Recently, liquid–liquid phase separation (LLPS) of secondary organic
aerosol (SOA) particles free of inorganic salts has been intensively studied
due to the importance of cloud condensation nuclei (CCN) properties.
In this study, we investigated LLPS in four different types of SOA particles
generated from α-pinene ozonolysis and α-pinene
photooxidation in the absence and presence of ammonia (NH3). LLPS was observed
in SOA particles produced from α-pinene ozonolysis at
∼95.8 % relative humidity (RH) and α-pinene
ozonolysis with NH3 at ∼95.4 % RH. However, LLPS was
not observed in SOA particles produced from α-pinene photooxidation
and α-pinene photooxidation with NH3. Based on datasets of the average
oxygen to carbon elemental ratio (O:C) for different types of SOA particles
from this study and from previous studies, there appears to be a relationship
between the occurrence of LLPS and the O:C of the SOA particles. When LLPS
was observed, the two liquid phases were present up to ∼100 % RH. This result can help more accurately predict the CCN
properties of organic aerosol particles.
Introduction
Secondary organic aerosol (SOA) particles in the atmosphere can be formed by the
oxidation of volatile organic compounds (VOCs) emitted from biogenic and
anthropogenic sources (Hallquist et al., 2009). These SOA particles may
comprise a large fraction of ultrafine aerosol particles depending on the
location (Zhang et al., 2007; Jimenez et al., 2009). They can affect the
energy balance of the Earth by scattering and absorbing solar radiation and
also by acting as nuclei for cloud formation (Kanakidou et al., 2005;
Hallquist et al., 2009; IPCC, 2013; Knopf et al., 2018). In addition, these
particles can affect air quality and human health (Kanakidou et al., 2005;
Jang et al., 2006; Forster et al., 2007; Baltensperger et al., 2008; Murray
et al., 2010; Wang et al., 2012; Pöschl and Shiraiwa, 2015; Shiraiwa et al.,
2017).
Many previous studies have shown that SOA particles can be formed more
efficiently in the presence of gaseous species such as ammonia (NH3)
(Zhang et al., 2004; Na et al., 2006, 2007; Laskin et al., 2014;
T. Y. Liu et al., 2015; Y. Liu et al., 2015; Babar et al., 2017). NH3 is one
of the abundant and reactive gaseous species in the atmosphere (Reis et al.,
2009; Heald et al., 2012; Reche et al., 2015; Zheng et al., 2015; Sharma et
al., 2016; Warner et al., 2016). The chemical composition of SOA particles
can be influenced by reaction with NH3 (Laskin et al., 2015; Y. Liu et
al., 2015b), but the chemical composition from the reaction is still poorly understood.
Aerosol particles containing SOAs can undergo phase transitions in the
atmosphere as relative humidity (RH) changes. Thus far, many researchers have
focused on phase transitions, especially liquid–liquid phase separation
(LLPS) in particles containing SOAs and inorganic salts during changes in RH
(Pankow, 2003; Marcolli and Krieger, 2006; Ciobanu et al., 2009; Bertram
et al., 2011; Krieger et al., 2012; Song et al., 2012a, b;
Zuend and Seinfeld., 2012; Ault et al., 2013; Veghte et al., 2013; O'Brien
et al., 2015). They established that LLPS always occurs in SOA particles
mixed with inorganic salts when the oxygen to carbon elemental ratio (O:C)
of the organic materials is lower than 0.56, whereas LLPS never occurs
when the O:C of the organic materials is higher than 0.80. LLPS commonly
occurs in the intermediate O:C range (Bertram et al., 2011; Krieger
et al., 2012; Song et al., 2012a, 2013; You et al., 2013, 2014). LLPS in a mixture of SOA particles and inorganic salts is
known to affect the optical properties (Fard et al., 2018), gas–particle
partitioning (Zuend et al., 2010; Zuend and Seinfeld, 2012; Shiraiwa et al.,
2013), reactivity (Kuwata and Martin, 2012), hygroscopic properties (Hodas
et al., 2016) and cloud condensation nuclei (CCN) properties of these
particles (Hodas et al., 2016; Ovadnevaite et al., 2017; Rastak et al.,
2017; Altaf et al., 2018).
