Approval and Promulgation of State Implementation Plans; State of Colorado; Interstate Transport of Pollution Revisions for the 1997 8-hour Ozone NAAQS: “Significant Contribution to Nonattainment” Requirement, 31306-31317 [2010-13050]

Agencies

• ENVIRONMENTAL PROTECTION AGENCY

[Federal Register Volume 75, Number 106 (Thursday, June 3, 2010)]
[Rules and Regulations]
[Pages 31306-31317]
From the Federal Register Online via the Government Printing Office [www.gpo.gov]
[FR Doc No: 2010-13050]


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ENVIRONMENTAL PROTECTION AGENCY

40 CFR Part 52

[EPA-R08-OAR-2007-1032; FRL-9155-5]


Approval and Promulgation of State Implementation Plans; State of 
Colorado; Interstate Transport of Pollution Revisions for the 1997 8-
hour Ozone NAAQS: ``Significant Contribution to Nonattainment'' 
Requirement

AGENCY: Environmental Protection Agency (EPA).

ACTION: Final rule.

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SUMMARY: EPA is partially approving State Implementation Plan (SIP) 
revisions submitted by the State of Colorado on June 18, 2009. These 
revisions, referred to as the Colorado Interstate Transport SIP, 
address the requirements of Clean Air Act section 110(a)(2)(D)(i)(I) 
for the 1997 8-hour ozone National Ambient Air Quality Standards 
(NAAQS). In this action EPA is approving the Colorado Interstate 
Transport SIP non-regulatory provisions that address the requirement of 
section 110(a)(2)(D)(i)(I) that emissions from the state's sources do 
not ``contribute significantly'' to nonattainment of the 1997 8-hour 
ozone NAAQS in any other state. EPA will act at a later date on the 
Colorado Interstate Transport SIP provisions that address the 
requirement of section 110(a)(2)(D)(i)(I) that emissions from the 
state's sources do not ``interfere with maintenance'' of the 1997 8-
hour ozone NAAQS in any other state. This action is being taken under 
section 110 of the Clean Air Act.

DATES: Effective Date: This final rule is effective July 6, 2010.

ADDRESSES: EPA has established a docket for this action under Docket ID 
No. EPA-R08-OAR-2007-1032. All documents in the docket are listed on 
the https://www.regulations.gov Web site. Although listed in the index, 
some information is not publicly available, e.g., Confidential Business 
Information (CBI) or other information whose disclosure is restricted 
by statute. Certain other material, such as copyrighted material, is 
not placed on the Internet and will be publicly available only in hard 
copy form. Publicly available docket materials are available either 
electronically through https://www.regulations.gov, or in hard copy at 
the Air Program, Environmental Protection Agency (EPA), Region 8, 1595 
Wynkoop Street, 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, 8:00 a.m. 
to 4:00 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 Street, Denver, Colorado 80202-1129, (303) 312-6416, 
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.

[[Page 31307]]

    (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 State or Colorado mean the State of Colorado, unless 
the context indicates otherwise.

Table of Contents

I. Background
II. Response to Comments
III. Section 110(l)
IV. Final Action
V. Statutory and Executive Order Reviews

I. Background

    Section 110(a)(2)(D)(i) of the CAA requires that a state's SIP must 
contain adequate provisions prohibiting any source or other type of 
emissions activity within the state from emitting any air pollutant in 
amounts which will: (1) Contribute significantly to nonattainment of 
the NAAQS in any other state; (2) interfere with maintenance of the 
NAAQS by any other state; (3) interfere with any other state's required 
measures to prevent significant deterioration of air quality; or (4) 
interfere with any other state's required measures to protect 
visibility. On March 31, 2010, EPA published a notice of proposed 
rulemaking (NPR) proposing partial approval of the State Implementation 
Plan (SIP) revision ``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,'' submitted by the 
State on June 18, 2009. As indicated by the title, this SIP addresses 
the first two of the four requirements listed above-i.e., (1), 
``significant contribution,'' and (2), ``interference with 
maintenance.'' EPA's proposed rule action reviewed and proposed 
approval of the Colorado SIP's section addressing only the 
``significant contribution'' requirement. EPA will act at a later date 
on the Colorado Interstate Transport SIP section that addresses the 
``interference with maintenance'' requirement.
    To assess whether emissions from Colorado contribute significantly 
to downwind nonattainment for the 1997 8-hour ozone NAAQS, EPA's 
technical analysis relied on EPA's 2006 Guidance, recommending 
consideration of available EPA modeling conducted in conjunction with 
CAIR,\1\ 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. 
Consistent with the NOX SIP Call and CAIR, our technical 
analysis assessed the extent of ozone transport from Colorado not just 
to areas designated nonattainment, but also to areas in violation of 
the NAAQS. Because EPA did not have detailed modeling for Colorado and 
nearby downwind states, our approach did not rely on a quantitative 
determination of Colorado's contribution but on a weight-of-evidence 
approach using quantitative information such as Colorado's distance 
from areas with monitors showing violations of the NAAQS, modeling 
results outlining wind vectors for regional transport of ozone on high 
ozone days, back trajectory analyses for the downwind nonattainment 
areas closest to the State, and results of modeling studies for the 
nonattainment areas specifying the range of wind directions along which 
contribution of ozone transport occurred. Given that the assessments 
for each of these pieces of evidence are not individually definitive or 
outcome determinative, EPA concluded in its proposed action that the 
various factual and technical considerations supported a determination 
of no significant contribution from Colorado emissions to the ozone 
nonattainment areas noted above. EPA did not receive comments that 
persuade the Agency that there is such significant contribution, and 
thus in today's final action EPA is making a final regulatory 
determination that Colorado emissions sources do not contribute 
significantly to violations of the 1997 8-hour ozone NAAQS in any other 
state.
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    \1\ In this action the expression ``CAIR'' refers to the final 
rule published in the May 12, 2005 Federal Register and entitled 
``Rule to Reduce Interstate Transport of Fine Particulate Matter and 
Ozone (Clean Air Interstate Rule); Revisions to Acid Rain Program; 
Revisions to NOX SIP Call; Final Rule'' (70 FR 25162).
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II. Response to Comments

