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Predicting Secondary Organic Aerosol Enhancement in the Presence of Atmospherically Relevant Organic Particles Jianhuai Ye, Paul Van Rooy, Cullen Adam, Cheol-Heon Jeong, Bruce Urch, David R. Cocker, Greg J. Evans, and Arthur W. H. Chan ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.8b00093 • Publication Date (Web): 24 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018
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ACS Earth and Space Chemistry
"!
Predicting Secondary Organic Aerosol Enhancement in the Presence of
#!
Atmospherically Relevant Organic Particles
$! %!
Jianhuai Ye1, Paul Van Rooy2, 3, Cullen H. Adam1, Cheol-Heon Jeong1, 4, Bruce Urch4, 5, David R. Cocker
&!
III2, 3, Greg J. Evans1, 4, 5, Arthur W.H. Chan1, 4, *
'! (!
1
Department of Chemical Engineering & Applied Chemistry, University of Toronto, Toronto, Ontario, Canada
)!
2
Department of Chemical and Environmental Engineering, University of California, Riverside, Riverside, CA, USA
*!
3
Center for Environmental Research and Technology, College of Engineering, University of California, Riverside,
"+!
Riverside, CA, USA
""!
4
Southern Ontario Centre for Atmospheric Aerosol Research, University of Toronto, Toronto, Ontario, Canada
"#!
5
Division of Occupational and Environmental Health, Dalla Lana School of Public Health, University of Toronto,
"$!
Toronto, Ontario, Canada.
"%! "&!
Abstract
"'!
Secondary organic aerosol (SOA) produced from atmospheric oxidation of organic vapors,
"(!
comprises a large portion of ambient particulate matter. Currently, SOA models typically assume
")!
that all organic species form a well-mixed phase as a simplification, which follows that SOA
"*!
formation is enhanced in the presence of pre-existing organic aerosol (OA) according to the
#+!
Raoult’s Law. In this work, we show through experiments with atmospherically relevant OA that
#"!
not all organic species are equally miscible, and the thermodynamics of mixing are composition
##!
dependent. SOA formation from !-pinene ozonolysis was investigated in the presence of OA that
#$!
was collected from Toronto ambient air and other sources including biomass burning, meat-
#%!
cooking emissions and diesel exhaust. Compared to experiments with ammonium sulfate seed
!
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#&!
particles, enhanced SOA yields were observed with particles from biomass burning, meat cooking,
#'!
but not with diesel exhaust and concentrated ambient particles. We demonstrate that both kinetic
#(!
(bulk diffusion-limitation) and thermodynamic (miscibility-limitation) factors are important in
#)!
determining atmospheric organic aerosol partitioning. We develop parameterization methods
#*!
using bulk elemental ratios (H/C and O/C) and functional group abundance (FOH and FCOOH) to
$+!
estimate average intermolecular interactions, which allow us to use Hansen Solubility framework
$"!
we had previously developed to predict atmospheric organic aerosol miscibility and SOA yield
$#!
enhancements in these complex mixtures. The framework has also been utilized to better
$$!
understand the liquid-liquid phase separation between organic aerosol and inorganic salts. Our
$%!
results show that a molecular description of thermodynamic forces is needed to describe aerosol
$&!
mixing in the atmosphere and accurately parameterize SOA formation.
$'! $(!
Keywords: aerosol miscibility; gas-particle partitioning; intermolecular interactions; kinetic and
$)!
thermodynamic limitations; Hansen Solubility Parameters; !-pinene ozonolysis
$*! %+!
1.! Introduction
%"!
Organic aerosol (OA) comprises a major fraction (20-90%) of submicron particulate matter (PM)1,
%#!
which is important for global climate change and human health2–4. A large fraction of OA is
%$!
secondary (known as secondary organic aerosol, or SOA), formed from oxidation of volatile
%%!
organic compounds followed by condensation of low-volatility products. Owing to the complexity
%&!
in oxidation precursors and atmospheric oxidation processes, the composition and the physical and
%'!
chemical properties of SOA remain poorly understood. These properties directly impact gas-
!
