Introduction
It is well established that biomass burning, as an important
source of atmospheric aerosol particles, has a wide range of climate effects
that can be classified into direct radiative effects through light-absorbing
carbon aerosol particles and indirect effects by impact on cloud condensation
nuclei (CCN) and cloud microphysics (Andreae and Gelencsér, 2006;
Moosmüller et al., 2009; Hecobian et al., 2010; Rizzo et al., 2011; Rose
et al., 2011; Cheng et al., 2012; Engelhart et al., 2012; Lack et al., 2012;
Jacobson, 2014; Liu et al., 2014; Saleh et al., 2013, 2014). Atmospheric
light-absorbing particles that arise from biomass burning play an important
role as a driver of global warming (Favez et al., 2009; Hegg et al., 2010;
Lack et al., 2012; Laborde et al., 2013; Srinivas and Sarin, 2013). According
to the IPCC report (Boucher and David, 2013; IPCC, 2013), the climate forcing
of black carbon aerosol particles may rival that of methane, with
a present-day global warming effect of up to 0.3–0.4 ∘C (Wang
et al., 2014). Also, certain types of aerosol particles emitted by biomass
burning, when immersed into cloud droplets, absorb solar radiation and
facilitate water evaporation and cloud dispersion, which indicates an
additional indirect aerosol effect that counteracts the cooling effect of
cloud droplets nucleated by aerosols (Powelson et al., 2014). Therefore,
a better understanding of the influence of aerosol particles from biomass
burning on cloud formation, precipitation, and Earth's radiative budget is
required to comprehend biomass burning aerosol properties and behavior.
The understanding of the aerosol–cloud–climate impact of a vast range of
organic compounds derived from biomass burning, however, is rather limited
due to the complexity of biomass burning emissions, gas- and aerosol-phase
processing, and the restricted availability of field measurements (Pratt
et al., 2011; Lei et al., 2014; Paglione et al., 2014; Srinivas and Sarin,
2014; Zhong and Jang, 2014; Arnold et al., 2015; Lawson et al., 2015; Gilman
et al., 2015). Moreover, biomass burning particles are often mixtures of
water-soluble organic carbon, black carbon, varying amounts of inorganic
components, and water-insoluble inclusions, such as mineral dust or poorly
soluble organics (Väkevä et al., 2002; Sadezky et al., 2005; Saarnio
et al., 2010). An appreciable number of organic compounds affect the
physicochemical properties of aerosols, such as hygroscopicity, liquid–solid
and liquid–liquid phase transitions, and chemical reactivity in liquid
phases and/or on particle surfaces (Shiraiwa et al., 2013). For example,
equilibrium between the variable environmental water vapor mixing ratio and
aerosol particles may lead to substantial changes in particle size and
chemical composition, all of which can influence light absorption and
scattering (Seinfeld and Pandis, 2006; Zhang et al., 2016). Transitions
between solid and liquid (aqueous) phases that are dependent on relative
humidity (RH) are also important in determining optical properties (Martin
et al., 2013; Wang et al., 2010; Kim et al., 2016; Wu et al., Denjean et al.,
2015, 2016; Hodas et al., 2015; Atkinson et al., 2015). Studies have shown
that water-soluble organic matter from biomass burning (approximately
70 % of total organic matter) can significantly suppress, enhance, or
have no effect on the deliquescence (e.g., the RH at which deliquescence
occurs at a certain temperature, the DRH) and efflorescence processes (e.g.,
the efflorescence RH, ERH) of present inorganic electrolytes. The effect
depends predominantly on the type of organics, mass fraction of organics
relative to inorganics, and particle size (Zawadowicz et al., 2015; Hodas
et al., 2015; Gupta et al., 2015). Whole particles, individual phases within
particles, or specific chemical compounds can undergo a range of phase
transitions including crystallization–efflorescence,
dissolution–deliquescence, and liquid–liquid phase separation as the RH
varies in the atmosphere. A number of laboratory studies have focused on
liquid–liquid phase separations within particles consisting of inorganic and
organic fractions (Svenningsson et al., 2006; Carrico et al., 2008; Dusek
et al., 2011; Hodas et al., 2015). For example, studies about liquid–liquid
separation occurring in mixed organic–inorganic aerosols were performed by
Song et al. (2012a, b) and You et al. (2013) using Raman and optical
microscopy, establishing that liquid–liquid phase separation typically
occurs in mixed organics + ammonium sulfate (AS) particles with an
average elemental oxygen-to-carbon (O : C) ratio of the organic fraction of
less than 0.6 and in some cases for 0.6 < O : C <0.8.
You et al. (2013) further found that for a O : C ratio between 0.5 and 0.8,
the occurrence of liquid–liquid phase separation at a moderate to high RH
depends on the types of inorganic salts present (i.e., the effective strength
of the salting-out effect), e.g., (NH4)2SO4≥NH4HSO4≥NaCl≥NH4NO3. Recently, the effect of a potential size-dependent
morphology and dependence of the phase separation mechanism on the
organic / inorganic mass ratio in mixed aerosol was studied for mixtures
of poly(ethylene glycol)-400 + ammonium sulfate using
cryogenic-transmission electron microscopy (Altaf et al., 2016). Therefore,
many independent studies suggest that the occurrence of solid–liquid and/or
liquid–liquid phase separations, as well as related (temperature-dependent)
RH levels of phase transitions (DRH, ERH, and onset of RH of liquid–liquid phase separation, SRH), depend on the relative
amounts of organic and inorganic aerosol components and their nonideal mixing
behavior.
Substances and their physical properties used in this
work.
Chemical compound
Chemical
Molar
Density in solid
Solubility
Solution
Manufacturer
formula
mass
or liquid state
(g(100cm3H2O)-1)
surface
(gmol-1)
(gcm-3)
tension
(Jm-2)
Ammonium
(NH4)2SO4
132.140
1.770a (solid),
74.400
0.072
Alfa Aesar,
sulfate
1.550a (liquid)
(at 20∘)
(0.001–10 mgmL-1)
99.95 %
Levoglucosan
C6H10O5
126.100
1.618b (solid)
0.073c
Aldrich,
1.512b (liquid)
(0.01–10 mgmL-1)
99 %
4-Hydroxybenzoic
C7H6O3
138.100
1.460 (solid)
0.675
0.070 e
Alfa Aesar,
acid
1.372d (liquid)
(at 25∘)
(> 10 mg mL-1)
99.99 %
Humic acid
NA
0.800 f (solid)
NA
0.073g
Aldrich, 99 %
a Clegg and Wexler (2011a). b Lienhard et al. (2012).
c Tuckermann and Cammenga (2004) at 293 K. d
Jedelsky et al. (2000). e Kiss et al. (2005).f
Yates III and Wandruszka (1999). g Mikhailov et al. (2008).
