Characteristics, behaviour and fluxes of microplastics in the coastal boundary zones of the Persian Gulf and Oman Sea

Relatively little is known about microplastics (MPs) in the atmosphere of the coastal zone, including their interaction with the sea surface. In this study, MPs have been determined and characterised in the lower atmosphere, advecting air, depositing dusts, coastal sediments and seawater of the Persian Gulf and Oman Sea using a variety of sampling techniques (including filtration of water and pumped air and deployment of a vertical array of sediment traps). MPs were detected in all samples and were dominated by fibres that were, in most cases, small (<100 μm) and black. MP numbers captured in advecting air showed no trends with height or differences between locations or deployments over land and sea, but a clear increase was observed during strong winds. MPs in atmospheric suspension and in deposited dusts, sediments and seawater were also heterogeneously distributed. Environmental or transport pathway fractionation was evident according to morphology and particle size (% fibres and % small fibres) and to polymer density. Regarding the latter, relatively low-density polymers (e.g., polyethylene, polypropylene) were more abundant in the atmosphere, advecting air and seawater, whereas higher density polymers (e.g., regenerated cellulosics, polyethylene terephthalate) were more abundant in settling dusts and sediments. Fluxes, based on particle counts and converted MP m h, revealed advection (up to ∼ 1.2 × 10) was greater than deposition (up to ∼ 400) by at least three orders of magnitude over land but only by a factor of a few hundred over sea. Neglecting any differences in resuspension, this suggests a greater net loss of airborne MPs over the ocean. Net settling velocities for the population of MPs in the lower atmosphere, derived from depositional fluxes and concentrations in air, ranged from about 1.2 to 13.1 m h, with residence times at an elevation of 10 m ranging from 45 min to 8.3 h. Our observations suggest that long-range transport of the type of MPs detected is constrained by a succession of deposition-resuspension cycles that must be factored into future modelling.