Review of „Bioaerosols as indicators of central Arctic ice nucleating particle sources“ by Kevin Barry and co-authors

The study presented by Barry et al. investigates the seasonal cycle of INP concentration and bioaerosols based on measurements taken onboard the research vessel Polarstern over a one-year period. They find significant variability in the bacterial and fungal composition, suggesting a mixture of local, regional, and long-range transported bioaerosols. Moreover, the authors state that biological particles contribute significantly to the INP population throughout the year, dominating it in summer.

The study is very well written and presents an interesting dataset from a unique campaign. However, my major concern is about the methods used to infer information about the biological, organic, and inorganic content of the INP samples using heat treatments and H₂O₂ digestion, and consequently, how the results of these treatments are discussed. I am aware that such treatments are frequently used nowadays to investigate contributing species to the INP population. However, recent studies have shown that wet heat treatments can also alter the ice nucleation ability of some mineral particles, while some biogenic INPs are not affected by heat treatments (Daily et al., 2022). Using this method alone to infer contributions from biological aerosols to INPs is therefore not sufficient

Minor

• Abstract: More information about the measurement methods could be given here (e.g., how INPs were measured, the temperature range and the time resolution of the measurements.
• Line 55: „campaign“ is double.
• Lines 62 – 64: Given that the abundance of INPs and bioaerosols might be different depending on the sampling location within the Arctic, a map of the legs could be helpful.
• Line 74: With a three-day time resolution of the filter samples, I assume that impact from the reserach vessel itself can occur. How could this impact the results of the INP and DNA analysis?
• Line 178: As part of the data is presented in Creamean et al. (2022), it might be worth to mention the difference between this and their study.
• Line 182: Are there measurement or modeling information about the Arctic haze occurrance during this year? And does it align well with the Fig. 2?
• 3: The marine impact in January is quite high as compared to the other winter months, is there an explanation for it?
• Line 180: Similar to the explanation of „heat labile (presumably biological)“, an explanation to heat stable organics and inorganics might help the reader to put this in context.
• Figure 2: Sample Aug 2 seems to be different to the other summer samples, any explanation for it?
• Figure 2: It is interesting to see that at T -25 °C in summer, also heat labile (biological) INPs are dominating the INP population, as this is a temperature range where mostly dust particles contribute to the INP population. Thus I am wondering if the treatments are really giving information about the inorganic or biological content (see my major concern). Are there other studies suggesting that dust (e.g., from local sources such as glacial dust) are not as important during this season? It is interesting as emissions of Arctic dust is largest in late spring summer, early autumn, depending on location (e.g., Bullard, 2012; Groot Zwaaftink et al., 2016). Is there any information about the abundance of dust particles during the here presented measurement period?
• Section 3.2: Are there studiesn about the ice nucleation activity of the discussed bacterial taxas?
• Section 3.2: How does the seasonal cycle of bioaerosol relate to the Arctic haze phenomena?
• Figure 3 and Figure 5: Why do these figures have headers?
• Lines 247 – 248: Only when reading the figure caption of Fig. 6 it became clear to me what you mean with the relative percentages of each zonal coverage, I suggest to explain it in more detail in the text.

References

Bullard, J. E.: Contemporary glacigenic inputs to the dust cycle, Earth Surface Processes and Landforms, 38, 71-89, https://doi.org/10.1002/esp.3315, 2013.

Daily, M. I., Tarn, M. D., Whale, T. F., and Murray, B. J.: An evaluation of the heat test for the ice-nucleating ability of minerals and biological material, Atmos. Meas. Tech., 15, 2635-2665, 10.5194/amt-15-2635-2022, 2022.

Groot Zwaaftink, C. D., Grythe, H., Skov, H., and Stohl, A.: Substantial contribution of northern high-latitude sources to mineral dust in the Arctic, Journal of Geophysical Research: Atmospheres, 121, 13,678-613,697, doi:10.1002/2016JD025482, 2016.

