Atmospheric transport and deposition of radon-222 and its progeny using flexpart

Precipitation routinely causes excursions in environmental gamma dose monitoring stations due to wet deposition of the progeny of airborne radon-222 (222Rn). Since such stations are used to monitor the environment for signs of aberrant radioactivity, it is important to understand these excursions lest they be erroneously designated as anomalies. While their
temporal evolution of such excursions can be theoretically explained using the Bateman equation, their magnitude is difficult to predict. The same precipitation intensity yields wildly varying dose excursions from one event to the next. Natural variations in the atmospheric concentration of 222Rn are one possible reason for this. While varying meteorological conditions at the measurement site are often named as a reason for observed variations, we
think that local meteorological information alone is not a good predictor. Owing to its 3.8-day half-life, 222Rn can be transported over considerable distances through the atmosphere.
If atmospheric conditions are at the root of the varying dose responses, one should instead consider atmospheric conditions over a much larger area. To investigate whether weather phenomena at the larger scale can account for these variations, we model the atmospheric transport of 222Rn in Flexpart. Flexpart is a Lagrangian particle model to calculate long-range atmospheric transport of pollutants. In this work, we adapt Flexpart to deal with the
dispersion of the parent nucleus (222Rn) and its progeny (218Po, 214Pb and 214Bi) in a single simulation under the assumption that parent and progeny are collocated in non-precipitating conditions. We prove that this collocation may be assumed despite the different dispersive properties that arise from the fact that only the parent is a noble gas. We present a case study using ECMWF reanalysis numerical weather data that covers the northern hemisphere for the year 2022. The entire land area of Europe is modelled as a heterogeneous plane source based on existing radon emanation data. We compare modelled to observed 222Rn concentrations, and modelled deposition to environmental gamma dose observations.
Observations come from Telerad, the Belgian radiological surveillance network and earlywarning system. The results of this case study will help us understand whether excursions in environmental gamma dose can be explained with the proposed modelling setup and, if successful, will help to better interpret environmental gamma dose measurements.