Article pubs.acs.org/JPCA
Methane Isotope Instrument Validation and Source Identification at Four Corners, New Mexico, United States Caleb Arata,† Thom Rahn, and Manvendra K. Dubey* Earth System Observations, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ABSTRACT: Measurements of δ13CH4 and CH4 concentration were made at a field site in Four Corners, New Mexico (FC), where we observed large sustained CH4 enhancements (2−8 ppm peaks for hours) during nocturnal inversions. Potential sources of this large CH4 signal at FC include (1) fugitive emissions from coal mining and gas processing that are thermogenic and isotopically 13C enriched relative to background atmosphere and (2) emissions from agriculture, ruminants, landfills, and coalbed biogenic methane that are13C depleted relative to background atmosphere. We analyze our measurements of methane concentration and δ13C during spring and summer of 2012 to identify fugitive methane sources. We find CH4 plumes that are both enriched and depleted in 13C relative to CH4 in background air. Keeling plots show a continuum of δ13C source compositions between −40‰ and −60‰ that are consistent with thermogenic and biogenic sources. The Picarro Mobile Methane Investigator (PMMI), a mobile δ13CH4 instrument platform, was deployed in the spring of 2013 and used to verify the isotopic enrichment of coal bed methane in the region. We combine our results with meteorological data to spatially separate these sources in the Four Corners regions. Using CO and CO2 data, along with meteorological data, we propose that the high methane concentration events ([CH4] > 3.5 ppm) are from both thermogenic and biogenic methane released from coal beds.
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INTRODUCTION Methane is a greenhouse gas 25 times more potent than carbon dioxide over a 100 year period, with a concentration in the atmosphere that is increasing.1 With an ambient background concentration of 1.8 ppm, methane is the most abundant green house gas after CO2 (excluding H2O).2,3 Methane has an atmospheric lifetime of 10 years.4 Sources of methane include biomass burning, fossil fuels, ruminants, and wetlands. Isolating these sources is key to understanding this increase in methane growth. Before the modern industrial era, global background atmospheric methane concentrations did not exceed 773 ppb in the last 650 000 years and never rose above 600 ppb in the last 420 000 years.5 Since the start of the industrial era, global methane concentrations are 2.5 times higher than observed in ice cores dated to 1000−1750 AD.3,5 The global methane concentration rose steadily throughout the second half of the twentieth century, leveled off in 1999, and began growing again in 2007.1 The period of stable concentration has been attributed to a reduction in biogenic methane production; Anthropogenic methane emissions continued to rise through this period.6,7 Methane sources have been successfully linked to emissions by examining the isotopic composition of methane. δ13C for methane, or δ13CH4, is a measure of the isotopic ratio of 13C to 12 C in methane, defined as ⎛ ⎜ δ13CH4 = ⎜ ⎜ ⎜ ⎝
13
( ) ( ) C C
12
sample
13
C 12 C
standard
⎞ ⎟ − 1⎟ × 1000‰ ⎟ ⎟ ⎠ © XXXX American Chemical Society
where the standard values come from the Vienna Pee Dee Belemnite (VPDP) standard. δ13CH4 is given in units of permil, ‰. Table 1 shows δ13CH4 values for various emission types.8 Table 1. Values for δ13C from Various Methane Emission Sources (Values from Quay et al.8 and References within)a methane source
range (‰) −24 −36 −43 −50 −60 −60
biomass burning coal mining natural gas landfills ruminants wetlands
± ± ± ± ± ±
3 7 7 2 5 5
a
Note that biogenic methane (ruminates and wetlands) is less isotopically enriched than fossil methane (coal mining and natural gas).
Biomass burning emissions are the heaviest, followed by fossil methane, whereas biogenic methane emissions from ruminants and wetlands have a lower δ13CH4.9−13 Analysis of ice core records shows that δ13CH4 was higher at the Late Glacial Maximum than it is today, with an average of −43‰,14 attributed to the decline of wetland emissions in colder climates.15 Preindustrial (∼1700 AD) methane was less isotopically enriched, at an average of ∼−49‰.16 Since preindustrial times, δ13CH4 has increased by 2‰, with 1.7‰ of the increase occurring in the in the last half of the 20th Special Issue: James G. Anderson Festschrift Received: February 1, 2016
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Figure 1. Allan variance plots for measurements of δ13C. log(σ2) decreases proportionally to the measurement integration time, implying that the signal noise is white over the period measured for this experiment.
