by Carol Rasmussen
In the Mojave Desert, where scientists found unexpectedly volatile salates, plant surfaces can reach temperatures as high as 45°C (113°F). (Photo courtesy Alex Guenther.)
We humans have to apply sunscreen to protect ourselves from ultraviolet rays. Plants are more efficient: they create their own sunscreen, which includes compounds known as salates. Like the sunscreens we use, these natural compounds were thought to stay on unless rubbed or washed off. Now, a team of chemists from NCAR, in collaboration with the University of Nevada's Desert Research Institute, has learned that salates can evaporate into the atmosphere. The volatilized salates could be the atmosphere's largest source of organic aerosols in hot climates.
"This is the first time that we've seen these compounds as a gas," says NCAR scientist Alex Guenther. "It's something that's been left out of all our models
Heavy, bulky, and sticky, salate molecules seem about as likely to become airborne as Godzilla. Most molecules in the atmosphere consist of two or three atoms and have a molecular weight under 60; in comparison, homosalate, one of the most common salate compounds, has a formula of C16H22O3 and a molecular weight over 262.
The scientists weren't looking for salates when they made the new finding. They were studying sesquiterpenes (C15H24), the heaviest class of terpenes that is known to become volatile. Terpenes, which are responsible for both the fragrance and the stickiness of pine resin, are found on many kinds of plants. Atmospheric chemists are interested in all kinds of terpenes because the compounds are an important source of secondary organic aerosols, or SOAs, which form when a gas condenses onto a particle or reacts with another gas to form a particle. Scientists are seeking to quantify the budget and reactions of sesquiterpenes and the other volatile organic compounds that form SOAs.
Although sesquiterpenes are a bit lighter than salates, they pose similar observational challenges. 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 and into detectors. Complicating things even further, the usual technique in atmospheric science of concentrating by freezing or absorbing, and then heating the concentrated sample, causes some of these compounds to break down into other molecules.
Alex Guenther. (Photo by Carlye Calvin.)
New technique yields new results
In July 2006, Guenther's team collected samples of air around ten species of plants growing in the Mojave Desert near Las Vegas, using enclosures from which the ozone had been scrubbed (see photo at right). Then, instead of using standard techniques, they analyzed the samples using a liquid-extraction technique that is common in plant biology. When ASP postdoctoral scientist 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.
Using mass spectroscopy, Matsunaga found and eventually confirmed that the compounds were salicylate esters, a type of salate. "I didn't know that such compounds could be emitted from plants," he says. The team also found more oxygenated sesquiterpenes, another extra-heavy compound, than expected. To their surprise, the emission rates for these superheavy 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 proved something else, Guenther adds: "In a cooler climate, these emissions probably are negligible. It has to be really hot to get them going." In the Mojave, leaf temperatures can reach 45°C (113°F).
Fitting salates into the global climate puzzle
A paper on their discovery is still under review, so Guenther and colleagues aren't ready to talk in depth about the significance of their findings. However, if salate emissions prove to be as prevalent elsewhere in the world as they are in the Mojave, they may require a rethinking of Earth's radiation budget. As Guenther points out, 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.
Since no one knew these compounds could become airborne, they are not included in any atmospheric chemistry model. Modelers may be able to simulate the salates' effects by lumping them in with those of some already-modeled species, such as the sesquiterpenes that the team was looking for in the first place. However, Guenther's initial guess is that this workaround won't prove adequate. "They're different enough that I think they will need to be included separately."
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 an SOA. The salate molecules are so big that they can turn into SOAs all by themselves, by simply condensing as they reach slightly cooler air (although they also can become aerosols in the usual way, by chemical reactions). Within the last decade or so, researchers have discovered that the size of the SOAs, 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. Guenther 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."
A sampling enclosure on a mesquite plant, one of the species that was found to emit salate compounds. (Photo courtesy Alex Guenther.)