    EPA received one letter from WildEarth Guardians (WG) commenting on 
EPA's Federal Register action proposing approval of the portion of the 
Colorado Interstate Transport SIP that addresses the ``significant 
contribution to nonattainment'' requirement of CAA Section 
110(a)(2)(D)(i)(I) for the 1997 8-hour ozone NAAQS. In this section EPA 
responds to the significant adverse comments made by the commenter.
    Comment No. 1--The commenter asserted that EPA's proposed approval 
was based on a ``flawed legal standard.'' According to the commenter, 
EPA erred in the proposal by explaining that various factual or 
technical assessments indicate that it is ``unlikely'' that emissions 
from Colorado sources significantly contribute to violations of the 8-
hour ozone NAAQS in other states. The commenter's position was that EPA 
cannot approve a SIP submission based upon ``unlikelihood'' because CAA 
Section 110(a)(2)(D)(i)(I) prohibits emissions that contribute 
significantly to nonattainment in other States and does not allow EPA 
to approve SIPs simply because a state's emissions are ``unlikely'' to 
contribute significantly to nonattainment.
    EPA Response--EPA disagrees with the commenter's characterization 
of EPA's analysis and the commenter's interpretation of the statutory 
requirements. First, EPA notes that the discussion in the proposal was 
intended to present the various factual and technical considerations 
available to assess whether there is or is not significant contribution 
to nonattainment in other states as a result of emissions from Colorado 
sources. Given that these assessments are not individually definitive 
or outcome determinative, EPA believes that it is entirely appropriate 
to present and describe the relative probative value of the various 
considerations accurately. Second, EPA notes that all such technical 
evaluations are by their nature subject to some degree of uncertainty. 
Indeed, the modeling that the commenter elsewhere contends should be 
the sole method for evaluating interstate transport is itself but one 
means of evaluating the real world impacts of emissions in light of 
meteorological conditions, wind direction, and other such variables and 
produces a result that is itself subject to some degree of uncertainty. 
Third, EPA believes that it was also appropriate to describe the 
various factual and technical considerations and whether they indicated 
a ``likelihood'' of significant contribution to nonattainment in 
another state because the proposal was seeking comment from the public 
upon whether these considerations together supported a determination of 
no such significant contribution. EPA did not receive comments that 
persuade the Agency that there is such significant contribution, and 
thus in today's final action EPA is making a final regulatory 
determination that Colorado emissions sources do not significantly 
contribute to violations of the 1997 8-hour ozone NAAQS in any other 
state, for the reasons explained elsewhere in this notice. In other 
words, EPA has concluded that the existing SIP for Colorado already 
contains adequate

[[Page 31308]]

provisions to prevent emissions from Colorado sources from 
significantly contributing to violations of the 1997 8-hour ozone NAAQS 
in other states and is therefore approving Colorado's submission for 
this purpose.
    Comment No. 2--The commenter argued that Colorado and EPA did not 
appropriately assess impacts to nonattainment in downwind states. 
According to the commenter, Colorado failed to assess significance of 
downwind impacts in accordance with EPA guidance and precedent. 
Although this is unclear from the comment, the commenter evidently 
believes that EPA's applicable guidance for this purpose appears only 
in the 1998 NOX SIP call. The commenter asserts that, based 
on the precedent of the NOX SIP Call, the following issues 
need to be addressed in determining whether or not an area is 
significantly contributing to nonattainment in downwind States: (a) The 
overall nature of the ozone problem; (b) the extent of downwind 
nonattainment problems to which upwind State's emissions are linked; 
(c) the ambient impact of the emissions from upwind States' sources on 
the downwind nonattainment problems; and (d) the availability of high 
cost-effective control measures for upwind emissions. (63 FR 57356-
57376, October 27, 1998).
    EPA Response--EPA disagrees with the commenter on this point. 
Section 110(a)(2)(D) does not explicitly specify how states or EPA 
should evaluate the existence of, or extent of, interstate transport 
and whether that interstate transport is of sufficient magnitude to 
constitute ``significant contribution to nonattainment'' as a 
regulatory matter. The statutory language is ambiguous on its face and 
EPA must reasonably interpret that language when it applies it to 
factual situations before the Agency.
    EPA agrees that the NOX SIP Call is one rulemaking in 
which EPA evaluated the existence of, and extent of, interstate 
transport. In that action, EPA developed an approach that allowed the 
Agency to evaluate whether there was significant contribution to ozone 
nonattainment across an entire region that was comprised of many 
states. That approach included regional scale modeling and other 
technical analyses that EPA deemed useful to evaluate the issue of 
interstate transport on that geographic scale and for the facts and 
circumstances at issue in that rulemaking. EPA does not agree, however, 
that the approach of the NOX SIP Call is necessarily the 
only way that states or EPA may evaluate the existence of, and extent 
of, interstate transport in all situations, and especially in 
situations where the state and EPA are evaluating the question on a 
state by state basis, and in situations where there is not evidence of 
widespread interstate transport.
    Indeed, EPA issued specific guidance making recommendations to 
states about how to address section 110(a)(2)(D) in SIP submissions for 
the 8-hour ozone NAAQS. EPA issued this guidance document, entitled 
``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 PM2.5 National Ambient Air Quality 
Standards'' on August 15, 2006.\2\ This guidance document postdated the 
NOX SIP Call, and was developed by EPA specifically to 
address SIP submissions for the 1997 8-hour ozone NAAQS.
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    \2\ Memorandum from William T. Harnett entitled 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 PM2.5 National Ambient Air Quality Standards 
(Aug. 15, 2006) (``2006 Guidance''); p. 3. An electronic copy is 
available for review at the regulations.gov web site as Document ID 
No. EPA-R08-OAR-2007-1032.0004.1.
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    Within that 2006 guidance document, EPA notes that it explicitly 
stated its view that the ``precise nature and contents of such a 
submission [are] not stipulated in the statute'' and that the contents 
of the SIP submission ``may vary depending upon the facts and 
circumstances related to the specific NAAQS.'' \3\ Moreover, within 
that guidance, EPA expressed its view that ``the data and analytical 
tools available'' at the time of the SIP submission ``necessarily 
affect[] the content of the required submission.'' \4\ To that end, EPA 
specifically recommended that states located within the geographic 
region covered by the ``Clean Air Interstate Rule'' (CAIR) \5\ comply 
with section 110(a)(2)(D) for the 1997 8-hour ozone NAAQS by complying 
with CAIR itself. For states outside the CAIR rule region, however, EPA 
recommended that states develop their SIP submissions for section 
110(a)(2)(D) considering relevant information.
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    \3\ Id. at 3.
    \4\ Id.
    \5\ In this action the expression ``CAIR'' refers to the final 
rule published in the May 12, 2005 Federal Register and entitled 
``Rule to Reduce Interstate Transport of Fine Particulate Matter and 
Ozone (Clean Air Interstate Rule); Revisions to Acid Rain Program; 
Revisions to NOX SIP Call; Final Rule'' (70 FR 25162).
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    EPA explicitly recommended that relevant information for section 
110(a)(2)(D) submissions addressing significant contribution to 
nonattainment ``might include, but is not limited to, information 
concerning emissions in the State, meteorological conditions in the 
State, the distance to the nearest nonattainment area in another State, 
reliance on modeling conducted by EPA in determining that such State 
should not be included within the ambit of the CAIR, or such other 
information as the State considers probative on the issue of 
significant contribution.'' \6\ In addition, EPA recommended that 
states might elect to evaluate significant contribution to 
nonattainment using relevant considerations comparable to those used by 
EPA in CAIR, including evaluating impacts as of an appropriate year 
(such as 2010) and in light of the cost of control to mitigate 
emissions that resulted in interstate transport.
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    \6\ Id. at 5.
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    The commenter did not acknowledge or discuss EPA's actual guidance 
for section 110(a)(2)(D) SIP submissions for the 1997 8-hour ozone 
NAAQS, and thus it is unclear whether the commenter was aware of it. In 
any event, EPA believes that the Colorado submission and EPA's 
evaluation of it was consistent with EPA's guidance for the 1997 8-hour 
ozone NAAQS. For example, as discussed in the proposal notice, the 
state and EPA considered information such as monitoring data in 
Colorado and downwind states, geographical and meteorological 
information, and technical studies of the nature and sources of 
nonattainment problems in various downwind states. These are among the 
types of information that EPA recommended and that EPA considers 
relevant. Thus, EPA has concluded that the state's submission, and 
EPA's evaluation of that submission, meet the requirements of section 
110(a)(2)(D) and are consistent with applicable guidance.
    Finally, EPA notes that the considerations the Agency recommended 
to states in the 2006 guidance document are consistent with the 
concepts that the commenter enumerated from the NOX SIP Call 
context: (a) The overall nature of the ozone problem; (b) the extent of 
downwind nonattainment problems to which upwind State's emissions are 
linked; (c) the ambient impact of the emissions from upwind States' 
sources on the downwind nonattainment problems; and (d) the 
availability of high cost-effective control measures for upwind 
emissions. The only distinction in the case of the Colorado submission 
at issue here would be that because the available evidence indicates 
that there is very little contribution from emissions from Colorado 
sources to nonattainment in other states, it is not necessary to

