Biosphere-atmosphere exchange of organic compounds : impact of intra-canopy processes

Project Coordinator

Juliette LATHIERE– juliette.lathiere (at)






The terrestrial biosphere is a key component of surface-atmosphere interactions, as demonstrated in a recent review by Fowler et al. (2009). As the main source of Volatile Organic Compounds (or so-called biogenic VOCs), ecosystems are involved in both gas-phase and particulate reactions in the troposphere, such as the ozone cycle or secondary organic aerosol formation. Moreover, vegetation is a sink for many compounds that are removed from the atmosphere through dry deposition processes, at the leaf and soil surfaces, or by chemical reactions inside the canopy, that will both contribute to determine the net flux of compounds into the atmosphere. On the other hand, the development and distribution of vegetation at the Earth surface are strongly affected by changes in climate (for instance modification of temperature or rainfall) and human activities (evolution of land-use, deforestation, reforestation…). The atmospheric chemical composition is also known to play a significant role in vegetation growth: an increase in carbon dioxide concentrations promotes photosynthesis while high concentrations of pollutants such as ozone or nitrogen oxides lead to leaves alteration. Hence, it is crucial to have a comprehensive picture of every interaction and process involved, to be able to fully analyze the role of the terrestrial biosphere in the Earth System.

Some of the interactions between the terrestrial biosphere and the atmospheric chemical composition are still poorly understood and quantified. This is the case of intra-canopy processes (chemical reactions and deposition inside the canopy) that are generally not considered in global vegetation or chemistry-transport models, despite their importance for the surface-atmosphere exchange of organic compounds, as been demonstrated in several studies (Ciccioli et al., 1999; Makar et al., 1999; Fuentes et al., 2000; Forkel et al., 2006). Therefore, there is really a need for more experimental analysis to quantify the impact of intra-canopy processes, together with a modelling approach in order to evaluate the related error in the estimates of net fluxes to the atmosphere.

The objective of our project is to understand and quantify the role of intra-canopy processes in the surface-atmosphere exchange of compounds, focusing on biogenic VOCs, and especially isoprene which is the first biogenic VOC emitted, and ozone. The novelty of our project is based on (1) the combination of new experimental and modelling approaches, and (2) a multi-disciplinary methodology including leaf-scale to canopy-scale measurements for the structural, chemical and ecological characteristics of the canopy.

A measurement campaign was performed at the Oak Observatory at the Observatoire de Haute Provence (O3HP) in May-June 2012, focusing on one particular tree species, the Quercus pubescens, which is recognized as a strong isoprene emitter, covering a large scale in the Mediterranean region (first tree species with 250 000 ha in the Provence-Alpes-Côte d’Azur region). Several devices were set up to investigate leaf-scale isoprene emission rates, the atmospheric chemical composition together with the canopy structure, measurements that will provide crucial information to improve our knowledge on biosphere-atmospheric chemistry interactions.