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.