Map of the Bakken region. The NOAA Twin Otter flight tracks are shown with thin black lines. Active oil and gas wells in North Dakota (ND), Montana (MT), Saskatchewan (SK), and Manitoba (MB) are shown as light blue circles. Natural gas was sampled from 710 wells, colored mustard yellow, and analyzed for CH, C, and other trace gas abundance []. Point sources from the 2013 EPA GHG Reporting Program inventory are shown as open dark blue circles sized by inventory CHemissions. Urban area outlines, including Minot and Williston, ND, are colored orange. The approximate extent of the Williston Basin is shaded gray in the overview map.

We estimate the total CH 4 emitted to the atmosphere from the Bakken region of North Dakota using measurements taken aboard a chemically instrumented National Oceanic and Atmospheric Administration (NOAA) DHC‐6 Twin Otter aircraft in May 2014 during the Twin Otter Projects Defining Oil/gas Well emissioNs (TOPDOWN) field project, based out of Minot, ND (Figure 2 ). The NOAA Twin Otter flew on 12 days in the Bakken region, with transects performed upwind, over, and downwind of this region. The Twin Otter typically flew at ~65 m/s, and full field transects normally took 45 min. A flight was typically 3 to 3.5 h in duration. On six of the flight days, the Twin Otter refueled and took off again for a second flight. Figure 2 shows all the flight tracks of the NOAA Twin Otter around the Bakken region in May 2014. Using the mass balance technique, we determine the CH 4 emissions from this region on 3 days with steady winds. We then attribute the CH 4 emissions in the region to oil and natural gas extraction operations based on the similarity of atmospheric C 2 H 6 to CH 4 enhancement ratios during the mass balance transects with the abundances of C 2 H 6 and CH 4 in natural gas extracted from the Bakken region. Finally, we estimate the natural gas loss rate to the atmosphere from oil and gas operations in the Bakken region by converting the CH 4 emission to a natural gas emission and dividing by the amount of natural gas extracted in the region in May 2014.

Schneising et al. [ 2014 ] reported an increase in CH 4 emissions from the Bakken region of northwestern North Dakota and southeastern Saskatchewan of 0.99 ± 0.65 Tg CH 4 /yr between the years 2006–2008 and 2009–2011 based on measurements from the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) instrument on board the Envisat satellite. Schneising et al. [ 2014 ] further estimated an “energy content leak rate” for the Bakken, i.e., the energy content of the natural gas emitted to the atmosphere divided by the sum of the energy content of the oil and natural gas extracted from the ground, of 10.1% ± 7.3% by comparing the increase in CH 4 emissions with the increase in oil and natural gas production from this region over the same time period. We interpret this as a loss of 44 ± 32% of the natural gas withdrawn, as discussed in section 5.5 . Several studies using in situ airborne measurements and the mass balance method have recently quantified CH 4 emissions to the atmosphere from oil and natural gas‐producing regions such as the Uinta Basin of Utah [ Karion et al. , 2013 ], the Denver‐Julesburg Basin of northeastern Colorado [ Pétron et al. , 2014 ], the Haynesville region of northwestern Louisiana and eastern Texas, the Arkoma Basin of Arkansas, the Marcellus region of northeastern Pennsylvania [ Peischl et al. , 2015 ], and the Barnett region of central Texas [ Karion et al. , 2015 ]. These aircraft studies report a variety of atmospheric CH 4 emission rates, ranging from (14–80) × 10 3 kg CH 4 /h, with rates of natural gas losses to the atmosphere ranging from 0.18% to 11.7% of the natural gas extracted from the ground from these regions. The variability in emissions and loss rates from these regions likely stems from a variety of factors, including whether oil is extracted in addition to natural gas, the amount of aboveground fossil fuel processing that occurs, the well count, the fossil fuel production volume, the age of the infrastructure, and the type of activity occurring on days when the aircraft studies occur. For comparison, the average hourly emission increase found by Schneising et al. [ 2014 ] was (110 ± 70) × 10 3 kg CH 4 /h over the years 2006–2008 to 2009–2011. This CH 4 emission rate is the largest emission reported from all oil and gas‐producing regions studied to date, which warrants further examination. Here we derive atmospheric CH 4 emission rates from the Bakken region of North Dakota using in situ airborne data and the aircraft mass balance technique and compare to the estimate by Schneising et al. [ 2014 ].

