Grimy windows share dirty little secret
Monday, 14 May 2007 Anna Salleh
ABCKeeping your windows clean may have an added benefit, researchers say (Image: iStockphoto) (Source: iStockphoto)Related Stories
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Dirty windows may worsen smog, according to Canadian chemists who say urban grime may help recycle nitrogen oxides into the air.
Susannah Handley and team from the University of Toronto report their findings online ahead of print in the journal Environment Science and Technology.
Window grime contains inactive nitrogen compounds that have been deposited from air pollution, say the researchers, as well as an oily mixture of organic compounds, including bird poo.
The researchers did a study suggesting that the nitrogen compounds in this grime can covert back into pollution-generating nitric oxides in the presence of sunlight.
"As far as I know no one's done that [study] before," says Ian Galbally, an Australian atmospheric chemist at CSIRO Marine and Atmospheric Research, commenting on the research.
"They find that there is significant recycling of the nitric oxides."
Motor vehicles and power plants produce nitric oxide and nitric dioxide, which help produce ozone, the main component of photochemical smog.
In the course of this reaction, these nitric oxides are converted to nitric acid and nitrates, which are no longer active, in terms of producing ozone.
Galbally says these can be blown away by wind or washed away by rain. Or they can settle on various surfaces.
If they settle on natural surfaces like plants or soils, they are absorbed or eaten by microbes, he says.
But if they settle on non-porous surfaces like windows they will stay there, caught in the grime, waiting to be recycled.
Galbally says we still don't know how important this recycling process is in contributing to urban pollution.
While the study suggests two-thirds of the nitrogen compounds landing on windows are recycled into nitric oxides, he says the seriousness of the issue will depend on how much of them land on windows in the first place.
Galbally says while planting more trees in cities helps absorb nitrogen, trees can also emit volatile organic compounds, which contribute to air pollution.
He says if nitrogenous fertilisers are used on trees, this can also cause the release of nitric oxide from the soil