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HETEROGENEOUS NUCLEATION OF NaCl DIHYDRATE IN SUPERCOOLED DROPLETS
OF SEA SALT ANALOG SOLUTION
A. KISELEV, A. PECKHAUS, R. WAGNER, D. DUFT, and T. LEISNER
Karlsruhe Institute of Technology, Institute for Meteorology and Climate Research, Atmospheric Aerosol
Research Department, Hermann-von-Helmholtz pl. 1, 76344 Eggenstein-Leopoldshafen, Germany.
Keywords: Sea salt aerosol, heterogeneous nucleation, sodium chloride dihydrate, Raman spectroscopy
INTRODUCTION
Seawater spray is the dominant source of atmospheric aerosol over oceans and in coastal areas. With its
global emission of 1733 Tg/y the see spray aerosol (SSA) compares only with mineral dust (Koch, 2006).
The phase state of sea spray particles controls their physical and chemical properties, for example, in
interaction with ozone (Koop, 2000) or in heterogeneous nucleation of ice (Wise, 2012; Wagner, 2013).
The NaCl aerosol can exist either as aqueous solution droplets or in one of the two crystalline forms:
anhydrous NaCl and NaCl dihydrate (NaCl·2H2O). The latter can serve as an efficient ice nucleating particle
and is essential for the microphysics of high altitude marine clouds (Wise, 2012; Schill, 2014). The
nucleation of NaCl dihydrate in sea spray aerosol may be further influenced by the presence of inorganic
(𝐶𝑎𝑆𝑂4 , 𝑀𝑔𝐶𝑙2 , 𝑀𝑔𝑆𝑂4 , 𝐾𝑀𝑔𝐶𝑙3 ) and organic substances. Some of these salts (e.g., gypsum or bassanite)
have lower solubility than NaCl and therefore would precipitate at humidity values higher than ERH of
crystalline NaCl (Tang, 1997; Koop, 2000; Xiao, 2008; Wise, 2009). Precipitated salts could facilitate
heterogeneous nucleation of hydrated NaCl. However, reliable prediction of effloresced fraction as a
function of temperature was impossible due to the absence of experimental data on key parameters such as
solid-liquid interfacial energy or diffusivities of water and ionic species in the low temperature range.
Here we report the measurements of nucleation rate coefficient of NaCl dihydrate in the aqueous
solution droplets of pure NaCl and sea salt analogue (SSA) solution suspended in an electrodynamic balance
(EDB) (Peckhau, 2016). We have derived the interfacial energy of crystalline NaCl dihydrate in a
supersaturated NaCl solution in the temperature range from 240 to 250 K. With this data, we have applied a
heterogeneous classical nucleation theory (CNT) model to explain the high temperature shift of the NaCl
dihydrate efflorescence for SSA droplets, assuming the presence of super-micron solid inclusions (hydrate
or hemihydrate of CaSO4). Scanning Electron Microscope (SEM) and energy dispersive X-ray analysis of
individual aerosol particles collected in the cloud chamber has confirmed the presence of such inclusions.
MATERIALS AND METHODS
We have used a humidity controlled electrodynamic balance coupled to the inverted Raman
Microscope (Peckhaus, 2016). The relative humidity (RH) inside the EDB has been set between 38% and
44%. The time between injection and efflorescence of NaCl solution droplets has been recorded for every
efflorescence event, allowing for determination of volume specific nucleation rate. After the efflorescence,
both Raman spectra and optical images have been recorded allowing for the detection of particle phase and
morphology. In total, we have studied over 600 NaCl solution droplets and over 250 sea salt analogue
droplets. The homogeneous nucleation rate coefficient of NaCl dihydrate was measured for 5 different
temperatures between 240 and 250 K by plotting the fraction of effloresced particles as a function of time
and accounting for concentration change in the evaporating droplets (Peckhaus, 2016).
An environmental scanning electron microscope (ESEM FEI, Quanta 650 FEG) equipped with the
energy dispersive X-ray (EDX) spectrometer (Bruker) have been used to record images and chemical maps
of individual SSA particles deposited on a silicon wafer. The particles for the ESEM study have been
produced by depositing droplets of sea salt analogue solution (Instant Ocean®) onto a silicon wafer and
allowing them to evaporate at room conditions. Atlantic sea salt particles have been sampled from the cloud
chamber AIDA on the Nuclepore membrane filters with the pore diameter of 200 nm.
