1 

MEMORANDUM 

From: Gail Tonnesen, EPA Region 8 Air Quality Modeler 

To: Fort Berthold FIP File 

Date: June 15, 2012 

Re: Impacts of Well Pad VOC Enclosed Combustors and Utility Flares on the Ozone and NO2

NAAQS 

Summary 

EPA evaluated the impacts of changes in Volatile Organic Compounds (VOC) and Nitrogen Oxides 

(NOX) emissions from enclosed combustors and utility flares used for control of VOC emissions at oil 

and natural gas production facilities on the Fort Berthold Indian Reservation (FBIR) as part of the 

technical analysis for the FBIR Federal Implementation Plan (FIP). Emissions categories that are 

affected by this FBIR FIP include VOC and NOX. Both VOC and NOX are precursors to ozone, a 

criteria pollutant, and NOX is a precursor to Nitrogen Dioxide (NO2), also a criteria pollutant. As 

discussed in the TSD, EPA reviewed data from synthetic minor permit applications1

 and determined that 

a typical uncontrolled well pad2 with two wells would have VOC emissions of about 4,330 tpy.3 After 

installing VOC control equipment, including a combination of enclosed combustors, pit flares, and 

utility flares with destruction efficiencies ranging from 90-98%, potential controlled VOC emissions 

from a typical well pad with two wells and no pipeline access were estimated to be 150 tpy. 

Additionally, NOX emissions resulting from the associated combustion of VOCs were estimated to be 

8.8 tpy4 for a two-well facility with no pipeline access. Actual emissions are expected to be much lower 

due to the anticipated development of pipeline infrastructure in future years. 

EPA performed a qualitative assessment of ozone impacts and current ozone air quality in the region and 

determined that the proposed action is not expected to cause violations of the 8-hour ozone NAAQS. As 

discussed below, EPA also evaluated two scenarios of NOX emissions from enclosed combustors and 

utility flares using the AERMOD nearfield dispersion model to evaluate impacts on the 1-hour average 

NO2 NAAQS. Input data to the AERMOD model for VOC enclosed combustors includes emission rates 

of NOX, stack height and diameter, stack gas exit velocity and temperature. Input data for utility flares 

includes flare height and calories per second generated by VOC combustion. The highest modeled 

impacts on NO2 were 27 ppb, or 27% of the 1-hour NO2 NAAQS. When combined with background 

concentrations of 11 ppb, the highest modeled concentrations were 38 ppb or 38% of the 1-hour NO2

NAAQS. Thus, the AERMOD modeling results do not indicate exceedances of the NO2 NAAQS for 

NOX emissions from enclosed combustors and utility flares. 

 

1

 The applications can be found in the docket for the interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be 

accessed at: http://www.regulations.gov. 

2

 A well pad is an oil and natural gas production facility that consists of one or more wells and production and storage 

operations. 

3

 A summary of VOC emissions from application data titled “VOC Emissions Analysis from Syn Minor Application Data” 

can be found in the docket for the interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: 

http://www.regulations.gov. 

4

 A summary of NOX emissions from application data titled “NOX Emissions Analysis from Syn Minor Application Data” 

can be found in the docket for the interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: 

http://www.regulations.gov. 
2 

Background Air Quality Data 

Impacts of the proposed action on ambient concentrations of NO2 and ozone must be evaluated relative 

to current background concentrations of NO2 and ozone in the region. The State of North Dakota (ND) 

operates 3 ambient monitoring sites in the western part of the state that are designed to characterize

regional background concentrations of criteria pollutants including ozone, NO2, SO2, and particulate 

matter. These sites are located at Dunn Center (Dunn), Theodore Roosevelt National Park-Northern Unit 

(TNRP), and Lostwood National Wildlife Refuge (LNWR). The State also operates 13 additional sites 

that are designed to characterize source specific impacts or significant source and population centers, 

and these sites are not appropriate for evaluating regional background air quality. 

 Figure 1. Locations of ambient air quality monitoring sites in ND. 