[[Page 31309]]

advance to the final step and evaluate whether the cost of controls for 
those sources is above or below a certain cost of control as part of 
determining whether the contribution constitutes ``significant 
contribution to nonattainment'' for regulatory purposes, as was 
necessary in the NOX SIP Call and in CAIR.
    Comment No. 3--The commenter argued that Colorado based its claim 
of no significant contribution ``primarily on attainment plan modeling 
for the Denver Metropolitan Area/North Front Range (DMA/NFR) 
nonattainment area'' and noted that EPA itself ``does not accept'' that 
modeling for purposes of assessing impacts on nonattainment in downwind 
States.
    EPA Response--EPA disagrees with the commenter's characterization 
of the state's submission and of EPA's evaluation of it. This comment 
reflects an incomplete reading of EPA's evaluation of how the results 
of Colorado's modeling analysis for the DMA/NFR relate to an assessment 
of whether emissions from Colorado sources contribute significantly to 
downwind nonattainment of the 1997 8-hour ozone NAAQS in other states.
    It is correct that the State relied upon this information in its 
submission to EPA. It is correct that EPA did not agree with Colorado's 
view that the modeling analysis results for the DMA/NFR attainment 
plan, in and of themselves, prove that there could be no significant 
contribution from Colorado sources to downwind ozone nonattainment in 
other states. EPA explicitly disagreed with the state's belief that: `` 
* * * these results [of the DMA/NFR modeling analysis] demonstrate that 
the magnitude of ozone transport from Colorado to other States is too 
low to significantly contribute to nonattainment. * * *.''
    Nevertheless, EPA did agree that these modeling results were a 
relevant piece of information that could be useful when considered in 
conjunction with other information. EPA stated that these modeling 
results do support the conclusion that there is not significant 
transport of ozone from Colorado to other states with violations of the 
NAAQS: `` * * * [h]owever, as a reflection of emission levels, the 
relatively (to the 1997 8-hour ozone NAAQS) moderate concentrations in 
eastern Colorado * * * somewhat reduce the probability of significant 
contribution from Colorado emission sources to considerably farther 
downwind nonattainment areas such as St. Louis, Missouri, and Chicago, 
Illinois.'' (See 75 FR 16034-35). The commenter suggests that EPA 
approved the State's submission based wholly upon technical support 
that EPA itself rejected and this is incorrect.
    Comment No. 4--The commenter reiterated its concern that the 
Colorado section 110(a)(2)(D) submission was deficient because it did 
not strictly follow the commenter's summary of the structure of the 
analysis of interstate transport in the NOX SIP Call: (a) 
The overall nature of the ozone problem; (b) the extent of downwind 
nonattainment problems to which upwind State's emissions are linked; 
(c) the ambient impact of the emissions from upwind States' sources on 
the downwind nonattainment problems; and (d) the availability of high 
cost-effective control measures for upwind emissions.
    EPA Response--EPA disagrees with the commenter's view that any 
analysis of interstate transport must follow a specific formulaic 
structure to be approvable. As noted above, EPA issued specific 
guidance to states making recommendations for section 110(a)(2)(D) SIP 
submissions for the 1997 8-hour ozone NAAQS. Within that guidance, EPA 
recommended various types of information that states might wish to 
consider in the process of evaluating whether their sources contributed 
significantly to nonattainment in other states. EPA has concluded that 
the submission from Colorado, augmented by EPA's own analysis, 
sufficiently establishes that Colorado sources do not significantly 
contribute to violations of the 1997 8-hour ozone NAAQS in other 
states. As noted above, EPA believes that the state's submission, and 
EPA's analysis of it, address the same conceptual considerations that 
the commenter advocated.
    Comment No. 5--The commenter asserted that Colorado and EPA 
provided ``no analysis'' of the contribution from Colorado to downwind 
states and no ``actual assessment'' of the significance of any such 
contribution.
    EPA Response--EPA disagrees with the commenter's position. The 
commenter again assumes that section 110(a)(2)(D) explicitly requires 
the type of modeling analysis that the commenter advocates throughout 
its comments. Because the commenter apparently views the NOX 
SIP Call as the applicable guidance, the commenter contends that any 
analytical approach that is not identical to that approach is 
impermissible. In addition, the commenter overlooks the fact that in 
other actions based upon section 110(a)(2)(D), EPA has also used a 
variety of analytical approaches, short of modeling, to evaluate 
whether specific states are significantly contributing to violations of 
the NAAQS in another state (e.g., the west coast states that EPA 
concluded should not be part of the geographic region of the CAIR rule 
based upon qualitative factors, and not by the zero out modeling EPA 
deemed necessary for some other States).
    In the proposed approval, EPA explained that other forms of 
available information were sufficient to make the determination that 
there is no significant contribution from Colorado sources to downwind 
nonattainment of the 1997 8-hour ozone NAAQS. As stated in the 
proposal:

    ``EPA's evaluation of whether emissions from Colorado contribute 
significantly to ozone nonattainment in these areas [St. Louis and 
Chicago] relies on an examination of a variety of data and analysis 
that provide insight on ozone transport from Colorado to these two 
areas. Because EPA does not have detailed modeling for Colorado and 
nearby downwind states, our approach does not rely on a quantitative 
determination of Colorado's contribution, as EPA did for other 
states in its CAIR rulemaking, but on a weight-of-evidence analysis 
based on qualitative assessments and estimates of the relevant 
factors. While conclusions reached for each of the factors 
considered in the following analysis are not in and by themselves 
determinative, consideration of all of these factors provides a 
reliable qualitative conclusion on whether Colorado's emissions are 
likely to contribute significantly to nonattainment in the St. Louis 
and the Illinois/Wisconsin areas.''

