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2

become available since the issuance of the 2007 PM2.5 Implementation Rule, this memorandum provides 

additional guidance to assist states with development of their SIPs for the 2006 24-hour PM2.5 NAAQS. 

Fine particle concentrations in many cities are affected by a combination of regional emissions (from 

sources like power plants) and local emissions (from sources like local industry, motor vehicles, and 

residential wood combustion). The regulatory impact analysis conducted for the 2006 24-hour PM2.5

NAAQS placed additional emphasis on the air quality improvements that could be achieved from local 

reductions. We have observed that some western nonattainment areas have elevated levels of organic 

carbon on high fine particle days in the winter time.5 Potential contributing sources of these emissions

include the combustion of wood and other types of biomass in stoves and the use of hydronic heaters for 

residential and institutional heating. For these areas, SIPs for the 2006 24-hour PM2.5 NAAQS may need 

to include a heavier reliance on local emission reduction measures than SIPs for the 1997 PM2.5

NAAQS. Therefore, this memorandum includes guidance for states that have nonattainment areas with 

exceedances occurring exclusively during one season of the year.

The first section of this memorandum reviews the overall framework and basic statutory requirements 

for SIPs to demonstrate attainment with the 2006 24-hour PM2.5 NAAQS, consistent with the 2007 PM2.5

Implementation Rule. This section also suggests additional considerations for states in the development 

of their SIPs. The next sections provide guidance to states regarding the use of emissions inventories 

(including seasonal inventories, for some areas) for SIP planning (e.g., reasonable further progress 

(RFP) plans and attainment demonstrations). The last section describes how specific interpretations of 

the statute as outlined in various provisions in the regulatory text of the 2007 PM2.5 Implementation Rule 

could help inform attainment plans for the 2006 24-hour PM2.5 NAAQS.

Please share this guidance with the state agencies in your Region. It is important to note that any 

guidance contained herein is not binding on the states, the public, or the EPA.

Overall Framework for State Implementation Plans to Demonstrate Attainment

SIPs for areas violating the 2006 24-hour PM2.5 NAAQS should meet the basic Clean Air Act (CAA) 

requirements set forth in section 172. The EPA outlined its interpretation of many of these provisions in 

the 2007 PM2.5 Implementation Rule. In addition to regulatory provisions, the EPA provided substantial 

general guidance for attainment plans for PM2.5 in the preamble to the final the 2007 PM2.5

Implementation Rule. This section of this document includes details regarding these interpretations, and 

provides additional guidance, where applicable for purposes of the 2006 24-hour PM2.5 NAAQS.

Under the CAA, each attainment plan needs to demonstrate that the area will attain the NAAQS “as 

expeditiously as practicable,” but no later than five years from the effective date of the designation of 

the area. See section 172(a)(2)(A). If attainment within five years is considered impracticable by a state 

due to the severity of an area’s air quality problem and the lack of available control measures, the state 

may propose in its attainment plan an attainment date of more than five years but not more than ten 

5 Frank, N. The Chemical Composition of PM2.5 to Support PM2.5 Implementation. 

http://www.epa.gov/ttn/naaqs/pm/presents/current_pm_monitoring_data_for_speciation-neil_frank.ppt,

presented at "The PM2.5 Implementation Program and the Area Designation Process for the 2006 PM2.5 Standards Training 

Workshop Presentations," Chicago IL, June 20-21, 2007.
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years from designation. The EPA will evaluate the appropriateness of the state’s proposed attainment 

date during its rulemaking action on the attainment plan submission.

To determine the most expeditious attainment date for an area, we recommend that states first identify 

emission reduction programs that have already been adopted and are being implemented at the federal, 

state, and local levels. States could then use this information to evaluate, through air quality modeling or 

other technical analyses, the amount of air quality improvement such programs are projected to provide 

for the nonattainment area within five years of the date of designation. For example, the EPA finalized 

the Transport Rule (also known as the Cross State Air Pollution Rule) in July 2011, which is expected to 

achieve significant reductions in sulfur dioxide (SO2) and nitrogen oxides (NOx) emissions from central 

and eastern states by 2014.6 In addition, the EPA has promulgated a number of regulations over the past 

decade to reduce emissions from many types of mobile sources (e.g., Tier 2 on-road vehicle and 

standards; the 2007 heavy duty highway vehicle, engine and on-road diesel fuel rule; various non-road 

engine, vehicle and equipment regulations; the locomotive and marine engine rule; and the ocean-going 

vessel rule). All of these regulations will provide emission reductions over a broad region, which are 

expected to help many nonattainment areas attain the 2006 24-hour PM2.5 NAAQS.7

Next, we recommend that states evaluate additional control measures and control technologies—

reasonably available control measures (RACM) and reasonably available control technology (RACT)—

for an area. Under the CAA, states are required to impose RACM and RACT that can be implemented 

on sources located in nonattainment areas and to adopt enforceable regulations to ensure these areas will 

attain as expeditiously as practicable. See section 172(c)(1). The 2007 PM2.5 Implementation Rule 

requires that states evaluate potential RACM/RACT control measures for sources of direct PM2.5, SO2,

and NOx.

8 We recommend that states follow a similar approach for identifying which pollutants to 

control in a given nonattainment area for the 2006 24-hour PM2.5 NAAQS. When acting upon a state’s 

attainment plan, the EPA will evaluate the state’s approach for determining which pollutants to control.

The 2007 PM2.5 Implementation Rule also requires that after January 1, 2011, for purposes of 

establishing emissions limits for RACT and RACM, states establish such limits taking into consideration 

the condensable fraction of direct PM2.5 emissions.9 The reason for this delay in connection with the 

1997 PM2.5 NAAQS was the need for EPA to reevaluate the test methods for measuring condensable 

emissions. On December 21, 2010 (75 FR 80118), the EPA published a final notice with the revised 

Methods 201A and 202 providing for a more accurate emissions test. Thus, the EPA believes that to 

6 The final Transport Rule was published in the Federal Register on August 8, 2011 (76 FR 48208). Through this rule, 

emission reductions are expected to be achieved beginning in 2012 (76 FR 48214). On December 30, 2012, the District of 

Columbia Circuit Court (the Court) issued an order addressing the status of the Transport Rule and the CAIR in response to 

motions filed by numerous parties seeking a stay of the Transport Rule pending judicial review. In that order, the Court 

stayed the Transport Rule pending resolution in 2012 of the petitions for review of that rule in EME Homer Generation, L.P. 

v. EPA (No. 11-1302 and consolidated cases).

7 Note that section 110(a)(2)(D)(i)(I) of the CAA requires a state to ensure that emissions from sources or activities within the 

state do not contribute significantly to nonattainment in, or interfere with maintenance by, any other state.

8 The 2007 PM2.5 Implementation Rule includes a presumption that sources of NOx must be evaluated for control measures, 

unless the state and the EPA demonstrates that NOx emissions from the state do not significantly contribute to PM2.5 

concentrations in the nonattainment area. It also includes a presumption that the state does not need to evaluate sources of 

volatile organic carbon (VOC) or ammonia for control measures, unless the state or the EPA demonstrates that VOC or 

ammonia emissions from the state do significantly contribute to PM2.5 concentrations in the nonattainment area. The EPA 

believes that these same presumptions would be appropriately applied with respect to the 2006 24-hour PM2.5 NAAQS as 

well. 

