Abstract
Abstract. NaCl in fresh sea-salt aerosol (SSA) particles can partially or fully react with atmospheric NOx/HNO3, so internally mixed NaCl and NaNO3 aerosol particles can co-exist over a wide range of mixing ratios. Laboratory-generated, micrometer-sized NaCl and NaNO3 mixture particles at 10 mixing ratios (mole fractions of NaCl (XNaCl) = 0.1 to 0.9) were examined systematically to observe their hygroscopic behavior, derive experimental phase diagrams for deliquescence and efflorescence, and understand the efflorescence mechanism. During the humidifying process, aerosol particles with the eutonic composition (XNaCl = 0.38) showed only one phase transition at their mutual deliquescence relative humidity (MDRH) of 67.9 (±0.5)% On the other hand, particles with other mixing ratios showed two distinct deliquescence transitions; i.e., the eutonic component dissolved at MDRH, and the remainder in the solid phase dissolved completely at their DRHs depending on the mixing ratios, resulting in a phase diagram composed of four different phases, as predicted thermodynamically. During the dehydration process, NaCl-rich particles (XNaCl > 0.38) showed a two stage efflorescence transition: the first stage was purely driven by the homogeneous nucleation of NaCl and the second stage at the mutual efflorescence RH (MERH) of the eutonic components, with values in the range of 30.0–35.5%. Interestingly, aerosol particles with the eutonic composition (XNaCl = 0.38) also showed two-stage efflorescence, with NaCl crystallizing first followed by heterogeneous nucleation of the remaining NaNO3 on the NaCl seeds. NaNO3-rich particles (XNaCl ≤ 0.3) underwent single-stage efflorescence transitions at ERHs progressively lower than the MERH because of the homogeneous nucleation of NaCl and the almost simultaneous heterogeneous nucleation of NaNO3 on the NaCl seeds. SEM/EDX elemental mapping indicated that the effloresced NaCl–NaNO3 particles at all mixing ratios were composed of a homogeneously crystallized NaCl moiety in the center, surrounded either by the eutonic component (for XNaCl > 0.38) or NaNO3 (for XNaCl ≤ 0.38). During the humidifying or dehydration process, the amount of eutonic composed part drives particle/droplet growth or shrinkage at the MDRH or MERH (second ERH), respectively, and the amount of pure salts (NaCl or NaNO3 in NaCl- or NaNO3-rich particles, respectively) drives the second DRHs or first ERHs, respectively. Therefore, their behavior can be a precursor to the optical properties and direct radiative forcing for these atmospherically relevant mixture particles representing the coarse, reacted inorganic SSAs. In addition, the NaCl–NaNO3 mixture aerosol particles can maintain an aqueous phase over a wider RH range than pure NaCl particles as SSA surrogate, making their heterogeneous chemistry more probable.
Highlights
Atmospheric aerosols play important roles in global climate change, directly by scattering or absorbing incoming solar radiation and indirectly by serving as cloud condensation nuclei (Pandis et al, 1995; Satheesh and Moorthy, 2005)
Aerosol particles generated from a pure NaCl aqueous solution showed typical hysteresis curves with DRH = 75.5 (±0.5) % and efflorescence RH (ERH) = 47.6–46.3 %, and these values were consistent with the reported values (Wise et al, 2007; Tang et al, 1997)
The hygroscopic behavior and microstructures of mixed NaCl–NaNO3 particles at various mixing ratios collected on transmission electron microscopy (TEM) grids were investigated by the use of optical microscopy and SEM/EDX
Summary
Atmospheric aerosols play important roles in global climate change, directly by scattering or absorbing incoming solar radiation and indirectly by serving as cloud condensation nuclei (Pandis et al, 1995; Satheesh and Moorthy, 2005). A detailed knowledge of the hygroscopic properties, mixing states, and the spatial distribution of the chemical components in NaCl– NaNO3 mixture particles, as partially or fully reacted SSA surrogates, can serve as a good preliminary step to a better understanding of the complex chemical/physical mixing states, hygroscopic behavior, and reactivity of ambient SSAs. Many studies have examined the hygroscopic properties of two-component inorganic salt particles as inorganic aerosol surrogates (Cohen et al, 1987a; Tang and Munkelwitz, 1993, 1994a, b; Tang et al, 1978; Ge et al, 1996, 1998; Chang and Lee, 2002). For two-component inorganic hygroscopic salt particles, the first transition generally occurs at their MDRH, and the aqueous phase resulting from the partial deliquescence has the eutonic composition. A recent experimental study of the two component NaCl–KCl mixture particles showed that during the dehydration process, the aqueous droplets of various mixing ratios underwent single step efflorescence (Li et al, 2014). NaCl–NaNO3 binary aerosol particles as reacted SSA surrogates at 10 different mixing ratios for the first time
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