Dear Dr. Abdelmonem,

I have received two referee reports of the revised manuscript. Both reviewers suggest major revisions before the manuscript can be accepted. I have listed the reviewers' comments below. If you feel you can address these comments, please submit a point-by-point response to the raised comments and a revised manuscript at your earliest convenience.

With kindest regards,

Daniel Knopf

Reviewer #1:
The author refers to surface hydroxyl groups on mica (p9, l10-20) although there are none. This is an important point, because it has a bearing on the on the discussion of the orientation of water molecules at the surface. Mica does not protonate by forming silanol groups, the effect of water at moderate or low pH is to replace the potassium ions with hydrogen ions. The author should consult the extensive literature on mica again, for although some is cited, the information has not been correctly reported.

Reviewer #2:
The revised paper still contains some technical issues which may affect the interpretation of the experimental results.

(1) Page 1 in the point-to-point response to Referee #2.
AC: "Before starting the measurements, the polarization of the water SHG signal was analyzed and found to have the expected maxima at S and P polarizations. (Has been mentioned in the revised version, P. 3, L. 35-37)"

Reviewer 2: It is not clear what are “the expected maxima at S and P polarizations.” Because mica is birefringent, the polarizations of the incident beam and the output SHG may be rotated. This effect is related to the incident plane, the incident angle, and the thickness of the individual mica used in the experiments.

(2) Page 4 in the point-to-point response to Referee #2.
AC: “The border between a film at the solid-air interface and a bulk (ice or water) was defined experimentally by the point where the intensity of a TIR reflected light from the solid-air interface drops due to the violation of TIR condition when the refractive index of the contact medium changes drastically from that of air (na=1) to one of those of water or ice (nw or i > 1.3).”

Reviewer 2: If this is the experimental setup, the SHG drop in Figure 4 is highly related to the Fresnel factors calculated by the author. As the Fresnel factors depend on the polarization of light, the birefringent property of mica has to be considered or ruled out. Data before and after the Fresnel factor correction should be shown in Figure 4.

Dear Author,

After evaluating your revised manuscript and consultation with the reviewers, I am happy to accept your manuscript for publication in Atmospheric Chemistry and Physics.

I have only a few minor technical corrections (such as typos) as given below, you might want to consider before publication.

With kindest regards,

Daniel Knopf

Technical corrections:
Page 2, line 5: What does “contains a water-soluble compound, almost potassium carbonate crystallites” mean? The word “almost” is distracting/confusing.

Page 2, line 40: I suggest the following change “The change of pH with temperature is known for neutral water (…)”.

Page 3, line 6: I suggest to discard “which will contribute to its rapid development in the next years.”

Page 5, line 20: Please omit “logical”.

Page 8, line 8: Omit “big”.

Page 10, l. 17: Correct “weak”.

SI:
Equation 1: I suggest to leave out the “dot” or place a “center dot” to indicate multiplication.

Figure caption S4: Period missing.