General comments

The manuscript by Sun et al. reports INP measurements at Changbai Summit in the summer of 2021. Using results from heat and H2O2 treatments, the authors distinguish between biological INPs, other organic INPs, and inorganic INPs and conclude that the majority of INPs in the temperature range studied were of biological origin. It is an interesting study that provides new information about biological INPs. They discuss correlations of biological INPs with meteorological data as well as with some chemical tracers, suggesting that biological INPs may originate from soil dust. In addition, possible sources and transport mechanisms of INPs are discussed based on knowledge of the height of the planetary boundary layer and the concentration-weighted backward trajectories of air masses. The authors have made an effort to explain their data, but given the small number of samples (22 in total, half of which were collected during the day and the other at night) and the lack of sample dilutions to extend the freezing spectra to lower temperatures, some sections require substantial revision in my opinion. I have major and minor comments that I hope will be helpful to the authors and can be addressed during the review process.

Specific comments

Abstract

Line 12: “modifying” is maybe not the right word here.

Lines 22-24: While this insight into atmospheric dynamics is welcome, I would note that this positive correlation between biological INPs with planetary boundary layer height is based on 7 data points only. 2 outliers were excluded and 2 missing data points were not mentioned. It is not stated which correlation analysis was done, something that persists throughout the study. Furthermore, it is unclear for the reviewer how these two cases of high concentrations of biological INPs differ from the other samples in terms of long-range transport. These results need thorough review, discussions about them need to be clarified, and the relevant conclusions probably need to be revised.

Introduction

The reviewer thinks that some parts of the introduction should be worded more precisely.

Line 40 and line 52: Please use “etc” carefully. Try to list fully instead.

Line 40: There are also efficient proteinaceous cell-free INPs (Pummer et al., 2015) that are not embedded in cell membranes. Please, reformulate the sentence.

Line 45: I am unsure what is meant by “and [in] agricultural soils”. Do you mean a site, where presumably many INPs come from particles from agricultural soils? Please reformulate.

Line 68: The tree sites that are referred to are not all located in the Swiss Alps, but are located across Switzerland.

Lines 70-71: Instead of “one order of magnitude”, you could keep the same units as in the previous sentence. Maybe summarize in percent per kilometer.

Line 72: The “atmosphere” can only be in singular.

Lines 72-75: Please mention that Schrod et al. 2017 findings were obtained during specific events, i.e., a series of elevated Saharan dust plumes.

Lines 75-77: The citation Conen et al. 2022 is missing at the end of the sentence. I would suggest adding here the following specification: “under free-tropospheric conditions”. Study could have been mentioned in L. 44-45 instead/as well.

Line 87: “INPs measurements” --> “INP measurements”

Methods

The reviewer found that the methods are well described, and that it was helpful that the authors provided the raw data for the reviewers.

Line 119: Please specify that your times are meant in local time or mention the correct time zone.

Line 120: A total of 24 samples were collected for INP analysis. The results of only 22 samples were reported in the provided data file. Could the authors please explain why the results of two samples are missing? Could it maybe be that 22 samples and 2 background filters were collected?

Line 130: It would be interesting to know how far the weather station is from the measurement site.

Lines 178-186: Please be more precise how you generated the air mass backwards trajectories and add information about starting time as well as number of trajectories per sample.

Line 188: Which data product from the Climate Data Store did you use for the PBL data?

Results and Discussion

The reviewer found that some parts of the results and discussion need thorough revision and much clearer statements.

Line 208 and lines 210-211: Wieder et al. (2022) contains INP concentrations from a mountain site called Weissfluhjoch (2693 m a.s.l.) and a valley site called Wolfgangpass (1631 m a.s.l.). The two sites in the Swiss Alps are only 4 km apart and are not necessarily representative of the entire Swiss Alps. Please be more specific: a) Swiss Alps and not any Alps, b) add the name of the site(s) you mean.

