Toward resolving the budget discrepancy of ozone-depleting carbon tetrachloride (CCl<sub>4</sub>): an analysis of top-down emissions from China

Abstract

Carbon tetrachloride (CCl₄) is a first-generation ozone-depleting substance, and its emissive use and production were globally banned by the Montreal Protocol with a 2010 phase-out; however, production and consumption for non-dispersive use as a chemical feedstock and as a process agent are still allowed. This study uses the high frequency and magnitude of CCl₄ pollution events from an 8-year real-time atmospheric measurement record obtained at Gosan station (a regional background monitoring site in East Asia) to present evidence of significant unreported emissions of CCl₄. Top-down emissions of CCl₄ amounting to 23.6±7.1 Gg yr⁻¹ from 2011 to 2015 are estimated for China, in contrast to the most recently reported, post-2010, Chinese bottom-up emissions of 4.3-5.2 Ggyr⁻¹. The missing emissions (~19 Ggyr ⁻¹) for China contribute to approximately 54 % of global CCl₄ emissions. It is also shown that 89 % ± 6% of CCl₄ enhancements observed at Gosan are related to CCl₄ emissions from the production of CH₃Cl, CH₂Cl₂, CHCl₃ and C₂Cl₄ and its usage as a feedstock and process agent in chemical manufacturing industries. Specific sources and processes are identified using statistical methods, and it is considered highly unlikely that CCl₄ is emitted by dispersive uses such as old landfills, contaminated soils and solvent usage. It is thus crucial to implement technical improvements and better regulation strategies to reduce evaporative losses of CCl₄ occurring at the factory and/or process levels.