Dynamics of microplastics across the air–sea interface: enrichment in the sea-surface microlayer, foam, and links to regional biogeochemistry

We used a simultaneous, multi-compartment sampling approach to quantify microplastics (MPs) in the coastal zone of the Persian Gulf during winter and summer. Samples were collected from subsurface seawater, the sea-surface microlayer (SML), sea foam, suspended atmospheric particles and deposited dust. MPs were dominated by fibres of varied sizes and colours and, based on µ-Raman analysis of a subset, comprised thermoplastics, thermoplastic elastomers, synthetic rubbers and resins. MPs were strongly enriched in the SML and in sea foam relative to underlying seawater (enrichment factors on the order of 10², using SML thickness estimates up to 1000 µm), indicating the SML is a key reservoir and mediator of air–sea exchange. Estimated settling velocities in the lower atmosphere (derived from suspended concentrations and depositional fluxes) ranged from ~28 to 47 m h⁻¹, and size- and shape-dependent fractionation was evident: the finest fraction (