Tracking volatile organic compounds

It is well-known that vegetation, and forests in particular, play an important role in the carbon cycle. Researchers have now become interested in other chemical compounds produced by plants which play an important role in atmospheric pollution and even in climate change: these are volatile organic compounds (VOCs). More than two thousand of these compounds have been identified. Some of them, such as isoprene, for example, which are emitted by forests in large quantities, are currently being studied by scientists. A recent study compiled 28 measuring programs which were conducted virtually all over the world in order to compare the modeling of isoprene emissions with actual emissions in nature in accordance with climate and the natural environment (tundra, temperate forests etc.). The measurements carried out by Bernard Heinesch and his collaborators at the Unit of Biosystems Physics at Gembloux Agro-Bio Tech (University of Liege) in the forest of Vielsalm have been included in this study. They have thus been able to demonstrate outside the laboratory that the production of isoprene is linked to photosynthesis and therefore to the quantity of carbon absorbed by the plant.

The method used could be loosely compared with that of an elevated blind used by hunters except that the top of the structure extends above the forest canopy and reaches a height of ten meters above the forest of Vielsalm, the “Grand Bois” site. Right at the top of this structure, some fifty meters high, the researchers from the Unit of Biosystems Physics at Gembloux Agro-Bio Tech (University of Liege) installed their measuring equipment with a view to understanding what happens on a chemical level where vegetation meets the atmosphere. They discovered a lot of things…

Vegetation certainly exchanges carbon dioxide (CO₂) with the atmosphere. The type of scientific equipment used makes it possible to have a better understanding of the natural carbon cycle and to answer this important question: are the forests carbon sinks? Also, to what extent are they involved in the fight against global warming? For quite a few years now, these “scientific hunting blinds” have also been used to track volatile organic compounds (VOCs) in order to understand their impact on the chemistry of the atmosphere. (1). “Laboratory equipment for measuring these emissions exists”, explains Bernard Heinesch, a lecturer at Gembloux Agro-Bio Tech and a researcher at the Unit of Biosystems Physics. “In simple terms, it consists in placing a plant under a bell and measuring at the exit of this bell what the plant releases into the air. This is a useful but somewhat simplistic method. A forest is much more complex than a potted plant. This is why it was important to take real-time measurements in the natural environment.”

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.

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.

The role of agriculture

Not only forests emit VOCs. Every kind of vegetation is a potential producer of these compounds. This is why researchers are interested in other environments such as, for example, cultivated land which often raises questions about the impact of farming on the composition of the atmosphere. The Unit of Biosystems Physics installed equipment in the middle of a field in Lonzée (province of Namur): this was an anemometer which measured the strength of the wind twenty times a second, and an air sample linked to instruments on the ground capable of measuring the chemical composition of the air. This equipment system made it possible to analyze the flow of the different compounds, CO2 in particular (Read : Forests and grasslands : carbon sinks), but also, over a two-year period, a bouquet of VOCs. In this semi-natural environment, where wheat and corn are growing, the researchers from Gembloux demonstrated that these plants emitted mostly methanol. While these two crops are the most important in terms of surface area on a global scale, they have remained virtually unstudied up to the present. These results are therefore very important for scientists attempting to estimate the quantity of VOCs released into the atmosphere on a global scale.

The researchers from Gembloux have now turned their attention to another environment: pastureland. They are going to deploy their equipment in a meadow in the Dorinne area, to the south of Namur. A farmer grazes a herd of  Belgian blue breed cattle on this land for six months of the year. The study aims to measure the flow of VOCs in this meadow, and the impact of certain stress-factors on the production of VOCs. For example, grazing by the cattle causes the emission of VOCs from the wounded plants when cows cut grass blades. The researchers want to know in what quantities VOCs are emitted during this activity. Droughts, heat-waves or peaks in the concentration of ozone gas can also modify the emissions. Expected climate change for the coming decades predicts more and more extreme events of the kind mentioned above. Will such events increase the emission of VOCs or reduce them? And by how much will they do so? Will the type of VOCs emitted change in these conditions? There are so many pressing questions with regard to the reaction of the ecosystems of our regions that can be answered by carrying out measurements in the natural environment in perfectly realistic conditions. The first measurements will be revealed in the spring of 2014. 

(1) http://www.gembloux.ulg.ac.be/physique-des-bio-systemes/research-projects/volatile-organic-compounds/

(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.