Approval and Promulgation of State Implementation Plans; State of Colorado; Interstate Transport of Pollution Revisions for the 1997 8-hour Ozone NAAQS: “Interference With Maintenance” Requirement

AGENCY:

Environmental Protection Agency (EPA).

ACTION:

Proposed Rule.

SUMMARY:

EPA is proposing to approve the “State of Colorado Implementation Plan to Meet the Requirements of Clean Air Act section 110(a)(2)(D)(i)(I)—Interstate Transport Regarding the 1997 8-Hour Ozone Standard” addressing the “interference with maintenance” requirement of section 110(a)(2)(D)(i)(I). On June 18, 2009 the State of Colorado submitted an interstate transport State Implementation Plan (SIP) addressing the interstate transport requirements under section 110(a)(2)(D)(i) of the Clean Air Act (CAA). In this action, EPA is proposing to approve the Colorado Interstate Transport SIP provisions that address the section 110(a)(2)(D)(i)(I) requirement prohibiting a state's emissions from interfering with maintenance of the 1997 8-hour ozone National Ambient Air Quality Standards (NAAQS) by any other state. This action is being taken under section 110 of the CAA.

DATES:

Comments must be received on or before October 18, 2010.

ADDRESSES:

Submit your comments, identified by Docket ID No. EPA-R08-OAR-2007-1035, by one of the following methods:

• http://www.regulations.gov. Follow the on-line instructions for submitting comments.
• E-mail: mastrangelo.domenico@epa.gov
• Fax: (303) 312-6064 (please alert the individual listed in the FOR FURTHER INFORMATION CONTACT if you are faxing comments).
• Mail: Callie Videtich, Director, Air Program, Environmental Protection Agency (EPA), Region 8, Mailcode 8P-AR, 1595 Wynkoop Street, Denver, Colorado 80202-1129.
• Hand Delivery: Callie Videtich, Director, Air Program, Environmental Protection Agency (EPA), Region 8, Mailcode 8P-AR, 1595 Wynkoop, Denver, Colorado 80202-1129. Such deliveries are only accepted Monday through Friday, 8 a.m. to 4:30 p.m., excluding Federal holidays. Special arrangements should be made for deliveries of boxed information.

Instructions: Direct your comments to Docket ID No. EPA-R08-OAR-2007-1035. EPA's policy is that all comments received 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 information claimed to be Confidential Business Information (CBI) or other information whose disclosure is restricted by statute. Do not submit information that you consider to be CBI or otherwise protected through http://www.regulations.gov or e-mail. The http://www.regulations.gov Web site is an “anonymous access” system, which means EPA will not know your identity or contact information unless you provide it in the body of your comment. If you send an e-mail comment directly to EPA, without going through http://www.regulations.gov,, your e-mail address will be automatically captured and included as part of the comment that is placed in the public docket and made available on the Internet. If you submit an electronic comment, EPA recommends that you include your name and other contact information in the body of your comment and with any disk or CD-ROM you submit. 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. Electronic files should avoid the use of special characters, any form of encryption, and be free of any defects or viruses. For additional instructions on submitting comments, go to Section I. General Information of the SUPPLEMENTARY INFORMATION section of this document.

Docket: All documents in the docket are listed in the http://www.regulations.gov index. Although listed in the index, some information is not publicly available, e.g., CBI or other information whose disclosure is restricted by statute. Certain other material, such as copyrighted material, will be publicly available only in hard copy. Publicly-available docket materials are available either electronically in http://www.regulations.gov or in hard copy at the Air Program, Environmental Protection Agency (EPA), Region 8, Mailcode 8P-AR, 1595 Wynkoop, Denver, Colorado 80202-1129. EPA requests that if at all possible, you contact the individual listed in the FOR FURTHER INFORMATION CONTACT section to view the hard copy of the docket. You may view the hard copy of the docket Monday through Friday, 8 a.m. to 4 p.m., excluding Federal holidays.

FOR FURTHER INFORMATION CONTACT:

Domenico Mastrangelo, Air Program, U.S. Environmental Protection Agency, Region 8, Mailcode 8P-AR, 1595 Wynkoop, Denver, Colorado 80202-1129, (303) 312-6436, mastrangelo.domenico@epa.gov.

