ENVIRONMENTAL PROTECTION AGENCY 

40 CFR part 52 

[EPA-R09-OAR-2011-0046; FRL-_________] 

Approval and Promulgation of Implementation Plans; State of 

California; Interstate Transport of Pollution; Significant 

Contribution to Nonattainment and Interference with Maintenance 

Requirements 

AGENCY: Environmental Protection Agency (EPA). 

ACTION: Proposed rule. 

SUMMARY: EPA is proposing to approve the State Implementation 

Plan (SIP) revision submitted by the State of California for the 

purpose of addressing the interstate transport provisions of 

Clean Air Act (CAA) section 110(a)(2)(D)(i)(I) for the 1997 8-

hour ozone National Ambient Air Quality Standards (NAAQS or 

standards) and the 1997 fine particulate matter (PM2.5) NAAQS. 

Section 110(a)(2)(D)(i) of the CAA requires that each state have 

adequate provisions to prohibit air emissions from adversely 

affecting air quality in other states through interstate 

transport. EPA is proposing to approve California’s SIP revision 

for the 1997 8-hour ozone and 1997 PM2.5 NAAQS as meeting the 

requirements of CAA section 110(a)(2)(D)(i)(I) to prohibit 

emissions that will contribute significantly to nonattainment of 
2 

the these standards in any other state and to prohibit emissions 

that will interfere with maintenance of these standards by any 

other state. 

DATES: Written comments must be received on or before [Insert 

date 30 days after publication in the Federal Register]. 

ADDRESSES: Submit your comments, identified by Docket ID Number 

EPA-R09-OAR-2011-0046, by one of the following methods: 

1. http://www.regulations.gov: Follow the on-line instructions 

for submitting comments. 

2. E-mail: mays.rory@epa.gov

3. Fax: 415-947-3579 

4. Mail or deliver: Rory Mays (AIR-2), U.S. Environmental 

Protection Agency, Region IX, 75 Hawthorne Street, San 

Francisco, CA 94105-3901. Deliveries are only accepted 

during the Regional Office’s normal hours of operation. 

Instructions: All comments will be included in the public 

docket without change and may be made available online at 

http://www.regulations.gov, including any personal information 

provided, unless the comment includes Confidential Business 

Information (CBI) or other information whose disclosure is 

restricted by statute. Information that you consider CBI or 

otherwise protected should be clearly identified as such and 

should not be submitted through http://www.regulations.gov or e-
3 

mail. http://www.regulations.gov is an anonymous access system, 

and EPA will not know your identity or contact 

information unless you provide it in the body of your comment. 

If you send email directly to EPA, your e-mail address will be 

automatically captured and included as part of the public 

comment. If EPA cannot read your comment due to technical 

difficulties and cannot contact you for clarification, EPA may 

not be able to consider your comment. 

Docket: The index to the docket for this action is 

available electronically at http://www.regulations.gov and in 

hard copy at EPA Region IX, 75 Hawthorne Street, San Francisco, 

California. While all documents in the docket are listed in the 

index, some information may be publicly available only at the 

hard copy location (e.g., copyrighted material), and some may 

not be publicly available in either location (e.g., CBI). To 

inspect the hard copy materials, please schedule an appointment 

during normal business hours with the contact listed directly 

below. 

FOR FURTHER INFORMATION CONTACT: Rory Mays, Air Planning Office 

(AIR–2), U.S. Environmental Protection Agency, Region IX, (415) 

972–3227, mays.rory@epa.gov. 

SUPPLEMENTARY INFORMATION: Throughout this document, the terms 

‘‘we,’’ ‘‘us,’’ and ‘‘our’’ refer to EPA. 
4 

TABLE OF CONTENTS 

I. Background 

II. What is the State process to submit these materials to EPA? 

III. What is EPA’s evaluation of the State’s submission? 

A. Evaluation of Significant Contribution to Nonattainment 

1. Significant Contribution to Nonattainment Evaluation 

for the 1997 8-hour Ozone NAAQS 

2. Significant Contribution to Nonattainment Evaluation 

for the 1997 PM2.5 NAAQS 

3. Conclusion Regarding Significant Contribution 

B. Evaluation of Interference with Maintenance 

1. Interfere With Maintenance Evaluation for the 1997 8-

hour Ozone NAAQS 

2. Interfere With Maintenance Evaluation for the 1997 

PM2.5 NAAQS 

3. Conclusion Regarding Interference With Maintenance 

IV. Proposed Action 

V. Statutory and Executive Order Reviews 

I. Background 

On July 18, 1997, EPA promulgated new standards for 8-hour 

ozone1 and fine particulate matter2 (PM2.5). This proposed action 

is in response to the promulgation of these standards (the 1997 

8-hour ozone NAAQS and 1997 PM2.5 NAAQS). This proposed action 

 

1 See 62 FR 38856. The level of the 1997 8-hour ozone NAAQS is 0.08 parts per 

million (ppm). 40 CFR 50.10. The 8-hour ozone standard is met when the 3-year 

average of the annual 4th highest daily maximum 8-hour ozone concentrations 

is 0.08 ppm or less (i.e., less than 0.085 ppm based on the rounding 

convention in 40 CFR part 50 Appendix I). This 3-year average is referred to 

as the "design value."

2 See 62 FR 38652. The level of the 1997 PM2.5 NAAQS are 15.0 micrograms per 

cubic meter (μg/m3) (annual arithmetic mean concentration) and 65 μg/m3 (24-

hour average concentration). 40 CFR 50.7. The annual standard is met when the 

3-year average of the annual mean concentrations is 15.0 μg/m3 or less (i.e.,

less than 15.05 μg/m3 based on the rounding convention in 40 CFR part 50 

Appendix N Section 4.3). The 24-hour standard is met when the 3-year average 

annual 98th percentile of 24-hour concentrations is 65 μg/m3 or less (i.e.,

less than 65.5 μg/m3 based on the rounding convention in 40 CFR part 40 

Appendix N Section 4.3). Id. These 3-year averages are referred to as the 

annual PM2.5 and 24-hour PM2.5 "design values," respectively.
5 

does not address the requirements of the 2006 PM2.5 NAAQS or the 

2008 8-hour ozone NAAQS; those standards will be addressed in 

future actions. 

Section 110(a)(1) of the CAA requires states to submit SIPs 

to address a new or revised NAAQS within three years after 

promulgation of such standards, or within such shorter period as 

EPA may prescribe. Section 110(a)(2) lists the elements that 

such new SIPs must address, as applicable, including section 

110(a)(2)(D)(i) which pertains to interstate transport of 

certain emissions. On August 15, 2006, EPA issued a guidance 

memorandum that provides recommendations to states for making 

submissions to meet the requirements of section 110(a)(2)(D)(i) 

for the 1997 8-hour ozone and 1997 PM2.5 standards (2006 

Guidance).3 

The transport SIP provisions in section 110(a)(2)(D)(i) 

(also called "good neighbor" provisions) require each state to 

submit a SIP that prohibits emissions that adversely affect 

another state in the ways contemplated in the statute. Section 

110(a)(2)(D)(i) identifies four distinct elements related to the 

evaluation of impacts of interstate transport of air pollutants. 

In this rulemaking, EPA is addressing the first two elements of 

 

3 Memorandum from William T. Harnett entitled “Guidance for State 

Implementation Plan (SIP) Submission to Meet Current Outstanding Obligations 

Under Section 110(a)(2)(D)(i) for the 8-hour ozone and PM2.5 National Ambient 

Air Quality Standards,” August 15, 2006.
6 

this section. This proposed action does not apply to the 

remaining two elements of CAA section 110(a)(2)(D)(i) regarding 

interference with measures required to prevent significant 

deterioration of air quality or to protect visibility in another 

state. We intend to evaluate and act upon the 2007 Transport SIP 

for purposes of these additional requirements of CAA section 

110(a)(2)(D)(i) in separate actions. 

The first element of section 110(a)(2)(D)(i) requires that 

a state's SIP for a new or revised NAAQS must contain adequate 

measures to prohibit emissions from sources within the state 

from “contribut[ing] significantly to nonattainment” of the 

NAAQS in another state. The second element of CAA section 

110(a)(2)(D)(i) requires that a state's SIP must prohibit any 

source or other type of emissions activity in the state from 

emitting pollutants that will "interfere with maintenance" of 

the applicable NAAQS in any other state. 

