Characteristics of Pu contamination in atmospheric particulate matter in Beijing: seasonal variation, particle size...

Plutonium (Pu) isotopes are anthropogenic radionuclides characterized by highly chemical toxicity and radiotoxicity that persist in the atmosphere. Monitoring atmospheric Pu concentration is important for understanding its environmental characteristic and source identification. In this study, the activity concentrations of ^239,240Pu and ²⁴⁰Pu/²³⁹Pu atom ratios in PM_2.5, PM₁₀, and total suspended particulate (TSP) collected in 2022 in Beijing were determined. The determined ²⁴⁰Pu/²³⁹Pu atom ratios were in the range of 0.136-0.222, with the average value of 0.176 ± 0.005, which was in accordance with the global fallout. The measured ^239,240Pu activity concentrations in PM_2.5, PM₁₀, and TSP were in the range of 3.89 × 10^-6-1.11 × 10^-3 Bq/g, 1.04 × 10^-5-1.02 × 10^-3 Bq/g, and 2.88 × 10^-5-9.92 × 10^-4 Bq/g, respectively. The seasonal variation trend affected by monsoon climate was observed, which was characterized by higher value in spring, followed by the winter, and lower value in autumn. Meanwhile, the particle size distribution of Pu isotopes was discussed. About 50% of Pu isotopes were enriched in fine particles (2-2.5 μm), only ∼4.8% in the fraction of 2.5-10 μm, and ∼45.2% in 10-100 μm range. Fine particles (2-2.5 μm) are the primary carrier of atmospheric Pu in Beijing. By comparing ^239,240Pu concentrations in TSP samples and in the surface soil, the long-range transport of resuspended soil was considered as the main source of Pu during sandstorm, and the local soil resuspension was regarded as the main source of Pu on non-sandstorm days.