Recent studies have highlighted the importance of the atmosphere in the long-range transport of microplastic fibres. However, their dry deposition in the atmosphere is not fully understood, with the common spherical-shape assumption leading to significant uncertainties in predicting their travel distance and atmospheric residence time. Shapes of microplastic fibres vary greatly, which can be as long as 100 μm and as thin as 2 μm. Shapes of microplastic fibres may greatly affect their dry deposition in the atmosphere. Here we develop a theory-based settling velocity model for simulating atmospheric transport of microplastic fibres in different sizes and shapes. The model predicts a smaller aerodynamic size of microplastic fibres than that estimated by using volumetrically equivalent spherical counterparts. We find that the treatment of flat fibres as cylindrical ones, due to uncertainty in dimensions of sampled microplastic fibres, would cause overestimation of their dry deposition rate. Accounting for fibre thickness in sampled microplastic fibres leads to a mean enhancement of residence time by more than 450% compared to cylindrical ones. The results suggest a much more efficient long-range transport of flat fibres than previously thought.
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Original data underlying the figures are available at https://github.com/a20070348/MPFs_Data.
Code for computing the settling velocity given different characteristics of the fibres is available at https://github.com/a20070348/MPs.
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We thank D.L. Koch for insightful scientific discussions and comments concerning this work. Our thanks also extend to Y. Guo and S. Yin for assistance with the illustrative figures. Q.L. acknowledges support from the US National Science Foundation (NSF-CAREER-2143664, NSF-AGS-2028633, NSF-CBET-2028842) and computational resources from the National Center for Atmospheric Research (UCOR-0049). J.B. acknowledges support from the US National Science Foundation (NSF-MSB-1926559).
The authors declare no competing interests.
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Nature Geoscience thanks Sajjad Abbasi, Cristian Marchioli, Masanori Saito, and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Primary Handling Editor: Xujia Jiang, in collaboration with the Nature Geoscience team.
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Settling velocity for round fiber and flat fiber with l = 1 mm and different β definition.
Settling velocity distribution for flat fiber with different β definition for field measurement data.
Comparison of aerodynamic size distributions for flat fibers with different prescribed thicknesses.
(a) the reduction in dry deposition rate for MPF with a flat bottom and (b) the reduction in dry deposition rate for MPF with round cross-section. Note that the reduction in a dry deposition for MPF with a flat bottom is evaluated between ws calculated from our model for MPF with a flat bottom and the one obtained from our model but with a presumption that the cross section is in a round shape based on the sampled fiber on site. The reduction in dry deposition for MPF with a round cross section is compared between ws calculated from our model and the one obtained from the volumetric spherical particle model. For all the subfigures, red dash–dot lines denote the mean quantities and black solid lines represent the median values.
Figure showing the size of the ocean source (Tg on the y axis) as a function of the size of plastics (um on x-axis) in sensitivity studies where all plastics are assumed to be one size for each sensitivity study.
Flowchart for implementing the proposed model for large-scale climate model.
Framework of our proposed settling velocity model for microplastic fiber.
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Xiao, S., Cui, Y., Brahney, J. et al. Long-distance atmospheric transport of microplastic fibres influenced by their shapes. Nat. Geosci. 16, 863–870 (2023). https://doi.org/10.1038/s41561-023-01264-6