Review for Measurement report: Secondary organic aerosols at a forested mountain site in southeastern China by Zhang et al.

General comment

The study is basically about the interactions between SOA and clouds at a high-altitude background area in China as well as characterizing aerosol chemical composition at the site. The study involves real-time measurements conducted with aerosol mass spectrometry (AMS + ACSM) at 1128 m a.s.l. in southeastern China, in November 2022. The results from the AMS measurements are well presented and the paper is generally well written and the figures are mostly informative and clear. The weakness of the paper is the connection of the AMS data to the cloud events and thereby the fact that the conclusions of the paper are, as I see it, not very well supported by the rest of the paper. Only two time periods are chosen from the time series based on relative humidity (P1 and P2) to the analysis. RH stays at 100 % during P1 indicating in-cloud conditions while during P2, RH drops to about 80% several times. P2 is referred to as an “evaporative event”. My main concern is that the authors draw rather strong conclusions on how droplet evaporation e.g., increases the abundance of less oxygenated organic aerosol, but a potential (perhaps likely) change in air mass is not discussed, which could also explain the finding. In addition, due to the inclusion of only two time periods in the analysis, the statistics are rather poor. More events/periods could certainly be included, if the ACSM data were utilized. I have added some comments below, which the authors can consider.

Comments:

The title of the paper does not reflect much the cloud processing angle that is brought up in the conclusions and abstract. A new title could perhaps be considered also after the authors work on providing more evidence supporting their conclusions.

L51: “grow via gas-to-aqueous partition” rephrase

L82: Surely PM_2.5 and PM₁₀ were not measured with gas analyzers. See also L96. Please clarify.

L76: Was ACSM really operated with <5 min time resolution?

L94: The RIE for sulfate is slightly low for the ACSM. Did you use a full scan mode (Freney et al., 2019) when calibrating for ammonium sulfate?

L97: PMF is not described appropriately. Figure S.2 is therefore not very useful, as the reader FPEAK,Q, Qexp, and residuals are not introduced anywhere.

Sect. 2.2.3: How well does HYSPLIT capture actual airmass movements over a complex mountain terrain?

L118: Maybe distinguish what is meant by “particle” here (aerosol particle vs the hybrid single-particle). I also suggest reformulating L117–L121. The sentence is slightly confusing to me.

L131–L132: Maybe include the standard deviation of the PM₁ concentrations from the other mountain sites (if possible) to make a fairer comparison.

L135: What about the role of ammonia in nitrate formation?

L135: “impact on regional scale”, please specify

L136: How does nitrate interact with clouds? Please specify.

L139: What do you mean by the highest two probabilities?

L144: do you mean “transport of nitrate”?

L154 can’t the OA afternoon peak be related to the development of a boundary layer (measuring air that has been in contact with airmasses below/potentially more polluted air masses?)

L146: P1 and P2 are not explained in the caption.

L163: “Previous studies have shown that aerosol mass generally increases on foggy days” → What do you mean? Please add more references that support this statement. What is meant by foggy days? Overall or during fog? Note also that when comparing your results to others, things might be different in areas where fog formation is driven by radiative cooling.

L168: What makes P1 “a typical cloud scavenging period”?

L169–L172: Description of P2 somewhat hard to follow. Could you rephrase?

L173: What would cause the evaporation? It looks like temperature goes slightly up. Why is that? Does your air mass change?

Sect. 3.2. It seems that an episode similar to P2 took place on 21.11. and conditions similar to P1 took place also before P2 and during the first week of November. Why are these not included? One could use ACSM data to test if your conclusions from the P1 and P2 comparisons hold outside the P1 and P2 time frames? Including more data into the cloudy and “evaporation” periods would make the paper stronger.

L182–183: did the mass fractions increase 17 and 19%?

L205: I wonder if what you see with NO⁺/NO₂⁺ is mostly noise due to low concentrations during P1? Could you show the HR fit of NO⁺ in the for P1 and P2? I am just wondering if you also fit CH₂O⁺ (see Graeffe et al., 2023).

