Oxidation flow reactors (OFRs) are frequently used to
study the formation and evolution of secondary aerosol (SA) in the
atmosphere and have become valuable tools for improving the accuracy of
model simulations and for depicting and accelerating realistic atmospheric
chemistry. Driven by rapid development of OFR techniques and the increasing
appreciation of their wide application, we designed a new all-Teflon
reactor, the Particle Formation Accelerator (PFA) OFR, and characterized it
in the laboratory and with ambient air. A series of simulations and
experiments were performed to characterize (1) flow profiles in the reactor
using computational fluid dynamics (CFD) simulations, (2) the UV intensity
distribution in the reactor and the influence of it and varying O3
concentration and relative humidity (RH) on the resulting equivalent OH
exposure (OHexp), (3) transmission efficiencies for gases and
particles, (4) residence time distributions (RTDs) for gases and particles
using both computational simulations and experimental verification, (5) the
production yield of secondary organic aerosol (SOA) from oxidation of
α-pinene and m-xylene, (6) the effect of seed particles on resulting
SA concentration, and (7) SA production from ambient air in Riverside, CA,
US. The reactor response and characteristics are compared with those of a
smog chamber (Caltech) and of other oxidation flow reactors: the Toronto
Photo-Oxidation Tube (TPOT), the Caltech Photooxidation Flow Tube (CPOT),
the TUT Secondary Aerosol Reactor (TSAR), quartz and aluminum versions of
Potential Aerosol Mass reactors (PAMs), and the Environment and Climate
Change Canada OFR (ECCC-OFR).
Our studies show that (1) OHexp can be varied over a range comparable
to that of other OFRs; (2) particle transmission efficiency is over 75 %
in the size range from 50 to 200 nm, after minimizing static charge on the
Teflon surfaces; (3) the penetration efficiencies of CO2 and SO2
are 0.90 ± 0.02 and 0.76 ± 0.04, respectively, the latter of
which is comparable to estimates for LVOCs; (4) a near-laminar flow profile
is expected based on CFD simulations and suggested by the RTD experiment
results; (5) m-xylene SOA and α-pinene SOA yields were 0.22 and 0.37,
respectively, at about 3 × 1011 molec. cm-3 s OH
exposure; (6) the mass ratio of seed particles to precursor gas has a
significant effect on the amount of SOA formed; and (7) during measurements
of SA production when sampling ambient air in Riverside, the mass
concentration of SA formed in the reactor was an average of 1.8 times that
of the ambient aerosol at the same time.
Introduction
Atmospheric aerosols play major roles in air pollution, global climate
change, and visibility reduction
(Seinfeld and Pandis, 2006).
The complex mixtures of inorganic and organic species present in atmospheric
aerosols originate from both direct, or primary, emissions and production of
secondary aerosol (SA) from atmospheric reactions. Organic aerosol (OA)
makes up a substantial fraction of atmospheric aerosols and is comprised of
primary OA (POA) that is directly emitted in the particle phase and
secondary OA (SOA) that is formed in the atmosphere through reactions of gas
phase precursors. SOA forms when the reaction of volatile organic compounds
(VOCs) with gas phase oxidants produces less-volatile functionalized
compounds
(Pankow,
1994; Jimenez et al., 2009; George and Abbatt, 2010) and when water-soluble
organics dissolve in the aqueous phase (aerosol water or cloud droplets) and
are subsequently oxidized
(Lim
et al., 2010; Ervens et al., 2011). However, the mechanisms of SOA formation
are still poorly understood and are continuously extended and refined. Part
of the complexity of SOA formation arises from the numerous oxidation
reactions involving the large number of VOCs in the atmosphere
(Aljawhary et al.,
2016). Additionally, after formation from precursor gases the SOA can evolve
through multiphase and multi-generational processes, forming more complex
distributions of compounds comprised of thousands of molecules
(Xu
et al., 2015; Chen et al., 2018; Shrivastava et al., 2019).
For decades, comprehensive laboratory studies on the sources, formation, and
aging of SOA have been conducted in batch-mode atmospheric reactors, which
are also known as environmental chambers or smog chambers
(Pandis
et al., 1991; Carter et al., 2005; Song et al., 2005; Weitkamp et al.,
2007). Though such chambers can create environments that closely simulate
the complexity of the atmosphere, results from their use are affected by the
loss of particles and semi-volatile compounds to the walls
(Zhang
et al., 2014; Nah et al., 2016, 2017), by limitations on experiment duration
and extent of reaction, and by potential leakage of room or outside air into
the Teflon bags
(Pierce
et al., 2008; Matsunaga and Ziemann, 2010; Krechmer et al., 2015). Moreover,
the size of common smog chambers, which typically range from about 5 to 250 m3
(Lonneman
et al., 1981; Mentel et al., 1996; Wang et al., 2014; Tkacik et al., 2017),
imposes a substantial restriction on their use for studying aerosol
formation in ambient air
(Bruns et al.,
2015). Oxidation flow reactors (OFRs) have been developed as a complement to
traditional smog chambers and offer advantages such as providing oxidant
exposure that can greatly exceed that possible in smog chambers and is
variable over a wide range
(Kroll et al., 2009),
portability for use in the field
(Wong et al., 2011), and
the ability to investigate time-varying sources of SA
(Kang et al., 2007).
Inside an OFR, extremely high concentrations of hydroxyl radical and/or
other oxidants are maintained (up to 1010 molec. cm-3 for OH),
such that sampled air experiences the equivalent of several hours to days or
even weeks of oxidative chemistry over the residence time of just a few
minutes. Shorter residence times minimize interaction of the gases and
particles with walls (Keller and Burtscher,
2012) and permit measurements of dynamic environments and sources. The
portability and flexibility of OFRs also make them versatile, with the same
experimental system applicable for a variety of laboratory and field
measurements. Their fast response also makes them better suited than smog
chambers for experiments probing the influence of a matrix of parameters on
SOA formation
(Slowik
et al., 2012; Palm et al., 2018).
