Isotopic signatures of anthropogenic CH4 sources in Alberta, Canada

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Isotopic signatures of anthropogenic CH4 sources in

Alberta, Canada

M. Lopez, O.A. Sherwood, E.J. Dlugokencky, R. Kessler, L. Giroux, D.E.J.

Worthy

To cite this version:

M. Lopez, O.A. Sherwood, E.J. Dlugokencky, R. Kessler, L. Giroux, et al.. Isotopic signatures of

anthropogenic CH4 sources in Alberta, Canada. Atmospheric Environment, Elsevier, 2017, 164,

pp.280-288. �10.1016/j.atmosenv.2017.06.021�. �hal-03226721�

Isotopic signatures of anthropogenic CH

₄

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 CH4source plumes in Alberta.

CH4isotopic signatures were accurately derived using an AirCore coupled to a CRDS instrument.

The enriched isotopic values of CH4from the natural gas industry show thermogenic origin.

The depleted isotopic values of CH4from the oil industry show microbial origin.

Isotopic signature information will have a profound implication on modelling activities for CH4emission estimates in Canada.

a r t i c l e i n f o

Article history:

Received 13 February 2017 Received in revised form 9 June 2017

Accepted 10 June 2017 Available online 12 June 2017 Keywords:

Methane isotopic signature AirCore

Oil and natural gas industry Plume mapping

a b s t r a c t

A mobile system was used for continuous ambient measurements of stable CH4isotopes (12CH4and 13_CH

4) and ethane (C2H6). This system was used during a winter mobile campaign to investigate the CH4

isotopic signatures and the C2H6/CH4ratios of the main anthropogenic sources of CH4in the Canadian

province of Alberta. Individual signatures were derived fromd13CH4and C2H6measurements in plumes

arriving from identifiable single sources. Methane emissions from beef cattle feedlots (n ¼ 2) and landfill (n¼ 1) hadd13_CH

4signatures of66.7 ± 2.4‰and55.3 ± 0.2‰, respectively. The CH4emissions

associated with the oil or gas industry had distinctd13CH4signatures, depending on the formation

process. Emissions from oil storage tanks (n¼ 5) hadd13CH4signatures ranging from54.9 ± 2.9‰

to60.6 ± 0.6‰and non-detectable C2H6, characteristic of secondary microbial methanogenesis in

oil-bearing reservoirs. In contrast, CH4emissions associated with natural gas facilities (n¼ 8) hadd13CH4

signatures ranging from41.7 ± 0.7‰to49.7 ± 0.7‰and C2H6/CH4molar ratios of 0.10 for raw natural

gas to 0.04 for processed/refined natural gas, consistent with thermogenic origins. These isotopic sig-natures and C2H6/CH4ratios have been used for source discrimination in the weekly atmospheric

measurements of stable CH4isotopes over a two-month winter period at the Lac La Biche (LLB)

mea-surement station, located in Alberta, approximately 200 km northeast of Edmonton. The average signature of59.5 ± 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/).

1. Introduction

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

2016. This agreement aims to reduce the risks and impacts of climate change by limiting the increase in global temperatures to less than 2C above pindustrial levels through aggressive re-ductions in greenhouse gas emissions. With a radiative forcing of 0.48± 0.05 W m2from 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 CH4emissions are attributed to

human activities (Bousquet et al., 2006). As such, mitigation of CH4

* Corresponding author. CEA LSCE - Orme des Merisiers, 91191 Gif-sur-Yvette, France.

E-mail address:morgan.lopez@lsce.ipsl.fr(M. Lopez).

1 _{Now at Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Unit
e}

mixte CNRS-CEA-UVSQ, 91191 Gif-sur-Yvette, France.

