Sunlight is causing smog-forming nitrogen oxide compounds to be released from the grime that typically coats buildings and other outdoor surfaces in urban areas, new research to be presented at a meeting of the American Chemical Society has found.

The finding confirms previous laboratory work using simulated sunlight and upends a long-held notion that nitrates become inactive in urban grime, caused by pollution from cars and factories, and are “locked” in place.

“The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces,” researcher Dr James Donaldson from the University of Toronto said. “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.”

The results are based on field studies conducted following previous research that found grime exposed to a “solar simulator” shed more nitrates than the grime left in the dark, suggesting that light can chemically convert nitrogen compounds back into active forms that can return to the atmosphere.

A field study in Leipzig, Germany found that grime collection devices in shaded areas contained 10 per cent more nitrates than grime exposed to natural sunlight, indicating that nitrates were being recycled back into the air. A field study in Toronto, Canada 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,” Dr Donaldson said. “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.”

The researchers plan to examine the effects of humidity, grime levels and various amounts of illumination on the recycling of nitrates back into the atmosphere.