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ACS Earth and Space Chemistry
%(!
particle partitioning of semivolatile organic compounds (SVOCs), and, in turn, regional and global
%)!
PM budgets5.
%*! &+!
Traditional atmospheric models assume that organic compounds in the atmosphere form a single
&"!
well-mixed phase and undergo semivolatile partitioning6. An important corollary of semivolatile
&#!
partitioning is that the presence of organic seed particles during SOA formation decreases the
&$!
activity of SOA in the particle phase according to the Raoult’s Law. This decrease in activity in
&%!
turn leads to reduced equilibrium partial pressures and an increase in SOA formation by shifting
&&!
equilibrium from the gas phase to the particle phase. Therefore, SOA yield (Y), that is the mass of
&'!
SOA generated per mass of hydrocarbon reacted, must increase with increasing the amount of
&(!
organic seed aerosol7, as described in Eqn. (1):
&)!
!"
&*!
where 78 and 981 are the mass stoichiometric coefficient and the effective saturation vapor pressure
'+!
of component i, respectively; :; is the mass concentration of total miscible organic aerosol in the
'"!
particle phase. Under the well-mixed assumption, :; refers to the total mass concentration of
'#!
organic seed aerosol :< and generated SOA (#=>?).
#$%&' #$()
+ - /0 1
+ 4- 3#%&'6/0 1
5 , . , , , $ " $ * 23$ " * 234$ /0 1 3#%&'6/0 1 .
,$
5
(1)
,$
'$! '%!
Recent studies have brought into question the validity of the well-mixed assumption. Song et al.
'&!
observed no SOA yield enhancement from !-pinene ozonolysis with dioctyl phthalate particles
''!
(DOP) seed, implying that SOA and DOP form separate organic phases8. Using single-particle
'(!
mass spectrometry, Robinson et al. demonstrated that SOA from !-pinene ozonolysis and liquid
')!
squalane particles are immiscible9, consistent with the lack of SOA enhancement observed in our
'*!
previous laboratory studies10. One approach used in recent SOA models is to separate OA into
!
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(+!
hydrophobic and hydrophilic phases, but this approach is still unable to reproduce all the laboratory
("!
observations10,11. For example, formation of !-pinene SOA (considered to be hydrophilic) was not
(#!
enhanced in the presence of adipic or fulvic acid seed11, or erythritol or levoglucosan seed10, all of
($!
which are oxygenated and hydrophilic.
(%! (&!
The phase state of atmospheric aerosol may also be more complex than previously thought8,10–13.
('!
Since the physical state of organic aerosol of atmospheric OA plays an important role in gas-
((!
particle partitioning14–17, the well-mixed assumption could lead to errors in SOA estimates10,18.
()!
SOA can be comprised of high molecular-weight oligomers, resulting in a semi-solid or solid
(*!
organic phase19. The formation of an amorphous glassy phase (with dynamic viscosity @$ A 102 Pa
)+!
s)14 may prevent partitioning equilibrium by suppressing SVOC evaporation. Recent laboratory
)"!
experiments have shown that the kinetic limitations to mixing vary between different SOA types:
)#!
Liu et al.20 and Ye et al.21 demonstrate that diffusion of SVOCs in the particle is not kinetically
)$!
limited for isoprene and monoterpene SOA under atmospherically relevant humidity conditions,
)%!
but the mixing timescales may be much longer for toluene20 and sesquiterpene SOA22 and phase
)&!
equilibrium may not be achieved within atmospheric time scales. Moreover, lower temperatures
)'!
(such as those in the upper troposphere23) may also hinder mixing in the organic particle.
)(! ))!