Previous investigations of potential SA formation using different types of
OFRs have resulted in optimized designs and strategies for studying specific
sources or processes, such as measuring time-resolved SOA formation from
gasoline vehicles during a transient driving cycle
(Karjalainen et al., 2016) and from rapidly
changing vehicular emission sources
(Simonen et al., 2017). Several
groups have employed OFRs to study SA formation from ambient air, with
examples including investigation of the variability of precursor gases and
the resulting SOA in a ponderosa pine forest
(Palm et al.,
2016), high-time-resolution quantification of SOA formation from ambient air
in central Amazonia
(Palm et al.,
2018), and observation of SOA formation and aging from urban air
(Ortega et al., 2016). For laboratory-based studies, the concentrations and SOA
yield (Y) for certain precursors can serve as a reference to estimate total
SOA potential
(Iinuma
et al., 2004; Loza et al., 2014). Yields determined for common precursors
can also provide a quantitative measure of performance of a reactor relative
to others of varying design and purpose
(Kang
et al., 2011; Lambe et al., 2015). Numerous studies have been conducted
investigating differences in the SOA yield between OFRs and large
environmental smog chambers
(Matsunaga
and Ziemann, 2010; Bruns et al., 2015; Lambe et al., 2011, 2015). For
example,
Lambe
et al. (2011) showed that the SOA yield they observed in a PAM reactor is
similar to that reported for the Caltech smog chamber. Other studies focused
on SOA produced from different precursor gas sources. For example,
Li et al. (2019) used a new OFR (the Environment and Climate Change Canada OFR;
ECCC-OFR) to evaluate the SOA yields of single compounds (alkanes and
α-pinene) and of complex precursor mixtures, such as emissions from
oil sands. Ahlberg et al. (2017) found that using single-precursor experiment yields could lead to
underestimated SOA mass loadings if aerosol dynamics is not properly
accounted for.
Cubison
et al. (2011) characterized the evolution of laboratory biomass burning
emissions using a PAM reactor, and
Kang et al. (2011)
estimated the SOA-forming potential of model organic compounds.
Reactor design is a critical step in the development of an OFR system and
determines overall applicability and performance. The geometry and
dimensions of the reactor have substantial impacts on velocity profiles,
residence time distributions, wall effects, and extent of reaction. The
reactor design mainly includes the selection of materials, the inlet
configuration, the diameter-to-length ratio, the body length, the strategies
for mixing the reactants, and the mode(s) of generating the hydroxyl radical
or other oxidant(s). For example, some inlet designs can lead to dead zones
near the reactor walls, increasing the difficulty of achieving laminar flow
in the entrance of the reactor and broadening the residence time distribution (RTD)
(Mitroo et al., 2018). The
position and power output of the UV lamp(s) are determined by the reactor
materials and their transparency and by temperature control requirements
during operation
(Kang
et al., 2007; Ezell et al., 2010). With most OFRs, the lamps are either
mounted on the inner surface for metal-wall reactors or outside for
quartz-based reactors. The emitted wavelengths and intensity uniformity of
the UV lamp(s) are also important considerations in reactor design
(Li et al., 2015).
Selection of wall materials and any surface treatments is guided by an
application-dependent balance of the importance of loss of gas-phase
compounds or delays in their transfer, loss of charged particles to
non-conductive materials, and UV transmittance for designs for which the
lamps are outside of the reactor. Common materials used in OFRs include
chromate-coated aluminum (e.g., PAM), silicon-coated stainless steel (e.g., TPOT),
and quartz (e.g., CPOT, TSAR, and ECCC-OFR). Recent studies of organic gas
transmission through common tubing types described by
Deming et al. (2019) suggest perfluoroalkoxy
(PFA) and fluorinated ethylene propylene (FEP) Teflon may be alternative
choices for applications for which minimizing wall losses of gases is a
priority. Kang et al. (2007) described the prototype PAM chamber, a 19 L cylinder made of Teflon
FEP film. To the best of our knowledge, there are no previous studies
describing that all-Teflon OFR, and no data are available that show the
advantages and disadvantages of the all-Teflon reactor compared with those
constructed from other materials such as quartz and metal.
Here we present the design of a new all-Teflon OFR called the Particle
Formation Accelerator or PFA OFR. The reactor consists of a vertically
oriented tube, with the inlet at the bottom and outlet at the top. One
notable design difference between the PFA OFR and other OFRs is its use of a
relatively small and low power lamp at the top of the reactor, which
promotes thermal stratification and minimizes convective mixing. We report
the results of computational, laboratory, and field studies through which it
was characterized. UV intensity and total OH exposure (OHexp) were
quantified inside the flow tube. The flow profile in the OFR was modeled and
the resulting residence time distributions of gases and particles were both
modeled and experimentally verified. Two precursor species were used to
investigate SOA yield and the dependence of that yield on variations in
parameters such as precursor concentrations, OH exposure, and the presence
and concentration of seed particles. SOA mass yields are compared with those
reported in the literature for the same VOCs. Field testing was conducted by
measuring SA formation in ambient air sampled in Riverside, CA, US.
Collectively, these tests confirm the utility of the PFA OFR for both
laboratory and field studies.
Design and experimental setupReactor designPFA OFR and flow dynamic characterization
A cutaway view of the PFA OFR is shown in
Fig. 1. It consists of a PFA-Teflon tube sealed between inlet and outlet end
caps that were machined from blocks of polytetrafluoroethylene (PTFE). The
reactor is oriented vertically, with the inlet at the bottom and outlet at
the top. The PFA tube has a volume of 7.5 L and dimensions of 151 cm length × 7.8 cm ID. Both end caps have an OD of 10.2 cm and are sealed
with the reactor tube by FEP-encapsulated O-rings. The small
diameter-to-length ratio of the reactor section of 0.052 was selected to
result in a narrow residence time distribution in the flow tube and a more
uniform OH exposure
(Lambe
et al., 2011). However, the small diameter also results in a reactor surface-area-to-volume ratio of 0.53 cm-1 that is higher than that of the TPOT
(0.33 cm-1) and PAM (0.23 cm-1).
Cutaway view of the PFA OFR.
