Hygroscopic behavior of NaCl-MgCl2 mixture particles as nascent sea-spray aerosol surrogates and observation of efflorescence during humidifying process

Abstract

NaCl and MgCl2 are the two major constituents of seawater, so NaCl-MgCl2 mixture particles can be a better representative of sea-spray aerosols (SSAs) 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, in-situ Raman microspectrometry (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 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 3 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.