the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Oct 2025
Two years of total column measurements of CO2, CH4 and CO in Paris, France
Abstract. Several cities have established atmospheric monitoring networks to track greenhouse gas emissions. In the Paris metropolitan area, continuous in-situ surface measurements have been conducted since 2015. To complement them, three ground-based solar Fourier Transform Infrared (FTIR) spectrometers provide total column concentrations of CO2, CH4, CO, and H2O (XCO2, XCH4, XCO, XH2O): Jussieu (JUS, since 2014), Saclay (SAC, since 2021), and Gonesse (GNS, since 2022) within the ICOS Cities project.
These total column estimates capture background variability and urban plumes but are influenced by calibration, measurement noise, solar geometry, a priori profiles, and surface pressure. Accounting for these factors, overall uncertainties are estimated at 0.2 ppm for XCO2, 1.2 ppb for XCH4, and 2 ppb for XCO (EM27/SUN instruments). Inter-station gradients reveal the influence of the Paris emission plume, with typical XCO2 gradients below 1 ppm (mean 0.51 ppm, 8.9 % above 1 ppm).
Observed XCO2 gradients were compared with WRF-Chem simulations driven by the dynamic Origins.earth emission inventory. Correlations are moderate – 0.47 (SAC-GNS), 0.43 (JUS-GNS), and 0.26 (SAC-JUS) – with regression slopes of 0.66, 0.72, and 0.44, respectively, suggesting a potential overestimation of emissions by about 35 %. However, the large spread between measured and modelled gradients limits the robustness of this conclusion.
The paper first describes the monitoring network and harmonized data processing, then evaluates measurement uncertainties, and finally compares observations with model simulations to assess the potential of FTIR column data for validating urban emission inventories.
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- RC1: 'Comment on egusphere-2025-4876', Anonymous Referee #1, 21 Jan 2026 reply
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- 1
This manuscript describes a network of ground-based Fourier transform spectrometers for measuring greenhouse gas columns at Paris and evaluates the measurement uncertainties. The measurements are compared with model simulations based on Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to assess the potential of column measurements in verifying urban emission inventories. Correlation analysis based on observed and simulated spatial CO2 gradients suggests that the emission inventory used in the simulation (i.e., the Origins.earth emission inventory) is overestimated, while also revealing the difficulty of using column data for emission estimates.
The topic of this paper is relevant to the scope of Atmospheric Chemistry and Physics. The analysis method is appropriate and the writing structure is well organized. However, I have a number of specific comments, which need to be addressed before the publication.
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Specific comments:
L21: What is the basis for an overestimation in emissions of 35%? This figure is not mentioned in the main text. In addition, the Conclusion section states that emissions are overestimated by 44%.
Table 1: Please include the serial numbers (#88, #103, and #179) of the EM27/SUNs used at each station, along with the corresponding periods.
Figure 2: Regarding XCO2, instrument #88, which was deployed at SAC before 2023-05-19 and at GNS after 2023-05-19, appears to have a low bias, while instrument #103, which was deployed at SAC after 2023-05-19, has a high bias. Thus, there seems to be device-specific biases. Can the effect of the exchange of instruments be considered negligible? Additionally, Paris TCCON data are biased low by approximately 0.5 ppm, compared with other EM27/SUN data. Is it acceptable not to consider this bias?
Figure 4 and L233: Could you please clarify whether the ∆XCO2 values were calculated for each instrument, as indicated in the legend of Figure 4, or for each station, as described in the main text?
L294–295: According to Table 2, a 0.0015 change in XAIR corresponds to a 0.14 ppm change in XCO2. Which is correct?
L331: Is the observed growth rate the same at the three stations?
L465–478: Please add discussion of the differences between the simulations and observations, especially why the measured XCO2 values at Gonessa were, on average, larger than those at Saclay, contrary to the simulations.
L579: What is the emission inventory used by CAMS? This information would be helpful in understanding the differences between the WRF-Chem and CAMS simulations.
L587–588: Because the errors in the slopes for JUS-GNS and SAC-JUS are almost the same between CAMS (9 km resolution) and WRF-Chem (1 km resolution) simulations, the difference in the spatial resolution seems to be irrelevant.
L589–590: Why is underestimation in the emissions inventory considered more reasonable than overestimation?
L615–619: In L402–404, it is stated that emission estimates using in situ surface measurements are subject to significant uncertainty due to the difficulty of modeling the vertical mixing, whereas column measurements are insensitive to this modeling. Could you discuss which approach is more accurate?
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Technical corrections:
L64: Are the EM27/SUN data at JUS used in this study? Otherwise, it would be better to either state that fact or delete it to avoid confusion.
Table 1: What is the QUALAIR platform?
L147: “to be used” is unnecessary?
L157: the radiative transfer code GGG2020 package -> the retrieval software GGG2020
L159: TCCON framework -> TCCON archive
L225–226: we define a reference as the mean XCO2 over 10:00-14:00 local time. -> we define the mean XCO2 over 10:00-14:00 local time as a reference.
L240: modelled radiative -> modelled radiative processes
Equations (3.1)–(3.3): What do the double arrows means?
Figure 5: Please change the x-axis label to English.
L266: rSAC = 0.76 et rGNS = 0.75 -> rSAC = –0.76 and rGNS = –0.75
L312: This sentence is almost identical to L314–315, so it is redundant.
Table 3: What is the difference between “-” and “X”?
L324: According to Section 2.2, TCCON spectra were analyzed with GGG2020.
L392: CO2 plus -> CO2 plumes?
L420: Figure 10 is cited before Figure 9 in the main text. Figure 10 should be changed to Figure 9 and vice versa.
L504 and 507: et -> and
L560–564: Please add the slopes of the fits as well as the correlation coefficient.
L584: discrepancies in the simulation of emissions or atmospheric transport -> discrepancies in the atmospheric transport simulations or emissions