Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2018: Updates to Natural Gas Gathering and Boosting Station Emissions

April 2020

Inventory of U.S. Greenhouse Gas Emissions and Sinks 1990-2018:

Updates to Natural Gas Gathering & Boosting Station Emissions

This memorandum documents the updates implemented in EPA's 2020 Inventory of U.S. Greenhouse Gas
Emissions and Sinks (GHGI) for gathering and boosting (G&B) stations. Additional considerations for G&B were
previously discussed in memoranda released November 2019 (Inventory of U.S. Greenhouse Gas Emissions and
Sinks 1990-2018: Updates Under Consideration for Natural Gas Gathering & Boosting Station Emissions),1 October
2018 (Inventory of U.S. GHG Emissions and Sinks 1990-2017: Updates Under Consideration for Natural Gas
Gathering & Boosting Emissions),2 and April 2019 (Inventory of U.S. GHG Emissions and Sinks 1990-2017: Updates
to Natural Gas Gathering & Boosting Pipeline Emissions).3

During the stakeholder process for developing the 2020 GHGI, stakeholders supported making updates to
estimate G&B station emissions using data from a Colorado State University-led field campaign conducted during
year 2017 to characterize emissions from G&B stations across the U.S. (Zimmerle et al. 2019)4 and Greenhouse
Gas Reporting Program (GHGRP) subpart W data.

1 2019 (Previous) GHGI Methodology

The GHGI emissions calculation methodology for G&B stations was last updated in the 2016 GHGI, wherein EPA
incorporated findings from the Marchese et al. 2015 study5 to estimate station-level emissions and national
activity data. EPA's April 2016 memo Inventory of U.S. GHG Emissions and Sinks 1990-2014: Revision to Gathering
and Boosting Station Emissions (2016 G&B memo)6 and April 2017 memo Inventory of U.S. Greenhouse Gas
Emissions and Sinks 1990-2015: Revisions to Natural Gas and Petroleum Systems Production Emissions (2017
Production memo)7 document the historical considerations and the full methodology used for G&B stations in the
current GHGI.

In the 2019 (previous) GHGI, estimates for G&B station emissions were based on estimated station counts in each
year paired with station-level emission factors (EFs) for normal events (documented in the 2016 G&B memo) and
EFs for episodic events, i.e., blowdown sources (documented in the 2017 Production memo). The total G&B
station count in each year of the time series was estimated as the marketed onshore gas production in the given
year (published by the Department of Energy's Energy Information Administration (DOE/EIA)) divided by the year
2012 throughput per station from the Marchese et al. 2015 study. The previous GHGI paired this station count
activity data with a station-level CH4 EF for normal vented and leak emissions calculated using data from the
Marchese et al. 2015 study. The previous GHGI separately estimated episodic event emissions using a station-level
CH4 EF from Marchese et al. 2015. The previous GHGI estimated C02 emissions from G&B station normal and
episodic events using C02 EFs developed by applying a default production segment ratio of C02-to-CH4 gas
content, and as such does not fully account for C02 from sources such as flaring or acid gas removal units
(AGRUs).

1 https://www.epa.gov/sites/production/files/2019-ll/documents/ghgi_nov2019workshop_memo.pdf.

2 https://www.epa.gov/sites/production/files/2018-10/documents/ghgi_2018stakeholders_boosting.pdf

3 https://www.epa.gov/sites/production/files/2019-04/documents/2019_ghgi_update_-_gb_segment_2019-04-09.pdf

4 Zimmerle, Daniel et al., Characterization of Methane Emissions from Gathering Compressor Stations. Available at
https://mountainscholar.org/handle/10217/195489. October 2019.

5 Marchese, A. J. et al., Methane Emissions from United States Natural Gas Gathering and Processing. Environmental Science & Technology,
49, 10718-10727. 2015.

6 https://www.epa.gov/sites/production/files/2016-08/documents/final_revision_gb_station_emissions_2016-04-14.pdf

7 https://www.epa.gov/sites/production/files/2017-04/documents/2017_ng-petro_production.pdf

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2 Available Data

This section summarizes data sources that EPA reviewed to develop the approach implemented into the 2020
GHGI and considerations toward updating the GHGI methodologies for G&B stations.

2.1 GHGRP Data

Subpart W of the EPA's GHGRP collects annual activity and emissions data on numerous sources from natural gas
and petroleum systems that meet a reporting threshold of 25,000 metric tons of C02 equivalent (mt C02e)
emissions. Facilities that meet the subpart W reporting threshold have been reporting since reporting year (RY)
2011; however, certain sources, including subpart W emissions from G&B facilities, were first required to be
reported in RY2016. Subpart W activity and emissions data are currently used in the GHGI to calculate CH4, C02,
and N20 emissions for many sources throughout natural gas and petroleum systems.

Subpart W specifies facility definitions specific to certain segments. G&B facilities in subpart W are each defined
as a unique combination of operator and basin of operation. Subpart W does not delineate data for G&B stations
versus pipelines. However, the data are reported on an emission source level, so each source can be assigned as
likely occurring at either G&B stations or pipelines. For the analyses in this memo specific to G&B station
estimates, EPA excluded emissions from blowdown vent stacks under the "pipeline venting" emission source and
from pipelines under the "equipment leaks" emission source, and considered all other data (including some types
of blowdown venting and numerous other sources) as occurring at G&B stations. Appendix A documents the
subpart W calculation methodologies for each G&B station emission source.

The GHGRP data used in the analyses discussed in this memo are those reported to the EPA as of August 4, 2019.
In previous years, stakeholders have suggested additional or alternate uses of GHGRP data, such as for certain
sources using measurement data only. Stakeholders have also suggested modifications to the reported GHGRP
data for use in the GHGI, such as through removal of stakeholder-identified outliers. In the current GHGI, EPA uses
the publicly available GHGRP data set without modification for the GHGI, to ensure transparency and
reproducibility of GHGI estimates. Prior to public release of the GHGRP data, the EPA has a multi-step data
verification process for the data, including automatic checks during data-entry, statistical analyses on completed
reports, and staff review of the reported data. Based on the results of the verification process, the EPA follows up
with facilities to resolve identified potential issues before public release.

Analyses of available GHGRP data are further detailed in Section 3.1.

2.2 Zimmerle et al. 2019 G&B Study

During June through November 2017, Zimmerle et al. carried out methane (CH4) measurements at G&B stations
aiming to better characterize emissions at the component, equipment, and national levels. In the Zimmerle et al.
study, the team obtained measurements at 180 facilities in 11 U.S. states. The study noted that these facilities
were operated by nine companies that represented 35% of G&B compressors reported to GHGRP at the time. The
study team aimed to select stations representative of the U.S. gathering sector in terms of size, geographic
distribution, gas composition, and equipment mix. For vented and leak emissions identification and measurement,
the team used optical gas imaging (OGI), Bacharach® HI FLOW® Sampler (BHFS), and bagging if flow exceeded
BHFS capacity (occurred for less than 1% of samples). The team conducted emission measurements on 1,938
major equipment units (compressors, dehydrators, separators, tanks, AGRUs, and yard piping); additionally, the
team counted components on 1,002 major equipment units. At the same time, under the same U.S. Department
of Energy funding program, a separate team led by GSI Environmental, Inc. conducted a smaller field study that

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entailed performing component-level measurements at four compressor stations in southeast Texas; the GSI
study team provided results that were incorporated into the larger Zimmerle et al. study.8

This study produced several products that were used to inform GHGI updates: component-level leaker and
population EFs (analogous to those prescribed in the current GHGRP); population EFs for major equipment; and a
calculated estimate for year 2017 national emissions, using both study results and GHGRP data. This study also
developed and field-tested two measurement methods to better characterize emissions from unburned CH4
entrained in compressor engine exhaust ("combustion slip") and vented and leak CH4 emissions from gas-
powered, pneumatically actuated valves and controllers. The Zimmerle et al. study results are further detailed in
Section 3.2.