The CAA does not specifically mandate how to determine 

significant contribution to nonattainment or interference with 

maintenance. Therefore, EPA has interpreted these terms in past 

regulatory actions, such as the 1998 NOx SIP Call, in which EPA 

took action to remediate emissions of nitrogen oxides (NOx) that 

significantly contributed to nonattainment of, or interfered 

with maintenance of, the then applicable ozone NAAQS through 
7 

interstate transport of NOx and the resulting ozone.4 The NOx SIP 

Call was the mechanism through which EPA evaluated whether or 

not the NOx emissions from sources in certain states had such 

prohibited interstate impacts, and if they had such impacts, 

required the states to adopt substantive SIP revisions to 

eliminate the NOx emissions, whether through participation in a 

regional cap and trade program or by other means.

After promulgation of the 1997 8-hour ozone NAAQS and the 

1997 PM2.5 NAAQS, EPA again recognized that regional transport 

was a serious concern throughout the eastern United States and 

therefore developed the 2005 Clean Air Interstate Rule (CAIR) to 

address emissions of sulfur dioxide (SO2) and NOx that exacerbate 

ambient ozone and PM2.5 levels in many downwind areas through 

interstate transport.5 Within CAIR, EPA interpreted the term 

"interfere with maintenance" as part of the evaluation of 

whether or not the emissions of sources in certain states had 

such impacts on areas that EPA determined would either be in 

violation of the NAAQS, or would be in jeopardy of violating the 

NAAQS, in a modeled future year unless action were taken by 

 

4 See 63 FR 57356 (October 27, 1998). EPA's general approach to section 

110(a)(2)(D) in the NOx SIP Call was upheld in Michigan v. EPA, 213 F.3d 663

(D.C. Cir. 2000), cert denied, 532 U.S. 904 (2001). However, EPA's approach 

to interference with maintenance in the NOx SIP Call was not explicitly 

reviewed by the court. See, North Carolina v. EPA, 531 F.3d 896, 907 -09 

(D.C. Cir. 2008).

5 See "Rule to Reduce Interstate Transport of Fine Particulate Matter and 

Ozone (Clean Air Interstate Rule); Revisions to Acid Rain Program; Revisions 

to the NOx SIP Call; Final Rule," at 70 FR 25162 at 25263-69 (May 12, 2005).
8 

upwind states to reduce SO2 and NOx emissions. Through CAIR, EPA 

again required states that had such interstate impacts to adopt 

substantive SIP revisions to eliminate the SO2 and NOx emissions, 

whether through participation in a regional cap and trade 

program or by other means. 

EPA's 2006 Guidance addressed CAA section 110(a)(2)(D)(i) 

requirements for the 1997 8-hour ozone NAAQS and 1997 PM2.5

NAAQS. For those states subject to CAIR, EPA indicated that 

compliance with CAIR would meet the two requirements of section 

110(a)(2)(D)(i)(I) for these NAAQS. For states outside of the 

CAIR region, the 2006 Guidance recommended various methods by 

which states might evaluate whether or not their emissions 

significantly contribute to nonattainment of the 1997 8-hour 

ozone or the 1997 PM2.5 NAAQS in another state. Among other 

methods, EPA recommended consideration of available EPA modeling 

conducted in conjunction with the CAIR, or in the absence of 

such EPA modeling, consideration of other information such as 

the amount of emissions, the geographic location of violating 

areas, meteorological data, or various other forms of 

information that would be relevant to assessing the likelihood 

of significant contribution to violations of the NAAQS in 

another state. 
9 

The assessment of significant contribution to nonattainment 

is not restricted to impacts upon areas that are formally 

designated nonattainment. Consistent with EPA's approach in CAIR 

and recently in the Transport Rule Proposal, as discussed 

further below, this impact must be evaluated with respect to 

monitors showing a violation of the NAAQS.6 Furthermore, although 

relevant information other than modeling may be considered in 

assessing the likelihood of significant contribution to 

nonattainment of the 8-hour ozone or PM2.5 NAAQS in another 

state, EPA notes that no single piece of information is by 

itself dispositive of the issue. Instead, the total weight of 

all the evidence taken together is used to evaluate significant 

contributions to nonattainment of the 1997 8-hour ozone or 1997 

PM2.5 NAAQS in another state. 

As to the second element of section 110(a)(2)(D)(i), for 

states not within the CAIR region, EPA recommended that states 

evaluate whether or not emissions from their sources would 

"interfere with maintenance" in other states following the 

conceptual approach adopted by EPA in CAIR. After recommending 

various types of information that could be relevant for the 

technical analysis to support the SIP submission, such as the 

 

6 See 63 FR 57371 (October 27, 1998), NOx SIP Call; 70 FR 25172 (May 12, 

2005), CAIR; and 75 FR 45210 (August 2, 2010), Transport Rule Proposal.
10 

amount of emissions and meteorological conditions in the state, 

EPA further indicated that it would be appropriate for the state 

to assess impacts of its emissions on other states using 

considerations comparable to those used by EPA "in evaluating 

significant contribution to nonattainment in the CAIR."7 EPA did 

not make specific recommendations for how states should assess 

interfere with maintenance separately, and discussed the first 

two elements of section 110(a)(2)(D)(i) together without 

explicitly differentiating between them. 

In 2008, the U.S. Court of Appeals for the D.C. Circuit 

found that CAIR and the related CAIR federal implementation 

plans were unlawful.8 Among other issues, the court held that EPA 

had not correctly addressed the second element of section 

110(a)(2)(D)(i)(I) in CAIR and noted that "EPA gave no 

independent significance to the ‘interfere with maintenance’ 

prong of section 110(a)(2)(D)(i)(I) to separately identify 

upwind sources interfering with downwind maintenance."9 EPA's 

approach, the court reasoned, would leave areas that are "barely 

meeting attainment" with "no recourse" to address upwind 

emissions sources.10 The court therefore concluded that a plain 

language reading of the statute requires EPA to give independent 

 

7 2006 Guidance at 5. 

8 See North Carolina v. EPA, 531 F.3d 896 (D.C. Circuit 2008).

9 531 F.3d at 909.

10 Ibid.
11 

meaning to the interfere with maintenance requirement of section 

110(a)(2)(D)(i) and that the approach used by EPA in CAIR failed 

to do so. In addition to affecting CAIR directly, the court's 

decision in the North Carolina case indirectly affects EPA's 

recommendations to states in the 2006 Guidance with respect to 

the interfere with maintenance element of section 

110(a)(2)(D)(i) because the agency's guidance suggested that 

states use an approach comparable to that used by EPA in CAIR. 

To address the judicial remand of CAIR, EPA has recently 

proposed a new rule to address interstate transport of air 

pollution pursuant to section 110(a)(2)(D)(i), the “Federal 

Implementation Plans to Reduce Interstate Transport of Fine 

Particulate Matter and Ozone” (Transport Rule Proposal).11 As 

part of the Transport Rule Proposal, EPA specifically reexamined 

the section 110(a)(2)(D)(i)(I) requirements that emissions from 

sources in a state must not “contribute significantly to 

nonattainment” or “interfere with maintenance” of the 1997 8-

hour ozone NAAQS and 1997 PM2.5 NAAQS in other states. In the 

proposal, EPA developed an approach to identify areas that it 

predicts to be violating the 1997 8-hour ozone and PM2.5 NAAQS, 

and areas that it predicts to be close to the level of these 

NAAQS and therefore at risk to become nonattainment unless 

 

11 See 75 FR 45210 (August 2, 2010).
12 

emissions from sources in other states are appropriately 

controlled. This approach starts by identifying those specific 

geographic areas for which further evaluation is appropriate, 

and differentiates between areas where the concern is 

significant contribution to nonattainment as opposed to 

interference with maintenance. 

