Organic compounds residing near the surface of atmospheric aerosol particles
are exposed to chemical reactions initiated by gas-phase oxidants, such as
hydroxyl (OH) radicals. Aqueous droplets composed of inorganic salts and
organic compounds can undergo phase separation into two liquid phases,
depending on aerosol composition and relative humidity (RH). Such phase
behavior can govern the surface characteristics and morphology of the
aerosols, which in turn affect the heterogeneous reactivity of organic
compounds toward gas-phase oxidants. In this work, we used an aerosol flow
tube reactor coupled with an atmospheric pressure ionization source (direct
analysis in real time) and a high-resolution mass spectrometer to
investigate how phase separation in model aqueous droplets containing an
inorganic salt (ammonium sulfate, AS) and an organic acid (3-methylglutaric
acid, 3-MGA) with an organic-to-inorganic dry mass ratio (OIR) of 1 alters
the heterogeneous OH reactivity. At high RH, 3-MGA/AS aerosols were aqueous
droplets with a single liquid phase. When the RH decreased, aqueous 3-MGA/AS
droplets underwent phase separation at
Organic compounds present at or near the surface of atmospheric aerosols can
be efficiently reacted with gas-phase oxidants, such as OH, ozone (O
A simplified diagram for the phase separation in aqueous droplets containing inorganic salts, organic compounds and water. At high humidity, aqueous droplets exist as homogeneously mixed liquid. When the humidity decreases, they undergo phase separation, leading to different morphologies such as core–shell and partially engulfed structures. Blue indicates the aqueous inorganic-rich phase; yellow indicates the organic-rich phase; bluish green represents the homogeneously mixed single liquid phase case.
The phase separation behavior could vary with the hydrophilicity of atmospheric organic compounds (or hydrophobicity). While the state of two separate liquid phases is the dominant aerosol phase state for hydrophobic organic compounds, a single liquid phase and two separate liquid phases are both important aerosol phase states for atmospheric aerosols with oxygenated organic compounds. Studies have demonstrated that aqueous organic–inorganic droplets containing a single or a mixture of oxygenated organic compounds (e.g., polyols and carboxylic acids) may undergo phase separation to form an organic-rich outer phase; typically, this transition occurs at a RH below 90 % because of the moderate solubility of organics in aqueous ionic solutions. Previous work has explored the heterogeneous reactivity of aqueous organic–inorganic droplets with hydrophobic organic shells (or coatings) (e.g., McNeill et al., 2007; Dennis-Smither et al., 2012). However, the effects of phase separation on reactivity in aqueous organic–inorganic droplets containing moderately oxygenated organic compounds remain largely unexplored.
In a previous study, we have investigated the heterogeneous OH oxidation of
aqueous droplets containing 3-methylglutaric acid (3-MGA) and AS with an OIR
of 2 at a relatively high RH of 85 % (Lam et al., 2019). AS is a typical
inorganic salt in atmospheric aerosols and was chosen to analyze the
salting-in/out effect on the heterogeneous reactivity. 3-MGA was chosen as a
model compound for small branched carboxylic acids, because 3-MGA and its
structural isomer (2-methylglutaric acid, 2-MGA) are two of the most
abundant methyl-substituted dicarboxylic acids that have been detected in
tropospheric aerosols (Kawamura and Kalpan, 1987; Li et al., 2015; Kundu et
al., 2016). Furthermore, these small branched dicarboxylic acids can induce
liquid–liquid phase separation in aqueous inorganic–organic droplets when
they are mixed with inorganic salts (Losey et al., 2016). These studies
suggest that small, branched diacids could play a role in determining both
the occurrence and compositional extent of phase separation as well as the
heterogeneous reactivity of aqueous organic–inorganic droplets. We found
that aqueous 3-MGA/AS droplets with OIR of 2 become initially phase
separated at an RH of
Chemical structure, effective heterogeneous OH rate constant, and effective uptake coefficient of aqueous droplets containing 3-MGA and
ammonium sulfate (AS) in an organic-to-inorganic dry mass ratio (OIR)
Note that
To better understand the role of aerosol phase state (single liquid phase versus two separate liquid phases) on the heterogeneous reactivity, we investigated how the phase separation in aqueous droplets containing an inorganic salt (AS) and an organic compound (3-MGA) with an OIR of 1 alters the kinetics and products upon heterogeneous OH oxidation (Table 1). First, the separation RH (SRH; the LLPS onset RH during dehumidification) and morphologies of phase-separated 3-MGA/AS droplets were determined using two distinct techniques: an optical microscopy setup and a linear-quadrupole electrodynamic balance. Second, the molecular composition of 3-MGA/AS droplets before and after OH oxidation was investigated using an aerosol flow tube reactor coupled with a direct analysis in real time (DART) ionization source and a high-resolution mass spectrometer at different RH levels. Based on these results, we attempt to explore and analyze quantitatively how phase separation in aqueous organic–inorganic droplets may determine the heterogeneous OH reactivity of methyl-substituted dicarboxylic acids.
