Secondary organic aerosols (SOAs) affect human health and
climate change prediction; however, the factors (e.g., temperature, acidity
of pre-existing particles, and oxidants) influencing their formation are not
sufficiently resolved. Using a compact chamber, the temperature and acidity
dependence of SOA yields and chemical components in SOA from α-pinene ozonolysis were systematically investigated under 278, 288, and
298 K temperatures using neutral ((NH4)2SO4) and acidic
(H2SO4+((NH4)2SO4)) seed aerosols. SOA components
with m/z less than 400 were analyzed using negative electrospray ionization
liquid-chromatography time-of-flight mass spectrometry. Based on the
slightly negative temperature dependence of the SOA yields, the enthalpies
of vaporization under neutral and acidic seed conditions were estimated to
be 25 and 44 kJ mol-1, respectively. In addition, SOA yields increased
with an increase in the acidity of seed particles (solid/near-solid state)
at low SOA mass loadings, when compared with the seed particle amounts.
Acidity dependence analysis of the chemical formula, molecular mass, and O:C
ratio of the detected compounds indicated the enhanced formation of multiple
oligomers in the wide molecular mass range with a wide range of O:C ratios
under acidic seed conditions. The peak abundances of some chemical compounds
increased with an increase in the acidity of seed particles (e.g., m/z 197,
311, 313, 339, 355, and 383), while decreases in the peak abundances of some
chemical compounds were observed (e.g., m/z 171, 185, 215, 343, and 357). The
acidity dependence could be explained by acid-catalyzed heterogeneous
reactions or acid-catalyzed decomposition of hydroperoxides. In addition,
organosulfate (OS) formation was observed under acidic seed conditions. Six
out of the 11 detected OSs were potentially formed via the aldehyde +
HSO4- pathway.
Introduction
Secondary organic aerosol (SOA) in the atmosphere is a complex set of
organic compounds which are formed through oxidation of precursor volatile
organic compounds (VOCs) of either biogenic (e.g., monoterpene and isoprene)
or anthropogenic (e.g., alkanes and aromatics) origins, or both (Hallquist
et al., 2009). SOA plays important roles in the aerosol effect on climate
(Tilmes et al., 2019), air quality (Parrish et al., 2011), and human
health (Shiraiwa et al., 2017). Nevertheless, it is noted in the IPCC Fifth
Assessment Report (AR5) that the formation of SOA has not been included in
the estimation of the radiative forcing from aerosols because the formation
is influenced by a variety of factors not yet sufficiently quantified
(Stocker et al., 2014). However, in association with the advance of research
technologies, processes that influence the growth of SOA particles to sizes
relevant for clouds and radiative forcing have been intensively investigated
(Shrivastava et al., 2017).
The importance of the formation of low-volatility (and/or extremely low-volatility)
organic compounds (LVOCs) with saturation concentrations less than
10-0.5µgm-3 through heterogeneous/multiphase accretion
processes has been highlighted in SOA formation mechanisms in recent studies
(Ziemann and Atkinson, 2012; Shrivastava et al., 2017). Semi-volatile
organic compounds (SVOCs) are generated from oxidation reactions of VOCs in
the gas phase. Heterogenous/multiphase reactions of SVOCs on particles are
thought to contribute to the formation of LVOCs in SOA. Earlier
acid-catalyzed heterogeneous reaction studies (Jang et al., 2002; Hallquist
et al., 2009; Ziemann and Atkinson, 2012) proposed the formation of
hemiacetals, aldol products, and organosulfates, among others, in the presence
of acidic seed particles, which prompted the notion that the acidity of
pre-existing particles is one of the key factors that influence SOA
formation.
The influence of the acidity of pre-existing particles on SOA formation has
been investigated in both chamber experiments and field measurements. From
chamber experiments, Eddingsaas et al. (2012) observed the clear uptake of
several SVOCs (e.g., α-pinene oxide and α-pinene hydroxy
hydroperoxides) after the injection of acidic particles into the α-pinene OH oxidation system under low-NOx conditions (photooxidation for
4 h, lights off, and contents in the dark for 2 h followed by
the injection). However, no apparent uptake was observed after the injection
of neutral particles. Shiraiwa et al. (2013a) observed the evident formation
of peroxyhemiacetals after gaseous tridecanal was injected into the dodecane
photooxidation system under dry, ammonium sulfate seed particles and
low-NOx conditions (photooxidation for 4 h, lights off, and contents
in the dark for 2 h followed by the injection), which was believed to
have been catalyzed by the presence of acids generated in the low-NOx
dodecane mechanism. However, the influence of the acidity of pre-existing
particles on SOA yields from chamber experiments is poorly understood.
Previous studies have reported complex results. For example, Eddingsaas et
al. (2012) reported greater SOA yields under acidic than under neutral seed
conditions from photooxidation of α-pinene under high-NOx
conditions and no influence of seed particle acidity on SOA yields under
low-NOx conditions. Field studies also reported inconsistent results on the
influence of acidity on SOA formation. Some researchers reported that SOA
formation was enhanced under more acidic conditions (Chu et al., 2004;
Lewandowski et al., 2007; Zhang et al., 2007; Hinkley et al., 2008;
Rengarajan et al., 2011; Zhou et al., 2012), whereas others reported little
or no enhancement under acidic conditions (Takahama et al., 2006; Peltier et
al., 2007; Tanner et al., 2009). Other factors, such as temperature,
humidity, NOx concentration level, and oxidation agents, might have
also affected the results of the aforementioned studies in addition to the
acidity of pre-existing particles (Jang et al., 2008), but they are also not
well understood. For example, the relative importance of the
temperature dependencies of the volatilities of oxidation products and of
the gas-phase and multiphase chemical reactions, which could result in
different temperature dependence of SOA yields (Pathak et al., 2007b; von
Hessberg et al., 2009), is still not well constrained. This motivated the
current study to develop a new compact chamber system, in which SOA
formation reactions under controlled temperature, humidity, oxidation
agents, and seed particle acidity can be easily performed.
Monoterpenes are known to be a large source of SOA in the global atmosphere
(Kelly et al., 2018). α-Pinene is the dominant monoterpene and the
second most emitted VOC following isoprene (Guenther et al., 2012; Messina
et al., 2016). It can react rapidly with atmospheric oxidants including
O3, OH, and NO3 radicals, and ozonolysis is the major atmospheric
oxidation pathway, which is estimated to account for 46 % of reacted
α-pinene (Capouet et al., 2008). Previous studies regarding α-pinene ozonolysis indicate that both SOA yields and chemical compositions
in SOA are influenced by the air temperature and aerosol acidity (Czoschke
et al., 2003; Gao et al., 2004; Iinuma et al., 2004, 2005; Czoschke and
Jang, 2006; Jang et al., 2006; Northcross and Jang, 2007; Surratt et al.,
2007, 2008; Hallquist et al., 2009; Saathoff et al., 2009; Kristensen et
al., 2014, 2017). Pathak et al. (2007b) found that the SOA yields show a
weak temperature dependence in the range of 15 to 40 ∘C
and a stronger temperature dependence between 0 and 15 ∘C.
Saathoff et al. (2009) parameterized the temperature dependence of the
two-product SOA yield parameters using a dataset including several previous
studies as well as their own comprehensive measurements from 243 to 313 K.
In general, a negative temperature dependence of the α-pinene
ozonolysis SOA yields could be confirmed. While enhancements of SOA yields
from α-pinene ozonolysis reactions under acidic seed aerosol
conditions compared with neutral seed conditions have been generally
observed in previous studies (Czoschke et al., 2003; Gao et al., 2004;
Iinuma et al., 2004, 2005; Czoschke and Jang, 2006; Jang et al., 2006;
Northcross and Jang, 2007), the degree of enhancement varied probably
because of the different experimental settings among the studies. For
example, the study of Czoschke et al. (2003), Iinuma et al. (2004),
Czoschke and Jang (2006), Jang et al. (2006), and Northcross and Jang (2007)
reported enhancement ranges of 21 %–87 % from weak acid seed conditions to
high acid seed conditions. Interestingly, studies of Gao et al. (2004)
reported smaller enhancement (8 %–15 %) of SOA yields when the initial
α-pinene concentrations were high, and the study of Iinuma et al. (2005)
reported both increases and decreases in SOA yields under acidic seed
conditions. It turned out that OH radical scavengers, which are known to
play an important role in influencing SOA yields (Iinuma et al., 2005; Na et
al., 2007), were applied in the studies of Gao et al. (2004) and Iinuma et
al. (2005), but not applied in the studies of others.
