Size Distributions of Airborne Radionuclides from the Fukushima Nuclear Accident at Several Places in Europe

Segregation and radioactive analysis of aerosols according to their aerodynamic size were performed in France, Austria, the Czech Republic, Poland, Germany, and Greece after the arrival of contaminated air masses following the nuclear accident at the Fukushima Dai-ichi nuclear power plant in March 2011. On the whole and regardless of the location, the highest activity levels correspond either to the finest particle fraction or to the upper size class. Regarding anthropogenic radionuclides, the activity median aerodynamic diameter (AMAD) ranged between 0.25 and 0.71 μm for ¹³⁷Cs, from 0.17 to 0.69 μm for ¹³⁴Cs, and from 0.30 to 0.53 μm for ¹³¹I, thus in the “accumulation mode” of the ambient aerosol (0.1–1 μm). AMAD obtained for the naturally occurring radionuclides ⁷Be and ²¹⁰Pb ranged from 0.20 to 0.53 μm and 0.29 to 0.52 μm, respectively. Regarding spatial variations, AMADs did not show large differences from place to place compared with what was observed concerning bulk airborne levels registered on the European scale. When air masses arrived in Europe, AMADs for ¹³¹I were about half those for cesium isotopes. Higher AMAD for cesium probably results from higher AMAD observed at the early stage of the accident in Japan. Lower AMAD for ¹³¹I can be explained by the adsorption of gaseous iodine on particles of all sizes met during transport, especially for small particles. Additionally, weathering conditions (rain) encountered during transport and in Europe in March and April contributed to the equilibrium of the gaseous to total ¹³¹I ratio. AMAD slightly increased with time for ¹³¹I whereas a clear decreasing trend was observed with the AMADs for ¹³⁷Cs and ¹³⁴Cs. On average, the associated geometric standard deviation (GSD) appeared to be higher for iodine than for cesium isotopes. These statements also bear out a gaseous ¹³¹I transfer on ambient particles of a broad size range during transport. Highest weighted activity levels were found on the 0.49–0.95 μm and on the 0.18–0.36 μm size ranges in France and in Poland, respectively. The contribution from resuspension of old deposited ¹³⁷Cs was assessed for the coarse particle fractions only for the first sampling week