Hitchhiking to the Arctic: Atmospheric Microplastic Deposition

Microplastics Hitching a Ride

The discovery of microplastics in the Arctic snow [1] was sobering, since it elucidated how plastic pollution is neither limited to human-inhabited areas, nor the marine environments where it is conventionally investigated. As it turns out, these particles were most likely transported via the atmosphere, which is alarming since such transboundary pollution would enable deposition of these particles in various land and aquatic environments, further complicating the plastic source-pathway-sink model [2]. Furthermore, the most common source of atmospheric microplastics is from heavy traffic in the form of tire wear particles (TWP) and break wear particles (BWP), with research showing that 84% of atmospheric microplastic pollution in North America came from roadways, both through resuspension of settled dust as well as the generation of new microplastics (TWP and BWP) [3].

Unfortunately, atmospheric transport provides opportunity for these road-associated microplastics to travel to remote areas.  Evangeliou et al (2020) simulated the dispersion of road microplastics particles, and found that the size of the particle played a key role in their deposition. They found that smaller road microplastic particles (PM2.5) were dispersed more widely than larger ones (PM10), the latter being dominant closer to their emission sources. This has important consequences for the trajectory of particles, since it implicates that finer particles have a higher potential to reach the most remote corners of our globe, such as the Arctic [4]. In fact, since the size, composition, and colour of microplastics influence their effects on the environment, the deposition of colourful particles in the Arctic could have similar effects on the albedo as black carbon, thereby accelerating the melting of ice [5].

Modelling Microplastic Trajectory

Since wind currents are a potential vector for the long-distance transport of airborne microplastics, it is also possible to track the trajectory of these particles through modelling tools. While investigating the deposition of airborne microplastics in remote regions of the Pyrenees mountain range, Allen et al (2019) utilised the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model coupled with the settling velocity of particles to discern their trajectory pathway [6]. This model is one of the most common tools used to track the source to sink movement of atmospheric pollutants. In their investigation, the researchers utilised a comprehensive approach for simulating the trajectory, accounting for factors such as:

• the potential horizontal trajectory of microplastic particle
• back-trajectory duration = the duration a microplastic particle is airborne
• wind speed = the maximum recorded wind speed during each rain, snow or wind event

Owing to the large variations in the size and composition of microplastic particles [7] , HYSPLIT on its own is unable to provide comprehensive data to simulate atmospheric microplastic dispersion pathways. Therefore, Allen et al (2019) also took into account the settling velocity of the particles, which would illustrate the time taken for different particles to rise or fall [8].

Atmospheric microplastic research still has a long way to go, with difficulties primarily arising owing to the discreet nature of the particles making them hard to track in real time. Nevertheless, tools such as HYSPLIT- utilised in conjunction with field observations and additional models- still provide useful information about the dispersal of these particles, thereby enabling informed decision making for management and mitigation strategies.

References

[1] Bergmann, M., Mützel, S., Primpke, S., Tekman, M. B., Trachsel, J., & Gerdts, G. (2019). White and wonderful? Microplastics prevail in snow from the Alps to the Arctic. Science Advances, 5(8), eaax1157. https://doi.org/10.1126/sciadv.aax1157

[2] Bank, M. S., & Hansson, S. V. (2019). The Plastic Cycle: A Novel and Holistic Paradigm for the Anthropocene. Environmental Science & Technology, 53(13), 7177–7179. https://doi.org/10.1021/acs.est.9b02942

[3] Brahney, J., Mahowald, N., Prank, M., Cornwell, G., Klimont, Z., Matsui, H., & Prather, K. A. (2021). Constraining the atmospheric limb of the plastic cycle. Proceedings of the National Academy of Sciences, 118(16). https://doi.org/10.1073/pnas.2020719118

[4] Evangeliou, N., Grythe, H., Klimont, Z., Heyes, C., Eckhardt, S., Lopez-Aparicio, S., & Stohl, A. (2020). Atmospheric transport is a major pathway of microplastics to remote regions. Nature Communications, 11(1). https://doi.org/10.1038/s41467-020-17201-9

[5] Evangeliou, N., Grythe, H., Klimont, Z., Heyes, C., Eckhardt, S., Lopez-Aparicio, S., & Stohl, A. (2020). Atmospheric transport is a major pathway of microplastics to remote regions. Nature Communications, 11(1). https://doi.org/10.1038/s41467-020-17201-9

[6] Allen, S., Allen, D., Phoenix, V. R., Le Roux, G., Durántez Jiménez, P., Simonneau, A., Binet, S., & Galop, D. (2019). Atmospheric transport and deposition of microplastics in a remote mountain catchment. Nature Geoscience, 12(5), 339–344. https://doi.org/10.1038/s41561-019-0335-5

[7] Zhang, Y., Kang, S., Allen, S., Allen, D., Gao, T., & Sillanpää, M. (2020). Atmospheric microplastics: A review on current status and perspectives. Earth-Science Reviews, 203, 103118. https://doi.org/10.1016/j.earscirev.2020.103118

[8] Finlay, W. H. (2019). Motion of a single aerosol particle in a fluid. The Mechanics of Inhaled Pharmaceutical Aerosols, 21–52. https://doi.org/10.1016/b978-0-08-102749-3.00003-8