3 Analysis of Available Data

This section summarizes EPA's analyses of GHGRP data, the Zimmerle et al. 2019 study, and other recent studies
to update the G&B station methodology in the 2020 GHGI.

3.1 Analysis of Available GHGRP Data

EPA's October 2018 and November 2019 G&B memos presented a detailed analysis of GHGRP data and
considerations for using GHGRP data to update the GHGI G&B station emissions calculation methodology. This
section summarizes the latest available GHGRP data.

Table 1 shows year 2017 reported subpart W G&B station source-level emissions (sorted descending by RY2017
CH4 emissions), activity, EFs calculated from the subpart W data, and compares the total reported subpart W
emissions and 2019 (previous) GHGI emissions. Although GHGRP RY2018 data became publicly available in
October 2019 (as reported to EPA as of August 2019), it is not discussed here. The Zimmerle et al. study and
national emissions model are based on year 2017 data and we present RY2017 GHGRP data (available as of
October 2019) for consistency. The GHGRP data used in the 2020 GHGI were from the October 2019 dataset,
including emissions through year 2018.

Table 1. G&B Station Source-Level Emissions and Activity Data and Calculated EFs from Subpart W and
G&B Station National Total Emissions from 2019 (Previous) GHGI, Year 2017

Emission Source

Reported Emissions and Activity

Calculated EFs (mt/yr/unit
activity)

CH4 Emissions
(mt)

C02 Emissions
(mt)

Count

CH4EF

C02 EF

EF Activity
Basis

Pneumatic Controllers

197,791

14,596

143,336

1.4

0.10

per controller

Low-bleed Controllers

5,901

371

35,214

0.17

0.011

per controller

Intermittent-bleed
Controllers

161,514

12,253

101,905

1.6

0.12

per controller

High-bleed Controllers

30,376

1,972

6,217

4.9

0.32

per controller

Equipment Leaksa

104,830

12,306

Meters/piping -161,100
Separators - 32,907
Compressors - 15,751
Dehydrators - 3,173
Heaters -1,688
Headers -110
Wellheads -17

n/a

Tanks

92,193

586,074

33,682

2.7

17.4

per tank

8 GSI Environmental, Inc. et al., Integrated Component-specific Measurements to Develop Emission Factors for Compressors and Gas
Gathering Lines. Available at https://www.netl.doe.gov/sites/default/files/2019-01/FE0029084_Flnal.pdf. 2019.

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Blowdown Vent Stacks b

79,147

5,297

490,160

0.12

0.010

per event

Dehydrators

48,722

711,251

3,070

15.9

231.7

per dehydrator

Large Dehydrators

48,063

710,074

2,959

16.2

240

per dehydrator

Small Dehydrators

659

1,178

111

1.3

2.3

per dehydrator

Centrifugal Compressors

39,629

4,795

172

230.4

27.9

per compressor

Combustion Slip c

28,593

n/a

2,095

13.6

n/a

per unit

Pneumatic Pumps

21,759

1,566

12,875

1.7

0.12

per pump

Flare Stacks

8,738

2,139,694

4,393

2.0

487

per stack

Reciprocating Compressors

2,849

434

15,670

0.18

0.028

per compressor

AGRUs

n/a

491,009

139

n/a

3,532

per AGRU

Desiccant Dehydrators

0.3

0.45

0.004

per dehydrator

Subpart W Reported Totald

624,287

3,967,022

n/a

National Total (2019 GHGI)e

2,218,773

239,459

n/a

n/a - Not applicable.

a - Includes all emissions reported by G&B facilities under the equipment leaks reporting section, except for emissions attributed to
gathering pipelines. The reported equipment counts are only for those facilities that rely on population EFs and do not include counts
from facilities that use leaker EFs.

b - Includes blowdown emissions reported by G&B facilities for: compressors, emergency shutdowns, facility piping,
scrubbers/strainers, pig launchers and receivers, all other equipment with a physical volume greater than or equal to 50 cubic feet, and
emissions reported with flow meters. For emissions reported with flow meters, facilities do not report the corresponding count of
blowdown events, thus the calculated EFs do not include the emissions reported with flow meters (4,641 mt CH4 and 2,665 mt C02).
c - Combustion C02 emissions are also reported, but such emissions are not within the scope of natural gas systems estimates in the
GHGI and are therefore not shown in this table. Emissions and activity shown are from "large" combustion units; "small" combustion
units are not required to report emissions, only counts. "Large" combustion units include internal fuel combustion units of any heat
capacity that are compressor-drivers, internal fuel combustion units that are not compressor-drivers with a rated heat capacity greater
than 1 mmBtu/hr, and external fuel combustion units with a rated heat capacity greater than 5 mmBtu/hr.

d - The G&B facility definition in subpart W does not delineate reporting by "station" versus "pipeline." Therefore, these emissions
equal the sum of reported subpart W emissions assigned to G&B stations (see footnotes a and b), as documented in Section 2.1.
e - Includes normal vented and leak emissions (2,018,566 mt CH4 and 231,123 mt C02 in 2017) and episodic event emissions (200,207
mt CH4 and 8,336 mt C02 in 2017).

3.2 Analysis of Zimmerle et al. 2019 G&B Study Data and Approach to Calculate
National Emissions

Zimmerle et al. presented an approach in their study to calculate national emissions for potential use in the GHGI.
The Zimmerle et al. approach relies on EFs developed from the study's field measurements, EFs developed from
subpart W data, and activity data derived from study partner and subpart W data. The November 2019 G&B
station memo documents EPA's complete analysis and considerations for incorporating the Zimmerle et al. study
into the GHGI methodology, while the sections below highlight the information used in the 2020 GHGI updates.

3.2.1 Source-level Emission Factors

Zimmerle et al. calculated major equipment EFs for six major equipment sources measured during the field
campaign: compressors, tanks, yard piping, dehydrators, separators, and AGRUs. Each of these sources generally
align with major equipment units reported to GHGRP for the G&B segment (exceptions noted below), and the
Zimmerle et al. study EFs include leak emissions and vented emissions.9 In developing EFs based on study
measurements, Zimmerle et al. accounted for contribution from emissions that were detected but were too large
to be measured in the field (referred to as "large emitters" or "super emitters" within the study) by referencing
emissions data from previous studies. Table 2 below presents the CH4 EFs (metric ton (mt) per year, per unit
activity) calculated from the study measurements data, for use in estimating national G&B station emissions.

