This study provides valuable insights into the atmospheric degradation pathways of linear perfluoroaldehydes (PFAs) via reactions with HO₂ radicals. The combination of high-level quantum chemical calculations, kinetic modeling, and atmospheric simulations offers a comprehensive understanding of the roles of HO₂ in modulating PFAs’ lifetimes. The work is timely and addresses a critical gap in the atmospheric chemistry of PFASs. The conclusions are well-supported by the data, and the manuscript is well-organized. However, several clarifications and minor revisions are required to enhance clarity, strengthen methodological justification, and improve readability.  I recommend acceptance after minor revisions.

Specific Comments

1. Abstract

(a) The statement “the atmospheric lifetimes for C₂F₅CHO and C₃F₇CHO, approximately 14.4-31.3 hours and 21.6-51.8 hours by HO₂ are much shorter than those via OH radical,”  it is not validated. (lines 15-16)

2. Introduction

(a) Alternating use of "perfluoroaldehydes" (in tittle) and "linear perfluoroaldehydes" may confuse readers.

(b) The transition from PFAS’s GWP to perfluoroaldehyde sources (lines 25-30) is unclear.

(c) The literature review focuses solely on OH and Cl radicals, omitting potential roles of other oxidants (e.g., O₃, NO₃). (lines 35-49)

(d) The long sentence "Moreover, the rate constant of Cl atoms with C_nF_2n+1CHO (1,2, 3, 4) is around 2 × 10^-12 cm³ molecule^-1s^-1, which is slightly faster than that of the OH radical reactions." (lines 46-47) has poor readability and unclear notes.

3.Computational Methods

(a) In section 2.1, the “CCSD(T)-F12a/cc-pVTZ-F12” method is mentioned, but the text fails to demonstrating its applicability to perfluorinated compound systems. (lines 75-78)

4. Results and Discussion

(a) Figure 1 contains no relevant information. “The result shows that the specific reaction scale factors are 0.955 for TS1 (See Figure 1) and 0.956 for TS2 (See Figure 1)”. (lines 95-96)

(b) Grammatical error in “The torsion of the C-C bond gives produces multiple conformers”.  (lines 152-153)

(c) Figure 3’s X-axis label (“Number of alkyl functional groups”) is misleading, as the compounds are perfluorinated. (lines 180-185)

(c) The calculated high-pressure limit rate constants (e.g., k₁=5.42×10⁻¹⁴-3.35×10 ⁻¹² cm³ molecule⁻¹ s⁻¹) lack comparison with experimental data or analogous systems (e.g., non-fluorinated aldehydes + HO₂), reducing confidence in the results. (lines 204-207)

(e) There is inconsistency in the units used for atmospheric lifetimes. Table 2 reports lifetimes in seconds, while the discussion section uses hours. The authors should standardize the units throughout the manuscript to avoid confusion. (lines 250-255)

5. Atmospheric Implications

(a) The GEOS-Chem simulation results (Lines 273–287) focus on HO₂/OH ratios but do not discuss diurnal variation, which could affect the dominance of HO₂ pathways.

(b) The prospect for future research is somewhat brief and doesn’t adequately take into account the current study’s limitations and possible areas for expansion. (lines 321-325)

6. References

(a) Some of the cited references are incomplete or incorrect.

Lee, B. H., Munger, J. W., Wofsy, S. C., Rizzo, L. V., Yoon, J. Y. S., Turner, A. J., Thornton, J. A., and Swann, A. L. S.: Sensitive Response of Atmospheric Oxidative Capacity to the Uncertainty in the Emissions of Nitric Oxide (NO) From Soils 450 in Amazonia, Geophysical Research Letters, 51, 1-10, https://doi.org/10.1029/2023GL107214, 2024. (lines 448-450)

Long, B., Bao, J. L., and Truhlar, D. G.: Rapid unimolecular reaction of stabilized Criegee intermediates and implications for atmospheric chemistry, Nature Communications, 10, 1–8, https://doi.org/10.1038/s41467-019-09948-7, 2019. (lines 473-474)

Xia, D., Zhang, H., Ju, Y., Xie, H., Su, L., Ma, F., Jiang, J., Chen, J., and Francisco, J. S.: Spontaneous Degradation of the “Forever Chemicals” Perfluoroalkyl and Polyfluoroalkyl Substances (PFASs) on Water Droplet Surfaces, https://doi.org/10.1021/jacs.4c00435, 2024. (lines 548-550)

Zegang Dong  and coworkers have carried out an extensive theoretical investigation on the gas phase reaction between two perfluoroaldehydes and hydroperoxyl radical and estimated their atmospheric implications. The work is extensive, the theoretical model is well chosen and already shown to be appropriate for similar class of reactions, the results are properly presented and explained.

However, the work falls severely short in originality and fails to provide any new insight, and is simply an extension of their earlier work (J. Am. Chem. Soc. 2022, 144, 19910−19920). Both the reactions studied here are close replica, mechanistically, energetically and kinetically, of the reaction between CF3CHO and HO2 studied in their previous work. As the reactions are always centered at the carbonyl group of reacting aldehydes, with no involvement of the side chains, this is very much expected that a mere elongation of the side chain would not have any dramatic effect on the reactions.     

The only new analysis that is available in this work is a GEOS-Chem based atmospheric modelling of the studied reactions to estimate their atmospheric implications. However, that analysis also does not provide any new information that was not known from the work cited above.  Similar to CF3CHO, the two larger perfluoroaldehydes studied here also show that reaction with HO2 is more dominant atmospheric removal process compared to reaction with OH.

Therefore, the conclusion that this work “provide new insight into atmospheric degradation of linear perfluorinated aldehydes by HO2 radical” is not supported by the results at all. At most, the study provides new data that shows the atmospheric degradations of larger perfluoroaldehydes by HO2 are very similar to that of CF3CHO which has already been reported earlier.

The most baffling aspect of this work is the sudden introduction of NO into the reaction scheme and an attempt to show the title reaction as a source of formic acid and COF2. However, there is no attempt to calculate the rate constant of the reactions involving NO, which would be required to have any realistic estimate of the importance of NO in determining the atmospheric fate of the studied perfluoroaldehydes. Therefore, the conclusion that “under NO conditions this pathway may be a source of HCOOH and COF2 in the troposphere” is completely unfounded without proper kinetic analysis, including lifetime calculations, of these reaction channels.

Based on the above observations, I cannot recommend this work for publication in Atmospheric Chemistry and Physics.