Terpenes and their oxidation products in the French Landes forest: insight from Vocus PTR-TOF measurements

Abstract

<p><strong>Abstract.</strong> The capabilities of the recently developed Vocus proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF) are reported for the first time based on ambient measurements. With the deployment of the Vocus PTR-TOF, we present an overview of the observed gas-phase (oxygenated) molecules in the French Landes forest during summertime 2018 and gain insights into the atmospheric oxidation of terpenes, which are emitted in large quantities in the atmosphere and play important roles in secondary organic aerosol production. Due to the greatly improved detection efficiency compared to traditional PTR instruments, the Vocus PTR-TOF identifies a large amount of gas-phase signals with elemental composition categories including CH, CHO, CHN, CHS, CHON, CHOS, and others. Multiple hydrocarbons are detected, with carbon numbers up to 20. Particularly, we report the first direct observations of low-volatility diterpenes in the ambient air. The diurnal cycle of diterpenes is similar to that of monoterpenes and sesquiterpenes, but contrary to that of isoprene. Various types of terpene reaction products and intermediates are also characterized. Generally, the more oxidized products from terpene oxidations show a broad peak in the day due to the strong photochemical effects, while the less oxygenated products peak in the early morning and/or in the evening. To evaluate the importance of different formation pathways in terpene chemistry, the reaction rates of terpenes with main oxidants (i.e., hydroxyl radical, OH; ozone, O<sub>3</sub>; and nitrate radical, NO<sub>3</sub>) are calculated. For the less oxidized non-nitrate monoterpene oxidation products, their morning peaks likely have contributions from both O<sub>3</sub>- and OH-initiated monoterpene oxidation. Due to the decreased OH concentration at night, monoterpene ozonolysis becomes more important in the evening. For the monoterpene-derived organic nitrates, oxidations by O<sub>3</sub>, OH, and NO<sub>3</sub> radicals all contribute to their formation, with their relative roles varying considerably over the course of the day. Through a detailed analysis of terpene chemistry, this study demonstrates the capability of the Vocus PTR-TOF in the detection of a wide range of oxidized reaction products in ambient and remote conditions, which highlights its importance in investigating atmospheric oxidation processes.</p>

Capability of CI-Orbitrap for Gas-Phase Analysis in Atmospheric Chemistry: A Comparison with the CI-APi-TOF Technique

“…1,2 Once emitted, VOC are rapidly oxidized to form oxidized VOCs (OVOCs) spanning a broad range of chemical formulas, and, thus, volatilities. 1,3,4 As a result, organic compounds play a key role in the formation of atmospheric aerosol by either condensing onto preexisting aerosol particles or by forming new particles. 1,2,5,6 VOC emissions strongly impact atmospheric chemistry, human health, and Earth's climate.…”

Development of a thermal desorption-gas chromatography-mass spectrometry method for the analysis of monoterpenoids, sesquiterpenoids and diterpenoids

“…However, DTs have been found in essential oils of some boreal forest tree species (Judžentienė et al, 2006;Kanerva et al, 2008;Kupcinskiene et al, 2008;Tumen et al, 2010), for example, in Scots pine needles grown in northern Europe the DTs comprised 6-14 % of the essential oil content (Judzentiene and Kupcinskiene, 2008). Even though diterpenoids have not been detected (or specifically studied) in boreal forest air, they have been recently observed in atmospheric air and branch enclosure emissions in other environments (Chan et al, 2016;Haberstroh et al, 2018;Li et al, 2019;Matsunaga et al, 2012;Yáñez-Serrano et al, 2018;Yee et al, 2018). For example, at a rural site in Brazil the mean concentrations of DTs in air varied between 10-86 ppqv (Yee et al, 2018), and in Mediterranean gum rockrose enclosure the emission rates of DTs were in the range of 8x10 -5 -14x10 -5 µg g -1 h -1 (Haberstroh et al, 2018).…”

“…Diterpenes have fairly low vapor pressures and they can be categorised as semi volatile organic compounds (SVOCs). In previous atmospheric studies, DTs have been analysed by using either conventional chromatographic techniques or online mass spectrometric techniques (such as proton-transfer-reaction mass spectrometry) (Chan et al, 2016;Haberstroh et al, 2018;Li et al, 2019;Matsunaga et al, 2012;Yáñez-Serrano et al, 2018;Yee et al, 2018). Many of those chromatographic studies used traditional sorbent tubes for sampling prior to thermal desorption (or solvent desorption)-gas chromatographymass spectrometry (TD-GC-MS) (Haberstroh et al, 2018;Matsunaga et al, 2012;Yáñez-Serrano et al, 2018).…”

Oxygenated products formed from OH-initiated reactions of trimethylbenzene: Autoxidation and accretion

“…Vocus PTR. The newly developed Vocus PTR has a high sensitivity to a wide range of VOCs and oxygenated volatile organic compounds (OVOCs) (Krechmer et al, 2018;Li et al, 2019;Riva et al, 2019).…”