Typical tropospheric temperatures render possible phase
states of amorphous organic aerosol (OA) particles of solid, semisolid, and
liquid. This will affect the multiphase oxidation kinetics involving the
organic condensed-phase and gaseous oxidants and radicals. To quantify this
effect, we determined the reactive uptake coefficients (γ) of
O3, NO3, and OH by substrate films composed of single and binary OA
surrogate species under dry conditions for temperatures from 213 to 313 K. A
temperature-controlled coated-wall flow reactor coupled to a chemical
ionization mass spectrometer was applied to determine γ with
consideration of gas diffusion transport limitation and gas flow entrance
effects, which can impact heterogeneous reaction kinetics. The phase state
of the organic substrates was probed via the poke-flow technique, allowing
the estimation of the substrates' glass transition temperatures. γ
values for O3 and OH uptake to a canola oil substrate, NO3 uptake
to a levoglucosan and a levoglucosan / xylitol substrate, and OH uptake to a
glucose and glucose / 1,2,6-hexanetriol substrate have been determined as a
function of temperature. We observed the greatest changes in γ with
temperature for substrates that experienced the largest changes in viscosity
as a result of a solid-to-liquid phase transition. Organic substrates that
maintain a semisolid or solid phase state and as such a relatively higher
viscosity do not display large variations in heterogeneous reactivity. From
213 to 293 K, γ values of O3 with canola oil, of NO3
with a levoglucosan / xylitol mixture, and of OH with a
glucose / 1,2,6-hexanetriol mixture and canola oil, increase by about a factor
of 34, 3, 2, and 5, respectively, due to a solid-to-liquid phase transition
of the substrate. These results demonstrate that the surface and bulk
lifetime of the OA surrogate species can significantly increase due to the
slowed heterogeneous kinetics when OA species are solid or highly viscous in
the middle and upper troposphere. This experimental study will further our
understanding of the chemical evolution of OA particles with subsequent
important consequences for source apportionment, air quality, and climate.
Introduction
Organic aerosol (OA) particles are ubiquitous and can represent
20 %–90 % of the mass fraction of the submicron aerosol (particles ≤1µm in diameter) in the atmosphere. The significance of OA has
long been established, influencing air quality, human health, cloud
formation processes, and the radiative budget, on a regional and global scale,
and thus climate (Hallquist et al., 2009; Jimenez et al., 2009; Seinfeld
and Pandis, 2016; Stocker et al., 2013; Knopf et al., 2018; Abbatt et al.,
2019; Shiraiwa et al., 2017b; Kanakidou et al., 2005; Pachauri et al.,
2014; Pöschl and Shiraiwa, 2015). Characterizing these impacts crucially
depends on our ability to determine and quantify aerosol sources and
strengths (Bai et al., 2013; Robinson et al., 2006; McFiggans et al.,
2019; Hopke, 2016) and to understand the physical and chemical transformation
of aerosol particles during atmospheric transport by multiphase chemical
processes (George and Abbatt, 2010; Rudich et al., 2007; Laskin et al.,
2015; Springmann et al., 2009; Kaiser et al., 2011; Ervens et al., 2011; Zhou et
al., 2019; Moise et al., 2015). Gas-to-particle, also termed heterogeneous,
reactions can involve organic components in the condensed-phase and
gas-phase oxidants such as O3, NO3, and OH. These reactions,
which can include multiple phases, change the physicochemical properties of
particles including composition, density, and hygroscopicity, thereby
defining their chemical lifetime, optical properties, and ice nucleating
ability (Jimenez et al., 2009; Kroll et al., 2015; Katrib et al.,
2005b; Knopf et al., 2018; Slade et al., 2017; Robinson et al., 2007; Shiraiwa
et al., 2012, 2017a; Moise et al., 2015; Murray et al.,
2010; Wang et al., 2012).
It is now well established that OA can exhibit amorphous phase states
(Mikhailov et al., 2009; Virtanen et al., 2010; Koop et al., 2011; Reid et
al., 2018; Renbaum-Wolff et al., 2013; Kidd et al., 2014). Depending on the
viscosity and microstructure, the amorphous phases can be classified as
glasses, rubbers, gels, or ultra-viscous liquids (Mikhailov et al.,
2009; Riemer et al., 2019). The glass transition temperature,
Tg, is a characteristic parameter to describe the viscosity of
a liquid on the order of 1012 Pa s (Koop et al., 2011; Angell,
1995; Zobrist et al., 2011; Dette and Koop, 2015; Zhang et al., 2019). At this
temperature, the molecular motion of the species is so slow that it can be
considered a solid. The phase state of OA can be modulated by particle
composition and environmental conditions such as relative humidity (RH) and
temperature (Koop et al., 2011; Zobrist et al., 2008; Shiraiwa et al.,
2017a; Petters et al., 2019). Particle viscosity will influence the diffusion
of atmospheric oxidants and other small gas molecules (e.g., O3, OH,
NO3, H2O) entering the organic matrix (Price et al.,
2015; Zobrist et al., 2011; Slade et al., 2017; Davies and Wilson,
2016; Moridnejad and Preston, 2016) as well as the transport and mixing of
the condensed-phase organic species (Rothfuss and Petters, 2017; Marsh et
al., 2018; Lienhard et al., 2015; Abramson et al., 2013; Chenyakin et al.,
2017; Kiland et al., 2019; Shiraiwa and Seinfeld, 2012; Renbaum-Wolff et al.,
2013). Therefore, it can be expected that the particle phase state will
significantly affect the rate of multiphase chemical kinetics (Slade and
Knopf, 2014; Kolesar et al., 2014; Knopf et al., 2005; Katrib et al.,
2005a; Pöschl and Shiraiwa, 2015; Davies and Wilson, 2015; Gaston et al.,
2014; Zhang et al., 2018; Houle et al., 2018b). The heterogeneous reaction
kinetics are often expressed by the reactive uptake coefficient (γ),
which represents the fraction of gas collisions with a substrate surface
that yield uptake or reaction (Pöschl et al., 2007; Schwartz,
1986).
As has been demonstrated for heterogeneous ozonolysis reactions, the
composition of the condensed phase can alter the phase state, thereby
significantly altering the reactive uptake kinetics (Knopf et al.,
2005; Hearn et al., 2005; de Gouw and Lovejoy, 1998; Ziemann, 2005). In
general, the solid condensed phase showed significantly lower
γ values compared to the
ozonolysis of the liquid phase. Recent experimental studies focused on how
relative humidity (RH) influences the reactive uptake of gas oxidants via
its impact on the phase state of the organic species (Berkemeier et al.,
2016; Steimer et al., 2015; Shiraiwa et al., 2011; Slade and Knopf, 2014; Li et
al., 2018; Davies and Wilson, 2015; Hu et al., 2016; Marshall et al.,
2018; Pajunoja et al., 2016). The heterogeneous oxidation of a typical
component of biomass burning aerosol (BBA) particles, levoglucosan (LEV) by
OH yielded γ values in the range from 0.008 to 1 with implications
for the lifetime of LEV ranging from weeks at dry conditions (Slade and
Knopf, 2013, 2014; Kessler et al., 2010; Arangio et al., 2015)
to a couple of days when LEV is in a more liquid-like state in response to
ambient RH (Yang et al., 2013; Bai et al., 2013; Slade and Knopf, 2014).
The general conclusion is that at lower RH, the reactive uptake can be
dominated by surface reactions when the condensed phase is in a solid state,
whereas at higher RH, the condensed phase can be semisolid or liquid and
the oxidation process can commence at the surface and in the bulk. For the
latter scenario, the gaseous oxidants can access greater depths of the
condensed phase, and the oxidized organic species at the surface can be
readily replenished by unreacted molecules from the bulk (Arangio et al.,
2015; Slade and Knopf, 2014). Davies and Wilson (2015) investigated
how the viscosity change in citric acid, due to changes in water content,
governs the reactive uptake of OH radicals. They observed that the depletion
of citric acid and the formation of reaction products are confined near the
aerosol–gas interface on the order of 8 nm at 20 % RH, and the reaction
depth increases to ∼50 nm at 50 % RH. Recently,
Li et al. (2018) observed that the heterogeneous aging of secondary organic aerosol (SOA) by
OH radicals at 89 % RH and 25 % RH resulted in ∼60 %
and 20 % loss of particle mass, respectively. The authors concluded that
this difference in particle mass degradation is attributed to a larger OH
uptake coefficient and/or larger fragmentation probability at higher RH.
