Accumulation of Airborne Microplastics on Forest Canopy Leaves: Insights from Trichomes and Epicuticular Waxes

Abstract Airborne microplastics (AMPs) on konara oak (Quercus serrata Murray) leaves collected at a small forest in the Tokyo metropolitan area were analyzed. The leaves underwent a three-step treatment process: (i) washing with Milli-Q water, (ii) sonication in Milli-Q water, and (iii) extraction with a 10% KOH solution. The average number of AMPs found in solutions (i), (ii), and (iii) were 0.01, 0.05, and 0.07 microplastics (MP) /cm2 leaf, respectively. More than 50% of the total AMPs recovered from the aqueous samples were detected in the KOH solution. Scanning electron microscopy (SEM) observation revealed that the KOH solution significantly altered the leaf surfaces, while rinsing and ultrasonication with Milli-Q water had little effect. These results emphasize the strong adsorption of AMPs to epidermal waxes and the importance of using KOH solutions in addition to conventional water washing methods to prevent underestimation of AMPs intercepted by forest canopy leaves. Using data from the growing season, it was estimated that a small forest in the metropolitan area captures approximately 2.2 billion AMP particles per year. This indicates that forests may play an important role as sinks for AMPs in the terrestrial environment.