Comment on acp-2021-486

Shuang Han and co-authors present an experimental study of organic aerosol hygroscopicity for 23 atmospherically-relevant compounds. The hygroscopicity (kappa) is derived from water uptake measurements using an HTDMA. Compounds are grouped as sugars, polyols, diand tri-carboxylic acids, and amino acids, and are purchased. Two publicly-available chemical equilibrium models are compared with the data. The authors find that, although hygroscopicity varies with organic functional groups, oxidation level (O:C ratio), and molecular weight, these molecular properties are imperfect predictors of water uptake. The models over-predicted hygroscopicity for some, but not all, of the studied compounds. Various possible reasons are discussed. The work is technically sound, and the measurements are neither better nor worse than those of previous studies. This authors have done a good job summarizing previous findings, and they contribute measurements and analysis for new compounds.

This authors have done a good job summarizing previous findings, and they contribute measurements and analysis for new compounds.
This manuscript is a good fit for Atmospheric Chemistry and Physics. The work is laboratory-based but has relevance to modelling and to field studies. The authors have done a good job discussing this relevance.
Although this is an interesting and well-written paper, I cannot recommend publication in its present form. The authors have not adequately described the novelty of the work, and some of their findings are not substantiated by the data. The potential impact is limited because authors have not explicitly identified their contributions. I believe they can nevertheless improve the writing before publication. These and other issues are discussed below. I recommend major revisions.

Major Comments
The authors fall short of establishing/communicating the work's novelty. Several findings have been explored in past works (functional groups, molar mass, O:C, solubility and deliquescence). The authors have done a good job finding these studies. However, authors should discuss and emphasize their own contributions.
Further comments on the abstract: As stated above, findings summarized here should emphasize the novelty of the current work. The abstract should end with implications.
17-with additional functional groups -addition to what? This is more complicated than just the addition of functional groups. Carbon number matters as well. 18 -It sounds like you mean isomers. This statement is ambiguous 23 -"moderately" Uncertainty estimates are needed. As it stands, scatter in the data is used to discuss morphology. Although this is a nice discussion, some error bars and acknowledgement of the limitations of the measurement would lend more credibility to these claims.
Some of the conclusions are not supported by the data. On line 184, the authors discuss the order in which the functional groups contribute to hygroscopicity. Is this statement quantitative? If so, what is the observed partial derivative of kappa with respect to each functional group? The statement seems to have little connection to the data presented in Figure 4.
Restructuring of particles was observed, and this resulted in a negative growth factor. This was shown and discussed in the main text and in Figure S3, which shows severe discontinuities in water uptake for amino acids. Some of the restructuring-sizing error could be avoided by sizing the particles wet, following Nakao et al. Regarding restructuring, the residence time of the HTDMA is mentioned (2.7 seconds) but the authors do not bring this into the discussion. The authors should mention how this 2.7 s residence time affects particle restructuring, and how this instrument compares to other works.
There is a severe disagreement between the model and the measurements for some compounds, but the authors do not attempt to improve the prediction of water uptake by any calculation or modification to the models.
The authors do not discuss the disagreement between UManSysProp and E-AIM. Why do these models behave differently? There is valuable information here and it should be discussed.

Minor Comments
Solubility: Line 219, 241, and elsewhere -there are too many "types" here, consider clarifying these paragraphs by using more specific and consistent terminology for solubility regimes. See Because growth factor as a function of RH is diameter dependent, the results would be more general if the RH is divided by the Kelvin term, here expressed Ke (on line 87). This means that instead of RH you have growth factor as a function of water activity. aw = RH/Ke. 33 -the citation does not match the statement in any way 147,147 -how do you know that the water mass fraction always increased when the growth factor "shrank or grew" ? This calculation should be described here or in the supplement. The table of contents in the supplement is very confusing. Why are figures each described twice, except for Figure S4? Also: there is no Figure S4. Please clarify this text.