Despite its importance, hygroscopicity of mineral dust aerosol remains
highly uncertain. In this work, we investigated water adsorption and
hygroscopicity of different mineral dust samples at 25 ∘C, via
measurement of sample mass at different relative humidity (RH, up to 90 %)
using a vapor sorption analyzer. Mineral dust samples examined (21 in total)
included seven authentic mineral dust samples from different regions in the
world and 14 major minerals contained in mineral dust aerosol. At
90 % RH, the mass ratios of adsorbed water to the dry mineral ranged from
0.0011 to 0.3080, largely depending on the BET surface areas of mineral dust
samples. The fractional surface coverages of adsorbed water were determined
to vary between 1.26 and 8.63 at 90 % RH, and it was found that the
Frenkel–Halsey–Hill (FHH) adsorption isotherm could describe surface
coverages of adsorbed water as a function of RH well, with AFHH and
BFHH parameters in the range of 0.15–4.39 and 1.10–1.91, respectively.
The comprehensive and robust data obtained would largely improve our
knowledge of hygroscopicity of mineral dust aerosol.
Introduction
Mineral dust aerosol mainly comes from arid and semiarid areas
(Ginoux et al., 2012), such as the Sahara, the Taklimakan
Desert, etc. Its annual flux and atmospheric loadings are estimated to
be ∼ 2000 Tg yr-1 and ∼ 19.2 Tg
(Textor et al., 2006; Huneeus et al., 2011), making mineral dust one of
the most important aerosols in the troposphere. Mineral dust aerosol has
significant impacts on atmospheric chemistry, climate and biogeochemical
cycles (Knippertz and Stuut, 2014). It can alter the radiative
forcing of the earth both directly (Balkanski et al., 2007; Huang et al.,
2014; Di Biagio et al., 2017) and indirectly (Cziczo et al., 2013;
Karydis et al., 2017). Mineral dust can also change the abundance of
reactive trace gases as well as aerosol compositions via heterogeneous
reactions (Usher et al., 2003; Dupart et al., 2012; He et al., 2014; Tang
et al., 2017; Yu and Jang, 2019). Furthermore, the deposition of mineral
dust will bring substantial amounts of nutrients (e.g., Fe and P) into some
marine and terrestrial ecosystems, thereby largely affecting biogeochemistry
in these regions (Jickells et al., 2005; Okin et al., 2011; Schulz et
al., 2012; Li et al., 2017; Tagliabue et al., 2017; Meskhidze et al., 2019).
Hygroscopicity largely determines the impacts of mineral dust aerosol on
atmospheric chemistry and climate. For example, many studies found that
relative humidity (RH) and thus the amount of water associated with mineral
dust have profound effects on the rates, mechanisms and products of
heterogeneous reactions (Vlasenko et al., 2009; Rubasinghege and
Grassian, 2013; Tang et al., 2014; Tang et al., 2017; Lasne et al., 2018;
Wang et al., 2018; Yu and Jang, 2018; Mitroo et al., 2019). In addition,
hygroscopicity of mineral dust aerosol plays important role in its optical
properties (and thus the direct radiative effect) and its ability to act as
cloud condensation nuclei and ice-nucleating particles (and thus the
indirect radiative effect) (Sorjamaa and Laaksonen, 2007; Kumar et al.,
2009; Garimella et al., 2014; Kreidenweis and Asa-Awuku, 2014; Laaksonen et
al., 2016; Tang et al., 2016; Tang et al., 2019a). Therefore, a number of
previous studies have investigated water adsorption and hygroscopic
properties of mineral dust aerosol at < 100 % RH, as reviewed by
Tang et al. (2016). However, different studies displayed considerable
discrepancies as large as a few orders of magnitude (Tang et
al., 2016), thus precluding a good understanding of the role mineral dust
aerosol plays in atmospheric chemistry and climate.
As pointed out by Tang et al. (2016), such discrepancies are largely due to
the non-sphericity and low hygroscopicity of mineral dust particles, making
it difficult to quantify the amount of water associated with them at
elevated RH. Instruments which measure mobility or optical diameters of
aerosol particles often found that the diameters of mineral dust particles
did not increase significantly (or even showed considerable decrease due to
particle restructuring during humidification) with increasing RH
(Gustafsson et al., 2005; Vlasenko et al., 2005; Herich et al., 2009;
Koehler et al., 2009; Attwood and Greenslade, 2011). Fourier transform
infrared (FTIR) spectroscopy is a sensitive method to detect adsorbed water
on mineral dust (Goodman et al., 2001; Ma et al., 2010a; Joshi et al.,
2017); however, it is not a trivial task to convert the intensity of its
infrared absorption to the amount of adsorbed water (Schuttlefield et
al., 2007b; Ma et al., 2010b; Tang et al., 2016). Quartz crystal
microbalance (QCM) is another sensitive technique to examine water
adsorption and absorption (Schuttlefield et al., 2007b; Navea et al.,
2010; Yeşilbaş and Boily, 2016); however, it is in doubt that the
underlying assumptions required to convert the change in resonance frequency
of the quartz crystal to the change in sample mass are always fulfilled
(Tang et al., 2016; Tang et al., 2019a).
