Comments:

1. Elaborate on how exactly INPs affect the surface energy budget via the Arctic Clouds
2. The sampling height at some sites was 1.5m and for some 10m. Are both representative of the surface? How did you compare the two heights?
3. In sample analysis section, please close the bracket for 20-fold dilutions (250 microL sample…
4. Why is there data gap for 3,4, 12 and 16 September? The sampling period is very small!
5. Make a table listing all the sources and their concentration of INPs
6. To divide INPs into heat labile and stable fractions, why only one day data was used? What was the rationale behind this?
7. Figure 7: choose different colour scale
8. Why is correlation weaker with increased homogeneity? Please elaborate.
9. INPs in the water are predominantly organic….are you referring to heat labile or stable organics?

Review of “Active thermokarst regions contain rich sources of ice nucleating particles” submitted by Barry et al.

For this study, the authors collected a number of samples from different environments in the Alaskan Arctic close to Utquagvik for further analysis of the content of ice nucleating particles (INPs). Samples included air samples collected on filters up- and downwind of thermo-karst lakes (TKLs) but also at the shore and at a DOE site. Water samples were collected from e.g., the TKL themselves and from the ocean, and furthermore samples were collected from soils, sediments, the ice wedge and from vegetation.

Based on INP concentrations obtained from these samples, it was then argued that TKLs and the ocean provide a source for atmospheric INP.

On the other hand, a principle component analysis (PCA) was done, which I highly welcome. However, results from this analysis were only shortly described, and not involved in the overall interpretation of the results. Particularly, PCA seems to suggest that INPs from aerosol samples have diverse sources including particles from long range transport.

Also, heat treatment results were then introduced. Again, results from that seem to rather point to different INPs being present in the different sample types, with differences even between samples from permafrost and TKLs and all aerosol samples, and even differences between permafrost and active layer. This, however, again was not included in the main discussion and was not considered for the main conclusions of this study.

This manuscript can not be excepted in its present form. Instead, all data should be presented first, and then a joint interpretation of the data needs to be done. Given all the experimental evidence, to my understanding, the data do not support statements as e.g. the following from the abstract: “Arctic water bodies were shown to be important links of sources to the atmosphere in thermokarst regions.”

After thorough revision, this study may become eligible for publication. However, currently I cannot support publication.

Major comments:

Lines 175-176: It is unclear to me how your data can provide this evidence. INP concentrations at DOE seem to be independent of airmass origin (and likewise of wind speed?). Passing TKLs does not add a lot of INP. For those examples where downwind and upwind measurements of TKLs were compared, wind speed was not discussed, although this is an important parameter for sea spray production.

Therefore, this conclusion seems to not be well informed and needs revision.

Line 220: “have” should be “having”. But the main point in this part of the text is, that it is unclear where this conclusion of “atmospheric importance” comes from. Just because there is some highly ice active material on vegetation, it does not mean that it is atmospherically relevant, as it is unclear how it would become airborne.

Lines 250-251: Here it is very clear that the sequence in your text is not optimal. The results presented here make it necessary that earlier conclusions need to be revised. This should have been shown earlier, before presenting some of the interpretations and conclusions given above.

It would be good to first introduce all results, including those from heating and PCA. And only THEN start interpretations, in a new chapter, in which all results are discussed together! Your results are valuable, even if you cannot pinpoint the INP sources! But do not make statements that are later on contradicted by some of your own results.

Lines 254-255: It is not clear how the presence of heat labile INPs across the examined temperature spectrum supports the PCA conclusion of multiple INP sources. Check out e.g., Kunert et al. (2019), where for a single type of fungal spores heating affected the INP concentration across the whole temperature spectrum.

Line 262 ff, including Figure 9: Water TOC concentrations were not introduced in your study so far, so that part came as a surprise. Introducing your results first would have been good.

However, it is somewhat alarming that the correlation between INP and TOC concentrations only seem to work when you use all data from different water sources together. This also includes the one highest point in Figure 9, which certainly has a large influence on your resulting correlation. This seems to be a much too weak base to then suggesting the use of TOC concentrations as a proxy, particularly as you clearly said above, that it has been shown in literature that this does not work.

Lines 269-270: Just a remark: It seems that some of your conclusions go back to the hypothesis from your earlier publication (that wave breaking and bubble bursting are main mechanisms for INP realease into the atmosphere). However, this is only a hypothesis! Be careful to discriminate between what you know and what you suggest, but also base that latter on the data you have.

Conclusions: Summarizing, the conclusions section gives statements which are not well rooted in the presented data. Much of it needs to be rewritten. This concerns specifically the following sentences:

Lines 284-285: There would only be rich potential for INP sources if you clarify how these INP may get airborne, by e.g., showing a relationship between INP concentrations and important parameters for sea spray generation via wind speed.