More recently, researchers have focused on LLPS in SOA particles in the
absence of inorganic salts (Petters et al., 2006; Renbaum-Wolff et al., 2016;
Rastak et al., 2017; Song et al., 2017, 2018) as it is
important to explore the CCN properties of the particles (Petters et al.,
2006; Hodas et al., 2016; Renbaum-Wolff et al., 2016; Ovadnevaite et al.,
2017; Rastak et al., 2017; Liu et al., 2018). Renbaum-Wolff et al. (2016)
and Song et al. (2017) observed LLPS at a high RH of ∼95 %–100 % in SOA particles produced from the ozonolysis of α-pinene,
β-caryophyllene and limonene. However, Rastak et al. (2017) and Song
et al. (2017) did not observe LLPS in SOA particles produced from the
photooxidation of isoprene and toluene. The occurrence of LLPS in SOA
particles free of inorganic salts was related to the average O:C of the
organic materials. When the average O:C of the SOA particle was less than
∼0.44, LLPS was observed in the SOA particles free of
inorganic salts (Renbaum-Wolff et al., 2016; Rastak et al., 2017; Song et
al., 2017). Song et al. (2018) studied organic particles consisting of one
and two commercially available organic species free of inorganic salts and
found that the average O:C of the organic material can be an important
parameter to predict LLPS. LLPS was observed in particles containing one
organic species at an O:C of ≤0.44 and in particles containing
two organic species at an O:C of ≤0.58. As few
systems have been studied thus far, more studies are needed to confirm the
effect of O:C on the LLPS in organic particles.
Herein, we investigated LLPS in SOA particles produced from the ozonolysis and
photooxidation of α-pinene. Moreover, we studied the effects of
NH3 on SOA particles produced from the ozonolysis and photooxidation of
α-pinene on the occurrence of LLPS.
ExperimentalProduction of SOA particles
Four different types of SOA particles were generated in the flow tube
reactor of Kyungpook National University (KNU), South Korea: those produced via
α-pinene ozonolysis and α-pinene photooxidation in the
absence of NH3 (Table 1), and those produced via α-pinene
ozonolysis and α-pinene photooxidation in the presence of NH3
(Table 2). The method of SOA particle generation was previously described by
Babar et al. (2017). The flow tube reactor was run at a flow rate of 4.0 L min-1, with a residence time of 3.63 min at
∼10 % RH.
Experimental conditions for production and collection of SOA
particles from α-pinene ozone (termed “α-pinene O3”)
and photooxidation (termed “α-pinene OH”). The separation relative
humidity (SRH) upon moistening and the merging relative humidity (MRH) upon
drying are listed. The SRH is the RH at which liquid–liquid phase separation
occurred. The MRH is the RH at which two phases merged into one phase. The
uncertainties indicate the 2σ from several humidity cycles for one
sample and from the uncertainty of the calibration. SRH and MRH values of zero indicate that phase separation was not observed. “conc.” refers to concentration.
* After dilution of 4 L min-1 mainstream with 7 L min-1 of
humidified air at 60 % RH.
α-pinene at a concentration of 1000 ppb was injected into the flow tube
reactor to produce SOA particles via ozonolysis without NH3. O3
was produced by passing high purity O2 through a UV lamp (λ=185 nm) and was injected into the flow tube reactor at a concentration
of 10 000 ppb. Table 1 presents the experimental conditions for the
ozonolysis.
To produce SOA particles via photooxidation in the absence of NH3,
1000 ppb of α-pinene was injected in the flow tube reactor (Table 1). The OH radical was produced by photodissociation of O3 by irradiating
O3 with UV (λ=254 nm) in the presence of water vapor. The
following photochemical reactions take place:
R1O3+hv→O2+OR2O+H2O→2OH
In the flow tube reactor, OH concentrations were determined from the
photochemical decay of toluene, as toluene is well known for its OH
reaction rate. The OH reaction rate constant (kOH) of toluene is 5.48×10-12 molecules cm-3 s-1 with an insignificant
reaction rate with O3 (Atkinson and Aschmann, 1989). The OH concentrations
were calculated by varying O3 and RH from 2000 to 8000 ppb and
10 % to 60 %, respectively. The OH concentrations were calculated by the first-order decay of toluene by reaction with OH radicals (Babar et al., 2017).
