Abstract
The size distribution of marine microplastics provides a fundamental data source for understanding the dispersal, break down, and biotic impacts of the microplastics in the ocean. The observed size distribution at the sea surface generally shows, from large to small sizes, a gradual increase followed by a rapid decrease. This decrease has led to the hypothesis that the smallest fragments are selectively removed by sinking or biological uptake. Here we propose a new model of size distribution, focusing on the fragmentation of marine plastics. The model is inspired by ideas from statistical mechanics. In this model, the original large plastic piece is broken into smaller pieces once by the application of “energy” or work by waves or other processes, under two assumptions, one that fragmentation into smaller pieces requires larger energy and the other that the occurrence probability of the “energy” exponentially decreases toward larger energy values. Our formula well reproduces observed size distributions over wide size ranges from micro- to mesoplastics. According to this model, the smallest fragments are fewer because large “energy” required to produce such small fragments occurs more rarely.
Highlights
A large fraction of the estimated billion tonnes of plastic waste that goes into the ocean [1] is found in a fragmented form, “microplastics”, with sizes of less than 5 mm [2] through photodegradation and weathering processes [3,4,5]
Such simulations generally assume the size distribution of microplastics and their results would depend on the assumption because sedimentation and biological uptake can depend on size [14, 16,17,18]
In this paper, assuming that “energy” needed to break down the plastic pieces acts as a constraint, we propose a new approach to modeling the fragmentation of marine plastics in relation to potential mechanical forces exerted on the plastic pieces borrowing ideas from statistical mechanics
Summary
A large fraction of the estimated billion tonnes of plastic waste that goes into the ocean [1] is found in a fragmented form, “microplastics”, with sizes of less than 5 mm [2] through photodegradation and weathering processes [3,4,5]. Our model agrees well with the observed size distributions in the Western-Pacific transoceanic section (Err = 22%) and the Seto Inland Sea (Err = 21%) (Fig 3c and 3d) with somewhat larger error than around Japan. Our model generally reproduces well the observed size distributions it has large relative error in the South Pacific Ocean (Fig 4b), where the least number of samples were collected in Cozar et al.’s study and the spectrum is the least smooth (barely visible in the plot), suggesting that the number of samples is not sufficient. A model for the size distribution of marine microplastics plastic waste is fragmented there, the large γ value for the North Pacific may be explained (Fig 4c). It is possible that some microplastics in this region originate from the Philippine Sea, where wave energy is large [57], as previous studies indicate that some waters from the Philippine Sea are transported into the Seto Inland Sea through the western boundary current [58,59,60]
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