The expected physical state and morphology of aerosol particles containing
mixtures of a wide range of organic and inorganic salts and acids can, in
principle, be predicted by a selection of specialized thermodynamic
equilibrium models. Such models include the Extended Aerosol Inorganic Model
(E-AIM) (Clegg et al., 1998; Clegg and Seinfeld, 2006; available online:
http://www.aim.env.uea.ac.uk/aim/aim.php), the Aerosol Diameter
Dependent Equilibrium Model (ADDEM) (Topping et al., 2005), the universal
quasi-chemical functional group activity coefficients (UNIFAC) method
(Fredenslund et al., 1975; Hansen et al., 1991), and the Aerosol
Inorganic–Organic Mixtures Functional groups Activity Coefficients (AIOMFAC)
model (Zuend et al., 2008, 2011, 2012). These models have all been used to
predict atmospheric aerosol thermodynamic equilibrium for a variety of
inorganic and organic systems, yet not all of them can be used to
compute nonideal mixing in organic–inorganic systems. AIOMFAC has
been used to predict the distribution of components in multiple phases
in a range of mixed organic–inorganic systems and demonstrated its
broad applicability in predicting liquid–liquid phase separation in
such mixtures (Zuend et al., 2010; Song et al., 2012; Zuend and
Seinfeld, 2012; Shiraiwa et al., 2013; Renbaum-Woff et al., 2016;
Rastak et al., 2017).
Several previous experimental studies using the hygroscopicity tandem
differential mobility analyzer (HTDMA) technique (e.g., Zardini et al., 2008;
Lei et al., 2014) show that the deliquescence of inorganic compounds is
affected by the presence of organic components, which manifests itself in
a shift in the DRH of a salt compared to the corresponding organic-free
system. For instance, a clear shift of AS DRH was observed in the case of the
levoglucosan + ammonium sulfate system (Lei et al., 2014). Here we focus
on investigating the morphology, hygroscopicity, and phase transitions of
relevant organic compounds found in biomass burning aerosol during the
dehydration–dehumidification process. Moreover, we study how the presence of
organic compounds affects the water loss behavior of mixed organic–inorganic
aerosols with AS in the supersaturated state as well as after efflorescence
of AS. In addition, we compare the measured hygroscopicity behavior of mixed
aerosol particles with predictions from the
Zdanovskii–Stokes–Robinson (ZSR) mixing rule, the E-AIM model, and the
AIOMFAC model.
Methods
Aerosol system
The three organic compounds levoglucosan, 4-hydroxybenzoic acid, and humic
acid were used as surrogates for the rich class of water-soluble organic
components in biomass burning aerosols. The influence of the distinct
chemical structure of these compounds was studied with regard to the water
uptake and evaporation of the pure organic compounds as well as for mixed
particles containing organics and AS. Furthermore, a comparison with field
data from the Amazon Basin was preformed to quantify the ability of mixtures
of these three organic compounds to mimic the hygroscopic behavior of complex
ambient organic particles originating from biomass burning emissions. Here we
focus on the characterization of hygroscopic growth factors (HGFs) as well as
solid–liquid and liquid–liquid phase transitions during the
dehumidification conditions. The chemical substances and their physical
properties are characterized in Table 1. All of the experimental solutions
were prepared by dissolving in Milli-Q water (resistivity ≥18.2 MΩ) and the experiments were conducted at room
temperature (∼ 298 K). The chemical
compositions of biomass burning model mixtures are introduced in Table 2.
The chemical composition of biomass-burning model mixtures studied
given as mass percentages (wt %). See Sect. 3.3 for more information.
Mixture name
Levoglucosan
4-Hydroxybenzoic acid
Humic acid
Ammonium sulfate
Mix-bio-dry
87.2 %
9.2 %
1.5 %
2.1 %
Mix-bio-wet
68.0 %
26.0 %
3.0 %
3.0 %
Instrument design
Figure 1 shows a schematic of the HTDMA instrument; more detailed information
about this instrument's setup, calibration and evaluation is described
elsewhere (Lei et al., 2014; Jing et al., 2016; Liu et al., 2016). Briefly,
polydisperse sub-micrometer aerosol particles are generated by atomizing
(MSP 1500, MSP) a 0.05 weight % aqueous solution consisting of
different mass fractions of inorganic and organic components, assuming that
the composition of the aerosol particles formed is initially the same as that
of the solution used in the atomizer. Aerosol particles from an atomizer are
routed though homemade silica diffusion dryers and then pass through a Nafion
gas dryer (Perma Pure Inc., USA). After aerosol particles were dried to below
5 % RH (RH set point 1, RH1), they are directed to the impactor; those
aerosols with a diameter less than 1 µm are allowed to pass and
subsequently pass through a 85Kr electric charger to reach
a near-Boltzmann distribution of charges (Liu et al., 1985). After charging,
the aerosol particles enter the first differential mobility analyzer (DMA1)
at a sheath flow-to-aerosol flow ratio of 4:0.3. The sheath flow is
circulated by the diaphragm pump in the first loop DMA1 system, and its RH is
kept constant at below 5 % RH. The resulting monodisperse particle
population, selected within uncertainty by the DMA1, is then exposed to
high-RH conditions during which the aerosol flow is humidified to 98 % RH
by mixing water through a Nafion membrane humidifier at 30 ∘C. After
passing through a saturator (Perma Pure Inc., USA), the aerosols are dried to
a target RH level (RH2) through a series of two single-Nafion tubes (Perma
Pure Inc., USA) with RH2 set to a value in the range of 90 to 5 % RH.
Here, a pulse width modulator circuit is used to regulate the sheath flow on
the basis of a proportional integral derivative system. When the second
Nafion membranes allow for regulating the sheath flow to a desired RH and for
controlled flow into the sample stream until the RH2 setting value is equal
to the excess RH of sheath flow value (RH3), the mobility diameter of the
dehumidified aerosols at target RH is measured with the second DMA (DMA2,
a scanning DMA) coupled with a condensation particle counter (model 1500,
MSP). In addition, the residence time between the humidifier and DMA2 is
around 5 s, which is estimated to be sufficient for aerosols to grow
or shrink to equilibrium size at a certain RH set point. Also, due to
recirculation of the sheath flow and the pre-humidification of the aerosol
flow, the sheath flow and aerosol sample flow are enabled to rapidly reach
the same RH.
Schematic of the hygroscopicity tandem differential mobility analyzer
(HTDMA) system. The sheath flow, aerosol flow, and water flow are
represented by the blue, black, and green lines, respectively. PWM: pulse width
modulator circuit.
Theory and modeling methods
Models were applied to explore the extent to which measured HGFs, particle phase states, and phase compositions
under subsaturation conditions can be predicted using thermodynamic equilibrium
models. For the AS-containing system studies, the current thermodynamic
equilibrium predictions account for a crystalline AS phase with solid–liquid
equilibria prior to the complete deliquescence of AS under hydration
conditions. Similarly, the crystallization point followed by a solid–liquid
equilibrium (SLE) of AS needs to be considered to predict the effect of organic
components in the mixed particles on the shift–suppression of AS
efflorescence during aerosol dehumidification, i.e., referring to processes
occurring along the dehydration branch of a HTDMA
humidification–dehumidification cycle. The calculation of the ERH of AS in an
organic–inorganic solution is thermodynamically related to the solubility
limit, but it is not strictly deterministic (unlike the DRH) due to the
stochastic nature of nucleation and growth of a crystal embryo. The molality
of pure AS at saturation in an aqueous solution is known, e.g., measured by
Apelblat (1993) at 298.15 K as mAS(sat)=5.790 molkg-1, while measurements are most often not available
for the solubility limit of AS in aqueous inorganic–organic systems.