With respect to chemical compositions, Kristensen et al. (2017) compared the
chemical compositions of α-pinene ozonolysis SOA formed at
temperatures of 293 and 258 K and found that the mass fraction of carboxylic
acids increased at 258 K compared to 293 K, while the formation of dimer
esters was suppressed at the sub-zero reaction temperature. Compared with
neutral seed conditions, enhanced formation of large molecules under acidic
seed conditions has been reported by Gao et al. (2004) and Iinuma et al. (2004). Gao et al. (2004) additionally reported less abundant small
oligomers (e.g., a compound with molecular mass of 358) under acidic seed
conditions. This research indicates that systematic studies of α-pinene ozonolysis SOA formation under specific experimental settings are
warranted to clarify temperature and acidity dependence.
Furthermore, enhanced formation of organosulfates (OSs) from α-pinene
oxidation under acidic seed conditions has been suggested by previous
studies (Surratt et al., 2007, 2008; Iinuma et al., 2009; Duporte et al.,
2020). OS has been regarded as an important aerosol component, accounting
for up to 30 % of organic mass in PM10 and also as an anthropogenic
pollution marker in the past 2 decades (Iinuma et al., 2007, 2009; Surratt
et al., 2007, 2008; Riva et al., 2015, 2016; Duporte et al., 2016, 2020;
Brüggemann et al., 2020). The formation mechanisms of OS
from α-pinene oxidations have been studied under different
experimental settings. For example, Surratt et al. (2007) proposed OS
formation through esterification of hydroxyl or carbonyl groups in
photooxidation experiments. Surratt et al. (2008) proposed nitrooxy
organosulfate formation through esterification of hydroxyl groups in
nighttime oxidation (i.e., NO3-initiated oxidation under dark
conditions). Iinuma et al. (2009) proposed the acid-catalyzed ring opening
of an epoxide mechanism through the α-pinene oxide–acidic sulfate
particle experiment. Nozière et al. (2010) proposed a sulfate-radical-initiated OS formation process of α-pinene in irradiated sulfate
solutions. For α-pinene ozonolysis experiments, the formation of OS
through reactions between SO2 and stabilized Criegee intermediates
under dry conditions or organic peroxides in the aqueous phase has been recently
suggested (Ye et al., 2018; Stangl et al., 2019; Wang et al., 2019).
However, based on our knowledge, no study concerning OS formation from
α-pinene ozonolysis in the presence of sulfate particles exists.
In this study, α-pinene ozonolysis experiments have been conducted
under dark conditions in the presence of seed particles and an OH scavenger
utilizing a self-made compact-type chamber system. The study aimed to
characterize the newly developed chamber as well as to study the temperature
and acidity dependence of the yield and chemical composition of α-pinene ozonolysis SOA. Moreover, OS compounds and their possible formation
mechanisms have been targeted during the analysis.
ExperimentalChamber description and operation
A temperature-controllable chamber system has been developed for the
simulation of SOA formation (Fig. S1 in the Supplement). The chamber is a cuboid shape Teflon
bag (FEP; 0.7 m3 volume, 900 mm × 600 mm × 1300 mm;
50 µm thickness; Takesue, Japan) contained in a constant temperature
cabinet (HCLP-1240; W1200 mm × D703 mm × H1466 mm; NK
System, Japan). The temperature inside the cabinet was measured using a
thermocouple attached to the inside of the cabinet (T3 in Fig. S1). The
achievable operating temperature range of the chamber was 5–40∘.
A detailed evaluation of the thermostat capacity of the chamber under dark
conditions is presented in Text S1 in the Supplement, which indicates that the temperature
inside the chamber was well controlled (varied within ±1 ∘C). The chamber is collapsible and is operated at atmosphere pressure.
For a typical experimental run, a total volume of 0.6 standard cubic meters
(sm3) of G3 pure air (CO2< 1 ppmv, CO < 1 ppmv, THC
< 1 ppmv, and dew temperature <-70 ∘C) was
introduced into the Teflon chamber at a flow rate of 20 slpm for 30 min. The relative humidity (RH) of the chamber air was
adjusted by passing the G3 pure air through Milli-Q water (resistivity of 18.2 MΩ cm, total organic carbon content ≤ 5 ppb) before
it entered the Teflon bag. The RH of the chamber air was measured after the
experimental run by pumping the remaining chamber air into a separate small
Teflon bag, to which a Vaisala RH&T probe (model, HMP76) equipped with a
measurement indicator (model, M170) was attached. Particle number
concentration and VOC mixing ratio measurements indicate that the chamber
background concentrations of particles and VOCs were negligible, and no
extra contamination was observed by the humidification process of the G3 air
(Text S2). α-Pinene liquid (Wako Chemicals, Japan) was injected into
the G3 pure air line through a septum equipped in a Swagelok PFA Tee
connector using a micro-syringe (ITO Corporation, Japan) in the middle of
the injection of dry pure G3 air. Diethyl ether in nitrogen gas (mass
fraction of 0.4 %; Takachiho, Japan) used as an OH radical scavenger was
introduced to the chamber in excess amounts (approximately 53 ppmv;
164–1963 times the initial concentration of α-pinene) after the
introduction of pure air. Seed aerosol particles generated by a commercial
atomizer (ATM220S, Topas GmbH, Germany) were subsequently introduced into
the Teflon chamber after being dried by a diffusion dryer containing silica
gel. Neutral seed aerosols were generated from a 0.6/0.3 mol L-1
(NH4)2SO4 solution, and acidic seed aerosols were generated
using a solution mixture of 0.25 mol L-1 (NH4)2SO4 and
0.25 mol L-1 H2SO4.
After 30 min of stabilization, the initial concentrations of α-pinene and seed aerosol particles were measured with a quadrupole-type
proton transfer reaction mass spectrometry instrument (PTR-QMS500, Ionicon
Analytik GmbH, Innsbruck, Austria) and a scanning mobility
particle sizer (SMPS, TSI classifier model 3082; differential mobility
analyzer model 3081; condensation particle counter model 3772; USA),
respectively. The PTR-MS was operated at a flow rate of approximately 250 cm3 s-1 under a field strength (E/N, where E is the electric field
strength (V cm-1) and N is the buffer gas number density (molecule cm-3) of the drift tube) of 106 Td. The length of the drift tube was
9.2 cm. The drift voltage was set to 400 V. The temperatures of the inlet
and drift tubes were set to 105 ∘C, and the pressure at the drift
tube was set to 2.1 mbar. The signal intensities of ions with m/z values of 21,
30, 32, 37, 45, 46, 75, 81, and 137 were recorded approximately every 4.5 s.
The detection sensitivity of α-pinene was 3.3 ± 0.6 ncps ppbv-1 (ncps means normalized counts per second to 106 cps of
H3O+). The SMPS was contained in a smaller constant temperature
cabinet (LP-280-E, NK System, Japan), whose temperature was adjusted to be
the same as the cabinet containing the Teflon chamber. The sheath and sample
flow rates of the SMPS were 3.0 and 0.3 L min-1, respectively. The
measured diameter range of the SMPS was 13.8–697.8 nm, and the data were
collected every 5 min.
After obtaining the initial concentration of α-pinene and seed
particles, excess ozone produced by irradiation of pure O2 with vacuum
ultraviolet light from a low-pressure mercury lamp ozone generator (model
600, Jelight Compony Inc., USA) was introduced into the chamber at a flow
rate of 200 slpm for 1.5 min, to initiate the ozonolysis
reactions. After the ozone generator was turned off, the introduction of
pure O2 continued for another minute to purge all generated ozone into
the Teflon bag. Subsequently, the G3 pure air was introduced for 1 min
to facilitate the mixing of the chamber air. The ozone concentration in the
chamber was measured with an ozone monitor (model 1200, Dylec, Japan)
immediately after its introduction. For some experiments, the order of the
introduction of α-pinene and O3 was inverted (see Tables S1 and
S2 in the Supplement). In the experiments in which O3 was first introduced, the
introduction of G3 pure air was sustained for one more minute after the
injection of α-pinene to purge all α-pinene into the Teflon
bag and to facilitate the mixing of the chamber air. In the latter case, the
maximum α-pinene concentrations appeared within 55 s of its
introduction, which indicated that the mixing by introducing air with a flow
rate of 20 slpm was probably completed within 55 s. The concentrations of
both the α-pinene and aerosol particles were continually measured
until the end of the experiment, which is defined in this study as 90 min
after the start of the α-pinene ozonolysis reaction. The
concentration of ozone after 90 min of ozonolysis reactions was also
measured.
