Calibration and Field Testing of Cavity Ring-Down Laser Spectrometers Measuring CH<sub>4</sub>, CO<sub>2</sub>, and δ<sup>13</sup>CH<sub>4</sub> Deployed on Towers in the Marcellus Shale Region
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. The calibration of the continuous isotopic methane analyzers used in this study required both a linear calibration and a mole fraction correction, and a correction for cross-interference from ethane. 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 calibrated using high methane mole fraction air bottles with various isotopic ratios, from biogenic to thermogenic source values, diluted in zero air. Furthermore, at each tower location, three field calibration tanks were employed, from ambient to high mole fractions, with various isotopic ratios. By testing multiple calibration schemes, we determined an optimized field calibration method. A method to correct for cross interference from ethane is also described. Using an independent field tank for evaluation, the standard deviation of 4-hour means of the isotopic ratio of methane difference from the known value was found to be 0.26 ‰ δ13CH4. Following improvements in the field calibration tank sampling scheme, the standard deviation of 4-hour 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 enhancements of isotopic ratio measured at three of the towers, compared to the background tower, were −0.15 to 0.12 ‰ with standard deviations of the 10-min isotopic ratio enhancements of 0.8 ‰. In terms of source attribution, analyzer compatibility of 0.2 ‰ δ13CH4 affords the ability to distinguish a 50 ppb CH4 peak from a biogenic source from one originating from a thermogenic source. 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 ‰, consistent with a deep-layer Marcellus natural gas source.