Natural radionuclides in atmospheric deposition and their responses to sandstorm events: A case study in Lanzhou.

The use of naturally occurring radionuclides as tracers in Earth science provides valuable insights into atmospheric processes and environmental dynamics. In this study, we conducted a one-year observation (September 2022-September 2023) of atmospheric deposition fluxes of natural radionuclides (including 7Be, 210Pb, 40K, 226Ra, 228Ra, and 228Th) in Lanzhou, Northwest China, in order to investigate their seasonal variations, controlling mechanisms, and potential applications in tracer studies. At the same time, the natural radionuclide activity ratios on fine soil particles (<75 μm) of the Northwestern China were also determined. The depositional results revealed significantly different variation patterns between airborne (7Be and 210Pb) and terrigenous (40K, 226Ra, 228Ra and 228Th) radionuclides. Deposition fluxes of 7Be and 210Pb were elevated during the rainy season and suppressed in the dry season, reflecting efficient scavenging by precipitation. In contrast, 40K, 226Ra, and 228Th exhibited pronounced springtime peaks, primarily associated with frequent sand/dust storm activities. Strong linear correlations between 7Be and 210Pb deposition fluxes, as well as their significant relationships with precipitation, indicate that rainfall is the dominant removal process for airborne radionuclides (e.g., 7Be and 210Pb). By comparison, we found that the activity ratios of natural radionuclides in the atmospheric deposition particles during the sand and dust weather in Lanzhou were close to those in the fine particle fraction of the Northwestern China soil that is prone to being lifted by wind, suggesting that there was a contribution from local and regional scale dust sources in the sandstorm events in Lanzhou. Furthermore, significant shifts in 232Th/226Ra and 40K/232Th activity ratios during sand/dust storm events, together with distinct isotopic signatures identified for East Asian, Arabian, and Saharan dust, demonstrate the possibility of using natural radionuclide ratios (232Th/226Ra, 40K/232Th, and 40K/226Ra) as fingerprints for dust provenance. Overall, this study enhances our understanding of natural radionuclide scavenging and deposition dynamics in arid and semi-arid regions, while also highlighting their potential application in tracing key atmospheric processes and long-range dust transport.