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 (CH₄). However, the regional and global CH₄ 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 (Cly), including (i) the oxidation of chlorocarbons of natural (CH₃Cl, CHBr₂Cl, CH₂BrCl, and CHBrCl₂) and anthropogenic (CH₂Cl₂, CHCl₃, C₂Cl₄, C₂HCl₃, and CH₂ClCH₂Cl) 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 CH₄ oxidation. In agreement with observations, simulated surface levels of hydrogen chloride (HCl), the most abundant Cly 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 × 104 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 × 103 atoms cm¯³ over large areas. Globally, we estimate a tropospheric methane sink of ~12–13 Tg CH₄/yr due the CH₄ + Cl reaction (~2.5% of total CH₄ oxidation). Larger regional effects are predicted, with Cl accounting for ~10 to >20% of total boundary layer CH₄ oxidation in some locations.