Study of biogenic volatile organic compound emissions and depositions over a mixed temperate forest by PTR-TOF-MS and eddy covariance

Volatile organic compounds (VOCs) play a key role in atmospheric chemistry. These gases impact air quality by participating in the formation of ozone and secondary organic aerosols and extend the lifetime of methane in the atmosphere. Approximately 90% of global VOC emissions are biogenic (BVOCs), and since forests are the main emitters of BVOCs, these ecosystems deserve special attention in order to better characterize BVOCs exchanges with the atmosphere.          

Traditionally, flux measurements were mainly limited to a few dominant BVOC species, such as isoprene, terpenes and methanol, due to technical measurement limitations. Most of the measured species were found to be emitted by vegetation, but some studies detected significant net depositions of compounds such as methanol, driven by environmental factors favouring the formation of surface wetness. This observation supports the need for a more detailed and complete picture of BVOC bidirectional exchanges at forest sites as well as the mechanisms controlling these fluxes, which are essential to better characterize the in-canopy atmospheric chemistry.

To address this gap, BVOC fluxes were measured in spring-summer 2022 using a PTR-TOF-MS instrument (PTR-TOF-4000, Ionicon Analytik GmbH) over a mixed temperate forest in the Belgian Ardenne (Vielsalm), which is part of the ICOS network. The use of a PTR-TOF-MS instrument, deployed at Vielsalm in the framework of ACTRIS, allows for the simultaneous detection of a very wide range of VOC-related ion masses with increased sensitivity (especially for ions at high m/z ratios) and higher mass resolving power compared to conventional PTR-Quad-MS instruments. O₃ fluxes were also simultaneously acquired using fast and slow ozone analysers in order to complete the BVOC fluxes dataset and BVOC+O₃ concentration profiles were frequently measured at seven levels along the flux tower from ground level up to 51 m.

(Un)calibrated BVOC mixing ratios were first derived from results of the Ionicon Data Analyzer software (IDA, Ionicon Analytik GmbH) obtained on a near-daily basis, and these concentrations were then used in a computational tool based on InnFLUX (Atmospheric Physics and Chemistry Group, University of Innsbruck) to compute fluxes by eddy covariance. Along with these fluxes, uncertainties and limits of detection (LODs) were estimated, and quality control statistical tests were performed. During the whole measurement campaign, about 570 m/z peaks were detected by the IDA software.

Based on this extensive dataset, our goals are to: (1) perform spectral analysis (which often turns out to be trickier for low signal-to-noise ratios), (2) determine the significant fluxes based on LODs, (3) analyse their response to meteorological variables, phenology and O₃ concentrations/fluxes, (4) attempt compound attribution based on scientific literature and site-related information, and (5) establish the budget of BVOC emissions and depositions for the forest site. The obtained results will be compared to BVOC studies previously conducted at the Vielsalm ICOS forest site with a PTR-Quad-MS instrument and should improve our previous BVOC budget estimates at the site.