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.

微塑料颗粒的形态描述主要基于主观描述。然而，数据比对需要明确的分类。这项工作提出了基于最小封闭正交平行六面体长度的形态学描述。三个无量纲参数，即均匀度、扁平度和伸长率，可以参考基本的 3D（球体）、2D（板状）和 1D（棒状）形状来描述任何颗粒形状。与微塑料的环境归宿直接相关的粒径是斯托克直径。解释了基于 3D 形态描述符的斯托克直径的推导，并讨论了如果只有 2D 投影图像可用时可以使用的代理。这项工作表明，使用与粒子相同体积的球体直径作为描述符，不能充分预测不规则粒子的行为。需要获得专门为塑料颗粒（尤其是纤维）和大气隔室开发的方程。