# Lagrangian Modeling of Marine Microplastics Fate and Transport: The State of the Science
## 5 Results and Discussion  
Despite the fact that the reviewed models have generally acceptable accuracy in simulating the physical transformational and transport processes, each has some noticeable limitations in this regard. In this section, we have tried to discuss the reviewed models' limitations in detail regarding the consideration of microplastics' physical properties and processes. It can be concluded that this section can give better insights into the current gaps in the reviewed models. D-WAQ PART and Ichthyop assume microplastics as passive particles without inertia when they are floated at the sea surface, which is different in real ocean conditions [58; 119] since the shape and geometry of microplastics have impacts on their behavior, particularly during the different physical processes that they undergo. TrackMPD considers plastic particles as both active and passive particles using different scenarios. It means TrackMPD can simulate microplastics with different shapes (spherical and cylindrical) and sizes [61]. CaMPSim-3D is more powerful in this regard in comparison with the aforesaid models. This is because it utilizes the shape factor (\(\Psi\)) which capable CaMPSim-3D in considering particles with different sphericity and circularity [98]. As stated in Section 4, D-WAQ PART and Ichthyop cannot simulate biofouling and degradation, while both TrackMPD and CaMPSim-3D are capable of simulating these processes using the same empirical relationships. It should be noted that TrackMPD and CaMPSim-3D cannot simulate biofouling and degradation together within the same simulation. On the other hand, there are some other transformational processes in the real ocean condition such as hetero- and homo-aggregation [124] and agglomeration (known as the formation of particle clusters in which weak physical interactions are dominant between particles [125]). In addition, microplastics undergo weathering, photodegradation, chemical and biological degradation, and defouling (known as the biofilm layer disintegration [44]) in real conditions. However, none of the reviewed models can simulate any of the aforementioned processes yet. Unlike Ichthyop, D-WAQ PART, TrackMPD, and CaMPSim-3D can predict beaching and washing-off using a probability empirical theory with the same accuracy. However, actual beaching and washing-off in the real condition are highly complex processes since they depend on many other factors, such as the characteristics of winds, tides, waves, and coastline structure [58]. Moreover, microplastics may undergo stranding at the shoreline and be washed off from it repeatedly in subsequent episodes. Accordingly, the aforesaid models are currently incapable of simulating beaching and washing-off more accurately. All of the reviewed models can predict advection and diffusion; however, their accuracy in this regard is different. All models can simulate advection and windage (except for Ichthyop). Moreover, all of them are capable of modeling horizontal and vertical diffusion, although some of them employ different methods and relationships in calculating the horizontal/vertical diffusion coefficient. However, CaMPSim-3D utilizes a modified relationship for the calculation of turbulent diffusion, which seems to be a more accurate approach in comparison with the methods employed by other models. Among the reviewed models, only CaMPSim-3D can predict resuspension. On the other hand, there are some additional important driving transport processes such as Stokes drift (as a result of wave action [58; 104]), burial into the deep sediment of the waterbodies [124], and remobilization [126], which none of the reviewed models can predict. All of the reviewed models (except for CaMPSim-3D) are incapable of reading hydrodynamic parameters from an unstructured mesh provided by an Eulerian model. It should be noted that although reading hydrodynamic data from an unstructured mesh leads to a decrease in interpolation errors, it increases the computational costs of particle-tracking simulations.