Sun and city dirt trigger smog

Sunny days have been found to trigger the release of potentially harmful smog from grime which coats buildings, statues, and other outdoor surfaces in city areas.

In the past, it had been thought that, once the airborne pollution which was emitted from cars and factories settled onto the various surfaces, it was absorbed into the grime there and became inactive and was no longer harmful.

Scientists have now discovered that, when exposed to sunlight, the grime releases “smog-forming” nitrogen oxide compounds.

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

“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.”

Dr Donaldson and colleagues set up studies in Germany and Toronto, placing “grime collectors” containing glass beads throughout both cities. The beads create more surface area for grime to gather on than a flat surface, such as a window.

Some of the collection devices were left in the sun, others in the shade but with adequate air flow so that grime could collect on their surfaces.

The researchers found that grime in shaded areas contained 10% more nitrates than grime exposed to natural sunlight.

They noted that Leipzig had far more grime than Toronto, based on the amount of material gathered at collection stations in both cities so far, a difference which could have a significant influence on the city’s air quality.