9 See section 51.1002(c) of the 2007 PM2.5 Implementation Rule.
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meet the statutory requirements of section 172, states should address condensable emissions in the 

context of evaluating RACM/RACT for affected sources for the 2006 24-hour PM2.5 NAAQS.

Section 172 requires states to adopt measures that are RACM/RACT for the specific nonattainment area. 

In accordance with section 172, the preamble of the 2007 PM2.5 Implementation Rule provides extensive 

guidance for considering technical and economic feasibility for potential control measures.10 We 

recommend that states follow a similar approach to evaluate RACM/RACT for the 2006 24-hour PM2.5

NAAQS. Potential measures that are reasonably available considering technological and economic 

feasibility would need to be adopted as RACM/RACT if, considered collectively, they would advance 

the attainment date by one year or more. The EPA has long taken the interpretation that section 172 of 

the CAA does not require adoption of measures that could not collectively advance attainment by at 

least a year.11

We recommend that a state’s plan indicates whether the area can attain within five years of designation 

on the basis of emission reductions from existing programs and any new RACM/RACT measures that 

could be adopted. The EPA believes that while areas projected to attain within five years of designation 

as a result of modeling of existing national measures should still be required to conduct a RACT and 

RACM analysis, such areas may be able to conduct a limited RACT and RACM analysis that does not 

involve additional air quality modeling. A limited analysis of this type could involve the review of 

available reasonable measures, the estimation of potential emissions reductions, and the evaluation of 

the time needed to implement these measures.12

If the state determines, based on the severity of its nonattainment problem and the feasibility of pollution 

control measures, that it cannot attain the NAAQS within five years, then it would need to conduct 

further analyses to determine what attainment date (e.g., between six and ten years) would represent 

attainment “as expeditiously as practicable.” In order to support a requested attainment date of more 

than five years after the effective date of designation, the state should ensure that it has done a thorough 

evaluation of potential RACM/RACT measures. The state analysis should demonstrate that sufficient 

control measures could not be adopted and implemented cumulatively in order to establish an attainment 

date at least one year earlier. 

The 2007 PM2.5 Implementation Rule identifies a number of key source categories that can contribute to 

fine particle concentrations across the country. It recognizes that each nonattainment area has its own 

unique characteristics and mix of sources contributing to the problem. For this reason, the 2007 PM2.5

Implementation Rule neither requires specific control measures to be implemented in every 

nonattainment area, nor includes a specific source size threshold for the RACM/RACT analysis. The 

rule recognizes, however, that a state needing significant emissions reductions to attain the NAAQS in a 

given area will likely need to evaluate controls for smaller sources, while a state with an area that 

10 In June 2007, Earthjustice filed a petition for reconsideration of the 2007 PM2.5 Implementation Rule, which asserts that the 

final rule revised well-established criteria used to determine the economic feasibility of controls being considered for RACT 

(i.e., that a source in a particular source category should be able to afford emission controls similar to controls applied by

other sources in that same category) without any explanation of the legal or policy arguments in support of the new 

interpretation of the statute. On April 25, 2011, EPA granted the petition for reconsideration on this issue and intends to 

address the petition soon. 

11 See 57 FR 13498, 13560 (April 15, 1992) and 44 FR 20372, 20374 (April 4, 1979). In addition, the EPA has consistently 

interpreted RACM as a collection of measures that would advance the attainment date by at least 1 year, and the courts have 

determined that the statutory RACM requirement is ambiguous and deferred to EPA’s interpretation of the requirement. See 

Sierra Club v. EPA, 314 F.3d 735, 744 (5th Cir. 2002) and Sierra Club v. EPA, 294 F.3d, 155 162 (D.C. Cir. 2002). 12 See 72 FR 20612-20613 of the 2007 PM2.5 Implementation Rule.
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exceeds the NAAQS by only one microgram per cubic meter may not. We believe that a similar 

approach would be appropriate for the 2006 24-hour PM2.5 NAAQS. Accordingly, we recommend that 

states with more severe nonattainment problems evaluate controls for smaller sources. 

Air quality modeling is commonly used to demonstrate the level of air quality improvement that is 

projected from implementation of existing emission reduction programs and additional RACM/RACT 

measures under consideration. The details of emission inventory processing for attainment 

demonstrations and air quality modeling approaches are addressed in the EPA’s modeling guidance.13

The preamble to the 2007 PM2.5 Implementation Rule includes the consideration of RACM/RACT for 

sources participating in a regional trading program. Specifically, the preamble established a presumption 

that compliance with the Clean Air Interstate Rule (the CAIR) would satisfy RACM/RACT 

requirements for SO2 and NOx emissions from electric generating units (EGUs) in states participating in 

the CAIR cap-and-trade program for such emissions.14 The rule indicated that states could presume that 

EGUs located within a given nonattainment area were meeting the RACM/RACT requirement, based 

solely upon a regional program that imposed controls on sources both within and outside designated 

nonattainment areas. 

In June 2007, Earthjustice filed a petition that raised objections to this guidance and asserted that under 

the CAA, compliance with a regional trading program, such as the CAIR, should not be presumed to 

satisfy RACM/RACT requirements for individual EGU sources located in nonattainment areas. On a 

related issue regarding whether compliance with the NOx SIP Call constituted RACT for sources in 

nonattainment areas, in 2009 the United States Court of Appeals for the District of Columbia Circuit 

held that allowing use of the NOx SIP call to constitute RACT without any locally applicable analysis 

regarding the equivalence of NOx SIP Call and RACT reductions: “is inconsistent with the Clean Air 

Act . . . in allowing participation in a regional cap-and-trade program to satisfy an area-specific statutory 

mandate.”15 In light of this decision, on April 25, 2011, the EPA granted the Earthjustice petition for 

reconsideration of the PM2.5 presumption that CAIR satisfied RACM/RACT requirements for individual 

EGU sources located in nonattainment areas. For purposes of establishing RACT for EGU sources for 

the 2006 24-hour PM2.5 NAAQS, states wishing to rely on regional trading programs should demonstrate 

that any such program produces RACT-level reductions within the nonattainment area. 

As mentioned earlier, it is likely that SIPs for the 2006 24-hour PM2.5 NAAQS may need to include a 

greater emphasis on reducing emissions from local sources as compared to plans to attain the 1997 PM2.5

NAAQS. The EPA recommends that states consider studies in the published literature, which have 

indicated that emission reductions of direct PM2.5 produce higher estimated health benefits per ton than 

reductions of other pollutants, such as SO2 or NOx.

16 In addition, there are studies available that provide 

13 See the EPA Office of Air Quality Planning and Standards, April 2007. Guidance on the Use of Models and Other 

Analyses for Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5, and Regional Haze. (EPA-454/B-07-002). 

See also “Update to the 24 Hour PM2.5 NAAQS Modeled Attainment Test” available at 

http://www.epa.gov/ttn/scram/guidance/guide/Update to the 24-hour PM2.5 Modeled Attainment Test.pdf. 14 See PM2.5 Implementation Rule, 72 FR 20623–5. 15 See NRDC v. EPA, 571 F.3d 1245 (D.C. Cir. 2009). 16 Fann, N., Fulcher, C., and B. Hubbell, 2009. The Influence of location, source, and emission type in estimates of the 

human health benefits of reducing a ton of air pollution. Air Quality, Atmosphere & Health. Volume 2, Number 3, 169-176, 

June 2009.
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6

methods for maximizing health benefits and minimizing risk inequality in designing local scale air 

quality plans.17

After conducting a RACM/RACT analysis, the state needs to include a plan showing that the area will 

make ongoing RFP toward attainment as required by section 172(c)(2), which is commonly expressed in 

terms of incremental emission reductions and air quality improvement by certain interim milestone 

years.18 (RFP issues are discussed in greater detail later in this guidance).

