Introduction
Organic aerosol (OA) constitutes a large proportion of ambient particulate
matter, affecting the Earth's radiation balance, cloud formation and human
health (Hallquist et al., 2009). Understanding and simulating the
concentration and composition of OA particles is one of the major challenges
of modern atmospheric chemistry. In the mid-2000s the discovery that
oxidized organic compounds dominate in concentration compared to that of
primary organic compounds outside urban areas (Zhang et al., 2007; Jimenez et
al., 2009), together with the understanding that the ambient organic aerosol
concentrations were systematically underpredicted by existing chemical
transport models (Heald et al., 2005), led to a reevaluation of the treatment
of secondary organic aerosol (SOA) formation processes in chemistry and
climate models. Since the model–measurement gap is mostly overcome by
subjecting the particles to “oxidative ageing”, understanding the nature of
ageing processes has become a primary objective of new generation SOA
studies. Experimental findings showing the existence of highly oxidized SOA
molecular tracers with a high oxygen-to-carbon (O / C) ratio (Szmigielski
et al., 2007) and molecular structures that are chemically distinct from
first-
and second-generation oxidation products of the same precursors (Jenkin et al.,
2000) have provided indirect confirmation of still unknown chemical processes
forming highly oxidized, low-volatility compounds.
The first formulations of SOA ageing into models were based on the chemistry
of saturated hydrocarbon oxidation by OH, for which a step-by-step process
with a slow, progressive increase in the oxidation state, along with a
decrease in volatility, can be proposed (Robinson et al., 2007). The
duration of such processes clearly exceeds the residence time of SOA in
traditional environmental chamber experiments with equivalent atmospheric
ageing times of less than 1 day. These limitations led to the emergence of
oxidation flow reactors that are capable of higher integrated oxidant
exposures, including the potential aerosol mass (PAM) oxidation flow reactor
(Kang et al., 2007; Lambe et al., 2011) and related techniques (Hall IV et
al., 2013; Keller and Burtscher, 2012; Slowik et al., 2012). Recent studies
suggest that flow-reactor-generated SOA particles have similar composition
to SOA generated in chambers (Lambe et al., 2015; Bruns et al., 2015).
Modeling work further suggests that flow reactors simulate tropospheric
oxidation reactions with minimal experimental artifacts (Li et al., 2015;
Peng et al., 2015, 2016). Recent applications of oxidation flow reactors in
field measurements showed that the maximum yields of SOA were attained at
approximately 2–3 days of equivalent atmospheric OH oxidation; at higher
photochemical age, SOA yields decrease substantially (Tkacik et al., 2014;
Ortega et al., 2016; Palm et al., 2016). Such observations demonstrate the
influence of fragmentation reactions in which oxidation leads to C–C bond
cleavage with the production of highly volatile products (Kroll et al.,
2009; Chacon-Madrid and Donahue 2011; Lambe et al., 2012).
The idea of a slow, multi-generation SOA ageing was recently challenged by
recent findings from reaction chamber experiments employing modern chemical
ionization mass spectrometric methods. For example, it was found that
oxidized gaseous compounds with O / C > 0.7 form readily
upon VOC oxidation (Ehn et al., 2012, 2014; Krechmer et al., 2015; Rissanen
et al., 2014) and that even the chemical tracers of “aged” SOA can be in
fact produced among second-generation oxidation products (Müller et al.,
2012). The quantification of highly oxidized SOA compounds in reaction
chambers is challenging because of significant vapor and particle wall losses
(Matsunaga and Ziemann, 2010; Zhang et al., 2014; Krechmer et al., 2016; Ye
et al., 2016), but these findings suggest that SOA ageing can be much faster
than previously thought (Hodzic et al., 2016). As a result of the diverse
implementations of SOA schemes in models, the quantification of SOA
production and concentration in the atmosphere is still highly uncertain: a
recent intercomparison between 20 state-of-the-art global models showed that
the estimated SOA annual production rates differ by 1 order of magnitude
(Tsigaridis et al., 2014). These results call for more experimental
observations for constraining the existing SOA parameterizations.
The present study focuses on laboratory production of SOA from three
different precursors using a PAM reactor. The novel feature of this work is
our application of two offline analytical techniques that provide valuable
insight in regards to SOA composition yet are rarely employed for SOA
characterization. The first technique, 1H-NMR spectroscopy, is a
universal technique in organic chemistry. It has been used to confirm the
molecular structures of many SOA tracers (Finessi et al., 2014) or for
following SOA reaction products in aqueous solution (Yu et al., 2011). The
few examples of 1H-NMR spectroscopy on SOA complex mixtures (Cavalli et
al., 2006; Baltensperger et al., 2008; Bones et al., 2010; White et al., 2014)
indicate that the technique can be very specific for distinguishing different
biogenic and anthropogenic SOA systems. The present study is among the first
applications of 1H-NMR spectroscopy to SOA samples produced from the OH
oxidation of biogenic and anthropogenic SOA in the laboratory. The
acquisition of 1H-NMR fingerprints for fresh and aged biogenic and anthropogenic
SOA can also be useful for interpreting factor analysis results obtained on a
timeline of 1H-NMR spectra of ambient aerosol extracts (e.g., Paglione et al.,
2014a). The second technique is an HPLC setup for the determination of
humic-like substances (HULIS). HULIS have been observed in ambient organic
aerosol for nearly two decades (Havers et al., 1998; Limbeck et al., 2003),
but their formation pathways aside from production in biomass burning plumes
remain unclear (Graber and Rudich, 2006). It is well known that
high-molecular-weight oxygenated organic compounds readily form by
heterogeneous reactions (Limbeck et al., 2003) or by gas-to-particle
conversion (Kalberer et al., 2006), but there is little evidence for their
identification with HULIS in ambient aerosols (especially if we base the
definition on the chromatographic behavior, as in Baduel et al., 2009).
