Introduction
The extent to which aerosol–cloud interactions impact the atmospheric
radiative budget and climate change is significant, but remains highly
uncertain . Attributed to this uncertainty is the difficulty
in quantifying the effects of chemical aging during atmospheric particle
transport by heterogeneous or multiphase chemical reactions between organic
aerosol particles and trace gas-phase oxidants and radicals
. Heterogeneous oxidation reactions between organic
aerosol particles and OH, O3, or NO3 can impact the
particles' physical and chemical properties , and have been shown
to impact particle hygroscopicity and cloud condensation nuclei (CCN)
activity and ice nucleation (IN) .
Cloud nucleation efficiency depends on the particle's water solubility,
hygroscopicity, size, and morphology . The majority of submicron aerosol particles are
comprised of organic material , which possess
a wide range of hygroscopicity (κ∼ 0.01–0.5) .
A significant portion of atmospheric organic aerosol (OA) is derived from
biomass burning (BB) emissions . BB plays an important role both regionally and globally
, accounting for an estimated 2.5 PgCyr-1
. Reflectance data from satellite retrievals indicate
that BB accounts for a global footprint of 464 Mhayr-1 or
roughly ∼ 36 % of cropland on earth . Biomass
burning aerosol (BBA) constitutes a significant fraction of primary organic
aerosol (POA) and secondary organic aerosol (SOA), derived
from oxidative aging of volatile and semi-volatile organic vapors emitted
from biomass burning plumes .
Molecular markers of BB POA include pyrolyzed forms of glucose such as
levoglucosan (LEV, 1-6-anydro-β-glucopyranose) and
potassium-containing salts such as potassium sulfate (KS, K2SO4)
. The photo-oxidation of m-cresol, which is
emitted at high levels from biomass burning , in the
presence of NOx, generates 4-methyl-5-nitrocatechol (MNC), which has
recently been recognized as a potentially important tracer for biomass
burning SOA . With the exception of MNC, the CCN activity
and hygroscopicity of LEV and KS, among other select BBA compounds and smoke
particles, have been determined .
derived κ values of 0.2 for the water-soluble
organic content (WSOC) in particles produced from controlled laboratory
burns. determined a mean κ of 0.1 for carbonaceous
particles sampled from open combustion of several biomass fuels. Hygroscopic
growth factors of LEV and other biomass burning derived organics range from
1.27 to 1.29 at relative humidity RH = 90 % . In situ field measurements of the CCN efficiency (ratio of
CCN to the available condensation nuclei, CN) of biomass burning smoke
particles is on the order of 50 % at 1 % supersaturation
. While inorganic ions have only a minor importance as an
atmospheric tracer for biomass burning, they can significantly influence the
CCN activity of BBA, even if their fractions are significantly less than the
organic fraction .
Heterogeneous OH oxidation of organic aerosol can initiate reactions that
result in the production of oxidized polar functional groups that can reduce
the particle's surface tension and increase water
solubility , enabling greater water uptake and CCN activity.
For example, demonstrated that unsaturated fatty acid
aerosol particles comprised of oleic acid became more CCN active in the
presence of high exposures to O3. In a follow-up study,
corroborated this finding, attributing the enhancement
in CCN activity to a combination of an increase in water-soluble material and
a decrease in surface tension of the aqueous droplet during activation.
demonstrated that the CCN activity of model saturated and
unsaturated OA compounds is enhanced following oxidation by OH and
NO3. showed that the hygroscopicity of model OA,
bis-ethyl-sebacate (BES) and stearic acid was enhanced following oxidative
aging by OH radicals, which was attributed to the formation of highly
water-soluble oxygenated functional groups. The hygroscopicity of OH-impacted
ambient biogenic SOA was shown to increase at higher OH exposures as a result
of an increasing oxygen-to-carbon (O : C) ratio .
In an effort to better understand the influence of chemical aging on the CCN
activity of BBA, recent studies have investigated the influence of oxidative
aging on particle hygroscopicity of either particles generated in the
laboratory from a specific emission source or particles collected in the field , which may include multiple emission sources. While
field-collected particle studies of hygroscopic growth and cloud formation
are advantageous because they capture the chemical and physical complexity of
ambient aerosol, they lack the specificity and control of laboratory studies
in order to fully understand the fundamental physico-chemical processes that
govern cloud formation. investigated the impact of
photo-oxidation on the hygroscopicity of wood burning particles and found
that after several hours of aging in a smog chamber there was a general
enhancement in κ; however, this was attributed to both condensation of
oxidized organic or inorganic matter and oxidation of the particulate matter
itself. However, the effects of OH-initiated oxidation on the hygroscopicity
of BBA particles have not been examined systematically. In this work, we
investigate the effects of heterogeneous OH oxidation of laboratory-generated
BBA surrogate particles on the particles' hygroscopicity. Here, κ is
evaluated for several pure-component and multicomponent aerosol particles
containing both sparingly soluble and highly water-soluble compounds,
representing the range and complexity of atmospheric aerosol in regards to
hygroscopicity and chemical composition. κ is evaluated as a function
of OH exposure (i.e., [OH] × time) and O3 exposure using
a custom-built aerosol flow reactor (AFR) coupled to a CCNc. The chemical
aging effects on the CCN activity of internally mixed and organic-coated
inorganic particles are presented.
Materials and methods
Aerosol generation, flow conditions, and measurement
Surrogate polydisperse BBA particles were generated by atomizing 1 wt %
aqueous solutions of single-component particles LEV, MNC, KS, and particle
mixtures of LEV : MNC : KS in 1:1:0, 0:1:1, 1:0:1, 1:1:1, and
1:0.03:0.3 mass ratios in a flow of ultra-high purity (UHP) N2
using a commercial atomizer (TSI Inc. model 3076). To simulate the
partitioning of MNC from the gas phase to the particulate phase, first
reagent MNC was heated (up to ∼ 70 ∘C) and volatilized, and
then condensed onto KS seed particles. Growth of the KS seed particles by MNC
condensation was achieved by gradually cooling the mixed
MNC–KS flow downstream of the heating section before
entering the flow reactor. The atomized particles were dried by passing the
atomized flow through two diffusion dryers prior to entering the AFR. After
exiting the AFR, the particles were subsequently dried in two additional
diffusion dryers, where the overall sample flow RH ≤5 %, before
the size analysis and CCN activity measurements. This second drying stage was
included in the experimental setup because the derivation of κ
requires knowledge of dry particle size. The dry particle size distribution
was determined with a differential mobility analyzer (DMA, TSI Inc. model
3081) and a condensation particle counter (CPC, TSI Inc. model 3772), and
sampled at a total flow rate of 1.3 standard liters per minute
(standard L min-1). Number-weighted mean
particle diameters, D‾p, for all of the particles
investigated in this study ranged from ∼ 40 to 150 nm.
