Seasonal variability, sources, and parameterization of ice-nucleating particles in the Rocky Mountain region

Zhou, Ruichen; Perkins, Russell; Juergensen, Drew; Barry, Kevin; Ayars, Kelton; Dutton, Oren; DeMott, Paul; Kreidenweis, Sonia

doi:https://doi.org/10.5194/egusphere-2025-4306

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https://doi.org/10.5194/egusphere-2025-4306

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16 Sep 2025

| 16 Sep 2025

Seasonal variability, sources, and parameterization of ice-nucleating particles in the Rocky Mountain region

Ruichen Zhou, Russell Perkins, Drew Juergensen, Kevin Barry, Kelton Ayars, Oren Dutton, Paul DeMott, and Sonia Kreidenweis

Abstract. Atmospheric ice-nucleating particles (INPs) significantly influence cloud microphysics and aerosol-cloud interactions. Given that mountainous regions are vital to water resources, understanding of INPs in these areas is important for predicting impacts on regional clouds and precipitation. In this study, we conducted comprehensive measurements of immersion-freezing INPs at Mt. Crested Butte in the Rocky Mountains from September 2021 to June 2023 as part of the Surface Atmosphere Integrated Field Laboratory (SAIL) campaign. The average number concentration of INPs active at −20 ℃ was 2 L⁻¹, with distinct seasonal variation characterized by high summer concentrations and low winter concentrations. INP concentrations were correlated with a coarse dust aerosol type, which dominates PM₁₀ in this region. Converting INP concentrations to IN active surface site densities (n_s) led to reduction in variability, further supporting a relationship between coarse dust and INPs. Reduction of INP concentrations following treatment with H₂O₂ indicated substantial contributions from organic INPs across all activation temperatures, suggesting that organic-containing soil dust dominates the INPs in this region. Heat-labile INPs, likely biological in origin, were identified as dominant at > −15 ℃ through heat treatment of samples and showed significantly lower contributions in winter. Parameterizations based on n_s for the INPs observed in this mountainous region were developed, which effectively reproduced measured INPs concentrations, particularly when accounting for seasonal differences. This study provides the first long-term, comprehensive characterization of INPs for the Upper Colorado River Basin region and offers a parameterization potentially useful for predicting INPs in other remote continental regions.

Received: 03 Sep 2025 – Discussion started: 16 Sep 2025

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Ruichen Zhou, Russell Perkins, Drew Juergensen, Kevin Barry, Kelton Ayars, Oren Dutton, Paul DeMott, and Sonia Kreidenweis

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-4306', Anonymous Referee #1, 09 Oct 2025
This study provides a thorough characterization of airborne INPs from a remote, alpine area at the Mt. Crested Butte study site in the Rocky Mountains. The long-term monitoring of INPs (almost 2 years) allowed for the emergence of trends and conclusions that can only be made from such a comprehensive data set. The study revealed a distinct seasonal variation in INP concentrations with a peak in the summer time. Further, different INP concentrations were correlated with various sources, with organic-containing soil dust dominating the INP population in this area. The parameterizations developed can be useful for predicting INPs in remote continental regions. This paper adds a valuable dataset to the field of ice nucleation and will be of interest to ACP readers. It is recommended that this paper should be accepted for publication after the authors address some minor revisions.
General comments:
The discussion on the influence of vegetation on the INP population could benefit from additional context and references. Could the authors expand on this a bit more? For example, did the authors consider the influence of pollen? Biological INPs were present in warm seasons and decreased in winter (line 612). Does that line up with the pollen season? Furthermore, it might be helpful if the authors elaborated a bit more on the possible source pathways that link vegetation and soil, since the main conclusion is that organic-containing soil was the dominant INP.
PMF and source apportionment: While some atmospheric scientists are very familiar with PMF, others may not be fully convinced by your claims without having prior knowledge of PMF. Therefore, including a short explanation of PMF targeted for non-experts would make this section more convincing.
This may stem from my lack of PMF knowledge, but it was not clear to me how sample classification rules were determined. For example, why was 40% chosen as the cutoff for coarse dust contribution to PM₁₀?
Ulbrich et al., 2009 provides useful PMF guidelines when working with AMS data. Can you provide a similar reference for the technique that was used for the IMPROVE dataset? This will be very useful for scientists who want to learn PMF and reproduce your work, especially considering the interpretation of factors and if that is approached differently for different techniques.
In addition, could the authors please mention the techniques used for the filter and elemental analysis that were used from the IMPROVE network.

Just recently, a preprint by Lacher et al. 2025 was released, which also studied INP concentrations in the U.S. Rocky Mountains. The sampling period from Lacher et al., 2025 has some overlap with the sampling period in this paper and the sampling sites are close together. It would be very beneficial if you can compare trends in your data set with those in the other study.
Specific comments:
Title: The paper might benefit from a more precise title. Adding in your main finding into the title would make it more targeted.

