Atmospheric Microplastics Under Rain: How Precipitation Alters Particle Accumulation and Polymer Composition in Porto, Portugal

Atmospheric microplastics (MPs), including synthetic fibres, are increasingly recognised as persistent airborne pollutants, yet the influence of deposition pathways on their transport, removal, and characterisation remains poorly understood. This study quantified and chemically characterised airborne microplastic particles and synthetic fibres in an urban residential area of Porto, Portugal, focusing on rainfall effects on abundance, morphology, polymer composition, and size distribution. Six atmospheric deposition samples (three dry and three wet) were collected during June–July 2024 using a passive NILU Microplastic Collector. Samples were size-fractionated into five classes (5 mm–125 μm, 125–63 μm, 63–25 μm, 25–12 μm, and 12–1.2 μm) through sequential filtration and H₂O₂ digestion. Particles were analysed by optical microscopy, SEM, and micro-Raman spectroscopy. Due to the degree of polymer degradation, Raman spectra were classified as unambiguously identified polymers, chemically similar polymer families, or indeterminate synthetic polymers. Hierarchical cluster analysis (HCA) evaluated compositional similarities among samples. Wet deposition showed substantially higher deposition rates of non-fibrous microplastic particles and synthetic fibres. Non-fibrous microplastic particle accumulation was approximately 8.5 times higher during rainfall events (3629 ± 449 particles·day⁻¹·m⁻²) than during dry periods (427 ± 190 particles·day⁻¹·m⁻²), while synthetic fibre accumulation was nearly three times higher (6515 ± 2683 vs. 2305 ± 829 fibres·day⁻¹·m⁻²). An exact Mann–Whitney U test showed no conventional statistical significance (p = 0.10 for both variables), reflecting the small sample size. Acryl (PMMA) was the dominant identified polymer, followed by polyester (PES) and polypropylene (PP). Wet deposition yielded a higher proportion of unambiguously identified polymers (65%) than dry deposition (32%). This difference may reflect variations in pigmentation, ageing, mineral interference, surface contamination, or other factors affecting Raman spectral quality. HCA revealed distinct compositional patterns between dry and wet deposition, indicating that precipitation may influence the abundance and chemical characteristics of atmospheric MPs.