Zhang et al used an oxidation flow reactor (OFR) to produce SOA at different relative humidities, and attempt to explain the changes in SOA loadings caused by the changing RH. Unfortunately, I do not find their explanations and conclusions to be based on solid evidence or argumentation. I also cannot envision that further analysis of their data would allow new insights into the topic of how humidity influences SOA formation. Therefore, I cannot suggest this manuscript for publication in ACP. I outline some of the main shortcomings below.

Major comments

1. The experiments were conducted at loadings far from atmospheric concentrations, yet this aspect, and how it might impact the relevance of the study to the atmosphere, was not discussed at all. The gas phase oxidation chemistry of monoterpenes is a complicated process, largely due to competing fates of RO2 radicals under different conditions, and further reactions are very likely to take place after condensation into SOA (e.g. Pospisilova et al., 2020; Kalberer et al., 2004). As such, using offline SOA composition data to infer something about the reactions taking place in the first milliseconds after oxidant attack is extremely challenging and would, at the very least, require detailed analyses to exclude that any of the other stages of potential reactions are negligible. This was not done, and I cannot see that it would be possible with the data available in this study. In fact, the offline MS data is even said to stay relatively unchanged with the change in RH (line 194), so the chemical insights will be very limited, and really the only data used to draw conclusions on is the SMPS data. This is simply not enough for reaching any conclusive chemical understanding of the processes.
2. There are several studies concluding that RH does not have a noticeable impact on the formation of the most oxidized products that are expected to contribute most to SOA (e.g. Surdu et al., 2023; Li et al., 2019). Surdu et al. also analyze the potential reasons for the RH-driven changes in relation to particle phase reactions and changes in partitioning.
3. In addition to their speculative nature, the chemical mechanisms drawn up and discussed are wrong/misleading concerning the sCI. Only a small part of the formed CI will stabilize (and thus be impacted by RH), as most of them will simply decompose through the typical vinyl hydroperoxide channel. Reaction with water vapor is normally only relevant for the stabilized CI. In this manuscript, it is proposed that ozonolysis produces sCI at a 100% yield (e.g. lines 206-208, Fig 3-4). This raises further questions concerning how well the authors have understood the reactions that they are using to explain their observations.
4. In addition to the major concerns above, there are various other question marks concerning the conclusions drawn. The reasoning is not very clear concerning how the effect of increased sCI+H2O reactions would cause the observed changes. The main argumentation seems to be that the yield of carbonyls increases, which also makes the oligomerization more efficient. However, for conclusions like this, there should be more clearly stated what the proposed reactions are and, even more importantly, what are the competing reaction pathways (which then should produce something else, with a lower SOA yield).

References

Kalberer, M., Paulsen, D., Sax, M., Steinbacher, M., Dommen, J., Prevot, A. S. H., Fisseha, R., Weingartner, E., Frankevich, V., Zenobi, R., & Baltensperger, U. (2004). Identification of polymers as major components of atmospheric organic aerosols. Science, 303(5664), 1659-1662.

Li, X. X., Chee, S., Hao, J. M., Abbatt, J. P. D., Jiang, J. K., & Smith, J. N. (2019). Relative humidity effect on the formation of highly oxidized molecules and new particles during monoterpene oxidation. Atmospheric Chemistry and Physics, 19(3), 1555-1570. doi:10.5194/acp-19-1555-2019

Pospisilova, V., Lopez-Hilfiker, F. D., Bell, D. M., El Haddad, I., Mohr, C., Huang, W., Heikkinen, L., Xiao, M., Dommen, J., Prevot, A. S. H., Baltensperger, U., & Slowik, J. G. (2020). On the fate of oxygenated organic molecules in atmospheric aerosol particles. Science Advances, 6(11). doi:ARTN eaax892210.1126/sciadv.aax8922

Surdu, M., Lamkaddam, H., Wang, D. S., Bell, D. M., Xiao, M., Lee, C. P., Li, D. D., Caudillo, L., Marie, G., Scholz, W., Wang, M. Y., Lopez, B., Piedehierro, A. A., Ataei, F., Baalbaki, R., Bertozzi, B., Bogert, P., Brasseur, Z., Dada, L., Duplissy, J., Finkenzeller, H., He, X. C., Hohler, K., Korhonen, K., Krechmer, J. E., Lehtipalo, K., Mahfouz, N. G. A., Manninen, H. E., Marten, R., Massabo, D., Mauldin, R., Petaja, T., Pfeifer, J., Philippov, M., Rorup, B., Simon, M., Shen, J. L., Umo, N. S., Vogel, F., Weber, S. K., Zauner-Wieczorek, M., Volkamer, R., Saathoff, H., Mohler, O., Kirkby, J., Worsnop, D. R., Kulmala, M., Stratmann, F., Hansel, A., Curtius, J., Welti, A., Riva, M., Donahue, N. M., Baltensperger, U., & El Haddad, I. (2023). Molecular Understanding of the Enhancement in Organic Aerosol Mass at High Relative Humidity. Environmental Science & Technology. doi:10.1021/acs.est.2c04587