Here we focus on SOA systems generated from three distinct precursors:
isoprene, α-pinene and naphthalene. Naphthalene is used as proxy for
anthropogenic aromatic intermediate-volatility organic compounds (IVOCs).
α-Pinene is the most studied biogenic monoterpene due to its global
importance as a biogenic SOA precursor (e.g., Pye et al., 2010), while
isoprene is the most abundant biogenic VOC, accounting for 44 % of global
emissions (Guenter et al., 1995). The discovery of isoprene SOA is relatively
recent (Claeys et al. 2004). In the presence of acidic wet aerosols, SOA
originates from the heterogeneous uptake of isoprene epoxides (“IEPOX
channel”; Lin et al., 2012). Since aerosol water and acidity are primarily
determined by anthropogenic mineral acids, the formation of SOA from isoprene
appears to be very much controlled by anthropogenic emissions. On the other
hand, recent experiments conducted at very low nitrogen oxide (NO)
concentrations, and in the absence of seed aerosols, have showed that SOA can
still form from isoprene (“non-IEPOX” SOA; Krechmer et al., 2015). Such
aerosols are more representative of the preindustrial world and their
characterization is of paramount importance for understanding the climate
radiative forcing of SOA at the global scale. Our results, obtained in the
PAM reactors in the absence of NOx, are representative for non-IEPOX isoprene
SOA.
Experimental methods
PAM oxidation flow reactor
The PAM oxidation flow reactor is a horizontal 13 L glass cylindrical
chamber that is 46 cm long × 22 cm ID. Carrier gas flows of
8.5 L min-1 N2 and 0.5 L min-1 O2 were used, with
8.5 L min-1 of flow pulled through the reactor and 0.5 L min-1
of excess flow removed prior to the reactor. Other experimental details are
fully described in Lambe et al. (2011). In this study, the PAM reactor was
connected to a scanning mobility particle sizer (SMPS), an Aerodyne
time-of-flight aerosol mass spectrometer (AMS), and a filter holder equipped
with 47 mm (prebaked) quartz-fiber filters. SOA concentrations calculated
from SMPS and/or AMS measurements averaged over filter collection times
provided an estimate of the organic matter loading on the filters.
During the first set of experiments involving α-pinene and
naphthalene as SOA precursors, by varying the concentrations of OH inside the
PAM reactor, SOA with a different oxidation state could be obtained. For
instance, the OH exposure varied from
2.0 × 1011 molec cm-3 × s to
2.1 × 1012 molec cm-3 × s between the α-pinene
experiments and the resulting SOA oxidation degree – traced by the “f44”
parameter (i.e., the fraction of the m/z 44 signal with respect to the
total OA) – increased from 0.05 to 0.24. A total of five α-pinene
and five naphthalene SOA samples were obtained (collection time between 3 and
20 h), with integrated OH exposures varying between 2 × 1011
and 2 × 1012 molec cm-3 × s, corresponding to a
photochemical age of 1.5 to 15 days assuming a 24 h average OH concentration
of 1.5 × 106 molec cm-3 (Mao et al., 2009).
During the second set of experiments, isoprene SOA samples were generated in
the reactor (Table 2). Due to the lower yields of SOA produced by isoprene
oxidation, samples were collected at OH exposure of approximately
8 × 1011 molec cm-3 s (corresponding to a photochemical
age of 6 days) at which the maximum SOA yield is obtained (Lambe et al.,
2015). The collection time was varied between 2 and 18 h.
PAM experimental conditions for naphthalene and α-pinene
SOA ageing studies.
Sample
Oxidation
PAM
OH exposure
Collection
Average
OM on filter
level
lamp
(molec cm-3 × s)
Time (h)
AMS OM
based on AMS
voltage
concentration
(RIE × CE = 1.4)
(V)
(RIE × CE = 1.4)
(µg)
(µg m-3)
α-Pinene
Pin#1
High (f44 = 0.24)
110
2.10 × 1012
18.5
46.1
384
Pin#2
Med. (f44 = 0.11)
75
1.10 × 1012
3.2
151.6
221
Pin#3
Med. (f44 = 0.11)
75
1.10 × 1012
20.2
14.1
129
Pin#5
Low (f44 = 0.05)
30
2.00 × 1011
20.5
7.5
69
Pin#6
Low (f44 = 0.05)
30
2.00 × 1011
7.1
50.3
161
Naphthalene
Nap#1
High f44 = 0.30)
110
2.10 × 1012
19.7
31.3
277
Nap#2
Med. (f44 = 0.19)
75
1.10 × 1012
7
55.3
174
Nap#3
Low (f44 = 0.084)
30
2.00 × 1011
6.6
16.9
50
Nap#4
Med (f44 = 0.20)
75
1.10 × 1012
6.6
42.9
127
Nap#5
Low (f44 = 0.074)
30
2.00 × 1011
15.8
34.7
246
Blanks
Blk#1
110
2.10 × 1012
6.1
0
0
Blk#2
110
2.10 × 1012
23.2
0
0
Blk#3
110
2.10 × 1012
6.1
0
0
PAM experimental conditions for isoprene SOA ageing studies.