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EXPERIMENTAL RESULTS AND CNT-BASED PARAMETERIZATION
The Raman spectra of the SSA solution droplets and residual particles have been used to identify the NaCl
phase after efflorescence (Fig. 1). The spectral features used for identification of crystalline phase of NaCl
were the two sharp peaks corresponding to the stretching vibrations of water at 3424 cm-1 and 3545 cm-1,
and the librational mode at 390 cm-1.. These features are unique for 𝑁𝑎𝐶𝑙 ∙ 2𝐻2 𝑂, and, if present, always
dominate the Raman spectrum in this region due to the prevalence of NaCl in the solution Wise (2012). The
hydrated salts other than NaCl could be identified based on their own characteristic H 2O stretching and
bending modes. For example, the spectrum shown in black in the Figure 4 is characteristic for carnallite
(𝐾𝑀𝑔𝐶𝑙3 ∙ 6𝐻2 𝑂), found previously in SSA particles (Xiao, 2008). All Raman spectra of SSA particles
contained the 𝜈1 (𝑆𝑂42− ) stretching mode peak at 984 cm-1 and an additional minor feature at 1008 cm-1
indicating the presence of aqueous 𝑆𝑂42− ions in the environment of 𝐶𝑎2+ and 𝑀𝑔2+ ions (Zhang, 2000). The
Figure 1. Raman spectra of the suspended
SSA solution droplet (green), SSA residual
particles containing 𝑁𝑎𝐶𝑙 ∙ 2𝐻2 𝑂 (red),
residual particle containing NaCl and
precipitated carnallite 𝐾𝑀𝑔𝐶𝑙3 ∙ 6𝐻2 𝑂 (black),
and residual particle containing precipitated
anhydrous NaCl in the presence of dissolved
ionic species (blue). The spectra are
normalized to the sulfate stretching mode
peak at 984 cm-1 and are vertically offset for
clarity.
Raman spectra clearly show that even after efflorescence a significant amount of hydration water is present
in the SSA residual particles (Tang, 1997).
The ESEM / EDX study of residual SSA particles deposited onto a Si wafer revealed their complex
morphology and chemical composition. SSA particles contained clearly recognizable cubic crystals of
anhydrous NaCl embedded into the crust of other inorganic components (Fig. 2).
A
C
B
D
Figure 2. ESEM and shadow images of see salt residual particles. Left panel: Atlantic sea salt residual particle sampled
from the cloud chamber AIDA on the Nuclepore membrane filter. Note the needle-like bassanite CaSO4·0.5H2O
crystals. Middle panel: ESEM image of SSA residual particle overlaid with the EDX color map of Ca, Mg, and S.
Right panel: Shadow images of (A) anhydrous NaCl, (B) NaCl dihydrate, (C) SSA particles containing anhydrous
NaCl, and (D) SSA particles containing NaCl dihydrate. The scale bars in the right panel are 20 µm.
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The EDX mapping of this crust revealed that Ca, S, and O were co-located (see Fig. 2, middle panel),
suggesting formation of CaSO4·2H2O (gypsum) or CaSO4·0.5H2O (bassanite), which can be part of a gypsum
formation pathway (Wang, 2012). Both gypsum and bassanite have solubility values lower than that of NaCl,
implying that they should precipitate prior to the efflorescence water activity of NaCl dihydrate is reached.
We argue that these precipitated salts serve as centers of heterogeneous nucleation of NaCl dihydrate.
The total volume specific homogeneous nucleation rate for NaCl dihydrate in the pure NaCl droplets
has been determined from the time resolved efflorescence measurements (Fig. 3, left). It shows that the
homogeneous nucleation rate coefficient of NaCl dihydrate increases with decreasing temperature, while the
total nucleation rate does not show temperature dependence in the investigated temperature range. These
measurements allowed us to derive the interfacial energy of the NaCl dihydrate crystal in the supersaturated
NaCl solution. Based on this data and also using the parameterization of water diffusivity in concentrated
NaCl solution based on the data of Garbacz (2014) and Koop (2016), we have constructed a CNT model of
the NaCl dihydrate nucleation taking into account the temperature and concentration dependence of the
volume specific nucleation rate. This model adequately describes our experimental data and the data
previously obtained by Wagner (2012) and Wise (2012), for details see Peckhaus (2016).
