Spectral changes
The composition of SOA generated from ozonolysis of α-pinene has
been well-studied using both online and offline mass spectrometry
(e.g., Tolocka et al., 2006; Shilling et al., 2009; Camredon et al.,
2010; Putman et al., 2012; Yasmeen et al., 2012; Hall et al., 2013; Kristensen
et al., 2014, 2016; Zhang et al., 2017). The mass spectrum
obtained depends on the measurement method used. For example, mass spectra
of α-pinene SOA obtained using high-temperature evaporation
(>600 ∘C) coupled with electron impact ionization at
70 eV (the typical operating conditions of the Aerodyne AMS) have very few
ions with m/z>100 having substantial intensity (Shilling et
al., 2009). In contrast, mass spectra obtained using electrospray ionization
(ESI), typically considered a softer ionization method, typically show that
dimers or oligomers (nominally those compounds having molecular weights more
than twice higher than that of the parent compound, 136 amu for α-pinene) comprise a substantial fraction of the particle (Kristensen
et al., 2014; Zhang et al., 2017). Mass spectra for α-pinene SOA
obtained using the VUV-AMS, used here, exhibit peaks spanning the range
m/z=50–300, although with much lower intensity ions at m/z>200 amu
(Fig. 4). In the VUV-AMS spectra, ions in
range m/z=50–200 are likely a combination of monomers (especially above
m/z=140), fragments of monomers (especially below m/z=140) and fragments
of dimers. For example, characteristic fragment ions of monomers such as
cis-pinonic acid and pinic acid, known products of α-pinene
ozonolysis (Yu et al., 1999; Kristensen et al., 2014), have been
previously identified in VUV-AMS mass spectra of nonoxidized particles
(Mysak, 2006) and are observed here with high intensity in the
nonoxidized particles. The characteristic ions are observed at m/z=98,
114, 125, 166 for cis-pinonic acid (m/z=184) and at m/z=100, 114, 140, 168
for pinic acid (m/z=186), respectively. However, some of the
odd-numbered, high-intensity ions that were observed in the
nonoxidized particles, such as m/z=125, 141 and 169
(Fig. 4), may be fragments of dimers based on
Kristensen et al. (2016), although they used a different mass
spectrometry technique (ESI). This is consistent with the peaks in the
VUV-AMS spectrum of α-pinene SOA consisting of contributions
of monomers, fragments of monomers and fragments of dimers. Dimers and
oligomers are likely underrepresented in VUV-AMS spectra of α-pinene SOA, although they may comprise more than 50 % of the aerosol
mass (Kristensen et al., 2016). This underrepresentation is probably
attributable to substantial fragmentation of dimers during the ionization
process and/or the slow desorption of dimers from the heater block, leading
to low sensitivity of VUV-AMS towards dimers and oligomers.
Example mass spectra of SOA as a function of OH exposure for (a) low-RH (red, left panels) and (b) high-RH (blue, right panels) conditions.
OH exposure increases from the bottom to the top panels, with values listed
in the panels. Vertical gray bars highlight four ions that exhibited a
dramatic decay at high RH, but only small changes at low RH. The
mass-to-charge ratio of these four ions are indicated in the figure.
The relative changes of the α-pinene SOA mass spectra are
considered as a function of OH exposure. We define nonoxidized particles
as particles that passed through the OH flow tube with the lights on but with
no added O3. As such, all changes to the observed spectra are
attributable to chemical oxidation of the particles, not photolysis. We note
that there is no difference in the mass spectra for nonoxidized particles
for tests done with the lights on versus lights off in the absence of
O3 for low-RH conditions (R2=0.997; Fig. S2). This indicates
that direct photolysis of the SOA had no influence on the SOA spectra for
the short exposure timescale (38 s) here. We also observe that the mass
spectra of nonoxidized particles at low and high RH are identical
(R2=0.999), indicating that photolysis does not differ between low-
and high-RH conditions in these experiments. Thus, the differences in
spectral changes between the two RH conditions are driven by chemical
oxidation only.
The mass spectra observed at four different OH exposures are shown for low
RH (Fig. 4a) and high RH (Fig. 4b). There are distinct RH-dependent
differences in the spectral evolution. At low RH, the four most abundant
ions in the nonoxidized particle spectra (m/z=98, 125, 141 and 169)
are still among the most abundant at the highest OH exposure. In contrast,
at high RH the relative signal intensities of these four ions decreases
substantially with increasing OH exposure.
The overall similarity of the oxidized spectra to the nonoxidized
spectra can be considered by calculating the R2 between the spectra
(Lanz et al., 2007). Smaller R2 values indicate greater
spectral differences (lower similarity). The R2 between the mass spectra
of oxidized particles and nonoxidized particles are calculated at each
OH exposure for both the low- and high-RH experiments. Only ions that
contribute more than 0.5 % to the total signals are included in the
calculation. As shown in Fig. 5, changes in the
R2 differ substantially between low and high RH. At low RH, the
R2 decreases reasonably rapidly when the OH exposure is less than 2×1012 molecule cm-3 s. However, the R2 plateaus well above zero (to ca. R2=0.78), and the overall extent of decline is relatively small with limited changes above this OH exposure. In
contrast, at high RH the R2 exhibits a rapid decrease to near zero when the OH
exposure is less than 1×1012 molecule cm-3 s. The very small
R2 values at high RH indicate that the composition of the oxidized
particles differs substantially from the nonoxidized particles.
Coefficient of determination (R2) between spectra of oxidized
SOA and nonoxidized SOA as a function of OH exposure for dry (red
circles) and wet (blue triangles) conditions. Spectra are filtered by
excluding ions that have a percentage contribution to the total signal below 0.5 %. Lines are exponential fits and presented only as visual guides.
