In recent years big cities have seen lower rates of crime. But there’s still plenty of grime. Combustion from cars, factories and fires spews out nitrogen oxides. Those compounds react with sunlight and air to form ozone—the main ingredient in smog. And certain nitrogen oxides called nitrates—same stuff you find in fertilizer—also settle onto buildings and other city surfaces. Scientists figured that was the end of the story. 

“Usually one thinks about pollutants such as nitrogen dioxide and nitrous acid as being lost from the atmosphere onto surfaces.” University of Toronto chemist Jamie Donaldson, at a meeting of the American Chemical Society in Boston on August 17th. “What we have been interested in is to see whether or not the influence of sunlight on urban grime material can in fact recycle these compounds and bring them back into active play in the atmosphere.” [D. James Donaldson et al, Composition and chemistry of urban grime: a field and laboratory study]

Donaldson and his team found that sunlight interacts with nitrates stuck in grime, kicking them back into the air, where they can contribute to smog. They verified that process by putting trays of glass balls—a proxy for window glass, but with more surface area—out in downtown Toronto, and Leipzig, Germany. The glass in sunny areas lost 10 percent more nitrogen compounds than did their shady counterparts. 

“The total amount of nitrogen oxides entering a city is probably well captured in models. What is not well captured is the fact that the losses are less than people had considered because a loss from the gas phase to the grime surface does not constitute a permanent loss.” 

So grime is not as inert as we thought it was. And city beautification efforts, aimed at making buildings more pleasing to the eye, might also make the urban environment more pleasant for the lungs.

—Christopher Intagliata

[The above text is a transcript of this podcast.]