Anthropogenic Volatile Organic Compound (AVOC) Autoxidation
as a Source of Highly Oxygenated Organic Molecules (HOM)
Gas-phase hydrocarbon autoxidation is a rapid pathway for the production of in situ aerosol precursor compounds. It is a highway to molecular growth and lowering of vapor pressure, and it produces hydrogen-bonding functional groups that allow a molecule to bind into a substrate. It is the crucial process in the formation and growth of atmospheric secondary organic aerosol (SOA). Recently, the rapid gas-phase autoxidation of several volatile organic compounds (VOC) has been shown to yield highly oxygenated organic molecules (HOM). Most of the details on HOM formation have been obtained from biogenic monoterpenes and their surrogates, with cyclic structures and double bonds both found to strongly facilitate HOM formation, especially in ozonolysis reactions. Similar structural features in common aromatic compounds have been observed to facilitate high HOM formation yields, despite the lack of appreciable O3 reaction rates. Similarly, the recently observed autoxidation and subsequent HOM formation in the oxidation of saturated hydrocarbons cannot be initiated by O3 and require different mechanistic steps for initiating and propagating the autoxidation sequence. This Perspective reflects on these recent findings in the context of the direct aerosol precursor formation in urban atmospheres.
Introduction<!><!>Introduction<!>AVOC HOM in the Literature<!><!>AVOC HOM in the Literature<!><!>Challenges
in Detection<!><!>Challenges
in Detection<!>Challenges in Field Observations<!>Challenges in Computation<!>Challenges in Pathways<!><!>Challenges in Pathways<!><!>Challenges in Pathways<!>Additional
Unknowns—The Darker Side<!>Outlook<!>
<p>Air
pollution shortens the lifespan and decreases the well-being
of an increasing number of lives.1−5 Especially airborne particulate matter is of great concern.6,7 The situation is worse in the megacities across the globe, where
more people are living than ever before.8,9 While the limiting
of emission sources is the most efficient way to mitigate air pollution,
a large fraction of the emissions will remain unavoidable. Part of
the inability to hinder the problem raises from our incomplete understanding
of volatile organic compound (VOC) oxidation in the gas phase. Perhaps
most alarmingly, the formation of particulate air pollution from gaseous
sources is particularly poorly understood.</p><p>Recently the rapid
gas-phase autoxidation of endocyclic alkenes
initiated by a reaction with ozone (i.e., ozonolysis) was shown to
yield highly oxygenated organic molecules (HOM).10−12 HOM were originally
found in the Finnish Boreal forest10,13,14 and were promptly connected to the "missing
organic compounds" expected to explain organic aerosol measurements.15−17 Consequently, they were shown to be the key compound class involved
in the formation and growth of atmospheric secondary organic aerosol
(SOA) mass.10,18 The current understanding of
HOM was recently reviewed by Bianchi and co-workers19 in order to synthesize the decade of observations into
a more meaningful format. They defined HOM as species that are (i)
formed via a sequential autoxidation of peroxy radicals (RO2) (ii) under atmospherically relevant conditions and (iii) contain
six or more oxygen atoms. This definition contains the gas-phase aerosol
precursors formed by a rapid sequential oxidation through consecutive
H-shift reactions, while also excluding common plant-synthesized,
highly oxygenated carbohydrates (e.g., sugars). The HOM contain several
hydrogen-bond-donating functional groups such as alcohols and hydroperoxides
and aggregate by a peroxide "dimer" formation, and their
general stability on surfaces and in condensed phase media is still
largely in doubt. This strict definition currently leaves out certain
important aerosol forming molecules with similar properties in clustering
and condensing reactions, such as multicarboxylic acids (e.g., citric
acid and 3-methyl-1,2,3-butanetricarboxylic acid [MBTCA]), which do
not have known fast gas-phase formation routes. Also, the oxygenated
compounds left out from the HOM definition (e.g., multicarboxylic
acids and polyalcohols) are considerably more stable in condensed
phase, and while they have a similar propensity for hydrogen bonding
and clustering reactions, it is possible that they do not react significantly
in the condensed phase.</p><p>Anthropogenic volatile organic compounds
(AVOC) are hydrocarbons
released by human activities. They are emitted from countless activities
of the society, such as traffic, energy production, industry, from
solvents, adhesives, paints, lubricants, and wear-reducing products,
from cosmetics and personal care-products, and so on, and so on (Figure 1).20−22 In practice,
AVOCs cover all hydrocarbon subclasses containing all functional groups
from aromatic to aliphatic compounds. In fact, the division to AVOC
and BVOC (i.e., biogenic VOC) is chemically completely arbitrary;
often, the same compounds are emitted by both anthropogenic and biogenic
processes (e.g., isoprene [CH2=CHC(CH3)=CH2] and ethylene CH2=CH221,23−25). Generally, AVOCs are
thought to consist mainly of aromatic compounds and alkanes, and common
for these species is that they do not react appreciably with O3. This more chemically rigorous classification of AVOC is
adopted here too to allow for an easier discussion, notwithstanding
the shortcoming that several common anthropogenic compounds do not