In addition to crude oil, natural gas is also extracted from the Bakken shale formation. The region was responsible for 1.4% of U.S. natural gas production in May 2014 (EIA). Natural gas is a mixture primarily composed of methane (CH 4 ), with ethane (C 2 H 6 ) typically the second largest component. Emissions of natural gas to the atmosphere may occur during various stages of oil and natural gas extraction and processing. Consequently, the U.S. Environmental Protection Agency (EPA) greenhouse gas (GHG) inventory estimates that CH 4 emissions to the atmosphere from the oil and gas industry account for 29% of 2013 U.S. anthropogenic CH 4 emissions (EPA 430‐R‐15‐004, http://www3.epa.gov/climatechange/Downloads/ghgemissions/US‐GHG‐Inventory‐2015‐Main‐Text.pdf ). Two recent studies have shown measurement‐based estimates of CH 4 emissions from the oil and gas industry are in agreement with the EPA GHG inventory for large gas‐producing regions [ Peischl et al. , 2015 ; Karion et al. , 2015 ]. However, other studies have shown that GHG inventories underestimate CH 4 emissions from other oil and natural gas‐producing regions [ Pétron et al. , 2012 ; Peischl et al. , 2013 ; Karion et al. , 2013 ; Pétron et al. , 2014 ; Brandt et al. , 2014 ]. Therefore, studies that estimate CH 4 emissions from the oil and gas industry are necessary to constrain regional and national GHG emissions inventories and ultimately inform decisions based on the climate impacts of U.S. fuel choices.

From 1981 to 2003, North Dakota accounted for less than 1.5% of United States (U.S.) crude oil production (U.S. Energy Information Administration (EIA), www.eia.gov ). However, the onset of directional drilling and hydraulic fracturing has led to a large increase in oil and natural gas extraction from shale formations in the U.S. in recent years. One such shale formation, the Bakken, is located in the Williston Basin of northwestern North Dakota. As a result, North Dakota accounted for 12.5% of the U.S. crude oil production in May 2014 (EIA), due mainly to oil extraction from the Bakken shale formation (Figure 1 ), surpassing the oil production from all U.S. regions except Texas and federal Gulf of Mexico (EIA).

CH, carbon dioxide (CO), carbon monoxide (CO), and water vapor (HO) were measured by wavelength‐scanned cavity ringdown spectrometry using a Picarro G2401 ‐m []. CHwas measured once every 2 s with an estimated accuracy of ±1.4 ppb (all uncertainties herein are 1); the precision of this measurement was ±0.2 ppb. All CHmeasurements herein are reported as dry air mole fractions expressed as nanomole/mole or parts per billion. Cwas measured by tunable infrared laser direct absorption spectroscopy using an Aerodyne mini ethane analyzer []. Cwas measured every second with an estimated accuracy of ±7%; 1 s precision was ± (0.02 to 0.21) ppb, depending on flight conditions. Additionally, black carbon was measured using a single particle soot photometer [], ozone was measured using a 2B Technologies analyzer, and flasks of air samples were later analyzed for hydrocarbons and other trace gases. Two differential GPS antennae were mounted above the fuselage of the Twin Otter and provided aircraft heading, altitude, latitude, longitude, ground speed, and course over ground. Ambient temperature was measured with a Rosemount deiced total temperature sensor, model number 102CP2AF, calibrated before and after the field project, with a precision of ±0.2°C and an estimated accuracy of ±1.0°C. Meteorological and navigational measurements were made once per second by various sensors aboard the NOAA Twin Otter. Potential temperature, θ, is calculated using the following equation:whereis ambient temperature in units of Kelvin andis ambient pressure is in units of millibar. The ambient wind speed and wind direction were derived from the true air speed, heading, speed over ground, and course over ground measurements aboard the Twin Otter using the technique of]. We estimate uncertainties in the wind speed of ±1 m/s and wind direction of ±6° during typical level flight conditions, based on an analysis of the wind calibration flight pattern data [].