In addition to kinetic considerations, thermodynamic driving forces also play important roles in
)*!
organic mixing and gas-particle partitioning6,24,25. For mixing of two organic materials to occur
*+!
spontaneously, the change in Gibbs free energy due to mixing #BC8D " $ #EC8D F G#=C8D $
*"!
must be less than or equal to zero. Therefore, either increasing the temperature (G6 and entropy of
*#!
mixing (#=C8D ) or decreasing the heat of mixing (#EC8D ) helps to decrease the free energy of
!
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ACS Earth and Space Chemistry
*$!
mixing. Among these three parameters, #EC8D depends strongly on the intermolecular interactions
*%!
between the solute and the solvent26,27. It is suggested that organic compounds with distinct
*&!
intermolecular forces, like methanol and hexane, form separated phases, and the resulting phase
*'!
separation could lead to reduced prediction of particle-phase SOA formation through gas-particle
*(!
partitioning8,11,28. In our previous work, we have developed a molecular framework to understand
*)!
organic mixing using Hansen Solubility Parameters (HSP) that represent favorability of
**!
thermodynamic interactions between organic compounds10. The applicability of this framework
"++!
was demonstrated for mixing between !-pinene ozonolysis SOA, but in order to accurately
"+"!
describe mixing between SOA and realistic atmospheric mixtures, parameterization of the
"+#!
chemical composition is needed. Here we demonstrate that HSP is a useful parameter to predict
"+$!
SOA mixing and yield enhancement, using !-pinene ozonolysis SOA as a model SOA, and organic
"+%!
seed aerosol from atmospherically relevant emission sources and from ambient air. Particle seeds
"+&!
used in this study span a wide range of polarities and water solubilities. The complex composition
"+'!
of realistic seeds necessitates the use of bulk elemental ratios (H/C and O/C) and the functional
"+(!
group information (-OH and -COOH groups) to calculate HSPs.
"+)! "+*!
2.! Experimental
""+!
Experiments were conducted in a 38-m3 Teflon chamber29 in University of California, Riverside
"""!
or a 1-m3 Teflon chamber10 in University of Toronto. All experiments were conducted under
""#!
moderately humid conditions with relative humidity ~ 40-60% at room temperature (22-26 H). !-
""$!
pinene (Sigma-Aldrich, 98%) was used as SOA precursor and monitored by gas chromatography-
""%!
flame ionization detector (GC-FID). Cyclohexane (Sigma-Aldrich, 99.5%) was injected at
""&!
sufficient amount to scavenge OH. Ozone was produced by passing dry, purified air through an
!
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""'!
ozone generator. In the experiments with ammonium sulfate (AS) seeds, the seed aerosol was
""(!
generated with a TSI 3076 Aerosol Generator and dried using a diffusional dryer before
"")!
introducing into the chambers and flow tube reactor.
""*! "#+!
2.1 Experiments with biomass burning or meat-cooking emissions in a 38-m3 chamber
"#"!
Aerosol from biomass burning emissions was generated in a woodstove. The fuels burned included
"##!
dry manzanita branches, pine needles, and pine wood (see Supporting Information, Fig. S1A).
"#$!
Emissions were introduced into the chamber while the burning was under mixed flaming and
"#%!
smoldering conditions. Meat-cooking particles were produced by grilling beef patty (85% lean and
"#&!
15% fat) on a barbecue stove (Grill Master, Model 720-0697) using propane as fuel. The use of
"#'!
propane fuel minimized particle formation from the fuel. The barbecue stove was preheated for 5-
"#(!
10 min. Beef patty was placed on a layer of aluminum foil without direct contact with the flames.
"#)!
Overcooking was avoided to minimize black carbon emissions during the experiments.
"#*! "$+!
For both biomass burning and meat-cooking experiments, emissions were introduced directly into
"$"!
the chamber without removing semivolatile gases. Control experiments with injection of ozone
"$#!
and emissions only, without !-pinene, were conducted to investigate the reactivity of emissions
"$$!
(including gases and particles) towards ozone. The bulk chemical composition of particles was
"$%!
monitored using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS,
"$&!
hereafter referred to as AMS). H/C and O/C ratios were calculated using PIKA v1.16I with
"$'!
