A top-down assessment of CO2 and CH4 atmospheric variability and emission sources in the Aix-Marseille metropolis area, France.

The Aix-Marseille metropolis is the second most populated urbanized area of France. It aims at reaching carbon neutrality in 2050. Located in the south-east of France under a Mediterranean climate, this area is known as a hot-spot regarding climate change. Its west part is strongly industrialized. The local air quality monitoring agency  ATMOSUD delivers a high resolved greenhouse gas emissions inventory that represents the reference for local stakeholders in matter of emissions trajectory. However, this inventory is still quite uncertain and requires independent assessments. In this aim, in the framework of the ANR COoL-AMmetropolis projet (2019-2025) we set-up a local greenhouse gas monitoring network based on Cavity Ring Down Spectrometry analyzers. This local network comprises the OHP ICOS-France station, located 80 km north of Marseille city. Local meteorological features such as sea and land breezes impact local greenhouse gas concentrations through advection and boundary layer dynamical processes. Isotopic analysis of 14C and 13C in CO2, as well as CO2 correlations with NOx, CO, black carbon and SO2, show a strong impact of fossil fuel emissions on the CO2 local urban greenhouse gas atmospheric plumes. The identified fossil sources are mostly traffic, building, industrial and maritime activities. Modern sources such as wood burning may account for a larger part than assessed by the local inventory.