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DRAFT 

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Demonstrating Noninterference Under Section 110(l) of the Clean Air Act 

When Revising a State Implementation Plan 

 

1.0 Introduction 

 Section 110(l) of the Clean Air Act (CAA) indicates that EPA cannot approve a State 

Implementation Plan (SIP) revision if the revision would interfere with any applicable 

requirement concerning attainment and reasonable further progress (RFP), or any other 

applicable requirement of the CAA. Therefore, the EPA will approve a SIP revision that 

removes or modifies control measure(s) in the SIP only after the State has demonstrated that 

such removal or modification will not interfere (“noninterference”) with attainment of the 

National Ambient air Quality Standards (NAAQS), Rate of Progress (ROP), RFP or any other 

applicable requirement of the CAA. 

 Specifically, section 110(l) states: 

 

Each revision to an implementation plan submitted by a State under this Act shall be adopted 

by such State after reasonable notice and public hearing. The Administrator shall not 

approve a revision of a plan if the revision would interfere with any applicable requirement 

concerning attainment and reasonable further progress (as defined in section 171), or any 

other applicable requirement of this Act. 

2.0 What is the purpose of this document? 

 This document provides guidance to EPA Regional, State, and Tribal air quality management 

authorities and the general public on how EPA intends to exercise its discretion in implementing 

CAA provisions and EPA regulations concerning the section 110(l) requirement. It does not 

impose binding, enforceable requirements on any party, nor does it assure that EPA will approve 

all instances of its application. Thus, the guidance may not apply to a particular situation based 

upon the circumstances. Although section 110(l) applies to all applicable CAA requirements, the 

focus of this guidance is on how a State will demonstrate noninterference relative to attainment 

and maintenance of the NAAQS. For the most part, it does not address noninterference 

demonstrations for other CAA requirements. For these requirements, States are advised to work 

closely with their respective EPA Regional Offices to design an appropriate 110(l) 

demonstration. 

 

 The EPA and State decision makers retain the discretion to adopt approaches on a case-bycase basis that differ from this guidance, where appropriate. Any decisions by EPA regarding a 

particular SIP demonstration will only be made based on the statute and regulations, and will 

only be made following notice and opportunity for public review and comment. Therefore, 

interested parties are free to raise questions and objections about the contents of this guidance 
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and the appropriateness of the application of this guidance to a particular situation. EPA will, 

and States should, consider whether or not the recommendations in this guidance are appropriate 

in that situation. This guidance is a living document and may be revised periodically without 

public notice. The EPA welcomes public comments on this document and will consider those 

comments in any future revisions of this guidance document, providing such approaches comply 

with all applicable statutory and regulatory requirements. 

 Users are cautioned not to regard statements recommending the use of certain procedures or 

defaults as either precluding other procedures or information or providing guarantees that using 

these procedures or defaults will result in actions that are fully approvable. As noted above, 

EPA cannot assure that actions based upon this guidance will be fully approvable in all 

instances, and all final actions may only be taken following notice and opportunity for public 

comment. 

 Documentation which describes each analysis (e.g., databases used, modeling methodology, 

results, etc.) and explains how it supports a determination of noninterference should be 

submitted to the appropriate Regional Office. States/Tribes applying this guidance are 

encouraged to work closely with their Regional Offices to develop a plan for applying the 

guidance. Obtaining early review of approaches being applied can help assure that resources are 

not wasted on analyses which may not provide meaningful results. 

3.0 Does section 110(l) apply to all areas and to all CAA requirements?

 Yes, section 110(l) applies to all requirements of the CAA and to all areas of the country, 

whether attainment, nonattainment, unclassifiable or maintenance1

 for one or more of the six 

criteria pollutants: ozone, particulate matter (PM), nitrogen dioxide (NO2), sulfur dioxide (SO2), 

carbon monoxide (CO) and lead (Pb). As stated earlier, although the focus of this guidance is on 

how a State can demonstrate noninterference relative to attainment and maintenance of the 

NAAQS, section 110(l) is not limited in scope to those SIP revisions that only impact ambient 

air quality. Thus, when making demonstration of noninterference under section 110(l), States 

should be mindful that a showing should be made be made not only for attainment and 

maintenance, but for all other applicable requirements including, but not limited to: 

 

1

Section 110(l) applies to both maintenance plans under section 175A and those under section 110(a)(1) that may be 

required following promulgation of a new NAAQS.

