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Responses to EPA Comments on NAAQS 

Compliance Modeling in Support of the FCPP 

and Mine EIS

Robert Paine and Mary Kaplan (AECOM)

June 20, 2013

Introduction

The Arizona Public Service Company (APS) operates the Four Corners Power Plant (FCPP), a 

coal-fired power plant located on the Navajo Nation, about 25 miles west of Farmington, New 

Mexico. APS contracted AECOM to provide support in preparation for the environmental 

impact statement (EIS) to address the renewal of the lease and rights-of-way for the FCPP, the 

associated nearby Navajo Mine coal supply, and associated transmission lines, for the 25-year 

period of 2016 through 2041.

One of the technical studies required as part of the National Environmental Policy Act (NEPA) 

process involves determining the impact of criteria pollutant emissions from the FCPP and the 

associated Navajo Mine with respect to the National Ambient Air Quality Standards (NAAQS). 

A draft NAAQS modeling study was submitted to EPA Region 9 for review in April 2013. This 

analysis is particularly important because the NAAQS for SO2 and NO2 have recently been 

strengthened. In addition to SO2 and NO2, NAAQS impacts have been assessed for PM2.5, 

PM10, CO, and lead in this study. The impacts of emissions from FCPP on the ozone NAAQS 

is being addressed separately using a photochemical modeling approach, and was not 

addressed in the recently submitted NAAQS modeling study.

Mr. Scott Bohning of EPA Region 9 provided comments via e-mail to Mr. Robert Paine of 

AECOM on May 31, 2013 after EPA’s review of the NAAQS modeling analysis. These 

comments are paraphrased below, along with a response from APS/AECOM that addresses 

each comment.

EPA Comment #1: AERMOD Modeling Approach

AECOM has selected a version of AERMOD 12345 with EPA-provided options/refinements

that are currently being “beta” tested. EPA Region 9 would appreciate additional justification of 

this approach.
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Response:

AERMOD version 12345 with several potential refinements was developed by EPA’s Office of 

Air Quality Planning and Standards to address concerns submitted at the 10th Modeling 

Conference in March 2012 (and subsequent written comments) related to AERMOD low wind 

speed over-predictions. Although this version is still being tested, we have good evidence that

AERMOD version 12345 with low-wind refinements will produce more realistic results for the 

FCPP EIS modeling. Since AERMOD version 12345 with these refinements is still undergoing 

evaluation by the AERMOD-user community, use of these improved formulation options is still 

considered a “beta” approach.

To provide the additional information requested, AECOM has provided EPA with a recentlycompleted 2013 Air & Waste Management Association paper that documents a review and 

independent evaluation of AERMOD version 12345 with research-grade low-level databases. 

AECOM has also provided to EPA an updated North Dakota SO2 evaluation study that 

includes consideration of AERMOD version 12345 beta options. The North Dakota study 

features coal-fired power plants with tall stacks, similar to the FCPP application, and it indicates 

superior performance for the proposed modeling approach. This study was also presented to 

EPA at the 10th EPA modeling conference and was also discussed at the 2013 EPA modeling 

workshop, at which Robert Paine made an invited presentation on AECOM’s review of the low 

wind speed refinements in AERMOD version 12345.

EPA Comment #2: Modeling the Auxiliary Boiler Impacts

EPA has noted that the auxiliary boiler was not included in the modeling because its emissions 

are much lower than those from Units 4 & 5. However, EPA is concerned about a possible 

near-field impact from the operation of this natural-gas fired source, especially for 1-hour NO2

impacts.

Response:

The auxiliary boiler would be normally used only for steam assist to the coal-fired units during 

startup, and therefore it is not expected to be run at the same time as Units 4 and 5 after they 

have achieved normal operations after a startup. However, to address EPA’s concerns, 

AECOM has rerun the modeling with the very conservative assumption that the auxiliary boiler 

runs continuously (for short-term NAAQS) along with Units 4 and 5 running continuously at full 

load. This modeling results in only a 1 μg/m3increase in the predicted 1-hour NO2 design 

concentration, so that even with this conservative assumption, attainment with the NAAQS is 

maintained. . 

EPA Comment #3: Use of Actual Emissions for SO2 NAAQS Modeling

The EIS modeling for 1-hour SO2 NAAQS compliance used three recent years of actual hourly 

emissions, which is consistent with the draft EPA guidance provided in EPA’s Technical 
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Assistance Document for modeling, available at 

http://epa.gov/airquality/sulfurdioxide/pdfs/SO2ModelingTAD.pdf. While this approach is 

probably acceptable for the EIS, it should not be used for the PSD application if there are major 

increases in SO2 emissions that would require a new modeling compliance demonstration.

Response:

APS does not plan any major increases in SO2 emissions at FCPP Units 4 and 5, so no new 

NAAQS compliance modeling for SO2 is expected to be needed for the PSD permit application. 

In fact, it is likely that SO2 emissions will decrease from Units 4 and 5 because of two factors: 

1) some SO2 will be oxidized to H2SO4 by the SCR catalyst, and 2) if installed, some SO2

would likely be removed by controls for H2SO4. These SO2 emission decreases have not been 

accounted for in the NAAQS compliance modeling for SO2.

EPA Comment #4: Combining PM2.5 Modeled and Background Concentrations

EPA needs clarification as to how PM2.5 background concentrations were combined with 

modeled impacts for compliance with the 24-hour NAAQS. EPA cites relevant guidance as the 

March 23, 2010 EPA/Stephen Page memo (add modeled maximum concentrations to 98th

percentile monitored values).