Terrain height below the Twin Otter aircraft is estimated using a digital elevation map (DEM) from the U.S. Geological Survey (GTOPO30, 1996, https://lta.cr.usgs.gov/GTOPO30 ). The DEM has a resolution of 30 arc sec (~900 m) and is based on World Geodetic System 1984 data. Hybrid Single‐Particle Lagrangian Integrated Trajectory (HYSPLIT) model back trajectories [ Stein et al. , 2015 ; https://ready.arl.noaa.gov/HYSPLIT.php ] are used to evaluate the evolution of wind fields over time and are run with the following settings: Meteorology: North American Mesoscale Forecast System, 12 km; Vertical Motion: Model vertical velocity (which uses the vertical velocity field from the meteorological data). County‐level livestock populations were downloaded from the U.S. Department of Agriculture (USDA) National Agriculture Statistics Service (NASS) ( http://www.nass.usda.gov ). We assume an average emission of 67.9 kg CH 4 /yr per head from enteric fermentation (EPA GHG inventory) and no emission of C 2 H 6 . Bison populations are estimated from a North Dakota Department of Agriculture website ( http://www.nd.gov/aitc/agfacts/Livestock/ ). We assume that bison populations scale with cattle populations and assume an average emission of 72 kg CH 4 /yr per head [ Kelliher and Clark , 2010 ]. County‐level manure emissions estimates were downloaded from the National Renewable Energy Laboratory ( http://maps.nrel.gov/biomass ) and were based on 2002 USDA data. In cases where the NOAA Twin Otter flight track transects a county, an apportionment based on geographical area is used to determine livestock and manure emissions within the study region.

Natural gas sample analysis data are used from the work of Brandt et al. [ 2015 ] by way of the North Dakota Industrial Commission, Department of Mineral Resources, Oil and Gas Division ( https://www.dmr.nd.gov/oilgas/ ). Samples were taken from 710 wells across the Bakken region (Figure 2 ) and analyzed for the molar composition of C 1 ‐C 6 alkanes, CO 2 , and other trace gases. North Dakota well locations were downloaded from the same website (accessed September 2014). North Dakota well production data are from the North Dakota State Industrial Commission May 2014 Oil and Gas Production Report ( https://www.dmr.nd.gov/oilgas/mprindex.asp ). Bakken production data were obtained from the EIA Web site ( http://www.eia.gov/petroleum/drilling/ , downloaded October 2015).

The following inventories are used in the current analysis: the EPA 2013 GHG Inventory (publication EPA 430‐R‐15‐004, downloaded September 2015, http://www3.epa.gov/climatechange/Downloads/ghgemissions/US‐GHG‐Inventory‐2015‐Main‐Text.pdf ), and the EPA GHG Reporting Program (GHGRP) inventory (downloaded November 2015, http://www2.epa.gov/ghgreporting/subpart‐w‐reported‐data ). The GHGRP data include the latitude, longitude, and estimated CH 4 emission of known point sources (Figure 2 ). Some latitude‐longitude coordinates in the 2013 EPA GHGRP inventory data set were modified to match the actual locations determined using satellite imagery.

The mass balance technique is used as follows. We calculate the horizontal flux of a speciesthrough a vertical plane defined by an aircraft transect:wherecos() is the magnitude of the component of the wind velocity normal to the flight track averaged over the course of a transect,is the terrain height in meters above sea level (masl),is the adjusted mixing height in meters above sea level, () is the CHenhancement above the background, andis the crosswind distance []. We define the background CHmixing ratios as those found immediately outside the plume downwind of the Bakken region. This is discussed in more detail, below. We define the adjusted mixing height in meters above ground level (magl) as described by]:= (3+ z)/4, whereis the well‐mixed planetary boundary layer (PBL) depth, andis the entrainment height or the altitude at which mixing from the PBL has reached estimated free tropospheric CHmixing ratios. Below, we compare this method of determining the adjusted mixing height with that of]. All other calculations are similar to those reported by].