Assuming an atmospheric OH concentration of 1.5×106 molecules cm-3, OH exposures were
8.2×1010 and 2.3×1011 molecules cm-3 s, corresponding to an atmospheric
aging time of 0.5 and 2.5 d, respectively, and the concentrations of
O3 in the reactor were 2000 and 6000 ppb at 10 % RH, respectively.
The same method was used for the SOA particle generation via ozonolysis and
photooxidation in the presence of NH3, the exception being that
NH3 was injected into the flow tube reactor during particle generation.
The concentration of NH3 injected was 2000 ppb for the ozonolysis and
photooxidation (Table 2).
Experimental conditions for the production and collection of SOA
particles from α-pinene ozone with NH3 (termed “α-pinene O3/NH3”) and photooxidation with NH3 (termed
“α-pinene OH/NH3”). The separation relative humidity (SRH) upon
moistening and the merging relative humidity (MRH) upon drying are listed.
The SRH is the RH at which liquid–liquid phase separation occurred. The MRH
is the RH at which two phases merged into one phase. The uncertainties
indicate the 2σ from several humidity cycles for one sample and from
the uncertainty of the calibration. SRH and MRH values of zero indicate that
LLPS was not observed.
* After dilution of 4 L min-1 mainstream with 7 L min-1 of
humidified air at 60 % RH.
The 4 L min-1 mainstream flow of SOA particles at the outlet
of the flow tube reactor was diluted by a humidified air stream (RH of
60 %) of 7 L min-1. A diffusion dryer loaded with silica
gel was used upstream of the scanning mobility particle sizer (SMPS+C,
Grimm, Germany) for the measurement of dry SOA mass concentrations. After
dilution, the mass concentrations of the SOA particles were measured to
range between ∼480 and
∼880µg m-3 using the SMPS for different
experimental conditions as presented in Tables 1 and 2. The sample and
sheath flow rates of the SMPS were 0.3 and 3.0 L min-1,
respectively. The SOA particles consisting of up to ∼5µm were collected at the outlet of the reactor on a siliconized substrate
(siliconized glass slides of 18 mm, Hampton Research, USA). Figure S1 in the Supplement is an
example image of collected SOA particles derived from α-pinene
ozonolysis (α-pinene O3 no. 1 in Table 1) on a hydrophobic
substrate at the outlet of the flow tube reactor.
For each experiment, the siliconized glass slide was initially cleaned three
times with water and methanol. It was then dried by purging N2 gas.
Finally, it was fixed in the Stage D collector plate of a Sioutas cascade
impactor (SKC cat no. 225–370, USA), operated at 9 L min-1.
Observation of liquid–liquid phase separation in SOA particles
The observation of LLPS in a particle requires particle diameters of 20–80 µm. In order to obtain the appropriate particle sizes for the LLPS
experiments, SOA particles sized up to 5 µm collected on the
siliconized substrate from the flow tube reactor were placed into a
RH-controlled flow-cell coupled to an optical microscope (Olympus BX43,
40× objective) (Parsons et al., 2004; Pant et al., 2006; Bertram et
al., 2011; Song et al., 2012b, 2018) at ∼100 %
RH; the particles then grew and coagulated for ∼60 min.
This process resulted in a particle size of 20–80 µm (Renbaum-Wolff et
al., 2016). Once the particle size was appropriate for the LLPS experiments,
humidity cycles were performed.
During a humidity cycle, the RH was reduced from ∼ 100 % to
∼5 %–10 % lower than the RH at which the two liquid phases
merged into one phase, followed by an increase to ∼100 % RH
at a rate of 0.1 %–0.5 % RH min-1. If LLPS was not
observed, the RH was reduced from ∼100 % to ∼0 %, and
then it was increased to ∼100 % at a rate of 0.5 %–1.0 % RH min-1. We did not observe a dependence of LLPS on
the humidity ramp rate. The optical images of the SOA particles during the
experiment were recorded every 5 s using a complementary metal oxide
semiconductor detector (DigiRetina 16, Tucsen, China). All of the experiments
were performed at a temperature of 289±0.2 K.
