Atmospheric particles, consisting of inorganic salts, organic compounds and
a varying amount of water, can continuously undergo heterogeneous oxidation
initiated by gas-phase oxidants at the particle surface, changing the
composition and properties of particles over time. To date, most studies
focus on the chemical evolution of pure organic particles upon oxidation. To
gain more fundamental insights into the effects of inorganic salts on the
heterogeneous kinetics and chemistry of organic compounds, we investigate
the heterogeneous OH oxidation of 3-methylglutaric acid (3-MGA) particles
and particles containing both 3-MGA and ammonium sulfate (AS) in an
organic-to-inorganic mass ratio of 2 in an aerosol flow tube reactor at a
high relative humidity of 85.0 %. The molecular information of the
particles before and after OH oxidation is obtained using the direct
analysis in real time (DART), a soft atmospheric pressure ionization source
coupled to a high-resolution mass spectrometer. Optical microscopy
measurements reveal that 3-MGA–AS particles are in a single liquid phase
prior to oxidation at high relative humidity. Particle mass spectra show
that C6 hydroxyl and C6 ketone functionalization products are
the major products formed upon OH oxidation in the absence and presence of
AS, suggesting that the dissolved salt does not significantly affect
reaction pathways. The dominance of C6 hydroxyl products over C6 ketone products could be explained by the intermolecular hydrogen
abstraction by tertiary alkoxy radicals formed at the methyl-substituted
tertiary carbon site. On the other hand, kinetic measurements show that the
effective OH uptake coefficient, γeff, for 3-MGA–AS particles
(0.99±0.05) is smaller than that for 3-MGA particles (2.41±0.13) by about a factor of ∼2.4. A smaller reactivity observed in 3-MGA–AS particles might be attributed to a higher surface
concentration of water molecules and the presence of ammonium and sulfate
ions, which are chemically inert to OH radicals, at the particle surface.
This could lower the collision probability between the 3-MGA and OH
radicals, resulting in a smaller overall reaction rate. Our results suggest
that inorganic salts likely alter the overall heterogeneous reactivity of
organic compounds with gas-phase OH radicals rather than reaction mechanisms
in well-mixed aqueous organic–inorganic droplets at a high humidity, i.e.,
85 % relative humidity (RH). It also acknowledges that the effects of inorganic salts on the
heterogeneous reactivity could vary greatly, depending on the particle
composition and environmental conditions (e.g., RH and temperature). For
instance, at lower relative humidities, aqueous 3-MGA–AS droplets likely
become more concentrated and more viscous before efflorescence, possibly
giving rise to diffusion limitation during oxidation under relatively dry or
cold conditions. Further studies on the effects of inorganic salts on the
diffusivity of the species under different relative humidities within the
organic–inorganic particles are also desirable to better understand the
role of inorganic salts in the heterogeneous reactivity of organic
compounds.
Introduction
Atmospheric particles are chemically complex and are comprised of a large
variety of organic compounds, inorganic salts and water. Organic compounds
contribute a significant mass fraction (20 %–90 %) of atmospheric particles
(Kanakidou et al., 2005; Zhang et al., 2007; Jimenez et al., 2009).
Laboratory and modeling studies have revealed that organic compounds present
at, or near, the particle surface can be efficiently oxidized by gas-phase
oxidants such as hydroxyl (OH), ozone (O3) and nitrate
radicals (Rudich et al., 2007; George and Abbatt, 2010; Kroll et al., 2015;
Estillore et al., 2016; Chapleski et al., 2016). The effective OH uptake
coefficient, γeff, defined as the fraction of OH collisions
with organic molecules that yields a reaction of a target organic molecule,
has been commonly used to describe the kinetics and has been reported for a variety
of pure organic compounds. The γeff in general has a value of
≥0.1 and even ≥1, indicating the occurrence of secondary
chemistry (e.g., chain reactions induced by the hydrogen abstraction of
organic molecules by alkoxy radicals) (Richards-Henderson et al., 2015).
These heterogeneous oxidative processes can continuously alter the surface
and bulk composition of the particles (Slade and Knopf, 2013; Li et al.,
2018), and thus modify particle properties such as light extinction,
hygroscopicity and cloud condensation nuclei activity (Petters et al., 2006;
George et al., 2007; Lambe et al., 2007, 2009; Cappa et al., 2011; Slade et
al., 2015, 2017).
