Introduction
Laboratory and field studies over the last decade have shown that
organic components of atmospheric particles constitute 20 to
50 % of the fine particle mass (PM) in the continental
mid-latitudes, though the organic content can be higher (up to
90 %) in tropical forested regions .
On a global scale, 50–90 % of submicron organic PM is composed
of oxygenated organic aerosol that is typically
associated with secondary organic aerosol (SOA) formed by condensation
of oxidized gas-phase species . Field studies
indicate that SOA particles may influence cloud formation
and may
be optically active in the UV/visible region of the electromagnetic
spectrum . These studies have also revealed the
complexity of organic aerosol compositions and their chemical
evolution via oxidative aging. The atmospheric lifetime of ambient
SOA ranges from hours to weeks, providing a wide range of atmospheric
exposures to a variety of oxidant species. Measured ambient SOA
chemical compositions range from hydrocarbon-like organic aerosol,
such as observed directly downwind of the Deepwater Horizon oil site
during the 2010 Gulf oil spill , to highly oxygenated OA, such as background SOA
observed worldwide . Much of this complexity is
due to the thousands of organic compounds found in atmospheric
particulate matter, specifically low volatility, highly functionalized
species .
Laboratory experiments conducted in environmental chambers have been
essential in providing SOA physical and chemical properties as well as
yield data for predicting the rate of atmospheric SOA formation due to
oxidation of biogenic and anthropogenic volatile organic compounds
(VOCs) . Substantial progress has been made in
understanding reaction mechanisms and the factors that influence SOA
yields and composition. For example, SOA yields appear to have
a complex dependence on VOC : NOx ratio , precursor concentration/volatility
, and oxidant exposure
. Modeling observed atmospheric SOA levels
therefore remains a challenge because of the large number
of modeling parameters and associated sensitivities that are required
to capture mechanistic details of SOA formation .
Oxidant exposure is the integral of the oxidant species concentration
and the sample residence time. Relatively low oxidant exposures are
a major limitation of current environmental chamber techniques, which
operate at OH concentrations ranging from approximately 106 to
107 moleccm-3 that are equal to or slightly more than
daytime atmospheric OH concentrations. Losses of oxidized vapors and/or particles to the
chamber walls, as well as chamber deflation, generally limit chamber experiments to residence times of several hours.
This combination of low OH
concentrations and residence time limits environmental chambers to
simulating atmospheric aerosol particle lifetimes only up to 1 or
2 days, including the characterization of SOA yields. This
limitation prevents the formation and the study of highly oxygenated
SOA that is characteristic of aged atmospheric organic aerosol PM
.
Recently, aerosol flow reactors have been developed to study SOA
formation and evolution equivalent to multiple days of atmospheric OH
exposure. In these reactors OH concentrations are typically ∼109 moleccm-3 or greater, with reactor residence times of
seconds to minutes . With this range of OH concentrations
and exposure times, flow reactors can simulate the full range of
ambient levels of oxidation, measuring changes in SOA composition and
yields over a wide range of equivalent atmospheric oxidation.
Furthermore, because of the short flow reactor residence times,
experimental runs can be conducted on the scale of minutes rather than
hours.
While flow reactors appropriately simulate the full range of
integrated atmospheric oxidant exposures, in view of their short
residence times and high oxidant concentrations it must be established
how well the atmospheric aerosol chemistry is simulated
. A growing set of studies indicates that flow
reactor-generated SOA particles have compositions similar to ambient
SOA, suggesting that the dominant oxidation reaction pathways in flow
reactors are similar to those in ambient conditions
. Other studies have used a combination of aerosol
flow reactors and environmental chambers to characterize heterogeneous uptake of
organics on seed particles , SOA formation potential
, and evolution of functional groups in SOA
with aging ; in general, similar results are obtained in reactors and chambers.
However, these comparisons need to be extended over a wider range of reactants and experimental conditions
than are currently available.
Here we describe systematic intercomparison studies of SOA chemistry
and yields generated from a common set of precursors in a Potential
Aerosol Mass (PAM) flow reactor (Lambe et al., 2011a) and four
environmental chambers. SOA precursors studied are gas-phase alkane,
biogenic, and aromatic compounds. SOA chemical composition and yield
were characterized as a function of OH exposure. Additionally, the
effect of sulfate seed particles on isoprene SOA yields was studied.
