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

Abstract

Abstract. The CANOPEE project aims to better understand the biosphere–atmosphere exchanges of biogenic volatile organic compounds (BVOCs) in the case of Mediterranean ecosystems and the impact of in-canopy processes on the atmospheric 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 carried out during spring 2012 at the field site of the Oak Observatory of the Observatoire de Haute Provence (O3HP) located in the southeast of France. The site is a forest ecosystem dominated by downy oak, Quercus pubescens Willd., a typical Mediterranean species which features large isoprene emission rates. Mixing ratios of isoprene, its degradation products methylvinylketone (MVK) and methacrolein (MACR) and several other oxygenated VOC (OxVOC) were measured above the canopy using an online proton transfer 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 isoprene emissions, with daily maximum ambient concentrations 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 cycles with midday net emission fluxes from the canopy ranging between 2.0 and 9.7 mg m−2 h1. 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 ranging 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 information 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 isoprene was found to be weak and did not seem to have a significant impact on isoprene concentrations and fluxes above the canopy.

Variability of BVOC emissions from a Mediterranean mixed forest in southern France with a focus on <i>Quercus pubescens</i>

“…Details on VOC determination using the PTR-MS can be found in Kalogridis et al (2014). Twelve masses were followed for both the Acer and the Quercus branch.…”

“…For BVOC screening, the PTR-MS was connected to the enclosure system with a 25 m length 1/4 in PTFE tubing (not heated) in order to follow, online, the rapid diurnal variations of BVOC emission rates from a Q. pubescens and an A. monspessulanum branch; flow rate entering the chamber was fixed at 14.7 L min −1 (for details of PTR-MS system see Kalogridis et al, 2014).…”

“…Based on our assessed Is range (31 to 138 µgC g −1 DM h −1 ) and using an average branch-scale Is value of 60 µgC g Kalogridis et al (2014) extrapolated a canopy isoprene emission flux of 15 mg m −2 h −1 , twice the mean canopy flux measured in June during this study by the disjunct eddy covariance technique (6.6 mg m −2 h −1 ). The authors pointed out that such a factor of discrepancy is reasonable since it is in the range of uncertainties typically obtained for upscaling exercises (see for example Guenther et al, 1995), and is within the range of the tree-to-tree variability observed for Q. pubescens Is at this site (a factor of 4.3).…”

“…Am and Qp4 were located in a clearing 40 m north of the O 3 HP scaffold (Fig. 1) close to where the PTR-MS system was set up during the CANOPÉE campaign (see Kalogridis et al, 2014).…”

“…One of the enclosures was maintained on the Qp1 branch during the whole campaign (15 days) in order to follow continuous diurnal variations of isoprene emission rates during the concomitant isoprene canopy flux measurements carried out by Kalogridis et al (2014). The second enclosure was used to alternatively investigate, over 1 to 2 days, isoprene emissions from the other eight branches selected (sunlit and shaded).…”