Microplastics
(MPs) are drawing increasing attention from the international
community due to their potential threats to the ecosystem and human
health. Although their occurrence and spatial distribution have been
extensively studied in recent years, the relationship between their
abundance and sizes remains unclear. Moreover, the underlying mechanisms
dominating their size distribution have rarely been explored. In the
present study, we developed a novel conditional fragmentation model
to describe MP size distribution in the soil environment. It is proposed
that the distribution of MPs is not a coincidence but controlled by
conditional aging. The applicability of this model was tested using
data collected from different land use settings in Beijing, China.
A distinct downsizing phenomenon from fibers, films, and fragments
to granules is observed. Undisturbed land use types accumulated larger
sized MPs with higher stability, while human interference accelerated
the fragmentation of MPs. Both morphological analysis and time-of-flight
secondary ion mass spectroscopy (TOF-SIMS) observations provided direct
evidence for the conditional fragmentation process. Furthermore, the
model has proven to be suitable for describing the size distribution
of MPs from various sources (including atmospheric deposition, transportation,
and agriculture) and aging processes (such as mechanical abrasion,
chemical oxidation, and photochemical transformation). It is proposed
that this model can be used for various purposes in MP-related studies,
especially source identification, transport modeling, and risk assessment.