SQUIRREL v1.57I30. An improved-ambient calibration for H/C and O/C ratios proposed by
"$(!
Canagaratna et al.31 was applied during the analysis. Particle size distribution (28-730 nm) and
"$)!
volume concentration was measured by a custom-built scanning mobility particle sizer (SMPS)
!
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ACS Earth and Space Chemistry
"$*!
with a differential mobility analyzer (TSI 3081) and a condensation particle counter (TSI 3772).
"%+!
Particles were collected onto Teflon filters or pre-baked quartz filters for further use.
"%"! "%#!
!-pinene/cyclohexane solution was injected into a flask which was heated to 60 °C. Purified air
"%$!
(10 L/min) was blown through the flask to carry !-pinene and cyclohexane vapor into the chamber.
"%%!
Ozone was then injected into the chamber until its concentration was more than three times higher
"%&!
than !-pinene concentrations to ensure complete consumption. The concentration of ozone was
"%'!
monitored by ozone analyzer (Model 1003, Dasibi Environmental Corp.).
"%(! "%)!
2.2 Experiments with diesel exhaust, Toronto ambient particles, or meat-cooking particles
"%*!
in a 1-m3 chamber
"&+!
Particles from diesel exhaust were generated using a 3600W diesel generator (E3750, PRAMAC)
"&"!
with a Yanmar 4-stroke single cylinder engine (LV70V6EF1C1AA). The generator was run at
"&#!
1000W (30% load). Due to the high concentrations of NOx in the diesel exhaust, NO3 can be
"&$!
formed in the presence of NO2 (200-300 ppb measured during the experiments, by Thermal
"&%!
Environmental Instruments NO-NO2-NOX Analyzer, Model 42C) and ozone, which may further
"&&!
react with !-pinene. To avoid complication by NO3 reactions, experiments with diesel exhaust
"&'!
were conducted using an extraction-atomization method. Particles were collected onto 47 mm
"&(!
Teflon filters or pre-baked quartz filters at a sampling flow rate of 28 L/min. Ambient particles
"&)!
were collected at street level at University of Toronto, St. George campus in downtown Toronto.
"&*!
Sample was collected onto 90 mm pre-baked quartz filters at a sampling flow rate of 120 L/min
"'+!
during the summer time (2016/05/28-2016/05/31) using a PM2.5 sampler (URG-3000DB). The
"'"!
chemical composition of ambient particles was monitored using Aerodyne aerosol chemical
!
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"'#!
speciation monitor (ACSM)32. Particles from meat-cooking emissions were collected using the
"'$!
experimental setup described in Section 2.1 and following the same manner as diesel exhaust
"'%!
collection. All the filters were stored at -20°C before use.
"'&! "''!
Particle filters were ultrasonicated in a 10 mL methanol/dichloromethane (7:3) solution at room
"'(!
temperature for 10 min. The extraction process was repeated for 3 times for each filter. And the
"')!
extraction efficiency was examined in separated experiments by comparing organic carbon (OC)
"'*!
and elemental carbon (EC) on each filter before and after extraction. OC/EC analyses were
"(+!
performed using OC-EC Aerosol Analyzer (Model 4L, Sunset Laboratory Inc.). After
"("!
ultrasonication, the suspended insoluble components (mostly black carbon) were filtered using a
"(#!
syringe filter (Chromafil® Xtra PET-45/25, pore size 0.45 µm). Aerosol particles were then
"($!
generated using an atomizer (TSI, Model 3076) and volatile solvents were removed from the
"(%!
particle stream by a charcoal denuder (Aerosol Dynamics Inc.) prior to introduction into a 1-m3
"(&!
chamber prefilled with ozone. !-pinene/cyclohexane solution was injected into a glass vessel and
"('!
introduced into the chamber by purified air at a flow rate of 10 L/min to initiate SOA formation.
