How reliable are process-based ²²²radon emission maps? Results from an atmospheric ²²²radon inversion in Europe

Abstract. The radioactive noble gas radon (²²²Rn) is a suitable tracer for atmospheric transport and mixing processes that can be used to evaluate and calibrate atmospheric transport models or to estimate greenhouse gas emissions using the so-called Radon-Tracer method. However, the prerequisite for these applications is a reliable estimate of the ²²²Rn fluxes from the soil. In this study, we evaluate two process-based ²²²Rn flux maps for Europe based on two different soil moisture reanalysis products (GLDAS-Noah and ERA5-Land) using the flux results obtained from a one-year ²²²Rn inversion performed with the CarboScope-Regional inversion system and ²²²Rn observations from 17 European sites. We observe that, in particular, the ERA5-Land based ²²²Rn flux map underestimates the data-driven fluxes from the inversion in Central Europe in 2021. Our inversion results yield ca. 20 % (GLDAS-Noah) to almost 100 % (ERA5-Land) larger ²²²Rn fluxes than the respective process-based a priori fluxes within a domain covering Germany. Also, the temporal variability seems to be underestimated by the process-based ²²²Rn flux maps. We found a significant anti-correlation of -0.6 and -0.8 between the posterior flux estimate using a flat (uniform) prior inversion and the GLDAS-Noah and ERA5-Land soil moisture estimates, respectively, indicating that soil moisture is an important driver for the temporal variability of the ²²²Rn fluxes. To investigate the impact of the modelled atmospheric transport on the inversion results, we performed sensitivity runs using two other Lagrangian transport models. The respective annual mean a posteriori fluxes agree within ca. 10 %.