Signal decay of all peaks observed above background in the
mass-to-charge range of 15–350 amu as a function of OH exposure for
(a) low-RH and (b) high-RH conditions. Colors denote mass-to-charge of
a given peak. The black open circles and lines denote the unweighted, averaged
decay of all the peaks. The dark gray open diamonds and lines denote the
total signal fraction remaining (i.e., the signal weighted average of all
peaks). Note the log scale for the y-axis.
The decay of each individual m/z ion in the mass spectrum (from m/z=15–350)
with OH exposure is shown in Fig. 6. The
intensity of each ion has been first normalized by the particle number
concentration to account for changes in intensity due to variations in the
physical loss of particles in the flow tube, which likely result from
changes in the particle size distribution with OH exposure. The
number-normalized intensities were then normalized by the observed intensity
at zero OH exposure to assess the relative change. These resulting normalized
intensities are also referred to here as the signal fraction remaining
(SFR). The decay of the average SFR (calculated from the individual SFR curves) and
the weighted-average SFR (equal to total signal fraction remaining) are also
shown in Fig. 6. The differences in the average
SFR decay between low- and high-RH conditions and the absolute magnitude of
decay are consistent with the observed volume loss. However, the
weighted-average SFR decays to a greater extent than the average SFR decay or mass
loss under both low- and high-RH conditions. Ideally, loss of total signal
should be equivalent to loss of particle mass if detection sensitivity
towards all the species are the same. One possible explanation of this
difference is that the VUV-AMS exhibits a lower sensitivity towards
product species compared to parent species. Another possibility is the
underrepresentation of dimers and oligomers in VUV-AMS spectra, as
mentioned above. If the latter is true, it also indicates that dimers should
decay more slowly than monomers to account for the difference between mass
loss and total signal loss.
There is substantially greater spread in the SFR between individual ions at
high RH compared to low RH at a given OH exposure. In other words, at low RH
the intensities of the individual ions all decay to a similar extent.
However, at high RH the intensity of some ions decrease by nearly a factor
of 100 at the highest OH exposure, while the intensity of others remains
similar to the nonoxidized particles. This is consistent with the
differences in R2 versus OH exposure between low and high RH, indicating
distinctly different compositional change.
We focus first on the high-RH ion decay curves. To further understand
differences between ions that decay rapidly versus slowly under high-RH
conditions, the ions have been grouped by their extent of decay
(Fig. 7a). Four groups have been established.
Group 1 includes ions that exhibit the fastest and most substantial decay,
with the average SFR for this group equal to 0.04 at the highest OH exposure.
This group contains most of the ions that have the greatest signal intensity
in the nonoxidized particle spectrum and includes markers from known
products of α-pinene ozonolysis, such as cis-pinonic acid and
pinic acid. Also, the odd ions with m/z=125, 141 and 169 that might be markers
of oligomers as discussed above are also in Group 1. Nearly all of the ions
in Group 1 have m/z<200 (Fig. 7b),
suggesting that Group 1 primarily consists of either monomeric species or
fragments thereof and signature fragments from dimers (or larger
oligomers). That these ions exhibit a rapid, continuous decay indicates that
they are chemically transformed, producing either new functionalized
products or fragments that evaporated and are not produced during
heterogeneous oxidation.
(a) Categorization of peaks according to their decay with OH
exposure for high-RH experiments. The peaks were classified into four
groups. Peaks in Group 1 (gray) exhibit the fastest decay, with the average
shown as the solid black curve. The peaks in Group 2 (red) exhibit the
second fastest decay, with the average shown in dark red curve. The peaks in
Group 3 (green) exhibit negligible decay, with the average shown in dark
green. Group 4 (blue) contains only one peak. (b) Spectrum of the peaks in
Group 1. Group 1 contains markers from cis-pinonic acid (black peaks) and
pinic acid (pink peaks). (c) Spectrum of the peaks in Group 2. Group 2
contains patterns of repeating peaks separated by Δm/z=14,
illustrated by dark red peaks. (d) A zoomed-in view of the peaks of Group
3 and Group 4. These peaks have very small intensities.
Group 2 includes the ions that decay relatively slowly, albeit still to a
substantial extent, with the average SFR equalling 0.3 at the highest
OH exposure. Group 2 has the greatest number of ions, containing many ions
with m/z<200 and, notably, almost all the ions with m/z>200 (Fig. 7c). The Group 2 ions with
m/z<200 have generally smaller percentage contributions to the
total signal in the nonoxidized particles than the Group 1 ions. This
suggests that they are either less characteristic of minor ions for major
parent compounds or characteristic of ions for minor parent compounds in the
nonoxidized particles. The Group 2 ions with m/z>200 are
contributed by very highly oxygenated products, dimer fragments and dimers.
The comparably slow decay of Group 2 ions indicates either that they react
substantially more slowly than the Group 1 species (which are likely
monomers) or that production offsets some of the chemical loss during the
aging process. It is difficult to determine which is more likely.
Group 3 includes ions that exhibited little decay with oxidation. The Group
3 ions all have m/z<50 (Fig. 7d), and
thus are small ion fragments that could result from any of the
particle-phase compounds, both parent and product species. The relative
intensity of the Group 3 ions is overall extremely small. That their
intensity remains relatively constant suggests that these ion fragments are
produced to a greater extent from oxidation products than from the molecules
comprising the nonoxidized SOA. In other words, production is offsetting
loss. Group 4 includes the one ion (m/z=17) that exhibits an increase in
signal intensity. The relative intensity of this ion is very small,
contributing only 0.01 % to total signal in nonoxidized particles.