• Reasonably Available Control Technology (RACT) requirements 

• Inspection and maintenance programs (I/M) 

• Major source applicability cut-offs for purposes of RACT 

• ROP (1-hour ozone)/RFP (8-hour ozone) 
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• Stage II vapor recovery2

• Clean fuels fleet program under section 183(c)(4) of the CAA 

• Clean fuels for boilers under section 183(e)(3) of the CAA 

• Transportation Control Measures (TCMs) during heavy traffic hours as provided under 

section 182(e)(4) of the CAA 

• Enhanced (ambient) monitoring under section 182(c)(1) of the CAA 

• Transportation controls under section 182(c)(5) of the CAA 

• Vehicle miles traveled provisions of section 182(d)(1) of the CAA 

• Nitrogen oxides (NOx) requirements under section 182(f) of the CAA 

• 1-hour attainment demonstrations 

• Reasonably Available Control Measures (RACM) 

• Contingency measures 

• New Source Review 3 

• Prevention of Significant Deterioration (PSD)4

• Air Toxics 5, 6

• Regional Haze under sections 169A and 169B of the CAA 

• Inter- and intrastate transport 

• New Source Performance Standards 

 For SIP revisions that will or could potentially lead to a change in emissions or ambient 

concentrations of a pollutant or its precursors, the section 110(l) demonstration should address 

all pollutants whose emissions and/or ambient concentrations may change as a result of the SIP 

revision. For example, while an I/M program originally may have been adopted to address a CO 

problem, such program may also reduce NOx and/or volatile organic compound (VOC) 

emissions. Thus, removal of the I/M program from the SIP may also cause an increase in NOx

and VOC emissions, which may impact ozone and PM2.5. These NAAQS would need to be 

addressed in the 110(l) demonstration, as well as CO. 

 

2

Per section 202(a)(6), this requirement may change as the use of Onboard Refueling Vapor Recovery technology 

becomes widespread throughout the vehicle fleet. 

3

Per Hall v. EPA, 273 F.3d 1146 (9th Cir. 2001), the Agency is currently reviewing how section 110(l) will be met 

with regard to NSR/PSD revisions. We will provide additional guidance once we have finalized our policy.

4

Class I impacts (Air Quality Related Values under section 165(d)(2)) as well as PSD increment consumption.

5

Since there are no ambient air quality standards for air toxics, the area’s compliance with any applicable MACT 

standards, as well as any Federal mobile source control requirements under sections 112 or 202(l) would constitute 

an acceptable demonstration of noninterference for air toxics.

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Section 110(l) does not require a demonstration of noninterference for changes to Federal requirements that are not 

included in the SIP. A revision to the SIP, however, cannot interfere with any Federally mandated program such as 

a MACT standard (or related section 112 requirements), or Reid Vapor Pressure. 
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Similarly, and as further discussed in section 3.2 below, although section 110(l) does not 

require a showing of noninterference with a revoked NAAQS, if there is a measure remaining in 

the SIP related to that NAAQS, the State will need to show noninterference with any other 

relevant or applicable CAA requirements, including other NAAQS that may be impacted, before 

EPA can approve removal of the measure from the SIP. 

3.1 Can any control measure be removed from a SIP? 

 Some control measures may not be removed from a SIP even if doing so would not interfere 

with the Clean Air Act’s air quality goals. These measures are often referred to as “mandatory” 

measures because the CAA requires that they be included in the SIP for an area based on the 

area’s designation status and classification. For example, the CAA specifies that an area 

classified as moderate for ozone must implement a basic tailpipe I/M program. An ozone 

nonattainment area classified as Serious is required to implement an enhanced ambient air 

monitoring system. 

