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Lawn clippings may help make smog

The odour from a freshly mown lawn may contribute to smog, Australian researchers say.

They say cut grass releases volatile organic compounds (VOCs) that contribute, in a small way, to the toxic white haze surrounding our cities on hot days.

And on summer weekends, when more people tend to mow their lawns, more VOCs are released into the atmosphere, say Wayne Kirstine of Monash University and CSIRO's Ian Galbally.

They have for the first time estimated the amount of VOCs grass releases, publishing their work in a recent issue of the Journal of the Air and Waste Management Association.

In urban areas VOCs, small molecules made of hydrogen and carbon, interact with other pollutants in the presence of sunlight, to produce ozone, the main component of photochemical smog.

Trees do it too

Scientists have known since the 1970s that trees contribute a significant amount of VOCs, on top of what industrial and transport sources produce.

But Kirstine and Galbally have found out that grass, especially cut grass, also emits its own fair share of VOCs.

"They produce grassy fresh-type odours," Kirstine told ABC Science Online. "They're actually very important compounds in the tea industry because they actually lend the aroma and flavour to different types of teas and all sorts of wines."

The researchers modelled the release of VOCs from grass and calculated how much is released into the air above Sydney and Melbourne.

They calculated that VOCs from grass contributes 2% of yearly VOC emissions in Sydney and 3% in Melbourne. And they estimated that on summer weekends these percentages rise to 4 and 5%, respectively.

"On smog alert days when we're often told not to use outdoor barbeques and incinerators we should also be indicated not to cut grass," Kirstine said.

Kirstine said apart from their contribution to smog, plant VOCs were generally not directly toxic to humans, unlike VOCs from petrochemicals or solvent-based compounds.

Not a significant source

The study was commissioned by the NSW Department of Environment and Conservation, which is updating its calculations on where VOCs come from.

Director of the department's air policy section, Nigel Routh, said the contribution of grass to overall VOC emissions was not significant compared with VOCs from other sources.

"It's a relatively small contribution, and there are more important things to worry about," he told ABC Science Online.

Routh said the department concentrated on reducing VOCs from human-generated or anthropogenic sources, which contribute 64% of total annual VOC emissions in Sydney.

He said little could be done about the main plant sources, which consisted of native vegetation, which was often in protected areas.

Trucks and vehicles made up 44% of annual anthropogenic VOC emissions in Sydney and large industry, 14%. Homes and small businesses made up 42%, with paints and cleaning products being the main culprits.

Routh said the contribution of VOCs from lawnmower exhaust fumes could make up to 10% of annual anthropogenic VOC emissions in Sydney.

He also said VOC emissions due to hand-held two-stroke garden equipment, such as leaf blowers, "whipper snippers" and hedge trimmers, were predicted to increase because more people were using them.

He said he hoped a rating scheme for such equipment would give consumers some guide on how polluting they are.