| 06 Jul 2026
Enhanced warm-temperature marine ice-nucleating particles over the Arctic Ocean versus Asian-dust outflow at Jeju Island from real-time shipborne observations
Abstract. Ice-nucleating particles (INPs) influence mixed-phase cloud microphysics and the Earth’s radiative budget, yet observational records over the remote Arctic Ocean and contrasting continental-outflow environments remain sparse. This study presents INP concentrations measured by the Portable Ice Nucleation Experiment (PINE) over the Arctic Ocean during two RV(Research Vessel) Araon campaigns (ARA15A in 2024 and ARA16A in 2025) and at the Korea-Cloud Physics Experiment Chamber (K-CPEC) on Jeju Island, Korea (February–May 2024), totalling 41,524 expansion runs of which 35,370 passed rigorous quality control. INP temperature spectra differed markedly among the three datasets. At a representative warm temperature (−20 °C), median concentrations were 3.68 L−1 (ARA15A), 3.64 L−1 (ARA16A), and 0.89 L−1 at Jeju (Clean-day subset), with Arctic values exceeding Jeju; at a representative cold temperature (−28 °C), the pattern reversed — the Jeju Clean-day median rose to 8.22 L−1 while Arctic medians remained comparatively flat (ARA15A: 7.02 L−1; ARA16A: 6.39 L−1). Campaign-wide medians were comparable between the two Arctic years (6.83 and 6.57 L−1) but differed approximately two-fold in the 75–80° N latitude band (ARA15A: 1.56 and ARA16A: 3.19 L−1). These spectral contrasts are consistent with a marine biogenic-influenced regime at the Arctic warm-temperature end and an Asian-dust-influenced regime at the Jeju cold-temperature end, although direct compositional confirmation (heat treatment, chemical speciation) was not available in this dataset; all source attributions should therefore be interpreted as spectrally-consistent hypotheses rather than established source identifications. A dedicated ship-exhaust quality control scheme applied to both campaigns provides a replicable contamination-removal framework for future shipborne INP studies, and the combined dataset delivers new observational benchmarks for Arctic INP variability across the sampled 2024–2025 cruise periods.
Review of “Enhanced warm-temperature marine ice-nucleating particles over the Arctic Ocean versus Asian-dust outflow at Jeju Island from real-time shipborne observations” by Joo Wan Cha and co-authors.
This paper investigates the distribution of INPs across contrasting remote-marine and continental-outflow environments. Using a Portable Ice Nucleation Experiment (PINE) online expansion chamber, researchers measured ambient INP concentrations over the Arctic Ocean during two shipborne research campaigns and compared them with observations on Jeju Island in South Korea. The authors contrast the measured INP concentrations at the two locations and provide analysis and discussion on the origin of the INPs (in the absence of compositional information). Finally, the authors include historical INP measurements from other studies to investigate a hypothesised relationship between sea ice retreat, marine activity, and INP concentrations in the Arctic. The paper is very well written — though perhaps a little verbose at times.
I have several major concerns with this study – to a degree that I will not provide a full review until my primary concerns have been addressed. I believe a portion of the PINE data measured in the Arctic may have been erroneously removed below an INP threshold. If this is the case, then the resulting dataset is severely biased, influencing all subsequent analysis, methodological choices, and conclusions. I also have major concerns about the justification for ‘correcting’ historical datasets to better fit with the PINE data. For these reasons, I do not recommend publication in its current form, but I look forward to the author’s responses in the hope that this can be published in the near future.
Data removal
For the Jeju expansions the procedure was 30s at a rate of 3Lmin-1, equating to around 1.5 L of air sampled. Assuming 1 ice crystal per volume sampled is the minimum that can be reported should give a minimum INP concentration of 0.66 L-1. This minimum is consistent with the typical reported minimum INP concentration reported by previous PINE users (see Herbert et al. 2026 ESSD – specifically Figure 2 with the ExINP-ZEP level 1 dataset). For the Araon expansions the procedure was 40s at a rate of 3Lmin-1, equating to around 2 L of air sampled per expansion and a minimum INP concentration of around 0.5 L-1. The two minimum measurable concentrations are similar, which is expected given the similar procedure.
