the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 08 Aug 2025
Extension of the Total Carbon Column Observing Network (TCCON) over the Eastern Mediterranean and Middle East: The Nicosia site in Cyprus
Abstract. Long-term greenhouse gas (GHG) measurements are essential for understanding the carbon cycle, detecting trends in atmospheric composition, and assessing the efficiency of climate change mitigation strategies. However, observational gaps over large geographic areas such as the Eastern Mediterranean and Middle East (EMME), a well-known regional GHG hotspot, are likely to increase uncertainties in estimations of their sources and sinks. Here, we describe a new Total Carbon Column Observing Network (TCCON) observatory for solar absorption spectroscopy measurements that has been operating in Nicosia, Cyprus, since September 2019. The site helps bridge a regional observational gap in the EMME, a strategic location at the crossroads of air masses from Europe, Asia, and Africa. Using near-infrared (NIR, InGaAs detector) solar absorption spectra, TCCON-Nicosia measures total column average dry-air mole fractions (Xgas) of key trace gases, including carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), carbon monoxide (CO), hydrogen fluoride (HF), water vapor (H2O), and semi-heavy water (HDO). These continuous observations, spanning more than four years, are presented along with a description of the quality control procedures, compliant with the TCCON standards, to ensure total column atmospheric data with minimal errors.
In 2023, observations were extended into the mid-infrared (MIR) spectrum with the addition of a liquid-nitrogen-cooled InSb (LN2-InSb) detector enabling the retrieval of additional trace gases such as formaldehyde (HCHO), carbonyl sulfide (OCS), nitrogen monoxide (NO), nitrogen dioxide (NO2), and ethane (C2H6), herewith further contributing to the global Network for the Detection of Atmospheric Composition Change (NDACC).
To tie the TCCON Nicosia with the WMO reference scale, an AirCore (AC) campaign conducted in June 2020 over Cyprus provided vertical in situ profiles, which were converted into total column quantities (AC.Xgas) and compared to TCCON observations (Xgas). The TCCON/in situ comparison showed agreement well within their respective error budget.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
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.- Preprint
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Status: open (until 11 Oct 2025)
- RC1: 'Comment on egusphere-2025-1442', Anonymous Referee #1, 26 Aug 2025 reply
Interactive computing environment
Supplementary material for TCCON Nicosia site description manuscript Constantina Rousogenous https://doi.org/10.5281/zenodo.15085719
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This paper describes a relatively new TCCON station located near Nicosia, Cyprus. This location fills an important gap in carbon cycle monitoring, and it is therefore a welcome addition. The paper describes the location, measurements to date, and some comparisons with coincident AirCore profiles. A reference for these measurements will be very helpful, so I recommend publication after addressing the following comments.
Specific comments:
TCCON typically chooses 45 cm OPD for its spectral resolution to allow the spectra to distinguish the absorption lines of interest from the interfering absorption lines across a broad wavelength range while maintaining a high signal-to-noise ratio. You’ve chosen 64 cm OPD, presumably as a compromise between the TCCON- and NDACC-style measurements you wish to collect. Do you have a sense of how that choice impacts the TCCON measurements? Have you truncated the interferograms to show whether the additional 20 cm OPD improves or degrades the CO2 retrieved? Please justify your choice of 65 cm OPD.
Figure 2: What is the cause of the substantial increase in Xluft spread (stdev) in late 2019, and early-to-mid 2023? Did the density of data decrease in mid-2021 when you began NDACC-like measurements?
Figure 3: What caused the sparsity in measurements in early 2021?
Section 3.2.2: There are several reasons proposed for the seasonal cycle in CH4, but earlier in the paper, it was stated that Nicosia measures outflow from Europe, Africa, and Asia. Could you perform an analysis that distinguishes airmasses from each of these continents to confirm your earlier assertion? A back-trajectory analysis or a climatological analysis would be helpful to interpret your results. Can you make use of your HCHO and HCN measurements to strengthen your argument that CH4 enhancements are caused by fire activity? Is CH4 expected from agricultural waste burning?
Figure 5: I find this to be a difficult way to visualize the comparison between the AirCore and TCCON measurements. It would be helpful to show the TCCON data time series spread out across each panel as a function of the hour of the day, with horizontal lines in black and red representing the medians of the public TCCON and custom TCCON retrievals, respectively. Then, the AirCore diamond (in blue) should be positioned at the time of the lowest altitude AirCore measurement, so we can compare any trends in the TCCON measurements over the +/- 1 hour with the AirCore columns. With the current visualization, the reader cannot see if there are trends in the TCCON measurements throughout the comparison period. Also, what is the cause of the disagreement in XCO2 on June 29? According to Figure 4, there’s a substantial near-surface CO2 enhancement on June 29, and the AirCore profile does not appear to provide data below ~2 km on that day. Do you have surface data to fill in the bottom of the profile on that day?
Figure S8: How are the profiles extended down to the surface? The text seems to imply that the lowest AirCore measurement is dropped straight to the surface (“flat-extrapolation”), but replaced in the bottom two grid levels by the surface in situ measurement. However, the right panel of Figure S8 does not appear to show that. Please clarify whether these are two different profiles, or why the near-surface assembled profile is >8 ppm different between the left and right panels.
Technical comments:
L30: “mid-infrared (MIR) spectrum” should be “mid-infrared (MIR) spectral region”
L74: “north hemisphere” should be “northern hemisphere”
L93: remove “shall”
L96: TCCON Network seems redundant -> use just TCCON or just Network
Equation (1): missing ‘gas’ subscript on the central equation
L182: It is not just spectroscopic errors that cancel in the ratio. Alignment errors, pointing errors, some spectroscopic uncertainties can partially cancel.
L187: The O2 is retrieved from a 250 cm-1 wide window that is centred at 7885 cm-1, and not from a single line. The retrievals are based on multiple O2 absorption lines. See Mendonca et al. (2019) for reference for this comment and the previous one (L182).
References:
Mendonca, J., Strong, K., Wunch, D., Toon, G. C., Long, D. A., Hodges, J. T., Sironneau, V. T., and Franklin, J. E.: Using a speed-dependent Voigt line shape to retrieve O2 from Total Carbon Column Observing Network solar spectra to improve measurements of XCO2, Atmos. Meas. Tech., 12, 35–50, https://doi.org/10.5194/amt-12-35-2019, 2019.