While the transformation of pure organic particles has become more
reasonably understood, the chemical transformation of organic particles in
the presence of dissolved inorganic salts is largely unclear. Only a few
laboratory studies have investigated the heterogeneous oxidation of
organic–inorganic particles (McNeill et al., 2007, 2008; Dennis-Smither et
al., 2012). In those studies, hydrophobic organic compounds (e.g., oleic acid
and palmitic acid) have been chosen as model compounds. Due to the
hydrophobic nature of these compounds, the particles tend to be
phase separated with a thin organic layer on the surface of the aqueous
inorganic core (e.g., aqueous sodium chloride or ammonium sulfate (AS)
phases). For these phase-separated particles, the molecular structure and
orientation of organic molecules at the particle surface are observed to
alter the reactive uptake of gas-phase oxidants such as O3 and OH
radicals. The reaction products formed from the ozonolysis of oleic acid–AS
particles are very similar to those found in pure oleic acid particles
(McNeill et al., 2007). These observations are consistent with the
hypothesis that a thin organic layer effectively shields the aqueous
inorganic core from being oxidized at the particle surface.
To date, there are still uncertainties on how salts affect the heterogeneous
reactivity of organic compounds, in particular the more oxygenated ones,
which exhibit moderate to high solubility in water. In this work,
experiments were conducted to investigate the evolution of molecular
composition of 3-methylglutaric acid (3-MGA) particles and particles
containing 3-MGA and AS in an organic-to-inorganic mass ratio (water-free)
(OIR) of 2 upon heterogeneous OH oxidation at a relative humidity (RH) of
85.0 % using an aerosol flow tube reactor coupled to the direct analysis
in real time (DART) mass spectrometer. Here, 3-MGA is chosen as a model compound
for methyl-substituted dicarboxylic acids (Table 1), while AS is chosen as
the model inorganic salt. The OIR of 2 used in this work is in the range of the OIR commonly observed in atmospheric particles (Jimenez et al., 2009). The
model system allows us to gain more insights into the physics and chemistry
of heterogeneous reactions. The physical state of the particle is known to
be a key factor in controlling the heterogeneous reactivity (Renbaum and
Smith, 2009; Shiraiwa et al., 2011; Chan et al., 2014; Slade and Knopf,
2014; Fan et al., 2015; Marshall et al., 2018). Recent laboratory and
modeling studies have shown that in addition to deliquescence and
efflorescence, particles containing organic compounds and inorganic salts
can undergo phase separation, depending on the particle composition and
environmental conditions such as RH and temperature (Ciobanu et al., 2009;
Reid et al., 2011; Song et al., 2012a, b; Zuend and Seinfeld, 2012; Qiu and
Molinero, 2015; Stewart et al., 2015; You and Bertram, 2015; Freedman,
2017; Losey et al., 2018). Since whether particles are well mixed or
phase separated governs the surface composition of particles and thus the
heterogeneous reactivity, the phase separation behavior of 3-MGA–AS particles was investigated using an aerosol flow cell coupled to an
optical microscope. By assessing the molecular transformation of 3-MGA and
3-MGA–AS particles upon oxidation together with phase separation data
obtained from optical microscopy measurements, the effects of AS on the
heterogeneous OH kinetics and chemistry of 3-MGA are examined. More
recently, we have measured the heterogeneous OH reactivity of pure 2-MGA
particles, a structural isomer of 3-MGA (Chim et al., 2017a). Given their
similar structures, the results of this work together with 2-MGA data might
provide some new aspects on how dissolved inorganic salts would alter the
heterogeneous kinetics and chemistry of methyl-substituted dicarboxylic
acids.
Chemical structure, properties, effective heterogeneous OH rate
constant, effective OH uptake coefficient of 3-methylglutaric acid (3-MGA)
and 3-MGA mixed with ammonium sulfate (AS) in an organic-to-inorganic dry
mass ratio (OIR) =2.
Chemical structureChemical formulaC6H10O4O/C ratio0.67 H/C ratio1.67 3-MGA3-MGA–AS(OIR =2)Separation RH (SRH)–72.7 %–73.6 %Mass fraction at 85 % RH 3-MGA0.7070.344AS00.172H2O0.2930.484Effective second-order heterogeneous OH3.26±0.0652.72±0.064rate constant, k(×10-12 cm3 molecule -1 s-1)Effective OH uptake coefficient, γeff2.41±0.130.99±0.05Experimental methodsHeterogeneous OH oxidation of 3-MGA and 3-MGA–AS particles
The OH-initiated heterogeneous oxidation of 3-MGA and 3-MGA–AS particles
was performed in an aerosol flow tube reactor at 85.0 % RH. Details of
the experimental methods have been described elsewhere (Chim et al.,
2017a, b). In brief, the particle stream did not pass through a diffusion
dryer and was directly mixed with O3, oxygen (O2), dry nitrogen
(N2), humidified N2 and hexane before being introduced into the
reactor. The RH within the reactor was controlled by varying the dry N2-to-humidified N2 ratio and was measured at the inlet of the reactor. A
water jacket around the reactor was used to maintain a constant temperature
of 20 ∘C inside the reactor. Inside the reactor, gas-phase OH
radicals were generated via the photolysis of O3 using UV lamps at 254 nm. The processes can be described by the following reactions:
R1O3→O(1D)+O2,R2O(1D)+H2O→2OH.
