| 16 Apr 2026
Advancing Halocarbon Radiative Efficiency Estimates by coupling radiative transfer and quantum chemical calculations: impact of updated spectroscopic parameters and low-frequency contributions
Abstract. We update a fast method for calculating the global mean radiative efficiency (RE) for weak absorbers (known as the Pinnock Curve). It is then employed to evaluate improved REs, for 30 halocarbons focusing particularly on the low-wavenumber (<500 cm-1) contributions where laboratory measurements of absorption cross-sections are scarce. The Pinnock Curve is updated using spectral line parameters from the HITRAN2020 database and the MT_CKD_4.3 water vapor continuum formulation. Halocarbon REs are evaluated by coupling this update with infrared absorption cross section spectra evaluated by means of a quantum chemical protocol featuring a non-empirical inclusion of anharmonic effects in both transition frequencies and intensities and accounting for conformer distributions. Recent revisions to the MT_CKD water vapor continuum coefficients decrease atmospheric opacity, producing a small increase in RE, which is counteracted by updates to line-by-line spectroscopic parameters in HITRAN2020. These compensatory effects result in a small increase in halocarbon REs, with an average rise of ~0.3 %. Analysis of the low-wavenumber region shows that for the targeted compounds it contributes no more than 5 mW m-2 ppb-1. On average, it amounts to 0.9 % of the total RE, but reaches about 3 % for HFC-152a and HFC-161. Despite this modest magnitude, accurate treatment of this spectral range is essential, as its impact is molecule-dependent and even small contributions can influence climate metrics. For seven of the 30 gases considered here, the low frequency contribution to RE is more than 10 times greater than that calculated in earlier work.
General Comments:
This preprint reports an update to the Pinnock curve reflecting changes in spectral line parameters in the HITRAN2020 database and the updated MT_CKD_4.3 water vapor continuum. The impact of these changes on the shape of the Pinnock curve and radiative efficiency values for 30 halocarbons is discussed. In particular, the manuscript focuses on wavenumbers below 500 cm-1 and employs a rigorous anharmonic quantum-chemistry protocol to calculate radiative efficiencies in silico.
The content of the preprint is scientifically sound, and the work significantly contributes to the fast calculation of climate metrics (radiative efficiencies and global warming potentials) workflow by updating the Pinnock curve and then providing this curve to the community in the Supplement. The manuscript is well organized and clearly written. It will be ready for publication after addressing the minor revisions listed below.
Specific Comments:
Introduction
Line #24: The phrase “the greenhouse capacity” is at odds with the following discussion of the RE climate metric in lines #24-26. Generally, the “greenhouse capacity” is associated more with the GWP of a gas. Perhaps a different phrase can be used instead.
Lines #54-57: The phrase "without any other change in the climate system" should be clarified because Shine and Myhre (2020) included the stratospheric temperature adjustment, which does account for some change in the climate system. This could be mentioned at the end of line #57.
Methods
Line #104: The sentence mentions the “most recent updates in the HITRAN and MT_CKD”. This sentence should be rephrased, given the release of HITRAN2024 and the use of HITRAN2020 in this work.
Lines #120-121: The use of different basis sets for C, H, F versus Br stands out and needs to be justified further. While it is understandable that these basis sets are mixed to be computationally cost-effective, the mixing of jun-cc-pV(T+d)Z and aug-cc-pVTZ-PP leads to a basis set imbalance. The resulting C-F and C-Br bonds can be too long/short, directly affecting energy derivatives used to calculate the harmonic and anharmonic spectra. A counterpoint could be that this QC approach yields relatively accurate RE values, as per Alvarado-Jiménez and Tasinato (2024); however, this argument should be made in text.
Line #132: The “relevant thermal conditions” should be defined by providing a temperature or temperature range used when accounting for conformer populations.
Line #133: The phrase “incorporating spectral data” should be qualified by mentioning that it is calculated spectral data.
Line #144-46: While it is understandable that using DSDPBEP86-D3 may be computationally impractical for an anharmonic calculation of HFC-43-10mee, the mixing of different levels of theory for the HFC-43-10mee calculation needs to be justified further. Paraphrasing the key points in the “Computational details” section of Alvarado-Jiménez et al. (2026) may help.
Line #150: The choice of HWHM of 5 or 30 cm-1 should be justified. Is the HWHM chosen to ensure the QC spectrum resembles/matches experimental spectra?
Line #165-69: A sentence or two on the exact atmospheric lifetime adjustment (degradation by OH or photolysis in the stratosphere) used for each family of compounds studied should be included.
Line #173: The sources of error that contribute to the “average uncertainty of about 5%” for QC cross-sections should be outlined. Is this based on the deviation values presented in Table 1 in Alvarado-Jiménez and Tasinato (2024)? If so, this should be clarified.
Results
Line #234: The QC calculations account for anharmonic corrections to wavenumbers; however, there can still be a mismatch between calculated IR absorption wavenumbers and peak locations in experimental data. Since the location of each QC peak affects the integrated band strength (Table 1) and RE values (Table 2), were QC peak locations checked versus available experimental data?
Line #283-285: The phrase “the proportion of the total RE” should be clarified. Does the total RE here refer to the value from QC calculations or experimental data? In addition, the statement “(here defined as 0-500 cm-1)” conflicts with the lower experimental wavenumber of 150 cm-1 in line #282. Perhaps this statement can be moved to the methods section, or the statement “the far-IR region (here defined as 0-500 cm-1)” could be replaced with “low-wavenumber region”.
Line #285-286: An explicit reference should be made to Table 2 by providing the exact QC calculation low-wavenumber contribution percentages when comparing to experimental percentages mentioned in line #284.
Code and data availability
Thank you for providing the updated Pinnock curve in an easily accessible format. The curve provided is at a resolution of 10 cm-1. If possible, please include an additional file with the Pinnock curve at 1 cm-1 resolution (as was done in past iterations of the Pinnock curve).
Technical Corrections:
Introduction
Line #43: There is a typo “double-hybdrid”.
Methods
Line #129: There is an error in the in-text citation “GAUSSIAN 16 (Frisch et al.)” where the year is missing. The Gaussian website (https://gaussian.com/citation/) mentions “year={2016}” under “BibTex Starting Point”.
Conclusions
Line #323: There is a typo “funtional in earlier”.
Figures
If possible, please increase the font size for Figures 1-4.
Figure 2: There is inconsistent spacing between MT_CKD and 3.2 or 4.3 in the legend.
Figure 3: A closing bracket “]” is missing in the top panel y-axis label. Both panels have background grids that are absent in Figures 1 and 2. The lines in both panels seem thicker than those in Figures 1 and 2. Line thickness and background grids should be consistent between figures.
Figure 4: The gray border around this figure should be removed. The green line is distracting; perhaps disconnected green markers could be used instead. The “Diff. RE(%)” text is too close to the “Methylene Bromide” label.
Tables
Table 2: Ideally, each table should be independent of the main text. As such, please provide the wavenumber range for the “total RE” in the table caption. The column labels “RE (this work)” and “REThis work / REVan Hoomissen” should explicitly indicate that these columns are restricted to the low-wave number region to avoid confusion with RE values in the last column at an initial glance.