A lot has been said about air pollution in urban cities. A new research has unveiled that dust and grime that we all can see coated on buildings of any city can create another source of pollution if sunlight falls on them.

The research paper presented at the American Chemical Society in Boston has found that when urban grime is exposed to sunlight then it releases nitrogen oxides, considered as a source of ground-level ozone and smog.

Nitrogen oxides are a group of compounds formed when nitrogen combines with oxygen. They can form as nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O). There are different ways of its production ranging from bacterial processes, biological growth and decay, forest fore and burning of fossil fuels.

Study researchers said that the findings may explain about a missing source of air pollution. Study's lead researcher James Donaldson of the University of Toronto was of the view, "The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces. But based on our field studies in a real-world environment, this is happening".

The researchers for this study have recreated some lab experiments in the field. They have placed shelves with beads of window glasses on rooftops in Leipzig, Germany. Some were in the sun and the others in shade, but both became coated with city grime.

The ones exposed to sunlight had 10% less nitrate than the ones in shaded. Similar experiment was conducted in Toronto.