The RH was controlled by the ratio of N2/H2O gas at a total flow
rate of 500 sccm. The RH inside the flow-cell was determined using a
temperature and humidity sensor (Sensirion SHT71, Switzerland) which was
calibrated by observing the deliquescence RH for the following pure
inorganic salts at 293 K: potassium carbonate (44 % RH), sodium chloride
(76 % RH), ammonium sulfate (80.5 % RH) and potassium nitrate particle
(93.5 % RH) (Winston and Bates, 1960). The uncertainty of the RH after
calibration was ±2.0 %.
Results and discussionSOA particles produced from α-pinene ozonolysis and α-pinene photooxidation
SOA particles generated by α-pinene ozonolysis with a mass
concentration of ∼500–1000 µg m-3
underwent humidity cycles at 289±0.2 K. Figure 1 shows examples of
optical images of a SOA particle (α-pinene O3 no. 1 in Table 1)
produced from α-pinene ozonolysis with increasing RH. Only one phase
was observed from 0 to ∼96 % RH (Fig. 1). At 96.6 % RH,
LLPS occurred via a spinodal decomposition mechanism, which distributes
many small inclusions (schlieren) throughout a particle (Ciobanu et al.,
2009; Song et al., 2012b). After phase separation, at ∼97.0 % RH, small droplets grew and coagulated to form inner and outer
phases in the particle. As the RH increased further, the SOA particle
displayed a core–shell morphology consisting of inner and outer phases. The
two liquid phases coexisted up to ∼100 % RH, as shown in
Fig. 1. When the RH decreased from ∼100 %, the inner phase
became smaller and merged into one phase at ∼95.0 % RH. We
assume that the inner phase is a water-rich phase and the outer phase is an
organic-rich phase as the size of the inner phase depends on changes in
RH (Renbaum-Wolff et al., 2016; Song et al., 2017, 2018).
Optical images of a SOA particle produced from α-pinene
ozonolysis (α-pinene O3 no. 1 in Table 1) with increasing RH.
The purpose of the illustrations is to clarify the images. Green is the SOA-rich phase, and blue
is the water-rich phase. The scale bar is 20 µm.
Table 1 summarizes the separation relative humidity (SRH) upon moistening
and the merging relative humidity (MRH) upon drying. In all cases, the SOA
mass concentration was ∼500–1000 µg m-3. LLPS was observed at 95.8±2.3 % RH for all SOA particles
derived from α-pinene ozonolysis, and the two phases merged into one
phase at 92.9±4.6 % RH. The uncertainties of the SRH and the MRH
indicate the 2σ from several humidity cycles for one sample and from
the uncertainty of the calibration.
Renbaum-Wolff et al. (2016) observed LLPS in SOA particles derived from
α-pinene ozonolysis at ∼95 % RH, which is consistent
with our result. They also showed that LLPS in the particles did not depend
on the SOA particle mass concentrations between 75 and 11 000 µg m-3. As the SOA particle mass concentration does not affect LLPS, in
this study, we only focused on SOA particle mass concentrations between
∼500 and 1000 µg m-3 for different types
of SOA particles.
We also performed humidity cycles for SOA particles with mass concentrations between
∼500 and 1000 µg m-3 derived from
α-pinene photooxidation. Table 1 summarizes the results of the
humidity cycles. None of the SOA particles from α-pinene
photooxidation underwent LLPS during the RH cycles. Figure 2 shows examples
of optical images of a SOA particle (α-pinene OH no. 2 in Table 1)
for increasing RH. From 0 % RH to 100 % RH, there was no evidence of the occurrence
of LLPS in the particles.
Optical images of a SOA particle produced from α-pinene
photooxidation (α-pinene OH no. 2 in Table 1) with increasing RH.
The purpose of the illustrations is to clarify the images. Green is the SOA-rich phase, and blue
is the water-rich phase. The scale bar is 20 µm.