 If an area is redesignated from nonattainment to attainment, specific control measures that 

applied based on the area’s classification i.e., mandatory measures, will no longer be required. 

At that point, it may be possible for the State to remove the measure from the SIP, provided 

noninterference is demonstrated under section 110(l).7

 Measures not tied to an area’s classification and not mandated by the CAA are often referred 

to as “discretionary” measures. States can remove discretionary measures from an attainment, 

nonattainment or maintenance plan. However, a section 110(l) demonstration of noninterference 

would still be required. 

 In addition, the following sections of the CAA may prohibit an area from completely 

removing a control measure: 

 

• “General Savings Clause” (Section 193). Control measures in effect or required to be 

adopted in nonattainment areas by an order, settlement agreement, or plan in effect before the 

1990 CAA amendments in nonattainment areas may not be removed or modified absent a 

SIP revision that ensures equivalent or greater emissions reductions. 

 

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It is important to note that section 175A(d) requires that any control measure removed from the SIP be retained as a 

contingency measure in the maintenance plan.

• “Significant Interstate Contribution” (Section 110(a)(2)(D)). Control measures approved 

into the SIP to meet Federal regulations, such as the 1998 NOx SIP Call under section 

110(a)(2)(D) may be substituted with other measures, but only if the requirements of the 

Federal regulations (e.g., the statewide NOx emission budget) continue to be met. 
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• “Interstate Ozone Transport Region” (OTR) (Section 176A and Section 184). Unless an 

area has fully or partially opted out from the OTR, control measures that apply in ozone 

transport regions are mandated controls and cannot be removed from the SIP. Areas that 

have opted out and wish to remove an OTR measure from the SIP must make a section 110(l) 

demonstration of noninterference before removing the measure. 

 States should keep in mind that EPA approval of a section 110(l) demonstration to remove a 

measure does not mean that additional control measures might not be needed in the future to 

demonstrate attainment or maintenance, or to meet RFP requirements. In particular, this will be 

the case where a 110(l) demonstration precedes an attainment demonstration that is required by 

the CAA based on an area’s classification. 

3.2 How does revocation of a NAAQS affect removal or modification of a measure? 

 

 The final 8-hour ozone implementation rule (40 CFR §51.905) ("Phase 1 Rule") contains 

anti-backsliding provisions that require areas that were designated as nonattainment or 

attainment but subject to a maintenance plan under section 175A for the 1-hour ozone standard 

at the time of designation for the 8-hour standard retain in their SIPs (or to submit, if not yet 

approved into the SIP) certain obligations that applied by virtue of their 1-hour ozone 

designation and classification. These requirements are called "applicable requirements" under 

the Phase 1 Rule. Areas remain subject to these obligations until they attain and are redesignated 

to attainment for the 8-hour ozone NAAQS.8

 EPA believes that a similar approach would be 

appropriate with respect to other revoked standards. 

 Obligations in an approved SIP that are not "applicable requirements" under the Phase 1 Rule 

may be modified or removed, but only in conjunction with a demonstration of noninterference 

under section 110(l). This guidance applies whenever a State modifies a SIP in a manner that 

requires a section 110(l) demonstration, to SIP changes that States pursue under 40 CFR §51.905 

(8-hour ozone implementation rule), as well as to any other subsequent rules to implement a new 

NAAQS. 