In our previous work (Gu et al., 2017),
we developed a new method to investigate hygroscopic properties of
atmospherically relevant particles using a vapor sorption analyzer, which
utilized a very sensitive balance to measure the mass of a sample (typically
with a dry mass of tenths of a milligram or a few milligrams) as different RH under isotherm
conditions. Comprehensive validation carried out confirmed the robustness of
this method (Gu et al., 2017), and this
instrument has been employed to study hygroscopic properties of various
particles, including nonspherical particles such as saline mineral dust and
pollen grains (Chen et al., 2019; Tang et al., 2019b; Tang et al.,
2019c). This instrument was used in the present work to quantitatively
investigate hygroscopic properties of a number of mineral dust particles,
including several authentic mineral dust samples from different regions in
the world and individual minerals commonly found in mineral dust aerosol. We
also attempted to figure out which theoretical models could describe
hygroscopic properties of mineral dust particles, and we examined the
dependence of mineral dust hygroscopicity on several parameters (such as
particle diameter, surface area and the mass fraction of soluble materials).
Experimental sectionSample information
In total 21 different types of mineral dust were investigated, including
14 major minerals commonly found in mineral dust aerosol (Formenti
et al., 2011; Nickovic et al., 2012; Journet et al., 2014; Scanza et al.,
2015; Engelbrecht et al., 2016) and seven authentic mineral dust samples,
and their information can be found in Table 1. The 14 major minerals
examined included four oxides (SiO2, TiO2, magnetite and
hematite), one oxyhydroxide (goethite), three feldspars (potassium feldspar,
albite and microcline), two carbonates (CaCO3 and dolomite) and four
clay minerals (montmorillonite, illite, kaolinite and chlorite). As shown in
Table 1, SiO2, montmorillonite and kaolinite were supplied by Sigma
Aldrich; TiO2 (P25) was supplied by Degussa; hematite and magnetite
were supplied by Strem Chemicals; goethite was provided by Santa Cruz; and microcline,
CaCO3 and dolomite were provided by Alfa Aesar. Potassium feldspar and
albite were obtained from the National Research Center of Testing Techniques for
Building Materials (NRCTTBM, Beijing, China), and illite (IMt-1) was
obtained from the Clay Mineral Society at Purdue University, Indiana, USA
(Schuttlefield et al., 2007b; Tang et al., 2014). In addition, chlorite
was collected by one co-author from Liaoning Province, China.
Measured BET surface areas (BET), average particle
diameters (dp) and sources of mineral dust samples examined in this
work.
The seven authentic mineral dust samples were obtained from Africa, Asia and
North America. As shown in Fig. 1, three authentic mineral dust samples
(M'Bour dust, Bordj dust and Saharan dust) were collected from topsoil in
Senegal, Algeria and Cabo Verde (Tang et al., 2012; Joshi et al.,
2017). QH dust (which is brown desert soil) and China loess,
collected from topsoil in Qinghai and Shaanxi, were supplied by the Chinese
Academy of Geological Science as certificated materials (GBW07448 and
GBW07454) (Tang et al., 2019c). TLF
dust as airborne dust particles collected on 23 April 2010 at an urban
site in Turpan (Xinjiang, China) during a major dust storm. In addition,
Arizona Test Dust (ATD, nominal 0–3 µm fraction), an authentic
mineral dust sample commercially available from Powder Technology Inc.
(Minnesota, USA) and widely used in atmospheric aerosol research
(Vlasenko et al., 2005; Sullivan et al., 2010a; Tang et al., 2016), was
also investigated in our work.
Locations where (a) African and (b) Asian authentic
mineral dust samples examined in this work were collected.
When received, three feldspars, dolomite, illite, chlorite and TLF dust
contained significant amounts of rock chips or giant particles; as a result,
they were pretreated using the procedure described in our previous work
(Tang et al., 2019c). In brief, these
samples were dried at 120 ∘C for 24 h using an oven; after that,
they were ground manually and then using a ball mill so that most particles
were < 74 µm in diameter; finally, these samples were dried
again at 120 ∘C for 24 h and then cooled down. All the samples were
stored in plastic bottles which were tightly sealed to prevent contamination
by lab air.
Sample characterization
Dynamic light scattering (JL-1177, Jingxin Powder Technologies Inc.,
Chengdu, Sichuan, China) was employed to measure size distributions of
mineral dust samples examined in our work. In addition,
Brunauer–Emmett–Teller (BET) surface areas of these samples were determined
using an accelerated surface area and porosimetry analyzer (ASAP 2020 PLUS,
Micromeritics, Georgia, USA), and N2 was used as the adsorbate. Details
on particle size and BET surface area measurements can be found elsewhere
(Li et al., 2020).
To measure their inorganic soluble compositions, each mineral dust sample
(∼ 10 mg) was mixed with 10 mL of ultrapure deionized water, and
the mixture was stirred for 2 h using an oscillating table. After
centrifugalization, the solution was filtered using a 5 mL syringe fitted
with a 0.2 µm PTFE membrane syringe and then analyzed using ion
chromatography (Metrohm model 761 Compact IC, Metrohm, Herisau,
Switzerland). More information on ion chromatography analysis can be found
in our previous work (Tang et al.,
2019c). We attempted to measure five cations (Na+, K+,
NH4+, Mg2+ and Ca2+) and seven anions (NO3-,
SO42-, Cl-, NO2-, Br-, F- and
PO43-), and their detection limits were estimated to be around
0.02 mg/L.
