Isotopic signatures of anthropogenic CH4 sources in Alberta, Canada

Atmospheric Environment 164 (2017) 280e288

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Isotopic signatures of anthropogenic CH4 sources in Alberta, Canada M. Lopez a, *, 1, O.A. Sherwood b, E.J. Dlugokencky c, R. Kessler a, L. Giroux a, D.E.J. Worthy a a Environment and Climate Change Canada, Climate Research Division, Climate Chemistry Research Section, 4905 Dufferin St., Toronto, Ontario M3H 5T4, Canada b Institute of Arctic and Alpine Research, University of Colorado, Boulder, CO, USA c US National Oceanic and Atmospheric Administration, Earth System Research Laboratory, Boulder, CO, USA

h i g h l i g h t s
Mobile and continuous measurements of stable carbon isotopes in specific CH4 source plumes in Alberta.
CH4 isotopic signatures were accurately derived using an AirCore coupled to a CRDS instrument.
The enriched isotopic values of CH4 from the natural gas industry show thermogenic origin.
The depleted isotopic values of CH4 from the oil industry show microbial origin.
Isotopic signature information will have a profound implication on modelling activities for CH4 emission estimates in Canada.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 February 2017 Received in revised form 9 June 2017 Accepted 10 June 2017 Available online 12 June 2017

A mobile system was used for continuous ambient measurements of stable CH4 isotopes (12CH4 and 13 CH4) and ethane (C2H6). This system was used during a winter mobile campaign to investigate the CH4 isotopic signatures and the C2H6/CH4 ratios of the main anthropogenic sources of CH4 in the Canadian province of Alberta. Individual signatures were derived from d13CH4 and C2H6 measurements in plumes arriving from identifiable single sources. Methane emissions from beef cattle feedlots (n ¼ 2) and landfill (n ¼ 1) had d13CH4 signatures of 66.7 ± 2.4‰ and 55.3 ± 0.2‰, respectively. The CH4 emissions associated with the oil or gas industry had distinct d13CH4 signatures, depending on the formation process. Emissions from oil storage tanks (n ¼ 5) had d13CH4 signatures ranging from 54.9 ± 2.9‰ to 60.6 ± 0.6‰ and non-detectable C2H6, characteristic of secondary microbial methanogenesis in oilbearing reservoirs. In contrast, CH4 emissions associated with natural gas facilities (n ¼ 8) had d13CH4 signatures ranging from 41.7 ± 0.7‰ to 49.7 ± 0.7‰ and C2H6/CH4 molar ratios of 0.10 for raw natural gas to 0.04 for processed/refined natural gas, consistent with thermogenic origins. These isotopic signatures and C2H6/CH4 ratios have been used for source discrimination in the weekly atmospheric measurements of stable CH4 isotopes over a two-month winter period at the Lac La Biche (LLB) measurement station, located in Alberta, approximately 200 km northeast of Edmonton. The average signature of 59.5 ± 1.4‰ observed at LLB is likely associated with transport of air after passing over oil industry sources located south of the station. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Keywords: Methane isotopic signature AirCore Oil and natural gas industry Plume mapping

1. Introduction The Paris Agreement of the United Nations Framework Convention on Climate Change entered into force on November 4,

* Corresponding author. CEA LSCE - Orme des Merisiers, 91191 Gif-sur-Yvette, France. E-mail address: [email protected] (M. Lopez). 1
Now at Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Unite mixte CNRS-CEA-UVSQ, 91191 Gif-sur-Yvette, France.

2016. This agreement aims to reduce the risks and impacts of climate change by limiting the increase in global temperatures to less than 2  C above pre-industrial levels through aggressive reductions in greenhouse gas emissions. With a radiative forcing of 0.48 ± 0.05 W m2 from 1750 to 2011 (Myhre et al., 2013), methane (CH4) is the second most important anthropogenic greenhouse gas, after carbon dioxide (CO2). The atmospheric CH4 burden has approximately doubled since the pre-industrial era (Mitchell et al., 2011) and currently, 60% of global CH4 emissions are attributed to human activities (Bousquet et al., 2006). As such, mitigation of CH4

http://dx.doi.org/10.1016/j.atmosenv.2017.06.021 1352-2310/© 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

