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October 2007

Surprise finding in the desert

Researchers discover plant sunscreen chemicals in atmosphere

alex guenther

Alex Guenther.

Like humans, plants are vulnerable to the Sun’s damaging rays. Unlike people, however, they are able to produce their own sunscreen.

The compounds in this botanical sunscreen are called salates. Salates were thought to stay on a plant’s surface unless rubbed or washed off. A team of researchers in ESSL/ACD has recently learned, however, that they can evaporate into the atmosphere. Even more, these chemicals could be the atmosphere’s largest source of organic aerosols (airborne particles) in hot climates.

“This is the first time that we’ve seen these compounds as a gas,” says Alex Guenther. “It’s something that’s been left out of all our models and inventories.”

The surprise discovery is important because salate emissions could affect Earth’s radiation budget and climate, as well as human health.

The scientists weren’t looking for salates when they made the new finding. Rather, they were studying sesquiterpenes, the heaviest class of terpenes that are known to become volatile. Terpenes, which are responsible for the fragrance and the stickiness of pine resin, are found on many kinds of plants. Atmospheric chemists are especially interested in them because the compounds are an important source of secondary organic aerosols.

mojave

In the Mojave Desert, where scientists unexpectedly found airborne salates, plant surfaces can reach temperatures as high as 113°F (45°C). (Photo courtesy Alex Guenther.)

Observing sesquiterpenes isn’t easy. Some of them react quickly with ozone, so they must be measured in an enclosure from which ozone has been eliminated. They are too sticky to get through many sampling systems. Complicating things even further, the common technique of concentrating compounds by freezing or absorbing them, followed by heating, causes some sesquiterpenes to break down into other molecules.

New technique yields surprising results

In July 2006, the ACD team collected samples of air around 10 species of plants in the Mojave Desert near Las Vegas. Then, instead of using standard techniques, they analyzed the samples using a liquid-extraction technique that is common in plant biology. When postdoctoral researcher Sou Matsunaga put the liquid through a gas chromatograph, he was surprised to see large peaks at unexpected locations on the chromatograph, indicating sizeable quantities of an unknown compound.

Sou eventually confirmed that the compounds were a type of salate. “I didn’t know that such compounds could be emitted from plants,” he says.

The researchers also found more oxygenated sesquiterpenes, an extra-heavy compound, than expected. To their surprise, the emission rates for these compounds were, in some cases, as high as the rates for the better-known monoterpenes and sesquiterpenes combined.

Follow-up measurements in cool temperate forests in Australia, Japan, and near Boulder revealed something else. “In a cooler climate, these emissions probably are negligible. It has to be really hot to get them going,” Alex says.

Fitting salates into the global climate puzzle

If salate emissions prove to be as prevalent elsewhere in the world as they are in the Mojave, they may spur a rethinking of Earth’s radiation budget. Salates work as sunscreens because they absorb ultraviolet radiation, so even in plant-bound form, they influence energy budgets. Their effect when volatilized is not yet known. And because researchers didn’t know that these compounds could become airborne, they are not included in any atmospheric chemistry models.

The emissions are also of concern because of their effects on air quality and possibly human health. The bigger a gas molecule is, the more likely it is to form a secondary organic aerosol. Within the last decade or so, researchers have discovered that the small size of these aerosols, which are on the scale of a nanometer, makes it particularly easy for them to lodge in human lungs or even to pass through the lungs and damage other parts of the body.

Finally, the scope of these emissions isn’t known in global terms. Alex expects that in other hot regions—both dry and humid—the emission patterns are likely to be similar. “But of course we have to go there and look.”

mesquite

A sampling enclosure on a mesquite plant, one of the species that was found to emit salate compounds. (Photo courtesy Alex Guenther.)


In this issue...

Ice in clouds

Surprise finding in the desert

Tackling disasters in an energy-restricted Boulder

Jeffco bears fruit

Short Takes

Getting their paws wet

Delphi Question

Random profile: Chris Golubieski

Just One Look


 

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