Lines 208-211: Please provide the time spans for collection and averaging for these other studies. For example, Wieder et al. (2022) collected samples over short time spans (i.e., 20 minutes) and averaged them into 2-hour bins. They found a peak at 19 h UTC (i.e., 21 h CET), which is defined as nighttime based on the definition used in this manuscript, if I am not mistaken. A possible diurnal cycle at Changbai Mountain may have been averaged out due to the long sampling time of 11 hours. Please mention specifically for Changbai Mountain only the comparison of daytime and nighttime, since only two samples are taken per day. In my opinion, a conclusion over the entire diurnal cycle cannot be made here.

Line 213: What is meant by “the temperature spectra showed a wider range”?

Line 217: What is meant by “the N_INP value was much larger at Mt. Huang than our results”? Can't the difference between both sites be explained by the fact that your results cover a warmer temperature range than the results from Mt. Huang? Please mention the freezing temperatures while comparing INP concentrations.

Line 256: Shouldn’t it be “bio-INP” instead of “INP”?

Line 254-256: A mean increase in F_INP-bio from 0.8 to 0.9 with decreasing temperatures between -16.5 °C and -20 °C doesn’t make much sense and could be due to the way the data were processed here. It looks like some untreated and heat-treated samples already reached the upper detection limit at about -15 °C or -16 °C. In order to draw conclusions about the proportion of bio-INPs, other org-INPs, and inorganic INPs below -15 °C, I believe dilutions of the untreated and heat-treated samples would have been required. The number of data points of F_INP-bio from maybe around -18 °C and lower is likely too low to draw any conclusions below that temperature threshold.

Lines 277-281: What about biological INPs originating from plants and oceans? Please discuss.

Lines 279-281: Maybe instead of citing the review, authors could cite the original studies e.g. Hill et al. 2016 or others.

Line 301: Maybe instead of citing the review, authors could cite the original studies.

Lines 315-321: Could the authors elaborate how the inclusion of these points would change the results and conclusion?

Line 320: Please indicate here that the “freezing” temperature is meant.

Lines 323-325: How does the CWT analysis on August 18 and 25 differ from the other days?

Conclusion

The reviewer believes that some rewording needs to be done, especially in the last paragraph.

Lines 365-368: A decreasing trend in F_INP-bio with decreasing temperature suggests that other organic or inorganic INPs become more important with decreasing temperatures. It does not suggest that the ice nucleation activity of bio-INPs decreases. In addition, due to the lack of data points in the low temperature regime, no conclusions can be drawn regarding F_INP-bio for that temperature range, in my opinion. Dilutions would have been required, as mentioned above.

Line 379: Did you mean to write that the INP number concentration is lower than the ice crystal number concentration, especially in the warm temperature range?

Line 381: Is there a word missing between “…is second only to…”?

Line 383: Please delete “through various collisions with pre-existing ice”. There are several multiplication processes, and they involve many more processes than only ice-ice collision. More information can be found for example in Korolev and Leisner, 2020.

Line 385: Bio-INPs do not impact secondary ice formation directly, please reformulate.

Figures

Fig.2(b)

Please indicate in the figure and legend the name of the mountain in the Swiss Alps from which the data originate similar to what you did with the Mt. Huang dataset.

Should the y-axis perhaps be extended to lower values to see the lower limit of the error bar?

Fig. 3(a)

Why are the error bars for N_INP-H2O2 below -15 °C extending to values outside the graph?

Please use the same axes in Fig. 3a than in Fig. 3b and Fig. 2b.

Fig.3(c)

I appreciate that there is an indication of the number of data points included in the analysis. Since the upper detection limit of about 2 INPs per liter is reached around -15 °C for N_INP-bio, dilutions would have been necessary to extend the plot from -15 to -20 °C as mentioned above.

How can the median fraction of inorganic INPs above -12 °C be more than 0.0 if the median fraction of bio-INPs is 1.0?

Fig.4

What kind of correlation analysis did you do? Please specify.

Fig.5(a-c)

What does “r” stand for? Please also describe what the red lines and the purple circles represent.

Is the x-axis showing the mean PBL height?

Maybe add “freezing” between “three” and “temperatures”.

