Cosmogenic beryllium-10 (¹⁰Be) and beryllium-7 (⁷Be) have clear sources, stable environmental behavior, and significant differences in half-life. These natural environmental tracers have great potential for application and have been used to study stratospheric ozone input from natural sources, aerosol generation and transport, and atmospheric circulation.

Micro/nano-plastics (MNPs) can travel thousands of kilometers through the atmosphere, reaching remote environments such as the Tibetan Plateau and the Arctic. However, direct observational evidence of the sources and atmospheric migration mechanisms of MNPs in high-altitude areas is lacking, especially regarding the involvement of stratospheric air in the long-distance transport of microplastics.

Now, a research group from the Institute of Earth Environment of the Chinese Academy of Sciences has introduced cosmogenic ¹⁰Be and ⁷Be into atmospheric MNPs. Using accelerator mass spectrometry, pyrolysis gas chromatography/mass spectrometry, laser direct infrared imaging, and numerical simulation, the researchers systematically revealed the process by which stratospheric air participates in the long-distance transport of MNPs in high-altitude areas in Lhasa, on the southern Tibetan Plateau.

They established a new method for synergistically tracing stratospheric air transport of MNPs using ¹⁰Be and ⁷Be was established, achieving the quantitative identification of the cross layer migration process of new pollutants. This method provides a new technological approach to studying stratospheric transport, which has been difficult to observe directly for a long time.

The researchers found that the MNP mass concentration over the southern Tibetan Plateau ranged from 0.0059 to 0.11 μg/m³, mainly composed of polyethylene terephthalate, polyvinyl chloride, and polycarbonate, accounting for over 60% of the total mass.

By analyzing the ¹⁰Be/⁷Be ratio changes, the researchers identified episodes in which stratospheric air descended into the lower atmosphere over the Tibetan Plateau. Their results suggest that these air masses can undergo large-scale downward transport in approximately one month, which accelerates the removal of MNPs from the upper atmosphere. Consequently, the atmospheric residence time of MNPs in the stratosphere decreases from an estimated one to two years to just one to three months. During these intrusion events, the dry deposition flux of MNPs increased approximately threefold, while the regional daily average ozone concentration increased by about 34% (14 ppm).

Further analyses suggest that the strong westerly jet stream in winter promotes stratospheric air folding at the top of the troposphere over the Tibetan Plateau, pushing high-altitude air to sink to the near ground. The atmospheric MNPs in high-altitude areas not only come from near ground emissions, but also play a key role in the sinking airflow in the stratosphere and cross-border long-distance transport. During the observation period, about 7% of the air masses that reached the sampling point came from the boundary area between the stratosphere and troposphere.

The findings also suggest that, due to the unique high-altitude terrain and atmospheric dynamic processes of the Tibetan Plateau, the atmospheric circulation system can accumulate and rediffuse MNPs during certain periods, forming a "secondary pollution source." Beryllium isotopes serve as unique identification signals for stratospheric air masses. Continually exploring their tracing value can help clarify the transport patterns of MNPs and other substances in the upper atmosphere.

This study demonstrates the potential for extending this approach to the environmental tracing of emerging contaminants, such as MNPs. In the future, researchers will construct a natural tracer system for pollutants based on cosmogenic nuclides and apply it to studying cross-layer migration, long-distance transport, and the environmental fate of different typical pollutants. This will provide new research tools for revealing the global circulation mechanisms of emerging contaminants and will support the precise prevention and control of these contaminants.

This work, published in Journal of Hazardous Materials on July 15, was supported by the National Natural Science Foundation of China.