The temperature in the troposphere ranges approximately from 200 to 300 K
(Wallace and Hobbs, 2006), with subsequent impact on the OA phase state
(Shiraiwa et al., 2017a; Koop et al., 2011) and multiphase reaction
kinetics. However, only a few laboratory studies investigated the
temperature dependence of heterogeneous oxidation reactions while
considering the phase transition of organic substrates and aerosol particles
(de Gouw and Lovejoy, 1998; Edebeli et al., 2019; Gross et al., 2009; Knopf
et al., 2005; Moise and Rudich, 2002, 2000; Slade et al., 2017; Moise et al., 2002). Moise and Rudich (2002) demonstrated that
the uptake of ozone by oleic and linoleic acid is a strong function of
temperature, where γ decreases by about an order of magnitude with
decreasing temperature when both substrates transformed from a liquid to a
solid state. Similarly, Gross et al. (2009) observed
a decrease of 70 % to 90 % in γ of NO3, as oleic acid,
diethyl sebacate, and conjugated linoleic acid substrates solidified as the
temperature decreased from 302 to 263 K. The authors suggested that the
net liquid phase reaction involves both a surface reaction and a bulk
reaction, whereas solidification of the substrate greatly minimizes the
significance of any bulk reactions. This is in line with results of a
kinetic flux model applied to NO3 exposure studies of solid LEV and
abietic acid substrates, further indicating that under shorter timescales
reactive uptake was dominated by surface reaction, whereas for timescales
greater than ∼100 s, even for a solid organic substrate, bulk
processes can impact the overall reactive uptake kinetics
(Shiraiwa et al., 2012) Very recently, Edebeli et al. (2019) investigated the temperature dependence of bromide oxidation by
ozone in a citric acid / bromide mixture and observed that γ decreases
from 2×10-6 at 289 K to 0.5×10-6 at 245 K.
Their analysis indicated that the humidity-driven acceleration in uptake
reactivity decreased due to the increased viscosities of the citric
acid / bromide mixture and decreased diffusivity of ozone as the temperature
was lowered. The effect of temperature on uptake kinetics via its impact on
the condensed-phase state has also been highlighted in a recent modeling
study (Mu et al., 2018), which showed that low-temperature
conditions can substantially increase the lifetime of polycyclic aromatic hydrocarbons (PAHs) against O3
and enhance their dispersion through both the planetary boundary layer and
the free troposphere. Temperature also impacts the other processes involved
in multiphase kinetics (Schwartz, 1986; Pöschl et al., 2007)
including the collision flux of gas-phase species, the desorption rate, and
surface and bulk reaction rates, both typically expressed by an Arrhenius
factor (Laidler et al., 1940; Pöschl et al., 2007; Baetzold and
Somorjai, 1976). Clearly, the temperature dependency of the reaction
kinetics and the role of the condensed-phase state pose a challenge for
detangling the underlying physicochemical processes governing multiphase
chemical kinetics of OA under typical tropospheric conditions.
To add to our understanding of multiphase chemical kinetics at low
temperatures, we determined the reactive uptake of O3, NO3, and OH
by OA surrogates for temperatures ranging from 213 to 293 K under dry
conditions. We employed a chemical ionization mass spectrometer (CIMS) coupled to a
coated-wall flow-tube reactor. As OA proxies, we applied substrates of
canola oil (CA), levoglucosan (LEV), a levoglucosan / xylitol mixture
(LEV / XYL), glucose (GLU), and a glucose/1,2,6-hexanetriol mixture (GLU / HEX).
CA is a mixture of multiple saturated and unsaturated fatty acids,
dominated by unsaturated oleic and linoleic acid however, and serves as an OA surrogate
for both marine and terrestrial organic aerosol including anthropogenic
emissions like cooking (Kawamura et al., 2003; Schauer et al.,
2002; Limbeck and Puxbaum, 1999; Liu et al., 2017; Rogge et al., 1991). LEV
serves as a surrogate for biomass burning aerosol (BBA) including natural
sources such as forest fires and anthropogenic sources such as heating and
cooking BBA (Schauer et al., 2001; Iinuma et al., 2007). GLU can serve as
a tracer to characterize and apportion primary biogenic organic aerosols
(PBOAs) (Zhu et al., 2015; Samaké et al., 2019). XYL and HEX are used
in compound mixtures to modify the glass transition temperature and thus
condensed-phase viscosity. O3 and OH radicals reflect typical gas
oxidants present in photochemically active regions and during long-range
transport. NO3 radicals reflect an effective oxidant participating in
the nighttime chemistry of typically polluted regions (Finlayson-Pitts
and Pitts, 1999; Brown et al., 2006). Some of the examined substrates
undergo a phase transition from liquid to solid in the probed temperature
regime with subsequent consequences for the heterogeneous uptake kinetics.
The substrate's phase state was qualitatively verified by conducting film
poke-flow experiments. The results of our reactive uptake experiments
emphasize the importance of the temperature-induced phase changes of the OA
surrogates when describing the chemical degradation of OA during atmospheric
transport.
Schematic of the low-temperature coated-wall flow reactor
coupled to the chemical ionization mass spectrometer (CIMS). The oxidant
generation region displays the generation methods for NO3 and OH
radicals.
Experimental methodsApparatus
The experimental system is based on our previous setups (Slade and Knopf,
2013; Knopf et al., 2005, 2011) and includes a
temperature-controlled coated-wall flow reactor and a custom-built CIMS, as shown in Fig. 1. It consists of
three parts: gas-phase oxidant generation, multiphase chemical reaction, and
oxidant detection. The gas-phase oxidant (O3, NO3, or OH) enters
the coated-wall flow reactor via a movable injector, where it can react with
the organic substrate film coating on the inner wall of a rotating tube. The
changes in oxidant concentration are measured by CIMS via soft ionization in
the chemical ionization region.
The uptake measurements are conducted in a temperature range of 213 to 313 K. The axial temperature variation in the flow reactor is smaller than 1 K
for all our experimental conditions, which is determined with a thermocouple
that is fixed at the tip of the injector, as discussed in the Supplement Sect. S1.
The rotating tube fits snugly inside the flow tube that is enclosed by a
cooling jacket for temperature control. The rotating speed of the substrate
tube is set to ∼5–10 rpm to keep the liquid film evenly
distributed on its inner surface. The pressure in flow reactor ranges from 2
to 5 hPa depending on the applied oxidant gases.
For OH radical uptake experiments, an additional rotatory pump is directly
connected to the flow reactor. The pump is used to lower the flow reactor
pressure to < 2.5 hPa, thus minimizing gas transport limitation due
to diffusion (Knopf et al., 2015; Zasypkin et al., 1997) while
simultaneously maintaining a high flow rate. Two sizes of rotating glass
cylinders with diameters of 1.75 and 1.2 cm are used in these uptake
experiments. The latter one is designed specifically for OH heterogeneous
reactions to avoid limitation of gas transport by diffusion. For O3 and
NO3 uptake experiments, gas transport limitations due to diffusion can
be neglected under applied flow conditions because of a slower reactive
uptake.
The pseudo-first-order loss of the oxidant species to the organic substrate
is determined by monitoring the loss of the gaseous oxidants as the injector
is pulled back in 1 or 2 cm increments until reaching 10 cm. The flow rate
of gas oxidants in the movable injector ranges from 2 to 100 cm3 min-1 STP (standard temperature and pressure). The flow rate of the
carrier gas He in the flow reactor ranges from 30 to 900 cm3 min-1 STP. As a result, the residence time varies 2 orders of
magnitude in these experiments, ranging from ∼2 ms for OH
uptake to 100 ms for O3 uptake. The Reynolds number (Re) for all
experiments is below 20, indicating laminar flow conditions. Care has been
taken to ensure a carrier-to-injector gas velocity ratio of > 1.33, in order to avoid the disruption of the gas flow by a fast gas flow
exiting the injector, as pointed out by Davis (2008).
This condition is also a necessity to accurately account for the gas flow
entrance effect, which is important for fast-uptake kinetics, as further
discussed in the Appendix. The γ value at each temperature reported
in this study is derived from at least six reactive uptakes, with a freshly
prepared organic substrate employed for each.
Oxidant formation, detection, and flow conditions
A carbon filter (Supelco Supelcarb HC hydrocarbon trap) and a Drierite cold trap cooled
with liquid nitrogen or an ethanol / dry ice mixture are used to purify
ultra-high-purity (UHP) gases of N2, He, and O2. O3 is
generated by introducing a flow of O2 through a UV source (Jelight,
model no. 600) prior to the injector. The flow rate of the O2/O3
mixture ranged between 2 and 7 cm3 min-1 (STP). A carrier He gas
flow of 30–85 cm3 min-1 (STP) enters the flow reactor, mixes
with the injector O2/O3 gas flow, and results in a laminar flow
with Re < 3. O3 is detected as
O3- after charge transfer reaction with
SF6-.
