Highly oxygenated organic molecule (HOM) formation in the isoprene oxidation by NO3 radical
Authors: Zhao Defeng , Pullinen Iida , Fuchs Hendrik , Schrade Stephanie , Wu Rongrong , Acir Ismail-Hakki , Tillmann Ralf , Rohrer Franz , Wildt Jürgen , Guo Yindong , Kiendler-Scharr Astrid , Wahner Andreas , Kang Sungah , Vereecken Luc , Mentel Thomas F. 
Published: June 2021

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 ozone
(O3) and hydroxyl radical (OH), HOM formation in the oxidation by
nitrate radical (NO3), an important oxidant at nighttime 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 (RO2), 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, 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=7–13), 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 the isoprene + NO3
reaction under our experimental 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, 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 %-0.7%+1.3%, 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.