In total, 40 experimental runs were executed under neutral or acidic seed
aerosol conditions at temperatures of 278, 288, or 298 K (Tables S1 and S2).
Notably, when the chamber temperature was set to 278 K, the temperature in
the small cabinet was set to 280 K, which is the lowest work temperature of
the SMPS. In the present study, we did not investigate the influence of
humidity on SOA yield. Since very dry conditions are not realistic in
ambient air, we carried out the experiments at medium humidity (26 %–55 %
RH, Tables S1 and S2). The differences in RH among experiments in the
present study would not influence SOA formation significantly, as explained
in the following section. First, nucleation would be negligible in all
experiments because of the high concentrations of seed particles applied.
Consequently, the influence of RH on SOA formation would be reflected in the
particle phase (Kristensen et al., 2014). In addition, because the seed
particles were dried into effloresced states before being introduced into
the chamber, all particles would be in solid (neutral seed conditions) or
near-solid states (acidic seed conditions) (Tang and Munkelwitz, 1977).
Therefore, the influence of water on the particle phase through physical
partitioning or chemical reactions would be minor (Faust et al., 2017).
Before each experimental run, the Teflon bag was cleaned by filling it with
pure G3 air and then evacuating all the air from the bag at least three
times, which took approximately 40 min. The very low chamber background
particle concentrations indicate that the bag was sufficiently cleaned (Text S2).
One Teflon filter (PF020, 47 mm diameter, Advantec MFS) aerosol sample was
collected for each different combination of seed and temperature conditions.
The sample volume was 0.5 m3 for each sample. In total, six aerosol
samples were collected, and they were subjected to negative electrospray
ionization liquid-chromatography time-of-flight mass spectrometry analyses
(Sect. 2.2). A blank filter was also analyzed using a procedure similar to
that of the sample filters. The results confirmed no substantial
contamination in the filter and the filter analysis procedure (Text S2). The
acidity of the seed particles was measured in a separate experiment where
the seed particles were sampled on a Teflon filter which was then extracted
into 10 mL of Milli-Q water. The pH of the water solution was measured with a pH
meter (FPH70, AS ONE, Japan). The H+ concentration was ∼ 220 nmol m-3 under the acidic seed conditions.
(-) ESI LC-TOF-MS analysis
Chemical composition analysis of the Teflon filter samples was conducted
using electrospray ionization liquid-chromatography time-of-flight mass
spectrometry (ESI LC-TOF-MS) (Agilent Technologies, UK) similarly to in
previous studies (Sato et al., 2018, 2019) except that negative mode was
used in this study whereas positive mode was used in those previous studies.
The key configuration parameter settings were as follows: nebulizer pressure
was 0.21 MPa, the voltage in the spray chamber was -3500 V, the drying
nitrogen gas temperature was 325 ∘C and flow rate was 5 L min-1, and the fragmentor voltage was 175 V. The mass calibration and
lock-mass correction were conducted using G1969-85000 and G1969-85001 tuning
mixtures (Agilent Technologies, UK), respectively. The mass resolution of
the mass spectrometer (full width at half maximum) was > 20 000.
For the analysis, the Teflon filter sample was sonicated in 5 mL methanol
for 30 min after the addition of internal standard (i.e., sodium ethyl-d5
sulfate methanol solution, Sect. 3.2). The filter extract was concentrated
to near dryness under a stream of nitrogen (∼ 1 L min-1). A 1 mL
formic-acid–methanol–water solution (v/v/v=0.05/100/99.95) was added to
the concentrated extract to obtain the analytical sample. A 10 µL
aliquot of the analytical sample was injected into the LC-TOF-MS instrument
and separated with an octadecyl silica gel column (Inertsil ODS-3; GL
Science, Japan; 0.5 µm× 3.0 mm × 150 mm). A
formic-acid–water solution (0.05 % v/v) and methanol were used as mobile
phases. The total flow of the mobile phases was 0.4 mL min-1. The
methanol fraction during each analysis was set at 10 % (0 min), 90 %
(30 min), 90 % (40 min), 10 % (45 min), and 10 % (60 min). As
reported previously (Sato et al., 2007), the recovery of malic acid, whose
saturation concentration was estimated to be 157 µgm-3, was
determined to be > 90 %, suggesting that evaporation loss
during pre-treatment is negligible for molecules with saturation
concentrations of ∼ 102µgm-3 or less. We
tentatively determined the molecular formulae and signal intensities of 362
products (including 11 organosulfates) with different m/z (Table S3) based on
retention times and interpretation of mass spectra. In addition, the
tentatively determined molecular structures and compound names of some major
products based on literature data and the results of this study (Sect. 4.2)
are presented in Table S4.
Evaluation of wall loss
The wall-loss rate of particles in the chamber was evaluated by measuring
the time evolution of the volume–size distributions of seed-only particles
using the SMPS. The measurements were carried out whenever a new Teflon bag
was used or the experimental conditions (i.e., temperature or seed particle
acidity) were changed under humid air conditions. The latest measured bulk
wall-loss rate (Sect. 3.1) was applied for each SOA formation experiment.
In Fig. S4, size-resolved particle wall-loss rates, which were determined
assuming first-order wall-loss constants (Wang et al., 2018a), were shown
for seed particles of different size distributions. Large wall loss was
observed for particles with mobility diameters less than 100 nm and larger
than 200 nm. The size distributions of the measured particle wall-loss rates
presented shapes similar to that of a 0.83 m3 Teflon chamber (Hu et
al., 2014), whereas in the latter, the lowest wall-loss rates appeared in
the smaller size end (∼ 70–110 nm) and were greater
(∼ 0.2 h-1) than those in the present study. The large
apparent wall-loss rates of sub-100 nm particles were also similar to those
of a 1.5 m3 Teflon reactor (Wang et al., 2018a). Model simulation (Text S3) and literature survey results revealed that the high wall-loss rates of
sub-100 nm particles were mainly caused by particle coagulation (Nah et al.,
2017; Wang et al., 2018a), and those of super-200 nm particles were likely
the result of turbulent deposition (Lai and Nazaroff, 2000). Figure S4 also
indicates that the wall-loss rates of super-200 nm particles were relatively
high when the mean diameter of the seed particles was relatively small.
Therefore, we waited for 30 min after the introduction of seed particles to
start the ozonolysis reaction so that the size distribution of the seed
particles could shift to the larger size end due to coagulation and loss of
small particles. In addition, we used high-concentration solutions for the
generation of seed particles to produce larger particles in this study
(Sect. 2.1).
Wall loss of gas-phase organic compounds in the Teflon chamber could also
cause the underestimation of SOA yields (Zhang et al., 2014; Krechmer et
al., 2016). Although not experimentally determined in the present study, the
influence of gas-phase wall loss on SOA yields will be discussed based on
the studies of Zhang et al. (2014) and Krechmer et al. (2016) in Sect. 4.1.
Data analysisDerivation of SOA yield
SOA yield (Y) is defined as the ratio of the mass concentration of SOA
(mSOA, µgcm-3) to that of the reacted α-pinene
(ΔVOC, µgcm-3) in each experimental run.
Y=mSOAΔVOC,
where mSOA was calculated as the product of the increased volume of
aerosol particles from the volume of seed particles and a SOA density of
1.34 g cm-3 (Sato et al., 2018), and the arithmetic mean of the last
three data of each experimental run was applied here. It was corrected for
particle-phase wall loss using the bulk-volume wall-loss rate, assuming a
first-order wall-loss constant which is independent of particle size and
reaction time (Pathak et al., 2007b). The size-resolved wall-loss rates were
not applied because the bulk wall-loss rates were very close to the
size-resolved rates at approximately 300 nm (Fig. S4). The mode diameters of
the volume–size distributions of the seed particles (Fig. S5) and of aerosol
particles at the end of the ozonolysis reactions were also approximately 300 nm (Fig. S5). A detailed explanation of the derivation of mSOA is
presented in Text S4. Note that when the mass loadings of SOA are low, the
obtained mSOA and related yields retain greater uncertainties because
the subtracted volume concentrations of seed particles from the measured
volume concentrations are large (Mei et al., 2013). The influence of
gas-phase wall loss on SOA yield is discussed qualitatively in Sect. 4.1.
