A global model of tropospheric chlorine chemistry: Organic versus inorganic sources and impact on methane oxidation

Chlorine atoms (Cl) are highly reactive toward hydrocarbons in the Earth's troposphere, including the greenhouse gas methane (CH 4 ). However, the regional and global CH 4 sink from Cl is poorly quantified as tropospheric Cl concentrations ([Cl]) are uncertain by ~2 orders of magnitude. Here we describe the addition of a detailed tropospheric chlorine scheme to the TOMCAT chemical transport model. The model includes several sources of tropospheric inorganic chlorine (Cl y ), including (i) the oxidation of chlorocarbons of natural (CH 3 Cl, CHBr 2 Cl, CH 2 BrCl, and CHBrCl 2 ) and anthropogenic (CH 2 Cl 2 , CHCl 3 , C 2 Cl 4 , C 2 HCl 3 , and CH 2 ClCH 2 Cl) origin and (ii) sea‐salt aerosol dechlorination. Simulations were performed to quantify tropospheric [Cl], with a focus on the marine boundary layer, and quantify the global significance of Cl atom CH 4 oxidation. In agreement with observations, simulated surface levels of hydrogen chloride (HCl), the most abundant Cl y reservoir, reach several parts per billion (ppb) over polluted coastal/continental regions, with sub‐ppb levels typical in more remote regions. Modeled annual mean surface [Cl] exhibits large spatial variability with the largest levels, typically in the range of 1–5 × 10 4  atoms cm −3 , in the polluted northern hemisphere. Chlorocarbon oxidation provides a tropospheric Cly source of up to ~4320 Gg Cl/yr, sustaining a background surface [Cl] of <0.1 to 0.5 × 10 3 atoms cm −3 over large areas. Globally, we estimate a tropospheric methane sink of ~12–13 Tg CH 4 /yr due the CH 4  + Cl reaction (~2.5% of total CH 4 oxidation). Larger regional effects are predicted, with Cl accounting for ~10 to >20% of total boundary layer CH 4 oxidation in some locations.