ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus GmbHGöttingen, Germany10.5194/acp-15-4197-2015Vapor wall deposition in Teflon chambersZhangX.https://orcid.org/0000-0003-1548-8021SchwantesR. H.McVayR. C.LignellH.CoggonM. M.FlaganR. C.https://orcid.org/0000-0001-5690-770XSeinfeldJ. H.seinfeld@caltech.eduhttps://orcid.org/0000-0003-1344-4068Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USADivision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USAJ. H. Seinfeld (seinfeld@caltech.edu)23April20151584197421423September201424October20147February201525March2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://www.atmos-chem-phys.net/15/4197/2015/acp-15-4197-2015.htmlThe full text article is available as a PDF file from https://www.atmos-chem-phys.net/15/4197/2015/acp-15-4197-2015.pdf
Teflon chambers are ubiquitous in studies of atmospheric chemistry.
Secondary organic aerosol (SOA) formation can be underestimated, owing to
deposition of SOA-forming vapors to the chamber wall. We present here an
experimental protocol and a model framework to constrain the
vapor–wall interactions in Teflon chambers. We measured the wall deposition rates of 25
oxidized organic compounds generated from the photooxidation of isoprene,
toluene, α-pinene, and dodecane in two chambers that had been
extensively used and in two new unused chambers. We found that the extent of
prior use of the chamber did not significantly affect the sorption behavior
of the Teflon films. Among the 25 compounds studied, the maximum wall
deposition rate is exhibited by the most highly oxygenated and least
volatile compounds. By optimizing the model output to the observed vapor
decay profiles, we identified that the dominant parameter governing the
extent of wall deposition of a compound is its wall accommodation
coefficient (αw,i), which can be correlated
through its volatility with the number of carbons and oxygens in the
molecule. By doing so, the wall-induced deposition rate of
intermediate/semi-volatile organic vapors can be reasonably predicted based
on their molecular constituency. The extent to which vapor wall
deposition impacts measured SOA yields depends on the competition between
uptake of organic vapors by suspended particles and the chamber wall. The
timescale associated with vapor wall deposition can vary from minutes to
hours depending on the value of αw,i. For
volatile and intermediate volatility organic compounds (small
αw,i), gas-particle partitioning will dominate
wall deposition for typical particle number concentrations in chamber
experiments. For compounds characterized by relatively large αw,i, vapor transport to particles is suppressed by competition
with the chamber wall even with perfect particle accommodation.
Introduction
Understanding of the mechanism and extent of secondary organic aerosol (SOA)
formation from oxidation of volatile organic compounds (VOCs) has been
derived largely from experiments in Teflon chambers. Chamber-measured SOA
yields (mass of SOA formed per mass of VOC reacted) have been widely
parameterized into regional/global atmospheric models, and chemical
mechanisms leading to SOA formation and aging have been derived based on the
gas/particle-phase identification of intermediate/semi/low-volatility
compounds generated in controlled chamber experiments. An unavoidable
consequence of the use of an environmental chamber is interaction of vapors
and particles with the chamber wall. It has been recently established that
SOA formation can be substantially underestimated due to deposition of
SOA-forming vapors to the chamber wall rather than growing particles (Zhang
et al., 2014a).
Chamber-wall-induced decay of organic vapors was reported 30 years ago.
Grosjean (1985) and McMurry and Grosjean (1985) measured wall
deposition rates of several volatile organic compounds in a chamber
constructed from Fluorinated ethylene propylene (FEP) Teflon film. The
lifetime of the VOCs, with respect to wall deposition, was found generally to
exceed ∼15h. Loza et al. (2010) found that
deposition of the isoprene oxidation product surrogate,
2,3-epoxy-1,4-butanediol (BEPOX), and glyoxal to the FEP Teflon chamber wall
is reversible on sufficiently long timescales. On the contrary, rapid
reversible gas–wall partitioning of n-alkanes, 1-alkenes, 2-alcohols,
2-ketones, monoacids, and 1,2-diols was universally observed by Matsunaga
and Ziemann (2010) and Yeh and Ziemann (2014). Following the same
experimental protocol, Kokkola et al. (2014) measured that the equilibrium
fractions of nopinone and pinanediol on the wall of a 4 m3 FEP Teflon
chamber are on average 0.4 and 0.8, respectively.
The extent to which vapors and the chamber wall interact is reflected by
properties such as the gas-phase transport rate of organic molecules,
affinity of the wall for various organic molecules, the degree of
reversibility of the vapor–wall partitioning, and the equilibrium solubility
of organic vapors in the wall. Organic materials generated in chamber
experiments can deposit on the chamber wall to form a coating that can act
as the primary absorbing medium, or the Teflon film itself could act as the
absorbing medium, in a process akin to the sorption of small molecules by
organic polymers. While measurement of vapor wall deposition rates for the
thousands of organic molecules that are produced from the oxidation of SOA
precursor VOCs is not presently possible, empirical expressions that
represent the deposition rates of organic vapors as a function of general
molecular properties would be highly useful.
Theoretical framework representing the vapor–wall interactions.
Concentrations of organic vapor i in the well-mixed core, in the boundary
layer, over the surface of the chamber wall, and in the chamber wall are
denoted by C¯v,i, Cv,i, C0,i,
C¯w,i, respectively. Vapor fluxes at the gas–wall
interface are denoted by Jv,i and Jw,i.
A prime goal of characterizing vapor wall deposition in a chamber is to
understand its impact on SOA formation and evolution. We present here an
experimental protocol to constrain the nature of organic vapor wall
deposition in Teflon chambers. We measured wall-induced dark decay rates of
25 intermediate/semi-volatility organic vapors, which span a range of
volatilities and oxidation states, in both unused and previously used
chambers constructed with FEP Teflon film. A temperature ramping program
(298–318 K) was implemented to study the reversibility of vapor–wall
partitioning. A model framework is developed to describe interactions
between organic vapors and the chamber wall following the theories for
particle wall deposition and gas-particle partitioning. We address the
following questions in the present study. (1) What is the physicochemical
nature of the chamber wall? (2) What are the key parameters that characterize
the vapor–wall interactions and how can these values be determined? (3) How
can one predict the wall deposition rate of a specific compound based on
its molecular properties?