    EPA acknowledged that the various forms of information considered 
in the proposal (such as distance, orientation of surface and regional 
transport winds, back trajectory analyses, monitoring data) were not 
individually outcome determinative, but concluded that when taken 
together served to establish that Colorado sources do not significantly 
contribute to downwind nonattainment of the 1997 8-hour ozone NAAQS in 
other states. Thus, contrary to the commenter's assertion, EPA did 
perform an ``analysis'' and an ``assessment'' that was a reasonable 
basis for its conclusion that emissions from Colorado do not contribute 
significantly to downwind ozone nonattainment, using a combination of 
quantitative data and qualitative analyses. EPA does not agree that 
only the type of analysis advocated by the commenter could adequately 
evaluate the issue and support a rational determination in this 
instance.
    Comment No. 6--The commenter objected to EPA's proposed approval 
because Colorado assessed impacts in downwind states by considering 
monitoring data in those states as a means of evaluating significant 
contribution to nonattainment. In other

[[Page 31310]]

words, the commenter is concerned that Colorado did not assess impacts 
in areas that have no monitor. The commenter likewise objected to EPA's 
``endorsement'' of this approach. The commenter argued that this 
reliance on monitor data is inconsistent with both section 110(a)(2)(D) 
and with EPA's guidance, by which the commenter evidently means the 
NOX SIP Call. In support of this assertion, the commenter 
quoted from the NOX SIP Call proposal in which EPA addressed 
the proper interpretation of the statutory phrase ``contribute 
significantly to nonattainment:''

    ``The EPA proposes to interpret this term to refer to air 
quality and not to be limited to currently-designated nonattainment 
areas. Section 110(a)(2)(D) does not refer to `nonattainment areas,' 
which is a phrase that EPA interprets to refer to areas that are 
designated nonattainment under section 107(section 
107(d)(1)(A)(I))''

    According to the commenter, this statement, and similar ones in the 
context of the final NOX SIP Call rulemaking, establish that 
states and EPA cannot utilize monitoring data to evaluate the existence 
of, and extent of, interstate transport. Furthermore, the commenter 
interprets the reference to ``air quality'' in these statements to 
support its contention, amplified in later comments, that EPA must 
evaluate significant contribution in areas in which there is no 
monitored nonattainment.
    EPA Response--EPA disagrees with the commenter's arguments. First, 
the commenter misunderstands the point that EPA was making in the 
quoted statement from the NOX SIP Call proposal (and that 
EPA has subsequently made in the context of CAIR). When EPA stated that 
it would evaluate impacts on air quality in downwind states, 
independent of the current formal ``designation'' of such downwind 
states, it was not referring to air quality in the absence of monitor 
data. EPA's point was that it was inappropriate to wait for either 
initial designations of nonattainment for a new NAAQS under section 
107(d)(1), or for a redesignation to nonattainment for an existing 
NAAQS under section 107(d)(3), before EPA could assess whether there is 
significant contribution to nonattainment of a NAAQS in another state.
    For example, in the case of initial designations, section 107(d) 
contemplates a process and timeline for initial designations that could 
well extend for two or three years following the promulgation of a new 
or revised NAAQS. By contrast, section 110(a)(1) requires states to 
make SIP submissions that address section 110(a)(2)(D) and interstate 
transport ``within 3 years or such shorter period as the Administrator 
may prescribe'' of EPA's promulgation of a new or revised NAAQS. This 
schedule does not support a reading of section 110(a)(2)(D) that is 
dependent upon formal designations having occurred first. This is a key 
reason why EPA determined that it was appropriate to evaluate 
interstate transport based upon monitor data, not designation status, 
in the CAIR rulemaking.
    The commenter's misunderstanding of EPA's statement concerning 
designation status evidently caused the commenter to believe that EPA's 
assessment of interstate transport in the NOX SIP Call was 
not limited to evaluation of downwind areas with monitors. This is 
simply incorrect. In both the NOX SIP Call and CAIR, EPA 
evaluated significant contribution to nonattainment as measured or 
predicted at monitors. For example, in the technical analysis for the 
NOX SIP Call, EPA specifically evaluated the impacts of 
emissions from upwind states on monitors located in downwind states. 
The NOX SIP Call did not evaluate impacts at points without 
monitors, nor did the CAIR rulemaking. EPA believes that this approach 
to evaluating significant contribution is correct under section 
110(a)(2)(D), and EPA's general approach to this threshold 
determination has not been disturbed by the courts.\7\
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    \7\ Michigan v. U.S. EPA, 213 F.3d 663, 674-681 (D.C. Cir. 
2000); North Carolina v. EPA, 531 F.3d 896, 913-916 (D.C. Cir. 2008) 
(upholding EPA approach to determining threshold despite remanding 
other aspects of CAIR).
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    Finally, EPA disagrees with the commenter's argument that the 
assessment of significant contribution to downwind nonattainment must 
include evaluation of impacts on non-monitored areas. First, neither 
section 110(a)(2)(D)(i)(I) provisions, nor the EPA guidance issued for 
the 1997 8-hour ozone NAAQS on August 15, 2006, support the commenter's 
position, as neither refers to any explicit mandatory or recommended 
approach to assess air quality in non-monitored areas.\8\ The same 
focus on monitored data as a means of assessing interstate transport is 
found in the NOX SIP Call and in CAIR. An initial step in 
both the NOX SIP Call and CAIR was the identification of 
areas with current monitored violations of the ozone and/or 
PM2.5 NAAQS.\9\ The subsequent modeling analyses for NAAQS 
violations in future years (2007 for the SIP Call and 2010 for CAIR) 
likewise evaluated future violations at monitors in areas identified in 
the initial step. Thus, the commenter is simply in error that EPA has 
not previously evaluated the presence and extent of interstate 
transport under section 110(a)(2)(D) by focusing on monitoring data. 
Indeed, such monitoring data was at the core of both of these efforts. 
In neither of these rulemakings did EPA evaluate significant 
contribution to nonattainment in areas in which there was no monitor. 
This is reasonable and appropriate, because data from a properly placed 
Federal reference method monitor is the way in which EPA ascertains 
that there is a violation of the 1997 8-hour ozone NAAQS in a 
particular area. Put another way, in order for there to be significant 
contribution to nonattainment for the 1997 8-hour ozone NAAQS, there 
must be a monitor with data showing a violation of that NAAQS. EPA has 
concluded that by considering data from monitored areas, its assessment 
of whether emissions from Colorado contribute significantly to ozone 
nonattainment in downwind States is consistent with the 2006 Guidance, 
and with the approach used by both the CAIR rule and the NOX 
SIP Call.
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    \8\ 2006 Guidance, p. 5.
    \9\ ``Based on this approach, we predicted that in the absence 
of additional control measures, 47 counties with air quality 
monitors [emphasis ours] would violate the 8-hour ozone NAAQS in 
2010 * * *.'' From the CAIR proposed rule of January 30, 2004 (69 FR 
4566, 4581). The NOX SIP call proposed rule action reads: 
``* * * For current nonattainment areas, EPA used air quality data 
for the period 1993 through 1995 to determine which counties are 
violating the 1-hour and/or 8-hour NAAQS. These are the most recent 
3 years of fully quality assured data which were available in time 
for this assessment,'' 62 FR 60336.
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    Comment No. 7--In support of its comments that EPA should assess 
significant contribution to nonattainment in nonmonitored areas, the 
commenter argued that existing modeling performed by another 
organization ``indicates that large areas of neighboring states will be 
likely to violate the ozone NAAQS.'' According to the commenter, these 
likely ```violations'' of the ozone NAAQS were predicted for the year 
2018, as reflected in a slide from a July 30, 2008 presentation before 
the Western Regional Air Partnership (``Review of Ozone Performance in 
WRAP Modeling and Relevant to Future Regional Ozone Planning''). The 
commenter asserted that: ``Slide 28 of this presentation displays 
projected 4th highest 8-hour ozone reading for 2018 and indicates that 
air quality in areas such as northern New Mexico, western Wyoming, 
southern Utah, and central Arizona will exceed and/or violate the 1997 
ozone