Background Ozone Levels

The NAAQS for ozone is 75 parts per billion (ppb), and compliance with the NAAQS is determined by 

comparison to a “design value” that is calculated as the 3-year average of the fourth highest daily 8-hour 

ozone concentration at each monitor site. Based on the most recent ambient monitoring data in ND, the 

2011 ozone design values at the Dunn, TRNP and LNWR sites are 55, 59, and 60 ppb, respectively.5

Thus, current ozone air quality for western ND is substantially lower than the ozone NAAQS. 

Background NO2 Levels

Compliance with the NAAQS for this pollutant is determined by calculating a “design value” which is 

the eighth highest daily 1-hour average concentration averaged over a 3-year period at each monitor site. 

Source specific model impacts must be combined with background NO2 concentrations to evaluate 

potential exceedances of the NO2 NAAQS. Current background concentrations of NO2 in the region are 

quite low. Air Quality System (AQS) data from 2009 through 2011 shows 1-hour NO2 design values for 

 

5

 The Air Quality System (AQS) – the EPA’s Repository of Air Quality Data, available online at: 

http://www.epa.gov/ttn/airs/airsaqs/. The AQS monitoring data for the ND sites can be found in the docket for the interim 

final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov. 
3 

the Dunn, LWNR and TNRP monitors at 11 ppb, 17 ppb, and 9 ppb, respectively, compared to the 

standard of 100 ppb. The Dunn monitor site is located about midway between the oil development along 

the North Dakota – Montana border and the seven coal conversion facilities to the east. EPA selected the 

Dunn monitor data as the most appropriate background site for the FBIR because it is a rural 

background monitor that is influenced by regional energy development activity. 

Expected Future Development in the FBIR 

In an effort to determine future development likely to occur on the FBIR, the EPA evaluated the October 

2, 2009 Bureau of Land Management (BLM) report titled “Reasonable Foreseeable Development 

Scenario for Oil and Gas Activities on Bureau managed Lands in the North Dakota Study Area”. This 

report was supplemented on February 25, 2011 with the document titled “Revised Activity and Surface 

Disturbance Projections for the Reasonable Foreseeable Development Scenario for Oil and Gas 

Activities on Bureau Managed Lands in the North Dakota Study Area”. Both documents are included in 

the docket for this rule and are publicly available at the following website: 

http://www.blm.gov/mt/st/en/fo/north_dakota_field/rmp/RFD.html. 

The FBIR is located entirely within the North Dakota study area that was evaluated by BLM in the 

report. Therefore, it was possible to obtain the oil and gas development potential for projected pad 

densities and associated wells for the FBIR from information in the report. Based on the projected pad 

densities in the 2011 supplemental report, EPA determined the potential for oil and gas development on

the FBIR was approximately 990 well pads during the report planning period of 2010 to 2029. Although 

the BLM report estimated an average well density of 1.5 wells per pad, EPA used a value of two wells 

per pad to provide a more conservative estimate of well pad density in future development. EPA 

believes the average of two wells per pad to be representative of typical operations on FBIR, based on

information from synthetic minor permit applications and environmental assessments conducted by the 

Bureau of Indian Affairs.6

 Thus, EPA concluded that reasonably foreseeable development within the 

exterior boundaries of the FBIR would consist of approximately 2,000 wells or 1,000 well pads between 

2010 and 2029. 

Qualitative Assessment of Ozone Impacts 

Ambient ozone is a secondary pollutant that is formed from precursor emissions of VOC and NOX. High 

ambient ozone levels are typically observed in urban areas during stagnant meteorological conditions in 

the summer, and more recently, in areas of intensive oil and gas production in mountain basins that 

experience strong inversion conditions and snow cover during winter. The primary air pollutants that are 

affected by this action are VOC and NOX, both of which are precursors to ozone formation. For a typical 

well pad consisting of two wells, EPA estimated that the use of enclosed combustors and/or utility flares 

would result in VOC reductions of about 4,180 tpy. Additionally, EPA projected an increase in NOX

emissions of 8.8 tpy due to the combustion of natural gas emissions at the well pad. EPA estimated the

reasonably foreseeable development of 1,000 well pads with an enclosed combustor and/or utility flare 

controls would result in a cumulative reduction of 4,180,000 tons of VOC and an increase of 8,800 tpy 

of NOX distributed over the FBIR region of 1,544 square miles. Because the proposed action would 

result in large VOC reductions and comparatively small NOX increases, the action could result in either 

net increases or net decreases in ambient ozone levels depending on the sensitivity of ozone formation to 