Schematic illustration of the experimental setup to examine the
effect of OH and O3 oxidation on the CCN activity of single-component
and multicomponent biomass burning aerosol surrogate particles. From top left
to bottom right: aerosol generation and drying stage, O3 production
and humidification (mixing vessel), the aerosol flow reactor, O3-free
ultraviolet lamp and O3 monitor, relative humidity probe (RH sensor),
O3 denuder, second drying stages, aerosol sizing by the DMA and
particle counting by the CPC, and determination of the CCN activity by the
CCNc.
OH generation, flow conditions, and measurement
OH radicals were generated via O3 photolysis in the presence of water
vapor in a 60 cm long and 5 cm inner diameter
(i.d.) temperature-controlled Pyrex flow reactor as shown in
Fig. .
O3 was produced by flowing 2–25 sccm (standard cubic centimeters
per minute) of UHP O2 through an O3-producing lamp (Jelight
model 600; emission wavelength λ=185 nm). O3
concentrations ranged from 250 ppb to 20 ppm and were
monitored throughout the experiment using an O3 photometric analyzer
(2B Technologies model 202), which sampled at ∼ 850 sccm. An
O3 denuder containing a Carulite 200 catalyst was connected to the
outlet of the AFR to convert O3 to O2 before entering the
aerosol charge neutralizer and other sensitive instrumentation.
A 50–600 sccm flow of UHP N2 was bubbled in a 500 mL
Erlenmeyer flask filled with distilled/deionized Millipore water
(resistivity >18.2 MΩ cm) to generate humidified
conditions in the AFR. The RH for all of the experiments was measured with an
RH probe (Vaisala model HM70) and varied from 30 to 45 %. The humidified
and O3 flows were mixed in a 4.5 L glass vessel before
entering with the particles into the AFR. The mixed
N2/O2/O3/H2O and particle flow was then
passed over a 60 cmO3-free quartz tube containing a 60 cm
long mercury pen-ray lamp (λ>220 nm) to photolyze
O3. The lamp was cooled with a flow of compressed air. Total flow
rates in the flow reactor ranged from ∼ 2.2 to
3 standard L min-1, corresponding to a range in residence times of
26–39 s. Flows were laminar, with Reynolds numbers between 60 and
80. OH concentrations were determined by applying a photochemical box model
validated based on isoprene loss measurements in the presence of OH as
described previously . OH concentrations ranged
from ∼ 0.2×1010 to 2×1010 moleculecm-3 and were varied by changing either RH or
[O3]. As previous studies have indicated, neither UV light nor
O3 introduction in this manner leads to particle degradation or
a significant change in particle mass or chemistry . The temperature inside the flow reactor was
maintained near 298 K by a cooling jacket. A slight temperature
gradient of ∼ 3 ∘C from the leading edge of the sheath flow
tube containing the lamp to the inner walls of the AFR was observed, but has
no significant effect on [OH]. OH equivalent atmospheric exposures were
determined from the product of the residence time in the AFR and applied
[OH], which was then normalized to a daily averaged ambient
[OH]=2×106 moleculecm-3. Using this method
allowed varying atmospheric OH exposures equivalent to <1 day up to
∼ 1 week. At the given [OH], residence time, total pressure of
1 atm, and particle sizes, we assume that OH mass transfer to the
particles is sufficiently fast to maximize the exposure. At 40 % RH, the
reactive uptake coefficient, γ, of LEV+OH would be 0.65 for
atmospheric OH concentrations . However, the presence of
higher [OH] in the AFR decreases γ to ∼ 0.2 .
OH diffusion impacts γ by only ∼ 7 % ,
implying that OH exposure is not diffusion limited. At RH >15 %, MNC
is less reactive with OH, exhibiting γ<0.07 due to competitive
co-adsorption of water and OH . Similar suppressions in gas
uptake, due to competitive adsorption processes, have been observed in the
case of OH and NO3 uptake by BBA surrogate films , O3 uptake by benzo[a]pyrene in the
presence of water vapor , and O3 and NO2
uptake by benzo[a]pyrene-coated soot . The presence of higher
[O3] may further decrease the OH reactivity of OA
. Under the applied experimental conditions, the
multiphase reaction kinetics involving highly viscous organic material are
likely limited by surface–bulk exchange .
CCN measurements
The CCNc and operating conditions are described in more detail in
. CCN activity data were acquired following procedures
similar to previous studies , whereby the dry
particle diameter is scanned while keeping the CCN chamber supersaturation
fixed. A more detailed description of this approach is given in
. Briefly, particles first passed through a Kr-85 aerosol
neutralizer (TSI 3077A), were size-selected using a DMA (TSI 3081), and
processed in a CCNc (Droplet Measurement Technologies, Inc., single-column
CCNc) , while in tandem the total
particle concentration was measured with a CPC. The CCNc was operated at a
0.3 standard L min-1 total flow rate and 10:1 sheath-to-sample flow
rate ratio. The total sample flow rate, which includes
a 1 standard L min-1 CPC flow rate, was 1.3 standard L min-1,
and a 10:1.3 sheath-to-sample flow rate ratio was applied for the DMA. The
temperature gradient in the CCNc column was set by custom-programmed Labview
software and operated at ΔT=6.5, 8, 10, and 12 K,
corresponding to chamber supersaturations S=0.2, 0.27, 0.35, and
0.425 %, based on routine calibrations applying atomized ammonium sulfate
particles. The temperature gradient was stepped successively, from 6.5 to
12 K and in reverse. Each temperature gradient was maintained for
a total of 14 min to allow an up and down scan of the particle size
distribution by the DMA. The aerosol size distributions and size-resolved CCN
concentrations were acquired by applying an inversion method described in
, which implicitly accounts for multiply charged
particles. The ratio of the aerosol size distribution and CCN size
distribution provided size-resolved CCN activated fractions (i.e., the
fraction of particles that become CCN at a given supersaturation and particle
size).
Hygroscopicity and CCN activity determination
The hygroscopicity and CCN activity can be described by κ-Köhler
theory (KT) , which relates dry and wet particle diameter to
the particle's critical supersaturation (RH above 100 %, at which the
particle grows to a cloud droplet size) based on a single hygroscopicity
parameter, κ. In κ-Köhler theory, the water vapor
saturation ratio over an aqueous solution droplet as a function of droplet
diameter, S(D), is given by
S(D)=D3-Dd3D3-Dd3(1-κ)exp4σMwRTρwD,
where D is wet particle diameter, Dd is dry particle
diameter, σ is droplet surface tension, Mw is the
molecular weight of water, R is the universal gas constant,
T is temperature, and ρw is density of water.
κ ranges typically from ∼ 0.5 to 1.4 for hygroscopic inorganic
species and from ∼ 0.01 to 0.5 for less hygroscopic organic species;
κ=0 represents an insoluble but wettable particle, and thus Eq. (1)
reduces to the Kelvin equation .
An alternative, approximate expression for determining κ is
given as follows :
κ=4A327Dd3ln2Sc,
where
A=4σs/aMwRTρw.
