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

NaCl in fresh sea-salt aerosol (SSA) particles can partially or fully reactwith atmospheric NO_x/HNO₃, so internally mixed NaCl and NaNO₃aerosol particles can co-exist over a wide range of mixing ratios.Laboratory-generated, micrometer-sized NaCl and NaNO₃ mixture particlesat 10 mixing ratios (mole fractions of NaCl (X_NaCl) = 0.1 to 0.9)were examined systematically to observe their hygroscopic behavior, deriveexperimental phase diagrams for deliquescence and efflorescence, andunderstand the efflorescence mechanism. During the humidifying process,aerosol particles with the eutonic composition (X_NaCl = 0.38) showedonly one phase transition at their mutual deliquescence relative humidity(MDRH) of 67.9 (±0.5)% On the other hand, particles with othermixing ratios showed two distinct deliquescence transitions; i.e., theeutonic component dissolved at MDRH, and the remainder in the solid phasedissolved completely at their DRHs depending on the mixing ratios, resultingin a phase diagram composed of four different phases, as predictedthermodynamically. 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 andthe second stage at the mutual efflorescence RH (MERH) of the eutoniccomponents, with values in the range of 30.0–35.5%. Interestingly,aerosol particles with the eutonic composition (X_NaCl = 0.38) alsoshowed two-stage efflorescence, with NaCl crystallizing first followed byheterogeneous nucleation of the remaining NaNO₃ on the NaCl seeds.NaNO₃-rich particles (X_NaCl ≤ 0.3) underwent single-stageefflorescence transitions at ERHs progressively lower than the MERH becauseof the homogeneous nucleation of NaCl and the almost simultaneousheterogeneous nucleation of NaNO₃ on the NaCl seeds. SEM/EDX elementalmapping indicated that the effloresced NaCl–NaNO₃ particles at allmixing ratios were composed of a homogeneously crystallized NaCl moiety inthe center, surrounded either by the eutonic component (for X_NaCl> 0.38) or NaNO₃ (for X_NaCl ≤ 0.38). During thehumidifying or dehydration process, the amount of eutonic composed partdrives particle/droplet growth or shrinkage at the MDRH or MERH (secondERH), respectively, and the amount of pure salts (NaCl or NaNO₃ inNaCl- or NaNO₃-rich particles, respectively) drives the second DRHs orfirst ERHs, respectively. Therefore, their behavior can be a precursor tothe optical properties and direct radiative forcing for theseatmospherically relevant mixture particles representing the coarse, reactedinorganic SSAs. In addition, the NaCl–NaNO₃ mixture aerosol particlescan maintain an aqueous phase over a wider RH range than pure NaCl particlesas SSA surrogate, making their heterogeneous chemistry more probable