O FFICE OF THE G OVERNOR 

S TAT E OF M ONTANA 

STEVE B ULLOCK J OH N W AI.SH
 

GOVER.'\JOR Lr. GOVEHNOI {
 

April 3,2013 

Howard Cantor 

Acting Regional Administrator 

United States Environmental Protection Agency 

Region VIII, 8P-AR 

1595 Wynkoop St. 

Denver, CO 80202-1129 

RE: Montana Sulfur Dioxide (S02) NAAQS Designations 

Dear Mr. Cantor: 

As you are aware , the U.S. Environmental Protection Agency (EPA) promulgated a revision to 

the National Ambient Air Quality Standards (NAAQS) for sulfur dioxide (S02) on June 2, 20 10 

(75 FR 35520). Pursuant to 42 USC §7407, Congress directs EPA to submit a list of areas 

designated as nonattainment, attainment, or unclassifiable with respect to a new or revised 

NAAQS. Such lists of designated areas are due no later than two years following the 

promulgation of a new or revised standard, or June 3, 2012 unless the administrator has 

insufficient information to promulgate the designations, at which time the period may be 

extended for up to one year to June 3, 2013. 

This letter and the information provided in the enclosed technical support documents outlines 

Montana 's rationale and request for reconsideration of the proposed Nonattainment designation 

for Yellowstone County. 

This response is the state's only opportunity to comment and to provide additional information. 

As such, should EPA disagree with Montana 's justification for reconsideration of its 

Nonattainment designation, I am submitting a five factor analysis supporting a smaller, 

administratively appropriate Nonattainment area. 

Finally, I ask the agency to also reconsider its decision to make no designation for the other 55 

counties in our state. Montana originally designated all 56 counties in the state unclassifiable or 

attainment for the revised S02 NAAQS, but the EPA chose only to address Yellowstone County. 

Montana requests EPA follow 42 USC §7407, et seq. and designate those 55 counties as 

attainment or unclassifiable as defined within the code. 

If you have any questions regarding the enclosed, please contact M. Eric Merchant, the 

S TATE Cxrrro r, • P.O. Box 200801 • IIELl' NA. MONTAtoii\ 59620 -0801
 

TELEPHONE : 406-444-311 1 • FAX: 406-444-5529 • W EBsm :: www.vr.oov
 
Mr. Howard Cantor 

April 3, 2013 

Page 2 

Department's Air Quality Policy and Planning Supervisor, by telephone at (406) 444-1457 or by 

email at emerchant@mLgov. 

Sincerel y, 

- .
 *
STEVE BULLOCK 

Governor 

SB:sj 

Enc. 

cc: Tracy Stone-Manning, Director, Department of Environmental Quality 

David Klemp, Chief, Air Resources Management Bureau 

M. Eric Merchant, Air Resources Management Bureau 
TECHNICAL SUPPORT DOCUMENT

Montana Response to EPA Proposed Nonattainment Area Designation 

June 2, 2010 Revised National Ambient Air Quality Standards for Sulfur Dioxide

I. EXECUTIVE SUMMARY

On June 2, 2010, EPA revised the National Ambient Air Quality Standards (NAAQS) for 

sulfur dioxide (SO2), adding a short-term 1-hour primary (health-based) standard of 75 parts 

per billion (ppb) expressed as the 3-year average of the 99th percentile of the annual 

distribution of daily maximum 1-hour average concentrations. The primary 24-hour and 

annual SO2 NAAQS have been repealed under the revised rule, except as applicable for 

existing SO2 Nonattainment areas in the state (Laurel, East Helena). The 3-hour secondary 

(welfare-based) NAAQS of 500 ppb has been retained. The revisions were published in the 

Federal Register (FR) on June 22, 2010 at 75 FR 35520. 

Pursuant to 42 USC §7407, et seq., Montana was obligated to submit to EPA an initial list of 

geographic areas that attain the standard, do not attain the standard, or that are otherwise 

unclassifiable based on available information. In a May 27, 2011 letter, Director Richard 

Opper, under the authority of Governor Brian Schweitzer, requested that all 56 counties in 

Montana be designated as Attainment or Unclassifiable in accordance with 42 USC §7407. 

On February 6, 2013, EPA responded to Montana’s initial designations disagreeing with 

Montana’s request to classify Yellowstone County as Unclassifiable but did not act on the 

remaining 55 counties. In its letter, EPA presents the case that all of Yellowstone County 

should be designated as Nonattainment for the 2010 1-hour SO2 standard. The following 

evaluation provides additional information in support of Montana’s original area 

designations, focusing on Yellowstone County as that is the only county EPA is acting on in 

its pending designation on June 3, 2013. Additionally, Montana requests EPA follow 42 USC 

§7407 and designate the remaining 55 counties as Attainment or Unclassifiable as defined 

within the code. 

With respect to Yellowstone County, EPA utilized the 2009-2011 design value of 79 ppb 

monitored at the Coburn Road monitor in the Billings/Laurel area and identified it as a

violation of the revised SO2 NAAQS. However, the purpose and intent of a “Nonattainment” 

designation is to initiate a process to incorporate necessary enforceable, permanent, and

quantifiable emissions reductions into the affected State Implementation Plan (SIP) to ensure 

that the area will achieve and maintain NAAQS attainment and adequately protect public 

health in a timely manner. If enforceable, permanent and quantifiable emissions reductions 

necessary to attain NAAQS compliance are already mandated under the existing SIP/Clean 

Air Act (CAA) or other federally enforceable mechanisms, a nonattainment designation is 

inappropriate. 

It should be noted that the most significant changes in Billings/Laurel area SO2 emissions 

during the 2008-2012 timeframe was an over 1,600 ton increase at the ExxonMobil – Billings 

Refinery (ExxonMobil) during 2010. This emissions increase was a direct result of catalyst 

performance testing under an SO2 additive testing schedule pursuant to an EPA Office of 

Enforcement and Compliance Assurance (OECA) consent decree. This OECA consent 

decree-driven increase in ambient SO2 in the Billings area during 2010 most certainly 

impacted the monitored exceedances at Coburn Road. It is reasonable to presume that the 

CAA does not expect states to revise SIPs when federal requirements in the consent decree 

interfere with attainment of the NAAQS (42 USC §7410(a)(3)(C)). 

1 April 3, 2013
As can been seen in Figure 1 below, 2008 and 2010 stand out as high emissions years. 2010 

emissions are high primarily due to uncontrolled testing as a result of the OECA consent 

decree. 2008 also includes early catalyst testing under that same consent decree. Years 2009, 

2011, and 2012 represent similar and consistent operating conditions, reflect future operations 

and clearly establish a trend maintaining the NAAQS. Both 2008 and 2010 are not 

representative of future and ongoing emissions or ambient concentrations. These existing 

reductions in SO2 emissions provide further assurance that the Billings/Laurel area will 

achieve compliance with the new 1-hour SO2 NAAQS well before the required compliance 

date of June 2018. See Figure 1 below showing the Billings area combined facility SO2

emissions for the past 5 years with the annual 99th percentile as monitored at Coburn Road. 

Figure 1. Combined Billings Area Sources with Monitored Annual 99th Percentile at 

Coburn Road.

In its February, 2013 “Next Steps for Area Designations and Implementation of the Sulfur 

Dioxide National Ambient Air Quality Standard” (Next Steps) document EPA outlines its 

strategy for designating all areas without violating monitors. EPA states that this Next Steps 

strategy “Provides incentives and time for air agencies and sources to reduce emissions early 

and potentially avoid nonattainment designation in certain areas, improving protection of 

public health sooner than would be otherwise required. This would occur if air agencies and 

sources take action to limit emissions (e.g., to comply with the Mercury and Air Toxics 

Standards (MATS) or other requirements) and demonstrate attainment with the 1‐hour SO2

NAAQS before future designation milestone dates pass.” EPA has chosen to only act on areas 

with violating monitors, designating these areas as Nonattainment in accordance with 42 USC 

§7407. However EPA has failed to act on all other areas as required in 42 USC §7407. In the 

Next Steps strategy EPA is extending designation dates for the all the other areas by four to 

seven years with the intent to encourage areas to have the time for “air agencies and sources 

to reduce emissions early and potentially avoid nonattainment designation in certain areas, 

improving protection of public health sooner than would be otherwise required.” Areas with 

existing violating monitors are not being afforded this same opportunity. 