Due to the limited oxidative exposure provided by the environmental
chambers, direct comparison between the two techniques is possible
only over a narrow range. However, reasonable extrapolations extend
the range of interest.
Experimental
This manuscript compares properties of SOA produced in the PAM reactor
to SOA produced in environmental chambers operated at the four
institutions: California Institute of Technology (Caltech),
Massachusetts Institute of Technology (MIT), Paul Scherrer Institute
(PSI), and Carnegie Mellon University (CMU). The PAM reactor is
a horizontal 13.3 L glass cylindrical chamber,
46cmlong×22 cm ID and is operated in
continuous flow mode with an average residence time of 100 s. The relative humidity (RH)
in the reactor was controlled in the range of 30–40 %. The
Caltech, MIT, PSI, and CMU Teflon chambers range from 7.5 to
28 m3 in volume and are operated in batch or semi-batch mode
with experimental residence times ranging from 4 to 10 h. The
RH in the Caltech, MIT and CMU chamber experiments
was less than 10 %, and the RH in the PSI chamber experiments
was controlled in the range of 40–50 %. A summary of the
methods used for OH radical generation, particle generation, and data
analysis is provided below.
OH radical generation
In the flow reactor, OH radicals were produced in the absence of
NOx via the reaction O(1D)+H2O→2OH, with O(1D) radicals produced from the reaction
O3+hν→O2+O(1D). O3 (15–30 ppm)
was generated by O2 irradiation with a mercury lamp (λ=185 nm) outside the flow reactor. The O(1D)
atoms were produced by UV photolysis of O3 inside the flow
reactor using four mercury lamps
which emit primarily at λ = 254 nm. Additional photons are emitted at the following
wavelengths with relative intensities of 1 % or more of the UV intensity at 254 nm: 185 nm
(1 %; ); 302 nm (1 %); 313 nm (1 %); 366 nm (1 %); 405 nm (1 %); 436 nm (10 %);
546 nm (1 %) (BHK Inc. product specifications).
At the highest UV intensity that was used in the reactor, we calculate upper-bound
JUV = 2 × 1013 and 2 × 1015 cm-2 s-1 at λ = 185 and 254 nm from ozone and OH exposure measurements.
Corresponding lower limit timescales for UV photolysis of several phenols, carboxylic acids,
aldehydes, and ketones range from 12 to 50 000 s for absorption cross sections ranging from
approximately 4 × 10-17 to 1 × 10-20 cm3 molec-1 s-1
(https://sites.google.com/site/pamwiki/ and references therein).
In offline calibrations, OH concentrations were varied by changing the UV light intensity
through stepping the lamp voltages between 0 and 110 V. SO2 was added to the carrier gas,
typically at mixing ratios ranging from 30 to 60 ppbv, and was used as an OH tracer. Calibrations
were conducted at the same H2O and O3 concentrations used in SOA experiments.
At each lamp setting, OH exposures were quantified by measuring the steady-state SO2
mixing ratio and normalizing to the SO2 mixing ratio obtained with the lamps turned off.
The corresponding OH exposure was quantified by normalizing the SO2 mixing ratio with the lamps on to the SO2
mixing ratio with the lamps off and applying the known OH+SO2 rate constant
, as shown in Eq. ():
OHexposure=-1kSO2OHln[SO2][SO2]i.
The concentrations ranged from approximately 2.0 × 108 to
2.2 × 1010 moleccm-3. The corresponding OH
exposures ranged from 2.0 × 1010 to
2.2 × 1012 moleccm-3s or approximately 0.2 to
17 days of equivalent atmospheric exposure.
Additional SO2 calibration measurements were conducted in the presence
and absence of a subset of precursors (isoprene and JP-10) to investigate reductions
in OH levels following addition of those precursors to the flow reactor at mixing
ratios that were used in SOA experiments. No change in SO2 decay was
observed upon addition of isoprene, but addition of JP-10 decreased OH levels by
approximately 10 (highest OH exposure) to 50 % (lowest OH exposure)
. Reductions in OH exposure following addition of
other VOCs will be investigated in future work using the methods of .