M. Lopez et al. / Atmospheric Environment 164 (2017) 280e288

emissions is an important part of meeting Paris agreement targets. According to the National Inventory Report (NIR) of Canada, CH4 emissions accounted for 15% of Canada's total anthropogenic greenhouse gas emissions in 2012. The CH4 emissions are unevenly distributed across the country with 40% attributed to Alberta, which accounts for only 6% of the national territory and 12% of the Canadian population. Anthropogenic emissions of CH4 in Alberta in 2012 comprise fugitive emissions from the oil and gas industry (71%), enteric fermentation (22%) and landfills (5%) (NIR, 2015). Over 350,000 oil, bitumen and gas wells were drilled in Alberta between 1955 and 2015 (CAPP, 2016). The percentage of wells with leakage or venting of natural gas to the atmosphere is 5% provincially and exceeds 15% in an area between Edmonton and Lloydminster (Watson and Bachu, 2009). Recent studies suggest that fugitive emissions of CH4 can be underreported by “bottom up”
tron emission inventories (Karion et al., 2013; Miller et al., 2013; Pe et al., 2012). This underestimation is attributed to a range of factors, including incomplete spatial and temporal scales of measurement, emissions factors that are poorly representative or have not kept pace with deployment of newer technologies, and skewed distributions in which a small number of “super emitters” account for a disproportionate share of total emissions (Brandt et al., 2014; Lyon et al., 2016). It is therefore important to develop and improve independent approaches capable of assessing CH4 emissions from multiple and unknown point sources over large/regional areas. Stable carbon isotopic measurements of atmospheric CH4 (d13CH4) provide a constraint on source attribution since d13CH4 signatures vary by source: thermogenic CH4 associated with oil and gas production is relatively enriched in 13C whereas microbial CH4 is strongly depleted in 13C (Whiticar, 1990). Measurement of atmospheric d13CH4 is analytically difficult and most of the available long time series are from flask-air measurements with relatively sparse spatio-temporal coverage (Miller et al., 2002; Quay et al., 1999). However, recent instrumental developments have allowed continuous measurements of atmospheric d13CH4 with precision better than 1‰ over 1 min averages. This precision level has been shown to be sufficient in deriving single isotopic source signatures €ckmann et al., 2016; in various other studies (Eyer et al., 2016; Ro Rella et al., 2015; Assan et al., 2017). Multiple papers have integrated d13CH4 data in atmospheric models to improve constraints on CH4 source attribution on regional and global scales (Bousquet et al., 2006; Mikaloff Fletcher et al., 2004; Nisbet et al., 2016; Schaefer et al., 2016; Schwietzke et al., 2016). The use of isotopic measurements for source attribution requires that the isotopic signatures of individual source categories and sinks are known. However, the isotopic value of a single source category can vary considerably, depending on the CH4 formation process, geographic origin, season and secondary alteration (Bergamaschi et al., 1998a; Fisher et al., 2011; Levin et al., 1993; Quay et al., 1999; Townsend-Small et al., 2012; Whiticar, 1990). Therefore, to improve constraints on CH4 source attribution in Alberta, we compile an inventory of d13CH4 signatures representing the major CH4 source categories. We describe a mobile system designed for continuous measurements of d13CH4 in the plume of the main anthropogenic sources of CH4. Isotopic signatures of CH4 related to the oil and gas industries, to a landfill and to beef cattle feedlots derived from a 10 day long campaign are presented. A case study illustrates how the derived d13CH4 signatures improve CH4 source attribution at a long term monitoring station at Lac La Biche, Alberta.

281

17 to 25, 2016. The objective was to determine the isotopic composition of the main anthropogenic CH4 sources in Alberta using a mobile platform for real-time, continuous measurement. 2.1. Description of the mobile system Measurements of CH4 abundance in units of mole fraction and isotopologues (12CH4 and 13CH4) in air were performed by cavity ring down spectroscopy (CRDS - Picarro G2201i) at 1 Hz. The CRDS also measures CO2, C2H6 and H2O mole fractions in air. For a more detailed description of the instrument see Rella et al. (2015). The CRDS was mounted in a vehicle with a GPS (Garmin 18x USB) used to track the vehicle location at 1 Hz. Electrical power was provided by a pack of four lead acid batteries connected to a convertor allowing the measurement system to run for up to 12 h. An air intake, consisting of a 3 m length of Nylon tube (6.35 mm outer diameter) was fixed on the roof of the vehicle, approximately 2 m above ground level, next to the GPS antenna. A manifold, coupled to the CRDS, was used to select between the air intake, calibration tanks or the AirCore (Fig. 1). The concept of storage tubes, or “AirCores”, was introduced by Tans (2009) to sample vertical profiles of the atmosphere before analyzing the tube content at the laboratory. This sample technique has been adapted and validated by Rella et al. (2015) for mobile atmospheric measurements. Here, the AirCore consists of a 20 m long Dekabon tube, having an inner diameter of 0.6 cm and therefore a volume of 565 mL. The AirCore is continuously flushed with ambient air at 700 mL min1 which is regulated by flow controller FC2. Thus, the last 48 s of ambient measurements are continuously stored in the AirCore. The default “monitoring mode” allows continuous atmospheric measurements. In this mode, ambient air is pumped from the air intake and injected into the CRDS system, without drying, via pump

2. Method and instrumentation The results presented in this study were obtained during an intensive measurement campaign in Alberta, Canada from February

Fig. 1. Schematic of the mobile measurement system used for atmospheric measurement of CH4, d13CH4 and C2H6.