2 April 3, 2013
Montana has been engaged with the Billings/Laurel area stakeholders for a number of years 

and is well aware of significant SO2 reductions made in the area. With those reductions and 

the completion of the OECA consent decree with respect to ExxonMobil, the Billings/Laurel

area is seeing monitored values well within the NAAQS. EPA is using the 2009-2011 period, 

including the non-representative 2010 data, to calculate the Design Value. The inclusion of 

2010 in the Design Value is the sole reason the area is showing a violation of the standard. 

EPA is using its discretion to extend the designation timelines well beyond what is outlined in 

42 USC §7407 for all areas without violating monitors. Montana requests the same 

opportunities afforded the rest of the country. 

Montana believes that 2010 Billings/Laurel area industrial SO2 emissions and associated 

ambient impacts are not representative “normal” operations during that period or of future 

conditions in the area. The Department expects further SO2 reductions in the future with PPL 

Montana’s announcement that the Corette power plant in Billings will be mothballed in April 

2015. Further, if EPA provides this area with more time under EPA’s Next Steps document, 

that time would be used to establish hourly emission limits for those sources and to evaluate 

where additional reductions need to be made to maintain air quality in Billings well below the 

SO2 standard. Therefore, Montana strongly concludes that an initial designation of 

“Unclassifiable” for Yellowstone County is appropriate.

II. BILLINGS AREA

A. Sources/History and Emissions 

In 1973, the first ambient SO2 monitor was located in the Billings/Laurel area in response 

to both national and Montana ambient air quality standards and known large industrial 

emitters of SO2 in the area. In 1976, EPA determined the SIP in the Billings/Laurel area 

was inadequate. In 1978, EPA designated the area surrounding the CHS Inc. Refinery 

(CHS, known at that time as Cenex) in Laurel as nonattainment based upon monitored 

violations of the NAAQS. In 1979, Montana submitted and EPA approved a SIP to 

address the violations and control ongoing emissions. Following modeled violations of 

the SO2 standard, EPA called the SIP in 1993. Extensive work between Montana and the 

affected industries led to SIP submittals in 1996, 1998, and 2000. EPA eventually 

approved the SIP with a few notable exceptions. To that end, EPA adopted its own 

Federal Implementation Plan (FIP) in 2008 that adds additional SO2 control requirements 

to the SIP. The FIP provisions may be delayed by litigation, but have not been rendered 

moot and will still apply to those facilities. As a result, it is logical to assume those 

requirements will be fulfilled and will result in a reduction of SO2 emissions in the 

Billings airshed. 

Seven industrial point sources that are significant emitters of SO2 are located in the 

Billings/Laurel area and have been extensively involved in efforts with Montana to 

reduce SO2 emissions. These facilities are listed below in Table 1 by city.

TABLE 1. YELLOWSTONE COUNTY, MONTANA SIGNIFICANT 

SOURCES OF SO2 BY LOCATION

Source City

CHS, Inc. Laurel

PPL Montana, LLC – J.E. Corette Billings

Phillips66 Company Billings

3 April 3, 2013
Source City

Montana Sulphur & Chemical Company Billings

ExxonMobil Refining & Supply Company Billings

Yellowstone Energy Limited Partnership Billings

The Western Sugar Company Billings

The CHS petroleum refinery is located in Laurel and the following sources are located in 

Billings: Montana Sulphur & Chemical Company (MSCC) sulfur recovery plant; 

ExxonMobil Refining & Supply Company (ExxonMobil) petroleum refinery; Phillips66 

Company (Phillips66) petroleum refinery; PPL Montana, LLC - Corette (PPL-Corette) 

coal-fired power plant; The Western Sugar Cooperative (Western Sugar) sugar beet 

processing plant; and Yellowstone Energy Limited Partnership (YELP) petroleum cokefired electrical/steam co-generation facility. 

The Billings/Laurel area has made substantial progress in reducing SO2 emissions. Some 

additional reductions are expected in the future based on efforts made to comply with 

forthcoming regulatory requirements (see discussion in Section II.C). Figure 2 illustrates 

the reductions realized in actual SO2 emissions in the Billings/Laurel area from 1983 to 

2012. In that timeframe, total emissions from the seven Billings/Laurel sources were 

reduced from approximately 35,000 tons per year to approximately 6,287 tons per year in 

2012, see Figure 2 below. 

FIGURE 2. HISTORICAL SO2 EMISSIONS FROM THE BILLINGS/LAUREL 

INDUSTRIES

4 April 3, 2013
Table 2 and Figure 3 below provide the estimated total combined emissions from the 

seven major facilities in the Billings/Laurel area for calendar years 2008 - 2012, 

including the years EPA proposes to use for the Design Value for designation purposes 

(2009-2011). 

TABLE 2. BILLINGS/LAUREL AREA COMBINED INDUSTRIAL SO2 EMISSIONS 

Emissions Year Total SO2 Emissions (tons/year)

2008 8,270

2009 7,596

2010 8,264

2011 7,322

2012 6,294

FIGURE 3. BILLINGS AREA MAJOR SOURCE COMBINED SO2 EMISSIONS

5 April 3, 2013
As provided in Table 3 below, although some facilities showed annual variations with 

increases, the overall airshed has seen a decrease of over 1,900 tons/year or a 31% 

decrease. A significant portion of that reduction is a result of full implementation of the 

OECA consent decree. Table 3 below provides a comparison of annual emissions and 

emissions changes at each of the seven facilities over the 2010-2012 time period. 

TABLE 3, COMPARISON OF 2010 TO 2012 SO2 EMISSIONS CHANGES BY

SOURCE

Source 2010

Emissions

(tons 

SO2/year)

2012

Emissions

(tons 

SO2/year)

Difference

(tons SO2/year) Percent 

Difference

(%)

ExxonMobil 2,398 257 -2,141 -833%

Phillips66 73 101 +28 +28%

CHS, Inc. 225 210 -15 -7%

MSCC 1,383 1,935 +552 +29%

PPL Corette 2,271 1,884 -387 -21%

YELP 1,816 1,801 -15 -1%

Western Sugar 98 106 +8 +8%

Total Emissions 8,264 6,294 -1,970 -31%

With respect to the “non-representative” emissions and ambient impacts, 2010 emissions 

at the ExxonMobil Billings Refinery increased more than 1,600 tons per year over either 

2009 or 2011 levels. These additional emissions were entirely attributed to catalyst 

testing as required by the OECA consent decree in 2010 (including uncontrolled 

operation as a requirement of the testing scheme). This activity (and its associated 

emissions and ambient impacts) were clearly not representative of normal operations at 

the ExxonMobil Billings Refinery. Full implementation of the consent decree

requirements through 2011 and 2012 saw dramatic decreases in emissions at 

ExxonMobil, decreases which are now federally enforceable under that consent decree. 

Montana is aware that the other six major industrial sources in the Billings/Laurel area 

contribute to the SO2 emissions being measured at the Coburn Road monitor. However, 

it makes no sense and provides no environmental benefit to the area to designate using a 

year of emissions data that are clearly not representative and will not reoccur. There is 

CAA precedent for program implementation using representative information. When 

determining the applicability of the New Source Review Program, proposed emissions 

increases are compared with “baseline actual emissions.” “Baseline actual emissions” is 

defined under 40 CFR 52.21(b)(48)(iii) with the inclusion of the following sentence, 

“The Administrator shall allow the use of a different time period upon a determination 

that it is more representative of normal source operation.” 

As an example if the ExxonMobil Billings Refinery, as a major stationary source, were to 

propose a PSD netting analysis or modification application using 2010 as a “baseline” 

year under PSD regulations, Montana would reject that notion soundly, as would EPA on 

that same basis. The non-representative emissions included in the 2010 monitoring year 

should be implemented consistently across the CAA, with respect to permitting and 

ambient designation. Therefore, the 2010 emissions and monitoring year should be 

discarded from consideration as being not representative.

