Characterization and sources of indoor and outdoor microplastics and PM_2.5: Possible relationships

Microplastics (MPs) were caused adverse impacts on the marine and terrestrial environments. MPs have been detected in atmospheric deposition, in both large cities and remote areas, raising concern for human health due to their potential intake. In this work, we report atmospheric deposition of MPs indoors and outdoors and explore the characteristics, and sources of atmospheric MPs. The characteristics of MPs in samples were detected via laser direct infrared (LDIR) spectroscopy and intelligent ultra-depth field microscopy. The potential sources of MPs were determined by backward trajectory analysis. All samples contained MPs, and the average concentrations were 241 ± 17 MPs/m²/d (outdoor) and 160 ± 7 MPs/m²/d (indoor). The observed MPs exhibited various colours, with black MPs accounting for the overwhelming majority (92%). The proportion of MPs with diameters smaller than 20 μm – 80 μm was 70%. Pellets were the most abundant shape of all MPs (60.4% ± 11%), followed by fragments (22.5% ± 3%) and fibres (17.1% ± 15%). In all samples, 7 polymers were detected, cellulose (average proportion: 66.1%) and polyamide (PA; average proportion: 12.5%) occupied the highest proportion. Follow by Fluorocarbon Rubber (FKM) 10.7%, Polyvinylchloride (PVC 8.4%), Polyethylene Terephthalate (PET 1.4%), Polyurethane (PU 0.5%), Polystyrene (PS 0.5%). Backward trajectory analysis revealed the influence of wind on MPs and PM_2.5 transport, and potential source contribution analysis demonstrated the potential sources of MPs. The results of this study indicated that PA accounted for the highest proportion of MPs (average proportion: 47.1%), followed by PVC 43.5%, PET 4.7%, PU 2.4%, PS 2.4%. Pellets were the most abundant of the three shapes. The influence of the airflow trajectory on MPs and PM_2.5 transport is remarkable, and the concentrations of these two pollutants were significantly correlated (r = 0.322, p < 0.01). Therefore, it is necessary to invest more effort in exploring the sources and transmission of MPs and developing routine MPs detection programs.