However, at the highest OH exposure the percentage contribution of this ion
increases to nearly 1 %. Since these experiments were performed under
NOx free conditions, m/z=17 most likely corresponds to OH+ ions.
In general, the formation of OH+ ions from photoionization is
unfavorable, which explains the extremely low intensity of this ion in the
mass spectra. However, the increase in the intensity of this ion with
oxidation is likely still meaningful and could be a result of product
species of increasing generation number having a greater number of -OH
groups. Alternatively, it could indicate that the functional groups produced
from oxidation are more likely to generate OH ions compared to the parent
species due to differences in bond dissociation energies. Most likely, the
increase in intensity of this ion is a result of functionalization.
Considering now the low-RH experiment (Fig. 6a),
all the ions visually appear to decay to similar extents. However, building
on the results from the high-RH experiment above, the mass spectrum has been
split into the same m/z groups (Fig. 8). It is found
that the Group 1 ions collectively decay to the greatest extent for the low-RH experiment, consistent with the high-RH experiment. This provides
additional evidence that Group 1 ions are mainly contributed from monomeric
species that are chemically degraded and are not produced through
heterogeneous oxidation. The Group 2 ions decay on average to a lesser
extent than Group 1 ions at low RH, consistent with the high-RH experiment.
There is, however, a fair amount of variability in the individual ions for
this group (mainly driven by low intensity ions). The Group 3 ions exhibit,
on average, slightly less decay than Group 2 ions, though with a fair
amount of variability driven by the low ion intensities. The one Group 4 ion
(m/z=17) may increase slightly with OH exposure, but the overall behavior
is difficult to discern given the small change and low ion intensity.
Signal fraction remaining of all the individual ions observed
above background in the range m/z=15–350 under low-RH conditions.
Ions are colored according to the groupings determined for the high-RH
conditions (cf. Fig. 7a).
The mean SFR of Group 1 ions equals 0.4 at low RH compared to 0.05 at high RH
at the same high OH exposure (=4.7×1012 molecule cm-3 s).
This difference is comparable to the observed differences in decay of parent
ions for model (single-component) OA particles upon heterogeneous
oxidation by OH under low- versus high-RH conditions (Chan et al.,
2014; Davies and Wilson, 2015). As discussed above, ions in Group 1 are
likely representative of monomeric species in parent compounds. With the
assumption that Group 1 ions are only lost through heterogeneous oxidation
and not formed, the mean signal decay of Group 1 ions could be considered as
the lower limit of decay of parent species for α-pinene SOA.
The lower limit of the reaction rate coefficient of α-pinene SOA
with OH, kSOA+OH, can be estimated based on the signal loss rate of
Group 1 ions by fitting the decay of the average SFR of Group 1 ions with an
exponential function:
SFR=exp(-kSOA+OH⋅OHexposure)
The derived kSOA+OH is 2.3×10-13 cm3 molecule-1 s-1
for low-RH and ∼1.3×10-12 cm3 molecule-1 s-1 for high-RH conditions. This difference in
rate constants is similar to the difference reported by Davies and
Wilson (2015) for citric acid and Hu et al. (2016) for IEPOX-SOA for a
similar RH difference. The latter study reported an approximately linear
relation between the rate coefficient and the wet particle surface area for
the RH conditions examined. However, as discussed above, α-pinene
SOA only takes up a small amount of water at RH equalling 90 %, leading to an
increase in surface area of only ∼20 %, which cannot
explain the more than 5 times higher rate constant under high RH.
Overall, it has been observed that substantially larger changes are observed
at high RH than at low RH, both in terms of the extent of mass loss and the
change in the particle composition with increasing OH exposure. It has also
been observed that under low-RH conditions the composition of SOA stops
changing (based on consideration of the R2) above a certain oxidation
level, while mass loss continues slowly. In contrast, both compositional
change and mass loss occur continuously and dramatically under high-RH
conditions.
Model simulation: dependence on RH
The model is used here to establish the factors that primarily lead to the
observed difference in heterogeneous oxidation under different RHs in terms
of both mass loss and compositional change. Heterogeneous oxidation of
organic aerosol has several key steps, as discussed above and shown in
Fig. 2. These include sticking and uptake of
gas-phase HOx radicals, chemical reactions of different species and
diffusion of the species in the bulk. All of the steps can be affected by
RH, and in particular the influence of RH on viscosity plays important roles
in these steps.
For the uptake process, there are a variety of reasons that the reactivity
of HOx radicals may differ between low- and high-RH conditions. The
existence of water at the surface might lead to more efficient sticking of
gas-phase HOx radicals due to enhanced hydrogen bonding to water
(Chan et al., 2014). However, the greater abundance of water vapor
at high RH also might decrease the available sites for adsorption of
gas-phase HOx radicals due to competitive co-adsorption
(Kaiser et al., 2011). After adsorbing and sticking to the surface,
gas-phase HOx radicals are mobile on surfaces prior to reaction
(Slade and Knopf, 2013). Since different sites have different
reactivities depending on their bonding environment (Kwok and
Atkinson, 1995), the reaction probability of gas-phase HOx radicals
with surface species can be affected by molecular orientation and the
mobility of the radicals. The orientation of the SOA molecules can
potentially be affected by RH. At low RH the SOA molecules are likely less
mobile and their particular orientation and alignment at the surface is
relatively static compared to high RH (Moog et al., 1982). It
is possible that a difference in the orientation and mobility of surface
molecules between low and high RH affects the probability that an OH radical
reacts versus desorbs after colliding with and sticking to the surface.
