POBox 52025 

# RECElVED Mail Station: PAB3S2 Phoenix, AZ 85072-2025 Phone: (602) 236-2968 (602) 236-5900 

## OCT 2 0 

## 2010 Fax: (602) 236-6690 wwwsrpnet.com Email: Kevin.Wanttaja@srpnet.com 

### Permits Office Air-3

October 15, 20 I0 

### U.S. 

### EPA. Region 9

Dr. Anita Lee
U.S. Environmental Protection Agency Region IX
75 Hawthorne Street
San Francisco, CA 94105



### Re: Additional Information Requested During Meeting on September 28,2010
Best Available Retrofit Technology Determination

- Navajo Generating Station

Dear Dr. Lee:

Salt River Project (SRP) would like to thank United States Environmental Protection Agency (EPA), the
National Park Service (NPS), and the United States Forest Service (USFS) for taking the time to meet with us on September 28, 20 I 

### O. The purpose of the meeting was to discuss the results of SRP's ammonia monitoring program and the potential implications of the results on the visibility modeling analysis that
will be performed as part of the pending Best Available Retrofit Technology (BART) proposal for the Navajo Generating Station (NGS).

One of the topics discussed during this meeting was the modeling analysis (hereafter referred to as the "Benchmarking Analysis") that was included in Attachment 2 of SRP's comments on the Advanced
Notice of Proposed Rulemaking (ANPR) issued by EPA in August 2009. In the ANPR, EPA used a back-calculation method to establish higher background ammonia concentrations than those that were used by SRP in the BART analysis submitted in January 2008. SRP performed the Benchmarking Analysis to determine which background ammonia inputs produce the most reasonable agreement between the model-predicted ammonium nitrate concentrations and the ammonium nitrate concentrations
measured at sites that are part of the Interagency Monitoring of Protected Visual Environments (IMPROVE) program. The Benchmarking Analysis showed that the use of EPA's back-calculated
ammonia concentrations produces order-of-magnitude overpredictions (i.e., by a factor of 10 or more in

many cases) of ammonium nitrate concentrations relative to measured values.

During the meeting on September 28 1 ", 

### EPA requested additional information on the Benchmarking Analysis. The analysis compared model-predicted and measured results on specific days on which
emissions from NGS were predicted by the model to reach the IMPROVE sites. EPA expressed concern that this part of the analysis might exclude days that would be relevant for consideration and that could change the conclusions of the Benchmarking Analysis.

EC 14144,1010Page 2

Dr. Anita Lee, U.S. EPA

October 15, 2010

In response to this question, several graphics are included in Attachment I of this letter that show how the days of interest were selected in the Benchmarking Analysis. A day was selected if the model-predicted
ammonium sulfate concentration, which is insensitive to background ammonia, overpredicted or was a significant percentage of the measured value, and the concentrations were sufficiently high (well above zero) to merit consideration. On the selected days, since the model predicts emissions from NGS to reach
the IMPROVE site, a comparison of model-predicted and measured ammonium nitrate values is appropriate and can be used to determine which set of ammonia inputs yields the best model performance.

It is important to note that the analysis of specific days was only one of three approaches that were presented in the Benchmarking Analysis. The other two approaches considered illof the days throughout
the 2001-2003 period. The same conclusion was reached using the other two approaches as in the dayspecific analysis: the use of EPA's back-calculated ammonia concentrations produces significant overpredictions of ammonium nitrate concentrations relative to measured values.

SRP urges EPA to consider the following points as the agency develops a BART proposal for NGS:

• The degree of visibility improvement predicted by the current regulatory CALPUFF model is critically dependent on the background ammonia concentration used as input to the model. Although the ranking of BART control options will not change as a result of changes to the
ammonia concentration inputs to CALPUFF, the magnitude of the predicted visibility improvement from each BART control option will change substantially.

• As discussed in the September 28 th meeting, the current regulatory CALPUFF model has a general tendency to overpredict visibility impacts in winter. To compensate for this tendency, the visibility benefits modeling should be conducted using the background ammonia values that were used by SRP in the December 2008 BART analysis.

• A sensitivity modeling analysis that uses EPA's back-calculated ammonia inputs would
overestimate the degree of visibility improvement predicted for more stringent BART control options. 

### It has been shown that the use of EPA's back-calculated ammonia inputs produces significant overestimates compared to measured values.

• The nitrate formation modules in CALPUFF should be updated in order to correct the general tendency of the model to overpredict nitrate concentrations in the winter. To facilitate the
CALPUFF model improvement effort, WEST Associates is sponsoring the incorporation of the
nitrate chemistry improvements into the model. SRP intends to run the updated version of the
model to provide additional information to EPA for use in the BART determination for NGS.