6 April 3, 2013
B. Monitoring Data 

Montana and a consortium of industries known as the Billings/Laurel Air Quality 

Technical Committee (BLAQTC) have conducted SO2 monitoring in the Billings/Laurel 

area for over 30 years. Over that period of time, SO2 has been monitored at 31 locations 

in the Billings/Laurel area. Currently, Montana and the local public health agency, 

RiverStone Health (RiverStone), operate a single monitoring site and local SO2-emitting 

industries operate four others. 

At Montana’s State and Local Air Monitoring Station (SLAMS) monitoring site located 

at Coburn Road in Billings, the 2009-11 design value of hourly concentrations of SO2 in 

the ambient air is higher than the new 1-hour SO2 NAAQS. As shown in Table 4 below 

the only annual 99th percentile value over the standard is the 91 ppb in 2010. The design 

value based on the 2009-11 time period is 79 ppb. 

TABLE 4. COBURN ROAD SLAMS 1-HOUR DAILY MAXIMUM SO2 VALUES

(* = 99th percentile value)

2009 2010 2011 2012

Date Hr ppb Date Hr ppb Date Hr ppb Date Hr ppb

9/22/09 9 107 7/8/10 6 111 9/5/11 8 142 9/13/12 6 129

2/5/09 2 83 12/24/10 9 101 9/11/11 7 113 3/11/12 10 93

9/25/09 9 83 7/9/10 6 92 10/30/11 7 85 7/18/12 8 84

1/20/09 5* 72 2/10/10 4 91* 09/30/11 7 74* 11/3/12 8 70*

8/12/09 7 69 10/22/10 9 89 11/20/11 8 66 1/4/12 2 55

A summary of monitored NAAQS exceedances by quarter over the 2008-12 time period 

is provided in Table 5 below. As can been seen Calendar year 2010 included 10 of the 16 

monitored NAAQS exceedances over the design value time period (2009-2011). 

TABLE 5. COBURN ROAD SLAMS MONITORED NAAQS EXCEEDANCES 2008-10 

Calendar Year Calendar Quarter Number of Exceedances

(> 75 ppb)

2008 

1 0

2 1

3 2

4 3

2008 Total Exceedances  6

2009 

1 1

2 0

3 2

4 0

2009 Total Exceedances  3

7 April 3, 2013
Calendar Year Calendar Quarter Number of Exceedances

(> 75 ppb)

2010 

1 1

2 0

3 4

4 4

2010 Total Exceedances  9

2011 

1 0

2 0

3 0

4 3

2011 Total Exceedances  3

2012 

1 0

2 1

3 1

4 1

2012 Total Exceedances  3

The following table (Table 6) provides a summary and comparison of the ten highest-per-day 1-

hour SO2 concentrations measured at the Coburn Road monitoring station for each year from 

2008 through 2012. The measured values are listed in order of rank from highest to lowest (1st

through 10th highest) for each year, and the highest value in each rank across all five years is 

shaded. Three parameters in the table demonstrate that the 2010 dataset was unique and not 

representative of normal ambient air quality in the monitored airshed. First, during the period 

from 2008 through 2012, seven of the highest ten monitored concentrations (70%) were measured 

within the single year of 2010. Second, the numbers of monitored hours that exceeded the 

NAAQS of 75 ppb were highest in 2010. Significantly, 2010 demonstrated 9 monitored 1-hour 

exceedances-- all but one of the listed values for the year, and a number equivalent to the years 

2009, 2011, and 2012 combined (3 exceedances each). Third, the average of the ten highest 

monitored values was significantly greater in 2010 than in the four surrounding years. Notably, 

the average for 2008, 2009, 2011, and 2012 was 76 ppb, while the average for 2010 was 89 ppb, a 

difference of 13 ppb and an additional indicator of the 2010 departure from normal ambient air 

quality in this airshed. 

8 April 3, 2013
Table 6, Summary of Top Ten Monitored Values by Year at Coburn Road 

Notes: • Values listed are the ten highest-hour-per-day values (not necessarily the ten highest monitored hours in a year -- i.e. 

several high hourly values may be measured in a 24-hour day, but only the highest is reported for NAAQS 

comparison purposes).

• The 4th High Rank is the 99th percentile value for each year and is used for NAAQS compliance comparisons in 

years with adequate data recovery per 40 CFR 50 Appendix T.

• Values listed are in units of parts per billion (ppb).

• Shaded values indicate the highest value in each rank (e.g. 95 is the highest 3rd-high value for the listed years, and it 

was recorded in 2008).

• 2012 data are not yet certified to EPA.

Montana operates an ambient air monitoring quality assurance and quality control 

(QA/QC) program for many reasons, among them to ensure the ambient air monitoring

data supports and fulfills the operational, informational, and regulatory needs of the state. 

As part of this objective, Montana established the Ambient Air Monitoring Program 

Quality Management Plan (QMP). The QMP guides ambient air monitoring data 

collection activities. Montana has developed and maintains an EPA-approved Quality 

Assurance Project Plan (QAPP) and associated Standard Operating Procedures (SOP) to 

gather, review, and to evaluate ambient monitoring data in a consistent and defensible 

manner, including the ambient SO2 monitoring in the Billings/Laurel area.

Rank

2008 2009 2010 2011 2012 Min Max Avg

1st 119 107 111 142 129 107 142 122

2nd 112 83 101 113 93 83 113 100

3rd 95 83 92 85 84 83 95 88

4th 89 72 91 74 70 70 91 79

5th 77 69 89 66 55 55 89 71

6th 76 69 87 65 54 54 87 70

7th 74 68 86 64 53 53 86 69

8th 63 67 83 63 52 52 83 66

9th 58 66 78 59 52 52 78 63

10th 58 64 75 59 49 49 75 61

Avg 82 75 89 79 69 66 94 78.9

# of NAAQS 

Exceedances 6 3 9 3 3

Year Summary by Rank

9 April 3, 2013
Figure 4 below shows the 3-yr average Design Values and the annual 99th percentile for 

the Coburn Road SLAMS. Both 2008 and 2010 stand out as odd years. 

FIGURE 4. COBURN ROAD SO2 ANNUAL 99th PERCENTILE and 3-yr AVG 

DESIGN VALUES

Table 7 below reviews and evaluates the past 6 years (07-12) of data at the Coburn Road 

SLAMS.

TABLE 7. EVALUATION OF THE MOST RECENT 6 YEARS AT THE 

COBURN ROAD SLAMS.

2007 2008 2009 2010 2011 2012 Average Standard 

Deviation

Annual 99th 

PCTL 58 89 72 91 74 70 75.7 12.4

The standard deviation of the annual 99th percentile over the past 6 years produces a 

statistical normal range of 63.2 ppb to 88.1 ppb. This range represents the reasonable 

values, where values outside of this range should be considered outliers or questionable 

data. Using this methodology years 2007, 2008 and 2010 are questionable values and do 

not fit the norm. This strongly supports the Departments original position that 2010 data 

is compromised and should not be used in the evaluation. 

The Department believes that the analysis of monitoring information and the emissions 

data provided above clearly demonstrates that 2010 is not representative of air quality in 

the Billings area. Therefore 2010 data is inappropriate to use in the designation of the 

Billings airshed.

C. Regulatory Analysis 

The original designation request included several regulatory programs that were in 

various stages of proposal and implementation as of May, 2011 (the date of that 

designation request). The following information provides more concrete information as 

those standards have evolved with respect to potential impacts on SO2 emissions in the 

10 April 3, 2013
Billings/Laurel area as well as responding to EPA’s comments in their Technical Support 

Document (TSD), as necessary. As stated by EPA in the preamble to the June 2, 2010 

SO2 standard revision, 

“EPA ... notes that it anticipates several forthcoming national 

and regional rules, such as the pending Industrial Boilers MACT 

standard under CAA section 112(d), that are likely to require 

significant reductions in SO2 emissions over the next several 

years. A limited qualitative assessment based on the results of 

preliminary modeling of some sample facilities indicates that 

well controlled sources should meet the new SO2 NAAQS.”

