explained below, we relied only on an IWAQM default value for BART
sources in eastern Wyoming.
---------------------------------------------------------------------------

    \92\ Interagency Workgroup On Air Quality Modeling (IWAQM) Phase
2 Summary Report And Recommendations For Modeling Long Range
Transport Impacts (EPA-454/R-98-019), EPA OAQPS, December 1998,
http://www.epa.gov/scram001/7thconf/calpuff/phase2.pdf.
---------------------------------------------------------------------------

    The 1998 IWAQM report is the only guidance available for choosing
ammonia background concentrations. The IWAQM Phase 2 report relied on a
1992 review of ambient monitoring data for ammonia by Langford et
al.\93\ and explains that: ``. . . the formation of particulate nitrate
is dependent on the ambient concentration of ammonia, which
preferentially reacts with sulfate. The ambient ammonia concentration
is an input to the model. Accurate specification of this parameter is
critical to the accurate estimation of particulate nitrate
concentrations. Based on a review of available data, Langford et al.
suggest that typical (within a factor of 2) background values of
ammonia are: 10 ppb for grasslands, 0.5 ppb for forest, and 1 ppb for
arid lands at 20 degrees Celsius. Langford et al. (1992) provide strong
evidence that background levels of ammonia show strong dependence with
ambient temperature (variations of a factor of 3 or 4) and a strong
dependence on the soil pH. However, given all the uncertainties in
ammonia data, IWAQM recommends use of the background levels provided
above, unless specific data are available for the modeling domain that
would discredit the values cited. It should be noted, however, that in
areas where there are high ambient levels of sulfate, values such as 10
ppb might overestimate the formation of particulate nitrate from a
given source, for these polluted conditions. Furthermore, areas in the
vicinity of strong point sources of ammonia, such as feed lots or other
agricultural areas may experience locally high levels of background
ammonia.'' \94\
---------------------------------------------------------------------------

    \93\ Langford, A.O., F.C. Fehsenfeld, J. Zachariassen, and D.S.
Schimel (1992), Gaseous ammonia fluxes and background concentrations
in terrestrial ecosystems of the United States, Global
Biogeochemical Cycles, 6, 459-483.
    \94\ Interagency Workgroup On Air Quality Modeling (IWAQM) Phase
2 Summary Report And Recommendations For Modeling Long Range
Transport Impacts, EPA-454/R-98-019, (December 1998) pages 14-15.
---------------------------------------------------------------------------

    The IWAQM Phase 2 report also states that ``[i]n a refined
analysis, ``the background concentrations of ozone and ammonia are
allowed to vary in time and space.'' \95\ In summary, given numerous
uncertainties in ammonia data, the IWAQM Phase 2 report recommends use
of the background values it provides for different land use categories,
unless specific data is available in the modeling domain as a more
accurate substitute for its recommended default values, and allows for
the consideration of background ammonia concentrations that vary
seasonally or spatially.
---------------------------------------------------------------------------

    \95\ IWAQM, Ibid., page 6.
---------------------------------------------------------------------------

    EPA has reviewed monitoring data for ammonia and ammonium that have
been collected at one site in western Wyoming since 2006.\96\ We have
determined that the monitoring data from this site are the most
representative monitoring data available for characterizing ammonia and
ammonium background levels in the modeling domains used for western
Wyoming as explained in detail below. Based on this analysis, EPA has
concluded that the constant 2 ppb background concentration used by the
State is substantially higher than the observed combined ammonia and
particulate ammonium concentrations at this monitoring site in western
Wyoming, especially during the winter season when the observed sum of
ammonia and particulate ammonium concentration were typically much
lower than 2 ppb.\97\ Therefore, for two BART sources in western
Wyoming (PacifiCorp's Naughton and Jim Bridger) in one of our modeling
runs we modeled using monthly varying ammonia background concentrations
based on the combined observed concentration of ammonia and particulate
ammonium at this monitoring site in western Wyoming, as described in
the EPA Protocol.\98\ In a second modeling run for these two BART
sources, we modeled using the default IWAQM ammonia concentration of
0.5 ppb for forested areas. Although western Wyoming includes a mixture
of arid grasslands and forested areas, we used the IWAQM default value
of 0.5 ppb for forested areas because the Class I areas in the modeling
domain are primarily forested, and because the monitoring data more
nearly matched the IWAQM recommendation for forests than the default
for the other land types.
---------------------------------------------------------------------------

    \96\ Li, Y., Schwandner, F.M., Sewell, H.J., Zivkovich, A.,
Tigges, M., Raja, S., Holcomb, S., Molenar, J.V., Sherman, L.,
Archuleta, C., Lee, T., Collett Jr., J.L., Observations of ammonia,
nitric acid, and fine particles in a rural gas production region,
Atmospheric Environment (2013), doi: 10.1016/j.atmosenv.2013.10.007.
    \97\ Li et al. U.S. EPA, December, 2013.
    \98\ Air Quality Modeling Protocol: Wyoming Regional Haze
Federal Implementation Plan.
---------------------------------------------------------------------------

    When Wyoming adopted its Protocol in 2006, the State explained
that: ``A constant background ammonia concentration of 2.0 ppb is
specified. This value is based upon monitoring data from nearby states
and IWAQM guidance. Experience suggests that 2.0 ppb is conservative in
that it is unlikely to significantly limit nitrate formation in the
model computations.'' \99\
---------------------------------------------------------------------------

    \99\ Wyoming Protocol, p. 12.
---------------------------------------------------------------------------

    The Wyoming Protocol specified a constant 2 ppb background ammonia
concentration, but the Protocol (addressing source owners performing
their own BART analyses) states that: ``[i]f you believe that ammonia
limiting is appropriate for a specific BART analysis, justification
should be discussed with the Division prior to its use.'' \100\ The
Wyoming Protocol in the text quoted here refers to a method for
correcting CALPUFF for ammonia limiting conditions, which indicates
that the State recognized the possibility that its protocol could be
overly conservative in its treatment of ammonia. Therefore, we believe
it is appropriate and consistent with the IWAQM Phase 2 report and the
intention of the State's Protocol to model a BART source in western
Wyoming using both the newly available monitoring data, and the default
concentration recommended in the IWAQM Phase 2 report, to represent
background ammonia concentrations more accurately than would be the
case if we retained the 2 ppb value specified in the State Protocol.
---------------------------------------------------------------------------

    \100\ Wyoming Protocol, p. 15.
---------------------------------------------------------------------------

    We describe here in more detail the ambient monitoring data from
the site in western Wyoming and our use of those

data. Li et al (2013) \101\ report on an analysis of ambient monitoring
data conducted from 2006 to 2011 at the Pinedale site in western
Wyoming in an area with significant oil and gas production. The
monitoring site included measurements of gaseous ammonia
(NH3) and particulate ammonium (NH4) and a
complete set of acidic species including gaseous nitric acid and
particulate nitrate and sulfate. The complete set of measurements makes
it possible to determine the total ammonia and ammonium concentration
(NHX=NH3+NH4) and to determine if
ammonium nitrate formation is limited by the availability of excess
NH3. Li et al found significant seasonal variability in
NH3 and NH4 concentrations at the site and
concluded that excess nitric acid was present in winter, while
NH3 concentrations were close to zero in winter, indicating
that formation of ammonium nitrate was limited by the availability of
ammonia at this site in winter.\102\ Because ammonia at this monitoring
site may have been affected by nearby sources of sulfuric acid and
nitric acid, which would deplete the gaseous ammonia concentration
locally, we used the combined gaseous and particulate measurement of
NHX to estimate monthly average background ammonia
concentrations, with a low concentration in January of 0.36 ppb and a
peak concentration in 1.12 ppb in August.\103\ The monitor is located
in an area that includes nearby sources of ammonia emissions from
livestock and other anthropogenic sources, including a nearby area of
oil and gas production activity, which could result in locally elevated
ammonia compared to the area more immediate to the BART source and to
the nearest Class I areas. Moreover, some of the particulate ammonium
observed at the site was irreversibly bound with sulfate and may have
had a non-local origin due to long range transport of ammonium sulfate.
These factors mean that this estimate of local background may tend to
overestimate the regional background ammonia concentration and thus
also overestimate the visibility benefit due to NOX
reductions at sources. There may be other unknown factors also working
in the same direction or in the other direction. For example,
monitoring methods for ammonia and quality systems for characterizing
monitoring accuracy have not been standardized to the extent that
methods for other ambient compounds have been, resulting in uncertainty
as to whether there is bias in the measurements.
---------------------------------------------------------------------------

    \101\ Li, Y., Schwandner, F.M., Sewell, H.J., Zivkovich, A.,
Tigges, M., Raja, S., Holcomb, S., Molenar, J.V., Sherman, L.,
Archuleta, C., Lee, T., Collett Jr., J.L., Observations of ammonia,
nitric acid, and fine particles in a rural gas production region,
Atmospheric Environment (2013), doi: 10.1016/j.atmosenv.2013.10.007.
    \102\ Li et al., Ibid.
    \103\ Air Quality Modeling Protocol: Wyoming Regional Haze
Federal Implementation Plan,U.S. EPA, December, 2013, page 18, Table
7.
---------------------------------------------------------------------------

    As discussed above, we also modeled the PacifiCorp Naughton and Jim
Bridger BART sources in western Wyoming using a constant background
ammonia concentration of 0.5 ppb, which is the IWAQM Phase 2 report
default recommendation for forested areas. Model results using either
approach to determine an ammonia background concentration support our
final BART determinations, as they both show similar visibility
benefits.
    We also remodeled the BART sources in eastern Wyoming including
Wyodak, Dave Johnston and Laramie River. The Class I areas most
impacted by these BART sources are Badlands and Wind Cave National
Parks in western South Dakota. The closest long-term ammonia monitoring
site to these Class I areas is at site at Beulah, North Dakota operated
by the State of North Dakota. This site is about 280 miles from
Badlands National Park and about 300 miles away from Wind Cave NP. The
area around Beaulah site includes a mix of agricultural lands and
grassland. Measured monthly average gaseous ammonia concentrations at
the Beulah site in central North Dakota vary from about 1 to 2 ppb
throughout the year, with the lowest values in fall and winter.\104\
Additionally, combined ammonia and particulate ammonium measurements
have been reported at Wind Cave National Park for an 8 month period by
Chen et al.\105\ This study measured NHX daily average
concentrations in the range of about 0.05 to 4 ppb, with an annual
average concentration of 0.75 ppb. We considered these monitoring data
sufficient to put into serious doubt the 2 ppb concentration specified
in the State's Protocol, but insufficient to support either a single or
an alternative modeling run based on monitoring data. Therefore, we
chose to rely on the IWAQM Phase 2 report for a single set of modeling
runs for the BART sources in eastern Wyoming. The area around Wind Cave
National Park includes forested areas, including Black Hills National
Forest, while the area around Badlands National Park includes a mix of
arid and grass lands. While there is uncertainty in the appropriate
background ammonia level in this region, we used the IWAQM Phase 2
report recommended value of 1 ppb for arid lands because it falls
within the range of the limited monitoring data available in nearby
regions and because it is represents an intermediate level for the
different land use types within the region.
---------------------------------------------------------------------------

    \104\ Protocol for BART-Related Visibility Impairment Modeling
Analyses in North Dakota, November, 2005, North Dakota Department of
Health, Division of Air Quality, 1200 Missouri Avenue, Bismarck, ND
58506. Page 33.
    \105\ Chen et al., A Pilot Monitoring Study of Atmospheric
NHX at Selected IMPROVE sites AWMA Aerosol and
Atmospheric Optics, Visibility & Air Pollution Conference, September
24-28, 2012, Whitefish, MT.
---------------------------------------------------------------------------

    For both the eastern and western Wyoming modeling domains and runs,
we corrected for ammonia limiting conditions. The correction for
ammonia limiting conditions is a post-processing step in POSTUTIL, one
component of the CALPUFF modeling system. Because CALPUFF simulates
each BART unit individually, the background ammonia concentration is
assumed by the model to be fully available to react with emissions from
each unit. In reality, the total emissions from the combined units
compete for the available ammonia. Also, because CALPUFF simulates
multiple parcels of air originating at each unit, there is the
possibility that different parcels can overlap at a Class I area. The
ammonia limiting correction in POSTUTIL is designed to repartition the
available ammonia to react with emissions from all of the units and
overlapping air parcels, thereby avoiding double counting of the
background ammonia. We used the same ammonia limiting correction in our
modeling for Montana BART \106\ sources, and this is a standard
configuration in the CALPUFF modeling system.
---------------------------------------------------------------------------

    \106\ Modeling Protocol: Montana Regional Haze Federal
Implementation Plan (FIP) Support, prepared for EPA Region 8 by
Alpine Geophysics, LLC, November 21, 2011.
---------------------------------------------------------------------------

