Hygroscopicity of nitrates and chlorides
DRH at different temperature
First we investigated the effect of temperature on the DRH of
Ca(NO3)2⚫4H2O, Mg(NO3)2⚫6H2O and MgCl2⚫6H2O, which are the most stable
forms of corresponding salts for the temperature range (5–30 ∘C)
considered in this work (Kelly and Wexler, 2005). Figure 1a
shows the change of RH and normalized sample mass as a function of time in
an experiment to measure the DRH of Mg(NO3)2⚫6H2O
at 25 ∘C. An abrupt and significant increase in sample mass was observed
when RH was increased from 52 % to 53 %, suggesting that the deliquescence
occurred between 52 % and 53 % RH. Therefore, its DRH was measured to be
52.5 ± 0.5 %; since RH for our VSA instrument had an absolute
uncertainty of ±1 % (as stated in Sect. 2.2), in our work an
uncertainty of ±1 %, instead of ±0.5 %, was assigned to
the measured DRH. It should be noted that the mass change was >15 % when RH was increased from 52 % to 53 %,
as shown in Fig. 1a; such
a large mass increase cannot be solely caused by water adsorption since the
mass of several monolayers of adsorbed water is estimated to be <1 % of the dry particle mass (Gu et
al., 2017b). The continuous but small decrease in sample mass (about 1 %
in total) with time (around 500–1000 min) before deliquescence took place,
as shown in Fig. 1a, was likely caused by desorption of residual water
contained by the sample under investigation.
Change of normalized sample mass (blue curve, right
y axis) and RH (black curve, left y axis) as a function of time. (a) A typical
experiment conducted to measure the DRH. (b) A typical experiment conducted
to measure mass hygroscopic growth factors. In the two experiments shown
here, Mg(NO3)2⚫6H2O was investigated at 25 ∘C. In this paper the sample mass was always normalized to its dry
mass.
Table 1 summarizes our measured DRH of Ca(NO3)2⚫4H2O,
Mg(NO3)2⚫6H2O and MgCl2⚫6H2O as a function of
temperature (5–30 ∘C). DRH values show a
strong dependence on temperature for Ca(NO3)2⚫4H2O
(decreasing from 60.5 % at 5 ∘C to 46.0 % at 30 ∘C) and a
weaker temperature dependence for Mg(NO3)2⚫6H2O
(decreasing from 57.5 % at 5 ∘C to 50.5 % at 30 ∘C); in
contrast, the DRH values of MgCl2⚫6H2O (31.5 %–32.5 %)
exhibit little variation with temperature (5–30 ∘C). Several previous
studies have reported the DRH of Ca(NO3)2⚫4H2O,
Mg(NO3)2⚫6H2O and MgCl2⚫6H2O,
and their results are compared with our work in the following paragraphs.
DRH (%) of Ca(NO3)2⚫4H2O,
Mg(NO3)2⚫6H2O and MgCl2⚫6H2O
measured in this work as a function of temperatures (5–30 ∘C).
Solubility data (mol kg water-1) compiled by Kelly and Wexler (2005)
were used to calculate solubilities in moles per mole of water. All the errors given
in this work are standard deviations. The A⋅ΔHs/R and ΔHs values were not
estimated for MgCl2⚫6H2O because the difference in its
measured DRH between 5 and 30 ∘C was very small or even insignificant.
Please refer to Sect. 3.1.1 for further details.
T (∘C)
Ca(NO3)2⚫4H2O
Mg(NO3)2⚫6H2O
MgCl2⚫6H2O
5
60.5±1.0
57.5±1.0
32.5±1.0
10
58.0±1.0
56.5±1.0
32.5±1.0
15
55.5±1.0
54.5±1.0
32.5±1.0
20
52.5±1.0
53.5±1.0
32.5±1.0
25
49.5±1.0
52.5±1.0
31.5±1.0
30
46.0±1.0
50.5±1.0
31.5±1.0
Solubility (mol kg water-1)
8.4
4.9
5.84
Solubility (A, mol mol water-1)
0.1512
0.0882
0.1051
A⋅ΔHs/R (K)
913±59
427±28
–
ΔHs (kJ mol-1)
50.2±3.3
40.3±2.6
–
Ca(NO3)2⚫4H2O
RH of air in equilibrium with saturated
Ca(NO3)2⚫4H2O solutions, i.e., the DRH values of
Ca(NO3)2⚫4H2O, was measured to be 55.9 %, 55.4 %,
50.5 % and 46.7 % at 15, 20, 25 and 30 ∘C (Adams and
Merz, 1929), and the absolute differences between DRH reported by Adams and
Merz (1929) and those measured in our work are <3 %. The water
vapor pressures of saturated Ca(NO3)2⚫4H2O
solutions were measured to be 0.693, 0.920, 1.253, 1.591 and 1.986 kPa at
10, 15, 20, 25 and 30 ∘C (Apelblat, 1992), corresponding to DRH
of 56 %, 54 %, 54 %, 50 % and 47 %, respectively; therefore, the absolute
difference between DRHs measured in our work and those derived from Apelblat
(1992) is <2 %. In another study (Al-Abadleh et al., 2003), RH over the saturated
Ca(NO3)2⚫4H2O solution was measured to be
57 ± 5 % at room temperature; in other words, Al-Abadleh et al. (2003)
reported a DRH of 57 ± 5 % for Ca(NO3)2⚫4H2O,
slightly larger than that (49.5 ± 1.0 % at 25 ∘C)
determined in our work.
Mg(NO3)2⚫6H2O
Water vapor pressures of saturated
Mg(NO3)2⚫6H2O solutions were determined to be
0.737, 1.017, 1.390, 1.813 and 2.306 kPa at 10, 15, 20, 25 and 30 ∘C
(Apelblat, 1992), giving DRHs of 60 %, 60 %, 59 %, 57 % and 54 % at
corresponding temperatures. The vapor pressure of saturated
Mg(NO3)2⚫6H2O solutions at 25 ∘C was
reported to be 1.674 and 1.666 kPa by another two studies
(Biggs et al., 1955; Robinson and Stokes, 1959),
corresponding to DRH of ∼ 53 %. In addition, the water
activity of the saturated Mg(NO3)2 solution was measured to be
0.528 at 25 ∘C (Rard et al., 2004), also suggesting
a DRH value of ∼ 53 %; similarly, RH over the saturated
Mg(NO3)2 solution was reported to be ∼ 53 % at
22–24 ∘C (Li et al., 2008b). Al-Abadleh and Grassian (2003)
investigated the phase transition of the Mg(NO3)2⚫6H2O
film, and its DRH was determined to be 49 %–54 % at 23 ∘C. As
shown in Table 1, DRHs measured in our work agree very well with those
reported by most previous studies (Biggs et al., 1955; Robinson and
Stokes, 1959; Al-Abadleh and Grassian, 2003; Rard et al., 2004), but are
always 3 %–5 % lower than those derived from Apelblat (1992). It is not
clear why DRH values measured by Apelblat (1992) at different temperatures
are always slightly higher than other studies.