The original analysis was developed based on that sentiment. Unfortunately, many of 

those regulations, as finalized, will have little or no impact on reducing SO2 emissions, 

with two important exceptions: the full implementation of EPA’s Refinery Initiative and 

the associated consent decrees and the expected implementation of EPA’s Billings/Laurel 

SO2 FIP. 

i. EPA Refinery Initiative and Associated Consent Decrees

As discussed in the previous submittal, between 2002 and 2005, the Billings 

refineries and their respective parent companies each entered into a consent decree 

with US District Court, EPA, and the State of Montana. Consent decree requirements 

and projects have largely been completed at Phillips66 and CHS; however, as each 

refinery is on its own time schedule, ExxonMobil’s consent decree involves changes 

at the refinery that were still being implemented during and after the 2008-2010 time 

period, which EPA describes as being “representative of normal emissions.” EPA 

even goes so far as to state that higher emissions are representative just because they 

are higher, not because of any underlying conditions that were present or absent 

during that period. (EPA letter to Governor Bullock February 6, 2013) 

The consent decree projects were and are critical to the emissions levels at 

ExxonMobil, both with respect to increases and decreases. During catalyst testing at 

the fluidized catalytic cracking unit (FCCU) in 2010, that included uncontrolled 

operation as a requirement of the testing scheme, emissions increased more than 

1,600 tons per year over either 2009 or 2011 levels, yet is noted to be “normal” 

operations by EPA. Full implementation of the consent decree requirements through 

2011 and 2012 saw dramatic decreases in emissions at ExxonMobil as shown in 

Table 8 below. 

TABLE 8. EXXONMOBIL SO2 EMISSIONS FROM 2008-2012.

Facility 2008 SO2

Emissions 

(tons/yr)

2009 SO2

Emissions 

(tons/yr)

2010 SO2

Emissions 

(tons/yr)

2011 SO2

Emissions 

(tons/yr)

2012 SO2

Emissions 

(tons/yr)

ExxonMobil 1,765 696 2,389 775 257

DEQ is well aware that other facilities contribute to the SO2 emissions in the 

Billings/Laurel area. However, the State also believes that the additional emissions 

resulting from OECA consent decree (from uncontrolled testing) contributed to high 

values on the monitor. DEQ also believes that, because of the consent decree

controls, that SO2 emissions are unlikely to return to 2010 levels and that the 2010 

year of emissions was an anomaly.

11 April 3, 2013
 

ii. Existing FIP Implementation

EPA states in their TSD, “The existing FIP has not been implemented due to ongoing 

litigation. Though the FIP has not been implemented, the State discusses aspects of 

the FIP as if it had been.” As EPA is aware, the FIP provisions may be delayed by 

litigation, but have not been rendered moot and will still apply to those facilities. As 

a result, it is logical to assume those requirements will be fulfilled and will result in a 

reduction of SO2 emissions (or they should, as that is the sole purpose for the 

existence of the FIP), even if they occur long after the original 1998 Montana SO2

SIP for the Billings/Laurel area was partially disapproved.

For example, the FIP primarily applies to flaring events at the refineries (CHS, 

ExxonMobil, and Phillips66) and MSCC. Such flaring events arise from startup, 

shutdown and malfunction periods. The FIP requires the development of Flare 

Monitoring Plans which have been developed and submitted to EPA by the involved 

facilities. EPA has yet to approve those plans, apparently due to litigation. As flaring 

is almost exclusively a short-term intermittent activity that is in response to an upset 

condition, the full impact of the FIP will likely reduce short-term (1-hour and shorter) 

emission rates, which would ultimately be reflected in 1-hour monitoring values 

corresponding to the ambient standard, even further. No mention is made of these 

future emission reductions in EPA’s TSD.

iii. Mercury Air Toxics Standard (MATS)/Utility MACT Implementation

EPA finalized and published the MATS, promulgated as 40 CFR 63, Subpart 

UUUUU, in the Federal Register on February 16, 2012. Two of the major facilities in 

the Billings/Laurel area would be subject to these requirements: PPL - Corette (as a 

“unit designed for coal > 8,300 British thermal units per pound (Btu/lb)” and YELP 

(as a “unit designed to burn solid oil-derived fuel”). The finalized MATS 

requirements included several emission limit options by category. The category of 

interest here is acid gas, in which the options include taking a hydrochloric acid limit 

or an SO2 limit, which can only be used if the facility has some form of flue gas 

desulfurization (FGD) system and SO2 CEMS installed.

Given the current lack of an FGD system in place and the low level of chlorine 

content in the Wyoming sub-bituminous coal burned, it is assumed that PPL – 

Corette would take a hydrochloric acid limit as opposed to an SO2 limit. Similarly, 

based on the pre-MATS acid gas emissions data in the MATS information collection 

request for petroleum coke facilities, it is assumed that YELP would also be likely to 

take the hydrochloric acid limit. Any controls installed associated with the MATS 

PM/metals limit will provide a co-benefit of additional SO2 control. 

iv. Regional Haze / BART Implementation 

EPA finalized and published Montana’s Regional Haze FIP in the Federal Register

on September 18, 2012. Two portions of this program had potential to impact 

industrial facilities in the Billings and Laurel areas: BART and reasonable progress. 

However, EPA’s analysis resulted in only PPL - Corette ending up with additional 

limitations based on the Regional Haze FIP. With respect to SO2, EPA stated in its 

response to comments in the final Regional Haze FIP, “...for Corette, the emission 

limits should reflect emission rates currently being achieved with existing controls.” 

12 April 3, 2013
Therefore, an additional SO2 limit was added (0.57 lb/MMBtu averaged over a 

rolling 30-day period), but no additional emissions reductions would result in the 

Billings/Laurel area as a result of the Regional Haze FIP. 

v. Boiler Maximum Achievable Control Technology (MACT) Implementation 

The reconsideration of EPA’s Boiler MACT standard for major sources was finalized 

and published as 40 CFR 63, Subpart DDDDD in the Federal Register on February 

31, 2013. The Boiler MACT requires emission limits for various categories of large 

and small boilers. The rule also requires various energy audits and tune-ups of the 

boilers regardless of size and fuel. The energy audits and tune-ups are designed to 

lower fuel consumption; thereby lowering emissions. In the Billings and Laurel area, 

MSCC, PPL - Corette, ExxonMobil, CHS and Phillips 66 have units that will be 

impacted by the newly promulgated Boiler MACT for major sources. MSCC, PPL, 

ExxonMobil and Phillips 66 have gas-fired (either natural gas or refinery fuel gasfired) boilers that appear to meet the applicability requirements of the finalized rule. 

As such, energy audits and tune-ups appear to be requirements of the rule for those 

units. The reduction in SO2 is not quantifiable at this early date, but improvements in 

efficiency and tune-up are likely to lead to slightly lower emissions.

The reconsideration of EPA’s Boiler MACT standard for area sources was finalized 

and published as 40 CFR 63, Subpart JJJJJJ in the Federal Register on February 1, 

2013. The three coal-fired boilers at Western Sugar would be subject to the Hg and 

CO (as a surrogate for polycyclic organic matter) limits in this standard. At this time 

it is unclear what steps Western Sugar may take to meet the Hg emission limits listed 

in the area source rule. The three coal-fired boilers at Western Sugar are currently 

controlled using wet scrubbers. If scrubber upgrades are considered for Hg control, a 

co-benefit of SO2 emissions reductions will be realized.

As stated, ongoing implementation of the national and local SO2 limiting programs 

identified above will continue to result in SO2 reductions from the affected SO2 sources, 

particularly with respect to high concentration, short-term events (flaring, specifically, 

addressed by the FIP) that have particular impacts on a 1-hour standard. 

D. Conclusion 

The 2009-2011 design value monitored in the Billings/Laurel area is 79 ppb, an 

“apparent” violation of the revised SO2 NAAQS. However, the purpose and intent of a 

“Nonattainment” designation is to initiate a process to incorporate necessary enforceable, 

permanent, and quantifiable emissions reductions into the affected state implementation 

plan (SIP) to ensure that the area will achieve NAAQS attainment and adequately protect 

public health in a timely manner. Montana recognizes the ongoing and near-future 

implementation of many existing federal SO2 emissions limiting programs impacting SO2

point source emissions in the Billings/Laurel airshed. These reductions in SO2 emissions 

have already resulted in the Billings/Laurel area monitoring annual compliance with the 

new 1-hour SO2 NAAQS. Years 2009, 2011, 2012 represent similar and consistent 

operating conditions, reflect future operations including enforceable, permanent, and 

quantifiable emissions reductions, and clearly establish a trend maintaining the NAAQS. 

