Surprise finding in the desert
Researchers discover plant sunscreen chemicals in atmosphere
Like humans, plants are vulnerable to the Sun’s damaging
rays. Unlike people, however, they are able to produce their
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
“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
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
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
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
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...
finding in the desert
disasters in an energy-restricted Boulder
their paws wet
profile: Chris Golubieski
Just One Look
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