Sample
Oxidation
PAM
OH exposure
Collection
Average
OM on filter
level
lamp
(molec cm-3 × s)
Time (h)
AMS OM
based on AMS
voltage
concentration
(RIE × CE = 1.4)
(V)
(RIE × CE = 1.4)
(µg)
(µg m-3)
Isoprene
Iso#1
Med (f44 = 0.046)
60
7.8 × 1011
3.7
409
651
Iso#2
Med (f44 = N.A.)
60
7.8 × 1011
2.8
575
700
Iso#3
Med (f44 = 0.039)
60
7.8 × 1011
2.2
678
656
Iso#4
Med (f44 = 0.037)
60
7.8 × 1011
2.9
551
684
Iso#5
Med (f44 = 0.040)
60
7.8 × 1011
16.1
685
4959
Iso#6
Med (f44 = 0.066)
60
7.8 × 1011
3.9
767
1280
Iso#7
Med (f44 = 0.059)
60
7.8 × 1011
3.8
593
986
Blanks
Blk#1
60
7.8 × 1011
15.9
0
0
Blk#2
60
7.8 × 1011
3.1
0
0
Blk#3
60
7.8 × 1011
16.3
0
0
To compare SOA formation processes occurring in oxidation flow reactors and
in the atmosphere, two primary assumptions are required. First, we assume the
kinetics of laboratory processes occurring at higher oxidant concentrations
and shorter exposure times can be extrapolated to atmospheric processes
occurring at lower oxidant concentrations and longer residence times. Second,
we assume that the extent of nucleation or phase partitioning of SOA is not
limited by the shorter residence time in flow reactors. The first assumption
is supported by Renbaum and Smith (2011), Bahreini et al. (2012), and Lambe
et al. (2015). The second assumption may introduce uncertainty depending on
the particle surface area available to promote condensation and the mass
accommodation coefficient of the oxidized vapors (Lambe et al., 2015;
Shantanu et al., 2017).
Extraction and offline sample characterization
Each filter was extracted with 5 mL of deionized ultra-pure water (Milli-Q)
in a mechanical shaker for 1 h and the water extract was filtered on PTFE
membranes (pore size: 0.45 µm) in order to remove suspended
particles. The water extracts were dried by rotary evaporator and were then
re-dissolved in 2.15 mL of D2O: 0.65 mL for proton-nuclear magnetic
resonance (1H-NMR) characterization (Decesari et al., 2000), and 1.5 mL
for HPLC analysis and total organic carbon (TOC) analysis (Mancinelli et al.,
2007). Tests of extraction using methanol instead of water were carried out
on three isoprene SOA samples. The 1H-NMR spectra of methanol extracts
were completely consistent with those obtained for the other three analyzed
in deuterated water (Fig. S2 in the Supplement), indicating that there were
no specific classes of water-insoluble compounds in the isoprene SOA under
the conditions used in this study. The following discussion will focus on the
water-soluble fraction for which spectroscopic and chromatographic data were
obtained for all three SOA systems.
<inline-formula><mml:math id="M140" display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula>H-NMR spectroscopy
The 1H-NMR spectra were acquired at 600 MHz with a Varian
600 spectrometer in a 5 mm probe with 0.65 mL of each sample re-dissolved
in D2O. Sodium
3-trimethylsilyl-(2,2,3,3-d4) propionate
(TSP-d4) was used as the referred
internal standard. A buffer of potassium deuterated formate/formic acid
(pH ∼ 3.8) was used in the second series of experiments (isoprene SOA)
to stabilize the chemical shift of hydrogen atoms in acyl functional groups,
while the extracts obtained during the first experiments (α-pinene
and naphthalene) were analyzed unbuffered. 1H-NMR spectroscopy of
low-concentration samples in protic solvents provides the speciation of
hydrogen atoms bound to carbon atoms (H-C). On the basis of the range of
frequency shifts (the chemical shift, in ppm) in which the 1H-NMR resonances
occur, they can be attributed to different H-C-containing functional groups
(Paglione et al., 2014a).
1H-NMR spectra of α-pinene SOA as a function of
increasing photochemical age in the potential aerosol mass (PAM) oxidation
flow reactor. The sharp singlet at 0 ppm represents the internal standard
(TSP-d4), while the broad peak at 4.8 ppm is the – partly instrumentally
suppressed – hydrogen deuterium oxygen (HDO) peak. The sharp
singlets between 0.9 and 2.2 ppm in the fresh SOA samples (Pin#5 and
Pin#6) are genuine bands of the samples and were identified as methyl
groups of pinic and pinonic acid.