Some design elements of the PFA OFR are similar to those of other recently
developed OFRs
(Kang
et al., 2007; Karjalainen et al., 2016; Huang et al., 2017), though there
are some important differences as well. The inlet end cap has a 5.1 cm length × 1.3 cm ID bore used as the main sample air injection port, two
side injection ports for introducing seed particles and O3, and a
cone-shaped diffuser. That cone, which serves as the transition between the
inlet injection port and the reactor tube, has an angle of 35∘, which
is close to that suggested by
Huang et al. (2017) for
minimizing recirculation. The sample flow gradually expands and is expected
to be fully developed shortly after entering the reactor tube. A single
length of PFA tube (Ametek FPP P/N 33HPSC40x3.00) is used as the main body
in order to simplify construction. Only the central ∼ 50 %
of the flow through the reactor is extracted and analyzed. That sample flow
converges through an exit cone in the outlet end cap that tapers at an angle
of 24∘ from an ID of 4 cm to the 0.33 cm ID of the outlet bore
through the top of the end cap. The outer ∼ 50 % of the
flow that is most influenced by interactions with the reactor walls flows
into an annulus surrounding it. From there, it is pulled through 12
uniformly spaced ∼ 0.15 cm ID pinholes drilled through the PFA
pipe about 3 cm from the top. The flow extracted through those pinholes
travels into a channel between the flow tube and the end cap and then
through a port on the top cap where it is purged by a vacuum pump. The
diameter of the opening into the sample exit cone was selected such that,
for the expected parabolic velocity profile, the nominal 1:1 sample–side
flow ratio does not perturb gas streamlines.
To characterize the flow field and velocity distribution profile inside the
PFA OFR, computational fluid dynamics (CFD) simulations were performed using
a 3D geometry model in COMSOL Multiphysics 5.4 software, which has been used
by several research groups to optimize and evaluate their reactor designs
and to explore suitability for applications in atmospheric and aerosol
chemistry studies
(Renbaum-Wolff
et al., 2013; Zhang et al., 2015; Huang et al., 2017). The 2D geometry
velocity profile simulation result is shown in Fig. 2. The simulation used
the actual design and dimensions of the PFA OFR. The flow at the entrance to
the sample outlet tube of the main body is assumed to be fully developed and
laminar, while an atmospheric pressure boundary condition at the annular
outflow boundary and the no-slip condition at all the other boundaries were
applied. Though high velocity extends into the central tube flow region
above the inlet, within 15 cm from the entrance of the diffuser cone the
velocity profile is nearly parabolic, with a decrease in the maximum
velocity over the entrance length from 12 to 3 cm s-1. The
simulation suggests that jetting is minimal and that the area influenced by
recirculation is negligible.
CFD simulation results of the velocity distribution in the PFA
OFR.
UV source and intensity profile
The outlet end cap has NPT thread ports to accommodate one or two 0.5 cm OD
lamps. For the results discussed here, one 5.1 cm length × 0.5 cm OD
ozone-free (254 nm only) low-pressure mercury lamp (BHK Inc; PN 80-1057-01)
was inserted into the cap. Though the lamp is not isolated from the flow,
its position in the side purge flow annulus prevents any contact between it
and the sampled flow. The handle of the UV lamp is secured and sealed with a
Swagelok male connector fitting. Use of a relatively small and low power
lamp at one end of the reactor is perhaps the most significant design
difference between the PFA OFR and others. One objective of the approach was
to promote thermal stratification caused by the hot lamp at the top of the
reactor in order to minimize convective mixing. An obvious complication is
that UV intensity, and therefore OH production, is expected to decay with
distance through the long reactor tube. To mitigate that decay, materials
were selected that are highly UV reflective, such that emitted photons
penetrate far down the reactor tube as they are repeatedly reflected by the
walls. The PFA tube is non-absorbing at 254 nm but is not opaque and would
allow UV to leak out. Thus, the tube is wrapped with an inner layer of a
highly reflective 0.32 cm thick expanded PTFE gasket (ePTFE; Inertech) and
an outer layer of aluminized Mylar (Vivosun), though the combination of
materials results in sufficiently high reflectance for the 254 nm emission
peak of a mercury lamp.
Silva et al. (2010) showed
that the reflectance of ePTFE at 175 nm is significantly lower, with the
difference thought to be due to absorption by O2 trapped in pores.
Reflectance at the 185 nm emission peak of a mercury lamp is expected to be
slightly higher than that at 175 nm, but it is likely that a significant
intensity gradient would still exist and so a 254 nm only lamp is used, and
ozone is generated externally and introduced with the sample flow. The high
reflectance of the ePTFE at 254 nm directs UV back into the reactor tube and
results in increased intensity and uniformity. This illumination approach
reduces power consumption and heat generation, thereby simplifying
temperature control and long-term deployment for use in the field.
Temperature control
Temperature uniformity within an OFR and temperature stability over time
impact overall performance. Incomplete removal of the lamp heat can cause
convective mixing through the reactor, resulting in increased loss of
particles and gases and broadening of the RTD
(Lambe
et al., 2011; Mitroo et al., 2018). Techniques used to minimize heating by
the lamps include enclosing them in sleeves that are continuously flushed
with N2, but continuously controlling temperature during long-term
field studies can still be challenging
(Li et al., 2019). The
PFA OFR assembly is protected by a shell made from 13 cm × 13 cm
square aluminum tube. Two U-bolts mounted through the surface of the
aluminum shell hold the reactor securely, preventing accumulation of static
charge that could otherwise result from shifting between the reactor body
and the ePTFE and Mylar layers. The shell also provides a barrier to reduce
the accumulation of static charge from inadvertent touching or other
contact. A total of four fans are mounted on opposite faces near the top and
bottom of the shell. The fans near the bottom bring air into the space
between the reactor and the shell and those near the top exhaust it, which
removes heat generated by the low-power UV lamp and weakens the temperature
gradient through the whole system. The average working temperature for the
tests reported below was approximately 23.6 ∘C, which is close
to the average room temperature of 22.7 ∘C. A temperature rise
of less than 2 ∘C was observed during continuous operation over several days.
Experimental setup
The PFA OFR is an OFR254-type oxidation flow reactor, in which O3 must
be generated externally and introduced with the sample flow
(Li et al., 2015). Among
the advantages of OFR185-type oxidation flow reactors is their ability to be
operated without an inlet, which is often desirable for field
investigations. As noted above, reflectance of 185 nm UV by the ePTFE is
insufficient to produce the intensity and spatial uniformity required to
rely on photolysis of O2 and H2O for generation of O3 and OH.
