A Summary of 

Historical Ambient SO2 

Monitoring in the Billings Area 

For 

Billings-Laurel Air Quality Technical Committee 

(BLAQTC) 

700 ExxonMobil Road 

P.O. Box 1163 

Billings, Montana 59103 

Prepared by: 

Bison Engineering, Inc. 

1400 11th Avenue 

Helena, MT 59601 

(406) 442-5768 

www.bison-eng.com

February 9, 2013
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EXECUTIVE SUMMARY 

A review of the historical ambient air quality monitoring for sulfur dioxide (SO2) in the 

Billings-Laurel area indicates a wealth of monitoring sites and data. Continuous 

monitoring for sulfur dioxide was established in the early 1970s and continues on today. 

The area has been heavily monitored over the past 4+ decades and has extended over 

a large geographical area (see Figure 2). Approximately 31 locations have been 

monitored over the years in this airshed. 

The monitoring over time has served two purposes: 

1) Measure concentrations within population areas; and 

2) Establish and measure areas of high concentration 

Sulfur dioxide monitoring has been conducted at many sites throughout Billings, Laurel, 

and Lockwood targeting populous or population sensitive areas. At one time or another, 

SO2 monitoring has been conducted at or near public schools, local businesses, major 

street intersections, city parks, county fairgrounds, etc. 

The preponderance of SO2 monitoring throughout the Billings/Laurel valley has targeted 

areas of high SO2 concentrations. Consistently and historically, the Coburn Road site 

has yielded the highest SO2 concentrations in the monitoring networks. 

Although the record is not well documented, the Coburn Road site was chosen based 

on a sulfation plate study (an objective screening method) and a general belief that 

areas above the valley floor, and near major industrial facilities, would be a prospective 

location for high concentrations. Dispersion modeling was also used to evaluate areas 

for high SO2 concentrations, but dispersion modeling has not proven to be a useful tool 

to predict high SO2 concentration. For example, the model did not predict Coburn Road 

as the highest SO2 concentration area. Sites that were established primarily based on 

dispersion modeling such as Sacrifice Cliff, Lower Coburn Board and Johnson Lane 

yielded lower concentrations than Coburn Road. Thus, Coburn Road appears to be the 

appropriate site to represent the highest concentrations in the airshed. 

The results of the entire monitoring effort, with one exception in the 1970s, indicate no 

violations of the National Ambient Air Quality Standards (NAAQS) as they have existed 

for the past 39 years (i.e., prior to the new 2010 1-hr SO2 ambient standard). However, 

values were recorded in excess of the more stringent Montana Ambient Air Quality 

Standards (MAAQS). These values were resolved during the mid-1980s. No 

exceedances of these standards have been recorded since. 
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1.0 INTRODUCTION 

The Billings-Laurel Air Quality Technical Committee (BLAQTC) has asked Bison 

Engineering, Inc. (Bison) to conduct a historical review of the SO2 ambient air quality 

stations in the Billings/Laurel area. The purpose of the study will be to create a history 

that describes the ambient monitoring that has been conducted in Billings over the past 

30+ years. In particular, the study will further attempt to decipher the reasoning and 

rationale for the selection of some of the monitoring sites, especially sites targeting 

concentration areas. The Coburn Road site is selected for review to evaluate why this 

site is the appropriate ‘high concentration’ site for this area. 

This compendium will provide evidence to the public, to the Montana Department of 

Environmental Quality (MDEQ) and to the Environmental Protection Agency (EPA) as to 

why the Coburn Road site, and others, is most appropriately sited at a high SO2

concentration area. The information is germane to the ongoing designation process 

before EPA for Yellowstone County and Montana as a whole. 
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2.0 REFERENCE DOCUMENTS 

The following is a list of documents and information sources reviewed for this report. 

Copies of all of the referenced documents can be found in the Bison Engineering files. 

Other documents may exist in the files of MDEQ; however, they are stored off-site from 

MDEQ and are thought to be incomplete and difficult to access. 

STATE OF MONTANA - 2010 AIR MONITORING NETWORK ASSESSMENT 

MONTANA AIR MONITORING NETWORK REVIEW 2001 

MONTANA AIR MONITORING NETWORK REVIEW FOR 1996 

MONTANA 1986 NETWORK REVIEW 

STATE OF MONTANA – AIR QUALITY CONTROL IMPLEMENTATION PLAN, 

1979 

BILLINGS SO2 STUDY – DATA ANALYSIS, MODEL VALIDATION AND 

SOURCE DETERMINATION, 1984 

MONTANA AIR QUALITY DATA & INFORMATION SUMMARY FOR:

o 1979-1980 

o 1982 

o 1983 

o 1984 

o 1985 

o 1986 

o 1987 

o 1988 & 1999 

MONTANA AIR POLLUTION STUDY FINAL REPORT JUNE 1981 

Phone Interview: Mike Machler: Formerly of the Air Quality Bureau of MDEQ1, 

GeoResearch Inc., and Bison Engineering, Inc. 