Hygroscopicity measurements
Hygroscopic properties of mineral dust samples were investigated using a
vapor sorption analyzer (Q5000SA, TA instruments, Delaware, USA). This
instrument, described in our previous work (Gu et al., 2017; Chen et al.,
2019; Tang et al., 2019b), measured sample mass as a function of RH under
isotherm conditions. Measurements could be conducted in the RH range of
0 %–98 % and in the temperature range of 5–85 ∘C. We routinely measured
the deliquescence RH of NaCl, (NH4)2SO4 and KCl at 25 ∘C, and the measured values differed from the actual values by
< 1 % RH. The sample mass could be measured with an accuracy of
±0.1 µg, and the uncertainties for temperature and RH were
±0.1 ∘C and ±1 %.
RH (black curve, left y axis) and mass of mineral dust
(normalized to that at < 1 % RH, blue curve, right y axis) as a
function of experimental time: (a) hematite and (b) montmorillonite.
In this work, the initial masses of mineral dust samples used typically
ranged from 5 to 15 mg. As displayed in Fig. 2, the sample under
investigation was first dried at < 1 % RH; after that, RH was
increased in a stepwise manner to 90 %, and at each step RH was increased
by 10 %; at last, the sample was dried again at < 1 % RH. At
each step we changed the RH only after the samples' mass became stable (in
other words, only after an equilibrium was reached between gaseous and
particulate water), and the sample mass was considered to be stable when the
mass change was < 0.05 % in 30 min. In some experiments the sample
was considered to reach the equilibrium only when the mass change was
< 0.05 % in 60 min, and no significant difference in results was
found for the two equilibrium criterions. All the experiments were carried
out in triplicate at 25 ∘C.
ResultsSample characteristics
As shown in Table 1, the BET surface areas were found to vary between
2.17 ± 0.01 m2/g (microcline) and 249.91 ± 0.42 m2/g
(montmorillonite), spanning over 2 orders of magnitude. Except for
montmorillonite, the BET surface areas were in the range of a few to tens of
square meters per gram. In addition, the average particle diameters (dp) were
determined to range from 0.80 µm (hematite) to 54.41 µm
(M'Bour dust), and their size distributions can be found in Figs. S1–S7 in the Supplement.
Tables S1–S2 show mass fractions of water-soluble inorganic ions for the 21
mineral dust samples considered in this study. Na+, K+, Ca2+,
Mg2+, F-, Cl- and SO42- were detected in most of
the samples, while NH4+ was above its detection limit only for two
samples. The total mass fractions of all the soluble inorganic ions were
found to be quite low, ranging from 0.16 mg/g for SiO2 to 12.55 mg/g
for Bordj dust.
Water uptake by different mineral dust
As described in Sect. 2.3, sample mass of mineral dust was measured at
different RH values in our work; therefore, the mass ratio of adsorbed water to the
dry mineral, mw/m0, could then be determined as a function of RH.
Furthermore, mw/m0 could be converted to fractional surface coverage
(abbreviated as surface coverage) of adsorbed water (θ), using Eq. (1) (Tang et al., 2016):
θ=mwm0⋅NA⋅AwMw⋅ABET,
where NA is Avogadro's constant (6.02×1023 mol-1),
Mw is the molar mass of water (18 g mol-1), Aw is the surface
area each adsorbed water molecule would occupy (assumed to be
1 × 10-15 cm2) (Schuttlefield et al., 2007a; Hatch et al., 2014; Tang
et al., 2016) and ABET is the BET surface (in cm2 g-1) of
the mineral dust under consideration. Tables 2–5 summarize
mw/m0 and θ as a function of RH for all the mineral dust
examined in our work. Please note that our previous work
(Tang et al., 2019c) discussed water
uptake by China loess and QH dust, and these results are included here to
compare with the other 19 mineral dust samples.
Mass ratios of adsorbed water to the dry mineral
(mw/m0) and surface coverages of adsorbed water (θ) as a
function of RH (%) for SiO2, TiO2, magnetite, hematite,
goethite and potassium feldspar.
Below we discuss hygroscopicity of mineral dust investigated, and we compare
our measured mw/m0 and θ with those reported in previous
work. As our work directly measured mass change of mineral dust due to water
uptake, we prefer to compare mw/m0 when such values were also
reported in previous studies; otherwise, we then choose to compare θ. As aerosol-based measurements are usually not sensitive enough and also
need the particle sphericity assumption (Tang et al., 2016), we
do not compare our results with those measurements.
SiO<inline-formula><mml:math id="M277" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula> and TiO<inline-formula><mml:math id="M278" display="inline"><mml:msub><mml:mi/><mml:mn mathvariant="normal">2</mml:mn></mml:msub></mml:math></inline-formula>
In our work mw/m0 was determined to be 0.0011, 0.0020 and 0.0058 for
SiO2 at 30 %, 60 % and 90 % RH, corresponding to θ of
0.55, 1.04 and 2.95, respectively. Figure 3a compares our work with previous
studies in which FTIR (Goodman et al., 2001; Ma et al., 2010a; Joshi et
al., 2017) and QCM (Schuttlefield et al., 2007a;
Yeşilbaş and Boily, 2016) were used to measure water uptake by
SiO2. At a given RH, θ values reported by the four previous
studies (Goodman et al., 2001; Schuttlefield et al., 2007a; Ma et al.,
2010a; Joshi et al., 2017) were generally larger when compared to our work,
and the difference usually did not exceed a factor of 3. Furthermore,
the differences between our work and the four previous studies became
smaller at higher RH. For example, at 80 % RH our measured θ was
very close to those reported by Ma et al. (2010a) and Joshi et al. (2017),
and at 90 % RH our measured θ was 20 %–30 % larger than those
reported by the two studies (Ma et al., 2010a; Joshi et al., 2017).