fit in (e.g., ethene, propene, and acetylene21). The ozonolysis of double bonds is an important initiation step
in the formation of atmospheric particulate matter, and thus a division
of compounds that either react or do not react with O3 is
useful. Moreover, most of the information we currently have on HOM
formation has been gained through ozonolysis oxidation experiments.</p><!><p>Examples
of common AVOC sources in our living environments.</p><!><p>The structural analogies of aromatic AVOCs to several endocyclic
BVOCs together with their known high SOA yields motivated early experiments
on AVOC HOM formation.26,27 Aromatic systems have double
bonds coupled to cyclic structures (although still no appreciable
reaction with O3), and the pathways up to five oxygen-containing
bicyclic peroxy radicals (BPR) are well-established (see, e.g., refs (28−30)). Subsequently, several aromatic systems were shown
to generate HOM with a high yield, in line with their reported high
SOA yields. In stark contrast, saturated acyclic alkanes are completely
devoid of these sites known to enable a gas-phase autoxidation propagation
by sequential H-shift reactions under atmospheric conditions but still
are reported to generate a significant SOA mass in chamber oxidation
experiments.26,31,32 Thus, the recent finding of alkane HOM formation by Wang et al.33 seemingly contradicts our basic understanding
of gas-phase organic oxidation chemistry,34,35 yet is fully in line with the view of HOM being the pathway to atmospheric
SOA. This is understandable, as the same principles govern the atmospheric
autoxidation sequence in any VOC system, the loosening of C–H
bonds due to adjacent and nearby functional groups,11 or by heat.33 In pure, naked
hydrocarbons the initiation of RO2 H-shift reactions is
very slow,36 but with a functionalization
of the molecule, or at elevated temperature, the initiation considerably
speeds up.33</p><p>The detailed pathways
to HOM in aromatic compounds remain far from
understood, notwithstanding the relatively many publications already
devoted to the topic. Currently only Wang et al.37 have inspected the first steps of alkylbenzene-derived
RO2 H-shifts by detailed quantum chemical computations,
whereas others have mainly reported product compositions and overall
HOM yields.38−41 It seems that AVOC HOM from both aromatic and alkane sources have
escaped observations due to the same reasons HOM were not found in
the first place: (i) The yields of HOM generally fit into common measurement
uncertainties (i.e., HOM yield ∼0.1–10%19), (ii) the sampling from experimental setups has not been
optimized for free radicals and sticky aerosol precursors (i.e., HOM
were lost to the walls), and (iii) oxidation studies and mass spectrometric
detection are performed at reduced pressure and low O2,
decreasing, or even preventing, the formation of HOM.</p><p>This perspective
concerns AVOC HOM formation by autoxidation with
a focus on the recent advances in our understanding and on the remaining
challenges in uncovering the underlying mechanistic steps leading
to these in situ aerosol precursors. The text is structured as follows.
First the AVOC HOM reported in recent literature is presented, followed
by the related challenges in instrumental detection methodologies,
field observations, and theoretical computations. Then the mere lack
of chemical intuition in finding the correct reaction sequences is
detailed and discussed, with the last chapters dedicated to an examination
of the burning questions that are difficult, or even impossible, to
answer with the current research methodologies.</p><!><p>The current literature of AVOC HOM formation is presented in Table 1. Aromatic systems
have received the most attention with benzene and several alkylbenzenes
being studied experimentally and theoretically by Wang et al.,37 a collection of compounds including polyaromatics
by Molteni et al.,38 the sequential and
auto-oxidation of benzene and HOM formation in several other systems
by Garmash et al.,39 toluene and 1,2,4-methylbenzene
by Zaytsev et al.,40 and the photo-oxidation
of toluene and naphthalene by Wang et al.41 Numerous earlier works have investigated the formation of bicyclic
peroxy radicals that are the starting material for HOM formation from
aromatics (e.g., refs (28−30)) yet have not
reported any HOM products. Currently, only Wang et al.33 have reported alkane-derived HOM, though several
papers have described important steps on the way to HOM formation.36,42−45</p><!><p>Computed product
pathways.</p><p>Proposed pathways
based on previous
literature.</p><!><p>While the amount
of different AVOC systems investigated to date
is already considerable, the detailed mechanisms for HOM formation
has not been obtained for any of them. In the more studied aromatics,
steps beyond BPR remain highly uncertain. An important complexity
arises from a similar formation of gradually oxidized species by accelerating
multigeneration oxidation (i.e., oxidation initiation, termination,
and again initiation) and was very recently inspected by Garmash et
al.39 (Figure 2; also noted in previous works, e.g., refs (27) and (46)). The OH reaction rate
coefficients of the primary oxidation products are considerably faster
than the parent rates, with a concomitant higher probability in initiating
autoxidation, as the oxidized functional groups ease the subsequent
H-shift reactions. Moreover, the isomeric products are not separable
with the current mass spectrometric techniques applied for HOM detection
and, thus, pose severe challenges for experimental product determinations,