SF6- is generated by passing a
minor amount of SF6 in N2(g) through a 210Po source. The
O3 concentrations ranged between 1.5×1011 and 2.1×1012 molec. cm-3, reflecting typical atmospheric
concentrations (Wallace and Hobbs, 2006; Finlayson-Pitts and Pitts,
1999).
NO3 is generated by thermal dissociation of N2O5 gas at 433 K
in a glass oven prior to the injector, as previously described (Knopf et al.,
2006, 2011). N2O5 is generated via reaction of
NO2 and O3 and stored as pure N2O5 crystals in a glass
container at 193 K. A gas flow of N2O5 in He of about 12–18 cm3 min-1 STP is further diluted by 100–120 cm3 min-1
(STP) He(g) flow before entering the injector. The flow reactor carrier gas
is composed of He and O2 at ∼500 and ∼100 cm3 min-1 (STP), respectively. This results in laminar flow
conditions (Re < 10). NO3 is detected as
NO3- after chemical ionization by
I-. NO3 concentrations are obtained via
titration of a known amount of NO, resulting in 8×109 to 7×1011 molec. cm-3, representing typical atmospheric
NO3 nighttime concentrations (Finlayson-Pitts and Pitts,
1999; Brown et al., 2006).
OH radicals are produced following previously established methods
(Bertram et al., 2001; Slade and Knopf, 2014) via the reaction of H atoms
with O2. A 2.45 GHz Evenson microwave cavity powered by a microwave
generator is utilized to maintain an H plasma. The H atoms are formed by
flowing a mixture of H2(g) (1–2 cm3 min-1, STP) and He(g)
(40–80 cm3 min-1, STP) through a 1/4 in. o.d. (outer diameter) Pyrex
tube that runs through the center of the microwave cavity. The generated H
atoms then pass through a 1/8 in. o.d. Pyrex tube directed down the center of a
1/4 in. o.d. Pyrex injector where OH production occurs with H radicals reacting
with O2(g) at 2–22 cm3 min-1 (STP). Complete
reaction to OH is ensured by monitoring the formation of
OH- via charge transfer with
SF6- in the CIMS. At colder flow
reactor temperatures, the usable injector length is shortened to avoid
artifacts in OH concentration due to temperature gradients in the injector
tube which can impact OH formation (Atkinson et al.,
2004). A He gas flow of 700–900 cm3 min-1 (STP) enters the
flow reactor and further mixes with the injector flow, yielding a laminar
gas flow with Re < 20. Employed OH concentrations ranged between
5×107 and 1×109 molec. cm-3, which are
higher than typical background OH concentrations of 106 molec. cm-3 (Finlayson-Pitts and Pitts, 1999).
Substrate film preparation and characterization
The film substrate preparation method when using a coated-wall flow reactor
has been discussed in detail in previous studies (Knopf et al.,
2011; Slade and Knopf, 2013). Canola oil is liquid and thus can be directly
applied to the inner surface of the rotating tube. The other organic
substrates, i.e., LEV, LEV / XYL mixture, GLU, and GLU / HEX mixture, are first
dissolved in water, and then 1–2 mL of the aqueous solution is evenly
distributed inside the rotating glass tube. The concentration of the LEV
solution is 5 % (w/w) due to its relatively lower solubility, and all
other solutions are 10 % (w/w). The organic mass ratios of the LEV / XYL and
GLU / HEX films are 1:1 and 4:1, respectively. As outlined further below,
these mass ratios were chosen to set the expected glass transition
temperature within the examined temperature range. During the entire
process, a dry He or N2 purging gas flow is applied to avoid
contamination by room air. The glass tube rotates in the flow reactor
until a smooth coating is established. To dry the substrate, the substrate
is exposed to rough vacuum conditions (1–2 hPa) resulting in
the evaporation of water. Surface solidification of single-component
saccharide films may result in trapping of condensed-phase water and thus
impact the film composition and its viscosity. The remaining water
content for all substrate films after the drying process was determined to
be 13 %–16 % (w/w) by measuring the mass change of the organic substrate
films before and after the drying process with an ultra-microbalance
(Mettler Toledo XP2U), as detailed in Sect. S2. The thickness of the
applied organic film is estimated to be around 50–100 µm. We assume a
smooth surface morphology and use the geometric surface area of the film to
derive γ.
The phase state of the applied organic substrates at different temperatures
can be qualitatively verified by using the poking experiments outlined
below. The “poke-flow” technique has been applied to quantitatively
estimate the viscosity of amorphous organic particles
(Murray et al., 2012) including secondary
organic material (SOM) produced by α-pinene ozonolysis
(Renbaum-Wolff et al., 2013). A similar setup is
used here to qualitatively investigate the phase state of applied amorphous
substrates at different temperatures ranging from 213 to 313 K. A
temperature-controlled cooling stage (Linkam BSC 196) is coupled to an
optical microscope (Olympus BX51) (Fig. 2). Images of the film substrate are
recorded at 230 magnification and the depth of field of the 10X objective is
15.9 µm. The organic film substrate, generated from a volume of 2 µL liquid applied to a glass slide, is first placed into the flow
reactor, undergoing the same preparation method as the substrate films
employed for reactive uptake experiments. This results in a film with a
thickness of about 50–100 µm. The substrate film is then moved into
the poke-flow experiment. In the cooling stage, the organic substrate film
is sealed against room air and resides in an atmosphere of dry N2 at
positive pressure during the entire poking process. After poking with a
needle, the phase states and flow characteristics of the organic substrate
films at different temperatures are determined under the microscope as a
function of time. Images are recorded with the film surface in focus to
monitor the flow of the organic substrate and to probe film viscosity. Under
the applied resolution, all film substrates appeared to be smooth and
transparent. The calibration of the cooling stage was performed via
measurements of the melting points of octane (216.35 K), decane (243.5 K),
and water (273.15 K). Before each experiment, the temperature was verified
by measuring the melting temperature of three 2 µL water droplets
(273.15 K). The difference from the expected melting temperature difference is
always smaller than 1 K. We measured Tg with at least
five independently prepared substrate films.
Schematic of the experimental poke-flow setup. This
includes a temperature-controlled Linkam cooling stage coupled to a
microscope equipped with a digital camera and an image analysis system.
Estimation of the phase state of the film substrate as
shown for a GLU / HEX mixture (mass ratio of 4:1). Deformation and recovery
were monitored for different time periods due to the poking of the substrate
at different temperatures. The microscope images are 200 µm wide.
Chemicals
Listed below are the chemicals we used in our study and corresponding
purities and manufactures. N2 (ultra-high purity, UHP), O2 (UHP), H2 (UHP), and He (UHP) were purchased from Airgas East.
SF6 (99.998 %) was acquired from Praxair. NO2 (99.5 %) was
purchased from Mathenson. 1,6-Anhydro-β-D-glucopyranose (99 %) was
purchased from Acros Organics. Glucose (99 %) and xylitol (99 %) were
acquired from Alfa Aesar. 1,2,6-Hexanetriol (96 %) and iodomethane
(99 %) were purchased from Sigma-Aldrich. Purity of canola oil was not
determined. Millipore water (resistivity > 18.2 MΩ cm)
was applied to prepare aqueous solutions for generation of organic
substrates.
Results and discussionCharacterization of substrate phase state
Images of the film surfaces in Fig. 3 show the flow characteristics and the
morphologies of a GLU / HEX substrate mixture when subject to poking by a
needle. In the experiment at T=293 K (Fig. 3a), the film surface is
deformed after being poked, leaving a black dent. The area around the dent
reflects the smooth morphology of the film substrate before poking it. The
slightly darker shades within the imaged area do not represent any
morphology features but the unfocused scratches from the silver substrate
holder beneath the film. The film flowed at an observable rate to restore a
smooth surface, minimizing the surface energy of the system (Fig. 3a). At
273 K, the same deformed film did not show observable changes within 30 min (Fig. 3b). Only after 8 h can minor changes in film morphology
be identified, clearly demonstrating greater substrate viscosity at this
temperature compared to 293 K. At 253 K, the notch by poking on the surface
maintains its shape for 8 h, indicative of a semisolid phase state
(Fig. 3c). At T=233 K, which is below the predicted Tg=248 K, the surface shattered when poked with a needle (Fig. 3d).
Furthermore, over the experimental observation time of 8 h, no restorative
flow was observed at 233 K. The fragments have clear glass-like cracks, and
no smoothing of the edges was observed over the course of the experiment.
Based on our poke-flow experiments as shown in Figs. S2–S7 in the Supplement, the applied GLU
and LEV substrates remain in a solid or semisolid phase state within the
temperature range of 213 to 313 K. However, the mixtures of GLU / HEX and
LEV / XYL exist as liquids at 293 K and experience a glass phase transition
with decreasing temperature.
