize this substitution test. If by substituting the highest reported da ily maximum 1-hour value for that quarter (year non-specific; e.g., for January-March over the three years under considerati on) for as much of the missing daily data in the deficient quarters as is needed to make them 100 percent complete, the computation yields a recalcu lated 3-year 1-hour standard “t est design value” below the level of the standard, then the 1-hour primary standard design value is deemed to have passed the diagnostic test and the level of the standard is d eemed to have been met in that 3-year period. As noted in section 3(c)(i), in su ch a case, the 3-year design value based on the data actually reported, not the ‘‘test design valu e’’ including the low values, shall be used as the design value.202 (d) Years that do not meet the completeness criteria stated in 3.2(b) and the associated 1-hour primary standard design values ma y also be considered complete and 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 concentrations in determining whether to use such data.
(f) The procedures for calculating the 1-hour prim ary standard design 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 values reported as parts per bi llion with one decimal digit are not rounded.
(b) The annual primary standard design value is the annual mean calculate d from hourly values shall be rounded to the nearest whole number or 1 ppb (decimals 0.5 and greater are rounded up to the nearest whole number, and any decimal lower than 0.5 is rounded down to the nearest whole number).
4.2 Rounding Conventions for the 1-hour Primary NO 2 NAAQS.
(a) Hourly values reported as parts per bi llion with one decimal digit are not rounded.
(b) Daily maximum 1-hour values and therefore the annual 99 th percentiles of those daily values are not rounded.203 (c) The 1-hour primary standard design value calculated from three annual 99 th percentile values shall be rounded to the nearest whole number or 1 ppb (decimals 0.5 and greater are rounded up to the nearest whole number, and any decimal lower than 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 th e data completeness requirements in section 3.1 of this appendix, the annual mean is simply the ar ithmetic average of all of the reported 1-hour values.
(b) The annual primary standard design valu e for a site is the valid annual mean.
(c) The annual primary standard design value is rounded according to the conventions in section 4.1 of this appendix before a compar ison with the standard is made.

5.2 Calculation Procedures for the 1-hour Primary NO 2 NAAQS.
(a) When the data for a particular site and year meet the data completeness requirements or when they meet one of the diagnostic tests in section 3.2 of this appendix, calculation of the 99 th percentile is accomplished by the steps provided in this subsection. Table 1 of this appendix shall be used to identify annual 99 th percentile values.
(1) Procedure for identifying annual 99 th percentile values. Identification of annual 98th percentile values will be based on th e number of days with valid data.204 Procedure: Sort all the valid daily valu es from a particular site and year by descending value. (For example: (x[1], x[2], x[3], * * *, x[n]). Inthis case, x[1] is the largest number and x[n] is the smallest value.) The 99 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 the annual number of days with valid data for year y (cn y ). The corresponding “n” value in the right colu mn identifies the rank of the annual 99 th percentile value in the descending sorted list of daily site values for year y. Thus, P 0.99, y = the nth largest value.

Table 1

Annual number of da y s with valid data for year “y” (cn y ) P 0.99, y is the nth maximum value of the y ear, where n is the listed number 1–100 1

101-200 2

201-300 3

301-366 4

(b) The 1-hour primary standard design value is then calculated by averaging the annual 99 th percentiles of the daily maximum 1-hour va lues for the most recent 3-year period.
(c) The 1-hour primary standard design value (3-year average of the 99 th percentile daily maximum 1-hour values) is rounded according to th e conventions in section 4.2 of this appendix before a comparison with the standard is made.205 PART 58--AMBIENT AIR QUALITY SURVEILLANCE 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 average 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 s ubsection (a)(5) and addi ng 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 accordan ce with the requirements of appendix D to this part shall be submitted to the Administrator by July 1, 2011. The plan shall provide for all required stations to be operational by January 1, 2013.206 * * * * *

(b) * * *

(12) The identificat ion of required NO 2 monitors as either near-road 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, operating under all of the requirements of th is part, including the requirements of appendices A, C, D, E, and G to this part.