![]()
HPLC-UV-TOC method
HULIS were determined using the ion exchange chromatographic method described
by Mancinelli et al. (2007). A HPLC system (Agilent model 1100) with gradient
elution was used. The subsequent elution of chemical compounds bearing zero,
one, two or more than two ionized groups per molecule (mainly carboxylate
ions at pH 7) is monitored by a UV detector at 260 nm. Downstream of the
detector, a fraction collector is programmed to sample separately “neutral
compounds” (NCs), “monocarboxylic acids” (MAs), “dicarboxylic acids” (DAs),
and “polycarboxylic acids” (PAs) or HULIS. The amount of WSOC recovered in
each fraction is determined offline by TOC analysis using an Analytik-Jena
multi-analyzer N/C (model 2100). The HPLC column and chromatographic
conditions used in this study were the same as in Mancinelli et al. (2007).
Further information on the nature of the chemical classes separated by the
HPLC method based on elution tests of standard compounds, including
discussion of possible misclassification, is reported by Decesari et
al. (2005).
1H-NMR spectra of naphthalene SOA as a function of increasing
photochemical age in the PAM reactor. The sharp singlet at 0 ppm
represents the internal standard (TSP-d4), while the broad peak at 4.8 ppm is the – partly instrumentally
suppressed – HDO peak.
![]()
Results
<inline-formula><mml:math id="M154" display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula>H-NMR results
<inline-formula><mml:math id="M155" display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula>H-NMR fingerprints of fresh and aged <inline-formula><mml:math id="M156" display="inline"><mml:mi mathvariant="italic">α</mml:mi></mml:math></inline-formula>-pinene SOA
Figure 1 shows the 1H-NMR spectra of α-pinene SOA with
increasing photochemical age. The first spectrum corresponding to a “low”
SOA oxidation level is similar to reported 1H-NMR spectra of
environmental chamber-generated α-pinene ozonolysis (Cavalli et al.,
2006). However, the 1H-NMR fingerprint of α-pinene SOA evolves
rapidly with further oxidation steps. A clear, progressive disappearance of
first-generation oxidation products (pinic and pinonic acid) with an
increasing O / C ratio can be observed. In the 1H-NMR spectra
corresponding to a “medium” SOA oxidation level, the resonance at 0.83 ppm
of chemical shift, arising from one of the two gem-methyls of pinonic acid,
accounts for only 0.3 % of the total integral of the spectrum, while it
represented 2–3 % in the fresh SOA samples. This indicates that α-pinene SOA composition evolves rapidly towards more highly oxidized
molecular structures, with little resemblance to first-generation oxidation
products. At “medium” and “high” oxidation levels, the unsubstituted
alkyl groups of the SOA mixture give rise to a broad Gaussian band between
1.1 and 1.8 ppm of chemical shift with a maximum at 1.4–1.5 ppm. The
middle point position showing a slight deshielding with respect to a purely
alkylic chain (∼ 1.3 ppm for fatty acids) indicates the presence of
electronegative groups (such as oxygen atoms) in beta or gamma position with
respect to these alkyl groups. The band between 1.9 and 3 ppm, attributable
to C–H groups of acyl groups (HC-C = O), also shows a transition towards
structures containing more deshielded H atoms. In all cases, the most
conspicuous band in this spectral region is found at 2.2–2.3 ppm of
chemical shift, which corresponds to acetyl and acyl groups of aliphatic
compounds with a low O / C ratio, like pinonic acid
(R-CH2-C = O and CH3-(C = O)-R, where R is mainly a
CxH2x+1 radical). Such a band persists in all SOA samples, but an
additional band between 2.5 and 2.9 ppm is observed in the two samples with
the highest O / C ratio, indicating that the aliphatic groups become more
and more substituted by electronegative groups: the keto and carboxylic
groups become spaced by no more than two methylene (or methyns) groups
(x(C = O)-CH-CH-(C = O)X, where X is a generic substituent).
Finally, the 1H-NMR resonances in the third important aliphatic region
of alkoxy
groups (CH-O), between 3.3 and 4.2 ppm of chemical shift, are always
relatively small, with peak intensity at intermediate photochemical age. SOA
species that contribute to 1H-NMR resonance in this region may be
correlated with semivolatile, highly functionalized species that contribute
to maximum SOA yields observed at intermediate OH exposures (Lambe et al.,
2015).
Overall, the 1H-NMR fingerprint of α-pinene SOA is highly dependent on
photochemical age, with a sharp change already at medium ageing. The most
oxidized samples show spectral features that have lost any clear similarity
with those of SOA sampled in reaction chambers experiments without ageing
(Cavalli et al., 2006).
(a) 1H-NMR spectrum of isoprene SOA generated in the
PAM reactor at an OH exposure of
8 × 1011 molec cm-3 s.
The bottom trace shows the same spectrum with enlarged the broad background
bands. Panels (b) and (c) show the methyltetrols resonances
between 3.4 and 3.9 ppm and 1.12–1.13 ppm, respectively.