Estimated Tg of the applied substrate films. Tgexp is the measured Tg using the poke-flow technique. Tglit is the literature-reported Tg. Tgpred is predicted Tg for mixtures with the Gordon–Taylor equation using Tglit and kGT. kGT is the solute-specific constant in the Gordon–Taylor equation. D is fragility.
a Lienhard et al. (2012). b DeRieux et al. (2018). c Elamin et al. (2012). d Interpolated values based on mass fraction. e Zobrist et al. (2008). f Nakanishi et al. (2011). g Rothfuss (2019). h Tombari and Johari (2015). i Dorfmüller et al. (1979). j Diogo and Ramos (2008).
By monitoring the substrate surface morphology, we can estimate
Tg of the applied substrates as the highest temperature when
shattering occurs. Viscosity greater than 1012 Pa s indicates the
presence of a glassy phase. Thus, we ascribe the observed shattering of the
substrate film a viscosity of 1012 Pa s, although the exact value
cannot be assessed with the poke-flow technique. All images of the poke-flow
experiment are documented in Figs. S2–S7. Table 1 gives
the estimated Tgexp values of applied substrates
compared to literature Tglit and predicted
Tgpred values. Tgpred
for substrate mixtures is derived from the Gordon–Taylor equation
(Gordon and Taylor, 2007) assuming literature Tg
values and no residual water present. The uncertainties in
Tgpred values are derived by Gaussian error
propagation. Except for the GLU substrate film, we achieve agreement or
close agreement with either Tglit or
Tgpred. Since Tg determination
depends on cooling or drying rates, we do not expect agreement with
literature values or predicted values. In addition, if 10 %–16 %
residual water is considered while using the Gordon–Taylor equation,
Tgpred would be roughly 30 K lower than
Tgexp, assuming the residual water is
homogeneously distributed in the film. This discrepancy is very likely due
to our substrate film preparation process where under slow drying
(evaporation) the outermost layers of the film contain less water than the
deeper layers. Thus, the substrate surface represents the
experimental conditions more closely. Furthermore, the microscope focus in our poke-flow
experiments is on the substrate surface, thereby monitoring the substrate
morphology that is governed by the film viscosity most closely representing the
desired conditions.
The Angell plot of viscosity as a function of temperature
for LEV, LEV / XYL, GLU, and GLU / HEX substrates. The lines represent predicted
viscosities as a function of Tgexp and applied D
values (see Table 1). PF-shattered and PF-liquid (blue circles) indicate the
conditions for which the poke-flow (PF) experiment detected a solid (glassy)
and liquid phase state of the substrate, respectively. The shaded areas
represent predicted viscosities by using the VTF equation with modified D
values as indicated in the panels based on the estimated viscosities derived
from the poke-flow experiment. The horizontal black line at η=102 Pa s indicates the threshold of liquid and semisolid phase states.
The temperature dependence of the substrate viscosity can be predicted by
using the modified Vogel–Tammann–Fulcher (VTF) equation (Angell,
1991):
logη=-5+0.434T0DT-T0,
where T0 is the Vogel temperature and T is the ambient temperature. The
fragility parameter, D, is defined in terms of the deviation of the
temperature dependence of the viscosity from the simple Arrhenius behavior.
Assuming T=Tg, η=1012 Pa s, T0 can be
represented by Tg (DeRieux et al., 2018):
T0=39.17TgD+39.17.
Therefore, the temperature dependence of substrate viscosity can be
predicted using our measured Tgexp as illustrated
in Fig. 4. For our calculations, we use literature D values for pure sugars
and a mass-weighted interpolation of D values for the mixtures as shown in
Table 1. In Fig. 4a, LEV maintains a relatively higher viscosity compared to
the LEV / XYL mixture throughout the studied temperature. The poke-flow
experiments indicate the presence of a liquid phase at 293 K and solid phase
at 238 K. In contrast to our poke-flow experiments, Fig. 4a suggests that
the LEV / XYL mixture does not show a liquid phase in the examined temperature
regime. Capturing this phase transition with the modified VTF equation
(Angell, 1991) can only be achieved when using lower D values of
5.7–7.6 (compared to the interpolated D value of the LEV / XYL
mixture), a reasonable assumption when considering the presence of residual
water in the substrate. The addition of water increases the steepness of the
glass transition and decreases D values by potentially reducing the thermal
energy necessary to promote the cooperative chain motions (Angell,
2002; Borde et al., 2002). Furthermore, a study of the trehalose–water system
corroborates that the presence of water lowers D values to 3.32–4.85 as a
lower limit (Elias and Elias, 1999). For GLU and the GLU / HEX
mixture shown in Fig. 4b, adding HEX to GLU greatly lowers the
Tg of the mixture and thus decreases its viscosity. Also, in
this case, the predicted substrate viscosity of GLU / HEX disagrees with our
observations when interpolating the D values of the pure compounds. A D value
between 4.3 and 5.7 better represents the observed phase transition of the
GLU / HEX mixture.
Reactive uptake experiments showing the change in the
normalized gas-phase oxidant signal as the reaction time is changed by
pulling back the injector incrementally. (a) Uptake of O3 by canola
oil (CA); (b) uptake of NO3 by levoglucosan (LEV); (c) uptake of OH
radical by glucose (GLU).
Reactive uptake kinetics
The uptake coefficient is determined experimentally from the loss of the gas-phase oxidant to the organic substrate as the substrate area or
corresponding reaction time is changed. Figure 5 shows three exemplary
uptakes of O3 by the canola oil (CA) film, NO3 by LEV film, and OH
radical by the GLU film, indicating the stepwise irreversible removal of the
gas oxidants. Taking O3 uptake as an example, the injector is pulled
back in 2 cm increments until reaching 10 cm and is then pushed back to its
original position (0 cm), with normalized O3 signals recovering to
unity.
Natural logarithm of the change in gas-phase oxidant
signal as a function of reaction time for (a)O3 uptake by CA, (b)NO3 uptake by the LEV / XYL mixture, and (c) OH uptake by the GLU / HEX mixture.
Open circles, triangles, squares, and diamonds correspond to uptake
measurements at different temperatures. Lines represent the corresponding
linear fits to the data and corresponding slopes. kobs values
are given in the legend.
From these data, the observed first-order wall loss rate,
kobs, is determined as the slope of the change in the natural
logarithm of the oxidant signal as a function of reaction time, i.e., the
residence time of the oxidants in the flow reactor (Knopf et al.,
2011; Slade and Knopf, 2013). Figure 6 shows exemplary kobs
derived for uptakes of O3, NO3, and OH for various temperatures.
kobs is obtained from the slope of the linear fit to the data.
The good linear regression indicates that the reported uncertainties for
γ are likely due to the variability of organic substrates and
uncertainty in the diffusion coefficient. In the case of O3 uptake
(Fig. 6a), we can identify two different regimes of kobs,
where the linear regression with a larger slope at higher temperatures
indicates greater reactivity. As discussed in detail further below, the
large difference in kobs between these two temperature regimes
coincides with the CA substrates being in a liquid and solid phase
state. For the cases of NO3 and OH (Fig. 6b and c), the
gradual change of kobs coincides with a phase transition for
both substrates from a highly viscous liquid to a semisolid and solid
(glassy) phase.
The change in concentration of oxidant X along the flow tube due to reactive
uptake can be expressed using the effective uptake coefficient
γeff,X=DtubeωX×ln[X]g,0[X]g/t=DtubeωX×kobs.
Following the Knopf–Pöschl–Shiraiwa (KPS) method
(Knopf et al., 2015), actual γ can be derived
as
γ=γeff,X1-γeff,X32NShweffKnX,
where Dtube is the diameter of the coated-wall tube, and
ωX (X=O3, NO3 or OH radical) is the mean
molecular velocity of the respective gas-phase oxidant.
NShweff is the effective Sherwood number which
represents an effective dimensionless mass-transfer coefficient to account
for changes in the radial concentration profile of the entry region
(Knopf et al., 2015; Davis, 2008), and KnX is the Knudsen
number that characterizes the flow regime (Wutz, 1989; Fuchs and
Sutugin, 1971). The Sherwood number departs from
NShweff=3.66 under conditions of fast flows and
short tubes. The KPS method is advantageous over the correction approach by
Brown (1978) since it accounts for entrance effects that can result
in the overestimation of reactive uptake coefficients as outlined by
Davis (2008) and discussed in more detail in the
Appendix. The Cooney–Kim–Davis (CKD) method (Cooney et al., 1974) also
accounts for the flow entrance effects as discussed in Murphy and
Fahey (1987). This is crucial for fast uptakes involving the OH radical,
however, less so for the uptakes involving O3 and NO3 radicals.
