Chemicals ejected from the tailpipes of cars and buses, factories and other sources of pollution cover city in a film of dirt. Researchers have said that sunlight breaks the layers of grit and grime and convert them into smog.

Dirt gets stuck to the surfaces of buildings, lampposts, sidewalks and statues and gets removed after rain. Now, a new study has found that on everyday basis, sunlight breaks down grime and releases smog-forming compounds.

Prof. James Donaldson from University of Toronto said, "The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces".

For now, the researchers are not aware till what extent the smog forming process takes place. But, the researchers do agree that it is an important thing that has remained unaccounted from quite a long time. Also, it adds to air pollution in cities.

The researchers had set up grime collectors made of glass beads across the cities of Toronto and Leipzig, Germany to know the role of sunlight in the case. After six weeks in Leipzig, beads in shady areas were having 10% more nitrates than those exposed to the sun. Experiment in Toronto is taking place.

Donaldson said there is a potential for recycling of nitrogen oxides into atmosphere from grime. Nitrogen oxides come from car exhaust, commercial manufacturing, electric utilities and use of synthetic fertilizer.

It accounts for around 5% of all US greenhouse gas emissions from human activities in the US. The researchers stated that when nitrogen oxides react with other compounds in the air known as volatile organic compounds (VOCs) then they produce ozone at ground level, which is the main component of smog.