Yeşilbaş and Boily (2016) employed a QCM to investigate water
adsorption on quartz (0.3–14 µm), and θ was determined to be
∼ 2300 at ∼ 70 % RH, almost 3 orders of
magnitude larger than those reported in our work and other previous studies
(Goodman et al., 2001; Schuttlefield et al., 2007a; Ma et al., 2010a;
Joshi et al., 2017); therefore, the results reported by Yeşilbaş and
Boily (2016) are not displayed in Fig. 3a.
Comparison of surface coverages of adsorbed water
(θ) measured in our work with those reported in previous studies for
(a) SiO2 and (b) TiO2. G01: Goodman et al. (2001); S07:
Schuttlefield et al. (2007a); M10: Ma et al. (2010a); J17: Joshi et al. (2017);
K07: Ketteler et al. (2007).
For TiO2, mw/m0 was determined to be 0.0072, 0.0135 and 0.0355
at 30 %, 60 % and 90 % RH, corresponding to θ of 0.44, 0.82
and 2.17, respectively. Water adsorption on P25 TiO2 was studied
previously using FTIR (Goodman et al., 2001; Ma et al., 2010a), and
another study (Ketteler et al., 2007) employed atmospheric
pressure X-ray photoelectron spectroscopy to explore interactions of water
vapor with the rutile single crystal surface (110). As shown in Fig. 3b,
when compared with our work, θ values reported by Ma et al. (2010a)
were higher across the entire RH range, and the relative differences between
our work and Ma et al. (2010a) were around a factor of 2 or smaller. The
relative differences between our work and the other two studies (Goodman
et al., 2001; Ketteler et al., 2007) were larger, being a factor of
∼ 5 at lower RH and becoming smaller at higher RH.
Hematite, goethite and magnetite
At 30 %, 60 % and 90 % RH, mw/m0 was measured to be 0.0029,
0.0047 and 0.0101 for hematite, corresponding to θ of 1.03, 1.72 and
3.68, respectively. Water adsorption on hematite was studied previously using FTIR
(Goodman et al., 2001; Ma et al., 2010a) and QCM
(Yeşilbaş and Boily, 2016). Figure 4a reveals that our
results agreed reasonably well with those reported by Goodman et al. (2001)
and Ma et al. (2010a), and the relative differences were found to be within
a factor of 2. In addition, our results agreed fairly well with those
reported for 10 nm hematite by Yeşilbaş and Boily (2016) but were
significantly smaller than their results for 50 nm hematite.
Yeşilbaş and Boily (2016) also studied water adsorption on 4 and 5 µm hematite particles, and θ values were reported to be
∼ 300 at ∼ 70 % RH, almost 2 orders of
magnitude larger than our results; therefore, their measured θ values for 4
and 5 µm hematite are not shown in Fig. 4a.
In our work, mw/m0 was measured to be 0.0029, 0.0052 and 0.0124 at
30 %, 60 % and 90 % RH for goethite, corresponding to θ of
0.73, 1.30 and 3.09. Yeşilbaş and Boily (2016) employed QCM to study
water adsorption on goethite, and their measured θ values are plotted in
Fig. 4b to compare to ours. When compared to our work, on average θ
values measured by Yeşilbaş and Boily (2016) were a factor of
∼ 2 larger. We also investigated water adsorption on
magnetite, and the results can be found in Fig. 4b. Compared to goethite,
θ values were generally 20 %–30 % smaller for magnetite. As far as
we know, water adsorption on magnetite was not quantitatively investigated
before.
Comparison of surface coverages of adsorbed water
(θ) measured in our work with those reported in previous studies for
(a) hematite and (b) goethite (θ values measured in our work for magnetite
are also plotted). G01: Goodman et al. (2001); M10: Ma et al. (2010a); Y16:
Yeşilbaş and Boily (2016).
Feldspars
Tables 2–3 show that the mass ratios of adsorbed water to the dry mineral
were determined to be 0.0056, 0.0060 and 0.0048 at 90 % RH for potassium
feldspar, albite and microcline, respectively; correspondingly, θ
values were found to be 4.73, 5.53 and 7.37. QCM was used by Yeşilbaş and
Boily (2016) to study water uptake onto microcline, and θ was
measured to be ∼ 300 at ∼ 70 % RH, about 2
orders of magnitude larger than our measurement. We are not aware of other
previous studies which investigated water adsorption on feldspars in a
quantitative manner.
Mass ratios of adsorbed water to the dry mineral
(mw/m0) and surface coverages of adsorbed water (θ) as a
function of RH (%) for albite, microcline, CaCO3, dolomite, illite
and kaolinite.
The mass ratio of adsorbed water to the dry mineral, mw/m0, was
measured in our work to be 0.0011 at 90 % RH for CaCO3, giving a
θ value of 1.73. Water adsorption on CaCO3 was investigated
previously, using thermogravimetric analysis
(Gustafsson et al., 2005), physisorption
analysis (Ma et al., 2012a) and QCM (Hatch et al.,
2008; Schuttlefield, 2008; Yeşilbaş and Boily, 2016). Hatch et al. (2008) and Ma et al. (2012a) reported mw/m0 as a function of RH.
