Gas emissions from mowed grass.

Lazy suburbanites now have one more excuse for letting their lawns grow wild. While pollution experts have long worried about the exhaust from gas-powered mowers, new research shows that the grass trimmings themselves release a burst of pollutants as they dry.

Past studies have shown that wounded grass leaves emit a long list of volatile organic compounds (VOCs), giving rise to the distinctive aroma that hangs over a freshly mowed lawn. Joost A. de Gouw, an atmospheric chemist at the University of Utrecht in the Netherlands, and his colleagues looked beyond the cutting to see what happens as grass dries. They clipped red rescue and Dutch white clover from a lawn, placed the clippings in a box under a 100-watt lightbulb, and measured the VOCs.

"The drying is more important in terms of emissions than the cutting process," says de Gouw. The drying grass gave off 10 times more VOCs than did freshly cut leaves, the researchers report in the April 1 GEOPHYSICAL RESEARCH LETTERS.

The grass released (Z)-3-hexenol, (Z)-3-hexenal, hexenyl acetate, formaldehyde, methanol, acetaldehyde, acetone, and butanone. Some of these VOCs react speedily with other chemicals to form ozone gas and other components of smog.

A lab experiment alone can't resolve whether mowing lawns gives off enough VOCs to increase air pollution noticeably, says de Gouw. Because metropolitan areas have many sources of pollution, the compounds from mowed lawns may contribute relatively little to the noxious stew. The researchers are planning a field experiment this summer to determine the importance of these emissions.

The new findings add to emerging evidence that plants emit substantial amounts of so-called biogenic hydrocarbons. "The biogenics are probably more important than were recognized," says Roger Atkinson of the University of California, Riverside.

The act of mowing may have its biggest impact in hay-producing areas. "When you start to crop millions of acres of alfalfa and it's drying, that could be quite a point source [of VOCs]," says biochemist Ray Fall of the University of Colorado at Boulder, who participated in the new study. Past experiments have measured some elements of smog over Indiana farmland during summer, a finding that puzzled researchers. The grass study may explain these results, says Fall.