City buildings literally ooze smog from the walls

© Jason Lee
The thin varnish of grime which usually covers outdoor surfaces, such as the walls of buildings or statues, releases smog-forming nitrogen oxide compounds when it is touched by natural sunlight, a first-of-its-kind Canadian study has found.

Field work conducted in the German city of Leipzig and Canadian city of Toronto has shown the impact of scores of chemical compounds, released into the air by cars or factories, as they settle on urban surfaces bathing in the sunlight in the form of grime.

Some of the pollutants, nitrogen oxides, get released back into the atmosphere whent touched by the sunlight, researchers at the University of Toronto (UToronto) have found out.

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“The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces,” a professor in the Chemistry department at UToronto, James Donaldson, said in the press 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.”

In a series of laboratory-based experiments, Donaldson and his colleagues compared the grime left in the dark to the grime exposed to a “solar simulator”. It revealed that the light is able to activate nitrogen compounds, returning them to the air.

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A six-week-long field study in Leipzig and a subsequent year-long still-ongoing study in Toronto showed the same tendency. Scientists placed grime collectors with glass beads in both cities, both in shadow and in the sun.

In Leipzig the difference between grime left in shade and in natural sunlight was the amount of nitrates – 10 percent. Donaldson said that “The fact that Leipzig appears to have 20 times more grime than Toronto suggests that there is a potential for 20 times more recycling of nitrogen oxides into the local atmosphere.”

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The scientists also plan to carry out other field experiments on the subject of pollution. They are set to compare the effects of humidity, as well as the differences between places that are “really grubby” and “really clean.”

“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.”

The research will be presented at a meeting of the American Chemical Society (ACS) on Thursday, alongside with other 9,000 reports.