How do Cl concentrations matter for the simulation of CH₄ and <i>δ</i>¹³C(CH₄) and estimation of the CH₄ budget through atmospheric inversions?

Abstract. Atmospheric methane (CH4) concentrations have been rising since 2007 due to an imbalance between CH4 sources and sinks. The budget is generally estimated through top-down approaches using chemistry transport models (CTMs) observations as constraints. atmospheric isotopic composition, δ13C(CH4), can also provide additional constraints helps discriminate emission categories. Nevertheless, be able use the information contained in these observations, must correctly account for processes influencing δ13C(CH4). oxidation by chlorine (Cl) likely contributes less than 5 % total of CH4. However, large kinetic isotope effect Cl sink produces a fractionation 13C, compared with 12C CH4, thus may strongly influence When integrating their setup constrain budget, which not yet standard, inversions prescribe different fields, therefore leading discrepancies flux estimates. To quantify on estimates, we perform sensitivity simulations four fields. We test removing tropospheric entire sink. find that fields tested here are responsible 0.3 8.5 chemical troposphere 1.0 1.6 stratosphere. Prescribing amounts lead differences up 53.8 Tg yr−1 global emissions 4.7 ‰ globally averaged signature source δ13C(CH4)source), although much smaller if only recent used. More specifically, each increase 1000 molec.cm-3 mean concentration would result adjustment +11.7 yr−1, emissions, −1.0 ‰, δ13C(CH4)source. Our study shows seasonal cycle amplitude modified 1 %–2 %, but δ13C(CH4) significantly 10 %–20 depending latitude. In inversion performed constraints, this significant posterior mixture. For example, contribution from wetland about 0.8 adjust δ13C(CH4)source, corresponding 15 change. This small current uncertainty, albeit far negligible. Finally, simulated vertical profiles above 30 km very impact profiles. Overall, our model captures observed well, especially troposphere, it difficult prefer one field over another based uniquely available