In addition, a four-product volatility basis-set (VBS) gas–particle
partitioning absorption model (Eq. 2, Donahue et al., 2006; Lane et al.,
2008) was applied to assist the interpretation of the observed responses of
Y to the chamber temperature and the acidity of the seed aerosol.
Y=∑iαi11+ci∗/mSOA,
where αi is the mass-based stoichiometric yield for product
i, and ci∗ is the effective saturation
concentration of i in µgm-3. In this study, αi is
assumed to be temperature-independent (Pathak et al., 2007a) whereas the
temperature dependence of ci∗ is accounted for
using the Clausius–Clapeyron equation as follows:
ci∗=ci,0∗T0TexpΔHvap,iR1T0-1T,
where T0 is the reference temperature, which is 298 K in this study;
ci,0∗ is the effective saturation
concentration of i at T0; R is the ideal gas constant; and ΔHvap,i is the effective enthalpy of vaporization. The temperature
dependence of Y is then represented by the substitution of Eq. (3) for
ci∗ in Eq. (2). We further assume a constant
effective ΔHvap for all condensable organic compounds. Thus, the
four-product basis set has five free parameters: α1, α2, α3, α4, and ΔHvap. Here,
c0∗={1,10,100,1000}µgm-3, which was set based on the measured
range of mSOA (Sect. 4.1) in this study. Microsoft Excel Solver GRG
Nonlinear engine was used for the derivation of the five parameters under
neutral or acidic seed particle conditions.
Derivation of the ethyl-d5-sulfate equivalent (EDSeq.) yield of OS
Before the extraction of filter samples, 20 µL of sodium ethyl-d5 sulfate (EDS) methanol solution (50 µgmL-1) was added to each sample filter as an internal standard for the quantification of OS. The ethyl-d5-sulfate equivalent (EDSeq.) masses of OSs were determined by comparing the total chromatographic peak areas of OSs to that of the EDS standard with known mass. The EDSeq. masses of OSs were divided by the
corresponding air volumes collected to obtain the EDSeq. concentrations of
OSs. The EDSeq. molecular yield of OS is defined as the ratio between the
estimated EDSeq. concentration of OS (mOS, µgcm-3) and the
reacted mass concentration of α-pinene (ΔVOC, µgcm-3). Note that the sensitivity of ESI mass spectrometry is compound-specific; therefore, the calculated EDSeq. yield includes the uncertainties
that result from compound-specific sensitivities.
Volatility distribution analysis
SOA compounds identified from the six filter samples through LC-TOF-MS
analysis were subjected to volatility distribution analysis. The saturation
concentration (C∗) of each chemical compound was calculated and then
ascribed to the volatility basis set (Donahue et al., 2006). Again, 298 K
was used as the reference temperature (T0). For compounds whose chemical
structures have been suggested by previous researchers, the SPARC online
calculator (Hilal et al., 2003; Sato et al., 2018) was used for the
derivation of their C0∗. For other compounds,
including organosulfates, the following equation from Li et al. (2016) was
applied:
log10C0∗=nC0-nCbC-nObO-2nCnOnC+nObCO-nSbS,
where nC0 is the reference carbon number;
nC, nO, and nS are the numbers of carbon, oxygen, and sulfur
atoms in the molecule, respectively; bC, bO, and bS are the
respective contribution of each atom to log10C0∗; and bCO is the carbon–oxygen
nonideality. For compounds containing only C, H, and O atoms, the values for
nC0, bC, bO, and bCO are 22.66,
0.4481, 1.656, and -0.7790, respectively. For OS compounds that contain C,
H, O, and S atoms, the values for nC0, bC,
bO, bCO, and bS are 24.06, 0.3637, 1.327, -0.3988, and 0.7579,
respectively. The log10C0∗ of the 362
compounds determined by LC-TOF-MS analysis are presented in Table S3. As has
been noted previously, the sensitivity of ESI mass spectrometry is compound-specific; thus the calculated distribution includes the uncertainties that
result from compound-specific sensitivities. The estimated volatility
distributions were further used to estimate the influence of gas-phase
wall loss on SOA yields following the method suggested by Krechmer et al. (2016) (Text S5).
Results and discussionPerformance of the Teflon chamber
An example experimental run of SOA formation from α-pinene
ozonolysis is presented in Fig. 1 (exp. no. 27). The initial concentrations
of α-pinene and ozone were 145 and > 824 ppbv,
respectively. The number concentration of seed aerosol particles was 9.6 × 103 cm-3, and they were concentrated in the diameter
range of 80–200 nm. SOA was formed while α-pinene was consumed
immediately at the introduction of excess O3. The mass concentration of
SOA reached its maximum while α-pinene was almost totally consumed
approximately 50 min after the introduction of ozone. The time variation in
α-pinene conforms to an α-pinene-limited first-order
chemical reaction (5τ=47 min). With the particle-phase wall-loss
correction, the SOA loading at the end of this experiment was calculated to
be 200 ± 34 µgm-3, which resulted in a final SOA yield of
26 ± 7 % (Table S2). As the corrected SOA particle concentration was
constant after 50 min, we consider the wall-loss correction applied here to be
reasonable (Ng et al., 2006). Without particle-phase wall-loss correction,
the concentration of SOA particles at 90 min would be underestimated by
approximately 40 %. The observed particle number–size distribution
shifted to a much greater but narrower size range of 200–300 nm at the end
of the experiment. Evolution of the particle number–size distribution is
presented in Fig. S6.
Example experimental run: (a) concentrations of α-pinene
(black solid curve), O3 (square markers), and SOA before (blue dashed
line) and after (circle markers) particle-phase wall-loss correction
compared with reaction time and (b) number–size distributions of seed
aerosol particles (black sticks) and aerosol particles at the end of the
experimental run (brown curve). The vertical dotted line in panel (a)
indicates the starting time that O3 was injected. Data shown in this
figure are from exp. no. 27 (Table S2).
The measured SOA mass yields at 298 K under neutral seed conditions were
compared with those of Pathak et al. (2007a) and other experimental studies
(Fig. 2). In this study, seven experiments with different initial α-pinene concentrations (54–323 ppbv) at 298 K and neutral seeds were
conducted under a RH condition of approximately 26 %–27 % (Table S1). In
Pathak et al. (2007a), the SOA mass yields from pre-existing studies under
low-NOx dark ozonolysis conditions were summarized and categorized into two
groups according to experimental RH conditions: RH < 10 % and
RH = 50 %–73 %. They further fitted the data in each group using the
multiple product basis-set approach (Pathak et al., 2007a). The four-product
basis-set fitting was adopted to compare with the experiment results of this
study (Fig. 2). The SOA yields in this study were 25 %–60 % lower than
that of Pathak et al. (2007a). Possible reasons may include lack of
consideration of the wall loss of oxidized organic vapors because the
surface-to-volume ratio of the chamber used in this study (7.1 m-1) was
much larger than those of previous studies (< 3 m-1; Pathak et
al., 2007a, and references therein). According to Zhang et al. (2014), the
vapor wall-loss bias factor, Rwall (defined as the ratio of the SOA mass
when the vapor wall loss was assumed to be zero to the SOA mass when the
optimal vapor wall loss rate was applied), was reported to be
∼ 4 at the initial seed surface area of ∼ 2 × 103µm2cm-3 (seed-to-chamber surface area
ratio ∼ 1 × 10-3) and ∼ 2 at the
initial seed surface area of > 6 × 103µm2cm-3 (seed-to-chamber surface area ratio > 3 × 10-3) during the photooxidation of toluene. As the initial
seed surface area in the present study was in the range of (1–3) × 103µm2cm-3 (seed-to-chamber surface area
ratio (1–4) × 10-4), Rwall in the Teflon bag might be at
least twice that of the large chambers. This leads to the underestimation of
the SOA yield of 50 % compared with the values obtained from the large
chambers. A low limit correction of the gas-phase wall-loss influence for
exp. no. 2 and other experimental runs in which chemical composition
analysis had been conducted based on the method of Krechmer et al. (2016)
(Text S5, Fig. 2), which confirmed that gas-phase wall loss is one reason
for the lower SOA yields in the present study compared with Pathak et al. (2007a) and other previous studies presented in Fig. 2. We note that this is
a shortcoming of the compact chamber with a 0.7 m3 volume used in this
study. We also note that the chamber is aimed at exploratory research,
where multiple experiments under different temperature, seed particle,
relative humidity, oxidant, and radiation conditions can be executed within
relatively short periods. Furthermore, the vapor wall-loss correction
factors of the SOA mass presented no obvious temperature dependence (Text S5). Therefore, the temperature dependence of SOA yields will be discussed,
assuming that the underestimation of the SOA yield due to the wall loss of
oxidized organic vapors does not affect the temperature dependence.