Sc represents the critical supersaturation, i.e., point of
supersaturation where more than 50 % of the initial dry particles are
activated to CCN. Here, we assume σs/a is
equivalent to that of water. While aqueous solutions of LEV and KS exhibit
surface tensions approximately equal to the surface tension of water
, to our knowledge no previous surface
tension measurements of MNC aqueous solutions have been made. Our assumption
applying the surface tension of water at all OH exposures could result in an
overestimation of κ since the presence of surface-active organics can
decrease σs/a . We do not have surface tension data of the
different mixtures applied in this study. However, we anticipate that
increasing OH exposure may decrease σs/a, thus
enhancing the particle's CCN activity as demonstrated in
and .
Hygroscopic growth of compounds exhibiting moderate to weak solubility in
water can be limited by their low water solubility , and
thus cannot be treated as either fully dissolvable or insoluble substances.
A theoretical treatment of κ, which includes solubility limitations,
has been detailed in . Here,
κ=εiκiH(xi)H(xi)=1if xi≥1xiif xi<1,
where ε is the volume fraction of the solute i in the dry
particle. κi is the theoretical κ of solute i in
the absence of solubility limitations and is given by
κi=νρimwρwmi,
where ν is the Van't Hoff factor, ρi is the density of the
solute, ρw is the density of water, mi is the molar
mass of the solute, and mw is the molar mass of water. xi
is defined as the dissolved volume fraction of the solute
and given as
xi=CiVwVi,
where Ci is the water solubility of the solute, expressed as the
solute volume per unit water volume at equilibrium with saturation,
and Vi is the volume of the solute. For complete dissociation,
xi is equal to unity. The parameters listed in Table 1 were used
in predicting κ.
Results and discussion
CCN activity of BBA surrogate particles
Exemplary activated fractions, i.e., fractions of initial dry particle sizes
activated to CCN, for LEV, MNC, KS, and the ternary particle mixtures at
a chamber supersaturation of 0.425 %, are shown in Fig. 2. The activated
fraction curves were fit to a cumulative Gaussian distribution function as
described in detail previously :
f(x)=12erfcx2,
where x=(Dd–Dd,50)/σD. In the fitting
procedure, Dd is the dependent variable and Dd,50
and σD are adjustable parameters to minimize the root mean square
error between f(x) and the data. Dd,50 is the dry diameter
interpreted as being where 50 % of the dry particles have activated into
cloud droplets, also referred to as the critical particle diameter,
Dp,c.
Chemical properties of the different particle types investigated in
this study and the parameters used in predicting κ.
Molecule
Structure
M (gmol-1)
ρ (gcm-3)
Solubility (gL-1)
Ci
ν
Levoglucosan
162.14
1.69
1000
0.592
1
4-methyl-5-nitrocatechol
169.13
1.5
4.8a
0.003
1
K2SO4
174.26
2.66
11
0.042
2b
a Estimated using the US Environmental Protection
Agency's Estimation Program Interface (EPI) suite .
b Taken from the reported Van't Hoff factor in
for (NH4)2SO4 assuming a solution droplet molality of approximately
0.2.
KS particles exhibit the smallest particle activation diameter of
∼ 50 nm, followed by LEV particles at ∼ 75 nm,
and MNC particles at ∼ 210 nm at S=0.425 %. In this
study, κ is derived from Eq. (2), where S is evaluated at
0.2, 0.27, 0.35, and 0.425 % is used in place of Sc, and
Dd is the determined Dp,c. At lower S, the
activated fraction curves are shifted to larger sizes since the smaller
particles do not activate at lower S.
Table 2 lists the derived κ values for all of the particle types
employed in this study in comparison to literature values. The reported
uncertainties in κ are ±1σ from the mean κ derived at
each S. The derived κ values for LEV and KS are consistent
with κ for LEV and KS given in the literature. The critical diameter
of LEV (∼ 70 nm at S=0.425 %) is in good agreement with
the critical diameter of LEV measured by at the same
S. κ ranges from 0.149 (±0.008) to 0.176 (±0.009)
for LEV over all S, in agreement with the humidified tandem DMA
(HT-DMA) derived κ=0.165 . Within experimental
uncertainty, κ for KS is in agreement with the value derived in
, but exhibits a marginal increase as a function of
S, possibly due to an increasing van't Hoff factor at higher
S, similar to ammonium sulfate. To our knowledge, no previous
hygroscopicity measurements of MNC have been made. κ ranges from
0.008 (±0.002) to 0.013 (±0.003) for MNC. However, κ could
not be derived for MNC at S = 0.2%, given the applied aerosol
size distribution. In other words, the Dp,c is likely too large,
and the scanned particle size range is not sufficient to give the complete
activation curve. As a result, the Dp,c could not be reliably
derived. For comparison, humic-like substances (HULIS), which is known to
contain nitrocatechols , exhibits a κ value of 0.05
. In addition, κ for NO3 oxidized oleic
acid particles, comprising similar chemical functionalities as MNC
(i.e., nitrogen oxides and conjugated double bonds), is ∼ 0.01
.
Activated fractions, i.e., fractions of the number of particles at
a given particle size activated to CCN as a function of the initial dry
particle diameter, for LEV (green), MNC (orange), KS (blue), 1:1:1 (red)
and 1:0.03:0.3 (black) particles at S=0.425 %. The dotted lines
correspond to the fits applying Eq. (8).
On average, κ for all of the binary and ternary mixed particles range
from 0.111 (±0.010) to 0.373 (±0.034). Due to constraints in water
uptake and water solubility, mixed particles comprising a significant
fraction of MNC (i.e., 1:1:0, 0:1:1, 1:1:1) exhibit relatively lower
κ than the 1:0:1 mixture. The 1:0.03:0.3 ternary-component
particles exhibit a slightly lower κ compared to the other particle
mixtures, due to the relatively low KS content. It is not entirely clear why
the particles exhibit lower κ at S=0.2 %. This implies that
either the supersaturation used to derive κ was artificially high, the
van't Hoff factor is lower, or limitations in solubility are more pronounced
at this lower supersaturation than at higher supersaturations. The
calibration data applying ammonium sulfate particles yield a variability in
the derived supersaturation at S=0.2 % of ∼ 5 %, which still
results in less variability in the derived κ values at that
supersaturation compared to what was observed.
Tabulated experimentally derived hygroscopicity parameters,
κ, for the various particle types investigated in this study before
oxidation.