However, crystalline AS in equilibrium with an aqueous mixture demonstrates
a specific molal ion activity product (IAP) in that solution at a given
temperature and atmospheric pressure. For example, in the case of a ternary
liquid mixture of levoglucosan + AS + water in SLE with a crystalline AS phase at a certain temperature T,
a constant molal ion activity product
IAPAS=IAPAS(sat)(T) is established (necessary
SLE condition). In this case the liquid mixture is a so-called saturated
solution with respect to AS. While the molar amount of AS in a saturated
solution depends on the other mixture constituents, the value of
IAPAS(sat)T is a function of temperature
only since it is derived from the fixed chemical composition and associated
chemical potential of the crystalline phase. A reference value for
IAPAS(sat)T can therefore be calculated with
the AIOMFAC model from an experimentally determined solubility limit of AS in known mixtures, such as the molality of AS at the point of saturation in
the binary aqueous system (water + AS). The RH at which full dissolution
of a solid phase upon humidification is just reached, the DRH, is directly
related to the conditions at which a saturated solution becomes subsaturated
upon addition of water. Here the degree of saturation with AS can be
determined unambiguously using the computed value of IAPAS as
a function of mixture composition and temperature. Making use of these
thermodynamic relationships, the AIOMFAC-based equilibrium model is used to
calculate the DRH and ERH of AS in the multicomponent system, as outlined
below. Detailed information on the modeling of SLE and
the IAP-based prediction of ERH is given in Zuend et al. (2011) and Hodas
et al. (2016). Briefly, the ERH is determined based on the following
equations:
IAPAS=aNH4+(m)2aSO42-(m)1IAPAS[crit]=cAS×IAPAS(sat).
Here aNH4+(m) and
aSO42-(m) are the molal activities of the
ammonium and sulfate ions in solution (Zuend et al., 2010). Molality basis is
indicated by the superscript “(m)” (which is not a mathematical exponent).
IAPAS(sat) denotes the molal IAP of AS at
salt saturation computed with the thermodynamic equilibrium model for any
aqueous AS system at a certain temperature (here 298.15 K). The
calculated molal IAP at saturation of the corresponding binary salt solution
is taken as the (known) reference value. The RH at which this
IAPAS[sat] value is just reached in certain bulk solution at
equilibrium with its environment (in contrast to
IAPAS<IAPAS(sat) at higher RH) is the
(bulk) DRH of AS. Similarly, the ERH is determined at the point of
crystallization by a critical IAP value denoted as
IAPAS[crit] (Hodas et al., 2016), the value of
IAPAS[crit]>IAPAS[sat] expresses
the need for reaching a critical IAP threshold (critical level of AS
supersaturation) for highly likely nucleation and growth of a new
crystalline AS phase. The multiplication factor cAS is used as
a constant coefficient relating the IAP at AS saturation to the one expected
at the point of crystallization in aqueous mixed particles. From the
comparison of laboratory measurement of ERH for aqueous AS solution to the
AIOMFAC-predicted IAPAS at that RH, the value of
cAS≈30 was determined; this value is in particular
applicable to submicron-sized AS droplets (Zardini et al., 2008; Ciobannu
et al., 2010).
An analogous approach is used for the ERH predictions with the E-AIM model;
however, since E-AIM provides activity coefficients and activities on a mole
fraction basis, denoted here by the superscript “(x)” (rather than molality
basis), the value of cAS(x) needs to be determined separately
for that model. Expressing Eq. (1) using mole-fraction-based activities of
NH4+ and SO42- and comparison to the
IAPAS(sat,x) and IAPAS(crit,x)
computed by E-AIM for AS at the experimental solubility limit and ERH in
aqueous AS solutions, a value of cAS(x)≈40 was
determined for the calculation with E-AIM.
As discussed by Lei et al. (2014), prediction of HGFs
with E-AIM includes a sophisticated composition-dependent solution density
model, which considers the nonideality effects on apparent molar volumes
used for the calculation of the solution density in mixed organic–inorganic
systems (Clegg and Wexler, 2011a, b). The AIOMFAC-based model applies
a simpler solution density treatment by assuming that the partial molar
volumes of solution species are independent of nonideal interactions, i.e.,
the mixed solution density is calculated based on linear additivity of pure
component solid or liquid volume contributions to obtain the HGF at a given
RH. Differences in the density models are expected to lead to relatively
small differences, typically on the order of the HTDMA measurement error or
less (e.g., Fig. 2a), in the application to HGF predictions, as demonstrated
by Lei et al. (2014) for the case of diameter vs. mass-based HGFs of AS
droplets. Both models include sophisticated sets of equations to compute
activity coefficients of all solution components in a thermodynamically
consistent manner.
Hygroscopic growth,
deliquescence, and efflorescence of aerosol particles. Hygroscopic growth
factors of (a) ammonium sulfate (AS), (b) levoglucosan,
(c) 4-hydroxybenzoic acid, and (d) humic acid aerosol
particles with a dry diameter of 100 nm (open, black square). In this
study, the green curves show E-AIM predictions, the red curves the AIOMFAC
predictions, and the blue lines the fitted expression (Eq. 5).
κ-Köhler theory and computation of the
hygroscopicity parameter κ
The hygroscopicity parameter, κ, is commonly used to characterize the
relative hygroscopicities of individual aerosol particles, known mixtures, or
complicated atmospheric aerosols (Petters and Kreidenweis, 2007) and to
model the composition dependence of the solution water activity. The
saturation ratio, S, in the traditional Köhler equation (Eq. 3) over
an aqueous droplet is calculated from
S=aw4σsMwRTρwDwet,
where aw is the mole-fraction-based water activity in solution
and
Mw and ρw are the molar mass of water and the
density of pure water in the liquid state at temperature T, respectively.
Dwet, the wet particle diameter at a given RH, is defined by
Dwet=HGF×D0. D0 denotes the diameter at dry
conditions at a RH below 5 %. The solution surface tension is denoted by
σs. In the κ-Köhler theory, the bulk
solution water activity is described by a single parameter κ, with the
hygroscopic parameter of the overall mixture related to Eq. (3) by
κHGF=1-HGF3+HGF3-1Sexp4σMwRTρwDwet.
This expression describes effective values of κHGF as
a function of droplet diameter and HGF at a certain
saturation ratio. In turn, known (measured) solution
κHGF values or component-specific κi values
can be used to parameterize or predict the HGF curve of
a mixture (Petters and Kreidenweis, 2007).
GF data fit
As described by Dick et al. (2000), the relationship between measured HGFs
and water activity can alternatively be parameterized by the following
expression:
HGF=1+c1+c2×aw+c3×aw2aw1-aw13.
By substitution of Eq. (3) for aw in Eq. (5) and a fit to the
measured HGF, the three adjustable coefficients c1,c2, and c3 of
Eq. (5) were determined. The coefficient values are given in Table 3 for the
different organic compounds considered.
Coefficients (c1, c2, c3) of the fitted growth factor
parameterization (Eq. 5) as follows.
Chemical compounds
c1
c2
c3
Levoglucosan
0.12868746
0.36582023
-0.39840382
4-Hydroxybenzoic acid
-1.389967 ×10-1
2.325586 ×10-1
-9.891943 ×10-2
Humic acid
-1.618304 ×10-2
2.202483 ×10-1
2.005134 ×10-2
GF prediction by ZSR
The ZSR mixing rule is widely used to approximate
the water uptake of mixed systems by assuming additivity of the water uptake
of each individual component in the mixed particles at a given RH (e.g., Malm
and Kreidenweis, 1997). HGFmix is based on the
HGFj of pure components j and their corresponding volume
fraction, εj, in the mixed particles.
