Measurement report: Molecular composition and volatility of gaseous organic compounds in a boreal forest – from volatile organic compounds to highly oxygenated organic molecules
Authors: Huang Wei , Li Haiyan , Sarnela Nina , Heikkinen Liine , Tham Yee Jun , Mikkilä Jyri , Thomas Steven J. , Donahue Neil M. , Kulmala Markku , Bianchi Federico 
Published: June 2021

Abstract. The molecular composition and volatility of gaseous organic
compounds were investigated during April–July 2019 at the Station for
Measuring Ecosystem – Atmosphere Relations (SMEAR) II situated in a boreal
forest in Hyytiälä, southern Finland. In order to obtain a more
complete picture and full understanding of the molecular composition and
volatility of ambient gaseous organic compounds (from volatile organic
compounds, VOCs, to highly oxygenated organic molecules, HOMs), two
different instruments were used. A Vocus proton-transfer-reaction
time-of-flight mass spectrometer (Vocus PTR-ToF; hereafter Vocus) was
deployed to measure VOCs and less oxygenated VOCs (i.e., OVOCs). In
addition, a multi-scheme chemical ionization inlet coupled to an atmospheric
pressure interface time-of-flight mass spectrometer (MION API-ToF) was used
to detect less oxygenated VOCs (using Br− as the reagent ion; hereafter
MION-Br) and more oxygenated VOCs (including HOMs; using NO3- as
the reagent ion; hereafter MION-NO3). The comparison among different
measurement techniques revealed that the highest elemental oxygen-to-carbon
ratios (O : C) of organic compounds were observed by the MION-NO3 (0.9 ± 0.1, average ± 1 standard deviation), followed by the MION-Br
(0.8 ± 0.1); lowest O : C ratios were observed by Vocus (0.2 ± 0.1). Diurnal patterns
of the measured organic compounds were found to vary among different
measurement techniques, even for compounds with the same molecular formula,
suggesting contributions of different isomers detected by the different
techniques and/or fragmentation from different parent compounds inside the
instruments. Based on the complementary molecular information obtained from
Vocus, MION-Br, and MION-NO3, a more complete picture of the bulk
volatility of all measured organic compounds in this boreal forest was
obtained. As expected, the VOC class was the most abundant (about 53.2 %), followed by intermediate-volatility organic compounds (IVOCs, about
45.9 %). Although condensable organic compounds (low-volatility organic
compounds, LVOCs; extremely low volatility organic compounds, ELVOCs; and
ultralow-volatility organic compounds, ULVOCs) only comprised about 0.2 %
of the total gaseous organic compounds, they play an important role in new
particle formation as shown in previous studies in this boreal forest. Our
study shows the full characterization of the gaseous organic compounds in
the boreal forest and the advantages of combining Vocus and MION API-ToF for
measuring ambient organic compounds with different oxidation extents (from
VOCs to HOMs). The results therefore provide a more comprehensive
understanding of the molecular composition and volatility of atmospheric
organic compounds as well as new insights into interpreting ambient
measurements or testing/improving parameterizations in transport and climate
models.