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 carbon tetrachloride formula. 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 Gg yr−1. The missing emissions (∼19 Gg yr−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 carbon tetrachloride formula occurring at the factory and/or process levels.
Carbon tetrachloride (CCl4) is a long-lived greenhouse gas and an ozone-depleting substance. Its emissive use, production and consumption are regulated under the Montreal Protocol on Substances that Deplete the Ozone Layer and its Amendments (MP). After reaching a peak in the early 1990s, the atmospheric abundance of carbon tetrachloride formula has been decreasing at a rate of ppt Cl yr−1 due to the phase-out of CCl4 use in MP non-Article 5 (developed) countries by 1995. MP Article 5 (developing) countries, including China, were required to cease CCl4 production and consumption for dispersive applications by 2010.
To verify these bottom-up estimates, independent top-down CCl4 emission studies have used the total lifetime of CCl4 with atmospheric observations and atmospheric transport models to derive top-down emission estimates. Using the most current estimates for the lifetime of CCl4 in the atmosphere, soil and ocean, global top-down emissions to the atmosphere were calculated as 40±15 Gg yr−1 from 2007 to 2014. A recent top-down study based on the observed temporal trend and interhemispheric gradient of atmospheric CCl4 consistently derived global CCl4 emissions of 30±5 Gg yr−1 from 2000 to 2012 when using the newly determined relative strength of oceanic sink versus soil loss. Therefore, the best estimate of global emissions from top-down methods is 35±16 Gg yr−1, which is significantly higher than reported emissions of 3 Gg yr−1, even when considering large uncertainties relating to soil and ocean CCl4 sinks (and how those sinks might change over time). Although the revised bottom-up estimate of 25 Gg yr−1 mentioned above contributes considerably to closing the gap between bottom-up and top-down emission estimates, this new bottom-up value is still lower than the average SPARC-merged top-down emission estimate of 35±16 Gg yr−1 (though the uncertainty is large). The discrepancy between bottom-up and top-down emission estimates implies the existence of unidentified sources and/or unreported industrial emissions.
