Dear authors,

One referee still had concerns about the revised manuscript. Please carefully read and respond accordingly.

Best regards

Hang Su

Dear authors,

I find that the referee's comments haven't been adequately adressed as listed below. Please carefully respond and revise accordingly. The paper may not be publised in ACP unless these concerns were properly adressed.

1. Page 8, line 175: The authors have suggested a “baseline drift” for the determination of the mass growth factor, or rather the DRH value. However, it is confusing to readers that how “a significant increase in sample mass” was defined. What is the corresponding reference value for the investigated salts? Does the baseline drift in sample mass always correspond to a same value for different types of Ca- and Mg- containing samples?
I think the referee was asking for the number or criteria, based on which you can conclude a “significant increase”. Please specify it in a quantitative way, e.g. > signal noise + baseline drift, etc.


3. Page 8, line 180: It is unclear that when did “an equilibrium” or “a new equilibrium” state actually reach during the experiments, at least it is difficult to tell from the corresponding figures, e.g., it is still able to expect some increase in the sample mass as displayed in Fig. 5. In addition, the determination of an equilibrium condition will directly influence the accuracy of water-tosolute ratio, mass growth factor, and DRH results reported in this work. In this sense, how to quantify the influence of this “equilibrium” state on measured change in m/m0 or in normalized sample mass with varying RH, and further on DRH and comparison of different hygroscopic results?

The referee is asking for the criteria to determine the equilibrium point, and how a non-equilibrium state could influence the accuracy of growth factor etc. This is in principle a general question for the proposed method and not specific for the investigated species, but hasn’t been adequately addressed in the last round. What’s the equilibrium time scale for your system and how did you estimate it? This is a very important issue, especially when viscous organics/semi-solid organics are involved or for measurements of new compounds, of which a reference gf is not available.

4. Page 9, line 188: Following the comment #3, there is no definition of “normalized sample mass”, which is also important for the determination of DRH. I feel lost in the authors’ response to Specific comment #15 of Ref #3, which explained that “ When plotting these three figures (i.e., Figs.1, 3, and 5), we did not always to normalize the sample mass to the dry particle mass. This is why the right y -axis in these three figures is labelled as normalized sample mass instead of m/m0.” How to quantify the change in sample mass accurately, if the “normalized sample mass” was not always normalized by the initial dry mass? Accordingly, which kinds of mass were used for the normalization if “not always” to normalize, and how should the readers evaluate the comparability among these normalized sample mass results?

The referee has asked for a definition of “normalized sample mass”, and how did you normalize your results if it is not normalized by the dry particle mass. These questions remain unanswered.
7. Page 12, line 242: In the expression of “could have unknown systematical errors”, I was confused by the “unknown” here, as this sentence was supposed to provide the specific reasons. Besides, why can you assume that it is due to systematical errors if it’s even unknown?
I agree with the referee and please rephrase the sentence.

Line 521: Why is the DRH determined to be 90.5±1.0% instead of ±0.5%? A similar issue existed in Line 533: “giving a measured DRH of 71.5±1.0%”. Please also have a check on the corresponding values such as in the Conclusion section.
I think the referee’s concern came from Page 9 “Therefore, its DRH was measured to be 52.5±0.5 %”.



Sincerely,

Hang Su

Dear authors,

I am glad to accept your manuscript for publication in ACP.
Congratulations!

Sincerely,

Hang Su