If volatile organic compounds interest researchers so much, it is because some of these compounds are chemically very reactive. The atmosphere is perturbed by their presence. For example, the chemical reactions they cause result either in the creation or the destruction of ozone in the air we breathe. In areas polluted by the combustion of fossil fuels (presence of nitrogen oxides), ozone gas will be created which has harmful effects on the quality of the air. In rural areas, on the other hand, as demonstrated by the American researcher Allen Goldstein (see Nature 459, May 2009), one of the pioneers of research in this area, certain VOCs can react with these ozone molecules and form aerosols that can form a kind of mist over the forest. According to the estimates of Goldstein, 150 to 200 million tonnes of VOCs around the planet are converted into aerosols. Today experts are investigating to what extent these aerosols have an impact on the climate. They could have a “cooling effect”, by blocking part of the solar radiation. A recent study (2), compiled 28 measurement programs across the globe in order to compare the modeling of isoprene emissions with actual emissions in nature according to climate and natural environment (tundra, temperate forests etc.). The measurements carried out by Bernard Heinesch and his collaborators in the Vielsalm forest were included in this study. “The key factor in relation to our data is the exceptionally long duration over which we carried out measurements: three years in total, which allowed us to monitor several cycles of vegetation”.

In the Vielsalm forest, researchers at the Unit of Biosystems Physics of Gembloux Agro-Bio Tech kept their equipment working for entire vegetation seasons, from April to October. They showed that the quantity of isoprene emitted increases with temperature and radiation and that when there is an equivalent amount of temperature and radiation the quantity of isoprene drops progressively from summer to autumn. The production of isoprene, conclude the researchers, is linked to photosynthesis and therefore the quantity of carbon produced by the plant. “We suspected as much, laboratory studies had already shown that isoprene is a by-product of photosynthesis. But it was still to be confirmed in the external environment”, explains Bernard Heinesch.

Another study (3) measured the production of another VOC, methanol (CH₃OH). This was identified as a by-product of leaf-growth, without having any known biological role. On the other hand, scientists know that it is a gas which plays an important role in the chemistry of the atmosphere. Emissions of methanol are abundant while leaves are growing at the beginning of the month of May, then they drop significantly and the forest can then become a methanol sink, that is to say, it can absorb more than it emits. The researchers noticed that the mechanism for the disappearance of methanol increases with humidity and they formed the theory that this volatile organic compound dissolves in the films of water present on the surfaces to be subsequently consumed by bacteria.

(2) Unger et al., Photosynthesis-dependent isoprene emission from leaf to planet in a global carbon-chemistry-climate model, in Atmospheric Chemistry and Physics (2013), 13.
(3) Laffineur et al., Abiotic and biotic control of methanol exchanges in a temperate mixed forest, in Atmospheric Chemistry and Physics (2012), 12.