Instead, the OH radicals are produced as the 254 nm UV radiation photolyzes
O3 introduced with the sample to generate excited oxygen atoms,
O(1D), which then react with H2O in ambient air or humidified
laboratory air. For the laboratory experiments described here, O3 and
humidified zero air were mixed with the tracer or precursor gas(es) prior to
being introduced into the reactor inlet. The schematic of the PFA OFR and
associated experimental equipment for laboratory and field experiments are
shown in Fig. 3a and b, respectively. Ozone was produced by
flowing zero air through an O3 generator (Jelight Company Inc. Model
610). The flow rate was controlled to 0.4 L min-1 and the O3 mixing ratio was monitored by an O3 analyzer (Teledyne Model T400U).
The resulting O3 concentration can be easily and precisely adjusted by
changing the position of a sleeve that covers a portion of the UV lamp or by
adjusting the flow rate of air through the generator. When used, seed
particles were generated using an atomizer and differential mobility
analyzer (DMA), as is described in the RTD experiment section. The flow at
the outlet of the reactor was split using a Swagelok tee. From one leg of
the tee a 150 cm length × 0.635 cm OD PFA tube was connected to gas
measurement instruments including the O3 analyzer, an SO2 analyzer
(Teledyne Model T100UP), and a gas chromatograph with flame ionization
detector (GC-FID, SRI Inc. Model 8610C). A 0.95 cm OD stainless tube was
connected to the other leg of the tee and carried the aerosol exiting the
reactor to a fabricated scanning mobility particle sizer (SMPS), which
measured the particle size distribution roughly once every 4 min. For the
ambient air experiments, outdoor air was brought inside the lab and to the
PFA OFR with a 200 cm length × 0.95 cm OD anti-static PFA tube
(Fluorotherm H2 PFA). A 150 cm × 0.95 cm OD length of copper tube
was used as a bypass in parallel with the OFR, with sampling alternated
between the two through the use of an automated three-way valve. The residence
time of the bypass line was approximately 2 s. Instrument operation and
experimental sequencing were controlled using National Instruments LabVIEW
software.
Schematic diagram of the PFA OFR and associated experimental setup
for (a) the laboratory and (b) field experiments.
The total flow rate for the laboratory tests was 3.5 L min-1,
corresponding to an average residence time of 130 s, while those of the PAM,
TPOT, and CPOT are about 100, 110, and 1500 s, respectively. A purge flow
rate of 1.5 L min-1 was extracted from the annulus outside of the
sample exit cone as described above. The ambient experiments were conducted
using a slightly lower flow rate of 3 L min-1, resulting in a residence
time of 150 s, and with a 1.5 L min-1 purge flow.
RTD experiments
The residence time distributions of particles and gases were experimentally
characterized and compared with results obtained from an ideal laminar flow
model simulation. The experimental configuration is illustrated in Fig. 3a. Monodisperse ammonium sulfate (AS) particles were generated by
atomizing a 0.04 M aqueous AS solution with an atomizer (TSI Inc. Model
3076). The atomized particles were dried by directing them through a silica
gel/molecular sieve diffusion column. The size of the particles was
selected using a differential mobility analyzer (DMA). The aerosol was
brought to a steady state charge distribution before and after size
classification by the DMA using soft X-ray neutralizers. The residence time
distributions (RTDs) for particles were characterized by introducing 30 s
pulses of 200 nm AS particles into the PFA OFR while measuring the particle
counts in the outlet flow with a condensation particle counter (CPC, TSI
Inc. Model 3762).
RTDs for gases were characterized by injecting 10 s pulses of SO2 and
CO2. Pulses of a compressed gas mixture containing 27.5 ppm SO2 in
nitrogen (Airgas) were injected into a continuous zero air flow, with the
pulse width controlled by opening and closing a mass-flow controller (Alicat
Scientific, PN MC-100SCCM-D/5M). The SO2 concentration was monitored
from the sampling outlet of the PFA OFR with an SO2 analyzer. Prior
to the measurements, the reactor was purged with zero air for as long as
required to reach a measured SO2 mixing ratio that was stable at less
than 0.5 ppb. To test the response function of a gas that would not react on
or be taken up by the walls, 10 s pulses of CO2 were injected from a
custom-made CO2 tank, with the pulses controlled by manually opening
and closing a valve. The CO2 concentration was measured at the outlet
of the PFA OFR by a CO2/H2O gas analyzer (Li-COR Biosciences,
Model Li-840A). A CO2 background of 400 ppm was subtracted from the
results because it was not removed by the zero air generator. The residence
time distributions of both gases and particles were determined with the UV
lamp turned on and turned off. The whole process described above
was repeated three times.
Gas and particle loss quantification
Particle losses in the reactor were characterized using AS particles within
the diameter range from 50 to 200 nm. The monodisperse AS particles were
size-selected by a DMA and then passed through a soft X-ray neutralizer
after size classification. Upon exiting the neutralizer, the size-dependent
fraction of particles that possess at least one positive or negative charge
varies from about 41 % for 50 nm particles to 71 % for 200 nm
particles (Wiedensohler, 1988). The flow rate through the
reactor was kept at 3.5 L min-1.
Particles were directed through the reactor or through a 150 cm length × 0.95 cm OD copper tube bypass, with sampling alternated between the two
through the use of an automated three-way valve. The particle transmission
efficiency was calculated from the ratio of the particle concentrations
measured at the outlets of the reactor and bypass using a CPC (TSI Inc.
Model 3760A). After a set of initial tests, the static charge on the PFA,
PTFE, and ePTFE surfaces was minimized by pushing concentrated bipolar ions
generated with an electronic ionizer (Simco-Ion Inc., Fusion) through and
around the flow tube for more than 12 h. Additional measurements of 50
and 100 nm particles were made after minimizing the static charge. The
measurements were repeated 2 or 3 times for each particle size, with
agreement between measurements found to be to within ±5 % when
sampling the same diameter.
Gas losses were determined by continuously injecting gas mixtures containing
CO2 and SO2 and measuring the ratio of the concentrations
downstream and upstream of the reactor with the CO2 and SO2 analyzers identified above. For the SO2 transmission efficiency
tests, the PFA OFR walls were first passivated by flowing SO2 gas
through the OFR for at least 15 min and until a stable concentration was
measured by the SO2 analyzer connected to the outlet.