Contents lists available atScienceDirect

Atmospheric Environment

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a t m o s e n v

http://dx.doi.org/10.1016/j.atmosenv.2017.06.021

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 CH4emissions 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 CH4in 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% provin-cially and exceeds 15% in an area between Edmonton and Lloyd-minster (Watson and Bachu, 2009). Recent studies suggest that fugitive emissions of CH4 can be underreported by“bottom up”

emission inventories (Karion et al., 2013;Miller et al., 2013; P
etron

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 distri-butions 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 in-dependent approaches capable of assessing CH4 emissions from

multiple and unknown point sources over large/regional areas. Stable carbon isotopic measurements of atmospheric CH4

(

d

13_CH

4) provide a constraint on source attribution since

d

13CH4

signatures vary by source: thermogenic CH4associated with oil and

gas production is relatively enriched in13C whereas microbial CH4

is strongly depleted in13C (Whiticar, 1990). Measurement of at-mospheric

d

13CH4is analytically difficult and most of the available

long time series are fromflask-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

d

13CH4 with precision

better than 1

‰

over 1 min averages. This precision level has been shown to be sufficient in deriving single isotopic source signatures in various other studies (Eyer et al., 2016; R€ockmann et al., 2016;

Rella et al., 2015; Assan et al., 2017). Multiple papers have inte-grated

d

13CH4data in atmospheric models to improve constraints

on CH4source 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 re-quires 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 CH4formation

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

d

13CH4signatures representing

the major CH4source categories.

We describe a mobile system designed for continuous mea-surements of

d

13CH4 in the plume of the main anthropogenic

sources of CH4. Isotopic signatures of CH4related 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

d

13_CH

4signatures improve CH4source attribution at a long

term monitoring station at Lac La Biche, Alberta. 2. Method and instrumentation

The results presented in this study were obtained during an intensive measurement campaign in Alberta, Canada from February

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 CH4abundance in units of mole fraction and

isotopologues (12CH4and13CH4) in air were performed by cavity

ring down spectroscopy (CRDS - Picarro G2201i) at 1 Hz. The CRDS also measures CO2, C2H6and H2O mole fractions in air. For a more

detailed description of the instrument seeRella 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) wasfixed 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 con-tent at the laboratory. This sample technique has been adapted and validated byRella et al. (2015)for mobile atmospheric measure-ments. 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 min1which is regulated byflow 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

Fig. 1. Schematic of the mobile measurement system used for atmospheric measure-ment of CH4,d13CH4and C2H6.

#1, following the green path shown in Fig. 1-A. Because of the known cross-sensitivity of water vapor with stable CH4isotopes,

the wet measurements performed in monitoring mode are not used for further analysis. The pump pressures the system to 20 psi above atmospheric pressure. Twofilters of 40 and 140

m

m pore size are placed in series after pump #1 to protect the system from dust (not shown inFig. 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 continuouslyflushed with the sample at a flow rate of 25 mL min1. An open end and aflow controller (FC1) are installed between the instrument and the valve #6 (seeFig. 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 min1and 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 CH4plume 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

m

mol mol1above the background), the system is manually switched to replay mode in which the contents of the AirCore is pushed through the drier (magnesium perchlorate car-tridge) to the analyzer using a separate pump (shown as pump #2 inFig. 1), and at a slowerflow rate of 40 mL min1_{. The slowerflow}

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. CH4isotopic signatures

Isotopic data are reported in delta notation (Eq.(1)) where R is the ratio of13C to12C. Values are expressed in

‰

and referenced to the Vienna Pee Dee Belemnite (VPDB) scale.

d

¼ _R sample Rstandard 1  *1000 ‰ (1)

Point source

d

13C 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 CH4sources (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:

d

13Cm¼ Cb* 

d

13Cb

d

13Cs  Cm þ

d

13_C s (2)

Thus, the intercept of the plot

d

13CH4vs 1/CH4from an

atmo-spheric sample gives the mean isotopic signature of CH4sources.

2.3. Instrument calibration and performance

Preliminary tests of the CRDS system in the laboratory showed that CH4,

d

13CH4 and C2H6 had linear responses in ranges of

1800e15000 nmol mol1 CH4,65.8 to 24.8

‰ d

13CH4and 0 to

15 000 nmol mol1C2H6. The CH4measurements 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

gas passed through the system. The system has an estimated un-certainty better than 0.8 nmol mol1and a 1 s repeatability of 0.2 nmol mol1(standard deviation at 1 sigma).