which are discussed next.</p><!><p>HOM concentrations (a) and yields (b) observed
in benzene (squares)
and phenol (circles) OH oxidation experiments. The dependence of benzene
HOM on OH and VOC oxidation rate uncovers the multigeneration oxidation
in competition with autoxidation through BPR. The color represents
the concentration of hydroxyl radicals. Reprinted with permission
from ref (39). Copyright
2020 Copernicus Publications.</p><!><p>All direct observations of gas-phase HOM have
been obtained with
atmospheric pressure controlled chemical ionization mass spectrometry
(APcCI-MS; also referred to with other names in the literature such
as selected ion CI-MS47). Most of the studies
have applied selective adduct-forming negative polarity reagent ions
for a sensitive detection of a wide variety of HOM. Especially the
utilization of nitrate ion (NO3–)-based
charging has been instrumental for the original discovery and subsequent
insights (e.g., refs (10, 11), and (48)). Recently,
also the usefulness of positive polarity adduct formation has been
demonstrated by Berndt et al.33,49 This very sensitive
and selective, controlled chemical analysis technique should not be
confused with the rough APCI-MS methods commonly used for detection
in conjunction with a chromatographic separation. The extreme selectivity
of cluster formation in the ion–molecule reactions of APcCI
charging means that the mass spectrum can be obtained almost devoid
of chemical noise, resulting in an extremely sensitive detection of
the HOM.</p><p>In APcCI methods either a single reagent ion (e.g.,
iodide and
bromide50−52) or a collection of reagent ions (e.g., nitrate and
acetate51,53) are produced in a controlled manner at
atmospheric pressure in a separate ion production unit, after which
the reagent ions and the targeted sample are mixed with the help of
electric fields in an ion–molecule reaction region. Subsequently
the charged sample is drawn into the mass spectrometer through an
orifice or a capillary, with practically no dilution, as the charging
happens before the entrance of the vacuum chambers. The careful control
over the ion pool that meets the sample improves the sensitivity and
reduces the secondary chemistry tremendously. Also other reagent ion
introduction designs have been implemented; for example, Sipilä
et al.54 used sulfuric acid injection directly
into the sample flow, to induce HSO4– formation in reactions with NO3–, followed
by its clustering with the target molecules. The controlled manner
by which well-chosen ions can be produced together with the extreme
selectivity of the clustering reaction make the APcCI currently an
unrivaled method for gas-phase HOM detection.</p><p>Novel proton-transfer-reaction
mass spectrometers (PTRMS) work
with largely the same principles as the APcCI-MS, and certain instrumental
configurations have shown promising developments in the detection
of HOM.55 However, PTRMS reaches inherently
lower detection sensitivities (i.e., higher detection limits) due
to low-pressure ionization sources (i.e., most of the sample is lost
before the sample is ionized) and due to nonselective ionization schemes,
making charge availability problematic. Nevertheless, with the novel
type of ion transfer optics, they have been shown useful in recent
HOM research and, with further optimization, could become of great
importance. Especially with several carefully chosen proton-transfer
schemes and the proton transfer being performed at atmospheric pressure
could allow for a selective detection of both the reagents and a wide
array of products with a single instrument. Yet, this type of an approach
remains to be demonstrated in atmospheric research.</p><p>Without
a doubt, the community would benefit from more instrumentation
capable of detecting these crucial, low-volatile, and reactive aerosol
precursors in situ. Arguably, the HOM research community suffers from
this scarcity of detection methods, and some have even criticized
HOM to be merely an artifact of the used instrumentation.56 Yet, the growing number of studies probing HOM
and subsequent new-particle and SOA formation in a laboratory and
in the field, and the increasing variety of reagent ion chemistries
used in HOM detection, are slowly unifying the perception of HOM importance
in gas-phase particulate matter formation.</p><p>In contrast to the
applied mass spectrometric detection methods,
spectroscopic and chromatographic techniques could supply direct information
on the product structure and would be highly beneficial for inferring
the presently uncertain HOM structures. However, their inherent experimental
limitations prevent a utilization of their benefits. In spectroscopic
techniques, the spectral overlaps of similar functional groups within
similar chemical neighborhoods constitute a tremendous problem for
speciation. On the one hand, the less-oxidized products and intermediates
present at higher concentrations than HOM will contain the exact same
functional groups in the exact same molecular positions (i.e., they
are gradually oxidized products from the same parent compound oxidation),
maximizing the spectral overlaps. In chromatographic techniques, on
the other hand, the transfer limitations of the elution columns generally
prevent the highly functionalized and polar HOM to pass through. Additionally,
the high surface activities and reactivity of polyperoxide HOM mean
that, if they can travel through the chromatographic column, they
are unlikely to do it without a change in their molecular structure.