MgCl2⚫6H2O
Kelly and Wexler (2005) calculated DRH of MgCl2⚫6H2O
from vapor pressures of saturated MgCl2⚫6H2O solutions
measured by previous work and found that DRH values were in the range of
33 %–34 % for temperatures at 0–40 ∘C. In addition, water activity of
the saturated MgCl2 solution was reported to be 0.3278 at 25 ∘C
(Rard and Miller, 1981), corresponding to a DRH value of
∼ 33 % for MgCl2⚫6H2O. The DRH values
of MgCl2⚫6H2O measured in our work, as summarized in
Table 1, show excellent agreement with those reported by previous work
(Rard and Miller, 1981; Kelly and Wexler, 2005). Phase transition and
deliquescence behavior of CaCl2⚫6H2O were also
investigated in our work and found to be very complex, and the result will
be discussed in Sect. 3.1.3.
Temperature in the troposphere varies from ∼ 200 to
>300 K, and it is thus warranted to explore the effects of
temperature on hygroscopic properties of atmospherically relevant particles.
The dependence of DRH on temperature can usually be approximated by the
Clausius–Clapeyron equation (Wexler and Seinfeld, 1991; Seinfeld and
Pandis, 2016; Jia et al., 2018):
lnDRHT=lnDRH298+A⋅ΔHsR1T-1298,
where T is temperature (K), DRH(T) and DRH(298) are the DRHs at T and 298 K, R is
the gas constant (8.314 J mol-1 K-1), and ΔHs is the
enthalpy of dissolution (J mol-1). The dimensionless constant, A, is
numerically equal to the water solubility of the salt under investigation in
the unit of moles per mole of water. Figure 2 shows the dependence of DRH values
on temperature for Ca(NO3)2⚫4H2O and
Mg(NO3)2⚫6H2O, confirming that Eq. (2) can indeed
approximate the temperature dependence. The slope, which is equal to
A⋅ΔHs/R, was determined to be 913 ± 59 K for
Ca(NO3)2⚫4H2O and 427 ± 28 K for
Mg(NO3)2⚫6H2O, and thus ΔHs was
derived to be 50.2 ± 3.3 kJ mol-1 for
Ca(NO3)2⚫4H2O and 40.3 ± 2.6 kJ mol-1
for Mg(NO3)2⚫6H2O. It should be noted that for Eq. (2) to
be valid, both the enthalpy of dissolution and the water solubility
are assumed to be constant for the temperature range considered. The
variation in DRH with temperature (5–30 ∘C) was very small and even
insignificant for MgCl2⚫6H2O; as a result, we did not
attempt to estimate the ΔHs value for MgCl2⚫6H2O
since such an estimation would have large errors.
Dependence of DRH on temperature for
Ca(NO3)2⚫4H2O and Mg(NO3)2⚫6H2O.
Water-to-solute ratios as a function of RH
The change of sample mass with RH (0 %–90 %) was measured at 5 and
25 ∘C for Ca(NO3)2⚫4H2O,
Mg(NO3)2⚫6H2O and MgCl2⚫6H2O,
using the VSA. The mass change, relative to that at
0 % RH, can be used to calculate water-to-solute ratios (WSRs, defined in
this work as the molar ratio of H2O to Ca2+ or Mg2+) for
deliquesced samples. Small increases in m/m0 (typically <2 %)
were observed for some compounds (as shown in Tables 2 and 6) when RH was
below corresponding DRH values, mainly due to water adsorption or desorption
and baseline drift. As summarized in Table 2, decrease in temperature would
lead to increase in WSR at a given RH: at 90 % RH for example, WSRs were
determined to be 28.78 ± 0.20 at 25 ∘C and 31.80 ± 0.96 at 5 ∘C for
Ca(NO3)2⚫4H2O, 36.87 ± 0.23 at 25 ∘C and 41.40 ± 1.36
at 5 ∘C for Mg(NO3)2⚫6H2O, and 36.26 ± 1.76 at 25 ∘C and
39.55 ± 2.43 at 5 ∘C for MgCl2⚫6H2O. As discussed in
Sect. 3.1.1, the enthalpies of dissolution (ΔHs) are negative
for these compounds, suggesting that their dissolution processes in water
are exothermic; therefore, dissolution is favored at lower temperatures and
at a given RH, decrease in temperature would lead to increase in WSR in the
aqueous solutions. Several previous studies have measured RH over aqueous
Ca(NO3)2, Mg(NO3)2 and MgCl2 solutions at given
concentrations, and their results are compared with our work, as discussed
below.
Mass growth factors (m/m0, defined as the ratio of
sample mass at a given RH to that at 0 % RH) and water-to-solute ratios
(WSRs) as a function of RH (0 %–90 %) at 25 and 5 ∘C for
Ca(NO3)2⚫4H2O, Mg(NO3)2⚫6H2O and MgCl2⚫6H2O. WSRs were only calculated for
RH exceeding the DRH (i.e., when the sample was deliquesced). All the errors
given in this work are standard deviations.
