Wet deposition modeling and behavior analysis of 137Cs aerosols using ten observation-based continuous size distributions following the Fukushima accident.

Cesium-137 is one of the most concerned radionuclides produced by fission energy, which has a long half-life, considerable biological toxicity and complex transport behaviors. Wet deposition is a key process in determining the atmospheric transport of 137Cs aerosols following the Fukushima Daiichi nuclear power plant (FDNPP) accident. However, our understanding of this process remains insufficient because of the complex size distributions of the 137Cs aerosols and related atmospheric transport processes. This study investigates the wet deposition behavior of observed 137Cs aerosols by developing a wet deposition model that considers multiple continuous size distributions in the online-coupled WRF-Chem. The wet deposition efficiency is calculated based on Köhler's theory. Ten log-normal size distributions are constructed, with geometric mean diameters (dm) derived from observation. These distributions are applied to simulate the 137Cs transport following the FDNPP accident. The results reveal that the established model successfully reproduces the 137Cs transport and wet deposition. Below-cloud deposition is dominant over in-cloud deposition for 137Cs aerosols with dm ≤ 2.0 μm. In-cloud deposition is also important when dm > 2.0 μm, whereas dry deposition is predominant for dm > 6.4 μm. As for the atmospheric concentration, the influence of aerosol size is substantial during weak rain near the source. 137Cs aerosols with dm ≈ 2.8 μm best reproduce both the cumulative deposition and atmospheric concentration at the same time. The results indicate the importance of considering multiple log-normal size distributions and fog deposition.