All SIPs also should include contingency measures that would take effect without further action by the 

state or the EPA in the event that the area fails to meet an RFP milestone or to attain by its attainment 

date. See CAA section 172(c)(9). Contingency measures should provide for emission reductions that 

would result in a level of air quality improvement comparable to one year of improvement to be 

achieved by the submitted SIP. (Contingency measures are discussed in greater detail later in this 

guidance).

Other programs required under the CAA to be implemented in any nonattainment area include new 

source review, transportation conformity, and general conformity. These programs are not discussed in 

detail in this memorandum. Each program is governed by a series of separate regulations and guidance 

documents. Information on new source review can be found at: http://www.epa.gov/nsr. In addition, 

information on transportation conformity, including the links to the current regulations and relevant 

guidance documents, can be found at: http://www.epa.gov/otaq/stateresources/transconf/index.htm. For 

information on general conformity, please visit: http://www.epa.gov/air/genconform/background.html.

Emission Inventory Components for SIP Development 

Emission inventory data serve as the foundation for various types of analyses that enable states to 

estimate the degree to which different source sectors contribute to the nonattainment problem, as well as 

to estimate the air quality improvement that can be achieved through different control measures. The 

EPA recommends that states use the best available, current emissions inventory information for SIP 

development. High quality emission inventory data are essential for the development of an effective 

attainment demonstration. Emission inventories should be consistent with the nature of the air quality 

problem and the types of emission reduction strategies that will be implemented and relied upon as part 

of the plan.

Emission inventory data element requirements are codified in 40 CFR Part 51, Subpart A. Under this 

regulation, states are required to provide annual statewide inventory data for selected categories, 

regardless of an area’s attainment status. Through 40 CFR Part 51, Subpart A, states are required to 

report primary (i.e., “direct”) PM2.5, which is defined as the sum of filterable PM2.5 and condensable 

PM.19 This regulation also requires reporting of SO2, NOx, volatile organic compounds (VOCs), and 

17 Fann et. al., 2011. Maximizing health benefits and minimizing inequality: incorporating local-scale data in the design and 

evaluation of air quality policies. Society for Risk Analysis, vol. 31, no. 6, p. 908–922, June 2011.

18 The 2007 PM2.5 Implementation Rule states that an approvable implementation plan (submitted three years after 

designation) which shows the area will attain within five years after designation will be deemed to meet the RFP requirement.

The EPA believes a similar interpretation would be appropriate for the 2006 24-hour PM2.5 NAAQS.

19 Under 40 CFR Part 51, Subpart A, filterable PM2.5 is defined as particles that are directly emitted by a source as a solid or 

liquid at stack or release conditions and captured on the filter of a stack test train. Filterable PM2.5 is particulate matter with 

an aerodynamic diameter equal to or less than 2.5 micrometers. Condensable PM is defined as material that is vapor phase at 

stack conditions, but which condenses and/or reacts upon cooling and dilution in the ambient air to form solid or liquid PM 
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ammonia—potential precursors to PM2.5. States submit these emissions data to EPA electronically, and 

the EPA merges these data with emissions data from other sources to form the national emissions 

inventory (NEI) every three years. 

CAA section 172(c)(3) includes the emissions inventory requirements for nonattainment areas. Under 

this section of the CAA, inventories for the nonattainment area must include a comprehensive, accurate, 

current accounting of actual emissions from all sources in the nonattainment area. The EPA believes that 

by requiring an accounting of actual emissions from all sources, the language in section 172(c)(3) is 

intended to include all emissions that contribute to the formation of a particular NAAQS pollutant. 

Therefore, for the 2006 24-hour PM2.5 NAAQS, this would include direct PM2.5 (both filterable PM2.5

and condensable PM), as well as potential precursors (SO2, NOx, VOCs, and ammonia). 

We expect that for many nonattainment areas, the annual statewide inventories that are developed under 

40 CFR Part 51, Subpart A will serve as an appropriate starting point for the emissions inventories used 

for SIP development. For example, a state may choose to refine the annual statewide inventory to focus 

on particular sources within a nonattainment area. Although annual statewide inventories are required 

under 40 CFR Part 51, Subpart A, we believe that it may be appropriate in certain circumstances for 

states to use seasonal inventories for SIP development. More information regarding the use of seasonal 

inventories is included in the section “The Use of Seasonal Versus Annual Inventories.”

There are several different types of inventories that are developed for SIP planning. States usually 

submit these inventories to the EPA in conjunction with the SIP revision they support, such as an 

attainment demonstration, an RFP plan, or a maintenance plan. If deemed appropriate, the EPA approves 

the inventory into the state’s SIP. In light of this formal approval process, the EPA requires these 

inventories to contain thorough documentation of how the emission estimates were prepared. 

A brief description of the different types of emission inventories used for SIP development is provided 

below. These inventories are not developed at the same time, but rather, are developed over a number of 

years as SIP revisions become due. 

ї Base Year Inventory for the Nonattainment Area. This inventory is commonly used as a starting 

point for both the attainment demonstration and the RFP plan for an area. As required under section 

172(c)(3), this inventory should include a comprehensive, accurate, current accounting of actual 

emissions from all sources in the nonattainment area (i.e., point, nonpoint area, on-road, and nonroad sources). The base year inventory should represent emissions for one of the three years on 

which the area was designated nonattainment. We recommend that to meet the statutory 

requirements of section 172(c)(3), the base year inventory for the nonattainment area includes 

emissions of direct PM2.5 (both filterable PM2.5 and condensable PM), as well as potential precursors 

(SO2, NOx, VOCs, and ammonia). 

o Projected Attainment Year Inventory for the Nonattainment Area. The projected attainment year 

inventory includes estimated emissions for all types of emission sources located in the nonattainment 

area for the year in which the area is expected to attain the NAAQS, consistent with the attainment 

demonstration due three years after nonattainment designation. This inventory includes projected 

emissions for the attainment year, taking into consideration assumptions about emission changes 

immediately after discharge from the stack. Note that all condensable PM, if measureable and present from a source, is 

typically in the PM2.5 size fraction, and therefore all of it is a component of both primary PM2.5 and primary PM10.
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expected since the base year, as well as any expected emission reductions from existing control 

measures, and any new measures that may be adopted as part of the local area attainment plan. The 

projected attainment year inventory for the nonattainment area can be used to calculate emissions 

milestones for RFP plans and motor vehicle emissions budgets, as needed for a particular area. As 

such, we recommend that the projected attainment inventory for the nonattainment area includes 

direct PM2.5, SO2, presumptively NOx, as well as other precursors that have been determined to be 

significant through the area’s SIP development process. 

