Modelling the seasonal cycle of atmospheric δ13C-CH4 using source specific δ13C-CH4 values

The atmospheric burden of methane (CH₄) has more than doubled since the 18^th Century. Currently the abundance of CH₄ in the atmosphere is well known, but emission rates from different source sectors are uncertain. CH₄ is emitted to the atmosphere from various sources. To better understand the changes in atmospheric CH₄ abundance before and after 2006, it is important to study the contribution from these different sources separately. Most CH₄ source have process specific δ¹³C-CH₄ values, which can be used to broadly identify source sectors.

This study examines the seasonal cycle of atmospheric δ¹³C-CH₄ in recent decades using the TM5 atmospheric transport. TM5 is driven by ECMWF ERA-Interim meteorological fields, and uses pre-calculated OH-fields and reaction rates with Cl and O(¹D) to account for the CH₄ sink processes in the atmosphere. TM5 is run at a 1^ox1^o resolution over Europe and globally at 6^ox4^o. Emissions for enteric fermentation and manure management, landfills and waste water treatment, rice cultivation, coal industry, oil and gas industry, and residential are taken from the EDGAR inventory. Natural emission for wetlands, peatlands and mineral soils, and soil sinks are taken from the LPX-Bern DYPTOP ecosystem model. Emissions for geological seeps including onshore hydrocarbon macro-seeps (including mud volcanoes), submarine (offshore) seeps, diffuse microseepage and geothermal manifestations are included. Emissions for fires (GFED v4), termites, wild animals and from the ocean are also included. Several sensitivity analyses are carried out. The sensitivity analyses include simulations with and without seasonal cycles in the anthropogenic emission fields (EDGAR v4.2 FT2010 vs EDGAR v4.3.2), and with and without spatial variations in source specific  δ¹³C-CH₄ values, which are used to calculate ¹³CH₄/CH₄ emission ratios. The effect of including the seasonal cycle in the anthropogenic emissions were not significant, which means natural sources probably play more important role in determining the seasonal cycle of δ¹³C-CH₄. The global observations of atmospheric CH₄ and δ¹³C-CH₄, provided by NOAA’s GMD, the INSTAAR and Royal Holloway, the University of London, are used for evaluation. We present the importance of having reasonable initial fields of atmospheric 13CH₄, which will be later used as inputs for CarbonTracker Europe-δ¹³CH₄ (CTE-δ¹³CH₄) data assimilation system to optimise CH₄ emissions by source category.