Figure 5a shows that when compared to our work, mw/m0 values
determined by Hatch et al. (2008) were significantly larger (by a factor of
10 or more), whereas the results reported by Ma et al. (2012a) were only
smaller by a factor of ∼ 2. We further compare our measured
θ values with those reported by another two studies (Gustafsson et al.,
2005; Schuttlefield, 2008). As shown in Fig. 5b, the results reported by
Gustafsson et al. (2005) and Schuttlefield (2008) were found to be larger
than ours, by a factor of 2–3. In addition, θ was measured to be
> 100 at ∼ 70 % RH for CaCO3
(Yeşilbaş and Boily, 2016), approximately 2 orders of
magnitude larger than our work.
As shown in Table 3, our work suggested that around 1.26 monolayers of
adsorbed water were formed on dolomite at 90 % RH, similar to that for
CaCO3. To our knowledge, water adsorption on dolomite has not been
quantitatively explored by previous work.
Comparison of water adsorption on CaCO3 examined in
different studies: (a) mass ratios of adsorbed water to the dry mineral
(mw/m0); (b) surface coverages of adsorbed water (θ). G05,
Gustafsson et al. (2005); H08, Hatch et al. (2008); S08, Schuttlefield
(2008); M12, Ma et al. (2012a).
Clay minerals
For illite, mw/m0 and θ were determined to be 0.0333 and
4.63 at 90 % RH in our study (Table 3). QCM was employed to study water
adsorption on illite, and mw/m0 was reported to be 0.28 at
∼ 90 % RH (Hatch et al., 2011) and
∼ 0.27 at 75 % RH (Schuttlefield et
al., 2007b), around 1 order of magnitude larger than our results. A recent
study (Yeşilbaş and Boily, 2016) also investigated water
uptake onto illite using QCM, and their reported θ values are compared with
our results in Fig. 6a. The relative differences between our and their
(Yeşilbaş and Boily, 2016) work were usually smaller than
a factor of 2 and became even smaller at higher RH.
Comparison of surface coverages of adsorbed water
(θ) measured by different studies for (a) illite and (b) kaolinite.
H15, Hung et al. (2015); Y16, Yeşilbaş and Boily (2016).
For kaolinite, mw/m0 and θ were determined in our work to be
0.0093 and 3.22 at 80 % RH and 0.0146 and 5.08 at 90 % RH, respectively.
A few previous studies investigated water adsorption on kaolinite using QCM
(Schuttlefield et al., 2007b; Hatch et al., 2011; Yeşilbaş and
Boily, 2016) and physisorption analysis (Hung et al., 2015).
Comparison of our measured θ values with those reported by Hung et al. (2015) is displayed in Fig. 6b for kaolinite, suggesting that the two
studies were in good agreement, and the relative differences were usually
within 30 %. At ∼ 80 % RH, mw/m0 values were determined
to be ∼ 0.03 for kaolinite provided by Alfa and
∼ 0.1 for kaolinite (KGa-1b) obtained from the Clay Mineral
Society (Schuttlefield et al., 2007b), around 3
and 10 times larger than our work. In the work by Hatch et al. (2011),
mw/m0 was determined to be ∼ 0.1 at ∼ 80 % RH for kaolinite (KGa-1b), about 1 order of magnitude larger than
our result. Yeşilbaş and Boily (2016) examined water adsorption on
two different kaolinite samples (kaolinite provided by Fluka and KGa-1), and
θ values were found to be up to 100 at ∼ 70 % RH,
> 30 times larger than our work.
Comparison of water adsorption on montmorillonite
examined in different studies: (a) surface coverages of adsorbed water
(θ); (b) the mass ratio of adsorbed water to the dry mineral
(mw/m0). F05, Frinak et al. (2005); S07, Schuttlefield et al. (2007b); H11, Hatch et al. (2011); H15, Hung et al. (2015); Y16,
Yeşilbaş and Boily (2016).
We also studied water adsorption on montmorillonite, and mw/m0 and
θ were measured to be 0.308 and 4.12 at 90 % RH. Physisorption
analysis (Hung et al., 2015) and QCM
(Yeşilbaş and Boily, 2016) were utilized to investigate
water uptake onto montmorillonite. As shown in Fig. 7a, our work agreed
well with Hung et al. (2015), and the results obtained by Yeşilbaş
and Boily (2016) for Ca and Na montmorillonite were much larger (by a
factor of > 10), when compared with our work. Figure 7b compares
our measured mw/m0 values with those reported in previous studies in which
FTIR (Frinak et al., 2005) and QCM (Schuttlefield et al., 2007b; Hatch
et al., 2011) were used. In general good agreement between our work and the
three previous studies was found, except for SAz-1 montmorillonite
(Schuttlefield et al., 2007b) obtained from the Clay
Mineral Society. One possible explanation for the observed discrepancy is
that montmorillonite samples from different sources may have different
hygroscopic properties. We note that prior to 2005, a few studies (Hall
and Astill, 1989; Cases et al., 1992; Xu et al., 2000; Zent et al., 2001)
also investigated water uptake by montmorillonite, and it was found that
these studies agreed well with Frinak et al. (2005); therefore, the four
studies conducted before 2005 should also be consistent with our work.