o RFP Inventories. Per section 172(c)(2), states must include a plan showing that the area will make 

ongoing RFP toward attainment, which is commonly expressed in terms of incremental emission 

reductions and air quality improvement by certain interim milestone years. The EPA recommends 

that the RFP inventories include direct PM2.5, SO2, and presumptively NOx, as well as other 

precursors that have been determined to be significant through the area’s SIP development process.20

The 2007 PM2.5 Implementation Rule provides guidance on determining target emission reduction 

levels to be achieved by interim milestone years for those pollutants included in the attainment 

demonstration. It also discusses an alternative approach in which a different mix of pollutant 

changes would be acceptable if it is expected to provide an approximately equivalent level of air 

quality improvement.

o Motor Vehicle Emissions Budgets. Motor vehicle emissions budgets (“budgets”) are used for 

transportation conformity determinations and typically reflect emissions from on-road mobile 

sources included in the attainment year and any RFP inventories for the nonattainment area. They 

are used in analyses to determine whether a transportation plan or transportation improvement 

program is consistent with the SIP and therefore would not cause or contribute to a violation of the 

NAAQS, delay timely attainment, or interfere with any interim milestones. The EPA recommends 

that motor vehicle emissions budgets include direct PM2.5 and presumptively NOx, as well other 

precursors that have been determined to be significant through the area’s SIP development process.21

For a further discussion on motor vehicle emissions budgets for PM2.5 precursors see 70 FR 24283-5

and 24287.

ї Attainment Demonstration Modeling Inventories. These inventories include detailed spatial and 

temporal elements needed to support air quality modeling, and in some cases, use methods that 

estimate emissions on a daily or hourly basis. There are two types of attainment demonstration 

modeling inventories:

1) Base Year (Baseline) Attainment Demonstration Inventory for Modeling. The base year 

inventory for attainment demonstration modeling commonly represents emissions from an 

area (sometimes a multi-state area) that may contribute to fine particle concentrations in the 

nonattainment area. The base year inventory should represent the best available inventory 

information, but the state has flexibility in determining the specific year to use.22 The 

20 The pollutant coverage of RFP assessments is determined on an area-specific basis according to each area’s attainment 

demonstration. In addition, for demonstrating RFP, states may choose to apply various control levels to various pollutants, so 

long as overall emission reductions are adequate. See 72 FR 20638-20639 of the 2007 PM2.5 Implementation Rule for more 

information.

21The EPA does not expect that states will establish motor vehicle emissions budgets for SO2 because on-road emissions of 

SO2 are at de minimis levels. 22 In some cases, the base year (baseline) attainment demonstration inventory selected for use in the attainment demonstration 

may be a different year from the base year inventory for the nonattainment area mentioned above. This could happen if, for 
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9

availability of good quality meteorological information and complete air quality data for the 

same base year is also an important consideration in selecting a base year to use. For 

purposes of photochemical modeling, we recommend that states include direct PM2.5 (both

filterable PM2.5 and condensable PM), as well as potential precursors (SO2, NOx, VOCs, and 

ammonia) in order to ensure that PM2.5 concentrations are accurately modeled.23

2) Modeled (Projected) Attainment Year Inventory for Demonstrating Attainment. This 

inventory represents projected future year emissions for the overall modeling domain 

(sometimes a multi-state area), taking into consideration assumptions about emission changes 

expected since the base year, as well as any expected emission reductions from existing 

control measures, and any new measures that may be adopted as part of the local area 

attainment plan. Air quality modeling analyses evaluate the projected change in future year 

air quality concentrations in nonattainment areas relative to air quality concentrations 

observed in the base year. For purposes of photochemical modeling, we recommend that 

states include direct PM2.5 (both filterable PM2.5 and condensable PM), as well as potential 

precursors (SO2, NOx, VOCs, and ammonia) in order to ensure that PM2.5 concentrations are 

accurately modeled.24

ї Maintenance Plan Inventories.25 When an area has attained the standard, the state may submit a 

request to redesignate that area from nonattainment to attainment. The requirements for a request for 

redesignation are outlined in section 107(d)(3)(E) of the CAA. 

These requirements include:

- The EPA determines that the area has attained the NAAQS (i.e., the most recent three years 

of complete quality assured data in AIRS-AQS show attainment);

- The EPA has fully approved the area’s applicable implementation plan under section 110(k) 

of the CAA; 

- The EPA determines the improvement in the area’s air quality is due to enforceable 

reductions in emissions resulting from implementation of the applicable implementation 

plan, applicable Federal air pollution control regulations, and other permanent enforceable 

reductions;

- The area has a fully approved maintenance plan meeting section 175A of the CAA; and

- The state has met all of the requirements applicable (for purposes of redesignation) to the 

area under section 110 (the applicable infrastructure SIP requirements) and Part D (the 

applicable nonattainment area SIP elements).

The maintenance plan provides for the maintenance of the NAAQS in the area for at least 10 years 

after the redesignation (i.e., for at least 10 years from the EPA’s final rule approving the 

example, meteorological conditions were abnormal during the selected base year inventory for the nonattainment area. 

Attainment demonstration modeling seeks to use meteorological years or episodes that are generally conducive to PM2.5 

formation. See the EPA’s PM2.5 modeling guidance for more information. 23 In some cases, where local primary PM2.5 emissions are the dominant source of PM2.5 exceedances, it may be appropriate 

for an attainment demonstration to rely solely on dispersion models or rollback techniques. In these cases, modeling of 

precursors may not be necessary as long as attainment of the NAAQS can be adequately demonstrated.

24 Ibid. 25 This guidance document does not, and is not intended to, provide specific details regarding redesignation requests and 

maintenance plans. However, we believe it is helpful to include information here regarding maintenance plan inventories.
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10

redesignation). Three sets of emissions inventories are used for a maintenance plan to demonstrate 

maintenance under section 175A: attainment year, interim year, and final year inventories. The first 

represents an actual attainment year inventory, while the second and third are projections of future 

emissions.

1) Maintenance Plan Attainment Year Inventory. As part of the maintenance plan, the state 

submits an attainment year inventory characterizing emissions in the maintenance area. This 

inventory is from one of the years in three-year period in which the area monitored 

attainment.

2) Maintenance Plan Interim Year Inventory. At least one interim year inventory is used to 

demonstrate that emissions in the area are not expected to exceed the attainment year 

inventory in the interim between the base year and the last year of the maintenance plan. The

demonstration, by means of an interim year inventory, that the area will maintain the 

standard throughout the maintenance period is derived from CAA section 175A, which states 

that maintenance in the area is to be provided “for at least ten years after the redesignation,” 

and not just in the final year. Thus, a maintenance plan includes at least one interim year 

inventory to establish that, during the period that maintenance is projected, emissions will 

remain at or below the level of the attainment year inventory. 

3) Maintenance Plan Projected Final Year Inventory. This inventory includes projected 

emissions for the maintenance area for a period ending 10 years after EPA’s final approval of 

the redesignation and maintenance plan. As stated above, section 175A requires a 

demonstration that the area will continue to attain the standard throughout the 10-year 

maintenance period. The maintenance plan final year inventory reflects assumptions about 

expected emissions changes in the maintenance area over the 10-year period due to ongoing 

and new national, state, and local requirements as well as other future conditions.26 In 

identifying an appropriate projection year, the state should factor in the time necessary for 

the EPA to review and take action on the redesignation request and maintenance plan, 

considering that CAA section 107(d)(3)(D) provides the EPA with up to 18 months after a 

complete redesignation request is submitted. The EPA recommends to states that the 

maintenance plan demonstrate maintenance for 12 years from the time of formal submittal to 

allow for completion of the redesignation rulemaking process. The EPA also recommends 

that when the state is in the initial stages of developing the maintenance plan, the state should 

consult with its EPA Regional Office on an appropriate year for the final year inventory. 

