Ozone-depleting Carbon Tetrachloride (CCl4) Emissions in China and Their Implications for the Global Emission Gap

Carbon tetrachloride (CCl₄) is a long-lived ozone-depleting substance whose production and consumption for dispersive uses have been phased out under the Montreal Protocol since 2010. However, substantial emissions of CCl₄ persist globally, potentially delaying ozone layer recovery, with ~30-40% of the sources of global emissions remaining largely unknown.

Here, we focus on CCl₄ emissions in China, a major contributor to global halocarbon emissions. Based on long-term atmospheric observations from a Chinese network and a top-down inverse modeling approach, we identified substantial CCl₄ emissions in China during 2011-2021, accounting for approximately half of the global emissions. By further incorporating industry information and a bottom-up approach, we find that although CCl₄ emissions from allowed feedstock uses — proposed to explain the majority of the global budget — were increasing, top-down CCl₄ emissions in China didn’t increase accordingly but instead slightly decreased (though statistically insignificant). A large portion of the top-down emissions in China likely originates from other unknown sources beyond allowed feedstock use, CFC-11 production related activities, and by-product emissions in chlorine-related processes. The unaccounted-for emissions in China could explain more than half of the reported global CCl₄ unknown gap in 2014 and 2019, although they may have decreased in China over 2011-2021. In addition, legacy emissions from contaminated sites due to historical use of CCl₄ in regions other than China are also substantial and warrant further attention.