Yield comparison. SOA mass yields measured at 298 K under neutral
seed conditions in the present study were compared to those of previous
studies. Colored markers represent the results of this study. Colored
circular markers represent the real-time SOA yields, i.e., the SOA yields
along with the α-pinene ozonolysis reactions from 0 to 90 min.
Different experimental runs are differentiated by colors. Red solid square
markers represent the final SOA yields of the seven experiments. Horizontal
error bars indicate the uncertainties of the final SOA concentrations;
vertical error bars indicate the uncertainties of the final SOA yields.
However, the systematic errors from vapor wall loss are not included. The
open square indicates the result of exp. no. 2 after gas-phase wall-loss
correction (Text S5). Black markers and curves represent results of previous
studies. Black markers represent experimental results. The solid and dotted
black curves represent the parameterized results from the four-product
volatility basis-set fittings of previous α-pinene ozonolysis
experiments under low-NOx and dark conditions summarized in Pathak et al. (2007a). The solid curve represents results under a 50 %–73 % RH range, and
the dotted curve represents results under RH < 10 %. The dashed
curve represents the results calculated using Eqs. (1), (6), (7), (10), and (11) at
303 K in Saathoff et al., 2009. The experiments of Saathoff et al. (2009)
were carried out at 303 K and 48 %–37 % and 0.02 % RH, without or with OH
scavenger (cyclohexane or 2-butanol); experiments of Wang et al. (2011) were
carried out at 295 K and < 1 % RH, without OH scavenger; experiments
of Wang et al. (2014) were carried out at 295 K and < 5 % RH,
without OH scavenger; experiments of Nah et al. (2016) were carried out at
298 K and < 5 % RH, with cyclohexane as OH scavenger; experiments of
Ye et al. (2018) were carried out at 296 K and 12 %–14 % and 48 %–49 % RH, with
cyclohexane as OH scavenger; experiments of Kenseth et al. (2020) were
carried out at 295 K and < 5 % RH, without OH scavenger; and
experiments of Czoschke and Jang (2006) were carried out at 294–300 K and
14 %–67 % RH, without OH scavenger. Note that all data presented in this
figure are normalized to unity density (1 g cm-3).
Temperature and acidity dependence of SOA yield
The yields of SOA from α-pinene ozonolysis under different
experimental conditions in this study are summarized in Fig. 3. Results
under neutral and acidic seed conditions are presented separately in panels
(a) and (b). In each panel, the measured SOA yields as a function of SOA
mass loadings are indicated by markers, and the four-product VBS model
fitting results are shown by curves. The fitted parameters with the
four-product VBS model under neutral and acidic seed conditions are
summarized in Table 1. Under neutral seed conditions, the fitted temperature-independent stoichiometric yields α={0.00,0.09,0.09,0.52} and ΔHvap is 25 kJ mol-1. Under
acidic seed conditions, α={0.00,0.14,0.05,0.43} and ΔHvap is 44 kJ mol-1. The fitting
results pointed out that most of the detected SOA compounds are of
relatively high saturation concentration (i.e., c∗ at 298 K of 1000 µgm-3) under both neutral and acidic seed conditions. Weak
increases in SOA yields with decreases in the chamber temperature can be
observed under both seed conditions. This is consistent with the result in
Pahtak et al. (2007b), which found a weak dependence of SOA yields on
temperature in the range of 288–303 K. As temperature decreases, SVOCs tend
to partition into the particle phase because of the lowering of their
volatilities.
Mass yields of SOA for increasing SOA mass loadings. Markers and
whiskers are measured data and their uncertainties (Text S4; the
uncertainties of the data during the reaction time of 85–90 min are
presented); dashed curves are fitting of measured data using a four-product
VBS model (Donahue et al., 2006; Lane et al., 2008). Panel (a) presents
results under neutral seed conditions, and panel (b) presents results under
acidic seed conditions.
The effective ΔHvap of α-pinene ozonolysis SOA derived
from the four-product VBS fitting in this study was compared with previous
studies (Table 2, Fig. S7). The ΔHvap values derived in this
study were comparable to those in Saathoff et al. (2009) and Pathak et al. (2007a) where the experiments were executed in large chambers of 10–200 m3. It may support our assumption that the temperature dependence of
SOA yields was not influenced by vapor wall loss. It is also in agreement
with the ΔHvap of 40 kJ mol-1 applied in the CMAQv4.7 model
(Carlton et al., 2010). However, they are lower than those of Saha and Grieshop (2016) and much lower than those of Epstein et al. (2010). While the ΔHvap in Saha and Grieshop. (2016) was derived based on thermodenuder
measurements which attributed 20 %–40 % of SOA mass to low-volatility
material (C∗< 0.3 µgm-3), most of the measured
SOA in this study is of relatively high volatility as aforementioned. The
differences in the volatility of SOA and the derived ΔHvap
between Saha and Grieshop (2016) and this study are consistent with the general
phenomenon that ΔHvap is conversely related with volatility
(Epstein et al., 2010). The ΔHvap in Epstein et al. (2010) was
derived from published experimental vapor pressure data of organic
compounds. The reason for the difference in ΔHvap between this
study and Epstein et al. (2010) cannot be currently explained. Notably,
sensitivity analyses achieved by fixing the stoichiometric yields αi while changing ΔHvap and comparing the resulting VBS
curves with measured data (Fig. S8) indicated that the effective ΔHvap could be in the 0 to 70 and 0 to 80 kJ mol-1 ranges
for neutral and acidic seed conditions, respectively.
Figure S9 presents the comparisons of the SOA yields under neutral and
acidic seed conditions at different temperatures. This indicates that the SOA
yields were enhanced under acidic seed conditions when the SOA loadings were
low. When the SOA loadings were high, the enhancement disappeared. This is
consistent with the results of Gao et al. (2004), which reported obvious
initial α-pinene concentration dependence of the enhancement of SOA
yields under acidic conditions when compared with neutral seed conditions.
For the initial α-pinene concentrations of 12, 25, 48, 52, 96, 120,
and 135 ppbv, the relative enhancements of SOA yields were 37 %, 34 %, 26 %, 24 %,
15 %, 10 %, and 8 %, respectively (Gao et al., 2004). This is probably
because the SOA components can be of high viscosity under conditions where
RH is smaller than around 50 %, and if high SOA mass loadings coated the
seed particles, the acid-catalyzed heterogeneous SOA formation reactions
could be impeded (Shiraiwa et al., 2013b; Zhou et al., 2013). In this study,
the initial concentrations of α-pinene were 54–323 ppbv at 298 K,
suggesting that the enhancement could be less than 24 %. When the SOA
volume loading was 50 µm3 cm-3, the fitted SOA yields under
acidic conditions were enhanced by 11 %, 17 %, and 25 % when compared to the
neutral seed conditions under 298, 288, and 278 K, respectively. This is
consistent with the findings of Gao et al. (2004) and is also comparable to
the results of Iinuma et al. (2005). In Iinuma et al. (2005), the experiment
with 2-butanol as an OH radical scavenger under room temperature (294–298 K) reported an enhancement of 19 %, with a final SOA volume concentration
of approximately 50 µm3 cm-3. Furthermore, the degree of
acidity of the seed aerosols could have also influenced the enhancement (Gao
et al., 2004; Czoschke and Jang, 2006). Further comprehensive studies are
warranted (including the consideration of the particle viscosity and phase
separation) on the influence of seed aerosol acidity on α-pinene
ozonolysis SOA formation.
Comparison of ΔHvap in this study with previous
studies.