Ca(NO3)2⚫4H2O, 25 ∘C
Ca(NO3)2⚫4H2O, 5 ∘C
RH (%)
m/m0
WSR
m/m0
WSR
0
1.000±0.001
–
1.000±0.001
–
10
1.000±0.001
–
1.001±0.001
–
20
1.014±0.005
–
1.005±0.003
–
30
1.016±0.007
–
1.005±0.002
–
40
1.017±0.009
–
1.009±0.003
–
50
1.237±0.006
7.10±0.03
1.032±0.005
–
60
1.363±0.008
8.76±0.05
1.041±0.002
–
70
1.550±0.009
11.22±0.06
1.610±0.010
12.00±0.07
80
1.897±0.012
15.77±0.10
1.979±0.027
16.85±0.23
90
2.889±0.020
28.78±0.20
3.119±0.095
31.80±0.96
Mg(NO3)2⚫6H2O, 25 ∘C
Mg(NO3)2⚫6H2O, 5 ∘C
RH (%)
m/m0
WSR
m/m0
WSR
0
1.000±0.001
–
1.000±0.001
–
10
1.000±0.001
–
1.000±0.001
–
20
1.000±0.001
–
1.000±0.001
–
30
1.001±0.001
–
1.000±0.001
–
40
1.001±0.001
–
1.000±0.001
–
50
1.000±0.001
–
1.000±0.001
–
60
1.503±0.001
13.15±0.01
1.539±0.003
13.67±0.03
70
1.724±0.001
16.30±0.01
1.773±0.007
16.99±0.07
80
2.121±0.001
21.94±0.01
2.203±0.021
23.11±0.22
90
3.171±0.029
36.87±0.23
3.489±0.114
41.40±1.36
MgCl2⚫6H2O, 25 ∘C
MgCl2⚫6H2O, 5 ∘C
RH (%)
m/m0
WSR
m/m0
WSR
0
1.000±0.001
–
1.000±0.001
–
10
1.000±0.001
–
1.000±0.001
–
20
1.000±0.001
–
1.000±0.001
–
30
1.001±0.001
–
1.000±0.001
–
40
1.344±0.057
9.89±0.42
1.327±0.082
9.69±0.60
50
1.489±0.062
11.52±0.48
1.473±0.090
11.34±0.69
60
1.677±0.072
13.65±0.58
1.667±0.100
13.52±0.82
70
1.951±0.084
16.74±0.72
1.950±0.117
16.72±1.00
80
2.433±0.117
22.18±1.06
2.465±0.148
22.54±1.35
90
3.681±0.178
36.26±1.76
3.972±0.244
39.55±2.43
Ca(NO3)2
Water
activities of Ca(NO3)2 solutions at 25 ∘C were measured to be
0.904, 0.812 and 0.712 when the concentrations were 2.0, 3.5 and 5.0 mol kg-1,
respectively (El Guendouzi and Marouani, 2003). Since
water activity of a solution is equal to the RH of air in equilibrium with
the solution, it can be derived that the molality concentrations of
Ca(NO3)2 solution were 2.0, 3.5 and 5.0 mol kg-1 when RH was
90.4 %, 81.2 % and 71.2 %; in other words, WSRs were found to be 11.1, 15.9 and
27.8 at 71.2 %, 81.2 % and 90.4 % RH, respectively (El Guendouzi and
Marouani, 2003). As shown in Table 2, in our work WSRs were determined to be
11.22 ± 0.06, 15.77 ± 0.10 and 28.78 ± 0.20 at 70 %, 80 % and
90 % RH for Ca(NO3)2 solutions at the same temperature,
suggesting good agreement with El Guendouzi and Marouani (2003).
Mg(NO3)2
Water
activities of Mg(NO3)2 solutions were reported to be 0.897, 0.812
and 0.702 when the concentrations of the bulk solutions were 1.6, 2.5 and
3.5 mol kg-1 at 25 ∘C, respectively (Rard et
al., 2004); this means that WSRs were equal to 15.9, 22.2 and 34.7 at 70.2 %,
81.2 % and 89.7 % RH. Ha and Chan (1999) fitted their measured water
activities of Mg(NO3)2 as a function of molality concentration at
20–24 ∘C with a polynomial equation, and WSRs were derived to be 12.93,
16.12, 21.50 and 36.09 at 60 %, 70 %, 80 % and 90 % RH. As shown in Table 2,
WSRs
were measured to be 13.15 ± 0.01, 16.30 ± 0.01, 21.94 ± 0.01
and 36.87 ± 0.23 at 60 %, 70 %, 80 % and 90 % RH for deliquesced
Mg(NO3)2 at 25 ∘C. Therefore, it can be concluded that for
WSRs of Mg(NO3)2 solutions at ∼ 25 ∘C, our work
shows good agreement with the two previous studies (Ha and Chan, 1999;
Rard et al., 2004).
MgCl2
Water activities of MgCl2
solutions were reported to be 0.909, 0.800, 0.692, 0.491 and 0.408 when the
concentrations were 1.4, 2.4, 3.2, 4.6 and 5.2 mol kg-1
(Rard and Miller, 1981); i.e., WSRs were equal to 10.7, 12.1,
17.4, 23.1 and 39.7 at 40.8 %, 49.1 %, 69.2 %, 80.0 % and 90.9 % RH. In another
work (Ha and Chan, 1999), an electrodynamic balance was used to
investigate hygroscopic growth of MgCl2 particles at 20–24 ∘C, and
the measured molality concentrations of MgCl2 solutions as a function
of water activity were fitted by a polynomial equation. It can be derived
from Ha and Chen (1999) that WSRs were equal to 10.65, 12.34, 14.29, 17.04,
22.24 and 34.78 when RHs were 40 %, 50 %, 60 %, 70 %, 80 % and 90 %, respectively.
WSRs measured in our work, as listed in Table 2, are 9.89 ± 0.42,
11.52 ± 0.48, 1.677 ± 0.072, 16.74 ± 0.72, 22.18 ± 1.06
and 36.26 ± 1.76 at 40 %, 50 %, 60 %, 70 %, 80 % and 90 % RH. As a result, our
work agrees well with the two previous studies (Rard and Miller, 1981; Ha
and Chan, 1999) for WSRs of MgCl2 solutions as a function of RH at
∼ 25 ∘C.
Phase transition of CaCl2⋅xH2O
The change in sample mass of CaCl2⚫6H2O with RH was
also investigated at 25 ∘C. As shown in Fig. 3, when dried at 0 %
RH, the sample mass was reduced by one-third (from ∼ 1.5 to
∼ 1.0), and it is speculated that CaCl2⚫6H2O was
converted to CaCl2⚫2H2O. When RH was
increased to 10 %, no significant increase in sample mass was observed. As
RH was further increased to 20 %, the sample mass was increased by
48 ± 7 %; this may indicate that CaCl2⚫2H2O was
converted to CaCl2⚫6H2O, as the ratio of molar mass of
CaCl2⚫6H2O (219 g mol-1) to CaCl2⚫2H2O (147 g mol-1)
is 1.49, approximately equal to the ratio of
sample mass at 20 % RH to that at 10 % RH. Further increase in RH to
30 % would lead to additional increase in sample mass, implying the
deliquescence of the sample and the formation of an aqueous CaCl2
solution.