To determine the type of noise in the measurement of δ13CH4, a plot of the Allan Variance was made. By plotting the variance of the measurements against the number of measurements used to calculate the variance, we can detect a bias in noise. Figure 1 shows both δ13CH4 and the Allan Variance observed for the high and low concentration standards. At both concentrations, as the log of the number of samples used to calculate the variance increases, the log of the variance decreases proportionally, indicating that the predominant noise in the measurements is white noise. Hence, an “optimal” sampling time over which to average is not dictated by the instrument’s precision over the time period for which we measured. The instrument was deployed to the LANL Four Corners Total Column Carbon Observing Network site, where a Picarro CRDS analyzer for CO/CO2/CH4/H2O was already deployed. Measurements of methane concentrations from both instruments were in agreement. An averaging time of 10 min was chosen for δ13CH4 measurements. Two concentration calibrations standards were run every 23 h at the site. Although these tanks were not certified for isotopic composition, the measured value of δ13CH4, as well as background δ13CH4 measurements, was used to monitor instrument drift. Long-term drift (25 days) was found to be ±2.5‰. Before deployment the short-term drift (24 h, peak-to-peak, 1 h interval average) was certified to be less than 2‰. Upon returning from the field site, the δ13CH4 measurement was verified using a secondary standard gas calibrated using a isotope ratio mass spectrometer tied to a WMO standard (standard, −38.55‰; measurement, −37.5 ± 1.0‰; 1σ, 1 Hz measurements over 5 min period). In the Spring of 2013, we conducted the field deployment and benchmark testing of the Picarro Mobile Methane Investigator (PMMI). The PMMI is a mobile platform that measures methane concentration (cavity ring-down spectroscopy), δ13CH4, wind speed and direction, and GPS location. On-board integrated software displays the data in realtime, allowing for quick and informed operator input resulting in the identification of surrounding methane sources. A gas “playback” system allows for precise sample analysis of plumes of interest: air enters the intake and is split between a path directly to the instrument and a path through a 46 m tube. When a plume of interest is detected, the sample inlet is closed and the instrument can slowly “playback” the air stored in the tube.
century, a rise attributed to an increase in anthropogenic sources.16,17 Methane stable isotope analysis has been used to constrain current emission sources. Bergamaschi et al. found methane along the Transiberian Railway to be isotopically depleted due to agricultural activity.18 Moriizumi et al. parsed fossil and nonfossil methane in Nagoya, Japan.19 Lowry et al. identified fugitive natural gas as contributing up to 20% of methane observed in the London Metro Area.20 Townsend-Small et al. identified fugitive natural gas as the dominant source of methane in the Los Angeles area.11 Although methods used in the aforementioned studies required air samples be transported to a laboratory before they could be analyzed, here we demonstrate the use of an online, high frequency, in situ measurement technique. Complicating our study, however, is the presence of biogenic methane in coal beds. This methane is produced by methanogen bacteria present in the coal bed.21 Though biogenic, this methane is still fugitive methane released from coal and natural gas extraction and so can be characterized as anthropogenic.
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RESULTS AND DISCUSSION Instrument and Validation. The Picarro G2123-i δ13C High Precision Isotopic CH4 CRDS analyzer utilizes cavity ring-down spectroscopy to measure the concentration of 12C methane, 13C methane, carbon dioxide, and gas phase water, along with calculating δ13CH4 for methane. Calibration of the instrument was performed by Picarro prior to the shipping of the instrument. Concentration calibration verification was performed at the LANL site, using two standards, 2.511 ± 0.025 (High Standard) and 1.492 ± 0.015 ppm (Low Standard) methane. Both standards contained 388 ± 8 ppm carbon dioxide. Averaging 1 s measurements over 5 min periods gave δ13CH4 precision of ±0.46‰ and ±0.89‰, for the High Standard and Low Standard, respectively. CO2 concentration was measured to be 382.8 ± 1.1 ppm, well within the nominal concentration value. Measurements of the total methane concentration were also in good agreement with the standards, at 2.49 ± 0.22 and 1.47 ± 0.22 ppm (1σ, 1 Hz measurements over 5 min period) for of the High Standard and Low Standard, respectively. B
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Figure 2. PMMI methane concentration measurements. Color-coded concentration measurements are plotted over location information. Blue indicated that the measurement was in “playback” mode, and the resultant δ13CH4 is plotted where the measurement was taken. This is also denoted in the bottom time series with a gray background. In this example, we measured concentrations between 1.8 and 11.6 ppm, with two isotope analyses yielding values of −45‰. The wind rose in the top right indicates wind was coming from the southwest direction.