Both 2008 and 2010 are not representative of future and ongoing emissions or ambient 

concentrations. These existing reductions in SO2 emissions provide further assurance that

the Billings/Laurel area will continue to show compliance with the new 1-hour SO2

NAAQS well before the required compliance date of June 2018. 

13 April 3, 2013
In fact, it should be noted that the most significant positive change in Billings/Laurel area 

SO2 emissions during the 2008-2012 timeframe was an over 1,900 ton decrease to areawide emission from 2010 to 2012. This significant reduction in emissions has been 

reflected on the monitor with reduction of the annual 99th percentile from 91 ppb to 70 

ppb in 2010 to 2012 respectively. This emissions decrease was a direct result of full 

implementation of the OECA consent decree. The OECA driven emissions increase in 

2010 corresponds directly to a majority of the documented NAAQS exceedances (10 of 

16) for the 2009-2011 time-period. One may reasonably presume that the Clean Air Act 

does not expect states to revise SIPs when federal requirements in the consent decree 

interfere with attainment of the NAAQS (42 USC §7410(a)(3)(C)). 

Further, the intent of ongoing and near-future implementation of federal SO2 emissions 

limiting programs would be to reduce industrial SO2 emissions nationwide and in the 

Billings/Laurel area to levels corresponding to compliance with the 2010 revised SO2

NAAQS. Implementation of regulatory programs including the flare provisions from the 

FIP (delayed by litigation) and ExxonMobil-specific consent decree requirements (now 

fully implemented), will continue to reduce Billings/Laurel area SO2 emissions and 

associated ambient concentrations. Additionally, PPL Montana has announced that the 

Corette power plant in Billings will be mothballed in April 2015. 

 

Therefore, Montana believes that 2010 Billings/Laurel area industrial SO2 emissions and 

associated ambient impacts are not representative of “normal” operating conditions or of 

future conditions in the area. Further, if EPA provides this area with more time under 

EPA’s Next Steps document, that time would be used to establish hourly emission limits 

for those sources and to evaluate where additional reductions need to be made to maintain 

air quality in Billings well below the SO2 standard. Therefore, Montana strongly 

concludes that an initial designation of “Unclassifiable” for Yellowstone County is 

appropriate. 

Montana maintains its position that the SO2 values for 2010 were influenced by an EPA 

OECA Consent Decree and are not representative of current or future ambient SO2

concentrations in the Billings area. The following five-factor analysis is presented as 

justification for an appropriately sized Nonattainment Area (NAA), should EPA maintain 

its position that 2010 monitoring is representative and should be included in the design 

value at Coburn Road. 

14 April 3, 2013
TECHNICAL SUPPORT DOCUMENT

Billings Area SO2 Five Factor Analysis

2010 SO2 1-hour Standard

April 3, 2013

Montana Department of Environmental Quality

Air Resource Management Bureau
Billings Area SO2 Five Factor Analysis

Table of Contents

A. Introduction .............................................................................................................. 1

B. Five Factor Analysis ................................................................................................ 1

C. Proposed NAA ....................................................................................................... 15

D. Conclusion ............................................................................................................. 16
A. Introduction

In June of 2010, the United States Environmental Protection Agency (EPA) promulgated new National 

Ambient Air Quality Standards (NAAQS) for sulfur dioxide (SO2). This action established a new 1-hour 

SO2 standard at a level of 75 parts per billion (ppb), based on the 3-year average of the annual 99th

percentile of 1-hour daily maximum concentrations. On May 27, 2011, Montana submitted its initial 

designation request to the EPA stating in part that Yellowstone County should be designated as 

unclassifiable due to unrepresentative monitoring data and pending Federal programs that will reduce SO2

in the airshed. On February 6, 2013, EPA responded stating that they disagreed with Montana’s initial 

designation and intends to designate all of Yellowstone County as Nonattainment for the 2010 1-hour SO2

standard. EPA based its decision in part on monitoring results from the 2009-2011 period. Montana 

maintains its position that the SO2 values for 2010 were influenced by an EPA Office of Enforcement and 

Compliance Assurance (OECA) Consent Decree and are not representative of current or future ambient 

SO2 concentrations in the Billings area. However, should EPA maintain its position that 2010 emissions 

and monitoring data are representative, the following five-factor analysis is presented as justification for an 

appropriately sized Nonattainment Area (NAA). 

Traditionally, absent other information, EPA has defaulted to the county boundary for defining the NAA. 

However, EPA has recognized larger or smaller areas based on an analysis of certain factors as defined in 

EPA’s March 24, 2011 Memorandum regarding “Area Designations for the 2010 Revised Primary Sulfur 

Dioxide National Ambient Air Quality Standards.” As a matter of general principle and because Montana 

counties cover such large geographic areas, the Montana Department of Environmental Quality 

(Department) has opted to define smaller NAAs whenever a scientific analysis supports it. In this case 

ambient SO2 concentrations are a direct result of industrial emissions confined to a small geographic area. 

Therefore, the Department has elected to conduct an analysis supporting designation of a NAA smaller than 

the county boundary proposed by EPA in their February 6, 2013 response to Montana’s initial designation. 

This analysis follows. 

On March 24, 2011, EPA released new SO2-specific guidance that contains five factors to be analyzed 

when initially designating new SO2 NAAs under the 2010 1-hour SO2 NAAQS. The five factors are: 

1. Air quality data 

2. Emissions-related data (location of sources and potential contribution to ambient SO2

concentrations)

3. Meteorology (weather/transport patterns)

4. Geography/topography (mountain ranges or other air basin boundaries)

5. Jurisdictional boundaries (e.g., counties, air districts, pre-existing NAAs, reservations, 

metropolitan planning organizations, etc.)

Because the Billings area is currently meeting the rescinded SO2 standards and only monitored values in 

excess of the new 1-hour SO2 standard, this document focuses only on issues related to the 2010 1-hour 

SO2 standard. 

B. Five Factor Analysis

1. Air Quality Data 

EPA states in their 2011 guidance document with respect to air quality data, 

“We intend to review SO2 air quality monitoring data, including the design value calculated for 

each monitor in the area, for the most recent 3-year period. Areas where monitoring data indicate a 

violation of the 1-hour, 75 ppb primary SO2 standard will be designated as “nonattainment.””...

1 April 3, 2013
SO2 has been monitored in the Billings area in one form or another for four decades. In accordance with 

EPA’s 2011 guidance the Department intends to use all monitoring data available in this analysis. 

Figure 1 below shows the spatial relationship between the Billings area sources, Laurel and the other 

communities in the surrounding area. Figure 1 also shows the location of the existing ambient air 

monitoring sites, including the Department’s site at Coburn Road, existing monitors in Laurel, Johnson 

Lane, the Brickyard monitor and the location of historical monitoring sites. 

Figure 1 – Billings Area SO2 major sources, historical and active monitoring locations.

Samples of ambient air are collected continuously every day at a Department run State and Local Air 

Monitoring Site (SLAMS) located at Coburn Road. Additional continuous SO2 monitoring has historically 

been conducted in the Billings area, and three industrial sites are currently active. The locations of these 

monitors are shown in Figure 1 above. The site designated as “Johnson Lane” is operated as required by 

Montana Air Quality Permit #2650-07 issued to the Yellowstone Electric Limited Partnership (YELP, 

PQAO 1197). The site is required to meet Prevention of Significant Deterioration (PSD) monitoring 

requirements as detailed in 40 CFR Parts 53 and 58, and as established in a Quality Assurance Project Plan 

(QAPP) submitted to the Department in 2006. The Department believes that the Johnson Lane site has been 

operated in conformance with the applicable requirements and that the resulting data are of sufficient 

quality and quantity to be used in this NAAQS designation evaluation.