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#1, following the green path shown in Fig. 1-A. Because of the known cross-sensitivity of water vapor with stable CH4 isotopes, the wet measurements performed in monitoring mode are not used for further analysis. The pump pressures the system to 20 psi above atmospheric pressure. Two filters of 40 and 140 mm pore size are placed in series after pump #1 to protect the system from dust (not shown in Fig. 1). Once pressurized, the sample is separated into two streams. One stream is directed to the CRDS. The second stream is directed through the AirCore and vented. The CRDS cavity is continuously flushed with the sample at a flow rate of 25 mL min1. An open end and a flow controller (FC1) are installed between the instrument and the valve #6 (see Fig. 1) to reduce the residence time of ambient air in the inlet line. In monitoring mode, flow controller 1 (FC1) is set to 1000 mL min1 and the total residence time is 30 s. The CRDS system can also be switched to “replay mode” for more precise analysis of isotopic signatures. Typically, the vehicle crosses a CH4 plume within a few seconds which, given the 1 Hz sampling frequency of the instrument, provides only a few data points to derive the source signature. When a large CH4 peak is detected (at least 1 mmol mol1 above the background), the system is manually switched to replay mode in which the contents of the AirCore is pushed through the drier (magnesium perchlorate cartridge) to the analyzer using a separate pump (shown as pump #2 in Fig. 1), and at a slower flow rate of 40 mL min1. The slower flow rate extends analysis time of individual peaks by a factor of 25. The sample is dried prior to analysis to avoid measurement bias potentially caused by water vapor interferences. Once the AirCore has been analyzed - approximately 15 min - the system is switched back to monitoring mode. 2.2. CH4 isotopic signatures Isotopic data are reported in delta notation (Eq. (1)) where R is the ratio of 13C to 12C. Values are expressed in ‰ and referenced to the Vienna Pee Dee Belemnite (VPDB) scale.



d¼

2.4. AirCore analysis

 Rsample  1 *1000 ‰ Rstandard

(1)

13

Point source d C signatures were derived using Keeling plots (Keeling, 1958). This approach assumes that i) the atmospheric measurements of CH4 (Cm) are the sum of all of the CH4 sources (Cs) in the measurement footprint added to the measured background (Cb), and ii) the carbon mass is conserved in the lower planetary boundary layer. From the previous assumptions, Eq. (2) can be derived:

d13 Cm ¼

  Cb * d13 Cb  d13 Cs Cm

gas passed through the system. The system has an estimated uncertainty better than 0.8 nmol mol1 and a 1 s repeatability of 0.2 nmol mol1 (standard deviation at 1 sigma). Calibration of the d13CH4 measurements was based on a twopoint calibration against tanks with d13CH4 values of 54.4‰ (7312.4 nmol mol1 CH4) and 38.8‰ (7763.2 nmol mol1 CH4). These tank d13CH4 values were measured at the Institute of Arctic and Alpine Research (INSTAAR, University of Colorado, Boulder, USA) using continuous flow isotope ratio mass spectrometry (CFIRMS) (Miller et al., 2002). Because the INSTAAR system is designed to measure small variations of less than 1‰ in the d13CH4 of background atmosphere, it employs one-point calibration against a series of tanks that have been tied to the primary reference material NBS-19 (þ1.92‰) (Tyler, 1986). Uncertainty for INSTAAR measurements beyond the range of ambient background of approximately 47‰, as assessed from measurements of four independently calibrated working standards (Isometric Instruments, Victoria, B.C., Canada) ranging from 66.5 to 23.9‰, is generally within 1‰. Given the large range in point source d13CH4 values encountered in this study, we consider ± 1‰ uncertainty to be adequate. The CRDS system was calibrated every two days for d13CH4 and showed a short term repeatability (1 min averages) better than 0.2‰ for CH4 greater than 7000 nmol mol1. The C2H6 measurements were calibrated in the laboratory before and after the campaign with a single tank supplied by Praxair containing 15 mmol mol1 of C2H6. Instrument response did not change between the two calibrations and the measured short term repeatability was 0.175 mmol mol1 at 15 mmol mol1. Rella et al. (2015) report interference between d13CH4 and C2H6: the instrument reports heavier d13CH4 when the sample contains C2H6. Following the protocol described by Rella et al. (2015), a correction factor of 36.6‰ mmol mol1 CH4 (mmol mol1 C2H6)1 was systematically applied on the d13CH4 measurements when C2H6 exceeded 0.2 mmol mol1.

13

þ d Cs

As explained previously, the AirCore is used to extend the analysis time of a CH4 plume. On the left panel of Fig. 2, a typical CH4 plume measured in monitoring mode downwind an identified CH4 source is shown. On the right panel, the CH4 signal was reanalyzed through the AirCore in replay mode. The solid black line represents the 1 s data and the blue dots are the 10 s averages. The higher flow rate used in monitoring mode results in most of the CH4

(2)

Thus, the intercept of the plot d13CH4 vs 1/CH4 from an atmospheric sample gives the mean isotopic signature of CH4 sources. 2.3. Instrument calibration and performance Preliminary tests of the CRDS system in the laboratory showed that CH4, d13CH4 and C2H6 had linear responses in ranges of 1800e15000 nmol mol1 CH4, 65.8 to 24.8‰ d13CH4 and 0 to 15 000 nmol mol1 C2H6. The CH4 measurements were calibrated every 3 days against a single tank calibrated on the NOAA-04 scale (Dlugokencky et al., 2005) with CH4 mole fraction of 2040 nmol mol1. The maximum drift observed between two successive calibrations was 0.3 nmol mol1. The evaluation of the system performance was based on injection of a calibrated target

Fig. 2. A typical CH4 plume measured during the monitoring phase (left panel) and analyzed through the AirCore (right panel).

