Aircraft‐derived particle fluxes distinguish entrainment zone and decoupled layer nucleation in marine boundary layers

Rasheeda Satheesh, Ajmal; Petters, Markus D.; Meskhidze, Nicholas

doi:10.5194/egusphere-2026-61

Preprints

https://doi.org/10.5194/egusphere-2026-61

Preprints

15 Jan 2026

| 15 Jan 2026

Aircraft‐derived particle fluxes distinguish entrainment zone and decoupled layer nucleation in marine boundary layers

Ajmal Rasheeda Satheesh, Markus D. Petters, and Nicholas Meskhidze

Abstract. New particle formation (NPF) in marine boundary layers plays a critical role in cloud condensation nuclei (CCN) budgets and aerosol–cloud interactions, yet the vertical distribution of NPF sources remains poorly constrained. We identified the vertical location of NPF events by deriving turbulent fluxes of 3–10 nm particles from aircraft measurements during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) campaign. To overcome stationarity limitations of traditional eddy covariance methods, we applied continuous wavelet transform analysis to data collected during June–July 2017 and January–February 2018 flights over the Azores. Our flux‐based analysis revealed two distinct NPF scenarios with fundamentally different vertical structures and spatial extents. The first scenario featured nucleation in the entrainment zone, where free tropospheric air entrains into the boundary layer. The second scenario showed nucleation in the decoupled layer, a stratified region between the well‐mixed surface layer and cloud-topped upper boundary layer. Both cases exhibited strong downward particle fluxes driven by similar mechanisms: air masses from different layers and mixing, which diluted aerosols to very low particle surface area, creating favorable nucleation conditions. NPF occurred in 15 % of flights, challenging prevailing theoretical expectations that NPF should rarely occur in marine boundary layers due to high condensation and coagulation sinks from sea spray aerosols. Aircraft‐derived aerosol fluxes provide essential observational constraints on the vertical distribution and source strength of new particle formation in marine environments, enabling improved representation of these processes in climate models.

Received: 05 Jan 2026 – Discussion started: 15 Jan 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics. The peer-review process was guided by an independent editor, and the authors also have no other competing interests to declare.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 15547 KB)

Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
Preprint (15547 KB)

Supplement (5939 KB)

Download & links

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Journal article(s) based on this preprint

09 Jul 2026

Aircraft-derived particle fluxes distinguish entrainment zone and decoupled layer nucleation in marine boundary layers

Ajmal Rasheeda Satheesh, Markus D. Petters, and Nicholas Meskhidze

Atmos. Chem. Phys., 26, 9657–9678, https://doi.org/10.5194/acp-26-9657-2026,https://doi.org/10.5194/acp-26-9657-2026, 2026

Short summary

Ajmal Rasheeda Satheesh, Markus D. Petters, and Nicholas Meskhidze

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2026-61', Anonymous Referee #1, 04 Mar 2026

General Comments
Overall, the paper is well written and clear. The two areas where <10 nm particles were measured provide important evidence that new particles can form in these boundary layer regions where this phenomenon is relatively unexplored. I was glad to see that the authors made corrections for in-cloud sampling and time lags, which show they have experience with aircraft measurements and have taken extra care. However, there are a few additional measurements that should be further discussed to strengthen the overall conclusions, as detailed below.
Specific Comments
Was there no measure of coarse-mode concentrations that are expected to be small in the SPE regions? This should be discussed, given that one of the main points of the paper seems to be to dispute the idea that sea-spray surface area inhibits new particle formation. As a proxy, even windspeeds (which aren’t discussed except in the framework of flux measurements), might be helpful.
Also, it would be useful to show time series of aerosol surface area, at least in the sub-micron sizes, as that looks to be a potentially important change that occurs in the SPE regions. Fig 4 indicates almost no particles in the 10-1000 nm range in the second SPE period—is that correct, or is that just missing data? (Stot is shown and briefly discussed in the Fig 4 b-d vertical profiles, but these are not from the actual SPE periods.)
Lines 428-436: Were these cases when clouds were present above? If so, did you check to rule out drizzle, which would breakup in the inlet and cause artifacts? Below-cloud drizzle would be independent of LWC, which is derived from the CDP (smaller sizes). Also, since droplet number concentrations are never given, it’s unknown whether or not these were clean clouds, which are more likely to precipitate. The sometimes high (up to 1 g m-3) LWCs would suggest drizzle as a possibility. Call me skeptical, but I’ve enough experience with in-situ cloud measurements from aircraft to know that artifacts are common. This point, as well as some discussion of which of the four source regimes (lines 75-78) that the presented measurements represent would be useful.
Fig 3a and discussion on lines 412-413—are these occasional spikes in 3-10 nm particles above the cloud layer--e.g., 11:20-11:25--real? Should be discussed to assure that some of the signals noted below are not from entrainment of particles from above. Unlikely, since the concentrations below are sometimes higher, and these are short spikes, so perhaps they are artifacts of some sort?
Minor Issues
Lines 43-35: “This expectation is based on the relatively high surface area of sea spray aerosols, which act as condensation and coagulation sinks for nucleating vapors and newly formed particles”. There are plenty of accumulation-mode sulfate/organic particles in most MBLs that also may act as condensation and coagulation sinks. As do clouds themselves. This should be mentioned.
Perhaps move some of the lengthy data and analysis criteria in Section 2 to the supplement, where some of the associated graphs already are?
Lines 340-342: “We examine two flight days as case studies of SPEs observed at varying altitudes above the ocean. Additional supporting flights are presented in the Supplementary Information for each case.” After this you go immediately into Table 1 that shows six different flight days, which I found confusing. Perhaps discuss Table first and then go into the case studies.
Figure 4a caption: “The main panel shows size-resolved particle number concentrations (10-600 nm) from FIMS as a function of time and altitude, while N3-10 concentrations in the lower strip.” I think “are shown” is missing before “in the lower strip”.
I would suggest making 4a and 4b-d (and 7a and 7b-d) into separate plots, as these are really different from each other and this is too much information to easily assimilate in a single plot.
Fig 7 has a lot of missing data, so I’d suggest explicitly adding the sentence from Fig 4 caption to Fig 7 caption as well. This confused me until I figured it out. (“Gaps in the time series indicate the missing data.“)

