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Methane, black carbon, and ethane emissions from natural gas flares in the Bakken Shale, ND Alexander Gvakharia, Eric A. Kort, Adam R. Brandt, Jeff Peischl, Thomas B. Ryerson, Joshua P. Schwarz, Mackenzie L Smith, and Colm Sweeney Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b05183 • Publication Date (Web): 12 Apr 2017 Downloaded from http://pubs.acs.org on April 13, 2017

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Methane, black carbon, and ethane emissions from natural gas ares in the Bakken Shale, ND Alexander Gvakharia,

∗, †

Eric A. Kort, † Adam Brandt, ‡ Je Peischl, ¶ § Thomas B. ,

Ryerson, § Joshua P. Schwarz, § Mackenzie L. Smith, † and Colm Sweeney ¶ †Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor,

Michigan, USA ‡Department of Energy Resources Engineering, Stanford University, Stanford, California,

USA ¶Cooperative Institute for Research in Environmental Science, University of Colorado

Boulder, Boulder, Colorado, USA §NOAA ESRL Chemical Sciences Division, Boulder, Colorado, USA E-mail: [email protected]

Phone: 757 232-3228 1

Abstract

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Incomplete combustion during aring can lead to production of black carbon (BC)

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and loss of methane and other pollutants to the atmosphere, impacting climate and

4

air quality. However, few studies have measured are eciency in a real-world setting.

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We use airborne data of plume samples from 37 unique ares in the Bakken region of

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North Dakota in May 2014 to calculate emission factors for BC, methane, ethane, and

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combustion eciency for methane and ethane. We nd no clear relationship between 1


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emission factors and aircraft-level wind speed, nor between methane and BC emission

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factors. Observed median combustion eciencies for methane and ethane are close

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to expected values for typical ares according to the US EPA (98%). However, we

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nd that the eciency distribution is skewed, exhibiting lognormal behavior. This

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suggests incomplete combustion from ares contributes almost 1/5 of the total eld

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emissions of methane and ethane measured in the Bakken shale, more than double the

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expected value if 98% eciency was representative. BC emission factors also have a

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skewed distribution, but we nd lower emission values than previous studies. The direct

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observation for the rst time of a heavy-tail emissions distribution from ares suggests

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the need to consider skewed distributions when assessing are impacts globally.

TOC Art

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Introduction

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Over 140 billion cubic meters (BCM) of gas is globally ared each year. 1 Flaring is used to

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dispose of gas at production and processing facilities that lack infrastructure and means to

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capture or use the gas. The United States ares about 8 BCM per year, with almost half of

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that coming from North Dakota alone. 2 From 2004 to 2014, the amount of gas annually ared

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in North Dakota increased from 0.08 BCM to 3.7 BCM, and in 2014 about 28% of North

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Dakota's total produced natural gas was ared. 3 Flaring has implications for the atmosphere.

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Although ideally gas would be captured instead of lost, it is preferable to are rather than 2



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vent, since aring destroys methane (CH 4 ) and volatile organic compounds which aect air

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quality, converting them to CO 2 . CH4 is a potent greenhouse gas, the 2nd most important

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anthropogenic greenhouse gas behind CO 2 based o integrated radiative forcing. 4,5 Flaring

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is not 100% ecient, and through incomplete combustion it can be a source for CH 4 and

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VOCs. 6,7 Flaring can also create black carbon (BC) as a byproduct, an anthropogenic forcer

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of climate with public health implications. 811 The World Bank recently introduced a "Zero

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Routine Flaring" initiative to end aring worldwide by 2030 through government incentives

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and institutional cooperation, hoping to mitigate economic losses due to aring and relieve

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its burden on the atmosphere. 12

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Inventories that account for aring often use a combustion eciency value of 98% of the

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initial gas, citing an EPA technical report. 13,14 This eciency value assumes are stability,

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and can decrease based on wind speed and other factors such as ow rate or aeration.

