Six years of greenhouse gas fluxes at Saclay, France, estimated with the Radon Tracer Method

Abstract. Here, we use carbon dioxide (CO₂), methane (CH₄), carbon monoxide (CO), nitrous oxide (N₂O) and radon (²²²Rn) data from the Saclay ICOS tall tower in France to estimate CO₂, CH₄ and CO fluxes within the station footprint from January 2017 to December 2022 and N₂O fluxes from February 2019 to December 2022 using the Radon Tracer Method (RTM).

We first performed a sensitivity study of this method applied to CH₄ and combined with different radon exhalation maps including the improved European process-based radon flux maps developed within 19ENV01 traceRadon and back-trajectories in order to optimize it. Then, radon exhalation maps from the 19ENV01 traceRadon project, STILT trajectories from the ICOS Carbon Portal, best estimate of radon activity concentration and greenhouse gas data have been used to estimate the surface emissions. To our knowledge, this is the first study using the latest radon exhalation maps and standardized radon measurements to estimate CO₂, CH₄, CO and N₂O surface emissions. We found that the average RTM estimates are 609 ± 402 mg m⁻² h⁻¹, 0.81 ± 0.66 mg m⁻² h⁻¹, 1.04±1.80 mg m⁻² h⁻¹ and 0.063 ± 0.079 mg m⁻² h⁻¹ for CO₂, CH₄, CO and N₂O respectively. These fluxes are in good agreement with the literature.

CH₄, N₂O and CO are in fair agreement with the inventories, though with higher values. CO₂ fluxes are about five times higher than modeled anthropogenic and biogenic fluxes combined. The differences mainly occur during summer, and the CO/CO₂ ratio points toward a misrepresentation of the biogenic fluxes at this time by the WRF-VPRM version used here.