Regional and climatic variations in atmospheric microplastic deposition

The atmosphere is a critical reservoir for and transporter of microplastics (MPs) but little is known about the nature and drivers of their regional and climatic variability. In this study, dry deposition of MPs is quantified simultaneously over a seven-day period in nine Iranian cities encompassing different populations and climates and relationships with meteorological conditions and gaseous and particulate air quality parameters investigated. Overall, deposition ranged from < 5 to > 100 MP m-2 h-1 and was dominated by fibres of various sizes and constructed of different polymers (mainly polyethylene, polyethylene terephthalate, polypropylene, polystyrene and nylon), and there were clear and significant differences in mean values between the different cities that were not a simple function of climate or population. On a local scale, both positive and negative relationships between MP deposition and various meteorological and air quality parameters were observed among the cities. However, the pooled depositional data for MPs and various shapes and sizes thereof exhibited significant inverse relationships with wind speed and specific measures of airborne particulate matter (e.g., dust, PM-2.5, PM-10). The results suggest that there is a broadly consistent, fibre-dominated regional population of MPs whose deposition (and presumably resuspension) is influenced by variations in wind speed, but additional location-specific factors and sources contribute to temporal variations within the different cities. Despite the relationships between deposition and some gaseous and particulate air quality parameters identified at specific locations, it may be difficult to introduce a sharp parameter that can be used as a regional proxy for MP deposition. Acknowledgements We thank Shiraz University and Klaipeda University for technical support. This project has received funding from the Research Council of Lithuania (LMTLT), agreement No. S-PD-24-51.