SUPPLEMENTARY INFORMATION:

Definitions

For the purpose of this document, we are giving meaning to certain words or initials as follows:

(i) The words or initials Act or CAA mean or refer to the Clean Air Act, unless the context indicates otherwise.

(ii) The words EPA, we, us or our mean or refer to the United States Environmental Protection Agency.

(iii) The initials SIP mean or refer to State Implementation Plan.

(iv) The words Colorado and State mean the State of Colorado.

Table of Contents

I. General Information

What should I consider as I prepare my comments for EPA?

II. Background

III. What action is EPA proposing?

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

V. EPA's Review and Technical Information

A. EPA's Evaluation of Interference with Maintenance

B. Colorado Transport SIP

VI. Proposed Action

VII. Statutory and Executive Order Reviews

I. General Information

What should I consider as I prepare my comments for EPA?

1. Submitting CBI. Do not submit CBI to EPA through http://www.regulations.gov or e-mail. Clearly mark the part or all of the information that you claim to be CBI. For CBI information in a disk or CD ROM that you mail to EPA, mark the outside of the disk or CD ROM as CBI and then identify electronically within the disk or CD ROM the specific information that is claimed as CBI. In addition to one complete version of the comment that includes information claimed as CBI, a copy of the comment that does not contain the information claimed as CBI must be submitted for inclusion in the public docket. Information so marked will not be disclosed except in accordance with procedures set forth in 40 CFR part 2.

2. Tips for Preparing Your Comments. When submitting comments, remember to:

Identify the rulemaking by docket number and other identifying information (subject heading, Federal Register date and page number).

Follow directions—The agency may ask you to respond to specific questions or organize comments by referencing a Code of Federal Regulations (CFR) part or section number.

Explain why you agree or disagree; suggest alternatives and substitute language for your requested changes.

Describe any assumptions and provide any technical information and/or data that you used.

If you estimate potential costs or burdens, explain how you arrived at your estimate in sufficient detail to allow for it to be reproduced.

Provide specific examples to illustrate your concerns, and suggest alternatives.

Explain your views as clearly as possible, avoiding the use of profanity or personal threats.

Make sure to submit your comments by the comment period deadline identified.

II. Background

On July 18, 1997, EPA promulgated new NAAQS for ozone and for fine particulate matter (PM_2.5). This action is being taken in response to the promulgation of the 1997 8-hour ozone NAAQS. This action does not address the requirements for the 1997 or 2006 PM_2.5, or the 2008 8-hour ozone NAAQS; those standards will be addressed in later actions.

Section 110(a)(1) of the CAA requires states to submit SIPs to address a new or revised NAAQS within 3 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 its “Guidance for State Implementation Plan (SIP) Submissions to Meet Current Outstanding Obligations Under Section 110(a)(2)(D)(i) for the 8-Hour Ozone and PM_2.5 National Ambient Air Quality Standards” (2006 Guidance). EPA developed the 2006 Guidance to make recommendations to states for making submissions to meet the requirements of section 110(a)(2)(D)(i) for the 1997 8-hour ozone standards and the 1997 PM_2.5 standards.

As identified in the 2006 Guidance, the “good neighbor” provisions in section 110(a)(2)(D)(i) 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) contains four distinct requirements related to the impacts of interstate transport. The SIP must prevent sources in the state from emitting pollutants in amounts which will: (1) Contribute significantly to nonattainment of the NAAQS in other states; (2) interfere with maintenance of the NAAQS in other states; (3) interfere with provisions to prevent significant deterioration of air quality in other states; or (4) interfere with efforts to protect visibility in other states.

On June 18, 2009, EPA received a SIP revision from the State of Colorado intended to address the requirements of section 110(a)(2)(D)(i)(I) for the 1997 8-hour ozone standards. In this rulemaking, EPA is addressing only the requirements that pertain to preventing sources in Colorado from emitting pollutants that will interfere with maintenance of the 1997 8-hour ozone NAAQS by other states. In its submission, the State of Colorado indicated that its current SIP is adequate to prevent such interference. With this submission, the state intended to meet the recommendations of the 2006 Guidance for SIP submissions to meet the second element of section 110(a)(2)(D)(i) for the 1997 8-hour ozone standard.

