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

Abstract

Abstract Dichloromethane (CH 2 Cl 2 ) and perchloroethylene (C 2 Cl 4 ) are chlorinated very short lived substances (Cl‐VSLS) with anthropogenic sources. Recent studies highlight the increasing influence of such compounds, particularly CH 2 Cl 2 , on the stratospheric chlorine budget and therefore on ozone depletion. Here, a multiyear global‐scale synthesis inversion was performed to optimize CH 2 Cl 2 (2006–2017) and C 2 Cl 4 (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 CH 2 Cl 2 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 CH 2 Cl 2 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 C 2 Cl 4 , 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 CH 2 Cl 2 and C 2 Cl 4 abundances and trends in good agreement to those observed (including independent observations to the inversion). A shorter C 2 Cl 4 lifetime, from including an uncertain Cl sink, leads to larger global C 2 Cl 4 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 CH 2 Cl 2 oceanic emissions, is discussed.