A Synthesis Inversion to Constrain Global Emissions of Two Very Short Lived Chlorocarbons: Dichloromethane, and Perchloroethylene

Dichloromethane (CH₂Cl₂) and perchloroethylene (C₂Cl₄) are chlorinated very short lived substances (Cl-VSLS) with anthropogenic sources. Recent studies highlight the increasing influence of such compounds, particularly CH₂Cl₂, on the stratospheric chlorine budget and therefore on ozone depletion. Here, a multiyear global-scale synthesis inversion was performed to optimize CH₂Cl₂ (2006–2017) and C₂Cl₄ (2007–2017) emissions. The approach combines long-term surface observations from global monitoring networks, output from a three-dimensional chemical transport model (TOMCAT), and novel bottom-up information on prior industry emissions. Our posterior results show an increase in global CH₂Cl₂ emissions from 637 ± 36 Gg yr⁻¹ in 2006 to 1,171 ± 45 Gg yr⁻¹ in 2017, with Asian emissions accounting for 68% and 89% of these totals, respectively. In absolute terms, Asian CH₂Cl₂ emissions increased annually by 51 Gg yr⁻¹ over the study period, while European and North American emissions declined, indicating a continental-scale shift in emission distribution since the mid-2000s. For C₂Cl₄, we estimate a decrease in global emissions from 141 ± 14 Gg yr⁻¹ in 2007 to 106 ± 12 Gg yr⁻¹ in 2017. The time-varying posterior emissions offer significant improvements over the prior. Utilizing the posterior emissions leads to modeled tropospheric CH₂Cl₂ and C₂Cl₄ abundances and trends in good agreement to those observed (including independent observations to the inversion). A shorter C₂Cl₄ lifetime, from including an uncertain Cl sink, leads to larger global C₂Cl₄ emissions by a factor of ~1.5, which in some places improves model-measurement agreement. The sensitivity of our findings to assumptions in the inversion procedure, including CH₂Cl₂ oceanic emissions, is discussed.