From the journal Environmental Science: Atmospheres Peer review history

02-Feb-2021

Dear Dr Sanchez-Marroquin:

Manuscript ID: EA-ART-01-2021-000003
TITLE: Mineral and biological ice-nucleating particles above the South East of the British Isles

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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Reviewer 1

In the work submitted, Sanchez-Marroquin et al. measured INP concentrations in the boundary layer in the southeast of UK, and analyzed size-resolved aerosol composition using SEM-EDS. As most INP measurements were conducted at the ground level, aircraft-based measurements presented by this work are very valuable to improve our knowledge in INP concentrations in the troposphere. Measurements were conducted using techniques well-established by this group, and the manuscript is well written. I am not very familiar with field measurements, and the editor may invite colleagues familiar with field measurements of INP to review this manuscript. In my opinion, this work can be published after technical corrections.
Figure 2: The authors may need to exchange Figure 2a with Figure 2b, in order to be consistent with the second paragraph of Section 3.1.
Figure 2: please change “Note that in some of the blanks, almost all the droplets froze about -23 oC” to “Note that in some of the blanks, almost all the droplets froze at about – 23 oC”.
The current version does not contain the reference list, which should be included in the revised manuscript.

Reviewer 2

Review of Sanchez-Marroquin et al., “Mineral and biological ice-nucleating particles above the South East of the British Isles”

Overview:
In the manuscript titled “Mineral and biological ice-nucleating particles above the South East of the British Isles” by Sanchez-Marroquiin et al., airborne aerosol measurements from opportunistic sampling over the British Isles are presented. In their study, they collected particles for immersion freezing ice nucleation activity and aerosol composition analyses in efforts to determine characteristics of ice nucleating particles in this region. The possible influence of dust particles on the collected aerosol samples is evident based on compositional measurements. These dust particles appear to be important for the INP population, with estimates of ice nucleation site densities using estimated dust surface area that are comparable or exceed previously reported data. However, the authors note that dust is likely not the only contributing particle type to the INP population, with some samples having elevated INP number concentrations particularly at warmer temperatures. Visual inspection of microscopic images revealed most samples contained large biological particles, suggesting biological particles may also contribute to the INP population.

Overall, these airborne measurements are important for the community in gaining additional insights into INP population in various environments. The data are presented in a clear manner, though I have noted several placed for the authors to clarify experimental details and possible limitations. One larger concern was a brief discussion on how INP measurements from two different methods resulted in very different INP concentrations for 5 of 10 samples compared. I think these differences should be more clearly articulated and explained.

I think the authors have made a nice study out of “opportunistic” sampling, which is a great achievement. The inferences of how dust and biological particles may have influenced the measurements are also interesting and I therefore recommend the manuscript for publication after the following concerns are addressed. I also highly recommend the resubmitted version and any future manuscript submission to include line numbers to make the revision process simpler for all parties. I did my best to clearly state what section of the paper my comments refer to by pulling text directly from the submitted version.

Comments:
Overall:
- Please make sure the use of INP versus INPs is correct throughout the paper
- The reference formatting throughout the paper includes a #, which I don’t think is needed; check format requirements from the journal.
- Check for typos throughout.

Abstract:
- “Biological aerosol particles were detected in all the analysed samples in concentrations of at least 10 to 100 L-1 in the boundary layer”
Because the biological particle counts are described in the main text as a lower estimate, this should be made more clear in the abstract by specifying that these concentrations were based on visual detection of biological morphological features for particles larger than 2 microns.
- There are a couple places without “ice” preceding the term “nucleation”; please use “ice nucleation” rather than just “nucleation”, as this may be confused for the aerosol nucleation process associated with new particle formation.
- Please make sure the temperatures include the degree sign.

Introduction:
- Last two sentences of 1st paragraph – Perhaps unnecessary because this study doesn’t include high-latitude measurements.
- “Sea-spray aerosol containing marine organic material has been found to compete with mineral dust at altitudes where mixed-phase clouds can occur, particularly in the remote oceans {Vergara-Temprado, 2017 #396; Huang, 2018 #667; McCluskey, 2019 #669}.” – This sentence is also not necessary since this isn’t relevant for this study. If you decide to include this, please clarify that these 3 studies are specifically modeling studies.
- I was surprised not to see reference to any previous airborne measurements of INPs; Can the authors include a description of where the current literature stands on INP measurements a higher altitudes and how the previous results influenced their study?