Compound
κa
κb
κc
0.2 %
0.27 %
0.35 %
0.425 %
LEV
0.149 (±0.008)
0.175 (±0.010)
0.172 (±0.009)
0.176 (±0.009)
0.188
0.165d
0.208 (±0.015)e
KS
0.525 (±0.052)
0.575 (±0.026)
0.563 (±0.024)
0.538 (±0.074)
0.55
0.52d
MNC
N/A
0.013 (±0.003)
0.012 (±0.005)
0.008 (±0.002)
0.16
LEV : MNC : KS
κa
κb
Mass ratio
0.2 %
0.27 %
0.35 %
0.425 %
1:1:0
0.114 (±0.010)
0.143 (±0.016)
0.131 (±0.005)
0.137 (±0.009)
0.173
1:0:1
0.310 (±0.047)
0.360 (±0.031)
0.373 (±0.029)
0.373 (±0.034)
0.329
0:1:1
0.239 (±0.030)
0.336 (±0.068)
0.331 (±0.014)
0.322 (±0.015)
0.300
1:1:1
0.216 (±0.029)
0.255 (±0.012)
0.270 (±0.013)
0.268 (±0.013)
0.256
1:0.03:0.3
0.209 (±0.010)
0.233 (±0.008)
0.232 (±0.005)
0.234 (±0.022)
0.241
a This study. Reported uncertainties are 1σ from the mean in the derived κ.b Predicted values applying the volume mixing rule without solubility limitations. c Literature-reported values. d .e .
As listed in Table 2, the derived κ values are reasonably predicted by
applying the volume mixing rule :
κ=κOrg×εOrg+κInorg(1-εOrg),
where κOrg and κInorg are the κ
values of the organic and inorganic particles, respectively, and
εOrg is the organic volume fraction of the particles.
However, the quality of the estimate depends on whether the effects of
solubility are to be included. For example, applying the experimentally
derived κ of MNC particles in the volume mixing rule results in
a significant underprediction of κ for the 1:1:0, 0:1:1, and
1:1:1 particle mixtures. This deviation in κ suggests that water
uptake by the pure-component MNC particles is mechanistically different than
water uptake by the mixed particles, which contain a significant MNC volume
fraction. The volume mixing rule is applicable over a range of mixtures and
hygroscopicity. However, when the particles contain both soluble and
sparingly soluble compounds, predicted κ can deviate significantly
from derived κ . MNC is significantly less
water-soluble than pure LEV and KS. During CCN activation, the most
water-soluble component preferentially dissolves, enhancing the solute effect
in the Köhler equation. Since water-soluble components in aerosol
particles can retain greater liquid water content during water uptake, the
less water-soluble component can more easily dissolve . This solubility constraint in the
volume mixing rule is described in more detail in . As
a result and depending on the volume fraction of the sparingly soluble
compounds in the particle, the peak of the Köhler curve may occur at
a sufficiently large droplet size when all compounds, including the sparingly
soluble compounds, are completely dissolved. The same can be applied here for
the mixtures containing an appreciable MNC volume fraction. The water
solubility of MNC is approximated as C=0.003, which is categorized as
sparingly water-soluble . To verify whether MNC behaves as
if it is infinitely soluble in a solution with KS, Fig.
shows derived Köhler curves of pure MNC and mixtures containing variable
KS volume fractions and the MNC dissolved fraction, xMNC. In
Fig. , the critical supersaturation, i.e., the maximum in
the Köhler curve, decreases with increasing KS volume fraction.
Accordingly, the MNC dissolved fraction increases with an increasing KS
volume fraction. At a KS volume fraction of ∼ 36 % (MNC volume
fraction of ∼ 64 %) indicated by the orange curves in
Fig. , the maximum in the Köhler curve corresponds to
xMNC≈1, implying that CCN activation is not limited by MNC
solubility. This MNC volume fraction corresponds to the 1:1 by mass
MNC : KS particles, which suggests that for this particular mixture, MNC
behaves as if there are no solubility limitations during CCN activation
(i.e., infinitely soluble, equivalent to C=∞) and κ of
MNC can be predicted using Eq. (6). This result is consistent with the
1:1:1 by mass LEV : MNC : KS particles. In the presence of LEV, alone,
MNC remains slightly insoluble during CCN activation.
Example Köhler curves (solid lines) calculated from
Eq. (1) for pure MNC (black), MNC mixed with 5 % (blue),
15 % (green), and 36 % (orange) by volume KS. The dotted
lines are the dissolved fractions of MNC, xMNC,
calculated from Eq. (7), corresponding to the different Köhler
curves. The vertical dashed lines indicate the maxima of the
different Köhler curves. The dry diameter applied is 110 nm.
Figure shows the predicted κ plotted against
experimentally derived κ for all of the particle mixtures and
S applied in this study. κ was predicted by applying the
volume mixing rule under three different scenarios: (1) calculated from
experimentally derived single-component κ (κ = limit;
color-scale symbols), (2) calculated from Köhler theory (κ= KT;
gray-scale symbols), and (3) predicted from Eq. (6), which assumes no
solubility limitations (i.e., C=∞; horizontal lines). For all particle
mixtures containing equal by mass MNC (i.e., 1:1:0, 0:1:1, and 1:1:1),
applying the experimentally derived single-component κ underpredicts
the experimentally derived κ of the mixtures. Note that when assuming
solubility limitations, κ could not be predicted at S=0.2 % for
the mixtures containing MNC since κ for MNC, alone, could not be
derived experimentally at that supersaturation. As discussed previously, this
underprediction is due to the enhancement in MNC water solubility when in the
presence of water-soluble LEV and KS, which is not accounted for applying the
experimentally derived single-component κ. Applying Köhler theory
results in better agreement with the experimentally derived κ,
particularly for the 1:1:0 particle mixture. Predicted κ assuming no
solubility limitations results in an overprediction for the 1:1:0 particle
mixture, but is in best agreement with the experimentally derived κ
for all other mixtures. The ability to predict experimentally derived
κ of the mixtures applying Köhler theory depends on the solubility
and volume fractions of the different particle components, which have not all
been measured. The solubility of MNC was estimated from the US Environmental
Protection Agency's Estimation Program Interface (EPI) suite .
Applying this estimated solubility results in an underprediction in the
0:1:1 and 1:1:1 κ, but is in best agreement for the 1:1:0
particle mixture. These results support the finding that the volume mixing
rule is most accurate when accounting for the changes to water solubility
when the components are mixed. In the presence of LEV, MNC remains slightly
insoluble. However, in the presence of KS, MNC behaves as if there are no
solubility limitations during CCN activation.
Predicted κ as a function of experimentally derived κ
at different supersaturation (S) for the binary and ternary particle
mixtures of LEV, MNC, and KS. κ is predicted by applying the volume
mixing rule and based on single-component experimentally derived κ at
each S including solubility limitations (κ= limit; color
scale), κ calculated from Köhler theory (κ= KT; gray
scale), and κ assuming no solubility limitations (horizontal lines).
Note that the horizontal lines span the range of experimentally derived
κ. The black diagonal line represents a slope of 1 in the derived
vs. predicted κ. The LEV : MNC : KS mass ratios are indicated in
the legend for 1:1:0 (circle), 0:1:1 (square), 1:0:1 (triangle),
1:1:1 (star), and 1:0.03:0.3 (diamond).
