Unraveling size-resolved 137Cs resuspension and deposition from the 2020 Chernobyl Wildfires via multi-component inversion and WRF-Chem simulation

Wildfires in radioactively contaminated regions, such as the Chernobyl Exclusion Zone, pose a growing environmental threat by resuspending long-lived radionuclides into the atmosphere. However, accurately quantifying the redistribution of these radionuclides remains challenging. Existing top-down inversion studies often oversimplify source terms by assuming fixed particle sizes and release altitudes, which hinders the precise evaluation of transport mechanisms and deposition footprints.

To address this gap, this study proposes a novel multi-component source term inversion framework to simultaneously reconstruct the time-varying release profiles of ¹³⁷Cs across multiple particle sizes (0.4, 8, and 16 μm) and seven vertical layers (0-3000 m). We improved the Projected Alternating Minimization with L1-norm and Total variation regularization (PAMILT) algorithm by incorporating a TV-regularized initialization and a Bayesian optimization scheme for hyperparameter tuning to ensure robust convergence. These retrieved source terms were then coupled with the WRF-Chem model using size-resolved microphysics to conduct a high-resolution simulation of the April 2020 Chernobyl wildfires.

Validation results demonstrated exceptional agreement between the simulated and observed concentrations, achieving a Pearson correlation coefficient of 0.996 and reducing maximum relative biases from over 10⁶ to generally below 10². The inversion estimates a total ¹³⁷Cs release of approximately 836 GBq. This release was dominated by fine particles (0.4 μm, ~54%) and low-altitude injections, with 58.1% occurring below 1 km. Crucially, our WRF-CHEM simulations reveal a decoupling between emission abundance and deposition impact. Although fine particles dominate the source term, coarse particles (16 μm) control the near-field deposition flux due to rapid gravitational settling. These coarse particles exhibit a "transport plateau" beyond roughly 800 km, whereas fine particles show a linear growth in transport distance constrained only by meteorological dispersion. Furthermore, we identified distinct deposition signatures. Dry deposition manifests as a continuous spatial accumulation or "creeping" effect. In contrast, wet deposition drives "step-wise" long-range transport, triggering sudden and pulse-like removal events far from the source.

These findings provide critical insights into the complex mechanics of radionuclide redistribution and offer a refined methodology for assessing the environmental impact of future wildfire events in contaminated zones.