Dirty windows may worsen smog, according to Toronto chemists who say urban grime may help recycle nitrogen oxides into the air.

Susannah Handley and her team from the University of Toronto recently reported their findings online ahead of an article in the journal Environment Science and Technology.

The researchers say window grime contains inactive nitrogen compounds that have been deposited from air pollution, 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 convert back into pollution-generating nitric oxides in the presence of sunlight.

"Given the significant amounts of nitrate and potential photosensitizers present in urban films, this renoxification could be an important source of photochemically active nitrogen oxides in urban environments," the researchers conclude.

Ian Galbally, an Australian atmospheric chemist at CSIRO Marine and Atmospheric Research, said in commenting on the research: "As far as I know no one's done that [study] before. They find that there is significant recycling of the nitric oxides."

Science of smog

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 In the course of this reaction, these nitric oxides are converted to nitric acid and nitrates, which are no longer active, in terms of producing ozone.

"These can be blown away by wind or washed away by rain. Or they can settle on various surfaces," Galbally said.

If they settle on natural surfaces like plants or soils, they are absorbed or eaten by microbes, he said. But if they settle on non-porous surfaces such as windows, they will stay there, caught in the grime and waiting to be recycled.

Galbally said scientists 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 lands on windows in the first place.

Galbally said while planting more trees in cities helps absorb nitrogen, trees can also emit volatile organic compounds, which contribute to air pollution.

He said that if nitrogenous fertilizers are used on trees, this can also cause the release of nitric oxide from the soil.