Researchers in North America believe the combination of smog-clad skyscrapers with sunshine could be a key factor in the exacerbation of urban air pollution.

A University of Toronto study, scheduled for presentation before the American Chemical Society, claims exposure to sunlight can trigger the release of nitrogen oxide compounds harboured by the film of pollution that accumulates on the surface of built structures in smoggy urban areas.

This finding contradicts the long-standing assumption that nitrates are neutralised once they are trapped by urban surface grime, in a phenomenon that would serve to reduce the harmful effects of smog in polluted cities.

“The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces, said Dr James Donaldson from the University of Toronto Chemical Faculty. “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 accounted for contributor to air pollution in cities.”

The field studies conducted by Donaldson and his team serve to confirm the result of previous laboratory research, which found that grime exposed to “solar simulators” released more nitrates than grime just left in darkness.

If the results of the studies prove accurate, it means exposure to light can trigger the reversion of nitrogen compounds back into active forms that can proliferate in the atmosphere, providing another major source of pollution in urban environments with a large volume of of building surface areas.

“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 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.”

Developers of building areas in urban areas nonetheless have a range of options at their disposal for mitigating air pollution, including smog-vacuming bricks and pavements as well as various types of pollution-reducing facades.