As described in more detail below, EPA evaluated data from 

existing monitors over three overlapping 3-year periods (i.e., 

2003-2005, 2004–2006, and 2005-2007), as well as air quality 

modeling data, in order to determine which areas are predicted 

to be violating the 1997 8-hour ozone and PM2.5 NAAQS in 2012, 

and which areas are predicted potentially to have difficulty 

maintaining attainment as of that date. In essence, if an area’s 

projected data for 2012 indicates that it would be violating the 

NAAQS based on the average of these three overlapping periods, 

then this monitor location is appropriate for comparison for 

purposes of the significant contribution to nonattainment 

element of section 110(a)(2)(D)(i). If, however, an area’s 

projected data indicate that it would be violating the NAAQS 

based on the highest single period, but not over the average of 

the three periods, then this monitor location is appropriate for 
13 

comparison for purposes of the interfere with maintenance 

element of the statute.12 

By this method, EPA has identified those areas with 

monitors that are appropriate “nonattainment receptors” or 

“maintenance receptors” for evaluating whether the emissions 

from sources in another state could significantly contribute to 

nonattainment in, or interfere with maintenance in, that 

particular area. EPA believes that this new approach for 

identifying areas that are predicted to be nonattainment or to 

have difficulty maintaining the NAAQS is appropriate to evaluate 

a state’s submission in relation to the elements of CAA section 

110(a)(2)(D)(i)(I) pertaining to significant contribution to 

nonattainment and interference with maintenance.13 EPA's 2006 

Guidance did not provide this specific recommendation to states, 

 

12 A memorandum in the docket for this action provides the information EPA 

used to identify monitors that are receptors for evaluation of significant 

contribution or interference with maintenance for certain states in the 

western United States. See Memorandum from Brian Timin, EPA Office of Air 

Quality Planning and Standards, “Documentation of Future Year Ozone and 

Annual PM2.5 Design Values for Monitors in Western States,” August 23, 2010 

(Timin Memo).

13 To begin this analysis, EPA first identifies all monitors projected to be 

in nonattainment or, based on historic variability in air quality, projected 

to have maintenance problems in 2012. Monitors projected to be in 

nonattainment are those with future year design values that violate the 

standard, based on the projection of 5-year weighted average concentrations. 

Monitors projected to have maintenance problems are those at risk of not 

staying in attainment because the air quality data is close enough to the 

level of the 1997 8-hour ozone and PM2.5 NAAQS that minor variations in weather 

or emissions could result in violations of the NAAQS in 2012.
14 

but in light of the court's decision on CAIR, EPA will itself 

follow this approach in acting upon the California submission.14

As explained in the 2006 Guidance, EPA does not believe 

that section 110(a)(2)(D)(i) SIP submissions from all states 

necessarily need to follow precisely the same analytical 

approach of CAIR. In the 2006 Guidance, EPA stated that: “EPA 

believes that the contents of the SIP submission required by 

section 110(a)(2)(D) may vary, depending upon the facts and 

circumstances related to the specific NAAQS. In particular, the 

data and analytical tools available at the time the state 

develops and submits a SIP for a new or revised NAAQS 

necessarily affects the contents of the required submission.”15

EPA also indicated in the 2006 Guidance that it did not 

anticipate that sources in states outside the geographic area 

covered by CAIR were significantly contributing to 

nonattainment, or interfering with maintenance, in other 

states.16 As noted in the Transport Rule Proposal, EPA continues 

to believe that the more widespread and serious transport 

 

14 By letter dated January 26, 2011, CARB acknowledged that the 2008 remand of 

CAIR and EPA’s Transport Rule Proposal would affect EPA’s review of the 2007 

Transport SIP. The letter states that based on EPA’s findings in the Timin 

Memo regarding pollution transport in the western states, ARB staff concludes 

that pollutants from California do not contribute to nonattainment or 

maintenance problems in other states. See letter dated January 26, 2011, from 

Douglas Ito, Chief, Air Quality and Transportation Planning Branch, CARB to 

Lisa Hanf, Chief, Air Planning Office, EPA Region 9.

15 2006 Guidance at 4.

16 Ibid. at 5.
15 

problems in the eastern United States are analytically 

distinct.17 For the 1997 8-hour ozone and PM2.5 NAAQS, EPA 

believes that nonattainment and maintenance problems in the 

western United States are relatively local in nature with only 

limited impacts from interstate transport. In the Transport Rule 

Proposal, EPA did not calculate the portion of predicted ozone 

or PM2.5 concentrations in any downwind state that would result 

from emissions from individual western states, such as 

California. 

Accordingly, EPA believes that section 110(a)(2)(D)(i) SIP 

submissions for states outside the geographic area of the 

Transport Rule Proposal may be evaluated using a “weight of the 

evidence” approach that takes into account the available 

relevant information, such as that recommended by EPA in the 

2006 Guidance for states outside the area affected by CAIR. Such 

information may include, but is not limited to, the amount of 

emissions in the state relevant to the NAAQS in question, the 

meteorological conditions in the area, the distance from the 

state to the nearest monitors in other states that are 

appropriate receptors, or such other information as may be 

probative to consider whether sources in the state may interfere 

with maintenance of the 1997 8-hour ozone and PM2.5 NAAQS in 

 

17 See Transport Rule Proposal, 75 FR 45210 at 45227 (August 2, 2010).
16 

other states. These submissions can rely on modeling when 

acceptable modeling technical analyses are available, but EPA 

does not believe that modeling is necessarily required if other 

available information is sufficient to evaluate the presence or 

degree of interstate transport in a given situation. 

II. What is the State process to submit these materials to EPA? 

CAA sections 110(a)(1) and (2) and section 110(l) require 

that revisions to a SIP be adopted by the state after reasonable 

notice and public hearing. EPA has promulgated specific 

procedural requirements for SIP revisions in 40 CFR part 51, 

subpart F. These requirements include publication of notices, by 

prominent advertisement in the relevant geographic area, of a 

public hearing on the proposed revisions, a public comment 

period of at least 30 days, and an opportunity for a public 

hearing. 

On November 16, 2007, the California Air Resources Board 

(CARB) submitted the “Proposed State Strategy for California’s 

2007 State Implementation Plan” to attain the 1997 8-hour ozone 

and PM2.5 NAAQS (2007 State Strategy).18 Appendix C of the 2007 

State Strategy, as modified by Attachment A,19 contains 

 

18 See transmittal letter dated November 16, 2007, from James N. Goldstene, 

Executive Officer, CARB, to Wayne Nastri, Regional Administrator, EPA Region 

9, with enclosures, and CARB Resolution No. 07–28 (September 27, 2007). 

19 See “Technical and Clarifying Modifications to April 26, 2007 Revised Draft 

Air Resources Board’s Proposed State Strategy for California’s 2007 State 

Implementation Plan and May 7, 2007 Revised Draft Appendices A through G,” 
17 

California’s SIP revision to address the Transport SIP 

requirements of CAA section 110(a)(2)(D)(i) for the 1997 8-hour 

ozone and PM2.5 NAAQS (2007 Transport SIP). CARB’s November 16, 

2007 submittal includes public process documentation for the 

2007 State Strategy, including the 2007 Transport SIP. In 

addition, the SIP revision includes documentation of a duly 

noticed public hearing held on September 27, 2007 on the 

proposed 2007 State Strategy. 

We find that the process followed by CARB in adopting the 

2007 Transport SIP complies with the procedural requirements for 

SIP revisions under CAA section 110 and EPA’s implementing 

regulations. 

III. What is EPA’s evaluation of the State’s submission? 

A. Evaluation of Significant Contribution to Nonattainment 

This proposed approval addresses the significant 

contribution to nonattainment element of section 

110(a)(2)(D)(i)(I) for the 1997 8-hour ozone and 1997 PM2.5 NAAQS 

in several ways. It takes into account California’s 2007 

Transport SIP, in which the state explains that meteorological 

and other characteristics in California and in the surrounding 

areas reduce the likelihood that emissions from sources in 

California contribute significantly to nonattainment of the 1997 

 

included as Attachment A to CARB’s Board Resolution 07-28 (September 27, 

2007).
18 

8-hour ozone or PM2.5 NAAQS in any downwind state. In addition, 

EPA has supplemented the state’s analysis with its own 

evaluation of the evidence to assess whether emissions sources 

in California contribute significantly to nonattainment of the 

1997 8-hour ozone or PM2.5 NAAQS in other states. First, EPA has 

evaluated the potential for ozone transport from California to 

specific locations identified in the Transport Rule Proposal by 

reviewing ozone back-trajectory analyses and other relevant 

information. Second, EPA has considered information in the Brian 

Timin Memo, which provides projected future year ozone and 

annual PM2.5 design values for monitors in the western U.S. based 

on the air quality modeling carried out in support of the 

Transport Rule Proposal. Finally, EPA has reviewed recent ozone 

and PM2.5 monitoring data for the states bordering California to 

consider whether California emissions could contribute to 

violations of the 1997 8-hour ozone or PM2.5 NAAQS in those 

states. Based on these analyses, we propose to conclude that 

emissions from California do not contribute significantly to 

nonattainment in any other state for the 1997 8-hour ozone or 

PM2.5 NAAQS, consistent with the requirements of CAA section 

110(a)(2)(D)(i)(I). 

