June 16, 2014 

MEMORANDUM 

SUBJECT: Full Technical Response to Modeling Comments for June 2014 Final Arizona 

Regional Haze FIP (Phase III) 

FROM: Colleen McKaughan, Associate Director, Air Division 

Scott Bohning, Modeler, Air Quality Analysis Office 

TO: Arizona Regional Haze FIP Docket EPA-R09-OAR-2013-0588 

Summary: In the final Federal Register notice, we respond to comments on two modeling 

issues: (1) the correct version of CALPUFF to use for Regional Haze modeling, and (2) the 

background ammonia concentration to use in Regional Haze modeling. Our responses in the 

Federal Register notice did not include the entire technical rationale underpinning our responses. 

Consequently, we are providing the entire technical rationale in the record for those who have an 

interest in the technical and scientific information that we relied on when performing our 

analysis. 

Comments and Responses:

Comment: TEP stated that EPA should adopt version 6.42 of CALPUFF as the approved 

regulatory version for modeling regional haze, since this version corrects deficiencies in the 

chemistry and the dispersion functions of CALPUFF version 5.8. TEP indicated that several 

studies conducted over the last few years demonstrate that the deficiencies in version 5.8 result in 

over-estimation of the visibility impacts of NOX emissions in Class I areas. This causes 

erroneous over-estimation of the visibility improvements from proposed BART controls leading 

to biased cost benefit values. 

Response: We disagree with the commenter for two reasons. First, CALPUFF 5.8 is 

approved and fully vetted as a regulatory model for use by EPA in regional haze determinations. 

CALPUFF 6.42 is not because CALPUFF 6.42 has not yet undergone adequate review. We 

relied on version 5.8 of CALPUFF because it is the EPA-approved version in accordance with 

the Guideline on Air Quality Models (“GAQM”, 40 CFR 51, Appendix W, section 6.2.1.e). We 

updated the specific version to be used for regulatory purposes on June 29, 2007, including 

minor revisions as of that date. EPA took into account limitations of Version 5.8 when it 

suggested use of the 98th percentile day versus the maximum day. 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. 

CALPUFF version 6.41incorporates a more detailed treatment of chemistry, but is not 

approved by EPA for use in regulatory purposes. Given the current regulatory status of 

 

1 The latest version of CALPUFF is 6.42. Versions 6.401, 6.402, 6.41, and 6.42 include minor changes to version 

6.4, which was the first to include the more complex chemistry module cited by the commenters. Our response 
CALPUFF, the use of version 6.4 would require either a new rulemaking, or an “alternative 

model” demonstration under GAQM 3.2.2. Our promulgation of CALPUFF (68 FR 18440, April 

15, 2003) as a “preferred” model approved it for use in analyses of Prevention of Significant 

Deterioration increment consumption and for complex wind situations, neither of which involve 

chemical transformations. For visibility impact analyses, which do involve chemical 

transformations, CALPUFF 5.8 is considered a “screening” model, under GAQM section 4.2.1, 

rather than a “preferred” model; this “screening” status is also described in the preamble to the 

BART Guidelines (70 FR 39123, July 6, 2005). A version of CALPUFF other than 5.8, and 

especially one with such fundamental differences in its handling of chemistry, would no longer 

be considered a “screening” model under GAQM section 4.2.1. Instead it 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, a meteorological preprocessor for CALPUFF, 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.2 The 

change to CALPUFF 6.4 is not a simple model update to address bug fixes, but a significant 

change in the model science that requires its own rulemaking with public notice and comment. 

Furthermore, it should be noted that the U.S. Forest Service and EPA review3 of 

CALPUFF version 6.4 results supplied within a limited set of BART applications showed that 

differences in its results from those of version 5.8 are driven by two input assumptions and 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. Thus, 

although some exercises using CALPUFF version 6.4 have given different results than version 

5.8 when using different input assumptions, that does not in itself prove that version 5.8 with our 

inputs is faulty, and does not change our view that CALPUFF 5.8 is appropriate for this action. 

Introducing a new regulatory model is a long process. EPA intends to conduct a 

comprehensive evaluation of the latest CALPUFF version along with other “chemistry” air 

quality models, including a full statistical performance evaluation, verification of its scientific 

basis, and determination of whether the underlying science has been incorporated into the 

modeling system correctly. To accommodate such a model, there would have to be an evaluation 

 

refers to version 6.4, but applies equally to all these versions. Version differences are described on the model 

developer’s web site “The CALPUFF Modeling System”, http://www.src.com/calpuff/calpuff1.htm, including in the 

model FORTRAN source code. 