    In summary, we concluded that it was more reasonable to model
visibility impacts in western Wyoming using both the default IWAQM
Phase 2 report recommendation for forested areas and using a seasonally
varying NH3 concentration in western Wyoming that was based
on long term monitoring of NH3 and NH4 from one
site, than to use the 2 ppb concentration specified in the State's
Protocol. We found that the visibility impacts predicted for the
various control levels at the PacifiCorp Naughton and Jim Bridger BART
sources were very similar with these two approaches and that either set
of results supports the same BART determination for these sources.
Therefore, we did not have to make a

determination that one approach was superior to the other, and we have
not done so. We also determined that it was appropriate to use the
default IWAQM Phase 2 report recommendation of 1 ppb for eastern
Wyoming.
    Comment: A commenter believes that the improvements from the
addition of SCR technology are based on highly conservative models
which overestimate the deciview reduction as compared to actual
monitored data collected at the IMPROVE stations. Subsequently, the
improvement in visibility provided by SCR is not supported by the
escalated costs of $600 million above the cost to install Wyoming's
proposed control technologies.
    Response: EPA disagrees with this comment to the extent that it
takes issue with EPA's modeling. As discussed in response to other
commenters, EPA recognized that the State's original modeling protocol
specified a fixed value of 2 ppb for background ammonia. EPA has
performed new modeling using lower background concentrations and using
a correction for ammonia limiting conditions when modeling multiple
units from a single BART source.
    EPA also used the most current regulatory approved versions of the
models in the updated modeling. EPA has recognized that the CALPUFF
model can be conservative in estimating visibility impairment, and
therefore, EPA has used the 98th percentile model results instead of
the maximum modeled visibility impairment to address the possibility of
model overpredictions. In recommending the use of CALPUFF for assessing
source specific visibility impacts, EPA recognized that the model had
certain limitations but concluded that ``[f]or purposes of the RHR's
BART provisions, CALPUFF is sufficiently reliable to inform the
decision making process, e.g., see 77 FR 39123.\107\ As discussed
above, to the extent that the comment takes issue with the use of the
98th percentile, the legal deadline for challenging the use of CALPUFF
has passed, but we encourage the commenter to provide input in the
event that EPA develops any new future visibility guidelines and
predictive models.
---------------------------------------------------------------------------

    \107\ 77 FR 39123: ``Because of the scale of the predicted
impacts from these sources, CALPUFF is an appropriate or a
reasonable application to determine whether such a facility can
reasonably be anticipated to cause or contribute to any impairment
of visibility. In other words, to find that a source with a
predicted maximum impact greater than 2 or 3 deciviews meets the
contribution threshold adopted by the States does not require the
degree of certainty in the results of the model that might be
required for other regulatory purposes. In the unlikely case that a
State were to find that a 750 MW power plant's predicted
contribution to visibility impairment is within a very narrow range
between exemption from or being subject to BART, the State can work
with EPA and the FLM to evaluate the CALPUFF results in combination
with information derived from other appropriate techniques for
estimating visibility impacts to inform the BART applicability
determination. Similarly for other types of BART eligible sources,
States can work with the EPA and FLM to determine appropriate
methods for assessing a single source's impacts on visibility.''
---------------------------------------------------------------------------

    Comment: The commenter states that EPA made five errors in its
visibility modeling, including (1) given the general inaccuracy in
CALPUFF unit-specific modeling, not allowing Wyoming the deference
accorded it under the CAA; (2) relying upon an outdated CALPUFF method
of visibility modeling, contrary to EPA precedent; (3) violating the
applicable modeling guidance, Appendix W, by not using the ``best''
science; (4) violating the Data Quality Act by not using the ``best''
science; and (5) failing to recognize the gross overestimations and
internal inconsistencies in EPA's modeling approach.
    Response: EPA disagrees with this comment. In response to item (1):
In promulgating the BART guidelines we made the decision in the final
BART Guideline to recommend that the model be used to estimate the 98th
percentile visibility impairment rather than the highest daily impact
value as proposed. We made the decision because ``there are other
features of our recommended modeling approach that are likely to
overstate the actual visibility effects of an individual source. Most
important, the simplified chemistry in the model tends to magnify the
actual visibility effects of that source. Because of these features and
the uncertainties associated with the model, we believe it is
appropriate to use the 98th percentile, a more robust approach that
does not give undue weight to the extreme tail of the distribution.''
77 FR 39121. In regard to deference to the state, as discussed in more
detail elsewhere in this document, Congress crafted the CAA to provide
for states to take the lead for implementing plans, but balanced that
decision by requiring EPA to approve the plans or prescribe a federal
plan should the state plan be inadequate to meet CAA and regulatory
requirements. Our action today is consistent with the statute and
regulations. In response to item (2): We initially relied on the
State's modeling Protocol, adopted in 2006, that specified model
versions available at that time, but that have since been updated. In
our original proposal we performed limited gap filling modeling that
was consistent with the State's Protocol and that used the same model
versions as the State. In this final action, as presented in more
detail in the Protocol in the docket, due to a number of other changes
in the protocol, we also updated the protocol to use the current
regulatory version of the CALPUFF modeling system. In response to items
(3) and (4): In recommending the use of CALPUFF for assessing source
specific visibility impacts, EPA recognized that the model had certain
limitations but concluded that ``[f]or purposes of the regional haze
rule's BART provisions . . . CALPUFF is sufficiently reliable to inform
the decision-making process.'' \108\ EPA accordingly appropriately used
CALPUFF in this action. EPA recognized that there were uncertainties in
the science of the CALPUFF modeling system, and therefore used the less
conservative 98th percentile value to model results to address the
possibility that the model was overly conservative. We address concerns
about the Data Quality Act (also referred to as the Information Quality
Act), elsewhere in this document. In response to item (5): EPA
recognized that the State's original Protocol was inconsistent with the
IWAQM report and monitoring data because of the use of a constant 2 ppb
ammonia concentration, and our modeling in this action relied on
ambient monitoring data and the default values consistent with IWAQM
Phase 2 report, to specify two alternatives for more realistic
background ammonia concentrations in western Wyoming. We also reduced
the background ammonia concentration from 2 to 1 ppb in eastern
Wyoming, as discussed previously. A seasonal ammonia concentration was
not adopted due to the lack of high quality monitoring data in eastern
Wyoming; however, as discussed previously, the 1 ppb background
estimate is consistent with IWAQM Phase 2 report. As discussed
elsewhere, we also used the ammonia limiting correction for modeling
multiple units from a single BART source to address concerns with the
model being overly conservative.
---------------------------------------------------------------------------

    \108\ 70 FR at 39123.
---------------------------------------------------------------------------

    Comment: Contrary to its own guidance, EPA failed to use the most
realistic model. 40 CFR part 51, Appendix W, EPA's modeling guidance,
demands that the ``best'' model should always be used. EPA failed to
use the ``best'' model in Wyoming, which is CALPUFF 6.4. Therefore, EPA
failed to follow Appendix W's requirements.
    Response: We disagree with this comment. As explained above, EPA
followed the RHR. Specifically, in recommending the use of CALPUFF for
assessing source specific visibility impacts, EPA recognized that the
model had certain limitations but concluded

that ``[f]or purposes of the regional haze rule's BART provisions . . .
CALPUFF is sufficiently reliable to inform the decision-making
process.'' \109\ EPA accordingly appropriately used CALPUFF in this
action. The use of CALPUFF is subject to GAQM requirements in section
3.0(b), 4, and 6.2.1(e) which includes an approved protocol to use the
current 5.8 version, which is the version we used for EPA's final
modeling analysis. We did not use CALPUFF Version 6.4 because this
version of the model has not been approved by EPA for regulatory use.
---------------------------------------------------------------------------

    \109\ 70 FR at 39123.
---------------------------------------------------------------------------

    We made the decision in the final BART Guidelines to use less than
the highest daily impact value for assessment of visibility impacts. We
made this decision in response to comments we received expressing
concern that the chemistry modules in the CALPUFF model are less
advanced and that use of the 1st High was conservative and the
knowledge that CALPUFF's simplified chemistry could lead to model
overpredictions and thus be conservative. We decided in the BART
Guideline to use the 98 percentile for CALPUFF based modeling results.
We also received comments opposed to using the day with the worst
meteorology, but the primary reason we changed to using a less
stringent metric than the day with the highest visibility impact was
because of concerns about overestimations in CALPUFF's simplified
chemistry. As a result, we determined that it was appropriate to use
the 98th percentile or 8th High value when modeling all days of the
year instead of the 1st High value, also described as the Highest Daily
impact level for each year modeled: ``Most important, the simplified
chemistry in the model tends to magnify the actual visibility effects
of that source. Because of these features and the uncertainty
associated with the model, we believe it is appropriate to use the 98th
percentile--a more robust approach that does not give undue weight to
the extreme tail of the distribution.'' \110\
    Comment: EPA's modeling for its regional haze FIP action was
inadequate and incomplete. (EPA failed to re-run WRAP regional modeling
due to ``time and resource constraints''). Therefore, EPA's regional
haze FIP action violates the ``Guideline on Air Quality Models,'' 40
CFR part 51 Appendix W, the Information Quality Act and the
implementing guidelines issued, respectively, by the U.S. Office of
Management and Budget (OMB) and the EPA which require information
disseminated by EPA to be accurate, complete, reliable and unbiased.
The Information Quality Act and EPA's ``Information Quality
Guidelines'' place a heightened standard on ``influential''
information, including scientific information regarding health, safety,
or environmental risk assessments. EPA's inaccurate and incomplete
visibility modeling is by definition ``influential,'' because EPA could
reasonably determine that dissemination of the information will have or
does have a clear and substantial impact on important public policies
or important private sector decisions, such as the BART NOX
determinations in EPA's regional haze FIP.
    Therefore, this ``influential'' information must be based on best
available science and data and supporting studies must be conducted in
accordance with sound objective scientific practices and methods. EPA's
Guidelines implementing the Information Quality Act expressly
contemplate the correction of information disseminated by EPA that
falls short of the ``basic standard of quality, including objectivity,
utility, and integrity,'' established by either EPA's own Guidelines or
those issued by OMB.
    The commenter seeks correction to a number of errors and omissions
in EPA's regional haze FIP with regard to CALPUFF modeling and EPA's
failure to re-run the WRAP model. The commenter requests that EPA
withdraw its regional haze FIP until these issues are resolved.
    Response: As EPA explained in our Information Quality Guidelines,
we believe ``that the thorough consideration provided by the public
comment process serves the purposes of the Guidelines, provides an
opportunity for correction of any information that does not comply with
the Guidelines, and does not duplicate or interfere with the orderly
conduct of the action.'' \111\ Therefore, we are responding to the
modeling comments and related comments regarding EPA's Guidelines and
the Information Quality Act in this document.
---------------------------------------------------------------------------

    \111\ Guidelines for Ensuring and Maximizing the Quality,
Objectivity, Utility, and Integrity of Information Disseminated by
the Environmental Protection Agency, EPA/260R-02-008 October 2002,
U.S. Environmental Protection Agency Office of Environmental
Information (2810) 1200 Pennsylvania Avenue NW., Washington, DC
20460.
---------------------------------------------------------------------------

    WRAP performed regional photochemical modeling using both the
Community Multi-Scale Air Quality modeling system (CMAQ) and
Comprehensive Air Quality Model (CAMx) air quality models to evaluate
progress toward attaining visibility goals using all projected emission
changes from all source categories throughout the United States. WRAP
did not perform regional photochemical modeling to evaluate the
visibility impacts of individual BART sources. While WRAP did make
assumptions regarding the level of emissions control that would be
adopted by BART sources as part of its analyses, no state or EPA region
has re-run the WRAP's regional photochemical models to assess
individual BART source contributions to visibility impairment. Instead,
the BART sources, the states, and EPA have used the CALPUFF model to
evaluate contributions to visibility impairment from individual BART
sources. This approach is consistent with the BART Guidelines that
recommend that the CALPUFF model should be used to evaluate visibility
impairment from individual BART sources. Additionally, while EPA
supported development of WRAP CMAQ modeling in order to assist states
in developing their RPGs and determining the cumulative benefit of an
overall control strategy vis-[agrave]-vis the URP on the 20% worst
days, our use of CALPUFF for evaluating visibility improvement from a
single BART source is consistent with the BART Guidelines and also
consistent with modeling performed by other states and EPA regional
offices for individual BART sources.
    We have responded to comments related to errors and omissions in
the CALPUFF modeling in separate response to comments.
    Comment: The commenter states that the revised EPA modeling, which
used new information on emissions rates, did not significantly change
the results identified in Wyoming's BART analyses. The commenter also
states that there are small differences between EPA's and Wyoming's
analyses which do not justify EPA rejecting Wyoming's BART
determinations.
    Response: We disagree that in all cases there are only small
differences in EPA revised modeling and the State's modeling.
Importantly, as described elsewhere in this document and in the docket
for this action, EPA revised and corrected various inputs to the BART
factors so that the analyses are consistent with the RHR and statutory
requirements. While the difference at the most impacted Class I area
from individual sources or units is some cases can be characterized as
small, the cumulative differences from many small improvements can be
significant. Whether such differences are significant will depend on
the overall consideration of the BART factors. Because of the flaws in
Wyoming's