Phone Interview: Steve Duganz, formerly of Yellowstone County Health 

Department 

Phone Interview, Bob Jeffrey, formerly of MDEQ, Air Quality Section 

Phone Interview, Hoby Rash, MDEQ Air Quality Monitoring Section 

Personal Interview, Hal Robbins, formerly Air Quality Bureau Chief of MDEQ2, 

currently at Bison Engineering, Inc. 

 

1

 The agency was known as the Department of Health and Environmental Sciences at this time. It was 

later to become the Department of Environmental Quality. 

2

 Ibid. 
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3.0 EARLY SO2 MONITORING HISTORY (UP TO ~ 1980) 

There appears to be little surviving written information surrounding early SO2 monitoring 

efforts (pre-1975). Brief oral histories were taken from people involved in these activities 

and were used in this report to fill in some gaps. 

The surviving written history of SO2 monitoring in Yellowstone County starts about 

1974-75. The earlier written accounts have very little detail or resulting data collected. 

As time progresses, the available written reports become more detail oriented. 

The 1979 State of Montana Air Quality Control Implementation Plan reports SO2 

monitoring stations3 were installed and operating in Billings on January 1, 1972. The 

Plan also shows three SO2 bubblers operating in the Billings/Laurel area in 1975 (City 

Hall, Laurel and “Refinery Area”). The results from the monitoring showed compliance 

with the National Ambient Air Quality Standards (NAAQS), although problems were 

identified in Laurel, about 15 miles west-southwest of downtown Billings. The Plan also 

shows one continuous analyzer in operation in 1975, at “Sewage Treatment Plant E. on 

road to Fairgrounds, Billings.”4

In addition, the Montana Department of Health and Environmental Sciences (the 

predecessor to MDEQ), Yellowstone County and the Crow Tribe operated an extensive 

network of sulfation plates throughout the Billings-Laurel area.5 Although MDEQ knew 

that the sulfation plates were not “exact”, air quality officials knew the plates provided an 

objective measurement of relative levels and trends of ambient SO2 at many locations 

throughout the valley. In fact, there was a Montana Ambient Air Quality Standard for this 

pollutant during this time period. The standard was known as “Reactive Sulfur.”6 These 

sulfation plates could be easily deployed and thus create a geographical picture of local 

trends. Little information was found on the time frames of operation, or results of the 

 

3

 The sampling methodology at that time used what was referred to as a “bubbler” sampler. This method 

was the official SO2 reference method (per 40 CFR 50, Appendix A) until only a few years ago. The 

method bubbles ambient air through a liquid solution of potassium tetrachloromercurate (0.4 M) for 

typically 24 hours. The solution is then analyzed to determine ambient SO2 concentrations. This method 

is sometimes referred to as the “Pararosaniline Method.” 

4

 Although the document provides no details, it is assumed (from a past MDEQ employee) that this 

continuous analyzer operated on the principal of coulimetric titration and was manufactured by Beckman

(Model 906A). 

5

 These “sulfation plates” are usually referred to as the “Huey Sulfation Plates.” These plates, as far as 

can be determined, were invented by Norm Huey who was an air quality engineer who worked for EPA in 

the Region 8 office in Denver. These are static samplers in which a plastic petri dish was coated with a 

layer of PbO. The device was mounted outdoors and allowed to react with ambient SO2 to form PbSO4. 

The samplers were usually exposed to the atmosphere for about a month and then analyzed for SO2. 

More information may be obtained from the following publications: N. A. Huey “The lead dioxide 

estimation of sulfur dioxide pollution,” J. Air Poll. Control Assoc. 18:610 (1968) and Paul E. Rider et al.

“Studies of Huey Sulfation Plates at High Ambient SO2 Concentrations,” J. Air Poll. Control Assoc, 27:10, 

1011-1013 (1977). 

6

 The standard was codified in 16-2.14(1)-S14040 (as was the rule structure at that time). 
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sulfation plate network. Figure 1 shows the network of sulfation plates in 1986. From 

vague references, it appears that the sulfation plates operated from approximately 1970 

through 1990. 