M. Lopez et al. / Atmospheric Environment 164 (2017) 280e288

283

February 17 to February 25, 2017. The locations of the plumes evaluated for source signature characterization are indicated with different colored circles. Zoomed-in mapping areas around Edmonton, Lloydminster and Esther are also included. 3.1. Isotopic CH4 signatures of beef cattle and landfill

Fig. 3. A typical Keeling plot obtained by monitoring mode (red triangles), 1 s data in replay mode (green symbols) and 10 s data in replay mode (blue dots). AC stands here for AirCore. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

signal from the plume being vented through the exhaust instead of being analyzed (see Fig. 1). The slower flow rate used in replay mode extends the analysis time and results in higher CH4 values. The linear interpolation of the Keeling plot is then more accurate given the larger DCH4 (difference between background and maximum value) and the larger number of analysis points obtained during replay mode. This is illustrated by the Keeling plot in Fig. 3: the uncertainty on the intercept calculated from monitoring mode data is 10 times larger than the uncertainty calculated from replay mode data (1 s data and 10 s average data). The 10 s average data are used to compute isotopic signatures in the rest of the study. Note that no isotopic fractionation was observed during the analysis of the AirCore. The vehicle was parked during replay because the d13CH4 measurements are noisier when the vehicle is in motion and the analyzer is vibrating. The same analysis was done for the C2H6/CH4 ratio. This ratio is also used to correct interference on the d13CH4 measurements (see section 2.3). 2.5. Flask-air samples The d13CH4 and C2H6/CH4 ratio of pipeline natural gas was measured in Edmonton in winter 2014. Eight flasks were sampled at the Concordia University of Edmonton from a gas tap connected to the natural gas distribution system. The gas tap was open in a closed room for a few seconds to allow the natural gas levels to build up in the room. Methane in room air was monitored continuously using a CRDS. Room air was sampled into flasks when CH4 levels dropped from 10 to 3 mmol mol1. The 8 flasks were analyzed for CH4 at NOAA (Boulder, CO, USA) (Dlugokencky et al., 2015), and C2H6 at INSTAAR (Boulder, CO, USA), using gas chromatography (Helmig et al., 2014). The flasks were measured for d13CH4 (White et al., 2016) at INSTAAR (Boulder, CO, USA), using CFIRMS. 3. Results and discussion In total, 36 plumes were analyzed in replay mode and subjected to the following data filtering criteria: 1) the plume could be attributed to a specific point source based on visual cues and wind direction; 2) the error on Keeling plot d13CH4 determinations was 2‰. After filtering, data from 21 plumes were used to establish d13CH4 signatures from four main CH4 source categories: natural gas industry (n ¼ 8), oil industry (n ¼ 8), beef cattle feedlots (n ¼ 3) and a landfill (n ¼ 2). Fig. 4 shows the location of the individual selected plumes along with a survey mapping of the in situ CH4 measurements conducted over the entire campaign period of

Beef cattle and the landfills emit CH4 through a similar microbial fermentation process (Coleman et al., 1995; Levin et al., 1993). This process involves strong isotopic fractionation leading to CH4 emissions depleted in 13C. Prior to its atmospheric emission, the isotopic composition of the CH4 can also be modified by a partial bacterial oxidation (Born et al., 1990; Whiticar, 1999.) Methane emissions from beef cattle are either directly eructated by the cattle or generated in organic waste. The isotopic signature of CH4 emitted by cattle eructation strongly depends on diet. According to Levin et al. (1993), d13CH4 values range between 65.1 ± 1.7‰ for cattle fed on a 100% C3 diet to 55.6 ± 1.4‰ for cattle fed on 60e80% C4 diet. The CH4 signature of manure generated in the feedlot can vary from 73.9 ± 0.3‰ (liquid manure) to 45.5 ± 1.3‰ (manure pile), the manure representing approximately 20% of the total CH4 emissions by the ruminants (Levin et al., 1993). Three plumes were sampled and analyzed downwind of beef cattle feedlots in Alberta: two were located south of Calgary (High River e feedlot 1) and one was located east of Calgary (Strathmore e feedlot 2) (see the purple circle in Fig. 4). The average isotopic signature derived from the High River feedlots was 64.2 ± 1.3‰ and the one from the Strathmore feedlot was 69.1 ± 2.0‰. The estimated d13CH4 values indicate that the cattle were most probably fed with a 100% C3 diet in the 2016 winter. However, the isotopic signatures might have a seasonal cycle depending on seasonal changes in cattle diets. Active landfill emits CH4 from ventilation pipes and from overlying soil. Plumes measured downwind of the Edmonton landfill had CH4 enhancements of up to 10 mmol mol1. Two of these plumes were sampled and analyzed via the AirCore seven days apart, on the 18th and the 25th of February. Isotopic analyses on these days were consistent at 55.5 ± 0.2‰ and 55.1 ± 0.5‰. The measured d13CH4 values are consistent with microbial methanogenesis via acetate fermentation, and are within range of previous measurements from USA (Chanton and Liptay, 2000; Coleman et al., 1995; Liptay et al., 1998) and Europe (Bergamaschi et al., 1998b, Zazzeri et al., 2015). Note that the d13CH4 values are 13C enriched when CH4 is oxidized by microbes in the soil, which could lead to a d13CH4 seasonal cycle with more depleted values in winter than in summer (Coleman et al., 1981; Whiticar, 1999). The cattle feedlot and landfill isotopic signatures presented above were derived from plume measurements. Thus, they are not representative of a single source but they integrate the different processes involved in CH4 production, i.e., cattle eructation and manure fermentation for the feedlots. These “integrated” isotopic signatures are essential for source apportionment from long or short term atmospheric d13CH4 measurements. 3.2. Distinct CH4 signatures associated with fossil fuel production Methane is the principal component of natural gas, which can have microbial or thermogenic origins. Both types of natural gas are produced as oil-associated or non-associated gas throughout Alberta. Microbial gas is formed in relatively shallow geological formations through breakdown of organic matter in rock or through microbial consumption of hydrocarbons. The latter process is particularly important in the bitumen deposits of the Cold Lake, Athabasca and Peace River oil sands regions extending

