Lawnful of pollution

Each time you mow your lawn you are making smogs worse

Published: Wednesday 15 July 1998

 Keep off the grass do you know that each time you mow that beautiful lawn in front of your house, you are contributing to one of the biggest nightmares of urban living -- smogs? Well, stay warned for a two-year Australian study blames you and every other lawn-owner for making these smogs worse. The study says that gases released when grass is cut or trampled can add significantly to urban pollution.

In summer, in a city of average size and population, mowing can account for up to 10 per cent of hydrocarbons entering the atmosphere. Some of these hydrocarbons react in sunlight with nitrogen oxides and several other gases to form photochemical smog.

"The contribution that cut grass makes to the mix of gases in the atmosphere has not been fully appreciated before," says Ian Galbally of the division of atmosphere research of csiro, Australia's national research agency, in Melbourne. "Not cutting grass when the city is experiencing a smog will not solve the problem but, when combined with other actions, such as car pooling, it will make a difference."

Galbally and Wayne Kirstine, an environmental scientist at Monash University's Churchill campus, 150 km east of Melbourne, used a gas chromatograph to analyse samples of air above lawns and pasture in the summer months of 1994 and 1995.

With no disturbance, either by mowing or trampling, grass gave off 650 micrograms of carbon compounds per square metre per hour. For clover, the figure was 2050 micrograms. These emissions went up by as much as 180 times after mowing.

Grass, like other vegetation, gives off hydrocarbons naturally, says Galbally. "But we found that, when spread out over a year, emissions of hydrocarbons double if grass is cut every seven weeks."

As much as 50 kg of volatile organic compounds are given per hectare annually from lawns and pastures. They include highly reactive hexanals -- oxygenated hydrocarbons with six atoms -- which are released when the enzymes in grass encounter oxygen.

Hydrocarbon emissions from grasses should be factored into computer models predicting smog levels in urban areas, say the researchers.

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