Plastic is falling from the sky. But where does it come from?

These microplastics are not only washed ashore and accumulate on beaches. When waves break and winds sweep across the ocean, they throw seawater droplets into the air. These obviously contain salt, but also organic matter and microplastics. “Then the water evaporates and you are left with just the aerosols,” or tiny floating particles of particles, says Natalie Mahowald, a researcher at Cornell University, who co-led the work with Brahney. “Classically, we atmospheric scientists have always known that sea salts come this way,” she continues. But last year another group of researchers demonstrated this phenomenon with microplastics, showing that they present themselves under the sea breeze.

This time, Mahowald and Brahney have thought bigger, using atmospheric models to show how far marine microplastics travel after they take flight. They also looked at other sources of microplastic emissions, such as roads, towns and agricultural fields. They knew, for example, the amount of dust generated by the fields and the amount of microplastic that could be in that dust.

The researchers then combined this atmospheric modeling with real-world data. Brahney used air samplers scattered around remote locations in the American West, so that at one point she could tell how many plastic particles had fallen from the sky. Mahowald’s modeling could also tell what the atmospheric and climatic conditions looked like at the time, allowing researchers to trace where the particles likely blew.

They found that agricultural dust provided only 5% of airborne microplastics in the West. And surprisingly, cities only provided 0.4%. “If you asked someone how plastics get into the atmosphere, they’d say urban centers,” Brahney says. “I like to think of it more as the roads that are exit the most important cities. “

When a car is driven on a road, tiny spots fly off its tires as part of normal wear and tear. This material is not pure rubber; it contains added synthetic rubbers and a host of other chemicals. Tire particles are therefore technically microplastics, and they are everywhere. A study carried out in 2019 calculated that 7 trillion microplastics wash in the San Francisco Bay area annually, most tires.

Cities actually produce an astonishing amount of microplastics through road traffic and decaying garbage, but it doesn’t seem to go up in the atmosphere. This is for two reasons, think Brahney and Mahowald: buildings prevent the wind from rubbing a city’s surfaces and propelling them, and people drive cars slower in metropolitan areas, so there is less wind. agitation of tire particles that end up on the roadway. But get out on interstate freeways and there’s a lot more open space where winds can whip up debris. Further, says Mahowald, “the cars go 60 miles an hour. It’s a lot of energy. And small, tiny particles can enter the atmosphere with this energy.