The Use of Seasonal Versus Annual Inventories

As mentioned earlier in this memorandum, statewide annual emission inventories are required under 40 

CFR Part 51, Subpart A. We expect that for many nonattainment areas, these annual inventories will 

serve as an appropriate starting point for the emissions inventories used for SIP development. In contrast 

with the 1997 annual PM2.5 NAAQS, where states rely only on annual inventories in the implementation 

process, the 2006 24-hour PM2.5 NAAQS is designed to protect against peak exposures. Thus, for the 

26 The maintenance plan would also establish motor vehicle emissions budgets for the last year of the maintenance period, 

and could include budgets for an interim year(s) if the area chooses to do so. Any budgets would address direct PM2.5, NOx, 

and any other precursors that the maintenance plan identifies as being significant in order for the area to maintain the 

NAAQS.
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11

2006 24-hour PM2.5 NAAQS, there are some circumstances in which the EPA believes that seasonal 

inventories may be useful for SIP planning purposes. For example, we have observed that in some 

nonattainment areas, all of the highest fine particle concentrations over the course of a year occur in one 

season.27

If exceedances occur during only one season for each of the years on which the nonattainment 

designation is based, and this is the case for all subsequent years, we recommend that states develop a 

seasonal inventory and that they use this inventory for SIP planning purposes. We believe that, for some 

areas, a seasonal approach will enable states to expend fewer resources to determine which controls may 

be most effective for reducing concentrations of concern. For example, emission reduction strategies 

would not need to be evaluated for all seasons. In addition, fewer modeling runs may be needed to 

establish mobile source emission inventories (see the next section regarding the use of the Motor 

Vehicle Emission Simulator (MOVES) model). The determination regarding whether or not to use a 

seasonal inventory will be area-specific, however. The EPA recommends that each state works in 

consultation with it EPA Regional Office to characterize an area and to assess whether seasonal 

inventories are needed for sources that cause violations in a particular season.

To consider whether a seasonal inventory would be appropriate for an area, the EPA recommends that 

states first assess the nature of the fine particle problem in that area, focusing on the seasonality of the 

problem. This assessment should include consideration of information about the months during which 

the area experiences its exceedances;28 the meteorology on the days with exceedances; and insights from 

fine particle “speciation” (chemical composition) data about key local and regional sources contributing 

to the area’s exceedances. For example, we have observed that a number of nonattainment areas have 

elevated levels of organic carbon on high fine particle days only in the winter.29 Common sources 

contributing to high organic carbon concentrations in this season include the combustion of wood and 

other types of biomass in stoves and the use of hydronic heaters for residential and institutional heating. 

Often these high PM2.5 levels occur during multi-day episodes characterized by cool temperatures, low 

wind speeds, and inversion conditions in which cool air traps pollutants close to the ground and limits 

mixing in the atmosphere. The EPA has developed the “Strategies for Reducing Residential Wood 

Smoke” resource guide to provide states, local, and tribal governments with information on education 

and outreach tools, as well as regulatory approaches (e.g., example ordinances) and incentive-based 

programs to reduce wood smoke. The resource guide also provides information on calculating emission 

reductions from wood stove and fireplace changeout/retrofit programs and wood smoke curtailment 

programs. For more information, visit: http://www.epa.gov/ttncaaa1/t1/memoranda/strategies-doc-8-11-

09.pdf.

27 For more information, see: 

http://www.epa.gov/pmdesignations/2006NAAQS/final/TSD/tsd_D_chemical_composition_pm25.pdf.

28 The word “season” is not intended to imply specific months out of the year. For example, in some areas, the winter season 

may span from October through March, where in other areas, it could span from December through February. Note also that 

if an area experiences a high PM2.5 event that occurs outside of the season being considered for a seasonal inventory, a 

seasonal inventory may still be appropriate if the event meets the definition of an exceptional event under 40 CFR 50.1 and is 

determined to be an exceptional event in accordance with 40 CFR 50.14.

29 Frank, N. The Chemical Composition of PM2.5 to Support PM2.5 Implementation. 

http://www.epa.gov/ttn/naaqs/pm/presents/current_pm_monitoring_data_for_speciation-neil_frank.ppt,

Presented at "The PM2.5 Implementation Program and the Area Designation Process for the 2006 PM2.5 Standards Training 

Workshop Presentations," Chicago IL, June 20-21, 2007.
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12

We have also observed that some areas in the northern states have elevated ammonium nitrate levels in 

the winter, as nitrate remains in particle form and does not volatilize at lower temperatures. High 

ammonium nitrate levels are caused by the reaction of NOx and ammonia gas in the atmosphere. NOx 

emissions result from many sources that burn fossil fuels, such as cars, diesel trucks, non-road engines, 

and various industrial sources. 

To further assess the nature of the fine particle problem in an area, we recommend that states investigate 

the source mix for the season in question versus a longer-time averaging period (e.g., an annual 

average). "Source mix" refers to the contribution of sources in question to the total PM2.5 mass for the 

time period in question. The source mix can be assessed using information such as inventory data and 

source attribution/source tagging studies, in which ambient PM2.5 data or model output is apportioned to 

various source category types.

When development of a seasonal inventory would be appropriate, the EPA recommends that states give 

additional attention to refining emissions estimates for key source categories which operate 

predominantly during the relevant season (e.g., woodstove emissions in winter).30 When following a 

seasonal approach, the EPA believes that the control strategy evaluation (based on seasonal emission 

reduction measures) and the assessment of future year air quality concentrations (through air quality 

modeling or other analyses) should be conducted for that season.31 In addition, if a seasonal inventory 

approach is used, then the transportation conformity budget should pertain to the relevant season. States 

should work with their EPA Regional Offices to develop their seasonal inventories. This collaboration, 

for example, may help states determine the appropriate temporal resolution that will correlate well with 

the types of days that exceedances are recorded in the area. 

Developing On-Road Mobile Source Emission Inventories 

The EPA recommends that when developing their SIPs for the 2006 24-hour PM2.5 NAAQS in areas 

outside of California, states use of the most recent version of the MOVES model to estimate on-road 

emissions of direct PM2.5, NOx and other precursors deemed significant by the state for SIPs.32 MOVES 

is a state-of-the-art upgrade to the EPA’s modeling tools for estimating emissions from highway 

vehicles, based on analysis of millions of emission test results and considerable advances in the 

Agency’s understanding of vehicle emissions. The MOVES model provides updated vehicle emission 

factors, with particular improvements in the characterization of vehicle emissions under varying 

temperature and vehicle speed and power conditions over its predecessor MOBILE. For example, 

30 If more information is desired on how various emission sectors are temporally allocated, please see 

http://www.epa.gov/ttn/chief/emch/index.html#related. If additional information is required, please contact Tesh Rao at 

rao.venkatesh@epa.gov. 31 Modeling should be limited to a single season only in cases where exceedances in other seasons have not occurred in the 

past and are not expected to occur in the future (based on ambient data and analysis of emissions). See the modeling guidance 

for more information. 