Moreover, HOM completely lack authentic standards, and, as a currently
fully gas-phase species, their stability in condensed phases is a
mystery. The HOM standards would have to be formed in situ with a
controlled gas synthesis, a chore that is in practice close to impossible
to accomplish without a multitude of secondary unwanted reaction products.</p><p>While the chromatographic and spectroscopic techniques would be
ideal in solving detailed HOM structures, their successful application
requires a surmounting of these technical challenges first. Recently,
gas chromatography has been used for the quantification of several
multifunctional aromatic oxidation products, which are the likely
prestages of HOM formation through a sequential oxidation.40,46,57 If the technical limitations
can be overcome, then a combination of chromatographic separation
and spectroscopic detection would provide a currently unrivaled structural
characterization for the HOM, yet nothing like this seems available
in the foreseen future. Another fundamental technical obstacle comes
from the detection limit. Individual HOM compounds are present from
below 105 cm–3 to up to ∼108 cm–3 gas-phase concentrations even in optimal
laboratory settings. Such minute quantities present challenges for
any detection methods notwithstanding the nature of the target material.
Thus perhaps a more fruitful approach is to develop chemical ionization
mass spectrometry toward a structural analysis methodology.</p><p>While mass spectrometers are generally far from ideal at extracting
three-dimensional molecular information, they are able to resolve
features of the molecular structure. Several methods can be used to
infer a deeper insight into the HOM construction. D2O-mediated
H to D exchange,11 ion-mobility spectrometry
(IMS),58 and MS-MS fragmentation experiments59 have all been used to verify the assumption
that several HOM isomers are formed from the oxidation sequences.
If single-oxidation pathways and their subsequent products could be
isolated, then a threshold photoionization (e.g., with vacuum ultraviolet60 or multiphoton infrared radiation61) and potentially other compound-specific characteristics
could be used for a molecular identification. However, currently it
seems that only in very specific cases a single oxidation pathway
could be responsible for the HOM formation, and generally isomers
are formed. For an understanding of the chemistry the most important
isomers to distinguish would be those that are susceptible to a very
different subsequent chemistry, for example, double bonds versus cyclic
structures and bridged oxygens (e.g., epoxide and endoperoxide) versus
carbonyls, which hinder and enhance the oxidation propagation in very
specific ways.</p><p>As yet unexplored, structural information on
oxidation products
could also be inferred indirectly by utilizing several well-chosen
reagent ion schemes. Only a certain type of reagent ions bind to certain
functional groups, and thus, a chemical filtering of the product distribution
becomes possible.50,51,53,62−64 For example, distinguishing
between carbonyl and alcohol groups with a reference to molecular
composition from the MS measurement. In such a technology, a rapid
application of several ionization schemes would be highly beneficial.
The recently developed multischeme chemical ionization inlet (MION50) type of approach would allow this, enabling
the rapid switching between multiple reagent ion schemes in both ion
polarities. Even the utilization of several concomitant proton-transfer
schemes could be envisioned, enabling an overcoming of the charge
balance problem mentioned above.</p><p>Currently the HOM structures
are only known by inference to computed
HOM formation mechanisms. In experimentally detailing these mechanisms,
time scales are crucial yet problematic. The isomeric complexity likely
rapidly increases with reaction time, and at short times only the
fastest pathways have reached to highly oxidized products. This is
amply exemplified in recent research on a-pinene HOM formation.59,65 The a-pinene product distribution considerably changes from subsecond
to tens of seconds reaction time. The endoperoxide bridge reportedly
important for the rapid early oxidation propagation seems to be absent
already at 30 s reaction time according to the MS-MS analysis. Similar
changes in peroxy radical distributions as a function of reaction
time in isoprene oxidation have been discussed by Wennberg et al.66 While the long reaction times are rather trivial
to obtain when larger reactor setups are utilized, the short reaction
times pose considerable challenges for the CIMS research methodology.
CIMS is poorly suited for the study of rapid chemical changes due
to the finite time needed to charge the sample by ion–molecule
reactions. Especially the inlets optimized for a sticky and reactive
reaction product detection (e.g., several inlets that mimic the pioneering
work by Eisele and Tanner47) have sheath
flows and characteristic long charging times. The faster CI-inlets,
the MION working by ion injection,50 and
the "cluster CIMS" with a transverse ionization geometry
by Zhao et al.67 are more suited for this
type of research (Figure 3). Yet even more direct, and faster, detection would be preferable
and potentially available by a direct photoionization, but this has
not been demonstrated yet.</p><!><p>Two examples of novel chemical ionization inlet
designs that allow
an investigation of the rapid HOM formation processes in an AVOC oxidation.
(a) MION with two ion injectors, the source 2 providing the short
ionization time. (b) The "cluster-CIMS" in (i) flow
tube mode and in (ii) transverse ion mode, the latter geometry being
suitable for the short reaction time experiments. (a) Reprinted in
part with permission from ref (50) (copyright 2019 Copernicus Publications). (b) Reprinted
in part with permission from ref (67) (copyright 2010 American Geophysical Union).</p><!><p>Thus, it is the all-important characteristics of
the APcCI-MS methodologies
that have offered the recent leaps in HOM detection. The selectivity,
accompanied by the inherent sensitivity, combined with the practically
wall-less sampling under normal atmospheric conditions. Most of the
classical information on oxidation pathways comes from studies performed
at low pressures, especially the related MS work, which partly explains
why the discovery of atmospheric autoxidation and the subsequent HOM
formation took us so long. Considering all the experimental challenges
in resolving the HOM chemistry, it seems clear that the APcCI techniques
will remain the sole detection methodology for HOM still for some
time.</p><!><p>The challenges in
lab observations become only worse in the field.