Sampling platform
- Were all measurements made out of cloud or screened for cloud influence? Were their relative humidity measurements to support that the samples were in low-RH conditions?
- What are the size ranges for the PCASP and CDP? Can the authors include references and/or manufacture details for the PCASP, CDP, TE49C and AERO AL5002?
- Can the authors comment on the size range measured by the PCASP? Did this PCASP have an inlet heater to dry particles or where particles hydrated?
- For the CDP, aren’t measured particles at ambient RH and therefore could be easily influenced from cloud passes or higher RH?

Aerosol Particle sampling
- The pore size indicated for the two filters were 0.4 and 0.45 microns. However, depending on the gas velocity and filter thickness, it’s likely for the filter to collect particles smaller than the pore size (Spurny and Lodge, 1972), which would explain the particles smaller than 0.4 microns detected from the SEM-EDS analysis (Figures 3-6). Can the authors comment on the estimated effective diameter collected from these filters based on the filter thickness and pump flow rates? Reference: Spurny, K. R., & Lodge, J. P. (1972). Collection Efficiency Tables for Membrane Filters Used in the Sampling and Analysis of Aerosols and Hydrosols (No. NCAR/TN-77+STR). University Corporation for Atmospheric Research. doi:10.5065/D6F769JJ
- Please include a reference for HYSPLIT and also include a few details on how the back trajectories were calculated (e.g., meteorological reanalysis data, how a midpoint for a given sample was determined for the endpoint, how many hours were simulated, etc.)
- In Table 1, “interrupted samples” are indicated without a description of what this means. Why is this important and what was involved with the interrupted samples?
- Also in Table 1, “Teflon position” is included without a clear description of how this may influence the results and why it’s important; is there a different pump used for the different positions?
- What was the process for collecting sampling blanks? There is mention of a blank for biological particles in Section 3.4, but it’s not clear how this was conducted. Can the authors add a description of what the handling blank process entailed?

Ice-nucleating particle measurements
- Last paragraph – how was only half of a sample analyzed with the filter washing droplet-based IN measurement? Was the filter physically divided? If so, how?
- The language used in the last paragraph is qualitative when it’s possible to be more specific and quantitative. For example, “Note that some of the polycarbonate filters (in most cases halves of these filters) were also analysed using a filter washing droplet-based ice-nucleation experiment similar to previously described methods {O'Sullivan, 2018 #469}.” – how many samples was this done to? “Most cases, halves of these filters” – how many and how was only half analysed?; additional vague statements are throughout this section that could benefit from being more specific
- Last paragraph – Overall this paragraph and corresponding data in Figure S2 are concerning. I understand that the washing technique will not be as sensitive due to the distribution of particles within each droplet; however, why is this feature only in 50% of the samples that were compared? This is a significant difference that I do not think is adequately addressed or explained other than stating that the washing method is “much less sensitivity to measured INPs” [compared to the droplet-on-filter assay technique]. However, it seems the washing technique is sensitive “enough” and simply detects lower INP concentrations, often up to an order of magnitude or more. Are there important differences between the Teflon and Polycarbonate materials that could explain this? Or differences in the position of the filters? Are particles effectively washed off the filter?

Scanning Electron Microscopy:
- “Filters were coated with 30 nm of Ir prior..” what is Ir?
- “…the analysis volume (typically 2 um).” – should the units for the number 2 be a volume?
- “Morphological information is summarized in size-distributions.” – what diameter is used? Is it a physical diameter estimated by the microscopy image? If so, can the authors comment on how this would be different from the particle diameters measured by the PCASP and CDP optical probe?
- “…particles of each size bin are…” – How were the particle bin sizes chosen? Also, in the figures displaying these data, is it possible to change the bar width sizes to indicate the varying bin widths?
- How was the total and dust-specific surface area estimated? Are they upper or lower estimates?