UV intensity profile and OH exposure level
The 254 nm intensity at multiple positions inside the reactor was examined
using a spectroradiometer (OceanView, Model USB4000 UV-FL) via a fiber-optic
cable. The influence of the reflective material(s) wrapped around the flow
tube was assessed by measuring when it was wrapped only with aluminum-coated
Mylar and when it was wrapped with a combination of a ePTFE gasket (inner
layer) and Mylar (outer layer). The OH production rate and corresponding
equivalent exposure was varied by changing the UV intensity, RH, and
injected O3 concentration. Here, OHexp is defined as the OH
concentration (molec. cm-3) multiplied by the mean residence time of
the sample in the reactor. The UV intensity from the lamp was controlled
over a range of 50 % to 100 % using a lamp manager (BHK. Inc, PN IM10003)
by stepping the control voltage from 0 to 5 V. The O3 concentration in
the reactor was varied by adjusting the position of a sleeve over the lamp
in the ozone generator. To quantify OHexp, SO2 was injected with
initial mixing ratios ranging from 150 to 250 ppb. For each test, the UV
lamp was initially off and was turned on only after the SO2
concentration measured at the outlet was stable. After the lamp was turned
on, the concentration of SO2 was monitored at the reactor outlet. The
distribution of the O3 and OH within the PFA OFR was not measured in
the current study. Future studies will be designed to investigate their
profiles that result from that of UV. The typical concentration pattern
observed is shown in Fig. S1. OH exposure was quantified for each UV lamp
intensity and O3 concentration combination using Eqs. (1) and (2)
(Davis et al.,
1979; Atkinson et al., 2004). The procedure was repeated 2 or 3 times
at each UV intensity.
1d[SO2]/dt=-kOH-SO2[OH][SO2]2OHexp=kOH-SO2-1×ln[SO2]0/[SO2]f
Here, kOH-SO2 is 9×10-13 cm3 molec.-1 s-1,
[SO2]0 and [SO2]f are SO2 concentrations measured at the reactor outlet without and with the UV lamp turned on.
Results and discussionsUV intensity distribution and OHexp level
The normalized UV intensity as a function of distance from the lamp located
at the top of the reactor is shown in Fig. 4a. The normalized UV
intensity is calculated as the intensity at a specified position divided by
the maximum measured inside the PFA OFR. As expected, an intensity gradient
exists, with decreasing intensity with distance from the lamp. The gradient
is much steeper when the flow tube is not wrapped with the ePTFE gasket.
Without the ePTFE gasket, the intensity near the bottom of the tube is only
15 % of that at the top. Adding the ePTFE resulted in an intensity 30 cm
from the bottom that was approximately 5 times higher than that with only
the Mylar. The relative UV intensity enhancement (ERIntensity) is shown
as a function of position in Fig. 4b. The UV intensity is enhanced by a
factor of between about 2 and 6 with the addition of the ePTFE layer. In
addition to increasing the average UV intensity, the use of the reflective
gasket reduced the gradient in intensity, resulting in more uniform OH
generation throughout the reactor.
Relative UV intensity profile (a) and intensity enhancement (b)
achieved when the flow tube was wrapped with a reflective ePTFE gasket.
The maximum photon flux (with the maximum lamp power) was estimated for
ozone concentration measured at the exit of the OFR and known RH using a
photochemical model used in previous studies
(Li
et al., 2015; Peng et al., 2015, 2019). The input photon flux of the model
was adjusted to match the measured ozone concentration and OH exposure,
which resulted in a maximum photon flux estimate of ∼ 2.4 × 1014 photons cm-2 s-1 and 1.1 × 1015 without
and with the ePTFE layer, resulting in about 1.5 and 7 times, respectively,
that calculated from the lamp output power when neglecting any reflection.
The OH concentration and resulting OHexp were varied by varying the UV
intensity, the added O3 concentration, and the RH. Figure 5 shows the
sensitivity of OH exposure as a function of photon flux at 254 nm with and
without ePTFE wrapped around the flow tube. Without the ePTFE wrap around
the reactor (black solid symbols), the OH concentration ranged from
approximately 1.3 × 108 to 2.2 × 109 molec. cm-3. The corresponding OHexp ranges from 2 × 1010
to 3.3 × 1011 molec. cm-3 s, which is approximately
equivalent to 0.15 to 2.5 d of atmospheric exposure based on the
reference average OH concentration of 1.5 × 106 molec. cm-3. The increased reflectance and UV intensity with the ePTFE wrap
(red solid symbols) resulted in a maximum OHexp of approximately 1.1 × 1012 molec. cm-3 s, equivalent to 8.5 d of
atmospheric OH exposure, for the same RH (40 %) and O3 mixing
ratio (3.3 ppm). Overall, the highly reflective (and non-absorbing)
materials used result in OH exposure comparable to that in other OFRs
despite the use of a relatively low power output lamp.
Variations in the concentration of OH as a function of photon flux
at 254 nm with (red solid points) and without ePTFE (black solid points)
wrapped around the flow tube. Both experiments were done with the same OH
reactivity.
Gas and particle transmission efficiency
Figure 6 shows the transmission efficiency of AS particles with mobility
diameter ranging from 50 to 200 nm. As stated above, particle transmission
efficiency is calculated as the ratio of the concentration exiting the
reactor to that exiting a copper tube bypass. Concentrations measured
upstream and downstream of the copper tube agreed within ±1 %,
confirming minimal loss in the bypass line. We performed two sets of tests:
first, following the removal of static charge on the inner surface of the
reactor tube (preliminary removal process), and second, following the
additional removal of static charge between the ePTFE/Mylar wrap and the
outer surface of the reactor tube (secondary removal process). The particle
transmission efficiency after removal of only the charge on the inner
surface of the tube was 0.39, 0.75, and 0.93 for 50, 80, and 100 nm
diameter particles, respectively. With the removal of the static charge on
the outer surface of the tube, the transmission efficiency of 50 nm and 80
nm particles increased from 0.39 to 0.75 and from 0.75 to 0.84,
respectively. Each experiment was repeated twice, with agreement within
±10 % when sampling the same particle size and with the same flow
rate. These results indicate that loss of small particles in the reactor can
be significantly reduced by minimizing the static charge on both the inner
and outer surfaces of the reactor tube. The similarity in the resulting 36 % of the 50 nm particles that are no longer lost and the 41 % of those
50 nm particles that are expected to be charged
(Wiedensohler, 1988) suggests electrostatic loss was
minimal after the static charge was minimized. Comparison with the particle
transmission efficiency of other types of flow tube reactors with
non-conductive wall materials is included in Fig. 6. The PAM reactor
referenced is the horizontal 46 cm length × 22 cm ID glass cylindrical
chamber with a volume of 15 L that was described by
Lambe
et al. (2011), hereafter referred to as the quartz PAM. The results show
that the particle transmission efficiencies through the PFA OFR, TSAR, and
ECCC-OFR are higher than those for the quartz PAM, TPOT, and CPOT reactors,
which may in part be due to their use of similar cone-shaped inlets and of
centerline sampling. The particle transmission efficiencies of the
quartz PAM, CPOT, and TPOT for 50–100 nm particles are 30 %–50 %, 15 %–25 %, and 35 %–65 % lower than that of the PFA-OFR, respectively. On the
other hand, the transmission efficiency through the TSAR and the ECCC-OFR is
5 %–25 % higher than for the PFA-OFR, though at least some of this
difference is caused by the longer residence time of the PFA OFR than that
of the other two reactors.
