This manuscript investigates the contribution of sulfuric acid and organic vapours (essentially alpha-pinene oxidation products) on the growth of sub-10 nm size particles as a function of particle size and relative humidity based on laboratory experiments. The investigation appears to be robust and novel enough to be published in a scientific publication. There are a few issues, however, that should be addressed before final acceptance of this work.

Scientific issues

Line 17-18: Hydration effects and changes in oxidation products are given as scientific facts for explaining the observations in the abstract. However, when reading the paper, it appears that these explanations, while likely, are speculations rather than facts. I recommend that the authors pay more attention what is interpreted as a scientific fact, a probable explanation, or speculation when discussing the observations.

Lines 59-66: This paragraph lists what will be done/investigated in the paper. The authors should also give either concrete scientific goals of the study or, alternatively, research questions aimed to be answered here.

Lines 249-250: This statement (This behaviour is attributed to …) is given without a proper justification. The authors should add some reasoning(s), or at least speculations.

Technical issues:

Lines 126-129 and 194-196: Although the procedure of determining the OOM mass fraction is relatively straightforward, the paper would benefit from having an example plot on how this works in practice for the data applied here.

Line 169: Differ from --> different from

The given values in percent on lines 198 and 218 are too accurate. I would recommend using an accuracy of 1% or, in maximum, the accuracy of 0.1%.

The manuscript by Zhang et al. discusses the organic contribution to nanoparticle growth based on laboratory measurements. Since direct composition measurements in this size range are difficult, there is still limited understanding of the relative roles of sulfuric acid and organics as a function of size. This study contributes to this understanding and is therefore suitable for publication in ACP after some revisions. The analysis and figures are presented very clearly, but the language should still be checked carefully.

Major comments:

I am puzzled by the large systematic difference between the SA K-values in this study and earlier studies (Table 1). In the text and conclusions (r272), those are attributed to “the nano size effect and different measurement techniques”. However, e.g. Kim et al. 2016 also looked at 10 nm particles, so the “nano size effect” cannot be the explanation there. The authors should thoroughly analyze and discuss the potential reasons for the differences.

Can there be ammonia and/or amine contamination in your system? If yes, how would that affect the results and would that be constant with RH?

In row 138 you mention “the mass spectrometry” which was not discussed anywhere else. What mass spectrometer was used and for which part of the experiment?

Minor:

What is the RH of the DMA sheat flow?

Abstract, row 21: more pronounced compared to what?

Zhang et al measured the hygroscopicity of aerosol particles containing sulfuric acid (SA), α-pinene-derived oxygenated organic molecules (OOMs), and their mixture in the size range of 3-10 nm. The measurement was conducted using a custom-designed scanning flow condensation particle counter (SFCPC). By doing this, the organic mass fraction for the mixture particle can be derived for the SA-OOM mixture particles that were generated at different SA/α-pinene ratios and humidity. The main claim is that OOMs' contribution to the particle phase becomes increasingly important as particle size increases. When the motivation of the study and the use of language are overall good, the current level of analysis is not convincing enough to back up the main claim. If the following comments could be adequately addressed, I would suggest considering the work for publication.

Major Comment

1. Novelty of the work: The SFCPC has been used to study the CCN activity of nanoparticles several times before. The authors must provide a summary of their previous work (Wang et al., 2015; Zhang et al., 2023; Zhang et al., 2025) based on the SFCPC in the introduction. Unfortunately, the authors barely mention their previous work using SFCPC, and therefore, readers most likely have no clue about the novelty of the current work.
2. Aerosol Generation: There are a couple of questions that need clarification with support from compositional measurement and/or modelling.
   1. What was the estimated OH concentration in the flow tube per experiment?
   2. How was α-pinene consumed via ozonolysis and OH oxidation pathways?
   3. Was the composition of OOM comparable between different sizes for the OOM-only particles?
   4. How can the authors be sure the composition of OOM was the same between the OOM-only particles and SA-OOM particles for a specific size under the same RH?
3. Determination of f_org: It looks like the eq (1) is based on the assumption of ideal mixing of SA and OOM. If so, can you discuss the uncertainty regarding this assumption and how it will impact the determination of f_org?

Minor Comment

1. Lines 114 – 116: The description for the use of the electrometer needs to be clarified. Was it used to count total particle concentrations?
2. Line 138: Where is the mass spectrometry data?
3. Line 144: If SA exist in the form as (H2SO4)m-(H2O)n, how will the m and n change regarding particle size and the presence of OOM?

Technical Comment

1. Was a double-charge correction applied when deriving S from the measurement data?
2. What was the value for surface tension used in the equation to derive kappa?
3. Line 161: Please clarify what the nano-size effect is.

References

Wang, Z., Su, H., Wang, X., Ma, N., Wiedensohler, A., Pöschl, U., and Cheng, Y.: Scanning supersaturation condensation particle counter applied as a nano-CCN counter for size-resolved analysis of the hygroscopicity and chemical composition of nanoparticles, Atmos. Meas. Tech., 8, 2161–2172, https://doi.org/10.5194/amt-8-2161-2015, 2015.

Zhang, K., Xu, Z., Pei, X., Zhang, F., Su, H., Cheng, Y., and Wang, Z.: Characteristics of scanning flow condensation particle counter (SFCPC): A rapid approach for retrieving hygroscopicity and chemical composition of sub-10 nm aerosol particles, Aerosol Sci. Technol., 57, 1031–1043, https://doi.org/10.1080/02786826.2023.2245859, 2023.

Zhang, K., Xiong, C., Cheng, Y., Ma, N., Mikhailov, E., Pöschl, U., Hang, S., and Wang, Z.: Assessment of hygroscopicity uncertainties associated with size and thermodynamic model: Implications for inferring chemical composition of sub‐10 nm particles. Journal of Geophysical Research: Atmospheres, 130(15), e2025JD043835, 2025.