 Additionally, under the Phase 1 Rule, an area that was designated attainment subject to a 

maintenance plan for the 1-hour standard at the time of designation to attainment for the 8-hour 

standard may modify its contingency measures in the 1-hour maintenance plan, but only in 

conjunction with a section 110(a)(1) maintenance plan for the ozone 8-hour standard (see 69 FR 

23999, April 30, 2004). The revision to remove or modify a contingency measure could occur at 

the same time that EPA approves the area’s 110(a)(1) maintenance plan, and EPA believes that 

the section 110(a)(1) maintenance plan showing maintenance without the measure at issue, or 

 

8

Note that §51.372(c) of EPA’s I/M rule (60 FR 1735) allows areas that would otherwise qualify for redesignation 

to attainment to convert their basic I/M program to a contingency measure as long as implementation authority is 

retained and an I/M program is not required under a new NAAQS. See memo from Tom Helms and Leila Cook, 

“1-Hour Ozone: Maintenance Plans: Basic Inspection and Maintenance Programs,” dated 5-12-04 at 

http://www.epa.gov/ttn/oarpg/t1pgm.html. 
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with the measure as modified, could serve as an acceptable 110(l) demonstration for the 8-hour 

ozone NAAQS. 

4.0 How can a State or Tribe demonstrate noninterference? 

As stated earlier, other CAA requirements may apply to a given SIP and will require a 

section 

110(l) demonstration. It would not be feasible for the Agency to develop a document that 

provides guidance on how to make a demonstration of noninterference for every applicable CAA 

requirement that might apply. For this reason, this document thus focuses on the means by which 

States or Tribes can demonstrate noninterference relative to the CAA requirements of 

attainment, RFP and maintenance of the NAAQS. To the extent that this guidance does not 

address other CAA requirements that may apply to an area, States and Tribes are advised to 

work closely with their respective EPA Regional Offices to determine an appropriate 110(l) 

demonstration.

 With respect to attainment, maintenance and RFP, the Agency interprets section 110(l) such 

that areas have two options available to demonstrate noninterference for the affected pollutant(s). 

As discussed below, these options are: 1) Substitution of one measure by another with equivalent 

or greater emissions reductions/air quality benefit; or 2) an air quality analysis showing that 

removing the measure will not interfere with other applicable requirements (i.e., without a 

substitute measure). These options, discussed in more detail in Sections 5 and 6 below, are 

available to all areas, whether the requested action is to remove or modify an area-wide control 

measure, a state-wide control measure or a control measure at a single source. 

 When demonstrating noninterference, attainment areas (including maintenance areas) have 

the same options as nonattainment areas. The scope and rigor of the demonstration will depend 

on the nature of the requirement, the air quality status of the area, the potential impact of the 

revision on air quality and the pollutant(s) affected. 

 EPA retains the discretion to adopt approaches on a case-by-case basis that may differ from 

the general parameters provided herein, but which still comply with the CAA. Any decisions 

regarding a particular determination will be made based on the statute, regulations and technical 

rationale. 

5.0 What is the substitution option?

 Under the substitution option, the State removes a measure from the SIP and substitutes it 

with another measure that achieves equivalent or greater emission reductions. The basic 

principle of the substitution approach is that as long as the status quo in air quality is preserved, 

noninterference is demonstrated. To support the substitution, the State should provide EPA with 

an analysis showing that removal or revision of the SIP measure does not interfere with any 

other applicable requirements of the Act. Substitute measures should be quantifiable, 

permanent, surplus, enforceable and contemporaneous (when the substitute is a new measure). 

 EPA does not believe that it is reasonable nor practical to require air quality modeling to 

support the substitution in all cases. The most straightforward approach would be “direct 
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substitution” in which increases in a pollutant resulting from the SIP revision are offset with 

reductions from a replacement measure that achieves equivalent or greater emission reductions 

of the same pollutant. Examples would include substitution of one VOC control measure for 

another, NOx for NOx, SO2 for SO2 and so on. A simple scenario would also involve substituting 

low-level emission reductions for low-level emissions reductions, or elevated emissions 

reductions for elevated emissions reductions. In such cases, substituting with an alternative 

control measure that obtains equivalent or greater emission reductions of the same pollutant 

could be considered an acceptable demonstration of noninterference. Additional technical 

analysis may not be needed. 

