0528 GMT November 22, 2019
The dark muck was known to absorb such gases from the air, but it appears the nitrogen does not stay locked away, BBC reported.
In rooftop experiments in Germany, the researchers tracked the content of grime in both sunshine and shade.
They say sunlit grime releases nitrogen in two forms: the toxic pollutant nitrogen dioxide (NO2), plus nitrous acid — a key driver of smog formation.
The findings, presented at a conference of the American Chemical Society in Boston, were welcomed by pollution experts — and may explain a ‘missing’ source of smog-producing gas in the skies of London.
"Rather than being a permanent sink for nitrogen oxide gases... grime exposed to sunlight can re-release some of these gases back into the urban atmosphere," said James Donaldson, a chemistry professor at the University of Toronto in Canada who led the research.
Some of Prof. Donaldson's previous work had already shown that in a laboratory, artificial sunlight can strip the nitrogen component from grime — which is essentially a cocktail of chemicals belched into city air by exhaust pipes and chimneys.
In the latest experiments he worked with colleagues in Leipzig, Germany, to shift the work outside.
On a tower above the city they set up two large shelves filled with beads of window glass.
Both sets of beads received the same air flow — and got thoroughly grimy — but only one was in the sun.
"The ones which were exposed to sunlight showed 10 percent less nitrate than the ones which were shaded, suggesting that there is a photochemical loss [of nitrogen] consistent with what we saw in the lab," Donaldson told journalists at the conference.
That 10 percent drop may seem like a small effect, but it reflects a ‘steady state’ difference: as the sunshine eats away at the grime's nitrogen content, fresh nitrogen-rich grime is constantly being deposited.