Dear authors,

Thank you for submitting a revised manuscript. A referee still has major concerns about your manuscript (see Report #1). In order for publication in ACP the comments in the referee report #1 need to be addressed adequately. Please see comments in Report #1 and consider each comment when revising your manuscript.

In addition to addressing the referee’s comments, I would also like the following comments to be addressed (these comments expand on the referee’s comment’s).

1) The authors comment that for an internal mixture of oxalic acid with levoglucosan or succinic acid, there is enhanced water uptake at high RH due to the chemical interactions such as hydrogen bonding between solutes. Could the enhanced water uptake at high RH be just due to a freezing point depression, resulting in a liquid state at lower RH values. See Hemming, B. L., and Seinfeld, J. H.: On the hygroscopic behavior of atmospheric organic aerosols, Ind. Eng. Chem. Res., 40, 4162-4171, doi:10.1021/ic000790l, 2001; and Marcolli, C., Luo, B. P., and Peter, T.: Mixing of the organic aerosol fractions: Liquids as the thermodynamically stable phases, J. Phys. Chem. A, 108, 2216-2224, doi:10.1021/jp036080l, 2004.

2) Could the enhanced water uptake at high RH in the internal mixture of oxalic acid with levoglucosan or succinic acid be explained by the Gibbs-Duhem relation and assuming ideality for all solutes (see Equation 5 in Marcolli, C., Luo, B. P., and Peter, T.: Mixing of the organic aerosol fractions: Liquids as the thermodynamically stable phases, J. Phys. Chem. A, 108, 2216-2224, doi:10.1021/jp036080l, 2004. If this is the case, is it accurate to say the enhanced water uptake at high RH is due to the chemical interactions such as hydrogen bonding between solutes.

Sincerely,
Allan Bertram

Dear authors,

Thank you for responding carefully to the referee's comments. After reading over your responses, and reading your manuscript again, I have a few comments that should be addressed before publication.

1) In response to the comments from the referee, you state that the ZSR predictions assuming oxalic acid in a liquid state underestimate the hygroscopic growth of equal mass levoglucosan/oxalic acid mixture (seen in Figure 3a). This assumes that the dihydrate forms at low RH values. However, the results in this manuscript do not seem to support the formation of the dihydrate. For example in Figure 4c, there does not appear to be any indication of the dihydrate forming at low RH (2%) in the Levoglucosan/oxalic acid mixture. Unless you can better justify the formation of the dihydrate in the levoglucosan/oxalic acid mixture, then equal weight should be given to the results in Figure 3a and 3b.

2. Related to the above comments, in Section 5, you state that the OA in the dry mixtures is more likely to exist in a dihydrate form rather than an anhydrous one due to residual water retained by coexisting species such as levoglucosan. However, the formation of a dihydrate does not depend on just the presence of residual water. The formation of a dihydrate also depends on the kinetic barrier to the nucleation of the dihydrate crystal. If the kinetic barrier is high, than the dihydrate will not form regardless of the presence of residual water. If the authors want to conclude that the dehydrate is more likely, then justification is needed.

3. Figure 12. For this figure did you assume that the oxalic acid is in a liquid state at high relative humidities like you did in Figure 3 (dashed lines). This was not clear from the description. Please clarify what is plotted in Figure 12 and if you did not assume oxalic acid was in a liquid state when doing the calculations for this figure, then please add some justification on why a liquid state was not assumed similar to Figure 3 (dashed lines).

Sincerely,
Allan

Dear authors,

Thank you for responding to my comments. However, I do not feel your responses have resolved my concerns or the referees concerns, and the modifications have led to sections in the manuscript that are confusing and contradictory. These issues need to be resolved before I am able to accept the manuscript for publication. See specific comments below:

1) Regarding justification for the formation of oxalic acid dihydrate. Based on the previous studies, I am fine with the assumption that oxalic acid dihydrate forms at low relative humidities in single component oxalic acid particles. However, I wonder about the validity of this assumption for particles containing oxalic acid and levoglucosan. You used previous studies of single component oxalic acid particles to argue that the oxalic acid dihydrate will form in your particles containing oxalic acid and levoglucosan. However, even though the dihydrate forms in single component oxalic acid particles, the dihydrate may not form in the two component oxalic acid-levoglucosan particles. The presence of the levoglucosan may completely shut off any crystallization. Just to be clear, I am suggesting the possibility that mixed levoglucosan and oxalic particles remain in a liquid (or amorphous solid) state even at the lowest RHs studied. In fact, I wonder if figure 4c provides some justification for this. In 4c there doesn’t appear to be any indication of crystallization even at the lowest relative humidity. The spectra in figure 4c were recorded with larger particles, so with smaller particles, as used in the HTDMA experiments, crystallization would be even less likely, suggesting that at the lowest RHs studied mixtures of oxalic acid and levoglucosan will be in the liquid state. Please discuss this possibility in your manuscript.

2) “Anhydrous OA” is used throughout the manuscript but it is not always clear if the authors are referring to “amorphous anhydrous OA” or “crystalline anhydrous OA”. Please go through the manuscript and make sure if is clear what type of anhydrous state you are referring to (i.e. crystalline or amorphous).

3) Issues that are now confusing in the manuscript after changes have been made:
a. Section 4.2.1. The authors indicate that “no deliquescence behavior is observed and the continuous water uptake starting at low RH indicates the initial particles appear to be in a liquid state and retain water.” Later they say that “OA in the initial mixed particles may exist in a liquid state and contributes to water uptake by mixed particles in the subsequent humidification”. This discussion is consistent with my comment (1) above. However, later the authors also state, “Considering hygroscopic species such as levoglucosan could retain some water at low RH, it can be expected that oxalic acid in the submicron mixed particles is also composed on stable dihydrate”. Since a dihydrate is a crystalline form, these statements are contradictory and confusing. How do the authors reconcile these contradictory statements? Please adjust the manuscript to remove this contradiction.

b. Section 4.2.1., the authors state “oxalic acid particles under dry conditions are most likely composed of dihydrate irrespective of various phase state”. What does “irrespective of various phase state” refer to? Since they are referring to the dihydrate there is only one phase state?

c. Section 4.2.1. In the newly added text, the authors state “Mikhailov also found that the solid amorphous oxalic acid particles under dry conditions consisted of oxalic acid dihydrate”. A solid amorphous oxalic acid particle cannot consist of oxalic acid dihydrate, since a dihydrate refers to a crystalline material while an amorphous material refers to a non-crystalline material.

Sincerely,
Allan Bertram

Dear authors,

Thank you for addressing my questions. It is clear now that my main concerns were due to a misunderstanding. I understand now, that you were using dihydrate to indicate the amount of water, but not the phase. I think the following very minor modifications would clear up most of the misunderstandings for readers of your manuscript:

1) When referring to the dihydrate add an appropriate adjective to specify the phase state. For example use 1) crystalline dihydrate, 2) amorphous dihydrate or 3) crystalline or amorphous dihydrate instead of just dihydrate.

2) I don't think the use of quotation marks around dihydrate is needed.

3) in the revised document you stated "OA dihydrate, i.e.,non-stoichiometric hydrates containing about two water molecules per oxalic acid molecule". I think "i.e." should be replaced with "or" since "i.e." is not appropriate here.

Thank you for considering these comments.

Sincerely,
Allan Bertram