The technique used to estimate CH 4 fluxes is the mass balance approach [ White et al. , 1976 ]. This technique has been used extensively to estimate CH 4 emissions from oil and gas‐producing regions [ Karion et al. , 2013 ; Pétron et al. , 2014 ; Peischl et al. , 2015 ; Karion et al. , 2015 ]. Further, the mass balance technique was used to estimate C 2 H 6 emissions from an oil and gas‐producing region [ Smith et al. , 2015 ] and black carbon and C 2 H 6 emissions from the Bakken region during the 2014 TOPDOWN project [ Schwarz et al. , 2015 ; Kort et al. , 2016 ].

5 Results and Discussion

5.1 CH 4 Emissions to the Atmosphere From the Bakken Study Region The NOAA Twin Otter aircraft flew three flights dedicated to determining the CH 4 mass balance from the Bakken region on 13, 14, and 22 May (Figure 3). Eight additional flight days were dedicated to characterizing emissions from CH 4 point sources, flaring, and general field emissions. One flight day, 21 May, was dedicated to mass balance, but because the wind speeds were light (~2–3 m/s), we cannot verify that they were steady over a full traverse of the Bakken region, and we therefore discard this day from our analysis. For the three mass balance flights used here, the winds measured aboard the Twin Otter were steady throughout the Bakken region, in agreement with HYSPLIT back trajectories. Figure 3 Open in figure viewer PowerPoint 4 fluxes were calculated. North Dakota natural gas production data are summed into a ~5.6 × 5.6 km grid. Flight tracks are colored and sized by CH 4 mixing ratio for the transects shown in Figure Map showing the five transects downwind of the central Bakken shale formation from which CHfluxes were calculated. North Dakota natural gas production data are summed into a ~5.6 × 5.6 km grid. Flight tracks are colored and sized by CHmixing ratio for the transects shown in Figure 4 . The arrow indicates the average wind speed and direction during the transects, where the length of the arrow is proportional to the wind speed. (a) The 13 May transect at approximately 350 m agl. (b) The 14 May northeast‐to‐southwest transect at approximately 350 m agl and southwest‐to‐northeast transect at approximately 650 m agl. (c) The 22 May east‐to‐west transect at approximately 500 m agl. (d) The 22 May west‐to‐east transect at approximately 1000 m agl. The 13, 14, and 22 May flight tracks sampled downwind of the majority of the highest‐producing wells in the Bakken region (Figure 3). The flight tracks in Figure 3 are colored by the CH 4 mixing ratio measured within the PBL along the transects used in the mass balance analysis. Figure 4 shows the time series of CH 4 and aircraft altitude for the downwind transects highlighted in Figure 3. Also plotted in Figure 4 is the background CH 4 mixing ratio used in equation 2. We use the C 2 H 6 measurement, which typically has a greater plume signal to background variability ratio than the CH 4 measurement, to help define the plume background, especially on days such as 22 May, when the CH 4 plume enhancements were only a factor of 2–4 greater than the CH 4 background variability. The CH 4 background is further selected to be consistent with the C 2 H 6 data, such that the ratio of their derived fluxes is equal to the molar enhancement ratio of C 2 H 6 to CH 4 observed on the downwind transects. Figure 4 Open in figure viewer PowerPoint 4 measurements (black) from the five transects downwind of the Bakken shale region shown in Figure 4 used for the flux calculations; the dashed red traces represent the estimated ±1σ uncertainty of the background. The shaded areas show the CH 4 enhancement above the background along the transects. Time series of CHmeasurements (black) from the five transects downwind of the Bakken shale region shown in Figure 3 . The blue trace shows the Twin Otter altitude above ground level. The red trace represents the background CHused for the flux calculations; the dashed red traces represent the estimated ±1uncertainty of the background. The shaded areas show the CHenhancement above the background along the transects. Examples of the mixing height determination are shown with three vertical profiles in Figure 5. Figure 5 (left) shows the vertical profile immediately before the transect shown in Figure 4a; Figure 5 (middle) shows the vertical profile immediately before the second transect shown in Figure 4b; Figure 5 (right) shows the vertical profile immediately after the first transect shown in Figure 4c. We calculate the adjusted