Change of normalized sample mass (blue curve, right
y axis) and RH (black curve, left y axis) as a function of time for
CaCl2⚫xH2O at 25 ∘C.
Assuming that CaCl2⚫6H2O was converted to
CaCl2⚫2H2O after being dried at 0 % RH, we could use
the change of sample mass as a function of RH to calculate WSR (defined as
molar ratio of H2O to Ca2+), and the results are listed in Table 3.
Please note that we did not calculate WSR at 20 % RH since it is
speculated that the significant mass increase at 20 % RH was caused by the
transformation of CaCl2⚫2H2O to CaCl2⚫6H2O,
as mentioned above. Water activities of aqueous CaCl2
solutions as a function of molality concentration reported in a previous
study (Rard et al., 1977) were used to calculate WSR as a
function of RH, and the results are also included in Table 3 for comparison.
As evident from Table 3, at the same or similar RH, WSRs measured in our work are in
good agreement with those derived from Rard et al. (1977), supporting our
assertion that CaCl2⚫6H2O was converted to
CaCl2⚫2H2O after being dried at 0 % RH. In fact,
theoretical calculations (Kelly and Wexler, 2005) and
experimental measurements (Gough et al., 2016) both suggested
that when RH is gradually increased, solid–solid phase transition from
CaCl2⚫2H2O to CaCl2⚫6H2O would
occur before deliquescence takes place.
Mass growth factors (m/m0, defined as the ratio of
sample mass at a given RH to that at 0 % RH) and water-to-solute ratios
(WSRs) as a function of RH (0 %–90 %) at 25 ∘C for CaCl2⚫xH2O.
WSRs derived from RH over aqueous CaCl2 solutions as a
function of concentration (mol kg-1) at 25 ∘C (Rard et al., 1977)
are also included for comparison. All the errors given in this work are
standard deviations.
Our work
Rard et al. (1977)
RH (%)
m/m0
WSR
RH (%)
Molality
WSR
0
1.000±0.001
–
–
–
–
10
1.000±0.001
–
–
–
–
20
1.448±0.072
–
–
–
–
30
1.724±0.007
7.97±0.03
31.2
7.0
7.94
40
1.929±0.008
9.64±0.04
39.2
6.0
9.26
50
2.144±0.010
11.40±0.05
49.9
5.0
11.11
60
2.408±0.012
13.55±0.07
–
–
–
70
2.786±0.015
16.64±0.09
70.1
3.4
16.34
80
3.448±0.020
22.05±0.13
79.8
2.6
21.37
90
5.194±0.030
36.30±0.21
89.9
1.6
37.72
Additional experiments, in which RH was stepwise increased from 0 % with
an increment of 1 % per step, were carried out in attempt to measure the
DRH of CaCl2⚫xH2O at 25 ∘C. In all of these
experiments, CaCl2⚫6H2O was always transformed to
CaCl2⚫2H2O after being dried at 0 % RH. In some of
these experiments the deliquescence took place at a RH of ∼ 28.5 %,
which is consistent with the DRH of CaCl2⚫6H2O reported in the literature (Kelly and Wexler,
2005), suggesting that CaCl2⚫2H2O was first transformed
to CaCl2⚫6H2O prior to deliquescence. However, in some
other experiments the deliquescence occurred at a RH of ∼ 18.5 %, corresponding
to the DRH of CaCl2⚫2H2O
reported previously (Kelly and Wexler, 2005), implying that
CaCl2⚫2H2O was deliquesced without being transformed to
CaCl2⚫6H2O. The dual deliquescence processes, i.e., (1) transformation
of CaCl2⚫2H2O to CaCl2⚫6H2O
prior to deliquescence and (2) direct deliquescence of
CaCl2⚫2H2O, were also observed using Raman spectroscopy
at low temperatures (223–273 K) (Gough et al., 2016). It seems
that the competition of these two mechanisms is both thermodynamically and
kinetically dependent. Since phase transitions of CaCl2 are not only
important for atmospheric aerosols but may also play a role in the existence
of liquid water in some hyperarid environments (Gough et al.,
2016), further investigation is being carried out by combining the VSA technique with vibrational spectroscopy.
Hygroscopic growth of aerosol particles
Hygroscopic GFs, which were measured using H-TDMA at room
temperature, are displayed in Fig. 4 for Ca(NO3)2, CaCl2,
Mg(NO3)2 and MgCl2 aerosols, and the results are also
compiled in Table 4. It was found in our work that all four types of
aerosols exhibit high hygroscopicity, with GF at 90 % RH being around 1.7
or larger. In addition, all the four types of aerosol particles, instead of
having distinct solid–liquid phase transitions, showed significant
hygroscopic growth at very low RH (as low as 10 %), and their GFs increased
continuously with RH. This phenomenon is due to the fact that these aerosol
particles, generated by drying aqueous droplets, were likely to be
amorphous. It was also observed in previous work that some types of
particles generated by drying aqueous droplets would be amorphous, such as
Ca(NO3)2 (Tang and Fung, 1997; Gibson et al., 2006; Jing et
al., 2018), Mg(NO3)2 (Zhang et al., 2004; Gibson et al., 2006;
Li et al., 2008a), CaCl2 (Park et al., 2009; Tobo et al., 2009) and
MgCl2 (Cziczo and Abbatt, 2000; Park et al., 2009).
Hygroscopic growth factors (GFs) of aerosol particles as a
function of RH measured using H-TDMA. (a) Ca(NO3)2 and
Mg(NO3)2; (b) CaCl2 and MgCl2.