In 1978, the Montana Air Pollution Study (MAPS) implemented an SO2 monitoring 

network. The network consisted of four continuous analyzers which operated from 

1978-1980. The locations were Central Park, Lockwood School, 27th and Montana, and 

Laurel Farm east of CHS (formerly Cenex). The Laurel site recorded exceedances of 

the NAAQS. This data, combined with previous analyzer data, resulted in a portion of 

the Laurel area (1 kilometer circle) being designated as non-attainment for SO2 in 

1978.7 

During this time period, no violations of the NAAQS were found at any other site. It is 

worth noting that in accordance with a 1977 stipulation8 between valley industries and 

MDEQ (DHES at that time); Cenex implemented some corrective measures aimed at 

bringing the area below the NAAQS. Once these efforts were concluded, no further 

violations of the NAAQS have been recorded. 

Starting in 1980, better written accounts of SO2 monitoring history have been made by 

the MDEQ. Section 4 provides more detail on this monitoring. 

 

7

 43 FR 8964 (3/3/78) and 43 FR 40427 (9/11/78) 

8

 This is reference to what is commonly referred to as the “1977 Stipulation” which was between the major 

industries (CHS, Phillips 66, Western Sugar, ExxonMobil, Montana Sulphur & Chemical Co., and PPL) 

and MDEQ. 
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Figure 1: Sulfation Plate Network - 1986 
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4.0 MDEQ BILLINGS MONITORING HISTORY (~ 1978) 

The following history is a collection of excerpts from the STATE OF MONTANA - 2010 

AIR MONITORING NETWORK ASSESSMENT, and the MONTANA AIR MONITORING 

NETWORK REVIEW FOR 1996. Bob Jeffrey recounted specifically compiling the 

monitoring network histories for the state, as he thought they would be useful as a 

historical reference. The following paragraphs are taken from these reports, and 

specifically deal with SO2 monitoring in Yellowstone County. 

In preparing this “Summary of Historical Ambient SO2 Monitoring in the Billings Area” 

document various comments and annotations are made as appropriate. 

“Yellowstone County

The Billings/Laurel area in Yellowstone County has dealt with SO2 issues for several decades 

dating back to the beginning of local and Montana air quality programs in the 1960s. There is 

a long history of SO2 monitoring in the cities of Billings and Laurel which are located in the 

Yellowstone River Valley, which runs southwest to northeast and has a major influence on 

the airflow in the area. To the west near Laurel, the valley is approximately 12 miles wide but 

heading east the valley narrows and on the east edge of Billings, the sandstone cliffs or 

rimrocks result in a constriction that is approximately one mile wide and which channels the 

air flows. The major industrial SO2 sources are all located in the Billings/Laurel area along the 

river valley and, as a result, the nearby elevated terrain is often higher than the tallest 

smokestacks.”9

“Wind roses for the area reflect the orientation of the Yellowstone River Valley. The 

predominant wind direction is from the southwest and the second most predominant direction 

is from the northeast; i.e., the wind flows up and down the river valley. As with many areas in 

Montana, the area experiences inversions in the fall and winter months which causes 

pollutants to become trapped and concentrated. The breakup of the temperature inversions 

right after sunrise can force those concentrated layers of SO2 to the ground, resulting in high 

SO2 concentrations being measured at the ground level monitoring stations.” 

While the observation offered by MDEQ has some validity, it seems that more recent 

data, at least, offers an equally valid alternative explanation. A review of the wind 

speeds and directions during periods of observed high concentrations at Coburn 

indicates that the high concentrations are more likely the result of convective air 

movement with perhaps some accumulation (but not always) from inversions or stable 

air masses aloft.10 The reasoning is that if, indeed, inversion (thermal) breakups were 

the cause of some of these relatively higher concentrations (after sunrise) then such 

events would likely be accompanied by low wind speeds. A typical inversion break-up 

(associated with sunrise) is typically caused by the sun heating the ground. This causes 

some vertical movement in the atmosphere which brings concentrations of SO2 in the 

 

9

 Note that this is especially true for the Coburn site, the primary subject of this report, and the Sacrifice 

Cliff site which is discussed later in this document. 

10 This is technically breaking up an inversion, but not in the more traditional sense when coupling it with 

an assumed sunrise (solar heating). 
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upper atmosphere (a few hundred vertical feet) down to the ground and thus breaks up 

the inversion.11

 

Observed high concentrations at Coburn are almost always associated with medium 

strength wind speeds; between 8 and 12 mph. Such an event will, of course, break up 

an inversion, but it does so by moving the air from one location to another and mixing 

the air in the interim (from turbulence). Other times during relatively higher SO2

concentrations observed at Coburn Road, the wind was also blowing at a steady 8-12 

mph from approximately 240 degrees. The Coburn Road SO2 monitor routinely records 

higher SO2 concentrations during this steady southwest wind pattern. 

That issue aside, MDEQ continues with their discussion of ambient monitoring. 