Measured particle transmission efficiency of the PFA OFR,
quartz PAM and TPOT (Lambe et al., 2011), CPOT (Huang et al., 2017), TSAR
(Simonen et al., 2017), and ECCC-OFR (Li et al., 2019) flow reactors as a
function of mobility diameter for bis(2-ethylhexyl) sebacate (BES) and
ammonium sulfate (AS). Our results are shown as blue squares and red
triangles.
The experimental configuration used to measure the loss of SO2 and
CO2 is similar to that used to characterize the gas RTD. The
penetration efficiencies of CO2 and SO2 were 0.90 ± 0.02 and
0.76 ± 0.04, respectively. The wall loss for most precursor species is
expected to be equal to or less than the 24 % found for SO2 because
it is a good surrogate for wall-adhering species
(Lambe
et al., 2011; Ahlberg et al., 2017; Huang et al., 2017). For comparison,
Lambe
et al. (2011) reported that the measured CO2 and SO2 transmission efficiencies for the TPOT were 0.97 ± 0.10 and 0.45 ± 0.13, respectively, and for the quartz PAM were 0.91 ± 0.09
and 1.2 ± 0.4, respectively.
The fate of low-volatility organic compounds (LVOCs) that can condense onto
particles, stick to the reactor walls, react with OH, or exit the reactor
before condensing can be evaluated using the approach described by
Palm et al. (2016). Based on the simple model they present, LVOC wall losses for the
PFA OFR have an upper limit of approximately 30 % for a residence time of
130 s, which is comparable to that observed for SO2 (24 %). Although
the LVOC fate method is strongly dependent on the design and the geometry of
the reactor, the consistency between the estimated loss and that measured
for SO2 suggests the value is a reasonable estimate of the vapor loss
for our design. Losses of some gases are expected to be greater in this OFR
than in most others because of its larger surface-area-to-volume (A/V) ratio
of 0.53 cm-1, which is greater than that of the PAM reactor, while the
mean residence times of the two are similar. However, losses of some gases
may be lower as well because only the central core flow is subsampled, all
Teflon materials are used, and, as is described in the next section, the RTD
is comparatively narrow, which suggests less mixing than in other OFRs.
Gas and particle residence time distributions
The residence time probability distribution functions for particles and
gases are shown in Fig. 7a and b. Reporting the results as normalized
distribution functions facilitates comparison of the flow characteristics of
reactors of different shapes and sizes. RTDs of idealized devices and those
reported for CPOT and quartz PAM are also shown in Fig. 7a and b for
comparison (PAMWiki, 2020). The residence time probability distribution
function is defined as the normalized measured concentration (Cout(t)) divided by the total area of the normalized pulse
(Fogler, 2006; Simonen
et al., 2017), as described in Eq. (3) below. The average residence time was
calculated as the summation of the product of the measured concentration and
the corresponding residence time, all divided by the total area of the
pulse.
PDFt=Cout(t)∫0∞Cout(t)dt
Residence time probability distribution functions of the PFA OFR,
CPOT (Huang et al., 2017), and quartz-PAM flow tubes (Lambe et al., 2011) as
a function of residence time for (a) particles and (b) gases.
The residence time distributions of particles and gases in the PFA OFR shown
in Fig. 7a and b approach those expected for laminar developed flow.
Measured RTDs for both particles and gases have relatively short tails at
longer times compared with the ideal laminar flow pulse, as is expected
because only the center ∼ 50 % of the sample flow is
subsampled and directed to the analyzers. Relative to the total flow through
the reactor, the subsampled core has a narrower velocity range and less
interaction with the walls. Extraction of the side purge flow also helps by
preventing recirculation near the outlet. The RTDs measured with the UV lamp
turned on are only slightly broader than those with it turned off. Previous
studies report that UV lamps broaden the RTD because they heat the reactor
walls and enhance convection inside the reactor
(Simonen et al., 2017). Significant
degradation is not observed in the PFA OFR, presumably because of the use of
a comparatively low-power light source, circulation of air through the
reactor housing, and the reactor being oriented vertically with the lamp at
the top to promote stratification and to minimize convective mixing.
Reversible uptake by the walls is responsible for the broader RTD for
SO2 relative to that for CO2.
We also investigated the effect of the ratio of the sample to side flow on
the RTD for AS particles. The condition without the side purge flow was
numerically simulated in COMSOL 5.4 by coupling the Laminar Flow and the
Transport in Dilute Species packages. This result is compared with the RTD
of that obtained experimentally with different sample–side flow ratios in
Fig. S2. The experimental results show the improvement in RTD response as
the sample–side flow ratio is decreased. We expect future efforts to include
simulation studies, such as the model-derived relationship between the
sample–side flow ratio and the losses of particles and gases.
SOA yield measurements
Secondary organic aerosol yields (Y) are defined as the mass of OA formed
(ΔCOA) per reacted precursor mass
(ΔHC) (Odum et al.,
1996). The measured yields of m-xylene and α-pinene SOA as a function
of OH exposure and organic aerosol concentration
(COA) are shown in Fig. 8a and b. Here, the SOA
yields are corrected for size-dependent gas and particle losses, with an
average magnitude of the combined correction of 25 %. For comparison, the
magnitude of the particle wall loss correction of the PAM reactor was 32 % ± 15 %
(Lambe et al., 2015).
