Li et al. studied the aerosol scattering hygroscopic growth factor (f(RH)) in a Chinese coastal city and compared f(RH) between NPF and non-NPF days. Based on the ACSM measurement, some analyses and explanations have been provided. The difference in f(RH) between NPF and non-NPF days is due to the variations in SO₄^2-, NO₃^- and the oxidation degree of OA.

The study falls into the scope of ACP. But, compared with existing work, I do not see too much novelty in the current version of the manuscript. The quality of the work has been significantly weakened by but not limited to 1) no detailed composition analysis from the ACSM data (e.g., no PMF analysis), 2) no systematic comparison with literature on f(RH), 3) too brief discussion many times, 4) inconsistency throughout the manuscript and 5) the use of language. The manuscript will require major revisions, and the comments below need to be fully addressed before the manuscript can be reconsidered for publication.

Major Comments

1. Lines 215 – 219: It is not straightforward to visualize the comparison in a table. It is recommended that the author use a figure to illustrate the comparison. If necessary, the figure should be considered to be placed in the main text. In addition, the author compares f(80%), f(85%), and f(70%) between studies. This needs to be clarified. What are the reasons behind the differences between f(RH) from different studies? The author needs to discuss that.

1. Lines 316 – 318: Was the wind direction from the major roads near the site? How might the increasing fraction of OM be attributed to the emission from heavy trucks? Did the authors observe any increase in traffic-related marker ions or PMF factors on ACSM?
2. Lines 320 – 323: The authors claim that the compared to non-NPF days, a higher mass fraction of sulfate on NPF days was possibly due to the nucleation of sulfuric acid. Have the authors considered other possibilities, e.g., the increased sulfate fraction can be attributed to the increase in other species? Have the authors observed in any absolute increase in SO₂ on NPF days?
3. Section 3.3: As Xiamen is a coastal city, the influence of sea air masses and the contribution of sea salt are expected to be considerable. As far as I know, the ACSM is not good at quantifying the refractory components (e.g., sea salt). When discussing the relationship between f(RH) and aerosol chemical composition, a thorough discussion on the influence of sea salt must be provided as well.
4. Lines 332 – 337: The author speculated on the condensation of large quantities of sulfuric acid and organic vapors on NPF days. Unless there was CIMS measurement, this statement is speculative and can be misleading.
5. Lines 338 -353: Nitrate was assumed to be the driving factor for the aerosol scattering hygroscopic growth. This is supported by a significant decline in f(RH) when both nitrate content and NOR were low. When NOR and RH were shown in Figure 4, there was no data for nitrate content and f(RH) in the same figure.
6. Lines 351 – 353: I understand that the SOR and NOR were higher on non-NPF days compared with NPF days. However, it is unclear to me how the enhanced SOR and NOR were possibly driven by aqueous phase reaction. A detailed analysis needs to be provided to support the statement.
7. Lines 384 – 386: The main drive for aerosol hygroscopicity on NPF days is ambiguous. In lines 345 – 347, nitrate was suggested to be essential for aerosol hygroscopicity on NPF days. Here, the sulfate was suggested to be important in the aerosol hygroscopicity on NPF days. Consistent statements need to be provided through the whole manuscript.
8. Lines 488 – 493: What are the reported k_OA values in other studies? Could the author use a table or figure to summarize the comparison? How different are the organic aerosol composition between studies?