3.1. Ice nucleating particle measurements
- Figure 1 – It is great to see the sampling locations and the HYSPLIT results. It is difficult to interpret these figures because the color meanings used in Figure 1a do not match that of Figures 1b and 1c and the scale/map projection also looks different between 1a and 1b. Figure 1b is busy and some of the colors are difficult to see over the cyan ocean and brown land. I would suggest making an individual map panel for each flight date with the trajectories colored by trajectory heights, if possible.
- I think the fraction frozen data (Figure 2b) are not necessary in the main text, but also aren’t hurting anything. Could move this to the SI.
- I think the correlation analysis is good to do for the initial analysis; I wonder why -19 deg C was used for this. I would expect colder temperature data for a correlation to dust area and warmer data for the correlation with biological particles. Can the authors explain?
- Missing here and also in the Abstract is the range of INP number concentrations observed from this study for a specific temperature. The sentence “Part of this low correlation is explained by the fact that the measured INP concentrations have a very low variability” is misleading I think, as there is quite a lot of interesting variability in INP number concentrations from these 14 samples, ranging from ~0.5 per L to ~7 per liter at -15 deg C and ~2 per L to 30 per L. I realize that this measured range is relatively normal for INP measurements, but it usually suggests that there are variable contributions from different aerosol types, as supported by the analysis in the following sections.

Aerosol with SEM
- Figures 3-6 – can the authors increase the label sizes for these figures?
- Figures 3-6 - What diameter is used for the x axis? Is it the same for each measurement type? Also, can the bar widths be changed to represent the varying bin widths?
- “… a second braod mode centered between ~1-5 μm.” - typo “braod”; Also, I don’t see a very clear mode exempt in a couple samples in Figure 6. Can the authors clarify what is meant?
- “The chemical composition of this sample (Fig 6b)…” - should this be Fig 4b?
- “ However, the total number of particles of this sample was smaller than most of the other SEM analysed samples, as shown in Fig S4.” – CO25_4 also had a small number of particles analyzed (N = 320, Figure 6d), right? Perhaps this isn’t necessary?

The contribution of mineral dust to the INP Population
- Check for typos: “As shown in Sect 3.3” I think should be Section 3.2?; “whereas if the ns for our sample” – format ns; Period after “…must control the sample’s ice nucleating ability”; “s is the surface areas” I think should be singular “surface area”;
- I think this analysis is compelling with some additional information on how dust surface area was estimated added to the methods with a note of if the estimated surface area is an upper or lower estimate (noted above).

Observation of primary biological aerosol particles
- I suggest changing this section title to “Estimates of primary…” since the approach isn’t quantitative, based on 1% of the filter area and only includes particles larger than 2 micron with visible “biological morphological features”.
- This section is disproportionally long, perhaps you could move the description of how the biological particles were detected in the experimental section?

Conclusions:
- Check for typos: “Particled up to” should be “particles”; “inlet system on board of the aircraft” grammar typo?; “and of and possessing an obvious biological” extra “and”?; “ability of our samples is consistent” I think “is” should be “were”; “However, they have been conducted” what is “they” in this sentence?
- “The measured INP concentrations were consistently relatively high during the campaign.” Relative to what?

28-Feb-2021
Manuscript ID: EA-ART-01-2021-000003
TITLE: Mineral and biological ice-nucleating particles above the South East of the British Isles
We would like to thank the reviewers and the editors for all the work they have done. We have tried to address all the comments suggested by the reviewers. An explanation of the changes done for each of the reviewer’s comments can be found below each point. The changes have been highlighted as revisions in the text.
Referee: 1
Comments to the Author
In the work submitted, Sanchez-Marroquin et al. measured INP concentrations in the boundary layer in the southeast of UK, and analyzed size-resolved aerosol composition using SEM-EDS. As most INP measurements were conducted at the ground level, aircraft-based measurements presented by this work are very valuable to improve our knowledge in INP concentrations in the troposphere. Measurements were conducted using techniques well-established by this group, and the manuscript is well written. I am not very familiar with field measurements, and the editor may invite colleagues familiar with field measurements of INP to review this manuscript. In my opinion, this work can be published after technical corrections.
Figure 2: The authors may need to exchange Figure 2a with Figure 2b, in order to be consistent with the second paragraph of Section 3.1.
The paragraph has been arranged so it is consistent with the figures now.
Figure 2: please change “Note that in some of the blanks, almost all the droplets froze about -23 oC” to “Note that in some of the blanks, almost all the droplets froze at about – 23 oC”.
Done.
The current version does not contain the reference list, which should be included in the revised manuscript.
Done.

