Abstract
Monoterpenes (MTs) represent an important family of biogenic volatile organic compounds (BVOCs) in terms of amount and chemical diversity. This family has been extensively studied using gas chromatography (GC) and proton transfer reaction-mass spectrometry (PTR-MS). Upon recent advances with Fast Gas Chromatography (FastGC), it was also commercialized with proton transfer reaction-time of flight-mass spectrometry (PTR-ToF-MS) instruments. The combination of both techniques showed promising results in the near real-time separation of isomers, with the need of further improvements. In this study, a FastGC prototype was coupled to a conventional PTR-MS (PTR-QuadMS). Extensive laboratory experiments were performed, in order to test the system’s performance and to optimize its operational parameters for MT separation. The detection limit was determined to be around 0.8–1.7 ppbv, depending on the MT. The system was afterwards deployed during a three-week field campaign in a mixed holm oak (Quercus ilex) forest known for its important MT emissions. MTs were measured in the incoming and the outgoing air of dynamic enclosures installed on the branches of four different trees. Three chemotypes of holm oak trees could be distinguished showing consistently different proportions of the emitted MTs throughout the measurement campaign: pinene-type, myrcene-type and limonene-type. Measurements showed a systematic diel variation in emissions typical of light and temperature-dependent, de novo-synthesized VOCs. The results demonstrated the feasibility of the FastGC/PTR-MS system for continuous measurements from dynamic chambers in the field, whereas further improvements would be necessary to lower the detection limit for ambient air measurements.
Highlights
Terrestrial vegetation, including forests, crops, grasslands and shrubs, represents the dominant source of volatile organic compounds (VOCs) released in the atmosphere
With coupling the fast gas chromatography (FastGC) to the conventional proton transfer reaction-mass spectrometry (PTR-MS), came some technical some technical challenges. They included the need of very low dwell times narrowing the amount of challenges. They included the need of very low dwell times narrowing the amount of monitored monitored masses, the higher dilution due to the higher flow inside the PTR-MS drift-tube and the masses, the higher dilution due to the higher flow inside the PTR-MS drift-tube and the necessity necessity to adapt a number of control elements, since the normal PTR-ToF-MS input/output system to adapt a number of control elements, since the normal PTR-ToF-MS input/output system is not is not available
The latter was the main adaptation made in this setup, compared with the one available. The latter was the main adaptation made in this setup, compared with the one described in described in Malásková et al (2019) [30], where this FastGC prototype was coupled to a PTR-ToFMalásková et al (2019) [30], where this FastGC prototype was coupled to a PTR-ToF-MS
Summary
Terrestrial vegetation, including forests, crops, grasslands and shrubs, represents the dominant source of volatile organic compounds (VOCs) released in the atmosphere. Plant-produced VOCs are of particular interest, seeing their abundance and their considerable role in gas phase and heterogeneous chemistry of the troposphere They are subject to photochemical processes, involving atmospheric oxidants like OH, O3 and NO that lead to the formation of a harmful tropospheric ozone and secondary organic aerosols (SOAs) [3,4]. Despite the considerable effort invested to better understand BVOC-mediated tropospheric photochemistry, substantial uncertainties still exist These uncertainties are highlighted by discrepancies often observed between measured total OH reactivity and the estimated OH reactivity derived from simultaneous VOC measurements. The total OH reactivity is defined as the sum of the concentration of each compound, multiplied by the respective rate coefficient of the reaction with OH These differences, noted as missing OH reactivity, are due to unmeasured or unidentified primary emitted and/or secondary formed reactive species. They highlight the need for a more detailed characterization of BVOCs, especially in forest environments where higher values of OH reactivity and missing OH reactivity were reported [5,6,7,8,9,10,11]
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