Characterisation of methane sources in Lutjewad, The Netherlands, using quasi-continuous isotopic composition measurements

Abstract

Despite the importance of methane for climate change mitigation, uncertainties regarding the temporal and spatial variability of the emissions remain. Measurements of CH₄ isotopic composition are used to partition the relative contributions of different emission sources. We report continuous isotopic measurements during 5 months at the Lutjewad tower (north of the Netherlands). Time-series of χ(CH₄), δ¹³C-CH₄, and δD-CH₄ in ambient air were analysed using the Keeling plot method. Resulting source signatures ranged from −67.4 to −52.4‰ vs V-PDB and from −372 to −211‰ vs V-SMOW, for δ¹³C and δD respectively, indicating a prevalence of biogenic sources. Analysis of isotope and wind data indicated that (i) emissions from off-shore oil and gas platforms in the North Sea were not detected during this period, (ii) CH₄ from fossil fuel related sources was usually advected from the east, pointing towards the Groningen gas field or regions further east in Germany. The results from two atmospheric transport models, CHIMERE and FLEXPART-COSMO, using the EDGAR v4.3.2 and TNO-MACC III emission inventories, reproduce χ(CH₄) variations relatively well, but the isotope signatures were over-estimated by the model compared to the observations. Accounting for geographical variations of the δ¹³C signatures from fossil fuel emissions improved the model results significantly. The difference between model and measured isotopic signatures was larger when using TNO-MACC III compared to EDGAR v4.3.2 inventory. Uncertainties in the isotope signatures of the sources could explain a significant fraction of the discrepancy, thus a better source characterisation could further strengthen the use of isotopes in constraining emissions.

Keywords:

• methane
• isotope ratio mass spectrometry
• source isotopic signatures
• emission inventories
• in-situ measurements

Data availability statement

The data that support the findings of this study are openly available in mamenoud/CH4-Lutjewad-2016-2017: First release (Version v1.0.0) at http://doi.org/10.5281/zenodo.3970888.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

Supplemental data for this article can be accessed here

Table 1. Initial δ¹³C and δD values from literature used in the models for the different emission sectors (Szénási 2019). They are derived from signatures found in the cited studies. The range of values is reported in the brackets. Only the δ¹³C value for fossil fuel emissions (bold) was modified from Szénási 2019 to better represent the emissions from this sector in the Netherlands.

Acknowledgements

We specially thank Bert Kers and Marcel de Vries at CIO for the technical support they provided during the measurements at Lutjewad.

Additional information

Funding

This work was supported by the ITN project Methane goes Mobile – Measurements and Modelling (MEMO²; https://h2020-memo2.eu/). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 722479. AGAGE is supported principally by NASA (USA) grants to MIT and SIO, and also by: BEIS (UK) and NOAA (USA) grants to Bristol University; CSIRO and BoM (Australia): FOEN grants to Empa (Switzerland); NILU (Norway); SNU (Korea); CMA (China); NIES (Japan); and Urbino University (Italy).