CCN activity of single-component BBA surrogate particles exposed to OH
Surrogate single-component BBA particles were oxidized in the presence of
O3 (mixing ratio, χO3 = 0.76–20 ppm) and
in the presence of OH radicals
(0.2×1010–2×1010 moleculecm-3),
corresponding to <1 day up to ∼ 1 week of a 12 h daytime OH
exposure at [OH]=2×106 moleculecm-3. κ
as a function of O3 exposure is presented in the Supplement. Upon
exposure to OH, both LEV and MNC particles exhibited significant chemical
erosion due to molecular fragmentation and volatilization . Figure shows the
evolution of LEV and MNC particle volume in the presence of OH, V
(Hg lamp on, with O3), normalized to the initial particle volume just
before switching on the Hg lamp, V0 (Hg lamp off, with
O3), as a function of OH exposure. Following OH exposure, the average
decrease in particle volume for all OH exposures for LEV and MNC particles
was 36 (±7) and 19 (±7) %, respectively. In general, OH
exposure led to an increase in LEV modal particle diameter and a decrease in
MNC modal particle diameter. The increase in LEV modal particle diameter in
combination with a decrease in total particle volume suggests the smallest
LEV particles experienced the most chemical erosion. Occasionally, a second
smaller size mode developed following OH oxidation of pure MNC particles.
While the exact mechanism for the formation of the smaller mode is not clear,
we speculate that OH oxidation of gas-phase MNC could lead to in situ
particle formation in the flow reactor. Particle size is not expected to
alter κ directly unless a change in particle size coincides with a
change in particle composition. Given that there are two different particle
populations and presumably two different particle compositions following OH
oxidation of MNC, the newly formed particles may affect the derived κ.
Clearly, more careful control and study of the particle size distribution are
needed to resolve the impacts of volatilization, but are beyond the scope of
this study.
κ was determined as a function of OH exposure and S for the
single-component organic particles LEV and MNC as shown in the top panels of
Fig. . The bottom panels of
Fig. correspond to the critical particle diameter
as a function of OH exposure. It should be noted that the critical particle
diameter decreases with increasing S. For the same exposure, smaller
particles become more oxidized due to their larger effective surface area to
volume ratio. As demonstrated in Fig. for both LEV
and MNC, at a fixed OH exposure, the largest κ corresponds to the
smallest critical particle diameter. While it is clear that κ depends
on the particle size at a fixed OH exposure, we are interested in the
resulting changes to κ due to increasing OH exposure at the applied
S. For both LEV and MNC, the trend in κ as a function of OH
exposure does not significantly deviate for the applied particle sizes. For
LEV particles, κ at the lowest OH exposure is not significantly
different to κ derived at the highest OH exposure. Conversely, MNC
κ increases significantly from ∼ 0.01 to ∼ 0.1 with
increasing OH exposure at all applied S.
LEV and MNC particle volume change as a function of OH exposure. The
measured particle volume in the presence of OH (V; Hg lamp on, with
O3) is normalized to the measured particle volume in the absence of
OH (V0; Hg lamp off, with O3).
The reactive uptake, condensed-phase reaction products, and volatilized
reaction products resulting from heterogeneous OH oxidation of LEV are well
documented . However, there are no direct measurements of its CCN activity
following OH oxidation. showed that following OH
exposure, particle volatilization accounts for a ∼ 20 % by-mass
loss of LEV. This suggests that the majority of the reaction products, which
include carboxylic and aldehydic species , remain in
the condensed phase. Although volatilization due to high OH exposures has
been linked to an increase in the critical supersaturation and thus
suppression in the CCN activity of oxidized squalane particles
, the results here suggest that, regardless of
volatilization, the condensed-phase reaction products are just as or somewhat
more active CCN than pure LEV. On average, there is only a slight increase in
κ for LEV particles, with increasing OH exposure as indicated by the
positive slope in the linear fit to the data at all applied S. Such
an incremental enhancement in κ may be a result of similar κ
between LEV and its oxidation products. The hygroscopicity of several
carboxylic acids that may represent levoglucosan OH oxidation products,
including malonic, glutaric, glutamic, succinic, and adipic acid, exhibits
κ values between 0.088 and 0.248 , similar to
κ of oxidized and pure LEV. Furthermore, the hygroscopicity of organic
compounds containing hydroxyl functionalities similar to LEV or carboxylic
groups are nearly equivalent . We also cannot rule out that
volatilization, while reducing particle mass, also removes newly formed
reaction products from the aerosol phase, leaving the parent organic
(i.e., LEV) and thus κ unchanged.
Derived κ (top) and critical particle diameter (bottom) for
LEV and MNC particles are shown as a function of OH exposure. As indicated in
the legend, the colors represent the different supersaturations (S)
accessed during this study. The vertical error bars represent ±1σ
from the mean of the data acquired at a given OH exposure and S.
Horizontal error bars correspond to the uncertainty in the OH exposure based
on a ±5 % drift in RH over the sampling period. The dotted lines show
the best linear fit to the OH exposure data as a function of S.
The CCN activity of MNC aerosol particles increases with OH exposure as shown
in the top right panel of Fig. . MNC becomes more
CCN active with increasing OH exposure and κ transitions from
∼ 0.01 in the absence of OH to ∼ 0.1 for OH exposures equivalent
to a few days in the atmosphere. Further exposure (≥4×1011 moleculescm-3s-1) does not significantly enhance
MNC κ, which suggests that MNC or the particle surface is fully
oxidized and that the reaction products reach a maximum in
κ. Similar enhancements in κ and subsequent constant κ
values with increasing OH exposure have been observed for organic aerosol
with initially low hygroscopicity . For
example, observed that κ of BES increased from
∼ 0.008 to ∼ 0.08 for an OH exposure of ∼ 1.5×1012 moleculecm-3 s, and κ of stearic acid increased
from ∼ 0.004 to ∼ 0.04 due to an OH exposure of ∼ 7.5×1011 moleculecm-3 s.
The enhancement in MNC κ following OH exposure may be linked to the
formation of more hydrophilic chemical functionalities. Strongly linked to
enhancements in OA hygroscopicity are larger O : C ratios
. Neglecting the
oxygen atoms in the -nitro functionality of MNC , the O : C
ratio of pure MNC is ∼ 0.29, close to the lower end in O : C where
transitions from low κ to high κ typically occur
. The presence of -methyl, unsaturated, and -nitro
functionalities are also linked to low hygroscopicity . As
proposed in and observed for other nitro-phenolic species,
OH oxidation of MNC can favor removal of the -nitro functionality by
electrophilic substitution of OH . OH
substitution at the -methyl position and addition to the double bonds is also
possible . OH addition to the -nitro or -methyl
functionality would increase O : C to ∼ 0.43 or ∼ 0.5,
respectively. OH substitution at both positions would enhance O : C to
∼ 0.67. showed that hydroxyl-dominated OA with an
O : C of less than ∼ 0.3 has an apparent κ of ≤10-3.
