‘First of its kind’ study claims that natural sunlight triggers release of nitrogen oxide compounds from grime on outdoor urban surfaces
Dirty city buildings, statues and other outdoor surfaces may be contributing to air pollution, according to US chemistry scientists.
Researchers from the University of Toronto presented a study in Boston, USA, this week explaining how natural sunlight can trigger the release of smog-forming nitrogen oxide compounds from the grime that typically coats outdoor surfaces in urban areas.
The scientists say their findings – based on field studies in the Leipzig in Germany and Toronto in Canada – confirm previous laboratory work using simulated sunlight and builds on the ‘long-held’ notion that nitrates in urban grime are ‘locked’ in place.
One of the report’s authors, James Donaldson, 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.â€?
According to Dr Donaldson, grime on urban surfaces is a mixture of thousands of chemical compounds – including nitrogen oxides – which are “spewed into the airâ€? from the likes ofroad vehicles and factories.
Nitrogen oxides in the air can combine with other pollutants – known as volatile organic compounds (VOCs) – to form ozone pollution, which is a major component of smog. However, it was previously suspected that nitrogen oxides become inactive when trapped in grime and settled on an urban surface.
But Dr Donaldson and his colleagues at the University of Toronto found in previous work that nitrate anions disappeared from grime at faster rates than possible simply by washing off in rainfall. And, they also found that nitrate disappeared from as much as grime 10,000 times faster when exposed to sunlight than from a water-based solution.
Finally, the scientists also discovered that grime exposed to a ‘solar simulator’ lost more nitrates in comparison to grime left in the dark, suggesting that light can chemically convert nitrogen compounds back into active forms that can return to the atmosphere.
To test the laboratory findings, the research team then carried out real-world field tests in Leipzig, Germany, for six weeks, and also in Toronto for 12 months, placing ‘grime collectors’ containing glass beads throughout both cities. The beads created more surface area for grime to gather than on a flat surface, such as a window.
The collection devices were left both in the sun and in the shade with adequate air flow to collect pollutants. And, although the Toronto study is still ongoing, the Leipzig study revealed that grime in shaded areas contained 10% more nitrates than grime exposed to natural sunlight.
Dr 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.â€?
He added that they found Liepzig had “far moreâ€? dirt on urban surfaces than Toronto, which could have a “significant influenceâ€? on the German city’s air quality: “The fact that Leipzig appears to have 20 times more grime than Toronto suggests that there is a potential for 20 times more recycling of nitrogen oxides into the local atmosphere.â€?
The scientists now plan to take the research further by conducting and comparing experiments in cities that are very clean and others which are much dirtier. They also plan to examine the effects of humidity, grime levels and various amounts of illumination on the recycling of nitrates back into the atmosphere.
The findings of the report were presented at the 250th National Meeting & Exposition of the non-profit American Chemical Society (ACS) in Boston, which took place throughout this week (August 16-20).
ACS claims to be the world’s largest scientific society, boasting more than 158,000 members, and the meeting featured more than 9,000 reports.