Therefore, it is needless to say that a quantification of individual
AVOC-derived HOM from the ambient gas-phase has not been demonstrated.
Their largely unknown molecular structures and, thus, unknown detection
sensitivities, combined with a diversity of chemicals leading to HOM
with similar compositions, will continue to make this task extremely
challenging. Fortunately, HOM formed by similar autoxidation sequences
are likely to share similar enough molecular properties that their
quantification as a group of compounds is still worthwhile for an
understanding of SOA budgets (i.e., HOM have low to extremely low
volatility18 and contain hydrogen-bonding
−OH and −OOH groups that enable clustering and tangling
to a substrate51,68). Thus, often it is likely enough
to merely estimate the sum of HOM from the mass spectrometer signal
from products with HOM compositions. This carries some extra uncertainty
due to the varying detection sensitivities for molecules with different
functional groups, yet the difference in charging efficiency is likely
relatively small for such a highly oxygenated reaction products.51 Furthermore, it is rather easy to propose HOM
compositions based on simple mechanistic rules concerning their formation
pathways. For example, if an OH H-abstraction reaction started the
oxidation sequence and the only bimolecular steps were O2 additions, then likely a succession of products with varying, high
oxygen content, the same amount of C atoms, and two fewer H atoms
are seen, the second H being lost during an −OH ejection after
an oxidation-terminating H-shift from a hydroperoxide C. However,
in reality every branching point, especially C–C bond scissions
and H-atom scrambling, complicate this task, leading to a wider array
of possible product molecular structures. However, currently most
of the observed HOM products pertain to certain oxidation propagation
rules (e.g., refs (19) and (69)), with few
C atoms lost on the way.</p><p>Most likely places for AVOC HOM observations
are polluted urban
atmospheres and places in close proximity to industrial activities
and combustion sources. These environments are characterized by a
vast number of different emitted chemicals and airborne particulate
surfaces where HOM can deposit on. Generally urban atmospheres can
have high AVOC HOM concentrations as exemplified recently by McDonald
et al.20 in considering the vast amount
of different petrochemically derived volatile chemical products (VCP)
emitted from the daily routines of the society.</p><!><p>Most of the information on HOM formation mechanisms and molecular
structures have been obtained by quantum-chemical computations.19 The inherent complication in this research is
a result of the rapidly growing number of potentially relevant unique
molecular structures that contribute to the important reaction steps.
Often it is not a simple task to decide which pathway to follow computationally,
and the previously generated rules of thumb slowly lose their meaning
as a function of molecular complexity. The substituent effects of
functional groups that affect the H-transfer rates are also not necessarily
additive. This rapidly increases the number of species to consider,
and an even larger number results from their mutual reactions—often
being referred to as the combinatorial problem of organic chemistry.
For example, isomers (i.e., molecules with the same functional groups
but distributed differently) are almost certainly also reacting differently,
yet even the conformation of the molecule (i.e., the same molecular
structure but different spatial structure) can have a strong influence
on its subsequent chemistry.42</p><p>A
coupled technical hurdle comes from the fact that several of
the key reactions require state-of-the-art computational tools, which
consume a vast amount of computational resources. For example, recently
the decades old mystery of gas-phase "dimerization"
of peroxy radicals into oxygen-bridged peroxides,70−72 which have
special importance in growing atmospheric SOA,48,73,74 was resolved only by an application of far
more sophisticated computational methodology (i.e., multireference
computations with intersystem crossing rates having their origin in
relativistic quantum mechanics). Thus, parts of the potential energy
surfaces (PES) must be described very accurately, which consumes a
lot of effort and, consequently, time. Very recently, the rapid formation
of HOM from α-pinene, the most studied HOM-forming system, was
uncovered by a more detailed look on the same ozonolysis-derived intermediates
that had been already considered several times.65,69,75 Specifically it was the improved description
of the excess energy partitioned into the postozonolysis vinoxy radical
intermediate that was required for uncovering this important oxidation
pathway.65</p><p>As the experimental speciation
and quantification of individual
HOM compounds will remain extremely challenging, the role of computations
will likely only increase in the future. Especially of importance
for the applied CIMS methodologies are the studies that associate
reagent ion and target binding energies with instrument sensitivity63,64 and the use of these with controlled fragmentation to obtain experimental
constraints.76 This also amply highlights
the general need for joint theoretical-experimental approaches, as
parts of the problem are easier to address computationally, whereas
other areas are easier to resolve in laboratory settings. The synergistic
application of both is likely to be the most fruitful one, in which
theory is used to guide the experimental design, and experimental
results are used to constrain the further theoretical argumentation.