As noted above, there are two emission sources that do not exactly align between the GHGRP and Zimmerle et al.
definitions: compressors and yard piping. For compressors, the Zimmerle et al. study encountered mostly gas-

9 The Zimmerle et al. study uses the acronym "F&V" (fugitive and vented) to describe these emissions data. However, for consistency with
GHGI terminology, the acronym "L&V" (leak and vented) is used throughout this memo.

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fueled engine-driven reciprocating compressors and developed a single EF from all measurements (approximately
four percent of which were made on centrifugal or screw compressors) that is applied to a GHGRP-based count of
total gas-driven compressors—expecting the mix of compressors in industry to not substantially differ from what
was encountered during the study. Therefore, the "Compressor L&V" emission source in Table 2 below aligns with
the combined GHGRP categories of reciprocating and centrifugal compressors shown in Table 1. For yard piping,
the Zimmerle et al. study developed a per-station EF; the "Yard Piping L&V" emission source in Table 2 below
aligns with a subset of the GHGRP category for equipment leaks (i.e., meters and piping emissions) shown in Table
1.

For blowdowns, dehydrator vents, and flares, Zimmerle et al. suggest application of EFs calculated from subpart
W data (see Table 1). For combustion slip, Zimmerle et al. collected study measurement data from which EFs were
calculated (refer to separate discussion in Section 3.2.3). For pneumatic controllers, Zimmerle et al. collected
study measurement data (see Section 3.2.4) but suggest using the subpart W rule-prescribed EFs. Zimmerle et al.
did not specifically address pneumatic pump and desiccant dehydrator emissions in their measurement campaign
or national emissions estimation approach, but EPA did include such emissions in the GHGI, see Section 3.2.5.

Table 2. Source-level EFs from the Zimmerle et al. Study

Emission Source

Source Description

ch4ef

(mt/yr/unit activity)

Compressor L&V

Leaks and vented emissions from the compressor, compressor
driver, and any auxiliary equipment attached to the skid

15.93 per compressor

Tank L&V

Leaks and normally operating vents

5.67 per tank

Yard Piping L&V

Fuel gas systems, station inlet and outlet headers, meter runs,
and pig launchers and receivers

12.55 per station

Dehydrator L&V

Leaks and normally operating vents (not including dehydrator
reboiler vents)

0.50 per dehydrator

Separator L&Va

Leaks and normally operating vents

0.09 per separator

AGRUL&V

Leaks and normally operating vents (not including heater
combustion stacks)

0.61 per AGRU

a - Accounts for emissions from separators that are not on a compressor skid, referred to as 'station' separators in the
Zimmerle et al. study. Emissions from separators that are on a compressor skid are included within the compressor L&V

EF.

3.2.2 National Activity Data

In the Zimmerle et al. study's recommended approach for estimating national emissions, the CH4 EFs are paired
with GHGRP-based activity data to estimate national G&B station emissions. To develop a national estimate using
GHGRP reported activity, the study developed two key methodological steps:

1. Estimating counts of stations and separators. Subpart W G&B segment facilities are not required to
report counts of stations and Zimmerle et al. examined two populations of separators (separators not on
a compressor skid and separators on a compressor skid). Therefore, to calculate activity data for these
sources, the study used partner data to develop activity factors (AFs). Taking into account various basin-
level considerations, the study estimated a national average AF of 2.8 compressors per station, leading to
an estimated 5,683 stations reporting under subpart W in RY2017. Also considering basin-level analyses,
the study data leads to an estimate of 2.04 separators per station.10 Note, this separator AF estimates the
number of 'station' separators and does not consider separators on a compressor skid (see footnote a to
Table 2).

2. Scaling reported counts to national total activity. Subpart W reporting reflects data only from facilities
that meet the reporting threshold (see Section 2.1). Therefore, Zimmerle et al. developed a factor to scale
up reported activity to estimate national total activity. Zimmerle et al. used basin-level production data

10 EPA calculated 2.04 separators per station by using two AFs from the Zimmerle et al. study: 0.73 separators per compressor times 2.8
compressors per station equals 2.04 separators per station.

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from subpart W (reported quantity of gas produced for sales) and Drillinglnfo to estimate that 7.5% of
stations are not reported to the GHGRP, for a scaling factor of 1.075. Note, this scaling factor approach is
similar to that previously developed by EPA; see Section 3.1 which documents an approach EPA
considered to scale RY2017 subpart W reported G&B station emissions by a factor of 1.07 to estimate
national emissions.

Table 3 below presents the estimated activity based on the study's approach for use in pairing with the applicable
EFs in Table 1 and Table 2 to estimate national G&B station emissions. The activity applicable to combustion slip
are discussed in Section 3.2.3.

Table 3. Source-level National Activity Based on Zimmerle et al. Study Approach (Year 2017)

Emission Source

Activity Basis

Reported
Activitya

National Activity
Estimateb

Compressor L&V

GHGRP reported # reciprocating and centrifugal compressors

15,842

17,030

Tank L&V

GHGRP reported # tanks

33,682

36,208

Pneumatic
Controllers

GHGRP reported # controllers subcategorized by bleed type:
low bleed, LB; intermittent bleed, IB; high bleed, HB

LB: 35,214
IB: 101,905
HB: 6,217

LB: 37,855
IB: 109,548
HB: 6,683

Blowdowns

GHGRP reported # unit blowdown events

490,160

515,211

Dehydrator Vents

GHGRP reported # dehydrator units

Large: 2,959
Small: 111

Large: 3,181
Small: 541

Yard Piping L&V

# Stations calculated from GHGRP reported # compressors and
study assumption of # compressors per station

5,683

6,111

Pneumatic Pumpsc

GHGRP reported # pumps

12,875

13,841

Flares

GHGRP reported # flare stacks

4,393

4,722

Dehydrator L&V

GHGRP reported # dehydrator units

3,070

3,722

Separator L&V

# Separators calculated from GHGRP reported # compressors
and study assumption of # separators per compressor

11,650

12,528

AGRUL&V

GHGRP reported ttAGRUs

139

149

Desiccant
Dehydratorsc

GHGRP reported # desiccant dehydrators

a - GHGRP reported counts as also shown in Table 1. Station and separator counts are not directly reported but are estimated using
reported compressor counts and study-developed ratios,
b - Scaling factor of 1.075 is applied to estimate national total activity.

c - Pneumatic pumps and desiccant dehydrators were not included in the Zimmerle et al. study approach, but activity data are provided
for reference because EPA included their emissions in the 2020 GHGI, see Section 3.2.5.

3.2.3 Combustion Slip

In addition to analyzing leak and vented emissions from certain sources as discussed above, the Zimmerle et al.
2019 study characterizes emissions from unburned CH4 entrained in G&B compressor engine exhaust
("combustion slip").