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Studies have investigated are eciency in laboratories using scaled-down are simulations

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in a controlled environment, reporting 98-99% are combustion eciency, 15,16 but there

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have been few eld studies done to assess are eciency and directly measure emissions

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in a real-world environment. Thus, scaled-up laboratory results may not be representative

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of real-world aring. A study of two are sites in Canada calculated an average observed

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combustion eciency of 68 ± 7%, much lower than the assumed eciency. 17 One remote

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sensing study in the Netherlands found high eciencies of 99% but only analyzed three

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ares, with up to 30% error in the measured gas concentrations, and noted the lack of in

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situ data. 18 There was also a comprehensive study to observe industrial are emissions and

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eciency but the tests were conducted at a are test facility, not directly at well sites. 19

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To our knowledge the only extensive study of in situ are eciency for CH 4 sampled ten

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ares in the Bakken Shale in North Dakota and one in western Pennsylvania. 20 This study

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reported high are eciencies up to 99.9% but based on their identication techniques, they

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acknowledged a possible bias towards larger, brighter burning, and thus more ecient, ares.

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Black carbon emissions from gas aring have been investigated, but there are not many

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studies that use direct observations of aring. Schwarz et al. (2015) 10 quantied total eld

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emissions of BC and derived an upper-bound on BC emission factor for aring from the

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Bakken using the same aircraft campaign data as used in this paper. Their emission factor

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was obtained using BC ux calculated with a mass balance technique for the entire eld.

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Hence, it did not target individual ares. It includes all BC sources in the region (e.g. diesel

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trucks, generators, limited agriculture, etc.) and is expected to provide an upper bound.

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Weyant et al. (2016) 21 calculated BC emission factors from targeted ares in the same

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region and found an average value well below the upper bound of Schwarz et al. (2015), and

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to our knowledge is the only previously published peer-reviewed study of BC emissions from

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aring that directly sampled ares. BC emission factors have been shown to vary based on

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fuel chemistry and stability of the are, necessitating the use of specic emission factors or

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a distribution rather than using a single average value as representative. 22

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The lack of direct, in situ observations of aring eciency suggests that estimates of

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emissions from incomplete combustion may be inaccurate. Also, using a single value for

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aring emission factors or combustion eciency does not take into account the various pa-

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rameters that may aect a are, 23 and a statistically robust sample of aring eciency

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would help identify a representative distribution. Total fugitive emissions from oil and gas

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production and leakage can be a substantial source of atmospheric CH 4 and are underrepre-

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sented in inventories. 24 Studies have observed non-normal distribution of CH 4 emissions in

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some elds, where less than 10% of sampled sources contributed up to 50% of the sampled

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emissions. 2529 A study of are emissions using Greenhouse Gas Reporting Program and Gas

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Emission Inventory data found that 100 ares out of 20,000 could be responsible for over

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half the emissions in the US, but this conclusion results from the non-normal distribution

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of gas volume ared and not from a skewed are combustion eciency (which is not repre-

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sented). 30 In addition to the non-normal distribution of gas volume ared, there may be a

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skewed distribution of emissions from incomplete combustion in ares based on eciency as

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well.

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We present an analysis of combustion eciency and emission factors of CH 4 , BC, and

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C2 H6 for thirty-seven distinct ares in the Bakken Shale Formation in North Dakota using

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data obtained during a May 2014 aircraft campaign, to our knowledge the largest study of

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aring emissions in the eld based on number of ares and the rst to include C 2 H6 . This

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gives us sucient statistics to obtain an eciency distribution and determine the implications

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for total fugitive emissions from incomplete combustion in actual eld conditions.

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Methods

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Flights and Instrumentation

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All observations used in this analysis were made as part of the TOPDOWN 2014 (Twin Otter

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Projects Dening Oil/gas Well emissioNs) study, and were collected on-board a National

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Oceanic and Atmospheric Administration (NOAA) DHC-6 Twin Otter aircraft. 10,31,32 This

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campaign focused on understanding the atmospheric impact of fossil fuel extraction activities.

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17 research ights were conducted on 11 separate days between May 12-26, 2014, totaling

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40 hours. Flights were typically 3-3.5 hours in duration, and were primarily conducted at

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low-altitudes (400-600 magl) within the planetary boundary layer at an average speed of 65

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m/s. Vertical proles were performed in each ight to dene the mixed layer height. Flights

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dedicated to mass balance conducted transects around the Bakken region, and although a

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few ares were sampled during these transects, most of the ares were identied on "mowing-

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the-lawn" ghts that swept across the region to target point sources as well as some ights

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dedicated to point source identication. Flares were circled multiple times during these

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ights between 400-600 magl although some were sampled higher up, around 1000 magl.