32The EPA has indicated that it expected all SIPs for areas outside of California for the 2006 24-hour PM2.5 NAAQS to 

include estimates of on-road emissions based on results from the MOVES model. (See question 6 of the guidance document 

titled, “Policy Guidance on the Use of MOVES2010 for State Implementation Plan Development, Transportation 

Conformity, and Other Purposes.” MOVES2010a is the current version of MOVES available for use in SIP preparation 

outside of California. However, state and local agencies should refer to the MOVES webpage to determine the version of 

MOVES to be used when they begin work on their SIP for this NAAQS. This web page also contains links to guidance 

documents on the use of MOVES. The MOVES web page is available at: http://www.epa.gov/otaq/models/moves/index.htm. 

State and local agencies in California would use the most recent version of EMFAC in the preparation of SIPs for this 

NAAQS.
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13

MOVES provides much better estimates of direct PM2.5 emissions from both diesel and gasoline 

vehicles and accounts for how emissions vary with temperature throughout the year. The contribution of 

on-road motor vehicles to a nonattainment area’s PM2.5 problem depends on numerous factors, including 

local diesel truck traffic, fleet mix, and vehicle-miles-traveled (VMT) growth. 

On-road inventories and motor vehicle emissions budgets for transportation conformity purposes would 

include direct PM2.5, NOx, and any other precursor emissions deemed significant by the state for their 

SIP.33 For each relevant year, areas should choose MOVES inputs for the season of the year when 

exceedances have occurred. If exceedances occurred in only one season for multiple years, then 

emissions should be estimated using MOVES inputs and other data that are relevant for that season. The 

resulting on-road inventories and motor vehicle emissions budgets would be for a 24-hour period 

(calculated in MOVES using the hourly time aggregation option with average hourly meteorology and 

activity) as representative days.34 For example, if the SIP demonstrates that exceedances occurred only 

during the winter, the inventory and motor vehicle emissions budgets should be developed for only that 

season using MOVES inputs for parameters such as temperature, humidity and fuel properties that are 

consistent with winter time conditions. In this example, the SIP’s inventory and motor vehicle emissions 

budgets would also be based on the best available information concerning winter time VMT, speeds, and 

other relevant data.

If exceedances occur in more than one season, then on-road inventories should be calculated for the full 

year using appropriate MOVES inputs and other data to accurately estimate on-road emissions. 

Based on the information provided above, we therefore recommend that motor vehicle emissions 

budgets are established in one of two ways:

1. The SIP could establish motor vehicle emissions budgets for direct PM2.5, NOx and any other 

relevant precursors for such areas on an annual basis consistent with the calculation of annual 

on-road inventories; or

2. The SIP could establish motor vehicle emissions budgets for direct PM2.5, NOx and any other 

relevant precursors for the one season of the year when on-road motor vehicles make the 

most significant contribution to an area’s PM2.5 exceedances.

For example, an area may choose to take the second approach in its SIP because it can show that motor 

vehicles make the most significant contribution to exceedances in the winter. This may be the case if 

direct PM2.5 emissions from gasoline powered vehicles are higher during that time of year and NOx

emissions from on-road vehicles contribute to nitrate concentrations that are higher in the winter than in 

the summer. Alternately, if an area had exceedances during the summer, the SIP may show that the 

exceedances in the summer are predominantly due to SO2 emissions from large point sources, such as 

EGUs and other sources of direct PM2.5 in the area, and that emissions from on-road motor vehicles are 

33 A small number of areas may determine that either re-entrained road dust or dust from the construction of transportation 

projects are significant contributors to their PM2.5 problems and include those emissions in the area’s motor vehicle emissions 

budgets. [40 CFR 93.102(b)(3) and 93 CFR 122(f)(2)]. Re-entrained road dust emissions would be calculated using the latest 

approved version of AP-42 or other local method that has been approved by EPA. Emissions of dust from construction of 

transportation projects would be calculated using AP-42 or other local method.

34 For additional guidance on creating daily emissions inventories, refer to EPA’s guidance documents located at: 

http://www.epa.gov/otaq/stateresources/index.htm.
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14

less important to the area’s PM2.5 problem during the summer. If an area chooses to establish motor 

vehicle emissions budgets for one season of the year, it should use MOVES inputs and other data that 

are relevant for the chosen season. In such a case, the resulting budgets would be for a 24-hour period 

calculated in MOVES on an hourly basis. 

Regardless of which approach is used, we recommend that the state base the motor vehicle emissions 

budgets on air quality data, including available speciation data from the years on which the SIP is based 

and any available modeling demonstration. Decisions on which approach to take should be made after 

consultation occurs between states and local air and transportation agencies (including the metropolitan 

planning organization), the Federal Highway and Transit Administration local offices, and the EPA 

Regional Office.

It is important to note that the regional emissions analyses for subsequent transportation conformity 

determinations would estimate emissions using inputs consistent with the inputs used to estimate the 

motor vehicle emissions budgets. For example, emissions for a regional emissions analysis would be 

calculated using winter MOVES inputs if the budgets for the area were established for the winter season. 

Developing Non-Road Emission Inventories

Areas are required to provide estimates of emissions from non-road vehicles and equipment. The EPA 

recommends the use of the most recent version of the EPA’s NONROAD model to estimate non-road 

emissions in areas outside of California.35 It should be noted that some categories of non-road sources 

will have higher emissions during one season as opposed to the others. For example, in northern states, 

emissions from construction equipment are likely to be higher in the summer as compared to the winter. 

This seasonal variation should be accounted for in modeling inventories and in any other seasonal 

inventories that are prepared.

Recommendations for the 2006 24-hour PM2.5 NAAQS in Comparison with the 2007 PM2.5

Implementation Rule

Attainment Dates: Section 51.1004

Under section 172(a)(2)(A) of the CAA, an area’s attainment date “shall be the date by which attainment 

can be achieved as expeditiously as practicable, but no later than 5 years from the date such area was 

designated nonattainment * * * , except that the Administrator may extend the attainment date to the 

extent the Administrator determines appropriate, for a period no greater than 10 years from the date of 

designation as nonattainment considering the severity of nonattainment and the availability and 

feasibility of pollution control measures.” For PM2.5, a state’s attainment demonstration should provide a 

thorough analysis of available control measures and provide a proposed attainment date supported by the 

analysis. Under the 2007 PM2.5 Implementation Rule, analytical and regulatory requirements are more 

stringent for areas that are unable to demonstrate attainment within the first five years. The EPA 

believes that these same approaches are reasonable interpretations of the statute for the 2006 24-hour 

PM2.5 NAAQS as well.