Vapor wall deposition – theory
Figure 1 depicts the steady-state concentration profiles of an organic
compound i in the well-mixed core of the chamber (C¯v,i),
in the boundary layer adjacent to the wall (Cv,i), at the wall surface
(C0,i), and in the chamber wall (C¯w,i). Vapor
molecules in the well-mixed core of a chamber are transported through a
boundary layer adjacent to the wall by a combination of molecular and
turbulent diffusion. The transport rate depends on both the molecular
properties of the individual organic compound (as characterized by the
molecular diffusion coefficient, Di), as well as the extent of
turbulent mixing in the chamber (as characterized by the coefficient of eddy
diffusion, Ke). As vapor molecules encounter the chamber wall, the
fraction of those encounters that lead to uptake is represented by the
accommodation coefficient (αw,i), and molecules
rebound with a probability of 1-αw,i. The
accommodation coefficient depends, in principle, on the nature of the wall
surface as well as the compound chemical composition. It is worth
emphasizing that αw,i characterizes imperfect
wall accommodation of the gas–wall interface. Molecules deposited on the
wall may re-evaporate at a rate that depends on their concentration in the
wall. In order to represent this process, we note that, at equilibrium, the
flux arriving from the gas phase (Jv,i) and the evaporation flux from
the wall (Jw,i) are equal. Thus, the evaporative flux from the wall
(Jw,i) can be expressed as a function of the accommodation coefficient
(αw,i), as described in Eqs. (7)–(9) later.
A conservation balance on C¯v,i, the concentration of
vapor i in the well-mixed core of a chamber that is subject only to the
deposition process, is given by
dC¯v,idt=-kw,depo,iC¯v,i+kw,evap,iC¯w,i,
where kw,depo,i (s-1) is the deposition rate coefficient to the
wall, kw,evap,i (s-1) is the evaporation rate coefficient from
the wall, and C¯w,i is the concentration of vapor i that
has accumulated on the chamber wall. The dynamic behavior of
C¯w,i is described by a corresponding balance:
dC¯w,idt=-kw,evap,iC¯w,i+kw,depo,iC¯v,i.
Note that C¯w,i is assumed to be zero at the onset of
vapor i generation, ultimately reaching equilibrium with
C¯v,i.
Vapor flux arriving from the gas phase (Jv,i)
For a chamber that is relatively well mixed, transport to the wall occurs by
molecular and turbulent diffusion across a thin boundary layer, of thickness
δ, adjacent to the chamber wall. The flux due to molecular diffusion
is given by -Di∇Cv,i, where Cv,i is the local
vapor i concentration in the boundary layer and Di is its molecular
diffusivity. The turbulent diffusion flux is expressed as -De∇Cv,i, where De is the eddy diffusivity. One can
invoke the Prandtl mixing length expression near a wall, De=Kex2, where x is the distance from the wall, and Ke is the
coefficient of eddy diffusion (Corner and Pendlebury, 1951; Crump and
Seinfeld, 1981). Owing to the small value of δ, a
quasi-steady state condition exists in the boundary layer, and the
concentration of vapor i within the boundary layer, 0≤x≤δ, is governed by
ddx(Kex2+Di)dCv,idx=0.
Introducing the
dimensionless variable z by setting x=(Di/Ke)1/2z, Eq. (3) becomes
z2+1d2Cv,idz2+2zdCv,idz=0,
subject to the boundary conditions,
x=0(z=0)→Cv,i=C0,i,x=δ(z=(Ke/Di)1/2δ)→Cv,i=C¯v,i,
where C0,i and C¯v,i are concentrations of vapor i
over the wall surface and in the well-mixed core of the chamber,
respectively. Note that the accommodation coefficient for particles on the
wall was assumed to be unity in previous theoretical studies (e.g., Crump
and Seinfeld, 1981; McMurry and Grosjean, 1985), meaning that particles that
encounter the wall will lead to 100 % uptake. This assumption is
reasonable, especially if particles are in a quasi-liquid state. On the
other hand, the accommodation coefficient for vapors on the wall
(αw,i) is likely less than unity, and the
steady-state concentration is then nonzero at the chamber wall surface. The
solution of Eq. (4) expressed in the original variables is
Cv,i=C0,i+(C¯v,i-C0,i)tan-1(Ke/Di)1/2xtan-1(Ke/Di)1/2δ≈C0,i+(C¯v,i-C0,i)tan-1(Ke/Di)1/2xπ/2.
Physically, turbulent diffusion dominates molecular diffusion at the outer
edge of the boundary layer, so that
(Ke/Di)1/2δ≫1.
The vapor flux arriving from the gas phase to the wall surface
(Jv,i) is derived from the kinetic theory of gases:
Jv,i=αw,iv¯iC0,i4,
where v¯i is the species mean thermal speed.
Vapor flux leaving from the wall due to evaporation (Jw,i)
Without loss of generality, vapor wall deposition can be assumed to be
reversible. The flux of molecules i that evaporate from the wall back to the
gas phase (Jw,i) depends on the concentration of i in the wall
(C¯w,i). So we can write Jw,i as a function
of C¯w,i:
Jw,i∝C¯w,i or Jw,i=λC¯w,i,
where λ is simply a quantity that reflects the positive correlation
between Jw,i and C¯w,i. If the gas and wall
phases are at equilibrium, then
Jv,i,(eq)=Jw,i,(eq).
Therefore,
λ=αw,iv¯iC0,i,eq4C¯w,i,eq=αw,iv¯i4Hi,
where Hi is the Henry's law constant of organic species i. Substitution
of Eq. (9) into Eq. (7) gives
Jw,i=αw,iv¯iC¯w,i4Hi.