[[Page 31311]]

NAAQS * * *.'' \10\ In short, the commenter argues that modeling 
performed by the WRAP establishes that there will be violations of the 
1997 8-hour ozone NAAQS in 2018 in non-monitored areas of states 
adjacent to Colorado.
---------------------------------------------------------------------------

    \10\ The presentation is available for review as Document ID 
 EPA-R08-OAR-2007-1032-0007.8 at Regulations.gov, Docket ID 
 EPA-R08-OAR-2007-1032.
---------------------------------------------------------------------------

    EPA Response--EPA disagrees with this comment on several grounds. 
First, as explained in response to other comments, EPA does not agree 
that it is appropriate to evaluate significant contribution to 
nonattainment for the 1997 8-hour ozone NAAQS by modeling ambient 
levels in areas where there is no monitor to provide data to establish 
a violation of the NAAQS in question. Section 110(a)(2)(D) does not 
require such an approach, EPA has not taken this approach in the 
NOX SIP Call or other rulemakings under section 
110(a)(2)(D), and EPA's prior analytical approach has not been 
disturbed by the courts.
    Second, the commenter's own description of the ozone concentrations 
predicted for the year 2018 as projecting ``violations'' of the ozone 
NAAQS is inaccurate. Within the same sentence, quoted above, slide 28 
is described as displaying the projected 4th max ozone reading for the 
year 2018, and as indicating that ``* * * air quality * * * will exceed 
or violate [emphasis ours] the 1997 ozone NAAQS.'' By definition, a one 
year value of the 4th max above the NAAQS only constitutes an 
exceedance of the NAAQS; to constitute a violation of the 1997 8-hour 
ozone NAAQS, the standard must be exceeded for three consecutive years 
at the same monitor. Thus, even if the WRAP presentation submitted by 
the commenter were technically sound, the conclusion drawn from it by 
the commenter is inaccurate and does not support its claim of projected 
violations of the NAAQS in large areas (monitored or unmonitored) of 
Colorado's neighboring States.
    Finally, EPA has reviewed the WRAP presentation submitted by the 
commenter, and believes that there was a substantial error in the WRAP 
modeling software that led to overestimation of ground level ozone 
concentrations. A recent study conducted by Environ for the Four 
Corners Air Quality Task Force (FCAQTF; Stoeckenius et al., 2009) has 
demonstrated that excessive vertical transport in the CMAQ and CAMx 
models over high terrain was responsible for overestimated ground level 
ozone concentrations due to downward transport of stratospheric 
ozone.\11\ Environ has developed revised vertical velocity algorithms 
in a new version of CAMx that eliminated the excessive downward 
transport of ozone from the top layers of the model. This revised 
version of the model is now being used in a number of applications 
throughout high terrain areas in the West. In conclusion, EPA believes 
that this key inadequacy of the WRAP model, noted above, makes it 
inappropriate support for the commenter's concerns about large expanses 
of 8-hour ozone nonattainment areas projected for 2018 in areas without 
monitors.
---------------------------------------------------------------------------

    \11\ Stoeckenius, T.E., C.A. Emery, T.P. Shah, J.R. Johnson, 
L.K. Parker, A.K. Pollack, 2009. ``Air Quality Modeling Study for 
the Four Corners Region,'' pp. ES-3, ES-4, 3-4, 3-12, 3-30, 5-1. 
Prepared for the New Mexico Environment Department, Air Quality 
Bureau, Santa Fe, NM, by ENVIRON International Corporation, Novato, 
CA.
---------------------------------------------------------------------------

    Comment No. 8--As additional support for its assertion that EPA 
should require modeling to assess ambient levels in unmonitored 
portions of other states, the commenter relied on an additional study 
entitled the ``Uinta Basin Air Quality Study (UBAQS).'' The commenter 
argued that UBAQS further supports its concern that Colorado and EPA, 
having limited the evaluation of downwind impacts only to areas with 
monitors, failed to assess ozone nonattainment in non-monitored areas. 
According to the commenter, UBAQS modeling results show that: (a) The 
Wasatch front region is currently exceeding and will exceed in 2012 the 
1997 8-hour ozone NAAQS; and (b) based on 2005 meteorological data, 
portions of the four counties in the southwest corner of Utah are also 
currently in nonattainment and will be in nonattainment in 2012.\12\
---------------------------------------------------------------------------

    \12\ The southwestern area referred to by the commenter includes 
portions of Washington, Iron, Kane, and Garfield Counties.
---------------------------------------------------------------------------