Citation: https://doi.org/10.5194/egusphere-2026-61-RC1
- AC1: 'Reply on RC1', Nicholas Meskhidze, 18 May 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-61/egusphere-2026-61-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2026-61-AC1
RC2:
'Comment on egusphere-2026-61', Anonymous Referee #2, 20 Mar 2026

see attached review

Citation: https://doi.org/10.5194/egusphere-2026-61-RC2
- AC2: 'Reply on RC2', Nicholas Meskhidze, 18 May 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-61/egusphere-2026-61-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2026-61-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2026-61', Anonymous Referee #1, 04 Mar 2026

General Comments
Overall, the paper is well written and clear. The two areas where <10 nm particles were measured provide important evidence that new particles can form in these boundary layer regions where this phenomenon is relatively unexplored. I was glad to see that the authors made corrections for in-cloud sampling and time lags, which show they have experience with aircraft measurements and have taken extra care. However, there are a few additional measurements that should be further discussed to strengthen the overall conclusions, as detailed below.
Specific Comments
Was there no measure of coarse-mode concentrations that are expected to be small in the SPE regions? This should be discussed, given that one of the main points of the paper seems to be to dispute the idea that sea-spray surface area inhibits new particle formation. As a proxy, even windspeeds (which aren’t discussed except in the framework of flux measurements), might be helpful.
Also, it would be useful to show time series of aerosol surface area, at least in the sub-micron sizes, as that looks to be a potentially important change that occurs in the SPE regions. Fig 4 indicates almost no particles in the 10-1000 nm range in the second SPE period—is that correct, or is that just missing data? (Stot is shown and briefly discussed in the Fig 4 b-d vertical profiles, but these are not from the actual SPE periods.)
Lines 428-436: Were these cases when clouds were present above? If so, did you check to rule out drizzle, which would breakup in the inlet and cause artifacts? Below-cloud drizzle would be independent of LWC, which is derived from the CDP (smaller sizes). Also, since droplet number concentrations are never given, it’s unknown whether or not these were clean clouds, which are more likely to precipitate. The sometimes high (up to 1 g m-3) LWCs would suggest drizzle as a possibility. Call me skeptical, but I’ve enough experience with in-situ cloud measurements from aircraft to know that artifacts are common. This point, as well as some discussion of which of the four source regimes (lines 75-78) that the presented measurements represent would be useful.
Fig 3a and discussion on lines 412-413—are these occasional spikes in 3-10 nm particles above the cloud layer--e.g., 11:20-11:25--real? Should be discussed to assure that some of the signals noted below are not from entrainment of particles from above. Unlikely, since the concentrations below are sometimes higher, and these are short spikes, so perhaps they are artifacts of some sort?
Minor Issues
Lines 43-35: “This expectation is based on the relatively high surface area of sea spray aerosols, which act as condensation and coagulation sinks for nucleating vapors and newly formed particles”. There are plenty of accumulation-mode sulfate/organic particles in most MBLs that also may act as condensation and coagulation sinks. As do clouds themselves. This should be mentioned.
Perhaps move some of the lengthy data and analysis criteria in Section 2 to the supplement, where some of the associated graphs already are?
Lines 340-342: “We examine two flight days as case studies of SPEs observed at varying altitudes above the ocean. Additional supporting flights are presented in the Supplementary Information for each case.” After this you go immediately into Table 1 that shows six different flight days, which I found confusing. Perhaps discuss Table first and then go into the case studies.
Figure 4a caption: “The main panel shows size-resolved particle number concentrations (10-600 nm) from FIMS as a function of time and altitude, while N3-10 concentrations in the lower strip.” I think “are shown” is missing before “in the lower strip”.
I would suggest making 4a and 4b-d (and 7a and 7b-d) into separate plots, as these are really different from each other and this is too much information to easily assimilate in a single plot.
Fig 7 has a lot of missing data, so I’d suggest explicitly adding the sentence from Fig 4 caption to Fig 7 caption as well. This confused me until I figured it out. (“Gaps in the time series indicate the missing data.“)