Summary of PAM reactor and environmental chamber OH exposure
conditions.
Precursor
PAM
Caltech
MIT
PSI
CMU
Refs
isoprene
1.6×1011–1.0×1012
9.5×1010
–
–
–
1–3, 8
α-pinene
2.0×1010–2.2×1012
5.4×1010–1.3×1011
–
9.0×1010–4.0×1011
–
1–4, 7–8, 10
toluene
1.6×1011–2.1×1012
1.5×1011
–
–
–
1–3, 8
m-xylene
4.1×1010–2.1×1012
7.7×1010–1.6×1011
–
–
–
1–3, 7–8
naphthalene
1.6×1011–2.1×1012
2.3×1011
–
–
–
2–3, 8, 10
n-C10
1.6×1011–2.1×1012
–
2.6×1011
–
–
1, 5, 9
cyclodecane
1.6×1011–1.9×1012
–
1.8×1011
–
2.2×1010
1, 5–6
JP-10
1.3×1011–2.0×1012
–
3.3×1011–5.8×1011
–
–
5, 9–10
References: 1 this work; 2 ; 3
; 4 ; 5
; 6 ; 7 ; 8 ; 9 ; 10
.
In the environmental chambers, OH radicals were generated by UV
photolysis (λ=350 nm) of hydrogen peroxide
(H2O2) with no added NOx or by UV photolysis of
nitrous acid (HONO) or methyl nitrite (CH3ONO) with
NOx. In the present studies, OH radicals generated in the
Caltech chamber were formed from photolysis of H2O2, HONO, or
CH3ONO, depending on the experiment, whereas OH radicals
generated in the MIT, PSI, and CMU chambers were formed exclusively
from HONO photolysis. Typical chamber OH concentrations were
approximately 2×106 moleccm-3 (H2O2) and
2×107 moleccm-3 (HONO) during the initial stage
of chamber experiments. Corresponding OH exposures ranged from
5.4×1010 to 4.0×1011 moleccm-3s
(Table ), equivalent to approximately 0.4 to 3 days of
atmospheric exposure at a typical 24 h average OH concentration of
1.5×106 moleccm-3 .
Particle generation
The gas-phase SOA precursors used in these
studies include two biogenic compounds (isoprene, α-pinene), three
aromatic compounds (toluene, m-xylene, naphthalene), and three
alkanes (n-C10, cyclodecane,
tricyclo[5.2.1.02,6]decane, a jet fuel also known as JP-10).
In the flow
reactor, SOA was generated via gas-phase OH oxidation of precursors
followed by homogeneous nucleation or by condensation onto sulfuric
acid or ammonium sulfate seed particles. The sulfuric acid seed
particles were generated by OH oxidation of SO2 together with
the SOA precursor, and ammonium sulfate seed particles were generated
by atomizing an ammonium sulfate solution. The particles were dried
and introduced continuously into the flow reactor (without radioactive charge neutralization) along with the
gas-phase SOA precursor. In environmental chambers, SOA was generated
via gas-phase OH oxidation of precursors usually followed by
condensation onto ammonium sulfate seed particles. For long residence
time chamber experiments, wall condensation of precursor gas-phase
species can be significant. Seed particles are used in chamber
studies to reduce wall effects. In some of the flow reactor
experiments seed particles were also used to study their effect on SOA
yields.
Particle monitoring and analysis
Particle number concentrations and size distributions were measured
with a TSI scanning mobility particle sizer (SMPS). Aerosol mass
spectra were measured with an Aerodyne time-of-flight aerosol mass
spectrometers (ToF-AMS) .
Elemental analysis was performed on the AMS data
to determine the bulk aerosol hydrogen-to-carbon (H/C)
and oxygen-to-carbon (O/C) ratios along with the
average aerosol carbon oxidation state (OSc‾)
. While AMS measurements provide basic
information about SOA composition, additional supporting measurements such as Fourier
transform infrared spectroscopy, nuclear magnetic resonance, gas chromatography mass
spectrometry, and chemical ionization mass spectrometry are required to investigate SOA chemistry at the molecular level.