visibility and cost analyses for many of its BART sources, EPA could
not be confident that Wyoming's BART determinations were reasonable
without undertaking an appropriate analysis of the statutory factors.
    Comment: The commenter states that Wyoming provided the required
modeled visibility improvement information for SCR, and quotes from the
State's analysis: ``Post-Control Scenario B is directly comparable to
Post-Control Scenario A as the only difference is directly attributable
to the installation of SCR.'' The commenter then concludes EPA did not
lack the required information to evaluate visibility improvements.
    Response: We have addressed this comment in a previous response,
citing the requirement in the BART Guidelines that visibility must be
assessed relative to the pre-control scenario (and not just incremental
to other control scenarios). Moreover, there remain deficiencies, as
presented elsewhere in this document and docket, with the State's BART
modeling analyses that justify our rejection of the State's BART
determinations for PacifiCorp. Most notably, as discussed in separate
responses, the State did not assess the visibility improvement of SNCR
as required by the CAA and BART Guidelines.
    Comment: An older version of the CALPUFF modeling suite was used by
EPA (CALPUFF model of March 2006 vintage and the CALPOST model of April
2006 vintage.) These older versions pre-date the latest Model Change
Bulletin (MCB-D) of June 23, 2007. Since the analysis for the five
Wyoming power plants was performed in February thru April 2012, we
question why the older version was used and not the current CALPUFF 5.8
version, which was approved as the guideline version in June of 2007.
We do not recommend use of the older versions of CALPUFF and CALPOST.
    Response: As described in this action, our previous modeling
continued to use the State's Protocol, including the older model
versions, to maintain consistency with the State's modeling results.
However, in this final action, we adopted a new modeling protocol that
uses the current regulatory versions of the models, including the Model
Change Bulletin suggested by the commenter. We determined that it was
appropriate to adopt an updated modeling protocol because we made other
significant changes in the State's modeling approach, and because we
remodeled all emissions scenarios, there was no longer a need to use
older model versions for consistency of comparison of our limited gap
filling model results to the State's original modeling.
    Comment: EPA should have used the most recent version of CALPUFF,
or at a minimum, should have used the version that EPA requires for
other regional haze SIPs. EPA has taken the position that CALPUFF
Version 5.8 must be used for regional haze modeling. 77 FR 42834,
42854. However, EPA's unit-specific CALPUFF modeling in Wyoming,
completed in April 2012, used CALPUFF Version 5.711a (originally
released in 2004). Version 5.711a is eight years old, and several
CALPUFF versions behind Version 5.8. While PacifiCorp believes the more
modern and realistic CALPUFF Version 6.42 should be used, at a minimum
EPA must abide by its own position and use Version 5.8 in evaluating
the Wyoming regional haze SIP, which it failed to do. According to
EPA's own statements, EPA's own modeling results should be discarded
because EPA used an improper ``alternative model'' in Wyoming.
    EPA should have used the most recent version of CALPUFF (Version
6.42) in Wyoming because it produces more realistic and accurate
results. Version 6.42 contains needed refinements, such as a better
``chemistry'' module known as ISORROPIA (Version 2.1). CALPUFF Version
6.42 is more accurate because, as the FLMs have noted, Version 5.8 does
not have the required settings to perform the new Method 8 visibility
analysis. Additionally, CALPUFF Version 6.42 has been maintained by
TRC, Inc., a private contractor, and has had many bug fixes and
enhancements not included in CALPUFF Version 5.8. Most importantly, the
previous chemistry modules used in Version 5.8 (and in the 5.711a
Version EPA used here) also have been shown to overestimate nitrate
concentrations in Wyoming by a factor of 3-4 and substantial
improvements have been made to eliminate this over-prediction using the
ISORROPIA module.
    Despite all these advancements in modeling and modeling science,
EPA conducted its modeling for its regional haze FIP in 2012 using the
same (now outdated) CALPUFF version that PacifiCorp and Wyoming used
five years ago, which has been shown to overestimate results by 300% to
400%. Rejecting Wyoming's modeling, and then using the same, outdated
modeling approach, is arbitrary and capricious.
    Response: As described in previous responses, we previously used
the same modeling protocol adopted by the State for the purpose of our
limited, gap filling modeling, so that we would have a consistent basis
for comparison with the State's modeling. In this action we have
updated the protocol to use the current regulatory versions of the
models including CALPUFF version 5.8.
    We did not use CALPUFF Version 6.42 because this version of the
model has not been approved by EPA for regulatory use. EPA relied on
version 5.8 of CALPUFF because it is EPA-approved version in accordance
with the Guideline on Air Quality Models (``GAQM'', 40 CFR 51, Appendix
W, section 6.2.1.e). EPA updated the specific version to be used for
regulatory purposes on June 29, 2007, including minor revisions as of
that date. The approved CALPUFF modeling system includes CALPUFF
version 5.8, level 070623, and CALMET version 5.8 level 070623. CALPUFF
version 5.8 has been thoroughly tested and evaluated, and has been
shown to perform consistently with the initial 2003 version in the
analytical situations for which CALPUFF has been approved. Any other
version, and especially one with such fundamental differences in its
handling of chemistry, would be considered an ``alternative model'',
subject to the provisions of GAQM section 3.2.2(b), requiring full
model documentation, peer-review, and performance evaluation. No such
information for the later CALPUFF versions that meet the requirements
of section 3.2.2(b) has been submitted to or approved by EPA.
Experience has shown that when the full evaluation procedure is not
followed, errors that are not immediately apparent can be introduced
along with new model features. For example, changes introduced to
CALMET to improve simulation of over-water convective mixing heights
caused their periodic collapse to zero, even over land, so that CALPUFF
concentration estimates were no longer reliable.\112\
---------------------------------------------------------------------------

    \112\ ``CALPUFF Regulatory Update'', Roger W. Brode,
Presentation at Regional/State/Local Modelers Workshop, June 10-12,
2008; http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2008/agenda.htm.
---------------------------------------------------------------------------

    The change from CALPUFF version 5.8 to CALPUFF 6.4 is not a simple
model update to address minor issues, but a significant change in the
model science that requires its own rulemaking with public notice and
comment before it can be relied on for regulatory purposes.
    Furthermore, it should be noted that the U.S. Forest Service and
EPA review of CALPUFF version 6.4 results for a limited set of BART
applications showed that differences in its results from those of
version 5.8 are driven by two input assumptions not associated

with the chemistry changes in 6.4. Use of the so-called ``full''
ammonia limiting method and finer horizontal grid resolution are the
primary drivers in the predicted differences in modeled visibility
impacts between the model versions. These input assumptions have been
previously reviewed by EPA and the FLMs and have been rejected based on
lack of documentation, inadequate peer review, and lack of technical
justification and validation.
    Comment: The commenter states that EPA treats the results from the
CALPUFF as being capable of accurately predicting visibility
improvements down to the tenths or hundredths of a deciview, but that
the model does not accurately predict visibility impacts at this level.
    Response: As described in response to other comments, EPA
recognized that there is uncertainty in the CALPUFF results, and EPA
addressed this uncertainty by using the modeled 98th percentile
visibility impairment rather than the maximum visibility impairment.
EPA considers model changes on the order of tenths of a deciview to be
useful for informing the BART decision process, consistent with BART
modeling performed by other EPA regions and states.
    Comment: PacifiCorp presented substantial information supporting
the need to use improved and updated versions of the models and
provided substantial information on the effects that the nitrogen
oxides to nitrogen dioxide conversion rate and background ammonia
concentrations have on modeled visibility impacts.
    Response: We agree with the comment that the background ammonia
concentration has a significant effect on model predicted visibility
impacts. As described elsewhere in this action, we reviewed recent
ambient monitoring data for ammonia and particulate ammonium, and
concluded that the original background ammonia concentration of 2 ppb
was inconsistent with the IWAQM Phase 2 report and monitoring data for
estimating visibility impacts in Wyoming, especially in the western
portion of the State. In the modeling results included in this action,
we considered the default value of 0.5 ppb and also applied a
seasonally varying background ammonia concentration in western Wyoming
that was based on measurements from 2006 to 2011 of total ammonia and
particulate ammonium at Pinedale, Wyoming.\113\ We also reduced the
background ammonia concentration to 1 ppb in the eastern portion of the
State, and for both parts of the State we used an ammonia limiting
correction for modeling multiple units from a single BART source to
avoid double counting of the available ammonia. The use of more
realistic ammonia background concentrations, the ammonia limiting
correction, and the use of the 98th percentile modeled impact address
the concern that the CALPUFF model could overestimate visibility
impacts.
---------------------------------------------------------------------------

    \113\ Li et al. 2013
---------------------------------------------------------------------------

    Comment: One commenter stated that we underestimated the background
ammonia concentration in the CALPUFF modeling, and cited the IWAQM
Phase 2 recommendations for default ammonia concentrations for
grasslands, forest lands and arid lands, respectively, of 10, 0.5 and 1
ppb, at 20 degrees Celsius. The commenter stated that, because land use
type can vary across the large domains used in the CALPUFF modeling, it
would be appropriate to calculate a weighted average of the background
ammonia based on the fractional land use type in the model domain.
    Response: We disagree with this comment. The IWAQM Phase 2 report
does not recommend calculating a weighted average of default ammonia
concentration based on regional variation in land use types. The
commenter provides no regulatory basis for use of a weighted average.
    Comment: One commenter stated that the IWAQM recommended default
background ammonia recommendations do not specifically account for
strong point/area sources of ammonia, such as cattle feedlots, which
are also scattered throughout the modeling domain and which generally
add to the background ammonia levels. This commenter stated that some
areas of the modeling domain, namely northeastern Colorado, are
described as ``ammonia rich''. For BART source analyses in Colorado,
the recommended background ammonia value from the Colorado Department
of Public Health and Environment (CDPHE) is 44 ppb, based on
measurements conducted during the Northern Front Range Air Quality
Study (NFRAQS), and therefore the Wyoming DEQ/EPA background ammonia
concentration of 2 ppb might not carry sufficient ammonia for an
accurate modeling assessment of visibility impacts within certain
Colorado Class I areas.
    Response: We disagree with the comment that large point/area
sources are not included in estimates of background ammonia
concentrations. While concentrations of ammonia of several hundred ppb
can be observed near a cattle feedlot, these concentrations are not
typical of regional background concentrations. Additionally, dispersion
and vertical mixing occur in plumes of air transported downwind of
large emissions sources, and the resulting dilution of ammonia results
in lower concentrations as the plume is transported downwind of the
source area. Therefore, ambient ammonia concentrations are generally
greatly reduced downwind from the source. Moreover, ammonia has a short
atmospheric lifetime of a few hours to a few days,\114\ and removal of
ammonia by deposition further reduces the concentration downwind of the
source area. Therefore, it would be inappropriate to estimate
background ammonia levels by measuring ammonia concentrations close to
a large ammonia emissions source. Background concentrations of air
pollutants are generally estimated using ambient monitoring data at
background sites that are specifically selected such that there is no
direct influence by large nearby point or areas sources.\115\
Therefore, background monitoring data do explicitly include the
contributions of emissions from large point and area sources by
providing a direct measurement of the ambient concentration after
transport, dilution and removal processes operate on the emissions from
the source.
---------------------------------------------------------------------------

    \114\ Langford et al.
    \115\ For example, see EPA guidance documents that discuss
methods for estimating background NO2 concentrations: ``Additional
Clarification Regarding Applicability of Appendix W Modeling
Guidance for the 1-hour NO2 NAAQS''
---------------------------------------------------------------------------

    The commenter also cites modeling performed by the CDPHE and
ammonia measurements made during the NFRAQS studies. As discussed in
another response, CDPHE performed a CALPUFF model sensitivity study to
evaluate the effect of background ammonia on model predicted nitrate
concentrations, and found that the CALPUFF model was insensitive to
variations in background concentrations greater than 10 ppb and became
progressively more sensitive to background NH3 as it was reduced from
10 to 0 ppb.\116\ The NFRAQS study reported measured ammonia
concentrations in the Denver metropolitan area, and these measurements
are not representative of background ammonia concentrations in

rural and remote areas of central Colorado or western Wyoming.
---------------------------------------------------------------------------

    \116\ Supplemental BART Analysis CALPUFF Protocol for Class I
Federal Area Visibility Improvement Modeling Analysis, DRAFT,
revised Aug 19, 2010, Colorado Department of Public Health and
Environment. (CDPHE) Air Pollution Control Division Technical
Services Program, 4300 Cherry Creek Drive South, Denver, Colorado
80246, pages 26-33.
---------------------------------------------------------------------------