“There are seven major SO2 sources in the Billings/Laurel area. In Billings, these include 

ExxonMobil (oil refinery), Phillips 66 (oil refinery), PPL-MT Corette (coal-fired electric power 

generating facility), Western Sugar (coal-fired sugar beet factory), Yellowstone Energy 

Limited Partnership (coke-fired cogeneration power plant), and Montana Sulphur & Chemical 

(sulfur recovery facility). In Laurel, the major source is the Cenex Harvest States (CHS - oil 

refinery). All of these sources combined contribute to the SO2 in the Billings/Laurel airshed. 

In March 1978, based on ambient air monitoring data, the Laurel area was designated 

nonattainment for SO2. 

12 In anticipation of the pending nonattainment designation, the 

Billings/Laurel industries and the State signed a stipulation requiring the industries to employ 

various control options in December 1977. This stipulation was incorporated into the SIP, and 

it provided the framework to bring the area into compliance. The 1977 stipulation also 

commissioned the Department (with industrial funding) to conduct an ambient air quality 

study to more fully describe the magnitude and geographical extent of the SO2 

concentrations. Another major goal of the study was to identify the relative source 

contributions from each industrial source.” 

Another related goal of the study was to investigate the validity of computer modeling in 

predicting concentrations at relevant sites. 

 “In 1981-82, the Department (through a cooperative effort with the industries) designed and 

installed a local SO2 monitoring network. The network consisted of eight SO2 monitoring sites 

with meteorological monitoring that included upper air measurements from acoustic radar and 

pilot balloons. Six of the SO2 sites were in Billings and two were in Laurel. Using the 

monitoring data, the Department attempted to validate a mathematical model13 that could 

predict concentrations from the various SO2 sources. The model failed for the following 

reasons:14

• a lack of good emissions data; 

 

11 As the air mixes vertically, temperature no longer increases with increasing elevation and thus an 

inversion no longer exists. 

12 The “area” referred to by MDEQ consisted only of a 1 kilometer circle around Tank 26 at the CHS 

refinery. 

13 This was, in fact, a “Gaussian” model. 

14 The reader is reminded that these are MDEQ conclusions. 
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• high concentrations caused by inversion breakup15 and direct plume impact which the 

model could not handle; 

• over-prediction of plume rise by the model; and 

• small changes in wind direction could cause large differences in predicted versus actual 

concentrations at receptors located close to the sources.” 

 

The reader is reminded that the conclusions above are those of MDEQ only and not the 

authors of this document. 

“A novel approach was developed using directional analysis and mass emissions to 

characterize source contributions to the annual average at the worst-case receptor (highest 

estimated ground-level SO2 concentration). This was useful but could not be applied as a 

predictive tool for short-term averages or at other receptors in the valley. 

Following the 1981-82 study, the S02 monitoring network was scaled down in various stages 

until it reached a minimum early in 1987. As mentioned previously, eight S02 monitoring 

stations were in operation in 1982 during the S02 study. Following the conclusion of that 

study, the network was scaled down to four sites in 1983. These included Coburn Road (30-

111-0066), Lockwood Park (30-111-0065), North Johnson Lane (30-111-0064) and Taft 

School (30-111-0064). It should be noted that no sites were kept in the Laurel area since the 

two sites in the 1981-82 study showed compliance with the NAAQS and MAAQS. The 

Department felt that greater emphasis was needed in the Billings/Lockwood area. In 

December 1983, the Taft School site was shut down to reduce the operator's workload in 

Billings. The network of three continued until June 1986 when a leasing problem caused the 

Department to shut down the North Johnson Lane site. In January 1987, the Department shut 

down its Lockwood Park site due to further cutbacks of personnel in the Billings office. At that 

time the network was reduced to a single site, Coburn Road, which has been the backbone of 

the network.” 

The Coburn Road site was chosen as what MDEQ describes as the “backbone of the 

network” because, based on opinion and experience Coburn was “the” monitoring site 

that yielded the most information for regulatory purposes. MDEQ clearly had an interest 

in monitoring in the Billings area for many reasons including population exposure. 

However, it was believed that if one continues to monitor at the “highest” reporting 

station and that station was reporting concentrations below the ambient standards, then 

it was reasonable to assume that the rest of the County was also below the ambient 

standard. 

“Following the 1987 Legislature session, a committee of the Billings/Laurel industries, the 

local chamber of commerce, and air pollution professionals united to monitor and report on 

the area's air quality. The Billings/Laurel Air Quality Technical Committee (BLAQTC) was 

formed in the spring of 1987. The Department was a non-paying member of BLAQTC and 

worked with the group on site selection, quality assurance, and ambient monitoring. BLAQTC 

hired a contractor, purchased equipment and installed three SO2 monitoring stations. Two of 

the sites were in Billings: Lockwood Park (30-111-1065) and Coulson Road (30-111-2004), 

 

15 MDEQ believed that these high concentrations were caused by inversion break-up, but this was not 

necessarily a known fact. 
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and one in Laurel: Laurel-BLAQTC (30-111-0016). By the end of 1987 the combined 

Department-BLAQTC network consisted of five sites. 