Minor Comments

1. Lines 46-47 and Lines 49 -50 : How does the aerosol hygroscopicity alter particle size and refractive index? How does the aerosol hygroscopicity affect chemical processes and air quality? This is difficult for readers without little background knowledge of hygroscopicity. Could the author give a better explanation of that?
2. How were the two nephelometers compared when operated under dry conditions?
3. Please give more information about the ACSM. Is it a ToF-ACSM or Q-ACSM? What are the RIEs used for different species?
4. Lines 177 – 181 should be rearranged into a place where γ and F₀ were in use. I suggest they should be moved closed to the paragraph in lines 403 – 417.
5. Lines 239-242: Why was hygroscopicity at RH >90% was lower than that at 80%<RH<90% in Zhao et al., (2019)? But why was it the same case in this study? More discussion need to be provided.
6. Lines 242 -244: To support the sentence “Secondly, the characteristics of f(RH)… were minor”, a statistical analysis (e.g., ANOVA) needs to be conducted.
7. Lines 249 – 252: Could the authors provide qualitative analysis about the fluctuations of f(RH) and the dramatic increase in particle number conc., variations in chemical composition?
8. Lines 262 -266: Why is the Eq. (1) the most suitable for the analysis? What is it based on? R²?
9. Lines 274 – 275: What are more complex factors? More complex to what? What was compared to?
10. Lines 301 – 302: The data showed in Deng et al., 2016 is one-year data but not decade-long data. The sentence needs to be rewritten to avoid misunderstanding.
11. Line 303: What are the control measures?
12. Line 304 -306: The statement “the prominent nitrate pollution” is based on a comparison between this study and another study focusing on winter in 2013. In my opinion, the statement completely lacks supporting evidence, unless the cited study focused on the long-term data.
13. Lines 328-329: The sentence “This illuminates that fewer hygroscopic particles were born during the NPF event in Xiamen” is not clear and needs revision.
14. Line 331: What were the difference precursors? Please give a brief explanation.
15. Line 376: What is uncertainty range for the linear regression? Please show the range as shaded areas in Figure 5. Same applies for Figure 6.
16. Lines 415 – 417: What are these studies about? Please provide more background information.
17. Line 457: What is the density used for organic when converting the mass fraction into volume fraction?
18. Lines 456 – 478: What is the defined RH for k_f(RH)?

Technical Comments

1. Please use “non-NPF days” instead of “Non-NPF days”.
2. To increase the readability, it will be good to use a large font size and put labels of “NPF” and “Non-NPF” in figures.
3. Line 52: What RH is classified as high ambient humidity? Could the author define a range?
4. Line 62: Lower than what? What is compared with mineral dust and biomass combustion aerosols here?
5. Lines 63 – 66: “Hydrophilic species such…”. Please add references.
6. Lines 70 – 71: “For a fixed chemical composition,”. Fierz-Schmidhauser et al., 2010b only measured the f(RH) of ammonium sulfate and sodium chloride. Please clarify the sentence.
7. Line 80: What does a relatively low level mean? Relative to what?
8. Line 96: I assume these studies point to studies conducted in China. If so, Titos et al., 2014 are miscited here.
9. Line 112: What is the meaning of enhanced observation?
10. Lines 132 -134: Instead of just citing a couple of papers, I suggest the author describe the principle and operation and include them in the supplement.
11. Line 138: What are the objectives of this study?
12. Line 156: Now the sentence reads like “In a 3-month observation, there are 85 days on which NPF can be identified”. This is a very high NPF frequency. In other words, only a few days are classified as non-NPF or unidentified days. Please rewrite the sentence to avoid misunderstanding.
13. Lines 182 -185: The sentence is too long and difficult to follow. Instead of just citing a paper, the calculation procedure must be detailed in the supplement.
14. Line 206: Please clarify the definition of low PM 2.5 pollution in the Chinese AQI standard.
15. Lines 208 – 209: “standard deviation” is redundant. The format needs to be consistent with other places in the manuscript, e.g., line 205.
16. Lines 232- 233: Standard deviation must be provided here for consistency.
17. Line 234: Please use “total number concentration”.
18. Figure 1: The diverging colour messes up the trend of f(RH) with increasing RH. Please use sequential colour for better visualization. What are the meanings of whisker and error bars? Can the median be included as well?
19. Lines 238 – 239: Reference(s) is needed for “… the interval of which was the most beneficial to the aerosol scattering hygroscopic growth.”
20. Line 264: Is Eq (1) in Chen et al., 2014 or in this study?
21. Eq (2): Is “a” or “α” in the equation? Is the same α as the one expressed in Eq (1)?
22. Lines 284 – 286: The sentence “It should be noted that the aerosol scattering… with relatively low levels of particle pollution.” is too hard to follow. What is the meaning of the diminishment of aerosol scattering hygroscopic growth?
23. Line 296: The sentence “Aerosol chemical compositions play a vital role in the aerosol hygroscopic growth” is redundant here.
24. Line 304: Where is the data for winter in 2013?
25. Line 312: What is the other event?
26. Figure 6: Why is the regression line in d) in red?
27. Please add the uncertainty ranges for the linear regression as shaded areas in Figures 2, 5, 6 and 7.
28. Lines 418 – 420: Please provide references for the sentence “Over the recent decades, the Chinese…”
29. Line 431: Please rephrase “that of Li’s report”.
30. Line 468: What does the word “donation” mean?
31. Lines 494 – 497: Please provide references for the sentence about f44.
32. Line 502: “uplift of oxidation degree of OA” is unclear.

