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

Abstract. 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 (X_NaCl) ≤ 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 X_NaCl = 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 ≤ X_NaCl ≤ 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 ≤ X_NaCl ≤ 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 X_NaCl = 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 X_NaCl = 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 (X_NaCl = 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.