In addition, water uptake by chlorite was explored in our work. As shown in
Table 4, mw/m0 and θ were measured to be 0.012 and 4.03 at
90 % RH. To our knowledge, hygroscopic properties of chlorite have not
been examined before.
Mass ratio of adsorbed water to the dry mineral
(mw/m0) and surface coverages of adsorbed water (θ) as a
function of RH (%) for montmorillonite, chlorite, ATD, M'Bour dust, Bordj
dust and Saharan dust.
ATD. Table 4 suggests that at 90 % RH, mw/m0 and
θ were measured in our work to be 0.0644 and 5.87 for ATD. Two
previous studies (Navea et al., 2010;
Yeşilbaş and Boily, 2016) employed QCM to investigate water
adsorption on ATD. In the first study (Navea et al.,
2010), mw/m0 was measured to be > 0.1 at 70 % RH,
2–3 times larger than our result (∼ 0.04 at 70 % RH); in the
second study (Yeşilbaş and Boily, 2016), θ was
measured to be > 200 at ∼ 70 % RH, almost 2
orders of magnitude larger than our work (∼ 3.6 at 70 % RH).
Gustafsson et al. (2005) used a thermogravimetric analyzer to study water
uptake by ATD, and as shown in Fig. 8a, their results agreed very well
with ours. A recent study (Joshi et al., 2017) investigated water
adsorption on ATD using FTIR; compared to our work, the values reported by
Joshi et al. (2017) were ∼ 30 % lower, suggesting fairly
good agreement between the two studies.
Comparison of surface coverages of adsorbed water
(θ) reported in different studies for (a) ATD and (b) Bordj dust and
M'Bour dust. G05, Gustafsson et al. (2005); J17, Joshi et al. (2017).
African dust. In our study, mw/m0 and θ were
measured to be 0.0192 and 3.91 for Bordj dust and 0.0152 and 3.51 for M'Bour
dust at 90 % RH, respectively. Joshi et al. (2017) employed FTIR to
investigate interaction of water vapor with Bordj dust and M'Bour dust. As
suggested by Fig. 8b, the relative differences between our and their work
(Joshi et al., 2017) were usually within a factor of 2 for the two
dust samples, and the discrepancy also became smaller at higher RH,
suggesting fair consistency between the two studies.
For Saharan dust, mw/m0 and θ were determined in our study
to be 0.0793 and 5.15 at 90 % RH. Water uptake onto Saharan dust was
studied using QCM (Navea et al., 2010), and their
results, as shown in Fig. 9a, were 2–3 times larger than our work.
(a) Comparison of mass ratios of adsorbed water to the
dry mineral (mw/m0) measured by our work and N10
(Navea et al., 2010) for Saharan dust. (b) Comparison of
surface coverages of adsorbed water (θ) for TLF dust measured in our
work with Beijing dust measured in M12 (Ma et al., 2012b).
Asian dust. Table 5 summarizes our results obtained for three Asian
mineral dust samples, including China loess, QH dust and TLF dust. It should
be pointed out that mw/m0 values have been reported in our previous work (Tang et al., 2019c) for China loess
and QH dust, and they are included here for comparison. As shown in Table 5,
the three Asian authentic dust samples exhibited very similar water uptake
properties, and their mw/m0 values were determined to be 0.021–0.022 at
90 % RH. Navea et al. (2010) employed QCM to study the interaction of water
vapor with China loess, and mw/m0 was reported to be ∼ 0.17 at 70 % RH, more than 1 order of magnitude larger than our result
(∼ 0.012 at 70 % RH).
As mentioned in Sect. 2.1, TLF dust examined in our work was airborne
dust particles collected during a dust storm in Turpan (Xinjiang, China)
which was very close to the dust source. In a previous study
(Ma et al., 2012b), dust particles (termed Beijing dust
here) were collected during a dust storm in Beijing (and thus these
particles had undergone atmospheric aging to some extent), and their
hygroscopic properties were then investigated using a physisorption
analyzer. As revealed by Fig. 9b, our work agreed fairly well with Ma et
al. (2012b) at high RH (70 %, 80 % and 90 %), though the differences
became considerably larger at lower RH.
Mass ratios of adsorbed water to the dry mineral
(mw/m0) and surface coverages of adsorbed water (θ) as a
function of RH (%) for QH dust, China loess and TLF dust.
To investigate water adsorption by mineral dust, one previous study
(Gustafsson et al., 2005) employed
thermogravimetric analysis which measured sample mass as a function of RH
(essentially the same to vapor sorption analyzer (VSA) used in our study), and another two groups
(Ma et al., 2012a, b; Hung et al., 2015) employed
physisorption analysis which measured change in water vapor pressure caused
by adsorption onto mineral dust (Ma et al., 2010b).
Thermogravimetric analysis, physisorption analysis and the VSA technique
used in our work can be considered absolutely quantitative, and as
discussed in Sect. 3.2.1–3.2.6, in general our work agreed well with
these four previous studies (Gustafsson et al., 2005; Ma et al., 2012a, b; Hung et al., 2015).