This organization known as “BLAQTC” continues today. Over time, the group has 

become a loose organization that primarily consists of six of the seven industrial 

facilities in the Billings/Laurel airshed. 

“The layout of the monitoring network remained the same until October 1989. Upon 

agreement between the Department and BLAQTC, the Scottish Rites site was moved to 

Ponderosa School (30-111-0076) and the Coulson Road site was moved to Brickyard Lane 

(30-111-2005). The group felt that the old sites had served their purpose and that higher SO2 

concentrations might be found at the new sites. The EPA approved all of these sites in a 

letter dated May 14, 1990. On June 30, 1992, the Ponderosa School site was shut down due 

to low readings. EPA approved this action in a letter dated June 22, 1992. The monitoring 

shelter from Ponderosa was moved and replaced the aging shelter at Coburn Road. 

Due to interest from EPA and environmental groups, in November 1988 the Department 

began collecting peak 5-minute SO2 averages for each valid hour of data.16 At a meeting with 

the Department and BLAQTC in February 1990, EPA offered two SO2 analyzers to conduct 

peak monitoring in the range from 1 to 5 ppm. In September 1990, the Department and 

BLAQTC installed these monitors at the Coburn Road and Lockwood Park sites. These 

monitors collected peak 5-minute data for 6 months until March 1991. Peak data was also 

collected at the other three sites with analyzers in the range of 0 to 1 ppm. After completion of 

the study, an EPA contractor reviewed and analyzed the data but the results were not 

conclusive. The Department continued to collect peak 5-minute SO2 data through 2003, but 

none was collected at the industry sites. It’s worth noting that for calendar year 2002, the 

Department collected and reported not just the maximum 5-minute value in each hour, but all 

of the 5-minute values in each hour. 

In 1990, E. H. Pechan and Associates (Pechan) developed an SO2 emission inventory for the 

Billings/Laurel area. The final report, submitted to EPA in 1991, indicated a large difference 

between actual and potential SO2 emissions for most sources. Also in 1990, the City of 

Billings contracted with GeoResearch Inc. to conduct an SO2 dispersion modeling study for 

the Billings area utilizing recently gathered meteorological data and the Pechan SO2 emission 

inventory. The dispersion modeling results predicted violations of the SO2 NAAQS at both the 

potential to emit and maximum actual emission rates.” 

The purpose of the study commissioned by the City of Billings was to identify areas in 

the city and its environs where new or expanding industry might be sited successfully. 

Given the use of emission rates (from the Pechan study) far beyond those reported in 

actual use, the study, not surprisingly, predicted ambient concentrations well beyond 

those actually observed at the monitoring sites. 

“The Department reviewed and refined the results of those latest modeling studies17 and 

concluded additional SO2 monitoring sites were needed. At the location of the modeled 

maximum SO2 "hot-spot" the Department installed an SO2 monitor. That max site, Sacrifice 

Cliff (30-111-0080), was approved by EPA effective May 19, 1993, and sampling began. 

 

16 MDEQ does not mention in this summary that the industries themselves, via BLAQTC, also participated 

in the collection of short-term (5 minute) data. 

17 MDEQ is primarily referring to dispersion modeling studies conducted by the “YELP-BGI” facility in 

support of its air quality permit application (1991). 
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Also, as a result of the permitting process, YELP-BGI was required to operate SO2 monitors 

at two other modeled "hot-spots" at Johnson Lane (30-111-2006) and Pine Hills (30-111-

2007). EPA approved both sites on March 7, 1994, with data collection starting on November 

2, 1993. Through 1996, the Sacrifice Cliff site measured the highest SO2 concentrations in the 

Billings/Laurel monitoring network but a change to low sulfur coal at the YELP-BGI facility18

resulted in a dramatic drop in ambient SO2 concentrations. After the fuel switch, the Sacrifice 

Cliff site consistently measured lower concentrations than the values at Coburn Road and the 

Sacrifice Cliff site was shut down in June 2001. 