Line 363: course dust was found to correlate with INPs at -10 °C (R² = 0.43). Is this R² value significant for INP characterization? Is this a typical value you expect in INP field studies? I feel like this value is a bit low to suggest a direct correlation.

Line 632-636: Can you give some example methods on how this could be done? Comments like these are very useful when planning future studies and suggesting example methods would be very beneficial to the community.

Line 153: please include the equation for INP concentration directly in the methods section in addition to your reference to Vali, 1971

Figure 3a: This graph is a little difficult to read. Could the lines be thicker to make it easier to follow? Additionally, the data presented in Figure 3a and Figure S3a are very similar. Is it necessary to show both versions? Please also remove the “a” from Figure S3 as there is only one panel.

Figure 7: Can you add a line to the figure caption to explain that the y-axis and x-axis is the same for the 2 plots in each column? It was a bit challenging to understand the figure right away.

Figure S1 and S2: Are the black and orange curves in Figure S1 the same as the curves shown in Figure S2? If so, please consider removing S1 as the data is repeated.

Figure S12: Please add a marker to show the location of the sampling site. Something like what was done in Figures 1 and S8.

Technical corrections:
Figure S8: Please add a legend or a line in caption to explain what the black star represents.

Figure S10: Please add x-axis label

Please ensure all figures are presented with adequate resolution.

There is inconsistent figure border formatting throughout the paper. And in some cases, the figure border cuts through text (S3, S4, S6, S7, S9, S13).

Line 210: typo in April

Text S1: link for the IMPROVE sop #351 says page not found. Please update.

References:
Lacher, L., Hallar, A. G., McCubbin, I. B., Bail, J., Froyd, K. D., Jacquot, J., ... & Cziczo, D. (2025). Strong springtime increase of ice-nucleating particle concentration in the Rocky Mountains. EGUsphere, 2025, 1-30. https://doi.org/10.5194/egusphere-2025-4492
Ulbrich, I. M., Canagaratna, M. R., Zhang, Q., Worsnop, D. R., & Jimenez, J. L. (2009). Interpretation of organic components from Positive Matrix Factorization of aerosol mass spectrometric data. Atmospheric Chemistry and Physics, 9(9), 2891-2918. https://doi.org/10.5194/acp-9-2891-2009
Citation: https://doi.org/10.5194/egusphere-2025-4306-RC1
RC2:
'Comment on egusphere-2025-4306', Anonymous Referee #2, 17 Oct 2025
Zhou et al. presented a study on the variability, source apportionment and parameterization of ice nucleating particles (INPs) in the Rocky Mountain during the Surface Atmosphere Integrated Field Laboratory (SAIL) campaign. INP number concentration was measured over nearly two years using an offline droplet freezing assay, which the authors used to analyse the variability of INPs abundance. In addition, positive matrix factorization analysis was performed to investigate the major sources for observed INPs. The contribution of heat-labile and organic materials to the observed INPs was also tested by testing the remained INP abundance of H₂O₂ and heat treated samples. The authors also proposed a two-equation parameterization considering the seasonal variability of biological INPs to improve INP prediction. In general, this paper reports significant data on INP abundance and variations in Rocky Mountain and presents important results on INP sources, which are significant to under aerosol-cloud interactions in this region. However, additional details on the measurement and data analysis are expected to be provided in the revised version, along with a more in-depth discussion of the correlations between INPs and aerosols to more effectively present the narrative of this study. We hope that our comments will help the authors improve the revised version of this preprint. We recommend the acceptance for publication in Atmospheric Chemistry and Physics (ACP) after appropriate revision.
General comments:
Abstract should be more concise by rephrasing the first three sentence. INP source apportionment should be stressed more. It should be clearly noted that the major/dominant INP sources are locally emitted coarse-sized dust particles and biological particles. This is the key.

The classification of five PMF factors should be clearly defined in the main text but not in the SI. What is the size difference between Coarse dust and Fine dust? More details about the difference between these two should be provided. Also, specific properties associated with these five factors should be provided. For example, if it is the case, one cloud state that ‘the mass concentration of elemental carbon and K element was used for traces for biomass burning aerosols (BBA)’. Also, the information of instruments/measurements for those properties to represent five factors should be provided.

BBA containing elemental carbon or black carbon particles are reported to be poor INPs in the mixed-phase cloud regime (Gao et al., 2025; Wieder et al., 2022). It should be BBA associated (co-emitted) soil dust or organic particles that contribute to observed INPs (Mccluskey et al., 2014). This is also supported by the ns results in Figure 4b where it shows the ns of BBA period samples is similar to those samples with dust as INP sources. This point should be delivered to the readers more clearly. Instead, it now reads like the authors classified BBA as one of the INP sources which seems all emitted particles contribute to the observed INPs. In addition, would it be possible if the authors can also provide fire maps (e.g., from NASA FIRMS) with HYSPLIT airmass back trajectories for INP samples influenced by BAA? This will provide further evidence of BAA.

