Regional sources drive atmospheric microplastic deposition at rural background sites

The growing focus on long-range atmospheric transport of microplastics (MPs) has overshadowed the importance of local sources. Here we investigated the abundance and sources of MP deposition on a regional scale (22,994 km²) using pleurocarpous moss collected from 33 background rural sites across Tuscany, Central Italy. A total of 288 MPs (>50-5000 μm) were found across all sites, dominated by fibres at 86.8 % and tire wear particles at 4.9 %. Given the dominance of textile fibres, polyethylene terephthalate was the dominant polymer at 29.2 %; nonetheless, the diversity of polymers also suggested local agricultural sources, such as plastic mulch (polyethylene and copolyester, both at 12.5 %) and agricultural superabsorbent hydrogel polymers (polyacrylic acid at 16.7 %). The accumulation of MPs ranged from 1.3 to 11.6 MPs per gram of moss dry weight (median 4.8 ± 2.3 MP/g) and estimated mass concentration ranged from 0.3 to 116.8 μg/g (median 2.9 ± 2.1 μg/g). Median particle length was 650 μm and median particle mass was 0.5 μg, suggesting that atmospheric transport was the primary pathway for these small lightweight particles. The population within a 10 km buffer, distance to urban centres, and moss tissue content of chromium (Cr) and nickel (Ni) were significantly associated with airborne MPs, suggesting that MP concentrations were primarily influenced by local and regional-scale anthropogenic factors within a range of 10-100 km, rather than long-range sources. The sources of Cr and Ni are primarily geogenic, originating from ultramafic rocks, particularly ophiolites, which are a unique indicator of Tuscan aeolian dust emissions from agricultural fields or wind-blown soil particles. These findings highlight the potential of moss biomonitoring as a practical and scalable tool for the source assessment of atmospheric MP contamination on a regional scale. Further, our results identify agricultural plastics and urban centres as important regional sources of microplastics.