The viscosity of particles could also have an influence on the chemical
pathways and products formed from RO2⚫+RO2⚫ reactions within the bulk by affecting the (i) reaction
probability, (ii) branching ratio or (iii) both. Denisov and Afanas'ev (2005)
proposed that RO2⚫+RO2⚫ react to form an unstable
tetroxide, which then decomposes to produce a ketone and an alcohol (the
Russell mechanism), two ketones and H2O2 (the Bennett–Summers
mechanism), or two RO⚫ radicals. They also indicated that the
decomposition rate constant of the tetroxide intermediate decreases with an
increase in viscosity of the surrounding phase. The products formed from
decomposition of the tetroxide depend on the structure of the transition
state and, in the condensed phase, on further reaction of radical products
within a solvent cage. The presence of a solvent cage and the stability of
the cage can potentially influence the branching ratio towards production of
RO⚫ radicals relative to stable, functionalized product species. This
is important because RO⚫ radicals are the only species that lead to
fragmentation.
Another impact of an RH-dependent viscosity is on the mixing timescales
for condensed-phase species. The mixing timescale for particles depends on
the condensed-phase diffusivity, which is related to viscosity. For example,
for Dorg=10-12 cm2 s-1, the mixing timescale
for a 100 nm particle is about 2 s (Koop et al., 2011).
Thus, the particle can be generally considered well-mixed and
liquid-like. However, for Dorg=10-15 cm2 s-1
the mixing timescale for a 100 nm particle is ∼40 min. In
this case, mixing is slow and the particle can be considered as having a
more semi-solid phase. As discussed above, the diffusion coefficient (and
viscosity) of α-pinene SOA vary continuously with RH. In general,
at >70 % RH, the diffusivity of α-pinene SOA is large
(Dorg>10-12 cm2 s-1), while at low RH
the diffusivity is comparably small (Dorg<10-14 cm2 s-1). Given this, for our experiments it is expected that
the particles at low RH (∼30 %) are semi-solid while at
high RH (∼90 %) they are liquid-like. When the particles
are more viscous (semi-solid), reaction of the organic radicals generated
from reaction with OH will be primarily constrained to surface layers. In
contrast, when the particles are less viscous (liquid-like), reaction of
radicals may occur throughout the entire particle. Therefore, there is
reason to think that the impact of heterogeneous oxidation on both
distributions of parent and product species in the particles and mass loss
should depend on RH.
In the model simulations, the influence of RH on the above processes is
examined by systematically varying five input parameters: (i) the growth
factor (GF), (ii) the OH uptake coefficient (γ), (iii) bulk
diffusivity (Dorg), (iv) the RO2⚫+RO2⚫ reaction
rate constant (kRO2+RO2) and (v) the combined branching ratio and
fragmentation probability (pfrag). A summary of simulation parameters is
shown in Table 2.
Summary of values or ranges of values for parameters used in the
model simulation.
Parameter
Description
Value
Condensed phase
Dp,S
Surface-weighted middle diameter of initial dry particles
125 nm (initial)
ρp
Density of SOA
1.3 g cm-3
MWp
Molecular weight of SOA
175 g mol-1
GF
Hygroscopic growth factor
1.1 (high RH) or 1.0 (low RH)
γ
Uptake coefficient of OH and HO2
0–1
pfrag
A combined probability for fragmentation
0–1
kRO2+RO2
Reaction rate coefficient of RO2⚫+RO2⚫
1×10-21–1×10-15 cm3 molecule-1 s-1
dSL
Depth of surface layer
0.76 nm
Dorg
Bulk diffusivity of SOA compound
1×10-16–1×10-11 cm2 s-1
Gas-phase
[OH]g
OH concentration along the flow tube
0–1.84×1011 molecule cm-3-air
[HO2]g
HO2 concentration along the flow tube
Same as [OH]
c‾OH
Mean speed of OH radicals
610 m s-1
c‾HO2
Mean speed of HO2 radicals
440 m s-1
Dg,OH
Diffusion coefficient of OH radicals
0.21 cm2 s-1
Dg,HO2
Diffusion coefficient of HO2 radicals
0.25 cm2 s-1
Impact of variations in bulk diffusivity
The ability of differences in the bulk diffusivity alone, and thus in the
location of reactions, to explain the observed low- versus high-RH dependence
is considered first. We perform simulations where γ,
kRO2+RO2 and pfrag are all held constant while Dorg is varied
over a range constrained by the literature, from 10-16 to
10-11 cm2 s-1 by factors of 10 (Song
et al., 2015). (The γ, kRO2+RO2 and pfrag were all specified
to produce model results generally consistent with the observations.) For
these simulations it is assumed that the size of the particles is
independent of RH (discussed in Sect. 3.3.2). We
find that substantial differences in the extent of mass loss between
simulations with low Dorg (corresponding to low RH) and high
Dorg (corresponding to high RH), as was observed, can only be achieved if
the kRO2+RO2 is assumed to be <10-21 cm3 molecule-1 s-1, an unreasonably low value
(Fig. 9). When kRO2+RO2 is this low, the
RO2⚫+HO2 reaction (which leads to
functionalization) becomes competitive in the surface layer at low RH and at
high OH (HO2) exposure. This leads to a build-up for ROOH species in
the surface layer and limits the fragmentation pathway. At high RH, the
RO2⚫ rapidly diffuse away from the surface into the bulk, where
they primarily react with each other and can produce RO⚫ radicals that
can subsequently decompose, and the fragments can evaporate. This leads to a
suppression of fragmentation at low RH compared to high RH, especially at
higher OH (HO2) exposure. However, the kRO2+RO2 in this case is
orders of magnitude lower than reported in the literature for various
RO2⚫+RO2⚫ reactions (Denisov and
Afanas'ev, 2005).