III. What action is EPA proposing?

EPA is proposing approval of a portion of the Colorado Interstate Transport of Air Pollution SIP addressing the requirements of CAA section 110(a)(2)(D)(i)(I) for the 1997 8-hour ozone NAAQS. On December 30, 2008, the Colorado Air Quality Control Commission (AQCC) adopted the “State of Colorado Implementation Plan to Meet the Requirements of the Clean Air Act Section 110(a)(2)(d)(i)(I)—Interstate Transport Regarding the 1997 8-Hour Ozone Standard” (Colorado Interstate Transport SIP). Colorado submitted this SIP revision to EPA on June 18, 2009. In this Federal Register action EPA is proposing to approve only the language and demonstration that, in this SIP revision, address the requirements of element (2), i.e., the prohibition of interference with maintenance of the 1997 8-hour ozone NAAQS by any other state.

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

Section 110(k) of the CAA addresses EPA's rulemaking action on SIP submissions by states. The CAA requires states to observe certain procedural requirements in developing SIP revisions for submittal to EPA. Section 110(a)(2) of the CAA requires that each SIP revision be adopted after reasonable notice and public hearing. This must occur prior to the revision being submitted by a state to EPA.

The Colorado AQCC held in early December 2008 a public hearing for the Colorado Interstate Transport SIP revision, adopted it on December 30, 2008, and the State submitted it to EPA on June 18, 2009.

On November 18, 2009, the AQCC provided EPA with an exact color duplicate of the SIP adopted by the AQCC on December 30, 2008 and included in the June 18, 2009 submittal to EPA. In the original submittal, AQCC provided a black and white copy. The SIP's color duplicate, available for review as part of the Docket, makes it easier to understand modeling results reported in several graphs that are part of the SIP technical demonstration.

EPA has reviewed the submittal from the State of Colorado and has determined that the State met the requirements for reasonable notice and public hearing under section 110(a)(2) of the CAA.

V. EPA's Review and Technical Information

A. EPA's Evaluation of Interference With Maintenance

The second element of 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 would “interfere with maintenance” of the applicable NAAQS by any other state. This term is not defined in the statute. Therefore, EPA has interpreted this term in past regulatory actions, such as the 1998 NO_X SIP Call, in which EPA took action to eliminate emissions of NO_X that significantly contributed to nonattainment, or interfered with maintenance of, the then applicable ozone NAAQS through interstate transport of NO_X and the resulting ozone. The NO_X SIP Call was the mechanism through which EPA evaluated whether or not the NO_X 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 NO_X 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 PM_2.5 NAAQS, EPA again recognized that regional transport was a serious concern throughout the eastern U.S. and therefore developed the 2005 Clean Air Interstate Rule (CAIR) to address emissions of SO₂ and NO_X that exacerbate ambient ozone and PM_2.5 levels in many downwind areas through interstate transport. Within CAIR, EPA likewise 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 upwind states to reduce SO₂ and NO_X emissions. Through CAIR, EPA again required states that had such interstate impacts to adopt substantive SIP revisions to eliminate the SO₂ and NO_X emissions, whether through participation in a regional cap and trade program or by other means.

EPA's 2006 Guidance addressed section 110(a)(2)(D) requirements for the 1997 8-hour ozone NAAQS and 1997 PM_2.5 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 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 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.” 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) together without explicitly differentiating between them.

In 2008, however, the U.S. Court of Appeals for the DC Circuit found that CAIR and the related CAIR federal implementation plans were unlawful. 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. The court 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.” EPA's approach, the court reasoned, would leave areas that are “barely meeting attainment” with “no recourse” to address upwind emissions sources. The court therefore concluded that a plain language reading of the statute requires EPA to give independent meaning to the interfere with maintenance requirement of section 110(a)(2)(D) 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)(I) because the agency's guidance suggested that states use an approach comparable to that used by EPA in CAIR. States such as Colorado developed and adopted their Interstate Transport SIPs not long after the Court's July 2008 decision, but well before EPA, in the Transport Rule Proposal (see below), was able to propose a new approach for the interference with maintenance element. Without recommendations from EPA, Colorado's SIP may not have sufficiently differentiated between the significant contribution to nonattainment and interference with maintenance elements of the statute, and relied in a general way on the difference between monitored concentrations and the 1997 8-hour ozone NAAQS to evaluate the impacts of State emissions on maintenance of the NAAQS in neighboring states. EPA believes that it is necessary to evaluate these state submissions 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, EPA believes it is necessary to use an updated approach to this issue and to supplement the technical analysis provided by the state in order to evaluate the submissions with respect to the interfere with maintenance element of section 110(a)(2)(D)(i).