 

6

 A document containing our analysis of well pad densities from multiple environmental assessments can be found in the 

docket for the interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: 

http://www.regulations.gov. 
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changes in VOC and NOX. Generally, a photochemical modeling analysis is needed to determine the 

extent to which VOC and NOX contribute to ozone formation, and photochemical model simulations 

would be required to predict whether the impacts of this action would increase or reduce ambient ozone

levels. However, significant resources and time are required to develop a photochemical modeling 

analysis. Because ND and the FBIR generally have meteorological conditions with high winds and good 

ventilation during the summer ozone season, the emissions changes from the proposed action are not 

expected to significantly increase or reduce ambient ozone levels. Elevated winter ozone concentrations 

have not been observed in ND. However, preliminary analysis of winter ozone in Wyoming indicates 

that winter ozone formation is more sensitive to VOC emissions than to NOX emissions. Thus, if high 

winter ozone conditions are observed in ND in the future, the large VOC reductions from the proposed 

action would be expected to result in net ozone reductions in the FBIR. Given that current ambient 

ozone levels in ND are substantially lower than the ozone NAAQS, small changes in ambient ozone 

levels that result from the proposed action are not expected to cause exceedances of the ozone NAAQS. 

Therefore, EPA does not propose to perform photochemical modeling for this analysis. Instead, EPA 

proposes to continue to monitor ambient ozone levels in the region, and if increasing levels of ozone are 

observed in the future, additional analysis of VOC and NOX impacts on ozone will be performed at that 

time. 

AERMOD Scenario Descriptions for NO2 Impacts 

EPA used the AERMOD nearfield dispersion model to evaluate several scenarios of NOX emissions 

from enclosed combustors and utility flares to evaluate nearfield impacts on the 1-hour average ambient 

NO2 NAAQS. The scenarios are outlined in Table 1 and discussed in more detail below. 

EPA staff analyzed permit application data for existing and proposed well pads on the FBIR and 

determined that VOC control at the well pad is implemented using either VOC enclosed combustors, a 

utility flare, or a combination of VOC enclosed combustors and utility flares. Enclosed combustors are

utilized to control low pressure VOC emissions from storage tanks. The utility flares are used to control 

the high pressure produced natural gas emissions from the heater treaters. At larger facilities with 

multiple wells and storage tanks, storage tank natural gas emissions may be routed to a dual-tip utility 

flare that is capable of handling higher volumes of storage tank natural gas emissions as opposed to 

having multiple enclosed combustors. Using data obtained from the permit applications and 

conversations with various operators7

, a typical VOC enclosed combustor was determined to have a 

stack height 25 feet, stack diameter of 48 inches, exit velocity of 1 meter per second (m/s) and an exit 

temperature between 400 to 800 F. A typical utility flare was determined to have a height of 25 ft and a 

heat release rate of 796,261 calories per second (cal/s). The heat release rate of 796,261 calories per 

second was determined by averaging the btu/scf values for heater-treater produced natural gas from six

permit applications, each from a different operator8. A corresponding calories per second value was also 

obtained for storage tank waste gas. However, since the heater-treater produced natural gas comprises

 

7

 Enclosed combustor specifications were included in the synthetic permit applications for some facilities. EPA obtained 

utility flare and enclosed combustor specifications during discussions with various operators. Copies of the specifications 

can be found in the docket for the interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: 

http://www.regulations.gov. EPA also discussed facility operations with various operators to further develop modeling 

scenarios. 

8

 A document containing the calories per second calculations can be found in the docket for the interim final rule, Docket ID: 

EPA-R08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov. 
5 

90% of the volume of total waste gas at the facility, the difference resulting from the lower 

calories/second value for storage tank natural gas emissions was assumed to be negligible9. 