The COA was calculated by multiplying the volume concentration measured
with an SMPS by an assumed density of 1.2 g cm-3. The mixing ratios of
m-xylene and α-pinene introduced into the PFA OFR were in the ranges
of 20–118 and 13–145 ppb, respectively. The OHexp was not measured
simultaneously during the yield experiments, but applying the OFR254 OH
exposure estimation equation as a function of O3, RH, and UV lamp power
(Peng et al.,
2015) and the assumption that the OH reactivity was the same for both sets
of measurements resulted in estimated values that are consistent with the
measured OHexp described in Sect. 3.1. As expected, the SOA yield was
observed to be dependent on OH exposure and aerosol mass concentration. The
m-xylene SOA yield was 0.22 at 3 × 1011 molec. cm-3 s
OH exposure and an OA mass concentration of 46 µg m-3 and the
α-pinene SOA yield was 0.37 at 3 × 1011 molec. cm-3 s OH exposure and a mass concentration of 178 µg m-3.
SOA yield as a function of organic aerosol concentration
(COA) for (a)m-xylene SOA and (b)α-pinene SOA generated in the
PFA OFR. Marker color reflects experimental combinations of UV intensity,
O3 mixing ratio, and RH. Each marker represents one VOC
concentration.
The measured yields are compared with those reported by
Lambe
et al. (2011) for the TPOT (for 262–263 ppb precursor mixing ratio), the
quartz PAM (78–88 ppb), and the Caltech environmental chamber (14–48 ppb),
and by Ahlberg et al. (2017)
for the aluminum PAM (14–179 ppb of α-pinene and 43–395 ppb of
m-xylene). The comparisons as a function of COA are shown in Fig. 9a
and b. The SOA yields are higher in the PFA OFR than those in the
quartz PAM and TPOT but lower than in the aluminum PAM. The α-pinene
SOA yields in the PFA OFR (0.37 ± 0.02) and Caltech chamber (0.42 ± 0.06) agreed within 12 % for comparable OH exposures (∼ 1011 molec. cm-3 s). A contributor to differences in yield among
the OFRs is variation in OHexp, which, as noted above, was not measured
during the yield experiments. Our estimates of OHexp neglect the impact
of varying OH reactivity (OHR), which is defined as the summation of the
product of the concentrations of species that react with OH and their
reaction rate constants
(Li
et al., 2015; Peng et al., 2015). During our experiments, the maximum OH
reactivities for the m-xylene and α-pinene experiments were 34 and 103 s-1, respectively, which is higher than the 5.5 s-1 estimated for the SO2 experiments that were used to determine
the dependence of OHexp on RH, O3 concentration, and lamp power.
This is also a source of uncertainty in PAM yields that were reported in
Lambe
et al. (2011) and is estimated by
Li et al. (2015) to result
in a factor of 2 uncertainty in OHexp obtained from their model-derived
equation. Differences in O3 concentrations and resulting partitioning
between reactions with O3 and OH are expected to be more important for
α-pinene than for m-xylene. The formed SOA is dependent on the
reactivity of one or more of the SOA-forming compounds and the oxidant
concentrations (McFiggans et al.,
2019). For the same O3 mixing ratio (3.3 ppm) and OH exposure (3 × 1011 molec. cm-3 s) described above, the
reactivities of α-pinene towards O3 and OH are estimated to be
6.8 × 10-3 and 111 × 10-3 s-1,
while that of m-xylene towards OH is estimated to be 50 × 10-3 s-1.
Comparison of SOA yields as a function of organic aerosol
concentration (COA) with those reported for other OFRs and one large
Teflon chamber. (a)m-Xylene SOA and (b)α-pinene SOA. Marker color
reflects the OHexp.
To investigate the effect of the RTD and wall interactions on aerosol
production, we repeated the α-pinene SOA yield experiments under the
same experimental conditions, but without extracting the side purge flow
and, therefore, without subsampling just the central core flow. Doing so
results in a broadened RTD and sampling of air that, on average, interacted
more with the flow tube walls. The resulting α-pinene SOA yields as
a function of organic aerosol concentration (COA) with (black squares)
and without (red triangles) extracting the side purge flow are shown in Fig. S3. The figure shows that the narrower RTD and reduced wall effects
accompanying subsampling of the central flow result in higher SOA yields,
which may partially explain why the yield presented in Fig. 9 is slightly
higher than that reported for other OFRs. Additional experiments are needed
to determine the relative importance of the RTD and the wall effects and to
evaluate any role of other differences between the experiments such as the
increased average residence time (and OHexp) when not subsampling just
the higher velocity central core flow.
Seed particle SOA enhancement
The influence of seed particle concentration was investigated by measuring
SOA yield for varying ratios of the mass concentrations of α-pinene
and AS seed. For all experiments a constant flow rate (0.7 L min-1)
containing the AS seed particles was introduced together with a varying
mixing ratio of α-pinene (8–30 ppb). Using the same method that was
presented in Sect. 2.4, a DMA generated a narrow mode of AS seed particles
centered at a diameter of 200 nm. The average mass concentration of the AS
aerosol throughout the experiments was 40 µg m-3. The O3
concentration, RH, and UV lamp power were the same for all measurements,
with a resulting OHexp of about 2 × 1011 molec. cm-3 s, which is consistent with the OHexp estimated from the model-derived
equation (Peng
et al., 2015). Measurements for each precursor concentration were repeated
2 or 3 times, with agreement between measurements to within ±10 %. Figure 10a and b show the volume size distributions for one set
of experiments with and without added AS particles. The results show that
the addition of seed particles suppresses the nucleation mode as
condensation on the larger particles is favored. The concentration of
α-pinene SOA increased with the addition of high concentrations of
seed particles, as is expected because the increased surface area promotes
condensation on the aerosol and decreases the fraction of low-volatility
oxidation products that reach and are lost to the walls or are further
oxidized in the gas phase. In these experiments, the yield increased by as
much as a factor of 3 at the minimum precursor–seed mass ratio of about 2.
The magnitude of the enhancement decreased with increasing precursor–seed
ratio and was within the run-to-run variability for ratios exceeding about
5, which is shown in Fig. S4.
Example sets of volume size distributions from experiments
evaluating the impact of adding AS seed particles on SOA yield. The
precursor : seed mass ratio is (a) 1.8 (b) 3.9.
