the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Feb 2025
Particle flux-gradient relationships in the high Arctic: Emission and deposition patterns across three surface types
Abstract. The Arctic is experiencing a warming much faster than the global average, and aerosol-cloud-sea-ice interactions are considered to be one of the key features of the Arctic climate system. It is therefore crucial to identify particle sources and sinks to study their impact on cloud formation and cloud properties in the Arctic. Near-surface particle and sensible heat fluxes were measured using the gradient method during the ARTofMELT Arctic Ocean Expedition 2023. A gradient system was deployed to calculate sensible heat and particle fluxes over three different surface conditions: wide lead, narrow lead, and closed ice. To evaluate the gradient measurements, sensible heat fluxes and friction velocities were compared with eddy covariance data. The strongest sensible heat fluxes, ranging from 24 W m−2 to 70 W m−2, were observed over wide lead surfaces, aligning with measurements from the icebreaker. In contrast, closed ice surfaces had weak, often negative sensible heat fluxes. Wide leads acted as a particle source, with median net particle emission fluxes of 0.09 106 m−2 s−1. Narrow lead surfaces exhibited both net emission and net deposition, though the particle fluxes were weaker. Closed ice surfaces acted as a particle sink, with normalized fluxes around 0.06 cm s−1. The gradient method was found to be effective for measuring both sensible heat and particle fluxes, allowing flexible deployment over different surface types. This study addresses the critical need for improved quantification of vertical turbulent particle fluxes and related processes that influence the local particle number budget in the central Arctic.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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 preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2025-183', Anonymous Referee #1, 09 Mar 2025
General comment
The paper reports measurements of particle fluxes using a gradient approach in Artic to investigate emission and deposition over different surfaces. Measurements were done during the ARTofMEL expedition in an environment that is difficult to characterise in terms of particle fluxes being challenging for the measurement setup. I believe that the results are interesting and may be of interest for the scientific community. There are a few aspects that should be improved as detailed in my specific comments.
Specific comments
It is used to mention the normalised flux that is essentially what is indicated in other studies as deposition velocity. Why not using the more common deposition velocity?
It seems that when it is mentioned net deposition or net emission it is referred to a single 20-minute period, it may create confusion with long-term average of fluxes.
In equations (1) to (5) it is used the capital U’ for fluctuations while this was not done for other velocity components, why?
Line 45. I would say ideally 10 Hz because very often it is a lower resolution, also in the EC measurement here. I also suggest to mention that in EC measurements involving particles, or more in general closed path sensors, it is important the first order time response of the inlet rather than the sampling frequency, because this is often a more limiting factor for fast instruments see for example the discussion in Conte et al (2018, Science of the Total Environment 622, 1067-1078).
Figure 2. It would be interesting to add the comparison among the two EC systems, ice mast and ship mast to discuss is the differences are due to the different location or to the different methods (EC and gradient). The same for Figure 3. Do you have an interpretation on why the comparison for H is significantly worse than that for u*?
Line 244. The uncertainties of fluxes are quite high, it would be useful a comment if this is enough to have a robust measurement.
Table 2. Better to write 0.03-0.04 in the first raw and 0.005 in the second because the inerval 0.05-0.05 is not clear.
Figure 6. What do you mean with normalised concentrations? Why not showing the size distribution with the typical normalisation using dLog?
Citation: https://doi.org/10.5194/egusphere-2025-183-RC1 -
RC2: 'Comment on egusphere-2025-183', Thomas Foken, 16 Mar 2025
This publication investigates particle deposition (emission) over ice surfaces and thus an extremely important problem of changes in surface albedo and possible influences on the Arctic climate. The measurement concept corresponds to the current technical possibilities and the authors are recognised experts in this field. The measurement under Arctic conditions is a particular challenge. The theory used is state of the art, but the restriction to neutral stratification (which may not really correspond to reality after all) would not have been necessary, as the curvature of the gradients can certainly be taken into account by universal functions when determining the gradient (Foken and Mauder, 2024).
The general verification of the systems by determining the friction velocity is very useful and should also be used for further classification of the measurements if necessary. The deviations shown in Fig. 2a are typical for the gradient-eddy-covariance comparison, but in Fig. 2b the measurements should be labelled with a different symbol if there are significant differences in the footprint of the two systems or if the eddy mast is located to leeward of the ship.