"((!
Particle size distribution (18-1040 nm) and volume concentration were monitored using SMPS.
"()! "(*!
A typical chamber experiment lasts 4-5 hr. In all experiments (both for experiments in 1-m3 and
")+!
38-m3 chambers), a diffusion dryer was placed inline to remove water in particles before SMPS
")"!
sampling. Particle volume concentrations were corrected for wall loss assuming a first-order loss
")#!
rate, calculated from particle volume concentration decay 30 min after !-pinene concentrations
")$!
fell below detection limits (~1 ppb). A density of 1.25 g/cm3 was applied to the calculation of !-
")%!
pinene SOA mass concentration33.
!
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ACS Earth and Space Chemistry
")&!
2.3 Experiments with concentrated Toronto ambient particles in a flow tube reactor
")'!
Toronto ambient particles were concentrated using a three-stage Harvard Ambient Particle
")(!
Concentrator. A detailed description of the concentrator can be found in Lawrence et al.34. Briefly,
"))!
ambient air was sampled into the concentrator system at a flow rate of 5.5 m3/min. Particles with
")*!
diameter larger than ~2.8 µm were removed by a modified Sierra-Anderson Hi-Vol PM15 inlet at
"*+!
this flow rate. 80% of the flow was split out of the system at the first concentration stage, leaving
"*"!
a minor flow (1.1 m3/min) carrying the majority of the particles to the next stage due to the physical
"*#!
momentum of the particles. Particles were further concentrated at the second and third stages in
"*$!
the same manner. At each stage, particle concentration was increased by a factor of ~3. In total,
"*%!
the concentration of particles at the outlet of concentrator was suggested to be ~ 30 times higher
"*&!
than that in ambient air.
"*'! "*(!
Ambient particles were introduced into the flow tube by drawing a total flow of 2.7 L/min at the
"*)!
outlet of the flow tube reactor (10 L with a residence time of 3.7 min) with a pump. The flow tube
"**!
reactor was operated in a continuous flow mode. !-pinene / cyclohexane solution was injected to
#++!
a 1 L/min carrier gas and introduced into the flow tube. Ozone was produced by passing 0.5 L/min
#+"!
compassed air (Praxair Inc., Canada) through an ozone generator (UVP 97006601). The remaining
#+#!
1.2 L/min was used to draw concentrated ambient air into the system which was equipped with a
#+$!
charcoal denuder at the inlet to remove O3, NOX and semi-volatiles. The humidity inside the flow
#+%!
reactor was measured below 40%. The experimental conditions were summarized in Table S2.
#+&! #+'!
2.4 Chemical composition analysis of particle filters
!
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#+(!
Particle composition was analyzed using a thermal desorption – gas chromatograph coupled with
#+)!
a mass-selective detector (TD-GC/MS, Gerstel TDS3 thermal desorption system; Agilent Model
#+*!
7890B GC and 5977A MS). Chemical analysis was performed under both non-derivatization and
#"+!
derivatization modes. Detailed GC/MS procedures are described in Ye et al.35. Briefly, a punch of
#""!
particle filter was placed and heated in a thermal desorption system to 300°C, followed by
#"#!
collection and re-concentration of desorbed organic compounds in a cooled liner at 20°C. The
#"$!
trapped organic compounds were then re-heated to 300°C and transferred to GC/MS for analysis.
#"%!
Under non-derivatization mode, desorbed organic compounds were carried by helium gas to
#"&!
GC/MS, whereas under derivatization mode, the helium carrier gas was first flowed over the
#"'!
headspace of the derivatization agent N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA),
#"(!
prior to being introduced into the thermal desorption system. Derivatization reactions occurred
#")!
within the thermal desorption system and in the cooled liner. Polar OH groups are converted to
#"*!
nonpolar trimethylsilyl groups and make these compounds amenable for GC/MS analysis.
##+! ##"!
3.! Results and Discussion
###!
