Attachment	A	 TSRIA	Petition	for	Reconsideration Page	A	- 1

*Analysis	by	Bison	Engineering,	Inc.

ATTACHMENT A*

DESIGN VALUE CALCULATIONS FOR SO2 AT COBURN ROAD

1. INTRODUCTION

EPA’s SO2 nonattainment designation for the area surrounding the Coburn Road monitor was 

based on a design value calculation using data collected during calendar years 2009 – 2011. 

TSRIA recognizes that 40 CFR Part 50.17, Appendix T prescribes that design values are to be 

calculated based on three calendar years. However, the 2010 SO2 NAAQS did not become 

effective until August 23, 2010. Therefore, it is appropriate to calculate a design value based 

entirely on data collected after the standard became effective. Since three full calendars years 

have not yet elapsed, it is necessary to either calculate these design values using a start date 

within the year or to use calendar year data to date. 

This Attachment describes the methodology used to calculate the design value using three 

alternatives. The first design value calculation is made based on data from the most recent 3 

calendar years with the caveat that calendar year 2013 is not quite complete. The data were 

made available by the Montana Department of Environmental Quality (MDEQ) and go through 

November 3, 2013. The second design value calculation was made using the same most recent 

data but covers the most recent 36-month period, again ending November 3, 2013. 

The third design value is based on the most recent 3-year period, ending September 30, 2013, 

with validated data. The third design value represents the first 36-month calculation that does 

not contain data from before the August 23, 2010 effective date of the standard. It is also the first 

three-year period where all data were collected from the Coburn site using a 500 ppb span range, 

rather than the less precise 1000 ppb span range used previously.

2. DESIGN VALUE – CALENDAR YEARS 2011, 2012 AND 2013

A design value was calculated for the most recent 3 calendar years, although the final 16% of 

year 2013 is not complete. Assuming no out-of-character values are encountered, the design 

value for 2011-2013 would be as shown in Table 1.

TABLE 1: DESIGN VALUE CALCULATION FOR 2011 – 2013*

Ranking 2011 2012 2013*

Value

(ppb)

Date Value

(ppb)

Date Value

(ppb)

Date

1

st high 142 09/05/11 129 09/13/12 73 10/25/13

2

nd high 113 09/11/11 93 03/11/12 51 01/16/13

3

rd high 85 10/30/11 84 07/18/12 46 07/21/13

4

th high 74 09/30/11 70 11/03/12 46 10/26/13

5

th high 66 11/20/11 55 01/04/12 42 01/04/13
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6

th high 65 10/19/11 54 9/28/12 42 09/10/13

99th-percentile value 74 70 46

Design Value = (74 + 70 + 46 ) / 3 = 63 ppb

* Through November 3, 2013

3. DESIGN VALUE – ENDING NOVEMBER 3, 2013

Coburn Road maximum daily SO2 concentration data through November 3, 2013 were recently

made available by MDEQ.1 While this new data are currently described as “preliminary,” any

subsequent changes to that dataset will involve the removal of suspect values, rather than the 

addition of new values that were not present in the initial compilation. Therefore, the dataset in 

its current form contains all of the potential “high” SO2 values that could be present after final 

review and validation; use of that dataset for purposes of this evaluation actually represents a 

conservative approach. The newly available data were used to develop a design value calculation 

using a November 1, 2010 – October 31, 2013 basis (36 consecutive months). Effectively, the 

calculation presented in Section 3 has been shifted forward by one month, making it even more 

representative of current conditions. 

Because the full hourly dataset for October is not yet published in a public EPA database and has 

only been provided by MDEQ, this design value analysis for November 2010 – October 2013 is 

presented in steps for clarity. First, Table 2 shows the six highest daily maximum values for the 

indicated November 1, 2010 – September 30, 2013 periods, for which all hourly data were 

available.2

TABLE 2: MAXIMUM DAILY SO2 VALUES FOR NOV 2010 – SEP 2013

Ranking Nov 2010 – Oct 2011 Nov 2011 – Oct 2012 Nov 2012 – Sep 2013

Value

(ppb)

Date Value

(ppb)

Date Value

(ppb)

Date

1

st high 142 09/05/11 129 09/13/12 70 11/03/12

2

nd high 113 09/11/11 93 03/11/12 52 12/21/12

3

rd high 101 12/24/10 84 07/18/12 51 01/16/13

4

th high 86 12/21/10 66 11/20/11 49 11/07/12

5

th high 85 10/30/11 55 01/04/12 46 07/21/13

6

th high 74 09/30/11 54 09/28/12 42 09/10/13

Next, in order to obtain a complete result for the November 1, 2012 – October 31, 2013 period, 

applicable values for October 2013 must be integrated into the data shown above. MDEQ has 

 

1

 Email from Stephen Coe (MDEQ) to James M Parker (PPL) dated November 6, 2013, attached at the end of this 

document.

2 Hourly data for October 2013 were not available at the time of this analysis. However, MDEQ has reported the 4 

highest daily maximums for January through October 2013 as outlined in Table 3.
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reported the following four highest daily one-hour maximums for January – October 2013 (Table 

3).

TABLE 3: MAXIMUM DAILY SO2 VALUES FOR JAN – OCT, 2013 *

 Ranking Jan 2013 – Oct 2013 *

Value

(ppb)

Date

1

st high 73 10/25/13

2

nd high 51 01/16/13

3

rd high 46 07/21/13

4

th high 46 10/26/13

* This includes data through November 3

In combination, the two datasets can now be summarized as shown in Table 4.