mixing height, z 1 , following the method of Peischl et al. [2015] to account for any transport of CH 4 through the top of the PBL. We increase the effective PBL depth by one quarter the distance between the well‐mixed PBL, defined by the dashed lines in Figure 5, and entrainment height, z e , defined by the dotted lines in Figure 5, and include an uncertainty equal to this same distance. This adjusted mixing height and uncertainty encompasses a range of possible sources for the CH 4 enhancement above the free tropospheric levels immediately above the PBL: z PBL if the CH 4 enhancements above the PBL are solely due to preexisting layers aloft, and halfway between z PBL and z e if all CH 4 enhancements above the PBL are due to detrainment from the PBL. The resulting adjusted mixing heights and their uncertainties from each vertical profile during a flight are fit with a line. We use the value of this fit at the time of the transect, along with the corresponding 1σ confidence interval of the fit, to determine z 1 for equation 2, and plot the resulting adjusted mixing heights as filled yellow circles in Figure 5. For comparison, the mixed height determined by Schwarz et al. [2015] using a different method are shown with orange diamonds. For each mass balance transect in this analysis, the mixed height determinations are well within the uncertainties of the two different estimation techniques. Figure 5 Open in figure viewer PowerPoint β) used by Schwarz et al. [ 2015 4 background; dashed vertical gray lines represent the ±1σ uncertainty of the background. One vertical profile is plotted from each of the three flight days used for the mass balance. Dashed horizontal black lines indicate the well‐mixed PBL depth. Dotted horizontal black lines indicate the entrainment height. The yellow circles indicate the average adjusted mixing heights used for the mass balance calculations. The orange diamonds indicate the average adjusted mixing heights (h ×) used by]. Solid vertical gray lines indicate the CHbackground; dashed vertical gray lines represent the ±1uncertainty of the background. Table 1 lists the parameters and their uncertainties used in equation 2 to calculate the CH 4 flux for each transect. The largest uncertainties for the 13 May flight were from the uncertainty of the background CH 4 value (±25%), the variability of the wind speed (±20%), and the uncertainty of z 1 (±18%). The largest uncertainties for the 14 May flight were from the variability of the wind speed (±34%), the uncertainty of the background CH 4 value (±30%), and the uncertainty of z 1 (±21%). Finally, the largest uncertainties for the 22 May flight were from the uncertainty of the background CH 4 value (±27%), the variability of the wind speed (±24%), and the uncertainty of z 1 (±21%). The CH 4 fluxes calculated from the five downwind transects are (36 ± 13), (27 ± 13), (27 ± 12), (27 ± 12), and (25 ± 10) × 103 kg/h. The two flux determinations from the 14 and 22 May flights did not depend on transect altitude, indicating the emissions were mixed vertically within the PBL by the time they were sampled by the Twin Otter. The 1/σ2‐weighted average of the five CH 4 emissions estimates for the Bakken region is 28 (±5) × 103 kg CH 4 /h (Table 2), suggesting relatively small day‐to‐day variability in the total emissions rate for this short study. We therefore assume that this emission rate represents the average May 2014 CH 4 emission from the Bakken region. Table 1. Data Used in Equation From Five Transects on the 13, 14, and 22 May 2014 Flights and the Resulting CH 4 Fluxes Transect Terrain Height (masl) z 1 , (masl) Wind Direction (°) Wind Speed (m/s) CH 4 Flux (1026 molecule/s) CH 4 Flux (103 kg/h) 13 May 680 ± 60 2810 ± 265 312 ± 9 13.3 ± 2.7 3.8 ± 1.4 36 ± 13 14 May, #1 695 ± 85 2545 ± 395 339 ± 14 7.7 ± 2.6 2.8 ± 1.4 27 ± 13 14 May, #2 705 ± 80 2655 ± 245 339 ± 8 8.0 ± 2.3 2.8 ± 1.3 27 ± 12 22 May, #1 655 ± 45 2340 ± 355 177 ± 10 7.5 ± 1.8 2.8 ± 1.2 27 ± 12 22 May, #2 660 ± 45 2320 ± 300 180 ± 9 7.4 ± 1.7 2.7 ± 1.1 25 ± 10 Table 2. Summary of CH 4 Emissions From the Bakken Study Region Quantity Evaluated Amount CH 4 flux (28 ± 5) × 103 kg/h CH 4 estimated from nonoil and gas point sources and livestock ~1.5 × 103 kg/h CH 4 emissions attributed to the oil and gas industry (20–33) × 103 kg/h Natural gas production in May 2014 (9.9 ± 0.3) × 108 m3 CH 4 abundance in natural gasa 47 ± 13% CH 4 extraction rate in May 2014 (420 ± 120) × 103 kg/h Natural gas loss rate to the atmosphere 6.3 ± 2.1%

