Review of “High ice-nucleating particle concentrations associated with Arctic haze in springtime cold-air outbreaks” by Raif et al.

This study focuses on sources and concentrations of ice nucleating particles (INPs) in cold air outbreaks using airborne measurements during the March 2022 Arctic Cold-Air Outbreak (ACAO) field campaign over the Norwegian and Barents seas. Methods involved filter measurements upwind, below, and above cloud decks. This topic is critically important as shallow cumulus clouds are not well understood and understanding the nature and behavior of INPs is needed – there are scarce reports of these types of data the authors report. A key finding is that air upwind of the CAO cloud decks had supermicron particles comprised predominantly of mineral dust. In contrast, sea salt was not found to be as important. One key conclusion is that likely mineral dust was mixed with other ice nucleating materials that may have had biogenic sources. Sources of the INPs are thought to not be local but rather transported from lower latitude regions and associated with aged Arctic haze aerosol.

Major Comments:

A strength of this work is the dataset that adds to a growing archive of data needed to better understand INPs around the Arctic region. With so many campaigns being conducted around that vicinity, this work will get good attention in my view. The paper and methodology are generally well constructed. There are some typos/errors in the writing which I point to a few of below. I recommend publication and only have a few minor comments below for the authors to address.

Specific Comments:

Line 17: a bit weird that the “b” version of the Fletcher paper shows up first before the “a” version. ­­This may just be a citation manager software type of detail and not a big deal but it caught my eye.

Line 58: “th” should be “the”

Line 61: the part starting with “understanding” should likely start as a new sentence.

Introduction: generally well written

Line 85: “subsequent” may work better than “later” here in the sentence. Not a big deal to change.

Line 92: would sound better to say “..each inlet so that different..”

Line 111: Can the aircraft really sample on a filter as low as 10 m? Seems highly unlikely for safety reasons so clarify better here what you mean about this lower bound of the altitude range.

Line 164: “Portions of filters…” might work better here

Line 196: “CDP data have”

Line 253: “plumes” spelled wrong

Line 277: “CDP data were…”

Line 283: “were approximately constant at..”

Line 297: “, .” needs to be fixed

Line 390: It’d be good to report which meteorological dataset was used in the HYSPLIT software to obtain trajectories and what the native spatial resolution is of that dataset. This can be helpful to others interested in doing similar analyses.

Line 489: “…measurements during specific meteorological..” may work better here

Line 503: “…campaign raise…”

Line 517: what are examples of these regions you are encouraging more work to be done for? Would be nice to share explicitly a few example.

Figure 1 = very nicely done!

Line 522: “Flight data from the …., are stored…”

Line 523: “…data….are stored…”

Table 2: I would suggest the authors define the four column headers on the far right of table in the caption.

Table A1: Suggest the authors define some of the column headers in the caption like a, b, v_INP.

Throughout paper, it seems that the flight numbers are written in a different font which should be fixed.

Review for High ice-nucleating particle concentrations associated with Arctic haze in springtime cold-air outbreaks by Raif et al.,

In this study, the authors conducted airborne measurements of ice-nucleating particle (INP) concentrations during the ACAO campaign over the Norwegian and Barents Sea. They complemented these measurements with aerosol size distributions and chemical composition. Their analysis shows that during CAOs with primarily northerly flow, highly efficient INPs are transported out of the Arctic.  They attribute these high INP concentrations to the long-range transport of midlatitude dust with a biological component into the Arctic, which once there, acts as a reservoir of these highly efficient INPs, which can then  be transported out of the Arctic in CAOs. I think this is an interesting hypothesis, a very well written study and I commend the authors on their efforts to make such difficult measurements. However, the way the results are presented do not always align with the hypothesis and are not always straightforward to follow. Therefore, to improve the manuscript, I have some recommendations and comments listed below. That said, once these comments are incorporated, I think this manuscript is an excellent fit for publication in ACP.

General comments:

To support the presence of the proposed efficient INP reservoir (other than higher concentrations than previously observed) and to improve the readability of the manuscript, the way the individual flight data is presented should be improved. More specifically, it is impossible to understand the significance of where the INP measurements were taken relative to the meteorological/cloud conditions with this flight nomenclature. To remedy this, please group the INP spectra and corresponding aerosol size distributions by location with respect to the cloud/ boundary layer with a similar color as was done in Panel a of Figure 2 or change the naming of the flights (I know it is nice to keep the naming convention for other ACAO papers, but like this is it very difficult to interpret the results). This type of grouping should be done consistently throughout the manuscript.