ReferencesΔHvap (kJ mol-1)C∗ ranges (µgm-3)Temp ranges (∘C)This work; ozonolysis25–44101–1035–25Saha and Grieshop (2016); ozonolysis80-11 log10C∗10-2–10430–120Saathoff et al. (2009); ozonolysis24–592.1 × 10-3–56-30–40Pathak et al. (2007a); ozonolysis30, 7010-2–1040–49Epstein et al. (2010); semiempirical correlation-based fit129-11 log10C∗10-2–101027Carlton et al. (2010); CMAQv4.7 SOA module4015, 13440Temperature and acidity dependence of SOA composition
Among the 362 compounds identified through LC-TOF-MS analysis in this study
(Table S3), 331 compounds were ascribed to VBS bin ranges of -8 to 3. The
other 31 compounds were ascribed to higher VBS bin ranges of 4–6. Only the
former 331 compounds are targeted in the following discussions for two
reasons. First, less than a half of the compounds that belong to VBS bins
4 or greater could exist in the particle phase (Donahue et al., 2006),
which would introduce large uncertainties for the estimation of the mass
concentrations of the compounds in the gas phase from the particle phase. In
addition, LC-TOF-MS analysis of pure compounds indicated that fragmentation
of high molecular compounds during the ionization could occur; e.g., pinic
acid (C9H14O4) could be fragmented into
C8H14O2, and the latter was assigned to VBS bin 6 (Fig. S10).
Note that due to the potentially high viscosity of the newly formed SOA,
high-volatility compounds formed inside the aerosol particle could have been
wrapped into the particle phase and detected (Shiraiwa et al., 2013b).
Figure 4 presents the volatility distributions of the identified compounds.
The measured intensities of particle-phase compounds were normalized by
their total intensity for each experiment and are presented in Fig. 4a.
Compounds that were attributed to VBS bins between 0 and 3 are known as SVOCs
(Li et al., 2016). Those in bins 2 and 3 generally presented a decreasing
tendency with the increase in experimental temperatures, and there was no
obvious temperature dependence for those in bins 0 and 1 (Fig. 4a). The
corresponding gas-phase concentrations of each compound were derived
assuming gas–particle partitioning equilibrium (Odum et al., 1996). The
ΔHvap values derived in this study (Table 1) were used to
calculate the saturation concentration under 278 and 288 K following Eq. (3). The intensities of both the particle and gas phases were then
normalized by their total amount and are presented in Fig. 4b. With the
inclusion of the corresponding gaseous phase compounds, the temperature
dependence of compounds in VBS bin 3 changed to positive (i.e., increased
with temperature) whereas the tendency in bin 2 remained negative (Fig. 4b).
As the α-pinene ozonolysis rate constant at the temperature range of
278–298 K does not vary considerably (within 15 %; IUPAC Task Group on
Atmospheric Chemical Kinetic Data Evaluation, http://iupac.pole-ether.fr,
last access: 1 February 2021), α-pinene was completely consumed at
the reaction time of 90 min, and the temperature dependence of gas-phase
wall loss was considered insignificant (Text S5), the total amounts of SVOCs
formed should be similar at the three temperatures. Hence, the total amounts
of gas- and particle-phase compounds in each VBS bin should be independent of
experimental temperatures. If a larger ΔHvap were applied for
the derivation of the intensity of gas-phase compounds (e.g., the
semiempirical equation in Epstein et al., 2010), the total amount in VBS
bin 3 at 298 K would be much higher than at other temperatures, which is
unreasonable. This suggested the appropriateness of the ΔHvap
values derived from the four-product VBS fitting of the SOA yields in Sect. 4.1. In addition, we note that the volatility distribution pattern derived
in this study is similar to that of the experimental runs 1 and 6 (derived
using the same method) of Sato et al. (2018), although positive electrospray
ionization analysis was used and the α-pinene ozonolysis experiments
were carried out under dry conditions in the latter. According to Morino et
al. (2020), both the root-mean-square errors between the observed and
simulated SOA concentrations for the formation experiments and between the
observed and simulated volume fraction remaining for the heating
experiments were minimized in the case of the C∗ distribution
reported by Sato et al. (2018). In Fig. 4b, the volatility distributions
determined by normalizing the stoichiometric SOA yields (Table 2) based on
the total mass fractions of compounds in VBS bins 0 to 3 are also presented.
The mass fractions determined from the LC-TOF-MS data at VBS bins 1 and 3
and 0 and 2 were smaller and larger, respectively, than those determined
from the SOA stoichiometric yields. These differences are probably caused by
uncertainties in saturation concentration and sensitivity parameterization
as well as the existence of undetected molecules in the LC-TOF-MS analysis
(Sato et al., 2018).
Volatility distributions of chemical components in (a) particle-only phase and (b) particle + gas phases. In panel (a), data were
normalized by the total signal intensity of the particle phase of each
experiment; in panel (b), data were normalized by the total intensity of the
particle phase and its equilibrium gas phase in each experiment. Curves and
markers imposed in panel (b) are the volatility distributions under neutral
(black) and acidic (grey) seed conditions determined by normalizing the
stoichiometric SOA yields αi by the total mass fraction of
compounds in VBS bins 0 to 3 (Sato et al., 2018).
To gain more insights into the acidity dependence of the α-pinene
ozonolysis SOA, the relative intensities of the compounds identified by
LC-TOF-MS under acidic and neutral seed conditions were compared. For
compounds whose intensity under acidic seed conditions was more than 1.1
times that of neutral seed conditions, monomers with a chemical formula of
C10H9-18O4-14 accounted for 32 %, and oligomers with a
chemical formula of C11-22H7-35O2-12 accounted for 68 % of
the total intensity. Conversely, for compounds whose intensity under acidic
conditions was less than 0.9 times that of neutral seed conditions, monomers
with a chemical formula of C6-10H7-15O4-6 accounted for 87 %, and oligomers with a chemical formula of
C11-21H9-27O5-10 accounted for 13 % of the total
intensity.
Figure 5 presents the particle-phase mass distributions of compounds whose
mass fractions under acidic seed conditions were more than 1.1 (or less than
0.9) times those of neutral seed conditions. The molecular mass of those
compounds with greater mass fractions under acidic seed conditions was
generally distributed in the higher mass end compared with those with
greater mass fractions under neutral seed conditions. The mass-fraction-weighted mean molecular mass of those compounds presenting greater
intensities under acidic seed conditions was 284 ± 14 (mean ± standard deviation) g mol-1, whereas that of those presenting greater
intensities under neutral seed conditions was 204 ± 4 g mol-1.
Mean particle-phase mass fraction distributions of compounds whose
particle-phase mass fractions under acidic seed conditions were more than
1.1 (red symbols) or less than 0.9 (black symbols) times those of neutral
seed conditions. Whiskers represent the standard deviations of the three
experiments at different temperatures.
Figure 6 presents the identified compounds whose intensity under acidic seed
conditions was more than 1.1 (or less than 0.9) times that of neutral seed
conditions in the atomic H-to-C ratio (H:C) versus O-to-C ratio (O:C) space.
The color scale indicates the mean mass fraction of identified compounds
across all six experiments. It is suggested that compounds with a lower
intensity under acidic seed conditions were concentrated in the O:C ratio
range of 0.4–0.75 and H:C ratio range of 1.1–1.7, whereas those with
higher intensity under acidic seed conditions were more broadly distributed
in the H:C versus O:C space. Compounds with O:C ratios less than 0.4 were
oligomers with a chemical formula of C11-22H19-35O2-7 and
were distributed in the VBS bins of -3 to 3. Compounds with O:C ratios
greater than 0.75 were likely to be highly oxidized molecules with a
chemical formula of C10-14H9-21O9-14 and were attributed to
VBS bins -8 to -2. Furthermore, oligomers with O:C ratios of less than
0.4 accounted for 61 % of those oligomers with high relative intensity
under acidic conditions, whereas those with O:C ratios of greater than 0.75
accounted for only 1 %. This, together with the aforementioned chemical
formula and molecular mass distributions, indicated that the formation of
many oligomers, especially with small O:C ratios, was enhanced under acidic
seed conditions.
Atomic H-to-C ratio (H:C) versus O-to-C ratio (O:C). Stars and
circles respectively indicate compounds whose particle-phase mass
fractions under acidic seed conditions were more than 1.1 and less than 0.9
times those of neutral seed conditions. The color scale indicates the mean mass
fraction of identified compounds across all six experiments. Notably,
organosulfates were not presented.