Figure 3. (a) Oil and natural gas exploration land leases in the four corners area. The San Juan and Four Corners Power Plants are marked, indicating their methane source strength, along with natural gas sources and landfills. Note that leases do not necessarily mean that exploration has taken place. (b) Land cover and vegetation in the Four Corners area. Wetlands (light blue) and grassland (light green) are possible sources of biogenic methane, the latter due to ruminant grazing.
Figure 4. Methane concentrations measured at the FC site in May and June of 2012. Concentrations regularly exceeded 3.5 ppm. The average background methane concentration for the sampling interval was measured to be 1.83 ± 0.03 ppm.
measurements. (CH4 range 1−20 ppm, σ < 5 ppb + 0.05%, δ13CH4 σ < 0.8‰, 5 min). 2D sonic anemometer (0−50 m/s, σ = 0.1 m/s or 5%, and 1°) mounted on top of the vehicle yields
This system is described in more detail in Rella [2015].22 Concentration measurements are made using a cavity ringdown spectrometer for methane concentration and δ13CH4 C
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Figure 5. (a) All wind from May and June of 2012. Wind is slightly favored to come from the west. (b) Wind from May and June of 2012 when methane concentrations exceeded 3.5 ppm. During high methane events, wind is either still ( 3.5 ppm). Methane clearly comes from east of the FC site. Along with elevated methane concentrations, the FC site also experiences high levels of CO and CO2, both of which are tracers for combustion. Methane is the most biased on the basis of wind direction with virtually no elevated concentrations coming from the west. Elevated levels of CO also occur predominantly with easterly winds; however, they are infrequent enough that they barely skew the average concentration from that direction. Elevated CO2 concentrations come predominantly from the east, southeast, and south, although the highest levels are detected from the west; westerly
A study of methane from the coal beds of the Four Corners region (Scott25) found methane from this region to have a δ13CH4 value between −41.0 and −54.8‰. This range of values for δ13CH4 is attributed to the presence of both thermogenic and biogenic methane in the coal bed.25 A 2007 study of atmospheric methane composition performed at Niwot Ridge, Colorado (approximately 400 miles NNE of the FC site), found that the area had an average background δ13CH4 of −47.22 ± 0.13‰ and an average background methane concentration of 1.785 ± 0.029 ppm.26 Background concentration of methane at the FC site over the two month period was found to be 1.83 ± 0.03 ppm, and the background δ13CH4 was found to be −47.3 ± 2.0‰. Meteorological data were also collected at the FC site and used in conjunction with the methane data. Figure 5a shows a E
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thermogenic and biogenic ranges, suggesting that they are composed of methane from multiple emission sources. In the Spring of 2013 we verified our measurements of δ13CH4 using the Picarro Mobile Methane Investigator (PMMI). Measurements were made by bringing the PMMI in close proximity of methane emitters such as oil well pump jacks, so that the source of the measured methane was wellknown. We find that many of oil/gas wells and facilities sampled had substantial fugitive emissions, and that the isotopic enrichment of those methane emissions varies. The average value of δ13CH4 from the seven measurements taken in the FC area was −39.3‰, ±6.7‰. The highest measured value was −30.1 ± 4.6‰, falling in the range of coal mining. The lowest value measured was −51.8 ± 3.5‰, falling in both the biogenic and thermogenic ranges. This is still heavier than than the lightest sample identified in the area by Scott, measured to be −54.8‰.25 Despite attempts, throughout the deployment of the PMMI in the FC area, no methane was detected that could be definitively attributed to a biogenic source.