SOA particles produced from α-pinene ozonolysis with NH3 and α-pinene photooxidation with NH3
Ammonia (NH3) is an abundant and reactive gaseous species in the atmosphere (Reis
et al., 2009; Heald et al., 2012; Reche et al., 2015; Zheng et al., 2015;
Sharma et al., 2016; Warner et al., 2016). Previous studies have shown that SOA particles can be formed more effectively in the presence of NH3
(Zhang et al., 2004; Na et al., 2006, 2007; T. Y. Liu et al., 2015; Y. Liu et al., 2015; Babar et al., 2017). To investigate the effect of
NH3 on LLPS in SOA particles, we studied LLPS in SOA particles using
α-pinene ozonolysis and photooxidation in the presence of NH3.
Table 2 presents the experimental conditions for the particle generation. We
used the experimental conditions of SOA particle generation via ozonolysis
and photooxidation (Table 1) in this case too, but we injected 2000 ppb of
NH3 into the flow tube reactor during particle generation (Table 2).
We performed humidity cycles for the SOA particles produced from α-pinene ozonolysis in the presence of NH3 for mass concentrations
between ∼500 and 1000 µg m-3. Figure 3 shows
examples of the optical images of SOA particles produced by α-pinene
ozonolysis in the presence of NH3 as a function of increasing RH
(α-pinene O3/NH3 no. 1 in Table 2). Upon moistening, only
one phase was present (Fig. 3). As RH increased, the one phase of the SOA
particle was separated into two phases at 95.3 % RH; the underlying
mechanism for this separation was spinodal decomposition. At 95.6 % RH, small inclusions in
the particle coagulated and grew; then, as RH increased further, a
core–shell morphology, with a shell consisting of an organic-rich phase and
the core consisting of a water-rich phase on a substrate, was observed. The
two liquid phases coexisted up to ∼100 % RH. When the RH
decreased from ∼100 % RH, the inner phase of the particle
became smaller, and, eventually, the inner phase merged into one phase at
94.4 % RH.
Optical images of a SOA particle produced from α-pinene
ozonolysis with NH3 (α-pinene O3/NH3 no. 1 in Table 2) with increasing RH. The purpose of the illustrations is to clarify the images. Green is the
SOA-rich phase, and blue is the water-rich phase. The scale bar is 20 µm.
Table 2 summarizes the results of average SRH and MRH during the humidity
cycles for the SOA particles produced by α-pinene ozonolysis in the
presence of NH3. LLPS occurred at 95.4±2.9 % RH, and the two
phases merged into one phase at 94.4±2.7 % RH for all of the
particles (Table 2).
For SOA particles derived from α-pinene photooxidation in the
presence of NH3, no LLPS was observed during changes in RH. Table 2
lists the results of SRH and MRH for two different SOA particles derived
from α-pinene photooxidation in the presence of NH3. Figure 4
shows the examples of the optical images of SOA particles produced by α-pinene with NH3 photooxidation for increasing RH (α-pinene OH/NH3 no. 2 in Table 2). Only one phase was observed
from 0 % RH to 100 % RH.
Optical images of a SOA particle produced from α-pinene
photooxidation with NH3 (α-pinene OH/NH3 no. 2 in Table 1) with increasing RH. The purpose of the illustrations is to clarify the images. Green is the
SOA-rich phase, and blue is the water-rich phase. The scale bar is 20 µm.
Phases of the four different types of SOA particles
Figure 5 shows the RH at which two liquid phases were observed during RH
scanning for the four different types of SOA particles. Circles represent the
MRH upon drying, and triangles represent the SRH upon moistening. In the figure,
the values of the SRH and MRH of SOA particles derived from α-pinene
ozonolysis by Renbaum-Wolff et al. (2016) are also included (in red). If the RH equals 0 %, no LLPS was observed.
Relative humidity (RH) at which the two phases were observed during RH
scanning as a function of the four different types of SOA particles. Blue and
red symbols are from this study and from Renbaum-Wolff et al. (2016),
respectively. Circles represent the merging RH (MRH) for RH decreasing, and
triangles represent the separation RH (SRH) for RH increasing. A RH of 0 % indicates no LLPS. The green shaded region indicates one phase present, and the
orange shaded region indicates two phases present in the SOA particles.
Among the four different types of SOA particles, two types of particles
underwent LLPS but the remaining particles did not (Fig. 5). For the SOA
particles derived from α-pinene ozonolysis, two liquid phases
existed at ∼95.8±2.3 % RH up to ∼100±2.0 % RH with increasing RH. For values lower than ∼92.9±4.6 % RH with decreasing RH, only one phase was observed. For
the SOA particles derived from α-pinene ozonolysis in the presence
of NH3, the RH ranges for the two liquid phases were ∼95.4±2.9 % and ∼100±2.0 % with increasing RH.
