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

Abstract

Abstract. 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 conventional PTR instruments, the Vocus PTR-TOF identifies a large number 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, O3; and nitrate radical, NO3) are calculated. For the less oxidized non-nitrate monoterpene oxidation products, their morning and evening peaks have contributions from both O3- and OH-initiated monoterpene oxidation. For the monoterpene-derived organic nitrates, oxidations by O3, OH, and NO3 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.

Source identification of atmospheric organic vapors in two European pine forests: Results from Vocus PTR-TOF observations

“…An overview of the Vocus PTR-TOF measurements in the Landes forest has been presented earlier by Li et al (2020). The ambient observations at the SMEAR Ⅱ station were performed during 18 June -18 July 2019.…”

“…The nearest urban area of the Bordeaux metropole is around 40 km to the northwest. A more detailed description of the measurement site can be found elsewhere (Kammer et al, 2018;Bsaibes et al, 2020;Li et al, 2020). Ambient meteorological https://doi.org/10.5194/acp-2020-648 Preprint.…”

“…A Vocus PTR-TOF was deployed in both forest ecosystems to characterize atmospheric organic vapors. Equipped with a new chemical ionization source with a low-pressure reagent-ion source and focusing ion-molecule reactor (FIMR), the Vocus PTR-TOF is able to measure organic vapors with a wide range of volatilities (Krechmer et al, 2018;Riva et al, 2019;Li et al, 2020). A quadrupole radio frequency (RF) field inside the FIMR focuses ions to the central axis and improves the detection efficiency of product ions.…”

“…With a high water mixing ratio (10% v/v-20% v/v) in the FIMR, the instrument shows no humidity dependence for sensitivity. More instrumental details have been provided elsewhere (Krechmer et al, 2018;Li et al, 2020).…”

“…Based on a laboratory comparison of different chemical ionization techniques, Riva et al (2019) revealed that Vocus PTR-TOF is sensitive to a large range of oxygenated VOCs. With the deployment of a Vocus PTR-TOF in the French Landes forest, Li et al (2020) observed various terpenes and terpene oxidation products, including a range of organic nitrates.…”