plumes, however, are infrequent, and do not affect the average concentration from that direction. Although these combustion tracers are found at elevated levels from the same direction as elevated concentrations of methane, they are not correlated with methane emissions. Parts a and b of Figure 6 show correlation plots of methane with CO2 and CO, respectively. With both combustion tracers, incidents of elevated methane concentrations only occurred when tracer concentrations were low. Additionally, high concentrations of combustion tracers correlate with low methane concentrations. Because there is no correlation between methane concentration and combustion tracer concentration, biomass burning can be ruled out as a methane emission source at the FC site. Sources of methane not associated with combustion include agriculture and wetlands, ruminants, landfills, and fossil fuel extraction. Utilizing measurements of δ13CH4 and methane concentration, we can discriminate sources of methane through Keeling plot analysis.27,28 Assuming a single source of methane and a background concentration, Keeling plot analysis gives the isotopic enrichment of the source methane, which can be compared to previous data. The following relationship is used: δ13CH4 = Mo(1/[CH4]) + δ13CPLUME
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CONCLUSION Methane stable isotope measurements made in the Summer of 2012 indicate large concentrations of methane in the Four Corners area of New Mexico are consistent with fugitive methane emissions from coal and natural gas extraction. By incorporating meteorological data taken at the field site, we are able to determine the direction, relative to the site area, of the methane source. Comparison of methane concentration measurements to CO concentration measurements rule out combustion as a major source of methane in the area. Although three Keeling analysis measurements fall in the biogenic range established by Quay,8 prior measurements of the area’s methane isotopic enrichment, as well as measurements made by the PMMI, indicate that coal bed methane at the site contains a significant fraction of biogenic methane produced by methanogen bacteria.
(2)
where δ CH4 is the measured isotopic enrichment (‰), Mo is a constant, [CH4] is the concentration of methane measured (ppm), and δ13CPLUME is the isotopic enrichment of the source plume (‰). Figure 7 shows measurements of methane and δ13C during May and June 2012. Peaks which reached a concentration of 3.5 ppm or greater are highlighted and were selected for Keeling analysis (Figure 8). Concentrations typically rose around 13
Figure 8. Results of Keeling analysis for the plumes highlighted in Figure 7. The values for δ13CH4 source attributions found in Table 1 are also plotted.
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AUTHOR INFORMATION
Corresponding Author
midnight and fell in the morning, consistent with concentration increases expected from the lowering of the boundary layer, although plumes did sometimes persist into the afternoon. During two separate days, May 19 and June 10, methane concentrations did not show the expected behavior of elevated nighttime concentrations persisting until the morning. On five separate days, examination of plume methane concentration in conjunction with the isotopic enrichment gave evidence that the plume had sections that were from an isotopic light source and other sections that were from an isotopically heavier source. For example, the plumes from 6/21/2012 show two distinct areas of correlation (labeled 8.1 and 8.3) separated by a period of anticorrelation (labeled 8.2). Areas such as these were analyzed separately for Keeling plot analysis. Fifteen separate plumes from 9 days were analyzed. Values for δ13CH4 sources were between −56.5‰ and −41.3‰, agreeing well with the known range of isotopic enrichment of the area’s coal bed methane.25 The three heaviest plumes are within the isotopic range for coal bed and natural gas methane given by Quay.8 The three lightest observed plumes fall within the biogenic methane ranges given for wetlands and ruminants. Two plumes fall only in the range of natural gas, three plumes fall in the range of both natural gas and landfills, and three other plumes fall between the
*M. K. Dubey. E-mail: [email protected]. Notes
The authors declare no competing financial interest. † Also generally affiliated with Los Alamos National Laboratory.
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ACKNOWLEDGMENTS This work was supported by LANL’s Laboratory Directed Research and Development (LDRD) project 20110081DR (PI MKD).