The temperature-dependent formation of NaCl dihydrate in sea salt analogue solution droplets is
shown in Fig. 3, right panel. The efflorescence curve is shifted to higher temperatures by approximately
5.5 K compared to the pure NaCl case. We suggest that heterogeneous nucleation of NaCl dihydrate might
be responsible for the enhanced formation of NaCl dihydrate in SSA solution droplets. The fraction of NaCl
dihydrate forming via heterogeneous nucleation has been calculated as:
𝑓ℎ𝑒𝑡 (𝑇) = 1 − 𝑒𝑥𝑝(−𝐽ℎ𝑒𝑡 (𝑇) ∙ 𝑠𝑖𝑛𝑐𝑙 ∙ 𝑡)
(1)
where 𝑠𝑖𝑛𝑐𝑙 is the surface area of a solid inclusion and the 𝐽ℎ𝑒𝑡 (𝑇) is the heterogeneous nucleation rate
coefficient calculated with account for the reduced energy of the critical nucleus formation. Note that 𝑓ℎ𝑒𝑡 (𝑇)
does not depend on the droplet size but only on the size of inclusion.
Figure 3. Left panel: Volume specific nucleation rates of NaCl dihydrate calculated from efflorescence experiment.
Right panel: Fraction of NaCl dihydrate in SSA particles and the CNT-based parametrization of heterogeneous
nucleation of NaCl dihydrate in SSA droplets (the solid black line and the shaded area). Experimental data and the CNT
parameterization of homogeneous nucleation of NaCl dihydrate in NaCl solution droplets are given in diamonds and
black dashed line respectively.
The CNT-based model for NaCl dihydrate is shown as the solid black line in Figure 3, right panel. We
calculate the fractions of precipitated NaCl dihydrate as a function of temperature assuming the size of an
inclusion 𝑑𝑖𝑛𝑐𝑙 = 3 µ𝑚 but otherwise keeping all experimental parameters as in the case of pure NaCl
solution. The calculated curve adequately reproduces the measurement data. By allowing a variability of
inclusion size within (3 ± 1) 𝜇𝑚 the spread of the experimental data can be covered as well (the grey shaded
area in Figure 3, right panel). The chosen size of inclusion roughly corresponds to the mass fraction of 𝐶𝑎𝑆𝑂4
in the Instant Ocean® sea salt analogue mixture, under assumption that all 𝐶𝑎𝑆𝑂4 in a droplet precipitates
into a single spherical particle.
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CONCLUSIONS
We report series of single droplet efflorescence experiments with micron-sized droplets of NaCl and sea salt
analogue (Instant Ocean®) solutions suspended in the EDB at constant temperature and humidity. Our
motivation was to establish a relationship between the crystalline phase of the effloresced particle and the
thermodynamic and kinetic conditions of a solution droplet. The temperature-dependent partitioning between
anhydrous NaCl and NaCl dihydrate has been observed in the temperature range from 240 K to 250 K, with
NaCl dihydrate mostly forming at lower temperatures. Our experimental results are in a good agreement with
the data of Wise et al. (2012) but differ from the observations of Wagner et al. (2012) where a strong increase
in dihydrate formation was observed at temperatures below 235 K. Using our nucleation rate measurements
and parameterization of water diffusivity of Garbacz (2014), we have constructed a CNT model of the NaCl
dihydrate nucleation that has resolved this apparent inconsistency.
Following the concept of heterogeneous nucleation, the CNT model was adjusted to account for the
presence of solid inclusions in natural seawater droplets and in SSA solution droplets. These inclusions,
presumably being inorganic salts of lower solubility, are capable of catalyzing the nucleation of NaCl
dihydrate at lower concentration and / or higher temperatures, or shorter induction times. Assuming the
presence of such super-micron inclusions, our model was able to predict the 5 K shift of dihydrate
efflorescence curve towards higher temperatures observed for the SSA solution droplets. An electron
microscope analysis of droplet residuals revealed the presence of CaSO4·2H2O (gypsum) or CaSO4·0.5H2O
(bassanite) crystals, potentially responsible for the enhanced nucleation rate of NaCl dihydrate.
ACKNOWLEDGEMENTS
This work was supported by the Helmholtz Association under Atmosphere and Climate Research Programme
(ATMO), and by Graduate School of Climate and Environment (GRACE) and Karlsruhe House of Young
Scientists (KHYS).
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