Comparison between observations (points) and simulations (lines)
at varying Dorg values of the dependence of the volume fraction
remaining (VFR) versus OH exposure for SOA for low-RH (red triangles) and high-RH (blue
circles) conditions. The simulations assumed γ=0.6, pfrag=0.5, GF =1.0 and kRO2+RO2=3×10-22 cm3 molecule-1 s-1. The Dorg range from 10-16 to
10-11 cm2 s-1 is denoted by colored lines.
When a more reasonable value of kRO2+RO2 is used (1×10-15 cm3 molecule-1 s-1), only very small differences in the
calculated VFR versus OH exposure curves between the two RH conditions
(i.e.,
when Dorg is varied) are found no matter the values of γ and
pfrag, assuming these are RH-independent
(Fig. 10a). For these calculations, the γ
and pfrag are selected to provide good model–measurement agreement with
the VFR versus OH exposure curve for high-RH conditions. (A similar
negligible dependence of the VFR on Dorg is predicted when γ and
pfrag are selected to fit the low-RH observations.) It should be noted
that the particular γ and pfrag used here do not provide a
unique solution; various combinations give similar results. Thus, we
conclude that differences in Dorg alone cannot explain the differences
between the low- and high-RH experiments in terms of the overall mass loss.
Simulations also show that the overall reaction rate of RO2⚫+RO2⚫ in the particle is at least 3 orders of magnitude higher
than the rate of RO2⚫+HO2, regardless of the bulk
diffusivity or γ and pfrag used. Even if we assume that
[HO2] =10×[OH] instead of [HO2] = [OH], as suggested by
some oxidation flow reactor studies (R. Li et al., 2015; Peng et al., 2015),
loss of RO2⚫ by reaction with HO2 is still negligible
compared to loss by reaction with RO2⚫. This result suggests that
the uptake of HO2 and formation of hydroperoxides are not particularly
important during the heterogeneous oxidation of the SOA system considered
here. This agrees with a previous study by Lakey et al. (2016), who put
an upper limit of 0.001 on the uptake coefficient of HO2 by α-pinene SOA.
However, although the VFR versus OH exposure curves using different
Dorg values are very similar, there are substantial differences in terms of the
calculated compositional changes for the different Dorg values considered
(Fig. 10b). In particular, the fractional
contribution of model parent species in the oxidized particles depends on
Dorg, with larger contributions at smaller Dorg. For example, when
Dorg=1×10-15 cm2 s-1 (low RH), parent
compounds make up more than 90 % of the total molecules at highest OH
exposure (Fig. 10c). In contrast, when
Dorg=1×10-11 cm2 s-1 (high RH), oxidation
products make up nearly all of the particles at the highest OH exposure
(Fig. 10g). This is consistent with the
observation of more dramatic spectral changes occurring under high-RH conditions
when particles are expected to have low viscosity. The reason for this
results from differences in the rate of exchange between the bulk and
surface at the different RH values. At high RH, parent molecules exchange
rapidly between the surface and the bulk layers. Therefore, OH can access
all of the parent molecules in the particle, leading to rapid change of
particle composition. In contrast, at low RH the OH radicals cannot
effectively access parent molecules in below-surface layers due to the
high viscosity and low diffusivity. In this case, the surface becomes highly
oxidized while the bulk of the particle volume remains largely unoxidized,
corresponding to minimal change of the spectra with OH exposure under dry
conditions. Large differences in the overall particle composition between
low- and high-RH conditions do not require additional differences in either
γOH or the fragmentation probability, unlike for volume loss.
This is because of the compensating effects of reaction location and
oxidation extent of the products. Every time a stable species reacts with
OH, there is a probability of fragmentation. Therefore, the net probability
of fragmentation increases with higher generations of products, i.e., when
species are more oxidized. At low RH, reactions are limited to the surface
layer, leading to more highly oxidized products that have gone through more
generations of reaction, with the difference more evident at lower OH
exposures in particular (Fig. 10). While the net
probability of fragmentation is therefore increased for these
surface-layer molecules, they also inhibit reactions with the bulk of the
particle and will be, on average, more oxidized. Ultimately, at low RH the
volume loss proceeds through layer-by-layer oxidation and evaporation
of the surface. In contrast, at high RH OH radicals can react with the
entirety of the particle bulk as these molecules mix to the surface. The
average extent of oxidation at high RH is therefore comparably lower than
that in the surface-layer at low RH. However, a greater fraction of
molecules have some probability of fragmenting and evaporating. The net
effect is that the overall extent of volume loss is reasonably independent
of RH and the particle bulk diffusivity, although clearly the reason for the
volume loss (highly oxidized surface versus lightly oxidized bulk) depends
on RH.
Simulated effect of variations in the diffusion coefficient on
mass loss and compositional change. For these simulations, the Dorg is
allowed to vary from 10-11 to 10-15 cm2 s-1
while all other parameters are held constant and chosen to give good
agreement with the high-RH observations (γ=0.50, pfrag=0.31). (a) The simulated volume fraction remaining (VFR) versus OH exposure.
Observations (symbols) are shown for reference for low-RH (red triangles)
and high-RH (blue circles) conditions. The simulation results overlap
because diffusivity has no effect on bulk mass loss. (b) The calculated
coefficient of determination (R2) between the molecular density of all
simulated species as a function of OH exposure, referenced to the nonoxidized case. The Dorg is indicated by the line color, with the purple
solid line denoting the fastest diffusion. (c–g) Simulated fractional
concentrations of each species as a function of OH exposure for different
Dorg values. Colors indicate different species and generation (see legend).