The Zimmerle et al. approach for characterizing combustion slip from G&B compressors included conducting
standard stack testing and a newly developed in-stack tracer measurement method. Zimmerle et al. conducted
measurements (generally "as found") on 116 reciprocating compressor drivers at 51 G&B stations, including 70
four-stroke lean burn (4SLB) engines and 46 four-stroke rich burn (4SRB) engines. These types of engines were
considered representative of the vast majority of compressor drivers at G&B stations (versus electric motors or
turbines). Zimmerle et al. compared their measured emission rates to EPA AP-42 EFs for 4SLB and 4SRB engines
and found general agreement. The Zimmerle et al. study did not provide an EF that could be directly applied in the
GHGI. As such, EPA calculated a combustion slip CH4 EF, shown in Table 4, by dividing the study's national
combustion slip emissions by its national engine estimate.

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Table 4. Compressor Engine Combustion Slip EF Calculated from Zimmerle et al. Study Data

Combustion Slip EF
(mt Cm/engine)

20.4

The Zimmerle et al. study's approach to estimate a national engine count relied on the reported subpart W
compressor counts and two assumptions (based on partner data):11

1. 50% of centrifugal compressors are driven by engines and the remainder by turbines

2. Screw compressors are not included in the subpart W compressor counts and engine-driven screw
compressors are equal to 5.18% of reported reciprocating plus centrifugal compressors

Table 5 presents the resulting national estimate of engines that would be applied with the combustion slip EF.

Table 5. Combustion Slip National Activity (Compressor Engines) Based on Zimmerle et al. Study

Approach(Year 2017)

Emission Source

Activity Basis

Reported/Estimated
Activitya

National Activity
Estimateb

Combustion Slip

Reciprocating compressor engines (GHGRP reported
# reciprocating compressors)

15,670

17,825

Centrifugal compressor engines (50% of GHGRP
reported # centrifugal compressors)

Screw compressor engines (5.18% of GHGRP reported
# reciprocating and centrifugal compressors)

821

Total Engines

16,577

a - Uses GHGRP reported counts as shown in Table 1.
b - Scaling factor of 1.075 is applied to estimate national total activity.

3.2.4 Pneumatic Controllers

Zimmerle et al. conducted long-term measurements (average of 76 hours each) of 72 pneumatic controllers at
G&B stations to better understand emission rates. A separate journal article (Luck et al. 2019) provides additional
details and discussion of the Zimmerle et al. study results.12 Of the measured population, 42% exhibited abnormal
emissions. Average emissions for abnormally operating controllers, normally operating controllers, and overall for
each type of pneumatic controller are summarized in Table 6. The authors recommended that the study data be
used for qualitative understanding of pneumatic controller behavior rather than for developing emission factors.
For the 2020 GHGI updates, EPA applied the pneumatic controller subpart W-based EFs in Table 1, as detailed in
Section 3.2.5.

Table 6. Zimmerle et al. 2019 Study Pneumatic Controller Measurements

Controller Bleed
Type

Measu red

Abnormally Operating
Controllers

Normally Operating Controllers

Overall Average
CH4 Emission
Rate (mt/yr)a

Number
[% of total]

CH4 Emission
Rate (mt/yr)a

Number
[% of total]

CH4 Emission
Rate (mt/yr)a

Low Bleed

5 [21%]

4.5

19 [79%]

0.1

1.0

Intermittent

25 [63%]

2.1

15 [37%]

0.4

1.5

High Bleed

0 [0%]

n/a

8 [100%]

2.6

n/a - Not applicable.

11 These assumptions were detailed in Supporting Volume 3 of the original Zimmerle et al. 2019 study, but were not included in the October
2019 Revision of Supporting Volume 3. EPA will also consider information in an upcoming combustion slip article from the study team
(Vaughn et al.) to estimate a national engine count.

12 B. Luck et al., Multiday Measurements of Pneumatic Controller Emissions Reveal the Frequency of Abnormal Emissions Behavior at
Natural Gas Gathering Stations. Environmental Science and Technology Letters, Apr. 2019.

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a - EPA converted from whole gas emission rate in scfh as reported in the study to CH4 emission rate in mt/yr using the GHGI default
onshore production segment methane content of 78.8% and 8,760 operating hours.

3.2.5 Considerations for Using Zimmerle et al. 2019 Data in the 2020 GHGI

For pneumatic controllers, EPA's approach for the 2020 GHGI used as-reported subpart W data from G&B facilities
to calculate year-specific EFs (see Table 1), in order to reflect gas CH4 content, operating hours, and year-to-year
variation. This approach is consistent with the Zimmerle et al. study recommended approach for calculating EFs
from reported subpart W data for other emission sources not measured in their study (i.e., blowdowns,
dehydrator vents, and flares). EPA additionally included pneumatic pump and desiccant dehydrator emissions
estimates (not addressed in the Zimmerle et al. study) in the GHGI based on reported subpart W data.

For compressors, as described in Section 3.2.1, Zimmerle et al. calculated a single leak and vent EF and suggested
applying the EF to the combined total of gas-driven compressors in the G&B segment. Table 7 below is a
replication of Table S3-18 from the Zimmerle et al. study supporting information that documents the types of
compressors and drivers measured during the study and underlying the suggested EF. The majority of emissions
measurement data underlying the EF were collected from reciprocating compressors and this new EF is likely
more representative of G&B reciprocating compressor emissions than the subpart W EF. However, G&B
centrifugal compressors subject to subpart W apply a higher CH4 EF (230 mt/compressor/yr, see Table 1)
compared to the Zimmerle et al. compressor L&V EF of 17.7 mt/compressor/yr (see Table 2). Few centrifugal
compressors operate within the G&B segment, approximately 1 percent of compressors reporting to subpart W
(see Table 1). Zimmerle et al. created a single compressor EF as a simplification step, due to a lack of data and the
likely minimal impact that applying a centrifugal compressor-specific EF would have based on the current G&B
compressor population. EPA applied the compressor EF from the Zimmerle et al. study in the 2020 GHGI updates.

Table 7. Compressor Driver Type by Compressor Type for Zimmerle et al. Measured Compressors

Compressor Driver

Compressor Type

Total

Percent of
Total

Centrifugal

Reciprocating

Screw

Electric motor

2.9%

Reciprocating engine

402

433

95%

Turbine

1.8%

Total

403

454

100%

Percent of Total

3.1%

89%

8.1%

100%

EPA incorporated the Zimmerle et al. study data and general approach to estimating national emissions into the
2020 GHGI to update G&B station emission estimates. Section 4 discusses additional considerations regarding
variability across the time series and geographic regions.

3.3 Other Recent Research Studies with G&B Station Emissions Data

In addition to analyzing subpart W and Zimmerle et al. 2019 study data for comparison to GHGI estimates, EPA
reviewed findings from recent research studies which provide station-level EFs that could be directly compared to
the previous GHGI EF:

• Vaughn et al. (2017). Comparing facility-level methane emission rate estimates at natural gas gathering
and boosting stations.

• Yacovitch et al. (2017). Natural gas facility methane emissions: measurements by tracer flux ratio in two
US natural gas producing basins.

• Alvarez et al. (2018). Assessment of methane emissions from the U.S. oil and gas supply chain.