The gas-phase concentration of OH radicals was controlled by varying the
O3 concentration introduced to the reactor and was determined by
measuring the decay of hexane, a gas-phase tracer of OH radicals, using a
gas chromatograph coupled with a flame ionization detector (GC–FID). The OH
exposure, a quantity defined as the product of OH concentration and particle
residence time (∼1.3 min), can be determined by the following
equation (Smith et al., 2009):
OHexposure=-lnHexHex0kHex=∫otOHdt,
where kHex is the rate constant for the reaction of OH radicals with
hexane (5.2×10-12 cm3 molecule-1 s-1),
[Hex]0 is the initial hexane concentration entering the reactor and
[Hex] is the concentration of hexane leaving the reactor. The OH exposure
was varied from 0 to a maximum of 4.06×1011 molecules cm-3 s in 3-MGA experiments and was varied from 0 to a maximum of
3.84×1011 molecules cm-3 s in 3-MGA–AS experiments.
The oxidation levels are equivalent to about 3 d in the atmosphere under
a moderate to high level of OH concentration (1.5×106 molecules cm-3). Since the OH exposure (and OH concentration) was
determined by the in situ measurement of the decay of hexane, the impacts of
RH and water uptake by particles inside the reactor on the generation and
concentration of gas-phase OH radicals have been taken into account. The
competitions between the heterogeneous oxidation and the gas-phase oxidation
have also been considered when quantifying OH concentration. Upon exiting
the reactor, the particle stream then passed through an annular Carulite®
catalyst denuder for O3 removal and an activated charcoal denuder for
the removal of organic gas-phase species remaining in the stream. Hence,
only particle-phase reaction products were analyzed. It is acknowledged that
Carulite® catalyst denuder can slightly decrease the RH of the particle
stream. However, this would not have a significant effect on the reaction
products analyzed because the decrease in RH after oxidation would not
significantly affect the formation of reaction products, which primarily
occurred inside the reactor. Size distribution of the particles was
determined by sampling a small portion of the particle stream using a
scanning mobility particle sizer (SMPS, TSI) after oxidation. The remaining
flow was directed to a heater at 250–300 ∘C to fully
vaporize the particles. Both 3-MGA and 3-MGA–AS particles were confirmed to be
fully vaporized upon heating at 250 ∘C or above by measuring the
size distribution of the particles leaving the heater with the SMPS in
separate experiments. The resulting gas-phase species were then directed to
an ionization region, a narrow open space between the DART ionization source
(DART SVP, IonSense Inc.) and the inlet of the high-resolution mass spectrometer (Q Exactive Orbitrap, ThermoFisher) (Chan et al., 2013; Nah et al., 2013).
Details of the DART operation have been described in the work of
Cody et al. (2005). The DART ionization source was operated in negative ionization mode
with helium (He) as the ionizing gas. The formation of gas-phase ions in the
ionization region can be described as below (Cody, 2009):
R3e-+O2g→O2-(g),R4O2-g+Mg→M-H-g+HO2(g).
Atmospheric O2 molecules abstract the electrons (e-) produced by
the Penning ionization of metastable He in the DART ionization source to
form anionic oxygen ions (O2-) which then react with the gas-phase
species (M) to form deprotonated molecular ions ([M-H]-) by proton
abstraction. Previous studies have shown that the acidic proton of the
carboxyl group can be abstracted by the O2- ions to generate the
[M-H]-, which was observed dominantly in the mass spectra (Cheng et
al., 2015; Chim et al., 2017a, b). In this work, it is likely that proton
abstraction from the carboxyl group of 3-MGA and its reaction products
occurred to produce the [M-H]-. These ions were sampled by the
high-resolution mass spectrometer. Mass spectra, over a scan range from
m/z 70 to 400 at a mass resolution of about 140 000, were collected. The mass
spectra were analyzed using the Xcalibur software (Xcalibur Software Inc.,
Herndon, VA, USA). It is acknowledged that thermal composition of reaction
products could possibly occur during the thermal desorption process (Stark
et al., 2017). The mass spectra of some organic acids and alcohols (e.g.,
succinic acid, ketosuccinic acid and tartaric acid) are available in the
work of Chan et al. (2014), showing insignificant thermal decomposition
during the DART analysis. In this study, the thermal decomposition of 3-MGA
was found to be insignificant as the deprotonated molecular ion of 3-MGA is
the dominant peak before oxidation in the mass spectra (Fig. 1). We
acknowledge that reactions between peroxy radicals may yield organic
peroxides and oligomers, which may decompose thermally. We cannot completely
rule out the possibility of such reactions, but there was no indication of
any of the fragment ions expected from the thermal decomposition in the mass
spectra. Together, these results suggest that the impact of thermal
decomposition on the observed product distribution is likely insignificant.