However, an increase in O : C to 0.4 or 0.6 due to the addition of
hydroxyl, aldehydic, or carboxylic functionalities results in an enhanced
κ of ∼ 0.1. Thus, small changes in O : C can significantly
affect κ. Pure MNC is also sparingly soluble in water and thus
κ is strongly dependent on its actual solubility, which can change
depending on the oxidation level and the presence of other compounds having
different solubility . Consequently, the conversion from
low to high κ following OH oxidation is consistent with the addition
of more hydrophilic functionalities and a molecular transition from sparingly
soluble to sufficiently water-soluble. Interestingly, MNC, while having a 10
times smaller OH uptake coefficient compared to LEV at the same RH
, exhibits a greater change in κ than LEV following
OH oxidation. Under dry conditions, we understand uptake is limited by
surface–bulk processes . In that case, due to
the low hygroscopicity and low water solubility of MNC, its viscosity may be
sufficiently high that oxidation is limited to the particle surface.
Consequently, MNC surface molecules may undergo several generations of
oxidation as opposed to LEV, which is known to undergo a semi-solid to
liquid-phase transformation at the same RH = 40 %
. However, assessing the effects of RH or bulk
diffusivity on hygroscopicity following OH exposure is beyond the scope of
the current work.
CCN activity of binary-component BBA surrogate particles exposed to OH
Binary-component particles consisting of LEV : MNC, LEV : KS, and
MNC : KS in 1:1 mass ratios were exposed to OH and analyzed for their
hygroscopicity as a function of OH exposure. The approach here is to
determine whether the presence of more than one component can influence the
hygroscopicity of another following OH and oxidation; i.e., are the observed
changes in hygroscopicity of the pure-component particles following OH
oxidation retained when mixed? Figure shows
κ and the critical particle diameter for the different binary aerosol
mixtures as a function of OH exposure for each applied S. κ
as a function of O3 exposure is presented in the Supplement. The
dotted and dashed lines in Fig. display the
predicted κ as a function of OH exposure using the volume mixing rule
including and excluding MNC solubility limitations, respectively, based on
the linear fits of κ as a function of OH exposure for pure LEV and MNC
particles (Fig. ) at each S. Modeled
κ as a function of OH exposure excluding MNC solubility limitations
(i.e., black dashed lines in Fig. ) assumes that
κ for MNC of the mixed particles is 0.16.
Derived κ (top) and critical particle diameter (bottom) for
the binary-component particles with 1:1 mass ratios are shown as a function
of OH exposure. As indicated in the legend, the colors represent the
different supersaturations (S) accessed during this study. Error
bars are calculated as in Fig. . The dotted lines
are modeled κ using the volume mixing rule as a function of OH
exposure applying the linear fit to the derived κ of pure MNC and LEV
as a function of OH exposure (Fig. ). The dashed
black lines are the modeled κ using the volume mixing rule and
assuming no solubility limitations.
There are two important points to be made regarding the results from
Fig. . (1) Hygroscopicity of the mixed particles
is virtually unchanged as a function of OH exposure; i.e., while OH exposure
significantly impacts MNC hygroscopicity alone, it does not significantly
influence κ for the binary-component particles containing MNC as
predicted by the volume mixing rule applying the single-component
experimentally derived κ for LEV, MNC, or KS. (2) κ and the
trend in κ with OH exposure are significantly underpredicted assuming
MNC solubility limitations are applicable in the volume mixing rule (dotted
lines in Fig. ). As discussed previously and
demonstrated in Fig. , the presence of either KS or LEV
influences the extent that MNC solubility impacts particle activation. We
have shown that MNC exhibits no solubility limitations for the volume
fractions applied here. Larger MNC volume fractions are expected to have
a greater influence on κ following OH exposure. The organic content of
BBA was shown to dominate hygroscopic growth, in particular the water-soluble
organic content (WSOC), which is largely levoglucosan .
Other studies have indicated that sparingly soluble organic compounds have
limited importance in atmospheric aerosol CCN activity , although they are, besides
completely insoluble organic material, the most likely class of compounds
susceptible to hygroscopic changes following oxidation, due to their low
water solubility. In other words, there is more room for an enhancement in
the solute effect of sparingly soluble organic particles than there is for
more water-soluble particles. Our results show that oxidative aging impacts
on the hygroscopicity of pure-component particles can be vastly different if
the particles are internally mixed with substances having different water
solubility.
CCN activity of ternary-component BBA surrogate particles exposed to OH
Here we investigate the CCN activity of internally mixed LEV, MNC, and KS
particles with 1:1:1 and with an atmospherically relevant mass ratio of
1:0.03:0.3 (LEV : MNC : KS) following exposure to OH. The results for
the OH exposure are shown in Fig. and κ
as a function of O3 exposure is presented in the Supplement.
Within the uncertainty of the derived κ values for the 1:1:1 and
1:0.03:0.3 ternary-component particles, their hygroscopicities are
virtually unaffected by OH exposure, similar to the binary mixtures. However,
on average the 1:1:1 particle mixture exhibits a slight enhancement in
hygroscopicity. The predicted κ values for the 1:1:1 mixture, which
include MNC solubility limitations (dotted lines), significantly
underestimate κ, and only after removing these limitations (black
dashed line) does the predicted κ agree with the experimentally
derived κ. This is not surprising, given that the more water-soluble
components LEV and KS are present at equal mass to MNC. Thus MNC behaves as
if it is infinitely soluble during CCN activation. One possible explanation
for the slight enhancement in κ with OH exposure, which differs from
the binary mixed particles, is the presence of both MNC and LEV, which both
exhibit enhancements in κ following OH oxidation. However, the range
in derived κ at a given OH exposure is sufficiently large that, within
experimental uncertainty, there is no significant trend in κ with OH
exposure.
The WSOC, mostly LEV, is known to dominate the BBA volume fraction
. MNC constitutes ≤5 % by mass of the BBA organic
fraction as determined from both field and lab chamber studies
. The remaining fraction can be largely
composed of inorganic salts, including KS . To
simulate atmospheric BBA, we atomized a mixed aqueous solution of LEV, MNC,
and KS in a mass ratio of 1:0.03:0.3 and determined its CCN activity
unexposed and after exposure to OH and O3. The resulting κ of
this mixture as a function of OH exposure is displayed in the top right panel
of Fig. . As anticipated, since pure LEV shows
little enhancement in CCN activity with OH exposure
(Fig. ) and dominates the volume fraction of this
mixture, and KS is unreactive to OH, no enhancements in κ following OH
exposure were observed. A similar observation was made from
laboratory-controlled burns, whereby following several hours of
photo-oxidation, there were very slight enhancements in κ of the
particles . Larger enhancements in κ were observed
only for the SOA particles generated from oxidative aging of gas-phase
volatiles emitted during the controlled burns, in the absence of seed
particles . This implies that photo-oxidative aging of BBA
may contribute little to changes in its hygroscopicity, unless the entire
aerosol population is comprised of SOA material (e.g., MNC). Furthermore,
both predicted κ including solubility limitations and without
solubility limitations are in agreement with the derived values. This is due
to the low mass fraction of MNC present, which has sufficiently low impact on
both the solubility and oxidation level of the mixed aerosol particles.
