Understanding gaseous organic compounds' molecular composition and volatility in a boreal forest is essential for refining parametrizations used in transport and climate models and interpreting ambient measurements.
Instruments:
Measurements of Volatile Organic Compounds (VOCs) and Highly Oxygenated Molecules (HOMs) were performed in Hyytiälä, Southern Finland, from 16 April to 26 July 2019.
A Multi-scheme chemical ionization inlet, MION2, was deployed for the first time to perform ambient measurements of oxygenated VOCs. It was coupled to an Atmospheric Pressure Interface Time-of-Flight mass spectrometer (MION-Api-ToF) switching between bromide (Br-) and nitrate (NO3-) as reagent ions to detect less oxygenated VOCs (OVOC) and more oxygenated VOCs (incl. HOMs), respectively. Another instrument, a Vocus Proton-Transfer-Reaction Time-of-Flight mass spectrometer (Vocus PTR-ToF) was used to measure VOCs and less oxygenated VOCs.
Highlighted results:
During three month of measurements, 687 various CHOX (Cx≥1Hy≥1 Oz≥1 X0−n ) compounds were detected by MION-NO3 and 567 by MION-Br. During this period, Vocus identified 431 CHOX compounds and 72 CH (Cx≥1Hy≥1) compounds.
There were overlaps in the measured compounds; however, every technique identified also distinct compounds. In general, MION-NO3 exhibited superior detection capabilities for organics characterized by larger molecular weights and the highest oxidation levels, whereas Vocus demonstrated better performance in observing organics with the lowest oxidation extent.
MION-NO3 detected compounds with the highest elemental oxygen-to-carbon ratios (O:C): 0.9 ±0.1; for MION-Br the ratio was 0.8 ±0.1 and for Vocus the lowest was 0.2±0.1.
The measured compounds were grouped according to their volatility for each measurement technique: MION-NO3 and MION-Br were dominated by IVOC (intermediate-volatility) yet ELVOC (extremely low volatility) was also detected (Fig. 1). Furthermore, MION-NO3 also observed ULVOC (ultralow-volatility). The dominating organic compounds measured by Vocus were VOC and IVOC. (* Vocus measurements might be biased towards higher volatilities due to fragmentation inside the instrument).
A combined volatility distribution for all measured organic compounds in a boreal forest was composed using complementary observations by all techniques (Fig. 2). The size of the markers corresponds to the concentrations, while the color indicates the measurement technique used. Integrating both methods enables the comprehensive coverage of compounds from VOCs to HOMs with varying O:C ratios. Within a boreal forest, VOCs are the most abundant compounds, while HOMs (ULVOC, ELVOC, LVOC), comprising only 0.2%, significantly contribute to new particle formation and growth.
Reference: Huang, W., Li, H., Sarnela, N., Heikkinen, L., Tham, Y. J., Mikkilä, J., Thomas, S. J., Donahue, N. M., Kulmala, M., and Bianchi, F.: Measurement report: Molecular composition and volatility of gaseous organic compounds in a boreal forest – from volatile organic compounds to highly oxygenated organic molecules, Atmos. Chem. Phys., 21, 8961–8977, https://doi.org/10.5194/acp-21-8961-2021, 2021.
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