Investigating Regional Halocarbon Emissions: The Seoul Tracer Release Experiment

Hydrofluorocarbons (HFCs) are used as refrigerants, propellants or insulating foams. They don’t deplete the ozone layer like their predecessors, (hydro)chlorofluorocarbons ((H)CFCs). However, HFCs are still potent greenhouse gases and are regulated under the Kyoto Protocol (1997) and, more recently, the Kigali Amendment to the Montreal Protocol. The Kigali amendment targets reductions in HFC production and consumption over the coming decades.^{1, 2} Observing halogenated substances in the atmosphere provides an independent means to verify compliance with these international treaties. From these observations, regional and global emission estimates can be obtained by combining them with atmospheric modelling or using a reference tracer with known emissions.^{3, 4} Due to rapid industrialization and high demand for refrigeration and air conditioning, the eastern Asian region contributes significantly to global HFC emissions. Therefore, it is crucial to understand the emission patterns in this region to assess global compliance.

We have conducted a large-scale controlled-release tracer experiment to estimate regional halocarbon emissions of the greater Seoul metropolitan area (South Korea). Ethyl fluoride (HFC-161)⁵ and hexafluorobutane (HFO-1336mzzE), which are virtually absent in the background atmosphere, were released at one location in the City of Seoul. Release times were selected to align with favorable meteorological conditions that allowed air masses to reach the AGAGE station Gosan (Jeju Island, 490 km south of Seoul). The site is equipped with an instrument for in-situ halocarbon measurements. Intermediately located along the path of air mass transport, sites at the Global Atmosphere Watch (GAW) Observatory Anmyeondo and Mokpo National University (138 km and 320 km from Seoul, respectively) were used for additional flask sampling. The atmospheric transport model FLEXPART⁶ was used to forecast the tracer plume's trajectory and dispersion, and the release and sampling times were adjusted accordingly.

During two releases in November 2024 and April 2025, both tracers were detected at the flask sampling sites Anmyeondo GAW Observatory and Mokpo National University, as well as at Gosan station. The measurements show a strong correlation of our tracer substances with various HFCs. Preliminary emission estimates for the greater Seoul metropolitan area are derived using the tracer ratio method, and its limitations are discussed. Finally, a comparison to a full regional inversion, based on the continuous observations at Gosan, is conducted.

References

[1] Kyoto Protocol to the United Nations Framework Convention on Climate Change. adopted on December 11th, 1997; Kyoto, 1998, 1-22.

[2] Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer. adopted on October 15th, 2016; United Nations, Kigali.

[3] Matt Rigby, Sunyoung Park, Takuya Saito, Luke M. Western, Alison L. Redington, et al., Nature, 2019, 569 (7757), 546-550.

[4] Peter G. Simmonds, Matthew Rigby, Alistair J. Manning, Sunyoung Park, Kieran M. Stanley, et al., Atmospheric Chemistry and Physics 2020, 20 (12), 7271-7290.

[5] Dominique Rust, Martin K. Vollmer, Stephan Henne, Arnoud Frumau, Pim van den Bulk, et al., Nature, 2024, 633, 96-100.

[6] Ignacio Pisso, Espen Sollum, Henrik Grythe, Nina I. Kristiansen, Massimo Cassiani, et al., Geoscientific Model Development, 2019, 12 (12), 4955-4997.