Atmospheric deposition of microplastics in Shiraz, Iran

The atmosphere plays a critical role in the regional and global transportation and redistribution of microplastics (MPs). However, the significance of rainfall and its means of scavenging MPs are not well understood. In this study, MP deposition was determined during successive dry and rainy events over eight consecutive days in the Shiraz region of Iran. Flux magnitudes and temporal distributions at six sites within and outside the city (including a remote, non-urbanised location) were similar and revealed a progressive increase in MP abundance and deposition during dry periods (up to about 50 MP m−2 h−1) and subsequent relatively rapid wet deposition (washout) by incipient rainfall (peaking at about 130 MP m⁻² h⁻¹). Wet deposition of MPs progressively decreased throughout the rainfall event, but with evidence of secondary peaks, before atmospheric accumulation and dry deposition increased during the next dry event and the cycle continued. Dry and wet deposition were dominated by fibres (that included polyester-polyethylene terephthalate, polystyrene, polyvinyl chloride and polyethylene) but the evolution of deposition did not appear to be associated with changes in MP size. These observations indicate that, as with other airborne pollutants, initial rainfall is an efficient scavenger of atmospheric MPs. Along with variations in methodology, this effect may contribute to the wide variation in MP fluxes reported in the literature.