Aerosol formed from oxidation of ambient air
Ambient air from outside our lab at the UCR College of Engineering – Center
for Environmental Research and Technology (CE-CERT) in Riverside, CA, was
processed by the PFA OFR for several days in January 2020. Figure 11a
and b show results for a 30 h period (7–8 January 2020) and a 6 h
period on 8 January 2020. Throughout the sampling period, the SMPS alternated
through sets of three measurements of the processed aerosol at the exit of
the reactor and sets of two measurements of unprocessed aerosol that
bypassed the reactor through a copper tube. Each cycle of 5 measurements
lasted 21 min. The OHexp during the sampling period estimated from the
model-derived equation introduced in Sect. 3.4 was in the range of 1–4 × 1011 molec. cm-3 s for the maximum lamp power and
measured ozone concentration and RH.
Example time series of size distributions of the aerosol
processed by the PFA OFR and that which bypassed it over (a) 30 h on
7–8 January 2020 and (b) 6 h on 8 January 2020. The bands of high
concentration were measured when the aerosol and ambient air were processed
through the reactor.
Time series of aerosol mass concentrations calculated from integration of
the SMPS size distributions are shown in Fig. 12a. The mass concentration
of the aerosol exiting the reactor was corrected for the fractional dilution
by the injected O3 flow and for size-dependent gas and particle
transmission efficiencies. The aerosol mass concentration increased
significantly in the reactor during the oxidation process. A relative SA
enhancement (ERSA) is defined here as the ratio of the mass
concentration of SA divided by that of the ambient (unprocessed) aerosol,
with the SA simply defined as the difference between the processed and
unprocessed aerosols. The ERSA for the same sampling period is shown in
Fig. 12b. A consistent diurnal pattern was not observed throughout the
sampling period. The SA mass concentration was an average of 1.8 times that
of the ambient aerosol during the selected period. More SA formation was
observed during nighttime on 8 January, while decreasing amounts formed until
around noon. The maximum enhancement due to SA formation was observed in the
late afternoon on 7 January, when the SA mass concentration was approximately 7
times that of the ambient aerosol. A small SA enhancement was also observed
during the late afternoon on 8 January. The overall temporal pattern likely
reflects the impact of traffic-related emissions from nearby roads,
including a major highway that is about 1.5 km away. In the future there is
a need to add more comprehensive measurements of the chemical composition of
the particulate and gaseous species.
Time series of mass concentrations of the aerosol exiting the PFA
OFR and that bypassing it (a), and (b) the relative enhancement of the mass
concentration due to SA formation.
Summary
A new all-Teflon reactor, the Particle Formation Accelerator (PFA) OFR, was
designed, constructed, and characterized using both experimental
measurements and CFD modeling. Its performance was examined and evaluated
through laboratory measurements and with ambient air. The reactor response
and characteristics were compared with those from a smog chamber (Caltech)
and other oxidation flow reactors: the Toronto Photo-Oxidation Tube (TPOT),
Caltech Photooxidation Flow Tube (CPOT), the TUT Secondary Aerosol Reactor
(TSAR), quartz and aluminum versions of Potential Aerosol Mass reactors
(PAMs), and the Environment and Climate Change Canada OFR (ECCC-OFR).
Our results show that OHexp can be varied over a range comparable to
that of other OFRs, with the dependence on UV lamp power, RH, and O3
concentration characterized and reported. The particle transmission
efficiency is over 75 % in the size range from 50 to 200 nm after
minimizing static charge on the PFA, PTFE, and ePTFE surfaces. The gas
transmission efficiencies of CO2 and SO2 are 0.90 ± 0.02
and 0.76 ± 0.04, respectively, with the latter comparable to estimated
transmission of LVOCs through the PAM reactor. Computational simulation and
experimental verification of particle and gas residence time distributions
(RTDs) show that the flow through the reactor is nearly laminar, with
narrower RTDs than reported for OFRs with greater diameter-to-length ratios,
making it better suited for measurements of dynamic sources with
time-varying composition or concentration.
The mass yields of SOA from the oxidation of α-pinene and
m-xylene, and the effect of seed particles on those yields, were
investigated. At comparable OH exposure, the m-xylene and α-pinene
SOA yields are slightly higher than those in the quartz PAM and TPOT, but
lower than in the aluminum PAM. A likely contributor to differences in
yields between the PFA OFR and other OFRs is the uncertainty in OHexp,
which was not measured simultaneously during the yield measurements and was
determined from separate experiments for which the OH reactivity differed.
The α-pinene SOA yields in the PFA OFR (0.37 ± 0.02) and
Caltech chamber (0.42 ± 0.06) agree within 12 % for comparable OH
exposures (∼1011 molec. cm-3 s). The presence and
concentration of seed particles was shown to have a significant effect on
SOA yield. At a nominally fixed OH exposure of 2 × 1011 molec. cm-3 s, the α-pinene SOA yield for the minimum precursor–seed
mass ratio of about 2 was about 3 times that when no seed particles were
added. The magnitude of the enhancement decreased with increasing
precursor : seed ratio and was within the run-to-run variability for ratios
exceeding about 5. The SA production from ambient air was studied in
Riverside, CA. The mass concentration of SA formed in the reactor was about
twice the mass concentration of the ambient aerosol at the same time.
Overall, the computational and experimental results indicate that the PFA
OFR is suitable for laboratory studies and for field use that includes
measurement of rapidly changing ambient concentrations. Future efforts will
include adding direct measurement of OHexp during measurements,
development of an OHexp estimation description for the PFA OFR
comparable to that reported for other OFRs, and further exploring the
influence of OH reactivity on OHexp and of seed particles on SOA yield.
We will also expand upon measurements of the composition of the particulate
products and gaseous precursors during one or more field studies to evaluate
how well the PFA OFR simulates atmospheric chemistry that typically requires
hours or days.
Data availability
Data presented in this work are available from the authors.
The supplement related to this article is available online at: https://doi.org/10.5194/amt-14-2891-2021-supplement.
Author contributions
DRC designed the reactor and edited the paper. NX performed the experiments and simulations, processed the data, and wrote the paper.
Competing interests
The authors declare that they have no conflict of interest.
Review statement
This paper was edited by Mingjin Tang and reviewed by three anonymous referees.
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