the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 17 Mar 2026
Further constraining the role of in-atmosphere production on the global HFC-23 budget
Abstract. A large discrepancy of at least 10 Gg yr-1 exists between reported emissions of the potent greenhouse gas HFC-23 (CHF3, trifluoromethane) and emissions derived from atmospheric measurements. In-atmosphere production of HFC-23 from the breakdown of fluorinated source gases such as hydrofluorocarbons and hydrofluoroolefins contributes to this gap, but the magnitude of this source is weakly constrained. This uncertainty is due, in part, to limited experimental measurements of the photolysis quantum yield of trifluoroacetaldehyde (CF3CHO), a key degradation product which forms HFC-23 via photolysis. The parameters governing CF3CHO deposition are also poorly understood. Previous work reported an upper limit of the contribution of the in-atmosphere source to the global HFC-23 burden. Here, we use a 3D chemistry and transport model to further constrain this contribution, using recent estimates of source gas emissions, kinetic rate constants, photolysis rates and deposition parameters, as well as considering the uncertainties in these values. We find that in-atmosphere production of HFC-23 is in the range 0.013–0.035 Gg yr−1, significantly lower than previous estimates. This accounts for <0.5 % of the discrepancy between reported emissions and those derived from atmospheric observations, suggesting that this source makes a negligible contribution to the overall HFC-23 budget. As part of this work, we also calculate indirect global warming potentials for the HFC-23 source gases HFO-1234ze(E), HFO-1336mzz(Z) and HCFO-1233zd(E) and find that their impact on climate is up to ten times higher than previously reported.
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- RC1: 'Comment on egusphere-2026-1230', Anonymous Referee #1, 06 May 2026 reply
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- 1
The manuscript, 'Further Constraining the Role of In-Atmosphere Production on the Global HFC-23 Budget', by Adam and co-authors, presents the findings of an atmospheric modelling study that investigated the potential production of HFC-23 (a highly potent greenhouse gas) from precursor gases within the atmosphere. The study uses a state-of-the-art atmospheric chemistry model updated for HFC and HFO chemistry, making use of updated photolysis data for the critical intermediate trifluoroacetaldehyde. A series of sensitivity simulations were performed to propagate the uncertainties associated with several significant atmospheric processes. The study is well designed and the manuscript is well written, with the results communicated clearly. Furthermore, the results are highly relevant as they rule out significant atmospheric production of HFC-23. This suggests that direct emissions (currently unreported/unknown) must be responsible for the growth of HFC-23 in the atmosphere. I would highly recommend publication of the manuscript once some minor points have been addressed/clarified.
P5, L126: Please distinguish the references for observations and global emission estimates. AGAGE provides observations; Western et al. provides global emission estimates based on these observations. A simple 'and .., respectively.' should do it.
P5, L138: When reading through this and the following paragraph, I stumbled several times over the assumptions made. Here: why GDP scaling, when we know that transitions are different in different world regions. Later on, the assumption is questioned and a link is made to the sensitivity runs that explore the importance of the spatial distribution of emissions. I think this section would be easier to follow if it is made clear from the beginning that HFO emission totals and distributions are not well known and therefore different sensitivity tests were carried out. Already mention why spatial distribution is more important for HFOs than for HFCs (currently stated in L250 and following). This can be followed by the current description of the base case emissions and finally the mention of alternatives emission distributions. Essentially, this suggests putting the last paragraph in the section forward.
Table 2: Are these the lifetimes calculated from the base simulation or literature values? Please clarify.
Section 2.3.1: Since this section takes up quite some space and is the basis for the re-evaluation of reaction pathways, I would suggest to also include Figure S1 in the main text. The overall length of the manuscript is rather short and there seems to be no need to hide the Figure (and also the SI tables) in a supplement. Alternatively, the section and figures could be moved into an appendix.
Equation 2 and Figure S1: Equation 2 for the product yield does not fit to the values displayed in Figure S1b. S1b shows PY=1 for lambda<266, the equation suggests 0.5. Please clarify.
P8 L196: How is the actinic flux calculated in STOCHEM? The wavelength dependency of the actinic flux will obviously impact the relative importance of pathway a vs b.
P8, L196: How is the actinic flux calculated in STOCHEM? The wavelength dependency of the actinic flux will obviously impact the relative importance of pathway a vs b.
P9, L222: The upper end of deposition rates seems to be rather extreme. Not even very water-soluble compounds like HNO3 have such high deposition velocities, especially not over the ocean or bare soils. I understand that extremes should be explored but at the same time these should be within physical bounds.
P10, L 246: Assuming that the transition to HFOs is progressing fastest in Europe and, hence, the base case already represents an upper-limit emission scenario: how realistic is the evaluated emission range 0.33 - 3 times the base case?
Section 3: When discussing the results, it should be clarified if the ensemble mean/median or the base case are considered to give the most realistic results. A statement on how much the two differ may be helpful as well. Since some of the sensitivity runs are leaning towards extreme parameter tests, it is not obvious that the ensemble mean is an unbiased, best estimate.
P13, L323: What is the uncertainty estimate on the global HFC emissions from Western et al. (2026)? Assuming linearity this could be propagated to evaluate its influence in comparison to the HFO emission and other sensitivity runs. However, I agree that it will most likely be smaller than for the HFO emission sensitivity, which covered a broad range.
P13/14: For understanding the comparison with the Van Hoomissen et al. (2025) results it would be helpful to repeat the kind of model that was used in their study.
P14, L353: While global total HFC-23 production is similar in these runs with different spatial distribution of HFO emissions: Do the runs lead to considerable regional differences in HFC-23 production and, hence, mole fraction differences? Would these differences be sufficiently large to compare simulated versus observed HFC-23 latitudinal gradients?
Section 3.4: Are the cited literature values based on global model simulations as well or simply on kinetic data and average OH assumptions? Please clarify if the Liang et al. (2022) citation also valid for the HFO-1336mzz(Z) and HCFO-1233zd(E) lifetimes.