The sites designated as “Brickyard” and “Laurel” have been operated by a consortium of Billings industries 

known as the Billings Laurel Air Quality Technical Committee (BLAQTC); PQAO: Bison Engineering, 

0102. The BLAQTC monitoring originally included a third site designated as “Lockwood” which was 

located between the Brickyard and Johnson Lane sites. This site suffered irreparable storm damage in May 

2 April 3, 2013
of 2011 and has not operated since. However, prior to its termination the site produced data that is of value 

to this designation evaluation and is referenced in this document. The Lockwood and Laurel sites began 

monitoring in 1987, followed by the Brickyard site in 1989. The three sites are operated in accordance with 

a QAPP submitted to the Department in 1992 and revised in 2004 which commits to operation conforming 

to PSD monitoring requirements. The Department believes that the BLAQTC sites have been operated in 

conformance with the applicable requirements and that the resulting data are of sufficient quality and 

quantity to be used in this NAAQS designation evaluation. Data from both the YELP and BLAQTC sites 

has historically been entered into AQS. This data is currently being entered into the Departments database 

and is readily available to be uploaded into AQS. 

As mentioned previously the Department has four decades of historical monitoring data collected at 

numerous sites in the greater Billings/Laurel urban area (see Appendix A – Billings Monitoring history – 

Bison). Those historical results indicate the Coburn Road SLAMS monitoring site tends to consistently 

monitor the highest SO2 concentrations, on the average, of any monitoring site in the area. 

The 2009-2011 Design Values from monitoring at the Coburn Road SLAMS and the other area sites are 

shown in Table 1 and Figure 2 below. 

Table 1. Billing’s Area Monitors 2009-2011 Design Values (3 year average of the 99th percentile of 

the Annual 1-Hour Daily Maximum SO2 Values (ppb))

Monitoring Location 2009-2011 Design Value 

(ppb)

Coburn Road 79

Johnson Lane 71

Brickyard 49

Lockwood* 67

Laurel 58

* The Lockwood monitor suffered irreparable storm damage in 2011 and has not been operated since. The 

Design Value used for Lockwood was 2008-2010 (including the higher emissions from both 2008 and 

2010). 

Figure 2. Billings area SO2 monitors design values.

3 April 3, 2013
Table 2 and Figure 3 below show data collected using continuous instrumentation located at the Coburn 

Road SLAMS just east of downtown Billings. 

Table 2. Billing’s Coburn Road Running 1-Hour SO2 Standard Design Values (ppb)

(Based on three year averages of annual 99th percentile 1-hour daily maximums)

2007-2009 2008-2010 2009-2011 2010-2012

73 84 79 78

Figure 3. Coburn Road Annual 99th percentile and Design Values (3-year average)

CAA sec. 107(d)(1)(A)(i) defines nonattainment as any area that does not meet the national primary or 

secondary ambient air quality standard for the pollutant. Further 40 CFR 50.17(b) states “The 1-hour 

primary standard is met at an ambient air quality monitoring site when the three-year average of the annual 

(99th percentile) of the daily maximum 1-hour average concentrations is less than or equal to 75 ppb...” 

As shown in Table 1 and Figure 2 above, the design values for all the monitors in the area with the 

exception of Coburn Road SLAMS demonstrate attainment with the 2010 1-hour SO2 standard. In 

accordance with 40 CFR 50.17(b), it is inappropriate to include areas with monitors demonstrating 

attainment with the standard within the boundary of a NAA. 

The existence of monitors demonstrating attainment with the standard leads the Department to conclude 

that a smaller area, excluding areas of Yellowstone County shown by those monitors to be in attainment, is 

appropriate. Further CAA 107(d)(1)(A)(iii) defines Unclassifiable areas as “any area that cannot be 

classified on the basis of available information as meeting or not meeting the national primary or secondary 

ambient air quality standard for the pollutant.“ Based on that definition the portion of Yellowstone County 

devoid of major sources of SO2 or monitoring information should be designated as unclassifiable or 

attainment. 

4 April 3, 2013
2. Emissions-Related Data (location of sources and potential contribution to ambient 

SO2 concentrations)

In 2012, there were approximately 24 industrial point sources in Yellowstone County but only seven 

significant sources of SO2 emissions existed in the Billings/Laurel area. The significant sources of SO2

emission are show in Figure 4 below and include the following sources in the Billings area: 

• Cenex Harvest States (CHS) (petroleum refinery) in Laurel,

• The Western Sugar Cooperative (sugar beet processing plant),

• Phillips-66 (petroleum refinery), 

• PPL – Corette (coal-fired power plant), 

• Montana Sulphur & Chemical Co. (sulfur recovery plant), 

• Exxon Mobil Corp. (petroleum refinery), and

• Yellowstone Energy Limited Partners (petroleum coke-fired electrical/steam co-generation 

facility).

Figure 4. Major SO2 point sources in Yellowstone County.

In 2012, the 24 industrial point sources in Yellowstone County emitted a total of approximately 6,300 tons 

of SO2 (estimated actual emissions). The majority of the small point sources in Yellowstone County release 

a very small amount of the total, annual, area-wide SO2 emissions. The seven major point sources 

mentioned in the preceding paragraph emitted an estimated 6,287 tons of SO2 or 99.5% of the total SO2

emissions in Yellowstone County in 2012. The eighth largest point source of SO2 in the Billings airshed

was the City of Billings waste water treatment plant at approximately 35 tons per year. 

5 April 3, 2013
With respect to sources of SO2 (both major and minor) nearby a nonattainment area, Montana’s SIPapproved minor source permitting program (codified under ARM 17.8, Subchapter 7 – Permit, 

Construction and Operation of Air Contaminant Sources) is more stringent at lower levels than Montana’s 

SIP-approved New Source Review (NSR) provisions (codified at ARM 17.8, Subchapters 9 and 10- Permit 

Requirements for Major Stationary Sources or Major Modifications Locating Within Nonattainment Areas 

(Subchapter 9) or Within Attainment or Unclassified Areas (Subchapter 10)). The provisions of the minor 

source permitting program take effect upon construction of a facility or emitting unit that has the potential 

to emit 25 tons per year (tpy) or the modification (construction or changed conditions of operation) of an 

existing facility that has the potential to emit greater than five tpy. NSR provisions take effect when a 

modification at a major stationary source results in a significant net emissions increase. Therefore, for a 

major stationary source emitting SO2, the Montana minor source permitting provisions would take effect at 

a potential emissions increase of greater than 5 tpy, long before NSR provisions at 40 tpy.

In addition, the Montana provisions for de minimis changes take effect below that 5 tpy threshold pursuant 

to ARM 17.8.745. Although ARM 17.8.745 allows for changes to occur without permitting, exceptions to 

the rule exist that include “any construction or changed conditions of operation at a facility that would 

affect plume rise or dispersion characteristics of the emissions in a manner that would cause or contribute 

to a violation of an ambient air quality standard...”

Those provisions provide substantial protection. Pursuant to ARM 17.8.749(3), “A Montana air quality 

permit may not be issued for a new or modified facility or emitting unit unless the applicant demonstrates 

that the facility or emitting unit can be expected to operate in compliance with the Clean Air Act of 

Montana and rules adopted under that Act, the Federal Clean Air Act and rules promulgated under that Act 

(as incorporated by reference in ARM 17.8.767), and any applicable requirement contained in the Montana 

State Implementation Plan (as incorporated by reference in ARM 17.8.767), and that it will not cause or 

contribute to a violation of any Montana or national ambient air quality standard [emphasis added].” A 

similar demonstration would need to be made for a potential de minimis change that affected plume rise or 

dispersion characteristics.

As a result, far below the NSR significance level of 40 tpy, DEQ would be prohibited from issuing a permit 

or allowing a de minimis change for a new or modified emitting unit unless an applicant made the cause or 

contribute demonstration. However, as shown during the history of the program, instead of denying permit 

applications on this basis, DEQ has worked with applicants to further control or reduce emissions (and 

make those controls or reductions federally enforceable) so the appropriate demonstration can be made and 

the permit can be issued in compliance with ARM 17.8.749. In effect, the NSR provisions of offsetting and, 

in some respects, the Lowest Achievable Emissions Rate, are being used at the minor source permitting 

level to minimize impacts upon nonattainment areas and to ensure that the ambient standards are protected.

