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 (<100 µm) showed greater affinity for the atmospheric phase, while larger particles were preferentially retained in aqueous reservoirs. Notably, the proportion of MPs as fibres correlated with concentration ratios involving Ca²⁺ (the only major seawater ion showing non-conservative behaviour), suggesting that regional biogeochemical processes (e.g., precipitation, flocculation, organic binding) may influence MP partitioning and fractionation. These observations point to coupled physical (bubble-mediated ejection, wave breaking, deposition) and biogeochemical controls on MP dynamics at the air–sea boundary. Our results highlight the SML and foam as critical compartments for MP accumulation and transfer and underscore the need for targeted laboratory and longer-term field studies to unravel mechanistic links between MP behaviour and coastal biogeochemistry.

 

Acknowledgements

This project received funding from the European Union’s Horizon Europe research and innovation program under the Marie Skłodowska-Curie Actions Grant Agreement No. 101153990.