This is scientific American's 60 second science. I'm Christopher Intalyata. Got a minute?

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, the same stuff you find in fertilizer, also settle

on to buildings and other city surfaces. And scientists figured that was the end of this story.

Usually one thinks about pollutants such as nitrogen dioxide and a nitrous acid

as being lost from the atmosphere on two 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 the

urban grime material can in fact recycle these compounds and bring them back into active play in the atmosphere.

Donaldson and his team found that sunlight interacts with nitrates stuck in grime, kicking

them back out 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 and sunny areas lost 10% 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 Grimes 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.

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