Characteristics of the urban CO2 plume from Marseille city in the southern France : variability and sources identification using co-emitted species and isotopic ratios.

Urban areas are large sources of greenhouse gases and pollutants. CO2 source apportionment are of prerequisite for defining efficient mitigation strategy to reach the regional goal of carbon neutrality in 2050. It is yet challenging to document especially in a large and complex megacity such as Aix Marseille Metropolis (the 2^nd biggest French city).  In the framework of the ANR COoL-AMmetropolis project, this work focuses on assessing the variability and composition of the CO2 urban plume in the Marseille city. Three years of continuous atmospheric measurements and one field campaigns carried on at the Longchamp station in Marseille, south-east of France (43° 18′ 20″ N, 5° 23′ 41″ E) are presented. This station is in an urban environment and is mainly influenced by traffic, residential and industrial emissions (source: ATMOSUD inventory). Beside air quality variables like Carbon dioxide (CO₂), methane (CH₄), black carbon, particulate matter (PM) chemical composition and nitrogen oxides (NOx) are continuously measured at this station to study the spatio-temporal variability of these compounds. A field campaign of one week in January 2020 has been performed to better infer the sources of CO₂. One continuous carbon monoxide instrument and two volatile organic compounds analysers were deployed. Furthermore, about 60 air samples were collected for analysing the isotopic ratio and radiocarbon content of atmospheric CO₂. Contributions from anthropogenic fossil fuel emissions and biogenic respiration are quantified. The analysis of the temporal co-variations of CO₂ with co-emitted species, enhancement ratios and ¹³C isotopic ratio provide the identification and the contribution of fossil fuel emissions sectors. These results are also be used to verify regional inventories independently and highlights the main emission sectors contributing to the Marseille city center.