EPA has recently proposed a new rule to address interstate transport 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), in order to address the judicial remand of CAIR. As part of the Transport Rule Proposal, EPA specifically reexamined the section 110(a)(2)(D)(i) requirement that emissions from sources in a state must not “interfere with maintenance” of the 1997 8-hour ozone NAAQS and 1997 PM_2.5 NAAQS by other states. In the proposal, EPA developed an approach to identify areas that it predicts to be close to the level of the 1997 8-hour ozone NAAQS and 1997 PM_2.5 NAAQS in the future, and therefore at risk to become or continue to be nonattainment for these NAAQS unless 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 with interference with maintenance, rather than with significant contribution to nonattainment.

As described in more detail below, EPA's analysis evaluates data from existing monitors over three overlapping three 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 PM_2.5 NAAQS in 2012, and which areas are predicted potentially to have difficulty with 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 comparison for purposes of the interfere with maintenance element of the statute.

By this method, EPA has identified those areas with monitors that are appropriate “maintenance sites” or maintenance “receptors” for evaluating whether the emissions from sources in another state could interfere with maintenance in that particular area. EPA then uses other analytical tools to examine the potential impacts of emissions from upwind states on these maintenance receptors in downwind states. EPA believes that this new approach for identifying those areas that are predicted to have maintenance problems is appropriate to evaluate the section 110(a)(2)(D)(i) SIP submission of a state for the interfere with maintenance element. EPA's 2006 Guidance did not provide this specific recommendation to states, but in light of the court's decision on CAIR, EPA will itself follow this approach in acting upon the Colorado submission.

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 as CAIR. In the 2006 Guidance, EPA stated that: “EPA believes that the contents of the SIP submission required by section 110(a)(2)(D)(i) 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.” 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. As noted in the Transport Rule Proposal, EPA continues to believe that the more widespread and serious transport problems in the eastern United States are analytically distinct. For the 1997 8-hour ozone NAAQS and the 1997 PM_2.5 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 interstate ozone or PM_2.5 contributions to or from western states.

Accordingly, EPA believes that section 110(a)(2)(D)(i) SIP submissions for states not evaluated in the Transport Rule Proposal may be evaluated using a “weight of the evidence” approach that takes into account 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 NAAQS in 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.

B. Colorado Transport SIP

To meet the requirements of section 110(a)(2)(D)(i)(I) for the 1997 8-hour ozone standard, the State of Colorado developed and submitted to EPA on June 18, 2009 an Interstate Transport SIP that focused primarily on the “significant contribution to nonattainment” requirement of section 110(a)(2)(D)(i) and, as noted earlier, addressed only in a limited way the interference with maintenance requirement of section 110(a)(2)(D)(i)(I). On June 3, 2010, EPA approved the Colorado Interstate Transport SIP provision that require that emissions from a state's sources do not significantly contribute to nonattainment of the 1997 8-hour ozone NAAQS in any other state. To demonstrate that emissions from Colorado do not interfere with maintenance of the 1997 8-hour ozone NAAQS in neighboring states, the Colorado Interstate Transport SIP uses results from Colorado's 2009 “8-Hour Ozone Attainment Plan” for the DMA/NFR nonattainment area, and a report from the Western States Air Resource (WESTAR) Council to underscore that: (a) Local anthropogenic ozone contribution to high ozone concentrations in Denver is only about 25%; and (b) on days of highest ozone concentrations (reflecting a design value of 84.9 ppb) in the DMA/NFR area, the projected design values decrease to 63 ppb or less for all downwind Colorado counties east of an imaginary north-south line approximately 70 miles east from Denver. EPA does not agree with the State of Colorado Interstate Transport SIP's assessment that these results demonstrate that “the magnitude of ozone transport from Colorado to other states is too low to * * * interfere with maintenance by any other state with respect to the 0.08 ppm NAAQS” as the sole basis for evaluating the state's interference. The limited contribution of local emissions to nonattainment in the DMA/NFR and the quick drop in ozone levels in the easternmost Colorado counties, in and by themselves do not exclude a potential for interference with maintenance of the 8-hour ozone NAAQS from Colorado emissions to downwind maintenance areas. Rather, as a reflection of emission levels, the relatively (to the 1997 8-hour ozone NAAQS) moderate ozone concentrations in eastern Colorado and in neighboring states somewhat reduce the probability that State emissions interfere with maintenance of the NAAQS by these states.