    Comment: The commenter states that Wyoming has conducted its
regional haze SIP based on the modeling protocols and versions
available at the time its regional haze SIP was completed. Because of
this, there are limitations associated with the results obtained.
However, in proposing its regional haze SIP, Wyoming has evaluated the
model output with an understanding of the model's limitations. Wyoming
then applied its judgment, as encouraged and required by EPA's
guidelines and the CAA, which helped to mitigate the issues associated
with models that over-predict the visibility improvement associated
with BART controls being added. In contrast, EPA gives no consideration
to the limitations of the models it uses. In the absence of using good
judgment to deal with over-predictive results, it is critical that EPA
use the most up-to-date and scientifically accurate models available.
    We also received related comments that states have significant
modeling discretion to which EPA failed to grant the proper deference.
One commenter pointed out that the BART Guidelines recognize that
states can make judgments regarding the use of modeling results due to
the very real problems with CALPUFF, including its overestimation of
visibility improvement. As EPA itself has stated, Wyoming should be
free to make its own judgment about which modeling approaches are valid
and appropriate. 70 FR 39123. Another commenter pointed to the
statement that ``we must permit States to take into account the degree
of improvement in visibility that would result from imposition of BART
on each individual source when deciding on particular controls.'' 70 FR
39107, 39129. Another commenter stated that EPA failed to allow Wyoming
to account for CALPUFF's overestimation of NOX impacts, and
therefore, EPA is not affording Wyoming's BART decisions the proper
deference when it comes to the modeling and applying the modeling
results.
    Response: As discussed elsewhere in this document in greater
detail, Congress crafted the CAA to provide for states to take the lead
for implementing plans, but balanced that decision by requiring EPA to
approve the plans or prescribe a federal plan should the state plan be
inadequate. Our action today is consistent with the statute. As also
discussed elsewhere in this document, we agree that there are
limitations in the original modeling performed by the state, and
therefore, it was necessary to perform new modeling using more
realistic background ammonia concentrations, default values, and
updated model versions to provide a sound basis for evaluating BART
source visibility impacts. Our revised modeling is consistent with the
BART Guidelines and with visibility modeling guidance in the IWAQM
Phase 2 report, and we believe that the revised modeling constitutes a
sound basis for evaluating visibility impacts of BART sources and in
fact is supportive of Wyoming's SIP with respect to sources where
ammonia background makes a significant differences.
    Comment: The commenter states that EPA should have used the most
recent version of CALPUFF (Version 6.42) in Wyoming because it produces
more realistic and accurate results and because Version 5.8 does not
have the required settings to perform the new Method 8 visibility
analysis.
    Response: As described in response to another comment, we used
CALPUFF version 5.8 because this is the approved regulatory version of
the model, while CALPUFF version 6.42 has not been approved. CALPUFF
version 5.8 does allow the option of using the Method 8 visibility
analysis, and as described in our modeling protocol, we used Method 8
for our analysis. The availability of Method 8 in CALPUFF version 5.8
was one of the reasons that we determined it was important to perform
new modeling using the current regulatory version of the model, rather
than continuing to rely on the original protocol adopted by the State
in 2006, as we had done in our previous proposal.
    Comment: EPA used a different background ammonia number for
modeling than it requires of the states, and ignored current science on
background ammonia. Regional haze modeling, and the resulting predicted
visibility improvement, is greatly influenced by the background ammonia
number used in the model. EPA improperly used a constant 2 ppb
background ammonia number for the Wyoming BART modeling. EPA has not
provided any scientific proof showing the constant 2 ppb ammonia number
is appropriate for Wyoming. The 2 ppb ammonia value overestimates
visibility improvement, contrary to the approach used by Wyoming Land
Use, IWAQM Guidance, WRAP protocols, and elsewhere.
    Commenter suggests that the WRAP recommended the use of 1 ppb of
ammonia year round for states in the region to account for seasonal
variability. EPA has required states to use 1 ppb of background ammonia
when conducting regional haze modeling. 76 FR 52434 (New Mexico
criticized for not using 1 ppb background ammonia). At a minimum EPA
should follow its own guidelines and use 1 ppb of background ammonia
when conducting CALPUFF unit-specific modeling.
    However, the ``best'' science requires the use of ``variable
ammonia'' background numbers. IWAQM recommends ammonia background
numbers of 0.5 ppb for forest, 1 ppb for dry/arid lands, and 10 ppb for
agriculture/grassland. Given its geographic location and elevation
levels, Wyoming undergoes seasonal swings of dry-hot summers and snow
covered ground in the winter. Therefore, the use of a single ammonia
concentration for the entire year in a state where the land use and
land cover changes significantly between seasons results in
overestimation of visibility improvements. This is particularly true in
winter when agricultural activity is minimal and meteorological
conditions make visibility calculations particularly sensitive to
ambient ammonia concentrations. EPA has approved the use of variable
gaseous ammonia concentrations before, including the ``Addendum to
Modeling Protocol for the Proposed Desert Rock Generating Station'' and
should have used them when conducting the CALPUFF modeling for Wyoming.
    Sensitivity tests on ambient ammonia concentrations were performed
by the CDPHE for an area in northwest Colorado. The analysis
demonstrated that visibility calculations performed at Mount Zirkel
Wilderness Area in northwest Colorado had limited impact when ambient
ammonia concentrations were reduced from 100 to 1 ppb, but there was a
significant reduction in visibility impacts when concentrations were
further reduced to 0.1 ppb.
    Given the evidence presented above, the use of the monthly varying
ammonia would provide accurate estimates of visibility impacts from the
PacifiCorp regional haze units. EPA's failure to use variable
background ammonia in its modeling is arbitrary and capricious.
    Response: We agree that the 2 ppb constant background ammonia
concentration is inconsistent with the IWAQM Phase 2 report default
values and monitored data. This value was adopted by the State in 2006
before more reliable ammonia and particulate ammonium measurements were
available in Wyoming. As described in this action, we modeled using
seasonally varying background ammonia concentrations in western Wyoming
based on 5 years of

monitoring data,\117\ and we also modeled using the IWAQM default value
of 0.5 ppb for forests. In eastern Wyoming we adopted a constant 1 ppb
ammonia concentration based on the IWAQM guidance. We used an ammonia
limiting correction for BART sources with multiple units throughout the
State. While robust, long term monitoring data of ammonia and
particulate ammonium are not available in eastern Wyoming, the BART
sources in eastern Wyoming, and the South Dakota Class I areas where
they contribute the greatest visibility impairment, are located closer
to areas of Nebraska and the Dakotas which have large agricultural
sources of ammonia emissions. Moreover, the IMPROVE monitoring at the
South Dakota Class I areas show much higher winter concentrations of
ammonium nitrate than do Class I areas in western Wyoming. Therefore,
we believe it is appropriate to adopt higher background ammonia
concentrations in eastern Wyoming than in western Wyoming, and we used
a constant 1 ppb ammonia concentration in eastern Wyoming, consistent
with the IWAQM guidance \118\ for arid lands and also consistent with
the WRAP Protocol.\119\
---------------------------------------------------------------------------

    \117\ Li et al., 2013.
    \118\ Interagency Workgroup On Air Quality Modeling (IWAQM)
Phase 2 Summary Report And Recommendations For Modeling Long Range
Transport Impacts (EPA-454/R-98-019), EPA OAQPS, December 1998.
    \119\ Tonnesen, G., Wang, Z., Morris, R., Hoats, A., Jia, Y.,
Draft Final Modeling Protocol, CALMET/CALPUFF Protocol for BART
Exemption Screening Analysis for Class I Areas in the Western United
States, Submitted to the Western Regional Air Partnership, August
15, 2006.
---------------------------------------------------------------------------

    Comment: EPA made a modeling error in Wyoming when it used CALPOST
version 5 with Method 6. FLMs recommended in 2000 the use of Method 6
to determine visibility impacts from BART eligible sources. However,
for any recent PSD application and BART modeling since 2010, EPA has
requested that Method 8 be used for determining impacts on visibility
at nearby Class I areas.
    The previously preferred Method 6 simply computes background light
extinction using monthly average relative humidity adjustment factors
particular to each Class I area applied to background and modeled
sulfate and nitrate. Six years after the development of Method 6 in
1999, EPA released enhancements to the background light extinction
equations, which use the revised IMPROVE variable extinction efficiency
formulation. These enhancements take into account the fact that
sulfates, nitrates and organics and other types of particles have
different light extinction coefficients. Also, the background
concentrations at each Class I area have been updated by EPA to reflect
natural background visibility condition estimates for each Class I area
for each type of particle. Additionally, relative humidity adjustment
factors have been tailored separately for small particles, large
particles, and to account for sea salt background concentrations.
    These new enhancements to the calculation method, called Method 8,
greatly improve the accuracy of the estimated visibility impact. Method
8 was added to CALPOST in 2008 and was adopted as the preferred option
for determining impacts on visibility by the FLMs in their ``Federal
Land Managers Air Quality Related Values Work Group Guidance Document''
in 2010 (FLAG 2010). The applicable background concentrations and
relative humidity adjustment factors using Method 8 for each Class I
area are identified in the FLAG 2010 document.
    Despite the update to Method 8 in 2008 and the stated preference by
the FLMs in 2010 to use Method 8, EPA conducted the Wyoming BART
modeling in 2012 using the long outdated and scientifically inferior
Method 6. EPA's use of Method 6, and not Method 8, is arbitrary and
capricious.
    Response: We agree that it is preferable to use Method 8 rather
than Method 6 for evaluating visibility impacts based on the
recommendation of the FLMs in FLAG 2010.\120\ The older CALPUFF version
5.711 that was adopted in the State's original modeling protocol in
2006 did not have the option of using Method 8. In our previous
modeling we adopted the State's original protocol to maintain a
consistent basis of comparison with the State's modeling results. In
this final action, we adopted an updated modeling protocol using the
current regulatory version of the model, which allows the use of Method
8, and we used Method 8 for the analysis of visibility impacts.
---------------------------------------------------------------------------

    \120\ Federal Land Managers' Air Quality Related Values Work
Group (FLAG) Phase I Report--Revised (2010).
    Natural Resource Report NPS/NRPC/NRR--2010/232.
---------------------------------------------------------------------------

    Comment: We received numerous comments that EPA incorrectly used
the maximum annual visibility impacts occurring during any given year
of the 2001-2003 baseline period over which the Wyoming visibility
models are run. Commenters asserted that standard practice has been,
and continues to be, to average the results over the three year period
as the three year average is a more robust value than the single year
value used by EPA, and thus EPA should use longer term data. One
commenter pointed out that consistent with the principle of using
longer-term averages, baseline visibility conditions under the RHR are
determined by taking the average degree of visibility impairment for
the most and least impaired days for each of calendar years 2000
through 2004, and averaging the five annual values.
    Response: We disagree with this comment. In our review of the
CALPUFF modeling results presented by Wyoming, we cited the change in
the maximum 98th percentile impact over the modeled three year
meteorological period (2001-2003). As the 98th percentile value is
intended to reflect the 8th high value in any year, it already
eliminates seven days per year from consideration in order to account
for short-term events, unusual meteorological conditions, and any over-
prediction bias in the model. We also note that our approach is
consistent with the method used by Wyoming in identifying subject-to-
BART sources, where a source is exempt from BART only if the modeled
98th percentile change is less than 0.5 deciview at all Class I areas
for each year modeled.\121\ That is, whether a source is subject to
BART is dependent on the maximum 98th percentile over the three year
modeled meteorological period, not the average across the three year
period. We find that it is reasonable to use the same approach when
considering the visibility improvements associated with control
options. Finally, we note that this approach is consistent with our
consideration of visibility improvement in other actions, such as our
FIP for Montana.
---------------------------------------------------------------------------

    \121\ Chapter 6, Section 9(d)(i)(C) of the Wyoming Air Quality
Standards and Regulations.
---------------------------------------------------------------------------

