A green, fast protocol to estimate the accumulation of airborne anthropogenic microfibers in <i>Pittosporum tobira</i> in urban areas: effects of season and rainfall

Plastics represent a major organic pollutant, but research focused on their biomonitoring in the air has only recently received attention. In the present work, we investigated the ability of <i>Pittosporum tobira</i> leaves to distinguish different levels of air contamination due to anthropogenic microfibers (MFs) in six urban sites characterized by different land uses (industrial, urban, and green), and the effect of wet/dry season on their accumulation. Moreover, the effect of pouring rain on MFs accumulation was estimated by transplants of <i>P. tobira</i>. Microfiber extraction was done by tape tearing on 1 g composite leaf samples on the leaf surface. In summer, the highest number of MFs were found in the leaves from the industrial site (160), followed by urban ones (84-125), and green parks (48-54). The accumulation of MFs was overall higher in summer than in winter, due to the rain-washing effect in the latter, and the different leaf traits observed in the two seasons. The development of glandular hairs during summer could contribute to increasing the accumulation of MFs observed in this period under conditions of reduced precipitation. In agreement, when comparing MFs fallout on leaves of sheltered and unsheltered transplants after a heavy rainfall, the number counted on the latter was significantly lower, suggesting that precipitations reduce MFs deposition. These findings reinforce the suitability of <i>Pittosporum tobira</i> leaves as a bioindicator for airborne anthropogenic MFs. Moreover, the pronounced seasonal differences, as well as the higher MFs loads during dry summer months, indicate that monitoring sensitivity is enhanced under low-rainfall conditions.