Multimodal Single-Particle Characterization of Airborne Microplastics Reveals Physicochemical Heterogeneity and Aging-Related Features

Abstract Airborne microplastics (AMPs) have emerged as an important atmospheric contaminant; however, their physicochemical characteristics at the individual particle level remain poorly understood. In this study, a multimodal single-particle analytical approach integrating fluorescence microscopy, Raman microspectrometry (RMS), optical photothermal infrared (O-PTIR) spectroscopy, and SEM/EDX was developed for the characterization of individual AMP particles. Fluorescence microscopy enabled screening of particles with high AMP potential, while RMS and O-PTIR provided complementary molecular and functional characterization, and SEM/EDX further provided morphological information. Among the fluorescent AMP candidates analyzed in a single aerosol sample, 18.0% and 19.7% were identified as AMPs by RMS and O-PTIR, respectively, whereas their combined application increased the identification fraction to 22.0%. The incomplete overlap between the two techniques demonstrates their complementary analytical capabilities and indicates that reliance on either technique alone may result in the underestimation of AMPs. Their combined use further enabled the detailed physicochemical characterization of individual AMPs. Notably, 86% of the identified AMPs exhibited spectral features distinct from those of standard polymers, including peak attenuation, broadening, and additional peaks, accompanied by worn and irregular morphologies. These differences suggest modifications associated with environmental aging processes, including oxidation, chain scission, and changes in aromatic structures. Substantial particle-to-particle variability within the same polymer type and similar size range further indicated heterogeneity in environmental exposure history and sources. Although the present study is limited to a single urban aerosol sample, the results demonstrate that multimodal single-particle analysis can reveal physicochemical heterogeneity and aging-related features of AMPs that may not be readily accessible using conventional single-technique approaches.