Two control experiments were conducted: one in the presence of O3
without the UV light and another one in the absence of O3 with the UV
light on. No compositional changes were observed for 3-MGA and 3-MGA–AS
particles in both control experiments, indicating that the reaction of 3-MGA
with O3 is insignificant and that 3-MGA is not likely to be
photolyzed.
The particle mass spectrum of (a, c) 3-MGA and (b, d) 3-MGA–AS before
(a, b) and after (c, d) heterogeneous OH oxidation at 85.0 % RH, respectively. The brown color represents organic species and the blue color represents inorganic species.
The hygroscopicity data of 3-MGA have been reported in the work of Marsh et
al. (2017) with a growth factor of ∼1.2 at 85 % RH. As
shown by the hygroscopicity curve measured, 3-MGA particles absorb and
desorb water reversibly as the RH increases or decreases, indicating that
they are likely aqueous droplets prior to oxidation. Optical microscopy
measurements have been carried out (Fig. S1 in the Supplement) and show that 3-MGA–AS
particles are in a single liquid phase prior to oxidation at 85.0 % RH as
the particles become phase separated when the RH is below the separation RH
(SRH =72.7 %–73.6 %) (Fig. S2). Details of the optical microscopy
measurements have been given in the Supplement. Since the particles were always
exposed to high humidity and the experiments were carried out at 85.0 % RH, which is higher than the SRH, 3-MGA–AS particles are likely to be
single-phase liquid droplets prior to oxidation.
Results and discussionParticle mass spectra of 3-MGA and 3-MGA–AS before and after OH
oxidation
Figure 1 shows the mass spectra of 3-MGA and 3-MGA–AS before and after OH
oxidation at 85.0 % RH, respectively. For 3-MGA, a dominant peak is
observed before oxidation at m/z=145, which corresponds to the deprotonated
molecular ion of 3-MGA (C6H9O4-). After oxidation, two
major product peaks evolve, corresponding to two C6 functionalization
products (C6 hydroxyl products (C6H10O5) and C6
ketone products (C6H8O5)). A few minor product peaks, such as
C4H5O3-, C5H5O3-,
C5H7O3-, C5H7O4- and
C5H7O5-, are also observed. Each of these peaks
contributes less than 2.5 % of the total ion signal at the maximum OH
exposure. The mass spectra of 3-MGA–AS particles in OIR = 2 are very
similar to those of 3-MGA particles, except for the two inorganic sulfate
peaks that originate from dissolved AS. Before oxidation (Fig. 1), three
peaks at m/z=97, 145 and 195 are observed, corresponding to the
bisulfate ion (HSO4-) and the deprotonated molecular ion of 3-MGA and
H3S2O8-, respectively. One possibility is that
HSO4- is likely the dissolved ion from aqueous AS that became
acidified by the evaporative loss of ammonia (NH3) into the gas phase and
can be detected via direct ionization in the negative ion mode (Hajslova et
al., 2011). HSO4- has been detected when aqueous AS particles are
ionized by the DART ionization source as well as when being a reaction
product formed in the heterogeneous OH oxidation of sodium methyl sulfate,
sodium ethyl sulfate and methanesulfonic acid particles (Kwong et al.,
2018a, b). However, we do not have a clear explanation for the formation of
H3S2O8-, which is likely an adduct of HSO4- and
H2SO4. After oxidation, the deprotonated ions of C6
hydroxyl (C6H9O5-) and C6 ketone products
(C6H7O5-) are observed in addition to the unreacted
3-MGA. Some small product peaks are detected (C4H5O3-,
C5H5O3-, C5H7O3-,
C5H7O4- and C5H7O5-), with each
contributing less than 2.5 % of the total ion signal.
As shown in Fig. 2, the chemical evolution of the composition of 3-MGA and
3-MGA–AS particles upon oxidation is very similar. At the maximum OH
exposure, the C6 hydroxyl products are the most abundant species, which
accounts for 38.0 %–48.2 % of the total organic ion signal, followed by
unreacted 3-MGA (37.3 %–47.9 %) and the C6 ketone products
(7.3 %–7.6 %). For 3-MGA–AS, the intensities of HSO4- and
H3S2O8- remain about the same before and after OH
oxidation (Fig. S3). This could be attributed to the fact that dissolved
AS does not react effectively with gas-phase OH radicals at the particle
surface (George and Abbatt, 2010). In general, the same reaction products
are observed for both 3-MGA and 3-MGA–AS particles after oxidation,
suggesting that AS does not significantly affect the reaction pathways.
