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Microplastics are pervasive, found everywhere on Earth, from the Sahara Desert to patches of Arctic sea ice. Yet despite these plastic particles’ ubiquity, scientists have struggled to determine exactly how many of them are in our atmosphere.

Now, a new estimate published in Nature suggests that land sources release about 600 quadrillion (600,000,000,000,000,000) microplastic particles into the atmosphere every year, about 20 times more than the number of particles contributed by oceans (about 26 quadrillion).

The median concentration of microplastics is 0.08 particle per cubic meter (m³) over land and 0.003 particle per m³ over sea, the study found.

These estimates are between 100 and 10,000 times lower than previous accountings of atmospheric microplastics—a discrepancy that the researchers behind the new study say underscores the need for better global measures of these pollutants.

“We knew that uncertainties of existing emission estimates were very large,” says Andreas Stohl, senior author of the study and an atmospheric scientist at the University of Vienna. “They are even still large after our study, but we could at least narrow down the uncertainty range, especially when it comes to the importance of land-based versus ocean-based emissions.”

A microplastic is any plastic particle sized between one micron and five millimeters. Easily swept up by wind and carried long distances by water, these tiny motes are also exceedingly difficult to detect and almost impossible to remove from the environment.

Past estimates have focused on accounting for microplastics generated by human activity or directly measuring their concentration in the air in any given area. But these measures are highly variable: along the southeastern coast of China, for example, atmospheric microplastic estimates have ranged from 0.004 to 190 particles per m³. To try and get at a more global estimate, Stohl and his team compiled 2,782 measurements collected at 283 locations worldwide.

The researchers hope the findings will act as a baseline for future studies of global microplastic levels, including new measures that will be able to account for even smaller particles than they did.

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