HGFmix=∑jεjHGFj313
Results and discussion
GF of single-compound systems
Figure 2a shows the measured diameter growth factors of AS particles as
a function of RH for both humidification and dehumidification conditions. The
measured ERH of 100 nm AS particles is approximately 35 % RH at
298.15 K. The model-predicted GF and solid–liquid phase
transition of AS are in relatively good agreement with the experimental data
and, in particular, the efflorescence (crystallization) of AS is captured by
the AIOMFAC and E-AIM models. The good model–measurement agreement for the
ERH is of course expected, since the aqueous AS system serves as the
reference system for determining the value pairs of
IAPAS(sat) and cAS on a molality and mole
fraction basis for use with AIOMFAC and E-AIM, respectively (Sect. 2.3). An
ERH of 31 to 40 % RH was reported by other groups for a range of particle
sizes and experimental techniques (Zardini et al., 2008; Ciobanu et al.,
2010). There are several factors that contributed to the variability in
reported ERH values, such as particle size, temperature, solution impurities,
and the stochastic nature of the homogeneous or heterogeneous nucleation of
a crystalline phase near ERH (Ciobanu et al., 2010).
In Fig. 2b, upon dehydration, no efflorescence of the levoglucosan aerosol
particles is observed even at a RH below 10 %. The agreement of the HGF
between the hydration and dehydration processes demonstrates that these
particles equilibrate with the surrounding water vapor under these moisture
conditions. For example, the measured diameter growth factors of levoglucosan
particles at 80, 60, and 30 % RH are 1.19, 1.09, and 1.03, respectively,
which are similar to results obtained for the hydration process of such
particles. Levoglucosan has a DRH of ∼ 80 to 83 % (for a bulk
system) at 293 to 298 K (Mochida and Kawamura, 2004; Zamora et al.,
2011). The similarity of diameter growth factors both under hydration and
dehydration conditions even below the DRH of levoglucosan is explained by the
lack of crystallization of levoglucosan upon drying to low RH and the
presence of a metastable supersaturated aqueous levoglucosan solution in both
the hydration and dehydration modes for experiments initiated with liquid
solution droplets (Mochida and Kawamura, 2004; Chan et al., 2005;
Svenningsson et al., 2006). A possible reason for a persistent metastable
supersaturated solution state is that levoglucosan particles remain liquid
(possibly a viscous liquid state) upon drying to below 5 % RH, which was
also observed previously with a reported ERH <4 % RH (Mochida and
Kawamura, 2004; Chan et al., 2005). Also, the measured diameter growth
factors of levoglucosan particles are in good agreement with those estimated
from the standard UNIFAC model within the E-AIM model and the AIOMFAC model,
within experimental uncertainty. The UNIFAC models within E-AIM and AIOMFAC
are based on the original model expressions by Fredenslund et al. (1975) and
both include the extensive parameter set by Hansen et al. (1991) as well as
revised parameters for certain group interactions of water with carboxyl and
hydroxyl groups by Peng et al. (2001). Of relevance for levoglucosan and
other sugar-like compounds, the AIOMFAC model also contains certain revised
group parameters for hydroxyl groups and special alkyl groups for their
interactions with water, introduced by Marcolli and Peter (2005) for polyols,
as further detailed in Zuend et al. (2011). However, the molecular structure
of levoglucosan with several polar functional groups in close vicinity may
account for a small deviation between modeled and measured HGFs at RH below
70 % because intramolecular interactions are not fully considered by
these models.
The measured diameter growth factors of 4-hydroxybenzoic acid particles shown
in Fig. 2c demonstrate an untypical increase in diameter of 4-hydroxybenzoic
acid particles during dehumidification from 90 to 10 % RH, which is
consistent with previous diameter growth factors for a few solid particles
(Mochida and Kawamura, 2004). The organic particles measured are likely
always in the effloresced, i.e., crystalline, state apparently even at high
RH. The apparent increase in diameter during dehumidification may be
explained by particle shape restructuring since the (poly)crystalline
particles are likely nonspherical at dry conditions but may become more
sphere-like in shape when exposed to higher RH (Mikhailov et al., 2004).
Also, no ERH of 4-hydroxybenzoic acid in the dehydration mode was observed
during the experiments; the likely reason is that the highest RH reached in
the humidifier was approximately 98 %, which may be below the ERH of
4-hydroxybenzoic acid, reported as above 98 % RH in another study
(Mochida and Kawamura, 2004). As discussed previously by Lei et al. (2014),
our HTDMA experiments are carried out such that RH =98 % is reached
initially before dehumidification to a series of RH values at set point RH2
(90–5 % RH); the crystallization of the organic, however, could occur at
above 90 % RH. In addition, deviations between measurements and model
prediction are obvious in Fig. 2c. The observations by far surpass the
expected error in model performance, which is typically less than 0.05 in HGF
units for RH <85 %, as also indicated by an intercomparison of the
AIOMFAC and E-AIM predictions in Fig. 2c and much-improved model–measurement
agreement for the case of mixed 4-hydroxybenzoic acid + AS particles
shown in Fig. 4 (discussed in Sect. 3. 2. 2). However, note that the validity
of the shown model predictions in Fig. 2c depends on whether the assumption
of a liquid solution droplet is plausible. Therefore, it is no surprise that
the model-predicted curves deviate from the experimental hygroscopic behavior
of 4-hydroxybenzoic acid particles. Morphology effects, such as the
restructuring of nonspherical polycrystalline particles over a certain RH
range or liquid–liquid phase-separated particles of nonspherical shapes,
have been discussed by several groups (Sjogren et al., 2007; Reid et al.,
2011; Lei et al., 2014). In the case of hygroscopic growth
of pure 4-hydroxybenzoic acid particles and mixtures of 4-hydroxybenzoic acid with ammonium sulfate, an offset between
measurement and model predictions was observed both in the RH range below the
deliquescence of the particles and above it, i.e., above 80 % RH (Lei
et al., 2014). It is suggested that deviations are primarily caused by
a change in solid-state particle morphology during hydration, leading to
a restructuring of the polycrystalline particle shape towards a more compact,
near-spherical shape as the RH approaches the particle deliquescence point.
This would explain rather uncommon HGF values of less than 1.0 at an elevated
RH, also shown in Fig. 2c. Similar behavior was found for experimental growth
factors of mixtures containing adipic acid and AS and systematic deviations
between the associated ZSR predictions and observations by Sjogren
et al. (2007). Thus, while experimental data hint to the possible influence
of nonspherical particles and their humidity-induced restructuring as
a source of uncertainty, model predictions of HGF, such as those with the
AIOMFAC model, assume by default a spherical particle shape even for solid
phases and/or in cases in which liquid–liquid phase separation (LLPS) is
present.
The measured HGF curves of humic acid aerosol particles during
dehumidification and humidification measurements do not agree very well
within experimental uncertainty, in particular above 70 % RH. For
instance, the growth factor of humic acid aerosol particles at 80 % RH is
1.2 according to the dehumidification measurement, which is higher than
the HGF of humic acid particles in the humidification mode
at the same RH. Humic acid aerosol particles shrink continuously due to loss
of water content in the range from 90 to 10 % RH. For example, a stepwise
change in the water absorption and desorption behavior within different RH
ranges was observed in the case of Nordic aquatic fulvic acid (NAFA) and
Suwannee River fulvic acid by Chan and Chan (2005). These hygroscopic
behaviors suggest that humic acid particles and structurally similar
compounds retain some water down to the low RH levels achieved in the
instruments (imperfect drying during particle residence in the instrument).
