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Grimy City Buildings Re-Pollute the Air

Filthy urban surfaces “breathe” pollutants when they’re exposed to sun

(Clemens v. Vogelsang - Flickr)
smithsonian.com

Scientists have long suspected that filthy urban buildings have an upside: grime that traps pollutants and prevents them from getting into the air. But new research suggests that in fact, the opposite is true, reports BBC News’ Jonathan Webb.

New research by a group of chemists shows that instead of serving as a sink for nitrogen oxide gas, city grime actually releases two forms of nitrogen: nitrogen dioxide and nitrous acid. These gases are key smog drivers, writes Webb — and they can have a big effect.

Chemistry World’s Matthew Gunther writes that the researchers went into the field — Leipzig, Germany — to collect urban grime and see how it reacts to sunlight. They set up a tower above the city and filled it with shelves of window glass. One shelf was shielded from the sun, but the other got direct sunlight.

Soon enough, the glass beads became filthy — and measurements showed that nitrate and ammonium levels in the shady grime were lower than those in the sun, writes Gunther. Direct comparisons of the samples revealed that the sun-soaked filth had less nitrate, too, which suggests that it was releasing nitrogen into the atmosphere.

After taking their experiments into the lab, the team used collected city grime to figure out exactly what happens to filth when it’s exposed to the sun. What they found was even worse, writes Webb: It appears that sunlight caused nitrogen dioxide and nitrous acid to escape.

By “breathing” pollution back into the environment when there’s sun, city filth could give sunny days a whole new meaning. It looks like there could be good reason to keep buildings and statues clean in urban areas — it could make breathing a little bit easier.

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