1. Significant Contribution to Nonattainment Evaluation for 

the 1997 8-hour Ozone NAAQS 
19 

To address whether emissions from California sources 

significantly contribute to nonattainment of the 1997 8-hour 

ozone NAAQS in another state, California argued in the 2007 

Transport SIP that meteorological conditions within the State 

and its existing air pollution control programs support a 

finding that emissions from California sources “[do] not 

significantly affect nonattainment areas in other states.”20

Specifically, the State’s submittal argues that ozone episodes 

in the southwestern U.S. are normally associated with 

meteorology that results in stagnant conditions (i.e., not 

conducive to ozone transport) and that, on occasion, those 

conditions are weakly impacted by migrating low pressure systems 

over the Pacific Ocean that push air high above the surface 

eastward.21 Even though acknowledging the occasional possibility 

of ozone being transported over long distances, the State 

asserted in the 2007 Transport SIP that California’s existing 

air quality programs (e.g., its motor vehicle emissions control 

program, consumer product regulations, stationary source permit 

programs, and other control measures) greatly reduce the 

likelihood that emissions from California sources will 

 

20 2007 State Strategy, Attachment A, page 20.

21 Ibid.
20 

contribute significantly to nonattainment in any downwind 

state.22 

Also in support of its conclusion, the State’s 2007 

Transport SIP references language in the preamble to CAIR citing 

EPA’s own statement that, given geography, meteorology, and 

topography in the western U.S., “PM2.5 and 8-hour ozone 

nonattainment problems are not likely to be affected 

significantly by pollution transported across [the western] 

states’ boundaries.”23 In sum, the State argues in the 2007 

Transport SIP that EPA’s statement in the CAIR rulemaking with 

respect to the likelihood of transport in western states, 

together with the meteorological and other information provided 

in California’s submittal, support the finding that emissions 

from California sources do not significantly affect 

nonattainment areas in other states.

EPA does not agree with California’s assessment in the 2007 

Transport SIP that these factors alone demonstrate that 

emissions from California sources do not contribute 

significantly to nonattainment of the 1997 8-hour ozone NAAQS in 

other states. Therefore, EPA is supplementing the State’s 

submittal with additional information in order to assess this 

issue more fully, and in light of more recent information. As 

 

22 Ibid.

23 See ibid. (quoting CAIR proposal, 69 FR 4566 at 4581, January 30, 2004).
21 

noted above, EPA is evaluating the 2007 Transport SIP taking 

into account the methodologies and analyses developed in the 

Transport Rule Proposal in response to the judicial remand of 

CAIR, as well as EPA’s projections of future air quality at 

monitors in western states in the Timin Memo and preliminary air 

quality data from monitors in the states bordering California. 

The Transport Rule Proposal includes an approach to 

determining whether emissions from a state contribute 

significantly to nonattainment of the 1997 8-hour ozone NAAQS in 

other states. Specifically, EPA used existing monitoring data 

and modeling to project future concentrations of ozone at 

monitors to identify areas that are expected to be violating the 

1997 8-hour ozone NAAQS in 2012, based on the 5-year weighted 

average design value. We call these monitors “nonattainment 

sites” or “nonattainment receptors.” To identify the states with 

emissions that may contribute significantly to ozone 

nonattainment in other states, the Transport Rule Proposal 

models the states' contributions to ambient ozone levels at 

these nonattainment receptors.24 Because the Transport Rule 

Proposal does not model the contribution of emissions from 

California (and other western states not fully inside the 

Transport Rule Proposal's modeling domain) to 8-hour ozone 

 

24 Transport Rule Proposal, 75 FR 45210 at 45253-45273.
22 

nonattainment receptors in other states, our assessment in this 

proposed action relies on a weight of evidence approach that 

considers relevant information from the Transport Rule Proposal 

pertaining to states within its modeling domain and additional 

material such as back-trajectory analyses, geographical and 

meteorological factors, EPA’s projections of future air quality 

at monitors in western states in the Timin Memo, and EPA’s Air 

Quality System (AQS)25 monitoring data. Although each of the 

factors considered in the following analysis are not in and of 

themselves determinative, consideration of these factors 

together provides a reliable qualitative conclusion that 

emissions from California sources are not likely to contribute 

significantly to nonattainment of the 1997 8-hour ozone NAAQS at 

monitors in other states. 

Our analysis begins by assessing California’s contribution 

to the closest nonattainment receptors for the 1997 8-hour ozone 

standard. The Transport Rule Proposal identifies, within its 

modeling domain (consisting of 37 states east of the Rocky 

Mountains, and the District of Columbia), 11 nonattainment 

receptors for the 1997 8-hour ozone standard. Of these, the 

nonattainment receptors closest to California are seven 

receptors in the Dallas-Fort Worth and Houston-Galveston-

 

25 AQS is EPA’s database repository of monitored ambient air quality data. See 

http://www.epa.gov/ttn/airs/airsaqs/. 
23 

Brazoria 8-hour ozone nonattainment areas in eastern Texas. The 

remaining four nonattainment receptors for the 1997 8-hour ozone 

NAAQS are in Louisiana, New York, and Pennsylvania.26 

The nonattainment receptors in Dallas-Fort Worth and 

Houston-Galveston-Brazoria areas are over 900 miles from the 

easternmost border of California, and the monitors in Louisiana, 

New York, and Pennsylvania are significantly farther away. 

Although distance alone is not determinative in the analysis of 

potential ozone transport, with increasing distance there are 

greater opportunities for ozone and NOx dispersion and/or removal 

from the atmosphere due to the effect of winds or chemical sink 

processes. Moreover, the intervening Rocky Mountains act as a 

natural barrier to air pollution transport. These factors 

together support a conclusion that California sources do not 

contribute significantly to nonattainment of the 1997 8-hour 

ozone NAAQS in the nearest areas with nonattainment receptors 

identified in the Transport Rule Proposal. 

In order to evaluate the potential impact of emissions from 

California sources on the nonattainment receptors identified in 

the Transport Rule Proposal, EPA evaluated air parcel pathways 

from California to these monitoring sites. Specifically, EPA 

reviewed the analysis of ozone transport by the Texas Commission 

 

26 See Transport Rule Proposal, Table IV.C-11, 75 FR 45210 at 45252.
24 

on Environmental Quality for each exceedance day in 2007, 2008, 

and 2009 for the seven nonattainment receptors in the DallasFort Worth and Houston-Galveston-Brazoria 8-hour ozone 

nonattainment areas in eastern Texas.27 Exceedance days were 

identified using the AQS Database. Back-trajectories28 were run 

for all of the days during the 2007-2009 period when ozone 

concentrations at these receptors exceeded the 1997 8-hour ozone 

NAAQS (i.e., monitored ozone concentrations were 85 parts per 

billion (ppb) or above). These back-trajectory maps indicate 

that air parcel pathways to nonattainment receptors in eastern 

Texas do not originate in California. 

Because back-trajectory analysis results map pathways of 

air parcels that may or may not transport pollutants, they 

cannot be considered determinative as to the transport of ozone 

and its precursors or the absence of such transport from 

California emission sources. However, the fact that the air 

parcel trajectories do not directly connect California to the 

nonattainment receptors in eastern Texas strongly supports the 

conclusion that emissions of ozone and its precursors from 

 

27 See Technical Support Document, California 2007 Transport SIP, Evaluation 

of Significant Contribution to Nonattainment and Interference with 

Maintenance for the 1997 8-hour Ozone NAAQS, U.S. EPA Region 9, February 25, 

2011.

28 Trajectories for each monitor were run backwards in time for 72 hours 

(three days), using a trajectory height at the starting point of 1,500 meters 

above ground level.
25 

California are not likely to contribute significantly to 

nonattainment of the 1997 8-hour ozone NAAQS at these receptors. 

To assist in the evaluation of the potential for ozone 

transport among western states not included in the modeling 

domain for the Transport Rule Proposal, EPA also developed an 

additional analysis in the Timin Memo identifying monitors 

projected to record violations of the 1997 8-hour ozone NAAQS 

within a modeling domain that includes the western states.29 The 

Timin Memo identified numerous nonattainment sites for the 1997 

8-hour ozone NAAQS in southern and central California.30 This 

analysis did not, however, identify any projected nonattainment 

receptors for the 1997 8-hour ozone NAAQS in any other western 

state. EPA’s analysis for western states therefore supports our 

proposal to conclude that California sources do not contribute 

significantly to nonattainment of the 1997 8-hour ozone NAAQS in 

other western states. 

