How tiny plastic particles enter the atmosphere is an important question, as numerous recent studies have shown that microplastics in the air pose a health threat. Using a global model for the transport of chemical substances, researchers have demonstrated that, contrary to previous claims, the ocean is not a major source of microplastics in the atmosphere, but rather a significant sink.
Plastic dust is polluting the environment around the globe: Microplastics – particles with a diameter of less than 5 mm – can be detected not only in soils, bodies of water, and the sea, but also in the air we breathe. Researchers have even found microplastics in the human brain. Studies repeatedly confirm that microplastics pose a health risk. Furthermore, atmospheric microplastics reach the most remote corners of the earth and settle there. But how do they even get into the atmosphere in the first place?
Generally, the sources of microplastics are on land: These include, for example, fibers from synthetic clothing in domestic wastewater or the abrasion of car tires on the roads. Previous studies assumed that a major route for microplastics to enter the atmosphere is via the ocean: microplastics are washed into rivers and carried to the sea, where they accumulate. Air bubbles created by spray, wind, and waves can carry the plastic particles from the water into the atmosphere. However, a new study led by researchers at the Max Planck Institute for Meteorology (MPI-M) shows that the ocean primarily acts as a sink—and not as a source, as previously assumed.
The assumption that the ocean is a source of atmospheric microplastics was based on inverse modeling. This method derives the sources of a substance from measurements of its atmospheric concentration distribution. Applied to microplastics, this led to the assumption of an oceanic source of atmospheric microplastics amounting to several hundred million or even several billion kilograms per year. The precise mechanism by which this transfer occurs was then investigated in laboratory experiments. These experiments led to a completely different conclusion: only a few thousand or one hundred thousand kilograms per year seemed plausible.
Using a global atmospheric chemical transport model, an international research team, including Shanye Yang, former visiting scientist at the MPI-M, and MPI-M group leader Guy Brasseur, investigated whether the assumption of a small oceanic source leads to an atmospheric distribution of microplastics consistent with observations. The result was positive: The ocean appears not to be a source, but rather a sink, where 15 percent of all microplastics in the air settle.
The study also shows how the size of microplastics determines their transport in the atmosphere: Larger particles settle relatively quickly, either on land or near the coast. Small microplastic particles can remain in the atmosphere for up to a year and are thus transported around the globe. The model shows, for example, that the small particles – although emitted on the continents – can reach as far as the Arctic, where they settle on snow and ice. This demonstrates the global impact of microplastics. The findings of the study can be incorporated into strategies to reduce pollution. These strategies should therefore focus on continental sources rather than the role of the ocean as a source of microplastics.
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