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Flares were not specically targeted for any particular characteristic such as size, brightness,

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or aring volume. Flares were sampled over the entire region rather than in a particular

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cluster, giving low spatial sampling bias. However, due to the nature of the sampling,

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brighter ares were more easily identiable from the plane and thus more likely to have been 5


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targeted. Not all passes by a are produced a well-dened peak that could be used in the

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eciency analysis. Many of the ares were sampled at a distance on the order of hundreds

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of meters to kilometers downwind. This gave the are plume time to disperse and allowed

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us to measure large plumes over a time period of 10-20 seconds, providing more data per

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plume than if we sampled closer and lower.

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CH4 , CO2 , carbon monoxide (CO), and water vapor (H 2 O) were measured with a Picarro

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2401-m cavity ringdown spectrometer with a sampling rate of 0.5 Hz. CH 4 was measured

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with an accuracy of ±1.4 ppb and precision of ±0.2 ppb; CO2 with an accuracy of ±0.15

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ppm and precision of ±0.03 ppm. 33,34 An Aerodyne mini direct absorption spectrometer was

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used to continuously measure C 2 H6 , deployed as described previously in literature 35,36 along

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with hourly measurements of a standard gas to conrm stability. 32 Sampling was conducted

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at 1 Hz with precision of < 0.1 ppb and average accuracy of ±0.5 ppb. 32 Due to the Aerodyne

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ethane instrument having a response time of 1 second, compared to the Picarro's 2 second

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response time, there were sharper, narrower peaks in C 2 H6 than CO2 and CH4 . To enable a

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point-by-point comparison of C 2 H6 to CO2 , a weighted moving average (WMA) was applied

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to the C2 H6 data. The total integrated value of the C 2 H6 peak did not signicantly change

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with the WMA lter, indicating conservation of mass with the method.

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All trace gases are reported as dry air mole fractions, converted from the measured

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wet air mole fractions using water vapor observations from the Picarro. A single-particle

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soot photometer (SP2 by Droplet Measurement Technology Inc., Boulder, CO) was used to

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measure refractory black carbon (rBC) for particles containing rBC in the mass range of 0.7-

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160 fg. The SP2 provided 1-second rBC mass-mixing ratios with systematic uncertainty of

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25%. 10,37 Two dierential GPS antennae on the fuselage of the Twin Otter provided aircraft

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heading, altitude, latitude, longitude, ground speed, and course over ground. Wind speed

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was calculated as described in Conley et al. (2014), 38 with estimated uncertainties of ±1

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m/s in magnitude and ±6◦ in direction. A Rosemount de-iced Total Temperature Sensor,

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model number 102CP2AF, measured ambient temperature. Calibration before and after the

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eld project indicate measurement performance with precision of ±0.2 ◦ C and accuracy of

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±1.0 ◦ C.

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Flare Identication

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We identied ares in the following ways. During the science ights, all signicant events

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were logged, including when the plane ew by a are. These ight notes thus provide times

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when a are was visually conrmed, and these are plumes were identied in the data for

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the corresponding ight and agged. After locating all the ares conrmed by the ight

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notes, we searched through the remaining data to nd plumes that could be possible ares

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but were not noted during the ight, such as smaller ares that might have been hard to

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see on the ground. To identify the other possible are plumes, we looked for peaks in CO 2

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where ∆CO2 , the peak enhancement, was greater at its maximum point than 4 σ of the CO2

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background variability, indicating a statistically signicant elevation of CO 2 as a result of

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combustion from a are. We also looked for a peak less than 20 seconds in time. At a mean

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ground speed of 65 m/s this corresponds to a source about 3 km away using Gaussian plume

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theory, 39 which is about the distance we tended to sample where the plume still presented

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a robust signal above background. Figure 1 shows the research aircraft ight paths, known

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are locations, 3 and where we sampled plumes.

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Figure 1: Left panel shows ight paths (black lines), wells with known aring (grey triangles), 3 and are plume locations (red points) from the TOPDOWN 2014 campaign in the Bakken eld in northwest North Dakota. Times when the plane circled around an area multiple times to repeatedly sample can be seen in the middle of the region. Right panel is zoomed in on a single are plume, with ight path (black line), are plume (red points), and wells with reported aring (lled triangles with corresponding monthly aring amounts). The arrows indicate the wind direction. We used the wind direction, distance from well, and aring amount to verify that the plume was caused by aring.