SOA yields were calculated from the ratio of aerosol mass formed to
precursor gas reacted. The aerosol mass was calculated from the
integrated particle volume and the effective particle density
(ρ=Dva/Dm), where Dva is the mean
vacuum aerodynamic diameter obtained from the ToF-AMS and
Dm is the electric mobility diameter obtained from the
SMPS. Flow reactor SOA yields were corrected using size-dependent
bis(2-ethylhexyl) sebacate wall-loss measurements
; the average magnitude of these corrections was
32 % (±15 %) and represents an upper limit as it
combines losses into and through the reactor. Caltech chamber yields
were corrected for particle wall losses using size-dependent
first-order loss coefficients determined from ammonium sulfate
wall-loss measurements (Keywood et al., 2004). The magnitude of these
particle wall loss corrections typically ranged from 10 to 30 %.
MIT chamber experiments were corrected for particle wall losses using
the AMS organic-to-sulfate ratio to generate an upper limit and SMPS
measurements of particle loss to generate a lower limit for aerosol
yield . In the MIT chamber, these
corrections were between a factor of 1.5 and 3.0 at the highest yields
and OH exposures. Although the residence time in the flow reactor is
much shorter than in the chambers, the surface-to-volume ratio in the
PAM reactor is much greater. As a result, particle losses are
comparable in the two systems. Flow reactor SOA yields were also
corrected for UV lamp-induced temperature increases by applying yield
corrections of -0.02 per K of temperature rise (Qi et al.,
2010; Stanier et al., 2007) relative to room temperature (∼293 K). These temperature corrections ranged from
0 to 28 % (mean correction ±1σ=7±7 %). In
the flow reactor, a known amount of precursor gas was introduced and
the mass of reacted precursor gas was estimated from the OH exposure
and known bimolecular rate constants . In
environmental chamber studies, the mass of the remaining precursor gas
was measured directly as a function of exposure time.
Aerodyne ToF-AMS spectra of SOA generated in the (a
and b) Caltech environmental chamber and (c and
d) PAM flow reactor from the OH oxidation of
α-pinene and naphthalene. Caltech chamber data obtained from
.
Van Krevelen diagrams showing H/C ratio as
a function of O/C ratio for SOA generated in the PAM
flow reactor and environmental chambers by OH oxidation of
(a) biogenic, (b) aromatic, and (c) alkane
precursors. Error bars indicate ±1σ uncertainty in
binned O/C and H/C ratio
measurements. Caltech, PSI, CMU, and MIT chamber data obtained from
, (binned averages of
O/C and H/C data from experiments
1–9), , and ,
respectively.
Results and discussion
Sample mass spectra of flow reactor- and chamber-generated SOA
Figure shows representative ToF-AMS spectra of SOA
generated in the PAM reactor and the Caltech chamber
from the OH oxidation of
α-pinene and naphthalene, used here as representative biogenic
and anthropogenic precursors, respectively. The flow reactor spectra
are obtained at an OH exposure of
1.6×1011 moleccm-3s, or ∼1 day of
equivalent atmospheric oxidation. The chamber spectra represent the
SOA composition at peak aerosol formation (∼9×1010 moleccm-3s OH exposure). In this range
the OH exposure for the PAM reactor and chamber are approximately the
same, allowing for direct comparison.
To quantify the similarity between mass spectra, we calculated the dot
product between SOA mass spectra generated in the PAM flow reactor and the Caltech chamber .
Using this approach, each mass spectral signal is normalized to the square root of the sum
of the squares of all signals in the mass spectrum.
Each spectrum is represented as a normalized vector A or B, with dot product
A ⋅ B = Σi=1naibi, where ai and bi are the normalized
signals at each m/z in the spectrum; A ⋅ B = 0 indicates the spectra
are orthogonal and A ⋅ B = 1 indicates the spectra are identical.
The top table inset in Fig. shows the calculated dot products between
each pair of mass spectra.
