Scientists had long assumed that the air pollution that turned into the grey grime found on buildings in urban environments across the globe was inert and no longer a danger, but a new study suggests that this is not the case.
This new study shows that sunlight seems to play a key role in cities’ smog levels, as sunlight apparently breaks down the pollutants in urban grime releasing nitrogen gases which eventually convert to smog.
Initial data for this new study were presented at an American Chemical Society meeting in Boston on August 17th.
Details on new study showing sunshine turns grime into air pollution
Dr. James Donaldson, a chemistry professor at the University of Toronto and lead author of the new study, and his colleagues have collected data that support the theory that sunlight breaks down grime into gases that become smog. The researchers conducted initial studies that showed grime was chemically active. Then, in another lab study, they found that nitrates were disappearing from grime much faster when exposed to artificial sunlight. Finally, they studied grime exposed to either artificial sunlight or kept in the dark. The grime exposed to “sunlight” lost a lot more nitrates than the grime in the dark, showing that that light can cause a chemical reaction to convert nitrogen compounds back into active gas forms.
Donaldson and team then tested this concept in the real world. The researchers undertook a six-week field study in Leipzig, Germany and a year-long study in Toronto. They set up grime collectors containing glass beads to collect grime in both cities.
Some of the grime-collecting beads were left exposed to the sun; others were placed in the shade, but with enough air flow so that grime could build up on the beads. In Leipzig, the study determined that grime in shaded areas contained 10& more nitrates than grime exposed to natural sunlight, similar to the results of the lab studies.
The Toronto study is going on, and it will be some time before all the data are collected and analyzed.
Statement from lead study researcher
“The current understanding of urban air pollution does not include the recycling of nitrogen oxides and potentially other compounds from building surfaces,” Donaldson noted in his comments at the ACS meeting. “But based on our field studies in a real-world environment, this is happening.”
“If our suspicions are correct, it means that the current understanding of urban air pollution is missing a big chunk of information,” Donaldson continued. “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.”