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To verify if these additional plumes identied in the data were indeed caused by aring, we

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co-plotted the locations of these events with all nearby wells with reported aring and other

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CO2 producers such as processing facilities and gas plants using the EPA GHG Reporting

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Program as seen in Figure 1. Certain are locations were cross-checked with additional data

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from the VIIRS Active Fire Map and North Dakota Oil and Gas ArcIMS Viewer. Then, using

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Gaussian plume theory, we estimated how far away the source of a plume was based on the

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plume width and wind conditions, matching the plume to a possible are source. 39 Although

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the science ights were conducted on days with steady winds, leading to low variability

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in wind speed and direction, we accepted plumes that were within 20% of the theoretical

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distance to account for deviation in other factors such as not ying directly in the center

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of the diused plume. If a plume was located downwind from a well with aring, was not

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downwind of another CO 2 source, and had a width and distance consistent within 20% of

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Gaussian plume theory, we considered it likely due to a are source. If a plume was not

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downwind of a are at a distance consistent with Gaussian plume theory, or had interference

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from another CO2 source, we omitted it from the analysis. 39 are plumes were identied

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with the ight notes, and out of 17 additional plumes in the data, 13 were accepted using

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our verication method and 4 rejected for a total of 52 are plumes from 37 unique ares.

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Other sources for methane or black carbon closely co-located with ares (such as diesel

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engines or fugitive losses from production wells) could contribute to the observations we are

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attributing to aring, and we assess their potential impact on our analysis here. Using gas

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composition data from over 550 samples, the average chemical plume from the Bakken is

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0.7% CO2 , 3.7% N2 , 49% CH4 , 21% C2 H6 , and the rest in higher-order hydrocarbons. 40 This

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results in a molecular weight of about 29 g/mol, close to that of air and nearly double the

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weight of natural gas from other elds with higher CH 4 ratios. 21 An unburned source of gas

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is therefore neutrally buoyant compared to a hot are exhaust plume which will rise in the

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atmosphere. 41 However, the are plume can entrain these other sources, mixing them as the

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buoyant plume rises in the atmosphere. If we assume a are converts 98% of its hydrocarbons

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to CO2 , and that enhancements near a well pad due to other emissions are 50 ppm CH 4 and

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415 ppm CO2 , then if the are plume entrained a volume equal to its own (50% dilution) the

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resulting CH4 /CO2 slope measured by the aircraft (see Figure 2) would change by less than

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1%, smaller than the uncertainty range in tting the slope. Considering typical values for

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methane and CO2 enhancement (40 ppb and 5 ppm on average, respectively), we estimate

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the slope error (and thus error on calculated emission factors) would be less than 1% with

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10% as a upper bound. Adjusting the are eciency in this estimation does not signicantly

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aect the result (using a 90% combustion eciency, all else equal, would also have an impact

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of 1% on the slope). Although we cannot denitively rule out all potential contributions for

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such sources to the plumes we are analyzing, these considerations of possible entrainment

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suggest it is not signicant in this analysis, though the potential impact would suggest our

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results may represent a lower bound for combustion eciency.

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Combustion Eciency

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Destruction eciency and emissions factors were calculated for each are sampled. Black car-

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bon emission factors were determined following the methodology of Weyant et al. (2016), 21

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using eq. 1

EFBC = 1000 × F

CCO2

CBC + CCH4 + CBC

(1)

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Here CCO2 , CCH4 , and CBC are the mass concentrations of carbon in g/m 3 for each

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product with the respective background removed and F is the ratio of carbon mass to total

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hydrocarbon mass, calculated to be 0.79 from gas composition data for the Bakken. 40 CO2

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and CH4 data were converted from molar ratios to g/m 3 using a molar volume at standard

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temperature and pressure (273K, 1013 mb) to match the conditions of the BC mass concen-

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trations. This EFBC value is given in grams of BC per kg of gas, and can be converted to

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g/m3 using a gas density of 1.23 ± 0.14 kg/m3 for the composition. 40 For some of the ares

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we did not detect a strong BC enhancement correlated with CO 2 , causing skewed or negative

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emission factor values. To account for this, when the peak enhancement ∆BC was below

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the detection limit of 4 σ of the background, we used a value of half the detection limit in

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the EF calculation as in Weyant et al. (2016); 21 this was observed in a third of the plumes.

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The measured BC concentrations were scaled up by 15% to account for accumulation-mode

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mass outside of the SP2 detection range, as described in Schwarz et al. (2015); 10 rBC mass

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in either the coarse mode or a sub-accumulation mode size range would not be accounted

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for by this adjustment. Generally, as in Schwarz et al. (2015), 10 the accumulation mode

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size distribution is well-t with a lognormal function, and any additional smaller or larger

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populations of BC particles are revealed by deviations from the lognormal t at the smaller

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or larger limits of the detection range respectively. Here, there was no evidence of additional

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non-accumulation modes.

