The atmospheric lifetime of methane is determined by reaction with the hydroxyl radical (OH•), which also controls the lifetime of many other trace gases. The chlorine (Cl•) radical also reacts with methane but is a minor sink because of its low atmospheric abundance. Increasing the abundance of Cl• has been proposed as a strategy to reduce atmospheric methane and global warming. The atmospheric sources and abundance of Cl• are highly uncertain. Cl• cannot be directly measured. The atmospheric chemistry responsible for its production is complicated and is not included in most models of atmospheric chemistry, inhibiting the quantification of its role in the methane lifetime. Relevant chemistry has recently been included in two global chemistry-climate models. These models suggest that the chemistry of reactive chlorine is tightly coupled to the chemistry of two other halogens, bromine and iodine, and is strongly influenced by anthropogenic emissions [Sherwen et al., 2017; Saiz-Lopez et al., 2023]. This is consistent with Arctic ice core observations of recent trends in chlorine [Zhai et al., 2021], bromine [Zhai et al., 2024], and iodine [unpublished data].

This work aims to quantify the influence of the anthropogenic-driven change in Cl• on the methane lifetime and isotopes. Methane isotopes are very sensitive to the CH₄ + Cl• reaction due to the large fractionation effect. Methane isotopes are used to constrain methane’s sources and sinks and inform the cause of the observed variability in methane abundance and growth rate over the last 40 years. The large anthropogenic-driven change in Cl• since 1940 has likely influenced methane isotopes but has not been considered because past variability in the abundance of Cl• was unknown. The key goals are 1) determine the anthropogenic-driven change in the atmospheric abundance of Cl• using a state-of-the-art chemical transport model, 2) calculate the impact of the change in Cl• on methane isotopes and lifetime, and 3) assess the influence of Cl•-fractionation on calculations of the methane budget. Success will be evaluated based on the ability of the models to simulate Arctic ice core observations of chlorine, bromine, and iodine and atmospheric observations of methane isotopes.

Improved understanding of past anthropogenic impacts on Cl• and the methane lifetime and isotopes will 1) advance understanding of recent variability in methane abundance, as its causes are currently unknown, 2) quantify the role that humans have already played in altering Cl•, 3) understand the influence of coupled chlorine- bromine-iodine chemistry on Cl• and OH•, and 4) inform potential future changes in Cl• abundance and its impacts on methane due to changes in climate and anthropogenic emissions. This will in turn inform our understanding of our ability to and the impacts of purposefully decreasing the methane lifetime. This project will also contribute to the development of the only publicly available global chemical transport model that contains a comprehensive representation of tropospheric reactive halogen chemistry, thus enabling future studies of the sinks of methane by the global atmospheric chemistry community.