the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 May 2026
Frequent occurrence of newly formed aerosol particles over wide geographical areas in the Arctic free troposphere and atmospheric boundary layer
Abstract. New particle formation (NPF) can impact the Arctic radiative energy budget since this region is particularly sensitive to changes in aerosol particle and cloud condensation nuclei concentrations. Prior studies have predominantly investigated NPF in the Arctic atmospheric boundary layer (ABL), concluding that this phenomenon primarily takes place close to the surface. However, this study shows that NPF may take place throughout the entire lower Arctic troposphere. We have reached this conclusion by analyzing particle number size distribution data collected during a springtime aircraft campaign in the vicinity of Svalbard, the Fram Strait, and northern Greenland. We detected newly formed aerosol particles at various altitudes ranging from about 60 m to 3900 m and identified three atmospheric scenarios for their occurrence: newly formed particles in the free troposphere, in the ABL over sea ice, and in the vicinity of clouds. Our results suggest that regional Arctic atmospheric processes as well as long-range transport play key roles in the formation of new particles. Based on our data, we furthermore conclude that NPF may be a frequent and geographically extended phenomenon in the Arctic free troposphere.
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Status: open (until 23 Jun 2026)
- CC1: 'Comment on egusphere-2026-2215', Farahnaz Khosrawi, 29 May 2026 reply
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RC1: 'Comment on egusphere-2026-2215', Anonymous Referee #1, 16 Jun 2026
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The authors present rare airborne measurements of aerosol and meteorological parameters over the Arctic ocean. Quantifying the relative role of the various sources of Arctic cloud condensation nuclei (CCN) is a challenging task, but necessary in order to improve model uncertainty. Overall, the paper is well-written and I think the results of this study are a valuable contribution to the field as they provide insight into the potential role of new particle formation over the sea ice surface as well as aloft.
However, I believe more transparency is needed regarding the methodology of the study in order for the results to be interpreted correctly by the reader. My suggestion is to consider this manuscript for publication after major revisions, as I believe significant changes need to be made. Most of the changes involve including analysis into the manuscript that I am pretty sure the authors have already conducted, but not shown, so the work needed is probably not extensive. I also think that there are some claims that need to be toned down, rephrased, or further contextualized, as the results of this study are not enough to support them.
General comments:
The entire analysis leans on the identification method of newly formed particles, which is nicely demonstrated and explained - in the SI. I absolutely believe that this explanation and exemplary figure are essential parts of the methods and thus belong in the actual manuscript, to convince the reader you have in fact correctly identified particles sourced by new particle formation (NPF).
The statistics of the observations are only poorly described, leaving an important lack of context for the reader and the potential risk for miss- and overinterpretation. In section 3.1 I suggest adding how much flight time was spent in the boundary layer (BL) and free troposphere (FT) respectively, at what frequency newly formed particles (NFP) were identified in the BL (regardless of surface type) and FT, and then the relative frequency of NFP over sea ice and in the vicinity of clouds. It is important to show to the reader how you identified the 3 atmospheric environments considered. Furthermore, it is stated more than once that no NFP were observed in the BL over open ocean, but this is kind of contradicted in the SI when explaining the 14% non-assigned observations. In figure 1 it is clear NFP were observed over open ocean, was this then not in the BL? More importantly, how much time was actually flown over the open ocean inside the BL? Again, Figure 1 indicates not much. Please clarify and revise, as this is stated as one of your conclusions.
The manuscript would benefit from a bit more discussion on sources of precursors. The seasonal aspect should be considered, the observations were made in early spring, has biological productivity in the ocean (and presumed production of precursor gases) begun? Have you checked e.g satellite products? The main conclusions should also be set in this context for future reference.
Figure 2d in its current form is highly misleading. Since the measurement time is not evenly distributed across altitudes, nor concurrent in time or space, these profiles do not make sense. If you want to show vertical distributions of number concentrations, I think it should be done in a way where the reader can distinguish between flights and where the number of observations (or flighttime) is reflected. However, the figure is barely mentioned and not discussed in the text, so it could also be omitted.
Detailed comments:
Line 34: I think a correlation should rather be referred to as indirect evidence.
Line 36: See also Heslin-Rees et al, 2020 (https://doi.org/10.5194/acp-20-13671-2020 ) who showed that changes in transport patterns into the Arctic increased the marine influence on the aerosol population, rather than sea ice retreat.
Line 59: I interpret Pilz et al (2024) in the exact opposite way; free tropospheric sources of CCN were more common. Please revisit.
Lines 82-83: Please add whether the aerosol data have also been corrected for diffusion and inertial deposition or not.
Line 114: I suggest removing the word frequently. In line 121 you state it was 24% of the flight time that you identified NFP, whether that is frequent or not is not obvious but subjective.
Line 129: “Newly formed particles detected in the free troposphere account for the majority of our observations” is a misleading statement and can be used to misquote this study. 45% of all PNSD featuring newly formed particles is actually not even the majority of those cases (that would require >50%), and definitely not the majority of your observations.
Line 130-132: The percentages presented here lack context, they do not constitute a result without knowing how much flight time was spent in each category.
Figure 2c: How many scans do the non-hatched bars represent? Preferably add the number for each bar, or at least include a minimum and mean value.
Line 142-146: I believe this is the only reference to figure 2d. Mentioning the peaks at 1000 and 3000 m should be followed by discussion, how much observational time does this reflect? Are these peaks representative? In figure 2b it looks like the NPF- occurrence was not very long at these heights.