General comments:

Aerosol hygroscopicity is a fundamental parameter in atmospheric physics and chemistry, making the results of this study highly relevant to the scope of ACP. While new particle formation (NPF) events are typically associated with active photochemistry and do not directly affect optical hygroscopicity (f(RH)), the unique condensation mode chemical reactions occurring during NPF events could have an impact. Therefore, NPF events offer a valuable opportunity to study how the composition of condensation mode aerosols evolves during NPF and how this affects aerosol hygroscopicity. This study presents comprehensive results on the differences in aerosol composition and f(RH) between NPF and non-NPF days, along with closure studies revealing changes in the hygroscopicity of organic aerosols. Overall, this study provides valuable comparisons between NPF and non-NPF days, such as differences in the RH dependence of f(RH) and organic aerosol hygroscopicity. As a measurement report, I recommend publication after the following comments are addressed.

Major comments:

1. In the abstract, authors stated that “ NPF occurs frequently and has an obvious effect on f(RH)”

does NPF could really impacts on fRH? How was this concluded? NPF means new particle formation, the aerosols with at least size of 150 nm could impact on aerosol scattering thus fRH, and with obvious impacts need aerosol size to be more than 200nm. Authors could estimate how long a newly formed particle could grow to size of more than 200 nm. If authors did the calculation, they would realize that fRH could not be affected by local NPF events, only chemical reactions happened on condensation mode could affect fRH, therefore, if authors find fRH changes during NPF events, it could be condensation mode reactions happened simultaneously with NPF events that changed fRH not the NPF reaction and formation chain.

       2. In the abstract, authors state that “main source (30.78%) of the hygroscopicity parameter κf(RH)”

κf(RH) is not a parameter with absolute amount, therefore, authors cannot say their source, only could state what its variations were driven by what kind of aerosol component.  For example, NH4NO3 played a dominant role in κf(RH) changes/enhancement. Your statement about influences of organic aerosol is correct.

      3. Sect 3.5, OA was not attributed as POA and SOA factors, discussions on OA density should be included.

Specific comments:

L21, bracket of f(RH) can be deleted

L22, describe the aerosol indirectly hygroscopicity

L22, varies greatly due to the influence of aerosol chemical composition and size

L91-93 Zhao et al. (2019) have summarized this in China

L97-98 “between f(RH) and NPF events”, as stated in the major comments, NPF itself does not affect, however the unique condensation mode chemical reactions during NPF events could affect.

L132 Nephelometers

L136, this is weird, you already have a “dry” nephelometer in the fRH system

L146, which type of ACSM, Q-ACSM, Tof-ACSM?

L147, how BC mass concentrations were corrected, because AE31 is not as AE33 which have auto correction for loading effect?

L298, SO₄^2- etc, use SO4 is fine, or sulfate, because the ACSM doe not measure the ion form of sulfate

L328, fewer hygroscopic particles were born is not correct, because NPF itself doe not affect fRH, as you can see from Fig.3 that fRH is lower before NPF event, therefore, it is the background aerosols during NPF events that have lower hygroscopicity

L 332-336, this is speculative, however, authors could state that “On the basis of aerosol chemical compositions during NPF, it could be speculated that ……”

L356-357, The ACSM measures bulk aerosol compositions, if authors calculated the volume size distribution from PNSD measurements, it could be found that newly formed particle rarely affect bulk ACSM mass, meaning that the different bulk chemical compositions between NPF and non-NPF days could not attributed to formation mechanisms of new particles, it could only be attributed to different background aerosol when NPF occur, and reactions happened on existing condensation mode particles.  This sentence could be rewritten as “indicating the remarkably different bulk aerosol compositions and condensation mode aerosol formation mechanisms between NPF and non-NPF days”

L422-425, the difference of organic and inorganic aerosol fraction is the reason, not sulfate. Indeed, kappa of ammonium sulfate is slightly lower than that of ammonium nitrate, however, not the major cause.

Zhao, C., Yu, Y., Kuang, Y., Tao, J., and Zhao, G.: Recent Progress of Aerosol Light-scattering Enhancement Factor Studies in China, Advances in Atmospheric Sciences, 36, 1015-1026, 10.1007/s00376-019-8248-1, 2019.