Since the international community adopted the Vienna Convention for the Protection of the Ozone Layer in 1985 and the Montreal Protocol on Substances that Deplete the Ozone Layer in 1987, global "sky-repairing" actions have been implemented, progressively phasing out the production and consumption of ozone-depleting substances (ODS) including chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and halons. Currently, with the exception of HCFCs, the majority of controlled ODS production has been successfully phased out; recent studies indicate the global ozone layer is now on a path to recovery.

However, carbon tetrachloride (CCl₄), one of the earliest key controlled substances, has shown a persistent discrepancy over the past two decades: its annual emissions, estimated based on atmospheric observations, have consistently exceeded those calculated from reported national production and consumption data by over ten thousand tonnes. The issue of CCl₄ emissions has consequently become a recurring agenda item at the Meetings of the Parties to the Montreal Protocol since 2005. Unlike CFCs and halons, which are largely considered delayed emissions, CCl₄ is categorized as having instantaneous emissions. Nevertheless, its emission levels have remained high even after its global production and consumption for dispersive uses was banned in 2010, making it one of the significant factors impeding further recovery of the ozone layer.

Figure 1. Contributions of Major Ozone-Depleting Substances (Halogen-Containing Source Gases) to Stratospheric Halogen Loading. Source: Scientific Assessment of Ozone Depletion: 2022.

To further explain the "unknown sources" of carbon tetrachloride (CCl₄) emissions, clarify potential threats to the ozone layer recovery process, and contribute to global ozone layer protection efforts, a research team led by Professor Hu Jianxin from Peking University, in collaboration with multiple scientific research institutions, has focused on studying the long-standing scientific question of the global CCl₄ "source gap."

The research team investigated China's CCl₄ emissions based on continuous atmospheric observation data from multiple monitoring sites across the country. Addressing the characteristics of the observational data—namely, "large concentration spans and varying sampling frequencies"—the team optimized and constructed a hierarchical Bayesian numerical inversion framework. This framework, combined with non-Gaussian distribution assumptions and Markov Chain Monte Carlo sampling methods, enabled a high-precision, top-down estimation of China's CCl₄ emissions from 2011 to 2021. The results indicate that China's annual CCl₄ emissions over the past decade have remained relatively stable, within the range of approximately 16-25 kilotonnes per year.

Previous global studies suggested that the global CCl₄ emission gap could largely originate from production activities related to chlorinated methanes. However, this study found that although China's chlorinated methane production and use of CCl₄ as a feedstock have increased significantly in recent years, the country's CCl₄ emissions have not increased correspondingly but have instead slightly decreased. The study also found that China's share of global total CCl₄ emissions is lower than its share of global chlorinated methane production. Furthermore, the research suggests that other countries may still contribute significant legacy emissions and by-product emissions from chlorine-related industries. For instance, the contribution from some developed countries with historically high consumption levels should not be overlooked. These emission sources warrant attention and consideration from the international community.

The study further integrated the latest industrial production, consumption data, and potential emission source information, adopting a bottom-up inventory methodology to systematically delineate the emission structure of carbon tetrachloride (CCl₄) in China. The results revealed that a considerable proportion of China's total CCl₄ emissions cannot yet be explained by any existing known sources—whether controlled or non-controlled. These unaccounted-for "unknown sources" constitute approximately half of the global total emission gap. Notably, the research found that these unexplained "unknown emissions" have demonstrated a gradual declining trend in recent years, decreasing to a relatively low level by 2021. However, while CCl₄ remains permitted as a feedstock for producing new-generation substitute substances (such as hydrofluorocarbons HFCs and hydrofluoroolefins HFOs), the study found that emissions from this source show a continuous upward trend, increasingly constituting one of the primary emission sources in recent years.

Figure 2. Source Apportionment of Carbon Tetrachloride (CCl₄) Emissions in China

This research provides critical evidence for assessing and addressing legacy carbon tetrachloride emissions in China and other countries worldwide. In April 2025, the Ministry of Ecology and Environment, in collaboration with multiple departments, issued the *National Implementation Plan for Phasing out Ozone-Depleting Substances in China (2025-2030)*, which further strengthens the management of production quotas for CCl₄ exempted as feedstock and proposes enhanced monitoring and control of by-product emissions, thereby advancing the continuous deepening of ozone layer protection compliance efforts.

These research findings were published under the title“Persistent emissions of ozone-depleting carbon tetrachloride from China during 2011-2021” in Nature Geoscience on June 23, 2025. An Minde, a 2022 PhD graduate from the College of Environmental Sciences and Engineering at Peking University (now a postdoctoral researcher at the Massachusetts Institute of Technology, USA), served as the paper's first author. Professor Hu Jianxin from Peking University, Research Fellow Yao Bo from Fudan University, and An Minde are co-corresponding authors of the article.

DOI:https://www.nature.com/articles/s41561-025-01721-4?utm_source=rct_congratemailt&utm_medium=email&utm_campaign=nonoa_20250623&utm_content=10.1038/s41561-025-01721-4