5.2 Oil and Natural Gas Production in the Bakken Study Region Monthly oil and natural gas production from the Bakken region is estimated for May 2014 using both EIA and North Dakota state data. The EIA reports gross gas withdrawal volumes in thousands of cubic feet (MCF) at 60°F and 6.681528 kg per square inch. We therefore adjust the volumes reported by the EIA by factor of 0.9483 to convert to standard temperature and pressure of 0°C and 1 atm then convert cubic feet to cubic meters. The Bakken natural gas production for May 2014 was 10.2 × 108 m3 according to the EIA when converted to standard cubic meters. Similarly, the sum of individual well production data from the North Dakota Oil and Gas Division is 9.7 × 108 m3, within 5% of the EIA estimate. Each airborne transect analyzed here was downwind of wells that accounted for greater than 97% of total production. We therefore estimate natural gas production upwind of any given transect in the region to be (9.9 ± 0.3) × 108 m3 for the month of May 2014 (Table 2).

5.3 Other Sources of CH 4 in the Bakken Study Region The Bakken region encompasses landfills and livestock; these sources emit CH 4 but are not known to emit C 2 H 6 . The total CH 4 emission from point sources in the 2013 EPA GHGRP inventory not related to oil and natural gas processing in the Bakken region, two natural gas‐fired electrical generating stations, is 0.23 × 103 kg/yr, which averages to 0.03 kg CH 4 /h, which we treat as negligible. Here we use bottom‐up methods to estimate CH 4 emissions from livestock in the Bakken region and then compare these emissions to the total CH 4 emission from the region to confirm that livestock emissions are also not a major source of CH 4 to the atmosphere in the Bakken region. We estimate that the Twin Otter flight track encompassed 9% of the cattle and calves in North Dakota, based on a geographical apportionment of USDA NASS county data. This results in an estimated enteric fermentation emission from 161,000 cattle and calves of 1.3 × 103 kg CH 4 /h. CH 4 emissions from manure are estimated at 0.003 × 103 kg CH 4 /h. We estimate approximately 32,000 bison emit 0.3 × 103 kg CH 4 /h, based on statewide bison populations scaled by estimated cattle and calf populations in the Bakken region. The combined point source and livestock emission, 1.5 × 103 kg CH 4 /h (Table 2), accounts for approximately 5% of the 28 × 103 kg CH 4 /h emission calculated for May. Coal mines, another possible source of CH 4 , are located southeast of the Bakken region. The state of North Dakota has four active coal mines (EIA, http://www.eia.gov/coal/annual/pdf/table2.pdf), all of which are surface mines. These mines are represented in the 2013 EPA GHGRP inventory and are located between 47.0° and 47.5°N latitude and 101° and 102°W longitude (Figure 2). CH 4 emissions from these mines were further downwind of the 13 and 14 May transects, and therefore do not affect the mass balance calculations on these days, but may have contributed to the CH 4 enhancements measured on the 22 May flight. This is discussed in more detail in the next section.