The reviewer cannot follow the discussion of the results of the sensible heat flux (Fig. 3 and 4). The gradient mast does not have a uniform footprint, i.e. the lowest height has a very small footprint which is probably exclusively ice in all situations. This means that the temperature is also very low in all situations and may ‘simulate’ stable stratification. This can be seen very clearly with ‘Narrow Lead’, where the eddy-covariance measurements correctly show a positive sensible heat flux, while the gradient mast always indicates stable stratification.
The situation becomes even more problematic with ‘Closed Ice’. The measurements are increasingly stable and can in no way be assigned to the neutral range (however, the specified range for z/L is also very narrowly defined). The gradient mast in particular measures relatively large downward sensible heat fluxes, i.e. the gradient is comparatively large. Various phenomena such as decoupling, counter-gradients and coherent structures occur particularly at very low friction velocities (Foken, 2023;de La Casinière, 1974;Grachev et al., 2005;Sodemann and Foken, 2005;Lüers and Bareiss, 2010). To discuss the data, they should be categorised into u* classes. With regard to the interpretation of the particle fluxes, fluxes with u*<0.10... 0.15 m/s must probably be excluded after this investigation. The discussion of all the phenomena mentioned is too complicated and the data set only allows this in part. It may be possible to estimate the possibility of decoupling with the Brunt-Väisälä frequency (Foken, 2023;Peltola et al., 2021).
The conditions of the sensible heat flux naturally influence the particle gradient in the same way. At the very least, the proposed classification should be adopted. In a further study, it might be useful to investigate whether particles accumulate in the shallow layer above the ice in the event of decoupling. The layer is probably emptied of particles again with a short-term emission event. Perhaps Fig. A5 should be included in the text and compared with Fig. 3.
In the conclusions, one would have to answer the question of why the ice surface is a sink for particles. Is the cause the surface itself or the stable stratification predominantly found there? Perhaps it is possible to subdivide the results into 2-3 stability classes (z/L) based on the eddy covariance data.
As is often the case with experimental studies, there are more questions at the end than were solved by the experiment. Thus, the manuscript should only be revised very carefully to the extent absolutely necessary, but problems should be pointed out. Possibly the discussion of the questions raised should be dealt with in another article.
Minor comments:
Line 148ff: Normalised size distribution should be defined or explained like all other normalisations.
Line 490: Please replace Foken (2017) with Foken and Mauder (2024)
References
de La Casinière, A. C.: Heat Exchange over a Melting Snow Surface, J. Glaciol., 13, 55-72, doi: 10.3189/S0022143000023376, 1974.
Foken, T.: Decoupling between the atmosphere and the underlying surface during stable stratification, Boundary-Layer Meteorol., 187, 117-140, doi: 10.1007/s10546-022-00746-1, 2023.
Foken, T., and Mauder, M.: Micrometeorology, 3 ed., Springer, Cham, XXI, 410 pp., doi: 10.1007/978-3-031-47526-9, 2024.
Grachev, A. A., Fairall, C. W., Persson, P. O. G., Andreas, E. L., and Guest, P. S.: Stable Boundary-Layer Scaling Regimes: The Sheba Data, Boundary-Layer Meteorol., 116, 201-235, doi: 10.1007/s10546-004-2729-0, 2005.
Lüers, J., and Bareiss, J.: The effect of misleading surface temperature estimations on the sensible heat fluxes at a high Arctic site – the Arctic Turbulence Experiment 2006 on Svalbard (ARCTEX-2006), Atmospheric Chemistry and Physics, 10, 157-168, doi: 10.5194/acp-10-157-2010, 2010.
Peltola, O., Lapo, K., and Thomas, C. K.: A physics-based universal indicator for vertical decoupling and mixing across canopies architectures and dynamic stabilities, Geophys. Res. Letters, 48, e2020GL091615, doi: https://doi.org/10.1029/2020GL091615, 2021.
