the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 10 Apr 2026
The ICOS Ecosystem Station Loobos: a pine forest site exposed to atmospheric pollution
Abstract. The Loobos Ecosystem Station (ICOS: NL-Loo) is in a pine forest on sandy soil, the most dominant forest type in the Netherlands. The station was first built in 1995 and the first atmospheric flux measurements were taken in 1997. The station was one of the first in EuroFlux and FLUXNET. Between 2021 and 2023, the station was rebuilt to meet the ICOS standards as an Ecosystem Class 2 station. The initial purpose of the station was to measure fluxes of heat, water and carbon dioxide. Over time, interest has increased in better understanding the forest water use and ecosystem dynamics and its interaction with air quality. This has resulted in additional concentration and flux measurements of ozone and volatile organic compounds. In the near future, we intend to include measurements of nitrogen species (ammonia and nitrogen oxides) to exploit Loobos’ unique location downwind of major anthropogenic emission hotspots. Documenting the instrumentation and exploring the quality of the data is important to understand the observational data. This paper therefore describes the station in terms of geography, ecosystem and instrumentation.
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Status: final response (author comments only)
- RC1: 'Comment on essd-2026-99', Anonymous Referee #1, 23 Jun 2026
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RC2: 'Comment on essd-2026-99', Simone Sabbatini, 13 Jul 2026
First of all no need to be anonymous: I am Simone Sabbatini working at the ICOS Ecosystem Thematic Centre. Knowing my role can be useful to understand my perspective on this station and its data.
The authors provide an interesting overview of the measurements taken at the Loobos site, an evergreen needle forest on the Netherlands, in the last approx 5 years, that is since when what they call "the second tower" was operational. The second tower was indeed built to comply to the ICOS standards, and it represents the legacy of the previous tower ("the first tower"), in place since the Nineties and described on a previous paper published in this same journal earlier this year. The manuscript represents an important illustration about the relevance and the potential of the ecosystem monitoring in terms of GHG and beyond. However, in some parts it seems to overlook some important information, while avoiding to go deeper in key aspects, like for example the legacy. Also, it presents additional measurements not (yet) part of the ICOS portfolio, but the link with ICOS is not always clear (what is ICOS, what is not, how it works). All in all this mixture of info and some lack of completeness can confuse the reader and make it difficult to grasp the importance of the measurements. Also, the readers target is should be clearly identified: for example, is it OK to assume they know what is a footprint climatology and to have confidence with atmospheric characteristics like (z-d)/L?
My recommendation is then to have a major revision, where the authors are requested to:
1. make clear what is part of ICOS measurements, and what is not, and restructure the text accordingly so that the reader can have a clear overview of what is done in ICOS and how (reference to the Instructions), and what is out of ICOS and how it is done. What was missing in the first tower to be ICOS-compliant? What is the importance of being part of a standardised network, and at what cost and drawbacks?
2. Give more room to quality of the data, and be more transparent when comparing with pre-ICOS data (stressing when methods and/or sampling strategy/location and/or processing were different, highlighting the uncertainties on both cases, etc.)
3. extend the "Instrumentation" section to (other) theoretical insights and processing summary, as done for example for WTD, in part the storage and Ozone. This should include a short overview of the eddy covariance CO2 processing, footprint calculation, and key parameters (e.g. [z-d]/L). This would also be beneficial in explaining the differences between ICOS and pre-ICOS CO2 fluxes, that is considering the different processing steps applied.
4. Enlarge the section on the continuity/legacy between first and second towers: not only a single example and a visual inspection, but something more extended (there are 2 months of overlap) and with some statistics. I think it could be great to the reader to see fig 9 of Zhao paper on the first tower, completed with the new period.
5. In order to make it more immediate for the reader to see the difference between first and second periods measurements, some of the findings published in Zhao et al 2026 could be reported here (e.g. table 10).
6. better clarify what has been done for ammonia and N deposition at Loobos, and what is only at the regional scale: in some parts this seems a key measurement, but it is only shortly mentioned in the text.
7. Increase the consistency between the outlook list, which I see as the milestone achieved or to be reached with the work done at Loobos, with what reported in the text, in particular expanding the parts that are reported as outlook but not explored in the text.
For more details, and a line-by-line list of changes requested, please see the attached pdf file.
