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

Sarah A. Strode,Sylvia E. Michel,James W. C. White,Ryan Hossaini,Christoph A. Keller,James S. Wang,Bryan Duncan,Michael Manyin

doi:10.5194/acp-20-8405-2020

Abstract

Abstract. The 13C isotopic ratio of methane, δ13C of CH4, provides additional constraints on the CH4 budget to complement the constraints from CH4 observations. The interpretation of δ13C observations is complicated, however, by uncertainties in the methane sink. The reaction of CH4 with Cl is highly fractionating, increasing the relative abundance of 13CH4, 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 δ13C of CH4. We use a set of GEOS global model simulations with different predicted Cl fields to test the sensitivity of the δ13C of CH4 to the diversity of Cl output from chemical transport models. We find that δ13C 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 δ13C 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 δ13C. 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 ‰ increase in δ13CH4 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 δ13C. The large effect of Cl on δ13C compared to total CH4 broadens the range of CH4 source mixtures that can be reconciled with δ13C observations. Stronger constraints on tropospheric Cl are necessary to improve estimates of CH4 sources from δ13C observations.

Highlights

The global budget of methane is of great interest due to methane’s role as a greenhouse gas, ozone precursor, and sink of the hydroxyl radical
The observations at the other stations lie within the range of simulations, with most simulations underestimating the observations at the south pole
We investigated the sensitivity of the surface δ13C distribution of CH4 to the intermodel diversity in tropospheric Cl using a series of sensitivity studies with a global 3D model

Summary

Introduction

The global budget of methane is of great interest due to methane’s role as a greenhouse gas, ozone precursor, and sink of the hydroxyl radical. The resumed increase of methane concentrations beginning in 2007 (Dlugokencky et al, 2009; Rigby et al, 2008) can be explained by multiple hypotheses including an increase in fossil fuel emissions (Turner et al, 2016; Thompson et al, 2015; Hausmann et al, 2016), an increase in fossil fuel emissions combined with a decrease in biomass burning (Worden et al, 2017), an increase in bio-. Strode et al.: Sensitivity of CH4 isotopic composition to tropospheric chlorine genic sources (Schaefer et al, 2016; Nisbet et al, 2016), or a decrease in hydroxyl concentrations (Turner et al, 2017; Rigby et al, 2017). Variations in hydroxyl concentrations may be important for the decrease in methane growth from 1999 to 2006 (McNorton et al, 2016)

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