If applying vapor–particle partitioning theory here, Eq. (10) can be
rewritten as
Jw,i=αw,iv¯iC¯w,i4Kw,iCw,
where Kw,i is vapor–wall partition coefficient (Matsunaga and Ziemann,
2010):
Kw,i=RTpL,i0γiMw¯,
and where
pL,i0 is the vapor pressure of compound i as a
liquid. We calculate pL,i0 by the average of
two group contribution methods, “SIMPOL.1” developed by Pankow and Asher
(2008) and “EVAPORATION” developed by Compernolle et al. (2011). γi, the activity coefficient in the wall layer on a mole
fraction basis, is assumed to be unity here, R is the gas constant, T is
temperature, and Mw¯ is the average molecular weight of the
absorbing organic material on the wall, which, following Matsunaga and
Ziemann (2010), is assumed to be 250 gmol-1. Cw (gm-3) is an
assumed equivalent mass of absorbing organic material on the chamber wall
(Matsunaga and Ziemann, 2010). It can be regarded as characterizing the
equilibrium solubility of individual organic molecules in FEP Teflon polymer
and, possibly, in other organic materials deposited on the wall. When
Cw→∞, the wall presents essentially an absorbing medium
of infinite extent, and vapor wall deposition is ultimately an irreversible
process. Note, however, that the concept of an “equivalent absorbing organic
mass” does not necessarily imply that an actual layer of organic material
exists on the chamber wall. Cw might well represent the accumulation of
deposited organic material from previous chamber experiments, or it could
reflect the absorption properties of FEP film itself. We will return to the
nature of Cw shortly.
Since the gas–wall interface is presumed to have no thickness, the net flux
across the interface results from the concentration gradient,
DidCv,idxx=0=Jv,i-Jw,i=αw,iv¯iC0,i4-αw,iv¯iC¯w,i4Kw,iCw.
Note that when equilibrium is established, the net flux becomes zero and the
concentration gradient no longer exists at the gas–wall interface. The LHS
of Eq. (13) is based on Fick's law of diffusion and leads to Eq. (5). In this
way, the quantity C0,i is expressed as a function of
C¯v,i and C¯w,i. Therefore, the
conservation equation for the change in the concentration of vapor i in the
well-mixed core of the chamber owing to wall deposition is given by
dC¯v,idt=AVαw,iv¯i/4παw,iv¯i/8(DiKe)1/2+1C¯w,iKw,iCw-C¯v,i,
where A and V are the surface area and volume of the chamber, respectively.
A rewrite of Eq. (14) gives
kw,depo,i=AVαw,iv¯i/4παw,iv¯i/8(DiKe)1/2+1,kw,evap,i=kw,depo,iKw,iCw.
Example of the experimental procedure to assess vapor wall
deposition using 3-nitrooxy-6-dodecanol (m/z=(-)332): period (1)
organic oxidation product generation; period (2) vapor wall deposition at
298 K in the dark; period (3) chamber temperature ramp from 298 to 318 K;
and Period (4) temperature held at 318 K in the dark.
Experimental conditions for production of oxidized organic vapors.
a The temperature is controlled at 298 K for the first
∼20h of the experiment, including ∼1–7 h
irradiation and ∼13–16 h darkness, and then ramped up to
318 K within ∼3h and held for ∼4–6 h.
Vapor wall deposition – experiment
Experiments were conducted in the Caltech dual 24 m3 Fluorinated
ethylene propylene (FEP) Teflon chambers that are suitable for pristine
(low-NO) and polluted (high-NO) conditions (Zhang and Seinfeld, 2013; Fahnestock
et al., 2014; Loza et al., 2014). Figure 2 shows a schematic of the
experimental protocol used to measure deposition of organic vapors to the
chamber wall. Oxidized organic vapors were generated via photooxidation of
four parent VOCs, isoprene, toluene, α-pinene, and dodecane, in the
absence of seed aerosol. Once a sufficient amount of oxidized products is
formed with none or limited aerosol formation via nucleation, irradiation is
ceased, and the ensuing wall-induced dark decay of the array of oxidation
products is monitored by chemical ionization mass spectrometry (CIMS).
Following this period, the chambers were heated to investigate the extent to
which vapor–wall partitioning is reversible. These experiments were carried
out in two chambers that had been used in past SOA studies. Two control
experiments were also conducted in two unused 24 m3 Teflon chambers
using identical experimental protocols, see Table 1.
CIMS traces of oxidized organic vapors generated from the
photooxidation of isoprene, toluene, α-pinene and dodecane under
high/low-NO conditions over the four chamber periods in Fig. 2. Colored
circles represent CIMS measured normalized signals during background (blue),
vapor generation (magenta), vapor wall deposition at 298 K (green),
temperature ramp (yellow), and vapor re-evaporation at 318 K (red). Black
dashed lines and gray solid lines represent the simulated deposition rates
generated from SIM.1 and SIM.2, respectively.
Vapor molecules representing SOA products were generated directly via VOC
photooxidation, as opposed to the external injection of commercially
available chemical standards. In this manner, uncertainty in the initial
vapor concentration due to filling and mixing is avoided. In order to
generate a spectrum of oxidized compounds characterized by a combination of
different carbon numbers and types of functional groups, isoprene, toluene,
α-pinene, and dodecane were chosen as the parent VOCs. Prior to each
experiment, the Teflon chambers were flushed with purified dry air for
12 h
at 45 ∘C, then “conditioned” by UV irradiation for 24 h in the
presence of 2 ppmH2O2, followed by purging with purified dry air
for ∼4 days at 25 ∘C. Experiments were carried out
under conditions in which the peroxy radicals formed from the initial
OH
reaction with the parent hydrocarbon react either primarily with NO
(so-called high-NO) or HO2 and RO2 (so-called low-NO). For
low-NO
conditions, hydrogen peroxide (H2O2) was used as the OH source by
evaporating 120 µL of 50 % wt aqueous solution into the chamber with
5 Lmin-1 of purified air for ∼110min, resulting in an
approximate starting H2O2 mixing ratio of 2.0 ppm. For
high-NO
conditions, nitrous acid (HONO) was used as the OH source by dropwise
addition of 15 mL of 1 wt% NaNO2 into 30 mL of 10 wt%
H2SO4 in a glass bulb and introduced into the chambers with 5 Lmin-1
of purified air for ∼40min. Ozone formation is
substantially limited in the presence of a high concentration of HONO, and
NO3 formation is negligible. A measured volume of hydrocarbon (isoprene/toluene/α-pinene/dodecane) was injected via a syringe into a
glass bulb, which was connected to the Teflon chamber. Heated
5 Lmin-1
of purified air flowed through the glass bulb into the chamber for 20 min,
introducing 25–200 ppb of hydrocarbon into the chamber. After
∼60min mixing, photooxidation was initiated by irradiating
the chamber with black lights with output wavelength ranging from 300 to 400 nm.