In addition, the experimental growth factor of humic acid aerosol particles
during dehumidification can be represented well by fitting Eq. (5) to the
measurements. The determined fit parameters are listed in Table 3. The humic
acid sample used (Aldrich, 99 %) is a mixture of different
polycarboxylic acids of undefined chemical structure. However, specific
information on the chemical structure and mixture composition is necessary
for corresponding model predictions with AIOMFAC and E-AIM. Therefore, no
such model calculations are shown in Fig. 2d.
GF of mixtures of organic surrogate
compounds + ammonium sulfate
Biomass burning aerosol particles are likely mixtures of a diversity of
inorganic constituents and organic compounds in the atmosphere. For example,
particles may consist of a combination of AS mixed with low-volatility
and semi-volatile organics from biomass burning emissions (Lee et al., 2003;
Zhang et al., 2007; Pratt and Prather, 2010). Different water solubilities
and hygroscopic behavior of distinct organic compounds may affect the
HGFs of mixtures of partially or fully dissolved
inorganic and organic components. For example, Bodsworth et al. (2010)
studied the effect of different mass fractions of citric acid on the
efflorescence properties of mixed citric acid–ammonium sulfate particles at
lower temperatures and concluded that adding citric acid decreases the ERH of
ammonium sulfate in the mixed aerosol particles. These hygroscopic behaviors
of mixed aerosol particles, including phase transition in the range from
moderate to low RH, are the focus of attention in this study.
Hygroscopic growth, efflorescence of aerosol particles, and model
predictions represent the diameter growth factor during dehydration
experiments in the range from 90 to 5 % RH at 298.15 K
(a, b, c). Hygroscopic growth curves of mixtures consisting of
levoglucosan and ammonium sulfate (solid symbols) at three different
dry-state mass fractions for particles of an initial dry diameter of
100 nm at RH <5 % as compared to that of pure ammonium
sulfate (open symbols, “AS, obs”). AIOMFAC-based model predictions for bulk
systems are shown in red. E-AIM predictions are shown in green.
Mixed system: levoglucosan + ammonium sulfate
Figure 3 shows measured growth factors of mixed aerosol particles containing
levoglucosan + ammonium sulfate with different dry-state
organic-to-inorganic mass ratios (1:3, 1:1, 3:1) in the RH range from
90 to 10 %. There is a reduction in the diameter growth factor of aerosol
particles containing levoglucosan and AS with increasing levoglucosan mass
fraction, as expected from a ZSR-like additivity concept of hygroscopicity.
When the concentration of levoglucosan is low (25 wt %), a clear
efflorescence signature of AS is found, within the ERH shifting to a higher
RH (40–45 %) in comparison to the ERH of pure AS occurring at
33–35 % RH (Fig. 3a). A similar phenomenon has been found for the
certain mixtures of NaCl and NAFA, in which the crystallization of NaCl
shifted to higher RH by mixing with NAFA at a mass ratio of 1:1 (Chan and
Chan. 2003). With an increasing mass fraction of levoglucosan (i.e., 50 and
75 wt %), the mixtures release water gradually and no crystallization
of AS was observed. Although a small step in the growth factor curve might
have occurred (indicative of the crystallization of AS), it cannot be
detected with sufficient certainty by our measurement setup. The rather high
viscosity of solutions containing levoglucosan is expected to increase
considerably toward RH (Marshall et al., 2016). This increase in viscosity
might impede the crystallization of AS in the mixed systems on the timescale
of the experiment. Mass transfer limitation effects on the deliquescence or
efflorescence process of crystalline organic particles and the water uptake
or evaporation have been investigated in several experimental studies (Peng
et al., 2001; Choi and Chan, 2002; Chan and Chan, 2005; Sjogren et al., 2007;
Zardini et al., 2008; Ciobanu et al., 2010; Smith et al., 2012; Mikhailov
et al., 2013; Hodas et al., 2015). Mass transfer limitations may impact the
outcome of experiments significantly if the characteristic timescales for
equilibration are similar to or larger than the residence time of particles
in the experimental setup. In this study, the total residence time of the
aerosol sample during the equilibration phase before entering the DMA2 is
about 8 s. In order to improve the probability that the particles
reach equilibrium with the target RH during this residence time, the
monodisperse aerosol selected by DMA1 is first humidified to 98 % RH. The
aerosol particles are then exposed to a lower target RH by a two-step process
using double Nafion tubes. Kerminen (1997) estimated the necessary residence
time for achievement of water equilibrium of aqueous droplets to be between
0.005 and 0.1 s (water uptake coefficient αw=0.001,
25 ∘C) for 100 and
500 nm particles, respectively. Therefore, the typical residence time
of a few seconds in the humidification or dehumidification section in a HTDMA
measurement is assumed to be sufficient for most equilibrium hygroscopicity
measurements (Brooks et al., 2004; Mikhailov et al., 2004). Moreover, our HGF
results for the three pure organic components are in good agreement with data
by Mochida and Kawamura, (2004), Brooks et al. (2004), and Chan and Chan
(2005) conducted with different techniques and/or residence times. However,
there are cases in which water equilibration could be impeded substantially
in the presence of highly viscous or glassy particles at low RH, e.g., for
ternary sucrose + NaCl + water particles of >6 µm in
diameter studied by Bones et al. (2012), who report an equilibration
timescale >1000 s for such particles. Note that, aside from
viscosity, there is an important size dependence of the particles on the
equilibration timescale (e.g., Koop et al., 2011). For aqueous 100 nm
particles used in HTDMA experiments at room temperature, Bones et al. (2012)
indicate that the equilibration timescale for water is likely only of concern
for RH <10 % in such an instrument. We therefore conclude that the
residence time of 8 s is very likely sufficient to allow for
equilibrium HGF measurements in dehydration mode, at least down to
10 % RH (when starting with aqueous solution droplets).
Mass transfer effects in hygroscopicity measurements of aerosol particles
during hydration conditions have been encountered previously, particularly
when a solid–liquid phase transition (deliquescence) is involved (Peng
et al., 2001; Chan and Chan, 2005). For example, Peng et al. (2001) observed,
in electrodynamic balance experiments under conditions of very slow
humidification, that glutaric acid aerosol particles showed a deliquescence
phase transition in the RH range from 83 to 85 % over the course of
several hours. This is a much longer time span than that of
∼ 40 min for the deliquescence of other super-micron sized
dicarboxylic acid particles (e.g., malonic acid) in electrodynamic balance
experiments. This observation indicates that the solid–liquid phase
transition of glutaric acid particles may likely be mass transfer limited
during the hydration process. In this context, it is possible that the
deliquescence of initially solid, pure 4-hydroxybenzoic acid particles at
RH >97 % is further impeded by slow dissolution, which could have
led to the absence of deliquesced particles (Fig. 2c) on an experimental
timescale.
In addition, the measured diameter growth factors of mixtures of levoglucosan
and AS are compared to calculations of hygroscopic growth by the E-AIM and
AIOMFAC models. The E-AIM prediction is in relatively good agreement with
results from the HTDMA measurement but typically overestimates the water
content of particles consisting of organic–AS mixtures at the RH range close
to the ERH of AS. The liquid–solid phase transition of AS in
the mixed particles is considered in the E-AIM assumptions as described in
Sect. 2.3. There is a more distinct shift in ERH of AS with higher mass
fractions of levoglucosan. In the case of the AIOMFAC and E-AIM model
predictions, it is assumed that the diameter growth factor contribution from
AS is zero below the predicted ERH, i.e., there the growth factor deviation
from 1.0 is solely due to the organic water uptake. The model prediction
shows a slight deviation from the measurements, which may be in part due to
(i) model uncertainty in the correct description of the hygroscopicity of
levoglucosan, (ii) incomplete representation of AS + levoglucosan
interactions in aqueous solutions, and (iii) measurement error.