    Comment: EPA's use of the maximum values in its BART NOX
determinations for its regional haze FIP causes inflated visibility
impacts and over-estimated improvements being used. For example, if EPA
were to run its approved models, used its approved ammonia values for
the western states, and used the average visibility impact over the
three years rather than a maximum impact for a single year, the
incremental visibility impact between installing LNB technology and
SNCR at Wyodak and Dave Johnston Unit 3 drops to just 0.09 deciview.
Instead, EPA has used an improper evaluation to create an inflated
visibility improvement of 0.15 and 0.17 delta deciview to justify the
installation of the SNCR. As a result, EPAs' BART NOX
evaluations are

invalid. The modeling results reported by Mr. Paine using the actual
BART limits include values for each of the meteorological years 2001,
2002 and 2003, as well as the average values for the three years. All
of the values signify a negligible visibility improvement from SNCR.
    Response: We have addressed each aspect of this comment in separate
responses to comments. In our previous proposal we performed CALPUFF
model simulations consistent with the approach specified in the Wyoming
protocol, but in this action we adopted updated model versions and used
lower ammonia background concentrations that are consistent with
monitoring data and IWAQM Phase 2 report. Regardless, as discussed in
section IV, in consideration of our revised cost of compliance and
visibility impact analyses, and of the remaining BART factors, we have
changed our final NOX BART determinations for both of the
units in question. We are no longer requiring SNCR for either Wyodak or
Dave Johnston Unit 4.
    Comment: EPA's use of the cumulative deciview improvement from
several Class I areas overestimates the visibility improvement which
may reasonably be anticipated because visibility impacts from a BART
source may occur on different days at each Class I area. Adding the
numbers in Tables 47, 54, and 56 of EPA's proposed regional haze FIP
leads to the impression that a perceptible visibility improvement will
occur, when in reality none of the modeled visibility improvements
would be perceptible to the human eye.
    Response: We disagree with this comment. In evaluating the
visibility improvement associated with various control options, EPA
interprets the CAA to require consideration of visibility improvement
at all impacted Class I areas. Consideration of improvement at multiple
Class I areas, as opposed to just benefits at the most impacted Class I
area, has often been described as ``cumulative visibility
improvement.'' Despite this terminology, however, an analysis of
cumulative visibility improvement does not necessarily require that the
deciview improvement at each area be summed together. While states or
EPA are free to take such a quantitative approach, they are also free
to use a more qualitative approach. Here, we chose to rely primarily on
the visibility improvement at the most impacted Class I area, while
also considering the number of additional Class I areas that would see
improvement, as well as the level of improvement at each area. We did
not expressly rely on a summation of visibility benefits across Class I
areas, as we have done in other regional haze actions, although, as the
commenter points out, this metric was included in some tables. Contrary
to the commenter's assertion, however, a summation of visibility
benefits is not intended to suggest that individually imperceptible
levels of improvement are somehow perceptible, but rather to provide a
single metric that can simultaneously capture both the number of Class
I areas affected and the magnitude of improvement at those areas for
comparison purposes. Moreover, we note that visibility improvement does
not need to be perceptible to be deemed significant for BART purposes.
We have responded in more detail to concerns regarding perceptibility
elsewhere in this document.
    Comment: The commenter states that EPA's cumulative visibility
analyses ignore the discretion given to states in 70 FR 39107; Id. at
39123 (emphasis added); see also 77 FR 24768, 24774 (Apr. 25, 2012)
    Response: As stated above, EPA primarily relied on the benefits at
the area with the greatest visibility improvement from controls, but we
also considered impacts and benefits at nearby areas included in the
modeling analysis. The consideration of visibility benefits over
multiple Class I areas is a useful metric that can further inform a
BART determination.
    Comment: The BART rule provides no support for EPA's ``summation of
cumulative impacts'' approach. Rather, the BART rule makes clear that
the initial focus is expected to be on the ``nearest Class I area'' to
the facility in question. 70 FR 39104, 39162 (Separt 6, 2005). The BART
rules indicate that it is appropriate to take account impacts at not
only the nearest Class I area but also impacts at other nearby Class I
areas, not for the purposing of summing impacts at all of those areas,
but rather for the purpose of ``determin(ing) whether effects at those
(other) areas may be greater than at the nearest Class I area.'' Id.
The BART rule states: ``If the highest modeled effects are observed at
the nearest Class I area, you may choose not to analyze the other Class
I areas any further as additional analyses might be unwarranted.'' Id.
    Response: See our response to comments above. In addition, the BART
Guidelines provide that states, or EPA in lieu of the state, have
discretion on how to assess visibility impacts.
    Comment: The commenter states that the BART rule does not preclude
a state from taking into account, as part of a BART assessment for a
given facility, visibility impacts projected to occur in two or more
Class I areas that are attributable to that facility's emissions.
However, nothing in the rules requires such an analysis, and such
analyses are deceptive when used in a cumulative fashion. EPA did not
have the authority to disapprove Wyoming's visibility improvement
analyses on the grounds that EPA prefers a different approach than the
lawful and permissible approach taken by Wyoming. See Train v. Natural
Res. Def. Council, Inc., 421 U.S. 60, 79 (1975).
    Response: See our response to comments above and elsewhere in this
document (e.g., Legal Issue section) regarding EPA's oversight
authority.
    Comment: EPA has improperly failed to account for the very few
number of days of visibility impacts or the seasonal timing of when
those few impacts occur.
    Response: EPA recognizes that the BART Guidelines allow states to
consider the timing of impacts in addition to other factors related to
visibility impairment. However, states are not required to do so, and
EPA is not required to substitute a source's desired exercise of
discretion for that of the states. Furthermore, when promulgating a
FIP, EPA stands in the shoes of the state. In that capacity, EPA is not
required to consider the seasonality of impacts and has chosen not to
do so here. Taking into account visitation contradicts the goal of the
regional haze rule of improving visibility on the 20 percent best and
worst days. Indeed, EPA believes that the experiences of visitors who
come to Class I areas during periods other than the peak visitation
season are important and should not be discounted.
    Comment: A review of the unit-specific CALPUFF EPA modeling results
developed for the Mount Zirkel Wilderness Area provides a vivid example
of the over-estimation of visibility improvement that EPA is relying on
to justify the installation of hundreds of millions of dollars in
additional controls. The unit specific CALPUFF modeling would indicate
that adding SCR to these units would improve visibility in Mount Zirkel
by over seven deciviews.
    However, the monitored data from 2001-2003 at Mount Zirkel tells a
completely different story. This is the same time period used in the
CALPUFF models to develop the deciview impacts for each Wyoming BART-
eligible unit and to project the visibility improvements associated
with the addition of control devices.
    Looking at the three-year average monitored results, and assuming
that

the nitrates associated with the emissions from all sources (not just
the BART-eligible EGUs) are completely eliminated, only a 0.94 deciview
improvement would be expected. EPA attempts to justify over a billion
dollars in controls at eight PacifiCorp units by assuming more than
seven deciviews of improvement could be obtained from these eight units
when the actual monitored data indicates that only a 0.94 deciview
improvement would be possible if all nitrate was removed from all
sources. In essence, EPA's regional haze FIP fails to recognize that,
given the monitored nitrate impacts, the modeled visibility impacts are
obviously grossly exaggerated.
    Response: We agree with some of these concerns--that the original
modeling performed by the State and EPA used a high background ammonia
concentration and did not correct for ammonia limiting conditions. This
particularly affected the model results in the western part of Wyoming
and Class I areas such as Bridger Wilderness Area.
    We have addressed this concern by adopting a new modeling protocol
that makes several improvements in the model results, including the use
of the current regulatory version of the model, the use of Method 8 to
assess visibility impacts, the use of lower background ammonia
concentrations, and ammonia limiting correction for BART sources with
multiple units. We note that the model overprediction in our re-
proposed modeling analyses occurred at Class I areas affected by BART
sources in western Wyoming, in the region in which monitoring data
showed strong seasonal variability in ammonia concentrations. In
contrast, modeled nitrate impacts from BART sources in eastern Wyoming
were significantly lower than observed nitrate concentrations at
IMPROVE sites at Wind Cave and Badlands in western South Dakota.
    There are several factors that make it challenging to directly
compare CALPUFF results to measured concentrations at IMPROVE
monitoring sites at Class I areas. Most significantly, the monitor
operates every third day, while the model predicts concentration each
day. Moreover, modeled visibility impacts from multiple BART sources
cannot be summed and directly compared to measured data as all BART
sources are unlikely to have their largest impacts on the same Class I
area on the same day. Additionally, the model 98th percentile impact
should be compared to the maximum observed monitoring data because the
highest 2% of model impacts are discarded to address concerns that the
model can overpredict visibility impacts.
    Comment: The commenter cites a study by Terhorst and Berkman which
compared CALPUFF model predicted impacts of the Mojave Power Station at
the Grand Canyon to observed impacts after the facility was closed in
2005. The study concluded that there was virtually no evidence that the
(Mojave) closure improved visibility in the Grand Canyon, and the
commenter cites this conclusion as evidence of the unreliability of the
CALPUFF model.
    Response: We disagree that the Mohave Power Plant study raises
questions about CALPUFF's reliability. The Nevada Division of
Environmental Protection performed CALPUFF modeling to estimate the
contribution of the Mohave Power Plant to visibility impairment at
Grand Canyon National Park. Consistent with the BART Guidelines, the
State used the CALPUFF model to evaluate the Mohave Power Plant
contribution to visibility impairment relative to natural visibility
conditions. Subsequently, after the Mohave Power Plant ceased
operating, Terhorst and Berkman analyzed changes in monitored sulfate
concentrations at the Grand Canyon and calculated the visibility
impacts of those changes relative to current degraded visibility
conditions. Terhorst and Berkman incorrectly concluded that the State's
previous CALPUFF modeling overpredicted the Mohave Power Plant
visibility impacts because Terhorst and Berkman failed to compare their
results to natural visibility conditions. EPA considered and rejected
comments on the proposed BART Guidelines that visibility impacts should
be evaluated to relative to current degraded visibility conditions and
concluded that ``[u]sing existing conditions as the baseline for single
source visibility impact determinations would create the following
paradox: the dirtier the existing air, the less likely it would be that
any control is required.'' (70 FR 39124). Because Terhorst and Berkman
failed to compare observed changes in sulfate concentration to natural
visibility conditions, their analysis does not support the commenter's
statement that CALPUFF is unreliable. This flaw in their analysis has
also been recognized in a paper that responded to their analysis.\122\
Finally, as presented above, the use of the CALPUFF model for regional
haze is a settled manner for which the time for judicial review has
passed.
---------------------------------------------------------------------------

    \122\ W.H. White, R.J. Farber, W.C. Malm, M. Nuttall, M.L.
Pitchford, B.A. Schichtel, Comment on ``Effect of coal-fired power
generation on visibility in a nearby National Park (Terhorst and
Berkman, 2010)'', Atmospheric Environment 55 (2012) 173-178.
doi:10.1016/j.atmosenv.2012.02.076.
---------------------------------------------------------------------------

    Comment: The commenter states that EPA's own studies document that
CALPUFF overstates results and cites a May 2012 EPA sponsored study of
CALPUFF that found ``the current and past CALPUFF model performance
evaluations were consistent with CALPUFF tending to overestimate the
plume maximum concentrations and underestimate plume horizontal
dispersion.''
    Response: In the BART Guidance, EPA recognized concerns that
CALPUFF can overpredict visibility impacts in some cases, and
therefore, as explained above, adopted the use of the 98th percentile
modeled impact, rather than the maximum modeled impact, to address this
concern.
    Comment: EPA appears to take contrary positions in Oklahoma, where
it modeled all visibility impairing pollutants together, and Wyoming,
where EPA said that, based on the State's modeling, EPA ``could not
ascertain what the visibility improvement would be from an individual
NOX or PM control option.''
    Response: It appears that the commenter has confused (1) whether
all pollutants were modeled together; and (2) whether all emission
reductions were modeled together. All pollutants were modeled together
both in modeling performed by Wyoming and by EPA for BART sources in
Wyoming and Oklahoma, consistent with IWAQM Phase 2 report
recommendations and with the State of Wyoming modeling protocol. The
additional modeling performed by EPA was designed to evaluate
visibility improvements from certain emissions reduction technologies;
specifically, to compare the incremental benefits of SCR and SNCR. Each
of these model simulations by EPA also included all other visibility
impairing pollutants, so the approach used by EPA in Wyoming and
Oklahoma is consistent.
    Comment: EPA found that SCR provided only a 0.36 delta deciview
incremental visibility improvement for Dave Johnston Unit 3, using EPA
modeling, with an incremental cost of $7,163.00 per ton. 78 FR 34777-
34778. EPA failed to justify in its proposed rule how a 0.36 delta
deciview improvement, or approximately one-third that humanly
detectible, justifies the tremendous cost of SCR. Likewise, EPA found
that installing SNCR at Dave Johnston Unit 4 results in an incremental
0.11 delta deciview improvement over Wyoming's BART determination at an
incremental cost of