Based on a subset of 64 permit applications, EPA also determined that on average, each well would have

maximum allowable emissions of 4.4 tpy of NOX. The subset of 64 applications was chosen from three 

operators that did not have full access to pipeline infrastructure, thus creating a worst-case scenario in 

which all of the waste natural gas would have to be flared. Since EPA’s reasonable foreseeable 

development (RFD) scenario proposed an average density of two wells per well pad, the average well 

pad will have a total of 8.8 tpy of allowable NOX emissions. Among well pads in the 64 permit 

applications surveyed, the largest number of wells per pad was 10 wells. EPA determined that well pads

larger than 10 wells would likely need to obtain a permit, and would therefore be subject to an 

independent air quality impact analysis (AQIA). Thus, using emissions data for a 10-well facility was 

appropriate for a maximum emissions scenario. 

Using data from the 64 permit applications and information provided by various operators, EPA 

developed three scenarios for modeling analyses. Table 1 below outlines the details of each scenario. 

Scenario 1 was assumed to be the worst-case condition and modeled as a utility flare with no enclosed 

combustor. Given that the largest number of wells per pad was 10, the NOX emission rate was assumed 

to be 44 tpy or the sum of 4.4 tpy from each of the 10 wells. Based on data provided to EPA staff, the

stack height of the utility flare was assumed to be 25 feet. The stack diameter for the utility flare was not 

specified for this scenario because the models use a parameterization to calculate the flare diameter.

Scenario 2 was assumed to be the average condition and modeled as the combination of a smaller utility

flare and an enclosed combustor. The stack height for the utility flare was assumed to be 9 feet. The 

stack height of the enclosed combustor was also assumed to be 25 feet with a stack diameter of 48 

inches. Based on the information provided to EPA staff, the average NOX emission rate for the average 

utility flare was determined to be 15 tpy, while 3 tpy, or the average per well of four wells was assumed 

for the enclosed combustor. Scenario 3 was assumed to have the lowest impacts and had NOX emissions 

less than the de minimis level of 10 tpy. As a result, no additional analysis was performed for Scenario 3. 

AERMOD Input Parameters for NO2 Impacts 

The AERMOD dispersion model (version 12060) was used to predict the maximum 1-hour average 

ambient NO2 concentration from utility flares and enclosed combustors at well pads on the FBIR in 

North Dakota. The enclosed combustors and utility flares were represented as point sources in 

AERMOD. Table 2 outlines the input parameters for the AERMOD model for each of the modeled 

scenarios of NOX emissions from enclosed combustors and utility flares. Scenario 1 included a single 

AERMOD simulation performed for a 25 foot flare with 44 tpy NOX emissions. North Dakota’s flare 

modeling procedure10 was used to determine the remaining model input parameters for Scenario 1. For a 

typical flare, North Dakota’s flare modeling procedure assumes a stack exit velocity of 40 m/s, stack gas 

temperature of 1000 K, and 55 percent heat loss is due to entrainment of ambient air. The total flare heat 

release was also assumed to be 796,261 calories per second. These assumptions were then used to 

calculate the effective height and diameter of a utility flare. Scenario 2 included two AERMOD 

simulations, one for a 9 foot flare with 15 tpy NOX emissions and one for enclosed combustor with 3 tpy 

 

9

 A summary of flare gas volumes from application data titled “Flare Gas Volumes from Syn Minor Application Data” can be 

found in the docket for the interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: 

http://www.regulations.gov 

10 North Dakota - Department of Health Memorandum to Air Quality Staff Regarding Model Input Parameters for Flares 

issued on 11/10/2010. 
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year NOX. Similar to Scenario 1, North Dakota’s flare modeling procedure was used to determine the 

model input parameters for the utility flare. The input parameters for the AERMOD model for 

simulating a typical enclosed combustor were based on data obtained from various operators. 

Table 1. Modeled scenarios of NOX emissions from enclosed combustors and utility flares. 