The Vaughn, et al. (2017) study calculated two station-level EFs, shown in Table 8. Both EFs are higher than the
previous GHGI EF, the degree to which depends on whether tank venting (that was observed at two stations) is
included in the Vaughn et al. station-level EF.

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The Yacovitch et al. (2017) study calculated EFs for two regions, the Fayetteville shale play and Denver-Julesburg
(DJ) Basin; Table 8 presents the study results. The station-level emission rate for the DJ Basin is lower than the
Fayetteville shale play (note that the statistical mode of the EFs were presented in the study, rather than average
EFs); this emphasizes the existence of regional variation in station emissions. Yacovitch et al. (2017) also
presented confidence intervals around their study data. The confidence intervals encompass the current GHGI EF.
The Yacovitch et al. (2017) study also summarized results from prior studies (shown as "Multi-Basin: Tracer Sites"
in Table 8), which are included for reference.

The Alvarez et al. (2018) study synthesized results from recent measurement studies to estimate national G&B
station emissions as 2,100 Gg CH4 in year 2015 (compared to the 2018 GHGI estimate of 1,968 Gg CH4). Their
approach analyzed data from the Mitchell et al. 2015 G&B study (underlying the Marchese et al. 2015 study) and
from a Zavala-Araiza et al. 2015 study to calculate an effective average EF that is approximately 10% higher than
the Marchese et al. EF used in the previous GHGI, as shown in Table 8 below.

Appendix A summarizes information on each study (e.g., measurement methods, representativeness).

Table 8. G&B Station CH4 Emission Rates from Recent Studies Compared to the Current GHGI

CH4 Emission Rate

Parameter

(kg/h)

Vaughn et al. 2017

Station EF, excluding tank venting

50.4

Station EF, including tank venting

74.5

Yacovitch et al. 2017

Multi-basin: tracer sites mode EF

[95% confidence interval]

[12 - 3,300]

Fayetteville study area mode EF

[95% confidence interval]

[15 - 730]

DJ study area mode EF

[95% confidence interval]

[4.5-75]

Alvarez et al. 2018a

Station EF, excluding episodic events

Station EF, including episodic events

Zimmerle et al. 2019

Station EF, including episodic events

24.2

2019 GHGI

Station EF, excluding episodic events

Station EF, including episodic events

a - Station-level factors not presented in Alvarez et al. 2018, estimated
here from discussion text in Alvarez et al. 2018.

4 Regional Variability and Time Series Considerations

Stakeholders have previously suggested that differences due to regional and temporal variability should be
considered when updating GHGI methodologies, particularly for sources where variation is expected. The EPA
similarly considered whether and how to represent regional variability in G&B emissions. EPA specifically
considered the following regarding EFs and activity data to estimate G&B station emissions in the GHGI.

Station and Separator Count Ratios. Subpart W does not contain station counts and Zimmerle et al. examines two
populations of separators (see separator discussion in Section 3.2.1 and 3.2.2). To estimate the applicable counts,
Zimmerle et al. developed ratios for compressors per station and separators per compressor at the basin-level
from partner data. These ratios reflect differences in station size and configuration between basins. Zimmerle et

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al. calculated a national average of 2.8 compressors per station, with a basin-level minimum and maximum of 1.8
and 5.1 compressors per station, and a national average of 0.73 separators per compressor, with a basin-level
minimum and maximum of 0.3 and 1.1 separators per compressor.13 No comparable ratio information exists
across the GHGI time series. EPA applied the Zimmerle et al. national average ratios in the 2020 GHGI updates to
estimate the number of stations and separators represented by subpart W data.

Scaling Factor. There are likely small-scale G&B facilities (based on the GHGRP definition of a single operator
within a single basin) that do not exceed the GHGRP emissions threshold and therefore are not reporting to
GHGRP. Zimmerle et al. conducted a basin-level analysis to develop a scaling factor to account for G&B stations
that are not reporting, and estimated that nationally, 7.5% of stations did not report to GHGRP in RY2017.
Zimmerle et al. used two key basin-level inputs in that estimate:

• Production ratio between GHGRP production and Drillinglnfo natural gas production. Considered to
estimate coverage of GHGRP data in those basins with GHGRP reporters.

• Basins with no GHGRP reports but some Drillinglnfo production. Zimmerle et al. counted 27 basins
without GHGRP reporters in RY2017, accounting for 0.63% of all G&B stations.

EPA applied the scaling factor of 1.075 in the 2020 GHGI updates, based on 2017 data, and did not evaluate the
scaling factor for other years. In future Inventories, EPA will consider applying either a simplified, national-level
approach or a basin-level approach to implement the Zimmerle et al. scaling factor. A basin-level approach has
potential utility to organizations conducting region-specific field studies. However, a national-level approach
would likely result in calculation national emissions very similar to those calculated using a basin-level approach.
EPA will also consider how and whether to account for basins with no GHGRP reporters, which has minimal
impact on the scaling factor.

EPA considered multiple options for implementing Zimmerle et al. data into the GHGI time series calculations to
update estimates previously based on the Marchese et al. data. To determine G&B station counts over the time
series, EPA considered: (1) applying a Zimmerle et al. derived per station volume across the time series (this would
increase station counts across the time series), (2) using data from both studies (e.g., using the Marchese et al.
data from 1990 through 2013, Zimmerle et al. data for years 2017 and forward, and interpolating between the
two for intermediate years), or (3) maintaining the production volume per station derived from Marchese et al.
There were similar considerations for applying the Marchese et al. based-EFs versus the Zimmerle et al. approach
to EFs over the time series. EPA did not retain the Marchese et al. data in the 2020 GHGI updates and applied
Zimmerle et al. data (in conjunction with subpart W data) over the time series.

EPA used subpart W data to account for G&B flaring C02 and N20 emissions. Flaring emissions data are only
available for recent years, and while this data may not be representative of emissions over the GHGI time series,
EPA applied the subpart W EFs to all years due to lack of other available data.

5 Updated Methodology and National Emissions Estimates for G&B Stations in
the 2020 GHGI

Based on the data sources and considerations discussed in Sections 3 and 4 and stakeholder feedback supporting
updates that incorporate available GHGRP data, this section summarizes the approach EPA implemented into the
2020 GHGI. This approach relies on applying a combination of GHGRP-based EFs (see Table 1), Zimmerle et al.
study measurement-based EFs (see Table 2), and the corresponding activity data in Table 3 and Table 5. EPA did
not retain data from the previous GHGI methodology. Further details regarding EFs and activity data are discussed
in the following bullets.

13 Reflects Zimmerle et al. partner equipment counts aggregated by basin and presented in Report Figures S3-10 and S3-11.

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Emission Factors

• EPA applied the Zimmerle et al. study CH4 EFs, see Table 2, for certain major equipment (compressors,
tanks, dehydrators, acid gas removal units, separators, and yard piping) to all years of the time series.

o For a subset of these sources (compressors, dehydrators, separators, and yard piping), EPA

applied a default C02-to-CH4 gas content ratio to calculate the corresponding C02 EF for leak and
vent emissions (similar to the current GHGI methodology, discussed in Section 1).