Derived κ (top) and critical particle diameter (bottom) for
the ternary-component particles with LEV : MNC : KS mass ratios 1:1:1
(left) and 1:0.03:0.3 (right) are shown as a function of OH exposure. As
indicated in the legend, the colors represent the different supersaturations
(S) accessed during this study. The dashed black lines are
calculated as in Fig. and error bars and dotted
lines are calculated as in Figs.
and , respectively.
Mixing state effects on κ
Internally mixed organic–inorganic atmospheric aerosol particles can exhibit
phase separations, i.e., a core-shell structure, which often contains an
insoluble or solid inorganic core with a less viscous organic outer layer
. The presence of an organic coating
has been shown to impact CCN activity and water uptake , ice nucleation efficiency , and heterogeneous
chemistry . Because
MNC originates from gas-phase chemical reactions, and its concentration
determined in BBA particles, MNC must partition from the gas to the
particulate phase. In this section, we investigate whether the mixing state
of mixed MNC and KS particles has an effect on its CCN activity following OH
exposure by the application of MNC-coated particles in comparison to the
atomized MNC : KS binary-component particles. For example,
observed a complete deactivation in the CCN activity of
ammonium sulfate particles when thickly coated with stearic acid.
The CCN activity of KS particles coated with MNC was derived as a function of
the organic volume fraction (Vf,org) of MNC, and before and after
OH exposure as shown in Figs. a–c.
Figure a displays a color map of the dry KS particle
size distribution evolution following exposure to MNC in the absence of OH,
where time = 0 min is the point at which KS particle growth by
MNC condensation begins. The 25th, 50th, and 75th percentiles of the
number-weighted particle size distribution grew in size by ∼ 20 nm as
indicated by the red, black, and blue circles in Fig. a,
respectively. For the 50th percentile, this corresponds to an enhancement in
the MNC Vf,org from 0 % at time = 0 min to ∼ 70 % shortly after, close to the Vf,org of the atomized
MNC : KS binary-component particles of 64 %. The similar
Vf,org between the atomized and coated MNC-KS particles enables
a direct intercomparison of their hygroscopicity, since relatively larger MNC
Vf,org would bias towards lower κ and vice versa, as
indicated in the colored dashed and dotted lines in
Fig. b.
CCN activity of MNC-coated KS particles before and after exposure to
OH. Panel (a) shows a color map of the number-weighted particle size
distribution (dN) of KS and MNC-coated KS particles plotted as a function
of MNC coating before exposure to OH. The open circles in panel (a)
refer to the measured percentiles of the total particle population (25th:
red; 50th: black; 75th: blue). Panel (b) displays the change in
particle hygroscopicity (filled circles) and MNC volume fraction
(Vf,org, open circles) with time as a function of S
given as a black solid line corresponding to the data presented in
panel (a). The dotted lines show the predicted κ using the
volume mixing rule corresponding to the Vf,org at a given time
and based on the experimentally derived κ for KS and MNC given in
Table 1 as a function of S. The dashed lines represent the predicted
κ using the volume mixing rule corresponding to the Vf,org
at a given time and assuming the CCN activity of MNC is not limited by its
solubility (i.e., MNC κ=0.16 calculated from Eq. 6).
Panel (c) displays the change in κ for the MNC-coated KS
particles as a function of Vf,org, OH exposure, and S.
OH unexposed particles are plotted as open circles. Filled circles correspond
to particles exposed to OH at
3.3 × 1011 moleculecm-3 s. The error bars
represent 1σ from the mean in κ. The colored dotted lines show
predicted κ as a function of Vf,org at different
S using the volume mixing rule assuming the CCN activity of MNC is
limited by its solubility. The black dashed line shows the predicted κ
using the volume mixing rule assuming the CCN activity of MNC is not limited
by its solubility.
The particles' hygroscopicity was analyzed throughout the period of
condensational growth as demonstrated in Fig. b and
shown as the black circles. It is important to note that the particle size
distribution is scanned (up and down voltage scans) at four different
S (0.2, 0.27, 0.35, and 0.425 %) in ascending and descending
order, a process that takes roughly 90 min. In contrast to the atomized
binary-component particles, here KS particles grow due to MNC condensation
over this experimental time period. Hence, the DMA and CCNc capture the size
distribution and CCN activity of a time-dependent and compositionally
different particle population at each scan. The black line in
Fig. b displays the steps in S over the course
of the experiment. The first two κ values are of pure KS particles
evaluated at S=0.2 %. Subsequent κ values are of MNC-coated KS
particles, which increase in Vf,org with time. The change in
Vf,org with time is indicated by the red, black, and blue circles
according to the 25th, 50th, and 75th percentiles of the particle population,
respectively. Vf,org allows one to compare derived κ with
that predicted using the volume mixing rule. As previously discussed, the
solubility limitations of pure MNC can be neglected when predicting κ
of the atomized 1:1 mass ratio MNC : KS binary-component particles. To
determine whether the solubility of MNC impacts the MNC-coated KS particles
similarly to the atomized mixture, κ is predicted using the volume
mixing rule and applying the experimentally derived pure MNC κ
corresponding to a specific S (i.e., including solubility
limitations), as indicated by the dotted lines in
Fig. b, and compared to predicted κ applying
a pure MNC κ of 0.16 (i.e., pure MNC κ in the absence of
solubility limitations calculated from Eq. 6), as indicated by the dashed
lines in Fig. b. The predicted κ with increasing
Vf,org generally captures the trend in experimentally derived
κ with increasing Vf,org; however, similarly to the
atomized MNC : KS binary-component particles, assuming MNC CCN activity is
limited by its solubility, the volume mixing rule underpredicts derived
κ. When applying a pure MNC κ=0.16 in the absence of solubility
limitations, the volume mixing rule is in slightly better agreement with the
derived κ values. However, there are notable deviations between
derived κ and predicted κ in both cases, which depend on
S. For example, in Fig. b, the predicted
κ including MNC solubility limitations (dotted line) is in better
agreement with the derived κ at S=0.425 % than at lower
S. At higher S, the particles that activate first are
smaller in diameter than the particles that activate first at lower
S. Assuming differently sized KS particles were exposed to an equal
quantity of gas-phase MNC, the larger particles, having relatively larger
surface areas than the smaller KS particles, would acquire a thinner organic
coating, and thus relatively smaller Vf,org. As a result, the
particles that activate at S = 0.425 % possess a larger
Vf,org compared to the particles that activate at,
e.g., S = 0.2 %. This corresponds to a decrease in derived κ
at S = 0.425 % (i.e., better agreement with predicted κ
including MNC solubility limitations) relative to other S as
indicated in Fig. b. The generally better agreement in
the predicted κ excluding MNC solubility limitations with the
experimentally derived κ indicates that MNC is sufficiently
water-soluble to not deactivate KS, in contrast to the particle systems
studied by . One possible explanation for the higher than
expected κ when KS is coated with MNC is that the particle-phase
diffusivity is sufficiently high to allow water molecules to penetrate the KS
core .