The uncertainty in derived γ values represents the greater value of
either a 20 % uncertainty in the diffusion coefficient or variation in
measurements expressed as ±1σ.
The gas-phase diffusion coefficient of O3 in He
(DO3-He) is taken as 394 Torr cm2 s-1 at 298 K (Moise and Rudich, 2000). Diffusion coefficients of
NO3 in He and O2 are taken as 345 and 80 Torr cm2 s-1 at
273 K, respectively (Rudich et al., 1996). The method
introduced by Fuller et al. (1966) was utilized for O3 and
NO3 to derive the diffusion coefficients at other temperatures. The
diffusion coefficients of OH in He and O2 gas (DOH-He and
DOH-O2) for different temperatures are
theoretically calculated based on a method reported by Mason and
Monchick (1962). A reference value of DOH-He measured at room
temperature is 662±33 Torr cm2 s-1 (Ivanov et al.,
2007; Liu et al., 2009). To calculate the diffusion coefficient of an oxidant
(DX) in a mixture of He and O2, we applied the following
equation with PHe and PO2
representing pressures derived from experimental flow rates and pressure
measurements (Hanson and Ravishankara, 1991):
DX=PHeDX-He+PO2DX-O2-1.
The pressure in the flow reactor is maintained at less than 2.5 hPa to avoid
transport limitations by diffusion of OH radicals that undergo fast reactive
uptake kinetics. Diffusion limitation can be estimated by using the
additivity formula for kinetic resistances as (Gershenzon et
al., 1995)
61kobs=1kk+1kd7kk=ωX×γR×(2-γ)8kd=3.66DXR2,
where kobs (s-1) is the observed wall loss rate of
heterogeneous uptake, and kk and kd represent
its kinetic and diffusion limits, respectively. R is the radius of the flow
tube. Gershenzon et al. (1995) proposed that kk/kd should be < 3–5 to obtain an accurate γ value. Some of the
highest temperature OH uptake experiments involving liquid canola oil were
conducted under diffusion limitation with kk/kd≅15–30.
However, for all the remaining experiments kk/kd<5 and, thus, are not diffusion
limited.
As outlined in Sect. S4, for semisolid and solid substrate
films, the fraction of the unoxidized reaction sites is always larger than
90 % due to short reaction time and low concentrations of gas oxidants
(Bertram et al., 2001), implying no significant
surface saturation effect on γ. In other words, during the typical
duration of a reactive uptake experiment, the oxidant exposure does not lead
to complete oxidation of the substrate surface.
Figure 7 shows γ of O3 reacting with canola oil as a function
of temperature. The reactive uptake is determined under dry conditions in
the presence of O2. Both Brown and KPS methods were used to derive
γ (Brown, 1978; Knopf et al., 2015). Both methods yield the same
results due to the slow uptake kinetics as further discussed in the
Appendix. Around 90 % of canola oil is made up of oleic acid, linoleic
acid, and α-linolenic acid (Ghazani and Marangoni, 2013),
and it solidifies readily around its melting point (de Gouw and
Lovejoy, 1998). γ decreases sharply by a factor of ∼18 from (5.65±0.89) ×10-4 to (3.23±0.74)
×10-5 as the temperature decreases from 267 to 258 K, a temperature range in which the canola oil experiences a phase
transition from liquid to solid (Fasina et al., 2008). In
this temperature regime, the bulk diffusion coefficients of both ozone and
canola oil are expected to decrease by several orders of magnitude due to
the transition from a viscous liquid to a solid
(Koop et al., 2011). Our O3 reactive uptake
coefficients agree with previous literature data by de Gouw and
Lovejoy (1998) and extend those to lower temperatures. A study by
Berkemeier et al. (2016) showed that the
uptake of O3 by shikimic acid is ∼5×10-6
at low RH conditions and increases by a factor of ∼16 at
higher RH due to a decrease in viscosity as a result of a RH-induced phase
transition, similar to our observations. Our results are consistent with
previous studies (Shiraiwa et al., 2009, 2010; Steimer et
al., 2015), which showed that the O3 uptake is slower and restricted to
near the particle surface if the organic is in a solid phase, while the
uptake is dominated by bulk reaction if the organic is in a liquid phase.
Reactive uptake coefficients of O3 by canola oil as
a function of temperature. Squares and red circles represent γ
values derived using the KPS and Brown methods, respectively. See text for
more details. γlit represents values reported by de Gouw
et al. (1998).
In the low-temperature regime, from 213 to 258 K, γ only increases
by about a factor of 1.7 (Fig. 6). If we consider the gas-phase reaction
kinetics of O3 with an unsaturated bond having an activation energy of
15 kJ mol-1 (Atkinson et al., 2006), an
increase in the reaction rate by a factor of 4.4 would be expected over a
temperature of 45 K. This less-than-expected temperature dependency of
γ may be due to the combined temperature effects on both the
underlying reaction kinetics and the desorption lifetime (Pöschl
et al., 2007). As the temperature decreases and reactivity decreases, the
desorption lifetime increases, potentially resulting in a compensating
effect on the overall uptake kinetics. In the temperature regime above the
phase transition, between 267 and 293 K, γ is significantly larger
than when the canola oil substrate is solid, likely due to greater reaction
rates and greater O3 and condensed-phase diffusion coefficients.
The uptake coefficient of NO3 by levoglucosan (LEV) decreases from
(4.2±0.6) ×10-4 to (2.8±0.3) ×10-4 as the temperature decreases from 293 to 213 K, thus displaying
only a slight change in γ within the experimental temperature range
(Fig. 8a). Based on our poke-flow experiment, LEV substrates maintained a
semisolid or solid phase state for all examined temperatures. The slight
increase in γ with temperature may be due to the combined effects of
changes in substrate viscosity (see Fig. 4), the counteracting effect of the
temperature dependence of the reaction rate
(Atkinson et al., 2006), and the desorption lifetime
(Pöschl et al., 2007). Assessment of these various impacts on the
observed uptake kinetics necessitates an in-depth analysis, e.g., by
application of kinetic flux modeling (Shiraiwa et al., 2012; Arangio et
al., 2015). Note the γ of NO3 by LEV in this
experiment is smaller than the value of (1.3±0.9) ×10-3 determined in our previous study at 298 K, although the data agree
within uncertainties (Knopf et al., 2011). A
possible explanation for the lower γ value in this study is that the
substrates have undergone longer drying to remove residual water. Presence
of water would render the film less viscous, thereby increasing reactivity.
Reactive uptake coefficients, γ, of NO3
reacting with (a) levoglucosan (LEV) and (b)1:1 by mass LEV and xylitol
(XYL) mixture as a function of temperature. The dashed line represents the
glass transition temperature (Tg) measured in the poke-flow
experiment.
Comparison of uptake coefficients of NO3 on liquid and solid substrates of saturated and unsaturated organics.
SurfaceLiquid surface Solid surface T (K)γliquidT (K)γsolidLEV293(4.2±0.6)×10-4,aLEV213(2.8±0.3)×10-4,aLEV / XYL293(8±2)×10-4,a213(3.10±0.9)×10-4,aGlycerol293(1.4±0.3)×10-3,bGlycerol268(8.3±0.5)×10-4,b1-Octanol258(7.1±1.6)×10-3,c248(4.1±1.0)×10-3,cDES298(4.1±0.3)×10-3,b263(2.5±0.2)×10-4,bn-Hexadecane293(2.6±0.8)×10-3,c283–289(3.8±1.0)×10-4,c
a This study. b Gross et al. (2009). c Moise et al. (2002).
For the mixture of LEV / XYL (Fig. 8b), γ at 293 K is 8.5×10-4, about a factor of 2 larger than for pure LEV (see Table 2). As
the temperature decreases, γ reaches a value of 3.1×10-4 at 213 K, similar to the γ value derived for solid LEV
(Fig. 8a). Our poke-flow experiment yielded Tg=238 K for
the applied LEV / XYL film. The largest change of γ is observed above
the expected Tg (Fig. 8b). Therefore, the significant change in
NO3 uptake reactivity in the investigated temperature range can be
attributed to the transition of a solid or highly viscous substrate film to a liquid
substrate film. As such, the underlying reaction mechanism changes; i.e., at
higher temperatures, the kinetic uptake is likely governed by surface and
bulk reactions and at lower temperatures only surface reaction dominates. As
shown in Shiraiwa et al. (2012), for highly viscous and solid LEV
films, the transport of bulk LEV towards the interface is strongly limited,
slowing the reactive uptake kinetics. At higher temperatures, with
decreasing viscosity of LEV, the diffusivity of LEV and NO3 increases,
likely facilitating transport and thus reaction.
