Urban grime 'breathes' pollution when exposed to sunlight

Urban grime 'breathes' pollution back into the environment when exposed to sunlight, according to researchers. Grime, in this case, is the slurry of chemicals pumped into the air from common city trappings: car tail pipes, house chimneys, factories, and more. These chemicals build up on surfaces, forming a distinct dark "grime" over time. This grime has been known to absorb pollution out of the atmosphere, but according to a study conducted in Germany, it can also release some nitrogen pollution back into the environment.

The research was recently detailed at the American Chemical Society conference held in Boston, and was conducted using grime found on rooftops in Germany. Researchers looked at the content of this grime when it was in sunlight, and when it was not exposed to sunlight.

The research revealed that grime that is exposed to sunlight releases nitrous acid and the pollutant nitrogen dioxide back into the environment. As this nitrogen-filled grime is depleted, new nitrogen rich grime is deposited. Nitrous acid in particular can contribute to the formation of smog.

The research was helmed by University of Toronto chemistry professor James Donaldson, who said, "Rather than being a permanent sink for nitrogen oxide gases ... grime exposed to sunlight can re-release some of these gases back into the urban atmosphere."