The Role of Emission Sources and Atmospheric Sink in the Seasonal Cycle of CH₄ and δ¹³-CH₄: Analysis Based on the Atmospheric Chemistry Transport Model TM5

This study investigates the contribution of different CH₄ sources to the seasonal cycle of δ¹³C during 2000–2012 by using the TM5 atmospheric transport model, including spatially varying information on isotopic signatures. The TM5 model is able to produce the background seasonality of δ¹³C, but the discrepancies compared to the observations arise from incomplete representation of the emissions and their source-specific signatures. Seasonal cycles of δ¹³C are found to be an inverse of CH₄ cycles in general, but the anti-correlations between CH₄ and δ¹³C are imperfect and experience a large variation (ρ = −0.35 to −0.91) north of 30° S. We found that wetland emissions are an important driver in the δ¹³C seasonal cycle in the Northern Hemisphere and Tropics, and in the Southern Hemisphere Tropics, emissions from fires contribute to the enrichment of δ¹³C in July–October. The comparisons to the observations from 18 stations globally showed that the seasonal cycle of EFMM emissions in the EDGAR v5.0 inventory is more realistic than in v4.3.2. At northern stations (north of 55° N), modeled δ¹³C amplitudes are generally smaller by 12–68%, mainly because the model could not reproduce the strong depletion in autumn. This indicates that the CH₄ emission magnitude and seasonal cycle of wetlands may need to be revised. In addition, results from stations in northern latitudes (19–40° N) indicate that the proportion of biogenic to fossil-based emissions may need to be revised, such that a larger portion of fossil-based emissions is needed during summer.