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REFERENCES
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(20) Lowry, D.; Holmes, C. W.; Rata, N. D.; O’Brien, P.; Nisbet, E. G. London Methane Emissions: Use of Diurnal Changes in Concentration and δ13C to Identify Urban Sources and Verify Inventories. J. Geophys. Res. 2001, 106, 7427−7448. (21) Beckmann, S.; Krüger, M.; Engelen, B.; Gorbushina, A. A.; Cypionka, H. Role of Bacteria, Archaea and Fungi involved in Methane Release in Abandoned Coal Mines. Geomicrobiol. J. 2011, 28, 347−358. (22) Rella, C. W.; Tsai, T. R.; Botkin, C. G.; Crosson, E. R.; Steele, D. Measuring Emissions from Oil and Natural Gas Well Pads Using the Mobile Flux Plane Technique. Environ. Sci. Technol. 2015, 49, 4742−4748. (23) Lindenmaier, R.; Dubey, M. K.; Henderson, B. G.; Butterfield, Z. T.; Herman, J. R.; Rahn, T.; Lee, S.-H. Multiscale observations of CO2, 13CO2, and pollutants at Four Corners for emission verification and attribution. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 8386−8391. (24) Kort, E. A.; Frankenberg, C.; Costigan, K. R.; Lindenmaier, R.; Dubey, M. K.; Wunch, D. Four corners: The largest US methane anomaly viewed from space. Geophys. Res. Lett. 2014, 41, 6898−6903. (25) Scott, A. R. Coalbed Gas Composition, Upper Cretaceous Fruitland Formation, San Juan Basin, Colorado and New Mexico; Colorado Geological Survey, Division of Minerals and Geology, Department of Natural Resources: Golden, CO, 1994. (26) Tyler, S. C.; Rice, A. L.; Ajie, H. O. Stable Isotope Ratios in Atmospheric CH4: Implications for Seasonal Sources and Sinks. J. Geophys. Res. 2007, 112, D03303. (27) Keeling, C. D. The Concentration and Isotopic Abundances of Atmospheric Carbon Dioxide in Rural Areas. Geochim. Cosmochim. Acta 1958, 13, 322−334. (28) Keeling, C. D. The Concentration and Isotopic Abundances of Carbon Dioxide in Rural and Marine Air. Geochim. Cosmochim. Acta 1961, 24, 277−298.
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Methane Isotope Instrument Validation and Source Identification at Four Corners, New Mexico, United States Caleb Arata,† Thom Rahn, and Manvendra K. Dubey* Earth System Observations, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States ABSTRACT: Measurements of δ13CH4 and CH4 concentration were made at a field site in Four Corners, New Mexico (FC), where we observed large sustained CH4 enhancements (2−8 ppm peaks for hours) during nocturnal inversions. Potential sources of this large CH4 signal at FC include (1) fugitive emissions from coal mining and gas processing that are thermogenic and isotopically 13C enriched relative to background atmosphere and (2) emissions from agriculture, ruminants, landfills, and coalbed biogenic methane that are13C depleted relative to background atmosphere. We analyze our measurements of methane concentration and δ13C during spring and summer of 2012 to identify fugitive methane sources. We find CH4 plumes that are both enriched and depleted in 13C relative to CH4 in background air. Keeling plots show a continuum of δ13C source compositions between −40‰ and −60‰ that are consistent with thermogenic and biogenic sources. The Picarro Mobile Methane Investigator (PMMI), a mobile δ13CH4 instrument platform, was deployed in the spring of 2013 and used to verify the isotopic enrichment of coal bed methane in the region. We combine our results with meteorological data to spatially separate these sources in the Four Corners regions. Using CO and CO2 data, along with meteorological data, we propose that the high methane concentration events ([CH4] > 3.5 ppm) are from both thermogenic and biogenic methane released from coal beds.
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INTRODUCTION Methane is a greenhouse gas 25 times more potent than carbon dioxide over a 100 year period, with a concentration in the atmosphere that is increasing.1 With an ambient background concentration of 1.8 ppm, methane is the most abundant green house gas after CO2 (excluding H2O).2,3 Methane has an atmospheric lifetime of 10 years.4 Sources of methane include biomass burning, fossil fuels, ruminants, and wetlands. Isolating these sources is key to understanding this increase in methane growth. Before the modern industrial era, global background atmospheric methane concentrations did not exceed 773 ppb in the last 650 000 years and never rose above 600 ppb in the last 420 000 years.5 Since the start of the industrial era, global methane concentrations are 2.5 times higher than observed in ice cores dated to 1000−1750 AD.3,5 The global methane concentration rose steadily throughout the second half of the twentieth century, leveled off in 1999, and began growing again in 2007.1 The period of stable concentration has been attributed to a reduction in biogenic methane production; Anthropogenic methane emissions continued to rise through this period.6,7 Methane sources have been successfully linked to emissions by examining the isotopic composition of methane. δ13C for methane, or δ13CH4, is a measure of the isotopic ratio of 13C to 12 C in methane, defined as ⎛ ⎜ δ13CH4 = ⎜ ⎜ ⎜ ⎝
13
( ) ( ) C C
12
sample
13
C 12 C
standard
⎞ ⎟ − 1⎟ × 1000‰ ⎟ ⎟ ⎠ © XXXX American Chemical Society
where the standard values come from the Vienna Pee Dee Belemnite (VPDP) standard. δ13CH4 is given in units of permil, ‰. Table 1 shows δ13CH4 values for various emission types.8 Table 1. Values for δ13C from Various Methane Emission Sources (Values from Quay et al.8 and References within)a methane source
range (‰) −24 −36 −43 −50 −60 −60
biomass burning coal mining natural gas landfills ruminants wetlands
± ± ± ± ± ±
3 7 7 2 5 5
a
Note that biogenic methane (ruminates and wetlands) is less isotopically enriched than fossil methane (coal mining and natural gas).