• EPA calculated year-specific G&B station source-level CH4, C02, and N20 EFs (as applicable), see Table 1,
for RY2016 through RY2018 from subpart W data for emission sources that were not measured in the
Zimmerle et al. study and for sources that were measured in the Zimmerle et al. study but which had
flaring emissions or significant process C02 emissions (i.e., AGRUS). EPA applied the subpart W RY2016 EFs
to all prior years of the time series.

• Table 10 summarizes the data source used for each emission source and pollutant.

Table 9. EF Data Source Used for Each Emission Source in the 2020 GHGI Update

Emission Source

CH4 EF Source

C02 EF Source

N20 EF Source

Combustion Slip

Zimmerle study
measurements

n/a

Compressor L&V

Zimmerle study
measurements

Default C02:CH4 ratio

n/a

Tank L&V + Flaring

Zimmerle study
measurements

GHGRP

Pneumatic Controllers

GHGRP

n/a

Blowdowns

GHGRP

n/a

Dehydrator Vents

GHGRP

Yard Piping L&V

Zimmerle study
measurements

Default C02:CH4 ratio

n/a

Pneumatic Pumps

GHGRP

n/a

Flares

GHGRP

Dehydrator L&V

Zimmerle study
measurements

Default C02:CH4 ratio

n/a

Separator L&V

Zimmerle study
measurements

Default C02:CH4 ratio

n/a

AGRUL&V

Zimmerle study
measurements

GHGRP

n/a

Desiccant Dehydrators

GHGRP

n/a

n/a - Not applicable.

Activity Data

• EPA calculated a ratio of the Zimmerle et al. study's estimated 2017 G&B station count and the 2017 total
US gas production from Drillinglnfo and applied the ratio to calculate station counts across the time
series.

• EPA applied the Zimmerle et al. study national-level scaling factor (1.075) to inflate all reported GHGRP
equipment counts to the national-level for RY2016 through RY2018 to account for GHGRP non-reporters.

• EPA calculated ratios for all GHGRP G&B emission sources for RY2016 using the national-level emission
source counts and the estimated year 2016 G&B station count (e.g., tanks per station). EPA used the
RY2016 ratios for all prior years of the time series, coupled with the G&B station count, to estimate the
number of emission sources each year.

• Three emission sources have no GHGRP reporting requirements and EPA applied Zimmerle et al. activity
observations from RY2017 partner equipment inventories, as follows:

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o Combustion slip count is a function of the GHGRP reciprocating compressor count, the Zimmerle
et al. observed fraction of centrifugal compressors driven by engines, and the Zimmerle observed
fraction of screw compressors. See Section 3.2.3 and Table 5.
o Separator units per station is a function of the GHGRP compressor count and the Zimmerle

observed ratios of separator per compressor and compressors per station, as discussed in Section
4.

o One unit of yard piping exists at each station.

Table 10 presents CH4, carbon dioxide (C02), and nitrous oxide (N20) national emissions using the 2020 GHGI
updated methodology and a comparison to the 2019 (previous) GHGI.

Table 10. G&B Station National Emissions Estimates for the 2020 GHGI Update and 2019 GHGI, Year

2017

Emission Source

CH4 Emissions
(mt)

C02 Emissions
(mt)

N20 Emissions
(mt)

Combustion Slip

363,534

n/a

n/a

Compressor L&V

271,238

32,690

n/a

Tank L&V + Flaring

205,261

633,931

2.2

Intermittent Bleed Pneumatic Controllers

173,628

13,172

n/a

Blowdowns

63,823

4,923

n/a

Dehydrator Vents

52,376

764,595

6.2

Dehy Vents - Large units

51,668

763,329

0.3

Dehy Vents - Small units

708

1,266

2.4

Yard Piping L&V

76,709

9,245

n/a

High-bleed Pneumatic Controllers

32,654

2,120

n/a

Pneumatic Pumps

23,391

1,683

n/a

Flares

9,394

2,300,171

4.3

Low-Bleed Pneumatic Controllers

6,344

399

n/a

Dehydrator L&V

1,852

223

n/a

Separator L&V

1,152

139

n/a

AGRUL&V

91

527,835

n/a

Desiccant Dehydrators

38

0



Total

1,281,446

4,291,126

10.6

2019 GHGI Total

2,218,773

239,459

0

n/a - Not applicable.

6 Requests for Stakeholder Feedback

EPA sought stakeholder feedback on the approaches under consideration through a 2019 workshop, in the
November 2019 memo, and in the public review draft of the GHGI. Feedback received at the workshop generally
supported the update. EPA did not receive stakeholder comment letters in response to the November memo.
Comments received on the GHGI public review draft are summarized here:

•	Feedback from three stakeholder comment letters supported the update to gathering and boosting. Of
these stakeholder comments, one also specifically supported the use of the Zimmerle et al. approach to
developing the national-level scaling factor to account for GHGRP non-reporters, and another suggested
that the scaling factor and national average ratio of compressors per station be updated annually in future
Inventories if data are available to do so.

•	One stakeholder comment letter did not support the update. The comment letter noted discrepancies
found between site-level and component-level emissions data in recent studies (citing work primarily
focusing on the onshore production segment). For comparison with an alternative national-level gathering

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and boosting estimate, the letter references an estimate in Alvarez et al., which relied primarily on the
Marchese et al. study (previous Inventory data source), and the application of an adjustment factor of 10
percent. The comment letter recommended retaining the previous (Marchese et al.) data source. In their
paper, Zimmerle et al. discussed differences between the Zimmerle et al. study (current data source) and
the Marchese et al. Study (previous data source). The differences noted in Zimmerle et al. are: (1) the
Zimmerle et al. study uses an updated and likely more representative mix of stations in terms of
throughput and complexity, (2) the Zimmerle et al. study accessed component level activity and emissions
data from the GHGRP, which were not available at the time of the Marchese et al. study, and which
represented data from a large set of operators for the entire U.S., (3) the two studies utilized different
measurement methods, and (4) there may have been operational improvements to G&B stations and/or
construction of new lower-emitting stations during the intervening years between studies due to
increased attention to CH4 emissions across the natural gas value chain.

• The stakeholder comment letter that did not support the update to gathering and boosting also expressed
concern about the potential omission of "super-emitters." The Zimmerle et al. study detected a number
of large emitters. For example, the study noted that "For most leaker factors, 50% or more of emissions
are due to the largest 5% of emitters." The set of emission factors developed in the Zimmerle et al. study
which were used to calculate emissions in the GHG Inventory include estimates for all emissions detected
in the field campaign, including estimates for large emitters, and the study notes that these "Large
emitter emissions have substantial impact on major equipment emission factors, adding 70% - 83% to the
impacted major equipment factors."

• The stakeholder comment letter that did not support the update to gathering and boosting also sought
additional information justifying the use of the Zimmerle et al. (measurements conducted in 2017) and
GHGRP (data available starting in 2016) data across the time series as opposed to using data from
Marchese et al. (measurements from 2013 and 2014) for previous years. EPA considered this approach
but did not implement it in the Inventory due to incongruencies between the studies noted in the
previous paragraph. If the Marchese et al. study in emissions and activity data were used for early years of
the time series (e.g., 1990-2014) and the Zimmerle et al. and GHGRP data were used in more recent years
(e.g. 2016-2017), there would be a large decrease in emissions over a short period of time due to this
transition. Some fraction of the decrease would likely be attributable to improvements in technologies
and industry practices. However, as noted above there are other differences between the studies such as
study representativeness and the difference between the two is likely not entirely due to changes in
technologies (or any other single factor). For this reason, EPA did not implement an approach that uses
data from both of the studies in different parts of the time series.

