Hygroscopic properties of NaCl and NaNO<sub>3</sub> mixture particles as reacted inorganic sea-salt aerosol surrogates

Abstract

Abstract. NaCl in fresh sea-salt aerosol (SSA) particles can partially or fully react with atmospheric NO x /HNO 3 , so internally mixed NaCl and NaNO 3 aerosol particles can co-exist over a wide range of mixing ratios. Laboratorygenerated, micrometer-sized NaCl and NaNO 3 mixture particles at 10 mixing ratios (mole fractions of NaCl (X NaCl ) = 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 (X NaCl = 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 (X NaCl > 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 (X NaCl = 0.38) also showed two-stage efflorescence, with NaCl crystallizing first followed by heterogeneous nucleation of the remaining NaNO 3 on the NaCl seeds. NaNO 3 -rich particles (X NaCl ≤ 0.3) underwent singlestage efflorescence transitions at ERHs progressively lower than the MERH because of the homogeneous nucleation of NaCl and the almost simultaneous heterogeneous nucleation of NaNO 3 on the NaCl seeds. SEM/EDX elemental mapping indicated that the effloresced NaCl-NaNO 3 particles at all mixing ratios were composed of a homogeneously crystallized NaCl moiety in the center, surrounded either by the eutonic component (for X NaCl > 0.38) or NaNO 3 (for X NaCl ≤ 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 NaNO 3 in NaCl-or NaNO 3 -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-NaNO 3 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.

Hygroscopic behavior of NaCl–MgCl<sub>2</sub> mixture particles as nascent sea-spray aerosol surrogates and observation of efflorescence during humidification

“…For the NaCl-rich particles with compositions of X NaCl ≥ 0.3, the MDRH is independent of the compositions. On the other hand, the second DRHs were dependent on the compositions and shift toward a pure NaCl limit (DRH = 75.1 (±0.5) %) with increasing NaCl mole fraction, as observed for NaCl-KCl and NaCl-NaNO 3 mixture particles Gupta et al, 2015).…”

“…and a eutonic composition (X NaCl = 0.026, which was calculated from the ionic activity products predicted by the AIOMFAC model). Unlike the eutonic compositions of other binary mixture particles, such as NaCl-KCl (X NaCl = 0.7) and NaCl-NaNO 3 (X NaCl = 0.38) (Gupta et al, 2015), the NaCl-MgCl 2 mixture system has a MgCl 2 -dominant eutonic composition. In the context of thermodynamics, particles with a composition of X NaCl > 0.026 were defined as NaCl-rich and those with X NaCl < 0.026 were defined as MgCl 2 -rich.…”

“…The theoretical aspects of the hygroscopic properties of binary mixture particles are discussed in detail elsewhere Gupta et al, 2015). In general, for two-component inorganic hygroscopic salt particles, the equilibrium thermodynamics predict two-stage deliquescence and efflorescence transitions.…”

“…After the hygroscopicity measurements of the individual particles, SEM/EDX was performed on the effloresced NaClMgCl 2 mixture particles to determine the morphology and spatial distribution of the chemical elements Gupta et al, 2015). The SEM/EDX measurements were carried out using a Jeol JSM-6390 SEM equipped with an Oxford Link SATW ultrathin window EDX detector.…”

Climate engineering by mimicking natural dust climate control: the iron salt aerosol method

“…Known salt aerosol generation methods by vapor condensation or nebulization (Biskos et al, 2006;Gupta et al, 2015) allow not only the flame-descending ISA type 1 (Oeste, 2004) but also the condensation-and nebulizationdescending ISA variants 2 and 3 (see Sect. 7) to be produced with aerosol particle diameters between 0.1 and 0.01 µm.…”