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Fig. 4. Survey of in situ CH4 measurements conducted in Alberta, Canada, from February 17 to February 25, 2017 using a mobile platform (A). The locations of the plumes evaluated for source signature characterization are indicated with the colored circles: the light blue circles show the locations of the three gas plants (GP), the two gas wellheads (WH) and the compressor station (CS). The two feedlots are indicated with purple circles. Figure (B) shows a zoomed-in mapping of the Edmonton area. The landfill on the west side of Edmonton is indicated with a green star. Two CH4 plumes emanating from this landfill were characterized separately. The first evaluation occurred on 18th of February during an eastern wind flow condition (the plume was sampled west of the landfill) and the second 25th of February, occurred when the wind direction was originating from the south-west. For this latter case, the plume was sampled on the northeast side of the landfill. Both these plumes are shown within the green circle. The two plumes originating from the natural gas distribution system (NGD) were sampled at the same location, within the centre core of the city. The location of the NGD samples are shown within the blue circle. The plume source originating from the oil storage tanker (tank 1), shown within the red circle, is located in the Eastern side of the city. Figures (C) and (D) zoom-in on Lloydminster and Esther, respectively. Individual source characterizations were conducted on four oil storage tanks and are shown using red circles: two are located on the south-eastern side of Lloydminster and two are located near Esther, respectively 2.5 km south and 22 km northeast of the Esther measurement station. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

approximately between Lloydminster, Fort McMurray and Grand Prairie, and in the heavy oil fields around the Lloydminster area (Jones et al., 2008; Larter et al., 2006). Natural gas of microbial origin contains trace amounts (< 0.05%) of C2þ alkanes and d13CH4 values < 60‰ (Schoell, 1983). Thermogenic gas is generated in deeper geological formations under heat and pressure. Thermogenic gas typically contains > 2% C2þ alkanes, and d13CH4 values > 55‰ (Schoell, 1983). The C2H6/CH4 ratio decreases and d13CH4 increases with the level of thermal maturity. Thus, owing to a wide range of C2H6/CH4 and d13CH4 signatures among different types and maturity of natural gas, measurements of C2H6/CH4 and d13CH4 in air can be used to trace the origin of natural gas CH4 emissions to specific formations and/ or oil and gas-producing infrastructure. Fig. 4 shows that most of the CH4 plumes associated with natural gas are located in northwest Alberta, along the Canadian Rockies, whereas the CH4 plumes associated with oil production were sampled between Lloydminster and Esther, in heavy oil fields. Point-source CH4 emissions for the natural gas and oil industry include: 1) Surface casing vent flow or leaks at well heads (Rowe and Muehlenbachs, 1999b); 2) Well pad equipment, including pneumatic controllers, flow lines, separators, combustors, flares, and liquids (water, condensate and oil) storage tanks (Allen et al.,
tron et al., 2014); 3) Gas processing and compressor sta2013; Pe tions; 4) Transmission pipelines; and 5) Refineries and upgrader