Fig.5 (d-f)

Trajectories of which samples were included in the CWT analysis? Are the trajectories of all the samples included or only those of the daytime samples? Were the trajectories of the two outliers excluded or included here? Please be more specific.

Also describe in the legend what the star and triangle represent.

Fig. S1

Please describe in the legend precisely what is shown and what is meant by "R". Is the large "R" here similar to the small "r" in Fig. 5?

Fig. S2

Authors could add the star and the triangle in these maps, similar to Fig. 5 (d-f).

References:

Conen, F., Einbock, A., Mignani, C., and Hüglin, C.: Measurement report: Ice-nucleating particles active  ≥ −15 °C in free tropospheric air over western Europe, Atmos. Chem. Phys., 22, 3433–3444, https://doi.org/10.5194/acp-22-3433-2022, 2022.

Korolev, A. and Leisner, T.: Review of experimental studies of secondary ice production, Atmos. Chem. Phys., 20, 11767–11797, https://doi.org/10.5194/acp-20-11767-2020, 2020.

Hill, T. C. J., DeMott, P. J., Tobo, Y., Fröhlich-Nowoisky, J., Moffett, B. F., Franc, G. D., and Kreidenweis, S. M.: Sources of organic ice nucleating particles in soils, Atmos. Chem. Phys., 16, 7195–7211, https://doi.org/10.5194/acp-16-7195-2016, 2016.

Pummer, B. G., Budke, C., Augustin-Bauditz, S., Niedermeier, D., Felgitsch, L., Kampf, C. J., Huber, R. G., Liedl, K. R., Loerting, T., Moschen, T., Schauperl, M., Tollinger, M., Morris, C. E., Wex, H., Grothe, H., Pöschl, U., Koop, T., and Fröhlich-Nowoisky, J.: Ice nucleation by water-soluble macromolecules, Atmos. Chem. Phys., 15, 4077–4091, https://doi.org/10.5194/acp-15-4077-2015, 2015.

Schrod, J., Weber, D., Drucke, J., Keleshis, C., Pikridas, M., Ebert, M., Cvetkovic, B., Nickovic, S., Marinou, E., Baars, H., Ansmann, A., Vrekoussis, M., Mihalopoulos, N., Sciare, J., Curtius, J., and Bingemer, H. G.: Ice nucleating particles over the Eastern Mediterranean measured by unmanned aircraft systems, Atmos Chem Phys, 17, 4817-4835, https://doi.org/10.5194/acp-17-4817-2017, 2017.

Wieder, J., Mignani, C., Schär, M., Roth, L., Sprenger, M., Henneberger, J., Lohmann, U., Brunner, C., and Kanji, Z. A.: Unveiling atmospheric transport and mixing mechanisms of ice-nucleating particles over the Alps, Atmos. Chem. Phys., 22, 3111–3130, https://doi.org/10.5194/acp-22-3111-2022, 2022.

Review of Sun et al. 2023: Measurement report: Atmospheric Ice Nuclei ar Changbai Mountain (2623 m a.s.l.) in Northeastern Asia

Sun et al. 2023 present INP measurement results from the Changbai mountain in summer 2021. Changes in INP concentration are investigated towards diurnal variability, composition, source, and transport mechanism. It is an interesting study given the location of the measurement site, methodology and efforts taken in the analysis. However, major adjustments are needed to streamline and support the claims of the manuscript.

General Comments:

Major doubts concern the influence of the PBL height to Changbai mountain in Section 3.4. To the reviewer the analysis and interpretation are inconclusive. Especially it is not entirely traceable how the PBL height was derived and the involved error. Given the points raised below, the reviewer is not entirely sure if some claims need to be removed if not being support by further evidence from in-situ observations. In addition, the local wind system and dynamics should be better described or referenced.

Overall, it appears that frequently there are statements in the manuscript that should be supported by a reference. The authors may want to consider this aspect, while working through the manuscript. In addition, the figures of the manuscript could also be linked to the made statements more often. Furthermore, there appear some side information at some point, which not necessarily add to the flow of the manuscript. The authors may want to critically read through the paper, deciding which information is needed to reach the presented conclusions.