Sodemann, H., and Foken, T.: Special characteristics of the temperature structure near the surface, Theor. Appl. Climat., 80, 81-89, doi: 10.1007/s00704-004-0092-1, 2005.
Citation: https://doi.org/10.5194/egusphere-2025-183-RC2 -
CC1: 'Comment on egusphere-2025-183', Piotr Markuszewski, 28 Mar 2025
Comment on the preprint
“Particle flux-gradient relationships in the high Arctic: Emission and deposition patterns across three surface types”
by Piotr Markuszewski1,2 and Monica Mårtensson2
- Institute of Oceanology, Polish Academy of Sciences
- Stockholm University
- Uppsala University
This manuscript presents a well-executed field study investigating near-surface aerosol particle fluxes and sensible heat fluxes over different Arctic surface types using a novel gradient-based measurement system during the ARTofMELT expedition. The comparison with eddy covariance data enhances the credibility of the measurements. The study provides valuable insight into particle source/sink behavior over wide leads, narrow leads, and closed ice, contributing to our understanding of aerosol-cloud-sea-ice interactions in the central Arctic.
The paper is timely, methodologically sound, and clearly structured, but there are several areas where the manuscript could be improved for clarity, completeness, and scientific robustness.
The manuscript would benefit from a significantly broader and more critical engagement with the prior literature on gradient-based flux measurements, particularly over marine and ice surfaces. While the authors cite several key studies related to Arctic fluxes (e.g., Nilsson et al. 2001, Held et al. 2011a,b), the discussion omits earlier foundational work using gradient methods to estimate aerosol and heat fluxes in polar and marine environments.
For instance, the first gradient-based aerosol flux measurements conducted by Petelski (2003), Petelski et al. (2005), and further refined in Petelski and Piskozub (2006), including the later comment given by Andreas (2007), are highly relevant and should be referenced. Additionally, Savelyev et al. (2014) addressed fluxes under low-turbulence regimes using similar profile techniques, which is particularly relevant for the stable and weakly turbulent conditions encountered over closed ice. Authors may also find recent publication dedicated to the gradient method (Markuszewski et al., 2024).
In its current form, the manuscript gives the impression that the gradient method is underexplored in this field, which is not accurate. A proper contextualization would strengthen the justification for the study, allow a more meaningful comparison of uncertainties and assumptions, and acknowledge the methodological evolution within the field of flux-gradient applications.
The manuscript lacks of presentation of the raw or processed aerosol concentration profiles that underpin the flux-gradient calculations. Since the fluxes are derived from linear regressions across vertical gradients of particle number concentrations, it is essential to show representative examples of these vertical profiles to assess the validity of the method. Including a few example profiles—either in the main text or as supplementary material—would serve as a critical demonstration that the gradient system performs as intended. For instance, plots showing aerosol number concentrations at each height, along with fitted regression lines and R² values, would give readers confidence in the robustness of the derived fluxes. It is also unclear what the range of correlation coefficients (e.g., R² of the linear fit) was across the dataset, or how often profiles were rejected due to poor fits. Providing a histogram or table of regression diagnostics (e.g., slope, R², residuals) would help clarify the quality and reliability of the profiles used in flux calculations. Furthermore, it would be useful to include a discussion of profile curvature, measurement noise, or transient concentration spikes, and how these were addressed during preprocessing and averaging. Without this level of transparency, the central assumption—that vertical particle concentration gradients are well-defined and resolved—is insufficiently supported.
Another methodological weakness stems from the limited footprint characterization and the difference between systems used for intercomparison. Yet, the paper compares their fluxes as though they were co-located. While some differences are acknowledged qualitatively, there is no attempt to assess or estimate the footprints, nor to indicate whether the observed discrepancies fall within expected spatial variability. The authors should include either a footprint model (even a simplified 1D footprint estimate based on surface roughness and stability) or a discussion of fetch dependence to justify the comparability of the datasets. This is particularly important for interpreting disagreements in flux direction and magnitude over mixed or narrow lead surfaces.