facilities (Chambers et al., 2008). Note that these point-source emissions can be intentional (e.g, surface casing vent flow from wellheads, equipment maintenance, safety requirements) or unintentional (e.g., leaking wellheads, faulty equipment, incomplete combustion) (Lyon et al., 2016). 3.2.1. Thermogenic natural gas Samples of pipeline natural gas collected in flasks in Edmonton ranged from 10 to 3 mmol mol1 CH4 and 169.5 to 52.3 nmol mol1 C2H6. Keeling plots of d13CH4 vs. 1/CH4 (n ¼ 8) indicate 46.1 ± 0.1‰ for the isotopic signature for the pipeline natural gas. A plot of CH4 versus C2H6 is non-linear for C2H6 > 125 nmol mol1 which could be attributed to a non-linearity in the C2H6 measurement method (see part 2.5). Flasks having C2H6 > 125 nmol mol1 (n ¼ 3) are removed from the plot. Results show a C2H6/CH4 ratio for pipeline natural gas in Edmonton of 0.04 ± 0.01. Together, these data indicate a thermogenic origin for pipeline natural gas. During the mobile campaign, natural gas plume emission signatures were measured at three gas plants (GP), two gas wellheads (WH) and one gas compressor station (CS). The gas plants and gas wellheads were located in northwest Alberta, along the Canadian Rocky Mountains (Fig. 4). The compressor station was located 3.5 km east of the town of McNeill (AB), in southeast Alberta on the border with Saskatchewan (Fig. 4). In addition, two plumes from

M. Lopez et al. / Atmospheric Environment 164 (2017) 280e288

the natural gas distribution (NGD) system were sampled in Edmonton. The four types of natural gas facilities had d13CH4 signatures that ranged from 41.7 ± 0.7‰ to 49.7 ± 0.7‰ and C2H6/ CH4 ratios that ranged from 0.03 ± 0.01 to 0.12 ± 0.01 (Table 1). A plot of d13CH4 vs C2H6/CH4 from individual sample plumes associated with the natural gas industry is shown in Fig. 5. The colored lozenges are the averaged signatures calculated for the defined sources: the mean d13CH4 and C2H6/CH4 are respectively 49.1 ± 0.6‰ and 0.1 for the wellheads, 43.3 ± 1.2‰ and 0.08 for the gas plants, and 46.2 ± 0.4‰ and 0.05 for the natural gas distribution network. Note that the determination of natural gas distribution system signatures are in net agreement with the signatures measured from the gas tap in Edmonton in winter 2014, demonstrating the robustness of the sampling and analytical methods used during the mobile campaign. The enriched d13CH4 values and the level of C2H6 (Fig. 5) indicate a thermogenic origin for natural gas plumes measured in Alberta. The range of d13CH4 and C2H6/CH4 signatures for the gas plants and wellheads can be attributed primarily to differences in thermal maturity and d13C of source rocks feeding the hydrocarbon reservoirs from which these specific gases were sourced. Because of this variability, additional measurements would be needed to more fully characterize the d13CH4 and C2H6/CH4 signatures of natural gas plumes across the province. By contrast, NGD system gas should represent an integrated average of producing gas reservoirs in Alberta. Pipeline natural gas in Edmonton and the compressor station plume in SW Alberta had identical (within measurement uncertainty) d13CH4 signatures, which approximate the average plume d13CH4 signatures from the three gas plants and two wellheads. Thus, for the purposes of regional scale CH4 emissions attribution (section 3.3), we characterize natural gas as having a d13CH4 signature of around 46‰. Note that, whereas processing has little effect on natural gas d13CH4 signatures, it has an obvious large impact on C2H6/CH4 ratios as C2H6 is stripped from the gas stream prior to distribution, as shown by values of 0.01e0.05 for the pipeline and compressor station gas, to 0.07e0.12 for the gas plants and wellheads (Fig. 5). 3.2.2. CH4 associated with oil production During the mobile campaign, CH4 emissions from five oil storage tanks were characterized. These emissions are the product of volatilization when oil depressurizes upon reaching ambient conditions above-ground; for safety reasons, the volatiles are vented intentionally. Emissions from other oil-related infrastructure such as well pads and refineries/upgraders were either not encountered or the specific point-sources were ambiguous. The finding that oil tanks were the only unambiguous source of oil-related CH4 emissions concurs with recent aerial surveys showing that oil storage tanks account for > 90% of CH4 emissions across seven oil producing basins in the USA (Lyon et al., 2016). The intercepts of the respective Keeling plots for oil tanks ranged between 60.6 ± 0.6‰ and 54.9 ± 2.9‰ (Table 2). The d13CH4 signature of Tank 1 (located close to a refinery

285

Fig. 5. d13CH4 from Keeling plots C2H6/CH4 ratios derived from different sources related to the gas industry. The dashed lines and the crosses are the respective average for each source.