Over the course of the irradiation period, the maximum particle mass
concentration formed via nucleation ranged from 0.3 to 2 µgm-3,
corresponding to a particle surface area to chamber wall area ratio of
<10-5. Under these conditions, the surface area of particles
present in the chamber is sufficiently low that partitioning of organic
vapors onto particles is negligible. After ∼1–7 h of
reaction, UV lights were turned off and the decay of oxidation products due
to wall deposition was monitored for ∼13–16 h at 25 ∘C.
The chamber temperature was then ramped up to 45 ∘C during the remaining ∼4–6 h of the experiment with other
conditions held constant.
Comparison of vapor–wall interactions for α-pinene + OH
products under controlled experimental conditions in used (triangle)
vs. unused (circle) Teflon chambers. 30-min averaged data are shown
here for clarity. Colored bands denote successive experimental periods:
vapor generation (magenta), vapor wall deposition at 298 K (green),
temperature ramp (yellow), and vapor re-evaporation at 318 K (red).
Gas-phase organic compounds were monitored using a custom-modified Varian
1200 triple-quadrupole CIMS (Crounse et al., 2006; Paulot et al.,
2009). In negative-mode operation, CF3O- was used as the
reagent ion to cluster with analytes [R] with strong fluorine affinity such
as hydroperoxide, producing [R⚫CF3O]- or m/z=[M+85]-, where M is the molecular weight of the analyte. For more
strongly acidic species [X], the transfer product, [X[H]⚫HF]- or m/z=[M+19]-, is formed during ionization. Carboxylic
acids tend to have contributions to both the transfer and cluster products,
in which case the trace with higher signal-to-noise ratio is considered.
Prior to each experiment, the purified air in the chamber was sampled, and
this is subtracted off as the CIMS background signal. The background signal
is fairly consistent between the masses and over time. However, this
background subtraction does not guarantee that the background for every m/z
signal is absolutely zero, as noted in Fig. 3 that the CIMS background for
certain ions is hovering around zero. Identification of products by CIMS
from the photooxidation of isoprene, α-pinene, and dodecane in our
laboratory has been previously reported (Paulot et al., 2009; Eddingsaas et
al., 2012; Yee et al., 2012; Zhang et al., 2014b).
Absorbing organic mass on the chamber wall (Cw)
Figure 3 shows the continuous dark decay of the 25 organic vapors generated
from the photooxidation of isoprene, toluene, α-pinene, and dodecane
under high/low-NO conditions. In contrast to the behavior in Fig. 3,
Matsunaga and Ziemann (2010) and Yeh and Ziemann (2014) observed rapid
equilibrium established within less than an hour for vapor wall losses of
n-alkanes, 1-alkenes, 2-alcohols, 2-ketones, monoacids, and 1,2-diols in both
1.7 and 5.9 m3 Teflon chambers. The organic vapor generation
period in the present study ranges from 1 to 7 h, thus precluding the
possibility of observing more rapid partitioning that may have occurred. In
view of this, we carried out one vapor wall deposition experiment in the
α-pinene+OH low-NO system, with the experimental procedures
identical to those in Sect. 3, but with lights on for only 10 min. We also
increased the initial mixing ratios of α-pinene and OH radical
precursor H2O2 to 1 and 4 ppm, respectively, in order to
generate sufficient organic vapor CIMS signals during the short irradiation
period. Prompt formation of two ions, m/z 269 (–) and m/z 285 (–), was
observed on the CIMS after 10 min of photochemistry. These are assigned to
be two first-generation products, pinonic acid (C10H16O3) and
pinonic peroxy acid (C10H16O4), respectively (see Table 2 for
the proposed chemical structures). Owing to the short photochemical reaction
timescale, the other four possible products in Table 2 were not found in
this experiment. Figure 3 (bottom panel) shows the wall induced dark decay
of m/z 269 (–) and m/z 285 (–) at 298 K. The best-fit first-order decay
rates lie within the same order of magnitude as those reported in Table 2,
i.e., 7.61×10-6s-1 vs. 8.95×10-6s-1
for m/z 269 (–) and 1.67×10-6s-1 vs. 2.98×10-6s-1 for m/z 285 (–). No rapid vapor wall loss was
found immediately after lights off, and the deposition rates for both ions
were pretty consistent over the course of ∼15h dark decay.
Note that m/z 285 (–), although having a higher molecular weight, decays
more slowly than m/z 269 (–). We will demonstrate later that the
wall-induced decay rate depends inversely on the vapor pressure, which is a
function of the molecule size and functionalities. The addition of a
carboxylic acid group, as in m/z 269 (–), leads to a greater decrease in
volatility than that resulting from the addition of a peroxy carboxylic acid
group, as in m/z 285 (–). Our observations for these two compounds are
consistent with the observed behavior of the other 23 compounds. There are
three considerations regarding equipment setup and experimental protocol
that potentially contribute to the differences between the present study and
Ziemann and co-worker's work: (1) chamber size and depletion rate; (2) mixing
status, i.e., actively mixed vs. static; and (3) definition of the starting
point of the gas-phase vapor concentration.
Best-fit values of vapor–wall accommodation coefficient (αw,i) and calculated equivalent absorbing organic mass (Cw) on the
chamber wall for vapors with structure proposed based on the CIMS
measurement.
a Vapor pressures are estimated from the average of predictions from
the two group contribution methods, “SIMPOL.1” (Pankow and Asher, 2008) and
“EVAPORATION” (Compernolle et al., 2011).b The vapor wall deposition rate (kw,i) is calculated by Eq. (22b).c The accommodation coefficient (αw,i) is calculated via
optimal fitting of Eq. (22b) to the CIMS measured vapor decay rate assuming
first-order kinetics and irreversible gas–wall partitioning.dCw is calculated from the combination of Eqs. (16) and (17) as an
equation set.
