The intoxicatingly fresh aroma of pine forests is the result of organic compounds called terpenes that are released into the atmosphere by coniferous trees. Terpenes belong to a large group of volatile organic compounds (VOCs) thought to have a key role in modulating local climate. As VOCs waft into the atmosphere, they react with hydroxyl radicals (OH) and ozone (O3), contributing to the formation of aerosol particles. These aerosols serve as nuclei for water droplets to form into clouds, which ultimately have a cooling effect on the climate.

Researchers have long been puzzled by a mismatch between the amounts of organic aerosols measured in the atmosphere and those predicted to be there by models of plant emissions and subsequent reactions. To get to the bottom of this apparent incongruity, Astrid Kiendler-Scharr, an atmospheric chemist who recently established her lab at the Jülich Research Centre in Germany, teamed up with a Jülich colleague, physicist Jürgen Wildt, to demonstrate that an abundant VOC called isoprene may provide the missing link.

Wildt had developed specialized 'plant chambers' to study the types of emissions given off by plants. He, Kiendler-Scharr and their colleagues spent two years refining their experimental design of these chambers so that they could isolate VOC emissions and the aerosols produced as VOCs mingle with OH and O3 in the atmosphere.

Their system includes a pair of chambers with volumes of 1.1 and 1.3 cubic metres. The first chamber houses a small mixed stand of different saplings. VOCs emitted by plants in this chamber are transferred to the second chamber and there combined with O3 and OH. The researchers then apply a suite of analytical chemistry instruments to determine the exact size and number of aerosols formed.

In an earlier study, Kiendler-Scharr's team experimented with spruce and pine, which emit a class of terpenes known as monoterpenes. In that experiment, they were able to accurately predict how much aerosol should form based on the plants' VOC emissions.

For the current study (see page 381), they added to the plant chamber an oak sapling, which emits a different VOC called isoprene. This molecule accounts for a third of global VOC emissions from both natural sources such as vegetation and anthropogenic emissions such as traffic. When they first observed that the addition of an isoprene emitter to their experimental plant chamber led to fewer aerosol particles forming, they did not trust their results. “We went through the details of our experimental set-up for a week to figure out what mistake we were making,” says Kiendler-Scharr.

The team found that isoprene scavenges OH radicals and suppresses the formation of new aerosols. In the atmosphere, fewer aerosols would lead to less cloud formation and less of a cooling effect on climate. Thus, isoprene emissions could lead to increased global-warming trends, says Kiendler-Scharr.

She hopes that further collaborative efforts within her institute will make it possible to study atmospheric simulations over a period of two to three days in much larger chambers.

It is estimated that 10,000–100,000 different VOCs exist in the atmosphere, resulting in thousands of oxidation products. The lingering question, says Kiendler-Scharr, is how these complex mixtures of molecules interact during oxidation.