• In the meantime, given the tendency of the currently approved version of the CALPUFF model to overpredict nitrate concentrations and the degree of visibility improvement associated with more stringent BART control options, SRP urges EPA to proceed with a BART determination that is
based on SRP's ammonia inputs.

EC 14144.1010Page 3

Dr. Anita Lee, U.S. EPA

October 15, 2010



### SPR appreciates EPA's consideration of this additional information. Please do not hesitate to call Grant Smedley at (602) 236-2928 if you have any questions.

Sincerely,


# )""-- /I- 

# 1.1

Kevin Wanttaja Manager, Environmental Services

Attachments

EC 14144.1010

### Attachment 1: Selected Days for Benchmarking Analysis0 0 1/1/2001 1/15/2001 1/29/2001 2/12/2001 2/26/2001 3/12/2001 3/26/2001 4/912001 4/23/2001 51712001 5/21/2001 6/412001 6/18/2001 7/212001 7/16/2001 7/30/2001 8/13/2001 8/27/2001 9/10/2001 9/24/2001 10/8/2001 10/22/2001 11/512001 11/19/2001 12/312001 12/17/2001

AmmSulfate 24-hour Concentration (ugfm 3 ) o N N a a (JI t + s:: \J (I) <D III Cl. tJ) c <D Cl. Cl. » » 3 3 3 3 (/) (/) c c ro ro c c (Q (Q 3' 3'**~ ~**

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111/2002 1115/2002 1/29/2002 2/12/2002 2/26/2002 3/12/2002 3/26/2002 4/912002 4/23/2002 5/7/2002 5/21/2002 6/412002 6/18/2002 71212002 7116/2002 7/30/2002 8/13/2002 8/27/2002 9/10/2002 9/24/2002 10/8/2002 10/22/2002 11/5/2002 11119/2002 12/3/2002 12/17/2002 12/31/2002

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» 3 3 Dl 0 .... ::::s mc' -< 3 (")CJ) (I) r:::: 0­ Dlii1' ::::s .... '< (I) o 0 ::::s Dl :;'-<**~O** Dl 0 .., ::::s N(") 0(1) oa WQl !:!: 0 ::::s t/) 11/19/2003SELECTEDDATESFORBENCHMARKINGANALYSISBRYCECANYONDate10/16/200 I1/8/20023/21/200211/22/200212/28/200210/18/2003MeasuredAmmSulfate(ue:/m3)0.40.190.480.220.330.75PredictedAmmSulfate(ue:lm3)0.4020.1960.7840.5210.4040.500Cl Cl 1/1/2001

Cl (Jl

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..... ..... 6 c.r. N (Jl w Cl 1/15/2001 1/29/2001 2/12/2001 2/26/2001 3/12/2001 3/26/2001 4/912001 4/23/2001 5/7/2001 5/21/2001 6/412001 6/18/2001 71212001 7116/2001 7/30/2001 8/13/2001 8/27/2001 9/10/2001

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AmmSulfate 24-hour Concentration (ug/m 3 ) 0 N N <n 0 <.n o <n ..... --= ..

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# '" '" N N a aAmmSulfate 24-hour Concentration (ug/m 3 ) o o -" N N eN o 01 6 01 o 01 o

4/11/2003 _ 4/25/20031: 5/9120031-**-~** 5/23/2003 ......- • 6/612003 i

I» ..... -» 5.3 ;'3 0 G)**~.** I» c: ... ., 3 Q. (I) CJ> ./ c: G,ii;' ., ..... 6/20/2003 -I ./ I» (I) C Q.I» 7/412003 i ./ ("):: 1»'<**~(")** 7/18/2003 i .-.. '<0 o 0 8/1/2003i / -'(1) -< .....

8/15/2003 i (I) OJ I» ::::!: ., 0 ----- f +**~ ~** \)**8/29/2003~** .......... s::**~~** - CD iii °Ul III c- o - VI U c 9/12/20031- CD (,J iii c. - c- » 9/26/2003 i / -........ » 3 333 (j) (j) 1011 0/2003 c ---- ::;; ;;

c III CD 10/24/2003""---- "-J _ ..- c CD C <0 <0 3" 3" 1117120031 - ;S 11/21/2003 1215/2003 12119/2003SELECTEDDATESFORBENCHMARKINGANALYSISINDIANGARDENPredictedMeasuredDateAmmSulfateAmmSulfate(ulV'm-)(ulV'm3)8/26/20010.6050.8952/4/20020.2280.51811119/20020.2240.50112/4/20020.2350.7421/18/20030.2210.2758/25/20030.3730.99210/21/20030.3550.914