TABLE 4: DESIGN VALUE CALCULATIONS FOR NOV 2010 – OCT 2013

Ranking Nov 2010 – Oct 2011 Nov 2011 – Oct 2012 Nov 2012 – Oct 2013

Value

(ppb)

Date Value

(ppb)

Date Value

(ppb)

Date

1

st high 142 09/05/11 129 09/13/12 73 10/25/13

2

nd high 113 09/11/11 93 03/11/12 70 11/03/12

3

rd high 101 12/24/10 84 07/18/12 52 12/21/12

4

th high 86 12/21/10 66 11/20/11 51 01/16/13

5

th high 85 10/30/11 55 01/04/12 49 11/07/12

6

th high 74 09/30/11 54 09/28/12 46 07/21/13 &

10/26/13

99th-percentile value 86 66 51

Design Value = (86 + 66 + 51 ) / 3 = 68 ppb

4. DESIGN VALUE – ENDING SEPTEMBER 30, 2013

An updated design value was also calculated for the Coburn Road monitor with respect to the 

2010 SO2 NAAQS of 75 ppb based on three consecutive years ending before October 4, 2013, 

consisting of the following periods:

 Year 1: October 1, 2010 – September 30, 2011

 Year 2: October 1, 2011 – September 30, 2012

 Year 3: October 1, 2012 – September 30, 2013

Table 5 of this Attachment lists the six highest daily maximum one-hour SO2 concentrations 

observed at Coburn Road for each of those years. The fourth highest value within each year 

represents the 99th-percentile value that is the basis for design value calculations. Analysis of the 

data yields a design value of 74 ppb, as shown at the bottom of the table.
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TABLE 5: DESIGN VALUE CALCULATION FOR OCT 2010 – SEPT 2013

Ranking Oct 2010 – Sep 2011 Oct 2011 – Sep 2012 Oct 2012 – Sep 2013

Value

(ppb)

Date Value

(ppb)

Date Value

(ppb)

Date

1

st high 142 09/05/11 129 09/13/12 70 11/03/12

2

nd high 113 09/11/11 93 03/11/12 52 12/21/12

3

rd high 101 12/24/10 85 10/30/11 51 01/16/13

4

th high 89 10/22/10 84 07/18/12 49 11/07/12

5

th high 87 10/03/10 66 11/20/11 46 07/21/13

6

th high 86 12/21/10 65 10/19/11 42 09/10/13

99th-percentile value 89 84 49

Design Value = (89 + 84 + 49 ) / 3 = 74 ppb

The calculated design value of 74 ppb for the three-year period of October 2010 – September 

2013 is below the 75 ppb standard, and shows the Coburn Road monitor location meets the 

ambient standard. 

As a side note, if this analysis had been conducted from August 22, 2010 (the effective date of 

the rule) through August 21, 2011 (and so forth), the same results (< 75 ppb design value) would 

be observed. In fact, this is true for any continuous 36-month period following the effective date 

of the standard. 

The analyses presented in this Attachment show that if the most recently available three calendar 

year data period is used for a design value calculation,3 Yellowstone County is presently in 

attainment of the 2010 SO2 NAAQS of 75 ppb by a considerable margin. Based on past history

and the 2013 monitor readings through the first week of November, it is highly probable that the 

three calendar years following the effective date of the 2010 SO2 NAAQS will be well below 75 

ppb.

The results from all three methods demonstrate attainment and support TSRIA’s contention that 

EPA should reconsider and repeal the Yellowstone SO2 Designation. The continuing reduction in 

emissions coupled with clear evidence that ambient concentrations are below the 2010 SO2 NAAQS 

support such a request. 

 

3

The 2013 calendar year is based on data through November 3 only.
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DEQ-Supplied Coburn Data – 2013*

Jan 1 through November 3

 * Email from Stephen Coe (MDEQ) to James M Parker (PPL) dated November 6, 2013
1

THIS EMAIL INCLUDED WITH PERMISSION FROM STEPHEN COE OF MDEQ

From: Coe, Stephen [mailto:SCoe@mt.gov] 

Sent: Wednesday, November 06, 2013 8:31 AM 

To: Parker, James M 

Cc: Merchant, Eric 

Subject: Coburn info

  

Jim,

I just wanted to share with you the monitoring data I have been tracking and bring you up to speed with

where we are tracking with the 2013 data and 11‐13 design value.  Below are a couple graphs showing

all the hourly data, and pointing out the High 4 days (99th percentile) and a showing the 4th highest and

the resulting design value should that 4th highest stay the same.  We still have two months to go, so

anything could happen.  I have also included a table with the wind direction and speed for the high

values, as well as pollution roses for the high days, because the pollution roses are for the entire 24 hour

period they are not as accurate as the table below, but are still good information.

DISCLAIMER:  Some of the data presented in this email is unqualified and has not yet been quality

assured, these data are for discussion only and should not be used for official determinations.

The wind direction is coming from between 232.7 – 239.4 degrees and wind speed between 12 and 16

mph.  

CB_Blgs Coburn Rd 12_SPEED

001h

CB_Blgs Coburn Rd 13_WDIR

001h

CB_Blgs Coburn Rd 21_SO2‐

1HRB 001h

Date Final Value Final Value Final Value

10/25/2013 2:00 AM 16.0293144 232.726698 73.2670833

1/16/2013 8:00 AM 15.0486642 239.353242 51.9269444

7/21/2013 7:00 AM 11.9557505 233.207781 46.62875

10/26/2013 9:00 AM 13.3493935 234.593146 45.6081944

  
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