Finally, in addition to the information in the 2007 

Transport SIP, our review of air parcel pathways to the nearest 

nonattainment receptors identified from the modeling analyses 

conducted for the Transport Rule Proposal, and EPA’s projections 

of future air quality in the western states in the Timin Memo, 

 

29 See fn. 12 above.

30 See Timin Memo at Appendix B (“Base year 2003-2007 and Future Year 2012 8-

Hour Average Ozone Design Values – Western States”).
26 

EPA evaluated preliminary air quality monitoring data for the 

areas in states bordering California that are designated 

nonattainment for the 1997 8-hour ozone NAAQS. Although 

significant contribution must be measured not just against 

designated nonattainment areas but also against areas with 

monitors showing violations of the NAAQS, nonattainment areas 

are a convenient starting point for the analysis. The 2007 

Transport SIP identifies two areas in states bordering 

California that are currently designated nonattainment for the 

1997 8-hour ozone standard: the Las Vegas area in Clark County, 

Nevada, and the Phoenix-Mesa area in Arizona. EPA designated 

both of these areas as nonattainment for the 1997 8-hour ozone 

standard in 2004. See 69 FR 23858 (April 30, 2004); 40 CFR 

81.303 and 81.329. Both of these areas, however, have current 

design values indicating attainment of the 1997 8-hour ozone 

NAAQS. Our review of preliminary monitoring data for the 2007-

2009 period available in EPA’s AQS Database indicates that the 

8-hour ozone design values for Las Vegas and Phoenix-Mesa during 

this period were 78 ppb and 76 ppb, respectively.31 Thus, we 

believe it is reasonable to conclude that California sources are 

not contributing significantly to nonattainment of the 1997 8-

hour ozone NAAQS in either the Las Vegas, Nevada or Phoenix-

 

31 See U.S. EPA AQS, “Preliminary Design Value Report,” 2007-2009, for Nevada, 

Arizona.
27 

Mesa, Arizona nonattainment areas. No other area in the states 

bordering California (Oregon, Nevada, or Arizona) is currently 

designated nonattainment for the 1997 8-hour ozone NAAQS. 

As mentioned above, EPA considers not only significant 

contribution to designated nonattainment areas, but also to 

areas with monitor readings showing violations of the NAAQS. A 

review of the AQS monitoring data for adjacent states shows that 

it is highly unlikely that emissions from California contribute 

significantly to violations of the 1997 8-hour ozone NAAQS in 

any downwind state. Specifically, EPA’s observed maximum design 

values at monitors in the western states during the 2003-2007 

period were generally well below the 1997 ozone NAAQS (except in 

California), and the 2012 modeling results at these western 

monitors (where a future year design value could be estimated) 

show a downward trend in ozone.32

Additionally, we evaluated ozone monitoring data from the 

2007-2009 period from each of the ozone monitoring sites in 

Oregon, Nevada, and Arizona, to determine whether the ozone 

levels in any of these states violate or potentially violate the 

1997 8-hour ozone NAAQS.33 The highest ozone design value at 

these monitoring sites during the 2007-2009 period was 78 ppb 

 

32 See Timin Memo at Appendix B (“Base year 2003-2007 and Future Year 2012 8-

Hour Average Ozone Design Values – Western States”).

33 See U.S. EPA AQS, “Preliminary Design Value Report,” 2007-2009, for Oregon, 

Nevada, Arizona.
28 

(in the Las Vegas, Nevada area), and most monitors recorded 

significantly lower ozone levels.34 We have found no violations 

of the 1997 8-hour ozone NAAQS at any of the monitors in states 

bordering California, nor any indication that emissions from 

California sources contribute significantly to nonattainment of 

the 1997 8-hour ozone NAAQS in these adjacent states. 

The fact that monitors in these nearby areas are not 

registering violations of the NAAQS does not in itself 

conclusively establish that emissions from California could not 

contribute in the aggregate to violations in any other state. 

But this fact combined with our evaluation of the nearest 

nonattainment receptors in eastern Texas, taking into account 

distance, topographical barriers, and typical meteorological 

conditions, supports California’s conclusion that emissions from 

its sources do not contribute significantly to nonattainment of 

the 1997 8-hour ozone NAAQS in other states, in accordance with 

section 110(a)(2)(D)(i)(I). 

2. Significant Contribution to Nonattainment Evaluation for 

the 1997 PM2.5 NAAQS 

In its 2007 Transport SIP, California argues that distance 

to the nearest designated PM2.5 nonattainment area, topographical 

features and meteorology support a finding that California 

 

34 Ibid.
29 

sources do not significantly contribute to nonattainment of the 

1997 PM2.5 NAAQS in another state. The 2007 Transport SIP also 

references EPA’s technical support document (TSD) for the PM2.5

NAAQS nonattainment designations (PM2.5 Designations TSD),35 which 

identifies Libby, Montana (in Lincoln County), as the area 

closest to California that is designated nonattainment for the 

1997 PM2.5 standards.36 As EPA noted in the PM2.5 Designations TSD, 

PM2.5 in Libby is predominantly local in origin (e.g., 

residential wood-burning stoves during the winter time, when 

frequent and persistent temperature inversions occur, were 

specifically identified as a key source of particulate emissions 

in the area). Thus, California correctly noted that EPA 

concluded that PM2.5 pollution in Libby is a localized problem.37

 

35 See Technical Support for State and Tribal Air Quality Fine Particle (PM2.5) 

Designations, “EPA 9-Factor Analyses for Montana for the Designation of PM2.5 

Nonattainment Areas,” Chapter 6.8.1, December 17, 2004.

36 EPA designated this area as nonattainment for the 1997 PM2.5 NAAQS in 2005. 

70 FR 944 (January 5, 2005) and 40 CFR 81.305.

37 “Factor 6” of this 9-Factor Analysis describes the meteorology in the Libby 

area as follows: 

“Libby Montana is located in the northwestern part of the state in a narrow 

north-south oriented valley. The ridgetops surrounding Libby are 

approximately 4,000 feet higher than the town. There are no other towns or 

large emissions sources immediately upwind, so transport of high background 

concentrations into Libby is considered unlikely. The highest PM2.5 

concentrations in Libby generally occur during the months of November through 

February. During the summer months concentrations typically average less than 

half the level of the annual PM2.5 NAAQS, while winter concentrations may 

double the NAAQS. The much higher concentrations in winter are related to 

stagnant weather conditions dominated by light winds and strong temperature 

inversions. These meteorological conditions may trap emissions within the 

valley for many days. No recent meteorological data is available for Libby, 

however, data from Kalispell, MT show calm wind conditions occur 35 percent 

of the time in the winter months and only 15 percent of the time in the 

spring and summer. Vertical temperature soundings at Great Falls in Western 
30 

The fact that nonattainment in a given area is primarily 

the result of local emissions sources does not exclude the 

possibility of significant contribution to nonattainment from 

interstate transport. This fact and other evidence, however, 

support the conclusion that emissions from California sources 

are not significantly contributing to violations in Libby, 

Montana. That area is more than 900 miles away from California 

and is on the other side of the Sierra Nevada Mountains, a 400-

mile-long north-south range of mountains that act as a natural 

barrier to air movement between California and Montana.38 In 

addition, Libby is not in the predominant direction of winds 

from California, as transport winds generally flow from west to 

east, and not toward the north. Given the relatively long 

distance between California and Libby, Montana, the intervening 

mountainous topography, the localized nature of the PM2.5

nonattainment problem in Libby, and the general west-to-east 

direction of transport winds across California, EPA believes it 

is reasonable to conclude that California sources do not 

contribute significantly to nonattainment of the 1997 PM2.5 NAAQS 

 

MT also show a very high frequency of surface temperature inversions in the 

winter.

Due to the meteorology conditions in the town and surrounding vicinity 

of Libby and due to the topographical features within Lincoln County and more 

specifically around Libby, that create stagnant weather conditions, EPA feels 

the adjacent counties do not impact the PM2.5 monitor located at the Libby 

Courthouse Annex and that the nonattainment problem is a localized PM2.5 

problem.” PM2.5 Designations TSD at Chapter 6.8.1.