Referee: 2
Comments to the Author
Review of Sanchez-Marroquin et al., “Mineral and biological ice-nucleating particles above the South East of the British Isles”

Overview:
In the manuscript titled “Mineral and biological ice-nucleating particles above the South East of the British Isles” by Sanchez-Marroquin et al., airborne aerosol measurements from opportunistic sampling over the British Isles are presented. In their study, they collected particles for immersion freezing ice nucleation activity and aerosol composition analyses in efforts to determine characteristics of ice nucleating particles in this region. The possible influence of dust particles on the collected aerosol samples is evident based on compositional measurements. These dust particles appear to be important for the INP population, with estimates of ice nucleation site densities using estimated dust surface area that are comparable or exceed previously reported data. However, the authors note that dust is likely not the only contributing particle type to the INP population, with some samples having elevated INP number concentrations particularly at warmer temperatures. Visual inspection of microscopic images revealed most samples contained large biological particles, suggesting biological particles may also contribute to the INP population.

Overall, these airborne measurements are important for the community in gaining additional insights into INP population in various environments. The data are presented in a clear manner, though I have noted several placed for the authors to clarify experimental details and possible limitations. One larger concern was a brief discussion on how INP measurements from two different methods resulted in very different INP concentrations for 5 of 10 samples compared. I think these differences should be more clearly articulated and explained.
I think the authors have made a nice study out of “opportunistic” sampling, which is a great achievement. The inferences of how dust and biological particles may have influenced the measurements are also interesting and I therefore recommend the manuscript for publication after the following concerns are addressed. I also highly recommend the resubmitted version and any future manuscript submission to include line numbers to make the revision process simpler for all parties. I did my best to clearly state what section of the paper my comments refer to by pulling text directly from the submitted version.
Thank you very much for the detailed review. We have addressed each of the specific comments below.

Comments:
Overall:
- Please make sure the use of INP versus INPs is correct throughout the paper
Done.
- The reference formatting throughout the paper includes a #, which I don’t think is needed; check format requirements from the journal.
Done.
- Check for typos throughout.
Done.
Abstract:
- “Biological aerosol particles were detected in all the analysed samples in concentrations of at least 10 to 100 L-1 in the boundary layer”
Done.
Because the biological particle counts are described in the main text as a lower estimate, this should be made more clear in the abstract by specifying that these concentrations were based on visual detection of biological morphological features for particles larger than 2 microns.
Done.
- There are a couple places without “ice” preceding the term “nucleation”; please use “ice nucleation” rather than just “nucleation”, as this may be confused for the aerosol nucleation process associated with new particle formation.
Done.
- Please make sure the temperatures include the degree sign.
Done.
Introduction:
- Last two sentences of 1st paragraph – Perhaps unnecessary because this study doesn’t include high-latitude measurements.
Our measurements are at a mid latitude (~52 oN), a relevant region for the cloud-phase feedback (Murray et al., 2021)
- “Sea-spray aerosol containing marine organic material has been found to compete with mineral dust at altitudes where mixed-phase clouds can occur, particularly in the remote oceans (Vergara-Temprado et al., 2017; Huang et al., 2018; McCluskey et al., 2019).” – This sentence is also not necessary since this isn’t relevant for this study. If you decide to include this, please clarify that these 3 studies are specifically modeling studies.
Done.
- I was surprised not to see reference to any previous airborne measurements of INPs; Can the authors include a description of where the current literature stands on INP measurements a higher altitudes and how the previous results influenced their study?
Added to the third paragraph of the introduction.
Sampling platform
- Were all measurements made out of cloud or screened for cloud influence? Were their relative humidity measurements to support that the samples were in low-RH conditions?
The sampling strategy involves systematically avoiding sampling in cloudy conditions. The inlet line was closed immediately whenever we had online indication of the presence of liquid water or ice (via the cloud-physics instruments). The dew point temperature was monitored, being always at least a few degrees below the true air temperature.
- What are the size ranges for the PCASP and CDP? Can the authors include references and/or manufacture details for the PCASP, CDP, TE49C and AERO AL5002?
A reference with a detailed description of the PCASP-CDP and its use in the aircraft is included (Rosenberg et al., 2012). The full name and manufacturer of the O3 and CO instruments have been included. See Sect 2.1.
- Can the authors comment on the size range measured by the PCASP? Did this PCASP have an inlet heater to dry particles or where particles hydrated?
An approximated range has been included. The PCASP and CDP are located in an underwing probe, hence they sample straight from the air, without any inlet.
- For the CDP, aren’t measured particles at ambient RH and therefore could be easily influenced from cloud passes or higher RH?
Yes. However, all the used CDP data corresponds to the out of cloud runs. The signal in the CDP is very different than when sampling in cloud. Further discussion of the data produced by the instrument can be found in Rosenberg et al. (2012) and Sanchez-Marroquin et al. (2019).