5.4 Source Attribution of CH 4 Emissions The small contribution of livestock to the total CH 4 emission in the Bakken region suggested by the USDA inventory is confirmed using airborne data. Enhancements of C 2 H 6 in the boundary layer along the mass balance transects correlate with enhancements of CH 4 similar to the composition ratios of C 2 H 6 to CH 4 in natural gas extracted from the region (Figure 6). The lines representing the composition ratios in Figure 6 are offset to 1900.5 ppb CH 4 and 1.5 ppb C 2 H 6 , which represents the approximate background mixing ratios for the mass balance transects on the 13 and 14 May flights. For the 22 May flight, we plot the CH 4 enhancement above the sloping background in Figure 4 then offset the difference to the background values as for the composition ratios. The C 2 H 6 to CH 4 enhancement ratio was on the order of 0.4–0.5, an atypically high ratio compared to previously studied oil and gas‐producing regions. As a result, this ratio provides a unique source fingerprint and distinguishes Bakken region oil and gas emissions from natural gas in the supply chain or coal mine emissions, which typically have C 2 H 6 to CH 4 ratios of less than 0.05. Finally, an approximately 10 km wide CH 4 plume on 22 May sampled shortly before 17:00 CST not highly correlated with C 2 H 6 represents 6% of the CH 4 flux calculation for that day. These data were sampled along the eastern edge of the 22 May transects, where emissions from coal mines to the south‐southeast may have influenced the CH 4 measurements. Figure 6 Open in figure viewer PowerPoint 2 H 6 and CH 4 are plotted for each of the five transects used to calculate a CH 4 flux downwind of the Bakken region. Also, plotted are lines with a slope equal to the C 2 H 6 to CH 4 molar composition ratio (C 2 /C 1 ) from 710 samples of natural gas from the Bakken region [Brandt et al., 2015 2 H 6 and 1900.5 ppb CH 4 . The slope of the yellow line represents the geometric mean C 2 H 6 to CH 4 composition ratio of the samples. and CHare plotted for each of the five transects used to calculate a CHflux downwind of the Bakken region. Also, plotted are lines with a slope equal to the Cto CHmolar composition ratio (C/C) from 710 samples of natural gas from the Bakken region [], assuming a background of 1.5 ppb Cand 1900.5 ppb CH. The slope of the yellow line represents the geometric mean Cto CHcomposition ratio of the samples. We attribute CH 4 emissions to the oil and gas industry in the Bakken region as follows. We subtract the estimated livestock emission and the plume not correlated with C 2 H 6 on the 22 May flight from the total CH 4 emission estimated for the region. Since we cannot confirm quantitatively with our measurements the impact that livestock and coal mine emissions may have had on the CH 4 emissions, we assign a 100% uncertainty to these subtractions. Consequently, CH 4 emissions attributed to the oil and gas industry have larger error bars than the estimate for the total CH 4 emissions from the region, but both emissions estimates share the same upper limit (Table 2). Our estimate for oil and gas emissions of (20–33) × 103 kg CH 4 /h is a factor of 5–8 greater than the 2013 EPA GHGRP estimate for the entire Williston Basin of 0.0047 Tg CH 4 /yr, which averages to 4.1 × 103 kg CH 4 /h. However, under this program, emissions from many small facilities may not meet the threshold required to report (http://www2.epa.gov/sites/production/files/2015‐07/documents/subpartwinformationsheet.pdf), so we would expect the GHGRP value to be a lower limit to the total emissions. However, the EPA also estimates U.S. CH 4 emissions from petroleum systems of 1.009 Tg CH 4 /yr (2013 EPA GHG inventory, Tables 3–37). Assuming inventory CH 4 emissions scale with oil production, the Bakken region would account for 12.5% of the total U.S. emission, which averages to 14.4 × 103 kg CH 4 /h. Therefore, our estimate of CH 4 emissions from the Bakken region oil and gas industry is a factor of 1.4–2.3 greater than the 2013 EPA GHG inventory's emissions from petroleum systems scaled by oil production.