Airborne and marine microplastics in the Northern Indian Ocean: decoupled pathways traced by black carbon

Abstract. Microplastics (MPs) are increasingly detected in marine and atmospheric particulate matter, but their sources, transport, and air–sea exchange remain poorly constrained in remote marine regions. We investigated airborne microplastics (AMPs) and marine microplastics (MMPs) during a 1-month campaign at the Maldives Climate Observatory Hanimaadhoo and in adjacent coastal waters. Airborne particles were collected with a deposition box, and seawater was sampled at coastal sites. Particle morphology, size, and polymer composition were determined by optical microscopy and µ-Raman spectroscopy and evaluated alongside aerosol scattering and equivalent black carbon (eBC). AMP concentrations ranged from 0 to 10.3 MP m^-3 (mean: 3.1 ± 2.8 MP m^-3). MMP concentrations ranged from 120 to 2656 MP m^-3 (mean: 876 ± 804 MP m^-3), exceeding typical open-ocean values and indicating substantial local influence from inhabited islands and maritime activity. Significant differences in size distributions and pronounced compositional differences of AMPs and MMPs suggest weak coupling between the atmospheric and marine reservoirs. Low dry-season wind speeds probably suppressed wave breaking, bubble bursting, and sea-spray-mediated transfer from ocean to atmosphere. Instead, AMP variability coincided with polluted air-mass advection, elevated eBC, and enhanced aerosol scattering, indicating stronger control by regional atmospheric transport than by local marine emissions. These results show that remote island atmospheres are exposed to anthropogenic plastic aerosol transport and demonstrate the value of combining MP observations with established aerosol tracers to resolve plastic-particle pathways in marine environments.