![]()
<inline-formula><mml:math id="M192" display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula>H-NMR fingerprints of fresh and aged naphthalene SOA
The 1H-NMR spectra of naphthalene SOA samples with an increasing
O / C ratio are presented in Fig. 2. The extract of the least-oxidized
sample (on the top) shows broad resonances between 0 and 3 ppm probably due
to the effect of colloidal hydrophobic material in solution. Despite such an
artifact, all spectra of fresh and moderately aged SOA show clear signals
from aromatic structures (region at chemical shift from 6.5 to 8.5 ppm) and
alkenes (approximately between 5 and 7 ppm). The 1H-NMR spectra of
naphthalene SOA are very different than 1H-NMR spectra of SOA produced
from the OH oxidation of one-ring aromatic VOCs (Baltensperger et al., 2008),
which have mostly aliphatic groups originating from ring opening reactions.
Our data are in agreement with molecular speciation studies (Lee and Lane
2009), indicating that naphthalene is oxidized to form one- or two-ring
aromatic compounds such as naphthol, as well as substituted benzoquinones,
cinnamic acid, and phthalic acid (Chhabra et al., 2015). The chemical shift
range of the main aromatic band, between 7.4 and 8.1 ppm, indicates that
aromatic rings are substituted prevalently by electron-withdrawing groups,
such as carbonyl and carboxyls. At moderate ageing states, a small band at
6.9–7.1 ppm indicates the formation of phenolic structures. The 1H-NMR
spectra of fresh naphthalene SOA show several singlets in the aromatic
region, indicating a diversity of individual compounds occurring in
relatively high concentrations, while moderately aged SOA show mainly the two
singlets of phthalic acid. All spectra contain signals at lower chemical
shifts with respect to the aromatics, mainly between 3.5 and 6.0 ppm (with
the interference of the partly suppressed peak of water in the middle):
several functional groups can give rise to these bands, including
alkoxyls, peroxides, esters,
hemiacetals and acetals, and vinyls.
The spectrum of the most aged sample (NAPTH 1), which is also the one with
the lowest SOA concentration, shows an interference from α-pinene SOA
in the aliphatic region. This feature is likely due to experimental setting
contamination from a previous α-pinene SOA experiment. Despite the
low naphthalene SOA concentration, a broad aromatic band between 6.5 and
8.5 ppm and the same signals found between 3.5 and 6.0 ppm seen in the
samples with a medium O / C ratio are still visible in this most aged
naphthalene SOA spectrum. However, the band from oxygenated functional groups
between 3 and 4.5 ppm becomes relatively more intense with respect to
aromatics compared to SOA samples of smaller ageing state. Compared to
α-pinene SOA, the 1H-NMR fingerprint of naphthalene SOA appears
less sensitive to variations in the OH exposure between the low and the
medium level of exposure. More substantial changes can be found for the most
oxidized sample, which are only partly visible due the low signal-to-noise
ratio of the spectrum.
<inline-formula><mml:math id="M203" display="inline"><mml:msup><mml:mi/><mml:mn mathvariant="normal">1</mml:mn></mml:msup></mml:math></inline-formula>H-NMR fingerprints of non-IEPOX isoprene SOA
The samples of isoprene SOA were obtained from the same OH exposure
(corresponding approximately to a “medium” exposure in the α-pinene
and naphthalene experiments) and differed only for collection time and sample
quantity loaded on the filter. The isoprene SOA 1H-NMR spectra profiles
were all very similar (an example is provided in Fig. 3). The comparison with
literature data (Budisulistiorini et al., 2015) led to the unambiguous
identification of 2-methyltetrols, clearly responsible for the two singlets
at 1.12 ppm (methylic H atoms of methylerythritol) and 1.13 ppm (methylic H
atoms of methylthreitol) and for a series of multiplets between 3.4 and
3.9 ppm. Methylerythritol is more abundant (60 % of the sum of the two,
as an average between two samples extracted in water and three extracted in
methanol) than methylthreitol. The spectra show the occurrence of only two
diastereomers among the possible four ones (González et al., 2011),
indicating that the formation of methyltetrols is stereoselective, as already
proposed by Cash et al. (2016) on the basis of a theoretical analysis of the
IEPOX chemistry, and in contrast with the conclusions of González et
al. (2011), claiming that methyltetrols are produced in laboratory conditions
only in racemic mixtures. The two methyltetrols account for 65 % of the
total 1H-NMR signal, the rest being characterized by broad background
signal with very few sharp resonances, indicating that the isoprene SOA
samples are composed mainly of methyltetrols together with a significant
amount of mass composed of a very complex mixture of products. The unresolved
background resonances are located below the peaks of the methyltetrols,
suggesting that the complex mixtures (which can include also oligomeric
species) encompass molecular species (or monomers) similar to methyltetrols
(at least in their C–H backbone). However, the range of chemical shifts of
the background bands characterize molecular species with more electronegative
groups (leading to more deshielded H atoms) than methyltetrols: the band of
methylic protons extends to 1.7 ppm (with respect to 1.12–1.13 of
methyltetrols) and the band of alkoxy groups (HC-O) extends to 4.3 ppm. The results of Liu et al. (2016),
indicating that non-IEPOX isoprene SOA includes peroxide equivalents of
methyltetrols, are in agreement with these findings. However, Riva et
al. (2016) reported only peroxides for non-IEPOX SOA in unseeded experiments
and no methyltetrols. It is possible that peroxides decomposed to tetrols in
our filter samples during collection downstream the PAM or afterwards during
storing. The actual stability of isoprene hydroperoxides in the aerosol
itself is largely uncertain; therefore the discrepancy between the findings
presented in this study and the results of Riva et al. (2016) cannot be
clarified at this stage. In addition, neither Liu et al. (2016) nor Riva et
al. (2016) reported the presence of carboxylic or keto groups, while our data
clearly indicate that these (and/or other acyl groups) are found in the
unresolved mixtures of non-IEPOX isoprene SOA and are responsible for the
signal band between 2.0 and 2.6 ppm. Still, this band is much less intense
than that of alkoxyls, which is
opposite to what observed for α-pinene SOA, where acyls are by far
the main oxygenated aliphatic functional group. Thus, 1H-NMR
spectroscopy provides a distinct fingerprint for isoprene and monoterpene
SOA.