Aqueous solutions containing 3-MGA and AS with OIR
A linear-quadrupole electrodynamic balance (LQ-EDB) was used to levitate
micron-sized droplets containing 3-MGA and AS (OIR
An atmospheric pressure aerosol flow tube reactor was used to investigate
the heterogeneous OH oxidation of 3-MGA/AS droplets (OIR
Inside the reactor, the formation of OH radicals follow the photolysis of
O
Image sequences of liquid–liquid phase separation and
efflorescence leading to
Generally, the phase separation behaviors of different aqueous 3-MGA/AS
droplets determined by the two methods agree well. We acknowledge that the
phase separation and droplet morphology characteristics studied here using
the optical microscopy setup and the single particle levitation method were
both conducted on super-micron-sized droplets; the phase separation of
submicron-sized 3-MGA/AS droplets has not been measured directly. O'Brien et
al. (2015) reported that the phase separation behavior (i.e., SRH) of
droplets with diameter of
Predicted equilibrium phase diagram of the aqueous 3-MGA/AS system
as a function of RH (or water activity,
Thermodynamic phase equilibrium calculations were also performed using the
Aerosol Inorganic–Organic Mixtures Functional groups Activity Coefficients
(AIOMFAC) liquid–liquid equilibrium (LLE) model, hereafter referred to as
the AIOMFAC-LLE model, to compare the results of the experimentally observed
LLPS range and onset mechanism (Zuend et al., 2008, 2010, 2011; Zuend and
Seinfeld, 2013). As shown in Fig. 4, 3-MGA/AS droplets with OIR
Figure S2 shows the simulated equilibrium aerosol composition for 3-MGA/AS
droplets (OIR
The viscosity of an 3-MGA/AS droplet with a single liquid phase (RH
Figure S4 shows the aerosol-DART mass spectra over a range of RH. Very
similar mass spectra were observed at different RH. Before oxidation, two
major peaks at
The heterogeneous OH oxidation of aqueous 3-MGA/AS aerosols, consisting of
either a single liquid phase (RH
To quantify the kinetics, the effective heterogeneous OH rate constant (
To investigate how the phase separation affects the kinetics, we attempt to
compare the heterogenous reactivity of aqueous droplets with a single liquid
phase (RH
As shown in Fig. 5, when the RH decreases from 88 % to 55 %, the
Previous studies have revealed that the alkoxy radicals are likely generated
from the hydrogen abstraction at the tertiary carbon upon the heterogeneous
OH oxidation of methyl-substituted dicarboxylic acids (e.g., 2-MGA, 3-MGA
and 2,3-dimethylsuccinic acid) and could play a significant role in
governing the heterogeneous kinetics and chemistry (Cheng et al., 2015; Chim
et al., 2017; Lam et al., 2019). Upon oxidation, 3-MGA molecules
(
When the RH is above the determined SRH, 3-MGA/AS aerosols are likely
aqueous droplets with a single liquid phase (Fig. 1). As shown in Fig. 4, when the RH decreases from 88 % to 75 %, the determined
We would like to note that at 85 % RH,
For phase-separated 3-MGA/AS droplets, the
For phase-separated droplets, the diffusivity of organic molecules across
the organic-rich outer phase has been suggested to influence the
heterogeneous kinetics (Zhou et al., 2019). Additionally, when the
organic-rich phase becomes more viscous at lower RH, the oxidation can be
limited by the slow diffusion (Davies and Wilson, 2015). We carried out a
simple analysis to estimate the timescale for diffusive mixing of 3-MGA
within the organic-rich phase to investigate whether the molecular diffusion
of organic molecules (i.e., 3-MGA) across the organic-rich phase (outer
shell) controls the heterogeneous reactivity of phase-separated droplets
with a fully developed core–shell structure. The diffusion coefficient
(
In addition to measurements and thermodynamic phase equilibrium
calculations, we carried out MD simulations to probe the distribution of the
species within 3-MGA/AS droplets at different RH (for details see MD
simulations in the Supplement). In the MD simulations, 3-MGA molecules
tended to stay near the surface while the NH
In this work, we investigated the heterogeneous kinetics of aqueous 3-MGA/AS
droplets with an OIR of 1. LLPS onset by spinodal decomposition is revealed
to occur at 74.6 % RH (by optical microscopy) and
