promulgating the modified designation, and EPA must provide the State an opportunity to comment on the potential modification. The Clean Air Act requires EPA to complete the designation process within two years of the effective date of the standard unless the
Administrator finds that additional information is needed to make these decisions. In such a case, EPA may take up to an additional year to make the designations, i.e., no later than three years after the effective date of the standard. While the langua ge of Section 107 specifically addresses States, EPA will follow the same proce ss for Tribes to the extent practicable, pursuant to Sections 110(o) and 301(d) of the Act and the Tribal Auth ority Rule, or TAR (see 63 FR 7254).

EPA recommends that States and Tribes identify violating areas using the most recent three years of air quality data. In most cases, we expect these to be data from calendar years 2004-2006 that are stored in the EPA Air Quality Sy stem (AQS). In general, violations are identified using data from Federal referen ce method (FRM) and Federal equivalent method (FEM) monitors that are sited and operated in accordance with 40 CFR Part 58, as revised on October 17, 2006 (see 71 FR 61236). Procedures for using these data to determine whether a violation has occurred are given in 40 CFR Part 50 Appendix N, as revised on October 17, 2006 (see 71 FR 61144).

Air quality monitoring data affected by excep tional events may be excluded from use in identifying a violation if they m eet the criteria for such an excl usion, as specified in the Final
Rule on the Treatment of Data Influenced by Exceptional Events (72 FR 13560). For determining violations of the 24-hour PM 2.5 NAAQS, States must ensure that any 2004-2006 monitoring data affected by an exceptional event are flagge d in AQS by October 1, 2007. Further, documentation to support the determination that the data were influenced by exceptional events must be noticed for public comment and submitted to EPA no later than the submittal of
the Governor’s recommendation letter on nonatt ainment areas, which is due no later than December 18, 2007.

EPA believes that, in making their boundary recommendations for nonattainment areas,
States and Tribes should evaluate each area on a case-by-case basis. The CAA requires that a nonattainment area must include not only the area that is violating the st andard, but also nearby
areas that contribute to the viol ation. Thus, for each monitor or group of monitors that indicate violations of a standard, EPA will establish non attainment boundaries that cover a sufficiently large area to include both the area that violates th e standard and the areas that contribute to the
violations. EPA recommends th at States and Tribes base th eir boundary recommendations for violating areas on an evaluation of th e nine factors used in the prior PM 2.5 designations process, as well as on any other relevant factors or ci rcumstances specific to a particular area.

Two attachments provide additional inform ation. Attachment 1 is a time line of important dates in the revised 24-hour PM 2.5 NAAQS designation process. Attachment 2 includes a list of the nine factors that EPA plans to consider in evaluating and making decisions
on nonattainment area boundaries. When determini ng boundaries in urban areas for the annual
PM 2.5 NAAQS, EPA applied a presumption that the boundaries for urban nonattainment areasshould be based on metropolitan area boundaries as defined by the U.S. Office of Management and Budget. For the PM 2.5 24-hour NAAQS, EPA is establis hing no such presumption. EPA anticipates that the same boundaries esta blished for implementing the annual PM 2.5 standard may also be appropriate for implementing the 24-hour PM 2.5 NAAQS in areas where both standards are violated. Adopting this appr oach may more easily facilitate overall air quality planning for attaining the suite of PM 2.5 standards.

Staff in EPA’s Office of Air Quality Planning and Standards are available for assistance and consultation throughout the designation process. Questions on this guidance may be directed to Amy Vasu at 919-541-0107, or Rich Damberg at 919-541-5592.

Attachments: 2

cc: Stephen D. Page, OAQPS
Air Division Dir ectors, Regions I-X Margo Oge, OTAQ Brian McLean, OAP Elizabeth Cotsworth, ORIAATTACHMENT 1

TIME LINE FOR REVISED 24-HOUR PM 2.5 NAAQS DESIGNATION PROCESS

Milestone Date

Effective date of revised 24-hour PM 2.5 NAAQS December 18, 2006

State and Tribal recommendations due for 24-hour PM 2.5 designations. December 18, 2007

EPA notifies States and Tribes concerning any
modifications to their recommendations. No later than August 20, 2008 (120 days prior to final designations)

EPA issues final 24-hour PM 2.5 designations. No later than December 18, 2008*

* In the event the Administrator has insufficien t information to promulgate the designations by December 18, 2008, the date of fi nal designations may be extende d up to one year, but no later than December 18, 2009.ATTACHMENT 2

Factors EPA Will Consider as the Basis for Nonattainment Area Boundaries

EPA believes that certain f actors are appropriate to cons ider in making nonattainment area boundary recommendations and final boundary determinations. EPA will consider these same factors, 1 along with any other relevant informa tion, in evaluating modifications to the boundary recommendations from States and Tribes. EPA recommends that States and Tribes consider the following nine factor s in assessing whether to incl ude an area in the designated nonattainment area boundary:

C Emission data

C Air quality data

C Population density and degree of urbani zation (including commercial development)

C Traffic and commuting patterns

C Growth rates and patterns

C Meteorology (weather/transport patterns)

C Geography/topography (mountain ranges or other air basin boundaries)

C Jurisdictional boundaries (e.g., counties, ai r districts, Reservations, metropolitan planning organiza tions (MPOs))

C Level of control of emission sources

This list of recommended factors is not inte nded to be exhaustive, and States and Tribes may submit additional information on factors they believe are releva nt for EPA to consider. In general, a State’s or Tribe’s demonstration s upporting the boundary reco mmendation for an area should show that: 1) violations are not occurr ing in the excluded por tions of the recommended area, and 2) the excluded portions do not contain emission sources th at contribute to the observed
violations. A State or Tribal submittal that only addresses whether monitored violations are occurring in an area will not suffice as the sole justification for designating the boundaries of a
nonattainment area.

1 An explanation of each of these nine factors is provided in Chapter 5 of the Technical Suppo rt Document for December 17, 2004 designations and Apr il 2005 modifications, available at: http://www.epa.gov/pmdesignations/tech.htm