Air-sea exchange and coastal transport dynamics of microplastics around a Caribbean Island

Air-sea exchange represents a key yet insufficiently quantified pathway for microplastic (MP) transport, particularly in island environments where oceanic exposure and atmospheric forcing interact. In this work, we aimed to quantify the distribution and polymer composition of MP in both the atmosphere and seawater around a Caribbean island while simultaneously evaluating their transport pathways through Lagrangian drift modeling. MP distribution, polymer composition, and transport dynamics were examined through coordinated atmospheric and surface-water sampling, micro-FTIR analysis, chemometric discrimination, and Lagrangian drift modeling (OpenDrift). Polyethylene (PE) dominated airborne MP (34 %), while polyester (PES) prevailed in seawater (54 %), indicating selective partitioning driven by density, morphology, and surface chemistry. Morning airborne concentrations were 37 % higher than afternoon values, consistent with sea-breeze circulation patterns. Seawater MP concentrations increased from 5 MP L^-1 to 35 MP L^-1 toward the continental shelf, a spatial gradient reproduced by drift simulations showing > 40 % nearshore retention within 24 h and rapid northward export via the Yucatán Current. By integrating polymer-specific characterization with physical transport modeling, the present study provides mechanistic insight into how intrinsic material properties and local hydrodynamics jointly determine MP fate in tropical island systems, offering a framework for targeted monitoring and mitigation in coastal environments.