Biomass burning emissions are the heaviest, followed by fossil methane, whereas biogenic methane emissions from ruminants and wetlands have a lower δ13CH4.9−13 Analysis of ice core records shows that δ13CH4 was higher at the Late Glacial Maximum than it is today, with an average of −43‰,14 attributed to the decline of wetland emissions in colder climates.15 Preindustrial (∼1700 AD) methane was less isotopically enriched, at an average of ∼−49‰.16 Since preindustrial times, δ13CH4 has increased by 2‰, with 1.7‰ of the increase occurring in the in the last half of the 20th Special Issue: James G. Anderson Festschrift Received: February 1, 2016
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Figure 1. Allan variance plots for measurements of δ13C. log(σ2) decreases proportionally to the measurement integration time, implying that the signal noise is white over the period measured for this experiment.
To determine the type of noise in the measurement of δ13CH4, a plot of the Allan Variance was made. By plotting the variance of the measurements against the number of measurements used to calculate the variance, we can detect a bias in noise. Figure 1 shows both δ13CH4 and the Allan Variance observed for the high and low concentration standards. At both concentrations, as the log of the number of samples used to calculate the variance increases, the log of the variance decreases proportionally, indicating that the predominant noise in the measurements is white noise. Hence, an “optimal” sampling time over which to average is not dictated by the instrument’s precision over the time period for which we measured. The instrument was deployed to the LANL Four Corners Total Column Carbon Observing Network site, where a Picarro CRDS analyzer for CO/CO2/CH4/H2O was already deployed. Measurements of methane concentrations from both instruments were in agreement. An averaging time of 10 min was chosen for δ13CH4 measurements. Two concentration calibrations standards were run every 23 h at the site. Although these tanks were not certified for isotopic composition, the measured value of δ13CH4, as well as background δ13CH4 measurements, was used to monitor instrument drift. Long-term drift (25 days) was found to be ±2.5‰. Before deployment the short-term drift (24 h, peak-to-peak, 1 h interval average) was certified to be less than 2‰. Upon returning from the field site, the δ13CH4 measurement was verified using a secondary standard gas calibrated using a isotope ratio mass spectrometer tied to a WMO standard (standard, −38.55‰; measurement, −37.5 ± 1.0‰; 1σ, 1 Hz measurements over 5 min period). In the Spring of 2013, we conducted the field deployment and benchmark testing of the Picarro Mobile Methane Investigator (PMMI). The PMMI is a mobile platform that measures methane concentration (cavity ring-down spectroscopy), δ13CH4, wind speed and direction, and GPS location. On-board integrated software displays the data in realtime, allowing for quick and informed operator input resulting in the identification of surrounding methane sources. A gas “playback” system allows for precise sample analysis of plumes of interest: air enters the intake and is split between a path directly to the instrument and a path through a 46 m tube. When a plume of interest is detected, the sample inlet is closed and the instrument can slowly “playback” the air stored in the tube.
century, a rise attributed to an increase in anthropogenic sources.16,17 Methane stable isotope analysis has been used to constrain current emission sources. Bergamaschi et al. found methane along the Transiberian Railway to be isotopically depleted due to agricultural activity.18 Moriizumi et al. parsed fossil and nonfossil methane in Nagoya, Japan.19 Lowry et al. identified fugitive natural gas as contributing up to 20% of methane observed in the London Metro Area.20 Townsend-Small et al. identified fugitive natural gas as the dominant source of methane in the Los Angeles area.11 Although methods used in the aforementioned studies required air samples be transported to a laboratory before they could be analyzed, here we demonstrate the use of an online, high frequency, in situ measurement technique. Complicating our study, however, is the presence of biogenic methane in coal beds. This methane is produced by methanogen bacteria present in the coal bed.21 Though biogenic, this methane is still fugitive methane released from coal and natural gas extraction and so can be characterized as anthropogenic.
