Homeowners in urban areas might find grime on their walls more cumbersome that initially thought as a new study found that the soot is capable of releasing dangerous nitrogen gases when it comes into contact with sunlight.

In a research presented at the American Chemical Society conference, scientists from the University of Toronto examined the content of grime collected through experiments on rooftops in Germany. They tested the reaction of the samples to both shade and sunshine.

The researchers discovered that while grime has been known to absorb various types of gases, the dark muck does not seem to keep nitrogen locked away for long.

When the material comes into contact with sunlight, it releases the nitrogen it absorbed in two different forms. The first one is in the form of the highly toxic nitrogen dioxide (NO2) pollutant and the second is the key component for the formation of smog known as nitrous acid (HONO).

Toronto chemistry professor and lead author James Donaldson said that instead of serving as simply a repository for nitrogen oxide, grime can re-release this gas back into the atmosphere when exposed to the light from the sun.

In his earlier work, Donaldson showed during a laboratory experiment that exposing grime to artificial sunlight can effectively cause the material to release its nitrogen component back into the air.

For this recent experiment, however, he tested the same effect in an outdoor setting through the help of colleagues in Germany.

The researchers placed two shelves filled with window glass beads on a tower above Leipzig. They then allowed the shelves to be exposed to same amount of air flow so that they both become grimy. Only one of the shelves, however, was placed under the sun.

Donaldson said that the shelf that was exposed to sunlight was a 10 percent lower nitrate content compared to the one that was kept under shade. This suggests that the loss of nitrogen through a photochemical process observed in the recent experiment was consistent with the result they saw during the laboratory test.

The 10 percent nitrogen loss may seem to have a small effect, but according to the researchers, it represents a steady state difference. They said that as the light from the sun causes the grime to release its stored nitrogen content, newer grime rich with nitrogen is being deposited.

The two forms of nitrogen produced through this process also pose a threat to the environment. The researchers noted that while nitrogen dioxide (NO2) is the more toxic of the two gases, nitrous acid (HONO) causes the production of ozone, which in turn forms smog in urban areas.

Photo: Dean Souglass | Flickr