Optimal network designs of in situ atmospheric CO2 stations over continental France

The global Stocktake, a fundamental component of the Paris Agreement tracking progress on national mitigation actions, collects the Nationally Determined Contributions (NDCs) generated through the means of annual national inventories. Greenhouse gases (GHG) inventories are prone to uncertainties, especially when considering sub-national scales, sub-annual frequencies, or the natural component of GHG budgets, lacking verification and transparency. Atmospheric observations assimilated through the inverse approach can help constrain both the natural and anthropogenic components of national carbon budgets. Here, we aim at quantifying the carbon dioxide (CO₂) fluxes over continental France by combining atmospheric greenhouse gas concentrations from the ICOS (Integrated Carbon Observation System) measurement network and a high-resolution inversion system.

We present an assessment of the observational constraint from the current ICOS network. We also determine the optimal locations and number of additional stations to monitor CO₂ fluxes from human activities and different ecosystems. The CO₂ concentration measurements influenced by surface CO₂ fluxes are analyzed using a Lagrangian Particle Dispersion (LPDM) model. LPDM is run backward in time with meteorological inputs from the Weather Research Forescating (WRF) model, at 3km resolution over continental France. We infer the origin of the CO₂ using the TNO high-resolution fossil fuel inventory and biogenic CO₂ fluxes produced by the Vegetation Photosynthesis Respiration Model (VPRM). The VPRM model simulates both the CO₂ uptake from photosynthesis and the release from respiration using meteorological re-analysis products and surface remote sensing data.

We start by evaluating the improved model performances at high resolution compared to low resolution simulations. Then we analyze the influence of biogenic and fossil fuel sources at each tower of the ICOS network, and finally we explore which areas are constrained by atmospheric stations using different criteria: by ecosystem type, by land cover, and in terms of net carbon fluxes and fossil fuel emissions. We discuss here how our future inversion system could help constrain the regional distribution of CO₂ fluxes, sub-annual variations at seasonal and monthly timescales to track current climate change impacts (forest fires, droughts), and the effects of emission mitigation policies.