Tracing the air–sea exchange of microplastics over the Caspian Sea

The global proliferation of microplastics (MPs) is increasingly recognized as a transboundary environmental issue. At the air–ocean interface, MPs can be emitted via sea spray and transported back to land, while terrestrial MPs can likewise be advected and deposited over the oceans. However, the long-term net exchange of MPs between land and ocean via the atmosphere remains poorly constrained. Here, we investigate coastal atmospheric MPs and their near-surface landward and seaward transport over the southern Caspian Sea. Using a combination of passive air sampling (at seven heights with MWAC collectors) and active sampling (vacuum pump) over periods of 3 days and 2 months, respectively, together with coastal surface sediment samples, we quantified MP concentrations and assessed the influence of meteorological and environmental factors on their distribution. Fibrous MPs dominated all compartments, with airborne concentrations averaging 3.85 MP m−3 and sediment concentrations ranging from 507 to 1476 MP kg−1 (dry weight). Estimated near-surface horizontal fluxes were comparable in magnitude, with a landward influx of ~6566 MP m−2 h−1 and a seaward outflux of ~8039 MP m−2 h−1, indicating broadly balanced coastal transport during the 72 h campaign. To support source attribution, we evaluated co-trapped particulate proxies (sea salt and ash) and combined them with FLEXPART modelling. Trajectory modelling and proxy evidence indicate that most airborne MPs originated from inland sources (e.g., road dust and textile-related fibres), while marine sea-spray contributions were minor during the sampling period. These findings highlight the importance of long-range atmospheric transport in coastal MP pollution and demonstrate how integrating proxy observations with dispersion modelling can help constrain likely source regimes.