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RESULTS AND DISCUSSION Instrument and Validation. The Picarro G2123-i δ13C High Precision Isotopic CH4 CRDS analyzer utilizes cavity ring-down spectroscopy to measure the concentration of 12C methane, 13C methane, carbon dioxide, and gas phase water, along with calculating δ13CH4 for methane. Calibration of the instrument was performed by Picarro prior to the shipping of the instrument. Concentration calibration verification was performed at the LANL site, using two standards, 2.511 ± 0.025 (High Standard) and 1.492 ± 0.015 ppm (Low Standard) methane. Both standards contained 388 ± 8 ppm carbon dioxide. Averaging 1 s measurements over 5 min periods gave δ13CH4 precision of ±0.46‰ and ±0.89‰, for the High Standard and Low Standard, respectively. CO2 concentration was measured to be 382.8 ± 1.1 ppm, well within the nominal concentration value. Measurements of the total methane concentration were also in good agreement with the standards, at 2.49 ± 0.22 and 1.47 ± 0.22 ppm (1σ, 1 Hz measurements over 5 min period) for of the High Standard and Low Standard, respectively. B
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Figure 2. PMMI methane concentration measurements. Color-coded concentration measurements are plotted over location information. Blue indicated that the measurement was in “playback” mode, and the resultant δ13CH4 is plotted where the measurement was taken. This is also denoted in the bottom time series with a gray background. In this example, we measured concentrations between 1.8 and 11.6 ppm, with two isotope analyses yielding values of −45‰. The wind rose in the top right indicates wind was coming from the southwest direction.
Figure 3. (a) Oil and natural gas exploration land leases in the four corners area. The San Juan and Four Corners Power Plants are marked, indicating their methane source strength, along with natural gas sources and landfills. Note that leases do not necessarily mean that exploration has taken place. (b) Land cover and vegetation in the Four Corners area. Wetlands (light blue) and grassland (light green) are possible sources of biogenic methane, the latter due to ruminant grazing.
Figure 4. Methane concentrations measured at the FC site in May and June of 2012. Concentrations regularly exceeded 3.5 ppm. The average background methane concentration for the sampling interval was measured to be 1.83 ± 0.03 ppm.
measurements. (CH4 range 1−20 ppm, σ < 5 ppb + 0.05%, δ13CH4 σ < 0.8‰, 5 min). 2D sonic anemometer (0−50 m/s, σ = 0.1 m/s or 5%, and 1°) mounted on top of the vehicle yields
This system is described in more detail in Rella [2015].22 Concentration measurements are made using a cavity ringdown spectrometer for methane concentration and δ13CH4 C
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Figure 5. (a) All wind from May and June of 2012. Wind is slightly favored to come from the west. (b) Wind from May and June of 2012 when methane concentrations exceeded 3.5 ppm. During high methane events, wind is either still ( 3.5 ppm). Methane clearly comes from east of the FC site. Along with elevated methane concentrations, the FC site also experiences high levels of CO and CO2, both of which are tracers for combustion. Methane is the most biased on the basis of wind direction with virtually no elevated concentrations coming from the west. Elevated levels of CO also occur predominantly with easterly winds; however, they are infrequent enough that they barely skew the average concentration from that direction. Elevated CO2 concentrations come predominantly from the east, southeast, and south, although the highest levels are detected from the west; westerly
A study of methane from the coal beds of the Four Corners region (Scott25) found methane from this region to have a δ13CH4 value between −41.0 and −54.8‰. This range of values for δ13CH4 is attributed to the presence of both thermogenic and biogenic methane in the coal bed.25 A 2007 study of atmospheric methane composition performed at Niwot Ridge, Colorado (approximately 400 miles NNE of the FC site), found that the area had an average background δ13CH4 of −47.22 ± 0.13‰ and an average background methane concentration of 1.785 ± 0.029 ppm.26 Background concentration of methane at the FC site over the two month period was found to be 1.83 ± 0.03 ppm, and the background δ13CH4 was found to be −47.3 ± 2.0‰. Meteorological data were also collected at the FC site and used in conjunction with the methane data. Figure 5a shows a E
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thermogenic and biogenic ranges, suggesting that they are composed of methane from multiple emission sources. In the Spring of 2013 we verified our measurements of δ13CH4 using the Picarro Mobile Methane Investigator (PMMI). Measurements were made by bringing the PMMI in close proximity of methane emitters such as oil well pump jacks, so that the source of the measured methane was wellknown. We find that many of oil/gas wells and facilities sampled had substantial fugitive emissions, and that the isotopic enrichment of those methane emissions varies. The average value of δ13CH4 from the seven measurements taken in the FC area was −39.3‰, ±6.7‰. The highest measured value was −30.1 ± 4.6‰, falling in the range of coal mining. The lowest value measured was −51.8 ± 3.5‰, falling in both the biogenic and thermogenic ranges. This is still heavier than than the lightest sample identified in the area by Scott, measured to be −54.8‰.25 Despite attempts, throughout the deployment of the PMMI in the FC area, no methane was detected that could be definitively attributed to a biogenic source.
