Introducing 𝞵betr Global: A global-scale, geographically explicit, multimedia microparticle transport and fate model

Introducing µBETR Global, a global scale, geographically explicit, multimedia microparticle transport and fate model
Marianne Seijo¹, Mick Whelan², Todd Gouin³ and Antonia Praetorius^1
1University of Amsterdam, NL
²University of Leicester, UK
³ TG Environmental Research, UK E-mail contact: m.x.seijo@uva.nl

1. Introduction
The production and use of plastics have drastically increased over the last decades and are still on the rise: Since the 1950s, 8.3 billion tons of plastics have been produced [1]. This is mainly due to the materials’ high versatility and low cost. But the reverse of the medal is that 79% accumulated in landfills and the natural environment [1], which is impacted by adverse effects. In fact, plastic pollution is feared to represent an irreversible planetary boundary threat [2]. A particular issue is the formation of microplastics (MPs), ranging from 5 mm down to 1 μm (and < 1μm nanoplastics), via fragmentation of larger plastic waste. The MP (and nanoplastic) size range encompasses different dynamics processes from Newton to Brownian: It makes them readily available for ingestion and/or inhalation for a plethora of organisms, including humans [3]. MPs have been found across the globe in all environmental compartments, from soils to air, from freshwaters to oceans. However, limited mechanistic understanding is available on specific transport pathways and the complex interaction and transformation processes that MPs can undergo in different environmental media over time.
Environmental exposure models play an essential role in understanding the emissions, transport, and complex transformation processes of MPs in air, freshwater, soils, sediments, and the oceans. Adequate modelling tools are needed to support exposure and risk assessment. The particulate nature of MPs and their heterogeneity in terms of sources and physicochemical properties requires data-intensive modelling approaches, accounting for MP-specific transport and fate processes. Here we introduce a recently developed global-scale multimedia transport and fate model for MPs, focusing on the MP fate in the air.
 

2. Materials and Methods
The μBETR Global model is based on a recently developed open-source multimedia mass-balance modeling framework for MPs, the Full Multi [4]. The Full Multi is a modular mass-balance framework for MP fate in surface water systems. It contains mechanistic process descriptions for MP transformation and fate, such as fragmentation, biofouling, heteroaggregation, and sedimentation. MPs are represented in different size classes and different forms (e.g., pristine, biofouled, heteroaggregated). Our μBETR Global model expands the MP process descriptions to cover additional environmental compartments (air and soil) and implements the multimedia fate model at the global scale. Similar to the gridded multimedia model approach in the BETR Global model for organic pollutants [5], μBETR Global represents worldwide fate and transport by linking grid cells (model unit cells) built from well-mixed boxes of different environmental media (air, different types of soil, freshwater, ocean water) representing the global environment (Fig.1). μBETR Global operates at a higher spatial resolution (0.5° grid cells, equivalent to 55.0 km) and a real-time scale. Environmental system parameters to describe the multimedia environment as a function of space and time are obtained at high resolution from the Copernicus observation program, the European Centre for Medium-Range Weather Forecasts' (ECMWF's), and from Era5 reanalysis data. In addition, MP-specific property and fate data are obtained from the literature where available or estimated based on current scientific understanding.
Microplastic-specific fate processes implemented in the air include advection, wet and dry deposition, heteroaggregation with aerosols, degradation, fragmentation, vertical air exchange, and resuspension from land surface or via sea spray aerosol formation. Model equations are adapted to represent different MP sizes (from nano- to millimeter scale) and different shapes (e.g., spheres, plates, fibers). Emission estimates for different model scenarios are obtained from the literature based on measured data or material flow models.

3. Results and Discussion
The μBETR Global model is able to represent MP fate and transport and high spatial and temporal resolution. Here we present a first implementation of the μBETR Global model focusing on the air compartment. Different model scenarios are presented with emissions into the air at selected locations
around the globe. We show how the specific properties of MPs affect their potential for long-range environmental transport. For example, smaller size and lower density lead to a longer lifetime in the atmosphere, leading to deposition in remote regions. In contrast, larger and dense particles (including also heteroaggregates) are deposited more quickly towards land and water surface in areas close to emissions. We also demonstrate the impact of MP properties and emission location on long-range transport.

 

4. Conclusions
The μBETR Global model builds upon regional scale MP-specific fate and transport models and existing multimedia mass balance models for organic chemicals to achieve the assessment of MP fate in a global environment at very high spatial and temporal resolution. In addition, the air compartment, rarely considered in previous multimedia models for MPs, is described in μBETR Global with mechanistic process descriptions accounting for MP size, shape, and density. As a result, the model shows great potential for gaining new insights into global microplastic transport and fate, including assessing the potential for long-range environmental transport and assessing the relative importance of different emissions. This provides a much-needed basis to determine the effectiveness of global mitigation efforts toward reducing plastic pollution.

5. References
[1] Geyer R, Jambeck J. R., and Law K. L., 2017. Production, use, and fate of all plastics ever made. Science Advances 3: 1700782
[2] MacLeod M, Arp HPH, Tekman MB, Jahnke A. 2021. The Global Threat from Plastic Pollution. Science 373: 61–65.
[3] Amato-Lourenço LF, Carvalho-Oliveira R, Ribeiro Júnior G, dos Santos Galvão L, Ando RA, Mauad T. 2021. Presence of Airborne Microplastics in Human Lung Tissue. Journal of Hazardous Materials 416: 126124.
[4] Domercq P, Praetorius A, MacLeod M. 2022. The Full Multi: An Open-Source Framework for Modelling the Transport and Fate of Nano- and Microplastics in Aquatic Systems. Environmental Modelling & Software 148: 105291.
[5] MacLeod M, von Waldow H, Tay P, Armitage JM, Wöhrnschimmel H, Riley WJ, McKone TE, Hungerbuhler H. 2011. BETR Global – A Geographically-Explicit Global-Scale Multimedia Contaminant Fate Model. Environmental Pollution 159: 1442–45.

Acknowledgement - Funding to support this work came from the European Chemical Industry Council (CEFIC) through the Long-Range Research Initiative LRI-ECO57.