Radiocarbon-based fossil fuel CO2 emissions for the Paris region

Urban/industrial areas are responsible for the majority of the global fossil fuel CO₂ (ffCO₂) emissions. Inventories of the emissions can be, however, largely uncertain. Radiocarbon (¹⁴C), a natural radioactive isotope of carbon, is absent in fossil fuels, thus, being the most direct tracer to distinguish ffCO₂ emissions from natural carbon fluxes. In the framework of the PAUL (ICOS-Cities) project, ambient air sampling for ¹⁴C-based estimates of ffCO₂ has been taking place since March 2023 in Paris, the second-largest megacity in Europe. Samples are collected from two stations, one serving as a background (Meudon, MEU) located in a forested area southwest of Paris, and the other (Romainville, ROV), positioned approximately 20 km northeast of MEU, acting as a signal station based on wind direction. Samples are collected into flasks by automated samplers (MEU: RINGO by UHEI; ROV: ICOS flask sampler) from inlets at 90 m a.g.l and 103 m a.g.l. at MEU and ROV, respectively. The sampling strategy follows a Lagrangian approach and samplings are conducted when an air mass passes by both sites, thus crossing the urban area of Paris within a radius of 5 km. High-precision GHG measurements, co-emitted species (e.g. CO), and ∆¹⁴C-CO₂ are performed on the sample pairs. Based on ∆¹⁴C-CO₂ and CO₂ concentrations for pairs collected between March and September 2023, ffCO₂ estimates vary between 0.5 and 5.4 ppm (median = 1.6 ppm) in both spring (March, April, and May) and late summer (September). The variation in CO₂ concentration ranged from 1 to 6.8 ppm. In June/July, ffCO₂ estimates vary from negative values to up to ca. 4 ppm. Especially for the winter samples, i.e. during the reduced photosynthesis, we expect to better identify the ffCO₂ emission patterns and compare them with the emission inventories.