 In other cases, due to the uncertainty of predicting pollutant interactions, even when 

substituting with emissions reductions of the same pollutant or precursor, the EPA may 

determine that air quality modeling is necessary. Factors that could necessitate modeling include: 

• A large distance between affected sources 

• A large discrepancy in elevation of emissions between the affected sources 

•An update of emissions data in the SIP results in a large difference between the existing 

emissions inventory9

 and the emissions inventory that is presented in the noninterference 

demonstration 

• A significant portion of the area’s total emissions is under consideration for substitution 

 If an area chooses to substitute with a different pollutant or precursor, additional analysis 

would be necessary. The type of analysis warranted by interpollutant and interprecursor 

substitutions is discussed further in section 5.3. 

5.1 Substitution with existing measures

 An area can use either an existing or a new control measure as a substitute however, in order 

to use an existing measure (i.e., one that is already obtaining emissions reductions) as a 

substitute, the area should have an approved attainment/maintenance demonstration in order to 

ascertain that the emissions reductions from the existing control measure are indeed surplus and 

haven’t been relied on previously for attainment or maintenance of the NAAQS,10 or to meet 

ROP/RFP requirements. 

 As is the case with existing local or State measures, Federal measures can serve as 

substitution measures as well, provided their reductions were not previously relied on for credit 

 

9

Note that the Consolidated Emissions Reporting Rule (CERR) (67 FR 39602) requires all States and territories, 

irrespective of air quality status, to submit 3-year inventories for the following pollutants: SOx, NOx, VOC, CO, 

PM10, PM2.5, NH3. Areas using the substitution option but which have an outdated inventory can use a 3-year 

inventory under CERR as part of their section 110(l) demonstration. 

10In addition, a demonstration is needed to show that the increase in emissions would not interfere with attainment 

or maintenance of any affected NAAQS not covered by the attainment/maintenance plan. 
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toward attainment, maintenance or ROP/REF purposes (i.e., the reductions are surplus). 

5.2 Substitution with new measures

 EPA believes that if emissions reductions removed from the SIP are replaced with new 

control measures that achieve equivalent or greater emissions reductions, the SIP revision will 

not interfere with the area’s ability to continue to attain or maintain the affected NAAQS or other 

CAA requirements.11

 Timing. New substitute control measures should be contemporaneous to the time when the 

emissions reductions from the measure that was removed or modified cease occurring. The EPA 

recognizes that, in reality, implementation of a substitute control measure may not coincide 

exactly with removal or modification of the “old” measure. Keeping this in mind, the substitute 

control measure should go into effect close enough in time to the SIP revision such that the 

timing is reasonable enough to be considered contemporaneous, thus preserving the status quo in 

air quality. For this reason, the Agency advises States to build into their demonstration a 

reasonable amount of time to terminate old measures and implement any new replacement 

measures. Determination of “contemporaneous” will be made case-by-case within the context of 

a rulemaking approving the substitution. Note that the further apart in time implementation of 

the substitute measure is from removal of the old measure(s), the more difficult it will be to 

defend the substitution as contemporaneous. 

 

5.3 When substituting one pollutant or precursor for another 

 When offsetting an increase in a pollutant or precursor with reductions of another pollutant 

or precursor, States should provide a more detailed technical analysis that includes air quality 

modeling12 to show that the substitution will provide equivalent or greater air quality benefit for 

each NAAQS that is affected by the SIP revision. 

 For ozone, substitutions involving VOC and NOX measures can be complex, depending on 

many case-specific factors such as whether the area is NOX or VOC-limited, for example. For 

PM2.5, the interaction between SO2, NOx, VOC, ammonia and direct PM2.5 emissions in the 

formation of fine particulate matter can be even more complex, and much is still not known 

about the pollutant interactions that form PM2.5. Currently, relative effects on PM2.5

concentrations due to changes in SO2 and NOX emissions are generally better understood than 

the effects on PM2.5 concentrations resulting from changes in VOC and ammonia emissions. 

