Strong sensitivity of the isotopic composition of methane to the plausible range of tropospheric chlorine

The C-13 isotopic ratio of methane, delta C-13 of CH4, provides additional constraints on the CH4 budget to complement the constraints from CH4 observations. The interpretation of delta C-13 observations is complicated, however, by uncertainties in the methane sink. The reaction of CH4 with Cl is highly fractionating, increasing the relative abundance of (CH4)-C-13, but there is currently no consensus on the strength of the tropospheric Cl sink. Global model simulations of halogen chemistry differ strongly from one another in terms of both the magnitude of tropospheric Cl and its geographic distribution. This study explores the impact of the intermodel diversity in Cl fields on the simulated delta C-13 of CH4. We use a set of GEOS global model simulations with different predicted Cl fields to test the sensitivity of the delta C-13 of CH4 to the diversity of Cl output from chemical transport models. We find that delta C-13 is highly sensitive to both the amount and geographic distribution of Cl. Simulations with Cl providing 0.28 % or 0.66 % of the total CH4 loss bracket the delta C-13 observations for a fixed set of emissions. Thus, even when Cl provides only a small fraction of the total CH4 loss and has a small impact on total CH4, it provides a strong lever on delta C-13. Consequently, it is possible to achieve a good representation of total CH4 using widely different Cl concentrations, but the partitioning of the CH4 loss between the OH and Cl reactions leads to strong differences in isotopic composition depending on which model's Cl field is used. Comparing multiple simulations, we find that altering the tropospheric Cl field leads to approximately a 0.5 parts per thousand increase in delta(CH4)-C-13 for each percent increase in how much CH4 is oxidized by Cl. The geographic distribution and seasonal cycle of Cl also impacts the hemispheric gradient and seasonal cycle of delta C-13. The large effect of Cl on delta C-13 compared to total CH4 broadens the range of CH4 source mixtures that can be reconciled with delta C-13 observations. Stronger constraints on tropospheric Cl are necessary to improve estimates of CH4 sources from delta C-13 observations.