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Emission factors for CH 4 and C2 H6 were obtained by rst calculating the peak enhance-

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ment of CH4 , C2 H6 , and CO2 . We calculated a mean background value for each plume

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using the concentration data from 5-10 seconds before the start and after the end of the

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plume, and then subtracted the background from the plume values to obtain ∆CH4 , ∆C2 H6 ,

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and ∆CO2 . ∆CH4 and ∆C2 H6 were t with a Reduced Major Axis (RMA) regression to

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∆CO2 for each peak to obtain the emission factor, in ppm CH 4 or C2 H6 per ppm CO2 . 20

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Figure 2 shows an example plume from a are and its CH 4 regression. Regressions were

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well-correlated with 10-20 data points in each are plume. Uncertainty in EF for CH 4 and

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C2 H6 was given by 95% condence intervals from the regression. For all plumes, EF BC from

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eq. 1 linearly correlated with the slope of BC vs CO 2 with an R2 of 0.97. This t was used

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to derive uncertainty in EF BC from 95% condence intervals of the regression of BC and

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CO2 .

Figure 2: Example of a aring plume with CO 2 , CH4 , C2 H6 time series and regression to nd CH4 EF.

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We calculated the destruction removal eciency (DRE) following the methodology of

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Caulton et al. (2014) 20 using eq. 2, with a small correction to report the value as the

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fraction of gas destroyed rather than remaining.

DRE(%) = 1 − 226

µCH4 ∗ 100 ((X) ∗ µCO2 ) + µCH4

(2)

µCH4 and µCO2 are the gas concentrations in ppm and X is the carbon fraction of CH 4 11


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in the total fuel gas before combustion. From gas composition data for the eld, 40 the value

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of X is 0.26 ± 0.05 for CH4 .

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This DRE calculation was done two ways. First, by integrating over the entire peak

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to obtain a DRE value from the total integrated amount of CH 4 and CO2 . Second, by

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calculating the DRE value for each point in the peak individually to get an aggregate DRE

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dataset as seen in Caulton et al. (2014). 20 The respective baseline values were removed

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from each gas concentration in both methods. Since the integral method calculates DRE

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using the average concentration over the sampling time of the gases in the plume, and

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the point-by-point mean represents the average instantaneous DRE, a signicant divergence

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between the results would be indicative of a potential problem with the approach. For all

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ares the integrated DRE diered from the mean point-by-point DRE by 1% on average,

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demonstrating robustness between the two methods. C 2 H6 DRE was also calculated using

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both methods, with X = 0.23 ± 0.03 for C2 H6 . The eect of X's variability on the DRE is

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small and within the calculated uncertainty for DRE.

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Detection Threshold

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We compared the standard deviation of CH 4 background and the maximum peak CO 2 en-

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hancement to calculate a "noise DRE" using eq. 2 to assess the impact of a potential signal

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produced by background variability on the DRE. The distribution suggests a sensitivity

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threshold around 99%. We compared the sensitivity distribution to the measured DRE

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distribution, and an analysis of variance between the two produced a p value of 9 ×10−7 ,

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suggesting they are statistically signicantly dierent. Thus it would be dicult to dis-

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tinguish measured DRE values of greater than 99% as signicant compared to background

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variability, but values less than 99%, as we have observed, are robustly detectable with our

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approach. There is a trade-o between our sampling approach and the one used by Caulton

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et al. (2014), 20 where they ew lower and closer to the ares. With our ights, we obtained

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more points in each plume, allowing us to calculate regression lines for emission factors. 12



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However, we encountered a lower signal-to-noise ratio, making it more dicult to precisely

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measure the DRE of very ecient ares. We used the dierence between 100% and the DRE

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calculated using the sensitivity as a proxy for DRE uncertainty in each individual are.