New information on important PM-related research (such as the role of specific VOC species in 

PM2.5 formation and better quantification of ammonia inventories) continues to emerge. For 

 

11In some cases, additional factors need to be considered e.g., interprecursor substitution.

12 Appropriate air quality models for predicting changes in ozone and PM2.5 concentrations include CMAQ and 

CAMx. States should also consult the air quality modeling tools and resources available for download at 

www.epa.gov/scram001/tt25.htm#guidance.
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these reasons, substitution decisions should be made on a case-by-case basis. States should 

always take into account the latest scientific information when considering inter-pollutant 

substitution. 

 In general, direct PM2.5 emission reductions may be substituted for PM2.5 precursors. It 

would be appropriate to substitute emissions reductions from ammonia or VOC sources only 

where an area provides an adequate technical demonstration showing that such pollutants 

contribute to PM2.5 levels in the area. 

 The technical demonstration for complex substitutions such as these should include these 

principles: 

• The substitution should be consistent with any existing attainment demonstrations and plans 

to address rate-of-progress or reasonable further progress requirements for the area. 

 

• The State should provide exchange ratios13 for the affected pollutants that are supported 

either by modeling or by other technical analyses.14 

• In addition, for substitution cases involving PM2.5 precursors: 

 

13Exchange ratios indicate the relative effect of different precursors on air quality in a specific nonattainment area.

14 NOx Substitution Guidance at http://www.epa.gov/ttn/naaqs/ozone/ozonetech/#nox (scroll down to item 6.s., 

“NOx Substitution Guidance 12-15-93"). This policy offers one method of determining ratios for precursor 

substitution that may be appropriate, depending on the characteristics of the area in question. Substitution ratios 

may also be estimated using various air quality modeling tools by assessing the relative impact of reducing one 

PM2.5 precursor versus another by equal amounts or percentages.

Any proposed substitution of an existing measure with a measure achieving SO2, 

NOX, or direct PM2.5 emissions reductions should show that the substitute measure 

achieves equivalent or greater air quality benefit. 

 Any proposed substitution of an existing measure with a measure achieving VOC 

and/or ammonia emissions reductions should show that 1) VOC and/or ammonia are 

significant contributors to the area’s PM2.5 concentration; and 2) that the substitute 

measure achieves equivalent or greater air quality benefit. 

 Any proposed substitution that removes one measure and substitutes it with a new 

measure that reduces a PM2.5 precursor should also address the same PM2.5 precursor in 

its PM2.5 control strategy. For example, if an area chose to demonstrate noninterference 
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for PM2.5 by substituting NOX reductions with all VOC reductions, the EPA would 

expect the area to include VOC in the area’s overall PM2.5 control strategy. 

 If a proposed substitution measure includes VOC reductions for purposes of 

addressing ozone but the State finds that VOC are not a significant contributor to PM2.5

in the area, then the State should also obtain reductions of a different PM2.5 precursor 

(such as SO2) to address noninterference for PM2.5. Another option would be for the area 

to substitute with direct PM2.5 emissions reductions. 

 

5.4 Can substitute emissions reductions be obtained from outside the nonattainment or 

maintenance area boundary? 

 Yes. As discussed in EPA’s Clean Air Interstate Rule proceedings,15 PM2.5 and ozone are 

the result of emissions from many sources over a multi-State region. Emissions of NOx and 

VOC (as ozone precursors) and emissions of NOx and SO2 (as PM2.5 precursors), are well 

quantified for many urban and regional sources. Furthermore, the transport of these emissions 

can also be quantified with a fair degree of certainty. Because both ozone and PM2.5 formation 

occur as the result of emissions from many sources over a multi-State region, regional controls 

for these emissions can result in significant benefits. The Agency thus concludes that it is 

appropriate to allow ozone and PM2.5 areas to obtain substitute emissions reductions from 

sources located outside the nonattainment or maintenance area boundary, provided the State can 

demonstrate that the substitute measure provides the same air quality benefit as that being 

replaced. 