2

“CALPUFF Regulatory Update” Roger W. Brode, Presentation at Regional/State/LocalModelers Workshop, June 

10-12, 2008, available at 

http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2008/agenda.htm. 

3

“CALPUFF Status and Update” Tyler J. Fox, Presentation at Regional/State/LocalModelers Workshop, April 30-

May 4, 2012, available at 

http://www.cleanairinfo.com/regionalstatelocalmodelingworkshop/archive/2012/agenda.htm. 
of the effect on the regulatory framework for its use, including in NSR permitting, and also 

changes to the Guideline on Air Quality Models and other modeling guidance, in consultation 

with the FLMs. CALPUFF version 5.8 has already gone through this comprehensive evaluation 

process and remains the EPA-approved version. We believe its chemistry algorithms and 

performance were adequately taken into account in choosing it for BART modeling. We believe 

CALPUFF 5.8 is thus the appropriate version for EPA’s BART determinations for this action. 

TEP claimed that several studies show that CALPUFF 5.8 overestimates the visibility 

impacts of NOX emissions. An EPA presentation at the 2012 EPA Regional/State/Local 

Modelers Workshop did include the statement “CALPUFF was known to overestimate nitrates 

when it was promulgated as a regulatory model [68 FR 18440, April 15, 2003] and in the BART 

Guidelines [70 FR 39104, July 6, 2005], therefore EPA chose the 98th percentile (8th High) for 

regulatory evaluations.” EPA already considered the limitations of CALPUFF and its included 

chemistry scheme when selecting it for BART modeling. That regulatory action is past, and 

cannot be revisited in this action. Of the two studies cited by the commenter, one predated EPA’s 

promulgation of CALPUFF 5.8 in the BART Guidelines, and indeed was cited by EPA as an 

evaluation study when EPA promulgated CALPUFF as a regulatory model. 4 The other study5

relies on the same Southwest Wyoming Technical Air Forum (SWWYTAF) database of 

meteorological and emissions data. Even with the POSTUTIL NO3/HNO3 repartitioning applied 

in the Karamchandani et al. work cited in the comment, which allows for some competition for 

ammonia between overlapping source plumes, the assumption of 1 ppb ammonia everywhere for 

all source plumes would strongly accentuate any CALPUFF tendency toward over-prediction. 

The approach was quite different than BART modeling, where only a single source’s emissions 

are interacting with the ammonia. Overpredictions in the multi-source exercise do not directly 

apply to the single source case. 

Comment: TEP commented that the background ammonia concentration used in visibility 

modeling is critical because ammonia is a precursor to particulate ammonium nitrate. EPA's use 

of 1.0 parts per billion (ppb) for ammonia background concentration for all months of the year 

will tend to overestimate the visibility benefits associated with reductions of NOX, particularly in 

the winter months. TEP noted that monthly ammonia measurement data from the IMPROVE 

monitoring network site in southern Arizona (Chiricahua) indicate that ammonia concentrations 

below 1.0 ppb (e.g., 0.5 ppb) are present at this site during the winter months. TEP asserted that 

use of those values will more accurately predict the visibility improvements expected from the 

reductions in NOX emissions. 

Although TEP has not performed any new modeling for comparison to EPA’s results in 

the proposal, TEP sent a letter to EPA in May 2013 that provided clarification relating to certain 

 

4 TEP comments attachment B contains “Visibility Improvement Analysis for Candidate BART Control Options: 

Sundt Unit 4”, AECOM, November 2013. At p.4-2 this attachment cites Scire, J.S., Z-X Wu, D.G. Strimais and 

G.E. Moore, 2001. The Southwest Wyoming Regional CALPUFF Air Quality Modeling Study - Volume I. Prepared 

for the Wyoming Dept of Environmental Quality. This same study is cited in 40 CFR 51 Appendix W, A.4.n., as an 

evaluation study for CALPUFF. 