38 See PM2.5 Designations TSD at Chapter 6.8.1.
31 

in Libby, Montana. We note also that preliminary data available 

in EPA’s AQS Database for the 2007-2009 period indicate that the 

Libby, Montana nonattainment area is currently attaining the 

1997 PM2.5 standards.39 

EPA does not agree with California’s assessment in the 2007 

Transport SIP that these factors alone demonstrate that 

emissions from California sources do not contribute 

significantly to nonattainment of the 1997 PM2.5 NAAQS in any 

other states. Therefore, EPA is supplementing the state’s 

submission with additional information in order to assess this 

issue more fully, and in light of more recent information. As 

noted above, EPA is evaluating the 2007 Transport SIP taking 

into account the methodologies and analyses developed in the 

Transport Rule Proposal in response to the judicial remand of 

CAIR, as well as EPA’s projections of future air quality at 

monitors in western states in the Timin Memo and preliminary air 

quality data from monitors in the states bordering California. 

Specifically, we identified the nonattainment receptors for 

the 1997 annual PM2.5 NAAQS closest to California to evaluate 

whether emissions from California sources contribute 

significantly to nonattainment of the 1997 PM2.5 NAAQS in any 

 

39 This data indicates the annual PM2.5 design value for the Libby, Montana 

area during the 2007-2009 period was 12.2 μg/m3. See U.S. EPA AQS, 

“Preliminary Design Value Report,” 2007-2009, for Montana.
32 

other state.40 For the 1997 annual PM2.5 NAAQS, the nonattainment 

receptors closest to California that EPA identified from the 

modeling analyses conducted for the Transport Rule Proposal are 

all east of the Mississippi River.41 Given the significant 

distance between California and these nonattainment receptors, 

and the intervening mountainous terrain, we believe it is 

reasonable to conclude that California sources do not contribute 

significantly to nonattainment of the 1997 annual PM2.5 NAAQS in 

any of these areas. 

To address the potential for impacts on states not included 

in the modeling domain for the Transport Rule Proposal, we also 

evaluated whether there are monitors suitable for consideration 

as nonattainment receptors in western states outside of the 

geographic area covered by the Transport Rule Proposal. We note 

that EPA’s analysis in the Timin Memo for western states 

identified numerous nonattainment sites for the 1997 annual PM2.5

NAAQS in southern and central California.42 This analysis did 

 

40 For PM2.5, the Transport Rule Proposal identified nonattainment receptors 

for the 1997 annual PM2.5 NAAQS and the 2006 24-hour PM2.5 NAAQS. See 75 FR 

45210 at 45212. Because our proposal on California’s 2007 Transport SIP 

addresses requirements of CAA section 110(a)(2)(D)(i) only for purposes of 

the 1997 ozone and PM2.5 NAAQS, for PM2.5 purposes we consider only the 

nonattainment receptors for the 1997 annual PM2.5 NAAQS identified in the 

Transport Rule Proposal. 

41 Specifically, the nonattainment sites for the 1997 annual PM2.5 standard are 

located in Alabama, Georgia, Illinois, Indiana, Kentucky, Michigan, Ohio, 

Pennsylvania, and West Virginia. See Transport Rule Proposal, 75 FR 45210 at 

45247-45248 (August 2, 2010).

42 See Timin Memo at Appendix A (“Base year 2003-2007 and Future Year 2012 

Annual Average PM2.5 Design Values – Western States”).
33 

not, however, identify any projected nonattainment receptors for 

the 1997 annual PM2.5 NAAQS in any other western state. Thus, we 

believe it is reasonable to conclude that California sources do 

not contribute significantly to nonattainment of the 1997 PM2.5

NAAQS in other states. 

The analysis for the Transport Rule Proposal did not 

identify any nonattainment receptors for the 1997 24-hour PM2.5

NAAQS in the portions of the U.S. covered by the Transport Rule 

Proposal modeling domain (i.e., the 12 kilometer (km) grid 

covering the continental U.S. east of the Rockies).43 Recent 

monitoring data in EPA’s Air Quality System (2007-2009 design 

values that are under final EPA review) indicate that the 

highest 24-hour PM2.5 design value in the 47 states of the 

continental U.S. (excluding California) is 50 μg/m3,

44 which is 

well below the level of the 1997 24-hour PM2.5 NAAQS of 65 μg/m3. 

This data further supports our proposed finding that California 

sources do not contribute significantly to nonattainment of the 

1997 24-hour PM2.5 NAAQS in any other state. 

Finally, EPA evaluated PM2.5 air quality data for areas in 

the states bordering California to determine whether California 

 

43 75 FR 45210 at 45249-45251 (August 2, 2010).

44 These values were recorded at monitors in Liberty-Clairton, Pennsylvania 

and Provo, Utah. See http://epa.gov/airtrends/pdfs/PM2.5%202007-

2009%20design%20value%20update.pdf. Data from EPA’s Air Quality System can be 

viewed at http://www.epa.gov/ttn/airs/airsaqs/. 
34 

sources might contribute significantly to violations of the 1997 

PM2.5 NAAQS in these nearby areas. No areas in Oregon, Nevada, or 

Arizona are currently designated nonattainment for the 1997 PM2.5

NAAQS. As mentioned above, however, EPA considers not only 

significant contribution to designated nonattainment areas, but 

also to areas with monitoring data showing violations of the 

NAAQS. A review of the AQS monitoring data for adjacent states 

shows that it is highly unlikely that emissions from California 

contribute significantly to violations of the 1997 annual PM2.5

NAAQS in any downwind state. 

Specifically, we reviewed preliminary PM2.5 monitoring data 

for the 2007-2009 period available in EPA’s AQS Database from 

all PM2.5 monitoring sites in Oregon, Nevada, and Arizona, to 

determine whether the PM2.5 design values in any of these states 

potentially violate the 1997 annual PM2.5 NAAQS.45 During this 

period only one monitor in these adjoining states, the “Cowtown” 

monitor in Casa Grande, Arizona (monitor ID 04-021-3013), has a 

PM2.5 design value exceeding the 1997 annual standard of 15.0 

μg/m3.

46 EPA has separately determined, however, that this 

monitor is not suitable for determining compliance with the 1997 

annual PM2.5 standard because the monitor functions as a 

 

45 See U.S. EPA AQS, “Preliminary Design Value Report,” 2007-2009, for Oregon, 

Nevada, and Arizona.

46 The Cowtown monitor had a PM2.5 design value of 18.8 μg/m3. See U.S. EPA 

AQS, “Preliminary Design Value Report,” 2007-2009, for Arizona.
35 

population-oriented microscale (i.e., localized hot spot) 

monitor.47 No other PM2.5 monitor in the three states bordering 

California recorded a violation of the 1997 annual or 24-hour 

PM2.5 NAAQS during the 2007-2009 period.48 

The fact that monitors in these nearby areas are not 

registering violations of the 1997 PM2.5 NAAQS does not in itself 

conclusively establish that emissions from California could not 

contribute in the aggregate to violations in other states. But 

this fact combined with our evaluation of the nearest 

nonattainment receptors in states east of the Mississippi River, 

taking into account distance, topographical barriers, and 

typical meteorological conditions, supports California’s 

conclusion on PM2.5 contribution for the 1997 NAAQS. 

3. Conclusion Regarding Significant Contribution to 

Nonattainment 

Based on the weight of evidence discussed above, including 

the location of the nearest projected nonattainment sites, 

distance to the nearest designated PM2.5 nonattainment area, 

 

47 See 76 FR 6056 (February 3, 2011); see also “Technical Support Document for 

Determination that the Cowtown Monitor is Ineligible for Comparison with the 

Annual PM2.5 NAAQS,” April 26, 2010.

48 Our review of AQS data for the 2007-2009 period in the three states 

bordering California indicated the highest valid annual PM2.5 design value was 

12.8 μg/m3 (monitor ID 04-023-0004 in Nogales, Arizona) and the highest valid 

24-hour PM2.5 design value was 47 μg/m3 (monitor ID 41-035-0004 in Klamath 

Falls, Oregon). See U.S. EPA AQS, “Preliminary Design Value Report,” 2007-

2009, for Oregon, Nevada, and Arizona. 
36 

meteorology, topography, and recent air quality monitoring data, 

we propose to determine that California’s 2007 Transport SIP is 

adequate to ensure that emissions from California do not 

significantly contribute to nonattainment in any other state for 

the 1997 8-hour ozone or 1997 PM2.5 NAAQS, consistent with the 

requirements of CAA section 110(a)(2)(D)(i)(I). Thus, we propose 

to determine that California’s SIP includes the measures 

necessary to prevent such prohibited interstate transport 

impacts for these NAAQS. 

