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

RYAN HOSSAINI; MARTYN CHIPPERFIELD; ALFONSO SAIZ-LOPEZ; RAFAEL PEDRO FERNÁNDEZ; SARAH MONKS; PETER BRAUER; ROLAND VON GLASOW

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2016 vol. 121 p. 1 - 27

0148-0227

Chlorine atoms (Cl) are highly reactive towards 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, CHBrCl2) and anthropogenic (CH2Cl2, CHCl3, C2Cl4, C2HCl3, 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. Modelled annual mean surface [Cl] exhibits large spatial variability with the largest levels, typically in the range 1-5×104 atoms cm−3, in the polluted northern hemisphere. Chlorocarbon oxidation sustains a background surface [Cl] of <0.1 to 0.5×103 atoms cm−3 over large areas, and provides a tropospheric Cly source of up to ~4320 Gg Cl/year. Globally, we estimate a tropospheric methane sink of ~12-13 Tg CH4/year 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.