The monitoring network of greenhouse gas (CO₂, CH₄) in the Paris' region

Abstract. There is a growing interest for the study of greenhouse gas emissions over urban areas. In this context, a network for measuring greenhouse gas concentrations was set up in Paris in 2015. Since then, seven stations located in and around Paris and equipped with cavity ring-down spectrometers (CRDS) have been monitoring gas concentrations of different species (CO₂, CH₄, CO) on a continuous basis. Procedures for maintenance, calibration, data processing have been adapted to ensure that the network is operational, providing good quality data in near-real time (NRT) and with high availability. The CO₂ and CH₄ concentrations show a growth rate of the baseline (linear trend of the averaged concentrations over the Paris region) concentrations (+2.34 ppm CO₂/year and +11.1 ppb CH₄/year) between 2015 and 2022 consistent with that observed at remote observatories such as Jungfraujoch (Switzerland). The amplitude of the CO₂ seasonal cycle is around 20 ppm (i.e. around 5 %) while that of CH₄ is 40 ppb (i.e. around 2 %). The concentration gradients calculated as the differences between up- and down-wind concentrations, can be used to infer the emissions for long lived species. We use the measurements from the 100 m Saclay tower, outside of Paris, as a regional background during suitable wind direction conditions. The largest differences with these background measurements are observed in the two stations that are located within Paris (JUS and CDS), with urban offset up to ~ 50 ppm CO₂ and ~ 100 ppb CH₄ in winter and ~ 10 ppm CO₂ and ~ 50 ppb CH₄ in summer. In winter, the co-variability of CO and CO₂ hourly measurements (correlation coefficient r ~ 0.9 in all stations) indicates that the concentration variability is driven by anthropogenic emissions. Conversely, in summer, lower correlations between these two gases concentration (r ~ 0.3 in peri-urban stations and r ~ 0.6 at CDS and JUS) shows the more dominant role of vegetation fluxes.