Challenges of using leaves as a biomonitoring system to assess airborne microplastic deposition on urban tree canopies

Monitoring microplastic concentrations in the atmosphere is critical for assessing their inhalation risk in urban areas and other ecological impacts on the natural environment. To assess these risks, leaves in tree canopies could be used as a biomonitoring system, but this requires knowledge of the potential variability of microplastic concentrations based on leaf properties and position in the tree. This study aims to quantify this potential variability by analyzing the concentration of airborne microplastics on leaves in Los Angeles as a function of leaf height, leaf surface hydrophobicity, and land use. Microplastic concentrations on leaves varied between 0.14 and 25 particles(n) cm −2, but the concentration peaked within 0.6–1.2 m height above ground, indicating the position of the leaf above the ground could matter. Microplastic concentrations varied significantly between leaf types indicating leaf surface properties could influence the retention of microplastics. Contact angle measurements revealed that the hydrophilicity of leaves had a weak correlation with microplastic concentrations on leaves, indicating other factors such as surface roughness or leaf features could be important for microplastic retention. Land-use type did not affect microplastic concentration. The results confirmed a high uncertainty in predicting microplastic concentration on leaves in the urban canopy, which limits their ability to be used as biomonitoring systems for microplastic pollution in urban areas.