35 More information and guidance on NONROAD is available at: http://www.epa.gov/otaq/nonrdmdl.htm. Information on 

emissions from marine sources and locomotives are available at http://www.epa.gov/otaq/marine.htm and 

http://www.epa.gov/otaq/locomotives.htm, respectively. Areas in California should consult with EPA Region 9 to determine 

the appropriate model for estimating non-road emissions.
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15

Because the designation of nonattainment areas for the 2006 24-hour PM2.5 NAAQS became effective 

on December 14, 2009, the presumptive five-year attainment date for many areas would be no later than 

December 14, 2014, just two weeks short of the end of the calendar year. The form of the 24-hour PM2.5

NAAQS relies on evaluating three full calendar years of air quality data to calculate the design value for 

the area. To determine attainment, the EPA commonly uses the three most recent calendar years of 

complete air quality data that are available. Because calendar year 2014 would not be complete on 

December 14, 2014, this could be read to suggest that control measures taking effect at the beginning of 

2014, and the resulting air quality improvements, could not be considered in the attainment 

determination. However, we note that states can take 2014 into account in their attainment 

demonstrations, and the EPA will consider the approvability of attainment demonstration SIPs on a 

case-by-case basis. For example, an attainment demonstration that is intended to show attainment within 

five years of designation may be approvable if control measures to reach attainment are scheduled to be 

operational at the beginning of 2014.36 Under this example, this approach would provide states with the 

opportunity to use air quality data from 2012 through 2014 to assess attainment for the 2006 24-hour 

PM2.5 NAAQS.

One-Year Extension of the Attainment Date: Section 51.1005

Section 51.1005 of the 2007 PM2.5 Implementation Rule describes the criteria governing when an area 

may apply for a one-year extension of the attainment date pursuant to section 172(a)(2)(C)(ii) of the 

CAA. The EPA believes that an appropriate interpretation of this provision with respect to the 2006 24-

hour PM2.5 NAAQS would be that if an area has a design value which exceeds the standard based on 

three consecutive years of air quality data, but the third year’s data indicate improved air quality such 

that the PM2.5 concentration for that one year is below the level of the standard (e.g., the 98th percentile 

24-hour concentration for that one year is equal to or less than 35 ug/m3

), then the area has improving air 

quality and may apply to the EPA for a one-year extension of the attainment date.

The regulatory text of the 2007 PM2.5 Implementation Rule (see paragraphs 51.1005(a)(2) and (b)(2))

provides that a state “may apply” for a one-year attainment date extension if the 98th percentile 

concentration for the previous year was equal to or less than the 1997 24-hour PM2.5 NAAQS of 65 

ug/m3

. With respect to implementing the 2006 24-hour PM2.5 NAAQS, if states choose to apply for a 

one-year extension, we recommend that they show that the area’s 98th percentile concentration for the 

previous year is less than the 2006 24-hour PM2.5 NAAQS, i.e., 35 ug/m3.

Emission Inventory Requirements: Section 51.1008

The regulatory text of section 51.1008(b) of the 2007 PM2.5 Implementation Rule states that “[F]or 

inventories required for submission under paragraph (a) of this section, a baseline emission inventory is 

required for the attainment demonstration required under section 51.1007 and for meeting RFP 

requirements under section 51.1009. As determined on the date of designation, the base year for this 

inventory shall be the most recent calendar year for which a complete inventory was required to be 

submitted to the EPA pursuant to subpart A of this part. The baseline emission inventory for calendar 

36 See EDF v. EPA, 369 F.3d 193 (D.C. Circuit 2004); Sierra Club v. EPA, 356 F3d (D.C. Circuit 2004) amended 2004 WL 

877850 (D.C. Circuit 2004).
Withdrawn by the EPA on June 6, 2013

16

year 2002 or other suitable year shall be used for attainment planning and RFP plans for areas initially 

designated nonattainment for the PM2.5 NAAQS in 2004-2005.” 

The overall intent of this provision is for states to use the best available information in their analyses. 

States should use the most recently updated information when developing their emission inventories for 

the nonattainment area for use in their attainment plans (and for RFP plans for any area that needs to 

submit one). For purposes of developing plans to attain the 2006 24-hour PM2.5 NAAQS, the EPA 

believes that the statute provides adequate flexibility to states to use the most recent “suitable year” for 

their analyses. 

As mentioned earlier, if a seasonal inventory approach is followed, the EPA recommends that states give 

additional attention to refining emissions estimates for key source categories which operate 

predominantly during the relevant season (e.g., woodstoves in winter) in order to identify which 

emissions sources are contributing to the season-specific nonattainment. The emission inventory used 

for SIP purposes would pertain to the particular season in which exceedances occur for multiple years. If 

exceedances occur in more than one season, then we recommend that states use an annual inventory.

Reasonable Further Progress: Section 51.1009

This section addresses two issues related to RFP: the specific years for emission reduction milestones to 

be included in an RFP plan for the 2006 24-hour PM2.5 NAAQS; and the geographic region to be 

represented in the plan. 

Milestone Years. Section 51.1009 of the 2007 PM2.5 Implementation Rule requires any area that submits 

an approvable demonstration for an attainment date of more than five years from the effective date of 

designation to also submit an RFP plan. The 2007 PM2.5 Implementation Rule includes two approaches 

for demonstrating compliance with interim RFP milestones: either by showing that emission reductions 

provide for “generally linear progress,” or alternatively by showing that air quality improvement 

provides for “generally linear progress.” The EPA believes this interpretation of the statute would also 

be appropriate for the 2006 24-hour PM2.5 NAAQS. We recommend that the state develop an RFP plan 

that uses emission inventory information and related data consistent with what is used for the attainment 

demonstration. We further recommend that the plan includes emission reduction milestones showing 

that in each applicable milestone year, emissions will be at a level consistent with “generally linear 

progress” in reducing emissions between the base year and the attainment year. 

Section 51.1009(c)(1) of the 2007 PM2.5 Implementation Rule requires that an area with an attainment 

date of more than 5 years and less than 9 years includes 2009 emission milestones for direct PM2.5 and 

PM2.5 attainment plan precursors. Section 51.1009(c)(2) states that an area with an attainment date of 9 

or 10 years from designation must include 2009 and 2012 emission milestones for direct PM2.5 and 

PM2.5 attainment plan precursors. 

In other words, for the 1997 PM2.5 NAAQS, the 2007 PM2.5 Implementation Rule requires an area with 

an attainment date of 6-8 years from the date of designation (i.e., 2011 to 2013) to include an emission 

milestone in 2009. The 2007 PM2.5 Implementation Rule requires an area with an attainment date of 9 or 

10 years from the date of designation (i.e., 2014 to 2015) to include emission milestones in 2009 and in 

2012. The EPA recommends that the framework of the 2007 PM2.5 Implementation Rule and the 
Withdrawn by the EPA on June 6, 2013

17

associated plan submission timelines be similarly applied and interpreted in relative terms for purposes 

of developing RFP plans for the 2006 24-hour PM2.5 NAAQS. 

Therefore, for purposes of implementation of the 2006 24-hour PM2.5 NAAQS, EPA believes that it 

would be appropriate to apply an analogous framework. For an area with an attainment date of 6-8 years 

from the date of designation (i.e., 2015 to 2017 if the area was designated in 2009), under this 

interpretation the RFP plan would need a milestone for one interim year, in this case, 2014. For an area 

with an attainment date of 9 or 10 years from the date of designation (i.e., 2018 to 2019 if the area was 

designated in 2009), the RFP plan would need milestones for two interim years, 2014 and 2017. Thus, in 

reviewing implementation plans, submitted by areas that were designated nonattainment in 2009 for the 

2006 24-hour PM2.5 NAAQS, EPA intends to consider the appropriate interim milestone years to be 

2014 and (if necessary) 2017.

We recognize that each nonattainment area has its own unique characteristics (e.g., physical features, 

source population, seasonal nature of exceedances, types of emission reduction programs adopted) 

which will affect how the area can demonstrate reasonable further progress toward attainment.