HPLC chromatograms of α-pinene SOA water extracts.
Chromatographic features are grouped into neutral, mono-/dicarboxylic acid,
and polycarboxylic acid classes based on their affinity for the column
phase. Sample identifications are provided in Table 1.
![]()
HPLC results
The HPLC analysis of fresh α-pinene SOA extracts shows the presence
of compounds unretained by ion-exchange columns (neutral compounds) or weakly
retained (mono- and diacids) with a small contribution from compounds having
a high retention factor (polyacids, PA, or HULIS), in agreement with previous
results obtained from α-pinene SOA samples generated in environmental
chamber experiments (unpublished data). It should be noted, however, that the
chromatographic analysis of SOA compounds in water extracts generally does
not allow for recovery of high-molecular-weight organic oligomers susceptible to
hydrolysis reactions (e.g., polyacetals; Kroll and Seinfeld, 2008). The HULIS
determined by our method are essentially only the non-hydrolyzable ones,
stable in aqueous solutions. The HULIS content increases only moderately with
ageing, while the yield/fraction of diacids increases significantly with
respect to monoacids and neutral/basic compounds (Fig. 4). With increasing
photochemical age, the total organic carbon (TOC) mass fractions of
monoacids, diacids and HULIS increase from 20 to 34 % and 7 to
16 %, respectively, whereas the mass fraction of neutral compounds
decreases from 19 to 9 % (Fig. S3). These results are in qualitative
agreement with the known chemistry of α-pinene SOA, in which mono-
and dicarboxylic acids are the most characteristic condensable first-generation
products (Jenkin et al., 2000; Jaoui and Kamens, 2001), while tricarboxylic
acids such as 3-methyl-1,2,3-butanetricarboxylic acid or pinyl-diaterpenyl
ester (Szmigielski et al., 2007; Yasmeen et al., 2010) are present in lesser
amounts and can contribute the observed concentrations of HULIS in this
study.
The HPLC fractionation of naphthalene SOA (Fig. 5) shows that fresh samples
are characterized by a mixture of neutral compounds and mono- and diacids,
completely consistent with the molecular compositions reported in the
literature (Lee and Lane, 2009). However, a net increase in acidic compounds
with photochemical age can be clearly observed. However, the HULIS content,
initially small, increases substantially and progressively with ageing. With
increasing photochemical age, the TOC mass fraction of mono- and diacids
decreases from 33 to 18 % and from 34 to 33 %, respectively, while the
fraction of PA/HULIS increases from 11 to 30 % (Fig. S4). This is the
first evidence of HULIS formation (determined with the ion-exchange method)
in laboratory-generated SOA. The formation of polyacidic molecules with three
or more carboxylic groups implies the opening of the second aromatic ring in
the naphthalene precursor backbone, and/or oligomerization reactions. In both
cases, products of such oxidation reactions cannot be explained by current
naphthalene SOA molecular speciation studies (e.g., Kautzman et al., 2010).
HPLC chromatograms of naphthalene SOA water extracts.
Chromatographic features are grouped into neutral, mono-/dicarboxylic acid,
and polycarboxylic acid classes based on their affinity for the column
phase. Sample identifications are provided in Table 1.
![]()
HPLC chromatograms of isoprene SOA water extracts. Chromatographic
features are grouped into neutral, mono-/dicarboxylic acid, and
polycarboxylic acid classes based on their affinity for the column phase.
Sample identifications are provided in Table 2.
![]()
Pearson correlation coefficient between 1H-NMR spectra of
PAM-generated SOA and ambient PEGASOS WSOC.
![]()
Finally, the HPLC analysis of isoprene SOA shows that neutral
compounds (NCs) were dominant in all sample extracts (Fig. 6). NCs accounted
for 59 % of the TOC content of the sum of the HPLC fractions. The second
most abundant fraction (34 %) is the monoacids, while diacids accounted
for a much smaller fraction of TOC, and polyacids were almost absent (ca.