As a follow up, it would also be nice to present the spectra from a flight in series if possible ie from the upwind and downwind of the cloud as well as the cloud top temperature if available to understand how the INP were processed by the cloud/ removed by precipitation. At the moment, only a comparison at -15C is done but the entire spectra would be interesting to see. This would also help support the argument that washout/INP removal is the reason for the lower INP concentrations observed at the Norwegian coast by Geerts et al., (2022)

It is my understanding that there were other INP measurements ongoing during ACAO related to HALO AC3 and ISLAS. Is it possible to compare with any of the INP measurements during those simultaneous campaigns rather than relying on past measurements from previous years? That said, there are several other INP measurements from the Arctic that should be added to the discussion including some that do not rely on the filter washing technique (e.g.  Gjelsvik et al., 2024; Li et al., 2022, 2023; Rinaldi et al., 2021), or more generally that have taken place in the last few years in the study region (e.g. Freitas et al, 2023). These studies should be included in the discussion as they both support the authors hypothesis and offer conflicting results.

I understand that the authors would like to include some fits to their data for implementation into models, but I found the location of this a bit distracting. I would consider moving the parameterization section to the Appendix or maybe move it to later on in the discussion?  From my point of view, there is already enough really nice science in this paper and if the focus were to shift solely on the analysis on the INP variability with the CAO and meteorology/cloud fields, that would be enough for a great paper.

Minor comments:

Line 28-30: Is it really necessary that there are more cloud droplets than ice crystals for WBF to occur? Consider rephrasing this.

Line 55: Could cite e.g. Pereira Freitas et al., (2023) here

Line 58 – 60: Could be worth mentioning other studies who have looked at INPs in CAOs like during AGASP2  (Borys, 1989; Borys and Grant, 1982) already here.

Line 66: Consider adding Gjelsvik et al., (2024) here

Line 80-84: Consider including a table or something describing the flights and the associated meteorology instead of a list like this. Also as previously mentioned, would be nice to use a different naming convection that is more easily linked to meteorology or date etc. I now realize this shows up in Table 1, it would be really nice if this could be made a bit more compact and moved into the text instead of at the end of the manuscript.

Figure 1b: Was this a flight day? If yes can you add the corresponding flight track and sampling locations? If not, can you choose an example when the samples were being taken? I think it would be very helpful to have context about how the cloud field looked and where the sampling took place with respect to it. I know this can be tricky with MODIS overpasses but it should be doable this far north within an hour or two of the actual flight time.

Figure 1c: Is there some purpose to this flight coloring? Please choose a color scheme that is meteorologically/time relevant etc and then stick with it throughout for the flight tracks/ filter samples if possible

Line 104: I know you normalize for the air sampled, but how do your maximum and minimum volume sampled filters compare with the other filer volumes sampled in terms of INP concentration? Is it completely random or does it look like there is some relationship between INP conc and volume of air sampled?

Line 105-109: For the handling blank, was this done while in flight or at the ground i.e. was the filter exposed to the setup under flight conditions? If not, could there be any sources of contamination that would be identified at flight speed that is not found while sitting at the ground?

Line 113: If the sample time was set for 20 minutes, why did the volumes vary by so much? Did you have a different flow rate at different altitudes? It would be nice to have a table describing the sampling settings etc for each filter. I see this is in Table 1, but it might be nice to have more on this if it does seem to matter.

Line 118: But there was a flight where the filter was processed three days later, maybe mention that as it is the only filter with that kind of info in the table.

Line 140-145: How about liquid impinger techniques? There are several studies in the Arctic that have used this method and been compared in Li et al., (2023).

Line 158: Why is Af calculated and not a known quantity? Also, how much does Ad vary due to the precision of the pipettor used? Is the pipettor manual or automatic?

Line 191:  Is it possible to compare the PCASP aerosol concentration/ surface area with that of the SEM analysis? Shouldn’t these match quite closely?

Line 192-195: Seems a bit out of place and redundant. Consider removing this as it is already state earlier in the methods.

Figure 2: Panel b shows the INP concentration at -15 C. Also the lines are really small as are the flight labels.

Figure 3: As previously mentioned, this coloring is really difficult to interpret. Please consider presenting the results grouped by conditions or something to make the interpretation cleaner.

Line 234-245: As previously mentioned, there are also other ground-based measurements from Ny-Alesund, and in the same region as COMBLE (see major comments) that do not observe such high INP concentrations during this time of year. It would be worth mentioning these as they do not use the wash off method.

Line 245-246: Consider citing Lacher et al., (2024) and Li et al., (2023) here as well as they also compared various offline measurement techniques.

Line 275-276: You could mention the aerosol size distribution during COMBLE and the comparison to Ny-Alesund in Williams et al., (2024) here

Line 282-285: Here is another example where I think the way the flights are labelled and discussed is a bit hard to follow. It would be much nicer if it was clearer which day corresponded to which flow regime etc. Even adding the flow regime on the title of the subplots of Figure 6 would help.

Section 3.3: Was it not possible to take comparison filters farther south? It would have been really nice to see if there is a difference between the composition before and after precip etc. Were these two filters at different locations relative to the boundary layer? It looks like one was very low and the other at 1750 m. It would be interesting to know if there were differences in this regard. If there are, it might be worth mentioning the studies by Knopf et al., (2023) and Moore et al., (2024). I see there is a discussion on being within or above the cloud layer later but maybe it should already be mentioned here?  