Aerosol Particle sampling
- The pore size indicated for the two filters were 0.4 and 0.45 microns. However, depending on the gas velocity and filter thickness, it’s likely for the filter to collect particles smaller than the pore size (Spurny and Lodge, 1972), which would explain the particles smaller than 0.4 microns detected from the SEM-EDS analysis (Figures 3-6). Can the authors comment on the estimated effective diameter collected from these filters based on the filter thickness and pump flow rates? Reference: Spurny, K. R., & Lodge, J. P. (1972). Collection Efficiency Tables for Membrane Filters Used in the Sampling and Analysis of Aerosols and Hydrosols (No. NCAR/TN-77+STR). University Corporation for Atmospheric Research. doi:10.5065/D6F769JJ
The fact that the flow rate changes makes it difficult to give a specific value for the collection efficiency. The given reference gives values in between 0.400 and 0.666 for the collection of 0.2 µm particles at the right conditions (flow rates in between 15 and 40 LMP, 0.4 membrane filter, ~10 µm filter). The smallest particle size we detect (0.3 µm) will have an even higher collection efficiency. The collection efficiency of the Teflon filters, which are not membranes but fibrous filters will be even larger.
However, we assumed a collection efficiency of 100% through the whole size range. The decision was taken based on the recent literature and it is explained in the appendix A of Sanchez-Marroquin et al. (2019), which is the reference paper for the aerosol collection and SEM analysis technique. We included a sentence about this in the first paragraph of Sect 2.2 “This system and the used set up has been described and characterised previously (Sanchez-Marroquin et al., 2019).”

- Please include a reference for HYSPLIT and also include a few details on how the back trajectories were calculated (e.g., meteorological reanalysis data, how a midpoint for a given sample was determined for the endpoint, how many hours were simulated, etc.) .
Information added in Sect 2.2.
- In Table 1, “interrupted samples” are indicated without a description of what this means. Why is this important and what was involved with the interrupted samples?
The description has been improved in the caption of Table 1. The sampling normally takes place during one or more straight legs at a constant altitude. Sometimes, a filter is exposed to the air during more than one leg. In this case, the sampling is stopped during the change in direction or altitude in order to avoid sampling during these conditions, since we don’t know how this would affect the sampling efficiency.
- Also in Table 1, “Teflon position” is included without a clear description of how this may influence the results and why it’s important; is there a different pump used for the different positions?
The two inlets are identical, connected to the same pump. There might be non-consistent differences in the flow rate in each line. A discussion on this is included in the Sect. 2.3 of Sanchez-Marroquin et al. (2019), the reference paper of the technique. This has now been added in the caption of Table 1.
- What was the process for collecting sampling blanks? There is mention of a blank for biological particles in Section 3.4, but it’s not clear how this was conducted. Can the authors add a description of what the handling blank process entailed?
Blanks on the INP experiment were performed with clean filters straight from the filter box. This has been better explained in the third paragraph of Sec 2.3. For the biological particle analysis, an specific handling blank has been performed during a flight test. A description of the handling blank for biological particles is explained now in the fourth paragraph of Sect 3.4.

Ice-nucleating particle measurements
- Last paragraph – how was only half of a sample analyzed with the filter washing droplet-based IN measurement? Was the filter physically divided? If so, how?
Yes, the filters were physically divided with a blade, which did not introduce any contamination in terms of INP or aerosol particles detected with the SEM-EDS.

- The language used in the last paragraph is qualitative when it’s possible to be more specific and quantitative. For example, “Note that some of the polycarbonate filters (in most cases halves of these filters) were also analysed using a filter washing droplet-based ice-nucleation experiment similar to previously described methods (O'Sullivan et al., 2018).” – how many samples was this done to? “Most cases, halves of these filters” – how many and how was only half analysed?; additional vague statements are throughout this section that could benefit from being more specific
The text has been modified to include more detail.
- Last paragraph – Overall this paragraph and corresponding data in Figure S2 are concerning. I understand that the washing technique will not be as sensitive due to the distribution of particles within each droplet; however, why is this feature only in 50% of the samples that were compared? This is a significant difference that I do not think is adequately addressed or explained other than stating that the washing method is “much less sensitivity to measured INPs” [compared to the droplet-on-filter assay technique]. However, it seems the washing technique is sensitive “enough” and simply detects lower INP concentrations, often up to an order of magnitude or more. Are there important differences between the Teflon and Polycarbonate materials that could explain this? Or differences in the position of the filters? Are particles effectively washed off the filter?
The text in the end of Sect 2.3 has been modified. The text in Sect S2 has been also modified to better explain the discrepancies.