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

§ 58.16 Data submittal and archiving requirements.

* * * * *
(a) The State, or where appropr iate, local agency, shall reportto the Administrator, via AQS all ambient air quality data and a ssociated quality assurance data for SO 2 ; CO; O 3 ; NO 2 ;207 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, 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; meteorologica l data from NCore, PAMS, and nearroad NO 2 monitoring sites; average daily temperature and average daily pressu re for Pb sites if not already reported from sampler generated r ecords; and metadata records and information specified by the AQS Data Coding Manual (http://www.epa.gov/ttn/airs/airsaqs/manuals/manua ls.htm ). The State, or where appropriate, local agency, may report site specific meteor ological measurements generated by onsite equipment (meteorological instruments, or sample r generated) or measurem ents from the nearest airport reporting ambient pressure and temperature. Such air qua lity data and information must be submitted directly to the AQS via electronic tran smission on the specified quarterly schedule described in paragraph (b) of this section.

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Appendix C to Part 58 is amended as by adding paragraph 2.1.1 to read as follows:

Appendix C to Part 58—Ambient Ai r Quality Monitoring Methodology * * * * *

2.1.1 Any NO 2 FRM or FEM used for making pr imary NAAQS decisions must be capable of providing hourly av eraged concentration data.

* * * * *208 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

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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 expected maximum hourly concentrations located near a major road with high AADT counts as specified in paragraph 4.3.2(a)(1) of this appendix. A second near-road NO 2 monitoring station is required fo r 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.
(1) The near-road NO 2 monitoring stations shall be sel ected 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 where maximum hourly NO 2 concentrations are expected to be highest and siting criteria can be met in accordance to appe ndix E of this part. In situations where one CBSA is required to have two near-road NO 2 monitoring stations, the sites shall be differentiated209 from each other by one or more of the following factors: fleet mix; congesti on patterns; terrain; geographic area within the CBSA; or different route, interstate, or freeway designation.
(b) Measurements at required near-road NO 2 monitor sites must include at a minimum: NO, NO 2 , NO X , wind vector data in the hor izontal and vertical planes, ambient temperature, and ambient relative humidity.
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 repres enting the neighborhood or larger spatial scales. PAMS sites collecting NO 2 data that are situated inan area of expected high
NO 2 concentrations at th e neighborhood or larger spatial scal e 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 identify the appropriate locations within a CBSA for locating required area-wide NO 2 monitoring stations. CBSA populations shall be based on the latest av ailable census figures.
4.3.4 Regional Administrator Required M onitoring. (a) The Regi onal Administrator may require additional NO 2 monitoring stations above the mini mum requirements to monitor in locations where required near-road monitors do no t represent a location or locations where the expected maximum hourly NO 2 concentrations exist in a CB SA. The Regional Administrator may also require additional near-road NO 2 monitoring stations above the minimum required in situations where the minimum monitori ng requirements are not sufficient.210 (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 m onitoring requirements are not suff icient to meet monitoring objectives for an area, such as supporting phot ochemical pollutant a ssessment, air quality forecasting, PM precursor analysis, and characterizi ng impacts of NO 2 sources on certain communities. The Regional Admin istrator and the 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.

4.3.5 NO 2 Monitoring Spatial Scales. (a) The most important spatial scale for near-road
NO 2 monitoring stations to effectively ch aracterize 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 stations character izing maximum expected hourly
NO 2 concentrations are the microscale and middle s cale. The most import ant spatial scale for area-wide monito ring of high NO 2 concentrations is the neighborhood scale.
(1) Microscale —This scale would typify areas in close proximity to major roadways or point and area sources. Emissions from roadways result in high ground level NO 2 concentrations at the microscale, where concentration gradie nts generally exhibit a marked decrease with increasing downwind distance from major roads. Emissions from stationary point and area sources, and non-road sources may, under certainplume conditions, resu lt in high ground level concentrations at the microscale. In these cases , the microscale would represent an area impacted by the plume with dimensions exte nding up to approximately 100 meters.211 (2) Middle scale —This scale gene rally represents air quality le vels in areas up to several city blocks in size with dimensions on the or der 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 neighbor hood scale would characterize air quality conditions throughout some relatively uniform land use areas with dimensions in the 0.5 to 4.0 kilometer range. Emissions from stationary point and area s ources may, under certain plume 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 scale would be used to estimate concentrations over large portions of an urban area with dimensions from 4 to 50 kilometers. Such measurements would be useful for assessing tren ds in area-wide air quality, and hence, the effectiveness of large scale air pollution control strategies. Ur ban scale sites may also support other monitoring objectives of the NO 2 monitoring network identif ied in paragraph 4.3.4 above.