5

Ibid., citing Karamchandani, P., S. Chen, A. Bronson, and D. Blewitt, 2009. “Development of an Improved 

Chemistry Version of CALPUFF and Evaluation Using the 1995 SWWYTAF Data Base.” Presented at the Air & 

Waste Management Association Specialty Conference on Guideline on Air Quality Models: Next Generation of 

Models, October 28-30, 2009, Raleigh, NC. 
modeling parameters and the results of modeling performed by AECOM. According to TEP, the 

modeling performed by AECOM includes a BART control scenario involving SNCR and DSI, 

similar to EPA's proposed BART determination for Sundt Unit 4. The results of AECOM's 

modeling is a maximum visibility improvement of 0.16 dv at Saguaro National Park-East 

compared to the baseline case. The commenter noted that EPA's modeling representing the same 

control configuration (SNCR and DSI) reported a maximum visibility improvement of 0.49 dv. 

TEP acknowledged that these differences in modeling results have little practical effect, as EPA 

has proposed that its results support a BART determination involving application of SNCR and 

DSI on Sundt Unit 4, and the commenter does not dispute that overall finding. However, should 

EPA find a need to do additional modeling to support its final BART determination for Sundt 

Unit 4, the commenter recommended that the EPA incorporate the modeling improvements 

suggested in TEP’s letter of May 10, 2013. 

Response: We disagree that the 1 ppb ammonia background we assumed for CALPUFF 

modeling is too high. It is consistent with EPA guidance given that some ammonia 

measurements are higher than 1 ppb, and the available ammonia data is variable over the areas 

included in the visibility modeling. The uncertainty over appropriate ammonia values leaves us 

without a reasonable basis for choosing a different constant value, or a more complex monthly 

varying scheme as recommended by the commenter. Ambient ammonia measurements for use as 

input to modeling are scarce, and measurements that include it in the form of ammonium still 

scarcer. In the absence of compelling ammonia background estimates, the EPA IWAQM Phase 2 

guidance recommends the use of a 1 ppb ammonia background for arid lands, which includes 

Arizona.6 This is the only guidance available on this issue. 

There are measurements of gaseous ammonia (NH3) that by themselves are close to or 

greater than 1 ppb, even in winter. Winter measurements included in Sather et al.7ranged from 

1.1 ppb to 1.8 ppb for a monitor at the Farmington Airport in northwestern New Mexico. Since 

these were 3-week averages, the value for individual days would have been higher still. Winter 

2011-2013 measurements from the AMoN Network8range from 1.1 to 1.3 ppb for Farmington, 

and 0.7 to 0.9 ppb for Chiricahua, in southeastern Arizona. These are higher than the 0.5 ppb 

values for Chiracahua cited byTEP. Overall, these values are greater than or comparable to the 1 

ppb IWAQM default used by EPA, which appears to be a reasonable intermediate value between 

these and the lower values cited by commenters.9 Therefore, we consider the1 ppb ammonia 

background that we used to be appropriate for this action. 

 

6

 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. 

7

 Mark E. Sather et al., 2008. “Baseline ambient gaseous ammonia concentrations in the Four Corners area and 

eastern Oklahoma, USA”. Journal of Environmental Monitoring, 2008, 10, 1319-1325, DOI: 10.1039/b807984f. 

8 Atmospheric Ammonia Network, National Atmospheric Deposition Program, http://nadp.sws.uiuc.edu/amon/; data 

available in docket spreadsheet AMoN_network_AZ_NM_NH3_average.xlsx. 

9

 Incidentally, we note that the commenter’s recommended values capped the Chiricahua summer ammonia 

measurements at 1 ppb, whereas the diagram from the source paper had measurements for June, July, and August of 

roughly 2.5, 3.8, and 1.6 μg/m3, respectively, or about 3.6, 5.5, and 2.3 ppb. Since they occur in the summer, they 

are less important for visibility than the values during winter when nitrate mainly forms, but they do illustrate that 

ammonia measurements are variable and that the 1 ppb is not overly high. The measurements are from Figure 4-1 of 

an analysis by AECOM, in Attachment D to TEP’s comments. The paper cited by AECOM was Chen, X., D. Day, 
Finally, we agree with the commenter that the recommended modeling changes would 

have little practical effect on the BART determination for Sundt Unit 4. 

 

B. Schichtel, W. Malm, J. Mojica, C. McDade, E. Hardison, S. Kreidenweis, and J. Collett, 2012. “A Pilot 

Monitoring Study of Atmospheric NHx at Selected IMPROVE Sites.” Extended Abstract # 19, presented at the 2012 

Aerosol and Atmospheric Optics: Visibility & Air Pollution Specialty Conference, held September 25-28th, 2012 in 

Whitefish, MT. Air & Waste Management Association, Pittsburgh, PA.