$4,655. 78 FR 34781-34782. The alleged incremental visibility benefit
of installing SNCR at Wyodak is 0.12 delta deciview at an incremental
cost of $3,725 per ton. 78 FR 34784-34785. EPA provides no
justification for requiring such tremendous costs for such an
inconsequential visibility improvement that likely falls within
CALPUFF's margin of error. EPA's modeling approaches are inconsistent
because EPA has determined in other states that visibility improvements
greater than those used to justify SNCR at Wyodak are too small or
inconsequential to justify additional pollution controls. See 77 FR
24794 (0.27 deciview improvement termed ``small'' and did not justify
additional pollution controls in New York); 77 FR 11879, 11891 (0.043
to 0.16 delta deciview improvements considered ``very small additional
visibility improvements'' that did not justify NOX controls
in Mississippi); 77 FR 18052, 18066 (agreeing with Colorado's
determination that ``low visibility improvement (under 0.2 delta
deciview)'' did not justify SCR for Comanche units)). Tellingly, the
``low visibility improvements'' that Colorado found at the Comanche
units not to justify post-combustion NOX controls, as agreed
to by EPA, were 0.17 and 0.14 delta deciview. 77 FR 18066. In Montana,
where EPA issued a regional haze FIP directly, it found that a 0.18
delta deciview improvement to be a ``low visibility improvement'' that
``did not justify proposing additional controls'' for SO2 on
the source. 77 FR 23988, 24012. Here, EPA's actions requiring
additional NOX controls based on little to no additional
visibility improvement are arbitrary and capricious, especially when
EPA did not require additional NOX controls in other states
based on similar visibility improvements. This is particularly true in
Montana where EPA had direct responsibility for the regional haze
program.
    Response: We disagree that visibility improvements at Dave Johnston
Unit 3 and Wyodak are ``too small or inconsequential to justify
additional pollution controls.'' While the visibility benefits at these
units are less than what is generally considered perceptible (1.0
deciview), they are not so low as to preclude selection of the
associated controls without any consideration of the remaining BART
factors. The BART Guidelines are clear that states should consider
visibility impacts that are less than perceptible: ``Even though the
visibility improvement from an individual source may not be
perceptible, it should still be considered in setting BART because the
contribution to haze may be significant relative to other source
contributions in the Class I area. Thus, we disagree that the degree of
improvement should be contingent upon perceptibility. Failing to
consider less-than-perceptible contributions to visibility impairment
would ignore the CAA's intent to have BART requirements apply to
sources that contribute to, as well as cause, such impairment.'' 70 FR
39129. When the visibility improvements are considered, we continue to
find that this level of improvement, when considered along with the
other statutory factors, justifies the selected BART controls.
    Finally, regarding commenter's assertions that we are being
inconsistent, because the commenter is only specific about visibility
improvement, it is not possible for EPA to address in this response any
specific concerns. As articulated in our proposed rulemakings and
further explained in our responses to other comments, EPA's partial
approval and partial disapproval of the Wyoming Regional Haze SIP is
consistent with the CAA, the RHR, BART Rule, and EPA guidance. Our
determinations considered all five factors, not just visibility
improvement.
    Comment: Although it is true that Wyoming did not model the
visibility impact of SNCR, that fact is no justification for
disapproving Wyoming's BART. Nothing in the BART Guidelines or
Wyoming's BART Modeling Protocol demands modeling of SNCR, and EPA
points to nothing in either document that requires modeling of SNCR.
    Response: We disagree with this comment. The BART selection process
requires a comparison between all technically feasible control options,
not the evaluation of individual control technologies in isolation.
While the BART Guidelines do not specify the order in which control
options must be evaluated (e.g., beginning with the most stringent or
beginning with least stringent control), they do specify that the CAA
factors must be considered for all options: ``In the final guidelines,
we have decided that States should retain the discretion to evaluate
control options in whatever order they choose, so long as the State
explains its analysis of the CAA factors.'' 70 FR 39130. The only
exceptions are ``. . . if you find that a BART source has controls
already in place which are the most stringent controls available . .
.'', or ``. . . . if a source commits to a BART determination that
consists of the most stringent controls available . . .'' 70 FR 39165.
In these situations, it is not necessary to complete an analysis of all
five BART factors. Therefore, because neither of these criteria was
met, the State was required to perform an analysis of all five BART
factors for all technically feasible control options. As such, the
State's failure to consider the visibility impacts of SNCR did in fact
serve as appropriate grounds for EPA's disapproval of Wyoming's BART
determination.
    Comment: EPA modeling shows no significant visibility improvement
from SNCR and without a significant improvement there can be no
justification for disapproving the State's BART for Laramie River
Station, and, to the contrary, EPA's modeling supports Wyoming's
rejection of SNCR and choice of LNB/OFA because SNCR provides
negligible visibility improvement.
    Response: We have required new LNBs with OFA and SCR for the
Laramie River Station, not new LNBs with OFA and SNCR, which is the
control option addressed by the commenter. Accordingly, we do not find
that the comment is relevant to our action. Our revised modeling shows
that the visibility benefit of new LNBs with OFA and SCR for Laramie
River Units 1-3 is 0.57 deciviews, 0.53 deciviews, and 0.52 deciviews,
respectively. We continue to find that the visibility benefit, when
taking into consideration the remaining BART factors, justifies
installation of new LNBs with OFA and SCR.
    Comment: Basin Electric submitted results based on more accurate
modeling than EPA, which show that actual visibility improvement from
SNCR would be substantially lower than assumed by EPA. There is no
justification for disapproving Wyoming's BART based on a modeled
visibility improvement that is such a small fraction of what is humanly
perceptible.
    Response: As described in response to other comments, we agree that
the original modeling protocol adopted by the State was inconsistent
with the BART Guidelines, IWAQM Phase 2 report and newly available
ambient monitoring data, and in our revised modeling we adopted several
of the changes recommended by this commenter, including the use of
lower background ammonia concentration, a correction for ammonia
limiting conditions for multiple units located at a single BART source,
and the use of Method 8 for the evaluation of visibility impairment.
However, even using these model options, we still found significant
visibility impacts for SCR control at

Laramie River. Our results are generally consistent with the modeling
results submitted by the commenter which also show significant impacts.
As described in another response, while the visibility benefits at each
of these units individually are less than what is generally considered
perceptible (1 deciview), they are not so low as to preclude selection
of the associated controls without any consideration of the remaining
BART factors. The BART Guidelines are clear that States should consider
visibility impacts that are less than perceptible because these sources
may still contribute to cumulative visibility impairment.
    Comment: EPA did not assert a failure to model NOX
impacts separately was a flaw in the Laramie River Station modeling,
although EPA did identify this as a flaw in PacifiCorp modeling.
    Response: We agree that the State evaluated NOX impacts
separately for the control technologies that the State included in its
modeling, however, the State did not evaluate SNCR. The other
deficiencies in the State's visibility analysis, including the failure
to consider the visibility impacts of SNCR, were appropriate grounds to
disapprove the State's BART determination.
    Comment: The Wyoming modeling did in fact isolate the impact on
visibility for NOX control alternatives. Wyoming held
SO2 and PM emissions constant at baseline levels while
modeling varying NOX emission rates for each of the
NOX control options.
    Response: We agree with this comment. Nonetheless, as stated above,
the other deficiencies in the State's visibility analysis, which were
inconsistent with the BART Guidelines, including the failure to
consider the visibility impacts of SNCR, were appropriate grounds to
disapprove the State's BART determination.
    Comment: EPA claims that Wyoming modeled the wrong emission rates.
EPA notes that in its cost analysis it calculated a new removal
efficiency for NOX control options that was different than
the removal efficiency calculated by Wyoming, and claims that
visibility modeling should have used the EPA efficiencies. However, EPA
does not explain how modeling with the different removal efficiencies
conflicts with the BART Guidelines or the CAA. As to SNCR, EPA argues
that the State assumed a higher removal efficiency and thus,
paradoxically, modeling with the State's removal assumption would yield
greater visibility improvement than modeling with EPA's values. No such
modeling was done, however. The State did no modeling for SNCR, so the
State's removal efficiency was never modeled. It is an enigma how EPA
can disagree with modeling with the different SNCR removal values when
such modeling was never performed.
    Response: We disagree with this comment. The BART Guidelines are
clear on how removal efficiencies should be considered in the
visibility evaluation: ``Post-control emission rates are calculated as
a percentage of pre-control emission rates. For example, if the 24-hr
pre-control emission rate is 100 lb/hr of SO2, then the post
control rate is 5 lb/hr if the control efficiency being evaluated is 95
percent.'' 70 FR 39170. Therefore, because the control efficiencies
assumed by the State differed from those found by the EPA, they
affected the calculation of post-control emission rates for modeling
purposes (and thereby the consideration of visibility impacts).
    In regard to SNCR, as conceded by the commenter, the State did not
provide the visibility impacts associated with the control option. As
discussed elsewhere, failure to assess the visibility impacts of a
technically feasible control option is in clear conflict with the
requirements of the CAA and BART Guidelines. This failure alone,
regardless of the control efficiency assumed for SNCR, was sufficient
grounds for us to reject the State's BART determination. Moreover, the
incorrect removal efficiency for SNCR assumed by the State adversely
affected their analysis of cost of compliance, another statutorily
required BART factor.
    To put it simply, the State failed in the first instance by not
considering the visibility improvement of SNCR as required by the CAA
and BART Guidelines. When EPA corrected this deficiency by performing
the modeling ourselves, it was necessary for us to correct the removal
efficiency of SNCR (as discussed in response to other comments).
    Comment: The State assumed that SCR would reduce NOX
emissions from 0.21 lb/MMBtu to 0.07 lb/MMBtu--a reduction of 0.14 lb/
MMBtu. EPA assumes SCR would reduce NOX emissions from 0.19
lb/MMBtu to 0.05 lb/MMBtu, a reduction of the same 0.14 lb/MMBtu. All
other things being held constant, the 0.14 lb/MMBtu reduction will in
both cases yield an identical reduction in the visibility impairing
concentration of nitrate particulate in a Class I area.
    Response: We disagree with this comment. The CALPUFF model
simulations estimate the visibility impairment attributed to the
emissions in each control scenario, not the relative reduction in
different control scenarios. Therefore, an emissions rate of 0.07 lb/
MMBtu will have 40% greater total emissions and a larger visibility
impact than an emissions rate of 0.05 lb/MMBtu.
    Comment: EPA argues that Wyoming should have used a baseline of the
maximum 24-hour average NOX emission rate during the
baseline years of 2001-2003, and instead used an annual average
baseline rate. The BART Guidelines do not mandate the use of the 24-
hour maximum but, rather, ``recommend that the State use the highest
24-hour average actual emission rate'' and that the states should have
flexibility when evaluating the fifth statutory factor. The BART
Guidelines by their express terms authorize states to use baseline
emissions other than the 24-hour maximum rate. Use of the 24-hour
maximum baseline is not mandatory, and not using that baseline is not a
failure to comply with any requirement in the Guidelines. EPA itself
used annual average pre-control and post-control emission rates to
model visibility impacts in its Nevada FIP rulemaking.
    Response: We disagree with this comment. As stated in the preamble
to the BART Guidelines, ``the emissions estimates used in the
[visibility] models are intended to reflect steady-state operating
conditions during periods of high capacity utilization.'' 70 FR 39120.
As such, the BART Guidelines recommend excluding emissions during
periods of start-up, shutdown, or malfunction or estimating visibility
impacts based on a source's allowable emissions as this could inflate
the visibility impacts of a source. Rather, for sources such as power
plants where States have information on a source's daily emissions, the
BART Guidelines explains that an emission rate based on a source's
maximum actual emissions over a 24-hour period is an appropriate gauge
of a source's potential impact as it ensures that peak emission
conditions are reflected but would be unlikely to lead to an
overestimation of a source's potential impacts. Id. The BART Guidelines
state that in developing a modeling protocol, States should ``[u]se the
24-hour average actual emission rate from the highest emitting day of
the meteorological period modeled (for the pre-control scenario).'' Id.
and 70 FR 39170.
    Wyoming did not do this. Instead, in assessing the improvement in
visibility associated with the use of controls in its BART
determinations, Wyoming used the visibility modeling performed by
PacifiCorp and Basin Electric for their facilities. Although these
companies used very different approaches to estimating the baseline
emission rate--

neither of which used the 24-hour average actual emission rate--the
State accepted the visibility modeling done by both and submitted the
results in the Wyoming SIP. Even if the commenter were correct that the
approach in the BART Guidelines is only recommended, the commenter has
not provided any explanation as to why the disparate approaches used in
the Wyoming SIP were appropriate for estimating the degree of
visibility improvement associated with controls. Wyoming similarly
provided no explanation as to why the varying approaches adopted by
Basin Electric and PacifiCorp were appropriate for assessing visibility
improvement. Moreover, the commenter has not established that the
baseline emission rates used by Wyoming would accurately reflect
visibility impacts associated with steady-state operating conditions
during periods of high capacity utilization. Obviously, baseline
emission rates reflecting periods of relatively lower capacity
utilization would tend to underestimate peak visibility impacts.
Consider for example the baseline emission rate used for Laramie River
Unit 1. There, the State used a daily emission rate equating to 6,320
tons per year,\123\ while, based on actual emissions data, the EPA used
a daily rate equating to 8,786 tons per year.\124\ Thus, the rate used
by the State reflects a period of considerably lower capacity
utilization that would therefore tend to underestimate peak impacts.
---------------------------------------------------------------------------

    \123\ Wyoming Department of Environmental Quality Air Quality
Division BART Application Analysis AP-6047, Table 16, May 28, 2009.
    \124\ Air Quality Modeling Protocol: Wyoming Regional Haze
Federal Implementation Plan, U.S. EPA, Table B.9, January, 2014.
---------------------------------------------------------------------------