For the time being, computations will likely remain as the most reliable
window we have on the structures of these highly functionalized gas-phase
aerosol precursors.</p><!><p>The biggest
obstacle in describing HOM formation in any VOC system
lies in the rapidly growing amount of potential branching pathways.
These happen both in the (pseudo-) unimolecular oxidation propagation
and in the cross combination of practically all the radical intermediates
involved (see the sketch of the complexity in Figure 4). The term autoxidation stands for "autocatalytic
oxidation" and is inherently a self-catalyzed phenomena.11,77 The gained oxidized functionalities help the subsequent hydrogen
abstraction reactions (i.e., H shifts) and thus enable a propagation
of the oxidation chain. With each added oxygen, the several sites
of potential H abstractions become more equal, increase the possibility
for isomeric reaction channels, and directly relate to the mutual
experimental-theoretical challenge of determining the HOM-forming
pathways and subsequent product structures. The fast radical rearrangements
that do not transform the product chemical composition but only tweak
the spatial structure occur very rapidly, and they are not easily
tracked by the available research methodologies.</p><!><p>A sketch of the complex
reaction pathways initiated by a single
OH reaction. (a) A schematic description of the first reaction steps
that propagate the oxidation chain, together with (b) a sketch of
the interconnections of the reaction network. After the oxidation
initiation cross reactions of the intermediate ROx radicals (=RO and RO2) and reactions with
NOx, (=NO and NO2) and
HOx (=OH and HO2) all
complicate the ultimate outcome. The detailed branching in the pathways
is dependent on the exact molecular structures of the reacting partners
and on the prevailing experimental conditions. Peroxy radicals were
marked in blue, alkoxy radicals are in white with a blue background,
and carbon-centered radicals have an asterisk.</p><!><p>Tightly linked to the combinatorial problem has been the lack of
a genuine chemical imagination. Before the pioneering theoretical
work by Vereecken et al.78 invoking the
possibility of polyoxygenates in a rapid atmospheric oxidation, and
the separate field experiments and subsequent lab works by Ehn et
al.13,14 that describe HOM for the first time as
a common constituent of the ambient gas phase, practically no one
thought such polyperoxide, gas-phase compounds could exist. Yet, it
had been noted several times that organic compounds likely have an
important role in the atmospheric particulate matter formation and
growth of ambient aerosols. This was already clear by considering
the mass balance of atmospheric molecules and, later, from measurements
by aerosol mass spectrometers.79</p><p>More details on HOM compositions and a solid hypothesis for the
chemical nature and formation mechanism of these compositions was
soon reported by Ehn and co-workers.10 Subsequent
studies confirmed these hypotheses and refined the formation pathways
through RO2 autoxidation.11,12 The mechanism
was guided by low-temperature autoignition research80 and a recent work by Crounse et al.77 However, the important difference between the autoignition
and Crounse works was that the oxidation was shown to propagate into
far higher oxidized intermediates and products and did not terminate
the oxidation by the common H-C(-OOH) abstraction and
subsequent ketohydroperoxide formation. Importantly, in autoignition
the loosening of the C–H bonds and subsequent H-abstraction
happens due to heat, whereas in an atmospheric autoxidation the loosening
is brought about by nearby functional groups. Very recently Wang et
al. showed how these effects can be additive and, thus, how the heat
can be used to initiate autoxidation in naked alkanes too, providing
a tangible segue between these two phenomenological VOC oxidation
regimes.33 While the previous low-temperature
combustion research could not observe the HOM formation due to pertaining
instrumental limitations, it is likely that considerable insight could
still be gained from a revisit to the extensive ignition delay and
cool flame literature, potentially providing important experimental
and theoretical constraints.80</p><p>Thus,
whether unimolecular or bimolecular, branching constitutes
the major challenge in describing autoxidation of (A)VOCs. Its involvement
is clear already from the first principles in considering the radical
intermediates propagating the sequence, but unfortunately it is a
known complexity we have not afforded to dwell into yet. The sophisticated
theoretical computations have simply been too resource-demanding,
and the mass spectrometric detection methodologies have not been able
to follow distinct reaction pathways. Thus instead the HOM research
has focused on finding a single, theoretically sound oxidation pathway
able to explain a certain prominent HOM formation. Nevertheless, it
has been clear from the beginning that other pathways must contribute
and lead into isomeric HOM products. Recently, Noziere and Vereecken36 reported how even the naked alkanes have the
potential for H-shift reactions, but they are too slow to provide
a significant competition for other RO2 loss processes.
Thus, bimolecular RO2 reactions yielding RO (e.g., with
NO,81 RO2, or even HO2,82 even multiple consecutive33) seem pivotal for the initiation of alkane autoxidation
and formation of HOM (Figure 5).</p><!><p>Proposed general HOM-forming pathways in alkane autoxidation. Note
the special role of RO radicals in propagating the oxidation sequence.
