Multi-scale modeling of microplastics transport in marine environments

Licentiatavhandling, 2025

Over the past seventy years, plastic has thrived across numerous industries thanks to its low cost and exceptional properties. Yet, those very qualities make plastic pollution a significant environmental concern. Microplastics refer to plastic particles of sizes between 1 micron and 1 mm. Due to their small sizes, once microplastics enter marine environments, they become essentially irretrievable. Hence, there is a growing urgency to assess the ecological impact of microplastic pollution. Numerical tools are widely used to enhance our understanding of the ocean's state and its influence on microplastic transport and fate. However, the challenge is accurately capturing the wide range of scales involved, which can range from microns to km. To address this issue, a multi-scale modeling framework is presented. At the “small scale,” direct numerical simulations are performed to resolve particles settling in turbulent marine environments and extract relevant correlations. These correlations are tested for suitability when scaling up, at an "intermediate scale", with controlled environmental conditions. Finally, numerical results from large-scale simulations, combined with field observations, are used to identify the dominant mechanisms in marine environments that govern the transport and ultimate fate of microplastics. The proposed approach extends the capabilities of numerical models beyond mere predictive tools, providing a robust foundation for conducting environmental impact assessments.