Monitoring of ultra-trace uranium and thorium in six-grade particles.

The natural radioactive elements uranium(U) and thorium(Th) in atmospheric environment should be given attention, and their particle size distribution and concentration are important for estimating their hazardous effects to the human body. The concentrations of U and Th in 360 aerosol samples collected using six-stage aerosol collector from June - December, 2016 in Beijing were determinated using ICP-MS after acid digestion. The mass concentration ranges of U in PM0.39-0.69, PM0.69-1.3, PM1.3-2.1, PM2.1-4.2, PM4.2-10.2, and PM10.2- reached 0.0030-0.079, 0.0020-0.069, 0.0015-0.095, 0.0053-0.054, 0.0039-0.098, and 0.0083-0.10 ng/m3, respectively. The mass concentration range of Th in PM0.39-0.69, PM0.69-1.3, PM1.3-2.1, PM2.1-4.2, PM4.2-10.2, and PM10.2- amounted to 0.011-0.11, 0.0065-0.11, 0.0026-0.18, 0.0078-0.14, 0.015-0.30, and 0.0021-0.19 ng/m3, respectively. The contents of U and Th in all PM increased from June to December, and the contents in PM2.1 increased more sharply compared with those in other PM. A positive correlation was observed between the concentrations of U and Th and air quality index and relative humidity, whereas a negative correlation was identified between temperature and PM2.1, PM10.2, and total suspended particulates (TSP) after the Spearman-rank correlation test. The formation of PM was affected by meteorological conditions, which concurrently influenced the contents of U and Th in PM. The atmospheric contents of U and Th at night were higher than those in daytime. Compared with Th, the dose contribution of U to the public can be negligible. The median effective dose in public owing to inhalation of natural radioactive U and Th in the atmosphere measures 1.206 μSv/a.