The uncertainty estimation methodology, though commendable in its use of Monte Carlo simulations, underrepresents potential systematic errors. For example, the correction for tubing losses is based on size distribution measurements from a ship-based DMPS located several meters above the sea surface, which may not accurately represent the near-surface environment sampled by the gradient system. Variability in vertical gradients of particle size, especially under stratified conditions or during local emissions, could lead to an incorrect penetration fraction estimate and a biased flux. A sensitivity analysis showing how variations in assumed size distributions affect the loss correction would greatly improve transparency. Additionally, more details on the impact of inlet length, orientation, and isokinetic sampling conditions should be included to assess sampling biases under varying wind regimes.
The use of Monte Carlo simulations to estimate uncertainty in particle and sensible heat fluxes is a commendable choice; however, the manuscript lacks a sufficiently detailed explanation of how this method was applied specifically in the context of aerosol flux measurements. In aerosol science, uncertainty propagation is particularly sensitive to input data variability, non-linearity in particle losses, and the low signal-to-noise ratios often encountered in Arctic conditions. The authors state that 10,000 random values were generated per profile, but it is unclear whether the distributions used were strictly Gaussian, whether input variances were assumed to be independent across heights, or how temporal autocorrelation in measurement noise was handled. Furthermore, the choice to summarize daily uncertainties using the 90th percentile of simulated fluxes lacks justification—was this percentile empirically chosen, or based on prior studies? Overall, a clearer explanation of the statistical assumptions, potential bias sources, and limits of the method’s sensitivity is needed to allow readers to evaluate how well the simulation captures real-world variability and uncertainty in the derived fluxes.
The comparison of friction velocity estimates between the gradient system and eddy covariance (Figure 2) shows substantial scatter, yet the potential causes of these discrepancies are not sufficiently explored. In particular, it is unclear whether inertial motion correction was applied to the eddy covariance measurements on the ship mast. Motion-induced errors are known to bias vertical velocity estimates on moving platforms, and the absence of an inertial measurement unit (IMU) or equivalent correction could lead to systematic over- or underestimation of u*, especially under low turbulence. I recommend that the authors clarify whether motion correction was applied and, if not, discuss this as a potential limitation. In addition, the eddy covariance post-processing methods (e.g., coordinate rotation, averaging strategy) should be described more explicitly, and their influence on the u* estimates should be assessed. Without such information, the comparability of the methods remains questionable, particularly over complex surfaces like narrow leads where the surface heterogeneity is likely to exacerbate footprint mismatches.
In terms of data analysis, the manuscript occasionally uses ambiguous terminology when referring to “net emission” and “net deposition”. It is not always clear whether these terms refer to statistically significant fluxes (i.e., exceeding uncertainty bounds) or to the algebraic sign of the estimated flux. In several figures, flux values that lie within the uncertainty range are still color-coded as deposition or emission. I suggest the authors define a threshold for meaningful flux detection (e.g., based on the daily maximum error estimate) and clearly distinguish between statistically significant fluxes and those near the noise level. This would reduce the risk of overinterpreting marginal cases.
Throughout the manuscript, the authors report both absolute particle fluxes in m⁻²·s⁻¹ and normalized fluxes in cm·s⁻¹, which is a standard approach in aerosol micrometeorology. However, the manuscript lacks a clear explanation of why and when each form is used. The definition of normalized flux (V_D = –P/C) is provided, but the rationale for expressing it in [cm/s] rather than [m/s] is not discussed. This unit switch can be confusing, particularly since figure axes and captions do not always indicate units explicitly. I recommend that the authors (1) clearly define both flux formats early in the Methods or Data Analysis section, (2) justify the choice of units for normalized flux, and (3) ensure that all figure axes and table entries explicitly label the flux units used. A brief explanation of the comparability advantage of normalized fluxes (especially under variable concentration regimes) would also strengthen the interpretation of results.
While the authors interpret many of the observed concentration and flux patterns in terms of local surface type (wide lead, narrow lead, closed ice), they do not apply any trajectory analysis to objectively evaluate the origin of the sampled air masses. This is a significant limitation. In polar environments, long-range transport can dramatically influence background particle concentrations, size distributions, and chemical composition. The manuscript refers to air mass changes (e.g., "southerly winds bringing humid air") and suggests terrestrial influence from Greenland or Svalbard, but these assertions are speculative without the support of backward trajectory modeling (e.g., HYSPLIT, FLEXPART). Including at least a qualitative or cluster-based trajectory analysis would strengthen the interpretation of aerosol variability and help distinguish between local emission/deposition processes and transported signals. I recommend the authors include such an analysis or, at minimum, acknowledge this limitation in the discussion section.
