Revised Review Comments:

The paper by Chen et al. provides a clear analysis of the molecular characterization of organic aerosols in urban and seaside environments within a coastal city in southeast China. The dataset is credible and thoroughly analyzed, enhancing our understanding of molecular-level organic aerosol characteristics in coastal areas. The manuscript is generally well written, though it contains several English errors that require attention. I recommend acceptance after minor revisions, as detailed below:

1. Lines 116-117: Rewrite the sentence to:
   
   “Potential influence from shipping activities, due to the proximity of Xiamen Port, may impact the seaside site.”

2. Line 149: Clarify what the correlation coefficients represent.

3. Lines 157-158: The BC-related data is not used in the article and could be deleted.

4. Lines 190-191: Rewrite the sentence as:
   
   “The sum of the four compound classes was denoted as CHOX, where X indicates the potential presence of N, S, or both.”

5. Line 228: Ensure consistent terminology for “O₃ concentrations and UVB intensity.”

6. Lines 343-345:
   
   “The signal proportion of compounds with DBE/C > 0.7 was comparable between the two sites (CHO: ~25%; CHON: ~16%)…”
   
   Question: Were low-carbon-number compounds (e.g., C₂H₂O₄) excluded from this analysis?

7. Lines 417-420:
   
   “…urban OA is more influenced by anthropogenic emissions, dominated by aromatic species, whereas marine-influenced OA exhibits higher proportions…”
   
   Note: Verbs should be in singular form for consistency.

8. Lines 434-435:
   
   “…owing to the identification of abundant CₙH₂ₙ₋₄OₓS and CₙH₂ₙ₊₂OₓS homologues…”
   
   Suggestion: Provide additional context on gas-phase contributions.

9. Lines 488-490:
   
   “…reactions of NO with organic peroxy radicals (generated from monoterpenes oxidation by OH radicals or O₃) during the daytime, or via nighttime NO₃ radical-initiated reactions…”
   
   Question: Were aqueous/heterogeneous phase reactions considered?

10. Lines 493-495:
    
    “…N₂O₅ signals in Case 1…”
    
    Question: How does O₃ concentration compare in Case 1? Is it lower?

The manuscript by Chen et al. investigates the molecular composition and potential chemistry processes of organic aerosol samples collected from urban and seaside sites of a coastal environment with the deployment of a FIGAERO-CIMS mass spectrometer. The manuscript is well written and the topic of this manuscript is interesting. However, some revisions with more discussions are needed before its possible publication on ACP. Please see my comments and questions below.

Specific:

Line 24. Higher than what/where? Non-coastal area?

Line 41. What do you mean “absorbed into”? Do you mean gas uptake onto aerosol particles? I would suggest change to “... could undergo a series...”.

Line 54-56. It seems your major findings are similar to what Siegel et al (2021) and Xin et al (2024) found. I think discussions on what these two previous findings are still lacking and what is unique in your study are missing.

Line 68-70. Do you mean the EESI-TOF (Lopez-Hilfiker et al., 2019)? If so, it is probably still quite new in the field for aerosol particle online analysis.

Line 85-86. I think this is probably due to not enough maintenance of the instruments? Please consider to rephase since this seems to infer that long term measurements are not reliable (?). Of course it is a challenge to run long term measurements.

Line 111-117. It seems the urban site and seaside site is very close to each other, only 18km away. How can you be sure that the urban site don’t have any coastal influences at all from the transport, and vice versa? It seems to be the case as you showed in Line 274-276 that more than half of the CHO and CHON molecules at both sites shared same molecular formula accounting for 86-94% of total signals. Also in Figure S4 their spectra look very similar. Maybe digging into these molecule characteristics (e.g. diurnals, mass spectra difference, or PMF) can help on finding out more site-specific differences.

Line 225. Are they “significantly” higher? Did you do t-test on that (like in Line 230)?

Line 309-313. More discussions on site-specific differences would be beneficial.

Line 335-336. High saturation can also come from saturated fatty acids from marine source.

Line 368-370. Would be nice to add more discussions on e.g. the potential mechanisms and/or sources for these CHON compounds.

Line 370-375. What’s the few dominant S-containing species? Did you detect any e.g. fatty acids which FIGAERO-CIMS should be able to?

Line 386-389. Does it mean in both the urban site and seaside site aromatic compounds are the dominating species? If so, how could this seaside site still be classified as a seaside site? Also, in this case it’s not very accurate for the conclusions in Line 404-406 where it says urban site is dominated by aromatic species while seaside OA aliphatic compounds.

Line 400. You may change “conductive” to “vulnerable”.

Line 418-420. Please add some literature to support this. Because I doubt these compounds are from vehicle or combustion emissions. This would be mean both sites are dominantly affected by vehicle or combustion emissions. One the one hand, the seaside site “should” by the classification be less affected by the traffic, but if they are indeed from traffic and also dominating the signals, it means the site selection is not representative for seaside. On the other hand, C6H5NO3 and C7H7NO3 are well known biomass burning markers, nitrophenols, while C8H11NO7 may be related to monoterpene oxidations.

Line 431-433. Would be nice to add more discussions e.g. on the potential mechanisms and/or sources for these S-containing molecules.

Line 487. Would be nice to add the labels of these compounds in Figure7, similar to the compounds mentioned in Line 516. Also would be beneficial to add more discussions on their potential mechanisms and/or sources, similarly to the texts in Line 519-523.

Same case for compounds in Lien 530 and 532.

Line 501-511. Some discussions to support explaining the observations are needed, similar to the texts in the previous and following paragraph.

Technical:

Line 327. Please delete “of”.

Line 502. Please delete "was".

Reference:

Lopez-Hilfiker, F. D., Pospisilova, V., Huang, W., Kalberer, M., Mohr, C., Stefenelli, G., Thornton, J. A., Baltensperger, U., Prevot, A. S. H., and Slowik, J. G.: An extractive electrospray ionization time-of-flight mass spectrometer (EESI-TOF) for online measurement of atmospheric aerosol particles, Atmos. Meas. Tech., 12, 4867–4886, https://doi.org/10.5194/amt-12-4867-2019, 2019.