Reprinted with permission from ref (33). Copyright 2021 Springer Nature.</p><!><p>The conversions between RO2 and RO radicals are
at the
heart of AVOC autoxidation (Figure 5). When RO2-mediated isomerizations (i.e.,
H shifts and endoperoxidation) and carbon-centered radical (C*) rearrangements
are inaccessible, alkoxy radicals are needed to propagate the oxidation
sequence. RO undergo much more rapid H-shift reactions than RO2, and their more energetic nature makes for more varied branching
pathways.83 Alkoxy radicals are also prone
to decompose, and their special role in the breaking of carbon rings
that hinder HOM formation has been noted previously.39,75 Especially in the naked, straight-chain, unbranched alkanes the
role of RO is heightened, as the parent compounds do not have feasible
RO2 hydrogen-shift reactions available.33,36 Alkoxy radicals are even indirectly related to the molecular mechanism
of peroxide "dimer" formation by RO2 + RO2 reactions, in which two ROs will stay entangled before the
O2 breaks apart and the peroxide −C-O-O-C–
bridge is formed.72 Very recently alkoxy
radicals were also shown to undergo bimolecular reactions even with
another (stabile) radical species such as RO2,84 which highlights the poor understanding we have
of their importance for the atmospheric particulate matter formation
and on atmospheric chemistry in general.</p><p>The HOM and linked
SOA research are rapidly approaching a status
where lumped rate coefficients and simplified mechanistic rules do
not suffice to explain the phenomena at the required detail. Very
recently McFiggans et al.85 compared this
to the point in which ozone-pollution research was some decades ago,
that is, the details do matter. Yet this common practice of lumping
in atmospheric and combustion chemistry modeling is likely still useful
for several analysis, but for ambient pollutant formation and removal,
especially relating to particulate matter formation, a number of molecules
and oxidation processes must be considered individually. With relatively
complicated atmospheric molecules such as mono- and sesquiterpenes
this constitutes a considerable task in considering the wide variety
of possible reaction pathways. Fortunately, the number of species
likely involved in the formation of most of the atmospheric SOA is
far more limited than the whole VOC pool, and a thorough knowledge
of some tens of atmospheric systems is probably enough to adequately
estimate the aerosol-forming potential of most gas-phase environments.
Likewise, the pool of small atmospheric radicals that react with these
aerosol prestages is dominated by a handful of species, decreasing
the amount of interactions that must be explicitly accounted for.
Yet, even this reduced number of chemical systems results in a large
number of reactions and even larger number of products to consider,
and thus sophisticated machine-learning methodologies or other similar
approaches can hopefully soon provide order to this apparent chaos.</p><!><p>Several mechanistic aspects
can be envisioned to influence the
likelihood of AVOC HOM formation, yet their relevance remains unclear
and prevents a determination of their ultimate importance. Among the
most significant uncertainties are (i) competition for oxidation initiation
and propagation, (ii) role of hydrogen scrambling, (iii) role of catalytic
and photolytic reaction steps, (iv) role of differing RO2 reactivity, and (v) role of apparent minor channels. This listing
is not meant to be exhaustive and surely excludes several other burning
issues that remain to be resolved.</p><p>The competition between oxidation
pathways begins from the initiation
of the oxidation chain starting from the primary radical production
rate, whether it is by photolysis or by reactions with oxidants (e.g.,
competition for OH). The OH abstraction rate depends on the chemical
neighborhood, and practically every C–H abstraction in a molecule
will occur at a different rate, with only certain abstractions leading
to subsequent HOM formation. The following steps are likewise prone
for a similar competition, and even with the same primary radical
production rate the following R + O2 propagation rates
can vary significantly. However, under atmospheric conditions nearly
all O2 reactions are fast due to oxygen being in a very
large excess, which is commonly exploited in theoretical computations
by omitting their calculation. Nevertheless, even the O2 addition rates can vary by several orders of magnitude (e.g., CH3CHNH2 + O2 at 5.5 × 10–11 cm3 s–1, while C3H5 + O2 at 1.6 × 10–13 cm3 s–1 under practically identical low-pressure reaction
conditions86,87), and in certain cases this additional
complexity could be important to account for. This competition for
the oxidation propagation continues throughout the whole HOM formation
sequence and is of special importance for each radical intermediate.
The competition between different RO2 H-shift sites is
likely the most determining factor in HOM formation and currently
very poorly constrained, as only a few RO2 H-shift rates
have been inspected experimentally (see, e.g., refs (19) and (43)).</p><p>The H-scrambling
reactions in the oxidized radicals (e.g., H-shift
between hydroperoxide and peroxy radical functional groups88,89) can severely complicate the assessment of correct pathways. It
leaves no marks to the transformed product chemical composition and
is generally a very fast reaction step. Previously it was noted to
yield peroxy acids at the expense of hydroperoxides, yet in the cyclohexene
ozonolysis system an NO2 addition to the reaction mixture
apparently leads to the formation of highly oxygenated acylperoxynitrates.81 Mechanistically the H-scrambling reaction is
very intriguing, as it can in a sense reverse the course of the reaction
and return the radical site to its previous location.</p><p>The role
of oxidation propagation, or even initiation, by catalytic
pathways is currently poorly understood. Recently Monge-Palacios et
al.90 showed how formic acid can catalyze
the conversion of stabilized Criegee intermediates to vinylhydroperoxides
(VHP), enabling a reinitiation of the oxidation sequence. Similarly
the role of photolysis in inducing HOM formation has not been assessed.