    EPA Response--As noted above, EPA does not agree that it is 
appropriate to assess significant contribution to nonattainment for the 
1997 8-hour ozone NAAQS in the way advocated by the commenter. Even 
taking the UBAQS modeling results at face value, however, EPA does not 
agree that the 8-hour ozone nonattainment (current and projected) in 
the Wasatch Front Range area supports the commenter's concerns about 
the need to evaluate the possibility of significant contribution to 
nonattainment in non-monitored areas. EPA sees several problems with 
the commenter's interpretation of the UBAQS analysis results for 
counties in Utah's southwestern corner: ``based on 2005 meteorological 
data, portions of Washington, Iron, Kane, and Garfield Counties are 
also in nonattainment and will be in nonattainment in 2012.''
    First, the commenter's interpretation of the predicted ozone 
concentrations shown in Figures 4-3a and 4-3b (pages 5 and 6 of the 
comment letter) is inaccurate. A close review of the legend in these 
figures indicates that the highest ozone concentrations predicted by 
the model for portions of the counties noted above are somewhere 
between 81.00 and 85.99 ppb, but it is not specified. If it is actually 
predicted smaller than or equal to 84.9 ppb then the area is attaining 
the 1997 8-hour ozone NAAQS, if it is predicted as greater than 84.9 
ppb then it is not attaining those NAAQS. Thus, the current and 
predicted design values for the southwestern Utah area identified in 
Figures 4-3a and 4-3b could both be in attainment or both in 
nonattainment, or one of them in attainment and the other in 
nonattainment, for the 1997 8-hour ozone NAAQS. EPA does not believe 
that this evidence adequately establishes that one or both areas 
definitely violate the NAAQS, even if the information were taken at 
face value.
    Second, even if the design values predicted for these unmonitored 
areas were at the top of the 81.00-85.99 ppb range, their reliability 
would remain questionable. The UBAQS itself identifies and illustrates 
major shortcomings of its modeling analysis, only to neglect assessing 
the impact of these shortcomings on the modeling results.\13\ The study 
deviates in at least two significant ways from EPA's 2007 guidance on 
SIP modeling.\14\ One issue is the UBAQS modeling reliance on fewer 
than the five years of data recommended by EPA to generate a current 8-
hour ozone design value (DVC). UBAQS relaxed this requirement so that 
sites with as little as 1 year of data were included as DVCs in the 
analysis. The other issue is the computation of the relative responsive 
factor (RRF), which directly affects the modeling's future design value 
(DVF).\15\ Again due to unavailability of data satisfying EPA's 
recommendation that the RRF be based on a minimum of five days of ozone 
concentrations above 85

[[Page 31312]]

ppb, UBAQS modeling uses RRFs based on one or more days of ozone 
concentrations above 70 ppb.\16\ EPA concludes that the modeling 
analysis results used by the WG are unreliable for projecting non-
attainment status and therefore do not support its comments.
---------------------------------------------------------------------------

    \13\ See UBAQS, pp. 4-27 to 4-29.
    \14\ EPA, Guidance on the Use of Models and other Analyses for 
Demonstrating Attainment of Air Quality Goals for Ozone, PM2.5 and 
Regional Haze. Office of Air Quality Planning and Standards, Air 
Modeling Group. Research Triangle Park, North Carolina (2007), 
available at https://www.epa.gov/scram001/guidance/guide/final-03-pm-rh-guidance.pdf.
    \15\ DVC x RRF = DVF.
    \16\ See UBAQS, p. 4-28
---------------------------------------------------------------------------

    Comment No. 9--In support of its arguments that EPA should not 
assess significant contribution to nonattainment through evaluation of 
impacts at monitors instead of modeling impacts where there is no such 
monitor, the commenter cited a past statement by EPA to the effect that 
the monitor network in the western United States needs to be expanded. 
The quoted statements included EPA's observation that ``[v]irtually all 
States east of the Mississippi River have at least two to four non-
urban O3 monitors, while many large mid-western and western 
States have one or no non-urban monitors.'' 74 FR 34525 (July 16, 
2009). From this statement, the commenter argues that it is not 
appropriate for EPA to limit evaluation of significant contribution to 
nonattainment of the ozone NAAQS in other states relying on monitoring 
data instead of modeling ambient levels.
    EPA Response--EPA does not disagree that there are relatively few 
monitors in the western states, and that relatively few monitors are 
currently located in non-urban areas of western states. However, the 
commenter failed to note that the quoted statement from EPA concerning 
the adequacy of western monitors came from the Agency's July 16, 2009, 
proposed rulemaking entitled ``Ambient Ozone Monitoring Regulations: 
Revisions to Network Design Requirements.'' This statement was thus 
taken out of context, because EPA was in that proposal referring to 
changes in state monitoring networks that it anticipates will be 
necessary in order to implement not the 1997 8-hour ozone NAAQS that 
are the subject of this rulemaking, but rather the next iteration of 
the ozone NAAQS for which there are concerns that there will be a need 
to evaluate ambient levels in previously unmonitored areas of the 
western United States. The fact that additional monitors may be 
necessary in the future for newer ozone NAAQS does not automatically 
mean that the existing ozone monitoring networks are insufficient for 
the 1997 8-hour ozone NAAQS, as the commenter implies. Indeed, states 
submit annual monitor network reports to EPA and EPA evaluates these to 
insure that they meet the applicable requirements.
    For example, Colorado itself submits just such a report on an 
annual basis, and EPA reviews it for adequacy.\17\ All other states 
submit comparable reports. Absent a specific concern that another 
state's current monitor network is inadequate to evaluate ambient 
levels of the 1997 8-hour ozone NAAQS, EPA has no reason to believe 
that the evaluation of possible significant contribution from Colorado 
sources in reliance on those monitors is incorrect.
---------------------------------------------------------------------------

    \17\ See, for example, ''Colorado Annual Monitoring Network 
Plan'' dated 2009-2010. Plan is available for review at the 
regulations.gov Web site under Docket ID No. EPA-R08-OAR-2007-1032.
---------------------------------------------------------------------------

