General comments

This manuscript titled “Predicting Hygroscopic Growth of Organosulfur Aerosol Particles Using COSMOtherm” by Zijun Li et al. estimated the HGF of OS compounds and their mixtures with ammonium sulfate using the conductor-like screening model (COSMOtherm) and compared the results with those from existing experimental studies. It claimed that the model-estimated and experimental HGFs for the studied OS compounds agreed well, based on which it proposed that the quantum-chemistry-based approach for HGF estimation will open up the possibility of investigating the hygroscopicity of other OS compounds present in the atmosphere. The addressed scientific question is well within the scope of ACP and the manuscript is well structured and well-written.

However, more explanation of the COMSMOtherm model and the appropriateness of using it in the study should be included. E.g, how this model has successfully estimated the water activity of other organic compounds other than OS? What are the merits of this model compared with other models in estimating the water activity of OS and mixtures of OS and AS? In addition, more discussions in Sect. 3 are necessary to help readers better understand the results obtained. Please refer to the specific comments for details.

Specific comments

1. Line 5: “a group of compounds”, more specific explanations on the OS compounds could benefit the readers.
2. Lines 33-36: what are the respective merits and demerits of AIOMFAC and COSMO-RS? Why did you choose to use COSMO-RS? Is there any existing study using AIOMFAC?
3. Lines 36-37: Is there any other successful usage of COSMO-RS, comparison between modeled and experimental water activities of other organic compounds indicating the appropriateness of COSMO-RS to be used for the prediction of HGF? A thorough explanation of this would enhance the scientific quality of the method.
4. Lines 126-130: what can you conclude from this? Can you do some interpretation of this result? What’s the relationship between this result and those in the following sections?
5. Line 134: Is there any method to quantify the similarities between the model and the experimental results? If yes, do the same for other comparisons between model and experimental results. Doing this will enhance the scientific quality of the study and help the comparison with possible future studies.
6. Line 136: “Note that the … measurement data.” The explanation here is quite vague.
7. Line 162: 1:3 → 1:5?
8. Sect. 4 Conclusions: I think it would be good to include a brief discussion of the performance of COSMOtherm with different cations.

Lie et al. unitized the COSMO0RS model to simulate the hygroscopicity growth factor of organic sulfate (OS) and organic sulfate and ammonium sulfate mixture. The study shows good agreement with some experimental results. This study has good potential to provide a way to better simulate hygroscopicity for climate models. However, I have some major questions that need further explanation from the authors.

General comments:

1. Could you discuss why you see a discrepancy between the model-predicted and experiment-measured HGF in Figure S4? Also, I suggest moving Figure S4 to the main text since it is essential.
2. I am not fully convinced by section 3.3. First, NaMS measurements data shows a gradual increase of HGF with an increase of RH, but your model predicted HGF shows a step increase. I think the OS and ammonium sulfate (AS) mixture should be amorphous, which should not have a delinquency point. Moreover, do you have the hygroscopicity parameter of OS? I expect their hygroscopicity is lower than AS, and mixing AS and OS should have hygroscopicity lower than AS but higher than OS.
3. What size of particles you used in your model? Do you consider diffusion limit?

Specific comments:

1. For equation 5, Have you considered the effect of volume loss due to molecular interactions (e.g., hydrogen bond) and packing efficiency (e.g., intermolecular space)?
2. Figure 1: do you have any explanation for why NaMS and NaES show a sigmoidal relationship?
3. Figure 2: Why do you only show NaMS, KGAS, and NH4HES?