It is good that the authors provided results on DNA analysis for Snomax influenced samples in the Supplementary. Why not also for heated and H₂O₂ treated and untreated samples? This will provide direct and strong evidence on the presence/contribution of biological particles to the observed INPs. In Section 3.5, the results of heated and H₂O₂ treated samples are in-direct evidence on the contribution of biological particles to INPs.

Some statements in Section 4 are repeating discussions in Section 3, which makes it unnecessary long and reads not very interesting. Also, section 4 should be divided into two sections, one for summary/conclusion and one for atmospheric implications. In the original manuscript, atmospheric implications are not well/sufficiently addressed as it appears in the section title.

Specific comments:
Line 27 & 30: Provide number/statistics for ‘Substantial’ and ‘significant’. This makes the abstract stronger.
Line 51-52: colder than which temperature?
Line 89-101: The paragraph develops general statements so it should be moved forward
Line 154: reference to Vali 1971 does not follow ACP guidelines. Alos correct others, like Line 250
Line 155-156: not clear about the field blank collection before removal and storage. Remove what? Do you have filed blank for every sample for background noise correction? Also details about INP data processing is missing. Simply referring to Vali 1971 is not enough. There are differences in calculating the INP numbers.
Line 167: how much of H2O2 was added? And samples should be termed H2O2-heat treated but not only H2O2 treated. This should be changed through the paper
Line 187-190: How long can it affect the campaign sampling? It may also influence subsequent sampling but not only overlapped samples? can you evaluate this? since you see the Snomax effects
Line 214: provide size ranges for Coarse dust and Fine dust? in SI text S1 you provided that they contain similar elements Al, Ca, Fe, Mg, and Si but what is their difference making it Coarse or Fine?
Line 273: should be ice nucleation site according to Vali et al. (2015)
Line 2801-281: Did the authors test the role of particles smaller than 500 nm? Gao et al. (2024) reported that particles smaller than 500 nm may contribute to INPs active in the mixed-phase cloud regime by comparing the correlations between INPs at -25C and SMPS+APS particles or APS particles.
Lien 285: maybe move S15 as S9? the previous reference is S8 in line 252
Line 296-300: prepare a figure and provide it in the SI? Showing/comparing INP abundance in previous studies, like Lacher et al. (2025) and Tobo et al. (2013)
Line 304-305: isn't it also the case for T-20 T-15 showing peaks in Sep 2021? and also T-10?
Line 308-309: An increase in total aerosols does not necessarily mean an increase in INPs. Like not all BBA particles like BC contribute to INPs at -25C. Please give in-depth insight.
Line 320: why emission decrease? Like because of snow cover and decreased metabolism
Line 323-324: evidence/relevant results on dust events?
Line 328: be more specific. Dust is more relevant INPs for T<-15C
Line 329: Does lower-temperature INPs refer to INPs at -20 and -25C?
Line 335: Section 3.2: Figure S6 for SAIL campaign should be moved to this section. Also consider that this section should be harmonized with Section 3.1 discussing also in a seasonal manner
Line 345-347: I am disappointed that a statement is missing. figure S6 BBA peak in Sep 2021 is a very strong evidence for INP peak in Figure 3b. A clear statement should be made
Line 364-366: We cannot agree with this statement. Coarse dust include dust particles and dust carrying biological materials. Dust is not effective INPs at -10C, but BIO-dust could be. Also not all coarse particles carrying biological materials and behave as active INPs. Also, airborne bacteria can be submicron as part of biological particles.
Line 392: how can you see it is the predominant source by referring to Figure S6? does S6 provide fraction or other results to support this argument?
Line 392-394: tune down. Statement is too strong, given that in Sep 2021 BBA associated aerosol is the controlling source
Line 394-395: does it mean the active IN of mineral and soil dust is because of organics and/or salts? I am afraid of that one will not agree. Also, what is point for putting many references? Suggest referring to a literature with clear rationales
Line 416-417: Again, high aerosol number concentration does not necessarily mean more INPs. It is probably because of soil dust and some organics in BBA plumes
Line 431-432: Don’t agree. This may suggest coarse dust is more active INPs. Major contributor means more abundant INP number concentrations
Line 440-441: What is the biomass burning mass concentration IN Figure S11? what does it contain? We only see a statement in Text S1: The biomass burning factor was strongly associated with organic and elemental carbon, which are mainly from combustion processes, and K, a tracer of biomass burning.
Line 464: why -18C? not -15C or -20C? It is not very clear for us to read the so-called ‘clear segregation’. Instead of cumulative INP and ns curves, we believe differentiate curves will help the authors present more pronounced results.
Line 469-470: In Kanji et al. (2017), the temperature is -15C but not -18C
Line 480: again, we hardly read this -18C in Figure 5
Line 510: what is the correlation coefficient? and p-value? is it significant?
Line 530: please refer to subpanels. Also for other statements for the discussion on results in Figure 7.
Line 576-577: change the wording ‘single’. This may cause misunderstanding because you have two equations in the parameterization
Line 590: Is there any result/evidence provided for showing intensive biological INP events?
Line 609-610: Is there any result presented for the correlations between fine dust and INPs?
Technical corrections:
Figure 3a colour is difficult to read. Suggest also to use different symbols or find other better ways to visualize the data