    Comment No. 10--The commenter objected to EPA's proposed approval 
of the Colorado SIP submission because neither Colorado nor EPA 
performed a specific modeling analysis to assure that emissions from 
Colorado sources do not significantly contribute to nonattainment in 
downwind States. According to the commenter, EPA's decision to use a 
qualitative approach to determine whether emissions from Colorado 
contribute significantly to downwind nonattainment is not consistent 
with its own preparation of a regional model to evaluate such impacts 
from other states as part of CAIR.
    EPA Response--EPA disagrees with the commenter's belief that only 
modeling can establish whether or not there is significant contribution 
from one state to another. First, as noted above, EPA does not believe 
that section 110(a)(2)(D) requires modeling. While modeling can be 
useful, EPA believes that other forms of analysis can be sufficient to 
evaluate whether or not there is significant contribution to 
nonattainment. For this reason, EPA's 2006 guidance recommended other 
forms of information that states might wish to evaluate as part of 
their section 110(a)(2)(D) submissions for the 1997 8-hour ozone NAAQS. 
EPA has concluded that its qualitative approach to the assessment of 
significant contribution to downwind ozone nonattainment is consistent 
with EPA's 2006 Guidance.
    Second, EPA notes that the commenter's position also reflects a 
misunderstanding of the approach EPA used in the remanded CAIR due to 
an exclusive focus on those States that were selected for the modeling 
analysis. A wider understanding of the CAIR approach would recognize 
that EPA decided, based on other criteria, that it was not necessary to 
conduct modeling for certain western states: ``[i]n analyzing 
significant contribution to nonattainment, we determined it was 
reasonable to exclude the Western U.S., including the States of 
Washington, Idaho, Oregon, California, Nevada, Utah, and Arizona from 
further analysis due to geography, meteorology, and topography. Based 
on these factors we concluded that the PM2.5 and 8-hour 
ozone nonattainment problems are not likely to be affected 
significantly by pollution transported across these States' boundaries 
* * *.'' (69 FR 4581, January 30, 2004).
    EPA has taken a similar approach to assess whether Colorado 
contributes significantly to violations of the 1997 8-hour ozone NAAQS 
in downwind states. In the proposed action, EPA explained several forms 
of substantive and technically valid evidence that led to the 
conclusion that emissions from the Colorado sources do not contribute 
significantly to nonattainment, in accordance with the requirement of 
Section 110(a)(2)(D).
    Comment No. 11--In further support of its argument that EPA must 
use modeling to evaluate whether there is significant contribution to 
nonattainment under section 110(a)(2)(D), the commenter noted that EPA 
itself asks other agencies to perform such modeling in other contexts. 
As examples, the commenter cited four examples in which EPA commented 
on actions by other agencies in which EPA recommended the use of 
modeling analysis to assess ozone impacts prior to authorizing oil and 
gas development projects. As supporting material, the comment includes 
quotations from and references to EPA letters to Federal Agencies on 
assessing impacts of oil and gas development projects.\18\ The 
commenter questioned why EPA's recommendation for such an approach in 
its comments to other Federal Agencies, did not result in its use of 
the same approach to evaluate the impacts from Colorado emissions and 
to insure compliance with Section 110(a)(2)(D)(i)(I). The commenter 
reasoned that the emissions that would result from the actions at issue 
in the other agency decisions, such as selected oil and gas drilling 
projects, would be of less magnitude and importance that the statewide 
emissions at issue in an evaluation under section 110(a)(2)(D).
---------------------------------------------------------------------------

    \18\ WG's April 9, 2010 comment letter, pp. 9-10. Complete 
versions of the EPA comment letters referenced here were attached to 
the comment as Exhibits 3 through 6, and are viewable on the 
Regulations.gov Web site as Documents ID No. EPA-R08-OAR-2007-1032-
0007.4 through 1032-0007.7.
---------------------------------------------------------------------------

    EPA Response--As explained above, EPA disagrees with the 
commenter's fundamental argument that modeling is mandatory in all 
instances in order to evaluate significant contribution to 
nonattainment, whether by section

[[Page 31313]]

110(a)(2)(D), by EPA guidance, or by past EPA precedent. EPA's 
applicable guidance made recommendations as to different approaches 
that could lead to demonstration of the satisfaction of the interstate 
transport requirements for significant contribution to nonattainment in 
other states. Even EPA's own CAIR analysis relied on a combination of 
qualitative and quantitative analyses, as explained above. EPA's CAIR 
analysis excluded certain western states on the basis of a qualitative 
assessment of topography, geography, and meteorology.\19\
---------------------------------------------------------------------------

    \19\ See 69 FR 4581, January 30, 2004.
---------------------------------------------------------------------------

    EPA believes that the commenter's references to EPA statements 
commenting on the actions of other agencies are inapposite. As the 
commenter is aware, those comments were made in the context of the 
evaluation of the impacts of various Federal actions pursuant to NEPA, 
not the Clean Air Act. As explained above, in the context of section 
110(a)(2)(D), EPA does not agree that modeling is always required to 
make that different evaluation, and EPA itself has relied on other more 
qualitative evidence when it deemed that evidence sufficient to reach a 
reasoned determination.
    Comment No. 12--In further support of its argument that EPA should 
always require modeling to evaluate significant contribution to 
nonattainment, the commenter referred to EPA regulations governing 
nonattainment SIPs. The commenter noted 40 CFR 51.112(a)(1), which 
states that: ``[t]he adequacy of a control strategy shall be 
demonstrated by means of applicable air quality models, data bases, and 
other requirements specified in appendix W of [Part 51] (Guideline on 
Air Quality Models).'' The commenter argues that this regulation 
appears to support the commenter's position that modeling is required 
to satisfy the significant contribution element of 110(a)(2)(D).
    Response: EPA disagrees with this comment. The cited language 
implies that the need for control strategy requirements has already 
been demonstrated, and sets a modeling analysis requirement to 
demonstrate the adequacy of the control strategy developed to achieve 
the reductions necessary to prevent an area's air quality from 
continuing to violate the NAAQS. EPA's determination that emissions 
from Colorado do not contribute significantly to nonattainment for the 
1997 8-hour ozone standard in any other state eliminates the need for a 
control strategy aimed at satisfying the section 110(a)(2)(D) 
requirements. Moreover, EPA interprets the language at 40 CFR 
51.112(a): ``[e]ach plan must demonstrate that the measures, rules, and 
regulations contained in it are adequate to provide for the timely 
attainment and maintenance of the national standard that it 
implements,'' to refer to modeling for attainment demonstrations, an 
integral part of nonattainment area SIPs under part D of the CAA. This 
interpretation was upheld by the Sixth Circuit Court of Appeals. Wall 
v. U.S. EPA, 265 F.3d 426, 436 (6th Cir. 2001). Thus, the commenter's 
cited regulation is not relevant to EPA's technical demonstration 
assessing whether emissions from Colorado contribute significantly to 
nonattainment in any other states under section 110(a)(2)(D).
    Comment No. 13--The commenter also objected to EPA's proposed 
approval of the Colorado submission on the grounds that it was based 
upon a ``weight-of-evidence analysis,'' and that no such weight of 
evidence test appears in the CAA generally, or in section 110(a)(2)(D) 
in particular. According to the commenter, there is no regulatory 
support for using a ``weight-of-evidence'' approach to assessing air 
quality impacts. The commenter asserted that EPA neither cited nor 
quoted regulations or policy that provides for this, and failed to lend 
any specific meaning to the phrase through its proposed approval. 
Finally, the commenter asserted, without explaining, its belief that 
EPA failed to address ``several relevant factors related to the 
determination of whether Colorado contributes significantly to 
nonattainment undermines the agency's reliance on any `weight-of-
evidence' approach.''
    EPA Response--The fact that neither the CAA generally, nor section 
110(a)(2)(D) specifically, include the explicit phrase ``weight of 
evidence'' does not mean that it is inappropriate for EPA to use such 
an approach in this context. As explained above, section 110(a)(2)(D) 
does not explicitly stipulate how EPA is to assess whether there is a 
significant contribution to nonattainment in other states. The proper 
consideration, therefore, is whether EPA has a rational technical basis 
for its decision. Even if the term ``weight of evidence'' does not 
appear in section 110(a)(2)(D) or elsewhere in the CAA, courts have 
recognized EPA's reliance on such an analytical approach where 
reasonable.\20\ As described above, EPA's guidance issued for the 1997 
8-hour ozone NAAQS, the Agency specifically recommended types of 
information that states might wish to rely upon to evaluate the 
presence of, and extent of, instate transport for this purpose. EPA 
believes that a weight of evidence approach that properly considers 
appropriate evidence is sufficient to make a valid determination, as in 
this case.
---------------------------------------------------------------------------