Lines 285-286: Without knowing the influence of wind speed for the TKLs, TKLs and the ocean gave quite distinct pictures: while an enhancement of atmospheric INP due to the ocean was described for the one SINGLE (!) oceanic case, the enhancement after crossing TKLs was much weaker. This may be due to a saturation effect, but then, maybe also not many INPs were emitted from TKLs. Also, the PCA and the heat lability seem to show that the INP in the air and the TKLs and ocean are different.

Therefore, your data does not show that clearly, hence this sentence needs revision.

Line 290: Concerning the relationship between TOC and INPs, see my comment above. This is weak, mainly rooted on throwing a bunch of data from different sources together and having one outlier. To my understanding, this is not strong enough to allow for this statement.

Minor and editorial comments:

Line 24: Better replace “earth” with “containing”.

Line 44: The lake spray aerosol production should be highly variable in time and depends on the amount of INPs entering TKLs, the drainage of the TKLs and factors of aerolization. I suggest to not mention such a precise time of “over 3 weeks” here.

Lines 55-56: Already Creamean et al. (2018) described a transition from low to high INP concentrations during an ongoing Arctic spring, and Wex et al. (2019) showed a seasonal dependence for annual samples from several Arctic stations, which then, more recently, was corroborated in Li et al. (2022) for spring and fall data from Svalbard. Giving some more background here would be good.

Line 59: It could be worthwhile adding, that the active layer is the top soil layer in permafrost which is thawing during the summer.

Line 73: Explain the acronym “ATV”.

Lines 83-84: “… filters were collected between 2 and 4 hours after deployment.” How can they be collected after deployment? Do you mean after precleaning?

Line 106: As ice wedges consist mostly of water, I wonder if it makes sense to treat as you did, i.e., treating them similar to the sediment, active layer and permafrost samples by giving their INP concentrations later on “per g of material”? Or might it be more useful to give them as “per L of water”?

Line 170: With “temperature dependent”, do you mean the freezing temperature, and not the temperature of the surroundings!? Clarify!

Line 211: Please write “Alaska” instead of “AK”, as not everyone might understand the acronym immediately.

Lines 221-223: (“Ice wedges (Figs. S6 and S7) had values comparable to TKLs”)

In your study, you related ice wedge and permafrost samples to the mass of collected material used for the INP measurements. On the other hand, INP concentrations from TKLs were related to the volume of collected water. Therefore: How can you then compare INP concentrations from TKLs to those from ice wedges? And how are they similar? Also, if what you wrote here were correct, the heat lability of the samples should be similar, which it only is to a certain extent. This needs revision.

Lines 233-234: Can you explain which parameters are included in PC1 and PC2?

Figure 7: Zoom in a little - there is ample empty space in your plot. It may be enough to use -4 to 4 for the PC2-axis, and -2 to 2 for the PC1-axis. Also, add lines for the two "0" values.

Line 280: Is it really still true that the Arctic has only a limited amount of previous observations? Just observing current developments in the past years, my impression is that more INP measurements were done in the Arctic than anywhere else. Better revise this sentence.

Fig S1: Use colors to discriminate between the different samples, such that the reader can see which samples were e.g. taken downwind of TKLs, at the DOE etc .

Fig. S4: A different choice of color would be good: tan and salmon are very close and not easy to distinguish. (Also in other figures where the same colors were used.)

Literature:

Creamean, J. M., R. M. Kirpes, K. A. Pratt, N. J. Spada, M. Maahn, G. de Boer, R. C. Schnell, and S. China (2018), Marine and terrestrial influences on ice nucleating particles during continuous springtime measurements in an Arctic oilfield location, Atmos. Chem. Phys., 18, 18023–18042, doi:10.5194/acp-18-18023-2018.

Kunert, A. T., M. L. Pohlker, K. Tang, C. S. Krevert, C. Wieder, K. R. Speth, L. E. Hanson, C. E. Morris, D. G. Schmale, U. Poschl, and J. Frohlich-Nowoisky (2019), Macromolecular fungal ice nuclei in Fusarium: effects of physical and chemical processing, Biogeosciences, 16(23), 4647-4659, doi:10.5194/bg-16-4647-2019.

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.

Wex, H., L. Huang, W. Zhang, H. Hung, R. Traversi, S. Becagli, R. J. Sheesley, C. E. Moffett, T. E. Barrett, R. Bossi, H. Skov, A. Hünerbein, J. Lubitz, M. Löffler, O. Linke, M. Hartmann, P. Herenz, and F. Stratmann (2019), Annual variability of ice nucleating particle concentrations at different Arctic locations, Atmos. Chem. Phys., 19, 5293–5311, doi:10.5194/acp-19-5293-2019.