4.3.6 NO y Monitoring. (a) NO/ NO ymeasurements are included within the NCore multipollutant site requirements and the PAMS program. These NO/ NO ymeasurements will produce conservative estimates for NO 2 that can be used to ensure tracking continued compliance with the NO 2 NAAQS. NO/ NO ymonitors are used at these sites because it is important to collect dataon total reactive nitrogen species for understanding O 3 photochemistry.212 * * * * *
Appendix E to Part 58 is amended by revisi ng paragraph 2, 4(b), 6.1, adding paragraph 6.4 and revising Table E-4 to read as follows:

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

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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 diox ide 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 sa mpler 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 concentrat ions near roadways must be
3+ 1⁄2 meters above ground level. The probe or at l east 90 percent of the monitoring path must be at least 1 meter vertically or horizontally away from any support ing structure, walls, parapets, penthouses, etc., and away from dusty or dirty ar eas. If the probe or a significant portion of the monitoring path is located near the side of a bu ilding or wall, then it should be located on the windward side of the building relative to th e prevailing wind directi on during the season of highest concentration potential for the pollutant being measured.213 * * * * *
4. Spacing from Obstructions

(a) * * *

(b) * * *

(c) * * *
(d) For near-road NO 2 monitoring stations, the monitor probe sha ll 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.

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6. Spacing from Roadways
6.1 Spacing for Ozone Probes and Monitoring 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 min imum separation distances between a roadway and a probe or, where applicable, at least 90 percent of a monitoring pa th 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 classified as middl e scale rather than neighborhood or urban scale, since the measurem ents 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.214 For those situations where a monitoring path cr osses a roadway with fe wer than 10,000 vehicles per day, one must consider the entire segment of the monitoring path in the area of potential atmospheric interference from automobile emissions . Therefore, this calcu lation must include the length of the monitoring path over the roadway plus any segments of the monitoring path that lie in the area between the roadway and minimum separation distance, as determined from the Table E-1 of this appendix. The sum of these distances must not be great er than 10 percent of the total monitoring path length.

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6.4 Spacing for Nitrogen Dioxide (NO 2 ) Probes and Monitoring Paths (a) In siting nearroad NO 2 monitors as required in paragraph 4.3.2 of appendix D of this pa rt, 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 great er than 50 meters, 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 monito ring, it is important to minimize near-road influences. Table E-1 of this appendix provides the required minimum separation distances between a roadway and a probe or, where applicable, at least 90 percent of a monitoring path for vari ous ranges of daily roadway traffi c. 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 middle scale rather than neighborhood or urban scale, since the measurements from such a site would more closel y represent the middle scale. If an open path analyzer is used at a site, the monitoring path(s) mu st not cross over a roadway with an average daily traffic count of 10,000 vehicles per day or more. For those si tuations where a monitoring path crosses a215 roadway with fewer than 10,000 vehicles per day, one must consider the entire segment of the monitoring path in the area of potential atmospheric interferen ce form automobile emissions. Therefore, this calculation must include the length of the monito ring path over the roadway plus any segments of the monitoring path that lie in the area be tween the roadway and minimum separation distance, as determined form the Table E-1 of this appe ndix. The sum of these distances must not be greater than 10 percent of the to tal monitoring path length.

