Research on the VOCs therefore, uses the same scientific infrastructures as those used for the carbon cycle, but requires more sensitive measuring equipment because the VOCs are only present in the atmosphere in much smaller quantities than carbon dioxide. Currently, there are around 400 moles of CO₂ per million moles of air in a sample of the atmosphere (the unit is ppm). To put this into context, it is equivalent to 40cl for a volume of one cubic meter. The VOCs are measured in parts per billion (ppb). This is a thousand times less than the figure for CO₂! These are called “trace gases” and sophisticated machines are required to quantify them. The researchers at Gembloux use a mass spectrometer, installed on a trailer and which can be moved to the different places being studied. The mass spectrometry team from the Institute for Space Aeronomy at Uccle (Crist Amelynck/Niels Schoon) is directly linked to this work. “Only around ten teams in Europe have the capability to conduct this type of study”, estimates Bernard Heinesch.

Very reactive chemical compounds

Volatile organic compounds are carbon and hydrogen-based molecules which are very volatile and therefore often in the gaseous state at ambient temperature. A certain number of VOCs are produced by human activity and are considered to as polluting agents, sometimes to the point of being dangerous for human health. This is the case with benzene (C₆H₆), for example, a product of fossil fuel combustion. However, the vast majority of VOCs (the ratio is ten to one), are of natural origin. The researchers have already counted nearly two thousand VOCs emitted by plants. Isoprene (C₅H₈), for example, is a VOC of plant origin which is emitted in great quantities. Some VOCs are odorless; others on the other hand, produce a particular odor. If a pine forest smells very nice in summer it is because pine trees emit alpha-pinene. The smell of freshly-cut grass on a summer Sunday is also due to an organic volatile component (its scientific name is “cis-3-hexen-1-ol”), just as the very fresh smell that emanates from a lemon tree is produced by limonene. The reason wine-lovers can appreciate the bouquet of a grand cru when they run the glass under their nose is also due to VOCs).

In nature, each plant emits dozens of VOCs. It can be supposed that most of these compounds fulfil a biological function. “For example, we believe that isoprene plays, among other, a protective role in photosynthesis. It enables the plant to continue the process of photosynthesis despite high leaf temperatures”, explains Bernard Heinesch. According to current knowledge, it is the VOC most emitted by plants on a global scale.