1 % of the TOC). The dominance of NCs is consistent with the high yield
methyltetrols and their analogues (see Sect. 3.1.3). Assuming that the
distribution of 1H-NMR functional groups approximately reflects their
carbon content, methyltetrols (accounting for 65 % of the total 1H-NMR
signal) can account for the whole of the HPLC neutral compounds and, as a
corollary, the complex mixtures of products detected by unresolved bands by
1H-NMR spectroscopy correspond to the mono- and diacids in HPLC. As
already noted in the previous section, the 1H-NMR analysis indeed shows
the occurrence of acyl groups which indicate/support the presence of
carboxylic acids. We cannot exclude, however, possible misclassification of
some neutral compounds into the monoacid fraction, as already observed for
some dicarbonyls (Decesari et al., 2005). It is worthwhile to clarify that
the definition of chemical classes is based uniquely on the retention factor
on the anion exchange and is therefore sensitive to chromatographic secondary
interactions and to chromatographic conditions. Such (unwanted) effects can
explain the difference in speciation between samples Pin#2 and Pin#3,
both obtained at medium oxidation state (Fig. S3). Pin#3 was injected at
significantly higher concentrations than the other samples (about 3 times
than Pin#2), which can have caused the elution of some acidic compounds
along with the unretained NC fraction. The results of HPLC analysis of this
particular sample are therefore excluded from the following discussion.
Distribution of HPLC fractions (total recovered TOC
content = 100 %) for α-pinene SOA (a), naphthalene
(b), and for ambient OA sampled in Cabauw, Netherlands
(c).
![]()
Correlation plot between the AMS f44 of SOA and the HULIS fraction
of HPLC-eluted WSOC.
![]()
Discussion and conclusions
In this section, the 1H-NMR and HPLC results obtained for the isoprene,
α-pinene and naphthalene SOA systems are compared with ambient OA
samples. First, we investigated the similarity between the 1H-NMR
spectral profiles of SOA with those “typical” of ambient
non-biomass-burning WSOC. For this purpose we used one sample of
PM1 collected during the 2012 PEGASOS field campaign (Sandrini et al.,
2016) in the rural Po Valley (Italy) which can be considered representative
for a continental rural “near-city” site (according to the criteria of
Putaud et al., 2010). A second PM1 sample was collected at a rural site in
the State of Rondônia (Brazil) during the 2002 SMOCC field campaign, and,
more precisely, during the early rainy season, when local biomass burning
sources had largely ceased and the organic composition of submicron particles
was dominated by biogenic emissions (Decesari et al. 2006; Tagliavini et al.,
2006). The ambient WSOC and laboratory SOA spectra were binned to 400 points
in order to remove the variability in chemical shifts due to, for example,
different pH conditions during the analyses of the samples. Figure 7 shows
the correlation between the SOA spectra and the reference spectra of ambient
WSOC: the α-pinene SOA and naphthalene SOA spectra were compared to
the Po Valley WSOC sample, while the isoprene SOA spectra were compared to
the Amazonian sample. There is good correlation
(0.62 < r < 0.92) between the 1H-NMR spectra of
α-pinene SOA, at all oxidation levels, with the spectrum of the Po
Valley PM1 sample. This finding is in line with modeling results and previous
experimental findings indicating that the organic composition in northern
Italy in the summertime is dominated by biogenic SOA (Bessagnet et al., 2008;
Gilardoni et al., 2011). A moderate positive correlation was also found
between the spectra of isoprene SOA and of the PM1 sample from rural Brazil
(r=0.52, 0.54). It should be noted that the relative humidity at this
pasture site is variable during the day and very high overnight during the
rainy season, based on the meteorological data presented by Betts et
al. (2009). Therefore, biogenic aerosols are expected to include also
isoprene SOA forming through the IEPOX route (Hu et al., 2015), which is not
accounted for by our laboratory experiments. Finally, the naphthalene SOA
spectra exhibit zero or negative correlations with the Po Valley WSOC
spectrum (-0.15 < r < -0.02). This result is
somewhat expected if we consider that ambient water-soluble aerosols are
characterized by acyl functional groups
(HC-C = O) in higher
concentrations with respect to alkoxy groups (HC-O) and by a smaller
aromatic content (Decesari et al., 2007), with a functional group pattern
that is well reproduced by α-pinene SOA and not by naphthalene SOA.
Clearly, naphthalene SOA alone, with a hydrogen-to-carbon
(H / C) ratio less than 0.9 due to relatively high aromatic content
(Lambe et al. 2011, 2015), does not mimic ambient OA bulk
composition. It should be noted, however, that naphthalene and other
polyaromatic hydrocarbons are co-emitted with many other anthropogenic IVOCs
and VOCs, including aliphatic compounds in the real atmosphere; therefore the
contribution of naphthalene SOA could be simply masked by the contribution of
aliphatic IVOC SOA in the 1H-NMR spectra of ambient WSOC. When limiting
the correlation analysis of Fig. 7 to the aromatic and vinyl region of the
spectra (> 6 ppm), Pearson r coefficients of 0.49 to 0.58 are
found between the naphthalene SOA and the ambient WSOC spectra, while small
values (between -0.2 and 0.4) are found for the α-pinene and
isoprene SOA. This result suggests that SOA produced from naphthalene or
similar precursors, including many other ring-retaining oxidation products
(Sect. 3.1.2), can explain the presence of aromatic moieties in ambient
water-soluble aerosols in areas not affected by biomass burning emissions.