In addition, any major sources outside of the NAA will be subject to EPA’s February 6, 2013 Policy titled 

“Next Steps for Area Designations and Implementation of the Sulfur Dioxide National Ambient Air 

Quality Standard.” Under this policy states must demonstrate maintenance of the 2010 SO2 1-hour Standard 

in all areas of the state. This demonstration would include a modeling and/or monitoring compliance 

demonstration for all sources exceeding emissions thresholds as defined in the policy. 

6 April 3, 2013
Figure 5 below shows the trends from 1983 through 2012 in the annual emissions from the seven largest 

SO2 point sources in the Billings/Laurel area. As can be seen in Figure 5 emissions from all major sources 

in the Billings area have been declining significantly. The combined SO2 emissions from all major sources 

have been reduced approximately 82% from over 35,000 tons/year in 1983 to 6,294 tons in 2012. 

Figure 5. Trends in annual actual SO2 emissions from major point sources in the Billings/Laurel area 

from 1983-2012. 

7 April 3, 2013
A pollution rose for the design period is shown below in Figure 6 below. This pollution rose shows the SO2

concentrations correlated with the direction the wind was blowing at the time of the reading. The pollution 

rose has been overlaid on top of an aerial photo of the Billings area. This figure clearly identifies that the 

upwind sources have the greatest impact at the monitor both in frequency and in concentration. 

Figure 6. Coburn Road SLAMS Pollution Rose for 2009-2011. 

The pollution rose above defines the direction of prevailing winds during high SO2 readings at the monitor. 

As EPA states in its February 6, 2013 Technical Support Document (TSD), “exceedances are primarily 

driven from emissions blown from the southwest, which is the predominate wind direction.” 

8 April 3, 2013
Figure 7 below shows the annual emission from the four major upwind sources during the Design Value 

Period at the Coburn Road SLAMS. With respect to orientation the sources are depicted by distance to the 

monitor from left to right on the graph, with CHS being approximately 17.4 miles and the remaining 

sources approximately 2 miles southwest of the monitor. The emissions from PPL - Corette during the 

Design Value period are between 4.5 and 6 times the combined emissions of all other upwind sources. 

Figure 7. Major Sources upwind of the Coburn Road SLAMS. 

Figure 8, 9, and 10 below show the monitored values at the Coburn Road SLAMS site compared with 

operating variances of all the upwind sources. These figures demonstrate that under a variety of operating 

scenarios the average ambient SO2 concentrations at Coburn Road SLAMS decrease by nearly 80% when 

PPL-Corette is not operating. For example in calendar year 2012, the average ambient concentrations when 

PPL-Corette is operating ranges between 3.48 ppb to 3.83 ppb. In comparison when PPL-Corette is not 

operating the average ambient SO2 concentrations is 0.93 ppb, an approximate 76% reduction. In addition, 

exceedances of the 2010 1-hour SO2 standard have occurred only during times when PPL-Corette is 

operating. 

9 April 3, 2013
Figure 8. Monitored hourly ambient monitored concentration at the Coburn Road SLAMS for calendar year 2010.

10 April 3, 2013
Figure 9. Monitored hourly ambient monitored concentration at the Coburn Road SLAMS for calendar year 2011.

11 April 3, 2013
Figure 10. Monitored hourly ambient monitored concentration at the Coburn Road SLAMS for calendar year 2011.

12 April 3, 2013
In summary the Department concludes that the prior discussion of Emissions Related Data demonstrates that 

ambient SO2 concentrations monitored at the Coburn Road SLAMS are primarily a result of PPL-Corette operations 

and resulting emissions. This conclusion supports including PPL-Corette within the NAA boundary. 

 

3. Meteorology (weather/transport patterns)

Ambient levels of SO2 in the Billings/Laurel airshed reflect the nature of the sources, terrain effects, and typical 

dispersion patterns. Most of the SO2 emitted into the airshed is released from tall stacks with varying amounts of 

plume rise. These separate plumes may either loft up and out of the airshed, impact on nearby high terrain, or mix 

down to the ground either in a direct fashion during unstable conditions or as a fumigation episode related to 

inversion break up.

Dispersion patterns in the Billings/Laurel airshed have been studied extensively for more than 30 years. Most 

notable was a field study conducted in the early 1980s which included an acoustic sounder, three times weekly pilot 

balloon/t-sonde launches, surface meteorological monitoring, and a network of ambient SO2 monitors. The 

dispersion in the area was re-visited in the 1990s during the development of the 1994 Billings/Laurel SO2 State 

Implementation Plan (SIP). These studies and the resulting knowledge of dispersion patterns have greatly influenced 

the siting of monitors in the area. 

The Design Value Period wind rose from the Coburn Road monitoring site shown in Figure 11 below reflects 

several important features that influence the wind patterns. The dominant and prevailing synoptic wind flow is from 

the west-southwest. This lines up almost exactly with orientation of the Yellowstone River Valley in this reach of 

the drainage. Although the sides of the valley are not as dramatic as those in the more mountainous areas to the west, 

they still provide for a noticeable channelization of the wind flow in the surface boundary layer of the atmosphere in 

this area. The wind rose below in Figure 11 characterizes the direction of prevailing winds readings at the monitor. 

Figure 11. Wind rose for the Design Value period from the Department’s Coburn Road monitoring site.

13 April 3, 2013
Billings sits at the juncture of both the mountains and the plains and exhibits some of the climatic features of both 

regimes. Clear skies and dry air are the usual pattern and nocturnal inversions form most nights so there is usually 

no intervening synoptic event. Without the geographic trapping common in mountain valleys to the west, day-long 

stagnation events are rare and multi-day events even rarer. During the winter of 2010-2011, which was the coldest 

and snowiest of the past decade, the airshed saw only one three-day stagnation episode and only three other 

individual days when the inversion lasted more than 24 hours. Outside of the winter season, stagnant periods which 

exceed 24 hours are very uncommon.

EPA states in its February 6, 2013 Technical Support Document (TSD), “exceedances are primarily driven from 

emissions blown from the southwest, which is the predominant wind direction.” The Department is in agreement 

with EPA and concludes that the sources upwind from the Coburn Road SLAMS predominantly drive the ambient 

SO2 concentrations.

4. Geography/topography (mountain ranges or other air basin boundaries). 

The terrain in the vicinity of Billings and Laurel is a typical eastern Montana landform with an upland bench about 

4,000 feet in elevation that is steeply cut by the Yellowstone River and its various tributaries, resulting in a rugged 

topography with a total vertical relief locally of 800 to 900 feet. Both Billings and Laurel are situated on the banks 

of the Yellowstone River which runs from west-southwest to east-northeast in this area.

Billings is located approximately 15 miles downstream (east-northeast) of Laurel. Laurel’s elevation is 

approximately 3,300 feet while Billings’ elevation is approximately 3,100 feet. The Yellowstone River valley is 

fairly broad near Laurel consisting of a mostly flat plain approximately 3 to 4 miles wide rising to elevations 

approximately 500 feet above the valley floor within about 1 mile to the north and about 2 miles to the south. The 

Clark’s Fork of the Yellowstone River joins the Yellowstone River at Laurel cutting into the valley at a southsouthwest angle. 

The city of Billings is located at a significant constriction of the valley terrain that at its narrowest is only 

approximately 4,000 feet wide. The elevated terrain to the south, locally known as Sacrifice Cliff, is a sheer slope 

500 feet higher than the valley floor. The elevated terrain on the north side of Billings, locally known as the 

Rimrocks, rises 400 feet just as steeply.

The bench land to the south is more rugged than the area to the north, rising to approximately 4,500 feet within 10 

miles with steep slopes where the bench has been cut by erosion. To the north, the terrain is noticeably flatter rising 

to approximately 4,600 feet within 30 miles. See Figure 12 below. 