Some strong statements are made without reference or presenting data. For additional support of some of the made claims particle size measurements may be helpful. Have there not particle size measurements been available for the study?

Changbai mountain is referred as 2623 m a.s.l. (Tianchi site) and 2740 m a.s.l. (highest point). To avoid confusion the two locations and heights should be consistently appear combined.

For deeper explanations, see specific comments below.

Specific Comments:

Introduction

L29: ‘As most precipitation in clouds initiates via the ice phase’, this statement could be refined in regard of which cloud types and regions are affected and direct studies could be cited.

L63: ‘At present, it is unclear whether…’, in the reviewer’s opinion the current knowledge gap is not whether but to which extend and through which transport pathway INPs are brought to MPC relevant heights.

L68: One station in Conen et al. 2017 was located outside the Swiss Alps.

L71: Wieder et al. 2022 sampled frequently for short time spans throughout the day, whereas Conen et al. 2017 used filters sampling over longer timespans. Wieder et al. 2022 observed also a diurnal cycle with INP concentrations seeming to equilibrate over the course of a day.  Further it may be important to point out to the reader that this was only observed in a wind direction where the topography promoted vertical transport of air masses from lower elevation.

L75-77: “For example, at the Jungfraujoch station (3580 m a.s.l.) in the Swiss Alps, approximately 80% of INPs were biological aerosols at freezing temperatures above −15 °C” – add reference.

L81-82: “… and establishing a parametric equation that depends on temperature and ice supersaturation for predicting the INPs concentration” – this information seems irrelevant for the current study.

L83: Here, Changbai mountain is attributed with a height of 2740 m a.s.l., whereas in L87 a height of 2623 m a.s.l. is given. If understood correct, the latter height refers to the INP measurement station of the current study. To avoid confusion the distinction between the peak and the measurement station should be made (also throughout the manuscript).

Methods

L102: Shouldn’t the elevations around the mountain decrease to all directions?

L104: Is 150m correct? Looking at maps it seems like more.

L106-112: Adding a timeseries plot (appendix or supplement) of the general meteorological parameters throughout the campaign including exemplary INP concentration would be helpful to understand the sampling conditions.

L112: What about touristic activities?

L114: How was differentiated whether the sampling site was in the free troposphere or influenced by the PBL? Please elaborate and add data or reference.

L116: A dedicated reader may be interested in a picture of the setup (also possibly in the appendix or supplement)

L117: Was there any pretreatment of the PCTE filters?

L117-124: For clarity, maybe specify what each filter sample type was used for, i.e. PCTE for INP analysis, PM2.5 for chemical composition.

L120: There were 24 samples taken covering roughly 30% of the entire measurement campaign. The authors may want to comment on the underlying measurement strategy and providing more concrete information under which meteorological conditions samples were taken.

L120: The chemical analysis was conducted on the samples collected using a PM2.5 inlet. Can the authors estimate the number of particles larger than 2.5 µm which contributed to the INP analysis samples, but would not be covered in the chemical analysis? Has there been any parallel size measurements supporting that claim?

L130: For consistency, specify the type of weather station.

L153: above -> at

L158: What was the value of V_air?

L159: If the INP concentrations were given in standard liters, the reviewer would advise to indicate this by using e.g., sL^-1, or stdL^-1 in Equation 3 and corresponding INP data plots.

L178: Which PBL data product was used? The reviewer could not find a unique record. Furthermore, what is the uncertainty and sensitivity of the PBL data product? Is Changbai mountain centered in a grid box that data was taken from or were different adjacent grid boxes averaged? Given the complex terrain of the mountainous region, how reliable does a 25km x 20km grid box represent the PBL height? Generally, the reviewer is a bit skeptical of the representativeness for the presented application. The authors should elaborate why this data is applicable. In addition, are there any direct meteorological (especially wind) observations along the mountain slope that would support the later claim of vertical transport due to orographic lifting?