(a, b, c) The fraction of the organic ion signal attributed to
the parent 3-MGA, the major C6 hydroxyl and C6 ketone products of
3-MGA particles during the heterogeneous OH oxidation shown against OH
exposure. (d, e, f) Analogous to the above, but for the case of mixed
3-MGA–AS particles.
Oxidative kinetics of 3-MGA and 3-MGA–AS
The normalized parent decay in 3-MGA and 3-MGA–AS particles as a function of
OH exposure at 85.0 % RH is shown in Fig. 3. For both systems, the
OH-initiated decay of 3-MGA follows an exponential trend and can be fit with
an exponential function to obtain an effective second-order OH reaction rate
constant (k):
lnII0=-kOHt,
where I0 is the ion signals of 3-MGA before oxidation, I is the ion
signals of 3-MGA at a given OH exposure, [OH] is the gas-phase concentration
of the OH radical and t is the reaction time. The fitted values of k for the 3-MGA
and 3-MGA–AS are (3.26±0.065)×10-12 cm3 molecule-1 s-1 and
(2.72±0.064)×10-12 cm3 molecule-1 s-1, respectively. Using the fitted k value, the
effective OH uptake coefficient, γeff, defined as the fraction
of OH collisions with particles that yields a reaction, can be computed by
the following equation (Davies and Wilson, 2015):
γeff=2ρ0D0mfsNAk3MwcOH‾,
where ρ0 is the density of particle, D0 is the particle
diameter, mfs is the mass fraction of 3-MGA in the particle, Mw is the
molecular weight of 3-MGA, NA is Avogadro's number and cOH‾
is the mean speed of gas-phase OH radicals. The mean surface-weighted
diameters prior to OH oxidation (203.0 nm for 3-MGA and 200.8 nm for
3-MGA–AS) are used in the calculation of γeff.
Upon oxidation, the mean surface-weighted diameter decreases from 203.0
to 170.7 nm for 3-MGA particles and decreases from 200.8 to 187.8 nm for
3-MGA–AS particles (Fig. S5). The decrease in the particle diameter upon
oxidation is likely attributed to the formation and volatilization of
fragmentation products and the associated evaporative loss of water
molecules. Vaden et al. (2011) have discussed that evaporation of highly
viscous particles is likely independent of particle size distribution and is
unlikely to significantly influence the overall evaporation behavior. As
the study of Vaden et al. (2011) focused on highly viscous particles, while
the focus of this study is more liquid-like particles, their results may not
be applicable in our study. Since 3-MGA–AS particles are more liquid-like
particles, the evaporation rate would scale with the total surface area of the
polydisperse particle population. Since the spread of the polydisperse particle population is small in this work, the size change is not likely to be substantial, especially in the determination of γeff. In the work of Meng and
Seinfeld (1996), the mixing timescales of volatile species were
evaluated. Although it was suggested by the study that the timescales may
increase with increasing particle size, the difference may not be that
significant in our study, as the span of the polydisperse particles is much
smaller than the difference between coarse particles and fine particles used
in Meng and Seinfeld (1996). We thus postulate that the spread of particle
size and the mixing timescale would not play a substantial role in the
evaporation of fragmentation products during oxidation. As the change in
particle size upon oxidation is not very significant, the change in particle
diameter was not accounted for in the γeff calculation. The
γeff may thus considered to be an initial uptake coefficient
(Chim et al., 2018). We acknowledge that the spread of particle size could
potentially affect the uncertainty and determination of γeff,
but we could not quantify it since the particles are polydisperse in our
study. Future investigations can be carried out to measure the γeff for both monodisperse (size selected) and polydisperse particle
populations. The γeff assembled from different monodisperse
particle sizes can be compared with that obtained from polydisperse
populations using the surface-weighted mean diameter in order to assess how
the spread and uncertainty in the particle size distribution of polydisperse
particle populations affect the determination of γeff. The
mfs values were obtained from equilibrium composition calculations using the
Aerosol Inorganic-Organic Mixtures Functional groups Activity Coefficients
(AIOMFAC) model available online (https://aiomfac.lab.mcgill.ca, last access: 5 March 2019) (Table 1) (Zuend et al., 2008, 2011). Based on the composition (i.e., mfs), the densities of 3-MGA and
3-MGA–AS particles were estimated using the volume additivity rule with a known
density of 3-MGA, AS and water (Chim et al., 2017a). Using Eq. (3), the
γeff values for 3-MGA and 3-MGA–AS are calculated to be 2.41±0.13 and 0.99±0.05, respectively. The value of γeff for 3-MGA particles is larger than that for 3-MGA–AS particles by a factor of 2.4. One possible explanation is that the mass fraction of 3-MGA in
3-MGA–AS particles (mfs =0.344) is smaller than that in 3-MGA particles
(mfs =0.707) at 85.0 % RH before oxidation (Table 1), and this likely resulted from
the presence of AS and the concomitant increase in particle hygroscopicity.
