Sunlight causes grime on city buildings, statues and other outdoor surfaces to release chemicals that create smog, according to a new study.
According to leader of the study, chemistry professor James Donaldson from the University of Toronto, many are still not aware that urban air pollution also includes recycling of nitrogen oxides where these compounds stick and accumulate onto building surfaces making buildings dirty.
Scientists used to think that once nitrous oxides get caked and locked onto surfaces as grime, they no longer contribute to air pollution.
After the six-month field test in Leipzig, the researchers conducted a similar yearlong trial in Toronto, which showed similar results.
The study finding is to be presented Monday at the annual meeting of the American Chemical Society in Boston. He also noted that if his research comes to a suspected conclusion, the understanding of air pollution today would be incomplete: “If our suspicions are correct, it means that the current understanding of urban air pollution is missing a big chunk of information”.
“We don’t know yet to what extent this is occurring, but it may be quite a significant contributor to air pollution in cities”.
Among the compounds are nitrogen oxides. Him and his team first got a hint when they found nitrate anions disappeared from grime at faster rates than could be explained by wash-off due to rainfall. For their tests, they shined artificial sunlight onto grime samples in order to determine the grime’s effect on chemical output. 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.
Working with colleagues in Leipzig in Germany, Donaldson recreated some laboratory experiments in the field, putting shelves with beads of window glass on building roofs in the German city. They hope to eventually expand their studies from glass to more chemically-reactive surfaces, such as concrete, asphalt, and brick, to include a total city surface, Donaldson said in a press conference. “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.”
After analyzing the composition of the grime, Donaldson’s team found that the beads exposed to sunlight contained about 10% fewer nitrates than those in the shade, suggesting that the sunlight zapped the nitrates out of the grime and back into the atmosphere.
Interestingly, Donaldson notes that Leipzig has far more grime than Toronto, based on the amount of material gathered at collection stations in both cities so far. Donaldson said that “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.”