Theoretical study on the impact of microplastics in seawater on water-leaving reflectance using Monte Carlo simulations.

Microplastics present in seawater significantly alter its radiative properties. This study investigates the impact of various types of microplastics on the inherent optical properties (IOPs) of seawater and examines how these changes influence the radiative transfer of solar energy within the ocean. The analysis utilizes the parameter spectral water-leaving reflectance in the visible region, with Monte Carlo simulations. The microplastics considered in this study are polystyrene (PS), dryer lint (DL), polyamide (PA), glycol-modified polyethylene terephthalate (PETG), polyester fiber (PEF1 and PEF2), polypropylene (PP), and polyvinyl chloride (PVC). The Monte Carlo simulations were conducted for varying seawater conditions, with the chlorophyll concentration ranging from 0.1 to 2.0mgm-3 and the microplastics concentration ranging from 0.01 to 1.0gm-3. The experimental measurements of IOPs of microplastics were used for the simulations. The simulation results demonstrate that the presence of microplastics in seawater enhances the water-leaving reflectance, with varying levels depending on the microplastic type and chlorophyll concentration. PS microplastics exhibited the lowest reflectance due to higher absorption, while PETG microplastics resulted in the highest reflectance due to their higher scattering coefficients. Additionally, reflectance increased with higher microplastic concentrations, irrespective of chlorophyll levels in the seawater. At higher microplastic concentrations (0.5-1.0gm-3), reflectance notably increases in the 550-700 nm range across all chlorophyll levels, with a more uniform enhancement across all wavelengths at lower chlorophyll concentrations. These findings will greatly influence future research on depth profiles of microplastic concentrations and the interactions between different microplastics in relation to water-leaving reflectance.