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Elevated wet deposition of micro- and nanoplastics in remote mountains driven by free tropospheric transport
Summary
Researchers measured micro- and nanoplastic (MNP) concentrations in rainfall at a remote high-elevation mountain site in southern China, finding concentrations nearly three times higher than at a nearby urban low-altitude site, demonstrating that the free troposphere acts as a long-range transport pathway that delivers significant plastic loads to pristine environments.
Airborne Micro- and nanoplastics (MNPs) can reach the free troposphere (FT) and transport to remote regions, however, their wet deposition within FT remains unclear. Here, we investigated MNP wet deposition at a high-elevation site within the FT (1700 m a.s.l.) in a remote mountain reserve in southern China, and compared it with a low-altitude site (60 m a.s.l.) influenced by urban outflow. MNPs were identified and quantified by pyrolysis-gas chromatography-mass spectrometry (Pyr-GC/MS), resolving polymer types from diagnostic pyrolysates with external calibration to mass concentrations. Event-mean MNP concentrations in mountain rainwater averaged 22.0 ± 20.5 μg L, nearly three times higher than at the low-altitude site (7.8 ± 4.3 μg L), suggesting substantial MNP loads in the FT. Back-trajectory analysis identified dominant sources as regional transport from the Pearl River Delta and long-range transport from the Indochina Peninsula. Seasonal patterns showed that rainfall scavenging dominated in spring, while wind-driven FT transport prevailed in summer. MNP wet deposition averaged 54 μg m event (event-mean concentration × rainfall depth), with an annual rate of 13 kg km, totaling 7.4 metric tons per year in the mountain reserve. Our results indicate that the FT acts as both a transport pathway and a temporary reservoir for airborne MNPs, highlighting its role in the global fate of atmospheric plastic.