China’s huge cities are infamous—not just for their 50-lane traffic jams, but for the air pollution that clogs city skies, endangering health, provoking red alerts and even obscuring the view of Beijing from space. So you’d think that a reduction in one of the things that fuels air pollution—dust—would ease pollution problems. But you’d be wrong: As the BBC’s Matt McGrath reports, it turns out that less dust actually worsens air pollution in China.
In a new study in the journal Nature Communications, researchers reveal that a lack of airborne dust makes China’s air quality even worse. It’s long been known that dust from the Gobi desert can flood Chinese skies and worsen pollution, as in April when air pollution monitors in Beijing went off the charts. But when researchers simulated how dust and wind has moved across Eastern China over the last 150 years, they learned that it actually improves air quality in the region.
Blame the sun for the counterintuitive finding. Dust influences air temperature, which in turn influences winds by causing temperature differentials between Earth and sea. And a bigger temperature differential means more wind.
When there’s a lot of dust in the air, the sun can’t reach Earth’s surface, so it stays cool. That kicks up wind that helps circulate out the dust and other pollutants. But when there’s less dust, Earth heats up more and the wind weakens, causing stagnation. Pollutants build up—and China’s air quality suffers.
These wind speed changes seem tiny. In wintertime, there’s 29 percent less dust, translating to a reduction of just over a tenth of a mile per hour in wind speed. But when you look at the change over an entire region, it adds up. Just that tiny change in wind speed increases air pollution by 13 percent during the winter months, researchers learned.
That doesn’t mean that dust doesn’t affect air pollution in China. But researchers say that they largely affect visibility, not breathability—and that during less dusty years, people breathe in more human-caused pollutants.
“This is not the result we expected,” says Lynn Russell, who co-authored the paper, in a press release. And her team warns that even though dust makes a difference in pollution levels, the majority of China’s pollution is caused by humans. But the more information about how those pollutants interact with natural ones like dust, the better.