3.1 SOA enhancements in the presence of atmospherically relevant particles
##$!
The amount of SOA formed (measured as total aerosol minus seed aerosol) as a function of !-
##%!
pinene reacted for all the seeded experiments is shown in Fig. 1 and summarized in Table 1.
##&!
Enhancement in SOA yields from !-pinene ozonolysis was observed in the presence of biomass
##'!
burning emissions or meat-cooking emissions/particles, but not with diesel exhaust or Toronto
##(!
ambient particles (the term “emissions” here refer to the emitted gas and particle mixtures, as
##)!
compared to particles only). No separate nucleation mode was observed over the course of the
##*!
experiments, suggesting that SOA condensed onto pre-existing seed particles (Fig. S2, Supporting
!
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ACS Earth and Space Chemistry
#$+!
Information). It is noted that experiments conducted in both chambers show excellent repeatability
#$"!
(within 5%) regarding !-pinene ozonolysis (Exp. #1-3 and Exp. #9-12). SOA enhancements are
#$#!
much larger than day to day chamber variability. Control experiments with injection of only ozone
#$$!
and primary emissions show ~3% particle volume growth for biomass burning emissions and
#$%!
negligible increase for meat-cooking emissions (Fig. S3), indicating that any reactions between
#$&!
primary emissions (e.g. unsaturated fatty acids) and ozone does not increase aerosol mass
#$'!
substantially.
#$(! #$)!
In addition to the reaction between seed and ozone, organic vapor wall loss may affect the degree
#$*!
of SOA enhancements36. For the same type of organic seeds, the degree of SOA enhancements
#%+!
was observed to increase with increasing seed aerosol loading (Table 1 and Supporting Information,
#%"!
Fig. S4). For example, SOA yields of !-pinene ozonolysis increased from 30.2% (Exp. #4) to 35.1%
#%#!
(Exp. #6) when the amount of seed aerosol increased from 30.3 to 368.1 µm3/cm3 for the similar
#%$!
set of initial conditions (34 ppb of !-pinene). Similar increase was observed with meat-cooking
#%%!
emissions (Exp. #7 and Exp. #8). However, this effect was observed to be less significant in the 1-
#%&!
m3 chamber (Supporting Information, Fig. S4B), likely due to the seed area concentration spanning
#%'!
a relatively narrow range in the 1-m3 chamber as compared to those in the 38-m3 chamber
#%(!
experiments. Despite the influence of organic vapor wall loss, we demonstrate that this wall loss
#%)!
effect plays a minor role in the relative SOA yield comparison. For example, compared to AS seed
#%*!
experiments, with smaller amounts of seed injection, greater SOA yields were observed in the
#&+!
presence of meat-cooking emissions in the 38-m3 chamber (Table 1, Exp. #3 vs. Exp. #7) and
#&"!
meat-cooking particles in the 1-m3 chamber (Table 1, Exp. #12 vs. Exp. #16). This clearly indicates
!
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#&#!
that the properties of the injected seed aerosol play a more crucial role in determining SOA
#&$!
enhancement.
#&%! #&&!
The use of different particle injection methods in the two chambers (direct injection in 38-m3
#&'!
chamber vs. extraction-atomization in 1-m3 chamber) may also affect the degree of SOA
#&(!
enhancements. Here, the experiments with meat-cooking emissions/particles were used to
#&)!
illustrate the validity of the injection methods. As shown in Fig. 1B and Fig. 1D, enhancement of
#&*!
SOA formation was observed in both cases, demonstrating that both methods yield qualitatively
#'+!
consistent SOA enhancements although the composition or the morphology of individual particle
#'"!
may change during the “extraction-atomization” process, at least for meat-cooking emissions.
#'#!
However, enhancements in the 1-m3 chamber were observed to be smaller than that in the 38-m3
#'$!
chamber. This difference can be attributed to two potential causes. First, particle and vapor losses
#'%!
to the chamber walls were likely greater in the smaller chamber with higher surface-to-volume
#'&!
ratio, leading to smaller amount of semivolatile vapors available for gas-particle partitioning.