The effects of OH exposure on the CCN activity of MNC-coated KS particles as
a function of Vf,org are given in Fig. c.
κ is plotted as a function of MNC Vf,org. The κ
values resulting from an OH exposure of
3.3 × 1011 moleculecm-3 s are given by the filled
circles, whereby the different colors represent the applied S during
the experiment. Open circles correspond to κ in the absence of OH. At
Vf,org = 0 %, κ is ∼ 0.55 and independent
of OH exposure. κ decreases when Vf,org≈70 %,
but undergoes a slight enhancement following OH exposure. The dotted lines
indicate the modeled change in κ as a function of Vf,org
applying the volume mixing rule and applying the experimentally derived
κ for MNC and KS at a given S (i.e., including MNC solubility
limitations). Similar to the atomized 1:1 MNC : KS binary-component
particles, modeled κ as a function of Vf,org underpredicts
the experimentally derived OH-unexposed κ, even after accounting for
the enhancements in pure-component MNC κ due to the high OH exposure.
This suggests that in the presence of KS at this
Vf,org≈70 %, MNC may not be limited by its solubility,
similar to the atomized 1:1 mass ratio MNC : KS binary-component
particles, and that OH exposure can have very little impact on the CCN
activity of sparingly soluble organics coated on water-soluble compounds.
However, the dashed black line shows the modeled κ as a function of
Vf,org applying the volume mixing rule and assuming MNC is not
limited by its solubility, i.e., MNC κ = 0.16, which slightly
overpredicts the derived κ, but is in better agreement with the trend
in experimentally derived κ for the OH-exposed particles (filled
circles, Fig. c). A reasonable explanation for this is
that Vf,org≈70 % is sufficiently large such that MNC
solubility limitations on the CCN activity of MNC-coated KS particles are
partially exhibited. While OH exposure has a significant impact on the CCN
activity of pure MNC, its impact on the CCN activity of MNC-coated KS
particles is significantly less. The higher water solubility of KS appears to
govern hygroscopic growth, similar to the atomized MNC : KS
binary-component particles. This suggests that the water solubility of the
more soluble component of mixed-component aerosol particles can be more
important for CCN activation than the actual mixing state of the particle.
Conclusions
To our knowledge, there are no studies that have explicitly investigated the
influence of OH-initiated oxidative aging on the hygroscopicity of organic
and mixed organic–inorganic BBA particles. Biomass burning can greatly
influence cloud formation and microphysical properties by increasing the
available CCN in the atmosphere . However, the efficiency
at which aerosol particles act as CCN depends on their water solubility,
hygroscopicity, and size, which can be altered by multiphase chemical
reactions with gas-phase oxidants. While it is recognized that a significant
fraction of BBA is comprised of organic material , most of
which is water-soluble , water uptake can be
sensitive to the inorganic mass fraction . In
this study we investigated how sensitive the CCN activity of single-component
and mixed water-soluble/water-insoluble compounds associated with BBA are to
OH oxidation. The important findings relevant to the atmosphere include that
(i) the hygroscopicity of water-soluble organic compounds is unaffected by
chemical aging, (ii) the hygroscopicity of single-component water-insoluble
organic compounds is affected by chemical aging as anticipated from previous
studies , and (iii) if considering mixtures of
water-soluble and insoluble materials, the effects of chemical aging by OH
are more complicated, and single-component-derived κ and changes to
κ as a function of OH exposure do not translate directly to mixtures.
WSOC constitutes a significant fraction of biomass burning OA
and
atmospheric OA in general . Water-soluble OAs
are effective CCN because they enhance the solute term in the Köhler
equation. Chemical aging is known to promote the solubility of initially
insoluble and sparingly soluble OA by yielding more water-soluble and
multifunctional reaction products . The question of atmospheric relevance depends on the
concentration or potency of a particular molecule in the atmosphere. MNC,
while contributing little to the mass fraction of BBA particles, is toxic to
forests and recognized as an important biomass burning
SOA molecular marker . An OH exposure equivalent to only
a few days of atmospheric exposure leads to an order of magnitude enhancement
in MNC hygroscopicity. This implies that aged MNC is more susceptible to wet
depositional losses over atmospherically relevant particle transport
timescales, e.g., through cloud formation, compared to fresh MNC.
Calculations from indicate that substantial wet
depositional losses can occur when κ>0.01. The question of the
utility of MNC as a molecular marker for source apportionment is raised since
molecular markers are assumed to be inert over the course of its lifetime in
the atmosphere. Clearly, OH oxidation of MNC influences its chemical
composition, but in doing so also decreases its atmospheric lifetime by
enhancing its CCN activity. However, our results strongly suggest that if the
OA is WSOC-dominated, e.g., by LEV, the reaction products likely have similar
CCN activity to the parent WSOC, and thus particle oxidation plays a very
minor role in enhancing the CCN activity of WSOC. Indeed, only a minor
enhancement in the hygroscopicity of BBA, produced from controlled wood
burning, was observed after several hours of photo-oxidation, likely a result
of significant BBA WSOC content .
Much less is known of the effects of chemical aging on the CCN activity of
internally mixed water-soluble and insoluble organic–inorganic particles.
While oxidative aging can enhance the hygroscopicity of single-component
particles with initially low water solubility, atmospheric aerosol particles
are not often pure and consist of both organic and inorganic compounds
.
Organic compounds alone can influence the hygroscopicity of inorganic aerosol
particles and
moderate amounts of water-soluble inorganics can render low-solubility
organics infinitely water-soluble . When mixed with LEV or KS (or both) in significant mass
fractions, the effects of OH oxidative aging on the hygroscopicity of
single-component MNC are not revealed in the derived κ for the
binary- or ternary-component particles. Furthermore, a thick coating of MNC
on KS particles had similar impacts on the CCN activity behavior with
increasing OH exposure as the atomized binary-component MNC : KS particles.
The water-soluble fraction (i.e., KS) was sufficiently large that MNC became
infinitely soluble. Our results indicate that it is the fraction of the
water-soluble component of internally mixed water-soluble and insoluble
organic–inorganic particles that dictates whether chemical aging will
enhance the particles' CCN activity. Chemical aging has no major impact on
the CCN activity of the mixed water-soluble and sparingly soluble BBA
compounds studied here, beyond the point that the less water-soluble
component becomes infinitely soluble. Below this point, chemical aging can
influence the CCN activity of the mixed particle. However, we caution against
extrapolating general atmospheric conclusions given the limited number of
compounds and mixtures studied here and suggest that similar work in the
future also consider more complex, atmospherically relevant particle systems.