Citation: https://doi.org/10.5194/egusphere-2026-61-RC1
- AC1: 'Reply on RC1', Nicholas Meskhidze, 18 May 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-61/egusphere-2026-61-AC1-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2026-61-AC1
RC2:
'Comment on egusphere-2026-61', Anonymous Referee #2, 20 Mar 2026

see attached review

Citation: https://doi.org/10.5194/egusphere-2026-61-RC2
- AC2: 'Reply on RC2', Nicholas Meskhidze, 18 May 2026
  
  The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-61/egusphere-2026-61-AC2-supplement.pdf
  
  Citation: https://doi.org/10.5194/egusphere-2026-61-AC2

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Nicholas Meskhidze on behalf of the Authors (18 May 2026) Author's response Author's tracked changes Manuscript

EF by Mario Ebel (18 May 2026) Supplement

ED: Referee Nomination & Report Request started (18 May 2026) by Radovan Krejci

RR by Anonymous Referee #2 (09 Jun 2026)

Suggestions for revision or reasons for rejection

The original manuscript has been extensively revised, which has definitely improved its quality and readability significantly. In some places, however, I would still choose my words more carefully, as one often gets the feeling that the authors are trying to give the impression that these observations are being made for the first time.
While other publications on this topic are mentioned, they are not really discussed in depth in the main discussion. However, this general criticism I still have should not detract from the effort the authors have put into the thorough data analysis and the revision of the first version. Below, I have a few specific minor revisions that should be considered before the final submission.

For my minor comments I refer to the line numbers of the “tracked changes” version!

Starting an abstract with a size range may not be the most elegant stylistic choice: Aerosol particles in the size range of...

Line 168: Please provide numbers instead of vague statements like “…with signal-to-noise characteristics suitable for detecting both remote continental and clean marine aerosol
concentrations..” Furthermore, this should be a question of absolute particle numbers due to Poisson statistics – right?

Line 180: What do you mean with “normal operating conditions” ? – Please specifiy! Furthermore, I don’t understand this argument concerning possible system malfunctions: the CPC measures in the size range between 10 nm up to about one micron whereas the FIMS is capturing particles between 10 and 600 nm. So I assume the CPC should always measure higher concentration compared to the FIMS – right? So why do you expect a malfunction when the CPC measures about 10% less compared to the FIMS? Sounds a little bit arbitrary…

I still have some reservations about the fact that Section 2.3.2 contains a discussion that relies entirely on figures in the appendix. In my view, while it makes sense to move lengthy explanations into a separate chapter in the appendix, simply moving the figures there just to keep the manuscript shorter isn’t ideal—but that’s for the publisher to decide.

Line 460: maybe a display issue but for me it reads: Where and are fitted parameters. => parameters are missing in the text. Within the following lines it seems there is a “copy-paste” issue or so. The definition of your LoD seems to be duplicated – right?

The table header line 649 should be separated from the text body —please check whether this issue occurs only in the version I am referring to here

In Fig 3d the parameter names at the y-axis are missing, furthermore, why do you display the LWC on a log-scale? There is no need because the PDF of LWC is not logarithmic.

Line 1072ff: I don't understand the reasoning that the strength of the turbulent particle flow is supposed to determine the distance between the aircraft and the source of nucleation—or have I misinterpreted that? Please explain it a little more clearly.

Line 1080 – 1083: This sentence is difficult to understand and somewhat speculative in parts. Please consider rephrasing it.

References. I was curious what Etling and Brown, 1993 contributed to this topic but I didn’t find an entry in the reference list – please carefully check all references again.

Hide

ED: Publish subject to minor revisions (review by editor) (10 Jun 2026) by Radovan Krejci

AR by Nicholas Meskhidze on behalf of the Authors (12 Jun 2026) Author's response Author's tracked changes Manuscript

ED: Publish subject to minor revisions (review by editor) (15 Jun 2026) by Radovan Krejci

Dear authors,
thank you for quick reply to last review.