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Results

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Emission Factors

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Figure 3 shows the calculated CH 4 and C2 H6 emission factors plotted against mean aircraft-

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level wind speed for all are plumes. Previous laboratory are studies have observed a strong

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nonlinear dependence of ineciency on crosswind speed, 15,16 and Caulton et al. (2014) 20 ob-

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served a weak relationship in the ares they sampled in the eld. Considering our observed

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emission factors and crosswind speeds we nd similar results to Caulton et al. (2014). An

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exponential t of our data suggests a weak dependence, with parts of the data possibly follow-

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ing dierent distinct curves. A Pearson correlation analysis of the data and the exponential

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t produced a weak correlation coecient (0.34). Gas exit velocity and are parameters like

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the stack diameter can aect the ineciency curve and may be the reason for the apparent

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presence of multiple curves, but unfortunately these values were not known for our sampled

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ares. More specic knowledge of the gas composition and ow rate would potentially be

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illuminating for the possible bimodal distribution in CH 4 EF of low-eciency emitters (>30

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ppb/ppm) and high-eciency emitters (0-20 ppb/ppm) but we can only hypothesize without

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detailed information on specic ares at time of our sampling.

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Some ares were circled repeatedly or revisited on dierent days, and so we transected

273

multiple plumes from the same are. The calculated EF for the are was not consistent

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between dierent plumes, suggesting uctuation in the eciency. Caulton et al. (2014)

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found large overall variability in CH 4 EF and inconsistency between sampling on dierent

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days, but attribute the variability to the small sample size of their plumes. 20 Weyant et

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al. (2016) reported inconsistent emissions of BC for ares sampled on dierent days, and 13


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observed large variability in BC EF for multiple passes of the same are, citing variability

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in gas ow rate and gas composition as possible sources. 21 From our data alone we cannot

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resolve the cause of same-are variability, but it is apparently a feature consistent across

281

studies.

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We did not observe a clear relationship between EF and wind speed for plumes from the

283

same are, possibly due to factors like ow rate or exit velocity. For some ares that were

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sampled multiple times, we did not get a sharp, identiable peak in CO 2 or CH4 on every

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pass, and so we were not able to analyze all possible passes. The EF calculation included

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background points in the regression, removing these points from the t and forcing the line

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through zero did not signicantly aect the results. Comparing CH 4 and C2 H6 emission

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factors for each plume, we found a linear relationship with a R 2 value of 0.57, as plumes

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with higher emissions of CH 4 had corresponding higher emissions of C 2 H6 , suggesting that

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combustion eciency is somewhat uniform across hydrocarbons.

14



Figure 3: CH4 and C2 H6 EF plotted against wind speed for all plumes, with an exponential t in red. Error bars represent 95% condence intervals in EF and 1 σ in wind speed.

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Like Weyant et al. (2016), 21 we did not observe a dependence between BC emission factor

292

and CH4 EF for each plume. Elevated CH 4 emissions from a are do not necessarily indicate

293

higher or lower BC emission. Adding in an ethane term to eq. 1 did not signicantly change

294

the BC EF calculation, as the ppm-order enhancement of CO 2 dominates the ppb-order

295

enhancement of C2 H6 and CH4 .

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Figure 4 shows the distribution and probability function of BC emission factor in g

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BC/kg gas. The distribution is right-skewed, matching the results of Weyant et al. (2016), 21

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and was t with a lognormal density using a maximum-likelihood method. The lognormal

15


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Page 17 of 29

299


distribution function is given by

f (x) = p

1

2 /(2σ 2 ))

(2π)σx

e−((logx−µ)

(3)

300

µ and σ are the mean and standard deviation of the logarithm. The Pearson correlation

301

coecient between the BC emission factor probability distribution and the lognormal dis-

302

tribution resulted in a correlation coecient of 0.96. We present the lognormal t as a way

303

to illustrate the skewed distribution and provide a quantitative representation. Results de-

304

rived from the combustion eciency distribution use the raw distribution, rather than an

305

approximation with the lognormal t.

Figure 4: On the left, a histogram of black carbon emission factor for all are plumes, with lognormal density (red line). On the right, distribution function of BC EF in black with lognormal distribution function in red.

306

We report BC EF from ares in g/kg, grams of BC produced per kilogram of hydrocarbons

307

in the fuel gas. The values ranged from 0.0004 to 0.287 g/kg. We can convert from g/kg

308

to g/m3 using a ared gas density of 1.23 ± 0.14 kg/m3 , 40 allowing us to express BC EF

309

in terms of gas ared volume and to compare the results with previous studies. Even with

310

the observation of a right-skewed distribution, our analysis nds lower BC emissions than

311

previously reported. Schwarz et al. (2015) 10 provide an estimate for all the BC sources in the

312

Bakken of 0.57 ± 0.14 g/m3 . This upper bound on aring is twice the highest emission value

313

we observed (Figure 4). Similarly, laboratory analysis by McEwen et al. (2012) 22 reported 16



314

emissions much larger than we observe (0.51 g/m 3 , o scale in Figure 4). The mean value of

315

0.13 ± 0.36 g/m3 measured with an SP2 by Weyant et al. (2016) 21 is within our observed

316

range, though it falls within the top 20% of emitters we observed. Our observed in-eld

317

ares thus appear to have produced less BC than would be predicted from previous studies.