Only the ROH species are readily visible.
The model results also indicate that there is a threshold Dorg above
which bulk compositional changes are large and below which they are small,
with the threshold Dorg∼10-13 cm2 s-1. Above or below this value the model results are relatively
insensitive to further changes in Dorg (e.g., similar results are
obtained for Dorg=10-15 and
10-14 cm2 s-1, or for Dorg=10-12 and 10-11 cm2 s-1). The particular
threshold is related to the mixing timescale of the particles. For a 125 nm
diameter particle (as used here), the mixing timescale when Dorg=10-13 cm2 s-1 is about 40 s, which is comparable
to the experimental timescale. Thus, above Dorg=10-13 cm2 s-1 the particles are effectively well-mixed and further
increases in Dorg have limited influence, and at lower Dorg the
particles do not mix.
Our observations and modeling indicate that the phase state affects the
distribution and oxidation level of the organic species in the particles
upon photochemical aging. Slade et al. (2017) oxidized Suwannee River
fulvic acid (SRFA) particles above and below the glass transition
temperature (Tg) up to OH exposures of 7×1011 molecule cm-3 s-1, an order of magnitude lower than our highest
value. Above Tg the Dorg is much larger than below the Tg, with
an approximate transition from highly viscous to liquid-like around 295–300 K.
As such, these T-dependent experiments are complementary to the
RH-dependent experiments here. While they did not measure the particle
composition, using the KM-GAP model (Shiraiwa et al., 2010, 2012) they concluded that oxidation at low temperatures (<295 K; semi-solid) engendered compositional changes primarily at the particle
surface, while at high temperatures (>300 K; liquid-like)
changes occurred throughout the particle, consistent with our results.
Consequently, for the low-T experiments they predicted that the extent of
oxidation of molecules in the near-surface layers, specifically, was much
greater than what occurred throughout the entire particle in the high-T
experiments, also consistent with our conclusions. Associated, we find that
the concentration of highly oxidized (third or higher generation) species
in the surface layer rapidly increases in the low-RH (low Dorg)
simulations (Fig. S3). In contrast, for the high-RH simulations the
concentration of highly oxidized molecules increases only slowly, but
continuously, and ultimately reaches a higher concentration than for the low-RH simulations (Fig. S3). However, we note that these highly oxidized
molecules in our low-RH experiments make up only a small fraction of the
total molecules comprising the particle (Fig. 10),
which is somewhat inconsistent with the conclusion of Slade et al. (2017) that large changes occur to the average molecular weight of the organic
species comprising the particle.
Impact of size change
Besides affecting particle phase (i.e., Dorg), uptake of water at high RH
also leads to an increase of particle size and dilution of organic
compounds. The growth factor (GF) of α-pinene SOA at RH equalling 90 %
is around 1.1 (Varutbangkul et al., 2006), corresponding to 21 % and
33 % increase in particle surface area and volume, respectively. The
impact of this size increase is examined here by assuming that GF =1.1 for
high-RH conditions and GF =1.0 for low-RH conditions, while all the other
parameters are assumed to be RH independent. Multiple simulations are run for
different combinations of γ, Dorg, kRO2+RO2 and pfrag
(Table 2). We find that there are negligible
differences between simulations that account for size growth (GF =1.1) and
those that do not (GF =1.0) in terms of both the volume loss and
compositional change of the particles for all combinations of the other
parameters (i.e., γ, Dorg, kRO2+RO2 and pfrag) explored.
Example results are shown in Fig. S4. The reason for the insensitivity to
GF is that the increase in the surface area is offset by the decrease in the
molecular density of organic compounds.
Influence of variations in OH uptake and condensed-phase
reactions
Given that variations in Dorg alone cannot simultaneously explain the
differences in observed mass loss and chemical changes at low versus high
RH, coupled with the insight that the difference in hygroscopic growth for
low and high RH has a negligible impact on the simulations, the impact of
varying either γ or pfrag with RH is considered. For all
calculations that follow it is assumed that Dorg=10-12 cm2 s-1
for high RH and Dorg=10-14 cm2 s-1 for low RH, based on the discussion in the
Sect. 3.3.1. It is also assumed that GF =1.1 for high RH
and that GF =1.0 for low-RH simulations. The model is successful in
predicting bulk behavior, i.e., mass loss of particles over the full range of
OH exposure (=0–7×1012 molecule cm-3 s), when
RH-specific values of Dorg and GF are used along with RH-specific
values of either γ or pfrag (Fig. 11a). For both conditions, kRO2+RO2=1×10-15 cm3 molecule-1 s-1
is used. For each condition, various
combinations of γ and pfrag yield the same VFR versus OH
exposure curves. As shown in Fig. 11b, γ
and pfrag have an inverse relationship, with increasing γ
accompanied by decreasing pfrag (to achieve the same fit to the
observations). It is also observed that slopes of linear fits to log(γ) versus log(pfrag) for high and low-RH conditions are identical,
although the absolute values of γ and pfrag differ. This
indicates that the relationship between either γdry and γwet or pfrag,dry and pfrag,wet required for good
model–measurement agreement is independent of the absolute γ and
pfrag. In other words, when pfrag is assumed to be RH-independent, the
γOH always needs to be nearly 5 times faster at high-RH
conditions to reproduce the observed greater mass loss compared to low RH.
Alternatively, when γ is assumed RH-independent, the combined
probability of fragmentation (pfrag) needs to be around 4 times
higher at high RH to explain the difference in VFR between the two conditions.
