Concentrations and fluxes of isoprene and oxygenated VOCs at a French Mediterranean oak forest

The CANOPEE project aims to better understand the biosphere-atmosphere exchanges of biogenic volatile or- ganic compounds (BVOCs) in the case of Mediterranean ecosystems and the impact of in-canopy processes on the at- mospheric chemical composition above the canopy. Based on an intensive field campaign, the objective of our work was to determine the chemical composition of the air inside a canopy as well as the net fluxes of reactive species between the canopy and the boundary layer. Measurements were car- ried out during spring 2012 at the field site of the Oak Ob- servatory of the Observatoire de Haute Provence (O3HP) lo- cated in the southeast of France. The site is a forest ecosys- tem dominated by downy oak, Quercus pubescens Willd., a typical Mediterranean species which features large iso- prene emission rates. Mixing ratios of isoprene, its degra- dation products methylvinylketone (MVK) and methacrolein (MACR) and several other oxygenated VOC (OxVOC) were measured above the canopy using an online proton trans- fer reaction mass spectrometer (PTR-MS), and fluxes were calculated by the disjunct eddy covariance approach. The O3HP site was found to be a very significant source of iso- prene emissions, with daily maximum ambient concentra- tions ranging between 2-16 ppbv inside and 2-5 ppbv just above the top of the forest canopy. Significant isoprene fluxes were observed only during daytime, following diurnal cy- cles with midday net emission fluxes from the canopy rang- ing between 2.0 and 9.7 mg m 2 h 1 . Net isoprene normalized flux (at 30 C, 1000 µmol quanta m 2 s 1 ) was estimated at 7.4 mg m 2 h 1 . Evidence of direct emission of methanol was also found exhibiting maximum daytime fluxes rang- ing between 0.2 and 0.6 mg m 2 h 1 , whereas flux values for monoterpenes and others OxVOC such as acetone and acetaldehyde were below the detection limit. The MVK+MACR-to-isoprene ratio provided useful in- formation on the oxidation of isoprene, and is in agreement with recent findings proposing weak production yields of MVK and MACR, in remote forest regions where the NOx concentrations are low. In-canopy chemical oxidation of iso- prene was found to be weak and did not seem to have a sig- nificant impact on isoprene concentrations and fluxes above the canopy.