There are two items of note regarding DEQ’s comments above. First, the agency states 

that the SO2 concentration dropped as a result of a fuel switch (to lower sulfur coal) at 

PPL-Montana’s Corette station.19 While indeed PPL emissions were reduced during this 

period, it is equally true that emissions from other industries also reduced their 

emissions as well. The emissions from the major nearby facilities during the same 

period (3 year average prior to 1996 versus the 3-year average beginning in 1996) were 

as follows: 

Emissions and Ambient Comparison

1993 - 1995 vs. 1996 - 1998 

Emissions a

 1993 – 1995 1996 – 1998 Difference 

PPL (Corette) 7,250 2,570 -65 % 

 ExxonMobil 10,300 5,600 -46 % 

Phillips 66 1,800 1,090 -40 % 

 CHS 5,100 3,150 -39 % 

Western Sugar 470 140 -70 % 

 MSCC 3,500 3,700 +8 % 

YELP n/a 2,000 n/a 

Ambient Data b

Coburn Road 87 40 -54 % 

 Sacrifice Cliff 61 33 -46 % 

Notes: 

a. Emission values are in tons/year and are rounded for convenience 

b. Ambient data units = ppb 

c. Ambient data averaging period = 2nd high 24-hour value (3-year average); consistent with 

one of the averaging period of the NAAQS at that time. 

d. Emission values are rounded for convenience 

e. YELP began operation in late 1995. 

Second, a review of the data during this period shows that the Sacrifice Cliff data indeed 

yielded among the highest concentrations in the valley. On the other hand, the pre-1996 

values were nearly identical to that of Coburn Road. Some statistics indicated pre-1996 

 

18 This is an apparent error on MDEQ’s part. YELP-BGI does not combust coal. It is assumed that MDEQ 

was, or should have been, referring to the Corette plant. 

19 See footnote above. MDEQ mistakenly refers to YELP-BGI when it should have stated PPL. 
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Sacrifice Cliff as the highest concentration while others show that Coburn ranks higher. 

Regardless, the SO2 concentrations at Sacrifice Cliff, after a short time, were reduced 

and thus the site no longer served its purpose. The site was no longer the highestreporting site and it (and Coburn Road) failed to show any values in excess of the 

ambient air quality standards. 

In order to characterize population exposure, BLAQTC installed an SO2 analyzer at the 

Mount Olive site in late 1995. This was an already established air monitoring site that 

was measuring continuous carbon monoxide values. The site was located near the 

corner of 24th and Central Avenue, two major thoroughfares in Billings. 

At about the same time, another station was started at Senior High School. This site 

was also established for population measurement coverage. The Senior High site 

included carbon monoxide and meteorological parameters (wind speed, wind direction 

and temperature). 

Due to measured concentrations well below the ambient air quality standards and 

values below Coburn Road, both sites were discontinued in 1997. 

Another monitoring site was installed near the Coburn Road site in 1999 in an attempt 

to find yet-higher reporting monitoring values than Coburn Road itself. The site was 

chosen, in part, from dispersion modeling data. This new site was located about 1⁄2 miles 

from the Coburn site, at an elevation well below Coburn, but elevated from the valley 

floor. This became known as the Lower Coburn site. Monitoring continued here for 

about three years. Again, monitoring was discontinued since no exceedances of any 

ambient air quality standards were observed and the monitored values were well below 

those measured at Coburn Road. 

MDEQ describes the next few years of monitoring below. As before, none of these sites, 

despite their geographical and population intent, yield ambient air quality values above 

ambient standards. All results remain well below those found at Coburn Road. 

(from DEQ) 

“To better document population exposure and monitor trends in the growing west end of 

Billings, the Mount Olive site (30-111-0079) was reopened and monitoring begun for SO2 and 

CO in October 2001. In July 2001, SO2 monitoring was added to the Bridal Shop CO site (30-

111-0082). The results from Mount Olive showed a substantial reduction in ambient SO2 

concentrations in the west end of Billings as compared to the earlier monitoring results from 

the late 1990s, so the Mount Olive site was shut down in September 2004. At the same time, 

a review of the SO2 results from the Bridal Shop site indicated very low concentrations with 

the occasional elevated reading only when winds were blowing directly from the 

ConocoPhillips refinery, so the Bridal Shop site was shut down in June 2004. 

Since the 1980s, there has been substantial residential and commercial development on the 

high bench located above and northeast of downtown Billings. As the population in Billings 

Heights increased so did the number of complaints about air pollution. The “Heights” is 

located just north of several SO2-emitting industrial sources in the Lockwood Park area, and 

given the frequency of south winds during the summer months, the Department decided to 
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install a new air monitoring site to investigate the public’s air pollution complaints. As a result, 

the Beartooth Elementary site (30-111-0084) began collecting SO2 data in August 2003. Over 

the next 3 years, the site did monitor a few periods of elevated SO2 but they were of short 

duration and very infrequent, so the site was shut down in December 2006.20

As of this writing21, there are six SO2 monitoring sites in the Billings/Laurel area; two operated 

due to permit requirements (Johnson Lane and Pine Hills), three by BLATQC (Brickyard 

Lane, Lockwood Park, Laurel), and one by the Department (Coburn Road). Over the years, 

SO2 has been monitored at 31 locations in the Billings/Laurel area. In summary, those sites, 

their AIRS codes and years of operation are as follows: 