    \20\ See, e.g., BCCA v. EPA, 355 F.3d 817 (5th Cir. 2003).
---------------------------------------------------------------------------

    Specifically, EPA's technical analysis in the March 31, 2010, 
proposed rule action underscores its reliance on implementation 
policies set in the EPA 2006 Guidance: ``EPA's August 15, 2006, 
guidance to states concerning section 110(a)(2)(D)(i) recommended 
various methods by which states might evaluate whether or not its 
emissions significantly contribute to violations of the 1997 ozone 
standards in another state. Among other methods, EPA recommended 
consideration of available EPA modeling conducted in conjunction with 
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 
[emphasis added].'' \21\ On the basis of this guidance, Colorado and 
EPA chose to assess the impacts of emissions from Colorado sources on 
the closest downwind nonattainment areas (St. Louis, Missouri, and 
Illinois/Wisconsin counties along the southwestern shore of Lake 
Michigan) through a weight of evidence approach using quantitative 
information such as Colorado's distance from areas with monitors 
showing violating the NAAQS, modeling results outlining wind vectors 
for regional transport of ozone on high ozone days, back trajectory 
analyses for the downwind nonattainment areas closest to Colorado, and 
results of modeling studies for the nonattainment areas specifying the 
range of wind directions along which contributing ozone transport 
occurred. EPA's use of a weight of evidence analysis is by no means 
unusual for the assessment of ozone impacts through long range 
transport. The same analytical framework was used in the 1998 
NOX SIP Call, as indicated under Section II.C., entitled 
``Weight-of-Evidence Determination of Covered States.'' \22\ The 
differences between the specific types of evidence used in the 
NOX SIP Call and

[[Page 31314]]

in EPA's analysis for this action do not invalidate the use of the 
weight-of-evidence approach.
---------------------------------------------------------------------------

    \21\ 75 FR 16034, March 31, 2010.
    \22\ ``As discussed above, EPA applied a multi-factor approach 
to identify the amounts of NOX emissions that contribute 
significantly to nonattainment * * *.'' 1998 SIP Call, 63 FR 57381, 
October 27, 1998.
---------------------------------------------------------------------------

    As for the commenter's argument that EPA ``fails to lend any 
specific meaning to the phrase through its proposed approval,'' the 
Agency's technical analysis described in the proposal did specify the 
characteristics, including limitations, of a weight of evidence 
analysis: ``[f]urthermore * * * EPA notes that no single piece of 
information in the following discussion is by itself dispositive of the 
issue. Instead, the total weight of all the evidence taken together 
supports the conclusion that emissions from Colorado sources are 
unlikely to contribute significantly to violations of the 1997 8-hour 
ozone standard in any other state.'' (75 FR 16034).
    Finally, as to the commenter's assertion that EPA failed to 
consider ``several relevant factors'' and thus failed to conduct an 
appropriate weight of evidence evaluation, EPA cannot weigh the 
validity of this comment in the absence of an explanation of what these 
factors might be.
    Comment No. 14--The commenter also objected to EPA's proposed 
approval of the Colorado submission on the grounds that EPA did not 
assess the potential impacts of Colorado sources of emissions on 
violations of the 1997 8-hour ozone NAAQS in Arizona (Phoenix area), 
and Utah (Davis County area.)
    EPA Response--EPA did not discuss or assess potential impacts of 
Colorado emissions on Arizona or Utah in the proposal. EPA first notes 
that, west of the Continental Divide the prevailing winds generally 
move from south-westerly or westerly directions, as indicated by the 
typical movement of weather systems.
    Also, EPA notes that Davis County had a monitor indicating a 
violation of the NAAQS in 2007, but has not since then. Thus, there are 
currently no monitors in Utah with data showing violations of the 1997 
8-hour ozone NAAQS and, as a consequence, there are no monitors for 
which it would be appropriate to evaluate the possibility of 
significant contribution to nonattainment from Colorado sources for the 
1997 8-hour ozone NAAQS. In Arizona, the Maricopa 8-hour ozone 
nonattainment area, which includes Phoenix, does have monitors 
indicating a violation of this NAAQS. However, Phoenix lies 
approximately 600 miles southwest of the Colorado DMA/NFR area, and 
this area is generally upwind from Colorado sources. Emissions from 
Colorado would have to be affected by strong winds from the northeast, 
which are very infrequent, in order to contribute significantly to 8-
hour ozone nonattainment in the Phoenix area. The rarity of 
northeasterly winds in Arizona may be gauged by images of wind roses 
for Phoenix and Tucson.\23\
---------------------------------------------------------------------------

    \23\ Reproductions of wind roses are available for review under 
Docket ID No. EPA-R08-OAR-2007-1032, and online at: https://home.pes.com/windroses/wrgifs/_6200.GIF; https://www.wrh.noaa.gov/twc/aviation/windrose_TUS.php; and https://www.wrcc.dri.edu/htmlfiles/westwinddir.html
---------------------------------------------------------------------------

    Comment No. 15--The commenter argued that both Colorado and EPA 
relied inappropriately on a flawed ozone ``nonattainment'' SIP for the 
DMA/NFR nonattainment area as a basis for the proposed approval. 
According to the commenter, EPA cannot approve Colorado's section 
110(a)(2)(d) submission because it relies heavily on the requirements 
of the ozone nonattainment area SIP for the DMA/NFR nonattainment area. 
The commenter argued that ``many'' of the provisions of the 
nonattainment area SIP are themselves flawed or deficient. As examples, 
the commenter outlined alleged deficiencies in the Colorado Air Quality 
Control Commission's Regulation No. 7, RACT requirements for 
NOX emissions, exemptions for certain source categories of 
NOX emissions, and other unspecified provisions in the DMA/
NFR nonattainment area SIP.
    EPA Response--EPA disagrees with the commenter's position that its 
proposed approval relied heavily on the nonattainment area SIP for the 
DMA/NFR area, and that as a consequence EPA cannot approve the Colorado 
section 110(a)(2)(D) submission for the significant contribution 
element for the 1997 8-hour ozone NAAQS. First, EPA notes that its 
reliance on material from, and related to, the ``8-Hour Ozone 
Attainment Plan'' was limited to considering the modeling results 
indicating a quick drop in ambient ozone levels from the DMA/NFR area 
to the easternmost Colorado counties. EPA did not purport to pass upon 
the adequacy or approvability of each and every aspect of that 
nonattainment area SIP by referring to the modeling results as a source 
of relevant facts to be taken into consideration.
    Second, the proposal made clear that EPA's interpretation of the 
significance of this information is different from Colorado's: ``EPA 
does not accept the State of Colorado Interstate Transport SIP 
assessment