Abstract
Abstract. In situ and simultaneous measurement of the three most abundant isotopologues of methane using mid-infrared laser absorption spectroscopy is demonstrated. A field-deployable, autonomous platform is realized by coupling a compact quantum cascade laser absorption spectrometer (QCLAS) to a preconcentration unit, called trace gas extractor (TREX). This unit enhances CH4 mole fractions by a factor of up to 500 above ambient levels and quantitatively separates interfering trace gases such as N2O and CO2. The analytical precision of the QCLAS isotope measurement on the preconcentrated (750 ppm, parts-per-million, µmole mole−1) methane is 0.1 and 0.5 ‰ for δ13C- and δD-CH4 at 10 min averaging time. Based on repeated measurements of compressed air during a 2-week intercomparison campaign, the repeatability of the TREX–QCLAS was determined to be 0.19 and 1.9 ‰ for δ13C and δD-CH4, respectively. In this intercomparison campaign the new in situ technique is compared to isotope-ratio mass spectrometry (IRMS) based on glass flask and bag sampling and real time CH4 isotope analysis by two commercially available laser spectrometers. Both laser-based analyzers were limited to methane mole fraction and δ13C-CH4 analysis, and only one of them, a cavity ring down spectrometer, was capable to deliver meaningful data for the isotopic composition. After correcting for scale offsets, the average difference between TREX–QCLAS data and bag/flask sampling–IRMS values are within the extended WMO compatibility goals of 0.2 and 5 ‰ for δ13C- and δD-CH4, respectively. This also displays the potential to improve the interlaboratory compatibility based on the analysis of a reference air sample with accurately determined isotopic composition.
Highlights
Methane (CH4 ) is the second most important anthropogenically emitted greenhouse gas after carbon dioxide (CO2 )
Eyer et al.: Real-time analysis of δ 13 C- and δD-CH4 in ambient air with laser spectroscopy composition is reported in the δ-notation, representing the relative difference in the amount of heavy vs. light isotope of a sample in relation to an international measurement standard (Brand and Coplen, 2012; Coplen, 2011; Urey, 1948): δ 13 C = Rsample /Rstandard, (1)
We present further improvements of coupling a preconcentration unit to quantum cascade laser absorption spectrometer (QCLAS) to achieve real-time, high-precision measurements of methane isotopic composition (δ 13 C-CH4, δD-CH4 ) in ambient air
Summary
Methane (CH4 ) is the second most important anthropogenically emitted greenhouse gas after carbon dioxide (CO2 ). We present further improvements of coupling a preconcentration unit (trace gas extractor, TREX) to QCLAS to achieve real-time, high-precision measurements of methane isotopic composition (δ 13 C-CH4 , δD-CH4 ) in ambient air. During the intercomparison campaign a measurement cycle of 220 min duration was applied (Fig. 5), including the measurement of three different types of calibration gases (CG 1 at 635 and 745 ppm, CG 2 at 635 ppm) as well as repeatability measurements with preconcentrated target gas (TG) This sequence allowed the measurement of up to 20 ambient air samples per day. To the international standard scales as well as correction factors to account for the dependence of isotope ratios on CH4 mole fractions were determined by taking the mean of the calibration gas measurements in intervals of 16 to 48 h and applying a linear regression analysis. Repeatability based on 10 consecutive analyses of standard air is ±0.05 ‰ or better. δ 13 CCH4 values of RHUL are offset corrected by −0.3 ‰ based on intercomparison measurements with NIWA (Lowe et al., 2004)
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