In the study by Zhang et al., the authors explore the effect of humidity (RH) on the formation of secondary organic aerosol (SOA) from ozonolysis of two structurally different monoterpenes; limonene and Δ³-carene, and the sesquiterpene β-caryophyllene. Experiments are performed at constant temperature in an oxidation flow reactor at RHs ranging from 1-60 % whilst monitoring SOA particle number and mass concentrations followed by off-line analyses of the SOA chemical composition using ultra-high performance liquid chromatography quadrupole time-of-flight mass spectrometry (LC-MS). The study reports large differences in the effect of RH on the SOA formation from limonene and Δ³-carene, with the former showing increasing SOA mass and particle number concentration at elevated RH whilst little or no effects are observed in the case of Δ³-carene. From the chemical composition of the formed SOA the authors explain these discrepancies by water-influenced reactions on exocyclic double bonds yielding lower volatile organic compounds under higher RH.

The topic is relevant and fall within the scope of ACP, however, the manuscript is in need of major revision before any consideration for publication in ACP  

Major concerns include the far from atmospheric relevant conditions applied in the study experiments, lack of validation of experimental approach, lack of discussion on contribution of other oxidizing agents, as well as scarce evidence of enhanced dimer formation at elevated RH from chemical analysis of formed the SOA. These are concerns that needs to be addressed if the manuscript is in any way to contribute to the work of the many previous studies reporting on the influence of RH on the formation of SOA from monoterpenes.  

Major comments:

• As I understand, this is the first publication using the custom-made oxidation flow reactor (OFR). With a length of 6.02 meters, have the authors validated that measurements of e.g. ozone, RH, temperature and VOC performed at the end of the OFR represent initial conditions at the point of injection and thus initial oxidation? Other OFRs, such as in Jonsson et al., (2006) and Li et al., (2019), is designed to ensure proper mixing of injected oxidant (e.g. O3) and VOCs at the initial stage of the OFR. When using OFRs the uniform distributions of O₃, VOCs and H₂Oin the tube should be confirmed by measuring O₃, VOC and RH at the different locations prior to the experiments. A particular concern is that the O3:VOC ratio and maybe RH may be different at the point of injection compared to the end of the 6.02 meter tube.
• The authors report SOA mass concentrations of 980-2200 ug/m3 from the oxidation of 321 ppb of limonene by 6 ppm of O3 with corresponding yields of 63-142% (table 1). These values are very high in comparison with other studies which should be made apparent by the authors. E.g. for clarification, please add mass concentrations and yields of all studies in table 2. Any explanation for these high yields?
• Also, I think the author should discuss the feasibility of extrapolating their flow tube results to the real environment. Limonene mixing ratios are at the sub-ppb level for forest and urban environments, thus the conditions applied in the current study seems far from atmospheric relevant. Could the authors explain the rational for using such high concentrations?
• Looking at Table 1, it seems that more O3 is consumed in limonene experiments than in Δ³-carene experiments (if reported O3 concentrations relates to measurement performed during the oxidation). To examine this, could the authors maybe report on the consumed O3 (ppb) in all experiments (e.g. concentration before and after the OFR). In relation, have the authors considered the influence of OH-radicals as possible explanation for the differences in SOA formation from limonene and Δ³-carene? I wonder to what extent the resulting SOA from limonene and Δ³-carene can be ascribed to oxidation by OH vs O3. I would expect that reaction with O3 is the dominating oxidation pathway for limonene, whilst reactions with OH-radicals may be more significant in Δ³-carene experiments. Especially without an added scavenger. Fick et al., 2002 showed that the Δ³-carene + O3 reaction is suspected to yield higher OH compared to that of Limonene +O3. Also, they report a negative effect of RH on the reaction of ozonolysis with Δ3-carene, whilst no such RH effect was observed for O3+limonene. Consequently, although all experiments in the current study are conducted as dark ozonolysis of limonene and Δ³-carene, it might be important to address that this does not rule out the influence of other oxidation pathways (e.g. OH-radical reactions) which may be less effective at producing SOA compared to ozonolysis and which also could exhibit different response to RH. For instance it may be that the Δ³-carene + OH reaction is unaffected (or enhanced relative to Δ³-carene + O3 reactions) by RH (e.g. Bonn et al 2002) in contrast to the Limonene + O3 reaction.
• The authors spend much effort on presenting and discussing the results related to the limonene experiments. However, in comparison, discussions on the Δ³-carene results seems lacking. In particular, results on the molecular analysis of the Δ³-carene SOA is lacking, e.g. comparison of mass spectrums recorded at different RH (such as in Figure 2), number and intensity proportion of the monomers, dimers, trimers and tetramers (as in Table S1).
• In relation, the observed increase in SOA mass in limonene experiments at elevated RH is proposed to arise from increased particle number concentration from nucleation promoted by low-volatile compounds such as dimers. To support this, the authors report 25 more dimers (187 vs 162) in limonene SOA formed at higher RH compared to low RH. This relatively small increase in LVOC species seems unlileky to account for the observed enhancements of SOA particle formation at high RH. At least the authors need to show that these extra dimers indeed contribute significantly to the formed SOA.  Also, Could the authors please provide similar results from Δ³-carene experiments; i.e. how many dimers where found in Δ³-carene SOA and do the number of dimers change with changes in RH?
• What is the detection limit of the analytical method i.e. could the observation of the additional dimers (and HOMs) merely be due to higher filter mass loadings in high RH experiments. Excluding dimers and HOMs not found in low RH conditions, very little evidence is presented showing increased dimer and HOM formation at high RH. Also, despite more than 160 dimers found in LC-MS analysis of collected SOA, intensities are only reported for 5 dimers in limonene SOA and 7 dimers in Δ³-carene SOA (table S2 and S5). At least it would be beneficial to report how the intensities of these dimers change as a function of RH (not only high vs low RH). Particularly in Limonene experiments performed at 30, 40, 50 and 60 % RH where the particle number do not seem to changes significantly between experiments