Calibration of the

d

13_CH

4measurements was based on a

two-point calibration against tanks with

d

13CH4 values of 54.4

‰

(7312.4 nmol mol1CH4) and38.8

‰

(7763.2 nmol mol1CH4).

These tank

d

13CH4values were measured at the Institute of Arctic

and Alpine Research (INSTAAR, University of Colorado, Boulder, USA) using continuousflow isotope ratio mass spectrometry (CF-IRMS) (Miller et al., 2002). Because the INSTAAR system is designed to measure small variations of less than 1

‰

in the

d

13CH4 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 mea-surements beyond the range of ambient background of approximately 47

‰

, as assessed from measurements of four independently calibrated working standards (Isometric In-struments, Victoria, B.C., Canada) ranging from66.5 to 23.9

‰

, is generally within 1

‰

. Given the large range in point source

d

13CH4

values encountered in this study, we consider± 1

‰

uncertainty to be adequate.

The CRDS system was calibrated every two days for

d

13CH4and

showed a short term repeatability (1 min averages) better than 0.2

‰

for CH4greater than 7000 nmol mol1. The C2H6

measure-ments were calibrated in the laboratory before and after the campaign with a single tank supplied by Praxair containing 15

m

mol mol1of C2H6. Instrument response did not change

be-tween the two calibrations and the measured short term repeat-ability was 0.175

m

mol mol1at 15

m

mol mol1.

Rella et al. (2015)report interference between

d

13_CH

4and C2H6:

the instrument reports heavier

d

13CH4when the sample contains

C2H6. Following the protocol described byRella et al. (2015), a

correction factor of 36.6

‰ m

mol mol1CH4(

m

mol mol1C2H6)1

was systematically applied on the

d

13CH4 measurements when

C2H6exceeded 0.2

m

mol mol1.

2.4. AirCore analysis

As explained previously, the AirCore is used to extend the analysis time of a CH4plume. On the left panel ofFig. 2, a typical

CH4plume measured in monitoring mode downwind an identified

CH4 source is shown. On the right panel, the CH4 signal was

re-analyzed 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 higherflow rate used in monitoring mode results in most of the CH4

Fig. 2. A typical CH4plume measured during the monitoring phase (left panel) and

signal from the plume being vented through the exhaust instead of being analyzed (see Fig. 1). The slowerflow rate used in replay mode extends the analysis time and results in higher CH4values.

The linear interpolation of the Keeling plot is then more accurate given the larger

D

CH4 (difference between background and

maximum value) and the larger number of analysis points obtained during replay mode. This is illustrated by the Keeling plot inFig. 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

d

13CH4measurements are noisier when the vehicle is

in motion and the analyzer is vibrating. The same analysis was done for the C2H6/CH4ratio. This ratio is also used to correct interference

on the

d

13CH4measurements (see section2.3).

2.5. Flask-air samples

The

d

13CH4 and C2H6/CH4 ratio of pipeline natural gas was

measured in Edmonton in winter 2014. Eightflasks 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 intoflasks when CH4 levels dropped from 10 to 3

m

mol mol1. The 8flasks were

analyzed for CH4at NOAA (Boulder, CO, USA) (Dlugokencky et al.,

2015), and C2H6 at INSTAAR (Boulder, CO, USA), using gas

chro-matography (Helmig et al., 2014). Theflasks were measured for

d

13_CH

4(White et al., 2016) at INSTAAR (Boulder, CO, USA), using

CF-IRMS.

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

d

13_CH

4determinations was

2

‰

. Afterfiltering, data from 21 plumes were used to establish

d

13CH4signatures from four main CH4source categories: natural

gas industry (n¼ 8), oil industry (n ¼ 8), beef cattle feedlots (n ¼ 3) and a landfill (n ¼ 2).Fig. 4shows 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

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 CH4signatures of beef cattle and landfill

Beef cattle and the landfills emit CH4through 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 CH4can 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 CH4emitted by cattle eructation strongly depends on diet.

