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In October 2017, most European countries reported unique atmospheric detections of aerosol-bound radioruthenium (106Ru). The range of concentrations varied from some tenths of µBq·m−3 to more than 150 mBq·m−3. The widespread detection at such considerable (yet innocuous) levels suggested a considerable release. To compare activity reports of airborne 106Ru with different sampling periods, concentrations were reconstructed based on the most probable plume presence duration at each location. Based on airborne concentration spreading and chemical considerations, it is possible to assume that the release occurred in the Southern Urals region (Russian Federation). The 106Ru age was estimated to be about 2 years. It exhibited highly soluble and less soluble fractions in aqueous media, high radiopurity (lack of concomitant radionuclides), and volatility between 700 and 1,000 °C, thus suggesting a release at an advanced stage in the reprocessing of nuclear fuel. The amount and isotopic characteristics of the radioruthenium release may indicate a context with the production of a large 144Ce source for a neutrino experiment.

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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 (137)Cs, from 0.17 to 0.69 μm for (134)Cs, and from 0.30 to 0.53 μm for (131)I, thus in the "accumulation mode" of the ambient aerosol (0.1-1 μm). AMAD obtained for the naturally occurring radionuclides (7)Be and (210)Pb 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 (131)I 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 (131)I 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 (131)I ratio. AMAD slightly increased with time for (131)I whereas a clear decreasing trend was observed with the AMADs for (137)Cs and (134)Cs. 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 (131)I 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 (137)Cs was assessed for the coarse particle fractions only for the first sampling week.

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Traces of particulate radioactive iodine (I) were detected in the European atmosphere in January/February 2017. Concentrations of this nuclear fission product were very low, ranging 0.1 to 10 μBq m except at one location in western Russia where they reached up to several mBq m. Detections have been reported continuously over an 8-week period by about 30 monitoring stations. We examine possible emission source apportionments and rank them considering their expected contribution in terms of orders of magnitude from typical routine releases: radiopharmaceutical production units > sewage sludge incinerators > nuclear power plants > spontaneous fission of uranium in soil. Inverse modeling simulations indicate that the widespread detections of I resulted from the combination of multiple source releases. Among them, those from radiopharmaceutical production units remain the most likely. One of them is located in Western Russia and its estimated source term complies with authorized limits. Other existing sources related toI use (medical purposes or sewage sludge incineration) can explain detections on a rather local scale. As an enhancing factor, the prevailing wintertime meteorological situations marked by strong temperature inversions led to poor dispersion conditions that resulted in higher concentrations exceeding usual detection limits in use within the informal Ring of Five (Ro5) monitoring network.

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