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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union
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Volume 11, issue 3 | Copyright

Special issue: The 10th International Carbon Dioxide Conference (ICDC10)...

Atmos. Meas. Tech., 11, 1273-1295, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 05 Mar 2018

Research article | 05 Mar 2018

Calibration and field testing of cavity ring-down laser spectrometers measuring CH4, CO2, and δ13CH4 deployed on towers in the Marcellus Shale region

Natasha L. Miles1, Douglas K. Martins1,a, Scott J. Richardson1, Christopher W. Rella2, Caleb Arata2,3, Thomas Lauvaux1, Kenneth J. Davis1, Zachary R. Barkley1, Kathryn McKain4, and Colm Sweeney4 Natasha L. Miles et al.
  • 1Department of Meteorology and Atmospheric Science, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
  • 2Picarro, Inc., Santa Clara, California 95054, USA
  • 3Department of Environmental Science, Policy, and Management, University of California, Berkeley, California 94720, USA
  • 4National Oceanic and Atmospheric Administration, University of Colorado, Boulder, Colorado 80305, USA
  • acurrent address: FLIR Systems, Inc., West Lafayette, Indiana 47906, USA

Abstract. Four in situ cavity ring-down spectrometers (G2132-i, Picarro, Inc.) measuring methane dry mole fraction (CH4), carbon dioxide dry mole fraction (CO2), and the isotopic ratio of methane (δ13CH4) were deployed at four towers in the Marcellus Shale natural gas extraction region of Pennsylvania. In this paper, we describe laboratory and field calibration of the analyzers for tower-based applications and characterize their performance in the field for the period January–December 2016. Prior to deployment, each analyzer was tested using bottles with various isotopic ratios, from biogenic to thermogenic source values, which were diluted to varying degrees in zero air, and an initial calibration was performed. Furthermore, at each tower location, three field tanks were employed, from ambient to high mole fractions, with various isotopic ratios. Two of these tanks were used to adjust the calibration of the analyzers on a daily basis. We also corrected for the cross-interference from ethane on the isotopic ratio of methane. Using an independent field tank for evaluation, the standard deviation of 4h means of the isotopic ratio of methane difference from the known value was found to be 0.26δ13CH4. Following improvements in the field tank testing scheme, the standard deviation of 4h means was 0.11‰, well within the target compatibility of 0.2‰. Round-robin style testing using tanks with near-ambient isotopic ratios indicated mean errors of −0.14 to 0.03‰ for each of the analyzers. Flask to in situ comparisons showed mean differences over the year of 0.02 and 0.08‰, for the east and south towers, respectively.

Regional sources in this region were difficult to differentiate from strong perturbations in the background. During the afternoon hours, the median differences of the isotopic ratio measured at three of the towers, compared to the background tower, were &minus0.15 to 0.12‰ with standard deviations of the 10min isotopic ratio differences of 0.8‰. In terms of source attribution, analyzer compatibility of 0.2δ13CH4 affords the ability to distinguish a 50ppb CH4 peak from a biogenic source (at −60‰, for example) from one originating from a thermogenic source (−35‰), with the exact value dependent upon the source isotopic ratios. Using a Keeling plot approach for the non-afternoon data at a tower in the center of the study region, we determined the source isotopic signature to be −31.2±1.9‰, within the wide range of values consistent with a deep-layer Marcellus natural gas source.

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Short summary
Analyzers measuring methane and methane isotopic ratio were deployed at four towers in the Marcellus Shale natural gas extraction region of Pennsylvania. The methane isotopic ratio is helpful for differentiating emissions from natural gas activities from other sources (e.g., landfills). We describe the analyzer calibration. The signals observed in the study region were generally small, but the instrumental performance demonstrated here could be used in regions with stronger enhancements.
Analyzers measuring methane and methane isotopic ratio were deployed at four towers in the...