Visit any city in the world and you’ll find grimy old buildings. The dirt and miscellaneous particles that inevitably accumulate and become embedded in hard urban surfaces seems to be a natural symptom of city life. But new research suggests that we may need to revisit what we thought we knew about grime. 

Scientists have long theorized that grime (a mix of thousands of chemical compounds emitted by cars, trucks, and factories) trapped in place chemical compounds like nitrogen oxides, which can combine with volatile organic compounds to create ozone, the main component of smog. To test this theory, researchers led by University of Toronto chemistry professor James Donaldson studied the filth on a rooftop in Leipzig, Germany, both in the sun and in the cover of shade, by leaving trays filled with window glass beads on the building for six weeks. (Because they have more surface area, the glass beads can accumulate more grime than a flat surface like a window can.)

The researchers found that the beads placed in the sun contained 10 percent less nitrate than the shaded beads. “Rather than being a permanent sink for nitrogen oxide gases, grime exposed to sunlight can re-release some of these gases back into the urban atmosphere,” Donaldson explained at an American Chemical Society conference in Boston.

These findings are consistent with findings Donaldson and his team had previously made in the lab. The idea that grime may be making city air even worse than we thought is troubling. “If our suspicions are correct, it means that the current understanding of urban air pollution is missing a big chunk of information,” Donaldson said. In their continuing research, he and his team plan to conduct similar tests in very dirty and very clean cities. They also want to learn how humidity and varying degrees of sunlight exposure contribute to the problem.