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© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  19 Nov 2020

19 Nov 2020

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This preprint is currently under review for the journal ACP.

Highly oxygenated organic molecules (HOM) formation in the isoprene oxidation by NO3 radical

Defeng Zhao1,2, Iida Pullinen2,a, Hendrik Fuchs2, Stephanie Schrade2, Rongrong Wu2, Ismail-Hakki Acir2,b, Ralf Tillmann2, Franz Rohrer2, Jürgen Wildt2, Yindong Guo1, Astrid Kiendler-Scharr2, Andreas Wahner2, Sungah Kang2, Luc Vereecken2, and Thomas F. Mentel2 Defeng Zhao et al.
  • 1Department of Atmospheric and Oceanic Sciences & Institute of Atmospheric Sciences, Fudan University, 200438, Shanghai, China
  • 2Institute of Energy and Climate Research, IEK-8: Troposphere, Forschungszentrum Jülich, 52425, Jülich, Germany
  • anow at: Department of Applied Physics, University of Eastern Finland, Kuopio, 7021, Finland
  • bnow at: Institute of Nutrition and Food Sciences, University of Bonn, Bonn, 53115, Germany

Abstract. Highly oxygenated organic molecules (HOM) are found to play an important role in the formation and growth of secondary organic aerosol (SOA). SOA is an important type of aerosol with significant impact on air quality and climate. Compared with the oxidation of volatile organic compounds by O3 and OH, HOM formation in the oxidation by NO3 radical, an important oxidant at night-time and dawn, has received less attention. In this study, HOM formation in the reaction of isoprene with NO3 was investigated in the SAPHIR chamber (Simulation of Atmospheric PHotochemistry In a large Reaction chamber). A large number of HOM including monomers (C5), dimers (C10), and trimers (C15), both closed-shell compounds and open-shell peroxy radicals, were identified and were classified into various series according to their formula. Their formation pathways were proposed based on the peroxy radicals observed and known mechanisms in the literature, which were further constrained by the time profiles of HOM after sequential isoprene addition to differentiate first- and second-generation products. HOM monomers containing one to three N atoms (1–3N monomers) were formed, starting with NO3 addition to carbon double bond, forming peroxy radicals (RO2), followed by autoxidation. 1N monomers were formed by both the direct reaction of NO3 with isoprene and of NO3 with first-generation products. 2N-monomers (e.g. C5H8N2On (n = 8–13), C5H10N2On (n = 8–14)) were likely the termination products of C5H9N2On•, which was formed by the addition of NO3 to C5-hydroxynitrate (C5H9NO4), a first-generation product containing one carbon double bond. 2N-monomers, which were second-generation products, dominated in monomers and accounted for ~34 % of all HOM, indicating the important role of second-generation oxidation in HOM formation in isoprene+NO3 under our reaction conditions. H-shift of alkoxy radicals to form peroxy radicals and subsequent autoxidation (alkoxy-peroxy pathway) was found to be an important pathway of HOM formation. HOM dimers were mostly formed by the accretion reaction of various HOM monomer RO2 and via the termination reactions of dimer RO2 formed by further reaction of closed-shell dimers with NO3 and possibly by the reaction of C5-RO2 with isoprene. HOM trimers were likely formed by the accretion reaction of dimer RO2 with monomer RO2. The concentrations of different HOM showed distinct time profiles during the reaction, which was linked to their formation pathway. HOM concentrations either showed a typical time profile of first-generation products, or of second-generation products, or a combination of both, indicating multiple formation pathways and/or multiple isomers. Total HOM molar yield was estimated to be 1.2 %+1.3 %−0.7 %, which corresponded to a SOA yield of ~3.6 % assuming the molecular weight of C5H9NO6 as the lower limit. This yield suggests that HOM may contribute a significant fraction to SOA yield in the reaction of isoprene with NO3.

Defeng Zhao et al.

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Defeng Zhao et al.

Defeng Zhao et al.


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Short summary
The reaction of isoprene, a biogenic volatile organic compound with the globally largest emission rates, with NO3, an nighttime oxidant influenced heavily by anthrogogenic emissions, forms a large number of highly oxygenated organic molecules (HOM). These HOM are formed via one or multiple oxidation steps, followed by autoxidation. Their total yield is much higher than that in the daytime oxidation of isoprene. They may play an important role in nighttime organic aerosol formation and growth.
The reaction of isoprene, a biogenic volatile organic compound with the globally largest...