Measurement report: Eight 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 October 2024 and N2O fluxes from February 2019 to October 2024 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 data have been used to estimate the surface emissions. To our knowledge, this is the first study in Europe 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 867 ± 565, 1.10 ± 0.89, 1.01 ± 1.05 and 0.094 ± 0.118 mgm-2h-1 for CO2, CH4, CO and N2O, respectively. These fluxes are in good agreement with the literature for the same site or for similar suburban sites in Europe. No significant trends are observed over time, except for CO, which shows a small decreasing trend especially over the last three years. CH4, N2O and CO are also in fair agreement with the inventories, though with higher values. CO2 fluxes are about 5 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 by the WRF-VPRM version used here.