Found caked on buildings, statues and other outdoor surfaces in cities, urban grime is the result of chemical compounds discharged from automobiles, factories and other sources.
At the 250th National Meeting & Exposition of the American Chemical Society, James Donaldson, of the Univ. of Toronto, presented his research, which found natural sunlight triggers the release of nitrogen oxide compounds from grime.
“In Toronto, first, we collected urban grime onto a substrate on which we could actually measure the infrared spectrum of the grime so we could analyze to some extent what it was composed of,” said Donaldson at a press conference held in Boston. “We exposed this substrate to artificial sunlight and we saw the nitrate component go away very, very quickly.”
The researchers found nitrate disappeared from grime 10,000 times faster than from a water-based solution, which was also exposed to artificial sunlight.
Afterwards, Donaldson set up a six-week field study in Leipzig, Germany, to test the effects of natural sunlight on the grime.
“What we did was have glass beads exposed to sunlight and shaded from sunlight collecting the same material. And the ones which were exposed to sunlight showed 10% less nitrate than the ones which were not exposed to sunlight, suggesting there is some photochemical loss consistent with what we saw,” he said.
“Back in Toronto we look at the gas phase products of the reaction of artificial sunlight with real grime and see nitrogen oxides expressed into the gas phase,” he added.
According to Donaldson, the current understanding of air pollution does not factor in the expulsion of nitrogen oxides and other compounds from building and structure surfaces. The researchers have yet to quantify the expulsion rate of nitrogen oxide.
Nitrogen oxide is among the compounds referred to as volatile organic compounds. Together, these compounds can produce ozone, a main component of smog.
“The fact that Leipzig appears to have 20 times more grime than Toronto suggests there is a potential for 20 times more recycling of nitrogen oxides into the local atmosphere,” Donaldson said.
The team plans to study the effects of humidity, grime levels and illumination amounts on the recycling of nitrates into the atmosphere.
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