15 Nov 2021

15 Nov 2021

Review status: this preprint is currently under review for the journal ACP.

Role of emission sources and atmospheric sink on the seasonal cycle of CH4 and δ13-CH4: analysis based on the atmospheric chemistry transport model TM5

Vilma Kangasaho1, Aki Tsuruta1, Leif Backman1, Pyry Mäkinen2, Sander Houweling3,4, Arjo Segers5, Maarten Krol6,7, Ed Dlugokencky8, Sylvia Michel9, James White9, and Tuula Aalto1 Vilma Kangasaho et al.
  • 1Finnish Meteorological Institute, P.O. Box.503, FI-00101, Helsinki, Finland
  • 2Aalto University, Finland
  • 3SRON Netherlands Institute for Space Research, the Netherlands
  • 4Vrije Universiteit Amsterdam, the Netherlands
  • 5Netherlands Organisation for Applied Scientific Research (TNO), Utrecht, the Netherlands
  • 6Wageningen University & Research, Meteorology and Air Quality, the Netherlands
  • 7IMAU, Utrecht University, the Netherlands
  • 8NOAA Global Monitoring Laboratory (GML), USA
  • 9NSTAAR, University of Colorado, USA

Abstract. This study investigates the contribution of different CH4 sources to the seasonal cycle of 𝛿13C during years 2000–2012 using the TM5 atmospheric transport model. The seasonal cycles of anthropogenic emissions from two versions of the EDGAR inventories, v4.3.2 and v5.0 are examined. Those includes emissions from Enteric Fermentation and Manure Management (EFMM), rice cultivation and residential sources. Those from wetlands obtained from LPX-Bern v1.4 are also examined in addition to other sources such as fires and ocean sources. We use spatially varying isotopic source signatures for EFMM, coal, oil and gas, wetlands, fires and geological emission and for other sources a global uniform value.

We analysed the results as zonal means for 30° latitudinal bands. Seasonal cycles of 𝛿13C are found to be an inverse of CH4 cycles in general, with a peak-to-peak amplitude of 0.07–0.26 ‰. However, due to emissions, the phase ellipses do not form straight lines, and the anti-correlations between CH4 and 𝛿13C are weaker (−0.35 to −0.91) in north of 30° S. We found that wetland emissions are the dominant driver in the 𝛿13C seasonal cycle in the Northern Hemisphere and Tropics, such that the timing of 𝛿13C seasonal minimum is shifted by ∼90 days in 60° N–90° N from the end of the year to the beginning of the year when seasonality of wetland emissions is removed. The results also showed that in the Southern Hemisphere Tropics, emissions from fires contribute to the enrichment of 𝛿13C in July–October. In addition, we also compared the results against observations from the South Pole, Antarctica, Alert, Nunavut, Canada and Niwot Ridge, Colorado, USA. In light of this research, comparison to the observation showed that the seasonal cycle of EFMM emissions in EDGAR v5.0 inventory is more realistic than in v4.3.2. In addition, the comparison at Alert showed that modelled 𝛿13C amplitude was approximately half of the observations, mainly because the model could not reproduce the strong depletion in autumn. This indicates that CH4 emission magnitude and seasonal cycle of wetlands may need to be revised. Results from Niwot Ridge indicate that in addition to biogenic emissions, the proportion of biogenic to fossil based emissions may need to be revised.

Vilma Kangasaho et al.

Status: open (until 27 Dec 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Vilma Kangasaho et al.

Vilma Kangasaho et al.


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Short summary
Understanding the composition of carbon isotopes can help to better understand the changes in methane budgets. This study investigates how methane sources affect the seasonal cycle of the methane carbon-13 isotope during 2000–2012 using an atmospheric transport model. We found that emissions from both anthropogenic and natural sources contribute. The findings raise a need to revise the magnitudes, proportion, and seasonal cycles of anthropogenic sources and northern wetland emissions.