A simple analysis shows that the surface coverage of 3-MGA in 3-MGA
particles and 3-MGA–AS particles is roughly estimated to be 51.4 % and
21.6 %, respectively (see Supplement). A smaller surface concentration of 3-MGA in
3-MGA–AS particles might reduce the collision probability between 3-MGA and
gas-phase OH radicals at the air–particle interface and thus lower the
overall reactivity in comparison to 3-MGA particles. With reference to the work of Jungwirth et al. (2003) and Jungwirth and Tobias (2006), it is acknowledged that dissolved inorganic ions (e.g., SO42-) may not be homogeneously distributed in the droplets and may be concentrated in the core, which may increase the surface concentration of 3-MGA. Furthermore, the
surface activity of 3-MGA is not known and slight surfactant behavior could
drastically alter the surface concentration. Thus the numbers presented here
are to serve as a first approximation illustrating the possible effect of AS
addition on the surface coverage of 3-MGA. Further investigations on the
surfactant properties of 3-MGA and molecular dynamic simulation would be
useful to better understand the surface composition of both 3-MGA and
3-MGA–AS particles.
The normalized parent decay for the heterogeneous OH oxidation of
3-MGA and 3-MGA–AS particles at 85.0 % RH. Note the logarithmic scale of
the ordinate.
A similar result has also been observed for the OH oxidation with
methanesulfonic acid (MSA) reported in the literature. Mungall et al. (2017)
have investigated the heterogeneous OH oxidation of MSA–AS particles with a
mass fraction of MSA =0.16 at 75 % RH. The γeff was
reported to be 0.05±0.03, which is smaller than that of pure MSA
particles (γeff=0.45±0.14) measured at a slightly
higher RH (90 %) (Kwong et al., 2018b). The results obtained in this work
and in the literature suggest that for a given RH, inorganic salts (e.g., AS)
might lower the heterogeneous reactivity of organic compounds toward
gas-phase OH radicals due to the smaller surface concentration of 3-MGA
resulting from the presence of AS and concomitant increase in water uptake.
It is acknowledged that ammonium (NH4+) and sulfate (SO42-)
ions, which are chemically inert to OH radicals, present at or near the
surface could lower the overall reaction rates by reducing the surface
concentration of organic compounds. However, the additional effects of
NH4+ and SO42- ions on the surface activity and
configuration of organic molecules, which could play a role in determining
the heterogeneous activity, are not yet well understood and warrant further
investigations.
Kinetic measurements show that γeff values for both 3-MGA and 3-MGA–AS particles are close to or greater than 1. This indicates that
more than one 3-MGA molecule is reacted away per OH radical collision with
the particle surface, suggesting the occurrence of secondary chemistry in
the particle phase. In the following sections, reaction mechanisms are
tentatively proposed and discussed to explain the formation of major
products detected in the particles' mass spectra and the reaction pathways
likely responsible for the secondary chemistry.
Reaction mechanisms
As shown in Fig. 2, the reaction products observed in 3-MGA and 3-MGA–AS
particles are about the same upon oxidation. A generalized reaction scheme
is thus proposed for both systems based on well-known particle-phase
reactions previously published in literature (Russell, 1957; Bennett and
Summers, 1974; George and Abbatt, 2010). As shown in Scheme 1, the OH
oxidation with 3-MGA can be initiated by the hydrogen abstraction at three
different carbon sites: tertiary backbone carbon site (Path A), secondary
backbone carbon site (Path B) and the primary carbon site of the branched
methyl group (Path C). Depending on the initial OH reaction site, a variety
of reaction products can be formed and are broadly classified into two groups:
functionalization and fragmentation products.
Proposed reaction mechanisms for the heterogeneous OH oxidation of
3-MGA and 3-MGA–AS particles.
Functionalization products
At the first oxidation step, a hydrogen atom is abstracted from a 3-MGA
molecule by an OH radical, forming an alkyl radical that reacts quickly with
an O2 molecule to form a peroxy radical. The major C6 hydroxyl
(C6H10O5) and C6 ketone (C6H8O5) products
can be generated from the self-reactions of two peroxy radicals through the
Russell mechanism (Reaction R1) and/or Bennett–Summers reactions (Reaction R2). Alternatively,
the self-reactions of two peroxy radicals can form two alkoxy radicals which
can then abstract hydrogen atoms from the neighboring organic molecules (Reaction R3)
to form C6 hydroxyl products, or react with O2 molecules (Reaction R4) to
form C6 ketone products. However, when the hydrogen abstraction occurs
at the tertiary carbon site (Scheme 1, Path A), only the C6 hydroxyl
product can be formed because only a hydroxyl group can be added to the
tertiary carbon site. Depending on initial reaction site, structural isomers
of these C6 hydroxyl and ketone products are likely formed during
oxidation.