    Regarding the emissions rates used in the Nevada regional haze SIP,
the State did not use the 24-hour average of actual emissions from the
highest emitting day in its BART determination for Reid Gardner
Generating Station. 77 FR 50936, 50944 (Aug. 23, 2012). As part of its
review of the Nevada SIP, EPA performed new visibility modeling. In
that modeling exercise, EPA used Nevada's emission rates based on
annual averages. Please refer to a related comment and response in the
final action for that rule for a full discussion. See 77 FR 50944).
Following our review of comments in that rulemaking--including comments
that we should have used the Guidelines maximum 24-hour average of
emissions in our visibility modeling--we scaled our estimates of the
visibility impacts of controls based on the source's emissions using
the Guidelines maximum 24-hour average. We took these scaled visibility
impacts into account in our final action. Id. at 50945.
    Comment: EPA did not use the 24-hour maximum rate for the modeling
it performed in 2012. As noted in Section VIII.C, it used the same
baseline emission rates used by the State. EPA-R08-OAR-2012-0026-0037.
EPA did not find the State's approach was a violation of the BART
Guidelines or was a reason to disapprove the State's modeling or BART
determination. Having sanctioned the use of a different baseline then,
EPA may not now claim it violates the BART Guidelines or a ground for
disapproving the State's modeling or BART determination.
    Response: We agree that we did not use the 24-hour maximum [actual]
emission rates for modeling purposes in our original proposed rule
published in 2012. However, we did not finalize that rule, at least in
part, for the very reason that the baseline emission rates calculated
by Wyoming, and subsequently used by EPA in the 2012 proposed rule,
were inconsistent with the BART Guidelines. As we never finalized the
original rule, we disagree with the commenter's suggestion that we
somehow sanctioned Wyoming's approach. A proposed rule does not
represent final agency action.
    Comment: The maximum improvement modeled by EPA that would be
achieved at any Class I area by adding SCR to the existing new LNB plus
OFA is 0.5 delta deciview. This is below the 1.0 delta deciview level
often cited as the lowest level of change that is humanly perceptible.
For EPA to propose disapproval of the State's BART based on an
imperceptible improvement is to propose disapproval based on a
nonmaterial factor.
    Response: We disagree that the visibility improvements for Laramie
River or Jim Bridger are de minimis or too small to just justify the
expense of requiring controls. As discussed in response to another
comment, the BART Guidelines are clear that it is not necessary for the
visibility improvement of a particular control option to be above the
perceptible threshold. The regional haze program is premised on the
fact that numerous sources are contributing to visibility impairment
and numerous sources will need to reduce emissions in order to improve
visibility. We continue to find that this level of improvement, when
considered along with the other statutory factors, justifies the
selected BART controls.
    Comment: The commenter states that EPA's modeled visibility
improvement overstates the improvement that would actually be achieved.
The commenter submitted new modeling showing that the visibility
improvement from further reductions of NOX emissions would
be much smaller than that predicted by EPA. AECOM corrected four of the
flaws in EPA's modeling and re-ran CALPUFF. The commenter submitted
refined modeling with four adjustments: 1. The use of seasonal
background ammonia concentrations; 2. Modeling of all units together
with correction for ammonia limited conditions; 3. Use of a post-
control emission rate of 0.07 lb/MMBtu, consistent with EPA's proposed
emission limit; and 4. The use of CALPOST Method 8. AECOM's revised
modeling was identical to EPA's in all other respects. The refined
modeling predicted that the incremental visibility benefit of SCR at
each of Laramie River Units 1, 2 and 3 would range between 0.20-0.24
delta deciview at either Badlands or Wind Cave National Park. The
actual visibility improvement of SCR would be even less than predicted
by the refined modeling because CALPUFF is known to substantially
overstate nitrate haze.
    Response: We agree that our proposed modeling was inconsistent with
the BART Guidelines, IWAQM Phase 2 report and monitored data, and in
our revised final modeling we adopted several of the changes
recommended by this commenter, including the use of lower background
ammonia concentration, a correction for ammonia limiting conditions for
multiple units located at a single BART source, and the use of Method 8
for the evaluation of visibility impairment. However, even using these
less conservative model options, we still found significant visibility
impacts for SCR control at the Basin Electric Laramie River EGUs. We
did not use the seasonal background ammonia concentration proposed by
the commenter because we did not have sufficient ambient monitoring
data to determine the seasonal background concentrations in eastern
Wyoming.
    Comment: The commenter states that nitrate haze occurs primarily in
the winter when few visitors are present in Class I areas. During the
peak summer visitation period, the impact of wildfires would overwhelm
any marginal visibility improvement that might be achieved by SCR. The
commenter cites an EPA report that stated ``[A] all else being equal,
impairment from anthropogenic sources is considerably more
objectionable during times of the year with greatest visitor attendance
(e.g., summer). Visibility objectives might, therefore, be stated in
terms of acceptable frequency distributions of visibility (e.g.,
contrast) over the course of a year.'' Source: Report to Congress under
CAA Section 169A(a)(3). The

commenter states that these factors further support Wyoming's decision
to reject SCR and SNCR as BART. These technologies would not improve
visibility on the worst haze days because Laramie River doesn't
contribute to haze on those days, and any slight visibility improvement
would occur in the winter season when few visitors enter the Class I
areas. Wyoming's decision to reject SCR as BART is therefore reasonable
and complies with the CAA.
    Response: As discussed in response to another comment, EPA agrees
that nitrate impacts are more dominant in the winter. Nonetheless,
daily nitrate impacts from April through October are not trivial. EPA
also agrees that the BART Guidelines allow states to consider the
timing of impacts in addition to other factors related to visibility
impairment. However, states are not required to do so, and EPA is not
required to substitute a source's desired exercise of discretion for
that of the states. Furthermore, when promulgating a FIP, EPA stands in
the shoes of the state. In that capacity, EPA is not required to
consider the seasonality of impacts and has chosen not to do so here.
Taking into account visitation contradicts the goal of the regional
haze rule of improving visibility on the 20 percent best and worst
days. Indeed, EPA believes that the experiences of visitors who come to
Class I areas during periods other than the peak visitation season are
important and should not be discounted.
    Comment: We received comments that our FIP was not warranted
because the cause of visibility impairment during the times of peak
visitation was wildfires and thus does not justify the control of
NOX from stationary sources.
    Response: See response above.
    Comment: Ammonia levels at the altitude of the plume would be lower
than the reported surface level ammonia concentrations, so less ammonia
would be available to form visibility-impairing nitrate.
    Response: We disagree that there is compelling evidence that
background ammonia levels are significantly different at the altitude
of the plume compared to the surface. While there are limited studies
showing vertical gradients of ammonia in the troposphere,\125\ these
studies do not show a strong gradient within the planetary boundary
layer where the plume is typically located. Moreover, as discussed in
the response to another comment, it is necessary to evaluate the
combined concentrations of gas ammonia and particulate ammonium to
estimate the background ammonia level, so vertical gradients in
measured ammonia alone are not sufficient to specify the vertical
gradient in background ammonia. It is possible that decreasing
temperature with altitude could affect the thermodynamic equilibrium
between gas ammonia and particulate ammonium and that this could
contribute to observed vertical gradients in ammonia. It is also
possible that dry deposition of ammonia at the surface could create a
negative vertical gradient in ammonia near the surface. We recognize
that there are limited measurement studies available for total gas
ammonia and particulate ammonium, and as a result there is uncertainty
in the estimate of background ammonia. Given this uncertainty, we
believe it is appropriate to rely on measurement studies of total gas
ammonia and particulate ammonium when available and reliable as
explained elsewhere in this document (along with the IWAQM Phase 2
report default values), and to rely on the IWAQM Phase 2 report where
monitoring data are not available.
---------------------------------------------------------------------------

    \125\ Levine et al., (1980) The Vertical Distribution of
Tropospheric Ammonia, Geophys. Res. Letters, vol. 7, No. 5, 17-32.
---------------------------------------------------------------------------

    Comment: The commenter states that inventories show very low
ammonia concentrations in the corridors between Laramie River Station
and the relevant Class I areas.
    Response: We disagree with the commenter's assertions. We note that
there is large uncertainty in estimates of ammonia emissions
inventories that are based on source activity data and emissions
factors. Moreover, even when more certain estimates of ammonia
emissions are available, it is not possible to estimate ambient ammonia
concentrations based on emissions inventory data alone. An estimate of
ambient ammonia levels would require an evaluation of modeled emissions
data and the effects of transport, dispersion and removal of ambient
ammonia. Direct measurements of ambient concentrations of gas ammonia
and particulate ammonium provide a more reliable estimate of background
ammonia than do model simulations of the emissions, transport,
dispersion and removal of ammonia.
    Comment: The commenter states that reliable ammonia measurements
from the IMPROVE monitor located in the Wind Cave National Park were
published in September 2012. Chen et al, available at AECOM Report.
This monitor provides actual ground level ammonia data that is
representative of the Class I areas that are relevant to Laramie River
Station. AECOM Report at 4. EPA has given no explanation for its
assumption of a constant 2.0 ppb background concentration in this case.
    Response: We evaluated the ammonia and ammonium monitoring data
submitted by the commenter in Exhibit 6, which is an extended abstract
presented at the 2010 conference in Whitefish, MT.\126\ The data are
from a pilot study conducted from April 2011 to January 2012 designed
to measure total NHX as the sum of ammonia and particulate
ammonium at 9 IMPROVE sites. The pilot study includes data for IMPROVE
monitoring sites at Wind Cave and Rocky Mountain National Park, which
are Class I areas for which we evaluated visibility impacts in this
action. We note that the pilot study data are for less than one full
year and are plotted in Figure 1 of the report as monthly average
concentrations. The measured values of NHX are not reported,
but the plot does show seasonal variation in NHX
concentrations, as expected, with higher NHX concentrations
in summer and lower concentrations in winter. Annual average
NHX concentrations cannot be estimated from the plot itself,
but they appear to be approximately consistent with the default IWAQM
ammonia background concentration of 0.5 ppb for forested areas. Given
that both the Rocky Mountain and Wind Cave Class I areas have
significant forest cover, the measurements in the pilot study appear to
be consistent with the IWAQM Phase 2 report.
---------------------------------------------------------------------------

    \126\ Chen et al., A Pilot Monitoring Study of Atmospheric
NHX at Selected IMPROVE sites AWMA Aerosol and
Atmospheric Optics, Visibility & Air Pollution Conference, September
24-28, 2012, Whitefish, MT.
---------------------------------------------------------------------------

    Measurements of NHx are not reported for Badlands
National Park, which is a mix of bare rock and mixed-grass prairie
ecosystems. Based on the IWAQM Phase 2 report, default background
ammonia concentrations in the range of 1 to 10 ppb at 20 degrees
Celsius would be appropriate for this region. We reviewed the ambient
ammonia monitoring data on which the IWAQM Phase 2 report was based,
and the data for grasslands were largely based on measurements at
Pawnee National Grassland, where average ammonia levels in summer were
10 ppb. Because the Pawnee National Grassland is located close to large
agricultural and livestock ammonia sources in eastern Colorado, it is
uncertain if the same ammonia levels would be appropriate for the more
Badlands area. Therefore, we selected a background ammonia
concentration of 1 ppb for CALPUFF modeling of BART sources that impact
the Wind Cave and Badlands Class I areas.

    Comment: The commenter states that if EPA uses the maximum 24-hour
NOX emissions rate when modeling baseline visibility
impacts, it should also use the maximum 24-hour SO2 and
PM10 emissions rates for the baseline. NOX
competes with SO2 for ammonia to make either ammonium
nitrate or ammonium sulfate. Setting the SO2 baseline rate
at a low concentration relative to NOX skews the model to
predict the formation of more ammonium nitrate and less ammonium
sulfate. This magnifies the modeled benefits of reducing NOX
emissions.
    Response: We agree that we did not use the maximum [actual] 24-hour
emission rates for SO2 and PM10 as we did for
NOX. However, we have not found based on our analysis, and
the commenter has not established, that doing so had any material
impact on the modeled benefits associated with NOX controls.
The BART sources in Wyoming that are covered in this action are
subject-to-BART only for NOX and PM. In addition, we
considered comments on, but did not question the validity of the
State's BART analyses for PM. In fact, as explained in detail elsewhere
in this document, with respect to the State's PM BART determinations,
the State's SIP and existing information was adequate to find that the
PM BART determinations were reasonable. Accordingly, the purpose of our
modeling effort was to identify the visibility improvement associated
with NOX controls, not SO2 or PM controls. And
so, in evaluating the visibility of NOX controls, we held
the SO2 and PM emissions constant at the rate associated
with the ``committed controls'' identified by the State. Therefore,
even if there was a discernible impact on the modeled visibility
benefit of NOX controls related to our treatment of modeled
emission rates for SO2 and PM, it would be common to all of
the modeled NOX control scenarios and would not have favored
one control option over another.
    Comment: The visibility improvement from SCR will be much less than
EPA claims. The modeling preformed by AECOM and Wyoming produced
similar results, and both predicted much less visibility improvement
than EPA.
    Response: The modeling performed by Wyoming used the 2 ppb
background concentration that was established in the State's protocol,
and this resulted in model visibility impacts that were significantly
greater than those estimated by AECOM in its modeling using lower,
seasonally varying background ammonia concentrations. The ammonia
concentrations in ppb used in the AECOM modeling for the months of
January December were as follows: 0.3, 0.9. 0.9, 1.0, 1.0, 2.0, 2.0,
1.0, 0.8, 0.8, 0.6, and 0.3. We note that our modeling results in our
original proposal also used the State's protocol, and our model results
were identical to the State's modeling results for the emissions
scenarios that both the State and EPA evaluated. The EPA modeling
results in our revised proposal showed larger visibility impacts
because we corrected the baseline emissions rates to make the emissions
consistent with the BART Guidance. In the new modeling results that we
performed using our revised final EPA Protocol and included in this
action, we used a model configuration that is generally consistent with
modeling submitted by the commenter. The revisions to the protocol
include reduced background ammonia, correction for ammonia limiting
conditions, updated regulatory versions of the model, and the use of
Method 8. The commenter did not submit model results for all emissions
scenarios in a format that can be directly compared to our tabulated
model results, but our revised model results in this action appear to
be generally consistent with the commenter's model results, and these
results do show that SCR at Basin Electric Laramie River has
appreciable visibility benefit at the Wind Cave and Badlands Class I
areas.
    Comment: The commenter states that the version of CALPUFF used by
Wyoming and EPA (version 5.711a) relies on simplified chemistry
algorithms that overstate nitrate formation and overpredict visibility
impacts, and that EPA acknowledges that ``the simplified chemistry in
the [CALPUFF] model tends to magnify the actual visibility effects of
[a] source.'' 70 FR 39121. Papers by Morris et al. and Karamchandani et
al. show that CALPUFF chemistry overpredicts nitrates by a factor of 2-
to-4 times in winter.
    Response: As described in responses to other comments and in our
modeling protocol, EPA used the currently approved CALPUFF version 5.8
for modeling used in this action. EPA has acknowledged in the BART
Guidelines that there is uncertainty in the CALPUFF modeled visibility
impacts. EPA recognized the uncertainty in the CALPUFF modeling results
when EPA made the decision, in the final BART Guidelines, to recommend
that the model be used to estimate the 98th percentile visibility
impairment rather than the highest daily impact value. While
recognizing the limitations of the CALPUFF model in the BART Guidelines
Preamble, EPA concluded that, for the specific purposes of the Regional
Haze Rule's BART provisions, CALPUFF is sufficiently reliable to inform
the decision making process.
    Comment: The commenter states that a study by the CDPHE showed
model results for nitrates will be skewed high by assuming higher than
actual background levels of ammonia.
    Response: The CDPHE completed a CALPUFF model sensitivity study
that evaluated the effect of the background ammonia concentration on
model predictions for ammonium nitrate and sulfate. The CDPHE found
that CALPUFF model predicted nitrate was insensitive to variations in
background concentrations greater than 10 ppb and became progressively
more sensitive to background ammonia as it was reduced from 10 to 0
ppb.\127\ We note that CDPHE performed a sensitivity study but did not
evaluate model performance and did not identify any particular case in
which model performance was skewed by the use of inappropriate
background ammonia concentrations. The conclusions of the CDPHE study
are fully consistent with the IWAQM Phase 2 report, which also
recognized that accurate specification of background ammonia ``is
critical to the accurate estimation of particulate nitrate
concentrations.'' \128\
---------------------------------------------------------------------------

