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Tick Tock, Modeling Emissions From Trees Around the Clock

25 September 2011 1:00 pm
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Ben Langford

Canopy clock. Trees, such as those in this Malaysian rainforest, control their gas emissions with an internal clock.

Gaze at the Blue Ridge Mountains of Virginia and you'll be looking into a hazy cloud hovering above the range's deciduous forests. But it's not all lovely: The cloud contains organic chemicals from trees, called isoprenes, that can be converted into ozone, a dangerous pollutant and greenhouse gas. Thanks to a new finding that trees have a biological clock that controls their isoprene emissions, scientists may be better able to model how ozone is produced and how it reacts in the atmosphere.

Emitting isoprenes helps trees protect themselves from insects, heat, and other enemies. But as former U.S. President Ronald Reagan famously claimed, these emissions are major contributors to ground-level ozone and smog, although human activity contributes more than its fair share. Although this pollution can harm trees themselves, plants must have done just fine before humans came along, says atmospheric chemist Nicholas Hewitt of Lancaster University in the United Kingdom. The interplay between isoprenes, pollutants from fossil fuel, and other atmospheric components is complex, and scientists have had a hard time modeling ground-level ozone. The most widely used model tends to predict 50% more ozone than sensor readings say is actually present.

Hewitt's group wanted to make more accurate predictions. They suspected that unknown factors in isoprene production weren't being accounted for. Previously, they had discovered that leaves emit different levels of isoprene depending on the time of day but did not know how that translated to a larger scale. So they set up two isoprene detection stations in Malaysia: one over the canopy of a rainforest and one over an oil palm plantation. Then they tracked isoprene levels hour by hour. Isoprene emissions, they found, peaked at midday and dropped to minimal levels at night. The pattern was especially prominent at the oil palm plantation, where all the plants were of a single species.

The researchers then corrected their ozone model to reflect the circadian pattern. Assuming that plant isoprene emissions might follow similar circadian rhythms across the world, they compared the corrected model with real data from 290 of the U.S. Environmental Protection Agency's ozone-monitoring stations in the southeastern United States. Adjusting for circadian rhythms increased the model's accuracy by 10%, the researchers report today in Nature Geoscience.

In an e-mail, biologist Russell Monson at the University of Colorado, Boulder, says that "we now must respect" trees' internal clock when making climate models. Incorporating this variable will be difficult because the biochemistry is complex and not well understood. But circadian control makes sense, he says, because making isoprene requires a lot of energy, and plants may not want to produce it around the clock.

Alastair Lewis, an atmospheric chemist at the University of York in the United Kingdom, who has worked alongside Hewitt on different projects, points out that the finding has implications for climate modeling of methane, another important greenhouse gas. Together, human production of methane and ozone "approach the impact that CO2 has" on the temperature of the planet, he says. Because current climate models are "crude, it's very important that we get this right" by incorporating factors like circadian rhythms when trying to understand how the atmosphere has changed over centuries.

Hewitt's group plans to take its isoprene-monitoring setup to Brazil to find out how the daily rhythm of the Amazon rainforest's affects ozone production there.

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