This manuscript presents hygroscopic properties of levoglucosan and D-glucose nanoparticles, mainly growth factors, through combined laboratory nano-HTDMA measurements and model predictions. Measurement data of hygroscopic properties of nanoparticles, especially for sizes down to 6 nm are scarcely lacking. The results from this study may make important contributions to improve our understanding of the hygroscopic properties of small nanoparticles and thus their important roles in new particle formation and growth, also in aerosol aging, in particular, the important roles of the two important biomass burning product in the above aerosol formation and aging processes. The following issues/concerns need to be fully resolved before the manuscript would be considered.

1. Abstract: The abstract is not concise and not getting to the main ideas of the paper. L37-39, levoglucosan is mentioned in the sentence, however, the RH examples are for D-glucose, why?
2. Introduction: What is the purpose for the authors to compare the availability of the thermodynamic data between highly supersaturated inorganic aqueous solutions and organic solutions such as levoglucosan and D-glucose (the paragraph in L88-97)? A clear connection and comparability between the inorganic and organic solutions is needed.
3. Methodology: There are too many subsections in this part. Some of them are very short. It is not necessary to divide this part into so many subsections. Better rearrangement for this section is needed.
4. In Fig.2, it is not clear where the observations are from, is it from this study or from Chan et al.? The authors ought to clarify the data source. If it is from this study, why to mention Chan et al.’s measurements. If it is from the latter, why a comparison between the two is not seen in the figure.
5. L287-309, it is quite interesting to note that the levoglucosan nanoparticles become significantly evaporated below 20 nm. The authors attribute this trend to the Kelvin effect. However, if it is because of Kelvin effect, why it does not occur for other organic nanoparticles such as D-glucose?
6. Conclusions: L408-415, it is abrupt to introduce the biomass burning and the mixing state in the conclusion. What is the point to present them as illustrated at the end of the paper?
7. Below are rather minor issues/concerns, mainly grammar problems etc

• L23, indicting? You mean “indicating”? L29, predicating? Predicted；L57，quantification of the biomass; L64-65, have focused on.
• L83-83, this sentence seems quite awkward and it is suggested to change to “It is not clear how the size effect influences the hygroscopic growth of organics, especially thos without DRH and ERH”.
• L85, organic, not organics; L90, “are limiting” changed to “limit” ；L173-174, please revise the ill sentence; L183, there is no A in eq. 4, please double check; L188, ideal, not idea; L195, Growth factor predicted by …; L202-203, no coma is needed after note that and “are to predict” here “are to” is redundant and should be deleted; L214, the equation (equation (8)) shows the relationship between…; L216, no capital for V of vapor.
• L228-230, no subject is found in this sentence; L237, become highly supersaturated and article “the” should not be added before highly; L261, those of this study.
• L243-261, are the notations between “deliquescence and efflorescence modes” and “hydration and dehydration modes” the same? Please clarify.
• L308, indicating, not indicting; L316, among them? L328, delete during.
• L342-344, this sentence is hard to understand, please revise it “To have a clear observation for size dependence of the hygroscopic growth factor of D-glucose aerosol nanoparticles with diameters down to 6 nm, Fig. 8b shows the change in the hygroscopic growth factor of D-glucose aerosol nanoparticles with diameters from 100 down to 6 nm. “
• L382, has been.

The manuscript by Lei et al. reported the HTDMA data for levoglucosan and D-glucose particles for the size range of 6 ~ 100 nm. Both of these compounds are atmospherically important chemical species. The data for 100 nm particles agreed well with literature.  In the case of levoglucosan particles, measurement of hygroscopicity for particles smaller than 15 nm was challenging due to evaporation. The issue was not observed for D-glucose. In the case of D-glucose particles, a significant size-dependence in hygroscopic growth was observed only for the high RH (RH > 80%) region. The data of the study will serve as a reference for future studies on hygroscopicity of atmospheric nanoparticles. The topic is obviously a part of the scope of the research community. The following points should be considered prior to the acceptance of the manuscript.

Major comments

The authors state that the UNIFAC model does not provide highly accurate results for organic compounds which contains many polar functional groups (e.g., the description starts from L354). However, the discussion is not well supported by references. Thus, it is difficult for a reader to evaluate the level of uncertainty. As D-glucose is an important chemical compound in various research areas, a numerous number of studies should have been conducted for investigating its interaction with water. It would be great if the authors could provide further detailed discussion using these references to support their argument.

The authors demonstrated that evaporation of levoglucosan inhibits accurate measurements of hygroscopicity for the nano-size range. However, it seems that the influence of evaporation is absent for the D-glucose data. It would be beneficial to describe the reason why the evaporation only occurred to levoglucosan. As the authors carefully quantified the residence time of particles in their HTDMA system, it could be possible to provide more quantitative information using the saturation vapor pressures of the two chemical species.

It is interesting that the authors only observed the size-dependence in growth factor of D-glucose for the range of RH > 80%. There seems to be no size dependences for the lower RH region. To the best of the reviewer’s knowledge, it is not a well-known phenomenon. Do the authors have any potential hypothesis for explaining it? I believe that it will be beneficial for future studies if the author could add some ideas that can explain the observation to the manuscript.

Minor/editorial comments

There are numerous typos and grammatical issues with the manuscript. The reviewer suggests the authors to be careful about these points in revising the manuscript.

Abstract

The current abstract is a little bit long and redundant. It describes what the authors have done, but it does not tell the key conclusions of the manuscript well. I suggest the authors to revise it.

L60 ~ 63

Mochida and Kawamura (2004) are cited for three times for describing the atmospheric significance of levoglucosan and D-sucrose. However, the paper by Mochida and Kawamura (2004) focused on hygroscopicity measurement of levoglucosan and other important marker compounds for biomass burning. Other references that directly supports the corresponding descriptions need to be provided.

L61

I agree that levoglucosan and D-glucose are important chemical species for biomass burning particles. However, they would probably be one of the most polar chemical species among constituents of biomass burning particles. It is not clear to me if they can be considered as representative compounds for hygroscopicity.

L123-125

Generally, the issue of multiple charge particles is not a significant concern for small (diameter < 30 nm) particles. Could the authors make the corresponding description to be more detailed?

 L169

The equation (2) looks like an equation for the ideal solution to me. Although the idea of ideal solution can occasionally be applied for the Kohler theory, the Kohler theory is not equivalent as the ideal solution. It would be better to change the name of the corresponding section.

L190

Are there any good references to support the idea that the partial molar volume of water does not change in solution? If it changes, how much it could change? How would it affect the interpretation of the data?

L235

I wonder what the author’s definition of ‘diluted aqueous droplet’ is.  20 mol kg-1 is highly concentrated. Please clarify.

L237

Please indicate the saturation concentration of levoglucosan in water before mentioning about supersaturation.

L248 (and other places)

No deliquescence/efflorescence were observed in the study. It may not be appropriate to call the operation modes of the HTDMA as ‘deliquescence/efflorescence modes’ under this condition. Hydration/dehydration might be a better expression.

References

The reference list needs to be carefully checked. There are numerous issues. For instance, I do not believe that the names of the authors of the following paper accurately represented in the current version of the manuscript.

‘Chan, M. N. C. a. C. K.: Mass transfer effects in hygroscopic measurements of aerosol particles, Atmos. Chem. Phys., 5, 2703–2712,’