Dear Authors,

After careful evaluating the reviewers’ comments and your responses, I have a few additional minor comments. I would like to ask you to respond to these comments before proceeding to the publication of your manuscript.

With kindest regards,

Daniel Knopf

Minor comments:
P. 2, Line 12-14: I recommend to re-evaluate the definition of biological vs biogenic particles/INPs. Biological particles are bacteria, fungal spores, pollen, diatoms, etc. (Fröhlich-Nowoisky et al., 2016;Despres et al., 2012). The list you give, is a listing of biogenic particles, i.e. from biological entities derived.
Also, if you keep this list, you could cite (Hiranuma et al., 2015) for cellulose and our studies (Rigg et al., 2013;Wang et al., 2011) for humid material acting as INPs.

P. 2, Line 22-24: I think it would be fair to add references to the role of the marine environment as source of INPs. Like (Schnell, 1975); one of our 2011 papers, maybe (Alpert et al., 2011) and the studies by (Ladino et al., 2016) and (Wilson et al., 2015).

Just a comment: If you want to emphasize the importance of polysaccharides (like on p. 3, line 24), in the studies by (Ladino et al., 2016;Wilson et al., 2015), the exudate material that acted as INP is polysaccharidic. (Aller et al., 2017) has shown the presence of polysaccharides and proteinaceous material in marine aerosol.

P. 2, Line 25: I believe, those would qualify as biological INPs. Biological material cannot act as biogenic particle. It is either biological or biogenic (=biologically derived).

I struggled to understand some of the interpretation of the refreeze experiments, described on p. 13, line 6-22:

You present a couple of parameters in Tables 2 and 3, however, most of these parameters are not discussed in text or supplement. It is also not entirely clear how some of the numbers are derived. I struggled to understand the difference of the derivation of FF “over all wells and all cycles” vs. “evaluation of well by well”? A few more words on this would be helpful. If it distracts the main text, it could go in the supplement.

However, my biggest struggle is the interpretation using FF as a kind of probability, where you take FF to the exponent of 4, trying to get the probability of for 4 trials/experiments. Like rolling the dice 4 times. Maybe I misunderstand something, but the FF value is not a probability. FF is a value of an accumulated distribution. If it would be a probability, then integration of the FF curve would result in 1, like for alpha-PDF etc. I try to come up with an example. If you roll a dice 10 times and normalize by, e.g., the maximum points (=60), you can get a “point fraction” as a function of throws. However, this is not a probability with which the result can be deduced. For each roll of the dice the probability for a given face is 1/6. Similarly, as you cooled the wells, each well has a chance to be frozen or liquid, i.e., 50%. So, I strongly doubt that you can use the FF value as a kind of probability and place the number of trials in the exponent, resulting in these very low (unrealistic) overall probabilities. Since I do not know how “well by well evaluation” is performed (what is the change in population/normalization numbers etc. compared to other method), for now it looks that there is overlap in the FF values from both methods. Frankly, speaking, it has to be like this, or? If not your data/figures would look rather different? This may change your final statement in this section. However, I find this a very valuable analysis. Of course, this entire discussion will be affected by uncertainties in FF but this is difficult to assess and likely beyond the scope of this study.

Technical correction:
P. 9, line 16: Missing space “…SI). Apoferritin…”

References
Aller, J. Y., Radway, J. C., Kilthau, W. P., Bothe, D. W., Wilson, T. W., Vaillancourt, R. D., Quinn, P. K., Coffman, D. J., Murray, B. J., and Knopf, D. A.: Size-resolved characterization of the polysaccharidic and proteinaceous components of sea spray aerosol, Atmos. Environ., 154, 331-347, 10.1016/j.atmosenv.2017.01.053, 2017.
Alpert, P. A., Aller, J. Y., and Knopf, D. A.: Initiation of the ice phase by marine biogenic surfaces in supersaturated gas and supercooled aqueous phases, Phys. Chem. Chem. Phys., 13, 19882-19894, 10.1039/C1CP21844A, 2011.
Despres, V. R., Huffman, J. A., Burrows, S. M., Hoose, C., Safatov, A. S., Buryak, G., Fröhlich-Nowoisky, J., Elbert, W., Andreae, M. O., Pöschl, U., and Jaenicke, R.: Primary biological aerosol particles in the atmosphere: a review, Tellus B, 64, 1-58, 10.3402/tellusb.v64i0.15598, 2012.
Fröhlich-Nowoisky, J., Kampf, C. J., Weber, B., Huffman, J. A., Pöhlker, C., Andreae, M. O., Lang-Yona, N., Burrows, S. M., Gunthe, S. S., Elbert, W., Su, H., Hoor, P., Thines, E., Hoffmann, T., Despres, V. R., and Poschl, U.: Bioaerosols in the Earth system: Climate, health, and ecosystem interactions, Atmos. Res., 182, 346-376, 10.1016/j.atmosres.2016.07.018, 2016.
Hiranuma, N., Möhler, O., Yamashita, K., Tajiri, T., Saito, A., Kiselev, A., Hoffmann, N., Hoose, C., Jantsch, E., Koop, T., and Murakami, M.: Ice nucleation by cellulose and its potential contribution to ice formation in clouds, Nat. Geosci., 8, 273-277, 10.1038/ngeo2374, 2015.
Ladino, L. A., Yakobi-Hancock, J. D., Kilthau, W. P., Mason, R. H., Si, M., Li, J., Miller, L. A., Schiller, C. L., Huffman, J. A., Aller, J. Y., Knopf, D. A., Bertram, A. K., and Abbatt, J. P. D.: Addressing the ice nucleating abilities of marine aerosol: A combination of deposition mode laboratory and field measurements, Atmos. Environ., 132, 1-10, 10.1016/j.atmosenv.2016.02.028, 2016.
Rigg, Y. J., Alpert, P. A., and Knopf, D. A.: Immersion freezing of water and aqueous ammonium sulfate droplets initiated by humic-like substances as a function of water activity, Atmos. Chem. Phys., 13, 6603-6622, 10.5194/acp-13-6603-2013, 2013.
Schnell, R. C.: Ice nuclei produced by laboratory cultures marine phytoplankton, Geophys. Res. Lett., 2, 500-502, 1975.
Wang, B., and Knopf, D. A.: Heterogeneous ice nucleation on particles composed of humic-like substances impacted by O3, J. Geophys. Res., 116, D03205, 10.1029/2010JD014964, 2011.
Wilson, T. W., Ladino, L. A., Alpert, P. A., Breckels, M. N., Brooks, I. M., Browse, J., Burrows, S. M., Carslaw, K. S., Huffman, J. A., Judd, C., Kilthau, W. P., Mason, R. H., McFiggans, G., Miller, L. A., Najera, J. J., Polishchuk, E., Rae, S., Schiller, C. L., Si, M., Temprado, J. V., Whale, T. F., Wong, J. P. S., Wurl, O., Yakobi-Hancock, J. D., Abbatt, J. P. D., Aller, J. Y., Bertram, A. K., Knopf, D. A., and Murray, B. J.: A marine biogenic source of atmospheric ice-nucleating particles, Nature, 525, 234-238, 10.1038/nature14986, 2015.

Dear Authors,

I am happy to accept your manuscript as publication in Atmospheric Chemistry and Physics.

With kindest regards,

Daniel Knopf