The Oxygenated Compounds in the Tropical Atmosphere: Variability and Exchanges (OCTAVE) campaign aimed to improve the assessment of the budget and role of oxygenated volatile organic compounds (OVOCs) in tropical regions, and especially over oceans, relying on an integrated approach combining in situ measurements, satellite retrievals, and modeling. As part of OCTAVE, volatile organic compounds (VOCs) were measured using a comprehensive suite of instruments on Reunion Island (21.07° S, 55.38° E) from 7 March to 2 May 2018. VOCs were measured at a receptor site at the Maïdo observatory during the entire campaign and at two source sites: Le Port from 19 to 24 April 2018 (source of anthropogenic emissions) and Bélouve from 25 April to 2 May 2018 (source of biogenic emissions) within a mobile lab. The Maïdo observatory is a remote background site located at an altitude of 2200 m, whereas Bélouve is located in a tropical forest to the east of Maïdo and Le Port is an urban area located northwest of Maïdo. The major objective of this study was to understand the sources and distributions of atmospheric formaldehyde (HCHO) in the Maïdo observatory on Reunion Island. To address this objective, two different approaches were used to quantify and determine the main drivers of HCHO at Maïdo. First, a chemical-kinetics-based (CKB) calculation method was used to determine the sources and sinks (biogenic, anthropogenic/primary, or secondary) of HCHO at the Maïdo site. The CKB method shows that 9% of the formaldehyde formed from biogenic emissions and 89% of HCHO had an unknown source; that is, the sources cannot be explicitly described by this method. Next, a positive matrix factorization (PMF) model was applied to characterize the VOC source contributions at Maïdo. The PMF analysis including VOCs measured at the Maïdo observatory shows that the most robust solution was obtained with five factors: secondary biogenic accounting for 17%, primary anthropogenic/solvents (24%), primary biogenic (14%), primary anthropogenic/combustion (22%), and background (23%). The main contributions to formaldehyde sources as described by the PMF model are secondary biogenic (oxidation of biogenic VOCs with 37%) and background (32%). Some assumptions were necessary concerning the high percentage of unknown HCHO sources of the CKB calculation method such as the biogenic emission factor resulting in large discrepancies between the two methods.

A Flowing Afterglow-Tandem Mass Spectrometer (FA-TMS) was used to investigate the feasibility of selective on-line detection of a series of seven sesquiterpenes (SQTs). These SQTs were chemically ionized by either H₃O⁺ or NO⁺ reagent ions in the FA, resulting among others in protonated SQT and SQT molecular ions, respectively. These and other Chemical Ionization (CI) product ions were subsequently subjected to dissociation by collisions with Ar atoms in the collision cell of the tandem mass spectrometer. The fragmentation spectra show similarities with mass spectra obtained for these compounds with other instruments such as a Proton Transfer Reaction-Linear Ion Trap (PTR-LIT), a Proton Transfer Reaction-Mass Spectrometer (PTR-MS), a Triple Quadrupole-Mass Spectrometer (QqQ-MS) and a Selected Ion Flow Tube-Mass Spectrometer (SIFT-MS). Fragmentation of protonated SQT is characterized by fragment ions at the same masses but with different intensities for the individual SQT. Distinction of SQTs is based on well-chosen intensity ratios and collision energies. The fragmentation patterns of SQT molecular ions show specific fragment ion tracers at m/z 119, m/z162, m/z 137 and m/z 131 for α-cedrene, δ-neoclovene, isolongifolene and α-humulene, respectively. Consequently, chemical ionization of SQT by NO⁺, followed by MS/MS of SQT⁺ seems to open a way for selective quantification of SQTs in mixtures.

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.