Scanning Electron Microscopy:
- “Filters were coated with 30 nm of Ir prior..” what is Ir?
Iridium. The word has been added to the text.
- “…the analysis volume (typically 2 um).” – should the units for the number 2 be a volume?
Corrected.
- “Morphological information is summarized in size-distributions.” – what diameter is used? Is it a physical diameter estimated by the microscopy image? If so, can the authors comment on how this would be different from the particle diameters measured by the PCASP and CDP optical probe?
Added to second paragraph of Sect. 2.4: “Morphological information is summarised in size-distributions, using the equivalent circular diameter of each particle, calculated form its cross-sectional area.”
The equivalent circular diameter and the optical diameter measured by the PCASP-CDP are different. However, we do not have any tool to find out more about this. Hence, we haven’t attempted to correct any diameter and the comparison of one with another is qualitative, as explained in Sanchez-Marroquin et al. (2019).
- “…particles of each size bin are…” – How were the particle bin sizes chosen? Also, in the figures displaying these data, is it possible to change the bar width sizes to indicate the varying bin widths?
There are logarithmic 3 bins for the 0.3 to 1 µm range and 7 logarithmic bins for the 1 to 20 µm range (the data in each range is acquired at different magnifications (Sanchez-Marroquin et al., 2019)). The bar width is proportional to the actual bin width in a logarithmic scale (for each of the ranges).
- How was the total and dust-specific surface area estimated? Are they upper or lower estimates?
Better explanation and error added to table 1 and caption: “The given value of dust surface area is calculated by integrating the mineral dust surface area (total surface area multiplied by the fraction of particles in the categories Si rich, Si only, Al-Si rich and Ca rich in each bin). The number in the bracket after the dust surface area corresponds to the error of the magnitude calculated using Poisson counting statistics.”

3.1. Ice nucleating particle measurements
- Figure 1 – It is great to see the sampling locations and the HYSPLIT results. It is difficult to interpret these figures because the color meanings used in Figure 1a do not match that of Figures 1b and 1c and the scale/map projection also looks different between 1a and 1b. Figure 1b is busy and some of the colors are difficult to see over the cyan ocean and brown land. I would suggest making an individual map panel for each flight date with the trajectories colored by trajectory heights, if possible.
Figure 1b has been separated into 4 figures corresponding to each day. The trajectory has been coloured based on altitude.
- I think the fraction-frozen data (Figure 2b) are not necessary in the main text, but also aren’t hurting anything. Could move this to the SI.
Done.
- I think the correlation analysis is good to do for the initial analysis; I wonder why -19 deg C was used for this. I would expect colder temperature data for a correlation to dust area and warmer data for the correlation with biological particles. Can the authors explain?
We could not go to lower temperatures because -19 deg C was the coldest temperature for which there is data in all the runs.
- Missing here and also in the Abstract is the range of INP number concentrations observed from this study for a specific temperature. The sentence “Part of this low correlation is explained by the fact that the measured INP concentrations have a very low variability” is misleading I think, as there is quite a lot of interesting variability in INP number concentrations from these 14 samples, ranging from ~0.5 per L to ~7 per liter at -15 deg C and ~2 per L to 30 per L. I realize that this measured range is relatively normal for INP measurements, but it usually suggests that there are variable contributions from different aerosol types, as supported by the analysis in the following sections.
In Fig 2a one can see the whole range of the INP measurements is within the error bars of a measurement which is in the middle. Hence, the variability could be produced by the uncertainties associated to the experiment (better explained in Harrison et al. (2016) and Harrison et al. (2019)). The only exception to this are the three highest measurements, which correspond to the C024 flight.
Added to the text: “Part of this low correlation is explained by the fact that the measured [INP] have a relatively low variability, with most of the measured concentrations being consistent to each other considering the uncertainties. The only exception to this are the enhanced levels of O3, submicron aerosol and mineral dust the C024 flight, …”
Aerosol with SEM
- Figures 3-6 – can the authors increase the label sizes for these figures?
Done.
- Figures 3-6 - What diameter is used for the x axis? Is it the same for each measurement type? Also, can the bar widths be changed to represent the varying bin widths?
Optical diameter for the PCASP-CDP and Equivalent circular diameter for the SEM data. This is explain now in Sect 2.1 and 2.4.
As previously mentioned, bar widths are proportional to the logarithmic width of the bin, for each of the two ranges used.

