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), pivotal in atmospheric chemistry. While emissions of isoprene and monoterpenes, the most emitted BVOCs at the global scale, are generally well-estimated, uncertainties persist 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 ICOS station, a Belgian mixed temperate forest, during spring-autumn 2023, using a PTR-TOF-MS instrument (PTR-TOF-4000, Ionicon Analytik GmbH) and eddy covariance. In total, 33 VOC-related m/z values showed significant emissions or depositions.

Throughout the campaign, the net BVOC exchanges are positive, with the top 10 BVOCs accounting for 90% of observed BVOC emissions (in mass balance), led by monoterpenes, isoprene, and methanol. Isoprenoids and their derivates were emitted consistently, influenced by air temperature and solar radiation. Conversely, lighter oxygenated BVOCs like alcohols, organic acids, and aldehydes exhibited bidirectional net fluxes, sometimes favouring deposition, especially in lower photochemical and higher humidity conditions.

To investigate their impact on atmospheric reactivity, fluxes were multiplied by their corresponding reaction rate constants with OH radicals. Isoprene and monoterpenes were found to account for 70% and 22% of the total OH reactivity flux, respectively.

Further analysis will focus on bi-directional exchanges of VOCs between the forest canopy and the atmosphere. For this, we will start by comparing our results with existing emission models (e.g., MEGAN) and focus on deposition occurring in the canopy.