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 language of Section 107 specifically 

addresses States, EPA will follow the same process for Tribes to the extent practicable, pursuant 

to Sections 110(o) and 301(d) of the Act and the Tribal Authority 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 System (AQS). In general, violations are 

identified using data from Federal reference 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 exceptional events may be excluded from use in 

identifying a violation if they meet the criteria for such an exclusion, 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 PM2.5 NAAQS, States must ensure that any 2004-2006 

monitoring data affected by an exceptional event are flagged 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 nonattainment 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 standard, but also nearby 

areas that contribute to the violation. Thus, for each monitor or group of monitors that indicate 

violations of a standard, EPA will establish nonattainment boundaries that cover a sufficiently 

large area to include both the area that violates the standard and the areas that contribute to the 

violations. EPA recommends that States and Tribes base their boundary recommendations for 

violating areas on an evaluation of the nine factors used in the prior PM2.5 designations process, 

as well as on any other relevant factors or circumstances specific to a particular area. 

 Two attachments provide additional information. Attachment 1 is a time line of 

important dates in the revised 24-hour PM2.5 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 determining boundaries in urban areas for the annual 

PM2.5 NAAQS, EPA applied a presumption that the boundaries for urban nonattainment areas 
should be based on metropolitan area boundaries as defined by the U.S. Office of Management 

and Budget. For the PM2.5 24-hour NAAQS, EPA is establishing no such presumption. EPA 

anticipates that the same boundaries established for implementing the annual PM2.5 standard may 

also be appropriate for implementing the 24-hour PM2.5 NAAQS in areas where both standards 

are violated. Adopting this approach may more easily facilitate overall air quality planning for 

attaining the suite of PM2.5 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 Directors, Regions I-X 

 Margo Oge, OTAQ 

 Brian McLean, OAP 

 Elizabeth Cotsworth, ORIA 
ATTACHMENT 1

TIME LINE FOR REVISED 24-HOUR PM2.5 NAAQS DESIGNATION PROCESS

Milestone Date

Effective date of revised 24-hour PM2.5 NAAQS December 18, 2006 

State and Tribal recommendations due for 24-hour PM2.5

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 PM2.5 designations. No later than December 18, 2008*

* In the event the Administrator has insufficient information to promulgate the designations by 

December 18, 2008, the date of final designations may be extended 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 factors are appropriate to consider in making nonattainment 

area boundary recommendations and final boundary determinations. EPA will consider these 

same factors,1

 along with any other relevant information, in evaluating modifications to the 

boundary recommendations from States and Tribes. EPA recommends that States and Tribes 

consider the following nine factors in assessing whether to include an area in the designated 

nonattainment area boundary: 

C Emission data 

C Air quality data 

C Population density and degree of urbanization (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, air districts, Reservations, metropolitan 

planning organizations (MPOs))

C Level of control of emission sources 

 This list of recommended factors is not intended to be exhaustive, and States and Tribes 

may submit additional information on factors they believe are relevant for EPA to consider. In 

general, a State’s or Tribe’s demonstration supporting the boundary recommendation for an area 

should show that: 1) violations are not occurring in the excluded portions of the recommended 

area, and 2) the excluded portions do not contain emission sources that 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 Support Document for 

December 17, 2004 designations and April 2005 modifications, available at: 

http://www.epa.gov/pmdesignations/tech.htm