- “… a second braod mode centered between ~1-5 μm.” - typo “braod”; Also, I don’t see a very clear mode exempt in a couple samples in Figure 6. Can the authors clarify what is meant?
The mode can be better seen in the surface area size distribution, which has not been included. Hence, we removed the sentence for simplicity.
- “The chemical composition of this sample (Fig 6b)…” - should this be Fig 4b?
Yes, it is fixed now. Thank you for spotting the error.
- “ However, the total number of particles of this sample was smaller than most of the other SEM analysed samples, as shown in Fig S4.” – CO25_4 also had a small number of particles analyzed (N = 320, Figure 6d), right? Perhaps this isn’t necessary?
We refer to the fact that these sample (the only one above the boundary layer) exhibited less particles per cm-3 than the other samples, as shown in Fig S4. It has been better explained: “However, the particle number size distribution of this sample was smaller than most of the other SEM analysed samples, as shown in Fig S4”.


The contribution of mineral dust to the INP Population
- Check for typos: “As shown in Sect 3.3” I think should be Section 3.2?; “whereas if the ns for our sample” – format ns; Period after “…must control the sample’s ice nucleating ability”; “s is the surface areas” I think should be singular “surface area”;
Done.
- I think this analysis is compelling with some additional information on how dust surface area was estimated added to the methods with a note of if the estimated surface area is an upper or lower estimate (noted above).
Error in the dust areas has been added to Table 1. Explanation of dust calculations has been added to the end of Sec 2.4: “The surface area of mineral dust in our samples was calculated by multiplying the surface area size distribution by the fraction of dust particles in each bin and then integrating the resulting curve.“

Observation of primary biological aerosol particles
- I suggest changing this section title to “Estimates of primary…” since the approach isn’t quantitative, based on 1% of the filter area and only includes particles larger than 2 micron with visible “biological morphological features”.
Done
- This section is disproportionally long, perhaps you could move the description of how the biological particles were detected in the experimental section?
We agree that the section is quite long. We thought about doing this, but it would only slightly shorten the section (roughly one or two sentences less in each paragraph). Additionally it would make the section more difficult to follow.
Conclusions:
- Check for typos: “Particled up to” should be “particles”; “inlet system on board of the aircraft” grammar typo?; “and of and possessing an obvious biological” extra “and”?; “ability of our samples is consistent” I think “is” should be “were”; “However, they have been conducted” what is “they” in this sentence?
Done.
- “The measured INP concentrations were consistently relatively high during the campaign.” Relative to what?
Fixed: “The measured [INP] did not exhibit a large variability during the course of the campaign.”