April 2019 and October 2018 memos also covered the G&B station topic, and EPA sought stakeholder feedback in
those memos. The October 2018 memo summarizes feedback from two stakeholders regarding the June 2018
memo version. Two stakeholders responded to questions raised in the October 2018 memo (which outlined
potential approaches for using GHGRP data in GHGI updates and was released before publication of the 2019
GHGI and recent studies such as Zimmerle et al. 2019).

Summary of Stakeholder Feedback on EPA October 2018 G&B Memo

• Two stakeholders supported consideration of studies focused on G&B emission sources before developing
GHGI EFs based solely on GHGRP data. One stakeholder specifically expressed concerns that using only
GHGRP data to estimate G&B station emissions would underestimate CH4 emissions and suggested the
current GHGI approach be maintained in the 2019 GHGI.

• A stakeholder did not support using the processing or transmission segment-specific EFs to represent
compressor vented and leak emissions in the G&B segment but supported reviewing new information
from G&B source-specific measurements expected to become available in 2019.

• A stakeholder supported disaggregating emissions by source and using source-level EFs that represent
both routine emissions and large emissions caused by abnormal conditions.

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• A stakeholder supported the general approach of using GHGRP data as the basis for GHGI activity
estimates (e.g., estimating station count based on reported compressor counts).

• A stakeholder acknowledged difficulty in discerning the representation of G&B stations within reported
GHGRP data since there is no national count of G&B stations including no data on equipment at G&B
stations or their throughput. The stakeholder supported continued analysis to understand why nearly all
gathering pipeline mileage is represented by GHGRP reported information while G&B stations were
believed to be under-represented in GHGRP reporting.

• A stakeholder supported taking steps to accurately reflect CH4 emissions from compressor engine exhaust
in the GHGI.

• A stakeholder supported potential use of basin-level data for the highest emitting basins for reflecting
regional variability for G&B stations, and noted that temporal variability is reflected in many of the
emission sources reported under GHGRP (e.g., those requiring event-based data such as blowdowns).

The questions below were not updated for this memorandum and are copied from the November 2019 memo.

Questions to Stakeholders

1. EPA seeks feedback on applying the general approach outlined in the Zimmerle et al. 2019 study to
calculate G&B station emissions for the GHGI, including:

a. Applying EFs as presented in the Zimmerle et al. study that were calculated using recent field
measurement data and an approach for incorporating large emitters (see Section 3.2.1 and Table
2).

b. Applying EFs calculated from GHGRP data for emission sources that were not included in the
Zimmerle et al. study field campaign (blowdowns, dehydrator vents, flares, and pneumatic
controllers; see Section Error! Reference source not found, and Table 1).

c. The use of onshore production volumes to determine the coverage of reported subpart W G&B
data, used to develop a scaling factor (see Section 3.2.2).

2. EPA seeks feedback on whether it is appropriate to apply a single EF to estimate leak and vent emissions
from the total population of gas-driven reciprocating and centrifugal compressors (as suggested in the
Zimmerle et al. study; see Sections 3.2.1 and 3.2.5), versus having separate EFs for each compressor type
(as in the GHGRP and as generally used for other GHGI industry segments).

a. If a centrifugal compressor-specific EF is used, what EF should EPA apply (e.g., subpart W EF or an
EF from another data source)?

b. Few centrifugal compressors are currently used in the G&B segment, so EPA seeks feedback on
whether this is likely to change in the future to the extent that it is valuable to show centrifugal
compressors as a unique emission source in the GHGI.

3. EPA seeks feedback on how to consider regional variability for G&B stations in the GHGI, including
whether to apply a simplified, national-level approach to determine ratios and scaling factors versus the
detailed, basin-level approach the Zimmerle et al. study developed, as discussed in Section 4.

4. EPA seeks feedback on how to consider temporal variability for G&B station emissions in the GHGI,
including:

a. How to apply the Zimmerle et al. approach versus Marchese et al. EFs (the basis of the current
GHGI) over the time series, as discussed in Section 4. Differences between the Zimmerle et al. and
Marchese et al. study EFs are discussed in Section 3.2.5.

b. How to use Zimmerle et al. data versus Marchese et al. data (the basis of the current GHGI) to
determine G&B station counts over the time series, as discussed in Section 4.

c. How to use subpart W data to estimate flaring emissions over the time series, as discussed in
Section 4.

5. EPA seeks feedback on how to handle activity reported under the G&B segment in subpart W for which its
emissions may already be accounted for in the onshore production segment of the GHGI. The current
onshore production GHGI methodology relies on estimating leak emissions for well pad equipment

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(heaters, separators, dehydrators, meters/piping, compressors, pneumatic controllers, and pneumatic
pumps) using an equipment per well activity factor (e.g., 0.71 separators per gas well), and the activity
factors are multiplied by the applicable national gas well or oil well count. Well-pad activities and
emissions that are typically considered to be in the production segment may be reported under the G&B
segment in subpart W, due to the subpart W facility definitions for onshore production and G&B (i.e.,
subpart W onshore production "means all equipment on a single well-pad or associated with a single well-
pad" and thus subpart W G&B may include data for production equipment at a site associated with
multiple well-pads). EPA is considering subtracting all well-pad equipment counts from the G&B segment
reported data, based on applying equipment AFs to the number of wells reported under equipment leaks
for G&B. EPA acknowledges this consideration may only apply to 2016 and not future years of G&B data;
937 wellheads were reported for the G&B segment in RY2016, 17 wellheads in RY17, and 0 wellheads in
RY18. EPA also seeks feedback on why the number of wellheads reported under equipment leaks for the
G&B segment has declined over the first three years of reporting.

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Appendix A - Measurement Methodologies from Data Sources Considered for Updates

Emission Source

Measurement and/or Calculation Type

# Sources

Location &
Representativeness

EF Calculation Method

GHGRP Subpart W and Subpart C

G&B Acid gas removal (AGR)
vents

Emissions calculated from the available methods: (1) CEMS for
C02 with volumetric flow rate monitors, (2) Vent meter for C02
and annual volume of vent gas, (3) measured inlet (or outlet)
gas flow rate and inlet and outlet volumetric fraction of C02, or
(4) simulation software.

Emissions data (for 2017) are
available from 54 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Centrifugal Compressors

Emissions calculated using the count of centrifugal
compressors that have wet seal oil degassing vents multiplied
by default EF (annual volumetric flow per unit).

Emissions data (for 2017) are
available from 24 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Combustion

Emission calculations depend on the type of fuel burned:

•	If burning pipeline quality natural gas or the identified fuels
and blends (i.e., coal, coke, natural gas, petroleum
products, certain other solids and gaseous fuels,
solids/gaseous/liquid biomass fuels) then use default
subpart C EFs.