Figure 7 presents the compounds whose normalized intensity under acidic seed
conditions was less than 0.9 (panel a) or more than 1.1 (panel b) times that
of neutral seed conditions in the VBS space. The acidity dependence of the
major compounds in VBS bins was tentatively explained from the viewpoints of
acid-catalyzed decomposition reactions or acid-catalyzed heterogenous
reactions. Compounds that presented lower intensities under acidic than
neutral seed conditions were mainly distributed in VBS bins 3, 2, -1,
-4, and -6 (Fig. 7a). The respective compounds that presented the
highest intensity in VBS bins 3, 2, -1, -4, and -6 were m/z 215.091
(C10H15O5), 171.065 (C8H11O4), 185.081
(C9H13O4), 343.139 (C16H23O8), and 357.154
(C17H25O8), and they accounted for on average 25, 46, 62, 54,
and 99 % of the total intensity of the compounds determined in the
respective bins. The structures of m/z 215 have been proposed by Zhang et al. (2017) as hydroperoxides from α-pinene ozonolysis reactions. They
can decompose under acidic seed conditions to m/z 197.081
(C10H13O4, VBS bin 2) (Scheme 1a) as well as m/z 155.010
(C8H11O3, VBS bin 5) (Scheme 1b) (acid-catalyzed
decomposition of hydroperoxides; Seubold and Vaughan, 1953). Next, previous
studies indicate that m/z 171 and 185 were monomer precursors of dimers m/z 343
and 357, and all four products exist in both laboratory α-pinene
ozonolysis SOA samples and field samples (Gao et al., 2004; Yasmeen et al.,
2010; Kristensen et al., 2014; Zhang et al., 2015). The chemical structure
of m/z 185 has been proposed as pinic acid (Gao et al., 2004; Yasmeen et al.,
2010; Kristensen et al., 2014) and m/z 171 as terpenylic acid (Yasmeen et al.,
2010; Kristensen et al., 2014) and norpinic acid (Gao et al., 2004). In this
study, excess OH scavengers could have minimized the formation of an OH
functional group in the α-pinene ozonolysis products, whereas the
aldehydes should dominate the products (Gaona-Colmán et al., 2017).
Thus, it is possible that the acid-catalyzed heterogenous esterification
between acid products such as m/z 171 and 185 and aldehyde products such as
pinonaldehyde (Hallquist et al., 2009) led to the decreased intensity of
m/z 171 and 185 under acidic seed conditions, which further led to the
decreased intensity of the related dimers of m/z 343 and 357. Scheme 2 presents
a possible esterification reaction between terpenylic acid (a possible
isomer of m/z 171) and pinonaldehyde. The product, m/z 339.180
(C18H27O6), ascribed to VBS bin -1, presented higher
relative intensity under acidic seed conditions.
Volatility distribution of SOA compounds whose particle-phase mass
fractions under acidic seed conditions were (a) less than 0.9 and (b) more
than 1.1 times those of neutral seed conditions. Note the mass fractions
presented here are the same as those in Fig. 4a.
The compounds with higher contributions under acidic than neutral seed
conditions were mainly distributed in VBS bins -3, -2, 0, and 2 (Fig. 7b). The most abundant compounds of those whose intensity under acidic seed
conditions was more than 1.1 times that of neutral conditions in bins 2,
-2, and -3 were m/z 197 (C10H13O4), 355.175
(C18H27O7), and 383.170 (C19H27O8),
respectively. Their contributions to the total signal of their respective
bins were 14 %, 7 %, and 5 %, and the ratios of the intensities under acidic seed conditions to those under neutral seed conditions were 5.4, 1.8,
and 1.3, respectively. The higher intensity of m/z 197 under acidic than
neutral conditions might be explained partly by the acid-catalyzed
decomposition of m/z 215 (Scheme 1a). Zhang et al. (2015) proposed m/z 355 as a
dimer ester with a hydroperoxide function. Another possible assignment of
m/z 355 is an ester from the reaction of a Criegee intermediate
(C10H16O3) with terpenylic acid and/or norpinic acid
(C8H12O4) (Kristensen et al., 2017). The latter assignment of
the product also has a hydroperoxide group. The formation of hydroperoxides
involving Criegee intermediates has been observed in the gas phase (Sekimoto
et al., 2020) and on the surface of aqueous droplets (Enami and Colussi, 2017).
The relatively high intensity of m/z 355 under acidic seed conditions might be
explained by the fact that the hydrolysis of ester hydroperoxides becomes
slower under acidic conditions (Zhao et al., 2018). Kristensen et al. (2017)
proposed m/z 383 as a dimer ester, whereas the molecular structure is to be
identified. In bin 0, the intensities of m/z 311.149 (C16H23O6)
and m/z 313.165 (C16H25O6) were the highest (contributing on
average 10 % and 9 % of the total intensity of the bin, respectively), and
under acidic seed conditions were 4.8 and 1.5 times those of neutral seed
conditions, respectively. Although the structure of m/z 311 is yet to be
identified, m/z 313 was proposed as a dimer ester by Zhang et al. (2015). These
dimer esters at m/z 383 and 313 might be formed through acid-catalyzed
heterogeneous reactions.
In addition, the 11 identified organosulfate compounds were ascribed to
the VBS bins of -3 to 0 (Table 3). All of them presented greater intensity
on average under acidic conditions than under neutral seed conditions (Fig. S11). Four of the OSs were in VBS bin 0, which together contributed 9.2 %
of the intensity of that bin. The contributions of the two, three, and two
OS compounds (which respectively belong to VBS bins of -1, -2, and
-3) to their respective bins were 0.4 %, 4.7 %, and 2.6 %. The formation
mechanisms of those OS compounds will be discussed in the next section. It
should be noted that the intensities of OS compounds in this study might be
underestimated because methanol, which could react with carbonyl and
carboxylic compounds in the SOA (Bateman et al., 2008), was used as the
extraction solution. Acetonitrile should be used as the extraction solution
in future studies to confirm the influence (Bateman et al., 2008).
Organosulfates identified in this study, their respective m/z and
formula of [M-H]-, assigned VBS bin number, proposed OS structure,
and proposed precursor oxidation product structure.
a Selected laboratory and/or field studies where the corresponding OS
has been identified. b Hettiyadura et al. (2019). c Surratt et al. (2008).
d Yassine et al. (2012). e Zhang et al. (2015). f Tao et
al. (2014). g Ma et al. (2014). h Wang et al. (2018b). i Yu et al. (1999).
j Surratt et al. (2007). k Jackson et al. (2017). l Liggio
and Li (2006). m Kahnt et al. (2014a). n Kahnt et al. (2014b).
o Meade et al. (2016). p Reinnig et al. (2009). q Kristensen et
al. (2014). r Aschmann et al. (1998, 2002). s Brüggemann et al. (2019).
t Wang et al. (2016). u Brüggemann et al. (2017). v Ma
et al. (2008).
Proposed acid-catalyzed decomposition reactions of hydroperoxides
at m/z 215 in this study.
Possible acid-catalyzed esterification reaction of terpenylic acid
(m/z 171) with pinonaldehyde.
OS formation under acidic conditions
In this study, significant signals were identified for 11 organosulfates
under acidic seed conditions (Fig. S11). Figure 8 presents the
ethyl-d5-sulfate equivalent (EDSeq.) molecular yields of those OS compounds
estimated from the LC-TOF-MS analysis. m/z 223 (C7H12O6S) was
the most abundant OS identified at 298 k, followed by m/z 279
(C10H16O7S), 281 (C9H14O8S), 269
(C8H14O8S), 283 (C9H16O8S), 265
(C10H18O6S), 253 (C8H14O7S), 267
(C8H12O8S), 251 (C9H16O6S), 249
(C10H18O5S), and 247 (C10H16O5S).
The ethyl-d5-sulfate equivalent (EDSeq.) yield of the 11 OS
compounds and their arithmetic mean under acidic seed conditions at
different temperatures. The retention time range used for the calculation of
the intensity of each OS compound is indicated in Fig. S11.