Harrison, A.D., Lever, K., Sanchez-Marroquin, A., Holden, M.A., Whale, T.F., Tarn, M.D., McQuaid, J.B. and Murray, B.J. 2019. The ice-nucleating ability of quartz immersed in water and its atmospheric importance compared to K-feldspar. Atmospheric Chemistry and Physics Discussions. 2019, pp.1-23.
Harrison, A.D., Whale, T.F., Carpenter, M.A., Holden, M.A., Neve, L., O'Sullivan, D., Vergara Temprado, J. and Murray, B.J. 2016. Not all feldspars are equal: a survey of ice nucleating properties
across the feldspar group of minerals. Atmospheric Chemistry and Physics. 16(17), pp.10927-10940.
Huang, W.T.K., Ickes, L., Tegen, I., Rinaldi, M., Ceburnis, D. and Lohmann, U. 2018. Global relevance of marine organic aerosol as ice nucleating particles. Atmospheric Chemistry and Physics. 18(15), pp.11423-11445.
McCluskey, C.S., DeMott, P.J., Ma, P.L. and Burrows, S.M. 2019. Numerical Representations of Marine Ice‐Nucleating Particles in Remote Marine Environments Evaluated Against Observations. Geophysical Research Letters. 46(13), pp.7838-7847.
Murray, B.J., Carslaw, K.S. and Field, P.R. 2021. Opinion: Cloud-phase climate feedback and the importance of ice-nucleating particles. Atmospheric Chemistry and Physics. 21(2), pp.665-679.
O'Sullivan, D., Adams, M.P., Tarn, M.D., Harrison, A.D., Vergara-Temprado, J., Porter, G.C.E., Holden, M.A., Sanchez-Marroquin, A., Carotenuto, F., Whale, T.F., McQuaid, J.B., Walshaw, R., Hedges, D.H.P., Burke, I.T., Cui, Z. and Murray, B.J. 2018. Contributions of biogenic material to the atmospheric ice-nucleating particle population in North Western Europe. Sci Rep. 8(1), p13821.
Rosenberg, P.D., Dean, A.R., Williams, P.I., Dorsey, J.R., Minikin, A., Pickering, M.A. and Petzold, A. 2012. Particle sizing calibration with refractive index correction for light scattering optical particle counters and impacts upon PCASP and CDP data collected during the Fennec campaign. Atmospheric Measurement Techniques. 5(5), pp.1147-1163.
Sanchez-Marroquin, A., Hedges, D.H.P., Hiscock, M., Parker, S.T., Rosenberg, P.D., Trembath, J., Walshaw, R., Burke, I.T., McQuaid, J.B. and Murray, B.J. 2019. Characterisation of the filter inlet system on the FAAM BAe-146 research aircraft and its use for size-resolved aerosol composition measurements. Atmospheric Measurement Techniques. 12(11), pp.5741-5763.
Vergara-Temprado, J., Murray, B.J., Wilson, T.W., O'Sullivan, D., Browse, J., Pringle, K.J., Ardon-Dryer, K., Bertram, A.K., Burrows, S.M., Ceburnis, D., DeMott, P.J., Mason, R.H., O'Dowd, C.D., Rinaldi, M. and Carslaw, K.S. 2017. Contribution of feldspar and marine organic aerosols to global ice nucleating particle concentrations. Atmospheric Chemistry and Physics. 17(5), pp.3637-3658.

18-Mar-2021

Dear Dr Sanchez-Marroquin:

Manuscript ID: EA-ART-01-2021-000003.R1
TITLE: Mineral and biological ice-nucleating particles above the South East of the British Isles

Thank you for your submission to Environmental Science: Atmospheres, published by the Royal Society of Chemistry. I sent your manuscript to reviewers and I have now received their reports which are copied below.

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Reviewer 2

Dear Authors,

Thank you for addressing all of my concerns - the manuscript looks great. I only have a few minor comments that I think will add to readability and are simple to add.

Lines 76 – 78 – Is this a typo? What is meant by “mixed-phase clouds can occur in different modelling studies”?

Please add the details you mentioned in your response related to sampling non-cloudy conditions to Section 2.1.

Figure 1- I think the start colors correspond to the day color, correct? I suggest changing the legend to stars and/or add something to the figure caption to explain the colors indicated in the legend for each date correspond to the back trajectory endpoints denoted by the stars.

We would like to thank the editor and the reviewers much for accepting the manuscript. The suggested changes have been added (see below). High quality images are embedded in the text. Please let us know if something else is required.


Lines 76 – 78 – Is this a typo? What is meant by “mixed-phase clouds can occur in different modelling studies”?
The sentence has been modified so it makes more sense and it is easier to understand.

Please add the details you mentioned in your response related to sampling non-cloudy conditions to Section 2.1.
Done

Figure 1- I think the start colors correspond to the day color, correct? I suggest changing the legend to stars and/or add something to the figure caption to explain the colors indicated in the legend for each date correspond to the back trajectory endpoints denoted by the stars.
Done

25-Mar-2021

Dear Dr Sanchez-Marroquin:

Manuscript ID: EA-ART-01-2021-000003.R2
TITLE: Mineral and biological ice-nucleating particles above the South East of the British Isles

Thank you for submitting your revised manuscript to Environmental Science: Atmospheres. After considering the changes you have made, I am pleased to accept your manuscript for publication in its current form.

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With best wishes,

Dr Lin Wang
Associate Editor, Environmental Science: Atmospheres

Environmental Science: Atmospheres is accompanied by sister journals Environmental Science: Nano, Environmental Science: Processes and Impacts, and Environmental Science: Water Research; publishing high-impact work across all aspects of environmental science and engineering. Find out more at: http://rsc.li/envsci

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