Ca(NO3)2 and
Mg(NO3)2 aerosols
Two
previous studies (Gibson et al., 2006; Jing et al., 2018) employed H-TDMA
to examine hygroscopic growth of 100 nm Ca(NO3)2 aerosol particles
at room temperature. GF were determined to be 1.51 at 80 % RH and
∼ 1.77 at 85 % RH by Gibson et al. (2008). It should be
pointed out that though the DMA-selected dry particle diameters were 100 nm
for Ca(NO3)2 and Mg(NO3)2 aerosols, the dry diameters
used by Gibson et al. (2006) were 89 nm for Ca(NO3)2 and 77 nm for
Mg(NO3)2, being extrapolated to 0 % RH using the theoretical
growth curve based on the Köhler theory. The Köhler theory is based
on assumption of solution ideality and thus may not be applicable to highly
concentrated aerosol droplets at low RH (Seinfeld and Pandis,
2016). If the dry diameter selected using the DMA (i.e., 100 nm) was used in
GF calculation, GFs reported by Gibson et al. (2006) would be ∼ 1.34 at 80 % RH and ∼ 1.58 at 85 % RH; compared with our
results (1.51 ± 0.02 at 80 % RH and 1.62 ± 0.01 at 85 % RH),
GF reported by Gibson et al. (2006) are ∼ 11 % smaller at
80 % RH and only ∼ 3 % smaller at 85 %. In the second
study (Jing et al., 2018), GFs were determined to
be 1.56 at 80 % RH and 1.89 at 90 % RH; compared with our results
(1.51 ± 0.02 at 80 % RH and 1.79 ± 0.03 at 90 % RH), GFs
reported by Jing et al. (2018) were ∼ 3 % larger at 80 %
RH and ∼ 6 % larger at 90 % RH. Overall, our results show
reasonably good agreement with the two previous studies (Gibson et al.,
2006; Jing et al., 2018).
Hygroscopic growth factors (GFs) of Ca(NO3)2,
CaCl2, Mg(NO3)2 and MgCl2 aerosol particles
measured at room temperature using a H-TDMA. The absolute uncertainties in RH
were estimated to be within ±2 %. All the errors given in this
work are standard deviations.
RH
Ca(NO3)2
CaCl2
Mg(NO3)2
MgCl2
(%)
<5
1.00±0.01
1.00±0.01
1.00±0.01
1.00±0.01
10
1.09±0.01
1.05±0.01
1.05±0.02
1.03±0.01
20
1.17±0.02
1.11±0.02
1.10±0.01
1.08±0.01
30
1.20±0.02
1.17±0.01
1.41±0.01
1.15±0.01
40
1.23±0.02
1.22±0.01
1.18±0.01
1.18±0.01
50
1.28±0.03
1.27±0.01
1.22±0.01
1.23±0.01
60
1.34±0.01
1.33±0.01
1.27±0.01
1.29±0.01
70
1.40±0.03
1.40±0.01
1.34±0.02
1.36±0.01
75
1.45±0.02
1.45±0.01
1.38±0.02
1.41±0.01
80
1.51±0.02
1.51±0.01
1.45±0.04
1.46±0.01
85
1.62±0.01
1.59±0.02
1.53±0.03
1.57±0.02
90
1.79±0.03
1.71±0.03
1.67±0.03
1.71±0.03
To our knowledge, only one previous study investigated the hygroscopic
growth of Mg(NO3)2 aerosol (100 nm) using the H-TDMA
(Gibson et al., 2006), and GF was measured to be
1.94 ± 0.02 at 83 % RH. As stated above, the theoretical extrapolated
diameter (77 nm) at 0 % RH, instead of the dry diameter (100 nm) selected
using the DMA, was used as the dry diameter to calculate their reported GFs
(Gibson et al., 2006). If the DMA-selected dry
diameter (100 nm) was used in calculation, the GF reported by Gibson et al. (2006)
would be ∼ 1.49 at 83 % RH; for comparison, in our
work GF were determined to be 1.45 ± 0.04 and 1.53 ± 0.03 at 80 %
and 85 % RH, suggesting good agreement between the two studies if the
DMA-selected dry diameter was used to calculate GF reported by Gibson et al. (2006).
CaCl2 and MgCl2 aerosols
Hygroscopic growth of CaCl2 and MgCl2 aerosol particles
was explored using a H-TDMA (Park et al., 2009), and as far
as we know, this was the only study which reported the H-TDMA-measured
hygroscopic GFs of the two types of aerosols. Three dry diameters
(20, 30 and 50 nm) were used for CaCl2 and MgCl2 aerosol particles
(Park et al., 2009), and no significant size dependence of
their hygroscopic properties was observed. GFs were measured to be around
1.27, 1.38, 1.48 and 1.59 at 60 %, 75 %, 80 % and 90 % RH for CaCl2
(Park et al., 2009). For comparison, GFs were determined in
this work to be 1.33 ± 0.01, 1.45 ± 0.01, 1.51 ± 0.01 and
1.71 ± 0.03 at 60 %, 75 %, 80 % and 90 %, slightly larger than those
reported by Park et al. (2009), and the differences were found to be
<7 %.
At 50 %, 70 %, 80 %, 85 % and 90 % RH, GFs of MgCl2 aerosol were measured to
be about 1.17, 1.29, 1.47, 1.59 and 1.79 by Park et al. (2009); for
comparison, GFs were determined to be 1.23 ± 0.01, 1.36 ± 0.01,
1.46 ± 0.01, 1.57 ± 0.02 and 1.71 ± 0.03 in our work at the
same RHs. The differences did not exceed 6 % at any given RH, suggesting
good agreement between the two studies. Microscopy was used to investigate
the hygroscopic growth of micrometer-size MgCl2 particles deposited on
substrates (Gupta et al., 2015), and the ratios of 2-D
particle areas, relative to that at <5 % RH, were measured to be
around 1.65, 1.92, 2.02 and 2.28 at 60 %, 70 %, 75 % and 80 % RH, corresponding
to diameter-based GFs of approximately 1.28, 1.38, 1.42 and 1.51,
respectively. GFs of MgCl2 aerosol, as shown in Table 4, were determined
to be 1.29 ± 0.01, 1.36 ± 0.01, 1.41 ± 0.01 and 1.46 ± 0.01
at 60 %, 70 %, 75 % and 80 % RH in our work; therefore, the differences
between GFs reported in our work and those measured by Gupta et al. (2015)
were <4 %.
Comparison between hygroscopic growth with CCN activities
GF
measured using H-TDMA can be used to calculate the single hygroscopicity
parameter, κGF, using Eq. (3a) (Petters and Kreidenweis,
2007; Kreidenweis and Asa-Awuku, 2014; Tang et al., 2016a):
RHexpAKd0⋅GF=GF3-1GF3-(1-κGF),
where GF is the growth factor at a given RH; AK is a constant which
describes the Kelvin effect and is equal to 2.1 nm for a surface tension of
0.072 J m-2 (pure water) and temperature of 298.15 K (Tang
et al., 2016a). For a dry particle diameter (d0) of 100 nm, the
denominator in the left term of Eq. (3a) is not larger than 1.02; therefore,
the Kelvin effect is negligible and Eq. (3a) can be simplified to Eq. (3b):
RH=GF3-1GF3-(1-κGF).
