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​Grime on city streets becomes pollution in the air

Los Angeles, California, the nation's second largest city, typically ranks among the worst in the nation for ozone pollution.

AP Photo/Nick Ut

As if the city didn't feel dirty enough, new research finds that the layers of grit and grime covering the urban landscape get broken down by sunlight and converted into smog.

Thousands of chemicals spewed from the tailpipes of cars and busses, factories and other sources of nasty particulates shroud cities in a film of filth that scientists long thought stuck to the surfaces of buildings, lampposts, sidewalks and statues until rain provided a much needed power wash. But new research being presented this week at the National Meeting and Exposition of the American Chemical Society finds that every day, sunlight is breaking down grime and releasing smog-forming compounds.

"The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces," said University of Toronto chemistry professor James Donaldson, Ph.D., who performed the new study. "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."

Nitrogen oxides, including nitrogen dioxide (or nitrous oxide), come from car exhaust, commercial manufacturing, electric utilities and the use of synthetic fertilizer. They accounted for about five percent of all U.S. greenhouse gas emissions from human activities in the U.S. in 2013, according to the Environmental Protection Agency. When nitrogen oxides react with other compounds in the air, called volatile organic compounds (VOCs), they produce ozone at ground level -- the primary component of smog.

Research released by the American Lung Association last year found that almost half Americans live in areas where air pollution levels are often dangerously high. Elevated ozone levels put people at risk for premature death, aggravated asthma, difficulty breathing and future cardiovascular problems, according to the report. It can also shrink the brain. A study published online this week in the journal Plant, Cell and Environment found that nitrogen dioxide, in addition to causing smog, can make ragweed allergies stronger.

In his previous studies, Donaldson had found that nitrogen oxides disappeared from grime faster than could be explained by wash-off due to rain, and that more were released when exposed to artificial sunlight than when kept in the dark.

To test the role of sunlight in a real world scenario, Donaldson and his colleagues set up "grime collectors" made of glass beads throughout the cities of Toronto and Leipzig, Germany. After six weeks in Leipzig, beads in shady areas contained 10 percent more nitrates than those exposed to the sun. Work in Toronto is still ongoing.

"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 a statement. "In our work, we are showing that there is the potential for significant recycling of nitrogen oxides into the atmosphere from grime, which could give rise to greater ozone creation."

  • Amanda Schupak

    Amanda Schupak is the science and technology editor at CBSNews.com