Reactive uptake coefficients, γ, of OH radicals
reacting with (a) glucose (GLU), (b)1:4 by mass glucose and
1,2,6-hexanetriol (GLU / HEX) mixture, and (c) canola oil (CA) as a function of
temperature. Tg is the glass transition temperature determined
in the poke-flow experiment.
Gross et al. (2009) reported that γ of
NO3 by liquid glycerol (GLY) is (14±3) ×10-4 at
293 K and decreases to 8.3×10-4 at 268 K, representing a
decrease of 41 % in γ over 25 K. The Tg of glycerol
is 193 K, and the viscosity decreases from 20 Pa s at 268 K to 1.32 Pa s at
293 K (Schröter and Donth, 2000). For the same temperature
difference of 25 K, γ for the viscous liquid LEV / XYL substrate
decreases by about 24 %. The viscosity of LEV / XYL film is around 101
to 102 Pa s based on our poke-flow experiments and viscosity prediction
as outlined in Sect. 3.1. We hypothesize that the lesser change in γ with temperature for the LEV / XYL substrate compared to a GLY substrate
(Gross et al., 2009) may be due to the viscosity difference between both
substrates. Moise et al. (2002) studied the uptake of
NO3 by a saturated alcohol, 1-octanol, in the liquid and solid phases.
They derived γ=7.1×10-3 at 258 K for the liquid
phase of 1-octanol and 4.1×10-3 at 248 K for the solid phase
as shown in Table 2. The change in γ is a factor 1.73 over 10 K,
mainly because of the rapid phase transition of 1-octanol. In our LEV / XYL
uptake experiment, γ changes by a factor 2.74 over a temperature
range of 80 K; however, γ measured here is about 1 order of
magnitude lower. We suggest that the different sensitivity in γ over
these different temperature ranges is due to the continuous amorphous phase
transition of the LEV / XYL mixture compared to the abrupt phase transition of
1-octanol.
At low temperatures, γ for NO3 uptake by LEV and LEV / XYL films
is similar within our experimental uncertainties, as shown in Table 2. This
is in contrast to expectations derived from the gas-phase structure activity
relationships (SARs) (Atkinson, 1987; Kerdouci et al., 2014), which
predict that the reaction NO3+ LEV occurs 6.7 times faster than the
reaction of NO3+ XYL. Also, γ of solid LEV and LEV / XYL is
similar to NO3 uptake by solid alkane substrates and monolayers, e.g.,
n-hexadecane given in Table 2 (Knopf et al., 2006,
2011; Gross and Bertram, 2009; Moise et al., 2002). However, an alcohol is
expected to be more reactive with NO3 compared to an alkane, as the
α-hydrogen atom at the OH group is more labile and thus facilitates
the hydrogen abstraction. These measurements indicate that the presence of
labile α-hydrogen atoms in solid substrate films does not
significantly yield higher γ. A possible explanation may be that the
number density of labile α-hydrogen atoms at the interface is not
sufficiently different between LEV, the LEV / XYL mixture, and alkane
substrate films. Furthermore, considering that NO3 concentrations are
too low to saturate the substrate surface, the probability of collisions
with the most reactive hydrogen may also be low. This may result in similar
γ within our uncertainties, for both substrates. In conclusion, our
study demonstrates a strong positive correlation between temperature,
substrate phase state, and NO3 uptake reactivity for saturated
alcohols.
Temperature-modulated reactive OH uptake
Figure 9a shows γ of OH reacting with GLU as a function of
temperature, indicating no significant change in reactivity over the examined
temperature range. As the temperature increases from 250 K on, γ
appears to slightly decrease from 0.11±0.03 to 0.09±0.03,
although this change is within experimental uncertainties. GLU substrates
maintain a solid or highly viscous phase state for all examined
temperatures. The gas-phase structure activity relationship (SAR) predicts
that the gas-phase reaction rate of OH radicals with GLU
(kGlug) decreases by a factor of 2.5 from 213 to
298 K (Atkinson, 1987; Kwok and Atkinson, 1995). We also
observe a decrease in reactivity (Fig. 9a), however, not as strong as for
gas-phase reactions. This implies that other factors may also play a role in
the observed heterogeneous reactivity. Since different chemical bonds have
different reactivity and temperature dependence (Kwok and
Atkinson, 1995), the reaction probability of OH radicals can be affected by
the molecular orientation at the surface. SAR suggests that tertiary C–H
bonds contribute to this negative temperature dependency of the reactivity.
Hence, if more C–OH bonds are exposed to the surface compared to tertiary
C–H bonds, this negative temperature dependency of reactivity weakens. A
similar steric argument was given by Nah et al. (2014), who suggested that the increased surface density of C=C double
bonds due to the formation of oleic acid dimers possibly leads to a faster
surface reaction involving OH radicals. Also, the remaining amount of water
in these substrates may impact the surface structure and the viscosity of
the near-surface region, thereby potentially counteracting the decreasing
reaction rate with increasing temperature predicted with the SAR method.
For the GLU / HEX mixture (Fig. 9b), γ at 303 K is 0.13±0.03,
∼1.5 times larger than that at 233 K. The larger γ at
higher temperature can be attributed to the viscosity change of the
substrate film with increasing temperature. Tg is estimated as (248±3) K by our poke-flow experiment. Judging by the morphology and the
flow characteristics of the organic film, it experiences a glass phase
transition within the investigated temperature range. The major change in
γ occurs above the expected Tg of the GLU / HEX mixture. γ by GLU / HEX at low temperatures, i.e., between 213 and 233 K, is lower than γ by GLU, and this is likely related to the lower
reaction rate of HEX with OH radicals. For example, SAR predicts the
reaction rate for HEX + OH to be 2.6 times slower than that for GLU + OH
at 213 K.
Figure 9c shows that γ of CA increases by a factor of 5 within the
experimental temperature range, from 0.13±0.08 at 213 K to 0.66±0.24 at 293 K. This increase in γ is particularly
significant above the melting point of canola oil, Tm=258 K
(Fasina et al., 2008). Above the melting point, γ
increases by a factor of ∼2.4 between 273 and 293 K, where
the viscosity of canola oil changes from 0.185 to 0.079 Pa s
(Fasina et al., 2006). Therefore, we attribute this large
change in γ to the phase and viscosity change of the canola oil. We
observed that as the canola oil is in a solid or viscous liquid (grease)
state at temperatures between 213 and 253 K, γ is less sensitive to
temperature changes compared to when the canola oil is in a liquid phase.
γ by CA is larger than γ by GLU at low temperatures, although
both are in the solid phase, thus implying that OH reacts faster with C=C
bonds than with C–H bonds and OH groups present on solid surfaces.
Waring et al. (2011) measured the reactive loss of gas-phase OH
radicals on squalene surfaces at room temperature, reporting γ=0.39±0.07. These measurements suggest that γ by CA can be
larger than γ by squalene, although squalene has more double bonds
compared to canola oil and both organics have similar viscosity (e.g., μSqe is 0.012 Pa s at 298 K;
Comunas et al., 2013). Different
accessibility of C=C bonds on the substrate surfaces could be one reason
for different γ values. X-ray diffraction studies on liquid oleic
acid showed that the unsaturated acid molecules exist primarily as dimers
through hydrogen bonding of the carbonyl oxygen and the acidic hydrogen
(Iwahashi and Kasahara, 2011; Iwahashi et al., 2000), and this possibly
leads to more C=C double bonds at the substrate surface
(Hearn et al., 2005). However, whether or not this
phenomenon can compensate for the more numerous C=C double bonds present
in squalene molecules is uncertain. Lastly, gas-phase reaction kinetics
suggest that unsaturated oxygenated organics (e.g., acids, alcohols,
ketones) are more reactive toward OH radicals than their alkene equivalents
due to the formation of a hydrogen-bonded complex that facilities the
formation of stable reaction products via interactions between the OH
radical and the oxygenated functional group of the molecule (Mellouki et
al., 2003; Orlando et al., 2001).
Atmospheric implications
Temperature varies greatly in the troposphere, in both the latitudinal and
altitudinal directions. The zonal average surface temperature changes
approximately 0.86 K per degree latitude in the Northern Hemisphere
(Gates et al., 1999), and the vertical
temperature gradient is typically defined by the lapse rate of
∼10 K km-1 for dry air and ∼7 K km-1
for wet air (Wallace and Hobbs, 2006). Globally, the temperature will
greatly influence the phase state and the heterogeneous oxidation of OA
particles, especially for SOA particles that can exist in a liquid phase state
in the warmer planetary boundary layer but can be mostly solid in the colder
middle and upper troposphere (Shiraiwa et al., 2017a).
