Sunlight has been found to trigger the release of smog-forming compounds from the grime that builds up on city buildings. Scientists say they still do not know just to what extent this process is occurring, but it could be a significant contributor to air pollution in urban areas.

The findings were presented at the 250th National Meeting & Exposition of the American Chemical Society (ACS) by researchers from Leipzig in Germany and Toronto, Canada. Scientists note that urban grime is a mix of thousands of chemical compounds that are emitted into the air by cars, factories and other sources. These include nitrogen oxides, which when in the air combine with other air pollutants to produce ozone – the main component of smog.

The University of Toronto's James Donaldson, one of the study authors, said: "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. We don't know yet to what extent this is occurring, but it may be quite a significant, and unaccounted for, contributor to air pollution in cities."

Researchers had previously thought nitrogen oxides become active when trapped in grime, becoming "locked" in place. However, the team found evidence to contradict this theory after discovering nitrate anions disappear from grime at rates faster than can be explained through wash-off via rainfall.

Later studies showed the nitrate disappeared at a rate 10,000 times faster than from rainfall when they were exposed to artificial sunlight. After looking to see what happened to grime when kept in the dark or exposed to artificial sunlight, they found the solar simulated grime shed nitrates than that which had been left in the dark.

To test the idea that light chemically reverts nitrogen back into active forms, they set up a field study in Toronto using grime collectors – some were left in the sun while others were left in the shade. Findings showed that those in the shade contained 10% more nitrates than those placed in the sun.

Donaldson said: "If our suspicions are correct, it means that the current understanding of urban air pollution is missing a big chunk of information. In our work, we are showing that there is the potential for significant recycling of nitrogen oxides into the atmosphere from grime, which could give rise to greater ozone creation."