First Observations of Airborne Microplastics in Phnom Penh, Cambodia: Aerodynamic Size Distribution, Local Sources, and Monsoonal Transport

Airborne microplastics (AMPs) are increasingly recognized as an emerging air pollutant. However, observational data remains scarce in Southeast Asia. This study provides the first observations of AMPs in Phnom Penh, Cambodia, using µFTIR-ATR imaging. Number concentration, morphology, polymer composition, aerodynamic size distribution, Feret diameter, and surface aging characteristics were investigated together with meteorological parameters, gaseous pollutants, water-soluble ionic tracers, and HYSPLIT backward trajectories for source attribution. AMPs were dominated by polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET), with 52% classified as fragments and 82% having Feret diameters smaller than 30 µm. AMP concentrations ranged from 0.55 to 1.27 MP m-3 in TSP (mean: 0.97 ± 0.30 MP m-3, n = 134) and from 0.23 to 0.49 MP m-3 in PM2.5 (mean: 0.33 ± 0.10 MP m-3, n = 46). Carbonyl and hydroxyl indexes indicate that PE and PP were relatively fresh and in low-to-moderate photo-oxidative aging states. Correlation analysis further indicates that PE and PP were mainly associated with local waste fragmentation, whereas PET was linked to precipitation scavenging and resuspension processes. In addition, HYSPLIT backward trajectories suggest regional transport of AMPs by the Southwest Monsoon from marine source regions, including the Indian Ocean, Arabian Sea, and Andaman Sea. These findings provide the first baseline dataset for AMP pollution in Phnom Penh, Cambodia and highlight the combined importance of local emissions and regional atmospheric transport in Southeast Asia.