FTIR was widely employed in previous work (Goodman et al., 2001; Frinak
et al., 2005; Ma et al., 2010a; Joshi et al., 2017; Ibrahim et al., 2018) to
study water uptake onto mineral dust, although it is not straightforward to
convert IR absorption intensities of adsorbed water to its absolute amounts
(Schuttlefield et al., 2007a; Ma et al., 2010b; Tang et al., 2019a). The
relative differences between these studies and our work were typically
within a factor of 2–3; since even for dust samples with the same name,
samples examined in different studies may actually differ substantially in
composition and water uptake properties, the agreement between these studies
and our work can be considered fairly good.
QCM is another technique widely used to investigate water uptake onto
mineral dust (Schuttlefield et al., 2007b; Hatch et al., 2008;
Schuttlefield, 2008; Navea et al., 2010; Hatch et al., 2011;
Yeşilbaş and Boily, 2016). As shown in Sect. 3.2.1–3.2.6, though
good agreement was found for some mineral dust between our work and these
QCM studies, large discrepancies (up to 2–3 orders of magnitude) were
frequently observed. This implies that the underlying assumptions required
to convert the change in resonance frequency of the quartz crystal to the
change in sample mass may not be fulfilled, and as a result the QCM results
should be used with caution.
For the same dust (at least with the same name), different samples with
distinctive hygroscopicity may have been used in our work and previous
studies, contributing to the observed discrepancies; in addition, previous
work may adopt various pretreatment procedures, and it is difficult to
assess the effects of these pretreatments on dust hygroscopicity reported in
different studies. To further understand and resolve the discrepancies
identified, it will be very useful to distribute the same samples to
different groups (in which different techniques would be applied to study
their hygroscopic properties) and compare the results obtained; furthermore,
these samples should be pretreated with the same or very similar protocols after being
received by different groups. Similar strategies have already been adopted
before to compare different instruments used for ice nucleation research and
shown to be valuable (Hiranuma et al., 2015; DeMott et al., 2018).
Hygroscopicity parameterizations
It has been suggested that water adsorption and hygroscopicity of insoluble
particles can be parameterized as a function of RH by several theoretical
models, including (1) the Brunauer–Emmet–Teller (BET) adsorption isotherm
(Goodman et al., 2001; Ma et al., 2010a; Joshi et al., 2017; Ibrahim et
al., 2018), (2) the Freundlich adsorption isotherm (Hatch et al.,
2011), (3) the Frenkel–Halsey–Hill (FHH) adsorption isotherm (Kumar et al.,
2011b; Hatch et al., 2014; Hung et al., 2015; Hatch et al., 2019) and (4) the
κ-Köhler equation (Chen et al., 2019; Tang et al., 2019b). In
this work we attempted to use the aforementioned four models to fit our
experimental data. As shown in Fig. 10 (where SiO2, albite, kaolinite
and TLF dust are used as examples), our work suggested that the FHH
adsorption isotherm could describe the measured hygroscopicity of
mineral dust samples as a function of RH well. In addition, we found that the other
three parameterization methods could not fit our experimental data.
Surface coverages (θ) of adsorbed water on (a) SiO2, (b) albite, (c) kaolinite and (d) TLF dust as a function of RH
(0 %–90 %) at 25 ∘C. The experimental data were fitted with
Frenkel–Halsey–Hill adsorption isotherm model (solid curves).
The FHH adsorption isotherm, which describes surface coverages of adsorbed
water (θ) as a function of RH, is given by Eq. (2) (Sorjamaa and
Laaksonen, 2007; Tang et al., 2016):
θ=AFHH-ln(RH)BFHH,
where AFHH and BFHH are empirical parameters. We found that Eq. (2)
can fit θ versus RH well for all the 21 mineral dust samples
examined (R2 values were found to be in the range of 0.94–0.99), and the
generated AFHH and BFHH values are summarized in Table 6. As shown
in Table 6, AFHH values spanned from 0.15 ± 0.01 (dolomite) to
4.39 ± 0.81 (ATD), while the variation in BFHH was much smaller,
ranging from 1.10 ± 0.04 (for Bordj dust) to 1.91 ± 0.18 (for
ATD). Our results were largely consistent with the theoretical work by
Sorjamaa and Laaksonen (2007), who suggested from a theoretical view that
typical AFHH and BFHH values should be in the range of 0.1–3.0 and
0.5–3.0.
Comparison of AFHH and BFHH values determined in our
work for mineral dust with those reported in previous studies (a Kumar
et al., 2011a; b Hung et al., 2015; c Hatch et al., 2019).
A few previous studies investigated hygroscopic properties (Hung et al.,
2015; Hatch et al., 2019) and cloud condensation nuclei (CCN) activities (Kumar et
al., 2011b) of mineral dust and reported AFHH and BFHH values for
samples they examined. Their results are also compiled in Table 6. As
revealed by Table 6, BFHH values reported in our work were reasonably
consistent with previous studies, while larger differences were observed for
AFHH values, especially between our study and the work by Kumar et al. (2011a) for SiO2 and CaCO3. One reason for such a large difference
is that Kumar et al. (2011a) carried out their CCN activity measurements at
> 100 % RH whereas our work on hygroscopic growth was conducted
at < 100 % RH. Another study (Kumar et al.,
2011a) reported AFHH and BFHH values for wet-generated mineral dust
aerosols. Since the hygroscopicity of wet-generated mineral dust aerosols
could be very different from dry-generated aerosols (Sullivan et al.,
2010b; Kumar et al., 2011a), the results reported by Kumar et al. (2011b)
for wet-generated aerosols are not further discussed.