Other comments and suggestions:

• Please add to Figure S3 time evolution of SOA size and mass concentration from all Δ³-carene/O3 and limonene/O3 experiments to validate the stable conditions of the OFR
• Line 103-104: No description of materials are found in S2 (Figure?)
• Line 259-260: Note that HOMs are not all considered low-volatile (see Kurtén et al. (2016), entitled “α-Pinene Autoxidation Products May Not Have Extremely Low Saturation Vapor Pressures Despite High O:C Ratios”)

References

Jonsson, A. M., Hallquist, M., and Ljungstrom, E.: Impact of humidity on the ozone initiated oxidation  of limonene, Delta(3)-carene, and alpha-pinene, Environ. Sci. Technol., 40, 188-194, https://doi.org/10.1021/es051163w, 2006b

Li, X., Chee, S., Hao, J., Abbatt, J. P. D., Jiang, J., and Smith, J. N.: Relative humidity effect on the  formation of highly oxidized molecules and new particles during monoterpene oxidation, Atmos. Chem. Phys., 19, 1555-1570, https://doi.org/10.5194/acp-19-1555-2019, 2019b

Fick, J., Pommer, L., Andersson, B., and Nilsson, C.: A study of the gas-phase ozonolysis of terpenes: the impact of radicals formed during the reaction, Atmos. Environ., 36, 3299-3308, https://doi.org/10.1016/s1352-2310(02)00291-1, 2002.

Bonn, B. and Moortgat, G. K.: New particle formation during α- and β-pinene oxidation by O3, OH and NO3, and the influence of water vapour: particle size distribution studies, Atmos. Chem. Phys., 2, 183- 196, https://doi.org/10.5194/acp-2-183-2002, 2002.

Kurtén, T., et al. (2016): α-Pinene Autoxidation Products May Not Have Extremely Low Saturation Vapor Pressures Despite High O:C Ratios." The Journal of Physical Chemistry A 120(16): 2569-2582.

The manuscript by Zhang et al. investigated the effect of relative humidity (RH) on the formation of secondary organic aerosols (SOA) from structurally distinct monoterpenes from a molecular-level perspective. They observed a significant difference between limonene and Δ³-carene SOA formation on RH dependence. Further, they proposed potential chemical reaction mechanisms and pathways based on mass spectrometry analysis to explain the observed increase in limonene SOA and the decrease in Δ³-carene SOA. They suggested that the exocyclic double bond in limonene plays an important role in multi-generation reactions, contributing to the formation of lower volatile compounds under high RH conditions. Compared to many previous studies on the RH effects of SOA formation from monoterpenes, this study provides important insights into the multi-generation reactions that drive SOA formation by applying high-resolution MS analysis. The findings of this manuscript have significant implications for a better understanding of the mechanism of monoterpene oxidation reactions and the generation of secondary organic aerosols. This manuscript is well-written and I recommend publication in Atmospheric Chemistry and Physics after addressing the following minor concerns.

Specific comments：

• The authors used high precursor concentrations in the experiments. It would be more convincing if similar results can be found with lower precursor concentrations.
• The authors have extensively described the mass spectrometry analysis of limonene, while only briefly providing the distribution of high oxygenated compounds and dimers in Δ³-carene. It would be beneficial to include additional analysis of Δ³-carene mass spectrometry.
• Method: what is the temperature ramp program in liquid chromatography?
• Page 5, Line116: “limonene-” should be “limonene-SOA”.
• Page 7, Line 156: the authors have pointed out that the OH scavenger will produce additional products which may influence the reactions of target precursors, so what about the 2-butanol and cyclohexane discussed in this article?
• Page12, Line 238-239: specify the condition of the increase and decrease of C₉H₁₄O₃ and C₁₀H₁₆O₂, respectively.
• Supplement, Page 11: there were two (K) in Table S6.