L240: “Not surprise” could be reformulated

L244: I am surprised to see ON to go with MO-OOA and not LO-OOA due to the semi-volatile nature of ONs. Could you comment on that?

L256: How do you know it is evaporation and not a change in air mass?

L273–L274: Why would you only release LO-OOA from evaporating droplets and not MO-OOA, too? And what does "almost scavenged" mean?

L278: Why do you say LO-OOA contains HULIS?

L290: “high oxidative properties”, consider reformulating

L300: how do you determine the air mass clusters? How cohesive are these clusters? Consider showing all trajectories in a plot per cluster in the SI.

L322: Investigate further (using e.g. trajectory data if accurate enough) to see whether you can confirm that the air mass does not change during P2.

L324: “cloud evaporation released substantial amount of LO-OOA”. Try to provide more evidence on this. I am not convinced this is the full story. Why not including ACSM data into this (+ ACSM PMF ) and take at least 21.11. (no cloud), 15.–16.11. (no cloud), 13.–14.11 (cloud), 22.–26.11. (no cloud) into the analysis to improve statistics. Do your conclusions on the effects of droplet evaporation and LO-OOA change?

Figure 1: was there no BC at all in Mt. Tai?

Figure 4: why does ammonium disappear from the size distribution after 500 nm in panel a? Sulfate also looks odd. How long average is this and how much variability is there within P1? I can see that right before P2 there is a period when AMS was measuring. How does the mass distribution look then?

Figure 5: Could you explain for the bottom right plot what MSA and IN are in the caption? The arrow representation for those is also a bit hard to read. It is also stated in the text that 3.88 is obtained for AN calibration, but referred to as IN in the figure. Does the median for P1 equal the median of P2 in terms of the NO_x⁺ ions?

Figure 11: Why do you have “Events” here instead of P1 and P2? Event 3 seems to coincide with P2 and suggests an air mass change. I suggest you investigate this further to understand more what is due to the change of air mass and what due to droplet evaporation when you talk about differences in composition between P1 and P2.

Figure S.1: How do AMS and ACSM measurements agree for the different chemical species?

Figure S.3: are these all from the same exact measurement period? How come the diel cycle of PM_2.5 is so much less in terms of mass than PM₁? Almost as if the nitrate episode is not measured by the PM_2.5 instrument?

Figure S.5 easier to read if both y-axes had the same scale 

References

Freney, E., Zhang, Y., Croteau, P., Amodeo, T., Williams, L., Truong, F., Petit, J.-E., Sciare, J., Sarda-Esteve, R., Bonnaire, N., Arumae, T., Aurela, M., Bougiatioti, A., Mihalopoulos, N., Coz, E., Artinano, B., Crenn, V., Elste, T., Heikkinen, L., Poulain, L., Wiedensohler, A., Herrmann, H., Priestman, M., Alastuey, A., Stavroulas, I., Tobler, A., Vasilescu, J., Zanca, N., Canagaratna, M., Carbone, C., Flentje, H., Green, D., Maasikmets, M., Marmureanu, L., Minguillon, M. C., Prevot, A. S. H., Gros, V., Jayne, J., and Favez, O.: The second ACTRIS inter-comparison (2016) for Aerosol Chemical Speciation Monitors (ACSM): Calibration protocols and instrument performance evaluations, Aerosol Science and Technology, 53, 830–842, https://doi.org/10.1080/02786826.2019.1608901, 2019.

Graeffe, F., Heikkinen, L., Garmash, O., Äijälä, M., Allan, J., Feron, A., Cirtog, M., Petit, J.-E., Bonnaire, N., Lambe, A., Favez, O., Albinet, A., Williams, L. R., and Ehn, M.: Detecting and Characterizing Particulate Organic Nitrates with an Aerodyne Long-ToF Aerosol Mass Spectrometer, ACS Earth Space Chem., 7, 230–242, https://doi.org/10.1021/acsearthspacechem.2c00314, 2023.

Please find my detailed review report attached