EPA is evaluating the Colorado Interstate Transport SIP taking into account the methodologies and analysis results developed in the Transport Rule Proposal in response to the judicial remand of CAIR. As noted previously, the Transport Rule Proposal includes a new approach to determine whether emissions from a state interfere with maintenance of the 1997 8-hour ozone NAAQS and the 1997 PM_2.5 NAAQS by other states. In this action, EPA is using a comparable approach to that of the Transport Rule Proposal in order to determine if emissions from Colorado sources interfere with maintenance of the 1997 8-hour ozone NAAQS by other states.

To evaluate ambient impacts from upwind states to maintenance receptors, the Transport Rule Proposal evaluates, through air quality modeling of each state's emissions, the contribution from individual states to downwind maintenance receptors. States that contribute pollutant concentrations below the significance threshold for interference with maintenance, proposed at one percent of the NAAQS, are excluded from further analysis. For the 1997 8-hour ozone standard state contributions of 0.8 ppb and higher are considered above the threshold, while ozone contribution less than 0.8 ppb are below the threshold.

In the Transport Rule Proposal, EPA projected future concentrations of ozone at monitors to identify areas that are expected to be out of attainment with NAAQS or to have difficulty maintaining compliance with the NAAQS in 2012. To determine the states that may cause interference at the maintenance receptors, the Transport Rule Proposal models the states' ozone contribution to these maintenance receptors. Because the Transport Rule Proposal does not model the contribution of emissions from Colorado (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 (such as identification of maintenance receptors and estimates of ozone contributions) from the Transport Rule Proposal pertaining to states within its modeling domain, and additional material such as geographical and meteorological factors, modeling analysis results from other studies, back trajectory analyses, and AQS monitoring data. While conclusions reached for each of the factors considered in the following analysis are not in and by themselves determinative, consideration of these factors together provides a reliable qualitative conclusion that emissions from Colorado are not likely to interfere with maintenance of the 1997 8-hour ozone NAAQS at monitors in other states.

Our analysis begins by assessing Colorado's contribution to the closest maintenance 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) 16 maintenance receptors, among which the eight closest to Colorado are eight receptors in the Dallas Fort Worth (DFW) and Houston-Galveston-Brazoria (HGB) 8-hour ozone nonattainment areas.

Two of the three DFW area maintenance receptors are in Dallas County (Hinton Street and Dallas Executive Airport sites), and the third is in Tarrant County (Keller site). These monitors are at least 500 miles from Colorado. Distance by itself is not an obstacle to long range transport of ozone and/or its precursors. NO_X, the primary ozone precursor that is the object of the Transport Rule Proposal, may be transported for long distances, and contribute significantly to high ozone concentrations in other states. However, with increasing distance there are greater opportunities for ozone or NO_X dispersion and/or removal from the atmosphere due to the effect of winds or chemical sink processes. As a result, one may conclude that the approximately 500 miles from Colorado sources of_X emissions to the DFW area maintenance receptors reduces, but does not exclude, the possibility that the Colorado emissions interfere with maintenance of the NAAQS at these receptors.

Because pollutant transport is linked to wind direction, we examine how frequently air masses from Colorado pass through or end in the DFW area that includes the maintenance receptors identified above. The State of Texas' 2007 attainment demonstration for the DFW area points out, without quantifying contributions, how heavily the area's ozone concentrations are affected by substantial transport from other areas. Average ozone background levels for DFW (reflecting concentrations contributed to the area by emissions from sources within Texas but outside the nonattainment area, and from sources outside Texas) are estimated to range between 44 and 61 ppb, with peak averages between 64 and 68 ppb on days when 8-hour ozone concentrations exceed the 1997 standard.