Figure 4 (RHS – or 4b) confirms this – the lowest temperature-independent ‘stripe’ of INP data falls around 0.5 L-1 throughout both of the campaigns – which corresponds to 1 ice particle being measure during the expansion (The Jeju data minimum is lower than expected and will be discussed further down). So far, the Araon data presented looks sensible. However, comparing the Level 1 Araon data from Figure 4b with that of Figures 3b and 3d suggests that all Aaraon data below N_INP = 3 L-1 was removed – but only for the data measured within the Arctic (Figure 4 LHS – or Fig 4a). Section 2.6 alludes to the source of this – a Poisson-based LOD was applied to the data to ensure that more than 1 ice crystal was being measured during the 24 resampling period, which I presume ends up being around 3 L-1. If this is the case, then I have a few questions.
The removal of this data results in a very different distribution – as can be seen in Figure 3b and 3d – and influences the entire narrative of the paper. Removing the data below 3 L-1 strongly biases the dataset towards higher values – and may result in the largely temperature-independent INP spectrum used in all subsequent analysis in the study. The absence of strong temperature-dependence is routinely seen as evidence of biogenic influence, yet if the data is biased high then this may be masking the true temperature dependence. The authors need to look at this again and, if correct, provide more rationale as to why so much of this perfectly good looking data has been removed.
Conversely, the Jeju Level 1 dataset should have a similar INP distribution as that shown in Figure 4b for the Araon data – the lowest ‘stripe’ should fall around 0.75 L-1. However, inspecting Figures 3e to 3i and the box plots in Figure 4a suggest a lower limit of around 0.15 L-1. If this is from the Level 1 data then this implies a much larger volume was sampled (around 6 L). Can the authors check and confirm the data is correct? And can the level 1 Jeju data be included on Figure 4 as a third panel?
Herbert et al., 2026: https://essd.copernicus.org/preprints/essd-2026-41/
Including zero counts
The PINE expansions routinely result in an INP concentration of 0. This simply means that there were no ice crystals measured during the expansion. This is still useful data and should be included in the analysis – other PINE users have resampled the timeseries and taken means of the INP concentrations, including the zero INP expansions. Figure 2 from Herbert et al. 2026 demonstrates this. I suggest the authors consider performing this extra step before presenting their data as it will help remove some of the temperature independence that is seen in the dataset.
Correction factors
Although I agree that it would be very useful to ground the PINE observations with the historical record of filer-based methods, I don’t believe there is sufficient justification for combining INP datasets by including correction factors to bring them in line – to then look at something as fundamental as linking retreating sea ice with INP concentrations in the Arctic. The methodology is flawed and the current analysis does not belong in this paper.
From Section 4.4: "This integrated view frames the central methodological question of this section—how to place online expansion-chamber (PINE) and offline filter-based INP measurements on a common, quantitatively defensible scale — and both motivates and brackets the fixed harmonisation factor (CF = 16) derived in the remainder of this subsection." The simple answer is that they should not be manipulated and massaged to be put onto a common scale. If they do not match, then there are issues with one or more of the instruments, techniques, or analysis. I recommend the authors read Robinson et al. (2026) – they provide evidence for why PINE and other methods may give different results – especially at warmer temperatures where (for instance) the Wilbourn ENA comparison has the greatest discrepancy. Their results suggest there is a strong element of temperature-dependence to the differences, as well as from the INP species / material. If this is the case, then a systematic CF, as applied in this study, will not be able to account for these differences.
With regards to the justification in Section 4.4. “First, the PINE chamber samples a broader size and freezing-mode population than immersion-only offline filter assays”. What size ranges does PINE measure? How does this compare with filter-based sampling techniques? PINE has significant losses of particles around 5 micron (see Mohler et al., 2021 and Knopf et al., 2021 SI) – is this better than the filter-based methods? Is pore-condensation and deposition mode heterogenous nucleation truly likely to cause more freezing events in PINE than in other methods? This kind of statement needs to be supported by evidence.
Having said this – if the data below 3L-1 has been erroneously removed from the Araon datasets (see previous comment) then it might be that the PINE data falls closer to the filter-based data. I recommend the authors look into their data to ensure it is correct, then revisit this analysis to see whether CFs are even needed. I do not believe this paper will be publishable with its current use of correction factors.
Robinson et al., 2026: https://ar.copernicus.org/preprints/ar-2026-17/
Mohler et al, 2021: https://amt.copernicus.org/articles/14/1143/2021/
Knopf et al., 2021: https://doi.org/10.1175/BAMS-D-20-0151.1