The determined γ values can be used to estimate the oxidation
lifetime for a solid organic surface with the following equation
(Moise and Rudich, 2001):
τ=4NtotγωX[X]g,
where τ is the lifetime of one monolayer of coverage, i.e., how long
it takes for 63 % of surface molecules to be oxidized. Ntot
represents the concentration of reaction sites on the organic substrate
surface. Here, we apply this idealized approach to assess the degree of
surface oxidation of OA particles in a solid or highly viscous phase state
and when the oxidation reactions are confined to the surface. However, given
that the surface-active organics are ubiquitous in tropospheric aerosols and
organic films can exist on aerosol surfaces with potentially significant
effects on atmospheric chemistry and climate, e.g., heterogeneous reactions,
particle hygroscopicity, optical properties, and cloud-forming activity
(Jimenez et al., 2009; Kroll et al., 2015; Katrib et al., 2005b; Knopf et
al., 2007, 2018; Slade et al., 2017; Robinson et al., 2007; Shiraiwa et al.,
2017a; Cosman and Bertram, 2008; McNeill et al., 2006; Knopf
and Forrester, 2011; Moise et al., 2015), this approach can further our
understanding of the chemical evolution of atmospheric OA. We assume
Ntot=1015 cm-2 (Bertram et
al., 2001). The concentration of gas oxidants O3, NO3, and OH is
based on ambient conditions of 1×1012, 1.25×109, and 1×106 molec. cm-3, respectively
(Finlayson-Pitts and Pitts, 1999). Figure 10 shows the effect of
temperature on the surface species' lifetime for the different examined
heterogeneous oxidation reactions. Oxidation of CA by O3 proceeds
within minutes to hours for typical tropospheric temperatures. Thus,
degradation of unsaturated fatty acids is expected to proceed efficiently,
even at colder temperatures. Despite the OH radical being the most effective
oxidizer, Fig. 10 suggests that for middle- and upper-tropospheric
conditions, oxidation of the particle surface by OH can take 1 to 2 weeks,
emphasizing the slow physicochemical changes of the particle properties
during transport at high altitudes. However, closer to the surface,
degradation can proceed almost an order of magnitude faster. Degradation of
the particle surface by NO3 may proceed by about a factor of 2 slower at
the coldest tropospheric temperatures compared to boundary layer conditions.
Clearly, these datasets indicate that the topmost organic layers for most of
the investigated OA surrogates can be oxidized within 1 week for lower- and
middle-tropospheric conditions. However, as soon as OA particles reach
higher altitudes and lower temperatures by, e.g., pyro-convection
(Andreae et al., 2004; Jost et al., 2004; Fromm and Servranckx, 2003),
their atmospheric lifetime increases significantly. For example, aerosol
particles originating from extreme wildfires, like Australia bushfires, can
circumnavigate the globe in weeks and can even reach the stratosphere,
existing for weeks or months (Ribeiro et al., 2020).
Lifetime estimates of one monolayer of examined OA
surrogates for typical background concentrations of O3, NO3, and
OH. See text for more details.
Exposure- and temperature-dependent particle degraded
fraction (DF) for particles 200 nm in diameter for (a)O3 oxidation of
canola oil, (b) OH oxidation of canola oil, (c)NO3 oxidation of the
LEV / XYL mixture, and (d) OH oxidation of the GLU / HEX mixture. Note the different
scales on the time axis.
For OA particles, the lifetime (τp) and particle degraded
fraction (DF) can be estimated by the equations below (George
et al., 2007):
τp=43ρRNaγωX[X]gM,
and
DF=1-exp-tτp,
where R is the radius of the particle, chosen here as 100 nm, Na is
Avogadro's number, M is the molecular weight of the condensed-phase species,
and t is time. We derive DF as a function of temperature and oxidant exposure
time for oxidant concentrations given above. However, we note that this DF
estimate is a simplified approach, since we assume that measured oxidation
kinetics proceed throughout the entire particle with the same rate. This
ignores the slow gas diffusion in the condensed phase and the hindered
internal mixing of organic species, particularly when the OA particle is in
a solid or highly viscous phase state. For OH oxidation, previous studies
indicate that the oxidation reaction is confined to near the surface of a
liquid or solid organic substrate, even for longer OH exposure periods at
lower OH concentrations (Slade and Knopf, 2014; George and Abbatt,
2010; Lee and Wilson, 2016; Shiraiwa et al., 2011). However, whether bulk
processes may significantly change the reactivity under long oxidant
exposure as encountered in the atmosphere still needs to be examined. In the
case of O3 and NO3 oxidation of organic substrates, the results by
Shiraiwa et al. (2011, 2012) indicate that the reactive uptake coefficients decrease with
increasing exposure. Therefore, we probably underestimate the chemical
lifetime. As such, the DF values at lower temperatures likely represent upper
limits. In other words, degradation in ambient particles is expected to be
less. Keeping this limitation in mind, Fig. 11 displays the estimated DF for
the examined oxidant–surrogate systems. As expected, for the lowest
temperatures, the DF values are lowest, implying the longest lifetimes. The
stronger the particle viscosity change with temperature, the greater the
change in DF; e.g., viscosity of CA decreases from 1012 to 10-2 Pa s
over the temperature range from 213 to 293 K. This yields the largest change in DF over this temperature
range compared to the other investigated systems (considering the same
exposure time period). Figure 11c and d display a small DF within 1 month for LEV / XYL and GLU / HEX mixtures in the upper troposphere. This is
consistent with previous aircraft observations reporting a large number of
biomass burning aerosol particles in aged plumes at higher altitudes of the
troposphere (Cubison et al.,
2011). This also supports the hypothesis that aerosol particles originating
from large wildfires (e.g., Australian bushfires) can be transported into
the Northern Hemisphere from the stratosphere within a time period of 1 year (Deshler, 2008; Peterson et al., 2018). The presence of water vapor
will significantly impact the phase state, in particular, of hygroscopic
species such as LEV and GLU (Zobrist et al., 2008; Koop et al.,
2011; Mikhailov et al., 2009), where increasing humidity yields lower
condensed-phase viscosity and, in turn, faster reaction kinetics (Slade
and Knopf, 2014; Slade et al., 2017; Davies and Wilson, 2015). Neglecting this
effect will lead to an underestimation of DF. This discussion neglects the
chemical complexity of ambient OA where different condensed-phase species
can result in different reactivities and reaction pathways (Zhang et al.,
2015; Surratt et al., 2010; Ziemann and Atkinson, 2012; Knopf et al.,
2005; Davies and Wilson, 2015). Those in turn can change the multiphase
kinetics and its dependency on temperature and particle phase state.
Furthermore, heterogeneous particle composition and morphology can result in
matrix effects or liquid–liquid phase separation, where, for example, more reactive
organic species are shielded by less reactive species (Lignell et al.,
2014; Lee and Wilson, 2016; Charnawskas et al., 2017; Bertram et al., 2011).
Those effects were not assessed in this study but necessitate additional
experimental investigations.
Conclusions
In this study, we measured γ for several systems of oxidant and
organic aerosol surrogate combinations including O3 and OH+CA,
NO3+ LEV and LEV / XYL, and OH + GLU and GLU / HEX, under dry conditions
and for temperatures ranging from 213 to 313 K. For the case of OH, this
is the first low-temperature reactive uptake study of which the authors are
aware. The phase states of the organic substrate films were examined using
the poke-flow technique, allowing for an estimation of Tg and
substrate flow characteristics to constrain the magnitude of the substrate
viscosity at different temperatures using the VTF equation.
The strongest changes in heterogeneous reactivity observed for the examined
oxidant–substrate systems correlate with the largest change in organic
substrate viscosity with temperature associated with a solid-to-liquid phase
transition. The largest reactivity occurs between O3 and CA, exhibiting
a change by a factor of 34, due to the phase transition of CA. In general,
we attribute the faster heterogeneous kinetics in the semisolid and liquid
phase states to surface and bulk reactions, the latter enabled by increased
diffusion coefficients of gas and condensed species resulting from lower
viscosity. Furthermore, once in the liquid phase, as temperature increases
viscosity decreases and diffusivity increases, leading to potentially strong
increases in reactivity. LEV and GLU substrates display a semisolid and
solid phase state over the entire probed temperature range, with estimated
viscosities larger than 104 Pa s. As a result, the overall reactivity
is lower compared to liquid substrate films and does not change
significantly with temperature. Although viscosity in the semisolid phase
regime can change substantially with temperature, viscosity can still be too
high to allow for significant bulk processes to play a role. In this case,
surface reactions likely dominate, and replenishment of unoxidized molecules
to the surface is hindered. Application of organic substrate mixtures to
control Tg to induce a solid-to-liquid phase transition as
temperature increases is accompanied by an increase in reactivity.