B. Evaluation of Interference With Maintenance 

California’s 2007 Transport SIP relies upon the 

recommendations in EPA's 2006 Guidance and does not provide a 

specific analysis of the interference with maintenance element 

of section 110(a)(2)(D)(i). Given the court decision on CAIR in 

the interim, however, EPA believes that it is necessary to 

evaluate the submission for section 110(a)(2)(D)(i)(I) in such a 

way as to assure that the interfere with maintenance element of 

the statute is given independent meaning and is appropriately 

evaluated using the types of information that EPA recommended in 

the 2006 Guidance. To accomplish this, in this proposed action, 

EPA has supplemented California’s analysis with an approach 

comparable to that of the Transport Rule Proposal in order to 

adequately evaluate whether emissions from California sources 
37 

interfere with maintenance of these NAAQS in other states. As 

with the significant contribution to nonattainment analysis, we 

have evaluated the potential for transport of emissions from 

California sources to specific locations identified in the 

Transport Rule Proposal, EPA’s projected future year ozone and 

PM2.5 design values in the Timin Memo for monitors in the western 

U.S., and preliminary air quality data from monitors in the 

states bordering California. Based on these analyses, we propose 

to conclude that emissions from California sources do not 

interfere with maintenance of the 1997 8-hour ozone NAAQS or 

1997 PM2.5 NAAQS in any other state, consistent with the 

requirements of CAA section 110(a)(2)(D)(i)(I). 

1. Interfere with Maintenance Evaluation for the 8-hour 

Ozone NAAQS 

As discussed above, in the Transport Rule Proposal, EPA 

projected future concentrations of ozone at monitors to identify 

areas that are expected to be violating the NAAQS or to have 

difficulty maintaining compliance with the NAAQS in 2012. For 

purposes of the interfere with maintenance evaluation, EPA 

projected future concentrations of ozone at monitors to identify 

areas that are expected to have a maximum design value (based on 

a single 3-year period) that exceeds the 1997 8-hour ozone NAAQS 

and by 2012. EPA anticipates that these “maintenance receptors” 
38 

or “maintenance sites” will have difficulty in maintaining 

attainment of the NAAQS if there are adverse variations in 

meteorology or emissions. 

To identify the states with emissions that may cause 

interference with attainment of the NAAQS at the maintenance 

receptors, the Transport Rule Proposal models the states' 

contributions to ambient ozone levels at these maintenance 

receptors.49 Because the Transport Rule Proposal does not model 

the contribution of emissions from California (and other western 

states not fully inside the Transport Rule Proposal's modeling 

domain) to 8-hour ozone maintenance receptors in other states, 

our assessment relies on a weight of evidence approach that 

considers relevant information from the Transport Rule Proposal 

pertaining to states within its modeling domain and additional 

information such as back-trajectory analyses, geographical and 

meteorological factors, EPA’s projections of future air quality 

at monitors in western states in the Timin Memo, and AQS 

monitoring data. Although each of the factors considered in the 

following analysis is not in and of itself determinative, 

consideration of these factors together provides a reliable 

qualitative conclusion that emissions from California are not 

 

49 See Transport Rule Proposal, 75 FR 45210 at 45253-45273.
39 

likely to interfere with maintenance of the 1997 8-hour ozone 

NAAQS at monitors in other states. 

Our analysis begins by assessing California’s contribution 

to the closest maintenance receptors for the 1997 8-hour 

ozone standard. The Transport Rule Proposal identifies 16 

maintenance receptors for the 1997 8-hour ozone standard within 

its modeling domain (consisting of 37 states east of the Rocky 

Mountains, and the District of Columbia). Of these, the 

receptors closest to California are eight receptors in the 

Dallas-Fort Worth and Houston-Galveston-Brazoria 8-hour ozone 

nonattainment areas in eastern Texas. The remaining eight 

maintenance sites are located in Connecticut, Georgia, New York 

and Pennsylvania.50

As discussed above in section III.A.1, the Dallas-Fort 

Worth and Houston-Galveston-Brazoria areas are over 900 miles 

from the easternmost border of California. The maintenance 

receptor monitors located in Connecticut, Georgia, New York and 

Pennsylvania are significantly farther away. Although distance 

alone is not determinative in the analysis of potential ozone 

transport, with increasing distance there are greater 

opportunities for ozone and NOx dispersion and/or removal from 

the atmosphere. 

 

50 See Transport Rule Proposal, Table IV.C-12, 75 FR 45210 at 45252-45253.
40 

To evaluate further the potential for California emissions 

to interfere with maintenance at the closest maintenance 

receptor locations, EPA conducted an analysis of ozone transport 

for each exceedance day in 2005 and 2006 for the eight 

maintenance receptors in the Dallas-Fort Worth and HoustonGalveston-Brazoria 8-hour ozone nonattainment areas in eastern 

Texas.51 Exceedance days were identified using the AQS Database, 

EPA's repository of monitored ambient air quality data. EPA ran 

back-trajectories52 for those days during the 2005-2006 period 

when ozone concentrations at these receptors exceeded the 1997 

8-hour ozone NAAQS (i.e., monitored ozone concentrations were 85 

ppb or above). These back-trajectory maps indicate that air 

parcel pathways to maintenance receptors in eastern Texas do not 

originate in California. 

Because back-trajectory analysis results map pathways of 

air parcels that may or may not transport pollutants, they 

cannot be considered determinative as to the transport of ozone 

and its precursors or the absence of such transport from 

California emission sources. However, the fact that the air 

parcel trajectories do not connect California directly to the 

 

51 See Technical Support Document, California 2007 Transport SIP, Evaluation 

of Significant Contribution to Nonattainment and Interference with 

Maintenance for the 1997 8-hour Ozone NAAQS, U.S. EPA Region 9, February 25, 

2011.

52 For each monitor, EPA ran the trajectories backwards in time for 72 hours 

(three days), using a trajectory height at the starting point of 1,500 meters 

above ground level.
41 

maintenance receptors in eastern Texas strongly supports the 

conclusion that emissions of ozone and its precursors from 

California sources are not likely to interfere with maintenance 

of the 1997 8-hour ozone NAAQS at these receptors. The 

maintenance receptors for the 1997 ozone standard identified in 

the Transport Rule Proposal are in similar locations relative to 

California as are the nonattainment receptors discussed above in 

section III.A.1, and the same considerations regarding distance, 

topography, and meteorology therefore support our proposal to 

determine that emissions from California sources do not 

interfere with maintenance at the maintenance receptor sites. 

Thus, EPA believes it is reasonable to conclude that California 

sources do not interfere with maintenance of the 1997 8-hour 

ozone NAAQS in any other state. 

We note that EPA’s analysis in the Timin Memo, for western 

states not included in the modeling domain for the Transport 

Rule Proposal, identified four maintenance sites for the 1997 8-

hour ozone NAAQS in southern and central California.53 This 

analysis did not, however, identify any projected maintenance 

receptors for the 1997 8-hour ozone NAAQS in any other western 

state. The absence of monitors even suitable for comparison for 

this purpose indicates that emissions from California sources do 

 

53 See Timin Memo at Appendix B (“Base year 2003-2007 and Future Year 2012 8-

Hour Average Ozone Design Values – Western States”).
42 

not have such an impact in western states. Thus, EPA’s analysis 

for western states also supports our proposal to conclude that 

California sources do not interfere with maintenance of the 1997 

8-hour ozone NAAQS in other states. 

Finally, as discussed above in section III.A.1, EPA’s 

observed maximum design values at monitors in the western states 

during the 2003-2007 period were generally well below the 1997 

ozone NAAQS, and the 2012 modeling results at these western 

monitors (where a future year design value could be estimated) 

show a downward trend in ozone.54 Additionally, we evaluated 

ozone monitoring data from the 2007-2009 period from each of the 

ozone monitoring sites in Oregon, Nevada, and Arizona, and found 

no violations of the 1997 8-hour ozone NAAQS at any of these 

monitors during this period.55 The fact that monitors in these 

nearby areas are not registering violations of the NAAQS does 

not in itself conclusively establish that emissions from 

California could not interfere with maintenance of the 1997 8-

hour ozone NAAQS in any other state. But this fact combined with 

our evaluation of the nearest maintenance receptors in eastern 

Texas, taking into account distance, topographical barriers, and 

typical meteorological conditions, in addition to the back-

 

54 See Timin Memo at Appendix B (“Base year 2003-2007 and Future Year 2012 8-

Hour Average Ozone Design Values – Western States”).

55 See U.S. EPA AQS, “Preliminary Design Value Report,” 2007-2009, for Oregon, 

Nevada, and Arizona.
43 

trajectory analyses conducted to evaluate air parcel pathways to 

eastern Texas, support our proposal to conclude that California 

sources do not interfere with maintenance of the 1997 8-hour 

ozone NAAQS in any other state. 