The potential formation mechanisms of the OS compounds were proposed based
on literature data in combination with the results of the experiments of
this study. To our knowledge, while the formation of OS through α-pinene ozonolysis in the presence of an OH radical scavenger has been
reported very recently (Ye et al., 2018; Stangl et al., 2019; Wang et al.,
2019), these studies focused on the reactions between α-pinene
ozonolysis products and SO2 rather the identification of OS compounds.
Earlier studies on the identification and formation mechanisms of OS with
α-pinene as the precursor VOC were related to photooxidation (i.e.,
OH-initiated oxidation) or nighttime oxidation (i.e., NO3-initiated
oxidation under dark conditions) (Surratt et al., 2007, 2008). In this
study, the formation of OS from α-pinene ozonolysis in the presence
of an OH radical scavenger and acidic seed particles was investigated.
Table 3 summarizes the possible chemical structures of the 11 detected
OS compounds and their precursor α-pinene oxidation products. Among
the 11 OS compounds detected in this study, eight of them have been
detected in previous laboratory and/or field studies of α-pinene
oxidations, and the other three (m/z 251, 253, and 269) were detected for the
first time in this study (Table 3). While all 11 OS compounds detected
in this study are formed from α-pinene ozonolysis due to the
presence of excess OH radical scavenger, five of the previously identified
OS compounds (m/z 223, 247, 249, 265, and 279) were detected in α-pinene photooxidation and/or nighttime oxidation experiments (Surratt et
al., 2008), and the other three (m/z 267, 281, and 283) were only identified in
ambient aerosols.
The chemical structures of five of the OS compounds (i.e., m/z 223, 247, 249,
265, and 279) have been proposed in previous studies. Four of them (m/z 223,
247, 249, and 279) were likely to be sulfate esters formed from alcohols and
sulfate (Surratt et al., 2007, 2008; Zhang et al., 2015; Hettiyadura et al.,
2019), and one (m/z 265) was said to be from the sulfation of pinonaldehyde
(Liggio and Li, 2006; Surratt et al., 2007). The former is referred to
hereafter as the alcohol pathway and the latter the aldehyde pathway
(aldehyde + HSO4-; Surratt et al., 2007) (Scheme S1). Because
aldehydes should dominate the products from α-pinene ozonolysis in
the presence of an OH scavenger (Gaona-Colmán et al., 2017) and
pinonaldehyde has been detected in previous α-pinene ozonolysis
studies with an OH scavenger (Jackson et al., 2017), we propose that the
aldehyde pathway could be one of the dominant OS formation pathways in this
study. In addition to m/z 265, we propose possible aldehyde precursors formed
in α-pinene ozonolysis experiments for five other OS compounds
identified in this study (i.e., m/z 251, 253, 269, 279, and 283). Except for
the study where the aldehyde precursor of m/z 269 was found (Reinnig et al.,
2009), an OH scavenger was used in all other studies where the aldehyde
precursors were found (Yu et al., 1999; Ma et al., 2008; Jackson et al.,
2017). Presently, we suppose that the other five OS compounds were possibly
formed through the alcohol pathway although no relevant precursor
information has been found for m/z 223 and 247. Notably, another possible
precursor with a hydroxyl function, which is formed in α-pinene
ozonolysis experiments with an OH scavenger, has also been found for the OS
at m/z 269 (Kristensen et al., 2014). Additionally, the proposed precursor for
m/z 283 (Table 3) also contains a hydroxyl function, which can form another OS
of m/z 265.038 (C9H13O7S) through the alcohol pathway. A weak
signal of this OS was observed (not presented in Fig. S11).
As shown in Fig. 8, the ethyl-d5-sulfate equivalent (EDSeq.) yields of m/z 247,
249, and 265 decreased with the increase in temperature. In fact, the OS at
m/z 247 and 249 could only be observed at the lower temperatures. Conversely,
the EDSeq. yields of other OS compounds increased with the temperature, or
at least the yields at 278 K were the lowest among the three temperature
conditions. The temperature dependence of OS EDSeq. yields seems not to be
directly related to the formation mechanisms of either the alcohol pathway
or the aldehyde pathway. Nevertheless, the mean EDSeq. yields of the 11
OS compounds, which were dominated by m/z 223 and m/z 279, increased with the
increase in reaction temperature. As 11 OSs (including three unreported)
were observed in the α-pinene ozonolysis reactions with an OH
scavenger and acidic seed particles, the identification of the structures of
the OSs using high-resolution ion mobility mass spectrometry is planned as
the next step. When the structures of the OSs can be determined and the
precursor compounds can be assumed, the formation mechanisms of OSs in
α-pinene ozonolysis with acidic seeds may be confirmed.
Summary and conclusions
Using the compact chamber system, SOA formation from α-pinene
ozonolysis was studied with diethyl ether as an OH radical scavenger at
temperatures of 278, 288, and 298 K, with acidic and neutral seed aerosol
particles. The SOA yields and compounds with a molecular mass of less than
400 Da determined using a LC-TOF-MS were analyzed from the perspectives of
temperature and seed particle acidity dependence.
The SOA yield increased slightly with the decrease in chamber temperature.
The enthalpies of vaporization under neutral and acidic seed conditions were
estimated to be 25 and 44 kJ mol-1, respectively. The acidity
dependence of the SOA yields at low SOA loadings were comparable to those
reported by Gao et al. (2004) and Iinuma et al. (2005). However, the
enhancement of the SOA yields under acidic conditions would be limited if
the SOA mass loadings are much greater than the amounts of pre-existing
particles.
Among the 362 compounds identified, the volatility of 331 was distributed in
the VBS bins between -8 and 3. The temperature dependence of the
volatility distribution of those identified compounds (particle phase +
gas phase) could be consistently explained by the enthalpies of vaporization
derived in this study.
The compounds whose intensities under acidic seed conditions were less than
0.9 times those under neutral seed conditions were dominated by monomers,
whereas the compounds whose intensities under acidic seed conditions were
more than 1.1 times those under neutral seed conditions were dominated by
oligomers. The O:C ratios of the former were concentrated in the range of
0.4–0.75. The O:C ratios of the latter were broadly distributed. The
compounds with O:C ratios less than 0.4 were all oligomers, which accounted
for 61 % of the oligomers with high relative intensity under acidic
conditions, whereas those with O:C ratios of greater than 0.75 were highly
oxidized molecules and only contributed to 1 % of the oligomers. In
addition, the mean molecular mass of the former compounds (204 ± 4 g mol-1) was evidently lower than that of the latter (284 ± 14 g mol-1). The differences indicated that the formation of many oligomers,
especially with low O:C ratios, was enhanced under acidic seed conditions.
The acidity dependence of certain major compounds could be explained by
acid-catalyzed heterogenous reactions (e.g., m/z 171, 185, 343, and 357) or
acid-catalyzed decomposition reactions (e.g., m/z 215 and 197), which suggests
that little or no enhancement of SOA under acidic conditions in field
observations could occur when acid-catalyzed decomposition is dominant.
For the first time, organosulfate compounds were studied for α-pinene ozonolysis reaction in the presence of an OH scavenger and acidic
seed particles. Eleven OS compounds were determined from LC-TOF-MS analysis.
All of them on average presented higher yields under acidic than under
neutral seed conditions. Six of the OS compounds were potentially formed via
the aldehyde + HSO4- pathway, which should be confirmed in
future studies through high-resolution mass spectrometry analyses.
Finally, the organosulfates and the oligomers that increased with an
increase in acidity of the seed particles could be indicators of the acidity
of pre-existing particles in the field, and the new findings obtained from
this study should be confirmed using more complex and larger chambers.
Data availability
All the final data supporting the findings of this study are available in
the manuscript or in the Supplement. Raw data used to derive the final data
are available on request to the corresponding author.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-21-5983-2021-supplement.
Author contributions
SI, YD, SE, and HT performed chamber experiments. KS and SR carried out LC-TOF-MS
analyses. SI, YD, SE, and MY performed data analyses. YD, SI, and HT wrote the
manuscript, and all authors contributed to the revisions of the manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We thank Sumiko Komori, Yoshikatsu Takazawa, and Tomoharu Sano
for the technical support. This work was supported by NIES research funding
(Type A) and JSPS KAKENHI grant no. 19H01154.
Financial support
This research has been supported by the National Institute for Environmental Studies (NIES research funding (Type A)) and the Japan Society for the Promotion of Science (grant no. 19H01154).
Review statement
This paper was edited by Sergey A. Nizkorodov and reviewed by two anonymous referees.
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