 |
| Titel |
Hygroscopic behavior of NaCl–MgCl2 mixture particles as nascent sea-spray aerosol surrogates and observation of efflorescence during humidification |
| VerfasserIn |
D. Gupta, H.-J. Eom, H.-R. Cho, C.-U. Ro |
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| ISSN |
1680-7316
|
| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 19 ; Nr. 15, no. 19 (2015-10-12), S.11273-11290 |
| Datensatznummer |
250120089
|
| Publikation (Nr.) |
 copernicus.org/acp-15-11273-2015.pdf |
|
|
|
|
|
| Zusammenfassung |
| As Na+, Mg2+, and Cl− are major ionic constituents of
seawater, NaCl–MgCl2 mixture particles might represent sea-spray
aerosols (SSAs) better than pure NaCl. However, there have been very few
hygroscopic studies of pure MgCl2 and NaCl–MgCl2 mixture aerosol
particles despite the MgCl2 moiety playing a major role in the
hygroscopic behavior of nascent SSAs. Laboratory-generated pure MgCl2
and NaCl–MgCl2 mixture aerosol particles with 12 mixing ratios
(0.01 ≤ mole fraction of NaCl (XNaCl) ≤ 0.9) were
examined systematically by optical microscopy (OM), in situ Raman
micro-spectrometry (RMS), and scanning electron microscopy/energy dispersive
X-ray spectrometry (SEM/EDX) elemental X-ray mapping to observe their
hygroscopic behavior, derive the experimental phase diagrams, and obtain the
chemical micro-structures. Dry-deposited MgCl2 ⋅ 6H2O
particles exhibited a deliquescence relative humidity (DRH) of
~ 33.0 % and an efflorescence RH (ERH) of 10.8–9.1 %, whereas
the nebulized pure MgCl2 and MgCl2-dominant particles of
XNaCl = 0.026 (eutonic) and 0.01 showed single-stage transitions
at DRH of ~ 15.9 % and ERH of 10.1–3.2 %. The characteristic
OH-stretching Raman signatures indicated the crystallization of
MgCl2 ⋅ 4H2O at low relative humidities (RHs),
suggesting that the kinetic barrier to MgCl2 ⋅ 6H2O
crystallization is not overcome in the timescale of the dehydration
measurements. The NaCl–MgCl2 mixture particles of
0.05 ≤ XNaCl ≤ 0.9 generally showed two-stage
deliquescence: first at the mutual DRH (MDRH) of ~ 15.9 %; and
second with the complete dissolution of NaCl at the second DRHs depending on
the mixing ratios, resulting in a phase diagram composed of three distinct
phases. During dehydration, most particles of
0.05 ≤ XNaCl ≤ 0.9 exhibited two-stage efflorescence:
first, by the homogeneous nucleation of NaCl; and second, at mutual ERH
(MERH) of ~ 10.4–2.9 %, by the crystallization of the
MgCl2 ⋅ 4H2O moiety, also resulting in three distinct
phases. Interestingly, particles of XNaCl = 0.1 and 0.2
frequently showed three different types of mutual deliquescence behaviors.
The first type exhibited an MDRH at ~ 15.9 %. The second exhibited
the first MDRH at ~ 15.9 %, efflorescence to
MgCl2 ⋅ 6H2O (confirmed by in situ RMS) at RH of
~ 16.1–25.0 %, and a second MDRH at ~ 33.0 %. The third
showed an MDRH at ~ 33.0 %. Some particles of
XNaCl = 0.1 and 0.2 also exhibited higher MERHs
= 15.2–11.9 % and 23.7–15.3 %, respectively, forming
MgCl2 ⋅ 6H2O. These observations suggest that the
presence of sufficient condensed water and optimally sized crystalline NaCl
(XNaCl = 0.1 and 0.2) acting as heterogeneous nucleation seeds
helps overcome the kinetic barrier, leading to the structural growth and
crystallization of MgCl2 ⋅ 6H2O. SEM/EDX elemental X-ray
mapping showed that the effloresced NaCl-rich particles contain homogeneously
crystallized NaCl in the center, surrounded by
MgCl2 ⋅ 4H2O. The observation of an aqueous phase over a
wider RH range for NaCl–MgCl2 mixture particles indicates their more
probable heterogeneous chemistry compared to pure NaCl particles as a nascent
SSA surrogate. |
| |
|
| Teil von |
|
|
|
|
|
|
|
|