Ac-cording to Levin et al. (1993),

d

13_CH

4 values range between

65.1 ± 1.7

‰

for cattle fed on a 100% C3 diet to55.6 ± 1.4

‰

for cattle fed on 60e80% C4 diet. The CH4signature of manure

gener-ated in the feedlot can vary from73.9 ± 0.3

‰

(liquid manure) to45.5 ± 1.3

‰

(manure pile), the manure representing approxi-mately 20% of the total CH4emissions 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 inFig. 4). The average isotopic signature derived from the High River feedlots was64.2 ± 1.3

‰

and the one from the Strathmore feedlot was69.1 ± 2.0

‰

. The estimated

d

13_CH

4values indicate that the cattle were most

prob-ably 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 CH4from ventilation pipes and from

over-lying soil. Plumes measured downwind of the Edmonton landfill had CH4 enhancements of up to 10

m

mol 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 at55.5 ± 0.2

‰

and55.1 ± 0.5

‰

. The measured

d

13CH4 values are consistent with microbial

methano-genesis 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

d

13CH4values are13C enriched

when CH4is oxidized by microbes in the soil, which could lead to a

d

13_CH

4seasonal 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

d

13CH4measurements.

3.2. Distinct CH4signatures 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 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 thisfigure 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 C2þalkanes and

d

13CH4 values< 60

‰

(Schoell, 1983).

Ther-mogenic gas is generated in deeper geological formations under heat and pressure. Thermogenic gas typically contains> 2% C2þ

alkanes, and

d

13CH4values> 55

‰

(Schoell, 1983). The C2H6/CH4

ratio decreases and

d

13CH4 increases with the level of thermal

maturity. Thus, owing to a wide range of C2H6/CH4 and

d

13CH4

signatures among different types and maturity of natural gas, measurements of C2H6/CH4and

d

13CH4in air can be used to trace

the origin of natural gas CH4emissions to specific formations and/

or oil and gas-producing infrastructure.

Fig. 4shows that most of the CH4plumes associated with

nat-ural 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 oilfields. Point-source CH4 emissions for the natural gas and oil industry

include: 1) Surface casing ventflow 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., 2013; P
etron et al., 2014); 3) Gas processing and compressor sta-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 ventflow from wellheads, equipment maintenance, safety requirements) or un-intentional (e.g., leaking wellheads, faulty equipment, incomplete combustion) (Lyon et al., 2016).

3.2.1. Thermogenic natural gas

Samples of pipeline natural gas collected inflasks in Edmonton ranged from 10 to 3

m

mol mol1CH4and 169.5 to 52.3 nmol mol1

C2H6. Keeling plots of

d

13CH4 vs. 1/CH4 (n ¼ 8)

indicate46.1 ± 0.1

‰

for the isotopic signature for the pipeline natural gas. A plot of CH4 versus C2H6 is non-linear for C2H6 >

125 nmol mol1which could be attributed to a non-linearity in the C2H6measurement method (see part 2.5). Flasks having C2H6 >

125 nmol mol1(n¼ 3) are removed from the plot. Results show a C2H6/CH4ratio 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 sig-natures 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 Fig. 4. Survey of in situ CH4measurements 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 CH4plumes 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 thisfigure legend, the reader is referred to the web version of this article.)

the natural gas distribution (NGD) system were sampled in Edmonton. The four types of natural gas facilities had

d

13CH4

sig-natures that ranged from41.7 ± 0.7

‰

to49.7 ± 0.7

‰

and C2H6/

CH4ratios that ranged from 0.03± 0.01 to 0.12 ± 0.01 (Table 1).

A plot of

d

13CH4vs C2H6/CH4from 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

d

13CH4 and C2H6/CH4 are

respectively49.1 ± 0.6

‰

and 0.1 for the wellheads, 43.3± 1.2

‰

and 0.08 for the gas plants, and46.2 ± 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

d

13CH4values and the level of C2H6(Fig. 5) indicate

a thermogenic origin for natural gas plumes measured in Alberta. The range of

d

13CH4and C2H6/CH4signatures for the gas plants and

wellheads can be attributed primarily to differences in thermal maturity and

d

13_{C of source rocks feeding the hydrocarbon}

reser-voirs from which these specific gases were sourced. Because of this variability, additional measurements would be needed to more fully characterize the

d

13CH4and C2H6/CH4signatures 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)

d

13CH4 signatures, which approximate the average

plume

d

13CH4signatures from the three gas plants and two

well-heads. Thus, for the purposes of regional scale CH4 emissions

attribution (section3.3), we characterize natural gas as having a

d

13CH4signature of around46

‰

. Note that, whereas processing

has little effect on natural gas

d

13_CH

4signatures, it has an obvious

large impact on C2H6/CH4ratios as C2H6is 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. CH4associated with oil production