Also, in the case of mixtures consisting of AS and levoglucosan with an
organic-to-inorganic dry mass ratio of 3:1 (75 wt % levoglucosan of
dry particle composition), the underestimation of the growth factor by the
AIOMFAC model at RH <35 % in comparison to the measurements is
explained in part by the model prediction of AS efflorescence (which seems to
be absent in the measurements). However, with a decreasing AS mass fraction,
the hygroscopic behavior of levoglucosan dominates the diameter growth
factors of the mixtures, in relative agreement with the AIOMFAC-modeled
“dehydration branch” prediction. Minor differences in the AIOMFAC
prediction vs. measurement for diameter growth factors of mixed
levoglucosan and AS in the RH range of 35–25 % here might be attributed
to mixture viscosity effects at the higher levoglucosan contents, which may
suppress the efflorescence of AS in the mixed systems on an experimental
timescale or it could simply be due to sufficient miscibility of dissolved AS
in the aqueous levoglucosan solution (beyond that predicted by the model),
such that a small step change due to AS efflorescence could be beyond the
experimental detection range. As a result, accounting for the effect of the
organic components on the diameter growth factors of mixtures within aerosol
particles is crucial to accurately modeling the equilibrium hygroscopic
behavior.
Mixed system: 4-hydroxybenzoic acid + ammonium
sulfate
Mixtures of 4-hydroxybenzoic acid + AS with different organic mass
fractions (25, 50, 75 wt %) exhibit a gradual water desorption before
the AS fraction of the particle effloresces at a certain RH. With an increasing
4-hydroxybenzoic acid mass fraction, no discontinuity step at the
corresponding ERH in the dehydration curve of mixtures is observed. This
suggests the presence of 4-hydroxybenzoic acid in the liquid state retards or
offsets the efflorescence of AS in the mixtures. An interesting yet
contrasting phenomenon was observed for the hydration process of aerosol
mixtures containing 4-hydroxybenzoic acid and AS by Lei et al. (2014). For
the case of these mixtures during moistening, the deliquescence of AS in the mixed particles remains unaffected, within experimental
resolution, by the presence of 4-hydroxybenzoic acid (Lei et al., 2014).
Similar behavior has been observed for particles containing certain organic
acids of limited water-solubility mixed AS (Choi and Chan,
2002; Chan and Chan, 2003). For example, mixtures for succinic
acid + AS showed no substantial influence on the
deliquescence RH of AS in the hydration process (Choi and Chan,
2002). However, a clear RH shift of the deliquescence phase transition of
AS or sodium chloride was determined for mixed particles
containing organic acids of higher water solubility and O : C ratio, such
as citric acid and malonic acid (e.g., Choi and Chan, 2002). The DRH and ERH
of pure organics and AS in the mixed organic–AS particles are summarized in
Table 4; the measurements indicate that 4-hydroxybenzoic acid has
a significant effect on the efflorescence of AS when present in a sufficient
amount. Also, there is a clear reduction in the diameter growth factors prior
to crystallization for mixtures with increasing 4-hydroxybenzoic acid mass
fraction.
Experimental studies of organic and ammonium sulfate (AS)
deliquescence and efflorescence RH from this work and previous studies at
298 K.
Signal compound/mixture
Organic mass
Deliquescence relative
Efflorescence relative
fraction
humidity of AS or
humidity of AS or
(%)
organic in the
organic in the
mixed particle
mixed particle
Levoglucosan
–
80 %a,b
< 4 %a,b
82.8 %c
Levoglucosan +AS
25
80 %
45 %
50
–
–
75
–
–
4-Hydroxybenzoic acid
–
> 97 %a,b
< 4 %a,b
4-Hydroxybenzoic
25
80 %
35 %
acid +AS
50
80 %
25 %
75
80 %
–
Humic acid
–
–
–
Humic acid +AS
20
80 %
35 %
50
80 %
35 %
75
80 %
35 %
a The DRH and ERH of pure organic components.
b Mochida and Kawamura (2004).
c Zamora et al. (2011).
Hygroscopic growth factors, efflorescence of behavior, and
model predications for dehydration experiments in the range from 90
to 5 % RH at 298.15 K (a, b, c). Hygroscopic growth
curves of mixtures
consisting of 4-hydroxybenzoic acid and ammonium sulfate (solid
symbols) at three different dry-state mass fractions (initial dry
diameter of 100 nm at RH <5 %) as compared to those
of pure ammonium sulfate (open symbols). AIOMFAC-based model
predictions for bulk systems are shown in red; E-AIM-predictions are
shown in green for the case assuming that 4-hydroxybenzoic acid
remains in the liquid state. Shaded rectangle: RH range of gradual
crystallization of 4-hydroxybenzoic acid.
The measurements of mixtures consisting of 4-hydroxybenzoic acid and AS are
compared with model predictions based on different assumptions about the
phase state of the organic component since the deviation from measurements
might partly be explained by a transition in the physical state of the
organic component. The E-AIM model prediction refers to a system in which the
mixture of 4-hydroxybenzoic acid is assumed to be in the liquid state at all
RH levels and in which the efflorescence of AS is considered. Neglecting the
potential efflorescence of the organic component in the dehydration branch
makes a systematic offset more obvious prior to the efflorescence of AS.
A good E-AIM model–measurement agreement occurs below the predicted ERH of
AS for mixed particles. The overestimation of HGFs before the efflorescence
of AS is explained by the AIOMFAC model prediction with distinct assumptions
about the phase state of the organic component. A possible reason for
the departure of model–measurement agreement at RH <80 % is that
there two liquid-to-solid phase transitions occur in the mixed particles:
a gradual one for the organic component and a step-like one for AS at a lower
RH. This phenomenon is shown in the grid square range in Fig. 4 and is
supported by comparison of the measured HGF data with AIOMFAC-based
predictions for two assumptions about the organic phase state, especially in
the case of mixtures with 50 and 75 wt % organic. We acknowledge that
the model predictions of the HGF curves for the two organic phase state
assumptions differ within experimental error for the case shown in Fig. 4a,
indicating that alternative explanations, such as model–measurement
uncertainty in the absence of a liquid–solid phase transition, could explain
the observations. In the Fig. 4b, good agreement between measurements and the
AIOMFAC model prediction with liquid organic assumption is found for RH >65 %, while for RH ≤60 % the experimental data agree very well
with the dashed red model curve for the case with consideration of a solid
organic component. This suggests that crystallization followed by gradually
increasing partitioning of organic from the solution to the solid organic
phase occurs in the range from 70 to 60 % RH under conditions of
dehumidification. Similarly, a liquid-to-solid phase transition occurs for
the cases with an organic : AS mass ratio of 3:1 in the range from 80
to 50 % RH. Meanwhile, AS remains dissolved in a supersaturated aqueous
solution phase. Moreover, the AIOMFAC-based equilibrium model predicts a LLPS
to occur at a RH below ∼ 90 % for the calculation cases with the
assumption of the organic in the liquid state (for all three organic mass
fractions in Fig. 4). This prediction leads to a liquid phase enriched in
4-hydroxybenzoic acid with some water and AS dissolved and a coexisting
liquid phase enriched in AS and water. The onset of the LLPS during
dehumidification leads to the kink in the red model curve near 90 % RH
since the slope of the HGF curve with RH changes in a non-smooth manner at
the point of the LLPS phase transition. This change in slope is not
noticeable from the experimental data alone, but the model–measurement
comparison for the range above 80 % RH shows very good agreement. The two
liquid phases will likely remain separated until nucleation of a crystalline
4-hydroxybenzoic acid phase occurs followed by gradual partitioning of the
organic acid to the solid phase with decreasing RH (to ∼ 50 % RH),
at which point only a single liquid phase (an aqueous AS phase with a tiny
amount of dissolved humic acid) will remain until efflorescence of AS occurs.