Discussions
As shown in Tables 2–5, among the 21 mineral dust samples examined,
mw(90 %)/m0 (mass ratios of adsorbed water at 90 % RH to the dry
sample) was found to range from 0.0011 for CaCO3 to 0.0380 for
montmorillonite, and θ(90 %) (surface coverages of adsorbed water
at 90 % RH) varied between 1.26 for dolomite and 8.63 for TLF dust. It
appears that clay minerals and authentic mineral dust samples usually
exhibited larger hygroscopicity on a per mass basis, when compared to other
types of mineral dust. TiO2, for which mw(90 %)/m0 was only
lower than ATD, Saharan dust and montmorillonite, was an outstanding
exception, probably because of its very large BET surface area (54.6 m2 g-1).
One may expect that on a per mass basis, mineral dust samples with larger
surface area would have larger capacities to adsorb water. This was
supported by our results shown in Fig. 11a, which suggests that for mineral
dust samples considered in our study, overall mw(90 %)/m0
increased with the BET surface area. Nevertheless, not all the samples
obeyed this overall trend, as evident from Fig. 11a, indicating that other
factors would also play some roles in determining the ability of mineral
dust to adsorb water on a per mass basis. We also explored whether there was any
relationship between hygroscopicity of mineral dust samples and the soluble
materials they contained. It was found that for the 21 mineral dust samples
considered in our work, mw(90 %)/m0 did not show any general
dependence on the number of soluble inorganic ions.
(a) The dependence of mw(90 %)/m0 (mass ratios
of adsorbed water to the dry mineral at 90 % RH) on BET surface areas; (b)
the dependence of θ(90 %) (surface coverages of adsorbed water at
90 % RH) on average particle diameters.
Ibrahim et al. (2018) studied water adsorption on ATD particles with
different particle sizes, and they found that the RH at which one monolayer of
adsorbed water was formed increased with particle size; in other words, at
the same RH the surface coverages of adsorbed water would be higher for
smaller particles (Ibrahim et al., 2018). In contrast,
Yeşilbaş and Boily (2016) investigated water adsorption on different
mineral samples (21 in total) and suggested that at the same RH more
monolayers of adsorbed water would be formed on larger particles. However,
as shown in Fig. 11b in which our measured θ values at 90 % RH are
plotted versus particle size, our work revealed that surface coverages of
adsorbed water at 90 % RH showed no dependence on particle size for the 21
mineral dust samples examined in our work. This conclusion should be used
with caution as dust samples used in our work were far from being
monodisperse (see Figs. S1–S7).
Conclusions
Hygroscopicity largely determines environmental and climatic effects of
mineral dust aerosol, one of the most abundant tropospheric aerosols.
However, hygroscopic properties of mineral dust remain highly uncertain, due
to relatively low hygroscopicity of mineral dust and its non-sphericity. In
our work, a vapor sorption analyzer, which measured sample mass as a
function of RH (< 1 % to 90 %), was employed to investigate water
adsorption and hygroscopic properties of 21 different mineral dust samples,
including seven authentic mineral dust samples (from Africa, China and
the United States) and 14 major minerals found in tropospheric mineral dust
aerosol.
For all the mineral dust samples (21 in total) examined,
mw(90 %)/m0 was found to range from 0.0011 (CaCO3) to 0.3080
(montmorillonite), and θ(90 %) varied between 1.26 (dolomite) and
8.63 (TLF dust). When compared to other types of mineral dust, clay minerals
and authentic mineral dust samples usually exhibited larger hygroscopicity
on a per mass basis. Our work suggested that overall
mw(90 %)/m0 increased with the BET surface area, indicating that
on a per mass basis, mineral dust samples with larger surface area would
have larger capacities to adsorb water in general. Our results revealed no
dependence of mw(90 %)/m0 on the amount of soluble materials
contained or no dependence of θ(90 %) on particle size. In
addition, it was found in our work that the Frenkel–Halsey–Hill (FHH)
adsorption isotherm could describe surface coverages of adsorbed water
as a function of RH well for all the mineral dust investigated, and AFHH and
BFHH parameters were determined to be in the range of 0.15–4.39 and
1.10–1.91, respectively.
Data availability
Data used in this paper can be found in the main
text or Supplement of this paper.
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-13611-2020-supplement.
Author contributions
MT conceived this work; LC, CP, WG, HF and HZ carried out
experiments under the advice of XJ and MT; LC,
CP and MT analyzed the data; LC, CP and
MT wrote the manuscript with input from all the coauthors.
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
We would like to thank John Crowley (Max Planck
Institute for Chemistry, Germany) for providing Saharan dust, Pingqing Fu
(Tianjin University, China) for providing TLF dust, and Manolis Romanias
(Universiteì Lille, France) for providing M'Bour dust and Bordj dust.
Financial support
This work was sponsored by the National Natural
Science Foundation of China (42022050 and 91744204), Ministry of Science and
Technology of China (2018YFC0213901), Guangdong Foundation for Program of
Science and Technology Research (2020B1212060053 and 2019B121205006),
Guangdong Province (2017GC010501) and CAS Pioneer Hundred Talents
program.
Review statement
This paper was edited by Markus Ammann and reviewed by two anonymous referees.
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