Equation (4) can be derived by rearranging Eq. (3b):
κGF=(GF3-1)1-RHRH.
In our work, GF data at 90 % RH were used to derive κGF, as
usually done in many previous studies (Kreidenweis and Asa-Awuku,
2014). The single hygroscopicity parameter, κCCN, can also be
derived from experimental measurements or theoretical calculations of CCN
activities (Petters and Kreidenweis, 2007; Kreidenweis
and Asa-Awuku, 2014). Ideally aerosol–water interactions under both
subsaturation and supersaturation can be described by a constant single
hygroscopicity parameter (Petters and Kreidenweis, 2007).
Nevertheless, agreement and discrepancies between GF-derived and
CCN-activity-derived κ have been reported (Petters and
Kreidenweis, 2007; Petters et al., 2009; Wex et al., 2009), and several
factors can contribute to such discrepancies. First of all, the solutions
may not be ideal, and especially aerosol particles under subsaturation may
consist of concentrated solutions; secondly, some of the compounds may have
limited solubilities. As discussed previously (Petters and Kreidenweis,
2007; Prenni et al., 2007), both factors would lead to lower κGF,
compared to κCCN. The effect of reduced surface
tension, compared to pure water, should be negligible for the eight types of
aerosol particles considered in our work since none of these compounds are
known to be surface-active.
Comparison between κGF determined in our work and κCCN
measured in previous studies is summarized in Table 5 and
discussed below for Ca(NO3)2, CaCl2, Mg(NO3)2 and
MgCl2 aerosols. In previous work which measured CCN activities
(Sullivan et al., 2009; Tang et al., 2015; Gaston et al., 2017), the dry
particle diameters used were typically in the range of 50–125 nm. The
uncertainties in our derived κGF have taken into account the
uncertainties in measured GF at 90 % RH.
Comparison between κGF measured in our work
and κCCN measured in previous studies.
Aerosol
κGF (this work)
κCCN (previous studies)
Ca(NO3)2
0.49–0.56
0.44–0.64
(Sullivan et al., 2009)
0.57–0.59
(Tang et al., 2015)
Mg(NO3)2
0.38–0.43
Not measured yet
CaCl2
0.42–0.47
0.46–0.58
(Sullivan et al., 2009)
0.51–0.54
(Tang et al., 2015)
0.549–0.561
(Gaston et al., 2017)
MgCl2
0.42–0.47
0.456–0.464
(Gaston et al., 2017)
Ca(HCOO)2
0.28–0.31
0.47–0.52
(Tang et al., 2015)
Mg(HCOO)2
0.40–0.45
Not measured yet
Ca(CH3COO)2
0.09–0.13
0.37–0.47
(Tang et al., 2015)
Mg(CH3COO)2
0.28–0.29
Not measured yet
For Ca(NO3)2 aerosol, κCCN values were measured to be
0.44–0.64 by Sullivan et al. (2009) and 0.57–0.59 by Tang et al. (2015); in
our work GF at 90 % RH was measured to be 1.79 ± 0.03, giving a
κGF of 0.49–0.56, in good agreement with κCCN
reported by the two previous studies (Sullivan et al., 2009; Tang et al.,
2015).
For CaCl2 aerosol, κCCN values were measured to be 0.46–0.58
by Sullivan et al. (2009), 0.51–0.54 by Tang et al. (2015) and 0.549–0.561
by Gaston et al. (2017). GF at 90 % RH was determined to be 1.71 ± 0.03 in the present work, giving a κGF
of 0.42–0.47, slightly lower
than κCCN values measured previously (Sullivan et al., 2009;
Tang et al., 2015; Gaston et al., 2017).
In our work, GF was determined to be 1.71 ± 0.03 for MgCl2 at
90 % RH, giving a κGF of 0.42–0.47; a previous study
(Gaston et al., 2017) measured the CCN activity of
MgCl2 aerosol, and κCCN values were determined to be 0.456–0.464,
in good agreement with κGF measured in our work.
For Mg(NO3)2 aerosol, GF and κGF were determined
in our work to be 1.67 ± 0.03 and 0.38–0.43, respectively. To our
knowledge, CCN activities of Mg(NO3)2 aerosol have not been
experimentally explored yet, and κCCN values were predicted to be 0.8
for Mg(NO3)2 and 0.3 for Mg(NO3)2⚫6H2O
(Kelly et al., 2007; Kreidenweis and Asa-Awuku, 2014), exhibiting large
variation for the same compound with different hydrate states under dry
conditions. These calculations were performed using the Köhler theory,
assuming solution ideality (Kelly et al., 2007). As Kelly et al. (2007)
pointed out, the hydration states, which are not entirely clear for
Mg(NO3)2 aerosol particles under atmospherically relevant
conditions, can have large impacts on their hygroscopicity and CCN
activities.
Hygroscopicity of formates and acetates
DRH and water-to-solute ratios
We measured the mass change of Ca(HCOO)2, Mg(HCOO)2⚫2H2O and
Ca(CH3COO)2⚫H2O samples as a
function of RH at 25 ∘C and found that the sample mass remained
essentially constant for all three compounds when RH was increased from
0 % to 90 %. Therefore, a series of experiments in which RH was increased to
95 % were conducted, and for each compounds three duplicate experiments
were carried out. As shown in Fig. 5a, when RH was increased from 0 % to
95 %, a significant while small increase in sample mass (∼ 10 %) was observed
for Ca(HCOO)2. The average ratio of sample mass at
95 % RH to that at 0 % RH was determined to be 1.119 ± 0.036
for Ca(HCOO)2 and 1.064 ± 0.020 for Mg(HCOO)2⚫2H2O
(not shown in Fig. 5), probably indicating that the DRH values
were >95 % for both compounds at 25 ∘C.
Change of normalized sample mass (blue curve, right
y axis) and RH (black curve, left y axis) as a function of time at 25 ∘C.
(a) Ca(HCOO)2; (b) Ca(CH3COO)2⚫H2O.
When RH was increased from 0 % to 95 %, a large increase in sample mass
(almost by a factor of 6), as shown in Fig. 5b, was observed for
Ca(CH3COO)2⚫H2O. On average, the ratio of sample
mass at 95 % RH to that at 0 % RH was measured to be 5.849 ± 0.064,
corresponding to a WSR (defined as the molar ratio of H2O to Ca2+)
of 48.42 ± 0.53 for the aqueous Ca(CH3COO)2 solution at
95 % RH. This observation suggested that the deliquescence of
Ca(CH3COO)2⚫H2O at 25 ∘C occurred between 90 % and 95 % RH.
