BVOC and speciated monoterpene concentrations and fluxes at a Scandinavian boreal forest

Abstract. Boreal forests emit terpenoid biogenic volatile organic compounds (BVOCs) that significantly affect atmospheric chemistry. Our understanding of the variation of BVOC species emitted from boreal ecosystems is based on relatively few datasets, especially at the ecosystem-level. We conducted measurements to obtain BVOC flux observations above the boreal forest at the ICOS (Integrated Carbon Observation System) station Norunda in central Sweden. The goal was to study concentrations and fluxes of terpenoids, including isoprene, speciated monoterpenes (MTs), and sesquiterpenes (SQTs), during a Scandinavian summer. Measurements (10 Hz sampling) from a Vocus proton-transfer-reaction time-of-flight mass spectrometer (Vocus PTR-ToF-MS) were used to quantify a wide range of BVOC fluxes, including total MT (386 (± 5) ng m−2 s−1; β=0.1 °C−1), using the eddy-covariance (EC) method. Surface-layer gradient (SLG) flux measurements were performed on selected daytime sampling periods, using thermal-desorption adsorbent tube sampling, to establish speciated MT fluxes. The effect of chemical degradation on measured terpenoid fluxes relative to surface exchange rates (F/E) was also investigated using stochastic Lagrangian transport modeling in forest-canopy. While the effect on isoprene was within EC-flux uncertainty (FISO/EISO < 5 %), the effect on SQT and nighttime MT was significant, with average F/E ratios for nighttime FMT/EMT= ca. 0.9 (0.87–0.93), nighttime FSQT/ESQT=0.35 (0.31–0.41), and daytime FSQT/ESQT=0.41 (0.37–0.47). The main compounds contributing to MT flux were α-pinene and Δ3-carene. Summer shifts in speciated MT emissions for Δ3-carene were detected, featuring a decrease in its relative fraction among observed MT compounds from June to August sampling periods, indicating that closer attention to seasonality of individual MT species in BVOC emission and climate models is warranted.