318

The median, mean, and standard deviation of the mean BC emission factor we observed

319

were 0.021 g/m3 and 0.066 ± 0.009 g/m3 (or 0.017 and 0.053 ± 0.008 g/kg), respectively,

320

though given the skewed distribution care needs to be taken in interpreting these values.

321

Given that 3.7 BCM of gas was ared in the Bakken eld in 2014, 3 applying that to the

322

entire distribution of BC EF in g/m 3 suggests total BC emissions from aring of 0.24 Gg

323

BC/yr. However, the top quartile of ares contribute disproportionately, 0.17 Gg BC/yr,

324

which is 70% of the total emissions from ares. Overall, our emission rate of 0.24 Gg BC/yr

325

is two-thirds the rate of 0.36 Gg BC/yr calculated by Weyant et al. (2016) 21 for ares and

326

17% of the total Bakken emission rate (1.4 Gg BC/yr) reported by Schwarz et al. (2015). 10

327

Based on these results, using a single emission factor to estimate emissions from ares in

328

a region does not properly represent the wide variability in emissions that may be present.

329

Total emissions from aring could potentially be substantially reduced if the least ecient

330

ares alone are identied and addressed.

331

Combustion Eciency

332

For methane and ethane, the percent of gas remaining provides a useful metric for are

333

eciency - this is simply 100-DRE. In Figure 5 the distribution of percent remaining CH 4

334

and C2 H6 is illustrated and a lognormal relationship is apparent. As with emission factors,

335

we found a linear relationship between CH 4 and C2 H6 DRE for each plume, with a R 2 of

336

0.53.

17


Page 18 of 29

Page 19 of 29


Figure 5: Histogram of remaining CH 4 and C2 H6 (100-DRE) with density curve (dashed black) and lognormal t (red). These distributions were integrated to calculate the emissions due to incomplete combustion.

337

A Pearson correlation analysis of the DRE probability distributions and the lognormal

338

t distribution produced a correlation coecient of 0.99 for both CH 4 and C2 H6 . The

339

distribution of CH 4 and C2 H6 emission factors, which are theoretically consistent with the

340

DRE calculations, also exhibit a skewed distribution though a lognormal relationship is not

341

as apparent. The median DRE for CH 4 is 97.14 ± 0.37 using the integral method and 96.99

342

± 0.23 using the aggregate dataset. For C 2 H6 the median DRE is 97.33 ± 0.27 and 97.36

343

± 0.25 respectively. These median values are close to the expected eciency (98%), but the

344

right-skewed distribution indicates that 98% is not a representative destruction eciency

345

and would over-predict methane and ethane destruction.

346

We can assess the impact of this observed skewed distribution by considering the contri-

18



347

bution of incomplete are combustion to total eld methane and ethane emissions. Using

348

aircraft data and a mass balance technique, Peischl et al. (2016) 31 calculated a methane

349

ux for the Bakken region that extrapolates to an annual ux of 0.25 ± 0.05 Tg CH4 /yr.

350

As with black carbon, we can use reported aring gas volumes for North Dakota in 2014 3

351

and integrate the distribution of observed DRE values to produce an estimated emission of

352

methane from incomplete combustion of 0.052 Tg CH 4 /yr, or 21% ± 4% of the total emis-

353

sions reported by Peischl et al. (2016), using the uncertainty bound on the ux calculation.

354

This is more than double the contribution one would nd if the expected value of 98% was

355

assumed representative of the eld, which would predict emissions representing 8% ± 1.6% of

356

the total eld emissions. Caulton et al. (2014) reported much higher combustion eciencies,

357

and applying their median 99.98% value would suggest only a fraction of a percent (0.13%

358

± 0.03%) of the total eld emissions was from incomplete combustion in ares.

