Take old buildings in any big city, usually sporting a coating of dust and grime, add sunlight, and what do you get? Well, you may get yet another source of air pollution, researchers say.

Grimy buildings are more than just an eyesore, they explain; when sunlight falls on them, they can "breathe out" nitrogen oxides and nitrous acid they've absorbed over years, creating ground-level smog and ozone.

The finding may explain a "missing" source of air pollution, experts say, as the chemicals belched into city air by chimneys and vehicle exhausts and deposited on buildings is set free by sunlight to foul the air.

The finding goes against a long-held assumption that the nitrates contained in city grime were somehow "locked" in place and did not represent a pollution source, researchers told a 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," says study leader James Donaldson of the University of Toronto. "But based on our field studies in a real-world environment, this is happening."

Working with colleagues in Leipzig in Germany, Donaldson recreated some laboratory experiments in the field, putting shelves with beads of window glass on building roofs in the German city.

All the shelves eventually became coated with city grime, but only some were in the sun, with the others in shade.

"The ones which were exposed to sunlight showed 10 percent less nitrate than the ones which were shaded, suggesting that there is a photochemical loss [of nitrogen] consistent with what we saw in the lab," Donaldson told the ACS meeting.

Although the full extent of city grime's impact on air quality is unclear, "it may be quite a significant, and unaccounted for, contributor to air pollution in cities," he says.

After the six-month field test in Leipzig, the researchers conducted a similar yearlong trial in Toronto, which showed similar results.

Most previous studies of city grime coating buildings, statuary and other outdoor surfaces have been in a context of how windows get dirty and buildings get degraded, with little thought to other environmental impacts, Donaldson says.

"The chemistry part has been largely overlooked," he says. "If our suspicions are correct, it means that the current understanding of urban air pollution is missing a big chunk of information."

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