Therefore, each RFP plan will be evaluated on a case-by-case basis.

Geographic Region. Reasonable further progress is defined under CAA section 171(1) to mean “such 

annual incremental reductions in emissions of the relevant air pollutant as are required by this part or 

may reasonably be required by the Administrator for the purpose of ensuring attainment of the 

applicable national ambient air quality standard by the applicable date.” RFP for PM2.5 is required under 

section 172. Section 172(c)(3) states that nonattainment area plans must include a comprehensive 

inventory of emissions from “all sources . . . in [the nonattainment] area.” The 2007 PM2.5

Implementation Rule includes provisions for calculating emission reduction milestones using the base 

year and projected attainment year emission inventories for the relevant “geographic area” represented 

in the RFP plan. From the discussion of the geographic area in the preamble, one can infer that the 

default geographic area would be the nonattainment area. However, the preamble also includes 

discussion of a policy that allowed states to take credit for reductions of NOx and SO2 emissions up to 

200 kilometers (km) from outside the nonattainment area (and potentially for reductions of VOC or 

ammonia as well) only when certain conditions are met, including that when taking RFP credit for 

emissions reductions achieved outside of a nonattainment area, the baseline emissions inventory for the 

nonattainment area contain all, rather than a select few, sources in the outside area.

In 2007, the EPA received a petition for reconsideration of this policy for the 1997 PM2.5 NAAQS. In 

addition, in 2006, the EPA received a petition for reconsideration of a similar policy in the 

implementation rule for the 1997 8-hour ozone NAAQS.37 The petitions for both PM2.5 and ozone 

challenged the allowance of emissions reductions from outside of the nonattainment area for meeting 

RFP requirements. 

Additionally, in 2009, the United States Court of Appeals for the District of Columbia Circuit (the 

Court) examined the phrase “in the area” included in separate provisions relating to reductions from the 

application of RACT. In the Phase 2 Ozone Implementation Rule, EPA had explained that because an 

interstate emissions trading program would achieve beyond RACT-level NOx reductions regionally, 

37 In the preamble to the Phase 2 Ozone Implementation Rule, EPA stated that credit could be taken for VOC and NOx

emission reductions within 100 km and 200 km, respectively, from outside the nonattainment area under certain 

circumstances.
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18

areas did not have to meet the RACT-level reductions required under CAA section 182(b)(2) solely 

from within the nonattainment area. The Court, however, concluded that the phrase “in the area” means 

that reductions must occur from sources within the area and “reductions from outside the nonattainment 

area do not satisfy the requirement.”38 On December 22, 2010, (75 FR 80420), the EPA proposed a rule 

to reflect its reconsideration of the RFP policy for ozone. In light of the Court decision emphasizing that 

“in the area” means in the nonattainment area and due to the proposed action for ozone, the EPA granted 

the petition for reconsideration of the RFP policy for PM2.5 on May 13, 2010. 

The EPA acknowledges that attainment plans for some areas may include a certain level of emission 

reductions (and air quality improvement) from sources within the nonattainment area, and also a certain 

level of emission reductions (and air quality improvement) attributed to sources outside the 

nonattainment area. The EPA will be addressing this issue in response to the petition for reconsideration 

pertaining to the 2007 PM2.5 Implementation Rule. Therefore, for purposes of meeting the RFP 

requirement for the 2006 24-hour PM2.5 NAAQS, states should rely only on emission reductions

achieved from sources within the nonattainment area.

Contingency Measures: Section 51.1012

Contingency measures, required pursuant to section 172(c)(9) of the CAA, are emission reduction 

measures that are to be undertaken by a state if the area fails to attain the NAAQS by its attainment date 

or fails to make RFP toward attainment. These contingency measures must take effect without further 

action by the state. 

The preamble of the 2007 PM2.5 Implementation Rule (see 79 FR 20642-20645) notes that contingency 

measures “should provide for emission reductions equivalent to about one year of reductions needed for 

reasonable further progress (RFP).” The term “one year of reductions needed for RFP” requires 

clarification. This phrase may be confusing because all areas technically are not required to develop a 

separate RFP plan under the 2007 PM2.5 Implementation Rule. The basic concept is that an area’s set of 

contingency measures should provide for an amount of emission reductions that would achieve “one 

year’s worth” of air quality improvement proportional to the overall amount of air quality improvement 

to be achieved by the area’s attainment plan; or alternatively, an amount of emission reductions (for all 

pollutants subject to control measures in the attainment plan) that would achieve one year’s worth of 

emission reductions proportional to the overall amount of emission reductions needed to show 

attainment. Contingency measures can include measures that achieve emission reductions from outside 

the nonattainment area as well as from within the nonattainment area, provided that the measures 

produce the appropriate air quality impact within the nonattainment area.

The EPA believes a similar interpretation of the contingency measures requirements under section 

172(c)(9) would be appropriate for the 2006 24-hour PM2.5 NAAQS. Included below is an example to 

illustrate this concept. 

x Assume that the area in question was designated nonattainment in December 2009 with a 2006-

2008 design value of 41 ug/m3

.

38 See NRDC v. EPA, 571 F.3d 1245 at 1256 (D.C. Cir. 2009). 
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19

ї The SIP would be due in December 2012 and attainment would generally need to occur 

within five years, based on air quality data through December 2014. 

x Assume that the state analysis uses a 2008 base year emissions inventory and a future year 

projection inventory for 2014. 

ї To demonstrate attainment, the area needs to reduce its air quality concentration from 41 

ug/m3 in 2008 to 35 ug/m3 in 2014, equal to a rate of change of 1 ug/m3 per year. The 

attainment plan demonstrates that this level of air quality improvement would be achieved by 

reducing emissions between 2008 and 2014 by the following amounts: 1,200 tons of PM2.5;

6,000 tons of NOx; and 6,000 tons of SO2.

ї Thus, the target level for contingency measures for the area could be identified in two ways:

1) The area would need to provide an air quality improvement of 1 ug/m3 in the area, based 

on an adequate technical demonstration provided in the state plan. The emission reductions to 

be achieved by the contingency measures can be from any one or a combination of all 

pollutants addressed in the attainment plan, provided that the state plan shows that the 

cumulative effect of the adopted contingency measures would result in a 1 ug/m3

improvement in the fine particle concentration in the nonattainment area; and 2) The 

contingency measures for the area would be one-sixth (or approximately 17 percent) of the 

overall emission reductions needed between 2008 and 2014 to show attainment. In this 

example, these amounts would be the following: 200 tons of PM2.5; 1,000 tons of NOx; and 

1,000 tons of SO2.

Questions Regarding This Guidance

In general, questions may be directed to Rich Damberg at (919) 541-5592 or damberg.rich@epa.gov;

and Kristin Riha at (919) 541-2031 or riha.kristin@epa.gov. Specific questions on transportation 

conformity should be directed to Meg Patulski at (734) 214-4842 or patulski.meg@epa.gov. Any 

questions on the use of the MOVES model should be sent to: mobile@epa.gov.

cc: Anna Wood, OAQPS

Lydia Wegman, OAQPS

Richard Wayland, OAQPS

Greg Green, OAQPS

Peter Tsirigotis, OAQPS

Margo Oge, OTAQ

Karl Simon, OTAQ

Sarah Dunham, OAP