Dry and wet deposition of microplastics in a semi-arid region (Shiraz, Iran)

Microplastics (MP) have been retrieved from settled dusts and rainfall collected monthly over a period of a year and daily over an additional period of ten days in the city of Shiraz and at an elevated remote site (Mount Derak) in Iran. Total monthly deposition ranged from about 1000 to 3500 MP m at Shiraz and about 200 to 600 MP m at Mount Derak, with greater deposition taking place during the dry months (June to November) than in months when precipitation events occurred. Overall, MP deposition was dominated by dry deposition, with precipitation appearing to washout MP and inhibit local resuspension of material by dampening the ground. The majority of MP in dust and precipitation (>99%) was of a fibrous nature, with the finest particles measured (<100 μm in length) comprising ~20-40% of MP at Shiraz and ~70-80% of MP at Mount Derak and analysis of selected samples (n = 34) revealing the most abundant polymers to be polypropylene, polyethylene polystyrene and polyethylene terephthalate. While in Shiraz, local sources of MP are likely significant, the occurrence of MP at Mount Derak and back trajectories computed with HYSPLIT suggest that distal sources, mainly carried by air masses from the north and west, are also important. The observations here add to the growing evidence of the global ubiquity and long-range transport of MP in the atmosphere and reinforce calls for more research into the potential impacts of MP in this environmental compartment.