•	If burning field gas, process vent gas, or a gas blend then
determine volume of fuel combusted from company
records and use a continuous gas composition analyzer to
measure mole fraction of gas.

•	These sources are exempt: (1) external fuel combustion
sources with rated heat capacity < 5 MMBtu/hr, (2) internal
combustion sources, not compressor-drivers, with a rated
heat capacity < 1 MMBtu/hr (equal to 130 HP).

Emissions data (for 2017) are
available from 312 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Dehydrators

Emissions calculations depend on the daily throughput:

•	If daily throughput is > 0.4 million scf then use simulation
software.

•	If daily throughput is < 0.4 million scf then use EFs and a
dehydrator count

•	For dessicant dehys, use the amount of gas vented from
the dessicant vessel when it is depressurized

•	When a flare or a regenerator fire-box/fire tube is used
adjust the emissions to reflect the control efficiency.

Emissions data (for 2017) are
available from 276 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Equipment Leaks

Emissions calculated using: (1) default EFs, by source type; (2)
source type counts (rule provides default counts e.g., valves
per wellhead) including miles of gathering pipelines by
material type; (3) estimated time the source was operational;
and (4) concentration of C02 and CH4.

Emissions data (for 2017) are
available from 319 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Pneumatic Controllers

Emissions calculated using: (1) counts of continuous high
bleed, continuous low bleed, and intermittent bleed

Emissions data (for 2017) are
available from 289 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported

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Emission Source

Measurement and/or Calculation Type

# Sources

Location &
Representativeness

EF Calculation Method



controllers, (2) default EFs for each controller type, (3) annual
operating hours, and (4) GHG concentrations in vented gas.





emissions and activity data to
consider use in GHGI updates.

G&B Pneumatic Pumps

Emissions calculated using: (1) counts of pneumatic pumps, (2)
default EF, (3) annual operating hours, and (4) GHG
concentrations in vented gas.

Emissions data (for 2017) are
available from 218 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Reciprocating
Compressors

Emissions calculated using the count of reciprocating
compressors multiplied by default EF (annual volumetric flow
per unit).

Emissions data (for 2017) are
available from 313 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Tanks

Emissions calculations depend on the daily throughput:

•	If oil throughput is >10 bbl/d and the gas and liquid passes
through non-separator equipment (e.g., stabilizers, slug
catchers) before flowing to the tank, calculate C02 and CH4
emissions using simulation software or by assuming all C02
and CH4 is emitted.

•	If oil throughput is >10 bbl/d and the gas and liquid flows
directly to a tank without passing through a separator,
assume all C02 and CH4 is emitted.

•	If oil throughput is <10 bbl/d then calculate C02 and CH4
emissions from (1) counts of separators, wells, or non-
separator equipment that feed oil directly to the storage
tank and multiply by EF (annual volumetric flow per unit).

•	Subtract emissions if a VRU is used and if a flare is used
then use the flare calculation methodology.

Emissions data (for 2017) are
available from 231 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Flare Stacks

Emissions calculated using: (1) gas volume sent to the flare, (2)
combustion efficiency (from manufacturer or assume 98%),
fraction of feed gas sent to an un-lit flare, and (3) gas
composition forC02, CH4, and hydrocarbon constituents.

Emissions data (for 2017) are
available from 154 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

G&B Blowdown Vent Stacks

Emissions calculated from the available methods: (1) use
blowdown volumes, the number of blowdowns, and the ideal
gas law modified with a compressibility factor, or (2) used a
flowmeter to directly measure emissions for each equipment
type or all equipment associated with a blowdown event.

Emissions data (for 2017) are
available from 262 facilities.

Facilities in the U.S. that
exceed 25,000 mt C02e
reporting threshold.

For this memo, the EPA
evaluated the reported
emissions and activity data to
consider use in GHGI updates.

GRI/EPA 1996

Compressor exhaust

An average emission rate was calculated for each model of
compressor engine and turbine in the GRI TRANSDAT
Emissions Database, which is based on compressor tests
conducted by Southwest Research

Institute (SwRI). The emission rates were calculated from the
reported methane emissions per unit of fuel and the reported
fuel use rate for each compressor model.

86 turbines and 775
reciprocating engines

Natural gas value chain

TRANSDAT data were
combined to generate
emission factors by
correlating compressor driver
type, methane emissions, fuel
use rate, and annual
operating hours

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April 2020

Emission Source

Measurement and/or Calculation Type

# Sources

Location &
Representativeness

EF Calculation Method

Vaughn et al. 2017

G&B facilities

Dual-tracer measurements, aircraft measurements, and on-site
component-level measurements (direct measurements and
simulated direct measurements) coupled with engineering
estimates using Monte Carlo model.

36 gathering stations

•	Measurements conducted
September-October 2015

•	Eastern portion of the
Fayetteville shale play
(Arkansas)

Dual-tracer measurements,
including and excluding
significant tank venting

Yacovitch et al. 2017

Production, gathering,
processing, and transmission
facilities

Dual tracer flux ratio method

•	DJ study area: 12 gathering
stations, 5 wellpads, and 4
processing plants
measured.

•	FV study area: 31 gathering
stations, 18 wellpads, and 4
transmission stations
measured.

•	Two natural gas production
regions: Denver-Julesberg
(DJ) basin and Fayetteville
shale play (FV) in Arkansas

•	Nov 2014 for DJ basin

•	Sep-Oct 2015 for FV play

Dual-tracer measurements to
calculate facility-level
emission rates and
throughput-weighted
emissions

Alvarez et al. 2018

G&B stations

Synthesized data from 3 studies: Zavala-Araiza et al. 2015,
Mitchell et al. 2015, Marchese et al. 2015

National activity estimated as
5,122 stations in year 2015

Synthesized data from 3
studies: Zavala-Araiza et al.
2015, Mitchell et al. 2015,
Marchese et al. 2015
(measurements in multiple
U.S. basins)

Adjusted the Marchese et al.
central estimate loss rates by
the ratio of the Zavala-Araiza
et al. and Mitchell et al. EFs
(59.6/54) to better account
for heavy-tail emissions

Zimmerle et al. 2019

G&B stations

For vented and leak emissions identification and
measurement, optical gas imaging (OGI), Bacharach® HI
FLOW® Sampler (BHFS), and bagging if flow exceeded BHFS
capacity (occurred for «1% of samples).

•	Measurements taken at
180 facilities on 1,938
major equipment units

•	Components counted on
1,002 major equipment
units

•	National activity estimated
at a source-level based on
GHGRP data, for a total of
6,108 stations in year 2017

•	Study aimed to select
stations representative in
terms of size, geographic
distribution, gas
composition, and
equipment mix.

•	Measurements conducted
June - November 2017.

•	180 facilities in 11 U.S.
states.

•	Nine partner companies
represented 35% of G&B
compressors reported to
GHGRP at the time

•	Study measurements for
combustion slip,
compressors, tanks, yard
piping.

•	GHGRP data for pneumatic
controllers, blowdowns,
dehydrator vents, flares,
leaks from non-compressor
equipment (e.g.,
separators).

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