the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 14 Nov 2025
Evaluation of different sampling methods to determine the ice-nucleating particle concentration in the atmosphere using the GRAnada Ice Nuclei Spectrometer (GRAINS)
Abstract. This work deals with the analysis of different filter sampling methods to obtain INP concentration spectra using the GRAnada Ice Nuclei Spectrometer (GRAINS), a droplet freezing assay based on the design of the Colorado State University Ice Spectrometer (CSU-IS) with droplet volumes of 100 μL. GRAINS was first validated with NX Illite, showing spectra consistent with literature, and also compared with FrESH (Freezing Experiment Setup Helsinki), INSEKT (Ice Nucleation Spectrometer of the Karlsruhe Institute of Technology), and PINE (Portable Ice Nucleation Experiment), with results generally within confidence intervals or a factor of 5. To assess the filter sampling methods, we simultaneously sampled ambient aerosol on polycarbonate filters (commonly used for INP analysis) and microfiber quartz filters (used for chemical analysis) over three months, with 27 filters of each type. Three analysis approaches were tested: washing the polycarbonate filters (Polycarbonate method), randomly punching the quartz filters (Quartz 96-punch method), and washing a larger punch of the quartz filter (Quartz punch washed method). Our results showed a good performance of the three methods, obtaining similar results for the INP concentrations, with approximately 89 % of the data within a factor of 5. Differences between methods become more evident at lower temperatures, with lower INP concentrations detected with the Polycarbonate method compared to the other two, which could be related to the particle extraction efficiency of this method. Differences between the three methods varied depending on the sample, so these differences could originate from the nature of the particles being analyzed. Still, there is a clear correlation between the three methods, with Spearman's coefficients of around 0.9 (p < 0.05). The Quartz punch washed method allows to perform sample dilutions similar to the Polycarbonate method, making it a potentially better alternative to the Quartz 96-punch method for analyzing INP concentrations using quartz filters.
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Status: open (until 20 Dec 2025)
- RC1: 'Comment on egusphere-2025-5212', Anonymous Referee #1, 02 Dec 2025 reply
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This manuscript presents the development and validation of the GRAnada Ice Nuclei Spectrometer (GRAINS) and uses it to compare different filter substrates and extraction methods for offline INP analysis. The question of how sampling substrates and extraction procedures influence INP quantification is highly relevant to the community. In particular, the assessment of quartz filters is important and potentially valuable, as they are widely used in routine aerosol monitoring networks but rarely applied in INP studies. Identifying and validating new sampling substrates could have far-reaching implications for the INP community, where polycarbonate filters have long been the standard choice. Because of the potential impact, it is essential to examine the results with exceptional care. Overall, the study fits well within the scope of AMT, and it could become suitable for publication after the following comments are addressed.
Major Comments:
A substantial portion of the manuscript describing the instrument characterization and intercomparison is well written and technically solid. My main concerns focus on the key scientific question of the paper. Can quartz filters replace polycarbonate filters for offline INP sampling? Under what conditions would such a substitution be valid? And how should the two quartz-based extraction methods (quartz punch and punch-washed) be interpreted relative to each other? Unfortunately, in its current form, the presented dataset does not convincingly answer these questions. Several interpretations appear speculative or insufficiently supported by data or physical theory. I strongly encourage the authors to revisit these sections, refine the language, introduce clear limitations and uncertainties, and tone down the strength of the conclusions.
A first conceptual issue is that the filter-method evaluation relies almost entirely on ambient aerosol samples. While ambient samples are valuable for method demonstration, a rigorous assessment of sampling and extraction methods should begin with controlled tests using laboratory-generated standard INPs (e.g., mineral dust, biological particles). The suitability of each method depends on INP type and the freezing temperature range, and this dependence cannot be separated using ambient mixtures alone. At minimum, the ambient dataset should span a wider range of atmospheric conditions (e.g., clean days, dust events, heavy pollution, coastal influence). The manuscript should clearly acknowledge these limitations and specify the conditions under which the quartz-filter methods are applicable.
