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
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