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Australia's Blue Mountains. Virginia's Blue Ridge. Jamaica's Blue Mountain Peak. Places all over the world have acquired names evoking the bluish haze that hangs over wooded hills in summer. But what makes them hazy? In 1960, botanist F. W. Went suggested that simple hydrocarbon gases given off by trees were responsible, a phenomenon that 20 years later prompted the Reagan Administration to blame "killer trees" for air pollution. But the scientific story behind the emissions--and the implications for air quality--have remained, well, hazy.
An experiment reported in last week's issue of Nature dispels some of the haze by offering an answer to one question: Why do plants go to the trouble of producing it? The research implies that plants produce one major haze ingredient, isoprene, as a strategy for coping with heat. And that, together with recent evidence that these hydrocarbons are far more abundant than was thought, could change researchers' picture of the less benign haze that forms over urban and even rural areas on hot, sunny days. By understanding when and why plants give off hydrocarbons, explains Chris Geron, who does atmospheric modeling for the Environmental Protection Agency (EPA) in Research Triangle Park, North Carolina, "we can better understand the impact [of plant compounds] on our models of air quality."
Plant hydrocarbons, innocuous or even appealing on their own (the scent of a Christmas tree comes from one), are ingredients in photochemical smog. Along with hydrocarbons from cars and backyard grills, they combine with nitrogen oxides from combustion in engines and industry to generate low-level ozone--an irritant to lungs and to the plants themselves. "When plants did this 100 years ago, ozone formation was [probably] not being...