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The filthy urban grime contributes to air pollution

As if the cities aren’t already plagued with enough unhealthy vehicle emissions and dirt, new research suggests that the filthy, black grime that resides over buildings, statues and other outdoor surfaces in urban areas breathes out harmful nitrogen gases when exposed to sunlight. Researchers confirmed that natural sunlight triggers the release of nitrogen oxide compounds from the grime covering the urban landscape, eventually converting them into smog. They contend sunlit grime releases nitrogen in two forms – the toxic pollutant nitrogen dioxide (NO2), and nitrous acid, a pivotal driver of smog formation.

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

According to Donaldson, urban grime is a collection of thousands of chemical compounds spewed into the air by automobiles, factories and other sources of nasty pollutants. Among these compounds are nitrogen oxides. When in the air, these compounds combine with other air pollutants, known as volatile organic compounds to produce ozone which is the main component in smog. It can cause irritation to the nose, throat and eyes, as well as dizziness, nausea, and in extreme circumstances, pneumonia and bronchitis. However, Donaldson and his colleagues at the University of Toronto have collected data that proved sunlight triggers the release of nitrogen oxide compound from dirt, disapproving a long-held theory that nitrogen oxides become inactive when they are trapped in grime and settle on a surface.

In a previous experiment, professor Donaldson’s team discovered that nitrate anions stripped off from grime at faster rates than could be explained by wash-off due to rainfall. In a subsequent laboratory comparison, they found that nitrate disappeared from grime 10,000 times faster than from a water-based solution when both were exposed to artificial sunlight.

In a further study, researchers set up a six-week field study in Leipzig and a subsequent year-long study in Toronto to test their findings in the real world. Scientists placed grime collectors with glass beads throughout both cities. Some of the collection devices were left in the sun, while others were intentionally put in the shade. In Leipzig, the researchers found that the grime in shade areas contained 10 percent more nitrates than the grime exposed to natural sunlight, which was consistent with the laboratory’s findings. The study in Toronto is still ongoing.

“If our suspicions are correct, it means that the current understanding of urban air pollution is missing a big chunk of information,” said Donaldson. “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.”

The scientists also plan to carry out other field experiments on the subject of pollution in some location that is “really grubby” and some that is “really clean”. They plan to compare the effects of humidity, grim levels and various amounts of illumination on the recycling of nitrates back into the atmosphere.

The scientists will present their findings based on field studies conducted in Toronto and Leipzig, Germany, at the 250th National Meeting & Exposition of the American Chemical Society (ACS), the world’s largest scientific society on Monday, August 18. The meeting features more than 9,000 reports and is being held here through Thursday.

Scientists have studied the effects of ozone on health for decades. A research released by the American Lung Association last year found that almost half of the U.S. population lives in areas where air pollution levels are often dangerously high for them to breathe. The five communities with the worst air pollution were all in California, with Los Angeles-Long Beach at the top of the list with the highest ozone levels. Elevated ozone levels put people at risk with immediate health problems including shortness of breath, wheezing and coughing, asthma attacks, and risk of respiratory infections, in addition to increased risk of premature death, in extreme cases.