Fragmentation products
The fragmentation products can be generated from the decomposition of alkoxy
radicals (Reaction R5). For instance, when the initial hydrogen abstraction occurs at
the secondary carbon site (Scheme 1, Path B), the decomposition of the
secondary alkoxy radical can yield either a C2 (C2H2O3)
or a C5 fragmentation product (C5H8O3). On the other
hand, a C4 fragmentation product (C4H6O3) can be
yielded from the decomposition of the alkoxy radical formed at the tertiary
carbon site (Scheme 1, Path A) while oxidation at the primary carbon site
(Scheme 1, Path C) can yield a C3 fragmentation product
(C3H4O3). For both 3-MGA and 3-MGA–AS, the ion signal
intensity of fragmentation products is small (Fig. 1). For example, only a
small signal of C4 fragmentation product (C4H6O3),
which is formed from the oxidation at the tertiary carbon site (Scheme 1,
Path A), is detected. It contributes to less than 2 % of the total ion signal
at the maximum OH exposure. The observed low abundances of fragmentation
products could be explained by their higher volatilities (Tables S1 and S2) and some (e.g., C5H8O5) may be explained by the preference
for the initial OH reaction site as discussed below. It is noted that for
3-MGA–AS particles, the presence of AS increases the activity coefficients
of fragmentation products in the particle phase based on the thermodynamic
model predictions and thus increases their volatilities in general (Table S3).
Large C6 hydroxyl-to-C6 ketone product ratio:
implications for secondary chemistry
From the particle composition data, a large C6 hydroxyl-to-C6 ketone product ratio is observed. At the maximum OH exposure, the relative
abundance of C6 hydroxyl products is about 5.0–6.6 times that of
C6 ketone products for both 3-MGA and 3-MGA–AS particles. We
acknowledge that although the ionization efficiencies are not corrected for
these products in this study, the ionization efficiency of C4 hydroxyl
products is found to be about the same or even lower than that of C4
ketone products during the DART ionization processes (Chan et al., 2014).
The abundance of C6 hydroxyl products might be even larger than that
of C6 ketone products after correcting their ionization efficiencies,
supporting the statement above. One possible explanation for the dominance
of C6 hydroxyl products is that the OH abstraction may preferentially
occur at the tertiary carbon site (Scheme 1, Path A) since the tertiary
alkyl radicals are more stable than secondary and primary alkyl radicals
(Cheng et al., 2015). Only an addition of a hydroxyl group at the tertiary
carbon site is allowed via the alkoxy or peroxy radical reactions.
Another possibility is that the branched methyl group may sterically hinder
the two peroxy radicals from arranging into a cyclic tetroxide intermediate,
which is essential for the formation of hydroxyl and ketone
functionalization products through the Russell and the Bennett–Summers
mechanisms (Cheng et al., 2015). Alternatively, alkoxy radicals are more
likely formed through the self-reaction of two peroxy radicals and can react
with neighboring organic molecules (e.g., unreacted 3-MGA) by intermolecular
hydrogen abstraction to form the C6 hydroxyl products. Furthermore,
as proposed by Peeters et al. (2004) and Vereecken and Peeters (2009), the
strong hydrogen bonding among the two terminal carboxyl groups might lower
the decomposition rate of the alkoxy radical. This could increase the
competitiveness of the intermolecular hydrogen abstraction by the alkoxy
radicals. It is worthwhile to note that the intermolecular hydrogen
abstraction can regenerate an alkyl radical and eventually produce peroxy
radicals. These peroxy radicals can react again with other peroxy radicals
to regenerate alkoxy radicals. This allows the chain reactions to propagate
and increases the overall reactivity (i.e., more than one 3-MGA molecule can
be reacted per initial OH collision via secondary chemistry and, thus,
γeff is larger than 1). Overall, the alkoxy radical
chemistry, originating from the OH abstraction at the tertiary carbon site
at the first oxidation step (Scheme 1, Path A) is likely the important
reaction pathway for the OH reactions with 3-MGA.