During the mobile campaign, CH4emissions fromfive oil storage

tanks were characterized. These emissions are the product of volatilization when oil depressurizes upon reaching ambient con-ditions 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

emis-sions concurs with recent aerial surveys showing that oil storage tanks account for> 90% of CH4 emissions across seven oil

pro-ducing basins in the USA (Lyon et al., 2016).

The intercepts of the respective Keeling plots for oil tanks ranged between60.6 ± 0.6

‰

and54.9 ± 2.9

‰

(Table 2). The

d

13_CH

4 signature of Tank 1 (located close to a refinery

in Edmonton), derived from four individual plumes, averaged54.9 ± 2.9

‰

. The

d

13CH4signatures of the four other

tanks were derived from single plume analysis. Tanks 2 and 3 (

d

13CH4 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 (

d

13CH4¼ 60.6 ± 0.6

‰

and60.1 ± 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 C2H6signals above the

CRDS detection limit (approximately 0.1

m

mol mol1) were measured downwind of thesefive characterized oil storage tanks. Tanks 2, 3, 4 and 5 had virtually identical

d

13CH4signatures of

approximately 60

‰

. This

d

13CH4 value, along with a lack of

detectable C2H6, is characteristic of microbial CH4. Lloydminster

and Esther are located in an actively producing heavy oilfield. 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 CH4is generated during biodegradation

(Jones et al., 2008; Larter et al., 2006), thus accounting for the oil tank

d

13CH4signatures.

Tank 1 had a less negative

d

13CH4signature. This tank is located

in the refinery area of Edmonton, thus its

d

13CH4signature may

represent the integrated average of oils with different

d

13CH4

signatures. Table 1

d13_CH

4and C2H6/CH4ratios associated with natural gas industry.

Source Sampling data d13_CH

4(‰) signature C2H6/CH4

Distribution network Feb - 18 45.9 ± 0.2 & 46.6 ± 0.3 0.05± 0.01 & 0.05 ± 0.01

Gas Plant 1 Feb - 21 44.4 ± 0.9 0.09± 0.01

Gas Plant 2 Feb - 21 43.9 ± 1.5 0.07± 0.01

Gas Plant 3 Feb - 21 41.7 ± 0.7 0.08± 0.01

Wellhead 1 Feb - 21 48.5 ± 0.6 0.08± 0.01

Wellhead 2 Feb - 21 49.7 ± 0.7 0.12± 0.01

Compressor station Feb - 24 46.7 ± 0.7 0.03± 0.01

Fig. 5.d13_CH

4from 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

CH4isotopic signature associated with oil storage tanks.

Tank e Location area Sampling date d13_CH

4(‰) signature

Tank 1 - Edmonton Feb - 17 54.9 ± 2.9 Tank 2 - Lloydminster Feb - 19 59.9 ± 0.4 Tank 3 - Lloydminster Feb - 19 58.5 ± 0.4 Tank 4 - Esther Feb - 24 60.6 ± 0.6 Tank 5 - Esther Feb - 24 60.1 ± 1.3

3.3. Role of CH4signatures for source discrimination

The results from this work help to constrain

d

13CH4and C2H6/

CH4 signatures from a range of oil and gas infrastructure point

sources in Alberta. Using this knowledge, the atmospheric moni-toring of CH4, C2H6and

d

13CH4can help distinguish the

contribu-tion of specific source types to atmospheric CH4concentrations. For

reference, background ambient air typically has a

d

13CH4value of

around _47

‰

in the Northern Hemisphere (Dlugokencky et al., 2011). Deviations from the background level can be used to iden-tify sources of CH4present 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 ofFig. 6shows a short time series of hourly-averaged CH4for 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 CH4and 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 54.0N and 112.5W 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 CO2and 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 CH4time series inFig. 6shows broad increases

in CH4that range from 2 to 7 days, typical of synoptic variability. A

close visual inspection of the CH4isotope 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 CH4to 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-ronmental Multiscale (GEM) model (C^ot
e et al., 1998a; C^ot
e et al.,

1998b; D’amours, 1998). Analyses of these back-trajectories show that the episodic increases in CH4are most often associated with air

masses originating from the southern half of Alberta whereas, air masses originating from the north tend to show lower CH4levels.