    \127\ CDPHE, Ibid.
    \128\ IWAQM, page 14 and page 21
---------------------------------------------------------------------------

    Comment: The commenter cites several presentations and studies that
document flaws in CALPUFF's sulfate and nitrate chemistry: (1) It is
out of date, overly simplistic, and inaccurate; (2) CALPUFF greatly
overstated sulfate and nitrate in winter, overestimating visibility
impacts by 100-1000% in many cases; and (3) that the model understated
sulfate in summer; and that nitrate predictions were particularly
inaccurate, overstated, and unreliable.
    Response: EPA recognized the uncertainty in the CALPUFF model when
EPA made the decision, in the final BART Guidelines, to recommend that
the model be used to estimate the 98th percentile visibility impairment
rather than the highest daily impact value. While recognizing the
limitations of the CALPUFF model in the BART Guidelines, EPA concluded
that, for the specific purposes of the RHR's BART provisions, CALPUFF
is sufficiently reliable to inform the decision making process.
    Comment: NOX emissions control has little visibility
benefit during summer when visibility impairment is dominated by
wildfires.
    Response: EPA agrees that nitrate impacts are more dominant in the
winter. The CALPUFF model results are

consistent with these observations, with the largest modeled visibility
improvements occurring from late fall to early spring. NOX
emissions are precursors to ammonium nitrate, and high concentrations
of ammonium are typically observed from late fall to early spring when
cold temperatures and high relative humidity results in thermodynamic
conditions that favor the formation of ammonium nitrate. Observed
ammonium nitrate concentrations are typically low during summer because
warm temperatures result in thermodynamic conditions that are not
favorable to the formation of ammonium nitrate. Nonetheless, there may
be higher nitrate concentrations on colder days during this period.
    Comment: EPA's visibility-benefits analysis still is constrained in
the re-proposed Wyoming haze plan because EPA has not identified the
visibility benefits from BART controls across all of the Class I areas
affected by haze-causing pollutants from Wyoming sources. Wyoming EGUs
impact visibility over at least 18 Class I areas. While EPA's own
visibility modeling fully supports determinations that SCR is BART for
all Wyoming EGUs, the visibility benefits of SCR across all affected
Class I areas are cumulatively significant and, if the RHR's
fundamental purpose is to be fulfilled, they must not be ignored.
    Response: EPA disagrees with this comment. The commenter's number
of ``at least 18 Class I areas'' is derived by including Class I areas
that are more than 300 km from BART sources. EPA disagrees that these
Class I areas should be included in the visibility analysis. The IWAQM
Phase 2 report reviewed model performance evaluations of CALPUFF as a
function of distance from the source and concluded that:''Based on the
tracer comparison results presented in Section 4.6, it appears that
CALPUFF provides reasonable correspondence with observations for
transport distances of over 100 km. Most of these comparisons involved
concentration values averaged over 5 to 12 hours. The CAPTEX
comparisons, which involved comparisons at receptors that were 300 km
to 1000 km from the release, suggest that CALPUFF can overestimate
surface concentrations by a factor of 3 to 4. Use of the puff splitting
option in CALPUFF might have improved these comparisons, but there are
serious conceptual concerns with the use of puff dispersion for very
long-range transport (300 km and beyond). As the puffs enlarge due to
dispersion, it becomes problematic to characterize the transport by a
single wind vector, as significant wind direction shear may well exist
over the puff dimensions. With the above thoughts in mind, IWAQM
recommends use of CALPUFF for transport distances of order 200 km and
less. Use of CALPUFF for characterizing transport beyond 200 to 300 km
should be done cautiously with an awareness of the likely problems
involved.'' \129\ We present additional discussion of this issue in our
response to the following comment.
---------------------------------------------------------------------------

    \129\ IWAQM, p.18.
---------------------------------------------------------------------------

    Comment: EPA arbitrarily failed to model visibility impacts of the
various control options at all affected Class I areas, including those
that are beyond 300 km from the source. EPA recently responded to a
similar comment in its final action promulgating the Montana Regional
Haze FIP, 77 FR 57864, for the first time supporting its truncated
modeling by referencing a now-discredited 1998 report regarding CALPUFF
performance. Because EPA raised this issue only after the close of the
public comment period on its Wyoming regional haze action, EPA should
consider the Conservation Organizations' response. See 42 U.S.C.
7607(d)(4)(B)(i).
    In its response to public comments on the Montana FIP, EPA stated,
``The Interagency Workgroup on Air Quality Modeling (IWAQM) Phase 2
report (EPA, 1998) reviewed model performance evaluations of CALPUFF as
a function of distance from the source and concluded that: . . .[u]se
of CALPUFF for characterizing transport beyond 200 to 300 km should be
done cautiously with an awareness of the likely problems involved.'' 77
FR 57867-68. EPA then concludes, ``[t]herefore, given that the IWAQM
guidance provides for the use of the CALPUFF model at receptor
distances of up to 200 to 300 km, and given that EPA has already
addressed uncertainty in the CALPUFF model, we believe it is reasonable
to use CALPUFF to evaluate visibility impacts up to 300 km.'' Id. at
57868.
    We agree that CALPUFF is reliable at distances of 300 km. However,
EPA's use of the IWAQM Phase 2 report to support its decision to
exclude modeling at distances beyond 300 km is arbitrary. First,
changes to CALPUFF since 1998 may correct problems identified in the
IWAQM Phase 2 report with modeling accuracy in the 200-1,000 km range.
Second, a more recent study prepared for EPA called into question the
conclusions of the IWAQM Phase 2 report upon which EPA relies. See Long
Range Transport Models Using Tracer Field Experiment Data (May 2012)
(EPA Contract No: EP-D-07-102, Work Assignment No: 4-06). The May 2012
study concluded that ``The inability of most (~90%) of the current
study's CALPUFF sensitivity tests to reproduce the 1998 EPA study
tracer test residence time on the 600 km receptor arc is a cause for
concern.'' Not only were the authors of the May 2012 study unable to
reproduce the 1998 study's findings that CALPUFF overestimated
pollutant concentrations at distances of 600 km, the 2012 study
concluded that CALPUFF actually underestimates average pollutant
concentrations at 600 km. Accordingly, reliance on CALPUFF at long
distances would result in conservative estimates of visibility impacts.
It is not appropriate to assume, as EPA effectively did in its Wyoming
proposal, that such impacts are non-existent. EPA's failure to model
and consider visibility impacts at all affected Class I areas,
including those beyond 300 km, is not supported.
    Because the RHR, and SIPs and FIPs promulgated to implement it, are
to fulfill CAA requirements to mitigate and ultimately eliminate
anthropogenic sources of haze pollution at all Class I national parks
and wilderness areas, it is imperative that states and EPA use models
to completely and accurately depict the visibility impact of a source
to the region's Class I areas as well as projected benefits from BART.
In this regard, the conclusion of the May 2012 study that CALPUFF
reliably (if conservatively) identifies visibility impacts to Class I
areas beyond those previously evaluated are critical, and directs EPA
to supplement the incomplete analysis presented in its proposed action
on the Wyoming Regional Haze plan with additional modeling, or consider
the more complete modeling submitted by the conservation organizations
with their August 2, 2012 comments.
    Response: EPA disagrees with the commenter's assertion that changes
to CALPUFF now support modeling at distances greater than 300 km. The
commenter cited a May 2012 technical evaluation (Documentation of the
Evaluation of CALPUFF and Other Long Range Transport Models Using
Tracer Field Experiment Data \130\) that evaluates several long range
transport models based on several tracer studies. The report cited by
the commenter does not refute the IWAQM Phase 2 report which states
that ``IWAQM recommends use of CALPUFF for transport distances of order
200 km and less. Use of CALPUFF for characterizing transport

beyond 200 to 300 km should be done cautiously with an awareness of the
likely problems involved.'' \131\ In fact, the May 2012 report further
``emphasizes the need for a standardized set of options for regulatory
CALPUFF modeling.'' \132\ Given these findings, EPA does not agree, as
the commenter asserts, that it must consider CALPUFF modeling results
from Federal Class I areas beyond 300 km. EPA therefore believes that
the results of CALPUFF modeling beyond 300 km of the source should be
evaluated in light of the limitations discussed in the two guidance
documents cited above.
---------------------------------------------------------------------------

    \130\ http://www.epa.gov/scram001/reports/EPA-454-R-12-003.pdf.
    \131\ http://www.epa.gov/scram001/7thconf/calpuff/phase2.pdf,
page 18.
    \132\ http://www.epa.gov/scram001/reports/EPA-454-R-12-003.pdf,
page 10.
---------------------------------------------------------------------------

    Finally, we disagree that there is any notice issue with respect to
the commenter's allegations that EPA referenced the 1998 IWAQM study
for the first time in our response to comments in our Montana FIP
action. As quoted above, the BART guidelines specifically reference the
1998 IWAQM study with respect to CALPUFF settings.
    Comment: EPA modeled visibility benefits at four Class I areas, and
demonstrated visibility improvement due to SCR that approximately
doubled the improvement afforded by SNCR at every Class I area modeled.
78 FR 34775-34776. EPA properly took account of the cumulative
visibility improvement across all four modeled Class I areas for each
unit, id. at 34776, but in fact, as the Conservation Organizations
commented previously, see 8/2/2012 Conservation Organization Comments,
SCR affords visibility benefits across at least six Class I areas.
Thus, the cumulative visibility benefits are even greater than found by
EPA, and further support a determination that SCR is BART for Laramie
River Station Units 1-3.
    Response: We disagree that we should have evaluated visibility
impacts at all of the areas that the commenter considered in its
analysis. The commenter provided CALPUFF model results at 18 areas,
including areas that are not mandatory Class I areas, and at Class I
areas at distances greater than 300 km from Laramie River Station. In
our analysis of visibility impacts, we considered the visibility
improvement at four Class I areas within 300 km of the Laramie River
Station. Therefore, our modeling analyses did not ignore the visibility
improvement that would be achieved at areas other than the most
impacted Class I area, and we disagree with the assertion that we
should have evaluated all of the areas that the commenter considered.
    Comment: EPA's re-proposed Wyoming haze plan presents a unit-by-
unit analysis of the visibility benefits of the installation of various
BART control alternatives at Wyoming EGUs, and identifies benefits at
only a subset of the affected Class I areas. However, EPA did not
present evidence of the cumulative visibility benefits that would be
enjoyed by Class I areas from implementation of all of the BART
determinations in its 2013 re-proposal. To assess this shortcoming, the
Conservation Organizations contracted with Howard Gebhart to conduct a