In summary, this manuscript presents an important contribution to the field of Arctic micrometeorology and aerosol flux measurements. The deployment of a mobile gradient flux system under challenging field conditions, combined with extensive observational data across varied surface types, offers valuable insights into particle exchange processes in the central Arctic. However, to ensure scientific rigor and broader relevance, the manuscript requires substantial revision. Key areas for improvement include deeper engagement with prior literature on gradient methods, clarification of methodological details (particularly around uncertainty estimation and regression quality), and more cautious interpretation of flux patterns in the absence of chemical or trajectory data. Provided that these issues are thoroughly addressed, I believe the paper will make a strong and meaningful contribution and would ultimately be worthy of publication.
We fully understand that this is a comment in the discussion and that further development of the manuscript may already be underway. Nevertheless, we would be very grateful if the authors would consider addressing at least some of the issues and suggestions raised here. We believe that doing so would significantly enhance the clarity, robustness, and scientific value of this already promising contribution.
Piotr Markuszewski and Monica Mårtensson
References
Andreas, E. L.: Comment on “Vertical coarse aerosol fluxes in the atmospheric surface layer over the North Polar Waters of the Atlantic” by Tomasz Petelski and Jacek Piskozub, J. Geophys. Res.-Oceans, 112, C11, https://doi.org/10.1029/2007JC004191, 2007.
Held, A., Brooks, I. M., Leck, C., and Tjernström, M.: On the potential contribution of open lead particle emissions to the central Arctic aerosol concentration, Atmos. Chem. Phys., 11, 3093–3105, https://doi.org/10.5194/acp-11-3093-2011, 2011a.
Held, A., Orsini, D. A., Vaattovaara, P., Tjernström, M., and Leck, C.: Near-surface profiles of aerosol number concentration and temperature over the Arctic Ocean, Atmos. Meas. Tech., 4, 1603–1616, https://doi.org/10.5194/amt-4-1603-2011, 2011b.
Markuszewski, P., Nilsson, E. D., Zinke, J., Mårtensson, E. M., Salter, M., Makuch, P., and Piskozub, J.: Multi-year gradient measurements of sea spray fluxes over the Baltic Sea and the North Atlantic Ocean, Atmos. Chem. Phys., 24, 11227–11253, https://doi.org/10.5194/acp-24-11227-2024, 2024.
Nilsson, E. D., Rannik, Ü., Swietlicki, E., Leck, C., Aalto, P. P., Zhou, J., and Norman, M.: Turbulent aerosol fluxes over the Arctic Ocean: 2. Wind-driven sources from the sea, J. Geophys. Res.-Atmos., 106, 32139–32154, https://doi.org/10.1029/2000JD900747, 2001.
Petelski, T.: Marine aerosol fluxes over open sea calculated from vertical concentration gradients, J. Aerosol Sci., 34, 359–371, https://doi.org/10.1016/S0021-8502(02)00191-9, 2003.
Petelski, T., and Piskozub, J.: Vertical coarse aerosol fluxes in the atmospheric surface layer over the North Polar Waters of the Atlantic, J. Geophys. Res.-Oceans, 111, C06039, https://doi.org/10.1029/2005JC003295, 2006.
Petelski, T., Piskozub, J., and Paplińska‐Swerpel, B.: Sea spray emission from the surface of the open Baltic Sea, J. Geophys. Res.-Oceans, 110, C10023, https://doi.org/10.1029/2004JC002800, 2005.
Savelyev, I. B., Anguelova, M. D., Frick, G. M., Dowgiallo, D. J., Hwang, P. A., Caffrey, P. F., and Bobak, J. P.: On direct passive microwave remote sensing of sea spray aerosol production, Atmos. Chem. Phys., 14, 11611–11631, https://doi.org/10.5194/acp-14-11611-2014, 2014.
Citation: https://doi.org/10.5194/egusphere-2025-183-CC1
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