Table 2 CH4 isotopic signature associated with oil storage tanks. Tank e Location area

Sampling date

d13CH4 (‰) signature

Tank Tank Tank Tank Tank

Feb Feb Feb Feb Feb

54.9 59.9 58.5 60.6 60.1

1 2 3 4 5

-

Edmonton Lloydminster Lloydminster Esther Esther

-

17 19 19 24 24

± ± ± ± ±

2.9 0.4 0.4 0.6 1.3

in Edmonton), derived from four individual plumes, averaged 54.9 ± 2.9‰. The d13CH4 signatures of the four other tanks were derived from single plume analysis. Tanks 2 and 3 (d13CH4 ¼ 59.9 ± 0.4‰ and 58.5 ± 0.4‰, respectively) are located within 500 m of each other, in the Lloydminster area. Tanks 4 and 5 (d13CH4 ¼ 60.6 ± 0.6‰ and 60.1 ± 1.3‰, respectively) are located within 20 km of each other, near the unincorporated community of Esther, 170 km south of Lloydminster. Note that Tank 4 is located 2.4 km south of the Canadian continuous greenhouse gases measurement station at Esther. No C2H6 signals above the CRDS detection limit (approximately 0.1 mmol mol1) were measured downwind of these five characterized oil storage tanks. Tanks 2, 3, 4 and 5 had virtually identical d13CH4 signatures of approximately 60‰. This d13CH4 value, along with a lack of detectable C2H6, is characteristic of microbial CH4. Lloydminster and Esther are located in an actively producing heavy oil field. The heavy oil is a product of biodegradation promoted by the relatively shallow depth of the Mannville Group formations from which the oil is produced. Microbial CH4 is generated during biodegradation (Jones et al., 2008; Larter et al., 2006), thus accounting for the oil tank d13CH4 signatures. Tank 1 had a less negative d13CH4 signature. This tank is located in the refinery area of Edmonton, thus its d13CH4 signature may represent the integrated average of oils with different d13CH4 signatures.

Table 1 d13CH4 and C2H6/CH4 ratios associated with natural gas industry. Source

Sampling data

d13CH4 (‰) signature

C2H6/CH4

Distribution network Gas Plant 1 Gas Plant 2 Gas Plant 3 Wellhead 1 Wellhead 2 Compressor station

Feb Feb Feb Feb Feb Feb Feb

45.9 44.4 43.9 41.7 48.5 49.7 46.7

0.05 0.09 0.07 0.08 0.08 0.12 0.03

-

18 21 21 21 21 21 24

± ± ± ± ± ± ±

0.2 & 46.6 ± 0.3 0.9 1.5 0.7 0.6 0.7 0.7

± ± ± ± ± ± ±

0.01 & 0.05 ± 0.01 0.01 0.01 0.01 0.01 0.01 0.01

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Fig. 6. Left panel: hourly CH4 measurements (in gray) together with flask-air measurements of CH4 (in pink) and associated d13CH4 (in blue) at Lac La Biche station during winter, 2009. The green area refers to Fig. 7. Right panel: Keeling plot of the CH4 and d13CH4 measurements at LLB station in winter 2009 showing mean isotopic signature of 59.5 ± 1.4‰ (R2 ¼ 0.87, N ¼ 11). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

3.3. Role of CH4 signatures for source discrimination The results from this work help to constrain d13CH4 and C2H6/ CH4 signatures from a range of oil and gas infrastructure point sources in Alberta. Using this knowledge, the atmospheric monitoring of CH4, C2H6 and d13CH4 can help distinguish the contribution of specific source types to atmospheric CH4 concentrations. For reference, background ambient air typically has a d13CH4 value of around 47‰ in the Northern Hemisphere (Dlugokencky et al., 2011). Deviations from the background level can be used to identify sources of CH4 present in the sampled air, which can be either enriched or depleted in 13C depending on the source. As an example, to illustrate how distinct signatures can be detected in the in-situ measurements, the left panel of Fig. 6 shows a short time series of hourly-averaged CH4 for Lac La Biche (LLB) station, Alberta for January 1, 2009 to February 28, 2009. In addition to the continuous measurements, quasi-weekly discrete air samples were collected in flasks. Flask-air was sampled in the afternoon and analyzed at NOAA for CH4 and at the University of Colorado by INSTAAR (Institute of Arctic and Alpine Research) for stable CH4 isotope analysis and VOCs (including C2H6). The LLB observational site is one of ECCC's network of 20

