Low-frequency contributions in the radiative efficiencies of HFC-236fa, HFC-245fa and HFC-43-10mee over the 225 - 298 K temperature range

Abstract

Hydrofluorocarbons (HFCs) are used as substitutes for ozone-depleting substances regulated under the Montreal Protocol. While having zero ozone depletion potential, HFCs strongly absorb infrared radiation, making them potent greenhouse gases. Vibrational modes associated with C--F stretching absorb strongly within the atmospheric window (750 --1250 cm-1), contributing substantially to radiative forcing. The low-frequency region (< 500 cm-1), which accounts for approximately 16% of the Earth’s thermal emission, has however remained largely unexplored mainly due to instrumental challenges. Here, we present the first experimental measurements of infrared (IR) absorption cross-sections in the 150--500~cm-1 range for HFC-236fa, HFC-245fa, and HFC-43-10mee - three industrially relevant compounds with high global warming potentials (GWPs). The spectra were recorded at the Rutherford Appleton Laboratory using a high-resolution Fourier-transform infrared (FTIR) spectrometer in the temperature range between 225 and 298 K at resolution of 0.25 cm-1. In addition, IR cross section spectra were simulated through quantum chemical (QC) calculations including a non-empirical treatment of anharmonic effects.

From the experimental results, we derived radiative efficiencies (REs) in the low frequency region of 0.001, 0.005, and 0.003 Wm-2 ppbv-1 for HFC-236fa, HFC-245fa, and HFC-43-10mee, respectively, and revised global warming potentials over 20-, 100-, and 500-year time horizons. Comparison with values reported in the WMO Ozone Assessment Report 2022 reveals minor differences for HFC-245fa and HFC-43-10mee, whereas HFC-236fa shows a significant overestimation, corresponding to a discrepancy of approximately 700 units in the 100-year GWP. On the other hand, our theoretical predictions reproduced the experimental spectra with average deviations below 5%, confirming the reliability of the computational approach even in the low-frequency region.

These findings highlight that small variations in the treatment of low-frequency absorptions can propagate into substantial contributions in climate metrics, particularly for long-lived compounds. Overall, this study provides a consistent experimental--theoretical framework for quantifying the radiative forcing of HFCs and reducing current uncertainties in the estimation of their climate-relevant parameters.

The dataset comprises: (i) The corresponding raw transmission data of the HFCs measured by the spectrometer, for different sample pressures and temperatures. (ii) The derived experimental absorption cross-section data.

The full details are in the associated paper to the data repository.