Multi-decadal trend of HFC-23 from FTIR observations at the NDACC Junfraujoch station: Persistent increase and comparison with in situ AGAGE observations, TOMCAT model simulations, and ACE-FTS satellite data

Abstract. The gradual phase-out of stratospheric ozone-depleting substances (ODS), including chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), has led to the widespread adoption of hydrofluorocarbons (HFCs). HFC-23 (CHF₃) ranks third among HFCs in radiative forcing contribution. As a potent greenhouse gas, it falls under the Kyoto Protocol and Kigali Amendment, motivating careful monitoring. HFC-23 is not intentionally manufactured; its atmospheric release stems primarily from HCFC-22 production. This study presents the multi-decadal trend of HFC-23 derived from a newly developed retrieval methodology applied to high-resolution solar absorption spectra recorded by ground-based Fourier Transform InfraRed (FTIR) instruments. Measurements obtained at the Jungfraujoch high-altitude research station (Switzerland) within the Network for the Detection of Atmospheric Composition Change (NDACC) were combined across three successive FTIR instruments to produce a continuous and consistent time series from 1986 to 2024, including the earliest detected occurrence of HFC-23 at the site. The FTIR time series was compared with two independent observation datasets, surface in situ measurements from the Advanced Global Atmospheric Gases Experiment (AGAGE) network at Jungfraujoch and satellite retrievals from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS, L2 v5.3), as well as simulations from the TOMCAT global three-dimensional chemical transport model. The resulting annual relative trends (2008–2023) are 3.52 ± 0.37 %.yr⁻¹ (FTIR), 3.52 ± 0.15 %.yr⁻¹ (AGAGE), 3.66 ± 0.22 %.yr⁻¹ (ACE-FTS) and 3.53 ± 0.11 %.yr⁻¹ (model). The consistency among datasets supports our retrieval strategy. This methodology can be applied at additional NDACC sites, enabling broader spatial coverage for monitoring this compound using ground-based FTIR instrumentation.