Quantifying morphological complexity and wet deposition of suspected microplastics in rainwater: A case study of...

Despite growing concern regarding the wet deposition of atmospheric microplastics (MPs), the role of particle morphological complexity in controlling deposition efficiency and atmospheric transport remains insufficiently understood. Characterising MP geometry is essential for analysing aerodynamic behaviour and environmental interactions. This study presents the first quantitative assessment of the fractal dimension (FD) of particles exhibiting microplastic-like morphology (hereafter referred to as suspected microplastics, MPs) in the Central European city. Over a 14-month period, rainwater samples were collected from urban residential and traffic-influenced areas in Wroclaw, Poland, using a passive sampler positioned 5 m above ground level. Advanced morphological characterisation was conducted using scanning electron microscopy (SEM), followed by vector-based geometric analysis implemented in Python for particle classification and FD estimation. Particle identification is based on SEM-derived morphology and does not include chemical confirmation; therefore, classification as microplastics is indicative rather than definitive. Mean abundances of suspected microplastics (MPs) were 135 ± 89 particles L^-1 in the residential area and 168 ± 64 particles L^-1 in the traffic-influenced area. Fibres dominated wet deposition and exhibited a mean FD of 1.10 (σ = 0.15), indicating smooth, elongated geometries with low structural complexity. Fragments were observed less frequently and exhibited greater morphological variability; however, the analysis focuses primarily on fibres, as they are more common. These findings demonstrate that fractal dimension provides a quantitative descriptor of particle morphological complexity and may serve as a descriptor of aerodynamic behaviour and environmental fate in atmospheric systems.