Because the oil and gas industry accounts for around 75% of Alberta's reported emissions and with most of the re_{fineries being} located south of LLB, this source likely accounts for the variability observed in CH4at LLB. To illustrate, 10 days of hourly CH4

mea-surements in winter 2009 are plotted together with their respective 24 h back-trajectories inFig. 7. The chosen period is highlighted in green inFig. 6. It is clear from Fig. 7 that the large CH4 values

observed in the left panel (red dots) are associated to back-trajectories coming from the southern part of the region (right panel).

The Keeling plot of

d

13CH4vs 1/CH4from theflask-air samples,

plotted in the right panel ofFig. 6, shows a mean isotopic source signature of59.5 ± 1.4

‰

. This result, along with the C2H6/CH4

ratio of 0.010± 0.002 strongly indicates that the CH4sources being

observed at LLB in winter are speci_{fic to CH}4emissions 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

d

13_CH

4and wetter C2H6/CH4

ra-tios (Table 1). Specifically, the CH4enhancements are chemically

and isotopically consistent with plumes emanating from oil storage Fig. 6. Left panel: hourly CH4measurements (in gray) together withflask-air

mea-surements of CH4(in pink) and associatedd13CH4(in blue) at Lac La Biche station

during winter, 2009. The green area refers toFig. 7. Right panel: Keeling plot of the CH4

and d13_CH

4 measurements at LLB station in winter 2009 showing mean isotopic

signature of59.5 ± 1.4‰(R2_{¼ 0.87, N ¼ 11). (For interpretation of the references to}

colour in thisfigure legend, the reader is referred to the web version of this article.)

Fig. 7. Left panel: hourly CH4measurements over 10 days at Lac La Biche station. Right panel: 24 h-backward trajectories associated with hourly measurements at LLB (yellow

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 Colo-rado group gases also have

d

13CH4 signatures of around 60

‰

(Rowe and Muehlenbachs, 1999a), thus representing another possible source for the CH4enhancements 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 emis-sions 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 CH4sources 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

iso-topologues and C2H6 to identify various CH4 sources in Alberta,

Canada. Although broad signals of CH4 are typically required to

characterize the CH4source 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 (

d

13_CH

4and 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

d

13CH4 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 CH4signatures from oil production

activ-ities 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

d

13_CH

4signatures

of these plumes range from54.9 ± 2.9

‰

to60.6 ± 0.6

‰

. These

d

13CH4signatures, along with the absence of detectable C2H6, are

characteristic of secondary microbial methanogenesis during biodegradation of heavy crude oil. The CH4plumes originating from

natural gas activities (gas wellheads, gas plant, natural gas distri-bution network and compressor station) have

d

13CH4 signatures

ranging from41.7 ± 0.7

‰

to49.7 ± 0.7

‰

. The C2H6/CH4ratios

measured in these plumes ranged from 0.12± 0.01 to 0.03 ± 0.01. The enriched isotopic signatures and C2H6/CH4ratios indicates that

these CH4sources are thermogenic in origin.

The characterization of CH4signatures from various sources can

play an important role in understanding different CH4 emission

processes. As shown with the Lac La Bicheflask-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.

Acknowledgement

The authors would like to extend their gratitude to Drs. Karen McDonald and John Washington at the Concordia University of Edmonton for providing access to a laboratory to conduct natural gas signature characterization. These initial results forged the di-rection of the research campaign. We also thank Bruce Vaughn and Sylvia Michel (INSTAAR Stable Isotope Lab) for

d

13C analyses.

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