Our results are consistent with previous studies reporting the significance
of particle phase state for the reactive uptake kinetics (Arangio et al.,
2015; Knopf et al., 2005; Kolesar et al., 2014; Slade and Knopf, 2014; Davies
and Wilson, 2015; Shiraiwa et al., 2012). To resolve the molecular processes
of the measured heterogeneous kinetics, detailed modeling studies
(Pöschl et al., 2007; Shiraiwa et al., 2010; Arangio et al., 2015; Houle
et al., 2015; Houle et al., 2018a; Pöschl and Shiraiwa, 2015) extended to
lower temperatures are needed. A crucial aspect of this study is the
interplay between the temperature dependence of the reaction kinetics and
the desorption lifetime. At lower temperatures when an organic substrate is in
the solid state, over a wide temperature range the reactivity does not
change significantly. Desorption lifetime will likely increase significantly
with decreasing temperature with subsequent effects on the reaction
kinetics. Changes in substrate viscosity with temperature may also play a
role in the overall heterogeneous kinetics when the substrate is in a
semisolid phase state. However, to what extent the particle viscosity will
influence the diffusion and reaction kinetics is still not resolved. The
comprehensive dataset presented here will allow application of a more
detailed kinetic multi-layer model to constrain the temperature dependency
of reaction and transport parameters. This study did not address the role of
water vapor acting as a plasticizer concurrent to phase changes induced by
temperature changes (Zobrist et al., 2008; Koop et al., 2011; Mikhailov et
al., 2009). The role of humidity in amorphous phase state and resulting
multiphase kinetics has been studied at room temperature (Shiraiwa et
al., 2011; Slade and Knopf, 2014; Davies and Wilson, 2015; Li et al., 2018).
Most of these studies suggest that increasing humidity leads to faster
reactive uptake kinetics. However, at lower temperatures, diffusivity is
slower, leading to kinetically hindered adjustments of the condensed-phase
state (Berkemeier et al., 2014; Knopf et al., 2018; Charnawskas et al.,
2017; Wang et al., 2012). Future experimental studies should focus on how the
coupled effects of ambient temperature and humidity on the amorphous phase
state of OA particles modulate the multiphase oxidation kinetics.
Our study demonstrates unambiguously that the chemical reactivity of organic
matter towards atmospheric oxidants can vary significantly in response to
ambient temperature, which, in turn, modulates the organic phase state.
Ambient OA, however, displays greater chemical and morphological complexity
(Laskin et al., 2016, 2019), and as such we expect varying
multiphase reaction pathways having different reactivity towards atmospheric
gas-phase oxidants which will translate into different reactivity
dependencies on temperature and phase state. Despite this caveat, due to
lower temperatures at higher altitudes, we can expect OA particles during
transport in the free troposphere to have significantly longer lifetimes
with respect to chemical degradation. This is important
information for our understanding of the chemical evolution of OA particles
and their impact on source apportionment, air quality, and climate.
The impact of gas flow entrance effects and velocity profiles on
the reactive uptake kinetics
The KPS method (Knopf et al., 2015), similar to the Cooney–Kim–Davis (CKD)
method applied by Murphy and Fahey (1987), accounts for gas flow entrance
effects into the flow reactor, i.e., the establishment of concentration
profiles, that impact the derivation of the reactive uptake coefficient as
pointed out by Davis (2008). When considering the establishment of gas
concentration profiles for correction of observed pseudo-first-order wall
loss rates, slower uptake reaction kinetics (e.g., the uptake of O3)
typically represent conditions of either having a slow gas flow or long flow
tube, and thus the NShweff≈3.66 as
shown in Fig. A1 (Davis, 2008). In this case, the different approaches
(Brown, CKD, and KPS) to correct observed wall loss rate for transport
limitations by diffusion yield the same γ values as illustrated in
Fig. 7 (CKD approach is not shown but yields the same results as KPS). However,
reactive OH uptake exerts faster reaction kinetics and thus resembles
conditions of a fast gas flow or a short flow tube (Fig. A1) where
NShweff can significantly depart from 3.66 (Knopf
et al., 2015; Murphy and Fahey, 1987; Davis, 2008). In this case, γ
values derived by the Brown method always yield larger values compared to
those derived by the KPS method as illustrated in Fig. A2 for the uptake of
OH by glucose. Hence, for fast uptake kinetics, we recommend using either
the KPS or CKD methods to derive accurate uptake kinetics when using a
coated-wall flow reactor.
Dependence of the effective Sherwood number
(NShweff) on the dimensionless axial
distance of the flow reactor. Smaller dimensionless axial distance
represents the scenario of a fast flow or short tube. Adapted from Davies (2008) and Knopf et al. (2015).
Differences in reactive uptake coefficient (γ)
derived from two gas transport correction methods for fast heterogeneous
kinetics involving OH uptake by glucose as a function of flow velocity ratio
between mean gas flow velocity in the annular section of the flow reactor
(umeanann) and the gas flow exiting
the movable injector (umeaninj). Red squares and
blue diamonds represent the KPS and Brown methods, respectively.
The gas flow velocity ratio between mean flow velocities in the flow reactor
(umeanann) and the gas flow exiting the movable
injector (umeaninj) also impacts the derivation of
γ. If
umeanann<umeaninj,
reactive uptake experiments are ambiguous, potentially resulting in an
underestimation of the uptake kinetics, as pointed out by Davis (2008). This
flow condition can lead to a jet-like exit gas flow from the movable
injector. Figure A2 displays this effect when
umeanann/umeaninj < 1, where the measured γ values can deviate by a factor of 3 from the
actual value.
For all experimentally derived γ values, we set
umeanann/umeaninj > 1.27 for the flow tube with 1.758 cm inner diameter and
umeanann/umeaninj > 1.33 for the flow tube with an inner diameter of 1.2 cm, based on the
theoretical calculation below. The velocity profile and volume flow rate Q
of a Poiseuille flow in the annular section can be described by the
equations (Rosenhead, 1988)
ur=G4μR12-r2+R22-R12ln(r/R1)ln(R2/R1)
and
Q=∫R1R22πrurdr=Gπ8μR24-R14-(R22-R12)2ln(R2/R1),
where ur is the flow velocity profile as a function of the
radius r. G is a constant pressure gradient, μ is the dynamic viscosity,
R1 is the inner cylinder radius, R2 is the outer cylinder radius,
and r is a radius value between R1 and R2. If R1, R2, and
constant parameters (G and μ) are known, the maximum flow velocity
umax is obtained by setting dur/dr=0, and the
mean flow velocity in the annular section umean is derived
from Q/Aann, where Aann is the cross-sectional
area of the annular section.
Using the flow tube with a diameter of 1.758 cm as an example, and setting
R1=0.325 and R2=0.879 cm, the ratio of maximum flow
velocity over the average flow velocity in the annular section is
umaxann/umeanannul=1.575. The ratio of the mean flow velocity over the maximum velocity in a
tubular injector is umaxinj/umeaninj=2 as the injector flow is laminar
(Rogers, 1992). Therefore, even when adjusting the gas flows in the
injector and flow reactor to yield the same mean flow velocities, the
difference in the maximum flow velocities can still differ significantly. In
this case, the difference can be up to 27 %, potentially resulting in a
jet-like gas flow profile. For this reason, we set
umeanann/umeaninj > 1.27 to avoid this effect. In the case of using a flow tube
with a diameter of 1.2 cm,
umeanann/umeaninj > 1.33.
Data availability
All data needed to draw the conclusions in the present study are shown in the paper and/or the Supplement. For additional data related to this study, please contact the corresponding author.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-6055-2020-supplement.
Author contributions
JL conducted O3, NO3, and OH uptake experiments. SMF performed
N2O5 synthesis and conducted NO3 uptake experiments. JL
conducted the poke-flow experiments. JL performed all analysis of data. SMF
conducted analysis of NO3 uptake data and contributed to the writing of
the manuscript. JL led the writing of the manuscript. DAK oversaw the
project, envisioned the analysis, and contributed to the writing of the
manuscript.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Support from the National Science Foundation and partial support from the U.S.
Department of Energy, Office of Science (BER), Atmospheric System Research, are acknowledged.
Financial support
This research has been supported by the U.S. National Science Foundation (grant no. AGS-1446286) and the U.S. Department of Energy, Office of Science (BER), Atmospheric System Research (grant no. DE-SC0016370).
Review statement
This paper was edited by Jason Surratt and reviewed by two anonymous referees.
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