359

We performed the same analysis for ethane, and compareed with the total eld emissions

360

estimate of 0.23 ± 0.07 Tg C2 H6 /yr reported in Kort et al. (2016). 32 Again our observed

361

combustion eciencies suggest incomplete combustion from ares contributes substantially

362

to total eld emissions, 17% ± 5% of the total emissions (0.039 Tg/yr), more than double

363

that predicted by using 98% as a representative value.

364

The observed lognormal distribution results in a disproportionate impact from ares ex-

365

hibiting poor combustion eciencies. We nd the top quartile of methane emitters contribute

366

0.036 Tg CH4 /yr, which is 69% of total emissions from incomplete are combustion (and 14%

367

of the total eld emissions). Similarly, for ethane, the top quartile of emitters contributes

368

0.026 Tg C2 H6 /yr which is 66% of the total emissions from incomplete are combustion (and

369

12% of total eld emissions).

370

Why do we nd higher methane emissions and lower black carbon emissions than other

371

studies conducted in the Bakken shale? 10,20,21 We cannot denitively pinpoint the reason.

372

We sampled in the same subregion of the Bakken as Caulton et al. (2014), 20 though we

373

did not sample any of the same ares they did, and our campaign was 2 years after theirs.

19


Page 20 of 29

Page 21 of 29


374

Weyant et al. (2016) 21 did not report specic are locations but were likely in the same

375

subregion as well, 2 months before our campaign.

376

There is a dierence in sampling methods, which could contribute. Caulton et al. (2014)

377

ew low and close to the ares, though specic altitudes and distances are not reported. 20 As

378

we did not specically target larger (and so potentially more ecient) ares, our approach

379

makes it more likely to sample higher emitting ares. Weyant et al. (2016) also likely ew

380

closer to the ares than we did, though at a slower speed (45 m/s) than at which we typically

381

sampled (65 m/s). 21 We did not observe a clear correlation between sampling distance and

382

combustion eciency in our data, but it certainly aects variables such as plume entrainment,

383

other emissions sources, turbulence, and environmental factors.

384

The largest source of discrepancy in results is likely that relatively few ares have been

385

sampled: 26 (85 passes) by Weyant et al. (2016), 10 by Caulton et al. (2014), 20 and 37 (52

386

passes) in our study, and thus there is large representation error. In our study we attempt

387

a statistical sampling for greater representativeness, but given that there were over 5500

388

wells with reported aring in the Bakken in 2014, 3 37 independent ares only represents

389

0.6% of active ares. Thus, we think our results should be considered in concert with the

390

Weyant and Caulton analyses, and our data should be considered in aggregate. In doing so,

391

it would subtly change our total estimated contribution (lower for methane and higher for

392

black carbon), but the observed lognormal distribution result would not change.

393

Global Implications

394

Our sampling provides sucient statistics to observe a heavy-tail distribution of combus-

395

tion eciencies. This heavy-tail characteristic has been observed and reported for methane

396

emissions from the oil and gas sector, 2527,29,42 but this represents a rst observation of the

397

heavy-tail for aring emissions of methane and ethane. This has important implications

398

for current and future contributions from aring activities. To illustrate, let us consider if

20



399

our observed distribution were globally representative. Globally, 143 ± 13.6 BCM of gas

400

is ared annually. 43 If 98% destruction removal eciency were representative of every are,

401

that would correspond to a range in methane emissions of 1.14-1.90 Tg CH 4 /yr for a gas com-

402

position range of 60%-100% CH 4 . Applying our observed distribution, that range changes

403

to 2.78-4.64 Tg CH4 /yr, more than doubling the amount emitted. In assessing the climate

404

and air quality impacts of aring, it is critical that skewed distributions are accounted for in

405

the cases of methane, ethane, and black carbon. Although our specic observed emissions

406

factors and eciencies are likely only representative of the Bakken eld, the observations of

407

a skewed distribution is likely general.

21


Page 22 of 29

Page 23 of 29

408


Acknowledgement

409

Data from the aircraft campaign reported in the manuscript are archived and available

410

at http://www.esrl.noaa.gov/csd/groups/csd7/measurements/2014topdown/. This project

411

was supported by the NOAA AC4 program under grant NA14OAR0110139 and NASA grant

412

NNX14AI87G. J. Peischl and T. Ryerson were supported in part by the NOAA Climate Pro-

413

gram Oce and in part by the NOAA Atmospheric Chemistry, Carbon Cycle, and Climate

414

Program. We thank NOAA Aircraft Operations Center sta and ight crew for their eorts

415

in helping collect these data.

416

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