In further experiments a significant increase in sample mass
(by >10 %, and the sample was still increasing sharply when
the experiment was terminated) was observed when RH was increased from 90 % to
91 % for Ca(CH3COO)2⚫H2O at 25 ∘C,
suggesting a measured DRH of 90.5 ± 1.0 %. The DRHs of
Ca(CH3COO)2 and internally mixed CaCO3/Ca(CH3COO)2
particles were measured to be 85 % and 88 % at 5 ∘C
(Ma et al., 2012), using a modified physisorption
analyzer. Since in these two studies DRHs were measured at different
temperatures (25 ∘C in our work and 5 ∘C by Ma et al., 2012) and the
absolute difference in reported DRH was ∼ 5 %, the agreement
in reported DRH can be considered to be quite good for
Ca(CH3COO)2.
Table 6 summarizes the ratios of sample mass at a given RH to those at 0 %
RH for Mg(CH3COO)2⚫4H2O as a function of RH at
25 ∘C. Being different from Ca(HCOO)2, Mg(HCOO)2⚫2H2O
and Ca(CH3COO)2⚫H2O, for Mg(CH3COO)2⚫4H2O a large increase in
sample mass was observed when
RH was increased from 70 % to 80 %. This observation suggested that the
deliquescence of Mg(CH3COO)2⚫4H2O occurred between
70 % and 80 % RH. Further experiments were carried out to measure its DRH,
and a significant increase in sample mass occurred when RH was increased from
71 % to 72 %, giving a measured DRH of 71.5 ± 1.0 % at 25 ∘C.
The RH over the saturated Mg(CH3COO2)2 solution at
∼ 23 ∘C was measured to be 65 % (Wang et
al., 2005), slightly lower than the DRH determined in our work.
Mass growth factors (m/m0, defined as the ratios of
sample mass at a given RH to that at 0 % RH) and water-to-solute ratios
(WSRs) as a function of RH (0 %–90 %) at 25 ∘C for
Mg(CH3COO)2⚫4H2O. WSRs are only calculated for RH
exceeding the DRH (i.e., when the sample was deliquesced). All the errors
given in this work are standard deviations.
RH (%)
0
10
20
30
40
m/m0
1.000±0.001
1.012±0.021
1.012±0.022
1.013±0.022
1.013±0.022
WSR
–
–
–
–
–
RH (%)
50
60
70
80
90
m/m0
1.014±0.023
1.015±0.025
1.033±0.031
2.029±0.013
3.100±0.021
WSR
–
–
–
16.24±0.11
28.97±0.20
The ratios of sample mass, relative to that at 0 % RH, were measured to be
2.029 ± 0.013 and 3.100 ± 0.021 at 80 % and 90 % RH, corresponding
to WSRs of 16.24 ± 0.11 at 80 % RH and 28.97 ± 0.20 at 90 % RH
for aqueous Mg(CH3COO)2 solutions. A electrodynamic balance
coupled to Raman spectroscopy was employed to study the hygroscopic growth
of Mg(CH3COO)2 at ∼ 23 ∘C (Wang et
al., 2005), and WSR was determined to be ∼ 15.6 at 80 % RH,
in good agreement with our work. Ma et al. (2012) found that after
heterogeneous reaction with CH3COOH(g) at 50 % RH for 12 h, the
hygroscopicity of MgO particles, which was initially rather nonhygroscopic,
was substantially increased due to the formation of Mg(CH3COO)2.
The conclusion drawn by Ma et al. (2012) is qualitatively consistent with
the results obtained in our work.
Table 6 also reveals that a small increase in sample mass (by
∼ 3 %, relative to that at 0 % RH) was observed for
Mg(CH3COO)2⚫4H2O when RH was increased to 70 %
before the deliquescence of Mg(CH3COO)2⚫4H2O took
place. This could be due to the possibility that
Mg(CH3COO)2⚫4H2O samples used in our work may
contain a small fraction of amorphous Mg(CH3COO)2, which would
take up some amount of water at a RH below the DRH of
Mg(CH3COO)2⚫4H2O (Wang
et al., 2005; Pang et al., 2015).
Hygroscopic growth of aerosol particles
Figure 6 and Table 7 display hygroscopic GFs of Ca(HCOO)2,
Mg(HCOO)2, Ca(CH3COO)2 and Mg(CH3COO)2 aerosols,
measured in our work using a H-TDMA. To the best of our knowledge, this is the
first time that GFs of these four types of aerosols have been reported. For
Mg(HCOO)2, aerosol particles showed gradual while small growth for
RH of
up to 30 %, and a further increase in RH led to significant growth; the
average GF of Mg(HCOO)2 aerosol at 90 % RH was determined to be
1.69 ± 0.03, similar to those for Mg(NO3)2 (1.67 ± 0.03)
and MgCl2 (1.71 ± 0.03) at the same RH. For RH up to 85 %,
Ca(HCOO)2 aerosol particles exhibited gradual and small growth; when RH
was increased to 90 %, abrupt and large growth was observed, with the GF being
1.54 ± 0.02, significantly smaller than that for Mg(HCOO)2 aerosol
at the same RH. This is distinctively different from what was observed in
VSA experiments, in which the mass of Ca(HCOO)2 and
Mg(HCOO)2⚫2H2O powdered samples was only increased by
∼ 12 % and ∼ 6 % when RH was increased from
0 % to 95 %. This difference may be explained by different states of samples
used in these two types of experiments (i.e., crystalline samples in VSA
experiments, while likely amorphous aerosol particles in H-TDMA
measurements), leading to different hygroscopic behaviors.
Hygroscopic growth factors (GFs) of aerosol particles as a
function of RH measured using HTDMA. (a) Ca(HCOO)2 and
Mg(HCOO)2; (b) Ca(CH3COO)2 and Mg(CH3COO)2.
As shown in Fig. 6b, gradual and small growth was also observed for
Ca(CH3COO)2 and Mg(CH3COO)2 aerosols at low RH.