stations to accurately measure hourly atmospheric concentrations of carbon dioxide, methane and carbon monoxide from coastal, interior and arctic regions in Canada. The LLB site is at 54.0 N and 112.5 W in a region of peatlands and forest, approximately 200 km northeast of Edmonton, Alberta and 230 km due south of Fort McMurray, Alberta. The primary goal of the in-situ observational program at Lac La Biche is to apply data assimilation modelling techniques, together with long-term monitoring of CO2 and CH4, to independently quantify anthropogenic emissions for Alberta and neighboring provinces. Sample air is drawn from a sample line that extends to the top of a 50 m standalone steel tower. The two month CH4 time series in Fig. 6 shows broad increases in CH4 that range from 2 to 7 days, typical of synoptic variability. A close visual inspection of the CH4 isotope results (blue dots) show that the more negative values tend to coincide with these increases in methane and as such, may provide information on the regional source signals of the air masses that carry anthropogenic loadings of CH4 to the LLB site. Back-trajectories for this two month period at LLB were produced by the Canadian Meteorological Centre and calculated by the Canadian Meteorological Centre's Global Envi^ te
et al., 1998a; Co ^ te
et al., ronmental Multiscale (GEM) model (Co 1998b; D’amours, 1998). Analyses of these back-trajectories show that the episodic increases in CH4 are most often associated with air masses originating from the southern half of Alberta whereas, air masses originating from the north tend to show lower CH4 levels. Because the oil and gas industry accounts for around 75% of Alberta's reported emissions and with most of the refineries being located south of LLB, this source likely accounts for the variability observed in CH4 at LLB. To illustrate, 10 days of hourly CH4 measurements in winter 2009 are plotted together with their respective 24 h back-trajectories in Fig. 7. The chosen period is highlighted in green in Fig. 6. It is clear from Fig. 7 that the large CH4 values observed in the left panel (red dots) are associated to backtrajectories coming from the southern part of the region (right panel). The Keeling plot of d13CH4 vs 1/CH4 from the flask-air samples, plotted in the right panel of Fig. 6, shows a mean isotopic source signature of 59.5 ± 1.4‰. This result, along with the C2H6/CH4 ratio of 0.010 ± 0.002 strongly indicates that the CH4 sources being observed at LLB in winter are specific to CH4 emissions from the oil industry south of Lac La Biche (see part 3.2) and not fugitive emissions from natural gas exploration or refining, which is generally associated with heavier d13CH4 and wetter C2H6/CH4 ratios (Table 1). Specifically, the CH4 enhancements are chemically and isotopically consistent with plumes emanating from oil storage

Fig. 7. Left panel: hourly CH4 measurements over 10 days at Lac La Biche station. Right panel: 24 h-backward trajectories associated with hourly measurements at LLB (yellow circle). The color scale is the same for both panels. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

M. Lopez et al. / Atmospheric Environment 164 (2017) 280e288

tanks in the Lloydminster and Esther areas. Co-located with these storage tanks are oil wells, 15% of which have issues with gas leaking or venting (Watson and Bachu, 2009). The leaking/venting gas is sourced from Colorado Group formations overlying the heavy oil-producing Mannville Group. These low thermal maturity Colorado group gases also have d13CH4 signatures of around 60‰ (Rowe and Muehlenbachs, 1999a), thus representing another possible source for the CH4 enhancements at LLB. This knowledge of the type of CH4 sources impacting the methane observations at LLB is a key parameter, particularly in modelling activities that attempt to quantify anthropogenic emissions because the model optimization processes can focus on methane emissions from this single source. In summer, although not shown here, the methane isotope data along with trajectory analysis show evidence that summer atmospheric concentrations are also heavily influenced from contributions from wetlands sources. In short, the example shown for LLB, demonstrates that the discrimination of CH4 sources provided by isotopic measurements may be used to improve and better constrain prior source map distributions. And in turn, atmospheric modelling may help reduce uncertainties and produce better estimates in the methane budget (Bousquet et al., 2006; Dlugokencky et al., 2011). 4. Conclusion During February 17, 2016 to February 25, 2016, a mobile analytical system was used to measure continuous CH4 isotopologues and C2H6 to identify various CH4 sources in Alberta, Canada. Although broad signals of CH4 are typically required to characterize the CH4 source signatures, the coupling of a CRDS with an AirCore, permitted the analysis on smaller signals. This study showed the utility of using a mobile atmospheric measurement system to derive the signatures (d13CH4 and C2H6/CH4 ratio) of a landfill, of beef cattle feedlots and signatures from oil and natural gas activities. The signatures of an Edmonton landfill and beef cattle feedlots located in the southern part of Alberta were characterized from 2 and 3 plumes, respectively. The derived d13CH4 signatures are typical of microbial fermentation processes as they showed depleted values of 66.7 ± 2.4‰ for the feedlots and 55.3 ± 0.2‰ for the Edmonton landfill. It should be noted that these signatures typically have a seasonal cycle but the signatures from this study are for winter only. The CH4 signatures from oil production activities were characterized from the venting of accumulated natural gas in oil storage tanks. In total, five plumes emanating from oil storage tanks were sampled across Alberta. The d13CH4 signatures of these plumes range from 54.9 ± 2.9‰ to 60.6 ± 0.6‰. These d13CH4 signatures, along with the absence of detectable C2H6, are characteristic of secondary microbial methanogenesis during biodegradation of heavy crude oil. The CH4 plumes originating from natural gas activities (gas wellheads, gas plant, natural gas distribution network and compressor station) have d13CH4 signatures ranging from 41.7 ± 0.7‰ to 49.7 ± 0.7‰. The C2H6/CH4 ratios measured in these plumes ranged from 0.12 ± 0.01 to 0.03 ± 0.01. The enriched isotopic signatures and C2H6/CH4 ratios indicates that these CH4 sources are thermogenic in origin. The characterization of CH4 signatures from various sources can play an important role in understanding different CH4 emission processes. As shown with the Lac La Biche flask-air sampling record, with knowledge of the various source signatures in Alberta, it can be safely assumed that methane emissions from oil activities are impacting this site more than any other source in the region. In modelling activities, this understanding of sources impacting the site aids in the optimization process since only the known source types impacting the site can be constrained.

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