To evaluate the impact of wind direction on ozone transport from Colorado to the DFW maintenance receptors, we rely on the results of two back trajectory studies, including a set of trajectories with end points at the maintenance receptors in the DFW area. EPA generated these trajectories using the HYSPLIT 4.9 online computer application, selecting the archived Eta Data Assimilation System (EDAS) meteorological data sets with the highest degree of resolution (40 km). Back trajectories were run for the days of the 2005-2006 years in which ozone concentrations at these receptors exceeded the 1997 8-hour NAAQS—i.e., monitored ozone concentrations were 85 ppb or above. Exceedance days were identified using the Air Quality System (AQS), EPA's repository of monitored ambient air quality data. At each monitor, trajectories were started at 22 Coordinated Universal Time (UTC), equivalent to 4 p.m. CST, and were run backwards in time for 72 hours (three days). The trajectory height at the starting point is 1500 meters above ground level. From the individual back trajectories, “spider web” maps were generated for all three monitors combined and for each monitor (Figure 1.1 and Figures 1.1.a through 1.1.c in Appendix A of EPA's TSD). These maps indicate that air parcel pathways from Colorado and ending at maintenance receptors in Dallas and Tarrant Counties are rare during the three days preceding ozone exceedances at these receptors. On only one day, of the 35 exceedance days at maintenance receptors in 2005-2006, did the air mass pathway go through Colorado, and even in this one instance air parcels crossed the State along a short pathway through its northeast corner, before continuing on their southeastward course.

Back trajectory analysis results included in the May 23, 2007 DFW area Attainment Demonstration corroborate these conclusions. The analysis, also based on the HYSPLIT model, includes all days during the years 2001-2003, with trajectories of 48 hours (2 days) duration, heights of 100, 500 and 1300 meters, and start times of 20, 21 and 22 UTC (2, 3, and 4 p.m. CST). The resulting density plots in Figure 3-1 of the DFW attainment demonstration clearly show that during most of the ozone season, on high and low ozone days, air parcels from Colorado infrequently end in or pass through the DFW area.

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 of the absence of such transport, from Colorado emissions. However, the rarity of air parcel trajectories from Colorado to the DFW area and to its maintenance receptors strongly support the conclusion that emissions of ozone and its precursors from Colorado are not likely to interfere with maintenance of the 1997 ozone NAAQS at these receptors.

A final piece of evidence of a different type is found in a modeling analysis developed by EPA to assist the State of New Mexico in its assessment of ozone and PM_2.5 transport from New Mexico to other states. This modeling analysis, part of the New Mexico Interstate Transport SIP submission of July 30, 2007, relies on data developed by the Central Regional Air Planning Association (CENRAP) that includes a 2002 third quarter CENRAP modeling dataset. It is based on a 36 km national grid that includes Colorado, and uses the ozone source apportionment tool (OSAT) to determine potential linkages between state emissions and downwind states. Modeling results indicate that at the height of the 2002 ozone season, the highest ozone contribution from Colorado emissions to the DFW monitors (including its maintenance receptors) averaged 0.4 ppb or less. That is well below the contribution threshold of 0.8 ppb, used in the proposed Transport Rule.

The other five Texas monitors identified by the Transport Rule Proposal as maintenance-only receptors in Texas are located in Harris County, which lies within the HGB nonattainment area. This area is at least 700 miles from Colorado. General considerations on the effect of distance on ozone transport from Colorado to the DFW area, discussed above, also apply to the potential for transport from Colorado to the maintenance receptors in the HGB area. The greater distance (by about one third) between Colorado and the HGB area increases the chance for dispersion of any Colorado ozone during its transport to HGB maintenance receptors, and increases the odds for air masses from Colorado to pick up greater quantities of ozone and/or precursors during their longer travel through emissions rich Texas. Again, these considerations reduce, but do not exclude, the possibility of emissions from Colorado interfering with maintenance of the 8-hour ozone NAAQS at the HGB maintenance receptors.