L184: The authors may want to briefly describe the principle of CWT.

Results and Discussion

L190: “characterize situation of droplet freezing” sounds odd.

L192: As there are only two MilliQ water backgrounds displayed, rephrase to “were -30°C and -28.5°C”.

L193-196: This seems contradicting. If the contaminants could be ignored, one would not need to correct for it.

L195-196: Were averaged concentrations at each temperature step of the two blank filter samples subtracted as correction – please specify. As two samples are not many, the authors may want to comment on the overall obtained repeatability of blank filter measurements obtained which are not presented in this study.

L199: -26.0°C -> -20°C

L200: What it the values of T₅₀? -13°C from above? The authors should repeat this value here, or introduce another variable to avoid the nomenclature confusion of T₅₀ representing the result of one sample (L191) or the average of all samples (L194).

L201: It is not necessarily the diversity of different INPs but could also just relate to different emission strengths.

L202: What would that local source be?

L205-206: This statement cannot be made, as it seems that there have no dilutions of samples been made. This sets the upper limit in detectable N_INP = 1/V_air i.e., all droplets frozen.

L206: To the reviewer, the temperature dependence results especially from the fact that N_INP is cumulative. A direct connection to the complexity of sources cannot be made.

L207: What test was used to check for differences between daytime and nighttime samples?

L207-211: It remains questionable if the sampling intervals used in the study (maximally two per day, 20 samples total spread out unknown over a month) allow for the detection of a diurnal cycle. It is conceivable that if, e.g., the minimum in INP concentration occurs at 6:00 and the maximum at 18:00, no difference would be observed on the filters. The described scenario could be likely if the transport is facilitated by convection. In addition, are there any potential local sources located on Changbai mountain, e.g., from the lake, which could cover up a potential diurnal cycle? The authors should elaborate on these aspects.

L212-226: In the following discussion, references to the data figure could be beneficial. Furthermore, when discussing the obtained results to previous studies, more precision is needed, what is the difference between “narrowly” (L214) and “much narrower” (L216)? More quantitative expressions are needed. In addition, the present study motivated the need for measurements on high altitude sites. It is not entirely clear, how the sites of Cerro Mirador and Beijing relate to this. Maybe an extra motivation for this comparison would be needed. Studies from high altitude stations like Storm Peak (US), Jungfraujoch (Switzerland), or Altzomoni (Mexico) might be interesting additions, which could also complement Figure 2b, but the reviewer leaves this up to the authors.

L213: Precise to Swiss Alps.

L214: Shouldn’t it be high-temperature and low-concentration region and vice versa?

Section 3.2: The reviewer perceived the usage of the abbreviations bio-INPs, other org-INPs, and inorg-INPs at times a bit odd. If the authors want to keep them, they may want to consider introducing them already in the introduction.

L236: Specify type of H₂O₂

L254-256: Was this increase significant and isn’t it rather a bias due to the measurement limitations (no dilutions)?

L264: Add the actual number of samples that make up 50% (also in the caption of Fig. 3).

L269-270: Again, how significant was this increase and is the decrease to low temperatures owed to a measurement bias?

Section 3.3: The data used in this section is unavailable. As the raw data for creating Figure 4 is already quite digested in Figure 4, the authors may want to add a table with the data written out per sample in the appendix. In addition, the found correlations are described as “good” or “weak”. As the definitions for these terms may vary, the value and type of used correlation should be named throughout the manuscript.

L278-279: Even though the authors selected a mild formulation for their interpretation, this statement should be toned down a little bit more for having only found a good correlation with one element. Furthermore, could there be other sources as well?

L281: Kanji et al. 2017 is a summary, maybe use direct study for this reference.

L288-289: Is there reference for this methodology? What was the used threshold in concentration to come to this conclusion?

L306-309: Despite being textbook knowledge, the authors may want to give a reference to read up in these topics. Furthermore, a short description of the essential processes for Changbai mountain could be given in the introduction.

L313: Specify “moderate-to-good”.