Scenario Emission Rate

[tpy] 

Stack Height

[ft] 

Stack Diameter 

[inches] 

1 – Worst-Case Utility Flare 44 25 NSa

2 – Average Case Utility Flare 15 9 NSa

Enclosed Combustor 3 25 48 

3 – Lowest Impacts Utility Flare 7.4 25 NSa

Enclosed Combustor 1.4 12 48 

a

 Parameterizations were used to calculate this variable for AERMOD.

Table 2. Model input parameters assumed for each modeled scenario. 

Scenario 

Emission 

Rate 

[tpy] 

Stack

Height 

[ft] 

Stack/Flare 

Diameter 

[inches] 

Exit 

Velocity 

[m/s] 

Stack Gas 

Temperature 

[K] 

1 – Worst-Case Utility Flare 44 25 17.3 40 1000

2 – Average Case 

Utility Flare 15 9 17.3 40 1000

Enclosed 

Combustor 3 25 48 1 700 

To more realistically model NO2, the Plume Volume Molar Ratio Method (PVMRM) was also used for 

the AERMOD analyses. This method accounts for the ambient conversion of nitrogen oxide (NO) to 

NO2 in the presence of ozone. To apply the PVMRM option in AERMOD, an in-stack ratio of NO2/NOX

emissions of 0.5 was assumed for the utility flare simulation and enclosed combustor simulation. 

Typical values for the NO2/NOX ratio in combustion emissions are about 0.1, but because source test 

data for NO2/NOX ratios are not available for flares and enclosed combustors, we conservatively 

assumed an NO2/NOX ratio of 0.5. 

The AERMOD analyses also used 5 years of National Weather Service (NWS) meteorological data 

collected at the Bismarck, North Dakota station from 2004 to 2008. This meteorology station provided 

both surface and upper-air meteorological data11. Background ozone concentrations were based on 

hourly ozone data collected at the Hannover, North Dakota station between 2004 and 200812, and 

missing ozone background concentrations were assumed to be 55 ppb. The modeled scenarios also 

assumed flat terrain. 

AERMOD Results for NO2 Impacts 

Table 3 lists the AERMOD modeled direct impacts for each scenario. Table 3 also lists the background 

NO2 design value for the Dunn monitor site and the sum of the modeled impacts plus the background 

concentration. The 5 year average of the 8th highest daily modeled NO2 concentration was combined 

with the background 1-hr NO2 design value to evaluate the source impacts. The last row of Table 3 

shows the combined maximum impacts of the flare and the enclosed combustor for Scenario 2. For 

Scenario 1, the combined modeled and background NO2 design value was 28% of the NAAQS. For 

Scenario 2, the combined modeled and background NO2 design value was 38% of the NAAQS. 

 

11 Documents containing the surface and upper-air meteorological data obtained from ND can be found in the docket for the 

interim final rule, Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov. 

12 The document containing background ozone data obtained from ND can be found in the docket for the interim final rule, 

Docket ID: EPA-R08-OAR-2012-0479, which can be accessed at: http://www.regulations.gov 
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Although Scenario 1 had larger NOX emissions than did Scenario 2, with 44 tpy in Scenario 1 compared 

to 18 tpy in Scenario 2, Scenario 2 had a greater flare height and increased buoyancy of the plume 

because of the greater VOC mass combusted. As a result, the AERMOD model predicted lower surface 

level NO2 concentrations compared to Scenario 2. As described above, Scenario 3 had less than the 10 

tpy de minimis threshold, so modeling was not performed for Scenario 3. As shown in the last column of 

Table 3, the AERMOD NO2 concentrations were significantly lower than the 1-hour NO2 NAAQS. 

Table 3. AERMOD predicted impacts on NO2 from utility flare and enclosed combustor. 

Scenario 

Five year 

average of 8th

highest model 

impact 

(ppb)

Background 

1-hour 

average NO2

design value 

(ppb)

Combined 

Modeled and 

Background 

NO2

(ppb)

NO2

NAAQS 1-

hr 

Average 

(ppb)

Percent of 

NO2

NAAQS 

1 – Worst - 

Case Utility Flare 17 11 28 100 28% 

2 – Average 

Case 

Utility Flare 14 

Enclosed 

Combustor 13 

Sum of Flare 

and Combustor 27 11 38 100 38%