When considering the full spectral range, the 1H-NMR spectra of α-pinene SOA most closely mimic the functional group distributions of the
ambient WSOC sample obtained in PEGASOS (Fig. S1). Interestingly, similarity
between 1H-NMR spectra of α-pinene SOA and Po Valley WSOC
increases with increasing photochemical age. For the most oxidized α-pinene SOA samples, the functional group composition, characterized by
polysubstituted aliphatic compounds rich of acyls (carboxylic or keto
groups), overlaps well with that of ambient WSOC. A good fit between α-pinene SOA and ambient WSOC mass spectral features is already achieved at
medium oxidation conditions, in agreement with the results of Lambe et
al. (2011), showing that the correlation between the AMS spectra of
PAM-generated α-pinene SOA with the spectra of ambient oxygenated OA
(OOA)
increases up to an exposure of 1 × 1012
OH molec cm-3 × s and remains rather stable afterwards. These
results suggest that 1H-NMR-traced ageing processes reflect the same
chemical mechanisms already studied using high-resolution AMS techniques.
However, the correlation coefficients shown in Fig. 7 for the 1H-NMR
spectra of α-pinene vs. ambient WSOC (r2= 0.39 to 0.84) are
smaller than those between the HR-ToF-AMS spectra of PAM-generated SOA vs.
ambient OOA (r2= 0.7 to 0.9) (Fig. 9 in Lambe et al., 2011).
Apparently, the AMS features of ambient OA are more easily reproduced by PAM
experiments than the 1H-NMR composition, or, in other words, 1H-NMR
spectroscopy exhibits a greater selectivity for the OA components than AMS.
Specifically, 1H-NMR spectroscopy was able to resolve significant
changes in composition of α-pinene SOA with photochemical ageing in
great detail, especially at an OH exposure of
∼ 1.1 × 1012 molec OH cm-3 s equivalent to
multiple days of atmospheric ageing. It should be noted, finally, that a
comparison of the AMS and 1H-NMR techniques with respect to their ability
to trace chemical ageing in laboratory SOA and ambient oxidized aerosols is
challenged by the incomplete overlap between the classes of organic compounds
contributing to OOA and to WSOC (Xu et al., 2017).
A comparison of fresh and aged SOA with ambient WSOC samples with respect to
the HPLC fraction distributions is reported in Fig. 8. The distribution of
neutral vs. acidic classes of compounds in ambient WSOC refers to the average
of the samples collected at the rural background station of Cabauw in the
Netherlands (Paglione et al., 2014b). The station is located downwind from
anthropogenic sources and biogenic emissions (terpenes from deciduous
forests) over a large sector of northwest Europe (Henne et al., 2010). The
HULIS contribution in these samples varied between 15 and 20 %, in line
with previous results obtained in the Po Valley (Mancinelli et al., 2007),
but lower than in biomass burning aerosol samples (Decesari et al., 2006).
The α-pinene SOA generated in the PAM reactor at high photochemical
age and the fresh naphthalene SOA are characterized by a HULIS amount similar
to that of Cabauw samples, while the polyacidic content of aged naphthalene
SOA is higher than in the ambient samples. In the real atmosphere,
naphthalene is co-emitted with many other reactive VOC and IVOC with
potentially very diverse HULIS formation yields; therefore, the results
presented in Fig. 8 do not necessarily mean that the chemical composition of
ambient OA in Cabauw is better described by the monoterpene chemistry rather
than by anthropogenic IVOC oxidation. On the other hand, these results
demonstrate that laboratory experiments of SOA formation can generate complex
mixtures of products with the same chromatographic properties of HULIS
provided a sufficient extent of photochemical ageing using the PAM reactor or
related techniques. The HULIS fraction of WSOC is in fact proportional to the
AMS f44 for SOA (integrated over the filter sampling times) (Fig. 9)
irrespective of precursor type. Therefore, the formation of polycarboxylic
acids determined by the HPLC technique follows the same positive trend in
concentrations of the AMS proxy for C(O)OH groups with increasing OH
exposure. This is in contrast with the numerous observations of rapid
formation of SOA oligomers during reaction chamber experiments (Kalberer et
al., 2006; Reemtsma et al., 2006), indicating that oligomers do not account
for chromatographically defined HULIS. A survey of the laboratory studies on
the formation of humic material in secondary aerosol shows that evidence for
the formation of polycarboxylic acids comes from the reaction of phenolic
compounds in the presence of particulate water (Hoffer et al., 2004), while
little is known for unseeded, dry gas-to-particle formation experiments. With
the exception of the very preliminary data reported by Baltensperger et
al. (2008), our results – to our best knowledge – are the first showing the
occurrence of HULIS sensu stricto in monoterpene and aromatic
hydrocarbon SOA, and these HULIS are clearly shown to be a product of
photochemical ageing.
In conclusion, we observed that OA ageing reactions in the PAM reactor
produces water-soluble compounds of high complexity but with spectroscopic
and chromatographic properties that converge towards those characteristic of
ambient OA. Specifically, a good correlation between ambient HPLC/1H-NMR
samples and aged α-pinene SOA was observed in respect to HULIS
content and 1H-NMR functional group distribution, while aged aromatic IVOC SOA
shows clear potential for HULIS formation. The isoprene SOA samples do not
show compositional features with a clear overlap with those of ambient WSOC
obtained in the Cabauw and Po Valley samples that are representative of
continental polluted atmospheres, but they should serve as useful reference
spectra for future studies/environments impacted by non-IEPOX isoprene SOA.