SRH values of both SOA particles were very similar within the uncertainties
of the measurements. Furthermore, Fig. 5 showed that the values of SRH upon
moistening and MRH upon drying for the two types of particles were close
within the uncertainties of the measurements, suggesting that the kinetic
barrier to LLPS in the particles is low. Compared with the SOA particles
derived from α-pinene ozonolysis and from α-pinene
ozonolysis with NH3, LLPS was not observed in SOA particles derived
from α-pinene photooxidation without/with NH3 (Fig. 5). In
these cases, only one phase was present between 0 % RH and 100 % RH.
Relation between O:C ratio and LLPS
Recent studies have shown that the occurrence of LLPS in SOA particles free of
inorganic salts is related to the average O:C of the organic materials
(Renbaum-Wolff et al., 2016; Rastak et al., 2017; Song et al., 2017). These studies have shown that LLPS can occur in SOA particles derived from α-pinene,
limonene and β-caryophyllene for RH values between ∼95 %
and ∼100 % when the average O:C ranged between 0.34 and
0.44. LLPS was not observed in SOA particles derived from isoprene and
toluene when the average O:C was between 0.52 and 1.30. Figure 6 and
Table S1 in the Supplement show LLPS as a function of the average O:C of SOA particles
from previous studies (Lambe et al., 2015; Li et al., 2015; Renbaum-Wolff et
al., 2016; Song et al., 2017). Also presented in Table S2 are the O:C values
and experimental conditions for the SOA particles produced from α-pinene ozonolysis and photooxidation investigated in this study and
previous studies. In this study, data on the average O:C values were not
available; thus, we chose O:C values from the literature that were
closest to the experimental conditions (Table S2). The O:C values for the SOA
particles derived from α-pinene ozonolysis range from 0.42 to 0.44 as
per Li et al. (2015), whereas those for SOA particles derived from α-pinene photooxidation are between 0.40 and 0.90 according to Lambe et al. (2015).
According to the dataset of average O:C values of different types of SOA
particles from this study as well as previous studies, Fig. 6 shows that
LLPS occurred when the average O:C was between 0.34 and 0.44. This
range of the O:C required for the occurrence of LLPS in the SOA particles
is consistent with those from previous studies (Renbaum-Wolff et al., 2016; Rastak
et al., 2017; Song et al., 2017). However, LLPS did not occur when the
average O:C was between 0.45 and 1.30 in this study. Using a new type
of SOA particle generated from α-pinene photooxidation, we showed
that the absence of LLPS occurs over a wider range of O:C values than reported by a previous work (0.52–1.30; Song et al., 2017).
Similar to the results for LLPS in the SOA particles with respect to the O:C, bulk
solutions containing two organics and water also showed the miscibility gap
(Ganbavale et al., 2015). For example, bulk solutions of two organics with a
low O:C and water (e.g., a mixture of 1-butanol, 1-propanol and water)
formed two liquid phases (Ganbavale et al., 2015). However, bulk solutions
of two organics with a high O:C and water (e.g., a mixture of ethanol, acetic
acid and water) formed a single liquid phase.
Previous studies have found nitrogen-containing SOA species in the presence of
NH3 (Laskin et al., 2015; Y. Liu et al., 2015). These studies suggested that
ammonium carboxylates were formed by neutralization between carboxylic acid
and ammonia, and that amines were formed by carbonyl and ammonia via a Schiff
base reaction (Na et al., 2006, 2007; Laskin et al., 2015). The
nitrogen to carbon (N:C) ratio was reported to be 0.01–0.08 based on
aerosol mass spectrometry (AMS) and Fourier transform ion cyclotron
resonance (FT-ICR MS) (Laskin et al., 2014; Y. Liu et al., 2015). It is
noteworthy that ammonium carboxylates and amines are highly water soluble
compounds. However, more accurate data regarding O:C values of SOA particles
in the presence of NH3 are needed.
