This work measures the hygroscopic properties and cloud condensation nucleation (CCN) activities of model organosulfate aerosols and allow us to better understand the environmental fates and impacts of these compounds.  This work fits the scope of Atmospheric Chemistry and Physics. I have the following comments and suggestions for the authors' consideration.

Comments

Line 72, “As OS are ubiquitous and abundant in the troposphere, it is important to understand their hygroscopic properties and CCN activities in order to assess their environmental and climatic  effects.” As mentioned by the authors, the atmospheric abundance of organosulfates investigated in this work is in order of ng/m3. Can the authors further elaborate or comment the atmospheric significances of these investigated organosulfates? For instance, how the presence of these organosulfates would affect the physiochemical properties of atmospheric aerosols such as surface tension, hygroscopicity and CCN studied in this work?

Line 127, “As shown in Figure 2, the mass of OS samples at different RH was determined by the VSA using the following method.” In figure 2, only the results of sodium methyl sulfate were shown. What would be the VSA results for other species?

Line 197, “the increase in sample mass at 50% and 60% RH may be because ethyl-OS were partially deliquesced at this stage.” What causes the ethyl-OS partially deliquesce? Why other OSs do not deliquesce partially?

Line 276, “Figure 4 shows that obvious deliquescence transitions were observed for methyl-, ethyl-, and octyl-OS in the VSA experiments; in contrast, as revealed by Figure 5, continuous hygroscopic growth without obvious phase transitions was observed for methyl-, ethyl- and octyl-OS aerosol particles in H-TDMA measurements, suggesting that these aerosol particles may exist in amorphous state.” Can the authors discuss why methyl OS, ethyl-OS and octyl OS exhibited different deliquescence behaviours in H-TDMA and VSA measurements?

Figure 6, at the same RH the GFs of methyl-, ethyl- and octyl-OS derived from VSA measurements were found to be consistently smaller than those measured using H-TDMA. Can the authors elaborate this observation?

Line 351, “Such underestimation at 70% and 75% RH is likely to due to that inorganic compounds (AS, in our work) may dissolve partially in the organics/water solution (which can be formed at much lower RH due to continuous water uptake of organics) before the mixed particle is completely deliquesced (Svenningsson et al., 2006; Zardini et al., 2008; Wu et al., 2011);”  Can the authors comment if the amount of AS partially dissolved could be estimated based on their assumption and the hygroscopic data?

Line 392, “No significant difference was observed between κgf and κccn for five types of aerosol particles, and the relative differences between κccn and κgf values do not exceed 25%. However, octyl-OS appears to be an exception, and the average κccn value (0.206) was ~1.4 times larger than the average κgf value (0.086).” Given the differences between κgf and κccn were larger than the error bars (Figure 8), can the authors discuss why they would like to claim there is no significant difference between κgf and κccn.

Line 403, “Petters  and Kreidenweis. (2008) demonstrated that cloud droplet activation was highly sensitive to the solubility for sparingly soluble compounds in the range of 5×10-4-2×10-1, expressed as volume of solute per unit volume of water (Petters and Kreidenweis, 2008). Compared to the highly soluble methyl- and ethyl-OS, the solubility of octyl-OS (8.43×10-4-4.26×10-2) (Chemistry Dashboard, 2021) is rather limited.” What are the estimated water solubility for methyl- OSand ethyl-OS? Are they highly soluble or sparingly soluble?

Line 408, “incomplete dissolution at subsaturated condition in H-TDMA  measurements may lead to underestimation of κgf values for octyl-OS; as a result, the solubility limit may explain the observed difference between κgf and κccn for octyl-OS.” As mentioned by the authors, shall the authors estimate the κgf values for octyl-OS with solubility limit and surface tension correction?

The paper by Peng et al. addresses gap knowledge of organosulphates hygroscopic properties and CCN activity. Organosulphate hygroscopic properties have not been systematically studied to date and as such paper is a significant contribution to atmospheric science. The paper is generally well written although a moderate revision is needed to meet the publication standard of ACP.

Line 36. Maintain three significant digits for consistency. Even the third digit of the value is imprecise, because the standard error is changing the second digit.

Line 42. Quantitative numbers needed to illustrate qualitative terms like “reasonably well”, especially in the Abstract.

Line 157. delta GF should have reflected Kelvin effect which was not negligible for 100nm particles. The authors could use kappa method in estimating Kelvin effect against e.g. 300nm particle were Kelvin effect would be immeasurable.

Figure 3. Is Figure 3 meant for illustration purposes or is the result? It is unnecessary for the former and if for the latter it should be presented in terms of SScrit as a function of Dcrit along with ammonium sulphate.

Line 195. It is difficult to judge the significance of 11% without uncertainty error bars. Could it be due to physical spatial arrangement of 1mg mass lump?

Line 212. Measurable, not obvious. Nothing is obvious in scientific experiment.

Line 214. suggesting the occurrence of ...

Line 218. Interestingly, that in this case the authors discount 10-20% increase, contrary to ethyl-OS increase of 11%, mentioned earlier.

Table 1. Maintain three significant digits throughout.

Figure 5. I believe that a) and b) were split due to methyl and ethyl OS being similar and partly overlapping, but it is exactly for the same reason they should be on the same graph and if a single graph was bigger it would exhibit those differences clearly.

Line 254. DMA sizing precision is at best 5% (Wiedensohler et al. 2012, AMT) and, consequently, 7% of the two DMAs. Clearly 8-9% difference can be attributed to sizing uncertainty of different HTDMA systems.

Line 297. That is understandable as the bulk material is present in large lump of mass (1mg is huge when compared to single particle). In order for VSA to represent microscopic particles, one should use tiny amount of substance spread as e.g. 100nm film, which is challenging and impractical, thereby limiting the usefulness of VSA for atmospherically relevant studies.

Line 379. The authors should emphasize that reduced hygroscopicity was measured in supersaturated conditions while in subsaturated conditions hygroscopicity was higher as revealed by HTDMA (e.g. 60-70%).

Line 416. ...but much less pronounced for a mixture octyl-OS/AS. ("mixture" should be emphasized)

Line 417. What about the discrepancy of methyl and ethylOS despite both being very soluble? That should be noted and discussed, especially that their kappaGF are higher than kappaCCN.