A fast increase in GF started at about 80 % RH for Ca(CH3COO)2 aerosol,
and the GF was determined to be 1.26 ± 0.04 at 90 % RH. As discussed
in Sect. 3.2.1, in VSA experiments no significant increase in sample mass
was observed for Ca(CH3COO)2⚫H2O when RH was
increased from 0 % to 90 %, which is different from H-TDMA results. This
difference may again be explained (at least partly) by different states of
particles used in these two types of experiments, as mentioned above.
Careful inspection of Fig. 6b and Table 7 reveals a small decrease in
GF from 1.03 ± 0.01 to 1.00 ± 0.01 for Ca(CH3COO)2
aerosol when RH was increased from 50 % to 70 %. The decrease in GF may be
caused by restructuring of particles or change in particle morphology
(Vlasenko et al., 2005; Koehler et al., 2009); in addition, the small
change in GF (∼ 0.03) may not be significant when compared to
the uncertainties in our H-TDMA measurements.
When RH increased from 0 % to 70 %, small and gradual growth occurred for
Mg(CH3COO)2 aerosol particles, indicating that these particles
may contain some amount of amorphous materials. It was also found in
previous work (Li et al., 2008a, b) that
Mg(NO3)2 particles generated by drying aqueous droplets were
amorphous. Figure 6b reveals that a further increase in RH led to a large
increase in GFs, and this is largely consistent with the
occurrence of deliquescence at ∼ 71.5 % RH at 25 ∘C for
Mg(CH3COO)2⚫4H2O, as mentioned in Sect. 3.2.1.
At 90 % RH, the GF of Mg(CH3COO)2 aerosol was determined to be
1.53 ± 0.01, much larger than that for Ca(CH3COO)2
(1.26 ± 0.04).
At 90 % RH, for the four Ca-containing salts considered in our study and
nitrate and chloride aerosols have very similar GFs (1.79 ± 0.03 vs. 1.71 ± 0.03),
which are larger than that of formate (1.54 ± 0.02),
and acetate has the smallest GF (1.26 ± 0.04). For comparison, the
variation in GF at 90 % RH was found to be considerably smaller (from
∼ 1.53 to ∼ 1.71) for the four Mg-containing
salts studied herein.
Hygroscopic growth factors of Ca(HCOO)2,
Ca(CH3COO)2, Mg(HCOO)2 and Mg(CH3COO)2
aerosol particles measured using H-TDMA. The absolute uncertainties in RH
were estimated to be within ±2 %. All the errors given in this work
are standard deviations.
RH
Ca(HCOO)2
Ca(CH3COO)2
Mg(HCOO)2
Mg(CH3COO)2
(%)
5
1.00±0.01
1.00±0.01
1.00±0.01
1.00±0.01
10
1.01±0.01
1.01±0.01
1.02±0.01
1.01±0.01
20
1.01±0.01
1.01±0.02
1.02±0.01
1.01±0.01
30
1.01±0.01
1.01±0.01
1.02±0.01
1.02±0.01
40
1.01±0.01
1.02±0.01
1.04±0.01
1.02±0.01
50
1.02±0.01
1.03±0.01
1.11±0.01
1.04±0.01
60
1.02±0.01
1.01±0.01
1.18±0.01
1.04±0.01
70
1.03±0.01
1.00±0.01
1.27±0.01
1.10±0.02
75
1.04±0.01
1.02±0.02
1.33±0.01
1.16±0.02
80
1.04±0.01
1.07±0.01
1.41±0.01
1.25±0.01
85
1.01±0.01
1.13±0.01
1.52±0.02
1.37±0.01
90
1.54±0.02
1.26±0.04
1.69±0.03
1.53±0.01
According to Eq. (4), GF measured at 90 % RH can be used to calculate
κGF values, which were determined to be 0.28–0.31 for Ca(HCOO)2,
0.09–0.13 for Ca(CH3COO)2, 0.40–0.45 for Mg(HCOO)2 and
0.28–0.29 for Mg(CH3COO)2. A previous study
(Tang et al., 2015) investigated the CCN
activities of Ca(HCOO)2 and Ca(CH3COO)2 aerosols and reported
their single hygroscopicity parameters (κCCN), while the CCN
activities of Mg(HCOO)2 and Mg(CH3COO)2 have not been
explored yet. As summarized in Table 5, κCCN was reported to be
0.47–0.52 for Ca(HCOO)2 (Tang et al.,
2015), significantly larger than κGF (0.28–0.31) determined in
our work; for Ca(CH3COO)2, Tang et al. (2015) reported κCCN
to be in the range of 0.37–0.47, again much larger than κGF (0.09–0.13) derived from the present work.
As discussed in Sect. 3.1.4, for Ca(NO3)2 and CaCl2
aerosols, κGF values derived from H-TDMA experiments in the present
work show fairly good agreement with κCCN derived from CCN
activities measured in previous studies (Sullivan et al., 2009; Tang et
al., 2015); in contrast, for Ca(HCOO)2 and Ca(CH3COO)2
aerosols, κGF values derived from our H-TDMA experiments are
significantly smaller than κCCN reported by the previous study
(Tang et al., 2015). This can be largely
caused by the difference in water solubilities of Ca(NO3)2,
CaCl2, Ca(HCOO)2 and Ca(CH3COO)2.
Ca(NO3)2⚫4H2O and CaCl2⚫6H2O,
with solubilities being 1983 and 1597 g kg-1 of water at 25 ∘C
(Kelly and Wexler, 2005), can be considered to be highly
soluble; for comparison, the solubilities were reported to be 166 g kg-1
of water for Ca(HCOO)2 at 25 ∘C and 347 g kg-1 of water for
Ca(CH3COO)2⚫2H2O at 20 ∘C (Dean,
1973). Due to their limited water solubilities, Ca(HCOO)2 and
Ca(CH3COO)2 aerosol particles may not be fully dissolved at 90 %
RH in the H-TDMA experiments but would be dissolved to a larger extent (if
not completely) for RH > 100 % in CCN activity measurements
(Petters and Kreidenweis, 2008; Kreidenweis and
Asa-Awuku, 2014). Therefore, for Ca(HCOO)2 and Ca(CH3COO)2
aerosols, κGF derived from H-TDMA measurements would be smaller
than κCCN derived from CCN activity measurements. In fact, the
observation that κGF appeared to be significantly smaller
than κCCN, largely caused by limited water solubilities of
compounds under investigation, has been well documented in the literature
for laboratory-generated and ambient aerosol particles (Chang et al.,
2007; Prenni et al., 2007; Wex et al., 2009; Good et al., 2010; Massoli et
al., 2010).