Review for Barry et al., “Bioaerosols as indicators of central Arctic ice nucleating particle sources”, submitted to EGUsphere to be published in ACP

In this study, filters collected during the MOSAiC cruise in the Arctic were examined. DNA analysis was done to examine the present bioaerosols, together with measurements of INPs (ice nucleating particles). The data was amended with data from filters from other Arctic samples such as seawater, sea ice, snow etc. . Backward trajectories were also analyzed to zero in on the sources of bioaerosols and INPs.

As results, the by now well-known seasonal cycles of INP concentrations were presented, together with seasonal cycles for bioaerosols. Also, some ideas on possible sources, specifically for the bioaerosols, were given. For the latter, both long-rang transport as well as local influence was observed. While some terrestrial influence on the bioaerosols was seen throughout the year, in the summer month most bioaerosols clearly came from marine sources. Nevertheless, the fungal contributions predominantly point to terrestrial sources. Also, a comparison to INP concentrations from Svalbard shows closeness of the data for most of the time, which is interesting given that the distance between the ship and Svalbard was varying and sometimes large.

This all adds nice bits and pieces to things the community already understood about bioaerosols and INPs, and certainly merits publication. The methods are all sound, the text is well structured and well written. It only occurred to me if this is not better published as a measurement report rather than a full scientific publication. But this is a decision the editor should make.

There is one really unsettling information in the manuscript, which is the discrepancy between the INP concentrations published in Creamean et al. (2022) and in here, which both come from samples collected onboard the Polarstern simultaneously. I suggest below to include a comparison with other Arctic data to learn which of the two datasets may be closer to these. This may be included in the main text or the SI. But it should be done.

As the number of my comments and remarks below is rather small, they are all just given one after the other. And, as said, publication can certainly be granted once these few small and the one larger issue are taken care of.

Comments:

Line 185-186: There is a “,” missing between “productivity” and “sea”.  -  Also, you mention “less snow coverage” – where was that (as you were on a ship)? Are you referring to snow on land? Please clarify.

Line 209: Change “wasn’t” to “was not”.

Line 275: Please add a number for what you consider are “higher INP concentrations”.

Line 289: Add to this sentence where the Polarstern was, compared to Svalbard, in June and July, or at least point to Fig. 6 where the distance between both can be seen.

Line 291-292: The mentioned difference between your two datasets (one published earlier and the one included here) is unsettling. In the previous part you argue that INP concentrations quite similar between Svalbard and the Polarstern, no matter the distance between the two. And it is likely adequate to assume a somewhat repeatable annual cycle in INP concentrations. There were other Arctic long-term data published before, for land-based INP sampling, connected to parameterizations (e.g., Li et al., 20; Sze et al., 2023; the latter having data taken simultaneously to your measurements in northern Greenland). It would be very instructive to compare your data with these and see if either of your datasets matches, and which one. Please add this comparison to your manuscript or to the SI.

SI, line 20: Sampling time typically is taken care of when INP concentrations are calculated. That parameter should not show up in seasonal average values, unless there would be clogging of the filters for the longer sampling times, which, given the environment in which you took your samples, is unlikely. Remove the “sample integration time” from your list of possible differences, and maybe explain in a separate sentence that this should not influence the results if measurements are done properly.

Literature:

Creamean, J. M., K. Barry, T. C. J. Hill, C. Hume, P. J. DeMott, M. D. Shupe, S. Dahlke, S. Willmes, J. Schmale, I. Beck, C. J. M. Hoppe, A. Fong, E. Chamberlain, J. Bowman, R. Scharien, and O. Persson (2022), Annual cycle observations of aerosols capable of ice formation in central Arctic clouds, Nat. Commun., 13(1), doi:10.1038/s41467-022-31182-x.

Li, G. Y., J. Wieder, J. T. Pasquier, J. Henneberger, and Z. A. Kanji (2022), Predicting atmospheric background number concentration of ice-nucleating particles in the Arctic, Atmos. Chem. Phys., 22(21), 14441-14454, doi:10.5194/acp-22-14441-2022.

Sze, K. C. H., H. Wex, M. Hartmann, H. Skov, A. Massling, D. Villanueva, and F. Stratmann (2023), Ice-nucleating particles in northern Greenland: Annual cycles, biological contribution and parameterizations, Atmos. Chem. Phys., 23, 4741–4761, doi:10.5194/acp-23-4741-2023.