A second major concern relates to the extraction methodology. For a methods paper, key parameters such as droplet volume, extracted filter area, extraction time, and extraction technique require thorough justification. Many immersion-freezing instruments, including CSU-IS and INSEKT, use 50 μL droplets. The authors use 100 μL droplets, but the implications of this choice are not sufficiently discussed. Likewise, the manuscript states that particles were extracted using “manual agitation” for 60 seconds, but the procedure is not described in sufficient detail to evaluate its reproducibility or effectiveness. Such a short extraction time and uncontrollable extraction method is particularly problematic for quartz fiber filters, whose porous structure retains particles much more strongly than smooth PC filters. A clearer description, supporting evidence, and discussion of potential biases are needed.
Related to this, extraction efficiency is a central consideration in comparing sampling substrates. PC filters are widely used because they allow collected particles to be washed off efficiently. Quartz filters, in contrast, consist of a fibrous matrix in which particles can become embedded, resulting in lower extraction efficiency. The manuscript should discuss how this intrinsic structural difference may influence INP recovery, especially for the punch-washed method. In that method, lots of quartz fibers are inevitably transferred into the suspension and can themselves act as INPs at lower temperatures (Conen et al. 2012; Harrison et al., 2019). Their contribution must be carefully considered.
These methodological issues directly affect the interpretation of the key results. For example, below approximately -12 °C, the manuscript reports higher INP concentrations for the two quartz-based methods than for the PC method. The explanation offered in the manuscript remains speculative and lacks supporting evidence. A more plausible interpretation, consistent with Conen et al. (2012) and subsequent studies, is that the quartz filters introduce additional quartz fibers that act as INPs at colder temperatures. This naturally leads to increasing discrepancies among methods as temperature decreases. Indeed, Conen et al. (2012) concluded that quartz punch method is a better choice and should be restricted to temperatures ≥ -12 °C. This important limitation should be explicitly discussed. In this context, it is also worth noting that the quartz 96-punch method is likely the cleanest configuration, with minimal fiber contamination; the very short 60-second agitation step may, however, lead to insufficient particle extraction. This distinction is important because the 96-punch method is the one recommended by Conen et al. (2012) and Wex et al. (2019). Therefore, statements in the manuscript suggesting that the punch-washed method may be preferable are potentially misleading and require substantial reconsideration or additional solid experimental evidence.
In summary, while the instrument development and the general methodological framework are strong, several core conclusions regarding the equivalence and relative performance of the filter methods are currently not yet supported by the available data. I encourage the authors to substantially revise the manuscript, explicitly state the limitations, incorporate a more cautious interpretation of the results, and restrict conclusions to the conditions actually tested.
Specific comments:
References:
Conen, F.; Henne, S.; Morris, C. E.; Alewell, C., Atmospheric ice nucleators active ≥ −12 °C can be quantified on PM10 filters. Atmospheric Measurement Techniques 2012, 5, (2), 321-327.
Harrison, A. D.; Lever, K.; Sanchez-Marroquin, A.; Holden, M. A.; Whale, T. F.; Tarn, M. D.; McQuaid, J. B.; Murray, B. J., The ice-nucleating ability of quartz immersed in water and its atmospheric importance compared to K-feldspar. Atmospheric Chemistry and Physics 2019, 19, (17), 11343-11361.
Wex, H.; Huang, L.; Zhang, W.; Hung, H.; Traversi, R.; Becagli, S.; Sheesley, R. J.; Moffett, C. E.; Barrett, T. E.; Bossi, R.; Skov, H.; Hünerbein, A.; Lubitz, J.; Löffler, M.; Linke, O.; Hartmann, M.; Herenz, P.; Stratmann, F., Annual variability of ice-nucleating particle concentrations at different Arctic locations. Atmospheric Chemistry and Physics 2019, 19, (7), 5293-5311.