Please address correctly this reviewer comment:
#2. Line 168: Please provide numbers instead of vague statements like “…with signal-to-noise
characteristics suitable for detecting both remote continental and clean marine aerosol
concentrations..” Furthermore, this should be a question of absolute particle numbers due to
Poisson statistics – right?

The reviewer asked for numbers and you have provided just generic formula how to obtain them.

Your statement indicate that there can be significicant discrepancies in performance of the CPC and FIMS
Line 180: IMS-derived number concentration also served as a quality control flag for the CPC 3772. Since both instruments share an overlapping detection size range (10–600 nm for FIMS;>10 nm for CPC 3772), their total number concentrations should be broadly comparable (with CPC showing a higher. The FIMS provides high temporal resolution measurements with excellent sensitivity and counting statistics required for aircraft-based studies (Olfert et al., 2008) total number concentration than FIMS) under a laboratory setting where all the variables are controlled. But the CPC
concentration was found to be suspect at times, likely because of issues with the working fluid, or a change in flow
rate that is controlled using a critical orifice.

How does it correspond to your earlier statement starting at Line # 157.
Both flow rates remained stable across the sampling altitude range (Zheng et al., 2021). The airborne CPC configuration was validated for operation up to 4000 m altitude and across ambient relative humidity conditions of 0–90% RH. For a typical polluted environment (~ 5000 cm−3), CPC concentration measurements had an accuracy of 0.3 % (Kuang and Mei, 2019). All data used in this study passed instrument mentor specified quality control filters, which are distributed alongside the data.

Based on this, your instruments should perform much better than 10% margin. Can you please provide histogram distribution of the difference between integral FIMS and total CPC?

Thank you

Hide

AR by Nicholas Meskhidze on behalf of the Authors (22 Jun 2026) Author's response Author's tracked changes Manuscript

ED: Publish as is (23 Jun 2026) by Radovan Krejci

AR by Nicholas Meskhidze on behalf of the Authors (26 Jun 2026)

Journal article(s) based on this preprint

09 Jul 2026

Aircraft-derived particle fluxes distinguish entrainment zone and decoupled layer nucleation in marine boundary layers

Ajmal Rasheeda Satheesh, Markus D. Petters, and Nicholas Meskhidze

Atmos. Chem. Phys., 26, 9657–9678, https://doi.org/10.5194/acp-26-9657-2026,https://doi.org/10.5194/acp-26-9657-2026, 2026

Short summary

Ajmal Rasheeda Satheesh, Markus D. Petters, and Nicholas Meskhidze

Supplement

https://doi.org/10.5194/egusphere-2026-61-supplement

Ajmal Rasheeda Satheesh, Markus D. Petters, and Nicholas Meskhidze

Viewed

Total article views: 2,725 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	Supplement	BibTeX	EndNote
1,454	1,138	133	2,725	342	112	124

HTML: 1,454
PDF: 1,138
XML: 133
Total: 2,725
Supplement: 342
BibTeX: 112
EndNote: 124

Views and downloads (calculated since 15 Jan 2026)

Month	HTML	PDF	XML	Total
Jan 2026	591	385	72	1,048
Feb 2026	279	307	14	600
Mar 2026	425	309	35	769
Apr 2026	82	44	2	128
May 2026	48	58	6	112
Jun 2026	6	9	1	16
Jul 2026	23	26	3	52

Cumulative views and downloads (calculated since 15 Jan 2026)

Month	HTML	PDF	XML	Total
Jan 2026	591	385	72	1,048
Feb 2026	279	307	14	600
Mar 2026	425	309	35	769
Apr 2026	82	44	2	128
May 2026	48	58	6	112
Jun 2026	6	9	1	16
Jul 2026	23	26	3	52

Viewed (geographical distribution)

Total article views: 2,709 (including HTML, PDF, and XML) Thereof 2,709 with geography defined and 0 with unknown origin.

Country	#	Views	%

Cited

Latest update: 29 Jul 2026

Download

The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.

Preprint (15547 KB)
Metadata XML

Short summary

Using aircraft data collected over the Atlantic Ocean, we discovered that new particles form regularly in marine air at two different heights, challenging the belief that sea salt prevents this process. These particles can grow to form cloud droplets, affecting Earth's climate. We found particles forming at two heights: where upper atmosphere air enters the ocean boundary layer and where the well-mixed surface air meets the stable air layer above. This discovery will help improve climate models.


Total:	0
HTML:	0
PDF:	0
XML:	0

Aircraft‐derived particle fluxes distinguish entrainment zone and decoupled layer nucleation in marine boundary layers

Journal article(s) based on this preprint

Interactive discussion

Interactive discussion

Peer review completion

Suggestions for revision or reasons for rejection

Journal article(s) based on this preprint

Supplement

Viewed

Viewed (geographical distribution)

Cited

1 citations as recorded by crossref.