When the chamber temperature was increased from 25 to 45 ∘C,
with all the other experimental conditions held constant, the
concentrations of most compounds in the chamber increased to a minor degree
relative to the initial peak signal, reflecting modest desorption of vapors
from the chamber wall. As noted earlier, the chamber wall (in the used
chambers) might actually be coated with organic materials from previous
experiments, or the FEP Teflon film itself may act as an absorbing medium.
In view of the uncertain nature of the wall itself, two control experiments
were also conducted in the unused dual 24 m3 FEP Teflon chambers with
identical protocols: see Table 1. Organic vapor deposition and evaporation
rates between unused and used chambers are compared in Fig. 4. For all the
α-pinene photooxidation products studied here, their interaction
with the wall in the unused chambers is in general agreement with that in
the used chambers, except for a few oxidation products generated under
high-NO conditions. The fact that these particular compounds exhibit
slightly higher deposition rates in used chambers might be due to the
heterogeneous chemistry on the wall catalyzed by nitric acid, a product from
the NOx-O3 photochemical cycle. Overall, we conclude that the
extent to which chambers have been previously used is not a significant
factor in the sorption behavior of the FEP Teflon films.
Inferred total amount of (a) equivalent absorbing organic mass on
the chamber wall, Cw (gm-3), and (b) dimensionless Henry's law
constants, Hi, as a function of saturation concentration,
Ci∗ (µgm-3). Estimated vapor pressures of organic
compounds studied here are obtained from the average of predictions from the
two group contribution methods, “SIMPOL.1” (Pankow and Asher, 2008) and
“EVAPORATION” (Compernolle et al., 2011). The uncertainty bars give the
upper and lower limits of Cw values derived from Eq. (12), together with
Eqs. (16) and (17), when either “EVAPORATION” or “SIMPOL.1” is used to estimate
vapor pressures.
The equivalent absorbing organic mass parameter (Cw/gm-3) is
estimated using equilibrium partitioning theory. We show in the
Supplementary Materials that this theory is suitable for Cw estimation
after ∼18h of wall-induced vapor decay. The ratio of the
concentration of vapor i in the wall phase (C¯w,i) to that
in the gas phase (C¯v,i) is expressed as a function of
the corresponding gas–wall partitioning coefficient (Kw,i) and the total
amount of equivalent absorbing organic mass on the chamber wall (Cw).
Ideally, Cw can be obtained if the initial total concentration
(C¯tot,i) and equilibrium gas-phase concentration
(C¯v,i) of vapor i can be measured by CIMS. However, since
the fraction of organic compound i in the chamber wall at the onset of vapor wall deposition is unknown, we estimate Cw via the combination of
equilibrium partitioning expressions at two different temperatures, e.g.,
298 and 318 K:
C¯w,i@298KC¯v,i@298K=C¯tot,i-C¯v,i@298KC¯v,i@298K=Kw,i@298KCw,C¯w,i@318KC¯v,i@318K=C¯tot,i-C¯v,i@318KC¯v,i@318K=Kw,i@318KCw,
where C¯tot,i is the total initial concentration of vapor
i, C¯v,i@298/318K is the gas-phase concentration (as
indicated by the normalized CIMS signal with unit “a.u.”) of vapor i at
298/318 K, and Kw,i@T is the corresponding partitioning
coefficient at temperature T, see Eq. (12). In this manner, both
C¯tot,i and Cw can be calculated by solving the
equation set (16) and (17). Note that the product Kw,i@TCw is dimensionless, so that the normalized CIMS signal can
be directly substituted into Eqs. (16) and (17) as the actual gas-phase
concentration of organic vapor i. In the calculation,
C¯v,i@298K and C¯v,i@318K were
obtained by taking a 30 min average of the first-order extrapolation of the
normalized CIMS signals at 298 and 318 K, respectively, during the
temperature ramping period. The estimated Cw values vary by
approximately 5 orders of magnitude and exhibit a strong dependence on
the volatility of the organics, as shown in Table 2 and Fig. 5a. We will
address subsequently why the Cw values span such a wide range.
Vapor sorption into FEP Teflon films
It is instructive to consider possible mechanisms of organic vapor
interactions with Teflon films. Dual sorption mechanisms in glassy polymers
have been identified: ordinary dissolution and microvoid-filling (Meares,
1954; Paul, 1979; Paterson et al., 1999; Tsujita, 2003;
Kanehashi and Nagai, 2005). From the point of view of solubility
behavior, organic polymers such as FEP Teflon may be idealized as high
molecular weight organic liquids (Vieth et al., 1966). The
polymer rubbery state is hypothesized to represent a thermodynamic
equilibrium liquid state within which gas solubility obeys Henry's law. The
glassy state, on the other hand, is considered to comprise two components: a
hypothetical liquid state and a solid state, the latter containing a
distribution of microvoids/holes that act to immobilize a portion of the
penetrant molecules when the polymer is below its glass transition
temperature (Tg=339K for FEP, Kim and Smith, 1990). The
overall solubility of a gas molecule in a glassy polymer has been expressed
by (Barrer et al., 1958; Michaels et al., 1963; Vieth
et al., 1966; Kanehashi and Nagai, 2005):
C=CH+CL=kHp+CL′bp1+bp,
where C is the total
vapor concentration in the glassy polymer, CH is the
concentration based on Henry's law dissolution, CL is the
concentration based on Langmuir sorption, kH is the Henry's law
constant, p is the partial pressure in the gas phase,
CL′ is the hole saturation constant, and b is
the hole affinity constant. If bp≪1, Eq. (18) reduces to
C=(kH+CL′b)p.
The
condition of bp≪1 holds in the present situation because
the partial pressures of organic vapors generated in the chamber are
<10-7atm, and the derived hole affinity constants for small
organic molecules are <1atm-1 in glassy polymers (Vieth et
al., 1966; Sada et al., 1988; Kanehashi and Nagai, 2005). If Eq. (18)
holds for the equilibrium sorption behavior of organic vapors by FEP films,
then the dimensionless form of the effective Henry's law constant
(Hi) can be expressed as a function of the partitioning coefficient of
vapor i (Kw,i) and total absorbing organic mass on the chamber wall
(Cw):
Hi=C¯w,iC¯v,i=Kw,iCw∝(kH+CL′b).
