Elevated wet deposition of micro- and nanoplastics in remote mountains driven by free tropospheric transport

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^-1, nearly three times higher than at the low-altitude site (7.8 ± 4.3 μg L^-1), 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^-2 event^-1 (event-mean concentration × rainfall depth), with an annual rate of 13 kg km^-2, 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.