Above ∼ 90 % RH, a single, homogeneous liquid phase is predicted to
exist. Interestingly, this AIOMFAC model–measurement comparison (Fig. 4,
especially panels b and c) provides reasonable evidence that 4-hydroxybenzoic
acid remains dissolved and therefore in a liquid phase state at high RH in
the mixed particles upon dehumidification (it is present in both liquid
phases below 90 % RH, but highly enriched in the AS-poor phase). In
contrast, in the case of pure 4-hydroxybenzoic acid aerosol particles,
particles exposed to an initial RH of ≥90 % remain in the solid
state (or crystallize at RH >90 %) in the dehydration mode (Fig. 2c).
What factors contribute to keeping the organic in the liquid solution? It is
possible that the aerosols generated with those mixed solutions allowed the
4-hydroxybenzoic acid to fully dissolve as the AS provided substantial
particle-phase water content (within short time) into which the organic could
be dissolved and may have then further contributed to water uptake associated
with the organic's hygroscopicity (unlike in the case of the pure
4-hydroxybenzoic acid particles). The 4-hydroxybenzoic acid remains dissolved
in the mixture, possibly supersaturated with respect to the crystalline
organic state (similar to how AS stays supersaturated at RH below the DRH
during drying). We consider this a reasonable explanation for the observed
HGF data from the HTDMA in comparison to the different AIOMFAC-based curves.
Mixed system: humic acid + ammonium sulfate
Figure 5 shows that the experimental diameter growth factors of mixtures
consisting of humic acid and AS with dry mass ratios of 1:3, 1:1, or
3:1 decrease with an increasing mass fraction of humic acid at RH >35 %. For
example, at 35 % RH the measured HGF are 1.1, 1.05, and 1.05 for the
particles consisting of 25, 50, and 75 wt % humic acid. In comparison,
the diameter growth factor of pure supersaturated AS particles is
∼ 1.13 just prior to efflorescence of AS. Humic acid, unlike
levoglucosan and 4-hydroxybenzoic acid aerosol particles, has no noticeable
effect on the efflorescence point of AS in the mixed aerosol particles.
Results of the ZSR model agree well with measured hygroscopic growth for
mixtures within the experimental error. The ZSR curves shown in Fig. 5 are
based on the RH-dependent fitted HGFs of humic acid
with Eq. (5) and the AIOMFAC-predicted diameter growth factors of AS in the
dehydration mode. The success of the ZSR mixing rule for this system suggests
that interactions of organic molecules with AS ions in aqueous
solution will only marginally affect the HGFs of the
mixtures containing humic acid and AS. Due to the lack of detailed
information about the actual chemical structures of humic acid samples used,
it was not possible to perform E-AIM and AIOMFAC model predictions for
comparison with the measurement.
Hygroscopic growth factors and efflorescence of aerosol
particles and constituents consisting of humic acid and ammonium sulfate
at three different dry-state mass fractions with initial dry
diameter of 100 nm at RH <5 % as compared to that of
pure ammonium sulfate (open symbols). Colored curves: ZSR
predictions of diameter growth factors for dry particle compositions
corresponding to the experimental data during dehumidification in
the range from 90 to 5 % RH at 298.15 K.
Mixtures of biomass burning organic surrogate components
with ammonium sulfate
According to Decesari et al. (2006), sampling of aerosol particles, including
the water-soluble organic carbon (WSOC) fraction, was conducted from
9 September to 14 November 2002 in their field study; the sampling time was
subdivided into different periods. Despite significant changes in the
chemical composition of tracer compounds from the dry to the wet period, the
functional groups and general chemical classes of WSOC changed only to
a small extent in the Amazon Basin near Rondônia, Brazil. Model compounds
represent semiquantitatively (presence and abundance of functional groups)
and the chemical structure of WSOC can be used as surrogates in microphysical
models involving organic aerosol particles over tropical areas affected by
biomass burning scenarios (Andreae et al., 2002; Artaxo et al., 2002; Rissler
et al., 2006; Decesari et al., 2006). Here, we focus on experimental
observations and model calculations for relatively simple mixtures of
inorganic–organic surrogate components reflecting mixtures of aerosol
components found during different seasons during biomass burning events.
However, we are fully aware of the fact that actual biomass burning aerosols
are typically much more complex in terms of particle chemical composition.
Aerosol particle properties from biomass burning events depend on the type of
source, external–internal population mixing state, water-solubilities, and
phase state of the diversity of organic compounds and their mixing with
inorganic constituents during different time periods in the field (e.g.,
Decesari et al., 2006).
Hygroscopicity parameter, κ, representing mixed aerosol
particles consisting of organic surrogate components and ammonium sulfate at
different periods (initial dry diameter of 100 nm at RH <5 %). The black curves in both panels show the κ
prediction from
Eq. (4) with HGFmix calculated with
Eq. (6) using component volume fractions and the HGF of the
individual mixture components from a fit to the laboratory data
(using Eq. 5). The black symbols and error bars show field data from
the Amazon Basin during the dry and wet periods at conditions of water
vapor subsaturation (HTDMA measurement) and supersaturation
(κCNN) (Whitehead et al., 2016; Pöhlker et al., 2016).
Mixture system: mix-bio-dry and mix-bio-wet aerosol
particles
Figure 6a shows the small differences observed in the hygroscopicity
parameter κ for mixtures of organic surrogate components and AS
representing biomass burning particles during the dry and wet periods in the
Amazon, respectively. Hygroscopicity parameter values for bio-mix-dry aerosol
particles were determined to be between 0.16 and 0.18, with a decreasing RH
in the range from 90 to 40 % RH. The κ value representing the wet
period in the Amazon is shown in Fig. 6b, derived from laboratory HTDMA
measurements in the range from 90 to 40 % RH. A similar trend of an
increase in κ with a decrease in RH has also been observed by Cheung
et al. (2015). Their observation is based on ambient particle measurement
with a HTDMA in Hong Kong, therefore probing particles of more complex
compositions in the field campaign. The variability in the hygroscopicity
parameter in subsaturated conditions reveals some limitations of
a single-parameter hygroscopicity model for applications over a wide range of
RH values. At low, intermediate, and high RH levels, differing degrees of
solution nonideality, potential for liquid–liquid phase separation,
water-solubility limitations of organics in ambient organic–inorganic
particles, and assumptions about constant–variable surface tension may all
play a role (Mikhailov et al., 2009; Rastak et al., 2017; Ovadnevaite
et al., 2017). In the case of κ of organic surrogates mixed with AS,
the relevant κ value range is ∼ 0.12 to 0.15 obtained from 90 to
40 % RH. The measured κ values of the mixtures are compared to
field data of HTDMA and CCN measurements conducted at a remote rainforest
site in the central Amazon Basin during the dry and wet seasons (Whitehead
et al., 2016; Pöhlker et al., 2016), which are consistent with κ
obtained at similar field sites (within the uncertainty of experiments). The
likely reason for a relatively good agreement between the hygroscopicity of
the laboratory mixtures and the field data is that the organic mass fractions
of the mix-bio-dry and mix-bio-wet mixtures are chosen in our laboratory
experiments to be similar to those of the latest field data from the Amazon
Basin. For example, Pöhlker et al. (2016) obtained the effective
hygroscopicity parameters κ between 0.3±0.01 and 0.15±0.01
based on the organic mass fraction range from 0.65 to 0.97 in the dry season
using aerosol chemical speciation monitor and CCN measurements. The predicted
κ values of the mixtures at various RH levels shown in Fig. 6 (black
curves) are obtained through application of Eq. (4) with use of the
RH-dependent fitted HGFs of the organic surrogates (Eq. 5), the predicted
growth factor of AS from the AIOMFAC model (for the humidification case), and
the mixing rule based on volume fraction for a mixture's HGF (Eq. 6). For
these calculations, a solution surface tension of 0.072 Jm-2 was
assumed. These predictions agree relatively well with the experimental
κdry and κwet values obtained from the
HTDMA over a wide range in RH values referring to dehumidification conditions
(no solid AS). Furthermore, the combined approach of Eqs. (4)–(6) allows for
a prediction of the change in κ at high RH towards water vapor
supersaturation. A small difference in κ between sub- and
supersaturated conditions is observed for our mixed systems when comparing
the HTDMA data and predictions at 90 % RH with the predictions near
100 % RH and the κ values from the CNN field measurements. The
difference is more pronounced for the wet season case. Rastak et al. (2017)
observed a marked difference in apparent hygroscopicity and related mixture
κ of the organic aerosols (AS-free) occurring in the case of
monoterpene-derived secondary organic aerosol (SOA) for sub- vs.