The photolysis of several oxidation products,91 even RO2 radicals,92 could
provide an alternative route to the HOM formation. The aromatics and
their derivatives provide an exceptionally good example, as the absorption
spectra of the first- and second-generation oxidation products shift
to progressively longer wavelengths and higher absorption cross sections,
facilitating their photolysis under a solar illumination.91 However, the concentration of every new product
generation is likely to be lower than that of the parent, and thus
the influence is likely to stay minor. Nonetheless, this could prove
to be a significant source, for example, during early mornings when
solar irradiance quickly increases and the accumulated processed material
from the previous night photolyzes.93</p><p>In any real-life VOC oxidation system the cross reactions of different
RO2 leading to oxidation propagation and termination will
have a special importance, with the rates of common RO2 + R′O2 reactions spanning many orders of magnitude.94 Especially for the AVOC HOM formation the RO
involvement seems critical (Figures 4 and 5). Even HO2 reactions, which are generally waived by the implication that they
only lead to ROOH, can contribute to the RO budget, and in the need
for a truly realistic HOM formation description, they must be taken
into account.82 Critical are also the branches
to minor channels (e.g., minor H abstraction pathway to produce a
resonance-stabilized radical in competition with a ring closure by
endoperoxidation) and will remain difficult to account for. These
are problematic, especially in cases where the rest of the potential
energy surface of the minor channel would be far more attractive for
an HOM formation but is ruled out during an early inspection of the
mechanism due to computational limitations, as generally to find even
one pathway with rigorous treatments is a laborious task.</p><p>The
overarching realization is that HOM with a usual 0.1–10%
yield generally fit into uncertainties of the previous studies. Thus,
finding out the small differences in the pathways that enable HOM
formation will remain very challenging but is likely to be the key
in finding the HOM from the representative AVOCs. Furthermore, without
significant leaps in detection and computational methodologies, it
will remain very difficult to ascertain the importance of these less-clear
oxidation sequences. Nevertheless, with the recent surge on the computational
method development that exploits machine- and deep-learning strategies
enabled by ever-increasing computing power, the needed theoretical
advancements do not seem that science fiction anymore. Nonetheless,
the experimental techniques have more real-life physical limitations
to overcome and, thus, are likely to lag behind. Time will show.</p><!><p>While the decade of work on HOM formation by atmospheric autoxidation
has increased our understanding of complex organic oxidation in the
gas phase, several dark spots still exist and prevent us from describing
the formation of HOM from the diversity of AVOC. The lessons learned
about the likelihood of H-shift reactions are directly transferrable
between BVOC and AVOC, yet far more experimental-theoretical studies
are required to solidly anchor the underlying chemistry. As a topical
example, also the autoxidation of the first- and second-generation
reaction products must be studied (i.e., classical oxidation potentially
leading to autoxidation), not the least by remembering that the H-shift
reactions are faster in the oxidized intermediates than in the naked
parent hydrocarbons. Currently this has been only briefly inspected
by Wang et al.33 and Garmash et al.39</p><p>The rapid reaction sequence propagation
by alkoxy radicals and
carbon-centered radical (C*) rearrangements are likely of high importance
for an accurate description of the reaction mechanisms but lack methodology
for their experimental inspection. The commonly applied low-pressure
and low-O2 environments in VOC oxidation research have
a high potential to skew the results by limiting the collision frequency
and, in considering the real atmosphere, collisions with the correct
coreactants. The theoretical RO structure activity relationships (SAR)
by Vereecken et al.83,95 are a good starting point in
estimating the relevance of alkoxy radicals, whereas apparently nothing
similar exists for the C*. While SAR are a common and practical tool
in atmospheric chemistry research, such simplifications are also a
big part of the reason why it took such a long time to find the pathways
to HOM in the first place. In considering ambient HOM formation, we
are still far away from any useful SAR development, and thus system-specific
treatments are a must. Ultimately, this research will evolve enough
to allow for an SAR development, but when, and how, is still too early
to say.</p><!><p>The author declares no
competing financial interest.</p><p></p><p>Matti Rissanen is currently a tenure track assistant
professor
and the lead of the Radical Aerosol Physical Chemistry research group
in the Aerosol Physics laboratory of Tampere University. He obtained
his M.Sc. (Honors) and Ph.D. (Laudatur) from the Physical Chemistry
laboratory of the University of Helsinki studying the temperature-
and pressure-dependent kinetics of gas-phase free radical reactions.
He is an author of over 90 publications, and his main research focuses
on the in situ formation of atmospheric aerosol precursors and the
related development of research instrumentation. He is a Finnish Academy
Research Fellow and a holder of the prestigious European Research
Council Consolidator Grant.</p>