Elucidating microplastic's seasonal occurrence in urban indoor and outdoor aerosol

This study investigates the seasonal presence of microplastics (MPs) in indoor and outdoor urban aerosols in Madrid, Spain, for the first time ever. Compared with the research conducted in the hydrosphere and lithosphere, atmospheric MPs pollution is considered relatively unexplored. To address this gap, PM₁₀ aerosol samples were collected over one year using the standard EN 12341:2014 method. Two rapid quantification methods based on UV-assisted optical microscopy were used, with and without prior extraction. MPs were characterized morphologically using field-enhanced scanning electron microscopy with energy-dispersive X-ray detection (FE-SEM-EDX) and identified through μRaman spectroscopy and pyrolysis coupled with gas chromatography-mass spectrometry (PyGC-MS). The hybrid single-particle lagrangian integrated trajectory (HYSPLIT) modelling software was used to link MPs potential sources. The study found that indoor MPs exposure (124 ± 25 particles m³/day) were higher than outdoors levels (29 ± 7 particles m³/day). Indoors exposure was 88 ± 32 MPs m³/day during the warm season (April-September) and 122 ± 14 MPs m³/day in the cold season (October-March). Outdoors, exposure was relatively consistent year-round (29 ± 8 and 31 ± 6 particles m³/day, warm and cold respectively). MPs composition in indoor samples and outdoor samples during the warm season was similar, mainly polystyrene (45.8 %), polyethylene (22.8 %), polyethylene terephthalate (20.0 %), and polymethylmethacrylate (11.4 %). However, outdoor MPs during the cold season showed notable differences. Fibres dominated over fragments, mostly smaller than 100 μm. The highest fibre-to-fragment ratios occurred in December outdoors (0.76) and February indoors (0.88). Atmospheric back-trajectory analysis indicated that many MPs likely originated from distant, especially oceanic, sources, confirming their long-range transport. Overall, the findings highlight significant indoor MPs exposure, especially in colder months, and suggest a general increase in MPs exposure over time across both environments.