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There is a growing concern that nanoplastic pollution may pose planetary threats to human and ecosystem health. However, a quantitative and mechanistic understanding of nanoplastic release via nanoscale mechanical degradation of bulk plastics and its interplay with photoweathering remains elusive. We developed a lateral force microscope (LFM)-based nanoscratch method to investigate mechanisms of nanoscale abrasive wear of low-density polyethylene (LDPE) surfaces by a single sand particle (simulated by a 300 nm tip) under environmentally relevant load, sliding motion, and sand size. For virgin LDPE, we found plowing as the dominant wear mechanism (i.e., deformed material pushed around the perimeter of scratch). After UVA-weathering, the wear mechanism of LDPE distinctively shifted to cutting wear (i.e., deformed material detached and pushed to the end of scratch). The shift in the mechanism was quantitatively described by a new parameter, which can be incorporated into calculating the NP release rate. We determined a 10-fold higher wear rate due to UV weathering. We also observed an unexpected resistance to initiate wear for UV-aged LDPE, likely due to nanohardness increase induced by UV. For the first time, we report 0.4–4 × 10–3 μm3/μm sliding distance/μN applied load as an initial approximate nanoplastic release rate for LDPE. Our novel findings reveal nanoplastic release mechanisms in the environment, enabling physics-based prediction of the global environmental inventory of nanoplastics.