As shown in Fig. 5b, the derived Henry's law constants (Hi) for the organic oxidation products
span approximately 2 orders of magnitude and depend inversely on
saturation concentrations (Ci∗/µgm-3).
This behavior suggests that organic vapor solubility in FEP films
increases with decreasing volatility, i.e., increasing carbon number and
functionalization. This behavior provides a qualitative explanation for the
wide range of Cw values calculated for the 25 organic vapors studied
here. Although the solubility of low volatility vapors in the FEP Teflon
film is relatively high (large Hi), the total equivalent absorbing organic
mass on the wall required for gas–wall partitioning can still be low (small
Cw) because low volatility compounds tend to partition preferentially in
the wall phase (large Kw,i). As illustrated in Fig. 5b, the
dimensionless Henry's law constant of m/z=(-)303, a product from α-pinene low-NO photochemistry, is ∼20 times larger than that
of m/z=(-)185, a product from isoprene +OH under high-NO conditions.
The vapor pressure of m/z=(-)303, however, is ∼ 6 orders
of magnitude lower than that of m/z=(-)185. As a result, the Cw
value for m/z=(-)303 is ∼ 5 orders of magnitude smaller
than that for m/z=(-)185. One infers that the equivalent absorbing
organic mass on the chamber wall derived earlier is not constant but
specific to individual organic compounds, i.e., a function of volatility and
solubility in FEP Teflon polymer. We will show that Cw is not the most
dominant parameter, so the assumption of a single value for Cw, does not
invalidate the usefulness of the theory.
Accommodation coefficient on the chamber wall (αw,i)
One key parameter that emerges from the theory of vapor wall deposition, the
total equivalent absorbing organic mass (Cw), can be calculated based on
equilibrium gas–wall partitioning at two different temperatures. From this
information, we can estimate the other key parameter, the accommodation
coefficient (αw,i), by optimal fitting of the solution of Eq. (14) to CIMS measured organic vapor decay at 298 K:
dC¯v,idt=AVαw,iv¯i/4παw,iv¯i/8(DiKe)1/2+1⋅C¯tot,i-C¯v,iKw,iCw-C¯v,i.
Note that Eq. (21) is simply Eq. (14) in which C¯w,i has
been replaced with (C¯tot,i-C¯v,i). Thus, Eq. (21) constitutes a linear ODE
system with the one unknown (estimable) parameter, αw,i. The
Levenberg–Marquardt method implemented in MATLAB's “System Identification
Toolbox” was used for the nonlinear minimization at each time step of its
solution. The best-fit αw,i value obtained was
then substituted into Eq. (21) to give the simulated temporal profile of the
organic vapor i. Simulation results (SIM.1) are shown in Fig. 3.
The other limit of wall behavior is that of irreversible gas–wall
partitioning (Cw→∞). In this case, the accommodation
coefficient αw,i is the sole governing parameter and Eq. (14)
can be simplified as
dC¯v,idt=-AVαw,iv¯i/4παw,iv¯i/8(DiKe)1/2+1C¯v,i.
The overall wall loss rate of organic vapor i (kw,i) is
therefore
kw,i=AVαw,iv¯i/4παw,iv¯i/8(DiKe)1/2+1.
Results for irreversible gas–wall partitioning (SIM.2) are shown in Fig.. 3.
Simulations using both reversible (SIM.1) and irreversible (SIM.2) vapor wall deposition expressions match the experimental data. Outputs from SIM.1
tend to level off, whereas those from SIM.2 exhibit a continuous decreasing
trend at the end of ∼18h of vapor decay. The extent of
agreement between observations and simulations depends on the nature of
vapor wall deposition: most organic vapors in the Caltech Teflon chambers
exhibit a continuous decay. The agreement between SIM.1 and SIM.2 indicates
that the estimated Cw values are sufficiently large so that the
wall-induced vapor deposition in the Caltech chamber can be treated as an
irreversible process (Cw→∞) within a relatively long
timescale (<18h).
Inferred accommodation coefficients of organic oxidation products
on the chamber wall (log10(αw,i)) as a function of
saturation concentrations (log10(Ci∗)) and average carbon
oxidation state (OSC). Colored filled circles represent the best-fit
αw,i assuming irreversible gas–wall partitioning. The black
solid line represents the linear regression of log10(αw,i)
vs. log10(Ci∗) for all compounds.
Overall, results from the two simulations indicate that αw,i is
the more influential parameter than Cw in describing the wall-induced
deposition of semi-volatile organic vapors. The significance of αw,i is 2-fold: first, the accommodation coefficient for the
desorption of organic molecules from the gas–wall interface equals that for
the adsorption/uptake process, which together influence the time needed to
establish equilibrium; and second, diffusion in the chamber wall is not
considered in the theoretical framework; consequently, the best-fit
αw,i will reflect the mass transfer resistance in both the
gas–wall interface and the chamber wall layer. We suggest that the vapor wall deposition of individual compounds can be adequately parameterized
through the accommodation coefficient αw,i as the single
dominant variable. As shown in Table 2 and Fig. 6, for the compounds studied
here, estimated values of αw,i span approximately 2 orders of
magnitude (10-8–10-6) and depend inversely on volatility,
implying that more highly functionalized compounds dissolve more easily in
FEP Teflon film. The correlation of αw,i with the average
carbon oxidation state (OSC), however, is not strong due to the fact
that vapor pressures of molecules, although highly oxidized, are not
necessarily low.
Characterizing chamber vapor wall deposition rate
The wall-induced deposition of the 25 organic compounds investigated in the
present study can be sufficiently represented by a single parameter, the
wall accommodation coefficient (αw,i), which is observed to
exhibit a strong inverse dependence on Ci∗
(Fig. 6). It is possible to formulate an empirical expression for αw,i as a function of Ci∗, a parameter
that can be estimated by vapor pressure prediction models.
Linear regression was performed on log10αw,i vs. log10Ci∗ for the 25 organic vapors
studied:
log10αw,i=-0.1919×log10Ci∗-6.32.