Figure 

12. Billings area topography.

14 April 3, 2013
5. Jurisdictional Boundaries (e.g., counties, air districts, Reservations, etc.). 

On February 6, 2013, EPA stated that they disagreed with MT’s initial designation and intends to designate all of 

Yellowstone County as Nonattainment for the 2010 1-hour SO2 standard. The 2010 SO2 standard being a 1-hour 

limit based on the 99th percentile is designed to protect against localized, acute, short term exposures. Given the 

nature of SO2 emissions in the Billings/Laurel area as a localized, rather than a regional pollutant, the new SO2 NAA 

should protect against localized, acute, short-term exposures and should be significantly smaller than the entire 

county of Yellowstone as suggested by EPA. The logical approach is to define the new NAA using appropriate 

jurisdictional boundaries excluding areas with monitors demonstrating attainment with the 2010 1-hour SO2

standard. 

The 2011 SO2 Guidance allows the use of formal physical and jurisdictional boundaries, including, county 

boundaries, section quadrants, topography, roads and other physical/jurisdictional boundaries as appropriate. The 

Department will use the appropriate boundaries with consideration of the other four factors to define the area as 

administratively efficient as possible. 

C. Proposed NAA

Montana is proposing a NAA including approximately 10.5 square miles (mi2) as described below and shown in 

Figure 13. 

The proposed NAA originates at the point defined as the southwest corner of Section 11, Township 1S, 

Range 26E. From that point the NAA boundary proceeds north along the western section line of Section 11 

to the point of intersection with the midline of Interstate Highway 90. From that point the boundary 

follows the midline of Interstate Highway 90, across the Yellowstone River, to the point where the highway 

midline intersects the northern boundary of Section 35, Township 1N, Range 26E. From that point the 

boundary proceeds east along the northern section line of Sections 35 and 31 to the point where Old US 

87/Hardin Road leaves the section line and turns southeast. The boundary follows the midline of Old US 

87/Hardin Road southeast to the point where the road intersects the western boundary of the SE 1⁄4 of the 

SE 1⁄4 of Section 31, Township 1N, Range 27E. From that point the boundary proceeds south along the 1⁄4 

section line to the southern boundary of Township 1N, then east to the northeast corner of Section 5, 

Township 1S, Range 27E. The NAA boundary then proceeds south along the eastern section line of 

sections 5 and 9 to the southeast corner of Section 9, Township 1S, Range 27E, where it turns west and 

follows the south section line of Sections 9 and 7, Township 1S, Range 27E; and Sections 12 and 

11, Township 1S, Range 26E, back to the point of origin.

15 April 3, 2013
The NAA shown in Figure 13 below uses the appropriate jurisdictional boundaries and the prior five factor analysis 

to define the boundary of the proposed NAA. Montana’s proposed NAA includes the violating monitor (Coburn 

Road SLAMS) and the primary source impacting that monitor. Further, the proposed NAA excludes the monitors 

and associated areas demonstrating compliance with the 2010 1-hour SO2 Standard.

Figure 13. The proposed Billings 1-hour SO2 NAA. 

C. Conclusion

EPA has proposed in its February 6, 2013 letter to Governor Steve Bullock that it intends to designate all of 

Yellowstone County Montana as Nonattainment for the 2010 1-hour SO2 standard. The 2010 SO2 standard being a 

1-hour limit is designed to protect against localized, acute, short-term exposures. In support of this standard, the SO2

NAA boundary should be defined to protect against such exposures and therefore should be significantly smaller 

than the entire county of Yellowstone, as proposed by EPA. As previously stated, ambient SO2 is the direct result of 

industrial emissions and the significant sources of SO2 in Yellowstone County exist only in the Billings/Laurel area. 

Based on the prior five factor analysis, Montana has demonstrated that a smaller NAA protecting against impacts 

from the local industries is appropriate. 

Ambient air quality data collected at representative sites throughout the airshed over nearly four decades shows the 

Coburn Road SLAMS to be the highest concentration site. The other area monitors show compliance with the 

standard thereby limiting the extent. CAA sec. 107(d)(1)(A)(i) defines nonattainment as any area that does not meet 

16 April 3, 2013
the national primary or secondary ambient air quality standard for the pollutant. Further 40 CFR 50.17(b) states, 

“The 1-hour primary standard is met at an ambient air quality monitoring site when the three-year average of the 

annual (99th percentile) of the daily maximum 1-hour average concentrations is less than or equal to 75 ppb...” As 

shown in Table 1 and Figure 2 in the prior five factor analysis, the design values for all the monitors in the area with 

the exception of Coburn Road SLAMS demonstrate attainment with the 2010 1-hour SO2 standard. In accordance 

with 40 CFR 50.17(b), it would be inappropriate to include areas with monitors demonstrating attainment with the 

standard within the boundary of a NAA. 

The existence of monitors demonstrating attainment with the standard leads the Department to conclude that a 

smaller area, excluding areas of Yellowstone County shown by those monitors to be in attainment, is appropriate. 

Further CAA 107(d)(1)(A)(iii) defines Unclassifiable areas as “any area that cannot be classified on the basis of 

available information as meeting or not meeting the national primary or secondary ambient air quality standard for 

the pollutant.“ Based on that definition the portion of Yellowstone County devoid of major sources of SO2 or 

monitoring information should be designated as unclassifiable or attainment. 

Emissions related data and meteorology have shown and EPA also concludes in its February 6, 2013 response that 

impacts at the Coburn Road SLAMS are primarily the results of emissions from those sources located to the 

southwest (upwind) of the monitor most notably PPL-Corette. The average ambient SO2 concentrations at Coburn 

Road SLAMS decrease by nearly 80% when PPL-Corette is non-operational. For example in calendar year 2012, the 

average ambient concentrations when PPL-Corette is operating ranges between 3.48 ppb to 3.83 ppb. In comparison 

when PPL-Corette is not operating the average ambient SO2 concentrations is 0.93 ppb, an approximate 76% 

reduction. In addition, exceedances of the 2010 1-hour SO2 standard have occurred only during times when PPLCorette is operating. In summary, the Department concludes that ambient SO2 concentrations monitored at the 

Coburn Road SLAMS are primarily a result of PPL-Corette operations and resulting emissions. 

With respect to industrial sources outside of the proposed NAA boundary, permitting programs will protect against

potential impacts. Far below the NSR significance level of 40 tpy, DEQ would be prohibited from issuing a permit 

or allowing a de minimis change for a new or modified emitting unit unless an applicant demonstrates that it will not 

cause or contribute to a violation of any federal or state ambient air quality standard (ARM 17.8.745 and ARM 

17.8.749). As shown over the history of the program, the Department has worked with applicants to further control 

or reduce emissions (and make those controls or reductions federally enforceable) so the appropriate demonstration 

can be made and the permit can be issued in compliance with ARM 17.8.749. In effect, the NSR provisions of 

offsetting and the Lowest Achievable Emissions Rate are being used at the minor source permitting level to 

minimize impacts upon nonattainment areas and to ensure that the ambient standards are protected.

Throughout this document Montana has demonstrated, and EPA’s analysis appears to concur, that the predominant

wind patterns are from the southwest and the sources upwind of the violating monitor are primarily impacting the 

monitor. Monitoring data from all the monitors in the area, not just the Coburn Road monitor, shows that the 

impacts are highly localized and do not support EPA’s proposed designation of Nonattainment for the entire county. 

Given the nature of SO2 emissions in the Billings/Laurel area as a localized, rather than a regional pollutant, the new 

SO2 NAA should protect against localized, acute, short-term exposures and should be significantly smaller than the 

entire county of Yellowstone as suggested by EPA. The logical approach is to define the new NAA within the 

bounds of the monitors in the Billings area.

 

Montana’s five factor analysis demonstrates that a smaller NAA is appropriate as opposed to EPA’s default of the 

entirety of Yellowstone County. The proposed NAA includes the violating monitor (Coburn Road SLAMS) and the 

primary source impacting that monitor. Further, the proposed NAA excludes the monitors and associated areas 

demonstrating compliance with the 2010 1-hour SO2 Standard. 

Montana maintains its position that the SO2 values for 2010 were influenced by an EPA OECA Consent Decree and 

are not representative of current or future ambient SO2 concentrations in the Billings area. However the previous 

Five-Factor analysis is presented as justification for an appropriately sized NAA, should EPA maintain its position 

that 2010 monitoring is representative and should be included in the design value at Coburn Road. 

17 April 3, 2013