French ACROSS 2022 campaign – First results from CO2/H2O, energy and VOC fluxes measurements at the Rambouillet tower supersite

Volatile organic compounds (VOC) contribute to the production of pollutants harmful to human health and the environment. Most VOC (90 %) are biogenic. At the European scale, forests account for 55 % of the total VOC emissions, crops 27 %, and grasslands, wetlands and shrubs 18 % (Karl, 2009). These estimates are hampered by lack of data acquired at the ecosystem scale. In this work we measured VOC fluxes above the mixed (oaks and pines) forest of Rambouillet (about 60 km south west of Paris) during the ACROSS 2022 summer campaign. The objectives were (1) to quantify emission and deposition of constitutive VOC, and those occurring in response to heat and drought stresses, and (2) to study VOC formation in the canopy.

VOC fluxes were measured at the top of a 40 m-tall tower located at the Rambouillet ACROSS super site. A PTR-Qi-TOF-MS was installed in a shelter at the foot of the tower, and the air sucked through a 50 m heated line by a high-volume pump to ensure short residence time. The PTR-MS was synchronised at 10 Hz with an ultrasonic anemometer to compute fluxes by eddy covariance. Additionally, VOC concentrations were measured along a 5-level profile between 2m and 40m. Preliminary results show emission and deposition of more than 80 VOC compounds. The forest at this location was a strong emitter of monoterpenes, but also of isoprene and sesquiterpenes. A heatwave event has led to a clear decrease in photosynthesis and transpiration during week 28. This period also corresponded to the highest emissions observed during the whole campaign. Deposition of heavy VOC compounds (m/z  > 220) was also observed, possibly corresponding to nitrogen or sulphur containing compounds.