Volatile organic compounds (VOC) are starting to be quite well-known. Often integrated in cleaning products and solvents, they remain in gas phase in the air and it is regularly advised to ventilate the indoor air to get rid of them. But the vast majority of VOC’s are biogenic and naturally produced by plants. It includes a large diversity of compounds essential to the well-functioning of plants. “VOC’s have a strong role in thermotolerance (helps in case of high temperature rise), fight against oxidation, but also in communication among plants and between plants and insects” illustrates Juliette Lathière, researcher at LSCE-IPSL. “These components represent on average 2% to 3% of assimilated carbon, but it can jump to 10% when stressed, in particular hydric stress” she adds.

Biogenic VOCs are generally highly reactive compounds and play an essential role in atmospheric chemical interactions. They are quickly oxidized by the hydroxyl radical OH − the atmosphere’s cleaning agent − and react with other components leading to the production of ozone or secondary organic aerosols. The researcher says that “it’s a cooking recipe among various components that can bring about a rise or decrease of ozone”. There are hundreds of VOC’s, perhaps even thousands. Some with high emissions levels and other much lower, which makes them difficult to be observed in the atmosphere. The type of compounds and the emissions levels depend on the sort of vegetation it originates from. The most common biogenic VOCs worldwide are isoprene and monoterpene. But all are very dependant of their environmental conditions such as temperature, radiation, ground humidity or other compounds concentrations like carbon dioxide.

Different plants, different compounds

Components have different chemical reactivity and researchers are interested in their impact on the chemical composition of the atmosphere and ultimately on climate. These reactions affect ozone and methane concentrations and bring about new elements such as secondary organic aerosols that can have an effect on local temperature and precipitations. The decision to replace one species of plant by another – for agricultural productivity for instance – influences the type of biogenic VOC on the atmosphere. One particular plant species will not have the same VOC emissions than another, nor the same quantity. Our impact on soil use has thus a direct consequence on the type of compounds that gets in the air and the reactions that follows.

In Indonesia for instance, many original forests are being replaced by quick-maturing crops like oil palm. This change is followed by the development of a whole industrial and road network. “This automobile activity brings high emissions of nitrogen oxide and oil palm creates strong emission of isoprene, much more important than the vegetation it replaces. Together these compounds react and lead to substantial ozone production that can cause air quality problems for local populations” explains Juliette Lathière.

Another effect related to VOC emissions concerns cloud formation, but is more complex to quantify. “Secondary aerosols can constitute condensation nuclei and modify cloud formation, with a significate impact on climate” she details. If the role of biogenic VOC on the chemical composition of the atmosphere is indisputable, their impact on climate and the modification of radiative forcing remains small compared to greenhouse gases. This field of study gains ground yet many uncertainties persist on the relations between environmental conditions, types and quantities of biogenic VOC, the role of their oxidation on the formation of secondary organic aerosols and their impact on climate and their evolution.

A great variability

Gaining better insight on these links is crucial to understand the place of biogenic VOC on the atmospheric cycle and more generally on climate. But estimating these emissions and their formation is not an easy task, for they show a strong variability. Emissions can vary from one branch to another and it is extremely difficult to represent them accurately. “What we try to do first is to estimate these emissions on one site, on the scale of a forest” announces Juliette Lathière. A field campaign offers a punctual diagnostic of emissions fluxes at the scale of leaves and of concentrations at the canopy, where the most reactive biogenic VOC react. “VOCs are mainly emitted by leaves, and in a smaller manner by trunks, branches or even the ground and some components react in the canopy and will not interact in the atmosphere” details the researcher.

The challenge then remains in the attempt to extrapolate this information on site to a larger scale, all the while questioning if it is truly representative. The use of satellites data can also be an asset to evaluate compounds quantities in the atmosphere. “There is a consequent uncertainty factor but with field measures, flux measures on towers and satellites data we can have access to complementary information to evaluate our global models” she adds. A good understanding of the relation between compound emissions and environmental conditions and their place overall on the atmospheric chemistry and climate is a key step to evaluate their evolution and their role in the Earth System.

Stronger together

Biogenic VOC emissions have a lighter impact than greenhouse gases on the radiative budget. Nonetheless, their understanding is not less essential to draw a realistic painting of chemical and climatic processes and their evolutions, taking in consideration every cycle of possible retroactions. Researchers associate global models of climate (LMDZ), of the continental biosphere (ORCHIDEE, integrating the calculation of VOCB emissions) and of atmospheric chemistry (INCA) to make simulations with the whole of these biosphere-chemistry-climate interactions. The goal is then to use different scenarios of land use and environmental conditions to get insight on emissions and implications on chemistry and climate. “For instance a rise in temperature will have a positive impact on VOC emissions, that can also vary locally with the apparition of hot-spot in some regions” says Juliette Lathière.

Coupled models allow an analysis of the possible evolutions of interactions between atmospheric chemical composition, vegetation and future climate. “Experimental studies^[1] in laboratory have shown that a rise in temperature tends to lower the capacity of plant to emit biogenic VOC, especially in the case of isoprene” tell the researcher. Other factors are to be considered such as changes in temperature and solar radiation or changes in land use. Some increasing and others lowering biogenic VOC emissions, and the resulting effect is also under some uncertainty.

“We see today that this subject on interactions between continental biosphere, atmospheric chemistry and climate gains interest in the recent years. We are developing models comprising every existing interactions and retroactions, which are now also mentioned in the IPCC report” she highlights. There remain consequent uncertainties in this domain but associating experimental studies on site and modelling and using coupled models will help researchers to see more clearly. “By joining forces among scientists working on models and on-site measurement and laboratory and on satellite data we will be able to progress and lower uncertainties on biogenic VOC and their impacts” concludes Juliette Lathière.

^[1] Possell, M., & Hewitt, C. N. (2011). Isoprene emissions from plants are mediated by atmospheric CO2 concentrations. Global Change Biology, 17(4), 1595-1610.

Wilkinson, M. J., Monson, R. K., Trahan, N., Lee, S., Brown, E., Jackson, R. B., … & Fall, R. A. Y. (2009). Leaf isoprene emission rate as a function of atmospheric CO2 concentration. Global Change Biology, 15(5), 1189-1200.