Morphological description of microplastic particles for environmental fate studies

The morphological description of microplastic particles is mostly based on subjective descriptors. However, data intercomparison requires unambiguous classifications. This work presents a morphological description based on the lengths of the smallest enclosing orthogonal parallelepiped. Three dimensionless parameters, namely equancy, platiness and elongation describe any particle shape with reference on the basic 3D (sphere), 2D (plate) and 1D (rod) shapes. The particle size directly linked to the environmental fate of microplastics is the Stoke's diameter. The derivation of Stoke's diameter based on 3D morphological descriptors is explained and the proxies that can be used if only 2D projected images are available is discussed. This work shows that the behaviour of irregular particles is not adequately predicted using as descriptor the diameter of the sphere with the same volume as the particle. There is a need to obtain equations specifically developed for plastic particles, especially for fibres, and for the atmospheric compartment.