City dirt and sunlight is a toxic combination

Grime and sun lead to toxic pollution in cities

City dirt releases toxic nitrogen oxide compounds when illuminated by the Sun, increasing air pollution levels in urban areas, a new study has revealed.

The study conducted by researchers from the University of Toronto is the first of its kind, looking at the grime covering buildings, statues and outdoor surfaces in cities as a potential source of the toxic pollution. The ability of light to unlock nitrates in the grime was previously suggested in laboratory experiments.

"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 from 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."

Urban grime is a mixture of thousands of chemical compounds spewed into the air by automobiles, factories and a host of other sources. Among these compounds are nitrogen oxides. When in the air, these compounds may combine with other air pollutants, known as volatile organic compounds, producing ozone, which is the main component of smog.

As part of the study, the researchers distributed grime collectors throughout the German city of Leipzig and Toronto in Canada. Some collectors were placed in the sun while others in shaded areas. The collectors contained glass beads providing larger surface area for the grime to deposit on.

The Leipzig data, evaluated after six weeks, suggested that grime in shaded areas contained 10 per cent more nitrates than grime exposed to natural sunlight, meaning the nitrates from the insulated containers must have evaporated.

The findings were consistent with results of earlier laboratory experiments and refute the previously accepted theory that nitrogen oxides become inactive when they are trapped in grime and settle on a surface.

The laboratory experiments conducted by the same team earlier suggested that nitrate ions disappear from grime faster than would be explained by rainfall.

In a laboratory comparison, the researchers found that nitrate disappeared from grime 10,000 times faster than from a water-based solution when both were exposed to artificial sunlight. In yet another study, they exposed grime to either artificial sunlight or kept it in the dark. The grime exposed to a 'solar simulator' shed more nitrates than the grime left in the dark, suggesting that light can chemically convert nitrogen compounds back into active forms that can return to the atmosphere.

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

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