(a) Observed (points) and modeled (lines) VFR versus OH exposure
for the best-fit models for both low-RH (red) and high-RH (blue)
conditions. (b) Illustration of the relationship between γ and
pfrag that allow for a good fit to the observed VFR decay for low-RH
(red) and high-RH (blue) conditions. The dashed lines show the full
relationship, and the solid lines the best-estimate range. Four specific
combinations of γ and pfrag are indicated for consideration of
the associated composition change. (c–f) Simulated normalized composition
change as a function of OH exposure for the γ and pfrag
combinations indicated in panel (b). The colors correspond to different
species, with red indicating precursor (parent) species and other colors
indicating various oxidation products (cf. Fig. 10). Greater compositional changes for a given OH exposure result from
combinations with larger γ and smaller pfrag for a given RH
condition.
Although similar VFR versus OH exposure curves are obtained using various
(RH-specific) combinations of γ and pfrag, the predicted
variation in composition with OH exposure depends on the pair of γ
and pfrag used. At high RH when γ is small and pfrag is
large, the loss of parent molecules is relatively slow but most
reactions with OH lead to the fragmentation and evaporation of the products.
Consequently, there is little build-up of stable products in the
particles (Fig. 11c). In contrast, when γ
is large and pfrag is small the parent compounds react away faster but
with a much larger fraction of stable products forming. Consequently,
product species build up in the particles to a greater extent
(Fig. 11d). However, the net production rate of
fragments in these two cases are the same, with the decrease in reaction
rate offsetting the increase in fragmentation probability (or vice versa).
The covariation of γ and pfrag has a similar impact on the
compositional changes for low-RH conditions (Fig. 11e, f), although the overall compositional changes are much smaller due to
the high viscosity of the particles, as discussed above. Based on the above,
we conclude that for high-RH conditions the γ must be greater than
ca. 0.5 and pfrag must be less than ca. 0.3 to achieve both substantial
changes in the particle composition and substantial volatilization, as
observed. This range of γ, determined based on the overall bulk behavior, is
generally supported by a comparison of the modeled decay of the parent
compounds with the decay of the Group 1 ions (Fig. S5). If γwet is assumed to be ≤1, then pfrag,wet can be further
constrained to be >0.18. Given that γwet
∼5γdry or pfrag,wet∼4pfrag,dry, as established above, the parameters at low RH are
constrained to be 1.0>γdry>0.1 and
0.3>pfrag,dry>0.04.
Comparison with single-component studies
OH uptake
The results from our SOA experiments and model simulations can be compared
with OH oxidation experiments performed on various single-component
systems. Interestingly, some of these studies find that increasing RH can
enhance the effective uptake coefficient, while others find that increasing
RH reduces the effective uptake coefficient. The uptake coefficient can be
calculated in two ways: (i) by measuring the loss rate of reacting
particle-phase species, or (ii) by measuring the loss rate of
gas-phase OH radicals. The first method may include loss due to secondary
reactions in the condensed phase, in addition to direct loss from reaction
with OH, and is generally referred to as the effective OH uptake
coefficient, γOH,eff. The latter method characterizes loss of
OH only and is referred to here as the OH uptake coefficient, γOH.
Some studies have observed a general increase in γOH or γOH,eff with RH when going from very dry conditions to higher RH (40–70 %).
For example, Davies and Wilson (2015) reported that
γOH,eff for citric acid particles increases by a factor of 3
when RH is increased from 20 % to 50 %, although there is a slight
decrease in γOH,eff (by 30 %) as RH is increased further to
90 % RH. Chim et al. (2017b) similarly observed a
continuous increase in γOH,eff from RH equalling 30 % to 70 %
(from 1.9 to 2.6) and a small decrease at higher RH (to 2.4) for OH
oxidation of particles composed of 2-methylglutaric acid (2-MGA). The increase in
γOH,eff with RH at the lower-RH range was explained by a
decrease in viscosity and faster mixing. This allows OH radicals to react
directly more often with the parent species rather than producing highly
oxidized molecules at the surface. The decrease of γOH,eff at
the higher RH range was explained, in part, by a decrease in the relative
concentration of parent compounds due to dilution by water. Nevertheless,
both studies indicate γOH,eff is generally larger at high RH
(∼90 %) than at low RH (∼30 %).
Slade and Knopf (2014) determined that γOH (as opposed
to γOH,eff) for levoglucosan particles increased with RH over
the range 0–40 %, from approximately 0.2 to 0.7. While the exact reason
for this increase was not identified – although it was suggested to result from
differences in phase state and mixing timescale – if the reaction products
formed are less reactive towards OH than the levoglucosan, this could
explain the increase in γOH with RH because the products would
build up to a lesser extent at the surface when particle mixing is faster.
Alternatively, it may be that the reaction of OH with levoglucosan molecules
at the particle surface depends on the orientation of the levoglucosan
molecules. At very low RH the levoglucosan molecules may be fixed in
unfavorable orientations (on average), and as RH is increased the
levoglucosan may be able to adopt more favorable orientations. Finally, it
may be that the sticking probability of OH radicals increases with RH.
Regardless of the exact reason, the above single-component studies are
consistent with our determination that kSOA+OH for the Group 1 (parent)
ions is greater at high RH.
However, some single-component studies have reported a negative effect of
RH on the uptake coefficient or reaction rate constant, with increasing RH
leading to smaller γ or k. Slade and Knopf (2014) observed a
decrease in γOH from 0.2 to 0.05 for methyl-nitrocatechol
(MNC) particles when RH was increased from 0 % to 30 %. It was argued
that because MNC has a low solubility in water, the decrease was likely a
result of the competition for adsorption between OH radicals and water at
the surface; this differs from levoglucosan, which is moderately soluble.