#''!
Second, by using the “extraction-atomization” method, only particles were injected into the 1-m3
#'(!
chamber. Primary semivolatile vapors from the emissions may not be efficiently collected on the
#')!
filters, or may be removed by the charcoal denuder prior to the injection into the chamber, leading
#'*!
to smaller SOA yields. On the contrary, in experiments where emissions were directly introduced
#(+!
into the 38-m3 chamber, semivolatile vapors from the emissions were not removed and may
#("!
therefore condense upon mixing with !-pinene SOA. The condensation of primary semivolatile
#(#!
vapors may contribute to a larger total aerosol volume measured by the SMPS. Displayed in Fig.
#($!
2, SOA growth curves were fitted to the Volatility Basis Set (VBS) model25 (Eqn. (1)). Three
#(%!
volatility bins (981 = 1 µg/m3, 10 µg/m3, 100 µg/m3) were used to obtain the fit. The empirical
!
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ACS Earth and Space Chemistry
#(&!
fitting parameters (!1, !2 and !3) were obtained from the AS seed experiment (Exp. #10 in 1-m3
#('!
chamber, or Exp. #1 in 38-m3 chamber), where the concentration of organic seed, Mo, is 0. The
#((!
same parameters were then applied to Eqn. (1) with Mo = 47.6 µg/m3 (assuming a density of 1
#()!
g/cm3) and Mo = 78.9 µg/m3, the concentration of pre-existing meat-cooking particles in Exp. #17
#(*!
(1-m3 chamber) and Exp. #7 (38-m3 chamber), respectively. As shown in Fig. 2A, the resulting
#)+!
VBS growth curve matched the SOA growth observed in Exp. #17, indicating the miscibility of
#)"!
these two types of aerosols and the formation of a well-mixed phase. However, in Exp. #7 that is
#)#!
conducted in the 38-m3 chamber, since the semi-volatile vapors were not removed and injected
#)$!
together with particles, additional enhancement of particle volume concentration was observed
#)%!
besides those from SOA itself, which is suggested to be the condensation of the semi-volatile
#)&!
vapors from the emissions. Similar analysis has also been carried out for the experiments with
#)'!
biomass burning emissions and the results were shown in Fig. S5 (Supporting Information).
#)(! #))!
There may also be potential biases in chemical composition of the extracted material when organic
#)*!
seed particles were collected and introduced using the “extraction-atomization” method. To
#*+!
examine the extraction efficiency of this method, the organic carbon content of extracted material
#*"!
was examined using thermal-optical OC/EC analysis. As shown in Table S1, the extraction
#*#!
efficiency of organic carbon on all types of filters was > 90%, with cooking emission showing the
#*$!
highest extraction efficiency (~100%). The method used in this study is consistent with previous
#*%!
offline extraction methods for both polar and non-polar particulate matter, and exhibits high mass
#*&!
extraction efficiency. While it is not known if the chemical composition remains unchanged during
#*'!
this process, extraction of SOA has been used to examine properties such as viscosity of secondary
!
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Page 14 of 39
#*(!
organic material37, and oxidative potential38. In many cases, these properties have generally been
#*)!
preserved during the extraction process.
#**! $++!
3.2 Hansen Solubility Parameters (HSPs) and H/C and O/C ratios
$+"!
To explain organic aerosol miscibility and predict SOA yield enhancement, we have previously
$+#!
developed a method using the Hansen Solubility Parameter (HSP) and constrained the model using
$+$!
pure organic compounds to represent ambient primary organic aerosol (POA)10. In this study, this
$+%!
framework is extended to understand mixing with more realistic atmospheric mixtures. In brief,
$+&!
the miscibility between two organic materials can be described by comparing their total solubility
$+'!
parameter (I; ; total HSP) which is defined as the square root of the ratio between cohesive energy
$+(!
J0
