Sunlight causes grime on city buildings, statues and other outdoor surfaces to release chemicals that create smog, according to a new study.
City grime is a mixture of thousands of chemicals — including nitrogen oxides — emitted into the air by vehicles, factories and other sources. It had been believed that nitrogen oxides become inactive when they are trapped in grime on city surfaces.
Field studies conducted in Toronto and Leipzig, Germany, revealed that sunlight releases nitrogen oxides from grime. When in the air, nitrogen oxides can combine with other pollutants called volatile organic compounds to create ozone, which is the main component of smog.
The study finding is to be presented Monday at the annual meeting of the American Chemical Society in Boston.
“The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces,” study author James Donaldson, a University of Toronto chemistry professor, said in a society news release.
“But based on our field studies in a real-world environment, this is happening. We don’t know yet to what extent this is occurring, but it may be quite a significant, and unaccounted for, contributor to air pollution in cities,” he explained.
Research presented at medical meetings is typically considered preliminary until published in a journal that’s peer-reviewed.
The World Health Organization outlines the health risks of air pollution.