Dirty Buildings Are Creating Dirty Air

Researchers find that grime releases compounds into the air that create smog.
(Photo: David McNew/Getty Images)
Aug 17, 2015· 1 MIN READ
Katharine Gammon has written for Nature, Wired, Discover, and Popular Science. A new mom, she lives in Santa Monica.

The grime covering many buildings around the world isn’t just an eyesore; it could be harming your health, according to a new study.

Researchers have found that when sunlight strikes dirt, the chemical reaction creates smog-forming nitrogen oxide compounds.

“These compounds are getting removed from the atmosphere and stuck on the surfaces, and people think that’s the end of the story,” said James Donaldson, a chemistry professor at the University of Toronto and author of the new study. “But it now appears that when the sun shines on the buildings, this nitrate is not lost forever, but the active nitrogen can reengage with the local atmospheric chemistry.”

Donaldson and his colleagues took samples of urban grime during a year in Toronto and six weeks in Leipzig, Germany. He said that grime is made up of thousands of compounds, but each city has its own grime signature. For example, in Toronto, where roads are heavily salted in the winter, there is a leftover signal from sodium chloride in the grime.

Some of the compounds in urban grime are nitrogen oxides. When in the air, these compounds may combine with other air pollutants—known as volatile organic compounds—and light to produce ozone, which is the main component of smog.

Scientists had long thought that nitrogen oxides become inactive when they are trapped in grime and settle on a building’s surface, but Donaldson’s previous work indicated that some of the compounds reenter the atmosphere.

RELATED: Smog Makes It Hard to Breathe, and It Might Be Lowering Kids’ IQ Too

To test that hypothesis, Donaldson and his colleagues put out grime collectors containing small glass beads—the same sort of glass as windows—around the two cities. Some of the trays were shielded from light, and others were exposed to light.

The researchers then took some of the beads out of the trays and put them in a water-based solution to test what types of water-soluble chemical compounds were there.

They found 10 percent fewer nitrates in the samples exposed to sunlight compared with those gathering grime in the dark. Donaldson presented his results at the National Meeting & Exposition of the American Chemical Society on Monday.

Those compounds are turning into some kind of atmospheric nitrogen oxides, which could be creating more smog-causing ozone.

Donaldson hopes to do another set of experiments in a city with high levels of air pollution, such as Shanghai, and to quantify the nitrates returning to the air as nitrogen oxides in different cities.

Could self-cleaning buildings be a solution? Alcoa, for instance, has developed a building panel coated in titanium dioxide particles that, the company says, when hit by sunlight breaks down organic matter on the panel as well as nitrogen oxide in the surrounding atmosphere.