Variational inverse modeling within the Community Inversion Framework v1.1 to assimilate <i>δ</i><sup>13</sup>C(CH<sub>4</sub>) and CH<sub>4</sub>: a case study with model LMDz-SACS

Abstract. Atmospheric CH4 mole fractions resumed their increase in 2007 after a plateau during the 1999–2006 period, indicating relative changes sources and sinks. Estimating by exploiting observations within an inverse modeling framework (top-down approaches) is powerful approach. It is, nevertheless, challenging to efficiently differentiate co-located emission categories sinks using alone. As result, top-down approaches are limited when it comes fully understanding burden attributing these specific source variations. δ13C(CH4)source isotopic signatures of differ between (biogenic, thermogenic, pyrogenic) can therefore be used address this limitation. Here, new 3-D variational designed assimilate δ13C(CH4) together with presented. This system capable optimizing both emissions associated multiple at pixel scale. To our knowledge, represents first attempt carry out inversion assimilating chemistry transport model (CTM) independently optimize categories. We present technical implementation joint constraints analyze how sensitive setup controlling optimization LMDz-SACS CTM. find that allowing adjust provide relatively large differences global flux estimates for wetlands (−5.7 Tg yr−1), agriculture waste (−6.4 fossil fuels (+8.6 yr−1) biofuels–biomass burning (+3.2 compared results inferred without observations. More importantly, observations, but assuming perfectly known, factor 3–4, strengthening importance having as accurate signature possible. Initial conditions, uncertainties or number optimized have much smaller impact (less than 2 yr−1).