 For example, under current EPA policy, an ozone area can obtain replacement measures 

located up to 100 and 200 kilometers from the nonattainment area for VOC and NOx reductions, 

respectively.16 The distances in the guidance provide only a general policy presumption that, if 

used, would need to be supported by technical rationale showing that the result will produce 

equal or greater air quality benefit in the area and that the substitution will not interfere with any 

other area’s ability to attain or maintain the NAAQS. The Agency intends to reassess whether 

the policy should be revised in light of concerns recently brought to our attention pertaining to 

its technical feasibility. 

 

15http://www.epa.gov/cair/rule.html

16“Guidance for Implementing the 1-Hour Ozone and Pre-Existing PM10 NAAQS,” December 29, 1997, 

http://www.epa.gov/ttn/oarpg/t1/memoranda/iig.pdf 

6.0 What is the air quality analysis option? 

 
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 Under this option, the State would conduct air quality modeling or develop an attainment or 

maintenance demonstration based on EPA’s most recent technical guidance17,18 to show that 

even without the control measure or with the measure in its modified form, the area (as well as 

interstate and intrastate areas downwind) can continue to attain/maintain the NAAQS. An area 

seeking redesignation could also use this approach when removing a measure from the 

nonattainment SIP and adding it to the contingency portion of the maintenance SIP. Under this 

scenario, the maintenance demonstration submitted with the redesignation request would serve 

as the section 110(l) showing for the pollutant covered by the maintenance plan. The 

maintenance demonstration would need to show that the area can maintain the NAAQS without 

the measure in question. Additional demonstrations of noninterference would be required for any 

other affected NAAQS not covered by the maintenance plan. 

 Depending on the nature of the SIP revision, an area may need additional technical guidance. 

Consultation with the Regional Office and the Model Clearinghouse is recommended prior to a 

110(l) demonstration to support removal or revision of a SIP requirement under any of the 

options. In addition, Appendix W to 40 CFR Part 5119 (“Guideline on Air Quality Models”) 

contains general criteria that States should consider. 

6.1. Can I conduct alternative analyses in lieu of air quality modeling?

 

 For areas that don’t have an attainment demonstration, the EPA would consider alternative 

analyses such as sensitivity modeling using the Community Multiscale Air Quality (CMAQ) 

Modeling System, Comprehensive Air Quality Model with Extensions (CAMx) or other 

dispersion models on a case-by-case basis. In rare cases, other indicators such as ambient air 

quality analyses, air quality trends and emissions trends to create a base-case scenario may be 

considered.20 The level of rigor in the demonstration will vary depending on the nature of the 

requirement, its potential impact on air quality in the area and the air quality of the area in which 

the requirement applies. 

 For example, for certain pollutants such as CO, the Agency could determine that an increase 

in emissions is not interfering with attainment if ambient levels in an attainment area were well 

below the NAAQS and not trending upward. Under these conditions, the Agency could conclude 

that ambient levels are so low that a slight emissions increase is not likely to cause a violation of 

the NAAQS, hence noninterference has been demonstrated. 

 

17EPA modeling guidance for 8-hour ozone, PM2.5 and Regional Haze are available at 

http://www.epa.gov/scram001/tt25.htm#guidance. 

18“Procedures for Processing Requests to Redesignate Areas to Attainment,” Sept. 4, 1992 at 

http://www.epa.gov/ttn/oarpg/t5/memoranda/redesignmem090492.pdf.

19 Available at http://www.epa.gov/scram001/guidance/guide/appw_03.pdf. 

20One resource for PM10 demonstrations is “Critical Design Value Estimation and its Applications,” by Shao-hang 

Chu at http://www.epa.gov/ttn/oarpg/t1/memoranda/cdv.pdf. 
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6.2 Which NAAQS and precursors do I need to include in my air quality analyses? 