The PAM flow reactor and the chamber produce particles with similar mass spectra, as
indicated by dot products of 0.97 between spectra shown in Fig. a and c
(α-pinene SOA) and Fig. b and d (naphthalene SOA), suggesting
similar compositions.
Features unique to
α-pinene and naphthalene SOA are observed in both flow reactor-
and chamber-obtained spectra, with linear correlation coefficients of
r2=0.93 (α-pinene SOA) and r2=0.94 (naphthalene SOA) as
noted in the bottom table inset in Fig. . For example, α-pinene
SOA spectra are dominated by signals at m/z=43 (C2H3O+)
indicative of carbonyls and several ion clusters below m/z<100
containing signals that are indicative of cycloalkyl fragments such as
m/z=27, 41, and 55. However, AMS spectra of naphthalene
SOA are dominated by m/z=44 (CO2+), indicative of
carboxylic acids, as well as signals that are indicative of aromatic
compounds such as m/z=50–51, 65, and 76–77. As is evident from
Fig. , α-pinene and naphthalene SOA mass spectra
display pronounced differences, with dot products ranging from 0.42 to 0.63 and r2
ranging from 0.18 to 0.37 between spectra shown in Fig. a and b, a
and d, b and c, and c and d.
Average carbon oxidation state (OSc‾;
OSc‾=2×O/C-H/C) of flow reactor- and environmental
chamber-generated SOA. Error bars indicate ±1σ
uncertainty in binned OSc‾ measurements. Markers
indicate SOA precursor: TOL is toluene, NAP is naphthalene,
XYL is m-xylene, CYL is cyclodecane, JP is JP-10,
ISO is isoprene, AP is α-pinene, and
DEC = n-C10. Caltech, PSI, CMU, and MIT chamber
data obtained from
, , , and
, respectively.
H/C, O/C ratios for flow reactor- and chamber-generated SOA
H/C and O/C ratios obtained from mass
spectra such as shown in Fig. provide information about
the nature of SOA formation. Van Krevelen diagrams that show
H/C ratios as a function of O/C ratios
have been used to deduce oxidation reaction mechanisms for organic
aerosols . Typically, with oxidative aging the
O/C ratio increases and H/C ratio of
SOA decreases as oxygen-containing functional groups are added to
a carbon backbone. Here, we use Van Krevelen diagrams to compare the
composition of SOA formed in the flow reactor and environmental
chambers for the organic precursors studied. Direct comparisons are
possible in the overlapping OH exposure region. Typically the lowest
OH exposures attained in the flow reactor overlap (or nearly overlap)
with the highest OH exposures reached in environmental chambers
(Table ).
Figure shows Van Krevelen diagrams obtained from
laboratory SOA produced from the oxidation of gas-phase biogenic,
aromatic, and alkane precursors. To simplify presentation, the data
are displayed in three panels. Figure a shows biogenic
SOA generated from isoprene and α-pinene, Fig. b
shows SOA generated from aromatic compounds, and Fig. c
shows SOA produced from alkanes. In most cases, for a specific SOA
type the most-oxidized chamber SOA and the least-oxidized flow reactor
SOA have similar Van Krevelen plots at integrated OH exposures between
approximately 1×1011 and
2×1011 moleccm-3s, or about 1–2 days of
equivalent atmospheric oxidation. This observation suggests that in
the range of available OH exposure overlap for the flow reactor and
chambers, SOA elemental composition is similar whether the precursor
is exposed to low OH concentrations over long exposure times or high
OH concentrations over short exposures times. The flow reactor
studies were done without added NOx, whereas some of the
environmental chamber studies were conducted in the presence of
NOx. The similarity in compositional parameters shown in
Fig. (e.g., H/C, O/C)
were independent of the NOx levels used in the environmental
chambers in the region studied, as has been observed in previous
studies . The nitrogen-to-carbon
(N/C) ratio ranged from 0.031 to 0.054 for SOA
produced in the MIT chamber with added NOx
but was not characterized for other measurements
shown in Fig. .
Yields of SOA produced from photooxidation of (a) isoprene, (b) α-pinene, and (c)
tetracyclo[5.2.1.02,6]decane (JP-10) in environmental chambers and PAM reactor as a function of OH exposure.
