Actinides Th and U in Atmospheric Particulate Matter in Yakutsk

The distribution of Th and U was studied in the near-ground atmosphere of the city of Yakutsk. The chemical compositions of particulate matter (PM) in the summer atmosphere and in the solid and soluble phases of the snowpack were studied in 2019–2020. In the summer atmosphere, Th and U are concentrated mostly in the dust fraction of PM, together with a group of siderophile elements. The wintertime PM is characterized by a more homogeneous size of its particles. PM minerals with which concentrations of actinides correlate are amphiboles and pyroxene for Th and carbonates for U. The Th and U concentrations were found out to systematically decrease in the sequence soil–summer PM (dust)–winter PM (snow), at the dominance of Th, but the aqueous phase of the snow contains U concentrations one order of magnitude higher than those of Th. The bulk (close to 90%) of actinides is deposited during the warm season. The total amount of actinides falling out in Yakutsk is 63 mg/(m2 year), with two-thirds of this amount being Th. This value may locally increase to 200 mg/(m2 year) and more in areas of contrasting anomalies, with the predominance of Th. The main sources of actinides entering the near-ground atmosphere of Yakutsk are dusting from the soil surface and emissions from vehicles, with lesser amounts coming from energy-producing and construction-industry facilities.