Introduction
The molecular characterization of secondary organic aerosol (SOA) has been a
topic of interest in atmospheric chemistry for the last decades, owing to
the importance of organic aerosol in air quality and climate (for a review,
see Nozière et al., 2015). SOA comprises a large number of oxygenated
organic compounds and is a major constituent of submicrometer atmospheric
particulate matter (PM), and both biogenic (e.g., isoprene, monoterpenes,
sesquiterpenes) and anthropogenic (aromatics, n-alkanes) volatile organic
compounds (VOCs) serve as precursors for SOA. Abundant biogenic VOCs in the
terrestrial atmosphere are monoterpenes, having an annual global emission
rate of 155 Tg with α-pinene as the major terpene emitted
(Guenther et al., 2012). Several multifunctional SOA compounds, including
monomers and dimers from α-pinene oxidation have been structurally
identified (for a review, see Nozière et al., 2015). Recently,
“extremely low-volatility organic compounds” (ELVOCs), currently termed
“highly oxygenated molecules” (HOMs), originating from α-pinene
ozonolysis (α-pinene/O3) have been detected in both laboratory and field experiments by
chemical ionization–atmospheric pressure ionization–time-of-flight (CI-APi-TOF)
mass spectrometry with nitrate clustering (Ehn et al., 2012,
2014; Zhao et al., 2013) and have received much attention because of their
role in driving new particle formation and growth in pristine forested
environments. Molecular characterization of α-pinene SOA
constituents is needed to elucidate the underlying formation mechanism and
establish its link with gas-phase HOMs, and efforts in this direction have
recently been undertaken (Mutzel et al., 2015; Zhang et al., 2015, 2017; Krapf et
al., 2016). However, the relationship of HOMs detected
in the gas phase upon α-pinene ozonolysis with stable
high-molecular-weight SOA constituents is unclear, so that there is still a
missing element in closing the α-pinene SOA system.
High-molecular-weight esters have been reported in α-pinene/O3
SOA but their detailed chemical structures are only partially elucidated and
their mechanism of formation is still elusive. A high-molecular-weight
compound with a molecular weight (MW) of 358 was reported for the first
time by Hoffmann et al. (1998) in α-pinene/O3 SOA using off- and
online mass spectrometry (MS). With online atmospheric pressure chemical
ionization (APCI) MS it was shown that this compound is formed concomitantly
with two monomers, i.e., cis-pinic acid and a MW 172 compound that was
tentatively identified as norpinic acid. Tandem MS on the deprotonated
compound (m/z 357) revealed that it has a cis-pinic acid residue (m/z 185) as well as
a m/z 171 residue. Later work by Müller et al. (2008) focused on the
structure of the MW 368 compound. It was shown that this compound is
composed of cis-pinic and hydroxypinonic acid parts, which are linked together
by an ester bridge. The structure of the MW 358 compound was also addressed
by Yasmeen et al. (2010), who revised the structure of this compound and
presented evidence that it is a diaterpenylic ester of cis-pinic acid. The same
conclusion was reached by Gao et al. (2010), who also showed that the MW 358
ester is a major product in α-pinene ozonolysis experiments
performed at low mass loadings. Recent work by Beck and Hoffmann (2016),
where use was made of derivatization to the n-butylesters and subsequent
tandem MS analysis of the lithiated and ammoniated molecules, supported the
structure of the MW 358 ester as a diaterpenylic ester of cis-pinic acid.
Furthermore, the MW 358 ester was detected as a major tracer in
β-pinene ozonolysis SOA characterization studies (Iinuma et al., 2007;
Yasmeen et al., 2010).
It is noted that prior to the studies by Müller et al. (2008) and
Yasmeen et al. (2010) several studies dealt with the molecular
characterization of high-molecular-weight compounds and that very different
possible structures have been advanced. Gao et al. (2004) assigned the
MW 358 α-pinene/O3 compound to a dehydration product formed
between the gem-diol forms of two norpinonic acid molecules. Iinuma et al. (2004)
reported MW 354 and 370 α-pinene/O3 products that were
enhanced in acidic conditions and tentatively assigned them to reaction
products between the gem-diol of pinonaldehyde and pinonaldehyde, and
between pinonaldehyde and hydroxypinonaldehyde, respectively. Docherty et
al. (2005) proposed peroxycarboxylic acid dimers for the structure of
higher-MW SOA products from the ozonolysis of α-pinene in which
peroxypinic acid and the gem-diol of a keto or aldehydic compound are
connected via a peroxy bridge. Tolocka et al. (2004) characterized
high-molecular-weight compounds in α-pinene ozonolysis SOA and
suggested that the products were most likely formed by aldol and/or gem-diol
formation. In addition, Witkowski and Gierczak (2014) explained the
formation of MW 338 and 352 compounds in α-pinene ozonolysis as
aldol reaction products of α-acyloxyhydroperoxy aldehydes. All the
abovementioned studies thus provide evidence that the structure elucidation
of high-molecular-weight α-pinene/O3 compounds has turned out
to be very challenging.
With regard to the structure elucidation of the MW 358 ester there is still
ambiguity, in that two positional isomers are possible (Fig. 1), and that
different positional isomers have been proposed by Yasmeen et al. (2010)
(structure a), Gao et al. (2010) (structure b), and Beck and Hoffmann (2016)
(structure b). Based on the MS data obtained it is not possible to
unambiguously support the structure of one or the other positional isomer.