Response:

The guidance in the March 23, 2010 EPA/Stephen Page memo was followed; the peak 24-

hour modeled concentration was added to the 98th percentile background concentration. This 

procedure has been recently proposed to be replaced by EPA’s draft PM2.5 modeling 

procedures posted on March 4, 2013 (available at 

http://www.epa.gov/ttn/scram/guidance/guide/Draft_Guidance_for_PM25_Permit_Modeling.pdf). In

that draft guidance, EPA indicates that the 98th percentile modeled 24-hour concentration 

should be added to the 98th percentile background concentration when emissions are largely 

due to fugitive or non-combustion sources. Due to the fact that the peak modeled PM2.5

concentration for FCPP is due to non-combustion sources (no precursor emissions), the 

modeling results using the 2010 EPA/Stephen Page memo are very conservative.

EPA Comment #5: Representativeness of Background Monitors

EPA has expressed some general concern over the representativeness of the background 

monitors.

Response:

EPA did not provide concerns on specific monitors or pollutants in this comment. The 

background monitors are generally located in suburban or urban areas that would tend to 

provide higher concentration levels than that expected in the rural environment of the FCPP 
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and the Navajo Mine. In addition, for most pollutants (except SO2 and PM2.5), the design 

concentration is well below the NAAQS, so that even factor-of-two uncertainties in the 

background concentration would not change the overall result.

For SO2, the 99th percentile background monitor value represents a conservative Tier 1 

treatment at a site near a background source. For PM2.5, we are using a monitor that shows 

higher design concentrations than most in the general area (see Figure 1). Therefore, we 

contend that the monitored background concentrations are likely to be conservatively high for 

SO2 and PM2.5.

EPA Comment #6: Significant Concentration Gradient Analysis for San Juan 

Generating Station

EPA indicated that its review of the figure showing the relatively flat gradient of the impact of 

the San Juan Generating Station beyond the location of the Four Corners Power Plant 

indicates support for the position that this background source will not cause a significant 

concentration gradient for FCPP. However, EPA is not sure that the quantitative criterion of +/-

30% for the variation of impacts as being within the “noise level” is the correct approach.

Response:

APS agrees with EPA that there are no SO2 sources in the vicinity of FCPP that have a 

significant concentration gradient and are required to be modeled. Reliance on the quantitative 

criterion of +/- 30% is apparently not needed in this case. 

EPA Comment #7: SO2 and PM2.5 Modeling Results are Close to the NAAQS

EPA is concerned that the 1-hour SO2 and 24-hour PM2.5 modeling results are close to the 

NAAQS. 

Response:

The reported concentrations are 194.5 μg/m3for 1-hour SO2 (the NAAQS is 196.5 μg/m3) and 

33.16 μg/m3for 24-hour PM2.5 (the NAAQS is 35 μg/m3), which are indeed close to the 

respective NAAQS. However, the modeling procedures used for both 1-hour SO2 and 24-hour 

PM2.5 have inherent conservative features, as explained below. 

For SO2, the principle issue is that the highest predicted concentrations are caused by building 

downwash in high winds due to the influence of the Unit 4 and 5 boiler buildings. However, 

those boiler buildings are not solid structures (see Figures 2 and 3), although the modeling 

treats them as being solid. For a solid building, the modeled (and actual) plume rise would be 

depressed over that from a porous structure (see example in Figure 4), and the modeling 

would then result in a plume rise that is too low and a modeled impact that is too high. We 
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note that aside from a narrow area of downwash impacts due to NW winds (see Figure 5), the 

remaining design 1-hour SO2 concentrations are under 150 μg/m3, well below the NAAQS.

For PM2.5, as explained in the response to Comment #4 above, we followed the guidance in 

the March 23, 2010 EPA/Stephen Page memo for adding modeled to background 

concentrations. This guidance indicates that the peak 24-hour modeled concentration should 

be added to the 98th percentile background concentration. However, the new EPA draft 

procedure for PM2.5 modeling released on March 4, 2013 indicates that the 98th percentile 

modeled 24-hour concentration should be added to the 98th percentile background 

concentration in certain situations. This proposed procedure would be applicable in the FCPP 

case due to the fact that the peak modeled concentration is due to fugitive emissions (no 

precursor emissions). Using the recent draft guidance, the 24-hour PM2.5 design concentration 

would be reduced from about 33 μg/m3to about 26 μg/m3. 

Therefore, for both SO2 and PM2.5, we believe that the more likely design concentrations are on 

the order of 75% of the NAAQS, leaving considerable room for uncertainties in background 

concentrations.
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Figure 1 Available PM2.5 Monitors and 24-hour Design Concentrations in the Area of the Four Corners Power Plant
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Figure 2 View of Units 4 and 5 Boiler Buildings Looking Northeast
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Figure 3 View of Units 4 and 5 Boiler Buildings Looking Southwest
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Figure 4 Wind Tunnel Depiction of Plume Rise From Actual Porous Structure (Top) and 

Assumed Solid Structure (Bottom)*

*From Slide #9 at http://www.epa.gov/scram001/10thmodconf/presentations/1-11-

10hModelingConference_AWMA_AB3_Petersen_final.pdf. 
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Figure 5 1-Hour SO2 Design Concentrations Predicted by AERMOD Near FCPP Using Actual Hourly Emissions