We employ a group-contribution expression for
log10Ci∗ as a function of carbon number
(nCi) and oxygen number
(nOi) developed by Donahue et al. (2011):
log10Ci∗=nC0-nCibC-nOibO-2nCinOinCi+nOibCO,
where nC0 is the carbon number of 1 µgm-3
alkane (nC0=28.0483), bC is the
carbon–carbon interaction term (bC=0.4015), bO is the
oxygen–oxygen interaction term (bO=2.3335), and bCO is the
carbon–oxygen nonideality term (bCO=-0.4709). Best-fit values of
nC0, bC, bO, and bCO are obtained by
optimal fitting Eq. (24) to the saturation concentrations of 110 species,
including C5-C14n-alkanes, C5-C14 carbonyls,
C5-C14 di-carbonyls, C5-C14 alcohols, C5-C14
diols, C5-C14 carboxylic acids, C5-C14 di-carboxylic
acids, C5-C14 peroxides, C5-C14 di-peroxides,
C5-C14 nitrates, and C5-C14 di-nitrates. Vapor pressures
of these species are estimated by taking the average of predictions from the
two group contribution methods, “SIMPOL.1” and “EVAPORATION”.
Combining Eqs. (22), (23), and (24), the vapor wall deposition rate of any
intermediate/semi/low-volatility compound (kw,i/s-1) can be
ultimately related to its carbon and oxygen numbers. This vapor wall loss rate estimation approach, although simplified, proves to be quite useful
considering the limited knowledge of the chemical structures of the
thousands of ions detected by mass spectrometry during an experiment. The
proper guess of a molecular formula would be able to constrain the
wall-induced decay rate of each ion, and thus provide information to better
understand its formation and removal dynamics. In this way, one can
reasonably constrain the wall-induced organic vapor deposition rate based on
only two measurable or predictable properties, volatility and the extent of
oxygenation.
Predicted vapor wall deposition rate (kw,i/s-1) of
organic compounds in a Teflon chamber as a function of carbon number
(nC) and oxygen number (nO).
As shown in Fig. 7, within a certain volatility range, kw,i
increases with decreasing Ci∗, implying that
highly functionalized compounds tend to deposit on the chamber wall more
efficiently. The maximum value of vapor wall deposition rate is eventually
approached for highly oxygenated and extremely low-volatility compounds
(which, of course, are precisely those compounds that are most prone to form
SOA). Revisiting Eq. (22) reveals that the deposition rate of organic vapors
is limited either by gas phase transport (molecular diffusion and turbulent
mixing) or wall surface accommodation. For extremely small αw,i (large Ci∗),
kw,i becomes
kw,i=AVαw,iv¯i4.
In this case, the organic vapor wall deposition rate is governed by the
chamber wall accommodation process. On the other hand, if αw,i is sufficiently large (small Ci∗), kw,i is approximately given by
kw,i=π2AV(DiKe)1/2.
In this case, the vapor wall deposition rate is ultimately controlled by the
mixing state in the chamber. Equation (26) provides an expression for the upper
limit of vapor wall deposition rate in a chamber, which is a manifestation
of the extent of turbulent mixing in the chamber. One can determine which
process is the limiting step in governing the overall wall deposition rate
by referring to Eqs. (25) and (26). The threshold value of αw,i, at which gas phase transport (molecular diffusion and
turbulence mixing) and wall surface accommodation contribute equally to the
vapor wall deposition rate, is 6.8×10-6 in the Caltech
chamber.
Comparison of estimated gas-particle equilibration timescale
(τg/p,i) as a function of the gas-particle mass accommodation
coefficient (αp,i, lower x axis) and the ratio of total
particle surface area to the chamber wall area (Ra, color bar), and
vapor wall deposition timescale (τg/w,i) as a function of
gas–wall mass accommodation coefficient (αw,i, upper x axis).
The red solid line represents the gas-particle equilibration time for a
typical chamber experiment with seed surface area of ∼1×10-3µm2cm-3. White solid and dashed lines
define the region where τg/p,i≅τg/w,i. For
example, the top dashed white line is a collection of data points for which the
equality τg/p,i=τg/w,i=1.3×103min
holds. τg/w,i is calculated by substituting αw,i=10-7 into Eqs. (22), (23), and (24). τg/p,i is calculated
from Eq. (27) by varying αp,i (10-4–10-3) and
Ra (0.01–0.02).
Impact of vapor wall deposition on SOA yields
The extent to which vapor wall deposition impacts measured SOA yields
depends on the competition between uptake of organic vapors by suspended
particles and the chamber wall. The timescale (τg/p,i)
associated with establishing equilibrium gas-particle partitioning is
governed by three transport processes: diffusion of vapor molecules from the
bulk gas phase to the surface of the particle, uptake of vapor molecules by
the particle surface, and diffusion of molecules in the bulk particle phase.
Depending on a given situation, any of these three transport processes can
be the limiting step in determining the overall equilibrium partitioning
timescale. Here we represent the diffusional transport processes across the
gas-particle interface and in the particle phase itself by a single
parameter, the accommodation coefficient of organic vapors on the particle
(αp,i). In doing so, the mass transfer
resistances at the gas-particle interface and in the particle phase are
reflected by the single parameter αp,i, and the
timescale to achieve gas-particle partitioning following a small
perturbation of the condensing species in the gas phase is given by
(Seinfeld and Pandis, 2006):
τg/p,i=(2πNpD¯pDif(Kn,αp,i))-1,
where Np is the total number concentration of suspended particles,
D¯p is the number mean particle diameter, Kn(=2λ/Dp) is the Knudsen number, and f(Kn,αp,i) is the correction factor for
noncontinuum diffusion and imperfect accommodation (Seinfeld and Pandis,
2006).
Figure 8 shows the predicted τg/p,i as a function of: (1) the
ratio of total particle surface area to chamber wall area (Ra) and (2) αp,i. The red solid line represents τg/p,i for a typical chamber experiment with seed surface area
of ∼1000µm2cm-3. In this case, equilibrium
vapor–particle partitioning is established within a few minutes in the
presence of perfect accommodation of organic vapors onto particles
(αp,i=1) or when a sufficiently large
concentration of suspended particles is present (e.g., COA>105µgm-3 when αp,i<10-4).