A similar conclusion is suggested by the EPA back trajectories mapped for the HGB maintenance receptors. Using the same online HYSPLIT 4.9 online computer application as for the DFW trajectories, EPA ran back trajectories from the HGB area maintenance receptors for all 2005-2006 ozone exceedances days. The pathways of air parcels ending at, or passing through, these monitors when ozone concentrations reached levels of 85 ppb or higher are shown in Figure 2.1 of Appendix A in EPA's supporting documentation. At each monitor, trajectories started at 22 Coordinated Universal Time (UTC), equivalent to 4 p.m. CST, and ran backwards in time for 72 hours (three days), at 1500 meters above ground level. Results show that air parcel pathways passing 1500 meters above the HGB maintenance receptors at 4 p.m. on exceedance days rarely came from Colorado. Figure 3 of the back trajectories report shows that only in one out of 53 exceedance days at the maintenance receptors did the air parcel's pathway go through Colorado. Even in this one instance, the pathway crossed Colorado for a very short distance through the State's northeast corner, before continuing on its southeastward course.

Back trajectory analysis results from a 2009 report, “Effects of Meteorology on Pollutant Trends” report, corroborate these conclusions. The analysis uses HYSPLIT with EDAS meteorological datasets to plot 72-hour back trajectories centered in Houston, at 300 meters height and for various times of the day. Trajectories are plotted for all days with available data between May 1 and October 31, 2000-2007. A clustering algorithm built into HYSPLIT is used to group individual back trajectories into several classes based on shape and direction. Due to the greater number of days plotted, the six clusters of trajectories shown in Figures 6-17 to 6-22 include a much larger number of air parcel pathways than EPA's back trajectory analysis referenced above, but still show similar results concerning trajectories from Colorado. Air parcels from Colorado to the Houston area are rare, as shown by the few trajectories from Colorado in cluster 3 (Figure 6-19) as compared with the total sample of 1416 trajectories included in the six clusters. Figure 6-15 summarizes effectively the overall scarcity of wind pathways from Colorado, and from the west/lower northwest sector in general, to the HGB area. It shows the mean centerlines for the six identified clusters, and at their closest point to Colorado's borders the mean centerline (number 3) is still at an estimated distance of approximately 200 miles.

Again, back trajectories map pathways of air parcels that may or may not transport pollutants, and they cannot be considered determinative as to the transport of ozone and its precursors. However, the infrequency of air parcels trajectories from Colorado to the HGB area in general, and to its maintenance receptors in particular, strongly support the conclusion that ozone precursors' emissions and ozone from Colorado are not likely to interfere with maintenance of the 1997 ozone NAAQS at these receptors.

The EPA modeling analysis referenced earlier (concerning contribution from Colorado sources to the DFW area) includes information on the contribution of the State emissions to the HGB area as well. The 2002 modeled contribution from Colorado ozone emissions to the HGB area is estimated at 0.3 ppb or less. This fraction of the significant contribution threshold of 0.8 ppb, set in EPA's Transport Rule Proposal of August 2, 2010, strengthens our assessment that Colorado emissions are unlikely to interfere with maintenance of the 1997 ozone NAAQS at the HGB maintenance receptors.

As noted previously, eight of the 16 maintenance receptors identified within the modeling by the Transport Rule Proposal analysis are in a handful of East Coast states: Connecticut, Georgia, New York and Pennsylvania. The westernmost states “linked” by the Transport Rule Proposal to the eight maintenance receptors in these states include Indiana, Kentucky, Tennessee, and Alabama. None of the 13 states west of these contributing states and east of Colorado (such as North and South Dakota and Nebraska) was found to contribute significantly to the maintenance receptors in the east. In addition, among the 13 non-contributing states closer than Colorado to the maintenance receptors in the east, there are states such as Illinois, Wisconsin, Iowa, Missouri, Arkansas, and Louisiana that in 2005 had NO_X emissions up to twice as high as Colorado's. Because the analysis for the Transport Rule Proposal found that these states with substantially larger NO_X emissions than Colorado, and closer than Colorado to the maintenance receptors in the east, do not to contribute significantly to maintenance receptors in Connecticut, Georgia, New York and Pennsylvania, it is logical to conclude that it is quite unlikely for Colorado emissions to interfere with maintenance of the 1997 8-hour ozone NAAQS at these same receptors.