• Lockwood School Billings (30-111-0006), 1975-81 

• Laurel BN (30-111-0015), 1981-82 

• Laurel BLAQTC (30-111-0016), 1987- present 

• Hi-Ball Trucking (30-111-0035), 1974-75 

• Division & Grand (30-111-0052), 1975 

• 27th & Montana (30-111-0053), 1975-78 

• Central Park Billings (30-111-0059), 1978-81 

• Metra Park Billings (30-111-0061), 1981-82 

• Taft School Billings (30-111-0062), 1980-83 

• Shawnee Park Billings (30-111-0063), 1982 

• North Johnson Lane DHES (30-111-0064), 1982-86 

• Lockwood County Park DHES (30-111-0065), 1981-87 

• Coburn Road (30-111-0066), 1981- present 

• Scottish Rites Billings (30-111-0073), 1987-89 

• Ponderosa School (30-111-0076), 1989-92 

• Mount Olive Billings (30-111-0079), 1995-97 & 2001-04 

• Sacrifice Cliff (30-111-0080), 1993-2001 

• Bridal Shop (30-111-0081), 2001-04 

• Lower Coburn Road (30-111-0083), 1999-2003 

• Beartooth Elementary (30-111-0084), 2003-06 

• Laurel Jr. High (30-111-1001), 1973 

• Laurel Water Plant (30-111-1005), 1976 

• East Laurel (30-111-1008), 1973-76 

• Laurel East Farm Cenex (30-111-1009), 1976-80 

• Laurel New Farm (30-111-1014), 1980-82 

 

20 A cursory review of the data indicates that values at this site were about 1⁄3 of the values recorded at 

Coburn. Since Coburn was, itself, below the ambient air quality standards, the results did not justify

continued monitoring. 

21 2010. 
A Summary of Historical Ambient Page 15 

SO2 Monitoring in the Billings Area 2/9/2013

• Lockwood Park BLAQTC (30-111-1065), 1987- present 

• Coulson Road BLAQTC (30-111-2004), 1987-89 

• Brickyard Lane BLAQTC (30-111-2005), 1989 - present

• Johnson Lane YELP (30-111-2006), 1993 - present 

• Pine Hills YELP (30-111-2007), 1993 – present 

• Senior High (30-111-2008), 1995-97 

A map depicting the locations of all 31 monitoring stations is included here as Figure 2. 

The reader will observe that continuous ambient sampling for SO2 has occurred 

throughout the Billings-Laurel-Lockwood area. The sampling includes not only a wide 

geographical area; it also includes locations of high population density, public interest 

(schools), industrial activity, road intersections, and various elevations. 
DRAFT
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SO2 Monitoring in the Billings Area 2/4/2013

5.0 HISTORY OF COBURN ROAD MONITOR 

The written records of Billings ambient monitoring, Coburn Road in particular, have 

been reviewed. The records indicate an extensive effort by MDEQ, the county and the 

industrial sector to pick ambient monitoring stations that represented a plethora of 

purposes. Those purposes ranged from health study investigations such as the 1981 

Montana Air Pollution Study, population exposure, special requests and peak impacts. 

A total of 31 sites, other than sulfation plate studies, have been monitoring since the 

early 1970s. 

The Coburn Road continuous SO2 analyzer was installed in 1981. In various forms, an 

ambient SO2 analyzer has been operating at that site ever since. 

Based on the review of the data no specific written record was found that describes the 

specific logic behind the location of the Coburn Road monitoring station. However, 

interviews from a number of persons who were involved, in one form or another, with 

either the original site selection or its operation yielded some common recollections. 

Bob Jeffrey and Steve Duganz had similar recollections in their interviews, as follows: 

The sulfation plate network, previously described, was used to locate a station at 

Coburn Road. This was based, in part, on the fact that this location routinely 

recorded among the highest sulfation rates. In addition, the site also had three 

other attributes that are required to establish an ambient monitoring station: a 

cooperative landowner, (easy) access, and available power. 

In addition to the sulfation plate information, the site also represented an area 

above the valley floor. This site seemed intuitive to MDEQ since it was suspected 

that the prevailing southwest wind would yield elevated SO2 concentrations from 

upwind industrial facilities. 

Bob Jeffrey recalled that the Coburn Road site was moved several times, and received 

new shelters several times. None of the moves was more than 100 yards from the 

current site. 

Some selected text from the MDEQ 1986 MONTANA NETWORK REVIEW about the 

Coburn Road site is copied here for further information. 

“8. The Coburn Road site is located on the top of a ridge which separates the city of 

Billings from the Lockwood area. The Conoco oil refinery, the Montana Power Co. 

