The impact of chlorocarbons on tropospheric composition: a global model study

Tropospheric chlorine has the potential to perturb atmospheric oxidation capacity, which plays an important role in climate change and air quality. Although sea-salt is the predominant source of tropospheric chlorine, oxidation of chlorocarbons could prove to be a more important source of tropospheric chlorine than previously thought due to their increasing abundances over the last 2 decades. The most abundant chlorocarbon, methyl chloride (CH₃Cl) is predominantly emitted from natural sources and has stayed relatively stable over the last 20 years. However, the concentrations of a range of chlorine-containing very short-lived substances (Cl-VSLS) have varied substantially over the same time period, particularly anthropogenically emitted Cl-VSLS: dichloromethane (CH₂Cl₂), 1,2-dichloroethane (C₂H₄Cl₂), perchloroethylene (C₂Cl₄), trichloromethane (CHCl3) and trichloroethylene (C₂HCl₃). Additionally, there are a number of bromine and iodine containing Cl-VSLS (e.g. CH₂BrCl) that are released from natural sources.

Here, the Frontier Research System for Global Change version of the University of California Irvine Chemical Transport Model is used to explore the impact of Cl-VSLS in the troposphere. A tropospheric chlorine chemistry scheme including appropriate sources, reactions and sinks of chlorine species was incorporated into the model and multi-year simulations were used to assess the spatio-temporal trends in Cl-VSLS. Two approaches were compared to assess the impact of Cl-VSLS. The first method involves constraining the chlorocarbons using latitude- and time-varying surface concentrations generated from measurement data, whilst the second method used fully geographically varying emissions. We consider a more comprehensive set of chlorocarbons than previous studies and explore how their abundances have changed over time. We find that the contribution of chlorocarbons to tropospheric inorganic chlorine has increased; from ~4100 Gg/year in 2000 to ~4600 Gg/year in 2022. The impact of chlorocarbons on tropospheric composition will also be presented.