The OH exposure in (c) is corrected for reductions in OH levels upon JP-10 addition (see Sect. ).
Error bars indicate ±1σ uncertainty in binned SOA yield measurements and ±34 % uncertainty in OH exposure values .
Black markers indicate data from , and
obtained in the Caltech chamber and data from obtained in the MIT chamber.
Carbon oxidation state for flow reactor- and chamber-generated SOA
Recently, the average carbon oxidation state, defined as OSc‾=2×O/C-H/C, was proposed as a more
accurate indicator of atmospheric oxidative aging processes than the
O/C ratio alone because this measure takes into
account the level of saturation of the carbon atoms in the SOA
. As will be demonstrated,
OSc‾ of lightly oxidized SOA is strongly
precursor-dependent. Figure shows a scatter plot of
OSc‾ for flow reactor and chamber SOA for the eight
gas-phase precursors studied. Different colored symbols are used to
represent each of the environmental chambers used in the
intercomparison. For each data point obtained from environmental
chamber measurements, we used data from the flow reactor obtained at
the OH exposure that were closest in magnitude. A total linear least
squares fit to the data presented in Fig.
(PAMOSc‾=1.1⋅ChamberOSc‾-0.16; r2=0.54)
indicates that there is no systematic OSc‾
difference observed across multiple SOA types produced in chambers and
in flow reactors. For a specific SOA type, Fig. shows that the chambers and
flow reactor provide OSc‾ with absolute differences ranging from
0.0040 to 0.60 (mean deviation = 0.10 ± 0.34) over the range of measured SOA composition for comparable OH
exposures. The observed deviations between PAM and chamber
OSc‾ are no larger than deviations between two
chambers (e.g., α-pinene SOA produced in Caltech and PSI
chambers and cyclodecane SOA produced in MIT and CMU chambers).
Experimental conditions for PAM reactor, Caltech chamber, and MIT chamber yield measurements shown in Figs. 5–7.
Seed concentration
Maximum [NOx]
[isoprene]
[α-pinene]
[JP-10]
Refs
(µgm-3)
added (ppb)
(ppb)
(ppb)
(ppb)
Caltech chamber
0–29a
0
49–91
13.8–52.4
–
1–3
MIT chamber
50–100a
475
–
–
42.9
4
PAM flow reactor
0–59a; 0–114b
0
462
41–100
55
5–7
a Ammonium sulfate seed; b sulfuric acid seed; 1 ; 2 ; 3 ; 4 ; 5 this work; 6 ;7 .
SOA yields obtained in the flow reactor and environmental chambers
Several factors can affect SOA yields, including precursor
concentration , NOx , UV
intensity/wavelength , seed particle
composition/loading , and
interactions between SOA and chamber walls
. For reference, the range of SOA precursor
concentrations, NOx levels, and seed particle concentrations
used in SOA yield studies is summarized in Table .
Isolation of individual factors is beyond the scope of this
intercomparison. Because mass spectra and elemental ratios of SOA are
similar whether it is generated in an environmental chamber or in
a flow reactor (Sect. ), we suggest that the differences in precursor
concentration and UV wavelength (e.g., λ=350 nm
vs. λ=254 nm) used in these studies have at most
a minor effect on bulk composition.
Yields of SOA produced from photooxidation of isoprene in the
PAM reactor as a function of OH exposure in the presence of
20 µgm-3 ammonium sulfate seed. Error bars
indicate ±1σ uncertainty in binned SOA yield measurements and ±34 % uncertainty in OH exposure values .
Next we evaluate the influence of oxidant concentration and residence
time on yields of SOA formed from common precursors in the PAM reactor
and the Caltech and MIT environmental chambers. Figure
shows yields of SOA as a function of OH exposure for isoprene SOA (no
added NOx), α-pinene SOA (no added NOx), and
tricyclo[5.2.1.02,6]decane (JP-10) SOA, respectively. These
precursors provide the broadest range of available yield values for
intercomparison. Oxidation of isoprene forms SOA with low yield
, whereas α-pinene forms
SOA with moderate yields and JP-10 forms SOA
with mass yields greater than unity . Yields of alkane SOA do not display
a systematic NOx dependence ; thus, to
first order we assume different NOx levels between the MIT
chamber and PAM reactor do not influence our comparison of measured
JP-10 SOA yields. The following features are evident in
Fig. :
SOA yields at comparable OH exposures are
lower in the flow reactor than in chambers, whereas the mass
spectra, O/C, and H/C of SOA
generated in the chambers and flow reactor are similar
(Figs. –). Flow reactor SOA yields are also lower in the
flow reactor than in chambers for the other precursors studied that are not shown in Fig. .
