Fragmentation-Driven Optical Darkening of Nanoplastics during Atmospheric Microplastic Photoaging

Atmospheric photoaging can fragment microplastics into secondary microplastics and nanoplastics with potentially important radiative effects, while the coupled changes in particle size, oxidation, and light absorption during this process remain poorly resolved. Here, we systematically investigate the photofragmentation of microplastics and characterize the resulting micro/nanoplastics to resolve the relationships among fragment size, oxygenation, and visible-light absorption. Photoaging promotes progressive fragmentation toward the submicron regime, with smaller initial particles tending to fragment more rapidly. At the same time, oxidation intensifies as particle diameter decreases, yielding a pronounced size-dependent increase in the O/C ratio below ∼500 nm. This chemical evolution is accompanied by strong optical darkening: the imaginary refractive index (k) and mass absorption cross-section (MAC) increase markedly with decreasing particle diameter and increasing oxygenation, while the single-scattering albedo (SSA) decreases. A strong relationship is observed between fragment size, oxidative aging, and light absorption across the visible spectrum (450-650 nm). Atmospheric photoaging transforms weakly absorbing microplastics into highly absorbing nanoplastics, whose size-dependent optical properties and chemical evolution provide a basis for integrating them into atmospheric and climate models.