L315: “exceptionally high N_INP-bio values” add e.g., ‘as discussed below’ for readability.

L315-316: Even though being only a suggestion, in the reviewer’s opinion this statement cannot be made given too many assumptions and misinterpretation. First, the two high INP cases are excluded and given the argument should coincide with height. Is that an indication for a potential strong but infrequent local source? This might well be pure coincidence, but given the dates being a week apart, was there some periodic event near the measurement site? Second, while the correlation (which type of coefficient?) for some temperature is comparably large, there does not seem to be a significant increase in INP concentration. In addition, if the transport of bio-INP was the underlaying process, one could expect that the correlation should be expressed at a broad range of INP concentrations at a wide range of temperature in the HTR – has this been observed and could the correlation at all temperature been shown, e.g., in a table? Lastly, the PBL height never extends to the mountain top. For transport there is further evidence needed like wind speed and direction along the slope to support this claim. Ultimately, the analysis also bases only on 6-9 datapoints and the uncertainty in the PBL product remains undiscussed.

L316-319: For this claim, data should be presented.

L328: Add reference for phytoplankton blooms occurrence.

L348: Were there still enough datapoints available below

L357-358: Following Figure S1, it appears to the reviewer, that the correlations for other org-INPs seem similarly or even more consistent throughout the temperature range than for the presented bio-INPs data.

Conclusion

L364-371: Maybe repeat introduced variables such as F_INP-bio, LTR, WS.

L377-378: “With larger contributions observed from local and oceanic sources” - was this shown in the results section?

L379-389: The reviewer is unsure whether diving in to the topic of secondary ice formation in the last paragraph without any prior mention of the topic is within the scope of this publication.

L384: “confirm” seems to be the wrong word, the statement should be weakened.

Figures

Figure 1: Is it essential to indicate Beijing? If so, it should be named in the caption. To stay consistent between (b) and (a) it could be beneficial to indicate Changbai mountain (check spelling in figure) in red in (a).

Figure 2a: What is the error in T and f?

Figure 2b: Specify the data of Wieder et al. 2022 to Weissfluhjoch instead of Alps. The data of Wieder et al. 2022 is not fully visible. Beijing data is hard to read. For better comparability, maybe an average or median of each data set could be added.

Figure 3a and 3b: Using thin lines instead of dots per spectrum may enhance the readability.

Figure 3a: Error bars extend beyond axis limits. As N_INP, N_INP-heat, and N_INP-H2O2 are only used to calculate N_INP-bio, N_{INP-other org}, and N_INP-inorg Figure 3a could be moved to appendix to focus on the essential plots 3b and 3c.

Figure 3b: Are all the calculated differences in concentration significant? Could (exemplary) error bars be added?

Figure 4a: Figure missing.

Figure 4: As all other figures temperature increases to the right, the x axis of Figure 4 could be flipped for consistency. Which correlation coefficient was used? Specify in caption.

Figure 5: What is the red star? Is there a specific reason for the y-axis not being logarithmic in contrast to the other plots of INP concentrations?

References

Conen, F., Yakutin, M. V., Yttri, K. E., and Hüglin, C.: Ice nucleating particle concentrations increase when leaves fall in autumn, Atmosphere, 8, 202, https://doi.org/10.3390/atmos8100202, 2017.

Kanji, Z. A., Ladino, L. A., Wex, H., Boose, Y., Burkert-Kohn, M., Cziczo, D. J., and Krämer, M.: Overview of Ice 525 Nucleating Particles, Meteorological Monographs, 58, 1.1-1.33, https://doi.org/10.1175/amsmonographs-d-16-0006.1, 2017.

Wieder, J., Mignani, C., Schär, M., Roth, L., Sprenger, M., Henneberger, J., Lohmann, U., Brunner, C., and Kanji, Z. A.: Unveiling atmospheric transport and mixing mechanisms of ice-nucleating particles over the Alps, Atmos. Chem. Phys., 22, 3111–3130, https://doi.org/10.5194/acp-22-3111-2022, 2022.