Abstract
AbstractChlorine 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.
Highlights
Atmospheric chlorine chemistry rose to prominence in the 1970s when it was discovered that Chlorine atoms (Cl) atoms, released from chlorofluorocarbons and other long-lived anthropogenic compounds, could catalyze ozone loss in the stratosphere [Molina and Rowland, 1974]
Most Cly in the troposphere resides as hydrogen chloride (HCl), which is directly emitted from combustion processes [e.g., McCulloch et al, 1999], released from sea-salt aerosol through acid displacement (e.g., Table 5, H10), and produced following the reaction of atomic Cl with volatile organic compounds (VOCs) (e.g., Table 1)
We have implemented a detailed representation of tropospheric chlorine chemistry and sources into the TOMCAT global 3-D chemical transport model
Summary
Atmospheric chlorine chemistry rose to prominence in the 1970s when it was discovered that Cl atoms, released from chlorofluorocarbons and other long-lived anthropogenic compounds, could catalyze ozone loss in the stratosphere [Molina and Rowland, 1974]. In addition to the above inorganic Cl precursors, chlorocarbons such as dichloromethane (CH2Cl2) and chloroform (CHCl3) are present in the troposphere These so-called very short lived substances (VSLSs) have, in recent years, been a major topic of stratospheric ozone-focused research, because industrial emissions of certain VSLS (not controlled by the UN Montreal Protocol) are increasing [e.g., Hossaini et al, 2015a]. Various moderately stable and intermediate-organic product gases, most of which are subject to deposition processes, may be formed [Hossaini et al, 2015b] The validity of this assumption is unclear, and more broadly, the contribution of VSLS to tropospheric Cly is poorly quantified based on present-day VSLS loadings.
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