Atmospheric transport modelling of radon-222 at European scale: description and validation against ICOS observations.

Atmospheric transport models used for nuclear emergency purposes are dedicated to simulating the atmospheric transport of radionuclides released from a damaged nuclear facility. The quality of this response is crucial and must be constantly improved. However, long-range measurement campaigns for validation are scarce, especially for radioactive pollutants. An effective way to do so is by simulating the radon-222 which is a non-reacting atmospheric tracer species with quite well-known exhalation rate and well-known nuclear transitions.

Radon-222 is naturally emitted. Its flux spatial variation is mainly due to the type of soil and rocks, rather than the vegetation or land use. Temporal variations are mainly led by soil humidity, leading to a monthly variation. The monthly surface radon flux map of Karstens and Levin, 2022 is used in this study.

The availability of the observations at suitable temporal and spatial scales is achieved in this study thanks to the Integrated Carbon Observation System, ICOS (Heiskanen et al. 2021). ICOS provides standardized and open data from more than 39 atmosphere stations that measures greenhouse gases concentrations in the atmosphere. Some stations also provide radon-222 concentrations measurements. Among them, some also include measurements at different heights - from ground level up to 200 meters – which is valuable to validate the vertical atmospheric transport modelling. The limited set of radon-222 stations is not a substitute for performing the comprehensive validation against a large variety of observations but gives valuable information on the performance of air concentration predictions. A previous study using the dose rate measurements network (Quérel et al. 2022), required in addition the need of an accurate deposition modelling to assess gamma dose rates at ground level due to wet deposition of radon-222 decay products.

We evaluate here the overall performance of an air concentration modelling chain: Karstens radon-222 fluxes, Météo-France ARPEGE numerical weather predictions and IRSN LdX operational atmospheric transport model. Simulated radon-222 air concentrations are compared with observations from the ICOS monitoring network over Europe, on an hourly frequency basis over one year. On initial examination, the model appears to under-predict radon-222 concentrations and some possible explanations and sources of improvement are identified.

 

References:

Heiskanen, J., C. Brümmer, N. Buchmann, C. Calfapietra, H. Chen, B. Gielen, T. Gkritzalis, S. Hammer, S. Hartman, M. Herbst, et al. (2021), The Integrated Carbon Observation System in Europe, Bulletin of the American Meteorological Society, 1 - 54, doi:10.1175/bams-d-19-0364.1.

Karstens, U. and Levin, I. (2022). traceRadon monthly radon flux map for Europe 2006-2022 (based on GLDAS-Noah v2.1 soil moisture), https://hdl.handle.net/11676/ge5vMeklvG_Qz43rzcS2wx0-

Quérel, A., Meddouni, K., Quélo, D., Doursout, T., and Chuzel, S. (2022). Statistical approach to assess radon-222 long-range atmospheric transport modelling and its associated gamma dose rate peaks. Advances in Geosciences. 57. 109-124. 10.5194/adgeo-57-109-2022.