SOA yields in the flow reactor and chambers track each other;
that is, the maximum yield of isoprene SOA is approximately 0.03 in
the flow reactor and 0.06 in the Caltech chamber
(Fig. a). Likewise, the maximum yield of JP-10 SOA is
1.4 in the flow reactor and 1.6 in the MIT chamber
(Fig. c).
In the flow reactor, in all cases the SOA yield first increases
as a function of OH exposure and then decreases. In some cases
there is also evidence of a slight decrease in SOA yields at higher
OH exposure in chambers (e.g., Fig. c).
One reason for the lower SOA yield in flow reactors may be the
relative timescales for oxidation in the gas-phase vs. condensation
onto pre-existing aerosols. The timescale for condensation of a gas-phase molecule onto
pre-existing seed particles (τcond) can be calculated using Eq. () :
τcond=1αAp2πMwkBT,
where Mw is the molecular weight of the condensing species, α is the mass accommodation coefficient,
and Ap is the particle surface area.
For example, over a representative range
of particle surface area concentrations used in the flow reactor (10
to 100 µm2cm-3), condensation timescales
range from approximately 2000 to 20 000 s assuming a mass
accommodation coefficient of 0.1 as has been measured for α-pinene ozonolysis SOA
and an average
SOA molecular weight of 150 gmol-1. A lower limit of τcond = 200 to 2000 s is calculated over the same range of Ap assuming α=1.
While our measurements do not constrain the mass accommodation coefficient,
these timescales suggest that the residence time in the flow reactor
(100 s) may not be adequate to allow complete condensation of
semivolatile organic gas-phase species into SOA, whereas residence times in environmental chamber experiments are typically 10 000 s or longer. Another factor in
causing the SOA yield difference may be due to the condensation
conditions. All the chamber experiments displayed in this work were
done in the presence of ammonium sulfate seed particles, whereas seed
particles were not normally used in our flow reactor studies. The
effect of seed particles on SOA yields in the flow reactor is examined
further in Sect. .
The observation that the yields track each other is a further
indication that the reactive chemistry in the two systems is similar.
The decrease in SOA yield subsequent to increase as a function of OH
exposure is possibly due to gas-phase species carbon–carbon bond
breaking from continued oxidation or heterogeneous OH oxidation
reactions at high OH exposure ; this trend is most clearly
evident in the flow reactor studies. These observations suggest that
the first step in SOA formation is oxidation of gas-phase species
leading to subsequent condensation. At low OH exposures equivalent to
1–2 days, heterogeneous reactions do not appear to play
a significant role in SOA chemistry .
Effect of seed particles on SOA yields
The chamber experiments discussed in
Sects. – were performed in the
presence of ammonium sulfate seeds to promote more rapid condensation
of gas-phase species into SOA. We investigated in more detail the
influence of sulfate seed particle loading and composition on SOA
yields produced in the flow reactor. We hypothesize that a “seed
effect” should be most pronounced for SOA types with low yields and
investigate this proposal for isoprene SOA, whereas the effect of
sulfate seeds on α-pinene SOA yields is minor
.
Figure shows isoprene SOA yields
in the flow reactor as a function of OH exposure using an ammonium
sulfate seed particle mass concentration of 20 µgm-3,
which is comparable to sulfate volume concentrations used by
. At an OH exposure of
2.9×1011 moleccm-3s or less in the flow
reactor, the isoprene SOA yield with seeds added to the flow reactor
compared to the SOA yield in the Caltech chamber is negligible. As
the OH exposure is increased in the flow reactor, the SOA yield rises
to a maximum value of 0.10 at
7.8×1011 moleccm-3s OH exposure and then
decreases. We note that the same trend was observed without adding
seed particles, although with lower SOA yields. The
O/C ratio of the flow reactor SOA decreases from 0.63
to 0.59 when seeds are added at
7.8×1011 moleccm-3s OH exposure.