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11. Summary
Table E-4 of this appendix presents a summary of the general requirements for probe and monitoring path siting criteria withrespect to distances and heights. It is apparent from Table E-4 that different elevation distan ces above the ground are shown fo r 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 heights are based on the vertical concentration gradients. For CO and near-road NO 2 monitors, the gradients in the vertical direction are very large for the microscale, so a small range of heights are used. The upper limit of 15 meters is specifi ed for the consistency between pollutants and to allow the use of a single manifold or monitoring path for monito ring more than one pollutant.216 Table E-4 of Appendix E to Part 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 (Nearroad [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 precursor s (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 E1 of this appendix for all other scales.217 N/A--Not applicable.

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 distance 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 path should be away from minor sources, such as furnace or incineration 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, ash, 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 meters apart for flow rates greater than 200 liters/min or at least 1 meter apart for samplers having flow rates less than 200 liters/min to preclude airflow interference.

Appendix G to Part 58 is amended as by revisi ng 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 transf orms ambient concentrations to a scale from 0 to 500. The AQI is keyed as appropriate to the national am bient air quality standa rds (NAAQS) for each pollutant. In most cases, the inde x value of 100 is associated with the numerical level of the short-term standard (i.e., averag ing time of 24-hours or less) fo r each pollutant. The index value of 50 is associated with the numerical level of th e annual standard for a po llutant, if there is one,218 at one-half the level of the short-term standard for the pollutant, or at the level at which it is appropriate to begin to provide guidance on cautionary language. Higher categories of the index are based on increasingly serious health effects and increasing pr oportions of the population that are likely to be affected. The i ndex is related to othe r air pollution concentr ations through linear interpolation based on these levels. The AQI is equal to the highest of the numbers corresponding to each pollutant. For the purposes of reporting the AQI, the sub-indexes for PM 10 and PM 2.5 are to be considered separately. The pollutant responsible for the highest index value (the reported AQI) is calle d the “critical” pollutant.
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TABLE 2.—BREAKPOINTS FOR THE AQI These breakpoints O 3 (ppm)

8-hour O 3 (ppm) 1-hour 1 PM 2.5 (μg/m 3 ) PM 10 (μg/m 3 ) CO (ppm) SO 2 (ppm) NO 2 (ppm) A
0.000-0.059 .................. 0.0-15.4 0-54 0.0-4.4 0.000-0.034 0 – (0.040 – 0.050) 0

- 0.060-0.075 .................. 15.5-40.4 55-154 4.5-9.4 0.035-0.144 (0.040 – 0.050) – (0.080 – 0.100) 51

- 0.076-0.095 0.125-0.164 40.5-65.4 155-254 9.5-12.4 0.145-0.224 (0.080 – 0.10) – (0.120 – 0.150) 10 1 0.096-0.115 0.165-0.204 3 65.5-150.4 255-354 12.5-15.4 0.225-0.304 (0.120 – 0.150) – (0.160 – 0.200) 15 1 0.116-0.374 0.205-0.404 3 150.5-250.4 355-424 15.5-30.4 0.305-0.604 (0.160 – 0.200) – (0.240 – 0.300) 20 1 ( 2 )............... 0.405-0.504 3 250.5-350.4 425-504 30.5-40.4 0.605-0.804 (0.240 – 0.300) – (0.320 – 0.400) 30 1 ( 2 )............... 0.505-0.604 3 350.5-500.4 505-604 40.5-50.4 0.805-1.004 (0.320 – 0.400) – (0.400 – 0.500) 40 1

1 Areas are generally required to report the AQI based on 8-hour ozone values. However, there are a small number of areas where an AQI based on 1-hour ozone values would be more precautionary. In these cases, in addition to calculating the 8-hour ozone index value, the 1-hour ozone index value may be calculated, and the maximum of the two values reported.219 2 8-hour O 3 values do not define higher AQI values ( ≥ 301). AQI values of 301 or greater are calculated with 1-hour O 3 concentrations. 3 If a different SHL for PM 2.5 is promulgated, these numbers will change accordingly.