Figure 4a may use shedding ranges to better present the data

Figure 7: what is the significance level of the calculated coefficients?

References:
Gao, K., Vogel, F., Foskinis, R., Vratolis, S., Gini, M., Granakis, K., Zografou, O., Fetfatzis, P., Papayannis, A., Möhler, O., Eleftheriadis, K., and Nenes, A.: On the drivers of ice nucleating particle diurnal variability in Eastern Mediterranean clouds, npj Clim. Atmos. Sci., 8, https://doi.org/10.1038/s41612-024-00817-9, 2025.
Gao, K., Vogel, F., Foskinis, R., Vratolis, S., Gini, M. I., Granakis, K., Billault-Roux, A.-C., Georgakaki, P., Zografou, O., Fetfatzis, P., Berne, A., Papagiannis, A., Eleftheridadis, K., Möhler, O., and Nenes, A.: Biological and dust aerosol as sources of ice nucleating particles in the Eastern Mediterranean: source apportionment, atmospheric processing and parameterization, Atmos. Chem. Phys., 24, 9939-9974, https://doi.org/10.5194/acp-24-9939-2024, 2024.
Kanji, Z. A., Ladino, L. A., Wex, H., Boose, Y., Burkert-Kohn, M., Cziczo, D. J., and Krämer, M.: Chapter 1 Overview of Ice Nucleating Particles, in: Meteorological Monographs, 1.1-1.33, 10.1175/amsmonographs-d-16-0006.1, 2017.
Lacher, L., Hallar, A. G., McCubbin, I. B., Bail, J., Froyd, K. D., Jacquot, J., Shen, X., Rapp, C., Möhler, O., and Cziczo, D.: Strong springtime increase of ice-nucleating particle concentration in the Rocky Mountains, EGUsphere [Preprint], 10.5194/egusphere-2025-4492, 2025.
McCluskey, C. S., DeMott, P. J., Prenni, A. J., Levin, E. J. T., McMeeking, G. R., Sullivan, A. P., Hill, T. C. J., Nakao, S., Carrico, C. M., and Kreidenweis, S. M.: Characteristics of atmospheric ice nucleating particles associated with biomass burning in the US: Prescribed burns and wildfires, J. Geophys. Res. Atmos., 119, 10458-10470, 10.1002/2014jd021980, 2014.
Tobo, Y., Prenni, A. J., DeMott, P. J., Huffman, J. A., McCluskey, C. S., Tian, G., Pöhlker, C., Pöschl, U., and Kreidenweis, S. M.: Biological aerosol particles as a key determinant of ice nuclei populations in a forest ecosystem, J. Geophys. Res. Atmos., 118, 10,100-110,110, https://doi.org/10.1002/jgrd.50801, 2013.
Vali, G., DeMott, P. J., Möhler, O., and Whale, T. F.: Technical Note: A Proposal for Ice Nucleation Terminology, Atmos. Chem. Phys., 15, 10263-10270, https://doi.org/10.5194/acp-15-10263-2015, 2015.
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.
Citation: https://doi.org/10.5194/egusphere-2025-4306-RC2

Ruichen Zhou, Russell Perkins, Drew Juergensen, Kevin Barry, Kelton Ayars, Oren Dutton, Paul DeMott, and Sonia Kreidenweis

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https://doi.org/10.5194/egusphere-2025-4306-supplement

Ruichen Zhou, Russell Perkins, Drew Juergensen, Kevin Barry, Kelton Ayars, Oren Dutton, Paul DeMott, and Sonia Kreidenweis

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Short summary

A small fraction of aerosol particles, microscopic pieces of solid or liquid in the air, are important for controlling the freezing processes in clouds, which in turn impacts rain and snow. This work examines how concentrations of these special and important particles change throughout the year at a measurement location in the Colorado Rocky Mountains. We find at this location, most of these special particles are associated with soil dusts in the air, and concentrations decrease in the winter.


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