Data sets
ETC L2 Archive from Loobos 2023-01-01–2025-10-01 M. van der Molen et al. https://doi.org/11676/RBbGtSvPuyjgktNpRDUq5fpw
ETC L2 Meteosens from Loobos 2022-12-31–2025-09-30 M. van der Molen et al. https://doi.org/11676/ue7IUwhI1eSbNEsFZtbvWjur
ETC L2 Meteo from Loobos 2022-12-31–2025-09-30 M. van der Molen et al. https://doi.org/11676/7OvfpkiXvumy_vSHQ6ds3Zbo
ETC L2 Fluxes from Loobos 2022-12-31–2025-09-30 M. van der Molen et al. https://doi.org/11676/Ww4zYPNKHkBUcjZxNS4CXN1j
ETC L2 Auxdata from Loobos 2023-05-10–2025-10-01 M. van der Molen et al. https://doi.org/11676/N3zBxhFvmOdlkXIW8jwryl-A
ETC NRT Meteosens from Loobos, 2025-09-30–2026-02-02 M. van der Molen et al. https://doi.org/11676/dsr4zZ3uWWcwSAHl5VnaPK2V
ETC NRT Meteo from Loobos, 2025-09-30–2026-02-02 M. van der Molen et al. https://doi.org/11676/ZYwePVBwSnQfO8buNlkj3r8f
ETC NRT Fluxes from Loobos, 2025-09-30–2026-02-02 M. van der Molen et al. https://doi.org/11676/-l_yUYF4afS0hfT__FLxpmNJ
ETC NRT AuxData from Loobos, 2025-09-30–2026-02-02 M. van der Molen et al. https://doi.org/11676/fnL4vgIPrGLXLD-O97gpZ9cB
ICOS Forest ancillary data A. Iserbyt et al. https://icos.uantwerpen.be/Content/AncillaryReports/NL-Loo_Ancillary_Report.html
NL-Loo_O3_2025: Ozone concentration and flux observations at 925 Loobos, NL – 2025 M. van der Molen et al. https://doi.org/10.17887/WUR01-6UGU9F
Model code and software
Loobos Second Tower Reference Paper M. van der Molen https://git.wur.nl/molen050/loobos-second-tower-reference-paper
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This paper is organized and surely planned as the future reference paper for the ICOS class 2 station NL-Loo. It contains an extensive description of the core Eddy Covariance as well as the many ancillary measurements required for an ICOS station of that class. This part is written in a fashion mainly documenting what instrumentation is present and where exactly (e.g. measurement heights along the tower), and is rather standard in the ICOS community. It is possible that this rather conventional way of presenting and documenting an ICOS site in ch. 1.2 to 2.5 is impacted by the use of the Copilot.
The Introduction provides the history of the site both from a paleoclimate as well as a management perspective. While not needed properly for the description of the measurement program, it is an interesting story demonstrating that the dunes, the podzolization of the soils and the pine forests growing there now is a result of specific landuse conditions and decisions previously made. The site is rather homogenous over quite some spatial distances and represents a forest not very typical for the Netherlands, as can be seen e.g. through the phenocam. NB at the time of writing this review, the phenocam had no updated imagery for almost a month. Beyond the daily delivery to the CP, the sites runs its own visualization website with updates every 30 mins for many of the variables.
Up to and including ch. 2.6, the manuscript is a standard documentation of an ICOS class 2 site in a non-spectacular manner. However, from 2.7 onwards, it offers new perspectives on the feasibility of flux (rather than only concentration) measurements for VOC and ozone, clearly the highlight of the paper: the site has non-ICOS equipment installed and thus observations available which are rare, if not unique within the set of ICOS ecosystem sites. The authors spend a significant amount of effort into the quality assessment of these non-standard fluxes, not the least since the VOC concentrations are obtained at a much lower sampling frequency compared to H2O, CO2 and T. Spectral analysis is used to convince the reader that for the VOCs, 1 Hz measurements are sufficient. The spectra reveal, however, a high-frequency noise part which is surprisingly increasing with frequency (or rather, the product of frequency times spectral power is increasing), in stark contrast to H2O, T, and CO2 which are closer obeying Kolmogorov's law or at least show a power-law attenuation at high frequencies. This should be the topic of further investigation.
Chapter 3 has a focus on additional campaigns or experiments indicating the potential of NL-Loo to become an ecosystem research hub interesting for a broader scientific community than only ICOS. This ambition is confirmed by the Outlook chapter.
The paper is well organized as a site reference. Still, the annotated pdf attached to this review contains 63 detailed comments pointing to some smaller errors or improvements which can be made, which, however, accumulate to only minor revisions. After implementing them, the paper should be acceptable for publication in ESSD.