Radionuclides released during the Fukushima Daiichi nuclear accident caused altitude-dependent surface contamination in the mountainous areas of Japan. To explore the possible cloudwater deposition that formed a distinctive contamination profile, data from pollen sensors deployed nationwide were analyzed. Utilizing the polarization of scattered light, Cedar pollen and water droplets were distinguished. On March 15, when surface contamination was simulated in previous studies, dense clouds with high droplet number concentrations were observed outside the 137Cs surface deposition areas, indicating that the sensor sites were immersed amid cloud layers. In contrast, cloud droplets with moderate number concentrations were measured at altitudes of approximately 570-840 m, which overlapped with the surface contamination areas. Considering the existing knowledge on vertical gradients of cloudwater composition, these suggest that contaminated cloud droplets were localized near the cloud base where a moderate number concentration of cloud droplets was measured. A formation process was proposed for the observed vertical distribution, that is, surface contamination occurred intensively at the contact line between the cloud base and mountain slopes via cloudwater deposition, and the descending cloud base formed the contamination zone. This study sheds light on the deposition processes of radionuclides, which have not previously been clarified.
Keywords: Cedar pollen; Fukushima Daiichi; cloud base; deposition mechanism; fogwater; low-cost sensor; nuclear accident; radioactive aerosol.