plumes, however, are infrequent, and do not affect the average concentration from that direction. Although these combustion tracers are found at elevated levels from the same direction as elevated concentrations of methane, they are not correlated with methane emissions. Parts a and b of Figure 6 show correlation plots of methane with CO2 and CO, respectively. With both combustion tracers, incidents of elevated methane concentrations only occurred when tracer concentrations were low. Additionally, high concentrations of combustion tracers correlate with low methane concentrations. Because there is no correlation between methane concentration and combustion tracer concentration, biomass burning can be ruled out as a methane emission source at the FC site. Sources of methane not associated with combustion include agriculture and wetlands, ruminants, landfills, and fossil fuel extraction. Utilizing measurements of δ13CH4 and methane concentration, we can discriminate sources of methane through Keeling plot analysis.27,28 Assuming a single source of methane and a background concentration, Keeling plot analysis gives the isotopic enrichment of the source methane, which can be compared to previous data. The following relationship is used: δ13CH4 = Mo(1/[CH4]) + δ13CPLUME
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CONCLUSION Methane stable isotope measurements made in the Summer of 2012 indicate large concentrations of methane in the Four Corners area of New Mexico are consistent with fugitive methane emissions from coal and natural gas extraction. By incorporating meteorological data taken at the field site, we are able to determine the direction, relative to the site area, of the methane source. Comparison of methane concentration measurements to CO concentration measurements rule out combustion as a major source of methane in the area. Although three Keeling analysis measurements fall in the biogenic range established by Quay,8 prior measurements of the area’s methane isotopic enrichment, as well as measurements made by the PMMI, indicate that coal bed methane at the site contains a significant fraction of biogenic methane produced by methanogen bacteria.
(2)
where δ CH4 is the measured isotopic enrichment (‰), Mo is a constant, [CH4] is the concentration of methane measured (ppm), and δ13CPLUME is the isotopic enrichment of the source plume (‰). Figure 7 shows measurements of methane and δ13C during May and June 2012. Peaks which reached a concentration of 3.5 ppm or greater are highlighted and were selected for Keeling analysis (Figure 8). Concentrations typically rose around 13
Figure 8. Results of Keeling analysis for the plumes highlighted in Figure 7. The values for δ13CH4 source attributions found in Table 1 are also plotted.
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AUTHOR INFORMATION
Corresponding Author
midnight and fell in the morning, consistent with concentration increases expected from the lowering of the boundary layer, although plumes did sometimes persist into the afternoon. During two separate days, May 19 and June 10, methane concentrations did not show the expected behavior of elevated nighttime concentrations persisting until the morning. On five separate days, examination of plume methane concentration in conjunction with the isotopic enrichment gave evidence that the plume had sections that were from an isotopic light source and other sections that were from an isotopically heavier source. For example, the plumes from 6/21/2012 show two distinct areas of correlation (labeled 8.1 and 8.3) separated by a period of anticorrelation (labeled 8.2). Areas such as these were analyzed separately for Keeling plot analysis. Fifteen separate plumes from 9 days were analyzed. Values for δ13CH4 sources were between −56.5‰ and −41.3‰, agreeing well with the known range of isotopic enrichment of the area’s coal bed methane.25 The three heaviest plumes are within the isotopic range for coal bed and natural gas methane given by Quay.8 The three lightest observed plumes fall within the biogenic methane ranges given for wetlands and ruminants. Two plumes fall only in the range of natural gas, three plumes fall in the range of both natural gas and landfills, and three other plumes fall between the
*M. K. Dubey. E-mail: [email protected]. Notes
The authors declare no competing financial interest. † Also generally affiliated with Los Alamos National Laboratory.
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ACKNOWLEDGMENTS This work was supported by LANL’s Laboratory Directed Research and Development (LDRD) project 20110081DR (PI MKD).
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REFERENCES
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