We have performed thermodynamic phase equilibrium calculations to understand the phase transition and composition of 3-MGA/AS aerosols. The AIOMFAC-LLE model simulations provide an explanation for the phase separation mechanism consistent with the observations. While the model slightly overpredicts the SRH compared to the measurements, further development of predictive group contribution models like AIOMFAC-LLE would be highly desirable to better predict phase transitions and other composition-dependent properties of the typically highly complex organic–inorganic mixtures representative of atmospheric aerosols. In addition to thermodynamic modeling, our MD simulations suggest that 3-MGA molecules have a propensity to partition to the near-surface layer of a droplet, even at high RH. This is in qualitative support of the finding that the 3-MGA reactivity between single-phase and LLPS states of the droplets might not be dramatically different. Further improvements of MD simulation details for the studied system will likely lead to a valuable, complementary tool, because of the available molecular details it can provide. The combination of MD simulations and equilibrium thermodynamic computations will allow us to better understand the phase separation, morphology and size properties of aerosols, which ultimately govern the heterogeneous reactivity and other atmospheric processes, e.g., dynamic gas–particle partitioning.
Laboratory studies have shown that an organic-rich outer shell is always formed for hydrophobic organic compounds in a LLPS scenario, possibly shielding the interior from surface reactions with gas-phase oxidants (McNeill et al., 2007; Li et al., 2020). The organic-rich outer shell (Arangio et al., 2015; Houle et al., 2018) could be viscous, and the heterogeneous reactivity could be limited by the diffusion of organic species or oxidants across the organic outer shell. On the other hand, our results show that aqueous organic–inorganic droplets with more hydrophilic organic compounds (e.g., 3-MGA) may not necessarily experience diffusion limitation during heterogeneous OH oxidation, even when phase-separated. The overall heterogeneous reactivity is likely governed by the surface concentration of organic molecules at room temperature. It acknowledges that when the temperature decreases, the aerosol viscosity generally increases (everything else being equal). This would lead to a decrease in the diffusion rate of species from the bulk to the surface where oxidation preferentially takes place, and the overall rate of the oxidation will become more likely controlled by the diffusion. This is an expected temperature effect in the boundary layer (e.g., in the cold season or cold climates). However, in the context of vertical air motions (e.g., when air parcels rise adiabatically), a decrease in temperature will be accompanied by changes in RH; in the case of adiabatic ascent, RH tends to increase. This in turn would potentially limit the increase in viscosity of hygroscopic aerosols or even lower it while RH remains high (Gervasi et al., 2020). Overall, this work further emphasizes that the effects of phase separation and potentially distinct aerosol morphologies add further complexity to the quantitative understanding of the heterogeneous reactivity of organic compounds in aqueous organic–inorganic droplets in the atmosphere, motivating further experimental and process modeling studies for a variety of aerosol systems.
Data are available upon request from the corresponding author.
The supplement related to this article is available online at:
HKL and MNC designed and ran the experiments. JC, JFD, DH and MS provided the phase separation data. AZ, WL and YLST provided the model simulations. HKL, RX and MNC prepared the manuscript. All co-authors provided comments and suggestions to the manuscript.
The authors declare that they have no conflict of interest.
This work is supported by the Hong Kong Research Grants Council (HKRGC) project ID 2191111 (ref. 24300516). Andreas Zuend acknowledges support by the Natural Sciences and Engineering Research Council of Canada (NSERC) (grant no. RGPIN/04315-2014). Mijung Song acknowledges support by the National Research Foundation of Korea Grant funded by the Government of South Korea (NRF-2019R1A2C1086187). James F. Davies acknowledges the support of UC Riverside through startup funding.
This research has been supported by the Hong Kong Research Grants Council (HKRGC) (project ID 2191111, ref. 24300516), the Natural Sciences and Engineering Research Council of Canada (NSERC) (grant no. RGPIN/04315-2014), and the National Research Foundation of Korea Grant funded by the Government of South Korea (grant no. NRF-2019R1A2C1086187).
This paper was edited by Markus Ammann and reviewed by two anonymous referees.