Figure supports the hypothesis that addition of seed
particles promotes condensation, leading to higher SOA yields. Higher
concentrations of seed particles may be required for condensation of
gas-phase oxidation products to compete with continued OH oxidation in
the gas phase.
Yields of isoprene SOA produced in the PAM reactor at an OH
exposure of 7.8×1011 moleccm-3s as
a function of seed particle concentration using ammonium sulfate and
sulfuric acid seeds. Error bars indicate ±1σ uncertainty
in binned measurements. Lines are power law fits to guide the eye.
To further investigate the effect of seeds on condensation, we
measured isoprene SOA yields as a function of ammonium sulfate and
sulfuric acid seed particle concentrations (Fig. ). An
OH exposure of 7.8×1011 moleccm-3s was used
in the flow reactor because this condition provided the best
signal-to-noise ratio. It is evident from these figures that adding
sulfate seeds significantly increases the SOA yield. At an OH
exposure of 7.8×1011 moleccm-3s and a sulfate
seed particle concentration of 20 µgm-3, the yield of
isoprene SOA increases from 0.032 to approximately 0.14 in the
presence of ammonium sulfate seeds and 0.25 in the presence of
sulfuric acid seeds. SMPS size distributions of the mixed particles suggest that
most of the particle mass is measured by the AMS . Increasing the seed particle concentration led
to a continued increase in the yield, along with a decrease in the
O/C ratio of the SOA as condensation of less-oxidized
products was enhanced. The influence of seed acidity on isoprene SOA
yields is well documented , and the magnitude of the SOA yield enhancement in
the presence of acidic seeds relative to neutral seeds in our work
(factor of 2–3) is similar to these previous studies. Because
a systematic isoprene SOA yield enhancement in the presence of neutral
seeds (relative to unseeded conditions) is not observed in chamber
studies , these
measurements suggest seed particles are required in flow reactor
measurements of isoprene SOA (and potentially other types of SOA as well)
in order to more closely simulate condensation conditions
in environmental chambers.
Conclusions
We performed a systematic intercomparison study of the chemistry and
yields of SOA generated from OH oxidation of a common set of gas-phase
precursors in several environmental chambers and a flow reactor. The
most significant experimental parameters that varied between chambers
and the flow reactor were OH concentration, residence time, and use of
seed particles to promote condensation of oxidized vapors. The OH
concentrations were 100 to 1000 times higher in the flow reactor, and
residence times were 100 to 400 times higher in the environmental
chambers. Within the range of approximate OH exposure overlap of
(1–4) ×1011 moleccm-3s, the SOA mass
spectra and oxidation state were similar in both systems. The SOA
yields for representative systems tracked each other but were lower in
the flow reactor, probably in part because chamber SOA experiments
were done with seed particles to promote condensation of oxidized
vapors. Because SOA composition appears to be governed primarily by
gas-phase OH oxidation processes, our results suggest that either flow
reactors or chambers are properly characterizing SOA oxidative aging
mechanisms representative of ambient conditions. However, SOA yields
appear highly sensitive to the relative timescales of gas-phase OH
oxidation and condensation processes. Simple calculations and
measurements with seed particles suggest that condensation processes
may be residence-time-limited in flow reactors, depending on the mass
accommodation coefficient of the oxidized vapors onto pre-existing
particles. This could lead to underestimation of SOA yields. However, running an environmental chamber or flow reactor under
conditions where partitioning is overestimated relative to atmospheric
conditions will lead to a corresponding overestimation of SOA yields.
Environmental chambers are commonly used to constrain SOA yields at
low OH exposures, but flow reactors are needed to constrain SOA yields
at higher OH exposures than 1–2 days of equivalent atmospheric
oxidation, because environmental chamber SOA yield measurements appear
to significantly underestimate atmospheric SOA formation rates when
extrapolated over multiple days of equivalent atmospheric oxidation.