Relative humidity in two phases as a function of the average O:C of
SOA particles derived from α-pinene ozonolysis (pink) and α-pinene photooxidation (cyan) in this study, β-caryophyllene
ozonolysis (black) from Song et al. (2017), α-pinene ozonolysis
(blue) from Renbaum-Wolff et al. (2016), limonene ozonolysis (orange) from
Song et al. (2017), toluene photooxidation (green) from Song et al. (2017)
and isoprene photooxidation (purple) from Rastak et al. (2017). The O:C and
related experimental conditions are summarized in Tables S1 and S2 in the Supplement.
Figure 6 also showed the range of the two liquid phases. The two phases
consisting of an organic-rich shell and water-rich core were observed at RH values as high as ∼100 % in all cases. Recent studies by Rastak et
al. (2017) and Liu et al. (2018) showed, using laboratory work and modeling
results, that the presence of LLPS in organic particles at ∼100 % RH can lead to lower surface tension, and finally a lower kinetic
barrier to CCN activation. Our result can also provide additional insight
into attempting more accurate predictions of the CCN properties of organic
particles (Petters et al., 2006; Hodas et al., 2016; Renbaum-Wolff et al.,
2016; Ovadnevaite et al., 2017; Rastak et al., 2017; Liu et al., 2018).
Summary
In this study, we investigated liquid–liquid phase separation of SOA
produced from both α-pinene ozonolysis and α-pinene
photooxidation in the presence or absence of NH3. We conducted
humidity cycles at a temperature of 289±0.2 K for four different SOA
particles derived from α-pinene ozonolysis, α-pinene
photooxidation, α-pinene ozonolysis with NH3 and α-pinene photooxidation with NH3, for particle mass concentrations of
∼500–1000 µg m-3. Among the four
different types of SOA particles, LLPS occurred in SOA particles produced
from α-pinene ozonolysis at 95.8±2.3 % RH with increasing
RH and in those produced from α-pinene ozonolysis with NH3 at
95.4±2.9 % RH with increasing RH. In both types of particles, the
two liquid phases coexisted up to ∼100 % RH. However, LLPS
was not observed in SOA particles produced from α-pinene
photooxidation and α-pinene photooxidation with NH3. LLPS
occurred in the SOA particles produced by α-pinene ozonolysis, whereas no LLPS was observed in the SOA particles produced by α-pinene
photooxidation. In addition, the occurrence of LLPS did not depend on the
presence or absence of NH3. Analysis of the dataset of the average O:C values of different types of SOA particles from this study and those from previous
studies indicated that LLPS occurred when the O:C was less than
∼0.44, and LLPS did not occur when the O:C was greater
than ∼0.40.
Considering the range of the O:C values of organic particles in the
atmosphere (0.2–1.0), these results provide additional evidence that LLPS
can occur in organic particles even without the presence of inorganic salts
in the atmosphere. Moreover, LLPS occurred in the SOA particles at high RH values (as high as ∼100 %), implying that these results can
provide additional information regarding the CCN properties of organic
particles. Additional studies are needed to confirm LLPS in SOA particles
produced using more atmospherically relevant VOC mass concentrations,
particle mass concentrations and submicron sizes.
Data availability
Underlying material and related items for this paper are located in the Supplement.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-9321-2019-supplement.
Author contributions
MS and HJL conceived and designed the experiments. SH, JBL and
ZBB performed the experiments and analyzed the data. SH and MS wrote
the paper, and JBL and HJL edited the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
This work was supported by the National Research Foundation of Korea grant
funded by the Korea Government (MSIP; grant no. 2016R1C1B1009243). This research was
supported by the National Strategic Project-Fine Particle of the National
Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT
(MSIT), the Ministry of Environment (ME), and the Ministry of Health and
Welfare (MOHW; grant no. 2017M3D8A1092015).
Financial support
This research has been supported by the National Research Foundation of Korea grant funded by the Korea Government (MSIP; grant no. 2016R1C1B1009243) and the National Strategic Project-Fine Particle of the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (MSIT), the Ministry of Environment (ME), and the Ministry of Health and Welfare (MOHW; grant no. 2017M3D8A1092015).
Review statement
This paper was edited by Jason Surratt and reviewed by three anonymous referees.
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