 Depending on the number of pollutants that are affected by removal or modification of the 

control measure in question, an area may need to demonstrate that the change would not interfere 

with attainment or maintenance for more than one NAAQS. For example, removal of an I/M 

program that tested for CO, hydrocarbons and NOx would require a demonstration of 

noninterference for the following NAAQS: CO, ozone, PM10 and PM2.5. As noted above, the 

State could demonstrate noninterference through air quality modeling, an attainment 

demonstration, or in some situations as discussed in section 6.1 above, an alternative analysis not 

involving air quality modeling. 

 The following guidance should be considered in cases where removal of a measure results in 

an increase in a PM2.5 precursor. If the State conducts air quality modeling to demonstrate 

noninterference, the state should include the projected change in all PM2.5 precursors, including 

VOC and ammonia, in the emission inventory used for the air quality modeling scenario 

simulating conditions after the measure has been removed. This approach would best represent 

the overall interaction of increased PM2.5 precursor levels and the resulting air quality 

concentration. If the State chooses to perform an alternative analysis to demonstrate 

noninterference however, then the State should evaluate the resulting increase in emissions of 

SO2, NOX, and direct PM2.5 presumptively. Regarding VOC and/or ammonia, the State should 

evaluate noninterference for these pollutants only if the State or EPA had provided a 

demonstration showing that VOC and/or ammonia was a significant contributor to PM2.5

concentrations in the area in question. States are advised to consult with their respective EPA 

Regional Office for additional guidance on these issues. 

6.3 Do I need to demonstrate noninterference relative to transport? 

 Yes. Section 110(l) requires a demonstration of noninterference with respect to section 

110(a)(2)(D) requirements relating to interstate transport. The Agency believes areas that are 

subject to Federal regulations under section 110(a)(2)(D), such as the NOx SIP Call Rule or the 

Clean Air Interstate Rule, satisfy section 110(l) with respect to interstate transport for that 

pollutant or precursor, where the requirements of the Federal regulations (i.e., the statewide 

emission budgets) are being met. 

 In addition, section 110(a)(1) requires a SIP to provide for implementation and maintenance 

of a primary standard within the State. To satisfy section 110(l) with respect to intrastate 

transport, EPA believes the State should show that the revision is consistent with any attainment 

or maintenance plan in a downwind area elsewhere in the State. 

 

7.0 Are there other considerations for either option? 

 Yes. The following guidance applies whether an area is demonstrating noninterference 

through substitution or through air quality analysis. 
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7.1 SIP revisions that are administrative in nature 

 Some SIP elements, such as public hearing requirements, are clearly administrative or 

procedural, and changes to them will not affect emissions. For SIP revisions of this nature, the 

two options may not be relevant. Instead, an acceptable 110(l) demonstration might only require 

written analysis explaining that the revision is: (a) administrative or procedural in nature; (b) 

will not affect emissions; and (c) will not interfere with requirements of the CAA related to such 

administrative or procedural provisions. 

7.2 Compliance assurance requirements 

 Other SIP requirements such as monitoring, record keeping, reporting and additional 

compliance assurance requirements, may have an impact on the amount of pollutants emitted 

because they are instrumental in detecting violations of emissions limits. However, it may be 

difficult to quantify the effects of revising these SIP elements or to use either of the options for 

demonstrating noninterference described above. For changes to these types of SIP elements, 

States are advised to work closely with their respective EPA Regional Offices to design an 

appropriate 110(l) demonstration. 

 Note that CAA section 110(a) requires enforceable emissions limits and monitoring and that 

EPA has long interpreted the CAA, through regulations and policies, to require monitoring, 

record keeping, reporting and other compliance assurance measures in SIPs. Thus, at a 

minimum, any 110(l) demonstration should show that revised compliance assurance provisions 

do not interfere with these CAA requirements. 

7.3 Other

 Changes to opacity regulations are subject to section 110(l) and will be addressed on a caseby-case basis as part of the Agency’s examination of each request. In addition, SIP revisions 

that extend compliance deadlines and revise motor vehicle emissions budgets for transportation 

conformity purposes are subject to section 110(l).