2. Interfere With Maintenance Evaluation for the 1997 PM2.5

NAAQS. 

The Transport Rule Proposal identifies, within its modeling 

domain, 16 maintenance receptors for the 1997 annual PM2.5 NAAQS. 

Of these, the closest to California is one receptor located in 

the Harris County PM2.5 nonattainment area in eastern Texas. The 

remaining 15 maintenance receptors for the 1997 annual PM2.5

NAAQS are all located in states east of the Mississippi River.56 

As discussed above in section III.A.1, the Dallas-Fort 

Worth and Houston-Galveston-Brazoria areas are over 900 miles 

from the easternmost border of California, and states with 

maintenance receptors east of the Mississippi River are even 

farther away. Because the maintenance receptors for the 1997 

PM2.5 standard identified in the Transport Rule Proposal are in 

similar locations relative to California as are the 

nonattainment receptors discussed above in sections III.A.1 and 

A.2, the same considerations regarding distance, topography, and 

 

56 Specifically, the remaining 15 maintenance sites for the 1997 annual PM2.5

NAAQS are located in Illinois, Kentucky, New York, Ohio, Pennsylvania, and 

West Virginia.
44 

meteorology support our proposal to determine that emissions 

from California sources do not interfere with maintenance at the 

maintenance receptor sites. EPA therefore believes it is 

reasonable to conclude that California sources do not interfere 

with maintenance of the 1997 PM2.5 NAAQS in any other state. 

We note that EPA’s analysis in the Timin Memo, for western 

states not included in the modeling domain for the Transport 

Rule Proposal, identified numerous maintenance sites for the 

1997 annual PM2.5 NAAQS in southern and central California.57 This 

analysis did not, however, identify any projected maintenance 

receptors for the 1997 annual PM2.5 NAAQS in any other western 

state. Thus, we believe it is reasonable to conclude that 

California sources do not interfere with maintenance of the 1997 

PM2.5 NAAQS in other states. 

Finally, as discussed above in section III.A.2, EPA 

reviewed PM2.5 monitoring data for the 2007-2009 period from all 

from all PM2.5 monitoring sites in states bordering California 

(Oregon, Nevada, and Arizona) and found no violations of the 

1997 annual PM2.5 standard. The fact that monitors in these 

nearby areas are not registering violations of the NAAQS does 

not in itself conclusively establish that emissions from 

California could not interfere with maintenance of the 1997 

 

57 See Timin Memo at Appendix A (“Base year 2003-2007 and Future Year 2012 

Annual Average PM2.5 Design Values – Western States”).
45 

annual PM2.5 NAAQS in any other state. But this fact combined 

with our evaluation of the nearest maintenance receptor in 

eastern Texas, taking into account distance, topographical 

barriers, and typical meteorological conditions, supports our 

proposal to conclude that California sources do not interfere 

with maintenance of the 1997 PM2.5 NAAQS in any other state. 

The analysis for the Transport Rule Proposal did not 

identify any maintenance receptors for the 1997 24-hour PM2.5

NAAQS in the portions of the U.S. covered by the Transport Rule 

Proposal modeling domain.58 Recent monitoring data in EPA’s AQS 

Database (2007-2009 design values that are under final EPA 

review) indicate that the highest 24-hour PM2.5 design value in 

the 47 states of the continental U.S. (excluding California) is 

50 μg/m3, which is well below the level of the 1997 24-hour PM2.5

NAAQS of 65 μg/m3.

59 This data further supports our proposed 

finding that California emission sources do not interfere with 

maintenance of the 1997 PM2.5 NAAQS in any other state. 

3. Conclusion Regarding Interference With Maintenance 

Based on the weight of evidence, including the location of 

the nearest projected maintenance sites, taking into account 

distance, meteorology, topography, and recent air quality 

 

58 75 FR 45210 at 45249-45251 (August 2, 2010). See also fn. 40 and fn. 48. 

59 Data from EPA’s Air Quality System can be viewed at 

http://www.epa.gov/ttn/airs/airsaqs/. 
46 

monitoring data, as discussed above, we propose to determine 

that California’s 2007 Transport SIP is adequate and that 

emissions from California do not interfere with maintenance in 

any other state for the 1997 8-hour ozone or 1997 PM2.5 NAAQS, 

consistent with the requirements of CAA section 110(a)(2)(D)(I). 

Thus, we propose to determine that California’s SIP includes the 

measures necessary to prevent such prohibited interstate 

transport impacts for these NAAQS. 

IV. Proposed Action 

Under section 110(k) of the Clean Air Act, EPA is proposing 

to approve the 2007 Transport SIP submitted by CARB on November 

17, 2007, as adequate to prohibit emissions from California 

sources that will contribute significantly to nonattainment of 

the 1997 8-hour ozone or 1997 PM2.5 NAAQS in any other state, as 

required by CAA section 110(a)(2)(D)(i)(I). EPA is also 

proposing to approve the 2007 Transport SIP as adequate to 

prohibit emissions from California sources that will interfere 

with maintenance of these NAAQS by any other state, as required 

by section 110(a)(2)(D)(i)(I). Accordingly, we propose to find 

that the California SIP contains provisions adequate to prevent 

significant contribution to nonattainment of, and interference 

with maintenance of, these NAAQS and does not require any 

additional measures for this purpose at this time. This proposed 
47 

action does not apply to the remaining two elements of CAA 

section 110(a)(2)(D)(i) regarding interference with measures 

required to prevent significant deterioration of air quality or 

to protect visibility in another state. We intend to evaluate 

and act upon the 2007 Transport SIP for purposes of these 

additional requirements of CAA section 110(a)(2)(D)(i) in 

separate actions. 

EPA is soliciting public comments on this proposal and will 

accept comments until the date noted in the “DATES” section 

above.

V. Statutory and Executive Order Reviews 

Under the Clean Air Act, the Administrator is required to 

approve a SIP submission that complies with the provisions of 

the Act and applicable Federal regulations. 42 U.S.C. 7410(k); 

40 CFR 52.02(a). Thus, in reviewing SIP submissions, EPA’s role 

is to approve state choices, provided that they meet the 

criteria of the Clean Air Act. Accordingly, this action merely 

proposes to approve state law as meeting Federal requirements 

and does not impose additional requirements beyond those imposed 

by state law. For that reason, this proposed action: 

• is not a "significant regulatory action" subject to review 

by the Office of Management and Budget under Executive 

Order 12866 (58 FR 51735, October 4, 1993); 
48 

• does not impose an information collection burden under the 

provisions of the Paperwork Reduction Act (44 U.S.C. 3501 

et seq.); 

• is certified as not having a significant economic impact on 

a substantial number of small entities under the Regulatory 

Flexibility Act (5 U.S.C. 601 et seq.); 

• does not contain any unfunded mandate or significantly or 

uniquely affect small governments, as described in the 

Unfunded Mandates Reform Act of 1995 (Public Law 104-4); 

• does not have Federalism implications as specified in 

Executive Order 13132 (64 FR 43255, August 10, 1999); 

• is not an economically significant regulatory action based 

on health or safety risks subject to Executive Order 13045 

(62 FR 19885, April 23, 1997); 

• is not a significant regulatory action subject to Executive 

Order 13211 (66 FR 28355, May 22, 2001); 

• is not subject to requirements of Section 12(d) of the 

National Technology Transfer and Advancement Act of 1995 

(15 U.S.C. 272 note) because application of those 

requirements would be inconsistent with the Clean Air Act; 

and 

• does not provide EPA with the discretionary authority to 

address, as appropriate, disproportionate human health or 
49 

environmental effects, using practicable and legally 

permissible methods, under Executive Order 12898 (59 FR 

7629, February 16, 1994). 

In addition, this proposed rule does not have tribal 

implications as specified by Executive Order 13175 (65 FR 67249, 

November 9, 2000), because the SIP is not approved to apply in 

Indian country located in the state, and EPA notes that it will 

not impose substantial direct costs on tribal governments or 

preempt tribal law. 

List of Subjects in 40 CFR Part 52

Air pollution control, Environmental protection, 

Intergovernmental relations, Nitrogen dioxide, Ozone, 

Particulate matter, Reporting and recordkeeping requirements, 

Volatile organic compounds. 

Authority: 42 U.S.C. 7401 et seq. 

___/March 11, 2011/___ ___/s/________________ 

Date Jared Blumenfeld, 

 Regional Administrator, 

Region IX