Line 195: Were researchers led to believe that? In recent years, a lot of effort has been put into making vertical measurements of aitken and nucleation mode particles (e.g Pohorsky et al (preprint at https://egusphere.copernicus.org/preprints/2026/egusphere-2026-1068/), need for vertical measurements discussed e.g in Schmale et al, 2021). Would this be the case if the research community believed it wasn’t important? Near-surface measurements are more accessible, and have therefore been conducted to a higher degree.
Line 197-207: The information given here is highly relevant, but I question the conclusion of this section. The cited Weber study observed one NPF events during one in 38 flights, at 4200m. Why do you say that your results (in contrast?) indicate that NPF occurs frequently in the Arctic FT, based on the 11 flights of which two above 3200m if I understand correctly? If you had not encountered NFP during these flights, would you have concluded that NPF may not occur at that height at all? I don’t think so. Similarly, you can also not claim that it happens all the time because you observed it. Overall, I think that a discussion on where NPF “primarily occurs” in the Arctic can not be settled based on this limited set of measurements.
Line 244: See also Khadir et al, 2023 (https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2023GL104325) for the link between NPF and precipitation found at Zeppelin Observatory. To me, it looks like something happens specifically around 24-30 hours back for the NFP trajectories, particularly in panel 5b,d,e,f,g and i. I think this could be investigated or at least discussed further.
LIne 346: In the ABL case study, figure 7c, The number concentration varied a factor of 2 within 30 minutes, a few times during the flight period. In figure 10c, the number concentration at cloud top is clearly actually the same as at cloud base at first, and goes up after a couple of scans. How can you be sure that the difference you see is actually because you are at cloud base or top, respectively, and not spatial/temporal variability?
Figure 12: Why not show the no NFP cases like in Figure 5?
Line 390: Context missing, how much flight time over ocean and what would you expect during this season?
Line 398: Where are the clear indications? I believe you have 2 arguments for this, one measurement of higher concentrations of small particles at cloud top (though not consistent throughout the measurement period) and a reference that cloud tops provide favourable conditions for NPF. I disagree that they constitute clear indications.
Lines 404-405: I think the measurements you present, and the analysis of the 3 scenarios are unique and highly valuable to this research field. However, I think this statement is not really grounded in your data. There is no reason to believe that NPF in the Arctic FT is not geographically widespread, but these measurements also don’t prove that they are. Also, what is geographically widespread? Be more precise, or less strong.
The figures 5, 2d, 8, 11 and 12 are difficult to read when printed out because of the grid. Perhaps it can be made softer or sparser.
Citation: https://doi.org/10.5194/egusphere-2026-2215-RC1
Data sets
Master tracks in different resolutions during POLAR 6 campaign P6-247_BACSAM2_2024 Z. Jurányi et al. https://doi.org/10.1594/PANGAEA.971763
Five-day backwards trajectories at one minute resolution along the flight tracks of the Polar 6 research aircraft during BACSAM II B. Kirbus and M. Wendisch https://doi.org/10.1594/PANGAEA.971694
Airborne in-situ measurements of aerosol particle number size distributions during the BACSAM II campaign in April 2024 D. J. Simon et al. https://doi.pangaea.de/10.1594/PANGAEA.993546
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- 1
I would like to raise the attention of the authors to the following study which I published a while ago on new particle formation in the Arctic free troposphere:
Khosrawi, F., Ström, J., Minikin, A., and Krejci, R.: Particle formation in the Arctic free troposphere during the ASTAR 2004 campaign: a case study on the influence of vertical motion on the binary homogeneous nucleation of H2SO4/H2O, Atmos. Chem. Phys., 10, 1105–1120, https://doi.org/10.5194/acp-10-1105-2010, 2010.
It would be worth mentioning this study in the introduction on e.g. P2, L50 and/or the paragraph below. Our study analysed measurements from the ASTAR 2004 campaign and frequent newly formed particles in the free troposphere up to 4000 m were there also reported.
On P6, L135 and/or 157: The Khosrawi et al. (2010) study may be mentioned here as well.
P9, L197: Here, it would be definitely worth to mention here the Khosrawi et al. (2010) study where we investigated a case where newly formed particles were observed at 7000 m.
Additional comments I have on your manuscript:
P3, L84: The distinction of NPF and newly formed particles is here a bit weird since reading the sentence it feels like the same. You later explain the difference and then it becomes clear. My suggestion would be to move this sentence after explaining the difference between NPF and newly formed particles.
P14, L314: Is descent here really correct? If precursors were emitted from the surface, I would expect that these are then transported upwards, thus that you observe an ascent of air masses.
P14, L316-318: Isn' that a bit too speculative? Are there any measurements or other studies that could support this hypothesis?
P18, 382: Add "free" so that it reads "Arctic free troposphere"?
P20, L405: I am not convinced of this statement. Why do you think that NPF could be a geographically widespread phenomen? How do you see that in your data/analysis? Aren't you focusing on a certain part of the Arctic (a certain distance around the flights that are only a minor part of the entire Arctic). However, why shouldn't NPF occur everywhere in the Arctic? There are so many possibilities to form new particles so that I would assume that NPF can occur nearly everywhere in the Arctic. Further, how you show the connection to warm air intrusions would be worth to be discussed/repeated here a bit more.
P20, L400ff: Are there already any studies or measurements that investigate the trends or changes in NPF due to climate change?