Conclusions and atmospheric implications
Atmospheric particles can keep colliding with gas-phase oxidants, allowing
continuous oxidation to occur at or near the particle surface. To better
understand how dissolved inorganic salts affect the heterogeneous chemistry
and kinetics of organic compounds with gas-phase OH radicals, we
investigated the kinetics, products and mechanisms of particles comprised of
3-MGA and 3-MGA–AS in an OIR of 2 upon heterogeneous OH oxidation
at 85.0 % RH. Optical microscopy measurements for the detection of phase
separation reveal that 3-MGA–AS particles exhibit a single liquid phase
prior to oxidation. Same major reaction products are formed as a result of
heterogeneous OH oxidation with both 3-MGA and 3-MGA–AS particles. These
data suggest that the presence of aqueous AS does not significantly affect
the formation pathways of major reaction products. On the other hand, in the
presence of AS, the heterogeneous reactivity of 3-MGA toward gas-phase OH
radicals is slower in 3-MGA–AS particles compared to that in 3-MGA
particles. It is likely attributed to a lower concentration of 3-MGA at the
surface of 3-MGA–AS particles relative to 3-MGA particles, reducing the
collision probability between 3-MGA and gas-phase OH radicals. The results
from this work and the literature suggest that the presence of dissolved
salts could reduce the overall heterogeneous reactivity of organic compounds
with gas-phase OH radicals at the surface by lowering the surface
concentration of organic compounds at a given RH and temperature. Until
recently, the kinetic parameters (e.g., γeff) reported in the
literature were mostly measured based on experiments with salt-free organic
particles. The chemical lifetime of organic compounds or chemical tracers
against heterogeneous OH reaction in the atmosphere could be longer than
expected when the salts are present. Further investigations on how the
amount and types of inorganic salts alter heterogeneous kinetics and
chemistry are highly desirable.
A simplified diagram illustrating the change in separation
relative humidity (SRH, orange solid line) and phase composition of droplets
containing inorganic salts and organic compounds (single liquid phase vs.
liquid–liquid phase separated) upon heterogeneous oxidation at two different
environmental RHs.
Over the past decade, laboratory and modeling studies have demonstrated that
atmospheric particles can undergo phase separation and exhibit different
morphologies, which play a role in many atmospheric processes. For example,
the inhomogeneous distribution of inorganic and organic species within
phase-separated particles can affect the reactive uptake of gas-phase
species (e.g., N2O5) (Gaston et al., 2014) and water uptake of
organic–inorganic particles (Chan and Chan, 2007; Zuend and Seinfeld, 2012;
Hodas et al., 2015). It is still unclear how the occurrence of liquid–liquid
phase separation alters the heterogeneous reactivity of organic–inorganic
particles over time. As shown in Fig. S2, 3-MGA–AS particles become
phase separated when the RH is below the SRH (72.7 %–73.6 %). The
phase-separated particles might exhibit different reactivity compared to
those in a single liquid phase investigated in this work since the surface
composition of the particles is different in these two phases. Furthermore,
there is a possibility that the phase separation behavior (e.g., SRH) of the
particles may change in response to the change in the particle composition
over time. Although the phase of oxidized 3-MGA–AS particles has not been
determined experimentally in this work, the overall
〈O/C〉 is found to increase slightly from 0.67 to ∼0.75
(see Supplement), and the SRH is expected to decrease slightly after oxidation (Bertram et
al., 2011; Song et al., 2012b; You et al., 2013, 2014). Since the
experimental RH inside the reactor was fixed at 85.0 %, it is very likely
that 3-MGA–AS particles remain in a single liquid phase state during
oxidation. During oxidation, the degree of aerosol oxidation state (e.g.,
expressed by 〈O/C〉) typically increases due to the formation of more oxygenated reaction
products and, consequently, the SRH is expected to decrease. As shown in
Fig. 4, it is hypothesized that initially phase-separated particles might
transition to a homogeneous, single liquid phase state, depending on the
extent of oxidation and the environmental thermodynamic conditions. Hence,
it is of interest to investigate how the phase separation characteristics of
organic–inorganic particles change in response to a change in the
composition upon oxidation (Slade et al., 2015, 2017).
Moreover, with respect to future work, it would be interesting to
understand the dynamic interplay between the particle composition,
heterogeneous reactivity, liquid–liquid phase separation and the effects on
particle morphology under different environmental conditions and extents of
oxidation.
Data availability
The underlying research data are available upon request from the
corresponding author (mnchan@cuhk.edu.hk).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-19-9581-2019-supplement.
Author contributions
HKL, SMS and MNC designed and ran the experiments. HKL, SMS and MNC
prepared the paper. All co-authors provided comments and suggestions on
the paper.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We would like to thank Kevin Wilson for
his insightful comments.
Financial support
This research has been supported by the Hong Kong Research Grants Council (HKRGC) (project ID: 2191111, ref 24300516).
Review statement
This paper was edited by Markus Ammann and reviewed by two anonymous referees.
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