Corette Plant, and the Western Sugar refinery are sulfur dioxide sources which are 

located on the Billings side of the ridge within two miles of the monitoring site. On the 

other side of the ridge in Lockwood are the Exxon oil refinery and the Montana Sulphur 

and Chemical Company. Both of these sulfur dioxide sources are approximately two 

miles northeast of the Coburn Road site. The site is at an elevation of 3380 feet MSL or 

approximately 280 feet above the surface of the Yellowstone Valley. Wind patterns at 

the Coburn Road site are somewhat different than at the other sites located within the 

valley. In general, the wind speed is 0.5 to 1.0 meter per second faster with a much 
A Summary of Historical Ambient Page 18 

SO2 Monitoring in the Billings Area 2/4/2013

greater predominance from the west-southwest. Apparently, terrain features have 

much less effect on wind at the Coburn Road site.” 

“10. The department suspects that the Coburn Road site is located at an optimum 

elevation, so that during periods of multiple layered inversions, the plumes of the 

various sources are trapped in a layer at the same height. This might explain why sulfur 

dioxide concentrations at Coburn Road are generally higher and often do not follow the 

concentrations at the other valley locations. 

11. Since Coburn Road consistently records high sulfur dioxide concentrations, the 

department intends to continue to monitor sulfur dioxide at this location. In addition, the 

site will be the key site in determining the amount of sulfur dioxide control necessary at 

each plant.” 

In addition to those comments directly from MDEQ, Hal Robbins22 offered some of his 

thoughts and recollections of this site. He recalls reviewing and discussing the selection 

of the site with various agency staff. In particular he recalls that there were two primary 

factors that led to the selection of this site (beyond site access, power availability, etc.). 

The first, and most important, was the results of the sulfation network previously 

described in this document. The sulfation network included a large geographical area 

which helped narrow where the ideal maximum concentration site might be located. The 

sulfation network indicated that the Coburn Road area consistently showed either the 

highest or near the highest concentrations of measured ambient SO2. 

The second factor was elevation. It was thought at the time that monitoring efforts in the 

valley may be useful, relative to population exposures and the geographical extent of 

the network, but the valley monitoring may not characterize the peak impacts. (MDEQ 

wanted, consistent with EPA policy, to be sure that the monitoring network included 

areas with the maximum concentration.) A site that was above the valley floor, and thus 

closer to stack height, coupled with high sulfation plate readings was thought to be a 

prudent location. 

Hal also recalls working with David (Dave) Maughan during this time period. Dave was 

in charge of special projects and conducted a number of investigations into the extent 

and nature of ambient readings. This included fluoride studies in Butte-Anaconda, SO2

mapping in Anaconda, lead (Pb) snow-deposition studies in East Helena, etc. Dave was 

involved in the sulfation plate analysis and selection discussions. The Yellowstone 

County Health Department played an important role in these discussions as well. 

 

22 Hal was the Bureau Chief of the Montana Air Quality Bureau from 1980 through 1987, worked in the air 

monitoring and dispersion modeling section of the agency from 1975 to 1980, and was directly involved in 

the selection of Coburn as a high concentration site. 
A Summary of Historical Ambient Page 19 

SO2 Monitoring in the Billings Area 2/4/2013

6.0 SUMMARY AND CONCLUSIONS 

This document has reviewed the historical ambient air quality monitoring for SO2 in the 

Billings-Laurel area. The review has indicated a large number of monitoring sites have 

been in place over the past 40 years. The sites have included places on high terrain, 

low terrain, near schools, adjacent to major intersections, high population areas, low 

population areas, and near major industrial facilities. With one exception in the late 

1970s, none of these monitoring sites yielded ambient concentrations above the 

National Ambient Air Quality Standards (NAAQS) as they existed prior to 2010. 

The siting of these stations was typically driven by a desire to capture the location that 

produces the highest concentration. However, in some cases, the monitoring was 

established to ascertain population exposure or for a specific study or request. 

As it regards siting a station at an area of highest concentration, a combination of 

passive monitoring (sulfation plate studies), professional judgment and dispersion 

modeling were employed. In retrospect, the sulfation plate study along with some 

professional judgment appears to have been the most successful means of achieving 

that goal. 

On a consistency basis, the Coburn site has recorded, and continues to record, the 

highest concentrations of SO2 in the Billings airshed since it was installed in 1981; 32 

years ago. This observation remains despite the fact that dispersion modeling has 

indicated other locations which should, according to the model, record higher 

concentrations. Modeling was used to site additional samplers (Lower Coburn, Sacrifice 

Cliff and others), but those stations did not record values higher than Coburn Road. 

The geographical distribution (see Figures 1 and 2) coupled with the length of the 

monitoring periods has well characterized the concentration of SO2 in the Billings-Laurel 

airshed.