Multi-year observations of BVOC and ozone: concentrations and fluxes measured above and below the canopy in a mixed temperate forest

Abstract. Volatile organic compounds (VOCs) and ozone (O₃) are key constituents of tropospheric chemistry, affecting both air quality and climate. Forests are major emitters of biogenic VOCs (BVOCs), yet large uncertainties remain regarding the diversity of exchanged compounds, the drivers of their bidirectional fluxes, and their in-canopy chemistry. Long-term and comprehensive in situ datasets remain scarce, limiting our understanding of these complex processes. We conducted a three-year field campaign (2022–2024) at the Integrated Carbon Observation System forest station of Vielsalm (BE-Vie), combining vertical concentration profile and eddy covariance flux measurements above and below the canopy. Using a PTR-ToF-MS and a dedicated open-source processing pipeline, we identified 51 significantly exchanged VOCs. The vertical and diurnal gradients of the mixing ratios reflected the interplay between emission, deposition, chemistry, and transport. Combined with a profile of turbulence statistics, these observations offer an opportunity to investigate their behaviour within the canopy. The forest acted as a net VOC source in summer (∼ 1.25 μg m⁻² s⁻¹), while deposition dominated in autumn. Many oxygenated VOCs displayed bidirectional exchange. Monoterpenes, isoprene, and methanol were the most abundant flux contributors, but 15–30 (30–43) compounds were needed to account for 90 % of total emissions (depositions), depending on the season. Below-canopy BVOC and O₃ fluxes reached ∼ 10 % of above-canopy ones, with proportionally enhanced below-canopy ozone uptake at night. This study provides one of the most detailed long-term datasets of VOC and O₃ exchange in a temperate forest and serves as a key reference for improving process-based models of biogenic, physical, and chemical exchange in forest ecosystems.