Line 287-290: It might be nice to highlight the INP concentrations over water vs. the marginal sea ice zone and Svalbard more clearly.  

Line 293: Please state that the size distributions are compared with the PCASP/CDP. Until I saw the Figure 7, I wondered why this wasn’t shown.

Figure 6 is a great figure. It would be nice if the INP concentration at -15 C for example could be added to the corresponding filter time. Also, it would be nice if the boundary layer height/ cloud layer height could be denoted. As a side note, it sounds like the SEM filter was run over Svalbard or close to it. Were those points omitted as per the caption? If so, I think they should be included as it represents the airmass before it interacts with the ocean. Either way please clarify this.

Line 313-314: This has been observed quite a lot in the Arctic recently and as such it might be worthwhile mentioning some of those previous studies here.

Line 349-351: It would be much more convincing if the clustering of the high, medium and low INP concentrations were somehow related to the meteorological scenario/ location in the CAO etc. Without this information, based on such a limited number of samples, and the fact that the observations don’t fit with what has been previously observed, it might be best to recommend the median for comparison with ground-based obs etc.

Line 359-360: It would be really nice if these were plotted in a way that clearly showed this somewhere? Maybe also including the entire spectra?

Line 361-362: It would be good to mention that this has also been observed elsewhere e.g. (Knopf et al., 2023; Moore et al., 2024).

Line 364: remove “are”

Line 367-368: No it is not clear at all as there is no way of easily knowing which number corresponds to what location relative to the cloud. Even in  Figure 6, there are only some instances where the aerosol number concentration drops off with height and then it also increases again. As such it would be hard to say there is a clear trend in terms of number at least, especially without the cloud layer or boundary layer noted.

Line 373-374: You could cite Williams et al., (2024) here to justify this.

Line 385-386: But the measurements by Geerts et al, (2022) are consistent with other measurements in the Arctic at higher latitudes such as in Ny Alesund. So how could those low concentrations observations be explained by precip etc?

Line 393-399: Initially these lines suggests that Asia is the source for these highly efficient INP but then finishes by stating that there is no clear relationship based on the back trajectories. Please rephrase this to be more consistent or at least clarify what is meant here.

Line 438: you could cite Gong et al., (2023) here as well.

Line 442-446: Figure 10 only goes up to 3 km. Also, without identifying the height of the boundary layer, it is hard to be sure that the convection within the CAO could not mix up the aerosol. Please consider adding the general PBL.

Line 474-476: There are also several space-borne remote sensing studies that do not observe this reservoir in a statistical sense (e.g. Carlsen and David, 2022; Dietel et al., 2024; Murray-Watson and Gryspeerdt, 2024). This, in combination with the ground-based observations, raises the question if these reservoirs are frequently there or not and if they aren’t, if it really makes sense to come up with parametrizations based on a few flights? As previously mentioned, it might make more sense to move the parametrizations to the appendix. Either way, it might be worthwhile mentioning the discrepancies to these studies as well.

Line 505-506: Is it really clear that the cloud-phase feedback would work in this direction in the Arctic during fall, winter and spring (e.g. Tan and Storelvmo, 2019)?

Editorial comments:

Line 58: th -> the

Line 80: Cold-air outbreak -> CAO

Line 143: that -> at

Line 169: Aa -> a

Line 171 – 175: there are some typos and some phrases are a bit awkward.

Line 246: spacial is usually spatial

Line 494: remove “to be”

Table 2: Please add what the variables stand for in the table caption.

References:

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Borys, R. D. and Grant, L. O.: Effects of long-range transport of air pollutants on Arctic cloud-active aerosol, The, 1982.

Carlsen, T. and David, R. O.: Spaceborne Evidence That Ice-Nucleating Particles Influence High-Latitude Cloud Phase, Geophys. Res. Lett., 49, e2022GL098041, https://doi.org/10.1029/2022GL098041, 2022.

Dietel, B., Sourdeval, O., and Hoose, C.: Characterisation of low-base and mid-base clouds and their thermodynamic phase over the Southern Ocean and Arctic marine regions, Atmospheric Chem. Phys., 24, 7359–7383, https://doi.org/10.5194/acp-24-7359-2024, 2024.

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Gjelsvik, A. B., David, R. O., Carlsen, T., Hellmuth, F., Hofer, S., McGraw, Z., Sodemann, H., and Storelvmo, T.: Using a region-specific ice-nucleating particle parameterization improves the representation of Arctic clouds in a global climate model, EGUsphere, 1–32, https://doi.org/10.5194/egusphere-2024-1879, 2024.

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Knopf, D. A., Wang, P., Wong, B., Tomlin, J. M., Veghte, D. P., Lata, N. N., China, S., Laskin, A., Moffet, R. C., Aller, J. Y., Marcus, M. A., and Wang, J.: Physicochemical characterization of free troposphere and marine boundary layer ice-nucleating particles collected by aircraft in the eastern North Atlantic, Atmospheric Chem. Phys., 23, 8659–8681, https://doi.org/10.5194/acp-23-8659-2023, 2023.

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