From forest to atmosphere: towards a more comprehensive assessment of BVOC exchanges in a mixed temperate forest

Forests are the primary global source of biogenic volatile organic compounds (BVOCs), which play a key role in atmospheric chemistry. While emissions of isoprene and monoterpenes, the two most emitted BVOCs at the global scale, are generally well estimated by state-of-the-art emission models, uncertainties remain regarding the diversity, magnitude, and temporal variability of other BVOC exchanges that impact the non-methane volatile organic compound (NMVOC) budget and atmospheric chemistry.
To address this, half-hourly net BVOC fluxes were measured at the Vielsalm station, a Belgian mixed temperate forest part of the ICOS network, during spring-autumn 2023, using a PTR-TOF-MS instrument (PTR-TOF-4000, Ionicon Analytik GmbH) and the eddy covariance technique. In total, ion signals at 33 VOC-related m/z values showed significant emissions or depositions, sometimes only for brief periods.
Throughout the entire campaign, the net above-canopy BVOC exchanges are positive (i.e. net emissions). The 10 most emitted BVOCs alone account for 90% of observed BVOC emissions (in mass balance), led by monoterpenes, isoprene and methanol fluxes. Fluxes of isoprenoids and their derivates (isoprene, monoterpenes, MACR+MVK, MBO, p-cymene, sesquiterpenes, etc.) are consistently positive, their dynamics being influenced by air temperature and solar radiation. On the other hand, lower weight oxygenated BVOCs such as alcohols (methanol, ethanol), organic acids (formic acid, acetic acid) and aldehydes (formaldehyde, acetaldehyde) present bidirectional net fluxes with depositions sometimes as high as emissions. This happens especially at night and during early spring and autumn, which are characterised by lower photochemical activity and higher relative humidity.
OH reactivity fluxes were obtained by multiplying the BVOC fluxes by the corresponding reaction rate constants with the hydroxyl radical to estimate their impact on the oxidative capacity of the atmosphere. Isoprene and monoterpenes respectively contribute to 70% and 22% of the total OH reactivity flux, while the chemical species that show high depositions contribute almost negligibly to a reduction of the total OH reactivity flux.
Further analysis will focus on bi-directional exchanges of BVOCs between the forest canopy and the atmosphere. For this, we will start by comparing our results with emission models (e.g., MEGAN) and focus on deposition occurring in the canopy.