supersaturated conditions. A smaller difference was reported for the
isoprene-derived SOA (Pajunoja et al., 2015: Rastak et al., 2017), more like
the difference observed here for the mixtures containing AS (and therefore
having overall higher κ values than typical salt-free organic
aerosols). Rastak et al. (2017) attribute the distinct difference in
κSOA of the monoterpene SOA to the limited mutual solubility
of certain SOA components in water because a single liquid organic phase of
monoterpene oxidation products is present at RH below 95 %, but over a RH
range above 95 %, liquid–liquid phase separation is observed using
optical microscopy as well as predicted by the AIOMFAC-based equilibrium
model. In the mix-bio-wet and mix-bio-dry cases shown in Fig. 6, the likely
reason for the change in characteristic mixture hygroscopicity is not
necessarily due to a liquid–liquid phase separation at high RH. For example,
the κ parameter obtained from field data is ∼ 0.15 ± 0.06
at 90 % RH, while its value reaches ∼ 0.18 ± 0.04 at
RH > 100 % (just prior to CCN activation). A likely reason for the
difference is that hygroscopic particles, especially those containing
sparingly soluble organics like 4-hydroxybenzoic acid, take up water
dramatically above 95 % RH when approaching 100 % RH (Hartz et al.,
2006; Chan et al., 2008; Rastak et al., 2017), which is clear from model
predictions, as demonstrated in Fig. 6 by application of Eq. (4). The
predicted curve in the mixture's effective κ parameter may well
capture the change in hygroscopicity under such high RH conditions.
Consequently, for a precise representation of the hygroscopic growth behavior
(e.g., HGF) at high RH (>95 %) by the κ-Köhler model, the
value of κ would need to be varied. While a variable κ value is
contrary to the attempted simplicity of the single-parameter
κ-Köhler model, it is at least advisable to consider that
κ values derived from HGF data at 80 or 90 % RH may not apply
accurately for the calculation of CCN activation properties of such biomass
burning aerosols.
To summarize, there is small difference in hygroscopicity parameters between
subsaturated measurement conditions at 90 % RH in the laboratory with
HTDMA and supersaturated conditions using CCN measurements, in agreement with
the findings of other studies. On a regional scale, in the dry and wet
periods, the hygroscopic behavior in some extent of the Amazon rainforest is
influenced significantly by the biomass burning emissions, which enhances CCN
activity and droplet number concentrations in warm clouds in that region and
influences the radiation balance and cloud lifetime (Pöschl et al.,
2010). Underestimation of organic surrogate component mass fractions in the
mixed particles or organic : sulfate mass ratios may be responsible for
the slight differences in the determined κ parameters of the
laboratory and field measurements.
Conclusions
A number of field-based hygroscopicity studies about biomass
burning aerosol focus on the growth factors of mixtures at high RH (e.g.,
90 % RH). However, less attention has been paid to the growth behavior at
low to moderate RH, limiting the database for accurate estimates of
particles'
optical and radiative properties over those lower RH ranges. However, this is
a RH range in which water uptake or release behavior demonstrates
a considerable variability among different organic–inorganic systems. The
occurrence or suppression of a liquid–solid phase transition affects the
physicochemical particle properties in a relatively narrow RH range,
potentially leading to particles of different morphology and physical states,
affecting effective particle size and density. In this work, measurements and
thermodynamic equilibrium predictions for organic–inorganic aerosols related
to components from biomass burning emissions demonstrate a diversity of
hygroscopic growth–shrinking behavior. For example, in the case of aerosol
mixtures containing levoglucosan and AS, the presence of
levoglucosan may cause the efflorescence of AS to occur at a higher RH than in
pure aqueous AS particles, or it may completely suppress AS efflorescence, as
observed for mixtures with a high levoglucosan mass fraction. The growth
curves predicted with an AIOMFAC-based thermodynamic equilibrium model
reproduce the observations in most cases reasonably well, and we demonstrate
the usefulness of predictions with different assumptions about the physical
state of the organic components for the interpretation of experimental data,
such as in the case of mixtures of 4-hydroxybenzoic acid and AS. However, the accurate prediction of AS efflorescence or its
suppression in mixed particles is difficult. The E-AIM-predicted growth
curves reproduce the measured hygroscopic behavior relatively well for the
consideration of the effect of 4-hydroxybenzoic acid on the hygroscopic
behavior of mixtures with AS, which leads to suppression of the
AS efflorescence. In the case of mixtures of humic acid and
AS, continuous water desorption of aerosol particles shows no
significant effect on the efflorescence of AS. Also, as
expected, there is a clear reduction in the diameter growth factor of mixed
systems, in comparison with that of pure AS particles. In addition, the small
difference in hygroscopicity parameters of mix-bio-dry and mix-bio-wet
systems between measured data in the laboratory using HTDMA and in the field
using CCN activity measurements is due to the similar O : C ratios of
organic surrogate compounds and AS mass fractions used in the
model mixtures when experimental κ data from sub- and supersaturated
water vapor conditions are compared.
The range of measurement–model comparisons presented in this study
indicate that providing accurate thermodynamic model predictions of
the hygroscopic growth behavior of mixed organic–inorganic systems
remains a challenging problem. At moderate and low RH levels, at which aerosol
solution phases become highly concentrated, step-like or gradual
crystallization and related solid–liquid equilibria may occur with
high sensitivity to the organic / inorganic mass ratio and the chemical
nature of the mixture constituents. To further improve thermodynamic
equilibrium models for the prediction of hygroscopicity and phase
transitions, controlled laboratory experiments with single solutes
and/or with mixed organic–inorganic systems of known phase state will
be useful to constrain model parameters. Ideally, such measurements
should cover the high, intermediate, and low RH ranges under
humidification and dehumidification conditions.