This issue will be further addressed in Sect. 3. The same ambiguity holds
for the MW 368 ester (Fig. 1). In addition to the MW 358 and 368 esters,
minor high-molecular-weight compounds (i.e., MWs 272, 300, 308, 312, 314,
326, 338, 344, 352, 356, 376, 378 and 400) have also been reported in
α-pinene/O3 SOA (Müller et al., 2008; Yasmeen et al., 2010;
Kourtchev et al., 2014; Witkowski and Gierczak, 2014; Zhang et al., 2015)
but these products will not be addressed in the present paper.
Overview of the proposed high-molecular-weight ester compounds present
in α-pinene/O3 SOA which were investigated in the present study.
The compounds present in underivatized α-pinene/O3 SOA are
highlighted in red color.
High-molecular-weight esters have been detected up until now in many field
studies that were conducted in forested regions. MW 358 and 368 esters were
first reported in ambient nighttime PM with an aerodynamic diameter ≤ 2.5 µM
(PM2.5) that was collected at K-puszta, Hungary, during a
2006 summer campaign (Yasmeen et al., 2010). They were later detected in
several field studies that were conducted in other forested environments
(Kristensen et al., 2013, 2016; Kourtchev et al., 2014, 2015). It was shown
by Kourtchev et al. (2016) that oligomers (i.e., hetero-oligomers) are of
climatic relevance in that elevated SOA mass is one of the key drivers of
oligomer formation not only in laboratory experiments but also in the
ambient atmosphere. It was also demonstrated in the latter study that the
oligomer content is strongly correlated with cloud condensation nuclei
activities of SOA particles. Furthermore, it could be demonstrated in
laboratory chamber experiments that the ratio of monomers to oligomers and the
oligomer content in α-pinene ozonolysis SOA are enhanced at low
temperature and low precursor concentrations, conditions that are relevant
for the upper troposphere (Huang et al., 2018).
Efforts to understand ester formation from α-pinene ozonolysis have
also been actively undertaken. Yasmeen et al. (2010) proposed that ester
formation took place in the particle phase by esterification of cis-pinic acid
with terpenylic acid but this mechanism was not retained in later studies.
Kristensen et al. (2014) demonstrated their formation through gas-phase
ozonolysis and supported the participation of a stabilized Criegee
intermediate, as previously suggested for the formation of unstable
high-molecular-weight compounds that play a role in new particle formation
(Ziemann, 2002; Bonn et al., 2002; Lee and Kamens, 2005). In a study by
Zhang et al. (2015), the dynamics of particle-phase components of
α-pinene SOA formation were investigated in detail. It was shown that
formation of monomeric products like cis-pinic acid is observed after the
consumption of α-pinene upon ozonolysis, which cannot be explained
solely by a gas-phase mechanism and points to a particle-phase mechanism. A
mechanism involving gas-phase radical combination of acyl peroxy radicals
and a condensed-phase rearrangement was proposed that potentially explains
the α-pinene SOA features in terms of molecular structure,
abundance, growth rates, evolution patterns, and responses to variations in
temperature, relative humidity, and oxidant type. Furthermore, a recent
study by Zhang et al. (2017), using ozonolysis of deuterium-labeled
α-pinene, demonstrated that hydroperoxy derivatives of pinonic acid
containing the hydroperoxy group at different positions are components of
HOMs that are present in the particle phase. In work prior to the above-cited investigations, other studies already suggested the involvement of acyl
peroxy radicals in the formation of HOMs upon α-pinene ozonolysis
(Ziemann, 2002; Docherty et al., 2005). In addition, the suggestion that
peroxy radicals are involved in the formation of dimers also fits to the
observation of a suppression of new particle formation from monoterpene
oxidation by NOx (Wildt et al., 2014). Furthermore, evidence for
peroxyhemiacetal formation upon α-pinene ozonolysis has also been
reported (Hall and Johnston, 2012a). All the above-cited studies thus
document that establishing the underlying molecular mechanism leading to
ester formation in α-pinene ozonolysis is challenging. This is
mainly due to a lack of knowledge (or only a partial knowledge, i.e.,
molecular formulae) of the molecular structures of both gas-phase
intermediates and particulate-phase end products.
In the present paper, we focus on the structural characterization of the
MW 358 and 368 esters that are present in α-pinene/O3 SOA. To this
aim, we have performed liquid chromatography with
electrospray ionization mass spectrometry (LC/ESI-MS) in the positive ion mode on
α-pinene/O3 SOA with and without derivatization into methyl esters. A
soft methylation procedure using ethereal diazomethane was selected to avoid
hydrolysis of the ester function present in the targeted hetero-dimers. The
aim of the methylation was twofold: on the one hand, to confirm the number
of free carboxyl functions, and on the other hand, to obtain mass
spectrometric fragmentation that is different from that of intact esters in
(+)ESI and to that obtained in previous studies on intact esters in
(-)ESI (Müller et al., 2008; Yasmeen et al., 2010; Zhang et al., 2015).
Led by the molecular structures of the MW 368 and 358 esters, we propose
a formation mechanism that takes into account the detection of a C19
HOM in the gas phase by CI-APi-TOF MS with nitrate clustering (Ehn et al.,
2012, 2014; Zhao et al., 2013) and involves the combination of an acyl
peroxy radical related to cis-pinic acid with an alkoxy radical related to
isomeric hydroxypinonic acids, which are, like cis-pinic acid, major monomers in
α-pinene SOA.