To assist in the evaluation of whether states' emissions interfere with maintenance of the NAAQS in western states, EPA has developed, independent of the Transport Rule Proposal, a modeling analysis identifying monitors at risk for maintenance of the NAAQS within a modeling domain that includes the western states. The analysis presented in the memo, “Documentation of Future Year Ozone and Annual PM_2.5 Design Values for Western States” (Western States Design Values), uses model results from the Transport Rule modeling Continental U.S. 36 km grid, which is coarser than the 12 km grid used in the Transport Rule, but does not necessarily yield less reliable results.

EPA's modeling analysis of western states to determine the monitors that are at risk for maintenance of the 1997 8-hour ozone NAAQS identifies only four such maintenance receptors, and all four are in California, in Mercer, Placer, Riverside, and Sacramento Counties. Distance and topography are not favorable to ozone transport from Denver, which is approximately 750 miles east of the monitors in Placer and Sacramento Counties, and 850 miles northeast to a Riverside County monitor. In the absence of significant northwesterly regional transport winds, mountain ranges between Denver and the California maintenance receptors, such as the Rocky Mountains, the Wasatch and the Sierra Nevada, present large obstacles to ozone transport from Colorado to California. Thus, geography and topography reduce the likelihood of transport from Colorado to California's maintenance receptors.

Prevailing wind orientation in fact strongly supports the conclusion that Colorado's emissions are unlikely to interfere with maintenance of the 1997 8-hour ozone standard in California. West of the Continental Divide the prevailing winds generally move from south-westerly, westerly, or north-westerly directions, as indicated by the typical movement of weather systems. To further evaluate the direction of regional transport winds affecting the California maintenance receptors, we have plotted back trajectories starting at each maintenance receptor on high ozone days. High ozone days include the top one-third of the exceedance days (for the 1997 8-hour ozone NAAQS) registered at each monitor in 2005 and 2006. As shown by the trajectories mapped for all four maintenance receptors in Figure 3.1, Appendix A of EPA's supporting documentation, on high ozone days air parcels converge on the Mercer, Placer, Sacramento and Riverside monitors from the northwest, south and southeast, but there are no pathways from the east/northeast directions reaching even as far as the eastern Nevada border, let alone Colorado.

For a large number of receptors in western states, EPA's modeling analysis could not calculate 2012 projected design values because these receptors did not have at least 5 days with base year concentrations equal to or greater than 70 ppb, as required by EPA's modeling guidance. However, the observed maximum design values at these sites in the 2003-2007 period were generally well below the 1997 ozone NAAQS. The highest (non-California ) site had a maximum design value of 77 ppb. Additionally, the 2012 modeling results at western monitors (where a future year design value could be estimated, shows a downward trend in ozone. There are no areas in the West where ozone is predicted to be higher in 2012 (without CAIR) compared to 2005. On these bases it is plausible to conclude that it is highly unlikely, but not impossible, for these monitors to be at risk for maintenance of the 1997 8-hour ozone NAAQS.

In conclusion, the variety of data and the weight of evidence analysis presented in this section support the position of the Colorado Interstate Transport SIP (adopted into the State SIP on December 30, 2008 and submitted to EPA June 18, 2009) that emissions from Colorado do not interfere with maintenance of the 1997 8-hour ozone NAAQS by any other state, consistent with the requirements of element (2) of CAA section 110(a)(2)(D)(i).

VI. Proposed Action

EPA is proposing partial approval of the Colorado SIP to meet the requirements of Section 110(a)(2)(D)(i)(I) regarding the 1997 8-hour ozone standard. Specifically, in this action EPA is proposing to approve only the language and demonstration that address the requirements of element (2): Prohibition of interference with maintenance of the 1997 8-hour ozone NAAQS by any other state. EPA approved in a June 3, 2010 final action the language and demonstration addressing element (1): Prohibition of significant contribution to nonattainment of the 1997 8-hour ozone NAAQS in any other state.

VII. Statutory and Executive Order Review

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 approves state law as meeting Federal requirements and does not impose additional requirements beyond those imposed by state law. For that reason, this 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);
• 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 (Pub. L. 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 environmental effects, using practicable and legally permissible methods, under Executive Order 12898 (59 FR 7629, February 16, 1994).

In addition, this 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

• Environmental protection
• Air pollution control
• Incorporation by reference
• Intergovernmental relations
• Nitrogen dioxide
• Ozone
• Particulate matter
• Reporting and recordkeeping requirements
• Volatile Organic Compounds

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