In situ observations of the isotopic composition of methane at the Cabauw tall tower site

Abstract

Abstract. High-precision analyses of the isotopic composition of methane in ambient air can potentially be used to discriminate between different source categories. Due to the complexity of isotope ratio measurements, such analyses have generally been performed in the laboratory on air samples collected in the field. This poses a limitation on the temporal resolution at which the isotopic composition can be monitored with reasonable logistical effort. Here we present the performance of a dual isotope ratio mass spectrometric system (IRMS) and a quantum cascade laser absorption spectroscopy (QCLAS)-based technique for in situ analysis of the isotopic composition of methane under field conditions. Both systems were deployed at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands and performed in situ, high-frequency (approx. hourly) measurements for a period of more than 5 months. The IRMS and QCLAS instruments were in excellent agreement with a slight systematic offset of (+0.25 ± 0.04) ‰ for δ13C and (−4.3 ± 0.4) ‰ for δD. This was corrected for, yielding a combined dataset with more than 2500 measurements of both δ13C and δD. The high-precision and high-temporal-resolution dataset not only reveals the overwhelming contribution of isotopically depleted agricultural CH4 emissions from ruminants at the Cabauw site but also allows the identification of specific events with elevated contributions from more enriched sources such as natural gas and landfills. The final dataset was compared to model calculations using the global model TM5 and the mesoscale model FLEXPART-COSMO. The results of both models agree better with the measurements when the TNO-MACC emission inventory is used in the models than when the EDGAR inventory is used. This suggests that high-resolution isotope measurements have the potential to further constrain the methane budget when they are performed at multiple sites that are representative for the entire European domain.

Interlaboratory comparison of <i>δ</i><sup>13</sup>C and <i>δ</i>D measurements of atmospheric CH<sub>4</sub> for combined use of data sets from different laboratories

“…Later, a method based on a continuous-flow gas chromatography isotope ratio mass spectrometry (GC-IRMS) technique combined with combustion and pyrolysis furnaces became available (Merritt et al, 1995;Burgoyne and Hayes, 1998;Hilkert et al, 1999), which dramatically reduced time and effort in the laboratory and likewise the amount of sample air required (now typically 100 mL STP ). Such systems are now used in most laboratories worldwide to acquire δ 13 C-CH 4 and δD-CH 4 data in the current and past atmosphere (Rice et al, 2001;Miller et al, 2002;Sowers et al, 2005;Ferretti et al, 2005;Morimoto et al, 2006;Fisher et al, 2006;Umezawa et al, 2009;Brass and Röckmann, 2010;Sperlich et al, 2013;Schmitt et al, 2014;Bock et al, 2014;Brand et al, 2016;Röckmann et al, 2016). Although these systems use a similar measurement principle, they vary in the use of pre-concentration of CH 4 in sample air, GC separation and combustion/pyrolysis, data corrections and in the specific IRMS instrument among laboratories (see Schmitt et al, 2013, Sect.…”

“…Incorporating δ 13 C-CH 4 and δD-CH 4 data sets in chemistry transport models is useful for quantitatively separating different CH 4 source categories and attempts have been made to reduce uncertainties in the global CH 4 budget (e.g. Hein et al, 1997;Mikaloff Fletcher et al, 2004a, b;Monteil et al, 2011;Kirschke et al, 2013;Ghosh et al, 2015;Rice et al, 2016;Schaefer et al, 2016;Schwietzke et al, 2016;Röckmann et al, 2016;Turner et al, 2017;Rigby et al, 2017). However, although an increasing number of δ 13 C-CH 4 and δD-CH 4 data have been reported over the last decades, significant measurement offsets among laboratories have been found for both δ 13 C-CH 4 (e.g.…”

Advancing Scientific Understanding of the Global Methane Budget in Support of the Paris Agreement

“…The potential for measurements by laser spectrometry in the midinfrared to become a routine methodology for ambient air sampling is significant Röckmann et al, 2016) . The major benefits of laser spectrometric techniques include their potential for automation and continuous measurement, allowing deployment outside a usual laboratory setting, and their ability to measure more than one isotope system simultaneously.…”

“…Isotopic measurements are more straightforward to interpret on regional scales rather than global scales as pollution episodes have a direct and linear impact on the observed isotopic composition, which can be related directly to the changes in mole fraction Fujita et al, 2018;Röckmann et al, 2016;Warwick et al, 2016). However, inadequate spatial and temporal coverage of measurements has meant that measurements have so far only been used to rule in or out a particular source type contribution rather to than make flux estimates Umezawa et al, 2012;Warwick et al, 2016).…”

“…However, inadequate spatial and temporal coverage of measurements has meant that measurements have so far only been used to rule in or out a particular source type contribution rather to than make flux estimates Umezawa et al, 2012;Warwick et al, 2016). Deploying field instruments to measure near-continuous and high-precision δ 13 C-CH 4 and δD-CH 4 alongside mole fraction measurements will drive development in regional flux estimation over the coming years (Rigby et al, 2012;Röckmann et al, 2016).…”