the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Feb 2026
Ice-nucleating particles in Greenlandic glacial outwash plains
Abstract. High-latitude dust (HLD) represents a source of ice-nucleating particles (INPs) with potential impacts on cloud formation and radiative forcing in the Arctic. Previous studies have shown that HLD can exhibit high ice-nucleating activity at high subzero temperatures, likely linked to a biological component. Yet, comprehensive assessments of HLD ice-nucleating characteristics and sources remain limited, especially in Greenland. Here, we show that glacial dust from three outwash plains in southwestern Greenland effectively nucleates ice at temperatures relevant for mixed-phase clouds, but with lower ice-nucleating activity than other HLD regions. Ice-nucleating activity of glacial dust shows high variability and is largely driven by small amounts of organic and biological material, as indicated by sample treatments and positive correlations of ice-active mass site densities with total organic carbon and microbial abundance. Atmospheric INP concentrations above -20 °C were higher at the outwash plain sites compared to a nearby fjord site, indicating localized influence under summertime background conditions. This is further supported by similarities between atmospheric and dust INP spectra, as revealed by principal component analysis. The atmospheric INP population was dominated by organic and biological contributions, with no clear role of local meteorology or long-range transport. Overall, the ice-nucleating activity of glacial dust in southwestern Greenland lies within the lower range of reported HLD INP activity, suggesting that highly active HLD parameterizations may overestimate INP concentrations in this region. This highlights the importance of region-specific dust characterizations for improving representation of cloud processes and climate impacts in the Arctic.
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- RC1: 'Comment on egusphere-2026-484', Anonymous Referee #1, 03 Mar 2026 reply
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- 1
The manuscript presents a comprehensive study in southern Greenland of ice nucleating particles (INPs) in surface material of glacial outwash plains. In addition, aerosol particles sampled at several locations on and near the plains provide for a thorough assessment of outwash plains as potential sources of atmospheric INPs. Analyses of sampled material include heat and hydrogen peroxide treatments to distinguish between different kinds of INPs. Various parameters, including organic carbon concentration, bacterial abundance and soil particle mineralogy provide further insights on the likely origin of INPs in outwash material. It is a technically sound study describing in great detail (28 pages main text, 73 pages in total) work that has led to new insights. Care has been taken to relate obtained results to those of similar published studies and to provide possible explanations for particular findings.
My reading of the results is that glacial outwash dust is at best a minor source of atmospheric INPs in southern Greenland during summer, for three reasons. First, the PCA analysis (Fig. 9) of cumulative INP spectra suggests that most filter samples (atmospheric INPs) contained an INP population different from bulk samples (glacial outwash dust INPs). Second, neither aerosol particle number (> 0.5 um) nor INP concentration increased at high wind speed (Fig. 8), perhaps not so surprising as Figure B1 shows biological crusts on soil surfaces. Such crusts efficiently suppress wind erosion (Zhang et al., 2006). Although it takes in the Arctic two centuries for a biological crust to develop fully, intermediate levels of development already form within a few decades (Heindel et al., 2019; Tanner, 2025). Only at Narsasuaq plain, where the surface is apparently crust-free sand and silt, as indicated by footprints left in loose material around the solar panels (Figure B1c), there is some overlap between filter and bulk samples along the PC1 axis in Fig. 9. Third, the strongest correlation of dust INPs was with colony forming units (CFU), i.e., viable microorganisms. These thrive better on biological surfaces than on loose glacial outwash material. Biological crusts, vegetation, and leaf litter seem to cover a substantial fraction of the surrounding land area, e.g., at Narsarsuaq col and NIRS (Fig B1c,). Taken together, the evidence suggests that dust from glacial outwash plains is not a major source of atmospheric INPs above these plains in summer, except perhaps within the internal boundary layer (e.g., Fig. 1 in Dupont et al., 2021) above larger patches of bare outwash material (Narsarsuaq plain). From my point of view, this outcome should guide future studies on atmospheric INPs towards focusing more on biological surfaces. Although there is tentative pointing in this direction (lines 512 and 513, 517 to 519, 533 to 535), I would recommend to put more emphasis on this issue.
Minor issues
I would move Tables 1 and 2 to the Appendix.
Line 201: "approximately" is not necessary because 98.4 °C is a precise temperature value.
Line 290: The choice of -15 °C as a good one is further supported by the findings of Hanna et al. (2008).
Lines 321/322: "... higher organic fractions than Tobo et al. (2019)...", change to "...higher organic fractions than reported by Tobo et al. (2019)..."
Lines 341/342: The statement that " ice-nucleating efficiency of TOC may differ across climatic regions" could be supported by referring to Schnell and Vali (1973).
Correlation analysis of INP with soil parameters is convincing in numbers. However, in Figure 5 only panel b (CFU) displays a visually convincing correlation. Perhaps, it is in the cluster of data points with low values that similar patterns are hidden in panels a and c? Transforming the scale of the x-axis to log-scale might render correlations visible.
In Figures 2, 6, and B12 there is a lot of overlap between symbols. Consider using open symbols to reduce cover up.
Figure B11: The morphology of the particles in panels a and b is not as typical for mineral dust as is that in panel c. Are EDX spectra available for these particles?
References
Dupont et al., 2021, https://doi.org/10.1029/2021JD034735
Hanna et al., 2008, https://doi.org/10.1175/2007JAMC1549.1
Heindel et al., 2019, https://doi.org/10.1007/s10021-018-0267-8
Schnell and Vali, 1973, https://doi.org/10.1038/246212a0.
Tanner, 2025, https://doi.org/10.3390/land14091827
Zhang et al., 2006, https://doi.org/10.1016/j.geod