Dear Gabor Vali,
thanks for your replies to the referees and the revisions made to the manuscript. As you see from the Referee Reports, the changes have been well received. There is just one suggestion for further changes to the new Section 8, which I think would be a nice improvement and I would like to ask you to consider before final publication of the manuscript.
Thanks and best wishes,
Wiebke Frey

Referee's suggestion:
One small suggestion is to reference where the concept of the /cumulative/ active site density (n_s) was first introduced, in the Section 8 added on this topic, so that readers less familiar with the history of these concepts can find this information. In fact several references to the development and use of the active surface site density concept (as well as some issues identified with this metric by e.g. Beydoun et al.) should be added in this new Section, which is certainly a nice addition to the paper. There are currently no references provided in Section 8, and not many references provided in the manuscript in general.

Beydoun, H.; Polen, M.; Sullivan, R. C. Effect of particle surface area on ice active site densities retrieved from droplet freezing spectra. Atmos. Chem. Phys. 2016, 16, 13359–13378, doi:10.5194/acp-16-13359-2016.