The SOA here is moderately soluble, due to the highly oxygenated nature of
the organic compounds, and thus most comparable to the levoglucosan
experiments. Lai et al. (2014) reported a strong, negative effect
of RH on the loss rate of levoglucosan after exposure to OH, in direct
contrast to Slade and Knopf (2014). Lai et al. (2015) separately reported that the loss rate of cis-pinonic acid
decreased, by a small amount, as RH increased from 20 to 80 %. However,
there is an important distinction between the Lai et al. (2014, 2015)
and Slade and Knopf (2014) experiments. Whereas Slade and
Knopf (2014) used suspended particles, Lai et al. (2014, 2015) exposed
thin films (<1 nm thickness) of the parent organic species to OH.
Because the film was so thin the impact of diffusive exchange of molecules
was effectively eliminated, and the increase in RH served only to dilute the
levoglucosan and decrease the loss rate. These results suggest that the
experimental method used (e.g., suspended particles versus thin films) can
strongly impact the apparent influence of RH on heterogeneous oxidation
processes. While experiments using suspended particles are more directly
relevant to the atmosphere, the combination of thin film and suspended
particle experiments can help to isolate the influence of mixing on reactive
uptake and loss.
Reaction pathways
The influence of RH on the fate of the parent species from heterogeneous
oxidation has also been considered by several studies. Following from faster
loss of the parent species, Chan et al. (2014) observed faster
formation of both functionalization and fragmentation products for reaction
of succinic acid in aqueous droplets compared to solid aerosol, i.e., at high
versus low RH. As a result, they observed a more dramatic change in the mass
spectrum after OH exposure under high-RH conditions, consistent with our
observation. However, the ions formed after oxidation were identical for the
aqueous and solid succinic acid particles. That is, while the product
species form faster at high RH, there was no indication that this had a
major influence on the reaction pathways. Chim et al. (2017b) and Davies and Wilson (2015) similarly observed that, even
though the reaction rates are different, the oxidation products formed are
independent of RH. Furthermore, Chim et al. (2017b) observed
that for 2-MGA the relative abundances of several important
functionalization and fragmentation ions were independent of RH when
considered at the same 2-MGA lifetime (which accounts for RH variations),
indicating similar chemical pathways of oxidation regardless of RH.
In a chemical system containing carboxylic acids that are hydrophilic in
nature, acid-base chemistry in the particles may need consideration in
addition to free-radical chemistry when an aqueous phase is present.
Liu et al. (2017) developed a model that couples both
acid-base and free-radical chemistry in the oxidation of aqueous
citric acid aerosol by OH radicals. They compared their simulation results
to the observations of Davies and Wilson (2015). The inclusion of
acid-base chemistry in the simulations did not alter the decay rate of
citric acid nor the variation in hydrogen-to-carbon and oxygen-to-carbon ratios as a function of OH exposure.
However, a significant increase in the abundance of fragmentation products
were predicted only when acid-base chemistry is considered. One key
reason is the enhanced unimolecular fragmentation rate of alkoxy radical
anions compared to the neutral form. Hydration in the aqueous phase can also
enable the decarboxylation of certain acyloxy radicals. Consistent with the
simulations, Davies and Wilson (2015) reported that the abundance
of Nc=6 product species (the same carbon number as citric acid) was
smaller while the abundance of Nc=3–5 product species (fragments
or decarboxylation compounds) was larger at RH equalling 65 % (corresponding
liquid phase) compared to RH equalling 20 %, although the differences were not
significant. It is possible that acid-base chemistry, which we do not
include in our simulations, contributes to the difference in oxidation
between the low- and high-RH SOA experiments here. Considering that
acids can comprise a substantial fraction of α-pinene SOA, it is
possible that the accelerated decomposition of alkoxy radicals in their anion
form contributes to greater fragmentation under high-RH conditions in the
chemically complex system studied here. However, citric acid is
substantially more hygroscopic than α-pinene SOA. The growth
factor for citric acid at RH equalling 20 % is similar to that of α-pinene SOA at RH equalling 90 %. As such, it is unclear to what extent
acid-base chemistry might be playing a role in the oxidation of the
α-pinene SOA at RH equalling 90 % and it may be negligible.
OH radicals can also be produced in the condensed phase through
decomposition of organic peroxides (ROOH). ROOH molecules can contribute as
much as 50 % to the total mass of α-pinene+O3 SOA
(Docherty et al., 2005) but are unstable and with some
observed to have half-lives of less than an hour with respect to decomposition at room temperature (Krapf et al., 2016). The
decomposition products of ROOH – RO and OH radicals – both facilitate the
formation of small molecular weight products that lead to volatilization.
Although this chemical pathway is unlikely to contribute substantially to
our experiments given the short timescale (38 s), it may influence the rate
of aerosol mass loss under ambient conditions and longer timescales,
especially under high-RH conditions where an appreciable amount of liquid water
in the particles accelerates the decomposition of ROOH
(Tong et al., 2016).
Regardless, RH-dependent differences in pfrag, and thus the
oxidation products formed, can explain the difference in VFR and in the
particle composition for SOA considered here based on our simulation
results. Differences in viscosity of the particles have the potential to
influence the reaction rate and product branching ratio, and the importance of
this may differ in the more chemically complex SOA compared to the
single-component experiments. Comparison with the single-component
experiments suggests that the key difference between the low- and high-RH SOA
experiments is more likely from the RH-dependence of γOH,
although the role of variations in oxidation pathways (pfrag) cannot be
completely ruled out.