By analogy with the treatment of gas-particle partitioning, the time scale
associated with vapor–wall interactions is presumably governed by gas-phase
diffusion of vapor molecules to the wall through a boundary layer adjacent
to the wall, uptake of vapor molecules at the wall surface, and,
potentially, diffusion of molecules in the wall. Again, a single parameter,
the accommodation coefficient on the wall (αw,i), is employed to represent the latter two processes. Thus,
the vapor wall deposition timescale is given by
τg/w,i=kw,i-1.
The white solid line in Fig. 8 represents the predicted τg/w,i, covering a range of several minutes to several hours, as
a function of the vapor accommodation coefficient on the chamber wall
(αw,i). The region to the left of the white
solid line is that in which τg/w,i and τg/p,i are competitive. For low αw,i (e.g., <10-8), τg/w,i is
comparable to τg/p,i only if the vapor has a low
accommodation coefficient on the particles (αp,i<10-4) or if a relatively small
concentration of particles is present in the chamber (Ra<10-4). For αw,i>10-4,
τg/w,i is estimated to be of the order of several minutes
and, as a result, vapor transport to particles is suppressed by competition
with the chamber wall, even with perfect particle accommodation
(αp,i=1) or high particle concentrations
(Ra>10-2).
Overall, in the region (confined by the white solid and dash lines in Fig. 8)
where gas–wall partitioning is competitive with gas-particle
partitioning, it is necessary to account for vapor wall deposition when
deriving SOA yields from chamber experiments. The theoretical framework
developed in this study suggests that the area of this region is ultimately
controlled by the accommodation coefficient of organic vapors on particles
(αp,i) vs. the chamber wall (αw,i).
Conclusions
The wall-induced decay of organic vapors is the result of coupled physical
processes involving transport of organic vapors from the well-mixed core of
a chamber to its wall by molecular and turbulent diffusion, uptake of
organic molecules by the Teflon film, and re-evaporation from the wall. The
wall-induced dark decay of 25 intermediate/semi-volatility organic compounds
generated from the photochemistry of four parent hydrocarbons was monitored
in the Caltech dual 24 m3 FEP Teflon chambers. The extent to which
organic vapors and the chamber wall interact was found to be similar in used
vs. unused Teflon chambers. Based on this observation, one concludes that
the Teflon film itself acts as an effective sorption medium, and organic
materials deposited from past chamber experiments, if they indeed exist, do
not significantly impact the sorption behavior of organic molecules.
Reversibility in gas–wall partitioning was observed: evaporation of all 25
compounds that had deposited on the wall during an 18 h deposition period
occurred when the chamber temperature was increased from 25 to
45 ∘C.
Based on a derived model that describes the dynamics of vapor deposition on
the chamber wall, a single parameter, the accommodation coefficient
(αw,i), emerges to govern the extent of the
vapor–wall mass transfer process. Moreover, αw,i exhibits a strong dependence on the molecular properties,
such as vapor pressure and oxidation state, of the 25 organics studied. We
present an empirical expression for αw,i as a
function of the compound vapor pressure, thus affording the possibility to
predict the wall deposition rate of intermediate/semi/non-volatility
compounds in a Teflon chamber based on their molecular constituency.
Previous studies have observed the chemical transformation of δ-hydroxycarbonyls to substituted dihydrofurans on the chamber wall (Lim and
Ziemann, 2005, 2009; Zhang et al., 2014b), suggesting the
potential occurrence of heterogeneous reactions on the chamber wall surface.
While the extent to which heterogeneous transformations proceed can be
potentially represented through the accommodation coefficient, the
occurrence of wall-induced chemistry adds another dimension of complexity in
predicting vapor wall deposition rates.
Quantifying the impact of vapor wall deposition on the chamber-derived SOA
yield is the next step in assessing the effect of vapor wall deposition of
SOA formation and evolution. Future studies will be directed at (1) experiments
to determine the accommodation coefficients of organic vapors on
particles for a variety of SOA systems, and (2) state-of-art SOA predictive
models that describe the dynamics of vapor–wall and vapor–particle
interactions to estimate the fraction of organic vapor fluxes transported to
the suspended particles vs. the chamber wall.
A (m2):Total surface area of the chamber wallαp,i (dimensionless):Accommodation coefficient of organic vapor i on particlesαw,i (dimensionless):Accommodation coefficient of organic vapor i on the chamber wallC0,i (gm-3):Concentration of organic vapor i over the gas–wall interfaceCi∗ (gm-3):Saturation concentration of organic vapor iC¯tot,i (gm-3):Total concentration of organic vapor i in the chamberC¯v,i (gm-3):Concentration of organic vapor i in the well-mixed core of the chamberCv,i (gm-3):Local concentration of organic vapor i in the boundary layer adjacent to the wallC¯w,i (gm-3):Concentration of organic vapor i that has accumulated on the chamber wallCw (gm-3):Equivalent mass of absorbing organic material on the chamber wallD¯p (m):Number mean particle diameterDe (m2s-1):Eddy diffusivityDi (m2s-1):Molecular diffusivity of organic vapor iδ (m):Thickness of the boundary layer adjacent to the wallHi (dimensionless):Henry's law constant of organic compound iJv,i (gm-2s-1):Vapor flux arriving at the gas–wall interfaceJw,i (gm-2s-1):Vapor flux evaporating from the wallKe (s-1):Eddy diffusion coefficientKw,i (m3g-1):Gas–wall partitioning coefficientkw,depo,i (s-1):Deposition rate coefficient to the wallkw,evap,i (s-1):Evaporation rate coefficient from the wallMw¯ (gmol-1):Average molecular weight of the absorbing organic material on the wallNp (m-3):Total number concentration of suspended particlespL,i0 (atm):Vapor pressure of organic compound i as a liquidγi (dimensionless):Activity coefficient in the wall layer on a mole fraction basisv¯i (ms-1):Mean thermal speedV (m3):Total volume of the chamber
The Supplement related to this article is available online at doi:10.5194/acp-15-4197-2015-supplement.
Acknowledgements
This study was supported by NOAA Climate Program Office AC4 program, award
# NA13OAR4310058 and State of California Air Resources Board agreement
13-321.
Edited by: V. F. McNeill
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